From 7c7d0ee25e8ad3f83d635dec264965d2d216d8f0 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:32 +0200
Subject: [PATCH 01/30] Fix ReadBufferFromFileView position tracking (B115)

ReadBufferFromFileView assumed the inner buffer keeps its working buffer
across setReadUntilPosition; ReadBufferFromS3 resets it, so getPosition
lied and seek logic could re-read a stale decompressed block. Recompute
the offset from the inner buffer after every right-bound change.
Includes the ReadBufferFromMemory counterpart and gtest batteries.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/IO/ReadBufferFromFileView.cpp             |  63 +++-
 src/IO/ReadBufferFromMemory.cpp               |   5 +-
 .../gtest_read_buffer_from_file_view.cpp      | 280 ++++++++++++++++++
 .../tests/gtest_read_buffer_from_memory.cpp   |  19 ++
 4 files changed, 357 insertions(+), 10 deletions(-)
 create mode 100644 src/IO/tests/gtest_read_buffer_from_file_view.cpp
 create mode 100644 src/IO/tests/gtest_read_buffer_from_memory.cpp

diff --git a/src/IO/ReadBufferFromFileView.cpp b/src/IO/ReadBufferFromFileView.cpp
index 1304f372df06..22de4731673f 100644
--- a/src/IO/ReadBufferFromFileView.cpp
+++ b/src/IO/ReadBufferFromFileView.cpp
@@ -19,11 +19,13 @@ ReadBufferFromFileView::ReadBufferFromFileView(
     , file_offset_of_buffer_end(left_bound_)
     , original_working_buffer(working_buffer)
 {
-    /// Seek to the begin of file.
+    /// Seek to the begin of file. The impl still owns its native buffer state here (no swap yet),
+    /// so its buffer-end offset can be read directly after the seek.
     impl->seek(left_bound, SEEK_SET);
+    const size_t impl_buffer_end = impl->getPosition() + impl->available();
     swap(*impl);
 
-    file_offset_of_buffer_end += available();
+    file_offset_of_buffer_end = impl_buffer_end;
     original_working_buffer = working_buffer;
     resizeWorkingBuffer();
 }
@@ -40,14 +42,31 @@ void ReadBufferFromFileView::setReadUntilPosition(size_t position)
         throw Exception(ErrorCodes::ARGUMENT_OUT_OF_BOUND,
             "Cannot read until position: {}. File size is {}", position, getFileSize());
 
-    executeWithOriginalBuffer([&]{ impl->setReadUntilPosition(*read_until_position); });
+    /// The impl is allowed to DISCARD its working buffer here (e.g. `ReadBufferFromS3` rebases its
+    /// offset to the consumer position and resets the buffer when the range changes), so the view's
+    /// buffer-end offset MUST be rebased from the impl's post-op state - keeping the stale value
+    /// over a replaced buffer silently shifts the reported position by the discarded bytes.
+    size_t impl_buffer_end = 0;
+    executeWithOriginalBuffer([&]
+    {
+        impl->setReadUntilPosition(*read_until_position);
+        impl_buffer_end = impl->getPosition() + impl->available();
+    });
+    file_offset_of_buffer_end = impl_buffer_end;
     resizeWorkingBuffer();
 }
 
 void ReadBufferFromFileView::setReadUntilEnd()
 {
     read_until_position.reset();
-    executeWithOriginalBuffer([&]{ impl->setReadUntilPosition(right_bound); });
+    /// Same rebase contract as setReadUntilPosition.
+    size_t impl_buffer_end = 0;
+    executeWithOriginalBuffer([&]
+    {
+        impl->setReadUntilPosition(right_bound);
+        impl_buffer_end = impl->getPosition() + impl->available();
+    });
+    file_offset_of_buffer_end = impl_buffer_end;
     resizeWorkingBuffer();
 }
 
@@ -63,11 +82,19 @@ bool ReadBufferFromFileView::nextImpl()
         return false;
 
     bool result = false;
-    executeWithOriginalBuffer([&] { result = impl->next(); });
+    size_t impl_buffer_end = 0;
+    executeWithOriginalBuffer([&]
+    {
+        result = impl->next();
+        impl_buffer_end = impl->getPosition() + impl->available();
+    });
 
     if (result)
     {
-        file_offset_of_buffer_end += available();
+        /// Rebase from the impl's own accounting instead of incrementing: the view's previous
+        /// buffer-end may have been clamped by resizeWorkingBuffer below the impl's real one, and
+        /// the impl continues from ITS position - incrementing would mislabel the new chunk.
+        file_offset_of_buffer_end = impl_buffer_end;
         resizeWorkingBuffer();
     }
 
@@ -87,7 +114,12 @@ off_t ReadBufferFromFileView::seek(off_t off, int whence)
         throw Exception(ErrorCodes::ARGUMENT_OUT_OF_BOUND, "ReadBufferFromFileView::seek expects SEEK_SET or SEEK_CUR as whence");
 
     off_t result = 0;
-    executeWithOriginalBuffer([&] { result = impl->seek(new_pos, SEEK_SET); });
+    size_t impl_buffer_end = 0;
+    executeWithOriginalBuffer([&]
+    {
+        result = impl->seek(new_pos, SEEK_SET);
+        impl_buffer_end = impl->getPosition() + impl->available();
+    });
 
     if (result < 0)
         throw Exception(ErrorCodes::SEEK_POSITION_OUT_OF_BOUND, "Seek position ({}) underflow", result);
@@ -96,7 +128,7 @@ off_t ReadBufferFromFileView::seek(off_t off, int whence)
         throw Exception(ErrorCodes::SEEK_POSITION_OUT_OF_BOUND,
             "Seek position ({}) is out of bound. Available range: [{}, {}]", result, left_bound, right_bound);
 
-    file_offset_of_buffer_end = result + available();
+    file_offset_of_buffer_end = impl_buffer_end;
     resizeWorkingBuffer();
 
     return result - left_bound;
@@ -110,7 +142,20 @@ void ReadBufferFromFileView::executeWithOriginalBuffer(Op && op)
 
     /// Set working buffer and other internal into impl.
     swap(*impl);
-    op();
+    try
+    {
+        op();
+    }
+    catch (...)
+    {
+        /// The swap MUST be undone even if `op` throws — otherwise `this` and `impl` are left holding
+        /// each other's working buffers (and a stale `original_working_buffer`), so any subsequent
+        /// read or seek over-reads / serves wrong bytes. `op` can throw (e.g. setReadUntilPosition /
+        /// seek bound checks), so restore-on-exception is required for the view to stay consistent.
+        swap(*impl);
+        original_working_buffer = working_buffer;
+        throw;
+    }
     swap(*impl);
 
     original_working_buffer = working_buffer;
diff --git a/src/IO/ReadBufferFromMemory.cpp b/src/IO/ReadBufferFromMemory.cpp
index 882f8b6a07d3..9f3c20fc51e2 100644
--- a/src/IO/ReadBufferFromMemory.cpp
+++ b/src/IO/ReadBufferFromMemory.cpp
@@ -76,7 +76,10 @@ ReadBufferFromMemoryFileBase::ReadBufferFromMemoryFileBase(bool owns_memory,
 {
     chassert(data.size() == internal_buffer.size());
 
-    if (owns_memory)
+    /// memcpy's pointers are __attribute__((nonnull)) even when the length is 0. An empty file yields
+    /// data.data() == nullptr, so guard on non-empty to avoid the nonnull-attribute UB the asan_ubsan
+    /// lane aborts on (STID 5930-5afa). Nothing to copy when empty.
+    if (owns_memory && !data.empty())
         std::memcpy(internal_buffer.begin(), data.data(), data.size());
 
     working_buffer = internal_buffer;
diff --git a/src/IO/tests/gtest_read_buffer_from_file_view.cpp b/src/IO/tests/gtest_read_buffer_from_file_view.cpp
new file mode 100644
index 000000000000..b4a433361856
--- /dev/null
+++ b/src/IO/tests/gtest_read_buffer_from_file_view.cpp
@@ -0,0 +1,280 @@
+#include <gtest/gtest.h>
+
+#include <IO/ReadBufferFromFileView.h>
+#include <IO/ReadBufferFromFileBase.h>
+
+#include <cstring>
+
+using namespace DB;
+
+namespace
+{
+
+/// How the inner buffer reacts to setReadUntilPosition - the axis that broke B115.
+enum class InnerMode : uint8_t
+{
+    /// Like local file descriptors: setReadUntilPosition is a no-op, the buffer is kept.
+    FileLike,
+    /// Like ReadBufferFromS3: a range change rebases the offset to the CONSUMER position and
+    /// DISCARDS the working buffer (the next nextImpl re-fetches from the consumer position).
+    RemoteLike,
+};
+
+/// A seekable ReadBufferFromFileBase over a string, reading at most `chunk` bytes per nextImpl,
+/// with selectable setReadUntilPosition semantics. Mirrors the state conventions of real
+/// implementations: `file_offset` is the absolute offset of working_buffer.end().
+class FakeInnerBuffer : public ReadBufferFromFileBase
+{
+public:
+    FakeInnerBuffer(String data_, size_t chunk, InnerMode mode_)
+        : ReadBufferFromFileBase(chunk, nullptr, 0)
+        , data(std::move(data_))
+        , mode(mode_)
+    {
+    }
+
+    String getFileName() const override { return "fake_inner"; }
+    std::optional<size_t> tryGetFileSize() override { return data.size(); }
+    size_t getFileOffsetOfBufferEnd() const override { return file_offset; }
+    off_t getPosition() override { return file_offset - available(); }
+
+    off_t seek(off_t off, int whence) override
+    {
+        EXPECT_EQ(whence, SEEK_SET);
+        const size_t target = static_cast<size_t>(off);
+        /// In-buffer seek (both real local and S3 buffers do this).
+        if (!working_buffer.empty() && target + working_buffer.size() >= file_offset && target < file_offset)
+        {
+            pos = working_buffer.end() - (file_offset - target);
+            return off;
+        }
+        resetWorkingBuffer();
+        file_offset = target;
+        return off;
+    }
+
+    void setReadUntilPosition(size_t position) override
+    {
+        if (read_until && *read_until == position)
+            return;
+        if (mode == InnerMode::RemoteLike)
+        {
+            /// ReadBufferFromS3: offset = getPosition(); resetWorkingBuffer(); impl.reset();
+            file_offset = getPosition();
+            resetWorkingBuffer();
+        }
+        read_until = position;
+    }
+
+    void setReadUntilEnd() override { setReadUntilPosition(data.size()); }
+
+private:
+    bool nextImpl() override
+    {
+        const size_t limit = read_until ? std::min(*read_until, data.size()) : data.size();
+        if (file_offset >= limit)
+            return false;
+        const size_t to_read = std::min(limit - file_offset, internal_buffer.size());
+        memcpy(internal_buffer.begin(), data.data() + file_offset, to_read);
+        working_buffer = Buffer(internal_buffer.begin(), internal_buffer.begin() + to_read);
+        file_offset += to_read;
+        return true;
+    }
+
+    String data;
+    InnerMode mode;
+    size_t file_offset = 0;
+    std::optional<size_t> read_until;
+};
+
+constexpr size_t kHeader = 256; /// the view's left bound (the CHCA envelope size in production)
+
+String makePayload(size_t size)
+{
+    String s(size, 0);
+    for (size_t i = 0; i < size; ++i)
+        s[i] = static_cast<char>((i * 131 + 7) % 251);
+    return s;
+}
+
+std::unique_ptr<ReadBufferFromFileView> makeView(const String & payload, size_t chunk, InnerMode mode)
+{
+    String object = String(kHeader, '\xee') + payload;
+    auto inner = std::make_unique<FakeInnerBuffer>(std::move(object), chunk, mode);
+    return std::make_unique<ReadBufferFromFileView>(std::move(inner), "viewed", kHeader, kHeader + payload.size());
+}
+
+String readExact(ReadBuffer & buf, size_t n)
+{
+    String out(n, 0);
+    buf.readStrict(out.data(), n);
+    return out;
+}
+
+struct Case
+{
+    size_t chunk;
+    InnerMode mode;
+};
+
+class ReadBufferFromFileViewTest : public ::testing::TestWithParam<Case>
+{
+};
+
+}
+
+TEST_P(ReadBufferFromFileViewTest, SequentialReadWholeView)
+{
+    const auto [chunk, mode] = GetParam();
+    const auto payload = makePayload(1000);
+    auto view = makeView(payload, chunk, mode);
+
+    EXPECT_EQ(readExact(*view, payload.size()), payload);
+    EXPECT_TRUE(view->eof());
+    EXPECT_EQ(view->getPosition(), static_cast<off_t>(payload.size()));
+}
+
+TEST_P(ReadBufferFromFileViewTest, SeekAndRead)
+{
+    const auto [chunk, mode] = GetParam();
+    const auto payload = makePayload(1000);
+    auto view = makeView(payload, chunk, mode);
+
+    for (size_t target : {size_t(0), size_t(700), size_t(20), size_t(21), size_t(999), size_t(5)})
+    {
+        EXPECT_EQ(view->seek(target, SEEK_SET), static_cast<off_t>(target));
+        EXPECT_EQ(view->getPosition(), static_cast<off_t>(target));
+        EXPECT_EQ(readExact(*view, 1), payload.substr(target, 1));
+        EXPECT_EQ(view->getPosition(), static_cast<off_t>(target + 1));
+    }
+}
+
+/// B115 regression. The in-order MergeTree reader adjusts the right mark (setReadUntilPosition)
+/// while the consumer is mid-buffer. A remote-like inner buffer legitimately discards its working
+/// buffer on the range change; the view MUST keep reporting the consumer's position - before the
+/// fix it teleported forward by the discarded bytes, so the next seek was treated as "already
+/// there" and a stale block was re-served (duplicated + missing granules at the SQL level).
+TEST_P(ReadBufferFromFileViewTest, SetReadUntilPositionMidBufferKeepsPosition)
+{
+    const auto [chunk, mode] = GetParam();
+    const auto payload = makePayload(1000);
+    auto view = makeView(payload, chunk, mode);
+
+    EXPECT_EQ(readExact(*view, 36), payload.substr(0, 36));
+    EXPECT_EQ(view->getPosition(), 36);
+
+    view->setReadUntilPosition(72);
+    EXPECT_EQ(view->getPosition(), 36) << "position must survive a right-bound change";
+
+    /// The consumer's next seek to its current position must be a no-op...
+    EXPECT_EQ(view->seek(36, SEEK_SET), 36);
+    /// ...and the bytes must continue from 36, not from a stale buffer.
+    EXPECT_EQ(readExact(*view, 36), payload.substr(36, 36));
+}
+
+/// Truncate-then-extend: the right bound shrinks below already-buffered data, the consumer reads
+/// up to it, the bound is extended again. The continuation must produce the file's real bytes
+/// (before the fix the view's incremental buffer-end accounting drifted from the inner buffer's).
+TEST_P(ReadBufferFromFileViewTest, SetReadUntilTruncateThenExtend)
+{
+    const auto [chunk, mode] = GetParam();
+    const auto payload = makePayload(1000);
+    auto view = makeView(payload, chunk, mode);
+
+    EXPECT_EQ(readExact(*view, 10), payload.substr(0, 10));
+
+    view->setReadUntilPosition(30);
+    EXPECT_EQ(view->getPosition(), 10);
+    EXPECT_EQ(readExact(*view, 20), payload.substr(10, 20));
+    EXPECT_TRUE(view->eof());
+    EXPECT_EQ(view->getPosition(), 30);
+
+    view->setReadUntilPosition(500);
+    EXPECT_EQ(view->getPosition(), 30);
+    EXPECT_EQ(readExact(*view, 100), payload.substr(30, 100));
+
+    view->setReadUntilEnd();
+    EXPECT_EQ(readExact(*view, payload.size() - 130), payload.substr(130));
+    EXPECT_TRUE(view->eof());
+}
+
+/// The exact shape of the failing compact-part in-order read: per granule, adjust the right
+/// mark, seek to the granule's block, read it. Every block must contain its own bytes.
+TEST_P(ReadBufferFromFileViewTest, GranulePatternRegression)
+{
+    const auto [chunk, mode] = GetParam();
+    constexpr size_t block = 36;
+    constexpr size_t blocks = 20;
+    const auto payload = makePayload(block * blocks);
+    auto view = makeView(payload, chunk, mode);
+
+    for (size_t g = 0; g < blocks; ++g)
+    {
+        view->setReadUntilPosition(std::min((g + 2) * block, payload.size()));
+        EXPECT_EQ(view->seek(g * block, SEEK_SET), static_cast<off_t>(g * block));
+        EXPECT_EQ(readExact(*view, block), payload.substr(g * block, block)) << "block " << g;
+    }
+}
+
+/// Randomized conformance battery against a golden model.
+TEST_P(ReadBufferFromFileViewTest, RandomizedOps)
+{
+    const auto [chunk, mode] = GetParam();
+    const auto payload = makePayload(2000);
+
+    for (unsigned seed = 1; seed <= 5; ++seed)
+    {
+        auto view = makeView(payload, chunk, mode);
+        size_t model_pos = 0;
+        size_t model_until = payload.size();
+        unsigned rng = seed;
+        auto next_rand = [&rng] { rng = rng * 1103515245 + 12345; return (rng >> 8) % 1000; };
+
+        for (int step = 0; step < 300; ++step)
+        {
+            switch (next_rand() % 3)
+            {
+                case 0: /// read up to the current until-bound
+                {
+                    const size_t want = next_rand() % 64;
+                    const size_t n = std::min(want, model_until - model_pos);
+                    if (n)
+                    {
+                        ASSERT_EQ(readExact(*view, n), payload.substr(model_pos, n)) << "seed " << seed << " step " << step;
+                        model_pos += n;
+                    }
+                    break;
+                }
+                case 1: /// seek (never beyond the current until-bound - the consumer contract:
+                        /// the right mark always covers the ranges being read)
+                {
+                    const size_t target = next_rand() % (model_until + 1);
+                    ASSERT_EQ(view->seek(target, SEEK_SET), static_cast<off_t>(target));
+                    model_pos = target;
+                    break;
+                }
+                case 2: /// move the right bound (never below the consumer position)
+                {
+                    const size_t until = model_pos + next_rand() % (payload.size() - model_pos + 1);
+                    view->setReadUntilPosition(until);
+                    model_until = until;
+                    break;
+                }
+                default:
+                    UNREACHABLE();
+            }
+            ASSERT_EQ(view->getPosition(), static_cast<off_t>(model_pos)) << "seed " << seed << " step " << step;
+        }
+    }
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    ChunksAndModes,
+    ReadBufferFromFileViewTest,
+    ::testing::Values(
+        Case{7, InnerMode::FileLike},
+        Case{7, InnerMode::RemoteLike},
+        Case{108, InnerMode::FileLike},
+        Case{108, InnerMode::RemoteLike},
+        Case{1 << 20, InnerMode::FileLike},
+        Case{1 << 20, InnerMode::RemoteLike}));
diff --git a/src/IO/tests/gtest_read_buffer_from_memory.cpp b/src/IO/tests/gtest_read_buffer_from_memory.cpp
new file mode 100644
index 000000000000..b7955f816e79
--- /dev/null
+++ b/src/IO/tests/gtest_read_buffer_from_memory.cpp
@@ -0,0 +1,19 @@
+#include <gtest/gtest.h>
+
+#include <IO/ReadBufferFromMemory.h>
+
+#include <string_view>
+
+using namespace DB;
+
+/// An empty file materialized into an OWNED in-memory buffer must construct without undefined
+/// behaviour: std::memcpy's pointer arguments are __attribute__((nonnull)), so memcpy(dst, nullptr, 0)
+/// -- which an empty std::string_view (data() == nullptr) produces -- is UB that the asan_ubsan lane
+/// aborts on (STID 5930-5afa, PR #2073). The buffer must construct and be immediately at EOF.
+TEST(ReadBufferFromMemoryFileBase, EmptyOwnedBufferConstructsWithoutUB)
+{
+    /// ReadBufferFromMemoryFileBase's constructor is protected; ReadBufferFromOwnMemoryFile is the
+    /// public concrete class that always passes owns_memory=true, exercising the guarded memcpy path.
+    ReadBufferFromOwnMemoryFile buf("empty", std::string_view{});
+    EXPECT_TRUE(buf.eof());
+}

From 3d86e1cee068ba60b98307bdf3969d873b890d44 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:32 +0200
Subject: [PATCH 02/30] Stop retrying S3 reads after query cancellation (B117)

processException kept retrying transient errors after KILL QUERY; check
CurrentThread::get().isQueryCanceled() in the outer retry loop.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/IO/ReadBufferFromS3.cpp | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/IO/ReadBufferFromS3.cpp b/src/IO/ReadBufferFromS3.cpp
index f2d184fd8e71..34f1f2261891 100644
--- a/src/IO/ReadBufferFromS3.cpp
+++ b/src/IO/ReadBufferFromS3.cpp
@@ -16,6 +16,7 @@
 #include <Common/FailPoint.h>
 #include <Common/ElapsedTimeProfileEventIncrement.h>
 #include <Common/CurrentThread.h>
+#include <Common/ThreadStatus.h>
 #include <base/sleep.h>
 
 #include <cstdint>
@@ -353,6 +354,12 @@ bool ReadBufferFromS3::processException(size_t read_offset, size_t attempt) cons
         bucket, key, version_id.empty() ? "Latest" : version_id, read_offset, attempt, request_settings[S3RequestSetting::max_single_read_retries].value,
         getCurrentExceptionMessage(/* with_stacktrace = */ false));
 
+    /// Stop retrying once the query is cancelled (B117): otherwise a killed query's reads keep
+    /// retrying a transient error (e.g. a dropped connection) for many attempts with backoff,
+    /// zombying for minutes and adding load. The SDK's own RetryStrategy makes the same check
+    /// (src/IO/S3/Client.cpp), but this outer ReadBufferFromS3 retry loop did not.
+    if (CurrentThread::isInitialized() && CurrentThread::get().isQueryCanceled())
+        return false;
 
     if (auto * s3_exception = current_exception_cast<S3Exception *>())
     {

From 002f387b4f6872cd8868e6bb7b8f347ca799b6f3 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:32 +0200
Subject: [PATCH 03/30] Retain parent ThreadGroup from borrowed children (B90)

A borrowed child ThreadGroup parents its trackers at the parent group via
raw pointers; background work outliving the query produced a use-after-free
in parent counters. The child now holds a shared_ptr to the parent group.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/Common/ThreadStatus.h            | 8 ++++++++
 src/Interpreters/ThreadStatusExt.cpp | 4 ++++
 2 files changed, 12 insertions(+)

diff --git a/src/Common/ThreadStatus.h b/src/Common/ThreadStatus.h
index 975349e2dde5..ef3b73048e9d 100644
--- a/src/Common/ThreadStatus.h
+++ b/src/Common/ThreadStatus.h
@@ -84,6 +84,14 @@ class ThreadGroup
 
     const Int32 os_threads_nice_value;
 
+    /// A borrowed child group (materialized view / async-insert flush) parents its `memory_tracker`
+    /// and `performance_counters` at the parent group's trackers via RAW pointers. Retain a shared_ptr
+    /// to the parent so those trackers cannot be freed while any thread is still attached to this child
+    /// group — otherwise a detached task (e.g. an S3 upload scheduled via `threadPoolCallbackRunnerUnsafe`)
+    /// that attaches the child group can walk a freed parent tracker chain (use-after-free, B90). Null for
+    /// a top-level query/background group, whose parent is a process-lifetime tracker (user/total/background).
+    ThreadGroupPtr parent_thread_group;
+
     MemorySpillScheduler::Ptr memory_spill_scheduler;
     ProfileEvents::Counters performance_counters{VariableContext::Process};
     MemoryTracker memory_tracker{VariableContext::Process};
diff --git a/src/Interpreters/ThreadStatusExt.cpp b/src/Interpreters/ThreadStatusExt.cpp
index 813254ef842b..1ac5c8aa6050 100644
--- a/src/Interpreters/ThreadStatusExt.cpp
+++ b/src/Interpreters/ThreadStatusExt.cpp
@@ -129,6 +129,8 @@ ThreadGroup::ThreadGroup(ThreadGroupPtr parent)
     , global_context(parent->global_context)
     , fatal_error_callback(parent->fatal_error_callback)
     , os_threads_nice_value(parent->os_threads_nice_value)
+    /// Keep the parent group alive: this child parents its trackers at the parent's via raw pointers (B90).
+    , parent_thread_group(parent)
     , memory_spill_scheduler(parent->memory_spill_scheduler)
     , performance_counters(VariableContext::Process, &parent->performance_counters)
     , memory_tracker(&parent->memory_tracker, VariableContext::Process, /*log_peak_memory_usage_in_destructor*/ false)
@@ -143,6 +145,8 @@ ThreadGroup::ThreadGroup(ContextPtr query_context_, ThreadGroupPtr parent)
     , global_context(query_context_->getGlobalContext())
     , fatal_error_callback(parent->fatal_error_callback)
     , os_threads_nice_value(parent->os_threads_nice_value)
+    /// Keep the parent group alive: this child parents its trackers at the parent's via raw pointers (B90).
+    , parent_thread_group(parent)
     , memory_spill_scheduler(parent->memory_spill_scheduler)
     , performance_counters(VariableContext::Process, &parent->performance_counters)
     , memory_tracker(&parent->memory_tracker, VariableContext::Process, /*log_peak_memory_usage_in_destructor*/ false)

From a5a76b9a12daecdec7cb086c7250f7ee748466d5 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:32 +0200
Subject: [PATCH 04/30] Fail closed on a null MergeTreeDeduplicationLog writer
 (B37)

addPart/dropPart guarded current_writer only with chassert (a release
no-op), so a missing writer meant a null dereference; throw LOGICAL_ERROR
instead. Also treats a missing logs_dir as normal for storages that do
not materialize empty directories (carries a CAS-related hunk; wired later).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../MergeTree/MergeTreeDeduplicationLog.cpp   |  29 +++-
 .../gtest_deduplication_log_null_writer.cpp   | 139 ++++++++++++++++++
 2 files changed, 164 insertions(+), 4 deletions(-)
 create mode 100644 src/Storages/MergeTree/tests/gtest_deduplication_log_null_writer.cpp

diff --git a/src/Storages/MergeTree/MergeTreeDeduplicationLog.cpp b/src/Storages/MergeTree/MergeTreeDeduplicationLog.cpp
index 9987e466c53b..a4ff5691bf4a 100644
--- a/src/Storages/MergeTree/MergeTreeDeduplicationLog.cpp
+++ b/src/Storages/MergeTree/MergeTreeDeduplicationLog.cpp
@@ -20,6 +20,7 @@ namespace DB
 namespace ErrorCodes
 {
     extern const int ABORTED;
+    extern const int LOGICAL_ERROR;
 }
 
 namespace
@@ -103,8 +104,14 @@ void MergeTreeDeduplicationLog::load()
     {
         if (auto * object_storage = dynamic_cast<DiskObjectStorage *>(disk.get()))
         {
-            // MetadataStorageType::Plain does not have directory concept. When checking `logs_dir` existence, it might return false.
-            if (object_storage->getMetadataStorage()->getType() != MetadataStorageType::Plain)
+            // Plain and ContentAddressed object storages do not materialize empty directories, so a
+            // missing logs_dir is normal for a fresh table: fall through so the current_writer is still
+            // created (an INSERT must have a writer, else addPart fails closed). For these types a
+            // missing dir is NOT evidence of nothing to do; iterateDirectory below finds any logs that
+            // already exist, and rotate() creates the writer when there are none. Any other object
+            // storage returns here: a missing dir means there is genuinely nothing and nowhere to write.
+            const auto type = object_storage->getMetadataStorage()->getType();
+            if (type != MetadataStorageType::Plain && type != MetadataStorageType::CAS)
                 return;
         }
     }
@@ -268,7 +275,15 @@ std::vector<MergeTreeDeduplicationLog::AddPartResult> MergeTreeDeduplicationLog:
         throw Exception(ErrorCodes::ABORTED, "Storage has been shutdown when we add this part.");
     }
 
-    chassert(current_writer != nullptr);
+    /// A disk that cannot host the append-mode log leaves current_writer null; the release-build
+    /// chassert above is a no-op, so dereferencing it would segfault. Fail closed with a clear
+    /// exception instead of crashing the server (B37).
+    if (!current_writer)
+        throw Exception(
+            ErrorCodes::LOGICAL_ERROR,
+            "MergeTree deduplication log has no writer (the disk does not support the on-disk "
+            "deduplication log); cannot add part {}",
+            part_info.getPartNameAndCheckFormat(format_version));
 
     for (const auto & block_id : block_ids)
     {
@@ -306,7 +321,13 @@ void MergeTreeDeduplicationLog::dropPart(const MergeTreePartInfo & drop_part_inf
         throw Exception(ErrorCodes::ABORTED, "Storage has been shutdown when we drop this part.");
     }
 
-    chassert(current_writer != nullptr);
+    /// As in addPart: a null writer must produce a clear exception, never a segfault (B37).
+    if (!current_writer)
+        throw Exception(
+            ErrorCodes::LOGICAL_ERROR,
+            "MergeTree deduplication log has no writer (the disk does not support the on-disk "
+            "deduplication log); cannot drop part {}",
+            drop_part_info.getPartNameAndCheckFormat(format_version));
 
     for (auto itr = deduplication_map.begin(); itr != deduplication_map.end(); /* no increment here, we erasing from map */)
     {
diff --git a/src/Storages/MergeTree/tests/gtest_deduplication_log_null_writer.cpp b/src/Storages/MergeTree/tests/gtest_deduplication_log_null_writer.cpp
new file mode 100644
index 000000000000..7a2f72313a19
--- /dev/null
+++ b/src/Storages/MergeTree/tests/gtest_deduplication_log_null_writer.cpp
@@ -0,0 +1,139 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskLocal.h>
+#include <Storages/MergeTree/MergeTreeDeduplicationLog.h>
+#include <Storages/MergeTree/MergeTreePartInfo.h>
+#include <Common/Exception.h>
+#include <base/defines.h>   /// DEBUG_OR_SANITIZER_BUILD
+
+#include <filesystem>
+#include <memory>
+#include <string>
+#include <unistd.h>
+
+using namespace DB;
+
+namespace DB::ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+
+namespace
+{
+constexpr auto FORMAT_VERSION = MERGE_TREE_DATA_MIN_FORMAT_VERSION_WITH_CUSTOM_PARTITIONING;
+
+/// B37 regression: a `MergeTreeDeduplicationLog` whose `current_writer` is null (the disk could not
+/// host the append-mode log -- see `MergeTreeDeduplicationLog::load()`'s early-return path for a
+/// `DiskObjectStorage` whose metadata storage type is neither `Plain` nor `ContentAddressed`) used to
+/// be dereferenced unconditionally by `addPart`/`dropPart`: a release-build `chassert` is a no-op, so
+/// this was a null-pointer dereference (segfault) rather than a handled error. The fix makes both
+/// throw a `LOGICAL_ERROR` `DB::Exception` instead.
+///
+/// There is no way to drive this from a stateless SQL test: every disk type that reaches production
+/// either materializes `logs_dir` (so `load()` takes the normal `rotate()` path and sets a writer) or
+/// is one of the two types (`Plain`, `ContentAddressed`) `load()` explicitly special-cases to still get
+/// a writer. So this test constructs the log directly and never calls `load()` -- `current_writer`
+/// simply stays at its default-constructed null value, which is the exact precondition the guard in
+/// `addPart`/`dropPart` exists for.
+struct DeduplicationLogNullWriterFixture : public ::testing::Test
+{
+    std::filesystem::path base_path;
+    DiskPtr disk;
+    std::unique_ptr<MergeTreeDeduplicationLog> log;
+
+    void SetUp() override
+    {
+        const auto unique = std::to_string(::getpid()) + "_" + std::to_string(reinterpret_cast<uintptr_t>(this));
+        base_path = std::filesystem::temp_directory_path() / ("dedup_log_null_writer_gtest_" + unique);
+        std::filesystem::create_directories(base_path);
+        disk = std::make_shared<DiskLocal>("test_disk_" + unique, base_path.string());
+
+        /// deduplication_window != 0 so addPart/dropPart don't bail out on the "deduplication is off"
+        /// fast path before ever reaching the null-writer guard. `load()` is deliberately NOT called:
+        /// that is what leaves `current_writer` null.
+        log = std::make_unique<MergeTreeDeduplicationLog>("deduplication_logs", /*deduplication_window_=*/4, FORMAT_VERSION, disk);
+    }
+
+    void TearDown() override
+    {
+        log.reset();
+        std::error_code ec;
+        std::filesystem::remove_all(base_path, ec);
+    }
+};
+
+}
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+/// gtest runs *DeathTest suites before others; reuse the same fixture via an alias so the death arm
+/// gets the same null-writer precondition.
+using DeduplicationLogNullWriterDeathTest = DeduplicationLogNullWriterFixture;
+#endif
+
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST_F(DeduplicationLogNullWriterFixture, AddPartThrowsLogicalErrorInsteadOfCrashing)
+{
+    /// LOGICAL_ERROR "no writer" is a broken-invariant guard (addPart on a null current_writer). Under
+    /// abort_on_logical_error it aborts at construction instead of being catchable -- the DeathTest
+    /// below proves the abort in those builds.
+    auto part_info = MergeTreePartInfo::fromPartName("all_0_0_0", FORMAT_VERSION);
+
+    EXPECT_THROW(
+        {
+            try
+            {
+                log->addPart({"block-1"}, part_info);
+            }
+            catch (const Exception & e)
+            {
+                EXPECT_EQ(e.code(), ErrorCodes::LOGICAL_ERROR);
+                EXPECT_NE(e.message().find("no writer"), std::string::npos);
+                throw;
+            }
+        },
+        Exception);
+
+    /// The object stays alive and usable after the guard fires: it isn't left half-corrupted by the
+    /// failed call, and repeating the same call (still no writer) throws again, cleanly, rather than
+    /// crashing or behaving differently the second time.
+    EXPECT_THROW(log->addPart({"block-1"}, part_info), Exception);
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST_F(DeduplicationLogNullWriterDeathTest, AddPartAborts)
+{
+    auto part_info = MergeTreePartInfo::fromPartName("all_0_0_0", FORMAT_VERSION);
+    EXPECT_DEATH({ log->addPart({"block-1"}, part_info); }, "no writer");
+}
+#endif
+
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST_F(DeduplicationLogNullWriterFixture, DropPartThrowsLogicalErrorInsteadOfCrashing)
+{
+    auto part_info = MergeTreePartInfo::fromPartName("all_0_0_0", FORMAT_VERSION);
+
+    EXPECT_THROW(
+        {
+            try
+            {
+                log->dropPart(part_info);
+            }
+            catch (const Exception & e)
+            {
+                EXPECT_EQ(e.code(), ErrorCodes::LOGICAL_ERROR);
+                EXPECT_NE(e.message().find("no writer"), std::string::npos);
+                throw;
+            }
+        },
+        Exception);
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST_F(DeduplicationLogNullWriterDeathTest, DropPartAborts)
+{
+    auto part_info = MergeTreePartInfo::fromPartName("all_0_0_0", FORMAT_VERSION);
+    EXPECT_DEATH({ log->dropPart(part_info); }, "no writer");
+}
+#endif

From 069f021bf71cc0859f26ed3467df3a6d372679c4 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:32 +0200
Subject: [PATCH 05/30] S3 conditional-write support and 412 no-retry policy

HTTP 412 on a conditional request is deterministic: never retry it
(S3Exception::isPreconditionFailed, RetryStrategy). Adds conditional
PUT/COPY (If-Match / If-None-Match) through the client, WriteBufferFromS3
and copyS3File, and the token-conditional operations on S3ObjectStorage.
Also carries the copyS3File message_format_string fix (PreformattedMessage
instead of a preformatted string) and CAS-facing write-settings plumbing
(wired later).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../ObjectStorages/S3/S3ObjectStorage.cpp     | 189 ++++++++++-
 .../ObjectStorages/S3/S3ObjectStorage.h       |  38 +++
 .../ObjectStorages/S3/diskSettings.cpp        |   2 +
 src/IO/S3/Client.cpp                          |  41 ++-
 src/IO/S3/Client.h                            |  17 +
 src/IO/S3/Requests.h                          |   2 +
 src/IO/S3/copyS3File.cpp                      |  73 +++-
 src/IO/S3/copyS3File.h                        |  14 +-
 src/IO/S3/tests/gtest_aws_s3_client.cpp       | 137 ++++++++
 src/IO/S3AuthSettings.cpp                     |   1 +
 src/IO/S3Common.cpp                           |  45 +++
 src/IO/S3Common.h                             |  53 ++-
 src/IO/S3Defines.h                            |   7 +
 src/IO/WriteBufferFromS3.cpp                  |  30 +-
 src/IO/WriteBufferFromS3.h                    |   8 +
 src/IO/tests/gtest_writebuffer_s3.cpp         | 320 ++++++++++++++++++
 16 files changed, 954 insertions(+), 23 deletions(-)

diff --git a/src/Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.cpp b/src/Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.cpp
index 34eb1eaebb9d..32aa5b34e4b3 100644
--- a/src/Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.cpp
+++ b/src/Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.cpp
@@ -17,6 +17,7 @@
 #include <IO/WriteBufferFromS3.h>
 #include <IO/ReadBufferFromS3.h>
 #include <IO/S3/getObjectInfo.h>
+#include <IO/S3/Requests.h>
 #include <IO/S3/copyS3File.h>
 #include <IO/S3/deleteFileFromS3.h>
 #include <Interpreters/Context.h>
@@ -33,6 +34,7 @@
 #include <Common/ProfileEvents.h>
 #include <Common/StringUtils.h>
 #include <Common/logger_useful.h>
+#include <Common/StackTrace.h>
 #include <Common/MultiVersion.h>
 #include <Common/Macros.h>
 
@@ -68,8 +70,13 @@ namespace Setting
 
 namespace S3RequestSetting
 {
+    extern const S3RequestSettingsBool allow_native_copy;
+    extern const S3RequestSettingsBool check_objects_after_upload;
     extern const S3RequestSettingsUInt64 list_object_keys_size;
     extern const S3RequestSettingsUInt64 objects_chunk_size_to_delete;
+    extern const S3RequestSettingsUInt64 max_single_part_upload_size;
+    extern const S3RequestSettingsUInt64 min_upload_part_size;
+    extern const S3RequestSettingsUInt64 max_unexpected_write_error_retries;
 }
 
 
@@ -77,6 +84,8 @@ namespace ErrorCodes
 {
     extern const int BAD_ARGUMENTS;
     extern const int LOGICAL_ERROR;
+    extern const int NOT_IMPLEMENTED;
+    extern const int S3_ERROR;
 }
 
 namespace
@@ -227,7 +236,8 @@ class S3IteratorAsync final : public IObjectStorageIteratorAsync
 bool S3ObjectStorage::exists(const StoredObject & object) const
 {
     auto settings_ptr = s3_settings.get();
-    return S3::objectExists(*client.get(), uri.bucket, object.remote_path, {});
+    const bool e = S3::objectExists(*client.get(), uri.bucket, object.remote_path, {});
+    return e;
 }
 
 std::unique_ptr<ReadBufferFromFileBase> S3ObjectStorage::readObject( /// NOLINT
@@ -312,6 +322,29 @@ std::unique_ptr<WriteBufferFromFileBase> S3ObjectStorage::writeObject( /// NOLIN
         request_settings.updateFromSettings(settings, /* if_changed */ true, settings[Setting::s3_validate_request_settings]);
     }
 
+    if (write_settings.s3_check_objects_after_upload_override)
+        request_settings[S3RequestSetting::check_objects_after_upload] = *write_settings.s3_check_objects_after_upload_override;
+
+    if (write_settings.s3_single_part_upload_max_bytes_override)
+    {
+        /// Keep the whole body in ONE buffered part so the single-PUT path stays available up to
+        /// the cap (conditional writes on generation-token stores; see WriteSettings).
+        request_settings[S3RequestSetting::max_single_part_upload_size]
+            = write_settings.s3_single_part_upload_max_bytes_override;
+        request_settings[S3RequestSetting::min_upload_part_size]
+            = write_settings.s3_single_part_upload_max_bytes_override;
+    }
+
+    if (write_settings.s3_max_unexpected_write_error_retries_override)
+    {
+        /// WriteBufferFromS3's OWN retry loop (makeSinglepartUpload/completeMultipartUpload) reissues
+        /// the identical request — WITH its If-None-Match/If-Match condition — on a NO_SUCH_KEY
+        /// response; this sits ABOVE the S3 client, so a client-level profile override does not bound
+        /// it. See WriteSettings.
+        request_settings[S3RequestSetting::max_unexpected_write_error_retries]
+            = write_settings.s3_max_unexpected_write_error_retries_override;
+    }
+
     ThreadPoolCallbackRunnerUnsafe<void> scheduler;
     if (write_settings.s3_allow_parallel_part_upload)
         scheduler = threadPoolCallbackRunnerUnsafe<void>(getThreadPoolWriter(), ThreadName::REMOTE_FS_WRITE_THREAD_POOL);
@@ -320,8 +353,18 @@ std::unique_ptr<WriteBufferFromFileBase> S3ObjectStorage::writeObject( /// NOLIN
     if (blob_storage_log)
         blob_storage_log->local_path = object.local_path;
 
+    /// The SingleAttempt profile (e.g. CAS conditional writes, RFC cas-s3-timeout-retry-control) rides
+    /// on WriteSettings instead of changing this disk's shared client — every other write keeps using
+    /// client.get() and its normal retry policy unchanged. getSingleAttemptClient() is only invoked
+    /// when actually selected, so a plain write never pays for building/locking the clone.
+    std::shared_ptr<const S3::Client> used_client;
+    if (write_settings.object_storage_retry_profile == ObjectStorageRetryProfile::SingleAttempt)
+        used_client = getSingleAttemptClient();
+    else
+        used_client = client.get();
+
     return std::make_unique<WriteBufferFromS3>(
-        client.get(),
+        used_client,
         uri.bucket,
         object.remote_path,
         write_settings.use_adaptive_write_buffer ? write_settings.adaptive_write_buffer_initial_size : buf_size,
@@ -443,6 +486,65 @@ void S3ObjectStorage::removeObjectsIfExist(const StoredObjects & objects)
     removeObjectsImpl(objects, true);
 }
 
+ConditionalRemoveResult S3ObjectStorage::removeObjectIfTokenMatches(const StoredObject & object, const std::string & etag)
+{
+    S3::DeleteObjectRequest request;
+    request.SetBucket(uri.bucket);
+    request.SetKey(object.remote_path);
+    request.SetIfMatch(etag);
+
+    ProfileEvents::increment(ProfileEvents::DiskS3DeleteObjects);
+
+    auto outcome = client.get()->DeleteObject(request);
+
+    /// Mirror removeObjectImpl (deleteFileFromS3): every conditional delete lands in
+    /// system.blob_storage_log too — GC reclaim was invisible there otherwise. TokenMismatch
+    /// and NotFound are routine protocol outcomes, recorded with the S3 error for filtering.
+    if (auto blob_storage_log = BlobStorageLogWriter::create(disk_name))
+        blob_storage_log->addEvent(BlobStorageLogElement::EventType::Delete,
+                                   uri.bucket, object.remote_path,
+                                   object.local_path, object.bytes_size,
+                                   /* elapsed_microseconds */ 0,
+                                   outcome.IsSuccess() ? 0 : static_cast<Int32>(outcome.GetError().GetErrorType()),
+                                   outcome.IsSuccess() ? "" : outcome.GetError().GetMessage());
+
+    if (outcome.IsSuccess())
+        return {ConditionalRemoveOutcome::Removed, outcome.GetResult().GetDeleteMarker()};
+
+    const auto & err = outcome.GetError();
+
+    /// The token did not match the current incarnation: the conditional delete is rejected with a 412
+    /// (see `S3::isPreconditionFailedError` for the one policy). Callers treat 'mismatch' and 'gone'
+    /// alike (re-validate); a genuine absence is disambiguated downstream by a HEAD re-check.
+    if (S3::isPreconditionFailedError(err))
+        return {ConditionalRemoveOutcome::TokenMismatch, false};
+
+    /// The object no longer exists (404). Protocol callers treat 'mismatch' and 'gone' alike (re-validate).
+    if (S3::isNotFoundError(err.GetErrorType()))
+        return {ConditionalRemoveOutcome::NotFound, false};
+
+    throw S3Exception(err.GetErrorType(),
+        "{} (Code: {}, S3 exception: '{}') while conditionally removing object with path {} from S3",
+        err.GetMessage(), static_cast<size_t>(err.GetErrorType()), err.GetExceptionName(), object.remote_path);
+}
+
+bool S3ObjectStorage::conditionalOpsUseGenerationTokens() const
+{
+    return client.get()->usesGcsConditionalDialect();
+}
+
+std::optional<bool> S3ObjectStorage::isBucketVersioningEnabled() const
+{
+    S3::GetBucketVersioningRequest request;
+    request.SetBucket(uri.bucket);
+
+    auto outcome = client.get()->GetBucketVersioning(request);
+    if (!outcome.IsSuccess())
+        return std::nullopt;
+
+    return outcome.GetResult().GetStatus() == Aws::S3::Model::BucketVersioningStatus::Enabled;
+}
+
 static void putObjectsTagOnS3(
     const std::shared_ptr<const S3::Client> & s3_client,
     const String & bucket,
@@ -667,6 +769,66 @@ void S3ObjectStorage::copyObject( // NOLINT
         object_to_attributes);
 }
 
+ConditionalCopyResult S3ObjectStorage::copyObjectConditional( // NOLINT
+    const StoredObject & object_from,
+    const StoredObject & object_to,
+    const ReadSettings & read_settings,
+    const WriteSettings &,
+    std::optional<ObjectAttributes> object_to_attributes)
+{
+    auto current_client = client.get();
+    auto settings_ptr = s3_settings.get();
+
+    /// `copyS3File`'s `If-None-Match` precondition is only honored on the native server-side copy
+    /// path (`CopyObject` / `CompleteMultipartUpload`): if native copy is disabled it silently falls
+    /// back to an unconditional read-write copy (`copyDataToS3File`), which would defeat the
+    /// write-once guarantee the content-addressed staging promote relies on. Fail closed instead of
+    /// racing an unconditional overwrite onto what may already be a live blob.
+    if (!settings_ptr->request_settings[S3RequestSetting::allow_native_copy])
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "Conditional (write-once) object copy requires the native S3 copy path, which is disabled "
+            "(allow_native_copy=false) for object storage {}", getName());
+
+    auto size = S3::getObjectSize(*current_client, uri.bucket, object_from.remote_path, {});
+    auto scheduler = threadPoolCallbackRunnerUnsafe<void>(getThreadPoolWriter(), ThreadName::S3_COPY_POOL);
+    const auto read_settings_to_use = patchSettings(read_settings);
+
+    String dest_etag;
+    try
+    {
+        copyS3File(
+            /*src_s3_client=*/current_client,
+            /*src_bucket=*/uri.bucket,
+            /*src_key=*/object_from.remote_path,
+            /*src_offset=*/0,
+            /*src_size=*/size,
+            /*dest_s3_client=*/current_client,
+            /*dest_bucket=*/uri.bucket,
+            /*dest_key=*/object_to.remote_path,
+            settings_ptr->request_settings,
+            read_settings_to_use,
+            BlobStorageLogWriter::create(disk_name),
+            scheduler,
+            [&, this]{ return readObject(object_from, read_settings_to_use);},
+            object_to_attributes,
+            /*if_none_match=*/"*",
+            /*out_dest_etag=*/&dest_etag);
+    }
+    catch (S3Exception & exc)
+    {
+        /// A `412 Precondition Failed` is the expected "lost the race" signal (the destination already
+        /// exists), not an error — see `S3Exception::isPreconditionFailed` for the one policy.
+        if (exc.isPreconditionFailed())
+            return {.created = false, .dest_etag = {}};
+
+        /// Any other failure (network error, access denied, etc.) is a real error and must propagate:
+        /// never silently treat it as "lost the race".
+        throw;
+    }
+
+    return {.created = true, .dest_etag = dest_etag};
+}
+
 void S3ObjectStorage::shutdown()
 {
     /// This call stops any next retry attempts for ongoing S3 requests.
@@ -756,6 +918,29 @@ std::shared_ptr<const S3::Client> S3ObjectStorage::tryGetS3StorageClient()
     return client.get();
 }
 
+std::shared_ptr<const S3::Client> S3ObjectStorage::getSingleAttemptClient() const
+{
+    auto base = client.get();
+    std::lock_guard lock(single_attempt_client_mutex);
+    if (single_attempt_client && single_attempt_client_base == base)
+        return single_attempt_client;
+
+    auto cfg = base->getClientConfiguration();
+    cfg.retry_strategy.max_retries = 0;
+    cfg.retryStrategy = std::make_shared<S3::SingleAttemptRetryStrategy>();
+
+    /// A server can reject an If-Match/If-None-Match request before accepting its body; waiting for
+    /// the 100-continue response avoids uploading a large body that cannot commit. Respect the
+    /// disk's configured expect_continue_min_bytes; if unset, use the established 1 MiB floor.
+    static constexpr uint64_t fallback_expect_continue_min_bytes = 1024 * 1024;
+    if (cfg.expect_continue_min_bytes == 0)
+        cfg.expect_continue_min_bytes = fallback_expect_continue_min_bytes;
+
+    single_attempt_client = base->cloneWithConfigurationOverride(cfg);
+    single_attempt_client_base = base;
+    return single_attempt_client;
+}
+
 bool S3ObjectStorage::tryRefreshCredentialsViaCallback()
 {
     fiu_do_on(FailPoints::object_storage_force_refresh_callback_success, { return true; });
diff --git a/src/Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.h b/src/Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.h
index 1760153a1219..d357c3874e78 100644
--- a/src/Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.h
+++ b/src/Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.h
@@ -6,6 +6,7 @@
 
 #include <Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h>
 #include <memory>
+#include <mutex>
 #include <IO/S3/S3Capabilities.h>
 #include <IO/S3Settings.h>
 #include <Common/MultiVersion.h>
@@ -108,6 +109,9 @@ class S3ObjectStorage : public IObjectStorage
     /// `DeleteObjectsRequest` does not exist on GCS, see https://issuetracker.google.com/issues/162653700 .
     void removeObjectsIfExist(const StoredObjects & objects) override;
 
+    /// Uses `DeleteObjectRequest` with `If-Match` (token-exact removal for content-addressed disks).
+    ConditionalRemoveResult removeObjectIfTokenMatches(const StoredObject & object, const std::string & etag) override;
+
     void tagObjects(const StoredObjects & objects, const std::string & tag_key, const std::string & tag_value) override;
 
     ObjectMetadata getObjectMetadata(const std::string & path, bool with_tags) const override;
@@ -121,6 +125,17 @@ class S3ObjectStorage : public IObjectStorage
         const WriteSettings & write_settings,
         std::optional<ObjectAttributes> object_to_attributes = {}) override;
 
+    /// Write-once conditional server-side copy (`CopyObject`/`CompleteMultipartUpload` with
+    /// `If-None-Match: *`). Only performed via the native copy path (`copyS3File`'s
+    /// `allow_native_copy` path); if native copy is not available this throws rather than silently
+    /// falling back to an unconditional overwrite (see `.cpp` for details).
+    ConditionalCopyResult copyObjectConditional(
+        const StoredObject & object_from,
+        const StoredObject & object_to,
+        const ReadSettings & read_settings,
+        const WriteSettings & write_settings,
+        std::optional<ObjectAttributes> object_to_attributes) override;
+
     void copyObjectToAnotherObjectStorage( /// NOLINT
         const StoredObject & object_from,
         const StoredObject & object_to,
@@ -149,6 +164,12 @@ class S3ObjectStorage : public IObjectStorage
 
     bool isReadOnly() const override { return s3_settings.get()->request_settings[S3RequestSetting::read_only]; }
 
+    bool conditionalOpsUseGenerationTokens() const override;
+
+    std::optional<bool> isBucketVersioningEnabled() const override;
+
+    bool supportsRetryProfile(ObjectStorageRetryProfile) const override { return true; }
+
     std::shared_ptr<const S3::Client> getS3StorageClient() override;
     std::shared_ptr<const S3::Client> tryGetS3StorageClient() override;
 
@@ -156,6 +177,12 @@ class S3ObjectStorage : public IObjectStorage
 
     S3::URI getURI() const { return uri; }
     S3Settings getS3Settings() const { return *s3_settings.get(); }
+
+    /// Lazily-built clone of the current disk client with the single-attempt retry profile
+    /// (SingleAttemptRetryStrategy, max_retries=0, Expect:100-continue floor). Rebuilt whenever the
+    /// disk client rotates (applyNewSettings/credentials refresh) — the cached clone is keyed by the
+    /// base client's identity, so a stale clone can never outlive a rotation.
+    std::shared_ptr<const S3::Client> getSingleAttemptClient() const;
 private:
     void removeObjectImpl(const StoredObject & object, bool if_exists);
     void removeObjectsImpl(const StoredObjects & objects, bool if_exists);
@@ -174,6 +201,17 @@ class S3ObjectStorage : public IObjectStorage
 
     const bool for_disk_s3;
     S3CredentialsRefreshCallback credentials_refresh_callback;
+
+    mutable std::mutex single_attempt_client_mutex;
+    mutable std::shared_ptr<const S3::Client> single_attempt_client;
+    /// The base client the cached clone above was built from. Deliberately held as a shared_ptr (not
+    /// a raw pointer): a raw pointer would be compared for identity AFTER the object it once pointed
+    /// to could have been freed and a new client reallocated at the same address by an unrelated
+    /// rotation (ABA), which would false-match and serve a stale clone (e.g. built from retired
+    /// credentials) indefinitely. Holding the shared_ptr pins at most one retired client version —
+    /// released as soon as the next rotation is observed and the clone is rebuilt — which is what
+    /// makes the identity comparison in getSingleAttemptClient sound.
+    mutable std::shared_ptr<const S3::Client> single_attempt_client_base;
 };
 
 }
diff --git a/src/Disks/DiskObjectStorage/ObjectStorages/S3/diskSettings.cpp b/src/Disks/DiskObjectStorage/ObjectStorages/S3/diskSettings.cpp
index f95f53763de4..a72f0eb02fed 100644
--- a/src/Disks/DiskObjectStorage/ObjectStorages/S3/diskSettings.cpp
+++ b/src/Disks/DiskObjectStorage/ObjectStorages/S3/diskSettings.cpp
@@ -51,6 +51,7 @@ namespace S3AuthSetting
     extern const S3AuthSettingsString access_key_id;
     extern const S3AuthSettingsUInt64 connect_timeout_ms;
     extern const S3AuthSettingsBool disable_checksum;
+    extern const S3AuthSettingsUInt64 expect_continue_min_bytes;
     extern const S3AuthSettingsUInt64 expiration_window_seconds;
     extern const S3AuthSettingsBool gcs_issue_compose_request;
     extern const S3AuthSettingsUInt64 http_keep_alive_max_requests;
@@ -178,6 +179,7 @@ getClient(const S3::URI & url, const S3Settings & settings, ContextPtr context,
 
     client_configuration.endpointOverride = url.endpoint;
     client_configuration.s3_use_adaptive_timeouts = auth_settings[S3AuthSetting::use_adaptive_timeouts];
+    client_configuration.expect_continue_min_bytes = auth_settings[S3AuthSetting::expect_continue_min_bytes];
 
     if (request_settings.proxy_resolver)
     {
diff --git a/src/IO/S3/Client.cpp b/src/IO/S3/Client.cpp
index af6f0a2e6894..d1c5c733ec9d 100644
--- a/src/IO/S3/Client.cpp
+++ b/src/IO/S3/Client.cpp
@@ -25,8 +25,10 @@
 
 #include <Poco/Net/NetException.h>
 #include <Poco/Exception.h>
+#include <Poco/String.h>
 
 #include <IO/Expect404ResponseScope.h>
+#include <IO/S3Common.h>
 #include <IO/S3/Requests.h>
 #include <IO/S3/PocoHTTPClientFactory.h>
 #include <IO/S3/AWSLogger.h>
@@ -65,6 +67,8 @@ namespace ProfileEvents
 
     extern const Event S3Clients;
     extern const Event TinyS3Clients;
+
+    extern const Event S3SingleAttemptRetryConsultations;
 }
 
 namespace CurrentMetrics
@@ -102,6 +106,13 @@ bool Client::RetryStrategy::ShouldRetry(const Aws::Client::AWSError<Aws::Client:
     if (error.GetResponseCode() == Aws::Http::HttpResponseCode::MOVED_PERMANENTLY)
         return false;
 
+    /// A 412 Precondition Failed is the DETERMINISTIC result of a conditional request (the
+    /// content-addressed backend's CAS/dedup writes). Retrying never changes it, and on a store that
+    /// returns a non-AWS body (RustFS) the SDK marks the 412 retryable and storms the write path — and
+    /// downstream SYSTEM SYNC REPLICA (B166). One 412 policy across the retry and CA conditional ops.
+    if (S3::isPreconditionFailedError(error))
+        return false;
+
     if (attemptedRetries >= config.max_retries)
         return false;
 
@@ -181,6 +192,13 @@ void Client::RetryStrategy::RequestBookkeeping(
     RequestBookkeeping(httpResponseOutcome);
 }
 
+/// NOLINTNEXTLINE(google-runtime-int)
+bool SingleAttemptRetryStrategy::ShouldRetry(const Aws::Client::AWSError<Aws::Client::CoreErrors> &, long) const
+{
+    ProfileEvents::increment(ProfileEvents::S3SingleAttemptRetryConsultations);
+    return false;
+}
+
 namespace
 {
 
@@ -289,7 +307,14 @@ Client::Client(
         /// find credential keys we can simply behave as the underlying storage is S3
         /// otherwise, we need to be aware we are making requests to GCS
         /// and replace all headers with a valid prefix when needed
-        if (credentials_provider)
+        if (Poco::toLower(client_configuration.http_client) == "gcs_hmac")
+        {
+            /// GOOG4-HMAC mode: all requests are re-signed with x-goog headers at the HTTP layer,
+            /// so the SDK-side GCS accommodations (x-amz header renames, x-amz-api-version
+            /// deletion) must be active even though credentials are present.
+            api_mode = ApiMode::GCS;
+        }
+        else if (credentials_provider)
         {
             auto credentials = credentials_provider->GetAWSCredentials();
             if (credentials.IsEmpty())
@@ -506,6 +531,12 @@ Model::GetObjectTaggingOutcome Client::GetObjectTagging(GetObjectTaggingRequest
         doRequest(request, [this](const Model::GetObjectTaggingRequest & req) { return GetObjectTagging(req); }));
 }
 
+Model::GetBucketVersioningOutcome Client::GetBucketVersioning(GetBucketVersioningRequest & request) const
+{
+    return processRequestResult(
+        doRequest(request, [this](const Model::GetBucketVersioningRequest & req) { return GetBucketVersioning(req); }));
+}
+
 Model::ListObjectsV2Outcome Client::ListObjectsV2(ListObjectsV2Request & request) const
 {
     return doRequestWithRetryNetworkErrors</*IsReadMethod*/ true>(
@@ -1274,6 +1305,14 @@ std::unique_ptr<S3::Client> ClientFactory::create( // NOLINT
 
     auto credentials_provider = getCredentialsProvider(client_configuration, credentials, credentials_configuration);
 
+    if (Poco::toLower(client_configuration.http_client) == "gcs_hmac")
+    {
+        client_configuration.gcs_conditional_dialect = true;
+        client_configuration.gcs_hmac_credentials_provider = credentials_provider;
+    }
+    else if (Poco::toLower(client_configuration.http_client) == "gcp_oauth")
+        client_configuration.gcs_conditional_dialect = true;
+
     /// Disable per-thread retry loops if global retry coordination is in use.
     if (client_configuration.s3_slow_all_threads_after_retryable_error)
     {
diff --git a/src/IO/S3/Client.h b/src/IO/S3/Client.h
index ad4d685d88a2..5a6847799c29 100644
--- a/src/IO/S3/Client.h
+++ b/src/IO/S3/Client.h
@@ -208,6 +208,7 @@ class Client : private Aws::S3::S3Client
 
     Model::HeadObjectOutcome HeadObject(HeadObjectRequest & request) const;
     Model::GetObjectTaggingOutcome GetObjectTagging(GetObjectTaggingRequest & request) const;
+    Model::GetBucketVersioningOutcome GetBucketVersioning(GetBucketVersioningRequest & request) const;
     Model::ListObjectsV2Outcome ListObjectsV2(ListObjectsV2Request & request) const;
     Model::ListObjectsOutcome ListObjects(ListObjectsRequest & request) const;
     Model::GetObjectOutcome GetObject(GetObjectRequest & request) const;
@@ -252,6 +253,10 @@ class Client : private Aws::S3::S3Client
 
     const PocoHTTPClientConfiguration & getClientConfiguration() const { return client_configuration; }
 
+    /// True when this client's HTTP layer runs the GCS conditional dialect (http_client =
+    /// gcs_hmac or gcp_oauth): conditional tokens are GCS generations riding the ETag plumbing.
+    bool usesGcsConditionalDialect() const { return client_configuration.gcs_conditional_dialect; }
+
     /// For testing purposes only
     ClientCache * getRawCache() const { return cache.get(); }
 
@@ -273,6 +278,7 @@ class Client : private Aws::S3::S3Client
     /// otherwise region and endpoint redirection won't work
     using Aws::S3::S3Client::HeadObject;
     using Aws::S3::S3Client::GetObjectTagging;
+    using Aws::S3::S3Client::GetBucketVersioning;
     using Aws::S3::S3Client::ListObjectsV2;
     using Aws::S3::S3Client::ListObjects;
     using Aws::S3::S3Client::GetObject;
@@ -346,6 +352,17 @@ class Client : private Aws::S3::S3Client
     LoggerPtr log;
 };
 
+/// Refuses every SDK-transparent retry and counts each consultation. Used by the
+/// ObjectStorageRetryProfile::SingleAttempt per-write profile (conditional writes whose retry
+/// decisions live ABOVE the SDK: the caller must resolve an uncertain PUT before reissuing).
+class SingleAttemptRetryStrategy final : public Aws::Client::RetryStrategy
+{
+public:
+    bool ShouldRetry(const Aws::Client::AWSError<Aws::Client::CoreErrors> &, long) const override; // NOLINT(google-runtime-int)
+    long CalculateDelayBeforeNextRetry(const Aws::Client::AWSError<Aws::Client::CoreErrors> &, long) const override { return 0; } // NOLINT(google-runtime-int)
+    long GetMaxAttempts() const override { return 1; } // NOLINT(google-runtime-int)
+};
+
 class ClientFactory
 {
 public:
diff --git a/src/IO/S3/Requests.h b/src/IO/S3/Requests.h
index aa21602674f0..0f55ac33296e 100644
--- a/src/IO/S3/Requests.h
+++ b/src/IO/S3/Requests.h
@@ -13,6 +13,7 @@
 #include <aws/s3/model/ListObjectsRequest.h>
 #include <aws/s3/model/GetObjectRequest.h>
 #include <aws/s3/model/GetObjectTaggingRequest.h>
+#include <aws/s3/model/GetBucketVersioningRequest.h>
 #include <aws/s3/model/AbortMultipartUploadRequest.h>
 #include <aws/s3/model/CreateMultipartUploadRequest.h>
 #include <aws/s3/model/CompleteMultipartUploadRequest.h>
@@ -158,6 +159,7 @@ using ListObjectsV2Request = ExtendedRequest<Model::ListObjectsV2Request>;
 using ListObjectsRequest = ExtendedRequest<Model::ListObjectsRequest>;
 using GetObjectRequest = ExtendedRequest<Model::GetObjectRequest>;
 using GetObjectTaggingRequest = ExtendedRequest<Model::GetObjectTaggingRequest>;
+using GetBucketVersioningRequest = ExtendedRequest<Model::GetBucketVersioningRequest>;
 
 class UploadPartRequest : public ExtendedRequest<Model::UploadPartRequest>
 {
diff --git a/src/IO/S3/copyS3File.cpp b/src/IO/S3/copyS3File.cpp
index 4a54d7d53ec9..f0aa8cde9b18 100644
--- a/src/IO/S3/copyS3File.cpp
+++ b/src/IO/S3/copyS3File.cpp
@@ -81,7 +81,9 @@ namespace
             const std::optional<ObjectAttributes> & object_metadata_,
             ThreadPoolCallbackRunnerUnsafe<void> schedule_,
             BlobStorageLogWriterPtr blob_storage_log_,
-            const LoggerPtr log_)
+            const LoggerPtr log_,
+            const std::optional<String> & if_none_match_ = {},
+            String * out_dest_etag_ = nullptr)
             : client_ptr(client_ptr_)
             , dest_bucket(dest_bucket_)
             , dest_key(dest_key_)
@@ -90,6 +92,8 @@ namespace
             , schedule(schedule_)
             , blob_storage_log(blob_storage_log_)
             , log(log_)
+            , if_none_match(if_none_match_)
+            , out_dest_etag(out_dest_etag_)
             , num_parts(0)
             , normal_part_size(0)
         {
@@ -107,6 +111,13 @@ namespace
         BlobStorageLogWriterPtr blob_storage_log;
         const LoggerPtr log;
 
+        /// If set, passed as the `If-None-Match` precondition on the destination write of a copy
+        /// (`CopyObject` and, for large objects, `CompleteMultipartUpload`), making the copy write-once
+        /// conditional. Only meaningful for copyS3File() (CopyFileHelper); unused by copyDataToS3File().
+        const std::optional<String> if_none_match;
+        /// If non-null, filled in with the destination object's ETag on a successful copy.
+        String * out_dest_etag;
+
         /// Represents a task uploading a single part.
         /// Keep this struct small because there can be thousands of parts.
         /// For example, `UploadPartTask` must not contain a read buffer or `S3::UploadPartRequest`
@@ -197,6 +208,9 @@ namespace
 
             request.SetMultipartUpload(multipart_upload);
 
+            if (if_none_match.has_value())
+                request.SetIfNoneMatch(*if_none_match);
+
             size_t max_retries = std::max<UInt64>(request_settings[S3RequestSetting::max_unexpected_write_error_retries].value, 1UL);
             for (size_t retries = 1;; ++retries)
             {
@@ -216,6 +230,8 @@ namespace
 
                 if (outcome.IsSuccess())
                 {
+                    if (out_dest_etag)
+                        *out_dest_etag = outcome.GetResult().GetETag();
                     LOG_TRACE(log, "Multipart upload has completed. Bucket: {}, Key: {}, Upload_id: {}, Parts: {}", dest_bucket, dest_key, multipart_upload_id, multipart_tags.size());
                     break;
                 }
@@ -228,10 +244,17 @@ namespace
                     continue; /// will retry
                 }
                 ProfileEvents::increment(ProfileEvents::WriteBufferFromS3RequestsErrors, 1);
+                /// Preserve the S3 exception name (e.g. `PreconditionFailed` for a rejected
+                /// `If-None-Match` on `CompleteMultipartUpload`) on the thrown exception: `GetErrorType()`
+                /// alone maps an unmodeled error like a 412 to `UNKNOWN`, so callers that need to detect
+                /// a specific condition (a write-once conditional copy losing the race) must be able to
+                /// read `S3Exception::getExceptionName()`, mirroring how `S3ObjectStorage::removeObjectIfTokenMatches`
+                /// reads `AWSError::GetExceptionName()` directly off the (not-yet-thrown) outcome.
                 throw S3Exception(
+                    PreformattedMessage::create("Message: {}, Key: {}, Bucket: {}, Tags: {}",
+                        outcome.GetError().GetMessage(), dest_key, dest_bucket, fmt::join(multipart_tags.begin(), multipart_tags.end(), " ")),
                     outcome.GetError().GetErrorType(),
-                    "Message: {}, Key: {}, Bucket: {}, Tags: {}",
-                    outcome.GetError().GetMessage(), dest_key, dest_bucket, fmt::join(multipart_tags.begin(), multipart_tags.end(), " "));
+                    outcome.GetError().GetExceptionName());
             }
         }
 
@@ -613,7 +636,9 @@ namespace
             const std::optional<ObjectAttributes> & object_metadata_,
             ThreadPoolCallbackRunnerUnsafe<void> schedule_,
             BlobStorageLogWriterPtr blob_storage_log_,
-            std::function<void()> fallback_method_)
+            std::function<void()> fallback_method_,
+            const std::optional<String> & if_none_match_ = {},
+            String * out_dest_etag_ = nullptr)
             : UploadHelper(
                 client_ptr_,
                 dest_bucket_,
@@ -622,7 +647,9 @@ namespace
                 object_metadata_,
                 schedule_,
                 blob_storage_log_,
-                getLogger("copyS3File"))
+                getLogger("copyS3File"),
+                if_none_match_,
+                out_dest_etag_)
             , src_bucket(src_bucket_)
             , src_key(src_key_)
             , offset(src_offset_)
@@ -676,6 +703,9 @@ namespace
                 request.SetMetadataDirective(Aws::S3::Model::MetadataDirective::REPLACE);
             }
 
+            if (if_none_match.has_value())
+                request.SetIfNoneMatch(*if_none_match);
+
             const auto & storage_class_name = request_settings[S3RequestSetting::storage_class_name];
             if (!storage_class_name.value.empty())
                 request.SetStorageClass(Aws::S3::Model::StorageClassMapper::GetStorageClassForName(storage_class_name));
@@ -698,6 +728,8 @@ namespace
                 auto outcome = client_ptr->CopyObject(request);
                 if (outcome.IsSuccess())
                 {
+                    if (out_dest_etag)
+                        *out_dest_etag = outcome.GetResult().GetCopyObjectResultDetails().GetETag();
                     LOG_TRACE(
                         log,
                         "Single operation copy has completed. Bucket: {}, Key: {}, Object size: {}",
@@ -715,6 +747,12 @@ namespace
                 {
                     if (!supports_multipart_copy || outcome.GetError().GetExceptionName() == "AccessDenied")
                     {
+                        if (if_none_match.has_value())
+                            throw S3Exception(
+                                outcome.GetError().GetMessage(),
+                                outcome.GetError().GetErrorType(),
+                                outcome.GetError().GetExceptionName());
+
                         LOG_INFO(
                             log,
                             "Multipart upload using copy is not supported, will try regular upload for Bucket: {}, Key: {}, Object size: "
@@ -753,13 +791,17 @@ namespace
                     continue; /// will retry
                 }
 
+                /// Preserve the S3 exception name for the same reason as the `CompleteMultipartUpload`
+                /// throw in `completeMultipartUpload()` above (a 412 on a conditional `CopyObject` maps
+                /// to `S3Errors::UNKNOWN`; the exception name is the only reliable discriminator).
                 throw S3Exception(
+                    PreformattedMessage::create("Message: {}, Key: {}, Bucket: {}, Object size: {}",
+                        outcome.GetError().GetMessage(),
+                        dest_key,
+                        dest_bucket,
+                        size),
                     outcome.GetError().GetErrorType(),
-                    "Message: {}, Key: {}, Bucket: {}, Object size: {}",
-                    outcome.GetError().GetMessage(),
-                    dest_key,
-                    dest_bucket,
-                    size);
+                    outcome.GetError().GetExceptionName());
             }
         }
 
@@ -774,6 +816,9 @@ namespace
                 if (e.getS3ErrorCode() != Aws::S3::S3Errors::ACCESS_DENIED)
                     throw;
 
+                if (if_none_match.has_value())
+                    throw;
+
                 tryLogCurrentException(log, "Multi part copy failed, trying with regular upload");
                 fallback_method();
             }
@@ -855,7 +900,9 @@ void copyS3File(
     BlobStorageLogWriterPtr blob_storage_log,
     ThreadPoolCallbackRunnerUnsafe<void> schedule,
     const CreateReadBuffer& fallback_file_reader,
-    const std::optional<ObjectAttributes> & object_metadata)
+    const std::optional<ObjectAttributes> & object_metadata,
+    std::optional<String> if_none_match,
+    String * out_dest_etag)
 {
     if (!dest_s3_client)
         dest_s3_client = src_s3_client;
@@ -895,7 +942,9 @@ void copyS3File(
         object_metadata,
         schedule,
         blob_storage_log,
-        std::move(fallback_method)};
+        std::move(fallback_method),
+        if_none_match,
+        out_dest_etag};
     helper.performCopy();
 }
 
diff --git a/src/IO/S3/copyS3File.h b/src/IO/S3/copyS3File.h
index c61437585d4d..a67dfaa9b4b3 100644
--- a/src/IO/S3/copyS3File.h
+++ b/src/IO/S3/copyS3File.h
@@ -30,6 +30,16 @@ using CreateReadBuffer = std::function<std::unique_ptr<SeekableReadBuffer>()>;
 /// (copyDataToS3File()).
 ///
 /// read_settings - is used for throttling in case of native copy is not possible
+///
+/// If `if_none_match` is set, it is passed as the `If-None-Match` precondition on the destination
+/// write (both the single-operation `CopyObject` request and, for large objects, the multipart
+/// `CompleteMultipartUpload` request), turning the copy into a write-once conditional copy. A `412
+/// Precondition Failed` response (destination already exists) is not swallowed: it surfaces as a
+/// thrown `S3Exception`. This precondition is only honored on the native server-side copy path; it
+/// is not applied if the copy falls back to the non-native read-write copy (`copyDataToS3File`).
+///
+/// If `out_dest_etag` is non-null, it is filled in with the ETag of the destination object as
+/// reported by the copy response, on success.
 void copyS3File(
     std::shared_ptr<const S3::Client> src_s3_client,
     const String & src_bucket,
@@ -44,7 +54,9 @@ void copyS3File(
     BlobStorageLogWriterPtr blob_storage_log,
     ThreadPoolCallbackRunnerUnsafe<void> schedule,
     const CreateReadBuffer& fallback_file_reader,
-    const std::optional<ObjectAttributes> & object_metadata = std::nullopt);
+    const std::optional<ObjectAttributes> & object_metadata = std::nullopt,
+    std::optional<String> if_none_match = {},
+    String * out_dest_etag = nullptr);
 
 /// Copies data from any seekable source to S3.
 /// The same functionality can be done by using the function copyData() and the class WriteBufferFromS3
diff --git a/src/IO/S3/tests/gtest_aws_s3_client.cpp b/src/IO/S3/tests/gtest_aws_s3_client.cpp
index b3d8f50135bf..d86eba30b75f 100644
--- a/src/IO/S3/tests/gtest_aws_s3_client.cpp
+++ b/src/IO/S3/tests/gtest_aws_s3_client.cpp
@@ -20,13 +20,16 @@
 #include <aws/core/client/AWSError.h>
 #include <aws/core/client/CoreErrors.h>
 #include <aws/core/client/RetryStrategy.h>
+#include <aws/core/http/HttpResponse.h>
 #include <aws/core/http/URI.h>
 
+#include <Common/ProfileEvents.h>
 #include <Common/RemoteHostFilter.h>
 #include <IO/ReadBufferFromS3.h>
 #include <IO/ReadHelpers.h>
 #include <IO/ReadSettings.h>
 #include <IO/WriteBufferFromS3.h>
+#include <IO/WriteSettings.h>
 #include <IO/S3Common.h>
 #include <IO/S3/Client.h>
 #include <IO/S3/PocoHTTPClient.h>
@@ -42,6 +45,11 @@ namespace DB::S3RequestSetting
     extern const S3RequestSettingsUInt64 max_unexpected_write_error_retries;
 }
 
+namespace ProfileEvents
+{
+    extern const Event S3SingleAttemptRetryConsultations;
+}
+
 /*
  * When all tests are executed together, `Context::getGlobalContextInstance()` is not null. Global context is used by
  * ProxyResolvers to get proxy configuration (used by S3 clients). If global context does not have a valid ConfigRef, it relies on
@@ -197,6 +205,135 @@ static void testServerSideEncryption(
     EXPECT_EQ(content, expected_headers);
 }
 
+TEST(IOTestAwsS3Client, DoesNotRetryPreconditionFailed)
+{
+    /// B166: a 412 Precondition Failed (conditional CAS/dedup writes of the content-addressed
+    /// backend) must NOT be retried, even when the SDK marks it retryable because an S3-compatible
+    /// server (e.g. RustFS) returned a body whose ExceptionName it could not parse. Retrying it is a
+    /// storm that stalls the write path.
+    DB::S3::Client::RetryStrategy strategy(DB::S3::PocoHTTPClientConfiguration::RetryStrategy{.max_retries = 10});
+
+    Aws::Client::AWSError<Aws::Client::CoreErrors> precondition(Aws::Client::CoreErrors::UNKNOWN, /*isRetryable=*/true);
+    precondition.SetResponseCode(Aws::Http::HttpResponseCode::PRECONDITION_FAILED);
+    EXPECT_FALSE(strategy.ShouldRetry(precondition, /*attemptedRetries=*/0));
+    EXPECT_TRUE(DB::S3::isPreconditionFailedError(precondition));       // one policy: agrees via response code
+
+    /// A genuinely transient error is still retried (the guard is specific to 412).
+    Aws::Client::AWSError<Aws::Client::CoreErrors> unavailable(Aws::Client::CoreErrors::SLOW_DOWN, /*isRetryable=*/true);
+    unavailable.SetResponseCode(Aws::Http::HttpResponseCode::SERVICE_UNAVAILABLE);
+    EXPECT_TRUE(strategy.ShouldRetry(unavailable, /*attemptedRetries=*/0));
+    EXPECT_FALSE(DB::S3::isPreconditionFailedError(unavailable));
+
+    /// The one 412 policy also matches on the canonical <Code> name / raw body (the two CA conditional
+    /// ops see an error whose ExceptionName the SDK DID parse, or whose body carries the token).
+    Aws::Client::AWSError<Aws::S3::S3Errors> named(Aws::S3::S3Errors::UNKNOWN, "PreconditionFailed", "precondition failed", false);
+    EXPECT_TRUE(DB::S3::isPreconditionFailedError(named));
+
+    /// Typed-exception surface (the conditional copy / finalize catch an S3Exception): name and message.
+    EXPECT_TRUE(DB::S3Exception("boom", Aws::S3::S3Errors::UNKNOWN, "PreconditionFailed").isPreconditionFailed());
+    EXPECT_FALSE(DB::S3Exception("boom", Aws::S3::S3Errors::NO_SUCH_KEY, "NoSuchKey").isPreconditionFailed());
+}
+
+/// Every consultation is counted, not just the first: simulating two retryable 5xx decisions in a row
+/// proves the counter tracks each SDK consultation rather than being fixed/clamped at 1, which is what
+/// makes it a live tripwire ("SDK-level retries must remain zero for conditional writes") rather than a
+/// value nothing ever touches.
+TEST(IOTestAwsS3Client, SingleAttemptRetryStrategyRefusesAndCounts)
+{
+    using ProfileEvents::global_counters;
+    const auto before = global_counters[ProfileEvents::S3SingleAttemptRetryConsultations].load();
+    DB::S3::SingleAttemptRetryStrategy strategy;
+    const Aws::Client::AWSError<Aws::Client::CoreErrors> retryable_5xx(
+        Aws::Client::CoreErrors::INTERNAL_FAILURE, /*isRetryable=*/true);
+    EXPECT_FALSE(strategy.ShouldRetry(retryable_5xx, /*attempted=*/0));
+    EXPECT_FALSE(strategy.ShouldRetry(retryable_5xx, /*attempted=*/1));
+    EXPECT_EQ(strategy.GetMaxAttempts(), 1);
+    EXPECT_EQ(global_counters[ProfileEvents::S3SingleAttemptRetryConsultations].load() - before, 2u);
+}
+
+/// Drive a single-part conditional PUT (`If-None-Match: *`) with `body_size` bytes through a real
+/// S3 client whose `expect_continue_min_bytes` gate is `threshold`, against the mock HTTP server, and
+/// report whether the request that reached the wire carried an `Expect: 100-continue` header.
+static bool conditionalPutNegotiatesExpectContinue(uint64_t threshold, size_t body_size)
+{
+    TestPocoHTTPServer http;
+
+    DB::RemoteHostFilter remote_host_filter;
+    DB::S3::URI uri(http.getUrl() + "/IOTestAwsS3ClientExpectContinue/test.txt");
+
+    DB::S3::PocoHTTPClientConfiguration client_configuration = DB::S3::ClientFactory::instance().createClientConfiguration(
+        "us-east-1",
+        remote_host_filter,
+        /* s3_max_redirects = */ 100,
+        DB::S3::PocoHTTPClientConfiguration::RetryStrategy{.max_retries = 0},
+        /* s3_slow_all_threads_after_network_error = */ true,
+        /* s3_slow_all_threads_after_retryable_error = */ true,
+        /* enable_s3_requests_logging = */ false,
+        /* for_disk_s3 = */ false,
+        /* opt_disk_name = */ {},
+        /* request_throttler = */ {},
+        uri.uri.getScheme());
+
+    client_configuration.endpointOverride = uri.endpoint;
+    client_configuration.expect_continue_min_bytes = threshold;
+
+    DB::S3::ClientSettings client_settings{
+        .use_virtual_addressing = uri.is_virtual_hosted_style,
+        .disable_checksum = false,
+        .gcs_issue_compose_request = false,
+        .is_s3express_bucket = false,
+    };
+
+    std::shared_ptr<DB::S3::Client> client = DB::S3::ClientFactory::instance().create(
+        client_configuration,
+        client_settings,
+        /* access_key_id = */ "ACCESS_KEY_ID",
+        /* secret_access_key = */ "SECRET_ACCESS_KEY",
+        /* server_side_encryption_customer_key_base64 = */ "",
+        /* sse_kms_config = */ {},
+        /* headers = */ {},
+        DB::S3::CredentialsConfiguration{
+            .use_environment_credentials = false,
+            .use_insecure_imds_request = false,
+        });
+
+    DB::S3::S3RequestSettings request_settings;
+    request_settings[DB::S3RequestSetting::max_unexpected_write_error_retries] = 1;
+
+    DB::WriteSettings write_settings;
+    write_settings.object_storage_write_if_none_match = "*";
+
+    DB::WriteBufferFromS3 write_buffer(
+        client,
+        uri.bucket,
+        uri.key,
+        DB::DBMS_DEFAULT_BUFFER_SIZE,
+        request_settings,
+        /* blob_log = */ nullptr,
+        /* object_metadata = */ std::nullopt,
+        /* schedule = */ {},
+        write_settings);
+
+    const std::string body(body_size, 'x');
+    write_buffer.write(body.data(), body.size());
+    write_buffer.finalize();
+
+    return http.getLastRequestHeader().has("Expect");
+}
+
+TEST(IOTestAwsS3Client, ExpectContinueOnlyWhenThresholdPositive)
+{
+    /// RExpect: `Expect: 100-continue` (B118) is scoped to CAS-owned conditional writes. A non-CAS S3
+    /// client carries the default threshold 0 (disabled) and must NOT negotiate Expect on a conditional
+    /// PUT — that is the upstream wire behaviour a non-CAS disk (e.g. Iceberg's If-None-Match commits)
+    /// must keep. A CAS conditional-write client raises the threshold (see the single-attempt client in
+    /// ObjectStorageBackend) and DOES negotiate it for a body at least that large.
+    EXPECT_FALSE(conditionalPutNegotiatesExpectContinue(/*threshold=*/0, /*body_size=*/64));
+    EXPECT_TRUE(conditionalPutNegotiatesExpectContinue(/*threshold=*/8, /*body_size=*/64));
+    /// A positive threshold still excludes a body below it (only large bodies warrant the round-trip).
+    EXPECT_FALSE(conditionalPutNegotiatesExpectContinue(/*threshold=*/128, /*body_size=*/64));
+}
+
 TEST(IOTestAwsS3Client, AppendExtraSSECHeadersRead)
 {
     /// See https://github.com/ClickHouse/ClickHouse/pull/19748
diff --git a/src/IO/S3AuthSettings.cpp b/src/IO/S3AuthSettings.cpp
index 2bcfe965e84c..7946dccbeb2c 100644
--- a/src/IO/S3AuthSettings.cpp
+++ b/src/IO/S3AuthSettings.cpp
@@ -25,6 +25,7 @@ namespace DB
     DECLARE(Bool, no_sign_request, S3::DEFAULT_NO_SIGN_REQUEST, "", 0) \
     DECLARE(Bool, use_insecure_imds_request, false, "", 0) \
     DECLARE(Bool, use_adaptive_timeouts, S3::DEFAULT_USE_ADAPTIVE_TIMEOUTS, "", 0) \
+    DECLARE(UInt64, expect_continue_min_bytes, S3::DEFAULT_EXPECT_CONTINUE_MIN_BYTES, "", 0) \
     DECLARE(Bool, is_virtual_hosted_style, false, "", 0) \
     DECLARE(Bool, disable_checksum, S3::DEFAULT_DISABLE_CHECKSUM, "", 0) \
     DECLARE(Bool, gcs_issue_compose_request, false, "", 0) \
diff --git a/src/IO/S3Common.cpp b/src/IO/S3Common.cpp
index be42b184589b..47f7c53386f9 100644
--- a/src/IO/S3Common.cpp
+++ b/src/IO/S3Common.cpp
@@ -43,6 +43,51 @@ bool S3Exception::isAccessTokenExpiredError() const
     return code == Aws::S3::S3Errors::INVALID_ACCESS_KEY_ID || code == Aws::S3::S3Errors::ACCESS_DENIED || code == Aws::S3::S3Errors::INVALID_SIGNATURE || code == Aws::S3::S3Errors::UNKNOWN;
 }
 
+bool S3Exception::isPreconditionFailed() const
+{
+    /// See `S3::isPreconditionFailedError`. The thrown exception no longer carries the HTTP status, so
+    /// only the name and raw message are available here — fail-safe: matching too broadly maps a hard
+    /// error to a retryable re-validate, never a false success.
+    return exception_name == "PreconditionFailed"
+        || message().find("PreconditionFailed") != std::string::npos;
+}
+
+namespace S3
+{
+
+/// A synchronous rejection PROVING the request was never applied — matched by the canonical S3 error
+/// code STRING (many of these are UNKNOWN in the SDK's modeled enum, mirroring
+/// ObjectStorageBackend::finalizeConditionalWrite's own name-first matching) plus the modeled enum
+/// value where one exists, belt-and-suspenders.
+bool isMalformedRequestError(const S3Exception & e)
+{
+    const String & name = e.getExceptionName();
+    return name == "MalformedXML" || name == "MalformedPOSTRequest" || name == "InvalidArgument"
+        || name == "InvalidRequest" || name == "InvalidBucketName" || name == "KeyTooLongError"
+        || e.getS3ErrorCode() == Aws::S3::S3Errors::INVALID_PARAMETER_VALUE
+        || e.getS3ErrorCode() == Aws::S3::S3Errors::INVALID_REQUEST
+        || e.getS3ErrorCode() == Aws::S3::S3Errors::VALIDATION;
+}
+
+bool isEntityTooLargeError(const S3Exception & e)
+{
+    /// No modeled enum value for this error — name-only match, same as PreconditionFailed elsewhere.
+    return e.getExceptionName() == "EntityTooLarge";
+}
+
+bool isAccessDeniedError(const S3Exception & e)
+{
+    const String & name = e.getExceptionName();
+    return name == "AccessDenied" || name == "InvalidAccessKeyId" || name == "SignatureDoesNotMatch"
+        || name == "InvalidToken" || name == "ExpiredToken" || name == "AccountProblem"
+        || e.getS3ErrorCode() == Aws::S3::S3Errors::ACCESS_DENIED
+        || e.getS3ErrorCode() == Aws::S3::S3Errors::INVALID_ACCESS_KEY_ID
+        || e.getS3ErrorCode() == Aws::S3::S3Errors::SIGNATURE_DOES_NOT_MATCH
+        || e.getS3ErrorCode() == Aws::S3::S3Errors::INVALID_CLIENT_TOKEN_ID;
+}
+
+}
+
 }
 
 #endif
diff --git a/src/IO/S3Common.h b/src/IO/S3Common.h
index 27532ca851e7..8c1a4d524294 100644
--- a/src/IO/S3Common.h
+++ b/src/IO/S3Common.h
@@ -37,9 +37,18 @@ class S3Exception : public Exception
     {
     }
 
-    S3Exception(const std::string & msg, Aws::S3::S3Errors code_)
+    S3Exception(const std::string & msg, Aws::S3::S3Errors code_, String exception_name_ = {})
         : Exception(msg, ErrorCodes::S3_ERROR)
         , code(code_)
+        , exception_name(std::move(exception_name_))
+    {}
+
+    /// Preserves the static format string (system.text_log / system.errors grouping) while also
+    /// carrying the canonical S3 error name — build msg with PreformattedMessage::create.
+    S3Exception(PreformattedMessage && msg, Aws::S3::S3Errors code_, String exception_name_)
+        : Exception(std::move(msg), ErrorCodes::S3_ERROR)
+        , code(code_)
+        , exception_name(std::move(exception_name_))
     {}
 
     Aws::S3::S3Errors getS3ErrorCode() const
@@ -47,15 +56,57 @@ class S3Exception : public Exception
         return code;
     }
 
+    /// The canonical S3 error code string from the response XML `<Code>` (e.g. "PreconditionFailed",
+    /// "NoSuchKey") as reported by `Aws::Client::AWSError::GetExceptionName`. Errors unmodeled by the
+    /// SDK (a conditional-PUT 412 is one) have `getS3ErrorCode` == UNKNOWN, so this name is the only
+    /// machine-readable discriminator. Empty when the throw site did not attach it.
+    /// Not `Exception::name`; this is the AWS `<Code>` string.
+    const String & getExceptionName() const
+    {
+        return exception_name;
+    }
+
     bool isRetryableError() const;
     bool isAccessTokenExpiredError() const;
 
+    /// True for a conditional-request 412 (a lost `If-Match`/`If-None-Match`). The thrown exception
+    /// discards the HTTP status, so it matches on the canonical `<Code>` name and the raw message —
+    /// see `S3::isPreconditionFailedError` for the full (response-code-aware) policy.
+    bool isPreconditionFailed() const;
+
     S3Exception * clone() const override { return new S3Exception(*this); }
     void rethrow() const override { throw *this; } /// NOLINT(cert-err60-cpp)
 
 private:
     Aws::S3::S3Errors code;
+    String exception_name;
 };
+
+namespace S3
+{
+
+/// One policy for "is this error a conditional-request 412 (`PreconditionFailed`)?", shared by the
+/// retry strategy and the CA conditional delete/copy paths. The HTTP status is authoritative — a
+/// non-AWS body (e.g. RustFS) leaves the SDK-parsed `ExceptionName` empty — with the canonical `<Code>`
+/// name and the raw message as fallbacks. Fail-safe by direction: over-matching only forces a caller
+/// re-validate, never a false success.
+template <typename ErrorType>
+inline bool isPreconditionFailedError(const Aws::Client::AWSError<ErrorType> & error)
+{
+    return error.GetResponseCode() == Aws::Http::HttpResponseCode::PRECONDITION_FAILED
+        || error.GetExceptionName() == "PreconditionFailed"
+        || error.GetMessage().find("PreconditionFailed") != std::string::npos;
+}
+
+/// Error-name classifiers factored out of the CAS conditional-write outcome mapping
+/// (`CasRequestControl.cpp`), so the name lists live next to the other S3 error classifiers here
+/// and are available for reuse.
+bool isMalformedRequestError(const S3Exception & e);
+bool isEntityTooLargeError(const S3Exception & e);
+bool isAccessDeniedError(const S3Exception & e);
+
+}
+
 }
 
 #endif
diff --git a/src/IO/S3Defines.h b/src/IO/S3Defines.h
index 228758e57f44..8d13dd036b9a 100644
--- a/src/IO/S3Defines.h
+++ b/src/IO/S3Defines.h
@@ -33,6 +33,13 @@ inline static constexpr uint64_t DEFAULT_LIST_OBJECT_KEYS_SIZE = 1000;
 inline static constexpr uint64_t DEFAULT_MAX_SINGLE_READ_TRIES = 4;
 inline static constexpr uint64_t DEFAULT_MAX_UNEXPECTED_WRITE_ERROR_RETRIES = 4;
 inline static constexpr uint64_t DEFAULT_MAX_REDIRECTS = 10;
+/// Gate for the `Expect: 100-continue` negotiation on a conditional write (If-None-Match / If-Match):
+/// `0` = disabled (never negotiate Expect); a positive `N` negotiates Expect for a conditional `PUT`
+/// whose body is at least `N` bytes, so the server can reject (e.g. 412) BEFORE the body is streamed
+/// (B118). The default is DISABLED: only a CAS conditional-write client raises this (see the
+/// single-attempt client built in `ObjectStorageBackend`), so non-CAS S3 traffic keeps upstream
+/// behaviour instead of negotiating Expect on large conditional PUTs it never negotiated before.
+inline static constexpr uint64_t DEFAULT_EXPECT_CONTINUE_MIN_BYTES = 0;
 inline static constexpr uint64_t DEFAULT_RETRY_ATTEMPTS = 500;
 inline static constexpr uint64_t DEFAULT_RETRY_INITIAL_DELAY_MS = 25;
 inline static constexpr uint64_t DEFAULT_RETRY_MAX_DELAY_MS = 5000;
diff --git a/src/IO/WriteBufferFromS3.cpp b/src/IO/WriteBufferFromS3.cpp
index 8021a1f6cd04..81677edf6ee5 100644
--- a/src/IO/WriteBufferFromS3.cpp
+++ b/src/IO/WriteBufferFromS3.cpp
@@ -64,6 +64,7 @@ namespace ErrorCodes
     extern const int S3_ERROR;
     extern const int INVALID_CONFIG_PARAMETER;
     extern const int LOGICAL_ERROR;
+    extern const int NOT_IMPLEMENTED;
 }
 
 struct WriteBufferFromS3::PartData
@@ -405,6 +406,15 @@ void WriteBufferFromS3::writeMultipartUpload()
 
 void WriteBufferFromS3::createMultipartUpload()
 {
+    if (write_settings.s3_force_single_part_upload)
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "A conditional write would start a MULTIPART upload, but the target store enforces no "
+            "preconditions on CompleteMultipartUpload (GCS, measured 2026-07-03) — refusing "
+            "(silent-data-loss risk). The single-PUT budget is governed by the disk setting "
+            "gcs_max_conditional_put_bytes; the production-grade path for bigger blobs "
+            "(unconditional multipart to a temp key + conditional Compose) is not implemented yet. {}",
+            getShortLogDetails());
+
     LOG_TEST(limited_log, "Create multipart upload. {}", getShortLogDetails());
 
     S3::CreateMultipartUploadRequest req;
@@ -678,6 +688,7 @@ bool WriteBufferFromS3::completeMultipartUpload()
 
         if (outcome.IsSuccess())
         {
+            object_etag = outcome.GetResult().GetETag();
             LOG_TRACE(limited_log, "Multipart upload has completed. {}, Parts: {}", getShortLogDetails(), multipart_tags.size());
             return true;
         }
@@ -692,10 +703,13 @@ bool WriteBufferFromS3::completeMultipartUpload()
         }
         else
         {
+            /// Pass the canonical S3 error name: a conditional-write 412 is UNMODELED for the SDK
+            /// (the error type is UNKNOWN), so the name is the caller's only typed signal.
             throw S3Exception(
+                PreformattedMessage::create("Message: {}, Key: {}, Bucket: {}, Tags: {}",
+                    outcome.GetError().GetMessage(), key, bucket, fmt::join(multipart_tags.begin(), multipart_tags.end(), " ")),
                 outcome.GetError().GetErrorType(),
-                "Message: {}, Key: {}, Bucket: {}, Tags: {}",
-                outcome.GetError().GetMessage(), key, bucket, fmt::join(multipart_tags.begin(), multipart_tags.end(), " "));
+                outcome.GetError().GetExceptionName());
         }
     }
 
@@ -770,6 +784,7 @@ void WriteBufferFromS3::makeSinglepartUpload(WriteBufferFromS3::PartData && data
 
             if (outcome.IsSuccess())
             {
+                object_etag = outcome.GetResult().GetETag();
                 LOG_TRACE(limited_log, "Single part upload has completed. {}, size {}", getShortLogDetails(), content_length);
                 return;
             }
@@ -785,17 +800,20 @@ void WriteBufferFromS3::makeSinglepartUpload(WriteBufferFromS3::PartData && data
             else
             {
                 /// PreconditionFailed is an expected response for conditional writes (e.g. If-None-Match: *),
-                /// not a genuine error — the caller handles it.
-                if (outcome.GetError().GetExceptionName() == "PreconditionFailed")
+                /// not a genuine error — the caller handles it (see `S3::isPreconditionFailedError`).
+                if (S3::isPreconditionFailedError(outcome.GetError()))
                     LOG_INFO(log, "S3Exception name {}, Message: {}, bucket {}, key {}, object size {}",
                               outcome.GetError().GetExceptionName(), outcome.GetError().GetMessage(), bucket, key, content_length);
                 else
                     LOG_ERROR(log, "S3Exception name {}, Message: {}, bucket {}, key {}, object size {}",
                               outcome.GetError().GetExceptionName(), outcome.GetError().GetMessage(), bucket, key, content_length);
+                /// Pass the canonical S3 error name: a conditional-write 412 is UNMODELED for the SDK
+                /// (the error type is UNKNOWN), so the name is the caller's only typed signal.
                 throw S3Exception(
+                    PreformattedMessage::create("Message: {}, bucket {}, key {}, object size {}",
+                        outcome.GetError().GetMessage(), bucket, key, content_length),
                     outcome.GetError().GetErrorType(),
-                    "Message: {}, bucket {}, key {}, object size {}",
-                    outcome.GetError().GetMessage(), bucket, key, content_length);
+                    outcome.GetError().GetExceptionName());
             }
         }
 
diff --git a/src/IO/WriteBufferFromS3.h b/src/IO/WriteBufferFromS3.h
index 6a5e88875f27..fbce538c0a43 100644
--- a/src/IO/WriteBufferFromS3.h
+++ b/src/IO/WriteBufferFromS3.h
@@ -51,6 +51,10 @@ class WriteBufferFromS3 final : public WriteBufferFromFileBase
     void preFinalize() override;
     std::string getFileName() const override { return key; }
     void sync() override { next(); }
+    /// The object ETag from the PutObject / CompleteMultipartUpload response, captured on a
+    /// successful upload. Lets content-addressed callers record the written incarnation's token
+    /// without a follow-up HEAD. Valid only after a successful finalize().
+    std::optional<std::string> getResultObjectETag() const override { return object_etag; }
 
 private:
     /// Receives response from the server after sending all data.
@@ -88,6 +92,10 @@ class WriteBufferFromS3 final : public WriteBufferFromFileBase
     const WriteSettings write_settings;
     const std::shared_ptr<const S3::Client> client_ptr;
     const std::optional<ObjectAttributes> object_metadata;
+    /// Set from the PutObject / CompleteMultipartUpload response ETag on a successful upload; read
+    /// by getResultObjectETag() after finalize(). Written by the upload worker, read after the
+    /// finalize barrier (happens-before), so no extra synchronization is needed.
+    std::optional<String> object_etag;
     LoggerPtr log = getLogger("WriteBufferFromS3");
     LogSeriesLimiterPtr limited_log = std::make_shared<LogSeriesLimiter>(log, 1, 5);
 
diff --git a/src/IO/tests/gtest_writebuffer_s3.cpp b/src/IO/tests/gtest_writebuffer_s3.cpp
index 997ff354a535..fb09907fe6bd 100644
--- a/src/IO/tests/gtest_writebuffer_s3.cpp
+++ b/src/IO/tests/gtest_writebuffer_s3.cpp
@@ -18,6 +18,9 @@
 #include <aws/s3/model/UploadPartRequest.h>
 #include <aws/s3/model/HeadObjectRequest.h>
 #include <aws/s3/model/GetObjectRequest.h>
+#include <aws/s3/model/CopyObjectRequest.h>
+#include <aws/s3/model/DeleteObjectRequest.h>
+#include <aws/s3/model/GetBucketVersioningRequest.h>
 #include <aws/s3/S3Client.h>
 #include <aws/s3/S3Errors.h>
 
@@ -28,12 +31,16 @@
 #include <IO/AsyncReadCounters.h>
 #include <IO/ReadBufferFromS3.h>
 #include <IO/S3/Client.h>
+#include <IO/S3/copyS3File.h>
+#include <IO/SeekableReadBuffer.h>
 
 #include <Disks/IO/ThreadPoolRemoteFSReader.h>
 #include <Disks/IO/ReadBufferFromRemoteFSGather.h>
 #include <Disks/IO/AsynchronousBoundedReadBuffer.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/S3/S3ObjectStorage.h>
 
 #include <Common/filesystemHelpers.h>
+#include <Common/tests/gtest_global_context.h>
 #include <Core/Settings.h>
 
 
@@ -184,6 +191,9 @@ struct EventCounts
     size_t multiUploadAbort = 0;
     size_t uploadParts = 0;
     size_t writtenSize = 0;
+    size_t copyObject = 0;
+    size_t deleteObject = 0;
+    size_t getBucketVersioning = 0;
 
     size_t totalRequestsCount() const
     {
@@ -208,6 +218,9 @@ struct InjectionModel
     DeclareInjectCall(CompleteMultipartUpload)
     DeclareInjectCall(AbortMultipartUpload)
     DeclareInjectCall(UploadPart)
+    DeclareInjectCall(CopyObject)
+    DeclareInjectCall(DeleteObject)
+    DeclareInjectCall(GetBucketVersioning)
 #undef DeclareInjectCall
 };
 
@@ -277,6 +290,7 @@ struct Client : DB::S3::Client
 
         Aws::S3::Model::PutObjectOutcome outcome;
         Aws::S3::Model::PutObjectResult result(outcome.GetResultWithOwnership());
+        result.SetETag("etag-singlepart-" + request.GetKey());
         return result;
     }
 
@@ -392,6 +406,7 @@ struct Client : DB::S3::Client
         bStore.CompleteMPU(request.GetKey(), request.GetUploadId(), etags);
 
         Aws::S3::Model::CompleteMultipartUploadResult result;
+        result.SetETag("etag-multipart-" + request.GetKey());
         return Aws::S3::Model::CompleteMultipartUploadOutcome(result);
     }
 
@@ -414,6 +429,69 @@ struct Client : DB::S3::Client
         return Aws::S3::Model::AbortMultipartUploadOutcome(result);
     }
 
+    Aws::S3::Model::CopyObjectOutcome CopyObject(const Aws::S3::Model::CopyObjectRequest & request) const override
+    {
+        ++counters.copyObject;
+
+        if (injections)
+        {
+            if (auto opt_val = injections->call(request))
+                return std::move(*opt_val);
+        }
+
+        /// CopySource is "<bucket>/<key>"; parse it back apart to look the source object up
+        /// (both source and destination live in the same S3MemStrore in these tests).
+        const std::string & copy_source = request.GetCopySource();
+        const size_t sep = copy_source.find('/');
+        chassert(sep != std::string::npos);
+        const std::string src_bucket_name = copy_source.substr(0, sep);
+        const std::string src_key = copy_source.substr(sep + 1);
+
+        auto & src_store = store->GetBucketStore(src_bucket_name);
+        const std::string data = src_store.objects.at(src_key);
+
+        auto & dst_store = store->GetBucketStore(request.GetBucket());
+        dst_store.PutObject(request.GetKey(), data);
+
+        Aws::S3::Model::CopyObjectResult result;
+        Aws::S3::Model::CopyObjectResultDetails details;
+        details.SetETag("etag-copy-" + request.GetKey());
+        result.SetCopyObjectResultDetails(details);
+        return Aws::S3::Model::CopyObjectOutcome(result);
+    }
+
+    Aws::S3::Model::DeleteObjectOutcome DeleteObject(const Aws::S3::Model::DeleteObjectRequest & request) const override
+    {
+        ++counters.deleteObject;
+
+        if (injections)
+        {
+            if (auto opt_val = injections->call(request))
+                return std::move(*opt_val);
+        }
+
+        auto & bStore = store->GetBucketStore(request.GetBucket());
+        bStore.objects.erase(request.GetKey());
+
+        Aws::S3::Model::DeleteObjectResult result;
+        return Aws::S3::Model::DeleteObjectOutcome(result);
+    }
+
+    Aws::S3::Model::GetBucketVersioningOutcome GetBucketVersioning(const Aws::S3::Model::GetBucketVersioningRequest & request) const override
+    {
+        ++counters.getBucketVersioning;
+
+        if (injections)
+        {
+            if (auto opt_val = injections->call(request))
+                return std::move(*opt_val);
+        }
+
+        Aws::S3::Model::GetBucketVersioningResult result;
+        result.SetStatus(Aws::S3::Model::BucketVersioningStatus::Enabled);
+        return Aws::S3::Model::GetBucketVersioningOutcome(result);
+    }
+
     std::shared_ptr<S3MemStrore> store;
     mutable EventCounts counters;
     mutable std::shared_ptr<InjectionModel> injections;
@@ -460,6 +538,39 @@ struct UploadPartFailIngection: InjectionModel
     }
 };
 
+/// Injects an arbitrary AWSError<S3Errors> on DeleteObject -- used to drive the conditional-remove
+/// (`removeObjectIfTokenMatches`) outcome mapping: a 412-shaped error (exception name "PreconditionFailed",
+/// matched by `S3::isPreconditionFailedError`) must map to `ConditionalRemoveOutcome::TokenMismatch`, and a
+/// 404-shaped error (a `NO_SUCH_KEY`/`RESOURCE_NOT_FOUND`/`NO_SUCH_BUCKET` error type, matched by
+/// `S3::isNotFoundError`) must map to `ConditionalRemoveOutcome::NotFound`.
+struct DeleteObjectErrorInjection: InjectionModel
+{
+    explicit DeleteObjectErrorInjection(Aws::Client::AWSError<Aws::S3::S3Errors> error_) : error(std::move(error_)) {}
+
+    std::optional<Aws::S3::Model::DeleteObjectOutcome> call(const Aws::S3::Model::DeleteObjectRequest & /*request*/) override
+    {
+        return error;
+    }
+
+    Aws::Client::AWSError<Aws::S3::S3Errors> error;
+};
+
+/// Injects an arbitrary AWSError<S3Errors> on CopyObject -- used to drive `copyObjectConditional` /
+/// `copyS3File`'s `If-None-Match` handling: a "PreconditionFailed" error is the expected "lost the
+/// race" signal, while an "AccessDenied" error must propagate as a genuine failure rather than being
+/// swallowed into the unconditional-copy fallback (see `copyS3File.cpp`'s `processCopyRequest`).
+struct CopyObjectErrorInjection: InjectionModel
+{
+    explicit CopyObjectErrorInjection(Aws::Client::AWSError<Aws::S3::S3Errors> error_) : error(std::move(error_)) {}
+
+    std::optional<Aws::S3::Model::CopyObjectOutcome> call(const Aws::S3::Model::CopyObjectRequest & /*request*/) override
+    {
+        return error;
+    }
+
+    Aws::Client::AWSError<Aws::S3::S3Errors> error;
+};
+
 struct BaseSyncPolicy
 {
     virtual ~BaseSyncPolicy() = default;
@@ -952,6 +1063,33 @@ TEST_F(WBS3Test, PrefinalizeCalledMultipleTimes) {
 #endif
 }
 
+// The object ETag from the PutObject / CompleteMultipartUpload response is surfaced via
+// getResultObjectETag() after a successful finalize() — lets content-addressed callers record the
+// just-written incarnation's token WITHOUT a follow-up HEAD (CA head-after-put elimination).
+TEST_F(WBS3Test, ResultObjectETagIsCaptured) {
+    // Singlepart upload: the PutObject response ETag.
+    {
+        auto buffer = getWriteBuffer("singlepart-file");
+        writeAsOneBlock(*buffer, 10);
+        getAsyncPolicy().setAutoExecute(true);
+        buffer->finalize();
+        ASSERT_TRUE(buffer->getResultObjectETag().has_value());
+        ASSERT_EQ(*buffer->getResultObjectETag(), "etag-singlepart-singlepart-file");
+    }
+
+    // Multipart upload: the final object ETag comes from CompleteMultipartUpload, NOT a per-part tag.
+    {
+        getSettings()[Setting::s3_max_single_part_upload_size] = 0; // no single part — force multipart
+        getSettings()[Setting::s3_min_upload_part_size] = 1;
+        auto buffer = getWriteBuffer("multipart-file");
+        writeAsOneBlock(*buffer, 10);
+        getAsyncPolicy().setAutoExecute(true);
+        buffer->finalize();
+        ASSERT_TRUE(buffer->getResultObjectETag().has_value());
+        ASSERT_EQ(*buffer->getResultObjectETag(), "etag-multipart-multipart-file");
+    }
+}
+
 TEST_P(SyncAsync, EmptyFile) {
     getSettings()[Setting::s3_check_objects_after_upload] = true;
 
@@ -1186,6 +1324,188 @@ TEST_P(SyncAsync, StrictUploadPartSize) {
     }
 }
 
+/// Mock-S3 coverage for the content-addressed conditional-write primitives: `removeObjectIfTokenMatches`
+/// (`If-Match` `DeleteObject`) and `copyObjectConditional` (`If-None-Match: *` `CopyObject`), plus the
+/// fallback-disable guarantee in `copyS3File` when a conditional copy is requested.
+class S3ObjectStorageConditionalOpsTest : public ::testing::Test
+{
+public:
+    const String bucket = "cond-ops-bucket";
+    const String disk_name = "cond-ops-disk";
+
+    std::shared_ptr<S3ObjectStorage> object_storage;
+    MockS3::Client * mock_client = nullptr;
+    std::shared_ptr<MockS3::S3MemStrore> store;
+
+protected:
+    void SetUp() override
+    {
+        /// removeObjectIfTokenMatches()/copyObjectConditional() unconditionally call
+        /// BlobStorageLogWriter::create(), which falls back to Context::getGlobalContextInstance()
+        /// when there is no query context. Force that global context to exist (harmless -- blob
+        /// storage logging stays off by default) regardless of which other gtest TU ran first.
+        (void)getContext();
+
+        store = std::make_shared<MockS3::S3MemStrore>();
+        store->CreateBucket(bucket);
+
+        auto owned_client = std::make_unique<MockS3::Client>(store);
+        mock_client = owned_client.get();
+
+        S3::URI uri;
+        uri.bucket = bucket;
+        S3Capabilities capabilities;
+        ObjectStorageKeyGeneratorPtr key_generator;
+
+        object_storage = std::make_shared<S3ObjectStorage>(
+            std::move(owned_client), std::make_unique<S3Settings>(), std::move(uri), capabilities, key_generator, disk_name);
+    }
+
+    void TearDown() override
+    {
+        object_storage.reset();
+        mock_client = nullptr;
+        store.reset();
+    }
+};
+
+TEST_F(S3ObjectStorageConditionalOpsTest, RemoveObjectIfTokenMatchesSuccess)
+{
+    store->GetBucketStore(bucket).PutObject("key1", "data");
+
+    auto result = object_storage->removeObjectIfTokenMatches(StoredObject("key1"), "etag-1");
+
+    ASSERT_EQ(result.outcome, ConditionalRemoveOutcome::Removed);
+    ASSERT_EQ(mock_client->counters.deleteObject, 1);
+}
+
+TEST_F(S3ObjectStorageConditionalOpsTest, RemoveObjectIfTokenMatchesPreconditionFailedIsTokenMismatch)
+{
+    mock_client->setInjectionModel(std::make_shared<MockS3::DeleteObjectErrorInjection>(
+        Aws::Client::AWSError<Aws::S3::S3Errors>(Aws::S3::S3Errors::UNKNOWN, "PreconditionFailed", "precondition failed", false)));
+
+    auto result = object_storage->removeObjectIfTokenMatches(StoredObject("key1"), "stale-etag");
+
+    ASSERT_EQ(result.outcome, ConditionalRemoveOutcome::TokenMismatch);
+}
+
+TEST_F(S3ObjectStorageConditionalOpsTest, RemoveObjectIfTokenMatchesNotFoundIsNotFound)
+{
+    mock_client->setInjectionModel(std::make_shared<MockS3::DeleteObjectErrorInjection>(
+        Aws::Client::AWSError<Aws::S3::S3Errors>(Aws::S3::S3Errors::NO_SUCH_KEY, "NoSuchKey", "not found", false)));
+
+    auto result = object_storage->removeObjectIfTokenMatches(StoredObject("missing-key"), "any-etag");
+
+    ASSERT_EQ(result.outcome, ConditionalRemoveOutcome::NotFound);
+}
+
+TEST_F(S3ObjectStorageConditionalOpsTest, CopyObjectConditionalSuccess)
+{
+    store->GetBucketStore(bucket).PutObject("src-key", "hello-world");
+
+    auto result = object_storage->copyObjectConditional(
+        StoredObject("src-key"), StoredObject("dst-key"), ReadSettings{}, WriteSettings{}, std::nullopt);
+
+    ASSERT_TRUE(result.created);
+    ASSERT_FALSE(result.dest_etag.empty());
+    ASSERT_EQ(store->GetBucketStore(bucket).objects.at("dst-key"), "hello-world");
+}
+
+TEST_F(S3ObjectStorageConditionalOpsTest, CopyObjectConditionalPreconditionFailedIsNotCreated)
+{
+    store->GetBucketStore(bucket).PutObject("src-key", "hello-world");
+
+    mock_client->setInjectionModel(std::make_shared<MockS3::CopyObjectErrorInjection>(
+        Aws::Client::AWSError<Aws::S3::S3Errors>(Aws::S3::S3Errors::UNKNOWN, "PreconditionFailed", "precondition failed", false)));
+
+    auto result = object_storage->copyObjectConditional(
+        StoredObject("src-key"), StoredObject("dst-key"), ReadSettings{}, WriteSettings{}, std::nullopt);
+
+    /// "Lost the race": not an error, just created == false and no destination etag.
+    ASSERT_FALSE(result.created);
+    ASSERT_TRUE(result.dest_etag.empty());
+}
+
+/// B166-adjacent: `copyS3File`'s `If-None-Match` conditional copy must not silently fall back to an
+/// unconditional read-write copy on an `AccessDenied` `CopyObject` response -- that would defeat the
+/// write-once guarantee the CA promote path relies on. The exception must propagate, and the fallback
+/// (an unconditional `PutObject` upload of the source data) must never run.
+TEST_F(WBS3Test, CopyS3FileConditionalAccessDeniedPropagatesWithoutFallback)
+{
+    client->store->GetBucketStore(bucket).PutObject("src-key", "hello");
+
+    setInjectionModel(std::make_shared<MockS3::CopyObjectErrorInjection>(
+        Aws::Client::AWSError<Aws::S3::S3Errors>(Aws::S3::S3Errors::ACCESS_DENIED, "AccessDenied", "access denied", false)));
+
+    client->resetCounters();
+
+    S3::S3RequestSettings request_settings;
+    ReadSettings read_settings;
+    bool fallback_called = false;
+    auto fallback_reader = [&]() -> std::unique_ptr<SeekableReadBuffer>
+    {
+        fallback_called = true;
+        return nullptr;
+    };
+
+    EXPECT_THROW({
+        try
+        {
+            String dest_etag;
+            copyS3File(client, bucket, "src-key", 0, 5, client, bucket, "dst-key",
+                       request_settings, read_settings, nullptr, getAsyncPolicy().getScheduler(),
+                       fallback_reader, std::nullopt, String("*"), &dest_etag);
+        }
+        catch (const DB::S3Exception & e)
+        {
+            EXPECT_FALSE(e.isPreconditionFailed());
+            EXPECT_EQ(e.getExceptionName(), "AccessDenied");
+            throw;
+        }
+    }, DB::S3Exception);
+
+    EXPECT_FALSE(fallback_called);
+    EXPECT_EQ(client->counters.copyObject, 1);
+    EXPECT_EQ(client->counters.putObject, 0);
+}
+
+/// The other half of the same guard: a losing conditional copy (412) is a distinct, recognizable
+/// outcome (`S3Exception::isPreconditionFailed()`), still without ever running the fallback.
+TEST_F(WBS3Test, CopyS3FileConditionalPreconditionFailedSurfacesAsException)
+{
+    client->store->GetBucketStore(bucket).PutObject("src-key", "world");
+
+    setInjectionModel(std::make_shared<MockS3::CopyObjectErrorInjection>(
+        Aws::Client::AWSError<Aws::S3::S3Errors>(Aws::S3::S3Errors::UNKNOWN, "PreconditionFailed", "precondition failed", false)));
+
+    client->resetCounters();
+
+    S3::S3RequestSettings request_settings;
+    ReadSettings read_settings;
+    auto fallback_reader = []() -> std::unique_ptr<SeekableReadBuffer>
+    {
+        ADD_FAILURE() << "fallback must not run on a losing conditional copy";
+        return nullptr;
+    };
+
+    EXPECT_THROW({
+        try
+        {
+            String dest_etag;
+            copyS3File(client, bucket, "src-key", 0, 5, client, bucket, "dst-key",
+                       request_settings, read_settings, nullptr, getAsyncPolicy().getScheduler(),
+                       fallback_reader, std::nullopt, String("*"), &dest_etag);
+        }
+        catch (const DB::S3Exception & e)
+        {
+            EXPECT_TRUE(e.isPreconditionFailed());
+            throw;
+        }
+    }, DB::S3Exception);
+
+    EXPECT_EQ(client->counters.putObject, 0);
+}
+
 [[maybe_unused]] static String fillStringWithPattern(String pattern, int n)
 {
     String data;

From f6816f566530d4cfa96d9c55ac985c09aeb46201 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:32 +0200
Subject: [PATCH 06/30] Use Expect: 100-continue for large conditional S3
 uploads

A conditional PUT that is doomed to 412 should not stream its whole body;
send Expect: 100-continue above a size threshold and peek the response.
Prevents mid-upload connection resets and retry storms on S3-compatible
stores. Carries the GCS dialect integration points (wired later).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/IO/S3/PocoHTTPClient.cpp        | 93 ++++++++++++++++++++++++++++-
 src/IO/S3/PocoHTTPClient.h          | 38 ++++++++++++
 src/IO/S3/PocoHTTPClientFactory.cpp |  3 +
 3 files changed, 133 insertions(+), 1 deletion(-)

diff --git a/src/IO/S3/PocoHTTPClient.cpp b/src/IO/S3/PocoHTTPClient.cpp
index 2b76300bbbc9..319d7375f62d 100644
--- a/src/IO/S3/PocoHTTPClient.cpp
+++ b/src/IO/S3/PocoHTTPClient.cpp
@@ -7,6 +7,8 @@
 #if USE_AWS_S3
 
 #include <IO/S3/PocoHTTPClient.h>
+#include <IO/S3/GCSConditionalDialect.h>
+#include <IO/S3/GOOG4Signer.h>
 #include <IO/S3/Requests.h>
 
 #include <algorithm>
@@ -24,6 +26,7 @@
 #include <IO/S3/ProviderType.h>
 #include <Interpreters/Context.h>
 
+#include <aws/core/auth/AWSCredentialsProvider.h>
 #include <aws/core/http/HttpRequest.h>
 #include <smithy/tracing/NoopTelemetryProvider.h>
 #include <aws/core/http/HttpResponse.h>
@@ -89,6 +92,7 @@ namespace DB::ErrorCodes
     extern const int DNS_ERROR;
     extern const int AUTHENTICATION_FAILED;
     extern const int BAD_ARGUMENTS;
+    extern const int LOGICAL_ERROR;
 }
 
 namespace HistogramMetrics
@@ -221,11 +225,13 @@ PocoHTTPClient::PocoHTTPClient(const PocoHTTPClientConfiguration & client_config
     , remote_host_filter(client_configuration.remote_host_filter)
     , s3_max_redirects(client_configuration.s3_max_redirects)
     , s3_use_adaptive_timeouts(client_configuration.s3_use_adaptive_timeouts)
+    , expect_continue_min_bytes(client_configuration.expect_continue_min_bytes)
     , http_max_fields(client_configuration.http_max_fields)
     , http_max_field_name_size(client_configuration.http_max_field_name_size)
     , http_max_field_value_size(client_configuration.http_max_field_value_size)
     , enable_s3_requests_logging(client_configuration.enable_s3_requests_logging)
     , for_disk_s3(client_configuration.for_disk_s3)
+    , gcs_conditional_dialect(client_configuration.gcs_conditional_dialect)
     , request_throttler(client_configuration.request_throttler)
     , extra_headers(client_configuration.extra_headers)
 {
@@ -617,6 +623,42 @@ void PocoHTTPClient::makeRequestInternalImpl(
 
             Stopwatch watch;
 
+            /// A conditional write (`If-None-Match` / `If-Match`) that loses its precondition can waste
+            /// a LARGE body: streaming multi-MB into a request the server has already decided to reject
+            /// makes some stores (e.g. RustFS) close mid-upload or answer a retryable 500, which the SDK
+            /// then RETRIES up to `s3_retry_attempts` (500) times — a ~40-min stall that hangs CA INSERTs
+            /// (see B118). `Expect: 100-continue` lets the server reject (e.g. 412) BEFORE the body, so we
+            /// skip the doomed upload.
+            ///
+            /// `expect_continue_min_bytes` is the negotiation gate: `0` (the default, carried by every
+            /// non-CAS S3 client) DISABLES it entirely — non-CAS conditional PUTs keep upstream wire
+            /// behaviour and this whole block, INCLUDING the body-size probe, is skipped. A positive value
+            /// negotiates Expect for a conditional PUT whose body is at least that many bytes; only a CAS
+            /// conditional-write client raises it (the single-attempt client built in `ObjectStorageBackend`),
+            /// so the scope is exactly CAS-owned conditional writes. `x-goog-if-generation-match` is the GCS
+            /// conditional dialect's rename of If-None-Match / If-Match (applied BEFORE this point).
+            bool conditional_write = false;
+            if (expect_continue_min_bytes > 0
+                && method == Poco::Net::HTTPRequest::HTTP_PUT
+                && (poco_request.has("if-none-match") || poco_request.has("if-match")
+                    || poco_request.has("x-goog-if-generation-match")))
+            {
+                size_t content_body_size = 0;
+                if (const auto & content_body = request.GetContentBody())
+                {
+                    content_body->clear();
+                    content_body->seekg(0, std::ios_base::end);
+                    const auto end_pos = content_body->tellg();
+                    content_body->clear();
+                    content_body->seekg(0, std::ios_base::beg);
+                    if (end_pos > 0)
+                        content_body_size = static_cast<size_t>(end_pos);
+                }
+                conditional_write = content_body_size >= expect_continue_min_bytes;
+            }
+            if (conditional_write)
+                poco_request.setExpectContinue(true);
+
             auto & request_body_stream = session->sendRequest(poco_request, &connect_time, &first_byte_time);
             /// We record connect time here and not earlier, so that if an exception occurs while sending a request,
             /// we won't record the same latency twice.
@@ -624,7 +666,20 @@ void PocoHTTPClient::makeRequestInternalImpl(
             observeLatency(request, first_byte_latency_type, static_cast<HistogramMetrics::Value>(first_byte_time));
             latency_recorded = true;
 
-            if (request.GetContentBody())
+            /// With `Expect: 100-continue`, peek the interim response after the headers. `true` means
+            /// the server sent `100 Continue` (proceed with the body); `false` means it already sent a
+            /// FINAL response (now in `poco_response`) and the body must NOT be sent. `receiveResponse`
+            /// below is still called in both cases (Poco contract) and skips re-reading the headers.
+            bool skip_body = false;
+            if (conditional_write)
+            {
+                setTimeouts(*session, getTimeouts(method, first_attempt, /*first_byte*/ true));
+                skip_body = !session->peekResponse(poco_response);
+                if (enable_s3_requests_logging)
+                    LOG_TEST(log, "Expect: 100-continue peek -> {}", skip_body ? "final response, skipping body" : "100 Continue");
+            }
+
+            if (request.GetContentBody() && !skip_body)
             {
                 if (enable_s3_requests_logging)
                     LOG_TEST(log, "Writing request body.");
@@ -693,6 +748,17 @@ void PocoHTTPClient::makeRequestInternalImpl(
             response->SetResponseCode(static_cast<Aws::Http::HttpResponseCode>(status_code));
             response->SetContentType(poco_response.getContentType());
 
+            auto apply_gcs_generation_etag_override = [&]
+            {
+                if (gcs_conditional_dialect)
+                {
+                    /// The generation IS the incarnation token on GCS: surface it as the ETag so the
+                    /// entire existing ETag/token plumbing works unchanged (see GCSConditionalDialect.h).
+                    if (auto etag_override = gcsGenerationETagOverride(poco_response))
+                        response->AddHeader("ETag", *etag_override);
+                }
+            };
+
             if (enable_s3_requests_logging)
             {
                 WriteBufferFromOwnString headers_ss;
@@ -701,12 +767,14 @@ void PocoHTTPClient::makeRequestInternalImpl(
                     response->AddHeader(header_name, header_value);
                     headers_ss << header_name << ": " << header_value << "; ";
                 }
+                apply_gcs_generation_etag_override();
                 LOG_TEST(log, "Received headers: {}", headers_ss.str());
             }
             else
             {
                 for (const auto & [header_name, header_value] : poco_response)
                     response->AddHeader(header_name, header_value);
+                apply_gcs_generation_etag_override();
             }
 
             /// Request is successful but for some special requests we can have actual error message in body
@@ -835,6 +903,9 @@ void PocoHTTPClientGCPOAuth::makeRequestInternal(
     Aws::Utils::RateLimits::RateLimiterInterface * readLimiter,
     Aws::Utils::RateLimits::RateLimiterInterface * writeLimiter) const
 {
+    if (gcs_conditional_dialect)
+        applyGcsConditionalDialectToRequest(request);
+
     {
         std::lock_guard lock(mutex);
         if (!bearer_token || std::chrono::system_clock::now() > bearer_token->is_valid_to)
@@ -922,6 +993,26 @@ PocoHTTPClientGCPOAuth::BearerToken PocoHTTPClientGCPOAuth::requestBearerTokenFr
     };
 }
 
+PocoHTTPClientGCSHMAC::PocoHTTPClientGCSHMAC(const PocoHTTPClientConfiguration & client_configuration)
+    : PocoHTTPClient(client_configuration)
+    , credentials_provider(client_configuration.gcs_hmac_credentials_provider)
+{
+    if (!credentials_provider)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "PocoHTTPClientGCSHMAC requires a credentials provider (http_client = gcs_hmac wiring bug)");
+}
+
+void PocoHTTPClientGCSHMAC::makeRequestInternal(
+    Aws::Http::HttpRequest & request,
+    std::shared_ptr<PocoHTTPResponse> & response,
+    Aws::Utils::RateLimits::RateLimiterInterface * readLimiter,
+    Aws::Utils::RateLimits::RateLimiterInterface * writeLimiter) const
+{
+    applyGcsConditionalDialectToRequest(request);
+    signRequestGOOG4(request, credentials_provider->GetAWSCredentials(), std::chrono::system_clock::now());
+    PocoHTTPClient::makeRequestInternal(request, response, readLimiter, writeLimiter);
+}
+
 }
 
 #endif
diff --git a/src/IO/S3/PocoHTTPClient.h b/src/IO/S3/PocoHTTPClient.h
index e3182a508be9..d3154892919f 100644
--- a/src/IO/S3/PocoHTTPClient.h
+++ b/src/IO/S3/PocoHTTPClient.h
@@ -30,6 +30,11 @@ namespace Aws::Http::Standard
 class StandardHttpResponse;
 }
 
+namespace Aws::Auth
+{
+class AWSCredentialsProvider;
+}
+
 namespace DB
 {
 class Context;
@@ -73,6 +78,14 @@ struct PocoHTTPClientConfiguration : public Aws::Client::ClientConfiguration
 
     HTTPHeaderEntries extra_headers;
     String http_client;
+    /// GCS conditional dialect (spec 2026-07-03-cas-gcs-generation-binding-design): translate
+    /// AWS-style conditional headers and x-amz-* prefixes to the x-goog dialect at the wire
+    /// boundary, and surface x-goog-generation as the response ETag. Set for http_client values
+    /// `gcs_hmac` and `gcp_oauth`; never set for plain AWS-compatible endpoints.
+    bool gcs_conditional_dialect = false;
+    /// Credentials for the GOOG4-HMAC signer (http_client = gcs_hmac only): the same provider
+    /// chain the AWS path builds (inline keys, use_environment_credentials, ...).
+    std::shared_ptr<Aws::Auth::AWSCredentialsProvider> gcs_hmac_credentials_provider;
     String service_account;
     String metadata_service;
     String request_token_path;
@@ -82,6 +95,10 @@ struct PocoHTTPClientConfiguration : public Aws::Client::ClientConfiguration
 
     /// See PoolBase::BehaviourOnLimit
     bool s3_use_adaptive_timeouts = true;
+    /// Conditional PUT (If-None-Match / If-Match) bodies >= this negotiate Expect: 100-continue (B118).
+    /// `0` (the default) disables it, so non-CAS S3 clients keep upstream behaviour; only a CAS
+    /// conditional-write client raises it (see the single-attempt client in `ObjectStorageBackend`).
+    size_t expect_continue_min_bytes = DEFAULT_EXPECT_CONTINUE_MIN_BYTES;
     size_t http_keep_alive_timeout = DEFAULT_HTTP_KEEP_ALIVE_TIMEOUT;
     size_t http_keep_alive_max_requests = DEFAULT_HTTP_KEEP_ALIVE_MAX_REQUEST;
 
@@ -224,11 +241,13 @@ class PocoHTTPClient : public Aws::Http::HttpClient
     const RemoteHostFilter & remote_host_filter;
     unsigned int s3_max_redirects = DEFAULT_MAX_REDIRECTS;
     bool s3_use_adaptive_timeouts = true;
+    size_t expect_continue_min_bytes = DEFAULT_EXPECT_CONTINUE_MIN_BYTES;
     const UInt64 http_max_fields = 1000000;
     const UInt64 http_max_field_name_size = 128 * 1024;
     const UInt64 http_max_field_value_size = 128 * 1024;
     bool enable_s3_requests_logging = false;
     bool for_disk_s3 = false;
+    bool gcs_conditional_dialect = false;
 
     HTTPRequestThrottler request_throttler;
 
@@ -268,6 +287,25 @@ class PocoHTTPClientGCPOAuth : public PocoHTTPClient
     BearerToken requestBearerTokenFromADC() const;
 };
 
+/// GCS with HMAC credentials over the XML API, signed with Google's native GOOG4-HMAC-SHA256 —
+/// the ONLY way HMAC credentials get enforced conditional semantics on GCS (the S3-compatible
+/// sigv4 surface silently ignores If-None-Match / If-Match; measured 2026-07-03). Applies the GCS
+/// conditional dialect, then signs. Selected by `http_client = gcs_hmac`.
+class PocoHTTPClientGCSHMAC : public PocoHTTPClient
+{
+public:
+    explicit PocoHTTPClientGCSHMAC(const PocoHTTPClientConfiguration & client_configuration);
+
+private:
+    void makeRequestInternal(
+        Aws::Http::HttpRequest & request,
+        std::shared_ptr<PocoHTTPResponse> & response,
+        Aws::Utils::RateLimits::RateLimiterInterface * readLimiter,
+        Aws::Utils::RateLimits::RateLimiterInterface * writeLimiter) const override;
+
+    std::shared_ptr<Aws::Auth::AWSCredentialsProvider> credentials_provider;
+};
+
 }
 
 #endif
diff --git a/src/IO/S3/PocoHTTPClientFactory.cpp b/src/IO/S3/PocoHTTPClientFactory.cpp
index 0fb1cf40d93c..b7599290adbf 100644
--- a/src/IO/S3/PocoHTTPClientFactory.cpp
+++ b/src/IO/S3/PocoHTTPClientFactory.cpp
@@ -23,6 +23,9 @@ PocoHTTPClientFactory::CreateHttpClient(const Aws::Client::ClientConfiguration &
         if (Poco::toLower(poco_client_configuration.http_client) == "gcp_oauth")
             return std::make_shared<PocoHTTPClientGCPOAuth>(poco_client_configuration);
 
+        if (Poco::toLower(poco_client_configuration.http_client) == "gcs_hmac")
+            return std::make_shared<PocoHTTPClientGCSHMAC>(poco_client_configuration);
+
         return std::make_shared<PocoHTTPClient>(poco_client_configuration);
     }
 

From 57bd41968cc22743cb18f0827c3aa303e8a984f0 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:32 +0200
Subject: [PATCH 07/30] GCS conditional-write dialect and GOOG4 signer

GCS XML API needs native GOOG4 signing and x-goog-if-generation-match for
generation-safe conditional writes; AWS SigV4 If-Match semantics are not
enough, and conditional multipart complete is silently ignored. Adds the
signer, the header dialect, and fixed-vector tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/IO/S3/GCSConditionalDialect.cpp           | 104 ++++++++++++
 src/IO/S3/GCSConditionalDialect.h             |  36 ++++
 src/IO/S3/GOOG4Signer.cpp                     | 149 ++++++++++++++++
 src/IO/S3/GOOG4Signer.h                       |  30 ++++
 .../tests/gtest_gcs_conditional_dialect.cpp   | 159 ++++++++++++++++++
 src/IO/S3/tests/gtest_goog4_signer.cpp        |  62 +++++++
 6 files changed, 540 insertions(+)
 create mode 100644 src/IO/S3/GCSConditionalDialect.cpp
 create mode 100644 src/IO/S3/GCSConditionalDialect.h
 create mode 100644 src/IO/S3/GOOG4Signer.cpp
 create mode 100644 src/IO/S3/GOOG4Signer.h
 create mode 100644 src/IO/S3/tests/gtest_gcs_conditional_dialect.cpp
 create mode 100644 src/IO/S3/tests/gtest_goog4_signer.cpp

diff --git a/src/IO/S3/GCSConditionalDialect.cpp b/src/IO/S3/GCSConditionalDialect.cpp
new file mode 100644
index 000000000000..5c153ce44c2e
--- /dev/null
+++ b/src/IO/S3/GCSConditionalDialect.cpp
@@ -0,0 +1,104 @@
+#include <IO/S3/GCSConditionalDialect.h>
+
+#if USE_AWS_S3
+
+#include <Common/Exception.h>
+#include <aws/core/http/HttpRequest.h>
+#include <Poco/Net/HTTPResponse.h>
+
+#include <algorithm>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+
+namespace DB::S3
+{
+
+namespace
+{
+
+bool isAllDigits(const std::string & s)
+{
+    return !s.empty() && std::all_of(s.begin(), s.end(), [](char c) { return c >= '0' && c <= '9'; });
+}
+
+std::string stripQuotes(const std::string & s)
+{
+    if (s.size() >= 2 && s.front() == '"' && s.back() == '"')
+        return s.substr(1, s.size() - 2);
+    return s;
+}
+
+}
+
+void applyGcsConditionalDialectToRequest(Aws::Http::HttpRequest & request)
+{
+    const auto query_params = request.GetUri().GetQueryStringParameters();
+    const bool is_complete_multipart = request.GetMethod() == Aws::Http::HttpMethod::HTTP_POST
+        && query_params.contains("uploadId") && !query_params.contains("partNumber");
+
+    /// --- Conditional headers -> x-goog-if-generation-match ---
+    std::optional<std::string> generation_match;
+    if (request.HasHeader("if-none-match"))
+    {
+        const auto value = request.GetHeaderValue("if-none-match");
+        if (value != "*")
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "GCS conditional dialect: If-None-Match with a value other than '*' has no GCS "
+                "equivalent (got '{}') — refusing to silently change semantics", value);
+        generation_match = "0";
+        request.DeleteHeader("if-none-match");
+    }
+    if (request.HasHeader("if-match"))
+    {
+        const auto value = stripQuotes(request.GetHeaderValue("if-match"));
+        if (!isAllDigits(value))
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "GCS conditional dialect: If-Match value '{}' is not a generation number — an "
+                "ETag-kind token leaked into a generation-dialect client (mixed-mode misconfiguration)",
+                value);
+        generation_match = value;
+        request.DeleteHeader("if-match");
+    }
+    if (generation_match)
+    {
+        if (is_complete_multipart)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "GCS conditional dialect: a CONDITIONAL CompleteMultipartUpload was about to be sent. "
+                "GCS silently ignores preconditions on CompleteMultipartUpload (measured 2026-07-03) — "
+                "this would be silent data loss. Conditional writes must use the single-PUT path.");
+        request.SetHeaderValue("x-goog-if-generation-match", *generation_match);
+    }
+
+    /// --- AWS auth artifacts: drop (the GCS-mode client re-authenticates after this call) ---
+    for (const auto * header : {"authorization", "x-amz-date", "x-amz-content-sha256",
+                                "x-amz-security-token", "x-amz-api-version"})
+        request.DeleteHeader(header);
+
+    /// --- Rename every remaining x-amz-* header to x-goog-* (mixing is rejected by GCS) ---
+    std::vector<std::pair<std::string, std::string>> renamed;
+    for (const auto & [name, value] : request.GetHeaders())
+    {
+        if (name.starts_with("x-amz-"))
+            renamed.emplace_back("x-goog-" + name.substr(6), value);
+    }
+    for (const auto & [goog_name, value] : renamed)
+    {
+        request.DeleteHeader(("x-amz-" + goog_name.substr(7)).c_str());
+        request.SetHeaderValue(goog_name.c_str(), value);
+    }
+}
+
+std::optional<std::string> gcsGenerationETagOverride(const Poco::Net::HTTPResponse & response)
+{
+    if (!response.has("x-goog-generation"))
+        return std::nullopt;
+    return "\"" + response.get("x-goog-generation") + "\"";
+}
+
+}
+
+#endif
diff --git a/src/IO/S3/GCSConditionalDialect.h b/src/IO/S3/GCSConditionalDialect.h
new file mode 100644
index 000000000000..f63f331ae508
--- /dev/null
+++ b/src/IO/S3/GCSConditionalDialect.h
@@ -0,0 +1,36 @@
+#pragma once
+#include "config.h"
+#if USE_AWS_S3
+
+#include <optional>
+#include <string>
+
+namespace Aws::Http { class HttpRequest; }
+namespace Poco::Net { class HTTPResponse; }
+
+namespace DB::S3
+{
+
+/// The GCS conditional dialect, request side (spec: 2026-07-03-cas-gcs-generation-binding-design).
+/// Applied at the wire boundary by the GCS-mode Poco HTTP clients, so everything above keeps
+/// speaking AWS. Translations:
+///   - AWS auth artifacts (`authorization`, `x-amz-date`, `x-amz-content-sha256`,
+///     `x-amz-security-token`, `x-amz-api-version`) are DROPPED (the caller re-authenticates);
+///   - every remaining `x-amz-*` header is renamed to `x-goog-*`;
+///   - `If-None-Match: *` becomes `x-goog-if-generation-match: 0`;
+///   - `If-Match: "<digits>"` (quotes optional) becomes `x-goog-if-generation-match: <digits>`.
+/// Fail-close guards (throw LOGICAL_ERROR, the request never leaves the process):
+///   - `If-None-Match` with any value other than `*` (no GCS equivalent);
+///   - a non-numeric `If-Match` (an ETag-kind token leaked into a generation dialect);
+///   - a CONDITIONAL CompleteMultipartUpload (POST with `uploadId` and no `partNumber`): GCS
+///     silently ignores preconditions there (measured live 2026-07-03) — silent data loss.
+void applyGcsConditionalDialectToRequest(Aws::Http::HttpRequest & request);
+
+/// The dialect, response side: when the response carries `x-goog-generation`, returns it QUOTED —
+/// the caller substitutes it for the `ETag` response header, making the generation ride the
+/// entire existing ETag/token plumbing unchanged. Returns nullopt when no generation is present.
+std::optional<std::string> gcsGenerationETagOverride(const Poco::Net::HTTPResponse & response);
+
+}
+
+#endif
diff --git a/src/IO/S3/GOOG4Signer.cpp b/src/IO/S3/GOOG4Signer.cpp
new file mode 100644
index 000000000000..740b65d85c44
--- /dev/null
+++ b/src/IO/S3/GOOG4Signer.cpp
@@ -0,0 +1,149 @@
+#include <IO/S3/GOOG4Signer.h>
+
+#if USE_AWS_S3
+
+#include <aws/core/http/HttpRequest.h>
+#include <aws/core/http/HttpTypes.h>
+#include <aws/core/auth/AWSCredentials.h>
+#include <aws/core/utils/StringUtils.h>
+#include <Common/Exception.h>
+#include <base/hex.h>
+
+#include <openssl/hmac.h>
+#include <openssl/sha.h>
+
+#include <fmt/format.h>
+#include <map>
+
+namespace DB::ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+
+namespace DB::S3
+{
+
+namespace
+{
+
+constexpr auto UNSIGNED_PAYLOAD = "UNSIGNED-PAYLOAD";
+
+std::string hmacSHA256(const std::string & key, const std::string & message)
+{
+    unsigned char out[SHA256_DIGEST_LENGTH];
+    unsigned int out_len = 0;
+    HMAC(EVP_sha256(),
+         key.data(), static_cast<int>(key.size()),
+         reinterpret_cast<const unsigned char *>(message.data()), message.size(),
+         out, &out_len);
+    return std::string(reinterpret_cast<char *>(out), out_len);
+}
+
+std::string sha256Hex(const std::string & data)
+{
+    unsigned char out[SHA256_DIGEST_LENGTH];
+    SHA256(reinterpret_cast<const unsigned char *>(data.data()), data.size(), out);
+    return hexString(out, SHA256_DIGEST_LENGTH);
+}
+
+}
+
+void signRequestGOOG4(
+    Aws::Http::HttpRequest & request,
+    const Aws::Auth::AWSCredentials & credentials,
+    std::chrono::system_clock::time_point now)
+{
+    const std::time_t now_t = std::chrono::system_clock::to_time_t(now);
+    std::tm tm_utc{};
+    gmtime_r(&now_t, &tm_utc);
+    const std::string timestamp = fmt::format(
+        "{:04}{:02}{:02}T{:02}{:02}{:02}Z",
+        tm_utc.tm_year + 1900, tm_utc.tm_mon + 1, tm_utc.tm_mday,
+        tm_utc.tm_hour, tm_utc.tm_min, tm_utc.tm_sec);
+    const std::string datestamp = timestamp.substr(0, 8);
+
+    request.SetHeaderValue("x-goog-date", timestamp);
+    request.SetHeaderValue("x-goog-content-sha256", UNSIGNED_PAYLOAD);
+
+    /// Canonical headers: `host` + every x-goog-* header, lowercase names, sorted.
+    /// std::map keeps them sorted for us.
+    std::map<std::string, std::string> signed_headers_map;
+    for (const auto & [name, value] : request.GetHeaders())
+    {
+        std::string lower = Aws::Utils::StringUtils::ToLower(name.c_str());
+        if (lower == "host" || lower.starts_with("x-goog-"))
+            signed_headers_map.emplace(std::move(lower), value);
+    }
+    if (!signed_headers_map.contains("host"))
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "GOOG4 signing requires a Host header on the request");
+
+    std::string canonical_headers;
+    std::string signed_headers;
+    for (const auto & [name, value] : signed_headers_map)
+    {
+        canonical_headers += name + ":" + value + "\n";
+        if (!signed_headers.empty())
+            signed_headers += ";";
+        signed_headers += name;
+    }
+
+    /// Canonical query string: URL-encoded key=value pairs sorted by key; a parameter without a
+    /// value still gets a trailing `=` (e.g. `versioning=`).
+    ///
+    /// `Aws::Http::URI` has no ready-made helper for this: `CanonicalizeQueryString` only rewrites
+    /// the query string when it already contains an `=`, so a bare flag like `?versioning` (no `=`)
+    /// passes through unsorted and unencoded. `GetQueryStringParameters` doesn't help either — for
+    /// a valueless flag it has no `=` to split on, so it treats the whole `key` as the `value` too
+    /// (`versioning` becomes `versioning=versioning`, not `versioning=`). Parse the raw query string
+    /// by hand instead, splitting each `key[=value]` pair on the first `=` with an empty value when
+    /// absent, then URL-encode and join sorted `key=value` pairs with `&`.
+    std::map<std::string, std::string> query_params;
+    {
+        const std::string raw_query = request.GetUri().GetQueryString();
+        size_t pos = raw_query.empty() ? std::string::npos : 1; /// skip leading '?'
+        while (pos != std::string::npos && pos < raw_query.size())
+        {
+            const size_t amp = raw_query.find('&', pos);
+            const std::string pair = raw_query.substr(pos, amp == std::string::npos ? std::string::npos : amp - pos);
+            const size_t eq = pair.find('=');
+            std::string key = eq == std::string::npos ? pair : pair.substr(0, eq);
+            std::string value = eq == std::string::npos ? std::string() : pair.substr(eq + 1);
+            query_params.emplace(
+                Aws::Utils::StringUtils::URLDecode(key.c_str()),
+                Aws::Utils::StringUtils::URLDecode(value.c_str()));
+            pos = amp == std::string::npos ? std::string::npos : amp + 1;
+        }
+    }
+    std::string canonical_query;
+    for (const auto & [key, value] : query_params)
+    {
+        if (!canonical_query.empty())
+            canonical_query += "&";
+        canonical_query += Aws::Utils::StringUtils::URLEncode(key.c_str()) + "=" + Aws::Utils::StringUtils::URLEncode(value.c_str());
+    }
+    const std::string canonical_uri = request.GetUri().GetURLEncodedPath();
+
+    const std::string method = Aws::Http::HttpMethodMapper::GetNameForHttpMethod(request.GetMethod());
+
+    const std::string canonical_request = fmt::format(
+        "{}\n{}\n{}\n{}\n{}\n{}",
+        method, canonical_uri, canonical_query, canonical_headers, signed_headers, UNSIGNED_PAYLOAD);
+
+    const std::string scope = fmt::format("{}/auto/storage/goog4_request", datestamp);
+    const std::string string_to_sign = fmt::format(
+        "GOOG4-HMAC-SHA256\n{}\n{}\n{}", timestamp, scope, sha256Hex(canonical_request));
+
+    std::string key = hmacSHA256("GOOG4" + credentials.GetAWSSecretKey(), datestamp);
+    key = hmacSHA256(key, "auto");
+    key = hmacSHA256(key, "storage");
+    key = hmacSHA256(key, "goog4_request");
+    const std::string signature = hexString(hmacSHA256(key, string_to_sign).data(), SHA256_DIGEST_LENGTH);
+
+    request.SetHeaderValue("authorization", fmt::format(
+        "GOOG4-HMAC-SHA256 Credential={}/{}, SignedHeaders={}, Signature={}",
+        credentials.GetAWSAccessKeyId(), scope, signed_headers, signature));
+}
+
+}
+
+#endif
diff --git a/src/IO/S3/GOOG4Signer.h b/src/IO/S3/GOOG4Signer.h
new file mode 100644
index 000000000000..4b1f4c1b89c0
--- /dev/null
+++ b/src/IO/S3/GOOG4Signer.h
@@ -0,0 +1,30 @@
+#pragma once
+#include "config.h"
+#if USE_AWS_S3
+
+#include <chrono>
+
+namespace Aws::Http { class HttpRequest; }
+namespace Aws::Auth { class AWSCredentials; }
+
+namespace DB::S3
+{
+
+/// Sign `request` in place with GOOG4-HMAC-SHA256 — Google Cloud Storage's native V4 HMAC scheme
+/// for the XML API. Structurally sigv4 with renamed constants: key prefix `GOOG4`, scope
+/// terminator `goog4_request`, headers `x-goog-date` / `x-goog-content-sha256`. Bodies are never
+/// hashed (`UNSIGNED-PAYLOAD`), so streaming uploads sign in O(1).
+///
+/// Signs the `host` header plus EVERY `x-goog-*` header present on the request (GCS requires all
+/// x-goog headers to be signed); other headers ride unsigned. `now` is injected so unit tests can
+/// pin the timestamp to fixed vectors.
+///
+/// Live-validated against GCS 2026-07-03 (see `utils/ca-soak/scripts/gcs_goog4_probe.py`, 12/12).
+void signRequestGOOG4(
+    Aws::Http::HttpRequest & request,
+    const Aws::Auth::AWSCredentials & credentials,
+    std::chrono::system_clock::time_point now);
+
+}
+
+#endif
diff --git a/src/IO/S3/tests/gtest_gcs_conditional_dialect.cpp b/src/IO/S3/tests/gtest_gcs_conditional_dialect.cpp
new file mode 100644
index 000000000000..30d1117a0c60
--- /dev/null
+++ b/src/IO/S3/tests/gtest_gcs_conditional_dialect.cpp
@@ -0,0 +1,159 @@
+#include "config.h"
+#if USE_AWS_S3
+#include <gtest/gtest.h>
+#include <IO/S3/GCSConditionalDialect.h>
+#include <Common/Exception.h>
+#include <aws/core/http/standard/StandardHttpRequest.h>
+#include <Poco/Net/HTTPResponse.h>
+#include <base/defines.h>   /// DEBUG_OR_SANITIZER_BUILD
+
+using namespace DB::S3;
+
+static Aws::Http::Standard::StandardHttpRequest makeRequest(
+    const char * url = "https://storage.googleapis.com/b/k",
+    Aws::Http::HttpMethod method = Aws::Http::HttpMethod::HTTP_PUT)
+{
+    Aws::Http::Standard::StandardHttpRequest request{Aws::Http::URI(url), method};
+    request.SetHeaderValue("host", "storage.googleapis.com");
+    return request;
+}
+
+TEST(GCSConditionalDialect, IfNoneMatchStarBecomesGenerationZero)
+{
+    auto r = makeRequest();
+    r.SetHeaderValue("if-none-match", "*");
+    applyGcsConditionalDialectToRequest(r);
+    EXPECT_FALSE(r.HasHeader("if-none-match"));
+    EXPECT_EQ(r.GetHeaderValue("x-goog-if-generation-match"), "0");
+}
+
+TEST(GCSConditionalDialect, IfMatchDigitsMappedQuotesStripped)
+{
+    auto r = makeRequest();
+    r.SetHeaderValue("if-match", "\"1783078552147137\"");
+    applyGcsConditionalDialectToRequest(r);
+    EXPECT_FALSE(r.HasHeader("if-match"));
+    EXPECT_EQ(r.GetHeaderValue("x-goog-if-generation-match"), "1783078552147137");
+}
+
+TEST(GCSConditionalDialect, IfMatchUnquotedDigitsAlsoAccepted)
+{
+    auto r = makeRequest();
+    r.SetHeaderValue("if-match", "1783078552147137");
+    applyGcsConditionalDialectToRequest(r);
+    EXPECT_EQ(r.GetHeaderValue("x-goog-if-generation-match"), "1783078552147137");
+}
+
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(GCSConditionalDialect, NonNumericIfMatchThrows)
+{
+    /// The guard throws LOGICAL_ERROR (a broken-invariant signal: an S3-style ETag reached a
+    /// generation-dialect client). Under abort_on_logical_error that aborts at construction instead of
+    /// being catchable, so GCSConditionalDialectDeathTest.NonNumericIfMatchAborts proves it there.
+    auto r = makeRequest();
+    r.SetHeaderValue("if-match", "\"6654c734ccab8f440ff0825eb443dc7f\"");  /// an ETag leaked into a generation dialect
+    EXPECT_THROW(applyGcsConditionalDialectToRequest(r), DB::Exception);
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(GCSConditionalDialectDeathTest, NonNumericIfMatchAborts)
+{
+    auto r = makeRequest();
+    r.SetHeaderValue("if-match", "\"6654c734ccab8f440ff0825eb443dc7f\"");
+    EXPECT_DEATH({ applyGcsConditionalDialectToRequest(r); }, "");
+}
+#endif
+
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(GCSConditionalDialect, NonStarIfNoneMatchThrows)
+{
+    /// LOGICAL_ERROR (broken invariant); aborts under abort_on_logical_error -- see the DeathTest below.
+    auto r = makeRequest();
+    r.SetHeaderValue("if-none-match", "\"123\"");
+    EXPECT_THROW(applyGcsConditionalDialectToRequest(r), DB::Exception);
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(GCSConditionalDialectDeathTest, NonStarIfNoneMatchAborts)
+{
+    auto r = makeRequest();
+    r.SetHeaderValue("if-none-match", "\"123\"");
+    EXPECT_DEATH({ applyGcsConditionalDialectToRequest(r); }, "");
+}
+#endif
+
+TEST(GCSConditionalDialect, AmzHeadersRenamedAuthArtifactsDropped)
+{
+    auto r = makeRequest();
+    r.SetHeaderValue("authorization", "AWS4-HMAC-SHA256 ...");
+    r.SetHeaderValue("x-amz-date", "20260703T000000Z");
+    r.SetHeaderValue("x-amz-content-sha256", "deadbeef");
+    r.SetHeaderValue("x-amz-security-token", "tok");
+    r.SetHeaderValue("x-amz-api-version", "2006-03-01");
+    r.SetHeaderValue("x-amz-meta-foo", "bar");
+    r.SetHeaderValue("x-amz-storage-class", "STANDARD");
+    applyGcsConditionalDialectToRequest(r);
+    EXPECT_FALSE(r.HasHeader("authorization"));
+    EXPECT_FALSE(r.HasHeader("x-amz-date"));
+    EXPECT_FALSE(r.HasHeader("x-amz-content-sha256"));
+    EXPECT_FALSE(r.HasHeader("x-amz-security-token"));
+    EXPECT_FALSE(r.HasHeader("x-amz-api-version"));
+    EXPECT_FALSE(r.HasHeader("x-amz-meta-foo"));
+    EXPECT_FALSE(r.HasHeader("x-amz-storage-class"));
+    EXPECT_EQ(r.GetHeaderValue("x-goog-meta-foo"), "bar");
+    EXPECT_EQ(r.GetHeaderValue("x-goog-storage-class"), "STANDARD");
+}
+
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(GCSConditionalDialect, ConditionalCompleteMultipartUploadThrows)
+{
+    /// GCS silently IGNORES preconditions on CompleteMultipartUpload (measured live 2026-07-03) --
+    /// sending one would be silent data loss, so the dialect fails closed client-side with a
+    /// LOGICAL_ERROR; aborts under abort_on_logical_error -- see the DeathTest below.
+    auto r = makeRequest("https://storage.googleapis.com/b/k?uploadId=abc", Aws::Http::HttpMethod::HTTP_POST);
+    r.SetHeaderValue("if-none-match", "*");
+    EXPECT_THROW(applyGcsConditionalDialectToRequest(r), DB::Exception);
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(GCSConditionalDialectDeathTest, ConditionalCompleteMultipartUploadAborts)
+{
+    auto r = makeRequest("https://storage.googleapis.com/b/k?uploadId=abc", Aws::Http::HttpMethod::HTTP_POST);
+    r.SetHeaderValue("if-none-match", "*");
+    EXPECT_DEATH({ applyGcsConditionalDialectToRequest(r); }, "");
+}
+#endif
+
+TEST(GCSConditionalDialect, UnconditionalCompleteMultipartUploadPasses)
+{
+    auto r = makeRequest("https://storage.googleapis.com/b/k?uploadId=abc", Aws::Http::HttpMethod::HTTP_POST);
+    EXPECT_NO_THROW(applyGcsConditionalDialectToRequest(r));
+}
+
+TEST(GCSConditionalDialect, UploadPartIsNotComplete)
+{
+    /// PUT ?partNumber=N&uploadId=... is an UploadPart, not a Complete — must not trip the guard.
+    auto r = makeRequest("https://storage.googleapis.com/b/k?partNumber=1&uploadId=abc", Aws::Http::HttpMethod::HTTP_PUT);
+    EXPECT_NO_THROW(applyGcsConditionalDialectToRequest(r));
+}
+
+TEST(GCSConditionalDialect, ResponseGenerationOverridesETag)
+{
+    Poco::Net::HTTPResponse response;
+    response.set("ETag", "\"6654c734ccab8f440ff0825eb443dc7f\"");
+    response.set("x-goog-generation", "1783078552147137");
+    auto override_etag = gcsGenerationETagOverride(response);
+    ASSERT_TRUE(override_etag.has_value());
+    EXPECT_EQ(*override_etag, "\"1783078552147137\"");
+}
+
+TEST(GCSConditionalDialect, ResponseWithoutGenerationNoOverride)
+{
+    Poco::Net::HTTPResponse response;
+    response.set("ETag", "\"abc\"");
+    EXPECT_FALSE(gcsGenerationETagOverride(response).has_value());
+}
+#endif
diff --git a/src/IO/S3/tests/gtest_goog4_signer.cpp b/src/IO/S3/tests/gtest_goog4_signer.cpp
new file mode 100644
index 000000000000..f3e1a95aad6a
--- /dev/null
+++ b/src/IO/S3/tests/gtest_goog4_signer.cpp
@@ -0,0 +1,62 @@
+#include "config.h"
+#if USE_AWS_S3
+#include <gtest/gtest.h>
+#include <IO/S3/GOOG4Signer.h>
+#include <aws/core/http/standard/StandardHttpRequest.h>
+#include <aws/core/auth/AWSCredentials.h>
+
+using namespace DB::S3;
+
+static std::chrono::system_clock::time_point fixedNow()
+{
+    /// 2026-07-03 00:00:00 UTC
+    return std::chrono::system_clock::from_time_t(1783036800);
+}
+
+TEST(GOOG4Signer, PutWithGenerationPrecondition)
+{
+    Aws::Http::Standard::StandardHttpRequest request(
+        Aws::Http::URI("https://storage.googleapis.com/test-bucket/dir/obj.txt"), Aws::Http::HttpMethod::HTTP_PUT);
+    request.SetHeaderValue("host", "storage.googleapis.com");
+    request.SetHeaderValue("x-goog-if-generation-match", "0");
+
+    signRequestGOOG4(request, Aws::Auth::AWSCredentials("GOOGTESTACCESSKEY", "testsecretkey"), fixedNow());
+
+    EXPECT_EQ(request.GetHeaderValue("x-goog-date"), "20260703T000000Z");
+    EXPECT_EQ(request.GetHeaderValue("x-goog-content-sha256"), "UNSIGNED-PAYLOAD");
+    EXPECT_EQ(request.GetHeaderValue("authorization"),
+        "GOOG4-HMAC-SHA256 Credential=GOOGTESTACCESSKEY/20260703/auto/storage/goog4_request, "
+        "SignedHeaders=host;x-goog-content-sha256;x-goog-date;x-goog-if-generation-match, "
+        "Signature=4f82e49c69753329afd4768ccf1db6b472dbbd86d082a08b5b9f9fe368fb6ef6");
+}
+
+TEST(GOOG4Signer, GetWithQueryString)
+{
+    Aws::Http::Standard::StandardHttpRequest request(
+        Aws::Http::URI("https://storage.googleapis.com/test-bucket/?versioning"), Aws::Http::HttpMethod::HTTP_GET);
+    request.SetHeaderValue("host", "storage.googleapis.com");
+
+    signRequestGOOG4(request, Aws::Auth::AWSCredentials("GOOGTESTACCESSKEY", "testsecretkey"), fixedNow());
+
+    EXPECT_EQ(request.GetHeaderValue("authorization"),
+        "GOOG4-HMAC-SHA256 Credential=GOOGTESTACCESSKEY/20260703/auto/storage/goog4_request, "
+        "SignedHeaders=host;x-goog-content-sha256;x-goog-date, "
+        "Signature=28a981c32acff334738b9ea1a0f82c28c9a1ccff5b6dc8fb92a2e6622c8db73f");
+}
+
+TEST(GOOG4Signer, NonGoogHeadersAreNotSigned)
+{
+    Aws::Http::Standard::StandardHttpRequest request(
+        Aws::Http::URI("https://storage.googleapis.com/test-bucket/dir/obj.txt"), Aws::Http::HttpMethod::HTTP_PUT);
+    request.SetHeaderValue("host", "storage.googleapis.com");
+    request.SetHeaderValue("x-goog-if-generation-match", "0");
+    request.SetHeaderValue("content-type", "binary/octet-stream");
+    request.SetHeaderValue("amz-sdk-invocation-id", "whatever");
+
+    signRequestGOOG4(request, Aws::Auth::AWSCredentials("GOOGTESTACCESSKEY", "testsecretkey"), fixedNow());
+
+    /// Unsigned headers must not perturb the signature: same vector as PutWithGenerationPrecondition.
+    EXPECT_NE(request.GetHeaderValue("authorization").find(
+        "Signature=4f82e49c69753329afd4768ccf1db6b472dbbd86d082a08b5b9f9fe368fb6ef6"), std::string::npos);
+}
+#endif

From 4fcc40f97d983f492d1b63119f61b3084a7f764a Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:32 +0200
Subject: [PATCH 08/30] LocalObjectStorage: snapshot listing semantics and
 hardening

Emulate object-store behavior under concurrent removal: a file vanishing
between listing and stat is skipped, not a filesystem_error; non-recursive
explicit-stack walk with error_code overloads and a symlink guard; fail
closed on an embedded-NUL path (AST fuzzer).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../ObjectStorages/Local/LocalObjectStorage.cpp             | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.cpp b/src/Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.cpp
index 11a7cbf29acc..4719e6ae9237 100644
--- a/src/Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.cpp
+++ b/src/Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.cpp
@@ -408,6 +408,12 @@ std::optional<ObjectMetadata> LocalObjectStorage::tryGetObjectMetadata(const std
         throw fs::filesystem_error("Got unexpected error while getting last write time", path, error);
     }
 
+    /// A directory is not an object: fs::file_size would throw "Is a directory". Treat it as a
+    /// missing object (nullopt) so callers probing whether a path is a readable object do not get
+    /// a raw filesystem error (B38: system.remote_data_paths traversal on a CAS pool).
+    if (fs::is_directory(path, error))
+        return {};
+
     object_metadata.size_bytes = fs::file_size(path, error);
     if (error)
     {

From 93d545b744627c3b05b0cb791444f3bdbd5b90cd Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:33 +0200
Subject: [PATCH 09/30] Disk-transaction contract: one logical part = one
 transaction

Projection sub-parts ride the parent whole-part transaction instead of
committing early; read-your-writes (in-flight resolve) becomes part of the
IDiskTransaction contract so staged state is visible before commit;
clone/freeze/restore wrap the whole part in one transaction; staged
operation order is explicit. Mixed-file note: these files also carry the
content-addressed capability surface and eager-dispatch branches that are
wired by the later CAS integration commits.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../DiskObjectStorageTransaction.cpp          | 109 ++++++++---
 .../DiskObjectStorageTransaction.h            |  20 +++
 src/Disks/IDiskTransaction.h                  |  17 ++
 .../MergeTree/DataPartStorageOnDiskBase.cpp   | 168 +++++++++++++++--
 .../MergeTree/DataPartStorageOnDiskBase.h     |   2 +
 .../MergeTree/DataPartStorageOnDiskFull.cpp   | 170 ++++++++++++++++--
 src/Storages/MergeTree/IDataPartStorage.h     |   9 +
 src/Storages/MergeTree/IMergeTreeDataPart.cpp |   8 +-
 .../MergeTree/MergeProjectionPartsTask.cpp    |   4 +
 src/Storages/MergeTree/MergeTask.cpp          |  16 +-
 src/Storages/MergeTree/MergeTask.h            |   4 +
 src/Storages/MergeTree/MergeTreeData.cpp      | 116 +++++++++++-
 src/Storages/MergeTree/MergeTreeData.h        |  14 +-
 .../MergeTree/MergeTreeDataWriter.cpp         |   2 +
 src/Storages/MergeTree/MutateTask.cpp         |   3 +
 .../gtest_projection_borrowed_transaction.cpp |  86 +++++++++
 16 files changed, 691 insertions(+), 57 deletions(-)
 create mode 100644 src/Storages/MergeTree/tests/gtest_projection_borrowed_transaction.cpp

diff --git a/src/Disks/DiskObjectStorage/DiskObjectStorageTransaction.cpp b/src/Disks/DiskObjectStorage/DiskObjectStorageTransaction.cpp
index 34d14edaffa5..6a52ca93e673 100644
--- a/src/Disks/DiskObjectStorage/DiskObjectStorageTransaction.cpp
+++ b/src/Disks/DiskObjectStorage/DiskObjectStorageTransaction.cpp
@@ -110,7 +110,7 @@ MultipleDisksObjectStorageTransaction::MultipleDisksObjectStorageTransaction(
 
 void DiskObjectStorageTransaction::createDirectory(const std::string & path)
 {
-    operations_to_execute.push_back([path](MetadataTransactionPtr tx)
+    dispatch([path](MetadataTransactionPtr tx)
     {
         tx->createDirectory(path);
     });
@@ -118,7 +118,7 @@ void DiskObjectStorageTransaction::createDirectory(const std::string & path)
 
 void DiskObjectStorageTransaction::createDirectories(const std::string & path)
 {
-    operations_to_execute.push_back([path](MetadataTransactionPtr tx)
+    dispatch([path](MetadataTransactionPtr tx)
     {
         tx->createDirectoryRecursive(path);
     });
@@ -126,7 +126,7 @@ void DiskObjectStorageTransaction::createDirectories(const std::string & path)
 
 void DiskObjectStorageTransaction::moveDirectory(const std::string & from_path, const std::string & to_path)
 {
-    operations_to_execute.push_back([from_path, to_path](MetadataTransactionPtr tx)
+    dispatch([from_path, to_path](MetadataTransactionPtr tx)
     {
         tx->moveDirectory(from_path, to_path);
     });
@@ -134,7 +134,7 @@ void DiskObjectStorageTransaction::moveDirectory(const std::string & from_path,
 
 void DiskObjectStorageTransaction::moveFile(const String & from_path, const String & to_path)
 {
-    operations_to_execute.push_back([from_path, to_path](MetadataTransactionPtr tx)
+    dispatch([from_path, to_path](MetadataTransactionPtr tx)
     {
         tx->moveFile(from_path, to_path);
     });
@@ -142,7 +142,7 @@ void DiskObjectStorageTransaction::moveFile(const String & from_path, const Stri
 
 void DiskObjectStorageTransaction::truncateFile(const String & path, size_t size)
 {
-    operations_to_execute.push_back([path, size](MetadataTransactionPtr tx)
+    dispatch([path, size](MetadataTransactionPtr tx)
     {
         tx->truncateFile(path, size);
     });
@@ -150,7 +150,7 @@ void DiskObjectStorageTransaction::truncateFile(const String & path, size_t size
 
 void DiskObjectStorageTransaction::replaceFile(const std::string & from_path, const std::string & to_path)
 {
-    operations_to_execute.push_back([from_path, to_path](MetadataTransactionPtr tx)
+    dispatch([from_path, to_path](MetadataTransactionPtr tx)
     {
         tx->replaceFile(from_path, to_path);
     });
@@ -158,7 +158,7 @@ void DiskObjectStorageTransaction::replaceFile(const std::string & from_path, co
 
 void DiskObjectStorageTransaction::removeFile(const std::string & path)
 {
-    operations_to_execute.push_back([path](MetadataTransactionPtr tx)
+    dispatch([path](MetadataTransactionPtr tx)
     {
         tx->unlinkFile(path, /*if_exists=*/false, /*should_remove_objects=*/true);
     });
@@ -166,7 +166,7 @@ void DiskObjectStorageTransaction::removeFile(const std::string & path)
 
 void DiskObjectStorageTransaction::removeSharedFile(const std::string & path, bool keep_shared_data)
 {
-    operations_to_execute.push_back([path, keep_shared_data](MetadataTransactionPtr tx)
+    dispatch([path, keep_shared_data](MetadataTransactionPtr tx)
     {
         tx->unlinkFile(path, /*if_exists=*/false, /*should_remove_objects=*/!keep_shared_data);
     });
@@ -177,14 +177,14 @@ void DiskObjectStorageTransaction::removeSharedRecursive(
 {
     if (!keep_all_shared_data && file_names_remove_metadata_only.empty())
     {
-        operations_to_execute.push_back([path](MetadataTransactionPtr tx)
+        dispatch([path](MetadataTransactionPtr tx)
         {
             tx->removeRecursive(path, /*should_remove_objects=*/nullptr);
         });
     }
     else
     {
-        operations_to_execute.push_back([path, keep_all_shared_data, file_names_remove_metadata_only](MetadataTransactionPtr tx)
+        dispatch([path, keep_all_shared_data, file_names_remove_metadata_only](MetadataTransactionPtr tx)
         {
             tx->removeRecursive(path, /*should_remove_objects=*/[keep_all_shared_data, file_names_remove_metadata_only](const std::string & relative_path)
             {
@@ -196,7 +196,7 @@ void DiskObjectStorageTransaction::removeSharedRecursive(
 
 void DiskObjectStorageTransaction::removeSharedFileIfExists(const std::string & path, bool keep_shared_data)
 {
-    operations_to_execute.push_back([path, keep_shared_data](MetadataTransactionPtr tx)
+    dispatch([path, keep_shared_data](MetadataTransactionPtr tx)
     {
         tx->unlinkFile(path, /*if_exists=*/true, /*should_remove_objects=*/!keep_shared_data);
     });
@@ -204,7 +204,7 @@ void DiskObjectStorageTransaction::removeSharedFileIfExists(const std::string &
 
 void DiskObjectStorageTransaction::removeDirectory(const std::string & path)
 {
-    operations_to_execute.push_back([path](MetadataTransactionPtr tx)
+    dispatch([path](MetadataTransactionPtr tx)
     {
         tx->removeDirectory(path);
     });
@@ -212,7 +212,7 @@ void DiskObjectStorageTransaction::removeDirectory(const std::string & path)
 
 void DiskObjectStorageTransaction::removeRecursive(const std::string & path)
 {
-    operations_to_execute.push_back([path](MetadataTransactionPtr tx)
+    dispatch([path](MetadataTransactionPtr tx)
     {
         tx->removeRecursive(path, /*should_remove_objects=*/nullptr);
     });
@@ -220,9 +220,9 @@ void DiskObjectStorageTransaction::removeRecursive(const std::string & path)
 
 void DiskObjectStorageTransaction::removeFileIfExists(const std::string & path)
 {
-    operations_to_execute.push_back([path](MetadataTransactionPtr tx)
+    dispatch([path](MetadataTransactionPtr tx)
     {
-        tx->unlinkFile(path, /*if_exists=*/true, /*should_remove_objects*/true);
+        tx->unlinkFile(path, /*if_exists=*/true, /*should_remove_objects=*/true);
     });
 }
 
@@ -231,7 +231,7 @@ void DiskObjectStorageTransaction::removeSharedFiles(const RemoveBatchRequest &
     for (const auto & [path, if_exists] : files)
     {
         const bool should_remove_objects = !keep_all_batch_data && !file_names_remove_metadata_only.contains(fs::path(path).filename());
-        operations_to_execute.push_back([path, if_exists, should_remove_objects](MetadataTransactionPtr tx)
+        dispatch([path, if_exists, should_remove_objects](MetadataTransactionPtr tx)
         {
             tx->unlinkFile(path, if_exists, should_remove_objects);
         });
@@ -267,6 +267,16 @@ std::unique_ptr<WriteBufferFromFileBase> DiskObjectStorageTransaction::writeFile
 
     WriteSettings enriched_settings = updateIOSchedulingSettings(settings, read_resource_name, write_resource_name);
 
+    /// [TXN-ONE-PIPELINE] Give the metadata storage a chance to own the write (e.g. a content-addressed
+    /// hash-on-write buffer whose blob key is known only after the last byte). It returns a fully-wrapped
+    /// buffer (hash-on-write + append RMW + inline/blob split + autocommit/lifetime pin using `owner`), or
+    /// nullptr to fall through to the generic up-front-key streaming path below. Called BEFORE the append
+    /// check so a CA storage (which reports no native append) can service a verbatim append via
+    /// read-modify-rewrite inside the hook and reject a part-file append there.
+    if (auto buffer = metadata_transaction->tryCreateWriteBuffer(
+            shared_from_this(), path, buf_size, mode, enriched_settings, autocommit))
+        return buffer;
+
     /// NOTE: We check it here and not after writing blob because in case of plain/plain-rewritable metadata storages
     ///       undo of disk tx will actually remove existing data.
     if (mode == WriteMode::Append && !metadata_storage->supportWritingWithAppend())
@@ -395,7 +405,10 @@ void DiskObjectStorageTransaction::writeFileUsingBlobWritingFunction(
     /// We always use mode Rewrite because we simulate append using metadata and different files
     object.bytes_size = std::move(write_blob_function)(blob_path, WriteMode::Rewrite, /*object_attributes=*/std::nullopt);
 
-    operations_to_execute.push_back([object, mode](MetadataTransactionPtr tx)
+    /// [TXN-ONE-PIPELINE] Routed through dispatch for uniformity. Unreachable on CA (Audit 6):
+    /// generateObjectKeyForPath above throws NOT_IMPLEMENTED first, so CA never reaches this metadata
+    /// effect and never queues. On ordinary storage dispatch queues exactly as before.
+    dispatch([object, mode](MetadataTransactionPtr tx)
     {
         if (mode == WriteMode::Rewrite)
         {
@@ -413,15 +426,47 @@ void DiskObjectStorageTransaction::writeFileUsingBlobWritingFunction(
 
 void DiskObjectStorageTransaction::createHardLink(const std::string & src_path, const std::string & dst_path)
 {
-    operations_to_execute.push_back([src_path, dst_path](MetadataTransactionPtr tx)
+    /// For CA `dispatch` runs eagerly (call-time), which is load-bearing for read-your-writes: a
+    /// carried-forward projection hardlinked into the open whole-part transaction during a mutation must
+    /// be visible to `loadProjections` (same finalize, before commit) via the directory overlay. Deferring
+    /// it to commit replay would hide it until after `loadProjections` ran (B58/B63). The metadata-level
+    /// `createHardLink` is an idempotent map assignment, so eager staging is equivalent to the queued
+    /// replay — commit publishes the manifest from the staging.
+    dispatch([src_path, dst_path](MetadataTransactionPtr tx)
     {
         tx->createHardLink(src_path, dst_path);
     });
 }
 
+std::optional<StoredObjects> DiskObjectStorageTransaction::tryGetInFlightStorageObjects(const std::string & path) const
+{
+    return metadata_transaction->tryGetInFlightStorageObjects(path);
+}
+
+std::unique_ptr<ReadBufferFromFileBase> DiskObjectStorageTransaction::tryReadFileInFlight(
+    const std::string & path, const ReadSettings & settings, std::optional<size_t> read_hint) const
+{
+    return metadata_transaction->tryReadFileInFlight(path, settings, read_hint);
+}
+
+std::optional<uint64_t> DiskObjectStorageTransaction::tryGetInFlightFileSize(const std::string & path) const
+{
+    return metadata_transaction->tryGetInFlightFileSize(path);
+}
+
+bool DiskObjectStorageTransaction::hasInFlightDirectory(const std::string & path) const
+{
+    return metadata_transaction->hasInFlightDirectory(path);
+}
+
+std::vector<std::string> DiskObjectStorageTransaction::listInFlightDirectory(const std::string & path) const
+{
+    return metadata_transaction->listInFlightDirectory(path);
+}
+
 void DiskObjectStorageTransaction::setReadOnly(const std::string & path)
 {
-    operations_to_execute.push_back([path](MetadataTransactionPtr tx)
+    dispatch([path](MetadataTransactionPtr tx)
     {
         tx->setReadOnly(path);
     });
@@ -429,7 +474,7 @@ void DiskObjectStorageTransaction::setReadOnly(const std::string & path)
 
 void DiskObjectStorageTransaction::setLastModified(const std::string & path, const Poco::Timestamp & timestamp)
 {
-    operations_to_execute.push_back([path, timestamp](MetadataTransactionPtr tx)
+    dispatch([path, timestamp](MetadataTransactionPtr tx)
     {
         tx->setLastModified(path, timestamp);
     });
@@ -437,7 +482,7 @@ void DiskObjectStorageTransaction::setLastModified(const std::string & path, con
 
 void DiskObjectStorageTransaction::chmod(const String & path, mode_t mode)
 {
-    operations_to_execute.push_back([path, mode](MetadataTransactionPtr tx)
+    dispatch([path, mode](MetadataTransactionPtr tx)
     {
         tx->chmod(path, mode);
     });
@@ -516,7 +561,11 @@ void DiskObjectStorageTransaction::copyFileImpl(
         return;
     }
 
-    operations_to_execute.push_back([blobs_to_create, missing_locations, to_file_path](MetadataTransactionPtr tx)
+    /// [TXN-ONE-PIPELINE] Routed through dispatch for uniformity. Unreachable on CA (Audit 6):
+    /// copyFileImpl calls generateObjectKeyForPath above, which throws NOT_IMPLEMENTED on CA before this
+    /// point (and the empty-source case returns via the real writeFile above), so CA never queues here.
+    /// On ordinary storage dispatch queues exactly as before.
+    dispatch([blobs_to_create, missing_locations, to_file_path](MetadataTransactionPtr tx)
     {
         for (const auto & blob : blobs_to_create)
             tx->recordBlobsReplication(blob, missing_locations);
@@ -537,6 +586,15 @@ void MultipleDisksObjectStorageTransaction::copyFile(const std::string & from_fi
 
 void DiskObjectStorageTransaction::commit()
 {
+    /// [TXN-ONE-PIPELINE] An eager staging-overlay transaction (e.g. CA) must route every mutating method
+    /// straight to the metadata transaction at call time and keep this queue empty. A non-empty queue here
+    /// means a mutating method bypassed `dispatch` — which would re-introduce the two-timeline split this
+    /// design eliminates. Fail closed with a real throw (NOT chassert, which is a no-op in release builds).
+    if (metadata_storage->transactionIsStagingOverlay() && !operations_to_execute.empty())
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "An eager staging-overlay transaction must not queue deferred operations "
+            "(a mutating method bypassed dispatch): {} queued", operations_to_execute.size());
+
     auto component_guard = Coordination::setCurrentComponent("DiskObjectStorageTransaction::commit");
     for (size_t i = 0; i < operations_to_execute.size(); ++i)
     {
@@ -581,6 +639,13 @@ void DiskObjectStorageTransaction::commit()
 
 TransactionCommitOutcomeVariant DiskObjectStorageTransaction::tryCommit(const TransactionCommitOptionsVariant & options)
 {
+    /// [TXN-ONE-PIPELINE] See commit(): an eager staging-overlay transaction must never queue deferred
+    /// operations. Fail closed (real throw, not chassert) if a mutating method bypassed `dispatch`.
+    if (metadata_storage->transactionIsStagingOverlay() && !operations_to_execute.empty())
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "An eager staging-overlay transaction must not queue deferred operations "
+            "(a mutating method bypassed dispatch): {} queued", operations_to_execute.size());
+
     for (size_t i = 0; i < operations_to_execute.size(); ++i)
     {
         try
diff --git a/src/Disks/DiskObjectStorage/DiskObjectStorageTransaction.h b/src/Disks/DiskObjectStorage/DiskObjectStorageTransaction.h
index 2322263a0e06..14424d480a4b 100644
--- a/src/Disks/DiskObjectStorage/DiskObjectStorageTransaction.h
+++ b/src/Disks/DiskObjectStorage/DiskObjectStorageTransaction.h
@@ -104,6 +104,15 @@ struct DiskObjectStorageTransaction : public IDiskTransaction, public std::enabl
     void setReadOnly(const std::string & path) override;
     void createHardLink(const std::string & src_path, const std::string & dst_path) override;
 
+    /// B59 in-flight read-your-writes: forward to the metadata transaction (e.g. a CA part-build
+    /// transaction resolving its own staged-but-uncommitted files).
+    std::optional<StoredObjects> tryGetInFlightStorageObjects(const std::string & path) const override;
+    std::unique_ptr<ReadBufferFromFileBase> tryReadFileInFlight(
+        const std::string & path, const ReadSettings & settings, std::optional<size_t> read_hint) const override;
+    std::optional<uint64_t> tryGetInFlightFileSize(const std::string & path) const override;
+    bool hasInFlightDirectory(const std::string & path) const override;
+    std::vector<std::string> listInFlightDirectory(const std::string & path) const override;
+
 protected:
     /// Shared between `DiskObjectStorageTransaction::copyFile` and
     /// `MultipleDisksObjectStorageTransaction::copyFile`. Reads source blobs from the
@@ -118,6 +127,17 @@ struct DiskObjectStorageTransaction : public IDiskTransaction, public std::enabl
         const ReadSettings & read_settings,
         const WriteSettings & write_settings);
 
+    /// [TXN-ONE-PIPELINE] Route one metadata effect either into the FIFO replay queue (ordinary object
+    /// storage) or straight to the metadata transaction at call time (eager staging overlay, e.g. CA).
+    template <typename Operation>
+    void dispatch(Operation && operation)
+    {
+        if (metadata_storage->transactionIsStagingOverlay())
+            operation(metadata_transaction);
+        else
+            operations_to_execute.emplace_back(std::forward<Operation>(operation));
+    }
+
 private:
     std::unique_ptr<WriteBufferFromFileBase> writeFileImpl( /// NOLINT
         bool autocommit,
diff --git a/src/Disks/IDiskTransaction.h b/src/Disks/IDiskTransaction.h
index 720de290a4b9..db6d5b615724 100644
--- a/src/Disks/IDiskTransaction.h
+++ b/src/Disks/IDiskTransaction.h
@@ -138,6 +138,23 @@ struct IDiskTransaction : private boost::noncopyable
 
     /// Truncate file to the target size.
     virtual void truncateFile(const std::string & src_path, size_t size) = 0;
+
+    /// In-flight read-your-writes for a part being assembled by THIS transaction (B59). Forwarded to the
+    /// metadata transaction by object-storage disk transactions; default (e.g. local disk) is no in-flight
+    /// visibility, so a reader falls through to the committed path.
+    virtual std::optional<StoredObjects> tryGetInFlightStorageObjects(const std::string & /*path*/) const { return {}; }
+    virtual std::unique_ptr<ReadBufferFromFileBase> tryReadFileInFlight(
+        const std::string & /*path*/, const ReadSettings & /*settings*/, std::optional<size_t> /*read_hint*/) const { return nullptr; }
+    virtual std::optional<uint64_t> tryGetInFlightFileSize(const std::string & /*path*/) const { return {}; }
+    /// In-flight read-your-writes at DIRECTORY granularity: true iff this transaction has STAGED at least one
+    /// file under `path` for `path`'s part (mirrors the file trio above). Forwarded to the metadata
+    /// transaction by object-storage disk transactions; default (e.g. local disk) is no in-flight directory
+    /// visibility, so a reader falls through to the committed path.
+    virtual bool hasInFlightDirectory(const std::string & /*path*/) const { return false; }
+    /// In-flight read-your-writes directory ENUMERATION: the immediate-child names this transaction has
+    /// STAGED directly under `path` (one level, the directory prefix stripped). Forwarded to the metadata
+    /// transaction; default (e.g. local disk) is empty.
+    virtual std::vector<std::string> listInFlightDirectory(const std::string & /*path*/) const { return {}; }
 };
 
 using DiskTransactionPtr = std::shared_ptr<IDiskTransaction>;
diff --git a/src/Storages/MergeTree/DataPartStorageOnDiskBase.cpp b/src/Storages/MergeTree/DataPartStorageOnDiskBase.cpp
index def7f6395860..5ab4565675a4 100644
--- a/src/Storages/MergeTree/DataPartStorageOnDiskBase.cpp
+++ b/src/Storages/MergeTree/DataPartStorageOnDiskBase.cpp
@@ -2,6 +2,7 @@
 #include <Backups/BackupEntryFromImmutableFile.h>
 #include <Backups/BackupEntryWrappedWith.h>
 #include <Backups/BackupSettings.h>
+#include <Core/Defines.h>
 #include <Disks/IDiskTransaction.h>
 #include <Disks/SingleDiskVolume.h>
 #include <Disks/TemporaryFileOnDisk.h>
@@ -14,6 +15,7 @@
 #include <IO/ReadPipeline.h>
 #include <IO/ReadHelpers.h>
 #include <IO/WriteBufferFromFileBase.h>
+#include <IO/WriteHelpers.h>
 #include <IO/copyData.h>
 #include <Interpreters/Context.h>
 #include <Interpreters/MergeTreeTransaction/VersionMetadataOnDisk.h>
@@ -38,6 +40,7 @@ namespace ErrorCodes
     extern const int LOGICAL_ERROR;
     extern const int FILE_DOESNT_EXIST;
     extern const int CORRUPTED_DATA;
+    extern const int SUPPORT_IS_DISABLED;
 }
 
 std::unique_ptr<ReadBufferFromFileBase> IDataPartStorage::readFile(
@@ -270,6 +273,16 @@ bool DataPartStorageOnDiskBase::isStoredOnRemoteDisk() const
     return volume->getDisk()->isRemote();
 }
 
+bool DataPartStorageOnDiskBase::isContentAddressed() const
+{
+    return volume->getDisk()->isContentAddressed();
+}
+
+bool DataPartStorageOnDiskBase::supportsAtomicFileWrites() const
+{
+    return volume->getDisk()->supportsAtomicFileWrites();
+}
+
 std::optional<String> DataPartStorageOnDiskBase::getCacheName() const
 {
     if (volume->getDisk()->supportsCache())
@@ -401,6 +414,18 @@ void DataPartStorageOnDiskBase::backup(
 
     auto disk = volume->getDisk();
 
+    /// B34: the temporary-hard-link BACKUP path (used for Ordinary, non-UUID databases) calls
+    /// disk->createHardLink with a non-part-shaped temp path, which on a CAS disk
+    /// would otherwise surface as a raw LOGICAL_ERROR. Fail closed with a clear message instead.
+    /// The pointer-holding path (make_temporary_hard_links=false, used by Atomic/UUID databases)
+    /// uses getStorageObjects and round-trips on a CAS disk, so it is left untouched.
+    if (make_temporary_hard_links && disk->isContentAddressed())
+        throw Exception(
+            ErrorCodes::SUPPORT_IS_DISABLED,
+            "BACKUP via temporary hard links is not supported on a CAS disk yet (B16/B34); "
+            "use an Atomic database (which backs up via pointer-holding) instead; disk '{}'",
+            disk->getName());
+
     fs::path temp_part_dir;
     std::shared_ptr<TemporaryFileOnDisk> temp_dir_owner;
     if (make_temporary_hard_links)
@@ -506,8 +531,20 @@ MutableDataPartStoragePtr DataPartStorageOnDiskBase::freeze(
     const ClonePartParams & params) const
 {
     auto disk = volume->getDisk();
-    if (params.external_transaction)
-        params.external_transaction->createDirectories(to);
+
+    /// A CAS disk models a part as one atomic unit (N files -> one manifest -> one ref).
+    /// The per-file createHardLink autocommit Backup uses with no enclosing transaction would publish a
+    /// one-file ref per file and overwrite the destination, leaving the clone with only its last file
+    /// (the B21 corruption mode — seen as system.detached_parts listing metadata_version.txt instead of
+    /// the detached part dir, B36). When the caller did not supply a transaction, run the whole clone
+    /// through ONE self-created disk transaction so all files land in a single content-addressed part.
+    DiskTransactionPtr owned_transaction;
+    if (!params.external_transaction && disk->isContentAddressed())
+        owned_transaction = disk->createTransaction();
+    const DiskTransactionPtr & clone_transaction = params.external_transaction ? params.external_transaction : owned_transaction;
+
+    if (clone_transaction)
+        clone_transaction->createDirectories(to);
     else
         disk->createDirectories(to);
 
@@ -522,17 +559,38 @@ MutableDataPartStoragePtr DataPartStorageOnDiskBase::freeze(
         /* max_level= */ {},
         params.copy_instead_of_hardlink,
         params.files_to_copy_instead_of_hardlinks,
-        params.external_transaction);
+        clone_transaction);
 
     if (save_metadata_callback)
         save_metadata_callback(disk);
 
-    if (params.external_transaction)
+    if (clone_transaction)
     {
-        params.external_transaction->removeFileIfExists(fs::path(to) / dir_path / "delete-on-destroy.txt");
-        params.external_transaction->removeFileIfExists(fs::path(to) / dir_path / VersionMetadata::TXN_VERSION_METADATA_FILE_NAME);
+        clone_transaction->removeFileIfExists(fs::path(to) / dir_path / "delete-on-destroy.txt");
+        clone_transaction->removeFileIfExists(fs::path(to) / dir_path / VersionMetadata::TXN_VERSION_METADATA_FILE_NAME);
         if (!params.keep_metadata_version)
-            params.external_transaction->removeFileIfExists(fs::path(to) / dir_path / IMergeTreeDataPart::METADATA_VERSION_FILE_NAME);
+            clone_transaction->removeFileIfExists(fs::path(to) / dir_path / IMergeTreeDataPart::METADATA_VERSION_FILE_NAME);
+
+        /// When the caller wants a fresh metadata version written into the clone (the Replicated queue
+        /// clone path — `executeReplaceRange`/`replacePartitionFrom`/`movePartitionToTable` set
+        /// `metadata_version_to_write`), write `metadata_version.txt` INSIDE the clone transaction so it
+        /// is part of the single whole-part commit. On a content-addressed disk the part is published
+        /// atomically at `commit`; a separate post-clone autocommit `writeFile` of this part file (what
+        /// `cloneAndLoadDataPart` does for non-CA disks) would hit the per-file-autocommit guard (B21).
+        /// `cloneAndLoadDataPart`'s own post-clone write now runs unconditionally (the freeze special
+        /// case was dropped with all-tree Task 10): identical bytes land as a byte-equal repoint
+        /// no-op, differing bytes as a legal repoint.
+        if (params.metadata_version_to_write.has_value())
+        {
+            chassert(!params.keep_metadata_version);
+            auto out_metadata = clone_transaction->writeFile(
+                fs::path(to) / dir_path / IMergeTreeDataPart::METADATA_VERSION_FILE_NAME,
+                4096,
+                WriteMode::Rewrite,
+                write_settings);
+            writeText(*params.metadata_version_to_write, *out_metadata);
+            out_metadata->finalize();
+        }
     }
     else
     {
@@ -542,6 +600,11 @@ MutableDataPartStoragePtr DataPartStorageOnDiskBase::freeze(
             disk->removeFileIfExists(fs::path(to) / dir_path / IMergeTreeDataPart::METADATA_VERSION_FILE_NAME);
     }
 
+    /// Commit the self-created transaction (the whole-part clone commit point for CA). An external
+    /// transaction is committed by its owner, as before.
+    if (owned_transaction)
+        owned_transaction->commit();
+
     auto single_disk_volume = std::make_shared<SingleDiskVolume>(disk->getName(), disk, 0);
 
     /// Do not initialize storage in case of DETACH because part may be broken.
@@ -605,6 +668,51 @@ MutableDataPartStoragePtr DataPartStorageOnDiskBase::freezeRemote(
     return create(single_disk_volume, to, dir_path, /*initialize=*/ !to_detached && !params.external_transaction);
 }
 
+namespace
+{
+
+/// Recursively copy every file under `source_path` on `src_disk` into `destination_path` through
+/// `dst_transaction`'s NON-autocommit `writeFile` (IDiskTransaction::writeFile, NOT
+/// writeFileWithAutoCommit) -- the same primitive `freeze` already uses for a single file (the
+/// metadata_version.txt write, DataPartStorageOnDiskBase::freeze). Cross-disk, so it cannot reuse
+/// Backup()/BackupImpl: that helper's transactional branch calls transaction->copyFile, which is
+/// SAME-disk only (DiskObjectStorageTransaction::copyFile throws NOT_IMPLEMENTED across disks on
+/// CA), and its non-transactional branch always autocommits per file via IDisk::copyFile /
+/// copyDirectoryContent. Sequential, not the parallel copyThroughBuffers thread pool: a
+/// content-addressed transaction batches every file into ONE eventual manifest, and its staging
+/// map is not mutex-guarded; MOVE is a background, latency-insensitive operation, so
+/// parallelizing this is a deferred optimization, not a correctness requirement.
+void copyDirectoryContentIntoTransaction(
+    IDisk & src_disk,
+    const String & source_path,
+    IDiskTransaction & dst_transaction,
+    const String & destination_path,
+    const ReadSettings & read_settings,
+    const WriteSettings & write_settings,
+    const std::function<void()> & cancellation_hook)
+{
+    dst_transaction.createDirectories(destination_path);
+    for (auto it = src_disk.iterateDirectory(source_path); it->isValid(); it->next())
+    {
+        auto source = it->path();
+        auto destination = fs::path(destination_path) / it->name();
+
+        if (src_disk.existsDirectory(source))
+        {
+            copyDirectoryContentIntoTransaction(
+                src_disk, source, dst_transaction, destination, read_settings, write_settings, cancellation_hook);
+            continue;
+        }
+
+        auto in = src_disk.readFile(source, read_settings);
+        auto out = dst_transaction.writeFile(destination, DBMS_DEFAULT_BUFFER_SIZE, WriteMode::Rewrite, write_settings);
+        copyData(*in, *out, cancellation_hook);
+        out->finalize();
+    }
+}
+
+}
+
 MutableDataPartStoragePtr DataPartStorageOnDiskBase::clonePart(
     const std::string & to,
     const std::string & dir_path,
@@ -624,18 +732,46 @@ MutableDataPartStoragePtr DataPartStorageOnDiskBase::clonePart(
                         dir_path, getRelativePath(), path_to_clone, fullPath(dst_disk, path_to_clone));
     }
 
-    try
+    if (dst_disk->isContentAddressed())
     {
-        dst_disk->createDirectories(to);
-        src_disk->copyDirectoryContent(getRelativePath(), dst_disk, path_to_clone, read_settings, write_settings, cancellation_hook);
+        /// L2 (MOVE-to-CA fix): a content-addressed disk models a part as ONE atomic unit (N
+        /// files -> one manifest -> one ref). The generic per-file autocommit path below would
+        /// publish a separate one-file ref per file -- colliding on the shared "moving" ref
+        /// before L1, and throwing NOT_IMPLEMENTED on a non-first content file even after L1
+        /// ("Autocommit writes are not supported for content part files"). Run the whole clone
+        /// through ONE self-created disk transaction instead, mirroring freeze's
+        /// owned_transaction shape -- but streaming cross-disk bytes, since freeze's Backup() is
+        /// same-disk hardlink/copyFile (throws NOT_IMPLEMENTED for CA cross-disk).
+        auto clone_transaction = dst_disk->createTransaction();
+        try
+        {
+            copyDirectoryContentIntoTransaction(
+                *src_disk, getRelativePath(), *clone_transaction, path_to_clone,
+                read_settings, write_settings, cancellation_hook);
+            clone_transaction->commit();
+        }
+        catch (...)
+        {
+            LOG_WARNING(log, "Rolling back transaction after failed attempt to move a data part to {}", path_to_clone);
+            clone_transaction->undo();
+            throw;
+        }
     }
-    catch (...)
+    else
     {
-        /// It's safe to remove it recursively (even with zero-copy-replication)
-        /// because we've just did full copy through copyDirectoryContent
-        LOG_WARNING(log, "Removing directory {} after failed attempt to move a data part", path_to_clone);
-        dst_disk->removeRecursive(path_to_clone);
-        throw;
+        try
+        {
+            dst_disk->createDirectories(to);
+            src_disk->copyDirectoryContent(getRelativePath(), dst_disk, path_to_clone, read_settings, write_settings, cancellation_hook);
+        }
+        catch (...)
+        {
+            /// It's safe to remove it recursively (even with zero-copy-replication)
+            /// because we've just did full copy through copyDirectoryContent
+            LOG_WARNING(log, "Removing directory {} after failed attempt to move a data part", path_to_clone);
+            dst_disk->removeRecursive(path_to_clone);
+            throw;
+        }
     }
 
     auto single_disk_volume = std::make_shared<SingleDiskVolume>(dst_disk->getName(), dst_disk, 0);
diff --git a/src/Storages/MergeTree/DataPartStorageOnDiskBase.h b/src/Storages/MergeTree/DataPartStorageOnDiskBase.h
index e1c55719954f..1bd66ff660a6 100644
--- a/src/Storages/MergeTree/DataPartStorageOnDiskBase.h
+++ b/src/Storages/MergeTree/DataPartStorageOnDiskBase.h
@@ -40,6 +40,8 @@ class DataPartStorageOnDiskBase : public IDataPartStorage
     std::string getDiskName() const override;
     std::string getDiskType() const override;
     bool isStoredOnRemoteDisk() const override;
+    bool isContentAddressed() const override;
+    bool supportsAtomicFileWrites() const override;
     std::optional<String> getCacheName() const override;
     bool supportZeroCopyReplication() const override;
     bool supportParallelWrite() const override;
diff --git a/src/Storages/MergeTree/DataPartStorageOnDiskFull.cpp b/src/Storages/MergeTree/DataPartStorageOnDiskFull.cpp
index 5503d11c58d9..fd25e35153c4 100644
--- a/src/Storages/MergeTree/DataPartStorageOnDiskFull.cpp
+++ b/src/Storages/MergeTree/DataPartStorageOnDiskFull.cpp
@@ -2,6 +2,7 @@
 
 #include <Disks/IDiskTransaction.h>
 #include <Disks/SingleDiskVolume.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/StoredObject.h>
 #include <IO/PackedFilesReader.h>
 #include <IO/ReadBufferFromFileBase.h>
 #include <IO/ReadHelpers.h>
@@ -9,14 +10,24 @@
 #include <IO/WriteBufferFromFileBase.h>
 #include <Interpreters/Context.h>
 #include <Storages/MergeTree/MergeTreeIndicesSerialization.h>
+#include <Common/FailPoint.h>
 #include <Common/typeid_cast.h>
 
+#include <set>
+#include <vector>
+
 namespace DB
 {
 
+namespace FailPoints
+{
+    extern const char part_storage_fail_commit_transaction[];
+}
+
 namespace ErrorCodes
 {
     extern const int LOGICAL_ERROR;
+    extern const int FAULT_INJECTED;
 }
 
 DataPartStorageOnDiskFull::DataPartStorageOnDiskFull(VolumePtr volume_, std::string root_path_, std::string part_dir_)
@@ -48,22 +59,39 @@ DataPartStoragePtr DataPartStorageOnDiskFull::getProjection(const std::string &
 
 bool DataPartStorageOnDiskFull::exists() const
 {
-    return volume->getDisk()->existsDirectory(fs::path(root_path) / part_dir);
+    auto path = fs::path(root_path) / part_dir;
+    /// CA read-your-writes: a part dir being assembled by this transaction (e.g. a carried-forward
+    /// projection dir staged into the open whole-part txn) is not on committed metadata yet. Mirrors
+    /// existsDirectory at directory granularity for the part's OWN directory.
+    if (transaction && transaction->hasInFlightDirectory(path))
+        return true;
+    return volume->getDisk()->existsDirectory(path);
 }
 
 bool DataPartStorageOnDiskFull::existsFile(const std::string & name) const
 {
+    auto path = fs::path(root_path) / part_dir / name;
+    /// B59: a part still being assembled by this transaction can have staged-but-uncommitted files
+    /// (e.g. projection temp blocks on a content-addressed disk). Consult the held transaction first.
+    if (transaction && transaction->tryGetInFlightFileSize(path).has_value())
+        return true;
     if (looksLikePackedSkipIndexFile(name))
     {
         if (auto reader = getSkipIndicesPackedReader(); reader && reader->exists(name))
             return true;
     }
-    return volume->getDisk()->existsFile(fs::path(root_path) / part_dir / name);
+    return volume->getDisk()->existsFile(path);
 }
 
 bool DataPartStorageOnDiskFull::existsDirectory(const std::string & name) const
 {
-    return volume->getDisk()->existsDirectory(fs::path(root_path) / part_dir / name);
+    auto path = fs::path(root_path) / part_dir / name;
+    /// CA read-your-writes: a part still being assembled by this transaction can have a staged-but-uncommitted
+    /// directory (e.g. a carried-forward projection hardlinked into the open whole-part txn) that committed
+    /// metadata cannot see yet. Mirrors existsFile (B59) at directory granularity.
+    if (transaction && transaction->hasInFlightDirectory(path))
+        return true;
+    return volume->getDisk()->existsDirectory(path);
 }
 
 class DataPartStorageIteratorOnDisk final : public IDataPartStorageIterator
@@ -85,11 +113,52 @@ class DataPartStorageIteratorOnDisk final : public IDataPartStorageIterator
     DirectoryIteratorPtr it;
 };
 
+/// CA read-your-writes directory enumeration: a merged view of the committed disk entries PLUS the
+/// immediate children this transaction has STAGED under the part dir (deduplicated). Used so
+/// loadProjections' withPartFormatFromDisk can iterate a staged-but-uncommitted projection directory and
+/// find its mark file. Mirrors existsFile/existsDirectory (B59) at the enumeration level; the committed
+/// entries dominate (a name present both on disk and staged appears once).
+class DataPartStorageMergedIterator final : public IDataPartStorageIterator
+{
+public:
+    DataPartStorageMergedIterator(DiskPtr disk_, std::string dir_path_, std::vector<std::string> names_)
+        : disk(std::move(disk_)), dir_path(std::move(dir_path_)), names(std::move(names_))
+    {
+    }
+
+    void next() override { ++pos; }
+    bool isValid() const override { return pos < names.size(); }
+    std::string name() const override { return names[pos]; }
+    std::string path() const override { return fs::path(dir_path) / names[pos]; }
+    bool isFile() const override { return isValid() && disk->existsFile(path()); }
+
+private:
+    DiskPtr disk;
+    std::string dir_path;
+    std::vector<std::string> names;
+    size_t pos = 0;
+};
+
 DataPartStorageIteratorPtr DataPartStorageOnDiskFull::iterate() const
 {
+    auto dir_path = fs::path(root_path) / part_dir;
+    if (transaction)
+    {
+        if (auto staged = transaction->listInFlightDirectory(dir_path); !staged.empty())
+        {
+            /// Union the committed entries with the staged children (set semantics, committed dominates).
+            std::set<std::string> names(staged.begin(), staged.end());
+            if (volume->getDisk()->existsDirectory(dir_path))
+                for (auto it = volume->getDisk()->iterateDirectory(dir_path); it->isValid(); it->next())
+                    names.insert(it->name());
+            return std::make_unique<DataPartStorageMergedIterator>(
+                volume->getDisk(), dir_path, std::vector<std::string>(names.begin(), names.end()));
+        }
+    }
+
     return std::make_unique<DataPartStorageIteratorOnDisk>(
         volume->getDisk(),
-        volume->getDisk()->iterateDirectory(fs::path(root_path) / part_dir));
+        volume->getDisk()->iterateDirectory(dir_path));
 }
 
 Poco::Timestamp DataPartStorageOnDiskFull::getFileLastModified(const String & file_name) const
@@ -99,12 +168,17 @@ Poco::Timestamp DataPartStorageOnDiskFull::getFileLastModified(const String & fi
 
 size_t DataPartStorageOnDiskFull::getFileSize(const String & file_name) const
 {
+    auto path = fs::path(root_path) / part_dir / file_name;
+    /// B59: see existsFile — the merge stats the staged temp files before reading them back.
+    if (transaction)
+        if (auto size = transaction->tryGetInFlightFileSize(path))
+            return *size;
     if (looksLikePackedSkipIndexFile(file_name))
     {
         if (auto reader = getSkipIndicesPackedReader(); reader && reader->exists(file_name))
             return reader->getFileSize(file_name);
     }
-    return volume->getDisk()->getFileSize(fs::path(root_path) / part_dir / file_name);
+    return volume->getDisk()->getFileSize(path);
 }
 
 UInt32 DataPartStorageOnDiskFull::getRefCount(const String & file_name) const
@@ -115,7 +189,17 @@ UInt32 DataPartStorageOnDiskFull::getRefCount(const String & file_name) const
 std::vector<std::string> DataPartStorageOnDiskFull::getRemotePaths(const std::string & file_name) const
 {
     const std::string path = fs::path(root_path) / part_dir / file_name;
-    auto objects = volume->getDisk()->getStorageObjects(path);
+
+    /// B59: a file staged by this transaction resolves to its already-uploaded blob object(s) before commit.
+    /// A mutable per-part file intentionally does NOT resolve here (tryGetInFlightStorageObjects returns
+    /// nullopt → falls through): it has no blob object and must be read via tryReadFileInFlight. The merge
+    /// reads projection column blocks (blob-backed) through this path, not mutable files.
+    StoredObjects objects;
+    if (transaction)
+        if (auto inflight = transaction->tryGetInFlightStorageObjects(path))
+            objects = std::move(*inflight);
+    if (objects.empty())
+        objects = volume->getDisk()->getStorageObjects(path);
 
     std::vector<std::string> remote_paths;
     remote_paths.reserve(objects.size());
@@ -141,6 +225,42 @@ void DataPartStorageOnDiskFull::prepareRead(
     std::optional<size_t> read_hint,
     ReadPipeline & pipeline) const
 {
+    auto path = fs::path(root_path) / part_dir / name;
+
+    /// B59: read-your-writes for a part still being assembled by this transaction. A projection
+    /// spill-and-merge reads its own temp blocks back before the parent part's single commit; on a
+    /// content-addressed disk those files are staged in the transaction (blob uploaded, no ref yet),
+    /// so the committed metadata path can't see them. If the held transaction resolves the file
+    /// in-flight, serve it via a custom pipeline source that reads through the transaction. Gated on
+    /// `transaction != nullptr` so committed-part reads (no open transaction) are unchanged.
+    if (transaction)
+    {
+        StoredObjects inflight_objects;
+        if (auto objs = transaction->tryGetInFlightStorageObjects(path))
+            inflight_objects = std::move(*objs);
+        else if (auto size = transaction->tryGetInFlightFileSize(path))
+            /// Mutable per-part file staged inline (no blob object); synthesize a placeholder so the
+            /// single-object pipeline is satisfied — the custom creator below ignores it and reads the
+            /// inline bytes through the transaction.
+            inflight_objects = StoredObjects{StoredObject(path, path, *size)};
+
+        if (!inflight_objects.empty())
+        {
+            /// Safe to capture the raw transaction pointer: no cache/gather/async stage is added on this
+            /// branch, so the custom source is consumed synchronously inside build() during this read and
+            /// the pointer is never retained past it.
+            auto * tx = transaction.get();
+            pipeline.setSource(
+                [tx, path](const StoredObject &, const ReadSettings & read_settings, bool /*use_external_buffer*/, bool /*restrict_seek*/)
+                {
+                    return tx->tryReadFileInFlight(path, read_settings, std::nullopt);
+                },
+                std::move(inflight_objects),
+                settings);
+            return;
+        }
+    }
+
     if (looksLikePackedSkipIndexFile(name))
     {
         if (auto reader = getSkipIndicesPackedReader(); reader && reader->exists(name))
@@ -161,7 +281,8 @@ void DataPartStorageOnDiskFull::prepareRead(
             return;
         }
     }
-    volume->getDisk()->prepareRead(fs::path(root_path) / part_dir / name, settings, read_hint, pipeline);
+
+    volume->getDisk()->prepareRead(path, settings, read_hint, pipeline);
 }
 
 std::unique_ptr<ReadBufferFromFileBase> DataPartStorageOnDiskFull::readFileIfExists(
@@ -169,6 +290,13 @@ std::unique_ptr<ReadBufferFromFileBase> DataPartStorageOnDiskFull::readFileIfExi
     const ReadSettings & settings,
     std::optional<size_t> read_hint) const
 {
+    auto path = fs::path(root_path) / part_dir / name;
+    /// B59: serve a file staged by this transaction (uploaded blob or inline mutable bytes) before commit.
+    /// This direct delegate bypasses prepareRead, so the in-flight guard must be repeated here; it is the
+    /// only path that reaches the inline-mutable case via a returned buffer.
+    if (transaction)
+        if (auto rb = transaction->tryReadFileInFlight(path, settings, read_hint))
+            return rb;
     if (looksLikePackedSkipIndexFile(name))
     {
         if (auto reader = getSkipIndicesPackedReader(); reader && reader->exists(name))
@@ -177,7 +305,7 @@ std::unique_ptr<ReadBufferFromFileBase> DataPartStorageOnDiskFull::readFileIfExi
                 fs::path(root_path) / part_dir / String(SKIP_INDICES_PACKED_FILENAME),
                 name, settings, read_hint);
     }
-    return volume->getDisk()->readFileIfExists(fs::path(root_path) / part_dir / name, settings, read_hint);
+    return volume->getDisk()->readFileIfExists(path, settings, read_hint);
 }
 
 std::unique_ptr<WriteBufferFromFileBase> DataPartStorageOnDiskFull::writeFile(
@@ -266,20 +394,38 @@ void DataPartStorageOnDiskFull::createProjection(const std::string & name)
 
 void DataPartStorageOnDiskFull::beginTransaction()
 {
+    /// A borrowed projection sub-part shares the PARENT part's whole-part transaction (on a
+    /// content-addressed disk a part is one atomic unit: one manifest + one ref). It must not open its
+    /// own — riding the parent transaction is the point (B58) — so begin is a no-op here. This
+    /// centralizes the rule the 6 merge/mutate call sites used to duplicate as
+    /// `if (!isContentAddressed()) beginTransaction()`.
+    if (has_shared_transaction)
+        return;
+
     if (transaction)
-        throw Exception(ErrorCodes::LOGICAL_ERROR,
-            "Uncommitted{}transaction already exists", has_shared_transaction ? " shared " : " ");
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "Uncommitted transaction already exists");
 
     transaction = volume->getDisk()->createTransaction();
 }
 
 void DataPartStorageOnDiskFull::commitTransaction()
 {
+    /// The mirror of beginTransaction: a borrowed projection sub-part rides the parent's transaction and
+    /// is published by the parent's single commit. Committing here would be committing someone else's
+    /// transaction, so it is a no-op.
+    if (has_shared_transaction)
+        return;
+
     if (!transaction)
         throw Exception(ErrorCodes::LOGICAL_ERROR, "There is no uncommitted transaction");
 
-    if (has_shared_transaction)
-        throw Exception(ErrorCodes::LOGICAL_ERROR, "Cannot commit shared transaction");
+    /// Regression gate for the part-durability-before-Keeper-commit invariant: lets a test fail the
+    /// close of the PART's deferred disk transaction specifically (autocommit one-shot disk ops are
+    /// not affected, unlike disk_object_storage_fail_commit_metadata_transaction).
+    fiu_do_on(FailPoints::part_storage_fail_commit_transaction,
+    {
+        throw Exception(ErrorCodes::FAULT_INJECTED, "part_storage_fail_commit_transaction");
+    });
 
     transaction->commit();
     transaction.reset();
diff --git a/src/Storages/MergeTree/IDataPartStorage.h b/src/Storages/MergeTree/IDataPartStorage.h
index 37f31ade32b9..37cc719215a8 100644
--- a/src/Storages/MergeTree/IDataPartStorage.h
+++ b/src/Storages/MergeTree/IDataPartStorage.h
@@ -189,6 +189,15 @@ class IDataPartStorage : public boost::noncopyable
     virtual std::string getDiskName() const = 0;
     virtual std::string getDiskType() const = 0;
     virtual bool isStoredOnRemoteDisk() const { return false; }
+    /// True when the underlying disk stores a part as one atomic content-addressed unit (one manifest
+    /// + one ref). On such disks a projection sub-part must be written through the PARENT part's
+    /// whole-part transaction rather than its own sub-transaction (otherwise the projection is lost
+    /// from the committed manifest — B58).
+    virtual bool isContentAddressed() const { return false; }
+    /// True when the underlying disk publishes a file write atomically in one shot (no partial
+    /// content ever becomes visible under the file's final name). Such disks do not need the
+    /// tmp-file + `replaceFile` crash-safety dance that plain local writes require.
+    virtual bool supportsAtomicFileWrites() const { return false; }
     virtual std::optional<String> getCacheName() const { return std::nullopt; }
     virtual bool supportZeroCopyReplication() const { return false; }
     virtual bool supportParallelWrite() const = 0;
diff --git a/src/Storages/MergeTree/IMergeTreeDataPart.cpp b/src/Storages/MergeTree/IMergeTreeDataPart.cpp
index 5a29ab7a5f91..465f64c91a80 100644
--- a/src/Storages/MergeTree/IMergeTreeDataPart.cpp
+++ b/src/Storages/MergeTree/IMergeTreeDataPart.cpp
@@ -1356,7 +1356,13 @@ MergeTreeDataPartBuilder IMergeTreeDataPart::getProjectionPartBuilder(
     const String & projection_name, ProjectionDescriptionRawPtr projection, bool is_temp_projection)
 {
     const char * projection_extension = is_temp_projection ? ".tmp_proj" : ".proj";
-    auto projection_storage = getDataPartStorage().getProjection(projection_name + projection_extension, !is_temp_projection);
+    /// On a content-addressed disk a part is one atomic unit, so a temp projection sub-part (written
+    /// during a merge/mutate rebuild under `<proj>.tmp_proj`) must share the PARENT part's whole-part
+    /// transaction — its files are re-keyed into the parent manifest when `<proj>.tmp_proj` is renamed to
+    /// `<proj>.proj` (B58). On a non-CA disk a temp projection keeps its own sub-transaction (the
+    /// historical behavior): `use_parent_transaction = !is_temp_projection`.
+    const bool use_parent_transaction = !is_temp_projection || getDataPartStorage().isContentAddressed();
+    auto projection_storage = getDataPartStorage().getProjection(projection_name + projection_extension, use_parent_transaction);
     MergeTreeDataPartBuilder builder(storage, projection_name, projection_storage, getReadSettings());
     return builder.withPartInfo(MergeListElement::FAKE_RESULT_PART_FOR_PROJECTION).withParentPart(this).withProjection(projection);
 }
diff --git a/src/Storages/MergeTree/MergeProjectionPartsTask.cpp b/src/Storages/MergeTree/MergeProjectionPartsTask.cpp
index 8956d382ef6b..cb8b7171ab7f 100644
--- a/src/Storages/MergeTree/MergeProjectionPartsTask.cpp
+++ b/src/Storages/MergeTree/MergeProjectionPartsTask.cpp
@@ -129,6 +129,10 @@ bool MergeProjectionPartsTask::executeStep()
 
         /// FIXME (alesapin) we should use some temporary storage for this,
         /// not commit each subprojection part
+        ///
+        /// A borrowed (CA) recursively-merged projection sub-part shares the parent part's whole-part
+        /// transaction (the nested MergeTask skipped its own begin), so it is committed by the parent's
+        /// single commit; the storage makes commitTransaction a no-op there, so this is unconditional (B58).
         next_level_parts.back()->getDataPartStorage().commitTransaction();
         next_level_parts.back()->is_temp = true;
         next_level_parts.back()->temp_projection_block_number = block_num;
diff --git a/src/Storages/MergeTree/MergeTask.cpp b/src/Storages/MergeTree/MergeTask.cpp
index 5e7d3c71435b..7a631e06baaf 100644
--- a/src/Storages/MergeTree/MergeTask.cpp
+++ b/src/Storages/MergeTree/MergeTask.cpp
@@ -559,7 +559,14 @@ bool MergeTask::ExecuteAndFinalizeHorizontalPart::prepare() const
     std::optional<MergeTreeDataPartBuilder> builder;
     if (global_ctx->parent_part)
     {
-        auto data_part_storage = global_ctx->parent_part->getDataPartStorage().getProjection(local_tmp_part_basename,  /* use parent transaction */ false);
+        /// On a content-addressed disk a part is one atomic unit (one manifest + one ref). The projection
+        /// sub-part must therefore be written through the PARENT part's whole-part transaction (mirroring
+        /// the INSERT path, `MergeTreeDataWriter::writeProjectionPartImpl` with `use_parent_transaction =
+        /// true`) so its files land in the parent manifest and survive a reload (B58). On a non-CA disk we
+        /// keep the historical behavior: the projection sub-part opens and commits its own sub-transaction.
+        global_ctx->projection_uses_parent_transaction = global_ctx->parent_part->getDataPartStorage().isContentAddressed();
+        auto data_part_storage = global_ctx->parent_part->getDataPartStorage().getProjection(
+            local_tmp_part_basename, /* use_parent_transaction */ global_ctx->projection_uses_parent_transaction);
         builder.emplace(*global_ctx->data, global_ctx->future_part->name, data_part_storage, getReadSettings());
         builder->withParentPart(global_ctx->parent_part);
     }
@@ -579,6 +586,8 @@ bool MergeTask::ExecuteAndFinalizeHorizontalPart::prepare() const
     if (data_part_storage->exists())
         throw Exception(ErrorCodes::DIRECTORY_ALREADY_EXISTS, "Directory {} already exists", data_part_storage->getFullPath());
 
+    /// A borrowed projection sub-part shares the parent's already-open transaction; the storage makes
+    /// beginTransaction a no-op in that case, so this can be called unconditionally.
     data_part_storage->beginTransaction();
 
     /// Background temp dirs cleaner will not touch tmp projection directory because
@@ -1314,6 +1323,9 @@ void MergeTask::ExecuteAndFinalizeHorizontalPart::calculateProjectionForBlock(
             *global_ctx->data, ctx->log, result, projection, global_ctx->new_data_part.get(), ++ctx->projection_block_num, global_ctx->context);
 
         tmp_part->finalize();
+        /// A borrowed (CA) temp projection sub-part rides the parent's whole-part transaction and is
+        /// committed by the parent's single commit; the storage makes commitTransaction a no-op there,
+        /// so this can be called unconditionally (B58).
         tmp_part->part->getDataPartStorage().commitTransaction();
         ctx->projection_parts[projection.name].emplace_back(std::move(tmp_part->part));
     }
@@ -1356,6 +1368,8 @@ void MergeTask::ExecuteAndFinalizeHorizontalPart::finalizeProjections() const
                 *global_ctx->data, ctx->log, result, projection, global_ctx->new_data_part.get(), ++ctx->projection_block_num, global_ctx->context);
 
             temp_part->finalize();
+            /// See the matching note above: a borrowed (CA) temp projection sub-part rides the parent
+            /// transaction, so commitTransaction is a no-op and can be called unconditionally.
             temp_part->part->getDataPartStorage().commitTransaction();
             ctx->projection_parts[projection.name].emplace_back(std::move(temp_part->part));
         }
diff --git a/src/Storages/MergeTree/MergeTask.h b/src/Storages/MergeTree/MergeTask.h
index 21d027b5db20..a982d9ba58d6 100644
--- a/src/Storages/MergeTree/MergeTask.h
+++ b/src/Storages/MergeTree/MergeTask.h
@@ -219,6 +219,10 @@ class MergeTask
         ProjectionDescriptionRawPtr projection{nullptr};
         /// This will be either nullptr or new_data_part, so raw pointer is ok.
         IMergeTreeDataPart * parent_part{nullptr};
+        /// True only when this MergeTask builds a projection sub-part (`parent_part != nullptr`) whose
+        /// parent lives on a content-addressed disk: the sub-part then shares the parent's whole-part
+        /// transaction and must NOT begin/commit its own (B58). False for non-CA disks and top-level parts.
+        bool projection_uses_parent_transaction{false};
         MergedPartOffsetsPtr merged_part_offsets;
         ContextPtr context{nullptr};
         time_t time_of_merge{0};
diff --git a/src/Storages/MergeTree/MergeTreeData.cpp b/src/Storages/MergeTree/MergeTreeData.cpp
index 0d46d17862db..0ffa9443770d 100644
--- a/src/Storages/MergeTree/MergeTreeData.cpp
+++ b/src/Storages/MergeTree/MergeTreeData.cpp
@@ -33,9 +33,11 @@
 #include <DataTypes/DataTypeUUID.h>
 #include <DataTypes/NestedUtils.h>
 #include <DataTypes/hasNullable.h>
+#include <Disks/IDiskTransaction.h>
 #include <Disks/SingleDiskVolume.h>
 #include <Disks/TemporaryFileOnDisk.h>
 #include <Disks/createVolume.h>
+#include <IO/copyData.h>
 #include <IO/Operators.h>
 #include <IO/S3Common.h>
 #include <IO/SharedThreadPools.h>
@@ -5915,6 +5917,27 @@ MergeTreeData::PartsToRemoveFromZooKeeper MergeTreeData::removePartsInRangeFromW
         MergeTreeData::Transaction transaction(*this, NO_TRANSACTION_RAW);
         renameTempPartAndAdd(new_data_part, transaction, lock, /*rename_in_transaction=*/ false);     /// All covered parts must be already removed
 
+        /// On a content-addressed disk a part directory becomes durable only when its disk-storage
+        /// transaction is committed (the ref to its manifest is published at commit, not at rename).
+        /// The flow below rolls back the in-memory MergeTreeData transaction (to keep the empty part
+        /// Outdated, not Active), which never calls commitTransaction on the disk storage — so on a CA
+        /// disk the empty covering part would leave NO on-disk ref and vanish on restart/reattach,
+        /// defeating its sole purpose (it exists only to cover the dropped parts on disk so a restart
+        /// does not treat them as uncovered unexpected parts and trip TOO_MANY_UNEXPECTED_DATA_PARTS).
+        /// On a plain disk the rename in renameTempPartAndAdd is already durable, so this is a no-op
+        /// there. Commit the disk storage transaction here (CA only) so the ref is published before the
+        /// in-memory rollback; the part still ends up Outdated, exactly as on a plain disk.
+        ///
+        /// [TXN-ONE-PIPELINE] (`2026-07-16-cas-txn-one-pipeline-design.md`, Audit 7 / Tension 2): this
+        /// hand-placed `commitTransaction()` is NOT made redundant by moving publication into `commit`
+        /// — it is the direct consequence of that design. There is no `precommit` phase under the
+        /// one-pipeline model, and this rollback path (by construction, to keep the part Outdated) never
+        /// reaches `MergeTreeData::Transaction::commit`, the only other place a disk transaction is
+        /// committed. So this call remains the ONLY thing that publishes the empty cover's ref. Keep it.
+        if (new_data_part->getDataPartStorage().isContentAddressed()
+            && new_data_part->getDataPartStorage().hasActiveTransaction())
+            new_data_part->getDataPartStorage().commitTransaction();
+
         /// It will add the empty part to the set of Outdated parts without making it Active (exactly what we need)
         transaction.rollback(&lock);
         new_data_part->remove_time.store(0, std::memory_order_relaxed);
@@ -6725,6 +6748,54 @@ void MergeTreeData::checkAlterPartitionIsPossible(
                     can_execute_alter_on_disk = std::ranges::contains(supported_commands, command.type);
                     break;
                 }
+                case MetadataStorageType::CAS:
+                {
+                    /// On a CAS disk a part clone is cheap: identical content has the same
+                    /// `part_id`, so cloning is publishing a ref (no byte copy). The clone path is now
+                    /// transactional — `DataPartStorageOnDiskBase::freeze` runs the whole clone through ONE
+                    /// CA transaction, and `moveDirectory` re-keys the detached-staging → active rename into
+                    /// a complete active ref — so these are SUPPORTED and verified (read back identical data,
+                    /// survive restart): `ATTACH PARTITION`/`ATTACH PART` (re-clone of the table's own
+                    /// detached parts), `REPLACE PARTITION`/`ATTACH PARTITION ... FROM` (parses to
+                    /// `REPLACE_PARTITION`), and `MOVE PARTITION ... TO TABLE`. The pointer-unlink commands
+                    /// `DROP PARTITION` / `DETACH PARTITION` / `DROP DETACHED PARTITION` are also fine.
+                    /// `FETCH PARTITION`/`FETCH PART` is also SUPPORTED — it is a `ReplicatedMergeTree` op
+                    /// (now supported on CA), and a `to_detached` fetch takes the byte-fetch path: the
+                    /// downloaded files content-address into the `detached/` namespace (relink-into-detached
+                    /// is deferred, see backlog). `ALTER ... FETCH PART` parses to the same `FETCH_PARTITION`
+                    /// command type (with `part=true`), so this entry covers both.
+                    /// `FREEZE PARTITION`/`FREEZE ALL` and `UNFREEZE PARTITION`/`UNFREEZE ALL` are now SUPPORTED:
+                    /// a freeze publishes each part as its own ref in the `shadow/` namespace (a GC root sharing
+                    /// the live blobs zero-copy — no byte copy); UNFREEZE removes the backup's refs.
+                    /// `FORGET PARTITION` is SUPPORTED on CA — it only manipulates ZooKeeper partition metadata
+                    /// (removes block-number nodes from ZooKeeper) and does not write, clone, or touch any part
+                    /// files on disk, so it is safe on a content-addressed disk.
+                    /// NOTE: `MOVE_PARTITION` also admits cross-disk
+                    /// `MOVE ... TO DISK/VOLUME` (this check cannot distinguish the destination); that uses
+                    /// the byte-copy `clonePart` path (NOT the corrupting per-file hardlink), but only
+                    /// same-disk `MOVE ... TO TABLE` is verified here — cross-disk is a follow-up to verify.
+                    const static auto supported_commands = {
+                        PartitionCommand::DROP_PARTITION,
+                        PartitionCommand::DROP_DETACHED_PARTITION,
+                        PartitionCommand::FORGET_PARTITION,
+                        PartitionCommand::ATTACH_PARTITION,
+                        PartitionCommand::REPLACE_PARTITION,
+                        PartitionCommand::MOVE_PARTITION,
+                        PartitionCommand::FETCH_PARTITION,
+                        PartitionCommand::FREEZE_PARTITION,
+                        PartitionCommand::FREEZE_ALL_PARTITIONS,
+                        PartitionCommand::UNFREEZE_PARTITION,
+                        PartitionCommand::UNFREEZE_ALL_PARTITIONS,
+                    };
+
+                    if (!std::ranges::contains(supported_commands, command.type))
+                        throw Exception(
+                            ErrorCodes::SUPPORT_IS_DISABLED,
+                            "Partition operation ALTER TABLE {} is not supported on a CAS disk yet "
+                            "(it clones parts file-by-file with no transaction, which would corrupt the clone); disk '{}'",
+                            command.typeToString(), disk->getName());
+                    break;
+                }
                 case MetadataStorageType::StaticWeb:
                 {
                     can_execute_alter_on_disk = false;
@@ -7465,6 +7536,14 @@ void MergeTreeData::restorePartFromBackup(std::shared_ptr<RestoredPartsHolder> r
     /// Copy files from the backup to the directory `tmp_part_dir`.
     disk->createDirectories(temp_part_dir);
 
+    /// A content-addressed disk publishes a part as ONE manifest (N files -> one ref) atomically, so the
+    /// per-file copyFileToDisk autocommit below is rejected for content part files. Route the restore
+    /// through one whole-part transaction (mirrors DataPartStorageOnDiskBase::freeze's owned_transaction):
+    /// all files land in a single content-addressed part at tmp_restore_<part>, published by tx->commit().
+    DiskTransactionPtr restore_tx;
+    if (disk->isContentAddressed())
+        restore_tx = disk->createTransaction();
+
     for (const String & filename : filenames)
     {
         /// Needs to create subdirectories before copying the files. Subdirectories are used to represent projections.
@@ -7485,10 +7564,24 @@ void MergeTreeData::restorePartFromBackup(std::shared_ptr<RestoredPartsHolder> r
             continue;
         }
 
-        size_t file_size = backup->copyFileToDisk(part_path_in_backup_fs / filename, disk, temp_part_dir / filename, WriteMode::Rewrite);
-        reservation->update(reservation->getSize() - file_size);
+        if (restore_tx)
+        {
+            auto in = backup->readFile(part_path_in_backup_fs / filename);
+            auto out = restore_tx->writeFile(temp_part_dir / filename, DBMS_DEFAULT_BUFFER_SIZE, WriteMode::Rewrite, getContext()->getWriteSettings());
+            copyData(*in, *out);
+            out->finalize();
+            reservation->update(reservation->getSize() - backup->getFileSize(part_path_in_backup_fs / filename));
+        }
+        else
+        {
+            size_t file_size = backup->copyFileToDisk(part_path_in_backup_fs / filename, disk, temp_part_dir / filename, WriteMode::Rewrite);
+            reservation->update(reservation->getSize() - file_size);
+        }
     }
 
+    if (restore_tx)
+        restore_tx->commit();
+
     if (auto part = loadPartRestoredFromBackup(part_name, disk, temp_part_dir, detach_if_broken))
         restored_parts_holder->addPart(part);
     else
@@ -8901,12 +8994,31 @@ void MergeTreeData::Transaction::clear()
 
 void MergeTreeData::Transaction::renameParts()
 {
+    /// Materialize every part of this transaction: perform the deferred tmp->final renames, then
+    /// close each part's disk-storage transaction, making the parts DURABLE on their disks.
+    ///
+    /// Contract: after renameParts returns, every part of this transaction is durable at its
+    /// final name. commit only flips in-memory visibility (its commitTransaction loop remains as
+    /// a safety net for paths that do not come through here); rollback compensates with new
+    /// operations over committed disk state (removing a rolled-back part reclaims its disk data;
+    /// on a content-addressed disk that drops the published ref).
+    ///
+    /// Ordering is load-bearing: every call site invokes renameParts BEFORE its external Keeper
+    /// commit decision. A part must be durable before its block_id/part-znode is registered,
+    /// otherwise a fault between the Keeper commit and the disk commit leaves a phantom part whose
+    /// surviving block_id silently dedups a byte-identical client retry (acked data loss). This
+    /// also keeps the disk commit (network I/O on object storages) off the data_parts lock, which
+    /// Transaction::commit holds.
     for (const auto & part_need_rename : precommitted_parts_need_rename)
     {
         LOG_TEST(data.log, "Renaming part to {}", part_need_rename->name);
         part_need_rename->renameTo(part_need_rename->name, true);
     }
     precommitted_parts_need_rename.clear();
+
+    for (const auto & part : precommitted_parts)
+        if (part->getDataPartStorage().hasActiveTransaction())
+            part->getDataPartStorage().commitTransaction();
 }
 
 MergeTreeData::DataPartsVector MergeTreeData::Transaction::commit()
diff --git a/src/Storages/MergeTree/MergeTreeData.h b/src/Storages/MergeTree/MergeTreeData.h
index 826f4eead8f5..349341d8a81f 100644
--- a/src/Storages/MergeTree/MergeTreeData.h
+++ b/src/Storages/MergeTree/MergeTreeData.h
@@ -366,9 +366,17 @@ class MergeTreeData : public WithMutableContext, public IStorage, public IBackgr
         DataPartsVector commit();
         DataPartsVector commit(DataPartsLock & lock);
 
-        /// Rename should be done explicitly, before calling commit(), to
-        /// guarantee that no lock held during rename (since rename is IO
-        /// bound, while data parts lock is the bottleneck)
+        /// Renames should be done explicitly, before calling commit, to
+        /// guarantee that no lock is held during the rename and the disk
+        /// commit (both are IO bound, while the data parts lock is the
+        /// bottleneck). Contract: after renameParts every part of this
+        /// transaction is durable on its disk at its final name; commit only
+        /// flips in-memory visibility, and rollback compensates via new disk
+        /// operations (part removal). Every caller runs this BEFORE its
+        /// external Keeper commit decision: a part must be durable before its
+        /// block_id/part-znode is registered in Keeper, otherwise a fault between the two commits
+        /// leaves a phantom part whose surviving block_id silently dedups a byte-identical client
+        /// retry (acked data loss).
         void renameParts();
 
         void addPart(MutableDataPartPtr & part, bool need_rename);
diff --git a/src/Storages/MergeTree/MergeTreeDataWriter.cpp b/src/Storages/MergeTree/MergeTreeDataWriter.cpp
index 8e64543510c3..aaf7ddc527a9 100644
--- a/src/Storages/MergeTree/MergeTreeDataWriter.cpp
+++ b/src/Storages/MergeTree/MergeTreeDataWriter.cpp
@@ -1053,6 +1053,8 @@ MergeTreeTemporaryPartPtr MergeTreeDataWriter::writeProjectionPartImpl(
     auto projection_part_storage = new_data_part->getDataPartStoragePtr();
     auto data_settings = data.getSettings(&projection.settings_changes);
 
+    /// A temp projection sub-part opens a transaction only if it owns one; a borrowed (CA) projection
+    /// storage makes beginTransaction a no-op, so the `isContentAddressed()` branch is no longer needed.
     if (is_temp)
         projection_part_storage->beginTransaction();
 
diff --git a/src/Storages/MergeTree/MutateTask.cpp b/src/Storages/MergeTree/MutateTask.cpp
index 918ead21658d..0eb164c8e7a7 100644
--- a/src/Storages/MergeTree/MutateTask.cpp
+++ b/src/Storages/MergeTree/MutateTask.cpp
@@ -1871,6 +1871,9 @@ void PartMergerWriter::writeTempProjectionPart(size_t projection_idx, Chunk chun
         ctx->context);
 
     tmp_part->finalize();
+    /// A borrowed (CA) temp projection sub-part shares the new (parent) part's whole-part transaction
+    /// (see `IMergeTreeDataPart::getProjectionPartBuilder`) and is committed by the parent's single
+    /// commit; the storage makes commitTransaction a no-op there, so this is called unconditionally (B58).
     tmp_part->part->getDataPartStorage().commitTransaction();
     projection_parts[projection.name].emplace_back(std::move(tmp_part->part));
 }
diff --git a/src/Storages/MergeTree/tests/gtest_projection_borrowed_transaction.cpp b/src/Storages/MergeTree/tests/gtest_projection_borrowed_transaction.cpp
new file mode 100644
index 000000000000..ae079a61450f
--- /dev/null
+++ b/src/Storages/MergeTree/tests/gtest_projection_borrowed_transaction.cpp
@@ -0,0 +1,86 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskLocal.h>
+#include <Disks/SingleDiskVolume.h>
+#include <Storages/MergeTree/DataPartStorageOnDiskFull.h>
+
+#include <filesystem>
+#include <memory>
+#include <string>
+#include <unistd.h>
+
+using namespace DB;
+
+namespace
+{
+    /// A DiskLocal-backed parent part storage. `DiskLocal::createTransaction` yields a real
+    /// transaction object, which is all `beginTransaction` needs to hand a NON-NULL transaction to a
+    /// borrowed projection sub-part (the `has_shared_transaction == true` case).
+    struct ParentStorageFixture
+    {
+        std::filesystem::path base_path;
+        DiskPtr disk;
+        VolumePtr volume;
+        MutableDataPartStoragePtr parent;
+
+        ParentStorageFixture()
+        {
+            const auto unique = std::to_string(::getpid()) + "_"
+                + std::to_string(reinterpret_cast<uintptr_t>(this));
+            base_path = std::filesystem::temp_directory_path() / ("proj_txn_gtest_" + unique);
+            std::filesystem::create_directories(base_path / "all_1_1_0");
+            disk = std::make_shared<DiskLocal>("test_disk_" + unique, base_path.string());
+            volume = std::make_shared<SingleDiskVolume>("test_volume", disk);
+            parent = std::make_shared<DataPartStorageOnDiskFull>(volume, /*root_path=*/"", "all_1_1_0");
+        }
+
+        ~ParentStorageFixture()
+        {
+            std::error_code ec;
+            std::filesystem::remove_all(base_path, ec);
+        }
+    };
+}
+
+/// A projection sub-part that BORROWS the parent's whole-part transaction (the CA-disk shape:
+/// getProjection(..., use_parent_transaction = true)) must let begin/commit be NO-OPS — it rides the
+/// parent's single commit. Before the encapsulation this threw "Uncommitted shared transaction already
+/// exists" / "Cannot commit shared transaction", forcing every caller to branch on isContentAddressed().
+TEST(ProjectionBorrowedTransaction, BorrowedStorageBeginCommitAreNoOps)
+{
+    ParentStorageFixture fx;
+
+    /// Parent opens the whole-part transaction (as MergeTask/writer do for a CA part).
+    fx.parent->beginTransaction();
+    ASSERT_TRUE(fx.parent->hasActiveTransaction());
+
+    /// Borrowed projection sub-part: shares the parent transaction (has_shared_transaction == true).
+    auto proj = fx.parent->getProjection("p.proj", /*use_parent_transaction=*/true);
+    EXPECT_TRUE(proj->hasActiveTransaction());
+
+    /// The encapsulated rule: begin/commit on the borrowed storage are silent no-ops (they must NOT
+    /// open a second transaction, nor commit the parent's).
+    EXPECT_NO_THROW(proj->beginTransaction());
+    EXPECT_NO_THROW(proj->commitTransaction());
+
+    /// The parent's transaction is untouched by the projection's no-ops and still commits cleanly.
+    EXPECT_TRUE(fx.parent->hasActiveTransaction());
+    EXPECT_NO_THROW(fx.parent->commitTransaction());
+    EXPECT_FALSE(fx.parent->hasActiveTransaction());
+}
+
+/// The non-CA temp-projection shape (use_parent_transaction = false) is unchanged: the sub-part OWNS
+/// its transaction, so begin creates it and commit commits it (has_shared_transaction == false, so the
+/// no-op path never triggers).
+TEST(ProjectionBorrowedTransaction, OwnedProjectionStorageStillBeginsAndCommits)
+{
+    ParentStorageFixture fx;
+
+    auto proj = fx.parent->getProjection("q.proj", /*use_parent_transaction=*/false);
+    EXPECT_FALSE(proj->hasActiveTransaction());
+
+    EXPECT_NO_THROW(proj->beginTransaction());
+    EXPECT_TRUE(proj->hasActiveTransaction());
+    EXPECT_NO_THROW(proj->commitTransaction());
+    EXPECT_FALSE(proj->hasActiveTransaction());
+}

From 3d35dfce282feea62675b53a4f18dfa9507233f6 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:33 +0200
Subject: [PATCH 10/30] Unwrap table proxies in single-table SYSTEM commands

Single-table SYSTEM commands (SYNC/RESTORE/RESTART/DROP REPLICA, WAIT
LOADING PARTS, PREWARM, ...) cast the storage directly and missed tables
behind a proxy; unwrap the proxy before the cast.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/Storages/StorageProxy.h      | 9 +++++++++
 src/Storages/StorageTableProxy.h | 8 ++++++++
 2 files changed, 17 insertions(+)

diff --git a/src/Storages/StorageProxy.h b/src/Storages/StorageProxy.h
index dc1c0570e9b6..fe5b91e3bfba 100644
--- a/src/Storages/StorageProxy.h
+++ b/src/Storages/StorageProxy.h
@@ -143,6 +143,15 @@ class StorageProxy : public IStorage
 
     void mutate(const MutationCommands & commands, ContextPtr context) override { getNested()->mutate(commands, context); }
 
+    /// Must forward alongside `mutate`: `IStorage`'s default throws NOT_IMPLEMENTED ("doesn't
+    /// support mutations"), so a non-forwarding proxy rejects every mutation on a wrapped table
+    /// even though the nested engine supports them (found via `ALTER TABLE ... MATERIALIZE TTL`
+    /// on a `lazy_load_tables = 1` table wrapped in `StorageTableProxy`).
+    void checkMutationIsPossible(const MutationCommands & commands, const Settings & settings) const override
+    {
+        getNested()->checkMutationIsPossible(commands, settings);
+    }
+
     CancellationCode killMutation(const String & mutation_id) override { return getNested()->killMutation(mutation_id); }
 
     void startup() override { getNested()->startup(); }
diff --git a/src/Storages/StorageTableProxy.h b/src/Storages/StorageTableProxy.h
index fa998bab42ac..e935de70e221 100644
--- a/src/Storages/StorageTableProxy.h
+++ b/src/Storages/StorageTableProxy.h
@@ -54,6 +54,14 @@ class StorageTableProxy final : public StorageProxy
     StoragePolicyPtr getStoragePolicy() const override { return nullptr; }
     bool isView() const override { return false; }
 
+    /// NOTE: this proxy deliberately does NOT forward `checkTableCanBeRenamed` to the nested engine.
+    /// Doing so would materialize the lazy table (`getNested`) while `DatabaseAtomic` holds its
+    /// non-recursive database mutex, and a schema-inferred lazy `Buffer` resolves its destination via
+    /// `DatabaseCatalog::getTable` in its constructor -- re-entering the same database and self-
+    /// deadlocking. Bypassing the nested engine's rename restriction for a lazy (never-accessed) table
+    /// is a pre-existing gap tracked in docs/superpowers/cas/BACKLOG.md; the correct fix is to
+    /// materialize before the database mutex is taken, at the interpreter level.
+    ///
     /// Startup is deferred until first access via `getNested`.
     void startup() override { }
 

From f85cb4330c88d7619228d2784f1658ea28240254 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:33 +0200
Subject: [PATCH 11/30] clickhouse-disks: non-interactive runs exit nonzero on
 a failed command

clickhouse-disks --query always exited 0 and reported failures only on
stderr, so scripts, cron jobs and CI could not gate on it at all. Record
each command's error code in processQueryText and return it as the
process exit code for non-interactive runs; interactive REPL sessions
keep exiting 0. Within one semicolon-separated batch a later success does
not clear an earlier failure.

Carries the two test fixes the contract exposed: a 2024 typo in
test_disks_app_func ("d/a" instead of "a/d/a" always failed inside the
tool and was swallowed), and test_replicated_table_structure_alter
reading metadata_path from system.tables after DETACH DATABASE (empty
path, the remove never ran, the recovery scenario was never exercised).
Mixed-file note: DisksApp.cpp also carries the cas-* command registration
and CA pool initialization, wired by the later CAS integration commits.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 programs/disks/DisksApp.cpp                   | 35 +++++++++++++++++++
 programs/disks/DisksApp.h                     |  4 +++
 tests/integration/test_disks_app_func/test.py | 10 +++---
 .../test_replicated_database/test.py          | 11 ++++--
 4 files changed, 52 insertions(+), 8 deletions(-)

diff --git a/programs/disks/DisksApp.cpp b/programs/disks/DisksApp.cpp
index dbde5cb4672c..871316672a43 100644
--- a/programs/disks/DisksApp.cpp
+++ b/programs/disks/DisksApp.cpp
@@ -25,6 +25,7 @@
 #include <Common/TerminalSize.h>
 
 #include <Common/logger_useful.h>
+#include <Core/ServerUUID.h>
 #include <Loggers/OwnFormattingChannel.h>
 #include <Loggers/OwnPatternFormatter.h>
 #include "config.h"
@@ -34,6 +35,7 @@
 #include <IO/SharedThreadPools.h>
 #include <Common/ThreadPool.h>
 #include <Common/scope_guard_safe.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
 
 #include <Poco/FileChannel.h>
 
@@ -44,6 +46,7 @@ namespace ErrorCodes
 {
     extern const int BAD_ARGUMENTS;
     extern const int LOGICAL_ERROR;
+    extern const int STD_EXCEPTION;
 };
 
 LineReader::Patterns DisksApp::query_extenders = {"\\"};
@@ -212,6 +215,8 @@ bool DisksApp::processQueryText(const String & text)
         return false;
     CommandPtr command;
 
+    last_command_exit_code = 0;
+
     auto subqueries = splitOnUnquotedSemicolons(text);
     for (const auto & subquery : subqueries)
     {
@@ -230,6 +235,7 @@ bool DisksApp::processQueryText(const String & text)
         {
             int code = err.code();
             error_string = getExceptionMessageForLogging(err, true, false);
+            last_command_exit_code = code;
             if (code == ErrorCodes::BAD_ARGUMENTS)
             {
                 if (command.get())
@@ -246,10 +252,12 @@ bool DisksApp::processQueryText(const String & text)
         catch (std::exception & err)
         {
             error_string = err.what();
+            last_command_exit_code = ErrorCodes::STD_EXCEPTION;
         }
         catch (...) // Ok: report unknown exception
         {
             error_string = "Unknown exception";
+            last_command_exit_code = ErrorCodes::STD_EXCEPTION;
         }
         if (error_string.has_value())
         {
@@ -334,6 +342,11 @@ void DisksApp::registerCommands()
     command_descriptions.emplace("switch-disk", makeCommandSwitchDisk());
     command_descriptions.emplace("current_disk_with_path", makeCommandGetCurrentDiskAndPath());
     command_descriptions.emplace("touch", makeCommandTouch());
+    command_descriptions.emplace("cas-fsck", makeCommandFsck());
+    command_descriptions.emplace("cas-gc-dryrun", makeCommandCaGcDryRun());
+    command_descriptions.emplace("cas-gc-rebuild", makeCommandCaGcRebuild());
+    command_descriptions.emplace("cas-inspect", makeCommandCaInspect());
+    command_descriptions.emplace("cas-drop-member", makeCommandCaDropMember());
     command_descriptions.emplace("read-checksums", makeCommandReadChecksums());
     command_descriptions.emplace("help", makeCommandHelp(*this));
 #if CLICKHOUSE_CLOUD
@@ -539,6 +552,13 @@ int DisksApp::main(const std::vector<String> & /*args*/)
         /*max_io_thread_pool_free_size*/ 0,
         /*io_thread_pool_queue_size*/ 10000);
 
+    /// `clickhouse-disks` loads no `ServerSettings`, so this can't read
+    /// `cas_blob_upload_pool_size`; 16 mirrors that setting's default
+    /// (`src/Core/ServerSettings.cpp`). A `write` command that commits through a
+    /// `cas` disk reaches `uploadPendingBlobs`, which calls this pool
+    /// unconditionally (see the analogous init in `Server.cpp`/`LocalServer.cpp`).
+    DB::Cas::initializeBlobUploadPool(16);
+
     registerCommands();
 
     registerDisks(/* global_skip_access_check= */ true);
@@ -571,6 +591,16 @@ int DisksApp::main(const std::vector<String> & /*args*/)
 
     global_context->setPath(path);
 
+    /// Load the server UUID so that live CA namespaces resolve correctly.
+    /// Only load when the uuid file already exists — clickhouse-disks inspects existing
+    /// pools and must NOT create or mutate the uuid file (the disk may be read-only).
+    /// If the file is absent, ServerUUID stays Nil and shadow/non-live navigation works.
+    {
+        fs::path uuid_file = fs::path(path) / "uuid";
+        if (fs::exists(uuid_file))
+            ServerUUID::load(uuid_file, &logger());
+    }
+
     client = std::make_unique<DisksClient>(config(), global_context);
 
     suggest.setCompletionsCallback([&](const String & prefix, size_t /* prefix_length */) { return getCompletions(prefix); });
@@ -587,6 +617,10 @@ int DisksApp::main(const std::vector<String> & /*args*/)
     if (log_file)
         log_file->close();
 
+    /// Non-interactive runs surface a failing command as a nonzero process exit (CI/cron gating,
+    /// e.g. `cas-fsck` reporting dangling objects). Interactive sessions are unaffected.
+    if (query.has_value() && last_command_exit_code != 0)
+        return last_command_exit_code;
     return Application::EXIT_OK;
 }
 
@@ -636,6 +670,7 @@ int mainEntryClickHouseDisks(int argc, char ** argv)
     /// That way, accesses happen-before destruction.
     SCOPE_EXIT_SAFE({
         DB::StaticThreadPool::shutdownAll();
+        DB::Cas::shutdownBlobUploadPool();
         GlobalThreadPool::shutdown();
     });
 
diff --git a/programs/disks/DisksApp.h b/programs/disks/DisksApp.h
index fbe0639e00f3..27d7e03a6720 100644
--- a/programs/disks/DisksApp.h
+++ b/programs/disks/DisksApp.h
@@ -90,6 +90,10 @@ class DisksApp : public Poco::Util::Application
 
     std::optional<String> query;
 
+    /// Set when a command threw during processQueryText; used to make non-interactive (--query)
+    /// runs exit nonzero (e.g. `fsck` reporting dangling objects). Reset per processQueryText call.
+    int last_command_exit_code = 0;
+
     const std::unordered_map<String, String> aliases = {
         {"cp", "copy"},
         {"mv", "move"},
diff --git a/tests/integration/test_disks_app_func/test.py b/tests/integration/test_disks_app_func/test.py
index 87ab2e24b9f3..b635f6aad682 100755
--- a/tests/integration/test_disks_app_func/test.py
+++ b/tests/integration/test_disks_app_func/test.py
@@ -178,7 +178,7 @@ def init_data_s3_rm_rec(source):
     write(source, "test3", "a/b/d")
     write(source, "test3", "a/b/e")
 
-    write(source, "test3", "d/a")
+    write(source, "test3", "a/d/a")
 
 
 def test_disks_app_func_ld(started_cluster):
@@ -330,22 +330,22 @@ def test_disks_app_func_rm_shared_recursive(started_cluster):
     out = ls(source, "test3", ". --recursive")
     assert (
         out
-        == ".:\na\n\n./a:\na\nb\nc\nd\n\n./a/a:\na\nb\nc\n\n./a/b:\na\nb\nc\nd\ne\n\n./a/c:\n\n./a/d:\n\n"
+        == ".:\na\n\n./a:\na\nb\nc\nd\n\n./a/a:\na\nb\nc\n\n./a/b:\na\nb\nc\nd\ne\n\n./a/c:\n\n./a/d:\na\n\n"
     )
 
     remove(source, "test3", "a/a --recursive")
     out = ls(source, "test3", ". --recursive")
     assert (
-        out == ".:\na\n\n./a:\nb\nc\nd\n\n./a/b:\na\nb\nc\nd\ne\n\n./a/c:\n\n./a/d:\n\n"
+        out == ".:\na\n\n./a:\nb\nc\nd\n\n./a/b:\na\nb\nc\nd\ne\n\n./a/c:\n\n./a/d:\na\n\n"
     )
 
     remove(source, "test3", "a/b --recursive")
     out = ls(source, "test3", ". --recursive")
-    assert out == ".:\na\n\n./a:\nc\nd\n\n./a/c:\n\n./a/d:\n\n"
+    assert out == ".:\na\n\n./a:\nc\nd\n\n./a/c:\n\n./a/d:\na\n\n"
 
     remove(source, "test3", "a/c --recursive")
     out = ls(source, "test3", ". --recursive")
-    assert out == ".:\na\n\n./a:\nd\n\n./a/d:\n\n"
+    assert out == ".:\na\n\n./a:\nd\n\n./a/d:\na\n\n"
 
     remove(source, "test3", "a --recursive")
     out = ls(source, "test3", ". --recursive")
diff --git a/tests/integration/test_replicated_database/test.py b/tests/integration/test_replicated_database/test.py
index 804398c8307d..8226c58aaa42 100644
--- a/tests/integration/test_replicated_database/test.py
+++ b/tests/integration/test_replicated_database/test.py
@@ -1379,6 +1379,14 @@ def test_replicated_table_structure_alter(started_cluster):
     )
 
     competing_node.query("SYSTEM SYNC DATABASE REPLICA table_structure")
+
+    # `system.tables` only lists an attached database, so the metadata path of `mem` must be read
+    # before the DETACH below; afterwards the SELECT returns nothing.
+    metadata_path = competing_node.query(
+        "SELECT metadata_path FROM system.tables WHERE database='table_structure' AND name='mem'"
+    ).strip()
+    assert metadata_path, "metadata_path of table_structure.mem is empty"
+
     competing_node.query("DETACH DATABASE table_structure")
 
     main_node.query(
@@ -1389,9 +1397,6 @@ def test_replicated_table_structure_alter(started_cluster):
     )
     main_node.query("INSERT INTO table_structure.rmt VALUES (1, 2, 3)")
 
-    metadata_path = competing_node.query(
-        "SELECT metadata_path FROM system.tables WHERE database='table_structure' AND name='mem'"
-    ).strip()
     db_disk_name = get_database_disk_name(competing_node)
     competing_node.exec_in_container(
         [

From 407935b893b29f048d2ac66e2c281e82c087b3c0 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:33 +0200
Subject: [PATCH 12/30] Fix MSan build: include DataTypesDecimal.h in
 IcebergWrites.cpp

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergWrites.cpp | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergWrites.cpp b/src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergWrites.cpp
index 5bc1fe22fbce..7a267e3b2b6d 100644
--- a/src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergWrites.cpp
+++ b/src/Storages/ObjectStorage/DataLakes/Iceberg/IcebergWrites.cpp
@@ -13,6 +13,7 @@
 #include <DataTypes/DataTypeNullable.h>
 #include <DataTypes/DataTypeString.h>
 #include <DataTypes/DataTypeTime64.h>
+#include <DataTypes/DataTypesDecimal.h>
 #include <DataTypes/DataTypesNumber.h>
 #include <DataTypes/IDataType.h>
 #include <Databases/DataLake/Common.h>

From 83a04256953c16e08bab20d8c3c3568f6af92261 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:33 +0200
Subject: [PATCH 13/30] CAS subsystem: Primitives layer

Core value types of the content-addressed storage subsystem: identifiers,
hashes, tokens, namespaces. No dependencies on other CAS layers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../Primitives/CasBlobDigest.cpp              |  47 +++
 .../Primitives/CasBlobDigest.h                | 247 +++++++++++++
 .../Primitives/CasBlobHashingWriteBuffer.cpp  | 260 ++++++++++++++
 .../Primitives/CasBlobHashingWriteBuffer.h    |  50 +++
 .../Primitives/CasCodecUtil.h                 | 118 +++++++
 .../ContentAddressed/Primitives/CasEvent.cpp  |  95 +++++
 .../ContentAddressed/Primitives/CasEvent.h    | 126 +++++++
 .../Primitives/CasNamespaceLifeId.h           | 116 ++++++
 .../ContentAddressed/Primitives/CasTypes.h    | 331 ++++++++++++++++++
 .../Primitives/CasXxh3Streamer.h              |  83 +++++
 10 files changed, 1473 insertions(+)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasNamespaceLifeId.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasXxh3Streamer.h

diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.cpp
new file mode 100644
index 000000000000..3c95ff171e4f
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.cpp
@@ -0,0 +1,47 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+
+namespace DB::Cas
+{
+
+std::string_view blobHashAlgoName(BlobHashAlgo algo)
+{
+    switch (algo)
+    {
+        case BlobHashAlgo::CityHash128:
+            return "ch128";
+        case BlobHashAlgo::XXH3_128:
+            return "xxh3";
+        case BlobHashAlgo::Sha256:
+            return "sha256";
+    }
+    throw Exception(ErrorCodes::BAD_ARGUMENTS, "blobHashAlgoName: unknown BlobHashAlgo {}", static_cast<int>(algo));
+}
+
+uint64_t blobHashLenFor(BlobHashAlgo algo)
+{
+    switch (algo)
+    {
+        case BlobHashAlgo::CityHash128:
+        case BlobHashAlgo::XXH3_128:
+            return 16;
+        case BlobHashAlgo::Sha256:
+            return 32;
+    }
+    throw Exception(ErrorCodes::BAD_ARGUMENTS, "blobHashLenFor: unknown BlobHashAlgo {}", static_cast<int>(algo));
+}
+
+BlobHashAlgo parseBlobHashAlgo(std::string_view config_value)
+{
+    if (config_value == "cityhash128")
+        return BlobHashAlgo::CityHash128;
+    if (config_value == "xxh3-128")
+        return BlobHashAlgo::XXH3_128;
+    if (config_value == "sha256")
+        return BlobHashAlgo::Sha256;
+
+    throw Exception(ErrorCodes::BAD_ARGUMENTS,
+        "parseBlobHashAlgo: unknown blob_hash config value '{}' (expected one of "
+        "cityhash128|xxh3-128|sha256)", config_value);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h
new file mode 100644
index 000000000000..cc557fd753e1
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h
@@ -0,0 +1,247 @@
+#pragma once
+#include <Common/Exception.h>
+#include <base/defines.h>
+#include <base/extended_types.h>
+#include <base/hex.h>
+#include <base/types.h>
+#include <fmt/format.h>
+#include <array>
+#include <compare>
+#include <cstdint>
+#include <cstring>
+#include <string_view>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+}
+
+namespace DB::Cas
+{
+
+/// Blob identity for the content-addressed pool: the hash-algorithm vocabulary (`BlobHashAlgo`),
+/// the digest value (`BlobDigest`), its width-aware representation converter (`DigestCodec`), and
+/// the complete identity pair (`BlobRef`). The streaming machinery that PRODUCES digests lives in
+/// `CasBlobHashingWriteBuffer.h`; this header is dependency-light on purpose — it is included by
+/// virtually every CAS translation unit through `CasTypes.h`.
+
+/// The content-address hash function selected for a blob pool. The numeric values are persisted as a
+/// byte in the binary source-edge run format and must remain stable; the textual name used in object
+/// paths and pool metadata is returned by `blobHashAlgoName`.
+///
+/// `CityHash128` and `XXH3_128` produce 16-byte digests, while `Sha256` produces a 32-byte digest.
+/// The digest representation and codec derive their width from this algorithm for each blob; there
+/// is no single pool-wide digest width when a pool admits more than one algorithm.
+enum class BlobHashAlgo : uint8_t
+{
+    CityHash128 = 1,
+    XXH3_128 = 2,
+    Sha256 = 3,
+};
+
+/// The blob PATH SEGMENT for `algo`, e.g. `<pool>/blobs/<algo>/<shard>/<hex>`: `"ch128"` | `"xxh3"` |
+/// `"sha256"`. Throws `BAD_ARGUMENTS` for an out-of-range enum value.
+std::string_view blobHashAlgoName(BlobHashAlgo algo);
+
+/// Returns the digest byte width for `algo`: 16 for `CityHash128` and `XXH3_128`, or 32 for
+/// `Sha256`. This is also the width used by `Cas::codecFor(algo)`'s `DigestCodec`; callers must
+/// derive it from the algorithm rather than from pool state. Throws `BAD_ARGUMENTS` for an
+/// out-of-range enum value, preserving the fail-closed contract of `blobHashAlgoName`.
+uint64_t blobHashLenFor(BlobHashAlgo algo);
+
+/// Parses the per-disk `blob_hash` CONFIG value: `"cityhash128"` | `"xxh3-128"` | `"sha256"`. Throws
+/// `BAD_ARGUMENTS` on any other value (fail-closed).
+BlobHashAlgo parseBlobHashAlgo(std::string_view config_value);
+
+/// A content digest whose width is selected by its hash algorithm. The fixed 32-byte big-endian
+/// storage accommodates the existing 128-bit algorithms (`cityHash128` and `xxh3-128`) and
+/// `sha256`; only the first `blobHashLenFor(algo)` bytes are meaningful and the remaining bytes
+/// must be zero. A fixed array avoids a per-manifest-entry allocation that a variable `String`
+/// would require for 32-byte digests.
+///
+/// This type is reserved for content hashes. Protocol identifiers such as `payload_digest`,
+/// `RunRef::checksum`, source-edge identifiers, lease owners, and cleanup shards remain
+/// `UInt128`, because widening the content digest does not change their separate wire or ordering
+/// contracts. A blob's complete identity is `BlobRef`, which pairs this digest with its algorithm.
+struct BlobDigest
+{
+    std::array<uint8_t, 32> bytes{};
+
+    auto operator<=>(const BlobDigest &) const = default;
+    bool operator==(const BlobDigest &) const = default;
+
+    /// Converts a 128-bit content hash to the common representation: big-endian in `bytes[0:16]`
+    /// and zero in the tail. This is the bridge for the 128-bit hash algorithms.
+    static BlobDigest fromU128(const UInt128 & v)
+    {
+        BlobDigest d;
+        for (int i = 0; i < 16; ++i)
+            d.bytes[static_cast<size_t>(i)] = static_cast<uint8_t>(static_cast<UInt8>(v >> (8 * (15 - i))));
+        return d;
+    }
+
+    /// Reads `bytes[0:16]` as big-endian into a `UInt128`. The conversion is meaningful only for a
+    /// 128-bit digest; the caller is responsible for selecting that width and the tail is ignored.
+    UInt128 toU128() const
+    {
+        UInt128 v = 0;
+        for (int i = 0; i < 16; ++i)
+            v = (v << 8) | static_cast<UInt8>(bytes[static_cast<size_t>(i)]);
+        return v;
+    }
+};
+
+/// Hasher for `BlobDigest` as an `unordered_map`/`unordered_set` key. This is an in-process hash
+/// table key, not a content address, so a cheap FNV-1a mix over the raw bytes is sufficient -- no
+/// cryptographic property is needed here.
+struct BlobDigestHash
+{
+    size_t operator()(const BlobDigest & d) const noexcept
+    {
+        size_t h = 1469598103934665603ull; /// FNV-1a 64-bit offset basis
+        for (uint8_t b : d.bytes)
+        {
+            h ^= b;
+            h *= 1099511628211ull; /// FNV-1a 64-bit prime
+        }
+        return h;
+    }
+};
+
+/// Converts a `BlobDigest` using one algorithm's width. A codec must be obtained from the algorithm
+/// through `codecFor`, never from a pool-wide width: a pool may contain multiple algorithms. Hex
+/// and raw-byte conversions accept and produce exactly the selected width. `shardOf` reads the
+/// first eight digest bytes in big-endian order, preserving the existing shard mapping for every
+/// 128-bit digest.
+class DigestCodec
+{
+public:
+    /// Creates a codec for a supported digest width: 16 bytes for the 128-bit algorithms or
+    /// 32 bytes for `sha256`. Any other width violates the per-algorithm representation invariant.
+    explicit DigestCodec(uint64_t digest_len_) : len(digest_len_)
+    {
+        chassert(len == 16 || len == 32, "DigestCodec: digest length must be 16 or 32 bytes");
+    }
+
+    /// Renders exactly `2 * len` lowercase hex chars.
+    String toHex(const BlobDigest & d) const
+    {
+        checkZeroTail(d, "toHex");
+        return hexString(d.bytes.data(), len);
+    }
+
+    /// Requires exactly `2 * len` hex chars; throws `BAD_ARGUMENTS` otherwise (wrong
+    /// width or a non-hex character). Zero-fills the tail beyond `len`.
+    BlobDigest fromHex(std::string_view hex) const
+    {
+        if (hex.size() != 2 * len)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS,
+                "DigestCodec::fromHex: expected {} hex chars for a {}-byte digest, got {}",
+                2 * len, len, hex.size());
+
+        for (char c : hex)
+        {
+            if (unhex(c) == 0xff)
+                throw Exception(ErrorCodes::BAD_ARGUMENTS, "DigestCodec::fromHex: invalid hex character '{}'", c);
+        }
+
+        BlobDigest d;
+        for (uint64_t i = 0; i < len; ++i)
+            d.bytes[i] = unhex2(hex.data() + i * 2);
+        return d;
+    }
+
+    /// Serializes exactly `len` raw bytes, big-endian (i.e. `bytes[0:len]`).
+    String toBytesBE(const BlobDigest & d) const
+    {
+        checkZeroTail(d, "toBytesBE");
+        return String(reinterpret_cast<const char *>(d.bytes.data()), len);
+    }
+
+    /// Requires exactly `len` bytes; throws `BAD_ARGUMENTS` otherwise. Zero-fills the
+    /// tail beyond `len`.
+    BlobDigest fromBytesBE(std::string_view b) const
+    {
+        if (b.size() != len)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS,
+                "DigestCodec::fromBytesBE: expected {} bytes for the pool's digest width, got {}", len, b.size());
+
+        BlobDigest d;
+        memcpy(d.bytes.data(), b.data(), len);
+        return d;
+    }
+
+    /// Returns the first eight digest bytes as a big-endian `uint64_t`. Keep this explicit rather
+    /// than using a native-endian `memcpy`: changing the byte order would silently remap shards on
+    /// little-endian hosts and break compatibility with the 128-bit hash mapping.
+    uint64_t shardOf(const BlobDigest & d) const
+    {
+        uint64_t v = 0;
+        for (int i = 0; i < 8; ++i)
+            v = (v << 8) | d.bytes[static_cast<size_t>(i)];
+        return v;
+    }
+
+private:
+    uint64_t len;
+
+    /// Checks the representation invariant that bytes beyond the selected width are zero. This is
+    /// a debug-only internal assertion; wrong-width external input is rejected by the decoding
+    /// methods above.
+    void checkZeroTail(const BlobDigest & d, [[maybe_unused]] const char * what) const
+    {
+        for (uint64_t i = len; i < d.bytes.size(); ++i)
+            chassert(d.bytes[i] == 0, fmt::format("DigestCodec::{}: non-zero byte at tail position {} (len={})", what, i, len));
+    }
+};
+
+/// The complete blob identity is the pair of hash algorithm and
+/// digest. A bare digest is NOT a blob identity anywhere -- `ch128` and `xxh3` digests are both
+/// 16-byte, so the same digest value under two algos names two DIFFERENT objects. BlobRef is
+/// constructed ONLY where algo and digest are born together (the write mint / the hasher) or read
+/// together (a durable form: settlement key, blob path, manifest entry, envelope). Every other
+/// site COPIES BlobRefs -- never assemble one from an algo and a digest obtained separately.
+struct BlobRef
+{
+    BlobHashAlgo algo = BlobHashAlgo::CityHash128;
+    BlobDigest digest{};
+
+    auto operator<=>(const BlobRef &) const = default;
+    bool operator==(const BlobRef &) const = default;
+};
+
+/// Hasher for unordered_map/unordered_set keys (in-process only, not a content address).
+struct BlobRefHash
+{
+    size_t operator()(const BlobRef & r) const noexcept
+    {
+        size_t h = BlobDigestHash{}(r.digest);
+        h ^= static_cast<size_t>(r.algo) + 0x9e3779b97f4a7c15ULL + (h << 6) + (h >> 2);
+        return h;
+    }
+};
+
+/// Returns the codec whose width belongs to `algo`; callers must not substitute a pool-wide width.
+inline DigestCodec codecFor(BlobHashAlgo algo)
+{
+    return DigestCodec(blobHashLenFor(algo));
+}
+
+/// Bare lowercase hex of the digest at the algo's width -- for OBJECT KEY construction only
+/// (the algo lives in the key's path segment `blobs/<algo>/...`).
+inline String blobHexOf(const BlobRef & r)
+{
+    return codecFor(r.algo).toHex(r.digest);
+}
+
+/// Human/log identity: "<algoName>:<hex>", e.g. "sha256:ab12...". Rendered ids must never be a
+/// bare hex (ambiguous across algos) -- events, inspect JSON and error messages use this.
+inline String blobIdOf(const BlobRef & r)
+{
+    return String(blobHashAlgoName(r.algo)) + ":" + blobHexOf(r);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.cpp
new file mode 100644
index 000000000000..b5c3d530e535
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.cpp
@@ -0,0 +1,260 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h>
+
+#include <IO/BufferWithOwnMemory.h>
+#include <IO/HashingReadBuffer.h>
+#include <IO/HashingWriteBuffer.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <Common/Exception.h>
+#include <base/hex.h>
+
+#include "config.h"
+
+#if USE_SSL
+#    include <Common/OpenSSLHelpers.h>
+#    include <openssl/evp.h>
+#endif
+
+/// `XXH_INLINE_ALL` renames every public symbol under the `XXH_INLINE_` prefix (`XXH_NAMESPACE`) and
+/// makes the whole library a header-only, static-inline implementation local to THIS translation
+/// unit -- no link dependency on the separately-compiled `ch_contrib::xxHash` object. This file is
+/// part of the `dbms` target (not `clickhouse_functions_obj`, which gets the flag via its own
+/// `target_link_libraries(... ch_contrib::xxHash)`), so the macro is defined locally here, same
+/// effect, same prefixed names as `Functions/FunctionsHashing.h` uses.
+/// xxHash is included through this wrapper (which marks it a system header) to suppress the vendored-C
+/// warnings from lz4's shadowing copy under `-Werror -Weverything`. See `CasXxh3Streamer.h`.
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasXxh3Streamer.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int CANNOT_ALLOCATE_MEMORY;
+    extern const int OPENSSL_ERROR;
+    extern const int SUPPORT_IS_DISABLED;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Thin adapter over the existing `HashingWriteBuffer` so `CityHash128` blob hashes stay
+/// byte-identical to today. Bytes written to `*this` alias directly into `hashing`'s own buffer
+/// (the same zero-copy trick `HashingWriteBuffer` itself uses against its nested sink), so this
+/// adds no extra copy and no change to the chunked `CityHash128WithSeed` chaining.
+class CityHash128BlobHashingWriteBuffer : public IBlobHashingWriteBuffer
+{
+public:
+    explicit CityHash128BlobHashingWriteBuffer(WriteBuffer & sink)
+        : IBlobHashingWriteBuffer()
+        , hashing(sink)
+    {
+        working_buffer = hashing.buffer();
+        pos = working_buffer.begin();
+    }
+
+    void sync() override
+    {
+        hashing.sync();
+    }
+
+    String getHashHex() override
+    {
+        next();
+        return getHexUIntLowercase(hashing.getHash());
+    }
+
+private:
+    void nextImpl() override
+    {
+        hashing.position() = pos;
+        hashing.next();
+        working_buffer = hashing.buffer();
+    }
+
+    void finalizeImpl() override
+    {
+        next();
+        hashing.finalize();
+    }
+
+    void cancelImpl() noexcept override
+    {
+        hashing.cancel();
+    }
+
+    HashingWriteBuffer hashing;
+};
+
+/// A hash-and-passthrough buffer over the xxhash library's streaming `XXH3_128bits` state. Unlike
+/// `CityHash128`, xxh3's streaming digest is defined to agree with its one-shot digest, so there is
+/// no chunked-convention to preserve -- this just needs to feed every byte to the streaming state
+/// (`update`) and forward the same bytes to `sink` unchanged.
+class Xxh3128BlobHashingWriteBuffer : public BufferWithOwnMemory<IBlobHashingWriteBuffer>
+{
+public:
+    explicit Xxh3128BlobHashingWriteBuffer(WriteBuffer & sink_, size_t buf_size = DBMS_DEFAULT_HASHING_BLOCK_SIZE)
+        : BufferWithOwnMemory<IBlobHashingWriteBuffer>(buf_size)
+        , sink(sink_)
+    {
+        if (!state.valid())
+            throw Exception(ErrorCodes::CANNOT_ALLOCATE_MEMORY, "Xxh3128BlobHashingWriteBuffer: failed to allocate the xxh3 streaming state");
+    }
+
+    void sync() override
+    {
+        sink.sync();
+    }
+
+    String getHashHex() override
+    {
+        next();
+        UInt64 low = 0;
+        UInt64 high = 0;
+        state.digest(low, high);
+        return getHexUIntLowercase(UInt128{low, high});
+    }
+
+private:
+    void nextImpl() override
+    {
+        const size_t len = offset();
+        if (!len)
+            return;
+
+        state.update(working_buffer.begin(), len);
+        sink.write(working_buffer.begin(), len);
+    }
+
+    WriteBuffer & sink;
+    Xxh3Streamer state;
+};
+
+#if USE_SSL
+/// A hash-and-passthrough buffer over OpenSSL's streaming EVP SHA-256 digest. Unlike the 128-bit
+/// hashes above, `Sha256` produces a 32-byte digest (64 lowercase hex chars, see `blobHashLenFor`).
+/// Every byte written is folded into the running EVP digest (`EVP_DigestUpdate`) AND forwarded
+/// unchanged to `sink`, exactly like `Xxh3128BlobHashingWriteBuffer` above -- streaming SHA-256 is
+/// defined to agree with the one-shot digest, so there is no chunked-convention to preserve either.
+class Sha256BlobHashingWriteBuffer : public BufferWithOwnMemory<IBlobHashingWriteBuffer>
+{
+public:
+    explicit Sha256BlobHashingWriteBuffer(WriteBuffer & sink_, size_t buf_size = DBMS_DEFAULT_HASHING_BLOCK_SIZE)
+        : BufferWithOwnMemory<IBlobHashingWriteBuffer>(buf_size)
+        , sink(sink_)
+        , ctx(EVP_MD_CTX_new(), &EVP_MD_CTX_free)
+    {
+        if (!ctx)
+            throw Exception(ErrorCodes::OPENSSL_ERROR,
+                "Sha256BlobHashingWriteBuffer: EVP_MD_CTX_new failed: {}", getOpenSSLErrors());
+
+        if (EVP_DigestInit_ex(ctx.get(), EVP_sha256(), nullptr) != 1)
+            throw Exception(ErrorCodes::OPENSSL_ERROR,
+                "Sha256BlobHashingWriteBuffer: EVP_DigestInit_ex failed: {}", getOpenSSLErrors());
+    }
+
+    void sync() override
+    {
+        sink.sync();
+    }
+
+    String getHashHex() override
+    {
+        next();
+
+        unsigned char digest[EVP_MAX_MD_SIZE];
+        unsigned int digest_len = 0;
+        if (EVP_DigestFinal_ex(ctx.get(), digest, &digest_len) != 1)
+            throw Exception(ErrorCodes::OPENSSL_ERROR,
+                "Sha256BlobHashingWriteBuffer: EVP_DigestFinal_ex failed: {}", getOpenSSLErrors());
+
+        chassert(digest_len == 32);
+        return hexString(digest, digest_len);
+    }
+
+private:
+    using EVP_MD_CTX_ptr = std::unique_ptr<EVP_MD_CTX, decltype(&EVP_MD_CTX_free)>;
+
+    void nextImpl() override
+    {
+        const size_t len = offset();
+        if (!len)
+            return;
+
+        if (EVP_DigestUpdate(ctx.get(), working_buffer.begin(), len) != 1)
+            throw Exception(ErrorCodes::OPENSSL_ERROR,
+                "Sha256BlobHashingWriteBuffer: EVP_DigestUpdate failed: {}", getOpenSSLErrors());
+
+        sink.write(working_buffer.begin(), len);
+    }
+
+    WriteBuffer & sink;
+    EVP_MD_CTX_ptr ctx;
+};
+#endif
+
+}
+
+std::unique_ptr<IBlobHashingWriteBuffer> makeBlobHashingWriteBuffer(BlobHashAlgo algo, WriteBuffer & sink)
+{
+    switch (algo)
+    {
+        case BlobHashAlgo::CityHash128:
+            return std::make_unique<CityHash128BlobHashingWriteBuffer>(sink);
+        case BlobHashAlgo::XXH3_128:
+            return std::make_unique<Xxh3128BlobHashingWriteBuffer>(sink);
+        case BlobHashAlgo::Sha256:
+#if USE_SSL
+            return std::make_unique<Sha256BlobHashingWriteBuffer>(sink);
+#else
+            throw Exception(ErrorCodes::SUPPORT_IS_DISABLED,
+                "blob_hash = 'sha256' requires ClickHouse built with SSL support");
+#endif
+    }
+    throw Exception(ErrorCodes::BAD_ARGUMENTS, "makeBlobHashingWriteBuffer: unknown BlobHashAlgo {}", static_cast<int>(algo));
+}
+
+String blobHashHexOneShot(BlobHashAlgo algo, std::string_view bytes)
+{
+    switch (algo)
+    {
+        case BlobHashAlgo::CityHash128:
+        {
+            /// Preserve the blob content-hash convention used by `poolContentHash`: hash through
+            /// `HashingReadBuffer` in `DBMS_DEFAULT_HASHING_BLOCK_SIZE` chunks, chaining
+            /// `CityHash128WithSeed`. A one-shot `CityHash128WithSeed` call would produce a different
+            /// result for payloads larger than one hash block.
+            ReadBufferFromMemory in(bytes.data(), bytes.size());
+            HashingReadBuffer hashing(in);
+            hashing.ignoreAll();
+            return getHexUIntLowercase(hashing.getHash());
+        }
+        case BlobHashAlgo::XXH3_128:
+        {
+            UInt64 low = 0;
+            UInt64 high = 0;
+            xxh3_128_oneshot(bytes.data(), bytes.size(), low, high);
+            return getHexUIntLowercase(UInt128{low, high});
+        }
+        case BlobHashAlgo::Sha256:
+        {
+#if USE_SSL
+            /// One-shot SHA-256 is defined to agree with the streaming EVP digest above (there is no
+            /// chunked convention to preserve, unlike `CityHash128`), so this can go straight through
+            /// `encodeSHA256`'s one-shot path instead of round-tripping through a streaming buffer.
+            unsigned char digest[32];
+            encodeSHA256(bytes.data(), bytes.size(), digest);
+            return hexString(digest, sizeof(digest));
+#else
+            throw Exception(ErrorCodes::SUPPORT_IS_DISABLED,
+                "blob_hash = 'sha256' requires ClickHouse built with SSL support");
+#endif
+        }
+    }
+    throw Exception(ErrorCodes::BAD_ARGUMENTS, "blobHashHexOneShot: unknown BlobHashAlgo {}", static_cast<int>(algo));
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h
new file mode 100644
index 000000000000..0779363a2f54
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h
@@ -0,0 +1,50 @@
+#pragma once
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <IO/WriteBuffer.h>
+#include <base/types.h>
+#include <memory>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// A streaming hash-and-passthrough `WriteBuffer`: every byte written is forwarded unchanged to the
+/// nested sink AND folded into the running digest, exposed as lowercase hex once flushed (32 hex
+/// chars for the 128-bit hashes, 64 hex chars for `Sha256` -- see `blobHashLenFor`) (`getHashHex`
+/// calls `next()` first, mirroring `HashingWriteBuffer::getHash`). It does not finalize or cancel the
+/// underlying sink -- that stays the caller's responsibility, the same contract `CaContentWriteBuffer`
+/// already relies on for `HashingWriteBuffer` and its nested sink.
+class IBlobHashingWriteBuffer : public WriteBuffer
+{
+public:
+    explicit IBlobHashingWriteBuffer(Position ptr = nullptr, size_t size = 0) : WriteBuffer(ptr, size) {}
+    ~IBlobHashingWriteBuffer() override = default;
+
+    /// Flushes any pending bytes (like `HashingWriteBuffer::getHash`) and returns the digest as
+    /// lowercase hex (length depends on `algo`, see `blobHashLenFor`). May be called only once useful
+    /// data has stopped flowing; does not itself finalize the buffer (mirrors
+    /// `HashingWriteBuffer::getHash`, which also does not finalize).
+    virtual String getHashHex() = 0;
+};
+
+/// Builds a streaming hash-and-passthrough buffer for `algo` over `sink`. Every byte written to the
+/// returned buffer is forwarded unchanged to `sink` and included in its digest. `sink` must outlive
+/// the returned buffer, as with `HashingWriteBuffer`.
+///
+/// `CityHash128` is a thin adapter over the existing `HashingWriteBuffer`: it retains the
+/// `DBMS_DEFAULT_HASHING_BLOCK_SIZE` chunks and chained `CityHash128WithSeed` convention, so the
+/// default algorithm produces byte-identical blob hashes. `XXH3_128` uses the xxhash library's
+/// streaming `XXH3_128bits` state, and `Sha256` uses OpenSSL's streaming EVP SHA-256 digest. Both
+/// of those algorithms define their streaming digest to agree with their one-shot digest.
+std::unique_ptr<IBlobHashingWriteBuffer> makeBlobHashingWriteBuffer(BlobHashAlgo algo, WriteBuffer & sink);
+
+/// Computes the lowercase-hex digest of `bytes` for `algo`. This is used by the re-hash and
+/// copy-forward path and by tests. For `CityHash128`, the implementation deliberately uses
+/// `HashingReadBuffer` so it follows the same chunked, chained convention as the streaming write
+/// path; a one-shot `CityHash128WithSeed` call would diverge for payloads larger than one hash block.
+/// The one-shot `XXH3_128bits` and OpenSSL `encodeSHA256` paths agree with their streaming
+/// counterparts.
+String blobHashHexOneShot(BlobHashAlgo algo, std::string_view bytes);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h
new file mode 100644
index 000000000000..53f14a120174
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h
@@ -0,0 +1,118 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <IO/ReadBuffer.h>
+#include <Common/Exception.h>
+#include <string_view>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+/// Shared low-level byte-encoding helpers for CAS: the big-endian `UInt128` wire form, exact reads,
+/// and validation of identifiers embedded in persisted data. They remain independent of any
+/// particular object format.
+
+/// On-disk UInt128 wire form: the 16-byte big-endian representation used by raw CAS byte fields and
+/// key components. It is FROZEN — changing the bytes breaks every object already written. (The
+/// lowercase-hex form lives in `CasTypes.h` as `u128ToHex` / `hexToU128` and is out of scope here.)
+///
+/// Converts `v` to the frozen 16-byte big-endian representation used in raw byte fields and keys.
+inline std::string u128ToBytesBE(const UInt128 & v)
+{
+    std::string out(16, '\0');
+    for (int i = 0; i < 16; ++i)
+        out[i] = static_cast<char>(static_cast<UInt8>(v >> (8 * (15 - i))));
+    return out;
+}
+
+/// Parses a frozen 16-byte big-endian value. `what` identifies the containing field in corruption
+/// diagnostics; any other length is malformed persisted data and raises `CORRUPTED_DATA`.
+inline UInt128 u128FromBytesBE(const std::string & b, std::string_view what)
+{
+    if (b.size() != 16)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: big-endian UInt128 field must be 16 bytes, got {}", what, b.size());
+    UInt128 v = 0;
+    for (int i = 0; i < 16; ++i)
+        v = (v << 8) | static_cast<UInt8>(b[i]);
+    return v;
+}
+
+/// Read exactly `n` raw bytes. The bounds check MUST precede the allocation: `n` typically comes
+/// from a length field just read off the wire, so on corrupted input it can be huge (a u32 field
+/// admits 4 GiB) — allocating first would mean a multi-GiB transient allocation, which under a
+/// memory tracker surfaces as MEMORY_LIMIT_EXCEEDED instead of the pinned CORRUPTED_DATA.
+/// Comparing against `available` as the exact remainder is valid because all CAS codec decoding
+/// reads from `ReadBufferFromMemory`: the whole object is in memory, so `available` is exactly
+/// the number of bytes left.
+/// Throws `CORRUPTED_DATA` before allocating when the encoded object cannot contain `n` bytes.
+inline String readFixedBytes(ReadBuffer & in, size_t n)
+{
+    if (n > in.available())
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS codec: truncated encoded data: need {} bytes, {} available", n, in.available());
+    String s(n, '\0');
+    in.readStrict(s.data(), n);
+    return s;
+}
+
+/// Canonical clean relative path for ref/file names: non-empty, no NUL byte, no backslash, and no
+/// segment that is empty (rejects a leading/trailing/doubled '/'), ".", or "..". Names in this
+/// family originate from part names -- a NUL byte is never legitimate there, so it fails closed
+/// rather than being silently truncated or passed through.
+/// Returns true only for a non-empty relative path whose slash-separated components are all normal
+/// names; it does not normalize or rewrite the input.
+inline bool isCanonicalRefName(std::string_view name)
+{
+    if (name.empty() || name.find('\0') != std::string_view::npos || name.find('\\') != std::string_view::npos)
+        return false;
+    size_t start = 0;
+    while (true)
+    {
+        const size_t end = name.find('/', start);
+        const std::string_view segment
+            = name.substr(start, end == std::string_view::npos ? std::string_view::npos : end - start);
+        if (segment.empty() || segment == "." || segment == "..")
+            return false;
+        if (end == std::string_view::npos)
+            break;
+        start = end + 1;
+    }
+    return true;
+}
+
+/// Throws CORRUPTED_DATA naming both `caller` (the codec, e.g. "RefLogTxn") and `what` (the field,
+/// e.g. "set_published_at ref_name") when `name` fails `isCanonicalRefName`.
+/// On success, the input is unchanged; on failure, no partial normalization is attempted.
+inline void checkCanonicalRefName(std::string_view name, std::string_view caller, std::string_view what)
+{
+    if (!isCanonicalRefName(name))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "{}: {} is not a canonical clean relative path: '{}'", caller, what, name);
+}
+
+/// `ManifestRef` field validity, shared by the ref codecs (`CasRefLogFormat`,
+/// `CasRefSnapshotFormat`): `writer_epoch`/`build_sequence` nonzero, `manifest_ordinal` in
+/// `[1, kMaxManifestOrdinal]` -- the same range `manifestOrdinalFileName` (`CasManifestId.h`) enforces
+/// at key-construction time. Throws CORRUPTED_DATA naming both `caller` (the codec) and `what` (the
+/// field, e.g. "set_published_at manifest_ref").
+/// This keeps the value-level invariant aligned with the range enforced by manifest-key construction
+/// before either a codec encoder or decoder accepts the reference.
+inline void checkManifestRef(const ManifestRef & ref, std::string_view caller, std::string_view what)
+{
+    if (ref.writer_epoch == 0 || ref.build_sequence == 0)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "{}: {} manifest_ref writer_epoch/build_sequence must both be nonzero, got {}-{}",
+            caller, what, ref.writer_epoch, ref.build_sequence);
+    if (ref.manifest_ordinal == 0 || ref.manifest_ordinal > kMaxManifestOrdinal)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "{}: {} manifest_ref manifest_ordinal {} out of range", caller, what, ref.manifest_ordinal);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.cpp
new file mode 100644
index 000000000000..a981d1cd0575
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.cpp
@@ -0,0 +1,95 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Common/Exception.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+String toString(CasEventType type)
+{
+    switch (type)
+    {
+        case CasEventType::BlobPut:               return "blob_put";
+        case CasEventType::BlobReuseAdopt:        return "blob_reuse_adopt";
+        case CasEventType::BlobReuseResurrect:    return "blob_reuse_resurrect";
+        case CasEventType::BlobRetire:            return "blob_retire";
+        case CasEventType::BlobRetireReplaced:    return "blob_retire_replaced";
+        case CasEventType::BlobDelete:            return "blob_delete";
+        case CasEventType::BlobForget:            return "blob_forget";
+        case CasEventType::ManifestPut:           return "manifest_put";
+        case CasEventType::ManifestDelete:        return "manifest_delete";
+        case CasEventType::RefPublish:            return "ref_publish";
+        case CasEventType::RefDrop:               return "ref_drop";
+        case CasEventType::RefRepoint:            return "ref_repoint";
+        case CasEventType::RootAdd:               return "root_add";
+        case CasEventType::RootRemove:            return "root_remove";
+        case CasEventType::RootRepoint:           return "root_repoint";
+        case CasEventType::IndegZero:             return "indegree_zero";
+        case CasEventType::GcFoldBegin:           return "gc_fold_begin";
+        case CasEventType::GcFoldEnd:             return "gc_fold_end";
+        case CasEventType::GcRetireObserve:       return "gc_retire_observe";
+        case CasEventType::GcRetireDecision:      return "gc_retire_decision";
+        case CasEventType::GcRecheckVerdict:      return "gc_recheck_verdict";
+        case CasEventType::GcFence:               return "gc_fence";
+        case CasEventType::GcCursorAdvance:       return "gc_cursor_advance";
+        case CasEventType::GcShardReclaim:        return "gc_shard_reclaim";
+        case CasEventType::GcFenceOut:            return "gc_fence_out";
+        case CasEventType::GcRebuild:             return "gc_rebuild";
+        case CasEventType::GcFoldClamp:           return "gc_fold_clamp";
+        case CasEventType::GcAnomaly:             return "gc_anomaly";
+        case CasEventType::GcLeaseAcquire:        return "gc_lease_acquire";
+        case CasEventType::GcLeaseSteal:          return "gc_lease_steal";
+        case CasEventType::GcLeaseHeartbeat:      return "gc_lease_heartbeat";
+        case CasEventType::BuildStart:            return "build_start";
+        case CasEventType::BuildPublish:          return "build_publish";
+        case CasEventType::BuildAbort:            return "build_abort";
+        case CasEventType::Precommit:             return "precommit";
+        case CasEventType::PrecommitRemoved:      return "precommit_removed";
+        case CasEventType::PrecommitReclaim:      return "precommit_reclaim";
+        case CasEventType::GateRevalidate:        return "gate_revalidate";
+        case CasEventType::GateResurrect:         return "gate_resurrect";
+        case CasEventType::WatermarkRenew:        return "watermark_renew";
+        case CasEventType::MountRemount:          return "mount_remount";
+        case CasEventType::MountClaim:            return "mount_claim";
+        case CasEventType::MountRelease:          return "mount_release";
+        case CasEventType::MountConflict:         return "mount_conflict";
+        case CasEventType::MemberDecommission:    return "member_decommission";
+        case CasEventType::ForeignInterference:   return "foreign_interference";
+        case CasEventType::RefResolve:            return "ref_resolve";
+        case CasEventType::ReadMissing:           return "read_missing";
+        case CasEventType::DanglingAccess:        return "dangling_access";
+        case CasEventType::CorruptDangle:         return "corrupt_dangle";
+        case CasEventType::CorruptDecode:         return "corrupt_decode";
+        case CasEventType::SnapJournalIncoherent: return "snap_journal_incoherent";
+        case CasEventType::Exception:             return "exception";
+    }
+    throw DB::Exception(
+        DB::ErrorCodes::LOGICAL_ERROR,
+        "CasEvent: unknown CasEventType value {}",
+        static_cast<int>(type));
+}
+
+String toString(CasEventObjectKind kind)
+{
+    switch (kind)
+    {
+        case CasEventObjectKind::None: return "none";
+        case CasEventObjectKind::Blob: return "blob";
+        case CasEventObjectKind::Manifest: return "manifest";
+        case CasEventObjectKind::Root: return "root";
+        case CasEventObjectKind::Snap: return "snapshot";
+    }
+    throw DB::Exception(
+        DB::ErrorCodes::LOGICAL_ERROR,
+        "CasEvent: unknown CasEventObjectKind value {}",
+        static_cast<int>(kind));
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h
new file mode 100644
index 000000000000..752ac5cf3f84
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h
@@ -0,0 +1,126 @@
+#pragma once
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <functional>
+#include <map>
+#include <utility>
+
+namespace DB::Cas
+{
+
+/// Names the append-only audit events emitted by content-addressed storage. The metadata storage
+/// converts each `CasEvent` into a `ContentAddressedLogElement` and forwards it to the `SystemLog`;
+/// this layer deliberately keeps only pure data so the core and its unit tests do not depend on
+/// system-log machinery. The event log reconstructs each entity's lifetime, so this taxonomy
+/// includes state-changing decisions, GC transitions, and errors or anomalies rather than only
+/// successful user-visible operations.
+enum class CasEventType
+{
+    BlobPut, BlobReuseAdopt, BlobReuseResurrect, BlobRetire, BlobRetireReplaced, BlobDelete, BlobForget,
+    ManifestPut, ManifestDelete,
+    RefPublish, RefDrop, RefRepoint, RootAdd, RootRemove, RootRepoint, IndegZero,
+    GcFoldBegin, GcFoldEnd, GcRetireObserve, GcRetireDecision, GcRecheckVerdict,
+    GcFence, GcCursorAdvance, GcShardReclaim, GcFenceOut, GcRebuild, GcFoldClamp,
+    /// One GC round anomaly, with the context of the decision it forced. `gc_fold_end` reports only a
+    /// COUNT of anomalies, which cannot even distinguish a ref-prefix enumeration disagreement from an
+    /// undecodable ref-log body -- so every investigation had to fall back on the rotated text log, which
+    /// is how a masked permission error once produced a confident "no occurrences". An anomaly aborts ref
+    /// folding; that decision deserves a queryable record, not a counter.
+    GcAnomaly,
+    GcLeaseAcquire, GcLeaseSteal, GcLeaseHeartbeat,
+    BuildStart, BuildPublish, BuildAbort, Precommit, PrecommitRemoved, PrecommitReclaim,
+    GateRevalidate, GateResurrect, WatermarkRenew, MountRemount,
+    MountClaim, MountRelease, MountConflict,
+    /// Operator-driven erasure of a dead pool member's namespace. `decommissionPoolMember` runs as
+    /// a writer, never as GC: it claims the member's mount, drains its namespaces and debris, and
+    /// deletes the mount slot only after the drain is confirmed. The slot is the interrupted-operation
+    /// resume anchor, so a failed drain leaves it terminated for a later retry. `outcome` is one of
+    /// "begin", "namespace_removed", or "end".
+    MemberDecommission,
+    /// Incidental-detection reaction to foreign bytes at a ref-log wedge key owned by this mount.
+    /// The mount lease makes the key exclusive, so this is impossible under legitimate
+    /// single-writer operation and indicates that the wedge hard contract was violated. The
+    /// reaction records the anomaly and fails the local write path closed; it does not treat the
+    /// foreign bytes as valid ref-log state.
+    ForeignInterference,
+    RefResolve, ReadMissing, DanglingAccess,
+    CorruptDangle, CorruptDecode, SnapJournalIncoherent, Exception,
+};
+
+/// Identifies the kind of object described by a `CasEvent`. `None` is used for events about a
+/// protocol action, mount, or anomaly that is not tied to one stored object.
+enum class CasEventObjectKind { None, Blob, Manifest, Root, Snap };
+
+/// Pure-data event passed from the content-addressed core to the metadata-storage audit-log sink.
+/// Fields that do not apply to an event remain empty or zero. `reason` is mandatory for decisions
+/// and must explain why the operation took its outcome; `detail` carries structured facts needed
+/// to reconstruct the event without parsing the free-form reason. Hashes are lowercase hexadecimal,
+/// tokens identify object incarnations, and the numeric fields identify GC rounds, snapshot
+/// generations, or the manifest journal version as applicable.
+struct CasEvent
+{
+    CasEventType type = CasEventType::BlobPut;
+    String namespace_;          /// roots/<ns> (empty if N/A)
+    String ref_name;            /// the ref name — a mutable directory handle, git-style (empty if N/A)
+    CasEventObjectKind object_kind = CasEventObjectKind::None;
+    String object_hash;         /// lowercase hex (empty if N/A)
+    String token;               /// incarnation token (empty if N/A)
+    UInt64 round = 0;
+    UInt64 gen = 0;
+    UInt64 at_version = 0;
+    String outcome;             /// e.g. "ok","adopt","deleted","zeroed" (empty if N/A)
+    String reason;              /// REQUIRED: the human-readable WHY of the decision
+    std::map<String, String> detail;
+};
+
+/// Receives events by value so emission sites can move the complete record, including its `detail`
+/// map, into the sink instead of deep-copying it on the emitter thread. Emission sites pass an rvalue
+/// for a completed event; the sink consumes that event while converting it to the system-log row.
+using CasEventSink = std::function<void(CasEvent)>;
+
+/// Builds and emits a `CasEvent` for a store that owns the event sink. The builder supplies the
+/// per-event fields; the emitter supplies the sink owner shared by all events from that store.
+///
+/// If the store has no sink, the builder is not invoked and no event is constructed; the disabled
+/// path is therefore only the sink-presence check. Otherwise, `emit` moves the completed event into
+/// `emitEvent`, preserving the fields supplied by the builder without adding a copy. This class is
+/// templated to avoid a header-layering cycle (`CasPool.h` includes this header); any `S` exposing
+/// `hasEventSink` and `emitEvent` with the expected contracts can be used.
+template <typename S>
+class EventEmitter
+{
+public:
+    /// Keeps a reference to the store; the store must outlive this short-lived emitter.
+    explicit EventEmitter(const S & store_) : store(store_) {}
+
+    template <typename Builder>
+    /// Invokes `build` only when the store has an enabled sink, then moves the resulting event into
+    /// the store. `Builder` must accept `CasEvent &` and may populate any applicable fields; any
+    /// exception from the builder or the store is propagated to the caller.
+    void emit(Builder && build) const
+    {
+        if (!store.hasEventSink())
+            return;
+        CasEvent event;
+        build(event);
+        store.emitEvent(std::move(event));
+    }
+
+private:
+    const S & store;
+};
+
+template <typename S>
+EventEmitter(const S &) -> EventEmitter<S>;
+
+/// Converts an event taxonomy value to the stable snake_case name stored in the `SystemLog`
+/// `event_type` column. Every enumerator must have a mapping because these names are queried by
+/// users and are part of the audit-log schema; an unknown value raises a logical-error exception.
+String toString(CasEventType type);
+
+/// Converts an object-kind value to the stable snake_case name stored in the `SystemLog`
+/// `object_kind` column. An unknown value raises a logical-error exception rather than silently
+/// producing an unrecognized schema value.
+String toString(CasEventObjectKind kind);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasNamespaceLifeId.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasNamespaceLifeId.h
new file mode 100644
index 000000000000..c2bab7f2bf8a
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasNamespaceLifeId.h
@@ -0,0 +1,116 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/Exception.h>
+#include <base/types.h>
+#include <optional>
+#include <string_view>
+#include <utility>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+/// Opaque pool-wide physical identity of one namespace life. The catalog's existing `incarnation`
+/// field is this value; the alias makes listed-key APIs explicit without renaming catalog wire data.
+using NamespaceLifePhysicalId = UInt128;
+
+/// Builds the key segment for `incarnation`: 32 fixed-width lower-case hex digits, so the segment has
+/// one canonical spelling and `<life_id>/` sorts stably. Throws `LOGICAL_ERROR` on a zero incarnation
+/// for the same reason `renderRefTxnId` does: this render becomes an object key, and an invalid
+/// identity must never silently produce a well-formed-looking one.
+inline String renderIncarnation(const UInt128 & incarnation)
+{
+    if (incarnation == 0)
+        throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR,
+            "NamespaceLifeId: incarnation must be nonzero -- 0 never names a life");
+    return u128ToHex(incarnation);
+}
+
+/// Inverse of `renderIncarnation` for ONE listed path segment. Accepts the canonical form only:
+/// exactly 32 lower-case hex digits encoding a nonzero value. Upper case, a short or long segment,
+/// non-hex characters and an all-zero segment all return `std::nullopt`; the CALLER decides whether
+/// that is "not one of our keys" or corruption, because only the caller knows whether the rest of the
+/// key already identified the object as ours.
+inline std::optional<UInt128> parseIncarnation(std::string_view s)
+{
+    constexpr size_t kHexLen = 32;
+    if (s.size() != kHexLen)
+        return std::nullopt;
+
+    /// `unhexUInt` also accepts upper case, which the canonical form must reject, so the digits are
+    /// validated by hand first and only then handed to it.
+    for (char c : s)
+    {
+        const bool canonical_digit = (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f');
+        if (!canonical_digit)
+            return std::nullopt;
+    }
+
+    /// `unhexUInt` reads exactly sizeof(UInt128)*2 == kHexLen bytes from the pointer, already
+    /// validated above; it needs no NUL terminator.
+    // NOLINTNEXTLINE(bugprone-suspicious-stringview-data-usage)
+    const UInt128 value = unhexUInt<UInt128>(s.data());
+    if (value == 0)
+        return std::nullopt;
+    return value;
+}
+
+class Layout;
+
+/// The logical and physical identity of ONE LIFE of a namespace: the catalog name plus its opaque,
+/// pool-wide incarnation. Physical life-owned keys use only `incarnation`; the name remains the
+/// catalog authority used after a listed physical id is joined through one immutable catalog cut.
+///
+/// Part manifests and loose mountpoint objects are deliberately NOT qualified (Constraint 12,
+/// directive §2): manifests already carry globally unique build identities, and a loose mountpoint
+/// object is outside namespace ownership altogether.
+///
+/// There is deliberately NO conversion from a bare `RootNamespace`, none to one, and no default
+/// construction: code holding only the name cannot name a ref object or a namespace file at all, so
+/// losing the incarnation is a compile error rather than a runtime aliasing bug (spec §2 r9-3). The
+/// call sites that legitimately need the bare name -- manifest identities, loose mountpoint objects --
+/// say `.ns`, and are visible in review because they say it.
+///
+/// Permanent logical/physical pairs come only from `fromCatalogEntry` (recovery, fold, fsck and the
+/// sweeps). A held reader (e.g. `RefTableRuntime`) keeps the `NamespaceLifeId` it resolved this way
+/// at table-open time and threads it through, rather than re-deriving it. `Layout` parsers
+/// deliberately return only an untrusted `NamespaceLifePhysicalId`; a listed key can name any
+/// physical id, including one no longer in the catalog, and only one immutable catalog cut may
+/// attach a logical name to it.
+struct NamespaceLifeId
+{
+    RootNamespace ns;
+    NamespaceLifePhysicalId incarnation;   /// 0 is INVALID -- never a wildcard, never "any life"
+
+    bool operator==(const NamespaceLifeId &) const = default;
+
+    /// The catalog is the universe authority (INV-3): a discovery path learns of a namespace ONLY
+    /// from a catalog entry, and takes BOTH fields from that same entry. Pairing a namespace with an
+    /// incarnation obtained from anywhere else re-opens the rebirth alias this type exists to close.
+    static NamespaceLifeId fromCatalogEntry(RootNamespace ns, const UInt128 & incarnation)
+    {
+        return NamespaceLifeId{std::move(ns), incarnation};
+    }
+
+private:
+    /// Kept private so only the explicit factories above can construct a logical/physical pair.
+    friend class Layout;
+
+    NamespaceLifeId(RootNamespace ns_, const UInt128 & incarnation_)
+        : ns(std::move(ns_)), incarnation(incarnation_)
+    {
+        if (incarnation_ == 0)
+            throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR,
+                "NamespaceLifeId: incarnation must be nonzero for namespace '{}' -- 0 never names a life",
+                ns.string());
+    }
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h
new file mode 100644
index 000000000000..84f24dba3f73
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h
@@ -0,0 +1,331 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <base/defines.h>
+#include <base/extended_types.h>
+#include <base/hex.h>
+#include <base/types.h>
+#include <Common/Exception.h>
+#include <array>
+#include <charconv>
+#include <compare>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <functional>
+#include <fmt/format.h>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <tuple>
+#include <utility>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+/// Strongly typed value types and codecs shared by the content-addressed metadata and object paths.
+///
+/// These types deliberately keep protocol identity, content digests, backend tokens, and their
+/// textual forms distinct. Their ordering and equality operators make them usable as ordered keys;
+/// hash specializations below support unordered containers. Conversion to a storage key or wire
+/// representation remains explicit at the boundary that owns that representation.
+///
+/// Opaque namespace under which root manifests live. The core never interprets its contents:
+/// the wiring composes strings like "srv1/<table_uuid>" or "shadow/<backup>/<table_uuid>".
+/// Layout only validates its shape (non-empty, no leading/trailing or empty segments,
+/// no segment equal to the reserved "_files").
+/// Construction from `String` is explicit, preventing unrelated identifier types from being mixed;
+/// the underlying string is exposed only through `string` at object-storage boundaries.
+class RootNamespace
+{
+public:
+    RootNamespace() = default;
+    explicit RootNamespace(String value_) : value(std::move(value_)) {}
+    const String & string() const { return value; }
+    auto operator<=>(const RootNamespace &) const = default;
+    bool operator==(const RootNamespace &) const = default;
+
+private:
+    String value;
+};
+
+/// Returns 32 lowercase hex chars encoding `v`.
+inline String u128ToHex(const UInt128 & v)
+{
+    return getHexUIntLowercase(v);
+}
+
+/// Parses 32-char lowercase (or uppercase) hex string to UInt128.
+/// Throws BAD_ARGUMENTS on wrong length or non-hex characters.
+inline UInt128 hexToU128(const String & hex)
+{
+    if (hex.size() != 32)
+        throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+            "hexToU128: expected 32 hex chars, got {}", hex.size());
+
+    // Validate each character is a valid hex digit (unhex(c) returns 0xff for invalid chars).
+    for (char c : hex)
+    {
+        if (::unhex(c) == 0xff)
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "hexToU128: invalid hex character '{}'", c);
+    }
+
+    return unhexUInt<UInt128>(hex.data());
+}
+
+}
+
+namespace std
+{
+
+template <>
+struct hash<DB::Cas::RootNamespace>
+{
+    size_t operator()(const DB::Cas::RootNamespace & value) const noexcept
+    {
+        return std::hash<std::string>{}(value.string());
+    }
+};
+
+}
+
+namespace DB::Cas
+{
+
+/// The compact reference a root journal stores for a part manifest. It is not a string key:
+/// `CasLayout::manifestKey` derives the object key from this reference and the owning namespace.
+/// The namespace is deliberately absent because it comes from the owning root context and must not
+/// be serialized into the journal reference.
+///
+///   writer_epoch         - durable monotone writer epoch allocated under the mounted `server_root_id`;
+///                          never reused for that server root.
+///   build_sequence       - monotone build sequence inside one writer incarnation; part of identity
+///                          and of the build-scoped debris prefix.
+///   manifest_ordinal     - monotone ordinal inside one build, rendered as `000001.zst` in the key.
+struct ManifestRef
+{
+    uint64_t writer_epoch = 0;
+    uint64_t build_sequence = 0;
+    uint32_t manifest_ordinal = 0;
+
+    bool operator==(const ManifestRef & o) const = default;
+
+    /// Total order for std::map / std::set keys. Field order is arbitrary but stable.
+    bool operator<(const ManifestRef & o) const
+    {
+        return std::tie(writer_epoch, build_sequence, manifest_ordinal)
+             < std::tie(o.writer_epoch, o.build_sequence, o.manifest_ordinal);
+    }
+};
+
+/// The namespace-qualified protocol identity used by GC for source edges, blob deltas, cleanup work,
+/// and addressing. It is the pair `(root_namespace, ManifestRef)`.
+/// Two namespaces may legally carry the same `ManifestRef` tuple without addressing the same object;
+/// therefore those structures must use `ManifestId`, never `ManifestRef` alone.
+struct ManifestId
+{
+    RootNamespace root_namespace;   /// owning namespace; NOT part of the journal ref
+    ManifestRef ref;
+
+    bool operator==(const ManifestId & o) const = default;
+
+    bool operator<(const ManifestId & o) const
+    {
+        if (root_namespace.string() != o.root_namespace.string())
+            return root_namespace.string() < o.root_namespace.string();
+        return ref < o.ref;
+    }
+};
+
+inline constexpr uint32_t kMaxManifestOrdinal = 999999;
+
+/// Six-digit filename for a per-build part-manifest ordinal: `000001.zst` through `999999.zst`
+/// (the registered v3 stored suffix for `FormatId::PartManifest`). `0` is reserved as an invalid
+/// sentinel and is never emitted.
+inline String manifestOrdinalFileName(uint32_t manifest_ordinal)
+{
+    if (manifest_ordinal == 0 || manifest_ordinal > kMaxManifestOrdinal)
+        throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+            "Manifest ordinal must be in [1, {}], got {}", kMaxManifestOrdinal, manifest_ordinal);
+    return fmt::format("{:06}{}", manifest_ordinal, storedSuffix(FormatId::PartManifest));
+}
+
+/// The canonical `writer_epoch:build_sequence:manifest_ordinal` text form of a manifest reference.
+/// It is not an object-key encoding: keys are derived by `CasLayout::manifestKey`. Besides logs and
+/// diagnostics it is the wire form of the relink confirm token (spec §confirm-primitive), so
+/// `tryParseManifestRef` below is its exact inverse and the two must be changed together.
+inline String manifestRefDebugString(const ManifestRef & ref)
+{
+    return fmt::format("{}:{}:{}", ref.writer_epoch, ref.build_sequence, ref.manifest_ordinal);
+}
+
+/// Parses the canonical text form produced by `manifestRefDebugString` back into a reference.
+///
+/// Everything that is not EXACTLY three colon-separated decimal fields in range is `nullopt`. The text
+/// arrives from a remote peer in the relink confirm token, so it is untrusted: `from_chars` is used
+/// precisely because it accepts no sign, no whitespace and no partial consumption, and the ordinal is
+/// range-checked the same way `manifestOrdinalFileName` checks it (`0` is the reserved invalid
+/// sentinel, never emitted, so a token carrying it can only be malformed or forged). A refusal here is
+/// never a `No`: the caller cannot tell what was asked, which is an ambiguity, not knowledge.
+inline std::optional<ManifestRef> tryParseManifestRef(std::string_view text)
+{
+    const auto parse_field = [](std::string_view field, auto & out) -> bool
+    {
+        if (field.empty())
+            return false;
+        const auto * const begin = field.data();
+        const auto * const end = field.data() + field.size();
+        const auto result = std::from_chars(begin, end, out);
+        return result.ec == std::errc{} && result.ptr == end;
+    };
+
+    const size_t first = text.find(':');
+    if (first == std::string_view::npos)
+        return std::nullopt;
+    const size_t second = text.find(':', first + 1);
+    if (second == std::string_view::npos || text.find(':', second + 1) != std::string_view::npos)
+        return std::nullopt;
+
+    ManifestRef ref;
+    if (!parse_field(text.substr(0, first), ref.writer_epoch)
+        || !parse_field(text.substr(first + 1, second - first - 1), ref.build_sequence)
+        || !parse_field(text.substr(second + 1), ref.manifest_ordinal))
+        return std::nullopt;
+    if (ref.manifest_ordinal == 0 || ref.manifest_ordinal > kMaxManifestOrdinal)
+        return std::nullopt;
+    return ref;
+}
+
+}
+
+/// Hash specializations for the identity types. Each combines exactly the fields used by its
+/// corresponding `operator==`, so equal keys always have equal hashes.
+namespace std
+{
+
+template <>
+struct hash<DB::Cas::ManifestRef>
+{
+    size_t operator()(const DB::Cas::ManifestRef & r) const
+    {
+        const size_t h1 = std::hash<uint64_t>{}(r.writer_epoch);
+        const size_t h2 = std::hash<uint64_t>{}(r.build_sequence);
+        const size_t h3 = std::hash<uint32_t>{}(r.manifest_ordinal);
+        size_t h = h1;
+        h ^= h2 + 0x9e3779b97f4a7c15ULL + (h << 6) + (h >> 2);
+        h ^= h3 + 0x9e3779b97f4a7c15ULL + (h << 6) + (h >> 2);
+        return h;
+    }
+};
+
+template <>
+struct hash<DB::Cas::ManifestId>
+{
+    size_t operator()(const DB::Cas::ManifestId & id) const
+    {
+        const size_t h1 = std::hash<::String>{}(id.root_namespace.string());
+        const size_t h2 = std::hash<DB::Cas::ManifestRef>{}(id.ref);
+        return h1 ^ (h2 + 0x9e3779b97f4a7c15ULL + (h1 << 6) + (h1 >> 2));
+    }
+};
+
+}
+
+namespace DB::Cas
+{
+
+/// How a backend identifies one physical incarnation of an object key.
+enum class TokenType : uint8_t
+{
+    ETag = 1,        /// S3-family / Azure
+    Generation = 2,  /// GCS (binding deferred; fail-closed until probed)
+    Emulated = 3,    /// test backends (in-memory fake, Local emulation)
+};
+
+/// A backend-native incarnation token. Opaque; sent back to the backend EXACTLY as observed.
+struct Token
+{
+    String value;
+    TokenType type = TokenType::ETag;
+
+    bool empty() const { return value.empty(); }
+    bool operator==(const Token &) const = default;
+};
+
+/// The ordered ref-transaction identifier. A successful writer mount establishes a strictly newer
+/// `writer_epoch`; within an epoch, one namespace's `ref_sequence` values are CONTIGUOUS from 1 --
+/// derived per append from the table's own greatest applied id (`nextRefTxnId`), not drawn from any
+/// counter, so two namespaces of the same mount both count 1, 2, 3... independently. Both fields
+/// are nonzero for a valid id -- {0, 0} is never a real transaction. `writer_epoch` is the primary
+/// ordering component, so tuple order matches the intended timeline even across an epoch restart that
+/// resets `ref_sequence` back to one.
+struct RefTxnId
+{
+    uint64_t writer_epoch = 0;
+    uint64_t ref_sequence = 0;
+
+    auto operator<=>(const RefTxnId &) const = default;
+};
+
+/// Renders the canonical form: two fixed-width, lower-case, 16-digit hexadecimal numbers joined by
+/// '-' (e.g. "0000000000000007-000000000000008e"). Lexical order of the render equals tuple order of
+/// `id`, because '-' (0x2d) sorts below every hex digit character and both fields are fixed-width.
+/// Throws LOGICAL_ERROR if either field is zero: this render becomes an object key, and an invalid id
+/// must never silently produce a well-formed-looking one.
+inline String renderRefTxnId(const RefTxnId & id)
+{
+    if (id.writer_epoch == 0 || id.ref_sequence == 0)
+        throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR,
+            "RefTxnId: writer_epoch and ref_sequence must both be nonzero, got {}-{}",
+            id.writer_epoch, id.ref_sequence);
+    return getHexUIntLowercase(id.writer_epoch) + "-" + getHexUIntLowercase(id.ref_sequence);
+}
+
+/// Parses the canonical form only: exactly 33 characters, '-' at index 16, exactly 16 lower-case hex
+/// digits ('0'-'9', 'a'-'f') either side, and both parsed fields nonzero. Any other shape -- short,
+/// long, upper-case, non-hex, misplaced separator, or a zero component -- returns nullopt rather than
+/// throwing, since parsing an untrusted listed key is an ordinary "is this ours" question.
+inline std::optional<RefTxnId> parseRefTxnId(std::string_view s)
+{
+    constexpr size_t kFieldLen = 16;
+    constexpr size_t kTotalLen = kFieldLen * 2 + 1;
+    if (s.size() != kTotalLen || s[kFieldLen] != '-')
+        return std::nullopt;
+
+    /// Strict lower-case-hex-only parse: `unhexUInt` (base/hex.h) also accepts upper-case, which the
+    /// canonical form must reject, so digits are validated and accumulated by hand here.
+    const auto parseField = [](std::string_view field) -> std::optional<uint64_t>
+    {
+        uint64_t value = 0;
+        for (char c : field)
+        {
+            uint64_t digit = 0;
+            if (c >= '0' && c <= '9')
+                digit = static_cast<uint64_t>(c - '0');
+            else if (c >= 'a' && c <= 'f')
+                digit = static_cast<uint64_t>(c - 'a') + 10;
+            else
+                return std::nullopt;
+            value = (value << 4) | digit;
+        }
+        return value;
+    };
+
+    const auto epoch = parseField(s.substr(0, kFieldLen));
+    const auto seq = parseField(s.substr(kFieldLen + 1, kFieldLen));
+    if (!epoch || !seq || *epoch == 0 || *seq == 0)
+        return std::nullopt;
+    return RefTxnId{*epoch, *seq};
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasXxh3Streamer.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasXxh3Streamer.h
new file mode 100644
index 000000000000..ff3b1e3c1a8d
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasXxh3Streamer.h
@@ -0,0 +1,83 @@
+#pragma once
+
+/// Isolated include wrapper + tiny helper API for the xxHash XXH3-128 hash used by `CasBlobHashingWriteBuffer`.
+///
+/// Two problems this header contains in one place:
+///   1. In the `dbms` target a plain `#include <xxhash.h>` resolves to lz4's bundled copy
+///      (`contrib/lz4/lib` is a higher-priority `-I` than the `-isystem contrib/xxHash`), and that copy
+///      provides only XXH32/64 — NOT the XXH3 API. So xxHash is referenced by an explicit repo-relative
+///      path (from this file's directory up to the repo root) which unambiguously picks the full
+///      standalone `contrib/xxHash` that has XXH3.
+///   2. `XXH_INLINE_ALL` makes xxHash a header-only static-inline implementation whose vendored C is not
+///      clean under the CAS `-Werror -Weverything` flags, and whose inline functions carry an "unused"
+///      attribute that trips `-Wused-but-marked-unused` at every call site. `#pragma clang system_header`
+///      marks the rest of THIS header (and everything it includes, plus the helper calls below) as a
+///      system header, so ALL of those warnings are suppressed here — without disabling warnings for any
+///      real `CasBlobHashingWriteBuffer` code, which only ever touches the clean `DB::Cas` helpers defined below.
+///
+/// The explicit include is a build-selection detail: it does not change the XXH3-128 algorithm or
+/// the digest representation exposed to CAS. Keeping the dependency and warning suppression here
+/// prevents callers from depending on either vendored implementation directly.
+#pragma clang system_header
+
+#include <base/types.h>
+#include <cstddef>
+
+#define XXH_INLINE_ALL
+#include "../../../../../../contrib/xxHash/xxhash.h"
+
+namespace DB::Cas
+{
+
+/// Owns one streaming XXH3-128 state and exposes only the operations needed by `CasBlobHashingWriteBuffer`.
+/// The state is allocated by the constructor and released by the destructor; the wrapper is
+/// deliberately non-copyable because copying an xxHash state would make ownership and continuation
+/// semantics ambiguous. Callers normally check `valid` immediately after construction, then feed
+/// the complete byte sequence with `update` before retrieving the digest with `digest`.
+///
+/// All raw xxHash symbols are confined to this system header, so callers see no xxHash warnings.
+class Xxh3Streamer
+{
+public:
+    /// Allocates and resets a fresh state. `valid` reports allocation failure; callers must not call
+    /// `update` or `digest` on an invalid wrapper.
+    Xxh3Streamer() : state(XXH3_createState()) { XXH3_128bits_reset(state); }
+
+    /// Releases the state owned by this wrapper. It does not perform or publish a digest.
+    ~Xxh3Streamer() { XXH3_freeState(state); }
+
+    Xxh3Streamer(const Xxh3Streamer &) = delete;
+    Xxh3Streamer & operator=(const Xxh3Streamer &) = delete;
+
+    /// Returns whether the constructor obtained an xxHash state successfully.
+    bool valid() const { return state != nullptr; }
+
+    /// Adds the next byte range to the running digest. The range must remain readable for the
+    /// duration of the call; it is consumed immediately and is not retained by the wrapper.
+    void update(const void * data, size_t len) { XXH3_128bits_update(state, data, len); }
+
+    /// Writes the current 128-bit digest into its low and high 64-bit halves. This does not reset
+    /// the state, so it can be used for inspection before the stream is destroyed; callers should
+    /// finish all `update` calls before relying on the result.
+    void digest(UInt64 & low, UInt64 & high) const
+    {
+        const XXH128_hash_t d = XXH3_128bits_digest(state);
+        low = d.low64;
+        high = d.high64;
+    }
+
+private:
+    XXH3_state_t * state;
+};
+
+/// Hashes one byte range with XXH3-128 and writes the digest into its low and high 64-bit halves.
+/// The input is consumed during the call and no state is retained. This is the one-shot counterpart
+/// to `Xxh3Streamer` and is used as the reference path for the streaming CAS hash.
+inline void xxh3_128_oneshot(const void * data, size_t len, UInt64 & low, UInt64 & high)
+{
+    const XXH128_hash_t d = XXH3_128bits(data, len);
+    low = d.low64;
+    high = d.high64;
+}
+
+}

From 7abc22a333ee5b129d8682fc77eed1af23f8335d Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:33 +0200
Subject: [PATCH 14/30] CAS subsystem: Formats layer

On-wire/on-disk encodings: manifests, ref-log records, GC state, seals,
codecs. Depends only on Primitives.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../Formats/CasBlobEnvelopeFormat.cpp         | 253 ++++++++
 .../Formats/CasBlobEnvelopeFormat.h           | 110 ++++
 .../Formats/CasBlobMetaFormat.cpp             |  94 +++
 .../Formats/CasBlobMetaFormat.h               |  46 ++
 .../ContentAddressed/Formats/CasByteBudget.h  |  57 ++
 .../Formats/CasFoldSealFormat.cpp             | 546 ++++++++++++++++++
 .../Formats/CasFoldSealFormat.h               | 225 ++++++++
 .../ContentAddressed/Formats/CasFormat.cpp    | 198 +++++++
 .../ContentAddressed/Formats/CasFormat.h      | 203 +++++++
 .../Formats/CasGcMaintenanceStateFormat.cpp   |  67 +++
 .../Formats/CasGcMaintenanceStateFormat.h     |  21 +
 .../Formats/CasGcOutcomesFormat.cpp           | 129 +++++
 .../Formats/CasGcOutcomesFormat.h             |  58 ++
 .../Formats/CasGcStateFormat.cpp              | 118 ++++
 .../Formats/CasGcStateFormat.h                |  71 +++
 .../ContentAddressed/Formats/CasLayout.cpp    | 345 +++++++++++
 .../ContentAddressed/Formats/CasLayout.h      | 480 +++++++++++++++
 .../Formats/CasPartManifestFormat.cpp         | 353 +++++++++++
 .../Formats/CasPartManifestFormat.h           | 120 ++++
 .../Formats/CasPoolMetaFormat.cpp             | 182 ++++++
 .../Formats/CasPoolMetaFormat.h               |  91 +++
 .../Formats/CasRecordStreamFormat.cpp         | 325 +++++++++++
 .../Formats/CasRecordStreamFormat.h           | 164 ++++++
 .../Formats/CasRefCatalogFormat.cpp           | 397 +++++++++++++
 .../Formats/CasRefCatalogFormat.h             | 168 ++++++
 .../Formats/CasRefCkptFormat.cpp              | 179 ++++++
 .../Formats/CasRefCkptFormat.h                | 124 ++++
 .../Formats/CasRefLogFormat.cpp               | 443 ++++++++++++++
 .../Formats/CasRefLogFormat.h                 | 177 ++++++
 .../Formats/CasRefSnapshotFormat.cpp          | 305 ++++++++++
 .../Formats/CasRefSnapshotFormat.h            |  95 +++
 .../Formats/CasRefWireVocab.cpp               |  47 ++
 .../Formats/CasRefWireVocab.h                 |  65 +++
 .../Formats/CasServerRootFormats.cpp          | 178 ++++++
 .../Formats/CasServerRootFormats.h            |  93 +++
 .../Formats/CasTextFormat.cpp                 | 414 +++++++++++++
 .../ContentAddressed/Formats/CasTextFormat.h  | 241 ++++++++
 .../ContentAddressed/Formats/CasWireVocab.cpp | 103 ++++
 .../ContentAddressed/Formats/CasWireVocab.h   |  62 ++
 .../ContentAddressed/Formats/README.md        |  63 ++
 40 files changed, 7410 insertions(+)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasByteBudget.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/README.md

diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.cpp
new file mode 100644
index 000000000000..65176572e896
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.cpp
@@ -0,0 +1,253 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+constexpr std::string_view kBlobType = "cas_blob";
+
+std::string_view opToWord(ProvenanceOp op)
+{
+    switch (op)
+    {
+        case ProvenanceOp::Other:    return "other";
+        case ProvenanceOp::Insert:   return "insert";
+        case ProvenanceOp::Merge:    return "merge";
+        case ProvenanceOp::Mutation: return "mutation";
+        case ProvenanceOp::Attach:   return "attach";
+        case ProvenanceOp::Repack:   return "repack";
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS blob envelope: unknown ProvenanceOp {}", static_cast<int>(op));
+}
+
+ProvenanceOp opFromWord(std::string_view w)
+{
+    if (w == "other")    return ProvenanceOp::Other;
+    if (w == "insert")   return ProvenanceOp::Insert;
+    if (w == "merge")    return ProvenanceOp::Merge;
+    if (w == "mutation") return ProvenanceOp::Mutation;
+    if (w == "attach")   return ProvenanceOp::Attach;
+    if (w == "repack")   return ProvenanceOp::Repack;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS blob envelope: unknown op '{}'", w);
+}
+
+/// The escaped byte-length of one raw ref char under the frozen envelope alphabet (see writeEnvelopeRefField).
+size_t escapedLen(char c)
+{
+    const unsigned char u = static_cast<unsigned char>(c);
+    if (c == '"' || c == '\\')
+        return 2;
+    if (u < 0x20)
+        return 6;   /// \uXXXX
+    return 1;       /// everything else, INCLUDING '/', verbatim
+}
+
+void appendEscaped(String & out, char c)
+{
+    const unsigned char u = static_cast<unsigned char>(c);
+    if (c == '"')  { out += "\\\""; return; }
+    if (c == '\\') { out += "\\\\"; return; }
+    if (u < 0x20)
+    {
+        static constexpr char hexd[] = "0123456789abcdef";
+        out += "\\u00";
+        out += hexd[(u >> 4) & 0xF];
+        out += hexd[u & 0xF];
+        return;
+    }
+    out += c;
+}
+
+/// The blob-envelope's OWN ref-string writer. DELIBERATELY NOT `writeStringValue` and MUST NOT be
+/// "unified" with it: the 256-byte header budget arithmetic and the stored blob bytes depend on this
+/// alphabet being codec-owned and FROZEN — only `"`, `\`, and control chars (< 0x20, as `\uXXXX`)
+/// escape; `/` and every other byte pass verbatim. (`writeStringValue`/`FormatSettings::JSON` may
+/// legitimately evolve for the control-plane formats; this codec must not inherit that.) Writes the
+/// opening quote, the ref content escaped and truncated to at most `budget` escaped bytes (stopping at
+/// the first char that would overflow — never splitting an escape), then the closing quote.
+void writeEnvelopeRefField(String & json, size_t budget, std::string_view raw_ref)
+{
+    json += '"';
+    size_t used = 0;
+    for (char c : raw_ref)
+    {
+        const size_t need = escapedLen(c);
+        if (used + need > budget)
+            break;
+        appendEscaped(json, c);
+        used += need;
+    }
+    json += '"';
+}
+
+}
+
+String encodeEnvelopeHeader(EnvelopeHeader & header, uint32_t blob_header_len)
+{
+    if (header.kind != ObjectKind::Blob)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS blob envelope: unexpected ObjectKind {}", static_cast<int>(header.kind));
+
+    /// Build every field EXCEPT `ref` into a buffer (small, bounded by blob_header_len). `ref` is the
+    /// only truncated field, appended last so the truncation never disturbs another field.
+    String json;
+    {
+        CasJsonWriter buf(256);
+        bool first = true;
+        writeKey(buf, "type", first); writeStringValue(buf, kBlobType);
+        writeKey(buf, "v", first);    writeIntText(currentCompatibilityVersion(), buf);
+        writeKey(buf, "tag", first);  writeHex128Value(buf, header.incarnation_tag);
+        writeKey(buf, "bld", first);  writeHex128Value(buf, header.build_id);
+        if (header.provenance)
+        {
+            writeKey(buf, "ts", first); writeIntText(header.provenance->created_at_ms, buf);
+            writeKey(buf, "by", first); writeHex128Value(buf, header.provenance->creator_server_id);
+            writeKey(buf, "op", first); writeStringValue(buf, opToWord(header.provenance->op));
+            writeKey(buf, "ch", first); writeIntText(header.provenance->ch_version, buf);
+        }
+        /// Test-only critical extension: an unknown `!`-key BEFORE `ref`.
+        if (header.emit_unknown_critical_key)
+        {
+            writeKey(buf, "!x", first); writeStringValue(buf, "1");
+        }
+        json = std::move(buf).take();   /// e.g. {"type":"cas_blob","v":3,...,"ch":26006001   (no ref, no closing brace)
+    }
+
+    /// Optional `ref`, truncated to the exact remaining budget. Layout after this block:
+    ///   json + `,"ref":` + `"` + <escaped ref, <= budget bytes> + `"` + `}`   must be <= blob_header_len-1
+    /// (byte blob_header_len-1 is reserved for '\n'; the pad zone fills the gap with spaces).
+    if (header.intended_ref)
+    {
+        static constexpr std::string_view ref_key = ",\"ref\":";
+        /// +3 = opening quote + closing quote + closing brace.
+        const size_t fixed = json.size() + ref_key.size() + 3;
+        if (blob_header_len < 1 || fixed > static_cast<size_t>(blob_header_len) - 1)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS blob envelope: non-ref fields ({} bytes) do not fit blob_header_len {} before the ref",
+                fixed, blob_header_len);
+        const size_t budget = (static_cast<size_t>(blob_header_len) - 1) - fixed;
+        json += ref_key;
+        writeEnvelopeRefField(json, budget, *header.intended_ref);
+    }
+    json += '}';
+
+    if (json.size() > static_cast<size_t>(blob_header_len) - 1)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS blob envelope: header object {} bytes exceeds blob_header_len {} - 1",
+            json.size(), blob_header_len);
+
+    /// Space pad to byte blob_header_len-2, then '\n' at byte blob_header_len-1.
+    String out = std::move(json);
+    out.append((static_cast<size_t>(blob_header_len) - 1) - out.size(), ' ');
+    out += '\n';
+
+    header.header_len = blob_header_len;
+    return out;
+}
+
+EnvelopeHeader decodeEnvelopeHeader(std::string_view head_bytes, uint64_t /*object_size*/, ObjectKind expected_kind)
+{
+    ReadBufferFromMemory in(head_bytes.data(), head_bytes.size());
+    JsonObjectReader r(in, KeyStrictness::Tolerant, "blob envelope");
+
+    EnvelopeHeader h;
+    h.kind = ObjectKind::Blob;
+    bool saw_type = false;
+    bool saw_v = false;
+    bool have_prov = false;
+    Provenance prov;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "type")
+        {
+            const String t = r.readString();
+            if (t != kBlobType)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS blob envelope: object is a '{}', not a '{}'", t, kBlobType);
+            saw_type = true;
+        }
+        else if (key == "v")
+        {
+            h.compatibility_version = r.readU32Number();
+            checkCompatibility(h.compatibility_version, "blob envelope");
+            saw_v = true;
+        }
+        else if (key == "tag")
+            h.incarnation_tag = r.readHex128();
+        else if (key == "bld")
+            h.build_id = r.readHex128();
+        else if (key == "ts")
+        {
+            prov.created_at_ms = r.readU64Number();
+            have_prov = true;
+        }
+        else if (key == "by")
+        {
+            prov.creator_server_id = r.readHex128();
+            have_prov = true;
+        }
+        else if (key == "op")
+        {
+            prov.op = opFromWord(r.readString());
+            have_prov = true;
+        }
+        else if (key == "ch")
+        {
+            prov.ch_version = static_cast<uint32_t>(r.readU64Number());
+            have_prov = true;
+        }
+        else if (key == "ref")
+            h.intended_ref = r.readString();
+        else
+            r.skipUnknown(key);   /// `!`-key -> UNKNOWN_FORMAT_VERSION; unknown plain key -> skipped (tolerant)
+    }
+    if (!saw_type)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS blob envelope: missing type");
+    if (!saw_v)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS blob envelope: missing v");
+    if (h.kind != expected_kind)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS blob envelope: kind {} does not match expected {}",
+            static_cast<int>(h.kind), static_cast<int>(expected_kind));
+    if (have_prov)
+        h.provenance = prov;
+
+    /// Pad-verify: JsonObjectReader consumed through the closing '}', so in.count() == json_len. Every
+    /// byte up to the terminating '\n' must be an ASCII space (no smuggling); header_len is DERIVED from
+    /// the '\n' position (blob_header_len is never passed to decode).
+    while (true)
+    {
+        if (in.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS blob envelope: pad zone has no '\\n' terminator");
+        const char c = *in.position();
+        ++in.position();
+        if (c == '\n')
+        {
+            h.header_len = static_cast<uint32_t>(in.count());
+            break;
+        }
+        if (c != ' ')
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS blob envelope: non-space byte 0x{:02x} in the header pad zone", static_cast<unsigned>(static_cast<unsigned char>(c)));
+    }
+
+    return h;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h
new file mode 100644
index 000000000000..19250fe69ddd
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h
@@ -0,0 +1,110 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// Identifies the kind of content represented by an envelope. `Blob` is currently the only envelope
+/// kind, but the enum remains a distinct switch-friendly type because callers use the kind as part of
+/// their object and audit-event vocabulary.
+enum class ObjectKind : uint8_t
+{
+    Blob = 1,
+};
+
+/// Describes the operation that produced an envelope's optional diagnostic provenance. It is metadata
+/// for inspection and attribution only; readers do not use it to make storage or compatibility
+/// decisions.
+enum class ProvenanceOp : uint8_t
+{
+    Other = 0,
+    Insert = 1,
+    Merge = 2,
+    Mutation = 3,
+    Attach = 4,
+    Repack = 5,
+};
+
+/// Optional diagnostic metadata recorded with an envelope. The fields identify when and where the
+/// incarnation was created, the ClickHouse build that wrote it, and the operation that produced it;
+/// none of them participates in object identity or a protocol decision.
+struct Provenance
+{
+    uint64_t created_at_ms = 0;
+    UInt128 creator_server_id{};
+    uint32_t ch_version = 0;
+    ProvenanceOp op = ProvenanceOp::Other;
+};
+
+/// The blob envelope is a fixed-size JSON descriptor followed by the raw payload. The JSON object
+/// occupies bytes [0, json_len), ASCII spaces occupy [json_len, blob_header_len-1), and '\n' is at byte
+/// blob_header_len-1. The payload therefore begins at the pool-wide constant offset
+/// `blob_header_len` (256 for blob pools, a `PoolMeta` parameter), allowing the locate path to use a
+/// constant shift without reading an object-specific header first. The header is also the incarnation
+/// zone: it may differ between incarnations of one logical object, and each upload attempt gets a fresh
+/// random u128 `tag`, which is used as the exact-token delete identity.
+///
+/// The envelope intentionally does not duplicate identity or unused integrity metadata. The identity
+/// algorithm and digest are already present in the object key and manifest reference, `domain_id` had
+/// no validating consumer, and `header_hash` had no consumer once the CityHash64 check left the
+/// envelope. Writer forensics are represented
+/// by `ch` and `bld`, so a separate `writer_version` is unnecessary. The `v` field is the sole format
+/// compatibility gate; a reader rejects a version it does not understand before interpreting the body.
+struct EnvelopeHeader
+{
+    ObjectKind kind = ObjectKind::Blob;
+    /// Set by decode from the header `v`; encode stamps `currentCompatibilityVersion`. A reader
+    /// fails closed (UNKNOWN_FORMAT_VERSION) when `v` exceeds what this build understands.
+    uint32_t compatibility_version = 0;
+    UInt128 incarnation_tag{};              /// `tag`
+    UInt128 build_id{};                     /// `bld`
+    std::optional<Provenance> provenance;   /// `ts` / `by` / `op` / `ch`
+    std::optional<String> intended_ref;     /// `ref` (diagnostic; truncated on encode to fit the header)
+    uint32_t header_len = 0;                /// filled by encode/decode = blob_header_len (payload offset)
+    /// Test-only knob: emit an unknown `!`-critical key. Decoding the resulting header must fail
+    /// closed with `UNKNOWN_FORMAT_VERSION`, exercising the compatibility rule for critical extensions.
+    bool emit_unknown_critical_key = false;
+};
+
+/// Builds the fixed-length header for a pool whose `blob_header_len` is `blob_header_len` (256 for blob
+/// pools). Sets `header.header_len = blob_header_len` and returns exactly that many bytes. The
+/// diagnostic `ref` is the only truncatable field and is shortened, never dropped, when necessary to
+/// preserve the fixed layout. The header is built without payload bytes, so an upload can stage the
+/// header before the payload is streamed.
+String encodeEnvelopeHeader(EnvelopeHeader & header, uint32_t blob_header_len);
+
+/// Parses and validates the JSON descriptor, its expected `type`, and its compatibility version.
+/// Derives `header_len` from the terminating '\n' and requires every preceding byte in the pad zone to
+/// be an ASCII space, preventing bytes from being smuggled between the descriptor and payload. Malformed
+/// type, padding, or truncation produces `CORRUPTED_DATA`; a future `v` or unknown `!`-prefixed critical
+/// key produces `UNKNOWN_FORMAT_VERSION`. `object_size` is accepted for symmetry with read and GC call
+/// sites; the payload length is derived downstream as `object_size - header_len`, so this function does
+/// not otherwise inspect it.
+EnvelopeHeader decodeEnvelopeHeader(std::string_view head_bytes, uint64_t object_size, ObjectKind expected_kind);
+
+/// Payload starts right after the header.
+inline uint64_t payloadOffset(const EnvelopeHeader & header)
+{
+    return static_cast<uint64_t>(header.header_len);
+}
+
+/// Map an internal `ObjectKind` to the audit-log `CasEventObjectKind`. Single source for the mapping
+/// previously open-coded as a ternary at each emission site. Lives here (Formats) rather than in
+/// CasEvent.h (Primitives) so the include direction Formats -> Primitives is respected and
+/// `CasEvent` stays dependency-free.
+inline CasEventObjectKind toEventKind(ObjectKind kind)
+{
+    switch (kind)
+    {
+        case ObjectKind::Blob: return CasEventObjectKind::Blob;
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.cpp
new file mode 100644
index 000000000000..b62fd3b82424
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.cpp
@@ -0,0 +1,94 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+std::string_view metaStateToWord(MetaState s)
+{
+    switch (s)
+    {
+        case MetaState::Clean:     return "clean";
+        case MetaState::Condemned: return "condemned";
+    }
+    // The enum is persisted as a closed vocabulary. Do not silently invent a spelling for a value
+    // added without a corresponding format decision: that would make the writer emit data older
+    // readers cannot classify.
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS blob meta: unknown MetaState {}", static_cast<int>(s));
+}
+
+MetaState metaStateFromWord(std::string_view w)
+{
+    if (w == "clean")     return MetaState::Clean;
+    if (w == "condemned") return MetaState::Condemned;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS blob meta: unknown state '{}'", w);
+}
+
+}
+
+String encodeBlobMeta(const BlobMeta & meta)
+{
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::BlobMeta);
+    // `version` is represented by the header line. The JSON body contains only fields that describe
+    // the current marker and its accounting data.
+    bool first = true;
+    writeKey(out, "st", first);
+    writeStringValue(out, metaStateToWord(meta.state));
+    writeKey(out, "cr", first);
+    writeU64StringValue(out, meta.condemn_round);
+    writeKey(out, "sz", first);
+    writeU64StringValue(out, meta.size);
+    closeObject(out, first);
+    writeChar('\n', out);
+    return std::move(out).take();
+}
+
+BlobMeta decodeBlobMeta(std::string_view bytes)
+{
+    ReadBufferFromMemory in(bytes.data(), bytes.size());
+    expectHeaderLine(in, FormatId::BlobMeta);
+    const String body = readLine(in, traitsFor(FormatId::BlobMeta).line_cap, "blob meta");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    JsonObjectReader r(body_in, KeyStrictness::Tolerant, "blob meta");
+
+    // Start with the documented defaults. In particular, `version` stays at 1 because the header's
+    // version is authoritative and is not copied into the body struct.
+    BlobMeta m;
+    bool saw_state = false;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "st")
+        {
+            m.state = metaStateFromWord(r.readString());
+            saw_state = true;
+        }
+        else if (key == "cr")
+            m.condemn_round = r.readU64String();
+        else if (key == "sz")
+            m.size = r.readU64String();
+        else
+            r.skipUnknown(key);
+    }
+    if (!saw_state)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS blob meta: missing st");
+    if (!body_in.eof() || !in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS blob meta: trailing bytes");
+    return m;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.h
new file mode 100644
index 000000000000..bf1fa832a3b5
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.h
@@ -0,0 +1,46 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <base/types.h>
+#include <cstdint>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// The two states of the per-hash freshness marker used by the writer's deduplication gate and by GC.
+/// This marker is only a point-read hint, not the linearization point for blob lifetime: the body's
+/// in-body `incarnation_tag` and the body's exact-token delete provide the safety guarantee. A stale
+/// marker can therefore make a writer re-upload conservatively, but it is never authority for deleting
+/// the body.
+enum class MetaState : uint8_t
+{
+    Clean = 0,       /// The body is present and may be referenced.
+    Condemned = 1,   /// GC observed zero in-degree; the body remains present until exact-token deletion,
+                     /// so a writer may resurrect it by replacing the body and updating this marker.
+};
+
+/// The durable per-hash meta record. Its text representation consists of a format header followed by
+/// one JSON object with the state word, the GC condemnation round, and the raw body size. `size` is
+/// retained for introspection, fsck, and GC accounting; reads of the blob never consult the meta.
+/// Lifecycle transitions are conditional on the backend etag, while the encoded bytes themselves are
+/// not compared. The body header's `v` is the authoritative format version, so `version` remains only
+/// for the inspection interface and is deliberately not serialized in the JSON body.
+struct BlobMeta
+{
+    uint8_t version = 1;
+    MetaState state = MetaState::Clean;
+    uint64_t condemn_round = 0;   /// The GC round that condemned this blob; distinguishes a stale
+                                  /// condemnation from a later spare-and-recondemn transition.
+    uint64_t size = 0;
+};
+
+/// Serialize `meta` as the header line and one JSON body line. Invalid `MetaState` values are rejected
+/// with `CORRUPTED_DATA`; the body does not contain `version` because the header owns format versioning.
+String encodeBlobMeta(const BlobMeta & meta);
+
+/// Decode a stored meta record. The header, required state, field types, and complete input are checked;
+/// malformed input, an unknown state, or trailing bytes throws `CORRUPTED_DATA`. Unknown JSON keys are
+/// tolerated so the format can add nonessential fields without breaking older readers.
+BlobMeta decodeBlobMeta(std::string_view bytes);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasByteBudget.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasByteBudget.h
new file mode 100644
index 000000000000..28e5900f7674
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasByteBudget.h
@@ -0,0 +1,57 @@
+#pragma once
+#include <cstdint>
+
+namespace DB::Cas
+{
+
+/// The byte arithmetic every write-once text control object owes BEFORE its PUT.
+///
+/// Two caps, deliberately kept apart (they answer different questions and fail differently):
+///
+///   * the LINE cap bounds ONE encoded record. A record longer than it is not merely large, it is
+///     UNREADABLE: the streaming reader refuses the line, so an object containing one can never be
+///     decoded again. This predicate belongs at the point a record is emitted.
+///   * the OBJECT cap bounds the whole object. It is the ADDITIVE question — a fixed frame (header,
+///     meta, trailer) plus the worst-case reservation of every entry — because a producer that must
+///     decide whether one MORE entry still fits cannot encode first and measure afterwards.
+///
+/// Both predicates accept at EQUALITY: a cap is the largest permitted value, not the first forbidden
+/// one, and the readers enforce it the same way.
+///
+/// Additions AND multiplications saturate. A modular sum or product that wrapped would answer "fits"
+/// for an object that does not, turning an overflow into a durable unreadable object — the one
+/// outcome the caps exist to prevent.
+
+/// `a + b`, clamped to `UINT64_MAX` instead of wrapping.
+constexpr uint64_t addByteBudget(uint64_t a, uint64_t b)
+{
+    return a > UINT64_MAX - b ? UINT64_MAX : a + b;
+}
+
+/// `a * b`, clamped to `UINT64_MAX` instead of wrapping -- the same saturation discipline as
+/// `addByteBudget`, one step earlier: a per-entry byte cost multiplied by an entry count before the
+/// caller adds it to a fixed frame (`fitsObjectCap`'s second argument) can overflow first, and a
+/// wrapped product can land BELOW the true fixed-plus-reservation sum even though the real total is
+/// far over it -- the exact "answers fits for an object that does not" failure this file exists to
+/// prevent, one arithmetic step upstream of the sum itself.
+constexpr uint64_t mulByteBudget(uint64_t a, uint64_t b)
+{
+    return a != 0 && b > UINT64_MAX / a ? UINT64_MAX : a * b;
+}
+
+/// LINE predicate: `encoded_row_bytes <= line_cap`, measured EXCLUDING the '\n' terminator (the same
+/// bytes `readLine` measures). `line_cap == 0` means the format declares no line cap.
+constexpr bool fitsLineCap(uint64_t encoded_row_bytes, uint64_t line_cap)
+{
+    return line_cap == 0 || encoded_row_bytes <= line_cap;
+}
+
+/// OBJECT predicate: `fixed_bytes + entries_reservation <= object_cap`, evaluated with saturating
+/// addition. `object_cap == 0` means the format declares no whole-object cap (a streamed format that
+/// is never materialized whole).
+constexpr bool fitsObjectCap(uint64_t fixed_bytes, uint64_t entries_reservation, uint64_t object_cap)
+{
+    return object_cap == 0 || addByteBudget(fixed_bytes, entries_reservation) <= object_cap;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.cpp
new file mode 100644
index 000000000000..b4bba2bff08f
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.cpp
@@ -0,0 +1,546 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasByteBudget.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <algorithm>
+#include <utility>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LIMIT_EXCEEDED;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+std::string_view holdReasonToWord(HoldReason r)
+{
+    switch (r)
+    {
+        case HoldReason::GapBelowWitness:        return "gap_below_witness";
+        case HoldReason::UnconsumedSealCrossing: return "unconsumed_seal_crossing";
+        case HoldReason::WitnessDisappeared:     return "witness_disappeared";
+        case HoldReason::BodyUndecodable:        return "body_undecodable";
+        case HoldReason::ManifestBodyMissing:    return "manifest_body_missing";
+        case HoldReason::CheckpointUndecodable:  return "checkpoint_undecodable";
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: unknown hold reason {}", static_cast<int>(r));
+}
+
+namespace
+{
+
+HoldReason holdReasonFromWord(std::string_view w)
+{
+    if (w == "gap_below_witness")        return HoldReason::GapBelowWitness;
+    if (w == "unconsumed_seal_crossing") return HoldReason::UnconsumedSealCrossing;
+    if (w == "witness_disappeared")      return HoldReason::WitnessDisappeared;
+    if (w == "body_undecodable")         return HoldReason::BodyUndecodable;
+    if (w == "manifest_body_missing")    return HoldReason::ManifestBodyMissing;
+    if (w == "checkpoint_undecodable")   return HoldReason::CheckpointUndecodable;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: unknown hold reason '{}'", w);
+}
+
+/// The classification set is CLOSED. Every consumer of a coverage row branches on exact values — the
+/// sweep's §6 deletion premise refuses a row by testing `== 4` and then `== 0` — so a value outside the
+/// set is not an unknown variant to be tolerated forward: it is a row that passes every refusal written
+/// in terms of the set and reaches the irreversible delete. One predicate, used by both directions, so
+/// the writer's self-check and the reader's fail-close can never name different sets.
+bool isKnownClassification(uint64_t classification)
+{
+    return classification == 0 || classification == 1 || classification == 2 || classification == 4;
+}
+
+/// A hold names a position the fold must resolve, and both components of that id are nonzero (the
+/// canonical `RefTxnId` rule `renderRefTxnId` enforces for every id that becomes a key). A zero
+/// component is not a weaker hold, it is a self-erasing one: no position sorts below `{0, 0}`, so the
+/// carry rule clears it on the first round that folds anything, and the durable evidence that the
+/// namespace ever stopped disappears without a single record having been resolved.
+bool isCanonicalHoldPosition(const RefTxnId & at)
+{
+    return at.writer_epoch != 0 && at.ref_sequence != 0;
+}
+
+/// Insert a decoded record under a key that must appear at most ONCE in the object. Plain
+/// `map[key] = value` is last-wins, and last-wins is not a lossy nicety here: a repeated coverage key
+/// lets a later, clean row overwrite the held one that a whole namespace's retention rests on, which is
+/// exactly how a forged or mis-merged seal would erase the only durable record of an unresolved
+/// position. One record per key, or the object is corrupt.
+template <typename Map, typename Key, typename Value>
+void insertRecordOnce(Map & map, const Key & key, Value && value, std::string_view what)
+{
+    if (!map.emplace(key, std::forward<Value>(value)).second)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS fold seal: a second {} record for '{}' — a key appears at most once, and accepting the "
+            "duplicate would silently overwrite the record already read",
+            what, key);
+}
+
+/// Emit one run record (`k` = "btr") WITHOUT its line terminator; the caller closes (and measures) the
+/// line, and sorts the vector by key first.
+void writeRun(CasJsonWriter & out, std::string_view kind, const RunRef & r)
+{
+    bool first = true;
+    writeKey(out, "k", first);     writeStringValue(out, kind);
+    writeKey(out, "key", first);   writeStringValue(out, r.key);
+    writeKey(out, "ck", first);    writeHex128Value(out, r.checksum);
+    writeKey(out, "shard", first); writeIntText(r.shard, out);
+    writeKey(out, "gen", first);   writeU64StringValue(out, r.generation);
+    closeObject(out, first);
+}
+
+void validateFoldSealStructure(
+    const CasFoldSeal & seal, const Layout & layout, uint64_t gc_shards,
+    int error_code, std::string_view source)
+{
+    if (gc_shards == 0)
+        throw Exception(error_code, "CAS fold seal {}: gc_shards must be nonzero", source);
+
+    std::vector<bool> run_seen(gc_shards, false);
+    for (const RunRef & run : seal.blob_target_runs)
+    {
+        if (run.key.empty() || run.generation == 0)
+            throw Exception(error_code,
+                "CAS fold seal {}: blob-target run requires a nonempty key and nonzero physical generation",
+                source);
+        if (run.shard >= gc_shards)
+            throw Exception(error_code,
+                "CAS fold seal {}: blob-target shard {} is outside [0, {})",
+                source, run.shard, gc_shards);
+        if (run_seen[run.shard])
+            throw Exception(error_code,
+                "CAS fold seal {}: duplicate blob-target shard {} -- at most one run per shard is allowed",
+                source, run.shard);
+        run_seen[run.shard] = true;
+
+        const auto parsed = layout.parseBlobTargetRunKey(run.key);
+        if (!parsed || parsed->generation != run.generation || parsed->shard != run.shard || parsed->seq != 0)
+            throw Exception(error_code,
+                "CAS fold seal {}: blob-target run key '{}' is not canonical for generation {}, shard {}, sequence 0",
+                source, run.key, run.generation, run.shard);
+    }
+
+    if (seal.condemned_summary.size() != gc_shards)
+        throw Exception(error_code,
+            "CAS fold seal {}: condemned summary has {} rows, but exactly {} shards are required",
+            source, seal.condemned_summary.size(), gc_shards);
+    for (uint64_t shard = 0; shard < gc_shards; ++shard)
+    {
+        const auto it = seal.condemned_summary.find(shard);
+        if (it == seal.condemned_summary.end())
+            throw Exception(error_code,
+                "CAS fold seal {}: condemned summary is missing shard {} from [0, {})",
+                source, shard, gc_shards);
+        const CondemnedSummary & summary = it->second;
+        if (summary.pending_total > summary.condemned_total)
+            throw Exception(error_code,
+                "CAS fold seal {}: shard {} has pending_total {} greater than condemned_total {}",
+                source, shard, summary.pending_total, summary.condemned_total);
+        const bool has_nonpending = summary.pending_total < summary.condemned_total;
+        const bool has_real_oldest = summary.oldest_nonpending_condemn_round != UINT64_MAX;
+        if (has_nonpending != has_real_oldest)
+            throw Exception(error_code,
+                "CAS fold seal {}: shard {} must carry a real oldest non-pending condemn round exactly when non-pending rows exist",
+                source, shard);
+    }
+}
+
+}
+
+FoldSealCaps foldSealCaps()
+{
+    const FormatTraits & t = traitsFor(FormatId::FoldSeal);
+    return FoldSealCaps{.line_cap = t.line_cap, .object_cap = t.object_cap};
+}
+
+void checkFoldSealObjectBytes(uint64_t encoded_bytes)
+{
+    const uint64_t object_cap = foldSealCaps().object_cap;
+    if (!fitsObjectCap(encoded_bytes, /*entries_reservation*/0, object_cap))
+        throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+            "CAS fold seal: the seal encodes to {} bytes, over the {}-byte object cap. Writing it "
+            "would leave a durable seal no later round can read; the round is refused before the PUT "
+            "and retries.",
+            encoded_bytes, object_cap);
+}
+
+String encodeFoldSeal(const CasFoldSeal & seal)
+{
+    const FoldSealCaps caps = foldSealCaps();
+    CasJsonWriter out(256);
+
+    /// EVERY line this encoder emits is measured against the LINE cap, on the bytes actually emitted --
+    /// escaping, framing and all -- rather than on an estimate of them. A line that does not fit is not
+    /// a large line, it is an UNREADABLE one: `readLine` refuses it, so the whole object is lost.
+    /// Refuse here, where nothing is durable yet. The header and trailer are measured too, even though
+    /// their lengths are bounded by construction — a gate with an unstated exception is one that a
+    /// later edit widens without noticing.
+    const auto checkLineBytes = [&](uint64_t bytes, std::string_view what)
+    {
+        if (!fitsLineCap(bytes, caps.line_cap))
+            throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+                "CAS fold seal: the {} line encodes to {} bytes, over the {}-byte line cap; a longer "
+                "line cannot be read back, so the seal is refused before it is written",
+                what, bytes, caps.line_cap);
+    };
+
+    writeHeaderLine(out, FormatId::FoldSeal);   /// emits its own terminator
+    checkLineBytes(out.size() - 1, "header");
+
+    size_t line_start = out.size();
+    const auto closeLine = [&](std::string_view what)
+    {
+        checkLineBytes(out.size() - line_start, what);
+        writeChar('\n', out);
+        line_start = out.size();
+    };
+
+    /// meta line
+    {
+        bool first = true;
+        writeKey(out, "g", first);  writeU64StringValue(out, seal.generation);
+        writeKey(out, "pg", first); writeU64StringValue(out, seal.parent_generation);
+        closeObject(out, first);
+        closeLine("meta");
+    }
+
+    uint64_t n = 0;
+
+    /// Ref-life rows (`std::map<UInt128, ...>` => opaque-id-sorted). This is the sole serialized
+    /// producer of ref coverage and removal evidence.
+    for (const auto & [life_id, life_state] : seal.ref_lives)
+    {
+        const RefCoverage & cov = life_state.coverage;
+        const String life_hex = u128ToHex(life_id);
+        if (life_id == 0)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS fold seal: a ref-life row has a zero life id -- 0 never names a life");
+        /// THE STRICT GRAMMAR, enforced where the bytes are produced. Every refusal below is
+        /// `LOGICAL_ERROR`: this row came from our own fold, so an ill-formed one is a bug in this
+        /// process, not corruption arriving from a store — and none of these shapes is repairable once
+        /// durable, so none is ever written.
+        ///
+        /// A classification outside the closed set first, because the two checks after it are stated in
+        /// terms of the set and a row they cannot classify makes their answers meaningless.
+        if (!isKnownClassification(cov.classification))
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS fold seal: coverage '{}' has classification {}, which is not one of the four the "
+                "fold grammar defines (0 absent, 1 unchanged, 2 folded, 4 clamped) — every consumer "
+                "branches on those exact values, so this row would pass refusals meant to stop it",
+                life_hex, cov.classification);
+        /// A classification-4 row whose hold was dropped is indistinguishable, once durable, from a
+        /// namespace that stopped for no reason — and a hold on any other classification claims a stop
+        /// that did not happen.
+        if ((cov.classification == 4) != cov.hold.has_value())
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS fold seal: coverage '{}' has classification {} and {} hold — the hold fields are "
+                "required for classification 4 and forbidden otherwise",
+                life_hex, cov.classification, cov.hold ? "a" : "no");
+        /// A hold that names no position resolves itself on the next round (nothing sorts below
+        /// `{0, 0}`) and cannot be rendered where the sweep reports why it retained a manifest.
+        if (cov.hold && !isCanonicalHoldPosition(cov.hold->offending_position))
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS fold seal: coverage '{}' is held at {}-{} — a hold's offending position has both "
+                "components nonzero; a zero one would be cleared by the first record the next round "
+                "folds, erasing the only durable evidence that the namespace stopped",
+                life_hex, cov.hold->offending_position.writer_epoch, cov.hold->offending_position.ref_sequence);
+
+        if (life_state.cleanup_evidence
+            && (life_state.cleanup_evidence->remove_txn_id.writer_epoch == 0
+                || life_state.cleanup_evidence->remove_txn_id.ref_sequence == 0))
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS fold seal: ref life '{}' carries cleanup evidence with non-canonical removal "
+                "transaction {}-{} -- both components are required and nonzero",
+                life_hex,
+                life_state.cleanup_evidence->remove_txn_id.writer_epoch,
+                life_state.cleanup_evidence->remove_txn_id.ref_sequence);
+
+        bool first = true;
+        writeKey(out, "k", first);    writeStringValue(out, "rfl");
+        writeKey(out, "life", first); writeHex128Value(out, life_id);
+        writeKey(out, "cls", first);  writeIntText(static_cast<uint32_t>(cov.classification), out);
+        writeKey(out, "lfe", first);  writeU64StringValue(out, cov.last_folded_ref_id.writer_epoch);
+        writeKey(out, "lfs", first);  writeU64StringValue(out, cov.last_folded_ref_id.ref_sequence);
+        if (cov.hold)
+        {
+            writeKey(out, "hr", first);  writeStringValue(out, holdReasonToWord(cov.hold->reason));
+            writeKey(out, "hpe", first); writeU64StringValue(out, cov.hold->offending_position.writer_epoch);
+            writeKey(out, "hps", first); writeU64StringValue(out, cov.hold->offending_position.ref_sequence);
+            writeKey(out, "hrc", first); writeIntText(cov.hold->retry_count, out);
+            writeKey(out, "hnr", first); writeU64StringValue(out, cov.hold->next_retry_round);
+        }
+        if (life_state.cleanup_evidence)
+        {
+            writeKey(out, "rte", first);
+            writeU64StringValue(out, life_state.cleanup_evidence->remove_txn_id.writer_epoch);
+            writeKey(out, "rts", first);
+            writeU64StringValue(out, life_state.cleanup_evidence->remove_txn_id.ref_sequence);
+        }
+        closeObject(out, first);
+        closeLine("rfl");
+        ++n;
+    }
+
+    {
+        std::vector<RunRef> runs = seal.blob_target_runs;
+        std::sort(runs.begin(), runs.end(), [](const RunRef & a, const RunRef & b) { return a.key < b.key; });
+        for (const RunRef & r : runs)
+        {
+            writeRun(out, "btr", r);
+            closeLine("btr");
+        }
+    }
+    n += seal.blob_target_runs.size();
+
+    /// condemned summary (std::map<uint64> => shard-sorted)
+    for (const auto & [shard, s] : seal.condemned_summary)
+    {
+        bool first = true;
+        writeKey(out, "k", first);     writeStringValue(out, "cnd");
+        writeKey(out, "shard", first); writeIntText(shard, out);
+        writeKey(out, "ct", first);    writeIntText(s.condemned_total, out);
+        writeKey(out, "pt", first);    writeIntText(s.pending_total, out);
+        writeKey(out, "ocr", first);   writeU64StringValue(out, s.oldest_nonpending_condemn_round);
+        closeObject(out, first);
+        closeLine("cnd");
+        ++n;
+    }
+
+    const size_t trailer_start = out.size();
+    writeTrailerLine(out, n);   /// emits its own terminator
+    checkLineBytes(out.size() - trailer_start - 1, "trailer");
+
+    String text = std::move(out).take();
+    checkFoldSealObjectBytes(text.size());
+    return text;
+}
+
+CasFoldSeal decodeFoldSeal(std::string_view data, std::optional<uint64_t> expected_generation)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::FoldSeal);
+    const uint64_t line_cap = traitsFor(FormatId::FoldSeal).line_cap;
+
+    CasFoldSeal seal;
+
+    /// meta line
+    {
+        const String meta = readLine(in, line_cap, "fold seal");
+        ReadBufferFromMemory m(meta.data(), meta.size());
+        JsonObjectReader r(m, KeyStrictness::Strict, "fold seal");
+        String key;
+        while (r.nextKey(key))
+        {
+            if (key == "g") seal.generation = r.readU64String();
+            else if (key == "pg") seal.parent_generation = r.readU64String();
+            else r.skipUnknown(key);   /// Strict => any unknown key is CORRUPTED_DATA
+        }
+    }
+
+    uint64_t seen = 0;
+    while (true)
+    {
+        const String line = readLine(in, line_cap, "fold seal");
+        ReadBufferFromMemory l(line.data(), line.size());
+        JsonObjectReader r(l, KeyStrictness::Strict, "fold seal");
+        String key;
+        if (!r.nextKey(key))
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: empty line");
+
+        if (key == "n")
+        {
+            const uint64_t n = r.readU64Number();
+            if (r.nextKey(key))
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: trailer has extra keys");
+            if (!l.eof() || !in.eof())
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: bytes after trailer");
+            if (n != seen)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS fold seal: trailer count {} != {} records", n, seen);
+            if (expected_generation && seal.generation != *expected_generation)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS fold seal: body generation {} does not match the requested generation {}",
+                    seal.generation, *expected_generation);
+            return seal;
+        }
+        if (key != "k")
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: record must start with \"k\"");
+        const String kind = r.readString();
+
+        if (kind == "rfl")
+        {
+            std::optional<UInt128> life_id;
+            RefCoverage cov;
+            /// Read WIDE and validated before it is narrowed to the persisted byte. `cls` is the field
+            /// every consumer branches on, and a plain `static_cast<uint8_t>` maps 258 onto 2 ("all
+            /// records through the cursor were folded") and 256 onto 0 — a forged or damaged seal would
+            /// buy full coverage with an integer no reader ever sees.
+            std::optional<uint64_t> classification;
+            /// The hold fields are read individually so the grammar can be checked on WHICH of them
+            /// arrived, not merely on how many. `JsonObjectReader` already rejects a duplicate key, so
+            /// a second `hr` can never quietly rewrite the reason.
+            std::optional<HoldReason> hold_reason;
+            std::optional<uint64_t> hold_epoch;
+            std::optional<uint64_t> hold_sequence;
+            std::optional<uint32_t> hold_retry_count;
+            std::optional<uint64_t> hold_next_retry_round;
+            std::optional<uint64_t> remove_txn_epoch;
+            std::optional<uint64_t> remove_txn_sequence;
+            while (r.nextKey(key))
+            {
+                if (key == "life") life_id = r.readHex128();
+                else if (key == "cls") classification = r.readU64Number();
+                else if (key == "lfe") cov.last_folded_ref_id.writer_epoch = r.readU64String();
+                else if (key == "lfs") cov.last_folded_ref_id.ref_sequence = r.readU64String();
+                else if (key == "hr") hold_reason = holdReasonFromWord(r.readString());
+                else if (key == "hpe") hold_epoch = r.readU64String();
+                else if (key == "hps") hold_sequence = r.readU64String();
+                else if (key == "hrc") hold_retry_count = r.readU32Number();
+                else if (key == "hnr") hold_next_retry_round = r.readU64String();
+                else if (key == "rte") remove_txn_epoch = r.readU64String();
+                else if (key == "rts") remove_txn_sequence = r.readU64String();
+                else throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: unknown rfl key '{}'", key);
+            }
+
+            if (!life_id || *life_id == 0)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS fold seal: a ref-life row is missing a nonzero opaque life id");
+            const String life_hex = u128ToHex(*life_id);
+
+            /// `cls` is required, not defaulted: an absent one would read as 0 ("no round folded this
+            /// namespace"), which is a claim about a fold, not the absence of one.
+            if (!classification)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: rfl '{}' missing cls", life_hex);
+            if (!isKnownClassification(*classification))
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS fold seal: coverage '{}' has classification {}, which is not one of the four "
+                    "the fold grammar defines (0 absent, 1 unchanged, 2 folded, 4 clamped)",
+                    life_hex, *classification);
+            cov.classification = static_cast<uint8_t>(*classification);   /// in range, so narrowing is exact
+
+            /// The same strict grammar the encoder enforces, applied to bytes we did not write. A
+            /// PARTIAL hold is corruption, never a hold with defaults: a hold whose offending position
+            /// defaulted to `{0,0}` would be cleared by the very first record the next round folds.
+            const bool any_hold_field = hold_reason || hold_epoch || hold_sequence
+                || hold_retry_count || hold_next_retry_round;
+            const bool every_hold_field = hold_reason && hold_epoch && hold_sequence
+                && hold_retry_count && hold_next_retry_round;
+            if (cov.classification == 4)
+            {
+                if (!every_hold_field)
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS fold seal: coverage '{}' is held (classification 4) but its hold is "
+                        "incomplete — reason, offending position, retry count and next retry round are "
+                        "all required", life_hex);
+                /// PRESENT is not enough: the position must be one a fold can actually retry. `{0, 0}`
+                /// (or either component zero) is the shape that quietly deletes the hold — the carry
+                /// rule keeps a hold only while the walk stops BELOW it, and nothing is below zero — and
+                /// it cannot be rendered where the sweep names the position it retained a manifest for.
+                if (!isCanonicalHoldPosition(RefTxnId{*hold_epoch, *hold_sequence}))
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS fold seal: coverage '{}' is held at {}-{} — a hold's offending position has "
+                        "both components nonzero; a zero one clears itself on the next round",
+                        life_hex, *hold_epoch, *hold_sequence);
+                cov.hold = RefHold{.reason = *hold_reason,
+                                   .offending_position = RefTxnId{*hold_epoch, *hold_sequence},
+                                   .retry_count = *hold_retry_count,
+                                   .next_retry_round = *hold_next_retry_round};
+            }
+            else if (any_hold_field)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS fold seal: coverage '{}' carries hold fields at classification {} — they are "
+                    "forbidden on anything but a held (classification 4) row",
+                    life_hex, cov.classification);
+
+            const bool any_cleanup_field = remove_txn_epoch || remove_txn_sequence;
+            const bool every_cleanup_field = remove_txn_epoch && remove_txn_sequence;
+            std::optional<RefCleanupEvidence> cleanup_evidence;
+            if (any_cleanup_field)
+            {
+                if (!every_cleanup_field || *remove_txn_epoch == 0 || *remove_txn_sequence == 0)
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS fold seal: ref life '{}' carries incomplete or zero cleanup evidence -- "
+                        "both removal transaction components are required and nonzero",
+                        life_hex);
+                cleanup_evidence = RefCleanupEvidence{
+                    .remove_txn_id = RefTxnId{*remove_txn_epoch, *remove_txn_sequence}};
+            }
+            if (!seal.ref_lives.emplace(
+                    *life_id, RefLifeFoldState{.coverage = cov, .cleanup_evidence = cleanup_evidence}).second)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS fold seal: a second ref-life record for '{}' -- a life id appears at most once",
+                    life_hex);
+        }
+        else if (kind == "btr")
+        {
+            std::optional<String> run_key;
+            std::optional<UInt128> checksum;
+            std::optional<uint64_t> shard;
+            std::optional<uint64_t> generation;
+            while (r.nextKey(key))
+            {
+                if (key == "key") run_key = r.readString();
+                else if (key == "ck") checksum = r.readHex128();
+                else if (key == "shard") shard = r.readU64Number();
+                else if (key == "gen") generation = r.readU64String();
+                else throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: unknown run key '{}'", key);
+            }
+            if (!run_key || !checksum || !shard || !generation)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS fold seal: btr requires key, ck, shard, and gen");
+            seal.blob_target_runs.push_back(RunRef{
+                .key = std::move(*run_key), .checksum = *checksum, .shard = *shard, .generation = *generation});
+        }
+        else if (kind == "cnd")
+        {
+            std::optional<uint64_t> shard;
+            std::optional<uint64_t> condemned_total;
+            std::optional<uint64_t> pending_total;
+            std::optional<uint64_t> oldest_nonpending_condemn_round;
+            while (r.nextKey(key))
+            {
+                if (key == "shard") shard = r.readU64Number();
+                else if (key == "ct") condemned_total = r.readU64Number();
+                else if (key == "pt") pending_total = r.readU64Number();
+                else if (key == "ocr") oldest_nonpending_condemn_round = r.readU64String();
+                else throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: unknown cnd key '{}'", key);
+            }
+            if (!shard || !condemned_total || !pending_total || !oldest_nonpending_condemn_round)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS fold seal: cnd requires shard, ct, pt, and ocr");
+            insertRecordOnce(seal.condemned_summary, *shard, CondemnedSummary{
+                .condemned_total = *condemned_total,
+                .pending_total = *pending_total,
+                .oldest_nonpending_condemn_round = *oldest_nonpending_condemn_round}, "condemned-summary");
+        }
+        else
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: unknown record kind '{}'", kind);
+
+        if (!l.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS fold seal: junk after record");
+        ++seen;
+    }
+}
+
+CasFoldSeal decodeFoldSeal(
+    std::string_view data, const Layout & layout, uint64_t gc_shards,
+    std::optional<uint64_t> expected_generation)
+{
+    CasFoldSeal seal = decodeFoldSeal(data, expected_generation);
+    validateFoldSealStructure(seal, layout, gc_shards, ErrorCodes::CORRUPTED_DATA, "adoption");
+    return seal;
+}
+
+void validateFoldSealForWrite(const CasFoldSeal & seal, const Layout & layout, uint64_t gc_shards)
+{
+    validateFoldSealStructure(seal, layout, gc_shards, ErrorCodes::LOGICAL_ERROR, "producer");
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h
new file mode 100644
index 000000000000..f6d00a6e3b50
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h
@@ -0,0 +1,225 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <map>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+class Layout;
+
+/// A reference to one write-once run object and its whole-object checksum. A retry can compare the
+/// checksum with the bytes already sealed before it adopts or consumes the run.
+///
+/// `shard` and `generation` are required on `blob_target_runs`. An idle shard can carry a parent's run
+/// into a newer seal, even though the object remains under the older generation's key namespace. The
+/// explicit fields therefore preserve both the shard association and the physical generation without
+/// making consumers parse a storage key. Run references for objects that are always local to the current
+/// generation may leave these fields at their defaults because their consumers resolve them by key.
+struct RunRef
+{
+    String key;
+    UInt128 checksum{};
+    uint64_t shard = 0;        /// gc-shard this run belongs to (REQUIRED for blob_target_runs)
+    uint64_t generation = 0;   /// generation whose key namespace physically holds the object (for retention)
+    bool operator==(const RunRef &) const = default;
+};
+
+/// Why one namespace is held below its ref-log frontier. A BOUNDED enum: every value is a shape the
+/// fold can name exactly, so an operator reading a seal learns what stopped the namespace without
+/// correlating logs. Persisted as a word, so an unknown word is `CORRUPTED_DATA` rather than a silently
+/// reinterpreted integer.
+///
+/// THESE ARE WIRE VALUES, AND THEY ARE APPEND-ONLY. A durable seal written by one build is read by
+/// another, so a renumbered value or a reused word makes an older seal describe a hold that is not the
+/// one it recorded — and a hold's whole job is to say truthfully what stopped a namespace and where.
+/// Add new reasons at the end; never renumber, never repurpose a retired word.
+enum class HoldReason : uint8_t
+{
+    GapBelowWitness = 1,        /// 404 at the expected id with a durable witness above it, same epoch
+    UnconsumedSealCrossing = 2, /// a later epoch is reachable but this epoch's closing seal was never consumed
+    WitnessDisappeared = 3,     /// an above-cursor record stopped answering GETs — corruption, never clearance
+    BodyUndecodable = 4,        /// the ref-log body at the position is present but cannot be decoded/extracted
+    ManifestBodyMissing = 5,    /// a part-manifest body the position's edges name is absent (the fold barrier)
+    CheckpointUndecodable = 6,  /// the namespace's `_ckpt` is present but its body cannot be decoded
+};
+
+/// The wire word one `HoldReason` is persisted as. Exported because the reason is operator-facing well
+/// beyond the codec — the sweep names it when the §6 deletion premise retains a manifest under a held
+/// namespace — and a second rendering of these words elsewhere would be a second place for them to drift.
+std::string_view holdReasonToWord(HoldReason r);
+
+/// The durable hold on one namespace. It rides `RefCoverage` across rounds and across `REBUILD`, and
+/// clears ONLY by folding through `offending_position` and adopting the result in `gc/state` — never by
+/// observing another absent, because an absent is exactly the observation a lying store produces.
+struct RefHold
+{
+    HoldReason reason = HoldReason::GapBelowWitness;
+
+    /// The exact position the fold must resolve before this namespace may advance. A carried hold makes
+    /// the next round read this key even when the round's hint omits the namespace entirely.
+    ///
+    /// A CANONICAL id: both components are nonzero, and both codecs enforce it. `{0, 0}` is not a
+    /// degenerate hold, it is a hold that ERASES ITSELF — every position compares at or above it, so the
+    /// first fold that reaches any record clears the hold as resolved, and the namespace advances with
+    /// nothing recording that it ever stopped. It is also unnameable: `renderRefTxnId` refuses a zero
+    /// component, so the sweep that retains a manifest "because the namespace is held at <position>"
+    /// cannot even state its reason. The decoder rejects it as `CORRUPTED_DATA` and the encoder as
+    /// `LOGICAL_ERROR`.
+    RefTxnId offending_position{};
+
+    /// How many rounds have retried `offending_position` without resolving it. Purely observational:
+    /// it is what distinguishes a transient barrier (a writer still uploading a manifest body) from a
+    /// namespace that has been stuck for hours.
+    uint32_t retry_count = 0;
+
+    /// The first round that retries `offending_position`. The fold retries every round today (a hold
+    /// costs one exact `GET`, and a transient barrier must clear the moment its body lands), so this is
+    /// always `current_round + 1`; it exists so a future backoff policy needs no format change.
+    uint64_t next_retry_round = 0;
+
+    bool operator==(const RefHold &) const = default;
+};
+
+/// Records what the current round did for one life-keyed `CasFoldSeal::ref_lives` row.
+/// `classification` is a persisted byte:
+/// 0 means absent, 1 means unchanged, 2 means all records through the observed cursor were folded, and 4
+/// means folding was clamped below the ref-log cursor. A clamped entry must be read again in the next
+/// round, because an unfolded event may become foldable by then.
+///
+/// THE SET {0, 1, 2, 4} IS CLOSED, and both codecs enforce it (decode `CORRUPTED_DATA`, encode
+/// `LOGICAL_ERROR`). Every consumer branches on exact values — the sweep's §6 deletion premise refuses a
+/// row by testing `== 4` and `== 0` — so an unrecognized byte is not a variant to tolerate: it passes
+/// every refusal stated in terms of the set and reaches the delete. The decoder also validates BEFORE
+/// narrowing to the byte, because a wide integer on the wire (258, say) truncates into the set and would
+/// otherwise claim a coverage the fold never proved.
+struct RefCoverage
+{
+    uint8_t classification = 0;
+
+    /// The greatest `RefTxnId` whose owner changes have contributed their manifest-edge deltas. There is
+    /// one ref-log stream per namespace life, so this cursor is stored in that life-keyed row.
+    /// `{0, 0}` means that no transaction has been folded yet. A clamp leaves the cursor below the
+    /// offending transaction so the complete transaction is retried rather than partially applied.
+    RefTxnId last_folded_ref_id{};
+
+    /// STRICT GRAMMAR: present if and only if `classification == 4`. Both directions enforce it — the
+    /// encoder refuses to write a classification-4 row without a hold (a clamp whose reason was lost is
+    /// indistinguishable from a clean cursor once it is durable) and refuses to write a hold on any
+    /// other classification (`LOGICAL_ERROR`); the decoder rejects both shapes as `CORRUPTED_DATA`. The
+    /// pairing lives in the type, not only in the codec, so no producer can construct the forbidden
+    /// combination by forgetting a field.
+    std::optional<RefHold> hold = std::nullopt;
+
+    bool operator==(const RefCoverage &) const = default;
+};
+
+/// Positive evidence that the terminal `remove_namespace` transaction for one life was folded into
+/// the adopted seal. The owning opaque life id is the `CasFoldSeal::ref_lives` map key; the evidence
+/// therefore carries no duplicate namespace or incarnation and has no pending/completed state.
+struct RefCleanupEvidence
+{
+    RefTxnId remove_txn_id{};
+
+    bool operator==(const RefCleanupEvidence &) const = default;
+};
+
+/// The complete durable fold state for one cataloged ref life. Coverage and optional terminal
+/// evidence live in the same row so neither can be admitted by an independent producer.
+struct RefLifeFoldState
+{
+    RefCoverage coverage;
+    std::optional<RefCleanupEvidence> cleanup_evidence = std::nullopt;
+
+    bool operator==(const RefLifeFoldState &) const = default;
+};
+
+/// Per-shard summary of condemned rows carried in the sealed source-edge run. It lets graduation and
+/// pure reference-carry decisions inspect the seal without reading a run. Every newly written seal has
+/// an entry for every shard in `0..gc_shards-1`: a folding shard computes its entry from its remaining
+/// condemned rows, while a pure-carry shard copies the parent's entry. Missing entries are invalid and
+/// must not be interpreted as zero.
+struct CondemnedSummary
+{
+    uint64_t condemned_total = 0;   /// count of `kCondemned` rows in this shard's sealed run
+    uint64_t pending_total = 0;     /// how many of those are `delete_pending` (a graduation is due)
+    uint64_t oldest_nonpending_condemn_round = UINT64_MAX;   /// min condemn_round over non-pending; UINT64_MAX = none
+    bool operator==(const CondemnedSummary &) const = default;
+};
+
+/// The write-once fold seal for one GC generation at
+/// `<prefix>/gc/gen/<generation>/attempt/<attempt>/fold_seal`. It is the generation's durable coverage
+/// record: it stores cursors and run references, not one record per edge, manifest, or candidate. A retry
+/// and the next round use the adopted seal to determine what was folded and which parent runs can be
+/// carried forward. Its run references and ref-life rows are also the durable inputs to retention.
+/// Manifest cleanup is intentionally not represented here: those cleanups execute
+/// inline from the in-memory cleanup map, and no durable cleanup-run reader exists.
+struct CasFoldSeal
+{
+    uint64_t generation = 0;
+    uint64_t parent_generation = 0;
+    /// Exactly one row per `Live` or `Removing` catalog life admitted by `buildRefWalkPlan`.
+    std::map<UInt128, RefLifeFoldState> ref_lives;
+    std::vector<RunRef> blob_target_runs;           /// the blob in-degree run segments this gen sealed
+    std::map<uint64_t, CondemnedSummary> condemned_summary;   /// gc-shard -> summary; TOTAL over gc_shards
+    bool operator==(const CasFoldSeal &) const = default;
+};
+
+/// The two byte caps a fold seal is measured against, read from the format registry so the writer's
+/// gate and its boundary tests can never drift from the reader's limits.
+struct FoldSealCaps
+{
+    uint64_t line_cap;     /// longest decodable record, excluding the '\n' terminator
+    uint64_t object_cap;   /// largest whole seal object
+};
+FoldSealCaps foldSealCaps();
+
+/// PRE-PUT GATE. A seal larger than the object cap is writable but not readable — nothing enforces the
+/// cap on the fold-seal read path, so an oversized PUT would leave the pool with a durable seal that no
+/// later round can decode, which is unrecoverable. `encodeFoldSeal` therefore refuses BEFORE returning
+/// any bytes, which is before either PUT site can issue its write. `LIMIT_EXCEEDED`, not
+/// `CORRUPTED_DATA`: the bytes are well formed, the round is over budget, and the round fails closed and
+/// retries. Equality fits; one byte more does not.
+void checkFoldSealObjectBytes(uint64_t encoded_bytes);
+
+/// Encodes a fold seal as a strict, raw text control object. The header and meta lines are followed by
+/// tagged records in the fixed `rfl`/`btr`/`cnd` order and a record-count trailer. Map iteration and
+/// run references are sorted so retries produce byte-identical output for write-once adoption.
+///
+/// Enforces the whole coverage grammar — the closed classification set, the classification-4 hold
+/// pairing, and the hold's canonical offending position — and BOTH byte caps: every emitted line against
+/// `line_cap` — header, meta, records and trailer alike, with no exception — and the whole object
+/// against `object_cap`. Both PUT sites go through this function, so the gate cannot be bypassed by
+/// adding a third one.
+///
+/// A grammar violation here is `LOGICAL_ERROR`, not `CORRUPTED_DATA`: these bytes do not come from a
+/// store, they come from THIS process, so the seal it is about to make durable is a bug in our own
+/// writer — the same code `encodeGcState` raises for its own impossible input. The caps stay
+/// `LIMIT_EXCEEDED` (a well-formed seal, over budget). Nothing is returned on any refusal, so no PUT
+/// site can issue the write.
+String encodeFoldSeal(const CasFoldSeal & seal);
+
+/// Decodes and validates a fold seal, rejecting unknown fields, malformed records, trailing bytes, a
+/// second record for a key already read, and a trailer count that differs from the records read.
+/// Invalid persisted data raises `CORRUPTED_DATA` — including every shape the encoder refuses, since
+/// these bytes may have been written by anything at all.
+CasFoldSeal decodeFoldSeal(std::string_view data, std::optional<uint64_t> expected_generation = std::nullopt);
+
+/// Decodes a seal at an adoption boundary and validates the shard-indexed structure against the
+/// pool-authoritative nonzero `gc_shards`: canonical blob-target keys, at most one run per shard,
+/// and an exactly total, semantically consistent condemned summary.
+CasFoldSeal decodeFoldSeal(
+    std::string_view data, const Layout & layout, uint64_t gc_shards,
+    std::optional<uint64_t> expected_generation = std::nullopt);
+
+/// Applies the same shard-indexed structural checks to an in-memory producer immediately before its
+/// seal PUT. Invalid producer state is a `LOGICAL_ERROR`; no bytes are written.
+void validateFoldSealForWrite(const CasFoldSeal & seal, const Layout & layout, uint64_t gc_shards);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.cpp
new file mode 100644
index 000000000000..cf46c2e51b62
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.cpp
@@ -0,0 +1,198 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Common/Exception.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int UNKNOWN_FORMAT_VERSION;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Generation-1 baseline for every class. A future format change appends to that class's array and
+/// bumps `G_BUILD`: additive changes use the previous reader floor, while breaking changes use the
+/// new generation as the floor. Existing entries are immutable history.
+constexpr FormatChangePoint BASELINE[] = {{1, 1}};
+
+/// The two ref classes changed at generation 4 (INV-1, per-namespace contiguous ids) AND AGAIN at
+/// generation 5 (Stage B's recreate-only "format bump B": the ref layer re-keyed under
+/// `<ns>/<incarnation>/`). Both changes are BREAKING even though not one byte of the encoding moved
+/// either time -- a generation-3 stream's ids came from a pool-wide counter and legitimately skip,
+/// which a generation-4 reader reports as corruption, and a generation-4 key names no incarnation at
+/// all, which a generation-5 reader also reports as corruption (`Layout::parseRefObjectKey`). Each
+/// floor is the change generation itself.
+constexpr FormatChangePoint REF_STREAM[] = {
+    {1, 1},
+    {kContiguousRefStreamsGeneration, kContiguousRefStreamsGeneration},
+    {kNamespaceLifeKeyedGeneration, kNamespaceLifeKeyedGeneration},
+    {kOpaqueNamespaceLifeLayoutGeneration, kOpaqueNamespaceLifeLayoutGeneration},
+};
+
+/// `cas_ref_ckpt` is BORN at generation 4, so it has no generation-1 baseline to inherit: there is no
+/// such thing as a generation-1 `_ckpt` object, and claiming one would say a generation-1 reader could
+/// read it. Generation 5 re-keys it under `<ns>/<incarnation>/` exactly like `REF_STREAM` above, for
+/// the same reason and with the same floor.
+constexpr FormatChangePoint REF_CKPT[] = {
+    {kContiguousRefStreamsGeneration, kContiguousRefStreamsGeneration},
+    {kNamespaceLifeKeyedGeneration, kNamespaceLifeKeyedGeneration},
+    {kOpaqueNamespaceLifeLayoutGeneration, kOpaqueNamespaceLifeLayoutGeneration},
+    {kCommittedRefFrontierGeneration, kCommittedRefFrontierGeneration},
+};
+
+/// `cas_ref_catalog` is BORN at generation 4, one generation BEFORE the bump that makes namespace
+/// existence catalog-authoritative (Stage B's Task 4, "format bump B" -- `kNamespaceLifeKeyedGeneration`):
+/// Task 2 introduced the catalog OBJECT while `G_BUILD` was still the value
+/// `kContiguousRefStreamsGeneration` names, and Task 4 is the later change that actually wires
+/// discovery to read it and bumps the floor. The catalog's own encoding is unaffected by that bump (it
+/// reuses `kContiguousRefStreamsGeneration` as its birth generation, not a second constant named after
+/// itself, for the same reason `REF_CKPT` originally did), so it carries no second change point here.
+constexpr FormatChangePoint REF_CATALOG[] = {{kContiguousRefStreamsGeneration, kContiguousRefStreamsGeneration}};
+constexpr FormatChangePoint GC_MAINTENANCE_STATE[] = {{kUnifiedRefLifeFoldGeneration, kUnifiedRefLifeFoldGeneration}};
+constexpr FormatChangePoint POOL_META[] = {
+    {1, 1},
+    {kPoolGcShardsGeneration, kPoolGcShardsGeneration},
+    {kCommittedRefFrontierGeneration, kCommittedRefFrontierGeneration},
+};
+
+}
+
+std::span<const FormatChangePoint> changePoints(FormatId id)
+{
+    switch (id)
+    {
+        case FormatId::RefLog:
+        case FormatId::RefSnapshot:
+            return REF_STREAM;
+        case FormatId::RefCkpt:
+            return REF_CKPT;
+        case FormatId::RefCatalog:
+            return REF_CATALOG;
+        case FormatId::GcMaintenanceState:
+            return GC_MAINTENANCE_STATE;
+        case FormatId::PoolMeta:
+            return POOL_META;
+        case FormatId::Blob:
+        case FormatId::GcState:
+        case FormatId::Roster:
+        case FormatId::GcOutcomes:
+        case FormatId::PartManifest:
+        case FormatId::RunFile:
+        case FormatId::FoldSeal:
+        case FormatId::Owner:
+        case FormatId::ServerEpoch:
+        case FormatId::MountLease:
+        case FormatId::BlobMeta:
+        case FormatId::GcHeartbeat:
+            return BASELINE;
+    }
+    throw Exception(ErrorCodes::LOGICAL_ERROR, "CasFormat: unknown FormatId {}", static_cast<int>(id));
+}
+
+uint32_t currentWriterVersion()
+{
+    return G_BUILD;
+}
+
+uint32_t currentCompatibilityVersion()
+{
+    /// Until roster-based write-down is implemented, every object carries the current build as its
+    /// compatibility floor.
+    return G_BUILD;
+}
+
+void checkCompatibility(uint32_t compatibility_version, std::string_view what)
+{
+    if (compatibility_version > G_BUILD)
+        throw Exception(ErrorCodes::UNKNOWN_FORMAT_VERSION,
+            "CAS {}: object requires reader generation {} but this build supports at most {}",
+            what, compatibility_version, G_BUILD);
+}
+
+namespace
+{
+constexpr uint64_t kKiB = 1024;
+constexpr uint64_t kMiB = 1024 * 1024;
+
+/// Caps are 100-1000x above realistic sizes; hitting one indicates a corrupt object or protocol bug.
+/// `RefLog` and `RefSnapshot` objects are read whole, so their 64 MiB decompressed object cap
+/// accommodates the JSON-inflated removal-class transaction and full snapshot. Their codecs
+/// independently enforce the existing `ref_txn_max_bytes` (20 MiB) and 64 MiB removal/snapshot budgets
+/// before sealing.
+///
+/// Their `line_cap` intentionally equals `object_cap`. A smaller per-line limit would add no memory
+/// protection to a whole-read format, while creating a write/read split: admission measures the whole
+/// transaction against the object budget, so a large individual ref payload could be accepted on
+/// write and rejected on decode, leaving a persisted ref object that cannot be read. The line cap is
+/// instead meaningful for streaming formats, where it bounds the resident O(line) record. Matching
+/// `line_cap` to `object_cap` lets any individually valid record consume the available object budget.
+///
+/// Compression is per type and deterministic, with no size threshold. `Always` types can grow large
+/// and use a `.zst` key suffix; `PinnedRaw` types need stable bytes for adoption; `Never` types are
+/// small raw singletons.
+constexpr FormatTraits TRAITS[] =
+{
+    {FormatId::Blob,         "cas_blob",          TextFamily::PayloadHybrid, KeyStrictness::Tolerant, CompressionPolicy::Never,     256,        256},
+    {FormatId::BlobMeta,     "cas_blob_meta",     TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Never,     1 * kMiB,   64 * kKiB},
+    {FormatId::PoolMeta,     "cas_pool_meta",     TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Never,     1 * kMiB,   64 * kKiB},
+    {FormatId::RefLog,       "cas_ref_log",       TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Always,    64 * kMiB,  64 * kMiB},
+    {FormatId::RefSnapshot,  "cas_ref_snap",      TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Always,    64 * kMiB,  64 * kMiB},
+    /// `cas_ref_ckpt` is a three-field mutable singleton read by a point GET on every recovery and on
+    /// every cleanup decision, so its caps are deliberately TIGHT (64 KiB / 4 KiB rather than the
+    /// megabyte scale its Control-family siblings use): nothing legitimate approaches them, and the cap
+    /// is the first thing that fires if a foreign object ever lands at the key. STRICT for the same
+    /// reason its decoder is -- every field changes what cleanup may delete, so nothing in it may be
+    /// skipped. Raw (`Never`): a small singleton, and `publishCkpt` re-encodes it on every attempt.
+    {FormatId::RefCkpt,      "cas_ref_ckpt",      TextFamily::Control,       KeyStrictness::Strict,   CompressionPolicy::Never,     64 * kKiB,  4 * kKiB},
+    /// `cas_ref_catalog` (INV-3): one object for the whole pool, token-CAS like `gc/state`, read on
+    /// every fold round and every recovery. STRICT for the same reason `cas_ref_ckpt` is -- every
+    /// field decides a namespace's lifecycle, so nothing in it may be skipped. Raw (`Never`): the
+    /// admission gate measures `encodeRefCatalog`'s own output directly, so a compressed size would
+    /// answer the wrong question. The object cap is the fold-seal's own 256 MiB (predicate (2) of the
+    /// additive admission check bounds it further via the entry count); the line cap is tight (4 KiB)
+    /// because one entry's record is ordinarily small -- but not always small enough: a namespace or
+    /// `server_root_id` near their own byte bounds, worst-case escaped, can push a single line past
+    /// 4 KiB, and `encodeRefCatalog` REFUSES that entry (`LIMIT_EXCEEDED`, `CasRefCatalogFormat.cpp`'s
+    /// `checkLineBytes`) rather than writing an object no reader could later decode.
+    {FormatId::RefCatalog,   "cas_ref_catalog",   TextFamily::Control,       KeyStrictness::Strict,   CompressionPolicy::Never,     256 * kMiB, 4 * kKiB},
+    {FormatId::GcMaintenanceState, "cas_gc_maintenance_state", TextFamily::Control, KeyStrictness::Strict, CompressionPolicy::Never, 512 * kKiB, 512 * kKiB},
+    {FormatId::PartManifest, "cas_part_manifest", TextFamily::PayloadHybrid, KeyStrictness::Tolerant, CompressionPolicy::Always,    256 * kMiB, 64 * kKiB},
+    {FormatId::RunFile,      "cas_run",           TextFamily::RecordStream,  KeyStrictness::Strict,   CompressionPolicy::PinnedRaw, 0,          4 * kKiB},
+    {FormatId::FoldSeal,     "cas_fold_seal",     TextFamily::Control,       KeyStrictness::Strict,   CompressionPolicy::PinnedRaw, 256 * kMiB, 64 * kKiB},
+    {FormatId::GcState,      "cas_gc_state",      TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Never,     1 * kMiB,   64 * kKiB},
+    {FormatId::GcHeartbeat,  "cas_gc_hb",         TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Never,     1 * kMiB,   64 * kKiB},
+    {FormatId::GcOutcomes,   "cas_gc_outcomes",   TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Always,    256 * kMiB, 64 * kKiB},
+    {FormatId::Owner,        "cas_owner",         TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Never,     1 * kMiB,   64 * kKiB},
+    {FormatId::ServerEpoch,  "cas_epoch",         TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Never,     1 * kMiB,   64 * kKiB},
+    {FormatId::MountLease,   "cas_mount_lease",   TextFamily::Control,       KeyStrictness::Tolerant, CompressionPolicy::Never,     1 * kMiB,   64 * kKiB},
+};
+}
+
+const FormatTraits & traitsFor(FormatId id)
+{
+    for (const FormatTraits & t : TRAITS)
+        if (t.id == id)
+            return t;
+    throw Exception(ErrorCodes::LOGICAL_ERROR, "CasFormat: no traits for FormatId {} (reserved?)", static_cast<uint16_t>(id));
+}
+
+const FormatTraits * traitsForType(std::string_view type)
+{
+    for (const FormatTraits & t : TRAITS)
+        if (t.type == type)
+            return &t;
+    return nullptr;
+}
+
+std::string_view storedSuffix(FormatId id)
+{
+    return traitsFor(id).compression == CompressionPolicy::Always ? ".zst" : "";
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h
new file mode 100644
index 000000000000..633f8f8fd7be
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h
@@ -0,0 +1,203 @@
+#pragma once
+#include <cstdint>
+#include <span>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// Shared vocabulary and policy registry for persisted content-addressed objects. The registry
+/// supplies the version gate and the per-object text-file traits used by the codecs in this
+/// directory; it does not own object bytes or storage lifecycle state.
+
+/// The highest pool-format generation this build understands. A build keeps every decoder for
+/// generations 1..G_BUILD (new code always reads old); an object is readable iff its
+/// compatibility_version <= G_BUILD. Bump this (and append a change-point in CasFormat.cpp) when a new
+/// format generation is introduced.
+///
+/// Generation 2 is the first generation that understands mixed-algorithm pools: the schema-3
+/// source-edge settlement key includes the algorithm prefix, so a generation-1 reader can open the
+/// pool but cannot decode its GC state. Pool admission CAS-raises `min_reader_generation` to this
+/// build's own floor (`G_BUILD`), and a persisted floor above `G_BUILD` fails closed.
+///
+/// Generation 3 replaced mutable ref-shard objects with immutable `_log` and `_snap` objects.
+///
+/// Generation 4 makes each namespace's ref-log ids per-namespace and CONTIGUOUS within a writer epoch
+/// (INV-1). The bytes of a `_log`/`_snap` object did not change, but their MEANING did: a generation-3
+/// pool's ids were drawn from a pool-wide counter and are full of legitimate holes, which this build
+/// reads as a truncated -- i.e. corrupt -- stream. The per-object forward gate cannot reject such a
+/// pool (its version is not in the future), so pool-meta decoding applies
+/// `kContiguousRefStreamsGeneration` as a backward floor. Pools below the floor must be recreated;
+/// there is no migration path in the pre-release format.
+///
+/// Generation 5 (Stage B's own recreate-only bump, the plan's "format bump B") re-keys the ref layer
+/// under `<ns>/<incarnation>/` (spec INV-3: the whole-pool namespace catalog mints the incarnation).
+/// Again the bytes of `_log`/`_snap`/`_ckpt` objects did not change, but the KEY SHAPE they live under
+/// did: a generation-4 key named a namespace directly (`cas/refs/<ns>/_log/<id>`), while this
+/// generation's reader recognizes only the incarnation-qualified shape
+/// (`cas/refs/<ns>/<incarnation>/_log/<id>`) -- `Layout::parseRefObjectKey`/`parseRefCkptKey` already
+/// refuse the un-incarnated shape with `CORRUPTED_DATA` (Stage B Tasks 1/1c landed that refusal ahead
+/// of this bump, deliberately: the pre-release format carries zero persisted data and zero compat
+/// obligation, so the key shapes and the bump that makes them the ONLY readable shape need not land in
+/// the same commit). `kNamespaceLifeKeyedGeneration` is the backward floor for this change, applied the
+/// same way `kContiguousRefStreamsGeneration` is.
+///
+/// Generation 6 replaces that namespace-bearing grammar with opaque pool-wide life identifiers and
+/// splits hot ref streams from point-read state: `cas/ns/stream/<life_id>/...` contains `_log`, `_snap`
+/// while `cas/ns/state/<life_id>/...` contains `_ckpt` and `_files`. A generation-5
+/// pool must be recreated; no dual parser or copy-forward path exists.
+///
+/// Generation 7 replaces the fold seal's independent namespace-keyed coverage and cleanup
+/// collections with one opaque-life-keyed row and removes the retired terminal-marker object class. A generation-6
+/// pool must be recreated; there is no dual reader for the split grammar.
+///
+/// Generation 8 persists the creation-time `gc_shards` authority in `_pool_meta`. Generation-7 pools
+/// must be recreated because namespace admission can precede creation of `gc/state`; accepting a
+/// metadata object without this field would leave different openers charging different seal bounds.
+/// Generation 9 adds `_ckpt.committed_through`, the exact recovery frontier. Generation-8 pools
+/// must be recreated: the absence of this field has the incompatible meaning that no transaction has
+/// entered durable logical history.
+constexpr uint32_t G_BUILD = 9;
+
+/// The pool-format generation at which ref-log ids became per-namespace and contiguous. Pool metadata
+/// below this value cannot be opened, because its ref streams carry holes this build reports as
+/// corruption; the backward-floor check is applied by `decodePoolMeta`. Named separately from `G_BUILD`
+/// so a later generation that CAN still read a generation-4 pool does not silently move the floor with
+/// it.
+constexpr uint32_t kContiguousRefStreamsGeneration = 4;
+
+/// The pool-format generation at which the ref layer (and, per Stage B's Task 4b, namespace files)
+/// became incarnation-scoped under `<ns>/<incarnation>/`. Pool metadata below this value cannot be
+/// opened: its ref-object keys carry no incarnation segment, which this build's parsers refuse as
+/// corruption rather than read as a compatibility case (see the `G_BUILD` doc above). The backward-
+/// floor check is applied by `decodePoolMeta`, exactly mirroring `kContiguousRefStreamsGeneration`;
+/// named separately for the same reason that one is -- so a later generation that can still read a
+/// generation-5 pool does not silently move this floor with it. Pools below the floor must be
+/// recreated; there is no migration path in the pre-release format.
+constexpr uint32_t kNamespaceLifeKeyedGeneration = 5;
+
+/// The recreate-only generation at which namespace text disappeared from physical life keys and hot
+/// ref streams were separated from point-read namespace state.
+constexpr uint32_t kOpaqueNamespaceLifeLayoutGeneration = 6;
+
+/// The recreate-only generation at which one unified ref-life row replaced the split coverage and
+/// namespace-cleanup grammar.
+constexpr uint32_t kUnifiedRefLifeFoldGeneration = 7;
+
+/// The recreate-only generation at which `_pool_meta` became the authority for `gc_shards`.
+constexpr uint32_t kPoolGcShardsGeneration = 8;
+
+/// The recreate-only generation at which `_ckpt` gained its exact committed-transaction frontier.
+constexpr uint32_t kCommittedRefFrontierGeneration = 9;
+
+/// Stable identifiers for every self-describing persisted object class. The text registry uses the
+/// corresponding `type` string as the on-disk identity. Numeric values are part of the format history:
+/// retired values remain unused so an old object can never be mistaken for a later class.
+enum class FormatId : uint16_t
+{
+    Blob = 1,
+    /// Values 2, 3, and 4 are retired. The former tree and GC-snapshot classes were replaced by the
+    /// root-local part manifest, while the former mutable `cas_ref_shard` class was replaced by the
+    /// immutable `RefSnapshot` and `RefLog` objects. Keep all three values unused.
+    GcState = 5,
+    /// Value 6 is retired: condemned state now rides source-edge runs and the fold-seal
+    /// `condemned_summary`. Keep it unused. Value 7 is also retired: the build-watermark floor is
+    /// carried by the `MountLease` beat rather than a standalone object.
+    PoolMeta = 8,
+    Roster = 9,
+    /// Value 10 is retired: discovery authority is the pool-wide `cas/ref_catalog` object rather
+    /// than a roots registry object or a physical stream listing.
+    GcOutcomes = 11,
+    PartManifest = 12,    /// Immutable root-local `cas_part_manifest` payload-hybrid text object.
+    RunFile = 13,         /// Deterministic, uncompressed `cas_run` GC source-edge NDJSON stream.
+    FoldSeal = 14,        /// Write-once `cas_fold_seal` coverage and blob-target/cleanup-run object.
+    /// Value 15 is retired: the fold seal is the sole per-generation coverage record after the
+    /// one-pass GC round. The following three classes are per-server-root mount-safety objects.
+    Owner = 16,           /// `cas_owner` anchor from server-root ID to server UUID.
+    ServerEpoch = 17,     /// `cas_epoch` writer-epoch fence carrying `next_writer_epoch`.
+    MountLease = 18,      /// Live `cas_mount_lease` object.
+    /// These identifiers cover objects that were added to the registry after their initial codecs:
+    /// the ref transaction log, complete ref snapshot, blob freshness sidecar, and GC heartbeat.
+    /// Their values are frozen and must never be reused.
+    RefLog = 19,          /// cas_ref_log     — ref transaction log object
+    RefSnapshot = 20,     /// cas_ref_snap    — complete per-namespace ref table
+    BlobMeta = 21,        /// cas_blob_meta   — per-blob freshness sidecar
+    GcHeartbeat = 22,     /// cas_gc_hb       — GC leader heartbeat
+    RefCkpt = 23,         /// cas_ref_ckpt    — per-namespace checkpoint (INV-4)
+    RefCatalog = 24,      /// cas_ref_catalog — the whole-pool namespace catalog (INV-3)
+    GcMaintenanceState = 25, /// cas_gc_maintenance_state — leak-only namespace-janitor cursor
+};
+
+/// Returns the writer generation stamped on newly written objects. The current pre-roster writer
+/// always stamps `G_BUILD`.
+uint32_t currentWriterVersion();
+
+/// Returns the compatibility generation stamped on newly written objects. Until the roster and
+/// write-down-to-floor policy exist, this is always `G_BUILD`; readers reject values above `G_BUILD`.
+uint32_t currentCompatibilityVersion();
+
+/// Applies the common fail-closed reader gate. If an object's `compatibility_version` exceeds
+/// `G_BUILD`, throws `UNKNOWN_FORMAT_VERSION` before the caller interprets the body; `what` identifies
+/// the object in the exception message.
+void checkCompatibility(uint32_t compatibility_version, std::string_view what);
+
+/// One append-only entry in a class's format history. At `generation`, the class's ENCODING or the
+/// MEANING of what it encodes changed, and a reader must understand at least `min_reader` to read an
+/// object written at that generation. Additive changes retain the previous reader floor; breaking
+/// changes set the floor to the change generation itself. Generation 4's ref-stream entry is the
+/// worked example of the second kind: not one byte of `cas_ref_log` moved, but its ids became dense,
+/// so an older stream is unreadable to this build and the floor is the change generation.
+struct FormatChangePoint
+{
+    uint16_t generation;
+    uint16_t min_reader;
+};
+
+/// Returns the append-only change-point history for `id`, oldest first. A class's history begins at
+/// the generation it was BORN in, not at 1: the classes that existed from the start carry the frozen
+/// `{1, 1}` baseline, while `RefCkpt` — introduced at generation 4 — begins at `{4, 4}`, because there
+/// is no such thing as a generation-1 `_ckpt` and claiming one would say a generation-1 reader could
+/// read it. Future changes append entries without editing old ones.
+std::span<const FormatChangePoint> changePoints(FormatId id);
+
+/// The text-format registry has one row per decodable persisted object. Each row is the single source
+/// for the header-line `type`, body family, unknown-key policy, compression policy, and fail-closed
+/// size caps. A format missing from this registry cannot be decoded.
+
+/// The shape of a text object body: a materialized control object, a streamed sorted record sequence,
+/// or a descriptor followed by raw payload bytes.
+enum class TextFamily : uint8_t { Control = 1, RecordStream = 2, PayloadHybrid = 3 };
+
+/// Whether a decoder skips unknown ordinary keys or rejects them. Critical keys prefixed with `!`
+/// are rejected by all families because they signal a required extension.
+enum class KeyStrictness : uint8_t { Tolerant = 1, Strict = 2 };
+
+/// Deterministic storage policy. `Always` uses whole-object zstd and a `.zst` key suffix; `Never`
+/// remains raw; `PinnedRaw` is raw because byte adoption compares the serialized bytes.
+enum class CompressionPolicy : uint8_t { Never = 1, Always = 2, PinnedRaw = 3 };
+
+/// Codec metadata for one registered text object. The byte caps apply to decompressed object content
+/// and individual text lines; `object_cap == 0` means a streamed format has no whole-object cap.
+struct FormatTraits
+{
+    FormatId id;
+    std::string_view type;      /// header-line "type" value
+    TextFamily family;
+    KeyStrictness strictness;
+    CompressionPolicy compression;
+    uint64_t object_cap;        /// max DECOMPRESSED object bytes; 0 = uncapped (streamed)
+    uint64_t line_cap;          /// max bytes of one text line
+};
+
+/// Returns the traits for `id`. Throws `LOGICAL_ERROR` for `FormatId::Roster`, which is reserved and
+/// has no codec or traits row yet.
+const FormatTraits & traitsFor(FormatId id);
+/// Looks up a header-line `type` string. Returns nullptr for an unregistered type; it does not throw
+/// because callers use this result to classify the input before decoding it.
+const FormatTraits * traitsForType(std::string_view type);
+/// Returns the storage-key suffix for `id`: `.zst` for `Always`, and an empty suffix otherwise.
+/// Key builders use this policy directly so a point lookup never has to inspect the object body or
+/// try multiple keys.
+std::string_view storedSuffix(FormatId id);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.cpp
new file mode 100644
index 000000000000..c5dda3286ad6
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.cpp
@@ -0,0 +1,67 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LIMIT_EXCEEDED;
+}
+
+namespace DB::Cas
+{
+
+String encodeGcMaintenanceState(const GcMaintenanceState & state)
+{
+    if (state.janitor_cursor.size() > kMaxGcMaintenanceCursorBytes)
+        throw Exception(ErrorCodes::LIMIT_EXCEEDED, "CAS gc maintenance state: cursor has {} bytes, limit {}",
+            state.janitor_cursor.size(), kMaxGcMaintenanceCursorBytes);
+
+    CasJsonWriter out;
+    writeHeaderLine(out, FormatId::GcMaintenanceState);
+    bool first = true;
+    writeKey(out, "cur", first);
+    writeStringValue(out, state.janitor_cursor);
+    closeObject(out, first);
+    writeChar('\n', out);
+    return std::move(out).take();
+}
+
+GcMaintenanceState decodeGcMaintenanceState(std::string_view data)
+{
+    const uint64_t object_cap = traitsFor(FormatId::GcMaintenanceState).object_cap;
+    if (data.size() > object_cap)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS gc maintenance state: object has {} bytes, limit {}", data.size(), object_cap);
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::GcMaintenanceState);
+    const String body = readLine(in, traitsFor(FormatId::GcMaintenanceState).line_cap, "cas_gc_maintenance_state");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    JsonObjectReader reader(body_in, KeyStrictness::Strict, "cas_gc_maintenance_state");
+
+    GcMaintenanceState result;
+    bool has_cursor = false;
+    String key;
+    while (reader.nextKey(key))
+    {
+        if (key == "cur")
+        {
+            result.janitor_cursor = reader.readString();
+            has_cursor = true;
+        }
+        else
+            reader.skipUnknown(key);
+    }
+    if (!has_cursor)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS gc maintenance state: missing cur");
+    if (!body_in.eof() || !in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS gc maintenance state: trailing bytes");
+    if (result.janitor_cursor.size() > kMaxGcMaintenanceCursorBytes)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS gc maintenance state: cursor has {} bytes, limit {}",
+            result.janitor_cursor.size(), kMaxGcMaintenanceCursorBytes);
+    return result;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.h
new file mode 100644
index 000000000000..49d495995698
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.h
@@ -0,0 +1,21 @@
+#pragma once
+#include <base/types.h>
+#include <cstddef>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+constexpr size_t kMaxGcMaintenanceCursorBytes = 64 * 1024;
+
+/// Durable, leak-only progress for namespace maintenance. It has no GC authority fields.
+struct GcMaintenanceState
+{
+    String janitor_cursor;
+    bool operator==(const GcMaintenanceState &) const = default;
+};
+
+String encodeGcMaintenanceState(const GcMaintenanceState & state);
+GcMaintenanceState decodeGcMaintenanceState(std::string_view data);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.cpp
new file mode 100644
index 000000000000..e69ea98a799e
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.cpp
@@ -0,0 +1,129 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+std::string_view outcomeKindToWord(OutcomeKind o)
+{
+    switch (o)
+    {
+        case OutcomeKind::Deleted:  return "deleted";
+        case OutcomeKind::Absent:   return "absent";
+        case OutcomeKind::Replaced: return "replaced";
+        case OutcomeKind::Spared:   return "spared";
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS outcome log: unknown OutcomeKind {}", static_cast<int>(o));
+}
+
+OutcomeKind outcomeKindFromWord(std::string_view w)
+{
+    if (w == "deleted")  return OutcomeKind::Deleted;
+    if (w == "absent")   return OutcomeKind::Absent;
+    if (w == "replaced") return OutcomeKind::Replaced;
+    if (w == "spared")   return OutcomeKind::Spared;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS outcome log: unknown outcome '{}'", w);
+}
+
+}
+
+String encodeOutcomeLog(const OutcomeLog & log)
+{
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::GcOutcomes);
+    for (const OutcomeEntry & e : log.entries)
+    {
+        bool first = true;
+        writeKey(out, "k", first);
+        writeStringValue(out, objectKindToWord(e.kind));
+        writeBlobRefFields(out, first, e.ref);   /// ha + h
+        writeTokenFields(out, first, e.token);   /// tt + tv
+        writeKey(out, "oc", first);
+        writeStringValue(out, outcomeKindToWord(e.outcome));
+        closeObject(out, first);
+        writeChar('\n', out);
+    }
+    writeTrailerLine(out, log.entries.size());
+    return std::move(out).take();
+}
+
+OutcomeLog decodeOutcomeLog(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::GcOutcomes);
+    const uint64_t line_cap = traitsFor(FormatId::GcOutcomes).line_cap;
+
+    OutcomeLog log;
+    while (true)
+    {
+        const String line = readLine(in, line_cap, "outcome log");
+        ReadBufferFromMemory line_in(line.data(), line.size());
+        JsonObjectReader r(line_in, KeyStrictness::Tolerant, "outcome log");
+
+        String key;
+        /// The first key distinguishes a trailer ("n") from a record ("k").
+        if (!r.nextKey(key))
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS outcome log: empty line");
+        if (key == "n")
+        {
+            const uint64_t n = r.readU64Number();
+            while (r.nextKey(key))
+                r.skipUnknown(key);
+            if (!line_in.eof() || !in.eof())
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS outcome log: bytes after trailer");
+            if (n != log.entries.size())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS outcome log: trailer count {} != {} records", n, log.entries.size());
+            return log;
+        }
+
+        OutcomeEntry e;
+        String ha;
+        String hhex;
+        String tv;
+        bool have_ha = false;
+        bool have_h = false;
+        bool have_tt = false;
+        TokenType tt{};
+        do
+        {
+            if (key == "k") e.kind = objectKindFromWord(r.readString(), "outcome log");
+            else if (key == "ha") { ha = r.readString(); have_ha = true; }
+            else if (key == "h") { hhex = r.readString(); have_h = true; }
+            else if (key == "tt") { tt = tokenTypeFromWord(r.readString(), "outcome log"); have_tt = true; }
+            else if (key == "tv") tv = r.readString();
+            else if (key == "oc") e.outcome = outcomeKindFromWord(r.readString());
+            else r.skipUnknown(key);
+        } while (r.nextKey(key));
+
+        if (!have_ha || !have_h || !have_tt)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS outcome log: record missing ha/h/tt");
+        const BlobHashAlgo algo = blobHashAlgoFromWord(ha, "outcome log");
+        /// Validate the digest width before `fromHex`: a width mismatch must surface as the
+        /// CORRUPTED_DATA required for malformed serialized input, not fromHex's BAD_ARGUMENTS.
+        if (hhex.size() != blobHashLenFor(algo) * 2)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS outcome log: digest width {} does not match algo '{}'", hhex.size(), ha);
+        e.ref = BlobRef{algo, codecFor(algo).fromHex(hhex)};
+        e.token = Token{tv, tt};
+        if (!line_in.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS outcome log: junk after record");
+        log.entries.push_back(std::move(e));
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.h
new file mode 100644
index 000000000000..09a850ee66ff
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.h
@@ -0,0 +1,58 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <base/types.h>
+#include <cstdint>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// The outcome log records observations made while settling GC candidates. Each log belongs to one
+/// attempt-scoped generation, round, and shard at
+/// `gc/gen/{g}/attempt/{a}/outcomes/{round}/{shard}`. It contains the results of exact-token deletes
+/// for entries that were already published as `delete_pending`, as well as candidates spared when
+/// the one-pass merge found a live in-degree. The log is written before the round's single state CAS;
+/// `putIfAbsent` adopts an existing durable log on replay rather than treating a byte difference as
+/// an error. The uncompressed payload is a header line, one flat JSON record per entry in insertion
+/// order, and an `{"n":count}` trailer. `FormatId::GcOutcomes` stores the sealed payload in one zstd
+/// frame, so its object-storage key has the `.zst` suffix.
+enum class OutcomeKind : uint8_t
+{
+    Deleted = 1,    /// The exact-token delete succeeded.
+    Absent = 2,     /// The object was already absent, for example after a prior round's delete.
+    Replaced = 3,   /// A 412 showed that a writer recreated the object with a new token.
+    Spared = 4,     /// The merge found a positive in-degree, so the candidate was kept alive.
+};
+
+/// One observation about a blob incarnation considered by GC. `token` identifies the exact
+/// incarnation that GC examined, while `ref` identifies the content address; retaining both lets
+/// replay and inspection distinguish an absent object from a replacement that won a race with GC.
+struct OutcomeEntry
+{
+    ObjectKind kind = ObjectKind::Blob;
+    BlobRef ref{};
+    Token token;
+    OutcomeKind outcome = OutcomeKind::Spared;
+};
+
+/// The ordered records for one GC outcome object. Encoding preserves this insertion order because
+/// the log is observation-bearing rather than a canonical deterministic artifact; decoding returns
+/// only after the trailer count matches the records that were read.
+struct OutcomeLog
+{
+    std::vector<OutcomeEntry> entries;
+};
+
+/// Encodes a log as the uncompressed `GcOutcomes` text payload. The caller is responsible for
+/// applying the format registry's compression policy before storing the returned bytes.
+String encodeOutcomeLog(const OutcomeLog & log);
+
+/// Decodes and validates a `GcOutcomes` text payload. The header, required record fields, supported
+/// enum words, line boundaries, trailer count, and end-of-object condition are checked; malformed
+/// input raises `CORRUPTED_DATA`.
+OutcomeLog decodeOutcomeLog(std::string_view data);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.cpp
new file mode 100644
index 000000000000..7012c6787f70
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.cpp
@@ -0,0 +1,118 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <base/defines.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+String encodeGcState(const GcState & state)
+{
+    if (state.gc_shards < 1)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "encodeGcState: gc_shards must be >= 1 -- refusing to persist an unreadable gc/state");
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::GcState);
+    bool first = true;
+    writeKey(out, "rnd", first); writeU64StringValue(out, state.round);
+    writeKey(out, "gcs", first); writeIntText(state.gc_shards, out);
+    writeKey(out, "sg", first);  writeU64StringValue(out, state.snap_generation);
+    writeKey(out, "spt", first); writeU64StringValue(out, state.snap_pruned_through);
+    writeKey(out, "sa", first);  writeU64StringValue(out, state.snap_attempt);
+    writeKey(out, "msc", first); writeStringValue(out, state.manifest_sweep_cursor);
+    writeKey(out, "lo", first);  writeHex128Value(out, state.lease.owner);
+    writeKey(out, "ls", first);  writeU64StringValue(out, state.lease.seq);
+    closeObject(out, first);
+    writeChar('\n', out);
+    return std::move(out).take();
+}
+
+GcState decodeGcState(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::GcState);
+    const String body = readLine(in, traitsFor(FormatId::GcState).line_cap, "gc/state");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    JsonObjectReader r(body_in, KeyStrictness::Tolerant, "gc/state");
+
+    GcState state;
+    bool saw_gcs = false;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "rnd") state.round = r.readU64String();
+        else if (key == "gcs") { state.gc_shards = r.readU64Number(); saw_gcs = true; }
+        else if (key == "sg") state.snap_generation = r.readU64String();
+        else if (key == "spt") state.snap_pruned_through = r.readU64String();
+        else if (key == "sa") state.snap_attempt = r.readU64String();
+        else if (key == "msc") state.manifest_sweep_cursor = r.readString();
+        else if (key == "lo") state.lease.owner = r.readHex128();
+        else if (key == "ls") state.lease.seq = r.readU64String();
+        else r.skipUnknown(key);
+    }
+    /// Fail closed on an absent gcs: the writer always emits it, so a missing key means a corrupt object.
+    /// Do NOT silently keep the struct default (1) — that would hide corruption (no-fallback principle).
+    if (!saw_gcs)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS gc/state: missing gcs");
+    if (state.gc_shards == 0)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS gc/state: gc_shards must be >= 1");
+    if (!body_in.eof() || !in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS gc/state: trailing bytes");
+    return state;
+}
+
+String encodeGcHeartbeat(const GcHeartbeat & hb)
+{
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::GcHeartbeat);
+    bool first = true;
+    writeKey(out, "by", first);  writeHex128Value(out, hb.owner);
+    writeKey(out, "seq", first); writeU64StringValue(out, hb.hb_seq);
+    closeObject(out, first);
+    writeChar('\n', out);
+    return std::move(out).take();
+}
+
+GcHeartbeat decodeGcHeartbeat(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::GcHeartbeat);
+    const String body = readLine(in, traitsFor(FormatId::GcHeartbeat).line_cap, "gc heartbeat");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    JsonObjectReader r(body_in, KeyStrictness::Tolerant, "gc heartbeat");
+
+    GcHeartbeat hb;
+    bool saw_by = false;
+    bool saw_seq = false;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "by")
+        {
+            hb.owner = r.readHex128();
+            saw_by = true;
+        }
+        else if (key == "seq")
+        {
+            hb.hb_seq = r.readU64String();
+            saw_seq = true;
+        }
+        else r.skipUnknown(key);
+    }
+    if (!saw_by || !saw_seq)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS gc heartbeat: missing identity field");
+    if (!body_in.eof() || !in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS gc heartbeat: trailing bytes");
+    return hb;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h
new file mode 100644
index 000000000000..88bce088ed13
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h
@@ -0,0 +1,71 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// The durable lease portion of the `gc/state` control object. `owner` identifies the current GC
+/// leader and `seq` changes when that leader renews the lease. A contender compares the pair across
+/// observations before stealing a stalled lease; a zero owner means that the lease has never been
+/// held.
+struct GcLease
+{
+    UInt128 owner{};  /// Random GC leader identifier; zero means the lease has never been held.
+    uint64_t seq = 0; /// Renewal counter observed by contenders when deciding whether a lease stalled.
+};
+
+/// The durable control state for GC rounds and snapshot generations. It is materialized as one
+/// versioned JSON control object by `encodeGcState` and read back by `decodeGcState`.
+///
+/// `round` and the snapshot fields are publication cursors: a committed round makes its retire sets
+/// durable, while `snap_generation` and `snap_attempt` identify the fold seal whose snapshot was
+/// adopted. The fold's per-namespace and per-shard cursor lives in that write-once fold seal, not in
+/// this object. `gc_shards` is chosen once and must remain at least one; decoders reject an absent or
+/// zero value instead of silently accepting the C++ default. The manifest sweep cursor is only a
+/// best-effort orphan-manifest cleanup position and is never used for reachability decisions.
+struct GcState
+{
+    uint64_t round = 0;            /// the highest GC round whose retire sets are durable
+    uint64_t gc_shards = 1;        /// GC blob-target-shard count; set once, immutable; must be >= 1
+    uint64_t snap_generation = 0;  /// monotone; the authoritative snap objects' generation
+    uint64_t snap_pruned_through = 0;   /// highest snap generation fully pruned (retention cursor)
+    uint64_t snap_attempt = 0;     /// adopted attempt id (folding leader's lease.seq) for snap_generation
+    String manifest_sweep_cursor;  /// best-effort orphan part-manifest cleanup cursor; reachability ignores it
+    GcLease lease;
+};
+
+/// Encode `state` as the canonical `cas_gc_state` text object: a versioned header line followed by
+/// one JSON body object. The writer always emits the complete field set and asserts the invariant
+/// that `gc_shards` is nonzero.
+String encodeGcState(const GcState & state);
+
+/// Decode a complete `cas_gc_state` text object. The header and size limits are checked before the
+/// body is parsed; unknown non-reserved fields are tolerated for forward evolution, but malformed
+/// input, trailing bytes, a missing `gcs`, or a zero shard count raises `CORRUPTED_DATA` rather than
+/// falling back to a default state.
+GcState decodeGcState(std::string_view data);
+
+/// Advisory liveness state for the GC leader lease. The leader increments `hb_seq` on a fast cadence
+/// independently of round progress, because its lease renewal counter can remain unchanged during a
+/// long fold. A follower that observes the heartbeat advance backs off from stealing the lease; this
+/// prevents mistaking an alive, mid-round leader for a stalled one. The value is persisted as the
+/// versioned `cas_gc_hb` text object, whose body contains `by` and `seq` string values, replacing the
+/// former unversioned 24-byte record.
+struct GcHeartbeat
+{
+    UInt128 owner{};
+    uint64_t hb_seq = 0;
+};
+
+/// Encode a complete heartbeat as a canonical versioned header line and one JSON body object.
+String encodeGcHeartbeat(const GcHeartbeat & hb);
+
+/// Decode a complete `cas_gc_hb` text object, rejecting malformed input and trailing bytes with
+/// `CORRUPTED_DATA`. Unknown non-reserved fields remain skippable for forward evolution.
+GcHeartbeat decodeGcHeartbeat(std::string_view data);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.cpp
new file mode 100644
index 000000000000..5bd928ec01a3
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.cpp
@@ -0,0 +1,345 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/Exception.h>
+#include <charconv>
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Parses a canonical unsigned-decimal path segment (the shape `std::to_string` produces): non-empty,
+/// digits only, no leading zero unless the segment is exactly "0", and fits in a `uint64_t`. Returns
+/// `std::nullopt` for anything else -- never throws, since key parsing classifies a foreign/malformed
+/// segment as debris, not an error (mirrors `parseManifestKey`'s ordinal parse).
+std::optional<uint64_t> parseCanonicalU64(std::string_view s)
+{
+    if (s.empty() || (s.size() > 1 && s[0] == '0'))
+        return std::nullopt;
+    for (char c : s)
+        if (c < '0' || c > '9')
+            return std::nullopt;
+    uint64_t v = 0;
+    const auto [ptr, ec] = std::from_chars(s.data(), s.data() + s.size(), v);
+    if (ec != std::errc{} || ptr != s.data() + s.size())
+        return std::nullopt;   /// overflowed uint64_t or otherwise not fully consumed
+    return v;
+}
+
+}
+
+/// Blob-key operations are kept together with their parsing inverse. `CasTypes.h` supplies the
+/// complete `BlobRef` type and the hash-algorithm helpers used to render and validate the path.
+String Layout::blobKey(const BlobRef & ref) const
+{
+    return shardedKey("blobs/" + String(blobHashAlgoName(ref.algo)), blobHexOf(ref));
+}
+
+String Layout::blobMetaKey(const BlobRef & ref) const
+{
+    return blobKey(ref) + ".meta";
+}
+
+std::optional<BlobRef> Layout::parseBlobKey(std::string_view key) const
+{
+    std::string_view rest = key;
+    static constexpr std::string_view kMetaSuffix = ".meta";
+    if (rest.size() >= kMetaSuffix.size() && rest.substr(rest.size() - kMetaSuffix.size()) == kMetaSuffix)
+        rest.remove_suffix(kMetaSuffix.size());
+
+    const String blobs_root = blobsPrefix();   /// "<prefix>/blobs/"
+    if (rest.size() <= blobs_root.size() || !rest.starts_with(blobs_root))
+        return std::nullopt;
+    rest.remove_prefix(blobs_root.size());
+
+    const size_t algo_sep = rest.find('/');
+    if (algo_sep == std::string_view::npos)
+        return std::nullopt;
+    const std::string_view algo_name = rest.substr(0, algo_sep);
+    rest.remove_prefix(algo_sep + 1);
+
+    const size_t shard_sep = rest.find('/');
+    if (shard_sep == std::string_view::npos)
+        return std::nullopt;
+    const std::string_view shard = rest.substr(0, shard_sep);
+    const std::string_view hex = rest.substr(shard_sep + 1);
+    if (shard.size() != 2 || hex.size() < 2 || shard != hex.substr(0, 2))
+        return std::nullopt;   /// malformed shard/hex shape -- not ours
+
+    /// `<algoName>` -> `BlobHashAlgo`: the small enum-value set makes a linear scan against
+    /// `blobHashAlgoName` (the ONE name authority) cheaper and safer than a second name table that
+    /// could drift from it.
+    std::optional<BlobHashAlgo> algo;
+    for (BlobHashAlgo candidate : {BlobHashAlgo::CityHash128, BlobHashAlgo::XXH3_128, BlobHashAlgo::Sha256})
+        if (algo_name == blobHashAlgoName(candidate))
+        {
+            algo = candidate;
+            break;
+        }
+    if (!algo)
+        return std::nullopt;   /// unknown/foreign algo segment -- debris, not ours
+
+    if (hex.size() != 2 * blobHashLenFor(*algo))
+        return std::nullopt;   /// a KNOWN algo but the wrong-width hex -- not ours either
+
+    try
+    {
+        return BlobRef{*algo, codecFor(*algo).fromHex(String(hex))};
+    }
+    catch (const Exception &)
+    {
+        return std::nullopt;   /// non-hex characters -- malformed, not ours
+    }
+}
+
+std::optional<ParsedRefObjectKey> Layout::parseRefObjectKey(std::string_view key) const
+{
+    const String base = casRefsPrefix();
+    if (!key.starts_with(base))
+        return std::nullopt;
+    std::string_view rest = key;
+    rest.remove_prefix(base.size());
+
+    const size_t id_sep = rest.rfind('/');
+    if (id_sep == std::string_view::npos)
+        return std::nullopt;
+    std::string_view id_part = rest.substr(id_sep + 1);
+    std::string_view before_id = rest.substr(0, id_sep);
+
+    const size_t kind_sep = before_id.rfind('/');
+    if (kind_sep == std::string_view::npos)
+        return std::nullopt;
+    const std::string_view kind_seg = before_id.substr(kind_sep + 1);
+    const std::string_view before_kind = before_id.substr(0, kind_sep);
+    if (before_kind.empty())
+        return std::nullopt;
+
+    RefObjectKind kind{};
+    if (kind_seg == "_log")
+        kind = RefObjectKind::Log;
+    else if (kind_seg == "_snap")
+        kind = RefObjectKind::Snap;
+    else
+        return std::nullopt;
+
+    std::string_view render = id_part;
+    /// `_log` and `_snap` are always-compressed text stored under a `.zst` suffix.
+    constexpr std::string_view kZstSuffix = ".zst";
+    if (!render.ends_with(kZstSuffix))
+        return std::nullopt;
+    render.remove_suffix(kZstSuffix.size());
+
+    const auto txn_id = parseRefTxnId(render);
+    if (!txn_id)
+        return std::nullopt;
+
+    if (before_kind.find('/') != std::string_view::npos)
+        return std::nullopt;
+    return ParsedRefObjectKey{namespaceLifePhysicalIdOf(key, before_kind), kind, *txn_id};
+}
+
+std::optional<NamespaceLifePhysicalId> Layout::parseRefCkptKey(std::string_view key) const
+{
+    const String base = namespaceStateRootPrefix();
+    if (!key.starts_with(base))
+        return std::nullopt;
+    std::string_view rest = key;
+    rest.remove_prefix(base.size());
+
+    /// `<life_id>/_ckpt<suffix>` -- the leaf is the fixed name plus whatever the registry's
+    /// compression policy appends. Built from the same pieces `refCkptKey` uses, so the two cannot
+    /// drift apart.
+    const String leaf = "_ckpt" + String(storedSuffix(FormatId::RefCkpt));
+    const size_t leaf_sep = rest.rfind('/');
+    if (leaf_sep == std::string_view::npos || rest.substr(leaf_sep + 1) != leaf)
+        return std::nullopt;
+    const std::string_view life_id = rest.substr(0, leaf_sep);
+    if (life_id.empty() || life_id.find('/') != std::string_view::npos)
+        return std::nullopt;
+
+    return namespaceLifePhysicalIdOf(key, life_id);
+}
+
+std::optional<ParsedNamespaceFileKey> Layout::parseNamespaceFileKey(std::string_view key) const
+{
+    const String base = namespaceStateRootPrefix();
+    if (!key.starts_with(base))
+        return std::nullopt;
+    std::string_view rest = key;
+    rest.remove_prefix(base.size());
+
+    /// The first `/_files/` separates the single `<life_id>` segment from the relative name. A
+    /// relative name may itself contain `_files`, so the first occurrence is the delimiter.
+    static constexpr std::string_view kFilesSegment = "/_files/";
+    const size_t files_pos = rest.find(kFilesSegment);
+    if (files_pos == std::string_view::npos)
+        return std::nullopt;   /// no reserved segment: a loose mountpoint object, not one of our files
+
+    const std::string_view life_id = rest.substr(0, files_pos);
+    const std::string_view relative_name = rest.substr(files_pos + kFilesSegment.size());
+    if (relative_name.empty())
+        return std::nullopt;   /// the files prefix itself names no file
+
+    if (life_id.empty() || life_id.find('/') != std::string_view::npos)
+        return std::nullopt;
+    /// Mirror `namespaceFileKey`'s writer-side grammar: a relative name no current writer could have
+    /// produced (a dirty path, `..` segment, doubled slash, ...) is not one of ours, even though the
+    /// `_files/` segment and life id both parse. `namespaceLifePhysicalIdOf` below throws
+    /// `CORRUPTED_DATA` for the sibling life-id case; this mirrors that severity for symmetry.
+    if (!isCleanRelativeNamespaceFileName(relative_name))
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "CasLayout: object '{}' names no clean namespace file: '{}' is not a relative path any "
+            "current writer could have produced", key, relative_name);
+    return ParsedNamespaceFileKey{namespaceLifePhysicalIdOf(key, life_id), String(relative_name)};
+}
+
+NamespaceLifePhysicalId Layout::namespaceLifePhysicalIdOf(std::string_view key, std::string_view segment) const
+{
+    const auto incarnation = parseIncarnation(segment);
+    if (!incarnation)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "CasLayout: object '{}' names no life: '{}' is not 32 lower-case hex digits of a nonzero "
+            "life id. Generation-5 namespace-bearing pools are rejected by the pool-metadata format "
+            "gate before this generation-6 physical-key parser is reached",
+            key, segment);
+    return *incarnation;
+}
+
+std::optional<ManifestId> Layout::parseManifestKey(std::string_view key) const
+{
+    const String base = casManifestsPrefix();
+    if (!key.starts_with(base))
+        return std::nullopt;
+    std::string_view rest = key;
+    rest.remove_prefix(base.size());
+
+    const size_t file_sep = rest.rfind('/');
+    if (file_sep == std::string_view::npos)
+        return std::nullopt;
+    const std::string_view file = rest.substr(file_sep + 1);
+    const std::string_view before_file = rest.substr(0, file_sep);
+
+    const size_t build_sep = before_file.rfind('/');
+    if (build_sep == std::string_view::npos)
+        return std::nullopt;
+    const std::string_view build_seg = before_file.substr(build_sep + 1);
+    const std::string_view ns_part = before_file.substr(0, build_sep);
+    if (ns_part.empty())
+        return std::nullopt;
+
+    const auto build = parseRefTxnId(build_seg);
+    if (!build)
+        return std::nullopt;
+
+    const std::string_view kManifestSuffix = storedSuffix(FormatId::PartManifest);
+    constexpr size_t kOrdinalDigits = 6;
+    if (file.size() != kOrdinalDigits + kManifestSuffix.size() || !file.ends_with(kManifestSuffix))
+        return std::nullopt;
+    const std::string_view ordinal_str = file.substr(0, kOrdinalDigits);
+    uint32_t ordinal = 0;
+    for (char c : ordinal_str)
+    {
+        if (c < '0' || c > '9')
+            return std::nullopt;
+        ordinal = ordinal * 10 + static_cast<uint32_t>(c - '0');
+    }
+    if (ordinal == 0 || ordinal > kMaxManifestOrdinal)
+        return std::nullopt;
+
+    ManifestId parsed;
+    parsed.root_namespace = RootNamespace{String(ns_part)};
+    parsed.ref.writer_epoch = build->writer_epoch;
+    parsed.ref.build_sequence = build->ref_sequence;
+    parsed.ref.manifest_ordinal = ordinal;
+    return parsed;
+}
+
+std::optional<ParsedBlobTargetRunKey> Layout::parseBlobTargetRunKey(std::string_view key) const
+{
+    const String base = prefix + "/gc/gen/";
+    if (!key.starts_with(base))
+        return std::nullopt;
+    std::string_view rest = key;
+    rest.remove_prefix(base.size());
+
+    /// Splits the next '/'-delimited segment off the front of `rest`, returning `std::nullopt` if
+    /// `rest` has no further '/' (an incomplete key shape).
+    auto takeSegment = [](std::string_view & s) -> std::optional<std::string_view>
+    {
+        const size_t sep = s.find('/');
+        if (sep == std::string_view::npos)
+            return std::nullopt;
+        const std::string_view seg = s.substr(0, sep);
+        s.remove_prefix(sep + 1);
+        return seg;
+    };
+
+    const auto generation_seg = takeSegment(rest);
+    if (!generation_seg)
+        return std::nullopt;
+    const auto generation = parseCanonicalU64(*generation_seg);
+    if (!generation)
+        return std::nullopt;
+
+    const auto attempt_lit = takeSegment(rest);
+    if (!attempt_lit || *attempt_lit != "attempt")
+        return std::nullopt;
+
+    const auto attempt_seg = takeSegment(rest);
+    if (!attempt_seg)
+        return std::nullopt;
+    const auto attempt = parseCanonicalU64(*attempt_seg);
+    if (!attempt)
+        return std::nullopt;
+
+    const auto blob_target_lit = takeSegment(rest);
+    if (!blob_target_lit || *blob_target_lit != "blob_target")
+        return std::nullopt;
+
+    const auto shard_seg = takeSegment(rest);
+    if (!shard_seg)
+        return std::nullopt;
+    const auto shard = parseCanonicalU64(*shard_seg);
+    if (!shard)
+        return std::nullopt;
+
+    /// `rest` is now the final segment (`seq`): reject trailing garbage (a further '/').
+    if (rest.find('/') != std::string_view::npos)
+        return std::nullopt;
+    const auto seq = parseCanonicalU64(rest);
+    if (!seq)
+        return std::nullopt;
+
+    return ParsedBlobTargetRunKey{*generation, *attempt, *shard, *seq};
+}
+
+/// A namespace must be non-empty, with no leading/trailing '/', no empty segment ("//"), and no
+/// segment equal to the reserved "_files".
+void Layout::checkNamespace(const RootNamespace & ns) const
+{
+    const String & s = ns.string();
+    if (s.empty())
+        throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS, "CasLayout: namespace must be non-empty");
+
+    size_t start = 0;
+    while (true)
+    {
+        size_t end = s.find('/', start);
+        const String segment = s.substr(start, end == String::npos ? String::npos : end - start);
+        if (segment.empty())
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "CasLayout: namespace '{}' has an empty segment (leading/trailing or doubled '/')", s);
+        if (segment == "_files")
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "CasLayout: namespace '{}' uses the reserved segment '_files'", s);
+        if (segment == "_manifests")
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "CasLayout: namespace '{}' uses the reserved segment '_manifests'", s);
+        if (end == String::npos)
+            break;
+        start = end + 1;
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h
new file mode 100644
index 000000000000..093f380bd0e2
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h
@@ -0,0 +1,480 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasNamespaceLifeId.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Common/Exception.h>
+#include <base/types.h>
+#include <optional>
+#include <string_view>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+/// Shared writer/parser grammar for a namespace file's relative name (the part after `_files/`):
+/// no empty name, no leading or trailing `/`, no `//`, and no `..` path segment anywhere in it.
+/// `Layout::namespaceFileKey` (the writer) and `Layout::parseNamespaceFileKey` (the reader) both
+/// call this so their definition of "clean" cannot drift apart.
+inline bool isCleanRelativeNamespaceFileName(std::string_view name)
+{
+    return !(name.empty() || name.front() == '/' || name.back() == '/'
+        || name.find("//") != std::string_view::npos || name == ".." || name.starts_with("../")
+        || name.ends_with("/..") || name.find("/../") != std::string_view::npos);
+}
+
+/// Which immutable ref-object kind a `_log` or `_snap` key names.
+enum class RefObjectKind : uint8_t
+{
+    Log,
+    Snap,
+};
+
+/// The result of `Layout::parseRefObjectKey`: the opaque physical life id, ref-object kind, and
+/// canonical transaction identifier recovered from a listed stream key. Logical resolution belongs
+/// to the immutable catalog cut held by the consumer, never to the key parser.
+struct ParsedRefObjectKey
+{
+    NamespaceLifePhysicalId life_id = 0;
+    RefObjectKind kind = RefObjectKind::Log;
+    RefTxnId txn_id;
+
+    bool operator==(const ParsedRefObjectKey &) const = default;
+};
+
+/// The result of `Layout::parseNamespaceFileKey`: the opaque physical life id that owns a listed
+/// verbatim file and the file name relative to that life's `_files/` prefix.
+struct ParsedNamespaceFileKey
+{
+    NamespaceLifePhysicalId life_id;
+    String relative_name;
+
+    bool operator==(const ParsedNamespaceFileKey &) const = default;
+};
+
+/// The result of `Layout::parseBlobTargetRunKey`: the (generation, attempt, shard, seq) coordinates
+/// recovered from a listed blob-target in-degree/delta run segment key.
+struct ParsedBlobTargetRunKey
+{
+    uint64_t generation = 0;
+    uint64_t attempt = 0;
+    uint64_t shard = 0;
+    uint64_t seq = 0;
+
+    bool operator==(const ParsedBlobTargetRunKey &) const = default;
+};
+
+/// Builds and parses object-storage keys for one content-addressed pool.
+///
+/// `Layout` owns only the pool prefix; it does not own storage, cache state, or a pool-wide blob hash
+/// algorithm. Callers use its pure methods to derive keys for blobs, manifests, refs, verbatim files,
+/// and GC control data, and to classify listed keys before acting on them. Logical-name key builders
+/// validate namespaces, while parsers deliberately return `std::nullopt` for foreign or
+/// malformed listed keys so sweeps can treat those keys as debris without exceptions.
+///
+/// Every key is built from a pool prefix and a stable path subtree. The main families are:
+///   - content objects:  POOL/blobs/ALGO/S/HEX
+///   - part manifests:   POOL/cas/manifests/NAMESPACE/BUILD/ORDINAL.zst
+///   - ref stream:       POOL/cas/ns/stream/LIFE_ID/_log|_snap/ID
+///   - namespace state:  POOL/cas/ns/state/LIFE_ID/_ckpt
+///                       POOL/cas/ns/state/LIFE_ID/_files/FILE_NAME
+///   - GC state:         POOL/gc/...
+///   - pool metadata:    POOL/_pool_meta
+///
+/// NAMESPACE is opaque to the core: the wiring composes strings like "srv1/<table_uuid>" or
+/// "shadow/<backup>/<table_uuid>". The reserved "_files" and "_manifests" segments cannot collide
+/// with root shard keys because shard names are numeric, and `checkNamespace` rejects both as
+/// namespace segments.
+///
+/// The 2-char shard is always the first two characters of the id string.
+/// This matches the protocol's fixed two-character shard layout.
+///
+/// Blob bodies carry their own `BlobHashAlgo` as a path segment:
+/// `POOL/blobs/<algo>/S/<hex>`, where `<algo>` is `blobHashAlgoName(ref.algo)` (`"ch128"`, `"xxh3"`,
+/// `"sha256"`) and `<hex>`/`S` are taken from `ref.digest` at the algo's own width. This applies to
+/// ALL algos, including the pool's default, so blob keys are uniformly self-describing and a pool may
+/// hold blobs under several algos at once. Trees/manifests/refs/gc keys are unaffected -- only
+/// blob-body keys (`blobKey`/`blobMetaKey`) gain the segment. `Layout` itself carries NO
+/// algo -- there is no pool-wide "the" algo anymore; every blob key is built from a `BlobRef` alone.
+class Layout
+{
+public:
+    explicit Layout(String prefix_) : prefix(std::move(prefix_)) {}
+
+    /// Content objects: POOL/blobs/<algo>/S/<hex>, with `<algo>`/`<hex>` taken from `ref` itself.
+    /// Defined out-of-line in `CasLayout.cpp`, where the key construction and parsing helpers remain
+    /// together.
+    String blobKey(const BlobRef & ref) const;
+    /// The per-hash meta descriptor sibling of the blob body.
+    String blobMetaKey(const BlobRef & ref) const;
+
+    /// Inverse of `blobKey`/`blobMetaKey`: parses a listed object key of the shape
+    /// `<prefix>/blobs/<algoName>/<shard2>/<hex>` (the `.meta` sibling is accepted identically -- its
+    /// trailing `.meta` is stripped first, so a body and its meta parse to the SAME `BlobRef`).
+    /// Returns `std::nullopt` for anything that is not one of OUR blob keys: a foreign top-level
+    /// prefix, a missing shard/hex segment, an `<algoName>` this build does not recognize
+    /// (`blobHashAlgoName` never rendered it), a hex payload of the wrong width for a KNOWN algo, or
+    /// non-hex characters -- every case is "debris, not ours", never an exception (callers classify
+    /// it as foreign/unaccounted, mirroring the LIST sweep's existing `catch (...) continue`
+    /// contract). Defined out-of-line in `CasLayout.cpp` alongside `blobKey` and `blobMetaKey`.
+    std::optional<BlobRef> parseBlobKey(std::string_view key) const;
+
+    /// Immutable stream prefix for one physical life.
+    String namespaceStreamPrefix(const NamespaceLifeId & life) const
+    {
+        return namespaceStreamRootPrefix() + renderIncarnation(life.incarnation) + "/";
+    }
+
+    /// Point/path-addressed state prefix for one physical life.
+    String namespaceStatePrefix(const NamespaceLifeId & life) const
+    {
+        return namespaceStateRootPrefix() + renderIncarnation(life.incarnation) + "/";
+    }
+
+    /// Pool-wide immutable stream prefix. GC's one hot namespace enumeration is scoped here.
+    String casRefsPrefix() const
+    {
+        return namespaceStreamRootPrefix();
+    }
+
+    String namespaceStreamRootPrefix() const { return prefix + "/cas/ns/stream/"; }
+    String namespaceStateRootPrefix() const { return prefix + "/cas/ns/state/"; }
+    String namespaceRootPrefix() const { return prefix + "/cas/ns/"; }
+
+    /// Immutable transaction log object at
+    /// `<prefix>/cas/ns/stream/<life_id>/_log/<render>.zst`. The log is the text `cas_ref_log` stored with
+    /// the format's always-compressed `.zst` suffix; readers construct the one canonical key and do
+    /// not try an uncompressed variant.
+    String refLogKey(const NamespaceLifeId & ns_id, const RefTxnId & id) const
+    {
+        return namespaceStreamPrefix(ns_id) + "_log/" + renderRefTxnId(id) + String(storedSuffix(FormatId::RefLog));
+    }
+
+    /// Writer-published table snapshot at `.../_snap/<render>.zst`. The snapshot
+    /// is the text `cas_ref_snap` stored with the format's always-compressed `.zst` suffix. Snapshot
+    /// `X` reuses the `RefTxnId` of the last log it covers.
+    String refSnapshotKey(const NamespaceLifeId & ns_id, const RefTxnId & id) const
+    {
+        return namespaceStreamPrefix(ns_id) + "_snap/" + renderRefTxnId(id) + String(storedSuffix(FormatId::RefSnapshot));
+    }
+
+    /// The life's checkpoint object (spec INV-4) at `<prefix>/cas/ns/state/<life_id>/_ckpt`. Unlike
+    /// immutable stream objects it is mutable (token-CAS), carries no transaction id, and therefore lives
+    /// in the point/path-addressed state tree rather than a `_log`/`_snap` directory --
+    /// which is also why `parseRefObjectKey` does not recognize it and `parseRefCkptKey` exists.
+    String refCkptKey(const NamespaceLifeId & ns_id) const
+    {
+        return namespaceStatePrefix(ns_id) + "_ckpt" + String(storedSuffix(FormatId::RefCkpt));
+    }
+
+    /// Inverse of `refCkptKey`: returns the opaque physical life id when `key` is exactly one of our `_ckpt`
+    /// keys, and `std::nullopt` when the key is not one at all (a foreign pool prefix, a different
+    /// leaf name, an id-bearing ref object) -- classifying an untrusted listed key stays an ordinary
+    /// "is this ours" question. Logical resolution is a separate join against one catalog cut.
+    ///
+    /// It REFUSES, with `CORRUPTED_DATA` naming the key, a key whose leaf IS `_ckpt` but whose
+    /// incarnation segment is missing, non-canonical or zero: behind Stage B's format bump the
+    /// un-incarnated (Stage A) shape names no live object, and treating it as foreign debris would let
+    /// it sit unnoticed under a live namespace.
+    ///
+    /// Stream sweeps never consult this parser: checkpoints are deliberately outside the hot
+    /// `casRefsPrefix` enumeration.
+    std::optional<NamespaceLifePhysicalId> parseRefCkptKey(std::string_view key) const;
+
+    /// Inverse of `refLogKey`/`refSnapshotKey`: classifies a LISTED key under `casRefsPrefix()` by its
+    /// kind directory (`_log` or `_snap`) and parses the trailing
+    /// `RefTxnId` and the life it belongs to. Strict: returns `std::nullopt` for anything that is not
+    /// one of our ref-object keys: a foreign top-level prefix, a missing life/kind/id segment, an
+    /// unrecognized kind directory, a `_log`/`_snap` id missing its
+    /// `.zst` suffix (both are always-compressed text), trailing garbage after the id, or a
+    /// non-canonical `RefTxnId` render (delegates to
+    /// `parseRefTxnId`).
+    ///
+    /// It throws `CORRUPTED_DATA` when the key otherwise has a recognized stream-object shape but its
+    /// physical life-id segment is missing, non-canonical or zero. Logical names never come from keys.
+    /// Defined out-of-line in `CasLayout.cpp`, where the key construction and parsing helpers remain
+    /// together.
+    std::optional<ParsedRefObjectKey> parseRefObjectKey(std::string_view key) const;
+
+    /// Prefix that covers all part manifests of a namespace: `<prefix>/cas/manifests/<ns>/`.
+    String manifestNamespacePrefix(const RootNamespace & ns) const
+    {
+        checkNamespace(ns);
+        return prefix + "/cas/manifests/" + ns.string() + "/";
+    }
+
+    /// Pool-wide part-manifest prefix: `<prefix>/cas/manifests/`.
+    String casManifestsPrefix() const
+    {
+        return prefix + "/cas/manifests/";
+    }
+
+    /// Verbatim (non-content-addressed) file stored under ONE LIFE of a namespace. Names may be nested
+    /// (relative sub-paths — the wiring stores table-level subdirectory files such as
+    /// deduplication_logs/deduplication_log_1.txt verbatim); empty segments, leading or
+    /// trailing '/', and '..' segments are rejected (no escaping the life's files prefix).
+    ///
+    /// The incarnation segment is what makes a file of a previous life structurally unreachable from
+    /// the next one (directive §2): once the catalog entry is gone, nothing can name that life's files
+    /// again, so a LIST that omitted an old file can only leak storage — it can no longer let the file
+    /// become visible under a reborn namespace of the same name.
+    String namespaceFileKey(const NamespaceLifeId & life, const String & file_name) const
+    {
+        if (!isCleanRelativeNamespaceFileName(file_name))
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "CasLayout: namespace file name must be a clean relative path, got '{}'", file_name);
+        return namespaceFilesPrefix(life) + file_name;
+    }
+
+    /// Prefix that covers all verbatim files of ONE LIFE of a namespace (for list). A LIST of it
+    /// enumerates that life's files and nothing of any earlier life of the same namespace name.
+    String namespaceFilesPrefix(const NamespaceLifeId & life) const
+    {
+        return namespaceStatePrefix(life) + "_files/";
+    }
+
+    /// Inverse of `namespaceFileKey`: classifies a listed key under `namespaceStateRootPrefix` by the reserved
+    /// `_files` segment and returns the life it belongs to plus the name relative to that life's files
+    /// prefix. Returns `std::nullopt` for anything that is not one of OUR namespace files: a foreign
+    /// top-level prefix, a key with no `_files` segment at all, or nothing after the reserved segment.
+    ///
+    /// It THROWS `CORRUPTED_DATA` naming the key in the same one situation the ref parsers do: the key
+    /// carries the reserved segment, but the segment where its incarnation belongs is missing,
+    /// non-canonical or zero. Behind Stage B's format bump that is the un-incarnated (Stage A) shape,
+    /// and classifying it as foreign debris would leave it sitting unnoticed under a live namespace.
+    ///
+    /// The first `_files` segment separates the single life-id segment from the relative name; a
+    /// relative name may itself contain `_files`. Defined out-of-line in `CasLayout.cpp`, where the key construction and
+    /// parsing helpers remain together.
+    std::optional<ParsedNamespaceFileKey> parseNamespaceFileKey(std::string_view key) const;
+
+    /// Part manifest body key, in canonical hex form:
+    ///   <prefix>/cas/manifests/<ns>/<epoch-hex>-<build-seq-hex>/<000001>.zst
+    /// The build-scoped directory reuses `RefTxnId`'s hex rendering for `{writer_epoch,
+    /// build_sequence}` (same durable-epoch fence and hex width as a ref transaction id -- a different
+    /// counter with different semantics, not the same identifier). `manifest_ordinal` is a
+    /// per-build ordinal rendered as a six-digit filename. `root_namespace_id` comes from the owning
+    /// context (the `ManifestId`), never from the journal ref.
+    String manifestKey(const ManifestId & id) const
+    {
+        checkNamespace(id.root_namespace);
+        return prefix + "/cas/manifests/" + id.root_namespace.string() + "/"
+             + renderRefTxnId(RefTxnId{id.ref.writer_epoch, id.ref.build_sequence}) + "/"
+             + manifestOrdinalFileName(id.ref.manifest_ordinal);
+    }
+
+    /// Inverse of `manifestKey`: parses `<prefix>/cas/manifests/<ns>/<epoch-hex>-<seq-hex>/<NNNNNN>.zst`.
+    /// Strict: rejects the old decimal directory shape (it is not two fixed-width hex fields joined by
+    /// '-'), a missing namespace/build/ordinal segment, trailing garbage, a file not ending in the
+    /// registered suffix or of the wrong width, and an out-of-range or non-canonical ordinal.
+    /// Foreign/malformed keys return `std::nullopt`, never throw -- LIST sweep / fsck classify by key
+    /// shape, not by validity. All manifest-path parsing (sweep, fsck) routes through this one function.
+    /// Defined out-of-line in `CasLayout.cpp`, where the key construction and parsing helpers remain
+    /// together.
+    std::optional<ManifestId> parseManifestKey(std::string_view key) const;
+
+    /// A plain mountpoint object is a loose, non-content-addressed file mirrored at its
+    /// ClickHouse path under `roots/`, with NO namespace and NO `_files` wrapper. `key` is the
+    /// server-prefixed mirrored path (e.g. `srv1/clickhouse_access_check_abc`). It must NOT end in a
+    /// reserved area. Namespace discovery is catalog-authoritative, not derived from loose roots.
+    /// The `_files`/`_pool_meta` reservations still apply to its segments via the path itself
+    /// (these never appear in a real ClickHouse loose-file path).
+    String mountpointObjectKey(const String & key) const
+    {
+        if (key.empty() || key.front() == '/' || key.back() == '/' || key.find("//") != String::npos)
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "CasLayout: mountpoint object key must be a clean relative path, got '{}'", key);
+        return prefix + "/roots/" + key;
+    }
+
+    /// GC keys.
+    String gcStateKey() const
+    {
+        return prefix + "/gc/state";
+    }
+
+    /// Leak-only namespace-janitor cursor. It is deliberately separate from GC authority state.
+    String gcMaintenanceStateKey() const
+    {
+        return prefix + "/gc/maintenance_state";
+    }
+
+    /// GC heartbeat (advisory liveness pulse): `<prefix>/gc/hb`.
+    String gcHbKey() const
+    {
+        return prefix + "/gc/hb";
+    }
+
+    /// Prefix that covers EVERY per-round artifact of one generation (all attempts): <prefix>/gc/gen/<generation>/
+    /// The wholesale retention prune reclaims a whole generation (every attempt's debris) by this prefix.
+    String gcGenPrefix(uint64_t generation) const
+    {
+        return prefix + "/gc/gen/" + std::to_string(generation) + "/";
+    }
+
+    /// Prefix that covers one (generation, attempt)'s artifacts: <prefix>/gc/gen/<generation>/attempt/<attempt>/
+    /// `attempt` is the folding leader's monotonic per-round id; only the adopted attempt is reader-visible.
+    String gcGenAttemptPrefix(uint64_t generation, uint64_t attempt) const
+    {
+        return gcGenPrefix(generation) + "attempt/" + std::to_string(attempt) + "/";
+    }
+
+    /// Per-(generation, attempt) FOLD seal (write-once): <prefix>/gc/gen/<generation>/attempt/<attempt>/fold_seal.
+    String foldSealKey(uint64_t generation, uint64_t attempt) const
+    {
+        return gcGenAttemptPrefix(generation, attempt) + "fold_seal";
+    }
+
+    /// One blob-target in-degree/delta run segment:
+    ///   <prefix>/gc/gen/<generation>/attempt/<attempt>/blob_target/<shard>/<seq>
+    String blobTargetRunKey(uint64_t generation, uint64_t attempt, uint64_t shard, uint64_t seq) const
+    {
+        return gcGenAttemptPrefix(generation, attempt) + "blob_target/"
+               + std::to_string(shard) + "/" + std::to_string(seq);
+    }
+
+    /// Inverse of `blobTargetRunKey`: parses
+    /// `<prefix>/gc/gen/<generation>/attempt/<attempt>/blob_target/<shard>/<seq>`. Strict: rejects a
+    /// missing/foreign top-level prefix, a missing `attempt`/`blob_target` literal segment, a missing
+    /// generation/attempt/shard/seq segment, a non-canonical decimal component (empty, non-digit, a
+    /// leading zero other than a bare "0", or a value that overflows `uint64_t`), or trailing garbage.
+    /// Foreign/malformed keys return `std::nullopt`, never throw -- callers (`cas-inspect`) classify by
+    /// key shape, not by validity. Defined out-of-line in `CasLayout.cpp`, where the key construction
+    /// and parsing helpers remain together.
+    std::optional<ParsedBlobTargetRunKey> parseBlobTargetRunKey(std::string_view key) const;
+
+    /// Outcomes key: <prefix>/gc/gen/<generation>/attempt/<attempt>/outcomes/<round>/<shard>.zst
+    /// The `.zst` suffix comes from the traits table (cas_gc_outcomes is the one Always-compressed
+    /// control object): a constructed key names the compressed object deterministically, no body sniff.
+    String outcomesKey(uint64_t generation, uint64_t attempt, uint64_t round, uint64_t shard) const
+    {
+        return gcGenAttemptPrefix(generation, attempt) + "outcomes/" + std::to_string(round) + "/" + std::to_string(shard)
+            + String(storedSuffix(FormatId::GcOutcomes));
+    }
+
+    /// Prefix that covers every root-shard manifest and namespace file (GC round discovery).
+    String rootsPrefix() const
+    {
+        return prefix + "/roots/";
+    }
+
+    /// Prefix that covers every content blob (raw object listing for fsck). Deliberately stays
+    /// `<prefix>/blobs/` (no algo segment) even though `blobKey` nests one level deeper under
+    /// `blobs/<algo>/S/ID`: a recursive LIST of this prefix still returns every blob object across all
+    /// algos in one sweep. Any code that PARSES a listed key back to a hash must take the LAST path
+    /// component (the hex digest), which stays correct regardless of the algo segment (`CasGc.cpp` /
+    /// `CasFsck.cpp` already do this via `rfind('/')`).
+    ///
+    /// The S3-staging area lives under `<prefix>/staging/<mount_id>/` — a distinct top-level sibling of
+    /// `blobs/`, `cas/ns/`, and
+    /// `cas/manifests/`, never a sub-path of any of them. Every GC blob-discovery LIST (`CasGc.cpp`,
+    /// `CasFsck.cpp`) enumerates ONLY this `blobsPrefix()`, so a `staging/` object can never be listed,
+    /// HEAD'd, or condemned as an orphan blob — `Cas::sweepOwnMountStaging` (`CasStagingSweeper.h`) is
+    /// the sole reclaimer of `staging/` debris.
+    String blobsPrefix() const { return prefix + "/blobs/"; }
+
+    /// Per-server-root control subtree, keyed by the configured `server_root_id`
+    /// (validated by `DB::Cas::validateServerRootId`). All four control objects live together under
+    /// `<prefix>/gc/server-roots/<server_root_id>/` so a server's mount-safety state is one subtree.
+    String serverRootPrefix(const String & server_root_id) const
+    {
+        return prefix + "/gc/server-roots/" + server_root_id + "/";
+    }
+
+    /// Pool-wide server-roots prefix: `<prefix>/gc/server-roots/`. The base of every
+    /// `serverRootPrefix`; the GC heartbeat gate LISTs it to enumerate all mount objects (it must
+    /// filter to keys ending in `/mount`, since `/owner` and `/epoch` objects share the subtree).
+    String serverRootsPrefix() const
+    {
+        return prefix + "/gc/server-roots/";
+    }
+
+    /// Owner anchor: `<prefix>/gc/server-roots/<srid>/owner`.
+    String ownerKey(const String & server_root_id) const
+    {
+        return serverRootPrefix(server_root_id) + "owner";
+    }
+
+    /// Writer-epoch fence: `<prefix>/gc/server-roots/<srid>/epoch`.
+    String epochKey(const String & server_root_id) const
+    {
+        return serverRootPrefix(server_root_id) + "epoch";
+    }
+
+    /// Mount lease: `<prefix>/gc/server-roots/<srid>/mount`.
+    String mountKey(const String & server_root_id) const
+    {
+        return serverRootPrefix(server_root_id) + "mount";
+    }
+
+    /// The data subtree owned by a server root: `<prefix>/roots/<srid>/`. The mount-safety empty-root
+    /// precondition lists this prefix before data, ref, or manifest writes are admitted.
+    String serverRootDataPrefix(const String & server_root_id) const
+    {
+        return prefix + "/roots/" + server_root_id + "/";
+    }
+
+    /// Per-server-root content-addressed manifest subtree: `<prefix>/cas/manifests/<srid>/`. It is
+    /// included in the mount-safety empty-root check even before the first manifest is written.
+    String casManifestsServerPrefix(const String & server_root_id) const
+    {
+        return prefix + "/cas/manifests/" + server_root_id + "/";
+    }
+
+    /// Pool-level metadata object.
+    String poolMetaKey() const
+    {
+        return prefix + "/_pool_meta";
+    }
+
+    /// The whole-pool namespace catalog (spec INV-3): one object, token-CAS like `gc/state`, read on
+    /// every fold round and every recovery.
+    String refCatalogKey() const
+    {
+        return prefix + "/cas/ref_catalog";
+    }
+
+    /// Public validator for a namespace reconstructed from an untrusted listed key (GC ref intake):
+    /// `parseRefObjectKey` returns the namespace without checking its shape, so a
+    /// consumer that will act on it must re-validate. Throws BAD_ARGUMENTS on a malformed namespace,
+    /// exactly as every key-building method does.
+    void validateNamespace(const RootNamespace & ns) const { checkNamespace(ns); }
+
+    /// The pool's root key prefix, i.e. the constructor's `prefix_` verbatim. Exposed for diagnostics
+    /// only (e.g. scoping a free-function's log line to the pool it operates on when no LoggerPtr is
+    /// threaded that deep) -- no key-building method needs this, they already have `prefix` in scope.
+    const String & poolPrefix() const { return prefix; }
+
+private:
+    String prefix;
+
+    /// A namespace must be non-empty, with no leading/trailing '/', no empty segment ("//"),
+    /// and no segment equal to the reserved "_files". Defined out-of-line in `CasLayout.cpp`, where
+    /// the key construction and parsing helpers remain together.
+    void checkNamespace(const RootNamespace & ns) const;
+
+    /// Parses the one physical-id segment after the rest of a life-owned key identified its family.
+    NamespaceLifePhysicalId namespaceLifePhysicalIdOf(std::string_view key, std::string_view segment) const;
+
+    /// Build <prefix>/<namespace>/<first2chars>/<id>.
+    /// Throws BAD_ARGUMENTS if id is shorter than 2 characters.
+    String shardedKey(const String & ns, const String & id) const
+    {
+        if (id.size() < 2)
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "CasLayout: id must be at least 2 characters, got '{}'", id);
+        return prefix + "/" + ns + "/" + id.substr(0, 2) + "/" + id;
+    }
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.cpp
new file mode 100644
index 000000000000..c944bce3d284
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.cpp
@@ -0,0 +1,353 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <city.h>
+#include <algorithm>
+#include <optional>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+std::string_view placementToWord(EntryPlacement p)
+{
+    switch (p)
+    {
+        case EntryPlacement::Inline: return "inline";
+        case EntryPlacement::Blob:   return "blob";
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: unknown placement {}", static_cast<int>(p));
+}
+
+EntryPlacement placementFromWord(std::string_view w)
+{
+    if (w == "inline") return EntryPlacement::Inline;
+    if (w == "blob")   return EntryPlacement::Blob;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: unknown placement '{}'", w);
+}
+
+/// One entry-record line: {"p","pm", then either the Blob's "ha"/"h"/"sz" or the Inline's "il"}.
+void writeEntryRecord(CasJsonWriter & out, const ManifestEntry & e)
+{
+    bool first = true;
+    writeKey(out, "p", first);
+    writeStringValue(out, e.path);
+    writeKey(out, "pm", first);
+    writeStringValue(out, placementToWord(e.placement));
+    if (e.placement == EntryPlacement::Blob)
+    {
+        writeBlobRefFields(out, first, e.ref);   /// ha + h
+        writeKey(out, "sz", first);
+        writeIntText(e.blob_size, out);
+    }
+    else
+    {
+        writeKey(out, "il", first);
+        writeIntText(e.inline_bytes.size(), out);
+    }
+    closeObject(out, first);
+    writeChar('\n', out);
+}
+
+/// The exact banner text for one Inline entry's payload-zone chunk: `==> <path> il=<n> <==`. Takes
+/// `path`/`n` explicitly (not a `ManifestEntry`): on decode, `inline_bytes` is not yet populated at
+/// the point the expected banner is computed (that's the whole point of reading it from here first).
+String bannerFor(std::string_view path, uint64_t n)
+{
+    return "==> " + String(path) + " il=" + std::to_string(n) + " <==";
+}
+
+}
+
+String encodePartManifest(const PartManifest & m)
+{
+    /// Canonical path order plus duplicate-path rejection makes the encoded record sequence
+    /// deterministic and establishes the ordering required by the lookup helpers.
+    std::vector<const ManifestEntry *> sorted;
+    sorted.reserve(m.entries.size());
+    for (const auto & e : m.entries)
+        sorted.push_back(&e);
+    std::sort(sorted.begin(), sorted.end(),
+              [](const ManifestEntry * a, const ManifestEntry * b) { return a->path < b->path; });
+    for (size_t i = 1; i < sorted.size(); ++i)
+        if (sorted[i]->path == sorted[i - 1]->path)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: duplicate path '{}'", sorted[i]->path);
+
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::PartManifest);
+
+    /// descriptor meta line: ManifestRef (me/mb/mo, shared rendering with refsnaplog) + root
+    /// namespace + payload digest.
+    {
+        bool first = true;
+        writeManifestRefFields(out, first, "", m.ref);
+        writeKey(out, "ns", first);
+        writeStringValue(out, m.root_namespace_id.string());
+        writeKey(out, "pd", first);
+        writeHex128Value(out, m.payload_digest);
+        closeObject(out, first);
+        writeChar('\n', out);
+    }
+
+    for (const ManifestEntry * e : sorted)
+        writeEntryRecord(out, *e);
+
+    writeTrailerLine(out, sorted.size());
+
+    /// payload zone: one banner + raw bytes + '\n' per Inline entry, in path order. Blob entries
+    /// carry no payload-zone bytes (their bytes live in a separately addressed CAS blob).
+    for (const ManifestEntry * e : sorted)
+    {
+        if (e->placement != EntryPlacement::Inline)
+            continue;
+        const String banner = bannerFor(e->path, e->inline_bytes.size());
+        out.append(banner);
+        writeChar('\n', out);
+        out.append(e->inline_bytes);
+        writeChar('\n', out);
+    }
+
+    return std::move(out).take();
+}
+
+PartManifest decodePartManifest(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::PartManifest);
+    const uint64_t line_cap = traitsFor(FormatId::PartManifest).line_cap;
+
+    PartManifest m;
+
+    /// descriptor meta line
+    {
+        const String meta = readLine(in, line_cap, "cas_part_manifest");
+        ReadBufferFromMemory mm(meta.data(), meta.size());
+        JsonObjectReader r(mm, KeyStrictness::Tolerant, "cas_part_manifest");
+        std::optional<uint64_t> me;
+        std::optional<uint64_t> mb;
+        std::optional<uint64_t> mo;
+        std::optional<String> ns;
+        std::optional<UInt128> pd;
+        String key;
+        while (r.nextKey(key))
+        {
+            if (key == "me") me = r.readU64String();
+            else if (key == "mb") mb = r.readU64String();
+            else if (key == "mo") mo = r.readU64Number();
+            else if (key == "ns") ns = r.readString();
+            else if (key == "pd") pd = r.readHex128();
+            else r.skipUnknown(key);
+        }
+        if (!me || !mb || !mo)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: descriptor missing me/mb/mo");
+        if (!ns)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: descriptor missing ns");
+        if (!pd)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: descriptor missing pd");
+        m.ref = manifestRefFromFields(*me, *mb, *mo, "PartManifest", "descriptor");
+        m.root_namespace_id = RootNamespace(*ns);
+        m.payload_digest = *pd;
+        if (!mm.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: junk after descriptor line");
+    }
+
+    /// entry record lines, until the trailer. Inline entries remember their declared `il` length so
+    /// the payload zone below can read exactly that many raw bytes back into `inline_bytes`.
+    /// Index-aligned with `m.entries` (Blob entries push an unused 0 placeholder).
+    std::vector<uint64_t> inline_lens;
+    while (true)
+    {
+        const String line = readLine(in, line_cap, "cas_part_manifest");
+        ReadBufferFromMemory l(line.data(), line.size());
+        JsonObjectReader r(l, KeyStrictness::Tolerant, "cas_part_manifest");
+        String key;
+        if (!r.nextKey(key))
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: empty line");
+
+        if (key == "n")
+        {
+            const uint64_t declared_n = r.readU64Number();
+            while (r.nextKey(key))
+                r.skipUnknown(key);
+            if (!l.eof())
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: junk after trailer");
+            if (declared_n != m.entries.size())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "PartManifest: trailer count {} != {} records", declared_n, m.entries.size());
+            break;
+        }
+
+        if (key != "p")
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: record must start with \"p\"");
+        ManifestEntry e;
+        e.path = r.readString();
+
+        /// Manifest bytes arrive over the interserver relink channel: enforce the same path hygiene
+        /// as CasLayout::checkNamespace so no future consumer can inherit a traversal. Relative,
+        /// no empty/'.'/'..' segments. (Syntactic only — legal projection subdirs pass.)
+        if (e.path.empty() || e.path.front() == '/')
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS part manifest: invalid entry path '{}'", e.path);
+        for (std::string_view rest = e.path; !rest.empty();)
+        {
+            const size_t slash = rest.find('/');
+            const std::string_view seg = rest.substr(0, slash);
+            if (seg.empty() || seg == "." || seg == "..")
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS part manifest: invalid entry path '{}'", e.path);
+            rest = (slash == std::string_view::npos) ? std::string_view{} : rest.substr(slash + 1);
+        }
+
+        std::optional<String> pm;
+        std::optional<String> ha;
+        std::optional<String> h;
+        std::optional<uint64_t> sz;
+        std::optional<uint64_t> il;
+        while (r.nextKey(key))
+        {
+            if (key == "pm") pm = r.readString();
+            else if (key == "ha") ha = r.readString();
+            else if (key == "h") h = r.readString();
+            else if (key == "sz") sz = r.readU64Number();
+            else if (key == "il") il = r.readU64Number();
+            else r.skipUnknown(key);
+        }
+        if (!l.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: junk after record");
+        if (!pm)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: entry '{}' missing pm", e.path);
+        e.placement = placementFromWord(*pm);
+
+        if (e.placement == EntryPlacement::Blob)
+        {
+            if (!ha || !h || !sz)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: blob entry '{}' missing ha/h/sz", e.path);
+            const BlobHashAlgo algo = blobHashAlgoFromWord(*ha, "PartManifest entry");
+            /// Validate the digest width before calling `fromHex`. A width mismatch otherwise
+            /// produces `BAD_ARGUMENTS` instead of the `CORRUPTED_DATA` required for malformed
+            /// serialized input, allowing an invalid manifest to escape the decoder's fail-closed
+            /// error contract.
+            const uint64_t expected_hex_len = blobHashLenFor(algo) * 2;
+            if (h->size() != expected_hex_len)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "PartManifest: entry '{}' digest hex width {} does not match algo width {}",
+                    e.path, h->size(), expected_hex_len);
+            e.ref = BlobRef{algo, codecFor(algo).fromHex(*h)};
+            e.blob_size = *sz;
+            inline_lens.push_back(0);   /// unused for Blob; keeps inline_lens index-aligned with entries
+        }
+        else
+        {
+            if (!il)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: inline entry '{}' missing il", e.path);
+            inline_lens.push_back(*il);   /// bytes filled from the payload zone below
+        }
+
+        /// Canonical ascending-order and no-duplicate-path enforcement: compare only against the
+        /// immediately preceding entry, requiring
+        /// strict '<'. This is sufficient to catch a NON-adjacent duplicate too (e.g. forging entry
+        /// c's path to equal entry a's path in an a<b<c stream): the forged c is compared against b,
+        /// and since originally a<b, the forged c(=a) is NOT greater than b, so the strict check
+        /// fires regardless of which earlier entry the duplicate collides with. This also keeps
+        /// findEntry/entryRange's binary-search precondition (strictly ascending m.entries) sound.
+        if (!m.entries.empty() && !(m.entries.back().path < e.path))
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "PartManifest: entries not in canonical ascending order (prev '{}', got '{}')",
+                m.entries.back().path, e.path);
+        m.entries.push_back(std::move(e));
+    }
+
+    /// payload zone: for each Inline entry, in the same order it appeared above, a banner line then
+    /// exactly `il` raw bytes then a terminating '\n'.
+    for (size_t i = 0; i < m.entries.size(); ++i)
+    {
+        if (m.entries[i].placement != EntryPlacement::Inline)
+            continue;
+        const uint64_t n = inline_lens[i];
+        const String banner_line = readLine(in, line_cap, "cas_part_manifest payload zone");
+        const String expected = bannerFor(m.entries[i].path, n);
+        if (banner_line != expected)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "PartManifest: payload-zone banner mismatch, expected '{}', got '{}'", expected, banner_line);
+        m.entries[i].inline_bytes = readFixedBytes(in, n);
+        const String terminator = readFixedBytes(in, 1);
+        if (terminator[0] != '\n')
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "PartManifest: payload-zone chunk for '{}' missing terminating newline", m.entries[i].path);
+    }
+
+    if (!in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "PartManifest: trailing bytes after the payload zone");
+
+    /// Recompute and verify `payload_digest` last, over the fully-decoded body. `computePayloadDigest`
+    /// builds its own probe copy with payload_digest zeroed before hashing (see below), so calling it
+    /// directly here on `m` (whose payload_digest is whatever was read off the wire) is safe and
+    /// matches encode's own computation exactly.
+    const UInt128 expected_digest = computePayloadDigest(m);
+    if (expected_digest != m.payload_digest)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "PartManifest: payload_digest mismatch, expected {}, got {}",
+            u128ToHex(expected_digest), u128ToHex(m.payload_digest));
+
+    return m;
+}
+
+UInt128 computePayloadDigest(const PartManifest & m)
+{
+    /// Digest the canonical encoding with payload_digest zeroed, so the digest does not depend on
+    /// itself (it would be circular otherwise) and is stable for identical bodies, changing whenever
+    /// ref / namespace / entries change. Uses the CAS content-hash primitive (CityHash128) over the
+    /// deterministic encodePartManifest bytes - the same primitive blob/tree hashing uses.
+    PartManifest probe = m;
+    probe.payload_digest = UInt128{};
+    const String bytes = encodePartManifest(probe);
+    const auto h = CityHash_v1_0_2::CityHash128(bytes.data(), bytes.size());
+    return (static_cast<UInt128>(h.high64) << 64) | static_cast<UInt128>(h.low64);
+}
+
+bool refMatchesBody(const ManifestRef & journal_ref, const PartManifest & body)
+{
+    return journal_ref == body.ref;
+}
+
+bool manifestNamespaceMatches(const RootNamespace & owning, const PartManifest & body)
+{
+    return owning == body.root_namespace_id;
+}
+
+const ManifestEntry * findEntry(const std::vector<ManifestEntry> & entries, std::string_view path)
+{
+    const auto it = std::lower_bound(entries.begin(), entries.end(), path,
+        [](const ManifestEntry & e, std::string_view p) { return std::string_view(e.path) < p; });
+    if (it == entries.end() || std::string_view(it->path) != path)
+        return nullptr;
+    return &*it;
+}
+
+std::pair<const ManifestEntry *, const ManifestEntry *>
+entryRange(const std::vector<ManifestEntry> & entries, std::string_view dir_prefix)
+{
+    if (dir_prefix.empty())
+        return {entries.data(), entries.data() + entries.size()};
+    /// Every path starting with `dir_prefix` compares >= `dir_prefix`, and prefixed paths form a
+    /// contiguous run from the first such position.
+    const auto first = std::lower_bound(entries.begin(), entries.end(), dir_prefix,
+        [](const ManifestEntry & e, std::string_view p) { return std::string_view(e.path) < p; });
+    auto last = first;
+    while (last != entries.end() && std::string_view(last->path).starts_with(dir_prefix))
+        ++last;
+    return {entries.data() + (first - entries.begin()), entries.data() + (last - entries.begin())};
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h
new file mode 100644
index 000000000000..ab91c709e468
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h
@@ -0,0 +1,120 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <string>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Text codec for the immutable, root-local `cas_part_manifest` body. It is a `PayloadHybrid` object
+/// made of JSON lines followed by a raw payload zone. The public types and helper signatures remain
+/// stable for the surrounding CAS protocol.
+///
+///   header line                 {"type":"cas_part_manifest","v":N}
+///   descriptor meta line         {"me","mb","mo"} (the ManifestRef, shared rendering with
+///                                refsnaplog, `CasWireVocab.h`) + "ns" (root namespace) + "pd"
+///                                (payload digest, 32 lowercase hex)
+///   one entry-record line each   {"p":path,"pm":placement-word, then either the Blob's
+///                                {"ha","h","sz"} or the Inline's {"il"}}, in canonical path order
+///   trailer line                 {"n":entry-count}
+///   PAYLOAD ZONE (raw, follows the trailer): for each Inline entry, in path order, a
+///                                `head -v`-style banner line `==> <path> il=<n> <==\n`, then
+///                                exactly `n` raw bytes, then `\n`. Blob entries carry no
+///                                payload-zone bytes — their bytes live in a separately addressed
+///                                CAS blob; the manifest carries only the `BlobRef` + size.
+///
+/// The payload zone is why this format is `PayloadHybrid` rather than a plain `RecordStream`/
+/// `Control` object: `inline_bytes` is arbitrary binary, not necessarily valid UTF-8, so it cannot be
+/// JSON-string-encoded safely and instead rides outside the JSON-line region entirely.
+
+/// Where a manifest entry's file bytes live. There are no nested tree objects: a directory is a path
+/// prefix, not a placement. `Inline` bytes belong to the manifest's raw payload zone; `Blob` bytes
+/// are stored separately under `blobKey`.
+enum class EntryPlacement : uint8_t
+{
+    Inline = 1,   /// bytes embedded in `inline_bytes`
+    Blob = 2,     /// bytes stored as a content-addressed blob at `blobKey`
+};
+
+/// One file entry inside a part manifest. `ref` is meaningful only for `Blob`; `inline_bytes` only
+/// for `Inline`. `blob_size` is the raw `Blob` byte count (0 for `Inline` — decode never fills it for
+/// an inline entry, since the wire format carries no redundant size for inline bytes). Use `size()`
+/// for the entry's logical file size regardless of placement; no consumer should branch on
+/// `placement` just to answer "how big is this file". `ref` is the full blob identity: the algorithm
+/// travels with the digest, per entry, so a manifest may mix hash algorithms. A bare digest is never
+/// the identity.
+struct ManifestEntry
+{
+    String path;
+    EntryPlacement placement = EntryPlacement::Inline;
+    BlobRef ref{};
+    uint64_t blob_size = 0;
+    String inline_bytes;
+    bool operator==(const ManifestEntry &) const = default;
+
+    /// The single source of truth for this entry's logical file size, independent of where its bytes
+    /// live. Decoding an `Inline` entry leaves `blob_size == 0`, because the wire record has no
+    /// redundant blob size; a carried-forward inline entry can therefore be non-empty even though
+    /// `blob_size` is zero. Consumers that need the logical file size must use this method rather
+    /// than inspecting the placement-specific fields themselves.
+    uint64_t size() const { return placement == EntryPlacement::Inline ? inline_bytes.size() : blob_size; }
+};
+
+/// The immutable body of one root-local part manifest. It repeats `ref` and `root_namespace_id` so
+/// readers can validate the journal reference and owning root namespace against the body; neither
+/// repetition is a second identity. `payload_digest` is integrity/debug metadata only: it is never a
+/// key, deduplication input, or in-degree. Mutable per-reference payload remains in the root
+/// `RefRecord`. Entries carry their own digest algorithm, so one manifest may mix digest widths.
+/// Entries are strictly ascending by `path` after decoding, which permits index-free binary-search
+/// and prefix-range lookup without adding a directory index to the immutable body.
+struct PartManifest
+{
+    ManifestRef ref;
+    RootNamespace root_namespace_id;
+    UInt128 payload_digest{};
+    std::vector<ManifestEntry> entries;
+    bool operator==(const PartManifest &) const = default;
+};
+
+/// Deterministic, streaming-capable encode. Entries are written in canonical path order (the encoder
+/// sorts them); a duplicate path throws `CORRUPTED_DATA`. Byte output is reproducible for identical
+/// input (no timestamps, no nondeterministic compression). Returns the canonical TEXT (NOT sealed);
+/// the caller compresses via `sealObject(FormatId::PartManifest, …)` on the persist path
+/// (`CompressionPolicy::Always`).
+String encodePartManifest(const PartManifest & m);
+
+/// Decode the canonical TEXT (the caller `openObject`s a stored `.zst` first). Throws `CORRUPTED_DATA`
+/// on a malformed header/descriptor/record/trailer/payload-zone shape or an unknown placement word;
+/// `UNKNOWN_FORMAT_VERSION` for a header `v` above this build.
+PartManifest decodePartManifest(std::string_view data);
+
+/// Content digest of the canonical encoded body, using the CAS content-hash primitive
+/// (`CityHash_v1_0_2::CityHash128`, the same one used for blob/tree hashing, not a second hash
+/// primitive). Callers set `PartManifest.payload_digest` from this. It is
+/// integrity/debug ONLY - never a key, never dedup, never in-degree. Stable for identical bodies;
+/// changes when any byte of the canonical encoding does, and is independent of the `payload_digest`
+/// field itself (computed with it zeroed).
+UInt128 computePayloadDigest(const PartManifest & m);
+
+/// Fail-closed identity checks used when reading or folding a manifest. The journal `ManifestRef`
+/// must equal the `ref` inside the decoded body.
+bool refMatchesBody(const ManifestRef & journal_ref, const PartManifest & body);
+/// The body `root_namespace_id` must equal the owning root namespace.
+bool manifestNamespaceMatches(const RootNamespace & owning, const PartManifest & body);
+
+/// Pure ordered-entry lookup primitives over a decoded manifest. `decodePartManifest` guarantees
+/// strict ascending order by `path`; `PartFolderView` composes these lookups with wiring policy.
+
+/// Binary search. Returns nullptr when absent. The pointer aliases `entries` — do not outlive it.
+const ManifestEntry * findEntry(const std::vector<ManifestEntry> & entries, std::string_view path);
+
+/// The contiguous [first, last) sub-span of entries whose path starts with `dir_prefix` (canonical
+/// order makes matches contiguous). Empty prefix = the whole span.
+std::pair<const ManifestEntry *, const ManifestEntry *>
+entryRange(const std::vector<ManifestEntry> & entries, std::string_view dir_prefix);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.cpp
new file mode 100644
index 000000000000..fab51431c308
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.cpp
@@ -0,0 +1,182 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int UNKNOWN_FORMAT_VERSION;
+}
+}
+
+namespace DB::Cas
+{
+
+/// Minimum `blob_header_len` that provably fits the v3 `cas_blob` JSON envelope's mandatory (always-
+/// written) non-ref fields, computed at type maxima from `encodeEnvelopeHeader` (CasBlobEnvelopeFormat.cpp):
+///   {"type":"cas_blob"                                        18
+///   ,"v":<u32>          5 + 10 (currentCompatibilityVersion)  15
+///   ,"tag":"<32 hex>"   7 + 34                                41
+///   ,"bld":"<32 hex>"   7 + 34                                41
+///   ,"ts":<u64>         6 + 20 (created_at_ms)                26
+///   ,"by":"<32 hex>"    7 + 34                                41
+///   ,"op":"<word>"      6 + 10 (longest op word "mutation")   16
+///   ,"ch":<u32>         6 + 10 (VERSION_INTEGER)              16
+///                                            non-ref JSON  = 214 bytes
+/// The encoder then always frames the ref: `,"ref":` (7) + `""` (2) + `}` (1), and reserves byte
+/// blob_header_len-1 for '\n' (1) = 11 bytes. So the mandatory content needs 214 + 11 = 225 bytes;
+/// below that, encodeEnvelopeHeader throws LOGICAL_ERROR on the FIRST blob write (the old drop-and-retry
+/// that used to mask this is gone). We floor at 240 (a multiple of 8 comfortably above 225, leaving
+/// >= 15 bytes for the diagnostic ref even at type maxima, and well under the 256 default) so a
+/// misconfigured pool fails at CREATION with BAD_ARGUMENTS, not at first write with LOGICAL_ERROR.
+static constexpr uint64_t kMinBlobHeaderLen = 240;
+
+void validatePoolBlobHeaderLen(uint64_t blob_header_len, int error_code, std::string_view what)
+{
+    if (blob_header_len < kMinBlobHeaderLen)
+        throw Exception(error_code, "CAS {}: blob_header_len must be >= {} (v3 envelope minimum), got {}",
+            what, kMinBlobHeaderLen, blob_header_len);
+    if (blob_header_len % 8 != 0)
+        throw Exception(error_code, "CAS {}: blob_header_len must be a multiple of 8, got {}", what, blob_header_len);
+    if (blob_header_len > 16 * 1024)
+        throw Exception(error_code, "CAS {}: blob_header_len must be <= 16384, got {}", what, blob_header_len);
+}
+
+void validatePoolAlgosUsed(const std::vector<uint8_t> & algos_used, int error_code, std::string_view what)
+{
+    if (algos_used.empty())
+        throw Exception(error_code, "CAS {}: algos_used must be non-empty", what);
+    for (size_t i = 0; i < algos_used.size(); ++i)
+    {
+        try
+        {
+            blobHashAlgoName(static_cast<BlobHashAlgo>(algos_used[i]));
+        }
+        catch (const Exception &)
+        {
+            throw Exception(error_code, "CAS {}: algos_used contains an unknown algo {}", what, algos_used[i]);
+        }
+        if (i > 0 && algos_used[i] <= algos_used[i - 1])
+            throw Exception(error_code,
+                "CAS {}: algos_used must be strictly sorted with no duplicates, got {} at index {} not after {}",
+                what, algos_used[i], i, algos_used[i - 1]);
+    }
+}
+
+String encodePoolMeta(const PoolMeta & pm)
+{
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::PoolMeta);
+
+    bool first = true;
+    writeKey(out, "pid", first);
+    writeHex128Value(out, pm.pool_id);
+    writeKey(out, "hln", first);
+    writeIntText(pm.blob_header_len, out);
+    writeKey(out, "gcs", first);
+    writeIntText(pm.gc_shards, out);
+    writeKey(out, "mrg", first);
+    writeIntText(pm.min_reader_generation, out);
+    writeKey(out, "alg", first);
+    {
+        /// Comma-joined algo words (tiny list, <=3): "ch128" or "ch128,sha256".
+        String joined;
+        for (size_t i = 0; i < pm.algos_used.size(); ++i)
+        {
+            if (i != 0)
+                joined += ',';
+            joined += blobHashAlgoName(static_cast<BlobHashAlgo>(pm.algos_used[i]));
+        }
+        writeStringValue(out, joined);
+    }
+    closeObject(out, first);
+    writeChar('\n', out);
+
+    return std::move(out).take();
+}
+
+PoolMeta decodePoolMeta(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    const TextHeader header = expectHeaderLine(in, FormatId::PoolMeta);
+
+    /// An older pool predates a breaking ref-layer change this build cannot reconcile, so
+    /// reject it before reading the metadata body. Writers always emit the current generation, while
+    /// `expectHeaderLine` separately rejects a future generation that this build cannot understand.
+    /// Generation 9 is the latest recreate-only authority floor and subsumes the earlier stream floors.
+    if (header.v < kCommittedRefFrontierGeneration)
+        throw Exception(ErrorCodes::UNKNOWN_FORMAT_VERSION,
+            "CAS pool format {} predates generation-9 exact _ckpt committed_through recovery frontier; recreate the pool. "
+            "This build requires the namespace-admission shard bound and exact recovery frontier "
+            "in the generation-9 format "
+            "(generation {}+), and CAS is pre-release: there is no in-place migration.",
+            header.v, kCommittedRefFrontierGeneration);
+
+    const String body = readLine(in, traitsFor(FormatId::PoolMeta).line_cap, "pool meta");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    JsonObjectReader r(body_in, KeyStrictness::Tolerant, "pool meta");
+
+    PoolMeta pm;
+    bool saw_pid = false;
+    bool saw_gc_shards = false;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "pid")
+        {
+            pm.pool_id = r.readHex128();
+            saw_pid = true;
+        }
+        else if (key == "hln")
+            pm.blob_header_len = r.readU64Number();
+        else if (key == "gcs")
+        {
+            pm.gc_shards = r.readU64Number();
+            saw_gc_shards = true;
+        }
+        else if (key == "mrg")
+            pm.min_reader_generation = r.readU64Number();
+        else if (key == "alg")
+        {
+            const String joined = r.readString();
+            size_t start = 0;
+            while (start <= joined.size())
+            {
+                const size_t comma = joined.find(',', start);
+                const String word = joined.substr(start, comma == String::npos ? String::npos : comma - start);
+                if (word.empty())
+                    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS pool meta: empty algo word in '{}'", joined);
+                pm.algos_used.push_back(static_cast<uint8_t>(blobHashAlgoFromWord(word, "pool meta algo")));
+                if (comma == String::npos)
+                    break;
+                start = comma + 1;
+            }
+        }
+        else
+            r.skipUnknown(key);
+    }
+    if (!saw_pid)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS pool meta: missing pid");
+    if (!saw_gc_shards || pm.gc_shards == 0)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS pool meta: missing or zero gcs");
+    if (!body_in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS pool meta: junk after body object");
+    if (!in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS pool meta: trailing bytes after body line");
+
+    validatePoolBlobHeaderLen(pm.blob_header_len, ErrorCodes::CORRUPTED_DATA, "pool meta");
+    validatePoolAlgosUsed(pm.algos_used, ErrorCodes::CORRUPTED_DATA, "pool meta");
+
+    if (G_BUILD < pm.min_reader_generation)
+        throw Exception(ErrorCodes::UNKNOWN_FORMAT_VERSION,
+            "CAS pool meta: pool requires reader generation {} but this build supports at most {}",
+            pm.min_reader_generation, G_BUILD);
+
+    return pm;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h
new file mode 100644
index 000000000000..2ca0894d2f01
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h
@@ -0,0 +1,91 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+class Backend;
+class Layout;
+
+/// `_pool_meta` — the pool identity and the pool-wide constants that every reader and writer must
+/// agree on. The v3 text representation is a header line followed by one JSON body object:
+/// {"pid":"<32hex>","hln":<blob_header_len>,"mrg":<min_reader_generation>,"alg":"<algo-words>"}.
+///
+/// The persisted object is authoritative after creation. On reopen, `createOrValidate` uses its
+/// `blob_header_len` and reader-generation floor rather than replacing them with local configuration;
+/// the configuration's hash algorithm may only be admitted through the explicit opt-in path. The
+/// `pool_id` is also the envelope `domain_id`, so it remains stable for the entire pool lifetime.
+struct PoolMeta
+{
+    UInt128 pool_id{};
+    uint64_t blob_header_len = 0;
+    uint64_t gc_shards = 1;
+    uint64_t min_reader_generation = 0;
+    /// Every hash algorithm ever admitted, encoded as `static_cast<uint8_t>(BlobHashAlgo)`, in strictly
+    /// increasing order. Admission only appends a new algorithm to this durable set.
+    std::vector<uint8_t> algos_used;
+
+    /// Creates the pool metadata if `_pool_meta` is absent, or validates and possibly admits the
+    /// configured hash algorithm if it already exists. Initial creation validates the supplied header
+    /// size and records this build's reader-generation floor. Reopen ignores the supplied header size,
+    /// because changing it would move the blob payload offset for existing objects; a new hash algorithm
+    /// is rejected unless `allow_new` is set, and concurrent admission is retried from fresh metadata.
+    ///
+    /// `allow_mint` (spec §2 [C4][D2]) gates the create-if-absent path: minting a fresh `_pool_meta` is a
+    /// consequential write that establishes a brand-new pool identity, so it is permitted ONLY on the
+    /// writable startup path that has just passed the zero-write residual proof (`Pool::open`). Every
+    /// non-bootstrap caller — a read-only/observe open, `openForDecommission` — passes `false`; an absent
+    /// `_pool_meta` then fails closed with `INVALID_STATE` instead of silently minting (which, on an
+    /// observe scan over a partially-erased pool, would poison the next writable mount). The validate path
+    /// (meta already present) never consults it.
+    ///
+    /// Defaults to `false` — a safety gate must fail CLOSED when a caller leaves it unstated, so a future
+    /// pool-lifecycle entry point cannot silently re-arm the observe-mint footgun by omission. The two
+    /// production callers pass it explicitly; only test minting sites opt in with `allow_mint=true`.
+    static PoolMeta createOrValidate(
+        Backend &, const Layout &, uint64_t blob_header_len, uint64_t gc_shards,
+        BlobHashAlgo blob_hash_algo = BlobHashAlgo::CityHash128, bool allow_new = false,
+        bool allow_mint = false);
+
+    /// Convenience for single-shard callers. Production pool opening passes the configured value to
+    /// the explicit overload above; this preserves compact single-shard codec/unit fixtures.
+    static PoolMeta createOrValidate(
+        Backend & backend, const Layout & layout, uint64_t blob_header_len,
+        BlobHashAlgo blob_hash_algo = BlobHashAlgo::CityHash128, bool allow_new = false,
+        bool allow_mint = false)
+    {
+        return createOrValidate(
+            backend, layout, blob_header_len, /*gc_shards=*/1, blob_hash_algo, allow_new, allow_mint);
+    }
+};
+
+/// Serializes valid pool metadata as the versioned `_pool_meta` text object. The output includes the
+/// format header, one JSON body line, and its terminating newline; it is suitable for a conditional
+/// backend write and preserves the sorted algorithm set as comma-separated vocabulary words.
+String encodePoolMeta(const PoolMeta &);
+
+/// Parses and validates a persisted `_pool_meta` object. Unknown JSON keys are tolerated for additive
+/// evolution, while missing required data, malformed values, invariant violations, an unsupported
+/// ref-state generation, or a pool requiring a newer reader produce an exception with the appropriate
+/// corruption or compatibility error code.
+PoolMeta decodePoolMeta(std::string_view);
+
+/// Checks the fixed blob-envelope size invariant. The length must be 8-byte aligned, at most 16 KiB,
+/// and at least 240 bytes: v3's mandatory envelope fields, framing, and newline consume 225 bytes at
+/// type maxima, while 240 leaves room for a diagnostic `ref`. The caller supplies the error code so
+/// persisted violations can be reported as `CORRUPTED_DATA` and bad creation arguments as
+/// `BAD_ARGUMENTS`.
+void validatePoolBlobHeaderLen(uint64_t blob_header_len, int error_code, std::string_view what);
+
+/// Checks that every admitted hash algorithm is known, that the set is non-empty, and that its numeric
+/// representation is strictly increasing with no duplicates. The caller supplies the error code to
+/// distinguish invalid persisted metadata from invalid creation or admission input.
+void validatePoolAlgosUsed(const std::vector<uint8_t> & algos_used, int error_code, std::string_view what);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.cpp
new file mode 100644
index 000000000000..b21458aaf6e2
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.cpp
@@ -0,0 +1,325 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+UInt128 toWideChecksum(CityHash_v1_0_2::uint128 h)
+{
+    /// Keep the high and low halves in the same order for the write-side helper and the streaming
+    /// reader. The value is compared internally rather than exposed as a separately interpreted wire
+    /// field, but both paths must use the same packing for a stored run to verify successfully.
+    return (static_cast<UInt128>(h.high64) << 64) | static_cast<UInt128>(h.low64);
+}
+
+int hexNibble(char c)
+{
+    if (c >= '0' && c <= '9') return c - '0';
+    if (c >= 'a' && c <= 'f') return c - 'a' + 10;
+    return -1;
+}
+
+BlobHashAlgo algoFromByte(uint8_t b, std::string_view what)
+{
+    switch (b)
+    {
+        case static_cast<uint8_t>(BlobHashAlgo::CityHash128): return BlobHashAlgo::CityHash128;
+        case static_cast<uint8_t>(BlobHashAlgo::XXH3_128):    return BlobHashAlgo::XXH3_128;
+        case static_cast<uint8_t>(BlobHashAlgo::Sha256):      return BlobHashAlgo::Sha256;
+        default:
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: unknown algo byte {} in record key", what, b);
+    }
+}
+
+/// `b` = the algo byte as two lowercase hex chars, then the digest hex at the algo's width. The algo
+/// byte leads so that string-sorting `b` reproduces the binary (algo, digest) byte order.
+String renderB(const BlobRef & ref)
+{
+    static constexpr char H[] = "0123456789abcdef";
+    const uint8_t a = static_cast<uint8_t>(ref.algo);
+    String b;
+    b.push_back(H[(a >> 4) & 0xF]);
+    b.push_back(H[a & 0xF]);
+    b += codecFor(ref.algo).toHex(ref.digest);
+    return b;
+}
+
+BlobRef parseB(std::string_view b)
+{
+    if (b.size() < 2)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: record key too short");
+    const int hi = hexNibble(b[0]);
+    const int lo = hexNibble(b[1]);
+    if (hi < 0 || lo < 0)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: non-hex algo byte in record key");
+    const BlobHashAlgo algo = algoFromByte(static_cast<uint8_t>((hi << 4) | lo), "cas_run");
+    const std::string_view digest_hex = b.substr(2);
+    if (digest_hex.size() != static_cast<size_t>(blobHashLenFor(algo)) * 2)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS cas_run: digest hex width {} does not match algo width {}", digest_hex.size(), blobHashLenFor(algo) * 2);
+    BlobRef ref;
+    ref.algo = algo;
+    ref.digest = codecFor(algo).fromHex(String(digest_hex));
+    return ref;
+}
+
+std::string_view markerToWord(char m)
+{
+    switch (m)
+    {
+        case kEdgeActive: return "edge";
+        case kZeroMarker: return "zero";
+        case kCondemned:  return "condemned";
+        default:
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: unknown row marker 0x{:02x}", static_cast<uint8_t>(m));
+    }
+}
+
+char markerFromWord(std::string_view w)
+{
+    if (w == "edge")      return kEdgeActive;
+    if (w == "zero")      return kZeroMarker;
+    if (w == "condemned") return kCondemned;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: unknown row marker '{}'", w);
+}
+
+}
+
+void writeRunHeaderLine(WriteBuffer & out, std::string_view kind)
+{
+    const FormatTraits & t = traitsFor(FormatId::RunFile);
+    CasJsonWriter line(64);
+    bool first = true;
+    writeKey(line, "type", first);
+    writeStringValue(line, t.type);
+    writeKey(line, "v", first);
+    writeIntText(currentCompatibilityVersion(), line);
+    writeKey(line, "kind", first);
+    writeStringValue(line, kind);
+    closeObject(line, first);
+    writeChar('\n', line);
+    const std::string_view line_view = line.view();
+    out.write(line_view.data(), line_view.size());
+}
+
+void expectRunHeaderLine(ReadBuffer & in, std::string_view expected_kind)
+{
+    const FormatTraits & t = traitsFor(FormatId::RunFile);
+    const String line = readLine(in, t.line_cap, t.type);
+    ReadBufferFromMemory buf(line.data(), line.size());
+    JsonObjectReader r(buf, KeyStrictness::Tolerant, t.type);
+
+    String key;
+    if (!r.nextKey(key) || key != "type")
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: header line must start with \"type\"");
+    const String type = r.readString();
+    if (type != t.type)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: object is a '{}', not a '{}'", type, t.type);
+
+    if (!r.nextKey(key) || key != "v")
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: header line must carry \"v\" second");
+    const uint32_t v = r.readU32Number();
+    checkCompatibility(v, t.type);
+
+    if (!r.nextKey(key) || key != "kind")
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: header line must carry \"kind\" third");
+    const String kind = r.readString();
+    if (kind != expected_kind)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: unknown run kind '{}'", kind);
+
+    while (r.nextKey(key))
+        r.skipUnknown(key);
+    if (!buf.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: junk after the header object");
+}
+
+SourceEdgeRunWriter::SourceEdgeRunWriter(WriteBuffer & out_)
+    : out(out_)
+{
+    writeRunHeaderLine(out, kSourceEdgeKindWord);
+}
+
+void SourceEdgeRunWriter::append(const SourceEdgeRecord & rec)
+{
+    if (finished)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS cas_run: append after finish");
+    /// Non-decreasing (ref, source_id) is a HARD writer contract (deterministic run + streaming merge).
+    /// A regression is a programming bug at the producer, not corrupt on-disk data => LOGICAL_ERROR.
+    if (have_prev)
+    {
+        const bool regressed = (rec.ref < prev_ref)
+            || (rec.ref == prev_ref && rec.source_id < prev_source_id);
+        if (regressed)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS cas_run: records appended out of (ref, source_id) order");
+    }
+    have_prev = true;
+    prev_ref = rec.ref;
+    prev_source_id = rec.source_id;
+
+    scratch.clear();
+    bool first = true;
+    writeKey(scratch, "b", first);
+    writeStringValue(scratch, renderB(rec.ref));
+    writeKey(scratch, "s", first);
+    writeHex128Value(scratch, rec.source_id);
+    writeKey(scratch, "m", first);
+    writeStringValue(scratch, markerToWord(rec.marker));
+    if (rec.marker == kCondemned)
+    {
+        writeKey(scratch, "pend", first);
+        writeBoolValue(scratch, rec.delete_pending);
+        writeTokenFields(scratch, first, rec.token);   /// tt + tv
+        writeKey(scratch, "sz", first);
+        writeIntText(rec.size, scratch);
+        writeKey(scratch, "cr", first);
+        writeU64StringValue(scratch, rec.condemn_round);
+        writeKey(scratch, "mc", first);
+        writeBoolValue(scratch, rec.marker_confirmed);
+    }
+    closeObject(scratch, first);
+    writeChar('\n', scratch);
+    {
+        const std::string_view scratch_view = scratch.view();
+        out.write(scratch_view.data(), scratch_view.size());
+    }
+    ++count;
+}
+
+void SourceEdgeRunWriter::finish()
+{
+    if (finished)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS cas_run: finish called twice");
+    scratch.clear();
+    writeTrailerLine(scratch, count);
+    const std::string_view scratch_view = scratch.view();
+    out.write(scratch_view.data(), scratch_view.size());
+    finished = true;
+}
+
+UInt128 sourceEdgeRunChecksum(std::string_view stored_bytes)
+{
+    /// Use the same chained `CityHash128` and default block size as the reader. A one-shot hash would
+    /// diverge from the streaming hash for sufficiently large input, so the producer and later fold
+    /// must both process exactly the stored bytes through `HashingReadBuffer`.
+    ReadBufferFromMemory mem(stored_bytes.data(), stored_bytes.size());
+    HashingReadBuffer hashing(mem);
+    hashing.ignoreAll();   /// drain the whole object through the hash
+    return toWideChecksum(hashing.getHash());
+}
+
+SourceEdgeRunReader::SourceEdgeRunReader(ReadBuffer & in_)
+    : hashing(in_)
+{
+    /// Typed open: gate type/v/kind (and hash the header bytes) before any record is interpreted.
+    expectRunHeaderLine(hashing, kSourceEdgeKindWord);
+}
+
+bool SourceEdgeRunReader::next(SourceEdgeRecord & rec)
+{
+    if (done)
+        return false;
+
+    const String line = readLine(hashing, traitsFor(FormatId::RunFile).line_cap, "cas_run");
+    ReadBufferFromMemory line_in(line.data(), line.size());
+    JsonObjectReader r(line_in, KeyStrictness::Strict, "cas_run");
+
+    String key;
+    if (!r.nextKey(key))
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: empty line");
+
+    if (key == "n")
+    {
+        const uint64_t n = r.readU64Number();
+        if (r.nextKey(key))
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: trailer has extra keys");
+        if (!line_in.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: junk after trailer object");
+        /// The trailer must be the last line of the object; hashing must be at EOF (this also drains and
+        /// hashes the final bytes so accumulatedChecksum covers the whole object).
+        if (!hashing.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: bytes after trailer");
+        if (n != seen)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS cas_run: trailer count {} != {} records (line truncation?)", n, seen);
+        done = true;
+        return false;
+    }
+
+    SourceEdgeRecord out;
+    String b;
+    String tv;
+    bool have_b = false;
+    bool have_s = false;
+    bool have_m = false;
+    bool have_pend = false;
+    bool have_tt = false;
+    bool have_tv = false;
+    bool have_sz = false;
+    bool have_cr = false;
+    bool have_mc = false;
+    TokenType tt{};
+    do
+    {
+        if (key == "b") { b = r.readString(); have_b = true; }
+        else if (key == "s") { out.source_id = r.readHex128(); have_s = true; }
+        else if (key == "m") { out.marker = markerFromWord(r.readString()); have_m = true; }
+        else if (key == "pend") { out.delete_pending = r.readBool(); have_pend = true; }
+        else if (key == "tt") { tt = tokenTypeFromWord(r.readString(), "cas_run"); have_tt = true; }
+        else if (key == "tv") { tv = r.readString(); have_tv = true; }
+        else if (key == "sz") { out.size = r.readU64Number(); have_sz = true; }
+        else if (key == "cr") { out.condemn_round = r.readU64String(); have_cr = true; }
+        else if (key == "mc") { out.marker_confirmed = r.readBool(); have_mc = true; }
+        else r.skipUnknown(key);   /// Strict => any unknown key is CORRUPTED_DATA
+    } while (r.nextKey(key));
+
+    if (!have_b || !have_s || !have_m)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: record missing b/s/m");
+    out.ref = parseB(b);
+    if (out.marker == kCondemned)
+    {
+        if (!have_pend || !have_tt || !have_tv || !have_sz || !have_cr || !have_mc)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: condemned record missing pend/tt/tv/sz/cr/mc");
+        out.token = Token{tv, tt};
+    }
+    else if (have_pend || have_tt || have_tv || have_sz || have_cr || have_mc)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: non-condemned record carries condemned fields");
+
+    if (!line_in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_run: junk after record object");
+
+    rec = std::move(out);
+    ++seen;
+    return true;
+}
+
+UInt128 SourceEdgeRunReader::accumulatedChecksum()
+{
+    return toWideChecksum(hashing.getHash());
+}
+
+void SourceEdgeRunReader::verifyAgainst(const UInt128 & expected)
+{
+    const UInt128 got = accumulatedChecksum();
+    if (got != expected)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS cas_run: whole-file seal-checksum mismatch (the run bytes do not match the fold seal's "
+            "RunRef.checksum); refusing to act on this run");
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h
new file mode 100644
index 000000000000..d5f9a4801caf
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h
@@ -0,0 +1,164 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <IO/ReadBuffer.h>
+#include <IO/WriteBuffer.h>
+#include <IO/HashingReadBuffer.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <memory>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// Row tags for the sealed source-edge run. These byte values are part of the source-edge payload
+/// format, shared by this codec and the GC fold that interprets the rows.
+///
+/// Source-edge rows use `source_id == 0` as a sentinel key. A real active edge must never use that key;
+/// both sentinel tags are restricted to it. `kZeroMarker` describes a zero transition for the current
+/// generation and is dropped when the row is carried forward. `kCondemned` carries the condemned
+/// incarnation at the sentinel key across generations until settlement; its payload contains the full
+/// deletion token and other condemned-row state. A condemned row subsumes the zero marker for that
+/// generation.
+constexpr char kEdgeActive = 0x01;
+constexpr char kZeroMarker = 0x00;
+constexpr char kCondemned  = 0x02;
+
+/// The `cas_run` codec represents the GC source-edge in-degree data plane as sorted NDJSON. This is
+/// the `RecordStream` family
+/// (`FormatId::RunFile`): unbounded-cardinality sorted records, `object_cap = 0` (NEVER materialized
+/// whole — streamed one line at a time over a `ReadBuffer`), `line_cap = 4 KiB`, `PinnedRaw` (no
+/// compression) + `Strict` (byte-deterministic for `putDeterministicArtifact` adoption).
+///
+/// This file is backend-free: it accepts caller-owned `ReadBuffer`/`WriteBuffer` objects and never
+/// includes backend or GC subsystem headers. The GC layer owns the stream lifetime and the bridge to
+/// packed keys and condemned rows; this codec owns only the durable text representation and its
+/// identifier-layer types. Keeping that boundary physical prevents storage or GC dependencies from
+/// leaking into the format implementation.
+///
+/// File shape:
+///   {"type":"cas_run","v":3,"kind":"source_edge"}                      header line (type + v + kind gate)
+///   {"b":"01<digest-hex>","s":"<32hex>","m":"edge"}                    an active-edge / zero-marker row
+///   {"b":"01<digest-hex>","s":"00000000000000000000000000000000","m":"condemned","pend":false,"tt":"etag","tv":"...","sz":123,"cr":"7","mc":false}
+///   {"n":184267}                                                       trailer: record count
+///
+/// The record key `b` is the algo BYTE as two lowercase hex chars followed by the digest hex at the
+/// algo's width; `s` is the 32-hex source id. String-sorting records by (b, s) reproduces the current
+/// `(algorithm, digest, source_id)` byte order (lowercase hex preserves unsigned byte order and the
+/// algorithm byte is emitted first) — the invariant the fold's two-cursor merge depends on. The row-tag word
+/// `m` maps to the `kEdgeActive`/`kZeroMarker`/`kCondemned` bytes; a `condemned` row additionally
+/// carries the retired incarnation (`pend`/`tt`/`tv`/`sz`/`cr`) and the durable condemn-marker
+/// confirmation bit (`mc`).
+
+/// One decoded source-edge row. All fields are identifier-layer types so the codec stays backend-free.
+/// The condemned-only fields (`delete_pending`/`token`/`size`/`condemn_round`/`marker_confirmed`) are
+/// meaningful only when `marker == kCondemned`.
+struct SourceEdgeRecord
+{
+    BlobRef ref{};
+    UInt128 source_id{};
+    char marker = kEdgeActive;
+    bool delete_pending = false;
+    Token token{};
+    uint64_t size = 0;
+    uint64_t condemn_round = 0;
+    bool marker_confirmed = false;   /// durable Condemned meta confirmed for this entry (graduation gate)
+};
+
+/// The header-line `kind` word for the only live `cas_run` kind.
+inline constexpr std::string_view kSourceEdgeKindWord = "source_edge";
+
+/// Write the typed header line `{"type":"cas_run","v":G_BUILD,"kind":"<kind>"}\n` with a fixed key
+/// order for byte-determinism. The `kind` field distinguishes the record schema within the run
+/// family, so a reader can reject a valid run of the wrong kind before interpreting any records.
+void writeRunHeaderLine(WriteBuffer & out, std::string_view kind);
+
+/// Read + gate the typed header line: `type` must be `cas_run`, `v` is gated by `checkCompatibility`
+/// (future `v` -> `UNKNOWN_FORMAT_VERSION`), and `kind` must equal `expected_kind` (else
+/// `CORRUPTED_DATA`, "unknown run kind"). This is the typed-open — all three are validated before any
+/// record is interpreted.
+void expectRunHeaderLine(ReadBuffer & in, std::string_view expected_kind);
+
+/// Sorted NDJSON writer over a caller-owned `WriteBuffer` (backend-free; writes plainly — the whole-
+/// object checksum is `sourceEdgeRunChecksum` over the finished bytes, which keeps this writer free of a
+/// HashingWriteBuffer finalize-ordering hazard). `append` asserts records arrive in non-decreasing
+/// (ref, source_id) order and throws on a regression (this replaces the old `prev_key` monotonicity
+/// check). `finish` writes the `{"n":count}` trailer.
+class SourceEdgeRunWriter
+{
+public:
+    /// Write the typed source-edge header immediately. The writer borrows `out` for its entire
+    /// lifetime; the caller must keep it alive and must call `finish` exactly once after the final
+    /// record so the count trailer is present.
+    explicit SourceEdgeRunWriter(WriteBuffer & out_);
+
+    /// Append one record in non-decreasing `(ref, source_id)` order. Equal keys are allowed because
+    /// the merge layer may produce multiple rows for the same key. A regression is a producer
+    /// programming error and raises `LOGICAL_ERROR`; no partial record is written for that call.
+    void append(const SourceEdgeRecord & rec);
+
+    /// Write the record-count trailer and mark the stream finished. Calling `finish` twice raises
+    /// `LOGICAL_ERROR`; appending after it is likewise rejected so a completed run cannot be extended.
+    void finish();
+
+private:
+    WriteBuffer & out;
+    uint64_t count = 0;
+    bool have_prev = false;
+    BlobRef prev_ref{};
+    UInt128 prev_source_id{};
+    bool finished = false;
+    /// Reused line-scratch: each record is assembled here, then bulk-written to `out` in one call.
+    /// `clear` keeps the buffer's capacity, so memory stays bounded by the largest line ever
+    /// assembled, never by record count.
+    CasJsonWriter scratch;
+};
+
+/// The whole-object seal-checksum (`RunRef.checksum`) of a stored `cas_run`: the chained CityHash128 a
+/// `HashingReadBuffer` computes over ALL the object bytes. The reader accumulates the IDENTICAL hash as
+/// it streams (`SourceEdgeRunReader::verifyAgainst`), so a run PUT by the producer and later read by the
+/// fold agree byte-for-byte. Computed over the finished bytes on the write side (the producer already
+/// holds them to PUT); streamed on the read side (the run is never materialized whole to verify).
+UInt128 sourceEdgeRunChecksum(std::string_view stored_bytes);
+
+/// Sequential streaming reader over a caller-owned `ReadBuffer` (backend-free, O(one 4 KiB line)
+/// resident). The ctor reads + gates the typed header line. `next` yields records in stored order and
+/// returns false once the `{"n"}` trailer is consumed (the count is verified there — the line-truncation
+/// guard). Every byte read is fed through a chained CityHash128; after the trailer, `verifyAgainst`
+/// compares the accumulated whole-object hash to the seal's `RunRef.checksum` and throws `CORRUPTED_DATA`
+/// on a mismatch — the caller calls it after draining and BEFORE acting on the records (the deletion
+/// decision). Non-movable/non-copyable (owns a `HashingReadBuffer`) — construct in place.
+class SourceEdgeRunReader
+{
+public:
+    /// Construct a reader that borrows `in`, hashes every byte read, and validates the typed header
+    /// before exposing any record. The caller must drain the reader through the trailer before using
+    /// `verifyAgainst`, because the seal covers the complete object rather than only decoded rows.
+    explicit SourceEdgeRunReader(ReadBuffer & in_);
+    SourceEdgeRunReader(const SourceEdgeRunReader &) = delete;
+    SourceEdgeRunReader & operator=(const SourceEdgeRunReader &) = delete;
+
+    /// Decode the next record in stored order. Returns `false` only after consuming and validating
+    /// the count trailer and confirming that it is the final line; malformed or truncated input
+    /// raises `CORRUPTED_DATA`.
+    bool next(SourceEdgeRecord & rec);
+
+    /// Compare the accumulated whole-object checksum with the seal recorded for the run. Call only
+    /// after `next` has returned `false`; a mismatch raises `CORRUPTED_DATA` so callers can verify
+    /// the run before acting on decoded condemned rows.
+    void verifyAgainst(const UInt128 & expected);
+
+    /// Return the whole-object hash accumulated so far. It is meaningful for seal verification only
+    /// after the trailer has been consumed, when the reader has hashed every byte of the object.
+    UInt128 accumulatedChecksum();
+
+private:
+    HashingReadBuffer hashing;
+    uint64_t seen = 0;
+    bool done = false;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.cpp
new file mode 100644
index 000000000000..c5b119ba44ca
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.cpp
@@ -0,0 +1,397 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasByteBudget.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <limits>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LIMIT_EXCEEDED;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+std::string_view nsStateToWord(NsState s)
+{
+    switch (s)
+    {
+        case NsState::Creating: return "creating";
+        case NsState::Live:     return "live";
+        case NsState::Removing: return "removing";
+    }
+    /// Every value reaching here came from a live `NsState` or from `nsStateFromWord`, which already
+    /// validated it on decode -- so this is a bug in THIS process, not corruption arriving from a
+    /// store.
+    throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS ref catalog: unknown ns state {}", static_cast<int>(s));
+}
+
+NsState nsStateFromWord(std::string_view w)
+{
+    if (w == "creating") return NsState::Creating;
+    if (w == "live")     return NsState::Live;
+    if (w == "removing") return NsState::Removing;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: unknown ns state '{}'", w);
+}
+
+namespace
+{
+
+/// `creator` is required iff `state == Creating`, forbidden otherwise -- one predicate, used by both
+/// directions of the codec, so the writer's self-check and the reader's fail-close can never disagree.
+bool creatorPairingOk(const CatalogEntry & e)
+{
+    return (e.state == NsState::Creating) == e.creator.has_value();
+}
+
+bool removalRoundPairingOk(const CatalogEntry & e)
+{
+    return (e.state == NsState::Removing) == e.removal_started_round.has_value();
+}
+
+/// Whether `entries` is already in the catalog's canonical shape: strictly ascending by namespace
+/// bytes, no duplicate namespace. A duplicate namespace fails the SAME check as an out-of-order pair
+/// (equal keys never compare strictly less), which is exactly right -- both are "not canonical".
+bool isCanonicalCatalogOrder(const std::vector<CatalogEntry> & entries)
+{
+    for (size_t i = 1; i < entries.size(); ++i)
+        if (!(entries[i - 1].ns.string() < entries[i].ns.string()))
+            return false;
+    return true;
+}
+
+}
+
+String encodeRefCatalog(const RefCatalog & catalog)
+{
+    const uint64_t line_cap = traitsFor(FormatId::RefCatalog).line_cap;
+    CasJsonWriter out(256);
+
+    /// EVERY line this encoder emits is measured against the LINE cap, on the bytes actually emitted
+    /// -- escaping, framing and all -- mirroring `encodeFoldSeal`'s `checkLineBytes` EXACTLY,
+    /// including its error code: `LIMIT_EXCEEDED`, not `LOGICAL_ERROR`. A line that does not fit is
+    /// not a large line, it is an UNREADABLE one: `readLine` refuses it, so the whole object is lost.
+    /// This is a capacity refusal on otherwise well-formed input (an admitted namespace name or
+    /// server_root_id near their own byte bounds, worst-case escaped, can reach ~4.7 KiB on its own --
+    /// reachable, not theoretical), not a bug in this process, so a caller catching `LIMIT_EXCEEDED`
+    /// for a capacity refusal must not see it misreported as a `LOGICAL_ERROR`. Refuse here, where
+    /// nothing is durable yet.
+    const auto checkLineBytes = [&](uint64_t bytes, std::string_view what)
+    {
+        if (!fitsLineCap(bytes, line_cap))
+            throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+                "CAS ref catalog: the {} line encodes to {} bytes, over the {}-byte line cap; a longer "
+                "line cannot be read back, so the catalog is refused before it is written",
+                what, bytes, line_cap);
+    };
+
+    writeHeaderLine(out, FormatId::RefCatalog);   /// emits its own terminator
+    checkLineBytes(out.size() - 1, "header");
+
+    /// This is our own state, about to become durable: an out-of-order or duplicate-keyed vector is a
+    /// bug in the writer, not corruption arriving from a store.
+    if (!isCanonicalCatalogOrder(catalog.entries))
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS ref catalog: entries are not canonically ordered -- strictly ascending by namespace "
+            "bytes with no duplicate namespace is required before a catalog may be encoded");
+
+    size_t line_start = out.size();
+    const auto closeLine = [&](std::string_view what)
+    {
+        checkLineBytes(out.size() - line_start, what);
+        writeChar('\n', out);
+        line_start = out.size();
+    };
+
+    for (const CatalogEntry & e : catalog.entries)
+    {
+        if (e.ns.string().empty())
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS ref catalog: an entry's namespace must not be empty");
+        if (e.ns.string().size() > kMaxNamespaceBytes)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS ref catalog: namespace '{}' is {} bytes, over the {}-byte admission bound",
+                e.ns.string(), e.ns.string().size(), kMaxNamespaceBytes);
+        if (e.incarnation == 0)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS ref catalog: namespace '{}' has a zero incarnation -- 0 never names a life",
+                e.ns.string());
+        if (!creatorPairingOk(e))
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS ref catalog: namespace '{}' is {} and {} a creator fence -- creator is required "
+                "iff state == Creating and forbidden otherwise",
+                e.ns.string(), nsStateToWord(e.state), e.creator ? "carries" : "lacks");
+        if (!removalRoundPairingOk(e))
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS ref catalog: namespace '{}' is {} and {} removal_started_round -- the field is "
+                "required iff state == Removing",
+                e.ns.string(), nsStateToWord(e.state), e.removal_started_round ? "carries" : "lacks");
+
+        bool first = true;
+        writeKey(out, "k", first);   writeStringValue(out, "ent");
+        writeKey(out, "ns", first);  writeStringValue(out, e.ns.string());
+        writeKey(out, "st", first);  writeStringValue(out, nsStateToWord(e.state));
+        writeKey(out, "inc", first); writeHex128Value(out, e.incarnation);
+        if (e.removal_started_round)
+        {
+            writeKey(out, "rsr", first); writeU64StringValue(out, *e.removal_started_round);
+        }
+        if (e.creator)
+        {
+            writeKey(out, "csr", first); writeStringValue(out, e.creator->server_root_id);
+            writeKey(out, "cwe", first); writeU64StringValue(out, e.creator->writer_epoch);
+            writeKey(out, "cfg", first); writeU64StringValue(out, e.creator->fence_generation);
+        }
+        closeObject(out, first);
+        closeLine("ent");
+    }
+
+    const size_t trailer_start = out.size();
+    writeTrailerLine(out, catalog.entries.size());   /// emits its own terminator
+    checkLineBytes(out.size() - trailer_start - 1, "trailer");
+
+    return std::move(out).take();
+}
+
+RefCatalog decodeRefCatalog(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::RefCatalog);
+    const uint64_t line_cap = traitsFor(FormatId::RefCatalog).line_cap;
+
+    RefCatalog catalog;
+    uint64_t seen = 0;
+    for (;;)
+    {
+        const String line = readLine(in, line_cap, "ref catalog");
+        ReadBufferFromMemory l(line.data(), line.size());
+        JsonObjectReader r(l, KeyStrictness::Strict, "ref catalog");
+        String key;
+        if (!r.nextKey(key))
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: empty line");
+
+        if (key == "n")
+        {
+            const uint64_t n = r.readU64Number();
+            if (r.nextKey(key))
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: trailer has extra keys");
+            if (!l.eof() || !in.eof())
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: bytes after trailer");
+            if (n != seen)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS ref catalog: trailer count {} != {} records", n, seen);
+            return catalog;
+        }
+        if (key != "k")
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: record must start with \"k\"");
+        const String kind = r.readString();
+        if (kind != "ent")
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: unknown record kind '{}'", kind);
+
+        String ns_str;
+        std::optional<String> st_word;
+        std::optional<UInt128> inc;
+        std::optional<String> csr;
+        std::optional<uint64_t> cwe;
+        std::optional<uint64_t> cfg;
+        std::optional<uint64_t> removal_started_round;
+        while (r.nextKey(key))
+        {
+            if (key == "ns") ns_str = r.readString();
+            else if (key == "st") st_word = r.readString();
+            else if (key == "inc") inc = r.readHex128();
+            else if (key == "csr") csr = r.readString();
+            else if (key == "cwe") cwe = r.readU64String();
+            else if (key == "cfg") cfg = r.readU64String();
+            else if (key == "rsr") removal_started_round = r.readU64String();
+            else throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: unknown ent key '{}'", key);
+        }
+        if (!l.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: junk after record");
+
+        if (!st_word)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: entry '{}' missing st", ns_str);
+        const NsState state = nsStateFromWord(*st_word);   /// throws CORRUPTED_DATA on an unknown word
+
+        /// A missing "ns" key reads as the same empty string a present-but-empty one would, and both
+        /// are refused identically here -- an empty namespace would sort first (every non-empty
+        /// namespace compares strictly greater than ""), passing the canonical-order check below, and
+        /// then wedge every later catalog-driven pass that tries to build a ref/namespace-file key
+        /// from it (`Layout::checkNamespace` refuses an empty namespace).
+        if (ns_str.empty())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: an entry's namespace must not be empty");
+        if (ns_str.size() > kMaxNamespaceBytes)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS ref catalog: namespace '{}' is {} bytes, over the {}-byte admission bound",
+                ns_str, ns_str.size(), kMaxNamespaceBytes);
+
+        if (!inc)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref catalog: entry '{}' missing inc", ns_str);
+        if (*inc == 0)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS ref catalog: namespace '{}' has a zero incarnation -- 0 never names a life", ns_str);
+
+        const bool any_creator_field = csr || cwe || cfg;
+        const bool every_creator_field = csr && cwe && cfg;
+        std::optional<CreatorFence> creator;
+        if (state == NsState::Creating)
+        {
+            if (!every_creator_field)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS ref catalog: namespace '{}' is Creating but its creator fence is incomplete -- "
+                    "server_root_id, writer_epoch and fence_generation are all required", ns_str);
+            creator = CreatorFence{.server_root_id = *csr, .writer_epoch = *cwe, .fence_generation = *cfg};
+        }
+        else if (any_creator_field)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS ref catalog: namespace '{}' carries a creator fence at state '{}' -- creator is "
+                "forbidden on anything but Creating", ns_str, *st_word);
+
+        if ((state == NsState::Removing) != removal_started_round.has_value())
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS ref catalog: namespace '{}' is {} and {} removal_started_round -- the field is "
+                "required iff state == Removing",
+                ns_str, *st_word, removal_started_round ? "carries" : "lacks");
+
+        /// Canonical order, checked incrementally as records stream in: a namespace that does not
+        /// compare strictly greater than the previous one is either a duplicate or out of order --
+        /// both are "not canonical", and this one check rejects either shape.
+        if (!catalog.entries.empty() && !(catalog.entries.back().ns.string() < ns_str))
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS ref catalog: namespace '{}' does not sort strictly after the previous entry "
+                "'{}' -- entries must be canonically ordered with no duplicate namespace",
+                ns_str, catalog.entries.back().ns.string());
+
+        catalog.entries.push_back(CatalogEntry{.ns = RootNamespace{ns_str}, .state = state,
+                                                .incarnation = *inc, .creator = creator,
+                                                .removal_started_round = removal_started_round});
+        ++seen;
+    }
+}
+
+void checkCatalogObjectBytes(uint64_t encoded_bytes, const RootNamespace & ns)
+{
+    const uint64_t cap = traitsFor(FormatId::RefCatalog).object_cap;
+    if (!fitsObjectCap(encoded_bytes, /*entries_reservation*/0, cap))
+        throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+            "CAS ref catalog: admitting '{}' would grow the catalog to {} bytes, over the {}-byte "
+            "object cap (predicate 1: encoded_catalog_bytes <= catalog_object_cap) -- refused before "
+            "the write", ns.string(), encoded_bytes, cap);
+}
+
+uint64_t foldSealFixedBytes()
+{
+    static const uint64_t bytes = []
+    {
+        CasFoldSeal seal;
+        seal.generation = std::numeric_limits<uint64_t>::max();
+        seal.parent_generation = std::numeric_limits<uint64_t>::max();
+        const uint64_t empty_bytes = encodeFoldSeal(seal).size();
+        /// The empty trailer is `{"n":0}`. A real seal may carry a 20-digit record count.
+        return addByteBudget(empty_bytes, std::numeric_limits<uint64_t>::digits10);
+    }();
+    return bytes;
+}
+
+uint64_t worstCaseEntryFoldReservationBytes()
+{
+    /// Measured through the REAL fold-seal encoder, never a hand-kept formula, so a later change to
+    /// the fold seal's wire shape is felt here automatically instead of silently drifting.
+    static const uint64_t bytes = []() -> uint64_t
+    {
+        constexpr uint64_t kU64Max = std::numeric_limits<uint64_t>::max();
+        constexpr uint32_t kU32Max = std::numeric_limits<uint32_t>::max();
+
+        CasFoldSeal seal;
+        /// One catalog entry admits exactly one ref-life row. Charge its widest legal form: a held
+        /// coverage record plus terminal cleanup evidence, all numeric fields at maximum width.
+        seal.ref_lives[std::numeric_limits<UInt128>::max()] = RefLifeFoldState{
+            .coverage = RefCoverage{
+                .classification = 4,
+                .last_folded_ref_id = RefTxnId{kU64Max, kU64Max},
+                .hold = RefHold{.reason = HoldReason::UnconsumedSealCrossing,
+                                 .offending_position = RefTxnId{kU64Max, kU64Max},
+                                 .retry_count = kU32Max,
+                                 .next_retry_round = kU64Max}},
+            .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{kU64Max, kU64Max}}};
+
+        return encodeFoldSeal(seal).size() - encodeFoldSeal(CasFoldSeal{}).size();
+    }();
+    return bytes;
+}
+
+uint64_t widestBlobTargetRunReservationBytes(const Layout & layout, uint64_t gc_shards)
+{
+    if (gc_shards == 0)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS ref catalog: gc_shards must be nonzero when reserving blob-target rows");
+
+    constexpr uint64_t max = std::numeric_limits<uint64_t>::max();
+    CasFoldSeal seal;
+    seal.blob_target_runs.push_back(RunRef{
+        .key = layout.blobTargetRunKey(max, max, gc_shards - 1, 0),
+        .checksum = std::numeric_limits<UInt128>::max(),
+        .shard = gc_shards - 1,
+        .generation = max});
+    return encodeFoldSeal(seal).size() - encodeFoldSeal(CasFoldSeal{}).size();
+}
+
+uint64_t widestCondemnedSummaryReservationBytes(uint64_t gc_shards)
+{
+    if (gc_shards == 0)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS ref catalog: gc_shards must be nonzero when reserving condemned-summary rows");
+
+    constexpr uint64_t max = std::numeric_limits<uint64_t>::max();
+    CasFoldSeal seal;
+    seal.condemned_summary.emplace(gc_shards - 1, CondemnedSummary{
+        .condemned_total = max,
+        .pending_total = max,
+        .oldest_nonpending_condemn_round = max});
+    return encodeFoldSeal(seal).size() - encodeFoldSeal(CasFoldSeal{}).size();
+}
+
+void checkFoldSealReservation(
+    uint64_t entry_count, uint64_t gc_shards, const Layout & layout, const RootNamespace & ns)
+{
+    const uint64_t cap = foldSealCaps().object_cap;
+    const uint64_t fixed = foldSealFixedBytes();
+    /// Saturating, like `fitsObjectCap`'s own addition one step later: an unsaturated product can
+    /// wrap to a remainder far smaller than the true reservation, which would answer "fits" for an
+    /// `entry_count` that plainly does not.
+    const uint64_t ref_lives = mulByteBudget(entry_count, worstCaseEntryFoldReservationBytes());
+    /// `validateFoldSealStructure` permits at most one canonical seq-0 `btr` per shard, so charging
+    /// one widest row for every shard covers the full legal run domain without per-entry arithmetic.
+    const uint64_t blob_target_runs = mulByteBudget(
+        gc_shards, widestBlobTargetRunReservationBytes(layout, gc_shards));
+    const uint64_t condemned_summaries = mulByteBudget(
+        gc_shards, widestCondemnedSummaryReservationBytes(gc_shards));
+    const uint64_t reservation = addByteBudget(
+        ref_lives, addByteBudget(blob_target_runs, condemned_summaries));
+    if (!fitsObjectCap(fixed, reservation, cap))
+        throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+            "CAS ref catalog: admitting '{}' would need a fold seal reserving fixed={} + ref_lives={} "
+            "+ blob_target_runs={} + condemned_summaries={} bytes (entries={}, gc_shards={}), over "
+            "the {}-byte fold-seal object cap (predicate 2) -- refused before the write",
+            ns.string(), fixed, ref_lives, blob_target_runs, condemned_summaries, entry_count, gc_shards, cap);
+}
+
+String checkCatalogAdmission(
+    const RefCatalog & candidate, uint64_t gc_shards, const Layout & layout,
+    const RootNamespace & admitting_ns)
+{
+    const String encoded = encodeRefCatalog(candidate);   /// grammar-checked; LOGICAL_ERROR on our own bug
+    checkCatalogObjectBytes(encoded.size(), admitting_ns);           /// predicate (1)
+    checkFoldSealReservation(candidate.entries.size(), gc_shards, layout, admitting_ns); /// predicate (2)
+    return encoded;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h
new file mode 100644
index 000000000000..ca1fbe5a6ddc
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h
@@ -0,0 +1,168 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <optional>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+class Layout;
+
+/// The byte bound every namespace name admitted into `ref_catalog` must satisfy (spec INV-3:
+/// "namespace names get a byte bound"). It keeps the catalog's operator-visible row and line grammar
+/// bounded, and both directions of the codec enforce it. Logical namespace bytes do NOT enter
+/// predicate (2): fold-seal `rfl` rows are keyed only by the fixed-width opaque life id.
+constexpr size_t kMaxNamespaceBytes = 512;
+
+/// One namespace's catalog lifecycle state (spec INV-3, §3). `Creating` blocks publication and
+/// requires a `creator` fence identity; `Live` is the steady state and forbids `creator`;
+/// `Removing` forbids new positive ownership and, like `Live`, forbids `creator` (a namespace at
+/// this state was already `Live`, so no creation fence identity applies to it any longer).
+///
+/// THESE ARE WIRE VALUES, AND THEY ARE APPEND-ONLY, exactly like `HoldReason`: a catalog object
+/// written by one build is read by another, so a renumbered or repurposed value would make an older
+/// catalog name a different lifecycle than the one it recorded. Add new states at the end; never
+/// renumber, never repurpose a retired word.
+enum class NsState : uint8_t
+{
+    Creating = 1,
+    Live = 2,
+    Removing = 3,
+};
+
+/// The wire word one `NsState` is persisted as. Exported so any other reader of a lifecycle state
+/// renders the SAME three words this codec does, rather than a second, independently drifting copy.
+/// Every value it is ever called with comes from either a live `NsState` (a compile-time-closed
+/// enumeration) or a value `nsStateFromWord` already validated on decode, so an unrecognized value
+/// reaching it is a bug in THIS process, not corruption arriving from a store -- `LOGICAL_ERROR`.
+std::string_view nsStateToWord(NsState s);
+/// Inverse of `nsStateToWord`; throws `CORRUPTED_DATA` for anything but the three registered words.
+NsState nsStateFromWord(std::string_view w);
+
+/// The fence identity of the mounted writer CREATING one namespace (spec §3): the server root plus
+/// the writer epoch and admission fence generation captured at the moment `Creating` was minted. It
+/// is what a reconciler compares against `CasServerRoot`'s liveness/fence machinery before a stalled
+/// `Creating` entry may be CAS-reconciled away (INV-3: "stalled creators occupy entries until
+/// fence-terminal reconciliation").
+struct CreatorFence
+{
+    String server_root_id;
+    uint64_t writer_epoch = 0;
+    uint64_t fence_generation = 0;
+
+    bool operator==(const CreatorFence &) const = default;
+};
+
+/// One namespace's catalog row. `incarnation` is the ref-layer-scoped life identity minted once, at
+/// `Creating` (spec INV-3; consumed as a `NamespaceLifeId` by every ref/namespace-file key helper --
+/// see `NamespaceLifeId::fromCatalogEntry`), and never changes for the rest of this row's life: a
+/// namespace dropped and recreated gets a FRESH row with a FRESH incarnation, never a reused one --
+/// that is what makes rebirth structurally inert instead of an alias. `incarnation == 0` is always
+/// invalid, at every state -- "0 never names a life", the same rule `NamespaceLifeId` enforces.
+///
+/// `creator` is a STRICT GRAMMAR pairing: REQUIRED iff `state == Creating`, FORBIDDEN otherwise.
+/// `removal_started_round` is similarly REQUIRED iff `state == Removing`: it is sampled once by the
+/// `Live -> Removing` catalog CAS and never changes, so diagnostics can measure removal age without
+/// inventing a caller-local epoch. Both pairings are enforced in both codec directions.
+struct CatalogEntry
+{
+    RootNamespace ns;
+    NsState state = NsState::Creating;
+    UInt128 incarnation = 0;
+    std::optional<CreatorFence> creator = std::nullopt;
+    std::optional<uint64_t> removal_started_round = std::nullopt;
+
+    bool operator==(const CatalogEntry &) const = default;
+};
+
+/// The whole-pool namespace catalog (spec INV-3): one object, key `cas/ref_catalog`
+/// (`Layout::refCatalogKey`), read on every fold round and every recovery, mutated by one token-CAS
+/// write per lifecycle transition. `entries` is CANONICALLY ORDERED by namespace bytes, strictly
+/// ascending -- no duplicate namespace -- and both directions of the codec enforce it, so an
+/// out-of-order or duplicate-keyed catalog can never become durable.
+struct RefCatalog
+{
+    std::vector<CatalogEntry> entries;
+
+    bool operator==(const RefCatalog &) const = default;
+};
+
+/// Encodes `catalog` as the canonical `cas_ref_catalog` text object: a header line, one "ent" record
+/// per entry in canonical (ns-sorted) order, and a record-count trailer -- the same tagged-record
+/// container `encodeFoldSeal` uses. Enforces the FULL strict grammar on the way out: canonical order
+/// and no duplicate namespace, a non-empty namespace within the `kMaxNamespaceBytes` bound, nonzero
+/// incarnation, and the `creator`/state pairing. This is our own state about to become durable, so a
+/// violation is `LOGICAL_ERROR`, not `CORRUPTED_DATA`. Also enforces the per-line `LIMIT_EXCEEDED`
+/// line-cap gate (mirroring `encodeFoldSeal`'s `checkLineBytes` exactly, including its error code --
+/// a caller catching `LIMIT_EXCEEDED` as a capacity refusal must not see a `LOGICAL_ERROR` bug report
+/// instead) -- but deliberately does NOT enforce the whole-object cap itself: that predicate must
+/// name the namespace under admission, which only a caller of `checkCatalogAdmission` knows.
+///
+/// These bytes go to and come from the backend DIRECTLY, exactly like `cas_ref_ckpt`: the Pool-side
+/// `CasRefCatalog::read`/`casUpdateImpl` (`Pool/CasRefCatalog.cpp`) bypass `sealObject`/`openObject`,
+/// which are the identity under this class's `CompressionPolicy::Never` and would add nothing. A
+/// policy flip to `Always` therefore breaks this silently -- and is caught, because `storedSuffix`
+/// would stop being empty and the registry test asserting `storedSuffix(FormatId::RefCatalog) == ""`
+/// fails. That assertion is the tripwire for this shortcut, not an incidental check of the key shape.
+/// One consequence of the bypass, stated rather than fixed here (pre-existing for `RefCkpt` too):
+/// `openObject`'s own object-cap enforcement is skipped on the read path, so nothing on either the
+/// plain write path or the read path enforces the 256 MiB object cap outside `checkCatalogAdmission`
+/// -- the cap is load-bearing only through THAT gate, never through the codec or the backend read.
+String encodeRefCatalog(const RefCatalog & catalog);
+
+/// Decodes and validates a `cas_ref_catalog` object, re-checking every grammar rule `encodeRefCatalog`
+/// enforces against bytes that may have come from anywhere: `CORRUPTED_DATA` on a duplicate namespace,
+/// non-canonical order, a missing, empty, or over-bound namespace, a zero incarnation, an incomplete or
+/// forbidden creator fence, an unknown state word, or trailing bytes.
+RefCatalog decodeRefCatalog(std::string_view data);
+
+/// PRE-PUT GATE, predicate (1) of INV-3's additive admission: `encoded_bytes <= catalog_object_cap`
+/// (the registry's own cap for `FormatId::RefCatalog`). Equality is accepted; refuses
+/// (`LIMIT_EXCEEDED`, naming `ns`) one byte over.
+void checkCatalogObjectBytes(uint64_t encoded_bytes, const RootNamespace & ns);
+
+/// Worst-case bytes for the fold-seal frame with no records: maximal generation fields plus the
+/// widest possible `uint64_t` trailer count. Measured through the real encoder.
+uint64_t foldSealFixedBytes();
+
+/// The worst-case bytes ONE admitted catalog entry could ever add to a fold seal: one ref-life row
+/// containing held coverage and terminal cleanup evidence at their widest legal shapes. Measured
+/// through `encodeFoldSeal` itself, like `foldSealFixedBytes`.
+uint64_t worstCaseEntryFoldReservationBytes();
+
+/// Worst-case incremental bytes for one canonical blob-target run row. The serialized physical key
+/// includes `layout`'s pool prefix, so layout is part of the bound.
+uint64_t widestBlobTargetRunReservationBytes(const Layout & layout, uint64_t gc_shards);
+
+/// Worst-case incremental bytes for one condemned-summary row at the greatest configured shard.
+uint64_t widestCondemnedSummaryReservationBytes(uint64_t gc_shards);
+
+/// PRE-PUT GATE, predicate (2) of INV-3's additive admission. Reserves the widest fixed frame, one
+/// widest ref-life row per candidate catalog entry, and one widest blob-target plus condemned-summary
+/// row per authoritative GC shard. The `btr` multiplier follows the authoritative fold-seal grammar:
+/// at most one canonical sequence-0 run is legal for each shard. Equality is accepted; refuses
+/// (`LIMIT_EXCEEDED`, naming `ns`) one entry over. Every multiplication and addition saturates, so an
+/// unreachable-in-practice count can never wrap into something that reads as "fits".
+void checkFoldSealReservation(
+    uint64_t entry_count, uint64_t gc_shards, const Layout & layout, const RootNamespace & ns);
+
+/// Runs BOTH admission predicates against `candidate` -- the catalog state as it would read
+/// immediately AFTER the admission under consideration -- naming `admitting_ns` in whichever
+/// predicate refuses (INV-3: "admission refuses loudly"; "TWO INDEPENDENT predicates"). `candidate`
+/// is grammar-checked first (via `encodeRefCatalog`; `LOGICAL_ERROR` on our own bug), then predicate
+/// (1) and predicate (2), in that order. Returns the encoded bytes on success, so a caller's `casPut`
+/// writes EXACTLY what admission checked -- never a second, independently re-encoded copy.
+///
+/// Constraint 13 (removal is never refused): this function is for entry-ADMITTING mutations only.
+/// A removal transition (`Live` -> `Removing`) must go through the catalog's plain update path
+/// instead, never through here.
+String checkCatalogAdmission(
+    const RefCatalog & candidate, uint64_t gc_shards, const Layout & layout,
+    const RootNamespace & admitting_ns);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.cpp
new file mode 100644
index 000000000000..6ff7fa5dda43
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.cpp
@@ -0,0 +1,179 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+void checkRefCkptInvariants(const RefCkpt & ckpt, std::string_view what)
+{
+    /// PRESENT means REAL. `life_epoch` may be absent (no writer of this object knew the namespace's
+    /// genesis epoch), but a present one is a `writer_epoch`, and `RefTxnId` forbids a zero epoch.
+    /// Accepting zero would give the field two meanings -- "unknown" and "epoch zero" -- on an object
+    /// that gates destructive cleanup.
+    if (ckpt.life_epoch && *ckpt.life_epoch == 0)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: a present life_epoch must be nonzero", what);
+
+    /// A PRESENT id must be a real one. Both components nonzero is `RefTxnId`'s own validity rule
+    /// (`renderRefTxnId` refuses to build a key from anything else), so a half-zero id here would name
+    /// an object that cannot exist.
+    const auto check_id = [&](const std::optional<RefTxnId> & id, std::string_view field)
+    {
+        if (id && (id->writer_epoch == 0 || id->ref_sequence == 0))
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS {}: {} fields must both be nonzero, got {}-{}",
+                what, field, id->writer_epoch, id->ref_sequence);
+    };
+    check_id(ckpt.checkpoint_snapshot_id, "checkpoint_snapshot_id");
+    check_id(ckpt.last_epoch_seal, "last_epoch_seal");
+    check_id(ckpt.committed_through, "committed_through");
+    if (!ckpt.committed_through && (ckpt.checkpoint_snapshot_id || ckpt.last_epoch_seal))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS {}: checkpoint_snapshot_id and last_epoch_seal require committed_through", what);
+    if (ckpt.committed_through)
+    {
+        if (ckpt.life_epoch && ckpt.committed_through->writer_epoch < *ckpt.life_epoch)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS {}: committed_through must not precede life_epoch", what);
+        if (ckpt.checkpoint_snapshot_id && *ckpt.committed_through < *ckpt.checkpoint_snapshot_id)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS {}: checkpoint_snapshot_id must not exceed committed_through", what);
+        if (ckpt.last_epoch_seal && *ckpt.committed_through < *ckpt.last_epoch_seal)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS {}: last_epoch_seal must not exceed committed_through", what);
+
+        /// The checkpoint carries the same finite proof as the ref-log chain. A current epoch is either
+        /// closed at the frontier itself, or its frontier follows the seal of exactly the preceding
+        /// numeric epoch. A seal from the same epoch below the frontier would claim that an epoch kept
+        /// accepting transactions after it was closed; a larger gap would let a missing epoch masquerade
+        /// as a proved boundary.
+        if (ckpt.last_epoch_seal)
+        {
+            if (*ckpt.last_epoch_seal != *ckpt.committed_through
+                && ckpt.last_epoch_seal->writer_epoch + 1 != ckpt.committed_through->writer_epoch)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS {}: last_epoch_seal must equal committed_through or close its immediately preceding writer epoch",
+                    what);
+        }
+        else if (ckpt.life_epoch && ckpt.committed_through->writer_epoch > *ckpt.life_epoch)
+        {
+            /// With a known genesis epoch, an unsealed later epoch has no chain evidence. Leave the
+            /// unknown-genesis contribution representable: another checkpoint writer may still merge
+            /// the genesis fact before this partial contribution is encoded as durable authority.
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS {}: committed_through after life_epoch requires last_epoch_seal", what);
+        }
+    }
+}
+
+String encodeRefCkpt(const RefCkpt & ckpt)
+{
+    checkRefCkptInvariants(ckpt, "cas_ref_ckpt encode");
+
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::RefCkpt);
+    bool first = true;
+    /// Every field is encoded WHEN SET and omitted otherwise, so "nobody knows the genesis epoch",
+    /// "no checkpoint yet", and "no epoch has been closed yet" are absences on the wire rather than
+    /// sentinel values a reader would have to interpret. The two ids are flat epoch/sequence PAIRS,
+    /// written by the one shared `RefTxnId` writer the `_log` and `_snap` formats also use, so the
+    /// three ref formats cannot disagree on the encoding.
+    if (ckpt.life_epoch)
+    {
+        writeKey(out, "le", first);
+        writeU64StringValue(out, *ckpt.life_epoch);
+    }
+    if (ckpt.committed_through)
+        writeRefTxnIdFields(out, first, "cte", "cts", *ckpt.committed_through);
+    if (ckpt.checkpoint_snapshot_id)
+        writeRefTxnIdFields(out, first, "cse", "css", *ckpt.checkpoint_snapshot_id);
+    if (ckpt.last_epoch_seal)
+        writeRefTxnIdFields(out, first, "lse", "lss", *ckpt.last_epoch_seal);
+    closeObject(out, first);
+    writeChar('\n', out);
+
+    String text = std::move(out).take();
+    /// The registry cap is a corruption brake, not a budget: this object has three fields and cannot
+    /// approach it. Checking on the WRITE side too means an encoder bug surfaces here rather than as an
+    /// object that was accepted on write and is unreadable on decode.
+    const uint64_t object_cap = traitsFor(FormatId::RefCkpt).object_cap;
+    if (text.size() > object_cap)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS cas_ref_ckpt encode: encoded size {} exceeds the object cap {}", text.size(), object_cap);
+    return text;
+}
+
+RefCkpt decodeRefCkpt(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::RefCkpt);
+    const String body = readLine(in, traitsFor(FormatId::RefCkpt).line_cap, "cas_ref_ckpt");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    /// STRICT: an unknown ordinary key is `CORRUPTED_DATA` and a `!`-prefixed one is
+    /// `UNKNOWN_FORMAT_VERSION`. `_ckpt` is a control object whose every field changes what cleanup is
+    /// allowed to delete, so a reader that silently ignored a key it did not understand would be
+    /// deciding deletions from a body it only partially read. Duplicate keys are rejected by
+    /// `JsonObjectReader` itself.
+    JsonObjectReader r(body_in, KeyStrictness::Strict, "cas_ref_ckpt");
+
+    RefCkpt ckpt;
+    std::optional<uint64_t> cse;
+    std::optional<uint64_t> css;
+    std::optional<uint64_t> lse;
+    std::optional<uint64_t> lss;
+    std::optional<uint64_t> cte;
+    std::optional<uint64_t> cts;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "le") ckpt.life_epoch = r.readU64String();
+        else if (key == "cte") cte = r.readU64String();
+        else if (key == "cts") cts = r.readU64String();
+        else if (key == "cse") cse = r.readU64String();
+        else if (key == "css") css = r.readU64String();
+        else if (key == "lse") lse = r.readU64String();
+        else if (key == "lss") lss = r.readU64String();
+        else r.skipUnknown(key);
+    }
+
+    /// TRUNCATION IS REJECTED. Half an id pair is the dangerous shape: silently dropping it would turn
+    /// a truncated body into a well-formed `_ckpt` with NO checkpoint, which reads as "nothing is
+    /// deletable" today and as "recovery has no base" tomorrow -- both of which a reader would trust.
+    /// Fail closed instead. (A missing whole field is a legitimate absence, not truncation: every field
+    /// of this object is optional, so there is nothing to miss.)
+    if (cse || css)
+    {
+        if (!cse || !css)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_ref_ckpt: checkpoint_snapshot_id needs both cse and css");
+        ckpt.checkpoint_snapshot_id = RefTxnId{*cse, *css};
+    }
+    if (cte || cts)
+    {
+        if (!cte || !cts)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_ref_ckpt: committed_through needs both cte and cts");
+        ckpt.committed_through = RefTxnId{*cte, *cts};
+    }
+    if (lse || lss)
+    {
+        if (!lse || !lss)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_ref_ckpt: last_epoch_seal needs both lse and lss");
+        ckpt.last_epoch_seal = RefTxnId{*lse, *lss};
+    }
+    if (!body_in.eof() || !in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS cas_ref_ckpt: trailing bytes");
+
+    checkRefCkptInvariants(ckpt, "cas_ref_ckpt");
+    return ckpt;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h
new file mode 100644
index 000000000000..d0331248db6f
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h
@@ -0,0 +1,124 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <base/types.h>
+#include <cstdint>
+#include <optional>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// One namespace LIFE's checkpoint object (spec INV-4), persisted as the mutable, token-CAS
+/// `cas_ref_ckpt` control object at `Layout::refCkptKey`, whose argument is a `NamespaceLifeId`.
+///
+/// It exists because prefix cleaning made the ref stream unreadable from a LIST alone: a cleaned
+/// prefix plus a hidden snapshot is indistinguishable from an empty one, so recovery cannot decide
+/// which snapshot is its base by enumerating keys. `_ckpt` is the point-read answer -- it NAMES the
+/// base -- and, being the only authority on it, it also becomes the gate on destructive cleanup:
+///
+///   - snapshots are deletable only STRICTLY BELOW `checkpoint_snapshot_id` (so a STALE pointer can
+///     only ever under-clean, never delete the base a live recovery is about to fetch);
+///   - a sampled base that 404s is adjudicated against this object's TOKEN, not its content: an
+///     advanced token means cleanup moved the base while we read (restart), an unchanged token means
+///     the base was deleted under a live checkpoint, which is corruption.
+///
+/// TWO writers update it -- the snapshot publisher and the sealer -- and both run the SAME algorithm
+/// (`mergeCkpt` + `publishCkpt` in `Pool/CasRefCkpt.h`): read the whole body, merge by SEMANTIC
+/// MAXIMUM per field, token-CAS. (`publishCkpt` adds one refusal the merge itself cannot express -- a
+/// `life_epoch` BELOW the durable one -- because only the publish site knows which side is durable.)
+/// Writing the whole body is what makes a stale field dangerous, and
+/// the merge is what contains it: a writer that skipped it and wrote back the value it sampled
+/// earlier would silently regress the OTHER writer's progress (TLC counterexample
+/// `_sab_sealclobbersbase`, which loses an acked transaction).
+///
+/// INVARIANT (Constraint 15), and it constrains what may ever be ADDED here, not merely what is here
+/// today: `_ckpt` is a fixed-size product of SCALAR MONOTONE FACTS. Its encoded size is `O(1)` in refs,
+/// files, transactions and writer epochs. Maps, collections and cardinality-growing fields belong in a
+/// separate immutable object or ledger -- never in this one, because this one is MUTABLE, is rewritten
+/// whole on every publish by two concurrent writers, and has NO REPAIR PATH: a `_ckpt` that grows with
+/// the table is a body that eventually cannot be rewritten atomically, on the single object that names
+/// recovery's base and gates destructive cleanup.
+///
+/// The four dimensions do not all hold the same way, and the difference is worth stating so the
+/// invariant is checkable rather than merely believed:
+///   - refs and files enter in NO form. Two namespaces differing only in how many refs they hold encode
+///     BYTE-IDENTICAL `_ckpt` bodies.
+///   - transactions and writer epochs enter as the DECIMAL WIDTH of the two id pairs -- four orders of
+///     magnitude of `ref_sequence` cost four bytes. That is `O(1)` because the components are
+///     `uint64_t`, so the width is ceilinged at twenty digits and the whole object at a constant far
+///     below `traitsFor(FormatId::RefCkpt).object_cap` (which stays what it is documented to be: a
+///     corruption brake this object cannot approach, never the thing that bounds its size).
+/// Both halves are fenced by `gtest_cas_ref_ckpt_join.cpp`, which also carries the compile-time
+/// `std::is_trivially_copyable_v<RefCkpt>` assertion that a heap-owning field would break.
+struct RefCkpt
+{
+    /// The namespace's birth epoch -- the `writer_epoch` of its `NamespaceBirth` record. It is what
+    /// makes the epoch-seal grammar checkable without walking to the beginning of the stream
+    /// (`validateEpochSealGrammarContextual` takes exactly this value).
+    ///
+    /// It is NOT a namespace-lifetime constant, and the previous version of this comment said it was --
+    /// which is how the merge rule below came to be described as "its semantic maximum is itself". TWO
+    /// writers know a `life_epoch` and they derive it from different epochs: `completeCreation` from the
+    /// catalog creator's `writer_epoch`, and `commitRefChunk`'s birth chunk from the `NamespaceBirth`
+    /// record's. Those differ whenever a stalled `Creating` entry is resumed by a later actor over the
+    /// same incarnation, and whenever the mount's writer epoch advances between the creation and the
+    /// first write -- CREATE TABLE, restart, INSERT. The value that must survive is the LATER one (the
+    /// grammar needs the epoch the birth record actually landed at), and it is always the later
+    /// contribution, because writer epochs are durable-monotone per server root. So the semantic maximum
+    /// is right, and what is refused is a DECREASE rather than a disagreement -- by `publishCkpt`, not by
+    /// `mergeCkpt`, since only the publish site can tell which of the two values is the durable one.
+    ///
+    /// OPTIONAL, and the option is load-bearing rather than a convenience. Exactly ONE writer knows
+    /// this value -- the transaction that births the namespace -- and a table recovered from durable
+    /// objects written before it existed has no way to learn it. Every OTHER writer therefore
+    /// contributes `nullopt` and the merge leaves whatever is there alone. Making it mandatory would
+    /// force those writers to supply a number they do not have, and the semantic-max merge can never
+    /// lower a wrong one: a guess here is permanent. A consumer that NEEDS the genesis epoch (Stage B's
+    /// cross-epoch GC fold) must fail closed on `nullopt`, never substitute a floor.
+    std::optional<uint64_t> life_epoch;
+    /// The greatest transaction admitted to durable logical history. Absent means this life has no
+    /// committed transaction; a snapshot or epoch seal is then invalid because neither can describe
+    /// history that was never committed.
+    std::optional<RefTxnId> committed_through = std::nullopt;
+    /// The snapshot recovery point-reads as its base, and the floor cleanup deletes strictly below.
+    /// `nullopt` until this namespace's first snapshot publication commits.
+    std::optional<RefTxnId> checkpoint_snapshot_id;
+    /// The `EpochSeal` transaction that closed the newest epoch known to have been closed. Consumed by
+    /// a later mount locating the previous epoch's terminating record and by the GC fold crossing
+    /// epochs. `nullopt` before this namespace has ever had an epoch closed.
+    std::optional<RefTxnId> last_epoch_seal;
+
+    bool operator==(const RefCkpt &) const = default;
+};
+
+/// Encode `ckpt` as the canonical `cas_ref_ckpt` text object: a versioned header line followed by one
+/// JSON body object. STRICT IN BOTH DIRECTIONS -- the same `checkRefCkptInvariants` that guards decode
+/// runs here first, so a struct this build would refuse to read can never be written by it either
+/// (`CORRUPTED_DATA`). Encoding is canonical and deterministic, which is what lets `publishCkpt`
+/// compare a merged result against what it read.
+///
+/// These bytes go to and come from the backend DIRECTLY: this pair bypasses `sealObject`/`openObject`,
+/// which are the identity under this class's `CompressionPolicy::Never` and would add nothing. A
+/// policy flip to `Always` therefore breaks this silently -- and is caught, because `storedSuffix`
+/// would stop being empty and the registry test asserting `storedSuffix(FormatId::RefCkpt) == ""`
+/// fails. That assertion is the tripwire for this shortcut, not an incidental check of the key shape.
+String encodeRefCkpt(const RefCkpt & ckpt);
+
+/// Decode a complete `cas_ref_ckpt` text object. STRICT (`KeyStrictness::Strict`): an unknown ordinary
+/// key, a duplicate key, a truncated object (a missing body line, or half of an optional
+/// id pair), or trailing bytes all raise `CORRUPTED_DATA` -- never a partially-populated struct. This
+/// object gates destructive cleanup and names recovery's base, so "decoded something" must mean
+/// "decoded exactly what a writer of this format wrote".
+RefCkpt decodeRefCkpt(std::string_view data);
+
+/// The shared field-level validity rule, applied on both encode and decode: every PRESENT field is a
+/// real value -- a set `life_epoch` is nonzero, and a present id has both components nonzero. When a
+/// frontier is present, its writer epoch may not precede `life_epoch`, and a snapshot may not exceed
+/// it. Its `last_epoch_seal` is either that exact frontier or closes the immediately preceding numeric
+/// writer epoch; without a seal, a known `life_epoch` permits only that genesis epoch. `what`
+/// identifies the direction in the exception message. Exposed so a caller that assembles a `RefCkpt`
+/// from several sources can fail closed before it reaches the wire.
+void checkRefCkptInvariants(const RefCkpt & ckpt, std::string_view what);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.cpp
new file mode 100644
index 000000000000..be7ee5567575
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.cpp
@@ -0,0 +1,443 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <algorithm>
+#include <optional>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+std::string_view opKindToWord(RefOpKind k)
+{
+    switch (k)
+    {
+        case RefOpKind::NamespaceBirth:  return "namespace_birth";
+        case RefOpKind::OwnerTransition: return "owner_transition";
+        case RefOpKind::SetPublishedAt:  return "set_published_at";
+        case RefOpKind::RemoveNamespace: return "remove_namespace";
+        case RefOpKind::EpochSeal:       return "epoch_seal";
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: unknown op kind {}", static_cast<uint8_t>(k));
+}
+
+RefOpKind opKindFromWord(std::string_view w)
+{
+    if (w == "namespace_birth")   return RefOpKind::NamespaceBirth;
+    if (w == "owner_transition")  return RefOpKind::OwnerTransition;
+    if (w == "set_published_at")  return RefOpKind::SetPublishedAt;
+    if (w == "remove_namespace")  return RefOpKind::RemoveNamespace;
+    if (w == "epoch_seal")        return RefOpKind::EpochSeal;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: unknown op kind '{}'", w);
+}
+
+/// Byte budget over the encoded text. A removal-class transaction uses the larger complete-table
+/// budget and has neither an op-count nor a per-op cap; normal transactions are bounded by
+/// `ref_txn_max_ops` and, per op, by `ref_op_max_bytes` (checked via `encodedOpSize`, one op at a
+/// time -- no accumulation).
+void checkBudget(const std::vector<RefOp> & ops, size_t encoded_bytes)
+{
+    const bool removal = refLogTxnIsRemovalClass(ops);
+    const size_t byte_limit = removal ? ref_removal_max_bytes : ref_txn_max_bytes;
+    if (encoded_bytes > byte_limit)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefLogTxn: encoded size {} exceeds the {}-class byte limit {}",
+            encoded_bytes, removal ? "removal" : "normal", byte_limit);
+    if (removal)
+        return;
+    if (ops.size() > ref_txn_max_ops)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefLogTxn: {} operations exceeds the normal-class op-count limit {}", ops.size(), ref_txn_max_ops);
+    for (const RefOp & op : ops)
+    {
+        const size_t op_bytes = encodedOpSize(op);
+        if (op_bytes > ref_op_max_bytes)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefLogTxn: op encoded size {} exceeds the normal-class per-op limit {}", op_bytes, ref_op_max_bytes);
+    }
+}
+
+void writeBindingFields(CasJsonWriter & out, bool & first, std::string_view prefix, const RefOwnerBinding & b)
+{
+    checkCanonicalRefName(b.ref_name, "RefLogTxn", "owner binding ref_name");
+    checkManifestRef(b.manifest_ref, "RefLogTxn", "owner binding manifest_ref");
+    out.key(prefix, "bk", first);
+    writeStringValue(out, refOwnerKindToWord(b.kind));
+    out.key(prefix, "rn", first);
+    writeStringValue(out, b.ref_name);
+    writeManifestRefFields(out, first, prefix, b.manifest_ref);
+}
+
+void writeOp(CasJsonWriter & out, const RefOp & op)
+{
+    bool first = true;
+    writeKey(out, "op", first);
+    writeStringValue(out, opKindToWord(op.kind));
+    switch (op.kind)
+    {
+        case RefOpKind::NamespaceBirth:
+        case RefOpKind::RemoveNamespace:
+        case RefOpKind::EpochSeal:
+            break;
+        case RefOpKind::OwnerTransition:
+            if (op.old_binding)
+                writeBindingFields(out, first, "o", *op.old_binding);
+            if (op.new_binding)
+                writeBindingFields(out, first, "n", *op.new_binding);
+            break;
+        case RefOpKind::SetPublishedAt:
+            checkCanonicalRefName(op.ref_name, "RefLogTxn", "set_published_at ref_name");
+            checkManifestRef(op.expected_manifest_ref, "RefLogTxn", "set_published_at manifest_ref");
+            writeKey(out, "rn", first);
+            writeStringValue(out, op.ref_name);
+            writeManifestRefFields(out, first, "", op.expected_manifest_ref);
+            writeKey(out, "ts", first);
+            writeIntText(op.published_at_ms, out);
+            break;
+    }
+    closeObject(out, first);
+    writeChar('\n', out);
+}
+
+/// Collector for a ManifestRef's three flat fields under an optional prefix.
+struct ManifestFields
+{
+    std::optional<uint64_t> me;
+    std::optional<uint64_t> mb;
+    std::optional<uint64_t> mo;
+
+    bool any() const { return me || mb || mo; }
+    ManifestRef build(std::string_view what) const
+    {
+        if (!me || !mb || !mo)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: {} manifest_ref missing me/mb/mo", what);
+        return manifestRefFromFields(*me, *mb, *mo, "RefLogTxn", what);
+    }
+};
+
+/// Collector for one binding (old/new) under a prefix.
+struct BindingFields
+{
+    std::optional<String> bk;
+    std::optional<String> rn;
+    ManifestFields mf;
+
+    bool any() const { return bk || rn || mf.any(); }
+    RefOwnerBinding build(std::string_view what) const
+    {
+        if (!bk || !rn)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: {} binding missing bk/rn", what);
+        RefOwnerBinding b;
+        b.kind = refOwnerKindFromWord(*bk, "RefLogTxn owner binding");
+        b.ref_name = *rn;
+        checkCanonicalRefName(b.ref_name, "RefLogTxn", "owner binding ref_name");
+        b.manifest_ref = mf.build(what);
+        return b;
+    }
+};
+
+/// The log transaction's header-object meta line (ns + txn_id + the optional `prev_epoch_seal`
+/// chain). Shared by `encodeRefLogTxn` and `removalFramingSize` so the two never disagree by a byte;
+/// `removalFramingSize` always passes `std::nullopt` -- a removal transaction is never a sequence-1
+/// epoch-transition record. Additive: the `"!pse"`/`"!pss"` pair is emitted only when
+/// `prev_epoch_seal` is set, so a body without it is byte-identical to the pre-EpochSeal wire shape.
+/// `!`-prefixed: `prev_epoch_seal` is INV-2 chain evidence, not cosmetic metadata -- a decoder that
+/// doesn't understand it must refuse the object rather than silently drop the chain link while
+/// otherwise passing the structural grammar (`JsonObjectReader::skipUnknown` rejects any unrecognized
+/// `!`-key with `UNKNOWN_FORMAT_VERSION`, tolerant or not; see task-1 review finding M4).
+void writeLogMeta(CasJsonWriter & out, const String & ns, const RefTxnId & txn_id, const std::optional<RefTxnId> & prev_epoch_seal)
+{
+    bool first = true;
+    writeKey(out, "ns", first);
+    writeStringValue(out, ns);
+    writeRefTxnIdFields(out, first, "we", "rs", txn_id);
+    if (prev_epoch_seal)
+        writeRefTxnIdFields(out, first, "!pse", "!pss", *prev_epoch_seal);
+    closeObject(out, first);
+    writeChar('\n', out);
+}
+
+RefOp readOpRecord(JsonObjectReader & r, RefOpKind kind)
+{
+    RefOp op;
+    op.kind = kind;
+
+    /// set_published_at fields
+    std::optional<String> sp_rn;
+    ManifestFields sp_mf;
+    std::optional<uint64_t> sp_ts;
+    /// owner_transition bindings
+    BindingFields ob;
+    BindingFields nb;
+
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "rn") sp_rn = r.readString();
+        else if (key == "me") sp_mf.me = r.readU64String();
+        else if (key == "mb") sp_mf.mb = r.readU64String();
+        else if (key == "mo") sp_mf.mo = r.readU64Number();
+        else if (key == "ts") sp_ts = r.readU64Number();
+        else if (key == "obk") ob.bk = r.readString();
+        else if (key == "orn") ob.rn = r.readString();
+        else if (key == "ome") ob.mf.me = r.readU64String();
+        else if (key == "omb") ob.mf.mb = r.readU64String();
+        else if (key == "omo") ob.mf.mo = r.readU64Number();
+        else if (key == "nbk") nb.bk = r.readString();
+        else if (key == "nrn") nb.rn = r.readString();
+        else if (key == "nme") nb.mf.me = r.readU64String();
+        else if (key == "nmb") nb.mf.mb = r.readU64String();
+        else if (key == "nmo") nb.mf.mo = r.readU64Number();
+        else if (key == "pl")
+            /// `"pl"` (payload) was removed from the op wire in stage-1 T12 (the `set_payload` op became
+            /// `set_published_at`). The retired op WORD is already rejected by `opKindFromWord`, but this
+            /// generic reader reads field keys before switching on kind, so a `"pl"` field paired with a
+            /// still-recognized op word would otherwise be `skipUnknown`'d. It is a KNOWN-removed field,
+            /// not a genuinely-unknown one -- reject it explicitly rather than silently discard it.
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefLogTxn: op record carries the removed \"pl\" (payload) field");
+        else r.skipUnknown(key);
+    }
+
+    switch (kind)
+    {
+        case RefOpKind::NamespaceBirth:
+        case RefOpKind::RemoveNamespace:
+        case RefOpKind::EpochSeal:
+            break;
+        case RefOpKind::OwnerTransition:
+            if (ob.any())
+                op.old_binding = ob.build("old");
+            if (nb.any())
+                op.new_binding = nb.build("new");
+            break;
+        case RefOpKind::SetPublishedAt:
+            if (!sp_rn || !sp_ts)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: set_published_at missing rn/ts");
+            op.ref_name = *sp_rn;
+            checkCanonicalRefName(op.ref_name, "RefLogTxn", "set_published_at ref_name");
+            op.expected_manifest_ref = sp_mf.build("set_published_at manifest_ref");
+            op.published_at_ms = *sp_ts;
+            break;
+    }
+    return op;
+}
+
+}
+
+bool refLogTxnIsEpochSeal(const RefLogTxn & txn)
+{
+    return txn.ops.size() == 1 && txn.ops.front().kind == RefOpKind::EpochSeal;
+}
+
+void validateEpochSealGrammarStructural(const RefLogTxn & txn)
+{
+    const bool has_seal_op = std::any_of(txn.ops.begin(), txn.ops.end(),
+        [](const RefOp & op) { return op.kind == RefOpKind::EpochSeal; });
+    if (has_seal_op && txn.ops.size() != 1)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefLogTxn: a transaction carrying an EpochSeal op must contain exactly that one op, got {} ops",
+            txn.ops.size());
+
+    if (txn.prev_epoch_seal)
+    {
+        checkRefTxnIdNonzero(*txn.prev_epoch_seal, "RefLogTxn", "prev_epoch_seal");
+        if (txn.txn_id.ref_sequence != 1)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefLogTxn: prev_epoch_seal is only allowed at sequence 1, got txn_id {}-{}",
+                txn.txn_id.writer_epoch, txn.txn_id.ref_sequence);
+        /// INV-2 materializes every global writer epoch for an existing life. A sequence-1 transaction
+        /// in E therefore chains to the closing seal of exactly E-1, not merely an arbitrary earlier
+        /// epoch. A skip would make a missing intermediate seal look like a proved boundary and let a
+        /// fold or destructive tail walk bypass it. This is a context-free property of one body, so
+        /// reject it in the codec before any walker can interpret the link as evidence.
+        if (txn.prev_epoch_seal->writer_epoch >= txn.txn_id.writer_epoch)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefLogTxn: prev_epoch_seal writer_epoch {} must be strictly less than this "
+                "transaction's writer_epoch {}", txn.prev_epoch_seal->writer_epoch, txn.txn_id.writer_epoch);
+        if (txn.prev_epoch_seal->writer_epoch + 1 != txn.txn_id.writer_epoch)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefLogTxn: prev_epoch_seal writer_epoch {} must immediately precede this "
+                "transaction's writer_epoch {}", txn.prev_epoch_seal->writer_epoch, txn.txn_id.writer_epoch);
+    }
+}
+
+void validateEpochSealGrammarContextual(const RefLogTxn & txn, uint64_t life_epoch)
+{
+    /// The unconditional "forbidden outside sequence 1" half of the rule is
+    /// `validateEpochSealGrammarStructural`'s job; this function only owns the required-iff rule,
+    /// which is meaningless off sequence 1.
+    if (txn.txn_id.ref_sequence != 1)
+        return;
+    const bool required = txn.txn_id.writer_epoch > life_epoch;
+    if (required && !txn.prev_epoch_seal)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefLogTxn: sequence-1 txn at writer_epoch {} (life_epoch {}) must carry prev_epoch_seal",
+            txn.txn_id.writer_epoch, life_epoch);
+    if (!required && txn.prev_epoch_seal)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefLogTxn: sequence-1 txn at writer_epoch {} (life_epoch {}) must not carry prev_epoch_seal",
+            txn.txn_id.writer_epoch, life_epoch);
+}
+
+String encodeRefLogTxn(const RefLogTxn & txn)
+{
+    checkRefTxnIdNonzero(txn.txn_id, "RefLogTxn", "txn_id");
+    validateEpochSealGrammarStructural(txn);
+
+    CasJsonWriter out(512);
+    writeHeaderLine(out, FormatId::RefLog);
+
+    writeLogMeta(out, txn.ns, txn.txn_id, txn.prev_epoch_seal);
+
+    for (const RefOp & op : txn.ops)
+        writeOp(out, op);
+
+    writeTrailerLine(out, txn.ops.size());
+    String text = std::move(out).take();
+    checkBudget(txn.ops, text.size());
+    return text;
+}
+
+RefLogTxn decodeRefLogTxn(std::string_view data, const String & expected_ns, const RefTxnId & expected_txn_id)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::RefLog);
+    const uint64_t line_cap = traitsFor(FormatId::RefLog).line_cap;
+
+    RefLogTxn txn;
+
+    /// meta line
+    {
+        const String line = readLine(in, line_cap, "cas_ref_log");
+        ReadBufferFromMemory m(line.data(), line.size());
+        JsonObjectReader r(m, KeyStrictness::Tolerant, "cas_ref_log");
+        bool saw_ns = false;
+        bool saw_we = false;
+        bool saw_rs = false;
+        std::optional<uint64_t> pse;
+        std::optional<uint64_t> pss;
+        String key;
+        while (r.nextKey(key))
+        {
+            if (key == "ns") { txn.ns = r.readString(); saw_ns = true; }
+            else if (key == "we") { txn.txn_id.writer_epoch = r.readU64String(); saw_we = true; }
+            else if (key == "rs") { txn.txn_id.ref_sequence = r.readU64String(); saw_rs = true; }
+            else if (key == "!pse") pse = r.readU64String();
+            else if (key == "!pss") pss = r.readU64String();
+            else r.skipUnknown(key);
+        }
+        if (!saw_ns || !saw_we || !saw_rs)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: meta line missing ns/we/rs");
+        /// Both-or-neither: `nextKey` already rejects a repeated "!pse"/"!pss" (duplicate-key check), so
+        /// this only guards against a body carrying exactly one of the pair.
+        if (pse || pss)
+        {
+            if (!pse || !pss)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: prev_epoch_seal needs both !pse and !pss");
+            txn.prev_epoch_seal = RefTxnId{*pse, *pss};
+        }
+        if (!m.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: junk after meta line");
+    }
+
+    checkRefTxnIdNonzero(txn.txn_id, "RefLogTxn", "txn_id");
+    /// The namespace and transaction id are duplicated in the body because the object key is the
+    /// source of truth for which transaction is being read. Reject a valid body copied under a
+    /// different key before accepting any of its operations.
+    if (txn.ns != expected_ns || txn.txn_id != expected_txn_id)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefLogTxn: body (ns='{}', txn_id={}-{}) does not match the key it was read from "
+            "(ns='{}', txn_id={}-{})",
+            txn.ns, txn.txn_id.writer_epoch, txn.txn_id.ref_sequence,
+            expected_ns, expected_txn_id.writer_epoch, expected_txn_id.ref_sequence);
+
+    /// op record lines, until the trailer
+    while (true)
+    {
+        const String line = readLine(in, line_cap, "cas_ref_log");
+        ReadBufferFromMemory l(line.data(), line.size());
+        JsonObjectReader r(l, KeyStrictness::Tolerant, "cas_ref_log");
+        String key;
+        if (!r.nextKey(key))
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: empty line");
+        if (key == "n")
+        {
+            const uint64_t n = r.readU64Number();
+            while (r.nextKey(key))
+                r.skipUnknown(key);
+            if (!l.eof() || !in.eof())
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: bytes after trailer");
+            if (n != txn.ops.size())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefLogTxn: trailer count {} != {} ops", n, txn.ops.size());
+            break;
+        }
+        if (key != "op")
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: record must start with \"op\"");
+        const RefOpKind kind = opKindFromWord(r.readString());
+        txn.ops.push_back(readOpRecord(r, kind));
+        if (!l.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefLogTxn: junk after op record");
+    }
+
+    /// Finalization: `txn.ops` is now complete, so the context-free seal grammar (which needs the
+    /// full op list) can run. Both directions of the codec enforce the identical rule -- see
+    /// `encodeRefLogTxn`.
+    validateEpochSealGrammarStructural(txn);
+    checkBudget(txn.ops, data.size());
+    return txn;
+}
+
+size_t encodedOpSize(const RefOp & op)
+{
+    CasJsonWriter out(256);
+    writeOp(out, op);
+    return out.size();
+}
+
+bool refLogTxnIsRemovalClass(const std::vector<RefOp> & ops)
+{
+    return std::any_of(ops.begin(), ops.end(), [](const RefOp & op) { return op.kind == RefOpKind::RemoveNamespace; });
+}
+
+size_t removalOpEncodedSize(RefOwnerKind owner_kind, const String & ref_name, const ManifestRef & manifest_ref)
+{
+    /// One exact owner-removal op, exactly as it appears in a hypothetical whole-namespace removal
+    /// transaction: an owner_transition with only an old binding, no new binding.
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{owner_kind, ref_name, manifest_ref};
+
+    CasJsonWriter out(256);
+    writeOp(out, op);
+    return out.size();
+}
+
+size_t removalFramingSize(const String & ns, const RefTxnId & txn_id, uint64_t op_count)
+{
+    /// Header + meta + the terminal remove_namespace op + trailer(op_count). `op_count` counts every op
+    /// including the remove_namespace op (i.e. committed + precommits + 1).
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::RefLog);
+    writeLogMeta(out, ns, txn_id, std::nullopt);
+    RefOp remove_op;
+    remove_op.kind = RefOpKind::RemoveNamespace;
+    writeOp(out, remove_op);
+    writeTrailerLine(out, op_count);
+    return out.size();
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h
new file mode 100644
index 000000000000..34347fb97aaf
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h
@@ -0,0 +1,177 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <base/types.h>
+#include <cstddef>
+#include <cstdint>
+#include <optional>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Text codec for `cas_ref_log`, the immutable object stored at `_log/<txn_id>`. Each object contains
+/// exactly one committed transaction: its namespace, transaction id, and the batch of `RefOp`s applied
+/// by that commit. The body has a header, a meta line `{"ns","we","rs",["!pse","!pss"]}`, one JSON
+/// record per op, and a `{"n":count}` trailer. Records are emitted in the transaction's stored order
+/// and contain no codec-generated timestamps, so encoding the same value is byte-identical. This
+/// determinism is a property of the representation, not an adoption gate: ref commits use
+/// `putIfAbsentControlled`, and the caller applies the `Always`/`.zst` storage policy by sealing the
+/// returned text.
+///
+/// `RefOpKind::EpochSeal` closes an epoch transition in-band (spec INV-2): a seal transaction contains
+/// exactly that one op, and the meta line's optional `prev_epoch_seal` (wire fields `!pse`/`!pss`,
+/// CRITICAL -- an unrecognized `!`-key fails closed with `UNKNOWN_FORMAT_VERSION` rather than being
+/// silently skipped, since dropping it would lose INV-2's chain evidence while still passing the
+/// structural grammar) chains to the transaction id of the seal that closed the PRECEDING epoch, and
+/// its own `writer_epoch` is exactly one below the referencing transaction's. `prev_epoch_seal` is
+/// required on exactly sequence 1 of every epoch above the namespace's genesis (`life_epoch`, the
+/// writer epoch of its `NamespaceBirth`) and forbidden elsewhere -- see
+/// `validateEpochSealGrammarStructural` and `validateEpochSealGrammarContextual`.
+
+/// One operation kind in a ref transaction log. The numeric values are part of the in-memory and
+/// serialized representation; unknown values are rejected by the decoder.
+enum class RefOpKind : uint8_t
+{
+    NamespaceBirth = 1,
+    OwnerTransition = 2,
+    SetPublishedAt = 3,
+    RemoveNamespace = 4,
+    EpochSeal = 5,
+};
+
+/// One operation inside a `RefLogTxn`. Only the fields documented next to `kind` are meaningful for
+/// that kind, and the codec never reads or writes the others. `OwnerTransition` optionally removes
+/// `old_binding` and/or installs `new_binding`; `SetPublishedAt` carries the expected manifest and the
+/// publication timestamp. `RefOwnerBinding` is shared with the snapshot format through
+/// `CasRefWireVocab.h`. `NamespaceBirth`, `RemoveNamespace`, and `EpochSeal` carry none of `RefOp`'s
+/// fields -- `EpochSeal`'s only payload, `prev_epoch_seal`, lives on the containing `RefLogTxn`, not
+/// here (it is a property of the transaction's position in the log, not of the op).
+struct RefOp
+{
+    RefOpKind kind = RefOpKind::NamespaceBirth;
+
+    std::optional<RefOwnerBinding> old_binding;    /// OwnerTransition: absent = pure add
+    std::optional<RefOwnerBinding> new_binding;    /// OwnerTransition: absent = pure removal
+
+    String ref_name;                               /// SetPublishedAt
+    ManifestRef expected_manifest_ref;             /// SetPublishedAt
+    uint64_t published_at_ms = 0;                  /// SetPublishedAt
+
+    bool operator==(const RefOp &) const = default;
+};
+
+/// The complete immutable body of one ref transaction log object. `ns` and `txn_id` are repeated in
+/// the body even though both are key-derived. `decodeRefLogTxn` compares them with the values supplied
+/// from the object key, rejecting a valid body copied under a different key as corruption.
+struct RefLogTxn
+{
+    String ns;
+    RefTxnId txn_id;
+    std::vector<RefOp> ops;
+    /// The id of the `EpochSeal` transaction that closed the PRECEDING epoch (spec INV-2's genesis
+    /// contract): required iff `txn_id.ref_sequence == 1` and `txn_id.writer_epoch` is above the
+    /// namespace's `life_epoch` (its `NamespaceBirth` writer epoch), forbidden at every other
+    /// sequence. Populated by both the recovery-minted seal that closes an empty epoch and the first
+    /// ordinary transaction a writer appends after a transition -- see
+    /// `validateEpochSealGrammarStructural` / `validateEpochSealGrammarContextual`. Declared LAST (not
+    /// grouped with `txn_id`) to keep it a strict append to `RefLogTxn`'s field order. This project's
+    /// `-Wmissing-field-initializers`/`-Wmissing-designated-field-initializers` still require every
+    /// pre-existing positional `RefLogTxn{ns, txn_id, ops}` aggregate-init call site across the tree to
+    /// spell out an explicit trailing `std::nullopt` -- appending is still the smaller, purely
+    /// mechanical diff (one token per call site) compared to inserting a field in the middle.
+    std::optional<RefTxnId> prev_epoch_seal;
+
+    bool operator==(const RefLogTxn &) const = default;
+};
+
+/// Hard limits enforced by the codec at both encode and decode, measured over the JSON text bytes.
+/// Normal transactions have an operation-count limit, a byte limit, and a per-op size limit. A
+/// transaction containing `RemoveNamespace` is "removal-class": it shares the larger complete-table
+/// byte budget and has neither a separate operation-count cap nor a per-op cap, because that byte
+/// budget alone bounds it.
+inline constexpr size_t ref_txn_max_ops = 5000;
+/// Admission is ops-only (no accumulated-size estimation); this stays as a decode-side acceptance
+/// bound plus a post-encode writer assert. The canonical writer cannot reach it: at most
+/// `ref_txn_max_ops` ops at `ref_op_max_bytes` each is well under this, with framing headroom to
+/// spare.
+inline constexpr size_t ref_txn_max_bytes = 20 * 1024 * 1024;
+inline constexpr size_t ref_removal_max_bytes = 64 * 1024 * 1024;
+/// Per-op size cap on normal-class ops, enforced exactly per op (one op encoded alone, no
+/// accumulation) both at admission and at decode. Not unreachable: `checkCanonicalRefName` imposes
+/// no length limit and ref/part names grow with partition-key values. Removal-class ops are exempt
+/// (they share the byte budget above and have no per-op cap).
+inline constexpr size_t ref_op_max_bytes = 4096;
+
+/// Encode the transaction to canonical, uncompressed text. The persist path seals this text according
+/// to the `Always`/`.zst` policy; keeping the codec unsealed lets the byte-budget check and preview
+/// callers measure the actual text, and lets the state machine validate an uncompressed round-trip.
+/// Operations retain their stored order. Throws CORRUPTED_DATA on a zero `txn_id` field, a
+/// non-canonical `ref_name`, an out-of-range `manifest_ref`, an unknown op kind, or an op-count/byte
+/// limit violation over the encoded text.
+String encodeRefLogTxn(const RefLogTxn & txn);
+
+/// Decode canonical text after the caller has opened the stored `.zst` object. `expected_ns` and
+/// `expected_txn_id` come from the object key; the decoded body must equal them, otherwise the body/key
+/// binding fails with CORRUPTED_DATA. Unknown non-critical fields are skipped so additive fields can be
+/// introduced without changing this reader, while a future header version is rejected with
+/// UNKNOWN_FORMAT_VERSION. Truncation, an unknown op or owner kind, a non-canonical `ref_name`, a zero
+/// transaction-id field, a body/key mismatch, and a limit violation are reported as CORRUPTED_DATA.
+RefLogTxn decodeRefLogTxn(std::string_view data, const String & expected_ns, const RefTxnId & expected_txn_id);
+
+/// Encoded byte size of exactly one exact-owner-removal op line: an `owner_transition` with only an old
+/// binding, no new binding, as it appears in a hypothetical whole-namespace removal transaction (one
+/// such op per committed ref and precommit, followed by a terminal `remove_namespace`), encoded via
+/// `encodeRefLogTxn`.
+size_t removalOpEncodedSize(RefOwnerKind owner_kind, const String & ref_name, const ManifestRef & manifest_ref);
+
+/// Encoded byte size of a removal transaction's framing (header + meta + terminal remove_namespace op +
+/// trailer) for `op_count` total ops, excluding the per-owner removal op lines. `removalFramingSize(...)
+/// + Σ removalOpEncodedSize` equals `encodeRefLogTxn(...)`'s size, for the hypothetical whole-namespace
+/// removal transaction described above, exactly.
+size_t removalFramingSize(const String & ns, const RefTxnId & txn_id, uint64_t op_count);
+
+/// Encoded byte size of exactly one op, on its own, as it appears inside a `RefLogTxn` body (the
+/// record line only -- no header, meta, or trailer framing). Used to enforce the per-op size cap at
+/// admission and at decode without ever accumulating a whole transaction's bytes.
+size_t encodedOpSize(const RefOp & op);
+
+/// The one canonical removal-class discriminator: a transaction (or a not-yet-encoded item's built
+/// ops) is removal-class iff `ops` contains a `RemoveNamespace` op. `MutationScope::Kind::WholeShard`
+/// is NOT a substitute -- the stale-precommit reclaim sweep is also `WholeShard`-scoped but is not
+/// removal-class. Every site that selects the removal byte budget (`ref_removal_max_bytes` vs
+/// `ref_txn_max_bytes`) or exempts an item from the normal-class op/per-op caps must call this, not
+/// re-derive its own predicate.
+bool refLogTxnIsRemovalClass(const std::vector<RefOp> & ops);
+
+/// True iff `txn` is exactly a well-formed epoch-seal transaction: one operation, and that operation
+/// is `EpochSeal`. Unlike `refLogTxnIsRemovalClass` (a class predicate that tolerates other ops
+/// alongside `RemoveNamespace`), a seal transaction's grammar forbids any companion op (spec INV-2),
+/// so this checks the exact shape rather than mere presence -- callers that decode untrusted bytes
+/// and branch on "is this a seal" (Task 2's slot-occupy walk) get a precise answer.
+bool refLogTxnIsEpochSeal(const RefLogTxn & txn);
+
+/// The context-free half of the INV-2 seal grammar: a transaction carrying an `EpochSeal` op contains
+/// EXACTLY that one op, and `prev_epoch_seal`, when present, is well-formed (both `RefTxnId`
+/// components nonzero), appears ONLY at `txn_id.ref_sequence == 1`, and names the IMMEDIATELY
+/// preceding epoch (`prev_epoch_seal->writer_epoch + 1 == txn_id.writer_epoch`) -- a self, forward,
+/// or skipping pointer is rejected even though it cannot arise from `writeLogMeta`'s own writer,
+/// because Tasks 2/6 walk this pointer backwards over untrusted decoded bodies. Called by both
+/// `encodeRefLogTxn` and
+/// `decodeRefLogTxn`, so a caller-built transaction and a decoded one are held to the identical rule.
+/// Throws CORRUPTED_DATA on violation. Does NOT check the required-iff rule -- that needs
+/// `life_epoch`, which the codec never sees; see `validateEpochSealGrammarContextual`.
+void validateEpochSealGrammarStructural(const RefLogTxn & txn);
+
+/// The contextual half of the INV-2 seal grammar: `prev_epoch_seal` is required on exactly
+/// `txn_id.ref_sequence == 1` of every epoch with `txn_id.writer_epoch > life_epoch` (a namespace's
+/// `life_epoch` is the writer epoch of its `NamespaceBirth` record -- its genesis, which may be above
+/// 1 for a namespace born after earlier epoch transitions) and forbidden at
+/// `txn_id.writer_epoch <= life_epoch`; a no-op for `txn_id.ref_sequence != 1` (the unconditional
+/// "forbidden elsewhere" half of the rule is `validateEpochSealGrammarStructural`'s job, not this
+/// function's). Callers own `life_epoch`: the apply layer and the writer-side encode call sites that
+/// mint a sequence-1 transaction. Throws CORRUPTED_DATA on violation.
+void validateEpochSealGrammarContextual(const RefLogTxn & txn, uint64_t life_epoch);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.cpp
new file mode 100644
index 000000000000..f31e9fed4ca2
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.cpp
@@ -0,0 +1,305 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <optional>
+#include <tuple>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+void checkCommittedSorted(const std::vector<RefCommittedRow> & rows)
+{
+    for (size_t i = 1; i < rows.size(); ++i)
+        if (!(rows[i - 1].ref_name < rows[i].ref_name))
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefTableSnapshot: committed rows are not strictly ascending by ref_name at '{}' -> '{}'",
+                rows[i - 1].ref_name, rows[i].ref_name);
+}
+
+void checkPrecommitsSorted(const std::vector<RefOwnerBinding> & rows)
+{
+    for (size_t i = 1; i < rows.size(); ++i)
+    {
+        const auto prev_key = std::tie(rows[i - 1].ref_name, rows[i - 1].manifest_ref);
+        const auto cur_key = std::tie(rows[i].ref_name, rows[i].manifest_ref);
+        if (!(prev_key < cur_key))
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefTableSnapshot: precommit rows are not strictly ascending by (ref_name, manifest_ref) at '{}' -> '{}'",
+                rows[i - 1].ref_name, rows[i].ref_name);
+    }
+}
+
+/// Whole-object validation: transaction IDs must be nonzero and both row vectors must be strictly
+/// sorted. Applying the same
+/// checks before encoding and after decoding keeps malformed caller state and malformed stored data
+/// subject to the same contract.
+void checkSnapshotInvariants(const RefTableSnapshot & snapshot)
+{
+    checkRefTxnIdNonzero(snapshot.snapshot_id, "RefTableSnapshot", "snapshot_id");
+
+    checkCommittedSorted(snapshot.committed);
+    checkPrecommitsSorted(snapshot.precommits);
+}
+
+void writeCommittedRow(CasJsonWriter & out, const RefCommittedRow & row)
+{
+    checkCanonicalRefName(row.ref_name, "RefTableSnapshot", "committed ref_name");
+    checkManifestRef(row.manifest_ref, "RefTableSnapshot", "committed");
+    bool first = true;
+    writeKey(out, "k", first);
+    writeStringValue(out, "c");
+    writeKey(out, "rn", first);
+    writeStringValue(out, row.ref_name);
+    writeManifestRefFields(out, first, "", row.manifest_ref);
+    writeKey(out, "ts", first);
+    writeIntText(row.published_at_ms, out);
+    closeObject(out, first);
+    writeChar('\n', out);
+}
+
+void writePrecommitRow(CasJsonWriter & out, const RefOwnerBinding & row)
+{
+    if (row.kind != RefOwnerKind::Precommit)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefTableSnapshot: precommits entry '{}' has kind {}, expected Precommit",
+            row.ref_name, static_cast<uint8_t>(row.kind));
+    checkCanonicalRefName(row.ref_name, "RefTableSnapshot", "precommit ref_name");
+    checkManifestRef(row.manifest_ref, "RefTableSnapshot", "precommit");
+    bool first = true;
+    writeKey(out, "k", first);
+    writeStringValue(out, "p");
+    writeKey(out, "rn", first);
+    writeStringValue(out, row.ref_name);
+    writeManifestRefFields(out, first, "", row.manifest_ref);
+    closeObject(out, first);
+    writeChar('\n', out);
+}
+
+/// The snapshot's header-object meta line (`ns`, `snapshot_id`, and the required generation-8
+/// `lc:"live"` constant). Shared by
+/// `encodeRefTableSnapshot` and `snapshotFramingSize` so the two never disagree by a
+/// byte. Assumes the caller has already validated the snapshot (or is measuring framing only).
+void writeSnapshotMeta(CasJsonWriter & out, const RefTableSnapshot & snapshot)
+{
+    bool first = true;
+    writeKey(out, "ns", first);
+    writeStringValue(out, snapshot.ns);
+    writeRefTxnIdFields(out, first, "we", "rs", snapshot.snapshot_id);
+    writeKey(out, "lc", first);
+    writeStringValue(out, "live");
+    closeObject(out, first);
+    writeChar('\n', out);
+}
+
+/// Collector for a ManifestRef's three flat fields (bare "me"/"mb"/"mo").
+struct ManifestFields
+{
+    std::optional<uint64_t> me;
+    std::optional<uint64_t> mb;
+    std::optional<uint64_t> mo;
+
+    /// Reconstruct a manifest reference after the tolerant reader has collected all three flat
+    /// fields. Missing fields are malformed input; `manifestRefFromFields` performs the remaining
+    /// range checks and reports the same corruption context as the row decoder.
+    ManifestRef build(std::string_view what) const
+    {
+        if (!me || !mb || !mo)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: {} manifest_ref missing me/mb/mo", what);
+        return manifestRefFromFields(*me, *mb, *mo, "RefTableSnapshot", what);
+    }
+};
+
+}
+
+String encodeRefTableSnapshot(const RefTableSnapshot & snapshot)
+{
+    checkSnapshotInvariants(snapshot);
+
+    CasJsonWriter out(256 + 128 * (snapshot.committed.size() + snapshot.precommits.size()));
+    writeHeaderLine(out, FormatId::RefSnapshot);
+
+    writeSnapshotMeta(out, snapshot);
+
+    for (const RefCommittedRow & row : snapshot.committed)
+        writeCommittedRow(out, row);
+    for (const RefOwnerBinding & row : snapshot.precommits)
+        writePrecommitRow(out, row);
+
+    writeTrailerLine(out, snapshot.committed.size() + snapshot.precommits.size());
+    String text = std::move(out).take();
+    if (text.size() > ref_snapshot_max_bytes)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefTableSnapshot: encoded size {} exceeds the snapshot byte limit {}", text.size(), ref_snapshot_max_bytes);
+    return text;
+}
+
+RefTableSnapshot decodeRefTableSnapshot(
+    std::string_view data, const String & expected_ns, const RefTxnId & expected_snapshot_id)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::RefSnapshot);
+    const uint64_t line_cap = traitsFor(FormatId::RefSnapshot).line_cap;
+
+    RefTableSnapshot snapshot;
+
+    {
+        const String line = readLine(in, line_cap, "cas_ref_snap");
+        ReadBufferFromMemory meta_buf(line.data(), line.size());
+        JsonObjectReader r(meta_buf, KeyStrictness::Tolerant, "cas_ref_snap");
+        bool saw_ns = false;
+        bool saw_we = false;
+        bool saw_rs = false;
+        bool saw_lc = false;
+        String key;
+        while (r.nextKey(key))
+        {
+            if (key == "ns") { snapshot.ns = r.readString(); saw_ns = true; }
+            else if (key == "we") { snapshot.snapshot_id.writer_epoch = r.readU64String(); saw_we = true; }
+            else if (key == "rs") { snapshot.snapshot_id.ref_sequence = r.readU64String(); saw_rs = true; }
+            else if (key == "lc")
+            {
+                const String lifecycle = r.readString();
+                if (lifecycle != "live")
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "RefTableSnapshot: lifecycle must be exactly 'live', got '{}'", lifecycle);
+                saw_lc = true;
+            }
+            else if (key == "rte" || key == "rts")
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableSnapshot: meta carries retired terminal field '{}'", key);
+            else r.skipUnknown(key);
+        }
+        if (!saw_ns || !saw_we || !saw_rs || !saw_lc)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: meta line missing ns/we/rs/lc");
+        if (!meta_buf.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: junk after meta line");
+    }
+
+    /// record lines (committed then precommit), until the trailer
+    while (true)
+    {
+        const String line = readLine(in, line_cap, "cas_ref_snap");
+        ReadBufferFromMemory l(line.data(), line.size());
+        JsonObjectReader r(l, KeyStrictness::Tolerant, "cas_ref_snap");
+        String key;
+        if (!r.nextKey(key))
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: empty line");
+
+        if (key == "n")
+        {
+            const uint64_t n = r.readU64Number();
+            while (r.nextKey(key))
+                r.skipUnknown(key);
+            if (!l.eof() || !in.eof())
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: bytes after trailer");
+            if (n != snapshot.committed.size() + snapshot.precommits.size())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableSnapshot: trailer count {} != {} rows", n, snapshot.committed.size() + snapshot.precommits.size());
+            break;
+        }
+        if (key != "k")
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: record must start with \"k\"");
+        const String k = r.readString();
+
+        std::optional<String> rn;
+        ManifestFields mf;
+        std::optional<uint64_t> ts;
+        while (r.nextKey(key))
+        {
+            if (key == "rn") rn = r.readString();
+            else if (key == "me") mf.me = r.readU64String();
+            else if (key == "mb") mf.mb = r.readU64String();
+            else if (key == "mo") mf.mo = r.readU64Number();
+            else if (key == "ts") ts = r.readU64Number();
+            else if (key == "pl")
+                /// `"pl"` (payload) was removed from the row wire in stage-1 T12. It is a KNOWN-removed
+                /// field, not a genuinely-unknown future one the tolerant reader may skip -- silently
+                /// discarding a persisted payload would lose data -- so reject it explicitly.
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableSnapshot: record carries the removed \"pl\" (payload) field");
+            else r.skipUnknown(key);
+        }
+        if (!l.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: junk after record");
+
+        if (k == "c")
+        {
+            if (!rn || !ts)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: committed row missing rn/ts");
+            RefCommittedRow row;
+            row.ref_name = *rn;
+            checkCanonicalRefName(row.ref_name, "RefTableSnapshot", "committed ref_name");
+            row.manifest_ref = mf.build("committed");
+            row.published_at_ms = *ts;
+            snapshot.committed.push_back(std::move(row));
+        }
+        else if (k == "p")
+        {
+            if (!rn)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: precommit row missing rn");
+            RefOwnerBinding row;
+            row.kind = RefOwnerKind::Precommit;
+            row.ref_name = *rn;
+            checkCanonicalRefName(row.ref_name, "RefTableSnapshot", "precommit ref_name");
+            row.manifest_ref = mf.build("precommit");
+            snapshot.precommits.push_back(std::move(row));
+        }
+        else
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableSnapshot: unknown row kind '{}'", k);
+    }
+
+    /// The object key is supplied separately from the body. Check the binding before accepting any
+    /// decoded state so bytes stored under one namespace or snapshot ID cannot be interpreted as
+    /// another object.
+    if (snapshot.ns != expected_ns || snapshot.snapshot_id != expected_snapshot_id)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefTableSnapshot: body (ns='{}', snapshot_id={}-{}) does not match the key it was read from "
+            "(ns='{}', snapshot_id={}-{})",
+            snapshot.ns, snapshot.snapshot_id.writer_epoch, snapshot.snapshot_id.ref_sequence,
+            expected_ns, expected_snapshot_id.writer_epoch, expected_snapshot_id.ref_sequence);
+
+    checkSnapshotInvariants(snapshot);
+    return snapshot;
+}
+
+size_t committedRowEncodedSize(const RefCommittedRow & row)
+{
+    CasJsonWriter out(256);
+    writeCommittedRow(out, row);
+    return out.size();
+}
+
+size_t precommitRowEncodedSize(const RefOwnerBinding & binding)
+{
+    CasJsonWriter out(256);
+    writePrecommitRow(out, binding);
+    return out.size();
+}
+
+size_t snapshotFramingSize(const String & ns, const RefTxnId & snapshot_id, uint64_t row_count)
+{
+    RefTableSnapshot meta_only;
+    meta_only.ns = ns;
+    meta_only.snapshot_id = snapshot_id;
+
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::RefSnapshot);
+    writeSnapshotMeta(out, meta_only);
+    writeTrailerLine(out, row_count);
+    return out.size();
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h
new file mode 100644
index 000000000000..07972a0d47ab
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h
@@ -0,0 +1,95 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <base/types.h>
+#include <cstddef>
+#include <cstdint>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Text codec for `cas_ref_snap`, the complete per-namespace ref table snapshot at
+/// `_snap/<snapshot_id>`. The object is read whole rather than streamed and belongs to the Control
+/// family: callers store the encoded text as an Always/`.zst` object. Its canonical text consists of
+/// a header, a metadata line, committed and precommit row lines, and a `{"n":count}` trailer.
+///
+/// There is no such thing as a "seal snapshot". An epoch is closed IN-BAND, by an `EpochSeal`
+/// transaction the recovery CAS-walk places at `{E, T+1}` in the `_log` stream (INV-2) -- the exact key
+/// a dying predecessor's in-flight PUT would have taken, so the store's write-once create is the fence.
+/// The retired alternative was a synthetic snapshot at `{E-1, UINT64_MAX}` carrying a `sealed_from`
+/// bound: it occupied no log key, so it fenced nothing and needed a separate after-the-fact detector for
+/// the writes it let through. All `RefTxnId` components are still encoded as decimal strings. Rows are
+/// emitted in canonical order, making re-encoding deterministic by construction; these objects are
+/// published through the ordinary single-owner `putIfAbsentControlled` path, not a
+/// `putDeterministicArtifact` byte-adoption gate.
+
+/// In-memory ref-table lifecycle. Only `Live` is serializable as a generation-8 snapshot; terminal
+/// state lives in the removal log and fold evidence and has no snapshot DTO representation.
+enum class RefLifecycle : uint8_t
+{
+    Live = 1,
+    Removed = 2,
+};
+
+/// One committed ref-name-to-manifest row in a `RefTableSnapshot`. `published_at_ms` is the only
+/// mutable metadata field a committed row carries.
+struct RefCommittedRow
+{
+    String ref_name;
+    ManifestRef manifest_ref;
+    uint64_t published_at_ms = 0;
+
+    bool operator==(const RefCommittedRow &) const = default;
+};
+
+/// The complete state of one namespace's ref table in one canonical snapshot object. `precommits`
+/// reuses `RefOwnerBinding` from `CasRefWireVocab.h`; every entry's `kind` must be `Precommit`.
+/// Generation 8 serializes only `Live` snapshots. Both row vectors must already be strictly sorted by
+/// their documented keys, because the codec
+/// validates and emits the caller-provided order rather than sorting it.
+struct RefTableSnapshot
+{
+    String ns;
+    RefTxnId snapshot_id;
+    std::vector<RefCommittedRow> committed;     /// sorted by canonical bytewise ref_name, no duplicates
+    std::vector<RefOwnerBinding> precommits;    /// sorted by (ref_name, manifest_ref), no duplicates
+
+    bool operator==(const RefTableSnapshot &) const = default;
+};
+
+/// Hard encoded-size limit over the uncompressed text. The snapshot reuses the removal-class
+/// complete-table budget from `CasRefLogFormat.h`.
+inline constexpr size_t ref_snapshot_max_bytes = ref_removal_max_bytes;
+
+/// Encode to the canonical text (not sealed): the caller compresses via
+/// `sealObject(FormatId::RefSnapshot, …)` on the persist path (Always/`.zst`), and the in-memory
+/// validation and `admits` size-estimate callers use the uncompressed text. Throws
+/// CORRUPTED_DATA on: a zero `snapshot_id`; a non-canonical `ref_name`; an
+/// out-of-range `manifest_ref`; non-strictly-ascending
+/// `committed` / `precommits`; a `precommits` entry not `Precommit`; or an over-budget object.
+String encodeRefTableSnapshot(const RefTableSnapshot & snapshot);
+
+/// Decode the canonical text (the caller `openObject`s the stored `.zst` first). `expected_ns` /
+/// `expected_snapshot_id` are recovered from the object key; the decoded body must equal them (the
+/// key↔body binding). Throws UNKNOWN_FORMAT_VERSION for a header `v` above this build, CORRUPTED_DATA
+/// for truncation, a missing/non-`live` lifecycle word, either retired `rte`/`rts` field, an unknown
+/// owner kind, or any validation failure listed above.
+RefTableSnapshot decodeRefTableSnapshot(
+    std::string_view data, const String & expected_ns, const RefTxnId & expected_snapshot_id);
+
+/// Encoded byte size of exactly one committed row line, as `encodeRefTableSnapshot` would emit it.
+/// Reuses the same writer, so it is byte-identical to that row's contribution to a full encode.
+size_t committedRowEncodedSize(const RefCommittedRow & row);
+
+/// Encoded byte size of exactly one precommit row line, as `encodeRefTableSnapshot` would emit it.
+size_t precommitRowEncodedSize(const RefOwnerBinding & binding);
+
+/// Encoded byte size of a snapshot's framing (header + meta line + trailer) for the given metadata and
+/// row count, excluding all row lines. `snapshotFramingSize(...) + Σ committedRowEncodedSize +
+/// Σ precommitRowEncodedSize` equals `encodeRefTableSnapshot(...).size()` exactly.
+size_t snapshotFramingSize(const String & ns, const RefTxnId & snapshot_id, uint64_t row_count);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.cpp
new file mode 100644
index 000000000000..bf1a5445e4df
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.cpp
@@ -0,0 +1,47 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.h>
+#include <Common/Exception.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+std::string_view refOwnerKindToWord(RefOwnerKind k)
+{
+    switch (k)
+    {
+        case RefOwnerKind::Committed: return "committed";
+        case RefOwnerKind::Precommit: return "precommit";
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS ref wire: unknown RefOwnerKind {}", static_cast<int>(k));
+}
+
+RefOwnerKind refOwnerKindFromWord(std::string_view w, std::string_view what)
+{
+    if (w == "committed") return RefOwnerKind::Committed;
+    if (w == "precommit") return RefOwnerKind::Precommit;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: unknown owner kind '{}'", what, w);
+}
+
+void checkRefTxnIdNonzero(const RefTxnId & id, std::string_view format, std::string_view field)
+{
+    if (id.writer_epoch == 0 || id.ref_sequence == 0)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "{}: {} fields must both be nonzero, got {}-{}", format, field, id.writer_epoch, id.ref_sequence);
+}
+
+void writeRefTxnIdFields(CasJsonWriter & out, bool & first, std::string_view epoch_key, std::string_view seq_key, const RefTxnId & id)
+{
+    writeKey(out, epoch_key, first);
+    writeU64StringValue(out, id.writer_epoch);
+    writeKey(out, seq_key, first);
+    writeU64StringValue(out, id.ref_sequence);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.h
new file mode 100644
index 000000000000..fdccef8f7fdb
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefWireVocab.h
@@ -0,0 +1,65 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h>
+#include <base/types.h>
+#include <cstdint>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// Shared identifier vocabulary for the ref-log and ref-snapshot text formats. `RefOwnerBinding` and
+/// `RefOwnerKind` are used in log `OwnerTransition` records and snapshot `precommits`, so neither
+/// format owns a duplicate definition. This header has no backend or storage dependencies; the
+/// manifest-reference JSON helpers are shared by both ref formats and part-manifest descriptors
+/// through `CasWireVocab.h`.
+
+/// Identifies which ownership slot a `RefOwnerBinding` occupies. The numeric values are the
+/// in-memory discriminators; text codecs render them as the words "committed" and "precommit".
+enum class RefOwnerKind : uint8_t
+{
+    Committed = 1,
+    Precommit = 2,
+};
+
+/// One ref-name-to-manifest ownership binding. Log transitions use it for the optional old and new
+/// owners; snapshots use it for precommit rows, whose `kind` must be `Precommit`. A precommit's build
+/// identity is the pair `{manifest_ref.writer_epoch, manifest_ref.build_sequence}`; the binding has
+/// no additional build token. `manifest_ref` is validated by the format that decodes the binding.
+struct RefOwnerBinding
+{
+    RefOwnerKind kind = RefOwnerKind::Committed;
+    String ref_name;
+    ManifestRef manifest_ref;
+
+    bool operator==(const RefOwnerBinding &) const = default;
+};
+
+/// Convert an owner-kind discriminator to its canonical text word. Throws `CORRUPTED_DATA` for a
+/// value not represented by this format; accepting an unknown value would produce an ambiguous wire
+/// record.
+std::string_view refOwnerKindToWord(RefOwnerKind k);
+
+/// Parse a canonical owner-kind word. `what` identifies the containing field in the
+/// `CORRUPTED_DATA` exception. Unknown words are rejected rather than treated as a default kind.
+RefOwnerKind refOwnerKindFromWord(std::string_view w, std::string_view what);
+
+/// Append two flat `RefTxnId` fields -- its `writer_epoch` and `ref_sequence` components -- to an
+/// in-progress JSON object, both as decimal STRINGS -- the representation is width-independent, so no
+/// consumer has to care how large a `ref_sequence` can get. `epoch_key`/`seq_key` name the two fields,
+/// letting each format distinguish its primary id from any secondary id it embeds (for example,
+/// `cas_ref_log`'s `we`/`rs` versus its `prev_epoch_seal` pair) while sharing one writer so the
+/// formats can never disagree on the representation.
+void writeRefTxnIdFields(CasJsonWriter & out, bool & first, std::string_view epoch_key, std::string_view seq_key, const RefTxnId & id);
+
+/// `RefTxnId`'s validity rule applied to ONE field of a decoded or about-to-be-encoded record: both
+/// components nonzero. `renderRefTxnId` refuses to build a key from anything else, so a half-zero id
+/// here would name an object that cannot exist -- `CORRUPTED_DATA`, in both directions.
+///
+/// Shared for the same reason `writeRefTxnIdFields` is: ref formats can embed ids besides their
+/// primary one (for example, `cas_ref_log`'s `prev_epoch_seal`), and a per-format copy of the rule is
+/// a rule that can drift per format. `format` names the record kind and `field` the member, so each
+/// format keeps its own exception text without owning its own check.
+void checkRefTxnIdNonzero(const RefTxnId & id, std::string_view format, std::string_view field);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.cpp
new file mode 100644
index 000000000000..a12062fd21cb
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.cpp
@@ -0,0 +1,178 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Read exactly the one JSON body line allowed by a server-root control object. `readLine` rejects a
+/// missing newline and a line over the format-specific cap; each decoder separately checks that no
+/// bytes follow this line, so a concatenated object cannot be accepted accidentally.
+String readBodyLine(ReadBuffer & in, FormatId id, std::string_view what)
+{
+    return readLine(in, traitsFor(id).line_cap, what);
+}
+
+}
+
+String encodeOwner(const OwnerObject & o)
+{
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::Owner);
+    bool first = true;
+    writeKey(out, "su", first);
+    writeHex128Value(out, o.server_uuid);
+    if (o.retired_at_ms)
+    {
+        writeKey(out, "rt", first);
+        writeIntText(*o.retired_at_ms, out);
+    }
+    closeObject(out, first);
+    writeChar('\n', out);
+    return std::move(out).take();
+}
+
+OwnerObject decodeOwner(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::Owner);
+    const String body = readBodyLine(in, FormatId::Owner, "owner");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    JsonObjectReader r(body_in, KeyStrictness::Tolerant, "owner");
+
+    OwnerObject o;
+    bool saw = false;
+    std::optional<uint64_t> rt;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "su")
+        {
+            o.server_uuid = r.readHex128();
+            saw = true;
+        }
+        else if (key == "rt")
+            rt = r.readU64Number();
+        else
+            r.skipUnknown(key);
+    }
+    o.retired_at_ms = rt;
+    if (!saw)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS owner: missing su");
+    if (!body_in.eof() || !in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS owner: trailing bytes");
+    return o;
+}
+
+String encodeServerEpoch(const ServerEpoch & e)
+{
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::ServerEpoch);
+    bool first = true;
+    writeKey(out, "nwe", first);
+    writeU64StringValue(out, e.next_writer_epoch);
+    closeObject(out, first);
+    writeChar('\n', out);
+    return std::move(out).take();
+}
+
+ServerEpoch decodeServerEpoch(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::ServerEpoch);
+    const String body = readBodyLine(in, FormatId::ServerEpoch, "server-epoch");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    JsonObjectReader r(body_in, KeyStrictness::Tolerant, "server-epoch");
+
+    ServerEpoch e;
+    bool saw = false;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "nwe")
+        {
+            e.next_writer_epoch = r.readU64String();
+            saw = true;
+        }
+        else
+            r.skipUnknown(key);
+    }
+    if (!saw)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS server-epoch: missing nwe");
+    if (!body_in.eof() || !in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS server-epoch: trailing bytes");
+    return e;
+}
+
+String encodeMountLease(const MountLease & m)
+{
+    CasJsonWriter out(256);
+    writeHeaderLine(out, FormatId::MountLease);
+    bool first = true;
+    writeKey(out, "su", first);  writeHex128Value(out, m.server_uuid);
+    writeKey(out, "we", first);  writeU64StringValue(out, m.writer_epoch);
+    writeKey(out, "hn", first);  writeStringValue(out, m.hostname);
+    writeKey(out, "pid", first); writeIntText(m.pid, out);
+    writeKey(out, "sat", first); writeIntText(m.started_at_ms, out);
+    writeKey(out, "seq", first); writeU64StringValue(out, m.seq);
+    writeKey(out, "eat", first); writeIntText(m.expires_at_ms, out);
+    writeKey(out, "ma", first);  writeU64StringValue(out, m.min_active);
+    writeKey(out, "fen", first); writeBoolValue(out, m.gc_fenced);
+    closeObject(out, first);
+    writeChar('\n', out);
+    return std::move(out).take();
+}
+
+MountLease decodeMountLease(std::string_view data)
+{
+    ReadBufferFromMemory in(data.data(), data.size());
+    expectHeaderLine(in, FormatId::MountLease);
+    const String body = readBodyLine(in, FormatId::MountLease, "mount-lease");
+    ReadBufferFromMemory body_in(body.data(), body.size());
+    JsonObjectReader r(body_in, KeyStrictness::Tolerant, "mount-lease");
+
+    MountLease m;
+    bool saw_su = false;
+    bool saw_we = false;
+    String key;
+    while (r.nextKey(key))
+    {
+        if (key == "su")
+        {
+            m.server_uuid = r.readHex128();
+            saw_su = true;
+        }
+        else if (key == "we")
+        {
+            m.writer_epoch = r.readU64String();
+            saw_we = true;
+        }
+        else if (key == "hn") m.hostname = r.readString();
+        else if (key == "pid") m.pid = r.readU64Number();
+        else if (key == "sat") m.started_at_ms = r.readU64Number();
+        else if (key == "seq") m.seq = r.readU64String();
+        else if (key == "eat") m.expires_at_ms = r.readU64Number();
+        else if (key == "ma") m.min_active = r.readU64String();
+        else if (key == "fen") m.gc_fenced = r.readBool();
+        else r.skipUnknown(key);
+    }
+    if (!saw_su || !saw_we)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS mount-lease: missing identity field");
+    if (!body_in.eof() || !in.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS mount-lease: trailing bytes");
+    return m;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h
new file mode 100644
index 000000000000..447827d70dde
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h
@@ -0,0 +1,93 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <optional>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// Text codecs for the three per-server-root control objects under
+/// `gc/server-roots/<server_root_id>/`. Each object is a CAS text object consisting of a type/version
+/// header line and one JSON body line; the codec layer only maps fields and checks the syntax that is
+/// local to that object. Coordination and CAS operations remain in the pool/server-root layer.
+///
+/// The owner object permanently binds a configured `server_root_id` to one server UUID. The epoch
+/// object stores the next writer epoch and is CAS-bumped so an epoch is never reused after a
+/// restart or supersession. The mount object is the expirable liveness lease for one writer
+/// incarnation; its `min_active` value carries the GC acknowledgement floor, while `gc_fenced` is a
+/// terminal fence-out marker for that incarnation.
+
+/// Permanent identity anchor for one configured server root. It is created with put-if-absent, never
+/// reassigned, and tombstoned in place when explicitly decommissioned, so `decodeOwner` returns both
+/// the UUID that the caller must compare with its local server identity and the retirement state.
+struct OwnerObject
+{
+    UInt128 server_uuid{};
+    /// Set when this identity was explicitly decommissioned by an operator (`CasDecommission`'s
+    /// final step tombstones the owner object in place rather than deleting it, since its content
+    /// alone cannot distinguish decommissioned debris from a legitimate successor's live anchor).
+    /// A normal claim (`claimOwnerOrThrow`) must refuse to silently resume a tombstoned identity.
+    std::optional<uint64_t> retired_at_ms;
+};
+
+/// Durable counter state used to allocate unique writer epochs. The stored value is the next epoch
+/// to allocate; the caller CAS-writes a larger value and adopts the previous value for its mount.
+struct ServerEpoch
+{
+    uint64_t next_writer_epoch = 0;
+};
+
+/// The current liveness lease for one `(server_uuid, writer_epoch)` writer incarnation. The pool
+/// layer renews and replaces this object with CAS/overwrite operations, and GC may fence an expired
+/// lease by setting `gc_fenced`; a fenced incarnation must not resume writing. `min_active` is the
+/// merged GC acknowledgement floor, with `UINT64_MAX` marking a clean farewell (retired lease).
+struct MountLease
+{
+    UInt128 server_uuid{};
+    uint64_t writer_epoch = 0;
+    String hostname;
+    uint64_t pid = 0;
+    uint64_t started_at_ms = 0;
+    uint64_t seq = 0;
+    uint64_t expires_at_ms = 0;
+    uint64_t min_active = 0;   /// UINT64_MAX = retired (farewell)
+    bool gc_fenced = false;    /// GC fence-out of an expired lease; terminal
+};
+
+/// Encode the owner anchor as canonical text with the `cas_owner` header and a final newline. The
+/// optional retirement timestamp is omitted for a never-retired owner, preserving its historical
+/// bytes. This function does not perform the put-if-absent operation or validate ownership; those
+/// decisions belong to the caller that coordinates the server-root object.
+String encodeOwner(const OwnerObject & o);
+
+/// Decode an owner anchor, requiring its `su` field, tolerating an absent optional `rt` retirement
+/// timestamp, and rejecting bytes after the body line. Unknown JSON fields are skipped for
+/// forward-compatible reads; malformed input, a missing `su`, and trailing data throw
+/// `CORRUPTED_DATA`.
+OwnerObject decodeOwner(std::string_view data);
+
+/// Encode the durable next-writer-epoch counter as canonical text with the `cas_epoch` header and a
+/// final newline. Allocation and the CAS loop that makes the counter monotone are outside this
+/// codec.
+String encodeServerEpoch(const ServerEpoch & e);
+
+/// Decode the epoch counter, requiring its `nwe` field and rejecting bytes after the body line.
+/// Unknown JSON fields are skipped for forward-compatible reads; malformed input, a missing `nwe`,
+/// and trailing data throw `CORRUPTED_DATA`.
+ServerEpoch decodeServerEpoch(std::string_view data);
+
+/// Encode the complete mount-lease body as canonical text with the `cas_mount_lease` header and a
+/// final newline. This preserves full-range `uint64_t` values such as `min_active` as decimal JSON
+/// strings and writes `gc_fenced` as a JSON boolean; lease renewal, fencing, and token checks remain
+/// in the caller.
+String encodeMountLease(const MountLease & m);
+
+/// Decode a mount lease and reject bytes after its body line. The reader accepts unknown JSON fields
+/// so newer lease metadata can be read by older code, while malformed field values and trailing data
+/// throw `CORRUPTED_DATA`; cross-field lease validity is enforced by the mount/GC protocol, not here.
+MountLease decodeMountLease(std::string_view data);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.cpp
new file mode 100644
index 000000000000..9814d5d14811
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.cpp
@@ -0,0 +1,414 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/Exception.h>
+#include <Formats/FormatSettings.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/ReadHelpers.h>
+#include <base/hex.h>
+#include <base/scope_guard.h>
+#include <algorithm>
+#include <cstring>
+#include <limits>
+#include <zstd.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int UNKNOWN_FORMAT_VERSION;
+    extern const int CANNOT_PARSE_INPUT_ASSERTION_FAILED;
+    extern const int CANNOT_PARSE_QUOTED_STRING;
+    extern const int CANNOT_PARSE_NUMBER;
+    extern const int CANNOT_READ_ALL_DATA;
+    extern const int ATTEMPT_TO_READ_AFTER_EOF;
+    extern const int INCORRECT_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+const FormatSettings::JSON & jsonReadSettings()
+{
+    static const FormatSettings::JSON settings;
+    return settings;
+}
+}
+
+/// ---- CasJsonWriter ----
+
+namespace
+{
+constexpr bool isSpecialJsonByte(unsigned char c)
+{
+    return c < 0x20 || c == '"' || c == '\\' || c == 0xE2;
+}
+}
+
+const char * findNextSpecialJsonByte(const char * pos, const char * end)
+{
+    for (; pos != end; ++pos)
+        if (isSpecialJsonByte(static_cast<unsigned char>(*pos)))
+            return pos;
+    return end;
+}
+
+void CasJsonWriter::stringValue(std::string_view s)
+{
+    appendChar('"');
+    const char * pos = s.data();
+    const char * const end = s.data() + s.size();
+    while (pos != end)
+    {
+        const char * next = findNextSpecialJsonByte(pos, end);
+        if (next != pos)
+        {
+            buf.append(pos, static_cast<size_t>(next - pos));
+            pos = next;
+            if (pos == end)
+                break;
+        }
+        const unsigned char c = static_cast<unsigned char>(*pos);
+        switch (c)
+        {
+            case '\b': append("\\b");   ++pos; break;
+            case '\f': append("\\f");   ++pos; break;
+            case '\n': append("\\n");   ++pos; break;
+            case '\r': append("\\r");   ++pos; break;
+            case '\t': append("\\t");   ++pos; break;
+            case '\\': append("\\\\");  ++pos; break;
+            case '"':  append("\\\"");  ++pos; break;
+            case 0xE2:
+                if (end - pos >= 3 && pos[1] == '\x80' && (pos[2] == '\xA8' || pos[2] == '\xA9'))
+                {
+                    append(pos[2] == '\xA8' ? std::string_view{"\\u2028"} : std::string_view{"\\u2029"});
+                    pos += 3;
+                }
+                else
+                {
+                    appendChar('\xE2');
+                    ++pos;
+                }
+                break;
+            default:
+            {
+                /// A control byte without a named escape: \u00XY with writeJSONString's exact
+                /// nibble rendering (uppercase A-F for the low nibble).
+                const unsigned char lower_half = c & 0xF;
+                append("\\u00");
+                appendChar(static_cast<char>('0' + (c >> 4)));
+                appendChar(static_cast<char>(lower_half <= 9 ? '0' + lower_half : 'A' + lower_half - 10));
+                ++pos;
+                break;
+            }
+        }
+    }
+    appendChar('"');
+}
+
+/// ---- read-side pull cursor ----
+
+/// A canonical-text parse failure is CORRUPTED_DATA regardless of which ReadHelpers primitive
+/// noticed it first; the primitives themselves throw a handful of parse-specific codes (assertion
+/// failure, quoted-string, number, EOF flavors). This is the single place that narrows all of them
+/// down to the two codes this format speaks: CORRUPTED_DATA (translated here) and
+/// UNKNOWN_FORMAT_VERSION (thrown deliberately by skipUnknown and passed through unchanged).
+template <typename F>
+auto JsonObjectReader::guarded(F && f)
+{
+    try
+    {
+        return f();
+    }
+    catch (const Exception & e)
+    {
+        if (e.code() == ErrorCodes::CORRUPTED_DATA || e.code() == ErrorCodes::UNKNOWN_FORMAT_VERSION)
+            throw;
+        if (e.code() == ErrorCodes::CANNOT_PARSE_INPUT_ASSERTION_FAILED
+            || e.code() == ErrorCodes::CANNOT_PARSE_QUOTED_STRING
+            || e.code() == ErrorCodes::CANNOT_PARSE_NUMBER
+            || e.code() == ErrorCodes::CANNOT_READ_ALL_DATA
+            || e.code() == ErrorCodes::ATTEMPT_TO_READ_AFTER_EOF
+            || e.code() == ErrorCodes::INCORRECT_DATA)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: {}", what, e.message());
+        throw;
+    }
+}
+
+JsonObjectReader::JsonObjectReader(ReadBuffer & in_, KeyStrictness strictness_, std::string_view what_)
+    : in(in_), strictness(strictness_), what(what_)
+{
+    guarded([&] { assertChar('{', in); });
+}
+
+bool JsonObjectReader::nextKey(String & key)
+{
+    return guarded([&]() -> bool
+    {
+        if (done)
+            return false;
+        if (first)
+        {
+            first = false;
+            if (checkChar('}', in))
+            {
+                done = true;
+                return false;
+            }
+        }
+        else
+        {
+            if (checkChar('}', in))
+            {
+                done = true;
+                return false;
+            }
+            assertChar(',', in);
+        }
+        readJSONString(key, in, jsonReadSettings());
+        assertChar(':', in);
+        if (std::find(seen_keys.begin(), seen_keys.end(), key) != seen_keys.end())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: duplicate key '{}'", what, key);
+        seen_keys.push_back(key);
+        return true;
+    });
+}
+
+String JsonObjectReader::readString()
+{
+    return guarded([&]
+    {
+        String s;
+        readJSONString(s, in, jsonReadSettings());
+        return s;
+    });
+}
+
+UInt128 JsonObjectReader::readHex128()
+{
+    return guarded([&]
+    {
+        const String hex = readString();
+        if (hex.size() != 32
+            || std::any_of(hex.begin(), hex.end(), [](char c) { return unhex(c) == 0xff || (c >= 'A' && c <= 'F'); }))
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: expected 32 lowercase hex chars, got '{}'", what, hex);
+        return unhexUInt<UInt128>(hex.data());
+    });
+}
+
+uint64_t JsonObjectReader::readU64String()
+{
+    return guarded([&]
+    {
+        const String s = readString();
+        ReadBufferFromMemory buf(s.data(), s.size());
+        uint64_t v = 0;
+        readIntText(v, buf);
+        if (s.empty() || !buf.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: expected decimal u64 string, got '{}'", what, s);
+        return v;
+    });
+}
+
+uint64_t JsonObjectReader::readU64Number()
+{
+    return guarded([&]
+    {
+        uint64_t v = 0;
+        readIntText(v, in);
+        return v;
+    });
+}
+
+uint32_t JsonObjectReader::readU32Number()
+{
+    const uint64_t v = readU64Number();
+    if (v > std::numeric_limits<uint32_t>::max())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: value out of uint32 range", what);
+    return static_cast<uint32_t>(v);
+}
+
+bool JsonObjectReader::readBool()
+{
+    return guarded([&]
+    {
+        if (checkString("true", in))
+            return true;
+        assertString("false", in);
+        return false;
+    });
+}
+
+void JsonObjectReader::skipUnknown(const String & key)
+{
+    guarded([&]
+    {
+        if (!key.empty() && key[0] == '!')
+            throw Exception(ErrorCodes::UNKNOWN_FORMAT_VERSION,
+                "CAS {}: critical key '{}' is not understood by this build", what, key);
+        if (strictness == KeyStrictness::Strict)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: unknown key '{}' in a strict format", what, key);
+        skipJSONField(in, key, jsonReadSettings());
+    });
+}
+
+/// ---- header line / trailer line / raw line access ----
+
+void writeHeaderLine(CasJsonWriter & out, FormatId id)
+{
+    const FormatTraits & t = traitsFor(id);
+    bool first = true;
+    writeKey(out, "type", first);
+    writeStringValue(out, t.type);
+    writeKey(out, "v", first);
+    writeIntText(currentCompatibilityVersion(), out);
+    closeObject(out, first);
+    writeChar('\n', out);
+}
+
+void writeTrailerLine(CasJsonWriter & out, uint64_t n)
+{
+    bool first = true;
+    writeKey(out, "n", first);
+    writeIntText(n, out);
+    closeObject(out, first);
+    writeChar('\n', out);
+}
+
+String readLine(ReadBuffer & in, uint64_t line_cap, std::string_view what)
+{
+    String line;
+    while (true)
+    {
+        if (in.eof())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: truncated object (line without terminator)", what);
+        const char c = *in.position();
+        ++in.position();
+        if (c == '\n')
+            return line;
+        line.push_back(c);
+        if (line.size() > line_cap)
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: line exceeds the {}-byte cap", what, line_cap);
+    }
+}
+
+namespace
+{
+TextHeader parseHeaderObject(std::string_view line, std::string_view what)
+{
+    ReadBufferFromMemory buf(line.data(), line.size());
+    JsonObjectReader r(buf, KeyStrictness::Tolerant, what);
+    String key;
+    if (!r.nextKey(key) || key != "type")
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: header line must start with \"type\"", what);
+    TextHeader h;
+    h.type = r.readString();
+    if (!r.nextKey(key) || key != "v")
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: header line must carry \"v\" second", what);
+    h.v = r.readU32Number();
+    while (r.nextKey(key))
+        r.skipUnknown(key);
+    if (!buf.eof())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: junk after the header object", what);
+    return h;
+}
+}
+
+TextHeader expectHeaderLine(ReadBuffer & in, FormatId id)
+{
+    const FormatTraits & t = traitsFor(id);
+    const String line = readLine(in, t.line_cap, t.type);
+    const TextHeader h = parseHeaderObject(line, t.type);
+    if (h.type != t.type)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: object is a '{}', not a '{}'", t.type, h.type, t.type);
+    checkCompatibility(h.v, t.type);
+    return h;
+}
+
+std::optional<TextHeader> sniffHeaderLine(std::string_view bytes)
+{
+    constexpr uint64_t kSniffLineCap = 64 * 1024;
+    try
+    {
+        ReadBufferFromMemory buf(bytes.data(), bytes.size());
+        const String line = readLine(buf, kSniffLineCap, "sniff");
+        TextHeader h = parseHeaderObject(line, "sniff");
+        if (traitsForType(h.type) == nullptr)
+            return std::nullopt;
+        return h;
+    }
+    catch (const Exception &)
+    {
+        return std::nullopt;
+    }
+}
+
+/// ---- the zstd arm ----
+
+bool looksZstd(std::string_view bytes)
+{
+    static constexpr char kZstdFramePrefix[4] = {'\x28', '\xB5', '\x2F', '\xFD'};
+    return bytes.size() >= 4 && memcmp(bytes.data(), kZstdFramePrefix, 4) == 0;
+}
+
+namespace
+{
+constexpr int kZstdLevel = 3;
+}
+
+String sealObject(FormatId id, String text)
+{
+    const FormatTraits & t = traitsFor(id);
+    if (t.compression != CompressionPolicy::Always)
+        return text;
+
+    ZSTD_CCtx * cctx = ZSTD_createCCtx();
+    if (cctx == nullptr)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: cannot create zstd context", t.type);
+    SCOPE_EXIT({ ZSTD_freeCCtx(cctx); });
+    ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, kZstdLevel);
+    ZSTD_CCtx_setParameter(cctx, ZSTD_c_checksumFlag, 1);
+
+    String out;
+    out.resize(ZSTD_compressBound(text.size()));
+    const size_t written = ZSTD_compress2(cctx, out.data(), out.size(), text.data(), text.size());
+    if (ZSTD_isError(written))
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: zstd compression failed: {}", t.type, ZSTD_getErrorName(written));
+    out.resize(written);
+    return out;
+}
+
+String openObject(FormatId id, std::string_view stored)
+{
+    const FormatTraits & t = traitsFor(id);
+    if (!looksZstd(stored))
+    {
+        if (t.object_cap != 0 && stored.size() > t.object_cap)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS {}: raw object size {} exceeds the {}-byte cap", t.type, stored.size(), t.object_cap);
+        return String(stored);
+    }
+    if (t.compression != CompressionPolicy::Always)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS {}: compressed object in a format whose policy is raw", t.type);
+
+    const uint64_t content = ZSTD_getFrameContentSize(stored.data(), stored.size());
+    if (content == ZSTD_CONTENTSIZE_UNKNOWN || content == ZSTD_CONTENTSIZE_ERROR)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: zstd frame without a declared content size", t.type);
+    if (t.object_cap != 0 && content > t.object_cap)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS {}: declared decompressed size {} exceeds the {}-byte cap", t.type, content, t.object_cap);
+
+    String out;
+    out.resize(content);
+    const size_t got = ZSTD_decompress(out.data(), out.size(), stored.data(), stored.size());
+    if (ZSTD_isError(got) || got != content)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: zstd decompression failed: {}",
+            t.type, ZSTD_isError(got) ? ZSTD_getErrorName(got) : "short output");
+    return out;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h
new file mode 100644
index 000000000000..50edd33bd4ba
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h
@@ -0,0 +1,241 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <IO/ReadBuffer.h>
+#include <base/hex.h>
+#include <base/itoa.h>
+#include <optional>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Shared container mechanics for versioned content-addressed text objects: a header line
+/// {"type":"cas_<x>","v":N}, a body, an optional trailer line, and, for formats whose registry
+/// policy requires it, one zstd frame around the whole object. This is the only code that knows
+/// that container shape; per-object codecs add only key mappings and object-specific invariants.
+///
+/// Writers produce canonical text without whitespace outside JSON strings, and readers reject such
+/// whitespace as `CORRUPTED_DATA`. Values that may span the full u64 range are decimal strings;
+/// hashes are 32-character lowercase hexadecimal strings. The JSON writer settings are pinned in
+/// the implementation so global `FormatSettings` changes cannot alter CAS bytes: slash-containing
+/// ref paths and deterministic artifacts must retain the same representation and golden files.
+
+/// Bulk-append writer for canonical CAS JSON text. Replaces WriteBuffer in every CAS encode
+/// path: appends are inline stores into an owned String (no per-call finalized/canceled
+/// lifecycle, no per-byte writes, no heap allocations per record). Two usage modes:
+/// whole-object assembly (bounded formats; `take` at the end) and line-scratch (RecordStream:
+/// assemble one line, bulk-write it to the surrounding WriteBuffer, `clear` — memory stays
+/// bounded by the largest line). The JSON escaping semantics of `stringValue` are statically
+/// fixed to the CAS canon (forward slashes NOT escaped); process-wide FormatSettings cannot
+/// influence CAS bytes.
+class CasJsonWriter
+{
+public:
+    explicit CasJsonWriter(size_t reserve_hint = 256)
+    {
+        buf.reserve(reserve_hint);
+    }
+
+    void append(std::string_view s)
+    {
+        buf.append(s.data(), s.size());
+    }
+
+    void appendChar(char c)
+    {
+        buf.push_back(c);
+    }
+
+    /// '{' on the first call, ',' after, then "name": . `name` must be plain ASCII (written raw).
+    void key(std::string_view name, bool & first)
+    {
+        appendChar(first ? '{' : ',');
+        first = false;
+        appendChar('"');
+        append(name);
+        append("\":");
+    }
+
+    /// Same, for the prefixed key vocabulary ("o"/"n" + "me"/"mb"/"mo"/"bk"/"rn") — the
+    /// prefix and name are appended back to back, no composed temporary.
+    void key(std::string_view prefix, std::string_view name, bool & first)
+    {
+        appendChar(first ? '{' : ',');
+        first = false;
+        appendChar('"');
+        append(prefix);
+        append(name);
+        append("\":");
+    }
+
+    /// Quoted JSON string with full escaping (bulk-run scan). Defined in CasTextFormat.cpp.
+    void stringValue(std::string_view s);
+
+    void u64Number(uint64_t v)
+    {
+        char digits[24];
+        char * end = itoa(v, digits);
+        buf.append(digits, static_cast<size_t>(end - digits));
+    }
+
+    void u64StringValue(uint64_t v)
+    {
+        appendChar('"');
+        u64Number(v);
+        appendChar('"');
+    }
+
+    void hex128Value(const UInt128 & v)
+    {
+        char hex[32];
+        writeHexUIntLowercase(v, hex);
+        appendChar('"');
+        buf.append(hex, sizeof(hex));
+        appendChar('"');
+    }
+
+    void boolValue(bool v)
+    {
+        append(v ? std::string_view{"true"} : std::string_view{"false"});
+    }
+
+    void closeObject(bool & first)
+    {
+        if (first)
+            appendChar('{');
+        first = false;
+        appendChar('}');
+    }
+
+    void newline()
+    {
+        appendChar('\n');
+    }
+
+    size_t size() const
+    {
+        return buf.size();
+    }
+
+    std::string_view view() const
+    {
+        return buf;
+    }
+
+    void clear()
+    {
+        buf.clear();
+    }
+
+    String take() &&
+    {
+        return std::move(buf);
+    }
+
+private:
+    String buf;
+};
+
+/// The write-side JSON primitives used by the format codecs. `CasJsonWriter` is the only CAS
+/// text writer; every codec assembles its object in one before handing bytes to the underlying
+/// `WriteBuffer`.
+inline void writeKey(CasJsonWriter & out, std::string_view key, bool & first) { out.key(key, first); }
+inline void writeStringValue(CasJsonWriter & out, std::string_view s) { out.stringValue(s); }
+inline void writeHex128Value(CasJsonWriter & out, const UInt128 & v) { out.hex128Value(v); }
+inline void writeU64StringValue(CasJsonWriter & out, uint64_t v) { out.u64StringValue(v); }
+inline void writeBoolValue(CasJsonWriter & out, bool v) { out.boolValue(v); }
+inline void closeObject(CasJsonWriter & out, bool & first) { out.closeObject(first); }
+/// Argument order mirrors the IO helpers so migrated codecs keep their call shapes.
+inline void writeChar(char c, CasJsonWriter & out) { out.appendChar(c); }
+inline void writeIntText(uint64_t v, CasJsonWriter & out) { out.u64Number(v); }
+void writeHeaderLine(CasJsonWriter & out, FormatId id);
+void writeTrailerLine(CasJsonWriter & out, uint64_t n);
+
+/// Pull cursor over one canonical JSON object.
+///
+/// The reader borrows the input buffer and records the object name for exception messages. It
+/// enforces unique keys and translates the several low-level parser exceptions into the CAS
+/// `CORRUPTED_DATA` contract. Unknown keys follow the supplied evolution policy: ordinary keys
+/// may be skipped in tolerant objects, while `!`-prefixed keys always fail with
+/// `UNKNOWN_FORMAT_VERSION`.
+
+class JsonObjectReader
+{
+public:
+    /// Consumes the opening `{`; throws `CORRUPTED_DATA` when the object does not start there.
+    JsonObjectReader(ReadBuffer & in_, KeyStrictness strictness_, std::string_view what_);
+    /// Advances to the next key; false when the closing '}' was consumed. The caller must
+    /// consume the value (one read* / skipUnknown) before the next call. Duplicate keys are
+    /// rejected with `CORRUPTED_DATA`.
+    bool nextKey(String & key);
+    /// Reads the value for the key returned by `nextKey` as a JSON string.
+    String readString();
+    /// Reads a quoted 32-character lowercase hexadecimal string as a `UInt128`.
+    UInt128 readHex128();
+    /// Reads a quoted decimal u64 string and rejects empty, trailing, or non-decimal text.
+    uint64_t readU64String();
+    /// Reads an unquoted JSON number into a u64; low-level parse failures become `CORRUPTED_DATA`.
+    uint64_t readU64Number();
+    /// Reads an unquoted JSON number into a u32; rejects a value that would silently narrow.
+    uint32_t readU32Number();
+    /// Reads the bare JSON literals `true` and `false`.
+    bool readBool();
+    /// Applies the evolution rule for an unrecognized key: `!`-prefixed keys produce
+    /// `UNKNOWN_FORMAT_VERSION`; strict objects produce `CORRUPTED_DATA`; tolerant objects skip
+    /// the value.
+    void skipUnknown(const String & key);
+
+private:
+    /// Runs one parser operation under the CAS error taxonomy while preserving version exceptions.
+    template <typename F>
+    auto guarded(F && f);
+
+    ReadBuffer & in;
+    KeyStrictness strictness;
+    String what;
+    std::vector<String> seen_keys;
+    bool first = true;
+    bool done = false;
+};
+
+/// Header, trailer, and raw-line access for the common text container.
+
+/// Header metadata returned after parsing a self-describing CAS object header line.
+struct TextHeader
+{
+    String type;
+    uint32_t v = 0;
+};
+
+/// Reads and gates line 1 against `id`'s registered type; wrong type -> CORRUPTED_DATA; v above
+/// what this build understands -> UNKNOWN_FORMAT_VERSION.
+TextHeader expectHeaderLine(ReadBuffer & in, FormatId id);
+/// Best-effort "is this a CAS object, and which one" for fsck/dispatch: swallows every failure and
+/// returns nullopt. Never the load-bearing gate — that is expectHeaderLine.
+std::optional<TextHeader> sniffHeaderLine(std::string_view bytes);
+/// Reads one line (excluding the '\n' terminator); CORRUPTED_DATA on missing terminator or a line
+/// longer than `line_cap`.
+String readLine(ReadBuffer & in, uint64_t line_cap, std::string_view what);
+
+/// Position of the next byte `stringValue` treats specially (control byte, '"', '\\', or the
+/// 0xE2 lead byte of the U+2028/U+2029 lookahead), or `end`. Scalar bulk-run scan: the win is on
+/// the short hot strings, so SIMD is deferred to a benchmark-gated contingency.
+const char * findNextSpecialJsonByte(const char * pos, const char * end);
+
+/// Zstd detection and per-format compression policy.
+
+/// True iff `bytes` starts with the zstd frame prefix 28 B5 2F FD.
+bool looksZstd(std::string_view bytes);
+/// Compression per the per-type policy: `Always` -> one zstd frame (any size, checksum on);
+/// everything else -> identity (returns `text` unchanged).
+String sealObject(FormatId id, String text);
+/// Inverse of sealObject. A compressed body is only legal when `id`'s policy is `Always`
+/// (declared content size checked against the cap before allocation); a raw body is accepted
+/// (repair path — e.g. an operator-restored uncompressed copy) subject to the SAME `object_cap` --
+/// skipping compression must never also skip the size cap.
+String openObject(FormatId id, std::string_view stored);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.cpp
new file mode 100644
index 000000000000..31d44f260121
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.cpp
@@ -0,0 +1,103 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Common/Exception.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+std::string_view tokenTypeToWord(TokenType t)
+{
+    switch (t)
+    {
+        case TokenType::ETag:       return "etag";
+        case TokenType::Generation: return "generation";
+        case TokenType::Emulated:   return "emulated";
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS wire: unknown TokenType {}", static_cast<int>(t));
+}
+
+TokenType tokenTypeFromWord(std::string_view w, std::string_view what)
+{
+    if (w == "etag")       return TokenType::ETag;
+    if (w == "generation") return TokenType::Generation;
+    if (w == "emulated")   return TokenType::Emulated;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: unknown token type '{}'", what, w);
+}
+
+BlobHashAlgo blobHashAlgoFromWord(std::string_view w, std::string_view what)
+{
+    if (w == "ch128")  return BlobHashAlgo::CityHash128;
+    if (w == "xxh3")   return BlobHashAlgo::XXH3_128;
+    if (w == "sha256") return BlobHashAlgo::Sha256;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: unknown blob hash algo '{}'", what, w);
+}
+
+std::string_view objectKindToWord(ObjectKind k)
+{
+    switch (k)
+    {
+        case ObjectKind::Blob: return "blob";
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS wire: unknown ObjectKind {}", static_cast<int>(k));
+}
+
+ObjectKind objectKindFromWord(std::string_view w, std::string_view what)
+{
+    if (w == "blob") return ObjectKind::Blob;
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS {}: unknown object kind '{}'", what, w);
+}
+
+void writeTokenFields(CasJsonWriter & out, bool & first, const Token & t)
+{
+    writeKey(out, "tt", first);
+    writeStringValue(out, tokenTypeToWord(t.type));
+    writeKey(out, "tv", first);
+    writeStringValue(out, t.value);
+}
+
+void writeBlobRefFields(CasJsonWriter & out, bool & first, const BlobRef & r)
+{
+    writeKey(out, "ha", first);
+    writeStringValue(out, blobHashAlgoName(r.algo));
+    writeKey(out, "h", first);
+    writeStringValue(out, codecFor(r.algo).toHex(r.digest));
+}
+
+void writeManifestRefFields(CasJsonWriter & out, bool & first, std::string_view prefix, const ManifestRef & r)
+{
+    /// Unlike the WriteBuffer overload, the two-part key() form appends the prefix and name back
+    /// to back with no composed String(prefix) + "..." temporary.
+    out.key(prefix, "me", first);
+    out.u64StringValue(r.writer_epoch);
+    out.key(prefix, "mb", first);
+    out.u64StringValue(r.build_sequence);
+    out.key(prefix, "mo", first);
+    out.u64Number(r.manifest_ordinal);
+}
+
+ManifestRef manifestRefFromFields(uint64_t writer_epoch, uint64_t build_sequence, uint64_t manifest_ordinal,
+                                  std::string_view caller, std::string_view what)
+{
+    /// Check the upper bound before narrowing the caller-supplied value to the in-memory ordinal
+    /// type. `checkManifestRef` then applies the shared nonzero and lower-bound checks.
+    if (manifest_ordinal > kMaxManifestOrdinal)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS {}: {} manifest_ordinal {} out of range", caller, what, manifest_ordinal);
+    ManifestRef r;
+    r.writer_epoch = writer_epoch;
+    r.build_sequence = build_sequence;
+    r.manifest_ordinal = static_cast<uint32_t>(manifest_ordinal);
+    checkManifestRef(r, caller, what);
+    return r;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h
new file mode 100644
index 000000000000..8727e6e219b6
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h
@@ -0,0 +1,62 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <cstdint>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// Shared JSON vocabulary for the value sub-types embedded by the CAS text codecs. These helpers
+/// keep the same sub-object key names and full-word enum values across outcome logs, record streams,
+/// ref logs, ref snapshots, part manifests, and blob envelopes. Every reverse map rejects an
+/// unrecognized value with `CORRUPTED_DATA`; silently choosing a default would turn malformed
+/// persisted data into a different valid-looking record.
+
+/// Convert a token discriminator to its canonical wire word. Throws `CORRUPTED_DATA` if `t` is not
+/// one of the token types understood by this build.
+std::string_view tokenTypeToWord(TokenType t);
+
+/// Parse a canonical token-type word. `what` identifies the containing codec or field in the
+/// `CORRUPTED_DATA` exception; unknown words are rejected rather than treated as a default type.
+TokenType tokenTypeFromWord(std::string_view w, std::string_view what);
+
+/// Parse a canonical blob-hash algorithm word. The write side uses `blobHashAlgoName` directly, so
+/// this is its fail-closed inverse. `what` identifies the containing codec or field in the
+/// `CORRUPTED_DATA` exception.
+BlobHashAlgo blobHashAlgoFromWord(std::string_view w, std::string_view what);
+
+/// Convert an envelope object-kind discriminator to its canonical wire word. Throws
+/// `CORRUPTED_DATA` if `k` is not represented by this format.
+std::string_view objectKindToWord(ObjectKind k);
+
+/// Parse a canonical envelope object-kind word. `what` identifies the containing codec or field in
+/// the `CORRUPTED_DATA` exception; unknown words are rejected rather than treated as a default kind.
+ObjectKind objectKindFromWord(std::string_view w, std::string_view what);
+
+/// Append the sibling fields `tt` and `tv` to an in-progress JSON object. The caller owns `first`,
+/// which must describe the fields already written to that object; the token value is JSON-escaped.
+void writeTokenFields(CasJsonWriter & out, bool & first, const Token & t);
+
+/// Append the sibling fields `ha` and `h` to an in-progress JSON object. The algorithm word and
+/// lowercase digest are canonical, and the digest is rendered at the width required by `r.algo`.
+void writeBlobRefFields(CasJsonWriter & out, bool & first, const BlobRef & r);
+
+/// Append the three flat `ManifestRef` fields `me`, `mb`, and `mo` to an in-progress JSON object.
+/// `prefix` is prepended to each key, allowing the ref codecs to distinguish old and new owner
+/// bindings (`ome`/`omb`/`omo` and `nme`/`nmb`/`nmo`) while part manifests and ordinary rows use an
+/// empty prefix. The two unbounded `uint64_t` values are decimal JSON strings; the bounded ordinal
+/// is a JSON number. All consumers use this exact spelling and representation.
+void writeManifestRefFields(CasJsonWriter & out, bool & first, std::string_view prefix, const ManifestRef & r);
+
+/// Construct a `ManifestRef` from decoded field values and validate the complete domain range:
+/// nonzero `writer_epoch` and `build_sequence`, and `manifest_ordinal` in
+/// `[1, kMaxManifestOrdinal]`. The upper bound is checked before narrowing to the in-memory
+/// `uint32_t` ordinal. `caller` and `what` identify the codec and field in `CORRUPTED_DATA`
+/// exceptions.
+ManifestRef manifestRefFromFields(uint64_t writer_epoch, uint64_t build_sequence, uint64_t manifest_ordinal,
+                                  std::string_view caller, std::string_view what);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/README.md b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/README.md
new file mode 100644
index 000000000000..abd32c43b499
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/README.md
@@ -0,0 +1,63 @@
+# CAS persisted formats — the living registry
+
+Every persisted CAS object is a text file: header line `{"type":"cas_<object>","v":N}`, body
+(one JSON object / sorted NDJSON records / raw payload zone), optional `{"n":…}` trailer.
+Can-grow-large types are stored under a **`.zst` key suffix** and are ALWAYS one zstd frame
+(checksum on; declared content size checked against the cap before allocation); always-small and
+deterministic types are raw. `CasTextFormat.{h,cpp}` is the only code that knows this shape.
+
+The object inventory is text end to end — there are no binary CAS formats and no protobuf
+dependency. The GC source-edge data plane (`cas_run`) is sorted NDJSON written and read as a
+stream (no seek); its integrity check is the whole-file seal checksum. The part manifest is the
+one `PayloadHybrid` object: text header + descriptor meta + sorted entry records + `{"n":…}`
+trailer, followed by a banner-framed raw payload zone for inline file bytes.
+
+**Rule:** any change to a persisted format lands in the SAME commit as its row here.
+
+## Bucket map
+
+| Key (under the pool prefix) | Object | Codec | Writer |
+|---|---|---|---|
+| `_pool_meta` | pool identity + floors | `CasPoolMetaFormat` | pool create/admit |
+| `cas/ns/stream/<life_id>/_log/…​.zst` | ref transaction log | `CasRefLogFormat` (`.zst`) | writer commit path |
+| `cas/ns/stream/<life_id>/_snap/…​.zst` | complete ref table | `CasRefSnapshotFormat` (`.zst`) | writer/GC fold |
+| `cas/ns/state/<life_id>/_ckpt` | mutable life checkpoint | `CasRefCkptFormat` | writer/GC fold |
+| `cas/ns/state/<life_id>/_files/…​` | namespace-owned raw files | — | upper layers |
+| `cas/manifests/<ns>/<epoch-hex>-<seq-hex>/<ordinal>.zst` | part manifest | `CasPartManifestFormat` | part build |
+| blob keys (`CasLayout::blobKey`) | blob envelope + payload | `CasBlobEnvelopeFormat` | uploads |
+| blob-meta keys (`CasLayout::blobMetaKey`) | freshness sidecar | `CasBlobMetaFormat` | dedup/GC |
+| `gc/state`, `gc/hb` | GC state / leader heartbeat | `CasGcStateFormat` | GC |
+| `gc/maintenance_state` | leak-only namespace-janitor cursor | `CasGcMaintenanceStateFormat` | future janitor |
+| `gc/gen/<g>/attempt/<a>/outcomes/…​.zst` | outcome log | `CasGcOutcomesFormat` (`.zst`) | GC |
+| `gc/gen/<g>/attempt/<a>/fold_seal` | fold seal (deterministic) | `CasFoldSealFormat` | GC |
+| `gc/gen/<g>/…​/runs` | GC source-edge record-stream runs | `CasRecordStreamFormat` | GC |
+| `gc/server-roots/<srid>/{owner,epoch,mount}` | server-root singletons | `CasServerRootFormats` | mount |
+| `roots/…` | raw passthrough (verbatim) | — (never interpreted) | upper layers |
+
+## Codec table
+
+Authoritative per-format traits (type string, family, strictness, compression policy, caps) live
+in `CasFormat.cpp` (`TRAITS`), asserted complete by `gtest_cas_text_format.cpp`. Key naming: keys
+2–5 chars; fixed-width `UInt128` identities = 32-char lowercase hex strings; blob digests =
+algo-width hex (two chars per digest byte), rendered with their algo name (`sha256:ab12…`) wherever
+a bare hex would be ambiguous; unbounded u64 = decimal strings; bounded counts/lengths/ms-timestamps
+= numbers; units documented here per object as codecs land.
+
+## Evolution rules (one screen)
+
+- `v` (header line) is the ONLY version field; reader gate: `v > G_BUILD` →
+  `UNKNOWN_FORMAT_VERSION`, checked before the body.
+- Additive change = new tolerant key, no `v` bump; on MUTABLE objects the field is best-effort
+  until the pool floor rises (an old writer's fresh re-encode drops it).
+- Breaking change = `v` bump + `changePoints` + write-down-to-floor; the floor raise is what
+  fences old builds out (mount gates: `min_reader_generation` forward, pool-meta `v` backward).
+- Deterministic formats (`cas_fold_seal`, `cas_run`): strict keys, pinned raw, and the adoption
+  pin — on a `putDeterministicArtifact` conflict, re-encode at the `v` of the EXISTING object.
+- A key prefixed `!` is critical: a reader that does not understand it fails closed.
+- Padding zones (blob header pad, manifest banners) are deterministic and verified — no
+  unaccounted bytes in any object.
+- `openObject` policy asymmetry: a compressed body under a raw-compression policy is rejected
+  (`CORRUPTED_DATA`), but a NON-zstd body under an `Always` policy is passed through verbatim as
+  canonical text — an intentional uncompressed-repair path, not an "`Always` ⇒ must be compressed"
+  enforcement. In practice the mismatch never arises: `Always` objects are read via a constructed
+  `.zst`-suffixed key, so a raw body is not GETtable at that key.

From ef2448c0336bd5432dde6fd145527fae25fb3e11 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:34 +0200
Subject: [PATCH 15/30] CAS subsystem: Backend layer

Object-storage access layer: the backend interface, the object-storage
adapter, the in-memory backend for tests, capability probing and request
control.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../ContentAddressed/Backend/CasBackend.h     |  438 ++++++
 .../Backend/CasInMemoryBackend.cpp            |  426 ++++++
 .../Backend/CasInMemoryBackend.h              |  172 +++
 .../Backend/CasInstrumentedBackend.cpp        |  176 +++
 .../Backend/CasInstrumentedBackend.h          |  202 +++
 .../Backend/CasObjectStorageBackend.cpp       | 1209 +++++++++++++++++
 .../Backend/CasObjectStorageBackend.h         |  249 ++++
 .../ContentAddressed/Backend/CasProbe.cpp     |  320 +++++
 .../ContentAddressed/Backend/CasProbe.h       |   62 +
 .../Backend/CasRequestControl.cpp             |  745 ++++++++++
 .../Backend/CasRequestControl.h               |  621 +++++++++
 .../Backend/CasSentinelProbe.cpp              |  110 ++
 .../Backend/CasSentinelProbe.h                |   55 +
 13 files changed, 4785 insertions(+)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.h

diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h
new file mode 100644
index 000000000000..3b284a452b17
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h
@@ -0,0 +1,438 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <IO/ReadBuffer.h>
+#include <IO/WriteBuffer.h>
+#include <Common/Exception.h>
+#include <base/types.h>
+#include <functional>
+#include <map>
+#include <memory>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <vector>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int NOT_IMPLEMENTED;
+}
+}
+
+namespace DB::Cas
+{
+
+/// User metadata carried alongside an object (S3 x-amz-meta-*). The CA store uses exactly one entry,
+/// "cas_owner" = "<server_id_hex>:<epoch>:<build_seq>" — the owner triple the GC watermark reads.
+using ObjectMeta = std::map<String, String>;
+
+/// A byte window requested from an object. An absent length means that the window extends to EOF.
+/// Backends use the same semantics for materialized and forward-only reads: the offset is exact,
+/// while a backend may expose an advisory end when its underlying read buffer cannot enforce one.
+struct Range
+{
+    uint64_t offset = 0;
+    std::optional<uint64_t> length;   /// nullopt => to the end
+    bool whole() const { return offset == 0 && !length; }
+};
+
+/// Materialized object bytes together with the incarnation and user metadata observed by the read.
+/// The token identifies the exact object version whose bytes are in `bytes`; callers may use it to
+/// validate a subsequent token-conditional mutation.
+struct GetResult
+{
+    String bytes;
+    Token token;       /// token of the incarnation the bytes came from
+    ObjectMeta attributes;
+};
+
+/// A forward-only read of a WRITE-ONCE object (runs, seals): nothing is materialized by the seam.
+/// MUTABLE objects (root shards, gc/state, mounts) MUST keep using `get` — their bytes may change
+/// under an open stream. `token` identifies the incarnation the stream reads, same as `get`.
+struct GetStreamResult
+{
+    std::unique_ptr<ReadBuffer> stream;
+    Token token;
+};
+
+/// Metadata returned by `Backend::head`. For an absent key, `exists` is false and the other fields
+/// retain their defaults; for a present key, `size`, `token`, and `attributes` describe one current
+/// incarnation as observed by the backend.
+struct HeadResult
+{
+    bool exists = false;
+    uint64_t size = 0;
+    Token token;
+    ObjectMeta attributes;
+};
+
+/// Outcome of a write-once create or a token-conditional overwrite. A precondition failure means
+/// that the backend preserved the existing object; it is an expected result, not an exception.
+enum class PutOutcome : uint8_t
+{
+    Done,                 /// object written; the returned PutResult.token is the new incarnation's token
+    PreconditionFailed,   /// If-None-Match hit an existing key / If-Match mismatched — nothing changed
+};
+
+/// Outcome of a compare-and-set write. `Conflict` means that the expected token (or expected
+/// absence) did not match and that the backend left the object unchanged.
+enum class CasOutcome : uint8_t
+{
+    Committed,
+    Conflict,             /// expected token (or absence) did not match — nothing changed
+};
+
+/// Result of a backend write: the outcome plus the resulting object token (previously a `Token * out_token`
+/// out-parameter). `token` is set ONLY when the write actually landed an incarnation (a `Done`/`Committed`
+/// outcome); on `PreconditionFailed`/`Conflict` nothing was written and `token` is left default-constructed,
+/// exactly mirroring the old contract where callers only read `*out_token` on success.
+template <typename Outcome>
+struct WriteResultT
+{
+    Outcome outcome;
+    Token token;
+};
+
+using PutResult = WriteResultT<PutOutcome>;
+using CasResult = WriteResultT<CasOutcome>;
+
+/// Result of deleting one exact incarnation. `TokenMismatch` and `NotFound` are deliberately
+/// distinct: the former proves that another incarnation is now current, while the latter means
+/// there is no object to remove. `created_delete_marker` exposes a storage-versioning behavior
+/// that is incompatible with current-object reclamation.
+struct DeleteOutcome
+{
+    enum class Kind : uint8_t { Deleted, TokenMismatch, NotFound } kind = Kind::NotFound;
+    /// TRUE if the backend reported a delete marker was created because versioning is enabled. The
+    /// capability probe rejects this for the current-object storage model: exact deletion must reclaim
+    /// the current object rather than archive a noncurrent version.
+    bool created_delete_marker = false;
+};
+
+/// A key returned by `Backend::list`. The `token` field is populated ONLY when the backend
+/// returns TRUE from `supportsListTokens` — it identifies the key's current incarnation, matching
+/// what `head` would return for the same key at that instant. Callers that do not need the token
+/// (e.g. GC fence sweep, orphan sweep) ignore the field; GC discover uses it to skip unchanged
+/// root shards.
+struct ListedKey
+{
+    String key;
+    uint64_t size = 0;
+    std::optional<Token> token;   /// present iff supportsListTokens() == true
+};
+/// One page returned by `Backend::list`. `keys` contains only the requested prefix and the cursor
+/// resumes strictly after the last returned key; an empty cursor marks the end of the enumeration.
+struct ListPage
+{
+    std::vector<ListedKey> keys;
+    String next_cursor;       /// Last returned key; empty => no more pages.
+};
+
+/// Typed erasure evidence for one key or one prefix. `head`/
+/// `get` deliberately flatten every kind of miss (a clean absence, a missing bucket/container, a
+/// permission failure, a transport fault) into one "not found" result, which is exactly right for
+/// their callers (a plain read) but wrong for lifecycle recovery, which must never treat a
+/// transport/permission failure as proof that data is gone. `ProbeOutcome` keeps the four cases
+/// distinct: only a backend's OWN authoritative "not found" evidence earns `KeyAbsent` — a timeout,
+/// a 5xx, or an unclassifiable error is ALWAYS `Indeterminate`, never promoted to absence.
+enum class ProbeOutcome : uint8_t
+{
+    Present,           /// the key (or, for a prefix probe, at least one object under it) exists
+    KeyAbsent,         /// authoritative miss: the container is alive, the key itself is not there
+    ContainerAbsent,   /// the bucket/prefix-parent itself is gone, not merely the key
+    AccessDenied,      /// the probe was rejected on permissions — absence was never established
+    Indeterminate,     /// a transport/timeout/unclassifiable error — absence was NEVER proven
+};
+
+/// Result of `Backend::probeSentinelRaw`. `body` carries the materialized bytes only when the outcome is
+/// `Present`.
+struct SentinelProbeResult
+{
+    ProbeOutcome outcome;
+    std::optional<String> body;
+};
+
+/// Streaming conditional create (If-None-Match:* semantics). The caller writes the FULL object body
+/// (envelope header + payload) into buffer, then calls finalize exactly once:
+///   - Done                ⇒ the object is durable; the returned PutResult.token is the new incarnation's token
+///   - PreconditionFailed  ⇒ the key already existed — NOTHING was changed (same contract as putIfAbsent)
+/// finalize may throw on storage errors; PreconditionFailed is an OUTCOME, never an exception.
+/// cancel (or destruction before finalize) abandons the upload: the key is never created by it.
+///
+/// MISUSE/LIFETIME CONTRACT: after finalize or cancel the sink is DEAD — any further finalize,
+/// cancel, or write into buffer is a programming error (finalize asserts on it in debug builds).
+/// The caller must not call the underlying buffer's own finalize/cancel directly — only through
+/// the sink. A sink is single-caller: it is NOT thread-safe (only Backend itself is), and it must
+/// not outlive the Backend that created it.
+class WriteSink
+{
+public:
+    virtual ~WriteSink() = default;
+    virtual WriteBuffer & buffer() = 0;
+    virtual PutResult finalize() = 0;
+    virtual void cancel() noexcept = 0;
+};
+
+using WriteSinkPtr = std::unique_ptr<WriteSink>;
+
+/// Token-aware storage seam used by the content-addressed pool. TOKEN SEMANTICS ARE THE CONTRACT:
+///   - every present key has exactly one current incarnation identified by an opaque Token;
+///   - putOverwrite/casPut succeed only against the expected current token (or expected absence);
+///   - deleteExact removes ONLY the incarnation whose token matches — wrong token MUST be a
+///     TokenMismatch with the object untouched (backends that silently ignore the condition are
+///     rejected by `Cas::Probe`);
+///   - conditional PUTs are protocol hygiene; casPut and deleteExact are SAFETY-critical.
+///
+/// TOKEN ⟹ CONTENT PRECONDITION (read-path caches depend on this): a token must uniquely identify
+/// the byte-content of the incarnation it labels — i.e. `head(k).token == prior get(k).token` MUST
+/// imply the bytes are unchanged. The protocol's SAFETY only needs the contrapositive (changed
+/// bytes ⟹ a new token, so a stale CAS/delete is rejected), but `Cas::Pool`'s read-path decode
+/// cache (`readShardDecoded`) skips a re-`get`+decode on a token match, so a backend whose token
+/// could REPEAT across different content would make it serve stale manifests (wrong results). Holds
+/// for every backend in use: S3 ETag is content-derived; the emulated/in-memory backends mint a
+/// strictly-monotonic sequence that is never reused. A backend with a weak/recycled token must NOT
+/// be used as a Cas pool. The capability probe currently verifies conditional-operation behavior but
+/// does not test token non-reuse across different contents, so this invariant remains a requirement
+/// of every backend implementation.
+///
+/// Most ops take/return whole `String` bodies — sufficient for manifests, trees, and probe/GC
+/// objects. LARGE content blobs stream through `putIfAbsentStream` (see `WriteSink`); reads stay
+/// String-based because blob payload reads go through the wiring's read stack, not this seam.
+class Backend
+{
+public:
+    virtual ~Backend() = default;
+
+    /// Reads the selected bytes and their token, or returns nullopt when the key is absent. For a
+    /// mutable object, callers must use this materialized form so the body is fixed before parsing.
+    virtual std::optional<GetResult> get(const String & key, Range range) = 0;
+    std::optional<GetResult> get(const String & key) { return get(key, {}); }
+
+    /// Forward-only stream over the object's `range` (default: whole object) for WRITE-ONCE objects
+    /// (runs, seals). The returned `stream` yields exactly the window's bytes and nothing is
+    /// materialized whole by the seam — the caller reads at its own pace. MUTABLE objects (root
+    /// shards, gc/state, mounts) MUST keep using `get`: their bytes can change under an open stream.
+    /// CAVEAT: the window END is advisory on storages where `setReadUntilPosition` is a hint
+    /// (LocalObjectStorage) — the stream may yield bytes past the window; consumers MUST bound their
+    /// own consumption (RunFileReader bounds to its data_end). The window START is always exact.
+    virtual std::optional<GetStreamResult> getStream(const String & key, Range range) = 0;
+    std::optional<GetStreamResult> getStream(const String & key) { return getStream(key, {}); }
+
+    /// Returns the current incarnation's existence, size, token, and metadata without reading its
+    /// body. The result describes one point-in-time observation; a later operation must use the
+    /// returned token when it needs to protect against replacement.
+    virtual HeadResult head(const String & key) = 0;
+
+    /// Creates `key` only when it is absent. `PreconditionFailed` leaves the existing object intact;
+    /// storage failures are reported as exceptions. On success, the returned token identifies the
+    /// newly created incarnation.
+    virtual PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) = 0;
+    PutResult putIfAbsent(const String & key, const String & bytes) { return putIfAbsent(key, bytes, {}); }
+    /// Streaming variant of putIfAbsent — see WriteSink. Large content blobs use this; whole-String
+    /// ops remain for manifests, trees, probe and GC objects.
+    virtual WriteSinkPtr putIfAbsentStream(const String & key, const ObjectMeta & meta) = 0;
+    WriteSinkPtr putIfAbsentStream(const String & key) { return putIfAbsentStream(key, {}); }
+
+    /// Replaces the current object only when its token equals `expected`. A mismatch leaves the
+    /// object unchanged and returns `PreconditionFailed`; the returned token is meaningful only on
+    /// `Done`.
+    virtual PutResult putOverwrite(const String & key, const String & bytes, const Token & expected,
+                                   const ObjectMeta & meta) = 0;
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected)
+    {
+        return putOverwrite(key, bytes, expected, {});
+    }
+    /// expected == nullopt => create-if-absent CAS (the first write of a root manifest).
+    /// A non-null expected token conditionally replaces that exact current incarnation. Conflicts
+    /// leave the object unchanged and are returned as an outcome rather than an exception.
+    virtual CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                             const ObjectMeta & meta) = 0;
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected)
+    {
+        return casPut(key, bytes, expected, {});
+    }
+
+    /// Deletes only the current incarnation identified by `token`. A token mismatch must leave the
+    /// object untouched; the result distinguishes that case from an already absent key.
+    virtual DeleteOutcome deleteExact(const String & key, const Token & token) = 0;
+
+    /// Lists one page of keys under `prefix`, starting after `cursor` and returning at most `limit`
+    /// entries. `ListPage::next_cursor` is the only supported continuation state.
+    virtual ListPage list(const String & prefix, const String & cursor, size_t limit) = 0;
+
+    /// Capability fact about the LIST seam: TRUE iff this backend can surface a per-key incarnation
+    /// token through `list` (i.e. each `ListedKey` carries a token that uniquely identifies the
+    /// current incarnation of that key, matching what `head` would return).
+    ///
+    /// Why this matters: S3 ETags are content-derived and are returned in list responses; the
+    /// in-memory backend mints a monotonic token it can also surface through `list`. A backend that
+    /// cannot surface per-key tokens through `list` MUST return FALSE.
+    ///
+    /// FALSE ⇒ GC `discover` must read every root-shard body to learn the current token (fail closed).
+    /// TRUE  ⇒ `discover` may skip an unchanged root-shard body read when the listed token equals
+    ///          the persisted folded token, saving a GET per unchanged shard.
+    virtual bool supportsListTokens() const = 0;
+
+    /// Pool-level preconditions beyond per-op conditional semantics — checked by the capability
+    /// probe BEFORE the op battery. Default: nothing to check. The S3 backend fails closed here
+    /// when a generation-dialect (GCS) bucket has object versioning enabled: every token-exact
+    /// DELETE would archive a noncurrent generation instead of reclaiming storage, so GC
+    /// "reclaim" would silently stop reclaiming.
+    virtual void checkPoolPreconditions() {}
+
+    /// Fail-closed precondition: a Native-mode backend MUST have a
+    /// working single-attempt conditional-write path before it coordinates a WRITABLE pool — silently
+    /// running CAS conditional writes under the disk's default (~500-attempt) transparent retry policy
+    /// is exactly the hazard this seam forbids. Checked by the capability probe alongside
+    /// checkPoolPreconditions. Default: nothing to check (EmulatedSingleProcess and non-S3 backends
+    /// are not gated here — see ObjectStorageBackend's override for the one backend that is).
+    virtual void checkConditionalWriteSingleAttemptSupport() {}
+
+    /// Authoritative, cache-bypassing probe of one key — see `ProbeOutcome`. DEFAULT (used by every
+    /// backend without sharper raw-error evidence, e.g. `InMemoryBackend`): derived from `head`/`get`
+    /// alone, so it can only distinguish `Present` from `KeyAbsent`, and ANY exception from either
+    /// call is `Indeterminate` — never promoted to `KeyAbsent`. A backend able to surface real
+    /// container/permission evidence (the S3-native and Local paths of `ObjectStorageBackend`)
+    /// overrides this to sharpen the classification.
+    virtual SentinelProbeResult probeSentinelRaw(const String & key)
+    {
+        try
+        {
+            const HeadResult hr = head(key);
+            if (!hr.exists)
+                return {ProbeOutcome::KeyAbsent, std::nullopt};
+            auto g = get(key);
+            /// Vanished between head and get: still a clean, authoritative miss, not an error.
+            if (!g)
+                return {ProbeOutcome::KeyAbsent, std::nullopt};
+            return {ProbeOutcome::Present, std::move(g->bytes)};
+        }
+        catch (...)
+        {
+            return {ProbeOutcome::Indeterminate, std::nullopt};
+        }
+    }
+
+    /// WRITE-ONCE conditional SERVER-SIDE COPY of `staging_key` to `blob_key` (`If-None-Match:*` on the
+    /// destination) — the S3-native staging promote's create primitive. `Done` + `token` = the
+    /// destination ETag (the new incarnation token, exactly the role the
+    /// streaming `putIfAbsentStream` PUT's ETag plays) when this call created `blob_key`;
+    /// `PreconditionFailed` when `blob_key` already existed — NOTHING was changed (same write-once
+    /// contract as `putIfAbsentStream`). No LIVE object is ever overwritten by this call.
+    ///
+    /// DEFAULT: fail closed (`NOT_IMPLEMENTED`) — a backend without a native, enforced conditional
+    /// server-side copy is NEVER selected for S3 staging (the mount-time probe fell back to Local
+    /// staging + `putIfAbsentStream`), so this must throw rather than silently degrade to an
+    /// unconditional overwrite. The caller must use local staging when this primitive is unavailable.
+    virtual PutResult promoteStaged(const String & /*staging_key*/, const String & /*blob_key*/)
+    {
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "Cas::Backend::promoteStaged (write-once server-side copy) is not implemented for this backend");
+    }
+
+    /// UNCONDITIONAL re-upload of the writer's OWN payload over `blob_key` under a FRESH-tagged
+    /// envelope header — the sanctioned condemned-object resurrection overwrite. Writes
+    /// `[fresh_header][payload]`, streaming `payload` from `reader`, and returns the fresh incarnation's
+    /// token. Blob bodies have no size cap, so a Native backend streams the payload and never
+    /// materializes it; the emulated backend materializes (its conditional ops are whole-`String` by
+    /// design) and serializes resurrections to bound the peak to one body at a time.
+    ///
+    /// The reader is the caller's: it is ALWAYS the writer's own source (a staging object or a local
+    /// staged file), NEVER a read of the condemned `blob_key`, and the caller has already skipped any
+    /// envelope header on it. `fresh_header` must carry a freshly-minted `incarnation_tag`, which is what
+    /// makes the resurrected body — and hence its ETag/token — differ from the condemned incarnation, so
+    /// a queued exact-token delete of that incarnation can never match the live resurrection
+    /// (`INV-NO-RETURN`).
+    ///
+    /// UNCONDITIONAL is deliberate. An `If-Match` on the condemned token would save a redundant
+    /// re-upload when another writer resurrects the same blob first, and would prevent nothing: two
+    /// racing resurrections write payload-identical bodies, no consumer reads a dep token's VALUE, and
+    /// durable references name content hashes rather than incarnations.
+    ///
+    /// The caller MUST have observed the current incarnation as `Condemned` (per-hash meta point-read)
+    /// before calling this. That observation is NOT re-checked at the write: two racing resurrections
+    /// of the same blob may both run, and the loser overwrites the winner's FRESH incarnation. That is
+    /// accepted, not prevented -- the payloads are content-identical by construction and durable
+    /// references name content hashes, so the overwrite rotates the envelope and token of an
+    /// equivalent body. What must never be overwritten is a live incarnation of DIFFERENT content,
+    /// and that is guaranteed by the content address itself, not by this call.
+    /// DEFAULT: fail closed (`NOT_IMPLEMENTED`), same rationale as `promoteStaged`.
+    /// `payload_size` is the payload byte count the caller verified at staging time. The write COUNTS
+    /// while streaming and MUST abort -- publishing nothing -- when the reader yields a different
+    /// number of bytes. With an unconditional write this is the last line of defence: a source
+    /// truncated after hashing would otherwise displace the condemned incarnation with a short body
+    /// that the content address does not match, and a post-write check can only detect it AFTER the
+    /// malformed incarnation became current (and can even inspect a racing writer's incarnation
+    /// instead of its own).
+    virtual Token resurrect(ReadBuffer & /*payload*/, uint64_t /*payload_size*/, const String & /*blob_key*/,
+                            const String & /*fresh_header*/)
+    {
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "Cas::Backend::resurrect is not implemented for this backend");
+    }
+};
+
+using BackendPtr = std::shared_ptr<Backend>;
+
+/// Walk every key under `prefix` exactly once, resuming by the backend's explicit last-returned-key
+/// cursor (`ListPage::next_cursor`, empty => done). This centralizes the pagination contract shared by
+/// GC, fsck, and cleanup sweeps: each returned key is delivered once, and the backend's cursor is the
+/// only state used to request the next page.
+///
+/// `on_page_fetched`, if set, fires exactly once per physical `backend.list` call (including an
+/// empty/undersized final page) — a GC-owned caller's hook for a page-level ProfileEvents counter,
+/// without misattributing a non-GC caller (e.g. fsck) that leaves it unset. Trails `page_limit`
+/// (rather than sitting before it) so the two existing callers that override `page_limit`
+/// (`Gc::fold`, `CasFsck.cpp`'s `listAll`) can override `page_limit` without changing callback order.
+inline void forEachListedKey(Backend & backend, const String & prefix,
+                             const std::function<void(const ListedKey &)> & cb,
+                             size_t page_limit = 1000,
+                             const std::function<void()> & on_page_fetched = {})
+{
+    String cursor;
+    for (;;)
+    {
+        const ListPage page = backend.list(prefix, cursor, page_limit);
+        if (on_page_fetched)
+            on_page_fetched();
+        for (const ListedKey & k : page.keys)
+            cb(k);
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+}
+
+/// The normalized verdict of a token-exact delete, unifying the DeleteOutcome::Kind three-way that GC
+/// (blob + manifest delete) and the orphan-manifest sweep each mapped by hand.
+enum class DeleteClass : uint8_t { Deleted, Absent, Replaced };
+
+/// Converts a backend-specific delete outcome into the three states used by cleanup callers. The
+/// default branch is fail-safe: an unknown value is treated as `Replaced`, so cleanup never reports
+/// an unverified deletion as successful.
+inline DeleteClass classifyDeleteOutcome(const DeleteOutcome & d)
+{
+    switch (d.kind)
+    {
+        case DeleteOutcome::Kind::Deleted:       return DeleteClass::Deleted;
+        case DeleteOutcome::Kind::NotFound:      return DeleteClass::Absent;
+        case DeleteOutcome::Kind::TokenMismatch: return DeleteClass::Replaced;
+    }
+    return DeleteClass::Replaced;   /// unreachable; fail-safe toward "leave it" (never a false Deleted)
+}
+
+/// Returns the stable lowercase label used when reporting a normalized delete result. Unknown enum
+/// values are labeled `replaced`, matching `classifyDeleteOutcome`'s fail-safe behavior.
+inline std::string_view deleteClassName(DeleteClass c)
+{
+    switch (c)
+    {
+        case DeleteClass::Deleted:  return "deleted";
+        case DeleteClass::Absent:   return "absent";
+        case DeleteClass::Replaced: return "replaced";
+    }
+    return "replaced";
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.cpp
new file mode 100644
index 000000000000..d8eca3d876d0
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.cpp
@@ -0,0 +1,426 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <IO/ReadBufferFromString.h>
+#include <IO/WriteBufferFromString.h>
+#include <IO/copyData.h>
+#include <Common/Exception.h>
+#include <base/defines.h>
+#include <algorithm>
+#include <stdexcept>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int FILE_DOESNT_EXIST;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Memory-buffered WriteSink: accumulates the body in a WriteBufferFromOwnString and delegates the
+/// conditional publish to InMemoryBackend::putIfAbsent at finalize — the single mutex acquisition
+/// inside putIfAbsent gives atomicity for free. Nothing is ever published on cancel/destruction.
+class InMemoryWriteSink final : public WriteSink
+{
+public:
+    InMemoryWriteSink(InMemoryBackend & backend, String key, ObjectMeta meta)
+        : backend_(backend)
+        , key_(std::move(key))
+        , meta_(std::move(meta))
+    {
+    }
+
+    WriteBuffer & buffer() override { return buf_; }
+
+    PutResult finalize() override
+    {
+        chassert(!done_);   /// finalize after finalize/cancel is a misuse — see the WriteSink contract
+        done_ = true;
+        return backend_.putIfAbsent(key_, buf_.str(), meta_);
+    }
+
+    void cancel() noexcept override
+    {
+        done_ = true;
+        buf_.cancel();
+    }
+
+    ~InMemoryWriteSink() override
+    {
+        if (!done_)
+            cancel();
+    }
+
+private:
+    InMemoryBackend & backend_;
+    String key_;
+    ObjectMeta meta_;
+    WriteBufferFromOwnString buf_;
+    bool done_ = false;
+};
+
+/// The windowed slice of `data` for `range`, with the same clamping `get` documents: an offset at or
+/// past EOF yields an empty result; an open-ended length runs to EOF. Shared by `get` and `getStream`
+/// so the two stay in lockstep.
+String sliceWindow(const String & data, Range range)
+{
+    const size_t offset = static_cast<size_t>(range.offset);
+    if (offset >= data.size())
+        return {};
+    if (range.length.has_value())
+        return data.substr(offset, static_cast<size_t>(*range.length));
+    return data.substr(offset);
+}
+
+}
+
+Token InMemoryBackend::mintToken()
+{
+    Token t;
+    t.value = std::to_string(++token_seq_);
+    t.type = TokenType::Emulated;
+    return t;
+}
+
+std::optional<GetResult> InMemoryBackend::get(const String & key, Range range)
+{
+    std::lock_guard lock(mutex_);
+    auto it = store_.find(key);
+    if (it == store_.end())
+        return std::nullopt;
+
+    GetResult gr;
+    gr.bytes = sliceWindow(it->second.bytes, range);
+    gr.token = it->second.token;
+    gr.attributes = it->second.meta;
+    return gr;
+}
+
+std::optional<GetStreamResult> InMemoryBackend::getStream(const String & key, Range range)
+{
+    std::lock_guard lock(mutex_);
+    auto it = store_.find(key);
+    if (it == store_.end())
+        return std::nullopt;
+
+    /// Copy the windowed bytes into an owning buffer — the in-memory backend has no separate storage
+    /// to stream from, so the "stream" reads from a private copy of exactly the requested window.
+    GetStreamResult sr;
+    sr.stream = std::make_unique<ReadBufferFromOwnString>(sliceWindow(it->second.bytes, range));
+    sr.token = it->second.token;
+    return sr;
+}
+
+HeadResult InMemoryBackend::head(const String & key)
+{
+    std::lock_guard lock(mutex_);
+    auto it = store_.find(key);
+    if (it == store_.end())
+        return HeadResult{};
+
+    HeadResult hr;
+    hr.exists = true;
+    hr.size = static_cast<uint64_t>(it->second.bytes.size());
+    hr.token = it->second.token;
+    hr.attributes = it->second.meta;
+    return hr;
+}
+
+PutResult InMemoryBackend::putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta)
+{
+    std::lock_guard lock(mutex_);
+
+    // One-shot injected ambiguous outcome: throw WITHOUT touching the store, modeling a request whose
+    // own attempt outcome never reached the caller (see the header doc for the classification this
+    // must produce). std::runtime_error, not DB::Exception, is deliberate: it dodges BOTH
+    // classification paths in BOTH build configurations -- dynamic_cast<const Exception *> fails (so
+    // isDeterministicLocalFailure is never consulted), and classifyConditionalWriteResult falls through
+    // to its Unresolved default because it isn't an S3Exception. A DB::Exception would have been
+    // fragile: picking a code outside isDeterministicLocalFailure's set is a landmine for the next
+    // person who extends that set.
+    auto ambiguous_it = ambiguous_put_keys_.find(key);
+    if (ambiguous_it != ambiguous_put_keys_.end())
+    {
+        ambiguous_put_keys_.erase(ambiguous_it);
+        throw std::runtime_error("InMemoryBackend: injected ambiguous putIfAbsent outcome for '" + key + "'");
+    }
+
+    if (store_.contains(key))
+        return {PutOutcome::PreconditionFailed, {}};
+
+    Token t = mintToken();
+    Object obj;
+    obj.bytes = bytes;
+    obj.token = t;
+    obj.meta = meta;
+    store_[key] = std::move(obj);
+    return {PutOutcome::Done, t};
+}
+
+WriteSinkPtr InMemoryBackend::putIfAbsentStream(const String & key, const ObjectMeta & meta)
+{
+    return std::make_unique<InMemoryWriteSink>(*this, key, meta);
+}
+
+PutResult InMemoryBackend::putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta)
+{
+    std::lock_guard lock(mutex_);
+    auto it = store_.find(key);
+    if (it == store_.end())
+        return {PutOutcome::PreconditionFailed, {}};
+
+    if (enforce_tokens_ && it->second.token != expected)
+        return {PutOutcome::PreconditionFailed, {}};
+
+    Token t = mintToken();
+    it->second.bytes = bytes;
+    it->second.token = t;
+    it->second.meta = meta;
+    return {PutOutcome::Done, t};
+}
+
+CasResult InMemoryBackend::casPut(const String & key, const String & bytes, const std::optional<Token> & expected, const ObjectMeta & meta)
+{
+    std::lock_guard lock(mutex_);
+
+    // One-shot injected conflict
+    auto fail_it = fail_next_cas_.find(key);
+    if (fail_it != fail_next_cas_.end())
+    {
+        fail_next_cas_.erase(fail_it);
+        return {CasOutcome::Conflict, {}};
+    }
+
+    auto it = store_.find(key);
+    bool exists = (it != store_.end());
+
+    if (!expected.has_value())
+    {
+        // create-if-absent CAS
+        if (exists)
+            return {CasOutcome::Conflict, {}};
+        Token t = mintToken();
+        Object obj;
+        obj.bytes = bytes;
+        obj.token = t;
+        obj.meta = meta;
+        store_[key] = std::move(obj);
+        return {CasOutcome::Committed, t};
+    }
+    else
+    {
+        // swap-if-current CAS
+        if (!exists)
+            return {CasOutcome::Conflict, {}};
+        if (enforce_tokens_ && it->second.token != *expected)
+            return {CasOutcome::Conflict, {}};
+        Token t = mintToken();
+        it->second.bytes = bytes;
+        it->second.token = t;
+        it->second.meta = meta;
+        return {CasOutcome::Committed, t};
+    }
+}
+
+DeleteOutcome InMemoryBackend::applyDelete(const String & key, const Token & token)
+{
+    // Caller holds the mutex.
+    auto it = store_.find(key);
+    if (it == store_.end())
+    {
+        DeleteOutcome d;
+        d.kind = DeleteOutcome::Kind::NotFound;
+        return d;
+    }
+
+    if (enforce_tokens_ && it->second.token != token)
+    {
+        DeleteOutcome d;
+        d.kind = DeleteOutcome::Kind::TokenMismatch;
+        return d;
+    }
+
+    store_.erase(it);
+    DeleteOutcome d;
+    d.kind = DeleteOutcome::Kind::Deleted;
+    d.created_delete_marker = simulate_delete_markers_;
+    return d;
+}
+
+DeleteOutcome InMemoryBackend::deleteExact(const String & key, const Token & token)
+{
+    std::lock_guard lock(mutex_);
+
+    if (hold_deletes_)
+    {
+        // Validate the key exists (and token matches if enforcing) before queuing,
+        // but don't remove yet — just enqueue.
+        auto it = store_.find(key);
+        if (it == store_.end())
+        {
+            DeleteOutcome d;
+            d.kind = DeleteOutcome::Kind::NotFound;
+            return d;
+        }
+        if (enforce_tokens_ && it->second.token != token)
+        {
+            DeleteOutcome d;
+            d.kind = DeleteOutcome::Kind::TokenMismatch;
+            return d;
+        }
+        PendingDelete pd;
+        pd.key = key;
+        pd.token = token;
+        pending_deletes_.push_back(std::move(pd));
+        DeleteOutcome d;
+        d.kind = DeleteOutcome::Kind::Deleted;
+        d.created_delete_marker = simulate_delete_markers_;
+        return d;
+    }
+
+    return applyDelete(key, token);
+}
+
+PutResult InMemoryBackend::promoteStaged(const String & staging_key, const String & blob_key)
+{
+    std::lock_guard lock(mutex_);
+    auto src = store_.find(staging_key);
+    if (src == store_.end())
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+            "InMemoryBackend::promoteStaged: staging object {} is absent", staging_key);
+
+    /// Write-once: a present destination is the "lost the race" signal, not an overwrite.
+    if (store_.contains(blob_key))
+        return {PutOutcome::PreconditionFailed, {}};
+
+    /// Server-side copy: the destination bytes ARE the staging bytes; a fresh monotone token stands in
+    /// for the destination ETag the real backend returns from the conditional copy.
+    const Token t = mintToken();
+    Object obj;
+    obj.bytes = src->second.bytes;
+    obj.token = t;
+    store_[blob_key] = std::move(obj);
+    return {PutOutcome::Done, t};
+}
+
+Token InMemoryBackend::resurrect(ReadBuffer & payload, uint64_t payload_size, const String & blob_key,
+                                 const String & fresh_header)
+{
+    /// Drain the reader BEFORE taking the lock: the reader may be backed by another object in this
+    /// same store, and reading it under our own mutex would deadlock.
+    String body = fresh_header;
+    {
+        WriteBufferFromString out(body, AppendModeTag{});
+        copyData(payload, out);
+        out.finalize();
+    }
+
+    if (body.size() - fresh_header.size() != payload_size)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "InMemoryBackend::resurrect: source yielded {} payload bytes for {}, declared {} -- nothing was published",
+            body.size() - fresh_header.size(), blob_key, payload_size);
+
+    /// The fresh header makes the resurrected body differ from the condemned incarnation for the same
+    /// payload, so a delayed exact-token delete for the old incarnation cannot remove the resurrection
+    /// (`INV-NO-RETURN`). The condemned `blob_key` is never read.
+    std::lock_guard lock(mutex_);
+    const Token t = mintToken();
+    Object obj;
+    obj.bytes = std::move(body);
+    obj.token = t;
+    store_[blob_key] = std::move(obj);
+    return t;
+}
+
+ListPage InMemoryBackend::list(const String & prefix, const String & cursor, size_t limit)
+{
+    if (limit == 0)
+        return {};
+
+    std::lock_guard lock(mutex_);
+    ListPage page;
+
+    // Cursor is the last key returned by the previous page.
+    auto it = cursor.empty() ? store_.lower_bound(prefix) : store_.upper_bound(cursor);
+
+    size_t count = 0;
+    while (it != store_.end() && count < limit)
+    {
+        if (!it->first.starts_with(prefix))
+            break;
+
+        ListedKey lk;
+        lk.key = it->first;
+        lk.size = static_cast<uint64_t>(it->second.bytes.size());
+        lk.token = it->second.token;   /// in-memory backend always surfaces the token (supportsListTokens == true)
+        page.keys.push_back(std::move(lk));
+        ++count;
+        ++it;
+    }
+
+    // Set next_cursor if there are more keys in this prefix
+    if (!page.keys.empty() && it != store_.end() && it->first.starts_with(prefix))
+        page.next_cursor = page.keys.back().key;
+
+    return page;
+}
+
+void InMemoryBackend::setHoldDeletes(bool hold)
+{
+    std::lock_guard lock(mutex_);
+    hold_deletes_ = hold;
+}
+
+size_t InMemoryBackend::pendingDeletes() const
+{
+    std::lock_guard lock(mutex_);
+    return pending_deletes_.size();
+}
+
+DeleteOutcome InMemoryBackend::landPendingDelete(size_t i)
+{
+    std::lock_guard lock(mutex_);
+    if (i >= pending_deletes_.size())
+    {
+        DeleteOutcome d;
+        d.kind = DeleteOutcome::Kind::NotFound;
+        return d;
+    }
+
+    PendingDelete pd = pending_deletes_[i];
+    pending_deletes_.erase(pending_deletes_.begin() + static_cast<ptrdiff_t>(i));
+
+    // Apply the token check at LAND time — the object may have been modified since the delete was enqueued.
+    return applyDelete(pd.key, pd.token);
+}
+
+void InMemoryBackend::failNextCasPut(const String & key)
+{
+    std::lock_guard lock(mutex_);
+    fail_next_cas_.insert(key);
+}
+
+void InMemoryBackend::injectAmbiguousPutIfAbsent(const String & key)
+{
+    std::lock_guard lock(mutex_);
+    ambiguous_put_keys_.insert(key);
+}
+
+void InMemoryBackend::setEnforceTokens(bool enforce)
+{
+    std::lock_guard lock(mutex_);
+    enforce_tokens_ = enforce;
+}
+
+void InMemoryBackend::setSimulateDeleteMarkers(bool simulate)
+{
+    std::lock_guard lock(mutex_);
+    simulate_delete_markers_ = simulate;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h
new file mode 100644
index 000000000000..5c5d5e906e57
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h
@@ -0,0 +1,172 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <map>
+#include <mutex>
+#include <set>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Thread-safe, token-enforcing in-memory `Backend` implementation used by CAS tests.
+///
+/// All successful writes mint a monotonically increasing token (`TokenType::Emulated`).
+/// Tokens NEVER repeat across the lifetime of a backend instance.
+///
+/// The backend also exposes fault-injection controls for probe tests and CAS correctness tests:
+///   - `setHoldDeletes` / `landPendingDelete`: simulate async/delayed conditional deletes
+///   - `failNextCasPut`:                      inject a one-shot conflict
+///   - `setEnforceTokens(false)`:             mimic a "dumb" backend that ignores token checks
+///   - `setSimulateDeleteMarkers`:            mimic S3 versioning-enabled buckets
+///
+/// Not `final`: tests subclass it to distort single behaviors (e.g. clamp list page size to force
+/// pagination) while delegating everything else to this base.
+class InMemoryBackend : public Backend
+{
+public:
+    InMemoryBackend() = default;
+
+    /// Unhide the base convenience overloads (omitted Range/ObjectMeta/expected-token forms): the
+    /// overrides below would otherwise shadow them for callers holding a concrete backend type.
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    // ---- Backend interface ----
+
+    /// Returns the requested byte window, current token, and metadata, or `nullopt` when the key is absent.
+    std::optional<GetResult> get(const String & key, Range range) override;
+
+    /// Returns a forward-only stream over the requested byte window, or `nullopt` when the key is absent.
+    /// The in-memory implementation copies the window into an owning read buffer while holding the
+    /// backend lock, so the returned stream remains independent of later backend mutations.
+    std::optional<GetStreamResult> getStream(const String & key, Range range) override;
+
+    /// Returns the current existence, size, token, and metadata without materializing the body.
+    HeadResult head(const String & key) override;
+
+    /// The in-memory backend mints a monotonic token it surfaces through `list` — TRUE.
+    bool supportsListTokens() const override { return true; }
+
+    /// Creates `key` only when it is absent. On success stores `bytes` and `meta` under a new token;
+    /// on a precondition failure leaves the existing object untouched.
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override;
+
+    /// Returns a single-use sink whose finalized body is published with the same atomic conditional
+    /// create semantics as `putIfAbsent`. Cancelling or destroying the sink before finalization does
+    /// not publish anything.
+    WriteSinkPtr putIfAbsentStream(const String & key, const ObjectMeta & meta) override;
+
+    /// Replaces the existing object only when `expected` is its current token. Token enforcement can
+    /// be disabled with `setEnforceTokens` to model a backend that incorrectly ignores this condition.
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected,
+                           const ObjectMeta & meta) override;
+
+    /// Performs create-if-absent when `expected` is empty, or replace-if-current-token otherwise.
+    /// Conflicts leave the store unchanged and are returned as an outcome rather than an exception.
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override;
+
+    /// Removes exactly the incarnation named by `token`, or queues that token check for a later
+    /// `landPendingDelete` when delete holding is enabled. A queued delete is reported as accepted,
+    /// but its token is rechecked when it is landed.
+    DeleteOutcome deleteExact(const String & key, const Token & token) override;
+
+    /// Lists up to `limit` keys under `prefix` in map order. `cursor` is the last key from the previous
+    /// page; returned tokens identify the listed incarnations and `next_cursor` is set only when more
+    /// matching keys remain.
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override;
+
+    /// Same-store copy operations used to exercise the S3-staging contracts in memory. `promoteStaged`
+    /// copies the complete staging object to `blob_key` only when the destination is absent and returns
+    /// `PreconditionFailed` without changing it otherwise. The new token models the destination ETag.
+    ///
+    /// `resurrect` is the one sanctioned unconditional overwrite: it reads only the writer's
+    /// staging object, skips its envelope header, prepends `fresh_header`, and writes the resulting
+    /// body over `blob_key`. The fresh header makes the resurrected body and token different from the
+    /// condemned incarnation, so a delayed exact-token delete for that old incarnation cannot remove
+    /// the resurrection (`INV-NO-RETURN`). The caller must already have established that the
+    /// destination is condemned.
+    PutResult promoteStaged(const String & staging_key, const String & blob_key) override;
+    Token resurrect(ReadBuffer & payload, uint64_t payload_size, const String & blob_key, const String & fresh_header) override;
+
+    // ---- Fault-injection controls ----
+
+    /// When true, `deleteExact` validates and enqueues deletes rather than applying them immediately.
+    /// The caller sees `Deleted` (the send was accepted), but the object remains until
+    /// `landPendingDelete`, where the token is checked again.
+    void setHoldDeletes(bool hold);
+
+    /// Returns the number of currently held deletes.
+    size_t pendingDeletes() const;
+
+    /// Applies and removes the held delete at index `i`. The token is evaluated against the current
+    /// object at land time; the queue entry is removed whether the result is `TokenMismatch` or
+    /// `Deleted`. An invalid index returns `NotFound`.
+    DeleteOutcome landPendingDelete(size_t i);
+
+    /// Injects a one-shot artificial `Conflict` on the next `casPut` for `key`.
+    void failNextCasPut(const String & key);
+
+    /// Injects a one-shot AMBIGUOUS outcome on the next `putIfAbsent` for `key`: instead of attempting
+    /// the write, that call throws a plain (non-`DB::Exception`) exception -- classified `Unresolved`,
+    /// never `DefiniteFailure`, by `classifyConditionalWriteResult` regardless of build flags -- and the
+    /// store is left exactly as it was. Models a request whose own HTTP attempt outcome is lost (a
+    /// timeout, a dropped connection) rather than a clean `PreconditionFailed`, for tests of controlled
+    /// ops (`CasRequestController::slotOccupy` and its callers) that must exercise the "ambiguous
+    /// attempt, resolve before deciding" path without a live network. One-shot, mirroring
+    /// `failNextCasPut`'s contract: consumed by the first matching `putIfAbsent` call, whether the key
+    /// was already present or not.
+    void injectAmbiguousPutIfAbsent(const String & key);
+
+    /// Enables or disables token checks for delete, overwrite, and CAS operations. Disabling checks
+    /// models a backend that reports every expected token as matching.
+    void setEnforceTokens(bool enforce);
+
+    /// When true, successful deletes report `created_delete_marker = true`, modelling a versioned S3
+    /// bucket whose delete creates a marker instead of reclaiming the current object.
+    void setSimulateDeleteMarkers(bool simulate);
+
+private:
+    /// Complete in-memory incarnation state for one key. All fields are read or modified while
+    /// `mutex_` is held; replacing `token` marks a new incarnation even when the bytes are unchanged.
+    struct Object
+    {
+        String bytes;
+        Token token;
+        ObjectMeta meta;
+    };
+
+    /// Token captured when a held delete is queued. It is intentionally checked again at land time so
+    /// a replacement between send and land produces `TokenMismatch` rather than deleting the new object.
+    struct PendingDelete
+    {
+        String key;
+        Token token;
+    };
+
+    /// Mints the next process-local token. Tokens are strictly increasing and never reused by this
+    /// backend instance, which also makes token equality a safe content-cache identity check in tests.
+    Token mintToken();
+
+    /// Applies an exact-token delete while `mutex_` is already held. Used by immediate deletes and by
+    /// `landPendingDelete` after its queue entry has been removed.
+    DeleteOutcome applyDelete(const String & key, const Token & token);
+
+    mutable std::mutex mutex_;
+    std::map<String, Object> store_;
+    uint64_t token_seq_ = 0;
+
+    // Fault-injection state. These fields are protected by `mutex_` just like `store_`.
+    bool hold_deletes_ = false;
+    std::vector<PendingDelete> pending_deletes_;
+    std::set<String> fail_next_cas_;
+    std::set<String> ambiguous_put_keys_;
+    bool enforce_tokens_ = true;
+    bool simulate_delete_markers_ = false;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.cpp
new file mode 100644
index 000000000000..81f69a83a11e
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.cpp
@@ -0,0 +1,176 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h>
+
+namespace ProfileEvents
+{
+/// The CA per-namespace and per-operation events declared in `ProfileEvents.cpp`.
+extern const Event CASBlobPut;
+extern const Event CASBlobPutDeduplicated;
+extern const Event CASBlobOverwrite;
+extern const Event CASBlobCompareSwap;
+extern const Event CASBlobCompareSwapConflict;
+extern const Event CASBlobHead;
+extern const Event CASBlobHeadMiss;
+extern const Event CASBlobGet;
+extern const Event CASBlobGetStream;
+extern const Event CASBlobDelete;
+extern const Event CASBlobList;
+
+extern const Event CASManifestPut;
+extern const Event CASManifestPutDeduplicated;
+extern const Event CASManifestOverwrite;
+extern const Event CASManifestCompareSwap;
+extern const Event CASManifestCompareSwapConflict;
+extern const Event CASManifestHead;
+extern const Event CASManifestHeadMiss;
+extern const Event CASManifestGet;
+extern const Event CASManifestGetStream;
+extern const Event CASManifestDelete;
+extern const Event CASManifestList;
+
+extern const Event CASRootPut;
+extern const Event CASRootPutDeduplicated;
+extern const Event CASRootOverwrite;
+extern const Event CASRootCompareSwap;
+extern const Event CASRootCompareSwapConflict;
+extern const Event CASRootHead;
+extern const Event CASRootHeadMiss;
+extern const Event CASRootGet;
+extern const Event CASRootGetStream;
+extern const Event CASRootDelete;
+extern const Event CASRootList;
+
+extern const Event CASGCPut;
+extern const Event CASGCPutDeduplicated;
+extern const Event CASGCOverwrite;
+extern const Event CASGCCompareSwap;
+extern const Event CASGCCompareSwapConflict;
+extern const Event CASGCHead;
+extern const Event CASGCHeadMiss;
+extern const Event CASGCGet;
+extern const Event CASGCGetStream;
+extern const Event CASGCDelete;
+extern const Event CASGCList;
+
+extern const Event CASServerPut;
+extern const Event CASServerPutDeduplicated;
+extern const Event CASServerOverwrite;
+extern const Event CASServerCompareSwap;
+extern const Event CASServerCompareSwapConflict;
+extern const Event CASServerHead;
+extern const Event CASServerHeadMiss;
+extern const Event CASServerGet;
+extern const Event CASServerGetStream;
+extern const Event CASServerDelete;
+extern const Event CASServerList;
+
+extern const Event CASOtherPut;
+extern const Event CASOtherPutDeduplicated;
+extern const Event CASOtherOverwrite;
+extern const Event CASOtherCompareSwap;
+extern const Event CASOtherCompareSwapConflict;
+extern const Event CASOtherHead;
+extern const Event CASOtherHeadMiss;
+extern const Event CASOtherGet;
+extern const Event CASOtherGetStream;
+extern const Event CASOtherDelete;
+extern const Event CASOtherList;
+}
+
+namespace DB::Cas
+{
+
+/// Maps `(CasNs, CasOp)` to the corresponding `ProfileEvents::Event`. The table is row-major: the
+/// outer index is the namespace and the inner index is the operation. Its rows and columns must stay
+/// in lockstep with the `CasNs` and `CasOp` enum orderings.
+static const ProfileEvents::Event cas_event_table[CAS_NS_COUNT][CAS_OP_COUNT] =
+{
+    /* Blob   */ {ProfileEvents::CASBlobPut, ProfileEvents::CASBlobPutDeduplicated, ProfileEvents::CASBlobOverwrite,
+                  ProfileEvents::CASBlobCompareSwap, ProfileEvents::CASBlobCompareSwapConflict, ProfileEvents::CASBlobHead,
+                  ProfileEvents::CASBlobHeadMiss, ProfileEvents::CASBlobGet, ProfileEvents::CASBlobGetStream,
+                  ProfileEvents::CASBlobDelete, ProfileEvents::CASBlobList},
+    /* Manifest */ {ProfileEvents::CASManifestPut, ProfileEvents::CASManifestPutDeduplicated, ProfileEvents::CASManifestOverwrite,
+                  ProfileEvents::CASManifestCompareSwap, ProfileEvents::CASManifestCompareSwapConflict, ProfileEvents::CASManifestHead,
+                  ProfileEvents::CASManifestHeadMiss, ProfileEvents::CASManifestGet, ProfileEvents::CASManifestGetStream,
+                  ProfileEvents::CASManifestDelete, ProfileEvents::CASManifestList},
+    /* Root   */ {ProfileEvents::CASRootPut, ProfileEvents::CASRootPutDeduplicated, ProfileEvents::CASRootOverwrite,
+                  ProfileEvents::CASRootCompareSwap, ProfileEvents::CASRootCompareSwapConflict, ProfileEvents::CASRootHead,
+                  ProfileEvents::CASRootHeadMiss, ProfileEvents::CASRootGet, ProfileEvents::CASRootGetStream,
+                  ProfileEvents::CASRootDelete, ProfileEvents::CASRootList},
+    /* Gc     */ {ProfileEvents::CASGCPut, ProfileEvents::CASGCPutDeduplicated, ProfileEvents::CASGCOverwrite,
+                  ProfileEvents::CASGCCompareSwap, ProfileEvents::CASGCCompareSwapConflict, ProfileEvents::CASGCHead,
+                  ProfileEvents::CASGCHeadMiss, ProfileEvents::CASGCGet, ProfileEvents::CASGCGetStream,
+                  ProfileEvents::CASGCDelete, ProfileEvents::CASGCList},
+    /* Server */ {ProfileEvents::CASServerPut, ProfileEvents::CASServerPutDeduplicated, ProfileEvents::CASServerOverwrite,
+                  ProfileEvents::CASServerCompareSwap, ProfileEvents::CASServerCompareSwapConflict, ProfileEvents::CASServerHead,
+                  ProfileEvents::CASServerHeadMiss, ProfileEvents::CASServerGet, ProfileEvents::CASServerGetStream,
+                  ProfileEvents::CASServerDelete, ProfileEvents::CASServerList},
+    /* Other  */ {ProfileEvents::CASOtherPut, ProfileEvents::CASOtherPutDeduplicated, ProfileEvents::CASOtherOverwrite,
+                  ProfileEvents::CASOtherCompareSwap, ProfileEvents::CASOtherCompareSwapConflict, ProfileEvents::CASOtherHead,
+                  ProfileEvents::CASOtherHeadMiss, ProfileEvents::CASOtherGet, ProfileEvents::CASOtherGetStream,
+                  ProfileEvents::CASOtherDelete, ProfileEvents::CASOtherList},
+};
+
+CasNs classifyCasNs(const String & key)
+{
+    if (key.find("/blobs/") != String::npos)
+        return CasNs::Blob;
+    /// Ref streams and namespace-owned state live under `cas/ns/`; part manifests are under
+    /// `cas/manifests/<namespace>/`. These paths must be classified before
+    /// the generic `roots/` and `Other` cases, otherwise the ref and manifest operation counters
+    /// silently accumulate in the wrong namespace.
+    if (key.find("/cas/ns/") != String::npos)
+        return CasNs::Root;
+    if (key.find("/cas/manifests/") != String::npos)
+        return CasNs::Manifest;
+    if (key.find("/roots/") != String::npos)
+        return CasNs::Root;
+    if (key.find("/gc/") != String::npos)
+        return CasNs::Gc;
+    return CasNs::Other;
+}
+
+void incrementCasEvent(CasNs ns, CasOp op)
+{
+    ProfileEvents::increment(cas_event_table[static_cast<size_t>(ns)][static_cast<size_t>(op)]);
+}
+
+namespace
+{
+
+/// Wraps an inner `WriteSink`. The namespace is captured at creation because the key is not available
+/// at `finalize`; the `Put` versus `PutDeduplicated` outcome is emitted only after the inner sink returns.
+/// Buffer access and cancellation delegate verbatim, while exceptions from the inner sink propagate.
+class InstrumentedWriteSink final : public WriteSink
+{
+public:
+    InstrumentedWriteSink(WriteSinkPtr inner_, CasNs ns_) : inner(std::move(inner_)), ns(ns_) {}
+
+    WriteBuffer & buffer() override { return inner->buffer(); }
+
+    /// Finalize the inner upload first, then count its returned outcome. No event is emitted if the
+    /// inner operation throws.
+    PutResult finalize() override
+    {
+        PutResult result = inner->finalize();
+        incrementCasEvent(ns, result.outcome == PutOutcome::Done ? CasOp::Put : CasOp::PutDeduplicated);
+        return result;
+    }
+
+    void cancel() noexcept override { inner->cancel(); }
+
+private:
+    WriteSinkPtr inner;
+    CasNs ns;
+};
+
+}
+
+WriteSinkPtr InstrumentedBackend::putIfAbsentStream(const String & key, const ObjectMeta & meta)
+{
+    WriteSinkPtr sink = inner->putIfAbsentStream(key, meta);
+    if (!sink)
+        return sink;
+    return std::make_unique<InstrumentedWriteSink>(std::move(sink), classifyCasNs(key));
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h
new file mode 100644
index 000000000000..b7d7e82a7f05
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h
@@ -0,0 +1,202 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Common/ProfileEvents.h>
+
+namespace DB::Cas
+{
+
+/// Per-namespace and per-operation instrumentation for the content-addressed storage seam.
+///
+/// Every content-addressed storage operation flows through the abstract `Backend` seam.
+/// `InstrumentedBackend` is a transparent decorator: it owns an inner `BackendPtr`, delegates every
+/// operation, and increments a `ProfileEvent` keyed by the key's namespace and the operation's
+/// outcome. The pool wraps its backend once in `Pool::open`, which includes operations issued by
+/// background writers and GC as well as foreground calls, for both object-storage and in-memory
+/// backends. This backend-level chokepoint is needed because background PUTs are not attributable
+/// through the foreground request that scheduled them.
+
+/// Namespace of a CA key, classified by substring of the key path (6 classes; `Server` is currently
+/// unreachable through this classifier — the per-server control subtree lives under
+/// `/gc/server-roots/<server_root_id>/...` and classifies as Gc).
+///   <prefix>/blobs/..        → Blob
+///   <prefix>/cas/ns/..        → Root  (immutable streams and point/path-addressed namespace state)
+///   <prefix>/cas/manifests/.. → Manifest
+///   <prefix>/roots/..        → Root  (loose mountpoint objects)
+///   <prefix>/gc/..           → Gc
+///   else (e.g. _pool_meta, _probe) → Other
+enum class CasNs : uint8_t
+{
+    Blob = 0,
+    Manifest,
+    Root,
+    Gc,
+    Server,
+    Other,
+};
+static constexpr size_t CAS_NS_COUNT = 6;
+
+/// Operation + outcome class (11 classes), mapped from the `Backend` method and its return value.
+///   putIfAbsent / putIfAbsentStream finalize → Done ⇒ Put ; PreconditionFailed ⇒ PutDeduplicated
+///   putOverwrite                              → Done ⇒ Overwrite ; PreconditionFailed ⇒ CasConflict
+///   casPut                                    → Committed ⇒ Cas ; Conflict ⇒ CasConflict
+///   head                                      → exists ⇒ Head ; !exists ⇒ HeadMiss (the 404 signal)
+///   get                                       → Get (all calls, hit or miss)
+///   getStream                                 → GetStream (all calls, hit or miss)
+///   deleteExact                               → Delete (all outcomes)
+///   list                                      → List
+enum class CasOp : uint8_t
+{
+    Put = 0,
+    PutDeduplicated,
+    Overwrite,
+    Cas,
+    CasConflict,
+    Head,
+    HeadMiss,
+    Get,
+    GetStream,
+    Delete,
+    List,
+};
+static constexpr size_t CAS_OP_COUNT = 11;
+
+/// Classify a key into its namespace by substring. The order is significant where a more specific
+/// layout such as `cas/ns/` must be recognized before a generic fallback; unknown key families
+/// are intentionally counted as `Other`.
+CasNs classifyCasNs(const String & key);
+
+/// Increment the `ProfileEvent` corresponding to `(ns, op)`. The row-major table is defined in the
+/// implementation and must remain aligned with the `CasNs` and `CasOp` enum values.
+void incrementCasEvent(CasNs ns, CasOp op);
+
+/// Transparent `Backend` decorator that records operation counts without changing the wrapped
+/// backend's results, exceptions, or state transitions. The inner backend is owned by this object.
+/// For streaming creates, namespace classification happens when the sink is created and the
+/// `Put`/`PutDeduplicated` event is emitted only when `finalize` returns, because the outcome is unavailable
+/// earlier.
+class InstrumentedBackend final : public Backend
+{
+public:
+    /// Unhide the base convenience overloads (omitted Range/ObjectMeta/expected-token forms): the
+    /// overrides below would otherwise shadow them for callers holding a concrete backend type.
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    explicit InstrumentedBackend(BackendPtr inner_) : inner(std::move(inner_)) {}
+
+    /// Capability checks are deliberately uninstrumented: they do not represent storage operations.
+    void checkPoolPreconditions() override { inner->checkPoolPreconditions(); }
+    void checkConditionalWriteSingleAttemptSupport() override { inner->checkConditionalWriteSingleAttemptSupport(); }
+
+    /// The typed sentinel probe is a diagnostic/authoritative read, not a routine storage operation —
+    /// deliberately uninstrumented (no ProfileEvent), like the capability checks above. MUST still be
+    /// forwarded explicitly: `Backend::probeSentinelRaw`'s generic default derives its classification from
+    /// THIS object's own `head`/`get` (virtual dispatch would otherwise resolve back to
+    /// `InstrumentedBackend`'s plain, non-typed overrides above), silently discarding whatever sharper
+    /// container/permission evidence the wrapped `inner` backend (e.g. `ObjectStorageBackend`'s S3/Local
+    /// classification) is able to provide.
+    SentinelProbeResult probeSentinelRaw(const String & key) override { return inner->probeSentinelRaw(key); }
+
+    /// Delegate the read and count it after the inner call succeeds or returns absent. Exceptions
+    /// propagate unchanged and therefore do not produce a separate outcome event.
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        auto result = inner->get(key, range);
+        incrementCasEvent(classifyCasNs(key), CasOp::Get);
+        return result;
+    }
+
+    /// Delegate a forward-only read stream and count the request after the stream is acquired.
+    std::optional<GetStreamResult> getStream(const String & key, Range range) override
+    {
+        auto result = inner->getStream(key, range);
+        incrementCasEvent(classifyCasNs(key), CasOp::GetStream);
+        return result;
+    }
+
+    /// Count `Head` or `HeadMiss` from the returned presence flag after delegating to the backend.
+    HeadResult head(const String & key) override
+    {
+        HeadResult result = inner->head(key);
+        incrementCasEvent(classifyCasNs(key), result.exists ? CasOp::Head : CasOp::HeadMiss);
+        return result;
+    }
+
+    /// Count a successful create as `Put` and an existing-key precondition result as `PutDeduplicated`.
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        PutResult result = inner->putIfAbsent(key, bytes, meta);
+        incrementCasEvent(classifyCasNs(key), result.outcome == PutOutcome::Done ? CasOp::Put : CasOp::PutDeduplicated);
+        return result;
+    }
+
+    /// Return a sink that records the create outcome when its `finalize` is called.
+    WriteSinkPtr putIfAbsentStream(const String & key, const ObjectMeta & meta) override;
+
+    /// Count a successful token-conditional overwrite as `Overwrite`; a precondition conflict is
+    /// counted as `CasConflict`.
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected,
+                           const ObjectMeta & meta) override
+    {
+        PutResult result = inner->putOverwrite(key, bytes, expected, meta);
+        incrementCasEvent(classifyCasNs(key), result.outcome == PutOutcome::Done ? CasOp::Overwrite : CasOp::CasConflict);
+        return result;
+    }
+
+    /// Count a committed compare-and-swap as `Cas`; conflicts are counted as `CasConflict`.
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        CasResult result = inner->casPut(key, bytes, expected, meta);
+        incrementCasEvent(classifyCasNs(key), result.outcome == CasOutcome::Committed ? CasOp::Cas : CasOp::CasConflict);
+        return result;
+    }
+
+    /// Delegate token-exact deletion and count every returned deletion outcome as `Delete`.
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        DeleteOutcome outcome = inner->deleteExact(key, token);
+        incrementCasEvent(classifyCasNs(key), CasOp::Delete);
+        return outcome;
+    }
+
+    /// Delegate one paginated listing and classify the prefix used for the request.
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = inner->list(prefix, cursor, limit);
+        incrementCasEvent(classifyCasNs(prefix), CasOp::List);
+        return page;
+    }
+
+    /// This capability is a property of the wrapped backend, not an operation to count.
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+
+    /// Count a successful staged promotion as a create of the destination blob; an existing
+    /// destination is the same deduplication outcome as `putIfAbsent`.
+    PutResult promoteStaged(const String & staging_key, const String & blob_key) override
+    {
+        PutResult result = inner->promoteStaged(staging_key, blob_key);
+        /// A write-once server-side copy is a create attempt on the BLOB key: Done ⇒ Put, 412 ⇒ PutDeduplicated.
+        incrementCasEvent(classifyCasNs(blob_key), result.outcome == PutOutcome::Done ? CasOp::Put : CasOp::PutDeduplicated);
+        return result;
+    }
+
+    /// Count a staged resurrection as an unconditional overwrite of the destination blob. The
+    /// wrapped backend remains responsible for its fresh-header and condemned-token guarantees.
+    Token resurrect(ReadBuffer & payload, uint64_t payload_size, const String & blob_key, const String & fresh_header) override
+    {
+        Token token = inner->resurrect(payload, payload_size, blob_key, fresh_header);
+        /// An unconditional resurrect re-upload overwrites the (condemned) BLOB key.
+        incrementCasEvent(classifyCasNs(blob_key), CasOp::Overwrite);
+        return token;
+    }
+
+private:
+    BackendPtr inner;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.cpp
new file mode 100644
index 000000000000..01eab7498a3b
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.cpp
@@ -0,0 +1,1209 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/ObjectStorageIterator.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/StoredObject.h>
+#include <Disks/WriteMode.h>
+
+#include <Core/Defines.h>
+#include <IO/ReadBufferFromString.h>
+#include <IO/ReadHelpers.h>
+#include <IO/ReadSettings.h>
+#include <IO/WriteBufferFromString.h>
+#include <IO/copyData.h>
+#include <IO/WriteSettings.h>
+
+#include <Common/Exception.h>
+#include <Common/logger_useful.h>
+
+#include <base/defines.h>
+
+#include "config.h"
+
+#if USE_AWS_S3
+#include <IO/S3Common.h>
+#endif
+
+#include <algorithm>
+#include <chrono>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int FILE_DOESNT_EXIST;
+    extern const int NOT_IMPLEMENTED;
+}
+}
+
+namespace DB::Cas
+{
+
+ObjectStorageBackend::ObjectStorageBackend(ObjectStoragePtr object_storage_, Mode mode_, uint64_t conditional_single_put_cap_)
+    : object_storage(std::move(object_storage_))
+    , mode(mode_)
+    , conditional_single_put_cap(conditional_single_put_cap_)
+    , emu_root(object_storage->getCommonKeyPrefix())
+{
+    if (mode == Mode::Native && object_storage->conditionalOpsUseGenerationTokens())
+        native_token_type = TokenType::Generation;
+}
+
+/// See Backend::checkPoolPreconditions. Only the Native, generation-dialect (GCS) combination has
+/// anything to check: a token-exact DELETE on a versioned bucket archives a noncurrent generation
+/// instead of reclaiming storage, so GC "reclaim" would silently stop reclaiming.
+void ObjectStorageBackend::checkPoolPreconditions()
+{
+    if (mode != Mode::Native || native_token_type != TokenType::Generation)
+        return;
+
+    const auto versioned = object_storage->isBucketVersioningEnabled();
+    if (!versioned.has_value())
+    {
+        /// The check itself could not be verified — either the GetBucketVersioning-equivalent call
+        /// failed (e.g. permissions) or the storage does not support answering it. We proceed on the
+        /// ASSUMPTION that versioning is off rather than fail-closing the mount on an unknown: a
+        /// confirmed Enabled below is what actually breaks reclaim, and an outright refusal to mount
+        /// whenever the check is inconclusive would be too aggressive. This is intentionally logged
+        /// (not silent) so an operator can confirm the bucket's real state.
+        LOG_WARNING(getLogger("CasObjectStorageBackend"),
+            "CAS on GCS: could not VERIFY the bucket-versioning precondition (the versioning check "
+            "request failed or is not supported by this backend) — proceeding on the assumption that "
+            "bucket versioning is OFF. If versioning is actually enabled, token-exact DELETEs will "
+            "archive noncurrent generations instead of reclaiming storage and GC will silently stop "
+            "reclaiming space. Please verify the bucket's versioning setting manually.");
+        return;
+    }
+
+    if (*versioned)
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "CAS on GCS: the bucket has object VERSIONING enabled. A token-exact DELETE on a "
+            "versioned bucket archives a noncurrent generation instead of reclaiming storage — GC "
+            "would silently stop reclaiming space. Disable versioning on the bucket (and prefer "
+            "soft-delete duration 0 for CAS pools) and retry the mount.");
+}
+
+/// See Backend::checkConditionalWriteSingleAttemptSupport. This is a MOUNT-TIME gate, deliberately
+/// separate from the ctor: narrow, targeted unit tests can keep constructing a raw Native-mode backend
+/// over a non-S3 IObjectStorage (LocalObjectStorage) to exercise OTHER behaviors in isolation — the
+/// established convention throughout this test suite (see e.g. gtest_cas_backend_generation.cpp). A
+/// REAL writable mount, by contrast, always reaches this check: runCapabilityProbe (CasProbe.cpp) calls
+/// it for every non-read-only Pool::open, so production never silently runs Native-mode conditional
+/// writes under the disk's default (~500-attempt) transparent retry policy.
+void ObjectStorageBackend::checkConditionalWriteSingleAttemptSupport()
+{
+    if (mode != Mode::Native)
+        return;
+
+    /// The property checked is now backend CAPABILITY, not client presence: whether this object
+    /// storage can honor the SingleAttempt retry profile at all (S3ObjectStorage always can; a non-S3
+    /// object storage like LocalObjectStorage cannot).
+    const bool single_attempt_supported = object_storage->supportsRetryProfile(ObjectStorageRetryProfile::SingleAttempt);
+    if (!single_attempt_supported)
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "CAS Native-mode conditional writes require an object storage that supports the "
+            "SingleAttempt retry profile (RFC cas-s3-timeout-retry-control), but this one does not "
+            "(IObjectStorage::supportsRetryProfile returned false, or this build has no AWS S3 "
+            "support) — refusing to mount writable. Native mode is designed for an S3-like "
+            "conditional dialect only; a non-S3 object storage should use EmulatedSingleProcess.");
+}
+
+/// =========================================================================================
+/// Native helpers
+/// =========================================================================================
+
+std::optional<HeadResult> ObjectStorageBackend::nativeHead(const String & key)
+{
+    auto metadata = object_storage->tryGetObjectMetadata(key, /*with_tags=*/false);
+    if (!metadata)
+        return std::nullopt;
+
+    HeadResult hr;
+    hr.exists = true;
+    hr.size = metadata->size_bytes;
+    hr.token = tokenForHead(metadata->etag);
+    hr.attributes = ObjectMeta(metadata->attributes.begin(), metadata->attributes.end());
+    return hr;
+}
+
+/// Finalize a conditional write (the condition rode on the buffer's WriteSettings) and map a
+/// precondition loss to an OUTCOME — anything else propagates.
+///
+/// A backend reports a lost condition as an `S3Exception` carrying the canonical S3 error code string
+/// from the response XML `<Code>` (`S3Exception::getExceptionName`); a conditional-write 412 is
+/// UNMODELED for the AWS SDK (its enum value is UNKNOWN), so `S3Exception::isPreconditionFailed` is the
+/// typed signal — the `PreconditionFailed` name, or that token in the raw body for S3-compatible stores
+/// (RustFS) whose non-AWS body the SDK cannot parse into a name. A `404 NoSuchKey` on an `If-Match` PUT
+/// (the key was deleted out from under us) is treated identically: protocol callers handle 'mismatch'
+/// and 'gone' the same way (re-validate), so both collapse onto `PreconditionFailed`. `NoSuchKey` IS
+/// modeled by the SDK, and `WriteBufferFromS3` retries it internally surfacing the exhaustion with the
+/// typed enum code (and no name), so the enum is matched as well as the name. The mapping is fail-safe in
+/// direction: a misread error becomes a retryable PreconditionFailed/Conflict, never a false success.
+///
+/// Native conditional writes require an S3-compatible integration environment for end-to-end
+/// coverage. Unit tests cover the emulated semantics, the typed exception path, and this classifier
+/// through the test-only `detail` declaration.
+#if USE_AWS_S3
+PutOutcome detail::finalizeConditionalWrite(WriteBuffer & buf)
+{
+    try
+    {
+        buf.finalize();
+    }
+    catch (const S3Exception & e)
+    {
+        if (e.isPreconditionFailed()
+            || e.getExceptionName() == "NoSuchKey"
+            || e.getS3ErrorCode() == Aws::S3::S3Errors::NO_SUCH_KEY)
+            return PutOutcome::PreconditionFailed;
+        throw;
+    }
+    return PutOutcome::Done;
+}
+#endif
+
+/// Build-dispatching shim for the write paths below: without the AWS SDK there is no S3Exception
+/// to classify, so the errors of finalize simply propagate.
+static PutOutcome finalizeConditionalWrite(WriteBuffer & buf)
+{
+#if USE_AWS_S3
+    return detail::finalizeConditionalWrite(buf);
+#else
+    buf.finalize();
+    return PutOutcome::Done;
+#endif
+}
+
+/// Instrument the same single `finalize` call used by both Native write paths without changing their
+/// `Done`/`PreconditionFailed`-or-rethrow contract. A classified precondition loss is `Unresolved`,
+/// not `Committed` or a definite exception, because the response does not prove who created or
+/// replaced the object; the higher-level request controller may then resolve it with exact-key state.
+static PutOutcome finalizeConditionalWriteInstrumented(WriteBuffer & buf)
+{
+    recordConditionalWriteAttemptStarted();
+    try
+    {
+        const PutOutcome legacy = finalizeConditionalWrite(buf);
+        recordConditionalWriteOutcome(
+            legacy == PutOutcome::Done ? classifyConditionalWriteResult() : CasWriteOutcome::Unresolved);
+        return legacy;
+    }
+    catch (const std::exception & e)
+    {
+        recordConditionalWriteOutcome(classifyConditionalWriteResult(e));
+        throw;
+    }
+}
+
+/// Issue a conditional PUT (the condition rides on `ws`) and map a precondition loss — see
+/// finalizeConditionalWrite. The condition is checked by the backend when the object is completed,
+/// so the precondition loss always surfaces from the buffer's finalize, never from write.
+PutResult ObjectStorageBackend::nativeConditionalPut(const String & key, const String & bytes, const WriteSettings & ws, const ObjectMeta & meta)
+{
+    std::optional<ObjectAttributes> attrs;
+    if (!meta.empty())
+        attrs.emplace(meta.begin(), meta.end());   /// ObjectMeta is the same map type as ObjectAttributes
+    auto buf = object_storage->writeObject(
+        StoredObject(key), WriteMode::Rewrite, attrs, DBMS_DEFAULT_BUFFER_SIZE, ws);
+    buf->write(bytes.data(), bytes.size());
+    if (finalizeConditionalWriteInstrumented(*buf) == PutOutcome::PreconditionFailed)
+        return {PutOutcome::PreconditionFailed, {}};
+
+    /// Record the token of the incarnation WE just wrote (model WCreate). The S3 write returns
+    /// its object ETag in the PutObject/CompleteMultipartUpload response, so no follow-up HEAD
+    /// is needed — this is ~73% of the CA backend's HEADs. A backend with no write-time ETag
+    /// (local files) returns nullopt and we fall back to the HEAD (a cheap local stat there).
+    Token token;
+    if (auto etag = buf->getResultObjectETag(); etag && !etag->empty())
+        token = tokenForHead(*etag);
+    else
+    {
+        /// No write-time ETag (local files) or an (anomalous) empty one: fall back to the HEAD —
+        /// the pre-existing behavior, so an empty-ETag server is never worse than before.
+        auto hr = nativeHead(key);
+        token = hr ? hr->token : Token{};
+    }
+    return {PutOutcome::Done, token};
+}
+
+namespace
+{
+
+/// True-streaming WriteSink for Native mode: the underlying object-storage write buffer was opened
+/// with `If-None-Match: *` riding on its WriteSettings, so bytes stream through it directly and the
+/// condition is checked when finalize completes the object — see finalizeConditionalWrite for the
+/// outcome mapping. Nothing is ever published on cancel/destruction.
+class NativeStreamingSink final : public WriteSink
+{
+public:
+    NativeStreamingSink(ObjectStorageBackend & backend_, String key_, std::unique_ptr<WriteBufferFromFileBase> write_buf_)
+        : backend(backend_)
+        , key(std::move(key_))
+        , write_buf(std::move(write_buf_))
+    {
+    }
+
+    WriteBuffer & buffer() override { return *write_buf; }
+
+    PutResult finalize() override
+    {
+        chassert(!done);   /// finalize after finalize/cancel is a misuse — see the WriteSink contract
+        done = true;
+        if (finalizeConditionalWriteInstrumented(*write_buf) == PutOutcome::PreconditionFailed)
+        {
+            /// Losing the condition is an ORDINARY outcome, not an error: another writer legitimately
+            /// took the slot. Abort HERE rather than leaving it to the buffer's destructor, which warns
+            /// "was neither finished nor aborted" on every occurrence -- and a server that writes that
+            /// to stderr fails the test around it -- while the uploaded parts stay billable until a
+            /// lifecycle rule reaps them.
+            write_buf->cancel();
+            return {PutOutcome::PreconditionFailed, {}};
+        }
+
+        /// Record the token of the incarnation we just wrote (model WCreate). The S3 write
+        /// returns its object ETag in the response, so no follow-up HEAD is needed (the bulk of
+        /// the CA backend's HEADs). Backends with no write-time ETag (local) return nullopt and
+        /// we fall back to the HEAD (a cheap local stat there).
+        Token token;
+        if (auto etag = write_buf->getResultObjectETag(); etag && !etag->empty())
+            token = backend.tokenForHead(*etag);
+        else
+        {
+            /// No write-time ETag (local) or an (anomalous) empty one: fall back to the HEAD.
+            auto hr = backend.head(key);
+            token = hr.exists ? hr.token : Token{};
+        }
+        return {PutOutcome::Done, token};
+    }
+
+    void cancel() noexcept override
+    {
+        done = true;
+        write_buf->cancel();
+    }
+
+    ~NativeStreamingSink() override
+    {
+        if (!done)
+            cancel();
+    }
+
+private:
+    ObjectStorageBackend & backend;
+    const String key;
+    std::unique_ptr<WriteBufferFromFileBase> write_buf;
+    bool done = false;
+};
+
+/// Memory-buffered WriteSink for EmulatedSingleProcess mode (unit tests only — buffering the whole
+/// body is acceptable and documented): accumulates into a WriteBufferFromOwnString and delegates the
+/// conditional publish to putIfAbsent at finalize, which provides atomicity under emu_mutex. Nothing
+/// is ever published on cancel/destruction.
+class EmulatedBufferedSink final : public WriteSink
+{
+public:
+    EmulatedBufferedSink(Backend & backend_, String key_, ObjectMeta meta_)
+        : backend(backend_)
+        , key(std::move(key_))
+        , meta(std::move(meta_))
+    {
+    }
+
+    WriteBuffer & buffer() override { return buf; }
+
+    PutResult finalize() override
+    {
+        chassert(!done);   /// finalize after finalize/cancel is a misuse — see the WriteSink contract
+        done = true;
+        return backend.putIfAbsent(key, buf.str(), meta);
+    }
+
+    void cancel() noexcept override
+    {
+        done = true;
+        buf.cancel();
+    }
+
+    ~EmulatedBufferedSink() override
+    {
+        if (!done)
+            cancel();
+    }
+
+private:
+    Backend & backend;
+    const String key;
+    const ObjectMeta meta;
+    WriteBufferFromOwnString buf;
+    bool done = false;
+};
+
+}
+
+/// True when an exception from `IObjectStorage::readObject` means "the object is simply not there".
+/// Two surfaces:
+///   1. S3/RustFS:        `S3Exception` with `S3Errors::NO_SUCH_KEY` (the modeled enum — the primary
+///      signal) or `getExceptionName() == "NoSuchKey"` (the canonical XML `<Code>` string, present
+///      when the SDK was able to parse it; mirrors `finalizeConditionalWrite`'s detection).
+///   2. Local / emulated: `DB::Exception` with `ErrorCodes::FILE_DOESNT_EXIST` (from
+///      `ReadBufferFromFile` when `open(2)` returns ENOENT).
+///
+/// Any other error (network, auth, throttle, corruption) propagates unchanged — fail-closed.
+static bool isObjectNotFound(const std::exception & e)
+{
+#if USE_AWS_S3
+    if (const auto * s3e = dynamic_cast<const S3Exception *>(&e))
+        return s3e->getS3ErrorCode() == Aws::S3::S3Errors::NO_SUCH_KEY
+            || s3e->getExceptionName() == "NoSuchKey";
+#endif
+    if (const auto * dbe = dynamic_cast<const Exception *>(&e))
+        return dbe->code() == ErrorCodes::FILE_DOESNT_EXIST;
+    return false;
+}
+
+/// Read `range` of the object at `path` as a TRUE ranged read: seek to the offset and bound the
+/// read window. Seek the storage buffer to the requested offset and bound the returned bytes instead
+/// of reading a whole snapshot run and slicing it afterward; snapshot runs can be gigabytes at scale,
+/// while the caller's memory budget is O(block).
+static String readObjectRanged(IObjectStorage & object_storage, const String & path, Range range,
+                               uint64_t known_size = 0)
+{
+    auto buf = object_storage.readObject(
+        StoredObject(path), casSizedReadSettings(getReadSettings(), known_size), /*read_hint=*/std::nullopt);
+    String content;
+    if (range.whole())
+    {
+        readStringUntilEOF(content, *buf);
+        return content;
+    }
+
+    /// An offset at or past EOF yields an empty result, matching the range contract of the previous
+    /// whole-read implementation.
+    /// `seek` past the object size may throw depending on the storage, so fail-close the window
+    /// against the known size before touching the buffer position.
+    /// Native callers already HEAD the key, so passing its size avoids another metadata round trip.
+    /// A zero size means the caller does not know it and metadata must be fetched here.
+    const uint64_t object_size = known_size != 0 ? known_size
+        : object_storage.getObjectMetadata(path, /*with_tags=*/false).size_bytes;
+    if (range.offset >= object_size)
+        return {};
+
+    /// The readable window, clamped to EOF. `setReadUntilPosition` is only a hint (not every object
+    /// storage honors it — LocalObjectStorage does not), so the exact byte count below is what bounds
+    /// the read; the hint lets storages that DO honor it avoid over-fetching.
+    const uint64_t available = object_size - range.offset;
+    const uint64_t to_read = range.length.has_value() ? std::min(*range.length, available) : available;
+
+    if (range.length.has_value())
+        buf->setReadUntilPosition(range.offset + *range.length);
+    buf->seek(static_cast<off_t>(range.offset), SEEK_SET);
+
+    content.resize(to_read);
+    const size_t got = buf->read(content.data(), to_read);
+    content.resize(got);
+    return content;
+}
+
+/// Open a forward-only stream over `range` of the object at `path`, positioned at the window's first
+/// byte and bounded to its last. Mirrors
+/// `readObjectRanged`'s seek + bound, but RETURNS the buffer instead of draining it — the caller reads
+/// at its own pace, so nothing is materialized whole. Returns nullptr when the offset is at or past EOF
+/// (the empty-window clamp), matching the ranged-get contract.
+static std::unique_ptr<ReadBuffer> openObjectRangedStream(IObjectStorage & object_storage, const String & path, Range range,
+                                                          uint64_t known_size = 0)
+{
+    auto buf = object_storage.readObject(
+        StoredObject(path), casSizedReadSettings(getReadSettings(), known_size), /*read_hint=*/std::nullopt);
+    if (range.whole())
+        return buf;
+
+    /// Clamp exactly like `readObjectRanged`: an offset at or past EOF yields an empty stream, and
+    /// `seek` past the object size may throw depending on the storage, so fail-close against the known
+    /// size before touching the buffer position.
+    /// As in `readObjectRanged`, a caller-supplied size avoids another metadata round trip; zero means
+    /// that the size is unknown and must be fetched.
+    const uint64_t object_size = known_size != 0 ? known_size
+        : object_storage.getObjectMetadata(path, /*with_tags=*/false).size_bytes;
+    if (range.offset >= object_size)
+        return std::make_unique<ReadBufferFromString>(std::string_view{});
+
+    /// `setReadUntilPosition` is only a hint (LocalObjectStorage does not honor it), but for a returned
+    /// stream it is the only bound available — the caller drains to EOF, so a storage that DOES honor
+    /// the hint stops at the window end, and one that does not over-reads only the trailing bytes.
+    if (range.length.has_value())
+        buf->setReadUntilPosition(range.offset + *range.length);
+    buf->seek(static_cast<off_t>(range.offset), SEEK_SET);
+    return buf;
+}
+
+ReadSettings casSizedReadSettings(const ReadSettings & base, uint64_t known_size)
+{
+    if (known_size == 0)
+        return base;
+    return base.adjustBufferSize(known_size + CAS_FOLD_READ_SLACK_BYTES);
+}
+
+/// =========================================================================================
+/// Emulated helpers (caller holds emu_mutex)
+/// =========================================================================================
+
+namespace
+{
+
+/// The mtime-quantum guard (emuMintToken) only needs a key's `emu_token_state` entry while a
+/// same-quantum tie is still POSSIBLE for a FRESH recreate — i.e. while the just-deleted
+/// incarnation's own etag (mtime-ns, see emuMintToken) is recent. Once it is comfortably behind
+/// "now", no later recreate can land in the same mtime quantum, so retaining the entry serves no
+/// purpose (codex-review-triage §3.18, Important #1). 2 seconds is far above any filesystem's mtime
+/// tick coarseness while still bounding the map to the recently-deleted-key population.
+constexpr uint64_t EMU_TOKEN_STALE_AGE_NS = 2'000'000'000ULL;
+constexpr size_t EMU_TOKEN_EXPIRY_SWEEP_SIZE = 16;
+
+/// True iff `etag` parses as a plain nanosecond count (emuMintToken's `.first` is always the BARE
+/// etag, never the `etag#N` disambiguated form) that is at least EMU_TOKEN_STALE_AGE_NS behind now.
+/// An etag that fails to parse (e.g. a test double's non-numeric stub) is conservatively treated as
+/// NOT stale — never erasing is always safe, merely un-bounded, so an unparseable value must not be
+/// mistaken for a recent one.
+bool etagComfortablyInThePast(const String & etag, uint64_t now_ns)
+{
+    if (etag.empty() || !std::all_of(etag.begin(), etag.end(), [](char c) { return c >= '0' && c <= '9'; }))
+        return false;
+
+    uint64_t etag_ns = 0;
+    try
+    {
+        etag_ns = std::stoull(etag);
+    }
+    catch (...)
+    {
+        return false;
+    }
+
+    return now_ns > etag_ns && (now_ns - etag_ns) >= EMU_TOKEN_STALE_AGE_NS;
+}
+
+}
+
+String ObjectStorageBackend::emuPath(const String & key) const
+{
+    if (emu_root.empty())
+        return key;
+    if (!emu_root.empty() && emu_root.back() == '/')
+        return emu_root + key;
+    return emu_root + "/" + key;
+}
+
+uint64_t ObjectStorageBackend::emuNowNs() const
+{
+    if (emu_now_ns_for_test != 0)
+        return emu_now_ns_for_test;
+    return static_cast<uint64_t>(
+        std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::system_clock::now().time_since_epoch()).count());
+}
+
+void ObjectStorageBackend::setEmuNowNsForTest(uint64_t now_ns)
+{
+    std::lock_guard lock(emu_mutex);
+    emu_now_ns_for_test = now_ns;
+}
+
+size_t ObjectStorageBackend::emuTokenStateSizeForTest() const
+{
+    std::lock_guard lock(emu_mutex);
+    return emu_token_state.size();
+}
+
+void ObjectStorageBackend::emuPruneTokenState(uint64_t now_ns)
+{
+    for (size_t checked = 0; checked < EMU_TOKEN_EXPIRY_SWEEP_SIZE && !emu_token_expiry.empty(); ++checked)
+    {
+        const auto & candidate = emu_token_expiry.front();
+        auto current = emu_token_state.find(candidate.key);
+
+        /// A later mint (including a delete+recreate in the same mtime quantum) supersedes this exact
+        /// deleted state. Its queue record can be discarded immediately without touching the map.
+        if (current == emu_token_state.end() || current->second != candidate.token_state)
+        {
+            emu_token_expiry.pop_front();
+            continue;
+        }
+
+        /// Deletion time is monotonic within this mutex-protected FIFO. If its oldest record has not
+        /// crossed the safety window, every later matching record is too recent as well.
+        if (now_ns <= candidate.queued_at_ns || now_ns - candidate.queued_at_ns < EMU_TOKEN_STALE_AGE_NS)
+            break;
+
+        /// The record has aged enough to inspect its etag. Unparseable or otherwise uncertain etags
+        /// stay in the map (fail safe), but their queue records cannot block pruning of later keys.
+        if (etagComfortablyInThePast(current->second.first, now_ns))
+            emu_token_state.erase(current);
+        emu_token_expiry.pop_front();
+    }
+}
+
+bool ObjectStorageBackend::emuExists(const String & key) const
+{
+    return object_storage->exists(StoredObject(emuPath(key)));
+}
+
+String ObjectStorageBackend::emuRead(const String & key, Range range) const
+{
+    return readObjectRanged(*object_storage, emuPath(key), range);
+}
+
+Token ObjectStorageBackend::emuWrite(const String & key, const String & bytes, const ObjectMeta & meta)
+{
+    std::optional<ObjectAttributes> attrs;
+    if (!meta.empty())
+        attrs.emplace(meta.begin(), meta.end());   /// ObjectMeta is the same map type as ObjectAttributes
+    auto buf = object_storage->writeObject(StoredObject(emuPath(key)), WriteMode::Rewrite, attrs);
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+
+    const auto metadata = object_storage->tryGetObjectMetadata(emuPath(key), /*with_tags=*/false);
+    return emuMintToken(key, metadata ? metadata->etag : String{}, /*just_wrote=*/true);
+}
+
+Token ObjectStorageBackend::emuObserveToken(const String & key)
+{
+    const auto metadata = object_storage->tryGetObjectMetadata(emuPath(key), /*with_tags=*/false);
+    return emuMintToken(key, metadata ? metadata->etag : String{}, /*just_wrote=*/false);
+}
+
+Token ObjectStorageBackend::emuMintToken(const String & key, const String & etag, bool just_wrote)
+{
+    emuPruneTokenState(emuNowNs());
+
+    /// Anomalous: the object storage reported no etag at all (LocalObjectStorage always does; this
+    /// guards a hypothetical future/test double). Mint a fresh, UNPERSISTED value — never worse than
+    /// the old counter for this case, but never masquerading as a real etag-derived identity.
+    if (etag.empty())
+        return Token{std::to_string(++emu_seq), TokenType::Emulated};
+
+    auto it = emu_token_state.find(key);
+    if (it != emu_token_state.end() && it->second.first == etag)
+    {
+        /// The etag has not advanced since the last token we minted for this key. For a read-only
+        /// observation that is expected (the object simply has not changed) and the SAME value must be
+        /// returned. For a just-completed WRITE it means this write's mtime landed in the same quantum
+        /// as the previous incarnation's — two DIFFERENT incarnations must still never mint identical
+        /// tokens, so bump a small per-key disambiguator (mtime-quantum guard, triage §3.18 19c step 4).
+        if (just_wrote)
+            ++it->second.second;
+        const String value = it->second.second == 0 ? etag : etag + "#" + std::to_string(it->second.second);
+        return Token{value, TokenType::Emulated};
+    }
+
+    /// The etag advanced (or this key is seen for the first time): the bare etag is the token, and any
+    /// previous disambiguator is dropped — a genuinely new incarnation starts clean.
+    emu_token_state[key] = {etag, 0};
+    return Token{etag, TokenType::Emulated};
+}
+
+/// =========================================================================================
+/// Backend interface
+/// =========================================================================================
+
+std::optional<GetResult> ObjectStorageBackend::get(const String & key, Range range)
+{
+    if (mode == Mode::Native)
+    {
+        auto hr = nativeHead(key);
+        if (!hr)
+            return std::nullopt;
+
+        /// The object may be deleted between the HEAD above and the GET below (a GC or concurrent
+        /// writer racing the read window). Catch the not-found signal and honor the `optional`
+        /// contract — callers such as `Pool::loadShardDecoded` already handle a nullopt return and
+        /// treat it as "raced a deletion, absent". Any other error (network, auth, corruption)
+        /// propagates unchanged — fail-closed by construction.
+        ///
+        /// A REPLACEMENT racing the same window (HEAD observes token A, GET reads the bytes of a
+        /// subsequently-written incarnation B) is likewise not a hazard: HEAD strictly precedes GET, so
+        /// the returned token is never NEWER than the returned bytes — a mixed pair is always
+        /// (bytes_newer, token_older), never the reverse. Every consumer of this token uses it as a
+        /// conditional precondition (`casPut`/`putOverwrite`/`deleteExact`), which fails closed EXACTLY
+        /// in the mixed case, so a stale token costs a retry, never lets a caller act on a
+        /// bytes/token pair that never coexisted. This also covers `known_size`: content-addressed blob
+        /// bodies are byte-identical across incarnations (a "replacement" only rotates envelope/token),
+        /// mutable control objects are read-modify-CAS loops that re-validate on conflict, and write-once
+        /// objects self-validate their contents on decode.
+        GetResult gr;
+        try
+        {
+            gr.bytes = readObjectRanged(*object_storage, key, range, hr->size);
+        }
+        catch (const std::exception & e)
+        {
+            if (isObjectNotFound(e))
+                return std::nullopt;
+            throw;
+        }
+        gr.token = hr->token;
+        return gr;
+    }
+
+    std::lock_guard lock(emu_mutex);
+    if (!emuExists(key))
+        return std::nullopt;
+
+    /// The emulated path holds emu_mutex across the exists-check and the read, so no concurrent
+    /// caller in this process can delete the file in between. External deletion (e.g. a test teardown
+    /// racing a read) is still handled: convert FILE_DOESNT_EXIST to nullopt rather than letting it
+    /// escape as an unexplained exception.
+    GetResult gr;
+    try
+    {
+        gr.bytes = emuRead(key, range);
+    }
+    catch (const std::exception & e)
+    {
+        if (isObjectNotFound(e))
+            return std::nullopt;
+        throw;
+    }
+    gr.token = emuObserveToken(key);
+    return gr;
+}
+
+std::optional<GetStreamResult> ObjectStorageBackend::getStream(const String & key, Range range)
+{
+    if (mode == Mode::Native)
+    {
+        auto hr = nativeHead(key);
+        if (!hr)
+            return std::nullopt;
+
+        /// Same HEAD-then-read race as `get`: the object may be deleted between the HEAD above and the
+        /// stream open below. Honor the `optional` contract on a not-found signal; any other error
+        /// (network, auth, corruption) propagates unchanged — fail-closed by construction.
+        GetStreamResult sr;
+        try
+        {
+            sr.stream = openObjectRangedStream(*object_storage, key, range, hr->size);
+        }
+        catch (const std::exception & e)
+        {
+            if (isObjectNotFound(e))
+                return std::nullopt;
+            throw;
+        }
+        sr.token = hr->token;
+        return sr;
+    }
+
+    std::lock_guard lock(emu_mutex);
+    if (!emuExists(key))
+        return std::nullopt;
+
+    /// The emulated path holds emu_mutex across the exists-check and the stream open, matching `get`.
+    /// External deletion still converts to nullopt rather than escaping as an unexplained exception.
+    GetStreamResult sr;
+    try
+    {
+        sr.stream = openObjectRangedStream(*object_storage, emuPath(key), range);
+    }
+    catch (const std::exception & e)
+    {
+        if (isObjectNotFound(e))
+            return std::nullopt;
+        throw;
+    }
+    sr.token = emuObserveToken(key);
+    return sr;
+}
+
+HeadResult ObjectStorageBackend::head(const String & key)
+{
+    if (mode == Mode::Native)
+    {
+        auto hr = nativeHead(key);
+        return hr ? *hr : HeadResult{};
+    }
+
+    std::lock_guard lock(emu_mutex);
+    if (!emuExists(key))
+        return HeadResult{};
+
+    auto metadata = object_storage->tryGetObjectMetadata(emuPath(key), /*with_tags=*/false);
+    /// A path that exists on the Local filesystem but yields no object metadata is a directory, not
+    /// an object (`tryGetObjectMetadata` returns nullopt for a directory). HEAD must report it as
+    /// not-an-object (exists=false) — otherwise existsFile/getStorageObjects treat a pool sub-dir (e.g.
+    /// `store`, traversed by system.remote_data_paths) as a file and a later body read throws EISDIR.
+    if (!metadata)
+        return HeadResult{};
+    HeadResult hr;
+    hr.exists = true;
+    hr.size = metadata->size_bytes;
+    hr.attributes = ObjectMeta(metadata->attributes.begin(), metadata->attributes.end());
+    hr.token = emuObserveToken(key);
+    return hr;
+}
+
+/// See Backend::probeSentinelRaw / CasBackend.h's ProbeOutcome for the semantics this classifies.
+SentinelProbeResult ObjectStorageBackend::probeSentinelRaw(const String & key)
+{
+    if (mode == Mode::Native)
+    {
+        try
+        {
+            /// `getObjectMetadata` (unlike `tryGetObjectMetadata`/`nativeHead`) is the THROWING raw-HEAD
+            /// primitive — it does NOT collapse NO_SUCH_KEY/NO_SUCH_BUCKET/RESOURCE_NOT_FOUND into one
+            /// `nullopt` before we get a chance to classify the S3 error. Its result is discarded here;
+            /// only whether (and how) it throws matters — the body comes from `get` below.
+            object_storage->getObjectMetadata(key, /*with_tags=*/false);
+
+            /// The raw HEAD proved the key present. Delegate the body read to the existing `get`, which
+            /// already HEADs again and reads — an extra round trip this authoritative, low-rate probe can
+            /// afford, in exchange for reusing its already-correct HEAD→GET race handling. Kept INSIDE
+            /// this try: a transient failure here must also classify Indeterminate, never escape unclassified.
+            auto g = get(key);
+            if (!g)
+                return {ProbeOutcome::KeyAbsent, std::nullopt};   /// raced a deletion right after the raw HEAD
+            return {ProbeOutcome::Present, std::move(g->bytes)};
+        }
+#if USE_AWS_S3
+        catch (const S3Exception & e)
+        {
+            switch (e.getS3ErrorCode())
+            {
+                case Aws::S3::S3Errors::NO_SUCH_KEY:
+                    return {ProbeOutcome::KeyAbsent, std::nullopt};
+                case Aws::S3::S3Errors::RESOURCE_NOT_FOUND:
+                    /// A HEAD response carries no body, so the SDK cannot parse a `NoSuchKey` `<Code>`
+                    /// and instead derives this generic code straight from the HTTP 404 status (see
+                    /// `isNotFoundError`, `src/IO/S3/getObjectInfo.cpp`) — this is what a REAL S3 HEAD
+                    /// on an absent key actually throws. The container/key distinction is deliberately
+                    /// NOT attempted here (a bodyless 404 cannot carry it).
+                    return {ProbeOutcome::KeyAbsent, std::nullopt};
+                case Aws::S3::S3Errors::NO_SUCH_BUCKET:
+                    return {ProbeOutcome::ContainerAbsent, std::nullopt};
+                case Aws::S3::S3Errors::ACCESS_DENIED:
+                    return {ProbeOutcome::AccessDenied, std::nullopt};
+                default:
+                    /// Everything else (timeouts, 5xx, throttling, an unmodeled code) is inconclusive —
+                    /// NEVER promoted to KeyAbsent, per the IAM permutation table in spec §2.
+                    return {ProbeOutcome::Indeterminate, std::nullopt};
+            }
+        }
+#endif
+        catch (...)
+        {
+            return {ProbeOutcome::Indeterminate, std::nullopt};
+        }
+    }
+
+    /// EmulatedSingleProcess (Local): stat the configured container directory FIRST — `emuExists`/`get`
+    /// alone cannot distinguish "this key is absent" from "the whole pool directory is gone" (Local
+    /// listing is best-effort and silently reports zero either way, see LocalObjectStorage::listObjects).
+    try
+    {
+        if (!object_storage->existsOrHasAnyChild(emu_root))
+            return {ProbeOutcome::ContainerAbsent, std::nullopt};
+
+        auto g = get(key);
+        if (!g)
+            return {ProbeOutcome::KeyAbsent, std::nullopt};
+        return {ProbeOutcome::Present, std::move(g->bytes)};
+    }
+    catch (...)
+    {
+        return {ProbeOutcome::Indeterminate, std::nullopt};
+    }
+}
+
+/// Base WriteSettings for every Native conditional write. CAS-mutable keys (shard manifests,
+/// gc/state, the registry) override check_objects_after_upload to `false` (see WriteSettings.h);
+/// this was observed live against RustFS: a publish's manifest CAS raced the GC fence and the
+/// mismatch terminated the server from the upload worker.
+///
+/// On a generation-token store (GCS), a conditional write must ALSO never take the multipart path:
+/// GCS enforces no preconditions on `CompleteMultipartUpload` (measured), so a lost
+/// precondition on a multipart write would silently overwrite instead of failing. Force single-PUT
+/// and raise the single-part cap to conditional_single_put_cap (RAM-buffered) to keep the fast path
+/// available for bodies up to that size; a bigger body throws NOT_IMPLEMENTED from
+/// WriteBufferFromS3::createMultipartUpload.
+WriteSettings ObjectStorageBackend::conditionalWriteSettings() const
+{
+    WriteSettings ws;
+    ws.s3_check_objects_after_upload_override = false;
+    if (native_token_type == TokenType::Generation)
+    {
+        ws.s3_force_single_part_upload = true;
+        ws.s3_single_part_upload_max_bytes_override = conditional_single_put_cap;
+    }
+    /// Exactly one attempt at the WriteBufferFromS3 layer too: makeSinglepartUpload/
+    /// completeMultipartUpload run their OWN retry loop above the S3 client, reissuing the identical
+    /// (conditional!) request on NO_SUCH_KEY — a client-level override alone does not bound it. Plain
+    /// size_t field, harmless (ignored) for a non-S3 write path.
+    ws.s3_max_unexpected_write_error_retries_override = 1;
+    /// Exactly one HTTP attempt for every conditional write: the object storage resolves the
+    /// profile to its own single-attempt client. A backend that cannot honor it is rejected for
+    /// writable Native mounts by checkConditionalWriteSingleAttemptSupport (fail closed).
+    ws.object_storage_retry_profile = ObjectStorageRetryProfile::SingleAttempt;
+    return ws;
+}
+
+PutResult ObjectStorageBackend::putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta)
+{
+    if (mode == Mode::Native)
+    {
+        WriteSettings ws = conditionalWriteSettings();
+        ws.object_storage_write_if_none_match = "*";
+        return nativeConditionalPut(key, bytes, ws, meta);
+    }
+
+    std::lock_guard lock(emu_mutex);
+    if (emuExists(key))
+        return {PutOutcome::PreconditionFailed, {}};
+
+    return {PutOutcome::Done, emuWrite(key, bytes, meta)};
+}
+
+WriteSinkPtr ObjectStorageBackend::putIfAbsentStream(const String & key, const ObjectMeta & meta)
+{
+    if (mode == Mode::Native)
+    {
+        /// Same WriteSettings construction as putIfAbsent — the condition rides on the write buffer
+        /// and is checked when finalize completes the object.
+        WriteSettings ws = conditionalWriteSettings();
+        ws.object_storage_write_if_none_match = "*";
+        std::optional<ObjectAttributes> attrs;
+        if (!meta.empty())
+            attrs.emplace(meta.begin(), meta.end());   /// ObjectMeta is the same map type as ObjectAttributes
+        auto buf = object_storage->writeObject(
+            StoredObject(key), WriteMode::Rewrite, attrs, DBMS_DEFAULT_BUFFER_SIZE, ws);
+        return std::make_unique<NativeStreamingSink>(*this, key, std::move(buf));
+    }
+
+    return std::make_unique<EmulatedBufferedSink>(*this, key, meta);
+}
+
+PutResult ObjectStorageBackend::putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta)
+{
+    /// §3.18 №19: reject a wrong-dialect expected token before it ever reaches the wire (Native) or
+    /// the emu compare (Emulated) — see mintingTypeMatches.
+    if (!mintingTypeMatches(expected.type))
+        return {PutOutcome::PreconditionFailed, {}};
+
+    if (mode == Mode::Native)
+    {
+        WriteSettings ws = conditionalWriteSettings();
+        ws.object_storage_write_if_match = expected.value;
+        return nativeConditionalPut(key, bytes, ws, meta);
+    }
+
+    std::lock_guard lock(emu_mutex);
+    if (!emuExists(key))
+        return {PutOutcome::PreconditionFailed, {}};
+    if (!tokenMatches(emuObserveToken(key), expected))
+        return {PutOutcome::PreconditionFailed, {}};
+
+    return {PutOutcome::Done, emuWrite(key, bytes, meta)};
+}
+
+CasResult ObjectStorageBackend::casPut(const String & key, const String & bytes, const std::optional<Token> & expected, const ObjectMeta & meta)
+{
+    /// §3.18 №19: a create-if-absent CAS (expected == nullopt) has no token to validate; only the
+    /// swap form carries one, and it must match this backend's own minting dialect before anything
+    /// else runs.
+    if (expected.has_value() && !mintingTypeMatches(expected->type))
+        return {CasOutcome::Conflict, {}};
+
+    if (mode == Mode::Native)
+    {
+        WriteSettings ws = conditionalWriteSettings();
+        if (expected.has_value())
+            ws.object_storage_write_if_match = expected->value;
+        else
+            ws.object_storage_write_if_none_match = "*";
+
+        /// The PUT-side outcomes (Done / PreconditionFailed) collapse onto CAS outcomes 1:1: a lost
+        /// condition — whether a mismatched If-Match or a 404 on an If-Match PUT — is a Conflict.
+        PutResult put = nativeConditionalPut(key, bytes, ws, meta);
+        return put.outcome == PutOutcome::Done
+            ? CasResult{CasOutcome::Committed, put.token}
+            : CasResult{CasOutcome::Conflict, {}};
+    }
+
+    std::lock_guard lock(emu_mutex);
+    const bool exists = emuExists(key);
+
+    if (!expected.has_value())
+    {
+        if (exists)
+            return {CasOutcome::Conflict, {}};
+    }
+    else
+    {
+        if (!exists)
+            return {CasOutcome::Conflict, {}};
+        if (!tokenMatches(emuObserveToken(key), *expected))
+            return {CasOutcome::Conflict, {}};
+    }
+
+    return {CasOutcome::Committed, emuWrite(key, bytes, meta)};
+}
+
+DeleteOutcome ObjectStorageBackend::deleteExact(const String & key, const Token & token)
+{
+    /// §3.18 №19: same local dialect guard as putOverwrite/casPut — never forward a foreign-dialect
+    /// value as the removeObjectIfTokenMatches argument.
+    if (!mintingTypeMatches(token.type))
+    {
+        DeleteOutcome d;
+        d.kind = DeleteOutcome::Kind::TokenMismatch;
+        return d;
+    }
+
+    if (mode == Mode::Native)
+    {
+        /// `removeObjectIfTokenMatches` maps onto `DeleteOutcome` one-to-one. `NOT_IMPLEMENTED` from a
+        /// backend that does not enforce conditional removal propagates — fail-closed by construction.
+        auto result = object_storage->removeObjectIfTokenMatches(StoredObject(key), token.value);
+        DeleteOutcome d;
+        d.created_delete_marker = result.created_delete_marker;
+        switch (result.outcome)
+        {
+            case ConditionalRemoveOutcome::Removed:
+                d.kind = DeleteOutcome::Kind::Deleted;
+                break;
+            case ConditionalRemoveOutcome::TokenMismatch:
+                d.kind = DeleteOutcome::Kind::TokenMismatch;
+                break;
+            case ConditionalRemoveOutcome::NotFound:
+                d.kind = DeleteOutcome::Kind::NotFound;
+                break;
+        }
+        return d;
+    }
+
+    std::lock_guard lock(emu_mutex);
+    DeleteOutcome d;
+    if (!emuExists(key))
+    {
+        d.kind = DeleteOutcome::Kind::NotFound;
+        return d;
+    }
+    if (!tokenMatches(emuObserveToken(key), token))
+    {
+        d.kind = DeleteOutcome::Kind::TokenMismatch;
+        return d;
+    }
+
+    object_storage->removeObjectIfExists(StoredObject(emuPath(key)));
+    /// Keep the deleted incarnation's last-minted etag around ONLY while a same-mtime-quantum
+    /// collision with an immediate recreate is still possible (emuMintToken) — once it is
+    /// comfortably old, erase it so `emu_token_state` does not grow for the lifetime of the backend
+    /// instance (codex-review-triage §3.18, Important #1).
+    if (auto it = emu_token_state.find(key); it != emu_token_state.end())
+    {
+        const uint64_t now_ns = emuNowNs();
+        if (etagComfortablyInThePast(it->second.first, now_ns))
+            emu_token_state.erase(it);
+        else
+            emu_token_expiry.push_back(EmuTokenExpiry{now_ns, key, it->second});
+    }
+    d.kind = DeleteOutcome::Kind::Deleted;
+    return d;
+}
+
+PutResult ObjectStorageBackend::promoteStaged(const String & staging_key, const String & blob_key)
+{
+    if (mode != Mode::Native)
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "ObjectStorageBackend::promoteStaged is Native-mode only (EmulatedSingleProcess has no "
+            "server-side conditional copy and is never selected for S3 staging)");
+
+    /// WRITE-ONCE conditional server-side copy staging -> blob (`If-None-Match:*` on the destination),
+    /// via `IObjectStorage::copyObjectConditional`. `created` ⇒ the destination ETag is the new
+    /// incarnation token; `!created` ⇒ the destination already existed = the "lost the race" 412 signal.
+    /// Counted with the same attempt/outcome counters as every other conditional write
+    /// (`finalizeConditionalWriteInstrumented`'s contract): the copy is a conditional
+    /// create attempt too, and it is initiated by the controlled content-addressed upload path — an
+    /// uncounted attempt would hide SDK-versus-controller retry accounting.
+    /// A resolved `!created` is counted `Unresolved` (the 412 does not prove who created the occupant),
+    /// mirroring the PUT paths.
+    recordConditionalWriteAttemptStarted();
+    ConditionalCopyResult res;
+    try
+    {
+        res = object_storage->copyObjectConditional(
+            StoredObject(staging_key), StoredObject(blob_key), getReadSettings(), WriteSettings{});
+    }
+    catch (const std::exception & e)
+    {
+        recordConditionalWriteOutcome(classifyConditionalWriteResult(e));
+        throw;
+    }
+    recordConditionalWriteOutcome(res.created ? classifyConditionalWriteResult() : CasWriteOutcome::Unresolved);
+    if (!res.created)
+        return {PutOutcome::PreconditionFailed, {}};
+    return {PutOutcome::Done, Token{res.dest_etag, native_token_type}};
+}
+
+Token ObjectStorageBackend::resurrect(ReadBuffer & payload, uint64_t payload_size, const String & blob_key,
+                                      const String & fresh_header)
+{
+    if (mode != Mode::Native)
+    {
+        /// EmulatedSingleProcess (local object storage): same unconditional semantics. The body is
+        /// materialized -- the emulated conditional ops are whole-`String` by design -- so resurrections
+        /// are SERIALIZED process-wide by their own mutex: the fan-out may run N resurrect tasks at
+        /// once, and without this the peak would be the SUM of the bodies. One at a time bounds the
+        /// peak to the largest single body, the same guarantee the byte-weighted admission's exclusive
+        /// arm used to give. A dedicated mutex, not `emu_mutex`: the drain may read through the same
+        /// store, and `emu_mutex` guards individual ops inside it.
+        static std::mutex emulated_resurrect_mutex;
+        std::lock_guard resurrect_lock(emulated_resurrect_mutex);
+        String body = fresh_header;
+        {
+            WriteBufferFromString out(body, AppendModeTag{});
+            copyData(payload, out);
+            out.finalize();
+        }
+        if (body.size() - fresh_header.size() != payload_size)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "resurrect: source yielded {} payload bytes for {}, declared {} -- nothing was published",
+                body.size() - fresh_header.size(), blob_key, payload_size);
+        std::lock_guard lock(emu_mutex);
+        return emuWrite(blob_key, body, /*meta=*/{});
+    }
+
+    /// Unconditional overwrite of the condemned body (plain WriteSettings — no If-Match/If-None-Match).
+    /// This is safe by three independent structural properties, not merely "no time to add a
+    /// precondition": (1) the key is content-addressed, so every incarnation ever written under it is
+    /// byte-identical in its PAYLOAD — an overwrite here rotates only the envelope/token; (2) an
+    /// adopted dependency token VALUE is never a promote gate, only `has_value()` is consulted
+    /// (tokenless-on-ref promote), so no consumer can observe or react to the specific bytes of the old
+    /// token; (3) the fresh-tagged `fresh_header` guarantees the resurrected incarnation's token differs
+    /// from the condemned one, so every already-queued exact-token GC delete of the condemned
+    /// incarnation mismatches and misses (`INV-NO-RETURN`). An `If-Match` on the condemned token would
+    /// only save a redundant re-upload on a lost race, never prevent data loss.
+    ///
+    /// Plain `WriteSettings` also means no forced single part: a conditional write on a
+    /// generation-token store is capped because GCS drops preconditions on multipart completion, and
+    /// this write carries none, so it may take the multipart path on every backend. The payload is
+    /// streamed from `payload` and never materialized — blob bodies have no size cap.
+    auto out = object_storage->writeObject(
+        StoredObject(blob_key), WriteMode::Rewrite, /*attributes=*/std::nullopt, DBMS_DEFAULT_BUFFER_SIZE, WriteSettings{});
+    out->write(fresh_header.data(), fresh_header.size());
+    const size_t before = out->count();
+    copyData(payload, *out);
+    const size_t streamed = out->count() - before;
+    if (streamed != payload_size)
+    {
+        /// Abort BEFORE finalize: the incomplete multipart upload is discarded and nothing becomes
+        /// current. This is what keeps the unconditional write fail-closed against a source truncated
+        /// after hashing -- a post-write check would run only after the short body had displaced the
+        /// condemned incarnation.
+        out->cancel();
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "resurrect: source yielded {} payload bytes for {}, declared {} -- upload aborted, nothing published",
+            streamed, blob_key, payload_size);
+    }
+    out->finalize();
+
+    /// The plain write does not reliably surface the destination ETag across dialects, so HEAD the
+    /// fresh incarnation to learn its token.
+    const auto hr = nativeHead(blob_key);
+    if (!hr)
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+            "ObjectStorageBackend::resurrect: blob {} is absent immediately after the resurrect "
+            "re-upload — failing closed", blob_key);
+    return hr->token;
+}
+
+ListPage ObjectStorageBackend::list(const String & prefix, const String & cursor, size_t limit)
+{
+    /// Use the lazy object-storage iterator instead of `listObjects(..., max_keys=0)`: the latter
+    /// materialized the whole prefix, then sliced client-side, so a paginated walk re-fetched the full
+    /// subtree for every page. The backend cursor is "last key returned" (exclusive on resume).
+    ///
+    /// Some backends ignore `start_after`; filtering `key <= cursor` keeps the contract correct there,
+    /// only losing the resume optimization. S3 honors `start_after` and avoids the hot-path re-scan.
+    if (limit == 0)
+        return {};
+
+    const String physical_prefix = (mode == Mode::EmulatedSingleProcess) ? emuPath(prefix) : prefix;
+    const String strip = (mode == Mode::EmulatedSingleProcess) ? emuPath("") : String{};
+    if (mode == Mode::EmulatedSingleProcess)
+    {
+        RelativePathsWithMetadata children;
+        object_storage->listObjects(physical_prefix, children, /*max_keys=*/0);
+
+        /// Hold emu_mutex across the whole scan: emuMintToken below reads/updates emu_token_state, the
+        /// same per-key state get/head/put*/delete* mutate under this lock (see the "caller holds
+        /// emu_mutex" contract on the private emu* helpers).
+        std::lock_guard lock(emu_mutex);
+
+        std::vector<ListedKey> all;
+        all.reserve(children.size());
+        for (const auto & child : children)
+        {
+            if (!child->relative_path.starts_with(physical_prefix))
+                continue;
+            ListedKey lk;
+            lk.key = child->relative_path.substr(strip.size());
+            lk.size = child->metadata ? child->metadata->size_bytes : 0;
+            /// §3.18 №18: mint DIRECTLY as TokenType::Emulated — do NOT call tokenForList, which always
+            /// stamps native_token_type (ETag/Generation) regardless of mode and would surface a token
+            /// of the wrong dialect for every Emulated consumer (head/get mint Emulated).
+            if (child->metadata)
+                lk.token = emuMintToken(lk.key, child->metadata->etag, /*just_wrote=*/false);
+            all.push_back(std::move(lk));
+        }
+        std::sort(all.begin(), all.end(), [](const ListedKey & a, const ListedKey & b) { return a.key < b.key; });
+
+        ListPage page;
+        auto all_it = cursor.empty()
+            ? std::lower_bound(all.begin(), all.end(), prefix, [](const ListedKey & a, const String & s) { return a.key < s; })
+            : std::upper_bound(all.begin(), all.end(), cursor, [](const String & s, const ListedKey & a) { return s < a.key; });
+        while (all_it != all.end() && page.keys.size() < limit)
+        {
+            page.keys.push_back(*all_it);
+            ++all_it;
+        }
+        if (!page.keys.empty() && all_it != all.end())
+            page.next_cursor = page.keys.back().key;
+        return page;
+    }
+
+    const std::optional<String> start_after = cursor.empty()
+        ? std::nullopt
+        : std::optional<String>(cursor);
+
+    ListPage page;
+    auto it = object_storage->iterate(physical_prefix, /*max_keys=*/0, /*with_tags=*/false, start_after);
+    for (; it->isValid(); it->next())
+    {
+        const auto child = it->current();
+        if (!child->relative_path.starts_with(physical_prefix))
+            continue;
+
+        ListedKey lk;
+        lk.key = child->relative_path.substr(strip.size());
+        if (!cursor.empty() && lk.key <= cursor)
+            continue;
+
+        lk.size = child->metadata ? child->metadata->size_bytes : 0;
+        /// Surface the per-key incarnation token (matching what `head` would return, see above) so the
+        /// `supportsListTokens() == true` capability is honest. A listing without an etag leaves the
+        /// token unset, which GC discover treats as Read (fail closed). The supportsListTokens()+
+        /// empty-etag gate now lives in tokenForList.
+        if (child->metadata)
+            lk.token = tokenForList(child->metadata->etag);
+
+        if (page.keys.size() == limit)
+        {
+            page.next_cursor = page.keys.back().key;
+            break;
+        }
+        page.keys.push_back(std::move(lk));
+    }
+
+    return page;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h
new file mode 100644
index 000000000000..f06bafa969c2
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h
@@ -0,0 +1,249 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h>
+#include <deque>
+#include <map>
+#include <mutex>
+
+#include "config.h"
+
+namespace DB::Cas
+{
+
+/// Fold and point GETs commonly read tiny bodies (about 3.7 KiB on the measured workload), while the
+/// default `ReadBufferFromS3` allocation is about 1 MiB. If the caller already knows the object size,
+/// use `ReadSettings::adjustBufferSize` to request a buffer of `known_size + slack`, without exceeding
+/// the caller's configured default. A zero `known_size` means that the size is unknown and preserves
+/// the supplied settings unchanged.
+constexpr uint64_t CAS_FOLD_READ_SLACK_BYTES = 4096;
+ReadSettings casSizedReadSettings(const ReadSettings & base, uint64_t known_size);
+
+#if USE_AWS_S3
+namespace detail
+{
+/// Finalize a conditional write (the condition rode on the buffer's WriteSettings) and map a
+/// precondition loss to an OUTCOME — anything else propagates. This is the classifier for the
+/// typed `S3Exception` signal; exposed here for unit tests only — production callers go through
+/// `ObjectStorageBackend`. See the definition for the exact matching rules.
+PutOutcome finalizeConditionalWrite(WriteBuffer & buf);
+}
+#endif
+
+/// Production Backend over IObjectStorage.
+///
+/// Native mode (S3-like): conditions ride the existing plumbing — WriteSettings
+/// object_storage_write_if_none_match / object_storage_write_if_match (consumed by WriteBufferFromS3)
+/// and IObjectStorage::removeObjectIfTokenMatches. Tokens are backend ETags from getObjectMetadata.
+/// Trust is NEVER assumed: Cas::Probe validates enforcement per pool at open.
+///
+/// EmulatedSingleProcess mode (LocalObjectStorage — tests and local development ONLY): the object
+/// storage has no conditional ops, so this adapter provides EXACT token semantics itself with a
+/// process-wide mutex and an in-memory per-key token MINTED FROM the object's own etag (mtime-ns on
+/// LocalObjectStorage) — see emuMintToken. Every emulated token IS the object's current etag (not
+/// merely "seeded" for pre-existing keys); this is what keeps token-exact semantics correct ACROSS a
+/// process restart, which a plain in-process counter cannot do (codex-review-triage §3.18, 19c): a
+/// counter restarts at 0 and can re-mint a value colliding with a persisted pre-restart delete token
+/// for a completely different incarnation, while a resurrected body's mtime is always later. Semantics
+/// otherwise hold within ONE process only — exactly what unit tests need.
+class ObjectStorageBackend final : public Backend
+{
+public:
+    /// Unhide the base convenience overloads (omitted Range/ObjectMeta/expected-token forms): the
+    /// overrides below would otherwise shadow them for callers holding a concrete backend type.
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    enum class Mode { Native, EmulatedSingleProcess };
+
+    /// Construct a backend over `object_storage`. Native mode uses the storage's conditional
+    /// operations and native token dialect; `EmulatedSingleProcess` serializes operations locally for
+    /// tests and local development. The generation-token store limit applies only to Native mode:
+    /// generation stores must use a single PUT because their multipart completion path does not enforce
+    /// the precondition.
+    ObjectStorageBackend(ObjectStoragePtr object_storage_, Mode mode_, uint64_t conditional_single_put_cap_ = 1ULL << 30);
+
+    /// Read an object or return `nullopt` if it is absent. Native mode HEADs first so the returned
+    /// token identifies the incarnation whose bytes are read; a not-found race is also reported as
+    /// `nullopt`, while unrelated storage errors propagate.
+    std::optional<GetResult> get(const String & key, Range range) override;
+    /// Open a forward-only ranged stream for a write-once object. The stream is not materialized in
+    /// memory; mutable objects must use `get` because their contents may change while it is open.
+    std::optional<GetStreamResult> getStream(const String & key, Range range) override;
+    /// Return the current size, attributes, and incarnation token, or an absent `HeadResult`.
+    HeadResult head(const String & key) override;
+    /// S3 ETags are content-derived and surfaced in list responses — TRUE for ETag-token Native
+    /// and EmulatedSingleProcess modes. FALSE on a generation-token store (GCS): the XML LIST
+    /// surfaces MD5-style ETags in the response BODY, which the conditional dialect's header-level
+    /// rewrite cannot map to generations. A list-derived token would therefore be an invalid
+    /// `If-Match` token; generation stores deliberately omit it and make GC re-read each shard.
+    /// Consumers already treat absent list tokens as Read/fail-closed (GC discover re-reads every
+    /// shard — a cost, not a correctness change).
+    bool supportsListTokens() const override { return native_token_type != TokenType::Generation; }
+
+    /// Create `key` only if it is absent. On a precondition failure the object is untouched and the
+    /// result has no token; on success the token identifies the newly written incarnation.
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override;
+
+    /// Native mode: true streaming — bytes flow straight into the object storage's write buffer with
+    /// `If-None-Match: *` riding on the request. EmulatedSingleProcess mode: memory-buffered delegation
+    /// to putIfAbsent (acceptable: this mode exists for unit tests only).
+    WriteSinkPtr putIfAbsentStream(const String & key, const ObjectMeta & meta) override;
+    /// Replace `key` only when its current token exactly equals `expected`; a mismatch leaves the
+    /// existing incarnation untouched. Storage exceptions propagate instead of being reported as a
+    /// successful or failed precondition.
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override;
+    /// Perform a compare-and-set: `expected == nullopt` means create-if-absent. A conflict leaves the
+    /// object untouched; a committed result carries the new incarnation token.
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected, const ObjectMeta & meta) override;
+    /// Remove only the incarnation matching `token`, preserving the object on a mismatch and exposing
+    /// whether the storage created a delete marker.
+    DeleteOutcome deleteExact(const String & key, const Token & token) override;
+    /// Return a page after `cursor`; the next cursor is the last returned key and is empty at the end.
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override;
+
+    /// `promoteStaged` (S3-native staging, Native mode only — EmulatedSingleProcess has no server-side
+    /// conditional copy and is never selected for S3 staging, so it throws `NOT_IMPLEMENTED` there):
+    /// WRITE-ONCE conditional copy via `IObjectStorage::copyObjectConditional`.
+    /// `resurrect` (every mode): prepends `fresh_header` and UNCONDITIONALLY writes
+    /// `[fresh_header][payload]` to `blob_key` (fresh tag ⇒ distinct ETag from the condemned
+    /// incarnation, INV-NO-RETURN), then a fresh HEAD for the ETag. Native streams with plain
+    /// `WriteSettings` — no forced single part, no size ceiling on any dialect; EmulatedSingleProcess
+    /// materializes and SERIALIZES resurrections process-wide, bounding the peak to one body.
+    PutResult promoteStaged(const String & staging_key, const String & blob_key) override;
+    Token resurrect(ReadBuffer & payload, uint64_t payload_size, const String & blob_key, const String & fresh_header) override;
+
+    /// Pool-level precondition: on a Native, generation-dialect (GCS) backend, reject the pool if the
+    /// bucket has object versioning enabled — see Backend::checkPoolPreconditions.
+    void checkPoolPreconditions() override;
+
+    /// Fail-closed precondition for writable Native mode: require that the object storage supports the
+    /// SingleAttempt retry profile (ObjectStorageRetryProfile), which disables transparent
+    /// conditional-write retries. Without it, an SDK retry could cross the mount lease boundary or turn
+    /// an uncertain result into a misleading precondition failure. A non-S3 object storage used for
+    /// test construction reports no support; this check is the mount-time gate. No-op for
+    /// `EmulatedSingleProcess`.
+    void checkConditionalWriteSingleAttemptSupport() override;
+
+    /// See Backend::probeSentinelRaw. Native: a raw HEAD via `IObjectStorage::getObjectMetadata` (the
+    /// THROWING variant — unlike `tryGetObjectMetadata`/`nativeHead`, it never swallows the S3 error),
+    /// classified by S3 error code. EmulatedSingleProcess (Local): stats the configured container
+    /// directory (`emu_root`) first — `ContainerAbsent` if it is gone — then the key.
+    SentinelProbeResult probeSentinelRaw(const String & key) override;
+
+    /// The token kind this backend's object storage mints: TokenType::ETag for AWS-compatible
+    /// stores, TokenType::Generation when the storage runs the GCS conditional dialect (the
+    /// generation rides the ETag plumbing; the VALUE stays opaque either way).
+    TokenType nativeTokenType() const { return native_token_type; }
+    void setNativeTokenTypeForTest(TokenType t) { native_token_type = t; }
+
+    /// ---- Token policy (single source of truth; see the .cpp) ----
+    /// Mint the incarnation token for a key we just HEAD'd or wrote: the object ETag/generation
+    /// string carried under this backend's native dialect (native_token_type).
+    Token tokenForHead(const String & etag) const
+    {
+        return Token{etag, native_token_type};
+    }
+
+    /// The token to surface for a LISTED key: present iff this backend surfaces per-key list tokens
+    /// (supportsListTokens — FALSE on a generation store, where a list-derived token is a poisoned
+    /// If-Match) AND the listing carried a non-empty etag. Matches what tokenForHead would return.
+    std::optional<Token> tokenForList(const String & etag) const
+    {
+        if (!supportsListTokens() || etag.empty())
+            return std::nullopt;
+        return Token{etag, native_token_type};
+    }
+
+    /// Whether an observed incarnation token satisfies an expected one: exact identity (value AND
+    /// type). Every conditional compare in this backend goes through here.
+    static bool tokenMatches(const Token & observed, const Token & expected)
+    {
+        return observed == expected;
+    }
+
+    /// Build settings shared by every Native conditional write. They skip the racy post-upload
+    /// existence/size check, force single-part uploads for generation-token stores, and select the
+    /// SingleAttempt object-storage retry profile.
+    WriteSettings conditionalWriteSettings() const;
+    WriteSettings conditionalWriteSettingsForTest() const { return conditionalWriteSettings(); }
+    /// Override the emulated backend's wall clock for deterministic expiry tests.
+    void setEmuNowNsForTest(uint64_t now_ns);
+    /// Return the guarded per-key token-state size for expiry tests.
+    size_t emuTokenStateSizeForTest() const;
+
+private:
+    const ObjectStoragePtr object_storage;
+    const Mode mode;
+    TokenType native_token_type = TokenType::ETag;
+    /// GCS single-PUT budget for conditional writes (generation-token stores only); see ctor.
+    const uint64_t conditional_single_put_cap;
+
+    /// EmulatedSingleProcess state: per-key {etag, disambiguator} — see emuMintToken. A successfully
+    /// deleted entry is retained only while its etag is recent enough that an immediate recreate could
+    /// land in the same mtime quantum. `deleteExact` erases already-old entries immediately and queues
+    /// recent ones for the bounded lazy sweep in emuMintToken, so a key need not be revisited to expire.
+    /// The queue records the exact state generation deleted; a subsequent re-mint makes the record
+    /// obsolete rather than allowing it to erase the live incarnation's token state.
+    mutable std::mutex emu_mutex;
+    std::map<String, std::pair<String, uint64_t>> emu_token_state;
+    struct EmuTokenExpiry
+    {
+        uint64_t queued_at_ns;
+        String key;
+        std::pair<String, uint64_t> token_state;
+    };
+    std::deque<EmuTokenExpiry> emu_token_expiry;
+    uint64_t emu_now_ns_for_test = 0;
+    /// Fallback nonce for the (anomalous) case where the object storage reports an EMPTY etag: mints a
+    /// fresh, unpersisted value each time — never worse than the old counter for that case, but never
+    /// masquerading as a real etag-derived identity either.
+    uint64_t emu_seq = 0;
+
+    /// Look up Native metadata and convert the storage ETag or generation to this backend's token.
+    std::optional<HeadResult> nativeHead(const String & key);
+    /// Write a body with the condition already encoded in `ws`, finalize it, classify a lost
+    /// precondition, and return the new token when the write succeeds.
+    PutResult nativeConditionalPut(const String & key, const String & bytes, const WriteSettings & ws, const ObjectMeta & meta);
+
+    /// §3.18 №19 hardening: whether `t` is the dialect this backend itself mints (native_token_type
+    /// for Native mode, always TokenType::Emulated for EmulatedSingleProcess). Every conditional
+    /// mutation checks this BEFORE touching the wire (Native forwards only Token::value as the
+    /// If-Match/removeObjectIfTokenMatches argument, blind to Token::type) or comparing values
+    /// (Emulated) — a foreign-dialect token is rejected locally rather than trusted to the remote
+    /// backend, or to a value-space that was never designed to discriminate it.
+    bool mintingTypeMatches(TokenType t) const { return t == (mode == Mode::Native ? native_token_type : TokenType::Emulated); }
+
+    /// ---- Emulated helpers (caller holds emu_mutex) ----
+    ///
+    /// EmulatedSingleProcess resolves logical keys under the object storage's common key prefix (its
+    /// root), so each backend instance is physically isolated — a real object store likewise scopes keys
+    /// to a bucket/prefix. The token map is keyed by the LOGICAL key (prefix-independent).
+    const String emu_root;             /// object_storage->getCommonKeyPrefix() captured at construction
+    String emuPath(const String & key) const;   /// logical key -> physical object-storage path
+
+    /// The caller holds `emu_mutex` for all five helpers below, preserving the exists/read and
+    /// observe/write checks as one process-local operation.
+    bool emuExists(const String & key) const;
+    String emuRead(const String & key, Range range) const;
+    /// Write a body as the new incarnation of `key` and return its freshly minted token (the
+    /// object's own post-write etag — see emuMintToken).
+    Token emuWrite(const String & key, const String & bytes, const ObjectMeta & meta);
+    /// Return the current emulated token for a key we just read/HEAD'd, reflecting its on-disk etag —
+    /// does NOT advance the same-etag disambiguator (that only applies to a just-completed write).
+    Token emuObserveToken(const String & key);
+    uint64_t emuNowNs() const;
+    /// Examine a fixed number of oldest deleted-state records, expiring only an exact current match.
+    void emuPruneTokenState(uint64_t now_ns);
+    /// Single source of truth for minting an emulated token from an observed `etag`: the wire value IS
+    /// the etag while it is the first thing minted for `key` at that etag, or `etag#N` once a SAME-etag
+    /// rewrite forces a disambiguator (`just_wrote` — see the mtime-quantum note in emu_token_state's
+    /// declaration and codex-review-triage §3.18 19c step 4). An empty `etag` (the storage could not
+    /// report one) falls back to a fresh, UNPERSISTED monotonic value from emu_seq.
+    Token emuMintToken(const String & key, const String & etag, bool just_wrote);
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.cpp
new file mode 100644
index 000000000000..6d2b31a74fb5
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.cpp
@@ -0,0 +1,320 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h>
+#include <Common/Exception.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int NOT_IMPLEMENTED;
+}
+}
+
+namespace DB::Cas
+{
+
+void runCapabilityProbe(Backend & backend, const String & probe_prefix)
+{
+    // Probe key used for the primary battery steps.
+    // Sub-directory style ("probe_prefix/token") ensures that list(probe_prefix, …) works for both the
+    // in-memory backend (prefix match) and the LocalObjectStorage backend (directory listing).
+    const String key = probe_prefix + "/token";
+    // Probe key used for the casPut chain.
+    const String cas_key = probe_prefix + "/cas";
+
+    // Best-effort cleanup — runs at function exit regardless of outcome.
+    // We capture the keys we need to clean up.
+    auto cleanup = [&]() noexcept
+    {
+        // Skip the delete when HEAD says the key is already gone (the happy path: step 8 deleted
+        // it). A deleteExact with the absent HeadResult's EMPTY token is a malformed conditional
+        // op — AWS S3 answers 400 InvalidArgument ("If-Match cannot be empty"), which lands as a
+        // scary AWSClient <Error> log line on every mount even though the catch swallows it.
+        for (const auto & k : {key, cas_key})
+        {
+            try
+            {
+                const auto h = backend.head(k);
+                if (h.exists)
+                    backend.deleteExact(k, h.token);
+            }
+            catch (...) {} /// NOLINT(bugprone-empty-catch)
+        }
+    };
+
+    try
+    {
+        // ---- Step 0: store-level preconditions (backend-specific; throws = mount refused). ----
+        backend.checkPoolPreconditions();
+
+        // ---- Step 0b: conditional writes must use one underlying HTTP attempt. Transparent SDK
+        // retries can outlive the writer's mount lease and hide whether a conditional operation
+        // committed; CAS retries must instead be explicit and state-aware. Throws = mount refused.
+        // Keep this separate from Step 0 so each precondition remains independently unit-testable. ----
+        backend.checkConditionalWriteSingleAttemptSupport();
+
+        // ---- Step 1: putIfAbsent fresh → Done; read-after-write returns the bytes. ----
+        Token t1;
+        {
+            const auto res = backend.putIfAbsent(key, "probe-v1");
+            t1 = res.token;
+            if (res.outcome != PutOutcome::Done)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: putIfAbsent on a fresh key returned PreconditionFailed — backend is unexpectedly occupied or broken");
+        }
+        {
+            const auto g = backend.get(key);
+            if (!g.has_value() || g->bytes != "probe-v1")
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: read-after-write failed — putIfAbsent succeeded but the object is not readable");
+        }
+
+        // ---- Step 2: putIfAbsent same key → PreconditionFailed; bytes intact. ----
+        {
+            const auto outcome = backend.putIfAbsent(key, "should-not-land").outcome;
+            if (outcome != PutOutcome::PreconditionFailed)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: putIfAbsent on an existing key was not rejected (PreconditionFailed expected) — "
+                    "backend does not enforce conditional create");
+            const auto g = backend.get(key);
+            if (!g.has_value() || g->bytes != "probe-v1")
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: putIfAbsent conflict was 'reported' but the original bytes were clobbered — "
+                    "backend does not enforce conditional create");
+        }
+
+        // ---- Step 3: putOverwrite wrong token → PreconditionFailed; bytes intact. ----
+        {
+            /// Wrong-token values are NUMERIC on purpose: a generation-dialect backend (GCS)
+            /// validates the If-Match FORMAT client-side and throws on a non-numeric value (an
+            /// ETag-kind token leaking into a generation dialect) — the probe's synthetic wrong
+            /// tokens must be format-valid for EVERY token kind, merely guaranteed-wrong. A huge
+            /// numeric is a wrong ETag on AWS (412), a wrong generation on GCS (412), and a wrong
+            /// sequence on the emulated backends (TokenMismatch).
+            ///
+            /// The TYPE must be the LIVE dialect (t1.type, just observed from this same backend),
+            /// never a hardcoded TokenType::Emulated: a backend that mints a different dialect
+            /// (e.g. Native/ETag) rejects a foreign-dialect token locally, before the wrong VALUE
+            /// ever reaches the wire — which would make this check pass vacuously against a
+            /// non-enforcing store instead of proving enforcement (codex-review-triage §3.18,
+            /// Critical: the №19 local dialect guard must not defeat this probe).
+            Token wrong_token{"900000000000000001", t1.type};
+            const auto outcome = backend.putOverwrite(key, "clobbered", wrong_token).outcome;
+            if (outcome != PutOutcome::PreconditionFailed)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: putOverwrite with a wrong token was not rejected (PreconditionFailed expected) — "
+                    "backend does not enforce conditional overwrite");
+            const auto g = backend.get(key);
+            if (!g.has_value() || g->bytes != "probe-v1")
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: putOverwrite with wrong token was 'rejected' but the original bytes were clobbered");
+        }
+
+        // ---- Step 4: putOverwrite correct token → Done; bytes replaced; token changed. ----
+        Token t2;
+        {
+            const auto res = backend.putOverwrite(key, "probe-v2", t1);
+            t2 = res.token;
+            if (res.outcome != PutOutcome::Done)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: putOverwrite with the correct token was rejected — backend does not accept valid overwrite");
+            if (t2 == t1)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: putOverwrite succeeded but did not mint a new token — tokens must change on every write");
+            const auto g = backend.get(key);
+            if (!g.has_value() || g->bytes != "probe-v2")
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: putOverwrite succeeded but the new bytes are not readable");
+        }
+
+        // ---- Step 5: casPut chain. ----
+        // 5a: create-if-absent (nullopt expected).
+        Token ct1;
+        {
+            const auto res = backend.casPut(cas_key, "cas-s1", std::nullopt);
+            ct1 = res.token;
+            if (res.outcome != CasOutcome::Committed)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: casPut create-if-absent (nullopt expected) was not committed — "
+                    "backend does not support CAS create-if-absent");
+        }
+        // 5b: conflict on existing (nullopt expected, but key exists).
+        {
+            const auto outcome = backend.casPut(cas_key, "cas-s1x", std::nullopt).outcome;
+            if (outcome != CasOutcome::Conflict)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: casPut with nullopt expected against an existing key was not Conflict — "
+                    "backend does not enforce create-if-absent semantics on casPut");
+        }
+        // 5c: conflict on stale token.
+        {
+            Token stale{"900000000000000002", ct1.type};   /// numeric + live dialect: see step 3
+            const auto outcome = backend.casPut(cas_key, "cas-s1y", stale).outcome;
+            if (outcome != CasOutcome::Conflict)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: casPut with a stale token was not Conflict — "
+                    "backend does not enforce token-exact CAS");
+        }
+        // Bytes must still be the original.
+        {
+            const auto g = backend.get(cas_key);
+            if (!g.has_value() || g->bytes != "cas-s1")
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: casPut conflicts were reported but the original bytes were altered");
+        }
+        // 5d: commit on current token.
+        {
+            const auto res = backend.casPut(cas_key, "cas-s2", ct1);
+            if (res.outcome != CasOutcome::Committed)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: casPut with the current token was not committed — "
+                    "backend does not honor casPut with matching token");
+            const auto g = backend.get(cas_key);
+            if (!g.has_value() || g->bytes != "cas-s2")
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: casPut committed but new bytes are not readable");
+        }
+
+        // ---- Step 6: deleteExact wrong token → TokenMismatch AND the object still readable. ----
+        {
+            Token wrong_token{"900000000000000003", t2.type};   /// numeric + live dialect: see step 3
+            const auto d = backend.deleteExact(key, wrong_token);
+            if (d.kind != DeleteOutcome::Kind::TokenMismatch)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: deleteExact with a wrong token was not TokenMismatch — "
+                    "delete with mismatching token was honored — backend does not enforce conditional deletes");
+            const auto g = backend.get(key);
+            if (!g.has_value())
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: deleteExact with a wrong token was rejected (correctly) but the object was deleted anyway — "
+                    "backend does not enforce conditional deletes");
+        }
+
+        // ---- Step 7: list(probe_prefix) contains the probe key (list-after-write). ----
+        {
+            const auto page = backend.list(probe_prefix, "", 100);
+            bool found = false;
+            for (const auto & listed : page.keys)
+            {
+                if (listed.key == key)
+                {
+                    found = true;
+                    break;
+                }
+            }
+            if (!found)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: list-after-write failed — the probe key '{}' is not visible in the listing under prefix '{}'",
+                    key, probe_prefix);
+        }
+
+        // ---- Step 8: deleteExact correct token → Deleted; object gone; no delete marker;
+        //              list no longer contains the key. ----
+        {
+            const auto d = backend.deleteExact(key, t2);
+            if (d.kind != DeleteOutcome::Kind::Deleted)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: deleteExact with the correct token was not Deleted — backend rejected a valid token-exact delete");
+            if (d.created_delete_marker)
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: deleteExact succeeded but created a versioning delete marker — the bucket has "
+                    "object VERSIONING enabled, and a content-addressed pool cannot run on a versioned bucket: "
+                    "every GC delete would archive a noncurrent version instead of reclaiming storage (the bucket "
+                    "grows forever), and the constantly-rewritten ref objects would pile up versions on every "
+                    "commit. This is NOT ignorable and has no override. Use a bucket where versioning was NEVER "
+                    "enabled — note that merely SUSPENDING versioning is not enough (deletes on a "
+                    "versioning-suspended bucket still mint delete markers, so this probe will refuse again)");
+            const auto g = backend.get(key);
+            if (g.has_value())
+                throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                    "CasProbe: deleteExact succeeded (Deleted) but the object is still readable — backend delete is not effective");
+            // List-after-delete.
+            const auto page = backend.list(probe_prefix, "", 100);
+            for (const auto & listed : page.keys)
+            {
+                if (listed.key == key)
+                    throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+                        "CasProbe: list-after-delete failed — the deleted probe key '{}' is still visible in the listing under prefix '{}'",
+                        key, probe_prefix);
+            }
+        }
+
+        // ---- Step 9: cleanup (best-effort; also deletes cas_key). ----
+        // cas_key is still alive — clean it up via its current token.
+        {
+            const auto h = backend.head(cas_key);
+            if (h.exists)
+                backend.deleteExact(cas_key, h.token);
+        }
+    }
+    catch (...)
+    {
+        // Best-effort cleanup on failure path before re-throwing.
+        cleanup();
+        throw;
+    }
+
+    // Normal-exit cleanup (cas_key was cleaned inside the try; key was deleted in step 8).
+    // Call cleanup anyway to handle any partial state edge cases — it is a no-op if keys are gone.
+    cleanup();
+}
+
+bool probeConditionalCopy(IObjectStorage & object_storage, const String & probe_prefix)
+{
+    const String src_key = probe_prefix + "/src";
+    const String dst_key = probe_prefix + "/dst";
+
+    // Best-effort cleanup — runs unconditionally, on every exit path (success, non-enforcing
+    // result, or exception), mirroring runCapabilityProbe's cleanup lambda.
+    auto cleanup = [&]() noexcept
+    {
+        for (const auto & key : {src_key, dst_key})
+        {
+            try
+            {
+                object_storage.removeObjectIfExists(StoredObject(key));
+            }
+            catch (...) {} /// NOLINT(bugprone-empty-catch)
+        }
+    };
+
+    try
+    {
+        // ---- Step 1: write a tiny throwaway source object. ----
+        {
+            auto buf = object_storage.writeObject(StoredObject(src_key), WriteMode::Rewrite);
+            static constexpr char payload[] = "cas-conditional-copy-probe";
+            buf->write(payload, sizeof(payload) - 1);
+            buf->finalize();
+        }
+
+        // ---- Step 2: conditional copy to a FRESH destination -> must be created. ----
+        const auto first = object_storage.copyObjectConditional(
+            StoredObject(src_key), StoredObject(dst_key), ReadSettings{}, WriteSettings{});
+        if (!first.created)
+        {
+            // The backend refused to create a fresh destination — broken or misbehaving in a
+            // different way than "does not enforce the precondition". Either way, fail closed.
+            cleanup();
+            return false;
+        }
+
+        // ---- Step 3: the SAME conditional copy onto the now-existing destination -> must be
+        //              REJECTED (created == false). A backend that reports created == true here
+        //              silently overwrote the destination — it does NOT enforce If-None-Match, and
+        //              the S3-native staging promote path would be unsafe on it (see CasProbe.h). ----
+        const auto second = object_storage.copyObjectConditional(
+            StoredObject(src_key), StoredObject(dst_key), ReadSettings{}, WriteSettings{});
+
+        cleanup();
+        return !second.created;
+    }
+    catch (...)
+    {
+        // Any exception (including the default NOT_IMPLEMENTED thrown by a backend that does not
+        // override copyObjectConditional) is fail-closed: conditional copy is NOT supported.
+        cleanup();
+        return false;
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h
new file mode 100644
index 000000000000..37313482be69
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h
@@ -0,0 +1,62 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h>
+#include <base/types.h>
+
+namespace DB::Cas
+{
+
+/// Run the capability battery against `backend`, using throwaway keys under `probe_prefix`.
+///
+/// The probe validates the backend preconditions required by a writable content-addressed pool:
+///   1. Store-level safety checks pass, including the requirement that conditional writes use one
+///      underlying HTTP attempt. Hidden SDK retries can outlive the writer's mount lease and obscure
+///      whether a conditional operation committed; retries must therefore be explicit CAS state-machine
+///      transitions rather than transparent client behavior.
+///   2. Conditional-create and conditional-overwrite are enforced (`putIfAbsent` prevents overwrites,
+///      and `putOverwrite` rejects a wrong-token update).
+///   3. `casPut` supports create-if-absent, conflict-on-existing, conflict-on-stale, and commit-on-current.
+///   4. Conditional-delete is enforced (`deleteExact` with a wrong token is rejected and the object survives).
+///   5. Listing reflects both creation and deletion of a probe object.
+///   6. Successful deletion does not create a versioning delete marker. A content-addressed pool cannot
+///      reclaim storage correctly from a versioned bucket: garbage-collection deletes would archive old
+///      versions instead of removing objects, and repeated ref updates would accumulate versions.
+///
+/// On any failed check, throws a DB::Exception(ErrorCodes::NOT_IMPLEMENTED) with a message naming the
+/// specific failed check. This is fail-closed: a backend that does not pass the battery MUST NOT be
+/// used to coordinate a content-addressed pool.
+///
+/// Cleanup of probe keys is best-effort and runs unconditionally: after the battery completes, or on
+/// the failure path immediately before the check-failing exception is rethrown. Cleanup itself suppresses
+/// exceptions so that it cannot hide the capability-check failure.
+void runCapabilityProbe(Backend & backend, const String & probe_prefix);
+
+/// Probe whether `object_storage` ENFORCES a write-once conditional server-side copy
+/// (`IObjectStorage::copyObjectConditional`, `If-None-Match: *`) — an OPTIONAL capability, unlike
+/// the mandatory battery above (`runCapabilityProbe`). It is meaningful only for a disk configured
+/// with `staging_backend=s3`. S3-native staging promotes a temporary object into a content-addressed
+/// blob with this copy; if the destination precondition is ignored, the copy can silently overwrite a
+/// live blob. Such a backend is unsafe for S3-native staging, so the metadata layer must fall back to
+/// local staging rather than refuse to mount.
+///
+/// Goes directly through `IObjectStorage`, not through `Backend`/`CasProbe`'s battery — this keeps
+/// the probe decoupled from the `Backend`'s content-addressed operations. The metadata layer uses the
+/// result only to decide whether the optional S3-native staging path is safe to enable.
+///
+/// Writes a tiny throwaway object at `<probe_prefix>/src`, conditionally copies it to
+/// `<probe_prefix>/dst` (expects `created == true` — a fresh destination), then repeats the SAME
+/// conditional copy onto the now-existing `dst` (expects `created == false` — the destination must
+/// be REJECTED, proving the backend enforces `If-None-Match`). Returns `true` only when both
+/// expectations hold.
+///
+/// Fail-close: ANY exception (including the default `NOT_IMPLEMENTED` a backend throws when it does
+/// not override `copyObjectConditional` at all) OR a non-enforcing result (the second copy also
+/// reports `created == true`, i.e. the backend silently overwrote the destination) returns `false`.
+/// This function never throws — the caller treats `false` as "fall back to local staging", never as
+/// a mount failure.
+///
+/// Cleanup of the probe objects (`src`, `dst`) is best-effort and runs unconditionally on normal and
+/// exceptional exits, mirroring `runCapabilityProbe`.
+bool probeConditionalCopy(IObjectStorage & object_storage, const String & probe_prefix);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.cpp
new file mode 100644
index 000000000000..e8f9ef24ab4b
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.cpp
@@ -0,0 +1,745 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+
+#include <Common/Exception.h>
+#include <Common/LoggingHelpers.h>
+#include <Common/ProfileEvents.h>
+#include <Common/logger_useful.h>
+
+#include "config.h"
+
+#if USE_AWS_S3
+#include <IO/S3Common.h>
+#endif
+
+#include <algorithm>
+#include <chrono>
+#include <thread>
+#include <utility>
+
+namespace ProfileEvents
+{
+    extern const Event CASConditionalWriteAttempts;
+    extern const Event CASConditionalWriteCommitted;
+    extern const Event CASConditionalWriteDefiniteFailure;
+    extern const Event CASConditionalWriteUnresolved;
+    extern const Event CASConditionalWriteFenceLostPostWrite;
+}
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+    extern const int NETWORK_ERROR;
+    extern const int NOT_IMPLEMENTED;
+}
+}
+
+namespace DB::Cas
+{
+
+CasWriteOutcome classifyConditionalWriteResult([[maybe_unused]] const std::exception & e)
+{
+#if USE_AWS_S3
+    /// `PreconditionFailed`/`NoSuchKey` (a lost If-None-Match/If-Match — see
+    /// ObjectStorageBackend::finalizeConditionalWrite for the exact matching), any 5xx
+    /// (InternalError/ServiceUnavailable/SlowDown/RequestTimeout), and any S3 error this function does
+    /// not recognize all fall through to the fail-safe default below: Unresolved. Only the WHITELIST
+    /// below proves the request was never applied.
+    if (const auto * s3e = dynamic_cast<const S3Exception *>(&e))
+    {
+        if (S3::isMalformedRequestError(*s3e) || S3::isEntityTooLargeError(*s3e) || S3::isAccessDeniedError(*s3e))
+            return CasWriteOutcome::DefiniteFailure;
+    }
+#endif
+    /// Poco::Net::NetException (connection loss) / Poco::TimeoutException (client-side timeout) and
+    /// every other error type: the request's fate is unproven — fail toward "resolve before
+    /// reissuing, never toward a false
+    /// DefiniteFailure.
+    return CasWriteOutcome::Unresolved;
+}
+
+void recordConditionalWriteAttemptStarted()
+{
+    ProfileEvents::increment(ProfileEvents::CASConditionalWriteAttempts);
+}
+
+void recordConditionalWriteOutcome(CasWriteOutcome outcome)
+{
+    switch (outcome)
+    {
+        case CasWriteOutcome::Committed:
+            ProfileEvents::increment(ProfileEvents::CASConditionalWriteCommitted);
+            return;
+        case CasWriteOutcome::DefiniteFailure:
+            ProfileEvents::increment(ProfileEvents::CASConditionalWriteDefiniteFailure);
+            return;
+        case CasWriteOutcome::Unresolved:
+            ProfileEvents::increment(ProfileEvents::CASConditionalWriteUnresolved);
+            return;
+    }
+}
+
+namespace
+{
+
+uint64_t steadyClockNowMs()
+{
+    return static_cast<uint64_t>(std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::steady_clock::now().time_since_epoch()).count());
+}
+
+/// The default inter-attempt backoff sleep. NOT a race-fix sleep: it is deliberate, bounded,
+/// fence-gated pacing of reissues toward a recovering object store, and it is injectable so tests
+/// never wait on it.
+void threadSleepMs(uint64_t ms)
+{
+    std::this_thread::sleep_for(std::chrono::milliseconds(ms));
+}
+
+/// Deterministic caller/local bugs the create retry loop must surface immediately:
+/// reissuing only replays the same failure — up to ~12 minutes of budget × putBlob's outer loop at
+/// the defaults — and buries the root cause behind a retryable ABORTED. The set:
+///   LOGICAL_ERROR   — a local invariant violation (e.g. uploadFromSource's source-size check; pinned
+///                     by `CasPartWriteTxn.PutBlobWrongSizeFailsClosed`, which caught exactly this class)
+///   NOT_IMPLEMENTED — a mode/capability guard (e.g. `promoteStaged` on a backend without a native
+///                     conditional server-side copy) — a deterministic configuration bug
+///   BAD_ARGUMENTS   — a deterministic encode/argument rejection (e.g. BAD_ARGUMENTS escaping
+///                     buildHeader's second, intended_ref-less encode)
+///   CORRUPTED_DATA  — integrity failure; retrying re-reads/re-streams the same bad bytes (the same
+///                     fail-fast rule the driver-side correctness markers enforce)
+/// Fail-safe either way: a propagated exception is never a false Committed.
+bool isDeterministicLocalFailure(int code)
+{
+    return code == ErrorCodes::LOGICAL_ERROR || code == ErrorCodes::NOT_IMPLEMENTED
+        || code == ErrorCodes::BAD_ARGUMENTS || code == ErrorCodes::CORRUPTED_DATA;
+}
+
+}
+
+void validateCasRequestBudget(const CasRequestBudget & budget, uint64_t mount_lease_ttl_ms, uint64_t mount_renew_period_ms)
+{
+    if (budget.max_attempts < 1)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "CAS request budget rejected: max_attempts must be at least 1 (got {}) — zero would let "
+            "putIfAbsentControlled return Unresolved without ever sending an attempt.",
+            budget.max_attempts);
+
+    /// Overflow-safe: `attempt_timeout_ms + lease_safety_margin_ms` could wrap uint64 for absurd config
+    /// values, which would make the sum spuriously small and the inequality below pass when it should
+    /// fail closed. Compare via subtraction against the (unsigned, so already non-negative) TTL instead
+    /// of computing the sum directly.
+    if (!(budget.attempt_timeout_ms < mount_lease_ttl_ms
+          && budget.lease_safety_margin_ms < mount_lease_ttl_ms - budget.attempt_timeout_ms))
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "CAS request budget rejected: attempt_timeout_ms ({}) + lease_safety_margin_ms ({}) must be "
+            "strictly less than the mount lease TTL ({} ms). A writable mount refuses to open with "
+            "this budget.",
+            budget.attempt_timeout_ms, budget.lease_safety_margin_ms, mount_lease_ttl_ms);
+    /// STRICTLY less, and the strictness is the load-bearing half. `attempt_timeout_ms >
+    /// operation_deadline_ms` is the obvious error — a single attempt cannot outlast the logical
+    /// operation it belongs to. EQUALITY is the subtle one, and it is worse than useless: the deadline
+    /// is captured as `now + operation_deadline_ms` and every pre-send gate below asks
+    /// `now + attempt_timeout_ms > deadline_ms`, so equal values collapse that to `now_2 > now_1` and
+    /// ONE elapsed millisecond between the two clock reads refuses the operation having sent NOTHING.
+    /// The resulting behaviour is "mostly works, occasionally refuses with nothing sent", decided by
+    /// the scheduler rather than by the budget — exactly the flakiness this validation exists to catch,
+    /// and observed three times in tests before it was forbidden. A caller that wants one attempt says
+    /// `max_attempts = 1`; the equality adds only the race.
+    if (!(budget.attempt_timeout_ms < budget.operation_deadline_ms))
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "CAS request budget rejected: attempt_timeout_ms ({}) must be strictly less than "
+            "operation_deadline_ms ({}) — equality turns the pre-send gate into a wall-clock race that "
+            "refuses after a single elapsed tick, having sent nothing. Use max_attempts to bound the "
+            "number of attempts.",
+            budget.attempt_timeout_ms, budget.operation_deadline_ms);
+    if (!(budget.retry_initial_backoff_ms <= budget.retry_max_backoff_ms))
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "CAS request budget rejected: retry_initial_backoff_ms ({}) must not exceed "
+            "retry_max_backoff_ms ({}) — the capped-exponential backoff cap cannot sit below its own "
+            "starting value. Set both to 0 to disable inter-attempt backoff.",
+            budget.retry_initial_backoff_ms, budget.retry_max_backoff_ms);
+
+    LOG_INFO(getLogger("CasRequestControl"),
+        "CAS request budget in effect: attempt_timeout_ms={} operation_deadline_ms={} max_attempts={} "
+        "lease_safety_margin_ms={} retry_initial_backoff_ms={} retry_max_backoff_ms={} "
+        "(mount_lease_ttl_ms={} mount_renew_period_ms={})",
+        budget.attempt_timeout_ms, budget.operation_deadline_ms, budget.max_attempts,
+        budget.lease_safety_margin_ms, budget.retry_initial_backoff_ms, budget.retry_max_backoff_ms,
+        mount_lease_ttl_ms, mount_renew_period_ms);
+}
+
+namespace
+{
+/// Shared by both public entry points below so the log line and the exception's message text can
+/// never drift apart. Rate-limited (not per-distinct-`why` -- `LogSeriesLimiter` keys on the LOGGER
+/// NAME only, so under a sustained outage where `why` keeps changing slightly, only the first message
+/// in each window prints; this is the intended throttle, not a bug). Warning-level visibility is
+/// intentional: this condition is expected to self-heal
+/// (the caller retries), but an operator watching CAS logs directly should see it without having to
+/// know to look at system.replication_queue.
+void logCasWriteRetryLater(const String & why)
+{
+    LogSeriesLimiter log(getLogger("CasWriteRetryLater"), /*allowed_count=*/1, /*interval_s=*/30);
+    LOG_WARNING(log, "CAS write could not be committed ({}); retrying later", why);
+}
+}
+
+[[noreturn]] void throwCasWriteRetryLater(const String & why)
+{
+    logCasWriteRetryLater(why);
+    throw Exception(ErrorCodes::NETWORK_ERROR, "CAS write could not be committed ({}); retrying later", why);
+}
+
+std::exception_ptr makeCasWriteRetryLaterExceptionPtr(const String & why)
+{
+    logCasWriteRetryLater(why);
+    return std::make_exception_ptr(
+        Exception(ErrorCodes::NETWORK_ERROR, "CAS write could not be committed ({}); retrying later", why));
+}
+
+[[noreturn]] void throwCasTransientUnavailable(const String & subject, const String & condition)
+{
+    /// The code is coarse (it shares a `system.errors` row with socket failures), so the MESSAGE must
+    /// carry the whole truth: which CA condition refused, and that the refusal is a state rather than
+    /// damage. Consumers key on the code; operators read this line.
+    ///
+    /// The shared suffix carries ONLY the classification, because that is the one claim true at every
+    /// site: retry-later is right even where the condition may turn out terminal, since the next attempt
+    /// re-decides against fresh state. Any promise about HOW the condition clears belongs in `condition`,
+    /// where the site that can actually prove it makes it -- `checkFenceOrThrow` provably cannot.
+    throw Exception(ErrorCodes::NETWORK_ERROR,
+        "{} -- {}; TRANSIENT unavailability, not damage", subject, condition);
+}
+
+CasRequestController::CasRequestController(BackendPtr backend_, CasRequestBudget budget_, std::function<uint64_t()> now_ms_,
+                                           std::function<void(uint64_t)> sleep_ms_)
+    : backend(std::move(backend_))
+    , budget(budget_)
+    , now_ms(now_ms_ ? std::move(now_ms_) : std::function<uint64_t()>(steadyClockNowMs))
+    , sleep_ms(sleep_ms_ ? std::move(sleep_ms_) : std::function<void(uint64_t)>(threadSleepMs))
+{
+}
+
+void CasRequestController::setSleepFnForTest(std::function<void(uint64_t)> sleep_ms_)
+{
+    sleep_ms = sleep_ms_ ? std::move(sleep_ms_) : std::function<void(uint64_t)>(threadSleepMs);
+}
+
+uint64_t CasRequestController::backoffBeforeAttempt(uint32_t next_attempt) const
+{
+    const uint64_t initial = budget.retry_initial_backoff_ms;
+    const uint64_t cap = budget.retry_max_backoff_ms;
+    if (initial == 0 || next_attempt < 2)
+        return 0;
+    /// Saturating `initial << doublings`: `initial > cap >> doublings` implies the unshifted product
+    /// already exceeds the cap, so return the cap without ever computing an overflowing shift.
+    const uint32_t doublings = next_attempt - 2;
+    if (doublings >= 63 || initial > (cap >> doublings))
+        return cap;
+    return std::min(initial << doublings, cap);
+}
+
+bool CasRequestController::pauseBeforeReissue(uint32_t completed_attempt, uint64_t deadline_ms,
+                                              const std::function<bool()> & fence_ok, CasUnresolvedReason * out_reason)
+{
+    /// Fence BEFORE the sleep (the pre-attempt fence rule applies to the whole loop, not just the
+    /// attempt): a fence lost mid-backoff aborts the operation instantly — sleeping first would keep a
+    /// fenced writer alive for up to a full backoff cap after it lost its right to write.
+    if (!fence_ok())
+    {
+        if (out_reason)
+            *out_reason = CasUnresolvedReason::FenceLostMidWay;
+        return false;
+    }
+    const uint64_t backoff = backoffBeforeAttempt(completed_attempt + 1);
+    if (backoff == 0)
+        return true;
+    /// Never serve a sleep the operation cannot afford: if the backoff plus one more attempt would
+    /// cross the operation deadline, give up NOW instead of sleeping into a guaranteed Unresolved.
+    if (now_ms() + backoff + budget.attempt_timeout_ms > deadline_ms)
+    {
+        if (out_reason)
+            *out_reason = CasUnresolvedReason::DeadlineMidWay;
+        return false;
+    }
+    sleep_ms(backoff);
+    return true;
+}
+
+CasWriteOutcome CasRequestController::resolveByExactGet(std::string_view key, std::string_view expected_bytes,
+                                                        Token * out_token)
+{
+    const String key_s{key};
+    std::optional<GetResult> got;
+    try
+    {
+        got = backend->get(key_s);
+    }
+    catch (const std::exception &)
+    {
+        /// The GET itself failed (network, auth, ...): the object's identity cannot be proven either
+        /// way — an unresolved read leaves this Unresolved, exactly like an absent read.
+        return CasWriteOutcome::Unresolved;
+    }
+
+    if (!got)
+        return CasWriteOutcome::Unresolved;   /// absent -> another attempt may still be legal
+
+    if (got->bytes == expected_bytes)
+    {
+        if (out_token)
+            *out_token = got->token;
+        return CasWriteOutcome::Committed;    /// identical deterministic bytes -> the earlier attempt DID commit
+    }
+
+    /// A DIFFERENT valid object at the exact key this create intended: a real conflict, not a retryable
+    /// ambiguity. Fail closed rather than silently treating it as Unresolved/DefiniteFailure.
+    throw Exception(ErrorCodes::CORRUPTED_DATA,
+        "CasRequestController: exact-key resolution at '{}' observed a DIFFERENT object than the one "
+        "this attempt intended to create — a real conflict, not a retryable ambiguity", key_s);
+}
+
+CasWriteOutcome CasRequestController::putIfAbsentControlled(
+    std::string_view key, std::string_view bytes, const std::function<bool()> & fence_ok, Token * out_token,
+    CasUnresolvedReason * out_reason)
+{
+    const String key_s{key};
+    const String bytes_s{bytes};
+    const uint64_t deadline_ms = now_ms() + budget.operation_deadline_ms;
+    /// Diagnostic bookkeeping only -- nothing below branches on it (finding #37 defect 3).
+    uint32_t attempts_sent = 0;
+    /// Does an EARLIER attempt of THIS call remain unresolved -- sent, and neither proven applied nor
+    /// proven refused? Set on the one path that produces exactly that state: an attempt whose outcome
+    /// was ambiguous and whose exact-key resolve came back absent or unreadable. The request may have
+    /// been received; an absent read now proves nothing about what materializes later, which is the
+    /// whole reason the reissue loop exists. This is NOT diagnostic: it decides the CALL's verdict at
+    /// the DefiniteFailure arm below. A pre-attempt gate refusal never sets it -- those return without
+    /// sending, so they leave nothing that could land.
+    bool earlier_attempt_unresolved = false;
+    const auto unresolved = [&](CasUnresolvedReason reason)
+    {
+        if (out_reason)
+            *out_reason = reason;
+        return CasWriteOutcome::Unresolved;
+    };
+    if (out_reason)
+        *out_reason = CasUnresolvedReason::NotUnresolved;
+
+    for (uint32_t attempt = 1; attempt <= budget.max_attempts; ++attempt)
+    {
+        /// Gate BEFORE every attempt: the
+        /// local mount fence must still hold, and there must be enough of the operation's own deadline
+        /// left for one more attempt to plausibly complete. Neither check sends anything to the backend.
+        if (!fence_ok())
+            return unresolved(attempts_sent == 0 ? CasUnresolvedReason::NoAttemptSent
+                                                 : CasUnresolvedReason::FenceLostMidWay);
+        if (now_ms() + budget.attempt_timeout_ms > deadline_ms)
+            return unresolved(attempts_sent == 0 ? CasUnresolvedReason::NoAttemptSent
+                                                 : CasUnresolvedReason::DeadlineMidWay);
+        ++attempts_sent;
+
+        /// The committed incarnation's token, filled by whichever leg proves Committed below.
+        Token committed_token;
+        CasWriteOutcome attempt_outcome{};
+        try
+        {
+            const PutResult put = backend->putIfAbsent(key_s, bytes_s);
+            /// PreconditionFailed here means only "the key already exists" — it does NOT prove who
+            /// created it (possibly OUR earlier unresolved attempt). Collapse it onto Unresolved so it
+            /// goes through the SAME resolve-before-reissue path as an ambiguous exception, never a
+            /// false DefiniteFailure/Committed.
+            attempt_outcome = put.outcome == PutOutcome::Done ? CasWriteOutcome::Committed : CasWriteOutcome::Unresolved;
+            if (put.outcome == PutOutcome::Done)
+                committed_token = put.token;
+        }
+        catch (const std::exception & e)
+        {
+            attempt_outcome = classifyConditionalWriteResult(e);
+        }
+
+        if (attempt_outcome == CasWriteOutcome::DefiniteFailure)
+        {
+            /// THIS attempt is proven never applied — but the verdict belongs to the CALL. An earlier
+            /// attempt that is still unresolved may yet materialize at the key, and a caller reading
+            /// `DefiniteFailure` acts on "the key is unwritten": `CasRefLedger::commitRefChunk` clears
+            /// its apply-pending marker and reports the txn id never used, so the next append re-derives
+            /// that id and a late-landing predecessor becomes an acked-then-lost transaction. Ambiguity
+            /// dominates a definite refusal that came after it; the caller wedges and resolves the key
+            /// instead. No resolve and no retry either way — this attempt has nothing left to settle.
+            if (earlier_attempt_unresolved)
+                return unresolved(CasUnresolvedReason::DefiniteFailureAfterAmbiguity);
+            return CasWriteOutcome::DefiniteFailure;   /// every attempt of this call was proven never applied
+        }
+
+        if (attempt_outcome == CasWriteOutcome::Unresolved)
+        {
+            /// Resolve-before-reissue. May throw CORRUPTED_DATA (a real
+            /// conflict) straight out of this call — that is never a retry signal.
+            attempt_outcome = resolveByExactGet(key_s, bytes_s, &committed_token);
+            if (attempt_outcome == CasWriteOutcome::Unresolved)
+            {
+                /// This attempt is now one that may still land: it was sent, and the resolve settled
+                /// nothing. Recorded BEFORE the exhaustion checks below so it is set no matter which of
+                /// them ends the loop, and read by the DefiniteFailure arm of every later attempt.
+                earlier_attempt_unresolved = true;
+                /// Absent/unreadable: another attempt of the SAME (key, bytes) may be legal — after the
+                /// fence-gated capped-exponential backoff (pauseBeforeReissue). No pause after the LAST
+                /// attempt: the budget is spent, sleeping would only delay the Unresolved verdict.
+                ///
+                /// Both refusals report through `unresolved`, never a bare `return Unresolved`: this is
+                /// the ordinary way a busy lane exhausts itself, so leaving `out_reason` at its initial
+                /// `NotUnresolved` here made the ref lane's wedge message read "is UNCERTAIN (not
+                /// unresolved)" for the single most common wedge there is.
+                if (attempt == budget.max_attempts)
+                    return unresolved(CasUnresolvedReason::AttemptsExhausted);
+                CasUnresolvedReason pause_reason = CasUnresolvedReason::AttemptsExhausted;
+                if (!pauseBeforeReissue(attempt, deadline_ms, fence_ok, &pause_reason))
+                    return unresolved(pause_reason);
+                continue;
+            }
+        }
+
+        /// attempt_outcome == Committed here (either the attempt's own 2xx, or resolution found
+        /// identical bytes). Final fence check before reporting success: a fence lost here means the
+        /// write may have landed but this call must never claim it did. Count this "response observed
+        /// after the local fence" leg separately from the generic Unresolved classifier so a cross-epoch
+        /// fence loss is visible rather than folded into ordinary retry-budget exhaustion.
+        if (!fence_ok())
+        {
+            ProfileEvents::increment(ProfileEvents::CASConditionalWriteFenceLostPostWrite);
+            return unresolved(CasUnresolvedReason::FenceLostPostWrite);
+        }
+        if (out_token)
+            *out_token = committed_token;
+        return CasWriteOutcome::Committed;
+    }
+
+    return unresolved(CasUnresolvedReason::AttemptsExhausted);   /// budget exhausted, no definite outcome
+}
+
+CasCreateResult CasRequestController::conditionalCreateControlled(
+    std::string_view key, const std::function<PutResult()> & attempt, const std::function<bool()> & fence_ok)
+{
+    const String key_s{key};
+    const uint64_t deadline_ms = now_ms() + budget.operation_deadline_ms;
+
+    for (uint32_t attempt_no = 1; attempt_no <= budget.max_attempts; ++attempt_no)
+    {
+        /// Same pre-attempt gates as putIfAbsentControlled.
+        if (!fence_ok())
+            return {CasCreateOutcome::Unresolved, {}};
+        if (now_ms() + budget.attempt_timeout_ms > deadline_ms)
+            return {CasCreateOutcome::Unresolved, {}};
+
+        std::optional<PutResult> put;
+        try
+        {
+            put = attempt();
+        }
+        catch (const std::exception & e)
+        {
+            /// A deterministic LOCAL bug surfaced by the attempt itself — a caller/config error, never
+            /// a wire ambiguity; reissuing would only replay it. Propagate unchanged: instant, loud,
+            /// exactly the pre-controller behavior (see isDeterministicLocalFailure for the set and the
+            /// per-code rationale). This deliberately differs from `putIfAbsentControlled`'s
+            /// everything-Unresolved:
+            /// that lane's byte-exact resolve makes retrying any unproven error harmless, while
+            /// retrying a broken source/mode/encode here is pure noise. Fail-safe either way — a
+            /// propagated exception is never a false Committed.
+            if (const auto * db_e = dynamic_cast<const Exception *>(&e); db_e && isDeterministicLocalFailure(db_e->code()))
+                throw;
+            /// A whitelisted synchronous rejection PROVES the request was never applied: surface the
+            /// original exception — the blob lane's callers always saw the raw storage error's root
+            /// cause, and losing it behind an outcome enum here would only degrade diagnostics
+            /// Anything else is ambiguous: fall through to the occupancy
+            /// resolve below.
+            if (classifyConditionalWriteResult(e) == CasWriteOutcome::DefiniteFailure)
+                throw;
+        }
+
+        if (put)
+        {
+            if (put->outcome == PutOutcome::PreconditionFailed)
+                return {CasCreateOutcome::Occupied, {}};
+
+            /// Done. Final fence check before reporting success: a fence
+            /// lost here means the write may have landed but this call must never claim it did.
+            if (!fence_ok())
+            {
+                ProfileEvents::increment(ProfileEvents::CASConditionalWriteFenceLostPostWrite);
+                return {CasCreateOutcome::Unresolved, {}};
+            }
+            return {CasCreateOutcome::Committed, put->token};
+        }
+
+        /// Ambiguous attempt: resolve by exact-key OCCUPANCY — one HEAD, never a body GET (the body
+        /// may be multi-GB, and reading a possibly-condemned occupant would flirt with the resurrect
+        /// invariant; the key's content-address IS the identity proof, see the header contract).
+        bool occupied = false;
+        bool head_answered = true;
+        try
+        {
+            occupied = backend->head(key_s).exists;
+        }
+        catch (const std::exception &)
+        {
+            /// The HEAD itself failed: occupancy unproven either way. Reissuing is still safe — an
+            /// occupant answers the reissued If-None-Match with PreconditionFailed (-> Occupied on
+            /// the next round) — so treat exactly like "absent" and let the budget bound the loop.
+            head_answered = false;
+        }
+        if (head_answered && occupied)
+            return {CasCreateOutcome::Occupied, {}};
+
+        if (attempt_no == budget.max_attempts || !pauseBeforeReissue(attempt_no, deadline_ms, fence_ok))
+            return {CasCreateOutcome::Unresolved, {}};
+    }
+
+    return {CasCreateOutcome::Unresolved, {}};   /// attempt budget exhausted without a definite outcome
+}
+
+CasOverwriteResult CasRequestController::putOverwriteControlled(
+    std::string_view key, std::string_view bytes, const Token & expected, const std::function<bool()> & fence_ok)
+{
+    const String key_s{key};
+    const String bytes_s{bytes};
+    const uint64_t deadline_ms = now_ms() + budget.operation_deadline_ms;
+
+    for (uint32_t attempt_no = 1; attempt_no <= budget.max_attempts; ++attempt_no)
+    {
+        if (!fence_ok())
+            return {CasOverwriteOutcome::Unresolved, {}};
+        if (now_ms() + budget.attempt_timeout_ms > deadline_ms)
+            return {CasOverwriteOutcome::Unresolved, {}};
+
+        std::optional<PutResult> put;
+        try
+        {
+            put = backend->putOverwrite(key_s, bytes_s, expected);
+        }
+        catch (const std::exception & e)
+        {
+            /// Same rethrow convention as conditionalCreateControlled: a deterministic local bug or
+            /// a whitelisted synchronous rejection PROVES no retry can help -- surface it unchanged.
+            if (const auto * db_e = dynamic_cast<const Exception *>(&e); db_e && isDeterministicLocalFailure(db_e->code()))
+                throw;
+            if (classifyConditionalWriteResult(e) == CasWriteOutcome::DefiniteFailure)
+                throw;
+            /// Else ambiguous -- fall through to resolve below.
+        }
+
+        if (put && put->outcome == PutOutcome::Done)
+        {
+            if (!fence_ok())
+            {
+                ProfileEvents::increment(ProfileEvents::CASConditionalWriteFenceLostPostWrite);
+                return {CasOverwriteOutcome::Unresolved, {}};
+            }
+            return {CasOverwriteOutcome::Committed, put->token};
+        }
+
+        /// Ambiguous: either a caught transient exception, or PreconditionFailed (which alone does
+        /// NOT prove a real conflict -- it may be our own earlier attempt's write landing under a
+        /// concurrent resolve). Resolve with one GET.
+        std::optional<GetResult> got;
+        try
+        {
+            got = backend->get(key_s);
+        }
+        catch (const std::exception &)
+        {
+            got.reset();   /// GET failed: still ambiguous, fall through to retry below.
+        }
+
+        if (got && got->token == expected)
+        {
+            /// The token we CAS'd against is STILL current: our attempt never applied. Fall through
+            /// to the pause-and-reissue gate below (same key, bytes, expected).
+        }
+        else if (got && got->bytes == bytes_s)
+        {
+            if (!fence_ok())
+            {
+                ProfileEvents::increment(ProfileEvents::CASConditionalWriteFenceLostPostWrite);
+                return {CasOverwriteOutcome::Unresolved, {}};
+            }
+            return {CasOverwriteOutcome::Committed, got->token};
+        }
+        else if (got)
+        {
+            /// A DIFFERENT token AND different bytes: a genuine competing write. Real conflict --
+            /// never collapsed into Unresolved/DefiniteFailure, never thrown.
+            return {CasOverwriteOutcome::Conflict, {}};
+        }
+        /// else: the GET itself failed or the key vanished -- still ambiguous, fall through to retry.
+
+        if (attempt_no == budget.max_attempts || !pauseBeforeReissue(attempt_no, deadline_ms, fence_ok))
+            return {CasOverwriteOutcome::Unresolved, {}};
+    }
+
+    return {CasOverwriteOutcome::Unresolved, {}};   /// attempt budget exhausted without a definite outcome
+}
+
+CasOverwriteResult CasRequestController::putIfAbsentControlledMutable(
+    std::string_view key, std::string_view bytes, const std::function<bool()> & fence_ok)
+{
+    const String key_s{key};
+    const String bytes_s{bytes};
+    const uint64_t deadline_ms = now_ms() + budget.operation_deadline_ms;
+
+    for (uint32_t attempt_no = 1; attempt_no <= budget.max_attempts; ++attempt_no)
+    {
+        if (!fence_ok())
+            return {CasOverwriteOutcome::Unresolved, {}};
+        if (now_ms() + budget.attempt_timeout_ms > deadline_ms)
+            return {CasOverwriteOutcome::Unresolved, {}};
+
+        std::optional<PutResult> put;
+        try
+        {
+            put = backend->putIfAbsent(key_s, bytes_s);
+        }
+        catch (const std::exception & e)
+        {
+            /// Same rethrow convention as putOverwriteControlled/conditionalCreateControlled.
+            if (const auto * db_e = dynamic_cast<const Exception *>(&e); db_e && isDeterministicLocalFailure(db_e->code()))
+                throw;
+            if (classifyConditionalWriteResult(e) == CasWriteOutcome::DefiniteFailure)
+                throw;
+            /// Else ambiguous -- fall through to resolve below.
+        }
+
+        if (put && put->outcome == PutOutcome::Done)
+        {
+            if (!fence_ok())
+            {
+                ProfileEvents::increment(ProfileEvents::CASConditionalWriteFenceLostPostWrite);
+                return {CasOverwriteOutcome::Unresolved, {}};
+            }
+            return {CasOverwriteOutcome::Committed, put->token};
+        }
+
+        /// Ambiguous: either a caught transient exception, or PreconditionFailed (which alone does
+        /// NOT prove a real conflict -- it may be our own earlier attempt's write landing under a
+        /// concurrent resolve, or a racing writer creating the identical value). Resolve with one GET.
+        std::optional<GetResult> got;
+        try
+        {
+            got = backend->get(key_s);
+        }
+        catch (const std::exception &)
+        {
+            got.reset();   /// GET failed: still ambiguous, fall through to retry below.
+        }
+
+        if (!got)
+        {
+            /// Still absent: our attempt never applied. Fall through to the pause-and-reissue gate
+            /// below (same key, bytes).
+        }
+        else if (got->bytes == bytes_s)
+        {
+            if (!fence_ok())
+            {
+                ProfileEvents::increment(ProfileEvents::CASConditionalWriteFenceLostPostWrite);
+                return {CasOverwriteOutcome::Unresolved, {}};
+            }
+            return {CasOverwriteOutcome::Committed, got->token};
+        }
+        else
+        {
+            /// Present with DIFFERENT bytes: something else already occupies the key with a
+            /// different value. For a MUTABLE marker this is a normal outcome, not corruption --
+            /// return it as a value, never thrown.
+            return {CasOverwriteOutcome::Conflict, {}};
+        }
+
+        if (attempt_no == budget.max_attempts || !pauseBeforeReissue(attempt_no, deadline_ms, fence_ok))
+            return {CasOverwriteOutcome::Unresolved, {}};
+    }
+
+    return {CasOverwriteOutcome::Unresolved, {}};   /// attempt budget exhausted without a definite outcome
+}
+
+SlotOccupyResult CasRequestController::slotOccupy(
+    std::string_view key, std::string_view bytes, const std::function<bool()> & fence_ok)
+{
+    const String key_s{key};
+    const String bytes_s{bytes};
+    const uint64_t deadline_ms = now_ms() + budget.operation_deadline_ms;
+
+    /// Pre-attempt gate ONLY -- the same two checks every controlled op runs before its first (here,
+    /// only) attempt: the mount fence must still hold, and there must be enough of the operation's own
+    /// deadline left for one attempt to plausibly complete. Neither check sends anything to the
+    /// backend, so a refusal here PROVES the key is untouched by this call. UNLIKE every sibling
+    /// controlled op, there is no post-write recheck below: fence_ok is evaluated once per attempt, and
+    /// the stronger post-I/O consistency check (fence generation together with wedge/txn identity) is
+    /// the CALLER's contract (Task 4/6's re-acquire-lock-and-checkFenceOrThrow step), not this raw
+    /// primitive's.
+    if (!fence_ok() || now_ms() + budget.attempt_timeout_ms > deadline_ms)
+        return {.kind = SlotOccupyResult::Kind::Unresolved, .occupant_bytes = {}, .occupant_token = {},
+                .unresolved_reason = CasUnresolvedReason::NoAttemptSent};
+
+    std::optional<PutResult> put;
+    try
+    {
+        put = backend->putIfAbsent(key_s, bytes_s);
+    }
+    catch (const std::exception & e)
+    {
+        /// Same rethrow convention as putOverwriteControlled/putIfAbsentControlledMutable: a
+        /// deterministic local bug, or a whitelisted synchronous rejection that PROVES the request was
+        /// never applied, surfaces unchanged -- SlotOccupyResult::Kind has no DefiniteFailure member to
+        /// carry either one. Anything else is ambiguous: fall through to the raw resolve GET below,
+        /// exactly like a clean PreconditionFailed -- this primitive cannot and does not distinguish
+        /// the two.
+        if (const auto * db_e = dynamic_cast<const Exception *>(&e); db_e && isDeterministicLocalFailure(db_e->code()))
+            throw;
+        if (classifyConditionalWriteResult(e) == CasWriteOutcome::DefiniteFailure)
+            throw;
+    }
+
+    if (put && put->outcome == PutOutcome::Done)
+        return {.kind = SlotOccupyResult::Kind::Created, .occupant_bytes = {}, .occupant_token = {},
+                .unresolved_reason = CasUnresolvedReason::NotUnresolved};
+
+    /// Ambiguous attempt or a clean conflict: resolve with exactly ONE raw exact GET -- no byte-compare,
+    /// no throw on a different occupant [codex finding 3: this is a DEDICATED slot operation, not
+    /// putIfAbsentControlled (which retries the same (key, bytes) internally) or resolveByExactGet
+    /// (which compares against an expected body and throws CORRUPTED_DATA on a mismatch) composed
+    /// together]. Adjudicating whether the occupant is "mine" is entirely the CALLER's job (the
+    /// CaCasMountCore `mine` contract), never this primitive's.
+    ///
+    /// WHOLE-OBJECT read, unlike conditionalCreateControlled's occupancy resolve (see that method's doc
+    /// in the header), which deliberately uses HEAD instead of GET because a blob body "may be
+    /// multi-GB". That reasoning does not apply here: slotOccupy is scoped by its callers (Task 4/6,
+    /// spec INV-2) to small, write-once CONTROL slots -- ref-log transactions and epoch seals -- whose
+    /// size is bounded by their own format's registry cap (the strict-grammar object caps
+    /// CasRefLogFormat/CasRefCkptFormat enforce on decode), never a data blob. slotOccupy itself stays
+    /// format-agnostic (it takes a raw key/bytes pair, per the "Interface handed to Stage B" contract in
+    /// the plan) and does not encode any format's cap here -- the size bound is a property of what
+    /// callers are allowed to pass it, enforced where the returned bytes are decoded, not by this seam.
+    std::optional<GetResult> got;
+    try
+    {
+        got = backend->get(key_s);
+    }
+    catch (const std::exception &)
+    {
+        got.reset();   /// the GET itself failed: still unresolved -- a one-shot primitive never retries
+    }
+
+    if (!got)
+        /// The occupant that caused the conflict vanished before this GET (or the GET itself failed):
+        /// the outcome is unknowable right now -- NEVER a fabricated Created.
+        return {.kind = SlotOccupyResult::Kind::Unresolved, .occupant_bytes = {}, .occupant_token = {},
+                .unresolved_reason = CasUnresolvedReason::AttemptsExhausted};
+
+    return {.kind = SlotOccupyResult::Kind::Occupied, .occupant_bytes = std::move(got->bytes),
+            .occupant_token = got->token, .unresolved_reason = CasUnresolvedReason::NotUnresolved};
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h
new file mode 100644
index 000000000000..730894cacd27
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h
@@ -0,0 +1,621 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <base/types.h>
+#include <cstdint>
+#include <exception>
+#include <functional>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// Outcome of ONE HTTP attempt for a CAS conditional write (`If-None-Match`/`If-Match`), issued with
+/// the generic S3 client's transparent retries disabled for that attempt. This is the seam the
+/// `CasRequestController` is built on: it decides whether another attempt is legal and how an
+/// uncertain result is resolved.
+///   - Committed: the attempt's own request completed successfully (2xx) — the object is durable.
+///   - DefiniteFailure: a synchronous rejection that PROVES the request was never applied server-side
+///     — a WHITELISTED malformed-request / entity-too-large / access-denied error ONLY. Never
+///     `PreconditionFailed`: a lost precondition means the key exists, not that the request failed.
+///   - Unresolved: everything else — `PreconditionFailed`/`NoSuchKey`, a client-side timeout, a
+///     connection loss, a 5xx, or any error this classifier does not recognize. The caller resolves
+///     the exact key before deciding whether another attempt is legal; ambiguity always
+///     resolves toward Unresolved, never toward a false DefiniteFailure or a false Committed.
+enum class CasWriteOutcome : uint8_t
+{
+    Committed,
+    DefiniteFailure,
+    Unresolved,
+};
+
+/// WHY a controlled write came back `Unresolved`. It exists because `Unresolved` covers two materially
+/// different states, and telling them apart is the difference between a five-minute triage and an hour
+/// of it — and, since finding #37 defect 3, between a table that keeps its write availability and one
+/// that loses it until remount.
+///
+/// `NoAttemptSent` is the one that carries real information: both pre-attempt gates (the mount fence
+/// and the operation deadline) reject BEFORE anything reaches the backend, so on the very first
+/// iteration the key is PROVABLY unwritten — there is no ambiguity to resolve, only a lost right to
+/// write. Every other reason leaves an object that may or may not be durable, which is what the
+/// wedge/resolve machinery exists for.
+///
+/// NOT purely diagnostic any more: `unresolvedProvesNothingWasSent` below turns this into the fact the
+/// ref append lane acts on (`CasRefLedger::commitRefChunk`'s `Unresolved` arm), so ADDING A MEMBER HERE
+/// IS A PROTOCOL DECISION — read that predicate before you do.
+enum class CasUnresolvedReason : uint8_t
+{
+    NotUnresolved,     /// the call did not return Unresolved
+    NoAttemptSent,     /// a pre-attempt gate rejected on the FIRST iteration: nothing was ever sent
+    FenceLostMidWay,   /// >= 1 attempt was sent, then the mount fence dropped
+    DeadlineMidWay,    /// >= 1 attempt was sent, then the operation deadline left no room for another
+    FenceLostPostWrite,/// an attempt COMMITTED but the fence had dropped by the time it returned
+    AttemptsExhausted, /// the genuine case the "retry budget exhausted" wording describes
+    /// A LATER attempt was definitively refused while an EARLIER one of the same call is still
+    /// unresolved. The refusal proves only its own attempt never applied; the earlier one may still
+    /// materialize at the key, so the CALL cannot report `DefiniteFailure` (see
+    /// `putIfAbsentControlled`). Reported instead of the definite verdict, never alongside it.
+    DefiniteFailureAfterAmbiguity,
+};
+
+/// Does this `Unresolved` PROVE that no attempt ever reached the network — i.e. that the key is
+/// unwritten and there is nothing for an exact-key resolution to settle?
+///
+/// True for exactly ONE value, and that is the whole design: `NoAttemptSent` is reported only when a
+/// pre-attempt gate rejected while `attempts_sent == 0`, so `backend->putIfAbsent` was never called
+/// (see `putIfAbsentControlled`). Every other value — including `NotUnresolved`, which a caller can
+/// still observe if some path returns `Unresolved` without recording a reason — leaves an object that
+/// MAY be durable, and callers that protect themselves against that (the ref lane's append wedge) must
+/// keep doing so.
+///
+/// Written as an allow-list: a switch with no `default` and a trailing `return false`, so a member
+/// added to `CasUnresolvedReason` later fails BOTH ways safely. The missing case is a `-Wswitch` build
+/// error, which forces the contributor to classify it deliberately; and if that diagnostic is ever
+/// silenced, the runtime answer for the unclassified member is "no, this does not prove anything",
+/// which is the conservative side. Never turn this into a deny-list — a new reason must not be able to
+/// claim "nothing was sent" by omission.
+constexpr bool unresolvedProvesNothingWasSent(CasUnresolvedReason reason)
+{
+    switch (reason)
+    {
+        case CasUnresolvedReason::NoAttemptSent:
+            return true;
+        case CasUnresolvedReason::NotUnresolved:
+        case CasUnresolvedReason::FenceLostMidWay:
+        case CasUnresolvedReason::DeadlineMidWay:
+        case CasUnresolvedReason::FenceLostPostWrite:
+        case CasUnresolvedReason::AttemptsExhausted:
+        /// The whole point of this value is that an earlier attempt WAS sent and may still land.
+        case CasUnresolvedReason::DefiniteFailureAfterAmbiguity:
+            return false;
+    }
+    return false;
+}
+
+/// Human-readable tail for an exception or log line, so the two states above stop reading alike.
+constexpr std::string_view describeUnresolvedReason(CasUnresolvedReason reason)
+{
+    switch (reason)
+    {
+        case CasUnresolvedReason::NotUnresolved:      return "not unresolved";
+        case CasUnresolvedReason::NoAttemptSent:      return "no attempt was sent (the mount fence or the "
+                                                             "operation deadline rejected before the first "
+                                                             "request) — the key is provably unwritten";
+        case CasUnresolvedReason::FenceLostMidWay:    return "the mount fence dropped after at least one "
+                                                             "attempt had been sent";
+        case CasUnresolvedReason::DeadlineMidWay:     return "the operation deadline ran out after at least "
+                                                             "one attempt had been sent";
+        case CasUnresolvedReason::FenceLostPostWrite: return "an attempt committed but the mount fence had "
+                                                             "dropped before it returned";
+        case CasUnresolvedReason::AttemptsExhausted:  return "the attempt budget was exhausted without a "
+                                                             "definite outcome";
+        case CasUnresolvedReason::DefiniteFailureAfterAmbiguity:
+                                                      return "a later attempt was definitively refused, but "
+                                                             "an earlier attempt of the same call is still "
+                                                             "unresolved and may yet land";
+    }
+    return "unspecified";
+}
+
+/// The success path: `buf.finalize()` returned without throwing. Always Committed — kept as a named,
+/// counted entry point so both paths of a classify-then-record call site read the same way (see the
+/// exception overload below).
+constexpr CasWriteOutcome classifyConditionalWriteResult()
+{
+    return CasWriteOutcome::Committed;
+}
+
+/// The exception path: classify what `buf.finalize()` threw for ONE CAS conditional-write HTTP
+/// attempt, according to the CAS conditional-write operation classes. Pure — never rethrows, never
+/// touches counters; see recordConditionalWriteOutcome for the counters hookup.
+CasWriteOutcome classifyConditionalWriteResult(const std::exception & e);
+
+/// Records the start of one HTTP attempt for a CAS conditional write (the attempts counter).
+void recordConditionalWriteAttemptStarted();
+
+/// Records one attempt's terminal outcome (the per-class outcome counters). Callers pass the result of
+/// whichever classifyConditionalWriteResult overload applies, or an outcome already known by
+/// construction (e.g. the legacy `PutOutcome::PreconditionFailed` path, which today resolves without
+/// throwing — see ObjectStorageBackend::nativeConditionalPut).
+void recordConditionalWriteOutcome(CasWriteOutcome outcome);
+
+/// The three separate limits a CAS-owned retry controller enforces for ONE logical conditional-write
+/// operation. Never represented by a single `request_timeout_ms` value — see `validateCasRequestBudget`
+/// for the relationship a writable mount enforces at startup, and `CasRequestController` for the
+/// runtime use.
+struct CasRequestBudget
+{
+    /// Maximum client wait budgeted for one HTTP attempt. `CasRequestController` uses this ONLY as a
+    /// per-attempt scheduling check (an attempt is not started unless it could still finish inside the
+    /// operation deadline) — the actual socket-level wait is configured on the object storage's client
+    /// (the object storage backend's single-attempt client), not by this struct.
+    uint64_t attempt_timeout_ms = 5000;
+    /// Maximum wall-clock time for the COMPLETE logical operation — every attempt, every exact-key
+    /// resolution, and every inter-attempt backoff sleep — counted from the first call to
+    /// `putIfAbsentControlled`. A DURATION, not an absolute deadline: each call establishes its own
+    /// `now + operation_deadline_ms` bound.
+    ///
+    /// This deadline is the authoritative bound on how long a CAS conditional write keeps riding an S3
+    /// disruption server-side before the caller sees an abort. 90s absorbs a ~60s object-store outage
+    /// with margin (see the arithmetic on `max_attempts` below) — PROVIDED the mount fence stays alive.
+    /// The fence, not this deadline, is
+    /// what binds under a TOTAL outage: lease renewals are conditional writes against the same store,
+    /// so when everything is unreachable the fence deadline freezes at `last_renew + mount_lease_ttl`
+    /// and `fence_ok` stops the loop ≈ TTL−attempt_timeout−margin (~23s) after the last successful
+    /// renewal — the required fail-closed behavior (never an attempt past the lease), not a
+    /// budget limitation. While renewals DO land (blips, throttling, partial outages — the renewer runs
+    /// on its own background thread and keeps extending the fence deadline), the op is NOT bounded by
+    /// the lease TTL and rides the full deadline here.
+    uint64_t operation_deadline_ms = 90000;
+    /// Maximum number of controlled attempts for one logical operation (the first attempt counts as 1).
+    /// Sized so the operation deadline above — never this count — is what binds under the observed
+    /// failure shape (~3s adaptive first-attempt PUT timeout per failed attempt + capped-exponential
+    /// backoff): 16 attempts × ~3s + Σ backoff (0.2+0.4+0.8+1.6+3.2 + 10×5 = 56.2s) ≈ 104s > 90s.
+    uint32_t max_attempts = 16;
+    /// Startup-only margin folded into `validateCasRequestBudget`'s inequality against the mount lease
+    /// TTL. Not consulted at runtime by the controller itself — the caller's `fence_ok` callback (backed
+    /// by the local write fence's own deadline) is what actually gates lease-relative timing per attempt.
+    uint64_t lease_safety_margin_ms = 2000;
+    /// Inter-attempt backoff (`cas_s3_retry_initial_backoff_ms` /
+    /// `cas_s3_retry_max_backoff_ms`): the sleep before reissuing
+    /// after an ambiguous attempt whose resolve observed the key absent, capped exponential —
+    /// `initial · 2^(reissues-1)`, never above `retry_max_backoff_ms`. 0 disables backoff (immediate
+    /// reissue — the pre-backoff behavior, and what most exhaustion-path unit tests configure). The
+    /// controller checks the fence BEFORE every sleep and never sleeps past the operation deadline.
+    uint64_t retry_initial_backoff_ms = 200;
+    uint64_t retry_max_backoff_ms = 5000;
+
+    /// Recovery-level retry (`CasRefLedger::ensureRefTableRecovered`): a whole ref-table recovery
+    /// attempt (LIST + snapshot/log GETs + seal PUT) that fails with a transient NETWORK_ERROR is
+    /// retried, with capped-exponential backoff, until this total wall-clock budget is spent — then the
+    /// error propagates and the table's load fails for this touch (the `lazy_load_tables` database
+    /// setting makes the NEXT touch retry). This sits ON TOP of the per-request `operation_deadline_ms`
+    /// envelope above: one recovery attempt may itself burn ~90s inside a single seal PUT. Independent
+    /// of the mount-lease invariants validated in `validateCasRequestBudget` — not part of that
+    /// inequality set.
+    uint64_t recovery_retry_budget_ms = 120000;
+    uint64_t recovery_retry_initial_backoff_ms = 1000;
+    uint64_t recovery_retry_max_backoff_ms = 30000;
+};
+
+/// Startup validation: a writable mount refuses to open with an inconsistent budget rather than
+/// silently falling back to an unbounded or unsafe retry policy. Throws
+/// `BAD_ARGUMENTS` unless ALL hold:
+///   attempt_timeout_ms + lease_safety_margin_ms < mount_lease_ttl_ms
+///   attempt_timeout_ms < operation_deadline_ms          (STRICTLY — see below)
+///   retry_initial_backoff_ms <= retry_max_backoff_ms
+///
+/// The middle one is strict on purpose. Equality does not mean "one attempt's worth of budget": the
+/// deadline is captured as `now + operation_deadline_ms` and each pre-send gate asks
+/// `now + attempt_timeout_ms > deadline_ms`, so equal values reduce it to `now_2 > now_1` and a single
+/// elapsed millisecond refuses the operation having sent NOTHING. Bound the attempt COUNT with
+/// `max_attempts`, never by starving the deadline.
+/// `mount_renew_period_ms` takes no part in the inequality (the renewer keeps the fence deadline
+/// refreshed well ahead of the TTL by construction) — it is accepted only so the effective-values log
+/// line records the full picture in one place.
+///
+/// A successor mounting over an unclean predecessor waits at least one lease TTL, plus its
+/// materialization grace period, before trusting recovery listings. This is long enough for any
+/// conditional PUT still in flight at the predecessor to either land or be abandoned by its own
+/// exhausted retry budget. The predecessor's budget is constrained by
+/// `attempt_timeout_ms + lease_safety_margin_ms < mount_lease_ttl_ms`, so no additional handover
+/// check is needed here.
+void validateCasRequestBudget(const CasRequestBudget & budget, uint64_t mount_lease_ttl_ms, uint64_t mount_renew_period_ms);
+
+/// Throw the recoverable "CAS write could not be committed, retry later" condition.
+///
+/// WHY NETWORK_ERROR (this replaces an earlier ABORTED throw):
+/// A content-addressed write can fail for a reason that is neither the caller's fault
+/// nor permanent: the mount-lease / write fence was lost (e.g. a renewal PUT timed out
+/// against a slow or throttling object store), or a conditional PUT exhausted its retry
+/// budget mid-outage. The right response is "abandon this attempt, try again later" --
+/// which is precisely what a transient error means.
+///
+/// It previously threw `ABORTED`, which was actively harmful to background merges:
+/// `ReplicatedMergeMutateTaskBase` treats `ABORTED` as "merge deliberately cancelled
+/// (shutdown / `DROP` / merges-blocker), not an error", so it neither records
+/// `last_exception_time_ms` nor lets `ReplicatedMergeTreeQueue`'s exponential backoff
+/// engage. Under a sustained store outage the queue re-executed the merge roughly every
+/// 2 seconds, recomputing the whole (possibly multi-GiB) output part every time for the
+/// entire outage -- hundreds of full recomputes, and invisible in system.replication_queue.
+///
+/// `NETWORK_ERROR` is the best-fitting EXISTING code:
+///   - it is NOT in the merge "retry silently, no backoff" exemption set (only `ABORTED`
+///     and `PART_IS_TEMPORARILY_LOCKED` are), so the existing backoff -- capped by
+///     `max_postpone_time_for_failed_replicated_merges_ms` -- engages automatically;
+///   - it is already in ClickHouse's transient/retryable taxonomy
+///     (`checkDataPart::isRetryableException` lists it beside `ABORTED`), so a part under
+///     verification is not misread as corrupted;
+///   - nothing on the merge / insert / replication commit path special-cases it in a way
+///     that would misfire (ZooKeeper retriability keys on `Coordination::Exception`, a
+///     different type), and it is not caught specially on the CAS write path.
+///
+/// Honest caveat: `NETWORK_ERROR` is coarser than the true condition. For the
+/// throttled-store / timed-out / lost-lease cases it is accurate; for a purely logical
+/// fence loss (e.g. the namespace is being dropped) it slightly overstates "network".
+/// The precise cause is always in the exception MESSAGE, never inferred from the code.
+///
+/// If that imprecision ever matters -- operator confusion, or a future upstream change
+/// that attaches merge-path handling to `NETWORK_ERROR` and reintroduces a collision --
+/// switch to a dedicated code (e.g. CAS_WRITE_RETRY_LATER) by changing the
+/// single throw below. A dedicated code is honest and collision-proof by construction
+/// (backoff still engages, since only `ABORTED` / `PART_IS_TEMPORARILY_LOCKED` are exempt);
+/// the only extra work is one appended line in `ErrorCodes.cpp` and, optionally, adding it
+/// to `checkDataPart::isRetryableException` and an HTTP-status mapping for the foreground
+/// `INSERT` client. We deliberately kept `NETWORK_ERROR` for now to add zero new coupling to
+/// generic ClickHouse code, consistent with the rest of the CAS layer.
+///
+/// SCOPE: only the ESCAPING retry-later throws route here (fence lost / write outcome
+/// uncertain / conditional-create Unresolved). The ABORTED values used as internal
+/// control-flow signals (the condemned/vanished "re-upload from source" signal caught
+/// inside putBlob), and the startup/decommission and generic live-lock-brake ABORTEDs,
+/// keep their meaning and are NOT rerouted here.
+[[noreturn]] void throwCasWriteRetryLater(const String & why);
+
+/// Same classification as `throwCasWriteRetryLater`, but returns the exception as a
+/// `std::exception_ptr` for call sites that fail a pending future/promise (`CasRefLedger`'s
+/// `complete_error`) rather than throw directly. Both entry points route through the SAME
+/// construction internally, so the error code / message shape has exactly one place that decides it.
+std::exception_ptr makeCasWriteRetryLaterExceptionPtr(const String & why);
+
+/// Throw the recoverable "this content-addressed disk cannot serve the request right now" condition.
+/// Sibling of `throwCasWriteRetryLater`, same class for the same reasons (see the long rationale above),
+/// differing only in what it describes: that one names a WRITE whose commit did not land, this one names
+/// a DISK STATE that refused the request before it started -- on either plane.
+///
+/// The class is load-bearing beyond CAS. `ReplicatedMergeTreePartCheckThread::checkPartImpl` rethrows
+/// (leaving the part queued for a later check) exactly when `checkDataPart::isRetryableException`
+/// recognises the error, and otherwise declares the part broken -- detach and re-fetch.
+/// `INVALID_STATE` is absent from that classifier, so a lease blip used to read as part corruption
+/// (BACKLOG `{#lease-blip-part-check-collapse}`). Re-coding the CA transients
+/// was chosen over widening the upstream classifier because `INVALID_STATE` is broad: widening it would
+/// also reclassify 18 unrelated TERMINAL sites, CA and non-CA alike.
+///
+/// SCOPE, narrow by design: a refusal routes here when it either names an AUTO-RECOVERING disk condition,
+/// or CANNOT ESTABLISH that its condition is terminal. `checkFenceOrThrow` is the second kind -- one guard
+/// trips for a lease blip and for a FORGET decommission alike and it cannot tell them apart -- and it is in
+/// scope for write-plane uniformity: its 32 sibling write-transient sites already mint this class, and an
+/// unproven condition must be retried rather than consumed as damage. What is NEVER in scope is a refusal
+/// whose condition is PROVEN terminal: `IdentityLost`, both `Vanished` flavours, a storage that is not
+/// started, an unbootstrappable prefix, a proven-absent pool identity, a closed writer epoch -- all keep
+/// `INVALID_STATE`. A proven-terminal state that read as retryable would make every consumer retry forever
+/// against a disk that is never coming back.
+///
+/// `subject` names the refusing disk or pool (e.g. "content-addressed disk 'ca'") and `condition` states
+/// the CA condition truthfully, INCLUDING any promise about how it clears -- only the site knows whether
+/// it can make one. What is appended HERE is the classification alone, so it cannot drift between call
+/// sites. Unlike `throwCasWriteRetryLater` this deliberately does not log: these sites fire once per
+/// refused operation (tens of thousands within a single observed lease gap) and every caller already
+/// reports the exception it receives.
+[[noreturn]] void throwCasTransientUnavailable(const String & subject, const String & condition);
+
+/// Outcome of a controlled CONTENT-ADDRESSED conditional create (`conditionalCreateControlled`):
+///   - Committed:  an attempt's own request completed (2xx) and the final fence check held — `token`
+///     names the created incarnation.
+///   - Occupied:   the key holds an object — either a genuine `PreconditionFailed` (a racing twin) or
+///     an earlier ambiguous attempt of THIS operation that actually landed. For a content-addressed
+///     key these are THE SAME situation: the key embeds the content hash, so any occupant is the
+///     intended content (the exact trust model the plain 412-adopt path already relies on) — the
+///     caller runs its ordinary occupant machinery (adopt live / displace condemned).
+///   - Unresolved: budget exhausted or fence lost without a definite outcome — the write may or may
+///     not have landed; the caller must not ACK (a late-landing body is inert unreferenced debris for
+///     the orphan sweep, exactly like a stageManifest Unresolved).
+enum class CasCreateOutcome : uint8_t
+{
+    Committed,
+    Occupied,
+    Unresolved,
+};
+
+/// Result of one `CasRequestController::conditionalCreateControlled` operation. `token` is meaningful
+/// only when `outcome` is `Committed`; it identifies the incarnation created by the successful attempt.
+/// An `Occupied` result deliberately carries no token because the caller must use its normal occupant
+/// handling, whether the occupant was created by a racing writer or by an earlier ambiguous attempt.
+struct CasCreateResult
+{
+    CasCreateOutcome outcome = CasCreateOutcome::Unresolved;
+    Token token;   /// set ONLY on Committed
+};
+
+/// Outcome of a controlled MUTABLE conditional overwrite (`putOverwriteControlled`) -- an If-Match
+/// replace whose caller can, unlike a content-addressed create, supply the intended bytes for
+/// GET-based resolution, because the payload here is deterministic (a pure function of the
+/// caller's record), not freshly minted per attempt.
+///   - Committed: an attempt's own request completed (2xx) and the final fence check held, or
+///     resolution proved the intended bytes are already what's currently stored -- `token` names
+///     that incarnation.
+///   - Conflict: resolution proved the key's CURRENT token AND bytes both differ from what this
+///     call intended -- a genuine competing write. Returned as a value, never thrown, never
+///     collapsed into Unresolved/DefiniteFailure -- mirrors the existing uncontrolled
+///     casMeta/CasResult contract (a conflict lets the caller reload and decide).
+///   - Unresolved: budget exhausted, fence lost, or the current token still equals `expected` (the
+///     attempt provably never applied) with the resolve unable to prove either outcome yet --
+///     caller must not ACK.
+enum class CasOverwriteOutcome : uint8_t
+{
+    Committed,
+    Conflict,
+    Unresolved,
+};
+
+/// Result of one `CasRequestController::putOverwriteControlled` operation. `token` is meaningful
+/// only when `outcome` is `Committed`.
+struct CasOverwriteResult
+{
+    CasOverwriteOutcome outcome = CasOverwriteOutcome::Unresolved;
+    Token token;   /// set ONLY on Committed
+};
+
+/// Result of one `CasRequestController::slotOccupy` operation — a WRITE-ONCE conditional create whose
+/// body is content-addressed or otherwise not byte-comparable across separate CALLS the way
+/// `putOverwriteControlled`'s deterministic marker is (each caller of `slotOccupy` — an epoch seal, a
+/// wedge retry — mints its own attempt and decides for itself, from `Occupied`'s bytes, whether the
+/// occupant is its own earlier write or something else entirely; see the adjudication note below).
+///   - Created:    this call's OWN conditional create committed — the key held nothing before it.
+///   - Occupied:   the key already holds an object, observed by ONE raw exact `GET` after the create
+///     conflicted — `occupant_bytes`/`occupant_token` name exactly what is there NOW. The primitive
+///     never compares these bytes against what this call attempted and never throws on a mismatch:
+///     unlike `resolveByExactGet` (whose caller supplies ONE expected body across every retry of the
+///     SAME logical attempt), `slotOccupy` never retries, so there is no "our earlier attempt" to
+///     distinguish from a genuine foreign occupant — that adjudication (the `CaCasMountCore` `mine`
+///     contract: an occupant is this caller's write only if the BYTES match, never a generation/shape
+///     match alone) is entirely the CALLER's job.
+///   - Unresolved: the outcome is unknowable right now — a pre-attempt gate refused (fence lost /
+///     deadline exhausted, `unresolved_reason == NoAttemptSent`, nothing was sent — `unresolvedProvesNothingWasSent`
+///     is TRUE only for this case), or the conditional create was itself ambiguous (a transient
+///     exception) and the follow-up resolve GET found nothing (the occupant that caused the conflict
+///     vanished before the GET, or the GET itself failed — BOTH of these report `unresolved_reason ==
+///     AttemptsExhausted`, for which `unresolvedProvesNothingWasSent` is FALSE) — NEVER fabricated into
+///     a false `Created`. CALLERS: do not log a bare `describeUnresolvedReason(AttemptsExhausted)` for
+///     this case — it reads "the attempt budget was exhausted", which is misleading for a primitive
+///     with no retry budget, and it silently folds "the resolve GET found nothing" together with "the
+///     occupant that caused the conflict was DELETED under a live epoch" (a GC-invariant alarm, not
+///     routine contention) into the same generic wording. `SlotOccupyResult` carries no discriminator
+///     between those two sub-cases — log the slot key plus "the resolve read found nothing"; the day a
+///     caller NEEDS the split is the trigger for adding a dedicated `CasUnresolvedReason` value (a
+///     gated protocol decision, not a drive-by).
+struct SlotOccupyResult
+{
+    enum class Kind : uint8_t { Created, Occupied, Unresolved };
+    Kind kind = Kind::Unresolved;
+    /// Occupied only: the occupant, fetched by exact GET after the conditional create conflicted.
+    String occupant_bytes;
+    Token occupant_token;
+    /// Unresolved only: why the attempt outcome is unknowable right now.
+    CasUnresolvedReason unresolved_reason{};
+};
+
+/// CAS-owned retry controller: the only place that decides whether a conditional-write attempt may be
+/// reissued. It does not touch a writer cache or return ACK. Callers update their cache and acknowledge
+/// the operation only after this controller has resolved the outcome and performed its final fence
+/// check, using the returned `CasWriteOutcome`.
+class CasRequestController
+{
+public:
+    /// `now_ms_`: monotonic-ish clock, defaulting to `std::chrono::steady_clock`; tests inject a fake
+    /// one to drive deadline behavior deterministically (no sleeps).
+    /// `sleep_ms_`: the inter-attempt backoff sleep, defaulting to a real `std::this_thread::sleep_for`;
+    /// tests inject a recorder/no-op to assert the backoff schedule without wall-clock waits. The
+    /// controller only ever sleeps BETWEEN attempts of one logical operation, on the calling thread,
+    /// with no Pool mutex held (every call site — the ref append lane's leader, `stageManifest`, blob
+    /// uploads, snapshot publishes — invokes the controller outside its locks; the append lane's
+    /// LEADERSHIP is deliberately held across the sleep: same-table appends must queue behind an
+    /// unresolved predecessor PUT anyway, preserving the writer's per-table ordering.
+    CasRequestController(BackendPtr backend_, CasRequestBudget budget_, std::function<uint64_t()> now_ms_ = {},
+                         std::function<void(uint64_t)> sleep_ms_ = {});
+
+    /// Controlled `putIfAbsent` with resolve-before-reissue. Performs at
+    /// most `budget.max_attempts` attempts of the exact SAME (key, bytes) — never a different key, never
+    /// a different body — bounded by `budget.operation_deadline_ms` measured from this call's own start,
+    /// with capped-exponential inter-attempt backoff (`retry_initial_backoff_ms`/`retry_max_backoff_ms`).
+    /// `fence_ok` is consulted before EVERY attempt (a false answer sends no further attempt), before
+    /// EVERY backoff sleep (a fence lost mid-loop aborts instantly, never after a pointless sleep), and
+    /// once more before a `Committed` return (a false answer there means the write may have landed but
+    /// this call reports `Unresolved`, never a false `Committed`). A sleep is
+    /// never entered when it (plus one more attempt) could not fit the operation deadline. An uncertain
+    /// attempt is resolved via `resolveByExactGet` before deciding whether to reissue.
+    /// Throws `CORRUPTED_DATA` if resolution ever observes DIFFERENT valid bytes at `key` — a real
+    /// conflict, never collapsed into `Unresolved`/`DefiniteFailure`. Returns `Unresolved` (never
+    /// throws) when the fence is lost or the budget is exhausted before a definite outcome is reached.
+    ///
+    /// THE VERDICT IS THE CALL'S, NOT THE LAST ATTEMPT'S. `DefiniteFailure` is returned only when EVERY
+    /// attempt this call sent was itself proven never applied. One attempt's whitelisted rejection
+    /// proves nothing about an EARLIER attempt of the same call that went ambiguous: that request may
+    /// have been received and may still materialize at `key` (an absent resolve GET is not evidence —
+    /// `unresolvedProvesNothingWasSent`). Any such attempt therefore dominates the result, which becomes
+    /// `Unresolved`/`DefiniteFailureAfterAmbiguity` — the wedge path — because a caller acting on
+    /// `DefiniteFailure` declares the key unwritten and reuses the id (`CasRefLedger::commitRefChunk`),
+    /// which an ambiguous predecessor can turn into an acked-then-lost transaction. Attempts a
+    /// pre-attempt gate refused never reach the backend, so they never make this call ambiguous.
+    /// `out_token` (optional): set ONLY on a `Committed` return, to the committed incarnation's token —
+    /// the attempt's own `PutResult` token, or the token the resolve GET observed when it proved an
+    /// earlier ambiguous attempt landed. Lets audit emitters (e.g. `PartWriteTxn::stageManifest`'s
+    /// `ManifestPut` event) keep the token without a follow-up HEAD. Untouched on any other return.
+    /// `out_reason` (optional): WHY an `Unresolved` was returned. Diagnostic only — the returned
+    /// outcome is unchanged, so no caller's decision depends on it. It exists because `Unresolved`
+    /// currently conflates two very different situations, and the resulting message
+    /// ("retry budget exhausted") is printed even where NOTHING was ever sent: finding #37 defect 3,
+    /// whose own note records that the opacity "plausibly fed 3 prior wrong analyses" — and it did so
+    /// again on 2026-07-24, when a sanitizer-slow unit test fenced itself and the text sent the CI
+    /// triage looking for a retry problem that did not exist. `NoAttemptSent` is the load-bearing
+    /// distinction: it means the key was provably never written, whereas the other reasons leave a
+    /// possibly-durable object behind.
+    CasWriteOutcome putIfAbsentControlled(std::string_view key, std::string_view bytes,
+                                          const std::function<bool()> & fence_ok, Token * out_token = nullptr,
+                                          CasUnresolvedReason * out_reason = nullptr);
+
+    /// One-shot exact-key resolution of an uncertain immutable create:
+    ///   - identical bytes observed at `key`  -> Committed (the earlier attempt DID commit)
+    ///   - DIFFERENT bytes observed at `key`  -> throws CORRUPTED_DATA (a real conflict, not a retry
+    ///     signal — never silently treated as ambiguous)
+    ///   - absent, or the GET itself fails    -> Unresolved (another attempt may still be legal)
+    /// NEVER returns DefiniteFailure: an absent or unreadable key proves nothing about whether the
+    /// original request will eventually be provably non-applied, so resolution alone can never produce
+    /// that verdict. `out_token` (optional): set ONLY on `Committed`, to the observed incarnation's token.
+    CasWriteOutcome resolveByExactGet(std::string_view key, std::string_view expected_bytes,
+                                      Token * out_token = nullptr);
+
+    /// Controlled conditional create for content-addressed write-once keys whose body CANNOT be
+    /// byte-compared across attempts — the blob-body `putIfAbsentStream` create and `promoteStaged`'s
+    /// conditional server-side copy (`PartWriteTxn::uploadFromSource`). Byte-exact resolve
+    /// (`resolveByExactGet`) is impossible there BY DESIGN: W-FRESH-TAG mints a fresh
+    /// `incarnation_tag` into the envelope header on every re-stream, so two attempts of the same
+    /// logical create legitimately differ in bytes. The identity authority is the KEY itself (it
+    /// embeds algo + content digest), so an uncertain attempt resolves by exact-key OCCUPANCY (one
+    /// HEAD — never a GET of a possibly-multi-GB body, and never a GET-revive):
+    ///   - occupant present  -> Occupied (definite; whether it is our own landed attempt or a twin is
+    ///     immaterial for a content-addressed key — the caller's ordinary 412 machinery takes over)
+    ///   - absent            -> another attempt may be legal (fence/deadline/backoff-gated, same
+    ///     schedule as putIfAbsentControlled); `attempt` re-streams from the caller's REPLAYABLE
+    ///     source — a fresh re-upload, honoring the resurrect invariant
+    ///   - the HEAD fails    -> still ambiguous; reissue is safe (an occupant just answers the reissue
+    ///     with PreconditionFailed -> Occupied on the next round)
+    /// `attempt` performs ONE conditional-create attempt of the same logical content and returns its
+    /// PutResult (Done/PreconditionFailed) or throws. A whitelisted DefiniteFailure classification
+    /// RETHROWS the original exception (the blob lane always surfaced the raw storage error's root
+    /// cause — unlike the ref lane's outcome mapping, nothing here needs the code collapsed), and a
+    /// deterministic LOCAL failure from the attempt — `LOGICAL_ERROR` (e.g. a source streaming a
+    /// different byte count than it declared), `NOT_IMPLEMENTED` (a mode/capability guard),
+    /// `BAD_ARGUMENTS` (a deterministic encode rejection), `CORRUPTED_DATA` (integrity) — propagates
+    /// unchanged too, on the FIRST attempt with no resolve and no backoff: a caller/config bug reissue
+    /// would only replay, never a wire ambiguity.
+    /// A Done attempt gets the final fence check before being reported Committed
+    /// Occupied needs none — it acks nothing of OUR write, and the
+    /// caller's occupant path gates its own adoption.
+    CasCreateResult conditionalCreateControlled(std::string_view key,
+                                                const std::function<PutResult()> & attempt,
+                                                const std::function<bool()> & fence_ok);
+
+    /// Controlled If-Match overwrite with resolve-before-reissue, for a MUTABLE marker whose bytes
+    /// are deterministic so GET-based resolution can compare them (unlike a content-addressed
+    /// create's freshly-minted-per-attempt body). Performs at most `budget.max_attempts` attempts of
+    /// the exact SAME (key, bytes, expected token), bounded by `budget.operation_deadline_ms`, with
+    /// the same fence/backoff/deadline gates as `putIfAbsentControlled`. An ambiguous attempt
+    /// (`PreconditionFailed`, or a transient exception classified `Unresolved`) is resolved with ONE
+    /// GET at `key`:
+    ///   - the current token still equals `expected`  -> the attempt provably never applied; another
+    ///     attempt of the SAME (key, bytes, expected) is legal (fence/backoff/deadline-gated)
+    ///   - the current bytes equal `bytes`             -> Committed (an earlier ambiguous attempt of
+    ///     THIS call already landed); `token` is the observed incarnation
+    ///   - neither                                      -> Conflict: a genuine competing write
+    ///     landed; returned as a value, never thrown
+    ///   - the GET itself fails                         -> still ambiguous; reissue is safe
+    /// A whitelisted `DefiniteFailure` classification, or a deterministic LOCAL failure
+    /// (`isDeterministicLocalFailure`), RETHROWS the original exception -- mirrors
+    /// `conditionalCreateControlled`'s convention, never `putIfAbsentControlled`'s (that method
+    /// predates this convention).
+    CasOverwriteResult putOverwriteControlled(std::string_view key, std::string_view bytes,
+                                              const Token & expected, const std::function<bool()> & fence_ok);
+
+    /// Controlled put-if-absent for a MUTABLE marker whose bytes are deterministic, where an
+    /// EXISTING DIFFERENT value at the key is a normal outcome (Conflict), not corruption. This is
+    /// the create-side sibling of `putOverwriteControlled` and deliberately does NOT reuse
+    /// `putIfAbsentControlled`: that method's resolve (`resolveByExactGet`) throws `CORRUPTED_DATA`
+    /// on any different bytes at the key, which is correct for the ref-log lane's immutable,
+    /// content-addressed keys (a different value there truly is impossible-by-construction) but
+    /// wrong for a mutable state marker (e.g. a blob's freshness-meta sidecar), where a
+    /// pre-existing DIFFERENT value is an expected, non-corrupt state a racing writer or GC pass
+    /// left behind. Performs at most `budget.max_attempts` attempts of the exact SAME (key, bytes),
+    /// bounded by `budget.operation_deadline_ms`, with the same fence/backoff/deadline gates as
+    /// `putIfAbsentControlled`. An ambiguous attempt (`PreconditionFailed`, or a transient exception
+    /// classified `Unresolved`) is resolved with ONE GET at `key`:
+    ///   - absent                          -> the attempt provably never applied; another attempt of
+    ///     the SAME (key, bytes) is legal (fence/backoff/deadline-gated)
+    ///   - present, bytes equal `bytes`     -> Committed (an earlier ambiguous attempt of THIS call,
+    ///     or a racing writer creating the identical value, already landed); `token` is the observed
+    ///     incarnation
+    ///   - present, bytes differ           -> Conflict: something else already occupies the key with
+    ///     a different value; returned as a value, never thrown
+    ///   - the GET itself fails            -> still ambiguous; reissue is safe
+    /// Same DefiniteFailure/deterministic-local-failure rethrow convention as `putOverwriteControlled`.
+    CasOverwriteResult putIfAbsentControlledMutable(std::string_view key, std::string_view bytes,
+                                                    const std::function<bool()> & fence_ok);
+
+    /// A DEDICATED RAW slot-occupy primitive [codex finding 3]: exactly ONE fence/deadline-gated
+    /// conditional create of `bytes` at `key`; on conflict, exactly ONE raw exact `GET` of the
+    /// occupant. NEVER retries internally, NEVER lists, and NEVER composes `putIfAbsentControlled`
+    /// (which retries the SAME (key, bytes) internally) or `resolveByExactGet` (which compares against
+    /// an expected body and throws `CORRUPTED_DATA` on a mismatch) — both contradict "one conditional
+    /// create" and `Occupied(bytes, token)` respectively. This is the primitive every seal writer and
+    /// wedge retry uses (spec INV-2): each CALL is one bounded attempt, and a caller that wants to keep
+    /// trying calls this again later, under its OWN fence/deadline/backoff discipline.
+    ///
+    /// Pre-attempt gate ONLY: `fence_ok` and the operation deadline are checked ONCE, before the (only)
+    /// attempt — a refusal there sends nothing and reports `Unresolved`/`NoAttemptSent`, exactly like
+    /// every other controlled op's FIRST iteration. UNLIKE `putIfAbsentControlled` /
+    /// `conditionalCreateControlled` / `putOverwriteControlled` / `putIfAbsentControlledMutable`, there
+    /// is deliberately no POST-write fence recheck here: `fence_ok` is evaluated once per attempt, and
+    /// since this primitive makes exactly one attempt, admission-fence discipline across an outer
+    /// caller-driven retry (including verifying a `Created`/`Occupied` result is still relevant after
+    /// the I/O) is the CALLER's contract (Task 4/6's post-I/O recheck under its own state lock).
+    ///
+    /// CONSEQUENCE, stated bluntly because it is the OPPOSITE of every sibling op's behavior: a
+    /// `Created` or `Occupied` returned here may come from a call whose fence was lost WHILE the PUT or
+    /// GET was in flight — this primitive does not know and does not check. Acting on either result
+    /// (adopting, acknowledging, installing) without the caller's OWN post-I/O
+    /// `checkFenceOrThrow(admitted_generation)` under its own lock is a correctness bug, not a missed
+    /// diagnostic — see Task 4's `resolveWedgeOnce` and Task 6's recovery CAS-walk in the plan for the
+    /// exact recheck shape.
+    ///
+    /// A whitelisted synchronous rejection (`classifyConditionalWriteResult`'s `DefiniteFailure`) or a
+    /// deterministic local failure (`isDeterministicLocalFailure`) RETHROWS the original exception
+    /// unchanged — the same convention as `conditionalCreateControlled`/`putOverwriteControlled`/
+    /// `putIfAbsentControlledMutable` (`SlotOccupyResult::Kind` has no `DefiniteFailure` member to carry
+    /// it). Any other exception, or a clean `PreconditionFailed`, is ambiguous and falls through to the
+    /// resolve GET identically — this primitive cannot and does not distinguish the two.
+    ///
+    /// Op-count contract (asserted by every `gtest_cas_slot_occupy.cpp` test): `Created` costs exactly
+    /// one backend op (the create); `Occupied` costs exactly two (the create, then the resolve GET);
+    /// `Unresolved` costs at most two (zero when a pre-attempt gate refuses, otherwise the create plus
+    /// a resolve GET that came up empty or failed).
+    SlotOccupyResult slotOccupy(std::string_view key, std::string_view bytes,
+                                const std::function<bool()> & fence_ok);
+
+    /// Test-only: replace the inter-attempt backoff sleep (e.g. with a no-op) on an already-constructed
+    /// controller — for tests that reach the controller only through a fully-wired Pool/disk and cannot
+    /// pass the ctor parameter (see `Pool::setCasRetrySleepForTest`). Passing an empty function restores
+    /// the real sleep. Not thread-safe: call before driving any traffic through the controller.
+    void setSleepFnForTest(std::function<void(uint64_t)> sleep_ms_);
+
+private:
+    /// The gate between a completed ambiguous attempt and its reissue: fence check FIRST (a fence lost
+    /// mid-loop must abort before any sleep), then the capped-exponential backoff sleep — skipped
+    /// entirely (returning false, no sleep served) when the sleep plus one more attempt could not fit
+    /// the operation deadline. Returns true when the loop may proceed to the next attempt; the loop
+    /// top's own pre-attempt fence/deadline checks re-run AFTER the sleep.
+    ///
+    /// `out_reason` (optional) receives WHICH of the two refusals returned false, so a caller reporting
+    /// an `Unresolved` from here does not have to guess between them. Both are mid-way by construction:
+    /// this gate is only reached once an attempt has been sent.
+    bool pauseBeforeReissue(uint32_t completed_attempt, uint64_t deadline_ms, const std::function<bool()> & fence_ok,
+                            CasUnresolvedReason * out_reason = nullptr);
+    /// The backoff scheduled before attempt `next_attempt` (attempt 2 sleeps `retry_initial_backoff_ms`,
+    /// doubling per reissue), saturating at `retry_max_backoff_ms`. 0 when backoff is disabled.
+    uint64_t backoffBeforeAttempt(uint32_t next_attempt) const;
+
+    BackendPtr backend;
+    CasRequestBudget budget;
+    std::function<uint64_t()> now_ms;
+    std::function<void(uint64_t)> sleep_ms;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.cpp
new file mode 100644
index 000000000000..af52f2854bfd
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.cpp
@@ -0,0 +1,110 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Common/Exception.h>
+#include <Common/logger_useful.h>
+
+namespace DB::Cas
+{
+
+SentinelProbeResult probeSentinel(Backend & backend, const String & key)
+{
+    return backend.probeSentinelRaw(key);
+}
+
+namespace
+{
+
+/// Is `key` structurally-valid `_probe/` capability-battery debris — i.e. an object strictly under the
+/// reserved `<prefix>/_probe/` subtree? `runCapabilityProbe` (invoked with `<prefix>/_probe/<u128hex>` by
+/// `Pool::open`) is the ONLY writer under `_probe/`; a content-addressed pool NEVER stores durable
+/// data/control state there, so the whole subtree is ephemeral capability-probe scratch that a crash or a
+/// concurrent fresh opener may leave behind ([D2]). Ignoring it can therefore never strand real data —
+/// `pool_prefix` is exclusively CAS-owned. The trailing `/` in `probe_root` keeps a mere sibling
+/// look-alike (`<prefix>/_probe`, `<prefix>/_probelike/…`) OUT of the reserved subtree, so it is still
+/// treated as genuine residual and fails the bootstrap closed.
+bool isProbeSubtreeDebris(const String & probe_root, const String & key)
+{
+    return key.starts_with(probe_root);
+}
+
+}
+
+BootstrapResidual probePoolBootstrapResidual(Backend & backend, const Layout & layout)
+{
+    const String pool_meta_key = layout.poolMetaKey();
+    const String catalog_key = layout.refCatalogKey();
+    const String prefix = layout.poolPrefix() + "/";
+    const String probe_root = layout.poolPrefix() + "/_probe/";
+
+    /// Classification is order-independent for correctness: every listed key is examined, and finding
+    /// `_pool_meta` anywhere is decisive. It relies on lexicographic LIST order only for COST — `_pool_meta`
+    /// sorts first under `<prefix>/`, so a healthy pool short-circuits on the first page rather than
+    /// enumerating its whole content on every open.
+    bool has_residual = false;
+    bool has_catalog = false;
+    try
+    {
+        String cursor;
+        for (;;)
+        {
+            const ListPage page = backend.list(prefix, cursor, 1000);
+            for (const ListedKey & listed : page.keys)
+            {
+                if (listed.key == pool_meta_key)
+                    return BootstrapResidual::PoolMetaPresent;   /// decisive — the pool is authoritative
+                if (isProbeSubtreeDebris(probe_root, listed.key))
+                    continue;   /// crash leftover / concurrent opener's battery — ignore ([D2])
+                if (listed.key == catalog_key)
+                {
+                    has_catalog = true;
+                    continue;
+                }
+                has_residual = true;   /// a non-`_probe` object, and no `_pool_meta` seen (so far)
+            }
+            if (page.next_cursor.empty())
+                break;
+            cursor = page.next_cursor;
+        }
+    }
+    catch (...)
+    {
+        /// The LIST failed: absence was never proven. Never mint a fresh identity under uncertainty —
+        /// but log the swallowed error so the fail-closed startup refusal is diagnosable (the root cause
+        /// must not vanish just because the verdict is "Indeterminate").
+        LOG_WARNING(getLogger("CasBootstrap"),
+            "Pool prefix '{}': the authoritative residual LIST failed; treating the bootstrap as "
+            "Indeterminate (fail-closed, will refuse to mint _pool_meta): {}",
+            prefix, getCurrentExceptionMessage(/*with_stacktrace=*/false));
+        return BootstrapResidual::Indeterminate;
+    }
+    if (has_residual)
+        return BootstrapResidual::ResidualWithoutMeta;
+    if (!has_catalog)
+        return BootstrapResidual::EmptyOrProbeOnly;
+
+    /// LIST is only a discovery hint. Before treating catalog-only residue as retryable, exact-read
+    /// the listed key and prove it is precisely the sole canonical empty authority. A missing object,
+    /// malformed body, alternate encoding, or nonempty catalog is ordinary residual data, never a
+    /// license to mint `_pool_meta`.
+    try
+    {
+        const auto got = backend.get(catalog_key);
+        if (!got)
+            return BootstrapResidual::ResidualWithoutMeta;
+
+        RefCatalog catalog = decodeRefCatalog(got->bytes);
+        const String canonical_empty = encodeRefCatalog(RefCatalog{});
+        if (catalog.entries.empty() && got->bytes == canonical_empty)
+            return BootstrapResidual::CanonicalEmptyCatalogOnly;
+        return BootstrapResidual::ResidualWithoutMeta;
+    }
+    catch (...)
+    {
+        LOG_WARNING(getLogger("CasBootstrap"),
+            "Pool prefix '{}': could not prove listed catalog '{}' is canonical empty; refusing bootstrap: {}",
+            prefix, catalog_key, getCurrentExceptionMessage(/*with_stacktrace=*/false));
+        return BootstrapResidual::ResidualWithoutMeta;
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.h
new file mode 100644
index 000000000000..c0d926f1c31d
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.h
@@ -0,0 +1,55 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+
+namespace DB::Cas
+{
+
+/// Authoritative, cache-bypassing probe of one key. NEVER conflates transport errors with absence:
+/// timeouts / 5xx / connection errors => Indeterminate; permission errors => AccessDenied;
+/// missing container/bucket/prefix-parent => ContainerAbsent; a clean authoritative miss => KeyAbsent.
+///
+/// Free-function entry point (spec §2) — a thin dispatch to the backend's own typed-evidence
+/// classification (`Backend::probeSentinelRaw`; see there for the per-backend semantics: the
+/// S3-native raw HEAD error, the Local container-directory stat, or the generic head/get-based
+/// default for a backend without sharper evidence).
+SentinelProbeResult probeSentinel(Backend & backend, const String & key);
+
+/// Verdict of the zero-write startup bootstrap residual check ("Startup ordered vs the capability
+/// probe"). Before a writable `Pool::open` runs
+/// the MUTATING `_probe/` capability battery, it must decide whether it is safe to bootstrap a missing
+/// `_pool_meta`. `pool_prefix` is EXCLUSIVELY CAS-owned: a fresh pool identity may be minted ONLY over a
+/// genuinely empty prefix — never over residual data an incomplete erase left behind (that would strand
+/// the old objects as orphans under a new, colliding identity, the "restart poisons a partially-erased
+/// pool" hole this check closes).
+enum class BootstrapResidual : uint8_t
+{
+    /// `<prefix>/_pool_meta` exists → the pool is authoritative; proceed with the normal open/validate.
+    PoolMetaPresent,
+    /// No `_pool_meta`, and every listed object is structurally-valid `_probe/` debris (or the prefix is
+    /// empty) → safe to bootstrap a fresh pool.
+    EmptyOrProbeOnly,
+    /// No `_pool_meta`, and the only durable object is a byte-for-byte canonical empty
+    /// `cas/ref_catalog` (plus structurally-valid `_probe/` debris). This is the sole retryable
+    /// pre-meta bootstrap residue: a prior opener made the mandatory catalog durable but did not
+    /// publish `_pool_meta`.
+    CanonicalEmptyCatalogOnly,
+    /// No `_pool_meta`, but at least one non-`_probe` object exists → refuse to bootstrap (typed startup
+    /// failure, zero writes performed).
+    ResidualWithoutMeta,
+    /// The authoritative LIST itself failed → emptiness could not be proven → fail-closed (never mint a
+    /// fresh identity while residual data cannot be ruled out).
+    Indeterminate,
+};
+
+/// Zero-write authoritative classification of a pool prefix for the startup bootstrap decision. A single
+/// paginated LIST of `layout.poolPrefix()`; each listed object is classified as the `_pool_meta`
+/// sentinel, capability-battery debris under the reserved `<prefix>/_probe/` subtree (a crash-mid-battery
+/// leftover OR a concurrent fresh opener's in-flight battery — [D2]), or genuine residual CAS state. It
+/// NEVER writes, and it IGNORES probe debris exactly so a normal restart after a crash-mid-battery still
+/// bootstraps cleanly. On non-strong-LIST backends this is the single best-effort authoritative check the
+/// weaker guarantee allows — still fail-closed on any residual object found. Used by `Pool::open` BEFORE
+/// the capability battery so that no probe write ever precedes the emptiness proof.
+BootstrapResidual probePoolBootstrapResidual(Backend & backend, const Layout & layout);
+
+}

From 58d4f0b9ee024b44684f8d7d3835d27c3a69f116 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:34 +0200
Subject: [PATCH 16/30] CAS subsystem: Pool layer

Pool identity and runtime: server root, mount lifecycle, pool metadata,
the ref ledger and ref protocol (publish/confirm, recovery, snapshots),
the part-write transaction (dedup gate, conditional create, promote),
staging and plain objects.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../ContentAddressed/Pool/CasBlobMeta.cpp     |   46 +
 .../ContentAddressed/Pool/CasBlobMeta.h       |   65 +
 .../Pool/CasBlobUploadPool.cpp                |   75 +
 .../ContentAddressed/Pool/CasBlobUploadPool.h |   43 +
 .../Pool/CasEventDispatcher.cpp               |   56 +
 .../Pool/CasEventDispatcher.h                 |   64 +
 .../Pool/CasManifestReader.cpp                |  170 +
 .../ContentAddressed/Pool/CasManifestReader.h |  101 +
 .../ContentAddressed/Pool/CasMountRuntime.cpp |  545 ++
 .../ContentAddressed/Pool/CasMountRuntime.h   |  431 ++
 .../ContentAddressed/Pool/CasPartWriteTxn.cpp | 1463 +++++
 .../ContentAddressed/Pool/CasPartWriteTxn.h   |  402 ++
 .../ContentAddressed/Pool/CasPlainObjects.cpp |  154 +
 .../ContentAddressed/Pool/CasPlainObjects.h   |  111 +
 .../ContentAddressed/Pool/CasPool.cpp         | 1851 ++++++
 .../ContentAddressed/Pool/CasPool.h           | 1159 ++++
 .../ContentAddressed/Pool/CasPoolMeta.cpp     |  168 +
 .../ContentAddressed/Pool/CasRefCatalog.cpp   |  654 +++
 .../ContentAddressed/Pool/CasRefCatalog.h     |  351 ++
 .../ContentAddressed/Pool/CasRefCkpt.cpp      |  354 ++
 .../ContentAddressed/Pool/CasRefCkpt.h        |  172 +
 .../Pool/CasRefCowManifestSet.cpp             |  119 +
 .../Pool/CasRefCowManifestSet.h               |  127 +
 .../ContentAddressed/Pool/CasRefCowMap.cpp    |  245 +
 .../ContentAddressed/Pool/CasRefCowMap.h      |  209 +
 .../ContentAddressed/Pool/CasRefLedger.cpp    | 5113 +++++++++++++++++
 .../ContentAddressed/Pool/CasRefLedger.h      | 1246 ++++
 .../ContentAddressed/Pool/CasRefProtocol.cpp  | 1108 ++++
 .../ContentAddressed/Pool/CasRefProtocol.h    |  794 +++
 .../ContentAddressed/Pool/CasServerRoot.cpp   | 1512 +++++
 .../ContentAddressed/Pool/CasServerRoot.h     |  774 +++
 31 files changed, 19682 insertions(+)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPoolMeta.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h

diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.cpp
new file mode 100644
index 000000000000..d1c44d2b168f
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.cpp
@@ -0,0 +1,46 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+
+#include <Common/ProfileEvents.h>
+
+namespace ProfileEvents
+{
+    extern const Event CASMetaPut;
+    extern const Event CASMetaCompareSwap;
+    extern const Event CASMetaDelete;
+}
+
+namespace DB::Cas
+{
+
+std::optional<LoadedMeta> loadMeta(Backend & backend, const Layout & layout, const BlobRef & ref)
+{
+    const String key = layout.blobMetaKey(ref);
+    auto got = backend.get(key);
+    if (!got)
+        return std::nullopt;
+    return LoadedMeta{.meta = decodeBlobMeta(got->bytes), .etag = got->token};
+}
+
+CasOverwriteResult putMetaIfAbsent(Pool & pool, const BlobRef & ref, const BlobMeta & meta)
+{
+    ProfileEvents::increment(ProfileEvents::CASMetaPut);
+    const String key = pool.layout().blobMetaKey(ref);
+    return pool.stagingPutIfAbsentMutable(key, encodeBlobMeta(meta));
+}
+
+CasOverwriteResult casMeta(Pool & pool, const BlobRef & ref, const Token & expected, const BlobMeta & meta)
+{
+    ProfileEvents::increment(ProfileEvents::CASMetaCompareSwap);
+    const String key = pool.layout().blobMetaKey(ref);
+    return pool.stagingConditionalOverwrite(key, encodeBlobMeta(meta), expected);
+}
+
+DeleteOutcome deleteMetaExact(Backend & backend, const Layout & layout, const BlobRef & ref, const Token & expected)
+{
+    ProfileEvents::increment(ProfileEvents::CASMetaDelete);
+    const String key = layout.blobMetaKey(ref);
+    return backend.deleteExact(key, expected);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h
new file mode 100644
index 000000000000..87b77d1d8657
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h
@@ -0,0 +1,65 @@
+#pragma once
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.h>
+
+#include <optional>
+
+namespace DB::Cas
+{
+
+class Pool;
+
+/// A decoded blob meta record together with the backend token observed for the same incarnation.
+/// The token is returned with the decoded record because the next conditional update or exact delete
+/// must be guarded by the version that was actually read; comparing encoded meta bytes would not
+/// provide that protection.
+struct LoadedMeta
+{
+    BlobMeta meta;
+    Token etag;
+};
+
+/// Shared lifecycle operations for the blob freshness marker used by the writer and GC. The key is
+/// built from the complete `BlobRef`, so each algorithm uses its own digest representation and no
+/// pool-wide digest width is threaded through these functions. The marker is a point-read hint rather
+/// than the blob lifetime's linearization point: the blob body's incarnation tag and exact-token body
+/// deletion provide the safety guarantee, while a stale marker can at most make a writer re-upload.
+///
+/// `loadMeta` is used in the adopt path, so its backend must provide strong read-after-write
+/// consistency: after a successful meta write, the one subsequent GET must observe that write.
+/// Conditional updates and deletion use the backend token, not the encoded meta bytes.
+///
+/// Returns the current decoded marker and its conditional token, or nullopt when the meta key is
+/// absent. Decoding errors propagate as exceptions.
+std::optional<LoadedMeta> loadMeta(Backend & backend, const Layout & layout, const BlobRef & ref);
+
+/// Creates the marker only when its key is absent, controlled: a SlowDown/429/5xx on the attempt is
+/// resolved-and-reissued within budget rather than escaping as a raw client error (triage: S22 RCA).
+/// A precondition failure (another
+/// writer already created the marker -- possibly with a DIFFERENT record, e.g. a stale `Condemned`
+/// marker still present when a vanished body is freshly re-uploaded) is reported as
+/// `CasOverwriteOutcome::Conflict`, never thrown -- this uses `putIfAbsentControlledMutable`, NOT the
+/// ref-log lane's `putIfAbsentControlled` (that method's resolve throws `CORRUPTED_DATA` on any
+/// different bytes at the key, which is correct for the ref-log's immutable content-addressed keys
+/// but wrong for this mutable marker, where a pre-existing different value is an expected, non-corrupt
+/// outcome).
+CasOverwriteResult putMetaIfAbsent(Pool & pool, const BlobRef & ref, const BlobMeta & meta);
+
+/// Replaces the marker only when its current backend token equals `expected`, controlled (same
+/// budgeted resolve-and-reissue as putMetaIfAbsent). A genuine conflict (current token AND bytes both
+/// differ from what this call intended) is reported as `CasOverwriteOutcome::Conflict`, never thrown --
+/// exactly like the previous uncontrolled `CasResult` contract -- so the caller's existing
+/// reload-and-retry loop (`writeResurrectMetaClean`) keeps working unchanged.
+CasOverwriteResult casMeta(Pool & pool, const BlobRef & ref, const Token & expected, const BlobMeta & meta);
+
+/// Deletes only the marker incarnation identified by `expected`. A token mismatch leaves the current
+/// marker untouched; `NotFound` is distinct from that case so callers can tell absence from a raced
+/// replacement. The backend's complete `DeleteOutcome` is returned, including any storage-specific
+/// delete-marker status.
+DeleteOutcome deleteMetaExact(Backend & backend, const Layout & layout, const BlobRef & ref, const Token & expected);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.cpp
new file mode 100644
index 000000000000..9573563a983a
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.cpp
@@ -0,0 +1,75 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
+
+#include <Common/CurrentMetrics.h>
+#include <Common/Exception.h>
+#include <Common/ThreadPool.h>
+
+#include <algorithm>
+#include <memory>
+#include <mutex>
+
+namespace CurrentMetrics
+{
+    extern const Metric CASBlobUploadPoolThreads;
+    extern const Metric CASBlobUploadPoolThreadsActive;
+    extern const Metric CASBlobUploadPoolThreadsScheduled;
+}
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int LOGICAL_ERROR;
+}
+
+namespace Cas
+{
+
+namespace
+{
+    std::mutex pool_mutex;
+    std::unique_ptr<ThreadPool> pool_instance;
+}
+
+void initializeBlobUploadPool(size_t size)
+{
+    if (size == 0)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas_blob_upload_pool_size must not be 0");
+
+    std::lock_guard lock(pool_mutex);
+    if (pool_instance)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "The CAS blob upload pool is initialized twice");
+
+    pool_instance = std::make_unique<ThreadPool>(
+        CurrentMetrics::CASBlobUploadPoolThreads,
+        CurrentMetrics::CASBlobUploadPoolThreadsActive,
+        CurrentMetrics::CASBlobUploadPoolThreadsScheduled,
+        size);
+}
+
+ThreadPool & blobUploadPool()
+{
+    std::lock_guard lock(pool_mutex);
+    if (!pool_instance)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "The CAS blob upload pool is not initialized");
+
+    return *pool_instance;
+}
+
+void shutdownBlobUploadPool() noexcept
+{
+    std::lock_guard lock(pool_mutex);
+    pool_instance.reset();
+}
+
+bool blobUploadPoolInitializedForTest()
+{
+    std::lock_guard lock(pool_mutex);
+    return pool_instance != nullptr;
+}
+
+
+}
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h
new file mode 100644
index 000000000000..1e8594b5594b
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h
@@ -0,0 +1,43 @@
+#pragma once
+
+#include <Common/ThreadPool_fwd.h>
+
+#include <condition_variable>
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+#include <mutex>
+
+namespace DB::Cas
+{
+
+/// Server-wide pool for the parallel intra-part blob upload fan-out (stage-1 design §1,
+/// "Parallel blob upload within a part"). Deliberately disjoint from
+/// `IObjectStorage::getThreadPoolWriter`: an upload task may itself submit to the writer pool (S3
+/// multipart), so nesting the fan-out on that same pool would risk the classic same-pool
+/// wait-on-self deadlock. The calling thread only submits tasks and joins them -- it never
+/// occupies a pool slot itself -- so pool size 1 is a valid (fully serial) configuration, never a
+/// deadlock risk.
+///
+/// Fail-loud lifecycle: the server (or a test) must call `initializeBlobUploadPool` before any
+/// `blobUploadPool` use. There is no lazy self-initialization on the production path.
+
+/// Throws `BAD_ARGUMENTS` if `size == 0`. Throws `LOGICAL_ERROR` if already initialized.
+void initializeBlobUploadPool(size_t size);
+
+/// Throws `LOGICAL_ERROR` if the pool has not been initialized. The returned reference is only
+/// valid while the pool stays initialized: callers must not race this against
+/// `shutdownBlobUploadPool` (in the server, shutdown runs after query drain; tests own the order).
+ThreadPool & blobUploadPool();
+
+/// Idempotent: safe to call multiple times, and safe to call even if never initialized. Joins all
+/// outstanding tasks before returning.
+void shutdownBlobUploadPool() noexcept;
+
+/// For tests only: true once `initializeBlobUploadPool` has run, false before that call and after
+/// `shutdownBlobUploadPool`.
+bool blobUploadPoolInitializedForTest();
+
+
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.cpp
new file mode 100644
index 000000000000..39152156476f
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.cpp
@@ -0,0 +1,56 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.h>
+
+#include <Common/Exception.h>
+
+#include <utility>
+
+namespace DB::Cas
+{
+
+void EventDispatcher::setSink(Sink sink_)
+{
+    std::lock_guard<std::mutex> lock(mutex);
+    sink = std::move(sink_);
+    has_sink.store(static_cast<bool>(sink), std::memory_order_release);
+}
+
+void EventDispatcher::emit(CasEvent event)
+{
+    std::unique_lock<std::mutex> lock(mutex);
+    /// Enqueue before deciding who drains. A `bad_alloc` here escapes, but `draining` is untouched and
+    /// the deque's strong guarantee leaves the queue intact -- the dispatcher is never left wedged.
+    queue.push_back(std::move(event));
+
+    /// A drain loop already owns delivery (this call is reentrant from inside the sink, or a concurrent
+    /// emitter is draining). Enqueue-and-return: the running loop will deliver what we just pushed.
+    if (draining)
+        return;
+
+    draining = true;
+    while (!queue.empty())
+    {
+        CasEvent next = std::move(queue.front());
+        queue.pop_front();
+        /// The sink runs OUTSIDE `mutex`: a reentrant `emit` can take the lock, and a concurrent
+        /// emitter can enqueue, neither blocked on this delivery.
+        lock.unlock();
+        try
+        {
+            /// `sink` is set pre-traffic and never swapped concurrently with delivery, so reading it
+            /// here without `mutex` is race-free.
+            if (sink)
+                sink(std::move(next));
+        }
+        catch (...)
+        {
+            /// Contain the sink failure: dropping one audit event must not abandon the queued
+            /// remainder nor leave `draining` stuck true (which would silently mute all future events).
+            DB::tryLogCurrentException("CasEventDispatcher",
+                "Content-addressed audit event sink threw; the event was dropped");
+        }
+        lock.lock();
+    }
+    draining = false;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.h
new file mode 100644
index 000000000000..4bfc8859f26a
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.h
@@ -0,0 +1,64 @@
+#pragma once
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+
+#include <atomic>
+#include <deque>
+#include <mutex>
+
+namespace DB::Cas
+{
+
+/// Serialized, reentrancy-safe delivery for `CasEvent`s. Every content-addressed component that emits
+/// audit events (the `Pool`, the ref ledger, the manifest reader, the mount renewer) routes through
+/// the ONE dispatcher owned by its `Pool` instead of calling an installed sink `std::function`
+/// directly. That gives two properties the parallel intra-part upload fan-out (stage-1 design §1)
+/// newly requires:
+///
+///  - Serialization: concurrent emitters never run the sink at the same time, so a sink that appends
+///    to state without a lock of its own (as every existing sink does) stays correct.
+///  - Reentrancy safety: an emission performed FROM INSIDE a sink callback -- the sink calls a ledger
+///    read that itself emits -- is queued and drained by the already-running dispatch loop instead of
+///    recursing or self-deadlocking on the dispatcher mutex.
+///
+/// Why one dispatcher and not a per-component locking wrapper: a wrapper that held its own mutex
+/// across each component's sink call would establish a `state_mutex -> event_mutex` lock order and
+/// deadlock the instant a reentrant sink took `state_mutex` again. The dispatcher never holds `mutex`
+/// across the sink call, so reentrancy cannot deadlock it. It is the necessary-but-not-sufficient half
+/// of the contract: ledger emission points are additionally restructured to emit AFTER releasing
+/// `state_mutex`, so a sink's own reentrant ledger read can take that lock freshly.
+class EventDispatcher
+{
+public:
+    /// The sink type is `CasEventSink` (`std::function<void(CasEvent)>`) unchanged: every existing
+    /// sink stays valid as-is, and the by-value event preserves the move-the-`detail`-map-into-the-sink
+    /// idiom (`CasEvent.h`) rather than forcing a copy at a `const &` boundary.
+    using Sink = CasEventSink;
+
+    /// Installs the delivery sink. Pre-traffic only, matching the contract of the setter it replaces
+    /// (`Pool::setEventSink`): intended for pre-open wiring or tests with no active mount thread. A
+    /// null sink disables delivery.
+    void setSink(Sink sink_);
+
+    /// Whether a delivery sink is installed. Lock-free: `sink` is set pre-traffic and never swapped
+    /// concurrently with `emit`, so the query-frequency disabled hot path pays no mutex.
+    bool hasSink() const noexcept { return has_sink.load(std::memory_order_acquire); }
+
+    /// Delivers `event`, serialized across threads and safe to call reentrantly from within the sink.
+    /// Does not propagate a sink exception: a throwing sink is contained per event so one bad event
+    /// neither abandons the queued remainder nor leaves the dispatcher wedged (a `draining` flag stuck
+    /// true would silently drop every future event). Takes the event by value so the completed record
+    /// is moved into the queue and then into the sink, never deep-copied on the emitter thread. The
+    /// only exception that can escape is an allocation failure while enqueuing, which happens before
+    /// any dispatcher state changes -- the invariant is never broken.
+    void emit(CasEvent event);
+
+private:
+    std::mutex mutex;
+    std::deque<CasEvent> queue;           /// guarded by `mutex`
+    bool draining = false;                /// guarded by `mutex`; the draining thread owns delivery
+    Sink sink;                            /// swapped under `mutex`; read on the delivery path without it (no concurrent swap in traffic)
+    std::atomic<bool> has_sink{false};    /// lock-free mirror of `sink` presence for `hasSink`
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.cpp
new file mode 100644
index 000000000000..778814997ca1
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.cpp
@@ -0,0 +1,170 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <Common/CurrentMetrics.h>
+#include <optional>
+
+namespace ProfileEvents
+{
+    extern const Event CASPartFolderManifestGets;
+}
+
+namespace CurrentMetrics
+{
+    extern const Metric CASManifestDecodeCacheBytes;
+    extern const Metric CASManifestDecodeCacheEntries;
+}
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int FILE_DOESNT_EXIST;
+    extern const int CORRUPTED_DATA;
+    extern const int BAD_ARGUMENTS;
+}
+}
+
+namespace DB::Cas
+{
+
+CasManifestReader::CasManifestReader(
+    Backend & backend_, const Layout & layout_, const PoolMeta & meta_,
+    const CasEventSink & event_sink_, size_t manifest_decode_cache_bytes)
+    : backend(backend_), layout(layout_), meta(meta_), event_sink(event_sink_)
+{
+    if (manifest_decode_cache_bytes > 0)
+        manifest_cache = std::make_unique<ManifestDecodeCache>(
+            "LRU", CurrentMetrics::CASManifestDecodeCacheBytes, CurrentMetrics::CASManifestDecodeCacheEntries,
+            manifest_decode_cache_bytes, /*max_count=*/16384, ManifestDecodeCache::DEFAULT_SIZE_RATIO);
+}
+
+size_t CasManifestReader::ManifestCacheKeyHash::operator()(const ManifestCacheKey & k) const
+{
+    /// Combine the manifest-id hash with the token's bytes + type. The token is part of the key so a
+    /// re-incarnation under the same id misses (the immutable bytes changed identity).
+    const size_t h1 = std::hash<ManifestId>{}(k.manifest_id);
+    const size_t h2 = std::hash<String>{}(k.token.value);
+    const size_t h3 = std::hash<uint8_t>{}(static_cast<uint8_t>(k.token.type));
+    size_t h = h1;
+    h ^= h2 + 0x9e3779b97f4a7c15ULL + (h << 6) + (h >> 2);
+    h ^= h3 + 0x9e3779b97f4a7c15ULL + (h << 6) + (h >> 2);
+    return h;
+}
+
+std::shared_ptr<const PartManifest> CasManifestReader::readManifestShared(const ManifestId & id)
+{
+    /// A live reference naming a missing manifest body is a dangling-reference violation
+    /// (`INV-NO-DANGLE`). Never substitute an empty manifest: callers must observe the missing object
+    /// as an exception.
+    const String key = layout.manifestKey(id);
+
+    /// `HEAD` is mandatory even on a cache hit. It proves that the live reference still names an
+    /// existing object and supplies the token that identifies the immutable bytes being reused.
+    const HeadResult head = backend.head(key);
+    if (!head.exists)
+    {
+        if (event_sink)
+        {
+            CasEvent _ev1;
+            _ev1.type = CasEventType::ReadMissing;
+            _ev1.object_kind = CasEventObjectKind::Manifest;
+            _ev1.object_hash = manifestRefDebugString(id.ref);
+            _ev1.outcome = "missing";
+            _ev1.reason = "live ref names manifest but its object is missing (INV-NO-DANGLE)";
+            _ev1.detail = {{"code", "FILE_DOESNT_EXIST"}, {"site", "readManifest"}};
+            event_sink(std::move(_ev1));
+        }
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+            "live ref names manifest at {} but its object is missing — INV-NO-DANGLE", key);
+    }
+
+    if (manifest_cache)
+        if (auto cached = manifest_cache->get(ManifestCacheKey{.manifest_id = id, .token = head.token}))
+            return cached;
+
+    std::optional<GetResult> object = backend.get(key);
+    if (!object)
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+            "manifest at {} vanished between head and get — INV-NO-DANGLE", key);
+    ProfileEvents::increment(ProfileEvents::CASPartFolderManifestGets);
+
+    PartManifest body = decodePartManifest(openObject(FormatId::PartManifest, object->bytes));
+
+    /// The journal reference must equal the body's self-described `ref`; otherwise the reference
+    /// addresses a different object and the decoded bytes cannot be trusted.
+    if (!refMatchesBody(id.ref, body))
+    {
+        if (event_sink)
+        {
+            CasEvent _ev2;
+            _ev2.type = CasEventType::CorruptDecode;
+            _ev2.object_kind = CasEventObjectKind::Manifest;
+            _ev2.object_hash = manifestRefDebugString(id.ref);
+            _ev2.outcome = "corrupt";
+            _ev2.reason = "manifest body `ref` does not match the journal ManifestRef (refMatchesBody)";
+            _ev2.detail = {{"code", "CORRUPTED_DATA"}, {"site", "readManifest"}};
+            event_sink(std::move(_ev2));
+        }
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS manifest at {} body ref does not match the journal ManifestRef — refMatchesBody", key);
+    }
+
+    /// The body's `root_namespace_id` must equal the owning namespace. A mismatch is a
+    /// cross-namespace dangling reference and would give cleanup the wrong ownership authority.
+    if (!manifestNamespaceMatches(id.root_namespace, body))
+    {
+        if (event_sink)
+        {
+            CasEvent _ev3;
+            _ev3.type = CasEventType::CorruptDecode;
+            _ev3.object_kind = CasEventObjectKind::Manifest;
+            _ev3.object_hash = manifestRefDebugString(id.ref);
+            _ev3.outcome = "corrupt";
+            _ev3.reason = "manifest body root_namespace_id does not match the owning namespace (manifestNamespaceMatches)";
+            _ev3.detail = {{"code", "CORRUPTED_DATA"}, {"site", "readManifest"}};
+            event_sink(std::move(_ev3));
+        }
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS manifest at {} body root_namespace_id does not match the owning namespace — manifestNamespaceMatches", key);
+    }
+
+    auto decoded = std::make_shared<PartManifest>(std::move(body));
+    if (manifest_cache)
+        manifest_cache->set(ManifestCacheKey{.manifest_id = id, .token = head.token}, decoded);
+    return decoded;
+}
+
+PartManifest CasManifestReader::readManifest(const ManifestId & id)
+{
+    return *readManifestShared(id);
+}
+
+BlobLocation CasManifestReader::locate(const ManifestEntry & entry) const
+{
+    /// A ranged read into the content object: the payload starts at a constant offset for blobs
+    /// (the pool's fixed blob_header_len — no per-object header read). Inline carries no standalone
+    /// object location (there is no Subtree placement on a part manifest).
+    switch (entry.placement)
+    {
+        case EntryPlacement::Blob:
+        {
+            /// The entry carries the complete blob reference (algorithm and digest), so the key is
+            /// derived directly from it. The payload starts at the pool's fixed envelope length;
+            /// no object-specific header read is needed before the ranged payload read.
+            return BlobLocation{
+                .key = layout.blobKey(entry.ref),
+                .offset = meta.blob_header_len,
+                .length = entry.blob_size,
+            };
+        }
+        case EntryPlacement::Inline:
+            throw Exception(ErrorCodes::BAD_ARGUMENTS,
+                "entry placement {} has no blob location", static_cast<int>(entry.placement));
+    }
+    throw Exception(ErrorCodes::BAD_ARGUMENTS,
+        "entry placement {} has no blob location", static_cast<int>(entry.placement));
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.h
new file mode 100644
index 000000000000..af5d31776857
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.h
@@ -0,0 +1,101 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Common/CacheBase.h>
+#include <memory>
+
+namespace DB::Cas
+{
+
+/// The object key and ranged payload window for a manifest entry stored as a separate blob. The
+/// offset is measured from the beginning of the blob object and skips its fixed envelope; `length`
+/// is the entry's raw file size. Inline entries do not have a `BlobLocation`.
+struct BlobLocation
+{
+    String key;
+    uint64_t offset = 0;                      /// payload start within the object
+    uint64_t length = 0;
+};
+
+/// Reads and validates part manifests, caches immutable decodes, and translates blob entries into
+/// ranged object reads. A read first obtains the object's current backend token, then reuses a
+/// decode only for the matching `(ManifestId, Token)` pair; a cache miss performs a `GET` and
+/// validates both the manifest reference and owning namespace before publication into the cache.
+/// Missing or changing objects and failed identity checks are surfaced as exceptions, never as an
+/// empty or partially trusted manifest.
+///
+/// The reader receives its backend, immutable layout and pool metadata, and event sink by reference;
+/// it has no `Pool` back-reference and owns no `Pool`-level mutex. The decode cache is a
+/// byte-weighted `CacheBase` LRU whose synchronization is internal to `CacheBase`; a null cache
+/// means caching is disabled (`manifest_decode_cache_bytes == 0`).
+class CasManifestReader
+{
+public:
+    /// Binds the reader to the pool environment. A positive cache budget creates the byte-weighted
+    /// LRU; zero disables caching while leaving the mandatory `HEAD` and validation sequence intact.
+    CasManifestReader(
+        Backend & backend_, const Layout & layout_, const PoolMeta & meta_,
+        const CasEventSink & event_sink_, size_t manifest_decode_cache_bytes);
+
+    /// Reads a manifest by value using the fail-closed sequence described above. A missing body,
+    /// disappearance between `HEAD` and `GET`, decode failure, or either identity mismatch throws;
+    /// only a fully validated decode can enter the cache.
+    PartManifest readManifest(const ManifestId & id);
+
+    /// Reads a manifest like `readManifest` but returns the immutable shared decode. This preserves
+    /// the cache's value on the part-folder path and avoids copying all manifest entries on success.
+    std::shared_ptr<const PartManifest> readManifestShared(const ManifestId & id);
+
+    /// Computes the object key and payload window for a `Blob` entry without performing I/O. An
+    /// `Inline` entry, or any unsupported placement value, throws `BAD_ARGUMENTS` because it has no
+    /// standalone object to read.
+    BlobLocation locate(const ManifestEntry & entry) const;
+
+    /// Test seam: retained bytes of the manifest decode cache (0 when disabled).
+    size_t manifestDecodeCacheBytes() const { return manifest_cache ? manifest_cache->sizeInBytes() : 0; }
+
+private:
+    /// The cache must include the backend token: a reused manifest identifier can refer to a new
+    /// object incarnation, and its immutable decoded bytes must not be reused across incarnations.
+    struct ManifestCacheKey
+    {
+        ManifestId manifest_id;
+        Token token;
+        bool operator==(const ManifestCacheKey &) const = default;
+    };
+
+    /// Hashes both identity components and the token type so cache lookup uses the same complete
+    /// identity as `ManifestCacheKey::operator==`.
+    struct ManifestCacheKeyHash
+    {
+        size_t operator()(const ManifestCacheKey & k) const;
+    };
+
+    /// Estimates retained decode memory from fixed object overhead plus entry path and inline-byte
+    /// storage. Weighting by bytes gives a server reading many parts an honest memory ceiling instead
+    /// of a count-only bound; the cache key still provides the fail-closed token semantics.
+    struct PartManifestWeight
+    {
+        /// Returns the approximate bytes retained for one decoded manifest by the cache.
+        size_t operator()(const PartManifest & m) const
+        {
+            size_t bytes = 256;
+            for (const auto & e : m.entries)
+                bytes += e.path.size() + e.inline_bytes.size() + 96;
+            return bytes;
+        }
+    };
+    using ManifestDecodeCache = CacheBase<ManifestCacheKey, PartManifest, ManifestCacheKeyHash, PartManifestWeight>;
+
+    Backend & backend;
+    const Layout & layout;
+    const PoolMeta & meta;
+    const CasEventSink & event_sink;
+    std::unique_ptr<ManifestDecodeCache> manifest_cache;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.cpp
new file mode 100644
index 000000000000..21fb2fdbcf96
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.cpp
@@ -0,0 +1,545 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Common/Exception.h>
+#include <Common/logger_useful.h>
+#include <Common/setThreadName.h>
+#include <Common/thread_local_rng.h>
+#include <algorithm>
+#include <chrono>
+#include <ctime>
+#include <thread>
+#include <vector>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace ProfileEvents
+{
+    extern const Event CASIdentityLost;
+    extern const Event CASDataRootVanished;
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+/// Wall-clock seconds since epoch — the `since` timestamp the lifecycle snapshot reports (spec §7). A
+/// wall clock, deliberately unlike the fence's `CLOCK_BOOTTIME`: this is an operator-facing DateTime, not
+/// an interval measured across a possible VM suspend.
+int64_t wallClockNowSeconds()
+{
+    return static_cast<int64_t>(std::chrono::duration_cast<std::chrono::seconds>(
+        std::chrono::system_clock::now().time_since_epoch()).count());
+}
+}
+
+CasMountRuntime::CasMountRuntime(
+    BackendPtr backend_ptr_,
+    const Layout & layout_,
+    MountConfig config_,
+    String server_root_id_,
+    const CasEventSink & event_sink_,
+    CasRequestBudget cas_request_budget_,
+    std::function<bool()> remount_attempt_)
+    : backend_ptr(std::move(backend_ptr_))
+    , layout(layout_)
+    , config(std::move(config_))
+    , server_root_id(std::move(server_root_id_))
+    , event_sink(event_sink_)
+    , cas_request_budget(cas_request_budget_)
+    , remount_attempt(std::move(remount_attempt_))
+{
+}
+
+uint64_t CasMountRuntime::bootMs()
+{
+    struct timespec ts{};
+    clock_gettime(CLOCK_BOOTTIME, &ts);
+    return static_cast<uint64_t>(ts.tv_sec) * 1000 + static_cast<uint64_t>(ts.tv_nsec) / 1000000;
+}
+
+uint64_t CasMountRuntime::bootMsNow() const
+{
+    return config.boot_ms_fn ? config.boot_ms_fn() : bootMs();
+}
+
+void CasMountRuntime::waitSleep(uint64_t ms) const
+{
+    if (config.wait_sleep_fn)
+        config.wait_sleep_fn(ms);
+    else
+        std::this_thread::sleep_for(std::chrono::milliseconds(ms));
+}
+
+bool CasMountRuntime::mayMutate() const
+{
+    return !mount_fence.lost.load(std::memory_order_acquire)
+        && bootMsNow() < mount_fence.deadline_boot_ms.load(std::memory_order_acquire);
+}
+
+void CasMountRuntime::tripMountLost()
+{
+    mount_fence.lost.store(true, std::memory_order_release);
+    /// A durable-effect caller admitted under the incarnation this trip just ended must never conclude
+    /// the fence is fine again just because a LATER `armMountFence` happens to re-arm it (rev.7 [C2]).
+    fence_generation.fetch_add(1, std::memory_order_acq_rel);
+    /// The lease-loss event is exactly the `Live -> TransientNotLive` transition of the §1 state model.
+    /// Idempotent and terminal-safe (a compare-exchange from `Live` only).
+    noteLeaseLost();
+}
+
+void CasMountRuntime::checkFenceOrThrow(uint64_t admitted_generation) const
+{
+    /// [D5]: tell only what is known here. A tripped fence (or a bumped generation) means this node no
+    /// longer holds the mount incarnation the caller was admitted under -- but this same guard trips for a
+    /// transient lease blip AND for a deliberate terminal decommission (FORGET) or a lost identity, and this
+    /// code cannot tell them apart. So the CONDITION must NOT promise recovery ("temporarily unreachable"
+    /// would misdiagnose the terminal case); it names both possibilities and points at the authoritative
+    /// lifecycle. The CLASS is the write plane's uniform transient one (its 32 sibling write-transient sites
+    /// already mint it): under genuine ambiguity the refusal must be retried, never consumed as damage.
+    if (!mayMutate() || fenceGeneration() != admitted_generation)
+        throwCasTransientUnavailable(
+            fmt::format("content-addressed pool '{}'", server_root_id),
+            "mount fence tripped: the durable write is refused because this node no longer holds the mount "
+            "incarnation it was admitted under -- either a lease loss the disk auto-recovers from, or a "
+            "FORGET decommission / lost identity that does NOT recover; consult "
+            "system.cas_mounts for the disk's lifecycle before retrying");
+}
+
+bool CasMountRuntime::refAppendFenceOk() const
+{
+    /// `mayMutate` checks the latch and deadline. The additional budget check prevents starting a
+    /// controlled request that cannot plausibly finish, including its safety margin, before expiry.
+    if (mount_fence.lost.load(std::memory_order_acquire))
+        return false;
+    const uint64_t now = bootMsNow();
+    const uint64_t deadline = mount_fence.deadline_boot_ms.load(std::memory_order_acquire);
+    if (now >= deadline)
+        return false;
+    const uint64_t margin = cas_request_budget.attempt_timeout_ms + cas_request_budget.lease_safety_margin_ms;
+    return margin < deadline - now;
+}
+
+void CasMountRuntime::setMountDeadline(uint64_t deadline_boot_ms)
+{
+    mount_fence.deadline_boot_ms.store(deadline_boot_ms, std::memory_order_release);
+}
+
+void CasMountRuntime::armMountFence(UInt128 server_uuid, uint64_t writer_epoch, uint64_t deadline_boot_ms)
+{
+    mount_fence.server_uuid = server_uuid;
+    mount_fence.writer_epoch = writer_epoch;
+    mount_fence.deadline_boot_ms.store(deadline_boot_ms, std::memory_order_release);
+    /// A fresh lease incarnation is a fresh generation too: a durable-effect caller admitted under the
+    /// PRIOR incarnation must re-check and abort rather than ride this re-arm through (rev.7 [C2]).
+    fence_generation.fetch_add(1, std::memory_order_acq_rel);
+    if (arm_mount_fence_interposition_hook_for_test)
+        arm_mount_fence_interposition_hook_for_test();
+    /// Open the gate LAST. A caller that observes `lost == false` with acquire semantics must also see
+    /// the fresh generation; publishing the latch first exposes one admission window in which the dead
+    /// generation looks live again.
+    mount_fence.lost.store(false, std::memory_order_release);
+}
+
+uint64_t CasMountRuntime::minActive()
+{
+    std::lock_guard lk(builds_mutex);
+    return active_build_seqs.empty() ? next_build_seq : *active_build_seqs.begin();
+}
+
+uint64_t CasMountRuntime::peekNextBuildSeq()
+{
+    std::lock_guard lk(builds_mutex);
+    return next_build_seq;
+}
+
+void CasMountRuntime::renewWatermarkOnce()
+{
+    /// A read-only runtime has no heartbeat to renew. Report that misuse instead of fabricating a keeper
+    /// or silently treating the call as successful.
+    if (!mount_keeper)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS heartbeat: renewWatermarkOnce on a read-only Pool");
+    mount_keeper->renewOnce();
+}
+
+uint64_t CasMountRuntime::allocateBuildSeq()
+{
+    std::lock_guard lk(builds_mutex);
+    const uint64_t s = next_build_seq++;
+    active_build_seqs.insert(s);
+    return s;
+}
+
+void CasMountRuntime::registerInflightBuild(uint64_t seq, const PartWriteTxnPtr & build)
+{
+    /// The caller owns the build's shared pointer. Keep only a weak reference here so the registry does
+    /// not extend the build lifetime; publication, abandonment, or destruction removes the entry.
+    std::lock_guard lk(builds_mutex);
+    inflight_builds[seq] = build;
+}
+
+void CasMountRuntime::retireBuildSeq(uint64_t seq)
+{
+    std::lock_guard lk(builds_mutex);
+    active_build_seqs.erase(seq);
+    inflight_builds.erase(seq);
+}
+
+void CasMountRuntime::cancelInflightBuildsForNamespace(const RootNamespace & ns)
+{
+    /// The removal callback is invoked only after the namespace-removal transaction is durable. Keep
+    /// cancellation outside `builds_mutex`; `cancelForNamespaceRemoval` changes the build's atomic
+    /// cancellation state and does not require the registry lock.
+    std::vector<PartWriteTxnPtr> builds_to_check;
+    {
+        std::lock_guard lk(builds_mutex);
+        for (const auto & entry : inflight_builds)
+            if (auto build = entry.second.lock())
+                builds_to_check.push_back(std::move(build));
+    }
+    for (const auto & build : builds_to_check)
+        build->cancelForNamespaceRemoval(ns);
+}
+
+void CasMountRuntime::mintRandomProcessEpoch()
+{
+    /// Mint a nonzero equality-only identity. Keep it away from the zero/unarmed and UINT64_MAX/retired
+    /// sentinels; 52 random bits are sufficient for the expected collision risk of this token.
+    constexpr uint64_t EPOCH_MASK = (1ULL << 52) - 1;
+    process_epoch.store(
+        (thread_local_rng() ^ (static_cast<uint64_t>(thread_local_rng()) << 32)) & EPOCH_MASK,
+        std::memory_order_relaxed);
+    if (process_epoch.load(std::memory_order_relaxed) == 0)
+        process_epoch.store(1, std::memory_order_relaxed);
+}
+
+void CasMountRuntime::setProcessEpoch(uint64_t v, std::memory_order order)
+{
+    process_epoch.store(v, order);
+}
+
+void CasMountRuntime::setLiveWriterEpoch(uint64_t v)
+{
+    live_writer_epoch.store(v, std::memory_order_release);
+}
+
+void CasMountRuntime::installKeeper(UInt128 our_uuid, uint64_t writer_epoch, const std::function<uint64_t()> & now_ms)
+{
+    /// The mount object already contains this runtime's live `(uuid, epoch)` body. Construct the keeper
+    /// to adopt that exact slot rather than triggering its double-start guard. The keeper reads the
+    /// build-watermark floor through `minActive` while preparing each renewal.
+    const uint64_t ttl_ms = static_cast<uint64_t>(config.mount_lease_ttl_ms.count());
+    mount_keeper = std::make_unique<MountLeaseKeeper>(
+        backend_ptr, layout, server_root_id, our_uuid, writer_epoch,
+        config.mount_lease_ttl_ms, now_ms,
+        [this] { return minActive(); },
+        [this](CasEvent e) { emitEvent(std::move(e)); },
+        std::chrono::milliseconds(cas_request_budget.lease_safety_margin_ms),
+        [this] { return bootMsNow(); });
+    /// Install the fence callbacks before any background renewal can run: successful renewals extend the
+    /// local BOOTTIME deadline, while a superseded or foreign renewal latches the fence and starts recovery.
+    mount_keeper->setFenceCallbacks(
+        [this, ttl_ms](uint64_t attempt_boot_ms) { setMountDeadline(attempt_boot_ms + ttl_ms); },
+        [this]
+        {
+            tripMountLost();
+            /// Recover as a fresh incarnation; a fenced `(uuid, writer_epoch)` pair is never resurrected.
+            scheduleRemount();
+        });
+}
+
+void CasMountRuntime::keeperStart()
+{
+    mount_keeper->start();
+}
+
+void CasMountRuntime::keeperRenewOnce()
+{
+    mount_keeper->renewOnce();
+}
+
+void CasMountRuntime::keeperReset()
+{
+    mount_keeper.reset();
+}
+
+void CasMountRuntime::keeperStartBackground(std::chrono::milliseconds period)
+{
+    mount_keeper->startBackground(period);
+}
+
+void CasMountRuntime::keeperStopBackground()
+{
+    mount_keeper->stopBackground();
+}
+
+bool CasMountRuntime::isVanished() const
+{
+    const PoolLifecycle s = lifecycle();
+    return s == PoolLifecycle::VanishedReplaced
+        || s == PoolLifecycle::VanishedForgotten;
+}
+
+void CasMountRuntime::setLifecycleForTest(PoolLifecycle lc)
+{
+    /// Direct store, no precondition — the test harness pins an exact cell of the class × state table.
+    /// A `Vanished*` value also latches `vanished_intent` so the forced terminal state matches what a
+    /// natural `enterVanished` would leave behind (its truth semantics never depend on how it was reached).
+    /// Stamp `since` to match a naturally-reached state (release-store before the state store below, so a
+    /// snapshot reader that acquire-observes the forced state also observes the timestamp): 0 for `Live`,
+    /// now for every non-`Live` value. Keeps the forced cell of the class × state table indistinguishable
+    /// from a real transition for the introspection snapshot.
+    lifecycle_since_wall_s.store(lc == PoolLifecycle::Live ? 0 : wallClockNowSeconds(), std::memory_order_release);
+    pool_lifecycle.store(lc, std::memory_order_release);
+    if (lc == PoolLifecycle::VanishedReplaced
+        || lc == PoolLifecycle::VanishedForgotten)
+    {
+        vanished_intent.store(true, std::memory_order_release);
+        /// Keep the terminal-state guard consistent with the forced state, so a later `enterVanished`
+        /// (unusual, but not forbidden) is a clean no-op rather than re-storing / re-logging.
+        terminal_state_published.store(true, std::memory_order_release);
+    }
+}
+
+void CasMountRuntime::noteLeaseLost()
+{
+    /// `Live -> TransientNotLive`, and nothing else. A compare-exchange FROM `Live` leaves every other
+    /// state untouched, so a terminal state is never downgraded and a repeated call is a no-op. This is
+    /// the only transition the keeper thread performs, and it needs no lock because of that discipline.
+    PoolLifecycle expected = PoolLifecycle::Live;
+    if (pool_lifecycle.compare_exchange_strong(
+            expected, PoolLifecycle::TransientNotLive, std::memory_order_acq_rel, std::memory_order_acquire))
+    {
+        /// The `since` the lifecycle snapshot reports for `not_live` — the wall-clock instant this became
+        /// non-`Live`. Only the winning transition writes it (the guard above), so it is not re-stamped.
+        lifecycle_since_wall_s.store(wallClockNowSeconds(), std::memory_order_release);
+    }
+}
+
+void CasMountRuntime::noteRemounted()
+{
+    /// `TransientNotLive -> Live` on a successful reclaim. A compare-exchange FROM `TransientNotLive`
+    /// never revives `IdentityLost` or a `Vanished` state ([D3]) and is a no-op if already `Live`.
+    PoolLifecycle expected = PoolLifecycle::TransientNotLive;
+    if (pool_lifecycle.compare_exchange_strong(
+            expected, PoolLifecycle::Live, std::memory_order_acq_rel, std::memory_order_acquire))
+    {
+        /// Back to `Live`: the lifecycle snapshot reports no `since` (0) for a live pool.
+        lifecycle_since_wall_s.store(0, std::memory_order_release);
+    }
+}
+
+void CasMountRuntime::enterIdentityLost()
+{
+    /// `TransientNotLive -> IdentityLost`, one way. The compare-exchange FROM `TransientNotLive` gives
+    /// the brief's "from TransientNotLive only" precondition, idempotency (a second call finds the state
+    /// already `IdentityLost` and its exchange fails), and safety against a concurrent keeper
+    /// `noteLeaseLost` (which only ever moves `Live -> TransientNotLive`, never away from it). It does NOT
+    /// set `vanished_intent` (that latch is reserved for the `Vanished*` idempotency/FORGET protocol);
+    /// rev.8 makes `IdentityLost` a fail-loud TERMINAL state through `remountTerminal()`, which folds it
+    /// into the observer-exit boundary alongside `vanished_intent`, so the remount/GC observer threads
+    /// self-exit rather than demote.
+    /// `since` for the `identity_lost` snapshot row — the wall-clock instant the observer proved the
+    /// sentinels gone. Stamped (release) BEFORE the CAS that publishes `IdentityLost`, so a reader that
+    /// acquire-observes `IdentityLost` is guaranteed to observe this timestamp too (the winning CAS's
+    /// release carries this prior store) — the same before-publish ordering `enterVanished` uses. Safe to
+    /// stamp before the CAS here, unlike the lock-free `noteLeaseLost`: this runs only from
+    /// `TransientNotLive` under `Pool::remount_mutex` (a `Vanished` pool bailed at the caller's
+    /// `isVanished()` gate and the caller guards `!= IdentityLost`), so the CAS wins deterministically and
+    /// the stamp can never land on a state we did not transition.
+    lifecycle_since_wall_s.store(wallClockNowSeconds(), std::memory_order_release);
+
+    PoolLifecycle expected = PoolLifecycle::TransientNotLive;
+    if (!pool_lifecycle.compare_exchange_strong(
+            expected, PoolLifecycle::IdentityLost, std::memory_order_acq_rel, std::memory_order_acquire))
+        return;
+
+    ProfileEvents::increment(ProfileEvents::CASIdentityLost);
+    LOG_WARNING(getLogger("CasPool"),
+        "Content-addressed pool '{}' entered IdentityLost: the pool sentinels (_pool_meta and the owner "
+        "anchor) are authoritatively absent (both KeyAbsent). This is a fail-loud TERMINAL state: "
+        "store-class access now fails loud and this pool's remount + GC threads self-exit. "
+        "Recover by restart or SYSTEM CAS FORGET — a matching-sentinel restore does NOT "
+        "auto-revive this disk.",
+        server_root_id);
+}
+
+void CasMountRuntime::enterVanished(PoolLifecycle which, const String & reason)
+{
+    /// Validate the target BEFORE mutating any state — `enterVanished` takes only the two `Vanished*`
+    /// values (`VanishedReplaced`/`VanishedForgotten`); fail loud on a call-site bug rather than store a
+    /// non-terminal value or mislabel it.
+    const char * label = nullptr;
+    switch (which)
+    {
+        case PoolLifecycle::VanishedReplaced:  label = "replaced"; break;
+        case PoolLifecycle::VanishedForgotten: label = "forgotten"; break;
+        case PoolLifecycle::Live:
+        case PoolLifecycle::TransientNotLive:
+        case PoolLifecycle::IdentityLost:
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CasMountRuntime::enterVanished called with a non-terminal lifecycle value");
+    }
+
+    /// Publish the terminal-intent latch (spec §3). For a natural transition this is the FIRST publish; for
+    /// FORGET, `publishVanishedIntent` already set it at step 1. Either way it is published before the state
+    /// store below.
+    vanished_intent.store(true, std::memory_order_release);
+
+    /// Idempotency guard for the STATE transition, keyed on a dedicated latch rather than
+    /// `vanished_intent` (which FORGET publishes early): the FIRST winner stores the state, records the
+    /// reason, and logs; a later call returns here without re-storing or re-logging.
+    if (terminal_state_published.exchange(true, std::memory_order_acq_rel))
+        return;
+
+    /// Record the reason BEFORE the state's release-store, so a reader that acquire-observes the terminal
+    /// state (e.g. `Pool::throwIfLifecycleTerminal`) also observes this string. Written exactly once.
+    vanished_reason = reason;
+
+    /// The `since` the lifecycle snapshot reports for the `vanished` row — the wall-clock instant of the
+    /// terminal transition. Written before the `pool_lifecycle` release-store below, same as the reason,
+    /// so an acquire-observer of the terminal state also observes it.
+    lifecycle_since_wall_s.store(wallClockNowSeconds(), std::memory_order_release);
+
+    /// An unconditional store is safe now: the guard above serializes terminal transitions, and no
+    /// non-terminal transition can move a `Vanished` state (their compare-exchanges are keyed on
+    /// `Live`/`TransientNotLive`), so this value is absorbing.
+    pool_lifecycle.store(which, std::memory_order_release);
+
+    ProfileEvents::increment(ProfileEvents::CASDataRootVanished);
+    LOG_WARNING(getLogger("CasPool"),
+        "Content-addressed pool '{}' entered Vanished({}): {}. The disk stays registered but store-class "
+        "access now fails loud with a typed error (truth); restart re-registers the name.",
+        server_root_id, label, reason);
+}
+
+void CasMountRuntime::publishVanishedIntent()
+{
+    /// spec §5 step 1: publish the terminal-intent latch WITHOUT settling the state. The keeper callback
+    /// (`scheduleRemount`) and the remount loop both consult `vanished_intent` at their step boundaries, so
+    /// this stops new remount scheduling and makes an in-flight remount loop bail at its next step —
+    /// bounding FORGET's subsequent joins to one step + one backend timeout. The state store + WARN follow
+    /// in `enterVanished` (step 6). Idempotent.
+    vanished_intent.store(true, std::memory_order_release);
+}
+
+void CasMountRuntime::scheduleRemount()
+{
+    /// Count every entry before checking whether background work is enabled. Tests can therefore observe
+    /// the keeper's loss callback without depending on a recovery thread being spawned.
+    schedule_remount_calls_for_test.fetch_add(1, std::memory_order_relaxed);
+    if (!config.background_watermark)
+        return;
+    /// A terminal pool never claims/allocates/writes again (spec §3): the keeper callback must not arm a
+    /// recovery thread. `remountTerminal()` covers a published terminal `Vanished` intent (`vanished_intent`,
+    /// set by `publishVanishedIntent` at spec §5 step 1 for FORGET, or as `enterVanished`'s first step for a
+    /// natural transition, and subsuming every settled `Vanished*` state) AND `IdentityLost` (rev.8: now a
+    /// fail-loud terminal state — no demoted observer).
+    if (remount_shutting_down.load() || remount_running.load() || remountTerminal())
+        return;
+    std::lock_guard g(remount_thread_mutex);
+    if (remount_shutting_down.load() || remount_running.load() || remountTerminal())
+        return;
+    if (remount_thread.joinable())
+        remount_thread.join();   /// Reap a previous recovery before starting a new one.
+    remount_running.store(true);
+    remount_thread = ThreadFromGlobalPool([this]
+    {
+        setThreadName(ThreadName::CAS_REMOUNT);
+        uint64_t backoff_ms = 1000;
+        /// Exit at any step boundary once the pool is (being driven) terminal (spec §3). `remountTerminal()`
+        /// bails on a published terminal `Vanished` intent (`vanished_intent` — by FORGET at step 1, before
+        /// it joins this thread, or as `enterVanished`'s first step for a natural transition, and subsuming
+        /// every settled `Vanished*` state) AND on `IdentityLost` (rev.8: a fail-loud terminal state whose
+        /// identity gate just set it — the thread self-exits at the next boundary, ending the observer).
+        while (!remount_stop.load() && !remountTerminal())
+        {
+            if (remount_attempt())
+                break;
+            std::unique_lock lk(remount_cv_mutex);
+            remount_cv.wait_for(lk, std::chrono::milliseconds(backoff_ms),
+                                [this] { return remount_stop.load(); });
+            backoff_ms = std::min<uint64_t>(backoff_ms * 2, 30000);
+        }
+        remount_running.store(false);
+    });
+}
+
+bool CasMountRuntime::scheduleRemountForTest()
+{
+    scheduleRemount();
+    std::lock_guard g(remount_thread_mutex);
+    return remount_thread.joinable();
+}
+
+void CasMountRuntime::beginShutdownForTest()
+{
+    std::lock_guard g(remount_thread_mutex);
+    remount_shutting_down.store(true);
+}
+
+void CasMountRuntime::stopRemountThread()
+{
+    /// Refuse further recovery arming under the same mutex used by `scheduleRemount`, before joining.
+    /// Thus a keeper callback racing with teardown cannot re-arm the recovery thread after the join.
+    {
+        std::lock_guard g(remount_thread_mutex);
+        remount_shutting_down.store(true);
+    }
+    /// Stop recovery first; it could otherwise recreate the keeper while the heartbeat is being retired.
+    remount_stop.store(true);
+    remount_cv.notify_all();
+    {
+        std::lock_guard g(remount_thread_mutex);
+        if (remount_thread.joinable())
+            remount_thread.join();
+    }
+}
+
+void CasMountRuntime::finishTeardown(bool drained)
+{
+    /// On a drained teardown, `stop` writes an already-expired lease and the watermark farewell
+    /// (`min_active = UINT64_MAX`). This lets the same server reclaim immediately while retaining the
+    /// durable owner and epoch. A failure, such as another incarnation touching the slot, must not escape
+    /// destruction; log it and continue teardown.
+    if (mount_keeper)
+    {
+        if (drained)
+        {
+            try
+            {
+                mount_keeper->stop();
+            }
+            catch (...)
+            {
+                tryLogCurrentException(getLogger("CasPool"), "CAS mount-lease: release during Pool teardown failed");
+            }
+        }
+        else
+        {
+            /// If draining did not certify that every in-flight PUT resolved, a clean farewell would be
+            /// false evidence. Stop background renewal without a terminal operation so the successor uses
+            /// the slower but safe observation-based reclaim path.
+            LOG_WARNING(getLogger("CasPool"),
+                "CAS store shutdown with an unresolved ref-log PUT: skipping the clean-release marker; "
+                "the next mount will treat this end as unclean");
+            mount_keeper->stopBackground();
+        }
+    }
+
+    /// The second join closes the residual window where a keeper loss callback observed the shutdown gate
+    /// late during the heartbeat stop operation.
+    {
+        std::lock_guard g(remount_thread_mutex);
+        if (remount_thread.joinable())
+            remount_thread.join();
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h
new file mode 100644
index 000000000000..81d9da0ba9b9
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h
@@ -0,0 +1,431 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Common/ThreadPool.h>
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <cstdint>
+#include <functional>
+#include <limits>
+#include <map>
+#include <memory>
+#include <mutex>
+#include <set>
+
+namespace DB::Cas
+{
+
+class PartWriteTxn;
+using PartWriteTxnPtr = std::shared_ptr<PartWriteTxn>;
+
+/// The pool-level lifecycle condition (rev.7 §1) a `Pool` moves through as its shared backing changes
+/// underfoot. It is distinct from the storage-level `Constructing/Started/ShutDown` lifecycle (a null
+/// published pool -- before `startup`/after `shutdown`) the metadata storage tracks. Ordering of the
+/// enumerators is not significant; membership tests do the work.
+///   - `Live`             — the steady state; the mount lease is (or was last) held.
+///   - `TransientNotLive` — the lease was lost; access is uncertain and a self-remount retries. The §2
+///                          `Present`+identity-match recovery rule fires only from here (or `Live`).
+///   - `IdentityLost`     — the pool sentinels are authoritatively absent (both KeyAbsent):
+///                          fail-loud and TERMINAL (rev.8). The remount/GC observer threads self-exit;
+///                          matching-sentinel reappearance does NOT auto-revive it ([D3]); recovery is a
+///                          restart or `SYSTEM CAS FORGET`.
+///   - `Vanished*`        — fully terminal truth: the data root was replaced by a foreign pool, or the
+///                          disk was decommissioned by `FORGET`. Store-class access fails loud from here.
+enum class PoolLifecycle : uint8_t
+{
+    Live,
+    TransientNotLive,
+    IdentityLost,
+    VanishedReplaced,
+    VanishedForgotten,
+};
+
+/// Configuration owned by `CasMountRuntime`. `PoolConfig::mountConfig` projects the flat pool settings
+/// into this value, keeping the pool's existing configuration interface unchanged while allowing this
+/// lower-layer header to describe its own dependencies.
+struct MountConfig
+{
+    std::chrono::milliseconds mount_lease_ttl_ms{30000};
+    std::chrono::milliseconds mount_renew_period{10000};
+    /// When false, tests drive `renewWatermarkOnce` explicitly. In production this flag enables both
+    /// the merged mount-lease/build-watermark heartbeat and self-remount recovery.
+    bool background_watermark = false;
+    std::function<uint64_t()> boot_ms_fn = {};
+    std::function<void(uint64_t)> wait_sleep_fn = {};
+};
+
+/// Local, in-memory write fence. It is deliberately not checked by reading the object store for every
+/// write: the `MountLeaseKeeper` is the sole lease reader/renewer. A successful renewal translates the
+/// durable `expires_at_ms` into `deadline_boot_ms`; a foreign owner, newer `writer_epoch`, or failed
+/// renewal latches `lost`. Mutable operations are allowed only while the latch is clear and the local
+/// deadline has not passed. The `writer_epoch` is the durable fencing token.
+///
+/// The fence uses `CLOCK_BOOTTIME`, not `CLOCK_MONOTONIC`: monotonic time does not advance while a VM is
+/// suspended, so a resumed sleeper would compute the same "not yet expired" verdict it had before the nap
+/// even though wall time (and the GC leader's fence-out) moved far ahead — it could mutate the shared state
+/// under a live writer.
+/// `CLOCK_BOOTTIME` includes suspend time, so a resumed sleeper sees its fence expired.
+/// Container pause is already safe under either clock (the process is frozen, so no local check runs).
+struct MountFence
+{
+    UInt128 server_uuid{};
+    uint64_t writer_epoch = 0;
+    /// Until something arms a real lease deadline, the permissive default allows mutations. UINT64_MAX =
+    /// unarmed (never expires); otherwise a CLOCK_BOOTTIME-milliseconds instant.
+    std::atomic<uint64_t> deadline_boot_ms{std::numeric_limits<uint64_t>::max()};
+    std::atomic<bool> lost{false};
+};
+
+/// Owns the live writer-incarnation mechanics shared by the pool's mount and recovery orchestration:
+/// the `MountLeaseKeeper`, local `MountFence`, build watermark and in-flight build registry,
+/// `live_writer_epoch`, unclean-boundary marker, and self-remount thread. `Pool` retains the higher-level
+/// claim/recovery sequence and its `remount_mutex`; in particular, the runtime does not acquire or own
+/// the ref-ledger locks. The runtime receives its backend, layout, configuration, event sink, request
+/// budget, and a callback that performs one pool-level remount attempt, so it has no `Pool` back-reference.
+/// `Pool` delegates preserve the existing callers and test seams.
+class CasMountRuntime
+{
+public:
+    CasMountRuntime(
+        BackendPtr backend_ptr_,
+        const Layout & layout_,
+        MountConfig config_,
+        String server_root_id_,
+        const CasEventSink & event_sink_,
+        CasRequestBudget cas_request_budget_,
+        /// One pool-level recovery attempt. The callback captures the owning `Pool` and is invoked only
+        /// after construction, from the recovery thread.
+        std::function<bool()> remount_attempt_);
+
+    /// ---- per-server watermark and identity ----
+    /// `process_epoch` is random and nonzero for this pool incarnation. GC compares it for equality,
+    /// never ordering; a different value means that the previous writer incarnation is no longer live.
+    uint64_t epoch() const { return process_epoch.load(std::memory_order_acquire); }
+    uint64_t writerEpoch() const { return process_epoch.load(std::memory_order_acquire); }
+    /// The GC floor: the oldest in-flight build_seq, or next_build_seq when no build is active (so a
+    /// quiescent server's watermark floor advances to the next-to-be-allocated seq). Locks builds_mutex.
+    uint64_t minActive();
+    /// Test/assertion accessor for the next-to-allocate build_seq under the lock.
+    uint64_t peekNextBuildSeq();
+    /// Renew the merged mount heartbeat once, including its build-watermark floor. A read-only runtime
+    /// has no keeper and fails with a logical exception rather than fabricating a heartbeat.
+    void renewWatermarkOnce();
+
+    /// ---- local write fence ----
+    /// Return whether a mutable operation may start under the locally observed lease state.
+    bool mayMutate() const;
+    /// Permanently latch the local fence as lost for this runtime incarnation.
+    void tripMountLost();
+    /// Publish the BOOTTIME deadline from a successful lease renewal.
+    void setMountDeadline(uint64_t deadline_boot_ms);
+    /// Arm a new lease incarnation and clear any loss latched for the prior incarnation.
+    void armMountFence(UInt128 server_uuid, uint64_t writer_epoch, uint64_t deadline_boot_ms);
+    /// Test-only interposition at the publication boundary between the re-armed generation and the
+    /// live fence. A caller admitted from this hook must be refused: the old generation is already
+    /// dead, while the new generation is not live until `lost` is cleared.
+    void setArmMountFenceInterpositionHookForTest(std::function<void()> hook)
+    {
+        arm_mount_fence_interposition_hook_for_test = std::move(hook);
+    }
+    /// The fence clock: `CLOCK_BOOTTIME` in milliseconds (includes VM-suspend time, unlike
+    /// CLOCK_MONOTONIC — see `MountFence`). Consults the injected `config.boot_ms_fn` if set (tests),
+    /// otherwise `bootMs`.
+    uint64_t bootMsNow() const;
+    /// The real boot clock: `CLOCK_BOOTTIME` in milliseconds. Static so tests can compose it.
+    static uint64_t bootMs();
+
+    /// ---- fence-generation admission (rev.7 [C2]/[D1]) ----
+    /// Bumped by EVERY `tripMountLost` (a fence loss) and EVERY `armMountFence` (a re-arm -- a fresh
+    /// lease incarnation, e.g. after a self-remount). A durable-effect caller captures this value once
+    /// at admission and compares it again immediately before its durable backend call: a DIFFERENT
+    /// value means the lease incarnation moved from under it since admission -- even when the fence
+    /// happens to be live again under a brand-new incarnation, the caller's write is stale and must not
+    /// land. See `checkFenceOrThrow`.
+    uint64_t fenceGeneration() const { return fence_generation.load(std::memory_order_acquire); }
+
+    /// Fence-generation admission check for every durable CAS/PUT/DELETE (the plain-object surface,
+    /// staging-buffer finalize): the caller captures `fenceGeneration()` once at admission and passes it
+    /// back here immediately before its durable backend call -- and again before EVERY conditional-retry
+    /// iteration, not just the first attempt. Throws the typed transient refusal
+    /// (`throwCasTransientUnavailable`) when the fence is not currently held or the generation moved since
+    /// admission; the caller's write must never reach the backend in either case.
+    void checkFenceOrThrow(uint64_t admitted_generation) const;
+
+    /// ---- pool lifecycle condition (rev.7 §1, spec §§1-3); enum at namespace scope below ----
+    /// Atomic read of the current lifecycle (acquire).
+    PoolLifecycle lifecycle() const { return pool_lifecycle.load(std::memory_order_acquire); }
+    /// Whether the pool has reached one of the two fully-terminal `Vanished` values
+    /// (`VanishedReplaced` / `VanishedForgotten`).
+    bool isVanished() const;
+    /// Whether the terminal-intent latch (`vanished_intent`) is published — set by a natural
+    /// `enterVanished`, OR EARLY (spec §5 step 1) by FORGET's `publishVanishedIntent`, and NEVER by the
+    /// non-absorbing `IdentityLost` ([C1]). This is the EARLIEST terminal signal: it can already be true
+    /// while the state is still pre-terminal (mid-FORGET). Consulted alongside `isVanished()` by every
+    /// background worker that must self-exit the moment the pool is (being driven) terminal — the keeper
+    /// callback (`scheduleRemount`), the remount loop, and the GC scheduler.
+    bool vanishedIntentPublished() const { return vanished_intent.load(std::memory_order_acquire); }
+
+    /// Non-terminal lease-loss transition: `Live -> TransientNotLive`. Idempotent and lock-free; a
+    /// compare-exchange FROM `Live` only, so it never downgrades a terminal state. `tripMountLost`
+    /// calls this (the lease-loss primitive), and the remount loop's identity gate calls it as its
+    /// first step so a direct/forced remount attempt has a valid non-terminal predecessor state.
+    void noteLeaseLost();
+    /// Non-terminal recovery transition: `TransientNotLive -> Live`. Called after a self-remount
+    /// reclaimed a fresh incarnation. A compare-exchange FROM `TransientNotLive` only, so it NEVER
+    /// revives `IdentityLost`/`Vanished` ([D3]).
+    void noteRemounted();
+
+    /// One-way terminal transition to `IdentityLost`, from `TransientNotLive` only (a compare-exchange
+    /// FROM `TransientNotLive`, so it is idempotent and cannot fire from `Live`/`Vanished`). On the
+    /// transition it emits ONE WARN and one `CASIdentityLost` ProfileEvent. rev.8: `IdentityLost` is a
+    /// fail-loud TERMINAL state — `remountTerminal()` reports it, so the remount observer thread self-exits
+    /// (and the GC scheduler self-exits, through `Pool`) at its next boundary; there is no demoted observer.
+    /// It deliberately does NOT publish the `vanished_intent` latch (which is reserved for the `Vanished*`
+    /// idempotency/FORGET protocol); `remountTerminal()` widens the observer-exit boundary to include it.
+    /// Must be called under the caller's remount serialization (Pool::remount_mutex).
+    void enterIdentityLost();
+    /// Test seam: force the lifecycle condition directly to `lc`, bypassing the natural transition
+    /// preconditions (used by the operation-gate tests to pin each class × state cell without driving a
+    /// full remount/erase sequence). For a `Vanished*` value it also latches `vanished_intent`, so the
+    /// forced state is indistinguishable from a naturally-reached one. Never used in production.
+    void setLifecycleForTest(PoolLifecycle lc);
+
+    /// Publish the terminal-intent latch (`vanished_intent`) WITHOUT settling the lifecycle state. This is
+    /// spec §5 step 1 of `SYSTEM CAS FORGET`: publishing the latch FIRST makes the keeper
+    /// callback stop arming remounts and the remount loop bail at its next step boundary, so FORGET's
+    /// subsequent thread joins are bounded to one step + one backend timeout. The state store + WARN happen
+    /// later, in `enterVanished` at step 6. Idempotent; lock-free (a single release store). A natural
+    /// terminal transition does NOT call this — its `enterVanished` publishes the latch itself.
+    void publishVanishedIntent();
+
+    /// One-way transition to a fully-terminal `Vanished` value (spec §3). Publishes the terminal-intent
+    /// latch (so the keeper stops scheduling remounts and the remount loop exits at its next step
+    /// boundary) if it is not already published, records `reason`, stores the state, then emits ONE WARN +
+    /// one `CASDataRootVanished` ProfileEvent. Idempotent: the first terminal STATE transition wins (a
+    /// dedicated latch keyed separately from `vanished_intent`, because FORGET publishes that intent latch
+    /// early at step 1). `which` MUST be one of the two `Vanished*` values (`VanishedReplaced` or
+    /// `VanishedForgotten`). `reason` is retained and
+    /// surfaced verbatim in the `VanishedForgotten` [D5] error message (see `vanishedReason`). Threads exit
+    /// their own loops; the joins happen in `~Pool` for a natural transition, or synchronously in
+    /// `Pool::forgetDisk` for FORGET. Must be called under the caller's remount serialization
+    /// (Pool::remount_mutex).
+    void enterVanished(PoolLifecycle which, const String & reason);
+
+    /// The reason string recorded by the winning `enterVanished`, or empty when none has run (a
+    /// forced-for-test terminal state, or a non-terminal pool). `Pool::throwIfLifecycleTerminal` reads it
+    /// to build the `VanishedForgotten` [D5] message (which carries the operator's decommission timestamp
+    /// authored by `forgetDisk`). Safe to read only AFTER observing a terminal state via `lifecycle()`
+    /// (acquire): the reason is written once, before the state's release-store, so a reader that
+    /// acquire-observes the terminal state also observes the reason (release/acquire handoff).
+    const String & vanishedReason() const { return vanished_reason; }
+
+    /// Wall-clock second (`system_clock`, seconds since epoch) at which the pool ENTERED its current
+    /// non-`Live` lifecycle state, or 0 while `Live`. This is the `since` the non-gated
+    /// `system.cas_mounts` lifecycle snapshot (spec §7) reports. Written (release) at each
+    /// lifecycle edge — `noteLeaseLost`/`enterIdentityLost`/`enterVanished` set it to now, `noteRemounted`
+    /// clears it to 0 — and by `setLifecycleForTest`, so a forced state carries a `since` indistinguishable
+    /// from a naturally-reached one.
+    ///
+    /// Ordering vs the `pool_lifecycle` transition it accompanies: the TERMINAL edges (`enterVanished`,
+    /// `enterIdentityLost`) publish this store BEFORE the state store, so a reader that acquire-observes a
+    /// terminal state is guaranteed (release/acquire handoff) to observe this timestamp. The lock-free
+    /// lease-loss/remount edges (`noteLeaseLost`, `noteRemounted`) stamp it in the compare-exchange's
+    /// SUCCESS branch — after the CAS — because they may run on an already-terminal pool (`noteLeaseLost` is
+    /// called before the caller's `isVanished()` gate), where a pre-CAS stamp would clobber the terminal
+    /// `since`; a reader may therefore momentarily observe a just-entered `not_live` with `since` not yet
+    /// updated, a benign introspection artifact that converges within nanoseconds.
+    time_t lifecycleSinceWallS() const
+    {
+        return static_cast<time_t>(lifecycle_since_wall_s.load(std::memory_order_acquire));
+    }
+
+    /// Extends `mayMutate` with a remaining-budget check. A ref-log attempt is refused unless the
+    /// current lease has room for its configured timeout and safety margin, so work is not started when
+    /// it cannot plausibly finish before the fence expires.
+    bool refAppendFenceOk() const;
+
+    /// The `writer_epoch` of the live mount incarnation. Bumped by `tryRemountOnce` (self-remount after a
+    /// GC fence-out) — a `PartWriteTxn` minted under an older epoch fails closed on its next step.
+    uint64_t liveWriterEpoch() const { return live_writer_epoch.load(std::memory_order_acquire); }
+
+    /// ---- build registry ----
+    /// Allocate a strictly-increasing `build_seq` and add it to the active set. A sequence is never
+    /// reused or lowered, which lets the GC watermark advance monotonically.
+    uint64_t allocateBuildSeq();
+    /// Register the in-flight build so `dropNamespace`'s post-durable cancellation can reach it (weak_ptr).
+    void registerInflightBuild(uint64_t seq, const PartWriteTxnPtr & build);
+    /// Remove a build_seq from the active set + inflight map; idempotent (safe from publish/abandon/dtor).
+    void retireBuildSeq(uint64_t seq);
+    /// After the namespace-removal transaction is durable, cancel every in-flight build targeting `ns`.
+    /// Live shared pointers are collected under `builds_mutex` and cancelled after releasing it, because
+    /// cancellation may take a different path and must not run under the registry lock.
+    void cancelInflightBuildsForNamespace(const RootNamespace & ns);
+
+    /// ---- process epoch (identity) ----
+    /// Mint the random nonzero process identity used by GC's equality check.
+    void mintRandomProcessEpoch();
+    /// Set `process_epoch` to the durable `writer_epoch`. The caller supplies the memory order because
+    /// the initial writable claim and a later self-remount have different publication requirements.
+    void setProcessEpoch(uint64_t v, std::memory_order order);
+    /// Publish the live-incarnation `live_writer_epoch` with release ordering.
+    void setLiveWriterEpoch(uint64_t v);
+
+    /// ---- mount-lease keeper (owned) ----
+    /// Construct the `MountLeaseKeeper` adopting (our_uuid, writer_epoch) and wire its fence callbacks
+    /// (renew-ok refreshes the fence deadline; on-lost latches the fence + arms a self-remount) plus its
+    /// build-watermark `minActive` reader, and event sink. `keeperStart` is separate so pool claim
+    /// orchestration can catch `MountFencedException`, discard the keeper, allocate a fresh epoch, and
+    /// retry the claim.
+    void installKeeper(UInt128 our_uuid, uint64_t writer_epoch, const std::function<uint64_t()> & now_ms);
+    /// Adopt the already-claimed mount slot; on return the adoption is durable.
+    void keeperStart();
+    /// Force one fresh conditional lease write on the already-adopted slot (fails closed, like any
+    /// other `renewOnce`, if the slot changed hands underfoot). Used to re-anchor the write-fence
+    /// arm after a materialization grace long enough to have consumed the lease TTL.
+    void keeperRenewOnce();
+    /// Discard a keeper after a refused adoption so the caller can retry with a fresh epoch.
+    void keeperReset();
+    /// Start periodic lease and watermark renewal.
+    void keeperStartBackground(std::chrono::milliseconds period);
+    /// Stop periodic renewal; safe to call more than once.
+    void keeperStopBackground();
+    bool hasKeeper() const { return static_cast<bool>(mount_keeper); }
+
+
+    /// ---- self-remount recovery ----
+    /// On a lost lease, arm a recovery thread when background operation is enabled. It retries the
+    /// pool-level remount callback with exponential backoff until success or teardown.
+    void scheduleRemount();
+    /// Test seam: drive the arm/refuse path directly. Returns true iff a recovery thread is armed after.
+    bool scheduleRemountForTest();
+    /// Test seam: latch the shutdown gate without joining or otherwise tearing down the runtime.
+    void beginShutdownForTest();
+    /// Return how many times `scheduleRemount` was entered, including calls refused by the background
+    /// setting. This is useful for testing the keeper's loss callback without starting a real recovery.
+    uint64_t scheduleRemountCallCountForTest() const
+    {
+        return schedule_remount_calls_for_test.load(std::memory_order_relaxed);
+    }
+
+    /// ---- teardown ----
+    /// Stop and join the self-remount thread before retiring the keeper; otherwise it could recreate the
+    /// keeper while teardown is in progress.
+    void stopRemountThread();
+    /// Retire the merged heartbeat. When `drained` is true, publish the clean farewell; otherwise stop
+    /// background renewal without writing a terminal marker, because unresolved writes must not be
+    /// certified as clean. Finish with a second recovery-thread join to close the final callback window.
+    void finishTeardown(bool drained);
+
+    /// Sleep through the injected test hook when present; otherwise use the production thread sleep.
+    /// `Pool` claim observation and materialization grace waits share this seam so tests control both.
+    void waitSleep(uint64_t ms) const;
+
+    /// Forward keeper events to the injected sink. The sink is held by reference so it observes the
+    /// owning pool's current event routing for the runtime's entire lifetime.
+    void emitEvent(CasEvent && e) const { if (event_sink) event_sink(std::move(e)); }
+
+private:
+    /// TRUE once the pool has reached — or is being driven toward — a state on which the self-remount
+    /// observer thread must stop: a published terminal `Vanished` intent (`vanished_intent` — set early by
+    /// FORGET, or by a natural `enterVanished`, and already subsuming every settled `Vanished*` state since
+    /// it is published before the state store) OR `IdentityLost` (rev.8: a fail-loud TERMINAL state — no
+    /// demoted observer; recovery is restart or FORGET). Consulted by `scheduleRemount` before arming and by
+    /// the remount loop at every step boundary. (The GC scheduler applies the same three-way test through
+    /// `Pool`, spec §9 rev.8 item 8.)
+    bool remountTerminal() const
+    {
+        return vanished_intent.load(std::memory_order_acquire)
+            || lifecycle() == PoolLifecycle::IdentityLost;
+    }
+
+    /// ---- injected environment (no `Pool` back-reference); initialized first, in this order ----
+    BackendPtr backend_ptr;
+    const Layout & layout;
+    MountConfig config;
+    String server_root_id;
+    const CasEventSink & event_sink;
+    CasRequestBudget cas_request_budget;
+    std::function<bool()> remount_attempt;
+
+    /// Per-server build watermark. `process_epoch` is a random
+    /// nonzero u64 minted once at open: GC checks it for EQUALITY (an object stamped with a different
+    /// epoch is from a dead incarnation), never for ordering. next_build_seq is a strictly-increasing
+    /// per-process counter (monotonicity is load-bearing — a seq is never reused or lowered);
+    /// active_build_seqs holds the seqs of in-flight builds, so `minActive` yields the GC floor. The floor
+    /// is published by the merged `mount_keeper`
+    /// beat (there is no standalone watermark object anymore). ATOMIC because a self-remount re-stamps it
+    /// (kept equal to `live_writer_epoch`) off the background remount thread while `epoch`/`writerEpoch`
+    /// may observe it; the ref-lane hot readers were moved to `liveWriterEpoch`, so this now backs only
+    /// the identity accessors.
+    std::atomic<uint64_t> process_epoch{0};
+    std::mutex builds_mutex;
+    uint64_t next_build_seq = 1;
+    std::set<uint64_t> active_build_seqs;
+    /// In-flight builds keyed by `build_seq`. `dropNamespace` upgrades these weak pointers only after its
+    /// removal transaction is durable and cancels those targeting the removed namespace. The wiring owns
+    /// the shared pointers, so an expired entry is simply skipped. Guarded by `builds_mutex`.
+    std::map<uint64_t, std::weak_ptr<PartWriteTxn>> inflight_builds;
+
+    /// Mount-lease heartbeat. Constructed and started on a writable
+    /// open AFTER the owner/epoch/mount startup protocol; renews the mount lease async off the write
+    /// path and drives the local write fence (deadline on each successful renew, `tripMountLost` on a
+    /// superseded/foreign touch). Teardown stops it, whose `terminate` retires the lease (so a
+    /// same-server reopen can immediately reclaim). Null on a read-only open.
+    std::unique_ptr<MountLeaseKeeper> mount_keeper;
+
+    std::atomic<uint64_t> live_writer_epoch{0};
+    std::mutex remount_thread_mutex;       /// guards the thread handle below
+    std::atomic<bool> remount_running{false};
+    std::atomic<bool> remount_stop{false};
+    std::atomic<bool> remount_shutting_down{false};   /// latched at teardown top; scheduleRemount refuses to re-arm during teardown
+    std::condition_variable remount_cv;
+    std::mutex remount_cv_mutex;
+    ThreadFromGlobalPool remount_thread;
+    /// Counted entries into `scheduleRemount`; retained as a test-only observability seam.
+    std::atomic<uint64_t> schedule_remount_calls_for_test{0};
+
+    /// Local write fence. The unarmed default (`deadline_boot_ms = UINT64_MAX`, `lost = false`) permits
+    /// mutation until a keeper supplies a real lease deadline or reports that the lease was lost. This
+    /// is the gate at the ref-append mutation chokepoint.
+    MountFence mount_fence;
+
+    /// Fence-generation token (rev.7 [C2]): bumped by `tripMountLost` and `armMountFence`. See
+    /// `fenceGeneration`/`checkFenceOrThrow`.
+    std::atomic<uint64_t> fence_generation{0};
+    std::function<void()> arm_mount_fence_interposition_hook_for_test;
+
+    /// The pool lifecycle condition (rev.7 §1). Starts `Live`. Non-terminal transitions
+    /// (`noteLeaseLost`/`noteRemounted`) are lock-free compare-exchanges guarded by their exact
+    /// predecessor state; the terminal transitions (`enterIdentityLost`/`enterVanished`) are serialized
+    /// by the caller's `Pool::remount_mutex` and made race-safe against the keeper thread's concurrent
+    /// `noteLeaseLost` by the compare-exchange/latch discipline in the .cpp.
+    std::atomic<PoolLifecycle> pool_lifecycle{PoolLifecycle::Live};
+    /// Terminal-intent latch (spec §3), published before the state store — by `enterVanished` for a
+    /// natural transition, or EARLY (step 1) by `publishVanishedIntent` for FORGET. Only the fully-terminal
+    /// `Vanished*` transition sets it — `IdentityLost` deliberately does NOT (rev.8 folds IdentityLost into
+    /// the observer-exit boundary via `remountTerminal()` instead). Consulted (with `IdentityLost`) by
+    /// `remountTerminal()`, so a terminal pool's keeper callback never schedules a remount and the remount
+    /// loop bails at its next step boundary — no claim/allocate/write after the pool is (being driven) terminal.
+    std::atomic<bool> vanished_intent{false};
+    /// Idempotency guard for the terminal STATE transition (`enterVanished`'s body). Distinct from
+    /// `vanished_intent`: FORGET publishes that intent latch at step 1, so it can no longer serve as the
+    /// "state transition already done" flag. The FIRST `enterVanished` to win this exchange stores the
+    /// state, records `vanished_reason`, and logs; every later call returns early.
+    std::atomic<bool> terminal_state_published{false};
+    /// The reason recorded by the winning `enterVanished` (see `vanishedReason`). Written once, BEFORE the
+    /// `pool_lifecycle` release-store, and immutable thereafter — so a reader that acquire-observes a
+    /// terminal state also observes this string. Empty when no terminal transition has run.
+    String vanished_reason;
+
+    /// Wall-clock second at which the current non-`Live` lifecycle state was entered; 0 while `Live` (see
+    /// `lifecycleSinceWallS`). Set at every lifecycle edge with a release-store ordered before the
+    /// `pool_lifecycle` transition it accompanies.
+    std::atomic<int64_t> lifecycle_since_wall_s{0};
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.cpp
new file mode 100644
index 000000000000..e0c2fd1e210d
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.cpp
@@ -0,0 +1,1463 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <IO/ReadBufferFromString.h>
+#include <IO/WriteBufferFromString.h>
+#include <IO/WriteHelpers.h>
+#include <Common/Exception.h>
+#include <Common/logger_useful.h>
+#include <Common/ProfileEvents.h>
+#include <Common/thread_local_rng.h>
+#include <Common/config_version.h>
+#include <base/defines.h>
+#include <fmt/format.h>
+#include <algorithm>
+#include <chrono>
+
+namespace ProfileEvents
+{
+    extern const Event CASBlobDeduplicationCacheHit;
+    extern const Event CASBlobHeadFirst;
+    extern const Event CASBlobBodyPutAvoided;
+    extern const Event CASBlobAdoptTrusted;
+    extern const Event CASMetaCreateClean;
+    extern const Event CASMetaAdoptBackfill;
+    extern const Event CASMetaResurrectClean;
+}
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+    extern const int BAD_ARGUMENTS;
+    extern const int FILE_DOESNT_EXIST;
+    extern const int NOT_IMPLEMENTED;
+    extern const int ABORTED;
+    extern const int CORRUPTED_DATA;
+    extern const int LIMIT_EXCEEDED;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Backpressure caps, enforced fail-closed in stageManifest before the body write returns.
+constexpr uint64_t kMaxManifestEntries = 1048576;
+constexpr uint64_t kMaxManifestEncodedBytes = 256ULL << 20;        /// 256 MiB
+constexpr uint64_t kMaxManifestInlineBytesTotal = 16ULL << 20;     /// 16 MiB
+constexpr uint64_t kMaxLargestInlineEntryBytes = 1ULL << 20;       /// 1 MiB
+
+/// Two thread_local_rng draws composed into a UInt128. Used both to mint build ids and to mint, on
+/// every upload/re-upload, a FRESH incarnation_tag (W-FRESH-TAG).
+UInt128 mintU128()
+{
+    const UInt64 hi = thread_local_rng();
+    const UInt64 lo = thread_local_rng();
+    return (static_cast<UInt128>(hi) << 64) | lo;
+}
+
+uint64_t nowMs()
+{
+    return static_cast<uint64_t>(std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::system_clock::now().time_since_epoch()).count());
+}
+
+}
+
+/// The pool-wide content-hash convention, with the selected `algo`: `Cas::blobHashHexOneShot` uses the same
+/// streaming/chunked convention the write path uses for `CityHash128` (`ContentAddressedWriteBuffers`: `getHashHex` ->
+/// `BlobId` -> `hexToU128`) and the one-shot `XXH3_128bits` call for `XXH3_128` (defined to agree
+/// with its own streaming digest). The core otherwise never re-hashes payloads; any copy-forward
+/// re-verification must use this convention. A one-shot `CityHash128` instead of the chunked convention
+/// diverges for payloads larger than one hash block and can report valid stored content as `CORRUPTED_DATA`.
+///
+/// Returns the full `BlobRef` pair at `algo`'s own width (via `codecFor`) — never a bare digest. Exported
+/// (declared in `CasPartWriteTxn.h`) so the wiring's inline-candidate hashing
+/// (`ContentAddressedTransaction.cpp`) can mint the same way the
+/// streaming blob path does, rather than reimplementing the hex round-trip inline.
+BlobRef poolContentHash(BlobHashAlgo algo, std::string_view payload)
+{
+    return BlobRef{algo, codecFor(algo).fromHex(blobHashHexOneShot(algo, payload))};
+}
+
+BlobSource BlobSource::fromString(String bytes)
+{
+    BlobSource source;
+    source.size = bytes.size();
+    /// Owning reader: `ReadBufferFromString` only borrows, and this factory outlives no single call
+    /// -- an owning copy per attempt removes the question entirely for a source that is small by
+    /// construction (this helper exists for tests and inline payloads).
+    source.open = [b = std::move(bytes)]() -> std::unique_ptr<ReadBuffer>
+    { return std::make_unique<ReadBufferFromOwnString>(b); };
+    return source;
+}
+
+PartWriteTxn::PartWriteTxn(PoolPtr store_, UInt128 build_id_,
+             uint64_t build_seq_, uint64_t epoch_, PartWriteInfo info_)
+    : store(std::move(store_))
+    , build_id(build_id_)
+    , build_seq(build_seq_)
+    , epoch(epoch_)
+    , info(std::move(info_))
+{
+    /// A build began. `build_id`, `build_seq`, and `epoch` identify it for token-join attribution
+    /// against the GC delete rows.
+    EventEmitter{*store}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::BuildStart;
+        e.ref_name = info.intended_ref.value_or("");
+        e.token = u128ToHex(build_id);
+        e.outcome = "started";
+        e.reason = "beginPartWrite: build in-flight";
+        e.detail = {{"build_seq", std::to_string(build_seq)}, {"epoch", std::to_string(epoch)}};
+    });
+}
+
+PartWriteTxn::~PartWriteTxn()
+{
+    /// An attempted precommit whose terminal operation did not settle may still be a durable owner.
+    /// Transfer that exact cleanup duty to the mount and KEEP the build active: the next mutation of
+    /// this namespace first resolves the every-attempt wedge, then removes the owner if it exists, and
+    /// retires the sequence only after that same state observation proves the duty discharged. If the
+    /// process ends first, no false clean farewell is written and successor recovery seals the old
+    /// epoch before sweeping its stale precommit.
+    if (precommit_state == PrecommitState::Uncertain || precommit_state == PrecommitState::Durable)
+    {
+        store->enqueueWriterCleanupDuty(
+            precommit_target_ns, precommit_final_ref, precommit_manifest, build_seq);
+        return;
+    }
+
+    /// No owner duty remains: promotion/abandon settled it, or no precommit append was attempted.
+    store->retireBuildSeq(build_seq);
+}
+
+void PartWriteTxn::requireAlive() const
+{
+    if (!alive)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "PartWriteTxn has been abandoned; no further operations allowed");
+    /// `dropNamespace` cancels in-flight builds once its removal transaction is
+    /// durable. A cancelled build must not promote/precommit a fresh owner into the just-removed namespace
+    /// (nor stage more debris there), so every further op fails closed here -- fast, before any backend
+    /// work, with a clear diagnostic (the append lane would reject a promote/precommit anyway).
+    if (cancelled.load(std::memory_order_acquire))
+        throwCasWriteRetryLater(
+            "PartWriteTxn cancelled: its owning namespace was removed (dropNamespace) while this build was "
+            "in flight; restart the build only after the namespace is recreated");
+    /// Self-remount (fence-out recovery) supersedes the mount incarnation this build was minted
+    /// under; its write fence already interrupted the build mid-flight, so every further step fails
+    /// closed and the caller restarts the build under the live epoch.
+    if (const uint64_t live = store->liveWriterEpoch(); epoch != live)
+        throwCasWriteRetryLater(fmt::format(
+            "PartWriteTxn (writer_epoch {}) belongs to a superseded mount incarnation (live epoch {}) — "
+            "the mount was fenced out and self-remounted; restart the build", epoch, live));
+}
+
+PutBlobResult PartWriteTxn::putBlob(const BlobRef & ref, BlobSource source)
+{
+    /// Serial API preserved for existing callers: run the transaction-detached primitive, then fold its
+    /// single result into `build` on this (the owning writer) thread. Semantics are byte-for-byte those
+    /// of the pre-carve `putBlob`. The fan-out path (spec §1) instead collects many results off-thread and
+    /// folds them together via `mergeBlobUploadResults`. `source.size` is the streaming byte authority, so
+    /// the request's `declared_size` mirrors it.
+    const uint64_t declared_size = source.size;
+    const BlobUploadResult r = uploadBlobDetached(BlobUploadRequest{ref, std::move(source), declared_size});
+    deps[r.ref] = r.dep;
+    return PutBlobResult{r.ref, r.dep.size};
+}
+
+BlobUploadResult PartWriteTxn::uploadBlobDetached(const BlobUploadRequest & req) const
+{
+    requireAlive();
+
+    const PoolConfig & cfg = store->poolConfig();
+    /// The caller (the write-mint site) already produced the full `BlobRef` pair (algo + digest), so there
+    /// is no hex round-trip here. `logical_ref` is the blob identity end-to-end: the dedup cache and every
+    /// downstream event render key off THIS value directly.
+    const BlobRef & logical_ref = req.ref;
+    const String key = store->layout().blobKey(req.ref);
+    const BlobSource & source = req.source;
+
+    /// The source is RE-READABLE (the caller's `open` re-reads a staged temp file, or re-emits a
+    /// captured String): it can be invoked MULTIPLE times — the primary streaming PUT plus any INV-1
+    /// re-upload — so we never materialize the whole blob into memory here. The byte count is verified
+    /// against `source.size` at each streaming write site (via the sink buffer's `count()`), not by a
+    /// full pre-materialization, so peak memory is bounded by the write-buffer, not the blob size.
+
+    /// HEAD-before-PUT on a likely dedup hit (cache says present) or a large body (where a
+    /// wasted body-PUT that 412s is expensive — and on a store that early-closes a doomed conditional
+    /// PUT, the broken-pipe and retry storm caused by early rejection). A present HEAD ⇒ admit without streaming the body;
+    /// a stale/absent HEAD ⇒ fall through to the normal conditional upload. SAFE by construction: we
+    /// always genuinely observe present-at-round before skipping the body, so the cache can never cause
+    /// a dangle (a stale hit just HEADs 404 and uploads). The cache membership is read ONCE here: it both
+    /// arms the HEAD-first gate and distinguishes the `DeduplicationCacheHit` outcome from a size-triggered `HeadHit`.
+    const bool cache_hit = store->dedupCacheContains(logical_ref);
+    const bool head_first =
+        cache_hit
+        || (cfg.deduplication_head_first_min_bytes > 0 && source.size >= cfg.deduplication_head_first_min_bytes);
+    if (head_first)
+    {
+        ProfileEvents::increment(ProfileEvents::CASBlobHeadFirst);
+        const HeadResult hr = store->backend().head(key);
+        if (hr.exists)
+        {
+            ProfileEvents::increment(ProfileEvents::CASBlobBodyPutAvoided);
+            if (cache_hit)
+                ProfileEvents::increment(ProfileEvents::CASBlobDeduplicationCacheHit);
+            try
+            {
+                const BlobDepRecord dep = observeAndAdmit(ObjectKind::Blob, logical_ref, key, hr);
+                store->dedupCacheAdd(logical_ref);
+                return BlobUploadResult{req.ref, dep,
+                    cache_hit ? BlobUploadOutcome::DeduplicationCacheHit : BlobUploadOutcome::HeadHit};
+            }
+            catch (const Exception & e)
+            {
+                /// INV-1: HEAD-first path hit a condemned token → re-upload from our OWN source bytes.
+                if (e.code() != ErrorCodes::ABORTED)
+                    throw;
+                /// Fall through to uploadFromSource below.
+            }
+        }
+        /// hr.exists == false OR condemned-ABORTED → fall through to uploadFromSource / fresh upload.
+    }
+
+    /// Fresh-upload path + condemned-dedup recovery under INV-1. Try the primary upload via
+    /// uploadFromSource (which handles condemned-present via putOverwrite and absent via putIfAbsentStream
+    /// without any backend().get). Bounded loop guards against rare concurrent-condemnation churn.
+    constexpr int max_attempts = 8;
+    for (int attempt = 0; attempt < max_attempts; ++attempt)
+    {
+        try
+        {
+            const BlobUploadResult r = uploadFromSource(ObjectKind::Blob, logical_ref, key, source);
+            /// This hash is now known-present — future writers can HEAD-first and skip the body.
+            store->dedupCacheAdd(logical_ref);
+            return r;
+        }
+        catch (const Exception & e)
+        {
+            /// ABORTED from uploadFromSource is retryable — re-upload by re-streaming from our re-readable
+            /// source (bounded). Two cases produce it:
+            ///   • a racing writer displaced the condemned token before our putOverwrite landed and
+            ///     their fresh incarnation is itself already condemned (observeAndAdmit → ABORTED); or
+            ///   • the object was GC-deleted during the post-412 revival re-observe:
+            ///     reviveObserve converts FILE_DOESNT_EXIST → ABORTED so the vanish re-uploads here
+            ///     rather than escaping FATAL. Both are rare races covered by the bounded loop.
+            if (e.code() != ErrorCodes::ABORTED || attempt + 1 == max_attempts)
+                throw;
+        }
+    }
+
+    /// Unreachable: the loop either returns or rethrows on the final attempt.
+    throw Exception(ErrorCodes::LOGICAL_ERROR, "uploadBlobDetached: exhausted retries for {}", key);
+}
+
+void PartWriteTxn::mergeBlobUploadResults(std::span<const BlobUploadResult> results)
+{
+    /// Prevalidate EVERYTHING first, touching nothing but locals: a result without a token is
+    /// incomplete (every `uploadBlobDetached` branch sets one; a tokenless dep only ever arrives
+    /// through `adoptEvidence`'s separate direct-fold path, never through this method) -- a caller bug,
+    /// failed closed rather than merged as a hole. Two results for the SAME ref must carry an
+    /// IDENTICAL dep record (the fan-out's one-task-per-unique-ref invariant, spec §1); a conflict --
+    /// most commonly a conflicting size -- means that invariant was violated upstream.
+    std::map<DepKey, const BlobDepRecord *> seen;
+    for (const auto & r : results)
+    {
+        if (!r.dep.token.has_value())
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "PartWriteTxn::mergeBlobUploadResults: incomplete result for {} (no token) -- every "
+                "uploadBlobDetached branch sets one; a tokenless dep must be recorded via adoptEvidence, "
+                "never merged here", blobIdOf(r.ref));
+        const auto [it, inserted] = seen.try_emplace(r.ref, &r.dep);
+        if (!inserted && !(*it->second == r.dep))
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "PartWriteTxn::mergeBlobUploadResults: dep records differ for {} (sizes {} vs {}) -- "
+                "the fan-out must launch exactly one task per unique ref and merge exactly one result; "
+                "records for one ref that differ in ANY field (size, token, kind, or adopted) are a "
+                "wiring error, so the sizes shown may be equal when the mismatch is in another field",
+                blobIdOf(r.ref), it->second->size, r.dep.size);
+    }
+
+    /// Build-and-swap: apply every result into a COPY of the live `deps`. The live member is touched
+    /// ONLY by the final `swap` below, which runs after every result has applied -- so a mid-loop
+    /// exception (a `bad_alloc` from map-node allocation, or one injected by `setMergeHookForTest`'s
+    /// hook) leaves `deps` byte-for-byte as it was before this call. Applying an already-validated
+    /// duplicate a second time is idempotent (identical value, same key).
+    std::map<DepKey, BlobDepRecord> candidate = deps;
+    size_t applied = 0;
+    for (const auto & r : results)
+    {
+        candidate[r.ref] = r.dep;
+        ++applied;
+        if (merge_hook_for_test)
+            merge_hook_for_test(applied);
+    }
+    deps.swap(candidate);
+}
+
+bool PartWriteTxn::isTrustedAdopt(const BlobRef & ref) const
+{
+    /// §4: a leaf trusted at promote iff this build holds a TOKENLESS dep recorded by adoptEvidence
+    /// (a committed-source W-EVIDENCE adopt: the source pins it, in-degree >= 1, not condemnable). A
+    /// tokenless PENDING-upload dep (recordPendingBlobDep, adopted=false) is NOT trusted — it must be
+    /// tokened by putBlob before promote; reaching promote un-tokened is a staging bug (fail closed).
+    auto it = deps.find(ref);
+    return it != deps.end() && !it->second.token.has_value() && it->second.adopted;
+}
+
+bool PartWriteTxn::depIsTokened(const BlobRef & ref) const
+{
+    /// Discriminator for B156b: a putBlob'd blob records a TOKENED dep (recreatable by retrying), an
+    /// adoptFromTree carry-forward records a TOKENLESS W-EVIDENCE dep (not recreatable — pinned by a
+    /// committed source). Returns false when this build has no dep for the ref (the caller decides
+    /// the default; not-tokened is the fail-loud, INV-NO-LOSS-safe choice).
+    auto it = deps.find(ref);
+    return it != deps.end() && it->second.token.has_value();
+}
+
+BlobDepRecord PartWriteTxn::observeAndAdmit(ObjectKind kind, const BlobRef & ref, const String & key) const
+{
+    /// EDGE-BEFORE-OBSERVE: the durable-precommit guard
+    /// lives in the 4-arg overload below, scoped to its ADOPT branch only — NOT here. A HEAD result
+    /// reached through this wrapper (e.g. `reviveObserve`'s post-race re-observe) can resolve to EITHER
+    /// the condemned/ABORTED branch or the adopt branch once the 4-arg overload point-reads the meta;
+    /// gating here (before that point-read) would wrongly block the condemned branch too, which
+    /// re-uploads under THIS build's own build_id via `uploadFromSource` and stays watermark-protected
+    /// pre-precommit.
+    const HeadResult hr = store->backend().head(key);
+    if (!hr.exists)
+        /// Object absent at observe time. The live caller of this overload is the revival re-observe in
+        /// `uploadFromSource` (`reviveObserve` below, retryable): a GC-deleted object at re-observe time
+        /// is a race under INV-3 — the caller HOLDS the source bytes, so it catches FILE_DOESNT_EXIST
+        /// here and re-throws it as ABORTED so the INSERT layer sees the uniform retryable error and
+        /// re-uploads from those bytes. This overload only ever raises FILE_DOESNT_EXIST; the caller
+        /// decides fail-closed vs retry.
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+            "PartWriteTxn::observeAndAdmit: object {} vanished (GC-deleted) before observe; "
+            "caller must re-upload/re-materialize from source", key);
+    return observeAndAdmit(kind, ref, key, hr);
+}
+
+BlobDepRecord PartWriteTxn::observeAndAdmit(ObjectKind kind, const BlobRef & ref, const String & key, const HeadResult & hr) const
+{
+    /// `hr.exists` is guaranteed by the caller (the 3-arg wrapper checked it; the putBlob HEAD-first
+    /// path only calls this on a present HEAD). Avoids a redundant second HEAD on the dedup-hit path.
+    /// Logical (payload) size = object size minus the pool's fixed blob header. GUARD against
+    /// unsigned underflow: a truncated/corrupt object whose size is below the header length must surface
+    /// as CORRUPTED_DATA, never wrap to a huge value. Mirrors the GC path's `retiredLogicalSize`.
+    const uint64_t header_len = store->poolMeta().blob_header_len;
+    if (hr.size < header_len)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "PartWriteTxn: {} object {} size {} is below the pool blob header length {}",
+            kind == ObjectKind::Blob ? "blob" : "manifest", key, hr.size, header_len);
+    const uint64_t logical_size = hr.size - header_len;
+
+    const CasEventObjectKind ev_kind = toEventKind(kind);
+
+    /// The condemned decision is a per-hash META POINT-READ, not
+    /// the retired-view snapshot. `absent` meta means "not condemned" — GC always writes a `Condemned`
+    /// meta BEFORE it ever deletes a body, so an absent meta is exactly as live as a `Clean` one.
+    /// The meta ops layer is `BlobRef`-keyed directly (derives its codec from `ref.algo`
+    /// internally). Every event-log render below uses `blobIdOf(ref)` ("<algoName>:<hex>"), never a
+    /// bare hex.
+    const auto lm = loadMeta(store->backend(), store->layout(), ref);
+    const bool condemned = lm && lm->meta.state == MetaState::Condemned;
+    if (condemned)
+    {
+        /// INV-1 (revival-from-source): the observed token is condemned — we must NOT read the dying
+        /// object via backend().get. Throw ABORTED so the caller can re-upload from its OWN source bytes.
+        ///   • putBlob (has BlobSource): catches ABORTED, calls uploadFromSource from held bytes.
+        ///   • a bodyless observe (no source): propagates ABORTED (retryable; caller retries op).
+        EventEmitter{*store}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::BlobReuseResurrect;
+            e.object_kind = ev_kind;
+            e.object_hash = blobIdOf(ref);
+            e.token = hr.token.value;
+            e.round = 0;   /// the round is no longer a writer concept (meta point-read replaces the retired view)
+            e.outcome = "condemned";
+            e.reason = "observed token is condemned (meta point-read); caller must re-upload from source (INV-1)";
+        });
+        throw Exception(ErrorCodes::ABORTED,
+            "PartWriteTxn::observeAndAdmit: condemned token for {} — caller must re-upload from source bytes (INV-1)",
+            key);
+    }
+
+    /// EDGE-BEFORE-OBSERVE: from here on we are about to
+    /// ADOPT the live (non-condemned) incarnation as our own dependency — safe ONLY under this build's
+    /// durable precommit closure, because an adopted blob carries the ORIGINAL writer's build_id, so the
+    /// newborn-debris watermark does not cover it. (The condemned branch above is unaffected — it
+    /// re-uploads under THIS build's own build_id via `uploadFromSource`, which stays watermark-protected
+    /// pre-precommit.) See the TLA+ order sabotage (Gate A). A4: a real throw, not chassert — chassert is
+    /// compiled out in release, and a wiring/retry bug that reached adopt without a durable precommit
+    /// would silently drop watermark protection (later dangling ref / data loss with no production
+    /// signal).
+    /// `Durable` and nothing else. `Uncertain` is a precommit that MAY be live and may equally not
+    /// exist at all, and "may" is not the closure this invariant needs: the adopted blob would carry
+    /// the original writer's build_id with only a possibly-absent edge protecting it. It fails closed
+    /// here exactly as `NotAttempted` does, and for the same reason.
+    if (precommit_state != PrecommitState::Durable)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "PartWriteTxn::observeAndAdmit: EDGE-BEFORE-OBSERVE invariant violated — adopting an existing "
+            "incarnation before this build's precommit is durable would admit {} ({}) under the original "
+            "writer's build_id with no newborn-debris watermark protection",
+            key, kind == ObjectKind::Blob ? "blob" : "manifest");
+
+    /// `!lm`: no meta yet for this hash (a pre-existing blob from before this protocol, or a lost race
+    /// with a concurrent fresh-uploader's own meta write). Best-effort create it as Clean so future
+    /// point-readers (writers and GC) never have to fall back to a HEAD-only guess. A Conflict here just
+    /// means a racing writer already created it — both agree on the same Clean steady state.
+    if (!lm)
+    {
+        ProfileEvents::increment(ProfileEvents::CASMetaAdoptBackfill);
+        putMetaIfAbsent(*store, ref,
+            BlobMeta{.state = MetaState::Clean, .condemn_round = 0, .size = logical_size});
+    }
+
+    /// Adopt the current incarnation — free, no bytes moved.
+    /// Reuse an ADOPTED existing incarnation's token as NOT condemned (per the meta point-read).
+    /// Was the CAREUSE adopt audit line. Token-join this against a later blob_delete
+    /// of the same hash/token to pin a reuse-of-an-object-being-deleted race.
+    EventEmitter{*store}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::BlobReuseAdopt;
+        e.object_kind = ev_kind;
+        e.object_hash = blobIdOf(ref);
+        e.token = hr.token.value;
+        e.round = 0;   /// the round is no longer a writer concept (meta point-read replaces the retired view)
+        e.outcome = "adopt";
+        e.reason = "observed token not condemned (meta point-read); adopted the live incarnation (no bytes moved)";
+    });
+    /// Build-neutral: RETURN the adopt dep (tokened, adopted=false) instead of folding it into `deps`.
+    return BlobDepRecord{kind, hr.token, logical_size, /*adopted=*/false};
+}
+
+BlobUploadResult PartWriteTxn::uploadFromSource(ObjectKind kind, const BlobRef & ref, const String & key, const BlobSource & source) const
+{
+    /// StagingPromoted vs FreshUpload discriminator for the write-once create terminals: the source
+    /// carries a server-side-copy descriptor iff the bytes already live in an S3 staging object.
+    const BlobUploadOutcome fresh_outcome
+        = source.server_side_copy_from ? BlobUploadOutcome::StagingPromoted : BlobUploadOutcome::FreshUpload;
+
+    /// INV-1 (revival-from-source): re-upload a condemned or absent object from the writer's OWN
+    /// re-readable source — NEVER calls backend().get to read the dying object. W-FRESH-TAG: fresh
+    /// incarnation_tag and this build's build_id so the new incarnation is owned by THIS live build
+    /// (the source-based resurrection rule closes the prior ownership gap). The payload is STREAMED into the put sink
+    /// (`source.open`), never materialized into a full in-memory copy on the common
+    /// If-None-Match path; `source.open` is re-invoked on each attempt (it re-reads the staged
+    /// temp file), which is exactly what preserves INV-1 across retries.
+    const PoolMeta & meta = store->poolMeta();
+    const PoolConfig & cfg = store->poolConfig();
+    /// `ref` is the full blob identity (algo + digest) end-to-end -- the `.meta` API is
+    /// `BlobRef`-keyed directly, and the dep map + every event render below key off `ref` too.
+
+    auto buildHeader = [&]() -> String
+    {
+        EnvelopeHeader header;
+        header.kind = kind;
+        header.incarnation_tag = mintU128();
+        header.build_id = build_id;
+        /// ch = the real ClickHouse VERSION_INTEGER (diagnostic-only; no decision reads it) — the v3
+        /// envelope drops writer_version/hash_algo/domain_id, so forensics ride on ch + bld.
+        header.provenance = Provenance{nowMs(), cfg.server_id, VERSION_INTEGER, info.op};
+        if (kind == ObjectKind::Blob)
+            header.intended_ref = info.intended_ref;
+        /// The v3 codec pads to the pool's fixed header length and TRUNCATES a too-long intended_ref
+        /// internally (it is diagnostic-only), so the old drop-and-retry is gone — one encode call.
+        return encodeEnvelopeHeader(header, static_cast<uint32_t>(meta.blob_header_len));
+    };
+
+    const CasEventObjectKind ev_kind = toEventKind(kind);
+
+    /// Revival-local wrapper for the post-412 re-observe (INV-3). On the post-412 path a
+    /// racing writer is assumed to have (re-)created the object, so we adopt its token via the 3-arg
+    /// observeAndAdmit. But the object can be GC-deleted in the window (present at the conditional PUT
+    /// → 412, gone at the subsequent HEAD), making the 3-arg overload throw FILE_DOESNT_EXIST. The
+    /// caller (putBlob's fresh-upload path, via `uploadFromSource`) HOLDS the source bytes, so a vanish
+    /// here is a retryable race: convert FILE_DOESNT_EXIST → ABORTED so putBlob's bounded retry loop
+    /// re-uploads from those bytes. Without this, FILE_DOESNT_EXIST escaped putBlob's ABORTED-only catch
+    /// as a FATAL INSERT failure — the sibling of the gate bug.
+    auto reviveObserve = [&](const String & k_) -> BlobDepRecord
+    {
+        try
+        {
+            return observeAndAdmit(kind, ref, k_);
+        }
+        catch (const Exception & e_)
+        {
+            if (e_.code() != ErrorCodes::FILE_DOESNT_EXIST)
+                throw;
+            throw Exception(ErrorCodes::ABORTED,
+                "uploadFromSource: object {} vanished (GC-deleted) during revival re-observe; "
+                "retry the operation — re-upload from held source bytes (INV-3)", k_);
+        }
+    };
+
+    /// Build-neutral: emit the BlobPut audit event and RETURN the fresh/resurrect dep record (tokened,
+    /// adopted=false) for the caller to compose into its `BlobUploadResult` — it folds nothing into `deps`.
+    auto makeDepAndEmit = [&](Token tok) -> BlobDepRecord
+    {
+        EventEmitter{*store}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::BlobPut;
+            e.object_kind = ev_kind;
+            e.object_hash = blobIdOf(ref);
+            e.token = tok.value;
+            e.round = 0;   /// the round is no longer a writer concept (meta point-read replaces the retired view)
+            e.outcome = "ok";
+            e.reason = "uploadFromSource: fresh incarnation streamed from writer's own re-readable source (INV-1)";
+            e.detail = {{"size", std::to_string(source.size)}, {"build_id", u128ToHex(build_id)}};
+        });
+        return BlobDepRecord{kind, tok, source.size, /*adopted=*/false};
+    };
+
+    /// Meta write for the RESURRECT (condemned-displacement) case: flip the now-stale Condemned meta
+    /// back to Clean now that a live incarnation has displaced the condemned body. `lm_before` is the
+    /// point-read taken just before the condemned decision — its etag is the CAS precondition (absent
+    /// when there is no prior point-read at all, the FRESH upload case below). Each outer attempt
+    /// routes through the Pool's request controller (putMetaIfAbsent/casMeta), which already absorbs
+    /// a transient transport error (SlowDown/429/5xx) within its own budget — this outer loop reacts
+    /// only to a genuine `Conflict` (the marker's current token AND bytes both differ from what this
+    /// call intended: a racing writer or GC re-condemning) by reloading and retrying against the
+    /// fresh state. `Unresolved` (the controller's own budget/fence exhausted for one attempt) is
+    /// retried the same way — reloading may simply observe the write actually landed. After
+    /// max_meta_attempts of this outer loop WITHOUT a Committed result, this is a persistent failure,
+    /// not a blip (each outer attempt already burned its own inner retry budget) — the RCA requires
+    /// this reach the caller as a controlled retry-later signal, never a silent skip: a dropped
+    /// freshness marker leaves stale state for the next point-reader.
+    auto writeResurrectMetaClean = [&](std::optional<LoadedMeta> lm_before)
+    {
+        ProfileEvents::increment(ProfileEvents::CASMetaResurrectClean);
+        const BlobMeta clean{.state = MetaState::Clean, .condemn_round = 0, .size = source.size};
+        constexpr int max_meta_attempts = 8;
+        for (int attempt = 0; attempt < max_meta_attempts; ++attempt)
+        {
+            const bool committed = lm_before
+                ? casMeta(*store, ref, lm_before->etag, clean).outcome == CasOverwriteOutcome::Committed
+                : putMetaIfAbsent(*store, ref, clean).outcome == CasOverwriteOutcome::Committed;
+            if (committed)
+                return;
+            lm_before = loadMeta(store->backend(), store->layout(), ref);
+        }
+        throwCasWriteRetryLater(fmt::format(
+            "writeResurrectMetaClean: freshness-meta transition to Clean for {} did not land within "
+            "{} attempts (each already budget-controlled) — the body incarnation is durable, but the "
+            "meta marker is stuck; retry", store->layout().blobMetaKey(ref), max_meta_attempts));
+    };
+
+    /// Meta write for the FRESH (absent -> present) upload cases: the body just transitioned via
+    /// If-None-Match, so the freshness meta is created as Clean. Delegates to writeResurrectMetaClean
+    /// with no prior point-read: a pre-existing marker there is NOT always "a racing writer creating
+    /// the same Clean state" (the retired comment this replaced assumed) -- it can be a stale
+    /// Condemned marker left over from before this exact body vanished and was freshly re-uploaded
+    /// (proven reachable by CasPartWriteTxn.PutBlobResurrectVanishedReUploadsHeldBody), which must be
+    /// reconciled to Clean the same way a resurrect does, not silently ignored.
+    auto writeFreshMetaClean = [&]()
+    {
+        ProfileEvents::increment(ProfileEvents::CASMetaCreateClean);
+        writeResurrectMetaClean(std::nullopt);
+    };
+
+    /// Stream header + payload into a fresh putIfAbsentStream sink WITHOUT materializing the whole blob.
+    /// `source.open` re-reads the staged temp file (INV-1: the writer's own source, never the
+    /// dying object). The payload byte count is verified against `source.size` via the sink buffer's
+    /// `count()` (total bytes written so far) — the streaming equivalent of the old pre-materialized
+    /// size check, with no full in-memory copy. A mismatch is a LOGICAL_ERROR (a buggy/racing source).
+    ///
+    /// The whole conditional create rides the Pool's shared request controller
+    /// (`conditionalCreateControlled` — the retry controller's "any other
+    /// controller-bypassing conditional-write call site"): budgeted attempts + fence-gated backoff +
+    /// exact-key OCCUPANCY resolve, replacing the old bare single attempt whose whole S3-blip tolerance
+    /// was ONE ~3s adaptive-timeout attempt. Reissue is sound for BOTH primitives: the streaming PUT
+    /// re-invokes `source.open` (the REPLAYABLE source contract — a fresh re-upload, never a
+    /// GET-revive), and the server-side copy re-reads the intact staging object. Each re-stream mints a
+    /// fresh incarnation_tag (W-FRESH-TAG), so byte-exact resolve is impossible by design and the
+    /// controller resolves by occupancy instead: an occupant at this content-addressed key IS the
+    /// intended content (whether our own landed ambiguous attempt or a twin), surfaced as
+    /// PreconditionFailed so every existing gate branch below is UNCHANGED.
+    ///
+    /// The size-check LOGICAL_ERROR below stays instant and loud: `conditionalCreateControlled`
+    /// propagates every deterministic local failure — LOGICAL_ERROR, NOT_IMPLEMENTED (the promoteStaged
+    /// mode guard), BAD_ARGUMENTS (escaping buildHeader's second encode), CORRUPTED_DATA — from the
+    /// attempt unchanged (a caller/config bug reissue would only replay — pinned by
+    /// `CasPartWriteTxn.PutBlobWrongSizeFailsClosed` and the controller-level
+    /// `DeterministicLocalFailuresPropagateInstantly`).
+    auto streamIfAbsent = [&]() -> PutResult
+    {
+        const auto one_attempt = [&]() -> PutResult
+        {
+            /// S3-native staging promote: when the source
+            /// carries a server-side-copy descriptor, the write-once CREATE primitive is a conditional
+            /// server-side copy of the staging object to `key` (`If-None-Match:*`) instead of a
+            /// client-side streaming PUT. Same write-once contract (`Done` + dest-ETag token on created,
+            /// `PreconditionFailed` when `key` already exists). The staging object IS the promote source
+            /// — no envelope is streamed here.
+            if (source.server_side_copy_from)
+                return store->backend().promoteStaged(*source.server_side_copy_from, key);
+
+            /// No owner metadata is needed: protection is the precommit edge — reachability, not `cas_owner`.
+            /// A throw mid-stream abandons the sink (its dtor cancels): nothing is ever published by a
+            /// failed attempt except via the storage's own late-landing ambiguity, which the controller
+            /// resolves.
+            WriteSinkPtr sink = store->backend().putIfAbsentStream(key);
+            WriteBuffer & out = sink->buffer();
+            writeString(buildHeader(), out);
+            const size_t before = out.count();
+            copyData(*source.open(), out);
+            const size_t written = out.count() - before;
+            if (written != source.size)
+                throw Exception(ErrorCodes::LOGICAL_ERROR,
+                    "uploadFromSource: source streamed {} bytes, declared {}", written, source.size);
+            return sink->finalize();
+        };
+
+        const CasCreateResult res = store->stagingConditionalCreate(key, one_attempt);
+        switch (res.outcome)
+        {
+            case CasCreateOutcome::Committed:
+                return PutResult{PutOutcome::Done, res.token};
+            case CasCreateOutcome::Occupied:
+                return PutResult{PutOutcome::PreconditionFailed, {}};
+            case CasCreateOutcome::Unresolved:
+                break;
+        }
+        /// Unresolved = budget exhausted or fence lost without a definite outcome. Nothing referenced
+        /// this incarnation (deps/meta are recorded only on a definite outcome); a late-landing body is
+        /// inert debris behind the content-addressed key — a future writer of the same content adopts
+        /// or displaces it through the normal occupancy machinery. NETWORK_ERROR = the same retryable
+        /// abort class stageManifest and the ref lane map their exhausted budgets to.
+        throwCasWriteRetryLater(fmt::format(
+            "uploadFromSource: conditional create at '{}' is UNCERTAIN (retry budget exhausted or mount "
+            "fence lost) — nothing was acknowledged; retry re-uploads from the writer's own source (INV-1)",
+            key));
+    };
+
+    /// Try the If-None-Match upload (object absent or race with another writer).
+    {
+        const PutResult res = streamIfAbsent();
+        if (res.outcome == PutOutcome::Done)
+        {
+            const BlobDepRecord dep = makeDepAndEmit(res.token);
+            writeFreshMetaClean();
+            return BlobUploadResult{ref, dep, fresh_outcome};
+        }
+    }
+
+    /// PreconditionFailed: an incarnation exists. HEAD it to check whether it is condemned or live.
+    /// We do NOT read the body (no backend().get) — we only need the token to decide:
+    ///   • live (not condemned)  → adopt; this is the standard dedup case.
+    ///   • condemned             → displace via putOverwrite(If-Match: current_token) by re-reading our
+    ///                            own source, so we never read the dying object. This is the
+    ///                            equivalent of the old `resurrect` minus the GET.
+    const HeadResult hr = store->backend().head(key);
+    if (!hr.exists)
+    {
+        /// Object vanished between putIfAbsentStream (412d) and our HEAD — a concurrent GC delete.
+        /// The object is now absent; re-stream from our re-readable source (bounded by caller).
+        const PutResult res2 = streamIfAbsent();
+        if (res2.outcome == PutOutcome::Done)
+        {
+            const BlobDepRecord dep = makeDepAndEmit(res2.token);
+            writeFreshMetaClean();
+            return BlobUploadResult{ref, dep, fresh_outcome};
+        }
+        /// Still 412 after the vanish-and-retry: a racing writer re-created it. Adopt their token.
+        /// reviveObserve converts FILE_DOESNT_EXIST (deleted again in the window) → ABORTED (retryable).
+        return BlobUploadResult{ref, reviveObserve(key), BlobUploadOutcome::HeadMissAdopted};
+    }
+
+    /// The condemned decision is a per-hash META POINT-READ, not
+    /// the retired-view snapshot. `lm` (and its etag) is reused below to flip a condemned meta back to
+    /// Clean once the resurrect displacement lands.
+    const auto lm = loadMeta(store->backend(), store->layout(), ref);
+    const bool condemned = lm && lm->meta.state == MetaState::Condemned;
+    if (!condemned)
+    {
+        /// Live (not condemned): adopt the current incarnation — free, no bytes moved.
+        return BlobUploadResult{ref, observeAndAdmit(kind, ref, key, hr), BlobUploadOutcome::HeadMissAdopted};
+    }
+
+    /// Condemned: displace the condemned incarnation with our fresh source.
+    ///
+    /// rev.7 [C2] (backlog {#c2-resurrect-putoverwrite-fence-check}): the two displacement calls below
+    /// (`resurrect` / `putOverwrite`) are RAW backend writes with NO controller/fence coupling —
+    /// unlike `streamIfAbsent`, which rides the request controller's fence gate. They were the only
+    /// durable-effect writes left outside Task 4's fence-generation gate. Capture the mount fence
+    /// generation now, at the displacement DECISION, and re-check it (and `mayMutate()`) immediately before
+    /// whichever raw write we issue: a lease lost — or re-armed under a fresh incarnation — since we
+    /// observed the condemned state aborts with the typed transient refusal before any
+    /// stale-incarnation displacement can land. Mirrors `CasPlainObjects::casPutObject`'s
+    /// capture-at-admission-then-check-before-the-durable-write shape.
+    const uint64_t displace_admitted_generation = store->fenceGeneration();
+    if (source.server_side_copy_from)
+    {
+        /// S3-native staging RESURRECT (INV-NO-RETURN): re-establish a fresh
+        /// incarnation by re-uploading OUR OWN staging PAYLOAD under a FRESHLY-tagged envelope header —
+        /// NEVER a read/copy of the condemned `key` (`feedback_ca_resurrect_invariant`). We reach here
+        /// ONLY after the per-hash meta point-read observed `Condemned` just above, so this overwrites a
+        /// condemned body -- or, on a lost race, an equivalent FRESH resurrection of it, which is
+        /// accepted: payloads are content-identical, so the overwrite rotates only envelope and token.
+        /// A live incarnation of DIFFERENT content is unreachable here by the content address itself.
+        ///
+        /// CRITICAL — a VERBATIM server-side copy of the create-time staging object would reproduce the
+        /// condemned incarnation's exact bytes ⇒ identical ETag ⇒ the queued exact-token delete of the
+        /// condemned incarnation would kill the live resurrection (data loss). `buildHeader` mints a
+        /// FRESH `incarnation_tag` distinct from the staging header's tag, so the resurrected body (and
+        /// hence its ETag) differs from the condemned incarnation regardless of edge-before-observe. The
+        /// backend reads the payload from the staging object skipping its own `blob_header_len` envelope
+        /// header and prepends this fresh header.
+        const String fresh_header = buildHeader();
+        /// rev.7 [C2]: a raw `backend()` call with NO controller fence coupling (unlike
+        /// `promoteStaged`/`putIfAbsentStream` above, reached only through `stagingConditionalCreate`'s
+        /// controller+fence_ok). The `checkFenceOrThrow` re-checks the fence generation captured at the
+        /// displacement decision, closing the same gap Task 4 closed on the plain-object surface.
+        Token tok{};
+        store->checkFenceOrThrow(displace_admitted_generation);
+        /// The staging object's own envelope header is skipped HERE, by whoever knows its shape: the
+        /// backend is handed a reader already positioned at the payload.
+        auto staged = store->backend().getStream(*source.server_side_copy_from);
+        if (!staged)
+            throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+                "CAS resurrect: staging object {} is absent", *source.server_side_copy_from);
+        staged->stream->ignore(meta.blob_header_len);
+        tok = store->backend().resurrect(*staged->stream, source.size, key, fresh_header);
+        const BlobDepRecord dep = makeDepAndEmit(tok);
+        writeResurrectMetaClean(lm);
+        return BlobUploadResult{ref, dep, BlobUploadOutcome::ResurrectedS3};
+    }
+
+    /// CRITICAL: we re-read the writer's OWN source (NOT backend().get) — no GET of the dying object.
+    /// W-FRESH-TAG: a fresh incarnation_tag minted inside buildHeader() ensures INV-NO-RETURN.
+    ///
+    /// UNCONDITIONAL, exactly like the staging arm above. An `If-Match` on the condemned token would
+    /// save a redundant re-upload when another writer resurrects the same blob first, and would prevent
+    /// nothing: two racing resurrections write payload-identical bodies, no consumer reads a dep token's
+    /// VALUE, and durable references name content hashes rather than incarnations. What protects the
+    /// resurrection is the fresh tag — it makes this body's ETag differ from the condemned one, so every
+    /// already-queued exact-token delete of that incarnation misses.
+    ///
+    /// Blob bodies have no size cap, so a body larger than memory has to remain writable here: a
+    /// Native backend streams the payload from the reader; the emulated (local) backend materializes
+    /// one body at a time, serialized inside `Backend::resurrect`.
+    auto payload = source.open();
+    /// rev.7 [C2]: a raw, uncoupled backend call; fence-checked against the displacement-decision
+    /// generation immediately before the durable write.
+    store->checkFenceOrThrow(displace_admitted_generation);
+    /// `source.size` rides into the write, which counts while streaming and aborts WITHOUT publishing
+    /// on a mismatch -- with an unconditional overwrite, a post-write check would fire only after a
+    /// truncated body had already displaced the condemned incarnation (and could even inspect a racing
+    /// writer's fresh incarnation instead of ours).
+    const Token resurrected = store->backend().resurrect(*payload, source.size, key, buildHeader());
+
+    const BlobDepRecord dep = makeDepAndEmit(resurrected);
+    writeResurrectMetaClean(lm);
+    return BlobUploadResult{ref, dep, BlobUploadOutcome::ResurrectedLocal};
+}
+
+void PartWriteTxn::adoptEvidence(const ManifestEntry & entry)
+{
+    requireAlive();
+
+    /// W-EVIDENCE: record a TOKENLESS dependency — liveness evidence is the live source manifest, not a
+    /// token. Inline entries reference no standalone object, so they record nothing. NO backend call
+    /// (no HEAD, no GET, no PUT) — the caller already holds the resolved entry. Part manifests have only
+    /// Inline / Blob placements (no Subtree): only blobs are content-addressed.
+    if (entry.placement == EntryPlacement::Blob)
+    {
+        /// Carry `entry.ref` WHOLE (the pair, never re-derived) — this is what makes a
+        /// mixed-algo manifest's entries each dep-track under their OWN algo. §4: adopted=true marks this a
+        /// committed-source W-EVIDENCE dep, trusted at promote via the durable manifest edge (no probe).
+        deps[entry.ref] = BlobDepRecord{ObjectKind::Blob, std::nullopt, entry.blob_size, /*adopted=*/true};
+    }
+}
+
+void PartWriteTxn::recordPendingBlobDep(const BlobRef & ref, uint64_t size)
+{
+    requireAlive();
+    deps[ref] = BlobDepRecord{ObjectKind::Blob, std::nullopt, size};
+}
+
+RootNamespace PartWriteTxn::manifestNamespace() const
+{
+    /// The wiring sets the owning namespace EXPLICITLY (PartWriteInfo::intended_namespace). This is the
+    /// authoritative source: a ref can itself contain `/` (the `detached/<part>` fold), so we
+    /// must NOT recover the namespace by splitting intended_ref on the last `/` — that would yield a
+    /// spurious `<ns>/detached` namespace and the precommit namespace-match check would throw.
+    if (info.intended_namespace)
+        return *info.intended_namespace;
+
+    /// Fallback (Core tests that set only the diagnostic intended_ref, where the ref has no `/`): the
+    /// owning namespace is intended_ref minus its last `/`-segment (the ref name). A CA namespace itself
+    /// contains `/` (e.g. "srv1/<uuid>@cas@"), so split on the LAST slash only.
+    const String intended = info.intended_ref.value_or("");
+    const size_t slash = intended.find_last_of('/');
+    if (slash == String::npos || slash == 0)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "stageManifest: intended_ref '{}' has no namespace/ref split", intended);
+    return RootNamespace{intended.substr(0, slash)};
+}
+
+ManifestId PartWriteTxn::stageManifest(std::vector<ManifestEntry> entries)
+{
+    requireAlive();
+
+    /// Fail-closed caps — checked BEFORE the body write so no owner transition can ever name a
+    /// manifest that breaches a cap. Inline payload is read on every part-open and every owner
+    /// transition, so cap the total, not only per-entry.
+    if (entries.size() > kMaxManifestEntries)
+        throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+            "stageManifest: {} entries exceeds cap {}", entries.size(), kMaxManifestEntries);
+    uint64_t inline_total = 0;
+    for (const ManifestEntry & e : entries)
+    {
+        if (e.placement == EntryPlacement::Inline)
+        {
+            if (e.inline_bytes.size() > kMaxLargestInlineEntryBytes)
+                throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+                    "stageManifest: inline entry '{}' of {} bytes exceeds cap {}",
+                    e.path, e.inline_bytes.size(), kMaxLargestInlineEntryBytes);
+            inline_total += e.inline_bytes.size();
+        }
+    }
+    if (inline_total > kMaxManifestInlineBytesTotal)
+        throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+            "stageManifest: total inline {} bytes exceeds cap {}", inline_total, kMaxManifestInlineBytesTotal);
+
+    /// Mint the identity. `epoch` is the Pool's durable writer_epoch; `build_seq` is monotone inside
+    /// that epoch; `manifest_ordinal` is monotone inside this PartWriteTxn. Together with the owning namespace
+    /// this gives NoManifestIdReuse by construction, with no random manifest instance id.
+    if (next_manifest_ordinal > kMaxManifestOrdinal)
+        throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+            "stageManifest: manifest ordinal cap {} exceeded for build_seq {}", kMaxManifestOrdinal, build_seq);
+    const ManifestRef ref{epoch, build_seq, next_manifest_ordinal++};
+
+    /// Build the body. payload_digest is integrity/debug only — never a key, never dedup, never
+    /// in-degree. The body repeats its own ref + namespace for fail-closed RefMatchesBody /
+    /// ManifestNamespaceMatches at read/fold/promote time.
+    const RootNamespace owning_ns = manifestNamespace();
+    PartManifest body;
+    body.ref = ref;
+    body.root_namespace_id = owning_ns;
+    body.entries = std::move(entries);
+    body.payload_digest = computePayloadDigest(body);
+    /// The cap is measured over the canonical text before sealing, not the compressed PUT bytes.
+    /// 256 MiB comfortably bounds the largest realistic manifest, including its JSON structure.
+    const String encoded_text = encodePartManifest(body);
+    if (encoded_text.size() > kMaxManifestEncodedBytes)
+        throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+            "stageManifest: encoded manifest {} bytes exceeds cap {}", encoded_text.size(), kMaxManifestEncodedBytes);
+    const String encoded = sealObject(FormatId::PartManifest, encoded_text);
+
+    const ManifestId id{owning_ns, ref};
+    const String key = store->layout().manifestKey(id);
+
+    /// Body PUT through the Pool's shared request controller:
+    /// budgeted attempts + resolve-before-reissue, replacing the old bare single-attempt write whose
+    /// whole S3-blip tolerance was ONE ~3s adaptive-timeout attempt (a 19s object-store pause killed an
+    /// INSERT through it while every plain read/write path survived — v3 soak evidence). Reissuing this
+    /// conditional PUT is sound: the body bytes are fixed for the whole operation (`encoded` is built
+    /// once; `encodePartManifest` is canonical/deterministic), so `resolveByExactGet` can prove whether
+    /// an ambiguous attempt landed. Still NO preliminary HEAD. A DIFFERENT object at this key is a
+    /// ManifestId collision — the controller's resolve raises CORRUPTED_DATA (a proven conflict,
+    /// fail-closed before any owner transition can name this id), subsuming the old
+    /// PreconditionFailed->LOGICAL_ERROR mapping.
+    ///
+    /// fence_ok is the ref lane's own mount predicate (`refAppendFenceOk`: fence not lost + enough
+    /// lease left for one more attempt): staging runs on this writable Pool under that same mount
+    /// lease, and a fenced writer must not keep PUTting bodies ahead of a precommitAdd that would fail
+    /// the same fence anyway. There is no ref-table runtime here, so the lane's extra
+    /// `superseded_by_remount` term does not apply.
+    Token manifest_token;
+    const CasWriteOutcome put_outcome = store->stagingPutIfAbsent(key, encoded, &manifest_token);
+    if (put_outcome == CasWriteOutcome::DefiniteFailure)
+        throwCasWriteRetryLater(fmt::format(
+            "stageManifest: part-manifest PUT at '{}' definitively failed (non-retryable rejection); "
+            "nothing was named — the caller re-stages with a fresh ManifestId", key));
+    /// Unresolved = budget exhausted (or fence lost) without a definite outcome. Unlike the ref-log
+    /// lane there is nothing to wedge: this id was never named by any owner transition
+    /// (`next_manifest_ordinal` is already past it, so no re-stage ever reuses the key), and a
+    /// late-landing body is inert unreferenced debris for the orphan-manifest sweep. NETWORK_ERROR =
+    /// the same retryable abort class the ref lane's exhausted budget maps to.
+    if (put_outcome == CasWriteOutcome::Unresolved)
+        throwCasWriteRetryLater(fmt::format(
+            "stageManifest: part-manifest PUT at '{}' is UNCERTAIN (retry budget exhausted) — "
+            "nothing conclusive was named; the caller re-stages with a fresh ManifestId", key));
+
+    EventEmitter{*store}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::ManifestPut;
+        e.namespace_ = owning_ns.string();
+        e.object_kind = CasEventObjectKind::Manifest;
+        e.object_hash = manifestRefDebugString(id.ref);
+        e.token = manifest_token.value;
+        e.reason = "stageManifest: part-manifest body written";
+    });
+
+    staged_manifests.push_back(id);
+    staged_manifest_ids.insert(id);   /// A3 mint-tightening: `precommitAdd`'s only legal fresh-ownership source
+    return id;
+}
+
+void PartWriteTxn::precommitAdd(const RootNamespace & target_ns, const String & final_ref_name, const ManifestId & id)
+{
+    requireAlive();
+
+    /// ManifestNamespaceMatches at the source: the precommit's manifest must belong to the target
+    /// namespace (its key is built from id.root_namespace). A cross-namespace precommit is a bug.
+    if (id.root_namespace != target_ns)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "precommitAdd: manifest namespace '{}' != target namespace '{}'",
+            id.root_namespace.string(), target_ns.string());
+
+    /// A3 mint-tightening (ABA barrier for the relink confirm's exact-`ManifestRef` equality): an
+    /// unowned `ManifestId` may enter ownership only from the transaction that freshly staged it.
+    /// Evaluated HERE (synchronously, off this transaction's own private `staged_manifest_ids` --
+    /// never mutated concurrently, so no snapshot races) rather than read live inside the closure
+    /// below, because it does not depend on ledger state at all. The bool is captured BY VALUE into
+    /// the closure -- consistent with every other capture there -- rather than capturing `this`,
+    /// which would let a closure that outlives this stack (see the capture comment below) dereference
+    /// a dead transaction. Enforcement, however, is NOT done here: see the closure for why.
+    const bool id_staged_by_this_txn = staged_manifest_ids.contains(id);
+
+    /// One `owner_transition` create-precommit op.
+    /// No body HEAD — a missing body is a legal fail-closed, non-activating intent, unchanged from the
+    /// old protocol. Precommit ownership carries NO separate build token: `id.ref` (writer_epoch,
+    /// build_sequence, manifest_ordinal) IS the build identity (the tuple
+    /// §Transaction Log Format: "there is no second build token"). `build_ops` is invoked from inside
+    /// the per-namespace flush, so it sees the table's CURRENT state including any earlier item of the
+    /// same batch; when that state is not `Live` (never born, or `Removed`), it prepends
+    /// `namespace_birth` in the SAME transaction (the birth transaction
+    /// normally also adds the first precommit").
+    ///
+    /// THE INTENT IS RECORDED BEFORE THE APPEND, and that ordering is the whole point: an `Unresolved`
+    /// append MAY HAVE LANDED (`CasRefLedger.cpp`, the `Unresolved` arm says so explicitly), so a
+    /// `precommitAdd` that throws can still have made `id.ref` a live precommit owner. Setting the
+    /// fields afterwards left that case with an object which believed it had never precommitted:
+    /// `abandon` queued no removal and `cleanupStagedManifestDebrisBestEffort` -- deciding from the same
+    /// unset state -- writer-DELETED the body, so a wedge that later resolved as committed installed a
+    /// live precommit with no cleanup owner and no body. Same discipline, and same reason, as the ref
+    /// lane's preconstructed wedge. The three fields are plain assignments of already-materialized
+    /// values, so this cannot throw between the record and the append.
+    precommit_target_ns = target_ns;
+    precommit_final_ref = final_ref_name;
+    precommit_manifest = id.ref;
+    precommit_state = PrecommitState::Uncertain;
+
+    store->appendRefOps(target_ns, MutationScope::ref(final_ref_name),
+        /// Capture everything the closure reads BY VALUE (the `store` handle, target namespace, ref name,
+        /// and manifest id) rather than `[&]`: defense-in-depth so a closure that ever outlives this
+        /// stack frame (e.g. if the append lane stranded its item) never dereferences a dead stack. The
+        /// ref-lane leadership guard is the real fix; this makes the closure self-contained regardless.
+        [pool = store, target_ns, final_ref_name, id, id_staged_by_this_txn](const RefTableState & state) -> std::vector<RefOp>
+        {
+            /// Idempotent re-add: the target ref is ALREADY committed to this EXACT manifest_ref (a
+            /// legitimate re-drive calling precommitAdd+promote again for content that is already
+            /// live -- see `promote`'s matching no-op guard). Nothing to append: re-adding a precommit
+            /// for an already-owned manifest would violate "no conflicting owner may name the same
+            /// manifest", which the state machine enforces for every OTHER case.
+            ///
+            /// A3 mint-tightening's enforcement lives HERE, gated on this SAME live-state read, rather
+            /// than unconditionally before the closure: a legitimate re-drive can be handed an id that
+            /// this exact `PartWriteTxn` object never staged (e.g. a fresh build re-precommitting
+            /// content a PRIOR, since-destroyed build already promoted -- `CasPromoteRepublish.
+            /// PromoteSameManifestIsIdempotent`). That case is not a fresh ownership grant: the manifest
+            /// is already `final_ref_name`'s live, currently-protected binding, so re-affirming it
+            /// creates no new ABA surface. Checking membership-or-committed-match atomically under the
+            /// SAME state read (rather than a separate pre-check against a possibly-stale resolve) is
+            /// what keeps this race-free: a snapshot taken before the closure could see "already
+            /// committed to id.ref" and let a later closure invocation append a fresh owner_transition
+            /// after a concurrent repoint moved the ref away from id.ref in between -- exactly the
+            /// re-ownership of a dropped identity this check exists to prevent.
+            if (const auto it = state.getCommitted().find(final_ref_name);
+                it != state.getCommitted().end() && it->second.manifest_ref == id.ref)
+                return {};
+            if (!id_staged_by_this_txn)
+                throw Exception(ErrorCodes::LOGICAL_ERROR,
+                    "precommitAdd: manifest '{}' was not staged by this transaction and is not the current "
+                    "committed manifest of ref '{}' -- refusing to re-own a foreign or previously-dropped identity",
+                    manifestRefDebugString(id.ref), final_ref_name);
+
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+            {
+                /// Reaching an empty runtime here means catalog resolution admitted a new life. A
+                /// predecessor still `Removing` was refused before recovery, so rebirth needs no
+                /// physical marker or empty-prefix proof.
+                RefOp birth;
+                birth.kind = RefOpKind::NamespaceBirth;
+                ops.push_back(birth);
+            }
+            RefOp add;
+            add.kind = RefOpKind::OwnerTransition;
+            add.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, final_ref_name, id.ref};
+            ops.push_back(add);
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Precommit);
+
+    /// The append returned: the precommit binding is durably this build's, so the duty settles from
+    /// `Uncertain` to `Durable` and `observeAndAdmit`'s EDGE-BEFORE-OBSERVE gate opens.
+    precommit_state = PrecommitState::Durable;
+
+    EventEmitter{*store}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::Precommit;
+        e.namespace_ = target_ns.string();
+        e.ref_name = final_ref_name;
+        e.token = u128ToHex(build_id);
+        e.outcome = "ok";
+        e.reason = "precommitAdd: build-intent owner add in the target shard (owner = precommit(build_id))";
+        e.detail = {{"build_seq", std::to_string(build_seq)}};
+    });
+}
+
+bool PartWriteTxn::promote(const RootNamespace & target_ns, const String & final_ref_name, UInt128 promote_build_id, const ManifestId & id, bool allow_repoint)
+{
+    requireAlive();
+
+    if (id.root_namespace != target_ns)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "promote: manifest namespace '{}' != target namespace '{}'",
+            id.root_namespace.string(), target_ns.string());
+
+    /// Read + validate the manifest body ONCE (O(manifest entries), one streaming read). Absent or
+    /// invalid ⇒ fail closed: a committed ref must never name a missing/mismatched manifest.
+    const String manifest_key = store->layout().manifestKey(id);
+    const auto body_got = store->backend().get(manifest_key);
+    if (!body_got)
+        throwCasWriteRetryLater(fmt::format(
+            "promote: manifest body absent at {} — failing closed (retry with a fresh ManifestId)", manifest_key));
+    const PartManifest body = decodePartManifest(openObject(FormatId::PartManifest, body_got->bytes));
+    if (!refMatchesBody(id.ref, body))
+        throwCasWriteRetryLater(fmt::format("promote: RefMatchesBody failed for {}", manifest_key));
+    if (!manifestNamespaceMatches(target_ns, body))
+        throwCasWriteRetryLater(fmt::format("promote: ManifestNamespaceMatches failed for {}", manifest_key));
+
+    /// The copy-forward pre-pass is removed — the
+    /// in-closure blob revalidation below is now the SINGLE copy-forward site. Trade-off: the rare
+    /// condemned-tokenless copy-forward (a GET+PUT) now runs inside the append lane's flush, briefly
+    /// blocking the per-namespace batching queue; it is idempotent under a re-run (a retry sees its own
+    /// fresh token). The meta CAS is the only remaining coordination for this rare case.
+
+    /// The intended-repoint operation is an atomic composition of `WDropRef` and `WPromote`: the
+    /// manifest the ref currently commits,
+    /// when this promote is retiring it via an intended repoint (`allow_repoint`) rather than the
+    /// ordinary first-time-commit path. Set inside the closure below; read after it returns to decide
+    /// whether the `RefRepoint` audit event fires.
+    std::optional<ManifestRef> repoint_old;
+    /// `created`: whether `final_ref_name` has NO committed row as of THIS builder's read of `state`
+    /// -- set as the very first statement of the closure (before any other branch), so it is correct
+    /// on every path: idempotent re-promote no-op (a committed row for `id.ref` already existed ->
+    /// false), an intended repoint (a committed row for a DIFFERENT manifest already existed -> false),
+    /// or a genuine first-time bind (no committed row -> true). Same in-closure-output pattern as
+    /// `repoint_old` immediately below; read by the caller only after `appendRefOps` returns.
+    bool created = false;
+    /// Recorded BEFORE the append for the same reason `precommitAdd` records its intent before its own:
+    /// the append is the point past which failure stops being proof of the negative. Everything above
+    /// this line is validation that rejects without publishing anything, so a throw there leaves
+    /// `NotAttempted` -- and a caller may safely conclude the ref was not committed. From here on it
+    /// may not.
+    commit_state = CommitState::Uncertain;
+    store->appendRefOps(target_ns, MutationScope::ref(final_ref_name),
+        /// Capture the closure's INPUTS by value (ref name, manifest id, promote build id, repoint flag,
+        /// and the manifest `body` it revalidates) rather than `[&]`, as defense-in-depth against a
+        /// closure outliving this stack. Unlike `precommitAdd`, this closure cannot be made fully
+        /// self-contained: `depIsTokened`/`isTrustedAdopt` read this build's `deps` member (so it must
+        /// keep `this`), and `repoint_old`/`created` are OUTPUTS read after the call returns (so they stay
+        /// references). The ref-lane leadership guard is the real fix; both residual by-reference captures
+        /// are safe because the guard guarantees the closure is never invoked after this frame unwinds.
+        [this, final_ref_name, id, promote_build_id, allow_repoint, body, &repoint_old, &created]
+        (const RefTableState & state) -> std::vector<RefOp>
+        {
+            created = !state.getCommitted().contains(final_ref_name);
+
+            /// Idempotent re-promote: the target ref is ALREADY committed to this EXACT manifest_ref --
+            /// a legitimate re-drive (a crash/retry between a prior promote and its caller's own
+            /// follow-up, or a direct repeat call) that must complete as a no-op, not require a live
+            /// precommit that a first successful call already consumed. Mirrors precommitAdd's matching
+            /// guard; the DIFFERENT-manifest case below remains the BUG-1a fail-closed leak guard.
+            if (const auto it = state.getCommitted().find(final_ref_name);
+                it != state.getCommitted().end() && it->second.manifest_ref == id.ref)
+                return {};
+
+            /// NO writer-side view refresh here:
+            /// Gate A): promote-time view freshness is not load-bearing — tokened leaves are edge-protected
+            /// (EDGE-BEFORE-OBSERVE) and the tokenless K3 gate below reads the live view, which the floor
+            /// guarantees contains every graduated entry (in EVERY view >= condemn round + 1).
+
+            /// The `WPromote` owner guard (`owner[m] = bld`): a promote is a PURE owner MOVE that emits
+            /// NO blob delta (Δ=0) — it restores no blob in-degree. It is therefore only sound when the
+            /// precommit is STILL the live owner of the ref: if an abandon or GC reclaim already appended
+            /// a removal of the precommit binding, the blobs' in-degree was already decremented and a Δ=0
+            /// move would re-publish a committed ref over to-be-deleted blobs ⇒ a dangling committed
+            /// manifest (INV_NO_DANGLE). `RefTableState::getPrecommits` materializes live ownership directly:
+            /// `id.ref` alone identifies the build (there is no second
+            /// build token"), so an exact-binding lookup answers the same question directly.
+            if (!state.getPrecommits().contains({final_ref_name, id.ref}))
+                throwCasWriteRetryLater(fmt::format(
+                    "promote: precommit owner binding for ref '{}' (build {}) was removed (abandon or GC "
+                    "reclaim) and is no longer the live owner — failing closed; the build must restart "
+                    "(WPromote owner==bld)",
+                    final_ref_name, u128ToHex(promote_build_id)));
+
+            /// Blob-leaf revalidation. TOKENED leaves are
+            /// edge-protected — EDGE-BEFORE-OBSERVE: the precommit closure was durable BEFORE putBlob
+            /// observed them, so a condemnation in the putBlob→promote window cannot graduate (the next fold
+            /// sees the edge, d >= 1, spared), and putBlob's gate already validated them against the installed
+            /// view under that edge. They are NOT re-checked here. A NON-tokened leaf is EITHER a
+            /// committed-source W-EVIDENCE adopt (adoptEvidence ⇒ adopted=true) or a no-dep / pending-upload
+            /// staging bug. There is NO per-file probe on this path: an adopted leaf is TRUSTED via the
+            /// durable manifest edge (the live source pins the blob, in-degree >= 1, not condemnable) and this
+            /// build's precommit edge is durable — matching the relink trust model (ordinary
+            /// ReplicatedMergeTree interserver trust). A genuinely-absent adopted blob is an invariant
+            /// violation caught by fsck, not here.
+            for (const ManifestEntry & e : body.entries)
+            {
+                if (e.placement != EntryPlacement::Blob)
+                    continue;
+                if (depIsTokened(e.ref))
+                    continue;   /// edge-protected (EDGE-BEFORE-OBSERVE); putBlob validated under the durable edge
+                /// §4 manifest-trust: a tokenless adoptEvidence leaf is trusted — no HEAD, no loadMeta, no
+                /// copy-forward; the durable manifest edge is the liveness evidence. EDGE-BEFORE-TRUST: this
+                /// build's precommit edge was durably appended (`precommitAdd`, the `Precommit`
+                /// `OwnerTransition` above) BEFORE we get here, and the owner-liveness check at the top of this
+                /// closure ("WPromote owner==bld") already re-proved it is the LIVE owner — so the dst manifest
+                /// is a live precommit owner input and GC's fold pins every blob it names at in-degree >= 1
+                /// (the barrier-activated create-precommit +1). GC is the sole deleter and respects
+                /// in-degree, so a trusted-promote leaf cannot have been condemned/deleted. The backstop for
+                /// the (production-unreachable) genuinely-absent case is fsck's reachable-but-absent scan
+                /// (`CasFsck.cpp`, `++report.dangling`), NOT this gate. A tokenless PENDING-upload dep
+                /// (adopted=false) or a no-dep leaf never reaches promote un-resolved legitimately: it is a
+                /// staging bug (a pending upload that never completed) and fails closed (LOGICAL_ERROR).
+                if (!isTrustedAdopt(e.ref))
+                    throw Exception(ErrorCodes::LOGICAL_ERROR,
+                        "promote: blob leaf {} has no tokened and no adopted dep at commit — a staging bug "
+                        "(a pending upload never completed); failing closed",
+                        store->layout().blobKey(e.ref));
+                ProfileEvents::increment(ProfileEvents::CASBlobAdoptTrusted);
+                EventEmitter{*store}.emit([&](CasEvent & ev)
+                {
+                    ev.type = CasEventType::BlobReuseAdopt;
+                    ev.object_kind = CasEventObjectKind::Blob;
+                    ev.object_hash = blobIdOf(e.ref);
+                    ev.outcome = "adopt";
+                    ev.reason = "manifest-trust";   /// distinguishable trusted-adopt class (empty token)
+                });
+            }
+
+            /// BUG 1a: refuse to overwrite a live committed ref that already names a DIFFERENT manifest —
+            /// that would orphan the old manifest (its owner-removal `-1` is never emitted) UNLESS the
+            /// caller opted into an intended repoint (`allow_repoint`, modeled as the `WDropRef`+`WPromote`
+            /// composition) -- a standalone
+            /// write/remove on an already-committed part. Without the flag
+            /// this enforces the model's `RefFreeFor` guard (`WPromote` requires it) exactly as before. A
+            /// re-promote of the SAME manifest_ref is idempotent and allowed regardless (the state
+            /// machine's own promote precondition below would reject it as "precommit absent" anyway once
+            /// idempotent republish skips this call — see `republishRef`). Fail-closed with ABORTED (not
+            /// LOGICAL_ERROR): a conflicting durable state the caller handles, never a must-not-happen
+            /// invariant.
+            if (const auto it = state.getCommitted().find(final_ref_name);
+                it != state.getCommitted().end() && !(it->second.manifest_ref == id.ref))
+            {
+                if (!allow_repoint)
+                    throwCasWriteRetryLater(fmt::format(
+                        "promote: ref '{}' already names a different committed manifest — refusing to overwrite "
+                        "(unique-ref invariant; use republishRef for an intended repoint)", final_ref_name));
+                repoint_old = it->second.manifest_ref;
+            }
+
+            /// Promotion is a PURE OWNER MOVE: the SAME manifest_ref T moves from
+            /// precommit to committed in one atomic transaction, together with the SetPublishedAt op
+            /// that stamps `published_at_ms` (the initial stamp arrives via a separate set_published_at
+            /// op, in the same transaction or a later one -- here, the same one). It emits NO blob
+            /// deltas; the activating `+1` came from GC's barrier-activation of the create-precommit op.
+            std::vector<RefOp> ops;
+            /// An intended repoint additionally retires the OLD committed
+            /// binding in this SAME ref-log record -- a separate OwnerTransition (old=Committed(repoint_old),
+            /// new=absent), since one RefOwnerBinding cannot carry both the retired-committed and the
+            /// promoted-precommit manifests at once (CasRefLogCodec.h). Together with the transition op
+            /// below, this record atomically composes two verified model shapes -- this removal is `WDropRef`'s
+            /// and the transition is `WPromote`'s; `applyRefLogTxn`'s whole-record scratch apply makes the
+            /// composition a sound refinement (no intra-record intermediate state is observable;
+            /// corrected note). NOT `WRepoint` -- that action requires an unowned destination, while this
+            /// trigger always promotes a live precommit. Ref still moves directly
+            /// from the old manifest to the new one. Mirrors `publishCommittedTransition`'s old-removal
+            /// shape (cas_test_helpers.h) minus the add-precommit step, which already landed earlier as
+            /// this build's own `precommitAdd`.
+            if (repoint_old)
+            {
+                RefOp old_removal;
+                old_removal.kind = RefOpKind::OwnerTransition;
+                old_removal.old_binding = RefOwnerBinding{RefOwnerKind::Committed, final_ref_name, *repoint_old};
+                ops.push_back(old_removal);
+            }
+            RefOp transition;
+            transition.kind = RefOpKind::OwnerTransition;
+            transition.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, final_ref_name, id.ref};
+            transition.new_binding = RefOwnerBinding{RefOwnerKind::Committed, final_ref_name, id.ref};
+            ops.push_back(transition);
+
+            RefOp set_published_at;
+            set_published_at.kind = RefOpKind::SetPublishedAt;
+            set_published_at.ref_name = final_ref_name;
+            set_published_at.expected_manifest_ref = id.ref;
+            set_published_at.published_at_ms = nowMs();
+            ops.push_back(set_published_at);
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Promote);
+
+    commit_state = CommitState::Durable;
+    /// The owed terminal operation is discharged. `Settled`, not `NotAttempted`: the manifest body now
+    /// belongs to a committed ref, so it stays out of `cleanupStagedManifestDebrisBestEffort`'s reach.
+    precommit_state = PrecommitState::Settled;
+    store->retireBuildSeq(build_seq);
+    try
+    {
+        EventEmitter{*store}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::BuildPublish;
+            e.namespace_ = target_ns.string();
+            e.ref_name = final_ref_name;
+            e.object_hash = manifestRefDebugString(id.ref);
+            e.token = u128ToHex(promote_build_id);
+            e.outcome = "promoted";
+            e.reason = "promote: atomic owner move precommit(build_id) -> ref(final_ref_name) after fail-closed reval";
+            e.detail = {{"build_seq", std::to_string(build_seq)}};
+        });
+    }
+    catch (...)
+    {
+        tryLogCurrentException(getLogger("CasPartWriteTxn"), "CAS event emission after durable promote");
+    }
+
+    /// The low-level audit event for an intended repoint. The ProfileEvent counter + LOG_WARNING are
+    /// owned by `CachedPartFolderAccess::repointRef`,
+    /// the user-facing primitive that calls into this promote -- adding them here too would double-fire
+    /// once the caller-level instrumentation is installed. This event alone is per-call-site accurate regardless of which caller opted
+    /// into `allow_repoint`.
+    if (repoint_old)
+    {
+        try
+        {
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::RefRepoint;
+                e.namespace_ = target_ns.string();
+                e.ref_name = final_ref_name;
+                e.object_kind = CasEventObjectKind::Manifest;
+                e.object_hash = manifestRefDebugString(id.ref);
+                e.outcome = "repointed";
+                e.reason = "promote(allow_repoint=true): committed ref retargeted from an old manifest to a "
+                           "new one in one ref-log record -- standalone write/remove on a committed part";
+                e.detail = {{"old_manifest", manifestRefDebugString(*repoint_old)}};
+            });
+        }
+        catch (...)
+        {
+            tryLogCurrentException(getLogger("CasPartWriteTxn"), "CAS event emission after durable promote");
+        }
+    }
+    return created;
+}
+
+void PartWriteTxn::abandon()
+{
+    if (cancelled.load(std::memory_order_acquire))
+    {
+        /// `dropNamespace` cancelled this build once its removal transaction became durable: that same
+        /// transaction already removed EVERY precommit binding for the (now Removed) namespace, so
+        /// re-appending a precommit removal here is redundant AND impossible (`owner_transition` on a
+        /// non-Live namespace is rejected by the state machine -- and `requireAlive` would throw first
+        /// anyway). Do ONLY the best-effort staged-debris cleanup + seq retire (both idempotent, both on
+        /// this build's OWN thread); leave the precommit body, if any, for GC's
+        /// delete-after-sealed-decrements exactly as the normal path does.
+        alive = false;
+        precommit_state = PrecommitState::Settled;
+        store->retireBuildSeq(build_seq);
+        cleanupStagedManifestDebrisBestEffort();
+        /// Audit emission is best-effort: a throwing sink (e.g. a bad_alloc growing the system-log
+        /// queue, or a Context/log-shutdown edge) must never turn the already-durable cleanup above
+        /// into a reported failure. Mirrors `promote`'s post-durable emit guard.
+        try
+        {
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::BuildAbort;
+                e.token = u128ToHex(build_id);
+                e.outcome = "cancelled";
+                e.reason = "cancelForNamespaceRemoval: owning namespace removed (dropNamespace); best-effort "
+                           "deleted staged _manifests debris; precommit body (if any) left for GC";
+                e.detail = {{"build_seq", std::to_string(build_seq)}, {"staged", std::to_string(staged_manifests.size())}};
+            });
+        }
+        catch (...)
+        {
+            tryLogCurrentException(getLogger("CasPartWriteTxn"), "CAS event emission after durable abandon");
+        }
+        return;
+    }
+
+    requireAlive();
+
+    /// BUG 2 / TLA+ `WAbandonPrecommit`: if this build made a manifest a LIVE precommit owner input
+    /// (`precommitAdd` ran), abandoning it must NOT writer-delete that body. Instead append an exact
+    /// precommit-removal ref-log transaction, mirroring EXACTLY the removal
+    /// shape `Pool::dropRef` and the fenced-successor stale-precommit sweep use (an old_binding naming
+    /// the exact precommit, no new_binding). GC then folds the `-1` blob decrements and deletes the
+    /// body only after they are sealed (delete-after-sealed-decrements). Deleting a live precommit body
+    /// here would strand GC's fold barrier (live precommit, missing body → clamp forever) or lose the
+    /// activating `+1`. This is the correctness-bearing step of abandon, so it runs through
+    /// `appendRefOps` (the reliable append lane), not best-effort. It precedes the best-effort debris
+    /// deletion below so a partial cleanup can never leave the precommit binding live without its
+    /// body's GC release queued.
+    ///
+    /// `Uncertain` is treated EXACTLY as `Durable` here, and that is the point of the state existing: a
+    /// precommit that may be live owes the same removal a precommit that is live owes. The one
+    /// difference is the closure's tolerance below.
+    if (precommit_state == PrecommitState::Uncertain || precommit_state == PrecommitState::Durable)
+    {
+        /// A removal whose `old_binding` names an ABSENT precommit is rejected by the state machine
+        /// (`RefTableState::applyOwnerTransition`, `RemovePrecommit`: "exact precommit binding to remove
+        /// is absent") -- so the removal is NOT unconditionally idempotent, and an `Uncertain` append
+        /// that in fact never landed would make every `abandon` of this build fail forever, destructor
+        /// retries included. Under `Uncertain` the closure therefore reads the CURRENT table state and
+        /// emits NO op when the binding is not there: same read, same flush, no race. Under `Durable`
+        /// the binding is known to have been appended, so an absent one is a genuine anomaly and the
+        /// strict form is kept -- a fail-closed error is the right report for it.
+        const bool tolerate_absent = precommit_state == PrecommitState::Uncertain;
+        store->appendRefOps(precommit_target_ns, MutationScope::ref(precommit_final_ref),
+            /// By-value capture of the values the closure reads (ref name, manifest and the tolerance
+            /// flag) rather than `[&]`: defense-in-depth so the closure is self-contained if it ever
+            /// outlives this stack.
+            [ref_name = precommit_final_ref, manifest = precommit_manifest, tolerate_absent]
+            (const RefTableState & state) -> std::vector<RefOp>
+            {
+                if (tolerate_absent && !state.getPrecommits().contains({ref_name, manifest}))
+                    return {};
+                RefOp op;
+                op.kind = RefOpKind::OwnerTransition;
+                op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref_name, manifest};
+                return {op};
+            },
+            RootMutationOrigin::Writer, RootMutationKind::Abandon);
+        precommit_state = PrecommitState::Settled;
+
+        try
+        {
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::PrecommitRemoved;
+                e.namespace_ = precommit_target_ns.string();
+                e.ref_name = precommit_final_ref;
+                e.object_kind = CasEventObjectKind::Root;
+                e.object_hash = manifestRefDebugString(precommit_manifest);
+                e.reason = "abandon: precommit binding removed";
+            });
+        }
+        catch (...)
+        {
+            tryLogCurrentException(getLogger("CasPartWriteTxn"), "CAS event emission after durable abandon");
+        }
+    }
+
+    /// `alive` flips to false only AFTER the correctness-bearing precommit removal above is durable: if
+    /// `appendRefOps` threw, `alive` stays true and `precommit_state` keeps owing the removal, so a
+    /// caller that catches the failure can retry `abandon()` on this same object. A retry after an
+    /// ambiguous already-appended failure re-validates `old_binding` and errors (or, under the
+    /// `Uncertain` tolerance above, no-ops) -- it never corrupts.
+    alive = false;
+
+    /// No longer in-flight: retire the seq so the per-server active-build floor (`min_active`) can advance
+    /// (idempotent). This runs AFTER the precommit removal above (mirrors `PartWriteTxn::promote`, which retires
+    /// after its commit) so the build stays active until its precommit binding's removal is durable:
+    /// retiring first would advance `min_active` past a build whose precommit binding is still live in the
+    /// ref log, letting a freshness-window consumer judge the manifest build-dead while an un-removed
+    /// precommit still names it. Ordering removal-before-retire keeps that happens-before clean.
+    store->retireBuildSeq(build_seq);
+
+    cleanupStagedManifestDebrisBestEffort();
+
+    /// The build was abandoned; its uploads become GC-reclaimable debris. Best-effort audit emission:
+    /// the durable work (precommit removal + seq retire) is already done, so a throwing sink must not
+    /// propagate a failure out of a successful abandon.
+    try
+    {
+        EventEmitter{*store}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::BuildAbort;
+            e.token = u128ToHex(build_id);
+            e.outcome = "abandoned";
+            e.reason = "abandon: appended a precommit-removal event for the live precommit (body left for GC); "
+                       "best-effort deleted the never-precommitted _manifests debris; remainder reaped by the orphan sweep";
+            e.detail = {{"build_seq", std::to_string(build_seq)}, {"staged", std::to_string(staged_manifests.size())}};
+        });
+    }
+    catch (...)
+    {
+        tryLogCurrentException(getLogger("CasPartWriteTxn"), "CAS event emission after durable abandon");
+    }
+}
+
+void PartWriteTxn::cleanupStagedManifestDebrisBestEffort()
+{
+    /// Best-effort writer cleanup of THIS build's pre-precommit/staged `_manifests` debris. The common case
+    /// is writer cleanup; a missed object is benign — the namespace-scoped orphan sweep reclaims it. Exact-token delete only; never
+    /// throws. SKIP the manifest that became a live precommit owner: its body is a live precommit input
+    /// whose deletion is GC's job after the sealed decrement (never writer-delete it).
+    ///
+    /// "Became a live precommit owner" is decided from the ATTEMPT, not from a confirmed append: any
+    /// state other than `NotAttempted` -- including `Uncertain`, where the precommit may or may not
+    /// exist -- protects the body. Deleting it under uncertainty is the unrecoverable direction (a
+    /// precommit that turns out to be live and whose body is gone clamps GC's fold barrier forever),
+    /// while keeping it is not: an unreferenced body is ordinary orphan-sweep debris.
+    const bool precommit_attempted = precommit_state != PrecommitState::NotAttempted;
+    for (const ManifestId & id : staged_manifests)
+    {
+        if (precommit_attempted && id.ref == precommit_manifest && id.root_namespace == precommit_target_ns)
+            continue;   /// the (possibly) live precommit body — left for GC (delete-after-sealed-decrements)
+        try
+        {
+            const String key = store->layout().manifestKey(id);
+            const HeadResult hr = store->backend().head(key);
+            if (hr.exists)
+                store->backend().deleteExact(key, hr.token);
+        }
+        catch (...) // NOLINT(bugprone-empty-catch)
+        {
+            /// Best-effort cleanup: the GC backstop sweep is the durable guarantee.
+        }
+    }
+}
+
+void PartWriteTxn::cancelForNamespaceRemoval(const RootNamespace & removed_ns)
+{
+    /// Only cancel a build that actually targets the removed namespace. `manifestNamespace` reads only
+    /// the immutable `info` set at construction, so this is safe to call from `dropNamespace`'s thread
+    /// while the build's own thread runs. A build whose owning namespace cannot be determined (a
+    /// malformed diagnostic-only PartWriteInfo) is left alone — the append lane still fails closed on any op
+    /// it later attempts against the now-Removed namespace.
+    std::optional<RootNamespace> mine;
+    try
+    {
+        mine = manifestNamespace();
+    }
+    catch (...)
+    {
+        return;
+    }
+    if (mine != removed_ns)
+        return;
+    cancelled.store(true, std::memory_order_release);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h
new file mode 100644
index 000000000000..2ef5a44df0af
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h
@@ -0,0 +1,402 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <atomic>
+#include <functional>
+#include <map>
+#include <optional>
+#include <set>
+#include <span>
+#include <vector>
+
+namespace DB { class WriteBuffer; }
+
+namespace DB::Cas
+{
+
+/// Re-readable source for one content-addressed blob upload.
+///
+/// `open` returns a FRESH reader over exactly `size` logical bytes and may be called more than once: an
+/// upload can race with another writer or with GC, in which case the transaction retries from the writer's
+/// own source and each attempt must read from the beginning. `server_side_copy_from` is set only for an S3
+/// staging object; the ordinary create then promotes it by a server-side copy instead of streaming through
+/// ClickHouse, and `open` reads that same staging object when a resurrection has to stream it. The staging
+/// object must remain available through a condemned-object resurrection.
+struct BlobSource
+{
+    uint64_t size = 0;
+    std::function<std::unique_ptr<ReadBuffer>()> open;   /// yields exactly `size` bytes, from the start
+    /// When set, the blob's bytes already live in an S3 staging object with this key, and `putBlob` promotes it by a
+    /// WRITE-ONCE conditional SERVER-SIDE COPY (`Backend::promoteStaged`) instead of streaming
+    /// `open` — and resurrects a condemned incarnation by an unconditional server-side copy
+    /// from the SAME staging object (`Backend::resurrect`), never a read of the condemned blob
+    /// (revival must always be a fresh write from the source). Unset (the default, `StagingBackend::Local`) ⇒ the local
+    /// streaming path is byte-for-byte unchanged and `open` is the source.
+    std::optional<String> server_side_copy_from;
+    /// Build a re-readable source backed by an owned string; intended for small payloads and tests.
+    static BlobSource fromString(String bytes);
+};
+
+/// putBlob's return value: the `BlobRef` it was addressed by (the write mint's algo + digest pair)
+/// plus the admitted logical size.
+struct PutBlobResult
+{
+    BlobRef ref;
+    uint64_t size = 0;
+};
+
+/// One blob dependency this build contributes — EXACTLY the record `putBlob` folds into `deps`.
+/// A token identifies an incarnation uploaded by this transaction and must be retained through
+/// promotion; a tokenless entry relies on the durable source-manifest edge instead. `adopted`
+/// distinguishes trusted committed-source evidence (`adoptEvidence`) from a pending upload that has
+/// not yet been completed. CAS-owned public value type so a transaction-detached upload can RETURN
+/// its complete dep effect instead of folding it as a side effect (spec §1: "no branch may leave its
+/// dep effect behind as a side effect").
+struct BlobDepRecord
+{
+    ObjectKind kind = ObjectKind::Blob;
+    std::optional<Token> token;                       /// nullopt = live-source evidence
+    uint64_t size = 0;
+    bool adopted = false;                             /// true only for `adoptEvidence`
+
+    bool operator==(const BlobDepRecord &) const = default;
+};
+
+/// Which branch of the upload primitive admitted a blob. Carried out of `uploadBlobDetached` so the
+/// merge/fan-out layers (and tests) can assert the branch taken without inferring it from the dep.
+enum class BlobUploadOutcome
+{
+    DeduplicationCacheHit,      /// dedup cache said present; HEAD-first confirmed a live incarnation; adopted
+    HeadHit,            /// size-triggered HEAD-first found a present live incarnation; adopted
+    HeadMissAdopted,    /// the write-once create 412'd on a live incarnation (or a racing writer's); adopted
+    FreshUpload,        /// the write-once conditional create streamed a fresh local body
+    StagingPromoted,    /// the write-once conditional server-side copy promoted an S3 staging object
+    ResurrectedLocal,   /// a condemned incarnation displaced by a fresh local `putOverwrite`
+    ResurrectedS3,      /// a condemned incarnation displaced by a fresh server-side copy from staging
+};
+
+/// Public, CAS-owned input to `uploadBlobDetached`; the transaction's private dep representation is not
+/// exposed. `source` mirrors `putBlob`'s re-readable `BlobSource` (local streaming / S3 staging copy).
+/// `declared_size` is the value the fan-out layer groups and conflict-checks on; it mirrors
+/// `source.size`, which stays the authority for the per-attempt streaming byte check.
+struct BlobUploadRequest
+{
+    BlobRef ref;
+    BlobSource source;
+    uint64_t declared_size = 0;
+};
+
+/// Complete result of one detached upload: the addressed ref, the COMPLETE dep effect the upload
+/// contributes (no side channel), and the branch outcome.
+struct BlobUploadResult
+{
+    BlobRef ref;
+    BlobDepRecord dep;
+    BlobUploadOutcome outcome = BlobUploadOutcome::FreshUpload;
+};
+
+/// Hash `payload` with `algo` using the same convention as the streaming blob writer and return the complete
+/// `BlobRef` identity. The algorithm travels with the digest; callers must not reconstruct a blob identity from
+/// a bare digest or from an independently supplied digest width.
+BlobRef poolContentHash(BlobHashAlgo algo, std::string_view payload);
+
+/// Coordinates one part write from manifest staging through blob admission and ref publication. The transaction
+/// owns the in-memory dependency set and the identities of manifests staged by this build; `Pool` owns the
+/// durable object store and ref-log operations. A transaction is normally used by one writer thread, while
+/// `cancelForNamespaceRemoval` may set its cancellation flag from the namespace-removal thread.
+///
+/// The durable write order is `stageManifest` → `precommitAdd` → `putBlob` → `promote`. The precommit edge
+/// must be durable before any existing blob incarnation is adopted, because the edge is what protects that
+/// incarnation from GC while this build is in flight. `promote` then moves the same manifest owner binding
+/// atomically from precommit to committed. A failed or abandoned transaction never resumes after a process
+/// restart; its precommit is removed by the live owner or a fenced successor, and GC reclaims the resulting
+/// debris only after the corresponding ref-log decrements are durable.
+class PartWriteTxn
+{
+public:
+    /// Start a build and emit its durable in-flight-build attribution. The identity arguments identify the
+    /// build for ownership and GC fencing; `info_` is retained as immutable build context.
+    PartWriteTxn(PoolPtr store_, UInt128 build_id_,
+          uint64_t build_seq_, uint64_t epoch_, PartWriteInfo info_);
+
+    /// Retire this build's sequence so the pool's active-build watermark can advance. This is idempotent when
+    /// `promote` or `abandon` already retired the sequence, and also covers destruction during unwinding.
+    ~PartWriteTxn();
+
+    /// Every upload attempt mints a fresh random `incarnation_tag`.
+    /// New content: streaming PUT If-None-Match:*; on PreconditionFailed ⇒ the cold-reuse rule
+    /// (observe current token; condemned ⇒ uploadFromSource — re-upload from the writer's source
+    /// bytes; else adopt — free).
+    /// Ordering: `putBlob` is always called after `precommitAdd` (the wiring order is
+    /// `stageManifest` → `precommitAdd` → `putBlob` → `promote`). Its
+    /// ADOPT paths observe an existing incarnation, so they are safe only under this build's durable
+    /// precommit closure — enforced by a fail-closed throw (LOGICAL_ERROR, not a `chassert`, which is
+    /// compiled out in release) in observeAndAdmit. A FRESH upload before precommit is legal
+    /// (newborn-debris watermark), but production never does it.
+    PutBlobResult putBlob(const BlobRef & ref, BlobSource source);
+
+    /// Transaction-DETACHED upload primitive (spec §1). Runs the SAME durable, ordering-sensitive pool
+    /// effects `putBlob` runs — the HEAD-first dedup gate, the write-once conditional create, condemned
+    /// resurrection (INV-1: never GET a condemned object), the freshness-meta `Clean` transition,
+    /// dedup-cache reads/inserts, event emission, ProfileEvents — but folds NOTHING into `build`
+    /// (`deps`), returning the complete dep effect + branch outcome as a value instead. It is therefore
+    /// safe to run off the owning writer thread while `PartWriteTxn` stays single-writer for `build`.
+    /// `putBlob` = this primitive + a single-result `deps` fold on the calling thread.
+    BlobUploadResult uploadBlobDetached(const BlobUploadRequest & req) const;
+
+    /// Applies a fan-out's `uploadBlobDetached` results into `deps` on the CALLING thread, after the
+    /// fan-out's join -- so this is an owning-writer-thread API exactly like `putBlob`, and MUST NOT be
+    /// called from a pool task. Merge failure must not leave a partially merged build (spec §1): every
+    /// result is prevalidated FIRST -- completeness (a result's dep must carry a token; every branch of
+    /// `uploadBlobDetached` sets one, so a tokenless result is a caller bug, not a valid dep-only-evidence
+    /// state, which is folded through `adoptEvidence` instead) and duplicate-grouping consistency
+    /// (two results for the same `BlobRef` must carry an identical dep record; a conflict -- most
+    /// commonly a conflicting size -- means the fan-out's one-task-per-unique-ref invariant was
+    /// violated) -- BEFORE any result is applied. Application then runs against a COPY of `deps` (a
+    /// "build"), so a mid-application exception (including one raised by `setMergeHookForTest`'s hook, or
+    /// a `bad_alloc` from map-node allocation) never touches the live `deps`; the copy is committed by a
+    /// single no-throw `swap` only after every result has applied. The build is therefore either fully
+    /// merged or byte-for-byte untouched -- never partially merged.
+    void mergeBlobUploadResults(std::span<const BlobUploadResult> results);
+
+    /// Test-only fault-injection seam (inert in production): invoked after each result has applied to
+    /// the in-progress merge copy, with the count of results applied so far (1-based). A hook that
+    /// throws (e.g. to model a `bad_alloc` mid-merge) aborts `mergeBlobUploadResults` before its final
+    /// swap, so the live `deps` stays untouched -- the seam exists to prove that all-or-nothing property
+    /// under injected failure at every application point, not just the first or the last.
+    void setMergeHookForTest(std::function<void(size_t applied_so_far)> hook) { merge_hook_for_test = std::move(hook); }
+
+    /// Test-only DEEP snapshot of this build's recorded deps, keyed by `BlobRef`. A plain copy of the
+    /// private `deps` map -- lets a test assert the whole build is byte-for-byte untouched after a
+    /// rejected or aborted merge, rather than probing one ref at a time via `depIsTokened`.
+    std::map<BlobRef, BlobDepRecord> depsSnapshotForTest() const { return deps; }
+
+    /// Return whether this build holds a TOKENED Blob dep for `ref` (`putBlob` ⇒
+    /// tokened) versus a tokenless evidence dep (`adoptEvidence` ⇒ tokenless)? False also when this
+    /// build has no dep for the ref at all.
+    bool depIsTokened(const BlobRef & ref) const;
+
+    /// Record a TOKENLESS evidence blob dep directly from a `ManifestEntry` — no HEAD or backend
+    /// call. Lets staging adopt sites record the dep by hash without asserting presence before
+    /// precommit; the promote gate observes/resurrects it post-precommit. Inline entries record nothing.
+    void adoptEvidence(const ManifestEntry & entry);
+
+    /// Record a TOKENLESS pending blob dep by ref (without a HEAD) for a blob whose bytes are staged locally and
+    /// will be putBlob'd post-precommit. putBlob later overwrites it with the tokened dep on upload.
+    void recordPendingBlobDep(const BlobRef & ref, uint64_t size);
+
+    /// Mint a root-local part `ManifestId`, write its body under
+    /// `cas/manifests/<ns>/<writer_epoch>/<build_sequence>/000001.zst` via the pool's shared request
+    /// controller. It uses budgeted attempts with resolve-before-reissue and performs no preliminary HEAD,
+    /// because `manifest_ordinal` is monotone within this build. It enforces manifest-size caps before the body
+    /// write returns and therefore before any owner transition is published. The body is not retained after a
+    /// successful write; on retry the caller re-stages from source. Every call uses a fresh manifest ordinal.
+    /// The id is recorded for best-effort `abandon` cleanup.
+    ManifestId stageManifest(std::vector<ManifestEntry> entries);
+
+    /// Add this transaction's precommit owner intent; there is no `_precommits` namespace. One
+    /// `appendRefOps` call appending an OwnerTransition `RefOp` (new_binding = {Precommit,
+    /// final_ref_name, id.ref}) to final_ref_name's ref-log entry, so the later promote is an atomic
+    /// owner move over that same entry. Needs NO body-exists HEAD as a safety authority: GC and
+    /// promotion handle a missing precommit manifest body by failing closed (a missing-body precommit
+    /// is a non-activating, non-promotable intent).
+    ///
+    /// A3 mint-tightening: `id` must either be one THIS transaction minted via `stageManifest`, or
+    /// already be `final_ref_name`'s current committed manifest (the idempotent re-drive -- see the
+    /// closure in the .cpp). Any other id names a manifest this transaction never staged and does not
+    /// currently own -- most commonly one a DIFFERENT, since-abandoned/dropped transaction once staged
+    /// -- and granting it fresh ownership here would let a later exact-`ManifestRef` equality check
+    /// (the relink confirm) compare true against a token whose blobs may already be reclaimed (an ABA).
+    /// Rejected with `LOGICAL_ERROR`: this is a programming-invariant violation, never an operational one.
+    void precommitAdd(const RootNamespace & target_ns, const String & final_ref_name, const ManifestId & id);
+
+    /// Atomically promote the precommit to the committed ref with one `appendRefOps` call on the target ref's
+    /// ref-log entry.
+    ///  1. tokened leaves are already protected by the durable precommit edge, so no writer-side retired-view
+    ///     refresh is needed;
+    ///  2. stream-read the precommit manifest body; validate RefMatchesBody / ManifestNamespaceMatches;
+    ///  3. the NON-tokened blob leaves (tokened leaves are edge-protected, not re-checked): a committed-source
+    ///     adoptEvidence leaf is TRUSTED via the durable manifest edge — NO per-file HEAD/loadMeta probe (§4
+    ///     manifest-trust: the live source pins the blob, in-degree >= 1); a genuinely
+    ///     absent adopted blob is an invariant violation caught by fsck, not here;
+    ///  4. a body-absent precommit or a lost owner-liveness ⇒ ABORTED; a non-tokened, non-adopted leaf (no
+    ///     tokened dep and no committed-source adopt — a staging bug) ⇒ LOGICAL_ERROR (fail closed);
+    ///  5. atomically replace precommit(build_id) owner with committed(final_ref_name) owner by appending
+    ///     ONE pure-move `RefOp` (old_binding={Precommit,final_ref_name,T}, new_binding={Committed,final_ref_name,T},
+    ///     same manifest_ref T) and setting refs[final_ref_name];
+    ///  6. promotion never emits blob deltas. A missing-body precommit
+    ///     is non-activating and was rejected at step 4 (the writer re-stages with a fresh ManifestId).
+    ///
+    /// PROCESS-RESTART INVARIANT: a `PartWriteTxn` is a plain in-memory C++ object owned by the wiring's
+    /// `ContentAddressedTransaction` — it is NEVER persisted and NEVER resumed across a process
+    /// restart. There is no "replay a precommit" code path anywhere in the core: `promote` is called
+    /// synchronously, in-process, strictly AFTER every referenced blob's `putBlob` (which for S3
+    /// staging drives `promoteStaged`'s conditional copy) has already returned successfully. If the
+    /// process exits between `precommitAdd` and `promote` (e.g. between staging a blob and its
+    /// server-side-copy promote completing), the `PartWriteTxn` object is simply lost with it: nothing ever
+    /// "wakes up" that precommit and finishes promoting it. The precommit's owner binding is left as a
+    /// dead intent in the ref log and is REMOVED (never promoted) by an exact precommit-removal ref-log
+    /// transaction -- the current writer's own `PartWriteTxn::abandon` if it is still mounted, otherwise a
+    /// fenced successor's stale-precommit sweep. `GC` folds the resulting
+    /// `-1` manifest edge but never detects or removes a dead precommit itself. So the
+    /// hazard — "promote a precommit whose copy did not complete" — has no code
+    /// path to occur through: promotion is not a recoverable/resumable operation, only a synchronous
+    /// one that either completes within the writing process or never happens at all.
+    /// `allow_repoint` opts into an intended
+    /// repoint of a committed ref that already names a DIFFERENT manifest -- a standalone write/remove
+    /// on an already-committed part (the committed-publish machinery's missing piece; the promote guard's
+    /// own error text already named it: "use republishRef for an intended repoint"). Default `false`
+    /// preserves the existing unique-ref guard byte-for-byte: a committed ref naming a different manifest
+    /// still throws ABORTED. With `true`, the guard is skipped and the old committed binding is retired
+    /// in the SAME ref-log record as the ordinary precommit->committed promotion, plus a
+    /// `CasEventType::RefRepoint` audit event -- every effective repoint is loud by construction.
+    ///
+    /// Returns `created`: whether `final_ref_name` had NO committed row before this call. Derived
+    /// INSIDE the `appendRefOps` builder (the same in-closure-output pattern the builder already uses
+    /// for `repoint_old`) as `!state.getCommitted().contains(final_ref_name)`, evaluated once at the
+    /// top of the closure -- so it is correct on every path the closure can take: a first-time bind
+    /// (true), a repoint of an existing binding (false), and the idempotent re-promote no-op (false,
+    /// since a committed row for this exact manifest already existed). `build_ops` runs at most once,
+    /// on the flush leader, so this single evaluation is authoritative.
+    bool promote(const RootNamespace & target_ns, const String & final_ref_name, UInt128 build_id, const ManifestId & id, bool allow_repoint = false);
+
+    /// Retire the build sequence so the GC watermark floor can advance; staged manifest debris is best-effort
+    /// cleaned, with the orphan sweep as the durable backstop.
+    void abandon();
+
+    /// Called by `Pool::dropNamespace` for every in-flight build once its namespace-removal transaction is
+    /// durable. If
+    /// this build's owning namespace equals `removed_ns`, mark it cancelled so every further operation
+    /// fails closed at `requireAlive` (ABORTED); a build in any other namespace is left untouched.
+    /// Cross-thread safe: reads only the immutable `info` (the owning namespace) and stores ONE atomic --
+    /// it touches no other member, so the build's own thread may keep running concurrently. Staged debris
+    /// is cleaned best-effort when the build's own thread later runs `abandon` (or via the GC backstop).
+    void cancelForNamespaceRemoval(const RootNamespace & removed_ns);
+
+    UInt128 buildId() const { return build_id; }
+    /// The strictly increasing per-process sequence used by the active-build watermark.
+    uint64_t buildSeq() const { return build_seq; }
+
+    /// Lifecycle of THIS build's create-precommit `owner_transition`.
+    ///
+    /// It is a state and not a bool because `appendRefOps` has three outcomes and only two of them are
+    /// knowledge: an `Unresolved` `PUT` MAY HAVE LANDED (`CasRefLedger.cpp`, the `Unresolved` arm), so a
+    /// `precommitAdd` that threw can still have made this build's manifest a LIVE precommit owner. The
+    /// intent is therefore recorded BEFORE the ambiguous append -- the same "preconstruct before the
+    /// PUT" discipline the ref lane's wedge uses -- and settled either way when it returns.
+    ///
+    ///   `NotAttempted` -- `precommitAdd` never reached its append; nothing can be owned, and this
+    ///                     build's staged manifest bodies are ordinary writer debris.
+    ///   `Uncertain`    -- the append was ATTEMPTED and its outcome is unknown. The build OWES the
+    ///                     precommit removal exactly as if it were durable, and its manifest body is
+    ///                     NOT writer-deletable: it may be a live precommit input whose deletion would
+    ///                     strand GC's fold barrier.
+    ///   `Durable`      -- the append returned, so the precommit binding is this build's.
+    ///   `Settled`      -- the owed terminal operation (`promote` or `abandon`) has discharged the duty.
+    ///                     The body stays non-deletable: after a promote it belongs to a committed ref,
+    ///                     and after an abandon it is GC's to reclaim after the sealed decrement.
+    enum class PrecommitState : uint8_t { NotAttempted, Uncertain, Durable, Settled };
+    PrecommitState precommitState() const { return precommit_state; }
+
+    /// Lifecycle of THIS build's promote (precommit -> committed) append, recorded for the same reason
+    /// and in the same way as `precommitState`: a promote whose append threw may still have COMMITTED
+    /// the ref. The distinction is load-bearing one layer up -- the interserver relink maps a promote
+    /// that definitely did not commit to "fetch the bytes from the same source instead", and doing that
+    /// after a commit that actually landed would publish the same part twice.
+    ///
+    ///   `NotAttempted` -- promote failed (or was never called) strictly BEFORE its append: a rejected
+    ///                     validation, an absent body, a lost owner liveness. Proof of the negative.
+    ///   `Uncertain`    -- the append was attempted and its outcome is unknown. Conservative: the ref
+    ///                     lane collapses `DefiniteFailure` and a pre-attempt refusal into the same
+    ///                     retry-later class as a genuinely ambiguous `PUT`, so both are reported here
+    ///                     as uncertain. That costs a retry, never correctness.
+    ///   `Durable`      -- the append returned; the ref is committed.
+    enum class CommitState : uint8_t { NotAttempted, Uncertain, Durable };
+    CommitState commitState() const { return commit_state; }
+
+private:
+    /// Keyed on the full `BlobRef` pair (algorithm + digest), because a bare digest is not a blob identity.
+    /// This remains an ordered `std::map` (not `unordered_map`): `BlobRef` already provides `operator<=>`, so
+    /// no hasher is needed here. `BlobRefHash` is for unordered dedup-cache/set consumers elsewhere. The
+    /// dependencies are blob-only, so `ObjectKind` is not part of the key.
+    using DepKey = BlobRef;
+
+    /// Apply the cold-reuse rule: HEAD the key; absent ⇒ FILE_DOESNT_EXIST;
+    /// condemned-at-current-token ⇒ throw ABORTED (caller must re-upload from its own source bytes);
+    /// else RETURN the adopt dep record (current token, admitted logical size). Build-neutral: it folds
+    /// nothing into `deps` (its callers compose the returned record into a `BlobUploadResult`).
+    BlobDepRecord observeAndAdmit(ObjectKind kind, const BlobRef & ref, const String & key) const;
+    /// Overload for callers that already hold a fresh, present HeadResult for `key` (the putBlob
+    /// HEAD-before-PUT path), avoiding a redundant second HEAD. `hr.exists` MUST be true.
+    BlobDepRecord observeAndAdmit(ObjectKind kind, const BlobRef & ref, const String & key, const HeadResult & hr) const;
+    /// INV-1 (revival-from-source): revive a condemned or absent object by re-uploading from the writer's
+    /// OWN re-readable source without reading the dying object (no backend().get). On a Native backend the
+    /// source is STREAMED into the put sink (header + `source.open`); the emulated backend materializes
+    /// one body at a time inside `Backend::resurrect`;
+    /// `source.open` may be re-invoked on each conditional-write attempt (it re-reads the staged
+    /// temp file / re-emits the captured String), so it is taken by const ref and not consumed. Build-neutral:
+    /// RETURNS the complete `BlobUploadResult` (dep + branch outcome); it folds nothing into `deps`.
+    BlobUploadResult uploadFromSource(ObjectKind kind, const BlobRef & ref, const String & key, const BlobSource & source) const;
+
+    /// The build's owning root namespace, derived from PartWriteInfo::intended_ref ("ns/ref" — the ref is the
+    /// last `/`-segment; the namespace is everything before it). Sets a manifest body's root_namespace_id.
+    RootNamespace manifestNamespace() const;
+
+    /// Reject operations after `abandon`, namespace cancellation, or writer-epoch fencing. These checks happen
+    /// before backend work so a stale transaction cannot publish a new owner or stage more debris.
+    void requireAlive() const;
+    /// Best-effort exact-token delete of THIS build's staged `_manifests` debris; the precommit body (if
+    /// any) is SKIPPED -- left for GC's delete-after-sealed-decrements. Never throws (the namespace-scoped orphan
+    /// sweep is the durable backstop). Shared by the normal and the namespace-removal-cancelled `abandon`
+    /// paths; only ever called on the build's OWN thread.
+    void cleanupStagedManifestDebrisBestEffort();
+
+    /// A leaf is trusted at promote iff this build holds a TOKENLESS dep recorded by
+    /// `adoptEvidence` (adopted=true) — a committed-source evidence adopt. The live source pins the blob
+    /// (in-degree >= 1, not condemnable) and this build's precommit edge is durable, so the durable manifest
+    /// edge is the liveness evidence: no HEAD, no loadMeta, no copy-forward. A tokened dep (edge-protected,
+    /// handled by `depIsTokened`), a tokenless PENDING-upload dep (adopted=false), or NO dep at all (a
+    /// staging bug) is NOT trusted — it fails closed. The single gate for the promote non-tokened leaf.
+    bool isTrustedAdopt(const BlobRef & ref) const;
+
+    PoolPtr store;
+    UInt128 build_id{};
+    uint64_t build_seq{};                                 /// per-process monotone sequence
+    uint64_t epoch{};                                     /// owning Pool's process_epoch
+    uint32_t next_manifest_ordinal = 1;                   /// per-build monotone manifest ordinal
+    PartWriteInfo info;
+    bool alive = true;
+    /// Set by `cancelForNamespaceRemoval` from `Pool::dropNamespace`'s thread
+    /// once this build's owning namespace is durably removed. Atomic because it is WRITTEN cross-thread
+    /// and READ by `requireAlive` on the build's own thread. Once cancelled, every further op fails closed.
+    std::atomic<bool> cancelled{false};
+    PrecommitState precommit_state = PrecommitState::NotAttempted;
+    CommitState commit_state = CommitState::NotAttempted;
+
+    /// The precommit's target, recorded by `precommitAdd` BEFORE its append and never cleared
+    /// afterwards. Two consumers with different lifetimes read them: `abandon` needs them while the
+    /// duty is owed (`Uncertain`/`Durable`), and `cleanupStagedManifestDebrisBestEffort` needs them for
+    /// as long as this object lives, because a body that was ever the target of a precommit attempt
+    /// must never be writer-deleted -- which is why the terminal operations move `precommit_state` to
+    /// `Settled` rather than resetting these back to their unset values.
+    RootNamespace precommit_target_ns;
+    String precommit_final_ref;
+    ManifestRef precommit_manifest;
+
+    std::vector<ManifestId> staged_manifests;             /// for best-effort abandon cleanup
+
+    /// Every `ManifestId` this transaction has minted via `stageManifest`, checked by `precommitAdd`:
+    /// an unowned id may enter ownership only from the transaction that freshly staged it. Kept separate from
+    /// `staged_manifests` above -- that vector's role (best-effort abandon cleanup) is unrelated and
+    /// could change independently; this set exists purely as the ABA barrier's identity check, so it
+    /// stays correct even if the cleanup vector's contents or lifetime ever change.
+    std::set<ManifestId> staged_manifest_ids;
+
+    std::map<DepKey, BlobDepRecord> deps;                 /// dependencies recorded by this build (blobs only)
+
+    /// Backing state for `setMergeHookForTest`; empty (no-op) in production.
+    std::function<void(size_t)> merge_hook_for_test;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.cpp
new file mode 100644
index 000000000000..511ff5cc2516
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.cpp
@@ -0,0 +1,154 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.h>
+#include <Common/Exception.h>
+#include <algorithm>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int ABORTED;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+    constexpr size_t MAX_CAS_ATTEMPTS = 100;
+}
+
+void CasPlainObjects::casPutObject(const String & full_key, const String & bytes)
+{
+    /// The read determines whether this is a conditional create or replacement. The token is only
+    /// valid for the incarnation returned by that head, so a precondition failure means another
+    /// writer won the race and the loop must observe the new incarnation before trying again.
+    ///
+    /// SINGLE-APPENDER INVARIANT: `bytes` is frozen by the caller before this loop starts (see the
+    /// append-base note at `ContentAddressedTransaction::writeFile`'s Append branch); the loop only
+    /// re-reads the TOKEN on conflict, never the base content. This is correct only while nothing
+    /// concurrently appends to the same key — a losing retry would overwrite the winner's bytes with a
+    /// stale, pre-conflict payload (a lost update). Implement a real `casAppendObject` (re-reading the
+    /// base content, not just the token, inside the loop) before adding any concurrent appender.
+    ///
+    /// rev.7 [C2]: the fence generation captured at admission is re-checked immediately before EVERY
+    /// durable PUT below, not just the first attempt. A mismatch (the mount lease was lost, or re-armed
+    /// under a fresh incarnation, since admission) aborts with the typed transient error before the backend
+    /// is ever touched.
+    const uint64_t admitted_generation = fence_generation_fn();
+
+    for (size_t attempt = 0; attempt < MAX_CAS_ATTEMPTS; ++attempt)
+    {
+        HeadResult head = backend.head(full_key);
+        check_fence_or_throw_fn(admitted_generation);
+        if (!head.exists)
+        {
+            if (backend.putIfAbsent(full_key, bytes).outcome == PutOutcome::Done)
+                return;
+        }
+        else
+        {
+            if (backend.putOverwrite(full_key, bytes, head.token).outcome == PutOutcome::Done)
+                return;
+        }
+        /// `PreconditionFailed` means the observed state changed under us; re-head and retry.
+    }
+    throw Exception(ErrorCodes::ABORTED, "object CAS contention on '{}'", full_key);
+}
+
+std::optional<String> CasPlainObjects::casGetObject(const String & full_key)
+{
+    std::optional<GetResult> result = backend.get(full_key);
+    if (!result)
+        return std::nullopt;
+    return result->bytes;
+}
+
+void CasPlainObjects::casRemoveObject(const String & full_key)
+{
+    /// Delete only the incarnation observed by the preceding head. A token mismatch leaves the
+    /// replacement untouched and is retried against a fresh observation; absence is a successful
+    /// no-op.
+    ///
+    /// rev.7 [C2]: same fence-generation admission as `casPutObject` -- the admitted generation is
+    /// re-checked immediately before every durable delete.
+    const uint64_t admitted_generation = fence_generation_fn();
+
+    for (size_t attempt = 0; attempt < MAX_CAS_ATTEMPTS; ++attempt)
+    {
+        const HeadResult head = backend.head(full_key);
+        if (!head.exists)
+            return;
+        check_fence_or_throw_fn(admitted_generation);
+        const DeleteOutcome outcome = backend.deleteExact(full_key, head.token);
+        if (outcome.kind == DeleteOutcome::Kind::Deleted || outcome.kind == DeleteOutcome::Kind::NotFound)
+            return;
+        /// `TokenMismatch` means a concurrent rewrite; re-head and retry.
+    }
+    throw Exception(ErrorCodes::ABORTED, "object CAS contention on '{}' (runaway live-lock brake)", full_key);
+}
+
+void CasPlainObjects::putNamespaceFile(const NamespaceLifeId & life, const String & name, const String & bytes)
+{
+    casPutObject(layout.namespaceFileKey(life, name), bytes);
+}
+
+std::optional<String> CasPlainObjects::getNamespaceFile(const NamespaceLifeId & life, const String & name)
+{
+    return casGetObject(layout.namespaceFileKey(life, name));
+}
+
+std::vector<String> CasPlainObjects::listNamespaceFiles(const NamespaceLifeId & life)
+{
+    const String prefix = layout.namespaceFilesPrefix(life);
+    std::vector<String> names;
+    String cursor;
+    while (true)
+    {
+        ListPage page = backend.list(prefix, cursor, /*limit*/ 1000);
+        for (const ListedKey & listed : page.keys)
+        {
+            /// Strip the storage prefix so callers receive the bare flat file name.
+            if (listed.key.starts_with(prefix))
+                names.push_back(listed.key.substr(prefix.size()));
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    /// Backends are not required to return pages in the same order, so make the public result
+    /// deterministic instead of relying on `InMemoryBackend` ordering.
+    std::sort(names.begin(), names.end());
+    return names;
+}
+
+void CasPlainObjects::removeNamespaceFile(const NamespaceLifeId & life, const String & name)
+{
+    casRemoveObject(layout.namespaceFileKey(life, name));
+}
+
+void CasPlainObjects::putMountpointObject(const String & key, const String & bytes)
+{
+    casPutObject(layout.mountpointObjectKey(key), bytes);
+}
+
+std::optional<String> CasPlainObjects::getMountpointObject(const String & key)
+{
+    return casGetObject(layout.mountpointObjectKey(key));
+}
+
+bool CasPlainObjects::mountpointObjectExists(const String & key)
+{
+    /// Use metadata rather than a body GET because a path probe may resolve to a directory, such as
+    /// the `store` pool subdirectory traversed by `system.remote_data_paths`. The local backend
+    /// treats a directory as not an object, so this returns false instead of attempting a body read
+    /// that would raise a filesystem exception for a directory.
+    return backend.head(layout.mountpointObjectKey(key)).exists;
+}
+
+void CasPlainObjects::removeMountpointObject(const String & key)
+{
+    casRemoveObject(layout.mountpointObjectKey(key));
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.h
new file mode 100644
index 000000000000..d1eae78d6e57
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.h
@@ -0,0 +1,111 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h>
+#include <functional>
+#include <optional>
+#include <string>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Provides the pool's plain-object surface: loose, non-content-addressed objects whose key is
+/// chosen by the caller. This covers namespace files under `cas/ns/state/<life_id>/_files/` -- keyed
+/// by the namespace LIFE, never by its bare name -- and mountpoint objects mirrored by path. The object
+/// bodies are raw passthrough bytes; this component does not decode them as CAS metadata.
+///
+/// The component holds references to the shared `Backend` and `Layout` only. It owns no pool mutex
+/// and has no pool back-reference, allowing `Pool` to retain thin forwarding methods with the same
+/// external interface. The private helpers implement the shared head-plus-conditional-write and
+/// head-plus-exact-delete protocols used by both object families. A conditional outcome means that
+/// the observed incarnation changed, so the helper re-reads the head and retries; the fixed bound
+/// prevents an unexpected continuous conflict from becoming an unbounded operation and reports
+/// `ABORTED` when it is reached.
+///
+/// Every durable write/delete on this surface is fence-generation-gated (rev.7 [C2]): `Pool` injects
+/// two callbacks that reach its `mount_runtime` (declared AFTER this member, hence constructed
+/// after it -- these callbacks capture `Pool` itself and are invoked only at runtime, post-
+/// construction, exactly like `ref_ledger`'s callbacks in `CasPool.cpp`, so referencing a
+/// not-yet-constructed sibling member through them is safe).
+class CasPlainObjects
+{
+public:
+    CasPlainObjects(
+        Backend & backend_, const Layout & layout_,
+        std::function<uint64_t()> fence_generation_fn_,
+        std::function<void(uint64_t)> check_fence_or_throw_fn_)
+        : backend(backend_), layout(layout_)
+        , fence_generation_fn(std::move(fence_generation_fn_))
+        , check_fence_or_throw_fn(std::move(check_fence_or_throw_fn_))
+    {
+    }
+
+    /// Stores the raw bytes under ONE LIFE's `_files/` prefix. Existing files are replaced
+    /// conditionally using the object incarnation observed by `Backend::head`; a storage failure or an
+    /// exhausted conflict-retry bound is propagated as an exception.
+    ///
+    /// `life` is supplied by the caller and never re-derived here, so this surface issues no catalog
+    /// request of its own. A stale writer therefore targets its own old incarnation's key and cannot
+    /// write into a newer life's prefix.
+    void putNamespaceFile(const NamespaceLifeId & life, const String & name, const String & bytes);
+
+    /// Reads a namespace file of ONE LIFE without interpreting its body. Returns `nullopt` when the
+    /// object is absent and propagates backend read failures. A stale reader may see stale bytes or
+    /// `NotFound`, never a newer incarnation's data: its key names the life it was given.
+    std::optional<String> getNamespaceFile(const NamespaceLifeId & life, const String & name);
+
+    /// Enumerates the file names directly below ONE LIFE's `_files/` prefix. Fetches all paginated
+    /// backend results, strips the prefix, and returns names in sorted order independent of backend
+    /// listing order.
+    std::vector<String> listNamespaceFiles(const NamespaceLifeId & life);
+
+    /// Removes the current OBJECT incarnation of one of a life's files, if any (the object token, not
+    /// the namespace incarnation, which `life` fixes). A concurrent replacement is never removed
+    /// accidentally: the exact-delete helper re-reads and retries with the new token.
+    void removeNamespaceFile(const NamespaceLifeId & life, const String & name);
+
+    /// Stores raw bytes for a loose mountpoint file at the path-derived object key. The key is
+    /// validated and constructed by `Layout`; this method applies the same conditional overwrite
+    /// protocol as namespace files.
+    void putMountpointObject(const String & key, const String & bytes);
+
+    /// Reads a path-mirrored mountpoint object as raw bytes. Returns `nullopt` for an absent object
+    /// and propagates backend read failures.
+    std::optional<String> getMountpointObject(const String & key);
+
+    /// Checks only object metadata, not the body. A directory at the path-derived key is therefore
+    /// reported as absent, matching object-store semantics and avoiding a filesystem exception from
+    /// attempting to read a directory as an object.
+    bool mountpointObjectExists(const String & key);
+
+    /// Removes the current path-mirrored mountpoint-object incarnation, if present, using exact-token
+    /// deletion so a concurrent rewrite remains intact.
+    void removeMountpointObject(const String & key);
+
+private:
+    /// Creates or conditionally replaces one raw object. The method re-heads after a conditional
+    /// conflict and throws `ABORTED` after the bounded retry loop cannot establish a stable token.
+    /// Fence-generation-gated (rev.7 [C2]): captures the fence generation at admission for the call's
+    /// whole retry loop; every iteration re-checks it immediately before its durable PUT.
+    void casPutObject(const String & full_key, const String & bytes);
+
+    /// Reads one raw object by its complete backend key and returns `nullopt` when it is absent. A read,
+    /// not a durable-effect operation -- NOT fence-gated (rev.7 [C2] scopes the gate to durable writes).
+    std::optional<String> casGetObject(const String & full_key);
+
+    /// Removes one raw object by exact token. Absence is a successful no-op; a token mismatch causes
+    /// a fresh head and retry, while a bounded retry failure throws `ABORTED`. Fence-generation-gated
+    /// the same way as `casPutObject`.
+    void casRemoveObject(const String & full_key);
+
+    Backend & backend;
+    const Layout & layout;
+
+    /// ---- fence-generation admission (injected by `Pool`; see the class doc comment) ----
+    std::function<uint64_t()> fence_generation_fn;
+    std::function<void(uint64_t)> check_fence_or_throw_fn;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.cpp
new file mode 100644
index 000000000000..d7334ecc2c96
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.cpp
@@ -0,0 +1,1851 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/ReadHelpers.h>
+#include <IO/WriteBufferFromString.h>
+#include <IO/WriteHelpers.h>
+#include <Common/Exception.h>
+#include <Common/logger_useful.h>
+#include <Common/setThreadName.h>
+#include <Common/thread_local_rng.h>
+#include <base/scope_guard.h>
+#include <fmt/format.h>
+#include <algorithm>
+#include <chrono>
+#include <ctime>
+#include <thread>
+#include <unordered_set>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int NOT_IMPLEMENTED;
+    extern const int ABORTED;
+    extern const int BAD_ARGUMENTS;
+    extern const int CORRUPTED_DATA;
+    extern const int FILE_DOESNT_EXIST;
+    extern const int LIMIT_EXCEEDED;
+    extern const int LOGICAL_ERROR;
+    extern const int INVALID_STATE;
+}
+}
+
+namespace ProfileEvents
+{
+    extern const Event CASPartFolderManifestGets;
+    extern const Event CASRemountHeldTransient;
+    extern const Event CASRefBatchFlushes;
+    extern const Event CASRefBatchedMutations;
+    extern const Event CASRefBatchScopeCuts;
+    extern const Event CASRefQueueWaitMicroseconds;
+    extern const Event CASRefRecoveryRestarts;
+    extern const Event CASRefRecoveryRetries;
+    extern const Event CASRefAppendWedged;
+    extern const Event CASRefAppendUnwedged;
+    extern const Event CASRefAppendDefiniteFailure;
+    extern const Event CASRefSweepDeferred;
+    extern const Event CASRefSweepRearmed;
+    extern const Event CASRefStalePrecommitsReclaimed;
+    extern const Event CASRefTableEvictions;
+    extern const Event CASRefSnapshotPutBytes;
+    extern const Event CASRefSnapshotTailLogs;
+    extern const Event CASRefSnapshotPublishDispatched;
+    extern const Event CASRefSnapshotPublishBackoff;
+    extern const Event CASDeduplicationCacheHits;
+    extern const Event CASDeduplicationCacheMisses;
+}
+
+namespace CurrentMetrics
+{
+    extern const Metric CASDeduplicationCacheBytes;
+    extern const Metric CASDeduplicationCacheEntries;
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// The verdict of the pool-lifecycle identity gate (step 0 of `tryRemountOnce`, spec §2). Exactly one
+/// `Recover` path falls through to the existing fresh-incarnation recovery; every other verdict is
+/// resolved by the gate itself (a terminal transition, or staying transient to retry).
+enum class LifecycleGateVerdict : uint8_t
+{
+    Recover,        /// `_pool_meta` present + identity matches: proceed with the existing recovery.
+    Replaced,       /// `_pool_meta` present + a FOREIGN pool_id: `Vanished(replaced)` immediately.
+    IdentityLost,   /// both sentinels (`_pool_meta` + owner) authoritatively absent: fail-loud terminal.
+    StayTransient,  /// a probe error, an undecodable meta, or any ambiguous observation: retry as today.
+};
+
+struct LifecycleGate
+{
+    LifecycleGateVerdict verdict;
+    String reason;   /// human-readable detail for the WARN / typed error (only meaningful for Replaced).
+};
+
+/// Authoritative, cache-bypassing evaluation of the §2 verdict table. Reads ONLY; never claims,
+/// allocates, or writes. `expected_pool_id`/`expected_blob_header_len` are the identity this Pool
+/// established at open — the comparison is over those two fields ONLY ([B6]); `algos_used` and
+/// `min_reader_generation` are legally mutable and are deliberately not compared (the format gate is the
+/// decode itself succeeding).
+LifecycleGate probePoolLifecycleGate(
+    Backend & backend, const Layout & layout, const String & srid,
+    UInt128 expected_pool_id, uint64_t expected_blob_header_len)
+{
+    const SentinelProbeResult meta_probe = probeSentinel(backend, layout.poolMetaKey());
+    switch (meta_probe.outcome)
+    {
+        case ProbeOutcome::Present:
+        {
+            /// Format gate = a successful, compatible decode. A present-but-undecodable body proves
+            /// neither identity nor a foreign pool_id, so it stays transient rather than being declared
+            /// replaced (throw-when-uncertain).
+            PoolMeta fresh;
+            try
+            {
+                if (!meta_probe.body)
+                    return {LifecycleGateVerdict::StayTransient, "_pool_meta probed Present without a body"};
+                fresh = decodePoolMeta(*meta_probe.body);
+            }
+            catch (...)
+            {
+                return {LifecycleGateVerdict::StayTransient, "_pool_meta present but could not be decoded"};
+            }
+            if (fresh.pool_id == expected_pool_id && fresh.blob_header_len == expected_blob_header_len)
+                return {LifecycleGateVerdict::Recover, {}};
+            return {LifecycleGateVerdict::Replaced,
+                    fmt::format("data root replaced by a foreign pool (pool_id {} != {})",
+                                u128ToHex(fresh.pool_id), u128ToHex(expected_pool_id))};
+        }
+        case ProbeOutcome::KeyAbsent:
+        {
+            /// `_pool_meta` is authoritatively gone. Require the OTHER sentinel (the owner anchor) to be
+            /// conclusively absent too before declaring identity lost — any surviving sentinel, or an
+            /// undecidable owner probe, keeps us transient (throw-when-uncertain). rev.8: BOTH sentinels
+            /// authoritatively absent ⇒ `IdentityLost` (a fail-loud terminal state), regardless of whatever
+            /// else remains under the prefix. Erasure is never PROVEN by the system — only asserted by the
+            /// operator's `FORGET` — so there is no prefix-emptiness leg here.
+            const SentinelProbeResult owner_probe = probeSentinel(backend, layout.ownerKey(srid));
+            if (owner_probe.outcome != ProbeOutcome::KeyAbsent)
+                return {LifecycleGateVerdict::StayTransient,
+                        "_pool_meta absent but the owner sentinel was not conclusively absent"};
+            return {LifecycleGateVerdict::IdentityLost,
+                    "pool sentinels (_pool_meta + owner) authoritatively absent"};
+        }
+        default:   /// ContainerAbsent / AccessDenied / Indeterminate — absence was never proven.
+            return {LifecycleGateVerdict::StayTransient, "pool-meta probe inconclusive"};
+    }
+}
+
+}
+
+Pool::Pool(BackendPtr backend_, PoolConfig config_, PoolMeta meta_)
+    : pool_backend(std::move(backend_))
+    , config(std::move(config_))
+    , meta(std::move(meta_))
+    /// Seed the monotone admitted-algo cache from the pool state `createOrValidate` already
+    /// established (fresh create, steady-state member, or a just-completed admission union) --
+    /// register-before-first-write means this Pool's own `writeAlgo()` is ALWAYS a
+    /// member by the time the constructor runs.
+    , admitted_algos(meta.algos_used)
+    /// `Layout` no longer captures a pool algo -- every blob key is built from a
+    /// `BlobRef` (algo + digest) directly, so the constructor takes only the pool prefix.
+    , pool_layout(config.pool_prefix)
+    /// Plain-object surface component: binds to this Pool's own backend + layout (declared after
+    /// both, so this reference-holding member is constructed last and destroyed first) plus two
+    /// fence-generation callbacks reaching `mount_runtime` (declared AFTER `plain_objects`, hence
+    /// constructed after it -- these callbacks capture `this` and are invoked only at runtime,
+    /// post-construction, exactly like `ref_ledger`'s callbacks below, so referencing a
+    /// not-yet-constructed sibling member through them is safe).
+    , plain_objects(
+          *pool_backend, pool_layout,
+          [this] { return mount_runtime.fenceGeneration(); },
+          [this] (uint64_t gen) { mount_runtime.checkFenceOrThrow(gen); })
+    /// Manifest reader component: backend/layout/meta by reference + the event-sink reference. The
+    /// sink is installed by the factory before writable mounting starts. Owns the decode cache,
+    /// built from the same config bytes the Pool ctor used before.
+    , manifest_reader(*pool_backend, pool_layout, meta, event_sink_, config.manifest_decode_cache_bytes)
+    /// Ref-log / ref-table subsystem. Injected with backend/layout + the
+    /// RefLedgerConfig slice + the event-sink reference + the pool `cas_request_budget` + the RAW mount
+    /// `boot_ms_fn` (for its retry controller), plus callbacks into the mount/watermark state that lives
+    /// on `mount_runtime` (reached through Pool delegates). The callbacks capture `this`; they are
+    /// invoked only at runtime (post-construction), so referencing `mount_runtime` (declared AFTER
+    /// `ref_ledger`, hence constructed after it) is safe -- exactly as the pre-3.5 layout referenced the
+    /// mount raw-members that also followed `ref_ledger`. Declared/constructed BEFORE `mount_runtime`,
+    /// preserving the original member order verbatim (see the header note).
+    , ref_ledger(
+          pool_backend, pool_layout, config.refLedgerConfig(), event_sink_, config.cas_request_budget,
+          config.server_root_id,
+          config.boot_ms_fn,
+          [this] { return liveWriterEpoch(); },
+          [this] { return refAppendFenceOk(); },
+          [this] { return mount_runtime.fenceGeneration(); },
+          [this] (uint64_t gen) { mount_runtime.checkFenceOrThrow(gen); },
+          [this] { return bootMsNow(); },
+          [this] { return mayMutate(); },
+          [this] (const String & key, const String & reason, const std::optional<String> & offending_ns)
+              { reportImpossibleInterference(key, reason, offending_ns); },
+          [this] { return std::static_pointer_cast<void>(shared_from_this()); },
+          [this] (const RootNamespace & ns) { cancelInflightBuildsForNamespace(ns); })
+    /// Mount / write-fence / build-watermark / self-remount runtime. Injected with
+    /// backend/layout + the `MountConfig` slice + `server_root_id` + the event-sink reference + the pool
+    /// `cas_request_budget` + the `remount_attempt` callback (== `Pool::tryRemountOnce`, whose claim/
+    /// recovery ORCHESTRATION stays on Pool). The callback captures `this`; it is invoked only at runtime
+    /// (post-construction). Declared/constructed AFTER `ref_ledger`, preserving the original member order
+    /// verbatim (mount destroyed first, ledger last; both orders proven safe -- see the header note).
+    , mount_runtime(
+          pool_backend, pool_layout, config.mountConfig(), config.server_root_id, event_sink_,
+          config.cas_request_budget,
+          [this] { return tryRemountOnce(); })
+{
+    if (config.deduplication_cache_bytes > 0)
+        dedup_cache = std::make_unique<DeduplicationCache>(
+            "LRU", CurrentMetrics::CASDeduplicationCacheBytes, CurrentMetrics::CASDeduplicationCacheEntries,
+            config.deduplication_cache_bytes, DeduplicationCache::NO_MAX_COUNT, DeduplicationCache::DEFAULT_SIZE_RATIO);
+}
+
+bool Pool::isAlgoAdmitted(BlobHashAlgo algo) const
+{
+    const auto v = static_cast<uint8_t>(algo);
+    std::lock_guard lock(admitted_algos_mutex);
+    return std::binary_search(admitted_algos.begin(), admitted_algos.end(), v);
+}
+
+std::vector<uint8_t> Pool::refreshAdmittedAlgos()
+{
+    /// A direct GET+decode of `_pool_meta`, not a re-run of `createOrValidate`'s admission logic --
+    /// this Pool's OWN algo is already admitted, so all this
+    /// needs is the CURRENT authoritative `algos_used`, unioned into the monotone cache.
+    const auto existing = pool_backend->get(pool_layout.poolMetaKey());
+
+    std::lock_guard lock(admitted_algos_mutex);
+    if (existing)
+    {
+        const PoolMeta fresh = decodePoolMeta(existing->bytes);
+        for (uint8_t v : fresh.algos_used)
+            if (!std::binary_search(admitted_algos.begin(), admitted_algos.end(), v))
+            {
+                admitted_algos.push_back(v);
+                std::sort(admitted_algos.begin(), admitted_algos.end());
+            }
+    }
+    return admitted_algos;
+}
+
+bool Pool::dedupCacheContains(const BlobRef & ref) const
+{
+    /// raw lookup counters on the presence cache itself, disabled
+    /// (nullptr `dedup_cache`) means neither counter moves -- the short-circuit below never reaches the
+    /// probe. `PartWriteTxn::putBlob` calls this seam up to twice on a genuine hit (once to pick the
+    /// HEAD-first branch, once more just to attribute `CASBlobBodyPutAvoided` to the cache -- see
+    /// CasPartWriteTxn.cpp), so `CASDeduplicationCacheHits` counts LOOKUPS, not distinct blobs or putBlob calls. A hit
+    /// does not itself skip the HEAD that follows in putBlob's HEAD-first branch -- it steers the call
+    /// onto that cheap branch instead of an unconditional body stream; the body PUT is what a hit
+    /// actually avoids.
+    if (!dedup_cache)
+        return false;
+    if (dedup_cache->contains(ref))
+    {
+        ProfileEvents::increment(ProfileEvents::CASDeduplicationCacheHits);
+        return true;
+    }
+    ProfileEvents::increment(ProfileEvents::CASDeduplicationCacheMisses);
+    return false;
+}
+
+void Pool::dedupCacheAdd(const BlobRef & ref)
+{
+    if (dedup_cache)
+        dedup_cache->set(ref, std::make_shared<DedupPresent>());
+}
+
+/// ==== mount-runtime delegates ==== The mount lease keeper, the local write
+/// fence, the per-server build watermark, the live-incarnation epoch, and the self-remount recovery
+/// thread live in the `mount_runtime` member (Pool/CasMountRuntime.h); Pool keeps these thin public
+/// forwarders so the wiring, PartWriteTxn, Gc, the ref-ledger callbacks, and every test call site are unchanged.
+uint64_t Pool::bootMs()
+{
+    return CasMountRuntime::bootMs();
+}
+
+uint64_t Pool::bootMsNow() const
+{
+    return mount_runtime.bootMsNow();
+}
+
+bool Pool::mayMutate() const
+{
+    return mount_runtime.mayMutate();
+}
+
+void Pool::tripMountLost()
+{
+    mount_runtime.tripMountLost();
+}
+
+bool Pool::refAppendFenceOk() const
+{
+    return mount_runtime.refAppendFenceOk();
+}
+
+void Pool::setMountDeadline(uint64_t deadline_boot_ms)
+{
+    mount_runtime.setMountDeadline(deadline_boot_ms);
+}
+
+void Pool::armMountFence(UInt128 server_uuid, uint64_t writer_epoch, uint64_t deadline_boot_ms)
+{
+    mount_runtime.armMountFence(server_uuid, writer_epoch, deadline_boot_ms);
+}
+
+String Pool::lifecycleReasonDetail(PoolLifecycle lc) const
+{
+    /// The [D5] per-reason detail (spec §1) — named once here so the typed error and the introspection
+    /// snapshot always agree. No `content-addressed pool '<id>' ` prefix (callers add it if they want one).
+    switch (lc)
+    {
+        case PoolLifecycle::Live:
+        case PoolLifecycle::TransientNotLive:
+            return {};
+        case PoolLifecycle::IdentityLost:
+            return "identity lost — the pool sentinels are absent; access fails loud. Recover by restart or "
+                   "SYSTEM CAS FORGET (a matching-sentinel restore does not auto-revive it).";
+        case PoolLifecycle::VanishedReplaced:
+            return "data root replaced by a foreign pool (pool_id mismatch) — our generation is gone; "
+                   "restart re-registers the name.";
+        case PoolLifecycle::VanishedForgotten:
+        {
+            /// The forgotten detail carries the operator's decommission TIMESTAMP, threaded through
+            /// `enterVanished`'s reason by `forgetDisk` (`vanishedReason()`). A forced-for-test
+            /// `VanishedForgotten` (no real FORGET ran) has no stored reason, so fall back to the static
+            /// [D5] text — it still names the sub-state and keeps "erasure was NOT verified".
+            const String & reason = mount_runtime.vanishedReason();
+            if (!reason.empty())
+                return reason;
+            return "decommissioned by SYSTEM CAS FORGET — erasure was NOT verified; if this "
+                   "was a mistake the data may be intact (restart re-registers the name).";
+        }
+    }
+    return {};   /// unreachable — every `PoolLifecycle` value is handled above (`-Wswitch` enforces it).
+}
+
+void Pool::throwIfLifecycleTerminal() const
+{
+    /// The typed error carries the sub-state in its message so a wrong diagnosis is impossible from the
+    /// first error line (spec §1 [D5]). `Live`/`TransientNotLive` proceed here — the transient class is
+    /// still gated only by the write fence in this task (the full six-class gate is Task 8).
+    const PoolLifecycle lc = mount_runtime.lifecycle();
+    if (lc == PoolLifecycle::Live || lc == PoolLifecycle::TransientNotLive)
+        return;
+    throw Exception(ErrorCodes::INVALID_STATE,
+        "content-addressed pool '{}' {}", config.server_root_id, lifecycleReasonDetail(lc));
+}
+
+Pool::LifecycleSnapshot Pool::lifecycleSnapshot() const
+{
+    /// Non-gated, I/O-free (spec §7). Read the lifecycle ONCE (acquire), then the detail/`since` it
+    /// implies. `lifecycleReasonDetail` reads `vanishedReason()` only for a terminal state we have already
+    /// acquire-observed here, and `since` was release-stored before the same transition — so this coherent
+    /// triple never mixes a terminal state with a pre-terminal detail/timestamp.
+    LifecycleSnapshot snap;
+    snap.lifecycle = mount_runtime.lifecycle();
+    snap.detail = lifecycleReasonDetail(snap.lifecycle);
+    snap.since = mount_runtime.lifecycleSinceWallS();
+    return snap;
+}
+
+PoolPtr Pool::open(BackendPtr backend, PoolConfig config)
+{
+    /// Wrap the pool backend once, transparently, so EVERY CA S3 op — probe, pool-meta,
+    /// writer, GC, watermark — flows through the per-namespace/op ProfileEvents chokepoint. The
+    /// decorator only delegates and counts; it changes no behavior (read-only opens stay write-free).
+    backend = std::make_shared<InstrumentedBackend>(std::move(backend));
+
+    /// FAIL-CLOSED: the capability probe throws NOT_IMPLEMENTED on any failed check, and
+    /// PoolMeta::createOrValidate is pool-authoritative — the config constants apply only at creation.
+    Layout layout(config.pool_prefix);
+    bool initialize_empty_catalog = false;
+    /// The probe writes and deletes throwaway keys to verify conditional-op enforcement. A read-only
+    /// open must never mutate the pool it inspects; fsck only reads, so skip it. (Pool meta below is
+    /// read-only when the pool already exists; a missing pool meta on a read-only backend fails closed.)
+    if (!config.read_only)
+    {
+        /// (0) [C4][D2] Zero-write residual check FIRST — before ANY probe write. `pool_prefix` is
+        /// EXCLUSIVELY CAS-owned; `createOrValidate` below may mint a fresh `_pool_meta` only over a
+        /// genuinely empty prefix. The MUTATING capability battery must run AFTER this proof (it writes
+        /// `_probe/` debris, which would itself make the prefix look non-empty), and the emptiness
+        /// classification IGNORES structurally-valid `_probe/` debris so a normal restart after a
+        /// crash-mid-battery still bootstraps cleanly. This closes the "restart poisons a
+        /// partially-erased pool" hole: a missing `_pool_meta` over residual data now fails startup loud
+        /// with zero writes, instead of minting a fresh identity on top of the old objects.
+        switch (probePoolBootstrapResidual(*backend, layout))
+        {
+            case BootstrapResidual::PoolMetaPresent:
+                break;   /// authoritative existing pool; its catalog is mandatory below.
+            case BootstrapResidual::EmptyOrProbeOnly:
+            case BootstrapResidual::CanonicalEmptyCatalogOnly:
+                initialize_empty_catalog = true;
+                break;   /// proven-new or canonical catalog-only pre-meta bootstrap state.
+            case BootstrapResidual::ResidualWithoutMeta:
+            {
+                /// RECREATION QUIESCE. Reaching here means the prefix holds objects but no authoritative
+                /// `_pool_meta` -- the shape a pool recreation leaves behind. Before telling the operator
+                /// anything about residual data, ask the one question whose answer changes the remedy: is
+                /// a writer still entitled to this prefix? Refusing an OLD-FORMAT open fences nothing --
+                /// a server that mounted before the erase is still running, still holds its lease, and
+                /// still has queued writes; if the operator answers the residual message below by
+                /// clearing the prefix and recreating, that writer's next flush lands its old-format
+                /// transactions inside the NEW pool. So a non-terminal slot fails closed with its own
+                /// message: stop the writer first.
+                ///
+                /// This gate is the PRIMARY defence, not a nicety, because the mount fence cannot be
+                /// relied on to catch a straggler afterwards. Clearing the prefix also destroys the
+                /// durable writer-epoch counter, so a recreation by the SAME server uuid is handed the
+                /// very `(uuid, epoch)` the survivor still holds -- and the two are then indistinguishable
+                /// to the lease protocol, which reads the survivor's renewal as its own keeper adopting a
+                /// refreshed body. The fence only bites when the recreating mount is DISTINGUISHABLE (a
+                /// different server uuid, or a surviving epoch counter): then the survivor's next renewal
+                /// finds a slot it cannot hold and its local fence latches shut.
+                ///
+                /// What catches the ambiguous case is INV-1, after the fact: the straggler's append lands
+                /// at `{E, 1}`-relative ids in a table the recreated pool sees as empty, and the first
+                /// recovery of that namespace refuses the stream as non-contiguous rather than absorbing
+                /// it. That is a loud post-mortem, not prevention -- which is why the refusal here, BEFORE
+                /// anything is cleared, is the one that matters.
+                ///
+                /// Only on this arm: `EmptyOrProbeOnly` proves there is no slot object to read (a mount
+                /// lease is itself residual), and `PoolMetaPresent` is not a recreation at all -- neither
+                /// pays for the scan.
+                const std::vector<NonTerminalMountSlot> held = probeNonTerminalMountSlots(*backend, layout);
+                if (!held.empty())
+                {
+                    String detail;
+                    for (const NonTerminalMountSlot & slot : held)
+                        detail += fmt::format("\n  server root '{}': {}", slot.server_root_id, slot.detail);
+                    throw Exception(ErrorCodes::INVALID_STATE,
+                        "content-addressed pool '{}' (prefix '{}'): missing _pool_meta, but {} mount "
+                        "lease(s) under this prefix are still held — refusing to recreate the pool while "
+                        "a writer may still be using it. Stop (or decommission) the holder(s) so their "
+                        "mount slots become terminal, then retry; do NOT clear the prefix first, which "
+                        "would leave the surviving writer appending into the new pool.{}",
+                        config.server_root_id, config.pool_prefix, held.size(), detail);
+                }
+                throw Exception(ErrorCodes::INVALID_STATE,
+                    "content-addressed pool '{}' (prefix '{}'): missing _pool_meta over a non-empty pool "
+                    "prefix — refusing to bootstrap over residual data; recreate the pool or restore "
+                    "_pool_meta. The pool prefix is exclusively CAS-owned.",
+                    config.server_root_id, config.pool_prefix);
+            }
+            case BootstrapResidual::Indeterminate:
+                throw Exception(ErrorCodes::INVALID_STATE,
+                    "content-addressed pool '{}' (prefix '{}'): could not authoritatively list the pool "
+                    "prefix to prove it is safe to bootstrap — refusing to create _pool_meta while "
+                    "residual data cannot be ruled out (fail-closed).",
+                    config.server_root_id, config.pool_prefix);
+        }
+
+        if (!config.skip_access_check)
+        {
+            /// Give each mount a PER-MOUNT UNIQUE probe key prefix so two servers mounting the SAME
+            /// shared pool concurrently never collide on the (formerly fixed) `<pool>/_probe/token` /
+            /// `<pool>/_probe/cas` keys. Without this, the loser of the `putIfAbsent` race aborts startup
+            /// with PreconditionFailed (and the winner's cleanup delete can cascade into the loser). With a
+            /// fresh random 128-bit id per `Pool::open`, each mounter validates conditional-op support
+            /// independently. A crashed mount leaves harmless `_probe/<rand>/...` debris under the `_probe/`
+            /// namespace only (never the content planes) — acceptable.
+            const UInt128 probe_uid = (static_cast<UInt128>(thread_local_rng()) << 64) | thread_local_rng();
+            runCapabilityProbe(*backend, config.pool_prefix + "/_probe/" + u128ToHex(probe_uid));
+        }
+        else
+        {
+            /// skip_access_check: skip the access-check-class probe I/O (store preconditions + the
+            /// `_probe/` round trip, both folded into runCapabilityProbe above) but NOT the
+            /// single-attempt conditional-write gate — see `PoolConfig::skip_access_check`. Run it
+            /// directly so a Native-mode backend with no working single-attempt client still fails
+            /// closed at open instead of silently corrupting CAS state under blind retries later.
+            /// The skipped store-precondition check also covers GCS bucket-versioning/delete-marker
+            /// detection — that risk is purely environmental (slower GC reclaim, not data loss) and
+            /// gets re-checked the next time this pool is opened without skip_access_check.
+            backend->checkConditionalWriteSingleAttemptSupport();
+        }
+    }
+    /// The catalog is mandatory for every minted pool. Make it durable before `_pool_meta`: otherwise
+    /// an acknowledgement-loss or definite catalog-write failure could strand an authoritative meta
+    /// that makes every later open refuse the absent catalog. The catalog-only residual proof above is
+    /// the narrowly-defined retry path when this opener (or a concurrent opener) completed this step
+    /// but did not reach the pool-meta create.
+    if (initialize_empty_catalog)
+        CasRefCatalog::initializeEmptyForNewPool(*backend, layout);
+    /// `allow_mint` = writable open only: a writable `Pool::open` reaches here having just passed the
+    /// zero-write residual proof above, so minting a missing `_pool_meta` is safe. A read-only/observe
+    /// open never ran that proof (and there is no truly-read-only backend — `openPoolView` opens the same
+    /// writable object storage and only sets `read_only`), so it must NEVER mint: an absent meta fails
+    /// closed instead (spec §2 [C4][D2]).
+    PoolMeta meta = PoolMeta::createOrValidate(
+        *backend, layout, config.blob_header_len, config.gc_shards, config.blob_hash_algo, config.blob_hash_allow_new,
+        /*allow_mint=*/!config.read_only);
+    config.gc_shards = meta.gc_shards;
+    const BlobHashAlgo write_algo = config.blob_hash_algo;   /// `config` is moved-from just below
+
+    /// Private ctor: make_shared cannot reach it.
+    PoolPtr store(new Pool(std::move(backend), std::move(config), std::move(meta)));
+    store->setEventSink(std::move(store->config.event_sink));
+
+    /// Register-before-first-write, belt-and-braces: `createOrValidate` above already
+    /// admitted/validated the write algo, so the freshly-seeded cache must already contain it -- a
+    /// violation here would mean a build/write could reach this Pool naming an algo that was never
+    /// durably admitted (the invariant this whole design rests on).
+    chassert(store->isAlgoAdmitted(write_algo));
+
+    /// Per-server watermark: mint the random NONZERO `process_epoch`
+    /// once per Pool (GC checks it for equality only -- a different epoch == a dead incarnation). The
+    /// masking/redraw detail lives in `CasMountRuntime::mintRandomProcessEpoch`.
+    store->mount_runtime.mintRandomProcessEpoch();
+
+    /// W-ANCHOR: the per-server watermark must be durable BEFORE any object PUT. A read-only open
+    /// must never mutate the pool (the probe is skipped above for the same reason), so the watermark
+    /// — which rides inside the `gc/server-roots/<server_root_id>/mount` lease object — is only
+    /// constructed and anchored on a writable open.
+    if (!store->config.read_only)
+        mountWritable(store, store->config.server_id, MountClaimPolicy::WaitForExpiry);
+
+    return store;
+}
+
+void Pool::mountWritable(PoolPtr & store, UInt128 our_uuid, MountClaimPolicy policy)
+{
+    /// === Mount-safety startup protocol ===
+    /// STRICT ORDER: validate id → claim owner (identity) → allocate durable writer_epoch → claim
+    /// the mount lease (liveness) + arm the local write fence → anchor the watermark. owner / epoch
+    /// / mount / watermark are BOOTSTRAP-CONTROL writes: they establish the very right to write and
+    /// run BEFORE the write fence gates ordinary data/ref/manifest mutations. Fail closed throughout.
+    /// Shared by `open` (`policy = WaitForExpiry`, `our_uuid = config.server_id`) and
+    /// `openForDecommission` (`policy = NoWait`, `our_uuid` = the impersonated victim owner uuid;
+    /// -- the two differ only in WHO they mount
+    /// as and whether a non-`Claimed`/`FencedSelf` mount result gets `open`'s bounded observation wait
+    /// or an immediate refusal.
+    const String & srid = store->config.server_root_id;
+
+    /// 1. The server_root_id is a clean relative path (mirrors the config-read validation; cheap to
+    ///    re-check here so a Pool opened directly in tests is held to the same contract).
+    validateServerRootId(srid);
+
+    const ObserveRefCatalog observe_catalog = [s = store.get()]()
+    {
+        CasRefCatalog::Snapshot snapshot = CasRefCatalog::read(*s->pool_backend, s->pool_layout);
+        snapshot.life_index.throwIfAmbiguous("CAS server-root mount safety");
+        return snapshot.catalog;
+    };
+    /// Existing matching owner and epoch objects take fast paths that do not need an emptiness
+    /// observation. Validate the mandatory authority object unconditionally before either fast path
+    /// can reach a slot mutation; the callback remains available for fresh conflict rechecks below.
+    (void)observe_catalog();
+
+    /// 2. Owner anchor — IDENTITY (clock-free). A foreign uuid fails closed; an absent owner over a
+    ///    non-empty subtree is CORRUPTED_DATA; a fresh empty root is claimed.
+    claimOwnerOrThrow(*store->pool_backend, store->pool_layout, srid, our_uuid, observe_catalog);
+
+    /// Wall-clock `now_ms`, hoisted above the writer_epoch allocation below: the absent-epoch
+    /// branch's `DecommissionRecovery` policy needs it to judge a surviving mount's liveness before
+    /// the mount-lease claim (step 4) gets its own use of the same clock.
+    const auto now_ms = []() -> uint64_t
+    {
+        return static_cast<uint64_t>(std::chrono::duration_cast<std::chrono::milliseconds>(
+            std::chrono::system_clock::now().time_since_epoch()).count());
+    };
+
+    /// 3. Durable-monotone writer_epoch — CAS-bump the sticky `epoch` object. THE BRIDGE: this
+    ///    durable value REPLACES the random `process_epoch` for identity, so the watermark + every
+    ///    manifest ref carries it (the random mint above stays for the read-only
+    ///    path, which never reaches here). The epoch-aware sweep reads this value.
+    /// Mutable: a GC fence of our fresh lease during open (expiry mid-open racing a GC round) is
+    /// recoverable — a fence costs an epoch, so the fence-recovery loop below re-allocates a fresh
+    /// writer_epoch and re-claims (the TLA+-checked `NoPermanentWedge` invariant).
+    /// `epoch_policy`: `openForDecommission`'s `NoWait` gates the absent-epoch branch's mount-probe
+    /// on a TERMINAL (not live) surviving mount instead of authoritative absence (Phase C) — passed
+    /// uniformly to every call below, including the fence-recovery re-allocations, where it is inert
+    /// because those run with the epoch object already present.
+    const EpochMintPolicy epoch_policy = (policy == MountClaimPolicy::NoWait)
+        ? EpochMintPolicy::DecommissionRecovery
+        : EpochMintPolicy::NormalMount;
+    uint64_t writer_epoch = allocateWriterEpoch(
+        *store->pool_backend, store->pool_layout, srid, epoch_policy, now_ms(), observe_catalog);
+    store->mount_runtime.setProcessEpoch(writer_epoch, std::memory_order_relaxed);
+
+    /// 4. Mount lease — LIVENESS. Decide over the current mount object using the wall-clock `now_ms`
+    ///    hoisted above.
+    const uint64_t ttl_ms = static_cast<uint64_t>(store->config.mount_lease_ttl_ms.count());
+
+    /// CAS request budget: a
+    /// writable mount refuses to open with a budget that could let a controlled attempt outlive the
+    /// mount lease it is fenced under. Throws BAD_ARGUMENTS and aborts open on an inconsistent
+    /// budget; logs the effective values once on success. The controller gates Pool's ref-mutation
+    /// paths; this validates the config invariant up front, before any attempt runs.
+    validateCasRequestBudget(store->config.cas_request_budget, ttl_ms,
+        static_cast<uint64_t>(store->config.mount_renew_period.count()));
+
+    /// Poll twice per renew period so a live holder's renewal is always observed within the
+    /// observation window. Derived from existing config — no new knob.
+    const uint64_t poll_interval_ms = std::max<uint64_t>(
+        1, static_cast<uint64_t>(store->config.mount_renew_period.count()) / 2);
+    /// routes through `mount_runtime.waitSleep` (which itself routes through
+    /// `config.wait_sleep_fn` when a test injected one) rather than a bare `sleep_for` directly, so
+    /// a test intercepting `wait_sleep_fn` observes every wait `open` can block on -- and since the
+    /// post-reclaim materialization grace was retired, this observation poll is the only one left.
+    const auto sleep_ms = [s = store.get()](uint64_t ms) { s->mount_runtime.waitSleep(ms); };
+    /// Operator-visible log the moment startup decides to watch a stale-looking self-mount (the
+    /// disk-open path blocks up to ~threshold_ms here, so a silent block would be confusing). May
+    /// fire more than once per open: the observation restarts (and re-logs) every time the watched
+    /// lease's write-token changes before the full threshold elapses.
+    const auto on_wait_start = [&srid](const MountLease & held, uint64_t threshold_ms)
+    {
+        LOG_INFO(getLogger("CasPool"),
+            "CAS mount '{}': a stale-looking mount lease is held by uuid={} epoch={} pid={} "
+            "hostname={} (expires_at_ms={}); observing its write-token for up to ~{} ms before "
+            "reclaiming. If a second server is genuinely live, its renewals will keep restarting "
+            "the observation and startup will eventually abort as a live double-start.",
+            srid, u128ToHex(held.server_uuid), held.writer_epoch, held.pid, held.hostname,
+            held.expires_at_ms, threshold_ms);
+    };
+
+    /// Mount-slot writer audit (the "foreign writer" instrument): route every mount-slot
+    /// write/conflict event through the Pool's own sink. The factory installs the configured sink
+    /// before this mount protocol starts, including before any renewal thread can emit.
+    /// `s` outlives the lambda: it is captured by raw pointer into the keeper, a member of
+    /// `Pool` destroyed before the `Pool` itself.
+    const auto emit_mount_event = [s = store.get()](CasEvent e) { s->emitEvent(std::move(e)); };
+
+    Pool * raw = store.get();
+
+    /// Crash-recovery (`WaitForExpiry` only): a hard-killed prior incarnation leaves a stale,
+    /// unreleased mount lease. Rather than aborting, OBSERVE that lease's write-token (never its
+    /// stamped `expires_at_ms` against our wall clock) until it has held stable for the full
+    /// rate-bound threshold, then reclaim it; a genuinely live second server keeps renewing the
+    /// token and is (after bounded restarts) reported as LiveDoubleStart. The reclaim is
+    /// token-guarded (see `claimMountAwaitingExpiry`), so a live twin is never stolen from.
+    /// `NoWait` skips this observation entirely (see the policy branch below).
+    ///
+    /// Fence-recovery loop: if the GC fences our own fresh lease while we are opening
+    /// (the lease expired mid-open — e.g. a slow first beat — and a GC round fenced it), that is a
+    /// RECOVERABLE state, not a wedge: a fence costs an epoch, so allocate a fresh writer_epoch and
+    /// re-claim. Bounded so a pathological fence storm still fails closed. The fence can surface two
+    /// ways: `claimMount` observes an already-fenced own slot (`FencedSelf`), or the keeper's adopt
+    /// races a fence between its GET and CAS (`MountFencedException` from `start()`).
+    /// which certificate of death (if any) justified the reclaim FINALLY adopted below
+    /// (the last iteration's `claim` before `break` -- `claim` itself is loop-scoped). Read after the
+    /// loop to classify (and log) an unclean reclaim.
+    MountPriorState claimed_prior = MountPriorState::None;
+    /// The pre-I/O boot-clock instant of the claim attempt FINALLY adopted below -- survives the
+    /// `break` so the arm below can detect a claim that consumed the lease TTL and re-anchor before
+    /// arming (rev.4 Phase B, round-3 finding 2).
+    uint64_t claim_anchor_boot_ms = 0;
+    constexpr int max_fence_recoveries = 3;
+    for (int fence_recovery = 0; ; ++fence_recovery)
+    {
+        MountClaimResult claim;
+        if (policy == MountClaimPolicy::WaitForExpiry)
+        {
+            claim = claimMountAwaitingExpiry(
+                *store->pool_backend, store->pool_layout, srid, our_uuid, writer_epoch,
+                [&now_ms]() { return now_ms(); }, [raw] { return raw->bootMsNow(); },
+                ttl_ms, poll_interval_ms, sleep_ms, on_wait_start, emit_mount_event);
+        }
+        else
+        {
+            /// NoWait (decommission gate): a single unobserved attempt -- no bounded wait-and-retry
+            /// for a stale-looking lease to lapse. Anything but Claimed/FencedSelf below is refused
+            /// immediately.
+            claim = claimMount(*store->pool_backend, store->pool_layout, srid, our_uuid, writer_epoch,
+                now_ms(), ttl_ms, /*proven_dead_token=*/{}, emit_mount_event);
+        }
+        if (claim.kind == MountClaimResult::FencedSelf)
+        {
+            if (fence_recovery >= max_fence_recoveries)
+                throw Exception(ErrorCodes::ABORTED,
+                    "CAS mount '{}': our own mount lease was GC-fenced repeatedly during open "
+                    "({} recoveries exhausted) — a fresh writer_epoch kept being fenced before we "
+                    "could adopt it. This should not persist; investigate GC fence-out timing.",
+                    srid, max_fence_recoveries);
+            writer_epoch = allocateWriterEpoch(
+                *store->pool_backend, store->pool_layout, srid, epoch_policy, now_ms(), observe_catalog);
+            store->mount_runtime.setProcessEpoch(writer_epoch, std::memory_order_relaxed);
+            continue;
+        }
+        if (claim.kind != MountClaimResult::Claimed)
+        {
+            if (policy == MountClaimPolicy::NoWait)
+                /// No FORCE variant, no wait-and-observe: the decommission gate treats any live-looking
+                /// or foreign-owner lease as an immediate refusal.
+                throw Exception(ErrorCodes::ABORTED,
+                    "CAS decommission '{}': pool member is alive or contended — mount lease held by "
+                    "uuid={} epoch={} pid={} hostname={} (expires_at_ms={}). Refusing (no FORCE variant "
+                    "exists; stop the server or wait for its lease to lapse).",
+                    srid, u128ToHex(claim.body.server_uuid), claim.body.writer_epoch, claim.body.pid,
+                    claim.body.hostname, claim.body.expires_at_ms);
+            /// LiveDoubleStart (waited out the bound → a live twin) or ForeignOwner → fail closed
+            /// with the actionable, multi-line startup error.
+            throw Exception(ErrorCodes::ABORTED, "{}", mountDoubleStartMessage(srid, claim.body));
+        }
+        claimed_prior = claim.prior;
+
+        /// The mount object now holds OUR live (uuid, epoch) body. `installKeeper` constructs the keeper
+        /// -- which ADOPTS that very (uuid, epoch) slot rather than self-tripping the double-start guard --
+        /// AND wires its `minActive` build-watermark reader, its event sink, and the fence-coupling
+        /// callbacks (renew-ok refreshes the monotonic deadline; on-lost latches the fence + arms a
+        /// self-remount), all captured on `mount_runtime` (see `CasMountRuntime::installKeeper`).
+        /// `keeperStart` is separate so this claim orchestration can catch `MountFencedException` and
+        /// retry with a fresh epoch.
+        store->mount_runtime.installKeeper(our_uuid, writer_epoch, now_ms);
+        claim_anchor_boot_ms = store->bootMsNow();   /// pre-I/O anchor of the claim attempt
+        try
+        {
+            store->mount_runtime.keeperStart();
+        }
+        catch (const MountFencedException &)
+        {
+            /// The GC fenced our fresh lease between the keeper's adopt GET and CAS. Recoverable:
+            /// drop this keeper, take a fresh epoch, and re-claim.
+            if (fence_recovery >= max_fence_recoveries)
+                throw;
+            store->mount_runtime.keeperReset();
+            writer_epoch = allocateWriterEpoch(
+                *store->pool_backend, store->pool_layout, srid, epoch_policy, now_ms(), observe_catalog);
+            store->mount_runtime.setProcessEpoch(writer_epoch, std::memory_order_relaxed);
+            continue;
+        }
+        break;
+    }
+
+    /// A reclaim over a predecessor whose death was NOT proven clean may still have a conditional PUT
+    /// from that predecessor in flight -- `Fenced` and `UncleanObserved` are exactly the two
+    /// `MountPriorState`s with no such proof (`Clean`, drained farewell, and `None`, a fresh mount /
+    /// same-epoch refresh with nothing to hand over, are the proven ones).
+    /// An EXHAUSTIVE switch, not a positive allowlist -- a future `MountPriorState`
+    /// enumerator with no proof of clean death must fail the BUILD (a missing `-Wswitch` case), never
+    /// silently fall through to "clean".
+    ///
+    /// THIS NO LONGER WAITS. The `materialization_grace_ms` (`T_mat`) sleep that used to sit here bought
+    /// one thing: time for that straggler to land (or exhaust its retries) BEFORE this incarnation began
+    /// trusting its recovery LISTINGS. Recovery does not trust listings any more. It walks each ref
+    /// stream arithmetically from `_ckpt` and closes every dead epoch with an in-band `EpochSeal` at
+    /// `{E, T+1}`, written as a conditional create -- so the straggler's own conditional create loses to
+    /// an occupied slot no matter when it arrives, and a wait can only make startup slower, never safer.
+    /// What survives is the CLASSIFICATION and saying it out loud: an unclean predecessor is worth an
+    /// operator-visible line, and the exhaustive switch is worth keeping as the build-time guard.
+    bool unclean_reclaim = false;
+    switch (claimed_prior)
+    {
+        case MountPriorState::None:
+        case MountPriorState::Clean:
+            unclean_reclaim = false;
+            break;
+        case MountPriorState::Fenced:
+        case MountPriorState::UncleanObserved:
+            unclean_reclaim = true;
+            break;
+    }
+    if (unclean_reclaim)
+    {
+        LOG_INFO(getLogger("CasPool"),
+            "Content-addressed mount {} follows a predecessor whose death was not proven clean "
+            "(writer_epoch {}). Opening without a grace period: a still-in-flight conditional PUT from "
+            "that predecessor is fenced by the recovery seal, whenever it arrives.", srid, writer_epoch);
+    }
+
+    /// Arm the local write fence: cache (uuid, epoch) and set the boottime deadline at the claim
+    /// attempt's anchor + ttl (NOT `bootMsNow()` here -- arming from a post-I/O instant would authorize
+    /// mutations under a deadline the durable lease never actually backs). From here ordinary ref
+    /// mutations (appendRefOps) are fence-gated via mayMutate.
+    const uint64_t ttl_ms_u = static_cast<uint64_t>(store->config.mount_lease_ttl_ms.count());
+    if (store->bootMsNow() >= claim_anchor_boot_ms + ttl_ms_u)
+    {
+        /// The claim path outlived the lease TTL: its anchor can no longer authorize an armed fence (a
+        /// successor may have legally started reclaiming). Re-anchor with ONE fresh conditional lease
+        /// write -- it fails closed (Phase A classification) if anything took the slot meanwhile -- and
+        /// arm from the new attempt's anchor (rev.4 Phase B, round-3 finding 2).
+        ///
+        /// The unbounded operator-configured wait this guard was written for (`T_mat`) is gone, so
+        /// reaching it now means the CLAIM ITSELF -- `keeperStart`'s GET+CAS -- outran the whole lease
+        /// TTL, which `validateCasRequestBudget` already refuses to configure. It stays because a stalled
+        /// socket can still outlive a budget, and its recovery is one conditional write that fails closed;
+        /// it is LOUD rather than fatal because a slow open under a healthy protocol is not a reason to
+        /// refuse to start.
+        LOG_WARNING(getLogger("CasPool"),
+            "Content-addressed mount {}: the mount claim consumed the lease TTL ({} ms) before the write "
+            "fence could be armed; re-writing the lease first", srid, ttl_ms_u);
+        claim_anchor_boot_ms = store->bootMsNow();
+        store->mount_runtime.keeperRenewOnce();
+    }
+    store->armMountFence(our_uuid, writer_epoch, claim_anchor_boot_ms + ttl_ms_u);
+    /// Gate the background renewer with `background_watermark`: it runs only in production
+    /// (`background_watermark` = context != nullptr && !read_only), never in unit tests — which
+    /// drive renewOnce (or renewWatermarkOnce) explicitly and rely on the armed sub-TTL deadline,
+    /// never on the loop. The keeper itself is still started above (it must claim/adopt the mount +
+    /// arm the fence on every writable open); only the renewal thread is conditional. The merged
+    /// heartbeat renews at `mount_renew_period` — one beat now renews the lease and the floor.
+    if (store->config.background_watermark)
+        store->mount_runtime.keeperStartBackground(store->config.mount_renew_period);
+
+    store->mount_runtime.setLiveWriterEpoch(writer_epoch);
+}
+
+PoolPtr Pool::openForDecommission(BackendPtr backend, PoolConfig config, const String & victim_srid)
+{
+    backend = std::make_shared<InstrumentedBackend>(std::move(backend));
+    validateServerRootId(victim_srid);
+
+    config.server_root_id = victim_srid;
+    config.read_only = false;
+    config.skip_access_check = true;   /// the pool exists (the calling disk validated it); no probe writes
+    /// The admin claim must be RENEWED like any writable mount: the host disk may be observe-only
+    /// (background_watermark=false), but an unrenewed claim (TTL ~30s) aborts any long drain midway.
+    config.background_watermark = true;
+
+    Layout layout(config.pool_prefix);
+
+    /// Impersonate the victim: decommission acts as "the next incarnation of that server". The claim
+    /// below is then EXACTLY the crash-recovery reclaim semantics (`MountClaimPolicy::NoWait`): a
+    /// fenced/terminated/clean-farewell lease reclaims; a live lease refuses immediately (no bounded
+    /// observation wait -- see `mountWritable`). Owner anchor absent + mount absent = nothing to
+    /// decommission.
+    std::optional<UInt128> victim_uuid = readOwnerUuid(*backend, layout, victim_srid);
+    if (!victim_uuid)
+    {
+        if (const auto mount = backend->get(layout.mountKey(victim_srid)))
+            victim_uuid = decodeMountLease(mount->bytes).server_uuid;   /// partial hand-cleanup: adopt from the lease
+        else
+            throw Exception(ErrorCodes::BAD_ARGUMENTS,
+                "CAS decommission '{}': unknown pool member (no owner anchor and no mount lease). "
+                "Nothing to decommission; if victim objects linger without a slot, run cas-fsck.",
+                victim_srid);
+    }
+    config.server_id = *victim_uuid;
+
+    backend->checkConditionalWriteSingleAttemptSupport();
+    /// Decommission operates on an EXISTING pool member (an owner anchor / mount lease was just found), so
+    /// `_pool_meta` must already be present. It never bootstraps: `allow_mint=false` so an absent meta
+    /// (a partially-erased pool whose owner anchor survives) fails closed with INVALID_STATE rather than
+    /// minting a fresh identity here (spec §2 [C4][D2]).
+    PoolMeta meta = PoolMeta::createOrValidate(
+        *backend, layout, config.blob_header_len, config.gc_shards, config.blob_hash_algo, config.blob_hash_allow_new,
+        /*allow_mint=*/false);
+    config.gc_shards = meta.gc_shards;
+    const BlobHashAlgo write_algo = config.blob_hash_algo;   /// `config` is moved-from just below
+
+    /// Private ctor: make_shared cannot reach it.
+    PoolPtr store(new Pool(std::move(backend), std::move(config), std::move(meta)));
+    store->setEventSink(std::move(store->config.event_sink));
+
+    /// Register-before-first-write, belt-and-braces: same invariant `open` asserts.
+    chassert(store->isAlgoAdmitted(write_algo));
+
+    /// No random `process_epoch` mint here: `open` pays that prologue because its read-only path
+    /// never reaches `mountWritable` and so needs SOME nonzero epoch, but this factory is
+    /// writer-only -- `mountWritable` below unconditionally overwrites `process_epoch` with the
+    /// freshly allocated durable `writer_epoch` before anything could observe the zero-initialized
+    /// default.
+    mountWritable(store, *victim_uuid, MountClaimPolicy::NoWait);
+    return store;
+}
+
+Pool::~Pool()
+{
+    /// Teardown order is load-bearing and unchanged from the pre-3.5 inline sequence (only the
+    /// mount/remount mechanics were relocated into `mount_runtime`):
+    ///
+    /// 1. Stop + join the self-remount recovery thread FIRST (it may otherwise re-create the keeper
+    ///    below us). `stopRemountThread` latches `remount_shutting_down` under the thread mutex before
+    ///    the join, so a keeper on_lost firing during teardown can never re-arm the thread after we join.
+    mount_runtime.stopRemountThread();
+
+    /// 2. The farewell marker the keeper's `stop()` writes is a
+    /// certificate that no in-flight ref-log conditional PUT from this incarnation can land after it --
+    /// a successor treats it as proof of a clean death (`MountPriorState::Clean`, no observation wait
+    /// needed). Writing it without an actual drain would be a protocol-safety bug: an uncertain PUT this
+    /// incarnation is still resolving could land AFTER the successor already reclaimed and started
+    /// mutating. `drainRefLanesForShutdown` is the drain; bounded by one attempt's worth of budget plus
+    /// the lease safety margin -- long enough for an in-flight attempt to resolve, never unbounded. It
+    /// stays on `Pool` (mediating the mount↔ledger coupling), sequenced between the two mount-runtime
+    /// teardown steps exactly as before.
+    const bool ref_lanes_drained = ref_ledger.drainRefLanesForShutdown(
+        config.cas_request_budget.attempt_timeout_ms + config.cas_request_budget.lease_safety_margin_ms);
+    const bool drained = ref_lanes_drained && !writerCleanupDutiesPending();
+
+    /// 3. Retire the merged heartbeat: `finishTeardown` runs the keeper's terminal op on a clean drain
+    /// (stamping the lease already-expired + folding in the watermark farewell so a SAME-server reopen
+    /// reclaims immediately) or the fail-closed no-terminal-op on an unresolved PUT, then does the
+    /// belt-and-suspenders remount-thread re-join. See `CasMountRuntime::finishTeardown`.
+    mount_runtime.finishTeardown(drained);
+}
+
+void Pool::forgetDisk(const std::function<void()> & stop_and_join_gc, const String & reason)
+{
+    /// Hazard C6: FORGET joins the self-remount thread (and, via `stop_and_join_gc`, the GC threads), so it
+    /// MUST run on the admin/query thread — never a pool thread, whose join of itself would deadlock. The
+    /// guard is a programming-error assertion (a self-join hangs; it never corrupts), so a chassert is the
+    /// right severity, not a release fail-close.
+    const ThreadName tn = getThreadName();
+    chassert(tn != ThreadName::CAS_REMOUNT && tn != ThreadName::CAS_GC_SCHEDULER
+             && tn != ThreadName::CAS_GC_HEARTBEAT
+             && "SYSTEM CAS FORGET must not run on a CAS pool thread (self-join deadlock)");
+
+    /// Idempotent: an already-terminal `Vanished` pool (a second FORGET, or a pool that naturally vanished
+    /// as replaced) is already the terminal truth — nothing to force, and re-running the teardown
+    /// would double-retire the keeper. `IdentityLost`/`TransientNotLive`/`Live` all proceed (FORGET is
+    /// their escape hatch). Reading `isVanished()` here without the lock is safe: only a terminal transition
+    /// sets it, terminal states are absorbing, and a natural transition that wins concurrently below merely
+    /// makes our own `enterVanished` a no-op (first terminal transition wins).
+    if (mount_runtime.isVanished())
+        return;
+
+    /// (1) Publish the terminal-intent latch FIRST (spec §5). The keeper callback stops arming remounts and
+    /// the remount loop bails at its next step boundary, so every join below is bounded to one step + one
+    /// backend timeout.
+    mount_runtime.publishVanishedIntent();
+
+    /// (2) Trip the local fence — the deliberate decommission act (allowed on a live disk). No durable-
+    /// effect write admits past this point (the fence-generation gate), and a live pool moves to
+    /// `TransientNotLive`, so store-class access already fails loud during the teardown window below.
+    mount_runtime.tripMountLost();
+
+    /// (3+4) Stop the GC scheduler (clears its leadership and JOINS its worker + heartbeat threads) BEFORE
+    /// the Pool-side teardown, so no round writes `gc/state` under a disk we are decommissioning. Injected
+    /// because the scheduler is owned above the Pool (a no-op in unit / read-only / clickhouse-disks
+    /// contexts that run none). Runs OUTSIDE `remount_mutex` (spec §3 join discipline).
+    if (stop_and_join_gc)
+        stop_and_join_gc();
+
+    /// (5a) Stop + join the self-remount thread. `stopRemountThread` latches the shutdown gate under the
+    /// thread mutex before joining, and the thread is already bailing on the intent latch (step 1) — so the
+    /// join is bounded and a keeper callback racing teardown can never re-arm it. Outside `remount_mutex`.
+    mount_runtime.stopRemountThread();
+
+    /// A remount attempt already IN FLIGHT when step 1 published the intent completes its current step
+    /// before the loop bails (the "one step + one backend timeout" bound of §5), and a successful reclaim in
+    /// that window re-arms the local fence (`lost = false`). Now that the remount thread is JOINED and can
+    /// never run again, re-latch the fence so the terminal `mayMutate() == false` holds regardless of any
+    /// such raced reclaim. Idempotent; the durable mount lease the reclaim wrote is retired by the
+    /// `finishTeardown` below (it operates on whatever keeper is current — the reclaimed one).
+    mount_runtime.tripMountLost();
+
+    /// (5b) Drain the ref lanes (bounded by one attempt's budget + safety margin) to learn whether a clean
+    /// farewell is EARNED — exactly the `~Pool` rule.
+    const bool ref_lanes_drained = ref_ledger.drainRefLanesForShutdown(
+        config.cas_request_budget.attempt_timeout_ms + config.cas_request_budget.lease_safety_margin_ms);
+    const bool drained = ref_lanes_drained && !writerCleanupDutiesPending();
+
+    /// (3+5c) Retire the merged heartbeat: a clean-release farewell ONLY if the lanes provably drained,
+    /// otherwise stop background renewal with NO terminal marker so the lease expires by observation (never
+    /// an unearned clean farewell). Also does the belt-and-suspenders remount rejoin. Outside `remount_mutex`.
+    mount_runtime.finishTeardown(drained);
+
+    /// The pool object OUTLIVES this FORGET (it stays registered, `Vanished(forgotten)`, until DROP/restart),
+    /// so `~Pool` will re-run the same teardown. Drop the keeper now so that later teardown finds none and
+    /// skips it: `MountLeaseKeeper::stop`'s terminal op is single-shot (`doTerminate` throws a `LOGICAL_ERROR`
+    /// on a second call — an ASan-abort at construction), so a keeper that already terminated here must not be
+    /// terminated again. `keeperReset` is safe now: every keeper-touching thread (renewal, remount) is joined.
+    mount_runtime.keeperReset();
+
+    /// (6) Publish the terminal state + WARN, under remount serialization — matching the natural-transition
+    /// contract. Every pool thread is already joined, so taking `remount_mutex` here cannot self-deadlock.
+    /// `reason` is the [D5] message (with the operator's decommission timestamp) that
+    /// `throwIfLifecycleTerminal` surfaces to store-class callers.
+    {
+        std::lock_guard g(remount_mutex);
+        mount_runtime.enterVanished(PoolLifecycle::VanishedForgotten, reason);
+    }
+}
+
+/// The plain-object surface (namespace files + mountpoint objects) is implemented by the stateless
+/// `plain_objects` component; these are thin delegates preserving the API.
+void Pool::putNamespaceFile(const NamespaceLifeId & life, const String & name, const String & bytes)
+{
+    plain_objects.putNamespaceFile(life, name, bytes);
+}
+
+std::optional<String> Pool::getNamespaceFile(const NamespaceLifeId & life, const String & name)
+{
+    return plain_objects.getNamespaceFile(life, name);
+}
+
+std::vector<String> Pool::listNamespaceFiles(const NamespaceLifeId & life)
+{
+    return plain_objects.listNamespaceFiles(life);
+}
+
+uint64_t Pool::minActive()
+{
+    return mount_runtime.minActive();
+}
+
+uint64_t Pool::peekNextBuildSeq()
+{
+    return mount_runtime.peekNextBuildSeq();
+}
+
+bool Pool::tryRemountOnce()
+{
+    std::lock_guard serialize(remount_mutex);
+
+    const String & srid = config.server_root_id;
+    const UInt128 our_uuid = config.server_id;
+    const auto now_ms = []() -> uint64_t
+    {
+        return static_cast<uint64_t>(std::chrono::duration_cast<std::chrono::milliseconds>(
+            std::chrono::system_clock::now().time_since_epoch()).count());
+    };
+    const uint64_t ttl_ms = static_cast<uint64_t>(config.mount_lease_ttl_ms.count());
+    const uint64_t poll_interval_ms = std::max<uint64_t>(
+        1, static_cast<uint64_t>(config.mount_renew_period.count()) / 2);
+
+    /// Best-effort round for the MountRemount audit event only (diagnostic, never correctness-relevant):
+    /// `currentGcRound` is a live `gc/state` GET, which may itself fail on the very backend trouble that
+    /// is causing this remount attempt to fail — never let that escalate into an uncaught throw out of
+    /// a function whose contract is "returns bool, never throws".
+    const auto round_for_event = [this]() -> uint64_t
+    {
+        try { return currentGcRound(); } catch (...) { return 0; }
+    };
+
+    /// ==== Step 0 (rev.7 §2): pool lifecycle identity gate — BEFORE any claim/allocate/mount write ====
+    /// A remount attempt means the lease is presumed lost, so first ensure we are at least transient
+    /// (production reaches here already transient via `tripMountLost`; a direct/forced call may still be
+    /// `Live`). Then authoritatively probe the pool sentinels and dispatch per the §2 verdict table. Only
+    /// `Recover` (a present `_pool_meta` whose identity matches, in a non-`IdentityLost` state) falls
+    /// through to the existing recovery below; every other verdict resolves here and returns false.
+    mount_runtime.noteLeaseLost();
+    /// A fully-terminal `Vanished` pool never probes/claims/writes again.
+    if (mount_runtime.isVanished())
+        return false;
+    {
+        const LifecycleGate gate = probePoolLifecycleGate(
+            *pool_backend, pool_layout, config.server_root_id, meta.pool_id, meta.blob_header_len);
+        switch (gate.verdict)
+        {
+            case LifecycleGateVerdict::Recover:
+                /// [D3] no auto-revival: a matching-sentinel observation while `IdentityLost` does NOT
+                /// bring the disk back — the state is terminal; only a restart recovers.
+                if (mount_runtime.lifecycle() == PoolLifecycle::IdentityLost)
+                    return false;
+                break;   /// fall through to the existing fresh-incarnation recovery.
+            case LifecycleGateVerdict::Replaced:
+                /// NOT while a FORGET is in progress (spec §9 rev.8 item 7). `forgetDisk` publishes the
+                /// terminal-intent latch at step 1 (`publishVanishedIntent`), then joins the remount thread;
+                /// a `tryRemountOnce` already IN FLIGHT — one that passed the step-0 `isVanished()` gate
+                /// BEFORE the intent was published, which that gate therefore cannot catch — could otherwise
+                /// settle `Vanished(replaced)` mid-FORGET, stranding FORGET's own
+                /// `enterVanished(VanishedForgotten)` (first terminal STATE transition wins) and mislabeling
+                /// the operator-visible reason. The bail lives HERE, at the terminal settle, so the
+                /// Recover/`armMountFence` reclaim path is untouched — its mid-FORGET fence re-arm is the
+                /// SEPARATE hazard `forgetDisk`'s post-join re-trip (trip#2) guards. Post-excision this is the
+                /// ONLY surviving mid-FORGET natural-terminal race (the old erasure-proof promotion is gone).
+                if (mount_runtime.vanishedIntentPublished())
+                    return false;
+                mount_runtime.enterVanished(PoolLifecycle::VanishedReplaced, gate.reason);
+                return false;
+            case LifecycleGateVerdict::IdentityLost:
+                /// Both sentinels authoritatively absent. Enter `IdentityLost` once (from `TransientNotLive`);
+                /// a repeat probe while already `IdentityLost` is a no-op. rev.8: `IdentityLost` is a
+                /// fail-loud TERMINAL state — the remount thread self-exits at its next boundary (see
+                /// `CasMountRuntime::remountTerminal`), so there is no demoted observer.
+                if (mount_runtime.lifecycle() != PoolLifecycle::IdentityLost)
+                    mount_runtime.enterIdentityLost();
+                return false;
+            case LifecycleGateVerdict::StayTransient:
+                /// Uncertain — remain transient and let the recovery loop retry. The probe's own reason is
+                /// the only account of WHY, and it is the difference between a transient store hiccup and
+                /// a pool this build can never open again: an undecodable `_pool_meta` is what a pool
+                /// predating the contiguous-ref-stream format floor looks like from here (`decodePoolMeta`
+                /// throws, the gate catches it), and silently retrying forever would leave that pool's
+                /// operator with a fenced mount and nothing to read. Say it, and count it.
+                ProfileEvents::increment(ProfileEvents::CASRemountHeldTransient);
+                LOG_WARNING(getLogger("CasPool"),
+                    "content-addressed pool '{}' (prefix '{}'): remount held TRANSIENT — {}. The mount "
+                    "stays fenced closed and the remount loop will retry. If this repeats, the pool "
+                    "metadata is unreadable to this build (a pool below its format floor must be "
+                    "recreated) rather than merely unavailable.",
+                    config.server_root_id, config.pool_prefix, gate.reason);
+                return false;
+        }
+    }
+
+    /// The same startup protocol as Pool::open steps 2-4, as a FRESH incarnation (the old one is
+    /// dead by the fence-out contract and its keeper never re-mints). Open THROWS on any failure
+    /// (startup is fail-closed); the remount RETURNS false instead — the recovery loop retries.
+    try
+    {
+        const ObserveRefCatalog observe_catalog = [this]()
+        {
+            CasRefCatalog::Snapshot snapshot = CasRefCatalog::read(*pool_backend, pool_layout);
+            snapshot.life_index.throwIfAmbiguous("CAS server-root remount safety");
+            return snapshot.catalog;
+        };
+        /// The present-owner/present-epoch fast path would otherwise skip the observer entirely.
+        /// Require the catalog before allocating a new epoch or replacing the mount slot.
+        (void)observe_catalog();
+        claimOwnerOrThrow(*pool_backend, pool_layout, srid, our_uuid, observe_catalog);
+        const uint64_t writer_epoch = allocateWriterEpoch(
+            *pool_backend, pool_layout, srid, EpochMintPolicy::NormalMount, 0, observe_catalog);
+
+        /// Mount-slot writer audit: `this` is already fully open (setEventSink ran long ago), so
+        /// unlike the initial `open`, every event fired below reaches the real sink immediately.
+        const auto emit_mount_event = [this](CasEvent e) { emitEvent(std::move(e)); };
+
+        const auto sleep_ms = [](uint64_t ms) { std::this_thread::sleep_for(std::chrono::milliseconds(ms)); };
+        const MountClaimResult claim = claimMountAwaitingExpiry(
+            *pool_backend, pool_layout, srid, our_uuid, writer_epoch,
+            now_ms, [this] { return bootMsNow(); }, ttl_ms, poll_interval_ms, sleep_ms,
+            [&srid](const MountLease & held, uint64_t threshold_ms)
+            {
+                LOG_INFO(getLogger("CasPool"),
+                    "CAS self-remount '{}': observing a stale-looking mount's write-token (uuid={} "
+                    "epoch={} expires_at_ms={}) for up to ~{} ms before reclaiming",
+                    srid, u128ToHex(held.server_uuid), held.writer_epoch, held.expires_at_ms, threshold_ms);
+            },
+            emit_mount_event);
+        if (claim.kind != MountClaimResult::Claimed)
+        {
+            LOG_WARNING(getLogger("CasPool"),
+                "CAS self-remount '{}': mount not claimable ({}); will retry", srid,
+                claim.kind == MountClaimResult::ForeignOwner ? "foreign owner — never taking over"
+                                                             : "a live twin holds the lease");
+            return false;
+        }
+
+        /// NO WAIT HERE, AND THE ASSERT IS WHAT REPLACES IT. This used to pay `materialization_grace_ms`
+        /// whenever this incarnation's own ref lanes had not provably settled before the fence tripped:
+        /// an unresolved (still-wedged) ref-log conditional `PUT` from the dying epoch could otherwise
+        /// land after recovery began trusting its listings. Recovery no longer trusts listings — it walks
+        /// arithmetically and closes every epoch below the live one with an in-band `EpochSeal` at
+        /// `{E, T+1}`, so the wedged `PUT` loses its own conditional create to that seal and the wait
+        /// bought nothing but latency on every fence recovery.
+        ///
+        /// That replacement has ONE precondition, and it is this one: the incarnation we are about to
+        /// install must outrank the dying one. If the epoch did not strictly advance, the straggler's
+        /// slot is not "below the live epoch", nothing seals it, and the hole the wait used to paper over
+        /// reopens silently. `allocateWriterEpoch` mints from a durable monotone counter, so this holds by
+        /// construction — which is exactly why it is worth asserting rather than assuming: it fails the
+        /// build's own tests the moment someone reuses an epoch across a remount.
+        chassert(writer_epoch > mount_runtime.liveWriterEpoch());
+
+        /// Swap the keeper for the new incarnation. The old keeper's renewal loop already stopped on
+        /// its failed renew; never run its terminal op (the slot now belongs to the new claim).
+        /// `installKeeper` re-wires the SAME fence/min-active/event callbacks on `mount_runtime` as the
+        /// initial `open` did -- so the granular mechanics here are the exact same primitives, in the
+        /// exact same order, as `mountWritable`'s.
+        if (mount_runtime.hasKeeper())
+            mount_runtime.keeperStopBackground();
+        mount_runtime.installKeeper(our_uuid, writer_epoch, now_ms);
+        /// Pre-I/O anchor of this remount's claim attempt (mirrors `mountWritable`'s
+        /// `claim_anchor_boot_ms`, captured at the identical point -- right after `installKeeper`,
+        /// right before the keeper's own adopt write). No wait can land between this anchor and the arm
+        /// below, so no TTL-consumed redo is needed here: the anchor alone suffices (rev.4 Phase B,
+        /// round-3 finding 2; the redo lives in `mountWritable`, which keeps it for a claim that stalls).
+        /// `quiesceRefTablesForRemount` below IS a wait, but a bounded one -- bounded by the same
+        /// `cas_request_budget` that `validateCasRequestBudget` already guarantees fits under `ttl_ms`.
+        const uint64_t remount_anchor_boot_ms = mount_runtime.bootMsNow();
+        mount_runtime.keeperStart();
+
+        /// Re-establish the ref-protocol incarnation BEFORE re-arming the fence. Order is load-bearing:
+        /// `keeperStart` refreshes the lease deadline
+        /// but does NOT clear `lost`, so the fence stays closed here and no append/publish can race the
+        /// swap.
+        /// 1. Bump the live epoch so every subsequent `allocateRefTxnId` sorts strictly above any older
+        ///    (dead-incarnation or twin) durable log. Do this BEFORE `armMountFence` so there is no window
+        ///    where the gate is open while the epoch is still stale. Keep `process_epoch` (the identity
+        ///    accessors) equal to it.
+        mount_runtime.setLiveWriterEpoch(writer_epoch);
+        mount_runtime.setProcessEpoch(writer_epoch, std::memory_order_release);
+        /// 2. CANCEL OR JOIN every in-flight ref-table recovery, and BLOCK here until none is left (spec
+        ///    §3: "self-remount cancels or waits out recovery before rearming"). A recovery admitted under
+        ///    the outgoing incarnation WRITES -- its seal CAS-walk mints epoch seals and advances the
+        ///    `_ckpt` -- so it must be stopped at this boundary rather than caught one site at a time
+        ///    after the incarnation has already changed underneath it. Strictly before the quiesce below
+        ///    so a cancelled walk unwinds while its runtime is still attached, and strictly before the
+        ///    re-arm so no old-generation write can straddle it.
+        ref_ledger.cancelRecoveriesAndAwaitQuiescence();
+        /// 3. Drain publishers and drop the cached tables so each re-recovers under the new epoch on next
+        ///    touch (and any leader still holding an orphaned runtime fails closed). While the fence is lost.
+        ref_ledger.quiesceRefTablesForRemount();
+        /// 4. Re-open the gate. Anchored at the claim attempt's pre-I/O instant (`remount_anchor_boot_ms`),
+        ///    never at response time -- see the comment above `keeperStart()`. From here appends allocate
+        ///    ids under the new epoch and touch fresh runtimes.
+        mount_runtime.armMountFence(our_uuid, writer_epoch, remount_anchor_boot_ms + ttl_ms);
+        if (config.background_watermark)
+            mount_runtime.keeperStartBackground(config.mount_renew_period);
+
+        LOG_INFO(getLogger("CasPool"),
+            "CAS self-remount '{}': recovered as writer_epoch {} (fresh incarnation; older builds fail closed)",
+            srid, writer_epoch);
+        EventEmitter{*this}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::MountRemount;
+            e.round = round_for_event();
+            e.outcome = "ok";
+            e.reason = "self-remount recovered a fresh mount incarnation after fence-out / renewal failure";
+            e.detail = {{"writer_epoch", std::to_string(writer_epoch)},
+                        {"server_root_id", srid}};
+        });
+        /// Recovery succeeded: `TransientNotLive -> Live` (never revives a terminal state — but the gate
+        /// above guarantees we only reach here from a non-terminal state anyway).
+        mount_runtime.noteRemounted();
+        return true;
+    }
+    catch (...)
+    {
+        tryLogCurrentException(getLogger("CasPool"), "CAS self-remount attempt failed; will retry");
+        EventEmitter{*this}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::MountRemount;
+            e.round = round_for_event();
+            e.outcome = "failed";
+            e.reason = "self-remount attempt failed; the recovery loop retries with backoff";
+            e.detail = {{"server_root_id", srid},
+                        {"error", getCurrentExceptionMessage(/*with_stacktrace*/ false)}};
+        });
+        return false;
+    }
+}
+
+/// The self-remount recovery thread + the merged-heartbeat renew live in `mount_runtime`
+/// (Pool/CasMountRuntime.h); these are thin delegates. `mount_runtime`'s `remount_attempt` callback is
+/// bound to `Pool::tryRemountOnce` (the claim/recovery orchestration that stays on Pool).
+bool Pool::scheduleRemountForTest()
+{
+    return mount_runtime.scheduleRemountForTest();
+}
+
+void Pool::beginShutdownForTest()
+{
+    mount_runtime.beginShutdownForTest();
+}
+
+void Pool::renewWatermarkOnce()
+{
+    mount_runtime.renewWatermarkOnce();
+}
+
+void Pool::retireBuildSeq(uint64_t seq)
+{
+    mount_runtime.retireBuildSeq(seq);
+}
+
+void Pool::enqueueWriterCleanupDuty(
+    const RootNamespace & ns, const String & ref_name, const ManifestRef & manifest, uint64_t build_seq) noexcept
+{
+    try
+    {
+        auto duty = std::make_shared<WriterCleanupDuty>(WriterCleanupDuty{
+            .ref_name = ref_name,
+            .manifest = manifest,
+            .build_seq = build_seq,
+        });
+        std::lock_guard lock(writer_cleanup_mutex);
+        writer_cleanup_queues[ns].pending.push_back(std::move(duty));
+        writer_cleanup_cv.notify_all();
+    }
+    catch (...)
+    {
+        /// The build deliberately remains active. Advancing `min_active` after losing the only cleanup
+        /// duty would make an uncertain owner grant look dead; pinning the floor until process exit is
+        /// the safe failure direction, and successor recovery handles the durable remnant.
+        writer_cleanup_queue_failed.store(true, std::memory_order_release);
+        try
+        {
+            tryLogCurrentException(
+                getLogger("CasPool"),
+                "CAS writer cleanup duty could not be queued; retaining the build in the active watermark");
+        }
+        catch (...) // NOLINT(bugprone-empty-catch)
+        {
+            /// `noexcept` destructor path: the sticky bit above is the safety mechanism.
+        }
+    }
+}
+
+bool Pool::writerCleanupDutiesPending() const
+{
+    if (writer_cleanup_queue_failed.load(std::memory_order_acquire))
+        return true;
+    std::lock_guard lock(writer_cleanup_mutex);
+    return std::any_of(
+        writer_cleanup_queues.begin(), writer_cleanup_queues.end(),
+        [](const auto & item) { return !item.second.pending.empty(); });
+}
+
+void Pool::drainWriterCleanupDuties(const RootNamespace & ns)
+{
+    {
+        std::unique_lock lock(writer_cleanup_mutex);
+        writer_cleanup_cv.wait(lock, [&]
+        {
+            const auto it = writer_cleanup_queues.find(ns);
+            return it == writer_cleanup_queues.end() || !it->second.draining;
+        });
+
+        const auto it = writer_cleanup_queues.find(ns);
+        if (it == writer_cleanup_queues.end() || it->second.pending.empty())
+            return;
+        it->second.draining = true;
+    }
+
+    try
+    {
+        while (true)
+        {
+            std::shared_ptr<const WriterCleanupDuty> duty;
+            {
+                std::lock_guard lock(writer_cleanup_mutex);
+                const auto it = writer_cleanup_queues.find(ns);
+                chassert(it != writer_cleanup_queues.end() && it->second.draining);
+                if (it->second.pending.empty())
+                {
+                    writer_cleanup_queues.erase(it);
+                    writer_cleanup_cv.notify_all();
+                    return;
+                }
+                duty = it->second.pending.front();
+            }
+
+            ref_ledger.appendRefOps(
+                ns,
+                MutationScope::ref(duty->ref_name),
+                [ref_name = duty->ref_name, manifest = duty->manifest]
+                (const RefTableState & state) -> std::vector<RefOp>
+                {
+                    /// Absence is a conclusive settlement, not an error: the original uncertain grant
+                    /// was rejected, a promote atomically consumed it, or another exact owner-removal
+                    /// path already discharged it. Presence owes one exact removal.
+                    if (!state.getPrecommits().contains({ref_name, manifest}))
+                        return {};
+                    RefOp op;
+                    op.kind = RefOpKind::OwnerTransition;
+                    op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref_name, manifest};
+                    return {op};
+                },
+                RootMutationOrigin::Writer,
+                RootMutationKind::Abandon);
+
+            /// Removal/absence is now durable in the same state observation. Only now may the active
+            /// build floor advance beyond the manifest's build sequence.
+            retireBuildSeq(duty->build_seq);
+
+            std::lock_guard lock(writer_cleanup_mutex);
+            const auto it = writer_cleanup_queues.find(ns);
+            chassert(it != writer_cleanup_queues.end() && it->second.draining);
+            chassert(!it->second.pending.empty() && it->second.pending.front() == duty);
+            it->second.pending.pop_front();
+        }
+    }
+    catch (...)
+    {
+        std::lock_guard lock(writer_cleanup_mutex);
+        const auto it = writer_cleanup_queues.find(ns);
+        if (it != writer_cleanup_queues.end())
+            it->second.draining = false;
+        writer_cleanup_cv.notify_all();
+        throw;
+    }
+}
+
+PartWriteTxnPtr Pool::beginPartWrite(PartWriteInfo info)
+{
+    /// Mint a globally-unique build id from two thread_local_rng draws (random u128).
+    const UInt64 hi = thread_local_rng();
+    const UInt64 lo = thread_local_rng();
+    const UInt128 build_id = (static_cast<UInt128>(hi) << 64) | lo;
+
+    /// Strictly-increasing per-process build_seq carried by the `PartWriteTxn`. The `PartWriteTxn` is
+    /// added to the active set here and retired on publish/abandon/dtor, so minActive — the GC floor
+    /// the Pool-owned watermark renews — tracks in-flight builds. The build registry lives on
+    /// `mount_runtime`.
+    const uint64_t seq = mount_runtime.allocateBuildSeq();
+    bool registered = false;
+    SCOPE_EXIT({ if (!registered) retireBuildSeq(seq); });
+
+    auto build = std::make_shared<PartWriteTxn>(shared_from_this(), build_id, seq, liveWriterEpoch(), std::move(info));
+    /// Register for `dropNamespace`'s post-durable build cancellation. weak_ptr:
+    /// the wiring owns the returned shared_ptr; `retireBuildSeq` (publish/abandon/dtor) removes the entry.
+    mount_runtime.registerInflightBuild(seq, build);
+    registered = true;
+    return build;
+}
+
+/// The manifest read path (readManifest / readManifestShared / locate) + its decode cache live in
+/// the `manifest_reader` component (Pool/CasManifestReader.h); these are thin delegates.
+std::shared_ptr<const PartManifest> Pool::readManifestShared(const ManifestId & id)
+{
+    return manifest_reader.readManifestShared(id);
+}
+
+PartManifest Pool::readManifest(const ManifestId & id)
+{
+    return manifest_reader.readManifest(id);
+}
+
+BlobLocation Pool::locate(const ManifestEntry & entry) const
+{
+    return manifest_reader.locate(entry);
+}
+
+namespace
+{
+/// a tolerant, read-only peek at the
+/// `cas_ref_log` TEXT object (codecs-v3 phase 3) WITHOUT `decodeRefLogTxn`'s expected-value cross-check
+/// -- the whole point of this diagnostic is that the body is NOT expected to match this key's identity.
+/// It `openObject`s the stored `.zst`, skips the header line, and reads `ns`/`we`/`rs` off the meta
+/// line (`we`/`rs` are decimal u64 strings). Never validates the header `v`, never reads past the meta
+/// line (the ops are irrelevant to identifying the writer), and swallows any truncation/garbage: this
+/// is a background diagnostic only, never a decode anything else depends on.
+struct ForeignRefLogHeaderPeek
+{
+    String ns;
+    uint64_t writer_epoch = 0;
+    uint64_t ref_sequence = 0;
+};
+
+std::optional<ForeignRefLogHeaderPeek> peekForeignRefLogHeader(const String & bytes)
+{
+    try
+    {
+        const String text = openObject(FormatId::RefLog, bytes);
+        ReadBufferFromMemory in(text.data(), text.size());
+        const uint64_t line_cap = traitsFor(FormatId::RefLog).line_cap;
+        readLine(in, line_cap, "cas_ref_log");   /// header line -- skip
+        const String meta = readLine(in, line_cap, "cas_ref_log");
+        ReadBufferFromMemory m(meta.data(), meta.size());
+        JsonObjectReader r(m, KeyStrictness::Tolerant, "cas_ref_log");
+        ForeignRefLogHeaderPeek peek;
+        bool saw_ns = false;
+        bool saw_we = false;
+        bool saw_rs = false;
+        String key;
+        while (r.nextKey(key))
+        {
+            if (key == "ns") { peek.ns = r.readString(); saw_ns = true; }
+            else if (key == "we") { peek.writer_epoch = r.readU64String(); saw_we = true; }
+            else if (key == "rs") { peek.ref_sequence = r.readU64String(); saw_rs = true; }
+            else r.skipUnknown(key);
+        }
+        if (!saw_ns || !saw_we || !saw_rs)
+            return std::nullopt;
+        return peek;
+    }
+    catch (...)
+    {
+        return std::nullopt;
+    }
+}
+}
+
+void Pool::reportImpossibleInterference(const String & key, const String & reason,
+                                          const std::optional<String> & offending_ns)
+{
+    LOG_ERROR(getLogger("CasPool"),
+        "CAS anomaly policy: impossible foreign interference for server_root '{}' (namespace='{}', key='{}'): "
+        "{} -- fencing this mount closed and scheduling a remount",
+        config.server_root_id, offending_ns.value_or(String{}), key, reason);
+
+    EventEmitter{*this}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::ForeignInterference;
+        if (offending_ns)
+            e.namespace_ = *offending_ns;
+        e.reason = reason;
+        e.detail = {{"key", key}, {"server_root_id", config.server_root_id}};
+    });
+
+    /// Incidental-only detection has a fail-closed reaction -- the SAME on_lost
+    /// mechanics a foreign/superseded lease renewal already drives (the keeper's `setFenceCallbacks`
+    /// lambda). The fence + self-remount now live on `mount_runtime`.
+    mount_runtime.tripMountLost();
+    mount_runtime.scheduleRemount();
+
+    /// Diagnosis off the critical path: a background task may spend a FEW
+    /// requests -- never the caller's thread, and never blocking this call's own return.
+    /// `shared_from_this()` keeps the Pool alive for the thread's lifetime (mirrors
+    /// `maybeScheduleSnapshotPublish`'s dispatch).
+    auto self = shared_from_this();
+    try
+    {
+        ThreadFromGlobalPool([self, key]
+        {
+            setThreadName(ThreadName::CAS_ANOMALY_DIAG);
+            try
+            {
+                const auto got = self->pool_backend->get(key);
+                if (!got)
+                {
+                    LOG_ERROR(getLogger("CasPool"),
+                        "CAS anomaly diagnostics: the offending object at '{}' had already vanished by the "
+                        "time the background diagnostic GET ran", key);
+                    return;
+                }
+                if (const auto peek = peekForeignRefLogHeader(got->bytes))
+                    LOG_ERROR(getLogger("CasPool"),
+                        "CAS anomaly diagnostics: offending object at '{}' ({} bytes) decodes as a ref-log "
+                        "header: namespace='{}', writer_epoch={}, ref_sequence={}",
+                        key, got->bytes.size(), peek->ns, peek->writer_epoch, peek->ref_sequence);
+                else
+                    LOG_ERROR(getLogger("CasPool"),
+                        "CAS anomaly diagnostics: offending object at '{}' ({} bytes) does not decode as a "
+                        "ref-log header -- raw and unidentified", key, got->bytes.size());
+            }
+            catch (...)
+            {
+                tryLogCurrentException(getLogger("CasPool"),
+                    "CAS anomaly diagnostics: background GET failed for '" + key + "'");
+            }
+        }).detach();
+    }
+    catch (...)
+    {
+        /// Pool exhaustion: best-effort diagnostics must never block the caller's own fail-closed throw.
+        tryLogCurrentException(getLogger("CasPool"), "CAS anomaly diagnostics dispatch failed to launch for '" + key + "'");
+    }
+}
+
+uint64_t Pool::currentGcRound() const
+{
+    /// Read `gc/state` once (no CAS loop — a point-in-time read is sufficient; a concurrent
+    /// GC advance only makes the returned round larger, which is strictly more conservative for the
+    /// `precommitAdd` self-floor). Returns 0 when absent (pool never GC'd — no round to floor to).
+    const auto state_bytes = pool_backend->get(pool_layout.gcStateKey());
+    if (!state_bytes)
+        return 0;
+    return decodeGcState(state_bytes->bytes).round;
+}
+
+void Pool::removeNamespaceFile(const NamespaceLifeId & life, const String & name)
+{
+    plain_objects.removeNamespaceFile(life, name);
+}
+
+void Pool::putMountpointObject(const String & key, const String & bytes)
+{
+    plain_objects.putMountpointObject(key, bytes);
+}
+
+std::optional<String> Pool::getMountpointObject(const String & key)
+{
+    return plain_objects.getMountpointObject(key);
+}
+
+bool Pool::mountpointObjectExists(const String & key)
+{
+    return plain_objects.mountpointObjectExists(key);
+}
+
+void Pool::removeMountpointObject(const String & key)
+{
+    plain_objects.removeMountpointObject(key);
+}
+
+NamespaceListing Pool::listNamespaces(const String & prefix)
+{
+    /// The catalog is the logical namespace authority. Physical stream/state keys expose only an
+    /// opaque life id and therefore cannot mint a namespace during discovery.
+    std::unordered_set<String> found;
+    std::vector<UnattributableNamespaceKey> skipped;
+    const CasRefCatalog::Snapshot cut = CasRefCatalog::read(*pool_backend, pool_layout);
+    for (const CatalogEntry & entry : cut.catalog.entries)
+    {
+        try
+        {
+            if (const auto life = cut.life_index.resolve(entry.incarnation);
+                life && life->ns.string().starts_with(prefix))
+                found.insert(life->ns.string());
+        }
+        catch (const Exception & e)
+        {
+            if (e.code() != ErrorCodes::CORRUPTED_DATA)
+                throw;
+            skipped.push_back(UnattributableNamespaceKey{
+                pool_layout.refCatalogKey() + "#" + renderIncarnation(entry.incarnation), e.message()});
+        }
+    }
+
+    return NamespaceListing{{found.begin(), found.end()}, std::move(skipped)};
+}
+
+std::vector<String> Pool::listMirroredChildren(const String & prefix)
+{
+    /// Namespace children come from the catalog. `roots/` is still listed for loose mountpoint files,
+    /// whose logical paths retain path identity.
+    std::unordered_set<String> children;
+    const CasRefCatalog::Snapshot cut = CasRefCatalog::read(*pool_backend, pool_layout);
+    for (const CatalogEntry & entry : cut.catalog.entries)
+    {
+        if (!entry.ns.string().starts_with(prefix))
+            continue;
+        const std::string_view rest(entry.ns.string().data() + prefix.size(), entry.ns.string().size() - prefix.size());
+        const size_t slash = rest.find('/');
+        const std::string_view segment = slash == std::string_view::npos ? rest : rest.substr(0, slash);
+        if (!segment.empty())
+            children.emplace(segment);
+    }
+
+    const String roots_full = pool_layout.rootsPrefix() + prefix;
+    {
+        String cursor;
+        while (true)
+        {
+            ListPage page = pool_backend->list(roots_full, cursor, /*limit*/ 1000);
+            for (const ListedKey & listed : page.keys)
+            {
+                const String & key = listed.key;
+                if (!key.starts_with(roots_full))
+                    continue;
+                const std::string_view rest(key.data() + roots_full.size(), key.size() - roots_full.size());
+                const size_t slash = rest.find('/');
+                const std::string_view seg = slash == std::string_view::npos ? rest : rest.substr(0, slash);
+                if (!seg.empty())
+                    children.emplace(seg);
+            }
+            if (page.next_cursor.empty())
+                break;
+            cursor = page.next_cursor;
+        }
+    }
+    return {children.begin(), children.end()};
+}
+
+
+/// ==== ref-ledger delegates ==== The ref-log / ref-table subsystem lives in the
+/// `ref_ledger` member (Pool/CasRefLedger.h); Pool keeps these thin public forwarders so the wiring,
+/// PartWriteTxn, Gc, and every test call site is unchanged.
+
+void Pool::setCasRetrySleepForTest(std::function<void(uint64_t)> sleep_fn)
+{
+    ref_ledger.setCasRetrySleepForTest(std::move(sleep_fn));
+}
+
+std::optional<Resolved> Pool::resolveRef(const RootNamespace & ns, const String & ref_name, bool allow_stale, ResolveAudit audit)
+{
+    return ref_ledger.resolveRef(ns, ref_name, allow_stale, audit);
+}
+
+std::map<String, Resolved> Pool::listRefs(const RootNamespace & ns)
+{
+    return ref_ledger.listRefs(ns);
+}
+
+bool Pool::hasAnyRefWithPrefix(const RootNamespace & ns, std::string_view prefix)
+{
+    return ref_ledger.hasAnyRefWithPrefix(ns, prefix);
+}
+
+void Pool::dropRef(const RootNamespace & ns, const String & ref_name)
+{
+    mutateRefsAfterWriterCleanup(ns, [&]
+    {
+        ref_ledger.dropRef(ns, ref_name);
+    });
+}
+
+void Pool::updateRefPublishedAt(const RootNamespace & ns, const String & ref_name,
+                             std::function<void(RefPublishedAtUpdate &)> mutator)
+{
+    mutateRefsAfterWriterCleanup(ns, [&]
+    {
+        ref_ledger.updateRefPublishedAt(ns, ref_name, std::move(mutator));
+    });
+}
+
+DropNamespaceStats Pool::dropNamespace(const RootNamespace & ns)
+{
+    return mutateRefsAfterWriterCleanup(ns, [&]
+    {
+        return ref_ledger.dropNamespace(ns);
+    });
+}
+
+DropNamespaceStats Pool::dropNamespace(const NamespaceLifeId & life)
+{
+    return mutateRefsAfterWriterCleanup(life.ns, [&]
+    {
+        return ref_ledger.dropNamespace(life);
+    });
+}
+
+NamespaceLifeId Pool::namespaceLife(const RootNamespace & ns)
+{
+    return ref_ledger.namespaceLife(ns);
+}
+
+std::optional<NamespaceLifeId> Pool::namespaceFilesLifeIfReadable(const RootNamespace & ns)
+{
+    return ref_ledger.namespaceFilesLifeIfReadable(ns);
+}
+
+bool Pool::namespaceStillLogicallyPresent(const RootNamespace & ns)
+{
+    return ref_ledger.namespaceStillLogicallyPresent(ns);
+}
+
+void Pool::invalidateRemovedCatalogLife(const NamespaceLifeId & life)
+{
+    ref_ledger.invalidateRemovedCatalogLife(life);
+}
+
+void Pool::reconcileRefCatalogCut(const CasRefCatalog::Snapshot & catalog_cut)
+{
+    ref_ledger.reconcileCatalogCut(catalog_cut);
+}
+
+RefTxnId Pool::appendRefOps(const RootNamespace & ns, MutationScope scope,
+                             std::function<std::vector<RefOp>(const RefTableState &)> build_ops,
+                             RootMutationOrigin origin, RootMutationKind kind,
+                             bool skip_stale_precommit_sweep)
+{
+    return mutateRefsAfterWriterCleanup(ns, [&]
+    {
+        return ref_ledger.appendRefOps(
+            ns, std::move(scope), std::move(build_ops), origin, kind, skip_stale_precommit_sweep);
+    });
+}
+
+bool Pool::tryPublishSnapshotAndAdvanceCheckpointOnce(const RootNamespace & ns)
+{
+    return mutateRefsAfterWriterCleanup(ns, [&]
+    {
+        return ref_ledger.tryPublishSnapshotAndAdvanceCheckpointOnce(ns);
+    });
+}
+
+size_t Pool::wedgedRefLaneCount()
+{
+    return ref_ledger.wedgedRefLaneCount();
+}
+
+CasWriteOutcome Pool::stagingPutIfAbsent(std::string_view key, std::string_view bytes, Token * out_token)
+{
+    return ref_ledger.stagingPutIfAbsent(key, bytes, out_token);
+}
+
+CasCreateResult Pool::stagingConditionalCreate(std::string_view key, const std::function<PutResult()> & attempt)
+{
+    return ref_ledger.stagingConditionalCreate(key, attempt);
+}
+
+CasOverwriteResult Pool::stagingConditionalOverwrite(std::string_view key, std::string_view bytes, const Token & expected)
+{
+    return ref_ledger.stagingConditionalOverwrite(key, bytes, expected);
+}
+
+CasOverwriteResult Pool::stagingPutIfAbsentMutable(std::string_view key, std::string_view bytes)
+{
+    return ref_ledger.stagingPutIfAbsentMutable(key, bytes);
+}
+
+void Pool::cancelInflightBuildsForNamespace(const RootNamespace & ns)
+{
+    /// Delegate to `mount_runtime`. Invoked by
+    /// `ref_ledger` through the `cancel_inflight_builds` callback once its removal transaction is durable,
+    /// so that in-flight local builds targeting the removed namespace are cancelled.
+    mount_runtime.cancelInflightBuildsForNamespace(ns);
+}
+
+uint64_t Pool::refRecoveryRestartsForTest(const RootNamespace & ns)
+{
+    return ref_ledger.refRecoveryRestartsForTest(ns);
+}
+
+bool Pool::refLaneWedgedForTest(const RootNamespace & ns)
+{
+    return ref_ledger.refLaneWedgedForTest(ns);
+}
+
+String Pool::wedgedKeyForTest(const RootNamespace & ns)
+{
+    return ref_ledger.wedgedKeyForTest(ns);
+}
+
+void Pool::forceWedgeForTest(const RootNamespace & ns, uint64_t writer_epoch, uint64_t ref_sequence,
+                              const String & key, const String & bytes,
+                              std::optional<uint64_t> admitted_generation)
+{
+    ref_ledger.forceWedgeForTest(ns, writer_epoch, ref_sequence, key, bytes, admitted_generation);
+}
+
+uint64_t Pool::wedgedAdmittedGenerationForTest(const RootNamespace & ns)
+{
+    return ref_ledger.wedgedAdmittedGenerationForTest(ns);
+}
+
+std::optional<RefTxnId> Pool::lastEpochSealForTest(const RootNamespace & ns)
+{
+    return ref_ledger.lastEpochSealForTest(ns);
+}
+
+void Pool::setLastEpochSealForTest(const RootNamespace & ns, const std::optional<RefTxnId> & seal)
+{
+    ref_ledger.setLastEpochSealForTest(ns, seal);
+}
+
+RefLaneState Pool::laneStateForTest(const RootNamespace & ns)
+{
+    return ref_ledger.laneStateForTest(ns);
+}
+
+bool Pool::needsStalePrecommitSweepForTest(const RootNamespace & ns)
+{
+    return ref_ledger.needsStalePrecommitSweepForTest(ns);
+}
+
+void Pool::waitForSnapshotPublishSettleForTest(const RootNamespace & ns)
+{
+    ref_ledger.waitForSnapshotPublishSettleForTest(ns);
+}
+
+int Pool::pendingSnapshotPublishesForTest(const RootNamespace & ns)
+{
+    return ref_ledger.pendingSnapshotPublishesForTest(ns);
+}
+
+std::optional<RefTxnId> Pool::newestPublishedSnapshotIdForTest(const RootNamespace & ns)
+{
+    return ref_ledger.newestPublishedSnapshotIdForTest(ns);
+}
+
+bool Pool::refTableRecoveredForTest(const RootNamespace & ns)
+{
+    return ref_ledger.refTableRecoveredForTest(ns);
+}
+
+bool Pool::refRecoveryCancelRequestedForTest(const RootNamespace & ns)
+{
+    return ref_ledger.refRecoveryCancelRequestedForTest(ns);
+}
+
+void Pool::cancelRefRecoveriesAndAwaitQuiescence()
+{
+    ref_ledger.cancelRecoveriesAndAwaitQuiescence();
+}
+
+size_t Pool::tailSinceSnapshotCountForTest(const RootNamespace & ns)
+{
+    return ref_ledger.tailSinceSnapshotCountForTest(ns);
+}
+
+size_t Pool::committedOverlayEntriesForTest(const RootNamespace & ns)
+{
+    return ref_ledger.committedOverlayEntriesForTest(ns);
+}
+
+std::set<std::pair<String, ManifestRef>> Pool::livePrecommitsForTest(const RootNamespace & ns)
+{
+    return ref_ledger.livePrecommitsForTest(ns);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h
new file mode 100644
index 000000000000..5016d5283979
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h
@@ -0,0 +1,1159 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPlainObjects.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasManifestReader.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h>
+#include <Common/CacheBase.h>
+#include <Common/CurrentMetrics.h>
+#include <Common/HashTable/Hash.h>
+#include <Common/ProfileEvents.h>
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <deque>
+#include <cstdint>
+#include <functional>
+#include <future>
+#include <limits>
+#include <map>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <set>
+#include <unordered_map>
+#include <utility>
+
+namespace DB::Cas
+{
+
+
+/// Configuration supplied when opening a content-addressed pool. The fields remain flat for the
+/// compatibility of existing wiring and tests, while `refLedgerConfig` and `mountConfig` project
+/// the fields owned by those subsystems into typed values passed by value. The projections avoid an
+/// include cycle and make it explicit that the subsystem objects do not retain a reference to this
+/// configuration object.
+struct PoolConfig
+{
+    String pool_prefix;
+    UInt128 server_id{};                      /// owner token (ServerUUID) — provenance + watermark
+    /// Explicit, configured identity of the layout subtree this server owns. Required + validated
+    /// (clean relative path); `ServerUUID`/`server_id` is demoted to an
+    /// owner token. Validated via `Cas::validateServerRootId`.
+    String server_root_id;
+    uint64_t blob_header_len = 256;           /// creation-time only; the pool is authoritative on reopen
+    /// CAS mixed-algo pools: the
+    /// NODE-LOCAL algo this Pool writes NEW content with (`Pool::writeAlgo()`). NOT durable pool
+    /// state -- two live nodes may intentionally write with different (already-admitted) algos, so
+    /// no single truthful pool-wide value exists. `PoolMeta::createOrValidate` accepts it with no
+    /// write when it is already a member of the pool's `algos_used`; otherwise it is admitted via
+    /// the CAS-union below (opt-in, `blob_hash_allow_new`) or refused (BAD_ARGUMENTS, the default --
+    /// a changed config alone must never silently turn a pool mixed). Default `CityHash128` keeps
+    /// every existing pool's hash byte-for-byte unchanged.
+    BlobHashAlgo blob_hash_algo = BlobHashAlgo::CityHash128;
+    /// Opt-in for `blob_hash_algo` to be ADMITTED into the pool's `algos_used` when it is not
+    /// already a member. Consulted only on
+    /// the FIRST open that would otherwise refuse -- once admitted, `algos_used` membership alone is
+    /// the steady-state check and this flag is not needed again for the same algo. Default `false`
+    /// (fail-closed, matching the default-refuse behavior).
+    bool blob_hash_allow_new = false;
+    /// Dedup cache: byte ceiling for the per-disk known-present blob-hash LRU set. 0 disables the
+    /// cache (every create misses → HEAD-before-PUT only). A hint cache; correctness never depends on
+    /// it (a stale hit is caught by the mandatory HEAD in putBlob).
+    uint64_t deduplication_cache_bytes = 64ULL << 20;        /// 64 MiB
+    /// HEAD-before-PUT: on a dedup-cache MISS, a blob whose body is >= this many bytes is written
+    /// HEAD-first (a cheap HEAD avoids streaming a body that would 412). 0 disables the size trigger.
+    uint64_t deduplication_head_first_min_bytes = 1ULL << 20;   /// 1 MiB
+    /// Part-folder cache: byte bound for the manifest DECODE cache. The old cache
+    /// was count-bounded only (16384 entries) — decoded manifests carry inline bytes, so the worst
+    /// case was multi-GB. 0 disables decode caching (every read decodes fresh — diagnostic mode).
+    uint64_t manifest_decode_cache_bytes = 128ULL << 20;
+    /// How many superseded snapshot generations to retain. After committing
+    /// generation G, generations <= G - this are pruned (bounded per round). 0 = keep ALL
+    /// (debug/forensics — replay GC's in-degree view as-of a past round). Default 3 = the safety
+    /// margin covering any in-flight/resuming leader (a leader more than `keep` generations behind
+    /// has lost its lease; its round-commit CAS fails).
+    uint64_t gc_snapshot_generations_to_keep = 3;
+    /// Blob target shards for GC. Default 1 (a single shard, i.e. no fan-out). Creation-time only;
+    /// the pool is authoritative on reopen. This is the BLOB-HASH-prefix reducer axis.
+    uint64_t gc_shards = 1;
+    /// Cursor-paced orphan part-manifest sweep. The LIST budget bounds cold-prefix enumeration
+    /// per completed GC round; the delete budget separately bounds exact-token destructive work.
+    uint64_t manifest_sweep_list_budget_keys = 1000;
+    uint64_t manifest_sweep_delete_budget_keys = 100;
+    /// Per-round blob-deletion work envelope: caps how many entries the fold's graduation
+    /// (condemned -> delete_pending) and redelete (exact-token delete of a prior delete_pending row)
+    /// arms move out of the durable retired pipeline in one round. Excess entries are carried
+    /// unchanged in `still_retired` and retried next round (never dropped). 0 = unbounded.
+    /// Default UNBOUNDED. A count cap here is not backpressure: it throttles the consumer while the
+    /// producer (inserts, merges) is unaware of it, and the excess is carried in `still_retired`, which
+    /// the next round reads in full -- so a round's cost grows with the debt while its useful work stays
+    /// capped. Under sustained load that is a feedback loop, not a delay.
+    uint64_t gc_round_graduation_budget = 0;
+    uint64_t gc_round_redelete_budget = 0;
+    /// Orphan-manifest sweep: caps on the expensive step the LIST/nomination budgets never covered —
+    /// building a namespace's protection view (catalog-authoritative table recovery + committed-tail
+    /// walk). `sweep_namespace_budget` bounds how many DISTINCT namespaces one page may build a view
+    /// for; `sweep_recovery_op_budget` bounds the total ref-log GET/decode ops the committed-tail walk
+    /// may spend across every namespace, cumulative for the round. Exhausting either retains every
+    /// remaining candidate of the affected namespace on this page (fail-closed; retention is always
+    /// safe) rather than deciding it without a complete protection view.
+    uint64_t gc_round_sweep_namespace_budget = 20;
+    uint64_t gc_round_sweep_recovery_op_budget = 5000;
+    /// Ref-object cleanup (covered log/snapshot exact deletes) and generation-prefix wholesale delete
+    /// (superseded-generation prune) caps, cumulative for the round. `prefix_wholesale_budget` is the
+    /// round's shared remainder every prune `deletePrefixWholesale` call draws from, so no single call
+    /// passes an unbounded `bounded_remaining`.
+    uint64_t gc_round_ref_cleanup_budget = 5000;
+    uint64_t gc_round_prefix_wholesale_budget = 20000;
+    /// The post-CAS hand-off reclaim draws from its OWN reserve, never from `prefix_wholesale_budget`
+    /// above: the prune safely under-serves and retries next round via its cursor, but the hand-off is a
+    /// one-shot event with no reclaimer besides `fsck`, so a prune-heavy round must never be able to
+    /// starve it to zero.
+    /// Default UNBOUNDED, and this one is not a tuning choice. The hand-off is one-shot: a generation it
+    /// cannot fully reclaim in its round is never revisited, because the parent-seal difference that
+    /// triggers it does not recur. A cap on work that has no second chance does not defer the work, it
+    /// leaks it -- the same shape as the manifest-cleanup cap that was removed outright.
+    uint64_t gc_round_handoff_prefix_wholesale_budget = 0;
+    /// `GcOutcomes` per-round entry cap across the redelete/spared audit log, cumulative for the round.
+    /// Bounds only the audit-log write -- the settlement decision it records already happened
+    /// unconditionally in the fold. 0 = unbounded.
+    /// Default UNBOUNDED: nothing is retried on exhaustion because there is nothing left to retry -- the
+    /// decision already happened -- so the only thing a cap drops is the audit row explaining it, and it
+    /// drops exactly the rows of the busiest rounds, which are the ones an investigation needs.
+    uint64_t gc_round_outcome_entry_budget = 0;
+    /// Frontier probes: how many KNOWN-BUT-UNHINTED namespaces one round may walk to prove their
+    /// frontier. A namespace this round's LIST hint still mentions is walked regardless (the round owes
+    /// its edges anyway), and a HELD one is always walked (its hold must be retried by exact key, spec
+    /// §5), so this bounds only the extra exact `GET`s the universe union introduced -- normally zero,
+    /// because ordinary active namespaces normally remain hinted. Running out is NOT an error: the
+    /// unprobed namespaces are simply unproven, which
+    /// suppresses all destruction for the round. 0 => probe none (the exhaustion path, which tests
+    /// drive directly).
+    ///
+    /// Default effectively unbounded, and it must be spelled as a huge number rather than `0`: unlike
+    /// every other budget here, `0` means "probe nothing", not "no cap" -- the tests drive that path
+    /// deliberately, so the sentinel cannot simply be redefined. Exhaustion is the worst failure shape
+    /// of any budget in this struct, because it does not defer work: unprobed namespaces are unproven,
+    /// and one unproven namespace suppresses ALL destruction for the round. A count that is fine for ten
+    /// namespaces silently becomes a permanent GC stop for a pool with enough tables.
+    /// The cost of removing the cap is round LENGTH (extra exact `GET`s, normally zero because active
+    /// namespaces stay hinted) -- which is bounded by nothing today, since rounds have no time deadline.
+    uint64_t gc_frontier_probe_budget = std::numeric_limits<uint64_t>::max();
+    /// skip-unchanged: a GC round may DEFER
+    /// (re-adopt the sealed in-degree generation instead of rebuilding it) when fewer than this many
+    /// shards changed since the last fold AND no destructive decision is due. Default 1 = fold as soon
+    /// as anything changed (batching off; only idle rounds defer). > 1 batches small deltas.
+    uint64_t gc_fold_threshold = 1;
+    /// Liveness bound for batching: force a FOLD after this many consecutive DEFER rounds even below
+    /// the threshold. Inert at gc_fold_threshold == 1 (an idle defer has nothing to fold). Default 8.
+    uint64_t gc_fold_max_defer_rounds = 8;
+    /// Diagnostic-only threshold for a `Removing` catalog life whose terminal cleanup evidence has
+    /// not appeared. Test/config struct seam only; no user-facing setting is registered.
+    uint64_t gc_stuck_removal_rounds = 10;
+    /// gc-rebuild: max in-memory edges per gc-shard batch during rebuildBaseline
+    /// (~32 B each => default ~256 MB); each full batch folds into the next attempt number with the
+    /// previous attempt's runs as priors, so memory is O(budget), never O(edges).
+    uint64_t rebuild_edge_budget = 8000000;
+    /// Bounded pool size for the per-hash freshness
+    /// meta writes GC schedules at condemn/spare/delete (mass-DROP: a round condemning ~1M blobs would
+    /// take hours sequential). Every job internally catches its own exceptions (never wedges the round;
+    /// feedback_ca_gc_never_throw_on_404) and `Gc::runRegularRound` waits for the round's whole batch
+    /// before the round's single gc/state CAS, so the meta writes are durable before that CAS commits.
+    uint64_t gc_meta_pool_size = 16;
+    bool background_watermark = false;       /// tests drive renewOnce explicitly; gates the merged heartbeat's background thread
+    /// Installed on the pool before a writable mount can start its renewal thread.
+    CasEventSink event_sink = {};
+
+    /// Mount-lease TTL: how long a freshly-renewed mount lease is valid. The local
+    /// write fence's monotonic deadline is `renew_time + this`, so a superseded/paused writer is fenced
+    /// once `this` elapses with no successful renew. The background renewer runs every
+    /// `mount_renew_period` (default ttl/3) so a healthy mount renews well before expiry.
+    std::chrono::milliseconds mount_lease_ttl_ms{30000};
+    std::chrono::milliseconds mount_renew_period{10000};   /// = ttl/3 by default
+    bool read_only = false;                   /// observe-only open: skip the mutating capability probe; reads only
+
+    /// Boot-time "start now, fix later": skip the access-check-class part of the capability probe
+    /// (the `_probe/` read/write/delete/list round trip and the store-precondition check) while
+    /// STILL opening writable — a mistyped bucket / transient DNS blip at mount should not hard-fail
+    /// the disk when the operator asked to defer the access check (U#5), mirroring `checkAccess`'s
+    /// `skip_access_check` gate for other disk types. Does NOT skip the single-attempt conditional-
+    /// write gate (`checkConditionalWriteSingleAttemptSupport`, cas-s3-timeout-retry-control):
+    /// that guards every conditional write this writable mount will ever issue against running under
+    /// the disk's ~500-attempt transparent retry policy, a correctness hazard rather than a preflight
+    /// convenience, so `Pool::open` still runs it unconditionally whenever the mount is writable.
+    bool skip_access_check = false;
+
+    /// The CAS retry controller's budget (cas-s3-timeout-retry-control), validated against
+    /// `mount_lease_ttl_ms` at writable open (`Pool::open` calls `validateCasRequestBudget`) — an
+    /// inconsistent budget refuses the mount rather than silently retrying unsafely. Defaults are
+    /// consistent with the default `mount_lease_ttl_ms` above; a caller that raises the lease TTL may
+    /// keep these defaults, but a caller that LOWERS it must revisit this budget too.
+    CasRequestBudget cas_request_budget{};
+
+    /// The write-fence deadline clock (CLOCK_BOOTTIME milliseconds; see `MountFence`). Empty = the real
+    /// boot clock (`Pool::bootMs`); injected by tests to drive the fence deadline deterministically.
+    std::function<uint64_t()> boot_ms_fn = {};
+
+    /// Test hook for open/remount waits: `Pool::waitSleep` -- the mount-claim observation loop's poll --
+    /// routes through this function when set instead of a real `std::this_thread::sleep_for`, so a test
+    /// observes every wait without actually blocking. Empty (the production default) sleeps for real.
+    ///
+    /// That poll is now the ONLY wait either path can block on. The post-reclaim materialization grace
+    /// (`T_mat`) that used to sit beside it is gone: it existed so a straggler conditional `PUT` from a
+    /// dying epoch would land (or exhaust its retries) before the successor started trusting its recovery
+    /// LISTINGS, and recovery does not trust listings any more -- it walks the stream arithmetically and
+    /// closes every dead epoch with an in-band `EpochSeal` at `{E, T+1}`, written as a conditional
+    /// create. The straggler's own conditional create then LOSES against that occupied slot, whenever it
+    /// arrives. Waiting for a race that the protocol already decides is not caution, it is latency.
+    std::function<void(uint64_t)> wait_sleep_fn = {};   /// test hook for open/remount waits
+
+    /// a table becomes a publish candidate once its retained
+    /// tail -- every applied txn strictly above the newest published snapshot, no age filter -- exceeds
+    /// either threshold (their count / the sum of their encoded bytes), or right after recovery replays
+    /// a tail already above one (the mount-time trigger). Publication is background and never blocks an
+    /// append (see `Pool::maybeScheduleSnapshotPublish`). The grace-age holdback this trigger used to
+    /// apply is gone by design: the recovery seal decides a late-arriving predecessor write outright --
+    /// its conditional create loses to the seal already occupying the slot -- so a young txn has nothing
+    /// left to wait out.
+    /// The count default trades write-side PUT volume against read-side cold-fold cost: every publish
+    /// re-encodes and PUTs the FULL snapshot, so a low threshold under sustained load degenerates into
+    /// a near-continuous full-snapshot PUT stream, while on the read side a cold fold pays one GET per
+    /// log the newest snapshot does not cover -- 256 bounds that at 256 extra GETs, each far cheaper
+    /// than the snapshot churn it avoids.
+    uint64_t snapshot_log_count_threshold = 256;
+    uint64_t snapshot_log_bytes_threshold = 1ULL << 20;   /// 1 MiB
+    /// bounded per-table backoff arming a dispatch cooldown after
+    /// a NON-Committed publish outcome (an S3 timeout / uncertain PUT). Without it, a saturated backend
+    /// turns every ref read into a re-dispatched full-snapshot encode+PUT (the read-triggered PUT storm),
+    /// since a non-Committed publish deliberately does not prune the tail (that would be data loss) and so
+    /// leaves the threshold trigger latched. The interval doubles from `initial` up to `max` per
+    /// consecutive failure and resets on the next durable publish; combined with the single-in-flight
+    /// gate, it bounds publish dispatch to O(failures), not O(reads).
+    uint64_t snapshot_publish_backoff_initial_ms = 200;
+    uint64_t snapshot_publish_backoff_max_ms = 30000;
+    /// Bounded per-table cooldown between FAILED stale-precommit sweep attempts (the dangling-precommit
+    /// hazard). A failed/partial sweep re-arms
+    /// `needs_stale_precommit_sweep` instead of consuming the once-per-mount shot (one attempt burned in
+    /// the post-restart error window used to leave a dead incarnation's precommit bindings -- and the
+    /// manifests they protect from the GC orphan sweep -- live forever on a long-lived mount); this
+    /// cooldown keeps the retry from storming a saturated backend, exactly like the publish backoff.
+    /// Doubles from `initial` up to `max` per consecutive failure; reset by a verified-clean sweep.
+    uint64_t precommit_sweep_backoff_initial_ms = 200;
+    uint64_t precommit_sweep_backoff_max_ms = 30000;
+
+    /// resident-memory ceiling for the writer's
+    /// whole-table ref cache (`CasRefLedger::ref_name_slots`). This implementation has no row overlay, so eviction is
+    /// WHOLE-TABLE: when the summed estimated weight of cached tables exceeds this, whole tables are
+    /// dropped (never rows) and the next touch re-recovers them from the durable snapshot+log objects
+    /// Evicting the table drops the entire object; the next access repeats
+    /// recovery"). A table with a wedged append lane, a nonempty pending queue, or any in-flight
+    /// caller/publish (its un-persisted lane/queue state is not reconstructable) is never evicted, and
+    /// neither is the table whose recovery just triggered the pass -- so the effective floor is one
+    /// table. 0 = unbounded (eviction disabled). The estimate is the base snapshot body size plus the
+    /// retained log-tail bytes; both are already tracked, so a mutation costs no extra encode.
+    uint64_t ref_table_cache_bytes = 256ULL << 20;   /// 256 MiB
+
+    /// Projection accessors: build the per-owner typed slice from
+    /// the flat fields above, for BY-VALUE injection into the ref-ledger / mount-runtime components.
+    /// The fields stay flat here so every external caller (wiring, tests) that sets them is unchanged;
+    /// the slices are derived, not stored. `boot_ms_fn` is intentionally in `MountConfig` and reaches
+    /// the ref-ledger as a ctor callback (not duplicated into `RefLedgerConfig`).
+    RefLedgerConfig refLedgerConfig() const
+    {
+        return RefLedgerConfig{
+            .server_root_id = server_root_id,
+            .gc_shards = gc_shards,
+            .snapshot_log_count_threshold = snapshot_log_count_threshold,
+            .snapshot_log_bytes_threshold = snapshot_log_bytes_threshold,
+            .snapshot_publish_backoff_initial_ms = snapshot_publish_backoff_initial_ms,
+            .snapshot_publish_backoff_max_ms = snapshot_publish_backoff_max_ms,
+            .precommit_sweep_backoff_initial_ms = precommit_sweep_backoff_initial_ms,
+            .precommit_sweep_backoff_max_ms = precommit_sweep_backoff_max_ms,
+            .ref_table_cache_bytes = ref_table_cache_bytes,
+        };
+    }
+
+    MountConfig mountConfig() const
+    {
+        return MountConfig{
+            .mount_lease_ttl_ms = mount_lease_ttl_ms,
+            .mount_renew_period = mount_renew_period,
+            .background_watermark = background_watermark,
+            .boot_ms_fn = boot_ms_fn,
+            .wait_sleep_fn = wait_sleep_fn,
+        };
+    }
+};
+
+
+struct PartWriteInfo
+{
+    std::optional<String> intended_ref;       /// "ns/ref" forensics for the envelope (diagnostic)
+    /// The owning root namespace, set EXPLICITLY by the wiring. When present it is authoritative for
+    /// the manifest's owning namespace (PartWriteTxn::manifestNamespace), so a ref that itself contains '/'
+    /// (the `detached/<part>` fold) is staged in the TABLE namespace — NOT in a spurious
+    /// `<ns>/detached` namespace produced by splitting intended_ref on the last '/'. Absent ⇒ fall
+    /// back to splitting intended_ref on the last '/' (the diagnostic-only path used by Core tests).
+    std::optional<RootNamespace> intended_namespace;
+    ProvenanceOp op = ProvenanceOp::Other;
+};
+
+class PartWriteTxn;
+using PartWriteTxnPtr = std::shared_ptr<PartWriteTxn>;
+class Gc;
+class Pool;
+using PoolPtr = std::shared_ptr<Pool>;
+
+/// One listed key `Pool::listNamespaces` could not attribute to a namespace, with the refusal message
+/// that stopped it. Behind Stage B's format bump the only such key is a ref object or a namespace file
+/// that names no LIFE (the un-incarnated Stage A shape), which the `Layout` parsers refuse by name.
+struct UnattributableNamespaceKey
+{
+    String key;
+    String reason;
+
+    bool operator==(const UnattributableNamespaceKey &) const = default;
+};
+
+/// What one `Pool::listNamespaces` enumeration observed: the namespaces it attributed, and the keys it
+/// could not attribute to any namespace at all.
+///
+/// The two halves are separate because a short `namespaces` list is not the same fact as a clean one,
+/// and the enumeration is not the place to decide what the difference means. A namespace disappears
+/// from `namespaces` only if EVERY key that would have named it is in `skipped`, since attribution is
+/// per key -- but "only if" is not "never", and each consumer's stakes differ: a browse probe can
+/// answer conservatively, while a caller that RETIRES a slot on the strength of an empty list cannot
+/// treat an incomplete universe as a drained one.
+struct NamespaceListing
+{
+    std::vector<String> namespaces;
+    std::vector<UnattributableNamespaceKey> skipped;
+};
+
+/// The façade for one content-addressed pool. `open` first validates the backend's conditional-write
+/// capabilities and the durable pool metadata, and refuses to mount when either check fails. A
+/// writable instance owns the mount lease and write fence; the read path uses immutable manifests
+/// and does not participate in GC token ownership. The façade delegates plain-object access,
+/// manifest reads, ref-log mutation, and mount lifecycle to the corresponding member components.
+class Pool : public std::enable_shared_from_this<Pool>
+{
+    /// PartWriteTxn/Gc reach the ref-log lane only through Pool's PUBLIC surface now (the ref subsystem moved
+    /// to the `ref_ledger` member): PartWriteTxn's staging PUTs go through `stagingPutIfAbsent`/
+    /// `stagingConditionalCreate` (which encapsulate the controller call + fence), its ref mutations
+    /// through the public `appendRefOps` delegate; Gc uses the public
+    /// `wedgedRefLaneCount`. No `friend` needed -- both prior friendships were removed when the
+    /// ref-ledger became a member component.
+
+public:
+    /// Construct a pool after backend capabilities and durable pool metadata have been validated.
+    /// For a writable configuration, `open` also claims the configured mount before returning; a
+    /// read-only configuration returns an instance that performs no mutating startup operations.
+    static PoolPtr open(BackendPtr backend, PoolConfig config);
+    /// Admin writer mount of the VICTIM `server_root_id`, for `SYSTEM CAS DROP POOL
+    /// MEMBER`. Impersonates the victim's
+    /// owner uuid (`readOwnerUuid`, or -- when the owner anchor itself is missing -- recovered from a
+    /// lingering mount lease) and mounts writable under `MountClaimPolicy::NoWait`: a live victim
+    /// lease is an immediate `ABORTED` refusal (no wait-and-observe, no FORCE variant), unlike the
+    /// bounded reclaim wait a normal `open` pays. Throws `BAD_ARGUMENTS` when there is nothing to
+    /// decommission (no owner anchor and no mount lease for `victim_srid`).
+    static PoolPtr openForDecommission(BackendPtr backend, PoolConfig config, const String & victim_srid);
+    /// Stop the mount-renewal and remount activity, drain ref-log work, and release the owned
+    /// backend-facing components in their dependency order. Destruction is also the clean-farewell
+    /// path for a writable mount, so it must complete before the owning backend is released.
+    ~Pool();
+
+    /// ---- per-server watermark surface ----
+    /// process_epoch: random nonzero per Pool (process). GC checks epoch EQUALITY, never ordering.
+    uint64_t epoch() const { return mount_runtime.epoch(); }
+    /// The durable-monotone writer_epoch allocated at writable open. On a
+    /// writable Pool this is the value bridged into `process_epoch` (so the watermark + the manifest
+    /// manifest ref carries it); on a read-only open the random `process_epoch` is unchanged and
+    /// no durable epoch is allocated. A self-remount re-establishes this to the fresh incarnation's
+    /// writer_epoch (kept equal to `liveWriterEpoch`). The epoch-aware sweep reads this value.
+    uint64_t writerEpoch() const { return mount_runtime.writerEpoch(); }
+    /// The GC floor: the oldest in-flight build_seq, or next_build_seq when no build is active (so a
+    /// quiescent server's watermark floor advances to the next-to-be-allocated seq). Locks builds_mutex.
+    uint64_t minActive();
+    /// Test/assertion accessor for the next-to-allocate build_seq under the lock.
+    uint64_t peekNextBuildSeq();
+    /// Renew the merged heartbeat once (bump seq, refresh min_active from the live callback, stamp a
+    /// fresh expires_at_ms). The build-watermark floor rides this beat. In production this is driven by
+    /// the background renewer (background_watermark).
+    void renewWatermarkOnce();
+
+    /// ---- local write fence ----
+    /// A purely local, in-memory check — NEVER a per-write S3 read. True iff the fence has not latched
+    /// `lost` and the monotonic deadline has not passed. Permissive until armed: a Pool that has not
+    /// armed the fence (the default deadline is steady_clock::time_point::max()) always allows mutations.
+    bool mayMutate() const;
+    /// Latch the fence to lost (once lost, stays lost). Called by the renewer on a superseded
+    /// or foreign observation; the gated mutate chokepoints then fail closed.
+    void tripMountLost();
+    /// Refresh the write-fence deadline (a CLOCK_BOOTTIME-milliseconds instant; release).
+    /// keeper renew calls this on success.
+    void setMountDeadline(uint64_t deadline_boot_ms);
+    /// Arm the fence at startup: set (uuid, epoch, deadline), clear `lost`.
+    void armMountFence(UInt128 server_uuid, uint64_t writer_epoch, uint64_t deadline_boot_ms);
+    void setArmMountFenceInterpositionHookForTest(std::function<void()> hook)
+    {
+        mount_runtime.setArmMountFenceInterpositionHookForTest(std::move(hook));
+    }
+    /// The fence clock: CLOCK_BOOTTIME in milliseconds (includes VM-suspend time, unlike
+    /// CLOCK_MONOTONIC — see `MountFence`). Consults the injected `config.boot_ms_fn` if set (tests),
+    /// otherwise `bootMs`.
+    uint64_t bootMsNow() const;
+    /// The real boot clock: CLOCK_BOOTTIME in milliseconds. Static so tests can compose it.
+    static uint64_t bootMs();
+
+    /// ---- fence-generation admission (rev.7 [C2]/[D1]; owned by `mount_runtime`) ----
+    /// Bumped on every `tripMountLost`/`armMountFence`. Forwarders used directly by the S3-native
+    /// staging-buffer finalize (`ContentAddressedTransaction::writeFile`) -- the durable-effect site
+    /// outside `CasPlainObjects` that needs to capture-then-recheck a fence-generation token across an
+    /// async, potentially long-running upload. `CasPlainObjects` reaches the same primitives via
+    /// injected callbacks (see its own constructor).
+    uint64_t fenceGeneration() const { return mount_runtime.fenceGeneration(); }
+    /// Throws the typed transient refusal (`throwCasTransientUnavailable`) unless the fence is currently
+    /// held AND still at `admitted_generation`.
+    void checkFenceOrThrow(uint64_t admitted_generation) const { mount_runtime.checkFenceOrThrow(admitted_generation); }
+
+    /// ---- pool lifecycle condition (rev.7 §1; owned by `mount_runtime`) ----
+    /// Atomic read of the current lifecycle condition. Thin forwarder; safe to call from any thread.
+    PoolLifecycle lifecycle() const { return mount_runtime.lifecycle(); }
+    /// Whether the pool has reached one of the two fully-terminal `Vanished` values
+    /// (`VanishedReplaced` / `VanishedForgotten`).
+    bool isVanished() const { return mount_runtime.isVanished(); }
+    /// Whether the terminal-intent latch is published — a natural `enterVanished`, OR FORGET's early
+    /// (spec §5 step 1) `publishVanishedIntent`, and NEVER `IdentityLost` ([C1]). See
+    /// `CasMountRuntime::vanishedIntentPublished`. The GC scheduler consults this ALONGSIDE `isVanished()`
+    /// to self-exit its loops the instant the pool is (being driven) terminal, at the earliest signal.
+    bool vanishedIntentPublished() const { return mount_runtime.vanishedIntentPublished(); }
+    /// The store()-class lifecycle gate: throws the typed `INVALID_STATE` error, whose message names the
+    /// terminal sub-state, when the pool has entered `IdentityLost` or any `Vanished` state; returns
+    /// silently while `Live`/`TransientNotLive`. This is the minimal "nothing silently proceeds" hook the
+    /// metadata storage's `poolAccess()` calls after its null-pool (`throwStorageNotStarted`) check. The
+    /// FULL six-class operation gate — which additionally throws in the transient state and answers
+    /// truth-absent on removes/enumeration — is `checkOpAdmitted`; this covers only the terminal states.
+    void throwIfLifecycleTerminal() const;
+
+    /// A non-gated, I/O-free lifecycle snapshot for `system.cas_mounts` (spec §7,
+    /// Factory class). Reads only the runtime's atomics — NO backend op — so it is truthful in EVERY
+    /// state, including the terminal ones the store()-class surface refuses. `detail` is the same [D5]
+    /// reason text `throwIfLifecycleTerminal` throws (empty while `Live`/`TransientNotLive`), which spec §1
+    /// requires appear verbatim in the snapshot; `since` is the wall-clock second the current non-`Live`
+    /// state was entered (0 while `Live`). The metadata-storage layer maps `lifecycle` to the operator
+    /// vocabulary and derives the enum-clean sub-state word separately (see `CasLifecycleSnapshot`).
+    struct LifecycleSnapshot
+    {
+        PoolLifecycle lifecycle = PoolLifecycle::Live;
+        String detail;
+        time_t since = 0;
+    };
+    LifecycleSnapshot lifecycleSnapshot() const;
+
+    /// `SYSTEM CAS FORGET` — the operator force-Vanish (spec §5). Drives THIS pool to
+    /// `Vanished(forgotten)` with the fence-first protocol, node-locally, regardless of the current
+    /// lifecycle (it works precisely on a NOT-live disk — a stuck transient/`IdentityLost` pool). In order:
+    /// (1) publish the terminal-intent latch FIRST (so the remount loop / keeper callback bail at their
+    /// next step boundary, bounding the joins below); (2) trip the local fence (the deliberate
+    /// decommission act, allowed on a live disk); (3+4) stop the GC scheduler via `stop_and_join_gc` —
+    /// injected because the scheduler is owned above the Pool, a no-op in contexts that run none — and stop
+    /// + join the self-remount thread; (5) drain the ref lanes (bounded) and retire the keeper WITHOUT an
+    /// unearned clean farewell (the lease expires by observation unless the lanes provably drained); then
+    /// (6) publish `Vanished(forgotten)` carrying `reason` (the [D5] message with the operator's decommission
+    /// timestamp). Idempotent: an already-`Vanished` pool returns immediately (first terminal transition
+    /// wins). MUST run on the admin/query thread, never a pool (remount/GC) thread — the joins would
+    /// otherwise self-deadlock (hazard C6).
+    void forgetDisk(const std::function<void()> & stop_and_join_gc, const String & reason);
+
+    /// Test seam: force the pool lifecycle condition directly to `lc` (see
+    /// `CasMountRuntime::setLifecycleForTest`). Lets the operation-gate tests pin each class x state cell
+    /// on a metadata-storage-owned pool without driving a full remount/erase sequence. Never used in
+    /// production.
+    void setLifecycleForTest(PoolLifecycle lc) { mount_runtime.setLifecycleForTest(lc); }
+
+    /// Test seam: publish the terminal-intent latch WITHOUT settling a terminal state (spec §5 step 1 of
+    /// FORGET), so a test can exercise the "FORGET intent published, state still pre-terminal" window — the
+    /// step-0 remount-observer bail (M1) and the GC scheduler's earliest-signal self-exit (C1). Never used
+    /// in production; FORGET reaches `publishVanishedIntent` through `forgetDisk`.
+    void publishVanishedIntentForTest() { mount_runtime.publishVanishedIntent(); }
+
+    /// ---- write side ----
+    PartWriteTxnPtr beginPartWrite(PartWriteInfo info);                          /// W-HEARTBEAT durable before return
+    /// Remove a build_seq from the active set; idempotent (safe from publish/abandon/dtor). Public
+    /// PartWriteTxn-facing surface: a `PartWriteTxn` retires its own seq on finalize/abandon/dtor (previously reached
+    /// via `friend class PartWriteTxn`, removed when the ref-ledger became a member component.
+    void retireBuildSeq(uint64_t seq);
+
+    /// Transfer a destroyed transaction's unresolved precommit-release duty to the mount. The build
+    /// sequence remains active until a later mutation resolves the namespace's every-attempt wedge and
+    /// proves the exact precommit absent or appends its exact removal. `noexcept`: a transaction
+    /// destructor must fail closed by retaining the active build, never terminate while trying to
+    /// allocate queue bookkeeping.
+    void enqueueWriterCleanupDuty(
+        const RootNamespace & ns, const String & ref_name, const ManifestRef & manifest, uint64_t build_seq) noexcept;
+
+    /// ---- read side ----
+    /// `audit` defaults to `Emit` so every existing caller keeps emitting `RefResolve` unchanged; see
+    /// `ResolveAudit`'s doc comment (`CasRefLedger.h`) for the one `Deferred` call site.
+    std::optional<Resolved> resolveRef(const RootNamespace & ns, const String & ref_name, bool allow_stale = false,
+                                       ResolveAudit audit = ResolveAudit::Emit);
+    /// Gate 1 of the relink confirm -- a thin forward to the ref ledger, whose declaration carries the
+    /// rules (`CasRefLedger::confirmExactRef`). Read-only and object-store-I/O-free by contract.
+    ConfirmAnswer confirmExactRef(const RootNamespace & ns, const String & ref_name,
+                                  const ManifestRef & manifest_ref) const
+    {
+        return ref_ledger.confirmExactRef(ns, ref_name, manifest_ref);
+    }
+    /// Read the single immutable part manifest named by `id`. Derives the key via CasLayout::manifestKey,
+    /// decodes the body, and fails CLOSED: a committed ref naming a missing body throws FILE_DOESNT_EXIST
+    /// (INV-NO-DANGLE surfaced on the read path); a body whose `ref` ≠ id.ref (refMatchesBody) or whose
+    /// `root_namespace_id` ≠ id.root_namespace (manifestNamespaceMatches) throws CORRUPTED_DATA — the
+    /// ref is addressing the wrong object, or a cross-namespace dangle. Token-gated decode cache below.
+    PartManifest readManifest(const ManifestId & id);
+    /// Identical to `readManifest` (same mandatory HEAD, same fail-closed validation, same decode
+    /// cache) but returns the SHARED immutable decode the manifest cache holds — no per-call copy.
+    /// The wiring read path uses this variant.
+    std::shared_ptr<const PartManifest> readManifestShared(const ManifestId & id);
+    BlobLocation locate(const ManifestEntry & entry) const;       /// Blob placement only
+    std::map<String, Resolved> listRefs(const RootNamespace & ns);
+    /// Pure existence probe: whether any committed ref name starts with `prefix`, without
+    /// materializing `listRefs`'s full map. Empty `prefix` means "any ref at all".
+    bool hasAnyRefWithPrefix(const RootNamespace & ns, std::string_view prefix);
+    /// Catalog-authoritative namespaces with the given logical prefix, returned unordered.
+    ///
+    /// The enumeration REPORTS the keys it could not attribute and DECIDES NOTHING about them: it
+    /// neither aborts nor silently drops them, because the right answer differs per consumer and only
+    /// the consumer knows its own stakes. See `NamespaceListing`.
+    NamespaceListing listNamespaces(const String & prefix);
+
+    /// Scoped LIST of the mirrored subtree: the distinct next-path-segment names under
+    /// `roots/<prefix>` (a loose LIST used by browse only; callers re-check `listRefs`/`getFileSize`
+    /// before showing an entry). Not authoritative — logical discovery uses the ref catalog. `prefix`
+    /// is a server-relative or shadow-relative path ending in '/'.
+    std::vector<String> listMirroredChildren(const String & prefix);
+
+    /// ---- ref lifecycle ----
+    void dropRef(const RootNamespace & ns, const String & ref_name);            /// one owner_transition removal txn
+    void updateRefPublishedAt(const RootNamespace & ns, const String & ref_name,
+                          std::function<void(RefPublishedAtUpdate &)> mutator);   /// one set_published_at txn
+    /// The catalog transition to `Removing` happens first, then one ref-log transaction naming every
+    /// owner's exact removal followed by `remove_namespace`. Performs no physical deletion: GC records
+    /// folded terminal evidence and later removes the exact catalog row, while the perpetual janitor
+    /// reclaims dead-life stream, checkpoint, and namespace-file bytes independently.
+    DropNamespaceStats dropNamespace(const RootNamespace & ns);
+    /// Decommission-only exact-life overload; never re-resolves by namespace name.
+    DropNamespaceStats dropNamespace(const NamespaceLifeId & life);
+
+    /// The catalog life this namespace's objects are keyed under, for a WRITER, and the only resolution
+    /// that CREATES one: minted if the catalog names none (a namespace's first namespace file births it
+    /// exactly as its first ref op would). Resolved once per table-open and cached, so this is not a
+    /// per-operation catalog request.
+    NamespaceLifeId namespaceLife(const RootNamespace & ns);
+
+    /// The life a READER or a REMOVER of this namespace's files must use, or `nullopt` when it has no
+    /// readable files -- a never-created namespace, one mid-creation and a dropped table all answer
+    /// alike. NEVER creates a namespace: an uncataloged one is answered from a catalog-only lookup that
+    /// writes nothing, so a probe or an `if_exists` unlink against a table that was never opened cannot
+    /// admit an entry into the pool-wide catalog. Replaces the older "is it removed?" predicate at every
+    /// namespace-file read: the life and the readability come from one observation, so a reader cannot
+    /// pair one with the other's stale answer, and an unreadable namespace yields no life to read with
+    /// rather than a wrong one. See `CasRefLedger`'s declaration.
+    std::optional<NamespaceLifeId> namespaceFilesLifeIfReadable(const RootNamespace & ns);
+
+    /// Thin forward to `CasRefLedger::namespaceStillLogicallyPresent`, whose declaration carries the
+    /// state matrix. The sole caller is `ContentAddressedMetadataStorage::existsDirectory`'s
+    /// `DirShape::TableDir` case.
+    bool namespaceStillLogicallyPresent(const RootNamespace & ns);
+
+    /// GC callback after a proved exact catalog deletion; see `CasRefLedger` for the in-place cached
+    /// runtime invalidation contract.
+    void invalidateRemovedCatalogLife(const NamespaceLifeId & life);
+
+    /// Reconciles cached removal-closed ref runtimes against a complete catalog cut.
+    void reconcileRefCatalogCut(const CasRefCatalog::Snapshot & catalog_cut);
+
+    /// ==== writer ref-log append lane ====
+    ///
+    /// The ONE entry point every ref mutation funnels through -- Pool's own dropRef/updateRefPublishedAt
+    /// above, and (as a friend) PartWriteTxn's precommitAdd/promote/abandon. This is the SOLE ref-persistence
+    /// lane now: the legacy per-(ns,shard) mutable manifest format was removed once GC/sweep/fsck/inspect
+    /// were rewired onto the snapshot+log ref protocol.
+    ///
+    /// `build_ops(state)` is invoked from the per-namespace flush leader with the table's CURRENT cached
+    /// state (reflecting every earlier item of the SAME batch already applied) -- exactly the atomicity
+    /// the old per-shard closure got from running inside the shard's own CAS loop. It may perform
+    /// arbitrary caller-side I/O (PartWriteTxn's blob revalidation) and throw to reject ONLY this item; a
+    /// LOGICAL_ERROR/ABORTED/etc it throws propagates to the item's own caller without touching any
+    /// other queued item. It returns the ops this call contributes to the batch's one transaction.
+    /// `scope` reuses `MutationScope` (Ref(name) may co-batch; WholeShard runs solo -- used here for
+    /// `namespace_birth`, which the flush forces automatically whenever the cached state is not `Live`).
+    ///
+    /// Wedge semantics: at most one unresolved `PUT` per table. An
+    /// `Unresolved` outcome wedges this namespace's lane -- no later id is allocated until that SAME
+    /// (key, bytes) reaches a conclusive outcome. There are exactly three, and the middle one is the
+    /// reason a wedge is no longer a one-way door:
+    ///   it resolves DURABLE -- either an earlier attempt landed, or the bounded retry's own
+    ///     conditional create lands it now -- and is applied to the cache before the next id;
+    ///   a successor's `EpochSeal` occupies the key, which PROVES our bytes never landed and never can:
+    ///     the wedge clears, its callers fail permanently (they were never acknowledged), and the lane
+    ///     resumes only under a later writer epoch;
+    ///   or the process unmounts.
+    /// Every item in the failing batch receives the SAME uncertainty exception (`NETWORK_ERROR`, the
+    /// retry-later class); items already wedged from an EARLIER flush are retried by the NEXT call into
+    /// this namespace's queue -- at most one bounded attempt per flush, under the fence generation the
+    /// wedge was ADMITTED under, never the current one.
+    ///
+    /// `skip_stale_precommit_sweep`:
+    /// suppresses the hoisted `maybeSweepStalePrecommits` call below for THIS call only. Set ONLY by
+    /// `dropNamespace`'s own removal call: that call's `build_ops` already names every current precommit
+    /// binding (stale or not) for removal via `RemoveNamespace`, so the ordinary maintenance sweep is
+    /// redundant there -- and, left enabled, would race it: the sweep runs FIRST (hoisted at this
+    /// function's top) and reclaims any epoch-stale binding in its OWN separate transaction, so
+    /// `dropNamespace`'s later `build_ops` would see it already gone and undercount
+    /// `DropNamespaceStats::precommits`. No other caller passes `true` -- the sweep's behavior for
+    /// ordinary writers is unchanged.
+    RefTxnId appendRefOps(const RootNamespace & ns, MutationScope scope,
+                         std::function<std::vector<RefOp>(const RefTableState &)> build_ops,
+                         RootMutationOrigin origin, RootMutationKind kind,
+                         bool skip_stale_precommit_sweep = false);
+
+    /// the synchronous core of one publish attempt -- copies
+    /// the live `RefTableState` ONCE under `state_mutex` (candidate `X` = the state's `greatest_applied`
+    /// at that instant, no replay), encodes it, and `putIfAbsentControlled`s it off the lock. Returns true iff
+    /// a NEW snapshot was confirmed durable this call (false covers "nothing eligible yet", "nothing
+    /// new to cover", and every non-Committed outcome -- all harmless per the Failure Handling table:
+    /// "Snapshot create fails: keep all logs; writer recovery remains unchanged"). Public so tests can
+    /// drive one attempt deterministically without depending on the background dispatch's timing;
+    /// production reaches it only through `maybeScheduleSnapshotPublish`.
+    bool tryPublishSnapshotAndAdvanceCheckpointOnce(const RootNamespace & ns);
+
+    /// ---- verbatim namespace files (format_version.txt, ...) — plain keys, never content-addressed ----
+    /// Every one of them names ONE LIFE of the namespace (directive §2): the caller passes the life it
+    /// already holds, and none of these issues a catalog request to obtain one.
+    void putNamespaceFile(const NamespaceLifeId & life, const String & name, const String & bytes);
+    std::optional<String> getNamespaceFile(const NamespaceLifeId & life, const String & name);
+    std::vector<String> listNamespaceFiles(const NamespaceLifeId & life);
+    /// Exact-token delete of one verbatim file (no-op when absent). Verbatim files are never
+    /// content-addressed, so a mid-life delete (a pruned mutation entry, a stale tmp) must reclaim
+    /// the object NOW - the reachability GC never scans them.
+    void removeNamespaceFile(const NamespaceLifeId & life, const String & name);
+
+    /// ---- plain mountpoint objects ----
+    /// A loose disk file (the startup write probe; anything written outside a `@cas@` archive) is a
+    /// plain object at its mirrored path `roots/<key>`. No manifest, no journal, no dedup. GC never
+    /// scans these (it deletes only content and folds only registered namespaces); they are owned by
+    /// their path and removed only by `removeMountpointObject`.
+    void putMountpointObject(const String & key, const String & bytes);
+    std::optional<String> getMountpointObject(const String & key);
+    /// Existence check for a loose mountpoint object WITHOUT reading its body. Directory-safe: a HEAD
+    /// routes through the backend's metadata path (a directory reports as not-an-object), so probing
+    /// a directory-shaped pool path (e.g. `store`, system.remote_data_paths traversal) returns false
+    /// instead of a body read that would throw "Is a directory" (EISDIR).
+    bool mountpointObjectExists(const String & key);
+    void removeMountpointObject(const String & key);
+
+    /// Internal surface for PartWriteTxn (same TU family; not for the wiring):
+    const PoolConfig & poolConfig() const { return config; }
+    const PoolMeta & poolMeta() const { return meta; }
+    const Layout & layout() const { return pool_layout; }
+    Backend & backend() { return *pool_backend; }
+    /// The owning `BackendPtr` itself (not just a reference into it): the decommission slot-retirement
+    /// decommission step (`CasDecommission.cpp`) must keep the backend alive across `admin.reset()` -- the graceful
+    /// close that stamps the mount's farewell -- to physically delete the control objects afterward. A
+    /// bare `Backend &` from `backend()` would dangle the instant the owning `Pool` is destroyed.
+    BackendPtr poolBackendPtr() const { return pool_backend; }
+
+    /// Staging PUT surface for `PartWriteTxn`: both wrap the ref-ledger's retry controller
+    /// AND the ref-lane fence predicate, so `PartWriteTxn` reaches neither directly (the `friend` is gone).
+    /// Behavior-identical to the previously-inlined controller+fence at CasPartWriteTxn.cpp stageManifest /
+    /// uploadFromSource; thin delegates to `ref_ledger`.
+    CasWriteOutcome stagingPutIfAbsent(std::string_view key, std::string_view bytes, Token * out_token = nullptr);
+    CasCreateResult stagingConditionalCreate(std::string_view key, const std::function<PutResult()> & attempt);
+    /// Same retry/fence policy as `stagingConditionalCreate`, for a mutable If-Match overwrite.
+    CasOverwriteResult stagingConditionalOverwrite(std::string_view key, std::string_view bytes, const Token & expected);
+    /// Same retry/fence policy as `stagingPutIfAbsent`, for a mutable marker where an existing
+    /// DIFFERENT value at the key is a normal Conflict outcome, not corruption.
+    CasOverwriteResult stagingPutIfAbsentMutable(std::string_view key, std::string_view bytes);
+
+    /// CAS mixed-algo pools:
+    /// the NODE-LOCAL algo this Pool mints NEW content with (`PoolConfig::blob_hash_algo` -- never
+    /// durable pool state, see the field comment). Every write-mint site uses this, never a bare
+    /// `poolMeta()` field (the pool no longer records one truthful write algo).
+    BlobHashAlgo writeAlgo() const { return config.blob_hash_algo; }
+
+    /// Whether `algo` is a member of the pool's `algos_used`, per this Pool's MONOTONE in-memory
+    /// cache (seeded from `algos_used` at open time, unioned by `refreshAdmittedAlgos` -- never
+    /// shrinks). This is the validation-protocol fast path: a hit needs no I/O. A miss
+    /// for an algo this build KNOWS about must be followed by `refreshAdmittedAlgos()` before
+    /// concluding the algo is genuinely not admitted (a long-running fold can overlap a later
+    /// registration by another node) -- callers at the manifest-read boundary do this.
+    bool isAlgoAdmitted(BlobHashAlgo algo) const;
+
+    /// Re-reads `_pool_meta` and unions its CURRENT `algos_used` into the in-memory admitted-algo
+    /// cache (mutex-guarded; monotone -- a concurrent shrink is impossible since `algos_used` is
+    /// itself append-only). Returns the refreshed cache as a sorted vector, for callers that want to
+    /// render it (error messages, diagnostics). THE stale-cache-race fix: call this
+    /// on every admission-check miss, not just once at open.
+    std::vector<uint8_t> refreshAdmittedAlgos();
+
+    /// The writer_epoch of the LIVE mount incarnation. Bumped by `tryRemountOnce` (self-remount
+    /// after a GC fence-out) — a `PartWriteTxn` minted under an older epoch fails closed on its next step.
+    uint64_t liveWriterEpoch() const { return mount_runtime.liveWriterEpoch(); }
+
+    /// Test seam: publish a new live-incarnation writer epoch WITHOUT running a self-remount -- the
+    /// epoch half of what `tryRemountOnce` does alongside its fence re-arm. Lets a test drive an epoch
+    /// transition's WRITER-side effects (INV-2's `prev_epoch_seal` on the first append of the new epoch)
+    /// without the claim machinery and, deliberately, without `quiesceRefTablesForRemount`, so the
+    /// cached ref runtimes survive the transition and the effect under test is isolated from recovery.
+    void setLiveWriterEpochForTest(uint64_t writer_epoch) { mount_runtime.setLiveWriterEpoch(writer_epoch); }
+
+    /// Self-remount after a GC fence-out (liveness counterpart of the fence-out safety rule): the
+    /// OLD incarnation may never write again (the keeper never re-mints), but a FRESH incarnation —
+    /// durable writer_epoch bump + mount reclaim + re-armed write fence — is exactly what a server
+    /// restart would create, so a live server may create it in place. Runs the same claim machinery as
+    /// `Pool::open`. Orchestration stays here; the owned mount primitives it drives (keeper swap,
+    /// epoch bump, fence re-arm) live on `mount_runtime`. Returns false (and changes nothing durable
+    /// beyond the epoch bump) when the
+    /// mount cannot be claimed (foreign owner / a genuinely live twin) — the caller retries. Safe to
+    /// call concurrently (serialized internally); also the synchronous test seam.
+    bool tryRemountOnce();
+
+    /// Test seam: drive the (private) self-remount arm/refuse path directly — in production the
+    /// keeper's on_lost callback calls `scheduleRemount`, otherwise reachable only via the background
+    /// renewer's cadence. Returns true iff a recovery thread is armed after the call.
+    bool scheduleRemountForTest();
+    /// Test seam: how many times `scheduleRemount` has been ENTERED, counted
+    /// unconditionally as its very first statement -- BEFORE the `background_watermark` early-return, so
+    /// this increments even under the default `background_watermark = false` (no thread ever spawns; a
+    /// test never pays for a real self-remount attempt racing this Pool's own still-live keeper, which
+    /// -- confirmed while building this seam -- reliably takes 30+ seconds per call and is not something
+    /// a fast unit test should be driving). Positively pins that a production call site (e.g.
+    /// `reportImpossibleInterference`) actually invoked `scheduleRemount`, as opposed to merely observing
+    /// `mayMutate() == false` (which `tripMountLost` alone already accounts for).
+    uint64_t scheduleRemountCallCountForTest() const { return mount_runtime.scheduleRemountCallCountForTest(); }
+    /// Test seam: latch `remount_shutting_down` exactly as `~Pool()` does at its top, WITHOUT tearing
+    /// the Pool down, so a test can assert `scheduleRemount` refuses to spawn once teardown has begun.
+    void beginShutdownForTest();
+
+
+    /// Known-present blob-hash cache. A HINT only — correctness never
+    /// depends on it: a hit just makes putBlob go HEAD-first, and a stale hit is caught by that HEAD.
+    /// No-ops when disabled (deduplication_cache_bytes == 0). Keyed on the full `BlobRef` pair:
+    /// a bare digest is never the blob identity, and the same digest value under two algos is two
+    /// different objects.
+    bool dedupCacheContains(const BlobRef & ref) const;
+    void dedupCacheAdd(const BlobRef & ref);
+    /// Test seam: retained bytes of the manifest decode cache (0 when disabled).
+    size_t manifestDecodeCacheBytesForTest() const { return manifest_reader.manifestDecodeCacheBytes(); }
+
+    /// ---- event audit (system.cas_log) ----
+    /// The wiring injects a sink (CasEvent -> SystemLog row) when the log is configured; null sink
+    /// (unit tests, log disabled) makes emitEvent a no-op single branch. PartWriteTxn/Gc reach this via
+    /// their owning Pool. `reason`/`detail` on the event carry the decision's full rationale.
+    /// Intended only for pre-open wiring or tests with no active mount thread; later installation races emitters.
+    /// Every component that emits (this `Pool`, the ref ledger, the manifest reader, the mount
+    /// renewer) holds a reference to `event_sink_`, so routing that ONE `std::function` through the
+    /// single `event_dispatcher_` funnels every emitter into serialized, reentrancy-safe delivery
+    /// (stage-1 §1, Task 2). The forwarder is installed only when a real sink is present so
+    /// `hasEventSink`/`event_sink_` stays a truthful "delivery enabled" predicate and the disabled hot
+    /// path still skips constructing events entirely.
+    void setEventSink(CasEventSink sink)
+    {
+        event_dispatcher_.setSink(std::move(sink));
+        if (event_dispatcher_.hasSink())
+            event_sink_ = [this](CasEvent e) { event_dispatcher_.emit(std::move(e)); };
+        else
+            event_sink_ = {};
+    }
+    /// Rvalue-only: forces every call site to `std::move` its (dead-after) `CasEvent` local, so a
+    /// site a future edit forgets to update is a COMPILE ERROR here rather than a silent deep copy.
+    void emitEvent(CasEvent && e) const { if (event_sink_) event_sink_(std::move(e)); }
+    /// Cheap predicate so query-frequency hooks can skip constructing the CasEvent (+ its detail map)
+    /// entirely when the log is disabled (sink null) — a true no-op on the production hot path.
+    bool hasEventSink() const noexcept { return static_cast<bool>(event_sink_); }
+
+    /// Read the current GC round from `gc/state`. Returns 0 when `gc/state` is absent (pool
+    /// never GC'd). Best-effort: its one remaining caller is `tryRemountOnce`'s MountRemount audit
+    /// event, which reports round 0 on any read failure rather than let the error escalate.
+    uint64_t currentGcRound() const;
+
+private:
+
+    /// Construct the in-memory façade from validated backend, configuration, and pool metadata.
+    /// `open` performs the checks and then moves these values here so no partially validated pool is
+    /// exposed to callers.
+    Pool(BackendPtr backend_, PoolConfig config_, PoolMeta meta_);
+
+    /// Mount-claim policy for `mountWritable`.
+    enum class MountClaimPolicy : uint8_t
+    {
+        WaitForExpiry,   /// normal server open — waits out a stale self-lease
+        NoWait,          /// decommission gate — a live lease is an immediate ABORTED refusal
+    };
+
+    /// The writable-mount startup tail shared by `open` and `openForDecommission`: owner claim →
+    /// writer_epoch → mount claim (+fence-recovery loop) → `MountLeaseKeeper` start → watermark
+    /// anchor. `our_uuid` is the identity to mount as -- `config.server_id` for a normal open, the
+    /// victim's owner uuid for decommission (impersonation). `policy` changes only what happens when
+    /// the mount claim does not resolve `Claimed`/`FencedSelf`: `WaitForExpiry` observes a stale-
+    /// looking lease and refuses (`mountDoubleStartMessage`) only once it proves genuinely live;
+    /// `NoWait` refuses immediately, with no observation wait.
+    static void mountWritable(PoolPtr & store, UInt128 our_uuid, MountClaimPolicy policy);
+
+    /// The single serialized, reentrancy-safe event funnel (Task 2). Declared BEFORE `event_sink_` so
+    /// it constructs first and destructs last: the forwarder stored in `event_sink_` references it, and
+    /// reverse-order destruction retires the forwarder before the dispatcher it captures.
+    EventDispatcher event_dispatcher_;
+    /// Null means delivery is disabled and `emitEvent` is a no-op. When a real sink is installed this
+    /// holds a thin forwarder into `event_dispatcher_` (set by `setEventSink`); every other component
+    /// references this member, so all emitters share the one dispatcher.
+    CasEventSink event_sink_;
+
+    /// ==== ref-ledger callbacks that stay on Pool (thin delegates onto `mount_runtime`) ==== The whole
+    /// ref-log / ref-table subsystem moved to the `ref_ledger` member (Pool/CasRefLedger.h) and the mount/
+    /// watermark/build-registry state to `mount_runtime` (Pool/CasMountRuntime.h); these remain here
+    /// because the ledger is injected with them as callbacks (`fence_ok_fn` / `cancel_inflight_builds` /
+    /// `on_impossible_interference`) at construction and they bind to `Pool`.
+
+    /// Delegate to `mount_runtime`: the build registry (`inflight_builds`/`builds_mutex`) moved there.
+    /// Cancel every in-flight build targeting `ns` once its removal transaction is durable; this
+    /// cancels local builds. Injected into `ref_ledger` as the
+    /// `cancel_inflight_builds` callback.
+    void cancelInflightBuildsForNamespace(const RootNamespace & ns);
+
+    /// Delegate to `mount_runtime`: the write fence moved there. pre-attempt fence check: extends
+    /// `mayMutate` with the REMAINING budget check -- an attempt is not even started unless there is
+    /// enough of the mount lease left for one more attempt_timeout plus the lease safety margin. Passed
+    /// as `fence_ok` to every `CasRequestController` call the ref-log writer path makes.
+    bool refAppendFenceOk() const;
+
+    /// incidental-detection reaction for a foreign-interference
+    /// anomaly -- a signal that arrives on an operation the writer already performs (never a dedicated
+    /// probe) and that is impossible under legitimate single-writer operation once the mount lease
+    /// makes `key` exclusively ours: foreign bytes observed at our own wedge key, or the wedge hard
+    /// contract itself violated at new-id-allocation time. LOG_ERROR with full context, emit a
+    /// `ForeignInterference` CasEvent, then fence this mount closed and arm the SAME bounded
+    /// self-remount a foreign/superseded lease renewal already drives (`tripMountLost`/
+    /// `scheduleRemount` -- see the keeper's `on_lost` callback). Diagnosis is strictly off the
+    /// critical path: ONE background GET of `key` (best-effort, single attempt), decoded as far as its
+    /// ref-log header parses, logged -- never blocking or throwing on the caller's thread. Does NOT
+    /// itself throw: every call site raises its OWN `LOGICAL_ERROR` immediately after this returns, so
+    /// the message can name the specific contract that broke.
+    void reportImpossibleInterference(const String & key, const String & reason,
+                                       const std::optional<String> & offending_ns = {});
+
+
+public:
+    /// Test seams: observe resident recovery/wedge state without a private-member friend hack. These
+    /// observers never resolve a name, recover a table, or materialize a runtime.
+    uint64_t refRecoveryRestartsForTest(const RootNamespace & ns);
+    bool refLaneWedgedForTest(const RootNamespace & ns);
+    /// (I1) The object key of the current wedge for `ns`, or empty when the lane is not wedged -- lets a
+    /// test land a DIFFERENT object at the exact wedged key to exercise resolve-time CORRUPTED_DATA.
+    String wedgedKeyForTest(const RootNamespace & ns);
+    /// test seam: force this table's wedge to a synthetic value directly under `state_mutex`,
+    /// bypassing every production trigger. The ONLY way to construct the provably-unreachable state the
+    /// release-mode wedge-contract guard in `flushRefBatch` defends against (a wedge still present at
+    /// the new-id-allocation point) -- combine with `setRefPreCarveHookForTest` to install it AFTER the
+    /// top-of-flush wedge-resolution check has already run clean but BEFORE the batch is carved.
+    void forceWedgeForTest(const RootNamespace & ns, uint64_t writer_epoch, uint64_t ref_sequence,
+                           const String & key, const String & bytes,
+                           std::optional<uint64_t> admitted_generation = std::nullopt);
+    /// test seam: the fence generation the current wedge was ADMITTED under (0 when not wedged) -- the
+    /// value every later retry of that wedge is gated on. See `CasRefLedger::RefAppendAttempt`.
+    uint64_t wedgedAdmittedGenerationForTest(const RootNamespace & ns);
+    /// test seams: this table's `prev_epoch_seal` source -- the seal that closed its previous writer
+    /// epoch, `nullopt` at genesis. The setter stands in for the recovery CAS-walk that produces it.
+    std::optional<RefTxnId> lastEpochSealForTest(const RootNamespace & ns);
+    void setLastEpochSealForTest(const RootNamespace & ns, const std::optional<RefTxnId> & seal);
+    /// Test seam: this table's append lane state.
+    RefLaneState laneStateForTest(const RootNamespace & ns);
+
+    /// Whether this resident table still owes a stale-precommit sweep (armed by recovery; re-armed by a
+    /// failed attempt; cleared permanently only by a verified-clean sweep).
+    bool needsStalePrecommitSweepForTest(const RootNamespace & ns);
+
+    /// Number of ref-append lanes currently wedged (an uncertain PUT exhausted its retry budget and
+    /// the lane blocks until the same key resolves durable or is conclusively rejected). Per-disk GC
+    /// health for system.cas_mounts. O(live tables); takes each runtime state lock.
+    size_t wedgedRefLaneCount();
+
+    /// test seam: blocks until every background snapshot-publish attempt dispatched so far for
+    /// `ns` has settled. Needed only by tests that exercise the REAL background dispatch (production
+    /// concurrency); tests that just want deterministic publish-logic coverage call
+    /// `tryPublishSnapshotAndAdvanceCheckpointOnce` directly instead.
+    void waitForSnapshotPublishSettleForTest(const RootNamespace & ns);
+
+    /// test seam: the count of in-flight background snapshot-publish attempts for resident `ns` (the
+    /// single-in-flight gate holds this at <= 1).
+    int pendingSnapshotPublishesForTest(const RootNamespace & ns);
+
+    /// test seam: the id of the newest snapshot this resident runtime has confirmed durable (recovered
+    /// or published), or `nullopt` if none.
+    std::optional<RefTxnId> newestPublishedSnapshotIdForTest(const RootNamespace & ns);
+
+    /// test seam: whether `ns` has a RECOVERED cached runtime, WITHOUT forcing a recovery to find out.
+    bool refTableRecoveredForTest(const RootNamespace & ns);
+    /// test seam: whether the self-remount barrier's cancellation request is visible for `ns`.
+    bool refRecoveryCancelRequestedForTest(const RootNamespace & ns);
+
+    /// The self-remount's cancel-or-join barrier over in-flight ref-table recoveries (spec §3).
+    /// `tryRemountOnce` runs it immediately before quiescing the tables and re-arming the mount fence;
+    /// exposed so the barrier itself can be driven directly by a test.
+    void cancelRefRecoveriesAndAwaitQuiescence();
+
+    /// test seam: count of applied txns retained above `newestPublishedSnapshotIdForTest` (the
+    /// tail a snapshot candidate would replay from).
+    size_t tailSinceSnapshotCountForTest(const RootNamespace & ns);
+    size_t committedOverlayEntriesForTest(const RootNamespace & ns);
+
+    /// test seam: the ledger's live precommit view for `ns` (see
+    /// `CasRefLedger::livePrecommitsForTest`) -- the durable-but-unpromoted owner bindings, which is
+    /// what an abandoned/aborted build must leave empty.
+    std::set<std::pair<String, ManifestRef>> livePrecommitsForTest(const RootNamespace & ns);
+
+    /// test seam: whether any writer cleanup duty is still owed (see `writerCleanupDutiesPending`) --
+    /// the direct signal that a settlement failure retained its duty for retry, independent of any
+    /// build-floor side effect that could have the same shape for an unrelated reason.
+    bool writerCleanupDutiesPendingForTest() const { return writerCleanupDutiesPending(); }
+
+    /// Test-only hook: called by `flushRefBatch`
+    /// right before it carves a batch, i.e. AFTER the table is already recovered -- the one otherwise
+    /// untestable timing window `BlockingGetBackend`-style backend tricks cannot reach, since a warm
+    /// flush performs no I/O between becoming leader and carving. A test blocks here to let a second
+    /// caller's item join `rt->pending` before the carve, forcing deterministic co-batching.
+    void setRefPreCarveHookForTest(std::function<void()> hook) { ref_ledger.setRefPreCarveHookForTest(std::move(hook)); }
+
+    /// Test-only: pre-tenure fault seam for the append-lane leadership acquisition; forwards to
+    /// `CasRefLedger::setRefPreTenureHookForTest` (see it for the baton-safety contract).
+    void setRefPreTenureHookForTest(std::function<void()> hook) { ref_ledger.setRefPreTenureHookForTest(std::move(hook)); }
+    void setAppendAfterRuntimeCaptureHookForTest(std::function<void()> hook)
+    {
+        ref_ledger.setAppendAfterRuntimeCaptureHookForTest(std::move(hook));
+    }
+    void setReadBeforeStateLockHookForTest(std::function<void()> hook)
+    {
+        ref_ledger.setReadBeforeStateLockHookForTest(std::move(hook));
+    }
+    void setReadableCatalogAfterObservationHookForTest(std::function<void()> hook)
+    {
+        ref_ledger.setReadableCatalogAfterObservationHookForTest(std::move(hook));
+    }
+    void setWedgeBeforeSlotOccupyHookForTest(std::function<void()> hook)
+    {
+        ref_ledger.setWedgeBeforeSlotOccupyHookForTest(std::move(hook));
+    }
+    void setNamespacePresenceProbeAfterFirstReadHookForTest(std::function<void()> hook)
+    {
+        ref_ledger.setNamespacePresenceProbeAfterFirstReadHookForTest(std::move(hook));
+    }
+    void setNamespacePresenceProbeAfterTerminalProvenHookForTest(std::function<void()> hook)
+    {
+        ref_ledger.setNamespacePresenceProbeAfterTerminalProvenHookForTest(std::move(hook));
+    }
+    uint64_t recoveryInstallCountForTest() const { return ref_ledger.recoveryInstallCountForTest(); }
+
+    /// Test-only: fault seam for the ref-flush two-phase carve/validation protocol; forwards to
+    /// `CasRefLedger::setCarveHookForTest` (see it for the phase-point contract).
+    void setCarveHookForTest(std::function<void(CasRefLedger::CarvePhaseForTest)> hook)
+    {
+        ref_ledger.setCarveHookForTest(std::move(hook));
+    }
+
+    /// Test-only: negative control for the post-durable install region; forwards to
+    /// `CasRefLedger::setInstallRegionProbeForTest` (see it for what an allocating probe must do).
+    void setInstallRegionProbeForTest(std::function<void()> probe)
+    {
+        ref_ledger.setInstallRegionProbeForTest(std::move(probe));
+    }
+    void setSnapshotAfterCaptureHookForTest(std::function<void()> hook)
+    {
+        ref_ledger.setSnapshotAfterCaptureHookForTest(std::move(hook));
+    }
+    void setSnapshotBeforeCkptCasHookForTest(std::function<void()> hook)
+    {
+        ref_ledger.setSnapshotBeforeCkptCasHookForTest(std::move(hook));
+    }
+
+    /// Test-only: replace the request controller's inter-attempt backoff sleep (e.g. with a no-op) —
+    /// for tests that drive a persistent conditional-write fault to budget exhaustion through a fully
+    /// wired Pool/disk and must not serve the production capped-exponential sleeps for real (see
+    /// `CasRequestController::setSleepFnForTest`). Call before driving traffic; empty restores the
+    /// real sleep.
+    void setCasRetrySleepForTest(std::function<void(uint64_t)> sleep_fn);
+
+    /// Queue depth for the ref-append-lane tests (mirrors `shardQueuePendingForTest`): how many
+    /// `appendRefOps` callers are enqueued for `ns` right now.
+    size_t refQueuePendingForTest(const RootNamespace & ns) { return ref_ledger.refQueuePendingForTest(ns); }
+
+    /// Test seam: whether `ns` currently has an active append-lane leader (the baton). Mirrors
+    /// `refQueuePendingForTest`; used to assert the baton is not stranded on a pre-tenure fault.
+    bool refLeaderActiveForTest(const RootNamespace & ns) { return ref_ledger.refLeaderActiveForTest(ns); }
+
+    /// Test seam: how many concurrent `ensureRefTableRecovered` callers for `ns` are
+    /// PARKED right now waiting on the leader's in-flight recovery (see `RefTableRuntime::
+    /// recovery_waiters_for_test`) -- lets a test `yield()`-poll for "a second caller actually reached
+    /// the wait" deterministically, mirroring `refQueuePendingForTest` above.
+    uint64_t refRecoveryWaitersForTest(const RootNamespace & ns) { return ref_ledger.refRecoveryWaitersForTest(ns); }
+
+    /// cache-eviction test seams: how many whole ref tables are cached right now, and whether a
+    /// specific table's runtime is currently materialized (recovered) in the cache -- a table that was
+    /// evicted reports false until its next touch re-recovers it.
+    size_t refTablesCachedCountForTest() { return ref_ledger.refTablesCachedCountForTest(); }
+    bool refTableCachedForTest(const RootNamespace & ns) { return ref_ledger.refTableCachedForTest(ns); }
+    uint64_t refTableRuntimeIdentityForTest(const RootNamespace & ns)
+    {
+        return ref_ledger.refTableRuntimeIdentityForTest(ns);
+    }
+    uint64_t refTableRuntimeAdmittedFenceGenerationForTest(const RootNamespace & ns)
+    {
+        return ref_ledger.refTableRuntimeAdmittedFenceGenerationForTest(ns);
+    }
+    std::optional<NamespaceLifeId> refTableLifeForTest(const RootNamespace & ns)
+    {
+        return ref_ledger.refTableLifeForTest(ns);
+    }
+
+    /// Recovery-publication inventory seams (forward to `CasRefLedger`): the seeded admission budgets,
+    /// the recovered base snapshot's encoded body size and the tail-since-snapshot byte sum.
+    uint64_t refSnapshotBudgetForTest(const RootNamespace & ns) { return ref_ledger.refSnapshotBudgetForTest(ns); }
+    uint64_t refRemovalBudgetForTest(const RootNamespace & ns) { return ref_ledger.refRemovalBudgetForTest(ns); }
+    uint64_t refBaseSnapshotBytesForTest(const RootNamespace & ns) { return ref_ledger.refBaseSnapshotBytesForTest(ns); }
+    uint64_t refTailBytesSinceSnapshotForTest(const RootNamespace & ns) { return ref_ledger.refTailBytesSinceSnapshotForTest(ns); }
+private:
+    struct WriterCleanupDuty
+    {
+        String ref_name;
+        ManifestRef manifest;
+        uint64_t build_seq = 0;
+    };
+
+    struct WriterCleanupQueue
+    {
+        std::deque<std::shared_ptr<const WriterCleanupDuty>> pending;
+        bool draining = false;
+    };
+
+    /// Drain `ns` before admitting its next ordinary mutation. Only one caller drains a namespace at a
+    /// time; concurrent callers wait so none can overtake a cleanup whose build still holds the active
+    /// watermark floor. The drain calls `ref_ledger` directly to avoid re-entering this Pool wrapper.
+    void drainWriterCleanupDuties(const RootNamespace & ns);
+    bool writerCleanupDutiesPending() const;
+
+    /// The single admission seam for durable ref mutations exposed by `Pool`. Keeping drain-before-call
+    /// here makes a new forwarding entry point visibly choose between servicing writer cleanup and
+    /// deliberately bypassing it; the cleanup implementation itself uses `ref_ledger` directly.
+    template <typename Mutation>
+    decltype(auto) mutateRefsAfterWriterCleanup(const RootNamespace & ns, Mutation && mutation)
+    {
+        drainWriterCleanupDuties(ns);
+        return std::forward<Mutation>(mutation)();
+    }
+
+    BackendPtr pool_backend;
+    PoolConfig config;
+    PoolMeta meta;
+
+    mutable std::mutex writer_cleanup_mutex;
+    std::condition_variable writer_cleanup_cv;
+    std::map<RootNamespace, WriterCleanupQueue> writer_cleanup_queues;
+    /// Sticky fail-close bit for the destructor's allocation-failure path. If a duty could not enter
+    /// the queue, no mount teardown may claim a clean farewell even though the guarded map is empty.
+    std::atomic<bool> writer_cleanup_queue_failed{false};
+
+    /// CAS mixed-algo pools: monotone in-memory cache of `algos_used`, seeded from
+    /// `meta.algos_used` at open. Guards `isAlgoAdmitted`/`refreshAdmittedAlgos` -- ITS OWN mutex,
+    /// not `meta`'s (there is no other mutable access to `meta` post-open; this avoids taking a
+    /// wider lock than the cache needs). Kept sorted (a plain vector; membership is a handful of
+    /// entries, no need for a set).
+    mutable std::mutex admitted_algos_mutex;
+    std::vector<uint8_t> admitted_algos;
+
+    /// Known-present cache: a bytes-bounded LRU set of blob hashes confirmed present in the pool.
+    /// Value is a 1-byte presence marker; DedupWeight charges a fixed per-entry byte estimate so the
+    /// configured `deduplication_cache_bytes` is an honest memory ceiling. nullptr ⇔ disabled.
+    /// Marker stored for a blob hash known to be present. The value has no payload; the cache key is
+    /// the complete `BlobRef`, including its hash algorithm.
+    struct DedupPresent {};
+
+    /// Fixed memory estimate used by `CacheBase` to enforce the configured byte ceiling. It is an
+    /// estimate rather than an allocation measurement, but keeps cache growth bounded predictably.
+    struct DedupWeight
+    {
+        size_t operator()(const DedupPresent &) const { return 64; }
+    };
+    using DeduplicationCache = CacheBase<BlobRef, DedupPresent, BlobRefHash, DedupWeight>;
+    std::unique_ptr<DeduplicationCache> dedup_cache;
+    Layout pool_layout;
+    /// The plain-object surface (namespace files + loose mountpoint objects), extracted from Pool.
+    /// Stateless over `Backend &` + `const Layout &`; declared AFTER
+    /// pool_backend and pool_layout so it is constructed after (and destroyed before) both.
+    CasPlainObjects plain_objects;
+    /// The manifest read path + decode cache + locate, extracted from Pool.
+    /// Injected with backend/layout/meta + the event-sink reference; owns the decode cache (whose
+    /// synchronization is CacheBase-internal). Declared after event_sink_, pool_backend, meta, and
+    /// pool_layout so it is constructed after (and destroyed before) all four.
+    CasManifestReader manifest_reader;
+    /// The writer ref-log / ref-table subsystem, extracted from Pool. Owns the
+    /// whole-table ref cache, the append lane + wedge protocol, snapshot publication, stale-precommit
+    /// sweep, cache-budget eviction, the remount/shutdown drain, and the CAS retry controller -- with
+    /// the two ref mutexes. Declared AFTER event_sink_, pool_backend, meta, pool_layout, plain_objects
+    /// and manifest_reader so it is constructed after (and destroyed before) every dependency it is
+    /// injected with; its callbacks reach mount/watermark state now owned by `mount_runtime` (declared
+    /// AFTER this member), but they capture `Pool` and run only at runtime after the Pool is fully
+    /// constructed -- exactly as in the pre-3.5 layout, where the mount raw-members these callbacks reach
+    /// were also declared after `ref_ledger`. `~Pool` still calls `ref_ledger.drainRefLanesForShutdown`
+    /// explicitly, sequenced between `mount_runtime.stopRemountThread()` and
+    /// `mount_runtime.finishTeardown()` exactly as before.
+    CasRefLedger ref_ledger;
+    /// The mount / write-fence / build-watermark / self-remount runtime, extracted
+    /// from Pool. Owns the `MountLeaseKeeper`, the local `MountFence`, the per-server
+    /// build watermark (`process_epoch` + the `builds_mutex`-guarded seq/registry) and its in-flight-build
+    /// map, the live-incarnation `live_writer_epoch`, the unclean-epoch high-water-mark, and the
+    /// self-remount recovery thread (with its own thread-lifecycle locks). Injected with backend/layout +
+    /// the `MountConfig` slice + `server_root_id` + the event-sink reference + the pool `cas_request_budget`
+    /// + a `remount_attempt` callback (== `Pool::tryRemountOnce`, which STAYS on Pool: the claim/recovery
+    /// ORCHESTRATION drives these owned primitives).
+    ///
+    /// Declared AFTER `ref_ledger` -- preserving the pre-3.5 relative order VERBATIM (the mount raw-members
+    /// this component replaces all sat after `ref_ledger`), so `mount_runtime` is destroyed FIRST and
+    /// `ref_ledger` LAST. Both orders were proven equally safe -- `~Pool`
+    /// quiesces both subsystems before ANY member dtor runs (stopRemountThread ->
+    /// ref_ledger.drainRefLanesForShutdown -> mount_runtime.finishTeardown), and the ledger's async paths
+    /// pin `Pool::shared_from_this`, so no ledger->mount callback can fire during destruction in either
+    /// order. Both safe ⇒ this is a pure behavior-preserving relocation, so the ORIGINAL order is kept and
+    /// NO member-order change is introduced. Declared after event_sink_, pool_backend, config and
+    /// pool_layout so it is constructed after every dependency it is injected with.
+    CasMountRuntime mount_runtime;
+
+    /// Serializes `tryRemountOnce` (whose claim/recovery ORCHESTRATION stays on Pool). STAYS here with
+    /// its guarded critical section: the self-remount thread-lifecycle locks + fence
+    /// atomics + build registry moved to `mount_runtime`, but the top-level remount serialization guards
+    /// the Pool-side orchestration, so it stays on Pool.
+    std::mutex remount_mutex;
+
+    /// The single home of the [D5] per-lifecycle reason detail (spec §1) — the human-readable text that
+    /// names the ACTUAL sub-state, WITHOUT the `content-addressed pool '<id>' ` prefix. Both
+    /// `throwIfLifecycleTerminal` (which prefixes it and throws) and the non-gated `lifecycleSnapshot`
+    /// (which surfaces it verbatim in the system table) read it, so the error message and the introspection
+    /// row can never drift. Empty for `Live`/`TransientNotLive` (no terminal detail); for
+    /// `VanishedForgotten` it prefers the stored `vanishedReason()` (carrying the operator's decommission
+    /// timestamp) and falls back to the static [D5] text when none was recorded (a forced-for-test state).
+    String lifecycleReasonDetail(PoolLifecycle lc) const;
+
+    /// NOTE (M-C2): the ref-log is never trimmed here — trimming needs GC's fold state
+    /// (`last_folded_ref_id`, INV-JOURNAL-COVERAGE), which is GC state landing in M-C3.
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPoolMeta.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPoolMeta.cpp
new file mode 100644
index 000000000000..b658d9458b12
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPoolMeta.cpp
@@ -0,0 +1,168 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Common/Exception.h>
+#include <Common/thread_local_rng.h>
+#include <algorithm>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int LOGICAL_ERROR;
+    extern const int INVALID_STATE;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Two `thread_local_rng` u64 draws composed into a 128-bit id.
+UInt128 mintPoolId()
+{
+    const UInt128 hi = thread_local_rng();
+    const UInt128 lo = thread_local_rng();
+    return (hi << 64) | lo;
+}
+
+/// Whether `config_algo` is already a registered member of `pool.algos_used`. Membership, not the
+/// opt-in flag, is the steady-state check. `algos_used` is kept sorted, so this is a binary search.
+bool isAlgoAdmittedIn(const PoolMeta & pool, BlobHashAlgo config_algo)
+{
+    const auto v = static_cast<uint8_t>(config_algo);
+    return std::binary_search(pool.algos_used.begin(), pool.algos_used.end(), v);
+}
+
+/// Renders `algos_used` as "ch128, sha256" for the refusal message below.
+String joinAlgoNames(const std::vector<uint8_t> & algos_used)
+{
+    String out;
+    for (size_t i = 0; i < algos_used.size(); ++i)
+    {
+        if (i != 0)
+            out += ", ";
+        out += blobHashAlgoName(static_cast<BlobHashAlgo>(algos_used[i]));
+    }
+    return out;
+}
+
+/// Fail-closed on a non-admitted algo without the opt-in flag: admission is EXPLICIT
+/// opt-in; the default stays fail-closed -- a changed config alone must never silently turn a pool
+/// mixed. Never touches the pool.
+[[noreturn]] void throwNotAdmitted(const PoolMeta & pool, BlobHashAlgo config_algo)
+{
+    throw Exception(ErrorCodes::BAD_ARGUMENTS,
+        "CAS pool blob_hash mismatch: pool has {{{}}}; config requests {}; set "
+        "<blob_hash_allow_new>1</blob_hash_allow_new> to admit a new algo into this pool",
+        joinAlgoNames(pool.algos_used), blobHashAlgoName(config_algo));
+}
+
+/// The relaxed admission check (replaces an earlier fail-close that required the single pool algo to match):
+/// `pm`/`token` are the most-recently-read `_pool_meta` state (present, decoded, valid). Already a
+/// member of `algos_used` => OK, no write (steady state). Not a member and `!allow_new` =>
+/// `BAD_ARGUMENTS` (the pool is never touched). Not a member and `allow_new` => CAS-union `config_algo`
+/// into `algos_used` (recomputed from the FRESH value on every retry -- union-only, so there is no
+/// ABA) and raises `min_reader_generation` to THIS build's own floor (`G_BUILD`, `CasFormat.h`) in
+/// the SAME write (first registration of a schema-3-bearing algo also raises
+/// `min_reader_generation` -- a build that cannot decode schema-3 settlement state has an OLDER
+/// `G_BUILD` and is correctly refused by the startup gate once a future generation bump lands here).
+/// On a CAS conflict, re-read and retry the whole decision (a concurrent admitter may have unioned a
+/// DIFFERENT algo, or the very one we wanted, in the meantime).
+PoolMeta admitOrValidate(
+    Backend & backend, const String & key, PoolMeta pm, Token token,
+    BlobHashAlgo config_algo, bool allow_new)
+{
+    for (;;)
+    {
+        if (isAlgoAdmittedIn(pm, config_algo))
+            return pm;
+
+        if (!allow_new)
+            throwNotAdmitted(pm, config_algo);
+
+        PoolMeta next = pm;
+        next.algos_used.push_back(static_cast<uint8_t>(config_algo));
+        std::sort(next.algos_used.begin(), next.algos_used.end());
+        next.min_reader_generation = G_BUILD;
+
+        const CasResult res = backend.casPut(key, encodePoolMeta(next), token);
+        if (res.outcome == CasOutcome::Committed)
+            return next;
+
+        auto fresh = backend.get(key);
+        if (!fresh)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS pool meta: '{}' vanished mid-admission (conflicting write then a concurrent delete)", key);
+        pm = decodePoolMeta(fresh->bytes);
+        token = fresh->token;
+        /// loop: re-evaluate membership against the FRESH pm (never re-encode the stale `next`)
+    }
+}
+
+}
+
+PoolMeta PoolMeta::createOrValidate(
+    Backend & backend, const Layout & layout, uint64_t blob_header_len, uint64_t gc_shards,
+    BlobHashAlgo blob_hash_algo, bool allow_new, bool allow_mint)
+{
+    /// The passed config is the caller's responsibility — reject bad values before any I/O.
+    validatePoolBlobHeaderLen(blob_header_len, ErrorCodes::BAD_ARGUMENTS, "pool meta");
+    if (gc_shards == 0)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "CAS pool meta: gc_shards must be >= 1");
+    /// Defense against a garbage `static_cast` past the caller's own boundary: `blobHashAlgoName`
+    /// throws BAD_ARGUMENTS for anything `BlobHashAlgo` does not actually admit.
+    blobHashAlgoName(blob_hash_algo);
+
+    const String key = layout.poolMetaKey();
+
+    /// Present => the pool is authoritative; ignore the passed config's blob_header_len and run the
+    /// flag-gated admission check rather than the old single-value fail-close.
+    if (auto existing = backend.get(key))
+    {
+        PoolMeta pm = decodePoolMeta(existing->bytes);
+        return admitOrValidate(backend, key, std::move(pm), existing->token, blob_hash_algo, allow_new);
+    }
+
+    /// Absent => mint a pool id and try to create the object with `algos_used = {blob_hash_algo}`.
+    /// Every pool this build creates is schema-3-shaped from birth (schemas 1/2 do not exist in this
+    /// build at all), so the reader-generation floor is stamped at THIS build's `G_BUILD` at
+    /// creation, not left at 0.
+    ///
+    /// BOOTSTRAP GATE (spec §2 [C4][D2]): minting is permitted ONLY on the verified bootstrap path. A
+    /// non-bootstrap caller (a read-only/observe open, `openForDecommission`) passes `allow_mint=false`
+    /// and fails closed here — never minting a fresh identity outside that path (an observe scan that
+    /// minted would poison the next writable mount's residual check). The residual EMPTINESS proof itself
+    /// cannot live here: it must precede ANY write (spec's "zero-write residual check FIRST, before ANY
+    /// probe write"), and by the time `createOrValidate` runs the capability battery has already written
+    /// to the prefix, so `Pool::open` runs `probePoolBootstrapResidual` up front and only then passes
+    /// `allow_mint=true`. `pool_prefix` is exclusively CAS-owned.
+    if (!allow_mint)
+        throw Exception(ErrorCodes::INVALID_STATE,
+            "CAS pool meta: _pool_meta absent — refusing to mint outside the verified bootstrap path; "
+            "run a writable mount (or recreate the pool)");
+
+    PoolMeta pm;
+    pm.pool_id = mintPoolId();
+    pm.blob_header_len = blob_header_len;
+    pm.gc_shards = gc_shards;
+    pm.min_reader_generation = G_BUILD;
+    pm.algos_used = {static_cast<uint8_t>(blob_hash_algo)};
+
+    if (backend.casPut(key, encodePoolMeta(pm), /*expected*/ std::nullopt).outcome == CasOutcome::Committed)
+        return pm;
+
+    /// Lost the race: the winner's object MUST be present now. The loser UNIONS its algo via the SAME
+    /// flag-gated admission path as a reopen, instead of the old unconditional fail-close.
+    auto winner = backend.get(key);
+    if (!winner)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS pool meta: create-if-absent reported Conflict but '{}' is absent on re-read", key);
+    PoolMeta winner_pm = decodePoolMeta(winner->bytes);
+    return admitOrValidate(backend, key, std::move(winner_pm), winner->token, blob_hash_algo, allow_new);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.cpp
new file mode 100644
index 000000000000..b9b952a7321a
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.cpp
@@ -0,0 +1,654 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Common/Exception.h>
+#include <Common/thread_local_rng.h>
+#include <fmt/format.h>
+#include <algorithm>
+#include <exception>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+
+{
+
+CasRefCatalog::Snapshot readOptionalForBootstrap(Backend & backend, const Layout & layout)
+{
+    const auto got = backend.get(layout.refCatalogKey());
+    if (!got)
+    {
+        RefCatalog empty;
+        return CasRefCatalog::Snapshot{
+            .catalog = empty, .token = std::nullopt, .life_index = CatalogLifeIndex(empty)};
+    }
+    RefCatalog catalog = decodeRefCatalog(got->bytes);
+    return CasRefCatalog::Snapshot{
+        .catalog = catalog, .token = got->token, .life_index = CatalogLifeIndex(catalog)};
+}
+
+}
+
+CasRefCatalog::Snapshot CasRefCatalog::read(Backend & backend, const Layout & layout)
+{
+    Snapshot snapshot = readOptionalForBootstrap(backend, layout);
+    if (!snapshot.token)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "Mandatory CAS ref catalog '{}' is absent -- refusing to interpret opaque life "
+            "objects as an empty ownership universe",
+            layout.refCatalogKey());
+    return snapshot;
+}
+
+CasRefCatalog::Snapshot CasRefCatalog::initializeEmptyForNewPool(Backend & backend, const Layout & layout)
+{
+    RefCatalog empty;
+    const String canonical_empty = encodeRefCatalog(empty);
+    const PutResult put = backend.putIfAbsent(layout.refCatalogKey(), canonical_empty);
+    if (put.outcome == PutOutcome::Done)
+        return Snapshot{.catalog = empty, .token = put.token, .life_index = CatalogLifeIndex(empty)};
+
+    /// A second opener can win after both proved the prefix empty. Decode its exact object before
+    /// accepting the race; conflict is never a license to continue with an assumed empty catalog or
+    /// arbitrary decoded body.
+    const auto got = backend.get(layout.refCatalogKey());
+    if (!got)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS ref catalog '{}' disappeared after bootstrap create conflict",
+            layout.refCatalogKey());
+    RefCatalog catalog = decodeRefCatalog(got->bytes);
+    if (!catalog.entries.empty() || got->bytes != canonical_empty)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS ref catalog '{}' conflicts with bootstrap's required canonical empty catalog",
+            layout.refCatalogKey());
+    return Snapshot{.catalog = std::move(catalog), .token = got->token, .life_index = CatalogLifeIndex(empty)};
+}
+
+std::optional<NamespaceLifeId> CasRefCatalog::lifeIfCataloged(
+    Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const Snapshot snap = read(backend, layout);
+    for (const CatalogEntry & entry : snap.catalog.entries)
+        if (entry.ns.string() == ns.string() && entry.state != NsState::Creating)
+            return snap.life_index.resolve(entry.incarnation);
+    return std::nullopt;
+}
+
+std::vector<NamespaceLifeId> CasRefCatalog::liveUniverse(Backend & backend, const Layout & layout)
+{
+    const Snapshot snap = read(backend, layout);
+    snap.life_index.throwIfAmbiguous("CAS live namespace discovery");
+    std::vector<NamespaceLifeId> universe;
+    universe.reserve(snap.catalog.entries.size());
+    for (const CatalogEntry & entry : snap.catalog.entries)
+    {
+        if (entry.state == NsState::Creating)
+            continue;
+        universe.push_back(NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation));
+    }
+    return universe;
+}
+
+namespace
+{
+
+/// Live-lock brake, the same shape and for the same reason as `publishCkpt`'s/`allocateWriterEpoch`'s
+/// on their own contended token-CAS singletons: the catalog is ONE object mutated by every lifecycle
+/// transition of every namespace in the pool, so persistent contention is a real, not theoretical,
+/// exit condition to plan for.
+constexpr size_t kMaxCatalogCasAttempts = 100;
+
+/// Shared body of `casUpdate`/`casAdmitEntry`. `encode` turns a freshly `mutate`d candidate into the
+/// bytes to write: the plain path just grammar-checks (`encodeRefCatalog`), the admitting path also
+/// runs both admission predicates (`checkCatalogAdmission`) first. Retries on `Conflict` against a
+/// FRESH read, exactly like `PoolMeta::admitOrValidate` -- never re-encoding the stale candidate.
+RefCatalog casUpdateImpl(
+    Backend & backend, const Layout & layout,
+    const std::function<RefCatalog(const RefCatalog &)> & mutate,
+    const std::function<String(const RefCatalog &)> & encode)
+{
+    const String key = layout.refCatalogKey();
+    CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+
+    for (size_t attempt = 0; attempt < kMaxCatalogCasAttempts; ++attempt)
+    {
+        snap.life_index.throwIfAmbiguous("CAS ref catalog mutation");
+        RefCatalog candidate = mutate(snap.catalog);
+        const String bytes = encode(candidate);
+        const CasResult res = backend.casPut(key, bytes, snap.token);
+        if (res.outcome == CasOutcome::Committed)
+            return candidate;
+
+        snap = CasRefCatalog::read(backend, layout);
+        /// `read` treats authoritative absence after a conflict as corruption. Therefore no retry
+        /// can turn a vanished mandatory catalog into a one-update replacement authority.
+    }
+
+    throwCasWriteRetryLater(fmt::format(
+        "CAS ref catalog '{}' did not converge after {} attempts", key, kMaxCatalogCasAttempts));
+}
+
+/// Thrown from inside a `casUpdate` `mutate` closure to signal a refusal that must STOP the attempt
+/// rather than be treated as a `Conflict` to retry: `casUpdateImpl` propagates whatever `mutate`
+/// throws straight out, uncaught, which is exactly the behavior these three need. Retrying any of them
+/// against a freshly re-read catalog would just re-decide against an entry that is, by definition, no
+/// longer `observed` -- token-exactness means the FIRST mismatch is final, not a reason to loop.
+/// Each is caught by its own exact type right where it is thrown; deriving from `std::exception`
+/// is only so the throw itself is well-formed, never so a caller catches these by base class.
+struct CatalogFenceMovedMarker : std::exception {};
+struct CatalogEntryMismatchMarker : std::exception {};
+struct CatalogCreatorStillLiveMarker : std::exception {};
+
+/// Two `thread_local_rng` draws composed into a `UInt128`, the same pattern already used throughout
+/// this tree to mint build ids and incarnation tags (`CasPartWriteTxn.cpp`'s `mintU128`,
+/// `ContentAddressedTransaction.cpp`'s `incarnation_tag`). Retried on the astronomically unlikely `0`
+/// draw: unlike those callers, this value must never be zero (`CatalogEntry::incarnation == 0` is
+/// always invalid -- "0 never names a life"), and this is the one mint site in that family with a
+/// grammar rule to uphold.
+UInt128 mintFreshIncarnation()
+{
+    UInt128 v = 0;
+    while (v == 0)
+        v = (static_cast<UInt128>(thread_local_rng()) << 64) | thread_local_rng();
+    return v;
+}
+
+/// Shared by every function below that needs "the current entry for this namespace, if any" --
+/// keeping ONE lookup rather than three independently-written `find_if`s that could drift apart on
+/// what counts as a match.
+std::vector<CatalogEntry>::const_iterator findEntry(const RefCatalog & catalog, const RootNamespace & ns)
+{
+    return std::find_if(catalog.entries.begin(), catalog.entries.end(),
+        [&](const CatalogEntry & e) { return e.ns.string() == ns.string(); });
+}
+
+/// Thrown from `createNamespaceStep1`'s own `mutate` (below) when a FRESH read -- the first one, or
+/// any `Conflict` retry's re-read -- already carries an entry for the namespace being admitted. Never
+/// thrown by the public `casAdmitEntry`: that function keeps its documented "already-present is a
+/// caller bug, let `encodeRefCatalog` abort" contract for its many single-namespace-per-catalog test
+/// callers -- it has no production caller at all; `createNamespaceStep1` below duplicates its
+/// admission shape rather than calling it, precisely so this recheck can be added without weakening
+/// `casAdmitEntry` itself. `createNamespace` alone needs the other answer, because ITS
+/// "already present" can be a sibling opener's OWN in-flight step 1 landing between createNamespace's
+/// pre-check read and this loop's read -- a race the design already names and resumes through
+/// `Superseded`, not a caller bug.
+struct CatalogEntryAlreadyPresentMarker : std::exception {};
+
+/// Fires once, synchronously, right before `createNamespaceStep1`'s own first catalog read -- i.e.
+/// after `createNamespace`'s pre-check read already observed no entry. Lets a test land a sibling
+/// opener's full `createNamespace` call in that exact window, driving the interleaving
+/// `CatalogEntryAlreadyPresentMarker` exists to catch instead of relying on real thread scheduling.
+/// Empty (no-op) in production, mirroring every other `*_hook_for_test` in this tree.
+std::function<void()> create_namespace_step1_pre_read_hook_for_test;
+
+/// Step 1 of `createNamespace`, split out so it can recheck presence on EVERY catalog read this loop
+/// performs (the first one, and any `Conflict` retry's re-read), not only the snapshot-in-time read
+/// `createNamespace` itself already did before calling in. That single upfront read cannot see a
+/// sibling opener's OWN step 1 landing between it and this loop's read; without the recheck here, this
+/// loop would blindly insert a second row for the same namespace and let `encodeRefCatalog`'s
+/// canonical-order/no-duplicate grammar check abort the process with `LOGICAL_ERROR` for what is, at
+/// this call site only, an ordinary race outcome.
+RefCatalog createNamespaceStep1(
+    Backend & backend, const Layout & layout, uint64_t gc_shards, const CatalogEntry & entry)
+{
+    /// Moved into a local before invoking, not called on the global directly: a hook that reassigns
+    /// `create_namespace_step1_pre_read_hook_for_test` from inside its own body (a test driving a
+    /// one-shot interleaving) would otherwise reassign the very `std::function` object whose `operator()`
+    /// is executing it -- undefined behavior, not merely untidy. The local copy is a distinct object the
+    /// hook body cannot reach.
+    if (create_namespace_step1_pre_read_hook_for_test)
+    {
+        std::function<void()> hook_to_run;
+        std::swap(hook_to_run, create_namespace_step1_pre_read_hook_for_test);
+        hook_to_run();
+    }
+
+    const auto mutate = [&entry](const RefCatalog & cur) -> RefCatalog
+    {
+        if (findEntry(cur, entry.ns) != cur.entries.end())
+            throw CatalogEntryAlreadyPresentMarker{};
+        RefCatalog next = cur;
+        const auto it = std::lower_bound(next.entries.begin(), next.entries.end(), entry,
+            [](const CatalogEntry & a, const CatalogEntry & b) { return a.ns.string() < b.ns.string(); });
+        next.entries.insert(it, entry);
+        return next;
+    };
+    return casUpdateImpl(backend, layout, mutate,
+        [&entry, gc_shards, &layout](const RefCatalog & c)
+        {
+            return checkCatalogAdmission(c, gc_shards, layout, entry.ns);
+        });
+}
+
+}
+
+RefCatalog CasRefCatalog::casUpdate(
+    Backend & backend, const Layout & layout, const std::function<RefCatalog(const RefCatalog &)> & mutate)
+{
+    const auto identity_preserving_mutate = [&](const RefCatalog & current) -> RefCatalog
+    {
+        RefCatalog candidate = mutate(current);
+        if (candidate.entries.size() != current.entries.size())
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CasRefCatalog::casUpdate cannot add or delete catalog entries -- use casAdmitEntry, "
+                "deleteCompletedRemoving, or cancelStalledCreating");
+        for (size_t i = 0; i < current.entries.size(); ++i)
+        {
+            if (candidate.entries[i].ns != current.entries[i].ns
+                || candidate.entries[i].incarnation != current.entries[i].incarnation)
+                throw Exception(ErrorCodes::LOGICAL_ERROR,
+                    "CasRefCatalog::casUpdate cannot replace catalog identity at row {} -- namespace "
+                    "and incarnation are immutable outside the narrow admission/deletion APIs",
+                    i);
+        }
+        return candidate;
+    };
+    return casUpdateImpl(
+        backend, layout, identity_preserving_mutate, [](const RefCatalog & c) { return encodeRefCatalog(c); });
+}
+
+RefCatalog CasRefCatalog::casAdmitEntry(
+    Backend & backend, const Layout & layout, uint64_t gc_shards, const CatalogEntry & entry)
+{
+    if (entry.state == NsState::Removing)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CasRefCatalog::casAdmitEntry cannot admit namespace '{}' directly as Removing -- "
+            "removal is an exact transition of an existing Live row",
+            entry.ns.string());
+
+    /// The mutation shape is FIXED (insert `entry` at its canonical position) rather than a
+    /// caller-supplied lambda -- see the header comment on why that is the point, not an
+    /// inconvenience. A namespace that already has an entry is not de-duplicated here: the insert
+    /// makes the candidate carry two adjacent equal-ns rows, and `encodeRefCatalog`'s own
+    /// canonical-order/no-duplicate check (run inside `checkCatalogAdmission` below) rejects that
+    /// shape -- one place owns that rule, not two.
+    const auto mutate = [&entry](const RefCatalog & cur) -> RefCatalog
+    {
+        RefCatalog next = cur;
+        const auto it = std::lower_bound(next.entries.begin(), next.entries.end(), entry,
+            [](const CatalogEntry & a, const CatalogEntry & b) { return a.ns.string() < b.ns.string(); });
+        next.entries.insert(it, entry);
+        return next;
+    };
+    return casUpdateImpl(backend, layout, mutate,
+        [&entry, gc_shards, &layout](const RefCatalog & c)
+        {
+            return checkCatalogAdmission(c, gc_shards, layout, entry.ns);
+        });
+}
+
+CasRefCatalog::BeginRemovingOutcome CasRefCatalog::beginRemoving(
+    Backend & backend, const Layout & layout, const CatalogEntry & observed,
+    uint64_t removal_started_round, uint64_t admitted_generation,
+    const std::function<void(uint64_t)> & check_fence_or_throw)
+{
+    if (observed.state != NsState::Live || observed.creator || observed.removal_started_round)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CasRefCatalog::beginRemoving: namespace '{}' is not an exact Live entry",
+            observed.ns.string());
+
+    const auto mutate = [&](const RefCatalog & cur) -> RefCatalog
+    {
+        try { check_fence_or_throw(admitted_generation); }
+        catch (...) { throw CatalogFenceMovedMarker{}; }
+
+        const auto it = findEntry(cur, observed.ns);
+        if (it == cur.entries.end() || *it != observed)
+            throw CatalogEntryMismatchMarker{};
+
+        RefCatalog next = cur;
+        CatalogEntry & entry = next.entries[it - cur.entries.begin()];
+        entry.state = NsState::Removing;
+        entry.removal_started_round = removal_started_round;
+        return next;
+    };
+
+    try
+    {
+        casUpdateImpl(backend, layout, mutate, [](const RefCatalog & c) { return encodeRefCatalog(c); });
+    }
+    catch (const CatalogFenceMovedMarker &)
+    {
+        return BeginRemovingOutcome::FencedOut;
+    }
+    catch (const CatalogEntryMismatchMarker &)
+    {
+        const Snapshot current = read(backend, layout);
+        const auto it = findEntry(current.catalog, observed.ns);
+        if (it != current.catalog.entries.end()
+            && it->incarnation == observed.incarnation
+            && it->state == NsState::Removing)
+            return BeginRemovingOutcome::AlreadyRemoving;
+        return BeginRemovingOutcome::EntryChanged;
+    }
+    return BeginRemovingOutcome::Transitioned;
+}
+
+CasRefCatalog::CompletedRemovingDeleteResult CasRefCatalog::deleteCompletedRemoving(
+    Backend & backend, const Layout & layout, const CatalogEntry & observed,
+    const CasFoldSeal & authoritative_parent, uint64_t admitted_generation,
+    const std::function<LeaderFenceStatus(uint64_t)> & check_fence)
+{
+    if (observed.state != NsState::Removing || !observed.removal_started_round)
+        return {
+            .outcome = CompletedRemovingDeleteOutcome::ProofRefused,
+            .invalidated_life = std::nullopt,
+            .catalog_snapshot = std::nullopt};
+
+    const auto parent_it = authoritative_parent.ref_lives.find(observed.incarnation);
+    if (parent_it == authoritative_parent.ref_lives.end()
+        || !parent_it->second.cleanup_evidence
+        || parent_it->second.coverage.hold)
+        return {
+            .outcome = CompletedRemovingDeleteOutcome::ProofRefused,
+            .invalidated_life = std::nullopt,
+            .catalog_snapshot = std::nullopt};
+
+    return deleteCompletedRemovingAtSnapshot(
+        backend, layout, read(backend, layout), observed, authoritative_parent,
+        admitted_generation, check_fence);
+}
+
+CasRefCatalog::CompletedRemovingDeleteResult CasRefCatalog::deleteCompletedRemovingAtSnapshot(
+    Backend & backend, const Layout & layout, Snapshot catalog_snapshot,
+    const CatalogEntry & observed, const CasFoldSeal & authoritative_parent,
+    uint64_t admitted_generation,
+    const std::function<LeaderFenceStatus(uint64_t)> & check_fence)
+{
+    if (observed.state != NsState::Removing || !observed.removal_started_round)
+        return {
+            .outcome = CompletedRemovingDeleteOutcome::ProofRefused,
+            .invalidated_life = std::nullopt,
+            .catalog_snapshot = std::nullopt};
+
+    const auto parent_it = authoritative_parent.ref_lives.find(observed.incarnation);
+    if (parent_it == authoritative_parent.ref_lives.end()
+        || !parent_it->second.cleanup_evidence
+        || parent_it->second.coverage.hold)
+        return {
+            .outcome = CompletedRemovingDeleteOutcome::ProofRefused,
+            .invalidated_life = std::nullopt,
+            .catalog_snapshot = std::nullopt};
+
+    const NamespaceLifeId old_life
+        = NamespaceLifeId::fromCatalogEntry(observed.ns, observed.incarnation);
+    const auto resolved_result = [&](CompletedRemovingDeleteOutcome outcome)
+    {
+        const auto current_it = findEntry(catalog_snapshot.catalog, observed.ns);
+        const bool old_life_still_cataloged = current_it != catalog_snapshot.catalog.entries.end()
+            && current_it->incarnation == observed.incarnation;
+        return CompletedRemovingDeleteResult{
+            .outcome = outcome,
+            .invalidated_life = old_life_still_cataloged
+                ? std::nullopt
+                : std::optional<NamespaceLifeId>{old_life},
+            .catalog_snapshot = std::move(catalog_snapshot)};
+    };
+
+    for (size_t attempt = 0; attempt < kMaxCatalogCasAttempts; ++attempt)
+    {
+        catalog_snapshot.life_index.throwIfAmbiguous("CAS completed-removal deletion");
+        const auto observed_it = findEntry(catalog_snapshot.catalog, observed.ns);
+        if (observed_it == catalog_snapshot.catalog.entries.end() || *observed_it != observed)
+            return resolved_result(CompletedRemovingDeleteOutcome::EntryChanged);
+
+        bool fence_lost = check_fence(admitted_generation) == LeaderFenceStatus::Moved;
+
+        std::optional<CasResult> cas_result;
+        std::exception_ptr attempt_failure;
+        if (!fence_lost)
+        {
+            RefCatalog candidate = catalog_snapshot.catalog;
+            candidate.entries.erase(candidate.entries.begin() + (observed_it - catalog_snapshot.catalog.entries.begin()));
+            try
+            {
+                cas_result = backend.casPut(
+                    layout.refCatalogKey(), encodeRefCatalog(candidate), catalog_snapshot.token);
+            }
+            catch (...)
+            {
+                attempt_failure = std::current_exception();
+            }
+        }
+
+        /// The response to a conditional erase is not authority for what became durable. Resolve
+        /// every attempted erase, and a pre-CAS fence refusal, through one complete catalog read.
+        /// This snapshot is also the next retry/selection cut, so no second read separates them.
+        catalog_snapshot = read(backend, layout);
+
+        if (!fence_lost)
+            fence_lost = check_fence(admitted_generation) == LeaderFenceStatus::Moved;
+        if (fence_lost)
+            return resolved_result(CompletedRemovingDeleteOutcome::FencedOut);
+
+        const auto current_it = findEntry(catalog_snapshot.catalog, observed.ns);
+        const bool old_life_still_cataloged = current_it != catalog_snapshot.catalog.entries.end()
+            && current_it->incarnation == observed.incarnation;
+        if (!old_life_still_cataloged)
+            return resolved_result(current_it == catalog_snapshot.catalog.entries.end()
+                ? CompletedRemovingDeleteOutcome::Deleted
+                : CompletedRemovingDeleteOutcome::EntryChanged);
+
+        if (attempt_failure)
+            std::rethrow_exception(attempt_failure);
+        if (cas_result && cas_result->outcome == CasOutcome::Committed)
+            throwCasWriteRetryLater(fmt::format(
+                "CAS ref catalog erase for namespace '{}' reported committed, but a complete resolution read "
+                "still observed incarnation {}",
+                observed.ns.string(), u128ToHex(observed.incarnation)));
+        /// A token conflict that leaves the exact old row present retries from this mandatory
+        /// resolution snapshot. The fence is checked again immediately before the next CAS.
+    }
+
+    throwCasWriteRetryLater(fmt::format(
+        "CAS ref catalog erase for namespace '{}' did not converge after {} attempts",
+        observed.ns.string(), kMaxCatalogCasAttempts));
+}
+
+CasRefCatalog::StalledCreatingCancelOutcome CasRefCatalog::cancelStalledCreating(
+    Backend & backend, const Layout & layout, const CatalogEntry & observed,
+    const std::function<bool(const CreatorFence &)> & is_creator_fence_terminal,
+    uint64_t admitted_generation, const std::function<void(uint64_t)> & check_fence_or_throw)
+{
+    if (observed.state != NsState::Creating || !observed.creator)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CasRefCatalog::cancelStalledCreating: namespace '{}' is not a Creating entry with a "
+            "creator fence",
+            observed.ns.string());
+
+    const auto mutate = [&](const RefCatalog & cur) -> RefCatalog
+    {
+        try { check_fence_or_throw(admitted_generation); }
+        catch (...) { throw CatalogFenceMovedMarker{}; }
+
+        const auto it = findEntry(cur, observed.ns);
+        if (it == cur.entries.end() || *it != observed)
+            throw CatalogEntryMismatchMarker{};
+        if (!is_creator_fence_terminal(*observed.creator))
+            throw CatalogCreatorStillLiveMarker{};
+
+        RefCatalog next = cur;
+        next.entries.erase(next.entries.begin() + (it - cur.entries.begin()));
+        return next;
+    };
+
+    try
+    {
+        casUpdateImpl(backend, layout, mutate, [](const RefCatalog & c) { return encodeRefCatalog(c); });
+    }
+    catch (const CatalogFenceMovedMarker &) { return StalledCreatingCancelOutcome::FencedOut; }
+    catch (const CatalogEntryMismatchMarker &) { return StalledCreatingCancelOutcome::EntryChanged; }
+    catch (const CatalogCreatorStillLiveMarker &) { return StalledCreatingCancelOutcome::CreatorFenceStillLive; }
+    return StalledCreatingCancelOutcome::Cancelled;
+}
+
+CasRefCatalog::NamespaceCreationOutcome CasRefCatalog::completeCreation(
+    Backend & backend, const Layout & layout, const CatalogEntry & observed,
+    uint64_t admitted_generation, const std::function<void(uint64_t)> & check_fence_or_throw,
+    const CkptDeadline & deadline)
+{
+    if (observed.state != NsState::Creating || !observed.creator)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CasRefCatalog::completeCreation: namespace '{}' is not a Creating entry with a creator "
+            "fence -- steps 2/3 only ever run over one of those", observed.ns.string());
+
+    /// Step 2 (spec §3): INV-4's first `_ckpt` writer for this incarnation, and the only writer that
+    /// will ever know its genesis epoch -- see `Pool/CasRefCkpt.h`'s `publishCkpt` doc for the merge
+    /// discipline this rides on unchanged. `FencedOut` here ends the attempt: nothing durable changed.
+    const RefCkpt contribution{.life_epoch = std::optional<uint64_t>{observed.creator->writer_epoch},
+                                .committed_through = std::nullopt,
+                                .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    if (publishCkpt(backend, layout, NamespaceLifeId::fromCatalogEntry(observed.ns, observed.incarnation),
+                     contribution, admitted_generation, check_fence_or_throw,
+                     deadline) == CkptPublishOutcome::FencedOut)
+        return NamespaceCreationOutcome::FencedOut;
+
+    /// Step 3. `mutate` is the fence re-check point `casUpdate`'s header doc names -- checked FIRST,
+    /// mirroring `publishCkpt`'s own "after the read, before the CAS, on every attempt" placement, so a
+    /// caller stale on BOTH axes between step 2 and here is reported `FencedOut`, never `Superseded`
+    /// (both are truthful refusals of a CAS that was never sent; this is only which one speaks first).
+    const auto mutate = [&](const RefCatalog & cur) -> RefCatalog
+    {
+        try { check_fence_or_throw(admitted_generation); }
+        catch (...) { throw CatalogFenceMovedMarker{}; }   /// typed, not propagated -- publishCkpt's own precedent
+
+        const auto it = findEntry(cur, observed.ns);
+        if (it == cur.entries.end() || *it != observed)
+            throw CatalogEntryMismatchMarker{};
+
+        RefCatalog next = cur;
+        next.entries[static_cast<size_t>(it - cur.entries.begin())].state = NsState::Live;
+        next.entries[static_cast<size_t>(it - cur.entries.begin())].creator = std::nullopt;
+        return next;
+    };
+
+    try
+    {
+        casUpdate(backend, layout, mutate);
+    }
+    catch (const CatalogFenceMovedMarker &) { return NamespaceCreationOutcome::FencedOut; }
+    catch (const CatalogEntryMismatchMarker &) { return NamespaceCreationOutcome::Superseded; }
+    return NamespaceCreationOutcome::Live;
+}
+
+CasRefCatalog::NamespaceCreationOutcome CasRefCatalog::createNamespace(
+    Backend & backend, const Layout & layout, uint64_t gc_shards,
+    const RootNamespace & ns, const CreatorFence & creator,
+    uint64_t admitted_generation, const std::function<void(uint64_t)> & check_fence_or_throw,
+    const CkptDeadline & deadline)
+{
+    /// Read-first, per the Task 2 review's own note on `casAdmitEntry`: a namespace that already
+    /// carries an entry is THIS function's job to reject with a clear message, not `casAdmitEntry`'s
+    /// duplicate-namespace grammar refusal (which would report a `LOGICAL_ERROR` about canonical order
+    /// -- true, but useless to a caller trying to understand why its create failed). A concurrent
+    /// insert of the SAME namespace between this read and step 1 is still caught -- `casAdmitEntry`'s
+    /// own grammar check is the backstop, not the only check.
+    const Snapshot snap = read(backend, layout);
+    const auto existing = findEntry(snap.catalog, ns);
+    if (existing != snap.catalog.entries.end())
+    {
+        /// `Creating` is not this function's problem to solve (the class-level doc above says so) --
+        /// it is exactly the race `resolveNamespaceLife`'s own loop is built to absorb: sibling openers
+        /// of the SAME namespace (e.g. concurrent per-part freeze threads of one query, which share one
+        /// mount's fence) can all observe "no entry" before any of them lands step 1, then race into
+        /// this call. Reporting `Superseded` sends the loser back through the loop, where it re-reads
+        /// and takes the documented resume path (its own fence: `completeCreation`; a foreign one:
+        /// `reconcileStaleCreator`) instead of aborting the server for an outcome the design already
+        /// names and handles. `Live`/`Removing` stay a `LOGICAL_ERROR`: `namespaceLife`'s caller filters
+        /// `Live` before ever reaching here and refuses `Removing` outright, so seeing either here means
+        /// a caller bypassed that dispatch, not a race.
+        if (existing->state == NsState::Creating)
+            return NamespaceCreationOutcome::Superseded;
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CasRefCatalog::createNamespace: namespace '{}' already carries a catalog entry (state "
+            "'{}') -- a stalled Creating entry is resumed through reconcileStaleCreator + "
+            "completeCreation, never a fresh createNamespace call; an existing Live or Removing "
+            "namespace must complete its current lifecycle before a fresh creation can be admitted",
+            ns.string(), nsStateToWord(existing->state));
+    }
+
+    const CatalogEntry entry{.ns = ns, .state = NsState::Creating,
+                              .incarnation = mintFreshIncarnation(), .creator = creator};
+    /// The read above is a snapshot in time, not a lock: a sibling opener of the SAME namespace that
+    /// also observed "no entry" can land its own step 1 between that read and this one. `casAdmitEntry`
+    /// itself cannot be the backstop for that shape -- it retries its own `Conflict`s by blindly
+    /// re-inserting `entry` into whatever it freshly reads, and a duplicate-namespace insert reaches
+    /// `encodeRefCatalog`'s grammar check as an unconditional `LOGICAL_ERROR` abort. `createNamespaceStep1`
+    /// is the same admission, but rechecks presence on every read this loop performs (not just the one
+    /// above) and reports the race as `Superseded` instead.
+    try
+    {
+        createNamespaceStep1(backend, layout, gc_shards, entry);   /// step 1
+    }
+    catch (const CatalogEntryAlreadyPresentMarker &)
+    {
+        return NamespaceCreationOutcome::Superseded;
+    }
+    return completeCreation(backend, layout, entry, admitted_generation, check_fence_or_throw, deadline);
+}
+
+void CasRefCatalog::setCreateNamespaceStep1PreReadHookForTest(std::function<void()> hook)
+{
+    create_namespace_step1_pre_read_hook_for_test = std::move(hook);
+}
+
+CasRefCatalog::ReconcileCreatorOutcome CasRefCatalog::reconcileStaleCreator(
+    Backend & backend, const Layout & layout, const CatalogEntry & observed, const CreatorFence & new_creator,
+    const std::function<bool(const CreatorFence &)> & is_creator_fence_terminal,
+    uint64_t admitted_generation, const std::function<void(uint64_t)> & check_fence_or_throw)
+{
+    if (observed.state != NsState::Creating || !observed.creator)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CasRefCatalog::reconcileStaleCreator: namespace '{}' is not a Creating entry with a "
+            "creator fence -- nothing to reconcile", observed.ns.string());
+
+    /// Review I6: the fence re-check is checked FIRST, on every fresh read this CAS retries -- the same
+    /// placement `completeCreation` uses for exactly the same reason (see that function's own doc).
+    /// Token-exactness (the catalog's own entry, by full value) comes next: it is the cheaper, purely
+    /// local comparison, and a mismatch here means the question "is the OLD creator's fence terminal" is
+    /// moot -- `observed` no longer describes anything live to reconcile.
+    const auto mutate = [&](const RefCatalog & cur) -> RefCatalog
+    {
+        try { check_fence_or_throw(admitted_generation); }
+        catch (...) { throw CatalogFenceMovedMarker{}; }   /// typed, not propagated -- completeCreation's own precedent
+
+        const auto it = findEntry(cur, observed.ns);
+        if (it == cur.entries.end() || *it != observed)
+            throw CatalogEntryMismatchMarker{};
+        if (!is_creator_fence_terminal(*observed.creator))
+            throw CatalogCreatorStillLiveMarker{};
+
+        RefCatalog next = cur;
+        next.entries[static_cast<size_t>(it - cur.entries.begin())].creator = new_creator;
+        return next;
+    };
+
+    try
+    {
+        casUpdate(backend, layout, mutate);
+    }
+    catch (const CatalogFenceMovedMarker &) { return ReconcileCreatorOutcome::FencedOut; }
+    catch (const CatalogEntryMismatchMarker &) { return ReconcileCreatorOutcome::EntryChanged; }
+    catch (const CatalogCreatorStillLiveMarker &) { return ReconcileCreatorOutcome::CreatorFenceStillLive; }
+    return ReconcileCreatorOutcome::Reconciled;
+}
+
+void CasRefCatalog::checkPublicationAdmittedOrThrow(const RefCatalog & catalog, const RootNamespace & ns)
+{
+    const auto it = findEntry(catalog, ns);
+    if (it != catalog.entries.end() && it->state == NsState::Creating)
+        throwCasWriteRetryLater(fmt::format(
+            "CAS ref catalog: namespace '{}' is still Creating -- no ref writes are admitted until "
+            "its creation completes or is reconciled away", ns.string()));
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h
new file mode 100644
index 000000000000..6eca5f4c985f
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h
@@ -0,0 +1,351 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <functional>
+#include <optional>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// The `cas/ref_catalog` object (spec INV-3) as seen from the pool side: reading the current
+/// catalog, and the generic token-CAS retry primitive every lifecycle transition rides. This class
+/// builds ONLY that primitive -- the actual lifecycle steps (the three-conditional-write creation
+/// sequence, the removal terminal-record-then-entry-delete sequence) are later tasks' job, built ON
+/// TOP of `casUpdate`/`casAdmitEntry`.
+class CasRefCatalog
+{
+public:
+    /// The catalog snapshot as read from the backend: the decoded object plus the token an update
+    /// must present to `casPut`. Operational reads always return a token because the catalog is a
+    /// mandatory control object after pool bootstrap.
+    struct Snapshot
+    {
+        RefCatalog catalog;
+        std::optional<Token> token;
+        CatalogLifeIndex life_index;
+    };
+
+    /// Reads and decodes the mandatory current catalog. Absence is corruption, never an empty
+    /// authority set: without the catalog, opaque life keys cannot prove ownership.
+    static Snapshot read(Backend & backend, const Layout & layout);
+
+    /// Materializes the explicit empty catalog for a prefix already proven new by
+    /// `probePoolBootstrapResidual`. This is the only absence-tolerant catalog operation: no
+    /// existing-pool caller can accidentally turn authoritative absence into an empty catalog. A
+    /// concurrent bootstrap winner is accepted only after its object is read and decoded.
+    static Snapshot initializeEmptyForNewPool(Backend & backend, const Layout & layout);
+
+    /// The catalog's life for `ns` if a `Live`/`Removing` entry names it, else `nullopt` -- ONE catalog
+    /// read and, crucially, NO WRITE OF ANY KIND. This is the resolution a READ or a REMOVAL uses: it
+    /// answers "this namespace does not exist" instead of making it exist, which is what
+    /// `CasRefLedger::resolveNamespaceLife` would do (it mints for an absent entry, correctly, on behalf
+    /// of a writer). A caller must not substitute that one here: a read or an unlink against a
+    /// never-opened table would then perform a catalog CAS and a `_ckpt` publish, growing the single
+    /// pool-wide catalog object -- which is under a capacity-admission predicate -- for a namespace
+    /// nobody ever created.
+    ///
+    /// `Creating` is excluded for the same reason `liveUniverse` excludes it: no publication can exist
+    /// under an entry still being created, so there is nothing to resolve to and nothing to read.
+    static std::optional<NamespaceLifeId> lifeIfCataloged(
+        Backend & backend, const Layout & layout, const RootNamespace & ns);
+
+    /// Every `Live`/`Removing` life the catalog currently names, from this call's own catalog `GET`.
+    /// This helper is for independent readers such as `CasFsck`; a GC fold instead keeps the immutable
+    /// post-LIST snapshot attached to its scan and reuses that exact cut throughout the round.
+    /// `Creating` is excluded: spec §3, no publication can exist yet.
+    static std::vector<NamespaceLifeId> liveUniverse(Backend & backend, const Layout & layout);
+
+    /// The generic token-CAS retry loop shared by every catalog mutation, mirroring
+    /// `PoolMeta::admitOrValidate`'s loop: read the current snapshot, apply `mutate` to obtain the
+    /// CANDIDATE next catalog, `casPut` it against the mandatory object's observed token, and on
+    /// `Conflict` re-read and re-apply `mutate` to the
+    /// FRESH snapshot -- never re-encoding the stale candidate. `mutate` must return a canonically
+    /// ordered, grammar-valid candidate; `encodeRefCatalog` (called internally) enforces that.
+    ///
+    /// Bounded (the same live-lock brake `publishCkpt`/`allocateWriterEpoch` use on their own
+    /// contended token-CAS singletons): after 100 conflicting attempts it gives up and raises the
+    /// typed retryable error `throwCasWriteRetryLater`, naming the key and the attempt count, rather
+    /// than spinning forever against a pathologically busy catalog.
+    ///
+    /// A re-read that finds the object genuinely ABSENT after it was previously observed present is
+    /// corruption, not a fresh bootstrap. The required `read` throws before another CAS attempt, so
+    /// no mutation can replace every other namespace with a one-update catalog.
+    ///
+    /// This primitive runs NO admission check: Constraint 13 (removal is never refused) means
+    /// whether a candidate must clear the additive predicate is the CALLER's decision, not this
+    /// loop's. A caller mutating an entry's state without growing the catalog (a removal transition)
+    /// uses this directly.
+    ///
+    /// THE FENCE OBLIGATION (Task 3 carry-over from the Task 2 review): this loop has no fence
+    /// parameter and performs no fence check of its own -- `publishCkpt`'s "AFTER the read, BEFORE the
+    /// CAS, on every attempt" discipline has no equivalent built in here. The seam a fenced caller
+    /// MUST use is `mutate` itself: it runs, fresh, after EVERY read this loop performs (the very
+    /// first one and every one after a `Conflict`), immediately before the candidate it returns is
+    /// encoded and `casPut`. A caller that needs its own write fenced (any catalog mutation minted
+    /// under a mount incarnation -- which is every one Task 3 onward adds) MUST throw from inside
+    /// `mutate`, checking on EVERY invocation, not once before calling `casUpdate`: checking once
+    /// before the call fences against the read this loop is *about* to perform, not the one it just
+    /// did, and a `Conflict` retry performs an entirely new read `mutate` is never told about except
+    /// by being called again. `completeCreation`'s own `mutate` (below) is the first production
+    /// caller to ride this seam, and does so by wrapping its `check_fence_or_throw` call at the top of
+    /// its `mutate`, exactly where `publishCkpt` places the identical check.
+    static RefCatalog casUpdate(
+        Backend & backend, const Layout & layout,
+        const std::function<RefCatalog(const RefCatalog &)> & mutate);
+
+    /// Admits exactly ONE new namespace into the catalog under INV-3's two-predicate gate, inserting
+    /// `entry` at its canonical (ns-sorted) position and running the SAME bounded `casUpdate` retry
+    /// loop. Takes the entry to insert rather than an arbitrary mutation, by design: an admission
+    /// entry point that accepted a free-form candidate could be handed a REMOVAL by a future caller
+    /// that reads as correct, silently reopening Constraint 13 (removal is never refused) behind a
+    /// name that says "admitting". A namespace `entry.ns` already carries an entry is a bug in the
+    /// caller (Task 3's creation lifecycle owns checking that first) and surfaces as
+    /// `encodeRefCatalog`'s own canonical-order/no-duplicate grammar check, inside
+    /// `checkCatalogAdmission`.
+    static RefCatalog casAdmitEntry(
+        Backend & backend, const Layout & layout, uint64_t gc_shards, const CatalogEntry & entry);
+
+    enum class BeginRemovingOutcome : uint8_t
+    {
+        Transitioned,
+        AlreadyRemoving,
+        EntryChanged,
+        FencedOut,
+    };
+
+    /// Exact `Live -> Removing` transition. The immutable observed row is compared by full value on
+    /// every catalog retry, and the mount fence is checked after every fresh read and before its CAS.
+    /// A row already `Removing` under the same namespace/life resolves an ambiguous or concurrent
+    /// transition positively; no caller may change its recorded start round afterward.
+    static BeginRemovingOutcome beginRemoving(
+        Backend & backend, const Layout & layout, const CatalogEntry & observed,
+        uint64_t removal_started_round, uint64_t admitted_generation,
+        const std::function<void(uint64_t)> & check_fence_or_throw);
+
+    /// Outcome of the only fold-authorized catalog deletion. A refusal never writes the catalog.
+    enum class CompletedRemovingDeleteOutcome : uint8_t
+    {
+        Deleted,
+        ProofRefused,
+        EntryChanged,
+        FencedOut,
+    };
+
+    /// Authority result for completed-removal erases. Only an explicit `Moved` is a fence outcome;
+    /// exceptions mean authority could not be evaluated and propagate to the caller.
+    enum class LeaderFenceStatus : uint8_t
+    {
+        Held,
+        Moved,
+    };
+
+    struct CompletedRemovingDeleteResult
+    {
+        CompletedRemovingDeleteOutcome outcome;
+        /// Present only when a mandatory fresh catalog read proves that the exact observed life is no
+        /// longer cataloged, whether this actor's erase committed or another actor removed/replaced it.
+        std::optional<NamespaceLifeId> invalidated_life;
+        /// The complete mandatory resolution snapshot after an attempted erase. The GC drain feeds
+        /// this directly into its next deterministic selection; proof refusal performs no read and
+        /// leaves it absent.
+        std::optional<Snapshot> catalog_snapshot;
+
+        bool operator==(CompletedRemovingDeleteOutcome expected) const { return outcome == expected; }
+    };
+
+    /// Exact-CAS-deletes `observed` only when it is a complete `Removing` row and the authoritative
+    /// adopted parent carries cleanup evidence, but no hold, in the row keyed by the same opaque life
+    /// id. The whole parent seal is consumed so a caller cannot separate the life id from its proof or
+    /// reduce the proof to a caller-computed boolean. The leader fence is checked after every fresh
+    /// catalog read and before every attempted CAS.
+    static CompletedRemovingDeleteResult deleteCompletedRemoving(
+        Backend & backend, const Layout & layout, const CatalogEntry & observed,
+        const CasFoldSeal & authoritative_parent, uint64_t admitted_generation,
+        const std::function<LeaderFenceStatus(uint64_t)> & check_fence);
+
+    /// Same exact deletion, using the caller's complete selected catalog snapshot and token for the
+    /// one CAS attempt. Its mandatory resolution snapshot is returned in the result so a catalog-only
+    /// drain can select the next row without an intervening read.
+    static CompletedRemovingDeleteResult deleteCompletedRemovingAtSnapshot(
+        Backend & backend, const Layout & layout, Snapshot catalog_snapshot,
+        const CatalogEntry & observed, const CasFoldSeal & authoritative_parent,
+        uint64_t admitted_generation,
+        const std::function<LeaderFenceStatus(uint64_t)> & check_fence);
+
+    /// Outcome of exact stalled-creation cancellation, the only other exported deletion shape.
+    enum class StalledCreatingCancelOutcome : uint8_t
+    {
+        Cancelled,
+        CreatorFenceStillLive,
+        EntryChanged,
+        FencedOut,
+    };
+
+    /// Exact-CAS-deletes one observed `Creating` row only after its complete creator fence is proven
+    /// terminal. This performs no `_ckpt` or other physical cleanup; debris belongs to the janitor.
+    static StalledCreatingCancelOutcome cancelStalledCreating(
+        Backend & backend, const Layout & layout, const CatalogEntry & observed,
+        const std::function<bool(const CreatorFence &)> & is_creator_fence_terminal,
+        uint64_t admitted_generation, const std::function<void(uint64_t)> & check_fence_or_throw);
+
+    /// === Task 3: the §3 creation lifecycle, built on the two primitives above ===
+
+    /// Outcome of the two-step tail every creation attempt ends in (`_ckpt` publish + `Creating ->
+    /// Live` CAS) -- shared by a fresh `createNamespace` and a reconciler that just adopted a stalled
+    /// entry via `reconcileStaleCreator`, since both resume from the identical point (an OBSERVED
+    /// `Creating` entry with a live creator identity that is now THIS caller's own).
+    enum class NamespaceCreationOutcome : uint8_t
+    {
+        Live,        /// the entry reached `Live`; `_ckpt` is durable with this creator's `writer_epoch`
+                      /// as `life_epoch` (spec INV-4: the genesis epoch, recorded nowhere else).
+        FencedOut,   /// this caller's OWN admitted generation moved before the `_ckpt` publish or the
+                      /// `Creating -> Live` CAS. Nothing more was written; the caller's own mount
+                      /// incarnation is gone, so it cannot be the one to retry.
+        Superseded,  /// the catalog entry no longer equals what this caller observed -- a concurrent
+                      /// reconciler stole it, or a race already carried it to `Live`/`Removing`. Nothing
+                      /// was written; a DIFFERENT actor now owns whatever happens to this namespace next.
+    };
+
+    /// Outcome of `reconcileStaleCreator` alone (see below) -- kept distinct from
+    /// `NamespaceCreationOutcome` because the two refusal reasons here are not interchangeable with
+    /// "fenced" / "superseded": one is "not yet permitted" (retry later, unconditionally on the SAME
+    /// entry), the other is "someone else already moved this entry" (retrying against the SAME
+    /// `observed` value can never succeed; the caller must re-read first).
+    enum class ReconcileCreatorOutcome : uint8_t
+    {
+        Reconciled,          /// `creator` is now `new_creator`; the caller may proceed as if it had
+                              /// just run step 1 itself, over the SAME (unchanged) incarnation.
+        CreatorFenceStillLive,  /// `is_creator_fence_terminal` said no -- the stalled creator might
+                              /// still complete this itself. Not written; retry later against a FRESH
+                              /// terminality read, not immediately.
+        EntryChanged,         /// the catalog's current entry for `observed.ns` no longer equals
+                              /// `observed` -- token-exactness failed. Not written; the caller must
+                              /// re-read the catalog before trying again.
+        FencedOut,            /// review I6: this caller's OWN admitted generation moved before the CAS
+                              /// -- nothing was written, and the caller's own mount incarnation is gone,
+                              /// so it cannot be the one to retry. Mirrors `NamespaceCreationOutcome::
+                              /// FencedOut`; without this check a deposed mount could still steal a
+                              /// `Creating` entry onto its own dead fence before the following
+                              /// `completeCreation` refuses it -- the catalog would be mutated by an
+                              /// actor this subsystem otherwise never lets touch it.
+    };
+
+    /// The full, fresh §3 sequence for a namespace that carries NO catalog entry yet: mints a random
+    /// nonzero incarnation (spec: "fresh_random_128"), runs step 1 (`casAdmitEntry` inserting `{ns,
+    /// Creating, incarnation, creator}`), then steps 2+3 via `completeCreation` below.
+    ///
+    /// Per the Task 2 review's own note on `casAdmitEntry` ("a namespace `entry.ns` already carries an
+    /// entry is a bug in the caller -- Task 3's creation lifecycle owns checking that first"): this
+    /// function reads the catalog FIRST rather than handing `casAdmitEntry` a doomed insert and letting
+    /// its own grammar check report a confusing duplicate-namespace message. A namespace already
+    /// `Creating` is not this function's problem to solve -- that is exactly what `reconcileStaleCreator`
+    /// + `completeCreation` are for, so this reports `Superseded` (never `LOGICAL_ERROR`) and sends the
+    /// caller back through its own resume loop: sibling openers of the same namespace that all observed
+    /// "no entry" before any of them landed step 1 race in here exactly this way. A namespace already
+    /// `Live`/`Removing` IS a caller bug (recreating an existing name is removal's business, not
+    /// creation's) and still throws `LOGICAL_ERROR` naming the observed state.
+    static NamespaceCreationOutcome createNamespace(
+        Backend & backend, const Layout & layout, uint64_t gc_shards,
+        const RootNamespace & ns, const CreatorFence & creator,
+        uint64_t admitted_generation, const std::function<void(uint64_t)> & check_fence_or_throw,
+        const CkptDeadline & deadline);
+
+    /// Fires once, synchronously, right after `createNamespace`'s own pre-check read observed no
+    /// entry and right before its step 1 performs its own (first) catalog read -- the exact window a
+    /// sibling opener of the same namespace can land its own step 1 in. Lets a test drive that
+    /// interleaving deterministically instead of relying on real thread scheduling. Empty (no-op) hook
+    /// in production; a stateless class-scope hook (rather than an instance member) because
+    /// `CasRefCatalog` itself carries no state.
+    static void setCreateNamespaceStep1PreReadHookForTest(std::function<void()> hook);
+
+    /// Steps 2 (`_ckpt` publish) + 3 (`Creating -> Live` CAS) alone, given an entry the caller already
+    /// owns as `observed` -- either the entry `createNamespace`'s own step 1 just inserted, or one a
+    /// caller just reconciled onto itself via `reconcileStaleCreator`. Exposed separately (rather than
+    /// folded invisibly into `createNamespace`) because reconciliation resumes exactly HERE, never
+    /// re-running step 1.
+    ///
+    /// Step 2: `publishCkpt` with a contribution carrying `observed.creator->writer_epoch` as
+    /// `life_epoch` -- INV-4's genesis record; `observed.creator` must be present (i.e. `observed.state
+    /// == Creating`), enforced with `LOGICAL_ERROR` since a caller reaching here with anything else is
+    /// this module's own bug, not a race. A `FencedOut` from `publishCkpt` ends the attempt here.
+    ///
+    /// Step 3: `CasRefCatalog::casUpdate`'s `mutate` is the fence re-check point (see the class-level
+    /// note below) -- `check_fence_or_throw(admitted_generation)` runs FIRST, on every fresh read this
+    /// retry loop performs, exactly like `publishCkpt`'s own re-check; a throw from it is caught and
+    /// reported as `FencedOut`, nothing else. ONLY THEN is the fresh entry for `observed.ns` compared
+    /// against `observed` by FULL VALUE equality (`CatalogEntry::operator==`) -- the value-CAS that
+    /// plays the role `publishCkpt`'s object token plays for `_ckpt`, since one catalog object holds
+    /// every namespace's entry and there is no separate per-entry token to CAS against. A mismatch
+    /// (stolen by a concurrent reconciler, or already carried to `Live`/`Removing`) is `Superseded`,
+    /// caught before any CAS is attempted -- not a retry against fresh state, because retrying here
+    /// would mean re-deciding against an entry that is no longer `observed`, which is precisely what
+    /// token-exactness forbids. Ordering the fence check before the entry check is deliberate (mirrors
+    /// `publishCkpt`); a caller that manages to make BOTH stale sees `FencedOut`, not `Superseded` --
+    /// both are truthful refusals of a CAS that was never sent.
+    static NamespaceCreationOutcome completeCreation(
+        Backend & backend, const Layout & layout, const CatalogEntry & observed,
+        uint64_t admitted_generation, const std::function<void(uint64_t)> & check_fence_or_throw,
+        const CkptDeadline & deadline);
+
+    /// Stale-`Creating` reconciliation (spec INV-3: "stalled creators occupy entries until
+    /// fence-terminal reconciliation"; TLA Task 3 obligation 1: "the call-site is where
+    /// token-exactness is enforced"). `observed` must be a `Creating` entry this caller read a moment
+    /// ago (`LOGICAL_ERROR` otherwise -- a caller mistake, not a race). Refuses, WITHOUT writing
+    /// anything, unless BOTH hold against a FRESH catalog read:
+    ///   - `is_creator_fence_terminal(*observed.creator)` -- injected rather than reaching into
+    ///     `CasServerRoot` directly, so this module (and its tests) stay independent of the mount-lease
+    ///     machinery; the real predicate a production caller wires in is `isCreatorFenceTerminal`
+    ///     (`Pool/CasServerRoot.h`, called as `isCreatorFenceTerminal(backend, layout,
+    ///     fence.server_root_id, fence.writer_epoch)` -- it takes those two scalars, not the whole
+    ///     `CreatorFence`, so the mount layer stays independent of the ref-catalog format), built from
+    ///     `writer_epoch` plus the mount-terminality certificates
+    ///     `probeNonTerminalMountSlots`/`computeHeartbeatFloor` already use -- NEVER from
+    ///     `CreatorFence::fence_generation`. That field IS persisted (Task 2 serializes it into the
+    ///     catalog entry), so it reaches the object store fine; what it is NOT is comparable across
+    ///     actors: it mirrors `CasMountRuntime::fence_generation`, an in-process atomic that each mount
+    ///     bumps from its OWN zero on every open, so a different actor's counter (or the SAME actor's
+    ///     after a restart) starts over at the same values and answers a different question than "is
+    ///     the incarnation that minted this entry still alive";
+    ///   - the catalog's CURRENT entry for `observed.ns` still equals `observed` exactly
+    ///     (token-exactness: a concurrent reconciler, or the original creator finishing on its own,
+    ///     invalidates this immediately).
+    /// On success, CASes `creator` to `new_creator` -- `state` and `incarnation` are UNCHANGED, so the
+    /// caller resumes with `completeCreation(backend, layout, {..., .creator = new_creator}, ...)` over
+    /// the SAME incarnation, never a fresh one (rebirth under a fresh incarnation is Task 5/removal's
+    /// business, not a live reconciliation's).
+    ///
+    /// `admitted_generation`/`check_fence_or_throw` (review I6): re-checked FIRST on every fresh read
+    /// this CAS retries, exactly like `completeCreation`'s own placement -- a caller whose OWN mount
+    /// fence has already moved must not be the one to steal a `Creating` entry onto its own (now dead)
+    /// fence, even though the following `completeCreation` would go on to refuse it as `FencedOut`
+    /// anyway: by then the catalog would already have been mutated by a deposed actor, the one posture
+    /// this subsystem otherwise refuses everywhere else.
+    static ReconcileCreatorOutcome reconcileStaleCreator(
+        Backend & backend, const Layout & layout, const CatalogEntry & observed,
+        const CreatorFence & new_creator,
+        const std::function<bool(const CreatorFence &)> & is_creator_fence_terminal,
+        uint64_t admitted_generation, const std::function<void(uint64_t)> & check_fence_or_throw);
+
+    /// Spec §3: "`Creating` forbids publication -- no ref writes admitted while the entry is
+    /// Creating." Throws `throwCasWriteRetryLater`'s class (transient: `Creating` resolves once the
+    /// creator finishes or is reconciled away) if `catalog`'s entry for `ns` is `Creating`; a no-op for
+    /// every other case -- no entry, `Live`, or `Removing` -- since this is ONLY the birth-lifecycle
+    /// gate on the catalog's own `Creating` state, never a general existence/removal check (that role
+    /// moves onto the catalog in Task 4/Task 6). Takes an already-read `RefCatalog` rather than
+    /// `Backend`/`Layout`, so a caller that is about to append anyway (and so already holds a fresh
+    /// read for its OWN purposes) pays no second GET here.
+    ///
+    /// Production publication is catalog-governed by construction: an ordinary `appendRefOps` first
+    /// resolves the namespace lifecycle, refuses a foreign live `Creating` row without writing, and
+    /// cannot publish the initial stream object until creation has published `_ckpt` and moved the row
+    /// to `Live`. This helper states the same admission rule for callers that already own a catalog cut;
+    /// it is not the production append path's enforcement seam.
+    static void checkPublicationAdmittedOrThrow(const RefCatalog & catalog, const RootNamespace & ns);
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.cpp
new file mode 100644
index 000000000000..af43807648bf
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.cpp
@@ -0,0 +1,354 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Common/Exception.h>
+#include <algorithm>
+#include <limits>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int INVALID_STATE;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Live-lock brake, the same shape and for the same reason as `CasPlainObjects`': the deadline is the
+/// real bound, and this only stops an unexpected continuous conflict from spinning until it elapses.
+constexpr size_t MAX_CKPT_CAS_ATTEMPTS = 100;
+
+/// The per-field semantic maximum for an OPTIONAL field: a present value beats an absent one (an
+/// absence is "this writer knew nothing", never "this writer says none"), and two present values
+/// resolve by the field's own order -- for `RefTxnId` that is writer_epoch then ref_sequence, the
+/// intended timeline even across an epoch restart that resets the sequence.
+template <typename T>
+std::optional<T> maxKnown(const std::optional<T> & a, const std::optional<T> & b)
+{
+    if (!a)
+        return b;
+    if (!b)
+        return a;
+    return std::max(*a, *b);
+}
+
+std::optional<RefTxnId> mergeCommittedThrough(const RefCkpt & a, const RefCkpt & b)
+{
+    if (!a.committed_through)
+        return b.committed_through;
+    if (!b.committed_through)
+        return a.committed_through;
+    if (a.committed_through->writer_epoch == b.committed_through->writer_epoch)
+        return std::max(*a.committed_through, *b.committed_through);
+
+    const RefCkpt & higher = *a.committed_through < *b.committed_through ? b : a;
+    const RefCkpt & lower = *a.committed_through < *b.committed_through ? a : b;
+    if (lower.committed_through->writer_epoch + 1 != higher.committed_through->writer_epoch
+        || !higher.last_epoch_seal
+        || *higher.last_epoch_seal < *lower.committed_through
+        || *higher.committed_through < *higher.last_epoch_seal)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS _ckpt: cross-epoch committed_through requires the immediately next writer epoch and "
+            "a seal covering the lower frontier without exceeding the higher frontier");
+    return higher.committed_through;
+}
+
+/// `life_epoch` MAY NOT DECREASE, and this is the whole of that rule. It lives HERE, at the publish
+/// site, and deliberately not inside `mergeCkpt`: the merge is commutative, which is the stated reason
+/// the two writers need no ordering between them, and a commutative function cannot even express this
+/// rule -- it does not know which of its two arguments is the durable one. Here that distinction
+/// exists, because `durable` came from this attempt's read and `contribution` is what the caller wants
+/// to add.
+///
+/// The refusal is narrow ON PURPOSE, and the narrowness is what makes it stronger rather than weaker
+/// than a rule against any disagreement. Two present-and-different values are ORDINARY: the two writers
+/// that know a `life_epoch` derive it from different epochs that legitimately differ --
+/// `completeCreation` from the catalog creator's `writer_epoch`, `commitRefChunk`'s birth chunk from the
+/// `NamespaceBirth` record's -- so a resumed creation (`reconcileStaleCreator` handing a stalled
+/// `Creating` entry to a later actor, same incarnation) and a plain restart between CREATE TABLE and the
+/// first INSERT both raise the value honestly. Refusing THAT would wedge the namespace permanently:
+/// `_ckpt` has no repair path and no writer may delete it outside namespace removal, so every retry
+/// would re-read the old value, re-contribute the new one and re-throw.
+///
+/// A DECREASE is the case that cannot happen honestly, which is precisely why it is worth detecting.
+/// `writer_epoch` is durable-monotone per server root (`allocateWriterEpoch` CAS-bumps
+/// `<prefix>/gc/server-roots/<srid>/epoch`), and every live namespace is rooted at its own member's
+/// `server_root_id`, so a namespace's creator and any actor that later reconciles it share ONE monotone
+/// counter and a live actor's epoch always exceeds a terminal one's. A contribution below what is
+/// durable therefore means a writer whose epoch is already superseded got its contribution through --
+/// a fence violation, the class this subsystem cares most about. The semantic maximum absorbs it
+/// silently, which is why the check cannot be left to the merge even setting commutativity aside.
+///
+/// (That argument is per-server-root. It would need revisiting if one namespace could ever be created
+/// by two DIFFERENT server roots, whose epoch counters are independent and so unordered.)
+///
+/// It is a PREDICATE rather than a check that throws, because the caller has to consult the mount fence
+/// between detecting this and classifying it: a writer the fence is about to refuse has landed nothing
+/// anywhere, and reporting corruption for it would turn an expected transient control signal into a
+/// permanent verdict. Only a STILL-ADMITTED writer contributing a superseded epoch is the violation this
+/// detects.
+bool lifeEpochWouldDecrease(const RefCkpt & durable, const RefCkpt & contribution)
+{
+    return durable.life_epoch && contribution.life_epoch && *contribution.life_epoch < *durable.life_epoch;
+}
+
+/// The verdict for a decrease by a writer that IS still admitted.
+///
+/// The message says the object cannot be repaired in place, because that is the part an operator cannot
+/// work out from the numbers and cannot afford to guess: nothing rewrites `_ckpt` downwards, and no
+/// writer deletes it outside namespace removal, so if the durable value is the wrong one then every
+/// honest writer from here on contributes something lower and hits this same refusal forever. The
+/// refusal is still right -- silently adopting a suspect genesis epoch would corrupt the epoch-seal
+/// grammar for the life of the namespace -- but a fail-closed state with no in-place exit has to say so
+/// where it fires, not leave the operator to discover it by retrying.
+[[noreturn]] void throwLifeEpochDecrease(const RefCkpt & durable, const RefCkpt & contribution, const String & key)
+{
+    throw Exception(ErrorCodes::CORRUPTED_DATA,
+        "CAS {}: life_epoch may not decrease -- {} is durable and a still-admitted writer contributed {}. "
+        "Writer epochs are monotone per server root, so a lower contribution means a superseded writer's "
+        "work reached this object; refusing rather than taking the maximum. This object has NO in-place "
+        "repair: it is never rewritten downwards and is deleted only by namespace removal, so if {} is "
+        "itself the wrong value then every later writer will hit this same refusal and the namespace "
+        "cannot be written again until it is recreated",
+        key, *durable.life_epoch, *contribution.life_epoch, *durable.life_epoch);
+}
+
+}
+
+RefCkpt mergeCkpt(const RefCkpt & a, const RefCkpt & b)
+{
+    RefCkpt merged;
+    /// `life_epoch` is merged like every other field, by the same semantic maximum: taking it from
+    /// either side by name is how a writer that knows nothing about it would erase it. In the steady
+    /// state one side knows it and the other does not, and the max keeps the one that does. (It is not
+    /// a namespace-lifetime constant -- see the field's own doc in `Formats/CasRefCkptFormat.h` -- but
+    /// the values it legitimately takes only ever RISE, which is what makes the max right here and what
+    /// lets `publishCkpt` refuse the fall separately.)
+    merged.life_epoch = maxKnown(a.life_epoch, b.life_epoch);
+    merged.committed_through = mergeCommittedThrough(a, b);
+    merged.checkpoint_snapshot_id = maxKnown(a.checkpoint_snapshot_id, b.checkpoint_snapshot_id);
+    merged.last_epoch_seal = maxKnown(a.last_epoch_seal, b.last_epoch_seal);
+    /// Contributions may omit independent facts, but the resulting durable shape may not. In
+    /// particular, if one writer supplies `life_epoch` and another supplies a later frontier, their
+    /// merge must carry the chain evidence before `publishCkpt` can encode it.
+    if (merged.committed_through)
+        checkRefCkptInvariants(merged, "_ckpt merge");
+    return merged;
+}
+
+RecoveryGrounding chooseRecoveryGrounding(const std::optional<CatalogEntry> & catalog_state,
+                                          const std::optional<RefCkpt> & ckpt)
+{
+    if (!catalog_state || catalog_state->state == NsState::Creating)
+        throw Exception(ErrorCodes::INVALID_STATE, "CAS recovery grounding: namespace is absent or Creating");
+    if (catalog_state->state != NsState::Live && catalog_state->state != NsState::Removing)
+        throw Exception(ErrorCodes::INVALID_STATE, "CAS recovery grounding: namespace has an unsupported lifecycle state");
+    if (!ckpt || !ckpt->life_epoch)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS recovery grounding: a Live or Removing namespace requires a readable _ckpt with life_epoch");
+
+    checkRefCkptInvariants(*ckpt, "recovery grounding");
+    RecoveryGrounding result;
+    result.committed_through = ckpt->committed_through;
+    if (!result.committed_through)
+    {
+        if (ckpt->checkpoint_snapshot_id || ckpt->last_epoch_seal)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS recovery grounding: a checkpoint without committed_through cannot name a snapshot or epoch seal");
+        return result;
+    }
+
+    if (ckpt->checkpoint_snapshot_id && ckpt->last_epoch_seal
+        && *ckpt->checkpoint_snapshot_id == *ckpt->last_epoch_seal)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS recovery grounding: checkpoint_snapshot_id must not name last_epoch_seal");
+
+    if (ckpt->checkpoint_snapshot_id)
+        result.base = ckpt->checkpoint_snapshot_id;
+    if (result.base)
+    {
+        if (result.base->ref_sequence == std::numeric_limits<uint64_t>::max())
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS recovery grounding: checkpoint base has no representable successor");
+        result.walk_from = RefTxnId{result.base->writer_epoch, result.base->ref_sequence + 1};
+    }
+    else if (!result.base)
+    {
+        result.walk_from = RefTxnId{*ckpt->life_epoch, 1};
+    }
+    return result;
+}
+
+std::optional<CkptSample> readCkpt(Backend & backend, const Layout & layout, const NamespaceLifeId & life)
+{
+    std::optional<GetResult> got = backend.get(layout.refCkptKey(life));
+    if (!got)
+        return std::nullopt;
+    /// Materialized read, then decode: the object is MUTABLE, so the body must be fixed before it is
+    /// parsed, and the token must be the one that labels exactly these bytes.
+    return CkptSample{decodeRefCkpt(got->bytes), got->token};
+}
+
+CkptPublishOutcome publishCkpt(Backend & backend, const Layout & layout, const NamespaceLifeId & life,
+                               const RefCkpt & contribution, uint64_t admitted_generation,
+                               const std::function<void(uint64_t)> & check_fence_or_throw,
+                               const CkptDeadline & deadline)
+{
+    const String key = layout.refCkptKey(life);
+    std::optional<CkptSample> current;
+    bool have_current = false;
+
+    for (size_t attempt = 0; attempt < MAX_CKPT_CAS_ATTEMPTS; ++attempt)
+    {
+        if (deadline.now_ms() >= deadline.deadline_ms)
+            break;
+
+        /// Read the WHOLE body every attempt. A retry after a conflict must merge against what is
+        /// there NOW: reusing the previous attempt's reading is precisely the read-modify-write with
+        /// the merge left out, one round later.
+        if (!have_current)
+        {
+            current = readCkpt(backend, layout, life);
+            have_current = true;
+        }
+
+        /// The one rule the commutative merge cannot state, and it has to be decided HERE, before the
+        /// merge: the semantic maximum turns a decrease into a body identical to the stored one, which
+        /// the identical-skip below would return as a successful no-op. Detecting it after the merge
+        /// would therefore detect nothing.
+        ///
+        /// The FENCE decides which of the two verdicts it gets, so it is consulted first. A writer the
+        /// fence is about to refuse has landed nothing anywhere and gets `FencedOut` -- the same
+        /// transient control signal every other refusal in this function returns rather than throws. A
+        /// writer that is still admitted and yet contributing a superseded epoch is the fence violation
+        /// this detects, and that one is corruption.
+        if (current && lifeEpochWouldDecrease(current->ckpt, contribution))
+        {
+            try
+            {
+                check_fence_or_throw(admitted_generation);
+            }
+            catch (...)
+            {
+                return CkptPublishOutcome::FencedOut;
+            }
+            throwLifeEpochDecrease(current->ckpt, contribution, key);
+        }
+
+        /// ANY writer may create the object; none of them may invent a field. An absent `_ckpt` is
+        /// created from the contribution as it stands, so a publisher that knows only the checkpoint
+        /// creates one that knows only the checkpoint, and the birth transaction's `life_epoch` merges
+        /// into it whenever it arrives -- in either order, because the merge is a per-field maximum.
+        const RefCkpt merged = current ? mergeCkpt(current->ckpt, contribution) : contribution;
+
+        /// Nothing new: return WITHOUT a CAS. This is not an optimization -- both writers publish on
+        /// every snapshot and every seal, and most of those carry a checkpoint the object already has,
+        /// so issuing the write anyway would mint a fresh token per no-op and turn every other writer's
+        /// in-flight CAS into a conflict, for a body byte-identical to the one already stored.
+        if (current && merged == current->ckpt)
+        {
+            try
+            {
+                check_fence_or_throw(admitted_generation);
+            }
+            catch (...)
+            {
+                return CkptPublishOutcome::FencedOut;
+            }
+            return CkptPublishOutcome::IdenticalSkip;
+        }
+
+        /// AFTER the read, BEFORE the CAS, on EVERY attempt (spec §3). A generation that moved since
+        /// admission means this writer's lease incarnation is gone and the body it just merged is
+        /// stale, so the CAS must never be sent -- and because the check precedes it, nothing was.
+        try
+        {
+            check_fence_or_throw(admitted_generation);
+        }
+        catch (...)
+        {
+            /// Typed, not propagated: the caller asked "did this land", and "the fence moved, so
+            /// nothing was sent" is an answer, not a failure of the operation. Only the fence check is
+            /// wrapped, so nothing else can be mistaken for it.
+            return CkptPublishOutcome::FencedOut;
+        }
+
+        const std::optional<Token> expected =
+            current ? std::optional<Token>{current->token} : std::nullopt;
+        /// Encode before entering the ambiguity catch. Allocation or invariant failures happen before
+        /// any request is sent and must propagate as themselves, not trigger a needless resolution GET.
+        const String merged_bytes = encodeRefCkpt(merged);
+        try
+        {
+            if (backend.casPut(key, merged_bytes, expected).outcome == CasOutcome::Committed)
+                return CkptPublishOutcome::Published;
+        }
+        catch (...)
+        {
+            /// A thrown CAS response does not say whether the object changed. Never retry its bytes
+            /// from memory: first point-read the exact mutable object, including its fresh token. If
+            /// that observation includes this contribution under the semantic join, the write is
+            /// resolved durable; otherwise that exact observation is the only valid base for a retry.
+            try
+            {
+                current = readCkpt(backend, layout, life);
+                have_current = true;
+            }
+            catch (...)
+            {
+                throwCasWriteRetryLater("CAS _ckpt for namespace '" + life.ns.string()
+                    + "': a CAS response was ambiguous and the mandatory exact-read resolution failed ("
+                    + getCurrentExceptionMessage(/*with_stacktrace*/ false) + ")");
+            }
+
+            try
+            {
+                check_fence_or_throw(admitted_generation);
+            }
+            catch (...)
+            {
+                return CkptPublishOutcome::FencedOut;
+            }
+
+            if (current && lifeEpochWouldDecrease(current->ckpt, contribution))
+                throwLifeEpochDecrease(current->ckpt, contribution, key);
+
+            const RefCkpt resolved_merge = current ? mergeCkpt(current->ckpt, contribution) : contribution;
+            if (current && resolved_merge == current->ckpt)
+                return CkptPublishOutcome::Published;
+
+            /// `current` is the exact observation made after the ambiguous response. The next loop
+            /// iteration retries the SAME contribution against its token (or expected absence), with
+            /// no blind CAS and no redundant intervening GET.
+            continue;
+        }
+        /// `Conflict`: the incarnation we read is no longer current, so another writer's merge landed
+        /// first. Nothing of ours was written; re-read and merge against the winner.
+        current.reset();
+        have_current = false;
+    }
+
+    /// Fail closed. Every attempt was all-or-nothing, so there is no partial state -- only an
+    /// unpublished contribution, which the caller must be told about rather than left to assume.
+    throwCasWriteRetryLater("CAS _ckpt for namespace '" + life.ns.string()
+        + "': persistent CAS contention, the checkpoint contribution was not published");
+}
+
+MissingBaseVerdict classifyMissingSampledBase(const Token & sampled_token, const std::optional<Token> & current_token)
+{
+    if (current_token && !(*current_token == sampled_token))
+        return MissingBaseVerdict::RestartRecovery;
+    return MissingBaseVerdict::Corrupted;
+}
+
+bool snapshotDeletableUnderCkpt(const RefTxnId & snapshot_id, const std::optional<RefTxnId> & checkpoint_snapshot_id)
+{
+    return checkpoint_snapshot_id.has_value() && snapshot_id < *checkpoint_snapshot_id;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h
new file mode 100644
index 000000000000..5bf3782503aa
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h
@@ -0,0 +1,172 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <cstdint>
+#include <functional>
+#include <optional>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// The pure, catalog-authoritative input to both recovery entry points. The checkpoint supplies every
+/// recovery boundary; `LIST` cannot supply a base, life epoch, frontier, or stopping condition.
+struct RecoveryGrounding
+{
+    std::optional<RefTxnId> base;
+    std::optional<RefTxnId> walk_from;
+    std::optional<RefTxnId> committed_through;
+};
+
+/// Choose recovery's finite exact range from catalog lifecycle authority and `_ckpt`. Throws
+/// `INVALID_STATE` for absent/Creating names and `CORRUPTED_DATA` when a Live/Removing life lacks
+/// the checkpoint or genesis fact it must have. This helper performs no I/O and never trusts LIST.
+RecoveryGrounding chooseRecoveryGrounding(const std::optional<CatalogEntry> & catalog_state,
+                                          const std::optional<RefCkpt> & ckpt);
+
+/// THE update and read algorithms for one namespace's `_ckpt` object (spec INV-4). The object itself
+/// and its codec live in `Formats/CasRefCkptFormat.h`; this header is what WRITES it and what a reader
+/// consults when the base it sampled turns out to be gone.
+
+/// The one semantic-maximum merge, shared by BOTH `_ckpt` writers (the snapshot publisher and the
+/// sealer). Per field: the greater `life_epoch`, the greater present `checkpoint_snapshot_id`, the
+/// greater present `last_epoch_seal`; an absent optional loses to a present one, and two absents stay
+/// absent.
+///
+/// There is deliberately ONE of these rather than a surgical per-writer update. A `_ckpt` write is a
+/// whole-body read-modify-write, so a writer that wrote back only the field it knows about would carry
+/// its STALE reading of the other field along with it and silently regress the other writer's
+/// progress. That regression is not hypothetical: it is TLC counterexample `_sab_sealclobbersbase`,
+/// where a sealer writing its sampled body back verbatim drops a concurrently published base and the
+/// next recovery loses an ACKED transaction.
+///
+/// Compatible contributions still merge commutatively, but the committed frontier is deliberately not
+/// an unconstrained CRDT maximum: a cross-epoch pair must be numerically adjacent and carry its seal
+/// evidence. That makes arbitrary regrouping of a corrupt historical set invalid, while the actual
+/// publish protocol remains simple: each token-CAS merges one contribution with the one durable body it
+/// just read.
+///
+/// It is therefore NOT where `life_epoch`'s may-not-decrease rule lives, and that is a placement
+/// decision rather than an omission: a commutative function does not know which of its arguments is the
+/// durable one, so it cannot tell a decrease from an increase. That rule belongs to `publishCkpt`, which
+/// does know (see `checkLifeEpochDoesNotDecrease` in the `.cpp`).
+RefCkpt mergeCkpt(const RefCkpt & a, const RefCkpt & b);
+
+/// What one `publishCkpt` call did.
+enum class CkptPublishOutcome : uint8_t
+{
+    Published,      /// the merged body is durable -- this call's CAS committed it
+    IdenticalSkip,  /// the contribution added nothing to what was already there; NO write was issued
+    FencedOut,      /// the admitted fence generation moved before the CAS; NOTHING was written
+};
+
+/// The retry bound for `publishCkpt`: an absolute point on a monotonic millisecond clock, plus that
+/// clock. Both are required and must be the SAME clock -- the caller passes its own injectable boot
+/// clock (`CasRefLedger`'s `boot_ms_fn`), so a test drives the exhaustion arm deterministically
+/// instead of sleeping, and a VM suspend cannot shorten the window.
+struct CkptDeadline
+{
+    std::function<uint64_t()> now_ms;
+    uint64_t deadline_ms = 0;
+};
+
+/// Merge `contribution` into `ns`'s `_ckpt` and make the result durable.
+///
+/// One attempt is: GET the object -> decode it -> merge -> (identical? return without a CAS) ->
+/// re-check the fence -> token-CAS. A CAS conflict means another writer's read-modify-write landed
+/// between our GET and our CAS, so the whole attempt repeats against the NEW body -- never against the
+/// one we already read, which is the point of re-reading rather than retrying the same bytes.
+/// A THROWN CAS response is ambiguous rather than a conflict: exact-read the object, validate its body
+/// and token, then check admission again. If the durable body semantically includes the contribution,
+/// the write is resolved; otherwise retry the same contribution against that exact-read token. An
+/// unreadable resolution fails retry-later, and no path issues two CAS attempts without an intervening
+/// exact observation.
+///
+/// An ABSENT object is created from `contribution` as it stands. Every writer may create it and none
+/// may complete it: a publisher that knows only the checkpoint creates one that knows only the
+/// checkpoint, and the field a different writer knows merges in whenever it arrives, in either order.
+/// That is the whole reason each field is optional rather than defaulted.
+///
+/// FENCE DISCIPLINE (spec §3, the same value at every site of the trio): `check_fence_or_throw` is
+/// re-run on EVERY attempt, AFTER that attempt's read and immediately BEFORE its CAS -- not once at
+/// entry. A generation that moved means the mount lease incarnation changed since this work was
+/// admitted, so this writer's body is stale even if the fence happens to be live again; the CAS must
+/// not be sent. That refusal is returned as `FencedOut` rather than thrown: it is an expected,
+/// transient control signal (the same class the request controller reports as `Unresolved`), and the
+/// snapshot publisher that calls this sits after a durable PUT where an exception would be worse than
+/// a value. NOTHING has been written when it is returned -- the check precedes the CAS.
+///
+/// FAILS CLOSED, never open:
+///   - an existing `_ckpt` that does not decode PROPAGATES `CORRUPTED_DATA` and is never overwritten.
+///     It is the only record of recovery's base and of what cleanup may delete; replacing it with a
+///     body derived from `contribution` alone would erase the base while leaving a well-formed object
+///     behind -- corruption laundered into something a reader would trust.
+///   - a contribution whose `life_epoch` is BELOW the durable one raises `CORRUPTED_DATA`, checked after
+///     that attempt's read and before its merge, so no body is built and no CAS is sent. This is the one
+///     refusal that HAS to live here rather than in `mergeCkpt`: only this function knows which side is
+///     durable. It is reported as corruption ONLY for a writer the fence still admits -- one the fence
+///     is about to refuse gets `FencedOut` like every other refusal here, since it landed nothing.
+///   - exhausting the deadline (or the live-lock brake) under persistent conflict throws the
+///     retry-later class. No partial state exists to clean up: every attempt either committed the
+///     complete merged body or changed nothing.
+///
+/// `admitted_generation` is the fence generation the CALLER captured when its work was admitted, and
+/// `check_fence_or_throw` is the callback the pool wires from `CasMountRuntime::checkFenceOrThrow`
+/// (the ledger never owns a `CasMountRuntime`; it receives the pair the way `CasPlainObjects` does).
+CkptPublishOutcome publishCkpt(Backend & backend, const Layout & layout, const NamespaceLifeId & life,
+                               const RefCkpt & contribution, uint64_t admitted_generation,
+                               const std::function<void(uint64_t)> & check_fence_or_throw,
+                               const CkptDeadline & deadline);
+
+/// One observation of a namespace's `_ckpt`: the decoded body and the incarnation TOKEN it was read
+/// at. The token is what the missing-base revalidation adjudicates against, so a reader that keeps
+/// only the body cannot apply the rule.
+struct CkptSample
+{
+    RefCkpt ckpt;
+    Token token;
+};
+
+/// Point-read of `life`'s `_ckpt`. `nullopt` means the object is absent (a namespace whose creation has
+/// not published one yet); a present-but-undecodable object throws `CORRUPTED_DATA`.
+std::optional<CkptSample> readCkpt(Backend & backend, const Layout & layout, const NamespaceLifeId & life);
+
+/// The verdict of INV-4's three-way revalidation, for the one leg that is not simply "it is there".
+enum class MissingBaseVerdict : uint8_t
+{
+    RestartRecovery,  /// the checkpoint moved under us -- re-read `_ckpt` and start over from the new base
+    Corrupted,        /// the base is gone under a checkpoint that still names it -- fail closed
+};
+
+/// Adjudicate a sampled recovery anchor that turned out to be unavailable, by comparing the `_ckpt`
+/// token this recovery sampled against the token a fresh re-read observes. The anchor is the
+/// checkpoint-named snapshot and its retained same-id non-seal log witness; the caller supplies this
+/// verdict after either exact GET is absent.
+///
+///   - token ADVANCED  -> `RestartRecovery`. Cleanup legitimately advanced the checkpoint and deleted
+///     the previous anchor while we were reading. Nothing is wrong; restart from the newer base
+///     (bounded by the caller's own restart budget).
+///   - token UNCHANGED -> `Corrupted`. The checkpoint still names an object that is not there, and
+///     the deletion gate makes that unreachable in an honest run: the named snapshot and matching log
+///     are both retained. Something deleted a live anchor.
+///   - `_ckpt` itself ABSENT on the re-read -> `Corrupted` for the same reason, and more bluntly: the
+///     namespace has a base we sampled and no checkpoint at all. `_ckpt` is deleted only as part of
+///     namespace removal, which cannot be racing a live recovery of that same namespace.
+///
+/// Pure, so it is decided the same way at every call site; the caller raises `CORRUPTED_DATA` on
+/// `Corrupted` with its own context.
+MissingBaseVerdict classifyMissingSampledBase(const Token & sampled_token, const std::optional<Token> & current_token);
+
+/// INV-4's snapshot-deletion gate: a snapshot is deletable only STRICTLY BELOW the checkpoint. Strict
+/// rather than at-or-below because the checkpoint names the snapshot a recovery is entitled to fetch
+/// by exact key; deleting THAT one is what turns a stale-but-harmless pointer into the corruption
+/// `classifyMissingSampledBase` has to report (TLC counterexample `_sab_staleckptcorruption`).
+///
+/// Fail-closed on `nullopt`: a namespace with no checkpoint yet has NOTHING deletable, because no
+/// snapshot has been established as a covering base.
+bool snapshotDeletableUnderCkpt(const RefTxnId & snapshot_id, const std::optional<RefTxnId> & checkpoint_snapshot_id);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.cpp
new file mode 100644
index 000000000000..cc3e448bee11
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.cpp
@@ -0,0 +1,119 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace ProfileEvents
+{
+    extern const Event CASRefMaterializeInPlace;
+    extern const Event CASRefMaterializeCopy;
+}
+
+namespace DB::Cas
+{
+
+bool RefCowManifestSet::contains(const ManifestRef & m) const
+{
+    const auto it = overlay.find(m);
+    if (it != overlay.end())
+        return it->second;
+    return base->contains(m);
+}
+
+void RefCowManifestSet::insert(const ManifestRef & m)
+{
+    /// Unconditional membership guard, in EVERY build (not a `chassert`): a duplicate insert means the
+    /// index has drifted from `committed`/`precommits`, and if it silently bumped `net_delta` the index
+    /// would report a manifest present that a single `erase` could then hide while another owner still
+    /// names it -- corrupting the add-precommit uniqueness invariant and GC's `+1/-1` edge accounting.
+    /// Fail closed instead. The caller's own uniqueness check is what enforces the invariant; this is the
+    /// last line that turns a maintaining-code bug into a caught exception rather than silent drift.
+    if (contains(m))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefCowManifestSet: inserting a manifest that already has an owner -- the owned-manifest "
+            "index has drifted from committed/precommits (a bug in the maintaining code)");
+    const auto it = overlay.find(m);
+    if (it != overlay.end())
+        it->second = true;   /// was a tombstone shadowing a base member -- revive it
+    else
+        overlay.emplace(m, true);
+    ++net_delta;
+}
+
+void RefCowManifestSet::erase(const ManifestRef & m)
+{
+    /// Same fail-closed rationale as `insert`: erasing an absent manifest would drift `net_delta` and,
+    /// worse, could shadow a still-live owner. Throw in every build rather than silently corrupting.
+    if (!contains(m))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefCowManifestSet: erasing a manifest with no current owner -- the owned-manifest index "
+            "has drifted from committed/precommits (a bug in the maintaining code)");
+    const auto it = overlay.find(m);
+    if (it != overlay.end())
+    {
+        if (base->contains(m))
+            it->second = false;   /// keep shadowing the base member
+        else
+            overlay.erase(it);    /// pure-overlay member: nothing left to shadow
+    }
+    else
+    {
+        overlay.emplace(m, false);   /// tombstone a base-only member
+    }
+    --net_delta;
+}
+
+void RefCowManifestSet::materialize()
+{
+    if (overlay.empty())
+        return;
+    /// Same two-path shape and exception-coherence contract as `RefCowMap::materialize` (see its
+    /// comment for the full argument). Uniquely-owned base: fold each overlay entry into `*base` IN
+    /// PLACE -- O(overlay), no O(N) `unordered_set` copy. Incrementally coherent: the base mutation (the
+    /// only throw point -- an `unordered_set` insert's alloc/rehash, which is strong) runs first, then
+    /// the non-throwing `net_delta` adjustment and `overlay.erase` commit the entry. Any escape leaves
+    /// (base, overlay, net_delta) exactly coherent and a later `materialize` resumable.
+    if (base.use_count() == 1)
+    {
+        ProfileEvents::increment(ProfileEvents::CASRefMaterializeInPlace);
+        for (auto it = overlay.begin(); it != overlay.end(); )
+        {
+            if (it->second)
+            {
+                const bool inserted = base->insert(it->first).second;   /// throw point (alloc/rehash, strong)
+                if (inserted)
+                    --net_delta;   /// a member absent from base counted +1 in net_delta; now it lives in base
+            }
+            else
+            {
+                base->erase(it->first);   /// a tombstone only ever shadows a base member: non-throwing erase
+                ++net_delta;              /// counted -1 in net_delta; now actually removed from base
+            }
+            it = overlay.erase(it);       /// non-throwing: retire this entry only after its base mutation stuck
+        }
+        return;   /// net_delta arithmetic above lands it back at 0 (base now holds the whole merged view)
+    }
+    /// A copy still shares this base, so fold into a FRESH one and swap -- strong guarantee, the shared
+    /// holder stays byte-unchanged.
+    ProfileEvents::increment(ProfileEvents::CASRefMaterializeCopy);
+    auto fresh = std::make_shared<Base>(*base);
+    for (const auto & [m, present] : overlay)
+    {
+        if (present)
+            fresh->insert(m);
+        else
+            fresh->erase(m);
+    }
+    base = std::move(fresh);
+    overlay.clear();
+    net_delta = 0;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.h
new file mode 100644
index 000000000000..097ebeec68bd
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.h
@@ -0,0 +1,127 @@
+#pragma once
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <base/defines.h>
+#include <cstdint>
+#include <map>
+#include <memory>
+#include <unordered_set>
+#include <utility>
+
+namespace DB::Cas
+{
+
+/// A copy-cheap membership set of every `ManifestRef` that currently has an owner (a committed row
+/// or a precommit binding). `RefTableState` (Pool/CasRefProtocol.h) uses it to hold the
+/// add-precommit uniqueness invariant ("no conflicting owner may name the same manifest") as a
+/// structure instead of `manifestAlreadyOwned`'s old linear scan over `committed` + `precommits`.
+///
+/// Same copy-on-write shape as `RefCowMap` (Pool/CasRefCowMap.h): copies share an immutable `base`
+/// set (a `shared_ptr` refcount bump, no per-element copy) and differ only in a per-copy `overlay`,
+/// so the copy-then-mutate-then-swap pattern `applyRefLogTxn`'s scratch copy uses stays O(overlay
+/// size), never O(table size) -- exactly the regression `RefCowMap` was already built to avoid, and
+/// a plain `std::set<ManifestRef>` member would have reintroduced here. Membership-only: unlike
+/// `RefCowMap` there is no ordered (or any) iteration, because nothing in `RefTableState` ever needs
+/// to enumerate owned manifests, only ask "does any owner already name this one". Not thread-safe;
+/// same ownership rules as `RefCowMap` (callers retain the state lock or detached-copy ownership
+/// rules of the state that contains it).
+///
+/// `base` is `std::unordered_set` (O(1) lookup, the point of this class at large table size), but
+/// `overlay` is deliberately `std::map`, not `std::unordered_map`, even though it holds the exact
+/// same key type: `overlay` is copied on EVERY `RefTableState` scratch copy (unlike `base`, which is
+/// shared), and libstdc++'s `unordered_map` copy constructor allocates a real bucket array even for
+/// an empty source (measured ~30ns/copy via `.claude/tools/cppexpr.sh`, versus effectively free for
+/// an empty `std::map` -- the same reason `RefCowMap`'s own `overlay` is a `std::map`, not an
+/// `unordered_map`). The common case is an empty-or-few-entries overlay between flushes, so this
+/// keeps the added cost of `owned_manifests` on `BM_ScratchCopy` to roughly one more `shared_ptr`
+/// copy, not one more `shared_ptr` copy plus a hidden allocation.
+///
+/// `insert`/`erase` throw `CORRUPTED_DATA` on a violated precondition (absence / presence,
+/// respectively) in EVERY build rather than merely `chassert`-ing it: the ref table's own uniqueness
+/// invariant already guarantees both, so a violation here means the index itself has drifted from
+/// `committed`/`precommits`. A `chassert` would let that drift through silently in a release build,
+/// after which a single `erase` could report a manifest absent while another owner still names it --
+/// corrupting the add-precommit uniqueness invariant and GC's `+1/-1` manifest-edge accounting
+/// downstream. Failing closed keeps `net_delta` from ever drifting. This is the same class of bug
+/// `RefTableState::debugAssertBodyCounters` cross-checks in debug/sanitizer builds; the throw extends
+/// the guarantee to release builds too.
+class RefCowManifestSet
+{
+public:
+    /// Bucket hashing comes from the existing `std::hash<ManifestRef>` specialization
+    /// (Primitives/CasTypes.h), picked up by default. Membership hashing only -- this set is never
+    /// exposed to attacker-chosen keys, only to manifest refs this process itself allocated, so
+    /// adversarial collision resistance is not a concern here.
+    using Base = std::unordered_set<ManifestRef>;
+
+    RefCowManifestSet() = default;
+
+    /// True iff `m` currently has an owner: present in the merged base+overlay view. An overlay
+    /// tombstone reports absent even when `base` still has `m`.
+    bool contains(const ManifestRef & m) const;
+
+    /// Records `m` as owned. `m` must be absent from the merged view or this throws `CORRUPTED_DATA`
+    /// (in every build) -- the caller's own uniqueness check is what actually enforces the invariant;
+    /// this only guards against the index drifting away from it, failing closed rather than silently.
+    void insert(const ManifestRef & m);
+
+    /// Records `m` as no longer owned. `m` must be present in the merged view or this throws
+    /// `CORRUPTED_DATA` (in every build), same rationale as `insert`.
+    void erase(const ManifestRef & m);
+
+    /// `base->size() + net_delta`, O(1).
+    size_t size() const { return static_cast<size_t>(static_cast<int64_t>(base->size()) + net_delta); }
+    bool empty() const { return size() == 0; }
+
+    /// Folds `overlay` into `base` and clears the overlay. Call this at the same state-install point
+    /// `RefCowMap::materialize()` is called from (once per ref-log flush, never once per batch item).
+    /// If the overlay is already empty, this is a no-op.
+    ///
+    /// When `base` is uniquely owned (`use_count() == 1`, the production flush case), the overlay is
+    /// folded into `*base` IN PLACE -- O(overlay), no O(N) `unordered_set` copy. When a copy still
+    /// shares `base`, a fresh merged base is built and swapped in, so the shared holder's view stays
+    /// byte-unchanged. The full ownership-and-coherence safety argument is `RefCowMap::materialize`'s
+    /// (the `use_count()` of 1 is stable against both a concurrent increment and any cross-thread
+    /// release, every such release being lock-ordered under `state_mutex`; the in-place fold is coherent
+    /// at every throw point; `base` is a non-const `shared_ptr` so no `const_cast` is needed). Both paths
+    /// leave an empty overlay and `net_delta == 0`.
+    void materialize();
+
+    /// Member-wise swap, guaranteed non-throwing AND allocation-free, with the same contract and the
+    /// same install-time purpose as `RefCowMap::swap` (Pool/CasRefCowMap.h): `shared_ptr::swap` and
+    /// `std::map::swap` exchange pointers only, `net_delta` is a POD. The swapped-out set keeps its
+    /// former base reference until it is destroyed, so a caller that folds the installed set must
+    /// destroy the swapped-out one first.
+    void swap(RefCowManifestSet & other) noexcept
+    {
+        base.swap(other.base);
+        overlay.swap(other.overlay);
+        std::swap(net_delta, other.net_delta);
+    }
+
+    /// Test-only: current overlay entry count (0 right after `materialize()`).
+    size_t overlayEntriesForTest() const { return overlay.size(); }
+    /// Test-only: `base`'s `shared_ptr::use_count()` -- a copy that shares `base` (no per-element
+    /// allocation) bumps this by exactly one.
+    int64_t baseUseCountForTest() const { return base.use_count(); }
+    /// Test-only: identity of the current `base` allocation. `materialize()` on a uniquely-owned base
+    /// folds the overlay in place and leaves this unchanged; on a base still shared with a copy it
+    /// swaps in a fresh base, changing it. Lets a test tell the fast (in-place) path from the copy path.
+    const void * baseIdentityForTest() const { return base.get(); }
+
+private:
+    /// Non-const so `materialize()` can fold the overlay into `*base` in place when it is the sole
+    /// owner (see `materialize`'s doc for the safety argument). It is never mutated while shared.
+    std::shared_ptr<Base> base = std::make_shared<Base>();
+    /// `true` = an overlay addition (present, whether or not `base` also has it); `false` = a
+    /// tombstone shadowing a `base` member. An overlay-only member that is erased is removed from
+    /// this map outright rather than tombstoned (nothing left to shadow), mirroring `RefCowMap`.
+    /// `std::map`, not `std::unordered_map`: see the class doc comment above -- this is what keeps an
+    /// empty overlay's copy cost negligible.
+    std::map<ManifestRef, bool> overlay;
+    /// `size() = base->size() + net_delta`, maintained in lock-step by `insert`/`erase` so
+    /// `size()`/`empty()` stay O(1). Counts live overlay changes relative to `base`.
+    int64_t net_delta = 0;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.cpp
new file mode 100644
index 000000000000..67cd1c007e74
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.cpp
@@ -0,0 +1,245 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.h>
+#include <Common/ProfileEvents.h>
+#include <stdexcept>
+
+namespace ProfileEvents
+{
+    extern const Event CASRefMaterializeInPlace;
+    extern const Event CASRefMaterializeCopy;
+}
+
+namespace DB::Cas
+{
+
+std::pair<const String &, const RefCommittedRow &> RefCowMap::const_iterator::operator*() const
+{
+    return at_overlay
+        ? std::pair<const String &, const RefCommittedRow &>(overlay_it->first, *overlay_it->second)
+        : std::pair<const String &, const RefCommittedRow &>(base_it->first, base_it->second);
+}
+
+void RefCowMap::const_iterator::normalize()
+{
+    /// Drop overlay tombstones (and the base row each one shadows) until the next live overlay
+    /// entry, or exhaustion. A tombstone is only actually consumed once it is next-in-merge-order
+    /// (its key <= base_it's current key, the same tie-break the final at_overlay check below
+    /// uses) -- a tombstone further ahead than base_it must stay put, or base_it would later walk
+    /// straight past its (still-hidden) target with nothing left in overlay to hide it.
+    while (overlay_it != overlay_end && !overlay_it->second.has_value()
+           && (base_it == base_end || overlay_it->first <= base_it->first))
+    {
+        const String key = overlay_it->first;
+        ++overlay_it;
+        if (base_it != base_end && base_it->first == key)
+            ++base_it;
+    }
+    /// Overlay wins ties: a live overlay entry at the same key as `base_it` is an override.
+    at_overlay = (overlay_it != overlay_end) && (base_it == base_end || overlay_it->first <= base_it->first);
+}
+
+RefCowMap::const_iterator & RefCowMap::const_iterator::operator++()
+{
+    if (at_overlay)
+    {
+        const String key = overlay_it->first;
+        ++overlay_it;
+        if (base_it != base_end && base_it->first == key)
+            ++base_it;   /// this overlay entry shadowed a base row of the same key: consume it too
+    }
+    else
+    {
+        ++base_it;
+    }
+    normalize();
+    return *this;
+}
+
+RefCowMap::const_iterator RefCowMap::begin() const
+{
+    const_iterator it;
+    it.base_it = base->begin();
+    it.base_end = base->end();
+    it.overlay_it = overlay.begin();
+    it.overlay_end = overlay.end();
+    it.normalize();
+    return it;
+}
+
+RefCowMap::const_iterator RefCowMap::end() const
+{
+    const_iterator it;
+    it.base_it = base->end();
+    it.base_end = base->end();
+    it.overlay_it = overlay.end();
+    it.overlay_end = overlay.end();
+    it.at_overlay = false;
+    return it;
+}
+
+RefCowMap::const_iterator RefCowMap::find(const String & key) const
+{
+    const auto ov = overlay.find(key);
+    if (ov != overlay.end())
+    {
+        if (!ov->second.has_value())
+            return end();   /// tombstoned: not present
+        const_iterator it;
+        it.base_it = base->lower_bound(key);   /// first base key >= this one: keeps the iterator mergeable
+        it.base_end = base->end();
+        it.overlay_it = ov;
+        it.overlay_end = overlay.end();
+        it.at_overlay = true;
+        return it;
+    }
+    const auto b = base->find(key);
+    if (b == base->end())
+        return end();
+    const_iterator it;
+    it.base_it = b;
+    it.base_end = base->end();
+    it.overlay_it = overlay.lower_bound(key);   /// first overlay key >= this one: keeps the iterator mergeable
+    it.overlay_end = overlay.end();
+    it.at_overlay = false;
+    return it;
+}
+
+const RefCommittedRow & RefCowMap::at(const String & key) const
+{
+    const auto it = find(key);
+    if (it == end())
+        throw std::out_of_range("RefCowMap::at: key not found: " + key);
+    return it->second;
+}
+
+void RefCowMap::insertLive(const String & key, RefCommittedRow row)
+{
+    const auto ov = overlay.find(key);
+    if (ov != overlay.end())
+    {
+        if (!ov->second.has_value())
+            ++net_delta;   /// tombstoned (dead) -> live again
+        ov->second = std::move(row);
+    }
+    else
+    {
+        if (!base->contains(key))
+            ++net_delta;   /// brand new key, absent from base too
+        overlay.emplace(key, std::move(row));
+    }
+}
+
+std::pair<RefCowMap::iterator, bool> RefCowMap::emplace(String key, RefCommittedRow row)
+{
+    if (contains(key))
+        return {find(key), false};
+    insertLive(key, std::move(row));
+    return {find(key), true};
+}
+
+std::pair<RefCowMap::iterator, bool> RefCowMap::insert_or_assign(String key, RefCommittedRow row)
+{
+    const bool was_present = contains(key);
+    insertLive(key, std::move(row));
+    return {find(key), !was_present};
+}
+
+size_t RefCowMap::erase(const String & key)
+{
+    const auto ov = overlay.find(key);
+    if (ov != overlay.end())
+    {
+        if (!ov->second.has_value())
+            return 0;   /// already tombstoned: no-op
+        if (base->contains(key))
+            ov->second.reset();   /// keep shadowing the base row
+        else
+            overlay.erase(ov);    /// pure-overlay key: nothing left to shadow
+        --net_delta;
+        return 1;
+    }
+    if (!base->contains(key))
+        return 0;
+    overlay.emplace(key, std::nullopt);   /// tombstone a base-only row
+    --net_delta;
+    return 1;
+}
+
+RefCowMap::iterator RefCowMap::erase(const_iterator pos)
+{
+    if (pos == end())
+        return end();
+    const String key = pos->first;
+    ++pos;
+    erase(key);
+    return pos;
+}
+
+bool RefCowMap::operator==(const RefCowMap & other) const
+{
+    if (size() != other.size())
+        return false;
+    auto a = begin();
+    auto b = other.begin();
+    for (; a != end() && b != other.end(); ++a, ++b)
+        if (a->first != b->first || !(a->second == b->second))
+            return false;
+    return a == end() && b == other.end();
+}
+
+void RefCowMap::materialize()
+{
+    if (overlay.empty())
+        return;
+    /// See the header for the full ownership-safety argument. The exception-coherence argument for the
+    /// in-place path is here, next to the code it governs.
+    ///
+    /// Uniquely-owned base (the production flush case): fold each overlay entry into `*base` IN PLACE --
+    /// O(overlay), no O(N) copy. The fold is INCREMENTALLY COHERENT: for every entry the base mutation
+    /// (its ONLY throw point -- a `std::map` node insert or a `RefCommittedRow` copy under memory
+    /// pressure) runs FIRST, and the matching `net_delta` adjustment and `overlay.erase` are both
+    /// non-throwing and run only AFTER it succeeds. So at every point an exception can escape, the
+    /// (base, overlay, net_delta) triple is exactly coherent: the merged view is unchanged, `size()` is
+    /// exact, and a later `materialize` resumes cleanly from the un-erased overlay tail. This is a
+    /// STRONGER guarantee than the copy path's (which only had to be strong for the shared holder): the
+    /// state is coherent even mid-fold, which matters because `materialize` runs AFTER the transaction is
+    /// durably PUT and applied -- an allocation throw here must never leave live bookkeeping wrong. (A
+    /// throwing key-assign can leave a partially-updated base row, but its still-present overlay entry
+    /// shadows that key in the merged view, so the partial row stays invisible until it is re-folded.)
+    if (base.use_count() == 1)
+    {
+        ProfileEvents::increment(ProfileEvents::CASRefMaterializeInPlace);
+        for (auto it = overlay.begin(); it != overlay.end(); )
+        {
+            if (it->second)
+            {
+                const bool inserted = base->insert_or_assign(it->first, *it->second).second;   /// throw point
+                if (inserted)
+                    --net_delta;   /// a key absent from base counted +1 in net_delta; now it lives in base
+            }
+            else
+            {
+                base->erase(it->first);   /// a tombstone only ever shadows a base member: non-throwing erase
+                ++net_delta;              /// counted -1 in net_delta; now actually removed from base
+            }
+            it = overlay.erase(it);       /// non-throwing: retire this entry only after its base mutation stuck
+        }
+        return;   /// net_delta arithmetic above lands it back at 0 (base now holds the whole merged view)
+    }
+    /// A copy still shares this base, so fold into a FRESH one and swap -- the shared holder must stay
+    /// byte-unchanged. Strong guarantee: a mid-fold throw discards `fresh` and leaves this container (and
+    /// every sharer) exactly as it was.
+    ProfileEvents::increment(ProfileEvents::CASRefMaterializeCopy);
+    auto fresh = std::make_shared<Base>(*base);
+    for (const auto & [key, maybe_row] : overlay)
+    {
+        if (maybe_row)
+            (*fresh)[key] = *maybe_row;
+        else
+            fresh->erase(key);
+    }
+    base = std::move(fresh);
+    overlay.clear();
+    net_delta = 0;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.h
new file mode 100644
index 000000000000..567dc044aef5
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.h
@@ -0,0 +1,209 @@
+#pragma once
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <base/types.h>
+#include <cstdint>
+#include <map>
+#include <memory>
+#include <optional>
+#include <utility>
+
+namespace DB::Cas
+{
+
+/// A value-semantic ordered map from `ref_name` to `RefCommittedRow`, designed as a drop-in
+/// replacement for the `std::map<String, RefCommittedRow>` held by `RefTableState::committed`.
+/// Copies share an ordered base and copy only a per-copy overlay, so the
+/// copy-then-mutate-then-swap operations used by `CasRefLedger` and `CasRefProtocol` cost
+/// O(touched rows), rather than copying every committed reference. The base is immutable WHILE SHARED
+/// (a copy exists, `use_count() > 1`); when uniquely owned it may be folded into IN PLACE by
+/// `materialize()` under the caller's exclusive access. The map is not thread-safe; callers retain the
+/// same state lock or detached-copy ownership rules as the state it contains.
+///
+/// - Keyed reads (`find`/`contains`/`at`/`count`) check `overlay` first (a tombstone there means
+///   "removed"; a present entry means "overridden"), falling back to `base`.
+/// - Point writes (`emplace`/`insert_or_assign`/`erase`) only ever touch `overlay`.
+/// - Ordered iteration (`begin`/`end`) merges `base` and `overlay` in sorted key order, applying
+///   overlay overrides/tombstones -- used only by the table's cold full-scan paths (`snapshotOf`,
+///   `CasRefLedger::listRefs`, `dropNamespace`, `CasFsck`/`CasGc` owner-set builders) once the map
+///   is integrated into the ref table. This merge preserves the canonical bytewise `ref_name`
+///   order required by snapshot encoding.
+/// - `materialize()` folds `overlay` into `base` (in place when uniquely owned, else into a fresh
+///   base), leaving an empty overlay before the next flush's trial copies begin. Ref-table integration
+///   must perform this at the state-install point, not once per batch item, so the hot copy path remains
+///   proportional to the rows touched by the flush.
+///
+/// Iterators are read-only even when obtained from a non-const map. A caller that needs to change a
+/// row must copy it and use `insert_or_assign`; exposing mutable references would allow a write to
+/// bypass the overlay and modify neither the owning map's accounting nor its copy-on-write state.
+class RefCowMap
+{
+public:
+    using Base = std::map<String, RefCommittedRow>;
+
+private:
+    using Overlay = std::map<String, std::optional<RefCommittedRow>>;
+
+public:
+    /// A read-only forward iterator over the merged base-and-overlay view, in sorted key order.
+    /// `iterator` is an alias of `const_iterator`, so erasing through an iterator cannot expose a
+    /// mutable reference into the immutable base.
+    class const_iterator
+    {
+    public:
+        const_iterator() = default;
+
+        /// Returns the current key and row. The references remain valid while the source map's
+        /// base and overlay entries used by this iterator are not erased or otherwise replaced.
+        std::pair<const String &, const RefCommittedRow &> operator*() const;
+
+        /// Temporary proxy that gives a merged pair-of-references iterator the usual `it->member`
+        /// syntax without exposing mutable storage.
+        struct ArrowProxy
+        {
+            std::pair<const String &, const RefCommittedRow &> value;
+            const std::pair<const String &, const RefCommittedRow &> * operator->() const { return &value; }
+        };
+        ArrowProxy operator->() const { return ArrowProxy{**this}; }
+
+        /// Advances to the next live entry, skipping tombstones and consuming shadowed base rows.
+        const_iterator & operator++();
+
+        bool operator==(const const_iterator & other) const
+        {
+            return base_it == other.base_it && overlay_it == other.overlay_it;
+        }
+        bool operator!=(const const_iterator & other) const { return !(*this == other); }
+
+    private:
+        friend class RefCowMap;
+
+        /// Advances the two sorted source iterators past tombstones and selects the next source;
+        /// overlay entries win when both sources contain the same key.
+        void normalize();
+
+        Base::const_iterator base_it{};
+        Base::const_iterator base_end{};
+        Overlay::const_iterator overlay_it{};
+        Overlay::const_iterator overlay_end{};
+        bool at_overlay = false;
+    };
+    using iterator = const_iterator;
+
+    RefCowMap() = default;
+
+    /// Returns an iterator to the first live entry in the merged view.
+    const_iterator begin() const;
+
+    /// Returns the past-the-end iterator for the merged view.
+    const_iterator end() const;
+
+    /// Looks up `key`, consulting overlay entries before the shared base. A tombstone is reported
+    /// as absent, and a successful iterator refers to the overlay row when one overrides the base.
+    const_iterator find(const String & key) const;
+
+    bool contains(const String & key) const { return find(key) != end(); }   // NOLINT(readability-container-contains): this is the container's contains implementation.
+    size_t count(const String & key) const { return contains(key) ? 1 : 0; }
+    /// Returns the row for `key`, or throws `std::out_of_range` when the key is absent or tombstoned.
+    const RefCommittedRow & at(const String & key) const;
+
+    size_t size() const { return static_cast<size_t>(static_cast<int64_t>(base->size()) + net_delta); }
+    bool empty() const { return size() == 0; }
+
+    /// Inserts `row` only when `key` is absent from the merged view. The new row is stored in the
+    /// overlay; the returned flag reports whether insertion happened.
+    std::pair<iterator, bool> emplace(String key, RefCommittedRow row);
+
+    /// Inserts or replaces `key` in the overlay. The returned flag is true only when the merged
+    /// view did not already contain the key.
+    std::pair<iterator, bool> insert_or_assign(String key, RefCommittedRow row);
+
+    /// Removes `key` from the merged view. A base row is retained behind an overlay tombstone;
+    /// an overlay-only row can be removed outright. Returns one when a live row was removed.
+    size_t erase(const String & key);
+
+    /// Removes the row referenced by `pos` and returns the following iterator. `pos` must belong to
+    /// this map and be dereferenceable, as with the corresponding `std::map` operation; `end()` is
+    /// accepted as a no-op for compatibility with existing callers.
+    iterator erase(const_iterator pos);
+
+    /// Compares the live merged views, including both keys and committed-row contents; the
+    /// representation of base and overlay storage does not affect the result.
+    bool operator==(const RefCowMap & other) const;
+
+    /// Folds `overlay` into `base` and clears the overlay. Call this after installing a completed
+    /// state, once per ref-log flush and never once per batch item. If the overlay is already empty,
+    /// this is a no-op.
+    ///
+    /// When `base` is uniquely owned (`use_count() == 1`, the production flush case: the live table's
+    /// base is not shared with any outstanding scratch copy at the install point), the overlay is
+    /// folded into `*base` IN PLACE -- O(overlay), no O(N) base copy. When a copy still shares `base`
+    /// (`use_count() > 1`), a fresh merged base is built and swapped in, so the shared holder's view
+    /// stays byte-unchanged. Both paths leave an empty overlay and `net_delta == 0`. The in-place path
+    /// is additionally coherent at every intermediate throw point (see the exception-coherence argument
+    /// in `CasRefCowMap.cpp`), so an allocation failure mid-fold -- possible because this runs AFTER a
+    /// durable commit -- can never leave `size()` or the merged view wrong.
+    ///
+    /// The in-place path is sound because this container is not thread-safe by contract (Pool/
+    /// CasRefProtocol.h: callers serialize all access through the state lock, or own a detached copy).
+    /// A `use_count()` of 1 observed by the sole owner is STABLE for the duration of the fold, against
+    /// both a concurrent refcount INCREMENT and a concurrent DECREMENT:
+    ///  - No increment: every other holder reaches a copy by copying THIS container, which needs access
+    ///    the caller's exclusivity denies while `materialize` runs -- so no new sharer can appear.
+    ///  - No racing decrement: every copy of a live state that lives on a DIFFERENT thread (the
+    ///    background snapshot publisher's `candidate_state`) is BOTH created and destroyed under the same
+    ///    `state_mutex` this fold holds (see
+    ///    `CasRefLedger::tryPublishSnapshotAndAdvanceCheckpointOnce`, which resets its copy under the lock
+    ///    rather than at function return). So no cross-thread `shared_ptr` release can run
+    ///    concurrently with -- and form a data race against -- this relaxed `use_count()` load. The
+    ///    flush's OWN same-thread scratch copy (`working`) is released BEFORE this fold by program order
+    ///    on the one thread, which is a happens-before all its own.
+    /// `base` is a `shared_ptr<Base>` (non-const) so the in-place fold needs no `const_cast`: the pointee
+    /// was never const-qualified at construction, so mutating it is defined. The base is still never
+    /// mutated while shared -- the `use_count() > 1` branch is what guarantees that. Iterators handed out
+    /// by `begin`/`find` are read-only and short-lived by contract (see the class-level note on iterator
+    /// validity); `materialize` runs at the install point where no iterator into `base` is live, so an
+    /// in-place fold cannot invalidate an outstanding one.
+    void materialize();
+
+    /// Member-wise swap, guaranteed non-throwing AND allocation-free: `shared_ptr::swap` exchanges two
+    /// pointers, `std::map::swap` exchanges the trees' internal pointers (the allocator is
+    /// `std::allocator`, so it is always-equal and the swap is `noexcept`), and `net_delta` is a POD.
+    /// This is what lets a completed candidate state be installed inside `DENY_ALLOCATIONS_IN_SCOPE`
+    /// after its transaction is durable -- see `RefTableState::swap` and `CasRefLedger::commitRefChunk`.
+    /// Note the deliberate consequence: the swapped-out map still OWNS its former base reference, so the
+    /// caller must destroy it before folding the installed map (`materialize` takes its O(overlay)
+    /// in-place path only while `base` is uniquely owned).
+    void swap(RefCowMap & other) noexcept
+    {
+        base.swap(other.base);
+        overlay.swap(other.overlay);
+        std::swap(net_delta, other.net_delta);
+    }
+
+    /// Test-only: current overlay row count (0 right after `materialize()`).
+    size_t overlayEntriesForTest() const { return overlay.size(); }
+    /// Test-only: `base`'s `shared_ptr::use_count()` -- a copy that shares `base` (no per-row
+    /// allocation) bumps this by exactly one.
+    int64_t baseUseCountForTest() const { return base.use_count(); }
+    /// Test-only: identity of the current `base` allocation. `materialize()` on a uniquely-owned base
+    /// folds the overlay in place and leaves this unchanged; on a base still shared with a copy it
+    /// swaps in a fresh base, changing it. Lets a test tell the fast (in-place) path from the copy path.
+    const void * baseIdentityForTest() const { return base.get(); }
+
+private:
+    /// Records a live overlay value and updates `net_delta` according to whether it replaces a
+    /// tombstone, overrides the base, or introduces a key absent from both sources.
+    void insertLive(const String & key, RefCommittedRow row);
+
+    /// Non-const so `materialize()` can fold the overlay into `*base` in place when it is the sole
+    /// owner (see `materialize`'s doc for the safety argument). It is never mutated while shared:
+    /// every write goes to `overlay`, and only the uniquely-owned branch of `materialize` touches it.
+    std::shared_ptr<Base> base = std::make_shared<Base>();
+    Overlay overlay;
+    /// size() = base->size() + net_delta, maintained in lock-step by every overlay-mutating op so
+    /// size()/empty() stay O(1). `net_delta` counts live overlay changes relative to `base`.
+    int64_t net_delta = 0;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.cpp
new file mode 100644
index 000000000000..b2a43bdbf718
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.cpp
@@ -0,0 +1,5113 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/ReadHelpers.h>
+#include <IO/WriteBufferFromString.h>
+#include <IO/WriteHelpers.h>
+#include <Common/Exception.h>
+#include <Common/logger_useful.h>
+#include <Common/MemoryTracker.h>
+#include <Common/ProfileEvents.h>
+#include <Common/setThreadName.h>
+#include <Common/ThreadPool.h>
+#include <base/sleep.h>
+#include <fmt/format.h>
+#include <algorithm>
+#include <chrono>
+#include <thread>
+#include <type_traits>
+#include <unordered_set>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int FILE_DOESNT_EXIST;
+    extern const int INVALID_STATE;
+    extern const int LIMIT_EXCEEDED;
+    extern const int LOGICAL_ERROR;
+    extern const int NETWORK_ERROR;
+    extern const int S3_ERROR;
+    extern const int POCO_EXCEPTION;
+    extern const int UNKNOWN_FORMAT_VERSION;
+    extern const int SOCKET_TIMEOUT;
+    extern const int CANNOT_READ_FROM_SOCKET;
+    extern const int TIMEOUT_EXCEEDED;
+}
+}
+
+namespace ProfileEvents
+{
+    extern const Event CASRefBatchFlushes;
+    extern const Event CASRefBatchedMutations;
+    extern const Event CASRefBatchScopeCuts;
+    extern const Event CASRefQueueWaitMicroseconds;
+    extern const Event CASRefRecoveryRestarts;
+    extern const Event CASRefRecoveryRetries;
+    extern const Event CASRefAppendWedged;
+    extern const Event CASRefAppendPreAttemptRefused;
+    extern const Event CASRefAppendUnwedged;
+    extern const Event CASRefAppendDefiniteFailure;
+    extern const Event CASRefAppendSealRejected;
+    extern const Event CASRefAppendOccupantUnreadable;
+    extern const Event CASRefNeedsRecovery;
+    extern const Event CASRefSweepDeferred;
+    extern const Event CASRefSweepRearmed;
+    extern const Event CASRefStalePrecommitsReclaimed;
+    extern const Event CASRefTableEvictions;
+    extern const Event CASRefSnapshotPutBytes;
+    extern const Event CASRefSnapshotTailLogs;
+    extern const Event CASRefSnapshotPublishDispatched;
+    extern const Event CASRefSnapshotPublishBackoff;
+    extern const Event CASRefCheckpointPublished;
+    extern const Event CASRefCheckpointIdenticalSkip;
+    extern const Event CASRefCheckpointNotAdvanced;
+    extern const Event CASRefRecoveryEpochSealed;
+    extern const Event CASRefRecoveryEpochSealAdopted;
+    extern const Event CASRefRecoveryStragglerAdopted;
+    extern const Event CASRefRecoveryCancelled;
+    extern const Event CASRefRecoveryStreamHole;
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+/// Classifies whether an exception thrown out of a ref-table recovery attempt (checkpoint/snapshot/log
+/// GETs, or the seal PUT) is a TRANSIENT object-store transport failure worth retrying,
+/// vs. a terminal condition (corruption, decode failure, logic error, resource limit) that must fail
+/// fast. The recovery reads call the backend directly (not through `ref_request_controller`), so a
+/// transient blip surfaces as the object storage's native code -- `S3_ERROR` for the S3 backend, or a
+/// socket/timeout/Poco transport code -- NOT the `NETWORK_ERROR` that only the seal PUT's controller
+/// re-mints. Retrying only `NETWORK_ERROR` would leave the LIST/GET legs unprotected, which is exactly
+/// the exact-read path the recovery retry boundary protects.
+bool isTransientRecoveryError(int code)
+{
+    return code == ErrorCodes::NETWORK_ERROR
+        || code == ErrorCodes::S3_ERROR
+        || code == ErrorCodes::POCO_EXCEPTION
+        || code == ErrorCodes::SOCKET_TIMEOUT
+        || code == ErrorCodes::CANNOT_READ_FROM_SOCKET
+        || code == ErrorCodes::TIMEOUT_EXCEEDED;
+}
+
+/// What sits at a ref-log key that our own conditional create just lost to.
+enum class Occupant : uint8_t
+{
+    NotOccupied,     /// the create won, or the outcome is unresolved -- nothing was read
+    Ours,            /// byte-for-byte the transaction this attempt intended: an earlier attempt landed
+    SuccessorSeal,   /// this namespace's epoch-closing record, written by a successor (spec INV-2)
+    Foreign,         /// none of the above -- impossible under mount-lease exclusivity
+};
+
+/// The `mine | successor's seal | foreign` adjudication both write sites owe (the primitive that read
+/// the occupant deliberately does none of it). "Mine" means BYTE EQUALITY -- never a shape or
+/// generation match, which is the aliasing the phase-0 model rejected.
+///
+/// The `catch` is narrow ON PURPOSE, and it is the whole reason this is a function rather than three
+/// lines inline. A blanket `catch (...)` here would launder a TRANSIENT failure -- an allocation
+/// failure or a memory-limit hit while decoding -- into a `Foreign` verdict, and `Foreign` fences the
+/// mount closed and raises a foreign-interference alarm. A perfectly ordinary successor handover plus a
+/// memory blip would then read as corruption. Only the two codes that actually mean "these bytes are
+/// not a well-formed ref-log transaction of this namespace at this id" are absorbed: the decode layer
+/// normalises every malformed-input class to `CORRUPTED_DATA` and passes `UNKNOWN_FORMAT_VERSION`
+/// through (an object this build cannot read is still not a seal it can consume). Everything else
+/// propagates, leaving the caller's lane exactly as it was.
+std::optional<RefTxnId> chainLinkFor(const RefTxnId & id, const std::optional<RefTxnId> & last_epoch_seal)
+{
+    return id.ref_sequence == 1 && last_epoch_seal
+        && last_epoch_seal->writer_epoch + 1 == id.writer_epoch
+        ? last_epoch_seal : std::nullopt;
+}
+
+/// The epoch-closing transaction INV-2 places at `{E, T+1}`: exactly one `EpochSeal` op, no table
+/// content, and the chain link on -- and only on -- sequence 1, where the grammar requires it.
+///
+/// The seal carries nothing about the table because its entire effect is POSITIONAL: it occupies the
+/// one key a dying predecessor's in-flight PUT would have taken, so the store's write-once create is
+/// what fences the ghost, rather than a detector noticing it afterwards.
+RefLogTxn makeEpochSealTxn(const RootNamespace & ns, const RefTxnId & id, const std::optional<RefTxnId> & prev_epoch_seal)
+{
+    RefLogTxn seal;
+    seal.ns = ns.string();
+    seal.txn_id = id;
+    RefOp op;
+    op.kind = RefOpKind::EpochSeal;
+    seal.ops.push_back(op);
+    /// Through `chainLinkFor`, NOT a local `ref_sequence == 1` test, so this is not a fifth site of a
+    /// rule whose four-site drift is what let the writer preview a transaction it would never send. The
+    /// two conditions happen to coincide for every id the walk can reach here -- its first dead epoch
+    /// always already holds the birth transaction, so that seal sits at sequence >= 2, and every later
+    /// dead epoch's seal is at sequence 1 with the immediately preceding epoch's held seal -- but
+    /// "happen to coincide" is exactly the property that rots silently. One rule, one caller shape.
+    seal.prev_epoch_seal = chainLinkFor(id, prev_epoch_seal);
+    return seal;
+}
+
+/// The `prev_epoch_seal` a transaction at `id` carries, given what this table knows about the seal that
+/// closed its previous epoch. INV-2's grammar in one place, because there are now FOUR constructions of
+/// the same transaction -- the real one and its three previews -- and the read side REJECTS a mismatch:
+/// a preview built without the link previews a transaction the writer would never send.
+///
+/// The epoch comparison is not belt-and-braces, it is the DEPOSED-LANE case. A successor that seals an
+/// EMPTY epoch writes its record at `{E, 1}`, and a lane still live at E re-derives exactly `{E, 1}`.
+/// Stamping the seal there would produce a self-pointer, which `validateEpochSealGrammarStructural`
+/// refuses at ENCODE: the lane would fail with a self-inflicted `CORRUPTED_DATA` on every attempt and
+/// never reach the collision that is supposed to fence it. Stamping nothing lets the attempt go out and
+/// meet the seal, which is the intended conclusive rejection.
+Occupant classifyRefLogOccupant(const RootNamespace & ns, const RefTxnId & id, const String & occupant_bytes,
+                                const String & expected_bytes)
+{
+    if (occupant_bytes == expected_bytes)
+        return Occupant::Ours;
+    try
+    {
+        return refLogTxnIsEpochSeal(decodeRefLogTxn(openObject(FormatId::RefLog, occupant_bytes), ns.string(), id))
+            ? Occupant::SuccessorSeal : Occupant::Foreign;
+    }
+    catch (const Exception & e)
+    {
+        if (e.code() != ErrorCodes::CORRUPTED_DATA && e.code() != ErrorCodes::UNKNOWN_FORMAT_VERSION)
+            throw;
+        return Occupant::Foreign;
+    }
+}
+}
+
+CasRefLedger::CasRefLedger(
+    BackendPtr backend_ptr,
+    const Layout & layout_,
+    RefLedgerConfig config_,
+    const CasEventSink & event_sink_,
+    CasRequestBudget cas_request_budget_,
+    String server_root_id_,
+    std::function<uint64_t()> controller_boot_ms_fn,
+    std::function<uint64_t()> live_epoch_fn_,
+    std::function<bool()> fence_ok_fn_,
+    std::function<uint64_t()> fence_generation_fn_,
+    std::function<void(uint64_t)> check_fence_or_throw_,
+    std::function<uint64_t()> boot_ms_now_fn_,
+    std::function<bool()> may_mutate_,
+    std::function<void(const String &, const String &, const std::optional<String> &)> on_impossible_interference_,
+    std::function<std::shared_ptr<void>()> pin_owner_,
+    std::function<void(const RootNamespace &)> cancel_inflight_builds_)
+    : backend(*backend_ptr)
+    , layout(layout_)
+    , config(std::move(config_))
+    , event_sink(event_sink_)
+    , cas_request_budget(cas_request_budget_)
+    , server_root_id(std::move(server_root_id_))
+    , live_epoch_fn(std::move(live_epoch_fn_))
+    , fence_ok_fn(std::move(fence_ok_fn_))
+    , fence_generation_fn(std::move(fence_generation_fn_))
+    , check_fence_or_throw(std::move(check_fence_or_throw_))
+    , boot_ms_now_fn(std::move(boot_ms_now_fn_))
+    , may_mutate(std::move(may_mutate_))
+    , on_impossible_interference(std::move(on_impossible_interference_))
+    , pin_owner(std::move(pin_owner_))
+    , cancel_inflight_builds(std::move(cancel_inflight_builds_))
+{
+    /// The ref-log writer path uses the same retry controller and clock seam as the mount's local
+    /// write fence, so deadline-sensitive tests exercise both paths with one monotonic clock.
+    /// The raw mount `boot_ms_fn` -- the SAME fake-clock seam the local write fence uses -- is reused
+    /// here rather than adding a second clock knob; both are monotonic-ms clocks and tests that need
+    /// deterministic deadline behavior already inject it.
+    ref_request_controller = std::make_unique<CasRequestController>(backend_ptr, cas_request_budget, controller_boot_ms_fn);
+
+    /// Default backoff sleep for the recovery retry loop (`ensureRefTableRecovered`): sleep in short
+    /// slices and stop early if the mount fence drops (shutdown / lease loss), so teardown never waits
+    /// out a full 30s backoff. This is deliberate, bounded backoff against external object-store I/O
+    /// failure -- NOT masking a race -- exactly like `CasRequestControl`'s own inter-attempt
+    /// `threadSleepMs`; the slice loop additionally makes it interruptible, which that one is not.
+    recovery_retry_sleep_fn = [this](uint64_t total_ms)
+    {
+        constexpr uint64_t slice_ms = 200;
+        uint64_t slept = 0;
+        while (slept < total_ms && fence_ok_fn())
+        {
+            const uint64_t chunk = std::min(slice_ms, total_ms - slept);
+            sleepForMilliseconds(chunk);
+            slept += chunk;
+        }
+    };
+}
+
+CasWriteOutcome CasRefLedger::stagingPutIfAbsent(std::string_view key, std::string_view bytes, Token * out_token)
+{
+    /// The ref lane's mount predicate (`fence_ok_fn` == `Pool::refAppendFenceOk`, with no per-table
+    /// runtime term) gates every attempt, matching the other staged writes.
+    return ref_request_controller->putIfAbsentControlled(key, bytes, fence_ok_fn, out_token);
+}
+
+CasCreateResult CasRefLedger::stagingConditionalCreate(std::string_view key, const std::function<PutResult()> & attempt)
+{
+    /// The supplied attempt is controlled by the same retry and mount-fence policy as other staged
+    /// writes.
+    return ref_request_controller->conditionalCreateControlled(key, attempt, fence_ok_fn);
+}
+
+CasOverwriteResult CasRefLedger::stagingConditionalOverwrite(std::string_view key, std::string_view bytes, const Token & expected)
+{
+    /// The supplied write is controlled by the same retry and mount-fence policy as other staged
+    /// writes.
+    return ref_request_controller->putOverwriteControlled(key, bytes, expected, fence_ok_fn);
+}
+
+CasOverwriteResult CasRefLedger::stagingPutIfAbsentMutable(std::string_view key, std::string_view bytes)
+{
+    return ref_request_controller->putIfAbsentControlledMutable(key, bytes, fence_ok_fn);
+}
+
+void CasRefLedger::setCasRetrySleepForTest(std::function<void(uint64_t)> sleep_fn)
+{
+    ref_request_controller->setSleepFnForTest(sleep_fn);
+    recovery_retry_sleep_fn = std::move(sleep_fn);
+}
+
+
+std::optional<Resolved> CasRefLedger::resolveRef(const RootNamespace & ns, const String & ref_name, bool /*allow_stale*/,
+                                                 ResolveAudit audit)
+{
+    /// The read side of the snapshot+log protocol has one authoritative cached table for this mounted
+    /// writer. The `allow_stale` staleness-tolerance knob no longer selects anything: this mounted writer is the
+    /// ONLY writer of `ns`'s ref state (no external CAS token to go stale against, unlike the old
+    /// per-shard decode cache), so the recovered-and-cached `RefTableState` is always this process's
+    /// authoritative view. Kept as a parameter so existing callers compile unchanged.
+    const auto rt = acquireReadableRefTableRuntime(ns);
+    /// A namespace the catalog does not name resolves nothing and is not born by being asked: a ref that
+    /// could exist would have needed a write to put it there, and that write would have birthed the
+    /// namespace first. The two maintenance calls below are skipped with it, and provably lose nothing --
+    /// see the note in `listRefs`.
+    if (!rt)
+        return std::nullopt;
+    ensureRefTableRecovered(ns, *rt);
+    /// A table this mount only ever READS (never mutates) would otherwise
+    /// never have its just-replayed tail/precommits checked -- `appendRefOps`'s own hoisted checks only
+    /// fire for a table this mount WRITES to. Both are cheap (lock + comparison) on the warm path (the
+    /// flag/threshold is already false after the table's first touch this mount); the sweep, if it DOES
+    /// fire, runs synchronously here (safe: this call is not nested inside any queue leader's stack).
+    /// Insulated (unlike appendRefOps's own hoisted call): a READ must not fail because a piggybacked
+    /// maintenance action hit an uncertain PUT -- see `sweepStalePrecommitsForRead`.
+    sweepStalePrecommitsForRead(ns, rt);
+    maybeScheduleSnapshotPublish(ns, rt);
+
+    if (read_before_state_lock_hook_for_test)
+        read_before_state_lock_hook_for_test();
+
+    /// Capture the resolved edge under `state_mutex`, but emit AFTER releasing it (Task 2): the audit
+    /// sink may re-enter a ledger read that itself takes `state_mutex` (e.g. `resolveRef`), so emitting
+    /// while holding the lock self-deadlocks that reentrant read on the same thread. The reentrancy-safe
+    /// dispatcher additionally serializes delivery, but the same-thread relock is prevented here by the
+    /// lock discipline, not the dispatcher.
+    ManifestRef resolved_ref;
+    uint64_t resolved_published_at_ms = 0;
+    std::optional<CasEvent> pending_event;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        const auto it = rt->state.getCommitted().find(ref_name);
+        if (it == rt->state.getCommitted().end())
+            return std::nullopt;
+
+        const RefCommittedRow & row = it->second;
+        resolved_ref = row.manifest_ref;
+        resolved_published_at_ms = row.published_at_ms;
+        /// A resolved ref points to its manifest (the read-path entry point). `object_hash` is the manifest
+        /// instance id the ref names; pairs with a later readManifest ReadMissing if that body is gone.
+        /// `Deferred` (used only by `CachedPartFolderAccess::resolve` on the `getView` call path) skips this
+        /// emit; the caller decides, once it knows whether the access as a whole did real resolve work,
+        /// whether to emit the identical event itself — see `ResolveAudit`'s doc comment.
+        if (audit == ResolveAudit::Emit && hasEventSink())
+        {
+            CasEvent _ev0;
+            _ev0.type = CasEventType::RefResolve;
+            _ev0.namespace_ = ns.string();
+            _ev0.ref_name = ref_name;
+            _ev0.object_kind = CasEventObjectKind::Manifest;
+            _ev0.object_hash = manifestRefDebugString(row.manifest_ref);
+            _ev0.outcome = "resolved";
+            _ev0.reason = "read-side resolve of a ref to its part manifest";
+            pending_event = std::move(_ev0);
+        }
+    }
+    if (pending_event)
+        emitEvent(std::move(*pending_event));
+    return Resolved{
+        .manifest_id = ManifestId{.root_namespace = ns, .ref = resolved_ref},
+        .manifest_size = 0,
+        .published_at_ms = resolved_published_at_ms,
+    };
+}
+
+std::map<String, Resolved> CasRefLedger::listRefs(const RootNamespace & ns)
+{
+    /// The whole ref set is a map iteration over this namespace's recovered-and-cached `RefTableState`:
+    /// an empty but existing namespace still pays one exact recovery pass and zero further requests;
+    /// a warm namespace costs nothing at all (replacing the old per-shard LIST-then-HEAD-present-shards
+    /// dance, since there is no longer a shard fan-out to rediscover on every call). A namespace that was
+    /// never born costs one catalog GET and stops there.
+    const auto rt = acquireReadableRefTableRuntime(ns);
+    /// A namespace the catalog does not name has no refs to list, and listing them is the wrong event to
+    /// bring one into existence on.
+    ///
+    /// Returning here also skips the two maintenance calls below, and that is not a lost obligation:
+    /// neither would do anything. The stale-precommit sweep runs only when `needs_stale_precommit_sweep`
+    /// is armed, which recovery and commit are the only things that arm; the snapshot publish is admitted
+    /// only for a `Live` lifecycle over a non-empty tail, and an unrecovered runtime is neither.
+    if (!rt)
+        return {};
+    ensureRefTableRecovered(ns, *rt);
+    /// Apply the same read-side maintenance policy as `resolveRef`; see `sweepStalePrecommitsForRead`.
+    sweepStalePrecommitsForRead(ns, rt);
+    maybeScheduleSnapshotPublish(ns, rt);
+
+    std::map<String, Resolved> result;
+    std::lock_guard lock(rt->state_mutex);
+    for (const auto [ref_name, row] : rt->state.getCommitted())
+        result.emplace(ref_name, Resolved{
+            .manifest_id = ManifestId{.root_namespace = ns, .ref = row.manifest_ref},
+            .manifest_size = 0,
+            .published_at_ms = row.published_at_ms,
+        });
+    return result;
+}
+
+bool CasRefLedger::hasAnyRefWithPrefix(const RootNamespace & ns, std::string_view prefix)
+{
+    /// Same non-minting recovery/maintenance preamble as `listRefs`; see there for what each shape of
+    /// namespace -- never born, empty, warm -- costs.
+    const auto rt = acquireReadableRefTableRuntime(ns);
+    if (!rt)
+        return false;
+    ensureRefTableRecovered(ns, *rt);
+    sweepStalePrecommitsForRead(ns, rt);
+    maybeScheduleSnapshotPublish(ns, rt);
+
+    std::lock_guard lock(rt->state_mutex);
+    for (const auto [ref_name, row] : rt->state.getCommitted())
+        if (prefix.empty() || std::string_view(ref_name).starts_with(prefix))
+            return true;
+    return false;
+}
+
+
+ConfirmAnswer CasRefLedger::confirmExactRef(const RootNamespace & ns, const String & ref_name,
+                                           const ManifestRef & manifest_ref) const
+{
+    /// Gate 1 of the relink confirm (spec §confirm-primitive). A `Yes` authorizes a REMOTE receiver to
+    /// promote a manifest whose blobs are protected only by this writer's committed binding of that
+    /// exact manifest, so a `Yes` is an assertion about the durable table, not about this cache. Every
+    /// rule below exists to make that assertion true; the answer to anything a rule cannot establish is
+    /// `Unknown`, which costs the receiver a retry and costs correctness nothing.
+    ///
+    /// Two structural properties, both load-bearing:
+    ///
+    ///   ZERO object-store I/O. This runs on an interserver request, so anything it could be made to
+    ///   do is something a remote peer can make this writer do. It therefore reads only what is already
+    ///   resident, never recovers, never resolves a wedge, and -- see the `find` below -- never even
+    ///   materializes a runtime. Deliberately absent for the same reason: `ensureRefTableRecovered`,
+    ///   `sweepStalePrecommitsForRead` and `maybeScheduleSnapshotPublish`, the three maintenance calls
+    ///   `resolveRef` performs and all three of which can do I/O.
+    ///
+    ///   ONE snapshot across BOTH lane mutexes. `pending`/`leader_active` live under
+    ///   `ref_queue_mutex`, the rows and the wedge under `state_mutex`, and the whole point of the
+    ///   rules is their CONJUNCTION -- read at different instants they would prove nothing. The lock
+    ///   ORDER is the one the rest of this file already establishes (`enforceRefTableCacheBudget`
+    ///   nests `state_mutex` under `ref_queue_mutex`, and nothing anywhere takes them the other way
+    ///   round). Because admission (`appendRefOps`' `pending.push_back`) happens under
+    ///   `ref_queue_mutex`, an append is either entirely before this snapshot -- and then visible as a
+    ///   pending item -- or entirely after it. There is no interleaving in which a removal is admitted
+    ///   and this function still answers `Yes`.
+    ///
+    /// What a `Yes` does NOT prove, stated so nobody has to rediscover it: that this runtime's
+    /// recovered view is a COMPLETE replay of the durable log. Completeness is recovery's contract, not
+    /// this function's, and it cannot be re-established here without I/O. Rules 2-4 exclude every way
+    /// this MOUNT can have fallen behind its own durable writes; a recovery that silently observed less
+    /// than it should have is a different defect, in a different component.
+    std::lock_guard<std::mutex> qlock(ref_queue_mutex);
+
+    /// Rule 2 (residency). Direct slot lookup, never a catalog observation or exact-runtime acquisition:
+    /// a read-only query must not let a peer grow this writer's cache or make the next reader pay for a
+    /// recovery it invented. A cold or evicted table is simply unknown here.
+    const auto it = ref_name_slots.find(ns.string());
+    if (it == ref_name_slots.end())
+        return ConfirmAnswer::Unknown;
+    if (!it->second.current)
+        return ConfirmAnswer::Unknown;
+    RefTableRuntime & rt = *it->second.current;
+
+    /// `try_to_lock`, not a blocking acquire: `ensureRefTableRecovered` holds `state_mutex` across its
+    /// whole exact replay, so blocking here would make a confirm WAIT on someone else's recovery --
+    /// up to the full retry envelope -- while holding `ref_queue_mutex`, which is pool-wide append
+    /// admission. That is the zero-I/O contract broken by proxy: the query would not issue a request,
+    /// it would merely be paid for by one, and it would stall every table's lane meanwhile. Failing to
+    /// take the lock is just one more ambiguity, so it answers like every other one. (Same technique,
+    /// and same non-blocking rationale, as `enforceRefTableCacheBudget`'s candidate loop.)
+    std::unique_lock<std::mutex> slock(rt.state_mutex, std::try_to_lock);
+    if (!slock.owns_lock())
+        return ConfirmAnswer::Unknown;
+
+    /// Rule 2 (warm). An unrecovered or mid-recovery runtime has an EMPTY `state`, which would read as
+    /// "the ref does not exist" -- knowledge it does not have. `superseded_by_remount` is the same
+    /// class: this runtime was detached by a self-remount and its view belongs to a dead incarnation.
+    /// Recovery publishes atomically (`installRecoveryResult` sets `recovered` LAST under this mutex),
+    /// so there is no half-recovered view to catch in between.
+    if (!rt.recovered || rt.recovery_in_progress
+        || rt.catalog_life_invalidated.load(std::memory_order_acquire)
+        || rt.superseded_by_remount.load(std::memory_order_acquire))
+        return ConfirmAnswer::Unknown;
+
+    /// Rule 3 (lane quiescent). A wedge is "an object that may be durable and is not applied" -- it may
+    /// BE the removal being asked about. A pending item or an active leader tenure is a mutation this
+    /// table has already admitted; mid-tenure, a chunked flush has committed some of its transactions
+    /// and not others, and `leader_active` spans the whole tenure, so that partially-durable window is
+    /// covered too. None of the three says anything about WHICH ref is affected, so all three are
+    /// table-scoped refusals.
+    if (rt.lane_state != RefLaneState::Ready || !rt.pending.empty() || rt.leader_active)
+        return ConfirmAnswer::Unknown;
+
+    /// Rule 5 (exact row equality) -- the only rule that can answer `No` at all. On a table that passed
+    /// rules 2-4 the committed map is this writer's view, so a missing row or a different `ManifestRef`
+    /// is a real disagreement rather than an ambiguity about this cache. It is NOT a proof about the
+    /// DURABLE table: the fence has not been checked yet (rule 6, below, states why that order is
+    /// deliberate and why it is sound). Equality is exact and total:
+    /// mint-tightening (spec §A3) guarantees a repoint or a recreation mints a fresh `ManifestRef`, so
+    /// there is no ABA to defend against here.
+    const auto & committed = rt.state.getCommitted();
+    const auto row = committed.find(ref_name);
+    if (row == committed.end() || !(row->second.manifest_ref == manifest_ref))
+        return ConfirmAnswer::No;
+
+    /// Rule 6 (mount fence), LAST and still under both locks -- the order the spec fixes. Everything
+    /// above describes what this process believes; this is the check that it is still entitled to
+    /// believe it: a fenced-out mount is no longer the namespace's single writer, so another writer may
+    /// already have repointed the ref. Being last means a token that does not match is reported as `No`
+    /// even under a lost fence: `No` and `Unknown` are the same outcome for the caller (both are
+    /// `SourceProofFailed`, spec §failure-taxonomy), and only `Yes` is gated on the fence.
+    /// Both monotone runtime-invalidations are folded in exactly as the mutation gates fold them: a
+    /// retired or remount-superseded runtime can never authorize a remote promotion.
+    if (!fence_ok_fn()
+        || rt.catalog_life_invalidated.load(std::memory_order_acquire)
+        || rt.superseded_by_remount.load(std::memory_order_acquire))
+        return ConfirmAnswer::Unknown;
+
+    return ConfirmAnswer::Yes;
+}
+
+std::shared_ptr<CasRefLedger::RefTableRuntime> CasRefLedger::lookupRefTableRuntime(const RootNamespace & ns) const
+{
+    std::lock_guard lock(ref_queue_mutex);
+    const auto it = ref_name_slots.find(ns.string());
+    return it == ref_name_slots.end() ? nullptr : it->second.current;
+}
+
+std::shared_ptr<CasRefLedger::RefTableRuntime> CasRefLedger::acquireRefTableRuntime(
+    const NamespaceLifeId & life, uint64_t admitted_generation)
+{
+    check_fence_or_throw(admitted_generation);
+
+    std::shared_ptr<RefTableRuntime> result;
+    bool generation_moved = false;
+    bool identity_conflict = false;
+    {
+        std::lock_guard lock(ref_queue_mutex);
+        generation_moved = fence_generation_fn() != admitted_generation;
+        if (!generation_moved)
+        {
+            const auto it = ref_name_slots.find(life.ns.string());
+            if (it != ref_name_slots.end() && it->second.current)
+            {
+                const auto & current = it->second.current;
+                if (current->life == life
+                    && current->admitted_fence_generation == admitted_generation
+                    && !current->catalog_life_invalidated.load(std::memory_order_acquire)
+                    && !current->superseded_by_remount.load(std::memory_order_acquire))
+                    result = current;
+                else
+                    identity_conflict = true;
+            }
+            else
+            {
+                result = std::make_shared<RefTableRuntime>(
+                    next_ref_runtime_id.fetch_add(1, std::memory_order_relaxed) + 1,
+                    life,
+                    admitted_generation);
+                ref_name_slots.emplace(life.ns.string(), RefNameSlot{.current = result});
+            }
+        }
+    }
+
+    if (generation_moved)
+        check_fence_or_throw(admitted_generation);
+    if (identity_conflict)
+        throwCasWriteRetryLater(fmt::format(
+            "CAS namespace '{}': the cached runtime identity changed while publishing catalog life {}; "
+            "retry from a fresh catalog observation",
+            life.ns.string(), renderIncarnation(life.incarnation)));
+    return result;
+}
+
+std::shared_ptr<CasRefLedger::RefTableRuntime> CasRefLedger::acquireReadableRefTableRuntime(
+    const RootNamespace & ns)
+{
+    /// A resident runtime is the process's already-held immutable life handle. Hot readers deliberately
+    /// pay no catalog request here: after rebirth the handle may return predecessor bytes or NotFound,
+    /// but its exact physical id can never alias successor bytes. A genuinely fresh logical-name
+    /// admission is the cold path below and resolves the current catalog life before publishing a
+    /// runtime.
+    if (auto current = lookupRefTableRuntime(ns))
+    {
+        check_fence_or_throw(current->admitted_fence_generation);
+        {
+            std::lock_guard queue_lock(ref_queue_mutex);
+            if (current->removal_admission_closed)
+                return nullptr;
+        }
+        if (current->catalog_life_invalidated.load(std::memory_order_acquire)
+            || current->superseded_by_remount.load(std::memory_order_acquire))
+            throwCasWriteRetryLater(fmt::format(
+                "CAS namespace '{}': its cached life was detached; retry against a fresh observation",
+                ns.string()));
+        return current;
+    }
+
+    const uint64_t admitted_generation = fence_generation_fn();
+    check_fence_or_throw(admitted_generation);
+    const CasRefCatalog::Snapshot first_catalog = CasRefCatalog::read(backend, layout);
+    check_fence_or_throw(admitted_generation);
+    first_catalog.life_index.throwIfAmbiguous("CAS cold readable runtime admission");
+    const auto it = std::find_if(first_catalog.catalog.entries.begin(), first_catalog.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    if (it == first_catalog.catalog.entries.end() || it->state != NsState::Live)
+        return nullptr;
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(it->ns, it->incarnation);
+
+    if (readable_catalog_after_observation_hook_for_test)
+        readable_catalog_after_observation_hook_for_test();
+
+    /// The backend token, unlike a process-local invalidation counter, observes catalog mutations by
+    /// every actor that shares this pool. Validate both the token and the decoded canonical value:
+    /// neither a token-reuse defect nor an unrelated catalog write may let a life derived from the
+    /// first cut become the first resident runtime. This second GET is deliberately immediately before
+    /// the queue-locked fence/slot recheck in `acquireRefTableRuntime`; the held-handle warm path above
+    /// pays none.
+    const CasRefCatalog::Snapshot second_catalog = CasRefCatalog::read(backend, layout);
+    check_fence_or_throw(admitted_generation);
+    const bool catalog_changed
+        = second_catalog.token != first_catalog.token || second_catalog.catalog != first_catalog.catalog;
+    if (catalog_changed)
+        throwCasWriteRetryLater(fmt::format(
+            "CAS namespace '{}': its catalog changed while a cold reader observed life {}; "
+            "retry from a fresh catalog observation",
+            ns.string(), renderIncarnation(life.incarnation)));
+
+    return acquireRefTableRuntime(life, admitted_generation);
+}
+
+std::shared_ptr<CasRefLedger::RefTableRuntime> CasRefLedger::acquireMutableRefTableRuntime(
+    const RootNamespace & ns)
+{
+    const NamespaceLifeId life = namespaceLife(ns);
+    if (const auto current = lookupRefTableRuntime(ns))
+    {
+        if (current->life == life)
+            return current;
+    }
+    const uint64_t admitted_generation = fence_generation_fn();
+    return acquireRefTableRuntime(life, admitted_generation);
+}
+
+void CasRefLedger::invalidateRemovedCatalogLife(const NamespaceLifeId & life)
+{
+    std::shared_ptr<RefTableRuntime> rt;
+    {
+        std::lock_guard lock(ref_queue_mutex);
+        const auto it = ref_name_slots.find(life.ns.string());
+        if (it == ref_name_slots.end())
+            return;
+        rt = it->second.current;
+        if (!rt)
+            return;
+    }
+
+    /// Invalidation must neither materialize a cache entry nor perform catalog I/O on GC's thread.
+    /// Re-check the exact resident life while holding its state lock: a delayed reconciliation for a
+    /// predecessor must not invalidate a successor. Publishing this bit makes existing holders inert;
+    /// the slot is then detached and a later name-based caller may publish a distinct runtime.
+    {
+        std::lock_guard state_lock(rt->state_mutex);
+        if (rt->life != life)
+            return;
+        rt->catalog_life_invalidated.store(true, std::memory_order_release);
+    }
+    {
+        /// Detach by POINTER identity, not by logical name. A concurrent fresh lookup may already
+        /// have installed a successor for the same name; a delayed predecessor invalidation must never
+        /// erase that successor's cache slot.
+        std::lock_guard queue_lock(ref_queue_mutex);
+        const auto it = ref_name_slots.find(life.ns.string());
+        if (it != ref_name_slots.end() && it->second.current == rt)
+            ref_name_slots.erase(it);
+    }
+    rt->cv.notify_all();
+    rt->recovery_cv.notify_all();
+    rt->publish_settle_cv.notify_all();
+}
+
+void CasRefLedger::reconcileCatalogCut(const CasRefCatalog::Snapshot & catalog_cut)
+{
+    catalog_cut.life_index.throwIfAmbiguous("CAS resident ref-runtime reconciliation");
+
+    std::vector<std::shared_ptr<RefTableRuntime>> closed_runtimes;
+    {
+        std::lock_guard queue_lock(ref_queue_mutex);
+        for (const auto & [_, slot] : ref_name_slots)
+            if (slot.current && slot.current->removal_admission_closed)
+                closed_runtimes.push_back(slot.current);
+    }
+
+    for (const auto & rt : closed_runtimes)
+    {
+        const NamespaceLifeId & resident_life = rt->life;
+        const std::optional<NamespaceLifeId> catalog_life
+            = catalog_cut.life_index.resolve(resident_life.incarnation);
+        if (!catalog_life || *catalog_life != resident_life)
+            invalidateRemovedCatalogLife(resident_life);
+    }
+}
+
+void CasRefLedger::checkRecoveryStillAdmitted(const RootNamespace & ns, RefTableRuntime & rt,
+                                              bool & cancelled) const
+{
+    /// Polled at EVERY I/O boundary of the walk, because every one of them is a point at which this
+    /// recovery may already have lost the right to continue -- and the walk WRITES, so "continue" is not
+    /// a read-only proposition.
+    ///
+    /// Cancellation is checked FIRST and reported as its own outcome: a self-remount asking recovery to
+    /// stop is an orderly hand-off, not a failure of the store, and the caller must not re-drive it
+    /// through the transient-retry loop the way it would an S3 blip.
+    if (rt.recovery_cancel_requested.load(std::memory_order_acquire))
+    {
+        cancelled = true;
+        ProfileEvents::increment(ProfileEvents::CASRefRecoveryCancelled);
+        throwCasWriteRetryLater(fmt::format(
+            "CAS ref-table recovery for namespace '{}' was cancelled by a self-remount before the mount "
+            "fence was re-armed; nothing was written and nothing installed — the next touch recovers under "
+            "the fresh incarnation", ns.string()));
+    }
+
+    if (rt.catalog_life_invalidated.load(std::memory_order_acquire))
+        throwCasWriteRetryLater(fmt::format(
+            "CAS ref-table recovery for namespace '{}': catalog retirement invalidated life {} "
+            "while recovery I/O was in flight — nothing further is written and nothing is installed",
+            ns.string(), renderIncarnation(rt.life.incarnation)));
+
+    /// The remount's OTHER publication, ordered before the fence re-arm: this runtime is detached, so
+    /// whatever it recovers belongs to a dead incarnation's cache. Checked separately from the
+    /// cancellation because the two are independent facts, exactly as `resolveWedgeOnce` checks them.
+    if (rt.superseded_by_remount.load(std::memory_order_acquire))
+        throwCasWriteRetryLater(fmt::format(
+            "CAS ref-table recovery for namespace '{}': this cached table was superseded by a self-remount "
+            "mid-recovery — retry against the fresh mount incarnation", ns.string()));
+
+    /// The FENCE is deliberately NOT checked here, and the omission is the point. `checkFenceOrThrow`
+    /// asks two things at once -- "is the fence held right now" and "is the generation still mine" -- and
+    /// the first has no business gating a READ. Most of this walk is reads, and a mount that has
+    /// transiently lost its lease can still honestly serve them from durable data; refusing at every GET
+    /// would turn a lease blip into "this table cannot be read at all".
+    ///
+    /// The fence gates exactly the three sites that spend it, which is the trio: every `slotOccupy`
+    /// (through its own `admitted_fence_ok`), the `_ckpt` CAS (inside `publishCkpt`), and the install.
+    /// A walk that keeps reading after the generation moved simply wastes its own I/O and is then refused
+    /// at the first of those -- bounded, and strictly better than refusing the reads themselves.
+}
+
+std::optional<RecoveryResult> CasRefLedger::runRecoveryWalkOnce(
+    const RootNamespace & ns, RefTableRuntime & rt, uint64_t admitted_generation, uint64_t live_epoch,
+    const std::optional<RefAppendAttempt> & retained_attempt, std::optional<String> & hole_detail,
+    bool & cancelled)
+{
+    /// Spec §4, one attempt. Runs with NO lock held: everything below is either read-only I/O or a
+    /// conditional create at a key this namespace owns, and the replayed candidate is PRIVATE until the
+    /// caller installs it. `recovery_in_progress` (not `state_mutex`) is what keeps a second caller for
+    /// this same table from racing an independent walk -- see its doc comment.
+    ///
+    /// Runtime construction already fixed the exact catalog life, so every key this walk builds remains
+    /// under the predecessor even if the same logical name is concurrently rebound.
+    const NamespaceLifeId life = rt.life;
+
+    /// ---- Step 2: immutable runtime authority and checkpoint ----
+    /// The runtime was admitted for this exact life before entering recovery, so this walk must not take
+    /// another catalog cut. Retirement invalidates the runtime through `catalog_life_invalidated`, which
+    /// `checkRecoveryStillAdmitted` and the recovery write/install fences observe below.
+    const CatalogEntry catalog_entry{
+        .ns = life.ns,
+        .state = NsState::Live,
+        .incarnation = life.incarnation};
+    const std::optional<CkptSample> sampled_ckpt = readCkpt(backend, layout, life);
+    std::optional<CkptSample> accepted_ckpt_sample = sampled_ckpt;
+    checkRecoveryStillAdmitted(ns, rt, cancelled);
+
+    /// ---- Step 3: the finite grounding and exact base ----
+    /// `chooseRecoveryGrounding` is the single policy boundary. The immutable checkpoint alone names
+    /// the base and inclusive frontier; recovery does not enumerate its own stream.
+    RecoveryGrounding grounding = chooseRecoveryGrounding(
+        catalog_entry,
+        sampled_ckpt ? std::optional<RefCkpt>{sampled_ckpt->ckpt} : std::nullopt);
+    std::optional<RefTxnId> base_id = grounding.base;
+
+    std::optional<RefTableSnapshot> base_snapshot;
+    uint64_t base_snapshot_bytes = 0;
+    if (base_id)
+    {
+        try
+        {
+            CheckpointSnapshotBase base = readCheckpointSnapshotBase(backend, layout, life, sampled_ckpt->ckpt);
+            base_snapshot = std::move(base.snapshot);
+            base_snapshot_bytes = base.bytes;
+        }
+        catch (const Exception & e)
+        {
+            if (e.code() != ErrorCodes::CORRUPTED_DATA || !sampled_ckpt)
+                throw;
+
+            /// A newer checkpoint may have atomically re-anchored recovery while cleanup retired this
+            /// old anchor's snapshot or witness log. Restart from that newer immutable sample; an
+            /// unchanged checkpoint turns every helper failure (missing, malformed, or seal) into the
+            /// fail-closed corruption it describes.
+            const std::optional<CkptSample> current = readCkpt(backend, layout, life);
+            if (classifyMissingSampledBase(sampled_ckpt->token,
+                                           current ? std::optional<Token>(current->token) : std::nullopt)
+                == MissingBaseVerdict::RestartRecovery)
+                return std::nullopt;
+            throw;
+        }
+    }
+    checkRecoveryStillAdmitted(ns, rt, cancelled);
+
+    /// The committed replay range comes only from the grounding. Writer recovery additionally probes
+    /// the single arithmetic successor: it is the durable-but-not-yet-frontiered transaction left by
+    /// a lost checkpoint response. With no committed transaction that successor is genesis itself.
+    /// Deliberately no enumerated-log fallback exists here.
+    std::optional<RefTxnId> walk_from = grounding.walk_from;
+    if (!walk_from && !grounding.committed_through)
+        walk_from = RefTxnId{*sampled_ckpt->ckpt.life_epoch, 1};
+
+    RefReplayBuilder builder(std::move(base_snapshot), base_snapshot_bytes);
+    std::optional<RefTxnId> private_frontier = base_id;
+
+    /// The chain link, threaded through the whole walk: the greatest seal this recovery has SEEN,
+    /// whether it read it out of the durable tail, adopted it from a concurrent recoverer, or minted it.
+    /// It is what a sequence-1 seal must name, and what the table's next sequence-1 append must name.
+    std::optional<RefTxnId> last_epoch_seal = sampled_ckpt->ckpt.last_epoch_seal;
+
+    /// Applies one decoded transaction to the private candidate, accounting its resident footprint to
+    /// the streaming-recovery memory probe for exactly the span it is held (no-op in production).
+    const auto apply_one = [&](RefLogTxn && txn, uint64_t encoded_bytes)
+    {
+        const int64_t footprint = static_cast<int64_t>(decodedRefLogTxnFootprint(txn));
+        reportReplayMemoryDelta(footprint);
+        SCOPE_EXIT({ reportReplayMemoryDelta(-footprint); });
+        if (refLogTxnIsEpochSeal(txn))
+            last_epoch_seal = txn.txn_id;
+        private_frontier = txn.txn_id;
+        builder.applyOne(std::move(txn), encoded_bytes);
+    };
+
+    const auto check_recovery_write_admitted = [this, &rt](uint64_t expected_generation)
+    {
+        check_fence_or_throw(expected_generation);
+        if (rt.catalog_life_invalidated.load(std::memory_order_acquire))
+            throwCasWriteRetryLater(fmt::format(
+                "CAS ref-table recovery for namespace '{}': catalog retirement invalidated life {} "
+                "before its checkpoint contribution",
+                rt.life.ns.string(), renderIncarnation(rt.life.incarnation)));
+    };
+    const auto publish_recovered_frontier = [&](const RefLogTxn & txn)
+    {
+        const RefCkpt contribution{
+            .life_epoch = std::nullopt,
+            .committed_through = txn.txn_id,
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = refLogTxnIsEpochSeal(txn)
+                ? std::optional<RefTxnId>{txn.txn_id} : txn.prev_epoch_seal};
+        if (publishCkptContribution(life, contribution, admitted_generation, check_recovery_write_admitted)
+            == CkptPublishOutcome::FencedOut)
+            throwCasWriteRetryLater(fmt::format(
+                "CAS ref-table recovery for namespace '{}': the mount incarnation moved before the "
+                "checkpoint could record recovered txn {}-{}; nothing is installed",
+                ns.string(), txn.txn_id.writer_epoch, txn.txn_id.ref_sequence));
+
+        /// `publishCkptContribution` correctly merges a concurrent winner, but recovery's private
+        /// candidate cannot silently inherit that winner's farther frontier. If it moved beyond this
+        /// one successor between lookahead and our CAS, restart from the exact newer checkpoint so the
+        /// installed state covers every transaction its frontier certifies.
+        std::optional<CkptSample> exact = readCkpt(backend, layout, life);
+        if (!exact || !exact->ckpt.committed_through || *exact->ckpt.committed_through < txn.txn_id)
+            throwCasWriteRetryLater(fmt::format(
+                "CAS ref-table recovery for namespace '{}': exact checkpoint read after publishing "
+                "recovered txn {}-{} did not certify that transaction; nothing is installed",
+                ns.string(), txn.txn_id.writer_epoch, txn.txn_id.ref_sequence));
+        if (*exact->ckpt.committed_through != txn.txn_id)
+            return false;
+
+        /// This recovery itself may advance `_ckpt`. The just-read token and decoded body are the
+        /// latest authority cut the private candidate has validated, so the final install boundary
+        /// compares against this sample rather than the original one.
+        accepted_ckpt_sample = std::move(exact);
+        return true;
+    };
+
+    const std::optional<RefTxnId> sampled_frontier = grounding.committed_through;
+
+    /// ---- Steps 5 and 6: the arithmetic tail and the seal CAS-walk, as ONE loop ----
+    /// They are the same walk seen from two sides. Reading `{E, S}` and finding it present is the tail;
+    /// finding it ABSENT is a decision point: the live epoch's stream simply ends there, while a DEAD
+    /// epoch's must be closed at that exact slot before this table may be trusted. Writing them as one
+    /// loop is not brevity -- it is what makes "the seal goes where the ghost's PUT would have gone"
+    /// true by construction rather than by two functions agreeing on an index.
+    if (walk_from)
+    {
+        uint64_t epoch = walk_from->writer_epoch;
+        uint64_t sequence = walk_from->ref_sequence;
+        size_t slot_attempts_this_epoch = 0;
+
+        for (;;)
+        {
+            checkRecoveryStillAdmitted(ns, rt, cancelled);
+            const RefTxnId id{epoch, sequence};
+
+            if (const auto got = backend.get(layout.refLogKey(life, id)))
+            {
+                /// `runRecoveryWalkOnce` is the writer recovery entry point even after a process
+                /// restart, when no in-memory attempt survives. A readable birth checkpoint with no
+                /// committed frontier makes its first `{epoch,1}` log the same one unfrontiered writer
+                /// successor as `F+1`; both are recovered from the exact checkpoint and stream alone.
+                const bool above_sampled_frontier = sampled_ckpt
+                    && (!sampled_frontier || *sampled_frontier < id);
+                if (above_sampled_frontier && retained_attempt && retained_attempt->txn_id == id)
+                {
+                    /// `NeedsRecovery` retains the writer's complete attempted bytes exactly for this
+                    /// adjudication. A storage violation can replace a write-once log object between the
+                    /// failed frontier publish and recovery; accepting another ordinary transaction here
+                    /// would acknowledge history the admitted writer never created. A successor seal is
+                    /// the one conclusive, already-defined loss case: it closes the old writer's epoch
+                    /// and is replayed below as the durable stream record.
+                    const Occupant occupant = classifyRefLogOccupant(ns, id, got->bytes, retained_attempt->bytes);
+                    if (occupant == Occupant::Foreign)
+                        throw Exception(ErrorCodes::CORRUPTED_DATA,
+                            "CAS ref-table recovery for namespace '{}': a DIFFERENT object occupies retained "
+                            "writer txn {}-{} above the exact checkpoint frontier; recovery must not publish "
+                            "or install it as that writer's history",
+                            ns.string(), id.writer_epoch, id.ref_sequence);
+                }
+                RefLogTxn txn = decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), id);
+                const bool is_seal = refLogTxnIsEpochSeal(txn);
+                const std::optional<RefTxnId> next_committed_id
+                    = sampled_frontier && id <= *sampled_frontier
+                    ? nextRefLogIdWithinCommittedFrontier(id, is_seal, *sampled_frontier)
+                    : std::nullopt;
+                const RefLogTxn frontier_txn = txn;
+                apply_one(std::move(txn), got->bytes.size());
+                if (above_sampled_frontier)
+                {
+                    /// Before changing the sampled checkpoint, prove that this is the ONLY object above
+                    /// it. The append lane cannot allocate a second unfrontiered id. If the checkpoint
+                    /// moved while we inspected the second slot, restart from that exact newer cut;
+                    /// otherwise two successors are durable corruption and F+1 must not be laundered
+                    /// into the frontier first.
+                    const RefTxnId following_id = is_seal
+                        ? RefTxnId{id.writer_epoch + 1, 1}
+                        : RefTxnId{id.writer_epoch, id.ref_sequence + 1};
+                    if (backend.get(layout.refLogKey(life, following_id)))
+                    {
+                        const std::optional<CkptSample> current = readCkpt(backend, layout, life);
+                        if (!sampled_ckpt || !current || current->token != sampled_ckpt->token)
+                            return std::nullopt;
+                        const String frontier_description = sampled_frontier
+                            ? fmt::format("{}-{}", sampled_frontier->writer_epoch, sampled_frontier->ref_sequence)
+                            : "with only a life epoch";
+                        throw Exception(ErrorCodes::CORRUPTED_DATA,
+                            "CAS ref-table recovery for namespace '{}': exact checkpoint {} "
+                            "had two durable successors through {}-{} while its token remained unchanged; "
+                            "the append lane permits at most one unfrontiered transaction",
+                            ns.string(), frontier_description,
+                            following_id.writer_epoch, following_id.ref_sequence);
+                    }
+
+                    /// The sole successor is valid in the private candidate. Publish its frontier under
+                    /// this recovery's current admission before exposing the candidate.
+                    if (!publish_recovered_frontier(frontier_txn))
+                        return std::nullopt;
+                    /// Publishing F+1 makes this transaction admitted history, not the end of the
+                    /// writer walk. A cold writer can still be above this epoch and must seal its
+                    /// now-dead stream at the following slot before the recovered table is installed.
+                    if (is_seal)
+                    {
+                        ++epoch;
+                        sequence = 1;
+                        slot_attempts_this_epoch = 0;
+                    }
+                    else
+                        ++sequence;
+                    continue;
+                }
+                if (next_committed_id)
+                {
+                    epoch = next_committed_id->writer_epoch;
+                    sequence = next_committed_id->ref_sequence;
+                    slot_attempts_this_epoch = 0;
+                }
+                else if (is_seal)
+                {
+                    /// This epoch is closed. Its stream cannot continue, so the next durable id of this
+                    /// namespace is sequence 1 of the next epoch -- including when that epoch is at or
+                    /// above our own live one, which is what a mount deposed by a higher-epoch successor
+                    /// sees. Reading on is honest (the transactions ARE this namespace's) and harmless:
+                    /// our own appends then collide with the seal and are conclusively rejected, which is
+                    /// exactly how INV-2 tells a deposed writer it has been deposed.
+                    ++epoch;
+                    sequence = 1;
+                    slot_attempts_this_epoch = 0;
+                }
+                else
+                    ++sequence;
+                continue;
+            }
+
+            /// ---- Absent. Hole, end of the live stream, or a dead epoch to close ----
+            if (sampled_frontier && id <= *sampled_frontier)
+            {
+                /// This id belongs to the inclusive committed range. A 404 cannot shorten that range:
+                /// re-read the exact mutable checkpoint to distinguish a concurrent frontier movement
+                /// from durable-data loss under an unchanged authority token.
+                const std::optional<CkptSample> current = readCkpt(backend, layout, life);
+                if (!current || current->token != sampled_ckpt->token)
+                    return std::nullopt;
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS ref-table recovery for namespace '{}': committed log id {}-{} is absent while "
+                    "the exact checkpoint frontier {}-{} and its token remain unchanged",
+                    ns.string(), id.writer_epoch, id.ref_sequence,
+                    sampled_frontier->writer_epoch, sampled_frontier->ref_sequence);
+            }
+
+            /// Only the exact checkpoint's recorded seal may witness a same-epoch hole. LIST log names
+            /// are diagnostics only; an omitted or stale name cannot change a correctness verdict.
+            if (sampled_ckpt->ckpt.last_epoch_seal
+                && sampled_ckpt->ckpt.last_epoch_seal->writer_epoch == epoch
+                && sequence < sampled_ckpt->ckpt.last_epoch_seal->ref_sequence
+                && backend.get(layout.refLogKey(life, *sampled_ckpt->ckpt.last_epoch_seal)))
+            {
+                ProfileEvents::increment(ProfileEvents::CASRefRecoveryStreamHole);
+                hole_detail = fmt::format(
+                    "id {}-{} is absent while the exact checkpoint records same-epoch seal {}-{} — the "
+                    "ref-log stream is dense by construction (INV-1), so this is a hole, not the end of "
+                    "the stream",
+                    id.writer_epoch, id.ref_sequence,
+                    sampled_ckpt->ckpt.last_epoch_seal->writer_epoch,
+                    sampled_ckpt->ckpt.last_epoch_seal->ref_sequence);
+                return std::nullopt;
+            }
+
+            if (epoch >= live_epoch)
+                break;   /// the LIVE epoch's stream ends here: this is where the next append goes
+
+            /// A seal closes the epoch of a LIVE stream, and a namespace that is Removed (or never
+            /// born) has none: its terminal record already closed its history, and `applyOp` refuses a
+            /// seal over it -- correctly, since such an object would be a statement about a stream that
+            /// does not exist. Both sides of that rule live here and in `applyOp`, and they have to
+            /// agree: minting a seal this build then cannot replay would leave a durable object that
+            /// makes the namespace permanently unrecoverable.
+            ///
+            /// Advance to the next epoch WITHOUT writing. Skipping the write is not skipping the walk:
+            /// a namespace removed at epoch 5 and RECREATED at epoch 7 still has durable transactions
+            /// above, and stopping here would silently truncate them. The recreation's chain link comes
+            /// from its own `life_epoch` (its birth is sequence 1 of its genesis epoch, where the
+            /// grammar forbids a `prev_epoch_seal`), not from a seal over the dead life.
+            if (builder.lifecycle() != RefLifecycle::Live)
+            {
+                ++epoch;
+                sequence = 1;
+                slot_attempts_this_epoch = 0;
+                continue;
+            }
+
+            /// A DEAD epoch, unclosed. Everything that can throw is prepared BEFORE the conditional
+            /// create, so a failure here happens while the slot is provably untouched.
+            if (++slot_attempts_this_epoch > kRefRecoveryMaxSlotAttemptsPerEpoch)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS ref-table recovery for namespace '{}': the seal slot of dead epoch {} was taken by a "
+                    "straggler {} times running. INV-1 permits at most one in-flight append per table per "
+                    "writer, so a store that keeps materializing objects underneath this walk is not a race "
+                    "this recovery may keep chasing",
+                    ns.string(), epoch, slot_attempts_this_epoch - 1);
+
+            const RefLogTxn seal_txn = makeEpochSealTxn(ns, id, last_epoch_seal);
+            /// The CONTEXTUAL half of the seal grammar, checked only when this recovery actually LEARNED
+            /// the namespace's `life_epoch`. There is no `value_or` here on purpose (task 5's interface
+            /// note): a substituted 0 would demand a `prev_epoch_seal` on every sequence-1 transaction
+            /// and reject a genesis birth. Unknown `life_epoch` therefore means the structural grammar
+            /// alone -- which `encodeRefLogTxn` enforces unconditionally on the very next line -- and the
+            /// walk's own construction rule, which is provably equivalent here (see `makeEpochSealTxn`).
+            if (sampled_ckpt && sampled_ckpt->ckpt.life_epoch)
+                validateEpochSealGrammarContextual(seal_txn, *sampled_ckpt->ckpt.life_epoch);
+            const String seal_bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(seal_txn));
+
+            /// Presented on EVERY attempt: the generation this recovery was admitted under, never the
+            /// current one. A seal written by an incarnation that no longer owns the namespace is a write
+            /// from a dead mount, and refusing pre-attempt leaves the slot provably untouched.
+            const auto admitted_fence_ok = [this, &rt, admitted_generation]
+            {
+                return fence_ok_fn()
+                    && !rt.catalog_life_invalidated.load(std::memory_order_acquire)
+                    && !rt.superseded_by_remount.load(std::memory_order_acquire)
+                    && fence_generation_fn() == admitted_generation;
+            };
+
+            const SlotOccupyResult occupied =
+                ref_request_controller->slotOccupy(
+                    layout.refLogKey(life, id), seal_bytes, admitted_fence_ok);
+
+            switch (occupied.kind)
+            {
+                case SlotOccupyResult::Kind::Created:
+                {
+                    /// The epoch is ours to close and now IS closed. Apply our own seal to the candidate:
+                    /// it is a durable transaction of this stream like any other, and the next recovery
+                    /// will read it back exactly where we put it.
+                    RefLogTxn applied = seal_txn;
+                    apply_one(std::move(applied), seal_bytes.size());
+                    ProfileEvents::increment(ProfileEvents::CASRefRecoveryEpochSealed);
+                    if (!publish_recovered_frontier(seal_txn))
+                        return std::nullopt;
+                    ++epoch;
+                    sequence = 1;
+                    slot_attempts_this_epoch = 0;
+                    break;
+                }
+                case SlotOccupyResult::Kind::Occupied:
+                {
+                    /// Someone reached this slot first. A DECODE FAILURE here propagates: an object at a
+                    /// key this namespace owns that is not a transaction of this namespace at this id is
+                    /// corruption or a protocol breach, and the one thing recovery must not do is guess
+                    /// past it.
+                    RefLogTxn occupant = decodeRefLogTxn(
+                        openObject(FormatId::RefLog, occupied.occupant_bytes), ns.string(), id);
+                    const bool occupant_is_seal = refLogTxnIsEpochSeal(occupant);
+                    const RefLogTxn frontier_txn = occupant;
+                    apply_one(std::move(occupant), occupied.occupant_bytes.size());
+                    if (!publish_recovered_frontier(frontier_txn))
+                        return std::nullopt;
+                    if (occupant_is_seal)
+                    {
+                        /// A concurrent recoverer closed this epoch (or our own earlier attempt did, and
+                        /// its acknowledgment was lost). Either way the epoch is closed by a seal that is
+                        /// as good as ours -- adopt it and continue. Contesting a peer's CORRECT write is
+                        /// how two recoverers of the same table turn a designed race into an incident.
+                        ProfileEvents::increment(ProfileEvents::CASRefRecoveryEpochSealAdopted);
+                        ++epoch;
+                        sequence = 1;
+                        slot_attempts_this_epoch = 0;
+                    }
+                    else
+                    {
+                        /// A STRAGGLER: an ordinary transaction of the dead epoch landed at `T+1` between
+                        /// our read and our create. Adopt it, advance `T` by exactly ONE, and try the seal
+                        /// again at the NEW `T+1`. Never mint `T+2` around it: ids are state-derived
+                        /// (INV-1/INV-2), and writing past an occupied slot puts a hole in the durable
+                        /// stream that no later reader can distinguish from a lost object.
+                        ProfileEvents::increment(ProfileEvents::CASRefRecoveryStragglerAdopted);
+                        ++sequence;
+                    }
+                    break;
+                }
+                case SlotOccupyResult::Kind::Unresolved:
+                {
+                    /// The store will not say whether our seal landed. There is no honest way to continue:
+                    /// exposing the table would publish a dead epoch that may or may not be closed, and
+                    /// re-deriving the slot later needs a fresh read anyway. Fail this attempt into the
+                    /// caller's transient-retry loop, which either succeeds on a later attempt or spends
+                    /// its budget and leaves the table unrecovered.
+                    throwCasWriteRetryLater(fmt::format(
+                        "CAS ref-table recovery for namespace '{}': the epoch seal at {}-{} is UNRESOLVED "
+                        "({}); the table stays unrecovered rather than being exposed with a dead epoch that "
+                        "may or may not be closed",
+                        ns.string(), id.writer_epoch, id.ref_sequence,
+                        unresolvedProvesNothingWasSent(occupied.unresolved_reason)
+                            ? "nothing was sent" : "the outcome of the attempt is unknown"));
+                }
+            }
+        }
+    }
+
+    /// The sealer's checkpoint contribution is durable before the authority cut is validated. This is
+    /// still one contribution for the current walk; the following exact read certifies the same private
+    /// frontier rather than assuming the CAS result and the candidate stayed aligned.
+    if (last_epoch_seal)
+    {
+        checkRecoveryStillAdmitted(ns, rt, cancelled);
+        const RefCkpt contribution{.life_epoch = std::nullopt,
+                                   .committed_through = last_epoch_seal,
+                                   .checkpoint_snapshot_id = std::nullopt,
+                                   .last_epoch_seal = last_epoch_seal};
+        if (publishCkptContribution(life, contribution, admitted_generation, check_recovery_write_admitted)
+            == CkptPublishOutcome::FencedOut)
+            throwCasWriteRetryLater(fmt::format(
+                "CAS ref-table recovery for namespace '{}': the mount incarnation moved before the "
+                "checkpoint could record the epoch seal {}-{}; nothing was written and nothing is installed",
+                ns.string(), last_epoch_seal->writer_epoch, last_epoch_seal->ref_sequence));
+
+        const std::optional<CkptSample> exact = readCkpt(backend, layout, life);
+        if (!exact || exact->ckpt.committed_through != private_frontier || exact->ckpt.last_epoch_seal != last_epoch_seal)
+            return std::nullopt;
+        accepted_ckpt_sample = exact;
+    }
+
+    /// Final authority validation is the recovery linearization point. The last exact log probe fixed
+    /// the private cut, but another actor could have changed `_ckpt` immediately afterwards. Install
+    /// only when both the exact object token and its complete decoded body remain equal to the latest
+    /// authority sample this private candidate accepted.
+    const std::optional<CkptSample> final_ckpt = readCkpt(backend, layout, life);
+    if (!final_ckpt || !accepted_ckpt_sample
+        || final_ckpt->token != accepted_ckpt_sample->token
+        || final_ckpt->ckpt != accepted_ckpt_sample->ckpt)
+        return std::nullopt;
+    checkRecoveryStillAdmitted(ns, rt, cancelled);
+
+    RecoveryResult result = std::move(builder).finish();
+
+    /// `finish` returns the candidate WITHOUT materializing: `stateFromSnapshot` loads every committed
+    /// row and owned-manifest entry into the COW OVERLAY, and no tail transaction materializes either.
+    /// This state is the table's long-lived working state, so fold both `committed` and `owned_manifests`
+    /// into fresh shared bases ONCE here -- rather than making the first flush's scratch copy (and every
+    /// per-item/shape-check copy on it) deep-copy an N-row overlay. The O(N) fold rides inside recovery,
+    /// which is already O(N).
+    result.state.materializeCommitted();
+    /// Stale-precommit cleanup is dispatched once, from `appendRefOps`'s top level (never from here --
+    /// this call may itself be nested inside a queue leader's flush stack).
+    result.needs_stale_precommit_sweep = true;
+    result.last_epoch_seal = last_epoch_seal;
+    /// Per-table admission budgets pre-subtract this table's own wire overhead (`4 + ns.size()`, once in
+    /// a snapshot body and once in a removal txn body) plus a fixed safety margin from the raw hard
+    /// limits, once, here.
+    const uint64_t overhead = 4 + ns.string().size() + kRefAdmissionSafetyMargin;
+    result.snapshot_budget = overhead < ref_snapshot_max_bytes ? ref_snapshot_max_bytes - overhead : 0;
+    result.removal_budget = overhead < ref_removal_max_bytes ? ref_removal_max_bytes - overhead : 0;
+
+    return result;
+}
+
+NamespaceLifeId CasRefLedger::resolveNamespaceLife(
+    const RootNamespace & ns, uint64_t admitted_generation, uint64_t live_epoch,
+    bool * lifecycle_refusal)
+{
+    /// Bounded exactly like `CasRefCatalog::casUpdateImpl`'s own live-lock brake, but against THIS
+    /// loop's re-read cycle only -- every primitive called below already bounds its OWN retry against
+    /// the catalog's single contended object. A duel between two openers (one creating, one
+    /// reconciling a stale creator) converges in a handful of rounds; this guards only against a
+    /// pathologically un-converging sequence of them.
+    static constexpr size_t kMaxResolveAttempts = 32;
+    const CkptDeadline deadline{boot_ms_now_fn, boot_ms_now_fn() + cas_request_budget.operation_deadline_ms};
+    const CreatorFence our_fence{server_root_id, live_epoch, admitted_generation};
+
+    for (size_t attempt = 0; attempt < kMaxResolveAttempts; ++attempt)
+    {
+        const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+        const auto it = std::find_if(snap.catalog.entries.begin(), snap.catalog.entries.end(),
+            [&](const CatalogEntry & e) { return e.ns.string() == ns.string(); });
+
+        if (it == snap.catalog.entries.end())
+        {
+            /// No entry at all: this open is the namespace's first-ever opener. `createNamespace`
+            /// mints a fresh incarnation and carries it all the way to `Live` (or reports why it could
+            /// not); either way it does not hand the incarnation back, so a `Live` outcome re-reads the
+            /// catalog on the next loop iteration to learn it -- one extra GET, paid once per birth,
+            /// never per write.
+            const auto outcome = CasRefCatalog::createNamespace(
+                backend, layout, config.gc_shards, ns, our_fence,
+                admitted_generation, check_fence_or_throw, deadline);
+            if (outcome == CasRefCatalog::NamespaceCreationOutcome::FencedOut)
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS ref-table recovery for namespace '{}': the mount incarnation moved while "
+                    "birthing its catalog entry; nothing was written and nothing installed", ns.string()));
+            continue;   /// Live or Superseded: re-read (Superseded means a DIFFERENT actor won birth)
+        }
+
+        if (it->state == NsState::Live)
+            return NamespaceLifeId::fromCatalogEntry(it->ns, it->incarnation);
+
+        if (it->state == NsState::Removing)
+        {
+            if (lifecycle_refusal)
+                *lifecycle_refusal = true;
+            throwCasWriteRetryLater(fmt::format(
+                "CAS namespace '{}' is Removing: creation waits for its terminal fold and catalog "
+                "removal to complete; retry later", ns.string()));
+        }
+
+        /// `Creating`, with a strict-grammar creator fence (`CatalogEntry`'s own invariant guarantees
+        /// `it->creator` is set whenever `state == Creating`). If it names THIS mount's own currently
+        /// live fence, an earlier attempt of this SAME open landed step 1 (`casAdmitEntry`) but not
+        /// steps 2/3 -- e.g. a transient failure inside `completeCreation`'s own `publishCkpt` retry --
+        /// and resuming is simply re-running steps 2/3 over the entry as observed just now. Reasoning
+        /// about fence terminality for our OWN live fence would never terminate (we are, by definition,
+        /// not dead), so this case is checked FIRST and unconditionally, before any terminality probe.
+        if (it->creator->server_root_id == server_root_id && it->creator->writer_epoch == live_epoch)
+        {
+            const auto outcome = CasRefCatalog::completeCreation(
+                backend, layout, *it, admitted_generation, check_fence_or_throw, deadline);
+            if (outcome == CasRefCatalog::NamespaceCreationOutcome::FencedOut)
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS ref-table recovery for namespace '{}': the mount incarnation moved while "
+                    "resuming its own stalled creation; nothing was written and nothing installed",
+                    ns.string()));
+            continue;   /// Live or Superseded: re-read either way
+        }
+
+        /// A DIFFERENT actor's `Creating` entry. It may still be mid-flight (retry later, against a
+        /// fresh read -- never busy-loop this instant) or provably dead, in which case reconciliation
+        /// steals it onto our own fence and this open resumes `completeCreation` itself.
+        const auto reconcile_outcome = CasRefCatalog::reconcileStaleCreator(
+            backend, layout, *it, our_fence,
+            [this](const CreatorFence & f) { return isCreatorFenceTerminal(backend, layout, f.server_root_id, f.writer_epoch); },
+            admitted_generation, check_fence_or_throw);
+        switch (reconcile_outcome)
+        {
+            case CasRefCatalog::ReconcileCreatorOutcome::FencedOut:
+                /// Review I6: our OWN mount fence moved before the steal CAS -- nothing was written, and
+                /// this mount is the wrong actor to retry (its incarnation is gone).
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS ref-table recovery for namespace '{}': the mount incarnation moved while "
+                    "reconciling a stalled foreign creator; nothing was written and nothing installed",
+                    ns.string()));
+            case CasRefCatalog::ReconcileCreatorOutcome::Reconciled:
+            {
+                CatalogEntry resumed = *it;
+                resumed.creator = our_fence;
+                const auto outcome = CasRefCatalog::completeCreation(
+                    backend, layout, resumed, admitted_generation, check_fence_or_throw, deadline);
+                if (outcome == CasRefCatalog::NamespaceCreationOutcome::FencedOut)
+                    throwCasWriteRetryLater(fmt::format(
+                        "CAS ref-table recovery for namespace '{}': the mount incarnation moved while "
+                        "completing a reconciled creation; nothing was written and nothing installed",
+                        ns.string()));
+                continue;   /// Live or Superseded: re-read either way
+            }
+            case CasRefCatalog::ReconcileCreatorOutcome::CreatorFenceStillLive:
+                if (lifecycle_refusal)
+                    *lifecycle_refusal = true;
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS ref-table recovery for namespace '{}': its catalog entry is still Creating "
+                    "under a creator fence that is not yet provably dead; retry later", ns.string()));
+            case CasRefCatalog::ReconcileCreatorOutcome::EntryChanged:
+                continue;   /// token-exactness failed: someone else already moved this entry; re-read
+        }
+    }
+
+    throwCasWriteRetryLater(fmt::format(
+        "CAS ref-table recovery for namespace '{}': its catalog entry did not converge to a resolvable "
+        "incarnation after {} attempts", ns.string(), kMaxResolveAttempts));
+}
+
+void CasRefLedger::ensureRefTableRecovered(const RootNamespace & ns, RefTableRuntime & rt)
+{
+    {
+    std::unique_lock lock(rt.state_mutex);
+    /// Every touch, warm or cold,
+    /// marks this table most-recently-used so `enforceRefTableCacheBudget` evicts idle tables first.
+    rt.last_touch_tick = ref_table_access_tick.fetch_add(1, std::memory_order_relaxed) + 1;
+
+    /// `NeedsRecovery` is a hard lane fence: a transaction is known durable but this cache could not
+    /// install it. Replaying the durable stream is the only transition back to `Ready`.
+    const auto needs_rerecovery = [&rt]
+    {
+        return rt.lane_state == RefLaneState::NeedsRecovery;
+    };
+    if (rt.recovered && !needs_rerecovery())
+        return;
+
+    /// A concurrent second caller waits here rather than racing an independent walk against the first
+    /// caller's unlocked I/O.
+    while (rt.recovery_in_progress)
+    {
+        ++rt.recovery_waiters_for_test;
+        rt.recovery_cv.wait(lock);
+        --rt.recovery_waiters_for_test;
+    }
+    if (rt.recovered && !needs_rerecovery())
+        return;   /// the caller we waited on already finished it
+
+    rt.recovery_in_progress = true;
+    /// Cleared + broadcast on every exit from here, success or exception -- so a parked waiter is never
+    /// left hanging, and so the self-remount barrier's JOIN completes on a failed attempt as surely as
+    /// on a successful one.
+    SCOPE_EXIT({
+        rt.recovery_in_progress = false;
+        rt.recovery_cv.notify_all();
+    });
+
+    /// ---- Step 1: capture the admitted generation, ONCE ----
+    /// The trio (spec §3, codex finding 7): this ONE value is what the walk's every `slotOccupy` and its
+    /// `_ckpt` CAS present, and what the install below presents one final time. One capture point, three
+    /// checks, no re-derivation -- a value re-read midway would let a recovery that lost the mount
+    /// "recover" its right to write by observing a fresh incarnation it was never admitted under.
+    ///
+    /// Captured for the WHOLE call, not per attempt, for the same reason: the transient-retry loop below
+    /// exists for object-store blips, and a generation that moved is not one. The loop refuses to
+    /// re-drive under a moved generation (below), so the budget is never burned on a doomed retry.
+    const uint64_t admitted_generation = rt.admitted_fence_generation;
+    check_fence_or_throw(admitted_generation);
+    /// Preserve this runtime's exact writer identity across the unlocked walk. The runtime stays in
+    /// `NeedsRecovery` until the same lock installs a result, so no later append can replace it here.
+    const std::optional<RefAppendAttempt> retained_attempt = rt.append_attempt;
+    /// The live writer epoch, likewise captured once: it decides WHICH epochs are dead and therefore what
+    /// the walk may seal. Re-reading it mid-walk would let the boundary move under the decision.
+    const uint64_t live_epoch = live_epoch_fn();
+
+    /// Outer transient-retry loop (Layer 1 of the stuck-table-load fix): a whole recovery attempt that
+    /// fails with a TRANSIENT object-store transport error (`isTransientRecoveryError` -- `S3_ERROR`/
+    /// socket/timeout from the direct LIST/GET backend calls, or a controller `NETWORK_ERROR`) is retried
+    /// with capped-exponential backoff until `recovery_retry_budget_ms` is spent, instead of propagating
+    /// and failing this table's async load permanently. Non-transient errors (corruption, decode, a moved
+    /// fence, logic, resource limits) fail fast; so do the two LATCHED terminal cases below.
+    const uint64_t recovery_start_ms = boot_ms_now_fn();
+    uint64_t recovery_retry_num = 0;
+    bool vanish_brake_tripped = false;
+    bool cancelled = false;
+    for (;;)
+    {
+        try
+        {
+            std::optional<String> hole_detail;
+            for (uint64_t attempt = 0; ; ++attempt)
+            {
+                if (attempt > 0)
+                {
+                    if (attempt > kRefRecoveryMaxRestarts)
+                    {
+                        /// Terminal, NOT a transient object-store outage: latch so the outer retry loop
+                        /// rethrows immediately instead of re-driving this brake for the whole budget.
+                        vanish_brake_tripped = true;
+                        if (hole_detail)
+                            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                                "CAS ref-table recovery for namespace '{}' found a hole in the durable ref-log "
+                                "stream that persisted across {} re-reads: {}",
+                                ns.string(), attempt - 1, *hole_detail);
+                        throwCasWriteRetryLater(fmt::format(
+                            "CAS ref-table recovery for namespace '{}' restarted {} times (a selected snapshot "
+                            "kept vanishing between the checkpoint that named it and its GET) — giving up; this "
+                            "bound is a runaway brake against a pathological cleanup race, not an expected "
+                            "steady state", ns.string(), attempt - 1));
+                    }
+                    ++rt.recovery_restarts;
+                    ProfileEvents::increment(ProfileEvents::CASRefRecoveryRestarts);
+                }
+
+                /// Each attempt derives its own restart reason, so a prior hole is not misreported as a
+                /// selected-base vanish after the next attempt takes a different path.
+                hole_detail.reset();
+
+                /// The whole walk runs UNLOCKED. Nothing it touches is shared: the candidate is private
+                /// and `rt` is read only through atomics the poll consults. Unlocking is what keeps a
+                /// recovery's full I/O envelope from stalling every reader of this table -- and what lets
+                /// the self-remount barrier take the lock promptly to JOIN us.
+                lock.unlock();
+                std::optional<RecoveryResult> walked;
+                try
+                {
+                    walked = runRecoveryWalkOnce(
+                        ns, rt, admitted_generation, live_epoch, retained_attempt, hole_detail, cancelled);
+                }
+                catch (...)
+                {
+                    /// Re-acquire BEFORE letting anything propagate: the SCOPE_EXIT above mutates
+                    /// `recovery_in_progress` and notifies `recovery_cv`, and it MUST run with
+                    /// `state_mutex` held -- unwinding through it unlocked would be a data race on the
+                    /// plain bool and an unlocked notify.
+                    lock.lock();
+                    throw;
+                }
+                lock.lock();
+
+                if (!walked)
+                    continue;   /// restart requested (vanished base, or a hole worth one more reading)
+
+                /// ---- Step 8: the install recheck, the LAST member of the trio ----
+                /// Under the re-acquired lock and IMMEDIATELY before the install, present the admitted
+                /// generation one final time. Everything above ran on I/O that took an unbounded amount
+                /// of time to come back, and a recovery whose window straddled a fence bump describes a
+                /// mount incarnation that no longer owns this namespace. It must publish NOTHING: the
+                /// table stays unrecovered and the next touch recovers it properly.
+                check_fence_or_throw(admitted_generation);
+                if (rt.catalog_life_invalidated.load(std::memory_order_acquire))
+                    throwCasWriteRetryLater(fmt::format(
+                        "CAS ref-table recovery for namespace '{}': catalog retirement invalidated life {} "
+                        "before recovery install — nothing is installed",
+                        ns.string(), renderIncarnation(rt.life.incarnation)));
+                if (rt.superseded_by_remount.load(std::memory_order_acquire))
+                    throwCasWriteRetryLater(fmt::format(
+                        "CAS ref-table recovery for namespace '{}': this cached table was superseded by a "
+                        "self-remount while the walk was in flight — nothing is installed",
+                        ns.string()));
+
+                /// One atomic publication under `state_mutex`: `installRecoveryResult` copies every seeded
+                /// field from the result and sets `recovered` LAST, so no waiter (woken only by the
+                /// function-scope SCOPE_EXIT's `recovery_cv` notify, which runs after this returns) ever
+                /// observes a partially-installed table.
+                installRecoveryResult(rt, std::move(*walked));
+                recovery_install_count_for_test.fetch_add(1, std::memory_order_relaxed);
+                break;
+            }
+            break;   /// recovery succeeded -> exit the outer retry loop
+        }
+        catch (...)
+        {
+            /// `catch (...)`, not `catch (const Exception &)`: recovery exact-GET failures can surface as
+            /// a raw object-storage transport exception (even a non-`DB::Exception` Poco timeout), which
+            /// a `catch (const Exception &)` would not even see. `getCurrentExceptionCode()` normalises
+            /// every exception (DB, Poco, std) to a code so the transient classifier can decide.
+            const int code = getCurrentExceptionCode();
+            /// `cancelled` is latched by the poll itself: a self-remount's cancellation uses the
+            /// retry-later class (the caller should retry, against the FRESH incarnation), so without the
+            /// latch this loop would read it as a transient blip and re-drive the very work the remount
+            /// just stopped.
+            if (vanish_brake_tripped || cancelled || !isTransientRecoveryError(code))
+                throw;   /// a latched terminal case, or a non-transient failure -- fail fast
+
+            const uint64_t elapsed_ms = boot_ms_now_fn() - recovery_start_ms;
+            /// Fail closed BEFORE sleeping: budget spent, mount fence lost, this runtime superseded by a
+            /// self-remount, or the incarnation that admitted this recovery has moved (retrying under a
+            /// generation that is already stale can only ever be refused at the install).
+            if (elapsed_ms >= cas_request_budget.recovery_retry_budget_ms
+                || !fence_ok_fn()
+                || rt.catalog_life_invalidated.load(std::memory_order_acquire)
+                || rt.superseded_by_remount.load(std::memory_order_acquire)
+                || fence_generation_fn() != admitted_generation)
+                throw;
+
+            /// Saturating `initial << recovery_retry_num` (mirrors `CasRequestController::backoffBefore
+            /// Attempt`): `initial > cap >> n` implies the unshifted product already exceeds the cap, so
+            /// return the cap without ever computing an overflowing/UB shift for large retry counts.
+            const uint64_t init_backoff = cas_request_budget.recovery_retry_initial_backoff_ms;
+            const uint64_t cap_backoff = cas_request_budget.recovery_retry_max_backoff_ms;
+            const uint64_t backoff_ms = (recovery_retry_num >= 63 || init_backoff > (cap_backoff >> recovery_retry_num))
+                ? cap_backoff
+                : std::min(cap_backoff, init_backoff << recovery_retry_num);
+            ++recovery_retry_num;
+            ProfileEvents::increment(ProfileEvents::CASRefRecoveryRetries);
+            LOG_WARNING(getLogger("CasRefLedger"),
+                "CAS ref-table recovery for namespace '{}' hit a transient object-store error "
+                "(code {}: {}); retry #{} after {}ms backoff (elapsed {}ms / budget {}ms)",
+                ns.string(), code, getCurrentExceptionMessage(/*with_stacktrace=*/false),
+                recovery_retry_num, backoff_ms, elapsed_ms, cas_request_budget.recovery_retry_budget_ms);
+
+            lock.unlock();
+            /// Re-acquire the lock before letting any exception from the sleep unwind, so the SCOPE_EXIT
+            /// (which mutates `recovery_in_progress` + notifies `recovery_cv` and MUST run under
+            /// `state_mutex`) never runs unlocked -- same obligation as the walk's window above.
+            try
+            {
+                recovery_retry_sleep_fn(backoff_ms);
+            }
+            catch (...)
+            {
+                lock.lock();
+                throw;
+            }
+            lock.lock();
+            /// The fence/supersession/budget can all change during the unlocked sleep -- re-check before
+            /// starting the next full attempt so we never re-drive recovery on an orphaned runtime, past
+            /// the budget, or under a lost or moved fence (the sliced sleep may have woken early on fence
+            /// loss).
+            if (boot_ms_now_fn() - recovery_start_ms >= cas_request_budget.recovery_retry_budget_ms
+                || !fence_ok_fn()
+                || rt.catalog_life_invalidated.load(std::memory_order_acquire)
+                || rt.superseded_by_remount.load(std::memory_order_acquire)
+                || fence_generation_fn() != admitted_generation)
+                throw;
+            /// loop: re-run recovery from a fresh checkpoint read, exact replay and walk
+        }
+    }
+    }
+
+    /// A NEW table was just materialized; enforce the whole-table cache budget, protecting this one
+    /// The pass runs OUTSIDE `rt.state_mutex` (that scope closed above) so
+    /// the pass -- which acquires `ref_queue_mutex` and try-locks other tables' `state_mutex` -- never
+    /// nests this table's `state_mutex` under `ref_queue_mutex`.
+    enforceRefTableCacheBudget(ns);
+}
+
+void CasRefLedger::installRecoveryResult(RefTableRuntime & rt, RecoveryResult && result)
+{
+    /// One place that seeds a recovered table's runtime, copying EVERY `RecoveryResult` field so the
+    /// publication cannot drift from the struct. `recovered` is set LAST: the caller holds `state_mutex`
+    /// throughout and the function-scope SCOPE_EXIT notifies `recovery_cv` only after this returns, so a
+    /// parked waiter re-checking `recovered` under the same lock sees a fully-installed table or none.
+    rt.state = std::move(result.state);
+    rt.newest_snapshot_id = result.newest_snapshot_id;
+    /// The chain link the CAS-walk ended on -- the `prev_epoch_seal` this table's next sequence-1 append
+    /// must name. This is the PRODUCTION producer of `last_epoch_seal` (the two writer-side arms record
+    /// only what they happened to observe): a real epoch change arrives with a self-remount, which
+    /// discards every cached runtime, so the fresh one gets its link from exactly here.
+    rt.last_epoch_seal = result.last_epoch_seal;
+    rt.tail_count_since_snapshot.store(result.tail_count, std::memory_order_relaxed);
+    rt.tail_bytes_since_snapshot.store(result.tail_bytes, std::memory_order_relaxed);
+    rt.base_snapshot_bytes.store(result.base_snapshot_bytes, std::memory_order_relaxed);
+    rt.snapshot_budget = result.snapshot_budget;
+    rt.removal_budget = result.removal_budget;
+    rt.needs_stale_precommit_sweep = result.needs_stale_precommit_sweep;
+    rt.append_attempt.reset();
+    rt.lane_state = RefLaneState::Ready;
+    rt.recovered = true;   /// set LAST
+}
+
+void CasRefLedger::cancelRecoveriesAndAwaitQuiescence()
+{
+    /// Snapshot the runtimes (the copies keep them alive across the wait, exactly as
+    /// `quiesceRefTablesForRemount` does).
+    std::vector<std::shared_ptr<RefTableRuntime>> tables;
+    {
+        std::lock_guard<std::mutex> qlock(ref_queue_mutex);
+        tables.reserve(ref_name_slots.size());
+        for (auto & [name, slot] : ref_name_slots)
+            if (slot.current)
+                tables.push_back(slot.current);
+    }
+
+    /// Publish the request to EVERY table first, then wait -- never table-by-table. Requesting and
+    /// waiting in one pass would let a recovery start on table B while we are still parked on table A,
+    /// and we would then join work that began after the cancellation was already under way.
+    for (auto & rt : tables)
+        rt->recovery_cancel_requested.store(true, std::memory_order_release);
+
+    for (auto & rt : tables)
+    {
+        std::unique_lock<std::mutex> slock(rt->state_mutex);
+        rt->recovery_cv.wait(slock, [&] { return !rt->recovery_in_progress; });
+    }
+
+    /// Released once nothing is in flight. Clearing here rather than after the fence re-arm keeps this a
+    /// self-contained barrier with no obligation on the caller to unwind: the window it opens (a recovery
+    /// starting between here and the re-arm) is closed twice over by the re-arm's own generation bump and
+    /// by `quiesceRefTablesForRemount`'s `superseded_by_remount`, both of which the walk polls.
+    for (auto & rt : tables)
+        rt->recovery_cancel_requested.store(false, std::memory_order_release);
+}
+
+
+void CasRefLedger::enforceRefTableCacheBudget(const RootNamespace & keep_ns)
+{
+    if (config.ref_table_cache_bytes == 0)
+        return;   /// 0 = unbounded: eviction disabled
+
+    /// Evicted runtimes are held alive here until AFTER every lock is released, so a runtime whose sole
+    /// owner is its map slot is never destroyed while we still hold its `state_mutex` (that would destroy
+    /// a locked mutex).
+    std::vector<std::shared_ptr<RefTableRuntime>> evicted;
+    {
+        std::lock_guard<std::mutex> qlock(ref_queue_mutex);
+
+        /// Relaxed atomic reads: the `total` loop below reads this for EVERY table, including hot ones a
+        /// concurrent append lane is mutating under `state_mutex` only (a cross-lock read). The gated
+        /// candidate loop reads it too, but only for `use_count()==1` tables (no concurrent writer).
+        const auto weightOf = [](const RefTableRuntime & rt)
+        {
+            return rt.base_snapshot_bytes.load(std::memory_order_relaxed)
+                 + rt.tail_bytes_since_snapshot.load(std::memory_order_relaxed);
+        };
+
+        uint64_t total = 0;
+        for (const auto & [name, slot] : ref_name_slots)
+            if (slot.current)
+                total += weightOf(*slot.current);
+        if (total <= config.ref_table_cache_bytes)
+            return;
+
+        /// Idle candidates, least-recently-touched first. Idle == the map holds the SOLE `shared_ptr`
+        /// (`use_count() == 1`: no in-flight caller, queued append, leader, or background publish holds a
+        /// copy), no active queue leader, an empty pending queue, and not the just-recovered `keep_ns`.
+        /// The `use_count() == 1` gate is what makes append-lane split-brain impossible: any thread that
+        /// fetched this runtime keeps it non-evictable for as long as it holds the copy.
+        struct Cand { String name; uint64_t tick; uint64_t weight; };
+        std::vector<Cand> cands;
+        for (const auto & [name, slot] : ref_name_slots)
+        {
+            if (name == keep_ns.string())
+                continue;
+            const auto & rt = slot.current;
+            if (!rt)
+                continue;
+            if (rt.use_count() != 1 || rt->leader_active || !rt->pending.empty())
+                continue;
+            cands.push_back(Cand{name, rt->last_touch_tick, weightOf(*rt)});
+        }
+        std::sort(cands.begin(), cands.end(),
+                  [](const Cand & a, const Cand & b) { return a.tick < b.tick; });
+
+        for (const Cand & c : cands)
+        {
+            if (total <= config.ref_table_cache_bytes)
+                break;
+            auto it = ref_name_slots.find(c.name);
+            if (it == ref_name_slots.end())
+                continue;
+            std::shared_ptr<RefTableRuntime> & rt = it->second.current;
+            if (!rt)
+                continue;
+            {
+                /// `use_count() == 1` guarantees no other thread holds the runtime, so this try_lock
+                /// cannot fail; the RAII scope releases `state_mutex` before `rt` is moved out. A wedged
+                /// append lane is never evicted -- its uncertain in-flight PUT is not reconstructable from
+                /// the durable objects, and re-recovery could re-allocate an id:
+                /// Linearization forbids this).
+                std::unique_lock<std::mutex> slock(rt->state_mutex, std::try_to_lock);
+                if (!slock.owns_lock() || rt->lane_state != RefLaneState::Ready)
+                    continue;
+            }
+            if (rt.use_count() != 1 || rt->leader_active || !rt->pending.empty())
+                continue;   /// re-check under the still-held ref_queue_mutex
+            total -= c.weight;
+            evicted.push_back(std::move(rt));   /// keep alive past the erase and lock release
+            ref_name_slots.erase(it);
+            ProfileEvents::increment(ProfileEvents::CASRefTableEvictions);
+        }
+    }
+    /// `evicted` destructs the dropped runtimes here, with no lock held.
+}
+
+
+void CasRefLedger::quiesceRefTablesForRemount()
+{
+    /// Snapshot the current runtimes (copies keep them alive across the drain). New dispatches are
+    /// already suppressed while the fence is lost (`maybeScheduleSnapshotPublish`'s fence guard), so the
+    /// only publishers to drain are those dispatched before the fence dropped.
+    std::vector<std::shared_ptr<RefTableRuntime>> tables;
+    {
+        std::lock_guard<std::mutex> qlock(ref_queue_mutex);
+        tables.reserve(ref_name_slots.size());
+        for (auto & [name, slot] : ref_name_slots)
+            if (slot.current)
+                tables.push_back(slot.current);
+    }
+
+    /// Wait for every in-flight background publisher to finish so none is mid-PUT when its runtime is
+    /// detached. A publisher observes the lost fence (`fence_ok` false) and returns without committing,
+    /// then decrements `pending_snapshot_publishes` under `state_mutex` and signals `publish_settle_cv`.
+    for (auto & rt : tables)
+    {
+        std::unique_lock<std::mutex> slock(rt->state_mutex);
+        rt->publish_settle_cv.wait(slock,
+            [&] { return rt->pending_snapshot_publishes.load(std::memory_order_relaxed) == 0; });
+    }
+
+    /// Detach every cached table. Mark it superseded FIRST (release, and before the caller re-arms the
+    /// fence): a leader that raced in and holds one of these orphaned runtimes then fails closed at the
+    /// `flushRefBatch` gate rather than allocating an id against a stale cache under the re-armed fence.
+    /// Queued callers self-drain -- each `flushRefBatch` for a superseded runtime completes its whole
+    /// carved batch with a retry error, so no caller hangs; the next touch creates a fresh runtime that
+    /// re-recovers from the durable snapshot+log objects under `live_writer_epoch`. Dropping the map slot
+    /// discards each runtime's in-memory wedge, and nothing about that drop needs to be certified here:
+    /// the undecided `PUT` the wedge describes is settled by the durable protocol rather than by
+    /// bookkeeping this process carries across the boundary. Recovery closes the dead epoch with an
+    /// in-band `EpochSeal` written as a conditional create, so the wedged write either already landed
+    /// (and the arithmetic walk reads it) or loses its own create to the seal.
+    std::vector<std::shared_ptr<RefTableRuntime>> detached;
+    {
+        std::lock_guard<std::mutex> qlock(ref_queue_mutex);
+        detached.reserve(ref_name_slots.size());
+        for (auto & [name, slot] : ref_name_slots)
+        {
+            auto & rt = slot.current;
+            if (!rt)
+                continue;
+            rt->superseded_by_remount.store(true, std::memory_order_release);
+            rt->cv.notify_all();   /// wake any waiter so it re-leads and fails closed against the flag
+            detached.push_back(rt);
+        }
+        ref_name_slots.clear();
+    }
+    /// `detached` releases the map's references here (with no lock held); each runtime lives on only as
+    /// long as an in-flight leader/caller still holds it.
+}
+
+
+uint64_t CasRefLedger::refRecoveryRestartsForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return 0;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->recovery_restarts;
+}
+
+bool CasRefLedger::refLaneWedgedForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return false;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->lane_state == RefLaneState::Wedged;
+}
+
+String CasRefLedger::wedgedKeyForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return {};
+    std::lock_guard lock(rt->state_mutex);
+    return rt->append_attempt ? rt->append_attempt->key : String{};
+}
+
+uint64_t CasRefLedger::wedgedAdmittedGenerationForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return 0;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->append_attempt ? rt->append_attempt->admitted_fence_generation : 0;
+}
+
+std::optional<RefTxnId> CasRefLedger::lastEpochSealForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return std::nullopt;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->last_epoch_seal;
+}
+
+void CasRefLedger::setLastEpochSealForTest(const RootNamespace & ns, const std::optional<RefTxnId> & seal)
+{
+    const auto rt = acquireMutableRefTableRuntime(ns);
+    ensureRefTableRecovered(ns, *rt);
+    std::lock_guard lock(rt->state_mutex);
+    rt->last_epoch_seal = seal;
+}
+
+void CasRefLedger::forceWedgeForTest(const RootNamespace & ns, uint64_t writer_epoch, uint64_t ref_sequence,
+                              const String & key, const String & bytes,
+                              std::optional<uint64_t> admitted_generation)
+{
+    const auto rt = acquireMutableRefTableRuntime(ns);
+    ensureRefTableRecovered(ns, *rt);
+    /// Read outside `state_mutex`: it is an atomic load on the mount runtime, and taking it here keeps
+    /// the seam's default identical to what a wedge born at this instant would carry.
+    const uint64_t generation = admitted_generation.value_or(fence_generation_fn());
+    std::lock_guard lock(rt->state_mutex);
+    rt->append_attempt = RefAppendAttempt{RefTxnId{writer_epoch, ref_sequence}, key, bytes, generation};
+    rt->lane_state = RefLaneState::Wedged;
+}
+
+RefLaneState CasRefLedger::laneStateForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return RefLaneState::Closed;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->lane_state;
+}
+
+bool CasRefLedger::needsStalePrecommitSweepForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return false;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->needs_stale_precommit_sweep;
+}
+
+
+size_t CasRefLedger::wedgedRefLaneCount()
+{
+    std::vector<std::shared_ptr<RefTableRuntime>> runtimes;
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        runtimes.reserve(ref_name_slots.size());
+        for (const auto & [_, slot] : ref_name_slots)
+            if (slot.current)
+                runtimes.push_back(slot.current);
+    }
+    size_t wedged = 0;
+    for (const auto & rt : runtimes)
+    {
+        std::lock_guard lock(rt->state_mutex);
+        if (rt->lane_state == RefLaneState::Wedged)
+            ++wedged;
+    }
+    return wedged;
+}
+
+
+bool CasRefLedger::drainRefLanesForShutdown(uint64_t wait_budget_ms)
+{
+    /// Latch FIRST, then snapshot under `ref_queue_mutex` (see the `shutting_down` member comment): this
+    /// ordering is what makes the check in `appendRefOps` -- performed inside the SAME critical section
+    /// as its `pending.push_back` -- race-free against the snapshot below, for both an already-cached
+    /// table and one whose very first touch races this call.
+    shutting_down.store(true, std::memory_order_release);
+
+    std::vector<std::shared_ptr<RefTableRuntime>> runtimes;
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        runtimes.reserve(ref_name_slots.size());
+        for (const auto & [_, slot] : ref_name_slots)
+            if (slot.current)
+                runtimes.push_back(slot.current);
+    }
+
+    /// Wait for every table's queue to go idle (no pending item, no active leader), bounded overall by
+    /// `wait_budget_ms` -- `cv.wait_until` slices against one shared deadline, never a sleep. All the
+    /// runtimes share the one `ref_queue_mutex` that guards `pending`/`leader_active` (see the
+    /// `RefTableRuntime` field comments), so a single `lk` covers every table in the loop below; each
+    /// table's OWN `cv` is what its leader/appendRefOps notifies on a state change, so the wait must
+    /// target that specific `cv`, one table at a time.
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::milliseconds(wait_budget_ms);
+    bool timed_out = false;
+    {
+        std::unique_lock<std::mutex> lk(ref_queue_mutex);
+        for (const auto & rt : runtimes)
+        {
+            while (!(rt->pending.empty() && !rt->leader_active))
+            {
+                if (rt->cv.wait_until(lk, deadline) == std::cv_status::timeout
+                    && !(rt->pending.empty() && !rt->leader_active))
+                {
+                    timed_out = true;
+                    break;
+                }
+            }
+            if (timed_out)
+                break;
+        }
+    }
+
+    /// A queue going idle does NOT by itself prove no PUT is in flight: a wedge is recorded (under
+    /// `state_mutex`) strictly BEFORE the wedged item's caller is completed and the leader bookkeeping
+    /// reset (see `flushRefBatch`'s `Unresolved` case), so this check -- performed AFTER the wait above
+    /// -- observes it whenever the queue-idle wait itself raced a wedge. Every table is checked
+    /// regardless of `timed_out`, purely for a complete diagnostic; the return value already fails
+    /// closed on either condition alone.
+    bool any_wedge = false;
+    for (const auto & rt : runtimes)
+    {
+        std::lock_guard<std::mutex> lock(rt->state_mutex);
+        if (rt->lane_state == RefLaneState::Writing || rt->lane_state == RefLaneState::Wedged)
+            any_wedge = true;
+    }
+
+    return !timed_out && !any_wedge;
+}
+
+
+RefTxnId CasRefLedger::appendRefOps(const RootNamespace & ns, MutationScope scope,
+                             std::function<std::vector<RefOp>(const RefTableState &)> build_ops,
+                             RootMutationOrigin origin, RootMutationKind kind,
+                             bool skip_stale_precommit_sweep)
+{
+    if (kind == RootMutationKind::DropNamespace)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS namespace '{}': the generic append surface cannot acquire removal ownership; "
+            "use the exact `dropNamespace` lifecycle operation",
+            ns.string());
+    const auto rt = acquireMutableRefTableRuntime(ns);
+    if (append_after_runtime_capture_hook_for_test)
+        append_after_runtime_capture_hook_for_test();
+    return appendRefOpsOnRuntime(
+        ns, rt, std::move(scope), std::move(build_ops), origin, kind, skip_stale_precommit_sweep,
+        /*terminal_removal_authorized=*/false);
+}
+
+
+RefTxnId CasRefLedger::appendRefOpsOnRuntime(
+    const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt, MutationScope scope,
+    std::function<std::vector<RefOp>(const RefTableState &)> build_ops,
+    RootMutationOrigin origin, RootMutationKind kind, bool skip_stale_precommit_sweep,
+    bool terminal_removal_authorized)
+{
+    const auto refuse_if_removing = [&]
+    {
+        std::lock_guard<std::mutex> lock(ref_queue_mutex);
+        if (rt->removal_admission_closed && !terminal_removal_authorized)
+            throwCasWriteRetryLater(fmt::format(
+                "CAS namespace '{}' is Removing: positive ref mutation admission is closed while "
+                "its terminal fold and catalog removal complete; retry later", ns.string()));
+    };
+    /// Check before recovery/maintenance so a known-Removing runtime cannot spend an object-store
+    /// mutation on behalf of an operation it is already required to refuse.
+    refuse_if_removing();
+    /// Hoisted here (rather than left to `flushRefBatch`'s own idempotent call) so both
+    /// triggers below run on the CALLING thread, strictly BEFORE this call enqueues its own item or
+    /// becomes a queue leader -- `maybeSweepStalePrecommits`'s own nested `appendRefOps` calls are
+    /// therefore always a fresh top-level invocation, never nested inside a leader's flush stack
+    /// (which would deadlock the leader against itself).
+    ensureRefTableRecovered(ns, *rt);
+    if (!skip_stale_precommit_sweep)
+        maybeSweepStalePrecommits(ns, rt);
+    maybeScheduleSnapshotPublish(ns, rt);
+
+    auto item = std::make_shared<RefMutationItem>();
+    item->scope = std::move(scope);
+    item->build_ops = std::move(build_ops);
+    item->origin = origin;
+    item->kind = kind;
+    item->terminal_removal_authorized = terminal_removal_authorized;
+
+    const auto enqueued_at = std::chrono::steady_clock::now();
+    std::unique_lock<std::mutex> lk(ref_queue_mutex);
+    /// Refuse admission once a clean-release drain has begun (`drainRefLanesForShutdown`).
+    /// Checked in the SAME critical section as the `pending.push_back` below -- the pairing that makes
+    /// this race-free against the drain's snapshot-and-wait (see the `shutting_down` member comment).
+    if (shutting_down.load(std::memory_order_acquire))
+        throwCasWriteRetryLater(fmt::format(
+            "CAS store is shutting down — refusing to append ref-log transactions for server_root '{}'",
+            config.server_root_id));
+    if (rt->removal_admission_closed && !terminal_removal_authorized)
+        throwCasWriteRetryLater(fmt::format(
+            "CAS namespace '{}' is Removing: positive ref mutation admission is closed while its "
+            "terminal fold and catalog removal complete; retry later", ns.string()));
+    rt->pending.push_back(item);
+
+    while (!item->done)
+    {
+        if (!rt->leader_active)
+        {
+            /// The set of items THIS leader is responsible for: its own enqueued `item` plus every item a
+            /// flush carves out of `pending` (recorded by `flushRefBatch` as it carves). Whatever the
+            /// leader loop does below -- return normally, or throw at ANY point including BEFORE it ever
+            /// carves -- every one of these items must leave here `done` (its waiter woken), never left
+            /// stranded in `pending` for a future leader to carve after this caller's stack (and its
+            /// `build_ops` closure) is gone: a use-after-free that concurrent per-part commit makes
+            /// immediate. `completeOwnedItemsAndReleaseLeadership` enforces that on EVERY exit and folds in
+            /// the `leader_active` release the old catch used to own. Items NOT owned by this leader (other
+            /// callers' still-queued items) are untouched -- they stay validly owned by their blocked
+            /// callers.
+            ///
+            /// Build the responsibility set (its own `item`) BEFORE publishing the baton, so becoming
+            /// leader contains NO throwing operation once `leader_active` is set: the only allocation is
+            /// this first `push_back`, done here while still holding `lk` and NOT yet leader. If it throws
+            /// (a `bad_alloc` at the pre-tenure point; codex stage-1 review, Important), the baton is never
+            /// taken -- but `item` is already in `pending` (pushed above), so it must be un-enqueued before
+            /// propagating, else a future leader would carve an item whose `build_ops` closure died with
+            /// this unwinding caller (the same use-after-free the exit guard prevents post-publication).
+            /// Publishing the baton and reaching the exit guard is then a pure no-throw sequence.
+            std::vector<std::shared_ptr<RefMutationItem>> owned_items;
+            try
+            {
+                if (ref_pre_tenure_hook_for_test)
+                    ref_pre_tenure_hook_for_test();
+                owned_items.push_back(item);
+            }
+            catch (...)
+            {
+                std::erase(rt->pending, item);
+                throw;
+            }
+
+            rt->leader_active = true;
+            lk.unlock();
+            std::exception_ptr flush_exception;
+            try
+            {
+                runRefQueueLeader(ns, rt, item, owned_items);
+            }
+            catch (...)
+            {
+                flush_exception = std::current_exception();
+            }
+            /// Single exit authority (normal AND exceptional): complete every still-incomplete owned
+            /// item with `flush_exception` (nullptr on the normal path -> a fail-closed LOGICAL_ERROR)
+            /// and release leadership. This does NOT rethrow. Under chunked flush the leader's OWN item
+            /// may already have succeeded in an earlier committed chunk, and a later exception -- from a
+            /// subsequent chunk, the reseed, or chunk-N processing -- must NOT be handed to this caller
+            /// whose mutation is already durable (tenure exception containment, spec §3): the guard
+            /// leaves such an item `done` with no error, and the loop re-check + tail below return its
+            /// `committed_id`. An item that genuinely failed carries `item->error` and the tail rethrows
+            /// it, exactly as the old unconditional rethrow did for the single-chunk case.
+            completeOwnedItemsAndReleaseLeadership(ns, rt, owned_items, flush_exception);
+            lk.lock();
+        }
+        else
+        {
+            rt->cv.wait(lk);
+        }
+    }
+    lk.unlock();
+
+    ProfileEvents::increment(ProfileEvents::CASRefQueueWaitMicroseconds,
+        std::chrono::duration_cast<std::chrono::microseconds>(
+            std::chrono::steady_clock::now() - enqueued_at).count());
+    if (item->error)
+        std::rethrow_exception(item->error);
+    return item->committed_id;
+}
+
+
+void CasRefLedger::runRefQueueLeader(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt,
+                              const std::shared_ptr<RefMutationItem> & own,
+                              std::vector<std::shared_ptr<RefMutationItem>> & owned_items)
+{
+    /// Fairness baton pass: serve flushes only until the caller's OWN item is done, then hand off to a
+    /// woken waiter.
+    while (true)
+    {
+        {
+            std::lock_guard<std::mutex> g(ref_queue_mutex);
+            if (own->done)
+                return;
+        }
+        flushRefBatch(ns, rt, owned_items);
+    }
+}
+
+void CasRefLedger::completeOwnedItemsAndReleaseLeadership(
+    const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt,
+    const std::vector<std::shared_ptr<RefMutationItem>> & owned_items,
+    std::exception_ptr flush_exception)
+{
+    std::lock_guard<std::mutex> g(ref_queue_mutex);
+    for (const auto & owned : owned_items)
+    {
+        if (!owned->done)
+        {
+            owned->error = flush_exception
+                ? flush_exception
+                : std::make_exception_ptr(Exception(ErrorCodes::LOGICAL_ERROR,
+                    "CAS ref-log append for namespace '{}': the append-lane leader exited without "
+                    "completing an owned queue item -- failing it closed rather than leaving it stranded "
+                    "in the pending queue for a future leader to carve", ns.string()));
+            owned->done = true;
+        }
+        /// Never leave an owned item in `pending`: a stranded item would be carved by a future leader
+        /// which would then invoke its (now dangling) `build_ops` closure -- the use-after-free this
+        /// guard exists to prevent. Carved items were already popped during the carve, so this is a
+        /// no-op for them; it only matters for an item the leader owned but never got to carve.
+        std::erase(rt->pending, owned);
+    }
+    rt->leader_active = false;
+    rt->cv.notify_all();
+}
+
+void CasRefLedger::requireRecovery(RefTableRuntime & rt, const RootNamespace & ns, std::string_view region) noexcept
+{
+    const bool entering = rt.lane_state != RefLaneState::NeedsRecovery;
+    rt.lane_state = RefLaneState::NeedsRecovery;
+    if (!entering)
+        return;
+    ProfileEvents::increment(ProfileEvents::CASRefNeedsRecovery);
+    try
+    {
+        LOG_ERROR(getLogger("CasPool"),
+            "CAS ref table '{}' NEEDS RECOVERY at {}: a transaction is known durable but could not be "
+            "installed in this cached table. New writes, snapshots, and confirmations are fenced until "
+            "recovery replays the durable log.",
+            ns.string(), region);
+    }
+    catch (...)   // NOLINT(bugprone-empty-catch)
+    {
+        /// The state transition above is the safety mechanism; logging must not replace the original
+        /// post-durable exception.
+    }
+}
+
+CasRefLedger::WedgeResolutionResult
+CasRefLedger::resolveWedgeOnce(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt)
+{
+    WedgeResolutionResult result;
+
+    /// WHY the lane is not proceeding, decided under `state_mutex` and rendered into a message AFTER it
+    /// is released. Building an exception is `fmt::format` plus a stack-trace capture, neither of which
+    /// belongs under a lock that readers and the snapshot publisher contend for.
+    enum class Reason : uint8_t
+    {
+        None,
+        FenceMoved,             /// the mount incarnation moved since this attempt was admitted
+        CatalogLifeRetired,     /// exact catalog retirement detached this immutable life
+        Superseded,             /// a self-remount detached this runtime
+        WedgeReplaced,          /// the result belongs to a wedge that is no longer installed
+        RefusedPreAttempt,      /// `slotOccupy` sent nothing
+        ResolveFoundNothing,    /// it sent an attempt and the follow-up read came up empty
+        StaleState,             /// the table advanced under a proven-durable object we cannot install
+    };
+    Reason reason = Reason::None;
+    std::optional<RefLaneState> invalid_lane_state;
+
+    /// ---- Read the wedge and prepare EVERYTHING that can throw, before any I/O (spec §A1, site 2) ----
+    RefAppendAttempt wedge;
+    std::optional<RefTableState> candidate;
+    RefTxnId candidate_base_id;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        if (rt->lane_state == RefLaneState::Ready)
+            return result;   /// NoWedge -- the ordinary flush pays nothing for any of this
+        if (rt->lane_state != RefLaneState::Wedged || !rt->append_attempt)
+        {
+            invalid_lane_state = rt->lane_state;
+        }
+        else
+        {
+            wedge = *rt->append_attempt;
+            candidate.emplace(rt->state);
+            candidate_base_id = rt->state.getGreatestApplied();
+        }
+    }
+
+    if (invalid_lane_state)
+    {
+        result.kind = WedgeResolution::StillWedged;
+        if (*invalid_lane_state == RefLaneState::NeedsRecovery)
+        {
+            result.survivor_error = makeCasWriteRetryLaterExceptionPtr(fmt::format(
+                "CAS ref-log append for namespace '{}': lane recovery is still in progress; retry after "
+                "recovery completes",
+                ns.string()));
+        }
+        else
+        {
+            result.survivor_error = std::make_exception_ptr(Exception(
+                ErrorCodes::INVALID_STATE,
+                "CAS ref-log append for namespace '{}': terminal lane state {} does not permit wedge "
+                "resolution",
+                ns.string(), static_cast<uint8_t>(*invalid_lane_state)));
+        }
+        return result;
+    }
+
+    /// Decode and apply BEFORE the I/O. The wedge carries the encoded body, so this costs no extra
+    /// round trip -- only the decode and the overlay build, both of which can throw (allocation) and
+    /// both of which MUST NOT run after the object is proven durable. A throw here happens while the
+    /// outcome is still unknown and the wedge is still set, i.e. it is indistinguishable from "the
+    /// resolution has not been attempted yet": the lane stays wedged and a later flush retries the whole
+    /// thing. It propagates to `appendRefOps`' catch, which completes every survivor.
+    ///
+    /// The candidate is deliberately NOT cached in the wedge across attempts: a wedge can live until a
+    /// remount, and retaining a full state copy for that long is a real memory cost on a path that is
+    /// rare by construction. Recomputing it per attempt is the cheaper trade.
+    const RefLogTxn wedged_txn = decodeRefLogTxn(
+        openObject(FormatId::RefLog, wedge.bytes), ns.string(), wedge.txn_id);
+    applyRefLogTxn(*candidate, wedged_txn);
+
+    /// ---- ONE bounded attempt, admitted under the wedge's ORIGINAL generation ----
+    /// Never the CURRENT generation: a retry that "passes" because the mount was re-armed under a new
+    /// lease incarnation is a write from an incarnation that never admitted this transaction. Refusing
+    /// pre-attempt leaves the key provably untouched, which is the only state a later recovery can
+    /// reason about.
+    const auto admitted_fence_ok = [this, &rt, admitted = wedge.admitted_fence_generation]
+    {
+        return fence_ok_fn()
+            && !rt->catalog_life_invalidated.load(std::memory_order_acquire)
+            && !rt->superseded_by_remount.load(std::memory_order_acquire)
+            && fence_generation_fn() == admitted;
+    };
+
+    SlotOccupyResult occupied;
+    try
+    {
+        if (wedge_before_slot_occupy_hook_for_test)
+            wedge_before_slot_occupy_hook_for_test();
+        occupied = ref_request_controller->slotOccupy(wedge.key, wedge.bytes, admitted_fence_ok);
+    }
+    catch (...)
+    {
+        /// `ambiguous-then-definite`, the model-proven control. `slotOccupy` rethrows only a definite
+        /// refusal of THIS attempt (a whitelisted synchronous rejection, or a deterministic local
+        /// failure) -- and a definite refusal of a LATER attempt proves nothing whatsoever about the
+        /// EARLIER ambiguous one, which may still be in flight or may already have landed. So the lane
+        /// stays wedged: unwedging here is exactly how an acked-then-lost transaction gets written
+        /// around. The id is not consumed either, so the next attempt re-derives the SAME one.
+        result.kind = WedgeResolution::StillWedged;
+        result.survivor_error = makeCasWriteRetryLaterExceptionPtr(fmt::format(
+            "CAS ref-log append for namespace '{}': the bounded retry of wedged txn {}-{} was definitively "
+            "refused ({}), which says nothing about the earlier ambiguous attempt — the lane stays wedged",
+            ns.string(), wedge.txn_id.writer_epoch, wedge.txn_id.ref_sequence,
+            getCurrentExceptionMessage(/*with_stacktrace*/ false)));
+        return result;
+    }
+
+    /// ---- Classify the occupant OFF the lock: pure, and the decode allocates ----
+    /// The three-way `mine | successor's seal | foreign` adjudication is the CALLER's job by
+    /// construction (`slotOccupy` never compares bytes), and "mine" means BYTE EQUALITY -- never a
+    /// shape or generation match, which is the aliasing the phase-0 model rejected.
+    const Occupant occupant = occupied.kind == SlotOccupyResult::Kind::Occupied
+        ? classifyRefLogOccupant(ns, wedge.txn_id, occupied.occupant_bytes, wedge.bytes)
+        : Occupant::NotOccupied;
+    const bool exact_attempt_is_durable
+        = occupied.kind == SlotOccupyResult::Kind::Created || occupant == Occupant::Ours;
+    /// Caller holds `state_mutex`. Keeping the identity predicate in one place is part of the safety
+    /// rule: adding a frontier must not create yet another subtly different notion of "same attempt".
+    const auto same_wedge_under_lock = [&]
+    {
+        return rt->append_attempt
+            && rt->lane_state == RefLaneState::Wedged
+            && rt->append_attempt->txn_id == wedge.txn_id
+            && rt->append_attempt->bytes == wedge.bytes
+            && rt->append_attempt->admitted_fence_generation == wedge.admitted_fence_generation;
+    };
+
+    /// A durable ref-log object is not yet admissible history. Exactly as on the ordinary committed
+    /// append path, publish its frontier under the SAME admission before the cached table can install
+    /// it, return to `Ready`, or wake a surviving caller. This deliberately runs only for `Created` or
+    /// byte-identical `Ours`: a successor seal is conclusive evidence that OUR transaction did not land
+    /// and retains the rejection path below without publishing our frontier.
+    bool same_wedge_before_frontier = false;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        same_wedge_before_frontier = same_wedge_under_lock();
+    }
+    if (exact_attempt_is_durable && same_wedge_before_frontier)
+    {
+        const auto check_wedge_admitted = [this, &rt, &same_wedge_under_lock](uint64_t expected_generation)
+        {
+            check_fence_or_throw(expected_generation);
+            if (rt->catalog_life_invalidated.load(std::memory_order_acquire)
+                || rt->superseded_by_remount.load(std::memory_order_acquire))
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS namespace '{}': its captured runtime was retired before wedged-frontier publication",
+                    rt->life.ns.string()));
+
+            bool same_wedge = false;
+            {
+                std::lock_guard lock(rt->state_mutex);
+                same_wedge = same_wedge_under_lock();
+            }
+            if (!same_wedge)
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS namespace '{}': the captured wedge changed before frontier publication",
+                    rt->life.ns.string()));
+        };
+
+        const RefCkpt frontier{
+            .life_epoch = std::nullopt,
+            .committed_through = wedge.txn_id,
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = refLogTxnIsEpochSeal(wedged_txn)
+                ? std::optional<RefTxnId>{wedge.txn_id} : wedged_txn.prev_epoch_seal};
+
+        CkptPublishOutcome frontier_outcome = CkptPublishOutcome::FencedOut;
+        try
+        {
+            frontier_outcome = publishCkptContribution(
+                rt->life, frontier, wedge.admitted_fence_generation, check_wedge_admitted);
+        }
+        catch (...)
+        {
+            result.kind = WedgeResolution::StillWedged;
+            result.survivor_error = std::current_exception();
+            std::lock_guard lock(rt->state_mutex);
+            if (same_wedge_under_lock())
+                requireRecovery(*rt, ns, "wedged-frontier publication");
+            return result;
+        }
+        if (frontier_outcome == CkptPublishOutcome::FencedOut)
+        {
+            result.kind = WedgeResolution::StillWedged;
+            result.survivor_error = makeCasWriteRetryLaterExceptionPtr(fmt::format(
+                "CAS ref-log append for namespace '{}': wedged txn {}-{} is durable, but its admitted "
+                "fence moved before checkpoint-frontier publication; the lane needs recovery",
+                ns.string(), wedge.txn_id.writer_epoch, wedge.txn_id.ref_sequence));
+            std::lock_guard lock(rt->state_mutex);
+            if (same_wedge_under_lock())
+                requireRecovery(*rt, ns, "wedged-frontier publication fence");
+            return result;
+        }
+    }
+
+    /// ---- POST-I/O RECHECK, then act, in ONE hold of `state_mutex` ----
+    /// Everything above ran on an I/O result that took an unbounded amount of time to come back. Before
+    /// ANY consequence follows from it -- adopting, acknowledging, unwedging, failing the survivors --
+    /// this runtime must still be the one the attempt was admitted for. Two independent things can have
+    /// made it not: the mount fence moved (a loss, or a re-arm under a fresh lease incarnation), or the
+    /// wedge itself was replaced. Both are checked; neither implies the other.
+    {
+        std::lock_guard lock(rt->state_mutex);
+
+        /// The generation this attempt was admitted under, presented back. `checkFenceOrThrow` reports a
+        /// moved incarnation by throwing; it is CAUGHT here rather than propagated, because the caller's
+        /// retry classification keys on the retry-later error class and a routine lease blip must not
+        /// reach it as a hard failure. Nothing is installed and nothing is unwedged either way, which is
+        /// the whole meaning of INERT here.
+        bool fence_moved = false;
+        try
+        {
+            check_fence_or_throw(wedge.admitted_fence_generation);
+        }
+        catch (...)
+        {
+            fence_moved = true;
+        }
+
+        /// The remount half of the same question, checked separately because the two are independent
+        /// facts even though today's ordering makes one imply the other: `quiesceRefTablesForRemount`
+        /// detaches this runtime BEFORE the fence is re-armed, so a detached runtime always has a moved
+        /// generation and the check above would already have caught it. Relying on that ordering
+        /// silently is how a future edit to the remount sequence turns into a stale install.
+        const bool superseded = rt->superseded_by_remount.load(std::memory_order_acquire);
+        const bool catalog_life_retired = rt->catalog_life_invalidated.load(std::memory_order_acquire);
+
+        /// All three components of the identity, because none of them alone identifies the attempt: two
+        /// attempts of one table can share an id and a generation and describe DIFFERENT bytes, and
+        /// installing one attempt's candidate because the other's key resolved is the acked-then-lost
+        /// class itself. One leader per table makes this unreachable today; it is checked, not assumed,
+        /// because the cost is a comparison and the failure mode is silent data loss.
+        const bool same_wedge = same_wedge_under_lock();
+
+        if ((fence_moved || superseded) && same_wedge && exact_attempt_is_durable)
+        {
+            requireRecovery(*rt, ns, "wedged attempt resolved after its fence moved");
+            reason = Reason::StaleState;
+            result.kind = WedgeResolution::StillWedged;
+        }
+        else if (fence_moved || catalog_life_retired || superseded || !same_wedge)
+        {
+            reason = fence_moved ? Reason::FenceMoved
+                : (catalog_life_retired ? Reason::CatalogLifeRetired
+                : (superseded ? Reason::Superseded : Reason::WedgeReplaced));
+            result.kind = WedgeResolution::StillWedged;
+        }
+        else if (occupied.kind == SlotOccupyResult::Kind::Unresolved)
+        {
+            /// Still uncertain. Do NOT clear the wedge: it describes an object that may well be durable,
+            /// and clearing it on a failed read is the one thing this path must never do. There is no
+            /// deadline reset and no background loop -- the next caller of this namespace, or a remount,
+            /// retries. That is register R6's ACCEPTED behaviour: a permanently quiet wedged namespace
+            /// waits, which costs nothing, because the wedged operation was never acknowledged.
+            reason = unresolvedProvesNothingWasSent(occupied.unresolved_reason)
+                ? Reason::RefusedPreAttempt : Reason::ResolveFoundNothing;
+            result.kind = WedgeResolution::StillWedged;
+        }
+        else if (occupant == Occupant::SuccessorSeal)
+        {
+            /// THE conclusive rejection (spec INV-2). The ref-log key is write-once and a successor put
+            /// its epoch-closing record there, so our bytes provably never landed and never can. The
+            /// operation was never acknowledged, so nothing is lost by failing it permanently -- and it
+            /// must be permanent, not "retry later": no later attempt in this epoch can ever succeed.
+            ///
+            /// The seal IS this namespace's epoch-closing record, so it is also the `prev_epoch_seal`
+            /// that the first transaction of a LATER epoch must name. In this runtime that record is
+            /// mostly introspection: a real epoch change arrives with a self-remount, which discards
+            /// this runtime, and the fresh one gets its chain link from recovery's CAS-walk (Task 6).
+            /// It is recorded anyway because it is durable evidence this runtime holds and nothing else
+            /// would, and because `commitRefChunk` consumes it the moment the live epoch does advance
+            /// past the seal's.
+            ///
+            rt->append_attempt.reset();
+            rt->last_epoch_seal = wedge.txn_id;
+            rt->lane_state = RefLaneState::Closed;
+            result.kind = WedgeResolution::Rejected;
+        }
+        else if (occupant == Occupant::Foreign)
+        {
+            /// Impossible under mount-lease exclusivity. The terminal state carries the verdict; no
+            /// uncertain attempt remains to be retried.
+            rt->append_attempt.reset();
+            rt->lane_state = RefLaneState::Faulted;
+            result.kind = WedgeResolution::Corrupted;
+        }
+        else if (!(rt->state.getGreatestApplied() == candidate_base_id))
+        {
+            /// ADOPTION, refused. Only this leader mutates `rt->state`, and the attempt above ran
+            /// without the lock, so this compares the state against the snapshot the candidate was built
+            /// from. A debug build also `chassert`s it inside the region below; this is the RELEASE-mode
+            /// counterpart, because the window it guards is a full network round trip and a silent swap
+            /// would discard whatever advanced the table.
+            ///
+            /// The object is proven durable and this runtime cannot record it. No later id may be
+            /// allocated from this cache; recovery is the only legal successor.
+            requireRecovery(*rt, ns, "wedge-resolution install");
+            reason = Reason::StaleState;
+            result.kind = WedgeResolution::StillWedged;
+        }
+        else
+        {
+            /// ---- ADOPTION: `Created`, or `Occupied` with our own bytes ----
+            /// Receives the resolved wedge so it is destroyed OUTSIDE the region: clearing `rt->append_attempt`
+            /// in place would free its two `String` bodies there, and the region's contract is that it
+            /// touches no allocator at all.
+            std::optional<RefAppendAttempt> displaced_wedge;
+            static_assert(std::is_nothrow_swappable_v<std::optional<RefAppendAttempt>>,
+                "the wedge hand-off below must be non-throwing: it runs after the wedged object is proven "
+                "durable, where a throw would re-apply the transaction on the next resolution");
+            /// One of the two post-durable install regions (spec §A2; the other is `commitRefChunk`'s
+            /// own, further below, sharing this same probe). The `catch` cannot fire while §A1 holds --
+            /// the body below allocates nothing -- and is what makes a violation of §A1 VISIBLE rather
+            /// than silent: the attempt proved the object durable, so an install that does not complete
+            /// leaves this table's cached state missing it. It rethrows unchanged, so the lane's error
+            /// handling is unchanged; the explicit lane state makes the missing install visible.
+            try
+            {
+                DENY_ALLOCATIONS_IN_SCOPE;
+                /// The negative control (`setInstallRegionProbeForTest`), fired with the guard already
+                /// armed, exactly as in `commitRefChunk`'s region.
+                if (install_region_probe_for_test)
+                    install_region_probe_for_test();
+                /// The debug-build twin of the refusal above. `chassert` stringifies its condition, so
+                /// it reads a short local rather than the comparison itself: a long condition would heap
+                /// allocate ON FAILURE inside the very region that must not allocate.
+                chassert(same_wedge);
+                /// The install, allocation-free by construction: a member-wise swap of pointers and
+                /// PODs, two atomic increments, and a second swap of pointers. The object is durable, so
+                /// the transaction MUST be recorded -- and recording it MUST be inseparable from clearing
+                /// the wedge, or a failure between them leaves the transaction applied with the wedge
+                /// still set and the next resolution re-applies it. A wedge-resolved transaction is a
+                /// commit like any other: it joins the applied-above-newest-snapshot tail counters
+                /// exactly as the ordinary commit arm's does, or the snapshot-publish threshold and the
+                /// resident-weight estimate undercount by one transaction per resolved wedge until the
+                /// next recovery reseeds.
+                rt->state.swap(*candidate);
+                rt->tail_count_since_snapshot.fetch_add(1, std::memory_order_relaxed);
+                rt->tail_bytes_since_snapshot.fetch_add(wedge.bytes.size(), std::memory_order_relaxed);
+                rt->append_attempt.swap(displaced_wedge);
+                rt->lane_state = RefLaneState::Ready;
+            }
+            catch (...)
+            {
+                requireRecovery(*rt, ns, "wedge-resolution install");
+                throw;
+            }
+            /// `candidate` now holds the DISPLACED state, which still shares the COW bases `rt->state`
+            /// uses; destroying it here restores unique base ownership so the fold below keeps its
+            /// O(overlay) in-place path instead of rebuilding the whole base. Both `reset`s only destroy:
+            /// they allocate nothing and cannot throw.
+            candidate.reset();
+            displaced_wedge.reset();
+            /// Fold the just-installed overlay back into the base right here, exactly as the ordinary
+            /// commit arm does at its install point, so `rt->state` returns to "base + empty overlay" and
+            /// the next flush's trial copies stay cheap. Cheap: no scratch copy shares the base at this
+            /// point in the flush (`working` is not taken until later), so this is the O(overlay) in-place
+            /// fold. Coherent-on-throw (see `CasRefCowMap.cpp`), and SWALLOWING, symmetrically with the
+            /// ordinary commit arm: the transaction is durable, installed and unwedged before this runs,
+            /// so a mid-fold allocation failure merely defers the fold to the next flush -- it must not
+            /// unwind past a completed install.
+            try
+            {
+                rt->state.materializeCommitted();
+            }
+            catch (...)
+            {
+                tryLogCurrentException(getLogger("CasPool"), fmt::format(
+                    "CAS ref-log append for namespace '{}': wedged txn {}-{} resolved durable and was "
+                    "installed, but the post-install overlay fold failed and was retained coherently for "
+                    "the next flush",
+                    ns.string(), wedge.txn_id.writer_epoch, wedge.txn_id.ref_sequence));
+            }
+            result.kind = WedgeResolution::Adopted;
+        }
+    }
+
+    /// ---- Everything that allocates or reacts, now that `state_mutex` is released ----
+    switch (result.kind)
+    {
+        case WedgeResolution::Adopted:
+            ProfileEvents::increment(ProfileEvents::CASRefAppendUnwedged);
+            break;
+        case WedgeResolution::Rejected:
+            result.survivor_error = std::make_exception_ptr(Exception(ErrorCodes::INVALID_STATE,
+                "CAS ref-log append for namespace '{}': writer epoch {} was CLOSED by a successor's epoch "
+                "seal at {}-{}, which conclusively rejects the wedged transaction (it was never "
+                "acknowledged). This mount's append lane resumes only under a later epoch",
+                ns.string(), wedge.txn_id.writer_epoch,
+                wedge.txn_id.writer_epoch, wedge.txn_id.ref_sequence));
+            break;
+        case WedgeResolution::Corrupted:
+            on_impossible_interference(wedge.key,
+                fmt::format("ref-log wedge resolution for namespace '{}' txn {}-{} observed a foreign object "
+                    "at the wedged slot: neither this attempt's own bytes nor an epoch seal of this namespace",
+                    ns.string(), wedge.txn_id.writer_epoch, wedge.txn_id.ref_sequence),
+                ns.string());
+            result.survivor_error = std::make_exception_ptr(Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS ref-log append for namespace '{}': impossible foreign interference observed at the "
+                "wedged slot '{}' — the mount is fenced closed and a remount is scheduled; the lane is "
+                "deliberately left wedged for inspection. See the anomaly diagnostics log",
+                ns.string(), wedge.key));
+            break;
+        case WedgeResolution::StillWedged:
+        {
+            const String txn = fmt::format("{}-{}", wedge.txn_id.writer_epoch, wedge.txn_id.ref_sequence);
+            String why;
+            switch (reason)
+            {
+                case Reason::FenceMoved:
+                    why = fmt::format(
+                        "CAS ref-log append for namespace '{}': the resolution of txn {} returned under a "
+                        "DIFFERENT mount incarnation than the one that admitted it — the result is inert "
+                        "and the lane keeps its wedge for whoever recovers it under the live incarnation",
+                        ns.string(), txn);
+                    break;
+                case Reason::CatalogLifeRetired:
+                    why = fmt::format(
+                        "CAS ref-log append for namespace '{}': catalog retirement detached life {} "
+                        "before the bounded retry could be adopted — the result is inert and the "
+                        "successor life is untouched",
+                        ns.string(), renderIncarnation(rt->life.incarnation));
+                    break;
+                case Reason::Superseded:
+                    why = fmt::format(
+                        "CAS ref-log append for namespace '{}': the resolution of txn {} returned for a "
+                        "table that a self-remount had already detached — the result is inert; the fresh "
+                        "incarnation re-derives this table from the durable log",
+                        ns.string(), txn);
+                    break;
+                case Reason::WedgeReplaced:
+                    why = fmt::format(
+                        "CAS ref-log append for namespace '{}': the resolution of txn {} returned for a "
+                        "wedge that is no longer installed — the result is inert and the lane keeps "
+                        "whatever wedge it has",
+                        ns.string(), txn);
+                    break;
+                case Reason::RefusedPreAttempt:
+                    /// The admission-generation half of a pre-attempt refusal never reaches this message:
+                    /// the recheck above presents the same generation and reports `FenceMoved` first.
+                    /// What is left are the two causes that leave the generation intact.
+                    why = fmt::format(
+                        "CAS ref-log append for namespace '{}': the bounded retry of wedged txn {} was "
+                        "refused BEFORE any request was sent — the mount lease is not healthy enough to "
+                        "start a write, or the operation deadline is exhausted — so the slot '{}' is "
+                        "provably untouched by this attempt and the lane stays wedged",
+                        ns.string(), txn, wedge.key);
+                    break;
+                case Reason::ResolveFoundNothing:
+                    /// Never a bare `describeUnresolvedReason` for this primitive (the `SlotOccupyResult`
+                    /// doc's explicit call-site rule): `AttemptsExhausted` reads "the retry budget ran
+                    /// out", which is nonsense for a primitive with no retry budget, and it folds two very
+                    /// different observations together. Both are named instead.
+                    why = fmt::format(
+                        "CAS ref-log append for namespace '{}': the bounded retry of wedged txn {} was sent "
+                        "and the resolve read found NOTHING at slot '{}' — either the read itself failed, "
+                        "or the occupant that rejected the create was DELETED under a live epoch, which is "
+                        "a GC invariant alarm rather than routine contention. The lane stays wedged until "
+                        "the SAME slot resolves durable or a conclusive rejection is observed",
+                        ns.string(), txn, wedge.key);
+                    break;
+                case Reason::StaleState:
+                    why = fmt::format(
+                        "CAS ref-log append for namespace '{}': wedged txn {} is DURABLE but this table "
+                        "advanced under it during the resolution, so it cannot be installed without "
+                        "discarding that advance — the lane NEEDS RECOVERY and refuses later writes until "
+                        "replay re-derives the cache from the durable log",
+                        ns.string(), txn);
+                    break;
+                case Reason::None:
+                    why = fmt::format("CAS ref-log append for namespace '{}': txn {} stays wedged",
+                                      ns.string(), txn);
+                    break;
+            }
+            result.survivor_error = makeCasWriteRetryLaterExceptionPtr(why);
+            break;
+        }
+        case WedgeResolution::NoWedge:
+            break;
+    }
+    return result;
+}
+
+void CasRefLedger::flushRefBatch(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt,
+                                 std::vector<std::shared_ptr<RefMutationItem>> & owned_items)
+{
+    /// One flush = one carved batch through one attempted append. Contract: every ORDINARY outcome
+    /// (validation reject, DefiniteFailure, Unresolved/wedge, Committed) lands in the affected items so
+    /// waiters always wake, and this does NOT throw for any of them. Neither `commitRefChunk` nor
+    /// `resolveWedgeOnce` throws past the point where its object is proven durable -- both installs are
+    /// allocation-free by construction (spec §A1) -- and `resolveWedgeOnce` reports every ORDINARY
+    /// outcome, including a fence that moved under it, through its own result rather than by throwing.
+    /// The paths that can still throw are the wedge-resolution candidate build, which runs BEFORE the
+    /// bounded retry and therefore before anything is proven; a TRANSIENT failure while decoding a
+    /// foreign occupant (deliberately not laundered into a verdict); and an allocation failure in this
+    /// function's own bookkeeping (e.g. the chunk-boundary reseed). All are contained by
+    /// `appendRefOps`' catch, which completes every still-unfinished survivor and restores the leader
+    /// bookkeeping, so no caller hangs.
+    auto complete_error = [&](const std::vector<std::shared_ptr<RefMutationItem>> & items, std::exception_ptr e)
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        for (const auto & it : items)
+        {
+            it->error = e;
+            it->done = true;
+        }
+        rt->cv.notify_all();
+    };
+    auto carve_all_pending = [&]() -> std::vector<std::shared_ptr<RefMutationItem>>
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        std::vector<std::shared_ptr<RefMutationItem>> all(rt->pending.begin(), rt->pending.end());
+        rt->pending.clear();
+        return all;
+    };
+
+    try
+    {
+        ensureRefTableRecovered(ns, *rt);
+    }
+    catch (...)
+    {
+        complete_error(carve_all_pending(), std::current_exception());
+        return;
+    }
+
+    /// The local write fence ensures that a superseded or paused writer cannot race the live one.
+    /// Fails the WHOLE queue -- every caller would have gotten the same refusal alone.
+    if (!may_mutate())
+    {
+        complete_error(carve_all_pending(), makeCasWriteRetryLaterExceptionPtr(fmt::format(
+            "CAS mount lost / lease expired — refusing to append ref-log transactions for server_root '{}'",
+            config.server_root_id)));
+        return;
+    }
+
+    /// Self-remount re-incarnation: this runtime was detached by a
+    /// `quiesceRefTablesForRemount` swap, so its cache is a stale (pre-remount) view. Fail the whole
+    /// carved batch closed -- allocating an id / applying against this orphaned runtime under the
+    /// re-armed fence would split-brain against the fresh runtime the next touch re-recovers. The
+    /// superseded flag is ordered before the fence re-arm (release/acquire through `mayMutate`), so
+    /// reaching this AFTER passing `mayMutate` above proves the swap happened.
+    if (rt->superseded_by_remount.load(std::memory_order_acquire))
+    {
+        complete_error(carve_all_pending(), makeCasWriteRetryLaterExceptionPtr(fmt::format(
+            "CAS ref-log append for server_root '{}': this cached table was superseded by a self-remount — "
+            "retry against the fresh mount incarnation",
+            config.server_root_id)));
+        return;
+    }
+
+    /// Resolve an outstanding wedge FIRST (spec INV-1): "It does not start a later ref-log PUT for
+    /// that table until the earlier result is resolved." One bounded attempt per flush, and the only
+    /// outcome that lets this flush continue is an adoption -- everything else either leaves the lane
+    /// uncertain or closes it deliberately, and in both cases every queued caller is told so.
+    {
+        const WedgeResolutionResult resolution = resolveWedgeOnce(ns, rt);
+        switch (resolution.kind)
+        {
+            case WedgeResolution::NoWedge:
+                break;
+            case WedgeResolution::Adopted:
+                /// The adoption's tail bump may have crossed the snapshot-publish threshold, and this
+                /// flush can still return early below WITHOUT reaching the post-commit scheduler -- an
+                /// empty carve, or an all-no-op survivor batch, both return before it. Trigger it HERE
+                /// so a resolved wedge never leaves the table over-threshold until some later unrelated
+                /// mutation happens to arrive. Idempotent with the post-commit call below (the
+                /// single-in-flight gate), and off-lock as that call requires.
+                maybeScheduleSnapshotPublish(ns, rt);
+                break;
+            case WedgeResolution::Rejected:
+            case WedgeResolution::StillWedged:
+            case WedgeResolution::Corrupted:
+                complete_error(carve_all_pending(), resolution.survivor_error);
+                return;
+        }
+    }
+
+    /// Test-only (see `setRefPreCarveHookForTest`): a no-op in production.
+    if (ref_pre_carve_hook_for_test)
+        ref_pre_carve_hook_for_test();
+
+    /// Carve a compatible batch. `lifecycle != Live` forces a solo carve:
+    /// `namespace_birth` must run alone, and the flush already KNOWS the table's current lifecycle
+    /// before carving (unlike a per-item property, which would need speculative undo).
+    RefTableState working;
+    bool table_live = false;
+    /// Captured in the SAME hold as `working`, for the same reason the id is derived there: a preview
+    /// must describe the transaction the writer will actually send, and INV-2's read side rejects one
+    /// that carries the wrong chain link as hard as it rejects a hole.
+    std::optional<RefTxnId> preview_epoch_seal;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        working = rt->state;
+        table_live = rt->state.getLifecycle() == RefLifecycle::Live;
+        preview_epoch_seal = rt->last_epoch_seal;
+    }
+
+    /// The supersession gate again, and this is NOT the same check as the one at the top of the flush.
+    /// A self-remount does not wait for leaders, so it can land in the window between them -- and after
+    /// it does, this runtime's cached view belongs to a dead incarnation while `live_epoch_fn` already
+    /// reports the NEW epoch. Everything derived below is then a transaction of an epoch this state has
+    /// no chain link for, which INV-2's read side (correctly) calls corruption.
+    ///
+    /// Checking here is what keeps that from being how an ORDINARY remount is reported. The condition is
+    /// a routine, retryable fact about the world -- the fresh runtime the next touch recovers has both
+    /// the epoch and the link -- so it must surface as the retry-safe supersession error, not as a
+    /// `CORRUPTED_DATA` about a stale premise this lane was never entitled to use.
+    if (rt->superseded_by_remount.load(std::memory_order_acquire))
+    {
+        complete_error(carve_all_pending(), makeCasWriteRetryLaterExceptionPtr(fmt::format(
+            "CAS ref-log append for server_root '{}': this cached table was superseded by a self-remount "
+            "before its batch was carved — retry against the fresh mount incarnation",
+            config.server_root_id)));
+        return;
+    }
+
+    /// THE DEPOSED LANE, recognised BEFORE spending a request on it (spec INV-2).
+    ///
+    /// The shape: this table's next id is sequence 1 of the live epoch, and the only seal this runtime
+    /// holds is of THAT epoch -- a successor closed it while we still believed we were writing in it.
+    /// There is then no legal transaction to construct at all. Stamping the seal we hold would be a
+    /// self-pointer the ENCODER refuses; stamping nothing leaves an uncertified epoch crossing the
+    /// READER refuses. Both are dead ends, and both are provable from what this lane already knows.
+    ///
+    /// The OUTCOME here must be the same one the collision produced, not merely "an error". Before this
+    /// gate the lane sent its create, met the successor's seal at the key, and took the conclusive
+    /// rejection arm; skipping the request must not skip the CONCLUSION. So this is a permanent
+    /// rejection in the same class and the same words -- the operation was never acknowledged, no later
+    /// attempt in this epoch can ever succeed, and the lane resumes only under a later epoch, which is
+    /// what tells the caller (and the operator) that this mount has been deposed rather than merely
+    /// delayed. A retry-later class here would be the real bug: every caller would re-derive the same
+    /// impossible transaction forever, and the deposition would never be visible anywhere.
+    ///
+    /// Deliberately NOT a remount trigger, exactly as the collision arm is not: a successor closing our
+    /// epoch is a legitimate handover, and the mount lease is what resolves it. See `resolveWedgeOnce`'s
+    /// `SuccessorSeal` arm, whose reasoning this mirrors.
+    if (table_live)
+    {
+        const RefTxnId next_id = working.nextTxnId(live_epoch_fn());
+        if (next_id.ref_sequence == 1 && !chainLinkFor(next_id, preview_epoch_seal))
+        {
+            /// The two causes are reported SEPARATELY. They are different facts about the world and the
+            /// message must not assert the one it did not observe: holding a seal OF this epoch is
+            /// positive evidence that a successor closed it, while holding no seal at all says only that
+            /// this runtime has no chain link -- a deposition may or may not have happened. The behaviour
+            /// is identical either way (a `Live` table at sequence 1 with no usable link can construct
+            /// nothing legal), so only the diagnosis differs, and only the diagnosis is at risk of
+            /// being wrong.
+            const String cause = preview_epoch_seal
+                ? fmt::format("the only seal it holds, {}-{}, is of that SAME epoch — a successor already "
+                              "closed it", preview_epoch_seal->writer_epoch, preview_epoch_seal->ref_sequence)
+                : String("it holds no epoch seal at all, so it has no chain link to name");
+            complete_error(carve_all_pending(), std::make_exception_ptr(Exception(ErrorCodes::INVALID_STATE,
+                "CAS ref-log append for namespace '{}': writer epoch {} cannot be opened by this mount — its "
+                "next transaction would be {}-{}, sequence 1 of an epoch it holds no closing seal BELOW: {}. "
+                "Nothing legal can be written here and nothing was sent. This mount's append lane resumes "
+                "only under a later epoch",
+                ns.string(), next_id.writer_epoch, next_id.writer_epoch, next_id.ref_sequence, cause)));
+            ProfileEvents::increment(ProfileEvents::CASRefAppendSealRejected);
+            return;
+        }
+    }
+
+    /// Two-phase carve (spec §2). The old carve popped from `pending` while interleaving the allocating
+    /// `seen_refs`/`batch` growth and only recorded the batch into `owned_items` afterwards, so any throw
+    /// after the first pop stranded already-popped items -- neither in `pending` nor in `owned_items` --
+    /// and their waiters hung forever. Instead:
+    ///   PLAN (may throw, mutates NOTHING): under `ref_queue_mutex`, scan `pending` WITHOUT popping and
+    ///   build the selection count, reserving every container (`batch`, `owned_items`) that the publish
+    ///   below grows. A throw here leaves `pending`/`owned_items` byte-for-byte unchanged, so the
+    ///   leadership-exit guard completes only the leader's own item and the untouched followers stay
+    ///   queued for a later leader.
+    ///   PUBLISH (no-throw): still under the SAME continuous `ref_queue_mutex` hold (no TOCTOU by
+    ///   construction), pop the selected front items and append them to `batch` and `owned_items` using
+    ///   only non-throwing operations (capacity pre-reserved; `shared_ptr` copies and `deque::pop_front`
+    ///   never throw). ProfileEvents increments are deferred past the plan so the plan is literally
+    ///   non-mutating.
+    std::vector<std::shared_ptr<RefMutationItem>> batch;
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        const size_t cap = table_live ? kMaxRefBatch : 1;
+
+        /// --- PLAN ---
+        std::set<String> seen_refs;
+        size_t selected = 0;         /// contiguous front items to carve
+        bool scope_cut = false;      /// a duplicate ref name ended the selection early
+        for (const auto & candidate : rt->pending)
+        {
+            if (selected >= cap)
+                break;
+            if (candidate->scope.kind == MutationScope::Kind::WholeShard)
+            {
+                /// A whole-shard mutation carves solo -- it may only be the FIRST (and then only) item.
+                if (selected != 0)
+                    break;
+                if (carve_hook_for_test)
+                    carve_hook_for_test(CarvePhaseForTest::PlanBatchGrow);
+                batch.reserve(1);
+                ++selected;
+                break;
+            }
+            if (carve_hook_for_test)
+                carve_hook_for_test(CarvePhaseForTest::PlanSeenRefs);
+            if (!seen_refs.insert(candidate->scope.ref_name).second)
+            {
+                scope_cut = true;
+                break;
+            }
+            if (carve_hook_for_test)
+                carve_hook_for_test(CarvePhaseForTest::PlanBatchGrow);
+            batch.reserve(selected + 1);
+            ++selected;
+        }
+        /// Reserve the leader's responsibility set for the whole selection BEFORE any pop, so the publish
+        /// append below cannot throw. `owned_items` already holds the leader's own item (recorded by
+        /// `appendRefOps`), which the carve re-adds as it re-appears at the front of `pending` -- a
+        /// harmless idempotent double-listing the guard tolerates, matching the pre-fix behavior.
+        if (carve_hook_for_test)
+            carve_hook_for_test(CarvePhaseForTest::PlanReserveOwned);
+        owned_items.reserve(owned_items.size() + selected);
+
+        /// --- PUBLISH (no-throw) ---
+        if (carve_hook_for_test)
+            carve_hook_for_test(CarvePhaseForTest::PublishPop);
+        for (size_t i = 0; i < selected; ++i)
+        {
+            batch.push_back(rt->pending.front());        /// shared_ptr copy, capacity reserved
+            owned_items.push_back(rt->pending.front());  /// same item into the responsibility set
+            rt->pending.pop_front();
+        }
+
+        /// Deferred past the plan (spec §2) so the plan phase performs no observable mutation.
+        if (scope_cut)
+            ProfileEvents::increment(ProfileEvents::CASRefBatchScopeCuts);
+    }
+    if (batch.empty())
+        return;   /// raced: everything was carved by a previous flush of this leader
+
+    /// Per-item validation, in order, against `working` (per-request undo via `item_scratch`):
+    /// business preconditions (thrown by `build_ops` itself) and the pre-encode admission budget both
+    /// fail ONLY the offending item; survivors' ops accumulate into `final_ops` for ONE CHUNK. When
+    /// admitting the next item's ops would exceed `ref_txn_max_ops`, the accumulated chunk is committed
+    /// as a complete ref-log transaction and validation continues into a fresh chunk against the
+    /// reseeded live state (spec §3 chunked flush): one tenure may emit several transactions.
+    std::vector<RefOp> final_ops;
+    std::vector<std::shared_ptr<RefMutationItem>> survivors;
+    /// Every preview below stamps its throwaway transaction with `nextRefTxnId` of the state it is about
+    /// to be applied to, under `live_epoch_fn()` -- the SAME rule and the same epoch source
+    /// `allocateRefTxnId` uses for the real id, so a preview can never be rejected for an id shape the
+    /// persisted transaction would have been given. Deriving each preview id from ITS OWN state, rather
+    /// than carrying a running counter across the loop, is what makes failure isolation hold under
+    /// INV-1: an item that throws part-way through its per-op previews leaves `working` untouched, and
+    /// the next item's preview is still the successor of `working` rather than of the abandoned item's
+    /// last trial id. These ids are never persisted or compared outside this loop.
+    for (size_t item_index = 0; item_index < batch.size(); ++item_index)
+    {
+        const auto & it = batch[item_index];
+
+        /// Step 1: build this item's ops and apply the counts-only per-item caps. `build_ops` runs at
+        /// most once per item, HERE -- the overflowing item's ops are built once and reused in the fresh
+        /// chunk it lands in (the at-most-once contract holds across a chunk boundary). A failure here
+        /// (a business precondition thrown by `build_ops`, or an over-cap item/op) fails ONLY this item;
+        /// the chunk in progress and the remaining items are untouched.
+        std::vector<RefOp> item_ops;
+        bool removal_class = false;
+        try
+        {
+            item_ops = it->build_ops(working);
+
+            /// Counts-only admission caps (spec §3), checked before any op is touched further so an
+            /// oversized item or op never reaches `working` or the state-machine preview below and
+            /// fails ALONE -- neighbors in this same batch are unaffected. Removal-class items are
+            /// exempt from both: they share the larger `ref_removal_max_bytes` byte budget instead
+            /// (`checkBudget`, `CasRefLogFormat.cpp`) and are already carved as singletons (`WholeShard`
+            /// scope forces a solo carve above). `refLogTxnIsRemovalClass` is the ONE canonical
+            /// discriminator (built ops contain `RemoveNamespace`) shared with the codec's own
+            /// `checkBudget` -- `WholeShard` scope alone is NOT a substitute (the stale-precommit
+            /// reclaim sweep is also `WholeShard`-scoped but is not removal-class).
+            removal_class = refLogTxnIsRemovalClass(item_ops);
+            if (!removal_class)
+            {
+                if (item_ops.size() > ref_txn_max_ops)
+                    throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+                        "ref mutation on namespace '{}' has {} operations, exceeding the normal-class "
+                        "per-item op-count cap {} — refusing before any object is created",
+                        ns.string(), item_ops.size(), ref_txn_max_ops);
+                for (const RefOp & op : item_ops)
+                {
+                    const size_t op_bytes = encodedOpSize(op);
+                    if (op_bytes > ref_op_max_bytes)
+                        throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+                            "ref mutation on namespace '{}' contains an op of encoded size {}, exceeding "
+                            "the normal-class per-op cap {} — refusing before any object is created",
+                            ns.string(), op_bytes, ref_op_max_bytes);
+                }
+            }
+        }
+        catch (...)
+        {
+            complete_error({it}, std::current_exception());
+            continue;
+        }
+
+        /// Step 2: chunk boundary (spec §3). If admitting this item's ops would push the current
+        /// (non-empty) chunk over `ref_txn_max_ops`, COMMIT the accumulated chunk now as a COMPLETE
+        /// ref-log transaction and start a fresh one. Removal-class items are always solo-carved
+        /// (`WholeShard` scope), so `final_ops` is empty when one is processed and this branch never
+        /// fires for them.
+        if (!removal_class && !final_ops.empty()
+            && final_ops.size() + item_ops.size() > ref_txn_max_ops)
+        {
+            /// Release the scratch `working` so `commitRefChunk`'s post-commit overlay fold is in place
+            /// (the E5 fast path), exactly as the single-chunk path does before its commit arm.
+            working = RefTableState{};
+            const bool committed = commitRefChunk(ns, rt, final_ops, survivors);
+            if (!committed)
+            {
+                /// Failure isolation (spec §3): chunk N's survivors were already failed inside
+                /// `commitRefChunk`. Fail THIS item and the entire not-yet-attempted remainder too, so no
+                /// owned item is left stranded (its waiter would hang and its `build_ops` closure become
+                /// unsafe). Earlier chunks that already committed keep their callers' success -- an
+                /// unresolved wedge from `commitRefChunk` therefore contains ONLY this chunk.
+                std::vector<std::shared_ptr<RefMutationItem>> remainder(batch.begin() + item_index, batch.end());
+                complete_error(remainder, makeCasWriteRetryLaterExceptionPtr(fmt::format(
+                    "CAS ref-log append for namespace '{}': a preceding chunk of this multi-transaction "
+                    "flush did not commit — this item was not attempted and can be retried", ns.string())));
+                return;
+            }
+            /// Reseed `working` from the now-live state: the speculative `working` with trial ids from
+            /// the just-committed chunk is discarded, so a later zero-op item is completed against the
+            /// REAL committed id, never a trial id that never persisted. The preview ids need no reseed
+            /// of their own -- each is derived from the state it is applied to, so re-seating `working`
+            /// re-seats them. A throw at the boundary -- the injected `ChunkReseed` fault, or a genuine
+            /// reseed allocation failure -- propagates to `appendRefOps`' tenure-containment catch, which
+            /// preserves the already-committed chunk's callers' success.
+            if (carve_hook_for_test)
+                carve_hook_for_test(CarvePhaseForTest::ChunkReseed);
+            {
+                std::lock_guard lock(rt->state_mutex);
+                working = rt->state;
+                preview_epoch_seal = rt->last_epoch_seal;
+            }
+            final_ops.clear();
+            survivors.clear();
+        }
+
+        /// Step 3: validate this item into the current (possibly fresh) chunk and, past all throwing
+        /// points, publish its effects into `working`/`final_ops`/`survivors`. A failure here fails ONLY
+        /// this item. `item_ops` was built in step 1 against the pre-boundary state; the carve
+        /// deduplicates ref names within a batch, so the overflowing item operates on a ref distinct
+        /// from the just-committed chunk's and re-validating it against the reseeded `working` is
+        /// consistent.
+        RefTableState item_scratch = working;
+        try
+        {
+            /// Whole-item shape validation (prerequisite to `dropNamespace`): the
+            /// per-op loop below previews each op as its OWN single-op trial transaction, so a
+            /// whole-transaction-shape rule like "remove_namespace must be the FINAL op" trivially
+            /// passes on every singleton slice regardless of this item's REAL combined shape -- a
+            /// malformed item (e.g. remove_namespace not last) would otherwise only be caught by
+            /// `commitRefChunk`'s candidate apply -- which fails the whole chunk, taking every innocent
+            /// co-batched item with it, and (before the candidate moved ahead of the `PUT`) did so only
+            /// after the object was already durable. Validate the item's COMPLETE
+            /// ops array as ONE combined transaction, against a throwaway copy of the pre-item state,
+            /// before doing any other per-op work -- exactly what the real persisted transaction will
+            /// contain, using only the public two-phase `applyRefLogTxn` entry point (no need to reach
+            /// into the state machine's private per-op helpers).
+            if (!item_ops.empty())
+            {
+                RefTableState shape_check = working;
+                const RefTxnId shape_id = shape_check.nextTxnId(live_epoch_fn());
+                applyRefLogTxn(shape_check, RefLogTxn{ns.string(), shape_id, item_ops,
+                                                      chainLinkFor(shape_id, preview_epoch_seal)});
+            }
+            if (removal_class && !it->terminal_removal_authorized)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "ref mutation on namespace '{}' attempted `RemoveNamespace` through the generic append "
+                    "surface; only exact catalog removal ownership may append the terminal transaction",
+                    ns.string());
+
+            for (const RefOp & op : item_ops)
+            {
+                /// Admission budget: only STATE-GROWING ops need the check --
+                /// an `owner_transition` installing a binding (add or promote) and `set_published_at`.
+                /// `namespace_birth` is exempt (it grows nothing, and a never-born state's preview has
+                /// no meaningful "current snapshot" to encode); `remove_namespace` and a pure
+                /// owner_transition removal shrink state and can never violate the budget.
+                const bool state_growing = (op.kind == RefOpKind::OwnerTransition && op.new_binding.has_value())
+                    || op.kind == RefOpKind::SetPublishedAt;
+                if (state_growing && !admits(item_scratch, op, rt->snapshot_budget, rt->removal_budget))
+                    throw Exception(ErrorCodes::LIMIT_EXCEEDED,
+                        "ref mutation on namespace '{}' would exceed the table's admission budget "
+                        "(snapshot_budget={} removal_budget={}) — refusing before any object is created",
+                        ns.string(), rt->snapshot_budget, rt->removal_budget);
+                /// Apply THIS op to item_scratch now (a single-op trial transaction) so a LATER op of
+                /// the SAME item (e.g. namespace_birth immediately followed by its first
+                /// owner_transition) is validated -- both here and by admits's own preview -- against
+                /// a state that already reflects it, exactly as the real combined transaction will.
+                const RefTxnId trial_id = item_scratch.nextTxnId(live_epoch_fn());
+                applyRefLogTxn(item_scratch, RefLogTxn{ns.string(), trial_id, {op},
+                                                       chainLinkFor(trial_id, preview_epoch_seal)});
+            }
+            /// Reserve the growth of BOTH accumulators BEFORE this item's effects are published. These
+            /// reservations are the ONLY remaining throwing steps; once they succeed the publish below is
+            /// no-throw -- `working`'s move-assignment is `noexcept`, and the `RefOp` moves and the
+            /// `shared_ptr` copy land in pre-reserved capacity. Before the fix, `working` was moved and
+            /// `final_ops` appended before these allocations, so a failure here left a failed item applied
+            /// to `working` (corrupting later items' validation) and -- when the throw fell between the
+            /// two accumulator writes -- its ops already in the durably-committed transaction while its
+            /// own caller was told the append failed.
+            final_ops.reserve(final_ops.size() + item_ops.size());
+            survivors.reserve(survivors.size() + 1);
+            if (carve_hook_for_test)
+                carve_hook_for_test(CarvePhaseForTest::ValidateFinalOps);
+            working = std::move(item_scratch);
+            for (RefOp & op : item_ops)
+                final_ops.push_back(std::move(op));
+            survivors.push_back(it);
+        }
+        catch (...)
+        {
+            complete_error({it}, std::current_exception());
+        }
+    }
+    if (final_ops.empty())
+    {
+        /// Either every item failed validation (already completed via complete_error above, nothing
+        /// left to do), or every survivor of the LAST chunk contributed ZERO ops (an idempotent no-op,
+        /// e.g. precommitAdd/promote re-targeting a manifest already exactly committed). Survivors of the
+        /// latter kind still need marking done -- with no new object created, `committed_id` is the
+        /// table's current high-water mark. After an earlier committed chunk, `working` was reseeded from
+        /// the live state, so that mark is the REAL id the earlier chunk persisted, never a discarded
+        /// trial id.
+        if (!survivors.empty())
+        {
+            std::lock_guard<std::mutex> g(ref_queue_mutex);
+            for (const auto & it : survivors)
+            {
+                it->committed_id = working.getGreatestApplied();
+                it->done = true;
+            }
+            rt->cv.notify_all();
+        }
+        return;
+    }
+
+    /// Commit the FINAL chunk of this tenure (spec §3): the remaining accumulated ops form the last --
+    /// possibly only -- ref-log transaction. Release the scratch `working` first so `commitRefChunk`'s
+    /// post-commit overlay fold is in place (the E5 fast path), then run the full committed arm. Its
+    /// survivors are completed (success or failure) inside it, so nothing is owed here on any outcome,
+    /// and it no longer throws past its durable `PUT` at all (spec §A1).
+    working = RefTableState{};
+    commitRefChunk(ns, rt, final_ops, survivors);
+}
+
+CasRefLedger::PreparedRefChunk CasRefLedger::prepareRefChunk(
+    const Layout & layout, const NamespaceLifeId & life, RefTableState state, const RefTxnId & id,
+    const std::optional<RefTxnId> & chain_link, std::span<const RefOp> ops, uint64_t admitted_generation)
+{
+    PreparedRefChunk prepared{
+        .candidate = std::move(state),
+        .candidate_base_id = {},
+        .chunk_txn = RefLogTxn{life.ns.string(), id, std::vector<RefOp>(ops.begin(), ops.end()), chain_link},
+        .prepared_attempt = {},
+        .birth_contribution = std::nullopt,
+        .commit_contribution = RefCkpt{
+            .life_epoch = std::nullopt,
+            .committed_through = id,
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = chain_link},
+    };
+    prepared.candidate_base_id = prepared.candidate.getGreatestApplied();
+
+    /// A throw here is a clean PRE-durability failure -- the same class as an ordinary validation
+    /// reject: no object exists yet, the cache is untouched, and the id is simply never used (the next
+    /// attempt re-derives it from this same unchanged state).
+    applyRefLogTxn(prepared.candidate, prepared.chunk_txn);
+
+    /// The COMPLETE attempt (spec §A1 site 3), so the request can read its key and body straight out of
+    /// it and the `Unresolved` arm only has to MOVE it into the runtime. Building it here rather than
+    /// after the `PUT` is what keeps an allocation failure from leaving a possibly-DURABLE object with
+    /// NEITHER the transaction nor the attempt recorded -- strictly worse than a wedge, because the next
+    /// append would then mint a fresh id against a state missing a landed transaction, which is the
+    /// divergence the candidate exists to prevent.
+    ///
+    /// A rename, not an extra copy: the seal writes its result directly into the attempt's body and the
+    /// key is computed directly into the attempt's key, so nothing is copied twice on the committed path
+    /// either.
+    ///
+    /// FAULT CLASS, stated because the extraction NARROWED it, and this is the whole of the change.
+    /// `RefLogTxn`'s construction above and the key computation below used to sit outside any local
+    /// `try` in `commitRefChunk`, so an allocation failure in either escaped `commitRefChunk` and
+    /// `flushRefBatch` entirely and was caught by `appendRefOps`' tenure catch, which completes every
+    /// still-INCOMPLETE owned item with that exception and releases leadership, ending the tenure (an
+    /// item already made durable by an earlier committed chunk keeps its success -- the guard leaves it
+    /// `done` with no error, deliberately). Inside this function both are covered by the caller's single
+    /// `catch`, so the same failure now completes `chunk_survivors` with that exception and returns
+    /// false, and the tenure continues. That is exactly what an apply failure at this same pre-durability
+    /// stage already did, which is the point: an allocation failure here is no longer the one fault whose
+    /// blast radius differs from its immediate neighbours'.
+    ///
+    /// Nothing is stranded either way, and there is no universal remainder handler to appeal to -- the
+    /// two call sites dispose of their own. On a chunk boundary `flushRefBatch` fails this item plus the
+    /// entire not-yet-attempted remainder retry-later and returns at once. On the FINAL chunk the tail
+    /// call ignores the return value because no remainder is left: every batch item is by then either
+    /// already completed (it failed its own validation, or it belonged to an earlier chunk that
+    /// `commitRefChunk` completed) or is in THIS chunk's `chunk_survivors`, which this path completes.
+    /// `chunk_survivors` is emphatically NOT the whole batch on that path -- `survivors.clear()` at each
+    /// chunk boundary leaves it holding only the last chunk's items.
+    ///
+    /// One reported error class does change on the boundary path: the not-yet-attempted remainder now
+    /// gets a retry-later refusal instead of the raw escaping exception. Those items were provably never
+    /// attempted, so retry-later is the truthful class, and it is the class they already got whenever the
+    /// preceding chunk failed for any other reason.
+    prepared.prepared_attempt.txn_id = id;
+    prepared.prepared_attempt.key = layout.refLogKey(life, id);
+    prepared.prepared_attempt.admitted_fence_generation = admitted_generation;
+    prepared.prepared_attempt.bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(prepared.chunk_txn));
+
+    /// INV-4's `life_epoch`, which ONLY this transaction knows: the writer epoch of the `NamespaceBirth`
+    /// being appended. Prepared as a VALUE; `commitRefChunk` publishes it, because for a birth chunk that
+    /// publish is the FIRST durable effect and preparation is everything strictly before it.
+    if (std::any_of(ops.begin(), ops.end(),
+                    [](const RefOp & op) { return op.kind == RefOpKind::NamespaceBirth; }))
+        prepared.birth_contribution = RefCkpt{.life_epoch = std::optional<uint64_t>{id.writer_epoch},
+                                              .committed_through = std::nullopt,
+                                              .checkpoint_snapshot_id = std::nullopt,
+                                              .last_epoch_seal = std::nullopt};
+
+    if (refLogTxnIsEpochSeal(prepared.chunk_txn))
+        prepared.commit_contribution.last_epoch_seal = id;
+
+    return prepared;
+}
+
+bool CasRefLedger::commitRefChunk(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt,
+                                  const std::vector<RefOp> & chunk_ops,
+                                  const std::vector<std::shared_ptr<RefMutationItem>> & chunk_survivors)
+{
+    /// Reconstructed locally so this arm has the SAME completion + fence semantics as when it lived
+    /// inline in `flushRefBatch`: `complete_error` wakes a chunk's waiters under `ref_queue_mutex`, and
+    /// `fence_ok` folds `superseded_by_remount` into the append fence so a self-remount landing between a
+    /// leader's pre-allocate re-check and its `PUT` reports Unresolved rather than committing against a
+    /// stale cache.
+    auto complete_error = [&](const std::vector<std::shared_ptr<RefMutationItem>> & items, std::exception_ptr e)
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        for (const auto & it : items)
+        {
+            it->error = e;
+            it->done = true;
+        }
+        rt->cv.notify_all();
+    };
+    const auto fence_ok = [this, &rt]
+    {
+        return fence_ok_fn()
+            && !rt->catalog_life_invalidated.load(std::memory_order_acquire)
+            && !rt->superseded_by_remount.load(std::memory_order_acquire);
+    };
+
+    const bool positive_append = std::any_of(chunk_ops.begin(), chunk_ops.end(), [](const RefOp & op)
+    {
+        return (op.kind == RefOpKind::OwnerTransition && op.new_binding.has_value())
+            || op.kind == RefOpKind::SetPublishedAt;
+    });
+    const bool removal_append = refLogTxnIsRemovalClass(chunk_ops);
+
+    /// The local capability prevents callers from reaching this point through the generic API; the
+    /// durable row is the independent final authority. Re-read it immediately before id allocation so
+    /// a stale exact runtime cannot append a terminal after the catalog life changed, and require the
+    /// positive lane to still be closed under the same queue lock that guards admission.
+    if (removal_append)
+    {
+        try
+        {
+            if (!std::all_of(chunk_survivors.begin(), chunk_survivors.end(),
+                    [](const auto & item) { return item->terminal_removal_authorized; }))
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS namespace removal '{}': terminal chunk lacks exact removal ownership",
+                    ns.string());
+
+            {
+                std::lock_guard queue_lock(ref_queue_mutex);
+                if (!rt->removal_admission_closed)
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS namespace removal '{}': terminal append reached an open positive lane",
+                        ns.string());
+            }
+
+            const uint64_t admitted_generation = rt->admitted_fence_generation;
+            const CasRefCatalog::Snapshot catalog = CasRefCatalog::read(backend, layout);
+            check_fence_or_throw(admitted_generation);
+            catalog.life_index.throwIfAmbiguous("CAS terminal removal append");
+            const auto entry_it = std::find_if(
+                catalog.catalog.entries.begin(), catalog.catalog.entries.end(),
+                [&](const CatalogEntry & entry)
+                {
+                    return entry.ns == ns && entry.incarnation == rt->life.incarnation;
+                });
+            if (entry_it == catalog.catalog.entries.end() || entry_it->state != NsState::Removing)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS namespace removal '{}': terminal append requires its exact catalog life to be Removing",
+                    ns.string());
+        }
+        catch (...)
+        {
+            complete_error(chunk_survivors, std::current_exception());
+            return false;
+        }
+    }
+
+    /// The pre-carve seam is above this call, so this is the final catalog admission observation before
+    /// id allocation. It closes the cached-runtime window in which another actor publishes `Removing`
+    /// after this writer's ordinary entry gates. A non-`Live` exact row permanently closes this local
+    /// positive lane; the terminal owner-removal transaction remains the one deliberate exception.
+    if (positive_append)
+    {
+        try
+        {
+            const uint64_t admitted_generation = rt->admitted_fence_generation;
+            const CasRefCatalog::Snapshot catalog = CasRefCatalog::read(backend, layout);
+            check_fence_or_throw(admitted_generation);
+            const NamespaceLifeId & life = rt->life;
+            const auto entry_it = std::find_if(catalog.catalog.entries.begin(), catalog.catalog.entries.end(),
+                [&](const CatalogEntry & entry)
+                {
+                    return entry.ns == ns && entry.incarnation == life.incarnation;
+                });
+            if (entry_it == catalog.catalog.entries.end() || entry_it->state != NsState::Live)
+            {
+                {
+                    std::lock_guard queue_lock(ref_queue_mutex);
+                    rt->removal_admission_closed = true;
+                }
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS ref-log append for namespace '{}': exact catalog life is no longer Live",
+                    ns.string()));
+            }
+        }
+        catch (...)
+        {
+            complete_error(chunk_survivors, std::current_exception());
+            return false;
+        }
+    }
+
+    /// Self-remount re-check BEFORE allocating an id: the top-of-flush gate
+    /// is passed once, but a leader can stall between it and here -- in `build_ops`' caller I/O -- across
+    /// the whole fence-loss + remount window, then resume after `armMountFence`. Allocating {new_epoch,
+    /// seq} now and PUTting it (its live `fence_ok` would pass) would persist a transaction validated
+    /// against this orphaned runtime's STALE cache -- the C1 data-loss class. `superseded_by_remount` is
+    /// published before the fence re-arm, so failing closed here (no id, no PUT, no wedge, cache
+    /// unchanged) keeps the durable log free of any stale-view transaction. The append `fence_ok`
+    /// (which also checks the flag) is the airtight backstop for the narrow window past this point.
+    if (rt->superseded_by_remount.load(std::memory_order_acquire))
+    {
+        complete_error(chunk_survivors, makeCasWriteRetryLaterExceptionPtr(fmt::format(
+            "CAS ref-log append for server_root '{}': this cached table was superseded by a self-remount "
+            "before id allocation — retry against the fresh mount incarnation",
+            config.server_root_id)));
+        return false;
+    }
+
+    /// `Ready` is the sole new-id admission state. The resolver either restores it or returns the whole
+    /// batch closed, so any other state here is an internal lifecycle violation. It can be reached only
+    /// by a bug or a test injection after the top-of-flush resolver gate; make that contradiction an
+    /// explicit `Faulted` state and route it through the same anomaly policy as foreign interference.
+    {
+        RefLaneState lane_state = RefLaneState::Faulted;
+        std::optional<String> attempt_key;
+        {
+            std::lock_guard lock(rt->state_mutex);
+            lane_state = rt->lane_state;
+            if (rt->append_attempt)
+                attempt_key = rt->append_attempt->key;
+        }
+        if (lane_state != RefLaneState::Ready)
+        {
+            {
+                std::lock_guard lock(rt->state_mutex);
+                rt->append_attempt.reset();
+                rt->lane_state = RefLaneState::Faulted;
+            }
+            on_impossible_interference(attempt_key.value_or(""), fmt::format(
+                "ref-log append reached new-id allocation while the lane was in state {} instead of Ready",
+                static_cast<uint8_t>(lane_state)), ns.string());
+            /// `Faulted` is a TERMINAL lane state (same as the two other `Faulted` arms further down this
+            /// function, which both report `CORRUPTED_DATA`) -- reporting it via
+            /// `makeCasWriteRetryLaterExceptionPtr` would tell the caller a state the lane can never leave
+            /// on its own is transient. The arm is self-limiting today (the next flush's
+            /// `resolveWedgeOnce` takes the `invalid_lane_state` `Reason` arm and re-reports it the same
+            /// way), but a terminal state must never be reported as retryable from ANY arm --
+            /// `gtest_cas_ref_writer.cpp`'s `CasAnomalyPolicy.NonReadyAtNewIdAllocationFaultsAndFailsClosed`
+            /// pins this arm specifically (it already drives this exact seam; only its expected error
+            /// class needed to change).
+            complete_error(chunk_survivors, std::make_exception_ptr(Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS ref-log append for namespace '{}': refusing to allocate a new ref-log id while the "
+                "lane is not Ready (state {}, attempt key '{}'); the lane is faulted until remount",
+                ns.string(), static_cast<uint8_t>(lane_state), attempt_key.value_or(""))));
+            return false;
+        }
+    }
+
+    /// ONE atomic reading of everything this transaction is derived from that the RUNTIME owns: the
+    /// state it will be applied to, the id, the mount incarnation that admitted it, and the seal that
+    /// qualifies the id's sequence number. `prepareRefChunk` below is a pure function of its arguments,
+    /// and every argument is either one of those four readings, a value DERIVED from one of them outside
+    /// the lock (`chain_link`, from `id` and the seal -- see `chainLinkFor` below), or an immutable input the
+    /// runtime does not own at all (the layout, the namespace, this chunk's ops).
+    ///
+    /// The candidate is built from this snapshot BEFORE the PUT (spec §A1), so that the region between
+    /// "this chunk's object is durable" and "the runtime records it" is allocation-free and therefore
+    /// cannot throw. It used to be the other way round -- PUT, then `applyRefLogTxn(rt->state, ...)` -- and
+    /// that apply CAN throw on an allocation failure (the COW containers allocate their overlays), which
+    /// left the transaction durable but invisible to the writer: the apply check of the day admitted any
+    /// strictly greater id, so a later transaction sailed over the hole and a snapshot published
+    /// afterwards was labelled with THAT id -- recovery then skips the stranded transaction forever
+    /// while GC, which folds the ref LOGS, still applies it. That divergence is a data-loss class, not a
+    /// stale cache. INV-1 now refuses the same hole from the read side too, so the accident this
+    /// ordering prevents would also have to survive the density check to do any damage.
+    ///
+    /// A throw during PREPARATION, by contrast, is a clean PRE-durability failure -- the same class as an
+    /// ordinary validation reject: no object exists yet, the cache is untouched, and the id is simply
+    /// never used (the next attempt re-derives it from this same unchanged state).
+    ///
+    /// The snapshot copy taken here is cheap because it SHARES the live state's COW bases; the apply
+    /// inside preparation allocates only the overlay. It is CONSUMED by preparation -- moved, not copied
+    /// again -- so this remains the single copy of `rt->state` the commit path makes, exactly as it was.
+    /// The candidate is deliberately NOT materialized: a state that shares its base cannot fold in place,
+    /// so folding now would rebuild the whole base (O(table) per chunk). The install below restores unique
+    /// base ownership before the existing post-install fold, which therefore stays O(overlay).
+    ///
+    /// The id is derived in the SAME critical section that snapshots the state (INV-1): it is a function
+    /// of `greatest_applied`, so reading it at a different instant than the state the transaction is
+    /// applied to would be deriving this chunk's id from a different stream. Only this leader mutates
+    /// `rt->state`, so the two reads cannot disagree today -- taking them together is what keeps that
+    /// from being an invariant a future edit has to rediscover. This is also why the id, the generation
+    /// and the seal are INPUTS to `prepareRefChunk` rather than things it derives: it has no lock to read
+    /// them under.
+    ///
+    /// The fence GENERATION and `last_epoch_seal` are read in the same hold for the same reason. The
+    /// generation is this transaction's ADMISSION token: one atomic reading of "which mount incarnation
+    /// allowed this attempt", which the attempt then carries and every later retry and install presents
+    /// back. The seal is what qualifies the id's sequence number, so reading it at a different instant
+    /// than the id would be describing a different transition.
+    std::optional<RefTableState> state_snapshot;
+    RefTxnId id;
+    uint64_t admitted_fence_generation = 0;
+    std::optional<RefTxnId> last_epoch_seal;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        state_snapshot.emplace(rt->state);
+        id = allocateRefTxnId(*rt);
+        admitted_fence_generation = rt->admitted_fence_generation;
+        last_epoch_seal = rt->last_epoch_seal;
+    }
+    /// INV-2's chain link, stamped exactly where the grammar requires it: sequence 1 of an epoch above
+    /// this namespace's genesis names the seal that closed the previous one, and nothing else carries it.
+    /// `last_epoch_seal` is `nullopt` precisely at genesis (see `RefTableRuntime::last_epoch_seal`), so
+    /// a genesis birth at sequence 1 finds nothing to name -- which is what makes "no seal" a fact about
+    /// the stream rather than a defaulted field. Derived outside the `try` below because `chainLinkFor`
+    /// neither allocates nor throws, so nothing is gained by covering it.
+    const std::optional<RefTxnId> chain_link = chainLinkFor(id, last_epoch_seal);
+
+    /// Preparation is ONE pure call, and it is everything decided before this chunk can have any durable
+    /// effect: the candidate, the transaction with its chain link, the complete attempt (key + sealed
+    /// bytes), the post-log committed-frontier contribution, and -- for a birth -- the pre-log
+    /// `life_epoch` contribution as VALUES.
+    ///
+    /// THE PLACEMENT IS THE CORRECTNESS ARGUMENT, and it has to hold for BOTH chunk shapes, because
+    /// `commitRefChunk` has two different first durable effects. An ordinary chunk's is the ref-log
+    /// `putIfAbsentControlled` far below; a `NamespaceBirth` chunk's is the `_ckpt` publish, which is
+    /// EARLIER. This call therefore sits above both, and every statement between here and the lock above
+    /// is in-memory only. A "pure" preparation that published the `_ckpt` itself would be a lie, and
+    /// moving that publish later would change fault semantics the directive says to preserve.
+    ///
+    /// Everything it can throw is a pre-durability rejection with the identical handler, which is why ONE
+    /// catch replaces the two that used to sit around those steps separately (see the FAULT CLASS note on
+    /// `prepareRefChunk` for the two statements whose catcher changed). The reachable ones are a rejected
+    /// apply and a failed seal; an allocation failure anywhere inside is the same class, and so is the
+    /// `BAD_ARGUMENTS` that `layout.refLogKey` -> `namespaceStreamPrefix` -> `checkNamespace` would raise
+    /// for a malformed namespace -- unreachable here, since a mounted table's namespace already passed
+    /// that check, but the list is not meant to read closed.
+    std::optional<PreparedRefChunk> prepared;
+    try
+    {
+        prepared.emplace(prepareRefChunk(layout, rt->life, std::move(*state_snapshot), id, chain_link,
+                                         chunk_ops, admitted_fence_generation));
+    }
+    catch (...)
+    {
+        complete_error(chunk_survivors, std::current_exception());
+        return false;
+    }
+    const RefTxnId candidate_base_id = prepared->candidate_base_id;
+    /// The candidate moves back into its own optional so the post-durable install region below is
+    /// textually untouched by this extraction: it still swaps `*candidate` in and still `reset()`s it in
+    /// the same place, for the same reason (releasing bases whose `use_count()` the fold then reads). The
+    /// move is COW-pointer-only and happens here, while nothing is durable.
+    std::optional<RefTableState> candidate{std::move(prepared->candidate)};
+
+    /// INV-4's FIRST `_ckpt` writer, and the ONLY writer anywhere that knows this namespace's
+    /// `life_epoch`: it is the writer epoch of its `namespace_birth`, which is this transaction. No
+    /// later writer can recover it (a table recovered from a snapshot never replays the birth), so if
+    /// it is not recorded here it is not recorded at all. Spec §3 orders creation `_ckpt` first, THEN
+    /// the namespace becoming Live, which is exactly this placement -- before the durable `PUT`, where
+    /// a failure is an ORDINARY pre-durability rejection: the id is not consumed, nothing landed, and
+    /// the next attempt re-derives the same id. A `_ckpt` for a birth that then failed is inert debris
+    /// (it names no checkpoint, so nothing is deletable and recovery has no base to prefer), and the
+    /// next attempt's merge adopts it unchanged.
+    ///
+    /// `FencedOut` is a rejection here rather than a shrug: unlike the publisher's, this contribution
+    /// carries the one fact nothing else can supply, so proceeding without it would put the namespace
+    /// Live with its genesis epoch permanently unknown.
+    ///
+    /// ORDERING, and it is the one deliberate reorder in this extraction: this publish is a birth chunk's
+    /// FIRST durable effect, so preparation -- INCLUDING the sealed bytes -- now completes ABOVE it,
+    /// where it used to run below. That matters more than "an allocation could fail earlier", because
+    /// sealing is not pure serialization -- it VALIDATES. `encodeRefLogTxn` runs `checkRefTxnIdNonzero`,
+    /// then `validateEpochSealGrammarStructural`, then `checkBudget` over the encoded text
+    /// (`CasRefLogFormat.cpp`), and throws `CORRUPTED_DATA` -- all three of `checkBudget`'s refusals use
+    /// that code too. NOT `LIMIT_EXCEEDED`: that code belongs to the carve-time admission caps in
+    /// `flushRefBatch`, a different check at a different stage. The candidate apply above runs, of INV-2's
+    /// grammar, only `validateEpochSealGrammarContextual`, which returns early off sequence 1 and owns
+    /// nothing but the required-iff rule. The two grammar halves are DISJOINT BY
+    /// DESIGN -- each function's own comment says the other owns the half it does not -- and the budget
+    /// check belongs to neither. So a chunk can pass the apply and still be refused by the seal, on
+    /// grammar, on id shape, or on size; after the reorder that refusal lands BEFORE this `_ckpt` is
+    /// durable instead of after it, so it leaves no inert `_ckpt` behind where it used to leave one.
+    /// Caller-visible outcome is identical WHEN ONE of the two steps refuses: both orders reach the same
+    /// already-possible rejection, complete the same survivors with the same error, return false, and
+    /// consume no id. A birth chunk that would fail BOTH -- an unsealable transaction AND a moved mount
+    /// fence -- reports a different CLASS after the reorder, because whichever step now runs first is the
+    /// one that speaks: the seal's `CORRUPTED_DATA` instead of the `FencedOut` retry-later below. Both are
+    /// truthful about a chunk that was never sent and consumed no id, and neither order can report both.
+    /// Strictly less durable debris, nothing new that can throw.
+    if (prepared->birth_contribution)
+    {
+        try
+        {
+            if (publishCkptContribution(rt->life, *prepared->birth_contribution,
+                                        admitted_fence_generation, check_fence_or_throw)
+                == CkptPublishOutcome::FencedOut)
+            {
+                complete_error(chunk_survivors, makeCasWriteRetryLaterExceptionPtr(fmt::format(
+                    "CAS ref-log append for namespace '{}': the mount fence moved while creating the "
+                    "namespace's _ckpt, so the birth transaction was not sent", ns.string())));
+                return false;
+            }
+        }
+        catch (...)
+        {
+            complete_error(chunk_survivors, std::current_exception());
+            return false;
+        }
+    }
+
+    /// [CKPT-FAILED-BIRTH-DEBRIS] REMOVED (increment review Critical B, BACKLOG `{#ckpt-failed-birth-debris}`
+    /// reopened, `{#ckpt-neverborn-gc-backstop}` filed): the best-effort cleanup that used to live here
+    /// deleted `_ckpt` by a FRESH `head()` read at cleanup time, never a token captured from this
+    /// attempt's own publish -- so it deleted WHATEVER was at the key when it ran, not proven still to
+    /// be this attempt's bytes. All three of its call sites sit inside the `CORRUPTED_DATA` path, below
+    /// `getCurrentExceptionCode() != ErrorCodes::CORRUPTED_DATA` -- i.e. every branch that called it had
+    /// just PROVEN a different object occupies the derived key, directly contradicting the safety
+    /// argument's premise ("reachable only while the namespace's ref-log has never durably held
+    /// anything"). A namespace's ref-log with a live occupant at the derived key is not empty by the
+    /// very fact that triggered the call. Concretely: a successor that legitimately owns the same live
+    /// incarnation (a remount, INV-2's ordinary epoch-seal handoff) may have already read `_ckpt` for
+    /// its own recovery before this cleanup ran, and the delete could destroy a genesis record
+    /// (`life_epoch`, recorded nowhere else, no repair path) that successor's own future recovery still
+    /// needs -- with no way to tell that case apart from ordinary debris at cleanup time. A captured
+    /// token does not close this either: a successor that only READ `_ckpt` (never re-wrote it) leaves
+    /// the token matching, so a token-gated delete would still succeed and destroy what that successor
+    /// leaned on. Removed rather than patched, per the project's own fail-close principle: never take a
+    /// destructive action on a fallback path when the safe alternative is to skip it and surface the
+    /// gap. The trade, named rather than left implicit: debris now SURVIVES (a drained server root
+    /// carrying it will refuse decommission, `claimOwnerOrThrow` -> `CORRUPTED_DATA`, until a backstop
+    /// exists) instead of risking an unrecoverable delete of a live successor's genesis record. The
+    /// backstop -- independently re-verifying emptiness with a real LIST, never inferring it from one
+    /// attempt's own conflict -- is `{#ckpt-neverborn-gc-backstop}`, not built here.
+
+    /// Install the exact attempt before the first possible send. This is NOT preparation -- it mutates
+    /// `RefTableRuntime` and takes `state_mutex` -- so it stays here, between preparation and the first
+    /// send. From this point every exit must make one explicit lane transition; there is no independent
+    /// marker to update or reconstruct later.
+    bool attempt_armed = false;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        const bool same_base = rt->state.getGreatestApplied() == candidate_base_id;
+        if (rt->lane_state == RefLaneState::Ready && !rt->append_attempt && same_base)
+        {
+            static_assert(std::is_nothrow_move_constructible_v<RefAppendAttempt>);
+            rt->append_attempt = std::move(prepared->prepared_attempt);
+            rt->lane_state = RefLaneState::Writing;
+            attempt_armed = true;
+        }
+    }
+    if (!attempt_armed)
+    {
+        /// NO cleanup call here (review C1, and the whole class removed by increment review Critical B):
+        /// this arm makes NO lane transition -- it is reached whenever the lane is not what THIS attempt
+        /// expected, including `getGreatestApplied() != candidate_base_id`, which means some OTHER
+        /// append for this table already advanced applied state. That other append can be the birth
+        /// itself, whose `_ckpt` a cleanup call here would then delete out from under it -- the same harm
+        /// class the ambiguous `Writing -> Wedged` branch is deliberately excluded to avoid, against an
+        /// object with no repair path (BACKLOG `{#ckpt-damage-no-repair-path}`). "`putIfAbsentControlled`
+        /// was never reached" is true of THIS attempt; it says nothing about whether a DIFFERENT attempt
+        /// for this same namespace already made the birth durable. Now that Critical B removed the other
+        /// call sites too, `_ckpt` debris from a never-born namespace is reclaimed only by the future
+        /// GC-level backstop (`{#ckpt-neverborn-gc-backstop}`), never here.
+        complete_error(chunk_survivors, std::make_exception_ptr(Exception(
+            ErrorCodes::LOGICAL_ERROR,
+            "CAS ref-log append for namespace '{}': lane changed before attempt {}-{} could be armed",
+            ns.string(), id.writer_epoch, id.ref_sequence)));
+        return false;
+    }
+    const RefAppendAttempt & active_attempt = *rt->append_attempt;
+
+    CasWriteOutcome outcome{};
+    /// WHY an Unresolved came back. Two jobs (finding #37 defect 3): the wedge message stops claiming an
+    /// exhausted retry budget when in fact no request was ever sent, and -- see the `Unresolved` arm --
+    /// the one reason that PROVES nothing was sent decides whether the lane wedges at all.
+    CasUnresolvedReason unresolved_reason = CasUnresolvedReason::NotUnresolved;
+    try
+    {
+        outcome = ref_request_controller->putIfAbsentControlled(
+            active_attempt.key, active_attempt.bytes, fence_ok, /*out_token=*/nullptr, &unresolved_reason);
+    }
+    catch (...)
+    {
+        const std::exception_ptr write_error = std::current_exception();
+        /// `putIfAbsentControlled` throws CORRUPTED_DATA when resolve-before-reissue observes a DIFFERENT
+        /// object already at this txn's key -- a proven different-object conflict, not an unresolved PUT.
+        /// Any other exception after the send boundary is ambiguous and therefore transfers ownership
+        /// from `Writing` to `Wedged`; the exact attempt remains installed.
+        if (getCurrentExceptionCode() != ErrorCodes::CORRUPTED_DATA)
+        {
+            {
+                std::lock_guard lock(rt->state_mutex);
+                if (rt->lane_state == RefLaneState::Writing && rt->append_attempt
+                    && rt->append_attempt->txn_id == id)
+                    rt->lane_state = RefLaneState::Wedged;
+            }
+            complete_error(chunk_survivors, write_error);
+            return false;
+        }
+        /// THREE-WAY ADJUDICATION, the same one the wedge resolution owes [review HIGH-2]. "A different
+        /// object at our derived key" is not one situation but two, and they call for opposite
+        /// reactions. One of them is EXPECTED: a successor that sealed our epoch put its epoch-closing
+        /// record at exactly the id we keep re-deriving, and INV-2 says we must keep re-deriving it
+        /// ("a dying lane that observes the seal retries T+1, never mints T+2"). Treating that as
+        /// foreign interference would fence the mount and raise an anomaly alarm on the designed path.
+        /// The other is a genuine breach of write-exclusivity and must be exactly as loud as before.
+        ///
+        /// The occupant is read once, by exact key, here -- `putIfAbsentControlled` proved the mismatch
+        /// but does not hand back what it saw. One extra request on a path that is already exceptional
+        /// and already fatal to this attempt.
+        Occupant occupant = Occupant::Foreign;
+        bool classified = false;
+        try
+        {
+            if (const auto got = backend.get(active_attempt.key))
+            {
+                occupant = classifyRefLogOccupant(ns, id, got->bytes, active_attempt.bytes);
+                classified = true;
+            }
+        }
+        catch (...)   // NOLINT(bugprone-empty-catch)
+        {
+            /// Left unclassified deliberately -- see below. The original conflict is what the survivors
+            /// are told about; this read's own failure is not their business.
+        }
+        if (!classified)
+        {
+            /// We could not learn WHICH of the two this is, so we decide NEITHER. Reporting foreign
+            /// interference would fence the mount on a guess, and reporting a conclusive rejection would
+            /// acknowledge a deposition we did not observe. The id is not consumed and nothing is
+            /// recorded, so the next append re-derives the same id, meets the same conflict, and
+            /// classifies again -- deferring costs one round trip and decides nothing wrongly.
+            ///
+            /// It must be COUNTED, because deferring is the one arm here that is quiet by construction:
+            /// the loud interference report is only reached once the occupant can be read, so a real
+            /// breach whose occupant keeps failing to read would otherwise show up as nothing but a
+            /// throttled log line under load. Sustained growth on this counter is the signal that the
+            /// loud path is being starved.
+            ProfileEvents::increment(ProfileEvents::CASRefAppendOccupantUnreadable);
+            {
+                std::lock_guard lock(rt->state_mutex);
+                rt->append_attempt.reset();
+                rt->lane_state = RefLaneState::Faulted;
+            }
+            complete_error(chunk_survivors, std::make_exception_ptr(Exception(
+                ErrorCodes::CORRUPTED_DATA,
+                "CAS ref-log append for namespace '{}': a DIFFERENT object occupies the id {}-{} this table "
+                "derived, and reading it to tell a successor's epoch seal from foreign interference did not "
+                "succeed — the lane is faulted until remount recovery adjudicates durable state",
+                ns.string(), id.writer_epoch, id.ref_sequence)));
+            return false;
+        }
+        if (occupant == Occupant::SuccessorSeal)
+        {
+            /// Conclusive rejection, identical in meaning to the wedge site's: our bytes provably never
+            /// landed and never can, the operation was never acknowledged, and the seal IS this
+            /// namespace's epoch-closing record. No anomaly, no fence -- this is the protocol working.
+            /// Counted anyway: "the protocol working" here means THIS writer was deposed, and a lane that
+            /// keeps landing on this arm is a mount that has lost its lease and does not know it yet.
+            ProfileEvents::increment(ProfileEvents::CASRefAppendSealRejected);
+            {
+                std::lock_guard lock(rt->state_mutex);
+                rt->last_epoch_seal = id;
+                rt->append_attempt.reset();
+                rt->lane_state = RefLaneState::Closed;
+            }
+            complete_error(chunk_survivors, std::make_exception_ptr(Exception(ErrorCodes::INVALID_STATE,
+                "CAS ref-log append for namespace '{}': writer epoch {} was CLOSED by a successor's epoch "
+                "seal at {}-{}, which conclusively rejects this transaction (it was never acknowledged). "
+                "This mount's append lane resumes only under a later epoch",
+                ns.string(), id.writer_epoch, id.writer_epoch, id.ref_sequence)));
+            return false;
+        }
+        /// A genuine breach. This table's appends are now BLOCKED, and that is the intended contract:
+        /// under mount-lease exclusivity this key is exclusively ours, so a foreign object at it is
+        /// corruption or a protocol breach, not a race. The id is not consumed, so the next attempt
+        /// derives the SAME id and hits the SAME conflict, loudly, until a remount-level recovery (a
+        /// fresh writer epoch is a fresh key namespace) clears it. Advancing past the occupant, which is
+        /// what the pool-wide allocator did, would have written this table's stream around a foreign
+        /// object and hidden the violation -- and produced the hole INV-1 exists to forbid.
+        ///
+        /// Route it through the anomaly policy, exactly as the wedge-resolution site does for the
+        /// identical observation [review I5]. Failing closed is right, but failing closed FOREVER is
+        /// not: without this the mount stays blocked on this table until somebody notices and remounts
+        /// by hand. One impossibility, one reaction. The report is deliberately BEFORE the survivors are
+        /// completed, so the fence is closed by the time any caller wakes and can retry.
+        const String attempt_key = active_attempt.key;
+        {
+            std::lock_guard lock(rt->state_mutex);
+            rt->append_attempt.reset();
+            rt->lane_state = RefLaneState::Faulted;
+        }
+        on_impossible_interference(attempt_key,
+            fmt::format("ref-log append for namespace '{}' txn {}-{} observed a DIFFERENT object already at "
+                "the id it derived, and it is not an epoch seal of this namespace ({})",
+                ns.string(), id.writer_epoch, id.ref_sequence,
+                getCurrentExceptionMessage(/*with_stacktrace*/ false)),
+            ns.string());
+        complete_error(chunk_survivors, write_error);
+        return false;
+    }
+    switch (outcome)
+    {
+        case CasWriteOutcome::Committed:
+        {
+            /// A durable log object is not yet admitted to logical history. Publish its exact frontier
+            /// under the SAME admission generation before any local consequence can make a later id
+            /// observable or wake a waiter. `Published` and `IdenticalSkip` both prove the contribution
+            /// durable. `FencedOut`, contention exhaustion, decode failure, or any other unresolved
+            /// publication leaves the log known durable but uninstalled, which is exactly
+            /// `NeedsRecovery`; recovery owns resolution of that window.
+            const auto check_commit_admitted = [this, &rt](uint64_t expected_generation)
+            {
+                check_fence_or_throw(expected_generation);
+                if (rt->catalog_life_invalidated.load(std::memory_order_acquire)
+                    || rt->superseded_by_remount.load(std::memory_order_acquire))
+                    throwCasWriteRetryLater(fmt::format(
+                        "CAS namespace '{}': its captured runtime was retired before committed-frontier publication",
+                        rt->life.ns.string()));
+            };
+            CkptPublishOutcome frontier_outcome = CkptPublishOutcome::FencedOut;
+            try
+            {
+                frontier_outcome = publishCkptContribution(
+                    rt->life, prepared->commit_contribution, admitted_fence_generation, check_commit_admitted);
+            }
+            catch (...)
+            {
+                const std::exception_ptr frontier_error = std::current_exception();
+                {
+                    std::lock_guard lock(rt->state_mutex);
+                    requireRecovery(*rt, ns, "committed-frontier publication");
+                }
+                complete_error(chunk_survivors, frontier_error);
+                return false;
+            }
+            if (frontier_outcome == CkptPublishOutcome::FencedOut)
+            {
+                {
+                    std::lock_guard lock(rt->state_mutex);
+                    requireRecovery(*rt, ns, "committed-frontier publication fence");
+                }
+                complete_error(chunk_survivors, makeCasWriteRetryLaterExceptionPtr(fmt::format(
+                    "CAS ref-log append for namespace '{}': txn {}-{} is durable, but the mount fence "
+                    "moved before its checkpoint frontier was published; the lane needs recovery",
+                    ns.string(), id.writer_epoch, id.ref_sequence)));
+                return false;
+            }
+
+            if (carve_hook_for_test)
+                carve_hook_for_test(CarvePhaseForTest::PostDurableInstall);
+            bool install_refused = false;
+            std::exception_ptr install_admission_error;
+            {
+                std::lock_guard lock(rt->state_mutex);
+                /// The checkpoint CAS can succeed and the fence can move before the state lock is
+                /// reached. Re-present the same admission INSIDE the install hold, immediately before
+                /// inspecting and swapping the candidate. A stale runtime may leave both log and
+                /// frontier durable, but it must neither install nor acknowledge them.
+                try
+                {
+                    check_commit_admitted(admitted_fence_generation);
+                }
+                catch (...)
+                {
+                    install_admission_error = std::current_exception();
+                    requireRecovery(*rt, ns, "post-frontier install admission");
+                }
+                /// Only this leader mutates `rt->state`, so the candidate's base snapshot is still the
+                /// current one: there is one append-lane leader per table at a time (the `leader_active`
+                /// baton), the wedge-resolution apply ran earlier in this same flush on this same thread,
+                /// recovery installs a state exactly once per runtime and has already completed for this
+                /// table, and every other consumer (readers, the snapshot publisher) only COPIES the
+                /// state under this mutex. Evaluated here, one statement before the install, and
+                /// asserted inside it: the comparison allocates nothing, and the identifier is short
+                /// enough that even the failure path's message is inline-buffered rather than heap
+                /// allocated, so no build can turn the assert itself into an allocation in the region.
+                const bool state_unchanged
+                    = !install_admission_error
+                    && rt->lane_state == RefLaneState::Writing
+                    && rt->append_attempt
+                    && rt->append_attempt->txn_id == id
+                    && rt->append_attempt->bytes == active_attempt.bytes
+                    && rt->state.getGreatestApplied() == candidate_base_id;
+                if (!install_admission_error && !state_unchanged)
+                {
+                    /// RELEASE-mode counterpart of the `chassert` inside the region below, which is a
+                    /// no-op in a release build and therefore no guard at all for a window that spans a
+                    /// full network round trip. Swapping the candidate in anyway would DISCARD whatever
+                    /// advanced the table. The object is durable and this runtime cannot record it,
+                    /// `LOGICAL_ERROR` here, where the wedge site's identical refusal reports the
+                    /// retry-later class, and the asymmetry is deliberate: THIS one is reachable only by
+                    /// a second writer inside one process -- a bug in this build, which a debug build
+                    /// should abort on and shout about. The wedge site's is reachable by an ordinary
+                    /// remount racing a slow resolution, which is a retryable fact about the world, not a
+                    /// bug. Same refusal, different provenance, so different loudness.
+                    requireRecovery(*rt, ns, "commitRefChunk install");
+                    install_refused = true;
+                }
+                else if (!install_admission_error)
+                {
+                    std::optional<RefAppendAttempt> completed_attempt;
+                    try
+                    {
+                        DENY_ALLOCATIONS_IN_SCOPE;
+                        if (install_region_probe_for_test)
+                            install_region_probe_for_test();
+                        chassert(state_unchanged);
+                        rt->state.swap(*candidate);
+                        rt->tail_count_since_snapshot.fetch_add(1, std::memory_order_relaxed);
+                        rt->tail_bytes_since_snapshot.fetch_add(active_attempt.bytes.size(), std::memory_order_relaxed);
+                        rt->append_attempt.swap(completed_attempt);
+                        rt->lane_state = RefLaneState::Ready;
+                    }
+                    catch (...)
+                    {
+                        requireRecovery(*rt, ns, "commitRefChunk install");
+                        throw;
+                    }
+                    candidate.reset();
+                    completed_attempt.reset();
+                    try
+                    {
+                        rt->state.materializeCommitted();
+                    }
+                    catch (...)
+                    {
+                        tryLogCurrentException(getLogger("CasPool"), fmt::format(
+                            "CAS ref-log append for namespace '{}': committed txn {}-{} was applied durably, but "
+                            "the post-commit overlay fold failed and was retained coherently for the next flush",
+                            ns.string(), id.writer_epoch, id.ref_sequence));
+                    }
+                }
+            }
+            if (install_admission_error)
+            {
+                complete_error(chunk_survivors, install_admission_error);
+                return false;
+            }
+            if (install_refused)
+            {
+                complete_error(chunk_survivors, std::make_exception_ptr(Exception(
+                    ErrorCodes::LOGICAL_ERROR,
+                    "CAS ref-log append for namespace '{}': txn {}-{} is durable but this table changed "
+                    "before installation; the lane needs recovery and refuses later writes until replay",
+                    ns.string(), id.writer_epoch, id.ref_sequence)));
+                return false;
+            }
+            if (carve_hook_for_test)
+                carve_hook_for_test(CarvePhaseForTest::PostInstallPreAck);
+            ProfileEvents::increment(ProfileEvents::CASRefBatchFlushes);
+            ProfileEvents::increment(ProfileEvents::CASRefBatchedMutations, chunk_survivors.size());
+            {
+                std::lock_guard<std::mutex> g(ref_queue_mutex);
+                for (const auto & it : chunk_survivors)
+                {
+                    it->committed_id = id;
+                    it->done = true;
+                }
+                rt->cv.notify_all();
+            }
+            /// The threshold trigger -- off the lane,
+            /// dispatched AFTER waking every waiter above so this commit's own callers are never
+            /// delayed by it. Per chunk (spec §3): each committed chunk schedules its own publication,
+            /// and settlement coalesces the triggers so a mid-tenure publisher never suppresses a later
+            /// chunk (`settleSnapshotPublish`).
+            maybeScheduleSnapshotPublish(ns, rt);
+            return true;
+        }
+        case CasWriteOutcome::DefiniteFailure:
+        {
+            /// Proof that nothing became durable returns the exact attempt to `Ready`.
+            {
+                std::lock_guard lock(rt->state_mutex);
+                if (rt->lane_state == RefLaneState::Writing && rt->append_attempt
+                    && rt->append_attempt->txn_id == id)
+                {
+                    rt->append_attempt.reset();
+                    rt->lane_state = RefLaneState::Ready;
+                }
+            }
+            ProfileEvents::increment(ProfileEvents::CASRefAppendDefiniteFailure);
+            complete_error(chunk_survivors, makeCasWriteRetryLaterExceptionPtr(fmt::format(
+                "CAS ref-log append for namespace '{}' definitively failed (non-retryable rejection); "
+                "cached state is unchanged and txn id {}-{} was never used (a retry re-derives it)",
+                ns.string(), id.writer_epoch, id.ref_sequence)));
+            return false;
+        }
+        case CasWriteOutcome::Unresolved:
+        {
+            /// The ONE `Unresolved` shape that must NOT wedge (finding #37 defect 3). The wedge exists
+            /// because an `Unresolved` PUT MAY HAVE LANDED: the durable log may or may not contain this
+            /// transaction, only `resolveByExactGet` on that exact key can settle it, and until it does,
+            /// minting a later id would build on a state that may be missing a landed transaction. All of
+            /// that presupposes an attempt was SENT.
+            ///
+            /// `unresolvedProvesNothingWasSent` is true only for `NoAttemptSent`, which
+            /// `putIfAbsentControlled` reports only when a pre-attempt gate -- the mount fence or the
+            /// operation deadline -- rejected while `attempts_sent == 0`, i.e. strictly before the first
+            /// `backend->putIfAbsent`. Nothing reached the network, so the key is provably unwritten:
+            /// there is nothing for a wedge to resolve, and wedging is pointless.
+            ///
+            /// It is no longer HARMFUL, and the difference is worth stating because the old comment here
+            /// rested on it: a wedge over a never-written key used to be unclearable, because resolution
+            /// was a bare read and a read can only ever report absent. The every-attempt rule replaced
+            /// that with a conditional CREATE, so such a wedge now clears on the next caller's flush by
+            /// landing the transaction. What remains is that this lane would be blocked until then for no
+            /// reason at all -- a transient fence blip in the pre-attempt gate would cost the table its
+            /// write availability, and buy nothing, since there is provably nothing to resolve.
+            ///
+            /// The counterexample this argument deliberately excludes: a fence lost or a deadline reached
+            /// AFTER at least one attempt is `FenceLostMidWay`/`DeadlineMidWay`, and an attempt that
+            /// COMMITTED but returned under a dropped fence is `FenceLostPostWrite`. Each of those may
+            /// have left a durable object, so each keeps wedging -- as does anything a future contributor
+            /// adds to the enum without classifying it (see the predicate's allow-list construction).
+            if (unresolvedProvesNothingWasSent(unresolved_reason))
+            {
+                {
+                    std::lock_guard lock(rt->state_mutex);
+                    if (rt->lane_state == RefLaneState::Writing && rt->append_attempt
+                        && rt->append_attempt->txn_id == id)
+                    {
+                        rt->append_attempt.reset();
+                        rt->lane_state = RefLaneState::Ready;
+                    }
+                }
+                /// Count it. Before this arm existed these refusals bumped `CASRefAppendWedged`, so
+                /// removing the wedge also removed the only signal they were happening at all -- and a
+                /// soak oracle watching that counter fall could not tell "the fix works" from "nothing
+                /// happened". A separate event keeps both readings available: the wedge counter now means
+                /// only genuinely ambiguous appends, and this one means availability preserved.
+                ProfileEvents::increment(ProfileEvents::CASRefAppendPreAttemptRefused);
+                /// The id is not consumed (INV-1): it was derived from `greatest_applied`, which this
+                /// refusal leaves exactly as it was, so the next caller on this table derives the SAME id
+                /// and the durable stream keeps no trace of the refusal. That is the free half of the
+                /// every-attempt rule -- an attempt that provably sent nothing owes nothing.
+                /// The installed attempt is retired below; no request was sent.
+                /// and is what makes the genuinely ambiguous path below allocation-free.
+                complete_error(chunk_survivors, makeCasWriteRetryLaterExceptionPtr(fmt::format(
+                    "CAS ref-log append for namespace '{}' txn {}-{} was refused BEFORE any request was "
+                    "sent ({}) — the append lane is NOT wedged (nothing can be durable, so there is "
+                    "nothing to resolve) and the txn id is not consumed (a retry re-derives it)",
+                    ns.string(), id.writer_epoch, id.ref_sequence,
+                    describeUnresolvedReason(unresolved_reason))));
+                return false;
+            }
+            {
+                std::lock_guard lock(rt->state_mutex);
+                if (rt->lane_state == RefLaneState::Writing && rt->append_attempt
+                    && rt->append_attempt->txn_id == id)
+                    rt->lane_state = RefLaneState::Wedged;
+            }
+            ProfileEvents::increment(ProfileEvents::CASRefAppendWedged);
+            complete_error(chunk_survivors, makeCasWriteRetryLaterExceptionPtr(fmt::format(
+                "CAS ref-log append for namespace '{}' txn {}-{} is UNCERTAIN ({}) — "
+                "the append lane is wedged until the SAME key resolves durable or a conclusive rejection "
+                "is observed; this outcome is unproven, not failure",
+                ns.string(), id.writer_epoch, id.ref_sequence,
+                describeUnresolvedReason(unresolved_reason))));
+            return false;
+        }
+    }
+    /// Unreachable: the switch above covers every `CasWriteOutcome`. Kept explicit so the function has a
+    /// defined return on all control-flow paths.
+    return false;
+}
+
+bool CasRefLedger::hasStateBearingSnapshotCandidateUnderStateLock(const RefTableRuntime & rt) const
+{
+    /// The newest snapshot must be strictly older than the candidate. A seal advances epoch geometry
+    /// but carries no table state, so it is never a snapshot candidate.
+    return rt.state.getLifecycle() == RefLifecycle::Live
+        && (!rt.newest_snapshot_id || *rt.newest_snapshot_id < rt.state.getGreatestApplied())
+        && (!rt.last_epoch_seal || *rt.last_epoch_seal != rt.state.getGreatestApplied());
+}
+
+bool CasRefLedger::admitSnapshotPublishUnderStateLock(RefTableRuntime & rt)
+{
+    /// Caller holds `rt.state_mutex` (the `may_mutate` fence check is the caller's responsibility, since
+    /// it is not held under `state_mutex`). The whole decision -- the threshold trigger, the
+    /// single-in-flight gate, the backoff deadline -- and the `pending_snapshot_publishes` increment all
+    /// happen under that ONE hold, so two racing dispatchers can never both admit a publish for this
+    /// table, and the settlement re-evaluation can decrement-and-re-admit without the count transiently
+    /// reaching zero.
+    const uint64_t now = boot_ms_now_fn();
+    if (!rt.catalog_life_invalidated.load(std::memory_order_acquire)
+        && !rt.superseded_by_remount.load(std::memory_order_acquire)
+        /// Use the execution predicate at admission too, so settlement cannot redispatch a recovered seal.
+        && hasStateBearingSnapshotCandidateUnderStateLock(rt)
+        /// Single-in-flight gate: at most one background publish per table.
+        && rt.pending_snapshot_publishes.load(std::memory_order_relaxed) == 0
+        /// Backoff deadline: after a non-Committed publish, a saturated backend is not re-dispatched
+        /// until the bounded backoff elapses (the read-triggered PUT-storm latch).
+        && now >= rt.publish_backoff_until_ms)
+    {
+        /// The threshold trigger reads the tail counters directly -- no walk, no age filter.
+        /// `tail_count_since_snapshot`/`tail_bytes_since_snapshot` count ONLY applied txns strictly above
+        /// `newest_snapshot_id` (maintained incrementally by every commit in `commitRefChunk` and by the
+        /// wedge-resolution apply in `flushRefBatch`), so `over_threshold` here is never true without a
+        /// real, immediately-coverable candidate.
+        const uint64_t publishable_count = rt.tail_count_since_snapshot.load(std::memory_order_relaxed);
+        const uint64_t publishable_bytes = rt.tail_bytes_since_snapshot.load(std::memory_order_relaxed);
+        const bool over_threshold = publishable_count > config.snapshot_log_count_threshold
+            || publishable_bytes > config.snapshot_log_bytes_threshold;
+        if (over_threshold)
+        {
+            rt.pending_snapshot_publishes.fetch_add(1, std::memory_order_relaxed);
+            return true;
+        }
+    }
+    return false;
+}
+
+void CasRefLedger::dispatchSnapshotPublisher(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt)
+{
+    /// `admitSnapshotPublishUnderStateLock` already incremented `pending_snapshot_publishes` for THIS
+    /// dispatch. Off the mutation hot path: `tryPublishSnapshotAndAdvanceCheckpointOnce` never touches
+    /// the append queue, so dispatching it onto an unrelated global-pool thread can never deadlock a flush leader.
+    /// `pin_owner()` (the Pool's `shared_from_this`) keeps the Pool -- and hence this ledger member --
+    /// alive for the thread's lifetime.
+    ProfileEvents::increment(ProfileEvents::CASRefSnapshotPublishDispatched);
+    auto owner = pin_owner();
+    try
+    {
+        ThreadFromGlobalPool([owner, this, ns, rt]
+        {
+            setThreadName(ThreadName::CAS_REF_SNAPSHOT_PUBLISH);
+            try
+            {
+                tryPublishSnapshotAndAdvanceCheckpointOnceOnRuntime(ns, rt);
+            }
+            catch (...)
+            {
+                tryLogCurrentException(getLogger("CasPool"), "CAS background snapshot publish attempt failed");
+            }
+            settleSnapshotPublish(ns, rt);
+        }).detach();
+    }
+    catch (...)
+    {
+        /// The `ThreadFromGlobalPool` ctor can throw (pool exhaustion) AFTER the count was incremented.
+        /// Undo the count WITHOUT the settlement re-evaluation (else a persistently-failing dispatch could
+        /// re-fire itself in a loop) and SWALLOW the failure: dispatching a background publish is a
+        /// best-effort maintenance trigger and must never fail an otherwise-successful read or mutation.
+        /// The next trigger reschedules.
+        {
+            std::lock_guard lock(rt->state_mutex);
+            rt->pending_snapshot_publishes.fetch_sub(1, std::memory_order_relaxed);
+        }
+        rt->publish_settle_cv.notify_all();
+        tryLogCurrentException(getLogger("CasPool"), "CAS background snapshot-publish dispatch failed to launch");
+    }
+}
+
+void CasRefLedger::settleSnapshotPublish(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt)
+{
+    /// Fence re-checked outside `state_mutex` (as in `maybeScheduleSnapshotPublish`): a fence lost
+    /// between this publish's dispatch and its settlement must suppress a follow-up.
+    const bool live_mount = may_mutate();
+    bool redispatch = false;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        /// Drop THIS publish's in-flight count and, under the SAME hold, re-evaluate the accumulated
+        /// tail. A chunked tenure (or any concurrent mutation) that raised more log above the newest
+        /// snapshot while this publish was capturing an earlier prefix had its trigger discarded by the
+        /// single-flight gate; settlement re-fires it here so chunks 2..N are not suppressed until an
+        /// unrelated later trigger (spec §3 snapshot coalescing). Re-admitting under the SAME lock as the
+        /// decrement means `pending_snapshot_publishes` never transiently reaches 0 across the handoff, so
+        /// `waitForSnapshotPublishSettleForTest` never observes a false "settled". A durable publish
+        /// already subtracted its captured tail, so this self-terminates once the tail is back at/under
+        /// threshold; a non-durable one armed the backoff, which `admit...` respects -- no PUT storm.
+        rt->pending_snapshot_publishes.fetch_sub(1, std::memory_order_relaxed);
+        if (live_mount)
+            redispatch = admitSnapshotPublishUnderStateLock(*rt);
+    }
+    if (redispatch)
+        dispatchSnapshotPublisher(ns, rt);
+    else
+        rt->publish_settle_cv.notify_all();
+}
+
+void CasRefLedger::maybeScheduleSnapshotPublish(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt)
+{
+    /// Never dispatch a publisher while the fence is lost: a publish is a
+    /// conditional PUT that would fail `fence_ok` and return non-Committed anyway, and dispatching one
+    /// during the self-remount window is exactly the stale-cache-publish race the remount quiesce closes
+    /// -- with no dispatch here, `quiesceRefTablesForRemount` only has to drain publishers already in
+    /// flight before the fence dropped, never a moving target.
+    if (!may_mutate())
+        return;
+
+    bool dispatch = false;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        dispatch = admitSnapshotPublishUnderStateLock(*rt);
+    }
+    if (dispatch)
+        dispatchSnapshotPublisher(ns, rt);
+}
+
+
+void CasRefLedger::advancePublishBackoff(RefTableRuntime & rt)
+{
+    /// Caller holds `rt.state_mutex`. Double the interval from `initial` up to `max` per consecutive
+    /// non-Committed publish outcome; arm the deadline off the boottime clock (`bootMsNow`), so an
+    /// injected test clock drives it deterministically and a VM-suspend cannot shorten it.
+    rt.publish_backoff_ms = rt.publish_backoff_ms == 0
+        ? config.snapshot_publish_backoff_initial_ms
+        : std::min<uint64_t>(rt.publish_backoff_ms * 2, config.snapshot_publish_backoff_max_ms);
+    rt.publish_backoff_until_ms = boot_ms_now_fn() + rt.publish_backoff_ms;
+    ProfileEvents::increment(ProfileEvents::CASRefSnapshotPublishBackoff);
+}
+
+void CasRefLedger::resetPublishBackoff(RefTableRuntime & rt)
+{
+    /// Caller holds `rt.state_mutex`. A durable publish clears the cooldown.
+    rt.publish_backoff_ms = 0;
+    rt.publish_backoff_until_ms = 0;
+}
+
+void CasRefLedger::waitForSnapshotPublishSettleForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return;
+    std::unique_lock lock(rt->state_mutex);
+    rt->publish_settle_cv.wait(lock, [&] { return rt->pending_snapshot_publishes.load(std::memory_order_relaxed) == 0; });
+}
+
+int CasRefLedger::pendingSnapshotPublishesForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return 0;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->pending_snapshot_publishes.load(std::memory_order_relaxed);
+}
+
+std::optional<RefTxnId> CasRefLedger::newestPublishedSnapshotIdForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return std::nullopt;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->newest_snapshot_id;
+}
+
+bool CasRefLedger::refRecoveryCancelRequestedForTest(const RootNamespace & ns)
+{
+    std::lock_guard<std::mutex> qlock(ref_queue_mutex);
+    const auto it = ref_name_slots.find(ns.string());
+    return it != ref_name_slots.end() && it->second.current
+        && it->second.current->recovery_cancel_requested.load(std::memory_order_acquire);
+}
+
+bool CasRefLedger::refTableRecoveredForTest(const RootNamespace & ns)
+{
+    /// Like every observational seam, deliberately does not recover: the fail-closed tests ask "did that
+    /// refused recovery install anything", and an observer that recovered on demand would answer its own
+    /// question with a yes.
+    std::lock_guard<std::mutex> qlock(ref_queue_mutex);
+    const auto it = ref_name_slots.find(ns.string());
+    if (it == ref_name_slots.end() || !it->second.current)
+        return false;
+    std::lock_guard lock(it->second.current->state_mutex);
+    return it->second.current->recovered;
+}
+
+size_t CasRefLedger::tailSinceSnapshotCountForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return 0;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->tail_count_since_snapshot.load(std::memory_order_relaxed);
+}
+
+size_t CasRefLedger::committedOverlayEntriesForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return 0;
+    std::lock_guard lock(rt->state_mutex);
+    return rt->state.getCommitted().overlayEntriesForTest();
+}
+
+std::set<std::pair<String, ManifestRef>> CasRefLedger::livePrecommitsForTest(const RootNamespace & ns)
+{
+    const auto rt = lookupRefTableRuntime(ns);
+    if (!rt)
+        return {};
+    std::lock_guard lock(rt->state_mutex);
+    return rt->state.getPrecommits();
+}
+
+namespace
+{
+/// A clamped-to-zero fetch-subtract for the tail counters.
+/// `tryPublishSnapshotAndAdvanceCheckpointOnce` is public and NOT serialized against itself (two
+/// overlapping attempts can finish out of order), so the monotonic guard
+/// below
+/// skips a stale (superseded) adoption's subtraction outright, but it cannot see a SMALLER-candidate
+/// attempt that lands its adoption BEFORE a larger-candidate one already in flight: that ordering would
+/// have the larger attempt's `captured_count`/`captured_bytes` double-count the smaller one's
+/// already-subtracted region. A plain `fetch_sub` would then underflow the unsigned counter, wrapping it
+/// to near `UINT64_MAX` and permanently re-latching the read-triggered PUT-storm trigger on every
+/// subsequent read -- a release-build regression of the exact bug this guard prevents. Clamping to
+/// zero instead settles for a
+/// benign, self-healing under-count (a delayed next dispatch; the NEXT publish always captures the true
+/// live state fresh, so snapshot CONTENT is never affected) over an unsafe wraparound.
+void clampedCounterSub(std::atomic<uint64_t> & counter, uint64_t amount)
+{
+    uint64_t old_value = counter.load(std::memory_order_relaxed);
+    while (!counter.compare_exchange_weak(old_value, old_value > amount ? old_value - amount : 0,
+        std::memory_order_relaxed))
+    {
+    }
+}
+}
+
+
+CkptPublishOutcome CasRefLedger::publishCkptContribution(const NamespaceLifeId & life, const RefCkpt & contribution,
+                                                         uint64_t admitted_generation,
+                                                         const std::function<void(uint64_t)> & check_admission)
+{
+    /// The retry window is the SAME budget every other CAS operation of this ledger rides, measured on
+    /// the ledger's own injectable boot clock -- so a test drives the exhaustion arm without sleeping,
+    /// and a VM suspend cannot shorten it.
+    const CkptDeadline deadline{boot_ms_now_fn, boot_ms_now_fn() + cas_request_budget.operation_deadline_ms};
+    const CkptPublishOutcome outcome = publishCkpt(
+        backend, layout, life, contribution, admitted_generation, check_admission, deadline);
+    if (outcome == CkptPublishOutcome::Published)
+        ProfileEvents::increment(ProfileEvents::CASRefCheckpointPublished);
+    else if (outcome == CkptPublishOutcome::IdenticalSkip)
+        ProfileEvents::increment(ProfileEvents::CASRefCheckpointIdenticalSkip);
+    return outcome;
+}
+
+bool CasRefLedger::tryPublishSnapshotAndAdvanceCheckpointOnce(const RootNamespace & ns)
+{
+    const auto rt = acquireMutableRefTableRuntime(ns);
+    return tryPublishSnapshotAndAdvanceCheckpointOnceOnRuntime(ns, rt);
+}
+
+
+bool CasRefLedger::tryPublishSnapshotAndAdvanceCheckpointOnceOnRuntime(
+    const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt)
+{
+    ensureRefTableRecovered(ns, *rt);
+
+    /// This attempt's ADMISSION token, captured once, before any of its I/O: which mount incarnation
+    /// allowed this publish. It is presented back on every `_ckpt` CAS attempt below (spec §3's
+    /// recheck discipline -- the same value at every site), so a publish admitted under an incarnation
+    /// that has since been replaced can advance nothing.
+    const uint64_t admitted_generation = rt->admitted_fence_generation;
+    const auto runtime_still_admitted = [this, &rt, admitted_generation]
+    {
+        return !rt->catalog_life_invalidated.load(std::memory_order_acquire)
+            && !rt->superseded_by_remount.load(std::memory_order_acquire)
+            && fence_ok_fn()
+            && fence_generation_fn() == admitted_generation;
+    };
+
+    /// ONE copy of the live state, at a transaction boundary -- no
+    /// replay, no per-entry retention. The tail counters are captured in the SAME critical section so
+    /// adoption below subtracts exactly what this attempt's candidate actually covers.
+    RefTableState candidate_state;
+    RefTxnId candidate_x;
+    uint64_t captured_count = 0;
+    uint64_t captured_bytes = 0;
+    std::optional<RefLaneState> blocked_lane;
+    {
+        std::lock_guard lock(rt->state_mutex);
+        /// Snapshot certification is a `Ready`-only operation. This read and the state copy are one
+        /// critical section, so no transition to `Writing`, `Wedged`, or `NeedsRecovery` can interleave
+        /// between certification and capture.
+        if (rt->lane_state != RefLaneState::Ready)
+            blocked_lane = rt->lane_state;
+        else if (!hasStateBearingSnapshotCandidateUnderStateLock(*rt))
+            return false;   /// shares admission: terminal, covered, and seal candidates are all inert
+        else
+        {
+            candidate_state = rt->state;
+            candidate_x = rt->state.getGreatestApplied();
+            captured_count = rt->tail_count_since_snapshot.load(std::memory_order_relaxed);
+            captured_bytes = rt->tail_bytes_since_snapshot.load(std::memory_order_relaxed);
+        }
+    }
+
+    if (blocked_lane)
+    {
+        LOG_WARNING(getLogger("CasPool"),
+            "CAS ref table '{}': refusing snapshot publication while the append lane is not Ready "
+            "(state {})",
+            ns.string(), static_cast<uint8_t>(*blocked_lane));
+        return false;
+    }
+
+    if (snapshot_after_capture_hook_for_test)
+        snapshot_after_capture_hook_for_test();
+
+    /// The candidate holds shared COW bases, so even this stale-runtime exit must release it under the
+    /// same mutex that protects materialization. More importantly, this is the last test-only pause
+    /// before the first durable effect: retirement/remount can invalidate the captured runtime while it
+    /// is paused, and the old holder then becomes inert instead of resolving the name to a successor.
+    if (!runtime_still_admitted())
+    {
+        std::lock_guard lock(rt->state_mutex);
+        candidate_state = RefTableState{};
+        return false;
+    }
+
+    const RefTableSnapshot snap = snapshotOf(candidate_state, ns.string());
+
+    /// `candidate_state` is a COW copy that SHARES `rt->state`'s committed/owned-manifest bases. It is
+    /// dead past `snapshotOf`. Destroy it HERE, under `state_mutex` -- not at function return outside any
+    /// lock. Its destruction is a `shared_ptr` release-DECREMENT of those shared bases; the flush thread's
+    /// in-place `materializeCommitted()` reads their `use_count()` (relaxed) under this same mutex. Doing
+    /// the release off-lock would leave that load racing this atomic decrement with no happens-before
+    /// (TSan-reportable) and could momentarily let a flush observe a `use_count()` of 1 while this
+    /// decrement is in flight. Under the lock the two are serialized. Every subsequent exit path (encode
+    /// failure, non-Committed PUT, the monotonic-guard early return, success) then destroys an already
+    /// empty `candidate_state`, which touches no shared base. See both COW headers' materialize safety
+    /// argument, which relies on exactly this: every cross-thread copy is created AND destroyed under the
+    /// state lock.
+    {
+        std::lock_guard lock(rt->state_mutex);
+        candidate_state = RefTableState{};
+    }
+
+    String bytes;
+    try
+    {
+        bytes = sealObject(FormatId::RefSnapshot, encodeRefTableSnapshot(snap));
+    }
+    catch (...)
+    {
+        /// Failure Handling: "Snapshot create fails: keep all logs; writer recovery remains unchanged."
+        /// Treat like any other non-Committed outcome: arm the backoff so a persistent encode failure
+        /// does not re-dispatch on every read.
+        std::lock_guard lock(rt->state_mutex);
+        advancePublishBackoff(*rt);
+        return false;
+    }
+    const String key = layout.refSnapshotKey(rt->life, candidate_x);
+    const CasWriteOutcome outcome
+        = ref_request_controller->putIfAbsentControlled(key, bytes, runtime_still_admitted);
+    if (outcome != CasWriteOutcome::Committed)
+    {
+        /// DefiniteFailure/Unresolved: DO NOT prune (no durable covering snapshot -- pruning the tail
+        /// without one is data loss). Arm the bounded per-table backoff so the read path does not
+        /// re-dispatch this full-snapshot encode+PUT until it elapses -- the read-triggered PUT-storm
+        /// latch breaker. A later trigger past the deadline retries.
+        std::lock_guard lock(rt->state_mutex);
+        advancePublishBackoff(*rt);
+        return false;
+    }
+    ProfileEvents::increment(ProfileEvents::CASRefSnapshotPutBytes, bytes.size());   /// account published bytes
+
+    /// INV-4's SECOND `_ckpt` writer, at exactly the point the spec puts it: the snapshot body is
+    /// durable, and it becomes CLEANUP-AUTHORITATIVE only once the checkpoint names it. Ordering the
+    /// two this way is what makes the intervening race harmless -- cleanup planned between the body PUT
+    /// and this CAS still reads the OLD checkpoint, and the deletion gate is "strictly below" it, so it
+    /// cannot delete the snapshot just published.
+    ///
+    /// The contribution is the checkpoint ALONE. A publisher does not know this namespace's
+    /// `life_epoch` (it may have recovered the table from a snapshot that never replayed the birth), so
+    /// it contributes NOTHING for it and the semantic-max merge preserves whatever a writer that did
+    /// know has already recorded -- in either order.
+    ///
+    /// A checkpoint that does NOT advance leaves the attempt unadopted: the backoff is armed and this
+    /// returns false, so a later trigger re-runs the whole publish. The re-run's body PUT resolves to
+    /// `Committed` against its own identical bytes, so retrying costs one conditional PUT and not a
+    /// second snapshot. Adopting instead would mark this snapshot as the newest -- suppressing every
+    /// later publish for it -- while the checkpoint still pointed below it, leaving recovery replaying
+    /// from an older base with nothing scheduled to fix it.
+    bool ckpt_advanced = false;
+    if (!runtime_still_admitted())
+        return false;
+    const auto check_runtime_admission = [this, &rt](uint64_t generation)
+    {
+        if (snapshot_before_ckpt_cas_hook_for_test)
+            snapshot_before_ckpt_cas_hook_for_test();
+        check_fence_or_throw(generation);
+        if (rt->catalog_life_invalidated.load(std::memory_order_acquire)
+            || rt->superseded_by_remount.load(std::memory_order_acquire))
+            throwCasWriteRetryLater(fmt::format(
+                "CAS namespace '{}': its captured runtime was retired before checkpoint publication",
+                rt->life.ns.string()));
+    };
+    try
+    {
+        ckpt_advanced = publishCkptContribution(rt->life, RefCkpt{.life_epoch = std::nullopt,
+                                                            .committed_through = candidate_x,
+                                                            .checkpoint_snapshot_id = candidate_x,
+                                                            .last_epoch_seal = std::nullopt},
+                                                admitted_generation,
+                                                check_runtime_admission) != CkptPublishOutcome::FencedOut;
+    }
+    catch (...)
+    {
+        /// Swallowed deliberately, and ONLY here: the snapshot body is already durable, so there is
+        /// nothing to undo and nothing for a caller to decide --
+        /// `tryPublishSnapshotAndAdvanceCheckpointOnce` is one best-effort attempt whose every other
+        /// failure arm also returns false with a backoff. The
+        /// counter and the log line are what keep it from being silent.
+        tryLogCurrentException(getLogger("CasPool"),
+            "CAS ref table '" + ns.string() + "': the snapshot body is durable but its _ckpt checkpoint "
+            "could not be advanced; the snapshot is not yet cleanup-authoritative and the publish will "
+            "be retried");
+    }
+    if (!ckpt_advanced)
+    {
+        ProfileEvents::increment(ProfileEvents::CASRefCheckpointNotAdvanced);
+        std::lock_guard lock(rt->state_mutex);
+        advancePublishBackoff(*rt);
+        return false;
+    }
+
+    {
+        std::lock_guard lock(rt->state_mutex);
+        if (!runtime_still_admitted())
+            return false;
+        /// A durable publish clears any backoff: progress was made this attempt (even if the
+        /// monotonic guard below skips the in-memory adoption because a newer snapshot already won).
+        resetPublishBackoff(*rt);
+        /// Monotonic adoption guard (CRITICAL): publishes are NOT serialized, so two
+        /// overlapping attempts can finish out of order (this OLDER-candidate attempt landing its PUT
+        /// after a NEWER one already adopted). Adopting the older `candidate_x` here would REGRESS
+        /// `newest_snapshot_id` below what a newer attempt already advanced it to -- the next published
+        /// snapshot would then omit committed transactions and recovery would lose refs. Skip the
+        /// in-memory adoption (and the counter subtraction below) whenever a newer-or-equal snapshot is
+        /// already adopted; the already-durable `_snap/<candidate_x>` object is harmless (readers pick
+        /// the greatest snapshot, GC reclaims covered ones).
+        if (rt->newest_snapshot_id && !(*rt->newest_snapshot_id < candidate_x))
+            return true;
+        /// Subtract exactly the counters captured at copy time
+        /// -- more appends (or even another publish's own commits) may have landed on the LIVE counters
+        /// since, and only those should remain uncovered. Clamped (see `clampedCounterSub`): an
+        /// out-of-order adoption ordering the guard above does not catch (a SMALLER candidate that
+        /// adopts before a LARGER one already in flight) could otherwise subtract an already-subtracted
+        /// region and underflow the unsigned counter.
+        clampedCounterSub(rt->tail_count_since_snapshot, captured_count);
+        clampedCounterSub(rt->tail_bytes_since_snapshot, captured_bytes);
+        /// logs-per-table-after-snapshot: the tail this publish compacted.
+        ProfileEvents::increment(ProfileEvents::CASRefSnapshotTailLogs, captured_count);
+        rt->newest_snapshot_id = candidate_x;
+        /// The new cache-weight base is exactly the snapshot
+        /// we just encoded and PUT, so its body size is the fresh base weight -- no re-encode needed.
+        rt->base_snapshot_bytes.store(bytes.size(), std::memory_order_relaxed);
+    }
+    return true;
+}
+
+
+void CasRefLedger::sweepStalePrecommitsForRead(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt)
+{
+    /// A read-only caller (resolveRef/listRefs) must not fail its OWN
+    /// otherwise-successful read because a piggybacked maintenance action (the stale-precommit sweep)
+    /// hit an uncertain PUT -- the read asked for none of that; a mutation path (appendRefOps's own
+    /// top-level hoisted call, which calls `maybeSweepStalePrecommits` directly, uncaught) keeps
+    /// propagating instead, since it must not proceed past a wedged lane anyway. Swallowing here does
+    /// Do NOT drop the sweep: the failed
+    /// attempt already re-armed `needs_stale_precommit_sweep` (with a bounded cooldown) inside
+    /// `maybeSweepStalePrecommits`, so a later read/mutation trigger on THIS mount retries until a
+    /// sweep completes verified clean -- the old drop-the-shot behavior left a dead incarnation's
+    /// precommit bindings (and the manifests they protect from the GC orphan sweep) live forever on a
+    /// long-lived mount whenever the single attempt burned in the post-restart error window.
+    try
+    {
+        maybeSweepStalePrecommits(ns, rt);
+    }
+    catch (...)
+    {
+        ProfileEvents::increment(ProfileEvents::CASRefSweepDeferred);
+        tryLogCurrentException(getLogger("CasPool"),
+            "CAS stale-precommit sweep deferred for namespace '" + ns.string()
+                + "' (a read-only caller observed the failure and is proceeding with its own read)");
+    }
+}
+
+void CasRefLedger::maybeSweepStalePrecommits(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt)
+{
+    {
+        std::lock_guard lock(rt->state_mutex);
+        if (!rt->needs_stale_precommit_sweep)
+            return;
+        /// A failed attempt armed a cooldown; do not re-attempt (and do not touch the flag)
+        /// until it elapses -- the bounded-backoff storm latch, same shape as `publish_backoff_until_ms`.
+        /// The boottime clock is injectable (`boot_ms_fn`), so tests drive this deterministically.
+        if (boot_ms_now_fn() < rt->precommit_sweep_backoff_until_ms)
+            return;
+        /// Cleared FIRST: `sweepStalePrecommitsNow`'s own `appendRefOps` calls re-enter this same
+        /// top-level check (via `appendRefOps`'s hoisted call), and must see it already cleared. This
+        /// clear is for RE-ENTRANCY only, never consumption: any non-clean outcome re-arms below.
+        rt->needs_stale_precommit_sweep = false;
+    }
+    try
+    {
+        sweepStalePrecommitsNow(ns, rt);
+    }
+    catch (...)
+    {
+        /// A failed or partial sweep
+        /// failed or partial sweep must NOT consume the shot. Under kill-chaos the single attempt lands
+        /// exactly inside the post-restart error window (an uncertain PUT, a fence blip), and with no
+        /// retry the dead incarnation's durable precommit bindings -- and the manifests
+        /// `activeManifestKeys` protects for them -- leaked forever on a long-lived mount (GC has no
+        /// backstop). Re-arm with a bounded backoff and rethrow: the
+        /// read path insulates the caller (`sweepStalePrecommitsForRead`), the mutation path propagates
+        /// as before.
+        {
+            std::lock_guard lock(rt->state_mutex);
+            rt->needs_stale_precommit_sweep = true;
+            advancePrecommitSweepBackoff(*rt);
+        }
+        throw;
+    }
+    /// Verified clean: `sweepStalePrecommitsNow` returns only after a full pass over the live state
+    /// found zero stale bindings, so the flag stays cleared for the rest of this mount; reset the
+    /// failure cooldown too.
+    std::lock_guard lock(rt->state_mutex);
+    resetPrecommitSweepBackoff(*rt);
+}
+
+void CasRefLedger::advancePrecommitSweepBackoff(RefTableRuntime & rt)
+{
+    /// Caller holds `rt.state_mutex`. Double the interval
+    /// from `initial` up to `max` per consecutive failed sweep attempt; arm the deadline off the
+    /// boottime clock (`bootMsNow`), so an injected test clock drives it deterministically and a
+    /// VM-suspend cannot shorten it.
+    rt.precommit_sweep_backoff_ms = rt.precommit_sweep_backoff_ms == 0
+        ? config.precommit_sweep_backoff_initial_ms
+        : std::min<uint64_t>(rt.precommit_sweep_backoff_ms * 2, config.precommit_sweep_backoff_max_ms);
+    rt.precommit_sweep_backoff_until_ms = boot_ms_now_fn() + rt.precommit_sweep_backoff_ms;
+    ProfileEvents::increment(ProfileEvents::CASRefSweepRearmed);
+}
+
+void CasRefLedger::resetPrecommitSweepBackoff(RefTableRuntime & rt)
+{
+    /// Caller holds `rt.state_mutex`. A verified-clean sweep clears the cooldown.
+    rt.precommit_sweep_backoff_ms = 0;
+    rt.precommit_sweep_backoff_until_ms = 0;
+}
+
+
+void CasRefLedger::sweepStalePrecommitsNow(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt)
+{
+    /// After a fresh mount fence and recovery, this writer
+    /// knows the exact stale precommit bindings -- their `manifest_ref.writer_epoch` predates this
+    /// incarnation's live writer_epoch, i.e. they belong to a build from a superseded incarnation that
+    /// can never be promoted. Removed with ordinary exact `owner_transition(old_binding, none)`
+    /// operations, chunked to `ref_txn_max_ops` per transaction. Interruption is harmless: each chunk
+    /// re-reads the LIVE state, so a partial sweep just leaves fewer stale bindings for the next chunk
+    /// (a later retry on this mount, or the next mount's recovery) to find; nothing here can loop
+    /// forever since only OLDER-epoch bindings ever qualify, and this writer's own new work always uses
+    /// `live_epoch_fn()` -- which a self-remount bumps in lockstep with the threshold below, so a
+    /// remount's fresh precommits survive.
+    ///
+    /// A GC-side backstop stays deliberately OUT: the responsibility boundary assigns
+    /// precommit-binding cleanup to the WRITER -- GC never mutates another writer's ref-table state, so
+    /// a leader-side reclaim would be a new protocol capability (a question about GC-authored
+    /// ref-log transactions and their fencing), not a bugfix. The retry-until-clean loop above is the
+    /// writer-side answer; the follow-up (a GC visibility counter for "live precommit binding below the
+    /// mount-lease epoch" would require a separate protocol decision.
+    const uint64_t live_epoch = live_epoch_fn();
+    while (true)
+    {
+        std::vector<std::pair<String, ManifestRef>> chunk;
+        {
+            std::lock_guard lock(rt->state_mutex);
+            for (const auto & [ref_name, mref] : rt->state.getPrecommits())
+            {
+                if (mref.writer_epoch >= live_epoch)
+                    continue;
+                chunk.emplace_back(ref_name, mref);
+                if (chunk.size() >= ref_txn_max_ops)
+                    break;
+            }
+        }
+        if (chunk.empty())
+            return;
+
+        appendRefOps(ns, MutationScope::wholeShard(),
+            [chunk](const RefTableState & state) -> std::vector<RefOp>
+            {
+                std::vector<RefOp> ops;
+                for (const auto & [ref_name, mref] : chunk)
+                    if (state.getPrecommits().contains({ref_name, mref}))
+                    {
+                        RefOp op;
+                        op.kind = RefOpKind::OwnerTransition;
+                        op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref_name, mref};
+                        ops.push_back(op);
+                    }
+                return ops;
+            },
+            RootMutationOrigin::Writer, RootMutationKind::ReclaimPrecommit);
+
+        /// Audit each binding this sweep reclaimed, so
+        /// `system.cas_log` records the reclaim and the "abandoned precommits
+        /// reclaimed" counter is falsifiable (it had ZERO emit sites before). A binding gathered above
+        /// that is GONE from the live state after the committed append was reclaimed by this sweep's
+        /// work -- either this chunk's own ops or this lane's just-resolved wedged predecessor txn (a
+        /// PRIOR attempt of this same sweep whose ack was lost); one still present was skipped by the
+        /// builder (raced by another owner transition) and will be gathered again next iteration.
+        /// Collected under the lock, emitted outside it (the sink forwards to the SystemLog).
+        std::vector<std::pair<String, ManifestRef>> reclaimed;
+        {
+            std::lock_guard lock(rt->state_mutex);
+            for (const auto & [ref_name, mref] : chunk)
+                if (!rt->state.getPrecommits().contains({ref_name, mref}))
+                    reclaimed.emplace_back(ref_name, mref);
+        }
+        ProfileEvents::increment(ProfileEvents::CASRefStalePrecommitsReclaimed, reclaimed.size());
+        for (const auto & [ref_name, mref] : reclaimed)
+        {
+            EventEmitter{*this}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::PrecommitReclaim;
+                e.namespace_ = ns.string();
+                e.ref_name = ref_name;
+                e.object_kind = CasEventObjectKind::Root;
+                e.object_hash = manifestRefDebugString(mref);
+                e.reason = "stale-precommit sweep: dangling precommit of a superseded writer incarnation "
+                           "reclaimed by the successor's fenced sweep";
+                e.detail = {{"stale_writer_epoch", std::to_string(mref.writer_epoch)},
+                            {"live_writer_epoch", std::to_string(live_epoch)}};
+            });
+        }
+    }
+}
+
+
+void CasRefLedger::dropRef(const RootNamespace & ns, const String & ref_name)
+{
+    /// One `owner_transition` removal ref-log transaction. The
+    /// exact committed binding must exist; `build_ops` reads it off the CURRENT batch-validation state,
+    /// so a concurrently-co-batched publish/drop of a DIFFERENT ref sees a consistent view.
+    ManifestRef dropped_ref;
+    const RefTxnId txn_id = appendRefOps(ns, MutationScope::ref(ref_name),
+        [&](const RefTableState & state) -> std::vector<RefOp>
+        {
+            const auto it = state.getCommitted().find(ref_name);
+            if (it == state.getCommitted().end())
+                /// Fail-closed (no silent no-op): this item's own exception, the batch survives.
+                throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+                    "dropRef: no such ref {} in namespace {}", ref_name, ns.string());
+
+            dropped_ref = it->second.manifest_ref;
+            RefOp op;
+            op.kind = RefOpKind::OwnerTransition;
+            op.old_binding = RefOwnerBinding{RefOwnerKind::Committed, ref_name, dropped_ref};
+            return {op};
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Drop);
+
+    /// The ref was dropped (a removal operation GC folds as a true removal). `object_hash` is the
+    /// manifest the ref named, so a part's "publish -> drop" life is reconstructable from the rows.
+    if (hasEventSink())
+    {
+        CasEvent _ev3;
+        _ev3.type = CasEventType::RefDrop;
+        _ev3.namespace_ = ns.string();
+        _ev3.ref_name = ref_name;
+        _ev3.object_kind = CasEventObjectKind::Manifest;
+        _ev3.object_hash = manifestRefDebugString(dropped_ref);
+        _ev3.at_version = txn_id.ref_sequence;
+        _ev3.outcome = "ok";
+        _ev3.reason = "dropRef: appended an owner_transition removal ref-log transaction";
+        emitEvent(std::move(_ev3));
+    }
+}
+
+
+void CasRefLedger::updateRefPublishedAt(const RootNamespace & ns, const String & ref_name,
+                             std::function<void(RefPublishedAtUpdate &)> mutator)
+{
+    /// One `set_published_at` ref-log transaction. EVERY change (even timestamp-only) is an explicit
+    /// logged operation -- the immutable append-only log has no other way to record it.
+    /// `published_at_ms` is the only metadata this op carries (the mutable-file map is gone; every
+    /// per-part file is an ordinary manifest tree entry now, republished via `repointRef`, never
+    /// through this side channel).
+    appendRefOps(ns, MutationScope::ref(ref_name),
+        [&](const RefTableState & state) -> std::vector<RefOp>
+        {
+            const auto it = state.getCommitted().find(ref_name);
+            if (it == state.getCommitted().end())
+                throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+                    "updateRefPublishedAt: no such ref {} in namespace {}", ref_name, ns.string());
+
+            /// The mutator edits only `published_at_ms`; the carrier deliberately carries no
+            /// `manifest_ref`, so a reachability change is structurally impossible here (it goes through
+            /// publish/drop/repoint instead).
+            RefPublishedAtUpdate update;
+            update.published_at_ms = it->second.published_at_ms;
+
+            mutator(update);
+
+            RefOp op;
+            op.kind = RefOpKind::SetPublishedAt;
+            op.ref_name = ref_name;
+            op.expected_manifest_ref = it->second.manifest_ref;
+            op.published_at_ms = update.published_at_ms;
+            return {op};
+        },
+        RootMutationOrigin::Writer, RootMutationKind::UpdateRefPublishedAt);
+}
+
+
+NamespaceLifeId CasRefLedger::namespaceLife(const RootNamespace & ns)
+{
+    auto rt = lookupRefTableRuntime(ns);
+    if (rt)
+    {
+        check_fence_or_throw(rt->admitted_fence_generation);
+        bool removal_closed = false;
+        {
+            std::lock_guard queue_lock(ref_queue_mutex);
+            removal_closed = rt->removal_admission_closed;
+        }
+        if (removal_closed)
+        {
+            /// A lost erase response can leave only the detached predecessor's close bit. Reconcile
+            /// before refusing so an absent/replaced row frees the logical name without rebinding it.
+            reconcileCatalogCut(CasRefCatalog::read(backend, layout));
+            const auto refreshed = lookupRefTableRuntime(ns);
+            if (refreshed == rt)
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS namespace '{}' is Removing: creation waits for its terminal fold and catalog "
+                    "removal to complete; retry later", ns.string()));
+            rt = refreshed;
+        }
+        if (rt)
+        {
+            ensureRefTableRecovered(ns, *rt);
+            return rt->life;
+        }
+    }
+
+    /// A cold mutation observes or births the durable identity before allocating any local state.
+    const uint64_t admitted_generation = fence_generation_fn();
+    check_fence_or_throw(admitted_generation);
+    const CasRefCatalog::Snapshot catalog = CasRefCatalog::read(backend, layout);
+    check_fence_or_throw(admitted_generation);
+    const auto entry_it = std::find_if(catalog.catalog.entries.begin(), catalog.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    if (entry_it != catalog.catalog.entries.end() && entry_it->state == NsState::Removing)
+        throwCasWriteRetryLater(fmt::format(
+            "CAS namespace '{}' is Removing: creation waits for its terminal fold and catalog "
+            "removal to complete; retry later", ns.string()));
+
+    const NamespaceLifeId life
+        = entry_it != catalog.catalog.entries.end() && entry_it->state == NsState::Live
+        ? NamespaceLifeId::fromCatalogEntry(entry_it->ns, entry_it->incarnation)
+        : resolveNamespaceLife(ns, admitted_generation, live_epoch_fn());
+    check_fence_or_throw(admitted_generation);
+    rt = acquireRefTableRuntime(life, admitted_generation);
+    ensureRefTableRecovered(ns, *rt);
+    return rt->life;
+}
+
+std::optional<NamespaceLifeId> CasRefLedger::namespaceFilesLifeIfReadable(const RootNamespace & ns)
+{
+    const auto rt = acquireReadableRefTableRuntime(ns);
+
+    /// A namespace the catalog does not name has no files to read, and a read or an unlink is the wrong
+    /// event to bring one into existence on. Fresh resolution admits only a catalog `Live` row;
+    /// `Creating`, `Removing` and absent all answer absent without recovery or mutation.
+    if (!rt)
+        return std::nullopt;
+    ensureRefTableRecovered(ns, *rt);
+
+    std::lock_guard<std::mutex> lock(rt->state_mutex);
+    /// A stale already-held runtime may have applied the terminal before catalog invalidation reaches
+    /// it. Preserve the stated stale-or-not-found contract by hiding that terminal view.
+    if (rt->state.getLifecycle() != RefLifecycle::Live && rt->state.getRemoveTxnId().has_value())
+        return std::nullopt;
+    return rt->life;
+}
+
+bool CasRefLedger::namespaceStillLogicallyPresent(const RootNamespace & ns)
+{
+    /// O(1) fast path: a resident runtime already proven `Live` under an unbroken fence answers without
+    /// any catalog fetch -- the common case (`existsDirectory` on a warm, ordinary table). Anything
+    /// short of that falls through to the exact cold-path observation below.
+    if (const auto current = lookupRefTableRuntime(ns))
+    {
+        check_fence_or_throw(current->admitted_fence_generation);
+        bool closed = false;
+        {
+            std::lock_guard<std::mutex> queue_lock(ref_queue_mutex);
+            closed = current->removal_admission_closed;
+        }
+        if (!closed
+            && !current->catalog_life_invalidated.load(std::memory_order_acquire)
+            && !current->superseded_by_remount.load(std::memory_order_acquire))
+        {
+            std::lock_guard<std::mutex> state_lock(current->state_mutex);
+            if (current->recovered && current->state.getLifecycle() == RefLifecycle::Live)
+                return true;
+        }
+    }
+
+    /// Cold path: an exact catalog observation. `Creating` and `Live` both answer present immediately --
+    /// `true` is always the safe direction, so no revalidation is needed for either. A missing row is
+    /// the one answer that must never be manufactured by a race, so it alone is re-confirmed against a
+    /// second read before being trusted (mirroring `acquireReadableRefTableRuntime`'s own token/value
+    /// revalidation).
+    const uint64_t admitted_generation = fence_generation_fn();
+    check_fence_or_throw(admitted_generation);
+    const CasRefCatalog::Snapshot first_catalog = CasRefCatalog::read(backend, layout);
+    check_fence_or_throw(admitted_generation);
+    if (namespace_presence_probe_after_first_read_hook_for_test)
+        namespace_presence_probe_after_first_read_hook_for_test();
+    const auto find_entry = [&ns](const CasRefCatalog::Snapshot & snap) -> const CatalogEntry *
+    {
+        const auto it = std::find_if(snap.catalog.entries.begin(), snap.catalog.entries.end(),
+            [&](const CatalogEntry & entry) { return entry.ns == ns; });
+        return it == snap.catalog.entries.end() ? nullptr : &*it;
+    };
+
+    const CatalogEntry * entry = find_entry(first_catalog);
+    if (!entry)
+    {
+        const CasRefCatalog::Snapshot second_catalog = CasRefCatalog::read(backend, layout);
+        check_fence_or_throw(admitted_generation);
+        if (second_catalog.token != first_catalog.token || second_catalog.catalog != first_catalog.catalog)
+            throwCasWriteRetryLater(fmt::format(
+                "CAS namespace '{}': its catalog changed while probing table-root cleanup completeness; "
+                "retry from a fresh observation", ns.string()));
+        return false;   /// no catalog row, twice-confirmed: proven absent
+    }
+
+    if (entry->state == NsState::Creating || entry->state == NsState::Live)
+        return true;
+
+    /// `Removing`: only the exact incarnation's own durable ref-log proves the terminal
+    /// `remove_namespace` transaction actually landed -- the catalog row alone cannot distinguish a
+    /// completed removal from one whose terminal append is still outstanding after a crash. This is the
+    /// same load-bearing distinction `dropNamespaceImpl` makes before returning early. Once durably
+    /// proven for this EXACT incarnation, that incarnation's own terminal can never be un-proven -- but
+    /// the recovery call above (`acquireRefTableRuntime`/`ensureRefTableRecovered`) is real I/O with no
+    /// upper bound on wall time, and GC deleting this incarnation's now-terminal catalog row plus a
+    /// same-name rebirth (an explicitly supported sequence -- see `CASRefWriterNamespaceRemoval`'s own
+    /// same-name-rebirth tests) can both land inside that window. Proving THIS incarnation terminal is
+    /// therefore not proof that the CURRENT logical namespace `ns` is absent; a fresh catalog read is
+    /// required before answering `false` for the name.
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry->ns, entry->incarnation);
+    const auto rt = acquireRefTableRuntime(life, admitted_generation);
+    ensureRefTableRecovered(ns, *rt);
+    bool this_incarnation_terminal = false;
+    {
+        std::lock_guard<std::mutex> lock(rt->state_mutex);
+        this_incarnation_terminal = rt->state.getLifecycle() != RefLifecycle::Live && rt->state.getRemoveTxnId().has_value();
+    }
+    if (!this_incarnation_terminal)
+        return true;   /// removal admitted but not yet terminal: cleanup work remains
+
+    if (namespace_presence_probe_after_terminal_proven_hook_for_test)
+        namespace_presence_probe_after_terminal_proven_hook_for_test();
+
+    check_fence_or_throw(admitted_generation);
+    const CasRefCatalog::Snapshot post_terminal_catalog = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * post_terminal_entry = find_entry(post_terminal_catalog);
+    if (!post_terminal_entry || post_terminal_entry->incarnation == entry->incarnation)
+        return false;   /// terminal durably proven and nothing has since occupied `ns` under a new life
+    /// A successor incarnation now occupies `ns` -- `Creating`, `Live`, or a fresh `Removing` all mean
+    /// the name is not the proven-absent predecessor this call observed. `true` is always the safe
+    /// direction; a caller that acts on it retries against the successor's own (correct) state rather
+    /// than being told the namespace is gone while something already occupies its name.
+    return true;
+}
+
+
+DropNamespaceStats CasRefLedger::dropNamespace(const RootNamespace & ns)
+{
+    return dropNamespaceImpl(ns, std::nullopt);
+}
+
+DropNamespaceStats CasRefLedger::dropNamespaceImpl(
+    const RootNamespace & ns, const std::optional<UInt128> & expected_incarnation)
+{
+    /// One body transaction naming an exact `owner_transition`
+    /// removal for every committed ref and precommit, followed by `remove_namespace` -- the removal
+    /// class shares the bigger complete-table byte budget (encodeRefLogTxn's own `checkBudget`, keyed
+    /// off the presence of a `RemoveNamespace` op) and is exempt from the ordinary per-op admission
+    /// check (it only ever shrinks state; see `flushRefBatch`'s `state_growing` filter).
+    const CasRefCatalog::Snapshot initial_catalog = CasRefCatalog::read(backend, layout);
+    const auto initial_it = std::find_if(initial_catalog.catalog.entries.begin(), initial_catalog.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    if (initial_it == initial_catalog.catalog.entries.end())
+        return {};
+    if (expected_incarnation && initial_it->incarnation != *expected_incarnation)
+        throwCasWriteRetryLater(fmt::format(
+            "CAS namespace '{}': exact removal life {} differs from current catalog life {}",
+            ns.string(), renderIncarnation(*expected_incarnation), renderIncarnation(initial_it->incarnation)));
+
+    if (initial_it->state == NsState::Creating)
+    {
+        const CatalogEntry & observed = *initial_it;
+        const uint64_t admitted_generation = fence_generation_fn();
+        switch (CasRefCatalog::cancelStalledCreating(
+            backend, layout, observed,
+            [this](const CreatorFence & creator)
+            {
+                return isCreatorFenceTerminal(
+                    backend, layout, creator.server_root_id, creator.writer_epoch);
+            },
+            admitted_generation, check_fence_or_throw))
+        {
+            case CasRefCatalog::StalledCreatingCancelOutcome::Cancelled:
+                invalidateRemovedCatalogLife(NamespaceLifeId::fromCatalogEntry(observed.ns, observed.incarnation));
+                return {};
+            case CasRefCatalog::StalledCreatingCancelOutcome::CreatorFenceStillLive:
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS namespace removal '{}': its catalog entry is still Creating under a creator "
+                    "fence that is not yet provably terminal; retry later", ns.string()));
+            case CasRefCatalog::StalledCreatingCancelOutcome::EntryChanged:
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS namespace removal '{}': exact Creating row changed before cancellation; retry later",
+                    ns.string()));
+            case CasRefCatalog::StalledCreatingCancelOutcome::FencedOut:
+                throwCasWriteRetryLater(fmt::format(
+                    "CAS namespace removal '{}': mount fence moved before stalled creation cancellation",
+                    ns.string()));
+        }
+    }
+
+    const NamespaceLifeId observed_life
+        = NamespaceLifeId::fromCatalogEntry(initial_it->ns, initial_it->incarnation);
+    const uint64_t runtime_generation = fence_generation_fn();
+    const auto rt = acquireRefTableRuntime(observed_life, runtime_generation);
+    ensureRefTableRecovered(ns, *rt);
+    {
+        /// A real terminal transaction is idempotent. The representation also calls an empty, never-born
+        /// stream `Removed`, but its absent `remove_txn_id` distinguishes that state: a cataloged life may
+        /// already own `_ckpt` or `_files`, so it still needs durable birth+terminal evidence before GC
+        /// may delete its catalog row.
+        std::lock_guard lock(rt->state_mutex);
+        if (rt->state.getLifecycle() != RefLifecycle::Live && rt->state.getRemoveTxnId().has_value())
+            return {};
+    }
+
+    /// Close the local positive lane BEFORE publishing `Removing`. Calls already admitted ahead of
+    /// this point drain first; later positive callers observe the flag in the same queue critical
+    /// section as enqueue and receive the typed retry-later refusal. The terminal item below is the one
+    /// deliberate exception.
+    {
+        std::unique_lock<std::mutex> queue_lock(ref_queue_mutex);
+        rt->removal_admission_closed = true;
+        rt->cv.wait(queue_lock, [&]
+        {
+            return !rt->leader_active && rt->pending.empty();
+        });
+    }
+
+    const uint64_t admitted_generation = fence_generation_fn();
+    std::optional<CatalogEntry> observed_live;
+    if (initial_it->state == NsState::Live)
+        observed_live = *initial_it;
+    bool removing_durable = false;
+    try
+    {
+        const CasRefCatalog::Snapshot snapshot = CasRefCatalog::read(backend, layout);
+        const auto entry_it = std::find_if(snapshot.catalog.entries.begin(), snapshot.catalog.entries.end(),
+            [&](const CatalogEntry & entry) { return entry.ns == ns; });
+        if (entry_it == snapshot.catalog.entries.end())
+            throwCasWriteRetryLater(fmt::format(
+                "CAS namespace '{}': its catalog row disappeared before removal admission closed",
+                ns.string()));
+
+        const NamespaceLifeId & life = rt->life;
+        if (entry_it->incarnation != life.incarnation)
+            throwCasWriteRetryLater(fmt::format(
+                "CAS namespace '{}': cached life {} differs from catalog life {} while beginning removal",
+                ns.string(), renderIncarnation(life.incarnation), renderIncarnation(entry_it->incarnation)));
+
+        if (entry_it->state == NsState::Removing)
+        {
+            removing_durable = true;   /// retry after an earlier terminal append failure/ambiguous CAS
+        }
+        else if (entry_it->state == NsState::Live)
+        {
+            observed_live = *entry_it;
+            uint64_t removal_started_round = 0;
+            if (const auto got = backend.get(layout.gcStateKey()))
+                removal_started_round = decodeGcState(got->bytes).round;
+
+            switch (CasRefCatalog::beginRemoving(
+                backend, layout, *observed_live, removal_started_round,
+                admitted_generation, check_fence_or_throw))
+            {
+                case CasRefCatalog::BeginRemovingOutcome::Transitioned:
+                case CasRefCatalog::BeginRemovingOutcome::AlreadyRemoving:
+                    removing_durable = true;
+                    break;
+                case CasRefCatalog::BeginRemovingOutcome::EntryChanged:
+                    throwCasWriteRetryLater(fmt::format(
+                        "CAS namespace '{}': its exact catalog row changed while beginning removal",
+                        ns.string()));
+                case CasRefCatalog::BeginRemovingOutcome::FencedOut:
+                    throwCasWriteRetryLater(fmt::format(
+                        "CAS namespace '{}': the mount fence moved while beginning removal",
+                        ns.string()));
+            }
+        }
+        else
+        {
+            throwCasWriteRetryLater(fmt::format(
+                "CAS namespace '{}': its catalog row is Creating while removal owns a recovered life",
+                ns.string()));
+        }
+    }
+    catch (...)
+    {
+        /// Resolve an ambiguous transition before deciding whether this lane may reopen. `Removing`
+        /// under the same life is conclusive success. Reopening is permitted only after a fresh exact
+        /// observation still proves the original `Live` row and the same mount fence; every unreadable,
+        /// changed or fenced case remains closed (fail-close) and propagates the original error.
+        try
+        {
+            const CasRefCatalog::Snapshot fresh = CasRefCatalog::read(backend, layout);
+            const auto fresh_it = std::find_if(fresh.catalog.entries.begin(), fresh.catalog.entries.end(),
+                [&](const CatalogEntry & entry) { return entry.ns == ns; });
+            if (observed_live
+                && fresh_it != fresh.catalog.entries.end()
+                && fresh_it->incarnation == observed_live->incarnation
+                && fresh_it->state == NsState::Removing)
+            {
+                removing_durable = true;
+            }
+            else if (observed_live && fresh_it != fresh.catalog.entries.end() && *fresh_it == *observed_live)
+            {
+                check_fence_or_throw(admitted_generation);
+                std::lock_guard<std::mutex> queue_lock(ref_queue_mutex);
+                rt->removal_admission_closed = false;
+                rt->cv.notify_all();
+            }
+        }
+        catch (...) // NOLINT(bugprone-empty-catch)
+        {
+            /// The original failure remains the caller-visible one. Failure to prove an exact fresh
+            /// `Live` row deliberately leaves admission closed.
+        }
+        if (!removing_durable)
+            throw;
+    }
+
+    chassert(removing_durable);
+
+    /// This call's own removal
+    /// transaction named, filled from the SAME `state` the ops below are built from -- a retried
+    /// `build_ops` (a wedge resolving under a resumed leader) simply overwrites it with the final
+    /// durable transaction's true counts.
+    DropNamespaceStats stats;
+    appendRefOpsOnRuntime(ns, rt, MutationScope::wholeShard(),
+        [&](const RefTableState & state) -> std::vector<RefOp>
+        {
+            if (state.getLifecycle() != RefLifecycle::Live && state.getRemoveTxnId().has_value())
+                return {};   /// raced: another caller already removed it since our check above
+
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+            {
+                RefOp birth;
+                birth.kind = RefOpKind::NamespaceBirth;
+                ops.push_back(std::move(birth));
+            }
+            for (const auto [ref_name, row] : state.getCommitted())
+            {
+                RefOp op;
+                op.kind = RefOpKind::OwnerTransition;
+                op.old_binding = RefOwnerBinding{RefOwnerKind::Committed, ref_name, row.manifest_ref};
+                ops.push_back(op);
+            }
+            for (const auto & [ref_name, mref] : state.getPrecommits())
+            {
+                RefOp op;
+                op.kind = RefOpKind::OwnerTransition;
+                op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref_name, mref};
+                ops.push_back(op);
+            }
+            RefOp remove;
+            remove.kind = RefOpKind::RemoveNamespace;
+            ops.push_back(remove);
+
+            stats.committed_refs = state.getCommitted().size();
+            stats.precommits = state.getPrecommits().size();
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::DropNamespace,
+        /// The operations above already name (and remove) every current precommit
+        /// binding regardless of epoch, making the ordinary stale-precommit maintenance sweep redundant
+        /// for THIS call -- and, left enabled, a race: the hoisted sweep runs first and would reclaim an
+        /// epoch-stale binding in its OWN transaction, so `state.getPrecommits()` above would already be
+        /// missing it and undercount `stats.precommits`. See `appendRefOps`'s doc comment.
+        /*skip_stale_precommit_sweep=*/true,
+        /*terminal_removal_authorized=*/true);
+
+    /// "After the transaction is durable, it applies the same
+    /// operations to memory, cancels local builds, and rejects further ordinary mutations." Reaching here
+    /// means the removal is durable (this call's, or a concurrent caller's whose durable result the append
+    /// lane observed) -- a FAILED append would have thrown above, so cancellation is only ever reached
+    /// after durability (a failed append leaves the namespace `Removing`, not `Live`, and propagates;
+    /// the catalog CAS above already made that transition durable before this append was attempted).
+    /// Cancel
+    /// every in-flight build TARGETING this namespace so its next op fails closed (`requireAlive`),
+    /// preventing it from promoting/precommitting a fresh owner into (or staging more debris in) the
+    /// just-removed namespace. The append lane is the real linearization authority (an `owner_transition`
+    /// on a non-Live namespace is rejected by the state machine regardless); this stops wasted work early
+    /// and surfaces a clear error. Builds in OTHER namespaces self-filter (no-op). The build registry
+    /// (`inflight_builds`) lives on the owning Pool, so the cancellation runs through the injected
+    /// `cancel_inflight_builds` callback (which collects the live shared_ptrs under `builds_mutex` and
+    /// cancels OUTSIDE it -- see `Pool::cancelInflightBuildsForNamespace`).
+    cancel_inflight_builds(ns);
+
+    /// No background publisher may carry the old runtime across the later catalog deletion and its
+    /// in-place reset. A removal has already succeeded here, so the cached lifecycle is no longer Live
+    /// and settlement cannot re-dispatch another publisher.
+    {
+        std::unique_lock state_lock(rt->state_mutex);
+        rt->publish_settle_cv.wait(state_lock, [&]
+        {
+            return rt->pending_snapshot_publishes.load(std::memory_order_acquire) == 0;
+        });
+    }
+
+    /// The writer performs no physical deletion of ref-log/snapshot objects or namespace files. Once
+    /// the catalog row is drained, those objects are dead-life debris for the perpetual janitor.
+    return stats;
+}
+
+DropNamespaceStats CasRefLedger::dropNamespace(const NamespaceLifeId & life)
+{
+    return dropNamespaceImpl(life.ns, life.incarnation);
+}
+
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h
new file mode 100644
index 000000000000..a42ec80efb89
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h
@@ -0,0 +1,1246 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <atomic>
+#include <condition_variable>
+#include <cstdint>
+#include <deque>
+#include <functional>
+#include <map>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <set>
+#include <span>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Controls whether `resolveRef` emits its `RefResolve` audit event. `Emit` (the default) preserves
+/// today's behavior for every existing caller (`listRefs`, `dropRef`, GC, and ordinary reads).
+/// `Deferred` is for a caller that itself decides, after inspecting the resolve outcome, whether the
+/// access as a whole did real resolve work worth auditing — see `CachedPartFolderAccess::getView`,
+/// which re-emits the identical event on every path except a warm view-cache hit that served without
+/// re-validating anything.
+enum class ResolveAudit : uint8_t { Emit, Deferred };
+
+/// The complete state of one table's append lane. It is guarded by `RefTableRuntime::state_mutex`;
+/// there is no independent apply marker or durable-id floor whose combinations form a second,
+/// implicit state machine.
+///
+/// `Ready` is the only state that admits a new append or certifies a cached row. `Writing` owns the
+/// exact attempt before its first possible send. `Wedged` owns that same attempt after an ambiguous
+/// result. `NeedsRecovery` means a transaction is known durable but cannot be installed in this cache;
+/// it is a hard write and certification fence until replay completes. `Closed` records a successor's
+/// epoch seal, and `Faulted` records foreign or internally inconsistent durable state.
+enum class RefLaneState : uint8_t
+{
+    Ready,
+    Writing,
+    Wedged,
+    NeedsRecovery,
+    Closed,
+    Faulted,
+};
+
+/// The answer of the relink confirm's gate 1 (`CasRefLedger::confirmExactRef`).
+///
+/// `Yes` is the only answer that AUTHORIZES anything, so it is the only one that must be earned: it is
+/// returned exclusively when every rule of the lane snapshot holds. `Unknown` is the catch-all for
+/// every ambiguity, and it is the answer this primitive is biased towards: a cold, evicted, recovering,
+/// busy or non-`Ready` table answers `Unknown` rather than doing any work to find out.
+///
+/// `No` means "this runtime's committed row for that ref is not the manifest you asked about" -- and
+/// nothing more. It is NOT a proof of the negative about the durable table, because the mount fence is
+/// evaluated LAST (rule 6, deliberately): a mount that has already lost its fence, and whose view may
+/// therefore be behind another writer's repoint, still answers `No` rather than `Unknown`. That is
+/// sound only because `No` and `Unknown` are the SAME outcome for the caller -- both are
+/// `SourceProofFailed` (spec §failure-taxonomy) -- so nothing is authorized by either. Do not build a
+/// consumer that treats `No` as knowledge; only `Yes` is gated on the fence.
+enum class ConfirmAnswer : uint8_t { Yes, No, Unknown };
+
+/// Coordinates the writer-side ref-log and ref-table protocol for all namespaces in one mounted pool.
+/// It owns the recovered whole-table cache, the flat-combining append lane and its unresolved-`PUT`
+/// wedge, snapshot publication, stale-precommit cleanup, cache-budget eviction, and remount/shutdown
+/// draining. `ref_queue_mutex` protects cache membership and queue leadership; each table's
+/// `state_mutex` protects its decoded state and per-table lifecycle. Network I/O is deliberately performed
+/// without holding `state_mutex`, so readers and other maintenance operations are not blocked by retries.
+///
+/// The ledger receives storage, configuration, event delivery, and retry-budget dependencies directly.
+/// Mount state remains owned by `Pool` and is exposed through callbacks: the live writer epoch, append
+/// fence, clocks, mutation gate, unclean-boundary observation, anomaly reaction, owner lifetime pin, and
+/// cancellation of in-flight builds. The detached snapshot publisher uses the lifetime pin; no
+/// `Pool &` back-reference is retained. `Pool` forwards its existing public operations to this component.
+class CasRefLedger
+{
+public:
+    CasRefLedger(
+        BackendPtr backend_ptr,
+        const Layout & layout_,
+        RefLedgerConfig config_,
+        const CasEventSink & event_sink_,
+        CasRequestBudget cas_request_budget_,
+        /// This mount's own `server_root_id` (spec §3's `CreatorFence`): the ledger mints its OWN
+        /// creator fence out of this plus the live writer epoch and admission fence generation when it
+        /// resolves a namespace's catalog life (`resolveNamespaceLife`) -- never injected as a callback,
+        /// unlike the mount-state functions below, because it is a fixed identity for this ledger's
+        /// whole lifetime (mirrors `CasMountRuntime`'s own by-value `server_root_id`).
+        String server_root_id_,
+        /// Monotonic mount clock used by the retry controller; it may be empty when the controller's
+        /// default clock is appropriate.
+        std::function<uint64_t()> controller_boot_ms_fn,
+        /// Callbacks into mount and watermark state owned by `Pool`, bound for this ledger's lifetime:
+        std::function<uint64_t()> live_epoch_fn_,
+        std::function<bool()> fence_ok_fn_,
+        /// The two fence-GENERATION primitives (`CasMountRuntime::fenceGeneration`/`checkFenceOrThrow`),
+        /// injected exactly as `CasPlainObjects` takes them. `fence_ok_fn` above answers "may this mount
+        /// write AT ALL, right now"; these two answer the different question an append lane must ask
+        /// across an I/O window: "is this still the SAME mount incarnation that admitted the transaction
+        /// I am about to act on?" A wedge captures the generation at admission and presents it back on
+        /// every later retry and before every install, so a result that returns after a fence loss or a
+        /// re-arm is inert for the superseded runtime instead of installing a stale view (spec §3,
+        /// "the mount-fence generation is captured at admission and required on every slot-occupy and
+        /// install").
+        std::function<uint64_t()> fence_generation_fn_,
+        std::function<void(uint64_t)> check_fence_or_throw_,
+        std::function<uint64_t()> boot_ms_now_fn_,
+        std::function<bool()> may_mutate_,
+        std::function<void(const String &, const String &, const std::optional<String> &)> on_impossible_interference_,
+        std::function<std::shared_ptr<void>()> pin_owner_,
+        std::function<void(const RootNamespace &)> cancel_inflight_builds_);
+
+    /// Recovers `ns` on first access and resolves `ref_name` from the authoritative cached table.
+    /// The optional staleness argument remains for API compatibility; this mounted writer has no
+    /// alternate shard cache, so the recovered table is always the view used for the result.
+    /// `audit` defaults to `Emit` so every existing caller keeps emitting `RefResolve` unchanged;
+    /// `Deferred` suppresses the emit for a caller that re-emits it conditionally itself.
+    std::optional<Resolved> resolveRef(const RootNamespace & ns, const String & ref_name, bool allow_stale = false,
+                                       ResolveAudit audit = ResolveAudit::Emit);
+
+    /// Recovers `ns` on first access and returns every committed ref in canonical name order. Read-side
+    /// maintenance may schedule snapshot publication and stale-precommit cleanup, but those actions do
+    /// not change the returned committed view or make a read fail when maintenance has an uncertain `PUT`.
+    std::map<String, Resolved> listRefs(const RootNamespace & ns);
+
+    /// Recovers `ns` on first access and reports whether any committed ref name starts with `prefix`,
+    /// without materializing the full ref map `listRefs` returns. An empty `prefix` means "any ref at
+    /// all" and short-circuits on the first entry, so this is O(1) for that (dominant, emptiness-probe)
+    /// case; a non-empty prefix still stays a no-allocation scan.
+    bool hasAnyRefWithPrefix(const RootNamespace & ns, std::string_view prefix);
+
+    /// Gate 1 of the relink confirm (spec §confirm-primitive): does `ref_name` in `ns` still name
+    /// EXACTLY `manifest_ref` in this writer's committed view, read under a lane snapshot that cannot
+    /// observe a stale cache?
+    ///
+    /// Performs ZERO object-store I/O: a cold, evicted or recovering table answers `Unknown` rather
+    /// than recovering from storage, and no runtime is created as a side effect of asking. That is a
+    /// contract, not an optimization -- the confirm is a read-only interserver query that a remote
+    /// receiver drives, so it must never be able to make this writer do work.
+    ///
+    /// The rules are evaluated as one snapshot spanning both lane mutexes, in this order: table warm
+    /// and resident; lane state `Ready`; exact committed-row equality; mount fence live last. Every
+    /// ambiguity answers `Unknown` -- see `ConfirmAnswer`, and the .cpp for why the order and the
+    /// two-mutex hold are what make a `Yes` a linearization point rather than a guess.
+    ConfirmAnswer confirmExactRef(const RootNamespace & ns, const String & ref_name,
+                                  const ManifestRef & manifest_ref) const;
+
+    /// Appends the transaction that removes one ref and waits for its durable result. A failed append
+    /// propagates its exception and does not apply the removal to the in-memory table.
+    void dropRef(const RootNamespace & ns, const String & ref_name);
+
+    /// Builds and appends a published_at_ms update for one ref. The mutator is invoked while
+    /// constructing the transaction, and its changes become visible only after the append is durable.
+    void updateRefPublishedAt(const RootNamespace & ns, const String & ref_name,
+                          std::function<void(RefPublishedAtUpdate &)> mutator);
+
+    /// Durably removes the complete namespace, including its current ref/precommit state, then performs
+    /// the associated cancellation work. The catalog transition to `Removing` happens first; a failed
+    /// terminal append after that leaves the namespace `Removing` (not `Live`) and propagates.
+    DropNamespaceStats dropNamespace(const RootNamespace & ns);
+
+    /// Decommission-only exact-life form. Pins recovery to the immutable catalog cut selected by the
+    /// admin command, so a same-name replacement can never redirect destructive work.
+    DropNamespaceStats dropNamespace(const NamespaceLifeId & life);
+
+    /// The catalog life every one of this namespace's objects -- ref-layer AND namespace-file -- is keyed
+    /// under, resolved ONCE per table-open and read from the cache afterwards. This is the WRITE-side
+    /// resolution, and the ONLY one that CREATES: recovery's step 0 (`resolveNamespaceLife`) mints a
+    /// life when the catalog names none, so the first namespace file a table ever writes births the
+    /// namespace exactly as its first ref op would. A read or a removal must not use this -- see the
+    /// sibling below for why that is a correctness matter and not a preference.
+    NamespaceLifeId namespaceLife(const RootNamespace & ns);
+
+    /// The life a READER (or a REMOVER) of this namespace's files must use, or `nullopt` when there are
+    /// no readable files at all -- which is the same answer for a namespace that never existed, one
+    /// still being created, and one whose catalog row is `Removing` or absent.
+    ///
+    /// IT NEVER CREATES A NAMESPACE, and that is the property the callers depend on rather than a
+    /// side-effect of how it happens to be written: for an uncataloged namespace it answers from a
+    /// catalog-only lookup and returns without recovering, so an `existsFile` or an
+    /// `unlinkFile(..., if_exists = true)` against a never-opened table cannot admit an entry into the
+    /// single pool-wide catalog object. See the implementation for how the guarantee survives a
+    /// concurrent removal.
+    ///
+    /// ONE call, not a predicate plus a resolution, and that is deliberate: readability and the life are
+    /// answered from the SAME `state_mutex` hold over the SAME recovered runtime, so a reader can never
+    /// pair "readable" from one observation with a life from another. Returning `optional` rather than a
+    /// life plus a bool also makes the unreadable case unusable by construction -- a caller that forgets
+    /// to check gets no life to read with, instead of a plausible-looking one that names the wrong
+    /// prefix. Absence is the fail-closed direction: only a KNOWN-readable namespace surfaces files, and
+    /// every failure mode of the underlying reads throws rather than degrading to `nullopt`, so "no life"
+    /// is only ever reached for a namespace whose absence is durable knowledge.
+    std::optional<NamespaceLifeId> namespaceFilesLifeIfReadable(const RootNamespace & ns);
+
+    /// Table-root cleanup-completeness probe: whether this logical namespace still has foreground
+    /// removal work outstanding, or has never proven that none remains. `true` for `Creating`, every
+    /// `Live` row (including zero refs and zero namespace files), and `Removing` before its terminal
+    /// `remove_namespace` transaction is durable; `false` for no catalog row at all, or for `Removing`
+    /// whose terminal is durably proven (a non-`Live` `RefLifecycle` WITH a `remove_txn_id` -- the same
+    /// distinction `dropNamespaceImpl` makes before returning early, because a files-only life that
+    /// never emitted a ref transaction can otherwise look `Removed` without ever having been removed).
+    ///
+    /// NEVER creates or mutates a catalog entry -- a probe is the wrong event to birth a namespace on --
+    /// and NEVER answers `false` for an unreadable, ambiguous, or changing observation: every such case
+    /// throws instead, because the caller (`existsDirectory`) uses `false` as permission to physically
+    /// remove a directory tree. A resident runtime already proven `Live` is trusted as an O(1) fast
+    /// path so an ordinary warm table does not pay a `ref_catalog` fetch per probe; every other shape
+    /// re-reads the exact catalog row.
+    bool namespaceStillLogicallyPresent(const RootNamespace & ns);
+
+    /// Called after a complete catalog observation proves an exact resident life absent or replaced.
+    /// It does not
+    /// destroy the runtime: existing callers that already captured the old physical life may finish
+    /// with their stale-or-not-found contract. The name slot is detached by exact pointer identity; a
+    /// later name-based touch may publish a distinct successor runtime.
+    void invalidateRemovedCatalogLife(const NamespaceLifeId & life);
+
+    /// Reconciles resident removal-closed runtimes against one complete catalog observation. An exact
+    /// life absent from or replaced in the cut is invalidated and exactly detached; a matching current
+    /// life stays closed. No runtime is reset or rebound.
+    void reconcileCatalogCut(const CasRefCatalog::Snapshot & catalog_cut);
+
+    /// Queues a mutation for flat-combining with compatible callers. `build_ops` runs at most once in
+    /// the flush leader and must return operations without writing storage itself. The leader validates
+    /// the complete batch, writes one ref-log object behind the append fence, and applies the batch to the
+    /// cache only after a durable result; an unresolved conditional `PUT` wedges the table and blocks
+    /// later appends until the same object is resolved or definitely rejected.
+    RefTxnId appendRefOps(const RootNamespace & ns, MutationScope scope,
+                         std::function<std::vector<RefOp>(const RefTableState &)> build_ops,
+                         RootMutationOrigin origin, RootMutationKind kind,
+                         bool skip_stale_precommit_sweep = false);
+
+    /// Attempts one snapshot publication from a copy of the live state. The copy is made under
+    /// `state_mutex`, the conditional `PUT` is performed without that mutex, and counters are adopted
+    /// only when this attempt successfully publishes the captured snapshot.
+    bool tryPublishSnapshotAndAdvanceCheckpointOnce(const RootNamespace & ns);
+
+    /// Counts tables with an unresolved append `PUT`; the walk takes each table lock briefly and never
+    /// waits for the network operation that caused a wedge.
+    size_t wedgedRefLaneCount();
+
+    /// Marks cached runtimes obsolete before a self-remount reopens the append fence. Leaders holding an
+    /// orphaned runtime therefore fail closed instead of mutating state from the previous epoch.
+    void quiesceRefTablesForRemount();
+
+    /// The self-remount's CANCEL-OR-JOIN barrier over in-flight recoveries (spec §3: "self-remount
+    /// cancels or waits out recovery before rearming"). Requests cancellation on every cached table,
+    /// then WAITS until no recovery attempt is in flight anywhere, then clears the request. Returns only
+    /// once that is true, so the caller may re-arm the mount fence knowing no recovery straddles it.
+    ///
+    /// Why this exists on top of the install recheck: the recheck protects the INSTALL, the barrier
+    /// protects the WINDOW. A recovery admitted under the outgoing incarnation would otherwise keep
+    /// issuing writes (its seal CAS-walk WRITES) across the whole re-arm, and each one would have to be
+    /// caught individually at its own site; here it is stopped once, at the boundary, before the
+    /// incarnation changes underneath it.
+    ///
+    /// The narrow window it does NOT close -- a recovery that starts after this returns and before the
+    /// fence is re-armed -- is closed by the other two members of the same guard: the re-arm bumps the
+    /// generation, so that recovery's `_ckpt` CAS and its install both refuse; and
+    /// `quiesceRefTablesForRemount` (which the caller runs next, BEFORE the re-arm) publishes
+    /// `superseded_by_remount`, which the walk polls at every I/O boundary.
+    void cancelRecoveriesAndAwaitQuiescence();
+
+    /// Closes admission, snapshots the current table set, and waits up to `wait_budget_ms` for queued
+    /// mutations and leaders to finish. The check and enqueue paths share `ref_queue_mutex`, so no new
+    /// mutation can appear after shutdown has taken its snapshot.
+    bool drainRefLanesForShutdown(uint64_t wait_budget_ms);
+
+    /// Performs a staged conditional create through the ledger's retry controller and append-fence
+    /// predicate. Callers do not access either dependency directly, so every attempt observes the same
+    /// mount admission rule.
+    CasWriteOutcome stagingPutIfAbsent(std::string_view key, std::string_view bytes, Token * out_token);
+
+    /// Performs a conditional staged create using `attempt`, applying the same retry controller and
+    /// append-fence policy as `stagingPutIfAbsent`.
+    CasCreateResult stagingConditionalCreate(std::string_view key, const std::function<PutResult()> & attempt);
+
+    /// Same retry/fence policy as `stagingPutIfAbsent`/`stagingConditionalCreate`, for a MUTABLE
+    /// If-Match overwrite whose bytes are deterministic (safe for GET-based resolution).
+    CasOverwriteResult stagingConditionalOverwrite(std::string_view key, std::string_view bytes, const Token & expected);
+
+    /// Same retry/fence policy as `stagingPutIfAbsent`, for a MUTABLE marker where an existing
+    /// DIFFERENT value at the key is a normal Conflict outcome, not corruption (see
+    /// `CasRequestController::putIfAbsentControlledMutable`).
+    CasOverwriteResult stagingPutIfAbsentMutable(std::string_view key, std::string_view bytes);
+
+    /// Hooks required by `EventEmitter`: events are delivered to the injected sink when one is present.
+    bool hasEventSink() const noexcept { return static_cast<bool>(event_sink); }
+    void emitEvent(CasEvent && e) const { if (event_sink) event_sink(std::move(e)); }
+
+    /// Replaces the retry controller's delay seam for deterministic tests; production callers leave it
+    /// untouched.
+    void setCasRetrySleepForTest(std::function<void(uint64_t)> sleep_fn);
+
+    /// Test-only observability and fault-injection seams for recovery, wedges, cleanup, and publication.
+    /// The counters expose recovery and publication progress; wedge methods create and inspect the
+    /// unresolved-`PUT` state; cleanup methods expose sweep eligibility; publication methods expose
+    /// settling, snapshot identity, and tail accounting. Every observer below is resident-only: it
+    /// performs no catalog/backend I/O and never materializes or recovers a runtime.
+    /// Returns the number of exact-read recovery restarts recorded for `ns`.
+    uint64_t refRecoveryRestartsForTest(const RootNamespace & ns);
+    /// Reports whether `ns` currently has an unresolved append `PUT`.
+    bool refLaneWedgedForTest(const RootNamespace & ns);
+    /// Returns the object key retained for the unresolved append of `ns`.
+    String wedgedKeyForTest(const RootNamespace & ns);
+    /// Returns the fence generation retained with the unresolved append of `ns` (0 when not wedged).
+    uint64_t wedgedAdmittedGenerationForTest(const RootNamespace & ns);
+    /// Installs a synthetic unresolved append for `ns` so callers can exercise resolution and blocking.
+    /// `admitted_generation` defaults to the CURRENT fence generation, which is what a real wedge born
+    /// now would carry; pass an explicit value to model a wedge admitted under an older incarnation.
+    void forceWedgeForTest(const RootNamespace & ns, uint64_t writer_epoch, uint64_t ref_sequence,
+                           const String & key, const String & bytes,
+                           std::optional<uint64_t> admitted_generation = std::nullopt);
+    /// Returns the seal that closed `ns`'s previous writer epoch -- the `prev_epoch_seal` its next
+    /// sequence-1 append will carry (`nullopt` at genesis). See `RefTableRuntime::last_epoch_seal`.
+    std::optional<RefTxnId> lastEpochSealForTest(const RootNamespace & ns);
+    /// Installs `seal` as `ns`'s last epoch seal, standing in for the recovery CAS-walk that produces it
+    /// in production (Task 6). Lets a writer-side test drive the ordinary post-transition append without
+    /// a whole recovery.
+    void setLastEpochSealForTest(const RootNamespace & ns, const std::optional<RefTxnId> & seal);
+    /// Returns the append lane state of `ns` without forcing recovery.
+    RefLaneState laneStateForTest(const RootNamespace & ns);
+    /// Reports whether recovery or a prior incomplete sweep requires stale-precommit cleanup.
+    bool needsStalePrecommitSweepForTest(const RootNamespace & ns);
+    /// Waits until all background snapshot publications for `ns` have completed.
+    void waitForSnapshotPublishSettleForTest(const RootNamespace & ns);
+    /// Returns the number of background snapshot publications currently in flight for `ns`.
+    int pendingSnapshotPublishesForTest(const RootNamespace & ns);
+    /// Returns the newest snapshot id adopted by the cached runtime, if any.
+    std::optional<RefTxnId> newestPublishedSnapshotIdForTest(const RootNamespace & ns);
+    /// Whether `ns` currently has a RECOVERED cached runtime -- WITHOUT forcing a recovery. That is the
+    /// whole point: the fail-closed tests assert that a refused recovery
+    /// installed nothing, and a seam that recovered on demand would answer its own question.
+    bool refTableRecoveredForTest(const RootNamespace & ns);
+    /// Whether the self-remount barrier has PUBLISHED its cancellation request for `ns` (also without
+    /// forcing a recovery). The barrier-blocks test needs this as a handshake: it must not release the
+    /// parked recovery until the request is actually visible to it, or the recovery races past a flag
+    /// that was set a moment too late and the test observes a completion instead of a cancellation.
+    bool refRecoveryCancelRequestedForTest(const RootNamespace & ns);
+    /// Returns the number of applied transactions newer than the adopted snapshot.
+    size_t tailSinceSnapshotCountForTest(const RootNamespace & ns);
+    /// Returns the number of committed entries in the mutable overlay, when the COW representation has one.
+    size_t committedOverlayEntriesForTest(const RootNamespace & ns);
+    /// Returns this table's LIVE precommit view: the exact `{ref_name, manifest}` owner bindings that
+    /// `precommitAdd` creates and that `promote` (move to committed) or `abandon` (exact precommit
+    /// removal) take away again. A leaked binding here is the same-epoch precommit leak the stale sweep
+    /// -- prior-epoch-scoped -- can never reclaim, so it is what an abandon-path test must assert on.
+    std::set<std::pair<String, ManifestRef>> livePrecommitsForTest(const RootNamespace & ns);
+    /// Installs the test hook invoked immediately before the leader carves a compatible batch.
+    void setRefPreCarveHookForTest(std::function<void()> hook) { ref_pre_carve_hook_for_test = std::move(hook); }
+
+    /// Test-only fault seam for the two-phase carve/validation protocol (same `*ForTest` pattern as
+    /// `setRefPreCarveHookForTest`). `flushRefBatch` fires the hook at each named point of the carve's
+    /// plan/publish phases and of the per-item validation loop, so a test can inject `std::bad_alloc`
+    /// and assert the append queue and the batch-validation `working` state stay intact. `PlanSeenRefs`,
+    /// `PlanBatchGrow` and `PlanReserveOwned` fire in the non-mutating PLAN phase (nothing has been
+    /// popped yet); `PublishPop` fires at the start of the no-throw PUBLISH phase; `ValidateFinalOps`
+    /// fires once per admitted item, at the last throwing point before that item's effects are published
+    /// into `working`/`final_ops`. `ChunkReseed` fires once at each chunk boundary of a chunked flush --
+    /// immediately AFTER the just-full chunk committed durably (its survivors already completed) and
+    /// BEFORE `working`/the trial-id high-water mark are reseeded from the now-live state; it is the
+    /// injection point for the tenure-exception-containment contract (a throw here, or from the reseed
+    /// itself, must leave the committed chunk's callers with their success). `PostDurableInstall` fires
+    /// inside `commitRefChunk` after that chunk's `PUT` returned `Committed` and BEFORE the prepared
+    /// candidate is installed into the live state -- the seam a test uses to prove that the region
+    /// between "durable" and "recorded" can no longer strand a transaction (a throw injected there is
+    /// the only way left to simulate the OLD post-durable apply failure, since the install itself is now
+    /// allocation-free and cannot throw). `PostInstallPreAck` fires after the candidate swap and overlay
+    /// materialization, outside the allocation-denied scope and state lock, but before any waiter is
+    /// marked done or notified. It is the deterministic acknowledgment-order seam. Null in production.
+    enum class CarvePhaseForTest
+    {
+        PlanSeenRefs,
+        PlanBatchGrow,
+        PlanReserveOwned,
+        PublishPop,
+        ValidateFinalOps,
+        ChunkReseed,
+        PostDurableInstall,
+        PostInstallPreAck,
+    };
+    void setCarveHookForTest(std::function<void(CarvePhaseForTest)> hook) { carve_hook_for_test = std::move(hook); }
+
+    /// Installs the negative control for the post-durable install region (see
+    /// `install_region_probe_for_test`).
+    void setInstallRegionProbeForTest(std::function<void()> probe) { install_region_probe_for_test = std::move(probe); }
+
+    /// Installs the pre-tenure fault seam (see `ref_pre_tenure_hook_for_test`).
+    void setRefPreTenureHookForTest(std::function<void()> hook) { ref_pre_tenure_hook_for_test = std::move(hook); }
+
+    /// Pauses an ordinary append after it captured an exact runtime but before recovery or enqueue.
+    void setAppendAfterRuntimeCaptureHookForTest(std::function<void()> hook)
+    {
+        append_after_runtime_capture_hook_for_test = std::move(hook);
+    }
+
+    /// Pauses `resolveRef` after it captured/recovered an exact runtime but before the result state lock.
+    void setReadBeforeStateLockHookForTest(std::function<void()> hook)
+    {
+        read_before_state_lock_hook_for_test = std::move(hook);
+    }
+
+    /// Pauses a cold read after its catalog `GET` returned but before the observed life can be
+    /// published into the local name slot.
+    void setReadableCatalogAfterObservationHookForTest(std::function<void()> hook)
+    {
+        readable_catalog_after_observation_hook_for_test = std::move(hook);
+    }
+
+    /// Pauses `namespaceStillLogicallyPresent`'s cold path after its FIRST catalog `GET` but before any
+    /// decision is made from it (including the "no row" revalidation's own second read).
+    void setNamespacePresenceProbeAfterFirstReadHookForTest(std::function<void()> hook)
+    {
+        namespace_presence_probe_after_first_read_hook_for_test = std::move(hook);
+    }
+
+    /// Pauses `namespaceStillLogicallyPresent`'s `Removing` branch after it has proven the observed
+    /// incarnation's terminal (the exact-life recovery/lock section is done) but before its
+    /// post-terminal catalog revalidation read.
+    void setNamespacePresenceProbeAfterTerminalProvenHookForTest(std::function<void()> hook)
+    {
+        namespace_presence_probe_after_terminal_proven_hook_for_test = std::move(hook);
+    }
+
+    /// Pauses a wedge retry after it captured the exact predecessor attempt but before the request
+    /// controller is allowed to send a retry.
+    void setWedgeBeforeSlotOccupyHookForTest(std::function<void()> hook)
+    {
+        wedge_before_slot_occupy_hook_for_test = std::move(hook);
+    }
+
+    /// Counts exact recovery-result publications.
+    uint64_t recoveryInstallCountForTest() const
+    {
+        return recovery_install_count_for_test.load(std::memory_order_relaxed);
+    }
+
+    /// Pauses a direct snapshot publisher after it captured the runtime state but before its first
+    /// durable effect. Used only to exercise predecessor deletion/rebirth races deterministically.
+    void setSnapshotAfterCaptureHookForTest(std::function<void()> hook)
+    {
+        snapshot_after_capture_hook_for_test = std::move(hook);
+    }
+
+    /// Pauses a snapshot publisher after its body PUT and at the admission check immediately before
+    /// each `_ckpt` CAS attempt. This is intentionally inside the retrying checkpoint primitive.
+    void setSnapshotBeforeCkptCasHookForTest(std::function<void()> hook)
+    {
+        snapshot_before_ckpt_cas_hook_for_test = std::move(hook);
+    }
+
+    /// Returns the number of queued mutations for `ns` under the queue mutex.
+    size_t refQueuePendingForTest(const RootNamespace & ns)
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        const auto it = ref_name_slots.find(ns.string());
+        return it == ref_name_slots.end() ? 0 : it->second.current->pending.size();
+    }
+
+    /// Reports whether `ns` currently has an active append-lane leader (the baton). Under the queue mutex.
+    bool refLeaderActiveForTest(const RootNamespace & ns)
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        const auto it = ref_name_slots.find(ns.string());
+        return it != ref_name_slots.end() && it->second.current->leader_active;
+    }
+
+    /// Returns the number of callers currently waiting for `ns` recovery under its state mutex.
+    uint64_t refRecoveryWaitersForTest(const RootNamespace & ns)
+    {
+        const auto rt = lookupRefTableRuntime(ns);
+        if (!rt)
+            return 0;
+        std::lock_guard<std::mutex> g(rt->state_mutex);
+        return rt->recovery_waiters_for_test;
+    }
+
+    /// Returns the number of namespace runtimes currently retained in the cache.
+    size_t refTablesCachedCountForTest()
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        return std::count_if(ref_name_slots.begin(), ref_name_slots.end(), [](const auto & entry)
+        {
+            return static_cast<bool>(entry.second.current);
+        });
+    }
+    /// Reports whether `ns` has a cached runtime whose recovery completed.
+    bool refTableCachedForTest(const RootNamespace & ns)
+    {
+        const auto rt = lookupRefTableRuntime(ns);
+        if (!rt)
+            return false;
+        std::lock_guard<std::mutex> g(rt->state_mutex);
+        return rt->recovered;
+    }
+    /// Stable identity of the cached runtime object, or zero when no slot exists. This distinguishes
+    /// explicit life invalidation from an eviction/remount that would make a rebirth test pass by
+    /// constructing a different cache object.
+    uint64_t refTableRuntimeIdentityForTest(const RootNamespace & ns)
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        const auto it = ref_name_slots.find(ns.string());
+        return it == ref_name_slots.end() || !it->second.current ? 0 : it->second.current->runtime_id;
+    }
+    uint64_t refTableRuntimeAdmittedFenceGenerationForTest(const RootNamespace & ns)
+    {
+        std::lock_guard<std::mutex> g(ref_queue_mutex);
+        const auto it = ref_name_slots.find(ns.string());
+        return it == ref_name_slots.end() || !it->second.current ? 0 : it->second.current->admitted_fence_generation;
+    }
+    /// The physical life currently pinned in the cached runtime, without resolving or recovering it.
+    std::optional<NamespaceLifeId> refTableLifeForTest(const RootNamespace & ns)
+    {
+        std::shared_ptr<RefTableRuntime> rt;
+        {
+            std::lock_guard<std::mutex> g(ref_queue_mutex);
+            const auto it = ref_name_slots.find(ns.string());
+            if (it == ref_name_slots.end())
+                return std::nullopt;
+            rt = it->second.current;
+            if (!rt)
+                return std::nullopt;
+        }
+        return rt->life;
+    }
+    /// Recovery-publication inventory accessors: the seeded per-table admission budgets, the recovered
+    /// base snapshot's encoded body size and the tail-since-snapshot byte sum. Together with
+    /// `newestPublishedSnapshotIdForTest`,
+    /// `tailSinceSnapshotCountForTest`, `needsStalePrecommitSweepForTest` and the resolved state, they
+    /// let a test assert EVERY `RecoveryResult` field the install seeds.
+    uint64_t refSnapshotBudgetForTest(const RootNamespace & ns)
+    {
+        const auto rt = lookupRefTableRuntime(ns);
+        if (!rt)
+            return 0;
+        std::lock_guard<std::mutex> g(rt->state_mutex);
+        return rt->snapshot_budget;
+    }
+    uint64_t refRemovalBudgetForTest(const RootNamespace & ns)
+    {
+        const auto rt = lookupRefTableRuntime(ns);
+        if (!rt)
+            return 0;
+        std::lock_guard<std::mutex> g(rt->state_mutex);
+        return rt->removal_budget;
+    }
+    uint64_t refBaseSnapshotBytesForTest(const RootNamespace & ns)
+    {
+        const auto rt = lookupRefTableRuntime(ns);
+        if (!rt)
+            return 0;
+        return rt->base_snapshot_bytes.load(std::memory_order_relaxed);
+    }
+    uint64_t refTailBytesSinceSnapshotForTest(const RootNamespace & ns)
+    {
+        const auto rt = lookupRefTableRuntime(ns);
+        if (!rt)
+            return 0;
+        return rt->tail_bytes_since_snapshot.load(std::memory_order_relaxed);
+    }
+    /// Describes the one conditional `PUT` whose outcome is still uncertain for a table. It is owned by
+    /// `Writing` or `Wedged` and by no other lane state; it stays installed until the object is confirmed
+    /// durable and applied to the cache, or definitely rejected.
+    ///
+    /// The four fields together are the attempt's IDENTITY. THREE of them are compared before any later
+    /// result is acted on -- `txn_id`, `bytes` and `admitted_fence_generation` (`resolveWedgeOnce`'s
+    /// post-I/O recheck, which calls them "all three components of the identity"). The key is not
+    /// compared because it adds nothing: `Layout::refLogKey` is a function of
+    /// `(NamespaceLifeId, RefTxnId)`, and the namespace life is fixed for the runtime that holds the
+    /// attempt, so within one runtime an equal `txn_id` already implies an equal key. (Should a runtime
+    /// ever span two incarnations of one namespace -- it does not today -- that implication is what
+    /// would need re-checking, not the comparison list.)
+    ///
+    /// Comparing the id alone -- or the admission generation alone -- is the aliasing bug the phase-0
+    /// model found: two attempts of the same table can carry the same id under the same generation and
+    /// describe DIFFERENT bytes, and installing one attempt's candidate because the other's key resolved
+    /// is precisely the acked-then-lost class this every-attempt rule exists to close.
+    ///
+    /// Public because it is a member of `PreparedRefChunk`, which `prepareRefChunk` returns.
+    struct RefAppendAttempt
+    {
+        RefTxnId txn_id;
+        String key;
+        String bytes;
+        /// `CasMountRuntime::fenceGeneration()` as read at this transaction's ADMISSION -- the same
+        /// critical section that snapshotted the state and derived the id, i.e. one atomic reading of
+        /// "what this attempt was allowed to do". Every later `slotOccupy` retry is gated on THIS value
+        /// (never the current one), and every install is preceded by presenting it back through
+        /// `checkFenceOrThrow`: a retry admitted under a dead incarnation must send nothing, and a
+        /// result that returns after a fence bump/re-arm must install nothing.
+        uint64_t admitted_fence_generation = 0;
+    };
+
+    /// Everything `commitRefChunk` DECIDES before this chunk can have any durable effect, as one value.
+    struct PreparedRefChunk
+    {
+        RefTableState candidate;         /// the snapshot with this chunk applied -- deliberately NOT materialized
+        RefTxnId candidate_base_id;      /// greatest-applied of the state prepared FROM
+        RefLogTxn chunk_txn;             /// includes INV-2's chain link
+        RefAppendAttempt prepared_attempt;  /// COMPLETE: txn id, canonical key, sealed bytes, admitted generation
+        /// Set iff this chunk births the namespace. The VALUE only: `commitRefChunk` still publishes it,
+        /// because for a birth chunk that publish IS the first durable effect and preparation is by
+        /// definition everything strictly before it.
+        std::optional<RefCkpt> birth_contribution;
+        /// Published after the log object commits and before this candidate may be installed or
+        /// acknowledged. Every transaction advances `committed_through`; an epoch-seal transaction
+        /// contributes the same id as `last_epoch_seal` in this one atomic checkpoint merge.
+        RefCkpt commit_contribution;
+    };
+
+    /// The pure half of `commitRefChunk`: derive the candidate state, the transaction, the canonical key
+    /// and sealed bytes, the complete attempt, a namespace birth's pre-log `_ckpt` contribution, and
+    /// the post-log committed-frontier contribution -- all decided before anything can be durable.
+    ///
+    /// `static` on purpose, and it is load-bearing rather than stylistic: with no `this` no MEMBER backend
+    /// is reachable, and `static` is what removes the injected one -- so "backend-free" is
+    /// CHECKABLE instead of promised. That is what lets the protocol arithmetic be swept exhaustively
+    /// with no store at all: `gtest_cas_ref_chunk_preparation.cpp` names no backend and constructs none,
+    /// and no future edit inside this function can quietly reach for one and still compile there.
+    ///
+    /// `state` is CONSUMED: the caller snapshots `rt->state` under `state_mutex` and hands the snapshot
+    /// over, so the candidate IS that snapshot rather than a second copy of it (the pre-extraction code
+    /// made exactly one copy, under the lock, and this keeps it at one). The snapshot deliberately shares
+    /// the live state's COW bases and is NOT materialized here -- folding a base-sharing state would
+    /// rebuild the whole base, O(table) per chunk, which the install path exists to avoid.
+    ///
+    /// `id`, `chain_link` and `admitted_generation` are INPUTS, not derived here, because each traces back
+    /// to the SAME critical section that snapshots the state (INV-1): `id` and `admitted_generation` are
+    /// read inside that hold, and `chain_link` is derived just outside it from the `last_epoch_seal` read
+    /// inside it. Deriving the id from a different instant than the state it is applied to would be
+    /// deriving it from a different stream. The critical section therefore stays in `commitRefChunk`, and
+    /// this is a pure function of its arguments -- what that one atomic reading saw, plus `layout`,
+    /// `life` and `ops`.
+    ///
+    /// Every throw out of here is an ordinary PRE-durability rejection: no object exists, the cache is
+    /// untouched, and the id is simply never used (the next attempt re-derives it from the same unchanged
+    /// state). The reachable ones are a rejected apply and a failed seal; an allocation failure anywhere
+    /// inside is the same class, and so is `checkNamespace`'s `BAD_ARGUMENTS` on the key-building path
+    /// (unreachable for a mounted table, listed so the enumeration does not read closed). See the FAULT
+    /// CLASS note on the definition for what changed about WHO catches them.
+    ///
+    /// `life` (Stage B, Task 4-C) is `RefTableRuntime::life` -- the caller's already-resolved catalog
+    /// life, read under the SAME critical section as `id`/`admitted_generation` (a caller with no
+    /// resolved life yet has no business preparing a chunk at all: `ensureRefTableRecovered` resolves
+    /// it before this table is exposed as recovered).
+    static PreparedRefChunk prepareRefChunk(const Layout & layout, const NamespaceLifeId & life,
+                                            RefTableState state, const RefTxnId & id,
+                                            const std::optional<RefTxnId> & chain_link,
+                                            std::span<const RefOp> ops, uint64_t admitted_generation);
+
+private:
+    /// Injected storage and mount environment. The member order is part of construction/destruction
+    /// behavior because the callbacks and references are used by the runtime owned below.
+    Backend & backend;
+    const Layout & layout;
+    RefLedgerConfig config;
+    const CasEventSink & event_sink;
+    CasRequestBudget cas_request_budget;
+    /// This mount's `server_root_id`; see the constructor parameter's doc for why it is a plain
+    /// member rather than an injected callback.
+    String server_root_id;
+    std::function<uint64_t()> live_epoch_fn;
+    std::function<bool()> fence_ok_fn;
+    std::function<uint64_t()> fence_generation_fn;
+    std::function<void(uint64_t)> check_fence_or_throw;
+    std::function<uint64_t()> boot_ms_now_fn;
+    std::function<bool()> may_mutate;
+    std::function<void(const String &, const String &, const std::optional<String> &)> on_impossible_interference;
+    std::function<std::shared_ptr<void>()> pin_owner;
+    std::function<void(const RootNamespace &)> cancel_inflight_builds;
+
+    /// Backoff sleep used by `ensureRefTableRecovered`'s transient-retry loop. Default is an
+    /// interruptible slice-sleep (bails early if `fence_ok_fn` drops, e.g. on shutdown/lease loss);
+    /// `setCasRetrySleepForTest` overrides it (a unit test injects a clock-advancing no-op).
+    std::function<void(uint64_t)> recovery_retry_sleep_fn;
+
+    /// One queued append caller. `build_ops` is invoked at most once by the flush leader and returns the
+    /// caller's operations rather than mutating storage directly. Completion fields are synchronized by
+    /// `ref_queue_mutex`.
+    struct RefMutationItem
+    {
+        MutationScope scope;
+        std::function<std::vector<RefOp>(const RefTableState &)> build_ops;
+        RootMutationOrigin origin = RootMutationOrigin::Writer;
+        RootMutationKind kind = RootMutationKind::Publish;
+        /// Capability carried only by `dropNamespaceImpl` after it closed the exact runtime's positive
+        /// lane. `RootMutationKind::DropNamespace` is descriptive metadata, not removal authority: the
+        /// public generic append surface accepts that enum and must not thereby gain terminal rights.
+        bool terminal_removal_authorized = false;
+        bool done = false;                       /// guarded by ref_queue_mutex
+        std::exception_ptr error;                /// guarded by ref_queue_mutex
+        RefTxnId committed_id{};                  /// written by the leader before done = true
+    };
+
+    /// One coherent decoded `RefTableState` and append runtime for a namespace. It is recovered lazily
+    /// and evicted only as a whole. `state_mutex` is separate from
+    /// `ref_queue_mutex` (which only ever guards `pending`/`leader_active`) so a reader (resolveRef/
+    /// listRefs) can observe `state` without contending with the flush leader's network round trip --
+    /// the leader only holds `state_mutex` for the brief copy-out-before-validate and the
+    /// apply-after-commit steps, never for the `putIfAbsentControlled` call itself.
+    struct RefTableRuntime
+    {
+        /// An allocator can reuse an evicted predecessor's address for its successor. This monotone id
+        /// is therefore the only diagnostic identity used to test exact detach/rebirth.
+        const uint64_t runtime_id;
+
+        /// Exact catalog identity accepted before publication. It is part of the runtime key and can
+        /// never be rebound; a same-name successor is a different runtime object.
+        const NamespaceLifeId life;
+
+        RefTableRuntime(uint64_t runtime_id_, NamespaceLifeId life_, uint64_t admitted_fence_generation_)
+            : runtime_id(runtime_id_)
+            , life(std::move(life_))
+            , admitted_fence_generation(admitted_fence_generation_)
+        {
+        }
+
+        /// A later re-arm cannot retarget this runtime; it creates a distinct object instead.
+        const uint64_t admitted_fence_generation;
+
+        std::mutex state_mutex;
+        bool recovered = false;
+        /// Gates the recovery-seal I/O that runs outside `state_mutex`. A second caller waits on
+        /// `recovery_cv` and rechecks `recovered` after the first caller finishes; otherwise it could
+        /// perform a competing LIST/replay/seal and misclassify the losing conditional `PUT` as failure.
+        /// Both this flag and the condition variable are guarded by `state_mutex`.
+        bool recovery_in_progress = false;
+        std::condition_variable recovery_cv;
+        /// The self-remount cancellation request (spec §3: "self-remount cancels or waits out recovery
+        /// before rearming"). Set by `cancelRecoveriesAndAwaitQuiescence` from the remount thread and
+        /// polled by the recovery walk at EVERY I/O boundary; a recovery that observes it abandons its
+        /// attempt having written nothing and installed nothing.
+        ///
+        /// ATOMIC, not `state_mutex`-guarded like its two neighbours, and that is the point: the
+        /// canceller must be able to publish the request WITHOUT queueing behind the very recovery it is
+        /// trying to stop. The condition variable is still the acknowledgment channel -- the canceller
+        /// waits for `recovery_in_progress` to fall under the mutex -- so the request is lock-free and
+        /// only the JOIN takes the lock.
+        std::atomic<bool> recovery_cancel_requested{false};
+        /// Test-only count of callers currently waiting for recovery; guarded by `state_mutex` so tests
+        /// can observe that a concurrent caller reached the wait without depending on scheduling.
+        uint64_t recovery_waiters_for_test = 0;
+        RefTableState state;
+        /// Exact attempt owned by `Writing` or `Wedged`, and retained by `NeedsRecovery` while an
+        /// otherwise-admitted writer recovery must still adjudicate the precise durable successor it
+        /// expected. It is cleared only when recovery installs its result or the attempt reaches a
+        /// conclusive terminal outcome; losing these bytes would turn a foreign replacement into an
+        /// indistinguishable ordinary recovery transaction.
+        std::optional<RefAppendAttempt> append_attempt;
+        RefLaneState lane_state = RefLaneState::Ready;
+        /// The `EpochSeal` transaction that closed this namespace's PREVIOUS writer epoch -- exactly the
+        /// `prev_epoch_seal` that this table's next sequence-1 transaction must carry (INV-2's grammar:
+        /// required on sequence 1 of every epoch above the namespace's genesis, forbidden everywhere
+        /// else). Guarded by `state_mutex`, and read in the SAME hold that derives the id, so the field
+        /// and the sequence number it qualifies are one reading.
+        ///
+        /// `nullopt` means GENESIS, and it means it exactly: the namespace's recovered state contains no
+        /// seal and its `greatest_applied.writer_epoch` is its own `life_epoch`, so its first
+        /// transaction opens the stream rather than continuing one across a transition. A namespace born
+        /// at global epoch 5 therefore appends `{5, 1}` with NO `prev_epoch_seal`; its first transition
+        /// (5 -> 6) seals `{5, T+1}`, and the `{6, 1}` that follows carries that seal.
+        ///
+        /// THREE producers set it, and only one of them is the one `commitRefChunk` normally reads from.
+        /// Recovery's CAS-walk installs the last seal of the chain it walked (Task 6): that is the
+        /// production path, because a real epoch change arrives with a self-remount, which DISCARDS every
+        /// cached runtime (`quiesceRefTablesForRemount`) and hands the fresh one its chain link through
+        /// recovery. The other two are the conclusive-rejection arms -- `resolveWedgeOnce`'s and
+        /// `commitRefChunk`'s -- which record a seal this runtime observed with its own eyes at a key it
+        /// owns. Within THIS runtime that record is mostly introspection (the seal it saw closes the
+        /// epoch it is still living in, so the stamp guard correctly suppresses it); it is written anyway
+        /// because it is durable evidence nothing else holds, and because it becomes the right answer the
+        /// moment the live epoch advances past the seal's. Nothing else writes it: a seal is durable
+        /// evidence, never a local guess.
+        std::optional<RefTxnId> last_epoch_seal;
+        uint64_t recovery_restarts = 0;           /// diagnostic: exact-GET restarts forced by a vanished object
+        /// Per-table admission budgets: raw configured limits minus the table's `4 + ns.size()` wire
+        /// overhead and the fixed safety margin, computed once at recovery.
+        uint64_t snapshot_budget = 0;
+        uint64_t removal_budget = 0;
+
+        /// Number and encoded-byte sum of applied transactions strictly newer than `newest_snapshot_id`.
+        /// The live `state` is the next snapshot candidate, so no separate tail replay is retained.
+        /// These counters are atomic because the cache-budget pass reads them while holding
+        /// `ref_queue_mutex`, whereas append and publication paths hold `state_mutex`; relaxed ordering
+        /// is sufficient because the counters do not publish any other state.
+        std::atomic<uint64_t> tail_count_since_snapshot{0};
+        std::atomic<uint64_t> tail_bytes_since_snapshot{0};
+        std::optional<RefTxnId> newest_snapshot_id;
+        /// Whole-table cache-weight bookkeeping for `enforceRefTableCacheBudget`.
+        /// `base_snapshot_bytes` is the encoded body size of the snapshot
+        /// at `newest_snapshot_id` (0 for a never-published table), captured for free from the
+        /// recovered/published snapshot body -- refreshed only when that snapshot changes (recovery +
+        /// each publish), never per mutation. The estimated resident weight is
+        /// `base_snapshot_bytes + tail_bytes_since_snapshot`. `base_snapshot_bytes` is ATOMIC (relaxed)
+        /// for the same cross-lock `total`-loop read as `tail_bytes_since_snapshot` above. `last_touch_tick`
+        /// is the monotonic access stamp (`Pool::ref_table_access_tick`) used to evict least-recently-
+        /// touched tables first; it is read only in the `use_count()==1`-gated candidate loop (no
+        /// concurrent writer there), so it stays a plain `uint64_t`.
+        std::atomic<uint64_t> base_snapshot_bytes{0};
+        uint64_t last_touch_tick = 0;
+        /// Set true by recovery; cleared when a sweep attempt is
+        /// dispatched (so the sweep's own nested `appendRefOps` calls do not recurse) and PERMANENTLY
+        /// only once an attempt completes VERIFIED CLEAN (a full pass over the live state found zero
+        /// stale bindings). Any failed or partial attempt re-arms it (with the
+        /// `precommit_sweep_backoff_*` cooldown), so a later read/mutation trigger retries until clean --
+        /// a single attempt burned in the post-restart error window must not leave a dead incarnation's
+        /// precommit bindings protected from GC forever on a long-lived mount.
+        bool needs_stale_precommit_sweep = false;
+        /// Per-table retry cooldown for the stale-precommit sweep (guarded by `state_mutex`),
+        /// mirroring `publish_backoff_*` below: `until` is the boottime instant before which
+        /// `maybeSweepStalePrecommits` refuses to re-attempt; `ms` is the current exponential interval
+        /// (0 = no failure yet, or reset by the last verified-clean sweep).
+        uint64_t precommit_sweep_backoff_until_ms = 0;
+        uint64_t precommit_sweep_backoff_ms = 0;
+        /// Test-observability + graceful settling for the background snapshot-publish dispatch (see
+        /// `maybeScheduleSnapshotPublish`): the count of in-flight publish attempts for this table, and
+        /// the condvar (guarded by `state_mutex`) a test waits on via `waitForSnapshotPublishSettleForTest`.
+        std::atomic<int> pending_snapshot_publishes{0};
+        std::condition_variable publish_settle_cv;
+        /// Per-table snapshot-publish dispatch backoff (guarded by
+        /// `state_mutex`). `publish_backoff_until_ms` is the boottime instant before which
+        /// `maybeScheduleSnapshotPublish` refuses to dispatch; `publish_backoff_ms` is the current
+        /// exponential interval (0 = not backing off), doubled on each consecutive non-Committed publish
+        /// outcome and reset to 0 on the next durable publish.
+        uint64_t publish_backoff_until_ms = 0;
+        uint64_t publish_backoff_ms = 0;
+
+        std::deque<std::shared_ptr<RefMutationItem>> pending;    /// guarded by ref_queue_mutex
+        bool leader_active = false;                               /// guarded by ref_queue_mutex
+        /// Set before the exact `Live -> Removing` catalog CAS and retained until that life is deleted.
+        /// New positive mutations check it in the same queue critical section as admission; the one
+        /// terminal `DropNamespace` item is the sole exception.
+        bool removal_admission_closed = false;
+        std::condition_variable cv;
+
+        /// Published after GC commits the exact catalog deletion. The exact cache pointer is detached;
+        /// old holders observe the flag and remain a predecessor, never a rebindable name handle.
+        std::atomic<bool> catalog_life_invalidated{false};
+
+        /// Set true when a self-remount detaches this runtime from the cache (`quiesceRefTablesForRemount`):
+        /// the fresh incarnation re-recovers each table under the new epoch on next touch, so any leader
+        /// still holding THIS (now-orphaned) runtime must fail closed instead of allocating an id / applying
+        /// against its stale cache once the re-armed fence re-opens the gate. Stored with release BEFORE the
+        /// remount re-arms the fence, so a lane that observes `mayMutate` true also observes this flag
+        /// (release/acquire through the fence) -- there is no interleaving where a stale runtime both passes
+        /// the fence and reads this flag false.
+        std::atomic<bool> superseded_by_remount{false};
+    };
+
+    /// Appends against an exact runtime already captured by a lifecycle operation. This is the sole
+    /// path used by namespace removal after its exact `Live -> Removing` catalog transition: resolving
+    /// the logical name again there would either refuse the required terminal append or, after a
+    /// replacement, retarget destructive work to the successor. Ordinary mutations enter through the
+    /// public name-based `appendRefOps` wrapper and can never select this path.
+    RefTxnId appendRefOpsOnRuntime(
+        const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt, MutationScope scope,
+        std::function<std::vector<RefOp>(const RefTableState &)> build_ops,
+        RootMutationOrigin origin, RootMutationKind kind, bool skip_stale_precommit_sweep,
+        bool terminal_removal_authorized);
+
+    /// Publishes only from the exact runtime captured by the caller. Background dispatch carries this
+    /// pointer across the thread hand-off; it must never resolve the logical name again and accidentally
+    /// publish a same-name successor while settling the predecessor's in-flight accounting.
+    bool tryPublishSnapshotAndAdvanceCheckpointOnceOnRuntime(
+        const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt);
+
+    /// The logical-name cache owns no lifecycle identity. It merely points at the runtime currently
+    /// admitted for that name; exact retirement/remount clears this pointer while external holders may
+    /// continue using the detached predecessor and fail closed against its immutable identity.
+    struct RefNameSlot
+    {
+        std::shared_ptr<RefTableRuntime> current;
+    };
+    static constexpr size_t kMaxRefBatch = 1000;
+    /// Recovery retries at most this many times when an object selected by LIST vanishes before GET.
+    /// A failed recovery-seal `PUT` is separate: it leaves `recovered` false and the next touch starts
+    /// a fresh LIST/replay/seal attempt rather than resuming this bounded vanish-retry loop.
+    static constexpr size_t kRefRecoveryMaxRestarts = 3;
+    /// How many times the CAS-walk may lose the SAME dead epoch's seal slot to a straggler before it
+    /// fails closed. Not an arbitrary round number: INV-1's every-attempt rule permits AT MOST ONE
+    /// in-flight conditional create per (table, writer), and there is one writer per mount, so an honest
+    /// run needs ONE retry. The margin covers a dying writer whose lane held several attempts across
+    /// distinct incarnations; anything beyond it is a store that keeps materializing objects underneath
+    /// a walk, which is a fact to report, never one to keep looping on.
+    static constexpr size_t kRefRecoveryMaxSlotAttemptsPerEpoch = 64;
+    /// Fixed safety margin subtracted (alongside the per-table `4 + ns.size()` overhead) from
+    /// the raw `ref_snapshot_max_bytes`/`ref_removal_max_bytes` hard limits before calling `admits`.
+    static constexpr uint64_t kRefAdmissionSafetyMargin = 4096;
+
+    /// `mutable` for `confirmExactRef`, the one CONST member function that needs the lane snapshot:
+    /// taking a mutex to read consistently does not make the read a mutation.
+    mutable std::mutex ref_queue_mutex;
+    std::map<String, RefNameSlot> ref_name_slots;
+    /// Monotonic access stamp for whole-table cache LRU eviction, bumped on every table touch and
+    /// recorded in `RefTableRuntime::last_touch_tick`.
+    std::atomic<uint64_t> ref_table_access_tick{0};
+    std::atomic<uint64_t> next_ref_runtime_id{0};
+    std::atomic<uint64_t> recovery_install_count_for_test{0};
+    /// Latched by `drainRefLanesForShutdown` before it
+    /// snapshots `ref_name_slots`/waits on each table's queue -- every ordinary ref mutation (`appendRefOps`)
+    /// checks this under the SAME `ref_queue_mutex` critical section it uses to enqueue its item, so the
+    /// check-and-enqueue is atomic with the drain's snapshot-and-wait: a caller either enqueues strictly
+    /// before the drain observes this table (and the drain then waits for it), or observes this flag
+    /// already true and never enqueues at all. No caller can land a NEW item after the drain has decided
+    /// this table is idle.
+    std::atomic<bool> shutting_down{false};
+
+    /// The id `rt`'s next transaction carries (INV-1): `RefTableState::nextTxnId` of the table's OWN
+    /// state under the live writer epoch. There is no counter behind this -- the id is a pure function
+    /// of the state the transaction will be applied to, which buys two properties a pool-wide counter
+    /// could not:
+    ///   - each namespace's ids are dense `1..T` within one epoch, so a reader holding a table's log
+    ///     ids can tell a COMPLETE stream from a truncated one without consulting anything else;
+    ///   - an attempt that provably sent nothing consumes nothing: the state it derived from is
+    ///     unchanged, so the next caller derives the SAME id and no hole is left behind.
+    ///
+    /// A post-durable install failure moves the lane to `NeedsRecovery`, so this function is not called
+    /// again until replay has installed that durable transaction and advanced `greatest_applied`.
+    ///
+    /// The epoch component is the live mount incarnation's writer epoch, not the open-time
+    /// `process_epoch`: a self-remount allocates a strictly-greater durable writer_epoch, so every ref
+    /// transaction stamped after the remount sorts strictly ABOVE any (dead-incarnation or twin) log
+    /// still durable under an older epoch. `RefTxnId` compares epoch first, so the epoch bump alone
+    /// guarantees that a new log is never inserted at or below an already durable table log id.
+    ///
+    /// MUST be called with `rt.state_mutex` held, and the caller must apply the transaction to the SAME
+    /// state it read here: an id derived from one snapshot of a table and applied to another is not that
+    /// table's successor, and the apply-side density check would (correctly) reject it.
+    RefTxnId allocateRefTxnId(const RefTableRuntime & rt) const
+    {
+        return rt.state.nextTxnId(live_epoch_fn());
+    }
+
+    /// The CAS-owned retry controller this Pool's ref-log writer path uses for every
+    /// conditional log/snapshot `PUT` and uncertain-result resolution. Also shared (via the PartWriteTxn
+    /// `PartWriteTxn::stageManifest`'s part-manifest body `PUT` and by `PartWriteTxn::uploadFromSource`'s
+    /// blob-body create — both the streaming
+    /// `putIfAbsentStream` PUT and `promoteStaged`'s conditional server-side copy — via
+    /// `conditionalCreateControlled`. The controller is stateless per call (immutable
+    /// budget/clock/sleep — the sleep fn mutates only through the test-only seam, before traffic), so
+    /// concurrent lanes and builds use the one instance safely.
+    std::unique_ptr<CasRequestController> ref_request_controller;
+
+    /// Test-only hook called before a compatible append batch is carved; null in production.
+    std::function<void()> ref_pre_carve_hook_for_test;
+
+    /// Test-only fault seam fired on the CALLING thread at the instant a queue caller takes append-lane
+    /// leadership -- i.e. at the FIRST allocation that builds the leader's responsibility set, the last
+    /// throwing point before the baton (`leader_active`) is published. A throw here must leave the lane
+    /// idle (baton un-taken, the caller's item un-enqueued), never a permanently non-idle namespace with
+    /// no live leader. Null in production. See `appendRefOps`.
+    std::function<void()> ref_pre_tenure_hook_for_test;
+
+    /// Test-only hook fired at each carve/validation phase point (see `CarvePhaseForTest`); null in
+    /// production.
+    std::function<void(CarvePhaseForTest)> carve_hook_for_test;
+
+    /// Test-only probe fired inside either post-durable state install, under
+    /// `DENY_ALLOCATIONS_IN_SCOPE`: the ordinary committed install and wedge-resolution adoption. The
+    /// exact attempt is installed before sending, so `Unresolved` needs no post-I/O object install. It is the negative
+    /// control for the guard: a probe that allocates must abort a debug build, which is what proves the
+    /// region is armed and actually entered -- so a future edit that adds an allocating statement there
+    /// cannot pass unnoticed. It is also the only way left to reach `NeedsRecovery` from one of these
+    /// otherwise non-throwing regions. A test that installs a throwing probe must therefore disarm it
+    /// after the region it targets, or every later install throws too. Null in production.
+    std::function<void()> install_region_probe_for_test;
+    std::function<void()> append_after_runtime_capture_hook_for_test;
+    std::function<void()> read_before_state_lock_hook_for_test;
+    std::function<void()> readable_catalog_after_observation_hook_for_test;
+    std::function<void()> namespace_presence_probe_after_first_read_hook_for_test;
+    std::function<void()> namespace_presence_probe_after_terminal_proven_hook_for_test;
+    std::function<void()> wedge_before_slot_occupy_hook_for_test;
+    std::function<void()> snapshot_after_capture_hook_for_test;
+    std::function<void()> snapshot_before_ckpt_cas_hook_for_test;
+
+    /// Non-materializing diagnostic/cache lookup. It never observes the catalog and never creates a
+    /// name slot or runtime.
+    std::shared_ptr<RefTableRuntime> lookupRefTableRuntime(const RootNamespace & ns) const;
+
+    /// Publishes or returns one runtime for an exact catalog-observed life and admitted mount-fence
+    /// generation. A conflicting attached identity is a stale observation and fails closed rather than
+    /// retargeting either runtime.
+    std::shared_ptr<RefTableRuntime> acquireRefTableRuntime(
+        const NamespaceLifeId & life, uint64_t admitted_generation);
+
+    /// Lookup-first non-minting read acquisition. A cold name consults the catalog and materializes only
+    /// an exact `Live` life; absence/`Creating`/`Removing` returns no runtime.
+    std::shared_ptr<RefTableRuntime> acquireReadableRefTableRuntime(const RootNamespace & ns);
+
+    /// Lookup-first mutation acquisition. A cold name resolves or births the catalog life before
+    /// constructing its runtime.
+    std::shared_ptr<RefTableRuntime> acquireMutableRefTableRuntime(const RootNamespace & ns);
+
+    /// Common removal implementation. `expected_incarnation` is present for the decommission-only
+    /// exact-life overload and is checked before every lifecycle branch, including stalled creation.
+    DropNamespaceStats dropNamespaceImpl(
+        const RootNamespace & ns, const std::optional<UInt128> & expected_incarnation);
+
+    /// Lazily recovers `ns` per spec §4: catalog life resolution (first table-open only) -> `_ckpt` ->
+    /// exact-key base snapshot -> ARITHMETIC tail -> seal CAS-walk -> `_ckpt` CAS -> install. It does not
+    /// expose the table as recovered until every dead epoch it discovered is durably closed; concurrent
+    /// callers serialize across the whole unlocked I/O window through `recovery_in_progress`.
+    void ensureRefTableRecovered(const RootNamespace & ns, RefTableRuntime & rt);
+
+    /// Stage B (spec INV-3, §3): resolves `ns`'s catalog life -- ONCE per table-open, from inside
+    /// `ensureRefTableRecovered`'s transient-retry envelope, never from a per-write path (a per-write
+    /// catalog GET is a protocol-step addition and is vetoed). Three cases, closing over the catalog's
+    /// own three-state grammar:
+    ///   - no entry at all: this call is the namespace's first-ever opener. Mints one via
+    ///     `CasRefCatalog::createNamespace` under a `CreatorFence` built from `server_root_id`,
+    ///     `live_epoch` and `admitted_generation`;
+    ///   - an entry already `Live`/`Removing`: adopts its incarnation directly
+    ///     (`NamespaceLifeId::fromCatalogEntry`) -- `Removing` is adopted exactly like `Live` because
+    ///     this call only needs A life to key objects with; refusing WRITES to a `Removing` namespace
+    ///     is a different mechanism's job (Task 6's read-side contract), not this resolution's;
+    ///   - an entry `Creating`: if its `creator` fence is THIS mount's own (a previous attempt of this
+    ///     same open landed step 1 but not steps 2/3, e.g. after a transient error), resumes
+    ///     `completeCreation` directly over the observed entry; otherwise reconciles it via
+    ///     `CasRefCatalog::reconcileStaleCreator` + `isCreatorFenceTerminal`, refusing retry-later while
+    ///     the old creator's fence is not yet provably dead.
+    /// Every `createNamespace`/`completeCreation`/`reconcileStaleCreator` outcome that writes nothing
+    /// (`FencedOut`, `Superseded`, a reconciled entry, `EntryChanged`) re-reads the catalog and loops;
+    /// `CreatorFenceStillLive` throws the retry-later class, which this function's caller (the transient
+    /// retry loop) or a higher one re-drives. Bounded against a pathological duel between two openers;
+    /// each primitive this loop calls has its OWN bounded retry against the catalog's single object, so
+    /// this bound is only against THIS loop's re-read cycle.
+    NamespaceLifeId resolveNamespaceLife(
+        const RootNamespace & ns, uint64_t admitted_generation, uint64_t live_epoch,
+        bool * lifecycle_refusal = nullptr);
+
+    /// ONE attempt of the recovery walk, run with NO lock held (the candidate is private; nothing
+    /// touches `rt` until the install). `nullopt` REQUESTS A RESTART from a fresh listing -- the two
+    /// innocent explanations, a base that vanished under a checkpoint that moved and a hole a racing
+    /// cleanup could account for. Everything terminal throws.
+    ///
+    /// `admitted_generation` is the ONE fence generation this whole recovery was admitted under: the
+    /// walk presents it to every `slotOccupy` and to the `_ckpt` CAS, and the caller presents the same
+    /// value once more immediately before installing.
+    /// `retained_attempt` is copied under `state_mutex` before the unlocked walk. It is evidence from
+    /// this runtime's admitted writer, not a second recovery authority: only the exact slot it names
+    /// is compared byte-for-byte, and a successor seal remains the existing conclusive-loss case.
+    /// `cancelled` is the CALLER's latch, threaded in rather than kept locally: a cancellation is
+    /// reported through the retry-later class (the caller should retry, against the FRESH incarnation),
+    /// so without it the transient loop reads the stop as a blip and re-drives the very work the remount
+    /// just stopped -- while the barrier blocks waiting for that recovery to finish.
+    std::optional<RecoveryResult> runRecoveryWalkOnce(
+        const RootNamespace & ns, RefTableRuntime & rt, uint64_t admitted_generation, uint64_t live_epoch,
+        const std::optional<RefAppendAttempt> & retained_attempt, std::optional<String> & hole_detail,
+        bool & cancelled);
+
+    /// The I/O-boundary poll of the walk: is this recovery still entitled to continue? Two independent
+    /// facts, each of which alone disqualifies it -- a self-remount asked it to stop, or a self-remount
+    /// already detached its runtime. Throws; cancellation raises the retry-later class and LATCHES
+    /// through `cancelled` so the caller's transient loop does not re-drive it.
+    ///
+    /// The FENCE is deliberately absent: it gates the three sites that spend it (every `slotOccupy`, the
+    /// `_ckpt` CAS, the install), not every read. See the definition for why.
+    void checkRecoveryStillAdmitted(const RootNamespace & ns, RefTableRuntime & rt, bool & cancelled) const;
+
+    /// Publishes a completed streaming recovery into the runtime in one atomic step: copies EVERY field
+    /// `RecoveryResult` carries into `rt` and sets `recovered` LAST, so a waiter woken after this returns
+    /// never observes a partially-installed table. Struct-driven so a future added publication field
+    /// cannot be silently dropped from a scattered assignment list (spec §5). MUST hold `rt.state_mutex`.
+    void installRecoveryResult(RefTableRuntime & rt, RecoveryResult && result);
+
+    /// Evicts least-recently-touched, idle whole-table runtimes until the configured cache budget is met,
+    /// retaining `keep_ns` even when it is the next candidate.
+    void enforceRefTableCacheBudget(const RootNamespace & keep_ns);
+
+    /// Runs the append queue leader for `ns`, completing its own item and any compatible items carved
+    /// into the same batch. Exceptions are stored for waiters and do not leave the leader flag latched.
+    /// Every item this leader becomes responsible for -- its own `own` plus each item a flush removes
+    /// from `pending` to form a batch -- is recorded into `owned_items`, so the caller's leadership guard
+    /// can complete + de-pend any that a flush leaves unfinished on an exceptional exit.
+    void runRefQueueLeader(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt,
+                           const std::shared_ptr<RefMutationItem> & own,
+                           std::vector<std::shared_ptr<RefMutationItem>> & owned_items);
+
+    /// Validates, durably appends, and applies one compatible batch while preserving copy-before-commit
+    /// and apply-after-commit ordering -- the LIVE state is still only ever advanced once the object is
+    /// durable; `commitRefChunk`'s pre-`PUT` apply targets a private candidate that nothing else can
+    /// observe. Every item it carves out of `pending` is appended to
+    /// `owned_items` (the leader's responsibility set) at the moment it is carved. When a batch's total
+    /// op count exceeds `ref_txn_max_ops`, the validation loop emits SEVERAL ref-log transactions in one
+    /// tenure via `commitRefChunk` -- each a complete commit boundary.
+    void flushRefBatch(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt,
+                       std::vector<std::shared_ptr<RefMutationItem>> & owned_items);
+
+    enum class WedgeResolution : uint8_t
+    {
+        NoWedge,
+        Adopted,
+        Rejected,
+        StillWedged,
+        Corrupted,
+    };
+
+    struct WedgeResolutionResult
+    {
+        WedgeResolution kind = WedgeResolution::NoWedge;
+        std::exception_ptr survivor_error;
+    };
+
+    /// ONE bounded resolution attempt for `rt`'s outstanding wedge (spec INV-1's every-attempt rule):
+    /// at most one `slotOccupy(wedge.key, wedge.bytes, ...)` per calling flush, gated on the wedge's
+    /// ORIGINAL `admitted_fence_generation` rather than the current one. There is deliberately NO
+    /// background retry thread and no deadline-resetting loop: a permanently quiet wedged namespace
+    /// waits for its next caller or for a remount, which is acceptable precisely because the wedged
+    /// operation was never acknowledged.
+    ///
+    /// The conditional CREATE is what makes the rule "every attempt has its own conclusive rejection"
+    /// affordable: the ref-log key is write-once, so a create either lands our exact bytes (the
+    /// transaction is durable -- and it is the SAME transaction, byte for byte) or conflicts with
+    /// whatever is there, which the follow-up read then names. A read alone could only ever report
+    /// "absent", which is not a rejection: the earlier ambiguous attempt could still land afterwards.
+    ///
+    /// Post-I/O recheck: the outcome is adjudicated on an I/O result, so before ANY action follows from
+    /// it (adopt, acknowledge, unwedge, fail the survivors) this re-acquires `state_mutex`, presents
+    /// `admitted_fence_generation` back through `checkFenceOrThrow`, and compares the full wedge
+    /// identity against what is still installed. A result that returns after a fence bump/re-arm, or
+    /// after the wedge it belonged to was replaced, is INERT for this runtime.
+    WedgeResolutionResult resolveWedgeOnce(
+        const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt);
+
+    /// Commits ONE chunk of a `flushRefBatch` tenure as a complete ref-log transaction: allocates the
+    /// real transaction id, PREPARES the chunk (`prepareRefChunk` -- candidate, transaction, key, sealed
+    /// bytes, complete attempt, birth contribution), durably `PUT`s the sealed bytes, installs the
+    /// candidate under `state_mutex` by a no-throw swap, advances the tail counters, records the
+    /// per-transaction metrics, completes exactly `chunk_survivors` with the real id (waking their
+    /// waiters), and schedules snapshot publication. Preparation completes BEFORE the first durable
+    /// effect of EITHER chunk shape -- an ordinary chunk's ref-log `PUT`, and a `NamespaceBirth` chunk's
+    /// earlier `_ckpt` publish -- so that nothing between "durable" and "recorded" can throw (spec §A1);
+    /// a preparation failure is an ordinary pre-durability rejection. For the same reason the
+    /// `RefAppendAttempt` is built COMPLETE before the `PUT` -- the request reads its key and body -- so
+    /// the `Unresolved` arm only has to move it into the runtime: the OTHER thing that must be recorded
+    /// once the object may be durable. Arming that attempt into the runtime is NOT preparation (it
+    /// mutates `RefTableRuntime`) and stays here, between preparation and the first send.
+    /// Returns true when the chunk committed durably; false on any non-throwing failure (a rejected
+    /// apply / DefiniteFailure / unresolved wedge / a conclusive PUT rejection / an encode failure),
+    /// after having already failed `chunk_survivors` with the appropriate error. Past the durable `PUT`
+    /// it does not throw at all.
+    /// PRECONDITION: the caller has already released its scratch `working` copy so the post-commit
+    /// overlay fold is in place (the E5 fast path).
+    bool commitRefChunk(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt,
+                        const std::vector<RefOp> & chunk_ops,
+                        const std::vector<std::shared_ptr<RefMutationItem>> & chunk_survivors);
+
+    /// Enters the hard recovery fence after a transaction is known durable but cannot be installed.
+    /// The exact attempt is discarded because replay, not another write, is now the only legal owner.
+    static void requireRecovery(RefTableRuntime & rt, const RootNamespace & ns, std::string_view region) noexcept;
+
+    /// Leadership-exit guard for `appendRefOps`: under `ref_queue_mutex`, completes every still-unfinished
+    /// item this leader owned (with `flush_exception` when unwinding, or a fail-closed `LOGICAL_ERROR`
+    /// otherwise), removes each owned item from `pending` so no future leader can carve it, and releases
+    /// leadership (`leader_active = false` + `cv.notify_all`). On the normal path every owned item is
+    /// already `done`, so only the leadership release has effect. This is the single authority that
+    /// resets `leader_active` on any exit from the leader loop.
+    void completeOwnedItemsAndReleaseLeadership(
+        const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt,
+        const std::vector<std::shared_ptr<RefMutationItem>> & owned_items,
+        std::exception_ptr flush_exception);
+
+    /// Schedules best-effort background publication when tail thresholds and backoff permit. The
+    /// dispatch is fenced and the detached task retains the owner pin until it finishes.
+    void maybeScheduleSnapshotPublish(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt);
+
+    /// Merges one contribution into `life`'s `_ckpt` (spec INV-4), presenting `admitted_generation` back
+    /// through the pool's fence callback on every CAS attempt. The ledger owns no `CasMountRuntime`, so
+    /// this is only the place that assembles the deadline from the ledger's own injectable boot clock
+    /// and CAS budget; the algorithm itself is `publishCkpt`, shared verbatim with every other writer.
+    ///
+    /// THREE call sites (corrected here from "two": `runRecoveryWalkOnce`'s own sealer contribution
+    /// below is a third and was missing from this count), and they contribute DISJOINT fields:
+    ///   - `commitRefChunk`'s namespace-birth transaction contributes `life_epoch` -- it is the only
+    ///     writer that knows it (this transaction's own writer epoch), and spec §3 has the `_ckpt`
+    ///     created before the namespace becomes Live. The contribution is no longer built at the call
+    ///     site: `prepareRefChunk` returns it as `PreparedRefChunk::birth_contribution` and
+    ///     `commitRefChunk` passes that prepared value here. The split is deliberate -- DECIDING the
+    ///     contribution is pure preparation, while THIS call is a birth chunk's first durable effect, so
+    ///     the publish had to stay behind when preparation moved out;
+    ///   - `tryPublishSnapshotAndAdvanceCheckpointOnce` contributes `checkpoint_snapshot_id` once the
+    ///     snapshot body is durable, and contributes NOTHING about `life_epoch` (an absence, which the semantic-max
+    ///     merge leaves alone) because the publisher does not know it and must never guess;
+    ///   - `runRecoveryWalkOnce` contributes `last_epoch_seal` once its own CAS-walk minted or adopted
+    ///     one -- it is the only writer that mints seals, so it is the only writer that can record
+    ///     where the chain now ends.
+    CkptPublishOutcome publishCkptContribution(const NamespaceLifeId & life, const RefCkpt & contribution,
+                                               uint64_t admitted_generation,
+                                               const std::function<void(uint64_t)> & check_admission);
+
+    /// Common candidate predicate for scheduler admission and execution after capture. Caller holds
+    /// `rt.state_mutex`; an epoch seal is not state-bearing and cannot be snapshotted.
+    bool hasStateBearingSnapshotCandidateUnderStateLock(const RefTableRuntime & rt) const;
+
+    /// The Live + single-in-flight-gate + backoff + tail-threshold admission decision, factored out so
+    /// both the trigger (`maybeScheduleSnapshotPublish`) and the settlement re-evaluation share ONE
+    /// authority. The caller MUST hold `rt.state_mutex`; on admission this increments
+    /// `pending_snapshot_publishes` and returns true (the caller then dispatches). The fence check
+    /// (`may_mutate`) is the caller's responsibility (it is not held under `state_mutex`).
+    bool admitSnapshotPublishUnderStateLock(RefTableRuntime & rt);
+
+    /// Launches one detached publish attempt. Assumes `pending_snapshot_publishes` was already
+    /// incremented for this dispatch (by `admitSnapshotPublishUnderStateLock`). The task settles through
+    /// `settleSnapshotPublish`; if the thread cannot even be constructed, the count is undone and the
+    /// settle waiter notified so a leaked in-flight count never wedges shutdown/settle.
+    void dispatchSnapshotPublisher(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt);
+
+    /// Runs at the end of one detached publish attempt: drops this attempt's in-flight count and, under
+    /// the SAME `state_mutex` hold, re-evaluates the accumulated tail so a trigger the single-flight gate
+    /// discarded during this attempt (e.g. chunks 2..N of a chunked tenure whose chunk-1 publish was
+    /// in flight) is re-fired instead of lost. Re-admitting under the same lock keeps the in-flight count
+    /// from transiently reaching zero across the handoff, so a settle waiter never observes a false
+    /// "settled". Notifies the settle condvar only when no follow-up publish is dispatched.
+    void settleSnapshotPublish(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt);
+
+    /// Advances the exponential delay after a non-durable snapshot publication outcome.
+    void advancePublishBackoff(RefTableRuntime & rt);
+    /// Clears the snapshot-publication delay after durable progress.
+    void resetPublishBackoff(RefTableRuntime & rt);
+
+    /// Checks whether recovery or a mutation requested stale-precommit cleanup and dispatches it when
+    /// its per-table cooldown permits.
+    void maybeSweepStalePrecommits(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt);
+
+    /// Performs one fenced stale-precommit sweep. A partial or failed sweep re-arms the requirement and
+    /// propagates its exception; only a verified-clean pass clears it.
+    void sweepStalePrecommitsNow(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt);
+
+    /// Runs the read-triggered sweep without allowing an uncertain maintenance append to fail the read.
+    void sweepStalePrecommitsForRead(const RootNamespace & ns, const std::shared_ptr<RefTableRuntime> & rt);
+
+    /// Advances the stale-precommit sweep's exponential cooldown after failure.
+    void advancePrecommitSweepBackoff(RefTableRuntime & rt);
+    /// Clears the stale-precommit sweep cooldown after a verified-clean pass.
+    void resetPrecommitSweepBackoff(RefTableRuntime & rt);
+
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.cpp
new file mode 100644
index 000000000000..36f126bad20c
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.cpp
@@ -0,0 +1,1108 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <Common/scope_guard_safe.h>
+#include <algorithm>
+#include <limits>
+#include <mutex>
+#include <type_traits>
+#include <utility>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+CatalogLifeIndex::CatalogLifeIndex(const RefCatalog & catalog)
+{
+    for (const CatalogEntry & entry : catalog.entries)
+    {
+        const NamespaceLifePhysicalId life_id = entry.incarnation;
+        if (auto ambiguous_it = ambiguous_names.find(life_id); ambiguous_it != ambiguous_names.end())
+        {
+            ambiguous_it->second.push_back(entry.ns.string());
+            continue;
+        }
+
+        const auto [it, inserted] = unique_lives.emplace(
+            life_id, NamespaceLifeId::fromCatalogEntry(entry.ns, life_id));
+        if (inserted)
+            continue;
+
+        std::vector<String> names;
+        names.push_back(it->second.ns.string());
+        names.push_back(entry.ns.string());
+        unique_lives.erase(it);
+        ambiguous_names.emplace(life_id, std::move(names));
+    }
+}
+
+bool CatalogLifeIndex::isAmbiguous(NamespaceLifePhysicalId life_id) const
+{
+    return ambiguous_names.contains(life_id);
+}
+
+std::optional<NamespaceLifeId> CatalogLifeIndex::resolve(NamespaceLifePhysicalId life_id) const
+{
+    if (const auto ambiguous_it = ambiguous_names.find(life_id); ambiguous_it != ambiguous_names.end())
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS ref catalog: life_id {} is shared by current namespaces '{}' and '{}' -- both rows are unresolvable",
+            renderIncarnation(life_id), ambiguous_it->second[0], ambiguous_it->second[1]);
+    if (const auto it = unique_lives.find(life_id); it != unique_lives.end())
+        return it->second;
+    return std::nullopt;
+}
+
+void CatalogLifeIndex::throwIfAmbiguous(std::string_view consumer) const
+{
+    if (ambiguous_names.empty())
+        return;
+    const auto & [life_id, names] = *ambiguous_names.begin();
+    throw Exception(ErrorCodes::CORRUPTED_DATA,
+        "{}: catalog life_id {} is shared by current namespaces '{}' and '{}' -- refusing a decision from an ambiguous cut",
+        consumer, renderIncarnation(life_id), names[0], names[1]);
+}
+
+namespace
+{
+
+/// Txn-wide structural check, NOT a per-op precondition: if `ops` contains a
+/// `RemoveNamespace`, it must be the last element, and every earlier op must be an exact
+/// owner-removal `owner_transition` (`old_binding` set, `new_binding` empty). The sole other legal
+/// form is `[NamespaceBirth, RemoveNamespace]`: a cataloged life may own `_ckpt`/`_files` without ever
+/// having emitted a ref transaction, and its empty birth+terminal must be one durable removal record.
+/// `CasRefLogCodec` deliberately does not check this shape -- this is the one place that does.
+void checkRemoveNamespaceOrdering(const std::vector<RefOp> & ops)
+{
+    const bool has_remove = std::any_of(ops.begin(), ops.end(),
+        [](const RefOp & op) { return op.kind == RefOpKind::RemoveNamespace; });
+    if (!has_remove)
+        return;
+
+    if (ops.empty() || ops.back().kind != RefOpKind::RemoveNamespace)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefTableState: remove_namespace must be the final operation of its transaction");
+
+    if (ops.size() == 2 && ops.front().kind == RefOpKind::NamespaceBirth)
+        return;
+
+    for (size_t i = 0; i + 1 < ops.size(); ++i)
+    {
+        const RefOp & op = ops[i];
+        const bool pure_removal = op.kind == RefOpKind::OwnerTransition
+            && op.old_binding.has_value() && !op.new_binding.has_value();
+        if (!pure_removal)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefTableState: every operation before remove_namespace must be an exact owner removal");
+    }
+}
+
+/// Installed test probe for the streaming-recovery memory invariant (see the header). Guarded by its
+/// own mutex so an install/clear from a test thread cannot tear against a concurrent recovery.
+std::mutex g_recovery_replay_memory_probe_mutex;
+std::function<void(int64_t)> g_recovery_replay_memory_probe;
+
+/// The four legal `owner_transition` shapes, decided purely from the (old_binding, new_binding)
+/// optionals and their `RefOwnerKind`s -- no state read. `RefTableState::applyOwnerTransition` (the
+/// writer/replay state machine) and `manifestEdgesOfTxn` (the GC fold's edge extractor) both switch
+/// over this single classification instead of each carrying their own shape predicates, so a shape
+/// neither consumer recognizes cannot silently acquire divergent meaning in one of them.
+enum class OwnerTransitionShape : uint8_t
+{
+    AddPrecommit,      /// no old_binding, new_binding.kind == Precommit
+    RemovePrecommit,   /// old_binding.kind == Precommit, no new_binding
+    RemoveCommitted,   /// old_binding.kind == Committed, no new_binding
+    Promote,           /// old_binding.kind == Precommit, new_binding.kind == Committed, SAME ref_name
+                       /// and manifest_ref
+};
+
+/// Classify `op`'s (old_binding, new_binding) shape into one of the four legal transitions. Anything
+/// else -- neither binding, old+new naming DIFFERENT manifests, a promote whose old/new ref_name
+/// disagree, or any other kind combination -- throws `CORRUPTED_DATA` naming the offending combination
+/// instead of falling through to a caller that would otherwise assign it accidental meaning.
+[[nodiscard]] OwnerTransitionShape classifyOwnerTransitionShape(const RefOp & op)
+{
+    const bool has_old = op.old_binding.has_value();
+    const bool has_new = op.new_binding.has_value();
+
+    if (!has_old && has_new && op.new_binding->kind == RefOwnerKind::Precommit)
+        return OwnerTransitionShape::AddPrecommit;
+
+    if (has_old && !has_new && op.old_binding->kind == RefOwnerKind::Precommit)
+        return OwnerTransitionShape::RemovePrecommit;
+
+    if (has_old && !has_new && op.old_binding->kind == RefOwnerKind::Committed)
+        return OwnerTransitionShape::RemoveCommitted;
+
+    if (has_old && has_new && op.old_binding->kind == RefOwnerKind::Precommit
+        && op.new_binding->kind == RefOwnerKind::Committed
+        && op.old_binding->ref_name == op.new_binding->ref_name
+        && op.old_binding->manifest_ref == op.new_binding->manifest_ref)
+        return OwnerTransitionShape::Promote;
+
+    throw Exception(ErrorCodes::CORRUPTED_DATA,
+        "owner_transition does not match any legal transition shape (has_old={}, old_kind={}, "
+        "has_new={}, new_kind={})",
+        has_old, has_old ? std::to_string(static_cast<uint8_t>(op.old_binding->kind)) : "n/a",
+        has_new, has_new ? std::to_string(static_cast<uint8_t>(op.new_binding->kind)) : "n/a");
+}
+
+}
+
+void setRecoveryReplayMemoryProbeForTest(std::function<void(int64_t delta_footprint_bytes)> probe)
+{
+    std::lock_guard lock(g_recovery_replay_memory_probe_mutex);
+    g_recovery_replay_memory_probe = std::move(probe);
+}
+
+void reportReplayMemoryDelta(int64_t delta_footprint_bytes)
+{
+    /// Reads the installed probe under the mutex and calls it OUTSIDE the lock so the probe body may
+    /// itself do arbitrary work. A no-op in production (no probe installed).
+    std::function<void(int64_t)> probe;
+    {
+        std::lock_guard lock(g_recovery_replay_memory_probe_mutex);
+        probe = g_recovery_replay_memory_probe;
+    }
+    if (probe)
+        probe(delta_footprint_bytes);
+}
+
+uint64_t decodedRefLogTxnFootprint(const RefLogTxn & txn)
+{
+    /// A deterministic proxy for the heap a decoded transaction keeps alive: the ns string, the op
+    /// vector's element storage (ops count x per-op record size), and every owned ref-name string. Uses
+    /// `size()` (not `capacity()`) so the value depends only on the decoded content, hence is identical
+    /// across a streaming decode and a materialising control's decode of the same object.
+    uint64_t bytes = txn.ns.size() + txn.ops.size() * sizeof(RefOp);
+    for (const RefOp & op : txn.ops)
+    {
+        bytes += op.ref_name.size();
+        if (op.old_binding)
+            bytes += op.old_binding->ref_name.size();
+        if (op.new_binding)
+            bytes += op.new_binding->ref_name.size();
+    }
+    return bytes;
+}
+
+/// Member-wise swap; see the header for the install-region contract it exists for. Every member is
+/// enumerated here by hand rather than swapped through a generated move, so a member added to the
+/// class without a line here is a silent state-corruption bug -- the `static_assert`s below are the
+/// type-level half of the guarantee (the macro at the call site proves the code path, these prove the
+/// contract of the types), and `debugAssertBodyCounters` cross-checks the counters this swap carries.
+void RefTableState::swap(RefTableState & other) noexcept
+{
+    static_assert(std::is_nothrow_swappable_v<RefLifecycle>);
+    static_assert(std::is_nothrow_swappable_v<std::optional<RefTxnId>>);
+    static_assert(std::is_nothrow_swappable_v<RefTxnId>);
+    static_assert(std::is_nothrow_swappable_v<std::set<std::pair<String, ManifestRef>>>);
+    static_assert(std::is_nothrow_swappable_v<uint64_t>);
+    static_assert(noexcept(std::declval<RefCowMap &>().swap(std::declval<RefCowMap &>())));
+    static_assert(noexcept(std::declval<RefCowManifestSet &>().swap(std::declval<RefCowManifestSet &>())));
+
+    using std::swap;
+    swap(lifecycle, other.lifecycle);
+    swap(remove_txn_id, other.remove_txn_id);
+    swap(greatest_applied, other.greatest_applied);
+    committed.swap(other.committed);
+    precommits.swap(other.precommits);
+    owned_manifests.swap(other.owned_manifests);
+    swap(snapshot_body_bytes, other.snapshot_body_bytes);
+    swap(removal_body_bytes, other.removal_body_bytes);
+}
+
+/// True iff `manifest_ref` already names an existing committed row or precommit binding under ANY
+/// ref_name (the add-precommit rule: "no conflicting owner may name the same manifest"). O(1) via
+/// `owned_manifests`, a COW membership index (Pool/CasRefCowManifestSet.h) that every ownership-
+/// changing arm below (and `stateFromSnapshot`) maintains in lock-step with `committed` and
+/// `precommits`. The old linear scan lives on, in debug/sanitizer builds only, as
+/// `debugAssertBodyCounters`'s cross-check that the index has not drifted from those two containers.
+bool RefTableState::manifestAlreadyOwned(const ManifestRef & manifest_ref) const
+{
+    return owned_manifests.contains(manifest_ref);
+}
+
+/// The `owner_transition` op kind: dispatches on the `(old_binding,
+/// new_binding)` shape to one of the four legal transitions (add precommit / remove precommit /
+/// remove committed / promote). Any other shape is not a recognized transition.
+void RefTableState::applyOwnerTransition(const RefOp & op)
+{
+    if (lifecycle != RefLifecycle::Live)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableState: owner_transition while namespace is not Live");
+
+    /// Shape legality is decided once, by the shared classifier; everything below is the per-shape
+    /// PRECONDITION check and effect, unchanged.
+    switch (classifyOwnerTransitionShape(op))
+    {
+        /// Add precommit: no old_binding, a fresh Precommit new_binding.
+        case OwnerTransitionShape::AddPrecommit:
+        {
+            const RefOwnerBinding & b = *op.new_binding;
+            if (precommits.contains({b.ref_name, b.manifest_ref}))
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableState: add precommit '{}' already exists for this exact manifest", b.ref_name);
+            /// Cross-owner uniqueness runs UNCONDITIONALLY, in every apply strategy (writer append AND
+            /// trusted replay). Since E2 this is an O(1) `owned_manifests` lookup, so the E1-era elision of
+            /// it under trusted replay (which downgraded it to a debug-only `chassert`) bought nothing
+            /// measurable while making a corrupted log/snapshot FAIL OPEN -- a double-owner input would drift
+            /// the index and let ordinary later writes append invariant-violating durable history. Keeping it
+            /// here is what makes replay fail CLOSED on a corrupted `manifest_ref` collision.
+            if (manifestAlreadyOwned(b.manifest_ref))
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableState: manifest already has a conflicting owner under another ref_name");
+            precommits.emplace(b.ref_name, b.manifest_ref);
+            snapshot_body_bytes += precommitRowEncodedSize(RefOwnerBinding{RefOwnerKind::Precommit, b.ref_name, b.manifest_ref});
+            removal_body_bytes  += removalOpEncodedSize(RefOwnerKind::Precommit, b.ref_name, b.manifest_ref);
+            owned_manifests.insert(b.manifest_ref);
+            return;
+        }
+
+        /// Remove precommit: an exact Precommit old_binding, no new_binding.
+        case OwnerTransitionShape::RemovePrecommit:
+        {
+            const RefOwnerBinding & b = *op.old_binding;
+            if (precommits.erase({b.ref_name, b.manifest_ref}) == 0)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableState: exact precommit binding '{}' to remove is absent", b.ref_name);
+            snapshot_body_bytes -= precommitRowEncodedSize(RefOwnerBinding{RefOwnerKind::Precommit, b.ref_name, b.manifest_ref});
+            removal_body_bytes  -= removalOpEncodedSize(RefOwnerKind::Precommit, b.ref_name, b.manifest_ref);
+            owned_manifests.erase(b.manifest_ref);
+            return;
+        }
+
+        /// Remove committed ref: an exact Committed old_binding, no new_binding.
+        case OwnerTransitionShape::RemoveCommitted:
+        {
+            const RefOwnerBinding & b = *op.old_binding;
+            const auto it = committed.find(b.ref_name);
+            if (it == committed.end() || !(it->second.manifest_ref == b.manifest_ref))
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableState: exact committed binding '{}' to remove is absent", b.ref_name);
+            const RefCommittedRow removed = it->second;
+            committed.erase(it);
+            snapshot_body_bytes -= committedRowEncodedSize(removed);
+            removal_body_bytes  -= removalOpEncodedSize(RefOwnerKind::Committed, removed.ref_name, removed.manifest_ref);
+            owned_manifests.erase(removed.manifest_ref);
+            return;
+        }
+
+        /// Promote: the SAME ref_name and manifest_ref move from Precommit to Committed
+        /// in one atomic step; the resulting row's `published_at_ms` starts unset (installed by the
+        /// companion set_published_at op in the same transaction, or a later one).
+        case OwnerTransitionShape::Promote:
+        {
+            const RefOwnerBinding & b = *op.old_binding;
+            if (precommits.erase({b.ref_name, b.manifest_ref}) == 0)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableState: exact precommit binding '{}' to promote is absent", b.ref_name);
+            snapshot_body_bytes -= precommitRowEncodedSize(RefOwnerBinding{RefOwnerKind::Precommit, b.ref_name, b.manifest_ref});
+            removal_body_bytes  -= removalOpEncodedSize(RefOwnerKind::Precommit, b.ref_name, b.manifest_ref);
+            /// `owned_manifests` is deliberately left untouched by a promote: `b.manifest_ref` moves from
+            /// precommit ownership to committed ownership without ever giving it up in between -- the
+            /// same "there is no moment at which the manifest has no owner" invariant this function's
+            /// header doc states for promote generally. The index tracks "does ANY owner currently name
+            /// this manifest", not which kind, so an erase-then-insert pair here would be pure overhead.
+            /// A DIFFERENT manifest already committed under this exact ref_name must be evicted by its
+            /// own explicit owner_transition(old=Committed, new=None) first (an earlier op of this same
+            /// transaction, or an earlier transaction) -- never silently here. `GC`'s manifest-edge delta
+            /// is read off the transaction's explicit ops, not a
+            /// before/after state diff; a promote that silently evicted a stale committed row would never
+            /// emit that manifest's "-1" edge, leaking it as phantom-alive forever.
+            if (committed.contains(b.ref_name))
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableState: promote '{}' would silently displace a different already-committed "
+                    "manifest -- remove it with an explicit owner_transition first", b.ref_name);
+            RefCommittedRow row;
+            row.ref_name = b.ref_name;
+            row.manifest_ref = b.manifest_ref;
+            snapshot_body_bytes += committedRowEncodedSize(row);
+            removal_body_bytes  += removalOpEncodedSize(RefOwnerKind::Committed, row.ref_name, row.manifest_ref);
+            committed.emplace(b.ref_name, std::move(row));
+            return;
+        }
+    }
+    /// Reachable only if a future `OwnerTransitionShape` enumerator is added without a matching `case`
+    /// (mirrors `applyOp`'s exhaustive-switch-then-throw shape below) -- `-Wswitch`/`-Werror` catches that
+    /// at compile time; this throw is the runtime backstop for builds without it.
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableState: unhandled owner_transition shape");
+}
+
+/// The `set_published_at` op kind: the committed ref must still name `expected_manifest_ref`; replaces
+/// `published_at_ms` without touching the manifest edge.
+void RefTableState::applySetPublishedAt(const RefOp & op)
+{
+    if (lifecycle != RefLifecycle::Live)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableState: set_published_at while namespace is not Live");
+
+    const auto it = committed.find(op.ref_name);
+    if (it == committed.end() || !(it->second.manifest_ref == op.expected_manifest_ref))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefTableState: set_published_at '{}' no longer names its expected_manifest_ref", op.ref_name);
+
+    /// `RefCowMap`'s iterator is read-only (Pool/CasRefCowMap.h): a write always goes through
+    /// `insert_or_assign`, never through the found iterator in place. Copy the row, apply the same
+    /// field mutation the old in-place code did, and write the whole row back -- this IS the COW
+    /// map's single-row copy-out, not a whole-table one.
+    RefCommittedRow updated = it->second;
+    const uint64_t old_row_bytes = committedRowEncodedSize(it->second);
+    updated.published_at_ms = op.published_at_ms;
+    snapshot_body_bytes -= old_row_bytes;
+    snapshot_body_bytes += committedRowEncodedSize(updated);
+    /// removal_body_bytes unchanged: set_published_at touches neither ref_name nor manifest_ref.
+    committed.insert_or_assign(op.ref_name, std::move(updated));
+}
+
+/// One operation's local preconditions and effect, shared by
+/// `applyRefLogTxn`'s per-op loop and by `admits`'s single-op preview. `txn_id` is only read by
+/// `RemoveNamespace` (it becomes the resulting `remove_txn_id`). Validation is identical regardless of
+/// which apply strategy reached here, so this takes no mode.
+void RefTableState::applyOp(const RefOp & op, const RefTxnId & txn_id)
+{
+    switch (op.kind)
+    {
+        case RefOpKind::NamespaceBirth:
+        {
+            /// Namespace birth is legal from an empty runtime admitted under a fresh catalog life,
+            /// never from `Live`. A predecessor still `Removing` is refused before recovery.
+            if (lifecycle == RefLifecycle::Live)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableState: namespace_birth while already Live");
+            lifecycle = RefLifecycle::Live;
+            remove_txn_id.reset();
+            return;
+        }
+        case RefOpKind::OwnerTransition:
+            applyOwnerTransition(op);
+            return;
+        case RefOpKind::SetPublishedAt:
+            applySetPublishedAt(op);
+            return;
+        case RefOpKind::RemoveNamespace:
+        {
+            /// Remove namespace: requires Live and both owner sets already empty -- true only
+            /// if this transaction's earlier ops (checked by `checkRemoveNamespaceOrdering`) actually
+            /// named every owner that existed when the transaction started.
+            if (lifecycle != RefLifecycle::Live)
+                throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableState: remove_namespace while not Live");
+            if (!committed.empty() || !precommits.empty())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableState: remove_namespace with nonempty owner sets");
+            /// `committed`/`precommits` empty implies `owned_manifests` empty too -- every entry in
+            /// the index is put there by an ownership change to one of those two containers. A
+            /// mismatch here means the index has drifted, not that this transaction is invalid.
+            chassert(owned_manifests.empty());
+            lifecycle = RefLifecycle::Removed;
+            remove_txn_id = txn_id;
+            return;
+        }
+        case RefOpKind::EpochSeal:
+            /// INV-2's in-band epoch closure. A seal carries NO table content, and that is its whole
+            /// design: its effect is that it OCCUPIES `{E, T+1}` -- the exact key a dying predecessor's
+            /// in-flight PUT would have taken, so the store's own write-once create becomes the fence --
+            /// and that it advances `greatest_applied`, which `applyTxnInPlace` does after this switch.
+            /// There is deliberately nothing to do here: a seal that changed a row would be a seal that
+            /// can lose one.
+            ///
+            /// `Live` is required because a seal closes the epoch of a LIVE stream. A never-born
+            /// namespace has no stream to close, and a `Removed` one already closed its own with the
+            /// terminal record -- in both cases a seal is a statement about a stream that does not
+            /// exist, i.e. an object built against a different table's history. Recovery's CAS-walk
+            /// enforces the identical gate on the minting side, so this is the read half of ONE rule
+            /// rather than a second one that can drift from it.
+            if (lifecycle != RefLifecycle::Live)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "RefTableState: epoch_seal at {}-{} while the namespace is not Live -- a seal closes the "
+                    "epoch of a live stream, and this table has none",
+                    txn_id.writer_epoch, txn_id.ref_sequence);
+            return;
+    }
+    /// Reachable only through a hand-corrupted RefOpKind (mirrors CasRefLogCodec.cpp's
+    /// exhaustive-switch-then-throw shape); every named enumerator returns above.
+    throw Exception(ErrorCodes::CORRUPTED_DATA, "RefTableState: unknown op kind {}", static_cast<uint8_t>(op.kind));
+}
+
+RefTableState stateFromSnapshot(const RefTableSnapshot & snapshot)
+{
+    const String bytes = encodeRefTableSnapshot(snapshot);
+    const RefTableSnapshot validated = decodeRefTableSnapshot(bytes, snapshot.ns, snapshot.snapshot_id);
+
+    RefTableState state;
+    /// A persisted snapshot is a materialization of a live stream only. `RefTableState` defaults to
+    /// `Removed`, so this assignment is deliberately explicit rather than relying on construction
+    /// defaults that have the opposite meaning.
+    state.lifecycle = RefLifecycle::Live;
+    state.greatest_applied = validated.snapshot_id;
+    /// The codec validated sortedness and no-duplicate ref_name/(ref_name, manifest_ref), but NEVER
+    /// cross-owner `manifest_ref` uniqueness -- a snapshot naming one manifest under two owners
+    /// (committed/committed, committed/precommit, or precommit/precommit) is semantically corrupt and
+    /// this is the one place that rejects it. The check runs before each `owned_manifests.insert` so it
+    /// reports "corrupt snapshot data" rather than the container's "index drifted = code bug" framing,
+    /// which would be the wrong diagnosis for malformed persisted data.
+    for (const RefCommittedRow & row : validated.committed)
+    {
+        if (state.owned_manifests.contains(row.manifest_ref))
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "stateFromSnapshot: snapshot names one manifest under two owners (committed ref '{}')", row.ref_name);
+        state.committed.emplace(row.ref_name, row);
+        state.snapshot_body_bytes += committedRowEncodedSize(row);
+        state.removal_body_bytes  += removalOpEncodedSize(RefOwnerKind::Committed, row.ref_name, row.manifest_ref);
+        state.owned_manifests.insert(row.manifest_ref);
+    }
+    for (const RefOwnerBinding & b : validated.precommits)
+    {
+        if (state.owned_manifests.contains(b.manifest_ref))
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "stateFromSnapshot: snapshot names one manifest under two owners (precommit ref '{}')", b.ref_name);
+        state.precommits.emplace(b.ref_name, b.manifest_ref);
+        state.snapshot_body_bytes += precommitRowEncodedSize(RefOwnerBinding{RefOwnerKind::Precommit, b.ref_name, b.manifest_ref});
+        state.removal_body_bytes  += removalOpEncodedSize(RefOwnerKind::Precommit, b.ref_name, b.manifest_ref);
+        state.owned_manifests.insert(b.manifest_ref);
+    }
+    return state;
+}
+
+#ifdef DEBUG_OR_SANITIZER_BUILD
+/// Debug/sanitizer-only: recompute both body totals from scratch and assert the incrementally
+/// maintained values match. This is what makes the incremental counters *provably* byte-exact rather
+/// than a drift-prone estimate -- the concern the old non-incremental admits() cited. O(N); compiled
+/// only in debug and sanitizer builds (`DEBUG_OR_SANITIZER_BUILD`, the same condition `chassert` fires
+/// under), so an ASan/TSan run exercises it too, not just a debug build.
+///
+/// Also rebuilds the expected `owned_manifests` membership by scanning `committed` + `precommits`
+/// (the same linear walk `manifestAlreadyOwned` used to do directly) and cross-checks it against the
+/// COW index: every scanned manifest must be present in the index, and the index's total size must
+/// equal the number of rows scanned -- together those two checks catch both a missing entry and a
+/// stale/extra one, which a size-only or membership-only check could each miss on their own.
+void RefTableState::debugAssertBodyCounters() const
+{
+    uint64_t snap = 0;
+    uint64_t rem = 0;
+    size_t owned_scanned = 0;
+    for (const auto [name, row] : committed)
+    {
+        snap += committedRowEncodedSize(row);
+        rem  += removalOpEncodedSize(RefOwnerKind::Committed, name, row.manifest_ref);
+        chassert(owned_manifests.contains(row.manifest_ref));
+        ++owned_scanned;
+    }
+    for (const auto & [name, mref] : precommits)
+    {
+        snap += precommitRowEncodedSize(RefOwnerBinding{RefOwnerKind::Precommit, name, mref});
+        rem  += removalOpEncodedSize(RefOwnerKind::Precommit, name, mref);
+        chassert(owned_manifests.contains(mref));
+        ++owned_scanned;
+    }
+    chassert(snapshot_body_bytes == snap);
+    chassert(removal_body_bytes == rem);
+    chassert(owned_manifests.size() == owned_scanned);
+}
+#endif
+
+void RefTableState::applyTxnInPlace(const RefLogTxn & txn)
+{
+    /// Txn-wide preconditions first, before any mutation.
+    if (!(greatest_applied < txn.txn_id))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefTableState: txn_id {}-{} is not strictly greater than the greatest applied {}-{}",
+            txn.txn_id.writer_epoch, txn.txn_id.ref_sequence,
+            greatest_applied.writer_epoch, greatest_applied.ref_sequence);
+
+    /// INV-1: within `(namespace, epoch)` the DURABLE ids are DENSE, so the only admissible id is the
+    /// one `nextTxnId` derives -- the same rule, on the same state, that the writer mints with. This is
+    /// what turns "these are the log ids I can see" into "this is the whole stream": a reader that finds
+    /// `1..T` knows nothing was lost, which no amount of strict-increase checking could tell it.
+    /// Enforcing it HERE, on the read side, is deliberate -- a hole cannot become durable even if some
+    /// future writer path forgets the rule, because every apply (writer candidate, recovery replay,
+    /// `fsck`'s oracle) runs it.
+    ///
+    /// The strict-increase check above is not subsumed by this one: it rejects an id under an OLDER
+    /// epoch, whose sequence would still look like a legitimate fresh-epoch `1`.
+    if (const RefTxnId expected = nextTxnId(txn.txn_id.writer_epoch); txn.txn_id != expected)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefTableState: txn_id {}-{} does not continue the ref-log stream — the greatest applied id "
+            "is {}-{}, so the only contiguous successor is {}-{}",
+            txn.txn_id.writer_epoch, txn.txn_id.ref_sequence,
+            greatest_applied.writer_epoch, greatest_applied.ref_sequence,
+            expected.writer_epoch, expected.ref_sequence);
+
+    /// INV-2's CONTEXTUAL seal grammar, on the read side. `prev_epoch_seal` is required on exactly
+    /// sequence 1 of a NON-genesis epoch and forbidden everywhere else; the structural half (shape,
+    /// well-formedness, strictly-earlier epoch) is the codec's, and this is the half that needs to know
+    /// whether this transaction OPENS a life or CONTINUES one across a transition.
+    ///
+    /// THE EQUIVALENCE ARGUMENT, because this is the load-bearing part. The answer is DERIVED from the
+    /// state, exactly and totally -- no `life_epoch` is plumbed in, and there is no optional to
+    /// substitute a zero for (which would demand a chain link on every sequence-1 transaction and reject
+    /// every genesis birth -- the trap task 5's interface note names):
+    ///
+    ///   - state NOT `Live` (never-born, or `Removed`) <=> this transaction can only be a BIRTH <=> the
+    ///     epoch it lands in IS this life's genesis epoch, so a chain link is FORBIDDEN. A link here
+    ///     would name a seal of a previous life this state has no trace of;
+    ///   - state `Live` <=> a prior life is PROVEN to exist below this epoch -- the namespace was applied
+    ///     at `greatest_applied.writer_epoch`, and the density check immediately above just proved this
+    ///     transaction's epoch is above it -- so the transition is non-genesis and a chain link is
+    ///     REQUIRED. Passing `greatest_applied.writer_epoch` yields the same verdict the true
+    ///     `life_epoch` would for EVERY value the true one can hold, since the true one is at or below it
+    ///     and the rule only compares "strictly above / at or below".
+    ///
+    /// The derivation is exact for every reachable state and no default exists anywhere in it.
+    ///
+    /// It is also STRICTLY BETTER than a plumbed-in `life_epoch` would be, and not only because it needs
+    /// no plumbing: `life_epoch` is a property of a LIFE, and a namespace can be removed and recreated.
+    /// A single global value carried alongside the table would answer for the wrong life after a
+    /// rebirth -- it would demand a chain link from a recreated namespace's very first transaction,
+    /// which by definition has none. Reading the lifecycle instead makes per-life semantics fall out for
+    /// free: a `Removed` state receiving a birth makes THAT epoch the NEW life's genesis, with no
+    /// catalog and no extra field to keep in step.
+    ///
+    /// Both arms go through the ONE shared validator rather than re-deriving its rule, so the writer's
+    /// encode-side check and this one cannot drift.
+    if (txn.txn_id.ref_sequence == 1)
+        validateEpochSealGrammarContextual(
+            txn, lifecycle == RefLifecycle::Live ? greatest_applied.writer_epoch : txn.txn_id.writer_epoch);
+
+    checkRemoveNamespaceOrdering(txn.ops);
+
+    /// Apply every op, in array order, IN PLACE. A throw leaves `*this` PARTIALLY APPLIED ("poisoned").
+    /// This is the poisoning strategy (E3, no scratch copy): sound ONLY on a state the caller discards
+    /// on any throw. The public `applyRefLogTxn` reaches it only through a scratch copy (turning it into
+    /// the strong guarantee); `replay` reaches it directly on its own local, discard-on-throw state,
+    /// which is what eliminates the per-transaction deep-copy of the replay tail's unbounded COW
+    /// overlays -- a K-transaction replay over an N-row base drops from O(K*N) to O(K + N).
+    for (const RefOp & op : txn.ops)
+        applyOp(op, txn.txn_id);
+    greatest_applied = txn.txn_id;
+#ifdef DEBUG_OR_SANITIZER_BUILD
+    /// Reached only on success, where `*this` is fully applied and its incremental body counters and
+    /// owned-manifest index are consistent -- the invariant this cross-check defends.
+    debugAssertBodyCounters();
+#endif
+}
+
+RefTxnId nextRefTxnId(RefTxnId greatest_applied, uint64_t live_epoch)
+{
+    return greatest_applied.writer_epoch == live_epoch
+        ? RefTxnId{live_epoch, greatest_applied.ref_sequence + 1}
+        : RefTxnId{live_epoch, 1};
+}
+
+void applyRefLogTxn(RefTableState & state, const RefLogTxn & txn)
+{
+    /// The one public apply entry point, ALWAYS the strong exception guarantee: validate and apply the
+    /// whole transaction against a scratch copy; replace `state` only once the whole transaction
+    /// succeeds, so a throw anywhere leaves `state` byte-for-byte unchanged and no intra-transaction
+    /// intermediate state is ever observable. This copy is cheap on every caller: each applies against a
+    /// materialized (empty-overlay) live state or a small bounded-overlay batch scratch, never the
+    /// unbounded replay tail (that is `replay`'s job, and it uses the private in-place strategy directly
+    /// on its own discard-on-throw local state).
+    RefTableState scratch = state;
+    scratch.applyTxnInPlace(txn);
+    state = std::move(scratch);
+}
+
+RefTableSnapshot snapshotOf(const RefTableState & state, const String & ns)
+{
+    if (state.getLifecycle() != RefLifecycle::Live)
+        throw Exception(
+            ErrorCodes::CORRUPTED_DATA,
+            "snapshotOf: terminal namespace state is not snapshot-serializable");
+
+    RefTableSnapshot snapshot;
+    snapshot.ns = ns;
+    snapshot.snapshot_id = state.getGreatestApplied();
+
+    snapshot.committed.reserve(state.getCommitted().size());
+    for (const auto [name, row] : state.getCommitted())
+        snapshot.committed.push_back(row);   /// RefCowMap iterates sorted by ref_name (Pool/CasRefCowMap.h)
+
+    snapshot.precommits.reserve(state.getPrecommits().size());
+    for (const auto & [name, manifest_ref] : state.getPrecommits())
+        snapshot.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, name, manifest_ref});
+        /// std::set<std::pair<String, ManifestRef>> iterates sorted by (ref_name, manifest_ref),
+        /// matching CasRefSnapshotCodec's required precommit sort order exactly.
+
+    return snapshot;
+}
+
+RefTableState replay(const std::optional<RefTableSnapshot> & snapshot, std::span<const RefLogTxn> tail)
+{
+    RefTableState state = snapshot ? stateFromSnapshot(*snapshot) : RefTableState{};
+
+    const String * expected_ns = snapshot ? &snapshot->ns : nullptr;
+    for (const RefLogTxn & txn : tail)
+    {
+        if (expected_ns && txn.ns != *expected_ns)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "RefTableState::replay: transaction ns '{}' does not match the table's ns '{}'",
+                txn.ns, *expected_ns);
+        expected_ns = &txn.ns;
+        /// The one place the poisoning in-place apply strategy is reached (E3): `state` is `replay`'s own
+        /// local, returned only after the WHOLE tail succeeds, so a mid-tail throw destroys it during
+        /// unwinding and no consumer ever observes a poisoned state. `applyTxnInPlace` still runs the
+        /// FULL validation `applyRefLogTxn` does -- including the cross-owner uniqueness check, which is
+        /// O(1) and no longer elided (post-consult) -- so a corrupted/collision-bearing tail fails closed
+        /// here rather than silently drifting the index.
+        state.applyTxnInPlace(txn);
+    }
+    return state;
+}
+
+RefReplayBuilder::RefReplayBuilder(std::optional<RefTableSnapshot> base, uint64_t base_encoded_bytes)
+{
+    if (base)
+    {
+        result.newest_snapshot_id = base->snapshot_id;
+        result.base_snapshot_bytes = base_encoded_bytes;
+        expected_ns = base->ns;
+        candidate = stateFromSnapshot(*base);   /// full snapshot revalidation, exactly as `replay`
+    }
+}
+
+void RefReplayBuilder::applyOne(RefLogTxn && txn, uint64_t encoded_bytes)
+{
+    /// The streaming-recovery memory probe is driven by the CALLER's loop (around GET->decode->this
+    /// call->discard), not here: the alive decoded-transaction set is a property of how the loop holds
+    /// its transactions, which `applyOne` -- seeing one at a time regardless -- cannot observe. See
+    /// `reportReplayMemoryDelta` / `decodedRefLogTxnFootprint`.
+    if (expected_ns && txn.ns != *expected_ns)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "RefReplayBuilder: transaction ns '{}' does not match the table's ns '{}'",
+            txn.ns, *expected_ns);
+    expected_ns = txn.ns;
+    /// The same private in-place poisoning path `replay` uses (E3): `candidate` is this builder's own
+    /// state, discarded on any throw (the builder is destroyed during unwinding), so a mid-tail
+    /// corruption fails closed here and no consumer ever observes a poisoned candidate. NOT the public
+    /// scratch-copying `applyRefLogTxn`, which would deep-copy the growing candidate once per
+    /// transaction and reintroduce the O(K*N) cost `replay` was written to avoid.
+    candidate.applyTxnInPlace(txn);
+    ++result.tail_count;
+    result.tail_bytes += encoded_bytes;
+}
+
+RecoveryResult RefReplayBuilder::finish() &&
+{
+    /// Matches `replay`: the candidate is returned WITHOUT `materializeCommitted` -- the writer's
+    /// recovery folds the COW overlays once on the result before installing it; the read-only consumers
+    /// (orphan sweep and fsck) do not need the fold at all.
+    result.state = std::move(candidate);
+    return std::move(result);
+}
+
+uint64_t encodedSnapshotBudgetSize(const RefTableState & state)
+{
+    /// `snapshotOf` uses `snapshot_id = state.greatest_applied` and an empty namespace here. Snapshot
+    /// lifecycle is fixed to live on the wire, so the framing depends only on those fields and the row
+    /// count.
+    const uint64_t rows = state.getCommitted().size() + state.getPrecommits().size();
+    return snapshotFramingSize("", state.getGreatestApplied(), rows)
+        + state.getSnapshotBodyBytes();
+}
+
+uint64_t encodedRemovalBudgetSize(const RefTableState & state)
+{
+    /// The hypothetical whole-namespace removal transaction uses a fixed {1,1} preview id, empty ns,
+    /// and one removal op per owner (committed + precommit) plus a terminal remove_namespace op -- so
+    /// op_count = committed + precommits + 1.
+    static constexpr RefTxnId kPreviewTxnId{1, 1};
+    const uint64_t rows = state.getCommitted().size() + state.getPrecommits().size();
+    return removalFramingSize("", kPreviewTxnId, rows + 1) + state.getRemovalBodyBytes();
+}
+
+bool admits(const RefTableState & state, const RefOp & op, uint64_t snapshot_budget, uint64_t removal_budget)
+{
+    /// A fixed nonzero placeholder id: this previews `op` in isolation and the scratch state is
+    /// discarded immediately after reading its (incrementally maintained) budget sizes.
+    static constexpr RefTxnId kPreviewTxnId{1, 1};
+
+    /// Previews an op that has not yet been validated or durably appended anywhere, so it gets the full
+    /// append-time check (the same `applyOp` the writer's apply path runs -- validation is strategy-
+    /// independent).
+    RefTableState scratch = state;
+    scratch.applyOp(op, kPreviewTxnId);   // throws exactly as before if `op` is not a legal transition
+#ifdef DEBUG_OR_SANITIZER_BUILD
+    scratch.debugAssertBodyCounters();
+#endif
+
+    if (encodedSnapshotBudgetSize(scratch) > snapshot_budget)
+        return false;
+    return encodedRemovalBudgetSize(scratch) <= removal_budget;
+}
+
+std::vector<RefManifestEdge> manifestEdgesOfTxn(const RefLogTxn & txn)
+{
+    std::vector<RefManifestEdge> edges;
+    edges.reserve(txn.ops.size());   /// every recognized op contributes at most one edge
+    const RootNamespace ns{txn.ns};
+
+    for (uint32_t op_ordinal = 0; op_ordinal < txn.ops.size(); ++op_ordinal)
+    {
+        const RefOp & op = txn.ops[op_ordinal];
+        if (op.kind != RefOpKind::OwnerTransition)
+            continue;
+
+        /// Shape legality is decided once, by the shared classifier -- the same one
+        /// `RefTableState::applyOwnerTransition` dispatches on -- so an unrecognized shape throws here
+        /// instead of silently acquiring accidental edge meaning (e.g. an old+new pair naming different
+        /// manifests, which the state machine never admits, used to read as a tolerated "replace").
+        switch (classifyOwnerTransitionShape(op))
+        {
+            case OwnerTransitionShape::AddPrecommit:
+                edges.push_back(RefManifestEdge{
+                    ManifestId{ns, op.new_binding->manifest_ref}, +1, op.new_binding->kind, op_ordinal, 1});
+                continue;
+            case OwnerTransitionShape::RemovePrecommit:
+            case OwnerTransitionShape::RemoveCommitted:
+                edges.push_back(RefManifestEdge{
+                    ManifestId{ns, op.old_binding->manifest_ref}, -1, op.old_binding->kind, op_ordinal, 0});
+                continue;
+            case OwnerTransitionShape::Promote:
+                /// Same-manifest owner move: the manifest keeps an owner the whole time, so there is no
+                /// net edge.
+                continue;
+        }
+        /// Reachable only if a future `OwnerTransitionShape` enumerator is added without a matching
+        /// `case` -- `-Wswitch`/`-Werror` catches that at compile time; this throw is the runtime
+        /// backstop for builds without it.
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "manifestEdgesOfTxn: unhandled owner_transition shape");
+    }
+
+    return edges;
+}
+
+std::optional<RefTxnId> removalTxnId(const RefLogTxn & txn)
+{
+    for (const RefOp & op : txn.ops)
+        if (op.kind == RefOpKind::RemoveNamespace)
+            return txn.txn_id;
+    return std::nullopt;
+}
+
+std::map<NamespaceLifePhysicalId, RefTableListing> groupRefKeys(
+    const Layout & layout, const std::vector<String> & listed_keys)
+{
+    const String base = layout.casRefsPrefix();
+    std::map<NamespaceLifePhysicalId, RefTableListing> out;
+
+    for (const String & key : listed_keys)
+    {
+        if (!key.starts_with(base))
+            continue;
+
+        const auto parsed = layout.parseRefObjectKey(key);
+        if (!parsed)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "groupRefKeys: key '{}' under the ref prefix is not a valid ref object -- aborting ref folding", key);
+
+        RefTableListing & table = out[parsed->life_id];
+        switch (parsed->kind)
+        {
+            case RefObjectKind::Log:
+                table.logs.push_back(parsed->txn_id);
+                break;
+            case RefObjectKind::Snap:
+                table.snapshots.push_back(parsed->txn_id);
+                break;
+        }
+    }
+
+    for (auto & [life_id, table] : out)
+    {
+        std::sort(table.logs.begin(), table.logs.end());
+        std::sort(table.snapshots.begin(), table.snapshots.end());
+    }
+
+    return out;
+}
+
+RefCleanupPlan planRefCleanup(const RefTableListing & listing, const RefTxnId & durable_cursor,
+                              std::optional<RefTxnId> checkpoint,
+                              std::optional<RefTxnId> retained_log_proof)
+{
+    RefCleanupPlan plan;
+
+    /// A physical `_snap` listed after its PUT but before the `_ckpt` CAS is not a recovery base.
+    /// The caller supplies `checkpoint` only after `readCheckpointSnapshotBase` has exact-read the
+    /// same-id non-seal `_log` and `_snap`. With no such validated triple cleanup has no coverage
+    /// authority and deliberately leaks every listed object.
+    if (!checkpoint)
+        return plan;
+
+    for (const RefTxnId & log_id : listing.logs)
+    {
+        if (durable_cursor < log_id)       /// L > cursor: its edge delta is not yet durable
+            continue;
+        if (*checkpoint <= log_id)         /// the exact witness and its successors remain
+            continue;
+        if (retained_log_proof == log_id)  /// a later-epoch base still needs its predecessor seal
+            continue;
+        plan.deletable_logs.push_back(log_id);
+    }
+
+    /// The checkpoint's same-id snapshot is the recovery base, so only strictly older listed snapshots
+    /// are deletion candidates.
+    for (const RefTxnId & snapshot_id : listing.snapshots)
+        if (snapshot_id < *checkpoint)
+            plan.deletable_snapshots.push_back(snapshot_id);
+
+    return plan;
+}
+
+EpochCrossResult crossEpochFromSeal(Backend & backend, const Layout & layout, const RootNamespace & ns,
+                                    const RefTxnId & from_seal, std::optional<bool> seal_proven,
+                                    const RefTxnId & witness, const NamespaceLifeId & life)
+{
+    EpochCrossResult result;
+    if (from_seal == RefTxnId{})
+    {
+        result.outcome = EpochCrossOutcome::NothingConsumed;
+        return result;
+    }
+    if (seal_proven && !*seal_proven)
+    {
+        result.outcome = EpochCrossOutcome::NotASeal;
+        return result;
+    }
+
+    /// `life`: REQUIRED, not resolved here (review NEW-3) -- an internal fallback resolve was tried
+    /// once already (review C3, `Gc::fold`) and once more here (fsck's own independent walk defaulted
+    /// to `nullopt` and re-resolved), and both times a caller that had already committed to one `life`
+    /// for the rest of its walk could silently diverge from this function's OWN resolution if the
+    /// namespace is dropped and recreated between the two reads. `CasFsck.cpp`'s stream walk resolves
+    /// `life` once, at the top of its own function, and must pass that SAME value here rather than let
+    /// this function re-derive it a second time.
+    uint64_t target_epoch = witness.writer_epoch;
+    while (target_epoch > from_seal.writer_epoch)
+    {
+        const RefTxnId start{target_epoch, 1};
+        result.probed = start;
+        const auto body = backend.get(layout.refLogKey(life, start));
+        if (!body)
+        {
+            ++result.absent_probes;
+            result.outcome = EpochCrossOutcome::StartAbsent;
+            return result;
+        }
+        ++result.body_gets;
+        RefLogTxn head;
+        try
+        {
+            head = decodeRefLogTxn(openObject(FormatId::RefLog, body->bytes), ns.string(), start);
+        }
+        catch (const Exception & e)
+        {
+            result.outcome = EpochCrossOutcome::StartInvalid;
+            result.detail = e.message();
+            return result;
+        }
+        if (!head.prev_epoch_seal || *head.prev_epoch_seal < from_seal)
+        {
+            result.outcome = EpochCrossOutcome::ChainDoesNotReach;
+            return result;
+        }
+        if (*head.prev_epoch_seal == from_seal)
+        {
+            result.outcome = EpochCrossOutcome::Proved;
+            result.start = start;
+            return result;
+        }
+        target_epoch = head.prev_epoch_seal->writer_epoch;
+    }
+    result.outcome = EpochCrossOutcome::ChainDoesNotReach;
+    return result;
+}
+
+std::optional<RefTxnId> nextRefLogIdWithinCommittedFrontier(
+    const RefTxnId & current, bool is_epoch_seal, const RefTxnId & committed_through)
+{
+    if (committed_through < current)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS checkpoint-bounded ref walk: current id {}-{} lies above committed_through {}-{}",
+            current.writer_epoch, current.ref_sequence,
+            committed_through.writer_epoch, committed_through.ref_sequence);
+    if (current == committed_through)
+        return std::nullopt;
+
+    if (is_epoch_seal)
+    {
+        if (committed_through.writer_epoch == current.writer_epoch)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS checkpoint-bounded ref walk: committed_through {}-{} lies after EpochSeal {}-{} in "
+                "the same numeric epoch",
+                committed_through.writer_epoch, committed_through.ref_sequence,
+                current.writer_epoch, current.ref_sequence);
+        if (current.writer_epoch == std::numeric_limits<uint64_t>::max())
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS checkpoint-bounded ref walk: EpochSeal {}-{} has no representable successor",
+                current.writer_epoch, current.ref_sequence);
+        return RefTxnId{current.writer_epoch + 1, 1};
+    }
+
+    if (current.ref_sequence == std::numeric_limits<uint64_t>::max())
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS checkpoint-bounded ref walk: log id {}-{} has no representable successor",
+            current.writer_epoch, current.ref_sequence);
+    return RefTxnId{current.writer_epoch, current.ref_sequence + 1};
+}
+
+CheckpointSnapshotBase readCheckpointSnapshotBase(
+    Backend & backend, const Layout & layout, const NamespaceLifeId & life, const RefCkpt & checkpoint)
+{
+    const RootNamespace & ns = life.ns;
+    if (!checkpoint.checkpoint_snapshot_id)
+    {
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS recovery for namespace '{}': checkpoint has no snapshot base",
+            ns.string());
+    }
+    if (!checkpoint.life_epoch)
+    {
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS recovery for namespace '{}': checkpoint-named snapshot base has no life_epoch context",
+            ns.string());
+    }
+    const RefTxnId snapshot_id = *checkpoint.checkpoint_snapshot_id;
+    const auto log = backend.get(layout.refLogKey(life, snapshot_id));
+    if (!log)
+    {
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS recovery for namespace '{}': checkpoint-named base snapshot {}-{} has no matching log under "
+            "the supplied immutable lifecycle authority",
+            ns.string(), snapshot_id.writer_epoch, snapshot_id.ref_sequence);
+    }
+
+    const RefLogTxn base_txn = decodeRefLogTxn(
+        openObject(FormatId::RefLog, log->bytes), ns.string(), snapshot_id);
+    if (refLogTxnIsEpochSeal(base_txn))
+    {
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS recovery for namespace '{}': checkpoint-named base snapshot {}-{} names an EpochSeal, "
+            "not a snapshot base",
+            ns.string(), snapshot_id.writer_epoch, snapshot_id.ref_sequence);
+    }
+    validateEpochSealGrammarContextual(base_txn, *checkpoint.life_epoch);
+    if (base_txn.prev_epoch_seal && checkpoint.last_epoch_seal && checkpoint.committed_through
+        && checkpoint.committed_through->writer_epoch == snapshot_id.writer_epoch
+        && checkpoint.last_epoch_seal->writer_epoch + 1 == snapshot_id.writer_epoch
+        && *base_txn.prev_epoch_seal != *checkpoint.last_epoch_seal)
+    {
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS recovery for namespace '{}': checkpoint-named base snapshot {}-{} refers to previous "
+            "epoch seal {}-{}, but checkpoint authority names {}-{}",
+            ns.string(), snapshot_id.writer_epoch, snapshot_id.ref_sequence,
+            base_txn.prev_epoch_seal->writer_epoch, base_txn.prev_epoch_seal->ref_sequence,
+            checkpoint.last_epoch_seal->writer_epoch, checkpoint.last_epoch_seal->ref_sequence);
+    }
+
+    std::optional<RefTxnId> predecessor_seal_id;
+    if (base_txn.prev_epoch_seal)
+    {
+        predecessor_seal_id = *base_txn.prev_epoch_seal;
+        const auto predecessor = backend.get(layout.refLogKey(life, *predecessor_seal_id));
+        if (!predecessor)
+        {
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS recovery for namespace '{}': checkpoint-named base snapshot {}-{} refers to absent "
+                "previous epoch seal {}-{}",
+                ns.string(), snapshot_id.writer_epoch, snapshot_id.ref_sequence,
+                predecessor_seal_id->writer_epoch, predecessor_seal_id->ref_sequence);
+        }
+        const RefLogTxn predecessor_txn = decodeRefLogTxn(
+            openObject(FormatId::RefLog, predecessor->bytes), ns.string(), *predecessor_seal_id);
+        if (!refLogTxnIsEpochSeal(predecessor_txn))
+        {
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS recovery for namespace '{}': checkpoint-named base snapshot {}-{} refers to non-seal "
+                "transaction {}-{}",
+                ns.string(), snapshot_id.writer_epoch, snapshot_id.ref_sequence,
+                predecessor_seal_id->writer_epoch, predecessor_seal_id->ref_sequence);
+        }
+    }
+
+    const auto snapshot = backend.get(layout.refSnapshotKey(life, snapshot_id));
+    if (!snapshot)
+    {
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS recovery for namespace '{}': checkpoint-named base snapshot {}-{} is absent under the supplied "
+            "immutable lifecycle authority",
+            ns.string(), snapshot_id.writer_epoch, snapshot_id.ref_sequence);
+    }
+    return CheckpointSnapshotBase{
+        .snapshot = decodeRefTableSnapshot(openObject(FormatId::RefSnapshot, snapshot->bytes), ns.string(), snapshot_id),
+        .bytes = snapshot->bytes.size(),
+        .predecessor_seal_id = predecessor_seal_id};
+}
+
+RecoveredRefTable recoverRefTableDetailedFromAuthority(
+    Backend & backend, const Layout & layout, const std::optional<CatalogEntry> & catalog_entry,
+    const std::optional<RefCkpt> & ckpt)
+{
+    /// The frozen catalog row and `_ckpt` supplied by the caller determine every recovery boundary;
+    /// this function must not re-read either mutable object, or enumerate the stream, because that
+    /// would splice unrelated physical observations into the caller's one authority cut.
+    RecoveryGrounding grounding = chooseRecoveryGrounding(catalog_entry, ckpt);
+    /// `chooseRecoveryGrounding` has just established that this is a Live/Removing row. Constructing
+    /// the life from that SAME value, rather than resolving the name again, preserves the caller's
+    /// catalog-cut join.
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(
+        catalog_entry->ns, catalog_entry->incarnation);
+    const RootNamespace & ns = life.ns;
+
+    std::optional<RefTxnId> base_id = grounding.base;
+    std::optional<RefTableSnapshot> base_snapshot;
+    uint64_t base_snapshot_bytes = 0;
+    if (base_id)
+    {
+        CheckpointSnapshotBase base = readCheckpointSnapshotBase(backend, layout, life, *ckpt);
+        base_snapshot = std::move(base.snapshot);
+        base_snapshot_bytes = base.bytes;
+    }
+
+    RefReplayBuilder builder(std::move(base_snapshot), base_snapshot_bytes);
+    if (grounding.walk_from && grounding.committed_through)
+    {
+        RefTxnId id = *grounding.walk_from;
+        while (id <= *grounding.committed_through)
+        {
+            const auto got = backend.get(layout.refLogKey(life, id));
+            if (!got)
+            {
+                /// `NamespaceLifeId` is opaque and unique to one logical life. A later birth has a
+                /// different stream prefix, so no absent slot at or below this life's exact frontier
+                /// can be explained as a rebirth; it is always durable-data loss.
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS read-only recovery for namespace '{}': committed log id {}-{} is absent under "
+                    "the supplied immutable checkpoint frontier {}-{}",
+                    ns.string(), id.writer_epoch, id.ref_sequence,
+                    grounding.committed_through->writer_epoch, grounding.committed_through->ref_sequence);
+            }
+
+            RefLogTxn txn = decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), id);
+            const bool is_seal = refLogTxnIsEpochSeal(txn);
+            const int64_t footprint = static_cast<int64_t>(decodedRefLogTxnFootprint(txn));
+            reportReplayMemoryDelta(footprint);
+            SCOPE_EXIT({ reportReplayMemoryDelta(-footprint); });
+            builder.applyOne(std::move(txn), got->bytes.size());
+
+            if (const std::optional<RefTxnId> next = nextRefLogIdWithinCommittedFrontier(
+                    id, is_seal, *grounding.committed_through))
+                id = *next;
+            else
+                break;
+        }
+    }
+
+    RecoveryResult result = std::move(builder).finish();
+    return RecoveredRefTable{
+        .state = std::move(result.state),
+        .newest_snapshot_id = result.newest_snapshot_id,
+        .last_epoch_seal = ckpt->last_epoch_seal};
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h
new file mode 100644
index 000000000000..d498a4eff062
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h
@@ -0,0 +1,794 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <base/types.h>
+#include <algorithm>
+#include <cstdint>
+#include <functional>
+#include <map>
+#include <optional>
+#include <set>
+#include <span>
+#include <string_view>
+#include <utility>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Reverse catalog index built from one decoded catalog cut. Every lifecycle state participates.
+/// A duplicated physical id remains represented as ambiguous so reporting tools can continue over
+/// unrelated unique ids; no caller can accidentally obtain a first-row-wins resolution.
+class CatalogLifeIndex
+{
+public:
+    explicit CatalogLifeIndex(const RefCatalog & catalog);
+
+    /// Unique logical life for `life_id`, absence for an id not present in the cut, and
+    /// `CORRUPTED_DATA` when multiple current rows share the id.
+    std::optional<NamespaceLifeId> resolve(NamespaceLifePhysicalId life_id) const;
+    bool isAmbiguous(NamespaceLifePhysicalId life_id) const;
+    bool hasAmbiguity() const { return !ambiguous_names.empty(); }
+
+    /// Destructive and catalog-mutating consumers require the whole cut to be unambiguous.
+    void throwIfAmbiguous(std::string_view consumer) const;
+
+private:
+    std::map<NamespaceLifePhysicalId, NamespaceLifeId> unique_lives;
+    std::map<NamespaceLifePhysicalId, std::vector<String>> ambiguous_names;
+};
+
+// Shared value types for the ref-ledger writer and the pure ref-log protocol helpers below. They live
+// in this protocol header so the ledger can depend on the carriers without making the pool and ledger
+// headers include one another.
+/// Whether a root-shard mutation originates from the writer path (user-visible publish/drop/precommit)
+/// or from GC/maintenance. Diagnostic-only (`toString`, event logging): recorded on the mutation item.
+enum class RootMutationOrigin : uint8_t
+{
+    Writer,
+    Gc,
+};
+
+/// The write-scope of one `appendRefOps` call (ref-append-lane batching): which part of the table the
+/// call touches. The flat-combining batch builder admits at most ONE mutation per ref name into a
+/// single flush (per-ref durable histories stay bit-identical to the unbatched protocol) and flushes
+/// `WholeShard` calls SOLO (dropNamespace and anything touching multiple refs wholesale).
+struct MutationScope
+{
+    enum class Kind : uint8_t { Ref, WholeShard };
+    Kind kind = Kind::WholeShard;
+    String ref_name;   /// set iff kind == Ref
+
+    /// Creates a scope for a mutation that touches exactly one ref name. The name is moved into the
+    /// scope because scopes are normally assembled as part of an append request.
+    static MutationScope ref(String name) { return {Kind::Ref, std::move(name)}; }
+
+    /// Creates a scope for a mutation that cannot safely share a batch with per-ref mutations.
+    static MutationScope wholeShard() { return {Kind::WholeShard, {}}; }
+};
+
+/// Kind of mutation being applied, used in diagnostic logging and metrics. Does not affect behaviour.
+enum class RootMutationKind : uint8_t
+{
+    Publish,
+    Drop,
+    Precommit,
+    Promote,
+    Abandon,
+    UpdateRefPublishedAt,
+    DropNamespace,
+    ReclaimPrecommit,
+};
+
+/// Human-readable name for `RootMutationOrigin` (diagnostic logging).
+inline std::string_view toString(RootMutationOrigin origin)
+{
+    switch (origin)
+    {
+        case RootMutationOrigin::Writer: return "Writer";
+        case RootMutationOrigin::Gc:     return "Gc";
+    }
+    return "Unknown";
+}
+
+/// Human-readable name for `RootMutationKind` (diagnostic logging).
+inline std::string_view toString(RootMutationKind kind)
+{
+    switch (kind)
+    {
+        case RootMutationKind::Publish:           return "Publish";
+        case RootMutationKind::Drop:              return "Drop";
+        case RootMutationKind::Precommit:         return "Precommit";
+        case RootMutationKind::Promote:           return "Promote";
+        case RootMutationKind::Abandon:           return "Abandon";
+        case RootMutationKind::UpdateRefPublishedAt: return "UpdateRefPublishedAt";
+        case RootMutationKind::DropNamespace:     return "DropNamespace";
+        case RootMutationKind::ReclaimPrecommit:  return "ReclaimPrecommit";
+    }
+    return "Unknown";
+}
+
+/// The result of resolving a ref: its namespace-qualified manifest identity, the manifest size, and
+/// the publication timestamp carried by the ref. A `Resolved` value does not own the manifest body.
+struct Resolved
+{
+    /// The namespace-qualified identity of the part manifest this ref names. The owning RootNamespace
+    /// + the ref's manifest_ref form the ManifestId (the ref carries no namespace itself — that comes
+    /// from the owning root context).
+    ManifestId manifest_id;
+    uint64_t manifest_size = 0;
+    uint64_t published_at_ms = 0;   /// publish wall-clock (epoch ms); 0 = unset
+};
+
+/// The non-identity portion of a committed-ref update. The ref's manifest identity is deliberately
+/// absent: changing reachability must use an owner transition, while this carrier is only for updating
+/// the publication timestamp without changing the manifest edge. In the current all-tree
+/// representation, per-part files are ordinary manifest entries rather than a separate mutable-file
+/// map, so `published_at_ms` is the remaining metadata that can be restamped in isolation.
+struct RefPublishedAtUpdate
+{
+    uint64_t published_at_ms = 0;   /// publish wall-clock (epoch ms); 0 = unset
+};
+
+
+/// Counts the committed refs and precommit bindings named by one namespace-removal transaction. The
+/// transaction contains one exact owner-removal operation for each count before its final
+/// `remove_namespace` operation; callers interested only in completion may ignore this summary.
+struct DropNamespaceStats
+{
+    uint64_t committed_refs = 0;
+    uint64_t precommits = 0;
+};
+
+/// Per-owner configuration passed by value to the ref ledger. It is a projection of the flat pool
+/// configuration: `server_root_id` is used in ref-lane diagnostics, while boot time and wait-sleep
+/// callbacks remain owned by the pool and are supplied separately because they describe live mount
+/// state rather than ref-ledger policy.
+struct RefLedgerConfig
+{
+    String server_root_id;
+    uint64_t gc_shards = 1;
+    uint64_t snapshot_log_count_threshold = 256;
+    uint64_t snapshot_log_bytes_threshold = 1ULL << 20;
+    uint64_t snapshot_publish_backoff_initial_ms = 200;
+    uint64_t snapshot_publish_backoff_max_ms = 30000;
+    uint64_t precommit_sweep_backoff_initial_ms = 200;
+    uint64_t precommit_sweep_backoff_max_ms = 30000;
+    uint64_t ref_table_cache_bytes = 256ULL << 20;
+};
+
+/// The ONE id-derivation rule (INV-1): the id that continues `greatest_applied`'s stream under
+/// `live_epoch`. Within an epoch a table's ids are dense -- the successor of the greatest applied one --
+/// and an epoch change restarts the sequence at 1, because density is a property of `(namespace, epoch)`
+/// and a fresh incarnation's stream is a fresh stream. There is no counter anywhere: the id is a pure
+/// function of the state it will be applied to, which is what makes an attempt that sent nothing
+/// consume nothing (the next caller derives the same id from the same unchanged state).
+///
+/// This is the rule itself, applied to an ANCHOR. Callers do not choose the anchor: they go through
+/// `RefTableState::nextTxnId`, which derives from `greatest_applied` -- the writer to
+/// mint an id, every trial preview to stamp its throwaway transaction, and `applyTxnInPlace` to decide
+/// whether a transaction's id is admissible. Sharing one rule is deliberate: an allocator and a checker
+/// that each spell it out separately can drift, and a drift here is either a durable hole or a table
+/// that refuses its own writes.
+///
+/// Total by construction (no throw): the one input it cannot serve, an exhausted `ref_sequence` under
+/// the live epoch, would need 2^64 transactions in one incarnation of one table. Should a corrupt
+/// persisted snapshot ever seed such a state, the wrap produces a `ref_sequence` of 0, which
+/// `applyTxnInPlace`'s strict-increase precondition rejects before anything is written.
+RefTxnId nextRefTxnId(RefTxnId greatest_applied, uint64_t live_epoch);
+
+/// The in-memory table state: `TableState = Replay(S_X.state, tail(X))`. This class, `applyRefLogTxn`, `snapshotOf`, and
+/// `replay` are the ONE shared implementation of that equation -- used verbatim by the writer, its
+/// own recovery path, `fsck`, and snapshot construction, so every consumer agrees on what a
+/// transaction sequence means. "Namespace" and "table" name the same entity throughout this file
+/// (and its callers): one `RefTableState` per `RootNamespace`.
+///
+/// Representation note (never-born vs removed): there is no separate "first birth" flag. Both
+/// "never born" and "Removed" default `lifecycle` to `RefLifecycle::Removed`; they are told apart by
+/// `remove_txn_id`: absent means the namespace has never completed a `remove_namespace` transaction
+/// (either truly never born, or -- from this class's point of view -- indistinguishable from it,
+/// which is fine because a `namespace_birth` op is legal from EITHER case and nothing else is legal
+/// from either). Present means a real removal happened and recorded its `RefTxnId`. `committed` and
+/// `precommits` are always empty while `lifecycle == Removed` (an invariant `applyRefLogTxn`
+/// maintains: `remove_namespace` only fires once both are already empty, and no other operation is
+/// legal until the next `namespace_birth`).
+class RefTableState
+{
+public:
+    RefTableState() = default;
+
+    RefLifecycle getLifecycle() const { return lifecycle; }
+    const std::optional<RefTxnId> & getRemoveTxnId() const { return remove_txn_id; }
+    const RefTxnId & getGreatestApplied() const { return greatest_applied; }
+    const RefCowMap & getCommitted() const { return committed; }
+    const std::set<std::pair<String, ManifestRef>> & getPrecommits() const { return precommits; }
+    uint64_t getSnapshotBodyBytes() const { return snapshot_body_bytes; }
+    uint64_t getRemovalBodyBytes() const { return removal_body_bytes; }
+
+    /// The id this state's next transaction must carry (INV-1) — the ONE derivation, called by the
+    /// writer to mint an id, by every trial preview to stamp its throwaway transaction, and by
+    /// `applyTxnInPlace` to decide whether a transaction's id is admissible. Three callers, one rule:
+    /// an allocator and a checker that each spell it out separately can drift, and a drift here is
+    /// either a durable hole or a table that refuses its own writes.
+    ///
+    RefTxnId nextTxnId(uint64_t live_epoch) const
+    {
+        return nextRefTxnId(greatest_applied, live_epoch);
+    }
+
+    /// State-install point only (once per ref-log flush, never per batch item): folds the committed
+    /// map's and the owned-manifest index's COW overlays into their bases -- in place when the base is
+    /// uniquely owned (the production flush case), else into a fresh base (see each container's
+    /// `materialize`).
+    void materializeCommitted() { committed.materialize(); owned_manifests.materialize(); }
+
+    /// Member-wise swap, guaranteed non-throwing and allocation-free: every member's own swap is both
+    /// (`shared_ptr::swap`, `std::map::swap`, `std::set::swap`, `std::optional::swap` over a trivially
+    /// swappable payload, plus PODs). This is the ONLY sanctioned way to install a prepared candidate
+    /// state after its transaction is already durable -- see `CasRefLedger::commitRefChunk`'s
+    /// post-durable install region, which runs under `DENY_ALLOCATIONS_IN_SCOPE`. Move-assignment would
+    /// ALSO be `noexcept` today, but it would destroy the displaced state (freeing every `precommits`
+    /// node) INSIDE that region; a swap hands the old state back to the caller, which destroys it
+    /// outside. That destruction is not merely a tidiness matter: the old state still shares the COW
+    /// bases, and `materializeCommitted` folds in place only while they are uniquely owned.
+    void swap(RefTableState & other) noexcept;
+
+private:
+    RefLifecycle lifecycle = RefLifecycle::Removed;   /// see representation note above
+    std::optional<RefTxnId> remove_txn_id;
+    RefTxnId greatest_applied{};                       /// {0, 0} = no transaction applied yet
+
+    RefCowMap committed;                                              /// keyed by ref_name
+    std::set<std::pair<String, ManifestRef>> precommits;             /// (ref_name, manifest_ref)
+
+    /// COW membership index of every `ManifestRef` with a current owner (a `committed` row or a
+    /// `precommits` binding), maintained O(1) per applied op by every arm of `applyOwnerTransition`
+    /// and `stateFromSnapshot` that changes ownership. Gives `manifestAlreadyOwned` O(1) instead of
+    /// a linear scan over `committed` + `precommits`. See Pool/CasRefCowManifestSet.h.
+    RefCowManifestSet owned_manifests;
+
+    /// Running byte totals of the two admission-budget encodings' *bodies* (row/op lines only, no
+    /// header/meta/trailer framing), maintained O(1) per applied op by `applyOp` and seeded by
+    /// `stateFromSnapshot`. A pure function of `(committed, precommits)`: `admits` reads
+    /// `framing + total` instead of re-encoding the whole table. See `admits`'s doc for why this is
+    /// byte-exact rather than a drift-prone estimate.
+    uint64_t snapshot_body_bytes = 0;   /// Σ committedRowEncodedSize + Σ precommitRowEncodedSize
+    uint64_t removal_body_bytes  = 0;   /// Σ removalOpEncodedSize(one per committed + one per precommit)
+
+    /// One operation's local preconditions and effect, shared by `applyRefLogTxn`'s per-op loop and by
+    /// `admits`'s single-op preview. `txn_id` is only read by `RemoveNamespace` (it becomes the
+    /// resulting `remove_txn_id`). Validation is identical no matter which apply strategy reaches here
+    /// (see `applyTxnInPlace`), so this takes no mode. Was free `applyOpInPlace`.
+    void applyOp(const RefOp & op, const RefTxnId & txn_id);
+
+    /// The `owner_transition` op kind: dispatches on the `(old_binding, new_binding)` shape to one of
+    /// the four legal transitions (add precommit / remove precommit / remove committed / promote). Any
+    /// other shape is not a recognized transition. The add-precommit arm's cross-owner uniqueness check
+    /// runs unconditionally (it is O(1) via `owned_manifests`). Was free.
+    void applyOwnerTransition(const RefOp & op);
+
+    /// The `set_published_at` op kind: the committed ref must still name `expected_manifest_ref`;
+    /// replaces `published_at_ms` without touching the manifest edge. Was free.
+    void applySetPublishedAt(const RefOp & op);
+
+    /// Applies the COMPLETE transaction to `*this` IN PLACE (the two txn-wide preconditions first, then
+    /// every op in array order), or throws `CORRUPTED_DATA` -- leaving `*this` PARTIALLY APPLIED
+    /// ("poisoned") on any throw. This is the poisoning apply strategy: it is sound ONLY on a state the
+    /// caller discards on any throw. It is deliberately private and reachable from OUTSIDE this
+    /// translation unit at exactly ONE place -- `replay`, its `friend`, which builds its `RefTableState`
+    /// locally and returns it only after the WHOLE tail succeeds (any throw destroys that local state
+    /// during unwinding, so no consumer ever observes a poisoned state). The public
+    /// `applyRefLogTxn` reaches it too, but only through a scratch copy that turns it into the strong
+    /// guarantee "throw => the caller's `state` is byte-for-byte unchanged". No caller can express the
+    /// dangerous combination -- poison a live state that must survive a throw -- because the poisoning
+    /// path is structurally unreachable except via `replay`.
+    void applyTxnInPlace(const RefLogTxn & txn);
+
+    /// True iff `manifest_ref` already names an existing committed row or precommit binding under ANY
+    /// ref_name (the add-precommit rule: "no conflicting owner may name the same manifest"). Was free.
+    bool manifestAlreadyOwned(const ManifestRef & manifest_ref) const;
+
+#ifdef DEBUG_OR_SANITIZER_BUILD
+    /// Debug/sanitizer-only: recompute both body totals from scratch and assert the incrementally
+    /// maintained values match. Was free.
+    void debugAssertBodyCounters() const;
+#endif
+
+    friend void applyRefLogTxn(RefTableState & state, const RefLogTxn & txn);
+    friend RefTableState stateFromSnapshot(const RefTableSnapshot & snapshot);
+    friend RefTableState replay(const std::optional<RefTableSnapshot> & snapshot, std::span<const RefLogTxn> tail);
+    friend bool admits(const RefTableState & state, const RefOp & op,
+                       uint64_t snapshot_budget, uint64_t removal_budget);
+    /// The streaming generalisation of `replay`: reaches the same private in-place poisoning path
+    /// (`applyTxnInPlace`) on its own discard-on-throw candidate, one decoded transaction at a time.
+    friend class RefReplayBuilder;
+};
+
+/// The inverse of `snapshotOf`: state from a snapshot's rows. `replay` may receive a hand-built
+/// `RefTableSnapshot` that never passed through `decodeRefTableSnapshot`, so this round-trips it
+/// through the codec's own `encodeRefTableSnapshot`/`decodeRefTableSnapshot` rather than
+/// re-implementing a second, independently-maintained copy of its validation (sortedness, no
+/// duplicates, canonical names, nonzero ids, and `manifest_ref` field validity) that could silently
+/// miss a case. A decoded snapshot always constructs a `Live` runtime state; terminal lifecycle and
+/// its removal evidence exist only in replayed log state. Concretely: a hand-built snapshot with two committed
+/// rows sharing one `ref_name` would otherwise DROP the second row via `RefCowMap::emplace` below
+/// (same no-overwrite-on-existing-key semantics as `std::map::emplace`) -- the same phantom-alive
+/// class of bug as a promote's silent displacement (see `applyOwnerTransition` above), just reached
+/// through snapshot loading instead of a transaction.
+///
+/// One check this does that the codec does NOT: cross-owner manifest uniqueness. `CasRefSnapshotCodec`
+/// only enforces sortedness and no-duplicate `ref_name` (committed) / `(ref_name, manifest_ref)`
+/// (precommits); it never checks that a `ManifestRef` has at most one owner across committed rows and
+/// precommits. A snapshot naming one manifest under two owners is semantically corrupt (it would
+/// double-count GC's `+1/-1` edges and violate the add-precommit uniqueness invariant `applyRefLogTxn`
+/// enforces), so as each row is loaded this throws `CORRUPTED_DATA` if the manifest already has an
+/// owner. This is the one place that enforces it; `owned_manifests.insert` would also throw, but the
+/// explicit check here reports "corrupt snapshot data" rather than the container's "index drifted =
+/// code bug" framing, which is the accurate diagnosis for a malformed persisted snapshot.
+///
+/// Promoted from `CasRefProtocol.cpp`'s anonymous namespace to the public protocol API: the ONE
+/// validated way to construct a state from rows -- tests and benchmarks use it instead of poking
+/// fields.
+RefTableState stateFromSnapshot(const RefTableSnapshot & snapshot);
+
+/// Applies the COMPLETE transaction to `state`, or throws `CORRUPTED_DATA` (each transition shape
+/// below has exactly one precondition enforced here) -- with the STRONG exception guarantee: a throw
+/// anywhere leaves `state` byte-for-byte unchanged. The txn-wide preconditions (a `txn_id` that is the
+/// contiguous successor, `remove_namespace` ordering) are checked before any mutation, and the
+/// whole apply runs two-phase against a scratch copy that replaces `state` only once the WHOLE
+/// transaction has succeeded, so no intra-transaction intermediate state (e.g. a manifest with its
+/// precommit already gone but its committed binding not yet installed) is ever observable to a caller
+/// -- matching the promote rule: "There is no moment at which the manifest has no owner." This is the
+/// only public apply entry point, and it is always the strong guarantee: the writer's append-time
+/// contract and every trial/shape-check preview use it as-is.
+///
+/// The poisoning in-place apply strategy (E3 -- no scratch copy, `state` partially applied on throw)
+/// is NOT reachable here: it is `RefTableState::applyTxnInPlace`, private, used only by `replay` (which
+/// discards its local state on any throw). There is no mode argument and no way for an external caller
+/// to select the poisoning path.
+///
+/// Enforced preconditions:
+///  - `txn.txn_id` must be strictly greater than `state.greatest_applied` AND must be exactly the
+///    contiguous successor `RefTableState::nextTxnId` derives (INV-1): the next sequence number within
+///    the same writer epoch, or 1 under a greater one. A hole in a table's DURABLE stream is corruption,
+///    not a tolerated allocation artefact.
+///  - `remove_namespace`, if present, must be the transaction's FINAL operation, and every earlier
+///    operation must be an exact owner-removal `owner_transition` (`old_binding` set, `new_binding`
+///    empty). The codec does not check this shape; this is the one place
+///    that does.
+///  - `namespace_birth`: legal only while `lifecycle != Live`. Catalog admission guarantees this is
+///    either a never-born runtime or a fresh physical life after predecessor deletion.
+///  - `owner_transition` add (no `old_binding`, `new_binding.kind == Precommit`): namespace must be
+///    `Live`; the exact `(ref_name, manifest_ref)` pair must be absent from `precommits`; AND no
+///    existing committed row or precommit binding, under ANY ref_name, may already name the same
+///    `manifest_ref` ("no conflicting owner may name the same manifest" -- this
+///    is what lets `GC`'s `+1/-1` manifest-edge delta treat one manifest as ever having at most one
+///    owner). The build-tuple-is-locally-active-build half of that same sentence is the writer's own
+///    concern -- `RefTableState` has no notion of "active builds".
+///  - `owner_transition` remove (an `old_binding`, no `new_binding`): the exact binding (Precommit or
+///    Committed, matching `ref_name` and `manifest_ref`) must exist.
+///  - `owner_transition` promote (`old_binding.kind == Precommit`, `new_binding.kind == Committed`,
+///    same `ref_name` and `manifest_ref` on both sides): the exact precommit must exist, AND
+///    `ref_name` must not already name a DIFFERENT committed manifest -- that stale row must be
+///    evicted by its own explicit `owner_transition(old=Committed, new=None)` first (an earlier op of
+///    the same transaction, so the two together read as one atomic replace, or an earlier
+///    transaction). Promote never displaces an existing committed row implicitly: `GC`'s
+///    manifest-edge delta is read off each transaction's explicit
+///    ops, not a before/after state diff, so a silent displacement would never emit the evicted
+///    manifest's "-1" edge -- it would leak as phantom-alive forever. On success the precommit is
+///    replaced by a committed row whose `published_at_ms` starts UNSET (the initial stamp arrives
+///    via a separate `set_published_at` op, in the same transaction or a later one).
+///  - Any other `old_binding`/`new_binding` combination is not a recognized transition shape.
+///  - `set_published_at`: namespace must be `Live`; the committed ref named by `ref_name` must still name
+///    `expected_manifest_ref`.
+///  - `remove_namespace`: namespace must be `Live` and both `committed` and `precommits` must already
+///    be empty at this point in the (in-array-order) replay -- which is only true if the transaction's
+///    earlier removal ops actually named every owner.
+///  - Any operation other than `namespace_birth` while `lifecycle == Removed` is rejected:
+///    "Any operation other than a valid later namespace_birth while state is
+///    Removed is corruption."
+///
+/// `CORRUPTED_DATA` throughout: every rejection above uses the same "is corruption" framing,
+/// extended uniformly to every precondition in this section, matching how
+/// `CasRefLogCodec`/`CasRefSnapshotCodec` already use `CORRUPTED_DATA` for "this data does not
+/// correspond to a valid state" one layer down (wire shape rather than transition legality). Recovery
+/// and `fsck` -- the primary callers replaying persisted logs -- want exactly that fail-closed
+/// framing; a writer that wants a friendlier user-facing rejection for an ordinary attempted mutation
+/// (e.g. "ref already exists") checks its own business state before ever building the op.
+void applyRefLogTxn(RefTableState & state, const RefLogTxn & txn);
+
+/// The canonical snapshot of `state` under `ns`: `committed` sorted by
+/// bytewise `ref_name` (guaranteed by `RefCowMap`'s sorted merge-iteration order,
+/// `Pool/CasRefCowMap.h` -- the same ordering `std::map<String, ...>` gave before it, by design)
+/// and `precommits` sorted by `(ref_name, manifest_ref)` (guaranteed by
+/// `std::set<std::pair<String, ManifestRef>>`'s iteration order, since `ManifestRef::operator<`
+/// matches the tuple order `CasRefSnapshotCodec` itself sorts by). `snapshot_id` is
+/// `state.greatest_applied`. A non-`Live` state is terminal replay evidence rather than snapshot
+/// state and is rejected with `CORRUPTED_DATA`. This does not otherwise enforce that the result is
+/// encodable (a never-born state's `snapshot_id` is `{0, 0}`, which `encodeRefTableSnapshot`
+/// already rejects) -- that check already lives in the codec and need not be duplicated here.
+RefTableSnapshot snapshotOf(const RefTableState & state, const String & ns);
+
+/// `TableState = Replay(S_X.state, tail(X))` in one call: starts from `snapshot`
+/// (or the empty/never-born state when absent) and applies every transaction in `tail`, in order, via
+/// `applyRefLogTxn`. A given `snapshot` is revalidated in full -- sortedness, no duplicates, canonical
+/// names, nonzero ids, and `manifest_ref` field validity, i.e. everything
+/// `CasRefSnapshotCodec` already enforces -- because `replay` may be handed
+/// a hand-built `RefTableSnapshot` that never passed through `decodeRefTableSnapshot` (`fsck`, most
+/// notably). Every entry of `tail` must also share one `ns` -- with `snapshot`'s `ns` when a snapshot
+/// is given, otherwise with each other. A mismatch (of either kind) throws `CORRUPTED_DATA`: silently
+/// accepting a malformed snapshot or replaying transactions from the wrong table would produce a
+/// wrong-but-plausible-looking state, exactly the class of bug this equation exists to make
+/// impossible.
+RefTableState replay(const std::optional<RefTableSnapshot> & snapshot, std::span<const RefLogTxn> tail);
+
+/// Everything a successful recovery of one ref table seeds. Produced by streaming replay
+/// (`RefReplayBuilder::finish`) rather than assigned field-by-field into the runtime, so the whole
+/// publication is one value installed atomically -- a prose field list would drift, but a struct that
+/// the install copies wholesale cannot silently lose a field (Codex review round 4, spec §5).
+///
+/// `finish` populates the fields that are a pure function of `(base snapshot, replayed tail)`: `state`,
+/// `newest_snapshot_id`, `tail_count`, `tail_bytes`, and `base_snapshot_bytes`. The
+/// remaining fields are recovery-context the streaming builder cannot know -- the writer's own recovery
+/// (`CasRefLedger::ensureRefTableRecovered`) fills the admission budgets,
+/// `needs_stale_precommit_sweep` and `last_epoch_seal`, before installing the whole struct under
+/// `state_mutex` with `recovered` set last. The read-only consumers
+/// (`recoverRefTableDetailedFromAuthority` for the orphan sweep and fsck) read only
+/// `state` (plus `newest_snapshot_id` for the sweep) and leave the rest at default.
+struct RecoveryResult
+{
+    RefTableState state;
+    /// Identity of the base snapshot this recovery replayed from: `nullopt` for a never-born table.
+    std::optional<RefTxnId> newest_snapshot_id;
+    /// Applied transactions strictly newer than `newest_snapshot_id`, and the sum of their stored
+    /// (sealed) object byte sizes -- the tail-since-snapshot accounting the runtime tracks.
+    uint64_t tail_count = 0;
+    uint64_t tail_bytes = 0;
+    /// Encoded (sealed) body size of the base snapshot; 0 for a never-born base.
+    uint64_t base_snapshot_bytes = 0;
+
+    /// Recovery-context fields (filled by `ensureRefTableRecovered`, default for other consumers):
+    uint64_t snapshot_budget = 0;
+    uint64_t removal_budget = 0;
+    bool needs_stale_precommit_sweep = false;
+    /// The `EpochSeal` that closed the last dead epoch the recovery CAS-walk crossed -- minted by it,
+    /// adopted from a concurrent recoverer, or read out of the durable tail. It is the `prev_epoch_seal`
+    /// this table's next sequence-1 append must carry, and `nullopt` means GENESIS exactly (see
+    /// `CasRefLedger::RefTableRuntime::last_epoch_seal`). Recovery-context rather than replay-derived
+    /// because only the walk knows which epochs were dead.
+    std::optional<RefTxnId> last_epoch_seal;
+};
+
+/// The streaming generalisation of `replay` (spec §5): owns a PRIVATE candidate `RefTableState` and
+/// applies decoded transactions into it ONE AT A TIME, in place, discarding the candidate on any throw.
+/// It is the memory fix for a long post-snapshot tail: `replay` takes the whole `tail` materialised in a
+/// vector (every decoded transaction resident at once, each up to the 20 MiB normal-class cap), whereas
+/// a caller that GETs+decodes+`applyOne`+discards one object at a time holds at most a single decoded
+/// transaction. `applyOne` reaches `RefTableState::applyTxnInPlace` directly -- the same private
+/// poisoning path `replay` uses -- NOT the public scratch-copying `applyRefLogTxn`, which would deep-copy
+/// the growing candidate once per transaction and reintroduce the O(K*N) cost `replay` was written to
+/// avoid. The candidate never touches any live runtime state; a throw destroys it during unwinding, so no
+/// consumer ever observes a poisoned candidate. All three full-tail materialisers stream through this:
+/// the writer's recovery, `recoverRefTableDetailedFromAuthority` (orphan sweep and fsck).
+class RefReplayBuilder
+{
+public:
+    /// Seeds the candidate from `base` (or the empty/never-born state when absent), revalidating the
+    /// snapshot in full exactly as `replay` does (`stateFromSnapshot`). `base_encoded_bytes` is the
+    /// stored (sealed) size of that snapshot object, carried through to `RecoveryResult::base_snapshot_bytes`
+    /// (0 when the caller does not track it -- the read-only consumers do not).
+    explicit RefReplayBuilder(std::optional<RefTableSnapshot> base, uint64_t base_encoded_bytes = 0);
+
+    /// Applies one decoded transaction to the candidate in place. `encoded_bytes` is the stored (sealed)
+    /// object size of `txn`, accumulated into the tail-byte total. A decode/apply corruption throws
+    /// `CORRUPTED_DATA` (the non-transient class recovery fails fast on), discarding the candidate.
+    void applyOne(RefLogTxn && txn, uint64_t encoded_bytes);
+
+    /// The candidate's lifecycle AS OF the transactions applied so far. Recovery's CAS-walk needs it at
+    /// each epoch boundary it reaches, and it must be the LIVE reading rather than one taken from the
+    /// base snapshot: a removal transaction in the replayed tail is exactly the case where the two
+    /// differ, and it is the case that decides whether the epoch below gets a seal.
+    RefLifecycle lifecycle() const { return candidate.getLifecycle(); }
+
+    /// Materialises nothing extra (matches `replay`: the writer's recovery folds the COW overlays via
+    /// `materializeCommitted` on the result; the read-only consumers do not) and returns the replay-derived
+    /// `RecoveryResult` fields, moving the candidate out. The builder must not be used afterwards.
+    RecoveryResult finish() &&;
+
+private:
+    RefTableState candidate;
+    std::optional<String> expected_ns;
+    RecoveryResult result;
+};
+
+/// Resident footprint, in bytes, of a decoded ref-log transaction: the heap it keeps alive while it is
+/// held in memory -- its op vector's element storage plus every owned string (the transaction `ns` and
+/// each op's ref-name strings). A deterministic function of the decoded CONTENT (unlike the compressed
+/// stored size, which understates a highly-compressible transaction), so a memory-bound test built on it
+/// is stable under ASan. This is what a whole-tail materialiser accumulates N-fold, while the streaming
+/// recovery loops hold exactly one decoded transaction resident at a time.
+uint64_t decodedRefLogTxnFootprint(const RefLogTxn & txn);
+
+/// Report a decoded-transaction memory delta to the installed streaming-recovery memory probe, if any
+/// (a no-op in production -- no probe is installed). Each recovery loop calls `+footprint` when a
+/// decoded transaction becomes resident and `-footprint` when it is discarded, so the probe observes the
+/// loop's real resident set. Exposed (rather than confined to one translation unit) because the three
+/// recovery loops live in three files and a memory-bound test's materialising control drives the
+/// identical seam.
+void reportReplayMemoryDelta(int64_t delta_footprint_bytes);
+
+/// Test-only observability for the streaming-recovery memory invariant (spec §5): while a probe is
+/// installed, each recovery loop reports the resident footprint of every decoded transaction it holds,
+/// for exactly the span it holds it (`reportReplayMemoryDelta` + `decodedRefLogTxnFootprint`). A
+/// memory-bound test tracks the peak of the summed reported footprint and asserts it stays within a
+/// single transaction, where the retired whole-tail materialiser -- and the test-local materialising
+/// control that stands in for it -- held the entire tail resident at once. Because the report spans the
+/// decoded transaction's whole GET->decode->apply->discard lifetime at the LOOP, not one apply in
+/// isolation, a regression that materialises the whole tail before applying it is caught. No probe
+/// installed => no accounting. Guarded by an internal mutex; install before driving recovery and clear
+/// afterwards.
+void setRecoveryReplayMemoryProbeForTest(std::function<void(int64_t delta_footprint_bytes)> probe);
+
+/// The exact encoded size of `state`'s canonical snapshot (`encodeRefTableSnapshot(snapshotOf(state,
+/// "")).size()`), computed in O(1) from the running body counter plus O(1) framing instead of a full
+/// re-encode. Used by `admits` and directly property-tested against the real encoder.
+uint64_t encodedSnapshotBudgetSize(const RefTableState & state);
+
+/// The exact encoded size of `state`'s hypothetical whole-namespace removal transaction, computed in
+/// O(1) from the running body counter plus O(1) framing. Used by `admits`.
+uint64_t encodedRemovalBudgetSize(const RefTableState & state);
+
+/// Admission budget: true iff applying `op` to a COPY of `state` (via the same
+/// per-operation validator `applyRefLogTxn` uses -- an `op` that is not itself a legal transition
+/// throws exactly as `applyRefLogTxn` would, since `admits` answers "would this legal op still fit
+/// the budget", not "is this op legal") keeps BOTH the resulting table snapshot and the resulting
+/// hypothetical complete-removal transaction within their respective byte budgets.
+///
+/// `RefTableState` carries no `ns` (it is per-table but not one of this class's fields), so both
+/// hypothetical encodings are measured with an empty `ns`. `ns` is constant for one table for its
+/// entire lifetime, so a caller computes its own table's `ns.size()` overhead once (the wire layout's
+/// `u32` length prefix itself is present in BOTH the empty-`ns` measurement here and the real encoding,
+/// so it cancels -- only the `ns` bytes themselves are the delta; repeated exactly once in a snapshot
+/// body and once in a removal-transaction body, see `CasRefSnapshotCodec` / `CasRefLogCodec`'s wire
+/// layout) and pre-subtracts it, together with its own safety margin, from the raw
+/// `ref_snapshot_max_bytes` / `ref_removal_max_bytes` hard limits before calling `admits`.
+///
+/// Implementation: sizes are computed incrementally. `RefTableState` carries running body-byte totals
+/// (`snapshot_body_bytes` / `removal_body_bytes`) maintained O(1) per applied op by `RefTableState::applyOp`;
+/// `admits` applies `op` to a scratch copy and reads `framing + total` via `encodedSnapshotBudgetSize`
+/// / `encodedRemovalBudgetSize`, making the whole check O(touched rows) instead of O(table size). This
+/// is byte-exact rather than a drift-prone estimate: both budget encodings are pure per-row sums, the
+/// per-row contributions come from the same codec primitives the full encoders use, and a
+/// debug/sanitizer-only recompute-and-compare `chassert` (`RefTableState::debugAssertBodyCounters`)
+/// reasserts equality on every applied transaction and every `admits` preview.
+bool admits(const RefTableState & state, const RefOp & op, uint64_t snapshot_budget, uint64_t removal_budget);
+
+/// Pure ref-log intake primitives for a GC round. None of these read a
+/// manifest body, a snapshot body, or `gc/state`: they turn a global `LIST cas/ns/stream/` result and the
+/// decoded bodies of new transactions into (a) the per-table log/snapshot/marker listing, (b) the
+/// deterministic manifest-edge delta, and (c) the exact ref-object cleanup plan. The GC round
+/// (`CasGc.cpp`) drives the manifest-body reads (`foldManifestEdges`), the fold barrier, the durable
+/// cursor, and the batch deletions around these functions. Keeping the delta and cleanup logic pure
+/// makes it directly unit-testable (`gtest_cas_ref_intake.cpp`) without a full round.
+
+/// One `+1`/`-1` manifest edge emitted by one ref-log operation.
+/// `manifest_id` is namespace-qualified (equal `ManifestRef` tuples under two tables stay distinct).
+/// The ordinals locate the exact operation and edge inside the transaction, giving the spec's
+/// `event_id = {namespace, RefTxnId, operation_ordinal, edge_ordinal}` its determinism: replaying the
+/// same logs yields byte-identical edges, so retry and competing GC attempts produce the same delta.
+struct RefManifestEdge
+{
+    ManifestId manifest_id;
+    int change = 0;              /// +1 activation | -1 removal
+    RefOwnerKind owner_kind = RefOwnerKind::Committed;   /// kind of the binding that produced this edge:
+                                 /// the `new_binding` kind for a `+1`, the `old_binding` kind for a `-1`.
+                                 /// The GC fold needs it to classify a missing manifest body -- a removed
+                                 /// precommit that never activated is skipped, every other missing body clamps.
+    uint32_t op_ordinal = 0;     /// index of the op within its transaction
+    uint32_t edge_ordinal = 0;   /// 0 = the removal edge, 1 = the activation edge of one op
+
+    bool operator==(const RefManifestEdge &) const = default;
+};
+
+/// The manifest edges of ONE decoded transaction, in operation order, reading NO manifest body.
+/// `owner_transition` recognizes EXACTLY the four shapes `classifyOwnerTransitionShape`
+/// (Pool/CasRefProtocol.cpp) also uses to drive `RefTableState::applyOwnerTransition` -- the SAME
+/// classification, not a second copy of the shape knowledge:
+///   - add precommit (no old_binding, new_binding.kind == Precommit)     => `+1` for `new.manifest_ref`
+///   - remove precommit / remove committed (old_binding set, no new_binding)
+///                                                                       => `-1` for `old.manifest_ref`
+///   - promote (old_binding.kind == Precommit, new_binding.kind == Committed, SAME ref_name and
+///     manifest_ref)                                                    => no edge (net zero: the
+///     manifest keeps an owner the whole time)
+///   - `namespace_birth` / `set_published_at` / `remove_namespace`      => no edge
+/// Any other `owner_transition` shape -- neither binding, old+new naming DIFFERENT manifests, a
+/// promote whose old/new ref_name disagree, or any other kind combination -- throws `CORRUPTED_DATA`.
+/// These are exactly the shapes `applyRefLogTxn`/`replay` already reject at the state-machine layer, so
+/// a hand-corrupted or adversarial log body is the only way this branch is reached; a legitimately
+/// written log never produces one. The GC fold (`CasGc.cpp`) extracts edges inside the same try-block
+/// as `decodeRefLogTxn`, so the throw gets the identical "ref log body invalid: ref folding aborted
+/// this round" treatment as an undecodable body -- no cursor advance, no deletions, an anomaly
+/// recorded. The orphan sweep (`CasOrphanManifestSweep.cpp`) catches it around the whole
+/// `activeManifestKeys` construction and skips (or marks errored) that namespace's deletions rather
+/// than trusting an incomplete protection view.
+/// The `remove_namespace` operation changes lifecycle only; the exact owner removals that must precede
+/// it in the same transaction already emit their own `-1` edges.
+std::vector<RefManifestEdge> manifestEdgesOfTxn(const RefLogTxn & txn);
+
+/// The GC fold consumes a table's new transactions ONE log at a time, in ascending id order, emitting
+/// each log's `manifestEdgesOfTxn` into `foldManifestEdges` and advancing the durable cursor per fully
+/// folded log (mirroring the legacy per-event journal fold, including its clamp-on-missing-body barrier).
+/// In-batch add+remove cancellation is therefore NOT done as a pre-fold net pass: the idempotent
+/// `(blob, source_id)` in-degree set-merge already cancels a `+edge` and matching `-edge` folded into one
+/// generation, and a pre-fold net pass would be unsafe -- a mid-batch clamp could split a cancelled pair
+/// across the advanced cursor, folding a spurious `-1` in a later round. So there is deliberately no
+/// `netManifestDelta` here.
+
+/// The `remove_txn_id` of a transaction that ends its namespace's life (contains a `remove_namespace`
+/// operation), or `nullopt`. The value equals `txn.txn_id`. The round routes it into the durable
+/// cleanup evidence of that life's fold-state row.
+std::optional<RefTxnId> removalTxnId(const RefLogTxn & txn);
+
+/// One table's surviving ref-object keys from this round's global `LIST`, split by kind and sorted
+/// ascending.
+struct RefTableListing
+{
+    std::vector<RefTxnId> logs;
+    std::vector<RefTxnId> snapshots;
+    bool operator==(const RefTableListing &) const = default;
+};
+
+/// Parse and group a global `LIST` of keys under `layout.casRefsPrefix` by physical life id. Every
+/// key is expected to be one of the three immutable stream-object kinds; checkpoints and namespace
+/// files live in the separate state tree and are never offered by this hot enumeration. An
+/// unrecognized stream key throws `CORRUPTED_DATA`, so the round cannot derive a partial delta or
+/// authorize destructive work from an incomplete classification.
+/// A key outside `casRefsPrefix` is ignored: the caller lists only the stream prefix, and a foreign key
+/// is not this format's concern.
+std::map<NamespaceLifePhysicalId, RefTableListing> groupRefKeys(
+    const Layout & layout, const std::vector<String> & listed_keys);
+
+/// The exact ref objects one round may delete for one namespace life.
+/// Pure; acts only on keys THIS round's scan returned, but the scan is never cleanup authority. The
+/// caller may supply `checkpoint` only after exact validation of the `_ckpt`-named recovery triple:
+/// `_ckpt` plus its same-id non-seal `_log` and `_snap`. A later-epoch base also returns the exact
+/// predecessor seal that proved its transition; `retained_log_proof` keeps that log outside the delete
+/// plan while the checkpoint remains authoritative. Without a validated base the plan is empty.
+/// With it, a log `L` is deletable only when `L < checkpoint` and `L <= durable_cursor`; a listed
+/// snapshot is deletable only when its id is `< checkpoint`. The base's same-id `_log` and `_snap`,
+/// and every newer stream object, are retained.
+struct RefCleanupPlan
+{
+    std::vector<RefTxnId> deletable_logs;
+    std::vector<RefTxnId> deletable_snapshots;
+
+    bool operator==(const RefCleanupPlan &) const = default;
+};
+RefCleanupPlan planRefCleanup(const RefTableListing & listing, const RefTxnId & durable_cursor,
+                              std::optional<RefTxnId> checkpoint = std::nullopt,
+                              std::optional<RefTxnId> retained_log_proof = std::nullopt);
+
+/// Why an epoch crossing failed, or that it was PROVED. See `crossEpochFromSeal`.
+enum class EpochCrossOutcome : uint8_t
+{
+    Proved,             /// `start` is sequence 1 of the epoch that chains from `from_seal`
+    NothingConsumed,    /// `from_seal` is `{0, 0}`: nothing has been consumed, so there is no seal to cross from
+    NotASeal,           /// the record at `from_seal` is KNOWN not to be an `EpochSeal`
+    StartAbsent,        /// an epoch-start record the back-chain needs is not there
+    StartInvalid,       /// an epoch-start record is undecodable (`detail` carries the codec's message)
+    ChainDoesNotReach,  /// the chain names no seal at `from_seal` -- a genesis record, or one that skips it
+};
+
+/// One epoch crossing's outcome plus what it cost, so a caller can attribute the reads it performed.
+struct EpochCrossResult
+{
+    EpochCrossOutcome outcome = EpochCrossOutcome::ChainDoesNotReach;
+    RefTxnId start{};            /// meaningful only on `Proved`
+    RefTxnId probed{};           /// the epoch-start id the walk last read -- names the object in a diagnostic
+    String detail;               /// the decode error, on `StartInvalid`
+    uint64_t body_gets = 0;      /// epoch-start bodies actually fetched
+    uint64_t absent_probes = 0;  /// epoch-start reads that came back absent
+
+    bool proved() const { return outcome == EpochCrossOutcome::Proved; }
+};
+
+/// Cross into the epoch that follows the one `from_seal` closed, and PROVE it rather than guess it.
+///
+/// A listing only NOMINATES a candidate epoch (`witness`). The proof is the back-chain: the target
+/// epoch's sequence-1 record names the seal that must have been consumed before it (INV-2). If it names
+/// a seal ABOVE `from_seal`, an epoch sits in between that the nomination omitted -- the chain is
+/// followed back one epoch and retried, which is what makes a crossing independent of any enumeration.
+/// Anything else -- no sequence 1, an undecodable one, a genesis record (no `prev_epoch_seal`) above a
+/// consumed seal, or a chain that skips the position -- is an unproven crossing and is reported as such.
+///
+/// WHAT THE CHAIN PROVES, EXACTLY: the IDENTITY of the position the next epoch chains from, not its
+/// KIND. Those come apart when a writer names an ordinary record as `prev_epoch_seal`, and the damage is
+/// the one the seal exists to prevent -- epoch `E` declared closed while its writer may still append, so
+/// a later `{E, k}` lands permanently below the cursor. So the kind is checked wherever it is knowable:
+/// `seal_proven` carries `refLogTxnIsEpochSeal` of the record the CALLER applied at `from_seal` (free --
+/// it decoded the body to apply it). Pass `nullopt` only when the caller applied nothing and `from_seal`
+/// is an inherited cursor whose record may since have been cleaned, so its kind is unknowable by any
+/// amount of reading here and the crossing rests on chain-trust.
+///
+/// Terminates: `validateEpochSealGrammarStructural` guarantees `prev_epoch_seal->writer_epoch + 1 ==
+/// txn_id.writer_epoch`, so the target epoch decreases one numeric epoch per link and is bounded below
+/// by `from_seal`'s own epoch. The record it proves is read once more by the caller's own walk: one
+/// redundant `GET` per epoch crossed, and crossings happen once per writer-epoch change.
+///
+/// READ-ONLY, and shared deliberately: the GC fold's intake and fsck's audit must not be able to
+/// disagree about when an epoch boundary has been proved -- a rule that says which records a cut
+/// contains cannot have two implementations.
+///
+/// `life`: the namespace's life, REQUIRED (review NEW-3 -- a `nullopt`-resolves-internally default was
+/// tried once and reintroduced the exact divergence review C3 removed from `Gc::fold`, just relocated
+/// into `CasFsck.cpp`'s independent walk, which had its OWN already-resolved `life` in scope one call
+/// site above and simply did not pass it). Every caller must resolve `life` itself, ONCE, and pass the
+/// SAME value here that it uses for every other read in its own walk -- this function no longer
+/// resolves anything on its own, so there is no second resolution left to disagree with the first.
+EpochCrossResult crossEpochFromSeal(Backend & backend, const Layout & layout, const RootNamespace & ns,
+                                    const RefTxnId & from_seal, std::optional<bool> seal_proven,
+                                    const RefTxnId & witness, const NamespaceLifeId & life);
+
+/// Return the one exact successor an immutable `_ckpt.committed_through` range permits after the
+/// decoded record at `current`, or `nullopt` when `current` is the inclusive frontier itself. An
+/// `EpochSeal` ends its numeric epoch, so a strictly-later frontier in that SAME epoch is corrupt:
+/// advancing to `{E+1,1}` and letting ordinary ordering terminate would silently discard the invalid
+/// part of the claimed range. Shared by every checkpoint-bounded reader so recovery, fsck, and the
+/// orphan protection walk cannot disagree about that malformed authority.
+std::optional<RefTxnId> nextRefLogIdWithinCommittedFrontier(
+    const RefTxnId & current, bool is_epoch_seal, const RefTxnId & committed_through);
+
+/// The result of `recoverRefTableDetailedFromAuthority`: the replayed table state plus the identity of
+/// the snapshot recovery actually selected as its base (`nullopt` when it found no snapshot at all).
+struct RecoveredRefTable
+{
+    RefTableState state;
+    std::optional<RefTxnId> newest_snapshot_id;
+    /// The exact lifecycle authority's last sealed epoch. The authoritative read-only entry point
+    /// copies this from its immutable `_ckpt` input.
+    std::optional<RefTxnId> last_epoch_seal;
+};
+
+/// Exact-read and decode the checkpoint-named recovery base. The anchor is the bounded triple
+/// `_ckpt` + same-id non-seal `_log` + same-id `_snap`: read and decode the log first, reject an
+/// `EpochSeal`, validate its contextual epoch backlink against `_ckpt.life_epoch` and the exact
+/// `_ckpt.last_epoch_seal` when that field describes the base's preceding epoch, exact-read and require
+/// the named predecessor to be an `EpochSeal`, then read the snapshot. This order prevents a forged
+/// snapshot at any historical seal or contextually invalid epoch start from becoming state. Cleanup
+/// retains both the matching log and returned predecessor proof while the checkpoint names this base.
+struct CheckpointSnapshotBase
+{
+    RefTableSnapshot snapshot;
+    uint64_t bytes = 0;
+    /// The exact `EpochSeal` named by a non-genesis sequence-1 base. Recovery needs that object as
+    /// durable transition proof, so cleanup must retain it together with the checkpoint base.
+    std::optional<RefTxnId> predecessor_seal_id;
+};
+
+CheckpointSnapshotBase readCheckpointSnapshotBase(
+    Backend & backend, const Layout & layout, const NamespaceLifeId & life, const RefCkpt & checkpoint);
+
+/// Recover a ref table from ONE immutable lifecycle authority cut supplied by the caller. `catalog_entry`
+/// is either the exact row from that caller's frozen catalog cut or absence from that same cut; `ckpt` is
+/// the exact decoded `_ckpt` that caller read for the row's `NamespaceLifeId`. This function does NOT
+/// GET the catalog or `_ckpt` itself: accepting a later, competing cut would make its result disagree
+/// with the caller's other decisions. `chooseRecoveryGrounding` makes absent/Creating names non-recoverable
+/// and requires a readable `_ckpt` with `life_epoch` for Live/Removing.
+///
+/// Recovery performs no stream `LIST`: its replay is exact point GETs from the checkpoint-named base
+/// through inclusive `committed_through`; a missing checkpoint-named base or committed log is corruption
+/// under this immutable authority. In particular, this read-only API never probes or adopts `F+1`. There
+/// is deliberately no self-resolving compatibility overload: every consumer must pass the row from its
+/// frozen catalog cut explicitly.
+RecoveredRefTable recoverRefTableDetailedFromAuthority(
+    Backend & backend, const Layout & layout, const std::optional<CatalogEntry> & catalog_entry,
+    const std::optional<RefCkpt> & ckpt);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.cpp
new file mode 100644
index 000000000000..58ca6559bd83
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.cpp
@@ -0,0 +1,1512 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Common/logger_useful.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <Common/setThreadName.h>
+#include <base/getFQDNOrHostName.h>
+#include <fmt/format.h>
+#include <magic_enum.hpp>
+
+#include <algorithm>
+#include <ctime>
+#include <limits>
+#include <set>
+#include <string_view>
+#include <unistd.h>
+
+namespace ProfileEvents
+{
+    extern const Event CASMountLeaseLost;
+    extern const Event CASMountReleaseSkippedForeignOccupant;
+    extern const Event CASMountExclusivityViolation;
+}
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int ABORTED;
+    extern const int CORRUPTED_DATA;
+    extern const int FILE_DOESNT_EXIST;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+/// The owner, epoch, and mount-lease wire codecs are implemented in
+/// `Formats/CasServerRootFormats`; this file contains the mount-safety protocol logic that uses
+/// those codecs.
+
+namespace
+{
+/// TRUE iff a `list(prefix, "", 1)` over `prefix` returns at least one key.
+bool prefixHasAnyKey(Backend & b, const String & prefix)
+{
+    return !b.list(prefix, /*cursor*/ "", /*limit*/ 1).keys.empty();
+}
+
+uint64_t defaultBootMs()
+{
+    struct timespec ts{};
+    clock_gettime(CLOCK_BOOTTIME, &ts);
+    return static_cast<uint64_t>(ts.tv_sec) * 1000 + static_cast<uint64_t>(ts.tv_nsec) / 1000000;
+}
+
+/// Forward declaration: defined below (same TU-unique anonymous namespace) — `allocateWriterEpoch`
+/// names the current mount holder in its DecommissionRecovery live-refusal message.
+String describeMountHolder(const MountLease & m);
+
+std::optional<OwnerObject> readOwnerObject(Backend & b, const Layout & l, const String & server_root_id)
+{
+    const auto got = b.get(l.ownerKey(server_root_id));
+    if (!got)
+        return std::nullopt;
+    return decodeOwner(got->bytes);
+}
+
+void throwIfOwnerRetired(const OwnerObject & owner, const String & srid)
+{
+    if (!owner.retired_at_ms)
+        return;
+
+    throw Exception(ErrorCodes::CORRUPTED_DATA,
+        "CAS server-root '{}' was explicitly decommissioned by an operator (tombstoned at {} ms) "
+        "and is refusing to silently resume — if you genuinely intend to bring this server-root "
+        "back, manually clear the owner object's tombstone field and restart "
+        "(same manual-recovery pattern as an owner anchor lost over existing data)",
+        srid, *owner.retired_at_ms);
+}
+}
+
+bool serverRootSubtreeEmpty(
+    Backend & b, const Layout & l, const String & srid, const RefCatalog & catalog_observation)
+{
+    const String owned_prefix = srid + "/";
+    for (const CatalogEntry & entry : catalog_observation.entries)
+        if (entry.ns.string() == srid || entry.ns.string().starts_with(owned_prefix))
+            return false;
+
+    /// Manifests and loose roots retain logical path identity. Opaque namespace stream/state debris
+    /// alone is not evidence that this server root owns live work.
+    if (prefixHasAnyKey(b, l.casManifestsServerPrefix(srid)))
+        return false;
+    if (prefixHasAnyKey(b, l.serverRootDataPrefix(srid)))
+        return false;
+    return true;
+}
+
+std::optional<UInt128> readOwnerUuid(Backend & b, const Layout & l, const String & server_root_id)
+{
+    const std::optional<OwnerObject> owner = readOwnerObject(b, l, server_root_id);
+    if (!owner)
+        return std::nullopt;
+    return owner->server_uuid;
+}
+
+void claimOwnerOrThrow(
+    Backend & b, const Layout & l, const String & srid, UInt128 our_uuid,
+    const ObserveRefCatalog & observe_catalog)
+{
+    if (!observe_catalog)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS server-root '{}': catalog observer is required", srid);
+    const String key = l.ownerKey(srid);
+
+    /// Owner present → it is identity: equal UUID is ok, a different UUID fails closed regardless
+    /// of any lease/clock state.
+    if (const std::optional<OwnerObject> owner = readOwnerObject(b, l, srid))
+    {
+        if (owner->server_uuid == our_uuid)
+        {
+            throwIfOwnerRetired(*owner, srid);
+            return;
+        }
+        /// Mirror mountDoubleStartMessage's operator guidance: the by-far most common cause is a
+        /// REGENERATED local ClickHouse uuid file (wiped /var/lib/clickhouse, a pod rescheduled
+        /// without a persistent volume) while the pool kept the old identity — name it and the
+        /// recovery options instead of a bare refusal.
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS server-root '{}' is owned by a different server (owner server_uuid={}, ours={}) — refusing to claim. "
+            "This usually means THIS server's local uuid file was regenerated (e.g. /var/lib/clickhouse was wiped, "
+            "or the container/pod was recreated without a persistent volume) while the pool kept the old identity. "
+            "Recover by restoring the old local uuid file; or configure a fresh <server_root_id> for this disk; "
+            "or — only after verifying that NO server uses this root — manually delete the owner object '{}' and restart.",
+            srid, u128ToHex(owner->server_uuid), u128ToHex(our_uuid), key);
+    }
+
+    /// Owner absent. Claiming is allowed ONLY over a provably-empty subtree; an absent owner over
+    /// existing data means the identity was lost and must never be silently re-claimed.
+    if (!serverRootSubtreeEmpty(b, l, srid, observe_catalog()))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS server-root '{}' has no owner anchor but its data subtree is non-empty "
+            "(identity lost over existing data) — refusing to re-claim",
+            srid);
+
+    const PutResult put = b.putIfAbsent(key, encodeOwner(OwnerObject{
+        .server_uuid = our_uuid,
+        .retired_at_ms = std::nullopt,
+    }));
+    if (put.outcome == PutOutcome::Done)
+        return;
+
+    /// The conditional create conflicted. Recompute the whole catalog + manifest + roots bundle;
+    /// no stale emptiness result is carried across the conflict.
+    if (!serverRootSubtreeEmpty(b, l, srid, observe_catalog()))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS server-root '{}' owner claim conflicted and newly visible owned work blocks recreation", srid);
+
+    /// Race: another process claimed between our get and our putIfAbsent. Re-read and compare.
+    const std::optional<OwnerObject> reread = readOwnerObject(b, l, srid);
+    if (!reread)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS server-root '{}' owner anchor vanished during claim", srid);
+    if (reread->server_uuid == our_uuid)
+    {
+        throwIfOwnerRetired(*reread, srid);
+        return;
+    }
+    throw Exception(ErrorCodes::CORRUPTED_DATA,
+        "CAS server-root '{}' was claimed by a different server during our claim (foreign owner) "
+        "— refusing to proceed",
+        srid);
+}
+
+uint64_t allocateWriterEpoch(
+    Backend & b, const Layout & l, const String & srid, EpochMintPolicy policy, uint64_t now_ms,
+    const ObserveRefCatalog & observe_catalog)
+{
+    if (!observe_catalog)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS server-root '{}': catalog observer is required", srid);
+    const String key = l.epochKey(srid);
+
+    static constexpr int max_attempts = 100;
+    for (int attempt = 0; attempt < max_attempts; ++attempt)
+    {
+        const auto got = b.get(key);
+
+        ServerEpoch current;
+        std::optional<Token> expected;
+        if (got)
+        {
+            current = decodeServerEpoch(got->bytes);
+            expected = got->token;
+        }
+        else
+        {
+            /// A missing `epoch` over a non-empty subtree is a reset hazard (durable monotone
+            /// counter cannot be reconstructed) — fail closed.
+            if (!serverRootSubtreeEmpty(b, l, srid, observe_catalog()))
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS server-root '{}' has no durable epoch object but its data subtree is "
+                    "non-empty (writer_epoch reset hazard) — refusing to proceed",
+                    srid);
+
+            /// Same hazard through the CONTROL objects (spec rev.4 Phase C): an absent epoch while
+            /// a mount object exists means epoch state was lost under a live/recent mount —
+            /// re-minting epoch 1 there is how a same-(uuid, epoch) twin is born. This is a
+            /// lifecycle decision, so it uses the authoritative probe, never get-absence.
+            const SentinelProbeResult mount_probe = b.probeSentinelRaw(l.mountKey(srid));
+            switch (mount_probe.outcome)
+            {
+                case ProbeOutcome::KeyAbsent:
+                    break;   /// authoritative absence — fresh-root bootstrap proceeds below
+                case ProbeOutcome::Present:
+                {
+                    if (policy == EpochMintPolicy::DecommissionRecovery)
+                    {
+                        chassert(now_ms != 0);   /// the decommission caller must pass its clock
+                        const MountLease surviving = decodeMountLease(*mount_probe.body);
+                        /// Deliberately weaker than claimMount's reclaim gate (this file, ~:370-380),
+                        /// which never trusts a bare wall-clock comparison alone (only gc_fenced /
+                        /// the clean-farewell min_active==UINT64_MAX marker / a caller-proven-dead
+                        /// token justify a reclaim there, because clock skew can misjudge liveness).
+                        /// This is still safe: (a) the mint below is DISTINCT from the survivor's
+                        /// epoch by construction, so no same-(uuid, epoch) pair is ever representable
+                        /// even if this liveness read is wrong; (b) claimMount right after this still
+                        /// applies its own STRONG liveness gate and refuses a genuinely live member
+                        /// regardless of what happens here. So a clock-skewed "terminal" misread can
+                        /// only burn one epoch number on a doomed decommission attempt that aborts at
+                        /// claimMount — it can never admit a claim over a live member.
+                        const bool live = !surviving.gc_fenced && surviving.expires_at_ms > now_ms;
+                        if (live)
+                            throw Exception(ErrorCodes::ABORTED,
+                                "CAS decommission '{}': epoch object missing but a LIVE mount lease "
+                                "exists ({}) — refusing to re-mint an epoch under a live member "
+                                "(stop the server or wait for its lease to lapse)",
+                                srid, describeMountHolder(surviving));
+                        /// Terminal mount: proceed, but mint an epoch DISTINCT from the survivor's
+                        /// by construction — the same-pair state is unrepresentable on this path.
+                        current.next_writer_epoch = std::max<uint64_t>(1, surviving.writer_epoch + 1);
+                        break;
+                    }
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS server-root '{}' has no durable epoch object but a mount lease exists — "
+                        "durable epoch state was lost while a mount is live or recently live; "
+                        "refusing to re-mint epoch 1. If no server is live on this root, "
+                        "decommission it or manually remove the stale mount object '{}'.",
+                        srid, l.mountKey(srid));
+                }
+                case ProbeOutcome::ContainerAbsent:
+                case ProbeOutcome::AccessDenied:
+                case ProbeOutcome::Indeterminate:
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS server-root '{}': cannot verify mount-lease absence before re-minting "
+                        "the writer epoch (probe outcome: {}) — absence was never proven; failing closed",
+                        srid, magic_enum::enum_name(mount_probe.outcome));
+            }
+
+            if (current.next_writer_epoch == 0)
+                current.next_writer_epoch = 1;
+        }
+
+        const uint64_t next = current.next_writer_epoch;
+        ServerEpoch new_state;
+        new_state.next_writer_epoch = next + 1;
+
+        const CasResult res = b.casPut(key, encodeServerEpoch(new_state), expected);
+        if (res.outcome == CasOutcome::Committed)
+            return next;
+        if (!got)
+        {
+            /// The absent-epoch create conflicted. A winner may have installed an epoch while owned
+            /// work became visible, so recompute the complete catalog + manifest + roots bundle
+            /// before the next iteration is allowed to accept either a present or absent epoch.
+            if (!serverRootSubtreeEmpty(b, l, srid, observe_catalog()))
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS server-root '{}' writer_epoch allocation conflicted and newly visible owned "
+                    "work blocks recreation",
+                    srid);
+        }
+        /// Conflict: someone else allocated concurrently — retry against fresh state only after the
+        /// absent-epoch safety bundle above has been recomputed when required.
+    }
+
+    throw Exception(ErrorCodes::CORRUPTED_DATA,
+        "CAS server-root '{}' writer_epoch allocation did not converge after {} attempts",
+        srid, max_attempts);
+}
+
+namespace
+{
+/// Build a fresh mount-lease body for (uuid, epoch) with the given seq, stamped from `now_ms`.
+MountLease makeMountBody(UInt128 uuid, uint64_t epoch, uint64_t seq, uint64_t now_ms, uint64_t ttl_ms)
+{
+    return MountLease{
+        .server_uuid = uuid,
+        .writer_epoch = epoch,
+        .hostname = getFQDNOrHostName(),
+        .pid = static_cast<uint64_t>(::getpid()),
+        .started_at_ms = now_ms,
+        .seq = seq,
+        .expires_at_ms = now_ms + ttl_ms,
+    };
+}
+
+/// Mirrors `mountDoubleStartMessage`'s identity fields. The mount-audit sink is not yet installed
+/// during `Pool::open`, so at first-open these refusal messages are the only holder-identity
+/// carrier in err.log — name the toucher inline rather than just the key.
+String describeMountHolder(const MountLease & m)
+{
+    return fmt::format("server_uuid={} hostname={} pid={} writer_epoch={} seq={} expires_at_ms={}",
+        u128ToHex(m.server_uuid), m.hostname, m.pid, m.writer_epoch, m.seq, m.expires_at_ms);
+}
+
+/// The mount-slot "foreign writer" audit instrument: every mount-slot WRITE
+/// (`MountClaim`/`MountRelease`) and every OBSERVED foreign/conflicting body (`MountConflict`)
+/// becomes one `system.cas_log` row. `observed` is the CURRENT decoded body at the
+/// point of decision — for a conflict it carries the identity that made us refuse (holder_uuid/
+/// hostname/pid/epoch/seq/expires); null when no body was observed (e.g. a bare CAS race).
+/// No-op when `sink` is unset, so a disabled log does no per-call work.
+void emitMountEvent(const CasEventSink & sink, CasEventType type, const String & srid,
+                    const String & branch, const MountLease * observed, const String & reason)
+{
+    if (!sink)
+        return;
+    CasEvent e;
+    e.type = type;
+    e.object_kind = CasEventObjectKind::None;
+    e.outcome = branch;
+    e.reason = reason;
+    e.detail["server_root_id"] = srid;
+    e.detail["branch"] = branch;
+    if (observed)
+    {
+        e.detail["holder_uuid"] = u128ToHex(observed->server_uuid);
+        e.detail["holder_hostname"] = observed->hostname;
+        e.detail["holder_pid"] = std::to_string(observed->pid);
+        e.detail["holder_epoch"] = std::to_string(observed->writer_epoch);
+        e.detail["holder_seq"] = std::to_string(observed->seq);
+        e.detail["holder_expires_at_ms"] = std::to_string(observed->expires_at_ms);
+    }
+    sink(std::move(e));
+}
+}
+
+MountClaimResult claimMount(
+    Backend & b, const Layout & l, const String & srid, UInt128 our_uuid, uint64_t our_epoch,
+    uint64_t now_ms, uint64_t ttl_ms, const std::optional<Token> & proven_dead_token,
+    const CasEventSink & sink)
+{
+    const String key = l.mountKey(srid);
+    const auto got = b.get(key);
+
+    /// Absent → fresh claim.
+    if (!got)
+    {
+        const MountLease body = makeMountBody(our_uuid, our_epoch, /*seq=*/ 1, now_ms, ttl_ms);
+        const PutResult put = b.putIfAbsent(key, encodeMountLease(body));
+        if (put.outcome != PutOutcome::Done)
+            /// Raced with a concurrent writer between get and putIfAbsent. Treat as a live double
+            /// start — fail closed; never overwrite a slot that appeared under us. No re-read was
+            /// done, so no conflicting identity is known to attach to an event.
+            return {.kind = MountClaimResult::LiveDoubleStart, .body = body, .token = std::nullopt};
+        emitMountEvent(sink, CasEventType::MountClaim, srid, "mint", nullptr, "fresh mount slot minted");
+        return {.kind = MountClaimResult::Claimed, .body = body, .token = std::nullopt};
+    }
+
+    const MountLease existing = decodeMountLease(got->bytes);
+
+    /// Foreign owner → fail closed regardless of expiry. (This runs after the owner gate, so a foreign
+    /// mount should not normally exist, but the lease must never be taken across UUIDs.)
+    if (existing.server_uuid != our_uuid)
+    {
+        emitMountEvent(sink, CasEventType::MountConflict, srid, "foreign_owner", &existing,
+            "mount slot is held by a foreign server_uuid — refusing to take over across identities");
+        return {.kind = MountClaimResult::ForeignOwner, .body = existing, .token = std::nullopt};
+    }
+
+    /// Same uuid + same epoch: it is OUR OWN claim — but a FENCED body is terminal for this
+    /// (uuid, epoch): the GC dropped its ack from the floor when it fenced. Refreshing it in place
+    /// would resurrect a fenced incarnation — a fence permanently consumes this `(server_uuid,
+    /// writer_epoch)` pair, so the caller must re-open with a fresh `writer_epoch`.
+    if (existing.writer_epoch == our_epoch)
+    {
+        if (existing.gc_fenced)
+        {
+            emitMountEvent(sink, CasEventType::MountConflict, srid, "fenced_by_gc", &existing,
+                "own (uuid, epoch) mount slot is GC-fenced — terminal for this incarnation; "
+                "recover with a fresh writer_epoch");
+            return {.kind = MountClaimResult::FencedSelf, .body = existing, .token = std::nullopt};
+        }
+        const MountLease body = makeMountBody(our_uuid, our_epoch, existing.seq + 1, now_ms, ttl_ms);
+        const PutResult put = b.putOverwrite(key, encodeMountLease(body), got->token);
+        if (put.outcome != PutOutcome::Done)
+            /// The mount changed under us between get and putOverwrite: `got->token` is now KNOWN
+            /// STALE (that mismatch is exactly why the put failed), not merely unknown -- leaving
+            /// `.token` unset (rather than handing back a token the caller would wrongly treat as
+            /// current) is deliberate, matching the identical race below.
+            return {.kind = MountClaimResult::LiveDoubleStart, .body = body, .token = std::nullopt};
+        emitMountEvent(sink, CasEventType::MountClaim, srid, "refresh", &existing,
+            "own claim replayed — refreshed seq + expiry");
+        return {.kind = MountClaimResult::Claimed, .body = body, .token = std::nullopt};
+    }
+
+    /// Same uuid, DIFFERENT epoch: reclaim ONLY on a certificate of death that needs no fresh
+    /// wall-clock trust — never by comparing `expires_at_ms` against `now_ms`:
+    ///   - `gc_fenced` → the fence-out is terminal for that incarnation by construction (its keeper's
+    ///     every renewal fails the token guard forever, so it can never write again) — there is no
+    ///     liveness left to wait for. This is what makes self-remount (and a fast restart after a
+    ///     fence-out) instant instead of an observation wait.
+    ///   - the clean marker (`min_active == UINT64_MAX`) → the predecessor's OWN graceful farewell
+    ///     (`MountLeaseKeeper::terminate`) — no observation needed either.
+    ///   - `proven_dead_token` matches the token we just read → the CALLER (`claimMountAwaitingExpiry`)
+    ///     already watched this exact token hold stable for the full observation threshold on its own
+    ///     clock; re-deriving that here from a bare wall-clock comparison would be exactly the
+    ///     cross-node trust would make a clock-skewed or delayed observer unsafe.
+    /// Anything else → `LiveDoubleStart` (do NOT write): a same-uuid, different-epoch, not fenced, not
+    /// clean-marked, not (yet) proven-dead lease may simply be a live twin, and `expires_at_ms` alone
+    /// can never distinguish that from a dead predecessor across two different clocks.
+    const bool clean_marker = existing.min_active == std::numeric_limits<uint64_t>::max();
+    const bool proven_dead = proven_dead_token && *proven_dead_token == got->token;
+    if (existing.gc_fenced || clean_marker || proven_dead)
+    {
+        const MountLease body = makeMountBody(our_uuid, our_epoch, existing.seq + 1, now_ms, ttl_ms);
+        const PutResult put = b.putOverwrite(key, encodeMountLease(body), got->token);
+        if (put.outcome != PutOutcome::Done)
+            /// The mount changed under us between get and putOverwrite — someone else is racing the
+            /// reclaim. Fail closed. `got->token` is now KNOWN STALE (that mismatch is exactly why the
+            /// put failed) -- leaving `.token` unset is deliberate, not an oversight.
+            return {.kind = MountClaimResult::LiveDoubleStart, .body = body, .token = std::nullopt};
+        const MountPriorState prior = existing.gc_fenced ? MountPriorState::Fenced
+                                     : clean_marker       ? MountPriorState::Clean
+                                                           : MountPriorState::UncleanObserved;
+        emitMountEvent(sink, CasEventType::MountClaim, srid, "reclaim", &existing,
+            existing.gc_fenced ? "same server_uuid, different writer_epoch, GC-fenced — reclaimed"
+            : clean_marker     ? "same server_uuid, different writer_epoch, clean farewell — reclaimed"
+                               : "same server_uuid, different writer_epoch, observed dead by "
+                                 "token-stability — reclaimed");
+        return {.kind = MountClaimResult::Claimed, .body = body, .prior = prior, .token = std::nullopt};
+    }
+
+    emitMountEvent(sink, CasEventType::MountConflict, srid, "live_double_start", &existing,
+        "same server_uuid, different writer_epoch, not fenced/clean/proven-dead — no wall-clock trust; "
+        "the caller must run the token-stability observation wait before reclaiming");
+    /// No write was attempted on this path -- `got->token` is exactly the CURRENT body's
+    /// token (what we just read is what's still there), so it is safe to hand back for the caller's
+    /// observation loop to compare across polls without a redundant re-GET.
+    return {.kind = MountClaimResult::LiveDoubleStart, .body = existing, .token = got->token};
+}
+
+String mountDoubleStartMessage(const String & srid, const MountLease & existing)
+{
+    return fmt::format(
+        "Content-addressed disk cannot start: server_root_id '{}' is actively mounted by another LIVE server.\n"
+        "  Existing mount: server_uuid={} hostname={} pid={} last_seq={} expires_at_ms={}\n"
+        "This server already waited for the mount lease to lapse, but it kept being renewed — a second\n"
+        "server is holding the same CAS namespace. This prevents two ClickHouse servers from writing it.\n"
+        " - If the other server is running intentionally, configure a unique <server_root_id> for this disk.\n"
+        " - If the other server is a stale/zombie process, stop it; this server will then reclaim the mount on restart.\n"
+        " - CLOCK SKEW CAVEAT: liveness is judged by comparing the lease's wall-clock expires_at_ms against\n"
+        "   THIS server's clock, so a large clock skew between the two servers can misjudge it (a healthy holder\n"
+        "   may look mounted here, or a dead one may look live). Verify both servers' clocks are in sync (NTP).\n"
+        " - If the local ClickHouse uuid file was regenerated, restore the old uuid file, or remove the stale\n"
+        "   owner object gc/server-roots/{}/owner only after verifying no server uses this root.\n"
+        " - As a LAST RESORT, after verifying that NO server is writing this root, manually delete the mount\n"
+        "   object gc/server-roots/{}/mount and restart; this server will then re-claim it.",
+        srid, u128ToHex(existing.server_uuid), existing.hostname, existing.pid,
+        existing.seq, existing.expires_at_ms, srid, srid);
+}
+
+namespace
+{
+/// Bounded number of observation restarts before giving up on a same-uuid slot whose write-token keeps
+/// changing: each restart means the token changed DURING our observation window — i.e. something is
+/// actively renewing it. A genuinely dead predecessor's token never changes again after its last
+/// renewal, so it is observed stable well within one window; only a truly LIVE writer (a real second
+/// incarnation, or the predecessor's own background renewer racing our first few polls) keeps resetting
+/// the clock. Bounding this converts "wait forever for a live twin" into the same bounded-then-report
+/// shape the old wall-clock wait had, without ever trusting a wall-clock deadline to get there.
+constexpr size_t kMaxObservationRestarts = 3;
+}
+
+uint64_t mountObservationThresholdMs(uint64_t ttl_ms, uint64_t cadence_ms)
+{
+    return ttl_ms + ttl_ms / 20 + cadence_ms;
+}
+
+MountClaimResult claimMountAwaitingExpiry(
+    Backend & b, const Layout & l, const String & srid, UInt128 our_uuid, uint64_t our_epoch,
+    const std::function<uint64_t()> & now_ms_fn,
+    const std::function<uint64_t()> & mono_ms_fn,
+    uint64_t ttl_ms, uint64_t poll_interval_ms,
+    const std::function<void(uint64_t)> & sleep_ms_fn,
+    const std::function<void(const MountLease &, uint64_t)> & on_wait_start,
+    const CasEventSink & sink)
+{
+    /// A zero poll interval would spin; a single-ms floor keeps the loop a real (bounded) wait.
+    const uint64_t poll = poll_interval_ms == 0 ? 1 : poll_interval_ms;
+
+    /// Rate-bound observation threshold: the full lease TTL, plus a 5% allowance for clock-rate
+    /// mismatch between the holder's and our own local clock, plus one poll interval for observation
+    /// discreteness. It is measured only with OUR OWN clock (`mono_ms_fn`); no cross-node wall-clock
+    /// comparison participates in this loop. The shared helper keeps the startup and GC thresholds
+    /// identical.
+    const uint64_t threshold_ms = mountObservationThresholdMs(ttl_ms, poll);
+
+    std::optional<Token> observed;
+    uint64_t observed_since = 0;
+    size_t restarts = 0;
+
+    while (true)
+    {
+        const bool threshold_met = observed && mono_ms_fn() - observed_since >= threshold_ms;
+        MountClaimResult r = claimMount(b, l, srid, our_uuid, our_epoch, now_ms_fn(), ttl_ms,
+            threshold_met ? observed : std::nullopt, sink);
+        if (r.kind != MountClaimResult::LiveDoubleStart)
+            return r;
+
+        /// `claimMount` already read the current body. Reuse `r.token` whenever `claimMount`
+        /// set it (the common case: no write was attempted, so what it read is still current) instead of
+        /// re-GETting the SAME key here. The rare stale-race branches deliberately leave `.token` unset
+        /// (see their own comments), so this still falls back to a fresh read exactly there.
+        std::optional<Token> current_token = r.token;
+        if (!current_token)
+        {
+            const auto got = b.get(l.mountKey(srid));
+            if (!got)
+            {
+                /// The slot vanished between claimMount's own GET and ours — normally self-resolving
+                /// within one more `claimMount` call (which re-mints fresh on an absent slot), but under
+                /// slot churn (something else concurrently removing/re-minting it) that resolution could
+                /// keep losing the same race. Pace this like every other iteration and
+                /// count it toward the SAME bounded restart budget the token-churn case below uses,
+                /// instead of spinning `get`/`claimMount`/`put` at backend RTT with no sleep and no bound
+                /// — a persistently vanishing slot is exactly as "alive and contended" as a persistently
+                /// renewing token.
+                if (++restarts > kMaxObservationRestarts)
+                    return r;
+                sleep_ms_fn(poll);
+                continue;
+            }
+            current_token = got->token;
+        }
+
+        if (!observed || *observed != *current_token)
+        {
+            if (observed && ++restarts > kMaxObservationRestarts)
+                /// The token kept changing across bounded restarts — the holder is genuinely alive
+                /// (actively renewing), not a dead predecessor. Report it rather than waiting forever.
+                return r;
+            observed = *current_token;
+            observed_since = mono_ms_fn();
+            if (on_wait_start)
+                on_wait_start(r.body, threshold_ms);
+            LOG_INFO(getLogger("CasMountLease"),
+                "Attempting to mount content-addressed server root {} after node change or hard "
+                "restart; waiting ~{} ms (token-stability observation) to confirm the previous "
+                "incarnation's operations are all finalized", srid, threshold_ms);
+        }
+
+        sleep_ms_fn(poll);
+    }
+}
+
+HeartbeatFloor computeHeartbeatFloor(Backend & b, const Layout & l, uint64_t now_ms,
+                                     uint64_t mono_now_ms, uint64_t stable_threshold_ms,
+                                     MountObservationMap & obs)
+{
+    HeartbeatFloor floor;
+
+    /// `obs` is keyed by every srid this leader has EVER observed, but a
+    /// srid removed from the LIST entirely (its `/mount` key gone -- e.g. `SYSTEM CAS
+    /// DROP POOL MEMBER`) is never visited by the loop below again, so its entry would otherwise linger
+    /// forever (~150-250 B/srid, worse on a long-lived leader across many decommissions). Track every
+    /// srid actually seen THIS pass and prune anything else out of `obs` at the end -- disjoint from the
+    /// mid-loop `obs.erase(srid)` calls below (those fire for a srid seen but now terminal/fenced/gone
+    /// this pass; this is for a srid not seen AT ALL).
+    std::set<String> seen_srids;
+
+    const String prefix = l.serverRootsPrefix();
+    String cursor;
+    while (true)
+    {
+        const ListPage page = b.list(prefix, cursor, /*limit*/ 1000);
+        for (const auto & listed : page.keys)
+        {
+            /// `/owner` and `/epoch` objects share the subtree — only mount bodies gate the floor.
+            static constexpr std::string_view mount_suffix = "/mount";
+            if (!listed.key.ends_with(mount_suffix))
+                continue;
+
+            const String & key = listed.key;
+
+            /// The srid is the path segment between `serverRootsPrefix()` and the `/mount` suffix
+            /// (`<prefix>/gc/server-roots/<srid>/mount`). Used both for observability (fenced) and as
+            /// the key into `obs`.
+            const String srid = key.substr(prefix.size(),
+                key.size() - prefix.size() - mount_suffix.size());
+            seen_srids.insert(srid);
+
+            /// Fence-out on PreconditionFailed re-GETs and reclassifies from the top; bound the retries
+            /// so a pathologically contended holder cannot spin forever. On exhaustion the entry is
+            /// counted as live (conservative — never excluded without a landed fence-out).
+            constexpr int max_reclassify = 4;
+            for (int attempt = 0; ; ++attempt)
+            {
+                const auto got = b.get(key);
+                if (!got)
+                {
+                    obs.erase(srid);
+                    break;   /// Raced away (deleted) — nothing to classify.
+                }
+
+                const MountLease m = decodeMountLease(got->bytes);
+
+                if (m.gc_fenced)
+                {
+                    ++floor.already_fenced;
+                    obs.erase(srid);   /// terminal — no further observation needed
+                    break;
+                }
+                if (m.min_active == std::numeric_limits<uint64_t>::max())
+                {
+                    ++floor.terminated;
+                    obs.erase(srid);   /// terminal — no further observation needed
+                    break;
+                }
+
+                /// Observation-based liveness: stable ONLY if the
+                /// SAME token was already being watched and has now held for the full threshold on our
+                /// OWN monotonic clock. Anything else — no prior observation, or a changed token (a
+                /// live renewal, including one raced against our own fence-out attempt below) —
+                /// (re)starts the observation window and counts as `live` this call.
+                const auto it = obs.find(srid);
+                const bool stable = it != obs.end() && it->second.token == got->token
+                    && mono_now_ms - it->second.first_seen_mono_ms >= stable_threshold_ms;
+
+                if (!stable)
+                {
+                    if (it == obs.end() || it->second.token != got->token)
+                        obs[srid] = MountTokenObservation{got->token, mono_now_ms};
+                    ++floor.live;
+                    break;
+                }
+
+                const bool exhausted = attempt >= max_reclassify;
+                if (exhausted)
+                {
+                    ++floor.live;   /// conservative — never exclude without a landed fence-out
+                    break;
+                }
+
+                /// Stable past the threshold, not yet fenced → token-guarded fence-out preserving the
+                /// whole body (gc_fenced = true, seq + 1).
+                MountLease fenced = m;
+                fenced.gc_fenced = true;
+                fenced.seq = m.seq + 1;
+                const PutResult res = b.putOverwrite(key, encodeMountLease(fenced), got->token);
+                if (res.outcome == PutOutcome::Done)
+                {
+                    ++floor.fenced_now;
+                    floor.fenced_srids.push_back(srid);
+                    obs.erase(srid);
+                    LOG_INFO(getLogger("CasHeartbeatFloor"),
+                        "CAS GC fenced out mount lease for content-addressed server root {} at "
+                        "wall-clock ms {}: its write token held unchanged for >= {} ms on the GC "
+                        "leader's own monotonic clock (token-stability observation)",
+                        srid, now_ms, stable_threshold_ms);
+                    break;
+                }
+                /// PreconditionFailed: the holder renewed between our GET and PUT — re-GET and
+                /// reclassify (the observation check above will see the new token and restart it).
+            }
+        }
+
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+
+    /// Prune every `obs` entry for a srid this pass's LIST never saw at all.
+    for (auto it = obs.begin(); it != obs.end(); )
+        it = seen_srids.contains(it->first) ? std::next(it) : obs.erase(it);
+
+    return floor;
+}
+
+std::vector<NonTerminalMountSlot> probeNonTerminalMountSlots(Backend & b, const Layout & l)
+{
+    std::vector<NonTerminalMountSlot> slots;
+
+    /// Same enumeration as `computeHeartbeatFloor`'s gate -- LIST the server-roots subtree, keep the
+    /// `/mount` bodies -- but read-only and without any observation state: this answers "is anyone
+    /// still entitled to write here", not "may I fence them out".
+    const String prefix = l.serverRootsPrefix();
+    String cursor;
+    while (true)
+    {
+        const ListPage page = b.list(prefix, cursor, /*limit*/ 1000);
+        for (const auto & listed : page.keys)
+        {
+            static constexpr std::string_view mount_suffix = "/mount";
+            if (!listed.key.ends_with(mount_suffix))
+                continue;   /// `/owner` and `/epoch` share the subtree; only the lease says "live".
+
+            const String srid = listed.key.substr(prefix.size(),
+                listed.key.size() - prefix.size() - mount_suffix.size());
+
+            const auto got = b.get(listed.key);
+            if (!got)
+                continue;   /// raced away between LIST and GET -- there is no slot to be held.
+
+            MountLease m;
+            try
+            {
+                m = decodeMountLease(got->bytes);
+            }
+            catch (...)
+            {
+                /// An undecodable lease is the WORST case for a recreation, not an ignorable one: it is
+                /// what a slot written by a format this build does not understand looks like, and the
+                /// holder of that slot is exactly the writer we must not run over.
+                slots.push_back(NonTerminalMountSlot{srid, fmt::format(
+                    "mount lease could not be decoded by this build ({})",
+                    getCurrentExceptionMessage(/*with_stacktrace=*/false))});
+                continue;
+            }
+
+            if (m.gc_fenced || m.min_active == std::numeric_limits<uint64_t>::max())
+                continue;   /// terminal: fenced out by GC, or the holder's own graceful farewell.
+
+            slots.push_back(NonTerminalMountSlot{srid, fmt::format(
+                "held by server uuid {} (writer_epoch {}, host '{}', pid {}, lease seq {}, stamped "
+                "expiry {} ms) with neither a graceful farewell nor a GC fence-out",
+                u128ToHex(m.server_uuid), m.writer_epoch, m.hostname, m.pid, m.seq, m.expires_at_ms)});
+        }
+
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+
+    return slots;
+}
+
+std::vector<MountInfo> listMounts(Backend & backend, const Layout & layout, uint64_t now_ms, uint64_t skew_margin_ms)
+{
+    std::vector<MountInfo> out;
+    const String prefix = layout.serverRootsPrefix();
+    String cursor;
+    while (true)
+    {
+        const ListPage page = backend.list(prefix, cursor, 1000);
+        for (const auto & k : page.keys)
+        {
+            static constexpr std::string_view suffix = "/mount";
+            if (!k.key.ends_with(suffix))
+                continue;
+            const auto got = backend.get(k.key);
+            if (!got)
+                continue;   /// raced a delete — read-only view, skip the row
+            MountInfo info;
+            /// The srid is the path segment between `serverRootsPrefix()` and the `/mount` suffix —
+            /// may itself contain `/` (e.g. `shard-01/replica-a`), so slice by prefix length rather
+            /// than `rfind('/')`, matching `computeHeartbeatFloor`'s extraction.
+            info.srid = k.key.substr(prefix.size(), k.key.size() - prefix.size() - suffix.size());
+            try
+            {
+                info.lease = decodeMountLease(got->bytes);
+            }
+            catch (...)
+            {
+                info.state = "corrupt";
+                out.push_back(std::move(info));
+                continue;
+            }
+            if (info.lease.gc_fenced)
+                info.state = "fenced";
+            else if (info.lease.min_active == std::numeric_limits<uint64_t>::max())
+                info.state = "terminated";
+            else if (now_ms <= info.lease.expires_at_ms + skew_margin_ms)
+                info.state = "live";
+            else
+                info.state = "expired";
+            out.push_back(std::move(info));
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    return out;
+}
+
+namespace
+{
+
+/// The three clock-free certificates `isCreatorFenceTerminal` recognises, plus `None` for a live body
+/// that carries none of them -- see the function's header doc for what each one proves and why
+/// `fence_generation` is not among them.
+enum class FenceCertificate : uint8_t
+{
+    None,
+    GcFenced,
+    CleanFarewell,
+    SupersededEpoch,
+};
+
+FenceCertificate classifyFenceCertificate(const MountLease & lease, uint64_t fence_writer_epoch)
+{
+    if (lease.gc_fenced)
+        return FenceCertificate::GcFenced;
+    if (lease.min_active == std::numeric_limits<uint64_t>::max())
+        return FenceCertificate::CleanFarewell;
+    if (lease.writer_epoch != fence_writer_epoch)
+        return FenceCertificate::SupersededEpoch;
+    return FenceCertificate::None;
+}
+
+}
+
+bool isCreatorFenceTerminal(Backend & backend, const Layout & layout, const String & server_root_id,
+                            uint64_t writer_epoch)
+{
+    const auto got = backend.get(layout.mountKey(server_root_id));
+    if (!got)
+        return false;   /// absence proves nothing about liveness -- see the header doc
+
+    MountLease lease;
+    try
+    {
+        lease = decodeMountLease(got->bytes);
+    }
+    catch (...)
+    {
+        return false;   /// undecodable -- fail closed, never wave through
+    }
+
+    /// EXHAUSTIVE switch, not a positive allowlist -- mirrors `CasPool.cpp`'s own exhaustive switch over
+    /// `MountPriorState` (`claimMount`, in this file, only PRODUCES that classification; the switch
+    /// consuming it lives in the caller) deliberately: a future `FenceCertificate` enumerator with no
+    /// verdict assigned here must fail the BUILD (a missing `-Wswitch` case), never silently read as
+    /// terminal (which would let a reconciler steal a namespace out from under a writer that might
+    /// still be alive) or as live (which would block a reconciliation the certificate already proves
+    /// is safe).
+    /// The initializer is dead: the exhaustive switch below assigns every enumerator, and a future
+    /// enumerator left unassigned fails the build via `-Wswitch`, not this value.
+    bool terminal = false;
+    switch (classifyFenceCertificate(lease, writer_epoch))
+    {
+        case FenceCertificate::None:
+            terminal = false;
+            break;
+        case FenceCertificate::GcFenced:
+        case FenceCertificate::CleanFarewell:
+        case FenceCertificate::SupersededEpoch:
+            terminal = true;
+            break;
+    }
+    return terminal;
+}
+
+MountLeaseKeeper::MountLeaseKeeper(
+    BackendPtr backend_, const Layout & layout_, const String & srid_, UInt128 server_uuid_,
+    uint64_t writer_epoch_, std::chrono::milliseconds ttl_, std::function<uint64_t()> now_ms_fn_,
+    std::function<uint64_t()> min_active_fn_,
+    CasEventSink event_sink_,
+    std::chrono::milliseconds lease_safety_margin_,
+    std::function<uint64_t()> boot_ms_fn_)
+    : SingleWriterSlot(std::move(backend_), layout_.mountKey(srid_), "mount-lease", "release", "CasMountLeaseKeeper")
+    , srid(srid_)
+    , server_uuid(server_uuid_)
+    , writer_epoch(writer_epoch_)
+    , ttl(ttl_)
+    , now_ms_fn(std::move(now_ms_fn_))
+    , min_active_fn(std::move(min_active_fn_))
+    , event_sink(std::move(event_sink_))
+    , lease_safety_margin(lease_safety_margin_)
+    , boot_ms_fn(boot_ms_fn_ ? std::move(boot_ms_fn_) : defaultBootMs)
+{
+}
+
+void MountLeaseKeeper::refreshConfirmedDeadline(uint64_t anchor_wall_ms)
+{
+    confirmed_deadline_ms = anchor_wall_ms + static_cast<uint64_t>(ttl.count());
+}
+
+bool MountLeaseKeeper::shouldFenceOnTransientRenewFailure()
+{
+    /// Defensive: should never observe 0 here (see the member's doc comment) -- fail closed if it ever did.
+    if (confirmed_deadline_ms == 0)
+        return true;
+    const uint64_t now = now_ms_fn();
+    const uint64_t margin = static_cast<uint64_t>(lease_safety_margin.count());
+    return now + margin >= confirmed_deadline_ms;
+}
+
+SingleWriterSlot::RenewPayload MountLeaseKeeper::prepareRenew() const
+{
+    /// Carry the two dynamic fields (both read OFF the state lock — the merged floor callback reaches
+    /// into the Pool's own lock): `value` = wall-clock `now_ms` (so `encodeBody` stamps a fresh
+    /// `expires_at_ms = now_ms + ttl`), `value2` = `min_active` (the build-watermark floor).
+    /// Pre-I/O anchors (spec rev.4 Phase B): both fence deadlines anchor at this instant — the
+    /// wall stamp doubles as the payload's now_ms, so anchor <= the durable stamp trivially.
+    last_attempt_wall_ms = now_ms_fn();
+    last_attempt_boot_ms = boot_ms_fn();
+    return {.value = last_attempt_wall_ms, .value2 = min_active_fn()};
+}
+
+String MountLeaseKeeper::encodeBody(uint64_t seq_, const RenewPayload & payload) const
+{
+    const uint64_t now_ms = payload.value;
+    const uint64_t ttl_ms = static_cast<uint64_t>(ttl.count());
+    return encodeMountLease(MountLease{
+        .server_uuid = server_uuid,
+        .writer_epoch = writer_epoch,
+        .hostname = getFQDNOrHostName(),
+        .pid = static_cast<uint64_t>(::getpid()),
+        .started_at_ms = now_ms,
+        .seq = seq_,
+        .expires_at_ms = now_ms + ttl_ms,
+        .min_active = payload.value2,
+    });
+}
+
+SingleWriterSlot::Token MountLeaseKeeper::claim(const String & body)
+{
+    /// ADOPT-aware claim. The normal flow is `claimMount` wrote the live mount under
+    /// (server_uuid, writer_epoch); `start` then adopts that very slot. We must NOT self-trip the
+    /// live-double-start guard on our own (uuid, epoch).
+    const HeadResult head = backend->head(key);
+    if (!head.exists)
+    {
+        /// Absent → put it ourselves (a fresh start that ran without a prior claimMount, or a slot
+        /// that lapsed and was swept). putIfAbsent fails closed if it appears under us; that race has
+        /// no re-read (no observed body), so there is nothing to attach to a conflict event.
+        const PutResult res = backend->putIfAbsent(key, body);
+        if (res.outcome != PutOutcome::Done)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS mount-lease: key '{}' appeared between head and putIfAbsent — concurrent writer on our mount slot", key);
+        emitMountEvent(event_sink, CasEventType::MountClaim, srid, "mint", nullptr,
+            "mount slot absent — keeper minted it directly (no prior claimMount)");
+        refreshConfirmedDeadline(last_attempt_wall_ms);
+        return res.token;
+    }
+
+    /// Read the observed slot to decide adopt vs fail-closed by the (uuid, epoch) discriminator.
+    const auto got = backend->get(key);
+    if (!got)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS mount-lease: key '{}' vanished between head and get while claiming", key);
+    const MountLease observed = decodeMountLease(got->bytes);
+
+    /// Foreign uuid → fail closed (no cross-UUID takeover, ever). The audit payload: the CURRENT decoded
+    /// body's identity is exactly WHO touched the slot.
+    if (observed.server_uuid != server_uuid)
+    {
+        emitMountEvent(event_sink, CasEventType::MountConflict, srid, "adopt", &observed,
+            "mount slot is held by a foreign server — failing closed, never taking over");
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS mount-lease: key '{}' is held by a foreign server ({}) — failing closed, never taking over",
+            key, describeMountHolder(observed));
+    }
+
+    /// Same uuid but a DIFFERENT epoch → a newer incarnation superseded us (or a concurrent
+    /// double-start). Fail closed.
+    if (observed.writer_epoch != writer_epoch)
+    {
+        emitMountEvent(event_sink, CasEventType::MountConflict, srid, "adopt", &observed,
+            fmt::format("mount slot is held by a different writer_epoch ({} != ours {}) — superseded, failing closed",
+                observed.writer_epoch, writer_epoch));
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS mount-lease: key '{}' is held by a different writer_epoch ({} != ours {}) — superseded, failing closed ({})",
+            key, observed.writer_epoch, writer_epoch, describeMountHolder(observed));
+    }
+
+    /// Same (uuid, epoch) but FENCED: the GC fenced our fresh lease before we adopted it (the
+    /// lease expired mid-open — e.g. a slow first beat). Terminal for THIS epoch; the open path
+    /// recovers by allocating a fresh `writer_epoch` and re-claiming.
+    if (observed.gc_fenced)
+    {
+        emitMountEvent(event_sink, CasEventType::MountConflict, srid, "fenced_by_gc", &observed,
+            "own mount slot fenced by GC after lease expiry — recoverable with a fresh writer_epoch");
+        throw MountFencedException(fmt::format(
+            "CAS mount-lease: key '{}' was fenced by GC after lease expiry ({}) — "
+            "recoverable: re-open with a fresh writer_epoch", key, describeMountHolder(observed)));
+    }
+
+    /// Same uuid AND same epoch → it is OUR OWN claim → ADOPT: overwrite against the observed token
+    /// to refresh seq/expiry. (`body` is encoded for seq=1 by the base `doStart`; that is fine —
+    /// renewals advance from there.)
+    const PutResult res = backend->putOverwrite(key, body, got->token);
+    if (res.outcome != PutOutcome::Done)
+    {
+        /// The slot moved between our GET and PUT. Diagnose by the CURRENT body, not the token
+        /// The current body is the useful diagnostic: a GC fence is the only same-(uuid, epoch)-preserving
+        /// touch that can normally occur during adoption.
+        const auto reread = backend->get(key);
+        if (reread)
+        {
+            const MountLease current = decodeMountLease(reread->bytes);
+            if (current.server_uuid == server_uuid && current.gc_fenced)
+            {
+                emitMountEvent(event_sink, CasEventType::MountConflict, srid, "fenced_by_gc", &current,
+                    "GC fenced our mount between the adopt's read and write — recoverable with a "
+                    "fresh writer_epoch");
+                throw MountFencedException(fmt::format(
+                    "CAS mount-lease: key '{}' was fenced by GC inside the adopt window ({}) — "
+                    "recoverable: re-open with a fresh writer_epoch", key, describeMountHolder(current)));
+            }
+            emitMountEvent(event_sink, CasEventType::MountConflict, srid, "adopt", &current,
+                "mount slot was touched while adopting our own mount slot — failing closed");
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS mount-lease: key '{}' was touched while adopting our own mount slot ({}) — failing closed",
+                key, describeMountHolder(current));
+        }
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS mount-lease: key '{}' vanished while adopting our own mount slot — failing closed", key);
+    }
+    emitMountEvent(event_sink, CasEventType::MountClaim, srid, "adopt", &observed,
+        "adopted our own already-live (uuid, epoch) mount slot");
+    refreshConfirmedDeadline(last_attempt_wall_ms);
+    return res.token;
+}
+
+void MountLeaseKeeper::onRenewCommitted()
+{
+    /// Anchor at the attempt start (stashed by prepareRenew), never at this ack instant — a slow
+    /// ack must not extend either fence past what the durable body it acknowledges authorizes
+    /// (spec rev.4 Phase B). Runs for EVERY successful `renewOnce` — background-driven or a direct
+    /// caller (e.g. the startup-arm redo in `CasPool.cpp`'s `mountWritable`) — so the wall deadline
+    /// never goes stale just because a renewal happened to be invoked outside the background loop.
+    refreshConfirmedDeadline(last_attempt_wall_ms);
+}
+
+void MountLeaseKeeper::onRenewSucceeded()
+{
+    /// `confirmed_deadline_ms` was already refreshed by `onRenewCommitted` above, which `renewOnce`
+    /// (base) calls right after recording the successful write — before the background loop reaches
+    /// this hook. This hook fires only the boot-domain write-fence callback.
+    if (on_renew_ok)
+        on_renew_ok(last_attempt_boot_ms);
+}
+
+void MountLeaseKeeper::onRenewFailed()
+{
+    /// This is THE point at which this runtime stops believing it owns the mount, so it is where the
+    /// release path's arm-A/arm-B split is decided (see `terminate`). Set BEFORE the fence callback, so
+    /// a teardown racing this observation reads the conservative value.
+    deposition_observed.store(true, std::memory_order_release);
+    /// Background renewal failed: `renewOnce` threw and the loop is stopping. Latch the local write
+    /// fence to lost so no further mutation proceeds — fail closed. The mismatch itself was already
+    /// classified and emitted by `onRenewMismatch` (fenced_by_gc / same_epoch_state_uncertain /
+    /// superseded / foreign_writer / vanished) just before this throw propagated here — this event is
+    /// only the fence-latch timeline marker.
+    emitMountEvent(event_sink, CasEventType::MountConflict, srid, "renew_failed", nullptr,
+        "renew mismatch — see the preceding classified mount_conflict event; write fence latched to lost");
+    if (on_lost)
+        on_lost();
+}
+
+void MountLeaseKeeper::onRenewMismatch(const String & mismatched_key)
+{
+    /// The base contract's PreconditionFailed just means "our token didn't match" — re-read the
+    /// CURRENT body and classify. All four body-present cases and the absent case are covered
+    /// below, each fail-closed and NONE constructing a `LOGICAL_ERROR`, which aborts debug/ASan
+    /// builds at exception construction — on this KEEPER THREAD, taking the whole process with it
+    /// (STID 3982-3b48; parts 1a/1b covered vanished/absent-at-release, this covers the rest).
+    const auto got = backend->get(mismatched_key);
+    if (got)
+    {
+        const MountLease current = decodeMountLease(got->bytes);
+
+        if (current.server_uuid == server_uuid && current.gc_fenced)
+        {
+            emitMountEvent(event_sink, CasEventType::MountConflict, srid, "fenced_by_gc", &current,
+                "own mount slot fenced by GC after lease expiry (late renewal) — recoverable with a "
+                "fresh writer_epoch");
+            throw MountFencedException(fmt::format(
+                "CAS mount-lease: key '{}' was fenced by GC after lease expiry (late renewal) ({}) — "
+                "recoverable: re-open with a fresh writer_epoch", mismatched_key, describeMountHolder(current)));
+        }
+
+        if (current.server_uuid == server_uuid && current.writer_epoch == writer_epoch && !current.gc_fenced)
+        {
+            /// The slot advanced past our held token under our OWN (uuid, epoch), unfenced. This is
+            /// state UNCERTAINTY, not proof of anything (spec rev.4): the common cause is our own
+            /// earlier renewal PUT that landed while its ack was lost to a client-side timeout; the
+            /// pathological one is a same-pair twin after durable epoch-state loss (narrowed by the
+            /// allocateWriterEpoch re-mint guard). Both recover identically and fail closed: stop
+            /// renewing, latch the write fence, self-remount under a fresh writer_epoch. Never a
+            /// LOGICAL_ERROR — this shape is reachable by an ordinary network timeout.
+            ProfileEvents::increment(ProfileEvents::CASMountLeaseLost);
+            emitMountEvent(event_sink, CasEventType::MountConflict, srid, "same_epoch_state_uncertain", &current,
+                "own mount slot advanced past our held token under our own (uuid, epoch) — state "
+                "uncertain (ambiguous prior renewal or epoch-state loss); fencing and self-remounting");
+            throw Exception(ErrorCodes::ABORTED,
+                "CAS mount-lease: key '{}' advanced past our held token under our own (uuid, epoch) — "
+                "state uncertain; fencing and recovering via self-remount (observed {} vs our seq={})",
+                mismatched_key, describeMountHolder(current), seq);
+        }
+
+        if (current.server_uuid == server_uuid && current.writer_epoch != writer_epoch)
+        {
+            ProfileEvents::increment(ProfileEvents::CASMountLeaseLost);
+            emitMountEvent(event_sink, CasEventType::MountConflict, srid, "superseded", &current,
+                "own mount slot is held by a different writer_epoch — superseded by a newer incarnation");
+            /// A normal fencing outcome (the model's localLost), not a programming assertion:
+            /// a suspended predecessor legitimately resumes into this after a successor reclaimed.
+            throw Exception(ErrorCodes::ABORTED,
+                "CAS mount-lease: key '{}' was superseded by a newer incarnation ({}) — fencing "
+                "(this incarnation is deposed; recovery is a fresh-epoch self-remount)",
+                mismatched_key, describeMountHolder(current));
+        }
+
+        /// `current.server_uuid != server_uuid` — a DIFFERENT server holds the slot we thought was ours.
+        /// The owner anchor refuses foreign claims at open and decommission impersonates the victim uuid
+        /// rather than manufacturing a foreign one, so this is not something a healthy protocol run
+        /// produces. It is still ENVIRONMENT-REACHABLE, and this comment used to claim otherwise: clear
+        /// the pool prefix and recreate under a different server id — an operator `rm -rf`, or a
+        /// recreation over a reused prefix — and the surviving writer's very next renewal lands exactly
+        /// here. `CasRefContiguousAlloc.SurvivingWriterIsFencedByTheRecreatedPoolsMount` drives it
+        /// deliberately, which is the plainest possible refutation of "unreachable".
+        ///
+        /// So it must not be a `LOGICAL_ERROR`. That class aborts debug/ASan builds at CONSTRUCTION, and
+        /// this runs on the keeper's background thread, so a condition the environment can create took
+        /// the whole process down. `ABORTED` instead — the same class the two sibling fencing arms above
+        /// use, and one the storage layer already treats as a retry-safe mount-lost signal. The OUTCOME
+        /// is unchanged and is the whole point: renewal stops, `onRenewFailed` latches the write fence,
+        /// and this incarnation never takes over the foreign holder's slot.
+        ///
+        /// NOT yet done at the RELEASE path (`terminate` below): its foreign-incarnation arm has the
+        /// same defect and is reached by the same test at teardown, but three `EXPECT_DEATH` tests
+        /// (`CasGcRound.OrphanManifestCursorSweepDeletesAndPersistsCursor`,
+        /// `CasMountStartup.StaleSelfMountReclaimedAfterWait`,
+        /// `CasPoolRemount.ForeignOwnerIsNeverTakenOver`) deliberately pin that abort, so changing it is
+        /// a ruled decision rather than a local fix.
+        ProfileEvents::increment(ProfileEvents::CASMountLeaseLost);
+        emitMountEvent(event_sink, CasEventType::MountConflict, srid, "foreign_writer", &current,
+            "mount slot is held by a foreign server — failing closed, never taking over");
+        throw Exception(ErrorCodes::ABORTED,
+            "CAS mount-lease: key '{}' is held by a foreign server ({}) — failing closed, never taking over",
+            mismatched_key, describeMountHolder(current));
+    }
+
+    /// The mount slot object VANISHED (backing store deleted under a live mount -- e.g. an
+    /// operator or test rm -rf'd the pool dir). This is an ENVIRONMENTAL condition, not a logic
+    /// error: there is no foreign writer to fail closed against. Stop renewing (fail-closed: the
+    /// write fence latches to lost, we never re-mint) WITHOUT aborting the server --
+    /// LOGICAL_ERROR here aborts debug/ASan builds at exception construction.
+    ProfileEvents::increment(ProfileEvents::CASMountLeaseLost);
+    emitMountEvent(event_sink, CasEventType::MountConflict, srid, "vanished", nullptr,
+        "mount slot object vanished (backing store deleted under a live mount) — stopping renewal, fail-closed");
+    throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+        "CAS mount-lease: key '{}' vanished (backing store deleted under a live mount) — "
+        "stopping renewal, fail-closed (never re-minting)", mismatched_key);
+
+    /// NOTE: the pre-rev.4 trailing `SingleWriterSlot::onRenewMismatch(mismatched_key)` call is
+    /// GONE — the five cases above are exhaustive for this keeper (body-present × {fenced,
+    /// same-pair-unfenced, superseded, foreign} + absent), so the base class's generic
+    /// LOGICAL_ERROR is unreachable here. The base implementation stays for other slot subclasses.
+}
+
+void MountLeaseKeeper::terminate()
+{
+    /// Terminal op: retire the lease by stamping it already-expired (expires_at_ms = started_at_ms),
+    /// seq+1, against the token we hold. This makes a same-uuid reopen immediately reclaimable. The
+    /// merged watermark farewell folds in HERE: `min_active = UINT64_MAX` is the retired sentinel the
+    /// GC floor treats as "every build_seq of this server is retired" — one release retires both the
+    /// mount lease and the build watermark.
+    const uint64_t now_ms = now_ms_fn();
+    const String body = encodeMountLease(MountLease{
+        .server_uuid = server_uuid,
+        .writer_epoch = writer_epoch,
+        .hostname = getFQDNOrHostName(),
+        .pid = static_cast<uint64_t>(::getpid()),
+        .started_at_ms = now_ms,
+        .seq = seq + 1,
+        .expires_at_ms = now_ms,
+        .min_active = std::numeric_limits<uint64_t>::max(),
+    });
+    const PutResult res = backend->putOverwrite(key, body, last_token);
+    if (res.outcome != PutOutcome::Done)
+    {
+        /// A foreign incarnation on OUR release path has one clean cause: GC fenced this mount out
+        /// after its lease expired (the `gc_fenced` stamp). The slot is already released-by-fence and
+        /// there is nothing left to retire.
+        if (const auto got = backend->get(key))
+        {
+            const MountLease current = decodeMountLease(got->bytes);
+            if (current.gc_fenced)
+            {
+                LOG_INFO(getLogger("CasMountLeaseKeeper"),
+                    "CAS mount-lease: '{}' was fenced out by GC (expired lease); release is a no-op", key);
+                return;
+            }
+
+            /// Everything else used to be one arm raising `LOGICAL_ERROR` — "the world is broken". It
+            /// is TWO situations, they mean opposite things, and neither may abort: this runs from
+            /// `~Pool` via `finishTeardown`, whose `catch` a `LOGICAL_ERROR` defeats by aborting at
+            /// CONSTRUCTION, so an ordinary failover took the process down.
+            ///
+            /// ARM A — this runtime has already stopped believing it owns the mount (renewal failed
+            /// and the write fence latched). A foreign occupant is then the EXPECTED end state of
+            /// failover: our successor owns the slot, and the farewell we were about to write would
+            /// stamp OUR identity over THEIRS. Skip it, leave the slot byte-for-byte untouched, and let
+            /// teardown finish quietly. Reached whenever a deposed writer shuts down.
+            ///
+            /// Nothing here is ABORT-CAPABLE, which is the property that matters on a destructor path —
+            /// not "nothing throws". `finishTeardown` wraps this call in a `catch` and logs, so a throw
+            /// is contained; what a `LOGICAL_ERROR` did instead was abort at CONSTRUCTION, before that
+            /// catch could ever run. This arm happens not to throw at all, but it is the exception CLASS
+            /// discipline, not the absence of a `throw`, that keeps teardown alive.
+            if (deposition_observed.load(std::memory_order_acquire))
+            {
+                ProfileEvents::increment(ProfileEvents::CASMountReleaseSkippedForeignOccupant);
+                emitMountEvent(event_sink, CasEventType::MountRelease, srid, "deposed_foreign_occupant", &current,
+                    "mount slot is held by our successor and this incarnation was already deposed — "
+                    "skipping the farewell, slot left untouched");
+                LOG_WARNING(getLogger("CasMountLeaseKeeper"),
+                    "CAS mount-lease: '{}' is held by {} and this incarnation was already deposed — skipping "
+                    "the farewell rather than stamping our identity over the successor's; release is a no-op",
+                    key, describeMountHolder(current));
+                return;
+            }
+
+            /// ARM B — we never observed a deposition, so this runtime still believed it owned the mount
+            /// and a DIFFERENT one is in the slot. That is the single-writer guarantee broken, and it
+            /// stays maximally loud: named identities on both sides, its own counter, the write fence
+            /// latched so the runtime stops trusting itself, and NO write (the occupant is left exactly
+            /// as found — we do not retire someone else's lease).
+            ///
+            /// Loud, but still not abort-capable. Logical errors are exceptions here, not crashes, and
+            /// this verdict rests on a READ of the slot: a stale or adversarial backend can fabricate it
+            /// from the environment, which is precisely the input class that must never be able to kill
+            /// the server. There is deliberately no `chassert` either — it would abort exactly the
+            /// debug/ASan runs of the tests that now have to prove teardown SURVIVES this.
+            ProfileEvents::increment(ProfileEvents::CASMountExclusivityViolation);
+            emitMountEvent(event_sink, CasEventType::MountConflict, srid, "exclusivity_violation", &current,
+                "mount slot is held by a foreign incarnation although this runtime never observed a "
+                "deposition — single-writer exclusivity is broken; refusing the release and fencing");
+            LOG_ERROR(getLogger("CasMountLeaseKeeper"),
+                "CAS mount-lease: release of key '{}' found a FOREIGN incarnation ({}) while this runtime "
+                "(server_uuid={} writer_epoch={} seq={}) still believed it owned the mount — single-writer "
+                "exclusivity is broken. Refusing to retire another incarnation's lease; the slot is left "
+                "untouched and this runtime's write fence is latched.",
+                key, describeMountHolder(current), u128ToHex(server_uuid), writer_epoch, seq);
+            if (on_lost)
+                on_lost();
+            throw Exception(ErrorCodes::ABORTED,
+                "CAS mount-lease: release of key '{}' found a foreign incarnation ({}) while this runtime "
+                "still believed it owned the mount — single-writer exclusivity is broken; the slot is left "
+                "untouched and this runtime is fenced", key, describeMountHolder(current));
+        }
+        /// The lease object is ABSENT: the backing store was deleted under us (rm -rf of the pool
+        /// dir -- the same environmental condition the renewal path classifies as "vanished").
+        /// The desired end state of a release is "no live lease object", which is already true, so
+        /// this is a clean no-op release, never a LOGICAL_ERROR (which aborts debug/ASan builds).
+        ProfileEvents::increment(ProfileEvents::CASMountLeaseLost);
+        emitMountEvent(event_sink, CasEventType::MountRelease, srid, "vanished", nullptr,
+            "mount slot object already gone at release (backing store deleted) — no-op release");
+        LOG_INFO(getLogger("CasMountLeaseKeeper"),
+            "CAS mount-lease: '{}' is already gone at release (backing store deleted); release is a no-op", key);
+        return;
+    }
+    emitMountEvent(event_sink, CasEventType::MountRelease, srid, "farewell", nullptr,
+        "graceful release — lease stamped already-expired, watermark farewell folded in");
+    recordWrite(seq + 1, res.token);
+}
+
+SingleWriterSlot::SingleWriterSlot(
+    BackendPtr backend_, String key_, std::string_view slot_name_, std::string_view terminal_verb_,
+    std::string_view logger_name_)
+    : backend(std::move(backend_))
+    , key(std::move(key_))
+    , slot_name(slot_name_)
+    , terminal_verb(terminal_verb_)
+    , log(getLogger(String(logger_name_)))
+{
+}
+
+SingleWriterSlot::~SingleWriterSlot()
+{
+    /// Stop the renewal thread only — deliberately NO terminal op. Destruction without a terminal op
+    /// leaves the slot object persisted with a frozen seq, which full GC observes as stale state.
+    stopBackground();
+}
+
+void SingleWriterSlot::recordWrite(uint64_t new_seq, const Token & token)
+{
+    seq = new_seq;
+    last_token = token;
+}
+
+void SingleWriterSlot::doStart()
+{
+    /// Compute the per-call payload BEFORE taking state_mutex: a subclass callback (the watermark's
+    /// min_active hook) may reach into the Pool's own lock, so we never hold state_mutex across it.
+    const RenewPayload payload = prepareRenew();
+
+    std::lock_guard lock(state_mutex);
+    if (dead)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS {}: start after {} on key '{}'", slot_name, terminal_verb, key);
+    if (seq != 0)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS {}: already started on key '{}'", slot_name, key);
+
+    const String body = encodeBody(/*seq=*/1, payload);
+    const Token token = claim(body);
+    recordWrite(/*new_seq=*/1, token);
+}
+
+void SingleWriterSlot::renewOnce()
+{
+    /// Compute the per-call payload BEFORE taking state_mutex (see doStart): never hold state_mutex
+    /// across the subclass callback.
+    const RenewPayload payload = prepareRenew();
+
+    /// INVARIANT: holding `state_mutex` across the PUT below is safe ONLY because (a) doTerminate
+    /// joins the renewal thread before taking this mutex and (b) renewOnce has a single driver.
+    /// Do NOT add new `state_mutex`-guarded accessors without revisiting this (they would stall for
+    /// a full network round trip); the prepareRenew pattern above shows the lock-free alternative.
+    std::lock_guard lock(state_mutex);
+    /// Reset BEFORE the guards below: a `dead`/`seq==0` throw (a programming-bug guard, not a backend
+    /// outcome) must not be misread as a CONFIRMED mismatch by `backgroundLoop` -- it falls into the
+    /// TRANSIENT bucket by leaving this false, exactly like a `putOverwrite` exception below.
+    last_renew_failure_was_confirmed_mismatch = false;
+    if (dead)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS {}: renew after {} on key '{}'", slot_name, terminal_verb, key);
+    if (seq == 0)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS {}: renew before start on key '{}'", slot_name, key);
+
+    const String body = encodeBody(seq + 1, payload);
+    const PutResult res = backend->putOverwrite(key, body, last_token);
+    if (res.outcome != PutOutcome::Done)
+    {
+        /// The PUT completed and observed a foreign token -- a CONFIRMED mismatch (proven
+        /// supersession), not a transient failure. Mark it BEFORE calling the hook, which always throws.
+        last_renew_failure_was_confirmed_mismatch = true;
+        onRenewMismatch(key);
+    }
+
+    recordWrite(seq + 1, res.token);
+    /// Reached only on success (the branch above always throws on a mismatch). Notify the subclass
+    /// EVERY successful renewal is committed — background-driven (`backgroundLoop`'s own call) or a
+    /// direct caller (a redo site invoking `renewOnce` outright) alike; the mount-lease keeper
+    /// refreshes its confirmed-lease wall deadline here regardless of who called us.
+    onRenewCommitted();
+}
+
+void SingleWriterSlot::onRenewMismatch(const String & mismatched_key)
+{
+    throw Exception(ErrorCodes::LOGICAL_ERROR,
+        "CAS {}: key '{}' was touched by a foreign writer — failing closed, never re-minting", slot_name, mismatched_key);
+}
+
+void SingleWriterSlot::doTerminate()
+{
+    /// Join the renewal thread before taking the state lock, so no renewal races the terminal op.
+    stopBackground();
+
+    std::lock_guard lock(state_mutex);
+    if (seq == 0)
+        /// Never started (e.g. `Pool::open` failed before/inside `doStart`) — nothing was claimed,
+        /// so there is nothing to release. A never-started slot is inert: BOTH/ALL terminate calls on
+        /// it are quiet no-ops, and — unlike the genuinely-started path below — we do NOT set `dead`,
+        /// so a second no-op call takes this same early-return rather than tripping the "double
+        /// terminate" throw below. Throwing here only turned an already-failing teardown into extra
+        /// `LOGICAL_ERROR` noise during teardown.
+        return;
+    if (dead)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS {}: double {} on key '{}'", slot_name, terminal_verb, key);
+
+    /// Dead regardless of what terminate does below: we attempted the terminal op, the keeper must
+    /// never renew this key again.
+    dead = true;
+    terminate();
+}
+
+void SingleWriterSlot::startBackground(std::chrono::milliseconds period)
+{
+    /// After a thread-side renewal failure the loop returns (see backgroundLoop) but the thread
+    /// handle stays joinable, so a subsequent startBackground throws "already running" until
+    /// stopBackground is called. Intentional fail-closed: we never silently re-arm renewal after it
+    /// has failed.
+    std::lock_guard lock(background_mutex);
+    if (thread.joinable())
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS {}: background renewal is already running for key '{}'", slot_name, key);
+    stop_requested = false;
+    thread = ThreadFromGlobalPool([this, period] { backgroundLoop(period); });
+}
+
+void SingleWriterSlot::stopBackground()
+{
+    ThreadFromGlobalPool to_join;
+    {
+        std::lock_guard lock(background_mutex);
+        if (!thread.joinable())
+            return;
+        stop_requested = true;
+        wakeup.notify_all();
+        to_join = std::move(thread);
+    }
+    to_join.join();
+}
+
+void SingleWriterSlot::backgroundLoop(std::chrono::milliseconds period)
+{
+    setThreadName(ThreadName::CAS_LEASE_KEEPER);
+    /// A CONFIRMED mismatch, or a TRANSIENT failure once `shouldFenceOnTransientRenewFailure` says the
+    /// lease deadline has neared, stops the loop for good: the slot's seq stops
+    /// advancing and GC observes the frozen seq. No retry, no re-mint. A TRANSIENT failure while the
+    /// deadline is still safely away keeps the loop alive -- the mount-lease protocol guarantees no
+    /// other writer can claim the slot before that deadline, so retrying is safe.
+    std::unique_lock lock(background_mutex);
+    while (!stop_requested)
+    {
+        if (wakeup.wait_for(lock, period, [this] { return stop_requested; }))
+            break;
+
+        lock.unlock();
+        try
+        {
+            renewOnce();
+        }
+        catch (...)
+        {
+            /// `renewOnce` and this loop run on the SAME background thread, sequentially -- no
+            /// synchronization needed to read the flag it just set.
+            const bool confirmed = last_renew_failure_was_confirmed_mismatch;
+            if (!confirmed && !shouldFenceOnTransientRenewFailure())
+            {
+                tryLogCurrentException(log, fmt::format(
+                    "CAS {}: background renewal failed transiently, retrying while the lease is still valid",
+                    slot_name));
+                lock.lock();
+                continue;
+            }
+
+            tryLogCurrentException(
+                log, fmt::format("CAS {}: background renewal failed, the {} stops advancing", slot_name, slot_name));
+            /// Notify the subclass that renewal failed and the loop is stopping (off `state_mutex`).
+            /// The mount-lease keeper latches its local write fence to lost here. Never let the hook's
+            /// own throw escape the loop — we are already stopping.
+            try
+            {
+                onRenewFailed();
+            }
+            catch (...) // NOLINT(bugprone-empty-catch)
+            {
+                /// The renewal loop is already stopping; a hook exception must not escape it.
+            }
+            return;
+        }
+        /// Successful renewal: notify the subclass (off `state_mutex`) before sleeping again. The
+        /// wall deadline was already refreshed inside `renewOnce` (`onRenewCommitted`); the
+        /// mount-lease keeper fires the boot-domain write-fence callback here.
+        try
+        {
+            onRenewSucceeded();
+        }
+        catch (...) // NOLINT(bugprone-empty-catch)
+        {
+            /// A notification hook cannot be allowed to stop the already-renewed lease loop.
+        }
+        lock.lock();
+    }
+}
+
+void sweepOwnMountStaging(IObjectStorage & object_storage, const String & mount_staging_prefix) noexcept
+{
+    try
+    {
+        /// max_keys=0 asks `listObjects` for the FULL listing under the prefix (it paginates until
+        /// exhausted rather than capping at some default page size) — see `IObjectStorage::listObjects`.
+        /// A mount's own staging debris is bounded (one mount's in-flight + leaked uploads), so a single
+        /// unbounded LIST at startup is acceptable; unlike GC's per-round budgets, this runs once per
+        /// mount, not on a recurring schedule.
+        RelativePathsWithMetadata children;
+        object_storage.listObjects(mount_staging_prefix, children, /*max_keys=*/0);
+
+        size_t removed = 0;
+        for (const auto & child : children)
+        {
+            try
+            {
+                object_storage.removeObjectIfExists(StoredObject(child->relative_path));
+                ++removed;
+            }
+            catch (...) // NOLINT(bugprone-empty-catch)
+            {
+                /// Best-effort: one stubborn key must not abort the sweep of the rest — it is retried
+                /// by a later mount's sweep.
+            }
+        }
+
+        if (removed)
+            LOG_INFO(getLogger("CasStagingSweeper"),
+                "Reclaimed {} leaked S3 staging object(s) under '{}' at mount start",
+                removed, mount_staging_prefix);
+    }
+    catch (...) // NOLINT(bugprone-empty-catch)
+    {
+        /// Best-effort: a LIST failure (a transient backend hiccup) at mount time must never fail the
+        /// mount — any leaked staging objects are bounded debris, reclaimed by a later mount's sweep.
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h
new file mode 100644
index 000000000000..051f6469cde3
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h
@@ -0,0 +1,774 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h>
+#include <Common/Exception.h>
+#include <Common/Logger.h>
+#include <Common/ThreadPool.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <chrono>
+#include <condition_variable>
+#include <cstdint>
+#include <functional>
+#include <limits>
+#include <map>
+#include <mutex>
+#include <optional>
+#include <string_view>
+#include <utility>
+#include <vector>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int ABORTED;
+}
+}
+
+namespace DB::Cas
+{
+
+/// Durable single-writer-slot state machine used by the per-server merged heartbeat
+/// (`CasServerRoot.h`, `MountLeaseKeeper`): anchors a key that has EXACTLY ONE writer, renews it
+/// asynchronously off the write path, and ends with a terminal op; any precondition failure during
+/// renewal means a foreign touch — fail closed with an exception, never re-mint.
+///
+/// This base owns the common machinery (the `seq`/`last_token`/`dead` state, the renewal thread and
+/// its loop, `renewOnce`/`startBackground`/`stopBackground`, and the start/terminal
+/// bookkeeping). The noun ("watermark") and verb ("farewell") used in every fail-closed message are
+/// passed to the base constructor. Subclasses differ ONLY in EXPLICIT policy hooks:
+///   - `prepareRenew`     — runs OFF the state lock before each body encode, returning a per-call
+///                          payload (the watermark reads `min_active` from a Pool callback here).
+///                          Keeping it off `state_mutex` is load-bearing: the watermark callback
+///                          reaches into the Pool's own lock.
+///   - `encodeBody`       — builds the slot's exact JSON bytes for a given `seq` and payload;
+///   - `claim`            — the slot-specific anchor sequence in `start` (the watermark's
+///                          head→putIfAbsent/putOverwrite dance), returning the token we now hold;
+///   - `terminate`        — the terminal op (the watermark's retiring putOverwrite), run under the
+///                          state lock after `dead` is set, owning its own fail-closed throws and
+///                          final bookkeeping.
+///
+/// Every observable op (the JSON body bytes, the anchor put sequence, the renew cadence, the
+/// foreign-touch fail-close conditions and message wording, the stop semantics) is reproduced
+/// exactly by the subclass hooks, with no conditionals in the base that would blur the
+/// single-writer/fail-closed contract.
+class SingleWriterSlot
+{
+public:
+    /// `slot_name_` is the noun in every message ("watermark"); `terminal_verb_` is the
+    /// verb used in the start/renew/terminal guards ("farewell").
+    SingleWriterSlot(
+        BackendPtr backend_, String key_, std::string_view slot_name_, std::string_view terminal_verb_,
+        std::string_view logger_name_);
+
+    /// Stops the background thread only (no terminal op). Destruction without a terminal op leaves
+    /// the slot persisted, with its seq no longer advancing, so full GC observes frozen state.
+    virtual ~SingleWriterSlot();
+
+    /// seq++ via putOverwrite against the last token we wrote; LOGICAL_ERROR on a foreign touch
+    /// (single-writer fail-closed contract).
+    void renewOnce();
+
+    /// Starts periodic renewal. The period controls only the wake-up cadence; a failed renewal stops
+    /// the loop and is not silently re-armed by a later call.
+    void startBackground(std::chrono::milliseconds period);
+
+    /// Requests renewal-thread termination and joins it. Safe to call repeatedly, including after a
+    /// renewal failure or from destruction; it does not perform the slot's terminal operation.
+    void stopBackground();
+
+protected:
+    /// Per-call payload prepared OFF the state lock and handed to `encodeBody`. Subclasses needing
+    /// dynamic values (the mount lease's `now_ms` + merged `min_active`) carry them through this opaque
+    /// token. `value2` is a second scalar for slots that renew more than one dynamic field per beat (the
+    /// merged heartbeat: `value` = now_ms, `value2` = min_active).
+    struct RenewPayload
+    {
+        uint64_t value = 0;
+        uint64_t value2 = 0;
+    };
+
+    using Token = ::DB::Cas::Token;
+
+    /// === policy hooks (see class comment) ===
+    virtual RenewPayload prepareRenew() const = 0;
+    virtual String encodeBody(uint64_t seq, const RenewPayload & payload) const = 0;
+    virtual Token claim(const String & body) = 0;
+
+    /// === optional background-renewal observation hooks (default no-op) ===
+    /// Called from `renewOnce` itself, right after a SUCCESSFUL write is recorded (under
+    /// `state_mutex`, on EVERY caller of `renewOnce` — the background loop AND any direct caller,
+    /// such as the startup-arm redo in `CasPool.cpp`'s `mountWritable`). The mount-lease keeper
+    /// refreshes its confirmed-lease wall deadline here, so a direct `renewOnce` (which never goes
+    /// through `onRenewSucceeded` below) still keeps that deadline current. The watermark keeper does
+    /// not override it (no-op), so its behavior is unchanged.
+    virtual void onRenewCommitted() {}
+    /// Called from the background loop after a SUCCESSFUL `renewOnce` (off `state_mutex`, AFTER
+    /// `onRenewCommitted` has already run); the mount-lease keeper fires the boot-domain
+    /// write-fence callback (`on_renew_ok`) here — this hook is background-loop-only, unlike
+    /// `onRenewCommitted` above. The watermark keeper does not override it (no-op), so its behavior
+    /// is unchanged.
+    virtual void onRenewSucceeded() {}
+    /// Called from the background loop when `renewOnce` THREW (the loop is about to stop, off
+    /// `state_mutex`); the mount-lease keeper latches the write fence to lost here. Default no-op.
+    virtual void onRenewFailed() {}
+
+    /// Called from the background loop when `renewOnce` threw and the
+    /// throw was NOT a confirmed mismatch (`onRenewMismatch` -- an observed PUT outcome proving
+    /// supersession; see the flag this checks, `last_renew_failure_was_confirmed_mismatch`, in the
+    /// private section below). Returning `true` means "treat as terminal now" -- fence immediately, the
+    /// legacy behavior and the correct default for a subclass with no lease-deadline concept.
+    /// `MountLeaseKeeper` overrides this to ride out a TRANSIENT exception (a `putOverwrite` that threw
+    /// before any outcome was observed -- a timeout, 5xx, or connection reset) while its last CONFIRMED
+    /// lease has not yet reached its safety-margin boundary: the mount-lease protocol guarantees no
+    /// other writer can claim the slot before that deadline, so continuing to retry (not fencing) is
+    /// safe. Called OFF `state_mutex`, same as `onRenewFailed`.
+    virtual bool shouldFenceOnTransientRenewFailure() { return true; }
+
+    /// Called when the token-guarded renew PUT hits PreconditionFailed. The base contract stays
+    /// fail-closed and LOUD; subclasses may re-read and throw a more precisely classified
+    /// exception (the mount keeper distinguishes a GC fence of our own expired lease from a
+    /// genuine foreign writer). MUST throw — a renew mismatch never continues.
+    virtual void onRenewMismatch(const String & mismatched_key);
+
+    /// Runs the slot's terminal op against the held `last_token`. Called under `state_mutex` with
+    /// `dead` already set (so renewal can never race it). Owns its own fail-closed throws and final
+    /// bookkeeping (the watermark bumps seq/last_token on its retiring putOverwrite).
+    /// May throw — `dead` stays set regardless.
+    virtual void terminate() = 0;
+
+    /// Anchors the slot for seq=1 — durable when `doStart` returns. Subclasses expose this under their
+    /// own public name (`start`). Computes the payload off the lock, then runs the policy `claim`.
+    void doStart();
+
+    /// Stops the background thread, takes the state lock, runs the dead/seq guards (e.g.
+    /// "double farewell" / "discard before start"), sets `dead`, and delegates the op to `terminate`.
+    /// Subclasses expose this under their own public name (`farewell`/`discard`).
+    void doTerminate();
+
+    /// Bookkeeping after a successful write of the given seq/token: records seq, the token we now
+    /// hold, and the local-clock renew time. Must be called under `state_mutex`.
+    void recordWrite(uint64_t new_seq, const Token & token);
+
+    BackendPtr backend;
+    String key;
+
+    mutable std::mutex state_mutex;
+    uint64_t seq = 0;            /// 0 = not started
+    Token last_token;            /// the incarnation WE wrote — the only one we ever renew
+    bool dead = false;           /// set by the terminal op
+
+private:
+    void backgroundLoop(std::chrono::milliseconds period);
+
+    std::string_view slot_name;
+    std::string_view terminal_verb;
+
+    /// Set immediately before `renewOnce` invokes `onRenewMismatch` (which always
+    /// throws) so `backgroundLoop`'s catch block can tell a CONFIRMED mismatch (the PUT completed and
+    /// observed a foreign token -- proven supersession) apart from a TRANSIENT exception
+    /// (`putOverwrite` itself threw before any outcome was observed, or a defensive `dead`/`seq==0`
+    /// guard fired). Reset to `false` at the top of every `renewOnce` call. `renewOnce` and
+    /// `backgroundLoop` run on the SAME background thread, sequentially, so no synchronization is
+    /// needed for this flag.
+    bool last_renew_failure_was_confirmed_mismatch = false;
+
+    std::mutex background_mutex;
+    std::condition_variable wakeup;
+    bool stop_requested = false;
+    ThreadFromGlobalPool thread;
+
+    LoggerPtr log;
+};
+
+/// Validate a `server_root_id` — the explicit, configured identity of the content-addressed layout
+/// subtree a server owns. It is a clean relative path: it composes into the
+/// object-key tree (`gc/server-roots/<srid>/...`, `roots/<srid>/...`), so the same hygiene the layout
+/// applies to a namespace applies here (mirrors `CasLayout.h::checkNamespace`):
+///   - non-empty;
+///   - no leading/trailing '/', no empty segment ("//");
+///   - no '.' or '..' segment;
+///   - total length <= 255;
+///   - no segment equal to the reserved "_files" / "_manifests".
+/// Throws `ErrorCodes::BAD_ARGUMENTS` on any violation. Fail closed — there is no sanitizing fallback.
+inline void validateServerRootId(const String & id)
+{
+    if (id.empty())
+        throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS, "server_root_id must be non-empty");
+
+    if (id.size() > 255)
+        throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+            "server_root_id '{}' is too long ({} > 255 bytes)", id, id.size());
+
+    size_t start = 0;
+    while (true)
+    {
+        size_t end = id.find('/', start);
+        const String segment = id.substr(start, end == String::npos ? String::npos : end - start);
+        if (segment.empty())
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "server_root_id '{}' has an empty segment (leading/trailing or doubled '/')", id);
+        if (segment == "." || segment == "..")
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "server_root_id '{}' uses a relative segment ('.' or '..')", id);
+        if (segment == "_files")
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "server_root_id '{}' uses the reserved segment '_files'", id);
+        if (segment == "_manifests")
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "server_root_id '{}' uses the reserved segment '_manifests'", id);
+        if (end == String::npos)
+            break;
+        start = end + 1;
+    }
+}
+
+/// The per-server-root control objects (owner, epoch, and mount lease) and their text codecs are kept
+/// in `Formats/CasServerRootFormats`. This header includes those definitions so the protocol logic
+/// below can use `OwnerObject`, `ServerEpoch`, `MountLease`, and their `encode`/`decode` functions
+/// without duplicating the wire-format implementation.
+
+class Backend;
+class Layout;
+
+/// Mount-safety claim logic. These are the identity and epoch-allocation steps a server
+/// runs at startup over its `server_root_id` subtree, BEFORE any ordinary data write. They fail
+/// closed (`ErrorCodes::CORRUPTED_DATA`); there is no re-mint or silent-recreate fallback.
+
+/// True iff the successfully decoded catalog names no current life owned by `server_root_id`, and
+/// exact-component probes find neither `cas/manifests/<srid>/` nor `roots/<srid>/` work. Opaque
+/// `cas/ns/` debris alone does not identify a logical owner.
+bool serverRootSubtreeEmpty(
+    Backend & b, const Layout & l, const String & srid, const RefCatalog & catalog_observation);
+
+/// Supplied by the pool layer so the low-level server-root protocol always observes the mandatory
+/// catalog. Every absent-control retry obtains a fresh, successfully decoded observation.
+using ObserveRefCatalog = std::function<RefCatalog()>;
+
+/// Read the owner anchor (`gc/server-roots/<srid>/owner`) WITHOUT claiming or validating identity —
+/// a plain GET+decode. nullopt = anchor absent. Pool-member decommission uses this to read the
+/// victim UUID before mounting writable; `claimOwnerOrThrow` below is the identity-claiming
+/// counterpart used by normal opens and reuses this GET+decode path.
+std::optional<UInt128> readOwnerUuid(Backend & b, const Layout & l, const String & server_root_id);
+
+/// Claim (or validate) the sticky owner anchor that binds `srid` to a server UUID (identity).
+///   - owner present, equal `our_uuid`, and not tombstoned → ok (return);
+///   - owner present and tombstoned → throw `CORRUPTED_DATA` (explicitly retired — fail closed);
+///   - owner present, different → throw `CORRUPTED_DATA` (foreign owner — fail closed);
+///   - owner absent AND the subtree is provably empty → `putIfAbsent` the owner (claim);
+///   - owner absent BUT the subtree is non-empty → throw `CORRUPTED_DATA` (identity lost over
+///     existing data — never silently re-claim).
+/// The owner object is never deleted and never reassigned to a different UUID.
+void claimOwnerOrThrow(
+    Backend & b, const Layout & l, const String & srid, UInt128 our_uuid,
+    const ObserveRefCatalog & observe_catalog);
+
+/// Which mint policy governs `allocateWriterEpoch`'s absent-epoch branch (see below).
+enum class EpochMintPolicy : uint8_t
+{
+    NormalMount,            /// absent-epoch re-mint requires authoritative mount absence
+    DecommissionRecovery,   /// absent-epoch re-mint requires a TERMINAL mount; mints a distinct epoch
+};
+
+/// Allocate the next durable-monotone `writer_epoch` by CAS-bumping the sticky `epoch` object
+/// (`ServerEpoch{next_writer_epoch}`), returning the value the caller adopts as its writer_epoch.
+///   - `epoch` absent AND the subtree is non-empty → throw `CORRUPTED_DATA` (missing epoch over
+///     data is a reset hazard);
+///   - `epoch` absent AND the subtree is empty → the absent-epoch branch is a LIFECYCLE decision,
+///     so it uses `probeSentinelRaw`'s authoritative outcomes, never plain `get`-absence (which
+///     flattens transport faults into "not found"), to decide whether the mount object is really
+///     gone:
+///       - `KeyAbsent` → authoritative absence, mint epoch 1 (fresh-root bootstrap);
+///       - `Present`, `policy == NormalMount` → throw `CORRUPTED_DATA` (durable epoch state was
+///         lost while a mount is live or recently live — refusing to re-mint epoch 1 there is how a
+///         same-(uuid, epoch) twin is avoided);
+///       - `Present`, `policy == DecommissionRecovery` → the surviving mount must be TERMINAL (not
+///         live); a live member throws `ABORTED`, otherwise mint `surviving.writer_epoch + 1` —
+///         distinct from the survivor's by construction (`now_ms` is required, nonzero, here);
+///       - anything else (`ContainerAbsent`/`AccessDenied`/`Indeterminate`) → throw
+///         `CORRUPTED_DATA` (absence was never proven; fail closed);
+///   - otherwise read `next = current.next_writer_epoch`, `casPut` `{next + 1}` against the
+///     observed token, retry on `Conflict` (bounded), and return `next`.
+uint64_t allocateWriterEpoch(Backend & b, const Layout & l, const String & srid,
+                             EpochMintPolicy policy, uint64_t now_ms,
+                             const ObserveRefCatalog & observe_catalog);
+
+/// Which certificate of death justified a same-uuid, different-epoch mount reclaim. `None` when no
+/// reclaim of that kind happened (a fresh claim, a
+/// same-epoch refresh, `LiveDoubleStart`, `ForeignOwner`, `FencedSelf`).
+enum class MountPriorState
+{
+    None,
+    Clean,             /// the predecessor's own graceful farewell (`min_active == UINT64_MAX`)
+    Fenced,            /// the GC leader's own (already threshold-gated) fence-out (`gc_fenced`)
+    UncleanObserved,   /// OUR observation watched the write-token hold stable for the full threshold
+};
+
+/// Startup decision for the mount lease (`gc/server-roots/<srid>/mount`), run AFTER the owner gate
+/// (so `our_uuid` is the established owner). The lease is LIVENESS, not identity — the owner object
+/// already settled who may write; the lease settles whether a live incarnation currently holds the
+/// slot. Decision over `get(mountKey)`:
+///   - absent → write our body via `putIfAbsent` → `Claimed`;
+///   - same `server_uuid` AND same `writer_epoch` as (our_uuid, our_epoch) → it is OUR OWN claim
+///     (a replay / the keeper adopting it):
+///       - `gc_fenced` → terminal for THIS (uuid, epoch) — a fence costs an epoch, so refreshing it
+///         in place would resurrect a fenced incarnation → `FencedSelf` (no write);
+///       - otherwise → refresh (`putOverwrite` to bump seq + fresh `expires_at_ms`) → `Claimed`;
+///   - same `server_uuid`, DIFFERENT `writer_epoch` → reclaimed ONLY on a certificate of death that
+///     needs no fresh wall-clock trust (see
+///     `claimMountAwaitingExpiry` below for how a plain "looks expired" reading is turned into one):
+///       - `gc_fenced` (the GC leader already, itself, threshold-gated this incarnation dead; a fence
+///         costs an epoch, so its keeper can never renew again) → reclaim, `prior = Fenced`;
+///       - the clean marker (`min_active == UINT64_MAX`, the predecessor's own graceful farewell) →
+///         reclaim, `prior = Clean`;
+///       - `proven_dead_token` matches the CURRENTLY OBSERVED token (the caller itself watched this
+///         exact token hold stable for the full observation threshold) → reclaim, `prior =
+///         UncleanObserved`;
+///       - none of the above → `LiveDoubleStart` (do NOT write). In particular `expires_at_ms <=
+///         now_ms` ALONE is never sufficient — comparing a predecessor's stamp against OUR wall clock
+///         is unsafe because a clock-skewed or merely late-observing
+///         caller must never conclude death from a bare timestamp read);
+///   - different `server_uuid` → `ForeignOwner` (do NOT write, regardless of expiry or prior state).
+struct MountClaimResult
+{
+    /// Plain (unscoped) enum: callers compare with `MountClaimResult::Claimed` directly.
+    enum Kind
+    {
+        Claimed,
+        LiveDoubleStart,
+        ForeignOwner,
+        /// Same (uuid, epoch) as ours, but the body is `gc_fenced`: terminal for THIS epoch — a fence
+        /// costs an epoch. The caller must re-open with a fresh `writer_epoch`; refreshing or adopting
+        /// a fenced body in place is never correct.
+        FencedSelf,
+    };
+    Kind kind = ForeignOwner;
+    MountLease body;
+    /// Which certificate of death justified a same-uuid, different-epoch `Claimed` reclaim (`None` for
+    /// every other `Kind`, and for the absent-slot / same-epoch-refresh `Claimed` cases).
+    MountPriorState prior = MountPriorState::None;
+    /// The backend token of the body this result observed, for
+    /// `LiveDoubleStart` only (a fresh `Claimed`/`FencedSelf`/`ForeignOwner` write/observe has no
+    /// separate "prior body's token to remember" use). `claimMountAwaitingExpiry`'s observation loop
+    /// used to re-GET the mount key itself just to recover this token that `claimMount` had already
+    /// read one line earlier and thrown away -- one wasted GET per iteration. Empty for every other
+    /// `Kind` (nothing to compare against).
+    std::optional<Token> token;
+};
+
+/// Thrown when a mount operation observes that OUR OWN (uuid, epoch) slot was `gc_fenced` by the GC
+/// after our lease expired — a RECOVERABLE state ("a fence costs an epoch"): the caller re-opens with
+/// a fresh `writer_epoch`. A CAS-local typed exception rather than a new `ErrorCodes` number: a fork
+/// carries these edits indefinitely and the numbered `ErrorCodes` list conflicts with upstream on
+/// every rebase. Catch sites match BY TYPE (`catch (const MountFencedException &)`), never by code;
+/// the base code is `ABORTED` so an uncaught one still surfaces as a clean startup abort.
+class MountFencedException : public DB::Exception
+{
+public:
+    explicit MountFencedException(const String & msg)
+        : DB::Exception(msg, DB::ErrorCodes::ABORTED) {}
+};
+
+/// `proven_dead_token`: the write-token of a same-uuid, different-epoch lease that the CALLER already
+/// proved dead by observation (see `claimMountAwaitingExpiry`) — matching it against the CURRENTLY
+/// observed token is the ONLY way (besides `gc_fenced` / the clean marker) a same-uuid different-epoch
+/// lease is ever reclaimed. Absent (`{}`, the default) for a bare claim attempt with no such proof.
+MountClaimResult claimMount(
+    Backend & b, const Layout & l, const String & srid, UInt128 our_uuid, uint64_t our_epoch,
+    uint64_t now_ms, uint64_t ttl_ms, const std::optional<Token> & proven_dead_token = {},
+    const CasEventSink & sink = {});
+
+/// Format the operator-actionable startup error shown when the mount lease is held by a genuinely
+/// live second server (the same `server_root_id` is mounted twice). Produced only AFTER this server
+/// has already waited for the lease to lapse (see `claimMountAwaitingExpiry`) and it did not — so the
+/// remediation is about a live twin, not about waiting.
+String mountDoubleStartMessage(const String & srid, const MountLease & existing);
+
+/// Observation-based mount claim for restart recovery.
+/// Wraps `claimMount` in a loop:
+///   - first attempt decided immediately for `Claimed` (fresh / refreshed / reclaimed via `Fenced` or
+///     `Clean`), `ForeignOwner`, or `FencedSelf`;
+///   - a `LiveDoubleStart` from OUR OWN uuid (a stale lease from a prior incarnation of this server,
+///     OR a genuinely live twin — the two are indistinguishable from a bare read) is resolved by
+///     WATCHING the lease's write-token on OUR OWN clock (`mono_ms_fn`), NEVER by comparing the
+///     lease's stamped `expires_at_ms` against any clock: once the observed token has held stable for
+///     the full rate-bound threshold (`ttl_ms + ttl_ms / 20 + poll_interval_ms` — the lease TTL, a 5%
+///     clock-drift allowance, and one poll interval of discreteness. This rate bound ensures that a
+///     holder which last renewed before the observation began can no longer be within its lease.
+///     that token is handed to `claimMount` as `proven_dead_token`, which then reclaims token-guarded
+///     (`prior = UncleanObserved`). If the token changes DURING the wait (the holder renewed, or a
+///     genuine twin is alive) the observation RESTARTS from the new token; bounded to a handful of
+///     restarts before giving up and returning the last `LiveDoubleStart` (a holder whose token keeps
+///     changing across that many restarts is alive, not dead).
+/// `now_ms_fn` is WALL clock, used only for stamping the body we (may) write / diagnostics — it never
+/// participates in the reclaim decision. `mono_ms_fn` is the OBSERVATION clock: monotonic on this
+/// process, never compared against any other node's clock, and the ONLY clock the threshold is
+/// measured against. `sleep_ms_fn` paces the poll. All three, plus `on_wait_start`, are injected so
+/// tests drive fake clocks with no real sleeping. `on_wait_start` (default no-op) fires once per
+/// observation-window start (including restarts), with the currently-observed lease and the
+/// threshold, for an operator-visible startup log.
+/// All callers use this shared formula so a future adjustment cannot silently leave the startup
+/// observation path and either GC heartbeat path with different thresholds. `cadence_ms` is the
+/// caller's own poll or heartbeat interval; the additional interval accounts for observation
+/// discreteness, while `ttl_ms / 20` allows for a five-percent clock-rate difference.
+uint64_t mountObservationThresholdMs(uint64_t ttl_ms, uint64_t cadence_ms);
+
+MountClaimResult claimMountAwaitingExpiry(
+    Backend & b, const Layout & l, const String & srid, UInt128 our_uuid, uint64_t our_epoch,
+    const std::function<uint64_t()> & now_ms_fn,
+    const std::function<uint64_t()> & mono_ms_fn,
+    uint64_t ttl_ms, uint64_t poll_interval_ms,
+    const std::function<void(uint64_t)> & sleep_ms_fn,
+    const std::function<void(const MountLease &, uint64_t)> & on_wait_start = {},
+    const CasEventSink & sink = {});
+
+/// One `server_root_id`'s cross-round token-stability observation,
+/// owned by the GC leader instance (`Cas::Gc::mount_obs`) and threaded through consecutive
+/// `computeHeartbeatFloor` calls — one GC round is one observation tick. Mirrors
+/// `claimMountAwaitingExpiry`'s observation loop, but at heartbeat-gate granularity rather than a
+/// tight poll loop.
+struct MountTokenObservation
+{
+    Token token;
+    uint64_t first_seen_mono_ms = 0;
+};
+
+/// Keyed by `server_root_id`. In-memory only: a fresh leader (after a steal, or a process restart)
+/// starts with an empty map, which only delays fencing an already-dead mount by one extra round while
+/// it (re)establishes the observation — safe (never fences early), never unsafe.
+using MountObservationMap = std::map<String, MountTokenObservation>;
+
+/// GC heartbeat gate (GC round protocol step 1). Run by the GC leader at the top of a round: LIST
+/// `gc/server-roots/` (O(servers), single-digit counts), GET each mount body, and classify + fence out
+/// dead mounts (liveness only — graduation itself paces on GC rounds, not on heartbeat acks).
+/// Classification per body:
+///   - `gc_fenced` already set → excluded (`already_fenced`); a fenced mount is terminal, no PUT;
+///   - terminated (`min_active == UINT64_MAX`, the farewell sentinel stamped by
+///     `MountLeaseKeeper::terminate`) → excluded (`terminated`). `expires_at_ms` alone cannot
+///     distinguish a graceful farewell from an unclean stop, so the sentinel — not the timestamps — is the
+///     terminated marker;
+///   - otherwise, observation-based liveness (the same
+///     principle `claimMountAwaitingExpiry` uses for a mount's OWN reopen, applied here to the GC's
+///     fence-out): `obs` remembers, per srid, the write-token last seen and the leader's OWN
+///     monotonic clock reading (`mono_now_ms`) at the moment it first saw that token. A body whose
+///     CURRENT token differs from (or is absent from) `obs` is (re)started fresh — counted `live`,
+///     never fenced this call, regardless of what its stamped `expires_at_ms` claims (a bare
+///     wall-clock stamp is never trusted — see `claimMount`'s "certificate of death" doc). Only once
+///     the SAME token has held for `>= stable_threshold_ms` OF THE LEADER'S OWN CLOCK does the body
+///     become FENCE-eligible;
+///   - FENCE-eligible → one token-guarded `putOverwrite` preserving the WHOLE body, setting
+///     `gc_fenced = true` and `seq + 1`. On `Done` → excluded (`fenced_now`); on `PreconditionFailed`
+///     (the holder renewed concurrently — a live token change) → re-GET and reclassify from the top
+///     (bounded retries; the reclassify sees the new token and restarts the observation, counting it
+///     `live` — conservative, never exclude a heartbeat without a landed fence-out).
+///
+/// `now_ms` is WALL clock, used only for the audit/diagnostic log line — it never participates in the
+/// fence decision (mirrors `claimMountAwaitingExpiry`'s `now_ms_fn` vs `mono_ms_fn` split).
+/// `mono_now_ms` is the OBSERVATION clock: the caller's OWN monotonic reading, never compared against
+/// any other node's clock. `obs` is owned by the caller and threaded across consecutive calls (one GC
+/// leader instance, `Cas::Gc::mount_obs`) — a fresh leader starts with an empty map (safe: delays
+/// fencing one round, never fences early).
+///
+/// The fence-out is BOTH safety and liveness. Safety: a sleeper's later renewal permanently fails
+/// (its `putOverwrite` now mismatches the fenced token → `tripMountLost`), so it can never re-arm
+/// without a fresh `open`. Liveness: a dead server's stale mount slot must not linger forever.
+/// Preserving the body keeps restart recovery intact: a same-uuid reopen reads the current body and
+/// reclaims through the normal expired-our-uuid branch.
+struct HeartbeatFloor
+{
+    size_t live = 0;
+    size_t terminated = 0;
+    size_t fenced_now = 0;
+    size_t already_fenced = 0;
+    /// The srids of every mount fenced-out THIS call (one GcFenceOut audit event each).
+    std::vector<String> fenced_srids;
+};
+
+HeartbeatFloor computeHeartbeatFloor(Backend & b, const Layout & l, uint64_t now_ms,
+                                     uint64_t mono_now_ms, uint64_t stable_threshold_ms,
+                                     MountObservationMap & obs);
+
+/// One `gc/server-roots/<srid>/mount` slot whose holder is not provably finished with the prefix.
+struct NonTerminalMountSlot
+{
+    String server_root_id;
+    /// What was read: the lease's identifying fields, or why the body could not be interpreted.
+    String detail;
+};
+
+/// Read-only scan of every mount slot under the pool prefix, answering ONE question: is some writer
+/// still entitled to this prefix? A slot counts as terminal on exactly the two clock-free certificates
+/// the mount protocol already recognises (`computeHeartbeatFloor`'s own classification): `gc_fenced`
+/// (the GC leader fenced that incarnation out, and a fence costs an epoch, so its keeper can never
+/// renew again) and `min_active == UINT64_MAX` (the holder's own graceful farewell). Everything else is
+/// reported, INCLUDING a body this build cannot decode -- an unreadable lease of some other format
+/// generation is precisely the case that must block, not the one to wave through.
+///
+/// Deliberately no wall-clock judgement: `expires_at_ms` alone never proves death (comparing another
+/// node's stamp against our clock is exactly what `claimMount` refuses to do), and this is not the
+/// place to run an observation window either -- the answer to "someone may still be writing here" is
+/// for the operator to stop that writer, not for us to wait it out.
+///
+/// The caller is pool RECREATION (`Pool::open`'s bootstrap over a prefix with no authoritative
+/// `_pool_meta`): minting a fresh pool identity while a live writer still holds a slot would leave that
+/// writer appending its old-format transactions into the new pool. Writes nothing.
+std::vector<NonTerminalMountSlot> probeNonTerminalMountSlots(Backend & b, const Layout & l);
+
+/// A read-only snapshot of one server's mount slot, for introspection (`system.cas_mounts`).
+/// state: `live` (lease within TTL+skew), `expired` (lease ran out; the next GC round's heartbeat floor
+/// will fence it), `terminated` (clean farewell: `min_active == UINT64_MAX`), `fenced` (`gc_fenced`),
+/// `corrupt` (body failed to decode — surfaced as a row, never an exception).
+struct MountInfo
+{
+    String srid;
+    MountLease lease;
+    String state;
+};
+
+/// Enumerate every mount slot under `gc/server-roots/`, decoded and classified — the read-only sibling
+/// of `computeHeartbeatFloor`: ZERO writes (no fence-out), per-row fail-open. One LIST + one GET per slot.
+std::vector<MountInfo> listMounts(Backend & backend, const Layout & layout, uint64_t now_ms, uint64_t skew_margin_ms);
+
+/// Whether the mounted writer identified by `(server_root_id, writer_epoch)` (the two fields of a
+/// `CatalogEntry::creator` / `CreatorFence`, `ref_catalog`'s spec INV-3 §3, that this predicate actually
+/// needs) is PROVABLY unable to complete anything more — the gate
+/// `CasRefCatalog::reconcileStaleCreator` requires before a stalled `Creating` entry may be stolen by a
+/// NEW actor.
+///
+/// Takes the two scalars rather than a whole `CreatorFence` (review C4): that type lives on the
+/// ref-catalog side (`Formats/CasRefCatalogFormat.h`), and this file is already widely included
+/// through `CasPool.h` (mount/server-root plumbing reaches nearly every CAS translation unit), so
+/// naming that type here would make the mount layer depend on the ref-catalog format instead of the
+/// other way round, for a struct this function reads only two fields of. A caller holding a
+/// `CreatorFence` passes `fence.server_root_id, fence.writer_epoch` directly.
+///
+/// `fence_generation` is deliberately NOT one of the two scalars this function takes, and not because
+/// it is unavailable -- the catalog persists it (`cfg` in `CasRefCatalogFormat.cpp`) -- but because it
+/// is not the property this predicate needs. It mirrors `CasMountRuntime::fence_generation`, an
+/// in-process atomic that every mount bumps from its OWN zero on every open, so a DIFFERENT actor's
+/// counter (or the SAME actor's after a restart) starts over at the same small values and cannot
+/// answer "is the incarnation that minted this entry still alive" -- comparing it across actors
+/// compares two unrelated counts that happen to share a name. This reads the durable, cross-process
+/// proof instead: `server_root_id`'s CURRENT mount slot (`Layout::mountKey`), classified by the SAME
+/// two clock-free certificates `probeNonTerminalMountSlots`/`computeHeartbeatFloor` already use for the
+/// identical question at pool-prefix and GC-heartbeat granularity —
+///   - `gc_fenced` (the GC leader already fenced this incarnation; a fence costs an epoch, so its
+///     keeper can never renew again),
+///   - the clean-farewell sentinel `min_active == UINT64_MAX`,
+/// PLUS one more certificate available here that neither of those needs: a DIFFERENT `writer_epoch`
+/// currently live at that slot proves `writer_epoch`'s specific incarnation is superseded regardless of
+/// its OWN certificate — `allocateWriterEpoch`/`claimMount` are why an epoch, once superseded, is never
+/// reclaimed by its former holder. The classification is an EXHAUSTIVE switch over these three
+/// certificates, not a positive allowlist -- mirrors `CasPool.cpp`'s own exhaustive switch over
+/// `MountPriorState` (the classification `claimMount`, in THIS file, only PRODUCES; the switch
+/// consuming it lives in the caller) deliberately: a future certificate with no verdict assigned here
+/// must fail the BUILD (a missing `-Wswitch` case), never silently read as terminal.
+///
+/// Deliberately conservative on the two cases that are NOT proof of death: an ABSENT mount slot
+/// (`Backend::get` returning `nullopt` answers nothing about liveness — it is not proof either way)
+/// and an UNDECODABLE body (an unreadable lease of some other format generation is precisely the case
+/// that must block, not the one to wave through, mirroring `probeNonTerminalMountSlots`'s own stated
+/// discipline for that case) both return `false` — refuse reconciliation rather than guess.
+/// A merely `expired` lease (a wall-clock reading past `expires_at_ms`) is likewise NEVER
+/// treated as a certificate, for the same reason `claimMount` itself refuses to trust one: comparing
+/// another node's stamp against a clock is exactly the unsafe comparison the mount protocol exists to
+/// avoid, and there is not even a caller-supplied clock offered here to make that comparison with.
+///
+/// WHAT THIS DOES NOT PROVE, stated rather than left implicit: `true` means the CURRENT mount-slot body
+/// carries a certificate against `writer_epoch` specifically -- it is not a claim about the server
+/// root's OTHER activity, about whether `server_root_id` will ever mount again, or about
+/// anything beyond this one slot's current body at the instant of this GET. A caller that needs a
+/// stronger, race-free guarantee (e.g. "and it will never come back") must build that from a WIDER
+/// observation, the way `claimMountAwaitingExpiry`'s token-stability window does for its own decision --
+/// this function performs no such window and answers from one point-in-time read alone. Answering
+/// "unknown" (`false`, refuse) is the fail-closed choice on every path already listed above; there is
+/// no path where this function answers `true` on evidence weaker than one of the three certificates.
+bool isCreatorFenceTerminal(Backend & backend, const Layout & layout, const String & server_root_id,
+                            uint64_t writer_epoch);
+
+/// Per-server MERGED heartbeat: one `SingleWriterSlot` over the per-server-root mount object carries
+/// the mount lease (liveness) AND the build-watermark floor (`min_active`). One renewal PUT stamps the
+/// clock and the build-watermark floor together. Anchors the slot synchronously on `start`, renews it
+/// async off the write path, and fails closed on any foreign touch (`renewOnce` throws on a
+/// precondition miss). `graceful stop` folds the watermark farewell (`min_active = UINT64_MAX`) into
+/// the terminal already-expired mount body.
+///
+/// ADOPT RULE (critical): the steady-state flow is `claimMount(...)` writes the live mount under
+/// (our_uuid, our_epoch), THEN `keeper.start()`. So `start`'s `claim` hook must ADOPT a live mount
+/// that is ALREADY ours — same `server_uuid` AND same `writer_epoch` — instead of self-tripping the
+/// live-double-start guard. The discriminator is the (uuid, epoch) pair:
+///   - same uuid + same epoch  → our own just-written claim (or a replay) → adopt: `putOverwrite`
+///     against the observed token to refresh seq/expiry (no fail);
+///   - same uuid + DIFFERENT live epoch → a newer incarnation superseded us → fail closed;
+///   - foreign uuid → fail closed;
+///   - absent → `putIfAbsent`; expired-our-uuid (any epoch) → `putOverwrite` reclaim.
+/// After `start`, `renewOnce` (base) keeps the slot alive and already fails closed on a foreign touch.
+class MountLeaseKeeper : public SingleWriterSlot
+{
+public:
+    /// `min_active_fn_` is read OFF the state lock on each beat (via `prepareRenew`) and stamped into
+    /// the mount body — the merged watermark floor. It reaches into the Pool's own lock, so it must
+    /// never run under `state_mutex`.
+    MountLeaseKeeper(
+        BackendPtr backend_, const Layout & layout_, const String & srid_, UInt128 server_uuid_,
+        uint64_t writer_epoch_, std::chrono::milliseconds ttl_, std::function<uint64_t()> now_ms_fn_,
+        std::function<uint64_t()> min_active_fn_,
+        CasEventSink event_sink_ = {},
+        std::chrono::milliseconds lease_safety_margin_ = std::chrono::milliseconds(2000),
+        /// boot-domain clock for the on_renew_ok anchor; empty = real CLOCK_BOOTTIME. Injectable for
+        /// tests and wired by CasMountRuntime::installKeeper.
+        std::function<uint64_t()> boot_ms_fn_ = {});
+
+    /// Claims (adopts) the mount slot for (server_uuid, writer_epoch) with seq following the observed
+    /// one — durable when `start` returns.
+    void start() { doStart(); }
+
+    /// Releases the mount: the terminal op. Stops the background thread first.
+    void stop() { doTerminate(); }
+
+    /// Join the renewal thread BEFORE this object's own `std::function` members (`on_renew_ok` /
+    /// `on_lost`, which reach back into the Pool) are destroyed. The base `~SingleWriterSlot` also
+    /// calls `stopBackground`, but it runs AFTER the derived members are gone — a renewal firing
+    /// `on_lost` in that window would call a destroyed `std::function`. Stopping here closes that window.
+    ~MountLeaseKeeper() override { stopBackground(); }
+
+    /// Local write-fence coupling (set once by `Pool::open` before `startBackground`): the keeper is
+    /// the ONLY thing that touches S3 for the lease, so the fence must reflect the live lease without a
+    /// per-write S3 read. On each SUCCESSFUL background renew the keeper calls `on_renew_ok` (the Pool
+    /// refreshes its monotonic fence deadline); when background renewal FAILS (a foreign/superseded
+    /// touch makes `renewOnce` throw and the loop stop) the keeper calls `on_lost` (the Pool latches
+    /// its fence to lost). Both default to no-op so a keeper used without a Pool is inert.
+    /// `on_renew_ok` receives the ATTEMPT-START boot-domain instant of the renewal it acknowledges —
+    /// the fence deadline must anchor there, never at response time (spec rev.4 Phase B).
+    void setFenceCallbacks(std::function<void(uint64_t)> on_renew_ok_, std::function<void()> on_lost_)
+    {
+        on_renew_ok = std::move(on_renew_ok_);
+        on_lost = std::move(on_lost_);
+    }
+
+protected:
+    /// Reads the current wall-clock stamp and watermark floor without holding the base state lock;
+    /// the callbacks can acquire the Pool lock, so reversing this order would deadlock renewal.
+    RenewPayload prepareRenew() const override;
+
+    /// Encodes one complete mount body, combining the sequence assigned by the base slot with the
+    /// dynamic values returned by `prepareRenew`.
+    String encodeBody(uint64_t seq_, const RenewPayload & payload) const override;
+
+    /// Adopts or claims the mount object for this `(server_uuid, writer_epoch)`, returning the token
+    /// from the durable write that the base slot must retain for its next token-guarded renewal.
+    Token claim(const String & body) override;
+
+    /// Stamps the terminal, already-expired mount body and folds the watermark farewell into it;
+    /// the base state lock and `dead` flag prevent another renewal from racing this write.
+    void terminate() override;
+
+    /// Refreshes `confirmed_deadline_ms` after EVERY successful renewal (background OR a direct
+    /// caller) — see `onRenewCommitted`'s base doc comment.
+    void onRenewCommitted() override;
+
+    /// Fires the boot-domain write-fence callback (`on_renew_ok`) after a confirmed BACKGROUND
+    /// lease renewal. `confirmed_deadline_ms` itself is refreshed by `onRenewCommitted` above (which
+    /// already ran, for this same successful renewal, before the background loop calls this).
+    void onRenewSucceeded() override;
+
+    /// Latches the Pool's local write fence when renewal can no longer prove that this incarnation
+    /// owns the mount slot.
+    void onRenewFailed() override;
+
+    /// Re-reads a failed renewal and classifies it five ways -- fenced_by_gc, same_epoch_state_uncertain
+    /// (this incarnation's own token was advanced past under our own (uuid, epoch): ambiguous, not
+    /// proof of anything, and NOT fatal), superseded (a newer epoch), foreign_writer, and vanished
+    /// (the backing object disappeared) -- every branch stays terminal and fail closed, and NONE
+    /// constructs a `LOGICAL_ERROR`: they throw non-aborting codes (`ABORTED`/`FILE_DOESNT_EXIST`),
+    /// because each is reachable by an ordinary network timeout or environmental condition rather than
+    /// a programming bug -- and because this runs on the keeper's background thread, where an abort at
+    /// exception construction takes the whole process with it.
+    void onRenewMismatch(const String & mismatched_key) override;
+    /// Fence immediately only once our last CONFIRMED lease (the last successful
+    /// `claim`/renew) has reached its safety-margin boundary -- `confirmed_deadline_ms - now <=
+    /// lease_safety_margin`. Until then the mount-lease protocol still guarantees exclusivity.
+    bool shouldFenceOnTransientRenewFailure() override;
+
+private:
+    /// Refreshes `confirmed_deadline_ms` from `anchor_wall_ms` + `ttl` — anchor = the pre-I/O wall
+    /// instant of the confirming attempt. Called on every point this keeper KNOWS it holds a live
+    /// lease: both success paths of `claim` (mint and adopt), and every successful `renewOnce`
+    /// (`onRenewCommitted`) — background-driven or a direct caller alike.
+    void refreshConfirmedDeadline(uint64_t anchor_wall_ms);
+
+    String srid;
+    UInt128 server_uuid;
+    uint64_t writer_epoch;
+    std::chrono::milliseconds ttl;
+    std::function<uint64_t()> now_ms_fn;
+    std::function<uint64_t()> min_active_fn;
+    std::function<void(uint64_t)> on_renew_ok;
+    std::function<void()> on_lost;
+    CasEventSink event_sink;
+    std::chrono::milliseconds lease_safety_margin;
+    /// boot-domain clock for the on_renew_ok anchor; empty = real CLOCK_BOOTTIME. Injectable for
+    /// tests and wired by CasMountRuntime::installKeeper.
+    std::function<uint64_t()> boot_ms_fn;
+    /// BOOTTIME-ms deadline (same clock as `now_ms_fn`/`MountFence`, and for the SAME suspend-safety
+    /// reason -- see `MountFence`'s doc comment in `CasMountRuntime.h`) of the last CONFIRMED lease.
+    /// 0 = none yet (`claim` always sets this before `startBackground` can run, so
+    /// `shouldFenceOnTransientRenewFailure` observing 0 is defensive, not an expected steady state).
+    uint64_t confirmed_deadline_ms = 0;
+    /// Whether this runtime has STOPPED BELIEVING IT OWNS THE MOUNT. Set at `onRenewFailed`, which is
+    /// that one point and is deliberately broader than "a mismatch was classified": the background loop
+    /// also reaches it when a TRANSIENT renewal failure persists past the confirmed lease's
+    /// safety-margin boundary (`shouldFenceOnTransientRenewFailure`), where nothing was classified at
+    /// all and the write fence latches anyway. Both are the same fact for the reader below, and the
+    /// broader one is the SAFE one: what the release path needs to know is whether this runtime still
+    /// claims the slot, not why it stopped.
+    ///
+    /// The release path reads it to tell two opposite situations apart — a deposed writer meeting its
+    /// successor in the slot (the expected end of a failover) from a writer that still believed it owned
+    /// the mount meeting a stranger there (single-writer exclusivity broken). Atomic because the
+    /// keeper's background thread sets it and teardown reads it.
+    std::atomic<bool> deposition_observed{false};
+    /// Pre-I/O anchors of the CURRENT attempt, stashed by prepareRenew (which runs at the start of
+    /// every doStart/renewOnce attempt, off the state lock) and consumed by the success hooks.
+    /// `mutable` + no synchronization is safe: prepareRenew, claim, and the hooks all run on the
+    /// single renewal driver thread (see renewOnce's single-driver invariant).
+    mutable uint64_t last_attempt_wall_ms = 0;
+    mutable uint64_t last_attempt_boot_ms = 0;
+};
+
+/// Mount-lease-scoped staging sweeper for objects left behind by S3-native staging.
+///
+/// A leaked S3 staging object happens two ways: (1) an exception between `promoteStaged` succeeding and
+/// `cleanupPendingTempFiles` deleting the staging key (`ContentAddressedTransaction.cpp`), or (2) an
+/// aborted/cancelled transaction whose pending blobs were staged but never promoted — by design,
+/// `cleanupPendingTempFiles` deliberately leaves an S3 staging object in place on the abort path (never a
+/// bare `fs::remove` on a remote key), so this sweeper is its ONLY reclaimer. Debris from either case is
+/// bounded to `staging/<mount_id>/` — the ONE mount that could ever have written under that prefix, since
+/// every staging key this mount ever mints comes from `ContentAddressedMetadataStorage::stagingKeyPrefix()`
+/// (`physicalKey(pool_prefix + "/staging/" + server_root_id)`), keyed by THIS mount's own `server_root_id`.
+///
+/// LEASE-FENCE (fail-closed, never fail-open): `sweepOwnMountStaging` removes ONLY objects whose key
+/// starts with the given `mount_staging_prefix` — pass your OWN mount's prefix, never another mount's.
+/// The caller (`ContentAddressedMetadataStorage::startup()`) invokes this exactly once, at mount start,
+/// with `stagingKeyPrefix() + "/"` — the SAME prefix construction the writer uses to mint staging keys, so
+/// this sweep can never reach a different mount's `staging/<other_mount_id>/` subtree: no other writer
+/// ever stages a key under THIS mount's own `server_root_id` prefix, and this function never lists or
+/// touches anything outside the prefix it is given.
+///
+/// Best-effort and NEVER THROWS: one stubborn key (or a LIST failure) must never abort the sweep of the
+/// rest, and must never fail the mount (mirrors `feedback_ca_gc_never_throw_on_404` — a throw here would
+/// only wedge startup, not GC, but the same fail-open-on-error discipline applies to any best-effort
+/// reclaim of debris).
+///
+/// GC excludes `staging/` entirely: GC blob discovery LISTs `Layout::blobsPrefix()`
+/// (`<pool_prefix>/blobs/`) — a distinct top-level prefix from `staging/`, `cas/ns/`, and
+/// `cas/manifests/` (see `CasLayout.h`) — so a `staging/` object is never listed, HEAD'd, or condemned by
+/// GC's fold. This sweeper is the ONLY reclaimer of `staging/` debris.
+void sweepOwnMountStaging(IObjectStorage & object_storage, const String & mount_staging_prefix) noexcept;
+
+}

From 9d08bc07f12ee0b6dec343f6257f0242f5c0996a Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:34 +0200
Subject: [PATCH 17/30] CAS subsystem: Parts layer

Part-path parsing and the part-folder access facade over manifests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../Parts/PartFolderAccess.cpp                | 762 ++++++++++++++++++
 .../ContentAddressed/Parts/PartFolderAccess.h | 411 ++++++++++
 .../ContentAddressed/Parts/PartPathParser.cpp | 400 +++++++++
 .../ContentAddressed/Parts/PartPathParser.h   | 146 ++++
 4 files changed, 1719 insertions(+)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.h

diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.cpp
new file mode 100644
index 000000000000..bce2e9423704
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.cpp
@@ -0,0 +1,762 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Common/DateLUT.h>
+#include <Common/MemoryTracker.h>
+#include <Common/ProfileEvents.h>
+#include <Common/logger_useful.h>
+#include <base/defines.h>
+#include <base/scope_guard.h>
+#include <algorithm>
+#include <chrono>
+#include <unordered_set>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int FILE_DOESNT_EXIST;
+    extern const int ABORTED;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace ProfileEvents
+{
+    extern const Event CASPartFolderViewHits;
+    extern const Event CASPartFolderViewValidationMismatches;
+    extern const Event CASPartFolderViewMisses;
+    extern const Event CASPartFolderViewOversizedBypasses;
+    extern const Event CASPartFolderViewInvalidations;
+    extern const Event CASRefRollbackBestEffortDropFailed;
+    extern const Event CASPartFolderValidateSkipped;
+    extern const Event CASRefRepoint;
+}
+
+namespace CurrentMetrics
+{
+    extern const Metric CASPartFolderCacheBytes;
+    extern const Metric CASPartFolderCacheEntries;
+}
+
+namespace DB::Cas
+{
+
+PartFolderView::PartFolderView(PartRefKey key_, Cas::ManifestId manifest_id_, uint64_t manifest_size_,
+                               std::shared_ptr<const Cas::PartManifest> manifest_, uint64_t validated_at_ms_)
+    : key(std::move(key_))
+    , manifest_id(std::move(manifest_id_))
+    , manifest_size(manifest_size_)
+    , manifest_body(std::move(manifest_))
+    , validated_at_ms(validated_at_ms_)
+{
+    chassert(manifest_body);
+    /// The binary-search contract: entries must be strictly ascending by `path` (sorted and unique) —
+    /// `decodePartManifest` enforces exactly this for every decoded body, and `findEntry`'s binary
+    /// search assumes uniqueness. `adjacent_find` with `!(a.path < b.path)` flags any adjacent pair
+    /// that is out-of-order OR duplicate (stronger than `is_sorted`, which permits duplicates); a
+    /// hand-constructed manifest (tests) must honor it too.
+    chassert(std::adjacent_find(manifest_body->entries.begin(), manifest_body->entries.end(),
+        [](const Cas::ManifestEntry & a, const Cas::ManifestEntry & b) { return !(a.path < b.path); })
+        == manifest_body->entries.end());
+}
+
+std::shared_ptr<const PartFolderView> PartFolderView::make(
+    PartRefKey key, const Cas::Resolved & resolved, std::shared_ptr<const Cas::PartManifest> manifest,
+    uint64_t validated_at_ms)
+{
+    return std::make_shared<const PartFolderView>(
+        std::move(key), resolved.manifest_id, resolved.manifest_size,
+        std::move(manifest), validated_at_ms);
+}
+
+std::optional<std::string> PartFolderView::projectionDirPrefix(const std::string & file)
+{
+    if (file.empty())
+        return std::nullopt;
+    const auto last_slash = file.find_last_of('/');
+    const std::string_view last_component
+        = last_slash == std::string::npos ? std::string_view(file) : std::string_view(file).substr(last_slash + 1);
+    if (last_component.ends_with(".proj") || last_component.ends_with(".tmp_proj"))
+        return file + "/";
+    return std::nullopt;
+}
+
+const Cas::ManifestEntry * PartFolderView::findFile(const String & path) const
+{
+    return Cas::findEntry(manifest_body->entries, path);
+}
+
+bool PartFolderView::hasFile(const String & path) const
+{
+    return findFile(path) != nullptr;
+}
+
+std::optional<uint64_t> PartFolderView::fileSize(const String & path) const
+{
+    if (const auto * e = findFile(path))
+        return e->size();
+    return std::nullopt;
+}
+
+std::optional<String> PartFolderView::inlineBytes(const String & path) const
+{
+    const auto * e = findFile(path);
+    if (e && e->placement == Cas::EntryPlacement::Inline)
+        return e->inline_bytes;
+    return std::nullopt;
+}
+
+std::vector<String> PartFolderView::listChildren(const String & dir_prefix) const
+{
+    /// Collapse each entry to its first child component. Projection folders are structurally flat in
+    /// `MergeTree`, so this produces the same names as the old projection-specific path handling while
+    /// keeping one directory-listing rule for all manifest folders.
+    std::unordered_set<String> names;
+    auto add = [&](const String & full)
+    {
+        if (!full.starts_with(dir_prefix) || full.size() <= dir_prefix.size())
+            return;
+        const std::string_view rest = std::string_view(full).substr(dir_prefix.size());
+        const auto slash = rest.find('/');
+        names.emplace(slash == std::string_view::npos ? rest : rest.substr(0, slash));
+    };
+    const auto [first, last] = Cas::entryRange(manifest_body->entries, dir_prefix);
+    for (const auto * e = first; e != last; ++e)
+        add(e->path);
+    return {std::make_move_iterator(names.begin()), std::make_move_iterator(names.end())};
+}
+
+bool PartFolderView::hasDirectory(const String & dir_prefix) const
+{
+    const auto [first, last] = Cas::entryRange(manifest_body->entries, dir_prefix);
+    return first != last;
+}
+
+size_t PartFolderView::estimatedBytes() const
+{
+    /// Conservative cache weight: fixed overhead plus `manifest_size`. This deliberately over-counts
+    /// the shared decode, which is safe because eviction should happen before the budget is exceeded.
+    return 256 + manifest_size;
+}
+
+CachedPartFolderAccess::CachedPartFolderAccess(Cas::PoolPtr store_)
+    : CachedPartFolderAccess(std::move(store_), CacheParams{})
+{
+}
+
+CachedPartFolderAccess::CachedPartFolderAccess(Cas::PoolPtr store_, CacheParams params_, std::function<uint64_t()> now_ms_fn_)
+    : store(std::move(store_)), params(params_), now_ms_fn(std::move(now_ms_fn_))
+{
+    if (!now_ms_fn)
+        now_ms_fn = []() -> uint64_t { return timeInMilliseconds(std::chrono::system_clock::now()); };
+    if (params.cache_bytes > 0)
+        view_cache = std::make_unique<ViewCache>(
+            "LRU", CurrentMetrics::CASPartFolderCacheBytes, CurrentMetrics::CASPartFolderCacheEntries,
+            params.cache_bytes, params.max_entries, ViewCache::DEFAULT_SIZE_RATIO);
+}
+
+std::shared_ptr<const PartFolderView>
+CachedPartFolderAccess::getView(const PartRefKey & key, Freshness freshness) const
+{
+    /// Resolve first on every access. Absence is never retained, and the same ref-resolution result
+    /// supplies the manifest ID used to validate a retained view. The emit is deferred: a warm
+    /// `CachedForLoad` hit below serves the call without doing any real resolve work worth auditing, so
+    /// this call site decides itself, per path, whether to re-emit the identical `RefResolve` event.
+    auto resolved = resolve(key, freshness, Cas::ResolveAudit::Deferred);
+    if (!resolved)
+        return nullptr;
+
+    /// Reuse one canonical key for the retained view and the optional diagnostic journal.
+    const String cache_key = key.cacheKey();
+
+    /// Retained views serve `CachedForLoad` directly only after their manifest ID matches the fresh
+    /// resolve. `ForceFresh` must re-prove the manifest body unless the configured validation policy
+    /// explicitly permits a recent retained view; a fresh ref resolve proves ref currency, not body existence.
+    if (freshness == Freshness::CachedForLoad && view_cache)
+    {
+        if (auto cached = view_cache->get(cache_key))
+        {
+            if (cached->manifestId() == resolved->manifest_id)
+            {
+                /// The warm hit: the retained view already reflects this exact manifest, so this access
+                /// did no real resolve work beyond a cache lookup — no `RefResolve` audit row for it.
+                ProfileEvents::increment(ProfileEvents::CASPartFolderViewHits);
+                recordDecision(cache_key, LastDecision::Hit, cached.get(), /*retained=*/true);
+                return cached;
+            }
+            ProfileEvents::increment(ProfileEvents::CASPartFolderViewValidationMismatches);
+            /// Rebuild below; the stale entry is superseded by the new view when retention is enabled.
+        }
+    }
+
+    /// With a non-`Always` validation policy, `ForceFresh` may serve a retained view without another
+    /// body HEAD when its manifest ID still matches and its validation timestamp is within the age
+    /// policy. `StrictValidate` bypasses retention. A manifest-ID mismatch always rebuilds, because all
+    /// part content is represented by the manifest.
+    if (freshness == Freshness::ForceFresh && view_cache && params.validate.mode != PartFolderValidate::Mode::Always)
+    {
+        if (auto cached = view_cache->get(cache_key);
+            cached && cached->manifestId() == resolved->manifest_id)
+        {
+            const bool fresh_enough = params.validate.mode == PartFolderValidate::Mode::Never
+                || (now_ms_fn() - cached->validatedAtMs()) < params.validate.age_seconds * 1000ULL;
+            if (fresh_enough)
+            {
+                ProfileEvents::increment(ProfileEvents::CASPartFolderViewHits);
+                ProfileEvents::increment(ProfileEvents::CASPartFolderValidateSkipped);
+                recordDecision(cache_key, LastDecision::Hit, cached.get(), /*retained=*/true);
+                emitResolveEvent(key, *resolved);
+                return cached;
+            }
+        }
+    }
+
+    auto view = buildView(key, *resolved, freshness);
+
+    /// Retain eligible views. `StrictValidate` never populates the cache, and oversized views are
+    /// served but not retained.
+    /// `oversized` is tracked separately from `retained`: with retention disabled (`view_cache ==
+    /// nullptr`), `retained` is also false, but that is an ordinary disabled-mode miss rather than
+    /// an oversized bypass.
+    bool retained = false;
+    bool oversized = false;
+    if (freshness != Freshness::StrictValidate && view_cache)
+    {
+        if (view->estimatedBytes() <= params.max_entry_bytes)
+        {
+            /// CacheBase stores mutable pointers; views are logically const (never mutated).
+            view_cache->set(cache_key, std::const_pointer_cast<PartFolderView>(view));
+            retained = true;
+        }
+        else
+        {
+            oversized = true;
+            ProfileEvents::increment(ProfileEvents::CASPartFolderViewOversizedBypasses);
+        }
+    }
+    ProfileEvents::increment(ProfileEvents::CASPartFolderViewMisses);
+    recordDecision(cache_key,
+        freshness == Freshness::CachedForLoad ? (oversized ? LastDecision::OversizedBypass : LastDecision::Miss)
+        : freshness == Freshness::ForceFresh  ? LastDecision::ForceFreshRead
+                                              : LastDecision::StrictBypass,
+        view.get(), retained);
+    emitResolveEvent(key, *resolved);
+    return view;
+}
+
+void CachedPartFolderAccess::emitResolveEvent(const PartRefKey & key, const Cas::Resolved & resolved) const
+{
+    /// Mirrors `CasRefLedger::resolveRef`'s own (deferred-here) emit exactly: same event type and
+    /// fields, built from the `Resolved` this call already holds.
+    Cas::EventEmitter{*store}.emit([&](Cas::CasEvent & e)
+    {
+        e.type = Cas::CasEventType::RefResolve;
+        e.namespace_ = key.ns.string();
+        e.ref_name = key.ref;
+        e.object_kind = Cas::CasEventObjectKind::Manifest;
+        e.object_hash = Cas::manifestRefDebugString(resolved.manifest_id.ref);
+        e.outcome = "resolved";
+        e.reason = "read-side resolve of a ref to its part manifest";
+    });
+}
+
+std::shared_ptr<const PartFolderView> CachedPartFolderAccess::buildView(
+    const PartRefKey & key, const Cas::Resolved & resolved, Freshness freshness) const
+{
+    /// Fresh modes do not coalesce: each `ForceFresh`/`StrictValidate` call owns its mandatory HEAD.
+    /// Only cold `CachedForLoad` builds use single-flight.
+    if (freshness != Freshness::CachedForLoad)
+        return PartFolderView::make(key, resolved, store->readManifestShared(resolved.manifest_id), now_ms_fn());
+
+    std::promise<std::shared_ptr<const PartFolderView>> promise;
+    std::shared_future<std::shared_ptr<const PartFolderView>> future;
+    bool leader = false;
+    {
+        std::lock_guard lock(inflight_mutex);
+        if (auto it = inflight.find(key.cacheKey()); it != inflight.end())
+            future = it->second;                          /// Follower: share the leader's build.
+        else
+        {
+            leader = true;
+            future = promise.get_future().share();
+            inflight.emplace(key.cacheKey(), future);
+        }
+    }
+    if (!leader)
+        return future.get();                              /// Rethrows the leader's exception, if any.
+
+    SCOPE_EXIT({
+        std::lock_guard lock(inflight_mutex);
+        inflight.erase(key.cacheKey());
+    });
+    try
+    {
+        auto view = PartFolderView::make(key, resolved, store->readManifestShared(resolved.manifest_id), now_ms_fn());
+        promise.set_value(view);
+        return view;
+    }
+    catch (...)
+    {
+        promise.set_exception(std::current_exception());  /// Followers see the leader's exception.
+        throw;
+    }
+}
+
+void CachedPartFolderAccess::eraseView(const PartRefKey & key)
+{
+    const String cache_key = key.cacheKey();
+    if (view_cache)
+        view_cache->remove(cache_key);
+    ProfileEvents::increment(ProfileEvents::CASPartFolderViewInvalidations);
+    recordDecision(cache_key, LastDecision::Invalidated, nullptr, /*retained=*/false);
+}
+
+std::optional<Cas::Resolved>
+CachedPartFolderAccess::resolve(const PartRefKey & key, Freshness freshness, Cas::ResolveAudit audit) const
+{
+    return store->resolveRef(key.ns, key.ref, /*allow_stale=*/freshness == Freshness::CachedForLoad, audit);
+}
+
+bool CachedPartFolderAccess::existsRef(const PartRefKey & key, Freshness freshness) const
+{
+    return resolve(key, freshness).has_value();
+}
+
+Cas::CommitOutcome CachedPartFolderAccess::promoteBuild(Cas::PartWriteTxn & build, const PartRefKey & key, UInt128 build_id,
+                                          const Cas::ManifestId & manifest_id, bool allow_repoint,
+                                          bool * commit_recorded)
+{
+    /// `build.promote` derives `created` INSIDE its own `appendRefOps` builder (the same in-closure
+    /// pattern as that builder's `repoint_old`) and returns it the instant the append confirms.
+    ///
+    /// The outcome's STRINGS are copied BEFORE the append, so the only thing the post-durable region
+    /// has left to do is store a bool -- which makes that region allocation-free, hence non-throwing.
+    /// This is Part A's rule applied one layer up: nothing after a durable commit may throw before the
+    /// caller has recorded it. Assembling the outcome afterwards (as this did) put two `String` copies
+    /// and a cache invalidation between "the ref is committed" and "the handle knows", so a
+    /// `MEMORY_LIMIT_EXCEEDED` there landed in the caller's failed-promote handler -- which abandons
+    /// the build and, one layer up again, reports a byte fallback for a relink that already published.
+    Cas::CommitOutcome outcome{key.ns, key.ref, manifest_id.ref, /*created=*/false};
+
+    const bool created = build.promote(key.ns, key.ref, build_id, manifest_id, allow_repoint);
+
+    /// POST-DURABLE REGION. Two plain stores, no allocation, no call that can fail.
+    {
+        DENY_ALLOCATIONS_IN_SCOPE;
+        outcome.created = created;
+        if (commit_recorded)
+            *commit_recorded = true;
+    }
+
+    /// Test-only (see `setPostCommitProbeForTest`): models an allocation failure in the throwable
+    /// post-commit work below. It fires AFTER the region above, because the property under test is that
+    /// the commit is already recorded by the time anything here can throw.
+    if (post_commit_probe_for_test)
+        post_commit_probe_for_test();
+
+    eraseView(key);
+    return outcome;
+}
+
+namespace
+{
+
+/// A failed publish must not leak a live-epoch precommit binding: only `abandon` removes it (the build
+/// destructor merely retires the build seq; the stale-precommit sweep is prior-epoch-scoped and GC
+/// never touches a live precommit). `abandon` may itself fail on the same broken backend -- log and let
+/// whatever error the caller is already carrying stay primary. Returns whether the abandon completed,
+/// so a caller tracking a terminal state can tell a finished transaction from one still owing cleanup
+/// (`PartWriteTxn::abandon` stays retryable after an append failure).
+bool abandonBuildBestEffort(Cas::PartWriteTxn & build, const char * context) noexcept
+{
+    try
+    {
+        build.abandon();
+        return true;
+    }
+    catch (...)
+    {
+        tryLogCurrentException(getLogger("CachedPartFolderAccess"), context);
+        return false;
+    }
+}
+
+}
+
+PreparedPartWrite::PreparedPartWrite(CachedPartFolderAccess & owner_, PartWriteTxnPtr build_,
+                                     PartRefKey key_, ManifestId id_)
+    : owner(&owner_), build(std::move(build_)), key(std::move(key_)), id(std::move(id_))
+{
+}
+
+PreparedPartWrite::PreparedPartWrite(PreparedPartWrite && other) noexcept
+    : owner(other.owner), build(std::move(other.build)), key(std::move(other.key)), id(std::move(other.id))
+    , terminal(other.terminal)
+{
+    /// The move takes over the owed terminal operation in full: the source must never run it again,
+    /// so it is left terminal (and holding no transaction) rather than merely emptied.
+    other.owner = nullptr;
+    other.terminal = true;
+}
+
+PreparedPartWrite::~PreparedPartWrite()
+{
+    if (terminal || !build)
+        return;
+    /// Last-resort guard. Reaching here means no terminal operation COMPLETED -- it was forgotten, an
+    /// exception skipped it, or one was attempted and its append failed -- so the precommit binding is
+    /// still live and its removal is appended here rather than left to leak. Best-effort and noexcept:
+    /// a destructor that propagated the append failure would terminate the process, and the durable
+    /// backstop for a removal that cannot land at all is the same one every other abandon path uses.
+    LOG_ERROR(getLogger("CachedPartFolderAccess"),
+              "PreparedPartWrite for '{}/{}' was destroyed while still owing its terminal operation; "
+              "aborting it now", key.ns.string(), key.ref);
+    abandonBuildBestEffort(*build, "aborting a prepared part write destroyed without a terminal operation");
+}
+
+bool PreparedPartWrite::commitIsUnresolved() const
+{
+    return build && build->commitState() == Cas::PartWriteTxn::CommitState::Uncertain;
+}
+
+Cas::CommitOutcome PreparedPartWrite::promote(bool allow_repoint)
+{
+    if (terminal)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "PreparedPartWrite for '{}/{}' has already been promoted or aborted; it owes exactly one "
+            "terminal operation",
+            key.ns.string(), key.ref);
+    try
+    {
+        /// `terminal` is set by `promoteBuild` itself, inside the allocation-free region immediately
+        /// after the durable append -- NOT after this call returns. The difference is the whole point:
+        /// the outcome assembly and the cache invalidation that follow the append can throw, and a
+        /// handle that had not yet recorded the commit would then take the catch below and abandon a
+        /// build whose ref is already published.
+        return owner->promoteBuild(*build, key, build->buildId(), id, allow_repoint, &terminal);
+    }
+    catch (...)
+    {
+        /// The commit is durable and this handle knows it, so the duty is discharged and there is
+        /// nothing to abandon -- the error is post-commit work failing, which the caller still hears
+        /// about, unchanged.
+        if (terminal)
+            throw;
+        /// The catch-abandon-rethrow discipline the atomic `publishEntries` has always owned. It stays
+        /// correct even when the append itself was ambiguous: `abandon` appends a PRECOMMIT removal,
+        /// and a promote that did land moved the binding to committed, so the removal is rejected by
+        /// the state machine rather than undoing a published ref. The terminal flag flips only if the
+        /// abandon actually landed: a failed abandon leaves the handle owing cleanup, which the
+        /// destructor then retries.
+        terminal = abandonBuildBestEffort(*build, "abandoning the build of a failed PreparedPartWrite::promote");
+        throw;
+    }
+}
+
+void PreparedPartWrite::abort()
+{
+    if (terminal)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "PreparedPartWrite for '{}/{}' has already been promoted or aborted; it owes exactly one "
+            "terminal operation",
+            key.ns.string(), key.ref);
+    /// `abandon` appends the EXACT precommit removal through the reliable append lane -- that append,
+    /// not the destruction of the transaction object, is what releases the manifest's `+1`. A failure
+    /// propagates: the caller may retry the same handle, and the destructor is the backstop.
+    build->abandon();
+    terminal = true;
+}
+
+PreparedPartWrite CachedPartFolderAccess::prepareEntries(const PartRefKey & dst,
+    const std::vector<Cas::ManifestEntry> & entries, Cas::ProvenanceOp op)
+{
+    auto build = store->beginPartWrite(Cas::PartWriteInfo{.intended_ref = dst.ns.string() + "/" + dst.ref,
+                                                  .intended_namespace = dst.ns, .op = op});
+    try
+    {
+        /// Record write evidence for each non-inline entry. No pool HEAD/GET is performed before
+        /// precommit; the promote path re-proves each dependency fail-closed. Inline entries need no evidence.
+        for (const auto & entry : entries)
+            build->adoptEvidence(entry);
+        /// Stage a fresh manifest over the same entries. Blobs are content-addressed, but each part owns
+        /// its manifest ID, so `dst` receives a distinct manifest before ownership moves to it.
+        const Cas::ManifestId id = build->stageManifest(entries);
+        build->precommitAdd(dst.ns, dst.ref, id);
+        return PreparedPartWrite(*this, std::move(build), dst, id);
+    }
+    catch (...)
+    {
+        /// No handle is returned on this path, so the cleanup cannot be deferred to one: abandon here.
+        abandonBuildBestEffort(*build, "abandoning the build of a failed prepareEntries");
+        throw;
+    }
+}
+
+Cas::CommitOutcome CachedPartFolderAccess::publishEntries(const PartRefKey & dst,
+    const std::vector<Cas::ManifestEntry> & entries, Cas::ProvenanceOp op, bool allow_repoint)
+{
+    /// The atomic form: prepare and promote back to back, with no window in between. `promote` carries
+    /// the catch-abandon-rethrow discipline, so a failure here behaves exactly as it did when this was
+    /// one function.
+    PreparedPartWrite prepared = prepareEntries(dst, entries, op);
+    return prepared.promote(allow_repoint);
+}
+
+bool CachedPartFolderAccess::republishRef(const PartRefKey & src, const PartRefKey & dst)
+{
+    /// Content addressing has no rename, so move a committed ref by reading the source body freshly,
+    /// publishing equivalent entries at the destination, and then dropping the source. The source
+    /// body is re-proved and is never taken from a retained view.
+    auto resolved = store->resolveRef(src.ns, src.ref);
+    if (!resolved)
+        return false;
+    const auto src_manifest = store->readManifestShared(resolved->manifest_id);
+
+    /// If `dst` is already committed, a previous attempt may have completed its promote before the
+    /// source drop. Compare content rather than the whole manifest, whose ref/namespace/digest
+    /// legitimately differ: equal content completes the move by dropping `src`; different content
+    /// is a conflict and leaves the source intact.
+    if (auto dst_resolved = store->resolveRef(dst.ns, dst.ref))
+    {
+        const auto dst_manifest = store->readManifestShared(dst_resolved->manifest_id);
+        if (dst_manifest->entries != src_manifest->entries)
+            throw Exception(ErrorCodes::ABORTED,
+                "republishRef: destination '{}' is already committed with different content — refusing "
+                "(rename/attach conflict)", dst.ns.string() + "/" + dst.ref);
+        dropRef(src);
+        return true;
+    }
+
+    publishEntries(dst, src_manifest->entries, Cas::ProvenanceOp::Other);
+    dropRef(src);
+    return true;
+}
+
+Cas::CommitOutcome CachedPartFolderAccess::repointRef(const PartRefKey & key, std::vector<Cas::ManifestEntry> entries, Cas::ProvenanceOp op)
+{
+    /// Compare the candidate `entries` against the currently committed manifest's
+    /// decoded entries. This must NOT stage a candidate manifest first: `stageManifest` mints a
+    /// non-content-derived `ManifestRef` (epoch/build_seq/ordinal) AND durably PUTs the encoded body
+    /// on every call (CasPartWriteTxn.cpp), so staging-then-comparing IDs would itself be a pool mutation on
+    /// the byte-equal path — violating the "ZERO pool mutations" contract this primitive exists to
+    /// provide.
+    ///
+    /// The comparison must be symmetric. `committed_manifest->entries` already went
+    /// through one `decodePartManifest` round-trip, which does not carry `blob_size` for Inline
+    /// entries on the wire (it is redundant with `inline_bytes.size()`, use `ManifestEntry::size()` for
+    /// the logical size instead, and it is excluded from both the canonical encoding and the payload
+    /// digest — see `CasPartManifestFormat.cpp`'s `writeEntryRecord`/`decodePartManifest`). The
+    /// freshly constructed `entries` may not have the same incidental fields (the inline write path no longer sets
+    /// `blob_size`), but a straight struct compare against them is still not guaranteed byte-identical
+    /// (canonical path ordering, etc.), so route the candidate through the identical encode/decode
+    /// round-trip before comparing regardless — this is the same content comparison used by `republishRef`,
+    /// which is symmetric for the same reason (both sides there are already decoded).
+    auto resolved = resolve(key, Freshness::ForceFresh);
+    if (resolved)
+    {
+        const auto committed_manifest = store->readManifestShared(resolved->manifest_id);
+        Cas::PartManifest probe;
+        probe.ref = committed_manifest->ref;
+        probe.root_namespace_id = committed_manifest->root_namespace_id;
+        probe.entries = entries;
+        probe.payload_digest = Cas::computePayloadDigest(probe);
+        const Cas::PartManifest canonical_candidate = Cas::decodePartManifest(Cas::encodePartManifest(probe));
+        if (committed_manifest->entries == canonical_candidate.entries)
+            /// ZERO pool mutations: the outcome describes the manifest ALREADY committed, unchanged.
+            return Cas::CommitOutcome{key.ns, key.ref, resolved->manifest_id.ref, /*created=*/false};
+    }
+    /// `publishEntries` takes `entries` by const reference, so the caller-owned vector remains valid.
+    /// Capture the exact outcome IMMEDIATELY -- before the ProfileEvent/logging below -- so it is
+    /// published ahead of any further (even if non-throwing in practice) post-commit work.
+    const Cas::CommitOutcome oc = publishEntries(key, entries, op, /*allow_repoint=*/true);
+    ProfileEvents::increment(ProfileEvents::CASRefRepoint);
+    if (resolved)
+    {
+        /// Repoint is the normal mechanism for effective standalone writes/removes on committed parts,
+        /// so this routine event is logged at debug level while the counter remains an operator-facing signal.
+        LOG_DEBUG(getLogger("CachedPartFolderAccess"),
+            "Repointed committed ref {}/{} ({} entries) — standalone write/remove on a committed part",
+            key.ns.string(), key.ref, entries.size());
+    }
+    else
+    {
+        /// A repoint normally targets an existing ref. Keep an unexpected create-shaped call visible
+        /// at warning level rather than silently treating it as ordinary publication.
+        LOG_WARNING(getLogger("CachedPartFolderAccess"),
+            "repointRef published {}/{} ({} entries) with no prior committed ref to repoint — "
+            "unexpected call shape (repointRef requires an existing committed ref)",
+            key.ns.string(), key.ref, entries.size());
+    }
+    return oc;
+}
+
+void CachedPartFolderAccess::dropRef(const PartRefKey & key)
+{
+    store->dropRef(key.ns, key.ref);
+    eraseView(key);
+}
+
+void CachedPartFolderAccess::dropRefIfPresent(const PartRefKey & key)
+{
+    /// resolveRef gates the common case (a temporary ref that was never committed is a no-op, not an
+    /// error); dropRef re-reads the shard inside its own CAS loop, so a concurrent drop can land in
+    /// the window between our resolve and that re-read — surfacing as FILE_DOESNT_EXIST. Removal is
+    /// replay-safe, so an already-gone ref is success; any other exception still propagates. The view
+    /// is also erased on the early-return absent path.
+    if (!store->resolveRef(key.ns, key.ref, /*allow_stale=*/true))
+    {
+        eraseView(key);
+        return;
+    }
+    try
+    {
+        store->dropRef(key.ns, key.ref);
+    }
+    catch (const Exception & e)
+    {
+        if (e.code() != ErrorCodes::FILE_DOESNT_EXIST)
+            throw;
+        eraseView(key);
+        return;   /// Raced away between the gate and dropRef; nothing was actually dropped here.
+    }
+    eraseView(key);
+}
+
+void CachedPartFolderAccess::dropRefBestEffort(const PartRefKey & key) noexcept
+{
+    try
+    {
+        store->dropRef(key.ns, key.ref);
+    }
+    catch (...)
+    {
+        /// Best-effort destructor/rollback cleanup: debris is GC-reclaimed, but swallowing the
+        /// exception without a diagnostic could leave a live phantom ref after a backend outage.
+        ProfileEvents::increment(ProfileEvents::CASRefRollbackBestEffortDropFailed);
+        tryLogCurrentException(getLogger("CachedPartFolderAccess"),
+            fmt::format("CA best-effort rollback dropRef failed (ns={} ref={}); the ref may remain live",
+                        key.ns.string(), key.ref));
+    }
+    /// In destructor/rollback context the ref's durable state is unknown, so invalidate the view even
+    /// after a swallowed cleanup exception.
+    eraseView(key);
+}
+
+bool CachedPartFolderAccess::dropRefIfMatches(const PartRefKey & key, const Cas::ManifestRef & expected) noexcept
+{
+    /// One `appendRefOps` builder does both the read and the conditional removal, mirroring
+    /// `CasRefLedger::dropRef`'s own protocol shape (read the committed binding, emit ONE
+    /// `OwnerTransition` removal op) but with the removal guarded on `expected` inside the SAME
+    /// closure -- the leader-thread read of `state.getCommitted()` is the authoritative committed
+    /// binding at append time, so this is race-free the same way `PartWriteTxn::promote`'s
+    /// `repoint_old`/idempotent-guard reads are: `build_ops` runs at most once, on the flush leader,
+    /// against the batch-validated state. A mismatch (repointed since `expected` was observed, or
+    /// already absent) returns an empty op list -- a legitimate no-op, not an error, exactly like
+    /// `promote`'s own idempotent-redrive branch.
+    bool removed = false;
+    try
+    {
+        const RefTxnId txn_id = store->appendRefOps(key.ns, MutationScope::ref(key.ref),
+            [&](const RefTableState & state) -> std::vector<RefOp>
+            {
+                const auto it = state.getCommitted().find(key.ref);
+                if (it == state.getCommitted().end() || !(it->second.manifest_ref == expected))
+                    return {};   /// absent, or repointed away from `expected` -- leave it alone
+
+                removed = true;
+                RefOp op;
+                op.kind = RefOpKind::OwnerTransition;
+                op.old_binding = RefOwnerBinding{RefOwnerKind::Committed, key.ref, expected};
+                return {op};
+            },
+            RootMutationOrigin::Writer, RootMutationKind::Drop);
+
+        /// Audit a successful conditional removal exactly like `CasRefLedger::dropRef` audits its own
+        /// unconditional one -- otherwise a rollback drop (this method's only caller, as of Task 3)
+        /// would be invisible in `system.cas_log`. Byte-neutral to the ref-log: this is
+        /// an audit event only, emitted after the removal is already durable.
+        if (removed && store->hasEventSink())
+        {
+            Cas::CasEvent ev;
+            ev.type = Cas::CasEventType::RefDrop;
+            ev.namespace_ = key.ns.string();
+            ev.ref_name = key.ref;
+            ev.object_kind = Cas::CasEventObjectKind::Manifest;
+            ev.object_hash = Cas::manifestRefDebugString(expected);
+            ev.at_version = txn_id.ref_sequence;
+            ev.outcome = "ok";
+            ev.reason = "dropRefIfMatches: conditional rollback removed the exact manifest this caller committed";
+            store->emitEvent(std::move(ev));
+        }
+    }
+    catch (...)
+    {
+        /// Best-effort rollback cleanup, like dropRefBestEffort: debris is GC-reclaimed, but swallowing
+        /// without a diagnostic could leave a live phantom ref after a backend outage.
+        removed = false;
+        ProfileEvents::increment(ProfileEvents::CASRefRollbackBestEffortDropFailed);
+        tryLogCurrentException(getLogger("CachedPartFolderAccess"),
+            fmt::format("CA conditional rollback dropRefIfMatches failed (ns={} ref={} expected={}); "
+                        "the ref may remain live", key.ns.string(), key.ref, Cas::manifestRefDebugString(expected)));
+    }
+    /// The read above is authoritative fresh state regardless of outcome, so any locally retained view
+    /// is invalidated unconditionally -- cheap and conservative, matching dropRefBestEffort.
+    eraseView(key);
+    return removed;
+}
+
+void CachedPartFolderAccess::dropNamespace(const Cas::RootNamespace & ns)
+{
+    store->dropNamespace(ns);
+    if (view_cache)
+    {
+        const String prefix = ns.string() + '\0';
+        view_cache->remove([&](const String & k, const auto &) { return k.starts_with(prefix); });
+    }
+    ProfileEvents::increment(ProfileEvents::CASPartFolderViewInvalidations);
+}
+
+void CachedPartFolderAccess::recordDecision(const String & cache_key, LastDecision decision,
+                                            const PartFolderView * view, bool retained) const
+{
+    if (!params.explain_enabled)
+        return;   /// Disabled diagnostics keep the read path free of journal locking and allocation.
+    std::lock_guard lock(explain_mutex);
+    if (explain_map.size() >= EXPLAIN_MAX_ENTRIES)
+        explain_map.clear();
+    auto & e = explain_map[cache_key];
+    e.last_decision = decision;
+    e.retained = retained;
+    if (view)
+    {
+        e.manifest_ref = Cas::manifestRefDebugString(view->manifestId().ref);
+        e.estimated_bytes = view->estimatedBytes();
+    }
+}
+
+size_t CachedPartFolderAccess::explainJournalSizeForTest() const
+{
+    std::lock_guard lock(explain_mutex);
+    return explain_map.size();
+}
+
+CachedPartFolderAccess::ExplainResult CachedPartFolderAccess::explain(const PartRefKey & key) const
+{
+    ExplainResult result;
+    {
+        std::lock_guard lock(explain_mutex);
+        const auto it = explain_map.find(key.cacheKey());
+        if (it != explain_map.end())
+            result = it->second;
+    }
+    /// `retained` is reported live against the cache, not from the decision snapshot: `dropNamespace`
+    /// erases every key of a namespace via one `CacheBase::remove` predicate sweep without a per-key
+    /// `recordDecision` call, so a snapshot value would go stale for every key it touches except the
+    /// one last read. A live membership check is authoritative for every eraser (write-through or
+    /// namespace-wide) and costs one more `CacheBase` lookup on this test/log-only path.
+    result.retained = view_cache && view_cache->get(key.cacheKey()) != nullptr;
+    return result;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h
new file mode 100644
index 000000000000..cfa81f83662a
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h
@@ -0,0 +1,411 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/CacheBase.h>
+#include <Common/CurrentMetrics.h>
+#include <base/types.h>
+#include <functional>
+#include <future>
+#include <map>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Stable identity of a committed part or projection folder: its owning root namespace and
+/// committed-ref name (for example, a part name or `detached/<part>`). The key is used for both
+/// storage operations and retained-view indexing, so its equality and cache-key representations
+/// must describe exactly the same ref.
+struct PartRefKey
+{
+    Cas::RootNamespace ns{""};
+    String ref;
+
+    bool operator==(const PartRefKey & o) const { return ns.string() == o.ns.string() && ref == o.ref; }
+
+    /// Canonical map key. '\0' cannot occur in namespace strings or ref names (both derive from
+    /// disk paths), so the join is unambiguous even though refs may contain '/'.
+    String cacheKey() const { return ns.string() + '\0' + ref; }
+};
+
+/// Exact record of what one commit-shaped primitive (`promoteBuild`/`repointRef`) durably committed,
+/// derived INSIDE the primitive's own `appendRefOps` builder rather than read back with a separate
+/// call afterward -- a post-read would race against a concurrent repoint of the SAME ref and could
+/// observe someone else's commit instead of this caller's own. `created` distinguishes a first-time
+/// bind (no prior committed row for `ref`) from a repoint of an already-committed one; `manifest_ref`
+/// is the manifest now bound to `ref` (unchanged from before the call on a `repointRef` byte-equal
+/// no-op). A caller that needs to roll back its own commit later can drop `ref` conditionally on this
+/// exact `manifest_ref` (`dropRefIfMatches`) instead of unconditionally (`dropRef`), which would
+/// remove whatever manifest currently occupies the name -- unsafe once a concurrent writer may have
+/// repointed it since.
+struct CommitOutcome
+{
+    RootNamespace ns;
+    String ref;
+    ManifestRef manifest_ref;
+    bool created = false;
+};
+
+/// Read-freshness policy at the part-folder access boundary. The
+/// mutable-read-vs-write-evidence distinction is carried by the METHOD, not a fourth value:
+/// mutable per-part reads call `resolve` (no manifest involved); write-path source reads call
+/// `getView`, which under ForceFresh always re-proves the manifest body (mandatory HEAD in
+/// `readManifestShared` — a fresh ref resolve alone proves ref currency, NOT body existence).
+enum class Freshness
+{
+    CachedForLoad,   /// repeated load-window reads; stale-tolerant resolve (allow_stale=true)
+    ForceFresh,      /// mutable per-part reads and write-path source reads; resolve fresh
+    StrictValidate,  /// fsck/debug: bypass retained views entirely; fresh resolve + validated read
+};
+
+
+/// Immutable snapshot of one resolved committed part/projection folder. Index-free: the decoder guarantees strictly
+/// ascending canonical path order, so file lookup is a binary search and directory listing is a
+/// contiguous range scan over the SHARED decode (`manifest` is the same object the Pool's
+/// manifest cache holds). No I/O; never mutated after construction. All answers are pure functions
+/// of the members. Every per-part file is an ordinary manifest tree entry, so a content change is
+/// a manifest change through `repointRef`; comparing manifest IDs is therefore sufficient to detect
+/// a stale retained view. The view never performs I/O or mutates the shared manifest.
+class PartFolderView
+{
+public:
+    /// Creates a view from a resolved ref and its validated shared manifest decode. The manifest
+    /// must be non-null and its entries must be strictly ascending by canonical path; this is the
+    /// ordering required by the binary-search and range-scan helpers.
+    PartFolderView(PartRefKey key_, Cas::ManifestId manifest_id_, uint64_t manifest_size_,
+                   std::shared_ptr<const Cas::PartManifest> manifest_, uint64_t validated_at_ms_);
+
+    /// Joins a fresh `Resolved` with its validated shared decode. `validated_at_ms` is supplied by
+    /// the caller after `readManifestShared` has proven the manifest body with a HEAD. Keeping the
+    /// timestamp outside this helper lets `CachedPartFolderAccess` use one injectable clock for both
+    /// the stamp and its age-window comparison.
+    static std::shared_ptr<const PartFolderView> make(
+        PartRefKey key, const Cas::Resolved & resolved,
+        std::shared_ptr<const Cas::PartManifest> manifest, uint64_t validated_at_ms);
+
+    /// Recognizes a projection directory by its last path component, `.proj` or `.tmp_proj`, and
+    /// returns the corresponding in-tree prefix. The input is the routed file path; unrelated paths
+    /// return nullopt.
+    static std::optional<std::string> projectionDirPrefix(const std::string & file);
+
+    const PartRefKey & refKey() const { return key; }
+    const Cas::ManifestId & manifestId() const { return manifest_id; }
+    const std::shared_ptr<const Cas::PartManifest> & manifest() const { return manifest_body; }
+    /// The wall-clock ms at which this view's manifest body was last proven live by a HEAD. A
+    /// refresh that changes only ref metadata carries the original stamp forward because it did not
+    /// re-prove the body.
+    uint64_t validatedAtMs() const { return validated_at_ms; }
+
+    /// Finds an entry by canonical path using the manifest's sorted-entry invariant.
+    const Cas::ManifestEntry * findFile(const String & path) const;
+    /// Returns whether the manifest contains an entry at `path`.
+    bool hasFile(const String & path) const;
+    /// Returns the logical size of an inline or blob entry, or nullopt when absent.
+    std::optional<uint64_t> fileSize(const String & path) const;
+    /// Returns inline bytes for an inline entry, or nullopt for absent and blob entries.
+    std::optional<String> inlineBytes(const String & path) const;
+    /// Lists immediate child names below `dir_prefix`; the result is not required to be sorted.
+    std::vector<String> listChildren(const String & dir_prefix) const;
+    /// Returns whether any manifest entry lies below `dir_prefix`.
+    bool hasDirectory(const String & dir_prefix) const;
+    /// Estimates the retained-cache weight, conservatively including the encoded manifest size.
+    size_t estimatedBytes() const;
+
+private:
+    PartRefKey key;
+    Cas::ManifestId manifest_id;
+    uint64_t manifest_size = 0;
+    std::shared_ptr<const Cas::PartManifest> manifest_body;
+    uint64_t validated_at_ms = 0;
+};
+
+}
+
+namespace DB::Cas { class PartWriteTxn; }
+
+namespace DB::Cas
+{
+
+/// Controls whether `ForceFresh` must re-prove the manifest body on every access. `Always` (the default)
+/// preserves the fail-closed body check; `Age` and `Never` may serve a retained view after a fresh ref
+/// resolve when its manifest ID matches. A ref resolve proves ref currency, but not that the manifest
+/// body still exists, so these modes trade that additional check for a bounded performance optimization.
+struct PartFolderValidate
+{
+    enum class Mode : uint8_t { Always, Age, Never };
+    Mode mode = Mode::Always;
+    uint64_t age_seconds = 0;    /// only meaningful for Mode::Age
+};
+
+class CachedPartFolderAccess;
+
+/// A part write that has been staged and PRECOMMITTED but not yet promoted -- the durable-but-
+/// unpromoted state made into an owned object rather than an interval inside one call
+/// (spec §relink-handle). It exists because the relink confirm has to interpose between the receiver's
+/// `+1` becoming durable and the promote, so that the source can be asked whether it still holds the
+/// manifest before the receiver commits to it.
+///
+/// The handle OWNS an open `PartWriteTxn`, and DESTRUCTION IS NOT CLEANUP at the transaction level:
+/// `~PartWriteTxn` only retires the build sequence, so a dropped precommit would keep a live-epoch
+/// binding -- one the stale-precommit sweep (prior-epoch-scoped) never reclaims and GC never touches --
+/// permanently retaining the manifest's blobs. Exactly one terminal operation is therefore owed:
+/// `promote` (commit) or `abort` (append the exact precommit removal).
+///
+/// Getting that wrong is made impossible rather than merely discouraged:
+///  - move-CONSTRUCT-only, and a move leaves the source terminal, so the duty is never held twice.
+///    Move ASSIGNMENT is deleted: overwriting a handle that still owes a terminal has no correct
+///    implementation. Discharging the duty first can FAIL (`abandon` appends through the ref lane, and
+///    the lane can be wedged or fenced), and the assignment cannot report that -- so it would either
+///    drop a cleanup owner permanently or refuse to complete an operation the language says cannot
+///    fail. Nothing needs it: the one handle that travels (the interserver relink's) is move
+///    CONSTRUCTED into place, and a contract that cannot be relied on is worse than no contract;
+///  - an explicit terminal flag, set only once the underlying operation has actually completed, so a
+///    second `promote`/`abort` is rejected with `LOGICAL_ERROR` instead of re-driving a dead
+///    transaction -- while a terminal that FAILED (an append the caller may legitimately retry) leaves
+///    the handle non-terminal;
+///  - the destructor is the last-resort guard: a handle that reaches it non-terminal aborts
+///    best-effort and logs, so a forgotten or exception-skipped terminal still appends the removal.
+class PreparedPartWrite
+{
+public:
+    PreparedPartWrite(const PreparedPartWrite &) = delete;
+    PreparedPartWrite & operator=(const PreparedPartWrite &) = delete;
+    PreparedPartWrite(PreparedPartWrite && other) noexcept;
+    PreparedPartWrite & operator=(PreparedPartWrite && other) = delete;
+    /// Best-effort abort of a handle that never reached a terminal state; never throws.
+    ~PreparedPartWrite();
+
+    /// Completes the write: the atomic precommit-to-committed owner move, plus the facade's cache
+    /// invalidation. The handle records the commit INSIDE the allocation-free region that immediately
+    /// follows the durable append, so nothing between "the ref is committed" and "this handle knows it"
+    /// can throw. On a failure that is PROVEN to have committed nothing the build is abandoned (the
+    /// catch-abandon-rethrow discipline the atomic `publishEntries` has always applied); the original
+    /// error propagates in either case.
+    CommitOutcome promote(bool allow_repoint = false);
+    /// Durably abandons the write: appends the EXACT precommit removal so the manifest's `+1` is
+    /// released. Propagates an append failure -- the caller may retry, and the destructor is the
+    /// backstop if it does not.
+    void abort();
+
+    /// Whether the owed terminal operation has already completed. A moved-from handle is terminal.
+    bool isTerminal() const { return terminal; }
+    /// Whether a `promote` attempt reached the ref lane's append and did not come back with a verdict,
+    /// i.e. the commit MAY be durable. Only meaningful after `promote` threw; false everywhere else,
+    /// including on a promote that failed its pre-append validation (proof of the negative) and on a
+    /// moved-from handle. A caller that treats a failed promote as "nothing was published" -- the
+    /// interserver relink's byte fallback -- must consult this first: doing that after a commit that
+    /// actually landed publishes the same part twice.
+    bool commitIsUnresolved() const;
+    /// The ref this write will commit to, and the manifest staged for it.
+    const PartRefKey & refKey() const { return key; }
+    const ManifestId & manifestId() const { return id; }
+
+private:
+    friend class CachedPartFolderAccess;
+    PreparedPartWrite(CachedPartFolderAccess & owner_, PartWriteTxnPtr build_, PartRefKey key_, ManifestId id_);
+
+    CachedPartFolderAccess * owner = nullptr;
+    PartWriteTxnPtr build;
+    PartRefKey key;
+    ManifestId id;
+    bool terminal = false;
+};
+
+/// Single facade for committed content-addressed part-folder access. Reads build immutable
+/// `PartFolderView`s; committed-ref mutations are facade methods so cache invalidation is write-through
+/// rather than a caller responsibility. A bounded retained-view map is consulted for `CachedForLoad`
+/// and checked against every fresh ref resolve. `cache_bytes == 0` disables retention while preserving
+/// the uncached read path. The facade is thread-safe and shared by all readers and transactions of one disk.
+class CachedPartFolderAccess
+{
+public:
+    /// Retention knobs. `cache_bytes == 0` (the unit-test
+    /// default) disables retention entirely — the disk factory default is 64 MiB.
+    struct CacheParams
+    {
+        uint64_t cache_bytes = 0;            /// 0 = retention disabled (unit-test default;
+                                             /// the DISK default is 64 MiB, set in the factory)
+        uint64_t max_entries = 10000;
+        uint64_t max_entry_bytes = 16ULL << 20;
+        /// The explain decision journal is test/log-only and its `recordDecision`
+        /// path takes a per-disk global mutex and allocates on EVERY read. Off by default so the read
+        /// hit path never pays for it; the disk factory / tests turn it on when they consult `explain`.
+        bool explain_enabled = false;
+        /// The `ForceFresh` manifest-body re-proof policy. `Always` is the fail-closed default.
+        PartFolderValidate validate;
+    };
+
+    /// `CacheParams params_ = {}` cannot be a default argument here — Clang's complete-class-
+    /// context rule requires the enclosing class (`CachedPartFolderAccess`) to be complete before a
+    /// nested class's (`CacheParams`) default member initializers can be evaluated, and a default
+    /// argument written inside the class body is evaluated too early. Two overloads sidestep it; the
+    /// single-arg form default-constructs `CacheParams` (retention disabled) out-of-line.
+    explicit CachedPartFolderAccess(Cas::PoolPtr store_);
+    /// `now_ms_fn_`: wall-clock ms, injected (tests) for the age-window comparison AND the
+    /// retained view's `validated_at_ms` stamp -- the SAME function drives both, so a test controls
+    /// each side of the comparison exactly. Defaults to `std::chrono::system_clock` (mirrors
+    /// `Cas::Gc`'s `now_ms_fn` convention) when empty.
+    CachedPartFolderAccess(Cas::PoolPtr store_, CacheParams params_, std::function<uint64_t()> now_ms_fn_ = {});
+
+    /// Resolves the ref and, when present, reads and validates its manifest into an immutable view.
+    /// `nullptr` means the ref is absent. Strict validation and the default `ForceFresh` policy reach
+    /// `readManifestShared`'s mandatory HEAD because a fresh ref resolve alone does not prove that the
+    /// manifest body still exists.
+    std::shared_ptr<const PartFolderView> getView(const PartRefKey & key, Freshness freshness) const;
+
+    /// Ref-only resolution (per-part reads, part-dir existence, publish stamps): no
+    /// manifest is read. `CachedForLoad` = stale-tolerant; other modes force-fresh. `audit` defaults to
+    /// `Emit` so every caller other than `getView` keeps emitting `RefResolve` unchanged; `getView`
+    /// passes `Deferred` and re-emits the event itself once it knows whether a warm view-cache hit
+    /// served the call without doing any real resolve work.
+    std::optional<Cas::Resolved> resolve(const PartRefKey & key, Freshness freshness,
+                                         Cas::ResolveAudit audit = Cas::ResolveAudit::Emit) const;
+    bool existsRef(const PartRefKey & key, Freshness freshness) const;
+
+    /// ==== committed part-ref writes ====
+    /// Each primitive performs the protocol operation and owns the cache side effect:
+    /// erase the affected view on success; on exception cache state is untouched — except
+    /// dropRefBestEffort, which erases even on a swallowed failure: in its destructor/rollback
+    /// context the ref's durable state is unknown, so dropping the view is the conservative
+    /// direction). Committed-ref mutations anywhere else in wiring are style-check failures.
+
+    /// Completes a staged transaction with the atomic owner move and invalidates the affected view.
+    /// `allow_repoint` permits replacing a committed ref that names a different manifest. The returned
+    /// `CommitOutcome` is exact: `created` is derived INSIDE `PartWriteTxn::promote`'s `appendRefOps`
+    /// builder (the same in-closure-output pattern as that builder's own `repoint_old`), not read back
+    /// afterward.
+    ///
+    /// `commit_recorded`, when supplied, is set to `true` inside the allocation-free region that
+    /// immediately follows the durable append -- before the `CommitOutcome` is finished and before the
+    /// cache invalidation, both of which allocate and may therefore throw. It exists so a caller
+    /// holding a terminal duty (`PreparedPartWrite`) can record "committed" with nothing throwable in
+    /// between; without it an allocation failure in the post-commit work lands in the caller's
+    /// failed-promote handler with the ref already published.
+    CommitOutcome promoteBuild(Cas::PartWriteTxn & build, const PartRefKey & key, UInt128 build_id,
+                      const Cas::ManifestId & manifest_id, bool allow_repoint = false,
+                      bool * commit_recorded = nullptr);
+    /// The first half of the committed-publish sequence: adopt evidence over `entries`, stage a fresh
+    /// manifest, and precommit it -- then STOP. The receiver's `+1` is durable, the promote is deferred
+    /// until the caller has proven the source still holds the manifest (spec §relink-handle). The
+    /// returned handle OWNS the open transaction and must be either `promote`d or `abort`ed --
+    /// destruction alone is not cleanup at the transaction level (`~PartWriteTxn` only retires the
+    /// build sequence), which is why the handle's own destructor aborts as a last resort. A failure
+    /// inside `prepareEntries` itself abandons the build before propagating, so no handle is returned
+    /// and no precommit is leaked.
+    PreparedPartWrite prepareEntries(const PartRefKey & dst, const std::vector<Cas::ManifestEntry> & entries,
+                        Cas::ProvenanceOp op);
+    /// Performs the shared committed-publish sequence: adopt evidence over
+    /// `entries`, stage a fresh manifest, precommit it, and promote it. The new manifest is fully
+    /// prepared before the committed ref is moved. Returns `promoteBuild`'s exact `CommitOutcome`.
+    /// Implemented as `prepareEntries` immediately followed by `promote`, so the atomic callers and the
+    /// confirm-interposed relink path share one protocol sequence.
+    CommitOutcome publishEntries(const PartRefKey & dst, const std::vector<Cas::ManifestEntry> & entries,
+                        Cas::ProvenanceOp op, bool allow_repoint = false);
+    /// Moves a committed ref by publishing the source entries at `dst` and then dropping `src`.
+    /// Returns false when the source is absent; a pre-existing destination with different content
+    /// is rejected rather than silently discarding the source.
+    bool republishRef(const PartRefKey & src, const PartRefKey & dst);
+    /// Republishes an already committed part with `entries` (for a standalone write or removal):
+    /// republishes `key`'s manifest with `entries`. Byte-equal candidate (same decoded entries as the
+    /// currently committed manifest) is a ZERO-pool-mutation no-op, returns false. Otherwise republishes
+    /// a byte-equal candidate returns false without pool mutation; an effective repoint publishes
+    /// through `publishEntries(allow_repoint=true)`, emits the repoint audit signals, invalidates the
+    /// cached view, and returns the exact `CommitOutcome` (`created` is false on both the byte-equal
+    /// no-op path -- `manifest_ref` names the manifest ALREADY committed, unchanged -- and on an
+    /// effective repoint of an existing ref). `key` must already resolve.
+    CommitOutcome repointRef(const PartRefKey & key, std::vector<Cas::ManifestEntry> entries, Cas::ProvenanceOp op);
+    /// Drops a committed ref and invalidates its retained view after the drop succeeds.
+    void dropRef(const PartRefKey & key);
+    /// Idempotent removal: absent ref is success; a drop racing between resolve and the shard
+    /// re-read (FILE_DOESNT_EXIST) is success too — the removal unit is replay-safe.
+    void dropRefIfPresent(const PartRefKey & key);
+    /// Best-effort destructor/rollback cleanup: never throws, logs failure, and relies on GC to reclaim
+    /// lingering debris. The view is invalidated even when the durable ref state becomes unknown.
+    void dropRefBestEffort(const PartRefKey & key) noexcept;
+    /// Conditional rollback drop: removes `key`'s committed ref ONLY if its CURRENT committed manifest
+    /// binding equals `expected` (typically the `manifest_ref` from a `CommitOutcome` this caller
+    /// itself just produced). A ref repointed by someone else since then -- or already absent -- is
+    /// left untouched; the caller's stale rollback attempt must never clobber a newer commit. Reads
+    /// the committed binding and emits the removal op inside ONE `appendRefOps` builder (mirrors
+    /// `dropRef`'s underlying protocol call, guarded by the equality check inside the same closure).
+    /// `noexcept`: best-effort rollback context, like `dropRefBestEffort` -- swallows and logs a
+    /// failure rather than propagating it. Returns whether it actually removed the ref.
+    bool dropRefIfMatches(const PartRefKey & key, const ManifestRef & expected) noexcept;
+    /// Drops all refs in a namespace and removes every retained view belonging to that namespace.
+    void dropNamespace(const Cas::RootNamespace & ns);
+
+    /// ==== diagnostics ====
+    enum class LastDecision : uint8_t
+    { Hit, Miss, OversizedBypass, StrictBypass, ForceFreshRead, Invalidated };
+    struct ExplainResult
+    {
+        bool retained = false;             /// whether the last-served view is currently retained
+        LastDecision last_decision = LastDecision::Miss;
+        String manifest_ref;               /// manifestRefDebugString of the last-served view
+        size_t estimated_bytes = 0;
+    };
+    /// Returns the test/log-only decision record for `key`; an absent key yields the default result.
+    ExplainResult explain(const PartRefKey & key) const;
+
+    /// Test-only seam (nothing installs it in production): fires in `promoteBuild` immediately after
+    /// the commit has been recorded and before the throwable post-commit work (`CommitOutcome`
+    /// assembly's copies, `eraseView`). A test uses it to model an allocation failure in exactly that
+    /// window and assert that the caller's terminal duty is already discharged.
+    void setPostCommitProbeForTest(std::function<void()> fn) { post_commit_probe_for_test = std::move(fn); }
+    /// Test-only: number of entries in the decision journal (0 whenever explain is disabled).
+    size_t explainJournalSizeForTest() const;
+
+private:
+    Cas::PoolPtr store;
+    CacheParams params;
+    /// Wall-clock milliseconds; see the constructor comment. `std::function::operator` is const, so this is
+    /// callable from const methods (`getView`, `buildView`) without a `mutable` qualifier.
+    std::function<uint64_t()> now_ms_fn;
+
+    /// Supplies the conservative encoded-manifest weight used by `CacheBase` for eviction decisions.
+    struct ViewWeight
+    {
+        size_t operator()(const PartFolderView & v) const { return v.estimatedBytes(); }
+    };
+    using ViewCache = CacheBase<String, PartFolderView, std::hash<String>, ViewWeight>;
+
+    /// nullptr <=> retention disabled (cache_bytes == 0): same call graph, no retained map.
+    std::unique_ptr<ViewCache> view_cache;
+
+    /// Single-flight per PartRefKey for the build path: concurrent cold builders of the same key
+    /// share ONE readManifestShared. NEVER held across I/O — the map only hands out futures.
+    mutable std::mutex inflight_mutex;
+    mutable std::unordered_map<String, std::shared_future<std::shared_ptr<const PartFolderView>>> inflight;
+
+    /// Reads a manifest and constructs a view. Cold `CachedForLoad` builds are single-flight per key;
+    /// fresh modes perform their own read so each call retains its validation guarantee.
+    std::shared_ptr<const PartFolderView> buildView(
+        const PartRefKey & key, const Cas::Resolved & resolved, Freshness freshness) const;
+    /// Removes a retained view and records the invalidation for diagnostics.
+    void eraseView(const PartRefKey & key);
+    /// Emits the same `RefResolve` `CasEvent` `CasRefLedger::resolveRef` would have emitted for
+    /// `resolved`, a no-op when no sink is installed. Used by `getView`, which defers the emit from its
+    /// own `resolve(..., ResolveAudit::Deferred)` call so it can skip it on a warm view-cache hit that
+    /// served without any real resolve work.
+    void emitResolveEvent(const PartRefKey & key, const Cas::Resolved & resolved) const;
+
+    /// Decision journal for `explain` (test/log-only). Bounded by wholesale
+    /// clear — debug state, never consulted by the read/write paths.
+    static constexpr size_t EXPLAIN_MAX_ENTRIES = 10000;
+    mutable std::mutex explain_mutex;
+    mutable std::unordered_map<String, ExplainResult> explain_map;
+    /// Records the latest diagnostic decision without affecting read or write behavior.
+    void recordDecision(const String & cache_key, LastDecision decision,
+                        const PartFolderView * view, bool retained) const;
+
+    /// See `setPostCommitProbeForTest`. Empty in production.
+    std::function<void()> post_commit_probe_for_test;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.cpp
new file mode 100644
index 000000000000..fc27132b211c
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.cpp
@@ -0,0 +1,400 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.h>
+#include <algorithm>
+#include <array>
+#include <string_view>
+#include <utility>
+
+namespace DB::Cas
+{
+
+/// Split a path into non-empty components, treating repeated or leading/trailing '/' characters as
+/// separators. The parser deliberately does not normalize or otherwise interpret components: path
+/// classification must be based on the names supplied by the disk layer.
+static std::vector<std::string> splitNonEmpty(const std::string & path)
+{
+    std::vector<std::string> parts;
+    std::string cur;
+    for (char c : path)
+    {
+        if (c == '/')
+        {
+            if (!cur.empty())
+                parts.push_back(std::move(cur));
+            cur.clear();
+        }
+        else
+        {
+            cur.push_back(c);
+        }
+    }
+    if (!cur.empty())
+        parts.push_back(std::move(cur));
+    return parts;
+}
+
+namespace
+{
+
+/// The split of a disk-relative path into non-empty components is the dominant allocation of every
+/// path classifier, and the CA metadata read path runs SEVERAL of them on the SAME raw path per
+/// logical file-open (each of `existsFile` / `getFileSize` / `getStorageObjects` first calls
+/// `isPartFilePath`, then `parsePartFilePath`). The split is a PURE function of the path, so a small
+/// thread-local FIFO ring cache keyed on the raw path is always correct, disk-agnostic and
+/// lock-free. It is a fixed-capacity round-robin ring, NOT an LRU/MRU: a hit does not move or
+/// promote its slot, so under sustained eviction pressure a path can be re-split on a later call
+/// even if it was seen recently — always still CORRECT (re-splitting just re-derives the same
+/// result), only less effective as a cache. The returned reference stays valid until the next
+/// `splitCached` call on the SAME thread; every classifier consumes its split before splitting
+/// again (none splits while holding another's split).
+struct SplitCache
+{
+    static constexpr size_t kCapacity = 8;
+    std::array<std::pair<std::string, std::vector<std::string>>, kCapacity> slots;
+    size_t count = 0;  /// populated slots (<= kCapacity)
+    size_t next = 0;   /// round-robin insertion cursor
+    size_t misses = 0; /// underlying `splitNonEmpty` invocations (observability / test oracle)
+
+    /// Return the cached split for `path`, or replace the next round-robin slot with a newly split
+    /// value. A hit does not promote its slot, so this is a fixed-capacity FIFO-style ring rather
+    /// than an LRU. The returned reference remains valid until the next `get` call on this thread.
+    const std::vector<std::string> & get(const std::string & path)
+    {
+        for (size_t i = 0; i < count; ++i)
+            if (slots[i].first == path)
+                return slots[i].second;
+        ++misses;
+        auto & slot = slots[next];
+        slot.first = path;
+        slot.second = splitNonEmpty(path);
+        next = (next + 1) % kCapacity;
+        if (count < kCapacity)
+            ++count;
+        return slot.second;
+    }
+};
+
+thread_local SplitCache tls_split_cache;
+
+const std::vector<std::string> & splitCached(const std::string & path)
+{
+    // Every caller consumes the returned components before asking for another split on this
+    // thread, so the cache may safely reuse its ring slots between classifier invocations.
+    return tls_split_cache.get(path);
+}
+
+}
+
+/// Whether every character of `s` is a lowercase hex digit.
+static bool isLowerHex(std::string_view s)
+{
+    return std::all_of(s.begin(), s.end(), [](char c) { return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f'); });
+}
+
+/// Whether `s` has the exact shape of a canonical UUID string: 36 characters, dashes at positions
+/// 8/13/18/23, lowercase hex everywhere else.
+static bool looksLikeUuidDirName(std::string_view s)
+{
+    if (s.size() != 36)
+        return false;
+    for (size_t i = 0; i < 36; ++i)
+    {
+        const bool dash_pos = (i == 8 || i == 13 || i == 18 || i == 23);
+        if (dash_pos != (s[i] == '-'))
+            return false;
+        if (!dash_pos && !((s[i] >= '0' && s[i] <= '9') || (s[i] >= 'a' && s[i] <= 'f')))
+            return false;
+    }
+    return true;
+}
+
+/// Locate the `<uuid[:3]>/<uuid>` anchor inside a split Atomic path. The leading prefix is normally
+/// `store`, but may be absent from a disk-relative path, so the parser identifies the pair by its
+/// shape instead of requiring a particular prefix. Return the index of the UUID component; the
+/// component immediately after it is the part directory when one exists.
+static std::optional<size_t> findTableUuidComponent(const std::vector<std::string> & p)
+{
+    for (size_t i = 1; i < p.size(); ++i)
+    {
+        const auto & prefix = p[i - 1];
+        const auto & uuid = p[i];
+        /// Shape-based on purpose (robust to a missing `store/`), but the shape is now the REAL
+        /// Atomic one: 3 lowercase-hex chars followed by a well-formed UUID sharing that prefix —
+        /// a 3-char database named like its table (`data/abc/abcxyz/...`) no longer false-anchors.
+        if (prefix.size() == 3 && isLowerHex(prefix) && looksLikeUuidDirName(uuid)
+            && uuid.compare(0, 3, prefix) == 0)
+            return i;
+    }
+    return std::nullopt;
+}
+
+/// Return whether a component has the MergeTree part-directory grammar. Non-Atomic layouts do not
+/// have an Atomic UUID anchor, so their part boundary is found from the final three non-empty
+/// decimal underscore-separated groups: `_min_max_level`, optionally preceded by a mutation number.
+/// The grammar also covers temporary and operation prefixes such as `tmp_insert_all_1_1_0` and
+/// `delete_tmp_all_1_1_0`; this helper has no Storage dependency and is used only as the non-Atomic
+/// fallback.
+static bool looksLikePartDir(const std::string & name)
+{
+    std::vector<std::string> groups;
+    std::string cur;
+    for (char c : name)
+    {
+        if (c == '_')
+        {
+            groups.push_back(cur);
+            cur.clear();
+        }
+        else
+            cur.push_back(c);
+    }
+    groups.push_back(cur);
+
+    // Need at least <partition>_<min>_<max>_<level>: a partition group plus 3 trailing numeric groups.
+    if (groups.size() < 4)
+        return false;
+
+    auto is_number = [](const std::string & s)
+    {
+        if (s.empty())
+            return false;
+        for (char c : s)
+            if (c < '0' || c > '9')
+                return false;
+        return true;
+    };
+
+    const size_t n = groups.size();
+    return is_number(groups[n - 1]) && is_number(groups[n - 2]) && is_number(groups[n - 3]);
+}
+
+/// Describes the boundary found by `findPartDirComponent`: components in [table_start, part_idx)
+/// form the table identifier, `part_idx` names the part or reserved part container, and all later
+/// components form the in-part file path. Atomic identifiers contain one UUID component; non-Atomic
+/// identifiers contain the complete `data/<db>/<tbl>` prefix.
+struct PartDirAnchor
+{
+    size_t table_start;
+    size_t part_idx;
+};
+
+/// Locate the part-directory component. Prefer the UUID anchor for Atomic paths. Without it, treat
+/// the first reserved `detached` or `moving` component after the table root as the boundary, then
+/// fall back to the rightmost component matching the part-directory grammar. The reserved-directory
+/// scan must precede the right-to-left grammar scan: otherwise the inner part name would be mistaken
+/// for the boundary and the reserved directory would become part of a spurious table namespace that
+/// table cleanup does not own. Return nullopt when the path has no part component.
+static std::optional<PartDirAnchor> findPartDirComponent(const std::vector<std::string> & p)
+{
+    if (auto uuid_idx = findTableUuidComponent(p))
+    {
+        const size_t part_idx = *uuid_idx + 1;
+        if (part_idx < p.size())
+        {
+            // The reserved deduplication-log directory is a table-level subdir, not a part dir
+            // (see kDeduplicationLogsDirName).
+            if (p[part_idx] == kDeduplicationLogsDirName)
+                return std::nullopt;
+            return PartDirAnchor{*uuid_idx, part_idx}; // table id = the single <uuid> component
+        }
+        return std::nullopt; // table dir, no part component after the uuid
+    }
+
+    // No uuid anchor: a non-Atomic table path. `detached` (data/<db>/<table>/detached/<part>/...)
+    // and `moving` (data/<db>/<table>/moving/<part>/...) are both reserved table-level subdirs,
+    // exactly like the Atomic layout where the uuid anchor makes them the part_name for free.
+    // Anchor on either FIRST (leftmost, index >= 1): the right-to-left part-dir scan below would
+    // otherwise anchor on the INNER <part>-shaped component and fold the reserved dir into a
+    // spurious table id (data/<db>/<table>/detached or .../moving) that DROP TABLE never cleans,
+    // orphaning a permanently-live ref. Mirrors
+    // route()'s part_name == kDetachedDirName / kMovingDirName folding.
+    for (size_t i = 1; i < p.size(); ++i)
+        if (p[i] == kDetachedDirName || p[i] == kMovingDirName)
+            return PartDirAnchor{0, i}; // table id = the whole path before the reserved dir
+
+    // A non-Atomic database or table literally named `detached` is necessarily interpreted as the
+    // reserved directory. The two shapes are indistinguishable from a path string alone; resolving
+    // the ambiguity requires caller-supplied knowledge of existing databases and tables, which this
+    // pure string parser intentionally does not have. The reserved interpretation is retained so
+    // ordinary detached-part paths continue to map into the real table namespace. The test
+    // `CasPartPathParser.DetachedNamedTableIsKnownAmbiguityFoldedAsReservedDir` pins this behavior so
+    // any future change here is a conscious one.
+
+    // The table identifier must be at least one component (a real table dir, never the bare disk
+    // root), so the part dir is at index >= 1. Scan right to left so a part-dir-shaped
+    // table/partition name earlier in the path cannot steal the anchor.
+    for (size_t i = p.size(); i-- > 1;)
+        if (looksLikePartDir(p[i]))
+            return PartDirAnchor{0, i}; // table id = the whole path before the part dir
+    return std::nullopt;
+}
+
+/// Join components [start, end) with '/' into the stable table identifier used by the routing layer:
+/// one UUID component for Atomic paths or the complete `data/<db>/<tbl>` path for non-Atomic paths.
+static std::string joinTableId(const std::vector<std::string> & p, size_t start, size_t end)
+{
+    std::string id;
+    for (size_t i = start; i < end; ++i)
+    {
+        if (!id.empty())
+            id += "/";
+        id += p[i];
+    }
+    return id;
+}
+
+/// Return the number of underlying `splitNonEmpty` invocations on the current thread for cache
+/// observability and tests.
+size_t splitCacheMissesForTest()
+{
+    return tls_split_cache.misses;
+}
+
+void resetSplitCacheForTest()
+{
+    tls_split_cache = SplitCache{};
+}
+
+std::optional<PartFilePath> parsePartFilePath(const std::string & path)
+{
+    const auto & p = splitCached(path);
+    auto anchor = findPartDirComponent(p);
+    if (!anchor)
+        return std::nullopt;
+
+    PartFilePath r;
+    r.table_uuid = joinTableId(p, anchor->table_start, anchor->part_idx);
+    r.part_name = p[anchor->part_idx];
+    if (anchor->part_idx + 1 < p.size())
+    {
+        std::string file = p[anchor->part_idx + 1];
+        for (size_t i = anchor->part_idx + 2; i < p.size(); ++i)
+            file += "/" + p[i];
+        r.file = file;
+    }
+    // FREEZE target: shadow/<backup_name>/.../<part>. Capture both the backup name (the component
+    // right after the reserved "shadow" root) and the literal shadow table dir — the joined
+    // components before the part dir — for the commit / read / remove routing. The inner uuid
+    // anchor above is unaffected by the prefix.
+    if (p.size() >= 2 && p[0] == kShadowDirName)
+    {
+        r.backup_name = p[1];
+        r.shadow_table_dir = joinTableId(p, 0, anchor->part_idx);
+    }
+    return r;
+}
+
+std::optional<std::string> parseTableUuid(const std::string & path)
+{
+    const auto & p = splitCached(path);
+
+    // Atomic layout: exactly the table dir <prefix...>/<uuid[:3]>/<uuid>[/] — nothing after the uuid.
+    if (auto uuid_idx = findTableUuidComponent(p); uuid_idx && *uuid_idx + 1 == p.size())
+        return p[*uuid_idx];
+
+    // Non-Atomic layout: a directory path with no part-dir component is the table dir
+    // data/<db>/<table>. Require at least two components so the bare disk root (or a single generic
+    // dir) is never taken as a table dir.
+    if (findTableUuidComponent(p))
+        return std::nullopt; // had a uuid anchor but something followed it: not a table dir
+    if (p.size() >= 2 && !findPartDirComponent(p))
+        return joinTableId(p, 0, p.size());
+    return std::nullopt;
+}
+
+bool isAtomicShardDir(const std::string & path)
+{
+    // The Atomic on-disk layout shards table dirs as `store/<u3>/<uuid>@cas@`, so `store/<u3>` is a
+    // pure intermediate shard directory: the literal `store` root followed by exactly one 3-char
+    // uuid-prefix component, with nothing after it. This is ambiguous with the non-Atomic
+    // data/<db> fallback (both are two non-part components with no uuid anchor), so the router uses
+    // this strict predicate to disambiguate before parseTableUuid.
+    const auto & p = splitCached(path);
+    return p.size() == 2 && p[0] == "store" && p[1].size() == 3;
+}
+
+bool endsWithTableUuidPair(const std::string & path)
+{
+    const auto & p = splitCached(path);
+    auto uuid_idx = findTableUuidComponent(p);
+    return uuid_idx && *uuid_idx + 1 == p.size();
+}
+
+bool isPartFilePath(const std::string & path)
+{
+    // A file inside a part dir: <table_path...>/<part>/<file> => at least one component after the
+    // part dir, for both the Atomic and non-Atomic layouts.
+    const auto & p = splitCached(path);
+    auto anchor = findPartDirComponent(p);
+    return anchor && anchor->part_idx + 1 < p.size();
+}
+
+std::optional<TableFilePath> parseTableFilePath(const std::string & path)
+{
+    const auto & p = splitCached(path);
+
+    // Atomic layout: a table-level file lives under the table dir, i.e. at least one component
+    // after the uuid. The tail is EVERYTHING after the uuid joined by '/', so a table-level file in
+    // a subdirectory (deduplication_logs/deduplication_log_1.txt) keeps its full sub-path. A part
+    // file is excluded earlier by isPartFilePath; this function is only reached for non-part paths.
+    if (auto uuid_idx = findTableUuidComponent(p))
+    {
+        if (*uuid_idx + 1 >= p.size())
+            return std::nullopt; // the bare table dir, no file tail
+        TableFilePath r;
+        r.table_uuid = p[*uuid_idx];
+        r.tail = joinTableId(p, *uuid_idx + 1, p.size());
+        return r;
+    }
+
+    // Non-Atomic layout: a path with no part-dir component whose last component is the table-level
+    // file, and the components before it are the table dir data/<db>/<table>. Require the table id
+    // to be at least one component (a real table, never the bare disk root).
+    if (p.size() < 2 || findPartDirComponent(p))
+        return std::nullopt;
+
+    // A reserved table-level subdirectory (deduplication_logs/) splits the path explicitly: the
+    // table id is everything before it, the tail is the reserved dir and everything under it.
+    // Without this the generic "last component is the file" rule would fold the subdir into the
+    // table id and mis-scope the log objects. Index >= 1 so the table id is never the bare root.
+    for (size_t i = 1; i + 1 < p.size(); ++i)
+    {
+        if (p[i] == kDeduplicationLogsDirName)
+        {
+            TableFilePath r;
+            r.table_uuid = joinTableId(p, 0, i);
+            r.tail = joinTableId(p, i, p.size());
+            return r;
+        }
+    }
+
+    TableFilePath r;
+    r.table_uuid = joinTableId(p, 0, p.size() - 1);
+    r.tail = p.back();
+    return r;
+}
+
+std::string mirroredArchiveNamespace(const std::string & table_uuid)
+{
+    if (table_uuid.find('/') == std::string::npos)
+    {
+        /// Atomic: a bare uuid; mirror ClickHouse's store/<u3>/<uuid> fanout.
+        const std::string u3 = table_uuid.substr(0, 3);
+        return "store/" + u3 + "/" + table_uuid + std::string(kCasArchiveSuffix);
+    }
+    /// Non-Atomic: a full data/<db>/<tbl> path already; append the suffix to the last segment.
+    return table_uuid + std::string(kCasArchiveSuffix);
+}
+
+bool isShadowPath(const std::string & path)
+{
+    size_t i = 0;
+    while (i < path.size() && path[i] == '/')
+        ++i;
+    const auto first_end = path.find('/', i);
+    const std::string_view first = first_end == std::string::npos
+        ? std::string_view(path).substr(i)
+        : std::string_view(path).substr(i, first_end - i);
+    return first == kShadowDirName;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.h
new file mode 100644
index 000000000000..ab509e837452
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.h
@@ -0,0 +1,146 @@
+#pragma once
+#include <optional>
+#include <string>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Classifies disk-relative ClickHouse paths for the content-addressed metadata-storage wiring.
+/// It handles Atomic and non-Atomic table layouts, FREEZE shadow trees, detached and moving part
+/// directories, table-level deduplication logs, projections, and temporary or operation part names.
+/// The functions are pure and side-effect-free: they identify the table, part, and file portions,
+/// while content-addressed key construction remains in `Cas::Layout`. The `Cas` core therefore
+/// never needs to understand ClickHouse path conventions.
+
+/// The literal first path component reserved for FREEZE snapshots
+/// (shadow/<backup>/store/<uuid[:3]>/<uuid>/<part>/...).
+inline constexpr std::string_view kShadowDirName = "shadow";
+
+/// The MergeTree detached-parts directory. parsePartFilePath reports a detached path with
+/// part_name == kDetachedDirName and the real detached part dir as the FIRST component of `file`.
+/// The transaction and read routing re-split that `file` value to recover the detached part's
+/// actual name and in-part path.
+inline constexpr std::string_view kDetachedDirName = "detached";
+
+/// The MergeTree part-mover staging directory (`MergeTreeData::MOVING_DIR_NAME`,
+/// `MergeTreeData.h`). A part being relocated to another disk (explicit `ALTER … MOVE
+/// PART|PARTITION`, or a background TTL/policy move) is cloned under TABLE/moving/<part>/
+/// before the atomic rename into its final place. `parsePartFilePath` reports such a path with
+/// part_name == kMovingDirName and the real part dir as the FIRST component of `file` -- the
+/// exact same shape `kDetachedDirName` already produces, for free, on the Atomic layout (no parser
+/// change is needed there: "moving" already lands on `part_idx`
+/// because it is the component right after the table <uuid>, same as "detached"). Mirroring
+/// detached, `route()` folds this onto a `moving/`-PREFIXED ref (kMovingRefPrefix) -- NOT the
+/// part's final ref directly. Publishing the clone under the final ref would break move
+/// crash-atomicity: a crash between the clone commit and the mover's rename would leave a
+/// committed live ref before the swap ever happened, and `moving/`'s own startup cleanup
+/// couldn't tell that premature ref apart from a real live part. The staging ref keeps the
+/// pre-swap clone un-live; the mover's rename does a real ref repoint moving/<part> -> <part>.
+inline constexpr std::string_view kMovingDirName = "moving";
+
+/// Detached parts live inside the table's own archive namespace as refs keyed by this prefix —
+/// `detached/PART` versus a live `PART`. One namespace per table; the live-vs-detached
+/// name collision is impossible because the ref names differ. The routing prepends this to the
+/// detached part name to form the ref, and the `TABLE/detached` container dir surfaces the
+/// table's refs filtered to this prefix (stripped for display). No parallel detached namespace
+/// exists anymore (the old `detachedNamespace` is gone).
+inline constexpr std::string_view kDetachedRefPrefix = "detached/";
+
+/// MOVE-to-CA fix: mirrors kDetachedRefPrefix exactly, but for the mover's `moving/` staging
+/// dir instead of `detached/`. Keeps a moved-but-not-yet-swapped part's ref distinct from its
+/// eventual live ref `<part>`, so the destination CA transaction (`clonePart`'s CA branch) can
+/// publish it WITHOUT prematurely making it live or colliding with an existing live part of the
+/// same name.
+inline constexpr std::string_view kMovingRefPrefix = "moving/";
+
+/// The content-addressing boundary marker: a SUFFIX on a table-dir segment (`…/<uuid>@cas@`), not a
+/// path segment. It marks where the mirrored ClickHouse path ends and the content-addressed archive
+/// begins — like a `.zip` extension (`foo.zip/inner/file`). `@` is S3-safe and never occurs in
+/// ClickHouse uuids, part names, detached prefixes, projection names, or column files, so it cannot
+/// collide with real path data. Namespace discovery comes from the catalog, not path classification.
+inline constexpr std::string_view kCasArchiveSuffix = "@cas@";
+
+/// Compose the mirrored content-addressed archive path for a table identifier as the parser reports
+/// it. Atomic tables report the bare `<uuid>` → reconstruct `store/<u3>/<uuid>@cas@` (u3 = first 3
+/// chars, matching ClickHouse's store fanout). Non-Atomic tables report the full joined
+/// `data/<db>/<tbl>` path → append `@cas@` to it verbatim. The `@cas@` suffix lands on the
+/// table-dir (last) segment in both cases. Pure; no ClickHouse dependency.
+std::string mirroredArchiveNamespace(const std::string & table_uuid);
+
+/// Reserved table-level subdirectory: TABLE_DIR/deduplication_logs/FILE is structurally
+/// indistinguishable from a part file in the Atomic layout, so the name is reserved — never a part
+/// dir; its contents are table-level verbatim files. ClickHouse part names never take this form.
+inline constexpr std::string_view kDeduplicationLogsDirName = "deduplication_logs";
+
+/// The result of splitting a part path. `table_uuid` is a bare UUID for an Atomic table and the
+/// joined `data/<db>/<tbl>` path for a non-Atomic table. For detached or moving paths,
+/// `part_name` is the reserved directory name and `file` begins with the actual part directory;
+/// this preserves the on-disk shape expected by the routing layer. FREEZE paths additionally
+/// retain both the backup name and the literal shadow table directory for shadow-specific routing.
+struct PartFilePath
+{
+    std::string table_uuid;
+    std::string part_name;
+    std::string file; /// empty when the path is a part directory
+    /// Set to the backup name when the path is a FREEZE target shadow/<backup_name>/.../<part>[/<file>].
+    /// Empty for a normal live-part path.
+    std::string backup_name;
+    /// Set (alongside backup_name) for a FREEZE target: the LITERAL shadow table dir under the disk
+    /// root excluding the part and file (shadow/<backup>/store/<uuid[:3]>/<uuid>). Empty otherwise.
+    std::string shadow_table_dir;
+};
+
+/// Parse a disk-relative ClickHouse path to its (table, part, in-part file) split. Anchors on the
+/// Atomic <uuid[:3]>/<uuid> pair anywhere in the path (robust to a leading store/ or shadow/
+/// prefix); falls back to the RIGHTMOST part-dir-shaped component for non-Atomic layouts
+/// (data/db/table/part/...). Returns nullopt for the table dir or shallower.
+std::optional<PartFilePath> parsePartFilePath(const std::string & path);
+
+/// Returns the table identifier iff path is exactly a table dir: the bare <uuid> for the Atomic
+/// layout, the full joined data/db/table path for non-Atomic.
+std::optional<std::string> parseTableUuid(const std::string & path);
+
+/// True iff the path is an Atomic-layout INTERMEDIATE shard directory `store/<u3>`, where <u3> is a
+/// 3-character uuid prefix (the only child it has on disk is a uuid-anchored `<u3>/<uuid>` table
+/// dir). This shape is ambiguous with the non-Atomic `data/<db>` fallback of parseTableUuid, so the
+/// metadata router must consult it FIRST and treat `store/<u3>` as a generic intermediate dir to be
+/// enumerated by a mirrored LIST — never as a non-Atomic table id.
+bool isAtomicShardDir(const std::string & path);
+
+/// Strict "this dir IS a uuid-anchored table dir" predicate: the path's LAST two components form
+/// an Atomic <uuid[:3]>/<uuid> pair. Unlike parseTableUuid it rejects the non-Atomic fallback —
+/// the shadow router uses it to tell a shadow TABLE dir from a shadow INTERMEDIATE dir.
+bool endsWithTableUuidPair(const std::string & path);
+
+/// True iff the path addresses a file INSIDE a part dir (content-addressed). Table-level files
+/// (format_version.txt, deduplication_logs/...) and generic disk files are excluded.
+bool isPartFilePath(const std::string & path);
+
+/// The result of splitting a table-level file path. `table_uuid` uses the same Atomic versus
+/// non-Atomic representation as `PartFilePath`, while `tail` preserves the complete path below
+/// that table directory, including a nested `deduplication_logs/` prefix when present.
+struct TableFilePath
+{
+    std::string table_uuid;
+    std::string tail; /// path beyond the table dir, full sub-path preserved
+};
+
+/// Parse a non-part table-level file path. Returns nullopt for the bare table dir, shallower
+/// paths, part files, and generic disk-root files (e.g. clickhouse_access_check_*).
+std::optional<TableFilePath> parseTableFilePath(const std::string & path);
+
+/// True iff the path's FIRST component is the reserved FREEZE shadow root. Routed BEFORE the
+/// live-table branches (a shadow table dir also satisfies parseTableUuid).
+bool isShadowPath(const std::string & path);
+
+/// Return the number of underlying `splitNonEmpty` invocations on the current thread. This is an
+/// observability seam for verifying that repeated classifiers reuse the thread-local split cache.
+size_t splitCacheMissesForTest();
+
+/// Clear the current thread's split cache and reset its miss counter. This is intended for tests;
+/// production callers must not rely on cache state surviving between operations.
+void resetSplitCacheForTest();
+
+}

From a840c03b425e61a570a69431af97cfca8eb11d04 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:35 +0200
Subject: [PATCH 18/30] CAS subsystem: GC layer

The garbage-collection round: fold, in-degree settlement, lease and
heartbeat, prune, baseline rebuild, ack-floor fencing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../ContentAddressed/Gc/CasBlobInDegree.cpp   |  723 +++
 .../ContentAddressed/Gc/CasBlobInDegree.h     |  413 ++
 .../ContentAddressed/Gc/CasGc.cpp             | 4661 +++++++++++++++++
 .../ContentAddressed/Gc/CasGc.h               | 1012 ++++
 .../Gc/CasGcMaintenanceState.cpp              |   40 +
 .../Gc/CasGcMaintenanceState.h                |   29 +
 .../ContentAddressed/Gc/CasGcPhaseTimer.h     |   86 +
 .../ContentAddressed/Gc/CasGcScheduler.cpp    |  419 ++
 .../ContentAddressed/Gc/CasGcScheduler.h      |  227 +
 .../ContentAddressed/Gc/CasGcShardPlan.cpp    |   65 +
 .../ContentAddressed/Gc/CasGcShardPlan.h      |  138 +
 .../Gc/CasNamespaceJanitor.cpp                |  143 +
 .../ContentAddressed/Gc/CasNamespaceJanitor.h |   34 +
 .../Gc/CasOrphanManifestSweep.cpp             |  915 ++++
 .../Gc/CasOrphanManifestSweep.h               |  206 +
 .../Gc/CatalogLifecycleReconciler.cpp         |  121 +
 .../Gc/CatalogLifecycleReconciler.h           |   64 +
 17 files changed, 9296 insertions(+)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcPhaseTimer.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.h

diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.cpp
new file mode 100644
index 000000000000..1702682a0003
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.cpp
@@ -0,0 +1,723 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/WriteBufferFromString.h>
+#include <base/defines.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <Common/logger_useful.h>
+
+namespace ProfileEvents
+{
+    extern const Event CASGCRetiredSparedByReref;
+    extern const Event CASGCUnmatchedRemoveDeltas;
+}
+#include <city.h>
+#include <algorithm>
+#include <cstring>
+#include <memory>
+#include <optional>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+    extern const int NOT_IMPLEMENTED;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+const UInt128 kZeroSourceId{0};
+
+/// Streams a shard's prior source-edge run at O(one block) resident memory: chains the run SEGMENTS the
+/// caller resolved from the parent seal (`blob_target_runs` filtered to one shard) and exposes a one-row
+/// lookahead for the fold merge. The prior run carries
+/// BOTH surviving edges (`kEdgeActive`) AND the retired `kCondemned` sentinel rows at the zero source id,
+/// so the cursor stops at edges AND at condemned rows (exposing the type via `rowType`), while zero-marker
+/// sentinels are dropped on carry (per-generation, never carried forward). Row/key invariants are enforced
+/// while streaming: `kEdgeActive` never at `source_id = 0`; sentinel rows (`kZeroMarker` /
+/// `kCondemned`) ONLY at `source_id = 0`; at most one sentinel per blob; an unknown value byte or an empty
+/// payload is `CORRUPTED_DATA`. Resolution uses the exact object references supplied by the caller, so a run
+/// sealed for generation G that physically lives under an older generation's key is reached
+/// without key construction. An empty `segments` is the fresh-pool / empty baseline. The row stream is
+/// globally sorted by (blob_hash, source_id), so `key()` values are non-decreasing.
+class PriorEdgeCursor
+{
+public:
+    /// The key codec is stateless and self-describing, so a run may freely mix supported hash algorithms.
+    PriorEdgeCursor(Backend & backend_, const std::vector<RunRef> & segments_)
+        : backend(backend_), segments(segments_)
+    {
+        advance();
+    }
+
+    bool valid() const { return has_current; }
+    const String & key() const { return current_key; }
+    /// The value byte of the current row: `kEdgeActive` (a surviving edge) or `kCondemned` (a retired
+    /// sentinel row). Zero markers are never surfaced (dropped on carry).
+    char rowType() const { return current_type; }
+    /// The decoded retired sentinel for the current row (only valid when `rowType() == kCondemned`).
+    const CondemnedRow & condemnedRow() const { return current_condemned; }
+
+    /// Advance to the next surviving edge OR retired sentinel, dropping zero markers, enforcing the
+    /// row/key invariants, and crossing segment boundaries.
+    void advance()
+    {
+        while (true)
+        {
+            /// Pull rows from the open segment until a surviving edge, retired sentinel, or the segment ends.
+            if (reader)
+            {
+                String k;
+                String p;
+                while (reader->next(k, p))
+                {
+                    BlobRef bh;
+                    UInt128 sid;
+                    /// `parse` throws CORRUPTED_DATA on a malformed size / NOT_IMPLEMENTED on an
+                    /// unknown algo byte (fail-closed).
+                    SourceEdgeKeyCodec::parse(k, bh, sid);
+                    if (p.empty())
+                        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS source-edge run: empty row payload");
+                    const char v = p[0];
+                    const bool sentinel_key = (sid == kZeroSourceId);
+
+                    if (sentinel_key)
+                    {
+                        /// A sentinel key carries exactly one row per blob and never an edge.
+                        if (v == kEdgeActive)
+                            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                                "CAS source-edge run: active edge at the reserved sentinel source_id 0");
+                        if (v != kZeroMarker && v != kCondemned)
+                            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                                "CAS source-edge run: unknown sentinel row type 0x{:02x}", static_cast<uint8_t>(v));
+                        if (have_sentinel_blob && sentinel_blob == bh)
+                            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                                "CAS source-edge run: duplicate sentinel row for one blob");
+                        have_sentinel_blob = true;
+                        sentinel_blob = bh;
+                        if (v == kZeroMarker)
+                            continue;   // A zero marker is per-generation and is dropped on carry.
+                        /// A retired sentinel: decode and surface it (settled at close-out, not an edge).
+                        current_condemned = decodeCondemnedRow(p);
+                        current_key = k;
+                        current_type = kCondemned;
+                        has_current = true;
+                        return;
+                    }
+
+                    /// A non-sentinel key must carry a surviving edge and nothing else.
+                    if (v != kEdgeActive)
+                        throw Exception(ErrorCodes::CORRUPTED_DATA,
+                            "CAS source-edge run: sentinel row type 0x{:02x} at a non-sentinel key",
+                            static_cast<uint8_t>(v));
+                    current_key = k;
+                    current_type = kEdgeActive;
+                    has_current = true;
+                    return;
+                }
+                /// Segment fully drained: verify its whole-file checksum against
+                /// the seal's RunRef.checksum BEFORE the fold acts on any of its rows. This is the fold —
+                /// the checksum's most important consumer (it decides deletions). Fail-closed on mismatch.
+                reader->verifyAgainst(segments[seg_idx].checksum);
+                reader.reset();
+                ++seg_idx;
+            }
+
+            /// Open the next resolved segment; the segment list is exhausted => the chain is done.
+            if (seg_idx >= segments.size())
+            {
+                has_current = false;
+                return;
+            }
+            /// Typed open validates the NDJSON header before any row is consumed. Each row carries its
+            /// own algorithm byte, so no separate width gate is needed.
+            reader = openSourceEdgeRun(backend, segments[seg_idx].key);
+        }
+    }
+
+private:
+    Backend & backend;
+    const std::vector<RunRef> & segments;
+
+    size_t seg_idx = 0;
+    std::optional<SourceEdgeRunView> reader;
+    String current_key;
+    char current_type = kEdgeActive;
+    CondemnedRow current_condemned;
+    bool has_current = false;
+
+    /// Duplicate-sentinel guard: the last blob for which a sentinel row was seen (across skipped zero
+    /// markers too). Rows are globally sorted, so two sentinels for one blob are adjacent.
+    bool have_sentinel_blob = false;
+    BlobRef sentinel_blob{};
+};
+
+}
+
+UInt128 sourceEdgeId(const ManifestId & id, const String & path)
+{
+    String canon;
+    canon += id.root_namespace.string();
+    canon += '\0';
+    auto beU64 = [&](uint64_t v) { for (int i = 7; i >= 0; --i) canon += static_cast<char>((v >> (8 * i)) & 0xFF); };
+    auto beU32 = [&](uint32_t v) { for (int i = 3; i >= 0; --i) canon += static_cast<char>((v >> (8 * i)) & 0xFF); };
+    beU64(id.ref.writer_epoch); beU64(id.ref.build_sequence); beU32(id.ref.manifest_ordinal);
+    canon += '\0';
+    canon += path;
+    const auto h = CityHash_v1_0_2::CityHash128(canon.data(), canon.size());
+    const UInt128 result = (static_cast<UInt128>(h.high64) << 64) | static_cast<UInt128>(h.low64);
+    if (result == UInt128{0})
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS source edge: hash collided with the reserved sentinel id 0");
+    return result;
+}
+
+void assertValidSourceEdgeId(const UInt128 & source_id)
+{
+    if (source_id == UInt128{0})
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "CAS source edge: source_id 0 is the reserved sentinel key");
+}
+
+String encodeCondemnedRow(const CondemnedRow & row)
+{
+    String out;
+    out.push_back(kCondemned);
+    out.push_back(static_cast<char>((row.delete_pending ? 1 : 0) | (row.marker_confirmed ? 2 : 0)));
+    out.push_back(static_cast<char>(row.token.type));
+    auto beU64 = [&](uint64_t v) { for (int i = 7; i >= 0; --i) out += static_cast<char>((v >> (8 * i)) & 0xFF); };
+    beU64(row.condemn_round);
+    beU64(row.size);
+    if (row.token.value.size() > 0xFFFF)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS condemned row: token too long ({})", row.token.value.size());
+    out += static_cast<char>((row.token.value.size() >> 8) & 0xFF);
+    out += static_cast<char>(row.token.value.size() & 0xFF);
+    out += row.token.value;
+    return out;
+}
+
+CondemnedRow decodeCondemnedRow(std::string_view p)
+{
+    /// [0]=0x02 [1]=flags [2]=token_type [3..10]=round [11..18]=size [19..20]=len [21..]=value
+    constexpr size_t kFixed = 21;
+    if (p.size() < kFixed || p[0] != kCondemned)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS condemned row: malformed header");
+    CondemnedRow row;
+    const uint8_t flags = static_cast<uint8_t>(p[1]);
+    if (flags > 3)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS condemned row: unknown flags 0x{:02x}", flags);
+    row.delete_pending = flags & 1;
+    row.marker_confirmed = flags & 2;
+    const uint8_t type = static_cast<uint8_t>(p[2]);
+    if (type < 1 || type > 3)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS condemned row: unknown token_type {}", type);
+    row.token.type = static_cast<TokenType>(type);
+    auto beU64 = [&](size_t off) { uint64_t v = 0; for (int i = 0; i < 8; ++i) v = (v << 8) | static_cast<uint8_t>(p[off + i]); return v; };
+    row.condemn_round = beU64(3);
+    row.size = beU64(11);
+    const size_t len = (static_cast<uint8_t>(p[19]) << 8) | static_cast<uint8_t>(p[20]);
+    if (p.size() != kFixed + len)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS condemned row: declared token_len {} vs payload {}", len, p.size() - kFixed);
+    row.token.value = String(p.substr(kFixed, len));
+    return row;
+}
+
+SourceEdgeRunView::SourceEdgeRunView(std::unique_ptr<ReadBuffer> stream_)
+    : stream(std::move(stream_))
+    , reader(std::make_unique<SourceEdgeRunReader>(*stream))   /// the reader borrows *stream; both are members
+{
+}
+
+bool SourceEdgeRunView::next(String & key, String & payload)
+{
+    SourceEdgeRecord rec;
+    if (!reader->next(rec))
+        return false;
+    /// Reconstruct the packed SourceEdgeKeyCodec key + the ORIGINAL payload bytes (a single marker byte,
+    /// or the encoded condemned row) so the fold / zeroInDegree / previewDeletes / fsck consumers keep
+    /// their exact parse/compare logic against the NDJSON codec.
+    key = SourceEdgeKeyCodec::key(rec.ref, rec.source_id);
+    switch (rec.marker)
+    {
+        case kEdgeActive:
+        case kZeroMarker:
+            payload = String(1, rec.marker);
+            break;
+        case kCondemned:
+            payload = encodeCondemnedRow(CondemnedRow{.delete_pending = rec.delete_pending,
+                                                      .token = rec.token, .size = rec.size,
+                                                      .condemn_round = rec.condemn_round,
+                                                      .marker_confirmed = rec.marker_confirmed});
+            break;
+        default:
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS source-edge run: unknown row marker 0x{:02x}", static_cast<uint8_t>(rec.marker));
+    }
+    return true;
+}
+
+void SourceEdgeRunView::verifyAgainst(const UInt128 & expected)
+{
+    reader->verifyAgainst(expected);
+}
+
+UInt128 SourceEdgeRunView::accumulatedChecksum()
+{
+    return reader->accumulatedChecksum();
+}
+
+SourceEdgeRunView openSourceEdgeRun(std::string_view bytes)
+{
+    return SourceEdgeRunView(std::make_unique<ReadBufferFromMemory>(bytes.data(), bytes.size()));
+}
+
+SourceEdgeRunView openSourceEdgeRun(Backend & backend, const String & key)
+{
+    /// Streaming: `getStream` is a forward-only read of the write-once run — nothing is
+    /// materialized whole (cas_run is object_cap = 0). Absent object => fail-closed.
+    auto sr = backend.getStream(key);
+    if (!sr)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS source-edge run: object {} is absent", key);
+    return SourceEdgeRunView(std::move(sr->stream));
+}
+
+namespace
+{
+/// len-drift guard (mirrors DigestCodec::checkZeroTail, CasBlobDigest.h): a caller passing a digest
+/// wider than the algo's own width is a programming bug, not corrupted on-disk data — chassert, not
+/// throw.
+void checkZeroTailForAlgo(const BlobDigest & d, uint8_t digest_len, [[maybe_unused]] const char * what)
+{
+    for (size_t i = digest_len; i < d.bytes.size(); ++i)
+        chassert(d.bytes[i] == 0, fmt::format("SourceEdgeKeyCodec::{}: non-zero byte at tail position {} (digest_len={})", what, i, digest_len));
+}
+}
+
+String SourceEdgeKeyCodec::key(const BlobRef & ref, const UInt128 & source_id)
+{
+    const uint8_t digest_len = static_cast<uint8_t>(blobHashLenFor(ref.algo));
+    checkZeroTailForAlgo(ref.digest, digest_len, "key");
+    String out;
+    out.push_back(static_cast<char>(static_cast<uint8_t>(ref.algo)));
+    out += String(reinterpret_cast<const char *>(ref.digest.bytes.data()), digest_len);
+    out += u128ToBytesBE(source_id);
+    return out;
+}
+
+void SourceEdgeKeyCodec::parse(std::string_view key, BlobRef & ref, UInt128 & source_id)
+{
+    if (key.empty())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "CAS source-edge run: empty key");
+    const uint8_t algo_byte = static_cast<uint8_t>(key[0]);
+    BlobHashAlgo algo{};
+    switch (algo_byte)
+    {
+        case static_cast<uint8_t>(BlobHashAlgo::CityHash128): algo = BlobHashAlgo::CityHash128; break;
+        case static_cast<uint8_t>(BlobHashAlgo::XXH3_128):    algo = BlobHashAlgo::XXH3_128; break;
+        case static_cast<uint8_t>(BlobHashAlgo::Sha256):      algo = BlobHashAlgo::Sha256; break;
+        default:
+            throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+                "CAS source-edge run: unknown algo byte {} in key", algo_byte);
+    }
+    const uint8_t digest_len = static_cast<uint8_t>(blobHashLenFor(algo));
+    if (key.size() != 1 + static_cast<size_t>(digest_len) + 16)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS source-edge run: malformed key ({} bytes, expected {})", key.size(), 1 + digest_len + 16);
+    ref = BlobRef{};
+    ref.algo = algo;
+    memcpy(ref.digest.bytes.data(), key.data() + 1, digest_len);
+    source_id = u128FromBytesBE(String(key.substr(1 + digest_len, 16)), "src-edge run key source_id");
+}
+
+void putDeterministicArtifact(Backend & backend, const String & key, const String & bytes)
+{
+    if (backend.putIfAbsent(key, bytes).outcome == PutOutcome::PreconditionFailed)
+    {
+        const auto existing = backend.get(key);
+        if (!existing || existing->bytes != bytes)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS gc: deterministic artifact at {} occupied by divergent bytes (impossible under "
+                "correct operation; refusing to proceed)", key);
+        /// byte-equal => our own deterministic replay; adopt (no-op).
+    }
+}
+
+void foldDeltasIntoGeneration(Backend & backend, const Layout & layout,
+                              const std::vector<RunRef> & prior_runs,
+                              uint64_t new_generation, uint64_t attempt,
+                              uint64_t shard,
+                              std::vector<BlobDelta> scattered, std::vector<RunRef> & out_runs,
+                              uint64_t current_round, uint64_t condemn_round,
+                              const std::function<std::optional<HeadResult>(const BlobRef &)> & head_blob,
+                              const std::function<std::optional<HeadResult>(const BlobRef &)> & peek_head,
+                              const std::function<bool(const RetiredEntry &)> & confirm_condemned_marker,
+                              RetiredMergeResult * out_retired,
+                              bool suppress_destructive,
+                              std::vector<uint8_t> * out_applied_by_txn_ordinal,
+                              std::vector<BlobSourceRetirement> source_retirements,
+                              GcRoundWorkBudget * work_budget)
+{
+    RetiredMergeResult sink;
+    RetiredMergeResult & rmr = out_retired ? *out_retired : sink;
+
+    // Deterministic input ordering produces a byte-reproducible run for safe retry and adoption.
+    // MUST be stable: for the same (ref, source_id) the journal ordering is
+    // activation-before-removal; "last wins" then correctly resolves to removal (edge absent).
+    // An unstable sort can put removal before activation => last=activation => false positive.
+    // The comparator is exactly (ref.algo, ref.digest, source_id) == BlobRef::operator< then
+    // source_id — the same order the raw keys sort in (`SourceEdgeKeyCodec::key`'s algo
+    // byte decides before any digest byte can), so the merge below stays a plain key comparison.
+    std::stable_sort(scattered.begin(), scattered.end(),
+        [](const BlobDelta & a, const BlobDelta & b)
+        {
+            if (a.ref != b.ref) return a.ref < b.ref;
+            return a.source_id < b.source_id;
+        });
+    std::sort(source_retirements.begin(), source_retirements.end(),
+        [](const BlobSourceRetirement & a, const BlobSourceRetirement & b)
+        {
+            if (a.ref != b.ref) return a.ref < b.ref;
+            return a.source_id < b.source_id;
+        });
+
+    PriorEdgeCursor cursor(backend, prior_runs);
+
+    DB::WriteBufferFromOwnString out;
+    SourceEdgeRunWriter writer(out);   // sorted NDJSON; byte-deterministic for write-once adoption
+
+    // Streaming two-cursor merge over the prior run (surviving edges AND retired kCondemned
+    // sentinel rows at the zero source id) and this round's edge deltas (by (blob_hash, source_id)). All
+    // rows for one blob are adjacent in both inputs; the sentinel key (source_id 0) sorts first. We resolve
+    // final presence per edge locally (idempotent: prior present + activate => present; any remove =>
+    // absent), settle each blob's carried retired row against its post-merge in-degree at close-out, and
+    // re-emit the surviving retired rows / zero-transition markers. O(block) IO + O(1) per current blob.
+    size_t di = 0;
+    size_t ri = 0;
+    BlobRef cur_blob{};
+    bool have_blob = false;
+    uint64_t cur_edges = 0;              // surviving edges of cur_blob so far
+    bool cur_touched = false;            // cur_blob had prior edges or deltas this generation
+    std::optional<CondemnedRow> cur_condemned;   // the retired sentinel carried on the prior run for cur_blob
+
+    auto toRetiredEntry = [](const BlobRef & ref, const CondemnedRow & r) -> RetiredEntry
+    {
+        RetiredEntry e;
+        e.kind = ObjectKind::Blob;
+        e.ref = ref;
+        e.token = r.token;
+        e.size = r.size;
+        e.condemn_round = r.condemn_round;
+        e.delete_pending = r.delete_pending;
+        e.marker_confirmed = r.marker_confirmed;
+        return e;
+    };
+    /// THE DELETE-SITE IN-DEGREE RE-READ IS NORMATIVE (spec §5, third arm). It is not an optimization
+    /// and not defense-in-depth: it is the last of the three things that keep a delete from racing a
+    /// live edge, and the only one that acts on THIS pass's freshly merged view.
+    ///
+    ///   1. the round-paced floor: a blob condemned in round R cannot graduate before R+1, so a `+1`
+    ///      that lands in the same round as the condemnation is always folded before any delete;
+    ///   2. the exact-token delete: a writer that resurrected the blob replaced its incarnation, so a
+    ///      stale token's delete finds a TokenMismatch and removes nothing;
+    ///   3. THIS: the entry is settled against `indeg` recomputed by the merge that just ran, so an edge
+    ///      folded after the condemnation but before the delete pass spares the blob outright --
+    ///      `indeg > 0` wins over `delete_pending`, unconditionally and past the floor.
+    ///
+    /// Arms 1 and 2 bound WHEN and WHAT a delete may remove; only this one asks whether the blob is
+    /// still referenced at the moment the pass decides. Removing it -- or reordering the branches so
+    /// that `delete_pending` is checked first -- silently deletes re-referenced blobs on exactly the
+    /// interleaving the other two arms do not cover. Any change here needs a test that fails without it.
+    auto settleEntry = [&](const RetiredEntry & e, uint64_t indeg)
+    {
+        chassert(e.kind == ObjectKind::Blob);   /// the in-degree merge settles Blob entries only
+        if (indeg > 0)
+        {
+            /// A delete_pending entry recovering in-degree is the expected shape of a dedup-adopt vs
+            /// condemn race, not an ack-floor violation: a graduated blob carries NO surviving prior
+            /// edges (see the comment on the sentinel emission below), so any edge that resurrects its
+            /// in-degree is necessarily a FRESH this-generation edge -- a writer's `observeAndAdmit`
+            /// point-read of the per-hash meta raced GC's `Condemned` write and adopted the (about to
+            /// be deleted) token instead of resurrecting from source. Spare it LOUDLY (never a
+            /// fail-closed abort, never a delete of a re-referenced blob), but at Debug: this is a
+            /// routine, safely-handled race, not something to page on.
+            if (e.delete_pending)
+            {
+                /// No LoggerPtr is threaded this deep (foldDeltasIntoGeneration is a free function
+                /// shared by the non-sharded fold and CasGcShardPlan's per-shard reduce); scope the
+                /// message with the pool's own key prefix instead so a multi-disk process's logs can
+                /// still be attributed.
+                LOG_DEBUG(getLogger("CasGcFold"),
+                    "CAS gc fold ({}): delete_pending blob {} (condemned at round {}, observed at round {}) "
+                    "recovered in-degree {} -- a fresh dedup-adopt raced the condemn; sparing (never a "
+                    "fail-closed delete)", layout.poolPrefix(), blobIdOf(e.ref), e.condemn_round, current_round, indeg);
+                ProfileEvents::increment(ProfileEvents::CASGCRetiredSparedByReref);
+            }
+            rmr.spared.push_back(e);            /// recovery wins, even past the floor
+        }
+        else if (e.delete_pending)
+        {
+            /// Excess past the round's redelete budget is carried unchanged (still `delete_pending`) —
+            /// exactly the suppressed-pass shape below — rather than skipped ahead in `scattered`, so a
+            /// budget-exhausted round retries the same entry next round instead of losing it.
+            if (suppress_destructive || (work_budget && !work_budget->redeleteAvailable()))
+                rmr.still_retired.push_back(e); /// clamp-suppressed or budget-exhausted: carry UNCHANGED
+            else
+            {
+                if (work_budget)
+                    ++work_budget->redeletes_used;
+                rmr.redelete.push_back(e);      /// published pending by a PRIOR pass — execute + drop
+            }
+        }
+        else if (!suppress_destructive && e.condemn_round < current_round)
+        {
+            /// Graduation gate (triage 2026-07-17 §3.4): publishing delete_pending is the one edge that
+            /// authorizes an irreversible delete, and it requires CONFIRMED durable Condemned evidence
+            /// for this exact (hash, token) — the marker is the writer's adopt gate, so an entry whose
+            /// marker write was swallowed could be same-token adopted invisibly to this fold's cut.
+            /// Unconfirmed => carry unchanged (fail-safe delay; the gate callback retries the marker so a
+            /// later pass can confirm). This gates a DELETE on missing evidence; it never throws.
+            if (e.marker_confirmed || !confirm_condemned_marker || confirm_condemned_marker(e))
+            {
+                /// Excess past the round's graduation budget carries the floor-passed entry unchanged
+                /// (still condemned, not yet delete_pending) — it re-evaluates the floor next round and
+                /// graduates then; nothing is lost, only delayed.
+                if (work_budget && !work_budget->graduationAvailable())
+                    rmr.still_retired.push_back(e);
+                else
+                {
+                    if (work_budget)
+                        ++work_budget->graduations_used;
+                    RetiredEntry pending = e;       /// newly floor-passed: publish pending; delete NEXT pass
+                    pending.delete_pending = true;
+                    pending.marker_confirmed = true;
+                    rmr.graduated.push_back(pending);
+                    rmr.still_retired.push_back(std::move(pending));
+                }
+            }
+            else
+                rmr.still_retired.push_back(e); /// no durable condemn-marker evidence yet — carried
+        }
+        else
+            rmr.still_retired.push_back(e);     /// carried unchanged until the floor passes it
+    };
+
+    auto closeBlob = [&]()
+    {
+        if (!have_blob)
+            return;
+        const size_t retired_before = rmr.still_retired.size();
+
+        /// Settle the retired row carried on the prior run for the blob being closed, against its
+        /// post-merge in-degree...
+        if (cur_condemned)
+        {
+            /// The retired row already identifies the blob with the native `BlobRef` used by the run.
+            const RetiredEntry stale = toRetiredEntry(cur_blob, *cur_condemned);
+            /// On a re-reference cycle (touched this window, net in-degree 0),
+            /// re-observe the CURRENT token. If it differs from the retired row's token, a resurrect
+            /// replaced the incarnation at this key — supersede the stale entry with a fresh condemn of the
+            /// current token so the replacement enters the pipeline (the stale token's exact-token delete
+            /// would only find the new token and no-op). Keyed on (hash, current token), matching GRetire.
+            /// `peek_head` is deliberately side-effect-free and is not `head_blob` —
+            /// `head_blob` is the fresh-condemn hook (emits `BlobRetire` + increments
+            /// `CASGCRetiredCondemned`); calling it here would double-emit `blob_retire` alongside the
+            /// `blob_retire_replaced` this supersede already produces below, and double-count the
+            /// condemned counter for one physical condemnation.
+            bool superseded = false;
+            if (cur_edges == 0 && cur_touched && peek_head)
+            {
+                if (const auto hr = peek_head(cur_blob);
+                    hr && hr->exists && hr->token != stale.token)
+                {
+                    RetiredEntry fresh;
+                    fresh.kind = ObjectKind::Blob;
+                    fresh.ref = cur_blob;
+                    fresh.token = hr->token;
+                    fresh.size = hr->size;
+                    fresh.condemn_round = condemn_round;
+                    ReplacedEntry re;
+                    re.old_token = stale.token;                 /// the stale token this supersede replaces
+                    re.fresh = fresh;
+                    rmr.replaced.push_back(std::move(re));       /// caller emits blob_retire_replaced
+                    rmr.still_retired.push_back(std::move(fresh));
+                    superseded = true;
+                }
+            }
+            if (!superseded)
+                settleEntry(stale, cur_edges);
+        }
+        /// ...or condemn a fresh transition-to-zero (no carried row). `head_blob` captures the exact
+        /// incarnation token for the later exact-token delete; an absent object needs no entry.
+        else if (cur_edges == 0 && cur_touched && head_blob)
+        {
+            if (const auto hr = head_blob(cur_blob); hr && hr->exists)
+            {
+                RetiredEntry fresh;
+                fresh.kind = ObjectKind::Blob;
+                fresh.ref = cur_blob;
+                fresh.token = hr->token;
+                fresh.size = hr->size;
+                fresh.condemn_round = condemn_round;
+                rmr.still_retired.push_back(std::move(fresh));
+            }
+        }
+
+        /// Emit at most one sentinel row per blob: the `kCondemned` row when the
+        /// blob is condemned/carried/graduated this pass (still_retired grew for it), else a per-generation
+        /// `kZeroMarker` when it transitioned to zero this pass but was not condemned (redelete-dropped or
+        /// absent-at-condemn). A blob with surviving edges (cur_edges > 0) emits neither — its edge rows
+        /// were appended inline, and a condemned/zeroed blob has NO surviving edges, so appending the
+        /// sentinel now (its key sorts first for the blob, and no edge rows precede it) keeps the run
+        /// sorted. `still_retired` therefore mirrors exactly the emitted `kCondemned` rows, in order.
+        if (rmr.still_retired.size() > retired_before)
+        {
+            const RetiredEntry & e = rmr.still_retired.back();
+            writer.append(SourceEdgeRecord{.ref = cur_blob, .source_id = kZeroSourceId, .marker = kCondemned,
+                                           .delete_pending = e.delete_pending, .token = e.token,
+                                           .size = e.size, .condemn_round = e.condemn_round,
+                                           .marker_confirmed = e.marker_confirmed});
+        }
+        else if (cur_edges == 0 && cur_touched)
+            writer.append(SourceEdgeRecord{.ref = cur_blob, .source_id = kZeroSourceId, .marker = kZeroMarker});
+    };
+    auto openBlobIfNeeded = [&](const BlobRef & b)
+    {
+        if (!have_blob || b != cur_blob)
+        {
+            closeBlob();
+            cur_blob = b; have_blob = true; cur_edges = 0; cur_touched = false; cur_condemned.reset();
+        }
+    };
+
+    while (cursor.valid() || di < scattered.size() || ri < source_retirements.size())
+    {
+        // Pick the smallest row key across the prior-run cursor and this round's deltas.
+        String key;
+        bool from_prior = false;
+        if (cursor.valid()) { key = cursor.key(); from_prior = true; }
+        if (di < scattered.size())
+        {
+            const String dk = SourceEdgeKeyCodec::key(scattered[di].ref, scattered[di].source_id);
+            if (!from_prior || dk < key) { key = dk; from_prior = false; }
+        }
+        if (ri < source_retirements.size())
+        {
+            const String rk = SourceEdgeKeyCodec::key(source_retirements[ri].ref, source_retirements[ri].source_id);
+            if ((!from_prior && key.empty()) || rk < key) { key = rk; from_prior = false; }
+        }
+
+        BlobRef blob_ref;
+        UInt128 source_id;
+        SourceEdgeKeyCodec::parse(key, blob_ref, source_id);   // throws CORRUPTED_DATA on a malformed key (fail-closed)
+        openBlobIfNeeded(blob_ref);
+
+        /// A retired sentinel row from the prior run: stash it for close-out settlement. It is not an edge
+        /// and NEVER a touch — a carried kCondemned row must not force a zero-marker or a peek_head HEAD
+        /// and never a touch. Deltas never key the zero source id, so no delta merges at this key.
+        if (from_prior && cursor.rowType() == kCondemned)
+        {
+            cur_condemned = cursor.condemnedRow();
+            cursor.advance();
+            continue;
+        }
+
+        bool present = false;
+        if (from_prior && cursor.key() == key) { present = true; cursor.advance(); cur_touched = true; }
+        while (di < scattered.size()
+               && scattered[di].ref == blob_ref && scattered[di].source_id == source_id)
+        {
+            /// An unmatched remove: `present` was false immediately before this remove delta is
+            /// applied, meaning neither the prior run nor an earlier delta in this same scattered run
+            /// for this key had activated it. The set semantics make this a harmless per-key no-op
+            /// (never a false deletion), but a persistent nonzero rate is a correctness signal — count
+            /// it and hand ONE example back to the caller, who logs once per round (never from this
+            /// hot inner loop; it runs over potentially millions of rows).
+            if (scattered[di].remove && !present)
+            {
+                ++rmr.unmatched_removes;
+                ProfileEvents::increment(ProfileEvents::CASGCUnmatchedRemoveDeltas);
+                if (!rmr.unmatched_remove_example)
+                    rmr.unmatched_remove_example = UnmatchedRemoveExample{blob_ref, source_id};
+            }
+            /// PROBE B2: this delta reached a reducer and is being CONSUMED. Marked here rather than
+            /// at run flush because the in-degree model is a SET — an unmatched `-1` and a duplicate
+            /// `+1` legitimately vanish inside the merge, so a flush-side mark would fire on healthy
+            /// rounds. See `Cas::TxnApplyLedger`.
+            if (out_applied_by_txn_ordinal)
+                (*out_applied_by_txn_ordinal)[scattered[di].txn_ordinal] = 1;
+            present = scattered[di].remove ? false : true;   // apply in order; last wins
+            cur_touched = true;
+            ++di;
+        }
+
+        /// Orphan nomination retires this exact manifest-source identity after ordinary ref deltas at
+        /// the same key. It is accounting-neutral: absence is an idempotent no-op, not an unmatched
+        /// ref removal, and there is no transaction ordinal to mark in B2's apply ledger.
+        while (ri < source_retirements.size()
+               && source_retirements[ri].ref == blob_ref && source_retirements[ri].source_id == source_id)
+        {
+            present = false;
+            cur_touched = true;
+            ++ri;
+        }
+
+        if (present)
+        {
+            writer.append(SourceEdgeRecord{.ref = blob_ref, .source_id = source_id, .marker = kEdgeActive});
+            ++cur_edges;
+        }
+    }
+    closeBlob();
+
+    writer.finish();
+    out.finalize();
+    const String run_bytes = out.str();
+    /// Whole-object streaming checksum: the same chained CityHash128 the reader
+    /// accumulates on the read path, replacing the retired one-shot cityHash128. Carried by the fold
+    /// seal's RunRef.checksum and verified before any consumer acts on the run.
+    const UInt128 run_checksum = sourceEdgeRunChecksum(run_bytes);
+    const String run_key = layout.blobTargetRunKey(new_generation, attempt, shard, 0);
+    putDeterministicArtifact(backend, run_key, run_bytes);
+    out_runs.push_back(RunRef{.key = run_key, .checksum = run_checksum,
+                              .shard = shard, .generation = new_generation});
+}
+
+std::vector<BlobCandidate> zeroInDegree(Backend & backend, const std::vector<RunRef> & runs)
+{
+    std::vector<BlobCandidate> result;
+    for (const RunRef & run : runs)
+    {
+        /// The caller passes the exact object key, so a run sealed
+        /// for a later generation but physically living under an older key is reached directly. The run is
+        /// streamed at O(one block) resident memory, never materialized whole. `openSourceEdgeRun` enforces
+        /// the run kind + key schema; `kCondemned` sentinel rows are skipped (only `kZeroMarker` counts).
+        SourceEdgeRunView r = openSourceEdgeRun(backend, run.key);
+        String k;
+        String p;
+        while (r.next(k, p))
+            if (!p.empty() && p[0] == kZeroMarker)
+            {
+                BlobRef bh;
+                UInt128 sid;
+                /// `parse` throws `CORRUPTED_DATA` on a malformed key; malformed rows must not be silently
+                /// treated as absent candidates.
+                SourceEdgeKeyCodec::parse(k, bh, sid);
+                result.push_back(BlobCandidate{.ref = bh});
+            }
+        /// Whole-file checksum: verify the drained run against the seal's
+        /// RunRef.checksum BEFORE its candidates feed a GC delete decision. Fail-closed on mismatch.
+        r.verifyAgainst(run.checksum);
+    }
+    return result;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h
new file mode 100644
index 000000000000..7067b71d8b9c
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h
@@ -0,0 +1,413 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <functional>
+#include <limits>
+#include <memory>
+#include <optional>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// In-memory description of a blob incarnation condemned by the in-degree merge. The exact token and
+/// size are captured from the blob HEAD so the GC caller can issue an exact-token deletion later;
+/// `condemn_round` controls round-paced graduation. Entries are decoded from `kCondemned` rows and are
+/// returned through `RetiredMergeResult`; the type itself has no serialized representation.
+struct RetiredEntry
+{
+    ObjectKind kind = ObjectKind::Blob;
+    BlobRef ref{};
+    Token token;          /// the exact incarnation token GC observed (exact-token delete)
+    uint64_t size = 0;
+    uint64_t condemn_round = 0;   /// the GC round that condemned this incarnation (round-paced
+                                  /// graduation: an entry graduates only once condemn_round < the
+                                  /// current round). Consulted by GC only; the writer never reads it.
+    bool delete_pending = false;  /// Two-phase graduation: floor-passed and
+                                  /// published for deletion; the NEXT pass executes the exact-token
+                                  /// delete (pre-CAS, safe at any leader staleness) and drops the entry.
+                                  /// Terminal: a pending entry is never un-pended (writers keep seeing
+                                  /// it condemned and recreate).
+    bool marker_confirmed = false;   /// Durable `Condemned` meta CONFIRMED for this entry — the
+                                     /// graduation gate (triage 2026-07-17 §3.4): the per-hash condemn
+                                     /// marker is the writer's adopt gate, so graduation to
+                                     /// delete_pending requires confirmed durable evidence; an
+                                     /// unconfirmed entry is CARRIED, never fail-open deleted. Set at
+                                     /// graduation (delete_pending rows always carry it).
+};
+
+/// Backend-independent codec for source-edge keys. A key is `algo` (u8), the digest at that algorithm's
+/// native width, and `source_id` (16 bytes, big-endian). The packed byte order is exactly
+/// `(BlobRef, source_id)` order, which lets the fold merge compare keys directly. The leading algorithm
+/// byte makes the digest width self-describing; supported algorithms may therefore be mixed in one run.
+class SourceEdgeKeyCodec
+{
+public:
+    SourceEdgeKeyCodec() = delete;
+
+    /// key = algo(u8) ++ digest[blobHashLenFor(algo)] ++ source_id(16 BE); 33 or 49 bytes.
+    static String key(const BlobRef & ref, const UInt128 & source_id);
+    /// Parse a key. Throws `NOT_IMPLEMENTED` on an unknown algo byte, `CORRUPTED_DATA` on a wrong
+    /// total length for a known algo. Zero-tails the digest (beyond the algo's own width).
+    static void parse(std::string_view key, BlobRef & ref, UInt128 & source_id);
+};
+
+/// Deterministic 16-byte id of a source edge (ManifestId, path). Distinctness only — not reconstructable.
+UInt128 sourceEdgeId(const ManifestId & id, const String & path);
+
+/// The zero source_id is the reserved sentinel key (used internally for the zero-marker row) —
+/// producers of real source edges must fail closed on a hash collision with it.
+void assertValidSourceEdgeId(const UInt128 & source_id);
+
+/// Serialized payload of a condemned source-edge sentinel. The payload retains the full incarnation
+/// token, including its type, because deletion must remain exact-token guarded. Its fixed prefix is
+/// `[0x02][flags][token_type][round BE64][size BE64][token_len BE16]`, followed by token bytes.
+/// `flags` bit 0 is `delete_pending`, bit 1 is `marker_confirmed`.
+struct CondemnedRow
+{
+    bool delete_pending = false;
+    Token token;                 // {value, type} — the full token required by exact-token deletion
+    uint64_t size = 0;
+    uint64_t condemn_round = 0;
+    bool marker_confirmed = false;   // durable Condemned meta confirmed (graduation gate)
+    bool operator==(const CondemnedRow &) const = default;
+};
+
+/// Encode a condemned-row payload. Throws `CORRUPTED_DATA` if the token cannot fit in its u16 length.
+String encodeCondemnedRow(const CondemnedRow & row);
+
+/// Decode and validate a condemned-row payload. Unknown flags, token types, or inconsistent lengths
+/// throw `CORRUPTED_DATA`.
+CondemnedRow decodeCondemnedRow(std::string_view payload);
+
+/// Bridges the backend-free `Formats/CasRecordStreamFormat` NDJSON reader to the
+/// `(key, payload)` BYTE interface the fold / `zeroInDegree` / `previewDeletes` / `fsck` consumers use:
+/// `next` reconstructs the packed `SourceEdgeKeyCodec` key and the original payload bytes (a single
+/// marker byte for an edge / zero row, or the `encodeCondemnedRow` blob for a condemned row) from the
+/// decoded NDJSON record. So the codec stays backend-free while the consumers keep their exact parse /
+/// compare logic. The whole-object chained CityHash128 is accumulated as the run streams; `verifyAgainst`
+/// checks it against the fold seal's `RunRef.checksum` AFTER the run is fully drained and BEFORE the
+/// caller acts on it (a deletion decision).
+class SourceEdgeRunView
+{
+public:
+    /// false once the run's `{"n"}` trailer is consumed (the trailer count is verified there). `key` is
+    /// the reconstructed `SourceEdgeKeyCodec::key(ref, source_id)`; `payload` is the original marker byte
+    /// or `encodeCondemnedRow` bytes.
+    bool next(String & key, String & payload);
+    /// Verify the accumulated whole-file checksum against the seal's `RunRef.checksum`; CORRUPTED_DATA on
+    /// mismatch. Call after draining the run and before acting on its records.
+    void verifyAgainst(const UInt128 & expected);
+    /// The accumulated whole-file checksum after draining the run — non-throwing, for a read-only auditor
+    /// (fsck) that catalogues a mismatch and continues instead of failing closed. Call after draining.
+    UInt128 accumulatedChecksum();
+
+private:
+    friend SourceEdgeRunView openSourceEdgeRun(std::string_view bytes);
+    friend SourceEdgeRunView openSourceEdgeRun(Backend & backend, const String & key);
+    /// Keep the underlying stream alive for the reader, which borrows it rather than owning it.
+    explicit SourceEdgeRunView(std::unique_ptr<ReadBuffer> stream_);
+
+    std::unique_ptr<ReadBuffer> stream;             /// owns the backend stream / memory buffer the reader borrows
+    std::unique_ptr<SourceEdgeRunReader> reader;    /// over *stream (non-movable => held by pointer, destroyed before stream)
+};
+
+/// Open a typed source-edge run. The NDJSON header must identify a `cas_run` of kind `source_edge`;
+/// otherwise opening fails closed. The memory overload borrows caller-owned bytes. The backend overload
+/// streams the write-once object through `getStream`, retaining only one record-sized buffer.
+SourceEdgeRunView openSourceEdgeRun(std::string_view bytes);
+SourceEdgeRunView openSourceEdgeRun(Backend & backend, const String & key);
+
+/// Store a deterministic write-once artifact (same inputs => byte-identical bytes): the blob in-degree
+/// runs and fold seals. `putIfAbsent`; on a `PreconditionFailed` the key is already
+/// occupied — `get` it and compare bytes: byte-equal means our own deterministic replay (adopt, no-op),
+/// divergent bytes are impossible under correct operation and we fail closed with `CORRUPTED_DATA`
+/// rather than let a divergent artifact disagree with the adopted snapshot. Deterministic artifacts are
+/// therefore byte-equal-or-`CORRUPTED_DATA`. It is
+/// NOT for observation-bearing artifacts (outcome logs) — those carry HEAD-observed
+/// tokens that two observers may legitimately differ on and keep first-durable-write-wins semantics.
+void putDeterministicArtifact(Backend & backend, const String & key, const String & bytes);
+
+/// One source-edge update before merging: the edge `(ref, source_id)`, and whether it is an activation
+/// (+edge) or a removal (−edge). Idempotent under re-fold at the merge (set membership, not a counter).
+struct BlobDelta
+{
+    BlobRef ref{};
+    UInt128 source_id{};   /// `sourceEdgeId(ManifestId, path)` — an edge identity, not a content hash
+    bool remove = false;
+    /// Round-local index of the ref transaction that emitted this delta, into `Cas::TxnApplyLedger`.
+    /// NEVER persisted and never part of any comparator: it exists only so the reducer can prove it
+    /// consumed at least one delta from every transaction the round declared covered (probe B2). It
+    /// lands in the struct's existing tail padding — see `kBlobDeltaSize` below.
+    ///
+    /// DECLARED LAST, and that is load-bearing: several tests brace-initialise a `BlobDelta`
+    /// POSITIONALLY as `{ref, source_id, remove}`. Inserting a field ahead of `remove` silently rebinds
+    /// that third initialiser to the ordinal and turns every removal delta into an activation — a
+    /// compiling, type-correct, wrong-answer change. Any future field goes after this one, or every
+    /// aggregate initialiser gets converted to designated form first.
+    uint32_t txn_ordinal = 0;
+};
+
+/// The fold's per-edge row runs over potentially millions of records per round, so its size is a
+/// deliberate property, not an accident. Pinned here (rather than left to a perf run to discover)
+/// because probe B2's `txn_ordinal` was added into the existing tail padding: the row did NOT grow.
+/// A future field that breaks this assertion is a hot-path change and must be argued as one.
+static constexpr size_t kBlobDeltaSize = 64;
+static_assert(sizeof(BlobDelta) == kBlobDeltaSize,
+              "BlobDelta is the fold's hot per-edge row; growing it is a hot-path change");
+
+/// One orphan-manifest source edge to retire without pretending it came from a ref transaction.
+/// The reducer applies this as an idempotent exact-key removal, but it deliberately consumes no B2
+/// transaction ordinal and an already-absent edge is not an unmatched-remove correctness signal.
+struct BlobSourceRetirement
+{
+    BlobRef ref{};
+    UInt128 source_id{};
+};
+
+/// A blob whose active source-edge set became empty this generation — a retire candidate.
+struct BlobCandidate
+{
+    BlobRef ref{};
+};
+
+/// Merge the prior generation's blob source-edge run for `shard` with `scattered` deltas, producing the
+/// new generation's write-once run under blobTargetRunKey(new_generation, attempt, shard, 0). Streaming:
+/// prior run + scattered deltas are sorted by (blob_hash, source_id) and merged via two-cursor scan; the
+/// source-edge set is idempotent under re-fold (identical (blob_hash, source_id) pairs deduplicate). A blob
+/// whose active edge set transitions to exactly empty this generation is written as an explicit zero-marker
+/// row so `zeroInDegree` can stream it; prior-generation zero markers are dropped. Appends the produced
+/// run's `RunRef` (key + footer checksum + `shard` + `new_generation`) to `out_runs` for the fold seal.
+///
+/// `prior_runs` are the parent generation's run segments for this shard, resolved by the caller from the
+/// parent fold seal's `blob_target_runs` (filtered to `shard`). An empty vector is the fresh-pool / empty
+/// baseline. A run sealed for one generation may physically live under an older generation's key, so the
+/// seal's exact reference is authoritative and key construction is not used. `new_generation`, `attempt`,
+/// and `shard` name only the output run's key namespace.
+/// The fresh entry that re-condemns the current
+/// token, paired with the STALE entry's token it superseded. Kept as its own struct (rather than a
+/// field bolted onto `RetiredEntry`) so the common merge element stays slim — only replaced entries
+/// carry the extra superseded token.
+struct ReplacedEntry
+{
+    RetiredEntry fresh;   /// the freshly condemned CURRENT token (also pushed into still_retired byte-identically)
+    Token old_token;      /// the superseded (stale) entry's token — what the resurrect replaced
+};
+
+/// One example of an unmatched-remove delta, kept for the caller's single once-per-round WARNING
+/// (see `RetiredMergeResult::unmatched_removes` below) — naming the blob and source id is enough to
+/// start an investigation without logging every occurrence from the hot inner loop.
+struct UnmatchedRemoveExample
+{
+    BlobRef ref{};
+    UInt128 source_id{};
+};
+
+/// Outcome of the retired merge: the same
+/// streaming pass that folds edges settles every prior retired entry and detects new candidates.
+struct RetiredMergeResult
+{
+    std::vector<RetiredEntry> still_retired;   /// carried + newly-condemned + newly-PENDING entries (the next list)
+    std::vector<RetiredEntry> graduated;       /// newly floor-passed this pass — published pending, deleted NEXT pass
+    std::vector<RetiredEntry> spared;          /// in-degree recovered — entry dropped
+    std::vector<RetiredEntry> redelete;        /// pending in the PRIOR list — execute deleteExact pre-CAS, drop
+    std::vector<ReplacedEntry> replaced;  /// re-condemned CURRENT tokens that superseded a stale entry (resurrect-replaced); caller emits blob_retire_replaced
+
+    /// Count of `remove == true` deltas that matched no presence for their `(BlobRef, source_id)` key —
+    /// neither the prior run nor an earlier delta in the same `scattered` batch had activated it. The
+    /// in-degree model is a SET, so an unmatched remove is a per-key no-op BY DESIGN (never a false
+    /// deletion), but a persistent nonzero rate means removal deltas are reaching the reducer without
+    /// their matching activation, which is a correctness signal worth paging on. See
+    /// `ProfileEvents::CASGCUnmatchedRemoveDeltas`.
+    uint64_t unmatched_removes = 0;
+    /// The first unmatched remove observed this merge, for the caller's WARNING (one example is enough
+    /// to start an investigation; logging every occurrence would flood a hot per-edge inner loop).
+    std::optional<UnmatchedRemoveExample> unmatched_remove_example;
+};
+
+/// Cumulative per-round cap over EVERY destructive-or-observability-write work family a round touches —
+/// blob graduation and redelete (this file), the orphan-manifest planner's namespace fan-out and recovery
+/// walk (`CasOrphanManifestSweep.cpp`), ref-object/generation-prefix cleanup, the post-CAS hand-off
+/// reclaim, the post-CAS manifest-body cleanup, and the `GcOutcomes` audit log (`CasGc.cpp`). One instance
+/// is owned by `Gc::runRegularRound` per round and passed by reference/pointer into each family's call
+/// in turn, so a cap is cumulative across the WHOLE round, not reset per shard or per namespace. `0` in
+/// any bound is unbounded — the same opt-out convention every other CAS round budget uses, so a
+/// default-constructed instance reproduces pre-budget behavior everywhere. Exhausting a bound never drops
+/// WORK: each destructive family's caller carries the excess in its own already-durable pipeline (still
+/// `delete_pending`/condemned rows, a retained sweep candidate, a ref object left for next round) and
+/// retries next round with a fresh budget, or -- where nothing durable is left to carry (manifest cleanup,
+/// the outcome log) -- exhaustion drops only the redundant record of a decision the round already made
+/// safely elsewhere (see each field's own comment). Reusable as-is by a future bounded-parallel-walk: give
+/// each worker an atomic increment (or a partitioned share of a bound) instead of inventing a new
+/// accounting shape.
+struct GcRoundWorkBudget
+{
+    uint64_t max_graduations = 0;
+    uint64_t max_redeletes = 0;
+    uint64_t graduations_used = 0;
+    uint64_t redeletes_used = 0;
+
+    bool graduationAvailable() const { return max_graduations == 0 || graduations_used < max_graduations; }
+    bool redeleteAvailable() const { return max_redeletes == 0 || redeletes_used < max_redeletes; }
+
+    /// Orphan-manifest planner (`planManifestCursorPage` / `activeManifestKeys`): how many DISTINCT
+    /// namespaces the sweep may build a fresh protection view for in total this round (cumulative
+    /// across pages, not per page), and how many ref-log GET/decode
+    /// operations the committed-tail recovery walk may spend in total across every namespace this
+    /// round. Both caps exist because building a namespace's protection view (a catalog-authoritative
+    /// table recovery plus a committed-tail walk) is the expensive, potentially-unbounded step the
+    /// nomination/list budgets never covered.
+    uint64_t max_sweep_namespaces = 0;
+    uint64_t max_sweep_recovery_ops = 0;
+    uint64_t sweep_namespaces_used = 0;
+    uint64_t sweep_recovery_ops_used = 0;
+
+    bool sweepNamespaceAvailable() const { return max_sweep_namespaces == 0 || sweep_namespaces_used < max_sweep_namespaces; }
+    bool sweepRecoveryOpAvailable() const { return max_sweep_recovery_ops == 0 || sweep_recovery_ops_used < max_sweep_recovery_ops; }
+
+    /// Cleanup families (`Gc::cleanupRefObjects`, `deletePrefixWholesale`'s caller in
+    /// `Gc::pruneSupersededGenerations`). `deletePrefixWholesale` already takes a `bounded_remaining`
+    /// count; `prefixWholesaleRemaining` turns the shared round budget into that same count (its ONE
+    /// unbounded sentinel, `0 == max_prefix_wholesale_objects`, maps to the function's own "no cap"
+    /// value, `UINT64_MAX`) so every caller can pass "the round's remainder" instead of `UINT64_MAX`
+    /// outright.
+    uint64_t max_ref_cleanup_objects = 0;
+    uint64_t ref_cleanup_objects_used = 0;
+    uint64_t max_prefix_wholesale_objects = 0;
+    uint64_t prefix_wholesale_objects_used = 0;
+
+    bool refCleanupAvailable() const { return max_ref_cleanup_objects == 0 || ref_cleanup_objects_used < max_ref_cleanup_objects; }
+    uint64_t prefixWholesaleRemaining() const
+    {
+        if (max_prefix_wholesale_objects == 0)
+            return std::numeric_limits<uint64_t>::max();
+        return prefix_wholesale_objects_used < max_prefix_wholesale_objects
+            ? max_prefix_wholesale_objects - prefix_wholesale_objects_used : 0;
+    }
+
+    /// The post-CAS hand-off reclaim (`Gc::runRegularRound`) draws from its OWN reserve, never from
+    /// `max_prefix_wholesale_objects` above. The prune is safe to under-serve in any one round -- its
+    /// cursor never regresses, so a partially-drained generation is simply finished next round -- but the
+    /// hand-off is a ONE-SHOT event with no reclaimer behind it besides `fsck`: a generation it cannot
+    /// fully reclaim this round is never revisited (the parent-seal difference that triggers it does not
+    /// recur once the ref has moved). Sharing one pool would let a prune-heavy round starve the hand-off
+    /// to zero every time; a separate reserve makes that impossible regardless of how much the prune
+    /// consumes.
+    uint64_t max_handoff_prefix_wholesale_objects = 0;
+    uint64_t handoff_prefix_wholesale_objects_used = 0;
+
+    uint64_t handoffPrefixWholesaleRemaining() const
+    {
+        if (max_handoff_prefix_wholesale_objects == 0)
+            return std::numeric_limits<uint64_t>::max();
+        return handoff_prefix_wholesale_objects_used < max_handoff_prefix_wholesale_objects
+            ? max_handoff_prefix_wholesale_objects - handoff_prefix_wholesale_objects_used : 0;
+    }
+
+    /// `GcOutcomes` per-shard body (`Gc::runRegularRound`'s `redelete`/`spared` loops): a pure
+    /// observability record of a settlement decision that has ALREADY happened -- the merge unconditionally
+    /// decides `spared` for any entry whose in-degree recovered (INV_NO_LOSS: a fresh dedup-adopt must
+    /// never be treated as still condemned, past any budget), so this cap governs only whether the decision
+    /// gets an audit-log row, never the decision itself. Nothing is retained or retried on exhaustion --
+    /// there is nothing left to retry, the entry already left the retired pipeline correctly.
+    uint64_t max_outcome_entries = 0;
+    uint64_t outcome_entries_used = 0;
+
+    bool outcomeEntryAvailable() const { return max_outcome_entries == 0 || outcome_entries_used < max_outcome_entries; }
+};
+
+/// Merge the prior generation's source-edge run with new deltas. The prior run's `kCondemned` rows RIDE
+/// the source-edge run itself at the zero-sentinel key (`source_id = 0`), so there is no separate
+/// `prior_retired` cursor — the prior run IS the retired input. `PriorEdgeCursor` decodes each sentinel
+/// `kCondemned` row and hands it to the per-blob close-out (in ascending hash order, exactly the order
+/// the old sorted vector had). Settlement rules, in order, per condemned row for blob `h` with post-merge
+/// in-degree `d`:
+///   delete_pending (prior pass)                -> redelete if d = 0 (the caller executes the exact-token
+///                                                 delete pre-CAS and the entry drops); d > 0 for a pending
+///                                                 entry is structurally impossible but reachable under
+///                                                 races — spared + a loud log, never a
+///                                                 fail-closed abort;
+///   d > 0                                       -> spared (recovery wins even past the floor);
+///   d = 0 and condemn_round < current_round     -> graduated: REPUBLISHED as delete_pending (two-phase
+///                                                 graduation) — deleted the NEXT pass. GATED on a
+///                                                 confirmed durable condemn marker (see
+///                                                 `confirm_condemned_marker` below): an unconfirmed
+///                                                 entry is carried unchanged instead;
+///   d = 0 otherwise                             -> still_retired, carried byte-unchanged.
+/// A carried `kCondemned` row is SETTLEMENT-ONLY: it never sets the blob's `cur_touched` bit, so a
+/// generation that only carries the row emits no zero-marker and pays no `peek_head` HEAD. The surviving
+/// `still_retired` entries are re-emitted as `kCondemned` sentinel rows into the OUTPUT run (one sentinel
+/// per blob, emitted before the blob's edges since the sentinel key sorts first), so the next generation
+/// reads them back — `still_retired` mirrors exactly those rows, in the same order.
+/// When the pass is clamped on any shard, landed-before-cut events may remain unfolded behind the clamp,
+/// so graduating or executing pending
+/// deletes over this pass's in-degrees can delete a blob whose +1 is pending behind the clamp (the
+/// model's SabotageSkipChangedShard counterexample, realized). With `suppress_destructive` the merge
+/// neither graduates nor redeletes: pending entries carry UNCHANGED (still delete_pending) and
+/// floor-passed entries stay condemned-only. Condemnation and sparing remain (both non-destructive).
+/// A blob that transitions to zero THIS pass with no prior entry is condemned: `head_blob` captures
+/// the exact incarnation token/size (absent object or empty head_blob -> nothing to delete, skipped)
+/// and the entry is minted at `condemn_round`. Entries for blobs the merged stream never visits
+/// (no edges, no deltas) settle at in-degree 0 by definition. The retired cursor never changes the
+/// snapshot run bytes. Defaults preserve the empty-retired behavior
+/// current_round 0 => nothing graduates, no head_blob => nothing condemned).
+///
+/// `peek_head` is a side-effect-free HEAD used only by the resurrect-supersede
+/// branch (a `prior_retired` entry whose blob re-touched this pass at net in-degree 0, current token
+/// differs from the stale entry's token). `head_blob` is the FRESH-CONDEMN observation hook — it emits
+/// the `IndegZero`/`GcRetireObserve`/`BlobRetire` trail and increments `CASGCRetiredCondemned`, which is
+/// wrong for a supersede (the supersede's own event is `blob_retire_replaced`, emitted once by the
+/// caller from `RetiredMergeResult::replaced`). Calling `head_blob` from the supersede branch used to
+/// double-emit `blob_retire` alongside `blob_retire_replaced` and double-count the condemned counter;
+/// `peek_head` is a plain HEAD with no events and no counters. No supersede detection happens if
+/// `peek_head` is unset (default `{}`), independent of whether `head_blob` is set.
+///
+/// `confirm_condemned_marker` is the GRADUATION GATE (triage 2026-07-17 §3.4): graduation to
+/// `delete_pending` is the one edge that authorizes an irreversible delete, and the per-hash condemn
+/// marker (`writeCondemnedMeta`) is the writer's adopt gate — an entry whose marker write was silently
+/// swallowed can be same-token adopted by a writer invisible to this fold's cut, so it must NOT
+/// graduate. The callback returns whether durable `Condemned` evidence is confirmed for the entry's
+/// exact (hash, token); on false the entry is carried unchanged (fail-safe delay — the caller is
+/// expected to retry the marker so a later pass can confirm). An entry whose `marker_confirmed` bit is
+/// already set skips the callback. Unset (default `{}`) means UNGATED — the pre-gate merge semantics,
+/// for merge-mechanics unit tests only; the real GC round always passes the gate.
+/// The merge comparator is exactly `(ref.algo, ref.digest, source_id)` (that is, `BlobRef::operator<`
+/// followed by `source_id`), which is also the raw key order produced by `SourceEdgeKeyCodec`.
+void foldDeltasIntoGeneration(Backend & backend, const Layout & layout,
+                              const std::vector<RunRef> & prior_runs,
+                              uint64_t new_generation, uint64_t attempt,
+                              uint64_t shard,
+                              std::vector<BlobDelta> scattered, std::vector<RunRef> & out_runs,
+                              uint64_t current_round = 0, uint64_t condemn_round = 0,
+                              const std::function<std::optional<HeadResult>(const BlobRef &)> & head_blob = {},
+                              const std::function<std::optional<HeadResult>(const BlobRef &)> & peek_head = {},
+                              const std::function<bool(const RetiredEntry &)> & confirm_condemned_marker = {},
+                              RetiredMergeResult * out_retired = nullptr,
+                              bool suppress_destructive = false,
+                              /// PROBE B2 (see `Cas::TxnApplyLedger`): when set, one byte is stored per
+                              /// CONSUMED delta at `(*out_applied_by_txn_ordinal)[d.txn_ordinal]`. A raw
+                              /// vector rather than a callback: this runs once per delta over a stream
+                              /// that can reach millions of rows, and a `std::function` call there is
+                              /// not free. Never read by the merge; write-only. The caller must size it
+                              /// to cover every `txn_ordinal` present in `scattered`.
+                              std::vector<uint8_t> * out_applied_by_txn_ordinal = nullptr,
+                              std::vector<BlobSourceRetirement> source_retirements = {},
+                              /// Shared by reference across every shard's call within one round;
+                              /// `nullptr` (default) is unbounded, matching every existing caller.
+                              GcRoundWorkBudget * work_budget = nullptr);
+
+/// Stream the sealed in-degree runs named by `runs` (the current seal's `blob_target_runs` filtered to one
+/// shard) and return every blob written at in-degree 0 (the candidates that transitioned to zero). An
+/// empty `runs` is an empty baseline. Each `RunRef` supplies the exact object key, so resolution never
+/// reconstructs a key from generation metadata.
+std::vector<BlobCandidate> zeroInDegree(Backend & backend, const std::vector<RunRef> & runs);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.cpp
new file mode 100644
index 000000000000..10f75d953dbc
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.cpp
@@ -0,0 +1,4661 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcPhaseTimer.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/WriteBufferFromString.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <Common/logger_useful.h>
+#include <base/defines.h>
+#include <unordered_set>
+#include <algorithm>
+#include <limits>
+#include <optional>
+#include <set>
+
+namespace ProfileEvents
+{
+    extern const Event CASGCClampSuppressedPasses;
+    extern const Event CASGCDeadPrecommitSkipped;
+    extern const Event CASGCRetiredCondemned;
+    extern const Event CASGCRetiredSpared;
+    extern const Event CASGCRetiredGraduated;
+    extern const Event CASGCRetiredRedeleted;
+    extern const Event CASGCRetireReplaced;
+    extern const Event CASGCCondemnMarkerUnconfirmedCarry;
+    extern const Event CASGCHeartbeatFenceOuts;
+    extern const Event CASGCMetaWriteAnomaly;
+    extern const Event CASGCMetaOps;
+    extern const Event CASGCEnumerationPages;
+    extern const Event CASGCRefWalkPlansBuilt;
+    extern const Event CASGCUnmatchedAdoptedParentLives;
+    extern const Event CASGCStuckRemovals;
+    extern const Event CASGCNamespaceCleanupLeaks;
+    extern const Event CASGCRebuildVirginByEnumeration;
+    extern const Event CASGCUnappliedFoldedTransactions;
+    extern const Event CASRefGlobalListPages;
+    extern const Event CASRefLogBodyGets;
+    extern const Event CASRefManifestBodyFoldGets;
+    extern const Event CASRefEmittedEdges;
+    extern const Event CASRefCleanupObjectsDeleted;
+}
+
+namespace CurrentMetrics
+{
+    extern const Metric LocalThread;
+    extern const Metric LocalThreadActive;
+    extern const Metric LocalThreadScheduled;
+}
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int ABORTED;
+    extern const int BAD_ARGUMENTS;
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// The `on_page_fetched` hook GC passes to every `forEachListedKey`/`recoverRefTable`
+/// call it owns (never passed by fsck/offline-repair callers of those shared helpers) -- one increment
+/// per physical LIST page, never per listed key.
+void onGcEnumerationPage()
+{
+    ProfileEvents::increment(ProfileEvents::CASGCEnumerationPages);
+}
+
+/// Defined below; forward-declared so the post-CAS hand-off delete in `runRegularRound` can
+/// reach the same wholesale LIST-delete helper the retention prune uses.
+uint64_t deletePrefixWholesale(Backend & backend, const String & prefix, uint64_t bounded_remaining,
+                               bool * out_fully_drained = nullptr);
+
+/// The per-hash freshness-meta operations GC schedules on the bounded pool are
+/// best-effort/idempotent by design. The meta is only a point-read freshness marker for the writer/
+/// promote gate; the ledger retired-set + the exact-token body delete remain the actual safety
+/// core. A lost CAS here is never a correctness problem, only a (rare,
+/// self-healing) staleness window for the NEXT point-reader — with ONE exception (triage 2026-07-17
+/// §3.4): the CONDEMN marker is load-bearing for the delete edge. The exact-token delete argument
+/// below assumes the marker was durably written before the delete fires; a swallowed condemn-marker
+/// write lets a writer observe absent/Clean meta and adopt the SAME token the graduated entry later
+/// deletes (a dangling manifest). Graduation to `delete_pending` is therefore GATED on confirmed
+/// durable Condemned evidence for the exact (hash, token) — recorded in-process when the scheduled
+/// `writeCondemnedMeta` reports success, or re-established by a synchronous `loadMeta` re-check at
+/// graduation time. An unconfirmed entry is CARRIED (fail-safe delay) and its marker write retried;
+/// the delete itself and every other meta op stay async/advisory.
+///
+/// GC freshness meta is ADD-ONLY: GC may publish `Condemned`, and may REMOVE the meta once the exact body
+/// token is confirmed deleted/absent (`deleteConfirmedMeta`), but it NEVER transitions `Condemned ->
+/// Clean` on a spare. The SOLE `-> Clean` transition is a WRITER that has already displaced the body with
+/// a fresh incarnation token (`PartWriteTxn::uploadFromSource` + `writeResurrectMetaClean`). Rationale: a deposed leader that cleared a
+/// spare's meta then lost its round CAS would leave a durable stray-`Clean` over a still-condemned body;
+/// a writer reading `Clean` would reuse the exact condemned token, which a stale pre-CAS exact-token
+/// redelete then deletes -- live-blob data loss (INV_NO_LOSS). Removing the clear restores the exact-token
+/// delete argument in full: once a hash is `Condemned`, observing `Clean` means EITHER the condemned body
+/// is absent OR a writer already changed its incarnation token, so every stale `deleteExact(t1)` finds the
+/// body absent or `TokenMismatch`.
+
+/// Write the per-hash meta to Condemned: a blob newly entering the retired set this round (either the
+/// fresh zero-in-degree condemn, or a resurrect-supersede re-condemn of the current token). Absent meta
+/// is created fresh; an already-Condemned meta (a racing condemn, or a replay of this same round) is left
+/// alone rather than clobbering a possibly-newer condemn_round.
+///
+/// Returns whether durable Condemned evidence exists after the call: the conditional write committed,
+/// or an already-Condemned meta was observed. A lost CAS reports false and writes nothing further (the
+/// loser re-reads next time); a thrown backend error propagates (the scheduling wrapper swallows it) —
+/// either way the entry stays UNCONFIRMED and the graduation gate carries it (triage §3.4).
+bool writeCondemnedMeta(Pool & pool, const BlobRef & ref, uint64_t condemn_round, uint64_t size)
+{
+    const auto lm = loadMeta(pool.backend(), pool.layout(), ref);
+    const BlobMeta desired{.state = MetaState::Condemned, .condemn_round = condemn_round, .size = size};
+    if (!lm)
+        return putMetaIfAbsent(pool, ref, desired).outcome == CasOverwriteOutcome::Committed;
+    if (lm->meta.state != MetaState::Condemned)
+        return casMeta(pool, ref, lm->etag, desired).outcome == CasOverwriteOutcome::Committed;
+    return true;
+}
+
+/// Drop the meta after its body was physically deleted (or already found absent) by the round's
+/// exact-token delete. NO tombstone -- an absent meta reads exactly like a Clean one (absent
+/// means not condemned"). Idempotent: an already-absent meta, or one a racing writer/GC pass already
+/// moved, is a silent no-op.
+void deleteConfirmedMeta(Backend & backend, const Layout & layout, const BlobRef & ref)
+{
+    const auto lm = loadMeta(backend, layout, ref);
+    if (!lm)
+        return;
+    deleteMetaExact(backend, layout, ref, lm->etag);
+}
+
+}
+
+std::set<UInt128> RefPlan::lifeIds() const
+{
+    std::set<UInt128> out;
+    for (const auto & [life_id, row] : rows)
+        out.insert(life_id);
+    return out;
+}
+
+std::vector<NamespaceLifeId> RefPlan::lives() const
+{
+    std::vector<NamespaceLifeId> out;
+    out.reserve(rows.size());
+    for (const auto & [life_id, row] : rows)
+        out.push_back(row.life);
+    return out;
+}
+
+std::map<UInt128, RefLifeFoldState> RefPlan::parentFoldStates() const
+{
+    std::map<UInt128, RefLifeFoldState> out;
+    for (const auto & [life_id, row] : rows)
+        if (row.has_parent_fold_state)
+            out.emplace(life_id, row.fold_state);
+    return out;
+}
+
+std::map<UInt128, RefLifeFoldState> RefPlan::successorFoldStates() const
+{
+    std::map<UInt128, RefLifeFoldState> out;
+    for (const auto & [life_id, row] : rows)
+        out.emplace(life_id, row.fold_state);
+    return out;
+}
+
+size_t RefPlan::changedRows() const
+{
+    return std::count_if(rows.begin(), rows.end(), [](const auto & item)
+    {
+        const RefWalkPlanRow & row = item.second;
+        return row.tail_observation
+            && row.fold_state.coverage.last_folded_ref_id < *row.tail_observation;
+    });
+}
+
+std::optional<String> stuckRemovalWarning(
+    const RefWalkPlanRow & row, uint64_t current_round, uint64_t threshold_rounds,
+    const Layout & layout)
+{
+    if (!row.removal_started_round || row.fold_state.cleanup_evidence)
+        return std::nullopt;
+    const uint64_t started = *row.removal_started_round;
+    if (current_round < started || current_round - started < threshold_rounds)
+        return std::nullopt;
+
+    const uint64_t age = current_round - started;
+    const std::optional<RefHold> & hold = row.fold_state.coverage.hold;
+    if (hold && hold->reason == HoldReason::BodyUndecodable)
+        return fmt::format(
+            "CAS GC namespace removal is stuck: namespace='{}', life_id={}, removal_started_round={}, "
+            "current_round={}, age_rounds={}; cleanup evidence is absent because ref-log body '{}' is unreadable; "
+            "restore the exact object or recreate the pool",
+            row.life.ns.string(), u128ToHex(row.life.incarnation), started, current_round, age,
+            layout.refLogKey(row.life, hold->offending_position));
+
+    return fmt::format(
+        "CAS GC namespace removal is stuck: namespace='{}', life_id={}, removal_started_round={}, "
+        "current_round={}, age_rounds={}; cleanup evidence is absent because terminal has not folded",
+        row.life.ns.string(), u128ToHex(row.life.incarnation), started, current_round, age);
+}
+
+RefPlan buildRefWalkPlan(RoundInput && round_input)
+{
+    ProfileEvents::increment(ProfileEvents::CASGCRefWalkPlansBuilt);
+    RefPlan plan{std::move(round_input.ref_scan), std::move(round_input.catalog_cut)};
+    const CasRefCatalog::Snapshot & catalog_cut = plan.catalog_cut;
+    const RefScanSummary & ref_scan = plan.ref_scan;
+    catalog_cut.life_index.throwIfAmbiguous("CAS ref walk plan");
+
+    /// The sole admission loop. Everything below can only find one of these rows.
+    for (const CatalogEntry & entry : catalog_cut.catalog.entries)
+    {
+        if (entry.state == NsState::Creating)
+            continue;
+        plan.rows.emplace(entry.incarnation, RefWalkPlanRow{
+            .life = NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation),
+            .fold_state = {},
+            .removal_started_round = entry.removal_started_round,
+            .has_parent_fold_state = false,
+            .listed_hint = false,
+            .checkpoint_observation = std::nullopt,
+            .tail_observation = std::nullopt});
+    }
+
+    for (const auto & [life_id, state] : ref_scan.parent_ref_lives)
+    {
+        const auto it = plan.rows.find(life_id);
+        if (it == plan.rows.end())
+        {
+            /// The ordinary end of a namespace's life: its removal completed, GC deleted the catalog
+            /// row, and the parent seal still carries the fold state of a life the current cut no
+            /// longer names. There is nothing to attach it to and nothing for anyone to do, so this
+            /// is counted, not narrated -- a per-drop log line would be pure noise, and it could not
+            /// discriminate the expected case from an illegitimately vanished row anyway: this site
+            /// sees only the absence, never the evidence. The signal lives in
+            /// `CASGCUnmatchedAdoptedParentLives` and in the round's own
+            /// `walk_plan_dropped_parent_rows` phase metric; proving that a row disappeared WITHOUT a
+            /// completed removal belongs to fsck, which can compare against the removal evidence.
+            ProfileEvents::increment(ProfileEvents::CASGCUnmatchedAdoptedParentLives);
+            ++plan.dropped_parent_rows;
+            continue;
+        }
+        it->second.fold_state = state;
+        it->second.has_parent_fold_state = true;
+    }
+    for (const UInt128 & life_id : ref_scan.listed_lives)
+    {
+        const auto it = plan.rows.find(life_id);
+        if (it == plan.rows.end())
+        {
+            ++plan.dropped_listed_lives;
+            continue;
+        }
+        it->second.listed_hint = true;
+    }
+    for (const auto & [life_id, hold] : ref_scan.holds)
+    {
+        const auto it = plan.rows.find(life_id);
+        if (it == plan.rows.end())
+        {
+            ++plan.dropped_holds;
+            continue;
+        }
+        it->second.fold_state.coverage.classification = 4;
+        it->second.fold_state.coverage.hold = hold;
+    }
+    for (const auto & [life_id, checkpoint] : ref_scan.checkpoint_observations)
+    {
+        const auto it = plan.rows.find(life_id);
+        if (it == plan.rows.end())
+        {
+            ++plan.dropped_checkpoints;
+            continue;
+        }
+        it->second.checkpoint_observation = checkpoint;
+    }
+    for (const auto & [life_id, tail] : ref_scan.max_log_by_life)
+    {
+        const auto it = plan.rows.find(life_id);
+        if (it == plan.rows.end())
+        {
+            ++plan.dropped_tails;
+            continue;
+        }
+        it->second.tail_observation = tail;
+    }
+    return plan;
+}
+
+namespace tests
+{
+
+RefPlan buildRefWalkPlanForTest(RefScanSummary ref_scan, CasRefCatalog::Snapshot catalog_cut)
+{
+    return buildRefWalkPlan(RoundInput{std::move(ref_scan), std::move(catalog_cut)});
+}
+
+}
+
+uint64_t retiredLogicalSize(ObjectKind kind, uint64_t object_size, uint64_t blob_header_len)
+{
+    if (kind != ObjectKind::Blob)
+        return object_size;
+    if (object_size < blob_header_len)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS gc retire: blob object of {} bytes is smaller than the pool's fixed blob header ({} bytes)",
+            object_size, blob_header_len);
+    return object_size - blob_header_len;
+}
+
+bool shouldDeferRound(size_t changed_shards, bool graduation_due, uint64_t rounds_since_last_fold,
+                      uint64_t fold_threshold, uint64_t fold_max_defer_rounds)
+{
+    if (graduation_due)
+        return false;
+    if (changed_shards >= fold_threshold)
+        return false;
+    if (rounds_since_last_fold >= fold_max_defer_rounds)
+        return false;
+    return true;
+}
+
+Gc::Gc(PoolPtr store_, UInt128 gc_id_, std::function<uint64_t()> now_ms_fn_,
+       std::function<uint64_t()> mono_ms_fn_, LoggerPtr log_)
+    : store(std::move(store_))
+    , gc_id(gc_id_)
+    , logger(log_ ? std::move(log_) : getLogger("CasGc"))
+    , now_ms_fn(std::move(now_ms_fn_))
+    , mono_ms_fn(std::move(mono_ms_fn_))
+{
+    if (!store)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Cas::Gc: store must not be null");
+    if (gc_id == UInt128(0))
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Cas::Gc: gc_id must not be 0 (reserved for 'lease never held')");
+    if (store->poolConfig().gc_stuck_removal_rounds == 0)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "Cas::Gc: gc_stuck_removal_rounds must be nonzero");
+    if (!now_ms_fn)
+        now_ms_fn = []() -> uint64_t
+        {
+            return static_cast<uint64_t>(std::chrono::duration_cast<std::chrono::milliseconds>(
+                std::chrono::system_clock::now().time_since_epoch()).count());
+        };
+    /// Use `store->bootMsNow()`, not the raw static `Pool::bootMs()`
+    /// -- the latter bypasses the Pool's own injectable `config.boot_ms_fn`, so a time-controlled test
+    /// that fakes the mount's clock via `boot_ms_fn` (but constructs a `Gc` without an explicit
+    /// `mono_ms_fn`) would silently run its GC-side threshold math against the REAL wall clock while the
+    /// mount side runs against the fake one -- two desynced clocks passing for the same test.
+    /// `bootMsNow()` already falls back to `bootMs()` itself when no `boot_ms_fn` is injected, so
+    /// production (no test seam in play) is unaffected.
+    if (!mono_ms_fn)
+        mono_ms_fn = [s = store]() -> uint64_t { return s->bootMsNow(); };
+    /// Build the bounded pool for this round's per-hash freshness-meta writes here (ctor body),
+    /// not a member-initializer, so it can safely read `store->poolConfig()` AFTER the null check above.
+    const uint64_t configured_pool_size = store->poolConfig().gc_meta_pool_size;
+    const size_t pool_size = static_cast<size_t>(std::max<uint64_t>(1, configured_pool_size));
+    meta_pool = std::make_unique<ThreadPool>(CurrentMetrics::LocalThread, CurrentMetrics::LocalThreadActive,
+                                             CurrentMetrics::LocalThreadScheduled, pool_size);
+}
+
+void Gc::scheduleMetaJob(std::function<void()> job)
+{
+    /// Wrap once: `run` is safe to invoke either on the pool or inline (the scheduling-failure fallback
+    /// below), and NEVER lets an exception escape: a per-hash meta
+    /// op is advisory; the ledger + exact-token body delete are the actual safety core).
+    /// Capture the logger by value under a distinct name (the pool job may outlive nothing here, but it
+    /// must not depend on `this`; the copy also keeps the capture from shadowing the `logger` member).
+    /// `completed` is the `meta_pool_wait` phase's only visible signal: that phase's work runs HERE, on
+    /// a pool thread, so it contributes nothing to the round thread's ProfileEvents delta. Captured as a
+    /// raw pointer to the atomic rather than `this`, keeping `run`'s existing "must not depend on `this`"
+    /// property -- the pool is a member of the same `Gc` and is joined by `~Gc` before the atomic dies.
+    auto run = [job, job_logger = this->logger, completed = &meta_jobs_completed_]()
+    {
+        /// Count one per-hash freshness-meta op EXECUTED (attempt, not success) on this
+        /// bounded pool. `run` is invoked on the pool thread (the common path below) or inline on the
+        /// round's own thread (the scheduling-failure fallback below) -- either way this is pool-scoped
+        /// work, so the counter is GLOBAL-only by design.
+        ProfileEvents::increment(ProfileEvents::CASGCMetaOps);
+        try
+        {
+            job();
+        }
+        catch (...)
+        {
+            ProfileEvents::increment(ProfileEvents::CASGCMetaWriteAnomaly);
+            tryLogCurrentException(job_logger,
+                "CAS gc: a per-hash freshness-meta op failed on the bounded pool (advisory-only; "
+                "never wedges the round)");
+        }
+        /// After the catch: a job that threw still FINISHED, and `meta_pool_wait` reports drain
+        /// progress, not success (the anomaly counter above is what reports failure).
+        completed->fetch_add(1, std::memory_order_relaxed);
+    };
+    meta_jobs_scheduled_.fetch_add(1, std::memory_order_relaxed);
+    try
+    {
+        meta_pool->scheduleOrThrowOnError(run);
+    }
+    catch (...)
+    {
+        /// Scheduling itself failed (e.g. resource exhaustion under a mass-DROP burst) -- run inline
+        /// rather than silently lose the meta write. `run` still never throws.
+        ProfileEvents::increment(ProfileEvents::CASGCMetaWriteAnomaly);
+        tryLogCurrentException(logger,
+            "CAS gc: meta pool scheduling failed; running the op inline on the round's own thread");
+        run();
+    }
+}
+
+void Gc::scheduleCondemnMarkerWrite(const BlobRef & ref, const Token & token,
+                                    uint64_t condemn_round, uint64_t size)
+{
+    scheduleMetaJob([this, ref, token, condemn_round, size]()
+    {
+        if (writeCondemnedMeta(*store, ref, condemn_round, size))
+            noteCondemnMarkerDurable(ref, token);
+        /// A lost CAS / thrown error leaves the (ref, token) UNCONFIRMED: the graduation gate then
+        /// carries the entry and retries this write on a later round (fail-safe delay, triage §3.4).
+    });
+}
+
+void Gc::noteCondemnMarkerDurable(const BlobRef & ref, const Token & token)
+{
+    std::lock_guard lock(condemn_marker_mutex);
+    condemn_markers_confirmed.emplace(ref, token.value);
+}
+
+bool Gc::condemnMarkerConfirmedInProcess(const BlobRef & ref, const Token & token)
+{
+    std::lock_guard lock(condemn_marker_mutex);
+    return condemn_markers_confirmed.contains({ref, token.value});
+}
+
+void Gc::forgetCondemnMarker(const BlobRef & ref, const Token & token)
+{
+    std::lock_guard lock(condemn_marker_mutex);
+    condemn_markers_confirmed.erase({ref, token.value});
+}
+
+void Gc::runNamespaceJanitorPage(
+    const GcState & leased_state, bool suppress_destructive, uint64_t cleanup_evidence_rows)
+{
+    GcPhaseTimer t(phase_sink, "namespace_cleanup");
+    t.metric("evidence_rows", cleanup_evidence_rows);
+    NamespaceJanitorResult janitor_result;
+    try
+    {
+        Backend & backend = store->backend();
+        const Layout & layout = store->layout();
+        NamespaceJanitor janitor(backend, layout, 1000);
+        const uint64_t admitted_generation = leased_state.lease.seq;
+        janitor_result = janitor.runOnePage(suppress_destructive, [&]
+        {
+            const auto got = backend.get(layout.gcStateKey());
+            if (!got)
+                return false;
+            const GcState current = decodeGcState(got->bytes);
+            return current.lease.owner == gc_id && current.lease.seq == admitted_generation;
+        });
+        for (const String & anomaly : janitor_result.anomalies)
+            LOG_WARNING(logger, "CAS namespace janitor: {}", anomaly);
+        if (janitor_result.leaked)
+            ProfileEvents::increment(ProfileEvents::CASGCNamespaceCleanupLeaks, janitor_result.leaked);
+    }
+    catch (const std::exception & e)
+    {
+        LOG_WARNING(logger, "CAS namespace janitor skipped this round: {}", e.what());
+    }
+    t.metric("janitor_pages", janitor_result.pages);
+    t.metric("janitor_keys", janitor_result.keys);
+    t.metric("janitor_deleted", janitor_result.deleted);
+    t.metric("leaked", janitor_result.leaked);
+}
+
+RoundReport Gc::runRegularRound(std::function<void()> on_lease_acquired, bool allow_steal, UniversePolicy policy)
+{
+    RoundReport report;
+    GcState state;
+    Token state_token;
+    /// PHASE 1/18 `lease`. Also the ONLY phase a `NotALeader` round emits, which is why the phase rows
+    /// are correlated by `round_id` and not by the round number a follower never learns.
+    {
+        GcPhaseTimer t(phase_sink, "lease");
+        report.acquired_lease = acquireOrRenewLease(state, state_token, allow_steal);
+        t.metric("acquired", report.acquired_lease ? 1 : 0);
+        t.metric("steal_allowed", allow_steal ? 1 : 0);
+    }
+    if (!report.acquired_lease)
+        return report;
+
+    /// Baseline for the `meta_pool_wait` phase's job counts (the pool is per-`Gc`, the counters
+    /// cumulative), taken before anything in this round can schedule a job.
+    const uint64_t meta_jobs_scheduled_at_round_start = meta_jobs_scheduled_.load(std::memory_order_relaxed);
+    const uint64_t meta_jobs_completed_at_round_start = meta_jobs_completed_.load(std::memory_order_relaxed);
+
+    /// Fire the acquire-time hook BEFORE the long fold below, not after the round
+    /// returns - a new leader's first round could otherwise run for the whole fold with no
+    /// heartbeat cover, letting a follower steal deterministically once it freezes (owner, seq)
+    /// across two of its own ticks.
+    if (on_lease_acquired)
+        on_lease_acquired();
+
+    /// ONE-PASS round. There is no crash-resume step anymore: the round commits everything in the
+    /// SINGLE gc/state CAS at the end, so a crashed pass leaves only attempt-scoped debris that is
+    /// never adopted (retention prunes it), and every destructive PRE-CAS action below is justified by
+    /// PREVIOUSLY PUBLISHED durable state only (delete_pending entries), so replay under a fresh
+    /// attempt is idempotent.
+
+    const Layout & layout = store->layout();
+    Backend & backend = store->backend();
+    const uint64_t new_round = state.round + 1;
+
+    /// ONE budget instance for the WHOLE round, threaded into every destructive-or-observability-write
+    /// family below: `fold` (blob graduation/redelete, the `GcOutcomes` audit rows, the orphan-manifest
+    /// planner), `pruneSupersededGenerations`, the post-CAS hand-off reclaim (its OWN reserve, never
+    /// `pruneSupersededGenerations`' shared remainder), the post-CAS manifest-body cleanup, and
+    /// `cleanupRefObjects`. A cap is therefore cumulative over the round, never reset between families or
+    /// shards. See `GcRoundWorkBudget`'s own comment for the fail-closed contract each family applies on
+    /// exhaustion.
+    GcRoundWorkBudget round_work_budget;
+    round_work_budget.max_graduations = store->poolConfig().gc_round_graduation_budget;
+    round_work_budget.max_redeletes = store->poolConfig().gc_round_redelete_budget;
+    round_work_budget.max_sweep_namespaces = store->poolConfig().gc_round_sweep_namespace_budget;
+    round_work_budget.max_sweep_recovery_ops = store->poolConfig().gc_round_sweep_recovery_op_budget;
+    round_work_budget.max_ref_cleanup_objects = store->poolConfig().gc_round_ref_cleanup_budget;
+    round_work_budget.max_prefix_wholesale_objects = store->poolConfig().gc_round_prefix_wholesale_budget;
+    round_work_budget.max_handoff_prefix_wholesale_objects = store->poolConfig().gc_round_handoff_prefix_wholesale_budget;
+    round_work_budget.max_outcome_entries = store->poolConfig().gc_round_outcome_entry_budget;
+
+    /// The helping barrier precedes heartbeat work, DEFER, the hot stream LIST, and every successor
+    /// artifact. A deferred invocation therefore cannot leave a row the adopted parent already proved
+    /// complete, and a folding invocation takes its catalog cut only after the deletion settles.
+    {
+        GcPhaseTimer t(phase_sink, "pre_fold_ref_drain");
+        const CatalogLifecycleReconcileResult drain_result = drainCompletedRemoving(state);
+        for (const NamespaceLifeId & retired_life : drain_result.retired_lives)
+            store->invalidateRemovedCatalogLife(retired_life);
+        if (drain_result.authority_status != AuthorityStatus::Authoritative
+            || drain_result.catalog_resolution != CatalogResolution::DrainComplete)
+            throwCasWriteRetryLater("CAS GC pre-fold drain lost authority before the catalog settled");
+        t.metric("deleted", drain_result.deleted);
+    }
+
+    /// Token-guarded fence-out of dead mounts (liveness only — graduation itself paces on GC
+    /// rounds via `new_round`, not on heartbeat acks). Fencing no longer trusts a predecessor's stamped
+    /// `expires_at_ms` against our wall clock — it
+    /// fences ONLY once `mount_obs` has watched the mount's write-token hold unchanged for the full
+    /// threshold on THIS leader's own monotonic clock (mirrors `claimMountAwaitingExpiry`'s identical
+    /// `TTL + Drift` threshold for a mount's own reopen).
+    const uint64_t ttl_ms = static_cast<uint64_t>(store->poolConfig().mount_lease_ttl_ms.count());
+    /// The formula is shared with `claimMountAwaitingExpiry` via
+    /// `mountObservationThresholdMs` -- see its doc comment (CasServerRoot.h).
+    const uint64_t stable_threshold_ms = mountObservationThresholdMs(
+        ttl_ms, static_cast<uint64_t>(store->poolConfig().mount_renew_period.count()));
+
+    /// PHASE 3/18 `heartbeat_floor`: one LIST of `gc/server-roots/`, one GET per mount slot, and a fence
+    /// PUT per newly-fenced mount.
+    {
+        GcPhaseTimer t(phase_sink, "heartbeat_floor");
+        const HeartbeatFloor floor = computeHeartbeatFloor(backend, layout, now_ms_fn(), mono_ms_fn(),
+                                                           stable_threshold_ms, mount_obs);
+        report.fence_outs = floor.fenced_now;
+        if (floor.fenced_now > 0)
+            ProfileEvents::increment(ProfileEvents::CASGCHeartbeatFenceOuts, floor.fenced_now);
+
+        /// GcFenceOut audit row per expired mount fenced-out this round: the round latched a fence-out to
+        /// re-arm a sleeper's write fence (its held token is now invalid). One row per srid so the log
+        /// reconstructs which mount was reclaimed.
+        for (const String & srid : floor.fenced_srids)
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::GcFenceOut;
+                e.object_kind = CasEventObjectKind::Snap;
+                e.round = new_round;
+                e.gen = state.snap_generation;
+                e.outcome = "fenced";
+                e.reason = "expired mount lease past skew margin; token-guarded fence-out re-arms the write "
+                           "fence (prevents a resumed sleeper from mutating)";
+                e.detail = {{"server_root_id", srid}};
+            });
+
+        /// Emit the round's heartbeat classification (what mounts are live/terminated/fenced this round).
+        EventEmitter{*store}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::GcFence;
+            e.object_kind = CasEventObjectKind::Snap;
+            e.round = new_round;
+            e.gen = state.snap_generation;
+            e.outcome = "floor";
+            e.reason = "R1: heartbeat classification (live/terminated/fenced mounts)";
+            e.detail = {{"live", std::to_string(floor.live)},
+                        {"terminated", std::to_string(floor.terminated)},
+                        {"fenced_now", std::to_string(floor.fenced_now)},
+                        {"already_fenced", std::to_string(floor.already_fenced)}};
+        });
+
+        t.metric("live", floor.live);
+        t.metric("terminated", floor.terminated);
+        t.metric("fenced_now", floor.fenced_now);
+        t.metric("already_fenced", floor.already_fenced);
+    }
+
+    /// Decide DEFER vs FOLD from cheap pre-fold signals.
+    /// A DEFER round re-adopts the sealed generation — no fold, no delete, no gc/state write — so a
+    /// slow idle/small-delta round no longer rebuilds the whole in-degree snapshot. Safety: a due
+    /// graduation forces a FOLD (graduationDue), so no destructive decision runs on a stale snapshot.
+    ///
+    /// `listRefPrefix` is the round's one full enumeration of `cas/ns/stream/`. Its result is retained
+    /// (rather than discarded once the defer decision is taken) because `fold` regroups the very same
+    /// keys instead of listing the prefix again. A deferred round simply drops it.
+    ///
+    /// PHASE 4/18 `defer_decision`. `ref_scan` OUTLIVES the timer because the fold consumes it, and
+    /// `report.deferred` is set INSIDE the scope so the row already reflects the verdict when the
+    /// timer's destructor fires on the deferred round's early return. This phase also performs TWO of
+    /// the round's reads of the adopted fold seal (`graduationDue` and `listRefPrefix` each read the
+    /// same key) -- see `fold_seal_reads` below.
+    std::optional<RefPlan> walk_plan;
+    bool defer_round = false;
+    {
+        GcPhaseTimer t(phase_sink, "defer_decision");
+        const bool graduation_due = graduationDue(state, new_round);
+        walk_plan.emplace(buildRefWalkPlan(listRefPrefix(state)));
+        reportStuckRemovals(*walk_plan, state.round);
+        const RefScanSummary & ref_scan = walk_plan->refScan();
+        const size_t changed = walk_plan->changedRows();
+        defer_round = shouldDeferRound(changed, graduation_due, rounds_since_last_fold_,
+                                       store->poolConfig().gc_fold_threshold,
+                                       store->poolConfig().gc_fold_max_defer_rounds);
+        uint64_t ref_log_keys = 0;
+        for (const auto & [scanned_life, ids] : ref_scan.logs_by_life)
+            ref_log_keys += ids.size();
+        t.metric("changed_shards", changed);
+        t.metric("namespaces_seen", ref_scan.max_log_by_life.size());
+        t.metric("ref_log_keys_listed", ref_log_keys);
+        t.metric("ref_keys_listed", ref_scan.keys.size());
+        t.metric("graduation_due", graduation_due ? 1 : 0);
+        t.metric("dead_life_debris", ref_scan.dead_life_debris);
+        t.metric("walk_plan_builds", 1);
+        t.metric("walk_plan_rows", walk_plan->size());
+        t.metric("walk_plan_dropped_parent_rows", walk_plan->droppedParentRows());
+        t.metric("walk_plan_dropped_listed_lives", walk_plan->droppedListedLives());
+        t.metric("walk_plan_dropped_tails", walk_plan->droppedTails());
+        t.metric("deferred", defer_round ? 1 : 0);
+        /// The number of consecutive rounds already deferred BEFORE this one (this round's own verdict is
+        /// `deferred` above), so the pair reads unambiguously against `gc_fold_max_defer_rounds`.
+        t.metric("rounds_deferred_before", rounds_since_last_fold_);
+        /// `graduationDue` and `listRefPrefix` each GET the adopted fold seal at the SAME
+        /// (generation, attempt). Recorded, not fixed -- see the `fold_seal_read` phase, which records
+        /// the other duplicate pair; the round GETs that one key FIVE times on a folding round.
+        t.metric("fold_seal_reads", 2);
+
+        if (defer_round)
+        {
+            ++rounds_since_last_fold_;
+            report.deferred = true;
+            /// A DEFER round mints no new round -- unlike the fold path below (CasGc.cpp:642), which sets
+            /// `report.round = state.round` only AFTER the round's single `gc/state` CAS has committed
+            /// `next.round = new_round` and `state` was reassigned to that committed `next` (so on that
+            /// path `state.round` reads the FRESH round number). Here the round CAS never runs, so `state`
+            /// is still the round that was already adopted BEFORE this round started: `state.round` is the
+            /// honest, already-durable round number, while `new_round` (`state.round + 1`) would report a
+            /// round that never actually happened. Use `state.round` so `RoundReport::round` and the
+            /// `system.cas_gc_log` row it feeds never print a fabricated
+            /// round number on a deferred round.
+            report.round = state.round;
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::GcFence;   /// reuse the Snap round-event channel; outcome = "deferred"
+                e.object_kind = CasEventObjectKind::Snap;
+                e.round = state.round;
+                e.gen = state.snap_generation;
+                e.outcome = "deferred";
+                e.reason = "skip-unchanged: no changed shard reached the fold threshold and no graduation "
+                           "is due; re-adopting the sealed generation (snapshot rebuild elided)";
+                e.detail = {{"changed_shards", std::to_string(changed)},
+                            {"rounds_since_last_fold", std::to_string(rounds_since_last_fold_)}};
+            });
+            /// Return after the timer scope so the independently timed janitor phase is not nested
+            /// inside `defer_decision`.
+        }
+        else
+            rounds_since_last_fold_ = 0;   /// this round folds
+    }
+
+    if (defer_round)
+    {
+        /// DEFER has no `FoldResult`, hence no complete global destructive verdict. The janitor still
+        /// takes its bounded page and catalog cut, but suppression keeps both deletes and valid-page
+        /// cursor progress at the same position for the bounded forced fold to retry.
+        runNamespaceJanitorPage(state, /*suppress_destructive=*/true, /*cleanup_evidence_rows=*/0);
+        return report;   /// no fold, no pre-CAS deletes, no gc/state CAS — sealed generation stays pinned
+    }
+
+    /// Emit that the round's single pass begins.
+    EventEmitter{*store}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::GcFoldBegin;
+        e.object_kind = CasEventObjectKind::Snap;
+        e.round = state.round;
+        e.gen = state.snap_generation;
+        e.reason = "R2: one-pass fold (edges x deltas x retired) into a new durable generation";
+    });
+
+    /// Capture the PARENT seal's run refs BEFORE fold mutates
+    /// `state.snap_generation`/`snap_attempt` in-memory (CasGc.cpp:838). We compare these against the
+    /// NEW seal's refs post-CAS to detect a ref that moved OFF an already-pruned generation (the
+    /// wholesale prune skipped it while it was still referenced and its cursor advanced past it), and
+    /// hand-off delete that generation's now-unreferenced leftover. Absent parent seal => empty.
+    ///
+    /// PHASE 5/18 `parent_seal_read`: the round's THIRD GET of the adopted fold seal (`graduationDue`
+    /// and `listRefPrefix` already read it in `defer_decision`, and `fold` reads it twice more). One
+    /// small GET, given its own row rather than left untimed, because "the same key, five times a round"
+    /// is only actionable if each read is attributable to a phase.
+    std::vector<RunRef> parent_seal_runs;
+    {
+        GcPhaseTimer t(phase_sink, "parent_seal_read");
+        if (const auto parent_seal = readFoldSeal(state.snap_generation, state.snap_attempt))
+            parent_seal_runs = parent_seal->blob_target_runs;
+        t.metric("parent_runs", parent_seal_runs.size());
+    }
+
+    /// The pass performs discovery, windowing, and the three-cursor merge (spare / graduate / condemn).
+    /// It emits phases 5..10 of its own.
+    FoldResult folded = fold(state, state_token, report, new_round, *walk_plan, policy, round_work_budget);
+
+    /// THE ROUND'S DESTRUCTIVE GATE, read once, here, and consulted at EVERY destructive site below.
+    /// It is available this early because `fold` computes it (see `FoldResult::suppress_destructive`),
+    /// and it has to be: the first destructive site of the post-CAS tail is the hand-off reclaim, which
+    /// used to run before this value was ever read.
+    const bool suppress_destructive = folded.suppress_destructive;
+
+    EventEmitter{*store}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::GcFoldEnd;
+        e.object_kind = CasEventObjectKind::Snap;
+        e.round = state.round;
+        e.gen = state.snap_generation;
+        e.outcome = "ok";
+        e.reason = "R2 complete";
+        e.detail = {{"shards", std::to_string(folded.root_shards.size())},
+                    {"anomalies", std::to_string(report.anomalies.size())}};
+    });
+
+    const uint64_t generation = state.snap_generation;   /// set in-memory by fold; committed below
+    const uint64_t attempt = state.snap_attempt;
+
+    /// PRE-CAS deletes affect ONLY entries the PREVIOUS pass published as delete_pending (justified by
+    /// durable state and safe at any leader staleness), plus outcome bookkeeping for
+    /// every settled entry. THE SINGLE CONTENT-DELETE SITE.
+    ///
+    /// PHASE 11/18 `pending_deletes`, covering both the exact-token delete loop and the outcome-log
+    /// writes it feeds. Held in an `optional` rather than a `{ }` scope purely so this long, delicate
+    /// block is not reindented wholesale; `reset()` below is what emits the row, and an exception
+    /// escaping before it still emits from the destructor.
+    std::optional<GcPhaseTimer> pending_deletes_timer;
+    pending_deletes_timer.emplace(phase_sink, "pending_deletes");
+    const uint64_t redeleted_before = report.redeleted;
+    const uint64_t graduated_before = report.graduated;
+    std::map<uint64_t, OutcomeLog> outcomes;
+    for (uint64_t shard = 0; shard < folded.retired_merge.size(); ++shard)
+    {
+        RetiredMergeResult & merge = folded.retired_merge[shard];
+        /// The gate, stated at the site, and scoped to the DELETES alone: the spare / graduate / replace
+        /// bookkeeping below is not destructive and must still run on a suppressed round (a suppressed
+        /// round still condemns, spares and carries -- only irreversible work stops). A suppressed pass
+        /// produces an EMPTY `redelete` by construction (`settleEntry` carries every pending entry
+        /// unchanged instead of promoting it), so this loop would already do nothing -- but "would
+        /// already do nothing" is a property of another file, and the content-delete site does not
+        /// delegate its own gate. If the two ever disagree, the round deletes nothing rather than
+        /// deleting on a frontier it cannot prove.
+        ///
+        /// WHAT THIS GUARD DOES AND DOES NOT COVER, measured rather than assumed: it stops the delete
+        /// I/O, and nothing else. `settleEntry`'s gate is the primary one because promoting an entry to
+        /// `redelete` also DROPS it from `still_retired` -- so a build with only this guard performs no
+        /// delete and still loses the entry from the pipeline, leaking the blob instead of reclaiming
+        /// it. Do not read this as a licence to relax the merge-side gate.
+        static const std::vector<RetiredEntry> kNothingToDelete;
+        const std::vector<RetiredEntry> & redelete_now =
+            suppress_destructive ? kNothingToDelete : merge.redelete;
+        for (const RetiredEntry & entry : redelete_now)
+        {
+            DeleteOutcome del = backend.deleteExact(layout.blobKey(entry.ref), entry.token);
+            if (del.created_delete_marker)
+                throw Exception(ErrorCodes::LOGICAL_ERROR,
+                    "CAS gc: delete of blob {} created a delete marker — versioning is enabled "
+                    "on the pool (mis-provisioned; the capability probe must reject this)", blobIdOf(entry.ref));
+
+            /// A RustFS quirk: a conditional delete (`If-Match`) against an ABSENT
+            /// object can answer HTTP 412 (precondition failed) instead of 404 — we map that 412 to
+            /// TokenMismatch. Backend-agnostically disambiguate here: a genuine TokenMismatch means the
+            /// object exists under a different (fresh) token; if a follow-up HEAD shows the object is
+            /// gone, the "mismatch" was actually the object being absent — treat it as Absent (NotFound)
+            /// end-to-end so the `.meta` cleanup below still runs.
+            bool absent_on_mismatch_quirk = false;
+            if (del.kind == DeleteOutcome::Kind::TokenMismatch)
+            {
+                const HeadResult head = backend.head(layout.blobKey(entry.ref));
+                if (!head.exists)
+                {
+                    del.kind = DeleteOutcome::Kind::NotFound;
+                    absent_on_mismatch_quirk = true;
+                }
+            }
+
+            const DeleteClass del_class = classifyDeleteOutcome(del);
+            const OutcomeKind outcome_kind = del_class == DeleteClass::Deleted ? OutcomeKind::Deleted
+                                            : del_class == DeleteClass::Absent ? OutcomeKind::Absent
+                                                                                : OutcomeKind::Replaced;
+            OutcomeEntry outcome{.kind = entry.kind, .ref = entry.ref, .token = entry.token, .outcome = outcome_kind};
+            const String del_outcome{deleteClassName(del_class)};
+            /// The single content-delete site is attributable per row. TokenMismatch (a writer
+            /// recreated the incarnation) is terminal-OK: the fresh incarnation is a live object.
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::BlobDelete;
+                e.object_kind = CasEventObjectKind::Blob;
+                e.object_hash = blobIdOf(entry.ref);
+                e.token = entry.token.value;
+                e.round = new_round;
+                e.gen = generation;
+                e.outcome = del_outcome;
+                e.reason = absent_on_mismatch_quirk
+                    ? "delete_pending published by a prior pass; exact-token delete (pre-CAS) "
+                      "(delete returned token-mismatch but the object is absent — backend 412-on-absent quirk)"
+                    : "delete_pending published by a prior pass; exact-token delete (pre-CAS)";
+                e.detail = {{"condemn_round", std::to_string(entry.condemn_round)},
+                            {"key", layout.blobKey(entry.ref)}};
+            });
+            /// The audit row is observability only -- the delete above already executed regardless of
+            /// this cap. Skipping it here bounds the per-shard `GcOutcomes` body without skipping or
+            /// deferring any destructive work.
+            if (round_work_budget.outcomeEntryAvailable())
+            {
+                outcomes[shard].entries.push_back(std::move(outcome));
+                ++round_work_budget.outcome_entries_used;
+            }
+            ++report.redeleted;
+            ProfileEvents::increment(ProfileEvents::CASGCRetiredRedeleted);
+            /// Drop the per-hash meta only on Deleted/NotFound — a Replaced (TokenMismatch) outcome
+            /// means a writer already resurrected a fresh incarnation at this hash (INV-1), and that
+            /// writer's own resurrect path already flipped the meta back to Clean; blindly deleting here
+            /// would race that legitimate Clean write for no reason (the meta is advisory, but there is no
+            /// reason to touch it on that path at all).
+            if (del_class == DeleteClass::Deleted || del_class == DeleteClass::Absent)
+            {
+                const BlobRef ref = entry.ref;
+                scheduleMetaJob([this, ref]() { deleteConfirmedMeta(store->backend(), store->layout(), ref); });
+            }
+            /// The entry left the pipeline — drop its in-process condemn-marker confirmation.
+            forgetCondemnMarker(entry.ref, entry.token);
+        }
+        for (const RetiredEntry & entry : merge.spared)
+        {
+            /// A fresh dedup-adopt raced the condemn (see the matching CasGcFold Debug log emitted
+            /// during the merge, which increments CASGCRetiredSparedByReref) -- not an ack-floor
+            /// violation, so this is Debug, not a page-worthy Warning.
+            if (entry.delete_pending)
+                LOG_DEBUG(logger,
+                    "CAS gc: delete_pending blob {} (condemned at round {}, this round {}) recovered "
+                    "in-degree -- a fresh dedup-adopt raced the condemn; spared (never a fail-closed delete)",
+                    blobIdOf(entry.ref), entry.condemn_round, new_round);
+            /// Emit the spare verdict — a publish re-pinned the candidate before graduation.
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::GcRecheckVerdict;
+                e.object_kind = CasEventObjectKind::Blob;
+                e.object_hash = blobIdOf(entry.ref);
+                e.token = entry.token.value;
+                e.round = new_round;
+                e.gen = generation;
+                e.outcome = "spared";
+                e.reason = "in-degree recovered in the pass merge; entry dropped";
+            });
+            /// The audit row is observability only -- `settleEntry` already unconditionally spared this
+            /// entry (INV_NO_LOSS: recovery wins past any budget), so this cap can never re-condemn it;
+            /// it only bounds whether the decision gets a `GcOutcomes` row.
+            if (round_work_budget.outcomeEntryAvailable())
+            {
+                outcomes[shard].entries.push_back(OutcomeEntry{.kind = entry.kind, .ref = entry.ref,
+                                                               .token = entry.token, .outcome = OutcomeKind::Spared});
+                ++round_work_budget.outcome_entries_used;
+            }
+            ProfileEvents::increment(ProfileEvents::CASGCRetiredSpared);
+            /// A spare does NOT touch the
+            /// meta. GC freshness meta is add-only — GC never publishes `Clean`. The in-degree recovered,
+            /// but the meta stays `Condemned` (conservative marker) until a WRITER displaces the body with
+            /// a fresh incarnation token (`uploadFromSource` + `writeResurrectMetaClean`) — the SOLE
+            /// `Condemned -> Clean` transition. Clearing here on a
+            /// deposed leader that then lost its round CAS would strand a stray-`Clean` over a still-live
+            /// condemned token and lose the reuse to a stale exact-token redelete (INV_NO_LOSS); see
+            /// The next `putBlob` self-heals
+            /// the marker: `observeAndAdmit` refuses same-token adoption on `Condemned` and resurrects.
+            /// The entry left the pipeline — drop its in-process condemn-marker confirmation.
+            forgetCondemnMarker(entry.ref, entry.token);
+        }
+        for (const RetiredEntry & entry : merge.graduated)
+        {
+            ++report.graduated;
+            ProfileEvents::increment(ProfileEvents::CASGCRetiredGraduated);
+            /// Floor-passed — republished pending; the NEXT pass executes the delete.
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::GcRecheckVerdict;
+                e.object_kind = CasEventObjectKind::Blob;
+                e.object_hash = blobIdOf(entry.ref);
+                e.token = entry.token.value;
+                e.round = new_round;
+                e.gen = generation;
+                e.outcome = "pending";
+                e.reason = "condemn_round < current_round; published delete_pending (two-phase graduation)";
+                e.detail = {{"condemn_round", std::to_string(entry.condemn_round)}};
+            });
+        }
+        for (const ReplacedEntry & replaced : merge.replaced)
+        {
+            const RetiredEntry & entry = replaced.fresh;
+            ProfileEvents::increment(ProfileEvents::CASGCRetireReplaced);
+            /// RESURRECT-REUPLOAD-ORPHAN: the current object token differed from a stale retired entry;
+            /// the fold superseded that entry and re-condemned the current token in the same window.
+            /// `detail["superseded_token"]` carries the STALE token the supersede
+            /// dropped — `entry.token` above is only the fresh CURRENT token, and without the old one
+            /// this event cannot tell an operator WHICH incarnation was replaced.
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::BlobRetireReplaced;
+                e.object_kind = CasEventObjectKind::Blob;
+                e.object_hash = blobIdOf(entry.ref);
+                e.token = entry.token.value;
+                e.round = new_round;
+                e.gen = generation;
+                e.outcome = "replaced";
+                e.reason = "current object token differs from the retired entry — resurrect replaced the "
+                           "incarnation; superseded the stale entry and re-condemned the current token";
+                e.detail = {{"superseded_token", replaced.old_token.value}};
+            });
+            /// The supersede is ALSO a blob entering the retired set fresh (a re-condemn of the
+            /// CURRENT token) — write the meta Condemned exactly like a fresh `head_blob` condemn would,
+            /// so a NEXT writer's point-read gate sees it (and the graduation gate gets its (hash, token)
+            /// confirmation on success). `peek_head` itself stays side-effect-free (it runs once per
+            /// closed candidate, not just on a real supersede — see its own comment). The SUPERSEDED
+            /// (stale) token's in-process confirmation is dropped — that entry left the pipeline.
+            scheduleCondemnMarkerWrite(entry.ref, entry.token, entry.condemn_round, entry.size);
+            forgetCondemnMarker(entry.ref, replaced.old_token);
+        }
+    }
+
+    /// Outcome logs: write-once + byte-adopt (observation-bearing HEAD tokens — never the
+    /// deterministic-artifact path). Tally the report from the FINAL durable logs.
+    for (auto & [shard, log] : outcomes)
+    {
+        const String key = layout.outcomesKey(generation, attempt, new_round, shard);
+        const String body = sealObject(FormatId::GcOutcomes, encodeOutcomeLog(log));
+        if (backend.putIfAbsent(key, body).outcome == PutOutcome::PreconditionFailed)
+        {
+            const auto existing = backend.get(key);
+            if (!existing)
+                throw Exception(ErrorCodes::ABORTED,
+                    "CAS gc: outcome log at {} vanished between putIfAbsent and read", key);
+            if (existing->bytes != body)
+            {
+                try { log = decodeOutcomeLog(openObject(FormatId::GcOutcomes, existing->bytes)); }
+                catch (const Exception & e)
+                {
+                    throw Exception(ErrorCodes::ABORTED,
+                        "CAS gc: undecodable outcome log at {} cannot be adopted: {}", key, e.message());
+                }
+            }
+        }
+        for (const OutcomeEntry & o : log.entries)
+        {
+            switch (o.outcome)
+            {
+                case OutcomeKind::Deleted: ++report.deleted; break;
+                case OutcomeKind::Absent: ++report.absent; break;
+                case OutcomeKind::Replaced: ++report.replaced; break;
+                case OutcomeKind::Spared: ++report.spared; break;
+            }
+        }
+    }
+    pending_deletes_timer->metric("redeleted", report.redeleted - redeleted_before);
+    pending_deletes_timer->metric("graduated", report.graduated - graduated_before);
+    pending_deletes_timer->metric("deleted", report.deleted);
+    pending_deletes_timer->metric("absent", report.absent);
+    pending_deletes_timer->metric("replaced", report.replaced);
+    pending_deletes_timer->metric("spared", report.spared);
+    pending_deletes_timer->metric("outcome_logs_written", outcomes.size());
+    pending_deletes_timer.reset();   /// emits the `pending_deletes` row
+
+    /// Wait for the round's whole batch of per-hash freshness-meta writes (condemned during the
+    /// fold above, spared/redeleted-confirmed during R3 above) BEFORE the round's retired-list publish and
+    /// its single gc/state CAS below — the writer's meta point-read gate must see this round's condemns
+    /// durable no later than the ledger it is paired with. `wait()` never throws here: every scheduled job
+    /// already caught its own exception (see `scheduleMetaJob`).
+    ///
+    /// PHASE 12/18 `meta_pool_wait`, AND THE ONE HONEST GAP IN THIS INSTRUMENTATION: the work being
+    /// waited on runs on `meta_pool` threads, so none of it appears in this thread's `ProfileEvents`
+    /// delta and the row's `ProfileEvents` map is EMPTY BY CONSTRUCTION. That is not a phase with no
+    /// cost -- it is a phase whose cost this mechanism cannot see, so it carries explicit job counts
+    /// instead: read `jobs_scheduled` / `jobs_completed` next to the duration to tell "the queue was
+    /// deep" from "the endpoint was slow". `jobs_completed` is sampled BEFORE the wait deliberately --
+    /// after it, it would always equal `jobs_scheduled` and say nothing.
+    {
+        GcPhaseTimer t(phase_sink, "meta_pool_wait");
+        const uint64_t scheduled = meta_jobs_scheduled_.load(std::memory_order_relaxed)
+            - meta_jobs_scheduled_at_round_start;
+        const uint64_t completed_on_entry = meta_jobs_completed_.load(std::memory_order_relaxed)
+            - meta_jobs_completed_at_round_start;
+        meta_pool->wait();
+        t.metric("jobs_scheduled", scheduled);
+        t.metric("jobs_completed_on_entry", completed_on_entry);
+        t.metric("jobs_completed", meta_jobs_completed_.load(std::memory_order_relaxed)
+                                   - meta_jobs_completed_at_round_start);
+    }
+
+    /// Retired-in-snapshot — there is NO separate retired-list object to publish anymore. The
+    /// round's surviving condemned entries were already sealed as `kCondemned` rows inside the fold's
+    /// `blob_target_runs` (durable before this CAS, via `putDeterministicArtifact`), and the per-shard
+    /// `condemned_summary` the seal carries makes the next round's graduation/carry decisions zero-I/O.
+
+    /// The SINGLE round CAS publishes the round, adopted (generation, attempt), and retention cursor.
+    ///
+    /// PHASE 13/18 `round_commit`. It deliberately covers BOTH the retention prune (heavy: LISTs and
+    /// wholesale deletes, bounded at 64 generations a round) and the single `gc/state` CAS (trivial),
+    /// because the prune's writes are only safe as a pre-CAS action and splitting the two would suggest
+    /// they are independently retryable. `generations_visited` vs the prune's actual deletes is what
+    /// separates the two costs inside the row.
+    std::optional<GcPhaseTimer> round_commit_timer;
+    round_commit_timer.emplace(phase_sink, "round_commit");
+    const String manifest_sweep_cursor_before = state.manifest_sweep_cursor;
+    GcState next = state;
+    next.round = new_round;
+    if (!suppress_destructive && store->poolConfig().manifest_sweep_list_budget_keys > 0)
+        next.manifest_sweep_cursor = folded.orphan_sweep.next_cursor;
+    /// The generations the adopted seal's runs physically live in (reference-parent carry can point a
+    /// current shard's run back at an older generation's key). Retention must never reclaim these.
+    std::set<uint64_t> referenced_generations;
+    for (const RunRef & r : folded.fold_seal.blob_target_runs)
+        referenced_generations.insert(r.generation);
+    /// ALSO protect every generation the PARENT (currently-adopted, pre-fold) seal references
+    /// (`parent_seal_runs`, captured above): this prune runs BEFORE the round's own gc/state CAS below, so
+    /// a losing leader must not destroy what the winning leader's already-adopted seal still points at —
+    /// pre-CAS destructive actions may only rely on PREVIOUSLY PUBLISHED state (triage #5).
+    for (const RunRef & r : parent_seal_runs)
+        referenced_generations.insert(r.generation);
+    /// Retention floor uses THIS round's (post-fold) `generation`, so `gc_snapshot_generations_to_keep`
+    /// keeps exactly that many generations back from the current one. If this round's `gc/state` CAS
+    /// then LOSES, the prune reclaimed one generation deeper than the durably-adopted generation would
+    /// imply -- an accepted forensics-window slack, not a data-loss risk: every still-reachable
+    /// run/blob is independently protected via `referenced_generations` (captured pre-fold above).
+    const uint64_t pruned_through_before = state.snap_pruned_through;
+    pruneSupersededGenerations(generation, attempt, next, referenced_generations, suppress_destructive,
+                               round_work_budget);
+    round_commit_timer->metric("generations_visited", next.snap_pruned_through - pruned_through_before);
+    round_commit_timer->metric("pruned_through", next.snap_pruned_through);
+    round_commit_timer->metric("generations_referenced", referenced_generations.size());
+    const CasResult res = backend.casPut(layout.gcStateKey(), encodeGcState(next), state_token);
+    if (res.outcome != CasOutcome::Committed)
+        throw Exception(ErrorCodes::ABORTED,
+            "CAS gc round: gc/state moved during the round (another leader advanced it); retry next round");
+    state = std::move(next);
+    state_token = res.token;
+    report.round = state.round;
+    round_commit_timer->metric("round", report.round);
+    round_commit_timer->metric("generation", generation);
+    round_commit_timer.reset();   /// emits the `round_commit` row
+
+    /// Task 7: the retire pipeline's REMAINING sizes, read from the seal this round's CAS just
+    /// committed (`folded.fold_seal.condemned_summary` is TOTAL over every gc-shard -- see its own doc
+    /// comment in `CasFoldSealFormat.h`). Zero-shard pools (never folded) leave these at 0.
+    for (const auto & [shard, summary] : folded.fold_seal.condemned_summary)
+    {
+        report.pending_retired += summary.pending_total;
+        report.pending_candidates += summary.condemned_total - summary.pending_total;
+        report.pending_condemned += summary.condemned_total;
+    }
+
+    /// Post-CAS reference-parent HAND-OFF DELETE. `pruneSupersededGenerations` SKIPS a
+    /// generation the live seal still references AND advances `snap_pruned_through` PAST it
+    /// (CasGc.cpp:1066 computes the cursor as `g - 1` after the loop increments `g` over every skipped
+    /// generation). So once a skipped generation is behind the cursor, the wholesale prune NEVER revisits
+    /// it — a ref that later moves off it would strand that generation's WHOLE prefix (fold seal, retired/
+    /// outcomes sets, all shards' runs), not just the single carried run object. Reclaim it HERE, now that
+    /// the ref has moved: for every parent ref whose generation is already pruned-through and whose
+    /// generation NO new live ref still references, wholesale-delete that generation's prefix — the exact
+    /// reclaimer the normal prune would have used, deferred until the ref finally moved off. Best-effort:
+    /// a crash between the CAS and here leaks the prefix to fsck (single-crash window, no permanent leak —
+    /// but note the cursor already advanced, so a plain retry will NOT re-attempt it; fsck is the backstop).
+    ///
+    /// PHASE 14/18 `handoff_reclaim`.
+    {
+        GcPhaseTimer t(phase_sink, "handoff_reclaim");
+        uint64_t objects_reclaimed = 0;
+        std::set<uint64_t> new_referenced_generations;
+        for (const RunRef & r : folded.fold_seal.blob_target_runs)
+            new_referenced_generations.insert(r.generation);
+
+        std::set<uint64_t> handed_off;   /// dedupe: multiple parent refs can share one generation
+        /// GATED like every other destructive site, and it is also the FIRST destructive site of the
+        /// post-CAS tail -- which is why the gate is read before the tail begins rather than partway
+        /// down it.
+        ///
+        /// UNLIKE EVERY OTHER GATED SITE, SUPPRESSION HERE LOSES THE WORK RATHER THAN POSTPONING IT.
+        /// The hand-off is a one-shot DIFFERENCE between the parent seal's runs and the new seal's, and
+        /// a suppressed round still FOLDS -- only the irreversible half stops -- so the ref moves off
+        /// the old generation on this very round and the next round's parent seal no longer names it.
+        /// Nothing revisits it: `snap_pruned_through` is already past that generation and the wholesale
+        /// prune only walks forward. The prefix is left to `fsck`, which is the same outcome this site
+        /// already documents for a crash in this window (see the PHASE 14/18 comment above). Bounded --
+        /// one small run per shard per occurrence of a suppressed round that also folded a delta -- and
+        /// not a correctness problem, but it is the one place where the gate costs something permanent,
+        /// so it is asserted rather than left to be discovered.
+        static const std::vector<RunRef> kNoRuns;
+        const std::vector<RunRef> & handoff_candidates =
+            suppress_destructive ? kNoRuns : parent_seal_runs;
+        for (const RunRef & old_ref : handoff_candidates)
+        {
+            /// Only generations the wholesale prune already passed AND that no live ref still pins.
+            if (old_ref.generation > state.snap_pruned_through)
+                continue;   /// not yet pruned-through: the normal prune will reclaim it when it ages out
+            if (new_referenced_generations.contains(old_ref.generation))
+                continue;   /// still referenced by a (possibly different-shard) live ref: keep it
+            if (!handed_off.insert(old_ref.generation).second)
+                continue;   /// already reclaimed this round via another shard's ref
+            /// `bounded_remaining` draws from the hand-off's OWN reserve, never `UINT64_MAX` and never
+            /// `pruneSupersededGenerations`' shared remainder: this hand-off is a ONE-SHOT event (see the
+            /// PHASE 14/18 comment above) -- a generation this call reclaims only PARTIALLY is left
+            /// exactly like a crash in this window already is -- to `fsck`, never revisited by a later
+            /// round's hand-off (the parent-seal difference that triggers it does not recur once the ref
+            /// has moved). The prune, by contrast, safely retries an under-served generation next round
+            /// via its cursor, so sharing one pool would let a prune-heavy round strand this one-shot
+            /// reclaim at zero every time; the separate reserve makes that impossible.
+            const uint64_t remaining = round_work_budget.handoffPrefixWholesaleRemaining();
+            if (remaining == 0)
+                break;
+            const uint64_t reclaimed = deletePrefixWholesale(
+                backend, layout.gcGenPrefix(old_ref.generation), remaining);
+            round_work_budget.handoff_prefix_wholesale_objects_used += reclaimed;
+            objects_reclaimed += reclaimed;
+            LOG_TRACE(logger,
+                "CAS GC hand-off: generation {} moved out of the live seal below the retention cursor "
+                "({} objects) — post-CAS wholesale reclaim (the prune had skipped it while referenced)",
+                old_ref.generation, reclaimed);
+        }
+        t.metric("generations_reclaimed", handed_off.size());
+        t.metric("objects_reclaimed", objects_reclaimed);
+        t.metric("suppressed", suppress_destructive ? 1 : 0);
+    }
+
+    /// Post-CAS: owner-removed manifest bodies — deleted ONLY now, after their decrements were
+    /// ADOPTED by the round CAS (delete-after-sealed-decrements). NOT durable across rounds: the
+    /// ref-log intake cursor that discovered each `-1` edge is committed by THIS round's CAS above,
+    /// so a log already folded is never re-visited and never re-populates `mf_cleanup`. This phase is
+    /// deliberately unbudgeted: a cap would leave a declined entry unreachable from any live ref AND
+    /// never re-derived by this pipeline, converting a bounded burst into a permanent leak. It drains
+    /// the whole of `folded.mf_cleanup` every round it runs; only a crash (or the destructive-suppression
+    /// gate below) leaves an entry for the orphan-manifest sweep to reclaim later.
+    ///
+    /// PHASE 15/18 `manifest_deletes`.
+    {
+        GcPhaseTimer t(phase_sink, "manifest_deletes");
+        const uint64_t manifests_deleted_before = report.manifests_deleted;
+        /// GATED. A manifest body is content the ref graph still describes until its decrements are both
+        /// sealed AND taken on a round that could prove its frontier -- an unprovable round's `-1` may
+        /// itself be the observation that is missing an owner elsewhere, so deleting the body on it is
+        /// exactly the irreversible step the gate exists to withhold.
+        static const std::map<ManifestId, Token> kNoManifestCleanup;
+        const std::map<ManifestId, Token> & mf_cleanup_now =
+            suppress_destructive ? kNoManifestCleanup : folded.mf_cleanup;
+        uint64_t attempted = 0;
+        for (const auto & [id, token] : mf_cleanup_now)
+        {
+            ++attempted;
+            const DeleteOutcome mdel = backend.deleteExact(layout.manifestKey(id), token);   /// NotFound/TokenMismatch tolerated
+            const DeleteClass mdel_class = classifyDeleteOutcome(mdel);
+            if (mdel_class == DeleteClass::Deleted)
+                ++report.manifests_deleted;
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::ManifestDelete;
+                e.namespace_ = id.root_namespace.string();
+                e.object_kind = CasEventObjectKind::Manifest;
+                e.object_hash = manifestRefDebugString(id.ref);
+                e.token = token.value;
+                e.round = new_round;
+                e.gen = generation;
+                e.outcome = String{deleteClassName(mdel_class)};
+                e.reason = "owner-removed manifest body; exact-token delete after decrements adopted";
+            });
+        }
+        t.metric("attempted", attempted);
+        t.metric("deleted", report.manifests_deleted - manifests_deleted_before);
+        t.metric("suppressed", suppress_destructive ? 1 : 0);
+    }
+
+    /// Removal completion has no physical pass. The terminal fold placed positive evidence in the
+    /// life row; a later invocation's catalog-only pre-fold drain owns lifecycle deletion, while the
+    /// perpetual janitor owns dead-life bytes.
+    uint64_t cleanup_evidence_rows = 0;
+    for (const auto & [life_id, ref_life_state] : folded.fold_seal.ref_lives)
+        cleanup_evidence_rows += ref_life_state.cleanup_evidence ? 1 : 0;
+    runNamespaceJanitorPage(state, suppress_destructive, cleanup_evidence_rows);
+    /// PHASE 17/18 `ref_object_cleanup`. Emitted even when the whole pass is skipped (`trim_enabled` is
+    /// a test seam, `suppressed` gates the deletes), because "this phase did nothing and why" is exactly
+    /// what a reader of a round that reclaimed nothing needs to see.
+    {
+        GcPhaseTimer t(phase_sink, "ref_object_cleanup");
+        if (trim_enabled)
+            cleanupRefObjects(folded, state.lease, suppress_destructive, round_work_budget);
+        t.metric("suppressed", suppress_destructive ? 1 : 0);
+        t.metric("trim_enabled", trim_enabled ? 1 : 0);
+        t.metric("namespaces_planned", folded.ref_tables.size());
+    }
+
+    /// Bounded orphan-manifest backstop. The fold already exact-read each candidate, retired its exact
+    /// source edges into the adopted runs and placed cursor progress in the SAME `gc/state` CAS above.
+    /// Only this post-CAS tail may delete candidate bodies.
+    /// PHASE 18/18 `orphan_sweep`.
+    {
+        GcPhaseTimer t(phase_sink, "orphan_sweep");
+        ManifestSweepResult & sweep = folded.orphan_sweep;
+        for (const ManifestSweepResult::Nomination & nomination : sweep.nominations)
+        {
+            const DeleteOutcome outcome = backend.deleteExact(nomination.key, nomination.token);
+            const DeleteClass outcome_class = classifyDeleteOutcome(outcome);
+            EventEmitter{*store}.emit([&](CasEvent & e)
+            {
+                e.type = CasEventType::ManifestDelete;
+                e.namespace_ = nomination.id.root_namespace.string();
+                e.object_kind = CasEventObjectKind::Manifest;
+                e.object_hash = nomination.key;
+                e.token = nomination.token.value;
+                e.round = new_round;
+                e.gen = generation;
+                e.outcome = String{deleteClassName(outcome_class)};
+                e.reason = "orphan-manifest sweep: source edges retired and adopted before exact-token delete";
+            });
+            if (outcome.kind == DeleteOutcome::Kind::TokenMismatch)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS orphan sweep: manifest key {} changed token after exact GET; immutable manifest "
+                    "identity suffered illegal ABA, retained replacement", nomination.key);
+            if (outcome_class == DeleteClass::Deleted)
+                ++sweep.deleted;
+            else
+                ++sweep.skipped;
+        }
+        reportSweepRetention(sweep);
+        t.metric("cursor_advanced", state.manifest_sweep_cursor != manifest_sweep_cursor_before ? 1 : 0);
+        t.metric("list_budget_keys", store->poolConfig().manifest_sweep_list_budget_keys);
+        t.metric("suppressed", suppress_destructive ? 1 : 0);
+        t.metric("listed", sweep.listed);
+        t.metric("deleted", sweep.deleted);
+        t.metric("skipped", sweep.skipped);
+        /// THE §6 PREMISE'S SHARE OF `skipped`, BY REASON CLASS. Rule (1) is satisfiable only for a
+        /// closed-and-folded epoch, so a pass in which everything examined was RETAINED is an ordinary
+        /// outcome, and without these numbers it is indistinguishable on the row from a pass that found
+        /// nothing to do.
+        /// A row where `deleted` is 0 and all four are 0 means the sweep genuinely had no candidates.
+        t.metric("retained_no_coverage", sweep.retained_no_coverage);
+        t.metric("retained_hold", sweep.retained_hold);
+        t.metric("retained_unconsumed_seal", sweep.retained_unconsumed_seal);
+        t.metric("retained_tail_removal", sweep.retained_tail_removal);
+    }
+
+    return report;
+}
+
+void Gc::reportStuckRemovals(const RefPlan & plan, uint64_t current_round)
+{
+    for (const UInt128 & life_id : plan.lifeIds())
+    {
+        const auto warning = stuckRemovalWarning(
+            plan.row(life_id), current_round, store->poolConfig().gc_stuck_removal_rounds,
+            store->layout());
+        if (!warning)
+            continue;
+        ProfileEvents::increment(ProfileEvents::CASGCStuckRemovals);
+        LOG_WARNING(logger, "{}", *warning);
+    }
+}
+
+bool Gc::foldManifestEdges(const ManifestId & id, int sign, std::vector<BlobDelta> & deltas,
+                           std::map<ManifestId, Token> & mf_cleanup, uint32_t txn_ordinal)
+{
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+
+    const String key = layout.manifestKey(id);
+    /// ONE ROUND TRIP PER EDGE. The GET alone carries the absence signal a HEAD would have carried, so
+    /// the HEAD that used to precede it bought nothing and cost a second serial round trip on the
+    /// hottest read path of the round (one per manifest edge, on every folded log). `!got` is the SAME
+    /// absent outcome the missing HEAD used to produce -- record-and-continue, and the caller decides
+    /// what an absent body means for that edge (a missing-body precommit is a barrier; a committed one
+    /// fails closed). Never a throw: a 404 during the fold is an observation, not an error.
+    const auto got = backend.get(key);
+    if (!got)
+        return false;   /// absent body: caller decides (missing-body precommit OK; committed => fail closed)
+    ProfileEvents::increment(ProfileEvents::CASRefManifestBodyFoldGets);   /// one body GET per manifest fold
+
+    const PartManifest body = decodePartManifest(openObject(FormatId::PartManifest, got->bytes));
+    if (!refMatchesBody(id.ref, body))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS gc fold: manifest body ref mismatch at {} (refMatchesBody fail-closed)", key);
+    if (!manifestNamespaceMatches(id.root_namespace, body))
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS gc fold: manifest body namespace mismatch at {} (manifestNamespaceMatches fail-closed)", key);
+    /// The manifest-wide `blob_hash_len` foreign-width gate is GONE (the field
+    /// itself was deleted — entries now carry their own per-entry algo/width). `decodePartManifest`
+    /// already fail-closes on an algo byte this BUILD does not know (`blobHashAlgoName` throws
+    /// CORRUPTED_DATA there) -- but a known algo may still not be ADMITTED to THIS pool yet (a stale
+    /// in-memory `admitted_algos` cache reading a manifest another node already admitted a new algo
+    /// for). Per-entry admission validation refreshes on miss BEFORE
+    /// failing closed, so a genuinely fresh admission is never mistaken for corruption.
+    for (const ManifestEntry & entry : body.entries)
+        if (entry.placement == EntryPlacement::Blob && !store->isAlgoAdmitted(entry.ref.algo))
+        {
+            const std::vector<uint8_t> refreshed = store->refreshAdmittedAlgos();
+            if (!store->isAlgoAdmitted(entry.ref.algo))
+            {
+                String names;
+                for (size_t i = 0; i < refreshed.size(); ++i)
+                {
+                    if (i != 0)
+                        names += ", ";
+                    names += blobHashAlgoName(static_cast<BlobHashAlgo>(refreshed[i]));
+                }
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS gc fold: manifest entry algo {} not admitted to this pool (algos_used {{{}}})",
+                    blobHashAlgoName(entry.ref.algo), names);
+            }
+        }
+
+    for (const ManifestEntry & entry : body.entries)
+        if (entry.placement == EntryPlacement::Blob)
+        {
+            /// The fold settles the FULL `BlobRef` pair natively -- no bare-digest bridge remains.
+            deltas.push_back(BlobDelta{
+                .ref = entry.ref,
+                .source_id = sourceEdgeId(id, entry.path),
+                .remove = (sign < 0),
+                .txn_ordinal = txn_ordinal});
+            /// A folded owner edge over this blob (the manifest-model analog of the old
+            /// `RootAdd`). +1 = the manifest's owner activated this blob's reference; -1 =
+            /// the owner was removed, dropping the reference. Reconstructs WHY a blob's in-degree moved.
+            EventEmitter{*store}.emit([&](CasEvent & ev)
+            {
+                ev.type = sign > 0 ? CasEventType::RootAdd : CasEventType::RootRemove;
+                ev.namespace_ = id.root_namespace.string();
+                ev.object_kind = CasEventObjectKind::Blob;
+                ev.object_hash = blobIdOf(entry.ref);
+                ev.outcome = sign > 0 ? "edge_added" : "edge_removed";
+                ev.reason = sign > 0
+                    ? "fold: manifest owner activated; +1 blob edge"
+                    : "fold: manifest owner removed; -1 blob edge";
+                ev.detail = {{"manifest_ref_instance", manifestRefDebugString(id.ref)},
+                             {"path", entry.path}};
+            });
+        }
+
+    if (sign < 0)
+        mf_cleanup.emplace(id, got->token);   /// owner removed: defer exact-token body delete to recheck
+    return true;
+}
+
+Gc::CheckpointWitnesses Gc::readCheckpointWitnesses(const std::map<String, RefTableListing> & ref_tables,
+                                                    const CasRefCatalog::Snapshot & catalog_cut)
+{
+    /// Read the checkpoint of every namespace in the round's catalog cut, every namespace `ref_tables`
+    /// names, PLUS every namespace a HELD row in `parent_cursors` names. A catalog-only namespace is the
+    /// one whose second witness matters MOST: a genuinely empty listing cannot distinguish a namespace
+    /// that has no records from one whose records the same enumeration missed.
+    ///
+    /// EXACT KEY, ALWAYS -- never `RefTableListing::has_ckpt`. Skipping the read because the listing did
+    /// not show a `_ckpt` would make the second witness a function of the first, which is precisely the
+    /// dependency it exists to break: the listing is a SNAPSHOT, and a `_ckpt` that became durable after
+    /// the enumeration is exactly the one whose namespace has records the same enumeration also missed.
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+
+    std::set<String> witness_namespaces;
+    for (const CatalogEntry & entry : catalog_cut.catalog.entries)
+        if (entry.state == NsState::Live || entry.state == NsState::Removing)
+            witness_namespaces.insert(entry.ns.string());
+    for (const auto & [ns_str, listing] : ref_tables)
+        witness_namespaces.insert(ns_str);
+
+    CheckpointWitnesses out;
+    for (const String & ns_str : witness_namespaces)
+    {
+        const RootNamespace ns{ns_str};
+        /// Review C3: use the SAME complete catalog cut the round's walk resolved, never an independent
+        /// catalog re-read. A namespace absent from the cut, or present only as a non-walkable
+        /// `Creating` row, has no admitted witness key to read this round.
+        const auto entry_it = std::lower_bound(
+            catalog_cut.catalog.entries.begin(), catalog_cut.catalog.entries.end(), ns,
+            [](const CatalogEntry & entry, const RootNamespace & needle) { return entry.ns < needle; });
+        if (entry_it == catalog_cut.catalog.entries.end() || entry_it->ns != ns
+            || (entry_it->state != NsState::Live && entry_it->state != NsState::Removing))
+            continue;
+        const String ckpt_key = layout.refCkptKey(NamespaceLifeId::fromCatalogEntry(entry_it->ns, entry_it->incarnation));
+        /// THE GET AND THE DECODE ARE SPLIT HERE, rather than taken together through `readCkpt`, so the
+        /// catch below can scope to the DECODE ALONE. Wrapping the read too would turn a transport
+        /// failure -- which says nothing about this object and everything about the round's ability to
+        /// read anything -- into a per-namespace hold, silently narrowing a pool-wide outage to one
+        /// namespace. A backend throw still propagates and fails the round, exactly as it always did.
+        const std::optional<GetResult> got = backend.get(ckpt_key);
+        /// ABSENT IS NORMAL AND IS NOT A WITNESS: a namespace has no `_ckpt` until its first snapshot
+        /// publication commits, and one that 404s mid-round is a namespace being reclaimed. Neither says
+        /// anything about which ids exist, so neither may hold the walk -- and neither may throw
+        /// (a GC fold never fails a round on a 404).
+        if (!got)
+            continue;
+
+        RefCkpt ckpt;
+        try
+        {
+            /// Materialized read, then decode (`readCkpt`'s rule): the object is MUTABLE, so the body
+            /// must be fixed before it is parsed.
+            ckpt = decodeRefCkpt(got->bytes);
+        }
+        catch (const Exception & e)
+        {
+            /// PER-NAMESPACE, NEVER ROUND-WIDE (spec §5: every per-namespace failure is a clamp or a
+            /// hold). This object belongs to exactly one namespace, so it can never be grounds for
+            /// refusing to fold another one -- and refusing the whole round is what a single unreadable
+            /// 4 KiB object used to do, stopping every namespace's cursor, seal and cleanup for as long
+            /// as it stayed unrepaired. It is recorded, named, and left to the walk to hold.
+            ///
+            /// NAME THE OBJECT. The decode's own message says what is wrong with the bytes and nothing
+            /// about WHICH bytes, so without the key an operator cannot find the object to repair.
+            out.undecodable.emplace(ns_str, ckpt_key + ": " + e.message());
+            continue;
+        }
+        /// A checkpoint without `checkpoint_snapshot_id` is silent rather than empty: the object exists
+        /// because some OTHER field (`life_epoch`, `last_epoch_seal`) was published into it first.
+        if (ckpt.checkpoint_snapshot_id)
+            out.witnesses.emplace(ns_str, *ckpt.checkpoint_snapshot_id);
+        if (ckpt.life_epoch)
+            out.life_epochs.emplace(ns_str, *ckpt.life_epoch);
+        out.recovery_checkpoints.emplace(ns_str, std::move(ckpt));
+    }
+    return out;
+}
+
+std::optional<std::pair<uint64_t, uint64_t>> Gc::newestFoldSealRef()
+{
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+    const String gen_prefix = layout.gcGenPrefix(0);
+    const String top = gen_prefix.substr(0, gen_prefix.size() - 2);   /// ".../gc/gen/"
+
+    /// THE WIDE ENUMERATION IS A HINT HERE TOO, exactly as it is in the ref walk. Trusting it for
+    /// NEWEST-ness would reopen the same hole one layer up: an enumeration that omits the true newest
+    /// seal hands back an older one, and every hold detected since that older seal is silently lost --
+    /// an under-carry, which is the failure this whole path exists to prevent.
+    std::set<uint64_t> listed_generations;
+    bool listed_anything = false;
+    std::optional<std::pair<uint64_t, uint64_t>> newest;
+    forEachListedKey(backend, top, [&](const ListedKey & k)
+    {
+        listed_anything = true;
+        const size_t from = top.size();
+        const size_t gen_end = k.key.find('/', from);
+        if (gen_end == String::npos)
+            return;
+        uint64_t generation = 0;
+        try
+        {
+            generation = std::stoull(k.key.substr(from, gen_end - from));
+        }
+        catch (...) // NOLINT(bugprone-empty-catch)
+        {
+            return;   /// foreign key shape under `gc/gen` is debris, not a generation number
+        }
+        listed_generations.insert(generation);
+    }, 1000, onGcEnumerationPage);
+    const uint64_t listed_max_generation = listed_generations.empty() ? 0 : *listed_generations.rbegin();
+
+    /// STEP DOWN THROUGH THE GENERATIONS THE LISTING ITSELF REPORTED until one carries a seal. The
+    /// newest generation routinely exists WITHOUT one: a round writes its runs during the reduce phase
+    /// and its fold seal only at phase 10/18, so an ordinary crash in between leaves exactly that
+    /// shape. Stopping at the maximum would then refuse a pool whose holds are sitting readable one
+    /// generation down -- turning a plain crash into "recreate the pool".
+    ///
+    /// Stepping down costs no trust that has not already been spent: these are the generations the wide
+    /// listing reported, and its maximum -- which the probes above are checking -- is one of them. What
+    /// is NOT weakened is the refusal above: a seal found ABOVE the maximum stays terminal, because
+    /// that is the listing being caught in a lie rather than merely being incomplete about seals.
+    ///
+    /// "Never step PAST an unreadable seal" falls out of returning the FIRST generation that carries
+    /// one: the caller decodes it and refuses if it cannot, so an undecodable seal ends the search
+    /// instead of being skipped over in favour of an older, readable one.
+    for (auto it = listed_generations.rbegin(); it != listed_generations.rend(); ++it)
+    {
+        if (const auto probe = probeGenerationForSeal(*it); probe.seal_attempt)
+        {
+            newest = std::make_pair(*it, *probe.seal_attempt);
+            break;
+        }
+    }
+
+    /// DETECTION, NOT PROOF -- and labelled as such deliberately, in the same spirit as probe A. Two
+    /// NARROW single-generation probes above the wide listing's maximum ask whether that maximum was a
+    /// lie. The generation half of the question is arithmetic (generations are dense in minting: a fold
+    /// takes `snap_generation + 1`, a rebuild `max_gen + 1`), but the attempt half is not and cannot be
+    /// made so -- `attempt` is `lease.seq`, a global counter that advances on EVERY round including
+    /// deferred ones, so consecutive generations carry attempts separated by unbounded gaps and there is
+    /// no `attempt + 1` to point-read. So the step is an enumeration WITHIN ONE DIRECTORY: strictly
+    /// narrower than the pool-wide listing whose maximum it is checking, and honestly not the exact
+    /// read the ref walk gets.
+    ///
+    /// A seal found above the maximum means the wide listing lied about the very thing this path is
+    /// deciding, so the answer is REFUSAL, not adoption of the newer seal: a store that misreports its
+    /// own enumeration DURING DISASTER RECOVERY does not get a second guess, and silently adopting
+    /// whatever the second query returned would just move the trust one query along.
+    static constexpr uint64_t kProbeGenerationsAbove = 2;
+    for (uint64_t above = 1; above <= kProbeGenerationsAbove; ++above)
+    {
+        const uint64_t generation = listed_max_generation + above;
+        const GenerationSealProbe probe = probeGenerationForSeal(generation);
+        if (!probe.seal_attempt)
+            continue;
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS GC rebuild: a fold seal exists at generation {} (attempt {}) while the pool-wide "
+            "enumeration of {} reported nothing above generation {}. The enumeration this rebuild "
+            "would have taken its baseline from is demonstrably incomplete, so the holds it carries "
+            "cannot be trusted to be all of them. GC refuses to rebuild; this pool must be recreated.",
+            generation, *probe.seal_attempt, top, listed_max_generation);
+    }
+
+    if (newest)
+        return newest;
+
+    /// THE VIRGIN VERDICT, and everything it rests on. `gc/state` is already known absent or
+    /// unreadable (the caller's precondition); the wide listing found nothing; and one narrow probe of
+    /// generation 1 -- the first generation any pool would ever mint -- also finds nothing. That is
+    /// three pieces of ENUMERATION evidence and no point read, because the seal key's attempt component
+    /// has no arithmetic successor to probe.
+    ///
+    /// NAMED RESIDUAL, and the generation-1 probe NARROWS it rather than closing it. On a pool that has
+    /// been pruned, generation 1 LEGITIMATELY does not exist -- `pruneSupersededGenerations` deletes
+    /// whole old generation prefixes once they age past `gc_snapshot_generations_to_keep` -- so an empty
+    /// generation-1 probe proves nothing there. A total enumeration blackout on a lived-in, pruned pool
+    /// therefore still reads virgin here, and grants it a clean slate with no holds. What the probe
+    /// does buy is the un-pruned case: a young pool whose seals the wide listing hid is caught.
+    ///
+    /// No closure exists in the current key shapes, because a fold seal cannot be point-read from its
+    /// generation alone. The fix is a derivable per-generation marker that CAN be point-read, and it
+    /// would survive the blackout precisely because it needs no enumeration (see the report and
+    /// `docs/superpowers/cas/BACKLOG.md`). Anything the listing DOES show above its own maximum is
+    /// caught by the refusal above instead.
+    const GenerationSealProbe genesis = probeGenerationForSeal(1);
+    if (listed_anything || genesis.generation_exists)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS GC rebuild: the pool-wide enumeration of {} yielded no fold seal, yet {} -- so the "
+            "pool is NOT provably new and its holds cannot be enumerated. GC refuses to rebuild; this "
+            "pool must be recreated.",
+            top,
+            listed_anything ? "that enumeration did return objects under it"
+                            : "a narrow probe of generation 1 found objects the wide listing omitted");
+
+    ProfileEvents::increment(ProfileEvents::CASGCRebuildVirginByEnumeration);
+    LOG_WARNING(logger,
+        "CAS GC rebuild PROCEEDING AS NEVER-SEALED: no fold seal was found by the broad listing of {} "
+        "or by the generation-1 probe, and gc/state is absent or unreadable, so NO durable hold is "
+        "carried forward. IMPLICATION: if this pool HAS sealed and then pruned, generation 1 is gone "
+        "legitimately and this verdict rests on a total enumeration blackout -- holds may be lost and "
+        "GC may reclaim blobs a held namespace still protects. The verdict rests on ENUMERATION ALONE: "
+        "no point read can prove it, because a fold seal key needs an attempt component that is a "
+        "lease sequence number. Verify with the store operator that the object listing is complete "
+        "before trusting this rebuild.",
+        top);
+    return std::nullopt;
+}
+
+Gc::GenerationSealProbe Gc::probeGenerationForSeal(uint64_t generation)
+{
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+
+    GenerationSealProbe probe;
+    forEachListedKey(backend, layout.gcGenPrefix(generation), [&](const ListedKey & k)
+    {
+        probe.generation_exists = true;   /// ANY object proves this generation was minted
+        /// Parse a candidate attempt out of the path and then PROVE it by rebuilding the key: only a
+        /// string `foldSealKey` itself would have produced is a fold seal. Everything else under a
+        /// generation -- run objects, outcome sets, debris of a lost era -- must not get to decide
+        /// which baseline this pool's holds are read from.
+        static constexpr std::string_view kAttempt = "/attempt/";
+        const size_t a_begin = k.key.find(kAttempt);
+        if (a_begin == String::npos)
+            return;
+        const size_t a_from = a_begin + kAttempt.size();
+        const size_t a_end = k.key.find('/', a_from);
+        if (a_end == String::npos)
+            return;
+        uint64_t attempt = 0;
+        try
+        {
+            attempt = std::stoull(k.key.substr(a_from, a_end - a_from));
+        }
+        catch (...) // NOLINT(bugprone-empty-catch)
+        {
+            return;   /// foreign key shape is debris, not an attempt
+        }
+        if (layout.foldSealKey(generation, attempt) != k.key)
+            return;
+        if (!probe.seal_attempt || *probe.seal_attempt < attempt)
+            probe.seal_attempt = attempt;
+    }, 1000, onGcEnumerationPage);
+    return probe;
+}
+
+uint64_t Gc::FoldResult::FrontierDeficit::total() const
+{
+    return checkpoint_unusable + checkpoint_frontier_empty + committed_below_cursor
+        + held + probe_budget + fold_aborted + unattributed;
+}
+
+String Gc::FoldResult::FrontierDeficit::describe() const
+{
+    String out;
+    const auto add = [&](const char * name, uint64_t count)
+    {
+        if (count == 0)
+            return;
+        if (!out.empty())
+            out += ", ";
+        out += fmt::format("{}={}", name, count);
+    };
+    add("checkpoint_unusable", checkpoint_unusable);
+    add("checkpoint_frontier_empty", checkpoint_frontier_empty);
+    add("committed_below_cursor", committed_below_cursor);
+    add("held", held);
+    add("probe_budget", probe_budget);
+    add("fold_aborted", fold_aborted);
+    add("unattributed", unattributed);
+    return out;
+}
+
+void Gc::FoldResult::FrontierDeficit::count(FrontierUnproven reason)
+{
+    switch (reason)
+    {
+        case FrontierUnproven::Proven: return;
+        case FrontierUnproven::CheckpointUnusable: ++checkpoint_unusable; return;
+        case FrontierUnproven::CheckpointFrontierEmpty: ++checkpoint_frontier_empty; return;
+        case FrontierUnproven::CommittedBelowCursor: ++committed_below_cursor; return;
+        case FrontierUnproven::Held: ++held; return;
+        case FrontierUnproven::Unattributed: ++unattributed; return;
+    }
+}
+
+Gc::FoldResult Gc::fold(GcState & state, Token & /*state_token*/, RoundReport & report,
+                        uint64_t current_round, const RefPlan & walk_plan, UniversePolicy policy,
+                        GcRoundWorkBudget & work_budget)
+{
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+    FoldResult result;
+
+    /// 1. Group the round's one enumeration of `cas/ns/stream/` (taken before the defer decision) into
+    /// per-table immutable-object listings. That single enumeration serves the defer signal, the
+    /// ref-log intake, and ref-object cleanup planning alike.
+    ///
+    /// THE ROUND LISTS THIS PREFIX ONCE. The intake does not need a second opinion about the listing
+    /// because it does not consult the listing for completeness at all -- it walks by exact key from
+    /// the cursor.
+    ///
+    /// PHASE 6/18 `fold_ref_group`: the strict regrouping -- what this round will fold, decided before it
+    /// reads a single body. No I/O: the keys are already in hand.
+    std::optional<GcPhaseTimer> ref_list_timer;
+    ref_list_timer.emplace(phase_sink, "fold_ref_group");
+    const RefScanSummary & ref_scan = walk_plan.refScan();
+    const std::vector<String> & ref_object_keys = ref_scan.keys;
+
+    /// Stage B (spec INV-3): the round's ONE catalog `GET`. `live_incarnation` names, for every
+    /// namespace the catalog admits as `Live`/`Removing`, the ONE incarnation the fold may act on --
+    /// `Creating` is excluded, matching `discoverUniverse` (no publication can exist yet). This is what
+    /// makes the fold catalog-authoritative rather than LIST-authoritative: the pool-wide ref LIST
+    /// remains the round's intra-namespace hint (what a namespace the catalog already named has
+    /// listed), never the source of WHICH namespaces exist. Reused below for the catalog-only walk
+    /// targets, so the round pays this GET once.
+    const CasRefCatalog::Snapshot & catalog_snapshot = walk_plan.catalogCut();
+    std::map<String, UInt128> live_incarnation;
+    /// Final review F1: a `Creating` life IS named by the catalog -- `live_incarnation` excludes it
+    /// only because it is not yet WALKABLE (spec §3, no publication can exist), not because the
+    /// namespace is unaccounted. `completeCreation` publishes `_ckpt` (step 2) strictly BEFORE the
+    /// `Creating -> Live` CAS (step 3), so every ordinary namespace creation has a real window --
+    /// crash-stalled or merely mid-flight -- where a `Creating` entry's own `_ckpt` is durable and
+    /// listed while the entry itself is absent from `live_incarnation`. R10's un-cataloged anomaly
+    /// below must tell that apart from genuine "nothing in the catalog names this at all" debris, or
+    /// an ordinary or stalled creation suppresses the whole round's reclamation until someone
+    /// recreates the exact name and drives `reconcileStaleCreator` -- unbounded in the stalled case.
+    for (const NamespaceLifeId & life : walk_plan.lives())
+        live_incarnation.emplace(life.ns.string(), life.incarnation);
+    /// Carry the complete cut on the result (review C3) so every later consumer this round --
+    /// `cleanupRefObjects` and terminal-evidence attribution -- retains both the chosen incarnation and
+    /// the lifecycle/absence distinction instead of re-reading or reducing the catalog independently.
+    result.catalog_cut = catalog_snapshot;
+    /// THE POSITIVE EMPTY-UNIVERSE PROOF (see the destructive gate below). `token` is guaranteed by
+    /// `CasRefCatalog::read` on every operational path -- absence there is `CORRUPTED_DATA`, never an
+    /// empty snapshot -- but the check stays here so this fails closed if a bootstrap/test snapshot
+    /// ever reaches this line. `entries` (not `live_incarnation`, which drops `Creating`) is the right
+    /// source: a catalog holding only `Creating` rows must NOT read as an empty universe, and `entries`
+    /// is the one view that still carries those rows.
+    result.catalog_cut_proved_empty = catalog_snapshot.token.has_value() && catalog_snapshot.catalog.entries.empty();
+
+    /// A malformed ref-object key or namespace aborts ref folding for the whole round: the
+    /// round produces no ref delta, advances no cursor, and authorizes no destructive work -- recorded as
+    /// an anomaly (which drives `suppress_destructive`), never a throw that wedges the round.
+    std::map<String, RefTableListing> ref_tables;
+    bool ref_folding_aborted = false;
+    try
+    {
+        const auto physical_tables = groupRefKeys(layout, ref_object_keys);
+        for (const auto & [life_id, listing] : physical_tables)
+        {
+            const auto life = catalog_snapshot.life_index.resolve(life_id);
+            if (!life)
+                continue;   /// absent from the post-LIST cut: inert dead-life debris
+            const auto live_it = live_incarnation.find(life->ns.string());
+            if (live_it != live_incarnation.end() && live_it->second == life_id)
+                ref_tables.emplace(life->ns.string(), listing);
+        }
+    }
+    catch (const Exception & e)
+    {
+        ref_folding_aborted = true;
+        report.recordAnomaly(RootNamespace{}, 0, ManifestId{},
+                             "malformed ref-object key: ref folding aborted this round");
+        LOG_WARNING(logger,
+                    "CAS GC ref intake: {} -- aborting ref folding for the round", e.message());
+    }
+    for (const auto & [ns_str, listing] : ref_tables)
+        result.root_shards.emplace_back(RootNamespace{ns_str}, 0);
+
+    ref_list_timer->metric("ref_keys_listed", ref_object_keys.size());
+    ref_list_timer->metric("namespaces_seen", ref_tables.size());
+    /// The round's only remaining whole-round ref abort: a key attributable to no namespace. Reported on
+    /// every round, healthy or not — a column that is always 0 is what makes the one round where it is
+    /// not stand out.
+    ref_list_timer->metric("ref_folding_aborted", ref_folding_aborted ? 1 : 0);
+    ref_list_timer.reset();   /// emits the `fold_ref_group` row
+
+    /// Parent cursors — the per-(ns,shard) cursors a prior round sealed. `listRefPrefix` read them from
+    /// the fold seal at the adopted (snap_generation, snap_attempt) before it built this round's one
+    /// walk plan (the fold seal IS the coverage record).
+    /// Absent => fresh pool (cursor 0). A folded event must never be re-folded from 0 (that double-counts
+    /// blob in-degree => silent over-pin/leak).
+    /// A live `gc/state` whose adopted
+    /// fold seal OBJECT is MISSING is corrupt bookkeeping, never an empty baseline — treating it as
+    /// empty would re-fold only journal tails and mass-condemn everything the lost snapshot
+    /// protected. NOTE the distinction from a PRESENT seal with empty `ref_lives` (a legitimate
+    /// empty-universe generation) — the audit keys on object absence, not coverage emptiness.
+    ///
+    /// PHASE 7/18 `fold_seal_read`. The scope reaches down to `discover_ref_seal` below, because that is
+    /// a SECOND GET of the SAME key at the SAME (generation, attempt) -- the two reads belong on one row
+    /// or the redundancy is invisible. Everything between them is I/O-free (a `resize`, three lambda
+    /// DEFINITIONS, and plain assignments), so the duration is honestly the two GETs and their decodes.
+    /// Instrumented, NOT fixed: removing the second read is a behaviour change the follow-up study
+    /// decides, and the `redundant_reads` metric is the evidence it will need.
+    std::optional<GcPhaseTimer> seal_read_timer;
+    seal_read_timer.emplace(phase_sink, "fold_seal_read");
+    const std::optional<CasFoldSeal> adopted_seal = readFoldSeal(state.snap_generation, state.snap_attempt);
+    if (!adopted_seal && state.snap_generation > 0)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS GC: the adopted fold seal (generation {}, attempt {}) is missing under a live "
+            "gc/state — GC bookkeeping is corrupt. GC refuses to run; recover with "
+            "SYSTEM CAS GC REBUILD.",
+            state.snap_generation, state.snap_attempt);
+    /// Every fold decision below starts from the immutable plan, never from the raw scan that supplied
+    /// it. The successor seal owns its mutable copy; this separate const snapshot remains the prior
+    /// coverage/hold view while the successor earns changes later in this fold.
+    const std::map<UInt128, RefLifeFoldState> parent_ref_lives = walk_plan.parentFoldStates();
+    const uint64_t dropped_parent_ref_lives = walk_plan.droppedParentRows();
+    result.fold_seal.ref_lives = walk_plan.successorFoldStates();
+
+    /// Retired-in-snapshot: the prior generation's condemned entries RIDE the source-edge run as
+    /// `kCondemned` sentinel rows, so the round no longer reads any separate retired-list object —
+    /// the parent seal's `blob_target_runs` ARE the retired input. The per-gc-shard `condemned_summary`
+    /// the seal carries below is distilled from the `still_retired` rows each shard re-emits, making the
+    /// next round's `graduationDue` / pure-carry decisions zero-I/O.
+    const uint64_t condemn_round = state.round + 1;
+    result.retired_merge.resize(state.gc_shards);
+
+    /// Condemn-time observation: ONE HEAD per new zero-transition captures the exact incarnation token
+    /// the eventual delete carries (absent => a prior landed delete => nothing to condemn). Emits the
+    /// Candidate trail (IndegZero / GcRetireObserve / BlobRetire) exactly where the decision is made.
+    const auto head_blob = [&](const BlobRef & ref) -> std::optional<HeadResult>
+    {
+        EventEmitter{*store}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::IndegZero;
+            e.object_kind = CasEventObjectKind::Blob;
+            e.object_hash = blobIdOf(ref);
+            e.round = condemn_round;
+            e.gen = state.snap_generation + 1;
+            e.reason = "last folded owner edge dropped; in-degree reached 0";
+        });
+        const HeadResult observed = backend.head(layout.blobKey(ref));
+        EventEmitter{*store}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::GcRetireObserve;
+            e.object_kind = CasEventObjectKind::Blob;
+            e.object_hash = blobIdOf(ref);
+            e.token = observed.exists ? observed.token.value : "";
+            e.round = condemn_round;
+            e.gen = state.snap_generation + 1;
+            e.outcome = observed.exists ? "present" : "absent";
+            e.reason = "zero-in-degree candidate; HEAD-observe the current token";
+        });
+        if (!observed.exists)
+            return std::nullopt;
+        ++report.candidates;
+        ++report.condemned;
+        ProfileEvents::increment(ProfileEvents::CASGCRetiredCondemned);
+        EventEmitter{*store}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::BlobRetire;
+            e.object_kind = CasEventObjectKind::Blob;
+            e.object_hash = blobIdOf(ref);
+            e.token = observed.token.value;
+            e.round = condemn_round;
+            e.gen = state.snap_generation + 1;
+            e.outcome = "retired";
+            e.reason = "condemned zero-in-degree candidate; entering the current retired list";
+        });
+        HeadResult adjusted = observed;
+        adjusted.size = retiredLogicalSize(ObjectKind::Blob, observed.size, store->poolMeta().blob_header_len);
+        /// This candidate unconditionally becomes a fresh `RetiredEntry` in `closeBlob` (the ONLY
+        /// caller of `head_blob`) whenever this lambda returns a value — so this is exactly the round's
+        /// side-effecting condemn site. Write the meta Condemned so the writer's point-read gate
+        /// sees it; a successful write records the in-process (hash, token) confirmation the graduation
+        /// gate consumes (`scheduleCondemnMarkerWrite` captures everything BY VALUE — never by reference
+        /// to `cur_blob`, which the fold's tight streaming loop mutates while the job is queued).
+        scheduleCondemnMarkerWrite(ref, observed.token, condemn_round, adjusted.size);
+        return adjusted;
+    };
+
+    /// Side-effect-free peek: the fold's resurrect-supersede branch (inside
+    /// `foldDeltasIntoGeneration`) needs the CURRENT token to detect that a resurrect replaced a stale
+    /// retired entry, but must NOT emit the fresh-condemn trail or bump `CASGCRetiredCondemned` — that
+    /// hook is `head_blob` above, reserved for a genuinely NEW zero-in-degree candidate. A supersede's
+    /// own event is `blob_retire_replaced`, emitted once below from `merge.replaced`. Plain HEAD, no
+    /// events, no counters.
+    const auto peek_head = [&](const BlobRef & ref) -> std::optional<HeadResult>
+    {
+        HeadResult hr = backend.head(layout.blobKey(ref));
+        if (!hr.exists)
+            return std::nullopt;
+        hr.size = retiredLogicalSize(ObjectKind::Blob, hr.size, store->poolMeta().blob_header_len);
+        return hr;
+    };
+
+    /// Graduation gate (triage 2026-07-17 §3.4): the merge consults this before publishing an entry
+    /// delete_pending. Confirmation sources, in order: the in-process (hash, token) record left by a
+    /// successful `writeCondemnedMeta` completion, then ONE synchronous `loadMeta` re-check — a durable
+    /// `Condemned` meta observed NOW is sufficient evidence, because a writer that same-token adopted
+    /// must have observed a non-Condemned meta EARLIER, its edge (EDGE-BEFORE-OBSERVE) landed before the
+    /// meta turned Condemned, and the redelete only fires from a LATER fold whose cut postdates this
+    /// round — that fold sees the edge and spares. (`BlobMeta` carries no token, so the re-check is
+    /// per-hash by design; the two-phase pipeline + the exact-token delete carry the rest.) No durable
+    /// evidence => count the carry, RETRY the marker write (liveness: a swallowed write would otherwise
+    /// carry forever), and refuse — never throw (an unreadable meta is missing evidence, not a wedge).
+    const auto confirm_condemned_marker = [&](const RetiredEntry & entry) -> bool
+    {
+        if (condemnMarkerConfirmedInProcess(entry.ref, entry.token))
+            return true;
+        try
+        {
+            if (const auto lm = loadMeta(backend, layout, entry.ref); lm && lm->meta.state == MetaState::Condemned)
+            {
+                noteCondemnMarkerDurable(entry.ref, entry.token);   /// memoize for a round-CAS-abort replay
+                return true;
+            }
+        }
+        catch (...)
+        {
+            ProfileEvents::increment(ProfileEvents::CASGCMetaWriteAnomaly);
+            tryLogCurrentException(logger,
+                "CAS gc: condemn-marker re-check failed to read the meta (treated as missing evidence; "
+                "the entry is carried, never wedges the round)");
+        }
+        ProfileEvents::increment(ProfileEvents::CASGCCondemnMarkerUnconfirmedCarry);
+        /// Accepted race: this retry writes `Condemned` per-hash, with no token check. If a writer
+        /// resurrected this exact hash under a FRESH token between the original swallowed write and this
+        /// retry, the retry stamps `Condemned` over that writer's live, uncondemned incarnation. This is
+        /// never destructive -- the eventual exact-token delete is a no-op against the fresh token
+        /// (`DeleteOutcome::TokenMismatch`/`NotFound`) -- worst case the resurrecting writer's later
+        /// same-token adopter sees a stale `Condemned` meta and re-uploads once (a spurious resurrect).
+        scheduleCondemnMarkerWrite(entry.ref, entry.token, entry.condemn_round, entry.size);
+        return false;
+    };
+
+    const uint64_t new_generation = state.snap_generation + 1;
+    /// The fold mints THIS round's attempt id from `lease.seq` (the renew/steal paths bump it every
+    /// round, so it is a fresh monotonic per-round id). EVERY fold-artifact WRITE below lands under this
+    /// attempt; the PARENT-generation READS keep using `state.snap_attempt` (the attempt the prior round
+    /// adopted). The fold-adopt CAS #1 then commits `(new_generation, attempt)` together — a deposed
+    /// leader's fold lands under its own unadopted attempt and is invisible to every reader.
+    const uint64_t attempt = state.lease.seq;
+    result.fold_seal.generation = new_generation;
+    result.fold_seal.parent_generation = state.snap_generation;
+
+    std::vector<BlobDelta> deltas;
+    /// PROBE B2's round-local ledger (see `TxnApplyLedger`). Grows one entry per ref log the intake
+    /// opens; the reducers mark `applied` through `&ledger.applied` at the point they consume a delta.
+    TxnApplyLedger ledger;
+    bool folded_any = false;
+
+    /// PROBE B1 -- intake-layer identity. `logs_applied` counts, at the SINGLE cursor-advance site
+    /// below, every log whose whole body folded. At seal time it is compared against a recomputation
+    /// from the sealed coverage and the listing. The two are derived differently (a running counter vs
+    /// a recomputation), so a control-flow bug that advances a cursor without folding breaks the
+    /// equality.
+    ///
+    /// REACH, stated so this is not over-trusted: the recomputation reads the SAME listing the intake
+    /// read, so B1 is BLIND to a record missing from that listing. It is a control-flow assertion, not
+    /// a detector for the skipped-transaction defect -- probe A covers the listing, probe B2 covers
+    /// everything below the intake.
+    uint64_t logs_applied = 0;
+
+    /// The adopted parent seal, read once at the ADOPTED (snap_generation, snap_attempt): it carries the
+    /// parent generation's `blob_target_runs` (resolved below into per-gc-shard prior runs) and the parent
+    /// `condemned_summary` (the pure-carry decision). A completed round leaves its fold seal there; a fresh
+    /// pool has none (empty seal). Under the snapshot+log ref model there is no per-shard token-diff Skip:
+    /// the "did this table change" signal is simply whether the global LIST returned any log id above the
+    /// table's durable cursor, which the per-table loop below tests directly.
+    /// SECOND read of the key `adopted_seal` already holds: same generation, same attempt, same bytes
+    /// (nothing between the two touches `state.snap_generation` / `snap_attempt` or writes that key).
+    /// One redundant GET per folding round, and the round's FIFTH GET of this one key overall --
+    /// `graduationDue` and `listRefPrefix` make two in `defer_decision`, `parent_seal_read` a third,
+    /// `adopted_seal` above a fourth. Recorded on this row, not fixed here.
+    CasFoldSeal discover_ref_seal;
+    if (const auto fold_seal = readFoldSeal(state.snap_generation, state.snap_attempt))
+        discover_ref_seal = *fold_seal;
+    seal_read_timer->metric("seal_reads", 2);
+    seal_read_timer->metric("redundant_reads", 1);
+    seal_read_timer->metric("parent_ref_lives", parent_ref_lives.size());
+    seal_read_timer->metric("dropped_parent_ref_lives", dropped_parent_ref_lives);
+    seal_read_timer->metric("parent_runs", discover_ref_seal.blob_target_runs.size());
+    seal_read_timer->metric("parent_cleanup_evidence", std::count_if(
+        discover_ref_seal.ref_lives.begin(), discover_ref_seal.ref_lives.end(),
+        [](const auto & row) { return row.second.cleanup_evidence.has_value(); }));
+    seal_read_timer.reset();   /// emits the `fold_seal_read` row
+
+    /// Each fully-folded `remove_namespace` transaction earns terminal cleanup evidence.
+    std::vector<std::pair<RootNamespace, RefTxnId>> new_removals;
+
+    /// 2-3. Ref-log intake, by ARITHMETIC (spec §5). For each table the walk steps
+    /// `expected = cursor + 1` WITHIN the cursor's epoch and reads that exact key: under INV-1 the ids of
+    /// one `(namespace, writer_epoch)` are dense `1..T`, so the next record's id is computable and the
+    /// round never asks the listing what to read. Each body is decoded+validated and every explicit
+    /// owner-change folds into `foldManifestEdges` (which reads the manifest body and appends per-blob
+    /// `BlobDelta`s to `deltas`). The durable cursor advances per FULLY folded log; a missing manifest body
+    /// clamps this table below the log (barrier, re-read next round) while other tables keep folding.
+    ///
+    /// THE LISTING IS A HINT, and demoting it is the point of this loop. It used to be the source of
+    /// truth for which records exist, so a store that omitted a durable key from an enumeration -- the
+    /// observed `0x1430c`/`0x1430d` shape -- made the round skip those records' owner edges and then seal
+    /// a cursor ABOVE them, which is unrecoverable (a record below the cursor is never re-read). Under
+    /// arithmetic intake such an omission is a NON-EVENT: the exact GET finds the record anyway. The hint
+    /// keeps exactly two jobs here:
+    ///   (a) the genesis start of a never-folded namespace (`cursor == {0,0}` has no arithmetic
+    ///       predecessor; Stage B's `_ckpt.life_epoch` is what finally supplies it -- see below), and
+    ///   (b) the WITNESS set that makes an absent expected-next decidable:
+    ///         absent, no listed id above it  => this namespace's frontier this round (normal end);
+    ///         absent, a listed id above it   => impossible under contiguity, so the store is lying or a
+    ///                                           durable record was lost: HOLD the namespace at
+    ///                                           classification 4 with its cursor unmoved.
+    ///
+    /// Epochs are crossed only over a consumed `EpochSeal` (INV-2): the seal folds as an applied table
+    /// no-op (probe B2 `produced=false`) and the next epoch's start is `{E', 1}`, reached through the
+    /// `prev_epoch_seal` back-chain rather than guessed -- so an epoch the hint omits entirely is still
+    /// walked, and a crossing with no consumed seal behind it is an impossible shape that holds. Read
+    /// `crossFromSeal` for what that is proved FROM: within a round the seal's kind is checked outright,
+    /// across rounds it rests on the chain until Task 8 carries the kind in the durable cursor.
+    ///
+    /// PER-NAMESPACE FAILURES ARE PER-NAMESPACE (spec §5): an unreadable or undecodable body belongs to
+    /// exactly one namespace and clamps only it. The whole-round abort survives ONLY for a key that cannot
+    /// be attributed to any namespace at all (`groupRefKeys` above), which is why nothing in this loop
+    /// sets `ref_folding_aborted` anymore.
+    ///
+    /// PHASE 8/18 `fold_ref_intake`: one GET per record (always owed -- the round read every body anyway),
+    /// plus one exact 404 probe per namespace to prove its frontier, one extra GET per epoch crossed, and
+    /// one GET per manifest edge, which on a busy pool is where the round's object-read budget goes.
+    /// It also carries probe B1's two numbers -- reported on EVERY healthy round, so
+    /// "logs_accounted always equals logs_applied" becomes an observable property of the table rather than
+    /// a claim in a comment.
+    std::optional<GcPhaseTimer> intake_timer;
+    intake_timer.emplace(phase_sink, "fold_ref_intake");
+    uint64_t intake_tables_changed = 0;
+    uint64_t intake_tables_clamped = 0;
+    uint64_t intake_tables_held = 0;
+    uint64_t intake_dead_precommits_skipped = 0;
+    uint64_t intake_absent_probes = 0;
+    uint64_t intake_epoch_crossings = 0;
+
+    /// Probe B1's raw material: per namespace, the CONTIGUOUS runs of ids this round walked, one per epoch
+    /// entered, recorded as `[first, last]`. The recomputation below turns them back into a count and
+    /// compares it with the counter the advance site incremented -- so a cursor that moved over a position
+    /// nothing applied inflates the first number and fails the round closed.
+    std::map<String, std::vector<std::pair<RefTxnId, RefTxnId>>> walked_segments;
+
+    /// The round's SECOND witness source, independent of the listing -- see `readCheckpointWitnesses`
+    /// for what it decides and why a listing alone cannot decide it. Its `undecodable` half names the
+    /// namespaces whose `_ckpt` is present and unreadable; each of those is HELD below, and only those.
+    const CheckpointWitnesses checkpoints = readCheckpointWitnesses(ref_tables, catalog_snapshot);
+    const std::map<String, RefTxnId> & checkpoint_witness = checkpoints.witnesses;
+
+    /// WHICH NAMESPACES THIS ROUND WALKS -- i.e. THE ROUND'S UNIVERSE, the set the destructive gate owes
+    /// a frontier proof for (spec §5; `UniversePolicy` for why only the catalog can bound it): EVERY
+    /// `Live`/`Removing` row of this round's frozen catalog cut, and nothing else.
+    ///
+    /// The listing is a HINT and cannot change that set in either direction. It cannot SHRINK it -- a
+    /// store that goes quiet about a namespace, or stops listing one to clear its hold, leaves the
+    /// catalog row and therefore the obligation untouched -- and it cannot GROW it either, since a
+    /// physical id the catalog does not name is inert debris rather than a namespace to prove.
+    ///
+    /// A namespace with no hint entry walks against an EMPTY listing: it has no hint witnesses, but it
+    /// still reads its expected-next by exact key -- ONE `GET`, whose absence IS the frontier proof and
+    /// whose PRESENCE means the namespace was wrongly quiet and gets walked properly this round. A
+    /// carried hold additionally supplies the witness that keeps an absent below its position from
+    /// reading as a frontier.
+    ///
+    /// COST: held namespaces are always walked (their retry is a liveness obligation, not a budgeted
+    /// nicety); the merely-QUIET ones are bounded by `gc_frontier_probe_budget`, and the ones the budget
+    /// does not reach are simply unproven, which suppresses the round's destruction. In a healthy pool
+    /// that budget is rarely touched; dead physical lives are excluded by the catalog-built plan.
+
+    /// THE ROUND'S WORK-SET IS FROZEN AT ROUND START, AND THAT IS WHAT MAKES A ROUND END.
+    ///
+    /// Arithmetic intake reads the next id by exact key, so on its own it walks WHILE RECORDS EXIST --
+    /// and a namespace whose writer appends concurrently therefore has no last record to reach. Round
+    /// time stopped being `backlog / walker_rate` and became `backlog / (walker_rate - writer_rate)`,
+    /// which diverges the moment a writer keeps up with the walker. Measured: zero completed rounds in
+    /// 42 minutes on a hot pool. Everything the round paces on rounds then stops too -- the fold seal and
+    /// its cursors, the sampled store-quality detector, ref-object cleanup -- so the backlog the round
+    /// was falling behind on grows without bound.
+    ///
+    /// The bound is `_ckpt.committed_through`, snapshotted once per namespace before the walk (see
+    /// `readCheckpointWitnesses`) and never re-read within the round: the ceiling test at the top of the
+    /// walk refuses every position above it, so the work is fixed at round start whatever the writer
+    /// does meanwhile. It is also the AUTHORITY ceiling -- a record above it is durable but is not
+    /// logical history yet -- so ONE comparison serves both purposes and there is no second bound that
+    /// could drift out of agreement with it. NO NEW PERSISTED STATE: the number is read from an object
+    /// the fold reads anyway.
+    ///
+    /// THE BOUND IS ON FOLDING, NOT ON READING, and that distinction is the whole design.
+    ///
+    /// The walk still reads its expected-next exactly as before -- one exact `GET` at `cursor + 1` --
+    /// because that read IS the frontier proof, and a namespace that stops being read stops being
+    /// provable. An unprovable namespace leaves `frontier_complete` false forever, which suppresses every
+    /// destructive decision, which stops ref-object cleanup, which means its listing never drains and it
+    /// never becomes provable by any other route either. So "skip reading a quiet namespace" is not a
+    /// cheaper version of this design; it is a GC that permanently reclaims nothing. The saving it
+    /// appears to offer is exactly one `GET` per namespace per round, and that `GET` is the proof.
+    ///
+    /// The listed tail bounds nothing, but it is still CLASSIFIED for the phase row, in three classes:
+    ///   * `tail > cursor` -- the round has records to fold;
+    ///   * `tail == cursor` -- the listing shows nothing new, so the walk folds nothing and its single
+    ///     read is the frontier probe. On a wide pool this is most namespaces, most rounds;
+    ///   * `tail < cursor` -- the listing's greatest id is BELOW a cursor we folded through. Cleanup
+    ///     deletes logs from the bottom up, so a listed log below the cursor with none at or above it is
+    ///     a stale or lying listing, not a shape cleanup produces. The cursor is the truth (the sampled
+    ///     store-quality detector is this class's observer).
+    struct WalkTarget
+    {
+        String ns;
+        UInt128 life_id{};
+        const RefTableListing * listing;
+    };
+
+    static const RefTableListing kNoListing;
+    std::vector<WalkTarget> walk_targets;
+    /// The three buckets classify the hinted, UNHELD namespaces -- the ones whose tail comparison
+    /// decided how much they could fold. A held namespace folds up to a bound its HOLD also determines
+    /// and is reported by `tables_held`, so counting it here would make `tails_unchanged` stop meaning
+    /// "namespaces with no work this round", which is the number the row exists to publish.
+    uint64_t intake_tails_advanced = 0;
+    uint64_t intake_tails_unchanged = 0;
+    uint64_t intake_tails_below_cursor = 0;
+    uint64_t intake_unhinted_held = 0;
+    uint64_t intake_unhinted_quiet = 0;
+    uint64_t frontier_probe_budget = store->poolConfig().gc_frontier_probe_budget;
+    uint64_t intake_unprobed_budget = 0;
+    uint64_t intake_catalog_only = 0;
+    for (const auto & [catalog_ns_str, catalog_incarnation] : live_incarnation)
+    {
+        const auto listing_it = ref_tables.find(catalog_ns_str);
+        const RefTableListing * listing = listing_it != ref_tables.end() ? &listing_it->second : &kNoListing;
+        const auto parent_it = parent_ref_lives.find(catalog_incarnation);
+        const RefCoverage * parent_cov = parent_it != parent_ref_lives.end() ? &parent_it->second.coverage : nullptr;
+
+        if (listing_it == ref_tables.end() && parent_cov)
+        {
+            if (parent_cov->hold)
+                ++intake_unhinted_held;
+            else if (checkpoints.recovery_checkpoints.contains(catalog_ns_str))
+                ++intake_unhinted_quiet;
+            else if (frontier_probe_budget == 0)
+            {
+                ++intake_unprobed_budget;
+                continue;
+            }
+            else
+            {
+                --frontier_probe_budget;
+                ++intake_unhinted_quiet;
+            }
+        }
+        else if (listing_it == ref_tables.end())
+            ++intake_catalog_only;
+
+        walk_targets.push_back({catalog_ns_str, catalog_incarnation, listing});
+        if (listing->logs.empty() || (parent_cov && parent_cov->hold))
+            continue;
+
+        const RefTxnId tail = listing->logs.back();
+        const RefTxnId cursor = parent_cov ? parent_cov->last_folded_ref_id : RefTxnId{};
+        if (cursor < tail)
+            ++intake_tails_advanced;
+        else if (cursor == tail)
+            ++intake_tails_unchanged;
+        else
+            ++intake_tails_below_cursor;
+    }
+
+    for (const WalkTarget & target : walk_targets)
+    {
+        const String & ns_str = target.ns;
+        const RefTableListing & listing = *target.listing;
+        if (ref_folding_aborted)
+            break;
+        const RootNamespace ns{ns_str};
+        /// Every walk target came out of this round's own catalog read, so its incarnation is the REAL
+        /// one and the life below is minted from a catalog entry rather than guessed from a key.
+        const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(ns, target.life_id);
+
+        /// Parent cursor = the durable last_folded_ref_id this table folded to (absent => {0,0}).
+        ///
+        /// Id ordering does NOT subsume remove+recreate: under INV-1 (`nextRefTxnId`) ids are derived per
+        /// namespace from that table's own state, so a namespace removed and recreated within one writer
+        /// epoch restarts at `{E, 1}`, at or below a cursor sealed for the PREVIOUS life -- and the walk
+        /// starts at `cursor + 1`, so those edges would never fold and the recreated refs' manifests
+        /// would look unreferenced. That is closed STRUCTURALLY, not by comparing ids: this map and the
+        /// walk targets are both keyed by CATALOG INCARNATION, so a rebirth is a different key and
+        /// inherits no cursor, and a life the catalog no longer names contributes no walk target and
+        /// therefore re-carries no cursor into the new seal. Do not "fix" it by comparing ids.
+        const auto cursor_it = parent_ref_lives.find(target.life_id);
+        const RefTxnId cursor = cursor_it != parent_ref_lives.end()
+            ? cursor_it->second.coverage.last_folded_ref_id : RefTxnId{};
+
+        /// Baseline guard: a table with NO sealed cursor whose logs at/below its
+        /// newest snapshot have all been cleaned means a prior fold advanced+cleaned them and then gc/state
+        /// was lost -- folding from {0,0} would miss those edges and mass-condemn their blobs. Fail closed;
+        /// recover with the explicit rebuild. A fresh table (writer already snapshotted, logs still present)
+        /// passes because its logs at or below the snapshot survive.
+        if (cursor_it == parent_ref_lives.end() && !listing.snapshots.empty()
+            && !checkpoints.recovery_checkpoints.contains(ns_str))
+        {
+            const RefTxnId newest_snapshot = listing.snapshots.back();
+            const bool logs_below_snapshot_gone = listing.logs.empty() || newest_snapshot < listing.logs.front();
+            if (logs_below_snapshot_gone)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS GC baseline guard: table {} has snapshot {} but no surviving log at or below it and "
+                    "no sealed fold cursor -- gc/state was lost after cleaning covered logs. GC refuses to "
+                    "run; recover with SYSTEM CAS GC REBUILD.",
+                    ns_str, renderRefTxnId(newest_snapshot));
+        }
+
+        /// The hold the PARENT seal left on this namespace, if any. It is three things at once: the
+        /// position this round must retry by exact key, a durable witness (see `witnessAbove`), and the
+        /// hold that rides forward unless this round resolves that position.
+        const std::optional<RefHold> carried_hold =
+            cursor_it != parent_ref_lives.end() ? cursor_it->second.coverage.hold : std::nullopt;
+
+        RefCoverage cov;
+        cov.classification = 0;
+        bool table_changed = false;
+        /// THE FRONTIER PROOF for this namespace, and there is exactly one thing that establishes it:
+        /// the walk read the expected-next position by exact key, found it ABSENT, and no witness put
+        /// anything above it. That is the honest end of the record stream. Every other way out of the
+        /// loop leaves the namespace unproven -- a hold (this round's or a carried one), and the walk
+        /// that never started because a never-folded namespace's hint offered no genesis position AND
+        /// no `_ckpt.life_epoch` was on record either (the `expected` initialization below reads
+        /// `checkpoints.life_epochs` for exactly this case; a namespace with neither has a genuinely
+        /// unknown genesis, so no probe is taken, nothing is proved, and fail-closed says unproven).
+        bool frontier_proven = false;
+        /// Which exit left this namespace unproven, for the round's deficit tally. It starts at
+        /// `Unattributed` so that an exit which forgets to name itself is reported as such instead of
+        /// being silently absorbed into some other bucket.
+        FoldResult::FrontierUnproven unproven_reason = FoldResult::FrontierUnproven::Unattributed;
+
+        /// Use the same frozen catalog row + decoded checkpoint authority as read-only recovery. The
+        /// ref LIST is only a bounded-work hint; it may neither choose this life's genesis nor extend
+        /// its committed frontier.
+        const auto catalog_entry_it = std::lower_bound(
+            catalog_snapshot.catalog.entries.begin(), catalog_snapshot.catalog.entries.end(), ns,
+            [](const CatalogEntry & entry, const RootNamespace & needle) { return entry.ns < needle; });
+        chassert(catalog_entry_it != catalog_snapshot.catalog.entries.end());
+        chassert(catalog_entry_it->ns == ns);
+        chassert(catalog_entry_it->incarnation == target.life_id);
+        std::optional<RefCkpt> checkpoint;
+        if (const auto checkpoint_it = checkpoints.recovery_checkpoints.find(ns_str);
+            checkpoint_it != checkpoints.recovery_checkpoints.end())
+            checkpoint = checkpoint_it->second;
+
+        std::optional<RecoveryGrounding> grounding;
+        String checkpoint_failure;
+        if (const auto bad_checkpoint = checkpoints.undecodable.find(ns_str);
+            bad_checkpoint != checkpoints.undecodable.end())
+        {
+            checkpoint_failure = bad_checkpoint->second;
+        }
+        else
+        {
+            try
+            {
+                grounding = chooseRecoveryGrounding(std::optional<CatalogEntry>{*catalog_entry_it}, checkpoint);
+            }
+            catch (const Exception & e)
+            {
+                checkpoint_failure = e.message();
+            }
+        }
+
+        /// The hold THIS round detected, at the position it stopped. It IS the clamp signal: there is
+        /// no separate boolean that could disagree with it about whether the namespace stopped.
+        std::optional<RefHold> fired;
+        RefTxnId resolved_through = cursor;   /// advances per fully-folded log; a clamp keeps it below the log
+
+        /// The witness role: the smallest id strictly above `id` that SOMETHING says exists. An absent
+        /// expected-next means "frontier" without a witness and "impossible shape" with one, so the
+        /// witness set decides whether the walk stops quietly or holds. THREE independent sources, none
+        /// of them authoritative alone:
+        ///   * the hint, which may omit durable records (that is the whole reason intake is arithmetic);
+        ///   * `_ckpt.checkpoint`, the namespace's own durable tail -- a listing is a SNAPSHOT, so a
+        ///     record durable after the enumeration is invisible to that round's probes, and this one
+        ///     is not (phase-0 model `_fix_ckptwitness`);
+        ///   * the CARRIED HOLD's offending position, which is durable proof that a previous round
+        ///     reached that position -- with ONE exception, `CheckpointUndecodable`, the only hold
+        ///     minted before the walk reads anything: its position is the walk's OWN next position, so
+        ///     the strict comparison below never lets it witness against a position this walk goes on
+        ///     to read. It weakens nothing, because a hold that proves nothing also claims nothing.
+        ///     Under contiguity everything at or below a REACHED position must exist, so an absent
+        ///     below it is a gap. This is what makes "retry the exact offending position" work
+        ///     for a hold that sits above an epoch boundary: the crossing needs a witness to aim at,
+        ///     and without this the walk would stop one position short and never re-read it.
+        /// The SMALLEST wins, because the nearest witness is the one that decides same-epoch gap versus
+        /// epoch crossing.
+        const auto witnessAbove = [&](const RefTxnId & id) -> std::optional<RefTxnId>
+        {
+            std::optional<RefTxnId> nearest;
+            const auto consider = [&](const RefTxnId & w)
+            {
+                if (id < w && (!nearest || w < *nearest))
+                    nearest = w;
+            };
+            const auto it = std::upper_bound(listing.logs.begin(), listing.logs.end(), id);
+            if (it != listing.logs.end())
+                consider(*it);
+            if (const auto ck = checkpoint_witness.find(ns_str); ck != checkpoint_witness.end())
+                consider(ck->second);
+            if (grounding && grounding->committed_through)
+                consider(*grounding->committed_through);
+            if (carried_hold)
+                consider(carried_hold->offending_position);
+            return nearest;
+        };
+
+        /// Record the position an impossible shape was detected at, hold the namespace, and stop walking
+        /// it. The reason is a BOUNDED enum because it is persisted in the seal: an operator reading a
+        /// held row learns what stopped the namespace and exactly where, without correlating logs.
+        const auto hold = [&](const RefTxnId & at, HoldReason reason, const char * message)
+        {
+            report.recordAnomaly(ns, 0, ManifestId{ns, {}}, message);
+            EventEmitter{*store}.emit([&](CasEvent & ev)
+            {
+                ev.type = CasEventType::GcFoldClamp;
+                ev.namespace_ = ns_str;
+                ev.object_kind = CasEventObjectKind::Root;
+                ev.outcome = "held";
+                ev.reason = message;
+                ev.detail = {{"expected", renderRefTxnId(at)},
+                             {"resolved_through",
+                              resolved_through == RefTxnId{} ? "none" : renderRefTxnId(resolved_through)}};
+            });
+            LOG_ERROR(logger,
+                "CAS GC ref intake: namespace {} HELD at {} -- {}. The cursor stays at {} and this "
+                "namespace folds nothing further this round.",
+                ns_str, renderRefTxnId(at), message,
+                resolved_through == RefTxnId{} ? "none" : renderRefTxnId(resolved_through));
+            fired = RefHold{.reason = reason, .offending_position = at,
+                            .retry_count = 0, .next_retry_round = 0};   /// retry fields filled in at the seal below
+        };
+
+        /// Cross into the epoch that follows the one `from_seal` closed. The rule itself is
+        /// `crossEpochFromSeal` (Pool/CasRefProtocol.cpp) -- read its doc comment for what the back-chain
+        /// proves and why the hint may only NOMINATE the target epoch. It lives there rather than here
+        /// because fsck's audit walks the same streams: if the round and the audit could disagree about
+        /// when an epoch boundary is proved, they would disagree about which records a cut contains.
+        ///
+        /// This wrapper adds only the round's own accounting -- the reads the crossing performed land on
+        /// the row that spent them, and an undecodable epoch-start record is logged with the key.
+        const auto crossFromSeal = [&](const RefTxnId & from_seal, const std::optional<bool> & seal_proven,
+                                       const RefTxnId & witness) -> std::optional<RefTxnId>
+        {
+            const EpochCrossResult crossing =
+                crossEpochFromSeal(backend, layout, ns, from_seal, seal_proven, witness, life);
+            intake_absent_probes += crossing.absent_probes;   /// a failed crossing pays its reads too
+            ProfileEvents::increment(ProfileEvents::CASRefLogBodyGets, crossing.body_gets);
+            if (crossing.outcome == EpochCrossOutcome::StartInvalid)
+                LOG_WARNING(logger, "CAS GC ref intake: epoch-start log {} invalid: {}",
+                            layout.refLogKey(life, crossing.probed), crossing.detail);
+            return crossing.proved() ? std::optional<RefTxnId>(crossing.start) : std::nullopt;
+        };
+
+        /// The first position is arithmetic from the sealed cursor, or from the exact checkpoint's life
+        /// epoch on the first fold. A listing never chooses a logical history start. An unusable
+        /// checkpoint still has a canonical retry position once a cursor exists, so its durable hold
+        /// remains observable rather than degrading into an anonymous anomaly.
+        std::optional<RefTxnId> expected;
+        if (cursor != RefTxnId{})
+            expected = RefTxnId{cursor.writer_epoch, cursor.ref_sequence + 1};
+        else if (grounding)
+        {
+            expected = RefTxnId{*checkpoint->life_epoch, 1};
+        }
+
+        /// An unavailable or invalid `_ckpt` quarantines its own namespace and nothing else (spec §5). The object
+        /// belongs to exactly one namespace and both of its consumers are keyed by that namespace --
+        /// `witnessAbove` above, and `cleanupRefObjects`' delete boundaries via `result.checkpoints` --
+        /// so the damage is confinable, and confining it is the difference between one namespace waiting
+        /// for a repair and the whole pool's cursors, seals and cleanup stopping on one 4 KiB object.
+        ///
+        /// FOLD NOTHING FOR IT, rather than fold and merely refuse the frontier proof. The cursor
+        /// advance is the one irreversible thing the walk does (nothing ever re-reads below it), and
+        /// this namespace has just proved that a piece of its own durable state cannot be read. Spending
+        /// that irreversible step against a namespace in that condition buys reclamation latency and
+        /// costs recovery options; the hold costs only the latency.
+        ///
+        /// The hold sits at the position the walk WOULD have read, which is canonical by construction.
+        /// A namespace with no such position is a never-folded one whose listing shows no log -- the
+        /// `_ckpt`-only "phantom table" `parseRefCkptKey` deliberately admits is exactly this shape --
+        /// and there is no walk to stop; the anomaly alone carries it, because the other consumer is
+        /// `cleanupRefObjects`, whose boundaries an ABSENT checkpoint WIDENS, and only the round's
+        /// destructive gate keeps the snapshot this unreadable object names from being deleted.
+        if (!grounding)
+        {
+            LOG_WARNING(logger,
+                "CAS GC ref intake: namespace {} has no usable checkpoint -- {}. This namespace "
+                "folds nothing and reclaims nothing until that object is repaired; every other "
+                "namespace folds normally.",
+                ns_str, checkpoint_failure);
+            if (expected)
+                hold(*expected, HoldReason::CheckpointUndecodable,
+                     "ref intake: the namespace's `_ckpt` is unavailable or invalid -- its durable "
+                     "checkpoint authority cannot be read, so nothing above the cursor is accountable");
+            else
+                report.recordAnomaly(ns, 0, ManifestId{ns, {}},
+                    "ref intake: the catalog life has no usable `_ckpt` (no authoritative walk position)");
+            unproven_reason = FoldResult::FrontierUnproven::CheckpointUnusable;
+            expected.reset();   /// the cursor rides verbatim into this round's seal
+        }
+
+        if (grounding && !grounding->committed_through)
+        {
+            if (cursor == RefTxnId{})
+                frontier_proven = true;
+            else
+            {
+                report.recordAnomaly(ns, 0, ManifestId{ns, {}},
+                    "ref intake: an empty checkpoint frontier cannot explain a nonzero sealed cursor");
+                unproven_reason = FoldResult::FrontierUnproven::CheckpointFrontierEmpty;
+            }
+            expected.reset();
+        }
+
+        /// Whether the record this round last applied (the one `resolved_through` names) was an
+        /// `EpochSeal`. `nullopt` = this round has applied nothing yet, so `resolved_through` is still
+        /// the inherited cursor and its kind is not knowable here -- see `crossFromSeal`.
+        std::optional<bool> last_applied_is_seal;
+
+        /// Probe B1's per-epoch contiguous run: opened at the first position walked in an epoch, closed
+        /// when the walk leaves that epoch or stops.
+        std::optional<RefTxnId> segment_first;
+        std::vector<std::pair<RefTxnId, RefTxnId>> segments;
+        const auto closeSegment = [&]()
+        {
+            if (segment_first)
+                segments.emplace_back(*segment_first, resolved_through);
+            segment_first.reset();
+        };
+
+        while (expected)
+        {
+            /// `_ckpt.committed_through` is the inclusive authority ceiling. Stop before reading a
+            /// durable but uncommitted `F+1`, so neither it nor a later 404 can advance this cursor or
+            /// establish the destructive frontier.
+            if (*grounding->committed_through < *expected)
+            {
+                if (resolved_through == *grounding->committed_through)
+                    frontier_proven = true;
+                else
+                {
+                    report.recordAnomaly(ns, 0, ManifestId{ns, {}},
+                        "ref intake: checkpoint committed_through precedes the sealed cursor");
+                    unproven_reason = FoldResult::FrontierUnproven::CommittedBelowCursor;
+                }
+                break;
+            }
+
+            /// GET + decode the expected record. Absence is the decision point of the whole walk, and
+            /// an invalid body is a per-namespace hold: the key belongs to exactly one namespace, so it
+            /// can never be grounds for discarding another namespace's fold.
+            const auto got = backend.get(layout.refLogKey(life, *expected));
+            if (!got)
+            {
+                ++intake_absent_probes;
+                const auto witness = witnessAbove(*expected);
+                if (!witness)
+                {
+                    /// The ceiling test above already refused everything past `committed_through`, so
+                    /// this absent position is committed and its record owes us an answer.
+                    hold(*expected, HoldReason::GapBelowWitness,
+                         "ref intake: a checkpoint-committed ref log is absent -- its authoritative "
+                         "frontier cannot be complete");
+                    break;
+                }
+
+                if (witness->writer_epoch == expected->writer_epoch)
+                {
+                    hold(*expected, HoldReason::GapBelowWitness,
+                         "ref intake: expected next id absent below a same-epoch witness -- "
+                         "contiguity says this cannot happen, so a durable record is missing");
+                    break;
+                }
+
+                const auto crossed = crossFromSeal(resolved_through, last_applied_is_seal, *witness);
+                if (!crossed)
+                {
+                    hold(*expected, HoldReason::UnconsumedSealCrossing,
+                         "ref intake: a later epoch's records are reachable but this epoch's "
+                         "closing seal was never consumed (or the position they chain from is "
+                         "not one) -- the crossing has no proof");
+                    break;
+                }
+                /// Every iteration must move `expected` strictly forward. A crossing that lands back on
+                /// the position just read as absent makes no progress and would spin: the epoch-start
+                /// record answered a GET inside `crossFromSeal` and then not here, so it is vanishing
+                /// under us. Treat it as the impossible shape it is.
+                if (!(*expected < *crossed))
+                {
+                    /// The record at `*crossed` answered the crossing's GET and then stopped answering
+                    /// the walk's: an ABOVE-CURSOR object that vanished under us. Nothing may
+                    /// legitimately remove one, so this is corruption, and it is the one hold shape no
+                    /// amount of waiting clears -- which is exactly why it must be named durably. A
+                    /// later round that read the namespace as merely quiet would otherwise grant it a
+                    /// frontier proof and license destruction against a cut that is missing records.
+                    hold(*expected, HoldReason::WitnessDisappeared,
+                         "ref intake: the epoch crossing resolved back to the position that "
+                         "just read absent -- the epoch-start record is not stably readable");
+                    break;
+                }
+                ++intake_epoch_crossings;
+                closeSegment();
+                expected = *crossed;
+                continue;
+            }
+            const RefTxnId log_id = *expected;
+
+            /// Probe B2: open this transaction's ledger entry. A clamped log is opened but never
+            /// committed, so it is correctly not reported unapplied.
+            const uint32_t txn_ordinal = ledger.open(ns, log_id);
+
+            ProfileEvents::increment(ProfileEvents::CASRefLogBodyGets);   /// one body GET per new log
+            RefLogTxn txn;
+            std::vector<RefManifestEdge> edges;
+            try
+            {
+                txn = decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns_str, log_id);
+                /// Extraction shares the decode try-block: an unrecognized owner_transition shape
+                /// (`manifestEdgesOfTxn` -> `classifyOwnerTransitionShape`, Pool/CasRefProtocol.cpp) is
+                /// exactly as untrustworthy as an undecodable body -- both mean this log cannot be
+                /// folded -- so both get the SAME per-namespace hold below. Do
+                /// NOT widen this catch over `foldManifestEdges` -- only intake, not the fold itself,
+                /// shares this discipline.
+                edges = manifestEdgesOfTxn(txn);
+            }
+            catch (const Exception & e)
+            {
+                LOG_WARNING(logger, "CAS GC ref intake: log {} invalid: {}",
+                            layout.refLogKey(life, log_id), e.message());
+                hold(log_id, HoldReason::BodyUndecodable, "ref log body invalid: namespace held below it");
+                break;
+            }
+
+            /// Fold every explicit manifest edge of the log. A transaction applies
+            /// ATOMICALLY ("either the complete transaction applies or none of it applies"): stage this
+            /// log's blob deltas and owner-removed manifest cleanup in PER-LOG buffers and merge them into
+            /// the round buffers only once the WHOLE log folds. A mid-log clamp DISCARDS the staged buffers
+            /// so the cursor stays coherent -- merging a partially folded log's `-1` cleanup would let the
+            /// post-CAS body delete remove a body whose edge is still unfolded behind the clamp, and the
+            /// re-fold would then clamp on that missing body forever. A missing manifest body is a per-table
+            /// CLAMP (barrier), never a round abort: keep the cursor below THIS log and re-read it next
+            /// round. A removed precommit whose body never existed emitted no edge -- skip, no clamp.
+            std::vector<BlobDelta> log_deltas;
+            std::map<ManifestId, Token> log_mf_cleanup;
+            for (const RefManifestEdge & edge : edges)
+            {
+                ProfileEvents::increment(ProfileEvents::CASRefEmittedEdges);   /// one manifest-edge event
+                if (foldManifestEdges(edge.manifest_id, edge.change, log_deltas, log_mf_cleanup,
+                                      txn_ordinal))
+                    continue;
+
+                if (edge.change < 0 && edge.owner_kind == RefOwnerKind::Precommit)
+                    continue;   /// removed precommit that never activated: no edge to mirror, no clamp
+
+                /// A `+1` precommit whose body is absent normally holds the fold barrier (the writer may
+                /// still be uploading it). But a precommit naming a build PROVABLY DEAD by the durable
+                /// watermark floor -- the SAME fact the orphan sweep uses to reclaim the body -- can never
+                /// activate, and its body will never return. Skip it (non-activating, advance the log) so
+                /// the barrier is not held forever on a body no writer will complete; otherwise this table
+                /// clamps every round with no terminal resolution. Fail-closed: no durable watermark (the
+                /// build cannot be proven dead) keeps the barrier.
+                if (edge.change > 0 && edge.owner_kind == RefOwnerKind::Precommit
+                    && prefixEligible(*store, ns,
+                           BuildPrefix{.writer_epoch = edge.manifest_id.ref.writer_epoch,
+                                       .build_sequence = edge.manifest_id.ref.build_sequence}))
+                {
+                    ProfileEvents::increment(ProfileEvents::CASGCDeadPrecommitSkipped);
+                    ++intake_dead_precommits_skipped;
+                    EventEmitter{*store}.emit([&](CasEvent & ev)
+                    {
+                        ev.type = CasEventType::GcFoldClamp;   /// reuse the fold-decision channel; outcome distinguishes
+                        ev.namespace_ = ns_str;
+                        ev.object_kind = CasEventObjectKind::Root;
+                        ev.object_hash = manifestRefDebugString(edge.manifest_id.ref);
+                        ev.outcome = "dead_precommit_skipped";
+                        ev.reason = "live precommit body absent AND its build is below the watermark floor "
+                                    "(provably dead); skip the non-activating edge instead of clamping forever";
+                        ev.detail = {{"log", renderRefTxnId(log_id)}};
+                    });
+                    continue;
+                }
+
+                const char * reason = edge.change > 0
+                    ? (edge.owner_kind == RefOwnerKind::Precommit
+                           ? "fold barrier: live precommit body not yet present (non-activating)"
+                           : "committed/promoted ref names a missing manifest body")
+                    : "owner-removal: edge-bearing committed body missing at removal-fold";
+                report.recordAnomaly(ns, 0, edge.manifest_id, reason);
+                EventEmitter{*store}.emit([&](CasEvent & ev)
+                {
+                    ev.type = CasEventType::GcFoldClamp;
+                    ev.namespace_ = ns_str;
+                    ev.object_kind = CasEventObjectKind::Root;
+                    ev.object_hash = manifestRefDebugString(edge.manifest_id.ref);
+                    ev.outcome = "clamped";
+                    ev.reason = reason;
+                    ev.detail = {{"log", renderRefTxnId(log_id)},
+                                 {"resolved_through",
+                                  resolved_through == RefTxnId{} ? "none" : renderRefTxnId(resolved_through)}};
+                });
+                /// The barrier is a HOLD like any other -- same durable shape, same clearing rule (fold
+                /// through `log_id`), and the same suppression consequence. Its reason is the one whose
+                /// ordinary cause is benign (a writer that appended its record before finishing the
+                /// manifest upload), and naming it durably is what lets an operator tell that apart
+                /// from a namespace that has been stuck for hours. The anomaly and event were already
+                /// emitted above with the manifest identity, which `hold` cannot carry, so this site
+                /// sets the hold directly instead of calling it.
+                fired = RefHold{.reason = HoldReason::ManifestBodyMissing, .offending_position = log_id,
+                                .retry_count = 0, .next_retry_round = 0};
+                break;   /// stop folding this log's edges; the cursor stays at resolved_through (< log_id)
+            }
+
+            if (fired)
+                break;   /// discard the staged log_deltas / log_mf_cleanup (never merged) and stop this table
+
+            /// The whole log folded: merge its staged transaction into the round buffers.
+            if (!log_deltas.empty())
+                ledger.markProduced(txn_ordinal);
+            ledger.markCommitted(txn_ordinal);
+            for (BlobDelta & d : log_deltas)
+                deltas.push_back(std::move(d));
+            for (const auto & [mid, tok] : log_mf_cleanup)
+                result.mf_cleanup.emplace(mid, tok);
+
+            /// A fully-folded `remove_namespace` transaction hands its `{ns, remove_txn_id}` to the
+            /// life row's cleanup evidence; its owner-removal edges were folded above.
+            if (const auto removal = removalTxnId(txn))
+                new_removals.emplace_back(ns, *removal);
+            if (!segment_first)
+                segment_first = log_id;    /// this epoch's contiguous run opens at the first applied id
+            resolved_through = log_id;     /// this log fully folded
+            /// The KIND of the record the cursor now sits on, remembered for the crossing below: the
+            /// chain check proves the identity of the position an epoch chains from, never that that
+            /// position is a seal, and this is the one place the answer is free (the body is decoded and
+            /// in hand). See the crossing's own comment for the half of this that cannot be re-checked.
+            last_applied_is_seal = refLogTxnIsEpochSeal(txn);
+            ++logs_applied;                /// probe B1: the SINGLE cursor-advance site
+            table_changed = true;
+
+            if (const std::optional<RefTxnId> next = nextRefLogIdWithinCommittedFrontier(
+                    log_id, *last_applied_is_seal, *grounding->committed_through))
+            {
+                if (*last_applied_is_seal && next->writer_epoch != log_id.writer_epoch)
+                {
+                    const auto crossed = crossFromSeal(log_id, last_applied_is_seal, *grounding->committed_through);
+                    if (!crossed)
+                    {
+                        hold(*next, HoldReason::UnconsumedSealCrossing,
+                             "ref intake: checkpoint frontier cannot prove the successor of the epoch seal "
+                             "just consumed");
+                        break;
+                    }
+                    ++intake_epoch_crossings;
+                    closeSegment();
+                    expected = *crossed;
+                }
+                else
+                    expected = *next;
+            }
+            else
+            {
+                frontier_proven = true;
+                expected.reset();
+            }
+        }
+        closeSegment();
+        if (!segments.empty())
+            walked_segments[ns_str] = std::move(segments);
+
+        cov.last_folded_ref_id = resolved_through;
+
+        /// THE CLEARING RULE (spec §5), and the only place it is decided. A hold clears by exactly one
+        /// event: this walk RESOLVING the offending position -- folding through it and sealing a cursor
+        /// at or above it. It never clears by observing another absent, because an absent is precisely
+        /// what a lying store produces and precisely what made the hold necessary; and it never clears
+        /// by the hint going quiet, because a quiet hint is not evidence about anything.
+        ///
+        /// Three cases, total and in order:
+        ///   * this round detected a hold -- adopt it. It sits at or above the cursor, so any carried
+        ///     hold at a HIGHER position is simply not reached yet and will be re-detected once this
+        ///     one clears; a carried hold at a LOWER position was folded through, which is its
+        ///     clearance.
+        ///   * no new hold, but the walk stopped BELOW a carried hold's position -- it did not resolve
+        ///     it, so the hold rides VERBATIM (same reason, same position).
+        ///   * otherwise -- either there was no hold, or the walk folded through it. Cleared.
+        std::optional<RefHold> effective;
+        if (fired)
+            effective = fired;
+        else if (carried_hold && resolved_through < carried_hold->offending_position)
+        {
+            effective = carried_hold;
+            /// The round produced no anomaly of its own for this namespace (the walk ended quietly),
+            /// yet the namespace IS held: record one so this round's destructive work is suppressed on
+            /// today's `anomalies`-based rule as well as on the hold set itself. Without it a hold
+            /// carried through a quiet round would suppress nothing, which is the hole the carry exists
+            /// to close.
+            report.recordAnomaly(ns, 0, ManifestId{ns, {}},
+                "ref intake: namespace still held below an unresolved position");
+            EventEmitter{*store}.emit([&](CasEvent & ev)
+            {
+                ev.type = CasEventType::GcFoldClamp;
+                ev.namespace_ = ns_str;
+                ev.object_kind = CasEventObjectKind::Root;
+                ev.outcome = "held";
+                ev.reason = "carried hold: the offending position did not resolve this round";
+                ev.detail = {{"expected", renderRefTxnId(carried_hold->offending_position)},
+                             {"resolved_through",
+                              resolved_through == RefTxnId{} ? "none" : renderRefTxnId(resolved_through)},
+                             {"retry_count", std::to_string(carried_hold->retry_count)}};
+            });
+        }
+
+        if (effective)
+        {
+            /// Retry bookkeeping. The count belongs to a POSITION, so it continues only while the hold
+            /// stays at the same one; a hold that moved is a different stop and starts over. The count
+            /// saturates rather than wrapping -- a wrapped counter would report a namespace stuck for
+            /// four billion rounds as freshly held.
+            const bool same_position = carried_hold
+                && carried_hold->offending_position == effective->offending_position;
+            effective->retry_count = same_position && carried_hold->retry_count < UINT32_MAX
+                ? carried_hold->retry_count + 1
+                : (same_position ? UINT32_MAX : 0);
+            effective->next_retry_round = current_round + 1;
+            cov.hold = effective;
+            cov.classification = 4;
+            ++intake_tables_held;
+            /// A held namespace is unproven BY DEFINITION -- the hold names a position the walk could
+            /// not resolve, so everything at or above it is unaccounted. Stated here rather than left to
+            /// the loop's control flow: a carried hold rides forward on a round whose own walk ended
+            /// quietly, and that quiet end must not be mistaken for a proof.
+            frontier_proven = false;
+            unproven_reason = FoldResult::FrontierUnproven::Held;
+        }
+        else
+            cov.classification = table_changed ? 2 : 1;
+
+        result.fold_seal.ref_lives.at(target.life_id).coverage = cov;
+        ++result.frontier_namespaces;
+        if (frontier_proven)
+            ++result.frontier_proven;
+        else
+            result.frontier_deficit.count(unproven_reason);
+        if (table_changed)
+        {
+            folded_any = true;
+            ++intake_tables_changed;
+        }
+        if (fired)
+            ++intake_tables_clamped;
+    }
+
+    /// Namespaces the round KNOWS about but never probed, because the frontier-probe budget ran out
+    /// before reaching them. Their cursors ride VERBATIM -- dropping a cursor because a round ran out of
+    /// budget would hand the next round a namespace to re-fold from `{0,0}`, which is a far worse
+    /// outcome than the unproven frontier this already is. They count toward the universe and NOT toward
+    /// the proofs, which is what suppresses the round.
+    ///
+    /// THE DENOMINATOR IS THE SEALED SET, BY CONSTRUCTION. `frontier_namespaces` is incremented by the
+    /// number of rows this loop actually ADDED to the seal, never by the count the skip loop predicted.
+    /// The two are the same set under the same filters, so they agree -- but agreeing "because both
+    /// filters were written the same way" is exactly the kind of coincidence that decays under editing,
+    /// and `frontier_namespaces` is the denominator an operator (and the integration test) reads as THE
+    /// universe. Deriving it from the seal removes the possibility of publishing a number that
+    /// describes a different universe than the round sealed. The `chassert` states the equality so a
+    /// future divergence surfaces in debug rather than silently; it is metric integrity, not a safety
+    /// gate, because both paths suppress the round either way.
+    if (intake_unprobed_budget > 0)
+    {
+        result.frontier_namespaces += intake_unprobed_budget;
+        result.frontier_deficit.probe_budget += intake_unprobed_budget;
+        LOG_WARNING(logger,
+            "CAS GC ref intake: the frontier-probe budget ({}) ran out with {} known namespace(s) "
+            "unprobed; their cursors ride unchanged and ALL destructive work is suppressed this round",
+            store->poolConfig().gc_frontier_probe_budget, intake_unprobed_budget);
+    }
+
+    /// All-or-nothing: a malformed key/body anywhere aborts the round's ref folding. Discard
+    /// every ref delta and cursor advance already accumulated, carry each table's parent cursor verbatim,
+    /// and let the recorded anomaly suppress destructive work.
+    if (ref_folding_aborted)
+    {
+        deltas.clear();
+        ledger = TxnApplyLedger{};   /// the deltas are gone, so nothing can be unapplied
+        result.mf_cleanup.clear();
+        folded_any = false;
+        /// An abort discards this round's walk, so it discards its proofs with it: nothing this round
+        /// observed may be offered as a frontier proof to the destructive gate.
+        result.frontier_proven = 0;
+        result.frontier_deficit = FoldResult::FrontierDeficit{};
+        result.frontier_deficit.fold_aborted = result.frontier_namespaces;
+        for (const WalkTarget & target : walk_targets)
+        {
+            RefCoverage cov;
+            cov.classification = 1;
+            if (const auto pit = parent_ref_lives.find(target.life_id); pit != parent_ref_lives.end())
+            {
+                cov.last_folded_ref_id = pit->second.coverage.last_folded_ref_id;
+                /// An abort discards this round's work; it does not resolve anything, so a hold it
+                /// found in the parent seal rides forward untouched -- not even the retry count moves,
+                /// because nothing was retried.
+                if (pit->second.coverage.hold)
+                {
+                    cov.hold = pit->second.coverage.hold;
+                    cov.classification = 4;
+                }
+            }
+            RefLifeFoldState & ref_life_state = result.fold_seal.ref_lives.at(target.life_id);
+            ref_life_state.coverage = cov;
+        }
+    }
+
+    /// PROBE B1's recomputation, taken HERE rather than at the seal write: every input it reads
+    /// (`walked_segments`, the sealed ref-life coverage) is final as of this line, and nothing
+    /// between here and the seal write touches any of them. Computing it inside the intake phase is what
+    /// lets the `fold_ref_intake` row carry both numbers; the comparison and its fail-closed throw stay
+    /// where they were, just before the seal write. Both stay 0 on a ref-folding abort -- that path
+    /// discards every cursor advance and carries the parent cursors, so the identity does not apply.
+    ///
+    /// It counts the CUT ARITHMETICALLY, not by listed ids. Under arithmetic intake a listed-id count is
+    /// not even the right question: a hint hole means a round legitimately applies records the listing
+    /// never mentioned, so the old recomputation would report fewer logs than folded and fail every
+    /// healthy round on a lying store -- it would have made this task's own fix unshippable.
+    ///
+    /// BE HONEST ABOUT WHAT IS LEFT. The old formula could disagree with reality because it was derived
+    /// from a different source (the listing) than the counter. This one is derived from the runs the
+    /// single advance site produced, so for the current code shape it is close to tautological, and B1's
+    /// discriminating power went DOWN with this change rather than up. What it still asserts is worth
+    /// keeping and is not free: THE SEALED CURSOR IS THE WALK'S CURSOR. The last run of each namespace is
+    /// measured against the DURABLE ref-life coverage the next round will trust, not against the
+    /// walk's own end, so a cursor sealed from anywhere other than this walk -- a stale carry, a mutated
+    /// coverage row, a future edit that advances the cursor away from the advance site -- either fails
+    /// the epoch/order check below or lands as a count that no longer matches `logs_applied`.
+    logs_accounted_this_round = 0;
+    logs_applied_this_round = 0;
+    if (!ref_folding_aborted)
+    {
+        uint64_t logs_accounted = 0;
+        for (const auto & [ns_str, segments] : walked_segments)
+        {
+            const UInt128 life_id = live_incarnation.at(ns_str);
+            RefTxnId sealed{};
+            if (const auto sit = result.fold_seal.ref_lives.find(life_id);
+                sit != result.fold_seal.ref_lives.end())
+                sealed = sit->second.coverage.last_folded_ref_id;
+
+            for (size_t i = 0; i < segments.size(); ++i)
+            {
+                const auto & [first, last] = segments[i];
+                /// The final run must end exactly where the seal says this namespace stopped, in the same
+                /// epoch. A disagreement is a sealed cursor that did not come from this walk, so it fails
+                /// closed here rather than travelling into the next round's baseline.
+                const RefTxnId end = i + 1 == segments.size() ? sealed : last;
+                if (end.writer_epoch != first.writer_epoch || end < first)
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS GC fold: namespace {} sealed a cursor at {} that does not close the run it "
+                        "walked (opened at {}). GC refuses to commit the round; recover with "
+                        "SYSTEM CAS GC REBUILD.",
+                        ns_str, end == RefTxnId{} ? "none" : renderRefTxnId(end), renderRefTxnId(first));
+                logs_accounted += end.ref_sequence - first.ref_sequence + 1;
+            }
+        }
+        logs_accounted_this_round = logs_accounted;
+        logs_applied_this_round = logs_applied;
+    }
+
+    intake_timer->metric("logs_accounted", logs_accounted_this_round);
+    intake_timer->metric("logs_applied", logs_applied_this_round);
+    intake_timer->metric("deltas_emitted", deltas.size());
+    intake_timer->metric("transactions_opened", ledger.txns.size());
+    intake_timer->metric("tables_scanned", ref_tables.size());
+    intake_timer->metric("tables_changed", intake_tables_changed);
+    intake_timer->metric("tables_clamped", intake_tables_clamped);
+    /// `tables_held` counts the coverage rows this round SEALED held -- the ones it detected plus the
+    /// ones it carried. It is what `suppress_destructive` keys on, so a round that reclaims nothing has
+    /// this column to explain itself. `unhinted_held_walked` is the subset the hint never mentioned:
+    /// nonzero means the store stopped listing a namespace that is still held, which is the shape that
+    /// used to clear a hold silently.
+    intake_timer->metric("tables_held", intake_tables_held);
+    intake_timer->metric("unhinted_held_walked", intake_unhinted_held);
+    /// THE FROZEN WORK-SET, ON THE ROW THAT PAID FOR IT. `tails_advanced` is how many hinted namespaces
+    /// had records to fold this round because their listed tail sat above their cursor -- the round's
+    /// real work -- and `tails_unchanged` is how many folded NOTHING and paid only their single frontier
+    /// probe, which on a wide pool with a few hot namespaces is most of them and is the number that
+    /// explains a short round. `tails_below_cursor` is the anomaly of the three: cleanup deletes logs
+    /// from the bottom up, so a listing whose greatest id is below a cursor we folded through is stale or
+    /// lying, and a column that is normally 0 is what makes the pool where it is not stand out.
+    intake_timer->metric("tails_advanced", intake_tails_advanced);
+    intake_timer->metric("tails_unchanged", intake_tails_unchanged);
+    intake_timer->metric("tails_below_cursor", intake_tails_below_cursor);
+    /// THE FRONTIER OBLIGATION, on the row that explains a round which reclaimed nothing:
+    /// `frontier_namespaces` is the round's universe (hint ∪ sealed cursors ∪ catalog `Live`/`Removing`
+    /// entries), `frontier_proven` the part of it that reached an honest end-of-stream, and the
+    /// remaining columns say where the rest went -- walked because the hint had gone quiet about them,
+    /// not walked at all because the probe budget ran out, or walked ONLY because the catalog named a
+    /// namespace neither the hint nor a carried cursor did (`catalog_only_walked` -- always 0 until a
+    /// namespace is admitted with no listed objects and no sealed cursor yet).
+    intake_timer->metric("frontier_namespaces", result.frontier_namespaces);
+    intake_timer->metric("frontier_proven", result.frontier_proven);
+    /// `catalog_entries` is the hot-scan catalog cut's own row count (every lifecycle state, `Creating`
+    /// included), and `catalog_proved_empty` is the derived verdict the destructive gate's non-vacuity
+    /// term consults. Together they let an operator tell a proved-empty `0/0` (success) apart from a
+    /// `Creating`-only or otherwise unprovable `0/0` (still suppressed) without re-deriving either fact.
+    intake_timer->metric("catalog_entries", catalog_snapshot.catalog.entries.size());
+    intake_timer->metric("catalog_proved_empty", result.catalog_cut_proved_empty ? 1 : 0);
+    intake_timer->metric("unhinted_quiet_walked", intake_unhinted_quiet);
+    intake_timer->metric("frontier_unprobed_budget", intake_unprobed_budget);
+    intake_timer->metric("catalog_only_walked", intake_catalog_only);
+    intake_timer->metric("dead_precommits_skipped", intake_dead_precommits_skipped);
+    /// The exact reads arithmetic intake pays that the listing-driven loop did not. `absent_probes`
+    /// counts EVERY read that came back absent: the expected-next of each namespace walked (at least one
+    /// -- the absent expected-next IS the frontier proof) and the epoch-start read of a crossing that
+    /// failed, which a namespace that holds every round pays every round. `epoch_crossings` counts the
+    /// successful ones. Both are on the row so the cost shows up where it is spent.
+    intake_timer->metric("absent_probes", intake_absent_probes);
+    intake_timer->metric("epoch_crossings", intake_epoch_crossings);
+    intake_timer->metric("namespace_removals", new_removals.size());
+    intake_timer->metric("ref_folding_aborted", ref_folding_aborted ? 1 : 0);
+    intake_timer.reset();   /// emits the `fold_ref_intake` row
+
+    result.frontier_unprobed_budget = intake_unprobed_budget;
+
+    /// Reuse the round's single ref LIST for post-CAS ref-object cleanup: one LIST serves
+    /// intake AND cleanup planning).
+    result.ref_tables = ref_tables;
+    /// Same reuse for the checkpoints: the intake walk already paid for them as its second witness, and
+    /// the cleanup ranges below are the other consumer of the same fact.
+    ///
+    /// An UNDECODABLE `_ckpt` contributes no entry here and therefore grants no cleanup authority. The
+    /// walk above also held (or recorded an anomaly for) every such namespace, so the round's destructive
+    /// gate is shut and `cleanupRefObjects` deletes nothing at all this round. Any future change that
+    /// narrows the gate from round-wide to per-namespace must carry this set with it.
+    result.checkpoints = checkpoints.recovery_checkpoints;
+
+    /// Folding the terminal record earns positive cleanup evidence directly on the catalog-admitted
+    /// life row. It does not claim that any physical debris was removed: `_ckpt`, stream and `_files`
+    /// residue belongs to the perpetual janitor, while orphan manifests belong to the manifest sweep.
+    /// Consequently namespace removal performs no physical LIST and has no Pending/Completed handshake.
+    if (!ref_folding_aborted)
+        for (const auto & [rns, remove_txn_id] : new_removals)
+        {
+            const auto life_it = live_incarnation.find(rns.string());
+            if (life_it == live_incarnation.end())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS GC fold: terminal removal for namespace '{}' has no row in the round catalog cut",
+                    rns.string());
+            auto row_it = result.fold_seal.ref_lives.find(life_it->second);
+            if (row_it == result.fold_seal.ref_lives.end())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS GC fold: terminal removal for namespace '{}' has no admitted ref-life row",
+                    rns.string());
+            row_it->second.cleanup_evidence = RefCleanupEvidence{.remove_txn_id = remove_txn_id};
+        }
+
+    /// The parent generation's per-shard run segments, resolved from
+    /// the parent fold seal's `blob_target_runs` and grouped by the ref's explicit `shard`. The same seal
+    /// `discover_ref_seal` the token-diff already read is the run source — consumers resolve runs THROUGH
+    /// refs (a run sealed for the parent generation may physically live under an older generation's key),
+    /// never by `blobTargetRunKey` construction.
+    std::map<uint64_t, std::vector<RunRef>> parent_runs_by_shard;
+    for (const RunRef & r : discover_ref_seal.blob_target_runs)
+        parent_runs_by_shard[r.shard].push_back(r);
+
+    /// PURE REF-CARRY: a gc-shard with an EMPTY delta bucket
+    /// AND an EMPTY retired input list neither reads nor writes its run — the new fold_seal copies the
+    /// parent's `RunRef`s VERBATIM (key/checksum/shard/generation) so the next round resolves them. This
+    /// is deterministic (same refs for the same inputs), so seal determinism / crash-replay adoption hold.
+    /// An empty delta with a NON-EMPTY retired list still runs the merge: settlement must happen every
+    /// pass (carried/graduated/redeleted entries), and that pass reads the run to recompute in-degrees.
+    /// Distill one shard's `condemned_summary` entry from the `kCondemned` rows it re-emitted this pass
+    /// (`still_retired` mirrors those rows exactly). Folding shards call this; it makes the next
+    /// round's `graduationDue` and pure-carry decisions read only the seal, never a run.
+    auto summarize = [](const std::vector<RetiredEntry> & still) -> CondemnedSummary
+    {
+        CondemnedSummary s;
+        s.condemned_total = still.size();
+        for (const RetiredEntry & e : still)
+        {
+            if (e.delete_pending)
+                ++s.pending_total;
+            else
+                s.oldest_nonpending_condemn_round =
+                    std::min(s.oldest_nonpending_condemn_round, e.condemn_round);
+        }
+        return s;
+    };
+    /// The parent seal's summary for a shard, used ONLY for the pure-carry DECISION (condemned_total==0).
+    /// Missing => zero: a fresh pool has no parent entry (correct baseline), and a snap_generation>0 seal
+    /// that is missing an entry we would pure-carry fails closed inside `carryParentRefs` when it copies.
+    auto summaryOfParent = [&](uint64_t shard) -> CondemnedSummary
+    {
+        const auto it = discover_ref_seal.condemned_summary.find(shard);
+        return it != discover_ref_seal.condemned_summary.end() ? it->second : CondemnedSummary{};
+    };
+    auto carryParentRefs = [&](uint64_t shard)
+    {
+        const auto it = parent_runs_by_shard.find(shard);
+        if (it != parent_runs_by_shard.end())
+            for (const RunRef & r : it->second)
+                result.fold_seal.blob_target_runs.push_back(r);   /// verbatim: parent key/checksum/gen
+        /// Totality: a pure-carry shard settled nothing, so its `condemned_summary` entry is the parent's
+        /// VERBATIM. On a fresh pool (no adopted parent seal) it is the explicit zero baseline; otherwise
+        /// the parent seal MUST carry the entry (a live seal is total over gc_shards) — a missing entry is
+        /// corrupt bookkeeping, never silently treated as zero.
+        if (state.snap_generation == 0)
+            result.fold_seal.condemned_summary[shard] = CondemnedSummary{};
+        else
+        {
+            const auto sit = discover_ref_seal.condemned_summary.find(shard);
+            if (sit == discover_ref_seal.condemned_summary.end())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS gc fold: parent fold seal (generation {}, attempt {}) lacks a condemned_summary "
+                    "entry for gc-shard {} — the seal is not total over gc_shards; GC bookkeeping is corrupt",
+                    state.snap_generation, state.snap_attempt, shard);
+            result.fold_seal.condemned_summary[shard] = sit->second;
+        }
+    };
+    auto priorRunsFor = [&](uint64_t shard) -> const std::vector<RunRef> &
+    {
+        static const std::vector<RunRef> empty;
+        const auto it = parent_runs_by_shard.find(shard);
+        return it != parent_runs_by_shard.end() ? it->second : empty;
+    };
+
+
+    /// CLAMP SUPPRESSION: any clamp
+    /// this pass means landed-before-cut events may be UNFOLDED behind a clamped cursor — the
+    /// floor's "landed before cut => folded before graduation" lemma does not hold, so this pass
+    /// must not graduate NOR execute pending deletes (the merge carries everything; condemnation
+    /// and sparing continue). Deletes resume on the first clamp-free pass. This is the honest-mode
+    /// counterpart of the model's SabotageSkipChangedShard counterexample.
+    ///
+    /// PHASE 9/18 `fold_reduce`: the prior-run streaming GETs, one HEAD per zero-transition candidate,
+    /// and the run PUTs -- the heaviest phase of a folding round on a pool with churn. It also carries
+    /// probe B2's verdict (`transactions_unapplied`), which is 0 on every committed round because a nonzero
+    /// value throws a few lines below; the row is therefore the forensic record of a round that failed.
+    std::optional<GcPhaseTimer> reduce_timer;
+    reduce_timer.emplace(phase_sink, "fold_reduce");
+    const uint64_t deltas_in = deltas.size();
+    const uint64_t condemned_before = report.condemned;
+    uint64_t shards_pure_carry = 0;
+    /// ============================ THE DESTRUCTIVE GATE ============================
+    ///
+    /// Computed ONCE, here, from three independent terms, and consulted at every destructive site of
+    /// the round (see `FoldResult::suppress_destructive`). It sits at this point in the fold because
+    /// every input is final: the coverage rows the seal will carry are written, the anomalies are
+    /// recorded, and the frontier tally is closed.
+    ///
+    /// Term 2 is STRUCTURAL. Today every hold also records an anomaly, so term 1 happens to imply it --
+    /// but that is a property of the current code, not the invariant, and a gate that relies on a
+    /// coincidence opens the day the coincidence stops holding. The hold SET is the invariant, so the
+    /// gate reads the seal it is about to make durable.
+    const std::vector<std::pair<UInt128, RefHold>> carried_holds = result.carriedHolds();
+
+    /// Term 3, the universe seam. `Authoritative` means the round's universe is the catalog's own
+    /// `Live`/`Removing` set, so the per-namespace proofs decide on their own; `StageA_Suppressed`
+    /// refuses outright, which is the posture a test asserting inertness selects. See `UniversePolicy`.
+    const bool universe_authoritative = policy == UniversePolicy::Authoritative;
+    /// `frontier_proven == frontier_namespaces` is `0 == 0` -- TRUE --
+    /// on an empty universe, which is not a proof of anything BY ITSELF: a fresh pool, a damaged
+    /// catalog, or a read that legitimately returns nothing all produce zero entries. `frontier_namespaces
+    /// > 0` closes that degenerate case for the ordinary, nonempty pool -- a nonzero count still needs
+    /// every namespace PROVEN, which the equality above still checks.
+    ///
+    /// That floor alone is unsound for a pool whose LAST namespace was removed: the catalog then reads
+    /// genuinely empty forever, `frontier_namespaces` can never again exceed 0, and the equality's
+    /// vacuous truth can never be licensed -- an emptied pool would stop reclaiming permanently. The
+    /// floor's job was never "reject `frontier_namespaces == 0`", it was "reject the UNSUPPORTED case of
+    /// it". `catalog_cut_proved_empty` is the supported case: the round's own hot-scan catalog cut,
+    /// read once and reused (never a second `GET`), decoded successfully, token-bearing, and holding
+    /// zero rows of every lifecycle state including `Creating`. Under this pool's protocol every live or
+    /// live-precommit edge requires an exact `Live` catalog row (INV-3), so that cut is a positive proof
+    /// that no namespace anywhere holds one -- not merely an absence of proof. A catalog holding only
+    /// `Creating` rows produces the SAME `frontier_namespaces == 0` but is a birth in progress, not an
+    /// empty universe, and `catalog_cut_proved_empty` is false for it (see `entries.empty()` above).
+    result.frontier_complete = universe_authoritative
+        && result.frontier_proven == result.frontier_namespaces
+        && (result.frontier_namespaces > 0 || result.catalog_cut_proved_empty);
+    const bool frontier_incomplete = !result.frontier_complete;
+
+    result.suppress_destructive =
+        !report.anomalies.empty() || !carried_holds.empty() || frontier_incomplete;
+    const bool suppress_destructive = result.suppress_destructive;
+    if (suppress_destructive)
+    {
+        ProfileEvents::increment(ProfileEvents::CASGCClampSuppressedPasses);
+        /// LEVEL SPLIT, deliberately. A pass suppressed by an anomaly, a hold, an unproven namespace or
+        /// an empty universe has a per-round cause an operator can chase, and that is a WARNING. A pass
+        /// suppressed because the CALLER refused to supply a universe carries no such cause -- nothing on
+        /// the pool explains it -- so it is reported at Info with the same numbers rather than raising an
+        /// alarm nobody can act on.
+        ///
+        /// An AUTHORITATIVE-but-EMPTY universe that the catalog cut did NOT prove empty
+        /// (`frontier_namespaces == 0 && !catalog_cut_proved_empty` -- e.g. a `Creating`-only catalog)
+        /// is a per-round cause and is named explicitly here, because the bare equality
+        /// `frontier_proven != frontier_namespaces` reads `0 != 0` (false) and would otherwise miss it.
+        /// A catalog the cut DID prove empty is not a suppression cause at all: it satisfies
+        /// `frontier_complete`, so a round reaching this block with one is suppressed by an anomaly or
+        /// a hold, never by the frontier term.
+        const bool per_round_cause = !report.anomalies.empty() || !carried_holds.empty()
+            || result.frontier_proven != result.frontier_namespaces
+            || (universe_authoritative && result.frontier_namespaces == 0 && !result.catalog_cut_proved_empty);
+        const char * const universe_note =
+            universe_authoritative ? "" : "; the caller supplied no universe";
+        /// Names the real reason a `Creating`-only (or otherwise unprovable) catalog is not an empty
+        /// universe, so the operator does not read a suppressed round with rows on file as "empty".
+        const String catalog_empty_note =
+            (universe_authoritative && result.frontier_namespaces == 0 && !result.catalog_cut_proved_empty)
+                ? fmt::format("; catalog holds {} row(s), none walkable/provable", catalog_snapshot.catalog.entries.size())
+                : String{};
+        /// The per-cause breakdown of the unproven namespaces. Without it "N of M proven" names a
+        /// deficit but not its cause, and the causes want opposite operator responses.
+        const String deficit_note = result.frontier_deficit.total() == 0
+            ? String{}
+            : fmt::format("; unproven: {}", result.frontier_deficit.describe());
+        if (per_round_cause)
+            LOG_WARNING(logger,
+                "CAS GC fold: destructive work SUPPRESSED this pass — {} anomaly(ies), {} held "
+                "namespace(s), frontier {} ({} of {} namespace(s) proven{}{}{}). Graduations and pending "
+                "deletes are carried; nothing irreversible runs until a pass that clears all three.",
+                report.anomalies.size(), carried_holds.size(),
+                result.frontier_complete ? "complete" : "INCOMPLETE",
+                result.frontier_proven, result.frontier_namespaces, universe_note, catalog_empty_note,
+                deficit_note);
+        else
+            LOG_INFO(logger,
+                "CAS GC fold: destructive work SUPPRESSED this pass — {} anomaly(ies), {} held "
+                "namespace(s), frontier {} ({} of {} namespace(s) proven{}{}{}). Graduations and pending "
+                "deletes are carried; nothing irreversible runs until a pass that clears all three.",
+                report.anomalies.size(), carried_holds.size(),
+                result.frontier_complete ? "complete" : "INCOMPLETE",
+                result.frontier_proven, result.frontier_namespaces, universe_note, catalog_empty_note,
+                deficit_note);
+    }
+
+    std::vector<BlobSourceRetirement> orphan_source_retirements;
+    if (!suppress_destructive && store->poolConfig().manifest_sweep_list_budget_keys > 0)
+    {
+        result.orphan_sweep = planManifestCursorPage(
+            *store,
+            state.manifest_sweep_cursor,
+            store->poolConfig().manifest_sweep_list_budget_keys,
+            store->poolConfig().manifest_sweep_delete_budget_keys,
+            /// The sweep may recover a catalog-named namespace's debris only from the same frozen catalog
+            /// cut and `_ckpt` frontier the round's own universe came from -- which is exactly what an
+            /// authoritative universe means, and is why this is the gate's term and not a separate one.
+            universe_authoritative,
+            &work_budget);
+        for (const ManifestSweepResult::Nomination & nomination : result.orphan_sweep.nominations)
+            orphan_source_retirements.insert(
+                orphan_source_retirements.end(),
+                nomination.source_retirements.begin(),
+                nomination.source_retirements.end());
+    }
+
+    if (state.gc_shards == 1)
+    {
+        /// SINGLE-SHARD PATH (gc_shards == 1). Every blob routes to shard 0, so the entire delta stream
+        /// folds into one `blobTargetRunKey(new_generation, 0, 0)` run.
+        if (!folded_any && orphan_source_retirements.empty() && summaryOfParent(0).condemned_total == 0)
+        {
+            /// Pure ref-carry: nothing changed and no condemned entries to settle => zero run I/O. Carry the
+            /// parent shard-0 refs + summary into the seal so coverage/resume/graduation stay durable.
+            carryParentRefs(0);
+            ++shards_pure_carry;
+        }
+        else
+        {
+            /// Either a real delta or a non-empty retired input: run the merge (empty deltas still settle
+            /// the kCondemned rows riding the parent run). The prior runs are the parent seal's shard-0 refs.
+            foldDeltasIntoGeneration(backend, layout, priorRunsFor(0),
+                                     new_generation, attempt, /*shard*/0,
+                                     std::move(deltas), result.fold_seal.blob_target_runs,
+                                     current_round, condemn_round, head_blob, peek_head,
+                                     confirm_condemned_marker,
+                                     result.retired_merge.data(), suppress_destructive,
+                                     &ledger.applied, std::move(orphan_source_retirements),
+                                     &work_budget);
+            result.fold_seal.condemned_summary[0] = summarize(result.retired_merge[0].still_retired);
+        }
+    }
+    else
+    {
+        /// SHARDED PATH (gc_shards > 1) — target-sharded reducers. Each blob's
+        /// `BlobDelta` carries its full signed edge stream; `blobShard(blob_hash, gc_shards)` partitions
+        /// the stream into `gc_shards` disjoint buckets. Each bucket folds via its own `ShardReducer`
+        /// into `blobTargetRunKey(new_generation, shard, 0)`. The `RootOwnerEvent`'s paired old/new
+        /// bindings produced the `-1`/`+1` deltas above, so a promote that displaces a blob's owner
+        /// emits BOTH the `-1` (old binding) and the `+1` (new binding) at the SAME source event. This
+        /// is why cross-shard displacement needs no special handling: each delta routes independently and
+        /// deterministically to whichever target shard owns its blob; the old/new pair is solved at the
+        /// source, not by a cross-shard fixup.
+        std::vector<std::vector<BlobDelta>> buckets(state.gc_shards);
+        for (BlobDelta & d : deltas)
+            buckets[blobShard(d.ref, state.gc_shards)].push_back(std::move(d));
+        std::vector<std::vector<BlobSourceRetirement>> retirement_buckets(state.gc_shards);
+        for (BlobSourceRetirement & retirement : orphan_source_retirements)
+            retirement_buckets[blobShard(retirement.ref, state.gc_shards)].push_back(std::move(retirement));
+
+        for (uint64_t shard = 0; shard < state.gc_shards; ++shard)
+        {
+            if (buckets[shard].empty() && retirement_buckets[shard].empty()
+                && summaryOfParent(shard).condemned_total == 0)
+            {
+                /// Pure ref-carry for this shard: empty delta + no condemned entries => zero run I/O.
+                carryParentRefs(shard);
+                ++shards_pure_carry;
+                continue;
+            }
+            /// A reducer owns exactly one disjoint shard. Two replicas may run reducers for DIFFERENT
+            /// shards concurrently (CasGcScheduler ownership); their run-key namespaces never collide.
+            std::vector<RunRef> shard_runs;
+            foldDeltasIntoGeneration(
+                backend, layout, priorRunsFor(shard), new_generation, attempt, shard,
+                std::move(buckets[shard]), shard_runs,
+                current_round, condemn_round, head_blob, peek_head,
+                confirm_condemned_marker,
+                &result.retired_merge[shard], suppress_destructive,
+                &ledger.applied, std::move(retirement_buckets[shard]),
+                &work_budget);
+            for (RunRef & r : shard_runs)
+                result.fold_seal.blob_target_runs.push_back(std::move(r));
+            result.fold_seal.condemned_summary[shard] = summarize(result.retired_merge[shard].still_retired);
+        }
+    }
+
+    /// Aggregate the unmatched-remove signal across every gc-shard this pass touched and log ONCE per
+    /// round with the total plus one example, rather than from the hot per-edge inner loop that detects
+    /// them (see `foldDeltasIntoGeneration`'s comment) — that loop runs over potentially millions of
+    /// rows, so it only counts (`ProfileEvents::CASGCUnmatchedRemoveDeltas`, incremented per occurrence)
+    /// and hands back one example; this is the bounded, once-per-round operator-visible trail.
+    {
+        uint64_t total_unmatched_removes = 0;
+        std::optional<UnmatchedRemoveExample> example;
+        for (const RetiredMergeResult & merge : result.retired_merge)
+        {
+            total_unmatched_removes += merge.unmatched_removes;
+            if (!example && merge.unmatched_remove_example)
+                example = merge.unmatched_remove_example;
+        }
+        if (total_unmatched_removes > 0 && example)
+            LOG_WARNING(logger,
+                "CAS GC fold: {} unmatched removal delta(s) this pass (matched no existing source edge; "
+                "a harmless per-key no-op by design, since the in-degree model is a set, not a counter — "
+                "but a persistent nonzero rate means removal deltas are reaching the reducer without "
+                "their matching activation, which is a correctness signal) — example: blob {} source {}",
+                total_unmatched_removes, blobIdOf(example->ref), u128ToHex(example->source_id));
+        reduce_timer->metric("unmatched_removes", total_unmatched_removes);
+    }
+
+    /// PROBE B2's ledger verdict, computed inside the reduce phase so the row carries it; the
+    /// fail-closed throw it drives stays below, at its original site before the seal write.
+    const std::vector<uint32_t> unapplied_txns = ledger.unapplied();
+    transactions_unapplied_this_round = unapplied_txns.size();
+    {
+        uint64_t graduated = 0;
+        uint64_t spared = 0;
+        uint64_t redelete_pending = 0;
+        for (const RetiredMergeResult & merge : result.retired_merge)
+        {
+            graduated += merge.graduated.size();
+            spared += merge.spared.size();
+            redelete_pending += merge.redelete.size();
+        }
+        reduce_timer->metric("shards_total", state.gc_shards);
+        reduce_timer->metric("shards_pure_carry", shards_pure_carry);
+        reduce_timer->metric("shards_reduced", state.gc_shards - shards_pure_carry);
+        reduce_timer->metric("deltas_in", deltas_in);
+        reduce_timer->metric("runs_written", result.fold_seal.blob_target_runs.size());
+        reduce_timer->metric("condemned", report.condemned - condemned_before);
+        reduce_timer->metric("graduated", graduated);
+        reduce_timer->metric("spared", spared);
+        reduce_timer->metric("redelete_pending", redelete_pending);
+        reduce_timer->metric("suppress_destructive", suppress_destructive ? 1 : 0);
+        /// Published separately from `suppress_destructive` so a reader can tell the frontier term apart
+        /// from the anomaly and hold terms without re-deriving the formula from the tally.
+        reduce_timer->metric("frontier_complete", result.frontier_complete ? 1 : 0);
+        reduce_timer->metric("transactions_unapplied", transactions_unapplied_this_round);
+    }
+    reduce_timer.reset();   /// emits the `fold_reduce` row
+
+    /// The part-manifest cleanup RUN + its fold-seal record are removed: the run
+    /// object had no reader — the manifest cleanups execute inline from `result.mf_cleanup` (below /
+    /// the recheck path), so the durable bundle was pure dead weight. `result.mf_cleanup` is unchanged.
+
+    /// Write-once CasFoldSeal: its existence marks fold complete. The fold seal is DETERMINISTIC (same
+    /// fold inputs => byte-identical seal), so it goes through `putDeterministicArtifact`: a byte-equal
+    /// occupant is our own crash/deterministic replay (adopt, no-op); divergent bytes are impossible
+    /// under correct operation and fail closed with `CORRUPTED_DATA`. A deposed leader writes under its
+    /// own unadopted attempt so it never collides with the adopted seal — the occupant here is only ever
+    /// our own prior attempt-scoped write.
+    /// PROBE B2's verdict. A committed transaction that produced deltas but whose deltas never
+    /// reached a reducer means this round LOST a durable record it had already read and decoded.
+    /// Unlike a 404 during a fold (missing evidence, which must never wedge the round), this is proof
+    /// of loss, so the round fails CLOSED: nothing is adopted, GC reclaims and deletes nothing, and an
+    /// operator has to intervene. Thrown before the seal write, and therefore long before the single
+    /// `gc/state` CAS, so the whole round evaporates.
+    /// `unapplied_txns` was computed in the reduce phase above (so its row could report it); the verdict
+    /// itself is unchanged and still fires here, before the seal write.
+    if (!unapplied_txns.empty())
+    {
+        String detail;
+        for (size_t i = 0; i < unapplied_txns.size() && i < 8; ++i)
+        {
+            if (i != 0)
+                detail += ", ";
+            detail += ledger.namespaces[unapplied_txns[i]] + "@"
+                + renderRefTxnId(ledger.txns[unapplied_txns[i]]);
+        }
+        ProfileEvents::increment(ProfileEvents::CASGCUnappliedFoldedTransactions, unapplied_txns.size());
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS GC fold: {} ref transaction(s) folded and merged into the round buffers but NONE of "
+            "their blob deltas reached a shard reducer ({}{}). The round would have advanced its "
+            "cursor past a transaction it never applied. GC refuses to commit the round; recover with "
+            "SYSTEM CAS GC REBUILD.",
+            unapplied_txns.size(), detail, unapplied_txns.size() > 8 ? ", ..." : "");
+    }
+
+    /// PROBE B1's comparison. Both terms were derived at the end of the ref intake (see the
+    /// recomputation there, which is also what the `fold_ref_intake` row reports) and are 0 on a
+    /// ref-folding abort, where the identity does not apply -- so the inequality below can only fire on
+    /// a round that actually folded.
+    if (logs_accounted_this_round != logs_applied_this_round)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS GC fold: the round sealed coverage over {} ref log(s) but only {} fully folded -- "
+            "a cursor advanced past a log this round never applied. GC refuses to commit the round; "
+            "recover with SYSTEM CAS GC REBUILD.",
+            logs_accounted_this_round, logs_applied_this_round);
+
+    /// PHASE 10/18 `fold_seal_write`: one PUT (or, on a deterministic replay, a byte-compare GET).
+    {
+        GcPhaseTimer t(phase_sink, "fold_seal_write");
+        validateFoldSealForWrite(result.fold_seal, layout, store->poolConfig().gc_shards);
+        const String seal_body = encodeFoldSeal(result.fold_seal);
+        t.metric("seal_bytes", seal_body.size());
+        t.metric("seal_runs", result.fold_seal.blob_target_runs.size());
+        t.metric("seal_ref_lives", result.fold_seal.ref_lives.size());
+        t.metric("seal_cleanup_evidence", std::count_if(
+            result.fold_seal.ref_lives.begin(), result.fold_seal.ref_lives.end(),
+            [](const auto & item) { return item.second.cleanup_evidence.has_value(); }));
+        putDeterministicArtifact(backend, layout.foldSealKey(new_generation, attempt), seal_body);
+    }
+
+    /// One-pass round: the fold NO LONGER CASes gc/state. (new_generation, attempt) are adopted
+    /// in-memory here and committed — together with the round, the retired refs, and the retention
+    /// cursor — by the SINGLE round CAS in runRegularRound. A deposed leader's whole pass therefore
+    /// evaporates at that one CAS; its attempt-scoped artifacts are never adopted.
+    state.snap_generation = new_generation;
+    state.snap_attempt = attempt;
+    return result;
+}
+
+void Gc::reportSweepRetention(const ManifestSweepResult & result)
+{
+    const auto top = result.topRetainReason();
+    if (top.second == 0)
+    {
+        /// Nothing retained by the premise. Re-arm, so that the next retention -- however far off --
+        /// is reported as the change it is rather than swallowed by a repeat counter.
+        last_retain_rollup.reset();
+        retain_rollup_passes_since_report = 0;
+        return;
+    }
+
+    const bool changed = !last_retain_rollup || *last_retain_rollup != top;
+    if (!changed && ++retain_rollup_passes_since_report < kRetainRollupRepeatPasses)
+        return;
+
+    /// INFO, not WARNING: retention is the CORRECT outcome whenever rule (1) is unsatisfiable (it is
+    /// satisfiable only for a closed-and-folded epoch), so warning here would alarm on healthy rounds. It is
+    /// still the operator's answer to "why is manifest debris not shrinking?", which is why it is not
+    /// left at DEBUG with the per-object sentences.
+    /// The "X of Y" denominator is the RETAINED total, not `skipped`. `skipped` is a strictly larger
+    /// population -- it also counts malformed keys, ineligible prefixes, protected owners and
+    /// budget-deferred candidates -- so measuring the top class against it would understate the class's
+    /// share of the very number this sentence just reported.
+    const uint64_t retained = result.retained_no_coverage + result.retained_hold
+        + result.retained_unconsumed_seal + result.retained_tail_removal;
+    LOG_INFO(logger,
+        "CAS gc orphan sweep: retained {} manifest body(ies) this pass, most of them ({} of {}) for "
+        "'{}' -- see the fold seal's coverage for that namespace; deleted {}, listed {}, skipped {}",
+        retained, top.second, retained, sweepRetainClassName(top.first),
+        result.deleted, result.listed, result.skipped);
+
+    last_retain_rollup = top;
+    retain_rollup_passes_since_report = 0;
+}
+
+void Gc::cleanupRefObjects(
+    const FoldResult & folded, const GcLease & adopted_lease, bool suppress_destructive,
+    GcRoundWorkBudget & work_budget)
+{
+    /// A clamp / ref-folding abort this round may leave landed-before-cut edges unfolded behind the clamp,
+    /// so a covered-log cleanup could delete a log whose delta is not yet durable -- defer to a clean pass.
+    if (suppress_destructive)
+        return;
+
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+    if (!folded.catalog_cut)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "CAS GC ref cleanup: fold result carries no catalog cut");
+
+    for (const auto & [ns_str, listing] : folded.ref_tables)
+    {
+        const RootNamespace ns{ns_str};
+        /// Review C3: look up the SAME complete cut the round's walk resolved, never re-resolve
+        /// independently. A fresh read here could see a namespace dropped and recreated since the
+        /// walk and delete the successor's objects using predecessor bounds. An absent or `Creating`
+        /// row is skipped rather than mapped to a fabricated key.
+        const auto entry_it = std::lower_bound(
+            folded.catalog_cut->catalog.entries.begin(), folded.catalog_cut->catalog.entries.end(), ns,
+            [](const CatalogEntry & entry, const RootNamespace & needle) { return entry.ns < needle; });
+        if (entry_it == folded.catalog_cut->catalog.entries.end() || entry_it->ns != ns
+            || (entry_it->state != NsState::Live && entry_it->state != NsState::Removing))
+            continue;
+        const CatalogEntry & observed_entry = *entry_it;
+        const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry_it->ns, entry_it->incarnation);
+
+        /// Current-life ref cleanup is not the dead-life janitor: every irreversible key delete must
+        /// still be licensed by the SAME complete catalog observation and GC lease that adopted the
+        /// fold. Re-read both after the target HEAD and immediately before `deleteExact`. A moved token,
+        /// changed row/life, missing or unreadable authority object, or changed owner/sequence stops the
+        /// whole cleanup pass. Continuing with another row/key would turn a refusal into a fallback.
+        const auto deleteRefObject = [&](const String & key)
+        {
+            const HeadResult h = backend.head(key);
+            if (!h.exists)
+                return true;
+
+            try
+            {
+                const CasRefCatalog::Snapshot current_catalog = CasRefCatalog::read(backend, layout);
+                current_catalog.life_index.throwIfAmbiguous("CAS GC ref cleanup revalidation");
+                const auto current_entry_it = std::lower_bound(
+                    current_catalog.catalog.entries.begin(), current_catalog.catalog.entries.end(), ns,
+                    [](const CatalogEntry & entry, const RootNamespace & needle) { return entry.ns < needle; });
+                const std::optional<NamespaceLifeId> current_life
+                    = current_catalog.life_index.resolve(life.incarnation);
+                if (current_catalog.token != folded.catalog_cut->token
+                    || current_entry_it == current_catalog.catalog.entries.end()
+                    || current_entry_it->ns != ns || *current_entry_it != observed_entry
+                    || !current_life || *current_life != life)
+                {
+                    LOG_DEBUG(logger,
+                        "CAS GC ref cleanup stopped before deleting '{}': catalog observation/life moved",
+                        key);
+                    return false;
+                }
+
+                const auto current_state_object = backend.get(layout.gcStateKey());
+                if (!current_state_object)
+                {
+                    LOG_WARNING(logger,
+                        "CAS GC ref cleanup stopped before deleting '{}': mandatory gc/state is absent",
+                        key);
+                    return false;
+                }
+                const GcState current_state = decodeGcState(current_state_object->bytes);
+                if (current_state.lease.owner != adopted_lease.owner
+                    || current_state.lease.seq != adopted_lease.seq)
+                {
+                    LOG_DEBUG(logger,
+                        "CAS GC ref cleanup stopped before deleting '{}': GC fence moved",
+                        key);
+                    return false;
+                }
+            }
+            catch (const std::exception & e)
+            {
+                LOG_WARNING(logger,
+                    "CAS GC ref cleanup stopped before deleting '{}': authority revalidation failed: {}",
+                    key, e.what());
+                return false;
+            }
+
+            backend.deleteExact(key, h.token);
+            ProfileEvents::increment(ProfileEvents::CASRefCleanupObjectsDeleted);   /// cleanup object deletion
+            return true;
+        };
+
+        const auto row_it = folded.fold_seal.ref_lives.find(entry_it->incarnation);
+        const RefTxnId durable_cursor = row_it != folded.fold_seal.ref_lives.end()
+            ? row_it->second.coverage.last_folded_ref_id
+            : RefTxnId{};
+
+        /// Only a checkpoint-named RECOVERY TRIPLE licenses deletion. A listed snapshot may have landed
+        /// before its publisher's checkpoint CAS, so it must never be promoted into cleanup authority.
+        /// Validate the exact same-id non-seal `_log` and `_snap` through recovery's one shared helper;
+        /// failure is confined to this namespace and leaks its listed objects for a later round.
+        std::optional<RefCkpt> checkpoint;
+        if (const auto ckit = folded.checkpoints.find(ns_str); ckit != folded.checkpoints.end())
+            checkpoint = ckit->second;
+        if (!checkpoint || !checkpoint->checkpoint_snapshot_id)
+            continue;
+        const RefTxnId checkpoint_snapshot_id = *checkpoint->checkpoint_snapshot_id;
+
+        std::optional<RefTxnId> retained_log_proof;
+        try
+        {
+            retained_log_proof = readCheckpointSnapshotBase(backend, layout, life, *checkpoint).predecessor_seal_id;
+        }
+        catch (const Exception & e)
+        {
+            LOG_WARNING(logger,
+                "CAS GC ref cleanup retained namespace '{}': checkpoint base {} is not a valid recovery triple: {}",
+                ns_str, renderRefTxnId(checkpoint_snapshot_id), e.message());
+            continue;
+        }
+
+        const RefCleanupPlan plan = planRefCleanup(
+            listing, durable_cursor, checkpoint_snapshot_id, retained_log_proof);
+        for (const RefTxnId & log_id : plan.deletable_logs)
+        {
+            /// Cumulative per-round cap, never amortized against the per-key fail-close
+            /// validation `deleteRefObject` performs (HEAD + catalog re-read + gc/state re-read before
+            /// every exact delete stays exactly as expensive per key as before). Exhaustion simply stops
+            /// the round's cleanup pass here; `planRefCleanup` recomputes the SAME remaining candidates
+            /// from durable state next round, so nothing here needs its own cursor.
+            if (!work_budget.refCleanupAvailable())
+                return;
+            if (!deleteRefObject(layout.refLogKey(life, log_id)))
+                return;
+            ++work_budget.ref_cleanup_objects_used;
+        }
+        for (const RefTxnId & snap_id : plan.deletable_snapshots)
+        {
+            /// Task 5's rule, asserted where it is acted on rather than only where it is computed: the
+            /// snapshot the checkpoint names is the one a recovering reader will sample, so it must
+            /// survive every cleanup that the same checkpoint authorized.
+            chassert(snap_id < checkpoint_snapshot_id);
+            if (!work_budget.refCleanupAvailable())
+                return;
+            if (!deleteRefObject(layout.refSnapshotKey(life, snap_id)))
+                return;
+            ++work_budget.ref_cleanup_objects_used;
+        }
+    }
+}
+
+namespace
+{
+/// GC-metadata wholesale delete of every object under `prefix`. Returns the number of objects deleted.
+/// `bounded_remaining` caps how many objects this call may delete (0 => stop immediately, deleting none).
+///
+/// Token source: the in-memory and S3 backends surface a per-key token through `list`
+/// (`supportsListTokens()`), so `deleteExact` straight from the listed token; otherwise HEAD first.
+///
+/// 404 / NotFound is FAIL-OPEN: an object that vanished between LIST and delete (a concurrent crashed
+/// attempt, or a racing prune) is already reclaimed — never throw on a benign missing GC-internal object
+/// during a prune (it would only wedge GC). A genuine TokenMismatch is
+/// likewise tolerated here: the object was rewritten under us (another attempt is live at this key) — the
+/// safe direction during a best-effort prune is to leave it for a later round, never to force-delete.
+/// `out_fully_drained`, when set, reports whether the WHOLE prefix was exhausted (every listed key
+/// visited) rather than the call stopping early because `bounded_remaining` ran out. A
+/// caller advancing a monotone cursor past this prefix must consult this: a `false` here means objects
+/// remain, and the cursor must stay put so a later round's fresh budget can finish the same prefix
+/// instead of stranding the remainder permanently. `bounded_remaining == 0` conservatively reports
+/// `false` (nothing was even examined, so completeness cannot be claimed).
+uint64_t deletePrefixWholesale(Backend & backend, const String & prefix, uint64_t bounded_remaining,
+                               bool * out_fully_drained)
+{
+    if (out_fully_drained)
+        *out_fully_drained = false;
+    static constexpr size_t kListPageLimit = 1000;
+    uint64_t deleted = 0;
+    String cursor;
+    while (deleted < bounded_remaining)
+    {
+        ListPage page = backend.list(prefix, cursor, kListPageLimit);
+        /// One page fetched, not one increment per listed key below.
+        ProfileEvents::increment(ProfileEvents::CASGCEnumerationPages);
+        for (const auto & listed : page.keys)
+        {
+            if (deleted >= bounded_remaining)
+                return deleted;
+            if (listed.token.has_value())
+            {
+                /// deleteExact tolerates NotFound (returns Kind::NotFound) and TokenMismatch — both are
+                /// benign here (already gone / rewritten by a live attempt); do not throw.
+                backend.deleteExact(listed.key, *listed.token);
+            }
+            else if (const auto head = backend.head(listed.key); head.exists)
+            {
+                backend.deleteExact(listed.key, head.token);
+            }
+            ++deleted;
+        }
+        if (page.next_cursor.empty())
+        {
+            if (out_fully_drained)
+                *out_fully_drained = true;
+            break;
+        }
+        cursor = page.next_cursor;
+    }
+    return deleted;
+}
+}
+
+void Gc::pruneSupersededGenerations(uint64_t adopted_generation, uint64_t attempt, GcState & next,
+                                    const std::set<uint64_t> & referenced_generations,
+                                    bool suppress_destructive, GcRoundWorkBudget & work_budget)
+{
+    /// GATED, and `snap_pruned_through` stays where it is. The cursor is a monotone high-water mark the
+    /// wholesale prune never revisits, so advancing it over a generation this round declined to delete
+    /// would strand that generation's whole prefix with no reclaimer left (the hand-off only covers
+    /// generations a LIVE ref moved off, not ones skipped for suppression).
+    if (suppress_destructive)
+        return;
+
+    const uint64_t keep = store->poolConfig().gc_snapshot_generations_to_keep;
+    if (keep == 0)
+        return;   /// keep ALL (debug/forensics — replay GC's in-degree view as-of a past round)
+
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+
+    static constexpr uint64_t kMaxPrunePerRound = 64;   /// bound the per-round prune burst
+
+    /// (1) WHOLESALE generation-retention (correctness). A single generation may hold artifacts under
+    /// MULTIPLE attempts: every round mints a fresh `lease.seq` (= attempt), and a deposed leader writes
+    /// its fold_seal/runs/cleanup AND its attempt-scoped retired/outcomes sets under its OWN unadopted
+    /// attempt before its CAS fails. The old per-key single-attempt prune (keyed on the final
+    /// snap_attempt) therefore leaked every non-adopted attempt's debris. Instead, LIST the whole
+    /// `gc/gen/<g>/` prefix and delete every listed object — reclaiming ALL attempts of `g`, including
+    /// the retired/ and outcomes/ sets that now live under `gc/gen/<g>/attempt/<a>/`. Bounded per round
+    /// by generation count (`kMaxPrunePerRound`) AND by the round's shared object-count work budget;
+    /// fail-open on 404. `snap_pruned_through` advances over every generation the loop
+    /// FULLY processes this round — ref-retained (skipped) generations count as fully processed (there
+    /// is nothing left for THIS loop to do to them), but a generation whose delete the work budget cut
+    /// short does NOT: the loop stops there, so the cursor never strands a partially-drained prefix
+    /// behind it. It is a monotone high-water cursor, NOT a proof that everything below it is gone.
+    if (adopted_generation > keep)
+    {
+        const uint64_t prune_floor = adopted_generation - keep;   /// prune generations <= prune_floor
+        uint64_t g = next.snap_pruned_through + 1;
+        uint64_t pruned = 0;
+        for (; g <= prune_floor && pruned < kMaxPrunePerRound; ++g, ++pruned)
+        {
+            /// A generation whose run the LIVE adopted seal still
+            /// references (reference-parent carry: an idle shard's current run physically lives at an
+            /// older generation's key) must NOT be reclaimed — deleting it would strand the live seal's
+            /// ref. Skip its prefix delete; the run stays alive as long as it is referenced. NOTE the
+            /// cursor still advances past this skipped generation (see the `g - 1` cursor note above), so
+            /// the wholesale prune NEVER revisits it once it is behind the cursor. LEAK-FREEDOM therefore
+            /// rests on the post-CAS hand-off in `runRegularRound`: the round that finally moves the
+            /// ref OFF this generation (a later delta writes a fresh run) wholesale-deletes this whole
+            /// prefix right after its CAS. So every formerly-referenced generation is eventually FULLY
+            /// reclaimed — either here (if the ref moved off before the cursor reached it, WholesalePrune*
+            /// test) or by the hand-off (if the cursor passed it while still referenced, HandOffDeletes*
+            /// test). Until the ref moves it persists safely (bounded: one small run per shard).
+            if (referenced_generations.contains(g))
+            {
+                LOG_TRACE(logger,
+                    "CAS GC prune: retaining generation {} — still referenced by the live adopted seal",
+                    g);
+                continue;
+            }
+            /// `bounded_remaining` is the round's remainder shared across every PRUNE
+            /// `deletePrefixWholesale` call this round (never `UINT64_MAX`; the post-CAS hand-off draws
+            /// from its own separate reserve, never this one). A generation whose prefix
+            /// this call cannot FULLY drain within the remaining budget must not let the cursor advance
+            /// past it -- `snap_pruned_through` is a monotone high-water mark this loop never revisits,
+            /// so stranding a partially-drained generation behind it would leak the remainder forever.
+            /// Stop the loop here; `g - 1` (the previous, fully-processed generation) is what gets
+            /// persisted below.
+            const uint64_t remaining = work_budget.prefixWholesaleRemaining();
+            if (remaining == 0)
+                break;
+            bool fully_drained = false;
+            const uint64_t reclaimed = deletePrefixWholesale(
+                backend, layout.gcGenPrefix(g), remaining, &fully_drained);
+            work_budget.prefix_wholesale_objects_used += reclaimed;
+            if (!fully_drained)
+                break;
+        }
+        next.snap_pruned_through = g - 1;   /// highest generation FULLY processed this round
+    }
+
+    /// (2) NO per-round current-generation attempt-sweep (KISS). A previous revision LISTed the FOLD
+    /// generation's `gc/gen/<G_f>/` prefix EVERY completed round to delete non-adopted attempts with
+    /// `a < snap_attempt` — debris a deposed leader of the just-completed round left under its own
+    /// (unadopted) `lease.seq`. That per-round LIST was steady-state S3 budget spent for the RARE case
+    /// of a concurrent-leader collision (the GC-DISCOVERY-LIST-QUADRATIC concern), so it is removed.
+    ///
+    /// The wholesale generation-retention prune in (1) is now the SOLE reclaimer of ALL attempt debris,
+    /// including a deposed leader's: every artifact of generation `g` — across every attempt — lives
+    /// under `gc/gen/<g>/`, and the prefix-delete in (1) reclaims the whole subtree once `g` ages past
+    /// `keep`. Deposed-leader current-generation debris is therefore BOUNDED space (one collision leaves
+    /// at most a handful of small objects per generation) that waits at most `keep` completion-advances
+    /// to be reclaimed. This trades ~`keep` rounds of reclaim latency on (rare) concurrent-leader
+    /// collisions for eliminating a per-round LIST on the common (single-leader) path. When `keep == 0`
+    /// (keep-all / forensics mode) nothing is reclaimed by design — same as before.
+    (void)attempt;
+}
+
+std::optional<CasFoldSeal> Gc::readFoldSeal(uint64_t generation, uint64_t attempt)
+{
+    if (const auto got = store->backend().get(store->layout().foldSealKey(generation, attempt)))
+        return decodeFoldSeal(
+            got->bytes, store->layout(), store->poolConfig().gc_shards, generation);
+    return std::nullopt;
+}
+
+namespace
+{
+
+/// `Layout::parseRefObjectKey` for a key coming from a global `cas/ns/stream/` enumeration, with the one
+/// refusal it can raise absorbed into the ordinary "unrecognized" answer.
+///
+/// A ref object naming no life (the un-incarnated shape) is the single malformed key the parser
+/// REFUSES by name instead of classifying as debris, and both global enumerations run OUTSIDE the
+/// fold's catch. Letting the refusal escape one of them would not merely lose a round: GC is the only
+/// thing that could ever delete the key, so every future round would die on it too, with nothing able
+/// to clear it. Absorbed here, the key stays in the enumeration's raw key list, unindexed, exactly
+/// like every other malformed shape, and `groupRefKeys` raises it once inside the fold's catch --
+/// louder than before (an anomaly plus `suppress_destructive`), and without the wedge.
+///
+/// Only `CORRUPTED_DATA` is absorbed: any other exception is a real failure of the enumeration itself.
+std::optional<ParsedRefObjectKey> parseRefObjectKeyForEnumeration(const Layout & layout, const String & key)
+{
+    try
+    {
+        return layout.parseRefObjectKey(key);
+    }
+    catch (const Exception & e)
+    {
+        if (e.code() != ErrorCodes::CORRUPTED_DATA)
+            throw;
+        return std::nullopt;
+    }
+}
+
+}
+
+std::vector<NamespaceLifeId> Gc::discoverUniverse()
+{
+    /// One catalog GET replaces the pool-wide `LIST(cas/ns/stream/)` this used to run to
+    /// discover WHICH namespaces exist. `Creating` entries are excluded -- spec §3, "no publication can
+    /// exist" while a namespace is still being created, so there is nothing here for a discovery path to
+    /// walk; `Live` and `Removing` entries are both returned. `fromCatalogEntry` mints each life directly
+    /// from the row that is its own authority for both fields, never from a listed key (which could name
+    /// a DEAD incarnation of the same namespace name).
+    ///
+    /// The filter itself lives in `CasRefCatalog::liveUniverse` (review Important C) -- fsck's own
+    /// reachability walk needed the identical catalog-authoritative set and is not this class, so the
+    /// filter moved to where both can share it rather than grow a second copy that could disagree.
+    return CasRefCatalog::liveUniverse(store->backend(), store->layout());
+}
+
+bool Gc::graduationDue(const GcState & state, uint64_t current_round)
+{
+    /// Retired-in-snapshot: the graduation signal is read from the adopted fold seal's per-shard
+    /// `condemned_summary` — ZERO backend I/O beyond the single seal read. A summary distilled from this
+    /// generation's `kCondemned` rows says, per shard, how many entries are `delete_pending` (a graduation
+    /// is already published) and the oldest non-pending condemn round (one crosses the floor once
+    /// `condemn_round < current_round`).
+    if (state.snap_generation == 0)
+        return false;   /// fresh pool: nothing condemned yet, nothing to graduate.
+
+    /// FAIL-CLOSED: a missing / undecodable seal, or a summary that is not TOTAL over gc_shards, is
+    /// corrupt GC bookkeeping — force a FOLD so the round's own fail-closed path surfaces it, never a
+    /// silent defer (matching the fold's throw-on-missing-adopted-seal treatment).
+    std::optional<CasFoldSeal> seal;
+    try
+    {
+        seal = readFoldSeal(state.snap_generation, state.snap_attempt);
+    }
+    catch (...)
+    {
+        return true;   /// undecodable seal => fail-closed force-fold
+    }
+    if (!seal)
+        return true;
+    for (uint64_t shard = 0; shard < state.gc_shards; ++shard)
+    {
+        const auto it = seal->condemned_summary.find(shard);
+        if (it == seal->condemned_summary.end())
+            return true;   /// summary not total over gc_shards => fail-closed force-fold
+        if (it->second.pending_total > 0 || it->second.oldest_nonpending_condemn_round < current_round)
+            return true;
+    }
+    return false;
+}
+
+RefScanSummary Gc::enumerateRefPrefix()
+{
+    /// One full enumeration of `cas/ns/stream/`: the raw keys, plus a lenient per-life index of the
+    /// Log-kind ids among them. Lenient is deliberate — a malformed key is kept in `keys` and left
+    /// unindexed, so the STRICT validation (and the round-abort it can raise) happens exactly once, in
+    /// the fold's `groupRefKeys`. That holds for EVERY malformed shape: the one the parser refuses by
+    /// name is absorbed per key by `parseRefObjectKeyForEnumeration`, which is what keeps this
+    /// enumeration -- which runs before the fold, outside its catch -- unable to wedge the round.
+    const Layout & layout = store->layout();
+    Backend & backend = store->backend();
+
+    RefScanSummary scan;
+    static constexpr size_t kListPageLimit = 1000;
+    size_t count_in_page = 0;
+    forEachListedKey(backend, layout.casRefsPrefix(), [&](const ListedKey & lk)
+    {
+        scan.keys.push_back(lk.key);
+        const auto parsed = parseRefObjectKeyForEnumeration(layout, lk.key);
+        if (parsed)
+        {
+            scan.listed_lives.insert(parsed->life_id);
+            if (parsed->kind == RefObjectKind::Log)
+            {
+                scan.logs_by_life[parsed->life_id].insert(parsed->txn_id);
+                RefTxnId & g = scan.max_log_by_life[parsed->life_id];
+                if (g < parsed->txn_id)
+                    g = parsed->txn_id;
+            }
+        }
+        if (++count_in_page == kListPageLimit)
+        {
+            count_in_page = 0;
+            ProfileEvents::increment(ProfileEvents::CASRefGlobalListPages);
+        }
+    }, kListPageLimit, onGcEnumerationPage);
+    /// The walk's `backend.list` lands at least once even for an empty/undersized final page --
+    /// count it (one increment per physical LIST call).
+    if (count_in_page > 0 || scan.keys.empty())
+        ProfileEvents::increment(ProfileEvents::CASRefGlobalListPages);
+    return scan;
+}
+
+RoundInput Gc::listRefPrefix(const GcState & state)
+{
+    /// The round's ONE hint enumeration, followed by the parent coverage and authoritative catalog cut
+    /// needed to build the DEFER/FOLD walk plan. The caller computes the DEFER signal from that frozen
+    /// plan. A listed id absent from the later cut is dead, inert debris: it contributes no work and
+    /// cannot force DEFER.
+    RefScanSummary scan = enumerateRefPrefix();
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(store->backend(), store->layout());
+    /// TEST SEAM: see `setPostHotScanCatalogReadHookForTest`. Moved into a local before invoking (the
+    /// same reason `create_namespace_step1_pre_read_hook_for_test` is swapped rather than called
+    /// directly): a hook that reassigns the member from inside its own body would otherwise reassign
+    /// the very `std::function` executing it.
+    if (post_hot_scan_catalog_read_hook_for_test)
+    {
+        std::function<void()> hook_to_run;
+        std::swap(hook_to_run, post_hot_scan_catalog_read_hook_for_test);
+        hook_to_run();
+    }
+    catalog_cut.life_index.throwIfAmbiguous("CAS GC hot scan");
+    store->reconcileRefCatalogCut(catalog_cut);
+
+    const std::optional<CasFoldSeal> seal = readFoldSeal(state.snap_generation, state.snap_attempt);
+    if (!seal && state.snap_generation > 0)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS GC hot scan: adopted fold seal (generation {}, attempt {}) is missing",
+            state.snap_generation, state.snap_attempt);
+    if (seal)
+        scan.parent_ref_lives = seal->ref_lives;
+
+    for (const NamespaceLifePhysicalId life_id : scan.listed_lives)
+        if (!catalog_cut.life_index.resolve(life_id))
+            ++scan.dead_life_debris;
+
+    return RoundInput{std::move(scan), catalog_cut};
+}
+
+RebuildReport Gc::rebuildBaseline(bool force)
+{
+    /// The `gc/state` disaster-recovery command.
+    /// DRY: the engine is the round's own bricks — one catalog cut for the universe,
+    /// foldManifestEdges(+1) for edge emission, foldDeltasIntoGeneration with EMPTY priors
+    /// (attempt-iterated for O(budget) memory), computeHeartbeatFloor for the round mint.
+    /// Writes ONLY the GC plane; namespace streams/state, manifests, and blobs are read-only inputs;
+    /// the rebuild never deletes them.
+    RebuildReport rep;
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+
+    /// Read bookkeeping health before the lease (the lease acquire on an absent state CREATES a
+    /// bootstrap body, which must not make scenario (а) look healthy). A generation-0 ref-baseline
+    /// check is deliberately postponed to the sole post-LIST work cut below.
+    /// The prior seal, when `gc/state` claims one. It is the ONLY place holds live, so the rebuild
+    /// either reads it and carries every hold forward, or refuses (see the refusal below).
+    std::optional<CasFoldSeal> prior_seal;
+    bool healthy = false;
+    bool validate_generation_zero_ref_baseline = false;
+    {
+        const auto got = backend.get(layout.gcStateKey());
+        /// The state's own decode stays inside its own try: an undecodable `gc/state` IS scenario (а),
+        /// the disaster this command exists for. The prior-seal refusal below must NOT be swallowed by
+        /// that catch, so the seal is read outside it.
+        std::optional<GcState> decoded;
+        if (got)
+        {
+            try
+            {
+                decoded = decodeGcState(got->bytes);
+            }
+            catch (...) // NOLINT(bugprone-empty-catch)
+            {
+                /// undecodable state = scenario (а)
+            }
+        }
+        if (decoded)
+        {
+            const GcState & st = *decoded;
+            healthy = true;
+            if (st.snap_generation == 0)
+            {
+                /// This check needs the rebuild universe. Delay it until after lease acquisition, the
+                /// zero-mutation generation-0 drain, the completed hot LIST and the sole fresh work cut.
+                /// No pre-lease catalog snapshot may become authority for successor construction.
+                validate_generation_zero_ref_baseline = true;
+            }
+            if (st.snap_generation > 0)
+            {
+                /// THE REFUSAL (spec r9-1). The prior seal is where every hold lives, and a rebuild
+                /// rewrites coverage from owner state -- so with no readable seal it would hand back a
+                /// baseline that LOOKS proven while silently discarding holds it cannot even enumerate.
+                /// The alternative, a "pool-wide hold" on the rebuilt baseline, is not representable:
+                /// every hold names a position the fold must fold THROUGH, and a pool-wide one would
+                /// need an invented position nothing could ever resolve. So the honest branch is the
+                /// already-safe one -- refuse, and name pool recreation. FORCE does not buy past this:
+                /// force means "rebuild deliberately", never "drop the holds". It THROWS rather than
+                /// returning `rep.refusal` because, unlike every other refusal here, no flag and no
+                /// retry makes it succeed.
+                std::optional<CasFoldSeal> seal;
+                try
+                {
+                    seal = readFoldSeal(st.snap_generation, st.snap_attempt);
+                }
+                catch (const Exception & e)
+                {
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS GC rebuild: the prior fold seal (generation {}, attempt {}) is UNDECODABLE "
+                        "({}), so the holds it carries cannot be read. A rebuild that dropped them would "
+                        "bless a baseline whose frontier is unproven. GC refuses to rebuild; this pool "
+                        "must be recreated.",
+                        st.snap_generation, st.snap_attempt, e.message());
+                }
+                if (!seal)
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS GC rebuild: the prior fold seal (generation {}, attempt {}) is MISSING under "
+                        "a gc/state that claims it, so the holds it carries cannot be read. A rebuild "
+                        "that dropped them would bless a baseline whose frontier is unproven. GC refuses "
+                        "to rebuild; this pool must be recreated.",
+                        st.snap_generation, st.snap_attempt);
+                for (const RunRef & r : seal->blob_target_runs)
+                    if (!backend.head(r.key).exists)
+                        healthy = false;
+                prior_seal = std::move(seal);
+                rep.adopted_seal_generation = st.snap_generation;
+            }
+        }
+
+        /// NO ADOPTED BASELINE IS NAMED: `gc/state` is absent, undecodable, or sits at generation 0.
+        /// The holds live in the SEAL, not in the pointer to it, so stopping here would make losing the
+        /// pointer -- the LESSER corruption -- produce a hold-free baseline, while an unreadable seal
+        /// refuses. That asymmetry is inverted, and it matters because holds are not re-derivable:
+        /// `WitnessDisappeared` names a record that is gone, so the next walk reads a clean frontier
+        /// and hands the namespace exactly the proof the hold exists to deny.
+        ///
+        /// So find the newest fold seal OBJECT by enumeration and carry ITS holds. This keeps the
+        /// pool's disaster recovery intact -- losing `gc/state` on a lived-in pool is the scenario this
+        /// command exists for -- while making a hold-free baseline over a pool that had holds
+        /// unreachable.
+        ///
+        /// An unadopted deposed-leader attempt can be the newest object, and carrying its holds
+        /// over-holds RATHER THAN under-holds -- but that claim needs its qualifier, because it is not
+        /// universal. It holds outside the lying-store corner. The exception is a deposed attempt that
+        /// FOLDED THROUGH a position on a read that was transient: its seal records the hold as
+        /// cleared, the adopted leader's seal still holds it, and adopting the deposed one loses that
+        /// hold. Bounded by the refusal above (a seal above the listing's maximum refuses outright) and
+        /// by the fact that both attempts walked the same cursor state.
+        if (!decoded || decoded->snap_generation == 0)
+        {
+            const auto newest = newestFoldSealRef();
+            /// Absent here is the VIRGIN verdict, and it is the one path left on which a durable hold
+            /// can still be dropped without anything failing. It is REPORTED on the command's own row,
+            /// not merely logged: this command is run by hand during a disaster, and its operator is
+            /// exactly the person who needs to know the clean slate was inferred rather than proved.
+            rep.virgin_by_enumeration = !newest.has_value();
+            if (newest)
+            {
+                std::optional<CasFoldSeal> seal;
+                try
+                {
+                    seal = readFoldSeal(newest->first, newest->second);
+                }
+                catch (const Exception & e)
+                {
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS GC rebuild: gc/state names no adopted baseline and the newest fold seal "
+                        "(generation {}, attempt {}) is UNDECODABLE ({}), so the holds this pool carries "
+                        "cannot be read. A rebuild that dropped them would bless a baseline whose "
+                        "frontier is unproven. GC refuses to rebuild; this pool must be recreated.",
+                        newest->first, newest->second, e.message());
+                }
+                if (!seal)
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS GC rebuild: gc/state names no adopted baseline and the newest fold seal "
+                        "(generation {}, attempt {}) vanished between the enumeration and the read, so "
+                        "the holds this pool carries cannot be read. GC refuses to rebuild; this pool "
+                        "must be recreated.",
+                        newest->first, newest->second);
+                prior_seal = std::move(seal);
+                rep.adopted_seal_generation = newest->first;
+            }
+            /// else: no fold seal object anywhere. That is the ONE proof that dropping nothing is safe
+            /// -- a pool that never sealed a baseline has no hold to lose.
+        }
+    }
+    if (healthy && !force && !validate_generation_zero_ref_baseline)
+    {
+        rep.refusal = "gc/state and every referenced artifact are healthy — a rebuild would discard "
+                      "live bookkeeping; re-run with FORCE to rebuild deliberately";
+        return rep;
+    }
+
+    /// Lease: single leader vs regular rounds and other rebuilds. On an absent state this CREATES
+    /// a lease-bearing bootstrap body whose token anchors our final CAS. allow_steal=false: this is a
+    /// manual disaster-recovery command, same reasoning as the manual GC round (runRegularRound's doc
+    /// comment) — though it is structurally moot here too (a fresh one-shot `this` with
+    /// has_observation==false always takes the non-steal branch on its one and only call), pass it
+    /// explicitly rather than rely on that invariant.
+    GcState state;
+    Token state_token;
+    if (!acquireOrRenewLease(state, state_token, /*allow_steal=*/false))
+    {
+        rep.refusal = "another GC leader holds the lease";
+        return rep;
+    }
+
+    /// Healthy `FORCE REBUILD` shares the same parent-authorized barrier as an ordinary round. A
+    /// damaged-state rebuild has no adopted parent (`snap_generation == 0`) and the barrier performs
+    /// zero catalog mutations. Only after that distinction is resolved do we complete the hot LIST and
+    /// take the sole fresh work cut.
+    const CatalogLifecycleReconcileResult drain_result = drainCompletedRemoving(state);
+    for (const NamespaceLifeId & retired_life : drain_result.retired_lives)
+        store->invalidateRemovedCatalogLife(retired_life);
+    if (drain_result.authority_status != AuthorityStatus::Authoritative
+        || drain_result.catalog_resolution != CatalogResolution::DrainComplete)
+        throwCasWriteRetryLater("CAS GC rebuild lost authority before the catalog settled");
+    const RefScanSummary rebuild_ref_scan = enumerateRefPrefix();
+    const CasRefCatalog::Snapshot rebuild_work_catalog_cut = CasRefCatalog::read(backend, layout);
+
+    RefScanSummary rebuild_round_scan = rebuild_ref_scan;
+    if (prior_seal)
+        rebuild_round_scan.parent_ref_lives = prior_seal->ref_lives;
+    const RefPlan rebuild_walk_plan = buildRefWalkPlan(
+        RoundInput{std::move(rebuild_round_scan), rebuild_work_catalog_cut});
+    const std::vector<NamespaceLifeId> rebuild_walk_universe = rebuild_walk_plan.lives();
+    /// The exact checkpoint sample is paired with the same frozen catalog cut that chose the rebuild
+    /// universe. `recoverRefTableDetailedFromAuthority` deliberately has no internal catalog or
+    /// checkpoint read: a later cut could admit a different life or frontier than the one every other
+    /// part of this rebuild is using.
+    const CheckpointWitnesses rebuild_checkpoints = readCheckpointWitnesses({}, rebuild_walk_plan.catalogCut());
+
+    if (validate_generation_zero_ref_baseline)
+    {
+        /// A generation-0 state is healthy only when no table proves that a now-lost cursor cleaned
+        /// covered logs. The check consumes the same catalog-built universe as reconstruction; it does
+        /// not own an earlier catalog cut or a second admission rule.
+        for (const NamespaceLifeId & life : rebuild_walk_universe)
+        {
+            std::vector<String> table_keys;
+            forEachListedKey(backend, layout.namespaceStreamPrefix(life),
+                [&](const ListedKey & lk) { table_keys.push_back(lk.key); }, 1000, onGcEnumerationPage);
+            std::map<NamespaceLifePhysicalId, RefTableListing> grouped;
+            try
+            {
+                grouped = groupRefKeys(layout, table_keys);
+            }
+            catch (const Exception & e)
+            {
+                if (e.code() != ErrorCodes::CORRUPTED_DATA)
+                    throw;
+                healthy = false;
+                break;
+            }
+            const auto grouped_it = grouped.find(life.incarnation);
+            if (grouped_it != grouped.end() && !grouped_it->second.snapshots.empty()
+                && (grouped_it->second.logs.empty()
+                    || grouped_it->second.snapshots.back() < grouped_it->second.logs.front()))
+            {
+                healthy = false;
+                break;
+            }
+        }
+        if (healthy && !force)
+        {
+            rep.refusal = "gc/state and every referenced artifact are healthy — a rebuild would discard "
+                          "live bookkeeping; re-run with FORCE to rebuild deliberately";
+            return rep;
+        }
+    }
+
+    /// Numbering, part 1: generation above ANY surviving gc/gen prefix (putDeterministicArtifact
+    /// must never collide with debris of the lost era).
+    uint64_t max_gen = state.snap_generation;
+    {
+        const String gen_prefix = layout.gcGenPrefix(0);
+        const String top = gen_prefix.substr(0, gen_prefix.size() - 2);   /// ".../gc/gen/"
+        forEachListedKey(backend, top, [&](const ListedKey & k)
+        {
+            const size_t from = top.size();
+            const size_t slash = k.key.find('/', from);
+            if (slash == String::npos)
+                return;
+            try
+            {
+                max_gen = std::max(max_gen, static_cast<uint64_t>(std::stoull(k.key.substr(from, slash - from))));
+            }
+            catch (...) // NOLINT(bugprone-empty-catch)
+            {
+                /// Foreign key shape under `gc/gen` is debris, not a numbering input.
+            }
+        }, 1000, onGcEnumerationPage);
+    }
+    const uint64_t generation = max_gen + 1;
+    const uint64_t budget = rebuild_edge_budget_override ? rebuild_edge_budget_override
+                                                         : store->poolConfig().rebuild_edge_budget;
+
+    /// Per-gc-shard attempt-iterated fold state: batch k folds with attempt k and the previous
+    /// attempt's runs as priors; the FINAL attempt's runs go into the seal.
+    const uint64_t gc_shards = state.gc_shards ? state.gc_shards : store->poolConfig().gc_shards;
+    std::vector<std::vector<BlobDelta>> buckets(gc_shards);
+    std::vector<std::vector<RunRef>> prior_runs(gc_shards);
+    std::vector<uint64_t> attempt_of(gc_shards, 0);
+    /// The fold is EDGE-ONLY here: a rebuild condemns nothing (spec §7, and the deletion below), so no
+    /// condemn round is stamped and no head source is supplied. `current_round` 0 graduates nothing and
+    /// `condemn_round` 0 with an empty `head_blob` mints no `kCondemned` row -- this call is
+    /// `foldDeltasIntoGeneration`'s pure edge form.
+    auto flush_shard = [&](uint64_t shard)
+    {
+        if (buckets[shard].empty())
+            return;
+        std::vector<RunRef> out;
+        foldDeltasIntoGeneration(backend, layout, prior_runs[shard], generation, ++attempt_of[shard],
+                                 shard, std::move(buckets[shard]), out,
+                                 /*current_round*/0, /*condemn_round*/0, /*head_blob*/{},
+                                 /*peek_head*/{}, /*confirm_condemned_marker*/{},
+                                 /*out_retired*/nullptr, /*suppress_destructive*/false,
+                                 /// Probe B2 does not apply to the rebuild: it derives edges from raw
+                                 /// owner STATE, not from a stream of ref transactions, so there is no
+                                 /// transaction whose deltas could go unapplied and no fold cursor to
+                                 /// advance past one. Every delta it emits carries ordinal 0.
+                                 /*out_applied_by_txn_ordinal*/nullptr);
+        buckets[shard].clear();
+        prior_runs[shard] = std::move(out);
+    };
+    auto route_deltas = [&](std::vector<BlobDelta> & deltas)
+    {
+        rep.edges += deltas.size();
+        for (BlobDelta & d : deltas)
+        {
+            const uint64_t shard = blobShard(d.ref, gc_shards);
+            buckets[shard].push_back(std::move(d));
+            if (buckets[shard].size() >= budget)
+                flush_shard(shard);
+        }
+        deltas.clear();
+    };
+
+    /// `rebuild_walk_plan` was frozen immediately after the completed hot LIST and sole fresh catalog
+    /// cut. Listed ids absent from that later cut are inert dead-life debris and cannot mint work or
+    /// refuse reconstruction.
+    std::set<String> seen_ns;
+    std::set<String> owned_manifest_keys;
+    CasFoldSeal seal;
+    seal.generation = generation;
+    seal.parent_generation = state.snap_generation;
+    seal.ref_lives = rebuild_walk_plan.successorFoldStates();
+    /// Life ids whose hold this rebuild MINTED (see `minted_here`); they are stamped with the retry
+    /// round once it is known, and nothing else is touched.
+    std::set<UInt128> minted_hold_lives;
+    uint64_t max_fence_round = 0;
+    std::map<ManifestId, Token> mf_cleanup_unused;
+
+    for (const NamespaceLifeId & life : rebuild_walk_universe)
+    {
+        const RootNamespace & ns = life.ns;
+        seen_ns.insert(ns.string());
+        ++rep.shards;
+
+        /// Recover from the exact catalog row and exact checkpoint paired with this plan. A visible
+        /// log above `committed_through` is not logical history yet, and a Live/Removing row without a
+        /// readable checkpoint has no bounded recovery frontier; both cases must refuse rather than
+        /// letting a stream LIST decide what this baseline protects.
+        const auto entry_it = std::lower_bound(
+            rebuild_walk_plan.catalogCut().catalog.entries.begin(), rebuild_walk_plan.catalogCut().catalog.entries.end(), ns,
+            [](const CatalogEntry & entry, const RootNamespace & needle) { return entry.ns < needle; });
+        chassert(entry_it != rebuild_walk_plan.catalogCut().catalog.entries.end());
+        chassert(entry_it->ns == ns);
+        chassert(entry_it->incarnation == life.incarnation);
+        if (const auto bad_checkpoint = rebuild_checkpoints.undecodable.find(ns.string());
+            bad_checkpoint != rebuild_checkpoints.undecodable.end())
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS GC rebuild: catalog life {} has an undecodable checkpoint: {}", ns.string(), bad_checkpoint->second);
+        std::optional<RefCkpt> checkpoint;
+        if (const auto checkpoint_it = rebuild_checkpoints.recovery_checkpoints.find(ns.string());
+            checkpoint_it != rebuild_checkpoints.recovery_checkpoints.end())
+            checkpoint = checkpoint_it->second;
+        const RecoveredRefTable recovered = recoverRefTableDetailedFromAuthority(
+            backend, layout, *entry_it, checkpoint);
+        const RefTableState & st = recovered.state;
+
+        RefCoverage cov;
+        cov.classification = 2;   /// Folded (full coverage) unless a bodiless precommit clamps
+        cov.last_folded_ref_id = st.getGreatestApplied();
+        /// Whether the hold on this row was minted BY THIS REBUILD (and so still owes a retry round)
+        /// rather than carried from the prior seal. Tracked explicitly instead of by looking for a
+        /// `next_retry_round` of 0: 0 is a perfectly good wire value, and a carried hold that happened
+        /// to hold it would have its backoff silently rewritten by the stamping pass.
+        bool minted_here = false;
+
+        std::vector<BlobDelta> deltas;
+
+        /// Committed owners: a missing/invalid body under a committed ref is DATA LOSS the rebuild must
+        /// not bless (INV_NO_DANGLE) -- refuse.
+        for (const auto [ref_name, row] : st.getCommitted())
+        {
+            const ManifestId id{ns, row.manifest_ref};
+            owned_manifest_keys.insert(layout.manifestKey(id));
+            if (!foldManifestEdges(id, +1, deltas, mf_cleanup_unused, /*txn_ordinal=*/0))
+            {
+                rep.refusal = "committed ref '" + ns.string() + "/" + ref_name
+                    + "' names a missing or invalid part manifest — that is DATA LOSS the rebuild "
+                      "must not bless; run fsck forensics first";
+                return rep;
+            }
+            ++rep.committed_refs;
+        }
+
+        /// Live precommits: a present body contributes edges; a bodiless one is non-activating and clamps
+        /// (the fold barrier -- the first regular round folds it once the body lands).
+        for (const auto & [ref_name, manifest_ref] : st.getPrecommits())
+        {
+            const ManifestId id{ns, manifest_ref};
+            owned_manifest_keys.insert(layout.manifestKey(id));
+            if (foldManifestEdges(id, +1, deltas, mf_cleanup_unused, /*txn_ordinal=*/0))
+                ++rep.live_precommits;
+            else
+            {
+                /// A bodiless live precommit leaves the rebuilt baseline INCOMPLETE for this namespace:
+                /// nothing can enumerate the blobs it pins, so nothing can protect them. That is a
+                /// durable hold, not a report field -- and it is the one hold the rebuild has to invent
+                /// a position for, because it derives edges from owner STATE and cannot name the log
+                /// that introduced the precommit (that log is below the rebuilt cursor and no round
+                /// re-reads it). It holds at the FIRST POSITION THE NEXT ROUND WILL READ, which makes a
+                /// quiet namespace stay held indefinitely -- the fail-close answer while the body is
+                /// still missing -- and clears once the namespace makes durable progress.
+                /// RESIDUAL, named rather than hidden: progress unrelated to this precommit also clears
+                /// it, and the precommit's edges stay missing until another rebuild.
+                cov.classification = 4;   /// Clamped
+                cov.hold = RefHold{.reason = HoldReason::ManifestBodyMissing,
+                                   .offending_position = RefTxnId{cov.last_folded_ref_id.writer_epoch,
+                                                                 cov.last_folded_ref_id.ref_sequence + 1},
+                                   .retry_count = 0,
+                                   .next_retry_round = 0};   /// stamped below, once `round` is minted
+                minted_here = true;
+                ++rep.clamped_shards;
+            }
+        }
+        route_deltas(deltas);
+
+        /// HOLDS RIDE THROUGH A REBUILD VERBATIM (spec §5/§7). The rebuild derives coverage from owner
+        /// state, which knows nothing about a ref-log position that would not resolve -- so without
+        /// this every held row would be overwritten by a clean one and the rebuilt baseline would claim
+        /// a frontier proof it does not have. `retry_count` and `next_retry_round` ride unchanged too:
+        /// the rebuild retried nothing, so it must not reset the count that says how long the namespace
+        /// has been stuck. The ordinary clearing rule then applies from the next round on.
+        if (prior_seal)
+        {
+            const auto pit = prior_seal->ref_lives.find(life.incarnation);
+            if (pit != prior_seal->ref_lives.end() && pit->second.coverage.hold)
+            {
+                cov.classification = 4;
+                cov.hold = pit->second.coverage.hold;
+                minted_here = false;   /// a carried hold rides VERBATIM; its retry fields are not ours
+            }
+        }
+        if (minted_here)
+            minted_hold_lives.insert(life.incarnation);
+
+        RefLifeFoldState & row = seal.ref_lives.at(life.incarnation);
+        row.coverage = cov;
+    }
+    rep.namespaces = seen_ns.size();
+
+    /// Trimmed-but-live precommits: a build alive across trim has NO journal
+    /// evidence; its manifests look unowned. Include edges of every manifest that is unowned AND
+    /// not provably build-dead (the watermark fact) — over-protect. An unowned manifest that later
+    /// dies without journal evidence leaks its edges until a future rebuild (documented, bounded,
+    /// fsck-visible); provably-dead ones stay excluded (the orphan sweep owns their bodies).
+    for (const String & ns_str : seen_ns)
+    {
+        const RootNamespace ns{ns_str};
+        std::vector<BlobDelta> deltas;
+        forEachListedKey(backend, layout.manifestNamespacePrefix(ns), [&](const ListedKey & k)
+        {
+            if (owned_manifest_keys.contains(k.key))
+                return;
+            /// The one shared manifest-path parser for the canonical hexadecimal manifest identifier,
+            /// also used by fsck's parseBuildPrefix and the orphan sweep's parseListedManifestObject.
+            const auto parsed = layout.parseManifestKey(k.key);
+            if (!parsed)
+                return;   /// foreign key shape — debris
+            const ManifestRef & mref = parsed->ref;
+            if (prefixEligible(*store, ns, BuildPrefix{mref.writer_epoch, mref.build_sequence}))
+                return;   /// provably dead — the orphan sweep's territory, never an edge
+            const ManifestId id{ns, mref};
+            if (foldManifestEdges(id, +1, deltas, mf_cleanup_unused, /*txn_ordinal=*/0))
+            {
+                ++rep.unowned_alive_manifests;
+                route_deltas(deltas);
+            }
+            /// A missing/invalid UNOWNED body is debris (no owner claims it) — skip, never refuse.
+        }, 1000, onGcEnumerationPage);
+    }
+
+    /// A REBUILD CONDEMNS NOTHING (spec §7).
+    ///
+    /// It used to end here with a LIST of `blobs/`, condemning every listed body its traversal had not
+    /// reached ("pipeline blindness repair": the fold discovers candidates by TRANSITIONS to zero, so a
+    /// blob whose edges were already gone by rebuild time would have no row and never be reclaimed).
+    /// The premise was that a full traversal knows every live blob. It does not. BOTH legs of the
+    /// traversal above are listing-driven -- the owner replay reads the ref prefix, the trimmed-but-live
+    /// pass reads the manifest prefix -- so a store that omits a durable key from one enumeration hides
+    /// a LIVE owner, and this pass would then condemn the very blob that owner pins. That is
+    /// r5-finding-4: one lying enumeration, and acked data is scheduled for deletion. Hiding is not
+    /// hypothetical here; it is the observed `0x1430c` shape that made every ref walk arithmetic.
+    ///
+    /// THE NAMED RESIDUAL this leaves (Stage-A staging contract, register R4): a blob whose manifest no
+    /// longer exists anywhere is unreclaimable -- nothing can enumerate it safely -- until the
+    /// build/upload registry can say which uploads are in flight. It is retention, not loss, and it is
+    /// bounded by that registry landing. NO substitute reclamation is added in its place: any cheaper
+    /// rule that reclaims from an enumeration is the same vector wearing a different hat, and a
+    /// fallback that deletes on incomplete evidence is precisely what "fail closed" forbids.
+    ///
+    /// Numbering, part 2: the round above every surviving fence/state/generation number.
+    const uint64_t round = std::max({max_fence_round, state.round, max_gen}) + 1;
+
+    /// Stamp the retry round on the holds THIS rebuild minted (the bodiless-precommit ones, left unset
+    /// because the round was not minted yet). Carried holds are named by no key here and are not
+    /// touched: their `next_retry_round` and `retry_count` ride verbatim, since a rebuild retries
+    /// nothing.
+    for (const UInt128 life_id : minted_hold_lives)
+        if (const auto it = seal.ref_lives.find(life_id);
+            it != seal.ref_lives.end() && it->second.coverage.hold)
+            it->second.coverage.hold->next_retry_round = round + 1;
+
+    for (uint64_t shard = 0; shard < gc_shards; ++shard)
+        flush_shard(shard);   /// real-edge rows only: a rebuild condemns nothing, so it seeds nothing
+
+    for (uint64_t shard = 0; shard < gc_shards; ++shard)
+        for (const RunRef & r : prior_runs[shard])
+            seal.blob_target_runs.push_back(r);
+
+    /// Also fence out any dead mounts as part of the disaster-recovery pass (liveness cleanup; the
+    /// returned classification counts are not needed for the round mint — graduation paces on rounds).
+    /// Use the same threshold/`mount_obs` as the regular round.
+    const uint64_t ttl_ms = static_cast<uint64_t>(store->poolConfig().mount_lease_ttl_ms.count());
+    /// Share the identical formula with `claimMountAwaitingExpiry` via
+    /// `mountObservationThresholdMs` -- see its doc comment (CasServerRoot.h).
+    const uint64_t stable_threshold_ms = mountObservationThresholdMs(
+        ttl_ms, static_cast<uint64_t>(store->poolConfig().mount_renew_period.count()));
+    computeHeartbeatFloor(backend, layout, now_ms_fn(), mono_ms_fn(), stable_threshold_ms, mount_obs);
+
+    /// Retired-in-snapshot: the rebuilt seal's `condemned_summary` must be TOTAL over gc_shards so a
+    /// subsequent regular round reads graduation/carry decisions zero-I/O off it (and its `carryParentRefs`
+    /// totality check does not fail closed). Every entry is EMPTY -- a rebuild condemns nothing -- and
+    /// the totality is still owed: an ABSENT row and a zero row are different claims, and the round that
+    /// reads this seal fails closed on the absent one.
+    for (uint64_t shard = 0; shard < gc_shards; ++shard)
+        seal.condemned_summary[shard] = CondemnedSummary{};
+
+    /// Seal (deterministic artifact) + the single state CAS. attempt = the max per-shard attempt
+    /// (>= 1 so the seal key is stable даже for an empty universe).
+    uint64_t seal_attempt = 1;
+    for (uint64_t a : attempt_of)
+        seal_attempt = std::max(seal_attempt, a);
+    validateFoldSealForWrite(seal, layout, gc_shards);
+    putDeterministicArtifact(backend, layout.foldSealKey(generation, seal_attempt), encodeFoldSeal(seal));
+
+    GcState next = state;
+    next.round = round;
+    next.snap_generation = generation;
+    next.snap_attempt = seal_attempt;
+    /// No retired set to publish: a rebuild condemns nothing (the deletion above), so there is nothing
+    /// to retire in the first place. Retired-in-snapshot removed the separate `RetiredSet` object
+    /// family independently of that, and the two reasons are stated apart on purpose — a future reader
+    /// must not take this line as evidence that REBUILD still produces condemnations somewhere.
+    next.manifest_sweep_cursor = "";
+    const CasResult res = backend.casPut(layout.gcStateKey(), encodeGcState(next), state_token);
+    if (res.outcome != CasOutcome::Committed)
+    {
+        rep.refusal = "gc/state changed under the rebuild (a competing writer) — re-run";
+        return rep;
+    }
+
+    rep.performed = true;
+    rep.round = round;
+    rep.generation = generation;
+    EventEmitter{*store}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::GcRebuild;
+        e.object_kind = CasEventObjectKind::Snap;
+        e.round = round;
+        e.gen = generation;
+        e.outcome = "performed";
+        e.reason = "raw baseline rebuild from owner state (gc/state disaster recovery)";
+        e.detail = {{"namespaces", std::to_string(rep.namespaces)},
+                    {"shards", std::to_string(rep.shards)},
+                    {"committed_refs", std::to_string(rep.committed_refs)},
+                    {"live_precommits", std::to_string(rep.live_precommits)},
+                    {"unowned_alive_manifests", std::to_string(rep.unowned_alive_manifests)},
+                    {"edges", std::to_string(rep.edges)},
+                    {"clamped_shards", std::to_string(rep.clamped_shards)},
+                    {"force", force ? "1" : "0"}};
+    });
+    return rep;
+}
+
+std::vector<Gc::PreviewEntry> Gc::previewDeletes()
+{
+    std::vector<PreviewEntry> out;
+
+    const auto state_bytes = store->backend().get(store->layout().gcStateKey());
+    if (!state_bytes)
+        return out;
+    const GcState state = decodeGcState(state_bytes->bytes);
+
+    const Layout & layout = store->layout();
+    Backend & backend = store->backend();
+
+    /// Resolve the run objects THROUGH the adopted seal's refs, never by
+    /// `blobTargetRunKey` construction: with reference-parent carry a shard's current run may physically
+    /// live under an older generation's key, and the seal ref is the only authority for the real key.
+    /// Group the adopted `blob_target_runs` by the ref's explicit `shard`. Absent seal => no candidates.
+    std::map<uint64_t, std::vector<RunRef>> runs_by_shard;
+    if (const auto adopted = readFoldSeal(state.snap_generation, state.snap_attempt))
+        for (const RunRef & r : adopted->blob_target_runs)
+            runs_by_shard[r.shard].push_back(r);
+
+    /// Scan every blob-target shard (see `retire`): a preview that only looked at shard 0 would miss the
+    /// zero-in-degree candidates owned by shards 1..N under `gc_shards > 1`.
+    for (uint64_t shard = 0; shard < state.gc_shards; ++shard)
+    {
+        const auto it = runs_by_shard.find(shard);
+        static const std::vector<RunRef> kEmptyRuns;
+        const std::vector<RunRef> & shard_runs = it != runs_by_shard.end() ? it->second : kEmptyRuns;
+        for (const BlobCandidate & cand : zeroInDegree(backend, shard_runs))
+        {
+            const HeadResult observed = backend.head(layout.blobKey(cand.ref));
+            if (!observed.exists)
+                continue;
+            PreviewEntry e;
+            e.kind = ObjectKind::Blob;
+            e.ref = cand.ref;
+            e.key = layout.blobKey(cand.ref);
+            e.size = observed.size;
+            e.reason = "unreachable";
+            out.push_back(std::move(e));
+        }
+
+        /// Retired-in-snapshot: stream the SAME adopted seal runs and emit every `kCondemned`
+        /// sentinel row. The stored token IS the authority — NO HEAD here (a HEAD would defeat the point
+        /// and cost I/O). `delete_pending` rows are deleted next fold; the rest await graduation. Preview
+        /// stays WRITE-FREE (`openSourceEdgeRun` is a pure reader). Output is a superset of the above.
+        for (const RunRef & run : shard_runs)
+        {
+            SourceEdgeRunView reader = openSourceEdgeRun(backend, run.key);
+            String key;
+            String payload;
+            while (reader.next(key, payload))
+            {
+                if (payload.empty() || payload[0] != kCondemned)
+                    continue;
+                BlobRef ref;
+                UInt128 source_id;
+                SourceEdgeKeyCodec::parse(key, ref, source_id);   // throws CORRUPTED_DATA on a malformed key
+                const CondemnedRow row = decodeCondemnedRow(payload);
+                PreviewEntry e;
+                e.kind = ObjectKind::Blob;
+                e.ref = ref;
+                e.key = layout.blobKey(ref);
+                e.size = row.size;
+                e.token = row.token;
+                e.condemn_round = row.condemn_round;
+                e.reason = row.delete_pending ? "delete_pending" : "awaiting_graduation";
+                out.push_back(std::move(e));
+            }
+            /// Whole-file seal-checksum: verify the drained run before its condemned
+            /// rows are trusted in the preview. Fail-closed on mismatch.
+            reader.verifyAgainst(run.checksum);
+        }
+    }
+    return out;
+}
+
+void Gc::rememberObservation(const GcLease & lease)
+{
+    has_observation = true;
+    last_seen_owner = lease.owner;
+    last_seen_seq = lease.seq;
+}
+
+void Gc::pulseHeartbeat(Pool & store, UInt128 gc_id)
+{
+    const String key = store.layout().gcHbKey();
+    const auto got = store.backend().get(key);
+    GcHeartbeat hb;
+    std::optional<Token> expected;
+    if (got)
+    {
+        hb = decodeGcHeartbeat(got->bytes);
+        expected = got->token;
+    }
+    hb.owner = gc_id;
+    ++hb.hb_seq;
+    store.backend().casPut(key, encodeGcHeartbeat(hb), expected);
+}
+
+bool Gc::acquireOrRenewLease(GcState & state, Token & state_token, bool allow_steal)
+{
+    const String key = store->layout().gcStateKey();
+
+    for (int attempt = 0; attempt < 2; ++attempt)
+    {
+        const auto got = store->backend().get(key);
+
+        if (!got)
+        {
+            if (has_observation)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS gc/state vanished after being observed (owner {}, seq {})",
+                    u128ToHex(last_seen_owner), last_seen_seq);
+
+            GcState fresh;
+            fresh.lease = GcLease{gc_id, 1};
+            /// Creation-time only: gc_shards is set ONCE on first-ever acquire; subsequent rounds read
+            /// the authoritative value from the persisted GcState (pool is authoritative on reopen).
+            /// PoolConfig carries the configured value from the disk XML.
+            fresh.gc_shards = store->poolConfig().gc_shards;
+            const CasResult acquire_res = store->backend().casPut(key, encodeGcState(fresh), std::nullopt);
+            if (acquire_res.outcome == CasOutcome::Committed)
+            {
+                rememberObservation(fresh.lease);
+                state = std::move(fresh);
+                state_token = acquire_res.token;
+                return true;
+            }
+            continue;
+        }
+
+        GcState current = decodeGcState(got->bytes);
+        if (current.gc_shards != store->poolConfig().gc_shards)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS gc/state gc_shards {} disagrees with the pool-authoritative _pool_meta value {}",
+                current.gc_shards, store->poolConfig().gc_shards);
+
+        if (current.lease.owner == gc_id)
+        {
+            GcState next = current;
+            ++next.lease.seq;
+            const CasResult renew_res = store->backend().casPut(key, encodeGcState(next), got->token);
+            if (renew_res.outcome == CasOutcome::Committed)
+            {
+                rememberObservation(next.lease);
+                state = std::move(next);
+                state_token = renew_res.token;
+                return true;
+            }
+            continue;
+        }
+
+        GcHeartbeat hb;
+        if (const auto hb_got = store->backend().get(store->layout().gcHbKey()))
+            hb = decodeGcHeartbeat(hb_got->bytes);
+        /// Observation-based heartbeat liveness, symmetric with the frozen-lease-tuple check below:
+        /// ANY movement of the observed (owner, hb_seq) pair between this contender's two ticks is
+        /// proof of life, and `hb_seq` values are comparable only under the SAME remembered hb owner.
+        /// Deliberately NOT compared against `current.lease.owner`: a deposed leader's heartbeat
+        /// thread keeps pulsing (with `owner = itself`) until its next round resets `i_am_leader`,
+        /// and its writes can race out the live new leader's pulses — an hb pair that keeps moving
+        /// under the OLD owner's name must still read as "alive", or a live, pulsing new leader gets
+        /// its lease stolen. An hb owner change re-arms the window (this tick's pair is remembered
+        /// below); a steal happens only once the lease tuple AND the hb pair are both frozen across
+        /// a full window.
+        const bool hb_alive = has_observation
+            && (hb.owner != last_seen_hb_owner || hb.hb_seq > last_seen_hb_seq);
+
+        const bool incumbent_renewed = !has_observation
+            || current.lease.owner != last_seen_owner
+            || current.lease.seq != last_seen_seq;
+        if (incumbent_renewed || hb_alive || !allow_steal)
+        {
+            /// Only ARM the steal-decision state (last_seen_owner/seq/hb_*) when this call is itself
+            /// allowed to act on a frozen tuple - i.e. the loop path. A caller with allow_steal=false
+            /// (manual `SYSTEM ... GC`) reads current state for its own acquire/renew/back-off decision
+            /// above, but must NOT record this foreign-incumbent observation: doing so would let the
+            /// loop's own very next tick treat THIS snapshot as one half of ITS two-observation window,
+            /// without the real wall-time gap (>= H) the window's safety argument requires between the
+            /// loop's own ticks (a manual command can land microseconds before a scheduled tick). Leaving
+            /// last_seen_* untouched here restores the pre-A7 invariant exactly: the frozen-tuple
+            /// comparison that can actually trigger a steal is only ever between two LOOP observations,
+            /// always >= interval apart.
+            if (allow_steal)
+            {
+                rememberObservation(current.lease);
+                last_seen_hb_owner = hb.owner;
+                last_seen_hb_seq = hb.hb_seq;
+            }
+            return false;
+        }
+
+        GcState next = current;
+        next.lease.owner = gc_id;
+        ++next.lease.seq;
+        const CasResult steal_res = store->backend().casPut(key, encodeGcState(next), got->token);
+        if (steal_res.outcome == CasOutcome::Committed)
+        {
+            rememberObservation(next.lease);
+            state = std::move(next);
+            state_token = steal_res.token;
+            return true;
+        }
+
+        if (const auto reread = store->backend().get(key))
+            rememberObservation(decodeGcState(reread->bytes).lease);
+        return false;
+    }
+
+    return false;
+}
+
+CatalogLifecycleReconcileResult Gc::drainCompletedRemoving(const GcState & leased_state)
+{
+    if (leased_state.snap_generation == 0)
+        return {
+            .authority_status = AuthorityStatus::Authoritative,
+            .catalog_resolution = CatalogResolution::DrainComplete,
+            .retired_lives = {},
+            .final_catalog_cut = std::nullopt,
+            .deleted = 0};
+
+    const std::optional<CasFoldSeal> parent = readFoldSeal(
+        leased_state.snap_generation, leased_state.snap_attempt);
+    if (!parent)
+        throw Exception(ErrorCodes::CORRUPTED_DATA,
+            "CAS GC pre-fold drain: adopted parent seal (generation {}, attempt {}) is missing",
+            leased_state.snap_generation, leased_state.snap_attempt);
+
+    Backend & backend = store->backend();
+    const Layout & layout = store->layout();
+    const uint64_t admitted_generation = leased_state.lease.seq;
+    const auto check_fence = [&](uint64_t expected_generation)
+    {
+        if (expected_generation != admitted_generation)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS GC pre-fold drain: internal leader generation mismatch (expected {}, admitted {})",
+                expected_generation, admitted_generation);
+        const auto got = backend.get(layout.gcStateKey());
+        if (!got)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS GC pre-fold drain: gc/state vanished while checking leader generation {}",
+                admitted_generation);
+        const GcState current = decodeGcState(got->bytes);
+        if (current.lease.owner != gc_id || current.lease.seq != admitted_generation)
+            return CasRefCatalog::LeaderFenceStatus::Moved;
+        return CasRefCatalog::LeaderFenceStatus::Held;
+    };
+
+    return CatalogLifecycleReconciler(
+        backend, layout, *parent, admitted_generation, check_fence).reconcile();
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h
new file mode 100644
index 000000000000..51d640f39665
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h
@@ -0,0 +1,1012 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Common/Logger.h>
+#include <Common/ThreadPool.h>
+#include <atomic>
+#include <functional>
+#include <map>
+#include <memory>
+#include <mutex>
+#include <set>
+#include <utility>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// WHOSE UNIVERSE THE DESTRUCTIVE GATE IS ALLOWED TO TRUST.
+///
+/// A round may destroy only while holding a FRONTIER PROOF for EVERY namespace that can hold a live
+/// edge — because reachability is a property of the whole pool, not of one namespace. The proof per
+/// namespace is cheap and exact (one `GET` at the cursor's arithmetic successor, `CasGc.cpp`'s intake
+/// walk). What those proofs cannot supply is the SET they must cover, and only the catalog supplies it:
+/// a listing may omit a durable namespace, and a sealed fold cursor names only namespaces some round
+/// already folded, so neither source bounds the set on its own.
+///
+/// The scenario that makes this a correctness question rather than a tidiness one: a hidden acked `+1`
+/// lands in a namespace absent from BOTH the hint and `gc/state`, while a visible `-1` elsewhere drives
+/// the shared blob's OBSERVABLE in-degree to zero. Every namespace the round knows about is walked to a
+/// proven frontier, every probe comes back clean, and the round would delete a blob a durable committed
+/// edge still owns. No amount of per-namespace proof detects it: the namespace is not in the set being
+/// proved.
+enum class UniversePolicy : uint8_t
+{
+    /// The caller supplies no universe, so `frontier_incomplete` is unconditionally true and every
+    /// destructive site in the round declines. Passed explicitly by a test whose SUBJECT is the
+    /// suppression; nothing in production selects it.
+    StageA_Suppressed = 0,
+    /// The round's universe is the catalog's own `Live`/`Removing` set, so the per-namespace frontier
+    /// proofs decide the gate on their own.
+    Authoritative = 1,
+
+    /// The gate opens only on a COMPLETE, CATALOG-PROVEN frontier: every namespace in the universe
+    /// reached a proven frontier, no anomaly was recorded, and no hold rides forward. A universe of size
+    /// zero is unproven by DEFAULT (`frontier_namespaces == 0` alone -- an empty-BY-COUNTER universe --
+    /// is never a proof; a fresh pool, a damaged catalog, and a legitimately empty pool all produce it
+    /// identically). It is proven only when the round's OWN hot-scan catalog cut positively demonstrates
+    /// it: present, token-bearing, decoded, and holding zero rows of every lifecycle state including
+    /// `Creating` (`FoldResult::catalog_cut_proved_empty`). Without that positive proof an emptied pool
+    /// would stop reclaiming permanently, because its universe can never again exceed zero. Any of these
+    /// failing suppresses every destructive site POOL-WIDE — narrowing the suppression to the offending
+    /// namespace would be unsound, because blob in-degree is a pool-wide property and no ownership
+    /// partition of the blob space exists.
+    kDefault = Authoritative,
+};
+
+/// The logical (GC-bookkeeping) size of a retired object: a blob subtracts the pool's fixed
+/// blob_header_len (a blob OBJECT smaller than the fixed header is corrupt — CORRUPTED_DATA,
+/// fail closed, never a wrapped-around size). Sizes feed cost/health accounting only — no protocol
+/// decision ever reads them.
+uint64_t retiredLogicalSize(ObjectKind kind, uint64_t object_size, uint64_t blob_header_len);
+
+/// Pure skip-unchanged decision. Returns true iff the current round may be
+/// DEFERRED (re-adopt the sealed generation, no fold/delete). A round MUST fold when: enough shards
+/// changed (>= fold_threshold), OR a destructive decision is due (graduation_due), OR the defer bound
+/// is reached (rounds_since_last_fold >= fold_max_defer_rounds). The graduation_due term is the
+/// load-bearing safety guard: no destructive decision ever runs on a not-fully-folded snapshot.
+bool shouldDeferRound(size_t changed_shards, bool graduation_due, uint64_t rounds_since_last_fold,
+                      uint64_t fold_threshold, uint64_t fold_max_defer_rounds);
+
+/// One anomaly the fold surfaced (a clamped cursor — a missing committed/removal body, or the fold
+/// barrier on a live missing-body precommit). Surfaced to fsck/logs; recording an anomaly never throws:
+/// the round records the problem and continues, while the affected shard remains conservatively clamped.
+struct RoundAnomaly
+{
+    RootNamespace ns;
+    uint64_t shard = 0;
+    ManifestId id;
+    String reason;
+};
+
+/// Outcome of `Gc::rebuildBaseline`: either a live-conservative baseline was published, with its minted
+/// numbering and coverage counters, or the rebuild was refused. Refusal is fail-closed: `refusal` says
+/// why, no partial baseline is committed, and `gc/state` remains untouched.
+struct RebuildReport
+{
+    bool performed = false;
+    String refusal;
+    uint64_t round = 0;
+    uint64_t generation = 0;
+    uint64_t namespaces = 0;
+    uint64_t shards = 0;
+    uint64_t committed_refs = 0;
+    uint64_t live_precommits = 0;
+    /// Trimmed-but-live manifests kept over-protected because ownership could not be proved.
+    uint64_t unowned_alive_manifests = 0;
+    uint64_t edges = 0;
+    uint64_t clamped_shards = 0;
+    /// This rebuild concluded FROM ENUMERATION ALONE that the pool had never sealed a baseline, and so
+    /// carried NO durable hold forward. It is the single route by which a hold can still be dropped
+    /// silently, so the hand-run disaster command REPORTS it rather than leaving it to a log line: on a
+    /// pool that has ever completed a GC round, a 1 here means the enumeration lied.
+    bool virgin_by_enumeration = false;
+    /// WHICH generation's fold seal this rebuild carried holds from; 0 when it carried none. The two
+    /// verdicts this command reaches about the pool's history are then both on its own row — whether a
+    /// baseline was found at all, and which one it was. Grepping that out of the logs is the thing an
+    /// operator should not have to do in the middle of a disaster, and it is also how a step-down past
+    /// a crashed newest generation becomes visible rather than invisible.
+    uint64_t adopted_seal_generation = 0;
+};
+
+/// Counters and diagnostics returned by one `runRegularRound`. These values describe work attempted or
+/// observed by the round; durable `gc/state` and the fold artifacts remain the protocol state.
+struct RoundReport
+{
+    bool acquired_lease = false;  /// false => another leader is alive; nothing else was done
+    /// True iff this round deferred (re-adopted the sealed
+    /// in-degree generation instead of folding). A deferred round performs no fold, no pre-CAS
+    /// deletes, and no gc/state CAS -- every other RoundReport field below is meaningless/zero on it,
+    /// EXCEPT `round` (below), which a deferred round still sets to the honest, already-adopted round
+    /// number (see the defer branch of runRegularRound, CasGc.cpp) -- it just never advances it.
+    bool deferred = false;
+    uint64_t round = 0;
+    uint64_t candidates = 0;      /// retired entries WRITTEN this round (absent candidates are skipped)
+    uint64_t deleted = 0;
+    uint64_t absent = 0;
+    uint64_t replaced = 0;        /// 412-saves — a health metric
+    uint64_t spared = 0;
+    uint64_t manifests_deleted = 0;  /// owner-removed manifest bodies deleted, distinct from blob deletes
+    /// Retired-cursor pipeline observability: the pipeline's per-round transitions.
+    size_t condemned = 0;         /// entries newly condemned into the retired list this round
+    size_t graduated = 0;         /// entries newly floor-passed (published delete_pending) this round
+    size_t redeleted = 0;         /// pending deletes executed this round (exact-token blob deletes)
+    size_t fence_outs = 0;        /// expired mounts fenced-out by the round's heartbeat floor
+    std::vector<RoundAnomaly> anomalies;   /// fold clamps surfaced this round (never wedge the round)
+
+    /// Retire-pipeline REMAINING sizes as of the gc/state this round's single CAS just published --
+    /// unlike `condemned`/`graduated`/`redeleted` above (this round's DELTAs), these are the pipeline's
+    /// current outstanding totals, summed over every gc-shard's sealed `CondemnedSummary`
+    /// (`Formats/CasFoldSealFormat.h`). `pending_retired` = still delete_pending (graduated, awaiting the
+    /// exact-token delete next pass); `pending_candidates` = condemned but not yet floor-passed;
+    /// `pending_condemned` = their total (candidates + retired), the overall pipeline gauge. Left at 0 on
+    /// a `!acquired_lease`/`deferred` round -- those flags already mark the row non-authoritative.
+    size_t pending_candidates = 0;
+    size_t pending_condemned = 0;
+    size_t pending_retired = 0;
+
+    /// Record a fold/recheck anomaly (a clamped cursor). Surfacing, never throwing.
+    void recordAnomaly(const RootNamespace & ns_, uint64_t shard_, const ManifestId & id_, const char * reason_)
+    {
+        anomalies.push_back(RoundAnomaly{.ns = ns_, .shard = shard_, .id = id_, .reason = reason_});
+    }
+
+    /// Return whether this report already contains an anomaly for the specified namespace and shard.
+    bool hasAnomaly(const RootNamespace & ns_, uint64_t shard_) const
+    {
+        for (const RoundAnomaly & a : anomalies)
+            if (a.ns.string() == ns_.string() && a.shard == shard_)
+                return true;
+        return false;
+    }
+};
+
+/// One phase of one GC round. Pure data with no Interpreters dependency -- the same discipline
+/// `GcRoundLogRecord` follows, so the round engine can be instrumented without linking the system-log
+/// machinery. `CasGcScheduler` converts it into one `Phase` row of
+/// `system.cas_gc_log`.
+///
+/// DELIBERATELY NO VERB COLUMNS. `profile_events` is this phase's delta of the ordinary process
+/// counters, so `ProfileEvents['S3ListObjects']` on a `Phase` row answers "which phase burns the LIST
+/// budget" with no schema invention; `metrics` carries only the semantic counts a phase computes for
+/// itself and no counter can supply.
+///
+/// This lives in `CasGc.h` rather than next to `GcRoundLogRecord` in `CasGcScheduler.h` because
+/// `Cas::Gc` holds the sink as a member, and `CasGcScheduler.h` already includes this header -- putting
+/// it there would make the include cycle.
+struct GcPhaseRecord
+{
+    String phase;
+    UInt64 duration_us = 0;
+    std::map<String, UInt64> metrics;
+    std::map<String, UInt64> profile_events;   /// this phase's ProfileEvents delta
+};
+
+/// Where a phase record goes. Installed for the duration of one round by `CasGcScheduler`; empty
+/// elsewhere (a unit test driving `Gc` directly emits nothing).
+using GcPhaseSink = std::function<void(const GcPhaseRecord &)>;
+
+/// The round's one hint enumeration of `cas/ns/stream/`, taken before the defer decision and consumed by
+/// everything downstream: the DEFER signal (`changed_shards`), and — on a folding round — the strict
+/// grouping the fold works from (`keys`, regrouped by `groupRefKeys`).
+///
+/// It is a HINT, never a census. The fold's intake walks the ref stream ARITHMETICALLY by exact key
+/// (`cursor + 1`, epoch crossings proved through the seal chain), so an id this enumeration omits is
+/// still folded, and the round listing the prefix a second time would buy the intake nothing.
+struct RefScanSummary
+{
+    size_t changed_shards = 0;                          /// tables with a log above their sealed cursor
+    std::vector<String> keys;                           /// every listed key, verbatim, for the fold's strict grouping
+    std::set<NamespaceLifePhysicalId> listed_lives;     /// every parsed stream kind, classified by the later catalog cut
+    std::map<NamespaceLifePhysicalId, std::set<RefTxnId>> logs_by_life;
+    std::map<NamespaceLifePhysicalId, RefTxnId> max_log_by_life;
+    /// The validated adopted parent's rows read for this scan. They enrich the one walk plan built
+    /// after the catalog cut; the fold consumes that plan instead of reading them into a second one.
+    std::map<UInt128, RefLifeFoldState> parent_ref_lives;
+    std::map<UInt128, RefHold> holds;
+    std::map<UInt128, RefTxnId> checkpoint_observations;
+    /// Listed ids absent from the `RoundInput` catalog cut are inert dead-life debris.
+    size_t dead_life_debris = 0;
+};
+
+class RefPlan;
+class RoundInput;
+RefPlan buildRefWalkPlan(RoundInput && round_input);
+
+namespace tests
+{
+RefPlan buildRefWalkPlanForTest(RefScanSummary ref_scan, CasRefCatalog::Snapshot catalog_cut);
+class GcRoundPlanSignatureAccess;
+}
+
+/// The one owned observation boundary between the reconciled hot LIST and every ref-plan consumer.
+/// `Gc` constructs it from the completed hot LIST and its later catalog cut, then the sole builder
+/// moves those observations into a `RefPlan`. The scan is never separately pairable with a plan
+/// downstream.
+class RoundInput
+{
+public:
+    RoundInput(const RoundInput &) = delete;
+    RoundInput(RoundInput &&) = default;
+    RoundInput & operator=(const RoundInput &) = delete;
+    RoundInput & operator=(RoundInput &&) = delete;
+
+private:
+    friend class Gc;
+    friend RefPlan buildRefWalkPlan(RoundInput && round_input);
+    friend RefPlan tests::buildRefWalkPlanForTest(RefScanSummary ref_scan, CasRefCatalog::Snapshot catalog_cut);
+
+    RoundInput(RefScanSummary ref_scan_, CasRefCatalog::Snapshot catalog_cut_)
+        : ref_scan(std::move(ref_scan_)), catalog_cut(std::move(catalog_cut_))
+    {
+    }
+
+    RefScanSummary ref_scan;
+    CasRefCatalog::Snapshot catalog_cut;
+};
+
+struct RefWalkPlanRow
+{
+    NamespaceLifeId life;
+    RefLifeFoldState fold_state;
+    std::optional<uint64_t> removal_started_round;
+    bool has_parent_fold_state = false;
+    bool listed_hint = false;
+    std::optional<RefTxnId> checkpoint_observation;
+    std::optional<RefTxnId> tail_observation;
+};
+
+/// Diagnostic-only classification of a catalog `Removing` life against the adopted round. Returns
+/// no value before the threshold, when terminal cleanup evidence exists, or for a non-removing row.
+/// It performs no I/O and changes no round decision.
+std::optional<String> stuckRemovalWarning(
+    const RefWalkPlanRow & row, uint64_t current_round, uint64_t threshold_rounds,
+    const Layout & layout);
+
+/// A frozen catalog-built row set. Enrichment is exposed only through exact lookup; there is no
+/// `operator[]`, insertion method, or mutable row map through which a hint, parent, hold, or checkpoint
+/// can mint a logical life.
+class RefPlan
+{
+public:
+    RefPlan(const RefPlan &) = delete;
+    RefPlan(RefPlan &&) = default;
+    RefPlan & operator=(const RefPlan &) = delete;
+    RefPlan & operator=(RefPlan &&) = delete;
+
+    bool contains(const UInt128 & life_id) const { return rows.contains(life_id); }
+    const RefWalkPlanRow & row(const UInt128 & life_id) const { return rows.at(life_id); }
+    std::set<UInt128> lifeIds() const;
+    std::vector<NamespaceLifeId> lives() const;
+    const RefScanSummary & refScan() const { return ref_scan; }
+    const CasRefCatalog::Snapshot & catalogCut() const { return catalog_cut; }
+    std::map<UInt128, RefLifeFoldState> parentFoldStates() const;
+    std::map<UInt128, RefLifeFoldState> successorFoldStates() const;
+    size_t size() const { return rows.size(); }
+    size_t changedRows() const;
+    uint64_t droppedParentRows() const { return dropped_parent_rows; }
+    uint64_t droppedListedLives() const { return dropped_listed_lives; }
+    uint64_t droppedHolds() const { return dropped_holds; }
+    uint64_t droppedCheckpoints() const { return dropped_checkpoints; }
+    uint64_t droppedTails() const { return dropped_tails; }
+
+private:
+    friend RefPlan buildRefWalkPlan(RoundInput && round_input);
+
+    RefPlan(RefScanSummary ref_scan_, CasRefCatalog::Snapshot catalog_cut_)
+        : ref_scan(std::move(ref_scan_)), catalog_cut(std::move(catalog_cut_))
+    {
+    }
+    RefScanSummary ref_scan;
+    CasRefCatalog::Snapshot catalog_cut;
+    std::map<UInt128, RefWalkPlanRow> rows;
+    uint64_t dropped_parent_rows = 0;
+    uint64_t dropped_listed_lives = 0;
+    uint64_t dropped_holds = 0;
+    uint64_t dropped_checkpoints = 0;
+    uint64_t dropped_tails = 0;
+};
+
+/// Builds the one authoritative ref-life key set used by ordinary GC and healthy `REBUILD`.
+/// Adapters run only after the catalog loop has frozen that set and attach observations by `at`-style
+/// lookup; unknown, absent, or `Creating` ids are counted and dropped.
+/// PROBE B2 — end-to-end transaction accounting for ONE round. Round-local, never persisted.
+///
+/// The naive "intended vs applied" counter pair is VACUOUS: in the intake both counts increment in the
+/// same basic block, so they cannot differ. This ledger separates them across the whole pipeline
+/// instead — a transaction is `committed` when the intake merges its staged buffers, `produced` when
+/// it emitted at least one `BlobDelta`, and `applied` only when a shard reducer actually CONSUMED one
+/// of its deltas. A delta lost between the intake and a reducer (routing across gc shards, a skipped
+/// bucket, a future filter) leaves a committed+produced transaction unapplied.
+///
+/// Marking happens at reducer CONSUMPTION, not at run flush: the in-degree model is a SET, so an
+/// unmatched `-1` and a duplicate `+1` legitimately vanish inside the reducer and a flush-side mark
+/// would fire on healthy rounds. Loss INSIDE the reducer's own set collapse is a different class,
+/// covered by `CASGCUnmatchedRemoveDeltas` and by the mirror safety test in
+/// `gtest_cas_holey_list_detector.cpp`; probe B2 does not claim it.
+///
+/// SINGLE-THREADED: the shard reducers run sequentially on the fold thread (`Gc::fold`), so `applied`
+/// needs no synchronisation. A future parallel reducer must revisit this.
+struct TxnApplyLedger
+{
+    std::vector<RefTxnId> txns;        /// ordinal -> the log id
+    std::vector<String> namespaces;    /// ordinal -> namespace, for the failure message
+    std::vector<uint8_t> produced;     /// this transaction emitted >= 1 BlobDelta
+    std::vector<uint8_t> committed;    /// this transaction folded fully and merged into the round buffers
+    /// >= 1 of this transaction's deltas was consumed by a reducer. Public and written through a raw
+    /// pointer by `foldDeltasIntoGeneration`: that write sits in a loop over potentially millions of
+    /// rows, where a `std::function` hop is not free.
+    std::vector<uint8_t> applied;
+
+    /// Open a transaction and return its round-local ordinal.
+    uint32_t open(const RootNamespace & ns, const RefTxnId & id)
+    {
+        const uint32_t ordinal = static_cast<uint32_t>(txns.size());
+        txns.push_back(id);
+        namespaces.push_back(ns.string());
+        produced.push_back(0);
+        committed.push_back(0);
+        applied.push_back(0);
+        return ordinal;
+    }
+    void markProduced(uint32_t ordinal) { produced[ordinal] = 1; }
+    void markCommitted(uint32_t ordinal) { committed[ordinal] = 1; }
+    void markApplied(uint32_t ordinal) { applied[ordinal] = 1; }
+
+    /// Ordinals that were committed AND produced deltas but whose deltas never reached a reducer.
+    /// Empty on a healthy round.
+    std::vector<uint32_t> unapplied() const
+    {
+        std::vector<uint32_t> out;
+        for (uint32_t i = 0; i < txns.size(); ++i)
+            if (committed[i] && produced[i] && !applied[i])
+                out.push_back(i);
+        return out;
+    }
+};
+
+/// Leader-paced regular GC: one pass per round — heartbeat
+/// ack floor -> fold (two-cursor merge) -> pre-CAS deletes of previously-published pending
+/// entries -> single `gc/state` CAS -> post-CAS cleanup/trim, over the root-local part-manifest model. The lease is work deduplication only —
+/// every step is idempotent and split-brain-safe (monotone `gc/state`, append-by-unique-path retire and
+/// outcome logs, exact-token deletes). These properties mean that a stale leader can duplicate work but
+/// cannot roll back state or delete a newer object incarnation; the safety argument does not depend on
+/// the lease being perfectly exclusive.
+///
+/// LEASE / STEAL WINDOW (deterministic — this class NEVER reads a clock). The lease lives inside
+/// `gc/state` as {owner, seq} and moves only by CAS on the whole `gc/state` object. The
+/// `acquireOrRenewLease` method implements the observation, renewal, and steal protocol.
+///
+/// NOT thread-safe: one pacing thread drives a Gc instance. gc_id uniqueness across instances
+/// (a random u128) is a CALLER obligation — duplicate ids make two leaders indistinguishable.
+class Gc
+{
+    friend class tests::GcRoundPlanSignatureAccess;
+
+public:
+    /// `now_ms_fn` is the WALL clock (injected for tests): audit/diagnostic stamps only (e.g. the
+    /// heartbeat floor's `now_ms` argument) — it never gates a fence decision. `mono_ms_fn` is the
+    /// OBSERVATION clock: monotonic on this
+    /// process, injected for tests, defaults to `Pool::bootMs()`, and the ONLY clock the heartbeat
+    /// gate's own fence-out threshold is measured against (mirrors `claimMountAwaitingExpiry`'s
+    /// `mono_ms_fn`, but at heartbeat-gate granularity — one GC round is one observation tick).
+    /// Everything else in the round stays deterministic/clock-free.
+    ///
+    /// `log_` is the logger every round-engine log line is emitted through; pass a disk/srid-scoped
+    /// logger (e.g. `CasGcScheduler`'s own `log`, built from a `fmt::format("{}::...", storage_path)`
+    /// name) so a multi-disk process's GC logs are attributable. Defaults to the process-global
+    /// `getLogger("CasGc")` for callers with no natural scope of their own (tests, one-shot commands).
+    Gc(PoolPtr store_, UInt128 gc_id_, std::function<uint64_t()> now_ms_fn_ = {},
+       std::function<uint64_t()> mono_ms_fn_ = {}, LoggerPtr log_ = nullptr);
+
+    /// One full round. Returns acquired_lease=false (nothing else done) if another leader is alive.
+    /// `on_lease_acquired`, if set, is invoked ONCE, synchronously, immediately after the lease is
+    /// acquired/renewed and BEFORE the (potentially long) fold begins - the scheduler uses this to
+    /// mark itself leader and fire the first advisory heartbeat pulse right away: a new
+    /// leader's first round must not run unprotected for the whole fold before the pacing thread's
+    /// post-round bookkeeping would otherwise have set it). Never called when the lease is not held;
+    /// exceptions from the callback propagate like any other round failure (the caller is expected to
+    /// keep it advisory/non-throwing, matching pulseHeartbeat's own contract).
+    ///
+    /// `allow_steal` (default true — the paced background loop's semantics, unchanged): gates the
+    /// observation-window protocol's steal branch (see acquireOrRenewLease). The window's safety
+    /// argument requires the two observations that flag an incumbent "frozen" to be spaced by real
+    /// wall time (>= the heartbeat cadence H) so a live incumbent gets a chance to pulse in between —
+    /// a guarantee only the loop's own interval-paced ticks provide. A caller with no such guarantee
+    /// (e.g. a manual `SYSTEM ... GC` command, where two calls can land microseconds apart) must pass
+    /// `false`: it may still acquire a FREE lease or renew ITS OWN, but never executes the steal CAS —
+    /// dead-incumbent recovery stays the loop's job.
+    ///
+    /// `policy` is the destructive gate's universe seam — see `UniversePolicy`. Production passes
+    /// nothing; a test whose subject is the suppressed gate passes `StageA_Suppressed` here, which is
+    /// the only way to reach that posture.
+    RoundReport runRegularRound(std::function<void()> on_lease_acquired = {}, bool allow_steal = true,
+                                UniversePolicy policy = UniversePolicy::kDefault);
+
+    /// Advisory heartbeat: bump <prefix>/gc/hb to {gc_id, hb_seq+1}. Best-effort (a lost CAS is
+    /// harmless — the next pulse retries). Touches NO Gc instance state. Static by design.
+    static void pulseHeartbeat(Pool & store, UInt128 gc_id);
+
+
+    /// One deletion that the next regular round would preview, with the reason it is eligible. This is
+    /// diagnostic data only and does not carry the durable token or authorization for a delete by itself.
+    struct PreviewEntry
+    {
+        ObjectKind kind = ObjectKind::Blob;
+        BlobRef ref{};
+        String key;
+        uint64_t size = 0;
+        String reason;          /// "unreachable" | "delete_pending" | "awaiting_graduation"
+        Token token;            /// stored condemn-time token (empty for "unreachable")
+        uint64_t condemn_round = 0;
+    };
+
+    /// WRITE-FREE preview of the next round's deletes, derived from the DURABLE sealed in-degree
+    /// generation + gc/state. Diagnostic / cross-check ONLY — its output must never feed a real delete.
+    /// It reads the durable generation WITHOUT folding new owner events, so at NON-QUIESCENCE it can
+    /// OVER-REPORT a blob a since-landed publish re-referenced (the real round folds first and spares
+    /// it). The {preview} ⊆ {genuinely-unreachable} guarantee holds ONLY at quiescence. No CAS/delete.
+    std::vector<PreviewEntry> previewDeletes();
+
+    /// Raw baseline rebuild — the `gc/state` disaster-recovery command. Recomputes
+    /// the in-degree snapshot from raw owner state (committed refs + live precommits + the unowned
+    /// not-provably-dead over-protection), mints round/generation above every surviving ack/number,
+    /// publishes EMPTY retired lists, and CASes gc/state. Live-conservative; fail-closed refusals.
+    RebuildReport rebuildBaseline(bool force);
+
+    /// Install (or clear) the per-phase sink for the round that is about to run. `CasGcScheduler`
+    /// installs it around one `runRegularRound` and clears it afterwards, so the sink never outlives the
+    /// scheduler locals it captures. Not thread-safe, like the rest of `Gc`: one pacing thread drives one
+    /// instance, and the scheduler holds `gc_round_mutex` across both the install and the round.
+    void setPhaseSink(GcPhaseSink sink) { phase_sink = std::move(sink); }
+
+    void setRebuildEdgeBudgetForTest(uint64_t n) { rebuild_edge_budget_override = n; }
+
+    /// TEST SEAM: disable the round's journal trim so a folded event stays in the journal
+    /// across rounds — exactly the lazy-trim / partial-trim-after-crash condition under which the
+    /// next round's fold MUST recover the exact sealed cursor (else it re-folds the event and double-counts
+    /// blob in-degree). Production never calls this; trim is always enabled.
+    void setTrimEnabledForTest(bool enabled) { trim_enabled = enabled; }
+
+    /// Fires once, synchronously, right after `listRefPrefix`'s hot-scan catalog `GET`
+    /// (`CasRefCatalog::read`) returns -- the exact instant the round's catalog cut is taken, before
+    /// the round does anything else with it. Lets a test land a real namespace birth (through the
+    /// production writer path) in the window between that cut and the round's later destructive work,
+    /// driving the interleaving deterministically instead of relying on real thread scheduling. Empty
+    /// (no-op) in production, mirroring `CasRefCatalog::setCreateNamespaceStep1PreReadHookForTest`.
+    void setPostHotScanCatalogReadHookForTest(std::function<void()> hook)
+    {
+        post_hot_scan_catalog_read_hook_for_test = std::move(hook);
+    }
+
+    /// Abandoned-precommit cleanup belongs to the writer: it appends exact `owner_transition` removals in
+    /// ref logs. GC never invents a ref transition, because doing so could make the fold disagree with the
+    /// writer's durable ownership history.
+
+    /// DELETE-SITE INVARIANT: the round's pre-CAS
+    /// redelete phase in `runRegularRound` holds the ONLY content (blob reachability) delete in
+    /// the whole core, restricted to previously-published delete_pending entries.
+    /// The recheck's manifest-body exact-token delete (after its decrements are sealed), the retired-set
+    /// drop, the resume path (GC metadata), dropNamespace (verbatim files), and the capability probe
+    /// (throwaway keys) remove non-content objects they own. Adding a second content-delete site is a
+    /// protocol defect.
+
+private:
+    /// Lease acquire/renew/steal per the documented observation protocol. On success `state` holds the
+    /// committed gc/state (with our lease) and `state_token` its backend token. `allow_steal=false`
+    /// suppresses only the steal CAS (see runRegularRound's doc comment) — acquiring a free lease and
+    /// renewing our own are unaffected.
+    bool acquireOrRenewLease(GcState & state, Token & state_token, bool allow_steal);
+
+    /// Catalog-only helping barrier run immediately after lease acquisition. It validates the adopted
+    /// parent and delegates deterministic `Removing`-row settlement to `CatalogLifecycleReconciler`.
+    /// It performs no physical LIST or delete.
+    CatalogLifecycleReconcileResult drainCompletedRemoving(const GcState & leased_state);
+
+    /// Run exactly one independently paced physical namespace-maintenance page. The caller supplies
+    /// the one round-wide destructive verdict when it exists; DEFER passes suppression because it has
+    /// no folded frontier verdict. This helper owns only janitor I/O and phase metrics, never lifecycle
+    /// transitions or the hot stream walk plan.
+    void runNamespaceJanitorPage(
+        const GcState & leased_state, bool suppress_destructive, uint64_t cleanup_evidence_rows);
+
+    void reportStuckRemovals(const RefPlan & plan, uint64_t current_round);
+
+    /// What one fold produced. The blob deltas are sealed
+    /// into a write-once generation; `fold_seal` is the durable index of WHAT WAS FOLDED (a CasFoldSeal),
+    /// `root_shards` the discovered universe, `mf_cleanup` the part-manifest cleanup work keyed by
+    /// ManifestId (owner-removed bodies whose exact-token delete is deferred until their decrements are
+    /// sealed), and `retired_merge` the per-gc-shard ack-floor retired-cursor outcome.
+    struct FoldResult
+    {
+        CasFoldSeal fold_seal;
+        std::vector<std::pair<RootNamespace, uint64_t>> root_shards;
+        std::map<ManifestId, Token> mf_cleanup;
+        /// Bounded orphan candidates exact-read before reduce. Their source retirements ride this
+        /// fold's runs; their manifest tokens become deletable only after the round CAS adopts them.
+        ManifestSweepResult orphan_sweep;
+        /// Ack-floor one-pass round: the retired-cursor outcome per gc-shard (settled entries, new
+        /// condemnations, floor-passed pendings, and the prior pendings to delete pre-CAS).
+        std::vector<RetiredMergeResult> retired_merge;
+        /// The round's one global ref LIST, grouped per table. Reused post-CAS for
+        /// ref-object cleanup (covered logs / superseded snapshots) so a second LIST is never issued.
+        std::map<String, RefTableListing> ref_tables;
+
+        /// The round's complete immutable post-LIST catalog cut (review C3). Every later consumer in
+        /// the SAME round (`cleanupRefObjects`) must look a namespace up here rather than issuing an
+        /// independent catalog re-read, which can see a DIFFERENT answer if the namespace was dropped
+        /// and recreated between the fold's walk and the later call -- the exact "delete plan computed
+        /// under one life, applied under another" shape C3 named.
+        /// Keeping the full cut also preserves the distinction between an absent namespace and a
+        /// cataloged but non-walkable `Creating` row; reducing it to a `Live`/`Removing` map loses that
+        /// lifecycle fact. A namespace absent from this cut has no legitimate destructive key space;
+        /// never assume the Stage-A sentinel applies.
+        std::optional<CasRefCatalog::Snapshot> catalog_cut;
+
+        /// The round's per-namespace decoded `_ckpt`, read ONCE by the intake walk (its second,
+        /// hint-independent witness) and reused post-CAS by `cleanupRefObjects` for its delete ranges --
+        /// the same DRY reason `ref_tables` is carried here rather than re-listed. A namespace whose
+        /// `_ckpt` is present but UNDECODABLE has no entry, and the walk either HELD it or -- when it
+        /// offered no position to walk from -- RECORDED AN ANOMALY for it. Either way an absent entry
+        /// grants no cleanup authority; the shut destructive gate additionally prevents every other
+        /// namespace from deleting against the incomplete round.
+        std::map<String, RefCkpt> checkpoints;
+        /// THE DESTRUCTIVE GATE for this round, computed ONCE in `fold()` and threaded everywhere so no
+        /// two destructive sites can disagree about it:
+        ///
+        ///     suppress_destructive = any anomaly this round
+        ///                         OR carriedHolds() is non-empty
+        ///                         OR the frontier is incomplete
+        ///
+        /// The second term is STRUCTURAL, not decorative. Every hold the fold seals also records an
+        /// anomaly today, so the first term happens to cover the second — but that is a coincidence of
+        /// the current code, not the invariant. The invariant is the HOLD SET: a hold means some
+        /// namespace's frontier is unproven, and no proof taken elsewhere can license destruction while
+        /// one stands. Reading the seal directly is what keeps a future change to anomaly recording from
+        /// silently opening the gate.
+        ///
+        /// STAGE B'S NARROWING OF THIS GATE TO PER-NAMESPACE MUST CARRY TWO THINGS, NOT ONE. The first
+        /// is the hold-set term just described. The second is the namespace whose `_ckpt` is
+        /// undecodable AND which offers the walk no position to hold at: it mints no hold on purpose (a
+        /// fabricated `offending_position` would become a durable false witness), so its fail-close
+        /// rests on `recordAnomaly` plus `frontier_incomplete` and on nothing else -- the only
+        /// per-namespace failure in this file whose gate rests entirely on that pair. A per-namespace
+        /// gate that carried only the hold set would license destruction against a namespace whose own
+        /// checkpoint could not be read.
+        bool suppress_destructive = false;
+
+        /// Whether EVERY namespace in this round's universe reached a proven frontier — see
+        /// `UniversePolicy` for what the universe is and why the catalog is the only source that can
+        /// bound it. False under `StageA_Suppressed` no matter what the per-namespace probes found.
+        bool frontier_complete = false;
+
+        /// Whether the round's OWN hot-scan catalog cut (`catalog_cut`, the same `GET` the walk plan
+        /// already paid for -- never a second one) is itself the positive proof that the universe is
+        /// empty: a present, token-bearing, successfully decoded catalog with zero rows of ANY
+        /// lifecycle state, `Creating` included. `frontier_namespaces == 0` alone is NOT this proof --
+        /// it is also what a catalog holding only `Creating` rows produces, and a `Creating` row is a
+        /// birth in progress, not an empty universe. This is the second, POSITIVE way the frontier's
+        /// non-vacuity term can be satisfied, alongside `frontier_namespaces > 0`.
+        bool catalog_cut_proved_empty = false;
+
+        /// The universe's size and how much of it this round proved, for the `fold_ref_intake` row: a
+        /// round that suppressed everything owes the reader the two numbers that explain why.
+        uint64_t frontier_namespaces = 0;
+        uint64_t frontier_proven = 0;
+        /// Namespaces the round KNEW about (a cursor in the adopted seal) but did not probe at all,
+        /// because the round's frontier-probe budget ran out first. Each one is an unproven frontier.
+        uint64_t frontier_unprobed_budget = 0;
+
+        /// The reason ONE namespace ended its walk unproven. `Proven` is the absence of a reason; every
+        /// other value names the exit that produced it. `Unattributed` is what an exit that forgets to
+        /// name itself is reported as, so it surfaces as a number instead of disappearing into some
+        /// other bucket. A value is added here only together with the exit that sets it — an enumerator
+        /// no exit can reach makes the deficit claim a cause the code cannot produce.
+        enum class FrontierUnproven : uint8_t
+        {
+            Proven,
+            CheckpointUnusable,
+            CheckpointFrontierEmpty,
+            CommittedBelowCursor,
+            Held,
+            Unattributed,
+        };
+
+        /// WHY the round's unproven namespaces are unproven, one bucket each, so the buckets sum to
+        /// `frontier_namespaces - frontier_proven`. Without it an operator reading "N of M proven" sees
+        /// that the round suppressed but not which of several unrelated causes did it, and the causes
+        /// want opposite responses: a hold is something to chase, an exhausted probe budget is something
+        /// to raise. `unattributed` is expected to be zero on every round; a nonzero value means the
+        /// reason enumeration has stopped being exhaustive over the walk's exits.
+        struct FrontierDeficit
+        {
+            uint64_t checkpoint_unusable = 0;
+            uint64_t checkpoint_frontier_empty = 0;
+            uint64_t committed_below_cursor = 0;
+            uint64_t held = 0;
+            uint64_t probe_budget = 0;
+            uint64_t fold_aborted = 0;
+            uint64_t unattributed = 0;
+
+            void count(FrontierUnproven reason);
+            uint64_t total() const;
+            /// The nonzero buckets, as `name=count` pairs, for the suppression warning.
+            String describe() const;
+        };
+
+        FrontierDeficit frontier_deficit;
+
+        /// Every hold this round SEALED, as `(life id, hold)` — both the ones it detected and the
+        /// ones it carried from the parent seal because their offending position is still unresolved.
+        /// It reads the seal that is about to become durable, so it is the round's final answer rather
+        /// than an intermediate one.
+        ///
+        /// This is the input to the destructive-round suppression rule (`suppress_destructive` =
+        /// current anomalies OR every carried hold): a hold means some namespace's frontier is
+        /// unproven, and no frontier proof taken this round can license destruction while one stands.
+        /// Every hold the fold seals also records an anomaly today, so the two agree; the accessor
+        /// exists because that coincidence is not the invariant — the hold set is.
+        std::vector<std::pair<UInt128, RefHold>> carriedHolds() const
+        {
+            std::vector<std::pair<UInt128, RefHold>> out;
+            for (const auto & [life_id, state] : fold_seal.ref_lives)
+                if (state.coverage.hold)
+                    out.emplace_back(life_id, *state.coverage.hold);
+            return out;
+        }
+    };
+
+    /// Per changed root shard, stream the one ordered
+    /// RootOwnerEvent journal in transition_version order and dispatch each event by comparing
+    /// old_binding.manifest_ref to new_binding.manifest_ref:
+    ///   - EQUAL (an owner move, e.g. a promote Precommit->Committed at the SAME ref) => NO blob delta,
+    ///     NO part-manifest cleanup (the activating PrecommitAdd was folded earlier — see the barrier);
+    ///   - TRUE REMOVAL (old present, the ref not owned afterwards) => read the OLD body, emit -1 per
+    ///     blob entry + queue the body for cleanup (an old precommit never activated emitted no edges);
+    ///   - ACTIVATION (new present) => read the NEW body, emit +1 per blob entry, SUBJECT TO THE FOLD
+    ///     BARRIER: do not advance the durable fold cursor past a `RootOwnerEvent` that
+    ///     leaves a LIVE precommit binding whose manifest body is not present+valid; re-read each round.
+    /// 404 RULE: a body that is PRESENT-but-invalid (ref/namespace mismatch) is genuine corruption =>
+    /// CORRUPTED_DATA (hard). A MISSING body (404) is handled by where it appears: a precommit
+    /// activation new missing body => no edges + barrier holds the cursor; a committed/promote new
+    /// missing body or a true-removal old body missing at removal-fold => fail-closed FOR THAT DECISION
+    /// (clamp the shard's last_folded_ref_id below it, record the anomaly, stop folding THIS shard) —
+    /// never guess a delta and never wedge the round on a missing body.
+    /// On success `state` carries the committed snap_generation and `state_token` the committed gc/state
+    /// token. The committed pair is THREADED into retire, never re-read (zombie-steal protection).
+    /// Round-paced graduation: `current_round` (= state.round + 1, the SAME basis condemn_round is
+    /// stamped at) is the threshold the fold's two-cursor merge graduates/condemns against — an entry
+    /// graduates once `condemn_round < current_round`, i.e. it survived at least one full round after
+    /// being condemned. The fold no longer CASes gc/state — it sets (snap_generation, snap_attempt)
+    /// in-memory; the SINGLE round CAS commits them.
+    /// `walk_plan` owns the round's one enumeration of `cas/ns/stream/` (see `RefScanSummary`) and
+    /// its catalog cut; the fold regroups those keys strictly rather than listing the prefix again.
+    FoldResult fold(GcState & state, Token & state_token, RoundReport & report, uint64_t current_round,
+                    const RefPlan & walk_plan, UniversePolicy policy,
+                    /// One instance for the WHOLE round, owned by `runRegularRound` and threaded through
+                    /// every destructive-work family the round touches — see `GcRoundWorkBudget`.
+                    GcRoundWorkBudget & work_budget);
+
+    /// The round's `_ckpt.checkpoint` witness per namespace — the SECOND, hint-independent witness the
+    /// walk decides its absents against. ONE call site, in the fold, right where the hint is grouped.
+    ///
+    /// The listing cannot be the sole witness source: it is a snapshot, so a record that became durable
+    /// after the enumeration is invisible to that round's probes, and an absent expected-next then reads
+    /// as a frontier when it is really a gap. `_ckpt.checkpoint` is the namespace's own durable tail,
+    /// read by the fold anyway for cleanup ranges, and it decides the same question without asking the
+    /// listing anything — so it is read by EXACT KEY, never gated on `RefTableListing::has_ckpt`.
+    ///
+    /// It takes BOTH of the round's namespace sources because it owes a witness to both: `ref_tables`
+    /// is the hint, and `parent_cursors` is where a carried hold names a namespace the hint may have
+    /// stopped mentioning — precisely the namespace whose witness matters most. An absent `_ckpt`, and a
+    /// present one with no `checkpoint_snapshot_id`, both contribute no entry.
+    struct CheckpointWitnesses
+    {
+        /// namespace -> `_ckpt.checkpoint_snapshot_id`, for the namespaces that published one.
+        std::map<String, RefTxnId> witnesses;
+
+        /// namespace -> `_ckpt.life_epoch`, for the namespaces that published one. Stage B (Task 4-C):
+        /// the walk's ONLY use is seeding `expected` for a namespace that has never folded a cursor AND
+        /// whose listing shows no log at all -- a `Live` catalog namespace the round has not yet touched.
+        /// Without this, such a namespace can never leave `expected == nullopt`, so its `while (expected)`
+        /// loop never runs and `frontier_proven` stays false forever, even though nothing above `{life_epoch,
+        /// 1}` can possibly exist yet. A namespace admitted without a `_ckpt` at all (the test-only
+        /// `casAdmitEntry` bridge, not `completeCreation`'s production path) has no entry here and stays
+        /// correctly unproven -- fail-closed on a genuinely unknown genesis, never a guessed one.
+        std::map<String, uint64_t> life_epochs;
+
+        /// The complete decoded checkpoint for each catalog-admitted Live/Removing life. `REBUILD`
+        /// passes this exact sample together with the same immutable catalog row to read-only recovery;
+        /// it must not re-read either authority object after its plan has been frozen.
+        std::map<String, RefCkpt> recovery_checkpoints;
+
+        /// namespace -> the decode failure's message, for the namespaces whose `_ckpt` is PRESENT and
+        /// UNREADABLE. Separate from an absent entry because the two mean opposite things: an absent
+        /// entry says "this namespace published no checkpoint", which the walk may treat as no witness,
+        /// while an entry here says "this namespace HAS a checkpoint and we cannot read it", which it
+        /// may not — an unread witness is not an absent one. Both consumers of `witnesses` read it per
+        /// namespace, so the damage is confined to the namespace that owns the object: the walk holds it
+        /// (`HoldReason::CheckpointUndecodable`) and folds every other namespace normally.
+        std::map<String, String> undecodable;
+    };
+    CheckpointWitnesses readCheckpointWitnesses(const std::map<String, RefTableListing> & ref_tables,
+                                                const CasRefCatalog::Snapshot & catalog_cut);
+
+    /// What ONE generation's prefix says about itself: whether the generation exists at all, and the
+    /// greatest attempt under it whose key is one `foldSealKey` would have produced.
+    struct GenerationSealProbe
+    {
+        bool generation_exists = false;
+        std::optional<uint64_t> seal_attempt;
+    };
+    GenerationSealProbe probeGenerationForSeal(uint64_t generation);
+
+    /// The newest fold-seal OBJECT in the pool by `(generation, attempt)`, or absent when the pool has
+    /// never sealed a baseline. Used whenever `gc/state` names no adopted baseline: holds live in the
+    /// SEAL, not in the pointer to it, so losing the POINTER must not silently produce a hold-free
+    /// baseline while an unreadable SEAL refuses.
+    ///
+    /// The pool-wide enumeration is a HINT, never the answer: one that omitted the true newest seal
+    /// would hand back an older one and lose every hold detected since — the same hole one layer up.
+    /// Two NARROW single-generation probes above the listing's maximum ask whether it lied, and a seal
+    /// found there THROWS a refusal rather than being adopted; a store that misreports its own
+    /// enumeration during disaster recovery does not get a second guess.
+    ///
+    /// That is DETECTION, not proof, and the implementation says so at the site: the generation half of
+    /// the question is arithmetic (generations are dense in minting), while the attempt half is an
+    /// enumeration within ONE directory, because a seal key's attempt component is a lease sequence
+    /// number with no arithmetic successor to point-read.
+    ///
+    /// THROWS `CORRUPTED_DATA` on that refusal, and on a pool whose enumeration yielded no seal but
+    /// which is not provably new. Returning `nullopt` is the VIRGIN verdict — logged at WARNING with
+    /// its evidence enumerated and counted by `CASGCRebuildVirginByEnumeration`, because it rests on
+    /// enumeration alone and no point read can prove it.
+    std::optional<std::pair<uint64_t, uint64_t>> newestFoldSealRef();
+
+    /// Read ONE part manifest named by `id`, validate it, and append sign*(+1) blob deltas for each
+    /// blob entry to `deltas`. On sign<0 queue (id -> token) into mf_cleanup. Returns whether a body was
+    /// read+validated: false => ABSENT body (404; the caller decides per the 404 rule). A body that is
+    /// PRESENT but fails refMatchesBody / manifestNamespaceMatches throws CORRUPTED_DATA.
+    /// `txn_ordinal` stamps every delta this call pushes with the round-local ordinal of the ref
+    /// transaction that emitted it (probe B2 — see `TxnApplyLedger`).
+    bool foldManifestEdges(const ManifestId & id, int sign, std::vector<BlobDelta> & deltas,
+                           std::map<ManifestId, Token> & mf_cleanup, uint32_t txn_ordinal);
+
+
+
+
+    /// Ref-object cleanup: delete each table's ref logs covered by BOTH the durable fold cursor and a
+    /// checkpoint-named validated recovery triple, and listed snapshots strictly older than that base,
+    /// in batches of <=1000 exact keys. A folded terminal's cleanup evidence carries its tail-covering
+    /// snapshot id directly on the same ref-life row. Runs post-CAS and only on a clamp-free round.
+    ///
+    /// A listed `_snap` never licenses cleanup on its own: only `folded.checkpoints` whose exact
+    /// same-id `_log` and `_snap` validate through `readCheckpointSnapshotBase` do. Logs and snapshots
+    /// are then deletable only strictly BELOW that checkpoint (and logs also at or below the durable
+    /// cursor). A namespace with no checkpoint base, or an invalid triple, is leak-only this pass.
+    void cleanupRefObjects(
+        const FoldResult & folded, const GcLease & adopted_lease, bool suppress_destructive,
+        GcRoundWorkBudget & work_budget);
+
+    /// Emit the sweep's per-pass retention rollup, throttled (see `last_retain_rollup`). Separate from
+    /// the pass itself so the phase row and the log line read the SAME counters rather than two
+    /// independently-derived views of the page.
+    void reportSweepRetention(const ManifestSweepResult & result);
+
+    /// Stage B (spec INV-3): the round's universe -- every namespace life the destructive gate may one
+    /// day owe a frontier proof for -- is catalog-authoritative, ONE `cas/ref_catalog` `GET` replacing
+    /// the pool-wide `LIST(cas/ns/stream/)` this used to run. `Creating` entries are excluded: no publication
+    /// can exist yet (spec §3), so there is nothing here for a discovery path to walk; `Live` and
+    /// `Removing` entries are both returned, each minted via `NamespaceLifeId::fromCatalogEntry` directly
+    /// from the row that is its own authority for both fields -- never from a listed key, which could
+    /// name a DEAD incarnation of the same namespace name. Fresh pool (no catalog entries yet) => empty
+    /// result.
+    ///
+    /// Used by the fold (as the R11-guarded frontier universe) and by `rebuildBaseline`'s
+    /// disaster-recovery scan. The pool-wide ref LIST (`enumerateRefPrefix`/`groupRefKeys`) remains the
+    /// INTRA-namespace hint -- what a namespace this call already named has listed -- never the
+    /// discovery source for WHICH namespaces exist.
+    std::vector<NamespaceLifeId> discoverUniverse();
+
+    /// The two cheap pre-fold GC round-defer signals — both computed from
+    /// state already reachable before the fold's snapshot merge (O(retired)/O(shards), no snapshot
+    /// read), so `runRegularRound` can decide DEFER-vs-FOLD before paying the fold's cost.
+    ///
+    /// True iff a graduation is due this round, read ZERO-I/O from the adopted fold seal's per-shard
+    /// `condemned_summary` (retired-in-snapshot): `∃ shard: pending_total > 0 ||
+    /// oldest_nonpending_condemn_round < current_round`. `snap_generation == 0` (fresh pool) => false.
+    /// This is the load-bearing safety signal: it forces a fold before any
+    /// destructive decision. FAIL-CLOSED: a missing / undecodable seal, or a summary not TOTAL over
+    /// gc_shards, is corrupt GC bookkeeping — returns TRUE (forces a fold so the round's own fail-closed
+    /// path surfaces it); a round must never silently defer on corrupt bookkeeping.
+    bool graduationDue(const GcState & state, uint64_t current_round);
+
+    /// One full enumeration of `cas/ns/stream/`: the raw keys plus a lenient per-life index of the
+    /// Log-kind ids among them. A malformed key lands in `keys` and is not indexed; `groupRefKeys` in
+    /// the fold does the strict validation and the round-abort. This never throws on a malformed key,
+    /// including the one shape `parseRefObjectKey` refuses by name -- see
+    /// `parseRefObjectKeyForEnumeration`.
+    RefScanSummary enumerateRefPrefix();
+
+    /// The round's ONE hint enumeration (`enumerateRefPrefix`), followed by its one fresh catalog cut
+    /// and the validated adopted-parent rows. Its `RoundInput` can only be moved into the authoritative
+    /// plan builder; the resulting `RefPlan` owns all three and is the only value that reaches consumers.
+    RoundInput listRefPrefix(const GcState & state);
+
+    /// (`reclaimDroppedShards` was removed with the snapshot+log ref model: there is no mutable
+    /// per-namespace shard object to tombstone and reclaim; the perpetual janitor owns dead-life bytes.)
+
+    /// Attempt-scoped generation retention. The sole reclaimer — bounded per round and fail-open on a benign
+    /// 404 (never throw during a prune — it would only wedge GC):
+    ///   WHOLESALE generation-retention: every generation at or below the retention floor
+    ///   (`adopted_generation - gc_snapshot_generations_to_keep`) is reclaimed by LISTing its whole
+    ///   `gc/gen/<g>/` prefix and deleting every object — ALL attempts, including the attempt-scoped
+    ///   retired/ and outcomes/ sets AND any deposed-leader debris under a non-adopted attempt. Walks
+    ///   forward from `next.snap_pruned_through`, advancing it over generations fully reclaimed (persisted
+    ///   by the round-commit CAS).
+    /// There is deliberately NO per-round current-generation attempt-sweep (it cost a per-round LIST for a
+    /// rare concurrent-leader collision, the GC-DISCOVERY-LIST-QUADRATIC concern). Deposed-leader
+    /// current-generation debris is bounded space that waits at most `keep` completion-advances for the
+    /// wholesale prune to reclaim it. keep==0 prunes nothing (keep-all forensics mode). `attempt` is the
+    /// adopted attempt (`next.snap_attempt`); it is currently unused (retention keys on generation alone).
+    ///
+    /// `suppress_destructive` is the round's gate: a suppressed round prunes NOTHING and leaves
+    /// `snap_pruned_through` where it was. The cursor must not advance either -- it is a monotone
+    /// high-water mark that the wholesale prune never revisits, so advancing it over a generation this
+    /// round declined to delete would strand that generation's whole prefix permanently.
+    void pruneSupersededGenerations(uint64_t adopted_generation, uint64_t attempt, GcState & next,
+                                    const std::set<uint64_t> & referenced_generations,
+                                    bool suppress_destructive, GcRoundWorkBudget & work_budget);
+
+    /// Read the fold seal for (generation, attempt) (nullopt when absent). Used by resume + parent-cursor reads.
+    std::optional<CasFoldSeal> readFoldSeal(uint64_t generation, uint64_t attempt);
+
+    /// Update the remembered observation (steal protocol step 3/4).
+    void rememberObservation(const GcLease & lease);
+
+    /// Submit one per-hash freshness-meta op (condemn/spare/delete) to the bounded `meta_pool`.
+    /// NEVER throws: `job` is wrapped in its own try/catch (an exception counts into the
+    /// `CASGCMetaWriteAnomaly` profile event + a log line); if scheduling itself fails (e.g. resource
+    /// exhaustion) the op runs inline rather than being silently lost. Callers must capture every value
+    /// `job` touches BY VALUE (never by reference to a loop-local like the fold's `cur_blob`, which
+    /// mutates across iterations while this job may still be queued).
+    void scheduleMetaJob(std::function<void()> job);
+
+    /// Schedule the async per-hash condemn-marker write for a (ref, token) entering or being carried in
+    /// the retired set; when `writeCondemnedMeta` reports durable Condemned evidence, the in-process
+    /// confirmation for the exact (ref, token) is recorded (the graduation gate's fast path). A swallowed
+    /// write records nothing — the entry stays unconfirmed and graduation carries it (triage §3.4).
+    void scheduleCondemnMarkerWrite(const BlobRef & ref, const Token & token,
+                                    uint64_t condemn_round, uint64_t size);
+
+    /// The in-process condemn-marker confirmation registry (see `condemn_markers_confirmed`).
+    void noteCondemnMarkerDurable(const BlobRef & ref, const Token & token);
+    bool condemnMarkerConfirmedInProcess(const BlobRef & ref, const Token & token);
+    void forgetCondemnMarker(const BlobRef & ref, const Token & token);
+
+    PoolPtr store;
+    /// Where `GcPhaseTimer` sends one record per GC phase. Empty unless a `CasGcScheduler` installed one
+    /// for the current round, in which case every phase of that round emits a row.
+    GcPhaseSink phase_sink;
+    UInt128 gc_id{};   /// this leader's identity (random u128, never 0)
+    /// Every round-engine log line goes through this logger (see the constructor's `log_` doc
+    /// comment) so multi-disk processes can attribute GC logs to the disk/srid that produced them.
+    LoggerPtr logger;
+    uint64_t rebuild_edge_budget_override = 0;   /// tests force tiny batches
+    std::function<uint64_t()> now_ms_fn;   /// wall-clock ms; injected (tests), defaults to system_clock
+    /// The heartbeat gate's own observation clock —
+    /// monotonic on this process, injected (tests), defaults to `Pool::bootMs()`. Never compared
+    /// against another node's clock; see `computeHeartbeatFloor`.
+    std::function<uint64_t()> mono_ms_fn;
+    bool trim_enabled = true;     /// TEST SEAM ONLY: production always trims; see setTrimEnabledForTest
+    /// TEST SEAM ONLY: see setPostHotScanCatalogReadHookForTest. Empty in production.
+    std::function<void()> post_hot_scan_catalog_read_hook_for_test;
+    /// Leader-local, in-memory count of consecutive deferred
+    /// rounds since the last FOLD. NOT persisted (a fresh/stolen leader starts at 0 -- conservative:
+    /// it may fold one round sooner than a long-lived leader would, never later). Reset to 0 whenever
+    /// a round folds; incremented on every DEFER. Bounds batching via `gc_fold_max_defer_rounds`.
+    uint64_t rounds_since_last_fold_ = 0;
+
+    /// the contender's observation window (steal protocol)
+    bool has_observation = false;
+    UInt128 last_seen_owner{};
+    uint64_t last_seen_seq = 0;
+    /// Heartbeat pair observed alongside the lease (gates the steal): a steal requires the lease
+    /// tuple AND this (owner, hb_seq) pair to be frozen across a full window. `hb_seq` is compared
+    /// only when the remembered hb owner matches; an owner change counts as movement (alive).
+    UInt128 last_seen_hb_owner{};
+    uint64_t last_seen_hb_seq = 0;
+
+    /// The heartbeat gate's cross-round, per-srid
+    /// write-token observation (`computeHeartbeatFloor`'s `obs` argument). In-memory only — a new
+    /// leader (after a steal, or a process restart) starts empty, which only delays fencing an
+    /// already-dead mount by one extra round (safe: never fences early).
+    MountObservationMap mount_obs;
+
+    /// Throttle for the orphan sweep's retention rollup. The premise retains on any pass whose epochs are
+    /// not closed-and-folded, so an unconditional `LOG_INFO` would repeat the same sentence for as long as
+    /// that lasts and train operators to filter out the channel carrying the answer they need. It is reported when the
+    /// verdict CHANGES (a different top reason class, or a different count), and otherwise re-stated
+    /// every `kRetainRollupRepeatPasses` passes so a newly-arrived operator is never left with silence
+    /// they would have to read as "nothing to report". Leader-owned and in-memory: a fresh leader
+    /// re-states once, which is the harmless direction.
+    static constexpr uint64_t kRetainRollupRepeatPasses = 64;
+    std::optional<std::pair<SweepRetainClass, uint64_t>> last_retain_rollup;
+    uint64_t retain_rollup_passes_since_report = 0;
+
+    /// Bounded pool for the round's per-hash freshness-meta writes (condemn/spare/delete);
+    /// sized from `PoolConfig::gc_meta_pool_size` (constructed in the ctor, after the null/id checks --
+    /// never touches a possibly-null `store` at member-init time). A `unique_ptr` (not a plain member)
+    /// so construction can happen in the ctor body, after validating `store`.
+    std::unique_ptr<ThreadPool> meta_pool;
+
+    /// Cumulative counts of the per-hash freshness-meta jobs this leader handed to `meta_pool` and of
+    /// those that finished. The `meta_pool_wait` phase reports the ROUND's deltas: that phase's work runs
+    /// on other threads, so its `ProfileEvents` delta is empty BY CONSTRUCTION, and these two numbers are
+    /// what distinguish "the queue was deep" from "the endpoint was slow" when read next to its duration.
+    /// Atomic because a pool thread increments `meta_jobs_completed_` while the round thread reads it.
+    std::atomic<uint64_t> meta_jobs_scheduled_{0};
+    std::atomic<uint64_t> meta_jobs_completed_{0};
+
+    /// In-process confirmations of durable condemn-marker writes, keyed (blob, exact incarnation-token
+    /// value): inserted by `scheduleCondemnMarkerWrite`'s completion (and the rebuild's synchronous
+    /// marker publish) when `writeCondemnedMeta` reports durable Condemned evidence; consulted by the
+    /// graduation gate (the delete-authorizing edge, triage §3.4); pruned when the entry settles
+    /// (redelete / spare / supersede). In-memory only — after a restart or leader change the graduation
+    /// gate falls back to a synchronous `loadMeta` re-check (the marker itself is durable). Guarded by
+    /// `condemn_marker_mutex`: meta-pool completions insert concurrently with the fold thread's reads.
+    std::mutex condemn_marker_mutex;
+    std::set<std::pair<BlobRef, String>> condemn_markers_confirmed;
+
+    /// Probe B1's two numbers for the round: the ref-log POSITIONS the sealed coverage declares covered
+    /// (counted arithmetically over each namespace's cut -- not by listed ids, which under arithmetic
+    /// intake say nothing about what was applied), and the ref logs that actually folded. They are EQUAL
+    /// on every committed round (the fold throws otherwise) -- carrying them is what turns "they are
+    /// always equal" from an assumption into an observable property once the per-phase GC row emitter
+    /// reports them on the `fold_ref_intake` row.
+    /// Both stay 0 on a ref-folding abort, where the identity does not apply.
+    uint64_t logs_accounted_this_round = 0;
+    uint64_t logs_applied_this_round = 0;
+
+    /// Probe B2's verdict for the round: ref transactions the round folded and merged but whose blob
+    /// deltas never reached a shard reducer. 0 on every COMMITTED round -- a nonzero value is
+    /// accompanied by the fold's fail-closed throw, so the value only ever exists as the forensic
+    /// record of a round that then failed.
+    uint64_t transactions_unapplied_this_round = 0;
+
+public:
+    /// TEST SEAM: expose catalog-based universe discovery so unit tests can assert it against a catalog
+    /// they construct, without driving a full round.
+    std::vector<NamespaceLifeId> discoverUniverseForTest()
+    {
+        return discoverUniverse();
+    }
+
+    /// TEST SEAM: expose the two cheap pre-fold GC round-defer signals so unit tests can
+    /// assert them directly without driving a full round.
+    bool graduationDueForTest(const GcState & state, uint64_t current_round)
+    {
+        return graduationDue(state, current_round);
+    }
+
+    /// Test-only access to the round's ref-prefix enumeration (its `changed_shards` is the defer signal).
+    RefScanSummary listRefPrefixForTest(const GcState & state)
+    {
+        const RefPlan plan = buildRefWalkPlan(listRefPrefix(state));
+        RefScanSummary scan = plan.refScan();
+        scan.changed_shards = plan.changedRows();
+        return scan;
+    }
+
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.cpp
new file mode 100644
index 000000000000..19ff16bb956c
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.cpp
@@ -0,0 +1,40 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h>
+#include <Common/Exception.h>
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+
+namespace DB::Cas
+{
+
+GcMaintenanceReadResult readGcMaintenanceState(Backend & backend, const Layout & layout)
+{
+    const auto got = backend.get(layout.gcMaintenanceStateKey());
+    if (!got)
+        return {.status = GcMaintenanceReadStatus::Absent, .state = std::nullopt, .token = std::nullopt, .diagnostic = {}};
+    try
+    {
+        return {.status = GcMaintenanceReadStatus::Valid, .state = decodeGcMaintenanceState(got->bytes),
+            .token = got->token, .diagnostic = {}};
+    }
+    catch (const DB::Exception & e)
+    {
+        if (e.code() != ErrorCodes::CORRUPTED_DATA)
+            throw;
+        return {.status = GcMaintenanceReadStatus::Corrupt, .state = std::nullopt,
+            .token = got->token, .diagnostic = e.message()};
+    }
+}
+
+GcMaintenanceCasResult casGcMaintenanceState(
+    Backend & backend, const Layout & layout, const std::optional<Token> & expected, const GcMaintenanceState & next)
+{
+    const CasResult result = backend.casPut(layout.gcMaintenanceStateKey(), encodeGcMaintenanceState(next), expected);
+    if (result.outcome == CasOutcome::Committed)
+        return {.outcome = GcMaintenanceCasOutcome::Committed, .token = result.token};
+    return {.outcome = GcMaintenanceCasOutcome::Conflict, .token = {}};
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h
new file mode 100644
index 000000000000..6d941177d19f
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h
@@ -0,0 +1,29 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <optional>
+
+namespace DB::Cas
+{
+
+enum class GcMaintenanceReadStatus : uint8_t { Absent, Valid, Corrupt };
+struct GcMaintenanceReadResult
+{
+    GcMaintenanceReadStatus status;
+    std::optional<GcMaintenanceState> state;
+    std::optional<Token> token;
+    String diagnostic;
+};
+enum class GcMaintenanceCasOutcome : uint8_t { Committed, Conflict };
+struct GcMaintenanceCasResult
+{
+    GcMaintenanceCasOutcome outcome = GcMaintenanceCasOutcome::Conflict;
+    Token token;
+};
+
+GcMaintenanceReadResult readGcMaintenanceState(Backend & backend, const Layout & layout);
+GcMaintenanceCasResult casGcMaintenanceState(
+    Backend & backend, const Layout & layout, const std::optional<Token> & expected, const GcMaintenanceState & next);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcPhaseTimer.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcPhaseTimer.h
new file mode 100644
index 000000000000..c0201e5fb651
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcPhaseTimer.h
@@ -0,0 +1,86 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Common/CurrentThread.h>
+#include <Common/ProfileEvents.h>
+#include <Common/Stopwatch.h>
+#include <base/types.h>
+#include <map>
+#include <utility>
+
+namespace DB::Cas
+{
+
+/// Times ONE GC phase and emits a `GcPhaseRecord` through the round's sink on destruction.
+///
+/// The `ProfileEvents` delta is a plain snapshot DIFFERENCE of whatever counters container is currently
+/// attached to this thread. It deliberately does NOT use a nested `ProfileEventsScope`: that RE-PARENTS
+/// the thread's counters, and the round-level scope `CasGcScheduler::runRoundLogged` installs is already
+/// holding that slot. A snapshot diff composes with the outer scope instead of fighting it, and degrades
+/// to an empty map on a thread with no `ThreadStatus` (a bare gtest thread) -- the same degradation the
+/// round-level capture already accepts and documents.
+///
+/// WHAT THE DELTA DOES NOT COVER: work this phase hands to another thread. `meta_pool_wait` is the one
+/// phase where that is the whole content of the phase, so its row carries explicit `jobs_scheduled` /
+/// `jobs_completed` metrics instead; see its instrumentation site in `Gc::runRegularRound`. Anywhere
+/// else, a phase that shows a long duration and an empty delta means the time went somewhere this
+/// instrumentation cannot see, and that is a finding rather than a blank to be ignored.
+///
+/// RAII AND FAILURE: the record is emitted from the destructor, so a phase that THREW still produces its
+/// row while the stack unwinds. That is deliberate -- a round that failed is the round a reader most
+/// needs, and it is why the correlator is `round_id` (which always exists) and not the round number
+/// (which a failed round never obtains).
+///
+/// Cost per phase: one `Stopwatch` and two counters snapshots, against phases that each perform network
+/// I/O. Always on; no setting, deliberately -- a knob whose default nobody remembers is how
+/// instrumentation degrades to silence.
+class GcPhaseTimer
+{
+public:
+    /// `sink_` is `Gc::phase_sink`, which outlives every timer of the round (the scheduler clears it only
+    /// after `runRegularRound` returns). `phase_` must be a string literal.
+    GcPhaseTimer(const GcPhaseSink & sink_, const char * phase_)
+        : sink(sink_), phase(phase_), attached(CurrentThread::isInitialized())
+    {
+        if (attached)
+            before = CurrentThread::getProfileEvents().getPartiallyAtomicSnapshot();
+    }
+
+    GcPhaseTimer(const GcPhaseTimer &) = delete;
+    GcPhaseTimer & operator=(const GcPhaseTimer &) = delete;
+
+    /// Record one phase-specific count. Overwrites a previous value for the same key.
+    void metric(const String & key, UInt64 value) { metrics[key] = value; }
+
+    ~GcPhaseTimer()
+    {
+        if (!sink)
+            return;
+        GcPhaseRecord rec;
+        rec.phase = phase;
+        rec.duration_us = watch.elapsedMicroseconds();
+        rec.metrics = std::move(metrics);
+        if (attached)
+        {
+            const auto after = CurrentThread::getProfileEvents().getPartiallyAtomicSnapshot();
+            for (ProfileEvents::Event e = ProfileEvents::Event(0); e < ProfileEvents::Counters::num_counters; ++e)
+            {
+                const auto delta = after[e] - before[e];
+                if (delta != 0)
+                    rec.profile_events.emplace(String(ProfileEvents::getName(e)), static_cast<UInt64>(delta));
+            }
+        }
+        /// Best-effort, exactly like the round-row sink: instrumentation must never break GC, and this
+        /// runs in a destructor that may already be unwinding a round exception.
+        try { sink(rec); } catch (...) {}   // NOLINT(bugprone-empty-catch)
+    }
+
+private:
+    const GcPhaseSink & sink;
+    const char * phase;
+    bool attached;
+    Stopwatch watch{CLOCK_MONOTONIC};
+    ProfileEvents::Counters::Snapshot before;
+    std::map<String, UInt64> metrics;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.cpp
new file mode 100644
index 000000000000..f265444576a8
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.cpp
@@ -0,0 +1,419 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/CurrentThread.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <Common/ProfileEventsScope.h>
+#include <Common/logger_useful.h>
+#include <Common/setThreadName.h>
+#include <Common/thread_local_rng.h>
+#include <base/scope_guard.h>
+#include <algorithm>
+#include <optional>
+
+namespace DB::Cas
+{
+
+namespace
+{
+    /// Non-zero events of a per-round snapshot, keyed by event name. The snapshot is already a
+    /// delta when produced by a ProfileEventsScope (its counters start at zero on this thread).
+    std::map<String, UInt64> snapshotToMap(const ProfileEvents::Counters::Snapshot & snap)
+    {
+        std::map<String, UInt64> out;
+        for (ProfileEvents::Event e = ProfileEvents::Event(0); e < ProfileEvents::Counters::num_counters; ++e)
+        {
+            const auto value = snap[e];
+            if (value != 0)
+                out.emplace(String(ProfileEvents::getName(e)), static_cast<UInt64>(value));
+        }
+        return out;
+    }
+}
+
+CasGcScheduler::CasGcScheduler(
+    Cas::PoolPtr store_,
+    std::chrono::seconds interval_,
+    const String & log_name,
+    String disk_name_,
+    GcRoundLogger logger_)
+    : store(std::move(store_))
+    , interval(interval_)
+    /// Keep the advisory pulse comfortably inside the follower's observation window. The lower
+    /// bound prevents an unusually small configured interval from creating an excessively busy
+    /// heartbeat worker.
+    , hb_interval(std::max<std::chrono::milliseconds>(
+          std::chrono::milliseconds(50),
+          std::chrono::duration_cast<std::chrono::milliseconds>(interval_) / 4))
+    , log(getLogger(log_name))
+    /// gc_id uniqueness across instances is the Gc caller obligation - a random u128 per scheduler.
+    , gc_id((static_cast<UInt128>(thread_local_rng()) << 64) | thread_local_rng())
+    , disk_name(std::move(disk_name_))
+    , logger(std::move(logger_))
+    /// Thread this scheduler's own disk/srid-scoped logger (built from `log_name` above) into the
+    /// round engine, so `Gc`'s log lines carry the same scope as the round-outcome log records.
+    , gc(store, gc_id, {}, {}, log)
+{
+}
+
+CasGcScheduler::~CasGcScheduler()
+{
+    stop();
+}
+
+void CasGcScheduler::start()
+{
+    std::lock_guard lock(mutex);
+    if (thread.joinable())
+        return;
+    stopping = false;
+    thread = ThreadFromGlobalPool([this] { loop(); });
+    hb_thread = ThreadFromGlobalPool([this] { heartbeatLoop(); });
+}
+
+void CasGcScheduler::stop()
+{
+    {
+        std::lock_guard lock(mutex);
+        stopping = true;
+    }
+    wake.notify_all();
+    if (thread.joinable())
+        thread.join();
+    if (hb_thread.joinable())
+        hb_thread.join();
+    /// Clear the in-process leadership hint AFTER both workers are joined, so it is final: no round can set
+    /// it again (a round in flight is joined above and clears `round_in_flight` on the way out; clearing
+    /// `i_am_leader` before the join would race a completing round that re-sets it). The durable `gc/state`
+    /// lease stays authoritative and untouched here -- a restarted scheduler re-enters leadership through the
+    /// next round's normal acquisition, never by inheriting a stale hint. This is what lets `SYSTEM CONTENT
+    /// ADDRESSED GC STOP` keep the scheduler object alive (restartable) yet report "no longer leading".
+    i_am_leader.store(false, std::memory_order_relaxed);
+}
+
+void CasGcScheduler::requestRoundSoon()
+{
+    {
+        std::lock_guard lock(mutex);
+        if (stopping || !thread.joinable())
+            return;
+        round_requested = true;
+    }
+    wake.notify_all();
+}
+
+void CasGcScheduler::onLeaseAcquired()
+{
+    i_am_leader.store(true, std::memory_order_relaxed);
+    try
+    {
+        Cas::Gc::pulseHeartbeat(*store, gc_id);
+    }
+    catch (...)
+    {
+        /// Advisory, same as heartbeatLoop's own pulse: a lost pulse is harmless, the next
+        /// one (from heartbeatLoop, hb_interval later) retries. Must never fail the round.
+        tryLogCurrentException(log, "CA GC acquire-time heartbeat pulse failed (advisory; will retry)");
+    }
+}
+
+Cas::RoundReport CasGcScheduler::runRoundLogged(Cas::Gc & round_gc, GcRoundLogRecord::Trigger trigger,
+                                                 std::function<void()> on_lease_acquired, bool allow_steal)
+{
+    using Rec = GcRoundLogRecord;
+
+    /// Mark a round in flight for the whole body (success AND exception paths). `isQuiescent` reads this;
+    /// the FORGET / `GC STOP` tests use it to prove the scheduler's workers were joined (no round can be
+    /// mid-flight once `stop()` returned). See `isQuiescent`.
+    round_in_flight.store(true, std::memory_order_release);
+    SCOPE_EXIT({ round_in_flight.store(false, std::memory_order_release); });
+
+    /// Best-effort: the table row must never break GC. A throwing sink is swallowed.
+    auto emit = [&](const Rec & r)
+    {
+        if (logger)
+        {
+            try
+            {
+                logger(r);
+            }
+            catch (...) // NOLINT(bugprone-empty-catch)
+            {
+            }
+        }
+    };
+
+    /// The correlator every row of this round attempt carries -- see `GcRoundLogRecord::round_id` for why
+    /// it is a fresh id rather than the round number.
+    const String round_id = Cas::u128ToHex(
+        (static_cast<UInt128>(thread_local_rng()) << 64) | thread_local_rng());
+
+    Rec start;
+    start.event_type = Rec::EventType::Start;
+    start.trigger = trigger;
+    start.disk_name = disk_name;
+    start.srid = store->poolConfig().server_root_id;
+    start.gc_id = Cas::u128ToHex(gc_id);
+    start.round_id = round_id;
+    emit(start);
+
+    /// The phase sink handed to the round engine for the duration of THIS round. Same best-effort
+    /// discipline as `emit`: a throwing sink must never break GC. Cleared on the way out (on the
+    /// exception path too), so it never outlives the locals it captures -- this guard is declared after
+    /// them, so it is destroyed before them.
+    round_gc.setPhaseSink([&](const Cas::GcPhaseRecord & p)
+    {
+        Rec row = start;
+        row.event_type = Rec::EventType::Phase;
+        row.phase = p.phase;
+        row.phase_duration_microseconds = p.duration_us;
+        row.phase_metrics = p.metrics;
+        row.profile_events = p.profile_events;
+        emit(row);
+    });
+    SCOPE_EXIT({ round_gc.setPhaseSink({}); });
+
+    const auto t0 = std::chrono::steady_clock::now();
+    /// ProfileEventsScope requires an attached ThreadStatus (CurrentThread::get throws otherwise).
+    /// On the server it always is: the Scheduled path runs on a ThreadFromGlobalPool, the Manual
+    /// path on the query thread. In unit tests calling runOneRoundNow on a bare gtest thread there
+    /// is none — skip the per-round delta there rather than fail the round.
+    std::optional<ProfileEventsScope> profile_scope;
+    if (CurrentThread::isInitialized())
+        profile_scope.emplace();
+
+    auto collect_profile_events = [&]() -> std::map<String, UInt64>
+    {
+        if (!profile_scope)
+            return {};
+        return snapshotToMap(*profile_scope->getSnapshot());
+    };
+
+    Rec fin = start;
+    fin.event_type = Rec::EventType::Finish;
+    try
+    {
+        const Cas::RoundReport rep = round_gc.runRegularRound(std::move(on_lease_acquired), allow_steal);
+        if (rep.acquired_lease)
+        {
+            /// Keep health state per scheduler. Process-global gauges cannot distinguish multiple
+            /// content-addressed disks in one server process.
+            pending_reclaim.fetch_add(
+                static_cast<Int64>(rep.condemned) - static_cast<Int64>(rep.redeleted),
+                std::memory_order_relaxed);
+            last_success_ms.store(
+                static_cast<UInt64>(std::chrono::duration_cast<std::chrono::milliseconds>(
+                    std::chrono::steady_clock::now().time_since_epoch()).count()),
+                std::memory_order_relaxed);
+        }
+        fin.outcome = !rep.acquired_lease ? Rec::Outcome::NotALeader
+                    : rep.deferred        ? Rec::Outcome::Deferred
+                                           : Rec::Outcome::Success;
+        fin.round = rep.round;
+        fin.candidates_marked = rep.candidates;
+        fin.objects_deleted = rep.deleted;
+        fin.objects_absent = rep.absent;
+        fin.objects_replaced = rep.replaced;
+        fin.objects_spared = rep.spared;
+        fin.manifests_deleted = rep.manifests_deleted;
+        fin.entries_condemned = rep.condemned;
+        fin.entries_graduated = rep.graduated;
+        fin.entries_redeleted = rep.redeleted;
+        fin.fence_outs = rep.fence_outs;
+        fin.anomalies = rep.anomalies.size();
+        fin.duration_ms = std::chrono::duration_cast<std::chrono::milliseconds>(
+            std::chrono::steady_clock::now() - t0).count();
+        fin.profile_events = collect_profile_events();
+        emit(fin);
+        return rep;
+    }
+    catch (...)
+    {
+        fin.outcome = Rec::Outcome::Failed;
+        fin.error = getCurrentExceptionMessage(false);
+        fin.duration_ms = std::chrono::duration_cast<std::chrono::milliseconds>(
+            std::chrono::steady_clock::now() - t0).count();
+        fin.profile_events = collect_profile_events();
+        emit(fin);
+        throw;
+    }
+}
+
+Cas::RoundReport CasGcScheduler::runOneRoundNow(GcRoundLogRecord::Trigger trigger)
+{
+    std::lock_guard round_lock(gc_round_mutex);
+    /// allow_steal=false: a manual round may acquire a FREE lease or renew ITS OWN, but must never
+    /// steal a live incumbent — see Cas::Gc::runRegularRound's doc comment. Dead-incumbent recovery
+    /// stays the loop's job (bounded ~2*interval; loop() below passes the default allow_steal=true).
+    const Cas::RoundReport report = runRoundLogged(gc, trigger, [this] { onLeaseAcquired(); }, /*allow_steal=*/false);
+    i_am_leader.store(report.acquired_lease, std::memory_order_relaxed);
+    return report;
+}
+
+void CasGcScheduler::loop()
+{
+    setThreadName(ThreadName::CAS_GC_SCHEDULER);
+    size_t consecutive_backoffs = 0;
+    while (true)
+    {
+        {
+            std::unique_lock lock(mutex);
+            wake.wait_for(lock, interval, [this] { return stopping || round_requested; });
+            if (stopping)
+                return;
+            round_requested = false;
+        }
+        /// rev.7 §3 [C1] + rev.8 §9 item 8: self-exit the pacing loop the moment the pool reaches — or is
+        /// being driven toward — ANY terminal state. A NATURAL terminal transition (`VanishedReplaced` after
+        /// a foreign pool took the prefix, or `IdentityLost` once the sentinels are gone) never calls
+        /// `stop()` on this scheduler: only `~Pool`/FORGET join it. Without this check the loop would tick
+        /// FOREVER — `acquireOrRenewLease` throws `CORRUPTED_DATA` against the vanished `gc/state` every
+        /// interval (the G2 zombie: an error-log line + a Failed round row each tick), and worse, after
+        /// `VanishedReplaced` the `allow_steal=true` rounds could STEAL the FOREIGN pool's `gc/state` lease
+        /// and fold/condemn/delete its objects. We also exit on a published FORGET intent
+        /// (`vanishedIntentPublished`, still pre-terminal) — earliest-signal discipline — and on `IdentityLost`
+        /// (rev.8: a fail-loud terminal state; the last G2-zombie case — eternal `CORRUPTED_DATA` retries
+        /// against a half-erased pool — closes with it). Clearing `i_am_leader` before returning keeps
+        /// `gcHealth` honest (a terminal, self-exited scheduler reports it no longer leads). The thread exits
+        /// its OWN loop here — no join from this context (C6-safe); `stop()`/`~CasGcScheduler` still join the
+        /// finished thread cleanly.
+        if (store->isVanished() || store->vanishedIntentPublished()
+            || store->lifecycle() == Cas::PoolLifecycle::IdentityLost)
+        {
+            i_am_leader.store(false, std::memory_order_relaxed);
+            {
+                std::lock_guard exit_lock(terminal_exit_mutex);
+                loop_exited_on_terminal_for_test.store(true, std::memory_order_release);
+            }
+            terminal_exit_cv.notify_all();
+            return;
+        }
+        try
+        {
+            /// LOW/benign: if stop() flips `stopping` while we're blocked here (a concurrent manual
+            /// round holds gc_round_mutex), we still run one more Scheduled round once it unblocks,
+            /// before the next wait_for() observes `stopping` - an accepted extra round, not a
+            /// correctness issue.
+            std::lock_guard round_lock(gc_round_mutex);
+
+            /// runRoundLogged emits the Start + Finish table rows (incl. the per-round
+            /// ProfileEvents delta) and rethrows on a round exception (after an Aborted Finish).
+            /// on_lease_acquired (onLeaseAcquired, shared with runOneRoundNow) fires the instant the
+            /// lease is (re)acquired, before the fold runs - a new leader's first round is otherwise
+            /// unprotected (i_am_leader would only flip below, AFTER the whole round returns), so a
+            /// follower observing the frozen (owner, seq) across two of its own ticks would steal
+            /// deterministically once that first round outlasts them. allow_steal defaults to true here
+            /// (the loop is the ONLY caller allowed to execute the steal CAS).
+            const Cas::RoundReport report = runRoundLogged(gc, GcRoundLogRecord::Trigger::Scheduled, [this] { onLeaseAcquired(); });
+            i_am_leader.store(report.acquired_lease, std::memory_order_relaxed);
+            if (report.acquired_lease)
+            {
+                consecutive_backoffs = 0;
+                /// A deferred round never folds -- every counter below stays zero by construction
+                /// (RoundReport's own doc comment), so printing them through the SAME line as a folding
+                /// round reads as "GC is dead / stuck", indistinguishable from a round that genuinely
+                /// folded and found nothing. Keep the deferred case on its own, clearly-worded line
+                /// instead of adding a boolean to the counters line.
+                if (report.deferred)
+                    LOG_DEBUG(log, "CA GC round {}: deferred (skip-unchanged; no changed shard reached the "
+                        "fold threshold and no graduation was due; sealed generation re-adopted, no fold ran)",
+                        report.round);
+                else
+                    LOG_DEBUG(log, "CA GC round {}: candidates={} deleted={} absent={} replaced={} spared={} manifests_deleted={}",
+                        report.round, report.candidates, report.deleted, report.absent,
+                        report.replaced, report.spared, report.manifests_deleted);
+            }
+            else
+            {
+                /// NEVER silent: a follower backing off is the normal multi-mounter state, but a
+                /// pool where this scheduler never leads must be observable. The lease layer
+                /// handles safety, while these messages expose a liveness problem such as every
+                /// round using a new scheduler identity and never accumulating the observations
+                /// needed for dead-incumbent recovery.
+                ++consecutive_backoffs;
+                if (consecutive_backoffs % 10 == 0)
+                    LOG_INFO(log, "CA GC: lease held by another mounter for {} consecutive ticks "
+                        "(normal for a follower; investigate if no mounter is reclaiming)", consecutive_backoffs);
+                else
+                    LOG_TRACE(log, "CA GC: lease held by another mounter (tick {})", consecutive_backoffs);
+            }
+        }
+        catch (...)
+        {
+            /// Idempotent round - the next tick retries; failures must never kill the pacing thread.
+            /// runRoundLogged already emitted the Aborted Finish row before rethrowing.
+            i_am_leader.store(false, std::memory_order_relaxed);
+            tryLogCurrentException(log, "CA GC round failed (will retry next tick)");
+        }
+    }
+}
+
+void CasGcScheduler::heartbeatLoop()
+{
+    setThreadName(ThreadName::CAS_GC_HEARTBEAT);
+    /// Advance the advisory heartbeat independently of round progress. A long round updates the
+    /// durable lease only when it completes, so without these pulses a follower could mistake a
+    /// live leader for a dead one during the observation window. A missed pulse is harmless because
+    /// the heartbeat is advisory and the next cadence retries it.
+    while (true)
+    {
+        {
+            std::unique_lock lock(mutex);
+            if (wake.wait_for(lock, hb_interval, [this] { return stopping; }))
+                return;
+        }
+        /// rev.7 §3 [C1] + rev.8 §9 item 8: self-exit on ANY terminal (or FORGET-intent) pool, same as
+        /// `loop()`. A terminal pool's advisory pulses would target a deleted `gc/hb` key (`IdentityLost`) or
+        /// a FOREIGN pool's key (`VanishedReplaced`) — stop pulsing the moment the pool goes terminal.
+        if (store->isVanished() || store->vanishedIntentPublished()
+            || store->lifecycle() == Cas::PoolLifecycle::IdentityLost)
+        {
+            {
+                std::lock_guard exit_lock(terminal_exit_mutex);
+                hb_exited_on_terminal_for_test.store(true, std::memory_order_release);
+            }
+            terminal_exit_cv.notify_all();
+            return;
+        }
+        if (!i_am_leader.load(std::memory_order_relaxed))
+            continue;
+        try
+        {
+            Cas::Gc::pulseHeartbeat(*store, gc_id);
+        }
+        catch (...)
+        {
+            tryLogCurrentException(log, "CA GC heartbeat pulse failed (advisory; will retry)");
+        }
+    }
+}
+
+CasGcScheduler::GcHealth CasGcScheduler::gcHealth() const
+{
+    GcHealth h;
+    h.is_leader = i_am_leader.load(std::memory_order_relaxed);
+    h.pending_reclaim = pending_reclaim.load(std::memory_order_relaxed);
+    const UInt64 last_ms = last_success_ms.load(std::memory_order_relaxed);
+    h.ever_succeeded = last_ms != 0;
+    if (last_ms != 0)
+    {
+        const UInt64 now_ms = static_cast<UInt64>(std::chrono::duration_cast<std::chrono::milliseconds>(
+            std::chrono::steady_clock::now().time_since_epoch()).count());
+        h.last_success_age_seconds = now_ms > last_ms ? (now_ms - last_ms) / 1000 : 0;
+    }
+    h.wedged_namespace_count = store->wedgedRefLaneCount();
+    return h;
+}
+
+bool CasGcScheduler::waitForTerminalSelfExitForTest(std::chrono::milliseconds timeout)
+{
+    std::unique_lock lock(terminal_exit_mutex);
+    return terminal_exit_cv.wait_for(lock, timeout, [this]
+    {
+        return loop_exited_on_terminal_for_test.load(std::memory_order_acquire)
+            && hb_exited_on_terminal_for_test.load(std::memory_order_acquire);
+    });
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h
new file mode 100644
index 000000000000..2cce48399b66
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h
@@ -0,0 +1,227 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Common/ThreadPool.h>
+#include <base/types.h>
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <functional>
+#include <map>
+#include <mutex>
+
+namespace DB::Cas
+{
+
+/// The decoupled, pure-data record the Disks-layer GC scheduler emits per round. It carries NO
+/// Interpreters dependency: the metadata storage converts it into a
+/// ContentAddressedGarbageCollectionLogElement and forwards it to the SystemLog. Keeping it a plain
+/// POD lets the scheduler (and its unit tests) stay free of the system-log machinery.
+struct GcRoundLogRecord
+{
+    /// `Phase`: one row per GC phase of the round, carrying that phase's own duration, semantic counts,
+    /// and `ProfileEvents` delta. Emitted between the round's `Start` and `Finish`, correlated with them
+    /// by `round_id`.
+    enum class EventType { Start, Finish, Phase };
+    /// `Deferred`: the round acquired the lease and took the skip-unchanged fast path (`RoundReport::deferred`)
+    /// -- no fold, no pre-CAS deletes, no `gc/state` CAS. Distinct from `Success` so a reader of
+    /// `system.cas_gc_log` (or this scheduler's own log line) can tell a round
+    /// that genuinely folded and found nothing apart from one that never folded at all.
+    enum class Outcome { Unknown, Success, NotALeader, Failed, Deferred };
+    enum class Trigger { Scheduled, Manual };
+
+    EventType event_type = EventType::Start;
+    Outcome outcome = Outcome::Unknown;   /// Unknown until a round finishes
+    Trigger trigger = Trigger::Scheduled;
+    String disk_name;
+    String srid;          /// server_root_id of the mount that ran this round (this pool's poolConfig())
+    String gc_id;        /// hex of the scheduler's gc_id
+    UInt64 round = 0;
+    UInt64 candidates_marked = 0;
+    UInt64 objects_deleted = 0;
+    UInt64 objects_absent = 0;
+    UInt64 objects_replaced = 0;
+    UInt64 objects_spared = 0;
+    UInt64 manifests_deleted = 0;   /// owner-removed manifest bodies deleted, distinct from blob deletes
+    /// Counts for the three-stage deletion pipeline reported by `Cas::RoundReport`.
+    UInt64 entries_condemned = 0;   /// entries newly condemned into the retired list this round
+    UInt64 entries_graduated = 0;   /// entries newly floor-passed (published delete_pending) this round
+    UInt64 entries_redeleted = 0;   /// pending exact-token blob deletes executed this round
+    UInt64 fence_outs = 0;          /// expired mounts fenced-out by the round's heartbeat floor
+    UInt64 anomalies = 0;           /// fold clamps surfaced (never wedging) this round
+    UInt64 duration_ms = 0;
+    String error;
+    /// On a `Start`/`Finish` row: the whole round's delta. On a `Phase` row: THAT PHASE's delta.
+    std::map<String, UInt64> profile_events;
+
+    /// Correlator for every row of ONE round attempt: a fresh random hex id minted per `runRoundLogged`
+    /// invocation and stamped on the `Start` row, every `Phase` row, and the `Finish` row.
+    ///
+    /// DELIBERATELY NOT `round`. `round` is 0 on `Start`, is only known after the round's single
+    /// `gc/state` CAS on a folding round, and does not exist AT ALL on a `NotALeader` round -- and the
+    /// rounds a reader most needs to reconstruct are exactly the ones that never got that far.
+    String round_id;
+    /// The GC phase this row describes; empty on `Start`/`Finish`. The literals are the ones passed to
+    /// `Cas::GcPhaseTimer` at each phase's single instrumentation site.
+    String phase;
+    /// Wall time of this phase. MICROseconds, not milliseconds: `meta_pool_wait`, `round_commit` and
+    /// `parent_seal_read` are routinely sub-millisecond and the whole point of the row is seeing when
+    /// they are not.
+    UInt64 phase_duration_microseconds = 0;
+    /// Phase-specific semantic counts (`Phase` rows only). The per-phase verb counts ride
+    /// `profile_events` above instead, so no verb columns are invented.
+    std::map<String, UInt64> phase_metrics;
+};
+
+using GcRoundLogger = std::function<void(const GcRoundLogRecord &)>;
+
+/// Paces regular content-addressed garbage-collection rounds for one pool. The scheduler does not
+/// implement the GC protocol: `Cas::Gc` owns lease acquisition, work deduplication, and the
+/// split-brain-safe round operations, so schedulers on different mounters may run independently.
+/// This class waits for a tick, runs one round, records its result, and retries after exceptions;
+/// round operations are idempotent, while `LOGICAL_ERROR` exceptions remain visible in the log.
+///
+/// The background and heartbeat workers are `ThreadFromGlobalPool` instances. The background worker
+/// therefore has the attached `ThreadStatus` required by `ProfileEventsScope` for per-round
+/// `ProfileEvents` deltas. A manual round runs on the caller's thread; if that thread has no
+/// `ThreadStatus`, the per-round delta is omitted rather than making the GC round fail.
+class CasGcScheduler
+{
+public:
+    CasGcScheduler(
+        Cas::PoolPtr store_,
+        std::chrono::seconds interval_,
+        const String & log_name,
+        String disk_name_,
+        GcRoundLogger logger_ = {});
+    ~CasGcScheduler();
+
+    /// Starts the periodic round and heartbeat workers. Calling `start` more than once while the
+    /// scheduler is running is a no-op; after `stop`, it may be started again on the SAME instance --
+    /// the persistent `gc` observer and `gc_id` are preserved, so the lease's observation-window
+    /// protocol continues across a stop/start (`SYSTEM CAS GC START`).
+    void start();
+
+    /// Stops both workers, wakes them if they are waiting, and joins them before returning. It is
+    /// safe to call `stop` when the scheduler is not running and from the destructor. It also clears
+    /// the in-process `i_am_leader` hint (after the joins), so a stopped scheduler reports it no longer
+    /// leads GC; the durable `gc/state` lease is untouched (`SYSTEM CAS GC STOP`).
+    void stop();
+
+    /// Wake the periodic worker so it starts the next ordinary round promptly. This does not create a
+    /// second execution path: the same worker, lease protocol, and `gc_round_mutex` remain the sole
+    /// scheduled-round authority. Requests coalesce into one boolean while a round is pending.
+    void requestRoundSoon();
+
+    /// Test/diagnostics hook: run ONE round synchronously on the caller's thread. Returns the round
+    /// report so the SYSTEM command / tests can inspect it. Emits a Start + Finish record.
+    Cas::RoundReport runOneRoundNow(GcRoundLogRecord::Trigger trigger = GcRoundLogRecord::Trigger::Manual);
+
+    /// Returns per-disk GC health for `system.cas_mounts`. The fields describing
+    /// rounds snapshot this scheduler's state, while `wedged_namespace_count` is read live from the
+    /// store's ref lanes; keeping the state here avoids process-global gauges colliding across disks.
+    struct GcHealth
+    {
+        bool is_leader = false;
+        bool ever_succeeded = false;
+        Int64 pending_reclaim = 0;             /// cumulative condemned - executed deletes (this process)
+        UInt64 last_success_age_seconds = 0;   /// seconds since the last led round (0 if never)
+        UInt64 wedged_namespace_count = 0;
+    };
+    /// Takes a consistent-enough atomic snapshot for diagnostics. The returned counters are local
+    /// health indicators rather than durable GC state, and the wedged-namespace count is queried
+    /// directly from the store.
+    GcHealth gcHealth() const;
+
+    /// Whether GC is quiescent right now: TRUE iff NO round is currently in flight on this scheduler
+    /// (`round_in_flight` is held for the WHOLE round body via `SCOPE_EXIT` in `runRoundLogged`, on both the
+    /// success and the exception path). Used by the `SYSTEM CAS FORGET` / `GC STOP` tests to
+    /// prove the scheduler's worker threads were joined — no round can be mid-flight once `stop()` returned.
+    bool isQuiescent() const { return !round_in_flight.load(std::memory_order_acquire); }
+
+    /// Test seam: force the in-flight-round flag `isQuiescent` reads, so a test can drive
+    /// "running round => not quiescent" without spinning up a real round against a live backend.
+    void setRoundInFlightForTest(bool v) { round_in_flight.store(v, std::memory_order_release); }
+
+    /// Test seam (rev.7 §3 [C1]): block up to `timeout` for BOTH the pacing and heartbeat loops to have
+    /// SELF-EXITED via the terminal-lifecycle check — a `Vanished` pool or a published FORGET intent — as
+    /// opposed to exiting through `stop()`'s `stopping` flag. Returns false on timeout. Predicate-based
+    /// wait (no sleeps); the loops set their flag under `terminal_exit_mutex` before notifying, so there is
+    /// no lost-wakeup window. Lets a test prove the self-exit path fired without relying on any wall-clock
+    /// delay.
+    bool waitForTerminalSelfExitForTest(std::chrono::milliseconds timeout);
+
+private:
+    /// Waits for the configured interval, runs scheduled rounds while the scheduler is active, and
+    /// logs exceptions before continuing with the next tick. The round lock serializes this worker
+    /// with `runOneRoundNow` because the persistent `gc` object is not thread-safe.
+    void loop();
+
+    /// While this scheduler believes it owns the lease, periodically advances the advisory
+    /// heartbeat independently of round progress. The cadence is shorter than the lease
+    /// observation window, so a long round does not look like a dead leader to another scheduler.
+    /// Heartbeat failures are advisory and are retried on the next cadence.
+    void heartbeatLoop();
+
+    /// Runs synchronously when `Cas::Gc` acquires or renews the lease, before the round's potentially
+    /// long fold begins. It marks this scheduler as the heartbeat owner and sends the first pulse
+    /// immediately; otherwise a new leader's first round could appear inactive until it returned.
+    /// The same hook is used by scheduled and manual rounds so both acquisition paths are protected.
+    void onLeaseAcquired();
+
+    /// Run one round through the full logging path (Start record, ProfileEventsScope, Finish
+    /// record). Used by BOTH loop() and runOneRoundNow. Logging is best-effort - the logger sink
+    /// never throws into the round. Rethrows a round exception (after emitting an Aborted Finish).
+    /// `allow_steal` is forwarded to `Cas::Gc::runRegularRound` verbatim (see its doc comment).
+    Cas::RoundReport runRoundLogged(Cas::Gc & round_gc, GcRoundLogRecord::Trigger trigger,
+                                     std::function<void()> on_lease_acquired = {}, bool allow_steal = true);
+
+    const Cas::PoolPtr store;
+    const std::chrono::seconds interval;
+    const std::chrono::milliseconds hb_interval;   /// advisory heartbeat cadence, interval / 4 with a 50 ms minimum
+    const LoggerPtr log;
+    const UInt128 gc_id;
+    const String disk_name;
+    const GcRoundLogger logger;
+
+    /// One persistent Gc for BOTH loop() and runOneRoundNow: the lease's observation-window steal
+    /// protocol REQUIRES a stable observer (it compares the lease across consecutive runRegularRound
+    /// calls of the same instance). A throwaway per call could never recover a dead-incumbent lease.
+    Cas::Gc gc;
+    /// Serializes the manual round against the background round so the two never touch the single
+    /// (not-thread-safe) `gc` concurrently. Distinct from `mutex`: the loop releases `mutex` before
+    /// the round so stop()/heartbeatLoop are not blocked, so the round cannot hold `mutex`.
+    std::mutex gc_round_mutex;
+
+    std::mutex mutex;
+    std::condition_variable wake;
+    bool stopping = false;
+    bool round_requested = false;   /// guarded by `mutex`; coalesced external wake request
+    ThreadFromGlobalPool thread;
+    /// Set by the round worker and read by the heartbeat worker. It is only an in-process hint: the
+    /// durable lease remains the authority, and a failed round clears the hint before retrying.
+    std::atomic<bool> i_am_leader{false};
+    ThreadFromGlobalPool hb_thread;
+
+    /// Set true for the whole body of one round (`runRoundLogged`, held across the `gc_round_mutex`
+    /// critical section a scheduled or manual round runs under) and cleared when it returns, on the
+    /// success AND exception paths. Read by `isQuiescent` (the FORGET / GC-STOP join-completion signal).
+    std::atomic<bool> round_in_flight{false};
+
+    /// rev.7 §3 [C1] test-observation seam: set (under `terminal_exit_mutex`) by `loop`/`heartbeatLoop`
+    /// respectively when they SELF-EXIT via the terminal-lifecycle check, NOT when `stop()` flips
+    /// `stopping`. `waitForTerminalSelfExitForTest` waits on `terminal_exit_cv` for BOTH, so a test proves
+    /// the self-exit path fired without any sleep. Purely diagnostic; production behavior never reads them.
+    std::atomic<bool> loop_exited_on_terminal_for_test{false};
+    std::atomic<bool> hb_exited_on_terminal_for_test{false};
+    std::mutex terminal_exit_mutex;
+    std::condition_variable terminal_exit_cv;
+
+    /// Cumulative condemned entries minus exact-token deletes completed by this scheduler while it
+    /// led. It is an approximate health gauge, not durable GC state.
+    std::atomic<Int64> pending_reclaim{0};
+    /// Steady-clock timestamp of the last round that acquired the lease; zero means never.
+    std::atomic<UInt64> last_success_ms{0};
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.cpp
new file mode 100644
index 000000000000..c3b44569d074
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.cpp
@@ -0,0 +1,65 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Common/Exception.h>
+#include <base/defines.h>
+#include <functional>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+}
+
+namespace DB::Cas
+{
+
+uint64_t manifestCleanupShard(const ManifestId & id, uint64_t gc_shards)
+{
+    /// gc_shards >= 1 is enforced by GcState decode (CORRUPTED_DATA on 0).
+    /// Hash the qualified id (namespace plus all three `ManifestRef` components) using the same
+    /// mixing as `std::hash<ManifestId>`. Two namespaces can legally carry the same
+    /// `ManifestRef` without addressing the same object, so their cleanup work must never be
+    /// merged.
+    const size_t h = std::hash<ManifestId>{}(id);
+    return static_cast<uint64_t>(h) % gc_shards;
+}
+
+ShardReducer::ShardReducer(uint64_t shard_, uint64_t gc_shards_)
+    : shard(shard_), gc_shards(gc_shards_)
+{
+    if (gc_shards_ == 0)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "ShardReducer: gc_shards must be >= 1");
+    if (shard_ >= gc_shards_)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "ShardReducer: shard {} is out of range [0, {})", shard_, gc_shards_);
+}
+
+bool ShardReducer::owns(const BlobRef & ref) const
+{
+    return blobShard(ref, gc_shards) == shard;
+}
+
+std::vector<RunRef> ShardReducer::reduce(Backend & backend, const Layout & layout,
+                                         const std::vector<RunRef> & prior_runs,
+                                         uint64_t new_generation, uint64_t attempt,
+                                         std::vector<BlobDelta> shard_deltas,
+                                         uint64_t current_round, uint64_t condemn_round,
+                                         const std::function<std::optional<HeadResult>(const BlobRef &)> & head_blob,
+                                         const std::function<std::optional<HeadResult>(const BlobRef &)> & peek_head,
+                                         const std::function<bool(const RetiredEntry &)> & confirm_condemned_marker,
+                                         RetiredMergeResult * out_retired,
+                                         bool suppress_destructive,
+                                         std::vector<uint8_t> * out_applied_by_txn_ordinal,
+                                         GcRoundWorkBudget * work_budget) const
+{
+    std::vector<RunRef> out_runs;
+    foldDeltasIntoGeneration(backend, layout, prior_runs, new_generation, attempt, shard,
+                             std::move(shard_deltas), out_runs,
+                             current_round, condemn_round, head_blob, peek_head,
+                             confirm_condemned_marker, out_retired,
+                             suppress_destructive, out_applied_by_txn_ordinal, {}, work_budget);
+    return out_runs;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h
new file mode 100644
index 000000000000..20edc54dfb88
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h
@@ -0,0 +1,138 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <base/types.h>
+#include <base/extended_types.h>
+#include <cstdint>
+#include <functional>
+#include <optional>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Blob-target shard for a blob identity.
+///
+/// Deterministic and total over all hashes: the blob-target sharding axis uses the high 64 bits of
+/// the 128-bit blob hash modulo `gc_shards`. The high bits are taken rather than the low bits so
+/// that blobs whose hashes differ only in the low 64 bits — an adversarial corner case — still
+/// spread across shards. `CityHash128` output has high entropy in BOTH halves, so both choices
+/// are equivalent for organic workloads; high bits are the documented canonical choice.
+///
+/// Properties:
+///   - Deterministic/stable: same inputs always yield the same shard.
+///   - Total: result is in [0, gc_shards).
+///   - Single-shard equivalence: gc_shards == 1 always returns 0.
+///
+/// The argument is the complete `BlobRef`, so callers cannot silently discard its algorithm while
+/// routing. The shard number deliberately uses only `ref.digest`: distribution comes from the
+/// digest bytes, while the algorithm remains part of the identity carried through the GC pipeline.
+inline uint64_t blobShard(const BlobRef & ref, uint64_t gc_shards)
+{
+    /// gc_shards >= 1 is enforced by GcState decode (CORRUPTED_DATA on 0).
+    /// BE-u64 of bytes[0:8] — an EXPLICIT big-endian read, bit-identical to the old
+    /// `static_cast<uint64_t>(blob_hash >> 64)` for every 128-bit digest (`fromU128` writes the
+    /// UInt128 big-endian into bytes[0:16], so bytes[0:8] IS the old high 64 bits). MUST stay an
+    /// explicit big-endian read, never a native-endian memcpy (would silently reshard on an LE host).
+    uint64_t high64 = 0;
+    for (int i = 0; i < 8; ++i)
+        high64 = (high64 << 8) | ref.digest.bytes[static_cast<size_t>(i)];
+    return high64 % gc_shards;
+}
+
+/// Route a part-manifest cleanup bundle to a worker by its namespace-qualified `ManifestId`. Workers
+/// own disjoint ranges; routing by `ManifestRef` alone would merge cleanup work from two namespaces
+/// that happen to reuse the same reference components. `gc_shards == 1` routes every `ManifestId`
+/// to owner shard 0.
+///
+/// The hash mixes both the `root_namespace` string and the three `ManifestRef` components — the
+/// same mixing used by `std::hash<ManifestId>`. Two `ManifestId`s that share the same `ManifestRef`
+/// but carry different namespaces produce independent hash values and may route to different shards.
+///
+/// Properties:
+///   - Deterministic/stable: same inputs always yield the same shard.
+///   - Total: result is in [0, gc_shards).
+///   - Single-shard equivalence: gc_shards == 1 always returns 0.
+uint64_t manifestCleanupShard(const ManifestId & id, uint64_t gc_shards);
+
+/// Per-shard in-degree reducer for the sharded GC fold.
+///
+/// `ShardReducer` owns exactly ONE target shard (`shard` in [0, `gc_shards`)). It accepts the
+/// caller's per-shard slice of `BlobDelta`s — produced by `foldManifestEdges` and bucketed by
+/// `blobShard` — and merges them into a per-shard `CasBlobInDegree` generation run via
+/// `foldDeltasIntoGeneration`.
+///
+/// Ownership invariant: a reducer touches ONLY blobs it owns — i.e. `blobShard(h, gc_shards) == shard`.
+/// Two reducers for DIFFERENT shards may run concurrently; their key namespaces are disjoint
+/// (`blobTargetRunKey(gen, shard0, seq)` vs `blobTargetRunKey(gen, shard1, seq)`).
+///
+/// The `reduce` method delegates to `foldDeltasIntoGeneration` (the same path the non-sharded fold
+/// uses with `shard == 0`), so `gc_shards == 1` with `shard == 0` reproduces the non-sharded fold
+/// byte-for-byte. This keeps the one-shard configuration compatible with the original fold path.
+///
+/// NOTE on durable writes: `reduce` writes the per-shard in-degree run directly via `backend`
+/// (under `blobTargetRunKey(new_generation, shard, 0)`), exactly as `foldDeltasIntoGeneration`
+/// does. Returning the durable write here (rather than an in-memory map) keeps the round driver
+/// stateless: it simply constructs a `ShardReducer` per shard, calls `reduce`, and the sealed
+/// run is already present for the `zeroInDegree` consumer (and the fold's two-cursor merge). An
+/// in-memory return value is unnecessary because the backend is directly queryable; tests read the
+/// sealed run back over an `InMemoryBackend`.
+class ShardReducer
+{
+public:
+    /// Construct a reducer that owns `shard` (in [0, `gc_shards`)).
+    ShardReducer(uint64_t shard_, uint64_t gc_shards_);
+
+    /// True iff this reducer owns `ref` — i.e. `blobShard(ref, gc_shards) == shard`.
+    bool owns(const BlobRef & ref) const;
+
+    /// Merge `shard_deltas` (the caller's per-shard `BlobDelta` slice produced by `foldManifestEdges`
+    /// and bucketed by `blobShard`) into a new in-degree generation for this shard. Writes the sealed
+    /// run under `blobTargetRunKey(new_generation, shard, 0)` via `backend`, appends its `RunRef` to
+    /// `out_runs`, and returns the `RunRef`. The call is idempotent (write-once via `putIfAbsent`).
+    ///
+    /// `prior_runs` are the parent generation's run segments for this shard, resolved BY THE CALLER from
+    /// the parent fold seal's `blob_target_runs` filtered to `shard`. An empty vector is the
+    /// fresh-pool / empty baseline.
+    ///
+    /// PRECONDITION: every `BlobDelta` in `shard_deltas` must be owned by this reducer
+    /// (`blobShard(d.ref, gc_shards) == shard`). This is a caller contract; there is no
+    /// underflow throw backstopping it — pass a misbucketed delta and the fold silently misroutes it.
+    std::vector<RunRef> reduce(Backend & backend, const Layout & layout,
+                               const std::vector<RunRef> & prior_runs,
+                               uint64_t new_generation, uint64_t attempt,
+                               std::vector<BlobDelta> shard_deltas,
+                               uint64_t current_round = 0, uint64_t condemn_round = 0,
+                               const std::function<std::optional<HeadResult>(const BlobRef &)> & head_blob = {},
+                               const std::function<std::optional<HeadResult>(const BlobRef &)> & peek_head = {},
+                               const std::function<bool(const RetiredEntry &)> & confirm_condemned_marker = {},
+                               RetiredMergeResult * out_retired = nullptr,
+                               bool suppress_destructive = false,
+                               /// PROBE B2: forwarded verbatim to `foldDeltasIntoGeneration` — see its
+                               /// declaration and `Cas::TxnApplyLedger`.
+                               std::vector<uint8_t> * out_applied_by_txn_ordinal = nullptr,
+                               /// Forwarded verbatim to `foldDeltasIntoGeneration`; the caller shares
+                               /// one instance across every shard's reduce within a round.
+                               GcRoundWorkBudget * work_budget = nullptr) const;
+
+private:
+    uint64_t shard;
+    uint64_t gc_shards;
+};
+
+/// Role split of a sharded GC round (`gc_shards > 1`):
+///
+///   - COORDINATOR (exactly one per round — the lease holder): owns input-seal, round-visibility,
+///     the single GLOBAL fence (over all LIST-discovered shards), and generation-advance. These
+///     steps span the whole fence universe and must NOT be sharded: a publish into one root shard
+///     can protect a blob assigned to ANY target shard, so an independent per-reducer fence is
+///     unsafe. `Gc::fence` therefore stays the single coordinator fence over the entire universe.
+///
+///   - REDUCERS / CLEANUP WORKERS (one per disjoint shard): own ONLY their shard's blob-target reduce
+///     (`ShardReducer`) or part-manifest cleanup (`manifestCleanupShard`). Their key namespaces are
+///     disjoint, so two replicas may reduce DIFFERENT shards concurrently. Reducer work needs NO lease:
+///     the lease is work-dedup only (see `CasGcScheduler`), not a coordination primitive.
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.cpp
new file mode 100644
index 000000000000..e2eb6fc761e4
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.cpp
@@ -0,0 +1,143 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Common/Exception.h>
+
+namespace DB::Cas
+{
+
+NamespaceJanitorResult NamespaceJanitor::runOnePage(
+    bool suppress_deletes, const std::function<bool()> & fence_held)
+{
+    NamespaceJanitorResult result;
+    const GcMaintenanceReadResult progress = readGcMaintenanceState(backend, layout);
+    if (progress.status == GcMaintenanceReadStatus::Corrupt)
+    {
+        result.anomalies.push_back(progress.diagnostic);
+        (void)casGcMaintenanceState(backend, layout, progress.token, GcMaintenanceState{});
+        return result;
+    }
+
+    const String cursor = progress.state ? progress.state->janitor_cursor : String{};
+    ListPage page;
+    try
+    {
+        page = backend.list(layout.namespaceRootPrefix(), cursor, page_budget);
+    }
+    catch (...)
+    {
+        (void)casGcMaintenanceState(backend, layout, progress.token, GcMaintenanceState{});
+        throw;
+    }
+    result.pages = 1;
+    result.keys = page.keys.size();
+
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(backend, layout);
+    bool ambiguous = false;
+    try
+    {
+        catalog_cut.life_index.throwIfAmbiguous("CAS namespace janitor");
+    }
+    catch (const DB::Exception & e)
+    {
+        result.anomalies.push_back(e.message());
+        ambiguous = true;
+    }
+    /// A valid page is complete only when the round had deletion authority for every dead-life
+    /// candidate on it. Advancing while the global gate is closed can phase-lock a dead page onto
+    /// every suppressed round and a different page onto every bounded forced fold. Ambiguous cuts and
+    /// observed fence loss have the same shape: retain the old cursor so an authoritative round
+    /// retries the exact page. Malformed keys, absent objects and token mismatches are final per-key
+    /// outcomes and therefore do not by themselves prevent progress.
+    bool page_decided = !ambiguous && !suppress_deletes;
+
+    for (const ListedKey & listed : page.keys)
+    {
+        std::optional<NamespaceLifePhysicalId> life_id;
+        try
+        {
+            if (listed.key.starts_with(layout.namespaceStreamRootPrefix()))
+            {
+                if (const auto parsed = layout.parseRefObjectKey(listed.key))
+                    life_id = parsed->life_id;
+            }
+            else if (listed.key.starts_with(layout.namespaceStateRootPrefix()))
+            {
+                if (const auto parsed = layout.parseRefCkptKey(listed.key))
+                    life_id = *parsed;
+                else if (const auto file_parsed = layout.parseNamespaceFileKey(listed.key))
+                    life_id = file_parsed->life_id;
+            }
+        }
+        catch (const DB::Exception & e)
+        {
+            result.anomalies.push_back(listed.key + ": " + e.message());
+            continue;
+        }
+
+        if (!life_id)
+        {
+            result.anomalies.push_back(listed.key + ": unrecognized namespace object key");
+            continue;
+        }
+        if (ambiguous || suppress_deletes || catalog_cut.life_index.resolve(*life_id))
+            continue;
+
+        std::optional<Token> token = listed.token;
+        if (!token)
+        {
+            try
+            {
+                const HeadResult current = backend.head(listed.key);
+                if (!current.exists)
+                    continue;
+                token = current.token;
+            }
+            catch (const std::exception & e)
+            {
+                ++result.leaked;
+                result.anomalies.push_back(
+                    "leaked dead-life object '" + listed.key + "': exact HEAD failed: " + e.what());
+                continue;
+            }
+        }
+        if (!fence_held())
+        {
+            page_decided = false;
+            break;
+        }
+        try
+        {
+            if (backend.deleteExact(listed.key, *token).kind == DeleteOutcome::Kind::Deleted)
+                ++result.deleted;
+        }
+        catch (const std::exception & e)
+        {
+            ++result.leaked;
+            result.anomalies.push_back(
+                "leaked dead-life object '" + listed.key + "': exact delete failed: " + e.what());
+        }
+    }
+
+    /// Recheck even when the page had no dead candidate. A tenure that observes fence loss after LIST
+    /// or after the last exact delete must not publish progress. Loss after this check may still race
+    /// with the leak-only maintenance CAS; already completed exact deletes remain safe to repeat.
+    if (page_decided && !fence_held())
+        page_decided = false;
+
+    if (page_decided)
+    {
+        const GcMaintenanceState next{.janitor_cursor = page.next_cursor};
+        try
+        {
+            (void)casGcMaintenanceState(backend, layout, progress.token, next);
+        }
+        catch (const std::exception & e)
+        {
+            result.anomalies.push_back("cursor publication failed: " + String(e.what()));
+        }
+    }
+    return result;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.h
new file mode 100644
index 000000000000..e60f87e2e6a0
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.h
@@ -0,0 +1,34 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <functional>
+#include <vector>
+
+namespace DB::Cas
+{
+
+struct NamespaceJanitorResult
+{
+    uint64_t pages = 0;
+    uint64_t keys = 0;
+    uint64_t deleted = 0;
+    uint64_t leaked = 0;
+    std::vector<String> anomalies;
+};
+
+/// Runs one bounded, leak-only page over the physical namespace ownership tree.
+class NamespaceJanitor
+{
+public:
+    NamespaceJanitor(Backend & backend_, const Layout & layout_, size_t page_budget_)
+        : backend(backend_), layout(layout_), page_budget(page_budget_) {}
+
+    NamespaceJanitorResult runOnePage(bool suppress_deletes, const std::function<bool()> & fence_held);
+
+private:
+    Backend & backend;
+    const Layout & layout;
+    size_t page_budget;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.cpp
new file mode 100644
index 000000000000..b019212e00a6
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.cpp
@@ -0,0 +1,915 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <Common/logger_useful.h>
+#include <algorithm>
+#include <map>
+#include <set>
+#include <limits>
+
+namespace ProfileEvents
+{
+    extern const Event CASGCEnumerationPages;
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Hook used by this file's GC-owned enumeration calls to `forEachListedKey` and `recoverRefTable`.
+/// It increments once per physical LIST page, never once per listed key.
+void onGcEnumerationPage()
+{
+    ProfileEvents::increment(ProfileEvents::CASGCEnumerationPages);
+}
+
+/// The durable build floor is stored in the per-server mount lease together with the writer epoch. A
+/// namespace is rooted by `server_root_id`, but that id is a clean relative path and can contain slashes.
+/// Try namespace prefixes from longest to shortest and accept the first durable mount body. Without a
+/// mount there is no deletion authority, so the caller must leave the prefix untouched. The mount's
+/// `writer_epoch` and `min_active` are the single durable epoch/floor pair used for eligibility, including
+/// across process replacement and the retired sentinel.
+std::optional<MountLease> floorForNamespace(Pool & store, const RootNamespace & ns)
+{
+    const String & value = ns.string();
+    size_t pos = value.size();
+    while (true)
+    {
+        pos = value.rfind('/', pos == 0 ? 0 : pos - 1);
+        if (pos == String::npos)
+            break;
+
+        const String server_root_id = value.substr(0, pos);
+        if (!server_root_id.empty())
+        {
+            if (const auto got = store.backend().get(store.layout().mountKey(server_root_id)))
+                return decodeMountLease(got->bytes);
+        }
+        if (pos == 0)
+            break;
+    }
+    return std::nullopt;
+}
+
+struct ListedManifestObject
+{
+    RootNamespace ns;
+    BuildPrefix prefix;
+    ManifestRef ref;
+    String key;
+};
+
+/// Delegates to the shared `Layout::parseManifestKey`, which validates the canonical manifest-key
+/// encoding. Keeping parsing in `Layout` avoids a second interpretation of the manifest path and ensures
+/// the sweep and filesystem checker derive the same namespace and build identity.
+std::optional<ListedManifestObject> parseListedManifestObject(const Layout & layout, const String & key)
+{
+    const auto parsed = layout.parseManifestKey(key);
+    if (!parsed)
+        return std::nullopt;
+
+    return ListedManifestObject{
+        .ns = parsed->root_namespace,
+        .prefix = BuildPrefix{.writer_epoch = parsed->ref.writer_epoch, .build_sequence = parsed->ref.build_sequence},
+        .ref = parsed->ref,
+        .key = key};
+}
+
+/// The fold seal `gc/state` currently adopts, or `nullopt` when the pool has no `gc/state` or no seal
+/// at `(snap_generation, snap_attempt)` — a pool whose GC has never completed a round. It is read ONCE
+/// per sweep pass; every namespace the pass touches takes its coverage row out of the same object.
+std::optional<CasFoldSeal> readAdoptedFoldSeal(Pool & store)
+{
+    const Layout & layout = store.layout();
+    const auto state_got = store.backend().get(layout.gcStateKey());
+    if (!state_got)
+        return std::nullopt;
+    const GcState state = decodeGcState(state_got->bytes);
+    const auto got = store.backend().get(layout.foldSealKey(state.snap_generation, state.snap_attempt));
+    if (!got)
+        return std::nullopt;
+    return decodeFoldSeal(got->bytes, state.snap_generation);
+}
+
+
+/// One catalog-named life row out of an adopted seal. The catalog cut supplies the name-to-id join;
+/// neither an absent name nor a `Creating` row may recover coverage from a historical life.
+std::optional<RefCoverage> coverageOf(
+    const std::optional<CasFoldSeal> & seal,
+    const CasRefCatalog::Snapshot & catalog_cut,
+    const RootNamespace & ns)
+{
+    if (!seal)
+        return std::nullopt;
+
+    const auto catalog_it = std::find_if(
+        catalog_cut.catalog.entries.begin(), catalog_cut.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    if (catalog_it == catalog_cut.catalog.entries.end() || catalog_it->state == NsState::Creating)
+        return std::nullopt;
+
+    const auto row_it = seal->ref_lives.find(catalog_it->incarnation);
+    if (row_it == seal->ref_lives.end())
+        return std::nullopt;
+    return row_it->second.coverage;
+}
+
+/// Return this operation's ONE catalog row for `ns`. Callers hold the enclosing `Snapshot` for their
+/// entire decision; a later name resolution would splice its life into an earlier coverage decision.
+const CatalogEntry * catalogEntryOf(const CasRefCatalog::Snapshot & catalog_cut, const RootNamespace & ns)
+{
+    const auto it = std::find_if(
+        catalog_cut.catalog.entries.begin(), catalog_cut.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    return it == catalog_cut.catalog.entries.end() ? nullptr : &*it;
+}
+
+/// The durable `last_folded_ref_id` of a coverage row, `{0, 0}` when there is none — as for a fresh
+/// pool. A manifest removed by a log above this cursor has not had its `-1` decrement folded, so its
+/// body remains load-bearing until the fold catches up.
+RefTxnId sealedRefCursor(const std::optional<RefCoverage> & coverage)
+{
+    return coverage ? coverage->last_folded_ref_id : RefTxnId{};
+}
+
+/// One namespace's protection view: the manifest object keys the sweep must never delete, split so the
+/// §6 premise can test the removal half on its own. `active` is the pre-existing union the delete sites
+/// consult; `tail_removal_targets` is the subset the unconsumed tail above the cursor names as removal
+/// targets, which is what rule (2) is stated over.
+struct NamespaceProtection
+{
+    std::set<String> active;
+    std::set<String> tail_removal_targets;
+    /// Set when the committed-tail recovery walk stopped early because `work_budget`'s recovery-op cap
+    /// was spent (see `activeManifestKeys` below), or the walk was never attempted because the cap was
+    /// already spent by an earlier namespace this round. `active`/`tail_removal_targets` are then a
+    /// PARTIAL view — the caller must never use them to authorize a deletion decision, only to retain
+    /// every one of this namespace's candidates on the current page.
+    bool recovery_incomplete = false;
+};
+
+
+/// The active manifest-object-key set for one namespace, built as the
+/// same complete view writer recovery uses:
+///   owners in the newest snapshot + owner changes in every later log (== `recoverRefTable`'s committed
+///   rows and live precommits) + manifests removed anywhere in the tail above the durable
+///   `last_folded_ref_id` (their `-1` is not yet folded, so the GC fold still needs the body).
+/// Keys (not ManifestIds) so a listed object key can be tested directly. Throws on a corrupt snapshot /
+/// invalid transaction (via the authority-grounded recovery / `decodeRefLogTxn`); the caller SKIPS the
+/// namespace's deletions on such a throw rather than substituting an empty owner set.
+///
+/// `work_budget`, when set, bounds the committed-tail walk below: each ref-log GET the walk issues
+/// consumes one unit of `GcRoundWorkBudget::sweep_recovery_op_budget`, shared with every other
+/// namespace this round touches. Exhaustion sets `NamespaceProtection::recovery_incomplete` and stops
+/// the walk — it is deliberately NOT plumbed into `recoverRefTableDetailedFromAuthority` itself: that
+/// function is a shared recovery primitive also used by `fsck` (which needs a COMPLETE table to audit)
+/// and the GC rebuild path (which needs a complete table to reconstruct in-degree from scratch), so
+/// capping its own internal cost is out of scope here — this file only bounds the walk it owns.
+/// Reaching the budget before even calling `recoverRefTableDetailedFromAuthority` (already spent by an
+/// earlier namespace) skips that call entirely and reports incomplete immediately.
+NamespaceProtection activeManifestKeys(
+    Pool & store, const CatalogEntry & catalog_entry, const RefCkpt & ckpt,
+    const std::optional<RefCoverage> & coverage, GcRoundWorkBudget * work_budget = nullptr)
+{
+    NamespaceProtection protection;
+    if (work_budget && !work_budget->sweepRecoveryOpAvailable())
+    {
+        protection.recovery_incomplete = true;
+        return protection;
+    }
+    std::set<String> & active = protection.active;
+    const Layout & layout = store.layout();
+    Backend & backend = store.backend();
+    const RootNamespace & ns = catalog_entry.ns;
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(catalog_entry.ns, catalog_entry.incarnation);
+
+    /// Current owners = snapshot + replayed tail (committed rows + live precommits).
+    /// The exact row and `_ckpt` come from the caller's frozen catalog cut. Do not resolve `ns` here:
+    /// a later catalog cut can name a reborn life and turn this old life into an apparent orphan.
+    const RecoveredRefTable recovered = recoverRefTableDetailedFromAuthority(
+        backend, layout, catalog_entry, ckpt);
+    if (work_budget)
+        ++work_budget->sweep_recovery_ops_used;   /// one coarse unit for the snapshot+tail recovery itself
+    const RefTableState & state = recovered.state;
+    for (const auto [ref_name, row] : state.getCommitted())
+        active.insert(layout.manifestKey(ManifestId{ns, row.manifest_ref}));
+    for (const auto & [ref_name, manifest_ref] : state.getPrecommits())
+        active.insert(layout.manifestKey(ManifestId{ns, manifest_ref}));
+
+    /// Tail-removal protection: every manifest removed by a log ABOVE the durable fold cursor stays active
+    /// until its `-1` folds. The exact checkpoint frontier is the finite upper bound; LIST cannot decide
+    /// whether a removal exists. A namespace-removal transaction names every removed owner explicitly, so
+    /// this also protects a whole removed namespace's bodies until the fold catches up.
+    const RefTxnId cursor = sealedRefCursor(coverage);
+    if (!ckpt.committed_through || !(cursor < *ckpt.committed_through))
+        return protection;
+
+    /// When cleanup has removed the inherited cursor body, only the exact next global epoch may prove
+    /// that cursor was a seal. Asking `crossEpochFromSeal` about `{E+1,1}` preserves its backlink/body
+    /// validation without allowing a later witness to jump over a missing empty-epoch seal.
+    const auto cross_from_missing_cursor = [&](const RefTxnId & from_cursor)
+    {
+        if (from_cursor.writer_epoch == std::numeric_limits<uint64_t>::max())
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS orphan sweep: folded cursor {} has no representable next epoch",
+                renderRefTxnId(from_cursor));
+        const RefTxnId exact_next_epoch{from_cursor.writer_epoch + 1, 1};
+        const EpochCrossResult crossing = crossEpochFromSeal(
+            backend, layout, ns, from_cursor, std::nullopt, exact_next_epoch, life);
+        if (!crossing.proved())
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS orphan sweep: exact next-epoch record {} does not prove that folded cursor {} was its seal "
+                "(cross outcome {})",
+                renderRefTxnId(exact_next_epoch), renderRefTxnId(from_cursor),
+                static_cast<uint8_t>(crossing.outcome));
+        return crossing.start;
+    };
+
+    RefTxnId id;
+    std::optional<RefTxnId> prior;
+    std::optional<bool> prior_is_seal;
+    if (cursor == RefTxnId{} || (ckpt.life_epoch && cursor.writer_epoch < *ckpt.life_epoch))
+    {
+        id = RefTxnId{*ckpt.life_epoch, 1};
+    }
+    else
+    {
+        /// A cursor can name any old epoch's `EpochSeal`, not just `ckpt.last_epoch_seal`. When its
+        /// body survives, decode it and cross with that direct kind evidence. If compaction removed the
+        /// cursor, first try its ordinary same-epoch successor; only a 404 there can ask the shared
+        /// chain proof to establish a cross with kind unknown. That preserves tails after a cleaned
+        /// cursor without guessing that a missing cursor was a seal.
+        const auto cursor_got = backend.get(layout.refLogKey(life, cursor));
+        if (cursor_got)
+        {
+            const RefLogTxn cursor_txn = decodeRefLogTxn(
+                openObject(FormatId::RefLog, cursor_got->bytes), ns.string(), cursor);
+            if (refLogTxnIsEpochSeal(cursor_txn))
+            {
+                if (cursor.writer_epoch == std::numeric_limits<uint64_t>::max())
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS orphan sweep: folded epoch seal {} has no representable exact successor",
+                        renderRefTxnId(cursor));
+                id = RefTxnId{cursor.writer_epoch + 1, 1};
+            }
+            else
+            {
+                if (cursor.ref_sequence == std::numeric_limits<uint64_t>::max())
+                    throw Exception(ErrorCodes::CORRUPTED_DATA,
+                        "CAS orphan sweep: folded cursor {} has no representable exact successor",
+                        renderRefTxnId(cursor));
+                id = RefTxnId{cursor.writer_epoch, cursor.ref_sequence + 1};
+                prior = cursor;
+                prior_is_seal = false;
+            }
+        }
+        else
+        {
+            if (cursor.ref_sequence == std::numeric_limits<uint64_t>::max())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS orphan sweep: folded cursor {} has no representable exact successor",
+                    renderRefTxnId(cursor));
+            id = RefTxnId{cursor.writer_epoch, cursor.ref_sequence + 1};
+            prior = cursor;
+            prior_is_seal = std::nullopt;
+        }
+    }
+
+    while (id <= *ckpt.committed_through)
+    {
+        /// UNCERTAINTY, work-budget arm: the committed-tail walk is a finite but potentially huge range
+        /// (up to `ckpt.committed_through`), and the round shares one recovery-op budget across every
+        /// namespace it touches. Stopping HERE — before the next GET — leaves `active`/
+        /// `tail_removal_targets` genuinely partial, so the caller must treat the whole namespace as
+        /// undecided this page (fail-closed retain), never authorize a deletion from what was collected
+        /// so far.
+        if (work_budget && !work_budget->sweepRecoveryOpAvailable())
+        {
+            protection.recovery_incomplete = true;
+            break;
+        }
+        const auto got = backend.get(layout.refLogKey(life, id));
+        if (work_budget)
+            ++work_budget->sweep_recovery_ops_used;
+        if (!got)
+        {
+            /// A known ordinary predecessor cannot cross an epoch. Only an inherited cursor whose body
+            /// was compacted has unknown kind, and even then INV-2 permits exactly `{E+1,1}` rather than
+            /// an arbitrary later witness.
+            if (!prior || prior_is_seal.has_value())
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "CAS orphan sweep: committed tail log {} is absent under the supplied _ckpt frontier",
+                    renderRefTxnId(id));
+            id = cross_from_missing_cursor(*prior);
+            prior.reset();
+            prior_is_seal.reset();
+            continue;
+        }
+        const RefLogTxn txn = decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), id);
+        for (const RefManifestEdge & edge : manifestEdgesOfTxn(txn))
+            if (edge.change < 0)
+            {
+                const String key = layout.manifestKey(edge.manifest_id);
+                active.insert(key);
+                protection.tail_removal_targets.insert(key);
+            }
+
+        const bool is_seal = refLogTxnIsEpochSeal(txn);
+        if (const std::optional<RefTxnId> next = nextRefLogIdWithinCommittedFrontier(
+                id, is_seal, *ckpt.committed_through))
+        {
+            if (is_seal)
+            {
+                prior.reset();
+                prior_is_seal.reset();
+            }
+            else
+            {
+                prior = id;
+                prior_is_seal = false;
+            }
+            id = *next;
+        }
+        else
+            break;
+    }
+    return protection;
+}
+
+}
+
+NamespaceFoldView namespaceFoldView(Pool & store, const RootNamespace & ns)
+{
+    NamespaceFoldView view;
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(store.backend(), store.layout());
+    catalog_cut.life_index.throwIfAmbiguous("CAS orphan manifest sweep");
+    view.coverage = coverageOf(readAdoptedFoldSeal(store), catalog_cut, ns);
+    return view;
+}
+
+std::string_view sweepRetainClassName(SweepRetainClass c)
+{
+    switch (c)
+    {
+        case SweepRetainClass::None:           return "none";
+        case SweepRetainClass::NoCoverage:     return "no_coverage";
+        case SweepRetainClass::Hold:           return "hold";
+        case SweepRetainClass::UnconsumedSeal: return "unconsumed_seal";
+        case SweepRetainClass::TailRemoval:    return "tail_removal";
+        case SweepRetainClass::WorkBudgetExhausted: return "work_budget_exhausted";
+    }
+    return "unknown";
+}
+
+std::pair<SweepRetainClass, uint64_t> ManifestSweepResult::topRetainReason() const
+{
+    /// Enum order, so a tie resolves the same way on every pass and an unchanged pool keeps reporting
+    /// the same verdict instead of alternating between two equally-large classes.
+    const std::pair<SweepRetainClass, uint64_t> candidates[] = {
+        {SweepRetainClass::NoCoverage, retained_no_coverage},
+        {SweepRetainClass::Hold, retained_hold},
+        {SweepRetainClass::UnconsumedSeal, retained_unconsumed_seal},
+        {SweepRetainClass::TailRemoval, retained_tail_removal},
+        {SweepRetainClass::WorkBudgetExhausted, retained_work_budget},
+    };
+    std::pair<SweepRetainClass, uint64_t> top{SweepRetainClass::None, 0};
+    for (const auto & c : candidates)
+        if (c.second > top.second)
+            top = c;
+    return top;
+}
+
+bool manifestDeletionPremise(const NamespaceFoldView & view, const ManifestKey & manifest,
+                             String * retain_reason, SweepRetainClass * retain_class)
+{
+    const auto retain = [&](SweepRetainClass klass, String why)
+    {
+        if (retain_reason)
+            *retain_reason = std::move(why);
+        if (retain_class)
+            *retain_class = klass;
+        return false;
+    };
+    if (retain_class)
+        *retain_class = SweepRetainClass::None;
+    const String build_epoch = std::to_string(manifest.prefix.writer_epoch);
+
+    /// UNCERTAINTY, and it comes first because it is the case where the predicate knows NOTHING. With no
+    /// sealed coverage row, no round has folded a ref cursor for this namespace, so no epoch's closing
+    /// seal can be shown consumed. The sweep's own protection view cannot stand in for the missing
+    /// proof: that view is assembled from the very enumeration arithmetic intake distrusts.
+    if (!view.coverage)
+        return retain(SweepRetainClass::NoCoverage,
+                      "no sealed fold coverage for the namespace: no round has folded a ref cursor for "
+                      "it, so epoch " + build_epoch + "'s closing seal cannot be shown consumed");
+
+    const RefCoverage & cov = *view.coverage;
+
+    /// UNCERTAINTY, hold arm. A hold names the exact position the fold could not resolve, and everything
+    /// at or above it is unaccounted -- including, for all this predicate can tell, the record that
+    /// grants or removes this very manifest. `classification == 4` is tested separately from the hold
+    /// even though the seal's strict grammar pairs them: the thing standing between a clamped namespace
+    /// and an irreversible delete must not be a codec invariant enforced somewhere else.
+    if (cov.hold)
+        return retain(SweepRetainClass::Hold, "namespace held at " + renderRefTxnId(cov.hold->offending_position) + " ("
+                      + String{holdReasonToWord(cov.hold->reason)} + ", retried "
+                      + std::to_string(cov.hold->retry_count) + " round(s)): every record at or above "
+                      "that position is unaccounted for");
+    if (cov.classification == 4)
+        return retain(SweepRetainClass::Hold,
+                      "namespace coverage is classified clamped (4) with no hold recorded: whatever "
+                      "stopped the fold was not carried, so nothing above its cursor is accounted for");
+    if (cov.classification == 0)
+        return retain(SweepRetainClass::NoCoverage,
+                      "namespace coverage is classified absent (0): no round folded it, so its cursor "
+                      "is not the result of any walk");
+
+    /// RULE 1 (spec §6). Grants do not cross epochs, so every `+1` that could name an epoch-`E` build
+    /// lives among epoch `E`'s own records; and an epoch is left ONLY over its consumed `EpochSeal`
+    /// (INV-2), so a sealed cursor in a STRICTLY HIGHER epoch is durable proof that every one of those
+    /// records has folded -- proof by arithmetic, which is what makes it independent of the listing.
+    ///
+    /// A cursor still INSIDE epoch `E` proves nothing of the sort, and that stays true even when it
+    /// happens to sit on `E`'s own seal: the durable cursor records a POSITION, never the KIND of the
+    /// record there, so "the cursor is the seal" is not a readable fact here. Retaining that case costs
+    /// one more round and resolves itself -- the next epoch's first record crosses the seal, and the
+    /// round after that sweeps the build.
+    if (!(manifest.prefix.writer_epoch < cov.last_folded_ref_id.writer_epoch))
+        return retain(SweepRetainClass::UnconsumedSeal,
+                      "epoch " + build_epoch + "'s closing seal is not consumed: the sealed cursor is at "
+                      + renderRefTxnId(cov.last_folded_ref_id) + ", so a grant naming this build may "
+                      "still be unfolded above it");
+
+    /// RULE 2 (spec §6). Removals DO cross epochs, so a record in a LATER epoch can name this build as a
+    /// removal target; deleting the body before that `-1` folds leaves the fold clamping forever on a
+    /// manifest it must read to emit the decrement (the GC-WEDGE-2026-07-10 shape).
+    ///
+    /// The sweep's protection set already spares a tail removal target before this predicate is
+    /// consulted, so this test is belt over suspenders BY DESIGN: the rule belongs to the premise, so
+    /// that a future sweep path that assembles its protection set differently cannot lose it. Note what
+    /// it is NOT: its negative direction is no proof, because the set comes from the same enumeration
+    /// rule (1) exists to stop trusting. Rule (1) is what makes the tail decidable; this makes the
+    /// decision explicit.
+    if (view.tail_removal_targets.contains(manifest.key))
+        return retain(SweepRetainClass::TailRemoval,
+                      "an unconsumed tail record above the cursor names this manifest as a removal "
+                      "target: its `-1` has not folded, so the fold still needs the body");
+
+    if (retain_reason)
+        retain_reason->clear();
+    return true;
+}
+
+bool prefixEligible(Pool & store, const RootNamespace & ns, const BuildPrefix & prefix)
+{
+    /// Eligibility comes only from the durable mount-lease floor. A missing floor means NOT eligible;
+    /// do not replace that authority check with a frozen-sequence or judged-dead guess. Compare
+    /// `writer_epoch` first, then `build_sequence`, so old-epoch
+    /// debris drains after a process restart even when its build_sequence is above the current min_active.
+    const auto floor = floorForNamespace(store, ns);
+    if (!floor)
+        return false;
+
+    const MountLease & w = *floor;
+    if (prefix.writer_epoch < w.writer_epoch)
+        return true;
+    if (prefix.writer_epoch > w.writer_epoch)
+        return false;
+    if (w.min_active == std::numeric_limits<uint64_t>::max())
+        return true;   /// farewell/retired sentinel: every seq is retired
+    return w.min_active > prefix.build_sequence;
+}
+
+uint64_t sweepNamespace(Pool & store, const RootNamespace & ns, const BuildPrefix & prefix,
+                        std::vector<String> * warnings)
+{
+    if (!prefixEligible(store, ns, prefix))
+        return 0;   /// not eligible by the durable watermark fact — delete nothing (controls #8/#9)
+
+    const Layout & layout = store.layout();
+    Backend & backend = store.backend();
+
+    /// Build the protection view. A missing snapshot body, an invalid transaction, or an incomplete
+    /// ordered view throws, causing the sweep to skip deletion and surface the error; it never substitutes
+    /// an empty owner set. Skip this namespace's deletions on such a throw rather than deleting against a
+    /// wrong (empty) view. When a
+    /// caller opted in (`warnings != nullptr`), this "cannot confirm emptiness" also lands in `*warnings`
+    /// -- not just the log -- so a decommission run does not silently report a clean drain that never
+    /// happened; the log-only default stays exactly as before for every other caller, which treats this
+    /// the same way the periodic sweep always has: skip and retry next round.
+    /// The §6 premise's durable half, read before the protection view so both share one seal read.
+    NamespaceFoldView view;
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(store.backend(), store.layout());
+    catalog_cut.life_index.throwIfAmbiguous("CAS orphan manifest sweep");
+    view.coverage = coverageOf(readAdoptedFoldSeal(store), catalog_cut, ns);
+    const CatalogEntry * catalog_entry = catalogEntryOf(catalog_cut, ns);
+    if (!catalog_entry || catalog_entry->state == NsState::Creating)
+        return 0;   /// absent/Creating names have no recovery authority and therefore no deletion authority
+
+    NamespaceProtection protection;
+    try
+    {
+        const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(catalog_entry->ns, catalog_entry->incarnation);
+        const std::optional<CkptSample> ckpt = readCkpt(backend, layout, life);
+        if (!ckpt)
+        {
+            const String warning = "CAS orphan sweep: namespace " + ns.string()
+                + " is catalog-named but its required _ckpt is absent; skipped, emptiness not confirmed";
+            LOG_WARNING(getLogger("CasOrphanManifestSweep"), "{}", warning);
+            if (warnings)
+                warnings->push_back(warning);
+            return 0;
+        }
+        protection = activeManifestKeys(store, *catalog_entry, ckpt->ckpt, view.coverage);
+        view.tail_removal_targets = protection.tail_removal_targets;
+
+    }
+    catch (const Exception & e)
+    {
+        LOG_WARNING(getLogger("CasOrphanManifestSweep"),
+                    "CAS orphan sweep: namespace {} protection view unavailable ({}); skipping its deletions",
+                    ns.string(), e.message());
+        if (warnings)
+            warnings->push_back("CAS orphan sweep: namespace " + ns.string() + " protection view unavailable ("
+                                 + e.message() + "); skipped, emptiness not confirmed");
+        return 0;
+    }
+
+    /// Enumerate the ONE build prefix: cas/manifests/<ns>/<epoch-hex>-<seq-hex>/ in the canonical
+    /// hexadecimal form -- the same rendering `Layout::manifestKey` uses.
+    const String prefix_key = layout.manifestNamespacePrefix(ns)
+        + renderRefTxnId(RefTxnId{prefix.writer_epoch, prefix.build_sequence}) + "/";
+
+    uint64_t deleted = 0;
+    forEachListedKey(backend, prefix_key, [&](const ListedKey & listed)
+    {
+        if (protection.active.contains(listed.key))
+            return;   /// owned by a committed or precommit owner — never sweep
+
+        /// THE §6 SAFETY FLOOR, under the watermark eligibility already established above. The
+        /// watermark says the build is retired; the premise says the ref stream can be shown not to
+        /// name this body. A refusal is recorded rather than silent (Constraint 10).
+        String retain_reason;
+        if (!manifestDeletionPremise(view, ManifestKey{listed.key, prefix}, &retain_reason))
+        {
+            LOG_DEBUG(getLogger("CasOrphanManifestSweep"),
+                      "CAS orphan sweep: retaining {} -- {}", listed.key, retain_reason);
+            if (warnings)
+                warnings->push_back("CAS orphan sweep: retained " + listed.key + " -- " + retain_reason);
+            return;
+        }
+
+        /// Exact-token delete: HEAD for the current token, then deleteExact. A 404 between HEAD and
+        /// delete (or a TokenMismatch — a fresh owner reclaimed it) is tolerated (record-and-continue),
+        /// same as always, regardless of `warnings` -- that is the normal "someone else already reclaimed
+        /// it" race, not a failure. A THROWN exception (a transient backend hiccup) is the one thing
+        /// `warnings` changes: opted-in (non-null), it is recorded and the sweep moves to the next key;
+        /// opted-out (nullptr, every pre-existing caller), it propagates exactly as before (fail-close).
+        try
+        {
+            const HeadResult head = backend.head(listed.key);
+            if (!head.exists)
+                return;
+            const DeleteOutcome outcome = backend.deleteExact(listed.key, head.token);   /// NotFound/TokenMismatch spared
+            if (classifyDeleteOutcome(outcome) == DeleteClass::Deleted)
+                ++deleted;
+        }
+        catch (...)
+        {
+            if (!warnings)
+                throw;
+            warnings->push_back("CAS orphan sweep: " + listed.key + " delete failed: "
+                                 + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+        }
+    }, 1000, onGcEnumerationPage);
+    return deleted;
+}
+
+ManifestSweepResult planManifestCursorPage(
+    Pool & store,
+    const String & cursor,
+    uint64_t list_budget,
+    uint64_t nomination_budget,
+    bool catalog_recovery_authoritative,
+    GcRoundWorkBudget * work_budget)
+{
+    ManifestSweepResult result;
+    result.next_cursor = cursor;
+    if (list_budget == 0)
+        return result;
+
+    Backend & backend = store.backend();
+    const Layout & layout = store.layout();
+    const ListPage page = backend.list(layout.casManifestsPrefix(), cursor, list_budget);
+    /// This pass fetches exactly one page per round (the cursor advances across rounds, not within this
+    /// call), so the metric increments once per call, not once per listed key.
+    ProfileEvents::increment(ProfileEvents::CASGCEnumerationPages);
+
+    /// Freeze every possible destructive candidate BEFORE the later catalog cut. A same-name rebirth can
+    /// replace this logical manifest key between the observations; classifying the old bytes against the
+    /// later lifecycle cut is safe only when deletion retains the old exact token, so the replacement
+    /// loses `deleteExact`. Do not take a fresh GET after the catalog read: that would splice new-life
+    /// bytes into old candidate selection and authorize their deletion with the new token.
+    ///
+    /// Bounded to `nomination_budget` well-formed keys — never the whole `list_budget`-sized
+    /// page — since `nomination_budget` is the hard ceiling on how many of them this call can ever
+    /// nominate. A well-formed key beyond this cap has no frozen body; it is retained where its absence
+    /// is discovered below, in the exact same "budget exhausted, cursor does not step over it" shape the
+    /// nomination-count exhaustion already uses.
+    std::map<String, std::optional<GetResult>> observed_candidates;
+    if (nomination_budget > 0)
+    {
+        uint64_t frozen = 0;
+        for (const ListedKey & listed : page.keys)
+        {
+            if (frozen >= nomination_budget)
+                break;
+            if (parseListedManifestObject(layout, listed.key))
+            {
+                observed_candidates.emplace(listed.key, backend.get(listed.key));
+                ++frozen;
+            }
+        }
+    }
+
+    /// One seal and one later catalog cut for the whole page; every namespace joins through those same
+    /// immutable observations.
+    const std::optional<CasFoldSeal> adopted_seal = readAdoptedFoldSeal(store);
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(backend, layout);
+    catalog_cut.life_index.throwIfAmbiguous("CAS orphan manifest sweep");
+
+    std::map<String, bool> eligible_by_prefix;
+    std::map<String, NamespaceFoldView> view_by_ns;
+    std::map<String, std::set<String>> active_by_ns;
+    std::set<String> errored_namespaces;   /// protection view unavailable => skip, never delete
+
+    /// The key of the last candidate this page actually DECIDED on. The cursor resumes strictly after
+    /// it (`ListPage::next_cursor` is the last returned key), so a candidate the page never decided on
+    /// stays ahead of the cursor and is examined next pass. See the budget rule below.
+    String decided_through;
+    bool budget_exhausted = false;
+
+    for (const ListedKey & listed : page.keys)
+    {
+        ++result.listed;
+
+        /// UNCERTAINTY, budget arm (§6). Once a NON-ZERO delete budget is used up, the page stops
+        /// deciding: the candidates behind it are retained AND the cursor does not step over them.
+        /// Advancing past a candidate nothing examined would turn "retained this round" into
+        /// "unexamined until the cursor wraps the whole `cas/manifests/` keyspace" -- the same trade
+        /// the round-level gate refuses when it freezes the cursor along with a suppressed sweep.
+        /// A budget of ZERO is not exhaustion but a list-only pass: nothing is ever deletable, so
+        /// freezing the cursor on it would make the sweep spin on one page forever. That pass keeps
+        /// its pre-existing behaviour and advances.
+        if (budget_exhausted || (nomination_budget > 0 && result.nominations.size() >= nomination_budget))
+        {
+            budget_exhausted = true;
+            ++result.skipped;
+            continue;
+        }
+
+        const auto parsed = parseListedManifestObject(layout, listed.key);
+        if (!parsed)
+        {
+            ++result.skipped;
+            decided_through = listed.key;
+            continue;
+        }
+
+        if (nomination_budget == 0)
+        {
+            /// The list-only pass: examined, not deletable, cursor advances (see the budget rule above).
+            ++result.skipped;
+            decided_through = listed.key;
+            continue;
+        }
+
+        const CatalogEntry * catalog_entry = catalogEntryOf(catalog_cut, parsed->ns);
+        if (catalog_entry)
+        {
+            const String eligibility_key = parsed->ns.string() + "\n"
+                + std::to_string(parsed->prefix.writer_epoch) + "\n"
+                + std::to_string(parsed->prefix.build_sequence);
+            auto [eligible_it, eligible_inserted] = eligible_by_prefix.emplace(eligibility_key, false);
+            if (eligible_inserted)
+                eligible_it->second = prefixEligible(store, parsed->ns, parsed->prefix);
+            if (!eligible_it->second)
+            {
+                ++result.skipped;
+                decided_through = listed.key;
+                continue;
+            }
+        }
+
+        auto [view_it, view_inserted] = view_by_ns.emplace(parsed->ns.string(), NamespaceFoldView{});
+        auto [active_it, inserted] = active_by_ns.emplace(parsed->ns.string(), std::set<String>{});
+        if (inserted)
+        {
+            /// UNCERTAINTY, work-budget arm: building a fresh namespace's protection view
+            /// is the expensive step (a catalog-authoritative table recovery plus a committed-tail
+            /// walk) the LIST/nomination budgets never bounded. Once the round's per-page namespace cap
+            /// is spent, a NEW namespace gets no view at all -- retained, exactly like every other
+            /// "cannot confirm emptiness" cause below, never a partial or best-effort one.
+            if (work_budget && !work_budget->sweepNamespaceAvailable())
+            {
+                LOG_DEBUG(getLogger("CasOrphanManifestSweep"),
+                    "CAS orphan sweep: retaining {} -- round's per-page namespace work budget exhausted",
+                    parsed->key);
+                errored_namespaces.insert(parsed->ns.string());
+                ++result.retained_work_budget;
+                ++result.skipped;
+                decided_through = listed.key;
+                continue;
+            }
+            if (work_budget)
+                ++work_budget->sweep_namespaces_used;
+            view_it->second.coverage = coverageOf(adopted_seal, catalog_cut, parsed->ns);
+            if (catalog_entry && !catalog_recovery_authoritative)
+            {
+                LOG_DEBUG(getLogger("CasOrphanManifestSweep"),
+                    "CAS orphan sweep: retaining {} -- caller did not authorize catalog-named recovery",
+                    parsed->key);
+                errored_namespaces.insert(parsed->ns.string());
+                ++result.retained_no_coverage;
+                ++result.skipped;
+                decided_through = listed.key;
+                continue;
+            }
+            if (catalog_entry && catalog_entry->state == NsState::Creating)
+            {
+                LOG_DEBUG(getLogger("CasOrphanManifestSweep"),
+                    "CAS orphan sweep: retaining {} -- catalog namespace is Creating and has no recovery authority",
+                    parsed->key);
+                errored_namespaces.insert(parsed->ns.string());
+                ++result.retained_no_coverage;
+                ++result.skipped;
+                decided_through = listed.key;
+                continue;
+            }
+            /// A corrupt snapshot or invalid transaction means the protection view is unavailable. Skip
+            /// this namespace's deletions and surface the error; never substitute an empty owner set.
+            try
+            {
+                if (catalog_entry)
+                {
+                    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(
+                        catalog_entry->ns, catalog_entry->incarnation);
+                    const std::optional<CkptSample> ckpt = readCkpt(backend, layout, life);
+                    if (!ckpt)
+                    {
+                        LOG_WARNING(getLogger("CasOrphanManifestSweep"),
+                            "CAS orphan sweep: namespace {} is catalog-named but its required _ckpt is absent; skipping",
+                            parsed->ns.string());
+                        errored_namespaces.insert(parsed->ns.string());
+                    }
+                    else
+                    {
+                        NamespaceProtection protection = activeManifestKeys(
+                            store, *catalog_entry, ckpt->ckpt, view_it->second.coverage, work_budget);
+                        if (protection.recovery_incomplete)
+                        {
+                            /// The committed-tail walk stopped early: `active`/`tail_removal_targets`
+                            /// are a PARTIAL view. Discard them and retain the whole namespace on this
+                            /// page instead of deciding from an incomplete protection set (fail-closed).
+                            LOG_DEBUG(getLogger("CasOrphanManifestSweep"),
+                                "CAS orphan sweep: retaining {} -- committed-tail recovery walk's work "
+                                "budget exhausted before it could confirm the namespace's protection view",
+                                parsed->key);
+                            errored_namespaces.insert(parsed->ns.string());
+                            ++result.retained_work_budget;
+                        }
+                        else
+                        {
+                            view_it->second.tail_removal_targets = std::move(protection.tail_removal_targets);
+                            active_it->second = std::move(protection.active);
+                        }
+                    }
+                }
+
+            }
+            catch (const Exception & e)
+            {
+                LOG_WARNING(getLogger("CasOrphanManifestSweep"),
+                            "CAS orphan sweep: namespace {} protection view unavailable ({}); skipping",
+                            parsed->ns.string(), e.message());
+                errored_namespaces.insert(parsed->ns.string());
+            }
+        }
+        if (errored_namespaces.contains(parsed->ns.string()))
+        {
+            ++result.skipped;
+            decided_through = listed.key;
+            continue;
+        }
+        if (catalog_entry && active_it->second.contains(parsed->key))
+        {
+            ++result.skipped;
+            decided_through = listed.key;
+            continue;
+        }
+
+        /// If the post-observation catalog cut has no row, this candidate was necessarily created by a
+        /// now-dead life: creation publishes its row before any life-owned object. No current life can
+        /// name it, and a concurrent later creation can only replace its token after this observation.
+        /// A catalog-named row instead needs the ordinary coverage/owner proof below.
+        if (catalog_entry)
+        {
+            /// THE §6 SAFETY FLOOR, the same predicate `sweepNamespace` calls, under the same watermark
+            /// eligibility. A refusal is recorded rather than silent (Constraint 10).
+            String retain_reason;
+            SweepRetainClass retain_class = SweepRetainClass::None;
+            if (!manifestDeletionPremise(view_it->second, ManifestKey{parsed->key, parsed->prefix},
+                                         &retain_reason, &retain_class))
+            {
+                /// The sentence goes to the debug log for whoever is chasing ONE object; the class goes to
+                /// a counter, which is the only form in which this path can report itself (see
+                /// `ManifestSweepResult`). Both come from the predicate; neither is derived from the other.
+                LOG_DEBUG(getLogger("CasOrphanManifestSweep"),
+                          "CAS orphan sweep: retaining {} -- {}", parsed->key, retain_reason);
+                switch (retain_class)
+                {
+                    case SweepRetainClass::NoCoverage:     ++result.retained_no_coverage; break;
+                    case SweepRetainClass::Hold:           ++result.retained_hold; break;
+                    case SweepRetainClass::UnconsumedSeal: ++result.retained_unconsumed_seal; break;
+                    case SweepRetainClass::TailRemoval:    ++result.retained_tail_removal; break;
+                    case SweepRetainClass::WorkBudgetExhausted: ++result.retained_work_budget; break;  /// unreachable: the premise never returns this class itself
+                    case SweepRetainClass::None:           break;   /// unreachable: the premise refused
+                }
+                ++result.skipped;
+                decided_through = listed.key;
+                continue;
+            }
+        }
+
+        /// This exact token and bytes were captured before the catalog cut (see above). A missing body
+        /// has no deletion authority; a later replacement loses the old token at `deleteExact`.
+        ///
+        /// A well-formed key can legitimately be ABSENT here: the freeze loop above caps
+        /// fan-out at `nomination_budget` candidates, so a key beyond that cap was never frozen. Treat
+        /// it exactly like nomination-count exhaustion -- retain, and do NOT advance the cursor past
+        /// it, so the very next page/round examines it with a fresh budget instead of losing it.
+        const auto observed_it = observed_candidates.find(parsed->key);
+        if (observed_it == observed_candidates.end())
+        {
+            budget_exhausted = true;
+            ++result.skipped;
+            continue;
+        }
+        const std::optional<GetResult> & got = observed_it->second;
+        if (!got)
+        {
+            ++result.skipped;
+            decided_through = listed.key;
+            continue;
+        }
+        const PartManifest body = decodePartManifest(openObject(FormatId::PartManifest, got->bytes));
+        const ManifestId id{parsed->ns, parsed->ref};
+        if (!refMatchesBody(id.ref, body) || !manifestNamespaceMatches(id.root_namespace, body))
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "CAS orphan sweep: manifest identity mismatch at {} while deriving exact source edges",
+                parsed->key);
+
+        ManifestSweepResult::Nomination nomination{
+            .id = id,
+            .key = parsed->key,
+            .token = got->token,
+            .source_retirements = {}};
+        for (const ManifestEntry & entry : body.entries)
+            if (entry.placement == EntryPlacement::Blob)
+                nomination.source_retirements.push_back(BlobSourceRetirement{
+                    .ref = entry.ref,
+                    .source_id = sourceEdgeId(id, entry.path)});
+        result.nominations.push_back(std::move(nomination));
+        decided_through = listed.key;
+    }
+
+    if (budget_exhausted)
+    {
+        /// Resume strictly after the last DECIDED key, leaving every undecided candidate ahead of the
+        /// cursor. `wrapped` stays false because this page did not reach the end of the keyspace — it
+        /// stopped early, and reporting a wrap would tell the caller the sweep had made a full circuit
+        /// over keys it never looked at.
+        result.next_cursor = decided_through;
+        result.wrapped = false;
+        return result;
+    }
+
+    result.next_cursor = page.next_cursor;
+    result.wrapped = page.next_cursor.empty();
+    return result;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h
new file mode 100644
index 000000000000..91714dacc7c8
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h
@@ -0,0 +1,206 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <base/types.h>
+#include <optional>
+#include <set>
+#include <utility>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// One writer build prefix under `cas/manifests/<ns>/`: the canonical hex `<epoch-hex>-<seq-hex>/`
+/// directory encoded as canonical hexadecimal epoch and sequence components.
+struct BuildPrefix
+{
+    uint64_t writer_epoch = 0;
+    uint64_t build_sequence = 0;
+};
+
+
+/// One manifest object the sweep is considering, carrying both halves the §6 deletion premise needs:
+/// the full object key (what the tail's removal targets are named by) and the build prefix it lives
+/// under (whose `writer_epoch` is the epoch whose closing seal must be consumed).
+struct ManifestKey
+{
+    String key;
+    BuildPrefix prefix;
+};
+
+/// WHY the §6 deletion premise refused one manifest, as a CLASS rather than a sentence. The sentence
+/// (`retain_reason`) names one object and is what an operator reads; the class is what a counter can
+/// aggregate, and aggregating is the only way "the sweep retained every manifest in the pool, all for
+/// the same reason" becomes visible at all. Deriving the class by matching substrings of the sentence
+/// would make the prose load-bearing, so the predicate reports both and neither is parsed out of the
+/// other.
+enum class SweepRetainClass : uint8_t
+{
+    None = 0,          /// the premise admitted the deletion; no retention happened
+    NoCoverage,        /// no sealed coverage row for the namespace (a classification-0 row counts here)
+    Hold,              /// the namespace is held, or is classified clamped
+    UnconsumedSeal,    /// rule (1): the cursor has not consumed the build epoch's closing seal
+    TailRemoval,       /// rule (2): an unconsumed tail record names this manifest as a removal target
+    WorkBudgetExhausted, /// the round's per-namespace or recovery-op work budget was spent before this
+                         /// namespace's protection view could be built (or built completely); retained
+                         /// rather than decided without a complete view (fail-closed, never a partial one)
+};
+
+/// The class as a short stable word, for log lines and metric names.
+std::string_view sweepRetainClassName(SweepRetainClass c);
+
+/// The durable per-namespace fold state the §6 deletion premise reads. It is taken from the ADOPTED
+/// fold seal — the `gc/state` -> `fold_seal` pair the sweep already read to learn its cursor, kept
+/// whole instead of reduced to that one field.
+///
+/// IT IS NOT DERIVED FROM A LISTING, and the type exists to keep it that way. Arithmetic ref intake
+/// demoted the listing to a hint precisely because a store may omit a durable ref-log key from an
+/// enumeration; a premise that re-derived its answer by listing the tail would inherit that hole and
+/// license exactly the deletion it exists to withhold.
+struct NamespaceFoldView
+{
+    /// The namespace's shard-0 coverage row. `nullopt` means the adopted seal carries no row for this
+    /// namespace at all: no round has ever sealed a ref cursor for it, so no epoch's closing seal is
+    /// proven consumed and every manifest under it is retained.
+    std::optional<RefCoverage> coverage;
+
+    /// Manifest object keys the tail ABOVE the cursor names as REMOVAL targets, as the namespace's
+    /// protection view collected them. Rule (2) is a POSITIVE test against this set: a key found here
+    /// retains. Its negative direction proves nothing on its own — the set is assembled from the same
+    /// enumeration arithmetic intake distrusts — which is why rule (1) and not this set is what makes
+    /// the tail decidable. See `manifestDeletionPremise`.
+    std::set<String> tail_removal_targets;
+};
+
+/// Spec §6, the sweep deletion premise, as ONE predicate both sweep paths call. A manifest of an
+/// epoch-`E` build is deletable only when:
+///   (1) the namespace cursor has consumed epoch `E`'s seal, AND
+///   (2) no unconsumed tail record above the cursor names it as a removal target
+///       (removals cross epochs; grants do not).
+/// ANY uncertainty — an unreached frontier, an exhausted budget, a hold — means RETAIN. Delay is never
+/// damage: a body kept one round longer costs storage, while a body deleted under an unproven cut is
+/// either data loss (an unfolded `+1` still names it) or a fold that clamps forever on the missing body.
+///
+/// Rule (1) is what makes rule (2) decidable rather than a second guess at the same enumeration.
+/// Grants do not cross epochs, so every `+1` that could name an epoch-`E` build lives among epoch `E`'s
+/// own records; and an epoch is left ONLY over its consumed `EpochSeal` (INV-2), so a sealed cursor in
+/// a STRICTLY HIGHER epoch is durable proof that every one of those records is folded. Removals do
+/// cross epochs, which is why rule (2) exists at all and why it is a separate test.
+///
+/// `retain_reason`, when non-null, receives the reason the premise refused — it feeds the sweep's
+/// `warnings` out-param so a retained manifest is a visible decision rather than a silent one.
+/// `retain_class`, when non-null, receives the same refusal as a `SweepRetainClass` — what the cursor
+/// page counts, since it has no `warnings` to carry the sentence.
+bool manifestDeletionPremise(const NamespaceFoldView & view, const ManifestKey & manifest,
+                             String * retain_reason, SweepRetainClass * retain_class = nullptr);
+
+/// Read one namespace's fold view out of the adopted fold seal. `tail_removal_targets` is left empty:
+/// the callers that have a protection view fill it from theirs, and a caller that has none gets the
+/// coverage half alone, which is the half rule (1) needs.
+NamespaceFoldView namespaceFoldView(Pool & store, const RootNamespace & ns);
+
+
+
+/// Counters returned by one bounded cursor page. `listed` counts keys in the backend page, `skipped`
+/// counts malformed, protected, ineligible, budget-exhausted, or race-spared keys, and `deleted` counts
+/// only successful exact-token deletions. `next_cursor` and `wrapped` describe the backend cursor; the
+/// cursor is a cleanup-progress hint and is never used as reachability authority.
+///
+/// THE `retained_*` COUNTERS ARE THIS PATH'S ONLY VOICE. They break the §6 premise's share of
+/// `skipped` out by reason class. Unlike `sweepNamespace`, the cursor page has no `warnings`
+/// out-param and nothing downstream of it reads a per-object sentence, so without these a background
+/// sweep retaining every manifest in the pool is indistinguishable, at any production log level, from
+/// a sweep that had nothing to do. In Stage A that is not an edge case: rule (1) is satisfiable only
+/// for a closed-and-folded epoch, so retention is the NORMAL outcome and these counters are very
+/// nearly the whole story of what the sweep did and why.
+struct ManifestSweepResult
+{
+    String next_cursor;
+    bool wrapped = false;
+    uint64_t listed = 0;
+    uint64_t deleted = 0;
+    uint64_t skipped = 0;
+    uint64_t retained_no_coverage = 0;
+    uint64_t retained_hold = 0;
+    uint64_t retained_unconsumed_seal = 0;
+    uint64_t retained_tail_removal = 0;
+    uint64_t retained_work_budget = 0;
+
+    /// Exact-GET/decode candidates. The reducer must adopt every `source_retirements` entry before the
+    /// caller may exact-token-delete `key` with `token`.
+    struct Nomination
+    {
+        ManifestId id;
+        String key;
+        Token token;
+        std::vector<BlobSourceRetirement> source_retirements;
+    };
+    std::vector<Nomination> nominations;
+
+    /// The reason class that retained the most manifests on this page and how many — the rollup that
+    /// answers "why is manifest debris not shrinking?". `{None, 0}` means the premise retained nothing.
+    /// Ties resolve to the first class in enum order, which is stable across passes so an unchanged
+    /// pool reports an unchanged verdict.
+    std::pair<SweepRetainClass, uint64_t> topRetainReason() const;
+};
+
+/// Per-namespace pre-precommit orphan sweep. Deletes
+/// manifest bodies written before `PrecommitAdd` and never named by any live owner, scoped to ONE
+/// namespace + ONE build prefix. Rules:
+///   - eligibility from the durable watermark fact only: the retired sentinel
+///     (`min_active == UINT64_MAX`), or `min_active > build_sequence`, or a replaced incarnation —
+///     NEVER a frozen-seq / judged-dead heuristic alone (a missing watermark => not eligible);
+///   - the active `ManifestId` set comes from the namespace's committed + live-precommit owner view;
+///   - delete only bodies whose `ManifestId` is ABSENT from the active set, by exact token;
+///   - emits NO blob deltas (a pre-precommit body never contributed `+1`);
+///   - a 404 between listing and deletion is record-and-continue, never a throw;
+///   - never GETs a condemned body to revive it — eligibility +
+///     exact-token delete only.
+/// Returns the number of bodies actually deleted (a `DeleteClass::Deleted`-classified exact-token
+/// delete only, never a spared `NotFound`/`TokenMismatch`) — the decommission manifest-debris drain
+/// (`Core/CasDecommission.cpp`) sums this across every eligible build prefix into
+/// `DecommissionReport::manifest_debris_removed`.
+///
+/// `warnings`, when non-null, opts in to the decommission drain's tolerate-and-continue contract: a
+/// per-key transient failure (a thrown backend exception on `head`/`deleteExact`)
+/// is pushed onto `*warnings` and the sweep continues with the next key, instead of throwing out of
+/// this call; likewise a protection-view-unavailable namespace (the pre-existing corrupt-snapshot skip
+/// below) also pushes a "cannot confirm emptiness" warning, not just a `LOG_WARNING`. `warnings ==
+/// nullptr` (the default, every pre-existing caller) preserves the original behaviour exactly: a
+/// per-key failure propagates as an exception (fail-close default), and the protection-view skip is
+/// log-only. `NotFound`/`TokenMismatch` delete outcomes stay silently spared either way — those are the
+/// normal "a fresh owner reclaimed it" race the periodic sweep expects, not a failure to warn about.
+/// This direct decommission path relies on the caller's held server-root claim/fence: while that claim
+/// is held, a same-server-root rebirth cannot become live between its catalog cut and exact-token delete.
+/// It still rejects every catalog cut with an ambiguous current life id before making any deletion decision.
+uint64_t sweepNamespace(Pool & store, const RootNamespace & ns, const BuildPrefix & prefix,
+                        std::vector<String> * warnings = nullptr);
+
+/// Whether `prefix` is sweep-eligible by the durable watermark fact alone. The floor is read from the
+/// mount lease identified by the namespace's server-root prefix, not inferred from the manifest key or a
+/// judged-dead heuristic. A missing lease provides no deletion authority, so the prefix is not eligible.
+bool prefixEligible(Pool & store, const RootNamespace & ns, const BuildPrefix & prefix);
+
+/// Plan one cursor page without deleting. Every candidate is exact-GET, decoded and identity-validated;
+/// its exact manifest-source edges are returned for accounting-neutral retirement in the next fold.
+/// Catalog-named namespaces are retain-only unless the caller explicitly authorizes recovery from its
+/// frozen catalog cut and the exact `_ckpt` frontier of the life named there.
+///
+/// `work_budget`, when set, bounds the body-GET/retention fan-out to `nomination_budget` well-formed
+/// candidates (never the whole `list_budget`-sized page), caps how many DISTINCT namespaces this page
+/// may build a fresh protection view for, and caps the committed-tail recovery walk's ref-log GET
+/// count cumulatively across the round (shared with every other destructive-work family via the same
+/// `GcRoundWorkBudget` instance). Exhausting either cap retains every remaining candidate belonging to
+/// the affected namespace on THIS page rather than deciding it without a complete protection view;
+/// `nullptr` (the default) reproduces the pre-budget unbounded behavior.
+ManifestSweepResult planManifestCursorPage(
+    Pool & store,
+    const String & cursor,
+    uint64_t list_budget,
+    uint64_t nomination_budget,
+    bool catalog_recovery_authoritative,
+    GcRoundWorkBudget * work_budget = nullptr);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.cpp
new file mode 100644
index 000000000000..899f93504fde
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.cpp
@@ -0,0 +1,121 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+
+#include <Common/Exception.h>
+#include <fmt/format.h>
+#include <algorithm>
+#include <utility>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+}
+}
+
+namespace DB::Cas
+{
+
+CatalogLifecycleReconciler::CatalogLifecycleReconciler(
+    Backend & backend_, const Layout & layout_, const CasFoldSeal & adopted_parent_,
+    uint64_t admitted_generation_,
+    std::function<CasRefCatalog::LeaderFenceStatus(uint64_t)> check_fence_)
+    : backend(backend_)
+    , layout(layout_)
+    , adopted_parent(adopted_parent_)
+    , admitted_generation(admitted_generation_)
+    , check_fence(std::move(check_fence_))
+{
+}
+
+std::optional<CatalogEntry> CatalogLifecycleReconciler::selectEligible(
+    const CasRefCatalog::Snapshot & catalog) const
+{
+    for (const CatalogEntry & entry : catalog.catalog.entries)
+    {
+        if (entry.state != NsState::Removing)
+            continue;
+
+        const auto parent_row = adopted_parent.ref_lives.find(entry.incarnation);
+        if (parent_row == adopted_parent.ref_lives.end()
+            || !parent_row->second.cleanup_evidence
+            || parent_row->second.coverage.hold)
+            continue;
+
+        return entry;
+    }
+    return std::nullopt;
+}
+
+CatalogResolution CatalogLifecycleReconciler::resolveExactRow(
+    const CasRefCatalog::Snapshot & catalog, const CatalogEntry & observed)
+{
+    const auto current = std::find_if(
+        catalog.catalog.entries.begin(),
+        catalog.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns.string() == observed.ns.string(); });
+    if (current == catalog.catalog.entries.end())
+        return CatalogResolution::ExactRowAbsent;
+    if (current->incarnation != observed.incarnation)
+        return CatalogResolution::ExactRowReplaced;
+    return CatalogResolution::ExactRowStillPresent;
+}
+
+CatalogLifecycleReconcileResult CatalogLifecycleReconciler::reconcile()
+{
+    CatalogLifecycleReconcileResult result{
+        .authority_status = AuthorityStatus::Authoritative,
+        .catalog_resolution = CatalogResolution::DrainComplete,
+        .retired_lives = {},
+        .final_catalog_cut = std::nullopt,
+        .deleted = 0};
+    CasRefCatalog::Snapshot catalog = CasRefCatalog::read(backend, layout);
+
+    for (;;)
+    {
+        const std::optional<CatalogEntry> eligible = selectEligible(catalog);
+        if (!eligible)
+        {
+            if (check_fence(admitted_generation) == CasRefCatalog::LeaderFenceStatus::Moved)
+            {
+                result.authority_status = AuthorityStatus::FencedOut;
+                return result;
+            }
+            result.catalog_resolution = CatalogResolution::DrainComplete;
+            result.final_catalog_cut = std::move(catalog);
+            return result;
+        }
+
+        CasRefCatalog::CompletedRemovingDeleteResult delete_result
+            = CasRefCatalog::deleteCompletedRemovingAtSnapshot(
+                backend, layout, std::move(catalog), *eligible, adopted_parent,
+                admitted_generation, check_fence);
+        if (!delete_result.catalog_snapshot)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS catalog lifecycle reconciliation returned no catalog resolution snapshot");
+
+        catalog = std::move(*delete_result.catalog_snapshot);
+        result.catalog_resolution = resolveExactRow(catalog, *eligible);
+        if (delete_result.invalidated_life)
+            result.retired_lives.push_back(*delete_result.invalidated_life);
+
+        if (delete_result.outcome == CasRefCatalog::CompletedRemovingDeleteOutcome::FencedOut)
+        {
+            result.authority_status = AuthorityStatus::FencedOut;
+            return result;
+        }
+        if (delete_result.outcome == CasRefCatalog::CompletedRemovingDeleteOutcome::ProofRefused)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "CAS catalog lifecycle reconciliation selected namespace '{}' without matching no-hold cleanup evidence",
+                eligible->ns.string());
+        if (result.catalog_resolution == CatalogResolution::ExactRowStillPresent)
+            throwCasWriteRetryLater(fmt::format(
+                "CAS catalog lifecycle reconciliation left completed-removal namespace '{}' present",
+                eligible->ns.string()));
+        if (delete_result.outcome == CasRefCatalog::CompletedRemovingDeleteOutcome::Deleted)
+            ++result.deleted;
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.h
new file mode 100644
index 000000000000..fd34011c64ef
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.h
@@ -0,0 +1,64 @@
+#pragma once
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+
+#include <cstdint>
+#include <functional>
+#include <optional>
+#include <vector>
+
+namespace DB::Cas
+{
+
+enum class AuthorityStatus : uint8_t
+{
+    Authoritative,
+    FencedOut,
+};
+
+enum class CatalogResolution : uint8_t
+{
+    DrainComplete,
+    ExactRowAbsent,
+    ExactRowReplaced,
+    ExactRowStillPresent,
+};
+
+/// The catalog-only result produced before a GC round may enumerate or publish successor state.
+struct CatalogLifecycleReconcileResult
+{
+    AuthorityStatus authority_status;
+    CatalogResolution catalog_resolution;
+    std::vector<NamespaceLifeId> retired_lives;
+    std::optional<CasRefCatalog::Snapshot> final_catalog_cut;
+    uint64_t deleted = 0;
+};
+
+/// Settles catalog rows that an already-adopted parent fold seal proved safe to remove.
+///
+/// This component owns only deterministic eligible-row selection and the catalog `N + 1` drain.
+/// It neither discovers the parent seal nor performs a hot ref LIST, ref walk, fold, publication,
+/// runtime invalidation, or physical deletion.
+class CatalogLifecycleReconciler
+{
+public:
+    CatalogLifecycleReconciler(
+        Backend & backend_, const Layout & layout_, const CasFoldSeal & adopted_parent_,
+        uint64_t admitted_generation_,
+        std::function<CasRefCatalog::LeaderFenceStatus(uint64_t)> check_fence_);
+
+    CatalogLifecycleReconcileResult reconcile();
+
+private:
+    std::optional<CatalogEntry> selectEligible(const CasRefCatalog::Snapshot & catalog) const;
+    static CatalogResolution resolveExactRow(
+        const CasRefCatalog::Snapshot & catalog, const CatalogEntry & observed);
+
+    Backend & backend;
+    const Layout & layout;
+    const CasFoldSeal & adopted_parent;
+    uint64_t admitted_generation;
+    std::function<CasRefCatalog::LeaderFenceStatus(uint64_t)> check_fence;
+};
+
+}

From 379e94821030fe68a2e5009c6149e4c6c20b30b2 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:35 +0200
Subject: [PATCH 19/30] CAS subsystem: Tools layer

fsck (integrity checking), inspect, and pool-member decommission.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../Tools/CasDecommission.cpp                 |  476 +++++++
 .../ContentAddressed/Tools/CasDecommission.h  |   55 +
 .../ContentAddressed/Tools/CasFsck.cpp        | 1180 +++++++++++++++++
 .../ContentAddressed/Tools/CasFsck.h          |  285 ++++
 .../ContentAddressed/Tools/CasInspect.cpp     |  638 +++++++++
 .../ContentAddressed/Tools/CasInspect.h       |   30 +
 6 files changed, 2664 insertions(+)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h

diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.cpp
new file mode 100644
index 000000000000..416f5c327663
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.cpp
@@ -0,0 +1,476 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Common/Exception.h>
+#include <Common/logger_useful.h>
+#include <base/scope_guard.h>
+#include <algorithm>
+#include <chrono>
+#include <limits>
+#include <set>
+#include <tuple>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+uint64_t nowMs()
+{
+    return static_cast<uint64_t>(std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::system_clock::now().time_since_epoch()).count());
+}
+
+/// Delete every object listed under `prefix` by its listed (or, absent a list-token backend, HEAD'd)
+/// token. This backs the staging and roots drain phases below: the victim's writers are fenced by the
+/// decommission claim (`Pool::openForDecommission`), so nothing should be racing these deletes, and a
+/// plain exact-token delete of every listed object is race-free.
+///
+/// A per-object failure — a backend exception, a `TokenMismatch` or `NotFound` outcome, or an object
+/// disappearing between `LIST` and `HEAD` — is recorded as a warning and does not prevent the remaining
+/// objects from being attempted. The caller keeps the pool slot whenever warnings are present, so the
+/// terminated slot remains available as a resume anchor instead of being deleted after an unconfirmed
+/// drain. Returns only the objects whose exact-token delete was reported as `Deleted`.
+uint64_t deleteListedPrefix(Backend & backend, const String & prefix, std::vector<String> & warnings)
+{
+    uint64_t deleted = 0;
+    forEachListedKey(backend, prefix, [&](const ListedKey & listed)
+    {
+        try
+        {
+            Token token;
+            if (listed.token)
+                token = *listed.token;
+            else
+            {
+                const HeadResult head = backend.head(listed.key);
+                if (!head.exists)
+                {
+                    warnings.push_back("decommission drain: " + listed.key + " vanished before delete");
+                    return;
+                }
+                token = head.token;
+            }
+
+            const DeleteOutcome outcome = backend.deleteExact(listed.key, token);
+            const DeleteClass outcome_class = classifyDeleteOutcome(outcome);
+            if (outcome_class == DeleteClass::Deleted)
+                ++deleted;
+            else
+                warnings.push_back("decommission drain: " + listed.key + " delete outcome "
+                                    + String(deleteClassName(outcome_class)));
+        }
+        catch (...)
+        {
+            warnings.push_back("decommission drain: " + listed.key + " delete failed: "
+                                + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+        }
+    });
+    return deleted;
+}
+
+/// Delete one slot control object by a token captured at the protocol-defined fence point. Slot
+/// retirement is fail-closed: unlike the debris drains above, any non-`Deleted` outcome or exception
+/// stops the tail before it can touch the next control object.
+bool deleteSlotObject(Backend & backend, const String & key, const Token & token, std::vector<String> & warnings)
+{
+    try
+    {
+        const DeleteOutcome outcome = backend.deleteExact(key, token);
+        const DeleteClass outcome_class = classifyDeleteOutcome(outcome);
+        if (outcome_class == DeleteClass::Deleted)
+            return true;
+
+        warnings.push_back("slot delete failed: " + key + ": delete outcome "
+                           + String(deleteClassName(outcome_class)));
+    }
+    catch (...)
+    {
+        warnings.push_back("slot delete failed: " + key + ": "
+                           + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+    }
+    return false;
+}
+
+}
+
+DecommissionReport decommissionPoolMember(BackendPtr backend, PoolConfig config,
+                                          const String & victim_srid, const CasEventSink & sink,
+                                          const std::function<void()> & request_gc_round)
+{
+    DecommissionReport report;
+    report.srid = victim_srid;
+    bool gc_round_needed = false;
+    /// A namespace may have reached `Removing` before a later namespace fails closed. Preserve the
+    /// already-earned liveness signal on every exit: the callback only wakes the existing serialized
+    /// GC worker and cannot perform catalog work itself.
+    SCOPE_EXIT({
+        if (gc_round_needed && request_gc_round)
+            request_gc_round();
+    });
+
+    /// Validate one required immutable ownership cut before impersonating the victim. The admin open
+    /// performs its own fresh catalog observation for mount safety, but namespace selection below
+    /// must reuse this exact pre-mutation decision rather than read a later authority set.
+    const Layout catalog_layout(config.pool_prefix);
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(*backend, catalog_layout);
+    catalog_cut.life_index.throwIfAmbiguous("CAS decommission");
+
+    config.event_sink = sink;
+    PoolPtr admin = Pool::openForDecommission(std::move(backend), std::move(config), victim_srid);
+
+    EventEmitter{*admin}.emit([&](CasEvent & e)
+    {
+        e.type = CasEventType::MemberDecommission;
+        e.outcome = "begin";
+        e.reason = "operator decommission of pool member";
+        e.detail = {{"server_root_id", victim_srid}};
+    });
+
+    /// The pre-impersonation catalog cut is the complete ownership universe. Physical life keys carry
+    /// no logical path, and raw string prefixes such as `victim` must not select the distinct owner
+    /// `victim2`; the slash makes `victim` one canonical path component.
+    const String victim_namespace_prefix = victim_srid + "/";
+    std::vector<std::pair<CatalogEntry, NamespaceLifeId>> owned_lives;
+    for (const CatalogEntry & entry : catalog_cut.catalog.entries)
+    {
+        if (entry.ns.string() != victim_srid && !entry.ns.string().starts_with(victim_namespace_prefix))
+            continue;
+        const auto life = catalog_cut.life_index.resolve(entry.incarnation);
+        if (!life)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "ca-decommission: catalog entry '{}' has no physical life resolution", entry.ns.string());
+        owned_lives.emplace_back(entry, *life);
+    }
+
+    for (const auto & [selected_entry, life] : owned_lives)
+    {
+        const RootNamespace & ns = life.ns;
+        const String & ns_str = ns.string();
+
+        /// Refuse a same-name lifecycle move that landed after the immutable selection cut. The
+        /// exact-life overloads below also pin recovery to `life`, closing the race after this check:
+        /// a later replacement can never redirect a removal to its new incarnation.
+        const CasRefCatalog::Snapshot current_catalog = CasRefCatalog::read(admin->backend(), admin->layout());
+        const auto current_entry = std::find_if(
+            current_catalog.catalog.entries.begin(), current_catalog.catalog.entries.end(),
+            [&](const CatalogEntry & entry) { return entry.ns.string() == ns_str; });
+        if (current_entry == current_catalog.catalog.entries.end() || *current_entry != selected_entry)
+            throw Exception(ErrorCodes::CORRUPTED_DATA,
+                "ca-decommission: namespace '{}' changed incarnation after the validated catalog cut; "
+                "refusing destructive work",
+                ns_str);
+
+        if (selected_entry.state == NsState::Removing)
+        {
+            if (!admin->backend().head(admin->layout().refCkptKey(life)).exists)
+                throw Exception(ErrorCodes::CORRUPTED_DATA,
+                    "ca-decommission: namespace '{}' is Removing but its exact checkpoint is absent; "
+                    "the catalog row remains owned and the victim slot cannot be retired",
+                    ns_str);
+
+            /// `dropNamespace` is the sole terminal writer. On an already-complete removal this is an
+            /// idempotent observation; on a pre-terminal `Removing` life it resumes the exact terminal
+            /// append under the administrative writer fence. Catalog deletion remains GC's job.
+            (void)admin->dropNamespace(life);
+            ++report.namespaces_already_removed;
+            gc_round_needed = true;
+            continue;
+        }
+
+        const auto stats = admin->dropNamespace(life);
+        ++report.namespaces_removed;
+        report.committed_refs_removed += stats.committed_refs;
+        report.precommits_removed += stats.precommits;
+        report.edge_deltas_emitted += stats.committed_refs + stats.precommits;
+        if (selected_entry.state != NsState::Creating)
+            gc_round_needed = true;
+
+        EventEmitter{*admin}.emit([&](CasEvent & e)
+        {
+            e.type = CasEventType::MemberDecommission;
+            e.outcome = "namespace_removed";
+            e.reason = "decommission dropped a victim namespace";
+            e.detail = {{"server_root_id", victim_srid}, {"namespace", ns_str},
+                        {"committed", std::to_string(stats.committed_refs)},
+                        {"precommits", std::to_string(stats.precommits)}};
+        });
+    }
+
+    /// Manifest debris must be removed before the mount slot: deleting the mount body removes the
+    /// watermark authority, after which `floorForNamespace` returns no value and the ordinary orphan
+    /// sweep cannot prove that old-epoch debris is eligible. The decommission claim has advanced the
+    /// writer epoch, so every build prefix with `prefix.writer_epoch < w.writer_epoch` is eligible here.
+    /// Group the listed keys by namespace and build prefix so each group can use the exact-token orphan
+    /// sweep while the mount body still supplies its authority.
+    {
+        const String debris_prefix = admin->layout().casManifestsServerPrefix(victim_srid);
+        std::set<std::tuple<String, uint64_t, uint64_t>> groups;   /// (namespace, writer epoch, build sequence)
+        forEachListedKey(admin->backend(), debris_prefix, [&](const ListedKey & listed)
+        {
+            if (const auto parsed = admin->layout().parseManifestKey(listed.key))
+                groups.emplace(parsed->root_namespace.string(), parsed->ref.writer_epoch, parsed->ref.build_sequence);
+        });
+        for (const auto & [ns_str, writer_epoch, build_sequence] : groups)
+            report.manifest_debris_removed += sweepNamespace(
+                *admin, RootNamespace(ns_str), BuildPrefix{writer_epoch, build_sequence}, &report.warnings);
+    }
+
+    /// Drain the victim's own `<pool_prefix>/staging/<srid>/` area. The live-mount staging helper uses
+    /// an `IObjectStorage`, while this command intentionally works at the `Backend` layer, so the same
+    /// prefix is listed and deleted directly. The claim fences the victim's writers during this sweep.
+    report.staging_objects_removed += deleteListedPrefix(
+        admin->backend(), admin->poolConfig().pool_prefix + "/staging/" + victim_srid + "/", report.warnings);
+
+    /// Drain the victim's mountpoint objects. These are loose, non-content-addressed files under
+    /// `Layout::serverRootDataPrefix`; they have no writer epoch of their own, so the claim is what
+    /// prevents a returning victim from racing this deletion.
+    report.mountpoint_objects_removed += deleteListedPrefix(
+        admin->backend(), admin->layout().serverRootDataPrefix(victim_srid), report.warnings);
+
+    /// The catalog, not physical debris, owns the slot-retirement decision. A terminal append only
+    /// moves a row to `Removing`; GC must fold/prune/delete it before the member's ownership anchor can
+    /// disappear. Capture one exact whole-catalog cut after every drain, then revalidate its token and
+    /// canonical value immediately before entering the retirement tail. The administrative claim fences
+    /// the victim writer between those observations.
+    std::optional<CasRefCatalog::Snapshot> retirement_catalog_cut;
+    if (report.warnings.empty())
+    {
+        retirement_catalog_cut = CasRefCatalog::read(admin->backend(), admin->layout());
+        const uint64_t victim_owned_count = std::count_if(
+            retirement_catalog_cut->catalog.entries.begin(), retirement_catalog_cut->catalog.entries.end(),
+            [&](const CatalogEntry & entry)
+            {
+                return entry.ns.string() == victim_srid
+                    || entry.ns.string().starts_with(victim_namespace_prefix);
+            });
+        if (victim_owned_count > 0)
+            report.warnings.push_back(
+                "pool member decommission underway: " + std::to_string(victim_owned_count)
+                + " namespace(s) are still owned by this member; upcoming GC rounds perform the final "
+                  "cleanup — re-run this command afterwards to retire the slot");
+    }
+
+    /// Retire the slot strictly last and only after a clean drain. Copy the layout and shared backend
+    /// before `admin.reset()`: graceful close destroys the `Pool`, while the backend must remain alive to
+    /// retire the slot objects afterwards.
+    const Layout layout = admin->layout();
+    const BackendPtr pool_backend = admin->poolBackendPtr();
+    if (report.warnings.empty())
+    {
+        const CasRefCatalog::Snapshot fresh_retirement_catalog
+            = CasRefCatalog::read(admin->backend(), admin->layout());
+        if (!retirement_catalog_cut
+            || fresh_retirement_catalog.token != retirement_catalog_cut->token
+            || fresh_retirement_catalog.catalog != retirement_catalog_cut->catalog)
+        {
+            report.warnings.push_back(
+                "catalog changed after the victim ownership check; refusing slot retirement against a stale cut");
+        }
+    }
+    if (report.warnings.empty())
+    {
+        const String mount_key = layout.mountKey(victim_srid);
+        const String epoch_key = layout.epochKey(victim_srid);
+        const String owner_key = layout.ownerKey(victim_srid);
+
+        /// Capture both the epoch value and its exact token while the decommission claim still fences
+        /// the victim. A successor can only bump this object after the farewell below releases the
+        /// claim, so this token is the epoch-side successor fence for the retirement tail.
+        std::optional<GetResult> claimed_epoch;
+        try
+        {
+            claimed_epoch = pool_backend->get(epoch_key);
+            if (!claimed_epoch)
+                report.warnings.push_back("slot capture failed: " + epoch_key + " is absent under the admin claim");
+        }
+        catch (...)
+        {
+            report.warnings.push_back("slot capture failed: " + epoch_key + ": "
+                                      + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+        }
+
+        /// Graceful close stamps an already-expired lease and the watermark farewell
+        /// (`min_active = UINT64_MAX`), making the slot `terminated` before its mutable control objects
+        /// are removed and its owner anchor is tombstoned.
+        admin.reset();
+
+        /// Read the farewell immediately after `finishTeardown` wrote it. Its exact token is the
+        /// mount-side fence: deleting by this token can remove only THIS decommission's farewell, not
+        /// a successor reclaim. Validate the body against the epoch value captured under the claim so
+        /// a successor that completed before this GET is also recognized and left untouched.
+        std::optional<GetResult> farewell_mount;
+        try
+        {
+            farewell_mount = pool_backend->get(mount_key);
+            if (!farewell_mount)
+                report.warnings.push_back("slot capture failed: " + mount_key + " farewell is absent");
+        }
+        catch (...)
+        {
+            report.warnings.push_back("slot capture failed: " + mount_key + ": "
+                                      + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+        }
+
+        bool captures_match = claimed_epoch && farewell_mount;
+        if (captures_match)
+        {
+            try
+            {
+                const ServerEpoch epoch_value = decodeServerEpoch(claimed_epoch->bytes);
+                const MountLease mount_value = decodeMountLease(farewell_mount->bytes);
+                captures_match = epoch_value.next_writer_epoch != 0
+                    && mount_value.writer_epoch == epoch_value.next_writer_epoch - 1
+                    && mount_value.min_active == std::numeric_limits<uint64_t>::max()
+                    && !mount_value.gc_fenced;
+                if (!captures_match)
+                {
+                    report.warnings.push_back(
+                        "slot capture failed: " + mount_key
+                        + " is not this decommission's farewell for the epoch captured under the admin claim");
+                }
+            }
+            catch (...)
+            {
+                report.warnings.push_back("slot capture failed while validating " + mount_key + " and " + epoch_key + ": "
+                                          + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+                captures_match = false;
+            }
+        }
+
+        /// Mount first: if a successor reclaimed it after the farewell capture, the stale farewell
+        /// token yields `TokenMismatch` and the tail stops before touching epoch or owner. Epoch second:
+        /// its under-claim token similarly detects a successor allocation. Before touching owner, re-read
+        /// both mutable objects: a same-UUID successor can recreate them after both deletes without
+        /// rewriting the owner identity anchor. Mere presence proves that the slot is live again. Every
+        /// delete must be explicitly confirmed as `Deleted`, and the final owner tombstone rewrite must
+        /// succeed against the exact token read immediately before it.
+        ///
+        /// ACCEPTED RESIDUAL WINDOW (final review, not closed by this recheck): a same-UUID successor
+        /// can still recreate epoch/mount in the narrow gap strictly AFTER this liveness recheck but
+        /// BEFORE the owner CAS below reads its own token -- the successor's owner anchor (same
+        /// server_uuid, not yet retired) then gets tombstoned by this decommission run. The successor's
+        /// live process is not deleted (only its owner anchor is marked retired), but a LATER restart of
+        /// that same identity would refuse to reclaim it (claimOwnerOrThrow's tombstone guard). This is
+        /// a narrow, low-probability window, deliberately not closed here: T5's owner-tombstone design
+        /// (finding #9) intentionally stopped short of making concurrent decommission-vs-recreate
+        /// airtight to the microsecond, since that was explicitly not the priority for this fix.
+        report.slot_removed = false;
+        if (captures_match && deleteSlotObject(*pool_backend, mount_key, farewell_mount->token, report.warnings)
+            && deleteSlotObject(*pool_backend, epoch_key, claimed_epoch->token, report.warnings))
+        {
+            std::optional<GetResult> current_mount;
+            std::optional<GetResult> current_epoch;
+            bool liveness_recheck_succeeded = true;
+            try
+            {
+                current_mount = pool_backend->get(mount_key);
+            }
+            catch (...)
+            {
+                report.warnings.push_back("slot liveness recheck failed: " + mount_key + ": "
+                                          + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+                liveness_recheck_succeeded = false;
+            }
+            try
+            {
+                current_epoch = pool_backend->get(epoch_key);
+            }
+            catch (...)
+            {
+                report.warnings.push_back("slot liveness recheck failed: " + epoch_key + ": "
+                                          + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+                liveness_recheck_succeeded = false;
+            }
+
+            if (liveness_recheck_succeeded && (current_mount || current_epoch))
+            {
+                report.warnings.push_back(
+                    "slot delete aborted: successor reappeared after mutable control-object deletion; owner kept");
+            }
+            else if (liveness_recheck_succeeded)
+            {
+                try
+                {
+                    if (const auto owner = pool_backend->get(owner_key))
+                    {
+                        OwnerObject tombstoned = decodeOwner(owner->bytes);
+                        tombstoned.retired_at_ms = nowMs();
+                        /// Controlled, not a bare putOverwrite: a transient transport error here (or
+                        /// one whose response was simply lost) must not be reported as a hard failure
+                        /// when the write actually landed. A standalone controller (decommission is an
+                        /// administrative, non-hot-path operation; no mount-lease fence applies to it
+                        /// -- the exact-token CAS itself is the safety mechanism, same as the mount/
+                        /// epoch deletes above) resolves an ambiguous attempt with one GET: unchanged
+                        /// token means the write never applied (legitimately retryable within budget);
+                        /// matching bytes means this exact tombstone already landed (Committed, not a
+                        /// failure); anything else is a genuine successor reclaim (Conflict).
+                        CasRequestController controller(pool_backend, CasRequestBudget{});
+                        const CasOverwriteResult result = controller.putOverwriteControlled(
+                            owner_key, encodeOwner(tombstoned), owner->token, [] { return true; });
+                        if (result.outcome == CasOverwriteOutcome::Committed)
+                            report.slot_removed = true;
+                        else if (result.outcome == CasOverwriteOutcome::Conflict)
+                            report.warnings.push_back(
+                                "slot tombstone failed: " + owner_key
+                                + ": successor reclaimed the owner anchor before this decommission's tombstone write");
+                        else
+                            report.warnings.push_back(
+                                "slot tombstone failed: " + owner_key
+                                + ": tombstone write outcome could not be resolved (retry budget exhausted "
+                                  "or the resolve GET itself failed) -- rerun the command to retry");
+                    }
+                    else
+                        report.warnings.push_back(
+                            "slot tombstone failed: " + owner_key + ": object absent before tombstone write");
+                }
+                catch (...)
+                {
+                    report.warnings.push_back("slot tombstone failed: " + owner_key + ": "
+                                              + getCurrentExceptionMessage(/*with_stacktrace=*/false));
+                }
+            }
+        }
+    }
+    else
+    {
+        report.slot_removed = false;
+        LOG_WARNING(getLogger("CasDecommission"),
+            "CAS decommission '{}': drain incomplete ({} warnings) — mount slot kept (terminated); "
+            "re-run the command to finish", victim_srid, report.warnings.size());
+        admin.reset();   /// Graceful close still stamps the farewell, leaving the slot `terminated`.
+    }
+
+    /// The `end` event is emitted via `sink` directly, not `EventEmitter{*admin}`: `admin` is gone by
+    /// now. This also means its `warnings` count reflects the FINAL total, including a slot-retirement
+    /// failure appended just above -- `EventEmitter`'s own zero-cost-when-absent guard is reproduced by
+    /// the `if (sink)` below.
+    if (sink)
+    {
+        CasEvent e;
+        e.type = CasEventType::MemberDecommission;
+        e.outcome = "end";
+        e.reason = "decommission finished";
+        e.detail = {{"server_root_id", victim_srid},
+                    {"namespaces_removed", std::to_string(report.namespaces_removed)},
+                    {"warnings", std::to_string(report.warnings.size())},
+                    {"slot_removed", report.slot_removed ? "1" : "0"}};
+        sink(std::move(e));
+    }
+    return report;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h
new file mode 100644
index 000000000000..a86edf9286c4
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h
@@ -0,0 +1,55 @@
+#pragma once
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+
+#include <cstdint>
+#include <functional>
+#include <string>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Counts the work performed by `decommissionPoolMember` for one pool member. The namespace counters
+/// describe metadata and ref-log transitions; the object counters describe physical objects deleted by
+/// the manifest, staging, and mountpoint drains. Blob bytes are intentionally not reported: removing
+/// ref edges makes them eligible for ordinary GC, but this operation does not synchronously reclaim
+/// shared content.
+///
+/// A decommission is resumable. A previous run may already have moved namespaces to `Removing`, and a
+/// warning means that the corresponding drain was not confirmed. In either case the report lets the
+/// caller distinguish work done by this invocation from work observed from an earlier invocation.
+struct DecommissionReport
+{
+    String srid;                                  /// The decommissioned member's `server_root_id`.
+    uint64_t namespaces_removed = 0;              /// Namespaces erased by this invocation.
+    uint64_t namespaces_already_removed = 0;      /// Namespaces already `Removing` on entry.
+    uint64_t committed_refs_removed = 0;          /// Committed ref records removed by namespace drops.
+    uint64_t precommits_removed = 0;              /// Precommit records removed by namespace drops.
+    uint64_t edge_deltas_emitted = 0;             /// The sum of `committed_refs_removed` and `precommits_removed`.
+    uint64_t manifest_debris_removed = 0;         /// Eligible manifest objects deleted from old build prefixes.
+    uint64_t staging_objects_removed = 0;         /// Objects deleted from the member's staging prefix.
+    uint64_t mountpoint_objects_removed = 0;      /// Objects deleted from the member's roots/mountpoint prefix.
+    bool slot_removed = false;                    /// Whether mount and epoch were deleted and the owner was tombstoned.
+    std::vector<String> warnings;                 /// Drain or slot-retirement failures; a non-empty list keeps the slot.
+};
+
+/// Erases all content owned by a permanently dead pool member. The operation first claims the member's
+/// slot as an administrative writer; a live lease is refused, and the claim fences the dead member from
+/// writing while cleanup runs. It then drops each table namespace through `Pool::dropNamespace`, drains
+/// eligible manifest debris, staging objects, and mountpoint objects, and retires the slot only after all
+/// drains are confirmed. Namespace drops are idempotent: a rerun resumes any missing terminal append
+/// and leaves exact catalog-row deletion to GC. The member slot remains while any catalog entry still
+/// belongs to the victim.
+///
+/// This is a writer operation, not GC: it emits the normal ref-edge deltas and does not invent ref
+/// transitions. Per-object drain failures are recorded in `DecommissionReport::warnings` and leave the
+/// terminated slot as a resume anchor; other failures, including refusal to claim the member, propagate
+/// as exceptions. When set, `sink` receives `MemberDecommission` audit events for the run's begin,
+/// per-namespace, and end milestones.
+DecommissionReport decommissionPoolMember(BackendPtr backend, PoolConfig config,
+                                          const String & victim_srid, const CasEventSink & sink = {},
+                                          const std::function<void()> & request_gc_round = {});
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.cpp
new file mode 100644
index 000000000000..7848ae591f27
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.cpp
@@ -0,0 +1,1180 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+
+#include <Common/Exception.h>
+#include <Common/HashTable/Hash.h>
+#include <Common/scope_guard_safe.h>
+
+#include <algorithm>
+#include <limits>
+#include <set>
+#include <sstream>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int TIMEOUT_EXCEEDED;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+constexpr uint64_t PROGRESS_PAGES = 16;
+
+using Deadline = std::optional<std::chrono::steady_clock::time_point>;
+
+/// Enforce the optional overall scan deadline between backend operations. A timeout is propagated as
+/// `TIMEOUT_EXCEEDED`; the public `runFsck` wrapper may convert that exception into a partial report when
+/// explicitly requested.
+void checkDeadline(const Deadline & deadline, std::string_view phase)
+{
+    if (deadline && std::chrono::steady_clock::now() > *deadline)
+        throw Exception(ErrorCodes::TIMEOUT_EXCEEDED,
+            "fsck: exceeded the deadline during '{}' — run against a QUIESCED pool or raise --timeout.", phase);
+}
+
+void listAll(Backend & backend, const String & prefix, std::unordered_map<String, uint64_t> & out,
+             const FsckProgress & on_progress, const Deadline & deadline, std::string_view phase)
+{
+    static constexpr size_t kPageLimit = 1000;
+    uint64_t pages = 0;
+    size_t count_in_page = 0;
+    forEachListedKey(backend, prefix, [&](const ListedKey & k)
+    {
+        out[k.key] = k.size;
+        if (++count_in_page == kPageLimit)
+        {
+            count_in_page = 0;
+            ++pages;
+            checkDeadline(deadline, phase);
+            if (on_progress && pages % PROGRESS_PAGES == 0)
+                on_progress(phase, out.size(), pages);
+        }
+    }, kPageLimit);
+    /// The walk's `backend.list` lands at least once even for an empty/undersized final page --
+    /// check it here, mirroring the original per-page loop (deadline checked after every physical page).
+    if (count_in_page > 0 || pages == 0)
+    {
+        ++pages;
+        checkDeadline(deadline, phase);
+    }
+    if (on_progress)
+        on_progress(phase, out.size(), pages);
+}
+
+/// Parse (writer_epoch, build_sequence) from a manifest object key. Delegates to the one shared
+/// `Layout::parseManifestKey` instead of hand-rolling a second parser; returns false on a
+/// malformed or foreign key.
+bool parseBuildPrefix(const Layout & layout, const String & key, BuildPrefix & out)
+{
+    const auto parsed = layout.parseManifestKey(key);
+    if (!parsed)
+        return false;
+    out.writer_epoch = parsed->ref.writer_epoch;
+    out.build_sequence = parsed->ref.build_sequence;
+    return true;
+}
+
+/// The ref-walk (which builds `reachable_blobs`/`blob_labels`) and the HEAD-confirm below run minutes
+/// apart with no snapshot between them. A ref that gets republished (now names a
+/// different manifest) or DROPPED in that window, combined with a legitimate GC delete of the OLD
+/// blob, makes the stale walk look like a genuine dangle (a "phantom dangling") — this made the fsck
+/// oracle dishonest and falsely report a dangle during long-running validation.
+///
+/// Before counting a HEAD-absent blob as `Dangling`, re-resolve every `"ns/ref"` label under the same
+/// immutable catalog row, using a fresh exact `_ckpt` from that original physical life. This admits a
+/// same-life repoint/drop while refusing a competing rebirth. `label` is split on the LAST '/' —
+/// mirroring exactly how the walk built it (`ns_str + "/" + ref_name`): `ref_name` never contains '/',
+/// but `ns_str` may, so the join separator is always the rightmost one.
+///
+/// Fails CLOSED on any ambiguity (a malformed label, a recovery error, a corrupt manifest): treated as
+/// "still referenced", i.e. the original conservative verdict.
+/// The fix can only SHRINK false positives — it must never hide a real one.
+struct FsckRecoveryAuthority
+{
+    NamespaceLifeId life;
+    CatalogEntry catalog_entry;
+    std::optional<RefCkpt> checkpoint;
+};
+
+using FsckRecoveryAuthorities = std::unordered_map<String, FsckRecoveryAuthority>;
+using RecordRecoveryUnchecked = std::function<void(const RootNamespace &, const String &, const String &)>;
+
+/// Recheck one ref table against a newer `_ckpt` from the SAME physical life selected by fsck's
+/// original catalog cut. The catalog row and life id never move; only the monotone checkpoint may
+/// advance, which is how a same-life drop/repoint that completed during a long scan becomes visible
+/// without admitting a competing rebirth. A missing or unreadable checkpoint cannot prove that an
+/// old owner went away, so the caller records lost coverage and keeps the conservative verdict.
+std::optional<RefTableState> recoverLateRefTable(
+    Backend & backend, const Layout & layout, const FsckRecoveryAuthority & authority,
+    const RecordRecoveryUnchecked & record_unchecked)
+{
+    try
+    {
+        const std::optional<CkptSample> sampled = readCkpt(backend, layout, authority.life);
+        if (!sampled)
+        {
+            record_unchecked(authority.life.ns, layout.refCkptKey(authority.life),
+                "late ref recheck: the original life checkpoint is absent");
+            return std::nullopt;
+        }
+        return recoverRefTableDetailedFromAuthority(
+            backend, layout, authority.catalog_entry, sampled->ckpt).state;
+    }
+    catch (const Exception & e)
+    {
+        record_unchecked(authority.life.ns, layout.refCkptKey(authority.life),
+            "late ref recheck: the original life checkpoint or replay is unreadable: " + e.message());
+        return std::nullopt;
+    }
+    catch (...)
+    {
+        record_unchecked(authority.life.ns, layout.refCkptKey(authority.life),
+            "late ref recheck: the original life checkpoint or replay could not be read");
+        return std::nullopt;
+    }
+}
+
+bool blobStillReferenced(Pool & store, const Layout & layout,
+                          const FsckRecoveryAuthorities & authorities, const String & bkey,
+                          const std::vector<String> & labels, const Deadline & deadline,
+                          const RecordRecoveryUnchecked & record_unchecked)
+{
+    if (labels.empty())
+        return true;
+    for (const String & label : labels)
+    {
+        checkDeadline(deadline, "re-resolving refs at HEAD-absent");
+        const size_t slash = label.rfind('/');
+        if (slash == String::npos)
+            return true;   /// malformed label — cannot re-resolve, fail closed
+        const String ns_part = label.substr(0, slash);
+        const String ref_name = label.substr(slash + 1);
+        try
+        {
+            /// Never read a second catalog cut here. A later rebirth may name the same logical namespace
+            /// but it is not the life whose original row made this blob reachable in this fsck pass.
+            const auto authority_it = authorities.find(ns_part);
+            if (authority_it == authorities.end())
+            {
+                record_unchecked(RootNamespace{ns_part}, layout.refCatalogKey(),
+                    "late blob recheck: no original Live/Removing authority was retained");
+                return true;   /// no original Live/Removing authority -- fail closed
+            }
+            const RootNamespace rns{ns_part};
+            const std::optional<RefTableState> table = recoverLateRefTable(
+                store.backend(), layout, authority_it->second, record_unchecked);
+            if (!table)
+                return true;
+            const auto rit = table->getCommitted().find(ref_name);
+            if (rit == table->getCommitted().end())
+                continue;   /// the ref was DROPPED since the walk — this label no longer applies
+            const PartManifest body = store.readManifest(ManifestId{rns, rit->second.manifest_ref});
+            for (const ManifestEntry & e : body.entries)
+            {
+                if (e.placement != EntryPlacement::Blob)
+                    continue;
+                if (layout.blobKey(e.ref) == bkey)
+                    return true;   /// an original-life ref still names this exact blob — a real dangle
+            }
+        }
+        catch (...)
+        {
+            return true;   /// cannot confirm the ref moved away — keep the conservative verdict
+        }
+    }
+    return false;   /// no original-life label names this blob — the stale-walk artifact is gone
+}
+
+/// The manifest sibling of the `blobStillReferenced` recheck above. The ref-walk captures each committed
+/// `(ref_name -> manifest_ref)` from a FRESH per-namespace recovery, but the `backend.get(mkey)` that
+/// confirms the manifest body runs LATER in the same (possibly long) namespace loop. A ref republished to
+/// a DIFFERENT manifest — or DROPPED — in that window, combined with a legitimate GC delete of the OLD
+/// manifest body, makes the stale captured row look like a committed ref over a missing manifest (a
+/// "phantom dangling manifest"), the same dishonest-oracle failure `blobStillReferenced` kills for blobs.
+///
+/// Before counting a missing manifest body as `Dangling`, re-resolve the EXACT ref from the SAME frozen
+/// catalog row with a fresh exact `_ckpt` from that original physical life, then check whether the
+/// committed row still names THIS exact manifest key. A later catalog cut must not replace that row,
+/// but a same-life checkpoint advance must be visible. Fails CLOSED on any ambiguity (a throw, a corrupt
+/// table): treated as "still referenced", the original conservative verdict — the fix can only SHRINK
+/// false positives, never hide a real loss.
+bool manifestStillReferenced(Backend & backend, const Layout & layout, const RootNamespace & ns,
+                             const FsckRecoveryAuthorities & authorities, const String & ref_name,
+                             const String & mkey, const Deadline & deadline,
+                             const RecordRecoveryUnchecked & record_unchecked)
+{
+    checkDeadline(deadline, "re-resolving ref at missing-manifest");
+    try
+    {
+        const auto authority_it = authorities.find(ns.string());
+        if (authority_it == authorities.end())
+        {
+            record_unchecked(ns, layout.refCatalogKey(),
+                "late manifest recheck: no original Live/Removing authority was retained");
+            return true;   /// no original Live/Removing authority -- fail closed
+        }
+        const std::optional<RefTableState> table = recoverLateRefTable(
+            backend, layout, authority_it->second, record_unchecked);
+        if (!table)
+            return true;
+        const auto rit = table->getCommitted().find(ref_name);
+        if (rit == table->getCommitted().end())
+            return false;   /// the ref was DROPPED since the walk — no longer a committed owner
+        /// A republish moved the ref to a different manifest key: this old key is no longer owned.
+        return layout.manifestKey(ManifestId{ns, rit->second.manifest_ref}) == mkey;
+    }
+    catch (...)
+    {
+        return true;   /// cannot confirm the ref moved away — keep the conservative verdict
+    }
+}
+
+String renderId(const RefTxnId & id)
+{
+    return std::to_string(id.writer_epoch) + "-" + std::to_string(id.ref_sequence);
+}
+
+/// Per-NAMESPACE verdicts of the stream audit. Both counters count namespaces, not rows: a namespace
+/// has exactly one answer about its stream even when several checks reach it.
+///
+/// A namespace PROVEN broken is never also counted `unchecked`. "Proved broken" and "could not prove"
+/// are different answers, and letting the second overwrite or accompany the first would turn a fatal
+/// into an ambiguity — the recovery path throws on a holed stream, so a chain-broken namespace reliably
+/// produces a downstream failure too, and that failure must not dilute the verdict that explains it.
+struct NsVerdicts
+{
+    std::set<String> chain_broken;
+    std::set<String> unchecked;
+
+    void recordChainBroken(FsckReport & report, const RootNamespace & ns, const String & key, String note)
+    {
+        chain_broken.insert(ns.string());
+        unchecked.erase(ns.string());
+        push(report, key, FsckClass::ChainBroken, std::move(note));
+    }
+
+    void recordUnchecked(FsckReport & report, const RootNamespace & ns, const String & key, String note)
+    {
+        if (chain_broken.contains(ns.string()))
+            return;
+        unchecked.insert(ns.string());
+        push(report, key, FsckClass::Unchecked, std::move(note));
+    }
+
+    /// Both classes are emitted in EVERY mode, not just `detail`: they are namespace verdicts, bounded
+    /// by the namespace count, and a summary run that hid them would report a number nobody could act on.
+    void push(FsckReport & report, const String & key, FsckClass cls, String note) const
+    {
+        FsckObject o;
+        o.key = key;
+        o.kind = ObjectKind::Blob;   /// ref objects have no ObjectKind; reuse Blob as the generic kind
+        o.size = 0;
+        o.cls = cls;
+        o.reachable_from = {std::move(note)};
+        report.objects.push_back(std::move(o));
+    }
+
+    void publish(FsckReport & report) const
+    {
+        report.chain_broken = chain_broken.size();
+        report.unchecked = unchecked.size();
+    }
+};
+
+/// THE ARITHMETIC STREAM WALK (spec §7). Read-only, one namespace.
+///
+/// The frozen catalog row and exact `_ckpt` define the complete finite walk. LIST supplies no genesis,
+/// witness, frontier or stop condition, and the walker never probes the position after
+/// `_ckpt.committed_through`. Every required id is point-read from the checkpoint base's successor (or
+/// `{life_epoch, 1}`) through that inclusive frontier. A missing required id is therefore a proven hole;
+/// no above-hole listing witness is needed. An epoch seal advances directly to the next epoch's first id,
+/// exactly as authoritative read-only recovery does.
+void checkRefStream(Backend & backend, const Layout & layout, const NamespaceLifeId & life,
+                    const CatalogEntry & catalog_entry, const std::optional<CkptSample> & checkpoint_sample,
+                    const Deadline & deadline, FsckReport & report, NsVerdicts & verdicts)
+{
+    checkDeadline(deadline, "ref stream");
+    const RootNamespace & ns = life.ns;
+    const std::optional<RefCkpt> checkpoint
+        = checkpoint_sample ? std::optional<RefCkpt>{checkpoint_sample->ckpt} : std::nullopt;
+    const RecoveryGrounding grounding = chooseRecoveryGrounding(catalog_entry, checkpoint);
+    if (grounding.base)
+    {
+        try
+        {
+            /// Even when the base IS the frontier and there is no replay tail, a checkpoint may not
+            /// turn an `EpochSeal` into a state snapshot by naming a same-id `_snap`.
+            (void)readCheckpointSnapshotBase(backend, layout, life, *checkpoint);
+        }
+        catch (const Exception & e)
+        {
+            const String key = layout.refSnapshotKey(life, *grounding.base);
+            const String note = "ref stream: checkpoint snapshot base " + renderId(*grounding.base)
+                + " is invalid: " + e.message();
+            if (e.code() != ErrorCodes::CORRUPTED_DATA)
+            {
+                verdicts.recordUnchecked(report, ns, key, note);
+                return;
+            }
+
+            /// A concurrent checkpoint advance may retire the sampled base between these exact reads.
+            /// Only the SAME checkpoint incarnation turns a missing/invalid member of its required
+            /// triple into durable corruption. A changed, absent, or unreadable authority proves no
+            /// such thing and remains the honest `Unchecked` answer.
+            checkDeadline(deadline, "checkpoint-base authority revalidation");
+            try
+            {
+                const std::optional<CkptSample> current = readCkpt(backend, layout, life);
+                if (!current || !checkpoint_sample || current->token != checkpoint_sample->token)
+                {
+                    verdicts.recordUnchecked(report, ns, key,
+                        note + "; checkpoint authority changed while validating its snapshot base");
+                    return;
+                }
+            }
+            catch (const Exception & revalidation_error)
+            {
+                verdicts.recordUnchecked(report, ns, key,
+                    note + "; checkpoint authority could not be revalidated: " + revalidation_error.message());
+                return;
+            }
+            catch (...)
+            {
+                verdicts.recordUnchecked(report, ns, key,
+                    note + "; checkpoint authority could not be revalidated");
+                return;
+            }
+
+            verdicts.recordChainBroken(report, ns, key, note);
+            return;
+        }
+    }
+    if (!grounding.walk_from || !grounding.committed_through)
+        return;
+
+    RefTxnId expected = *grounding.walk_from;
+    while (expected <= *grounding.committed_through)
+    {
+        checkDeadline(deadline, "ref stream");
+        const auto got = backend.get(layout.refLogKey(life, expected));
+        if (!got)
+        {
+            verdicts.recordChainBroken(report, ns, layout.refLogKey(life, expected),
+                "ref stream: checkpoint requires id " + renderId(expected) + " at or below inclusive frontier "
+                + renderId(*grounding.committed_through) + ", but its exact key is absent");
+            return;
+        }
+
+        bool is_seal = false;
+        try
+        {
+            is_seal = refLogTxnIsEpochSeal(
+                decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), expected));
+        }
+        catch (const Exception & e)
+        {
+            verdicts.recordUnchecked(report, ns, layout.refLogKey(life, expected),
+                "ref stream: the checkpoint-required record at " + renderId(expected)
+                + " could not be decoded: " + e.message());
+            return;
+        }
+        ++report.ref_records_walked;
+
+        try
+        {
+            if (const std::optional<RefTxnId> next = nextRefLogIdWithinCommittedFrontier(
+                    expected, is_seal, *grounding.committed_through))
+                expected = *next;
+            else
+                break;
+        }
+        catch (const Exception & e)
+        {
+            verdicts.recordChainBroken(report, ns, layout.refLogKey(life, expected),
+                "ref stream: " + e.message());
+            return;
+        }
+    }
+}
+
+/// Perform the scan and accumulate into `report`. This helper owns the read-only traversal: it first
+/// recovers authoritative refs, then checks physical objects and GC labels, while preserving the
+/// distinction between a missing live object and expected in-flight cleanup. Deadline exceptions are
+/// intentionally left to `runFsck`, which decides whether partial results were requested.
+void runFsckImpl(Pool & store, bool detail, const FsckProgress & on_progress, const Deadline & deadline,
+                  const String & namespace_prefix, FsckReport & report)
+{
+    const Layout & layout = store.layout();
+    Backend & backend = store.backend();
+    /// Path-derived per-object algorithm parsing: every listed blob-tree key -- across every
+    /// admitted algo, not just the pool's node-local write algo -- is classified via
+    /// `Layout::parseBlobKey`, which derives the `BlobRef` from the key's OWN `<algo>` path segment
+    /// (and its `.meta` sibling). A foreign/malformed key (unknown algo segment, wrong-width hex, a
+    /// non-`.meta`/non-blob shape) parses to `std::nullopt` and is classified as debris, never an
+    /// exception.
+
+    /// Reachability is recomputed from the authoritative refs (never from GC state):
+    /// for each namespace, each committed ref resolves to a ManifestId; read its body; a committed ref
+    /// naming a MISSING body is an ERROR (Dangling); a present body whose blobs are missing is an ERROR.
+    std::set<String> reachable_blobs;        /// blob object keys named by a live owner
+    std::set<String> owned_manifest_keys;    /// manifest object keys named by a committed owner
+    /// blob key -> "ns/ref" labels of the refs that named it. Always populated (not just under
+    /// `detail`) — the HEAD-absent re-resolve below needs it in every mode.
+    std::unordered_map<String, std::vector<String>> blob_labels;
+
+    uint64_t refs_walked = 0;
+    NsVerdicts verdicts;
+    SCOPE_EXIT({ verdicts.publish(report); });
+    const RecordRecoveryUnchecked record_recovery_unchecked =
+        [&](const RootNamespace & ns, const String & key, const String & detail_text)
+        {
+            verdicts.recordUnchecked(report, ns, key, detail_text);
+        };
+
+    /// RECORD AND CONTINUE for a key that belongs to no namespace at all. fsck is the forensic tool an
+    /// operator reaches for once something is already wrong, so a key it cannot attribute must become a
+    /// FINDING and not an abort: an audit that died on the first bad key would report nothing about the
+    /// healthy namespaces it never reached, which is the wrong failure order for a read-only diagnostic.
+    ///
+    /// `seen` is what makes the count a count of DEFECTS: each sweep below enumerates namespaces again
+    /// and sees the same offending key, and only the first sighting is recorded.
+    std::set<String> lifeless_seen;
+    auto recordLifelessKeys = [&](const NamespaceListing & listing)
+    {
+        for (const UnattributableNamespaceKey & bad : listing.skipped)
+        {
+            if (!lifeless_seen.insert(bad.key).second)
+                continue;
+            ++report.lifeless_keys;
+            FsckObject o;
+            o.key = bad.key;
+            o.kind = ObjectKind::Blob;   /// a lifeless key has no ObjectKind; reuse Blob as the generic kind
+            o.cls = FsckClass::LifelessKey;
+            o.size = 0;
+            o.reachable_from = {bad.reason};
+            report.objects.push_back(std::move(o));
+        }
+    };
+
+    /// One immutable cut owns every physical-id join in this walk. `Creating` participates in that
+    /// attribution (its physical keys may exist) but is never recovered: only Live/Removing rows have a
+    /// durable publication frontier. A diagnostic records duplicate ids and keeps walking unrelated
+    /// unique lives.
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(backend, layout);
+    struct FsckWalkLife
+    {
+        NamespaceLifeId life;
+        CatalogEntry catalog_entry;
+    };
+    std::vector<FsckWalkLife> walk_lives;
+    walk_lives.reserve(catalog_cut.catalog.entries.size());
+    for (const CatalogEntry & entry : catalog_cut.catalog.entries)
+    {
+        if (!entry.ns.string().starts_with(namespace_prefix))
+            continue;
+        if (entry.state == NsState::Creating)
+            continue;
+        try
+        {
+            if (const auto life = catalog_cut.life_index.resolve(entry.incarnation))
+                walk_lives.push_back(FsckWalkLife{.life = *life, .catalog_entry = entry});
+        }
+        catch (const Exception & e)
+        {
+            if (e.code() != ErrorCodes::CORRUPTED_DATA)
+                throw;
+            recordLifelessKeys(NamespaceListing{{}, {{
+                layout.refCatalogKey() + "#" + renderIncarnation(entry.incarnation), e.message()}}});
+        }
+    }
+
+    /// Physical life-owned keys carry no logical name. Classify each COMPLETE, canonical key against a
+    /// catalog cut taken AFTER this physical listing finishes (observe-then-cut), not the earlier
+    /// `catalog_cut` above: `NamespaceJanitor::runOnePage` (the only real deleter of this debris) uses
+    /// the identical ordering, and it is what makes "life absent from a LATER cut" sound -- creation
+    /// always admits a `Creating` catalog row before writing any life-owned object (spec §2), so a life
+    /// that is absent from a cut taken after the listing cannot be a concurrent birth this listing raced.
+    /// A malformed shape (the parser refuses, or the reserved segment names no clean relative file) is
+    /// classified immediately as it cannot become residue no matter which cut resolves it.
+    if (namespace_prefix.empty())
+    {
+        struct CanonicalNamespaceKey
+        {
+            String key;
+            uint64_t size;
+            NamespaceLifePhysicalId life_id;
+        };
+        std::vector<CanonicalNamespaceKey> canonical_candidates;
+
+        forEachListedKey(backend, layout.namespaceRootPrefix(), [&](const ListedKey & listed)
+        {
+            std::optional<NamespaceLifePhysicalId> physical_id;
+            try
+            {
+                if (const auto ref_object = layout.parseRefObjectKey(listed.key))
+                    physical_id = ref_object->life_id;
+                else if (const auto checkpoint = layout.parseRefCkptKey(listed.key))
+                    physical_id = *checkpoint;
+                else if (const auto namespace_file = layout.parseNamespaceFileKey(listed.key))
+                    physical_id = namespace_file->life_id;
+                else
+                {
+                    recordLifelessKeys(NamespaceListing{{}, {{listed.key, "unrecognized key under the namespace ownership tree"}}});
+                    return;
+                }
+            }
+            catch (const Exception & e)
+            {
+                if (e.code() != ErrorCodes::CORRUPTED_DATA)
+                    throw;
+                recordLifelessKeys(NamespaceListing{{}, {{listed.key, e.message()}}});
+                return;
+            }
+            canonical_candidates.push_back(CanonicalNamespaceKey{listed.key, listed.size, *physical_id});
+        });
+
+        /// The post-observation cut. All three catalog states -- `Creating`, `Live`, `Removing` --
+        /// protect a life for this purpose; only a life absent from every one of them is residue.
+        const CasRefCatalog::Snapshot post_listing_cut = CasRefCatalog::read(backend, layout);
+        std::unordered_set<UInt128> pending_lives;
+        for (const CanonicalNamespaceKey & candidate : canonical_candidates)
+        {
+            try
+            {
+                if (post_listing_cut.life_index.resolve(candidate.life_id))
+                    continue;   /// protected by some catalog state as of the later cut -- not residue
+            }
+            catch (const Exception & e)
+            {
+                /// The reverse life index throws `CORRUPTED_DATA` when the post-listing cut carries a
+                /// duplicated life id: a catalog defect, not evidence about THIS key. Record and keep
+                /// walking, same as every other catalog-authority failure in this scan -- an audit that
+                /// aborted on the first bad key would report nothing about the healthy candidates
+                /// still queued behind it.
+                if (e.code() != ErrorCodes::CORRUPTED_DATA)
+                    throw;
+                recordLifelessKeys(NamespaceListing{{}, {{candidate.key, e.message()}}});
+                continue;
+            }
+            ++report.namespace_janitor_pending;
+            report.namespace_janitor_pending_bytes += candidate.size;
+            pending_lives.insert(candidate.life_id);
+            FsckObject o;
+            o.key = candidate.key;
+            o.kind = ObjectKind::Blob;   /// no ObjectKind names namespace-life debris; reuse Blob as the generic kind
+            o.cls = FsckClass::JanitorPending;
+            o.size = candidate.size;
+            o.reachable_from = {"physical life id is absent from a catalog cut taken after this listing; "
+                                 "janitor-pending, not corruption"};
+            report.objects.push_back(std::move(o));
+        }
+        report.namespace_janitor_pending_lives = pending_lives.size();
+    }
+
+    /// Every replay and late recheck below reuses the same exact catalog row and physical life. The
+    /// primary walk also retains its checkpoint sample; a late recheck exact-reads `_ckpt` again at that
+    /// SAME life so a concurrent same-life drop/repoint is visible without ever accepting a rebirth.
+    FsckRecoveryAuthorities recovery_authorities;
+    recovery_authorities.reserve(walk_lives.size());
+
+    for (const FsckWalkLife & walk_life : walk_lives)
+    {
+        const NamespaceLifeId & life = walk_life.life;
+        const RootNamespace & ns = life.ns;
+        const String & ns_str = ns.string();
+        /// RECORD AND CONTINUE, NEVER WEDGE. Everything below is per-namespace, and every one of these
+        /// steps can raise `CORRUPTED_DATA` on a namespace whose stream is damaged — the replay refuses a
+        /// non-contiguous tail, the codecs refuse an invalid body. For RECOVERY that throw is the correct
+        /// fail-close; for a read-only diagnostic it is a bug, because the audit then reports NOTHING
+        /// about the namespaces it never reached, including the healthy ones. So one namespace's failure
+        /// becomes that namespace's verdict and the sweep goes on.
+        ///
+        /// `TIMEOUT_EXCEEDED` is deliberately NOT caught: the deadline is a property of the whole scan,
+        /// and `runFsck`'s `partial` handling owns it.
+        try
+        {
+            /// One materialized `_ckpt` body is part of this namespace's frozen audit authority. The
+            /// recovery API receives exactly these bytes; `checkRefStream` receives the same decoded
+            /// value, so the two legs cannot quietly choose different frontiers after a concurrent CAS.
+            const std::optional<CkptSample> checkpoint_sample = readCkpt(backend, layout, life);
+            const std::optional<RefCkpt> checkpoint
+                = checkpoint_sample ? std::optional<RefCkpt>{checkpoint_sample->ckpt} : std::nullopt;
+            const auto [authority_it, inserted] = recovery_authorities.emplace(
+                ns.string(), FsckRecoveryAuthority{
+                    .life = life, .catalog_entry = walk_life.catalog_entry, .checkpoint = checkpoint});
+            chassert(inserted);
+
+            /// The arithmetic stream audit runs FIRST, so a holed stream gets the verdict that EXPLAINS
+            /// it (`chain-broken`) rather than the downstream `CORRUPTED_DATA` the replay below would
+            /// raise about the same hole.
+            checkRefStream(
+                backend, layout, life, walk_life.catalog_entry, checkpoint_sample, deadline, report, verdicts);
+
+            /// This recovery's finite range comes from the original catalog row and exact `_ckpt`, never
+            /// from a stream listing, a self-resolved name, or an F+1 probe.
+            const RefTableState table = recoverRefTableDetailedFromAuthority(
+                backend, layout, authority_it->second.catalog_entry, authority_it->second.checkpoint).state;
+            for (const auto [ref_name, row] : table.getCommitted())
+            {
+                const ManifestId id{ns, row.manifest_ref};
+                const String mkey = layout.manifestKey(id);
+                owned_manifest_keys.insert(mkey);
+                const String label = ns_str + "/" + ref_name;
+
+                const auto got = backend.get(mkey);
+                if (!got)
+                {
+                    /// A committed ref naming a missing manifest body would be an INV-NO-DANGLE violation —
+                    /// but the per-ref GET runs later than the namespace's ref recovery, so a stale captured
+                    /// row plus a legitimate GC delete of a since-superseded manifest can masquerade as one,
+                    /// and a bare GET can lag a present object. Revalidate exactly like the blob `Dangling`
+                    /// recheck below: HEAD the exact object AND re-resolve under the original catalog row
+                    /// plus a fresh checkpoint from its physical life. Count the dangle ONLY when the exact
+                    /// object is HEAD-absent AND that life still names THIS exact manifest — otherwise it is
+                    /// LIST/GET lag or a phantom stale-row, never a loss.
+                    if (!backend.head(mkey).exists
+                        && manifestStillReferenced(backend, layout, ns, recovery_authorities, ref_name, mkey,
+                            deadline, record_recovery_unchecked))
+                    {
+                        ++report.dangling;
+                        FsckObject o;
+                        o.key = mkey;
+                        o.kind = ObjectKind::Blob;   /// manifests have no ObjectKind; reuse Blob as the generic kind
+                        o.size = 0;
+                        o.cls = FsckClass::Dangling;
+                        o.reachable_from = {label};
+                        report.objects.push_back(std::move(o));
+                    }
+                    /// A present object is GET lag. A row not named by the original-life authority is a
+                    /// stale-walk artifact, not a dangle; its original owner cannot contribute blobs.
+                    ++refs_walked;
+                    continue;
+                }
+
+                PartManifest body = decodePartManifest(openObject(FormatId::PartManifest, got->bytes));
+                if (!refMatchesBody(id.ref, body) || !manifestNamespaceMatches(id.root_namespace, body))
+                {
+                    ++report.dangling;
+                    FsckObject o;
+                    o.key = mkey;
+                    o.kind = ObjectKind::Blob;
+                    o.size = got->bytes.size();
+                    o.cls = FsckClass::Dangling;
+                    o.reachable_from = {label};
+                    report.objects.push_back(std::move(o));
+                    ++refs_walked;
+                    continue;
+                }
+
+                for (const ManifestEntry & e : body.entries)
+                {
+                    if (e.placement != EntryPlacement::Blob)
+                        continue;
+                    const String bkey = layout.blobKey(e.ref);
+                    reachable_blobs.insert(bkey);
+                    ++report.total_blob_refs;
+                    report.referenced_logical_bytes += e.blob_size;
+                    blob_labels[bkey].push_back(label);
+                }
+
+                ++refs_walked;
+                checkDeadline(deadline, "walking refs");
+                if (on_progress && refs_walked % 64 == 0)
+                    on_progress("walking refs", reachable_blobs.size(), refs_walked);
+            }
+        }
+        catch (const Exception & e)
+        {
+            if (e.code() == ErrorCodes::TIMEOUT_EXCEEDED)
+                throw;
+            verdicts.recordUnchecked(report, ns,
+                layout.namespaceStreamPrefix(life),
+                "fsck could not examine this namespace: " + e.message());
+        }
+    }
+    report.distinct_blobs = reachable_blobs.size();
+
+    /// Scoped mode skips the GLOBAL physical classification below: it is meaningless under a
+    /// filter (blobs owned by other namespaces would read as unreachable) and would cost a
+    /// pool-wide LIST for what should be O(scoped refs).
+    if (namespace_prefix.empty())
+    {
+    /// Physical listing: blobs + manifest bodies. The per-hash `.meta` descriptor sibling
+    /// (`blobMetaKey(id) == blobKey(id) + ".meta"`) lives under the SAME
+    /// `blobsPrefix()` as the body, so partition the raw LIST into bodies vs `.meta` objects up
+    /// front — a `.meta` key must never be classified as a content body (it would otherwise be
+    /// misread as an unreferenced blob and fall into the dangling/pending/unaccounted pipeline
+    /// below), and a body must never be misread as a `.meta`.
+    std::unordered_map<String, uint64_t> present_all;
+    listAll(backend, layout.blobsPrefix(), present_all, on_progress, deadline, "listing blobs");
+    std::unordered_map<String, uint64_t> present_blobs;
+    std::unordered_set<BlobRef, BlobRefHash> present_meta_hashes;
+    present_blobs.reserve(present_all.size());
+    for (const auto & [key, sz] : present_all)
+    {
+        if (key.ends_with(".meta"))
+        {
+            if (const std::optional<BlobRef> ref = layout.parseBlobKey(key))
+                present_meta_hashes.insert(*ref);
+            /// else: foreign key shape under blobs/ — not ours to pair
+        }
+        else
+            present_blobs.emplace(key, sz);
+    }
+    for (const auto & [_, sz] : present_blobs)
+        report.physical_bytes += sz;
+
+    /// Reachable blobs must be present (HEAD-confirm against LIST lag before declaring loss).
+    for (const String & bkey : reachable_blobs)
+    {
+        auto it = present_blobs.find(bkey);
+        bool exists = it != present_blobs.end();
+        uint64_t size = exists ? it->second : 0;
+        if (!exists)
+        {
+            const HeadResult h = backend.head(bkey);
+            if (h.exists)
+            {
+                exists = true;
+                size = h.size;
+                report.physical_bytes += h.size;
+            }
+        }
+
+        const auto lit = blob_labels.find(bkey);
+        if (!exists)
+        {
+            /// Before declaring a loss, re-resolve the referencing refs from the original audit
+            /// authority. A later rebirth must not replace the old owner while this verdict is being
+            /// decided.
+            const bool still_referenced = blobStillReferenced(store, layout, recovery_authorities, bkey,
+                lit != blob_labels.end() ? lit->second : std::vector<String>{}, deadline,
+                record_recovery_unchecked);
+            if (!still_referenced)
+                continue;   /// stale-walk artifact: neither reachable nor dangling — skip entirely
+        }
+
+        if (exists)
+            ++report.reachable;
+        else
+            ++report.dangling;
+        if (detail || !exists)
+        {
+            FsckObject o;
+            o.key = bkey;
+            o.kind = ObjectKind::Blob;
+            o.size = size;
+            o.cls = exists ? FsckClass::Reachable : FsckClass::Dangling;
+            if (detail && lit != blob_labels.end())
+                o.reachable_from = lit->second;
+            report.objects.push_back(std::move(o));
+        }
+    }
+
+    /// Present-but-unreferenced blobs: classify through the GC pipeline view instead of one
+    /// suspicious "unreachable" lump (the multi-stage graduation keeps a nonzero churning
+    /// set here on ANY active pool, and beta testers read "unreachable" as a leak). The GC state is
+    /// read for LABELING ONLY — reachability above never consults it.
+    std::unordered_map<BlobRef, RetiredEntry, BlobRefHash> retired_by_hash;
+    std::unordered_set<BlobRef, BlobRefHash> unref_hashes;
+    std::unordered_set<BlobRef, BlobRefHash> in_run_hashes;
+    /// The NON-SENTINEL source edges the snapshot still holds on each unreferenced blob, collected in
+    /// `detail` mode only. `in_run_hashes` alone answers "does GC still see this blob at all"; the
+    /// stale-edge cross-check below needs the edge IDENTITIES so it can ask whether their source
+    /// manifests still exist. Sentinel rows (`source_id == 0` — `kZeroMarker`/`kCondemned`) are not
+    /// edges and are excluded.
+    std::unordered_map<BlobRef, std::vector<UInt128>, BlobRefHash> unref_edge_sources;
+    bool have_gc_state = false;
+
+    for (const auto & [bkey, sz] : present_blobs)
+        if (!reachable_blobs.contains(bkey))
+        {
+            if (const std::optional<BlobRef> ref = layout.parseBlobKey(bkey))
+                unref_hashes.insert(*ref);
+        }
+
+    if (!unref_hashes.empty())
+    {
+        if (const auto state_got = backend.get(layout.gcStateKey()))
+        {
+            have_gc_state = true;
+            const GcState gc_state = decodeGcState(state_got->bytes);
+            /// The adopted fold seal names the snapshot runs; resolution is by ref, never by key
+            /// construction. Every row whose hash is in our candidate set marks "known to GC" —
+            /// edges still counted (drop unfolded), an explicit zero-marker mid-pipeline, or a
+            /// `kCondemned` sentinel row that carries the condemned state (retired-in-snapshot):
+            /// the `kCondemned` rows feed `retired_by_hash` (the `PendingGc` classification) in the
+            /// SAME pass, replacing the removed `retired_refs`/`decodeRetiredSet` loop.
+            ///
+            /// These sets are keyed by the full `BlobRef`, not a narrowed digest. The run's own
+            /// algorithm-prefixed key is parsed by `SourceEdgeKeyCodec` and compared directly with
+            /// the full identity parsed from the listed blob key. This is required for mixed-algorithm
+            /// pools: a 64-hex digest must not be truncated or compared as though it used the pool's
+            /// local write algorithm, or its true GC state could be hidden as `Unaccounted`.
+            if (const auto seal_got = backend.get(layout.foldSealKey(gc_state.snap_generation, gc_state.snap_attempt)))
+            {
+                uint64_t rows = 0;
+                for (const RunRef & run : decodeFoldSeal(seal_got->bytes, gc_state.snap_generation).blob_target_runs)
+                {
+                    checkDeadline(deadline, "reading gc snapshot runs");
+                    /// Typed open: the source-edge run reader goes through openSourceEdgeRun (the NDJSON
+                    /// header gates type == cas_run + kind == source_edge). Fsck keys off the row's hash
+                    /// (the record's own algo-prefixed key, never from pool meta).
+                    SourceEdgeRunView reader = openSourceEdgeRun(backend, run.key);
+                    String key;
+                    String payload;
+                    while (reader.next(key, payload))
+                    {
+                        BlobRef ref;
+                        UInt128 source_id;
+                        SourceEdgeKeyCodec::parse(key, ref, source_id);   // throws CORRUPTED_DATA on malformed (fail-closed)
+                        if (unref_hashes.contains(ref))
+                        {
+                            in_run_hashes.insert(ref);
+                            if (detail && source_id != UInt128{0})
+                                unref_edge_sources[ref].push_back(source_id);
+                            if (!payload.empty() && payload[0] == kCondemned)
+                            {
+                                const CondemnedRow row = decodeCondemnedRow(payload);
+                                RetiredEntry e;
+                                e.kind = ObjectKind::Blob;
+                                e.ref = ref;
+                                e.token = row.token;
+                                e.size = row.size;
+                                e.condemn_round = row.condemn_round;
+                                e.delete_pending = row.delete_pending;
+                                retired_by_hash.emplace(ref, std::move(e));
+                            }
+                        }
+                        if (on_progress && ++rows % 65536 == 0)
+                            on_progress("reading gc snapshot runs", in_run_hashes.size(), rows);
+                    }
+                    /// Whole-file seal checksum: compare the drained run's accumulated
+                    /// checksum to the seal's `RunRef::checksum`. Fsck is a read-only auditor — instead of
+                    /// throwing (which would abort the whole scan on the first corrupt run), catalogue the
+                    /// mismatch as a `CorruptedRun` finding (with the run key) and continue so the audit
+                    /// enumerates every problem in one pass. The deletion-deriving consumers
+                    /// (`fold`/`zeroInDegree`/`previewDeletes`) still fail closed on the same mismatch.
+                    if (reader.accumulatedChecksum() != run.checksum)
+                    {
+                        ++report.corrupted_runs;
+                        if (detail)
+                            report.objects.push_back(FsckObject{.key = run.key, .cls = FsckClass::CorruptedRun, .reachable_from = {}});
+                    }
+                }
+            }
+        }
+    }
+
+    /// STALE-EDGE cross-check. A residual `+1` whose matching `-1` never folded pins its blob at
+    /// in-degree 1 forever: every GC round recomputes the same nonzero in-degree and never nominates
+    /// the blob, so the `AwaitingGc` "expected, no action needed" label is a lie — nothing will ever
+    /// reclaim it. The edge names its source, so the check is to ask whether that source still exists:
+    /// build the set of source ids that every manifest body PRESENT in the pool would contribute, and
+    /// treat an edge outside that set as one whose source manifest is gone.
+    ///
+    /// COST: one LIST per namespace plus one GET per manifest body. It is therefore gated on `detail`
+    /// — the cheap summary path (the ca-soak fixpoint poll calls it in a loop) must not gain a single
+    /// extra request — and additionally on some unreferenced blob actually carrying a real edge, so a
+    /// pool with nothing to cross-check pays nothing.
+    ///
+    /// `stale_edge_check_available` is the fail-closed switch: a manifest body we cannot decode would
+    /// silently withhold its edges from the live set and turn every blob it owns into a false hard
+    /// finding, so one undecodable body disables the whole cross-check for this scan rather than
+    /// manufacture an error. The check may only ever SHRINK to silence, never invent a finding.
+    std::unordered_set<UInt128, UInt128Hash> live_source_ids;
+    bool stale_edge_check_available = detail && !unref_edge_sources.empty();
+    if (stale_edge_check_available)
+    {
+        const NamespaceListing stale_edge_listing = store.listNamespaces(namespace_prefix);
+        recordLifelessKeys(stale_edge_listing);
+        for (const String & ns_str : stale_edge_listing.namespaces)
+        {
+            const RootNamespace ns{ns_str};
+            std::unordered_map<String, uint64_t> manifest_bodies;
+            listAll(backend, layout.manifestNamespacePrefix(ns), manifest_bodies, on_progress, deadline,
+                    "listing manifests for the stale-edge check");
+            for (const auto & [mkey, _] : manifest_bodies)
+            {
+                checkDeadline(deadline, "reading manifests for the stale-edge check");
+                const std::optional<ManifestId> id = layout.parseManifestKey(mkey);
+                if (!id)
+                    continue;   /// foreign/malformed key under `manifests/` — contributes no source edge
+                const auto got = backend.get(mkey);
+                if (!got)
+                    continue;   /// gone between the LIST and the GET — genuinely not a live source
+                try
+                {
+                    const PartManifest body = decodePartManifest(openObject(FormatId::PartManifest, got->bytes));
+                    for (const ManifestEntry & e : body.entries)
+                        if (e.placement == EntryPlacement::Blob)
+                            live_source_ids.insert(sourceEdgeId(*id, e.path));
+                }
+                catch (...)
+                {
+                    stale_edge_check_available = false;   /// incomplete live set — do not accuse anyone
+                    break;
+                }
+            }
+            if (!stale_edge_check_available)
+                break;
+        }
+    }
+
+    for (const auto & [bkey, sz] : present_blobs)
+    {
+        if (reachable_blobs.contains(bkey))
+            continue;
+        ++report.unreachable;
+
+        /// A foreign/malformed key (`parseBlobKey` -> `nullopt`) falls back to the default `BlobRef{}`,
+        /// which cannot match a real `retired_by_hash`/`in_run_hashes` entry — it lands in the generic
+        /// `Unaccounted` bucket below, exactly the "debris, not ours" classification `parseBlobKey`
+        /// documents: foreign algorithm segments are debris, not pool objects.
+        const BlobRef hash = layout.parseBlobKey(bkey).value_or(BlobRef{});
+
+        FsckClass cls = FsckClass::Unaccounted;
+        String note;
+        if (const auto rit = retired_by_hash.find(hash); rit != retired_by_hash.end()
+            && backend.head(bkey).token == rit->second.token)
+        {
+            /// The PRESENT incarnation is the condemned one — deletion is scheduled. A token
+            /// mismatch means the listed entry belongs to a displaced older incarnation and says
+            /// nothing about this object; fall through to the snapshot check.
+            cls = FsckClass::PendingGc;
+            note = rit->second.delete_pending
+                ? "delete_pending: exact-token delete executes next GC round"
+                : "condemned at round " + std::to_string(rit->second.condemn_round)
+                    + "; graduates once every writer acks past it (expected)";
+        }
+        else if (in_run_hashes.contains(hash))
+        {
+            /// `in_run_hashes` only says the GC snapshot still holds SOMETHING for this blob. Split on
+            /// whether any of it is still actionable. One edge whose source manifest is PRESENT keeps
+            /// the ordinary `AwaitingGc` verdict — that manifest's removal still folds its `-1`, and an
+            /// unowned-but-present manifest is reclaimed by the orphan sweep, so the blob is genuinely
+            /// mid-pipeline. When EVERY edge names a manifest that no longer exists, no `-1` is left to
+            /// fold: the in-degree is pinned above zero for good and only a rebuild can clear it.
+            uint64_t stale_edges = 0;
+            bool all_edges_stale = false;
+            if (const auto eit = unref_edge_sources.find(hash);
+                stale_edge_check_available && eit != unref_edge_sources.end() && !eit->second.empty())
+            {
+                for (const UInt128 & source_id : eit->second)
+                    if (!live_source_ids.contains(source_id))
+                        ++stale_edges;
+                all_edges_stale = stale_edges == eit->second.size();
+            }
+
+            if (all_edges_stale)
+            {
+                cls = FsckClass::StaleEdge;
+                note = "all " + std::to_string(stale_edges) + " source edges name manifests that no longer "
+                       "exist — unreclaimable by the incremental GC (needs `cas-gc-rebuild`); NOT expected, investigate";
+            }
+            else
+            {
+                cls = FsckClass::AwaitingGc;
+                note = "edges still in the GC snapshot; the drop has not folded yet (expected)";
+            }
+        }
+        else if (!have_gc_state)
+        {
+            cls = FsckClass::AwaitingGc;
+            note = "GC has not run on this pool yet";
+        }
+        else
+        {
+            note = "not in the current GC view — transient for a fast create+drop between rounds; "
+                   "PERSISTENT occurrences violate INV-2 (reachability-before-content), investigate";
+        }
+
+        switch (cls)
+        {
+            case FsckClass::PendingGc:   ++report.pending_gc;   break;
+            case FsckClass::AwaitingGc:  ++report.awaiting_gc;  break;
+            case FsckClass::StaleEdge:   ++report.stale_edge;   break;
+            default:                     ++report.unaccounted;  break;
+        }
+        if (detail)
+        {
+            FsckObject o;
+            o.key = bkey;
+            o.kind = ObjectKind::Blob;
+            o.size = sz;
+            o.cls = cls;
+            o.reachable_from = {std::move(note)};
+            report.objects.push_back(std::move(o));
+        }
+    }
+
+    /// Meta <-> body pairing: a `.meta` object with no
+    /// body is an INV-META-BODY violation (the fixed meta/body lifecycle never leaves a meta
+    /// orphaned of its body) — a real ERROR, distinct from `dangling` (which is reachability-driven).
+    /// A body with no `.meta` is a benign not-yet-adopted (or interrupted-birth) artifact, NOT a dangle
+    /// — it still classifies through the ordinary present-but-unreferenced pipeline above.
+    std::unordered_set<BlobRef, BlobRefHash> present_body_hashes;
+    present_body_hashes.reserve(present_blobs.size());
+    for (const auto & [bkey, _] : present_blobs)
+        if (const std::optional<BlobRef> ref = layout.parseBlobKey(bkey))
+            present_body_hashes.insert(*ref);
+        /// else: foreign key shape under blobs/ — not ours to pair
+    for (const BlobRef & hash : present_meta_hashes)
+        if (!present_body_hashes.contains(hash))
+            ++report.meta_without_body;
+    for (const BlobRef & hash : present_body_hashes)
+        if (!present_meta_hashes.contains(hash))
+            ++report.body_without_meta;
+    }
+    else
+    {
+        /// Scoped mode: dangling-only for the selected namespaces. Each blob named by a scoped ref
+        /// is HEAD-verified (O(scoped refs), no pool-wide LIST); the unreachable/pending pipeline
+        /// classification needs the whole pool and is intentionally skipped.
+        for (const String & bkey : reachable_blobs)
+        {
+            checkDeadline(deadline, "head-checking scoped blobs");
+            const HeadResult h = backend.head(bkey);
+            const auto lit = blob_labels.find(bkey);
+            bool exists = h.exists;
+            if (!exists)
+            {
+                /// Use the same HEAD-absent re-resolve as the global-mode loop above.
+                const bool still_referenced = blobStillReferenced(store, layout, recovery_authorities, bkey,
+                    lit != blob_labels.end() ? lit->second : std::vector<String>{}, deadline,
+                    record_recovery_unchecked);
+                if (!still_referenced)
+                    continue;   /// stale-walk artifact — neither reachable nor dangling
+            }
+            if (exists)
+            {
+                ++report.reachable;
+                report.physical_bytes += h.size;
+            }
+            else
+                ++report.dangling;
+            if (detail || !exists)
+            {
+                FsckObject o;
+                o.key = bkey;
+                o.kind = ObjectKind::Blob;
+                o.size = exists ? h.size : 0;
+                o.cls = exists ? FsckClass::Reachable : FsckClass::Dangling;
+                if (detail && lit != blob_labels.end())
+                    o.reachable_from = lit->second;
+                report.objects.push_back(std::move(o));
+            }
+        }
+    }
+
+    /// Pre-precommit manifest debris: a `cas/manifests/` body with no committed owner. An ELIGIBLE prefix's
+    /// orphan is reclaimable debris => INFO (Unreachable); a non-eligible (in-flight) one is also info,
+    /// never an error. The owner-visible missing-body case is the error above.
+    const NamespaceListing manifest_debris_listing = store.listNamespaces(namespace_prefix);
+    recordLifelessKeys(manifest_debris_listing);
+    for (const String & ns_str : manifest_debris_listing.namespaces)
+    {
+        const RootNamespace ns{ns_str};
+        const String manifests_prefix = layout.manifestNamespacePrefix(ns);
+        std::unordered_map<String, uint64_t> manifest_bodies;
+        listAll(backend, manifests_prefix, manifest_bodies, on_progress, deadline, "listing manifests");
+        for (const auto & [mkey, sz] : manifest_bodies)
+        {
+            if (owned_manifest_keys.contains(mkey))
+                continue;   /// owned by a committed ref — accounted above
+            ++report.unreachable;
+            if (detail)
+            {
+                BuildPrefix prefix;
+                const bool parsed = parseBuildPrefix(layout, mkey, prefix);
+                FsckObject o;
+                o.key = mkey;
+                o.kind = ObjectKind::Blob;
+                o.size = sz;
+                o.cls = FsckClass::Unreachable;
+                if (parsed && prefixEligible(store, ns, prefix))
+                    o.reachable_from = {"reclaimable-pre-precommit"};
+                else
+                    o.reachable_from = {"in-flight-pre-precommit"};
+                report.objects.push_back(std::move(o));
+            }
+        }
+    }
+
+}
+
+}
+
+FsckReport runFsck(Pool & store, bool detail, FsckProgress on_progress,
+                   std::optional<std::chrono::steady_clock::time_point> deadline,
+                   bool partial_on_deadline, const String & namespace_prefix)
+{
+    FsckReport report;
+    try
+    {
+        runFsckImpl(store, detail, on_progress, deadline, namespace_prefix, report);
+    }
+    catch (const Exception & e)
+    {
+        if (!partial_on_deadline || e.code() != ErrorCodes::TIMEOUT_EXCEEDED)
+            throw;
+        report.partial = true;
+        report.partial_reason = e.message();
+    }
+    return report;
+}
+
+String formatFsckSummary(const FsckReport & report)
+{
+    /// Field order is load-bearing for humans only; every consumer parses `key=value` tokens. `partial`
+    /// and its free-text reason go LAST because the reason can contain spaces and quotes, so a parser
+    /// splitting on whitespace has to trim from the tail (see the harness's `parse_fsck_summary`).
+    /// `std::ostringstream`, not a ClickHouse write buffer: this reproduces the exact `std::cout`
+    /// formatting the line has always had, `dedup_ratio`'s default double precision included, so
+    /// extracting the line from the command changes nothing a parser can observe.
+    std::ostringstream out;   // STYLE_CHECK_ALLOW_STD_STRING_STREAM
+    out << "reachable=" << report.reachable
+        << " dangling=" << report.dangling
+        << " unreachable=" << report.unreachable
+        << " pending_gc=" << report.pending_gc
+        << " awaiting_gc=" << report.awaiting_gc
+        << " unaccounted=" << report.unaccounted
+        << " stale_edge=" << report.stale_edge
+        << " corrupted_runs=" << report.corrupted_runs
+        << " chain_broken=" << report.chain_broken
+        << " lifeless_keys=" << report.lifeless_keys
+        << " janitor_pending=" << report.namespace_janitor_pending
+        << " janitor_pending_bytes=" << report.namespace_janitor_pending_bytes
+        << " janitor_pending_lives=" << report.namespace_janitor_pending_lives
+        << " unchecked=" << report.unchecked
+        << " ref_records_walked=" << report.ref_records_walked
+        << " physical_bytes=" << report.physical_bytes
+        << " referenced_logical_bytes=" << report.referenced_logical_bytes
+        << " distinct_blobs=" << report.distinct_blobs
+        << " total_blob_refs=" << report.total_blob_refs
+        << " dedup_ratio=" << report.dedupRatio();
+    if (report.partial)
+        out << " partial=1 reason='" << report.partial_reason << "'";
+    return out.str();
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h
new file mode 100644
index 000000000000..620904f4c37f
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h
@@ -0,0 +1,285 @@
+#pragma once
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+
+#include <array>
+#include <chrono>
+#include <functional>
+#include <optional>
+#include <string_view>
+#include <vector>
+
+namespace DB::Cas
+{
+
+/// Optional progress sink for `runFsck`: called periodically during the listing and reachability
+/// walk so a long scan over a large/slow pool is visibly progressing (not hung). `phase` names the
+/// current step; `objects`/`pages` are running counts. Default {} = no progress (existing callers).
+using FsckProgress = std::function<void(std::string_view phase, uint64_t objects, uint64_t pages)>;
+
+/// Classification assigned to each object examined by `runFsck`.
+///
+/// The reachability classes are derived only from authoritative refs and the physical object listing.
+/// The GC-related classes are an additional explanation for present-but-unreferenced blobs; GC state
+/// is used for labeling only and can never make a referenced object appear safe. Integrity classes are
+/// hard findings: the report remains unclean when any of them is present.
+enum class FsckClass : uint8_t
+{
+    Reachable,     /// reachable from a live ref AND present in the object store
+    Dangling,      /// reachable from a live ref but the object is MISSING — INV-NO-LOSS violation
+    Unreachable,   /// pre-precommit manifest debris (labeled reclaimable / in-flight)
+    /// The GC pipeline deletes present-but-unreferenced blobs in explicit stages, so these classes
+    /// distinguish expected in-flight work from an object outside the GC view. They are labels only,
+    /// never inputs to reachability.
+    PendingGc,     /// listed in the retired set (condemned / delete_pending) — deletion is scheduled; EXPECTED
+    AwaitingGc,    /// edges still in the GC snapshot (drop/reclaim not folded yet) or GC never ran — EXPECTED
+    Unaccounted,   /// absent from the whole GC view — transient for a fast create+drop between rounds;
+                   /// PERSISTENT occurrences should be impossible (INV-2 reachability-before-content)
+    StaleEdge,     /// every source edge the GC snapshot still holds on this blob names a manifest that no
+                   /// longer exists anywhere in the pool, so the matching `-1` can never fold: the blob's
+                   /// in-degree can never reach zero and the incremental GC can never reclaim it. Only a
+                   /// full rebuild of the in-degree state can. ERROR — never an `AwaitingGc` "expected"
+                   /// backlog, which is exactly the label that used to hide it.
+    CorruptedRun,  /// a GC source-edge run's whole-file seal checksum (`RunRef::checksum`) disagrees with
+                   /// the stored bytes — cataloged so the read-only audit enumerates every finding in one
+                   /// pass; deletion-deriving consumers (`fold`, `zeroInDegree`, `previewDeletes`) still
+                   /// fail closed on the same mismatch. ERROR
+    /// The two verdicts of the arithmetic ref-stream walk (spec §7). They are about a NAMESPACE, not an
+    /// object; the row's `key` identifies the exact log where the walk stopped or the checkpoint-named
+    /// snapshot base whose required triple could not be validated.
+    ChainBroken,   /// the exact checkpoint authority is durably inconsistent: its required snapshot-base
+                   /// triple is corrupt, or a ref-log id is absent below its confirmed frontier. Ids are
+                   /// dense `1..T` within `(namespace, epoch)` (INV-1), so neither is a stream end. ERROR
+    Unchecked,     /// the walk could not prove this namespace's stream EITHER WAY (an unprovable epoch
+                   /// crossing, an undecodable body, or unstable authority/transport). Not a finding and
+                   /// not a clean bill of health: the honest third answer, reported so nobody reads a
+                   /// silence as a proof.
+    LifelessKey,   /// a namespace-tree key the `Layout` parsers refuse (a malformed/non-canonical shape,
+                   /// including the un-incarnated Stage A layout), OR a catalog incarnation that is
+                   /// ambiguous or otherwise unreadable. Neither a current writer nor the catalog's own
+                   /// reverse life index can produce this key's meaning, so it belongs to no namespace
+                   /// and no per-namespace verdict can carry it. ERROR
+    JanitorPending,/// a COMPLETE, canonical namespace-life key (parses via the exact writer grammar,
+                   /// nonzero 32-hex life id) whose life is simply absent from a catalog cut taken AFTER
+                   /// the physical listing. This is the protocol-produced interval between a fenced GC
+                   /// exact-deleting a `Removing` catalog row and the perpetual `NamespaceJanitor`
+                   /// reaching this key on a later bounded page -- inert debris, not damage. Reported
+                   /// as a soft finding: NOT in `kFsckHardFindings`, does not fail the report.
+};
+
+/// One object or integrity finding emitted in detailed mode, or emitted for every missing reachable
+/// object even in summary mode. `key` identifies the physical or logical object; `size` is its listed
+/// size and is zero for a missing object. `reachable_from` contains `"namespace/ref"` owners for
+/// reachable and dangling objects, or a diagnostic note for other classifications.
+struct FsckObject
+{
+    String key;
+    ObjectKind kind = ObjectKind::Blob;
+    uint64_t size = 0;                   /// on-disk object size (0 when dangling)
+    FsckClass cls = FsckClass::Reachable;
+    std::vector<String> reachable_from;  /// "ns/ref" labels (populated for reachable/dangling when detail)
+};
+
+/// Aggregate result of a read-only `runFsck` scan.
+///
+/// Reachability and byte counters describe the scan's authoritative-ref view. `unreachable` is the
+/// total of all present-but-unreferenced objects, including the GC pipeline classes and manifest debris,
+/// and is intentionally retained as one monotone number for residual-settling monitoring. The detailed
+/// `objects` list is populated according to the scan's `detail` mode. In partial mode all counters are
+/// lower bounds over the portion walked before the deadline; `clean` must not be used as a claim about
+/// the unvisited part of the pool.
+struct FsckReport
+{
+    uint64_t reachable = 0;
+    uint64_t dangling = 0;
+    /// TOTAL of everything present-but-unreferenced (blob pipeline classes below + manifest debris).
+    /// Kept as the sum so residual-settling loops (soak) keep one monotone number to watch.
+    uint64_t unreachable = 0;
+    uint64_t pending_gc = 0;     /// blobs in the retired set — deletion scheduled (expected)
+    uint64_t awaiting_gc = 0;    /// blobs whose drop is not folded yet / GC never ran (expected)
+    uint64_t unaccounted = 0;    /// blobs outside the GC view (transient or anomaly)
+    /// Blobs whose every remaining source edge names a manifest that no longer exists — permanently
+    /// stuck at a nonzero in-degree, unreclaimable by the incremental GC. A hard ERROR (see
+    /// `FsckClass::StaleEdge`). Populated only in `detail` mode: naming the live sources costs one GET
+    /// per manifest body, and the cheap summary path must stay request-for-request unchanged.
+    uint64_t stale_edge = 0;
+
+    /// The per-hash `.meta` descriptor sibling of a blob body:
+    /// pairing check between the `blobs/` physical listing's `.meta` keys and its body keys.
+    /// ADVISORY, not a hard finding: GC deletes the body FIRST and then drops the `.meta` on a bounded,
+    /// error-suppressed advisory pool that runs strictly after (and may drop the op — see `CasGc`), so a
+    /// single raw LIST legitimately observes a body-less `.meta` mid-graduation and NO finite grace makes
+    /// a persistent one hard evidence. Counted and reported; excluded from `clean()`.
+    uint64_t meta_without_body = 0;   /// a `.meta` object with no body — INV-META-BODY advisory
+    uint64_t body_without_meta = 0;   /// a body with no `.meta` — a not-yet-adopted or interrupted-birth
+                                       /// artifact; benign, NOT a dangle
+
+    /// GC source-edge runs whose whole-file seal checksum did not match the stored bytes. Cataloged
+    /// with the run key in `objects`; the audit CONTINUES — a read-only auditor
+    /// enumerates all problems in one pass rather than aborting on the first corrupt run.
+    uint64_t corrupted_runs = 0;
+
+    /// The arithmetic ref-stream walk (spec §7). fsck reads each namespace's stream by EXACT KEY from
+    /// `_ckpt.checkpoint`'s successor upward — never from a listing, which may omit durable records —
+    /// and reports one verdict per namespace.
+    ///
+    /// `chain_broken` counts namespaces with a proven hole (see `FsckClass::ChainBroken`) and is a HARD
+    /// ERROR: part of `clean`, and the command exits nonzero on it. `unchecked` counts namespaces the
+    /// walk could not prove either way; it is COVERAGE, not a finding, so it
+    /// is reported and printed but does not make a report unclean — exactly like `partial`. A pool with
+    /// nothing wrong reads `chain_broken=0 unchecked=0`, so `unchecked` is never a resting state.
+    ///
+    /// `ref_records_walked` is how many ref-log records the walk actually read and proved, summed over
+    /// namespaces. It is what makes "the tail above the checkpoint was walked" observable rather than
+    /// inferred from the absence of a complaint.
+    uint64_t chain_broken = 0;
+    uint64_t unchecked = 0;
+    uint64_t ref_records_walked = 0;
+
+    /// Keys the namespace enumeration could not attribute to any namespace, OR a catalog incarnation
+    /// that is ambiguous or unreadable (see `FsckClass::LifelessKey` and `Cas::NamespaceListing`). Does
+    /// NOT include a complete, canonical namespace-life key whose life is simply absent from the catalog
+    /// -- that is `namespace_janitor_pending`, counted separately and not a hard finding. Counted
+    /// DISTINCT by key: the scan enumerates namespaces several times and every sweep sees the same
+    /// offending key, so a per-sweep count would multiply one defect. A hard finding: no current writer
+    /// can produce this key's meaning, and an audit is where an operator finds out about it.
+    uint64_t lifeless_keys = 0;
+
+    /// Canonical namespace-life keys whose life is absent from a catalog cut taken AFTER the physical
+    /// listing (see `FsckClass::JanitorPending`). SOFT: never in `kFsckHardFindings`, never fails the
+    /// report. Persistent non-convergence across authorized janitor cycles is an operational leak
+    /// question (`CASGCNamespaceCleanupLeaks`, the `namespace_cleanup` GC-log phase), not an integrity
+    /// finding this counter can answer on its own -- one snapshot cannot prove an unbounded leak.
+    uint64_t namespace_janitor_pending = 0;
+    uint64_t namespace_janitor_pending_bytes = 0;
+    uint64_t namespace_janitor_pending_lives = 0;   /// distinct life ids counted above
+
+    uint64_t physical_bytes = 0;
+    uint64_t referenced_logical_bytes = 0;
+    uint64_t total_blob_refs = 0;
+    uint64_t distinct_blobs = 0;
+
+    /// Set when the scan hit its deadline in partial mode: counts cover only what was walked
+    /// before the deadline — a lower bound, not the pool truth.
+    bool partial = false;
+    String partial_reason;
+
+    std::vector<FsckObject> objects;
+
+    /// Return logical blob references per distinct reachable blob, or zero when no distinct blob was seen.
+    double dedupRatio() const { return distinct_blobs ? double(total_blob_refs) / double(distinct_blobs) : 0.0; }
+
+    /// Return whether the scan found no missing reachable object or hard integrity violation. Expected
+    /// GC backlog classes do not make a report unclean, and `meta_without_body` is advisory (see its
+    /// field: GC's body-then-meta delete ordering makes a body-less `.meta` a legitimate transient with
+    /// no finite hard horizon); a partial report only covers the visited subset. `stale_edge` is a hard
+    /// finding, but it is only ever nonzero in `detail` mode — a clean summary report says nothing about
+    /// stale edges, exactly as a partial report says nothing about the unvisited part of the pool.
+    /// `chain_broken` is a hard finding in every mode. `unchecked` deliberately is NOT one: it says the
+    /// walk proved nothing about those namespaces, which is a statement about COVERAGE, and folding it
+    /// in here would make "cannot prove" indistinguishable from "found broken".
+    /// Defined out-of-line below, over `kFsckHardFindings`, so that "a term of `clean`" and "a row of
+    /// that list" are the same thing rather than two lists that can drift.
+    bool clean() const;
+};
+
+/// ONE hard finding: the name every surface renders it under, and the counter it reads.
+struct FsckHardFinding
+{
+    std::string_view name;
+    uint64_t FsckReport::* value;
+};
+
+/// THE HARD FINDINGS, and the single authority on what they are. `FsckReport::clean` is computed from
+/// this list, so adding a term means adding a row here.
+///
+/// The name is the one the text summary line and the SQL result column both use, which is what lets a
+/// test check a rendering surface by iterating this list instead of restating its contents.
+/// The SIZE IS DEDUCED, deliberately. A fixed `std::array<FsckHardFinding, N>` rejects an added row with
+/// an "excess elements in ..." diagnostic -- which stops the build, but its text carries none of the
+/// guidance the assert below does, so the author learns only that they miscounted. (Which noun that
+/// diagnostic uses depends on the brace form, so it is not quoted here.) Deduced, an added row compiles
+/// and the assert is what speaks.
+inline constexpr std::array kFsckHardFindings{
+    FsckHardFinding{"dangling", &FsckReport::dangling},
+    FsckHardFinding{"corrupted_runs", &FsckReport::corrupted_runs},
+    FsckHardFinding{"stale_edge", &FsckReport::stale_edge},
+    FsckHardFinding{"chain_broken", &FsckReport::chain_broken},
+    FsckHardFinding{"lifeless_keys", &FsckReport::lifeless_keys},
+};
+
+/// TRIPWIRE. A hard finding has to reach three CODE surfaces, and each has been forgotten at least once:
+/// the text summary line (`formatFsckSummary`), `CommandFsck::executeImpl`'s nonzero-exit set, and the
+/// SQL result row (`contentAddressedFsckColumns` + `appendContentAddressedFsckRow`). It has happened
+/// repeatedly, on more than one occasion and to more than one term, each time with the rule written down
+/// in prose and each time the prose not holding. (No count is given: the records that document those
+/// episodes do not support one number, and a tally nobody can reconstruct is the same defect as the rest.)
+///
+/// ONE ROW OF THIS LIST IS DELIBERATELY NOT IN THE EXIT SET, so the "three surfaces" rule has a named
+/// exception rather than a silent violation: `stale_edge` is nonzero only under `--detail`, and
+/// `CommandFsck::executeImpl` prints it as a `note:` and never throws. What licenses that is the pair --
+/// a documented reason AND a compensating gate elsewhere (`stale_edge_verdict` in
+/// `utils/ca-soak/soak/fsck.py`, asserted by the soak checkpoint in `soak/run.py`, which fails closed
+/// when the key is absent). A new finding may take the same exception only WITH both halves; without
+/// them it belongs in the exit set.
+///
+/// WHAT THIS ASSERT CHECKS, precisely: that the number of hard findings still equals the number written
+/// here. Nothing more. It does NOT check that any surface renders them -- it cannot see the renderers,
+/// which is the whole reason it lives with the struct: this header is included by the summary formatter,
+/// by `programs/disks/CommandFsck.cpp`, and by `src/Interpreters/InterpreterSystemQuery.cpp`, so changing
+/// the list breaks the build in every TU that owes an update, including the two no unit test can reach.
+///
+/// The summary line is checked for real, by a test that iterates the list
+/// (`CasFsckSummary.EveryHardFindingAppearsOnTheSummaryLine`). The exit set and the SQL row are NOT --
+/// for those, this assert plus the list below it is the whole of the mechanism, so bumping the number
+/// without visiting them defeats it. Bump it only after all three are done.
+///
+/// AND IT REACHES NO PROSE. The rule is also restated in `docs/superpowers/cas/AGENTS.md`
+/// and in the soak harness's comments and messages; those restatements have gone stale before -- more
+/// than once, about the exit set -- and nothing here can break a build over them. (No count is given,
+/// for the same reason the paragraph above gives none: nobody keeping a tally of restatements can
+/// promise its own count will not go stale next.) They are a fourth surface, unfenced by construction.
+static_assert(kFsckHardFindings.size() == 5,
+    "A hard finding was added to or removed from `kFsckHardFindings`, which is `FsckReport::clean`. "
+    "Before updating this count, render it in ALL THREE code surfaces: `formatFsckSummary`'s line, "
+    "`CommandFsck::executeImpl`'s nonzero-exit set, and `contentAddressedFsckColumns` + "
+    "`appendContentAddressedFsckRow`. Two of the three have no test that can fail for you -- the "
+    "comment above this assert says which. A finding may be left out of the exit set only the way "
+    "`stale_edge` is: with a documented reason AND a compensating soak assert.");
+
+inline bool FsckReport::clean() const
+{
+    for (const FsckHardFinding & finding : kFsckHardFindings)
+        if (this->*finding.value != 0)
+            return false;
+    return true;
+}
+
+/// Independently recompute reachability from authoritative refs (never from GC state or snapshots) and
+/// diff it against a raw object listing. The operation is read-only; `detail` populates per-object rows.
+/// `deadline`, if set, bounds the WHOLE scan: it is checked between list pages and reachability
+/// refs, throwing `TIMEOUT_EXCEEDED` if exceeded (a slow-but-progressing scan surfaces a clear
+/// error instead of an opaque hang) — unless `partial_on_deadline` is set, in which case the
+/// accumulated lower-bound counts are returned instead, flagged via `FsckReport::partial`. A single
+/// LIST page stuck in S3-client retries is bounded separately by the disk's S3 retry/timeout
+/// settings, not here. `namespace_prefix`, if non-empty, scopes the scan to namespaces with this
+/// prefix and skips the pool-wide unreachable classification (dangling-only mode).
+FsckReport runFsck(Pool & store, bool detail, FsckProgress on_progress = {},
+                   std::optional<std::chrono::steady_clock::time_point> deadline = {},
+                   bool partial_on_deadline = false, const String & namespace_prefix = {});
+
+/// Render the single machine-parseable summary line (no trailing newline). This is the ONLY view of a
+/// report most consumers ever get -- the soak harness parses it, CI greps it, an operator reads it -- so
+/// it lives here, next to the report and under test, rather than inline in the command where nothing
+/// could reach it. Every term of `FsckReport::clean` MUST appear: a hard finding the line omits is a
+/// finding no run will ever report, which is how `corrupted_runs` stayed invisible from the day it was
+/// first counted. That requirement is CHECKED for this surface, not merely stated:
+/// `CasFsckSummary.EveryHardFindingAppearsOnTheSummaryLine` iterates `kFsckHardFindings` and looks for
+/// each name in the line, so a term added to the list and not rendered here fails that test. Zeros are
+/// printed, never omitted: "absent" and "zero" are different facts, and consumers (e.g. the harness's
+/// `stale_edge_verdict`) fail closed on absence by design.
+String formatFsckSummary(const FsckReport & report);
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.cpp
new file mode 100644
index 000000000000..1dbf597e8a9b
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.cpp
@@ -0,0 +1,638 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Common/Exception.h>
+#include <fmt/format.h>
+#include <cstdint>
+#include <set>
+#include <vector>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+/// Escapes `s` as a JSON string LITERAL (including the surrounding quotes). Handles the standard
+/// two-char escapes plus a `\uXXXX` fallback for any other control byte; everything else (including
+/// raw multi-byte UTF-8) passes through unchanged. This is a debug/inspection rendering, not a wire
+/// format, so it deliberately does not attempt full Unicode validation.
+String jsonEscape(std::string_view s)
+{
+    String out;
+    out.reserve(s.size() + 2);
+    out += '"';
+    for (unsigned char c : s)
+    {
+        switch (c)
+        {
+            case '"':  out += "\\\""; break;
+            case '\\': out += "\\\\"; break;
+            case '\b': out += "\\b"; break;
+            case '\f': out += "\\f"; break;
+            case '\n': out += "\\n"; break;
+            case '\r': out += "\\r"; break;
+            case '\t': out += "\\t"; break;
+            default:
+                if (c < 0x20)
+                    out += fmt::format("\\u{:04x}", c);
+                else
+                    out += static_cast<char>(c);
+        }
+    }
+    out += '"';
+    return out;
+}
+
+/// u128 fields (hashes, ids, tokens-as-u128) render as a lowercase-hex JSON string, matching
+/// `u128ToHex` — never as a nested {high,low} object or a decimal number.
+String jsonHex(const UInt128 & v) { return jsonEscape(u128ToHex(v)); }
+String jsonUInt(uint64_t v) { return std::to_string(v); }
+String jsonBool(bool b) { return b ? "true" : "false"; }
+
+/// A minimal JSON object builder: each `add` takes a key and an already-rendered JSON fragment
+/// (a quoted string, a number, `true`/`false`/`null`, or a nested `{...}`/`[...]`) and joins them
+/// with commas. No pretty-printing — this is a debug/inspection tool, not a wire format.
+class JsonObj
+{
+public:
+    JsonObj & add(std::string_view key, const String & raw_value)
+    {
+        if (!first)
+            out += ",";
+        first = false;
+        out += jsonEscape(key);
+        out += ":";
+        out += raw_value;
+        return *this;
+    }
+
+    String str() const { return "{" + out + "}"; }
+
+private:
+    String out;
+    bool first = true;
+};
+
+String jsonArray(const std::vector<String> & items)
+{
+    String out = "[";
+    for (size_t i = 0; i < items.size(); ++i)
+    {
+        if (i)
+            out += ",";
+        out += items[i];
+    }
+    out += "]";
+    return out;
+}
+
+String renderManifestRef(const ManifestRef & r)
+{
+    return JsonObj()
+        .add("writer_epoch", jsonUInt(r.writer_epoch))
+        .add("build_sequence", jsonUInt(r.build_sequence))
+        .add("manifest_ordinal", jsonUInt(r.manifest_ordinal))
+        .str();
+}
+
+/// Snapshot and log ref objects use `RefTxnId` values with `writer_epoch` and `ref_sequence` fields.
+/// `renderRefTxnIdObj` renders those raw numeric fields rather than the canonical hex form, which
+/// rejects a zero field, so inspection can dump any object, including a malformed one, without
+/// failing while rendering its identifiers.
+String renderRefTxnIdObj(const RefTxnId & id)
+{
+    return JsonObj()
+        .add("writer_epoch", jsonUInt(id.writer_epoch))
+        .add("ref_sequence", jsonUInt(id.ref_sequence))
+        .str();
+}
+
+String refOwnerKindName(RefOwnerKind k)
+{
+    switch (k)
+    {
+        case RefOwnerKind::Committed: return "Committed";
+        case RefOwnerKind::Precommit: return "Precommit";
+    }
+    return "Unknown";
+}
+
+String renderRefOwnerBinding(const RefOwnerBinding & b)
+{
+    return JsonObj()
+        .add("kind", jsonEscape(refOwnerKindName(b.kind)))
+        .add("ref_name", jsonEscape(b.ref_name))
+        .add("manifest_ref", renderManifestRef(b.manifest_ref))
+        .str();
+}
+
+String renderRefCommittedRow(const RefCommittedRow & r)
+{
+    return JsonObj()
+        .add("ref_name", jsonEscape(r.ref_name))
+        .add("manifest_ref", renderManifestRef(r.manifest_ref))
+        .add("published_at_ms", jsonUInt(r.published_at_ms))
+        .str();
+}
+
+String renderRefTableSnapshot(const RefTableSnapshot & s)
+{
+    std::vector<String> committed;
+    committed.reserve(s.committed.size());
+    for (const auto & row : s.committed)
+        committed.push_back(renderRefCommittedRow(row));
+
+    std::vector<String> precommits;
+    precommits.reserve(s.precommits.size());
+    for (const auto & b : s.precommits)
+        precommits.push_back(renderRefOwnerBinding(b));
+
+    return JsonObj()
+        .add("object", jsonEscape("ref_snapshot"))
+        .add("namespace", jsonEscape(s.ns))
+        .add("snapshot_id", renderRefTxnIdObj(s.snapshot_id))
+        .add("committed", jsonArray(committed))
+        .add("precommits", jsonArray(precommits))
+        .str();
+}
+
+/// The namespace's checkpoint (spec INV-4). Every field is optional and each absence means something
+/// different an operator needs to see: no `life_epoch` means no writer that knew this namespace's
+/// genesis epoch has written here yet, no `committed_through` means the life has no committed
+/// transaction, no `checkpoint_snapshot_id` means recovery has no snapshot base, and no
+/// `last_epoch_seal` means no epoch of this namespace has been closed. They are rendered as explicit
+/// `null`s rather than omitted keys so all four cases are visible.
+/// `ns` comes from the KEY -- unlike the log and snapshot objects, a `_ckpt` body does not name its
+/// namespace, so there is no key-to-body binding to cross-check here.
+String renderRefCkpt(const RootNamespace & ns, const RefCkpt & c)
+{
+    return JsonObj()
+        .add("object", jsonEscape("ref_ckpt"))
+        .add("namespace", jsonEscape(ns.string()))
+        .add("life_epoch", c.life_epoch ? jsonUInt(*c.life_epoch) : "null")
+        .add("committed_through", c.committed_through ? renderRefTxnIdObj(*c.committed_through) : "null")
+        .add("checkpoint_snapshot_id",
+             c.checkpoint_snapshot_id ? renderRefTxnIdObj(*c.checkpoint_snapshot_id) : "null")
+        .add("last_epoch_seal", c.last_epoch_seal ? renderRefTxnIdObj(*c.last_epoch_seal) : "null")
+        .str();
+}
+
+String refOpKindName(RefOpKind k)
+{
+    switch (k)
+    {
+        case RefOpKind::NamespaceBirth: return "NamespaceBirth";
+        case RefOpKind::OwnerTransition: return "OwnerTransition";
+        case RefOpKind::SetPublishedAt: return "SetPublishedAt";
+        case RefOpKind::RemoveNamespace: return "RemoveNamespace";
+        case RefOpKind::EpochSeal: return "EpochSeal";
+    }
+    return "Unknown";
+}
+
+String renderRefOp(const RefOp & op)
+{
+    return JsonObj()
+        .add("kind", jsonEscape(refOpKindName(op.kind)))
+        .add("old_binding", op.old_binding ? renderRefOwnerBinding(*op.old_binding) : "null")
+        .add("new_binding", op.new_binding ? renderRefOwnerBinding(*op.new_binding) : "null")
+        .add("ref_name", jsonEscape(op.ref_name))
+        .add("expected_manifest_ref", renderManifestRef(op.expected_manifest_ref))
+        .add("published_at_ms", jsonUInt(op.published_at_ms))
+        .str();
+}
+
+String renderRefLogTxn(const RefLogTxn & t)
+{
+    std::vector<String> ops;
+    ops.reserve(t.ops.size());
+    for (const auto & op : t.ops)
+        ops.push_back(renderRefOp(op));
+
+    return JsonObj()
+        .add("object", jsonEscape("ref_log"))
+        .add("namespace", jsonEscape(t.ns))
+        .add("txn_id", renderRefTxnIdObj(t.txn_id))
+        .add("ops", jsonArray(ops))
+        .add("prev_epoch_seal", t.prev_epoch_seal ? renderRefTxnIdObj(*t.prev_epoch_seal) : "null")
+        .str();
+}
+
+String placementName(EntryPlacement p)
+{
+    switch (p)
+    {
+        case EntryPlacement::Inline: return "Inline";
+        case EntryPlacement::Blob: return "Blob";
+    }
+    return "Unknown";
+}
+
+/// `inline_bytes` renders as its LENGTH only, not its content — an inline file's bytes are payload
+/// data, not part-manifest identity, and may be arbitrarily large / non-UTF8.
+String renderManifestEntry(const ManifestEntry & e)
+{
+    /// Render `blobIdOf(e.ref)` ("<algoName>:<hex>"). The algorithm must remain part of the
+    /// rendered identity: a bare digest is ambiguous in a pool containing algorithms with different
+    /// digest widths, and each entry's own `ref.algo` determines its width.
+    return JsonObj()
+        .add("path", jsonEscape(e.path))
+        .add("placement", jsonEscape(placementName(e.placement)))
+        .add("blob", jsonEscape(blobIdOf(e.ref)))
+        .add("blob_size", jsonUInt(e.blob_size))
+        .add("inline_bytes_size", jsonUInt(e.inline_bytes.size()))
+        .str();
+}
+
+String renderPartManifest(const PartManifest & m)
+{
+    std::vector<String> entries;
+    entries.reserve(m.entries.size());
+    for (const auto & e : m.entries)
+        entries.push_back(renderManifestEntry(e));
+
+    return JsonObj()
+        .add("ref", renderManifestRef(m.ref))
+        .add("root_namespace_id", jsonEscape(m.root_namespace_id.string()))
+        .add("payload_digest", jsonHex(m.payload_digest))
+        .add("entries", jsonArray(entries))
+        .str();
+}
+
+String renderMountLease(const MountLease & m)
+{
+    return JsonObj()
+        .add("server_uuid", jsonHex(m.server_uuid))
+        .add("writer_epoch", jsonUInt(m.writer_epoch))
+        .add("hostname", jsonEscape(m.hostname))
+        .add("pid", jsonUInt(m.pid))
+        .add("started_at_ms", jsonUInt(m.started_at_ms))
+        .add("seq", jsonUInt(m.seq))
+        .add("expires_at_ms", jsonUInt(m.expires_at_ms))
+        .add("min_active", jsonUInt(m.min_active))
+        .add("gc_fenced", jsonBool(m.gc_fenced))
+        .str();
+}
+
+String renderGcLease(const GcLease & l)
+{
+    return JsonObj()
+        .add("owner", jsonHex(l.owner))
+        .add("seq", jsonUInt(l.seq))
+        .str();
+}
+
+String renderGcState(const GcState & s)
+{
+    return JsonObj()
+        .add("round", jsonUInt(s.round))
+        .add("gc_shards", jsonUInt(s.gc_shards))
+        .add("snap_generation", jsonUInt(s.snap_generation))
+        .add("snap_pruned_through", jsonUInt(s.snap_pruned_through))
+        .add("snap_attempt", jsonUInt(s.snap_attempt))
+        .add("manifest_sweep_cursor", jsonEscape(s.manifest_sweep_cursor))
+        .add("lease", renderGcLease(s.lease))
+        .str();
+}
+
+String tokenTypeName(TokenType t)
+{
+    switch (t)
+    {
+        case TokenType::ETag:       return "ETag";
+        case TokenType::Generation: return "Generation";
+        case TokenType::Emulated:   return "Emulated";
+    }
+    return "Unknown";
+}
+
+/// `Token::value` is an opaque backend-native string (e.g. an S3 ETag) — NOT a 128-bit hash — so it
+/// renders verbatim (escaped), not hex-converted; `type` names which backend family minted it.
+String renderToken(const Token & t)
+{
+    return JsonObj()
+        .add("value", jsonEscape(t.value))
+        .add("type", jsonEscape(tokenTypeName(t.type)))
+        .str();
+}
+
+String objectKindName(ObjectKind k)
+{
+    switch (k)
+    {
+        case ObjectKind::Blob: return "Blob";
+    }
+    return "Unknown";
+}
+
+String renderRunRef(const RunRef & r)
+{
+    return JsonObj()
+        .add("key", jsonEscape(r.key))
+        .add("checksum", jsonHex(r.checksum))
+        .add("shard", jsonUInt(r.shard))
+        .add("generation", jsonUInt(r.generation))
+        .str();
+}
+
+String renderRefCoverage(const RefCoverage & c)
+{
+    return JsonObj()
+        .add("classification", jsonUInt(c.classification))
+        .add("last_folded_ref_id", renderRefTxnIdObj(c.last_folded_ref_id))
+        .str();
+}
+
+String renderFoldSeal(const CasFoldSeal & seal)
+{
+    JsonObj ref_lives;
+    for (const auto & [life_id, state] : seal.ref_lives)
+        ref_lives.add(renderIncarnation(life_id), JsonObj()
+            .add("coverage", renderRefCoverage(state.coverage))
+            .add("cleanup_evidence", state.cleanup_evidence
+                ? JsonObj().add("remove_txn_id", renderRefTxnIdObj(state.cleanup_evidence->remove_txn_id)).str()
+                : "null")
+            .str());
+
+    std::vector<String> blob_target_runs;
+    blob_target_runs.reserve(seal.blob_target_runs.size());
+    for (const auto & r : seal.blob_target_runs)
+        blob_target_runs.push_back(renderRunRef(r));
+
+    /// A fold seal carries per-GC-shard totals for `kCondemned` rows in its source runs. Render the
+    /// summary from the seal itself; the older separate retired-reference object is no longer part
+    /// of the current layout.
+    JsonObj condemned_summary;
+    for (const auto & [shard, cs] : seal.condemned_summary)
+        condemned_summary.add(std::to_string(shard), JsonObj()
+            .add("condemned_total", jsonUInt(cs.condemned_total))
+            .add("pending_total", jsonUInt(cs.pending_total))
+            .add("oldest_nonpending_condemn_round", jsonUInt(cs.oldest_nonpending_condemn_round))
+            .str());
+
+    return JsonObj()
+        .add("generation", jsonUInt(seal.generation))
+        .add("parent_generation", jsonUInt(seal.parent_generation))
+        .add("ref_lives", ref_lives.str())
+        .add("blob_target_runs", jsonArray(blob_target_runs))
+        .add("condemned_summary", condemned_summary.str())
+        .str();
+}
+
+String provenanceOpName(ProvenanceOp op)
+{
+    switch (op)
+    {
+        case ProvenanceOp::Other:    return "Other";
+        case ProvenanceOp::Insert:   return "Insert";
+        case ProvenanceOp::Merge:    return "Merge";
+        case ProvenanceOp::Mutation: return "Mutation";
+        case ProvenanceOp::Attach:   return "Attach";
+        case ProvenanceOp::Repack:   return "Repack";
+    }
+    return "Unknown";
+}
+
+String renderProvenance(const Provenance & p)
+{
+    return JsonObj()
+        .add("created_at_ms", jsonUInt(p.created_at_ms))
+        .add("creator_server_id", jsonHex(p.creator_server_id))
+        .add("ch_version", jsonUInt(p.ch_version))
+        .add("op", jsonEscape(provenanceOpName(p.op)))
+        .str();
+}
+
+String metaStateName(MetaState s)
+{
+    switch (s)
+    {
+        case MetaState::Clean:     return "clean";
+        case MetaState::Condemned: return "condemned";
+    }
+    return "unknown";
+}
+
+/// The per-hash `.meta` descriptor is the blob body's sibling and records its freshness state
+/// (`Clean` or `Condemned`), not its payload. It is rendered separately from `renderEnvelopeHeader`:
+/// the body remains an enveloped object, while the descriptor has its own format.
+String renderBlobMeta(const BlobMeta & m)
+{
+    return JsonObj()
+        .add("object", jsonEscape("blob_meta"))
+        .add("version", jsonUInt(m.version))
+        .add("state", jsonEscape(metaStateName(m.state)))
+        .add("condemn_round", jsonUInt(m.condemn_round))
+        .add("size", jsonUInt(m.size))
+        .str();
+}
+
+String renderEnvelopeHeader(const EnvelopeHeader & h)
+{
+    return JsonObj()
+        .add("kind", jsonEscape(objectKindName(h.kind)))
+        /// The blob identity is carried by the object key, so the envelope keeps only the provenance
+        /// fields needed for forensics (`ch` and `bld`) together with its compatibility version.
+        .add("compatibility_version", jsonUInt(h.compatibility_version))
+        .add("incarnation_tag", jsonHex(h.incarnation_tag))
+        .add("build_id", jsonHex(h.build_id))
+        .add("header_len", jsonUInt(h.header_len))
+        .add("provenance", h.provenance ? renderProvenance(*h.provenance) : "null")
+        .add("intended_ref", h.intended_ref ? jsonEscape(*h.intended_ref) : "null")
+        .str();
+}
+
+/// The word vocabulary a row's marker byte renders as, matching the `cas_run` NDJSON's own `"m"` field
+/// words (`CasRecordStreamFormat.cpp`'s private `markerToWord`) so cas-inspect speaks the same vocabulary
+/// as the on-disk format rather than inventing a second one.
+String sourceEdgeRowKindName(char marker)
+{
+    switch (marker)
+    {
+        case kEdgeActive: return "edge";
+        case kZeroMarker: return "zero";
+        case kCondemned:  return "condemned";
+        default: return "unknown";
+    }
+}
+
+String renderCondemnedRow(const CondemnedRow & r)
+{
+    return JsonObj()
+        .add("delete_pending", jsonBool(r.delete_pending))
+        .add("token", renderToken(r.token))
+        .add("size", jsonUInt(r.size))
+        .add("condemn_round", jsonUInt(r.condemn_round))
+        .add("marker_confirmed", jsonBool(r.marker_confirmed))
+        .str();
+}
+
+/// Renders one blob-target source-edge run segment (`Layout::blobTargetRunKey`): every row (edge,
+/// zero-marker, or condemned sentinel), plus a summary. `parsed` carries the run's own coordinates
+/// recovered from the key; `bytes` is decoded with the same typed `SourceEdgeRunView` reader the fold /
+/// `zeroInDegree` / `fsck` consumers use (the memory overload, since `caInspectToJson` is a pure
+/// function of (key, bytes) with no backend access here). A malformed key or payload propagates the
+/// codec's own `CORRUPTED_DATA` (`SourceEdgeKeyCodec::parse`, `decodeCondemnedRow`) -- rows are never
+/// silently skipped.
+String renderBlobTargetRun(const ParsedBlobTargetRunKey & parsed, std::string_view bytes)
+{
+    SourceEdgeRunView reader = openSourceEdgeRun(bytes);
+
+    std::vector<String> rows;
+    std::set<BlobRef> distinct_blobs;
+    uint64_t edge_count = 0;
+    uint64_t condemned_count = 0;
+    uint64_t zero_marker_count = 0;
+
+    String key;
+    String payload;
+    while (reader.next(key, payload))
+    {
+        BlobRef ref;
+        UInt128 source_id;
+        SourceEdgeKeyCodec::parse(key, ref, source_id);   // throws CORRUPTED_DATA on a malformed key (fail-closed)
+        if (payload.empty())
+            throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+                "cas-inspect: source-edge run row for blob {} has an empty payload", blobIdOf(ref));
+        const char marker = payload[0];
+
+        distinct_blobs.insert(ref);
+        JsonObj row;
+        row.add("blob", jsonEscape(blobIdOf(ref)))
+           /// `source_id` is a `CityHash128` of (namespace, writer_epoch, build_sequence,
+           /// manifest_ordinal, path) -- not invertible here, so it renders as plain hex, exactly like
+           /// every other opaque u128 identifier in this file.
+           .add("source_id", jsonHex(source_id))
+           .add("kind", jsonEscape(sourceEdgeRowKindName(marker)));
+
+        switch (marker)
+        {
+            case kEdgeActive:
+                ++edge_count;
+                break;
+            case kZeroMarker:
+                ++zero_marker_count;
+                break;
+            case kCondemned:
+                ++condemned_count;
+                row.add("condemned", renderCondemnedRow(decodeCondemnedRow(payload)));   // CORRUPTED_DATA on malformed (fail-closed)
+                break;
+            default:
+                throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+                    "cas-inspect: source-edge run row for blob {} has an unknown marker 0x{:02x}",
+                    blobIdOf(ref), static_cast<uint8_t>(marker));
+        }
+        rows.push_back(row.str());
+    }
+
+    return JsonObj()
+        .add("object", jsonEscape("blob_target_run"))
+        .add("generation", jsonUInt(parsed.generation))
+        .add("attempt", jsonUInt(parsed.attempt))
+        .add("shard", jsonUInt(parsed.shard))
+        .add("seq", jsonUInt(parsed.seq))
+        .add("rows", jsonArray(rows))
+        .add("summary", JsonObj()
+            .add("rows", jsonUInt(rows.size()))
+            .add("distinct_blobs", jsonUInt(distinct_blobs.size()))
+            .add("edges", jsonUInt(edge_count))
+            .add("condemned", jsonUInt(condemned_count))
+            .add("zero_markers", jsonUInt(zero_marker_count))
+            .str())
+        .str();
+}
+
+}
+
+String caInspectToJson(const Layout & layout, const String & key, std::string_view bytes,
+                       const std::optional<NamespaceLifeId> & resolved_life)
+{
+    /// Most-specific first: `cas/manifests/.../NNNNNN.zst` before the pool-wide `cas/ns/stream/`
+    /// prefix, the `/mount` and `/fold_seal` suffixes before the pool-wide `gc/state` exact match,
+    /// and the `.meta` sibling suffix before the bare `blobs/` prefix it also matches.
+    if (key.starts_with(layout.casManifestsPrefix()) && key.ends_with(storedSuffix(FormatId::PartManifest)))
+        return renderPartManifest(decodePartManifest(openObject(FormatId::PartManifest, bytes)));
+
+    const auto requireResolvedLife = [&](NamespaceLifePhysicalId life_id) -> const NamespaceLifeId &
+    {
+        if (!resolved_life || resolved_life->incarnation != life_id)
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "cas-inspect: life_id {} has no unique resolution in the supplied catalog cut",
+                renderIncarnation(life_id));
+        return *resolved_life;
+    };
+
+    if (key.starts_with(layout.namespaceStateRootPrefix()))
+    {
+        if (const auto life_id = layout.parseRefCkptKey(key))
+            return renderRefCkpt(requireResolvedLife(*life_id).ns, decodeRefCkpt(bytes));
+    }
+
+    if (key.starts_with(layout.casRefsPrefix()))
+    {
+
+        const auto parsed = layout.parseRefObjectKey(key);
+        if (!parsed)
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+                "cas-inspect: key under cas/ns/stream is not a recognized ref-object key '{}'", key);
+        const NamespaceLifeId & life = requireResolvedLife(parsed->life_id);
+        if (parsed->kind == RefObjectKind::Snap)
+            return renderRefTableSnapshot(decodeRefTableSnapshot(
+                openObject(FormatId::RefSnapshot, bytes), life.ns.string(), parsed->txn_id));
+        if (parsed->kind == RefObjectKind::Log)
+            return renderRefLogTxn(decodeRefLogTxn(
+                openObject(FormatId::RefLog, bytes), life.ns.string(), parsed->txn_id));
+        throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "cas-inspect: unhandled ref-object kind for key '{}'", key);
+    }
+
+    if (key == layout.gcStateKey())
+        return renderGcState(decodeGcState(bytes));
+
+    if (key.ends_with("/mount"))
+        return renderMountLease(decodeMountLease(bytes));
+
+    if (key.ends_with("/fold_seal"))
+        return renderFoldSeal(decodeFoldSeal(bytes));
+
+    /// Blob-target source-edge run segments (`Layout::blobTargetRunKey`) are the ground truth for
+    /// every in-degree question, so they get a typed decode too, not just the fold seal that names
+    /// them. Checked before the pool-wide `blobs/` prefix below (disjoint anyway -- these keys live
+    /// under `gc/gen/`, never `blobs/` -- but most-specific-first stays the dispatch's rule).
+    if (const auto parsed = layout.parseBlobTargetRunKey(key))
+        return renderBlobTargetRun(*parsed, bytes);
+
+    /// `blobMetaKey(id) == blobKey(id) + ".meta"`, so a meta descriptor also matches
+    /// `blobsPrefix()` below. Check it first or it would be decoded incorrectly as an envelope. A
+    /// non-`.meta` blob body still carries its envelope.
+    if (key.starts_with(layout.blobsPrefix()) && key.ends_with(".meta"))
+        return renderBlobMeta(decodeBlobMeta(bytes));
+
+    if (key.starts_with(layout.blobsPrefix()))
+        return renderEnvelopeHeader(decodeEnvelopeHeader(bytes, bytes.size(), ObjectKind::Blob));
+
+    throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS,
+        "cas-inspect: unrecognized key layout '{}' (recognized: cas/ns/stream, cas/ns/state, cas/manifests, "
+        "gc/server-roots/*/mount, gc/state, gc/gen/*/fold_seal, gc/gen/*/attempt/*/blob_target/*/*, "
+        "retired, blobs, blobs/*.meta)", key);
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h
new file mode 100644
index 000000000000..0c6bfa3e0cee
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h
@@ -0,0 +1,30 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <base/types.h>
+#include <string_view>
+
+namespace DB::Cas
+{
+
+/// Read-only decode-to-JSON dispatch for `clickhouse-disks cas-inspect` (and its unit tests): given
+/// any key that could live in a content-addressed pool plus the raw bytes stored at it, decode with
+/// the matching codec and render the struct's fields as human-readable JSON. `layout` supplies the
+/// pool's key shapes (there is no live pool/backend access here — pure function of (key, bytes)), so
+/// it can be exercised directly against encoder output in unit tests, with no disk / object storage
+/// involved.
+///
+/// Dispatch is by KEY SHAPE, most-specific first (`cas/manifests/.../NNNNNN.zst` before the
+/// `cas/ns/stream/` and `cas/ns/state/` roots, `/mount` and `/fold_seal` suffixes, the
+/// `gc/gen/*/attempt/*/blob_target/*/*` source-edge run segments, then the pool-wide `gc/state`
+/// and `blobs/` prefix). u128 and hash fields render as lowercase hex strings (matching
+/// `u128ToHex`), while backend-native `Token` values render as escaped strings. Neither is exposed
+/// as an array of bytes or a raw struct dump.
+///
+/// Throws `ErrorCodes::BAD_ARGUMENTS` when `key` matches none of the recognized CA layouts. Any
+/// decode failure of a matched key (invalid header, corrupted bytes, future format version, ...)
+/// propagates as-is from the underlying `decode*` function (typically `CORRUPTED_DATA` or
+/// `UNKNOWN_FORMAT_VERSION`) — this function performs no fallback decode and swallows nothing.
+String caInspectToJson(const Layout & layout, const String & key, std::string_view bytes,
+                       const std::optional<NamespaceLifeId> & resolved_life = std::nullopt);
+
+}

From 1c5ecf225111f387d94a9b37f44215173609239a Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:35 +0200
Subject: [PATCH 20/30] CAS subsystem: microbenchmarks

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../benchmarks/CMakeLists.txt                 |   4 +
 .../benchmarks/benchmark_cas_ref_protocol.cpp | 553 ++++++++++++++++++
 2 files changed, 557 insertions(+)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks/CMakeLists.txt
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks/benchmark_cas_ref_protocol.cpp

diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks/CMakeLists.txt b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks/CMakeLists.txt
new file mode 100644
index 000000000000..0f792624cea1
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks/CMakeLists.txt
@@ -0,0 +1,4 @@
+clickhouse_add_executable(benchmark_cas_ref_protocol benchmark_cas_ref_protocol.cpp)
+target_link_libraries (benchmark_cas_ref_protocol PRIVATE
+    ch_contrib::gbenchmark_all
+    dbms)
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks/benchmark_cas_ref_protocol.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks/benchmark_cas_ref_protocol.cpp
new file mode 100644
index 000000000000..f23f4dc06bae
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks/benchmark_cas_ref_protocol.cpp
@@ -0,0 +1,553 @@
+#include <benchmark/benchmark.h>
+
+#include <algorithm>
+#include <vector>
+
+#include <Common/PODArray.h>
+#include <IO/WriteBufferFromVector.h>
+#include <IO/WriteHelpers.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+
+/// Pure measurement, no pass/fail assertions -- see the cas-gc-rebuild BACKLOG.md entries
+/// "OPTIMIZATION OPPORTUNITY -- ref-ledger JSON encoding writes byte-by-byte" and the (now
+/// RESOLVED) "admits() re-encodes the WHOLE ref table once per state-growing op" entry for the
+/// investigation these benchmarks measure. Build with `-DENABLE_BENCHMARKS=ON` and run the
+/// resulting `benchmark_cas_ref_protocol` binary directly; never wired into `ninja test`
+/// or CI.
+///
+/// BM_Admits history (synthetic RefTableState, time/call, this binary):
+///   Before incremental admits() (2026-07-19) -- full O(N) rebuild+encode per call:
+///     N=100: 48.8 us    N=1,000: 476 us    N=10,000: 5,018 us    N=100,000: 55,976 us
+///     Google Benchmark complexity fit: O(N log N), RMS 2%.
+///   After incremental admits() (2026-07-20) -- O(1) via incremental body-byte counters on
+///   RefTableState:
+///     N=100: 1842 ns    N=1,000: 1875 ns    N=10,000: 1864 ns    N=100,000: 1919 ns
+///     Google Benchmark complexity fit: O(1), RMS 1-2%.
+///
+/// BM_EncodeRefLogTxn history (this binary; acceptance gate for the CasJsonWriter migration):
+///   Before CasJsonWriter, field-by-field WriteBuffer calls (baseline): 753 ns.
+///   After CasJsonWriter bulk-append migration (2026-07-20): 333 ns -- this is the shipped code.
+///   BM_MemcpyTxnBytes floor (same bytes, plain String appends of 16-byte fragments): 30.7 ns.
+///   Ratio EncodeRefLogTxn / MemcpyTxnBytes = 333 / 30.7 ~= 10.8x -- above the 3x acceptance gate.
+///   A `keyLiteral` "rung-1" contingency variant (merging separator+key text into one literal
+///   append for the fixed unprefixed keys in writeOp/writeCommittedRow) was also measured: 325 ns
+///   ~= 10.8x -- a negligible ~2.5% move, not worth a third key-rendering path. It was NOT shipped;
+///   writeOp/writeCommittedRow keep the single `writeKey` path for clarity. Per the contingency
+///   ladder, rung 2 was NOT attempted either (it trades readability and needs a human decision);
+///   reported as DONE_WITH_CONCERNS. CasEncodingPins.* stayed byte-identical (green) throughout.
+///
+/// Phase B baselines, 2026-07-21, pre-encapsulation (this binary; `--benchmark_repetitions=3
+/// --benchmark_report_aggregates_only=true`; medians reported). Recorded ahead of the
+/// `RefTableState` encapsulation refactor so later phases can re-run this exact suite unchanged and
+/// diff against these numbers.
+///   BM_Admits (promote op; stays O(1) via the incremental budget counters, untouched by this round):
+///     N=100: 963 ns    N=1,000: 979 ns    N=10,000: 988 ns    N=100,000: 1,029 ns
+///     Complexity fit: O(1), RMS 2%.
+///   BM_AdmitsAddPrecommit (add op -- THE production hotspot shape: `manifestAlreadyOwned`'s linear
+///   value scan AT THIS BASELINE; O(1) via the owned-manifest index since E2 -- see the Final block
+///   below):
+///     N=100: 995 ns    N=1,000: 4,266 ns    N=10,000: 38,771 ns    N=100,000: 400,222 ns
+///     Complexity fit: O(N), ~4.0 ns/row, RMS 2%.
+///   BM_ApplyRefLogTxn (scratch copy + validate + apply + install of one promote):
+///     N=100: 724 ns    N=1,000: 738 ns    N=10,000: 784 ns    N=100,000: 788 ns
+///     Complexity fit: O(1), RMS 4%.
+///   BM_ReplayHistory (fold/recovery profile: snapshot of size N, 256 tail txns, 2 ops each):
+///     N=100: 6.15 ms    N=1,000: 46.1 ms    N=10,000: 454.0 ms    N=100,000: 4.93 s
+///     Complexity fit: O(N), ~48,859 ns/row, RMS 3%.
+///   BM_ScratchCopy (one full RefTableState copy off a materialized state -- the isolation floor):
+///     N=100: 45.7 ns    N=1,000: 46.0 ns    N=10,000: 46.7 ns    N=100,000: 46.8 ns
+///     Complexity fit: O(1), RMS 1%.
+///   BM_SnapshotEncode (encodeRefTableSnapshot(snapshotOf(state))):
+///     N=100: 14,955 ns    N=1,000: 150,061 ns    N=10,000: 1,508,586 ns    N=100,000: 15,885,841 ns
+///     Complexity fit: O(N), ~159 ns/row, RMS 1%.
+///   BM_MergedIteration (full base + 10%-overlay merged iteration, post-copy pre-materialize shape):
+///     N=100: 759 ns    N=1,000: 7,719 ns    N=10,000: 81,073 ns    N=100,000: 864,552 ns
+///     Complexity fit: O(N), ~8.6 ns/row, RMS 4%.
+///   BM_Materialize (RefCowMap::materialize after one overlay insert on an N-row base):
+///     N=100: 12,069 ns    N=1,000: 126,687 ns    N=10,000: 1,296,326 ns    N=100,000: 18,145,559 ns
+///     Complexity fit: O(N log N), RMS 2%.
+///
+/// Final, 2026-07-21, shipped tree (post E1+E2+E3; E4 tried and REVERTED -- full per-phase tables in
+/// `bench_t5_e3.log`):
+///   BM_AdmitsAddPrecommit: ~692-714 ns FLAT across N=100..100,000 -- O(1), RMS 1%
+///     (the owned-manifest index replaced the linear scan; ~571x at N=100k).
+///   BM_ReplayHistory: 1,725.58 ns/row (was 48,859) -- in-place `TrustedReplay` apply, -96.5%.
+///   BM_ApplyRefLogTxn: ~778-822 ns O(1). BM_Admits (promote): ~996-1,056 ns O(1).
+///   BM_ScratchCopy: ~58 ns O(1) (+~11 ns vs baseline: one more shared_ptr copy for the index).
+///   BM_SnapshotEncode / BM_MergedIteration / BM_Materialize: unchanged from baseline (E4 reverted).
+///
+/// Implementation note for later phases: `makeSyntheticState` calls `RefCowMap::materialize()`
+/// after `replay` (which never does -- it is the pure state-machine equation, and
+/// `stateFromSnapshot` loads every row through `emplace`, which only ever touches the overlay).
+/// Skipping that call makes every `RefTableState` copy in this suite (including `admits`'s and
+/// `applyRefLogTxn`'s own internal scratch copies) an O(N) deep-copy of an un-materialized overlay
+/// map instead of an O(1) shared-base copy -- this was caught during this round because it made
+/// BM_Admits regress from the documented O(1) to visibly O(N log N), contradicting its own history
+/// above. Production's RETAINED states are all materialized before reuse (the live table materializes
+/// once per flush; post-consult the recovery-install site in CasRefLedger.cpp materializes the
+/// replayed state before retaining it -- it previously did not, which is the recovery-latency cliff
+/// BM_FlushInstall now measures against), so the fix was to materialize in the helper, not to accept
+/// the contaminated numbers. (replay's own internal per-txn states are never materialized mid-fold;
+/// BM_ReplayHistory models that path on purpose.)
+
+using namespace DB::Cas;
+
+namespace
+{
+
+/// A ref-ledger key shape as actually written on the wire: table_uuid + database + table + part_name.
+constexpr std::string_view kSafeKeyLikeString
+    = "eeeb74a2-606a-4ee9-840a-1aac7b5ac25b_ca_stress_default_part_20260719_0_89811_538";
+
+RefLogTxn makeSamplePromoteTxn()
+{
+    RefLogTxn txn;
+    txn.ns = "roots/ca_soak_ch1";
+    txn.txn_id = RefTxnId{1, 12345};
+
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "20260719_0_89811_538_89818", ManifestRef{1, 1, 999999}};
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "20260719_0_89811_538_89818", ManifestRef{1, 1, 999999}};
+    txn.ops.push_back(op);
+    return txn;
+}
+
+/// A synthetic snapshot of `n` committed rows plus one pending precommit ready to promote.
+/// Built as a RefTableSnapshot and materialized via the public `replay` entry point, so this
+/// helper keeps compiling unchanged when RefTableState's fields become private (Phase A).
+RefTableSnapshot makeSyntheticSnapshot(size_t n)
+{
+    RefTableSnapshot snapshot;
+    snapshot.ns = "roots/bench";
+    snapshot.snapshot_id = RefTxnId{1, 1};
+    for (size_t i = 0; i < n; ++i)
+    {
+        RefCommittedRow row;
+        row.ref_name = "part_" + std::to_string(i) + "_20260719_0_1000_1";
+        row.manifest_ref = ManifestRef{1, 1, static_cast<uint32_t>(i + 1)};
+        snapshot.committed.push_back(row);
+    }
+    std::sort(snapshot.committed.begin(), snapshot.committed.end(),
+              [](const auto & a, const auto & b) { return a.ref_name < b.ref_name; });
+    snapshot.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "new_part_x", ManifestRef{1, 1, 999999}});
+    return snapshot;
+}
+
+/// A synthetic committed-ref table of `n` rows, plus one pending precommit ready to promote --
+/// exactly the shape `admits()` previews on every state-growing ref op. Rebuilt through `replay`
+/// (the public state-machine entry point) rather than by poking `RefTableState` fields directly,
+/// so this helper survives Phase A's encapsulation of `RefTableState`.
+///
+/// `replay` (the pure state-machine equation) never materializes: `stateFromSnapshot` loads every
+/// committed row through `RefCowMap::emplace`, which only ever touches the overlay. Left alone,
+/// every subsequent `RefTableState` copy here (`admits`'s and `applyRefLogTxn`'s own internal
+/// scratch copies, and every benchmark's own scratch copy below) would deep-copy an N-row overlay
+/// map instead of sharing an immutable base pointer -- silently turning "the cost of the operation
+/// under test" into "the cost of copying an un-materialized map" and swamping the O(1) `admits`
+/// result the header history documents. The RETAINED long-lived states production keeps are all
+/// materialized: the writer's live table materializes once per flush, and -- post-consult -- the
+/// recovery-install site in `CasRefLedger.cpp` now calls `materializeCommitted()` on the replayed
+/// state before retaining it (it previously did NOT, so the first flush copied an N-row overlay --
+/// exactly the cliff this fix removed and the reason `BM_FlushInstall` below measures the fully
+/// materialized flush cost). So this helper materializes too, matching what every real caller does
+/// immediately after building or replaying a state it will keep. (Note that `replay`'s own INTERNAL
+/// per-transaction states are never materialized mid-fold -- `BM_ReplayHistory` deliberately models
+/// that, feeding `replay(snapshot, tail)` an un-materialized base on purpose.)
+RefTableState makeSyntheticState(size_t n)
+{
+    RefTableState state = replay(makeSyntheticSnapshot(n), {});
+    state.materializeCommitted();
+    return state;
+}
+
+}
+
+/// Floor comparison: writeJSONString's per-character escaping loop (WriteHelpers.h) on a string
+/// that needs no escaping at all (a real ref-ledger key shape) vs a raw bulk write of the same
+/// bytes. See BM_RawBulkWriteSafe below for the delta.
+static void BM_WriteJSONStringSafe(benchmark::State & state)
+{
+    DB::FormatSettings settings;
+    DB::PODArray<char> buf;
+    for (auto _ : state)
+    {
+        buf.clear();
+        DB::WriteBufferFromVector<DB::PODArray<char>> out(buf);
+        DB::writeJSONString(kSafeKeyLikeString, out, settings);
+        benchmark::DoNotOptimize(buf.data());
+    }
+}
+BENCHMARK(BM_WriteJSONStringSafe);
+
+static void BM_RawBulkWriteSafe(benchmark::State & state)
+{
+    DB::PODArray<char> buf;
+    for (auto _ : state)
+    {
+        buf.clear();
+        DB::WriteBufferFromVector<DB::PODArray<char>> out(buf);
+        DB::writeChar('"', out);
+        out.write(kSafeKeyLikeString.data(), kSafeKeyLikeString.size());
+        DB::writeChar('"', out);
+        benchmark::DoNotOptimize(buf.data());
+    }
+}
+BENCHMARK(BM_RawBulkWriteSafe);
+
+/// Absolute cost of encoding one ref-log transaction (a single promote op) with
+/// `encodeRefLogTxn`'s migrated `CasJsonWriter` bulk-append implementation (see the history
+/// comment at the top of this file and the BACKLOG resolution). `BM_MemcpyTxnBytes` right below
+/// is the floor to diff this against.
+static void BM_EncodeRefLogTxn(benchmark::State & state)
+{
+    const RefLogTxn txn = makeSamplePromoteTxn();
+    for (auto _ : state)
+        benchmark::DoNotOptimize(encodeRefLogTxn(txn));
+}
+BENCHMARK(BM_EncodeRefLogTxn);
+
+/// The "near-memcpy" floor for BM_EncodeRefLogTxn: the SAME encoded bytes assembled from
+/// precomputed 16-byte fragments by plain String appends -- approximating the writer's append
+/// granularity with zero formatting/escaping work. Originally an acceptance gate for the
+/// CasJsonWriter migration; measurement showed the <=3x-of-floor target is physically unreachable for a validating,
+/// JSON-escaping encoder (BM_EncodeRefLogTxn lands at ~10.8x this floor even after the 2.26x
+/// CasJsonWriter speedup -- see the BACKLOG resolution for the profiled breakdown). Kept as a
+/// documented reference floor, not a pass/fail gate.
+static void BM_MemcpyTxnBytes(benchmark::State & state)
+{
+    const RefLogTxn txn = makeSamplePromoteTxn();
+    const String encoded = encodeRefLogTxn(txn);
+    std::vector<std::string_view> fragments;
+    constexpr size_t kFragment = 16;
+    for (size_t off = 0; off < encoded.size(); off += kFragment)
+        fragments.push_back(std::string_view(encoded).substr(off, kFragment));
+
+    String buf;
+    buf.reserve(encoded.size());
+    for (auto _ : state)
+    {
+        buf.clear();
+        for (const auto f : fragments)
+            buf.append(f.data(), f.size());
+        benchmark::DoNotOptimize(buf.data());
+    }
+}
+BENCHMARK(BM_MemcpyTxnBytes);
+
+/// admits() used to re-derive and re-encode the WHOLE committed-ref snapshot on every call
+/// (CasRefProtocol.cpp), showing O(N log N) growth with table size; it now maintains
+/// incremental body-byte counters on RefTableState instead, so this should show flat (O(1))
+/// time/call across the range. ->Complexity() has Google Benchmark fit and print the
+/// empirical big-O across the range.
+static void BM_Admits(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+    const RefTableState table = makeSyntheticState(n);
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "new_part_x", ManifestRef{1, 1, 999999}};
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "new_part_x", ManifestRef{1, 1, 999999}};
+
+    for (auto _ : state)
+        benchmark::DoNotOptimize(admits(table, op, 1ull << 40, 1ull << 40));
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_Admits)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+/// THE production hotspot shape: add-precommit runs `manifestAlreadyOwned` (a linear value scan
+/// today). Expected O(N) before the experiments, O(1) after the winning combination. Unlike
+/// BM_Admits (a promote, which never calls `manifestAlreadyOwned`), this previews a pure add --
+/// the op every part publication starts with -- so it is the shape production traces show as
+/// linear even after the incremental-budget fix landed for BM_Admits' promote shape.
+static void BM_AdmitsAddPrecommit(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+    const RefTableState table = makeSyntheticState(n);
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "brand_new_part", ManifestRef{2, 1, 1}};
+
+    for (auto _ : state)
+        benchmark::DoNotOptimize(admits(table, op, 1ull << 40, 1ull << 40));
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_AdmitsAddPrecommit)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+/// One transaction end-to-end: scratch copy + validate + apply + install (a promote of the
+/// staged precommit). The copy is part of the measured cost on purpose -- it is what E3 attacks.
+static void BM_ApplyRefLogTxn(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+    const RefTableState table = makeSyntheticState(n);
+
+    RefLogTxn txn;
+    txn.ns = "roots/bench";
+    txn.txn_id = RefTxnId{1, 2};
+    RefOp promote;
+    promote.kind = RefOpKind::OwnerTransition;
+    promote.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "new_part_x", ManifestRef{1, 1, 999999}};
+    promote.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "new_part_x", ManifestRef{1, 1, 999999}};
+    txn.ops.push_back(promote);
+
+    for (auto _ : state)
+    {
+        RefTableState scratch = table;
+        applyRefLogTxn(scratch, txn);
+        benchmark::DoNotOptimize(&scratch);
+    }
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_ApplyRefLogTxn)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+/// End-to-end FLUSH-INSTALL cost: apply one state-growing transaction (add a fresh precommit, then
+/// promote it -- touching BOTH the committed map AND the owned-manifest index) and then
+/// `materializeCommitted()`, which folds BOTH COW overlays into fresh shared bases. THIS is the O(N)
+/// critical section production holds `state_mutex` for, once per ref-log flush -- the number the
+/// "writer path is flat" claim (drawn from `BM_ApplyRefLogTxn`, which stops before materialize) must be
+/// weighed against. `BM_ApplyRefLogTxn` measures apply-without-install; the shipped-report
+/// `BM_Materialize` measures only `RefCowMap`'s half; this measures the whole install including the
+/// second (`owned_manifests`) container the index added, over the same N range.
+static void BM_FlushInstall(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+    const RefTableState table = makeSyntheticState(n);   // materialized, as a live table is at a flush boundary
+
+    /// add + promote of a fresh ref: the add inserts into `owned_manifests`, the promote grows
+    /// `committed` -- so materialize below folds a nonempty overlay in BOTH containers. Manifest {4,1,1}
+    /// and ref name are unique against the synthetic snapshot's {1,1,*} rows and "new_part_x" precommit.
+    RefLogTxn txn;
+    txn.ns = "roots/bench";
+    txn.txn_id = RefTxnId{1, 2};
+    RefOp add;
+    add.kind = RefOpKind::OwnerTransition;
+    add.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "flush_install_new_part", ManifestRef{4, 1, 1}};
+    txn.ops.push_back(add);
+    RefOp promote;
+    promote.kind = RefOpKind::OwnerTransition;
+    promote.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "flush_install_new_part", ManifestRef{4, 1, 1}};
+    promote.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "flush_install_new_part", ManifestRef{4, 1, 1}};
+    txn.ops.push_back(promote);
+
+    for (auto _ : state)
+    {
+        RefTableState working = table;   // O(1): shared base
+        applyRefLogTxn(working, txn);     // O(ops): bounded overlay
+        working.materializeCommitted();   // O(N): the critical-section fold this benchmark exists to measure
+        benchmark::DoNotOptimize(&working);
+    }
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_FlushInstall)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+/// Same flush-install as `BM_FlushInstall`, but exercising the E5 uniquely-owned-base fast path that
+/// production actually hits. `BM_FlushInstall` copies a shared fixture (`working = table`), so at
+/// `materializeCommitted()` the base still has `use_count() == 2` and the fold must build a fresh
+/// base -- O(N). Production's live table has NO outstanding scratch copy at the install point:
+/// `CasRefLedger::flushRefBatch` EXPLICITLY releases its trial-validation copy (`working = RefTableState{}`)
+/// before allocating the id and doing the post-PUT install, so at `materializeCommitted()` the live
+/// base is uniquely owned and the fold happens in place -- O(overlay). This variant models that by
+/// rebuilding a private,
+/// materialized state each iteration (its base `use_count()` is 1), timing only the apply + in-place
+/// materialize. The per-iteration rebuild AND the prior iteration's O(N) teardown are excluded from
+/// the measurement by hoisting `working` out of the loop and rebuilding it via move-assignment under
+/// Pause/ResumeTiming (the reassignment both destroys the previous grown state and installs a fresh
+/// materialized one, all untimed). The residual per-iteration Pause/Resume overhead is a constant
+/// floor, so the signal to read is FLATNESS across N (O(overlay)), not the absolute small-N number.
+static void BM_FlushInstallUniqueOwner(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+
+    RefLogTxn txn;
+    txn.ns = "roots/bench";
+    txn.txn_id = RefTxnId{1, 2};
+    RefOp add;
+    add.kind = RefOpKind::OwnerTransition;
+    add.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "flush_install_new_part", ManifestRef{4, 1, 1}};
+    txn.ops.push_back(add);
+    RefOp promote;
+    promote.kind = RefOpKind::OwnerTransition;
+    promote.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "flush_install_new_part", ManifestRef{4, 1, 1}};
+    promote.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "flush_install_new_part", ManifestRef{4, 1, 1}};
+    txn.ops.push_back(promote);
+
+    /// Hoisted out of the loop so the O(N) teardown of the previous iteration's grown state is folded
+    /// into the untimed move-assignment below, not charged to the timed apply + materialize region.
+    RefTableState working;
+    for (auto _ : state)
+    {
+        state.PauseTiming();
+        working = makeSyntheticState(n);   // private, materialized: base use_count() == 1
+        state.ResumeTiming();
+
+        applyRefLogTxn(working, txn);      // O(ops): bounded overlay
+        working.materializeCommitted();    // O(overlay): uniquely-owned base folded IN PLACE (the E5 win)
+        benchmark::DoNotOptimize(&working);
+    }
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+/// Fixed iteration count: the E5 fast path makes the timed apply + in-place-materialize region tiny
+/// and N-independent, so google-benchmark's default min-time targeting would demand millions of
+/// iterations at every N -- each paying an untimed O(N) `makeSyntheticState` rebuild, which explodes
+/// at large N. A fixed, modest count keeps every point cheap while still averaging enough samples to
+/// read the flatness across N (the whole point of this variant).
+BENCHMARK(BM_FlushInstallUniqueOwner)->RangeMultiplier(10)->Range(100, 100000)->Iterations(500)->Complexity();
+
+/// The fold/recovery profile: K transactions replayed over a size-N snapshot. Each txn creates
+/// and promotes one new ref (two ops), so each add pays today's `manifestAlreadyOwned` scan.
+/// K fixed at 256; complexity fit is over N.
+static void BM_ReplayHistory(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+    const RefTableSnapshot snapshot = makeSyntheticSnapshot(n);
+
+    constexpr size_t kTailTxns = 256;
+    std::vector<RefLogTxn> tail;
+    tail.reserve(kTailTxns);
+    for (size_t k = 0; k < kTailTxns; ++k)
+    {
+        RefLogTxn txn;
+        txn.ns = "roots/bench";
+        txn.txn_id = RefTxnId{1, 2 + k};
+
+        /// Refs unique per k, and namespaced under writer_epoch 3 so they collide with nothing in
+        /// the snapshot's own {1,1,i} committed series or its {1,1,999999} precommit.
+        const String ref_name = "replay_part_" + std::to_string(k);
+        const ManifestRef manifest_ref{3, 1, static_cast<uint32_t>(k + 1)};
+
+        RefOp add;
+        add.kind = RefOpKind::OwnerTransition;
+        add.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref_name, manifest_ref};
+        txn.ops.push_back(add);
+
+        RefOp promote;
+        promote.kind = RefOpKind::OwnerTransition;
+        promote.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref_name, manifest_ref};
+        promote.new_binding = RefOwnerBinding{RefOwnerKind::Committed, ref_name, manifest_ref};
+        txn.ops.push_back(promote);
+
+        tail.push_back(std::move(txn));
+    }
+
+    for (auto _ : state)
+        benchmark::DoNotOptimize(replay(snapshot, tail));
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_ReplayHistory)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+/// The isolation primitive on its own: one full state copy (COW committed + std::set precommits
+/// + counters). Overlay is empty (state fresh from replay+materialize), so this is the floor.
+static void BM_ScratchCopy(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+    RefTableState table = makeSyntheticState(n);
+    table.materializeCommitted();   /// makeSyntheticState already materializes; repeated here
+                                     /// defensively (a no-op on an empty overlay) so this benchmark's
+                                     /// floor claim does not silently depend on that helper's internals.
+
+    for (auto _ : state)
+    {
+        RefTableState copy = table;
+        benchmark::DoNotOptimize(&copy);
+    }
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_ScratchCopy)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+/// Canonical snapshot encoding for size N (per-flush cost, expected O(N) -- the question is the
+/// constant, which E4's contiguous scan attacks).
+static void BM_SnapshotEncode(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+    const RefTableState table = makeSyntheticState(n);
+
+    for (auto _ : state)
+        benchmark::DoNotOptimize(encodeRefTableSnapshot(snapshotOf(table, "roots/bench")));
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_SnapshotEncode)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+/// Full merged iteration with a 10% overlay (post-copy, pre-materialize shape): an N-row
+/// materialized base, then a fresh overlay of N/10 rows layered on top with `materialize()`
+/// deliberately not called again -- so iteration must merge base and overlay in sorted order the
+/// way the cold full-scan paths (snapshotOf, listRefs, dropNamespace) do against an in-flight batch.
+/// Benchmarks `RefCowMap` directly (like `BM_Materialize` below) rather than through
+/// `RefTableState::getCommitted()`: this isolates the merge-iteration primitive itself, and building
+/// the overlay via `RefTableState`'s promote/precommit transactions would additionally measure the
+/// state machine's own per-op bookkeeping, which is not what this benchmark is about.
+static void BM_MergedIteration(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+
+    RefCowMap map;
+    for (size_t i = 0; i < n; ++i)
+    {
+        RefCommittedRow row;
+        row.ref_name = "part_" + std::to_string(i) + "_20260719_0_1000_1";
+        row.manifest_ref = ManifestRef{1, 1, static_cast<uint32_t>(i + 1)};
+        map.emplace(row.ref_name, row);
+    }
+    map.materialize();
+
+    const size_t overlay_n = std::max<size_t>(1, n / 10);
+    for (size_t i = 0; i < overlay_n; ++i)
+    {
+        RefCommittedRow row;
+        row.ref_name = "overlay_part_" + std::to_string(i) + "_20260719_0_1000_1";
+        row.manifest_ref = ManifestRef{2, 1, static_cast<uint32_t>(i + 1)};
+        map.insert_or_assign(row.ref_name, row);
+    }
+
+    for (auto _ : state)
+    {
+        size_t total = 0;
+        for (const auto [ref_name, row] : map)
+            total += row.ref_name.size();
+        benchmark::DoNotOptimize(total);
+    }
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_MergedIteration)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+/// RefCowMap::materialize after one overlay insert on an N-row base (per-flush install cost).
+/// Benchmarks RefCowMap directly -- it is a public class.
+static void BM_Materialize(benchmark::State & state)
+{
+    const size_t n = static_cast<size_t>(state.range(0));
+    RefCowMap base_map;
+    for (size_t i = 0; i < n; ++i)
+    {
+        RefCommittedRow row;
+        row.ref_name = "part_" + std::to_string(i) + "_20260719_0_1000_1";
+        row.manifest_ref = ManifestRef{1, 1, static_cast<uint32_t>(i + 1)};
+        base_map.emplace(row.ref_name, row);
+    }
+    base_map.materialize();
+
+    for (auto _ : state)
+    {
+        RefCowMap copy = base_map;
+        RefCommittedRow new_row;
+        new_row.ref_name = "brand_new_part_20260719_0_1000_1";
+        new_row.manifest_ref = ManifestRef{2, 1, 1};
+        copy.insert_or_assign(new_row.ref_name, new_row);
+        copy.materialize();
+        benchmark::DoNotOptimize(&copy);
+    }
+
+    state.SetComplexityN(static_cast<int64_t>(n));
+}
+BENCHMARK(BM_Materialize)->RangeMultiplier(10)->Range(100, 100000)->Complexity();
+
+BENCHMARK_MAIN();

From edb79bed0488dfad301ae31a1acfdbea7a184302 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:35 +0200
Subject: [PATCH 21/30] CAS integration: disk surface and registration

The content-addressed metadata storage, its disk transaction and part
staging (the top-level subsystem glue), registration of the
content_addressed metadata storage type, capability predicates on
IDisk/DiskObjectStorage/IMetadataStorage, the conditional-object-storage
API on IObjectStorage, the FileView read-pipeline stage, write-ETag
surfacing, cache-over-CA, and the atomic-file-write short-circuit for
txn_version.txt.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../DiskObjectStorage/DiskObjectStorage.cpp   |   52 +-
 .../DiskObjectStorage/DiskObjectStorage.h     |   13 +-
 .../DiskObjectStorageCache.cpp                |   14 +-
 .../MetadataStorageFromCacheObjectStorage.cpp |    7 +
 .../MetadataStorageFromCacheObjectStorage.h   |    1 +
 .../ContentAddressedExchange.cpp              |  170 ++
 .../ContentAddressedExchange.h                |  260 ++
 .../ContentAddressedMetadataStorage.cpp       | 2299 +++++++++++++++++
 .../ContentAddressedMetadataStorage.h         |  774 ++++++
 .../ContentAddressedSettings.cpp              |  214 ++
 .../ContentAddressedSettings.h                |   89 +
 .../ContentAddressedTransaction.cpp           | 1966 ++++++++++++++
 .../ContentAddressedTransaction.h             |  420 +++
 .../ContentAddressed/README.md                |  198 ++
 .../MetadataStorages/IMetadataStorage.h       |   51 +
 .../MetadataStorageFactory.cpp                |   39 +
 .../ObjectStorages/IObjectStorage.h           |   72 +
 .../RegisterDiskObjectStorage.cpp             |   23 +-
 src/Disks/DiskType.cpp                        |    2 +
 src/Disks/DiskType.h                          |    1 +
 src/Disks/IDisk.h                             |    7 +
 src/Disks/ReadOnlyDiskWrapper.h               |    5 +
 src/IO/ReadPipeline.cpp                       |   39 +-
 src/IO/ReadPipeline.h                         |   21 +-
 src/IO/WriteBufferFromFileBase.h              |    7 +
 src/IO/WriteBufferFromFileDecorator.h         |    9 +
 src/IO/WriteSettings.h                        |   39 +
 .../VersionMetadataOnDisk.cpp                 |   12 +
 src/Storages/StorageMergeTree.cpp             |   15 +-
 src/Storages/StorageReplicatedMergeTree.cpp   |   12 +
 30 files changed, 6816 insertions(+), 15 deletions(-)
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.cpp
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h
 create mode 100644 src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/README.md

diff --git a/src/Disks/DiskObjectStorage/DiskObjectStorage.cpp b/src/Disks/DiskObjectStorage/DiskObjectStorage.cpp
index 211f73468449..191fc83b9b65 100644
--- a/src/Disks/DiskObjectStorage/DiskObjectStorage.cpp
+++ b/src/Disks/DiskObjectStorage/DiskObjectStorage.cpp
@@ -13,6 +13,7 @@
 #include <Common/CurrentMetrics.h>
 #include <IO/CachedInMemoryReadBufferFromFile.h>
 #include <IO/ReadPipeline.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h>
 #include <Disks/DiskObjectStorage/ObjectStorages/Cached/CachedObjectStorage.h>
 #include <Interpreters/FileCache/FileCache.h>
 #include <Disks/IO/ReadBufferFromRemoteFSGather.h>
@@ -743,6 +744,7 @@ bool DiskObjectStorage::isSharedCompatible() const
     {
         case MetadataStorageType::Plain:
         case MetadataStorageType::PlainRewritable:
+        case MetadataStorageType::CAS:
         case MetadataStorageType::StaticWeb:
             return true;
         default:
@@ -752,9 +754,33 @@ bool DiskObjectStorage::isSharedCompatible() const
 
 bool DiskObjectStorage::supportsHardLinks() const
 {
+    /// MergeTree consults `supportsHardLinks` at exactly two pure capability gates
+    /// (`MergeTreeData::checkAlterIsPossible` and `checkMutationIsPossible`) to decide whether
+    /// mutations / lightweight `DELETE` / data-`ALTER`s are possible. On a content-addressed pool
+    /// these are supported: `MutateTask` builds the new part through ONE whole-part transaction in
+    /// which `createHardLink` carries unchanged columns forward BY REFERENCE (the new manifest entry
+    /// points at the same blob hash — no re-upload) and changed columns are written as fresh blobs,
+    /// committed atomically. No code branches on this flag to choose a per-file-autocommit path, so
+    /// advertising true does NOT open the per-file clone hazard: the corrupting whole-part clone
+    /// paths (partition clone, BACKUP hard-link, replication) are gated by their own independent
+    /// checks (`checkAlterPartitionIsPossible`, the BACKUP CA rejection, the `Replicated*MergeTree`
+    /// CA rejection), which remain in force.
+    if (metadata_storage->isContentAddressed())
+        return true;
+
     return !metadata_storage->isWriteOnce() && !metadata_storage->isPlain();
 }
 
+bool DiskObjectStorage::isContentAddressed() const
+{
+    return metadata_storage->isContentAddressed();
+}
+
+bool DiskObjectStorage::supportsAtomicFileWrites() const
+{
+    return metadata_storage->supportsAtomicFileWrites();
+}
+
 
 String DiskObjectStorage::getReadResourceName() const
 {
@@ -790,7 +816,26 @@ void DiskObjectStorage::prepareRead(
     std::optional<size_t> read_hint,
     ReadPipeline & pipeline) const
 {
-    const auto storage_objects = metadata_storage->getStorageObjects(path);
+    /// Content-addressed reads: in-manifest bytes come from memory (no object exists); a
+    /// blob-backed part file translates to its physical blob object + a payload window, which
+    /// rides the STANDARD pipeline below (gather/caches/async prefetch — same chain as plain
+    /// object-storage disks, so right-mark bounds reach the object reader and its range requests
+    /// stay drainable, B116) and is bounded by the FileView stage at the end.
+    std::optional<IContentAddressedExchange::BlobViewPlan> ca_blob_view;
+    if (metadata_storage->isContentAddressed())
+    {
+        const auto * ca = dynamic_cast<const IContentAddressedExchange *>(metadata_storage.get());
+        if (ca)
+        {
+            if (ca->prepareInManifestRead(path, settings, pipeline))
+                return;
+            ca_blob_view = ca->getBlobViewPlan(path);
+        }
+    }
+
+    const auto storage_objects = ca_blob_view
+        ? StoredObjects{ca_blob_view->object}
+        : metadata_storage->getStorageObjects(path);
 
     auto read_settings = updateIOSchedulingSettings(settings, getReadResourceName(), getWriteResourceName());
     auto global_context = Context::getGlobalContextInstance();
@@ -870,6 +915,11 @@ void DiskObjectStorage::prepareRead(
             global_context->getAsyncReadCounters(),
             global_context->getFilesystemReadPrefetchesLog());
     }
+
+    /// A content-addressed blob-backed file is a payload window inside its blob: bound the
+    /// chain to it (and skip the CHCA envelope header in front).
+    if (ca_blob_view)
+        pipeline.needFileView(path, ca_blob_view->payload_offset, ca_blob_view->payload_end);
 }
 
 std::unique_ptr<ReadBufferFromFileBase> DiskObjectStorage::readFileIfExists(
diff --git a/src/Disks/DiskObjectStorage/DiskObjectStorage.h b/src/Disks/DiskObjectStorage/DiskObjectStorage.h
index 663fb0c3f215..09ec478b02d0 100644
--- a/src/Disks/DiskObjectStorage/DiskObjectStorage.h
+++ b/src/Disks/DiskObjectStorage/DiskObjectStorage.h
@@ -48,7 +48,14 @@ friend class DiskObjectStorageReservation;
 
     DataSourceDescription getDataSourceDescription() const override { return data_source_description; }
 
-    bool supportZeroCopyReplication() const override { return metadata_storage->getType() != MetadataStorageType::Keeper; }
+    /// A content-addressed pool deduplicates blobs across parts and has no per-replica unique blob
+    /// ids; the zero-copy subsystem (B1) is explicitly out of scope for M1, so advertise it as
+    /// unsupported (honest capability — B31). Other object-storage metadata types keep the old rule.
+    bool supportZeroCopyReplication() const override
+    {
+        return metadata_storage->getType() != MetadataStorageType::Keeper
+            && metadata_storage->getType() != MetadataStorageType::CAS;
+    }
 
     bool supportParallelWrite() const override { return object_storages->takePointingTo(cluster->getLocalLocation())->supportParallelWrite(); }
 
@@ -210,6 +217,10 @@ friend class DiskObjectStorageReservation;
 
     bool supportsHardLinks() const override;
 
+    bool isContentAddressed() const override;
+
+    bool supportsAtomicFileWrites() const override;
+
     /// Get structure of object storage this disk works with. Examples:
     /// DiskObjectStorage(S3ObjectStorage)
     /// DiskObjectStorage(CachedObjectStorage(S3ObjectStorage))
diff --git a/src/Disks/DiskObjectStorage/DiskObjectStorageCache.cpp b/src/Disks/DiskObjectStorage/DiskObjectStorageCache.cpp
index 5f144c7dd993..f3588a84b6cb 100644
--- a/src/Disks/DiskObjectStorage/DiskObjectStorageCache.cpp
+++ b/src/Disks/DiskObjectStorage/DiskObjectStorageCache.cpp
@@ -18,10 +18,22 @@ DiskObjectStoragePtr DiskObjectStorage::wrapWithCache(FileCachePtr cache, const
     auto local_location = cluster->getLocalLocation();
     registry[local_location] = std::make_shared<CachedObjectStorage>(registry[local_location], cache, cache_settings, layer_name);
 
+    /// A content-addressed disk cannot be fronted by the generic MetadataStorageFromCacheObjectStorage
+    /// passthrough: that wrapper hides isContentAddressed and the concrete CA metadata/transaction
+    /// types the CA read/write paths dynamic_cast to, so a cache-wrapped CA disk would take the generic
+    /// write path and throw NOT_IMPLEMENTED at startup. Reuse the CA metadata storage directly; only the
+    /// object storage is cached. Safe because ContentAddressedMetadataStorage::startup()/shutdown() are
+    /// idempotent, so the base disk and this cache disk share one mount/lease with no conflict.
+    /// Immutable content-hash blobs then cache through the CachedObjectStorage above; the control plane
+    /// keeps using the CA metadata storage's own raw object-storage pointer and bypasses the cache.
+    MetadataStoragePtr cache_metadata_storage = metadata_storage->isContentAddressed()
+        ? metadata_storage
+        : std::make_shared<MetadataStorageFromCacheObjectStorage>(metadata_storage);
+
     auto cache_disk = std::make_shared<DiskObjectStorage>(
         layer_name,
         std::make_shared<ClusterConfiguration>(layer_name, cluster->getConfiguration()),
-        std::make_shared<MetadataStorageFromCacheObjectStorage>(metadata_storage),
+        cache_metadata_storage,
         std::make_shared<ObjectStorageRouter>(std::move(registry)),
         std::dynamic_pointer_cast<const DiskObjectStorage>(shared_from_this()),
         Context::getGlobalContextInstance()->getConfigRef(),
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/Cache/MetadataStorageFromCacheObjectStorage.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/Cache/MetadataStorageFromCacheObjectStorage.cpp
index f1d1af2ae2c2..783ebc6f174c 100644
--- a/src/Disks/DiskObjectStorage/MetadataStorages/Cache/MetadataStorageFromCacheObjectStorage.cpp
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/Cache/MetadataStorageFromCacheObjectStorage.cpp
@@ -169,6 +169,13 @@ bool MetadataStorageFromCacheObjectStorage::isReadOnly() const
     return underlying->isReadOnly();
 }
 
+bool MetadataStorageFromCacheObjectStorage::isContentAddressed() const
+{
+    /// Defense-in-depth: never lie about content-addressing if this wrapper is ever constructed over a
+    /// CA storage. The primary fix (DiskObjectStorage::wrapWithCache) bypasses this wrapper for CA disks.
+    return underlying->isContentAddressed();
+}
+
 bool MetadataStorageFromCacheObjectStorage::isTransactional() const
 {
     return underlying->isTransactional();
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/Cache/MetadataStorageFromCacheObjectStorage.h b/src/Disks/DiskObjectStorage/MetadataStorages/Cache/MetadataStorageFromCacheObjectStorage.h
index 5cc03281bcba..f5e70e1e16ff 100644
--- a/src/Disks/DiskObjectStorage/MetadataStorages/Cache/MetadataStorageFromCacheObjectStorage.h
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/Cache/MetadataStorageFromCacheObjectStorage.h
@@ -63,6 +63,7 @@ class MetadataStorageFromCacheObjectStorage : public IMetadataStorage
     std::optional<StoredObjects> getStorageObjectsIfExist(const std::string & path) const override;
 
     bool isReadOnly() const override;
+    bool isContentAddressed() const override;
     bool isTransactional() const override;
     bool isPlain() const override;
     bool isWriteOnce() const override;
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.cpp
new file mode 100644
index 000000000000..513b0ae0a753
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.cpp
@@ -0,0 +1,170 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h>
+
+#include <array>
+
+namespace DB
+{
+
+namespace
+{
+
+/// The version tag. A peer that knows a different token shape must be refused rather than
+/// misparsed, and a bare field count is not a version: two shapes can agree on it by accident.
+constexpr std::string_view kTokenVersion = "car1";
+/// `|` is a legal cookie octet (RFC 6265 excludes only CTLs, whitespace, `"`, `,`, `;` and `\`)
+/// and never appears in an encoded field, because encoding leaves only the unreserved set and `%`.
+constexpr char kFieldSeparator = '|';
+constexpr size_t kFieldCount = 6;
+/// Sized for the widest real field -- a namespace is `<server_root_id>/store/<u3>/<uuid>@cas@` and a
+/// ref name is a part name, possibly `detached/`-prefixed. The cap is a bound on what a peer can make
+/// this server hold and log, not a schema: a field that needs more than this is refused, and a refused
+/// token costs a byte fetch.
+constexpr size_t kMaxFieldBytes = 256;
+constexpr size_t kMaxTokenBytes = 1024;
+
+bool isUnreserved(unsigned char c)
+{
+    return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || (c >= '0' && c <= '9')
+        || c == '-' || c == '.' || c == '_' || c == '~';
+}
+
+/// Control characters never travel: they would break the cookie header and they are the classic way to
+/// smuggle a forged line into whatever log prints the token.
+bool isControl(unsigned char c)
+{
+    return c < 0x20 || c == 0x7F;
+}
+
+void appendPercentEncoded(std::string_view field, String & out)
+{
+    static constexpr std::string_view hex_digits = "0123456789ABCDEF";
+    for (const char ch : field)
+    {
+        const auto c = static_cast<unsigned char>(ch);
+        if (isUnreserved(c))
+        {
+            out += ch;
+        }
+        else
+        {
+            out += '%';
+            out += hex_digits[c >> 4];
+            out += hex_digits[c & 0x0F];
+        }
+    }
+}
+
+std::optional<unsigned char> hexNibble(char ch)
+{
+    const auto c = static_cast<unsigned char>(ch);
+    if (c >= '0' && c <= '9')
+        return static_cast<unsigned char>(c - '0');
+    if (c >= 'A' && c <= 'F')
+        return static_cast<unsigned char>(c - 'A' + 10);
+    if (c >= 'a' && c <= 'f')
+        return static_cast<unsigned char>(c - 'a' + 10);
+    return std::nullopt;
+}
+
+/// Strict inverse of `appendPercentEncoded`: a `%` must be followed by exactly two hex digits, and no
+/// byte outside the unreserved set may appear unescaped. Lenient decoding is how a percent-encoding
+/// pair stops being a bijection, and this one has to be a bijection -- the encoded form is what routes
+/// the confirm, the decoded form is what it compares.
+std::optional<String> percentDecodeStrict(std::string_view field)
+{
+    String out;
+    out.reserve(field.size());
+    for (size_t i = 0; i < field.size(); ++i)
+    {
+        if (field[i] != '%')
+        {
+            if (!isUnreserved(static_cast<unsigned char>(field[i])))
+                return std::nullopt;
+            out += field[i];
+            continue;
+        }
+
+        if (i + 2 >= field.size())
+            return std::nullopt;
+        const auto hi = hexNibble(field[i + 1]);
+        const auto lo = hexNibble(field[i + 2]);
+        if (!hi || !lo)
+            return std::nullopt;
+        const auto decoded = static_cast<unsigned char>((*hi << 4) | *lo);
+        if (isControl(decoded))
+            return std::nullopt;
+        out += static_cast<char>(decoded);
+        i += 2;
+    }
+    return out;
+}
+
+}
+
+std::optional<String> encodeCasRelinkSourceToken(const CasRelinkSourceToken & token)
+{
+    const std::array<const String *, kFieldCount> fields{
+        &token.pool_uuid, &token.server_root_id, &token.root_namespace,
+        &token.ref_name, &token.part_name, &token.manifest_ref_text};
+
+    String out{kTokenVersion};
+    for (const String * field : fields)
+    {
+        /// Every field is required: a token with a hole in it can only route somewhere it was not meant
+        /// to, and `resolveContentAddressedConfirm` would have to re-discover that for itself.
+        if (field->empty() || field->size() > kMaxFieldBytes)
+            return std::nullopt;
+        for (const char ch : *field)
+            if (isControl(static_cast<unsigned char>(ch)))
+                return std::nullopt;
+
+        out += kFieldSeparator;
+        appendPercentEncoded(*field, out);
+    }
+
+    if (out.size() > kMaxTokenBytes)
+        return std::nullopt;
+    return out;
+}
+
+std::optional<CasRelinkSourceToken> decodeCasRelinkSourceToken(std::string_view text)
+{
+    if (text.size() > kMaxTokenBytes)
+        return std::nullopt;
+
+    std::array<std::string_view, kFieldCount + 1> segments;
+    size_t count = 0;
+    size_t pos = 0;
+    while (true)
+    {
+        if (count == segments.size())
+            return std::nullopt;   /// more separators than the shape has fields
+        const size_t sep = text.find(kFieldSeparator, pos);
+        if (sep == std::string_view::npos)
+        {
+            segments[count++] = text.substr(pos);
+            break;
+        }
+        segments[count++] = text.substr(pos, sep - pos);
+        pos = sep + 1;
+    }
+
+    if (count != segments.size() || segments[0] != kTokenVersion)
+        return std::nullopt;
+
+    CasRelinkSourceToken token;
+    const std::array<String *, kFieldCount> fields{
+        &token.pool_uuid, &token.server_root_id, &token.root_namespace,
+        &token.ref_name, &token.part_name, &token.manifest_ref_text};
+
+    for (size_t i = 0; i < kFieldCount; ++i)
+    {
+        auto decoded = percentDecodeStrict(segments[i + 1]);
+        if (!decoded || decoded->empty() || decoded->size() > kMaxFieldBytes)
+            return std::nullopt;
+        *fields[i] = std::move(*decoded);
+    }
+    return token;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h
new file mode 100644
index 000000000000..ac05a13bc78d
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h
@@ -0,0 +1,260 @@
+#pragma once
+
+#include <base/types.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/StoredObject.h>
+#include <memory>
+#include <optional>
+#include <string_view>
+
+namespace DB
+{
+
+class ReadPipeline;
+struct ReadSettings;
+
+/// The answer of the relink confirm as it crosses the exchange seam (spec §confirm-primitive). Declared
+/// here, on the narrow interface, so `DataPartsExchange` can carry the answer without including any
+/// content-addressed header; `ContentAddressedMetadataStorage` maps `Cas::ConfirmAnswer` onto it.
+///
+/// ONLY `Yes` AUTHORIZES ANYTHING. `No` and `Unknown` are one outcome for every caller -- both mean
+/// "not proven", both are `SourceProofFailed` in the spec's failure taxonomy, and neither may be used
+/// to conclude anything about the source. That is not a simplification, it is a coupling: gate 1
+/// evaluates the mount fence LAST (`CasRefLedger::confirmExactRef` rule 6), so a mount that has already
+/// lost its fence -- and can therefore no longer speak for the namespace at all -- still answers `No`
+/// for a token that does not match its last-known row. Any code that ever treats `No` as authoritative
+/// knowledge (say, to skip a retry or to conclude the part is gone) makes that ordering wrong and must
+/// hoist rule 6 above the row comparison first.
+enum class CasConfirmAnswer : uint8_t
+{
+    Yes,
+    No,
+    Unknown,
+};
+
+/// The sender's confirm token for one relink offer (spec §wire-protocol). It rides back to the receiver
+/// as a response cookie on the offer and returns verbatim as the only argument of the confirm request,
+/// so the receiver never has to understand a field of it and the sender never has to remember anything
+/// between the two requests. Every field is minted by the sender out of its OWN committed state; when
+/// it comes back it is untrusted peer input and is used only as a lookup key -- `resolveContentAddressedConfirm`
+/// answers whatever the fields happen to select, and selects nothing at all when they match nothing.
+///
+/// `pool_uuid` and `server_root_id` route the question to the one mount entitled to answer it (a pool
+/// UUID is shared by every server root writing into the pool, so it cannot select a mount on its own);
+/// `root_namespace` and `ref_name` name the binding; `manifest_ref_text` is the exact manifest the
+/// offer carried; `part_name` is gate 0's key into the sender's parts set.
+struct CasRelinkSourceToken
+{
+    String pool_uuid;
+    String server_root_id;
+    String root_namespace;
+    String ref_name;          /// the ref the sender published the part under (`detached/<name>` for B66b)
+    String part_name;         /// the MergeTree part name
+    String manifest_ref_text; /// canonical `writer_epoch:build_sequence:manifest_ordinal`
+};
+
+/// The token's wire form: `car1|<f1>|…|<f6>`, each field percent-encoded down to the RFC 3986
+/// unreserved set. The encoding exists because two of the fields are not character-safe as they stand
+/// -- a namespace carries `/` and `@`, and `server_root_id` is whatever the operator configured -- and
+/// the token has to survive both an HTTP cookie value and a URL query parameter. Percent-encoding is
+/// what makes that true by construction, instead of by a character allowlist that would silently
+/// disable relink for a legal-but-unusual `server_root_id`.
+///
+/// `nullopt` in either direction means "not a token": an empty or over-long field on the way out, and
+/// on the way in a wrong version tag, a wrong field count, a malformed escape, a control character or
+/// an over-long field. A refusal is never an answer about the source -- the sender simply makes no
+/// offer, and the receiver's confirm is simply unproven.
+std::optional<String> encodeCasRelinkSourceToken(const CasRelinkSourceToken & token);
+std::optional<CasRelinkSourceToken> decodeCasRelinkSourceToken(std::string_view text);
+
+/// Receiver side, the outcome of staging a relink (spec §failure-taxonomy). Two values, because the
+/// receiver has exactly two safe responses to a relink that did not commit, and they are NOT
+/// interchangeable:
+///  - `Prepared` -- the receiver's `+1` is durable and the caller now OWES the handle a terminal
+///    operation. It has published nothing yet.
+///  - `MechanismFallbackAllowed` -- relink cannot work here, nothing was staged, and the SENDER STILL
+///    HAS THE PART, so re-requesting the bytes from that same sender is a sound recovery.
+///
+/// A failure to prove the source is deliberately NOT representable here. That is the whole reason this
+/// is typed at all: `adoptPartFromManifest` used to catch every `Exception` and return `false`, which
+/// turned "the source could not prove it still holds the manifest" into "ask the source for the bytes"
+/// -- the one recovery that is unsound, because the doubt is about the source itself.
+enum class CaRelinkPrepare : uint8_t
+{
+    Prepared,
+    MechanismFallbackAllowed,
+};
+
+/// Receiver side, the outcome of promoting a prepared relink. THREE values, because a promote has
+/// three outcomes and only two of them are knowledge:
+///  - `Committed` -- the receiver's ref is committed.
+///  - `MechanismFallbackAllowed` -- the promote is PROVEN to have committed nothing (it was rejected
+///    before its ref-log append: a body-absent precommit, a precommit that is no longer the live owner,
+///    a ref conflict), which leaves the sender's copy the only authority and its bytes a sound recovery.
+///  - `Unresolved` -- the ref-log append was attempted and did not come back with a verdict, so the ref
+///    MAY be committed. This is NOT a mechanism fallback and must never be reported as one: fetching
+///    the bytes after a relink that actually committed publishes the same part twice. The only sound
+///    recovery is to retry the whole fetch later, which is what the caller does with it.
+///
+/// Anything else PROPAGATES as an exception rather than becoming a value here -- an unclassified local
+/// failure is not evidence that a byte fetch would fare better, and hiding it is exactly the catch-all
+/// this boundary replaced.
+enum class CaRelinkPromote : uint8_t
+{
+    Committed,
+    MechanismFallbackAllowed,
+    Unresolved,
+};
+
+/// Receiver side: a relink that is DURABLE BUT NOT PROMOTED, as the exchange sees it. It exists because
+/// the confirm has to interpose between the receiver's `+1` becoming durable and the promote (spec
+/// §relink-handle), and it is abstract so `DataPartsExchange` can own that window without owning any
+/// content-addressed type.
+///
+/// Exactly one terminal operation is owed, `promote` or `abort`. Destruction is a backstop, not a
+/// substitute: the underlying transaction's precommit binding is live-epoch, so no sweep and no GC
+/// reclaims it -- only the removal that `abort` appends does.
+class ICaPreparedRelink
+{
+public:
+    virtual ~ICaPreparedRelink() = default;
+
+    /// Commits the receiver's ref over the shared-pool blobs. Call ONLY after the source has proven it
+    /// still holds exactly the offered manifest. `Unresolved` is a real outcome, not a defensive
+    /// leftover: the commit may be durable, so the caller must neither publish the part nor fetch its
+    /// bytes.
+    virtual CaRelinkPromote promote() = 0;
+
+    /// Releases the durable `+1` by appending the exact precommit removal. NEVER THROWS, and that is a
+    /// requirement rather than a convenience: this is what a scope guard runs while an exception is
+    /// already in flight (the receiver's own retry-later error), and it must neither replace that error
+    /// nor terminate the process. An append that fails is logged here and retried by the underlying
+    /// handle's own destructor. Calling it after a `promote` -- successful or not -- is a no-op: a
+    /// promote that failed has already discharged the duty on its way out.
+    virtual void abort() noexcept = 0;
+};
+
+/// Purpose-built seam for `DataPartsExchange`: it exposes everything replication needs from a
+/// content-addressed disk and nothing else. `ContentAddressedMetadataStorage` implements it, and
+/// the exchange obtains the interface by casting the disk's `IMetadataStorage` to this interface
+/// rather than depending on the concrete storage class. Keeping this boundary narrow prevents the
+/// replication path from becoming coupled to content-addressed storage internals.
+///
+/// Relink wire contract: the sender transmits `{pool_uuid, encoded PartManifest body, confirm token}`.
+/// The replica-internal `part_id` cookie carries the opaque manifest bytes and a response cookie carries
+/// the token, so this exchange adds no protocol field of its own. The manifest's sender-specific identity (`ManifestRef`, `root_namespace_id`,
+/// and `payload_digest`) is not authoritative: the receiver uses only the entries, adopts references
+/// to blobs in the shared pool by hash without reading blob bodies from the sender, and publishes a
+/// fresh manifest in its own namespace. Every per-part file is an ordinary manifest entry, so the
+/// manifest is self-contained and there is no separate sidecar or metadata-version wire field.
+/// When the local manifest cannot be committed — for example a required blob body is absent at
+/// precommit — adoption publishes nothing and the caller falls back to fetching the part bytes.
+class IContentAddressedExchange
+{
+public:
+    virtual ~IContentAddressedExchange() = default;
+
+    /// The pool's stable identity (Cas::PoolMeta::pool_id, hex). Two replicas may relink iff equal
+    /// — endpoint/prefix string-matching is unsafe (false positives => mis-relink). Empty before
+    /// the storage started up.
+    virtual const String & getPoolUUID() const = 0;
+
+    /// Sender side, routing predicate for the confirm action (spec §wire-protocol "Routing contract"):
+    /// does THIS instance own `root_namespace` under `server_root_id`? A pool UUID is shared by every
+    /// server root writing into the pool, so `getPoolUUID` alone cannot select the mount that is
+    /// entitled to answer for a namespace; the caller pairs the two and requires EXACTLY one match
+    /// (zero or several are both `Unknown`). I/O-free and never throws in any lifecycle state -- a
+    /// routing predicate that could fail would turn a misrouted question into an error instead of an
+    /// unproven answer.
+    virtual bool ownsNamespace(const String & server_root_id, const String & root_namespace) const = 0;
+
+    /// Sender side, gate 1 of the relink confirm (spec §confirm-primitive), forwarded to the ledger:
+    /// does `ref_name` in `root_namespace` still name EXACTLY the manifest rendered by
+    /// `manifest_ref_text` (the canonical `writer_epoch:build_sequence:manifest_ordinal` form) in this
+    /// writer's committed view? Read-only, performs ZERO object-store I/O, and creates nothing: a cold,
+    /// evicted, recovering, busy, wedged, poisoned or unfenced table answers `Unknown` rather than
+    /// doing work, because a remote peer drives this query. Never throws: an unparsable token and a
+    /// disk that is not started or has reached a terminal lifecycle are `Unknown` too.
+    virtual CasConfirmAnswer confirmExactRef(const String & root_namespace, const String & ref_name,
+                                             const String & manifest_ref_text) const = 0;
+
+    /// Sender side: everything one relink offer puts on the wire. The manifest body is opaque to the
+    /// exchange caller and is decoded by the receiver; the token is what the receiver hands back to
+    /// confirm the offer before it promotes.
+    struct RelinkOffer
+    {
+        String manifest_bytes;
+        String confirm_token;
+    };
+
+    /// Sender side: build the relink offer for this server's committed part at the given disk-relative
+    /// path. `nullopt` means the path is not a committed content-addressed part here, or the token
+    /// could not be minted, so the sender must make no offer and streams the part bytes instead.
+    ///
+    /// The manifest body and the token come out of ONE resolution of the part, and that is the point of
+    /// returning them together rather than as two calls: a repoint between them would hand the receiver
+    /// a token naming a manifest whose entries it never adopted, and a `Yes` for that manifest would
+    /// protect the wrong blobs.
+    virtual std::optional<RelinkOffer> getRelinkOffer(const String & part_path) const = 0;
+
+    /// Receiver side, the FIRST half of a relink: decode the transferred manifest, perform a normal
+    /// local build from shared-pool blob references without reading a single blob body from the sender,
+    /// stage a fresh manifest in the receiver namespace, `precommitAdd` it -- and STOP. The sender's
+    /// `root_namespace_id`, `ManifestRef` and `payload_digest` are ignored; only the entries are used.
+    ///
+    /// `part_path` is the RECEIVER's disk-relative path of the part directory being built, addressed the
+    /// same way `getRelinkOffer` addresses the sender's: the namespace and the ref name come from
+    /// routing it, so the caller never composes a ref name and a relink into `TABLE/detached/DIR`
+    /// (B66b) lands on the `detached/<dir>` ref for free, through the one router every other read and
+    /// write of that part uses. A path that does not route to a part DIRECTORY of a live table -- a
+    /// table dir, a file inside a part, a FREEZE shadow path -- is a caller error and throws.
+    ///
+    /// On `Prepared` the receiver's `+1` is DURABLE and `out` holds the handle that owes the terminal
+    /// operation; nothing is committed and no ref is live yet. The promote is deferred because the
+    /// receiver must first ask the source whether it still holds exactly this manifest (spec
+    /// §core-idea): the `+1` has to be durable BEFORE that question is asked, or the answer proves
+    /// nothing about the window that follows it.
+    ///
+    /// On `MechanismFallbackAllowed` nothing was staged and `out` is null: the manifest did not decode,
+    /// or the local staging hit the retryable class (a body-absent precommit, a precommit that is no
+    /// longer the live owner, a ref conflict -- `ABORTED`/`NETWORK_ERROR`). Any OTHER error propagates.
+    ///
+    /// Promotion trusts the adopted references through the durable manifest edge -- it does not re-read
+    /// each blob body -- the same interserver trust as an ordinary `ReplicatedMergeTree` fetch. A blob
+    /// that becomes absent or condemned after adoption is not caught here; it is an fsck-detectable
+    /// invariant violation.
+    virtual CaRelinkPrepare prepareAdoptFromManifest(
+        const String & part_path,
+        const String & manifest_bytes,
+        std::unique_ptr<ICaPreparedRelink> & out) = 0;
+
+    /// ==== `DiskObjectStorage::prepareRead` hooks ====
+    /// The two CA-only reads `DiskObjectStorage::prepareRead` needs before it composes the standard
+    /// object-storage pipeline. Exposed on this narrow seam (rather than only on the concrete
+    /// `ContentAddressedMetadataStorage`) so `prepareRead` casts to the interface instead of coupling
+    /// to the concrete storage class.
+
+    /// The CA read entry called by `DiskObjectStorage::prepareRead` before the generic
+    /// storage-objects path: serves in-manifest bytes (mutable per-part files, inline entries,
+    /// verbatim namespace files) from memory. Returns false when the path is not in-manifest.
+    virtual bool prepareInManifestRead(const std::string & path, const ReadSettings & settings, ReadPipeline & pipeline) const = 0;
+
+    /// Translates a blob-backed part file to the physical blob
+    /// object plus the payload WINDOW inside it (the CHCA envelope header occupies
+    /// [0, payload_offset)). DiskObjectStorage::prepareRead composes the STANDARD object-storage
+    /// pipeline over `object` (gather/caches/async prefetch — the same chain plain s3 disks get,
+    /// so `MergeTreeReaderStream` right-mark bounds reach the object reader and its range
+    /// requests stay drainable) and bounds it with the pipeline's FileView stage.
+    /// nullopt = the path is not a blob-backed part file (caller falls through; absent paths
+    /// then fail in getStorageObjects exactly as before).
+    struct BlobViewPlan
+    {
+        StoredObject object;        /// physical blob key; logical path; readable extent (envelope + payload)
+        size_t payload_offset = 0;  /// view left bound inside the blob
+        size_t payload_end = 0;     /// view right bound (payload_offset + payload length)
+    };
+    /// Resolves a blob-backed path to its physical object and payload window. Returns nullopt for
+    /// in-manifest, loose, directory, or otherwise unresolved paths.
+    virtual std::optional<BlobViewPlan> getBlobViewPlan(const std::string & path) const = 0;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.cpp
new file mode 100644
index 000000000000..31a988bfa038
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.cpp
@@ -0,0 +1,2299 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/StaticDirectoryIterator.h>
+#include <Disks/IDisk.h>
+#include <IO/ReadBufferFromFileView.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/ReadHelpers.h>
+#include <IO/ReadPipeline.h>
+#include <Interpreters/Context.h>
+#include <Interpreters/ContentAddressedGarbageCollectionLog.h>
+#include <Interpreters/ContentAddressedLog.h>
+#include <Common/CurrentThread.h>
+#include <base/getThreadId.h>
+#include <Common/DateLUT.h>
+#include <Common/Exception.h>
+#include <Common/LoggingHelpers.h>
+#include <Common/logger_useful.h>
+#include <Common/thread_local_rng.h>
+#include <Poco/Util/AbstractConfiguration.h>
+#include <base/sleep.h>
+#include <charconv>
+#include <chrono>
+#include <filesystem>
+#include <ctime>
+#include <unordered_set>
+
+namespace fs = std::filesystem;
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int FILE_DOESNT_EXIST;
+    extern const int LOGICAL_ERROR;
+    extern const int CORRUPTED_DATA;
+    extern const int READONLY;
+    extern const int BAD_ARGUMENTS;
+    extern const int ABORTED;
+    extern const int NETWORK_ERROR;
+    extern const int NOT_IMPLEMENTED;
+    extern const int INVALID_STATE;
+}
+
+namespace ContentAddressedSetting
+{
+    extern const ContentAddressedSettingsString scratch_path;
+    extern const ContentAddressedSettingsString server_root_id;
+    extern const ContentAddressedSettingsBool gc_enabled;
+    extern const ContentAddressedSettingsUInt64 gc_interval_sec;
+    extern const ContentAddressedSettingsUInt64 deduplication_cache_bytes;
+    extern const ContentAddressedSettingsUInt64 deduplication_head_first_min_bytes;
+    extern const ContentAddressedSettingsUInt64 gc_snapshot_generations_to_keep;
+    extern const ContentAddressedSettingsUInt64 gc_shards;
+    extern const ContentAddressedSettingsUInt64 manifest_sweep_list_budget_keys;
+    extern const ContentAddressedSettingsUInt64 manifest_sweep_delete_budget_keys;
+    extern const ContentAddressedSettingsUInt64 gc_round_graduation_budget;
+    extern const ContentAddressedSettingsUInt64 gc_round_redelete_budget;
+    extern const ContentAddressedSettingsUInt64 gc_round_sweep_namespace_budget;
+    extern const ContentAddressedSettingsUInt64 gc_round_sweep_recovery_op_budget;
+    extern const ContentAddressedSettingsUInt64 gc_round_ref_cleanup_budget;
+    extern const ContentAddressedSettingsUInt64 gc_round_prefix_wholesale_budget;
+    extern const ContentAddressedSettingsUInt64 gc_round_handoff_prefix_wholesale_budget;
+    extern const ContentAddressedSettingsUInt64 gc_round_outcome_entry_budget;
+    extern const ContentAddressedSettingsUInt64 gcs_max_conditional_put_bytes;
+    extern const ContentAddressedSettingsUInt64 part_folder_cache_bytes;
+    extern const ContentAddressedSettingsUInt64 part_folder_cache_max_entries;
+    extern const ContentAddressedSettingsUInt64 part_folder_cache_max_entry_bytes;
+    extern const ContentAddressedSettingsUInt64 manifest_decode_cache_bytes;
+    extern const ContentAddressedSettingsUInt64 gc_meta_pool_size;
+    extern const ContentAddressedSettingsBool blob_hash_allow_new;
+    extern const ContentAddressedSettingsBool skip_access_check;
+}
+
+namespace
+{
+/// The `lifecycle` column value for `system.cas_mounts` (spec §7): the pool lifecycle
+/// condition collapsed to the operator-facing vocabulary. The two `Vanished*` sub-states both map to the
+/// bare `vanished` -- the sub-state (replaced/forgotten) lives in the `lifecycle_reason` column,
+/// so a `NULL`-free `lifecycle` stays a small, stable enumerated set.
+const char * casLifecycleToString(Cas::PoolLifecycle lc)
+{
+    switch (lc)
+    {
+        case Cas::PoolLifecycle::Live:              return "live";
+        case Cas::PoolLifecycle::TransientNotLive:  return "not_live";
+        case Cas::PoolLifecycle::IdentityLost:      return "identity_lost";
+        case Cas::PoolLifecycle::VanishedReplaced:
+        case Cas::PoolLifecycle::VanishedForgotten: return "vanished";
+    }
+    return "unknown";
+}
+
+/// The ENUM-CLEAN `lifecycle_reason` word: the `vanished` sub-state (replaced/forgotten) so a
+/// downstream `lifecycle || '(' || lifecycle_reason || ')'` yields exactly e.g. `vanished(forgotten)`.
+/// Empty for every non-`vanished` state (the `lifecycle` column already fully names those). The rich [D5]
+/// text is carried separately in `lifecycle_detail`.
+const char * casLifecycleReasonWord(Cas::PoolLifecycle lc)
+{
+    switch (lc)
+    {
+        case Cas::PoolLifecycle::Live:
+        case Cas::PoolLifecycle::TransientNotLive:
+        case Cas::PoolLifecycle::IdentityLost:      return "";
+        case Cas::PoolLifecycle::VanishedReplaced:  return "replaced";
+        case Cas::PoolLifecycle::VanishedForgotten: return "forgotten";
+    }
+    return "";
+}
+}
+
+/// ============================================================================================
+/// The method -> operation-class inventory (rev.7 spec §1; the review artifact of Task 8).
+///
+/// EVERY public method of `ContentAddressedMetadataStorage` and `ContentAddressedTransaction` is listed
+/// with the `CasOpClass` it routes through `checkOpAdmitted` (or "Factory" for the never-gated I/O-free
+/// surface). The gate is consulted at the method's entry; `store()`/`partAccess()`/`poolAccess()` keep
+/// their own terminal check (Task 5) as low-level defense, reached only in the `Live` case.
+///
+/// ---- ContentAddressedMetadataStorage ----
+/// Factory (never gated): getType, getPath, supportsChmod, supportsStat, isReadOnly, isContentAddressed,
+///   transactionIsStagingOverlay, supportsAtomicFileWrites, supportsTransactionalMutableFiles,
+///   areBlobPathsRandom, getHardlinkCount, createTransaction (I/O-free -- allocates a txn), getPoolUUID,
+///   serverRootId, scratchPath, stagingBackend, conditionalCopySupported, objectStorage, gcHealth,
+///   lifecycleSnapshot (both non-store()-gated introspection reads for system.cas_mounts --
+///   readable in EVERY lifecycle state including a not-live/vanished/null pool, spec §7),
+///   parseStagingBackend/parsePartFolderValidate/
+///   tryFromDisk (static), checkNotReadOnly, the *ForTest seams, serverPrefix/liveNamespace/
+///   shadowNamespace/route/classifyDirectory (pure path computation, no pool I/O),
+///   ownsNamespace (the relink-confirm routing predicate -- a string comparison against
+///   `server_root_id`, deliberately answerable in EVERY lifecycle state).
+/// Probe:       existsFile, existsDirectory, existsFileOrDirectory, listDirectory, iterateDirectory,
+///              isDirectoryEmpty, getStorageObjectsIfExist, liveTreeDirHasChildren, listLiveTreeChildren.
+///              EMPTY-PROOF RULE (Task 9, spec §1 [B3]): on a NON-terminal (Live/read-only) pool, an
+///              empty `listDirectory` answer at a `TableDir`/`DetachedContainer` root additionally runs
+///              `confirmPoolIdentityForEmptyEnumeration` (one authoritative, UNCACHED `_pool_meta` probe)
+///              -- a `KeyAbsent`/`ContainerAbsent`/transport result throws the typed 668 instead of the
+///              empty answer. `iterateDirectory`/`isDirectoryEmpty` inherit it (both funnel through
+///              `listDirectory`). A `Vanished` pool never reaches it (the gate short-circuits `Probe`).
+/// ContentRead: getFileSize, getLastModified, getStorageObjects, getBlobViewPlan, readBlobPayload,
+///              prepareInManifestRead, tryGetInManifestBytes, getRelinkOffer, confirmExactRef
+///              (the ONE ContentRead entry that converts the gate's refusal into its own typed
+///              `Unknown` answer instead of propagating it -- a confirm never throws at its caller).
+/// Write:       adoptPartFromManifest.
+/// Admin:       runOneGcRoundForTest, runGarbageCollectionRoundNow, runGcRebuildNow, runFsckNow (the
+///              rev.8 FSCK-on-running path -- an FSCK of a not-live disk is refused by the Admin gate).
+/// Lifecycle/uncgated drivers (NOT op-gated -- they DRIVE the state): startup, shutdown, forgetDisk,
+///   gcStop, gcStart. store()/partAccess()/poolAccess() are the internal accessors, not public op entries.
+///   readableNamespaceFilesLife, stagingKeyPrefix, detachedRefNames, movingRefNames are post-gate helpers
+///   (their public callers gate first; they reach store() only in the Live case).
+///   confirmPoolIdentityForEmptyEnumeration is a post-gate helper too (the EMPTY-PROOF RULE, Task 9):
+///   listDirectory calls it only after the gate admitted a Probe and only on an empty table-root answer.
+///
+/// ---- ContentAddressedTransaction (routes through metadata_storage.checkOpAdmitted) ----
+/// Write:  writeFile (and tryCreateWriteBuffer, which funnels into it), createDirectory,
+///         createDirectoryRecursive, createHardLink, moveDirectory, moveFile, replaceFile,
+///         setLastModified, setReadOnly.
+/// Remove: removeDirectory, removeRecursive, unlinkFile.
+/// commit/tryCommit: Remove when there is nothing to publish (parts empty -- the DROP/rename path, which
+///         applied its ref mutations immediately), Write when it must publish staged parts. This is what
+///         lets a vanished-disk table's DROP finish (Remove -> no-op success) while a publishing commit
+///         throws the typed Vanished [D5] refusal.
+/// Unsupported (always throw, state-independent -- no gate needed): createMetadataFile,
+///   generateObjectKeyForPath, chmod, truncateFile.
+/// Factory / overlay-only (no committed-pool I/O): supportsChmod, getSubmittedForRemovalBlobs, and the
+///   read-your-writes overlay readers tryGetInFlightStorageObjects/tryReadFileInFlight/
+///   tryGetInFlightFileSize/hasInFlightDirectory/listInFlightDirectory (they read THIS transaction's own
+///   in-memory staging; a vanished-disk transaction has none because its writes threw at writeFile).
+///
+/// Null-pool rule: a storage with no published pool (before `startup` or after `shutdown` -- the
+/// storage-level Constructing/ShutDown lifecycle) fails loud for EVERY class, `Probe` included, via
+/// `throwStorageNotStarted`. There is no benign "absent" answer for a storage that has never published a
+/// pool (or torn one down); the pool lifecycle below is the sole authority for a published pool.
+/// ============================================================================================
+
+namespace
+{
+
+/// Canonical disk-relative path: components joined by single '/', no leading/trailing slashes.
+/// Callers hand paths in both shapes (the Unfreezer walks shadow dirs WITH a trailing slash);
+/// namespace strings and prefix matching need the canonical form.
+std::string canonicalDiskPath(const std::string & path)
+{
+    std::string result;
+    std::string component;
+    auto flush = [&]
+    {
+        if (component.empty())
+            return;
+        if (!result.empty())
+            result += '/';
+        result += component;
+        component.clear();
+    };
+    for (char c : path)
+    {
+        if (c == '/')
+            flush();
+        else
+            component.push_back(c);
+    }
+    flush();
+    return result;
+}
+
+/// "<first>/<rest...>" -> {first, rest} ({whole, ""} when there is no '/').
+std::pair<std::string, std::string> splitFirstComponent(const std::string & s)
+{
+    const auto slash = s.find('/');
+    if (slash == std::string::npos)
+        return {s, ""};
+    return {s.substr(0, slash), s.substr(slash + 1)};
+}
+
+void addFirstComponent(std::unordered_set<std::string> & out, const std::string & name)
+{
+    const auto slash = name.find('/');
+    out.emplace(slash == std::string::npos ? name : name.substr(0, slash));
+}
+
+/// Drop a trailing `@cas@` content-addressing boundary marker from a mirrored path segment, so a
+/// table-dir surfaces under its logical (unsuffixed) name in directory listings.
+std::string stripCasArchiveSuffix(std::string s)
+{
+    const auto & suffix = Cas::kCasArchiveSuffix;
+    if (s.size() >= suffix.size() && std::string_view(s).ends_with(suffix))
+        s.resize(s.size() - suffix.size());
+    return s;
+}
+
+std::vector<std::string> toVector(std::unordered_set<std::string> && set)
+{
+    return std::vector<std::string>(std::make_move_iterator(set.begin()), std::make_move_iterator(set.end()));
+}
+
+/// The server uuid string (with dashes) -> the core's UInt128 server id.
+UInt128 serverIdToU128(const std::string & server_id)
+{
+    String hex;
+    hex.reserve(32);
+    for (char c : server_id)
+        if (c != '-')
+            hex += c;
+    if (hex.size() == 32)
+        return Cas::hexToU128(hex);
+    /// Unit-test ids ("srv1") are not uuids — hash them stably.
+    UInt128 r{};
+    for (char c : server_id)
+        r = r * 131 + static_cast<unsigned char>(c);
+    return r == UInt128(0) ? UInt128(1) : r;
+}
+
+}
+
+ContentAddressedMetadataStorage::ContentAddressedMetadataStorage(
+    ObjectStoragePtr object_storage_,
+    String storage_path_prefix_,
+    String server_id_,
+    String disk_name_,
+    ContextPtr context_,
+    const ContentAddressedSettings & settings_)
+    : object_storage(std::move(object_storage_))
+    , storage_path_prefix(std::move(storage_path_prefix_))
+    , storage_path_full(fs::path(object_storage->getRootPrefix()) / storage_path_prefix)
+    , server_id(std::move(server_id_))
+    , server_root_id(settings_[ContentAddressedSetting::server_root_id].value)
+    , disk_name(!disk_name_.empty() ? disk_name_ : storage_path_prefix)
+    , local_scratch_path(settings_[ContentAddressedSetting::scratch_path].value)
+    , context(context_)
+    , gc_enabled(settings_[ContentAddressedSetting::gc_enabled].value)
+    , gc_interval(std::chrono::seconds(settings_[ContentAddressedSetting::gc_interval_sec].value))
+    , deduplication_cache_bytes(settings_[ContentAddressedSetting::deduplication_cache_bytes].value)
+    , deduplication_head_first_min_bytes(settings_[ContentAddressedSetting::deduplication_head_first_min_bytes].value)
+    , gc_snapshot_generations_to_keep(settings_[ContentAddressedSetting::gc_snapshot_generations_to_keep].value)
+    , gc_shards(settings_[ContentAddressedSetting::gc_shards].value)
+    , manifest_sweep_list_budget_keys(settings_[ContentAddressedSetting::manifest_sweep_list_budget_keys].value)
+    , manifest_sweep_delete_budget_keys(settings_[ContentAddressedSetting::manifest_sweep_delete_budget_keys].value)
+    , gc_round_graduation_budget(settings_[ContentAddressedSetting::gc_round_graduation_budget].value)
+    , gc_round_redelete_budget(settings_[ContentAddressedSetting::gc_round_redelete_budget].value)
+    , gc_round_sweep_namespace_budget(settings_[ContentAddressedSetting::gc_round_sweep_namespace_budget].value)
+    , gc_round_sweep_recovery_op_budget(settings_[ContentAddressedSetting::gc_round_sweep_recovery_op_budget].value)
+    , gc_round_ref_cleanup_budget(settings_[ContentAddressedSetting::gc_round_ref_cleanup_budget].value)
+    , gc_round_prefix_wholesale_budget(settings_[ContentAddressedSetting::gc_round_prefix_wholesale_budget].value)
+    , gc_round_handoff_prefix_wholesale_budget(settings_[ContentAddressedSetting::gc_round_handoff_prefix_wholesale_budget].value)
+    , gc_round_outcome_entry_budget(settings_[ContentAddressedSetting::gc_round_outcome_entry_budget].value)
+    , gcs_max_conditional_put_bytes(settings_[ContentAddressedSetting::gcs_max_conditional_put_bytes].value)
+    , cas_part_folder_cache_bytes(settings_[ContentAddressedSetting::part_folder_cache_bytes].value)
+    , cas_part_folder_cache_max_entries(settings_[ContentAddressedSetting::part_folder_cache_max_entries].value)
+    , cas_part_folder_cache_max_entry_bytes(settings_[ContentAddressedSetting::part_folder_cache_max_entry_bytes].value)
+    , manifest_decode_cache_bytes(settings_[ContentAddressedSetting::manifest_decode_cache_bytes].value)
+    , gc_meta_pool_size(settings_[ContentAddressedSetting::gc_meta_pool_size].value)
+    , staging_backend(settings_.stagingBackend())
+    , blob_hash_algo(settings_.blobHashAlgo())
+    , blob_hash_allow_new(settings_[ContentAddressedSetting::blob_hash_allow_new].value)
+    , skip_access_check(settings_[ContentAddressedSetting::skip_access_check].value)
+    , part_folder_validate(settings_.partFolderValidate())
+{
+}
+
+Cas::StagingBackend ContentAddressedMetadataStorage::parseStagingBackend(const std::string & value)
+{
+    if (value == "local")
+        return Cas::StagingBackend::Local;
+    if (value == "s3")
+        return Cas::StagingBackend::S3;
+    throw Exception(ErrorCodes::BAD_ARGUMENTS,
+        "Unknown staging_backend value '{}' (expected 'local' or 's3')", value);
+}
+
+Cas::StagingBackend ContentAddressedMetadataStorage::parseStagingBackend(
+    const Poco::Util::AbstractConfiguration & config, const std::string & config_prefix)
+{
+    return parseStagingBackend(config.getString(config_prefix + ".staging_backend", "local"));
+}
+
+Cas::PartFolderValidate ContentAddressedMetadataStorage::parsePartFolderValidate(const std::string & value)
+{
+    using PartFolderValidate = Cas::PartFolderValidate;
+    if (value == "always")
+        return {PartFolderValidate::Mode::Always, 0};
+    if (value == "never")
+        return {PartFolderValidate::Mode::Never, 0};
+    if (value.starts_with("age "))
+    {
+        /// `std::from_chars` against an UNSIGNED type never accepts a leading '-' (unlike
+        /// `std::stoull`, which silently negates modulo 2^64) -- a malformed/negative/non-digit/empty
+        /// suffix falls through to the terminal throw below instead of wrapping into an astronomical
+        /// age_seconds that behaves as skip-forever.
+        const std::string age_str = value.substr(4);
+        uint64_t age_seconds = 0;
+        const auto [ptr, ec] = std::from_chars(age_str.data(), age_str.data() + age_str.size(), age_seconds);
+        if (ec == std::errc{} && ptr == age_str.data() + age_str.size())
+            return {PartFolderValidate::Mode::Age, age_seconds};
+    }
+    throw Exception(ErrorCodes::BAD_ARGUMENTS,
+        "Unknown part_folder_validate value '{}' (expected 'always', 'never', or 'age <non-negative integer seconds>')", value);
+}
+
+Cas::PartFolderValidate ContentAddressedMetadataStorage::parsePartFolderValidate(
+    const Poco::Util::AbstractConfiguration & config, const std::string & config_prefix)
+{
+    return parsePartFolderValidate(config.getString(config_prefix + ".part_folder_validate", "always"));
+}
+
+ContentAddressedMetadataStorage * ContentAddressedMetadataStorage::tryFromDisk(const DiskPtr & disk)
+{
+    /// The cheap predicate FIRST, never an exception probe: for every non-object-storage disk
+    /// (DiskLocal & co.) `getMetadataStorage` throws NOT_IMPLEMENTED, and merely CONSTRUCTING that
+    /// exception increments `system.errors` even when the throw is caught. This function runs on
+    /// every asynchronous-metrics tick for every configured disk, so the old exception-as-control-
+    /// flow probe polluted `system.errors` with a steady stream of NOT_IMPLEMENTED on pure-local
+    /// servers — caught as a stray-error failure by strict-error tests (`test_cancel_backup`'s
+    /// NoTrashChecker, Altinity PR#2073). `isContentAddressed` is a throw-free virtual (IDisk
+    /// defaults to false; wrappers forward it — see ReadOnlyDiskWrapper).
+    if (!disk || !disk->isContentAddressed())
+        return nullptr;
+    /// A content-addressed disk always implements getMetadataStorage (it IS the CA metadata
+    /// storage), so no NOT_IMPLEMENTED handling is needed past the predicate.
+    return dynamic_cast<ContentAddressedMetadataStorage *>(disk->getMetadataStorage().get());
+}
+
+void ContentAddressedMetadataStorage::runOneGcRoundForTest()
+{
+    /// Admin class (rev.7 spec §1): refuse on a transient / IdentityLost / Vanished pool (a terminal disk
+    /// throws the typed Vanished [D5] refusal; an uncertain one throws 668) before touching the scheduler.
+    /// The later `!cas_store` re-check under `gc_scheduler_mutex` still guards the not-mounted race.
+    checkOpAdmitted(CasOpClass::Admin);
+    /// The pacing scheduler must be STABLE across calls: the lease's observation-window steal
+    /// protocol compares consecutive observations of the SAME observer (gc_id), so an ad-hoc
+    /// scheduler per call would acquire the lease on the first call and then back off forever
+    /// ("incumbent alive" - its own previous incarnation). Recreating the scheduler for every call
+    /// would therefore make every round after the first a silent no-op.
+    /// Hold gc_scheduler_mutex for the whole round: a concurrent `shutdown` waits for the round to
+    /// finish because clean GC completion takes priority over fast shutdown. pointer_mutex (a
+    /// separate, briefly-held mutex) only guards the scheduler snapshot/creation below, so
+    /// gcHealth/store/partAccess never block behind this round.
+    /// Test-only seam (inert in production): lets a test interleave a concurrent FORGET into the window
+    /// between the pre-lock admission check above and the lock acquisition below -- the exact I-1 TOCTOU.
+    if (gc_verb_admit_window_hook_for_test)
+        gc_verb_admit_window_hook_for_test();
+    std::lock_guard round_lock(gc_scheduler_mutex);
+    /// Re-run the admission gate UNDER `gc_scheduler_mutex` (mirroring `gcStart`'s lock-then-gate): the
+    /// pre-lock check above is only a fast-fail. A round admitted while `Live` can block on this mutex behind
+    /// a concurrent FORGET (which holds it for its whole teardown); once FORGET settles the pool `Vanished`,
+    /// this re-check throws the typed [D5] refusal instead of resurrecting a scheduler on a decommissioned
+    /// pool. `checkOpAdmitted` takes only the brief `pointer_mutex` -- lock order gc_scheduler -> pointer.
+    checkOpAdmitted(CasOpClass::Admin);
+    if (shutdown_called)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "Cannot run garbage collection after ContentAddressedMetadataStorage shutdown has begun");
+    std::shared_ptr<Cas::CasGcScheduler> snapshot;
+    {
+        std::lock_guard ptr_lock(pointer_mutex);
+        if (!gc_scheduler)
+        {
+            /// `checkOpAdmitted(Admin)` above already fails loud on a null pool; this is the defensive
+            /// re-check for a concurrent `shutdown` reset between the two `pointer_mutex` acquisitions.
+            /// Surface the same `INVALID_STATE` refusal `poolAccess()` gives, never `LOGICAL_ERROR`
+            /// (which would abort under debug/ASan builds).
+            if (!cas_store)
+                throwStorageNotStarted();
+            gc_scheduler = std::make_shared<Cas::CasGcScheduler>(
+                cas_store, gc_interval, fmt::format("{}::ContentAddressedGC", storage_path_full),
+                disk_name, makeGcRoundLogger());
+        }
+        snapshot = gc_scheduler;
+    }
+    snapshot->runOneRoundNow();
+}
+
+void ContentAddressedMetadataStorage::requestGcRoundSoon()
+{
+    std::shared_ptr<Cas::CasGcScheduler> snapshot;
+    {
+        std::lock_guard lock(pointer_mutex);
+        snapshot = gc_scheduler;
+    }
+    if (snapshot)
+        snapshot->requestRoundSoon();
+}
+
+std::optional<Cas::CasGcScheduler::GcHealth> ContentAddressedMetadataStorage::gcHealth() const
+{
+    /// A brief pointer_mutex snapshot only -- this must NEVER wait behind gc_scheduler_mutex (an
+    /// in-flight round can hold that for a long time; an unprivileged SELECT on
+    /// system.cas_mounts must not stall behind it). The snapshot keeps the scheduler
+    /// alive via its own refcount even if `shutdown` concurrently resets the member, and
+    /// CasGcScheduler::gcHealth() is itself lock-free (atomic reads), so calling it outside any lock
+    /// here is safe.
+    std::shared_ptr<Cas::CasGcScheduler> snapshot;
+    {
+        std::lock_guard lock(pointer_mutex);
+        snapshot = gc_scheduler;
+    }
+    if (!snapshot)
+        return std::nullopt;
+    return snapshot->gcHealth();
+}
+
+CasLifecycleSnapshot ContentAddressedMetadataStorage::lifecycleSnapshot() const
+{
+    /// Factory-class (spec §7): I/O-free and reachable in EVERY state. NEVER calls store()/poolAccess()
+    /// (which refuse a not-mounted disk) or touches the backend -- that is the whole point, so the disk
+    /// that vanished is still visible. Only a brief pointer_mutex snapshot of the pool pointer, plus reads
+    /// of storage members that are immutable after startup.
+    CasLifecycleSnapshot snap;
+    snap.server_root_id = server_root_id;
+    /// The last-known pool identity. `pool_uuid` is written once (single-threaded) at the end of `startup`
+    /// and never reset by `shutdown`, so it is empty ONLY before the first successful startup and stable
+    /// thereafter -- the disk stays introspectable under its identity even once the pool is torn down.
+    snap.pool_id = pool_uuid;
+
+    Cas::PoolPtr pool;
+    {
+        std::lock_guard lock(pointer_mutex);
+        pool = cas_store;
+    }
+    if (!pool)
+    {
+        /// No pool published: the storage-level lifecycle (spec §1's Constructing/ShutDown). Distinguish
+        /// the two by whether startup ever ran, which `pool_uuid` records (empty => never started). A
+        /// disk that was started then torn down (shutdown) reports `shutdown`. reason/detail/since stay
+        /// empty/0 -- no terminal cause.
+        snap.lifecycle = snap.pool_id.empty() ? "constructing" : "shutdown";
+        return snap;
+    }
+
+    const Cas::Pool::LifecycleSnapshot ps = pool->lifecycleSnapshot();
+    snap.lifecycle = casLifecycleToString(ps.lifecycle);
+    snap.reason = casLifecycleReasonWord(ps.lifecycle);
+    snap.detail = ps.detail;
+    snap.since = ps.since;
+    return snap;
+}
+
+Cas::GcRoundLogger ContentAddressedMetadataStorage::makeGcRoundLogger() const
+{
+    /// Unit tests pass a null context (no system logs); the scheduler then runs without a sink.
+    if (!context)
+        return {};
+    auto ctx = context;
+    /// The configured disk name (threaded from the metadata-storage factory); falls back to
+    /// storage_path_prefix for callers that don't supply one (e.g. unit tests).
+    const String disk = disk_name;
+    return [ctx, disk](const Cas::GcRoundLogRecord & r)
+    {
+        auto log = ctx->getContentAddressedGarbageCollectionLog();
+        if (!log)
+            return;
+        ContentAddressedGarbageCollectionLogElement e;
+        const auto now = std::chrono::system_clock::now();
+        e.event_time = std::chrono::system_clock::to_time_t(now);
+        e.event_time_microseconds = timeInMicroseconds(now);
+        switch (r.event_type)
+        {
+            case Cas::GcRoundLogRecord::EventType::Start:
+                e.event_type = ContentAddressedGarbageCollectionLogElement::START;
+                break;
+            case Cas::GcRoundLogRecord::EventType::Finish:
+                e.event_type = ContentAddressedGarbageCollectionLogElement::FINISH;
+                break;
+            case Cas::GcRoundLogRecord::EventType::Phase:
+                e.event_type = ContentAddressedGarbageCollectionLogElement::PHASE;
+                break;
+        }
+        e.disk_name = r.disk_name.empty() ? disk : r.disk_name;
+        e.srid = r.srid;
+        e.gc_id = r.gc_id;
+        e.trigger = r.trigger == Cas::GcRoundLogRecord::Trigger::Manual
+            ? ContentAddressedGarbageCollectionLogElement::MANUAL
+            : ContentAddressedGarbageCollectionLogElement::SCHEDULED;
+        switch (r.outcome)
+        {
+            case Cas::GcRoundLogRecord::Outcome::Unknown:
+                e.outcome = ContentAddressedGarbageCollectionLogElement::UNKNOWN;
+                break;
+            case Cas::GcRoundLogRecord::Outcome::Success:
+                e.outcome = ContentAddressedGarbageCollectionLogElement::SUCCESS;
+                break;
+            case Cas::GcRoundLogRecord::Outcome::NotALeader:
+                e.outcome = ContentAddressedGarbageCollectionLogElement::NOT_A_LEADER;
+                break;
+            case Cas::GcRoundLogRecord::Outcome::Failed:
+                e.outcome = ContentAddressedGarbageCollectionLogElement::FAILED;
+                break;
+            case Cas::GcRoundLogRecord::Outcome::Deferred:
+                e.outcome = ContentAddressedGarbageCollectionLogElement::DEFERRED;
+                break;
+        }
+        e.round = r.round;
+        e.candidates_marked = r.candidates_marked;
+        e.objects_deleted = r.objects_deleted;
+        e.objects_absent = r.objects_absent;
+        e.objects_replaced = r.objects_replaced;
+        e.objects_spared = r.objects_spared;
+        e.manifests_deleted = r.manifests_deleted;
+        e.entries_condemned = r.entries_condemned;
+        e.entries_graduated = r.entries_graduated;
+        e.entries_redeleted = r.entries_redeleted;
+        e.fence_outs = r.fence_outs;
+        e.anomalies = r.anomalies;
+        e.duration_ms = r.duration_ms;
+        e.error = r.error;
+        e.profile_events = r.profile_events;
+        e.round_id = r.round_id;
+        e.phase = r.phase;
+        e.phase_duration_microseconds = r.phase_duration_microseconds;
+        e.phase_metrics = r.phase_metrics;
+        /// Best-effort: SystemLog::add never blocks GC; a full queue drops the row with a warning.
+        log->add(std::move(e));
+    };
+}
+
+Cas::CasEventSink ContentAddressedMetadataStorage::makeCasEventSink() const
+{
+    /// Unit tests pass a null context (no system logs); the Pool then runs without a sink.
+    if (!context)
+        return {};
+    auto ctx = context;
+    /// The configured disk name (threaded from the metadata-storage factory); falls back to
+    /// storage_path_prefix for callers that don't supply one (e.g. unit tests).
+    const String disk = disk_name;
+    return [ctx, disk](Cas::CasEvent ev)
+    {
+        auto log = ctx->getContentAddressedLog();
+        if (!log)
+            return;
+        ContentAddressedLogElement e;
+        const auto now = std::chrono::system_clock::now();
+        e.event_time = std::chrono::system_clock::to_time_t(now);
+        e.event_time_microseconds = timeInMicroseconds(now);
+        e.event_type = toString(ev.type);
+        e.disk_name = disk;
+        e.namespace_ = std::move(ev.namespace_);
+        e.ref_name = std::move(ev.ref_name);
+        e.object_kind = toString(ev.object_kind);
+        e.object_hash = std::move(ev.object_hash);
+        e.token = std::move(ev.token);
+        e.round = ev.round;
+        e.gen = ev.gen;
+        e.at_version = ev.at_version;
+        e.outcome = std::move(ev.outcome);
+        e.reason = std::move(ev.reason);
+        e.thread_id = getThreadId();
+        e.query_id = CurrentThread::getQueryId();
+        e.detail = std::move(ev.detail);
+        /// Best-effort: SystemLog::add never blocks the Core; a full queue drops the row with a warning.
+        log->add(std::move(e));
+    };
+}
+
+Cas::RoundReport ContentAddressedMetadataStorage::runGarbageCollectionRoundNow()
+{
+    checkNotReadOnly("GC round");
+    if (!gc_enabled)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "Garbage collection is not enabled on this content-addressed disk");
+    /// Admin class (rev.7 spec §1): refuse on a transient / IdentityLost / Vanished pool before touching
+    /// the scheduler -- `SYSTEM CAS GC RUN` reaches here directly.
+    checkOpAdmitted(CasOpClass::Admin);
+    /// Mirror runOneGcRoundForTest: a STABLE scheduler instance across calls (the lease's
+    /// observation-window steal protocol compares consecutive observations of the same gc_id).
+    /// Hold gc_scheduler_mutex for the whole round: a concurrent `shutdown` waits for the round to
+    /// finish because clean GC completion takes priority over fast shutdown. pointer_mutex only
+    /// guards the scheduler snapshot/creation below, so gcHealth/store/partAccess never block behind
+    /// this round.
+    /// Test-only seam (inert in production): lets a test interleave a concurrent FORGET into the window
+    /// between the pre-lock admission check above and the lock acquisition below -- the exact I-1 TOCTOU.
+    if (gc_verb_admit_window_hook_for_test)
+        gc_verb_admit_window_hook_for_test();
+    std::lock_guard round_lock(gc_scheduler_mutex);
+    /// Re-run the admission gate UNDER `gc_scheduler_mutex` (mirroring `gcStart`'s lock-then-gate): the
+    /// pre-lock check above is only a fast-fail. A round admitted while `Live` can block on this mutex behind
+    /// a concurrent FORGET (which holds it for its whole teardown); once FORGET settles the pool `Vanished`,
+    /// this re-check throws the typed [D5] refusal instead of resurrecting a scheduler on a decommissioned
+    /// pool. `checkOpAdmitted` takes only the brief `pointer_mutex` -- lock order gc_scheduler -> pointer.
+    checkOpAdmitted(CasOpClass::Admin);
+    if (shutdown_called)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "Cannot run garbage collection after ContentAddressedMetadataStorage shutdown has begun");
+    std::shared_ptr<Cas::CasGcScheduler> snapshot;
+    {
+        std::lock_guard ptr_lock(pointer_mutex);
+        if (!gc_scheduler)
+        {
+            /// Same reasoning as `runOneGcRoundForTest` above: `checkOpAdmitted(Admin)` already failed
+            /// loud on a null pool, so this is the defensive re-check for a concurrent `shutdown` reset
+            /// -- a normal `INVALID_STATE` refusal, never a `LOGICAL_ERROR` abort.
+            if (!cas_store)
+                throwStorageNotStarted();
+            gc_scheduler = std::make_shared<Cas::CasGcScheduler>(
+                cas_store, gc_interval, fmt::format("{}::ContentAddressedGC", storage_path_full),
+                disk_name, makeGcRoundLogger());
+        }
+        snapshot = gc_scheduler;
+    }
+    return snapshot->runOneRoundNow(Cas::GcRoundLogRecord::Trigger::Manual);
+}
+
+Cas::RebuildReport ContentAddressedMetadataStorage::runGcRebuildNow(bool force) const
+{
+    checkNotReadOnly("GC rebuild");
+    if (!gc_enabled)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "Garbage collection is not enabled on this content-addressed disk");
+    /// Admin class (rev.7 spec §1): refuse on a transient / IdentityLost / Vanished pool. (`store()` below
+    /// already throws on a terminal pool, but not on the merely-transient one -- this closes that gap and
+    /// keeps the refusal uniform with the other GC entry points.) Pre-lock: only a fast-fail.
+    checkOpAdmitted(CasOpClass::Admin);
+    /// Serialize the whole rebuild under `gc_scheduler_mutex`, exactly as a synchronous round and FORGET's
+    /// teardown are (both hold this same mutex). Held for the rebuild's DURATION so a concurrent FORGET waits
+    /// it out (fail-closed) instead of reporting the disk decommissioned while the rebuild's one-shot
+    /// `Cas::Gc` is still issuing durable `gc/`-plane writes. `Gc::rebuildBaseline` holds only the
+    /// Pool/backend (no back-reference to this storage), so it never re-takes this mutex -- no deadlock; and
+    /// `store()` takes only the brief `pointer_mutex` (lock order gc_scheduler -> pointer), so
+    /// gcHealth/store/partAccess never block behind this rebuild.
+    std::lock_guard round_lock(gc_scheduler_mutex);
+    /// Re-run the admission gate under the lock (mirroring the round verbs): a rebuild admitted while `Live`
+    /// but blocked here behind a FORGET refuses once the pool is `Vanished` (the later `store()` is also
+    /// fail-closed, so this is a fast-fail before minting the GC identity).
+    checkOpAdmitted(CasOpClass::Admin);
+    /// Test-only seam (inert in production): fires WHILE this rebuild holds `gc_scheduler_mutex` -- the
+    /// in-flight window a concurrent FORGET must serialize behind (I-2). Lets a test hold the lock here and
+    /// observe that FORGET blocks until the rebuild releases it.
+    if (gc_verb_admit_window_hook_for_test)
+        gc_verb_admit_window_hook_for_test();
+    /// A one-shot Gc instance is fine here (unlike the scheduler's stable-instance requirement for
+    /// the lease's observation-window steal protocol): rebuildBaseline does its own lease
+    /// acquire/steal check internally and this command runs exactly one round.
+    const UInt128 gc_id = (static_cast<UInt128>(thread_local_rng()) << 64) | thread_local_rng();
+    const auto cas_store_snapshot = store();
+    Cas::Gc gc(cas_store_snapshot, gc_id, {}, {},
+        getLogger(fmt::format("CasGc({})", cas_store_snapshot->poolConfig().server_root_id)));
+    return gc.rebuildBaseline(force);
+}
+
+ContentAddressedMetadataStorage::PoolView ContentAddressedMetadataStorage::openPoolView() const
+{
+    /// Native mode rides real conditional ops (probed fail-closed by Pool::open); Local object
+    /// storage has none, so the backend emulates exact token semantics in-process (single server).
+    const auto mode = object_storage->getType() == ObjectStorageType::Local
+        ? Cas::ObjectStorageBackend::Mode::EmulatedSingleProcess
+        : Cas::ObjectStorageBackend::Mode::Native;
+    auto backend = std::make_shared<Cas::ObjectStorageBackend>(object_storage, mode, gcs_max_conditional_put_bytes);
+
+    /// EmulatedSingleProcess emulates the conditional-op / exact-token semantics in-process (local
+    /// object storage has none). That emulation is per-process: two servers pointed at the SAME local
+    /// pool (e.g. an NFS/shared mount) each keep independent token state and would silently violate
+    /// the CAS invariants — the capability probe cannot detect this (each process passes it alone).
+    /// Make a shared-pool misconfiguration visible at INFO, not WARNING.
+    /// An inline `disk = disk(... object_storage_type=local ...)` opens the disk on the QUERY thread, so
+    /// a WARNING is forwarded to the client at the functional-test default `send_logs_level=warning` and
+    /// fails EVERY such query (clickhouse-test fails a test on ANY client stderr). At INFO the message
+    /// still lands in the server log for operator visibility but is not forwarded to client queries, so
+    /// the ~15 CA-over-local stateless tests stop failing on a benign single-server note. (A genuinely
+    /// shared local pool is a niche risk that would also surface via CAS/GC corruption; a future
+    /// `system.warnings` entry could restore a louder, test-safe signal.)
+    if (mode == Cas::ObjectStorageBackend::Mode::EmulatedSingleProcess)
+        LOG_INFO(
+            getLogger("ContentAddressedMetadataStorage"),
+            "Content-addressed disk over LOCAL object storage uses emulated in-process conditional "
+            "operations — safe ONLY for a single server. Do NOT share this pool path between multiple "
+            "ClickHouse servers (e.g. a shared/NFS mount): the CAS/GC invariants would break silently. "
+            "Use an S3-backed pool for multi-server / shared deployments.");
+
+    /// Key spaces per mode: the Emulated (Local) backend maps bare pool keys under
+    /// getCommonKeyPrefix (the disk root dir), so the POOL prefix must be bucket-relative - strip
+    /// the common prefix when the configured prefix carries it (the local factory passes the root
+    /// path). Native passes keys through, so the configured prefix is used as-is (for S3 it
+    /// already embeds the endpoint sub-path).
+    String pool_prefix = storage_path_prefix;
+    /// The configured prefix is an endpoint sub-path and usually carries a TRAILING slash
+    /// ("content_addressed_s3/"); Cas::Layout joins components with '/', and a doubled slash in
+    /// keys is backend-hostile (RustFS rejects "p//_probe" LIST prefixes with InvalidArgument -
+    /// Some backends reject such prefixes while others merely tolerate them).
+    while (!pool_prefix.empty() && pool_prefix.back() == '/')
+        pool_prefix.pop_back();
+    String physical_key_prefix_local;
+    if (mode == Cas::ObjectStorageBackend::Mode::EmulatedSingleProcess)
+    {
+        physical_key_prefix_local = object_storage->getCommonKeyPrefix();
+        /// Slash-tolerant strip: the common prefix usually ends with '/', the pool prefix was
+        /// just trimmed of trailing slashes - compare canonical forms.
+        String common_trimmed = physical_key_prefix_local;
+        while (!common_trimmed.empty() && common_trimmed.back() == '/')
+            common_trimmed.pop_back();
+        if (!common_trimmed.empty())
+        {
+            if (pool_prefix == common_trimmed)
+                pool_prefix.clear();
+            else if (pool_prefix.starts_with(common_trimmed + "/"))
+                pool_prefix = pool_prefix.substr(common_trimmed.size() + 1);
+        }
+        if (pool_prefix.empty())
+            pool_prefix = "ca";
+    }
+
+    Cas::PoolConfig pool_config;
+    pool_config.pool_prefix = pool_prefix;
+    pool_config.server_id = serverIdToU128(server_id);
+    pool_config.server_root_id = server_root_id;
+    /// A read-only (`<readonly>`) disk opens with no background watermark and no write probe.
+    pool_config.background_watermark = (context != nullptr) && !read_only;
+    pool_config.read_only = read_only;
+    pool_config.skip_access_check = skip_access_check;
+    /// The node-local write algorithm: `PoolMeta::createOrValidate` accepts it with no write once
+    /// it is a member of the pool's `algos_used`; a not-yet-admitted algo is admitted via
+    /// `blob_hash_allow_new` or refused (BAD_ARGUMENTS, the default).
+    pool_config.blob_hash_algo = blob_hash_algo;
+    pool_config.blob_hash_allow_new = blob_hash_allow_new;
+    pool_config.deduplication_cache_bytes = deduplication_cache_bytes;
+    pool_config.deduplication_head_first_min_bytes = deduplication_head_first_min_bytes;
+    pool_config.manifest_decode_cache_bytes = manifest_decode_cache_bytes;
+    pool_config.gc_snapshot_generations_to_keep = gc_snapshot_generations_to_keep;
+    pool_config.gc_shards = gc_shards;
+    pool_config.manifest_sweep_list_budget_keys = manifest_sweep_list_budget_keys;
+    pool_config.manifest_sweep_delete_budget_keys = manifest_sweep_delete_budget_keys;
+    pool_config.gc_round_graduation_budget = gc_round_graduation_budget;
+    pool_config.gc_round_redelete_budget = gc_round_redelete_budget;
+    pool_config.gc_round_sweep_namespace_budget = gc_round_sweep_namespace_budget;
+    pool_config.gc_round_sweep_recovery_op_budget = gc_round_sweep_recovery_op_budget;
+    pool_config.gc_round_ref_cleanup_budget = gc_round_ref_cleanup_budget;
+    pool_config.gc_round_prefix_wholesale_budget = gc_round_prefix_wholesale_budget;
+    pool_config.gc_round_handoff_prefix_wholesale_budget = gc_round_handoff_prefix_wholesale_budget;
+    pool_config.gc_round_outcome_entry_budget = gc_round_outcome_entry_budget;
+    pool_config.gc_meta_pool_size = gc_meta_pool_size;
+    pool_config.event_sink = makeCasEventSink();
+
+    PoolView view;
+    view.physical_key_prefix = physical_key_prefix_local;
+    view.pool_prefix = pool_prefix;
+    view.pool = Cas::Pool::open(std::move(backend), std::move(pool_config));
+    return view;
+}
+
+void ContentAddressedMetadataStorage::startup()
+{
+    if (cas_store)
+        return;
+
+    /// Observe-only mode (the disk's <readonly> config): skip the probe (a probe write would fail on
+    /// a read-only backend), run no watermark, start no GC, and fail the mutating surface closed.
+    read_only = object_storage->isReadOnly();
+
+    /// Everything below builds into LOCALS -- nothing is published to `cas_store`/`part_access`/
+    /// `gc_scheduler`/`pool_uuid`/`conditional_copy_supported` until the single publish step at the
+    /// very end. This makes a mid-startup throw leave the object exactly as unstarted as it was on
+    /// entry (the `if (cas_store) return;` head above still sees an empty pool), so a caller can
+    /// retry `startup` after a transient failure instead of being stuck with a half-built mount.
+    PoolView view = openPoolView();
+    physical_key_prefix = view.physical_key_prefix;
+    auto pool = std::move(view.pool);
+    auto uuid = Cas::u128ToHex(pool->poolMeta().pool_id);
+    auto facade = std::make_shared<Cas::CachedPartFolderAccess>(pool,
+        Cas::CachedPartFolderAccess::CacheParams{
+            .cache_bytes = cas_part_folder_cache_bytes,
+            .max_entries = cas_part_folder_cache_max_entries,
+            .max_entry_bytes = cas_part_folder_cache_max_entry_bytes,
+            .validate = part_folder_validate});
+
+    /// The optional mount-time capability probe for a write-once conditional server-side copy.
+    /// Only relevant when this disk opted in to `staging_backend=s3`; `Local` (the default,
+    /// global constraint: OFF BY DEFAULT) takes NO probe here — `conditional_copy_supported` simply
+    /// stays at its `false` default and is never consulted on the local path. Skipped in
+    /// observe-only/readonly mode: a probe write would fail on a read-only backend, exactly like the
+    /// mandatory battery (`runCapabilityProbe`) above skips a read-only mount.
+    ///
+    /// Fail-close, never fail-open: an unsupported or non-enforcing backend just falls back to local
+    /// staging (`conditional_copy_supported` stays `false`) — this is NOT a mount failure, unlike the
+    /// mandatory battery, because `local` staging remains fully functional.
+    bool copy_supported = false;
+    if (staging_backend == Cas::StagingBackend::S3 && !read_only)
+    {
+        const String probe_prefix = physicalKey(view.pool_prefix + "/staging/" + server_root_id + "/probe");
+        copy_supported = Cas::probeConditionalCopy(*object_storage, probe_prefix);
+        if (!copy_supported)
+            LOG_INFO(
+                getLogger("ContentAddressedMetadataStorage"),
+                "staging_backend=s3 requested but the object storage does not enforce conditional "
+                "copy; falling back to local staging");
+
+        /// Reclaim this mount's own leaked `staging/<server_root_id>/` debris (a promote whose staging-delete never
+        /// ran, or an aborted transaction's never-promoted staging object — see
+        /// `cleanupPendingTempFiles`) at mount start. Only runs when the S3 path is actually usable
+        /// (`conditional_copy_supported`) — an unsupported/fail-closed-to-local mount never wrote any
+        /// S3 staging objects under this prefix in the first place. LEASE-FENCE: the prefix below is
+        /// keyed by THIS mount's own `server_root_id` (the SAME prefix construction the probe above
+        /// and every staging key this mount ever mints use -- and the same formula `stagingKeyPrefix()`
+        /// uses post-startup; computed from `view.pool_prefix` here rather than via
+        /// `stagingKeyPrefix()` itself, because that helper calls `store()`, which is fail-closed and
+        /// would throw before the pool is published), so this sweep can never reach a different
+        /// mount's in-flight staging (`Cas::sweepOwnMountStaging`'s own doc comment). GC excludes
+        /// `staging/` entirely (a distinct top-level prefix from `blobs/` — see `CasLayout.h`), so this
+        /// sweeper is the ONLY reclaimer of `staging/` debris.
+        if (copy_supported)
+            Cas::sweepOwnMountStaging(*object_storage, physicalKey(view.pool_prefix + "/staging/" + server_root_id) + "/");
+    }
+
+    /// The background GC scheduler runs only on the disk-factory path (context non-null) and when
+    /// enabled - the lease makes concurrent schedulers across mounters safe (work dedup), so no
+    /// further gating is needed because the scheduler's lease coordinates concurrent mounters.
+    /// `CasGcScheduler` holds its own `PoolPtr` (see its `store` member), so starting it against the
+    /// LOCAL `pool` before publish is safe -- the scheduler keeps the pool alive on its own. It is
+    /// also safe on the unwind path below: `scheduler` here is a local `shared_ptr`, so if something
+    /// after this point throws (only the fault-injection hook can, in production nothing does),
+    /// its destructor runs during stack unwinding, which drops the last reference and destroys the
+    /// `CasGcScheduler`; its destructor calls `stop()`, which joins both worker threads before the
+    /// exception continues propagating. No explicit `SCOPE_EXIT` is needed for that.
+    std::shared_ptr<Cas::CasGcScheduler> scheduler;
+    if (context && gc_enabled && !read_only)
+    {
+        scheduler = std::make_shared<Cas::CasGcScheduler>(
+            pool, gc_interval, fmt::format("{}::ContentAddressedGC", storage_path_full),
+            disk_name, makeGcRoundLogger());
+        scheduler->start();
+    }
+
+    /// Test-only: lets a test prove that a failure here (after everything above has succeeded, but
+    /// before publish) leaves nothing published and a retry can still succeed. A no-op in production.
+    if (startup_fault_injection_for_test)
+        startup_fault_injection_for_test();
+
+    /// The single publish step: as the LAST action of `startup`, atomically hand the fully-built
+    /// pool, part-folder facade, and GC scheduler to the members other threads observe through
+    /// `store`/`partAccess`/the GC entry points, in ONE `pointer_mutex` acquisition -- so no caller of
+    /// `poolAccess()` can ever observe a half-published mount. Everything above only ever touched locals,
+    /// so any throw before this point (including from the fault-injection hook above) leaves those
+    /// members (a null `cas_store`) exactly as they were on entry.
+    {
+        std::lock_guard lock(pointer_mutex);
+        cas_store = std::move(pool);
+        part_access = std::move(facade);
+        gc_scheduler = std::move(scheduler);
+    }
+    pool_uuid = std::move(uuid);
+    conditional_copy_supported = copy_supported;
+}
+
+void ContentAddressedMetadataStorage::shutdown()
+{
+    /// Wait for any in-flight synchronous round to finish cleanly first (gc_scheduler_mutex is held
+    /// for a round's whole duration) -- unchanged priority: clean GC completion over fast shutdown.
+    std::lock_guard round_lock(gc_scheduler_mutex);
+    shutdown_called = true;
+    std::shared_ptr<Cas::CasGcScheduler> old_scheduler;
+    {
+        std::lock_guard ptr_lock(pointer_mutex);
+        old_scheduler = std::move(gc_scheduler);
+        gc_scheduler.reset();
+        part_access.reset();
+        /// Terminal server-shutdown semantics: a one-way trip (no server-lifecycle "remount" after
+        /// shutdown). Nulling `cas_store` puts the storage back into the null-pool (ShutDown) lifecycle,
+        /// so `poolAccess()`/the gate report the same operational refusal post-shutdown as pre-startup.
+        cas_store.reset();
+    }
+    /// `stop` joins the background threads. Runs outside pointer_mutex (no reset left to race:
+    /// gc_scheduler is already null) but still inside round_lock, so a NEW round can't start here.
+    /// old_scheduler keeps the object alive regardless.
+    if (old_scheduler)
+        old_scheduler->stop();
+}
+
+namespace
+{
+/// A human-readable UTC decommission stamp for the FORGET [D5] message (an operator asserted this, so the
+/// message must be traceable to the audit log by wall time). Format: "YYYY-MM-DD HH:MM:SS UTC".
+String utcStampNow()
+{
+    const std::time_t now = std::time(nullptr);
+    std::tm tm_utc{};
+    gmtime_r(&now, &tm_utc);
+    char buf[32];
+    const size_t n = std::strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S UTC", &tm_utc);
+    return String(buf, n);
+}
+}
+
+void ContentAddressedMetadataStorage::forgetDisk()
+{
+    /// SYSTEM CAS FORGET (spec §5): the operator force-Vanish. A lifecycle verb, NOT a
+    /// store()-class op — it must work on a NOT-live disk (a stuck transient / IdentityLost pool), so it
+    /// reaches the pool DIRECTLY, never through `poolAccess()`/`checkOpAdmitted` (which refuse a not-live
+    /// disk). Serialized against FSCK / GC STOP / GC START by `lifecycle_mutex`, and against a concurrent
+    /// synchronous GC round by `gc_scheduler_mutex` (a round holds the latter, so this waits it out).
+    std::lock_guard lifecycle(lifecycle_mutex);
+    std::lock_guard round_lock(gc_scheduler_mutex);
+
+    Cas::PoolPtr pool;
+    std::shared_ptr<Cas::CasGcScheduler> scheduler;
+    {
+        std::lock_guard lock(pointer_mutex);
+        pool = cas_store;
+        /// Detach the scheduler from the member under `pointer_mutex` (as shutdown/unmount do): no new
+        /// synchronous round can adopt it, and `gcHealth` reports "no GC" for a forgotten disk immediately.
+        /// The actual stop()+join runs below, inside the pool's protocol
+        /// (OUTSIDE `pointer_mutex`, since it joins threads).
+        scheduler = std::move(gc_scheduler);
+    }
+
+    if (!pool)
+    {
+        /// No published pool to forget (never started / shut down). The disk is already not serving; a
+        /// restart re-registers the name. Idempotent no-op. (The detached `scheduler` is null here too —
+        /// `cas_store`/`gc_scheduler` are published and cleared together.)
+        LOG_WARNING(getLogger("ContentAddressedMetadataStorage"),
+            "SYSTEM CAS FORGET on content-addressed disk '{}': no published pool — nothing "
+            "to decommission (a restart re-registers the name).", disk_name);
+        return;
+    }
+
+    /// The [D5] forgotten message, carrying the actual decommission timestamp (an operator ASSERTION, not
+    /// an erasure proof — the wording says so). `Pool::throwIfLifecycleTerminal` surfaces it verbatim to
+    /// every store-class caller after the transition, and the WARN at the transition logs it too.
+    const String reason = fmt::format(
+        "decommissioned by SYSTEM CAS FORGET at {} — erasure was NOT verified; if this was a "
+        "mistake the data may be intact (restart re-registers the name)", utcStampNow());
+
+    /// Run the fence-first protocol on the pool. The GC-stop callback stops+joins the (detached) scheduler
+    /// at spec §5 step 3/4; the scheduler is destroyed when the local `scheduler` leaves this scope.
+    pool->forgetDisk([&scheduler] { if (scheduler) scheduler->stop(); }, reason);
+}
+
+void ContentAddressedMetadataStorage::gcStop()
+{
+    /// SYSTEM CAS GC STOP (spec §6): stop ONLY the background GC scheduler. STOP-IN-PLACE --
+    /// the scheduler object is RETAINED in the member (contrast `forgetDisk`/`shutdown`, which `std::move`
+    /// it out and destroy it): a later `gcStart` must re-enter the SAME instance so its `gc_id` + lease
+    /// observation history survive. Keeping it in the member also keeps `gcHealth` reading the (stopped)
+    /// state truthfully, rather than "no GC".
+    /// A lifecycle-control verb: serialized against FSCK / forget / GC START by `lifecycle_mutex`, and
+    /// against a concurrent synchronous GC round by `gc_scheduler_mutex` (a round holds the latter, so this
+    /// waits it out). It does NOT consult `checkOpAdmitted` -- stopping GC works on ANY disk state, including
+    /// a not-live/Vanished one (stopping the reclaimer on a sick disk is a legitimate operator action).
+    std::lock_guard lifecycle(lifecycle_mutex);
+    std::lock_guard round_lock(gc_scheduler_mutex);
+
+    /// Snapshot the scheduler under `pointer_mutex` by COPY (never `std::move`): leave it in the member.
+    std::shared_ptr<Cas::CasGcScheduler> snapshot;
+    {
+        std::lock_guard lock(pointer_mutex);
+        snapshot = gc_scheduler;
+    }
+    if (!snapshot)
+    {
+        /// No scheduler at all (GC disabled / read-only / not started / already forgotten). Stopping GC on a
+        /// disk that runs none is a no-op success -- the operator's intent ("no GC background activity")
+        /// already holds.
+        LOG_INFO(getLogger("ContentAddressedMetadataStorage"),
+            "SYSTEM CAS GC STOP on content-addressed disk '{}': no GC scheduler "
+            "(disabled/read-only/not started) -- nothing to stop.", disk_name);
+        return;
+    }
+    /// `stop()` joins the worker + heartbeat threads and clears the in-process leadership hint. Runs OUTSIDE
+    /// `pointer_mutex` (it joins threads). Idempotent: a second STOP finds an already-stopped scheduler and
+    /// `stop()` is a safe no-op.
+    snapshot->stop();
+}
+
+void ContentAddressedMetadataStorage::gcStart()
+{
+    /// SYSTEM CAS GC START (spec §6): restart the background GC scheduler stopped by `gcStop`.
+    /// Serialized like `gcStop`. Unlike it, START refuses on a decommissioned/uncertain pool: restarting GC
+    /// there would only spin failing rounds, so it goes through the uniform GC gate.
+    std::lock_guard lifecycle(lifecycle_mutex);
+    std::lock_guard round_lock(gc_scheduler_mutex);
+
+    checkNotReadOnly("GC start");
+    if (!gc_enabled)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "Garbage collection is not enabled on this content-addressed disk");
+    /// Admin class (rev.7 spec §1): refuse on a transient / `IdentityLost` / `Vanished` pool (typed 668 /
+    /// [D5]) and on a null pool (`throwStorageNotStarted`). Only a `Live` pool proceeds -- the same uniform
+    /// gate every GC entry point uses (`runGarbageCollectionRoundNow` / `runGcRebuildNow`).
+    checkOpAdmitted(CasOpClass::Admin);
+    if (shutdown_called)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "Cannot start garbage collection after ContentAddressedMetadataStorage shutdown has begun");
+
+    std::shared_ptr<Cas::CasGcScheduler> snapshot;
+    {
+        std::lock_guard ptr_lock(pointer_mutex);
+        if (!gc_scheduler)
+        {
+            /// `Live` + `gc_enabled` + not read-only but no scheduler: reachable only when the disk was
+            /// started in a context that started none (e.g. a unit-test null context) or after a GC RUN that
+            /// created a lazy one was never started. Create a STABLE instance now, mirroring the GC RUN entry
+            /// points, so START is meaningful. (`checkOpAdmitted` above already proved `cas_store` is live.)
+            if (!cas_store)
+                throwStorageNotStarted();
+            gc_scheduler = std::make_shared<Cas::CasGcScheduler>(
+                cas_store, gc_interval, fmt::format("{}::ContentAddressedGC", storage_path_full),
+                disk_name, makeGcRoundLogger());
+        }
+        snapshot = gc_scheduler;
+    }
+    /// `start()` is a no-op if already running (idempotent) and re-enters the SAME instance after a stop --
+    /// the persistent `gc` observer + `gc_id` are preserved, and leadership is re-acquired only by the next
+    /// round's normal `gc/state` acquisition, never restored here. Runs outside `pointer_mutex` for symmetry
+    /// with `stop()` (it spawns threads but joins nothing, so it does not block).
+    snapshot->start();
+}
+
+Cas::FsckReport ContentAddressedMetadataStorage::runFsckNow(bool detail) const
+{
+    /// Outermost lock (see its own doc comment): held for the WHOLE scan, so a concurrent lifecycle-control
+    /// verb (FORGET / GC STOP / GC START) cannot race the disk out from under an in-flight FSCK.
+    std::lock_guard lifecycle(lifecycle_mutex);
+
+    /// FSCK scans the LIVE running pool directly (rev.8). Admin class -- refuse on a transient /
+    /// IdentityLost / Vanished / null pool before touching it, exactly like the GC entry points
+    /// (`runGarbageCollectionRoundNow`/`runGcRebuildNow`). The scan is read-only and its findings are
+    /// revalidated against a fresh authoritative read (`CasFsck`'s Dangling / missing-manifest rechecks),
+    /// so concurrent writers never yield a phantom finding.
+    checkOpAdmitted(CasOpClass::Admin);
+    return Cas::runFsck(*store(), detail);
+}
+
+ContentAddressedMetadataStorage::PoolAccessSnapshot ContentAddressedMetadataStorage::poolAccess() const
+{
+    PoolAccessSnapshot snap;
+    {
+        std::lock_guard lock(pointer_mutex);
+        snap.pool = cas_store;
+        snap.part_access = part_access;
+    }
+    /// A null pool covers before-first-startup and after-shutdown uniformly -- the storage-level
+    /// Constructing/ShutDown lifecycle. Fail loud (spec §1's null-pool fail-loud); there is no benign
+    /// answer for a storage that has not published a pool.
+    if (!snap.pool)
+        throwStorageNotStarted();
+    /// rev.7 §1: a published pool that has entered a terminal lifecycle condition (`IdentityLost` or any
+    /// `Vanished`) must ALSO refuse store()-class access, so nothing silently proceeds against an erased
+    /// or replaced data root. This is the store()-class terminal check; the full six-class operation gate
+    /// (which also gates the transient state and answers truth-absent on removes/enumeration) is
+    /// `checkOpAdmitted`.
+    snap.pool->throwIfLifecycleTerminal();
+    return snap;
+}
+
+void ContentAddressedMetadataStorage::throwStorageNotStarted() const
+{
+    /// No pool is published: the storage-level lifecycle is Constructing (before `startup`) or ShutDown
+    /// (after `shutdown`). `pool_uuid` is empty ONLY before the first successful startup (written once at
+    /// its end, never reset by `shutdown`), so it distinguishes the two, exactly as `lifecycleSnapshot()`
+    /// reports `constructing`/`shutdown`. Immutable-after-startup, so read without `pointer_mutex`.
+    const char * phase = pool_uuid.empty() ? "constructing" : "shutdown";
+    throw Exception(ErrorCodes::INVALID_STATE,
+        "content-addressed disk '{}' is not started (storage lifecycle: {})", disk_name, phase);
+}
+
+Cas::PoolPtr ContentAddressedMetadataStorage::store() const
+{
+    return poolAccess().pool;
+}
+
+std::shared_ptr<Cas::CachedPartFolderAccess> ContentAddressedMetadataStorage::partAccess() const
+{
+    return poolAccess().part_access;
+}
+
+void ContentAddressedMetadataStorage::checkNotReadOnly(std::string_view what) const
+{
+    if (read_only)
+        throw Exception(ErrorCodes::READONLY,
+            "Content-addressed disk is opened read-only: {} is rejected", what);
+}
+
+CasOpAdmission ContentAddressedMetadataStorage::checkOpAdmitted(CasOpClass op) const
+{
+    /// `Factory` is never routed here (its call sites are I/O-free and work in every state); a Factory
+    /// arg is a call-site bug. This unreachable path is genuinely unreachable, so the LOGICAL_ERROR never
+    /// constructs (never aborts a debug/ASan build).
+    if (op == CasOpClass::Factory)
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "checkOpAdmitted must not be called for the Factory class");
+
+    Cas::PoolPtr pool;
+    {
+        std::lock_guard lock(pointer_mutex);
+        pool = cas_store;
+    }
+
+    /// A null pool = the storage-level lifecycle is Constructing (before `startup`) or ShutDown (after
+    /// `shutdown`): fail loud for EVERY class, `Probe` included. There is no benign "absent" answer for a
+    /// storage that has never published a pool (or torn one down) -- only a genuinely `Vanished` POOL
+    /// (below) answers truth-absent. This is the spec §1 null-pool fail-loud contract.
+    if (!pool)
+        throwStorageNotStarted();
+
+    /// The rev.7 six-class gate keyed on the pool lifecycle condition (spec §1).
+    const Cas::PoolLifecycle lc = pool->lifecycle();
+    if (lc == Cas::PoolLifecycle::Live)
+        return CasOpAdmission::Proceed;
+
+    if (lc == Cas::PoolLifecycle::TransientNotLive)
+        /// A lease blip: uncertain but AUTO-RECOVERING. No class but Factory proceeds, and the refusal is
+        /// minted TRANSIENT -- this is the READ plane, where a consumer that cannot tell unavailability
+        /// from damage acts destructively: `ReplicatedMergeTreePartCheckThread` detaches a part whose read
+        /// throws an error its retryable-classifier does not list. `IdentityLost` gets its own richer,
+        /// TERMINAL 668 below -- it does not auto-recover, so both "temporarily unreachable" and the
+        /// retryable class would misdiagnose it. The wait-and-retry guidance is actionable for every caller
+        /// here, and specifically for `SYSTEM CAS GC START` run mid-recovery by an operator
+        /// who STOPped GC pre-maintenance: this is a wait, not a dead end.
+        Cas::throwCasTransientUnavailable(
+            fmt::format("content-addressed disk '{}'", disk_name),
+            "mount lease not held; backing may be temporarily unreachable; the operation is admitted "
+            "again once the disk recovers to Live");
+
+    /// `IdentityLost` and the terminal `Vanished*` states carry the typed per-reason [D5] message the pool
+    /// owns (single source). A SETTLED `Vanished` pool answers `Probe`/`Remove` truthfully without touching
+    /// it; `IdentityLost` (sentinels absent, no auto-recovery) has NO benign answer -- every class
+    /// fails loud with the "recover by restart or FORGET; a matching-sentinel restore does not auto-revive"
+    /// diagnosis. So the truth-absent short-circuit is gated on the Vanished states specifically.
+    const bool settled_vanished = lc == Cas::PoolLifecycle::VanishedReplaced
+        || lc == Cas::PoolLifecycle::VanishedForgotten;
+    if (settled_vanished && (op == CasOpClass::Probe || op == CasOpClass::Remove))
+        return CasOpAdmission::TruthAbsent;
+    pool->throwIfLifecycleTerminal();
+    throw Exception(ErrorCodes::LOGICAL_ERROR,
+        "checkOpAdmitted: unreachable -- non-Live pool did not throw for content-addressed disk '{}'", disk_name);
+}
+
+void ContentAddressedMetadataStorage::confirmPoolIdentityForEmptyEnumeration(const std::string & path) const
+{
+    /// EMPTY-PROOF RULE (rev.7 spec §1 [B3]). Reached ONLY when `listDirectory` computed an EMPTY listing
+    /// at a `TableDir`/`DetachedContainer` root on a NON-terminal pool -- `checkOpAdmitted` already ran
+    /// (admitted as `Live`, and NOT a settled `Vanished` state, which would have short-circuited `Probe` to
+    /// `TruthAbsent` before any classification). This is the last silent-empty-load killer: an empty table
+    /// root is exactly what a silently-erased backing looks like, and a read-only pool (no lease, no
+    /// erasure observer) has no other line of defense. So the empty answer is authorized ONLY by an
+    /// AUTHORITATIVE, UNCACHED positive on the pool identity object -- a cached positive never suffices.
+    const Cas::PoolPtr pool = store();   /// Live here (past the op gate's Live, non-terminal admission).
+
+    ++empty_proof_probe_count_for_test;
+    const Cas::SentinelProbeResult probe = empty_proof_probe_override_for_test
+        ? empty_proof_probe_override_for_test()
+        : Cas::probeSentinel(pool->backend(), pool->layout().poolMetaKey());
+
+    switch (probe.outcome)
+    {
+        case Cas::ProbeOutcome::Present:
+            /// The pool identity is authoritatively present -- the empty listing is the truth.
+            return;
+        case Cas::ProbeOutcome::KeyAbsent:
+        case Cas::ProbeOutcome::ContainerAbsent:
+            /// A clean authoritative miss on `_pool_meta`: the backing is (or is being) erased. Refuse
+            /// the empty answer rather than silently attaching an empty table over an erased pool.
+            throw Exception(ErrorCodes::INVALID_STATE,
+                "content-addressed disk '{}' -- pool identity object absent while enumerating '{}' -- "
+                "refusing the empty answer; the backing may be erased",
+                disk_name, path);
+        case Cas::ProbeOutcome::AccessDenied:
+        case Cas::ProbeOutcome::Indeterminate:
+            /// Absence was NEVER established (a transport/permission fault). Fail closed and TRANSIENT --
+            /// never promote an unproven probe into an empty answer, and never let a consumer read an
+            /// unreachable pool identity as damage. The arm above, where absence IS proven, keeps its
+            /// terminal 668: an erased backing does not heal by retrying. This arm promises no particular
+            /// recovery either: `AccessDenied` is a credential/policy fault that a return to `Live` does
+            /// not clear, so "retry" is the only honest guidance for the pair.
+            Cas::throwCasTransientUnavailable(
+                fmt::format("content-addressed disk '{}'", disk_name),
+                fmt::format("pool identity object could not be confirmed while enumerating '{}' "
+                            "(transport or permission fault) -- refusing the empty answer; retry", path));
+    }
+}
+
+MetadataTransactionPtr ContentAddressedMetadataStorage::createTransaction()
+{
+    checkNotReadOnly("writes");
+    return std::make_shared<ContentAddressedTransaction>(*this);
+}
+
+String ContentAddressedMetadataStorage::stagingKeyPrefix() const
+{
+    /// Mirrors the probe's own prefix construction (`startup`'s `probe_prefix` above), minus
+    /// the probe's own `/probe` leaf — this is the writer-owned sibling subtree of the SAME
+    /// `staging/<server_root_id>/` area. `store()` throws INVALID_STATE when no pool is published
+    /// (pre-`startup`/post-`shutdown`); every caller (writeFile, via a transaction) runs post-startup.
+    return physicalKey(store()->poolConfig().pool_prefix + "/staging/" + server_root_id);
+}
+
+/// ==== namespace mapping ====
+
+std::string ContentAddressedMetadataStorage::serverPrefix() const
+{
+    /// Live namespaces and mirrored live-tree files are rooted by the configured
+    /// `server_root_id`, not by the ClickHouse ServerUUID-derived token. `ServerUUID` is only the
+    /// mount owner token; `server_root_id` is the persistent layout identity.
+    return server_root_id;
+}
+
+std::vector<std::string> ContentAddressedMetadataStorage::listLiveTreeChildren(const std::string & path) const
+{
+    /// Probe: a Vanished disk enumerates empty (truth). Its callers (`listDirectory`) already gate, but
+    /// this public helper is gated too so a direct call is truthful.
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return {};
+    const std::string canonical = canonicalDiskPath(path);
+    const std::string scope = serverPrefix() + "/" + (canonical.empty() ? "" : canonical + "/");
+    std::unordered_set<std::string> result;
+    for (const auto & child : store()->listMirroredChildren(scope))
+        result.emplace(stripCasArchiveSuffix(child));
+    return toVector(std::move(result));
+}
+
+bool ContentAddressedMetadataStorage::liveTreeDirHasChildren(const std::string & path) const
+{
+    /// Probe gate FIRST, ahead of the hardcoded disk-root short-circuit below (the rev.7 offender): on a
+    /// Vanished disk even the disk root reads absent (truth), never the unconditional `true`.
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return false;
+    const std::string canonical = canonicalDiskPath(path);
+    /// The disk root always exists; otherwise a non-empty server-root-scoped mirrored LIST is the signal.
+    if (canonical.empty())
+        return true;
+    const std::string scope = serverPrefix() + "/" + canonical + "/";
+    return !store()->listMirroredChildren(scope).empty();
+}
+
+Cas::RootNamespace ContentAddressedMetadataStorage::liveNamespace(const std::string & table_uuid) const
+{
+    /// Path mirroring: the namespace is the table's canonical disk path with the
+    /// content-addressed boundary marked by `@cas@` on the table-dir segment, prefixed by the
+    /// configured `server_root_id`. e.g. `<server_root_id>/store/3f2/3f2a…@cas@`.
+    return Cas::RootNamespace{serverPrefix() + "/" + Cas::mirroredArchiveNamespace(table_uuid)};
+}
+
+std::optional<Cas::NamespaceLifeId>
+ContentAddressedMetadataStorage::readableNamespaceFilesLife(const Cas::RootNamespace & ns) const
+{
+    return store()->namespaceFilesLifeIfReadable(ns);
+}
+
+Cas::RootNamespace ContentAddressedMetadataStorage::shadowNamespace(const std::string & shadow_table_dir)
+{
+    /// The LITERAL shadow table dir (shadow/<backup>/store/<u3>/<uuid> or .../data/<db>/<tbl>):
+    /// bijective with the disk path for both layouts, pool-global (backups are read by any
+    /// replica), and the shadow tree enumerates from Pool::listNamespaces("shadow/").
+    /// Canonicalize because the unfreezer can hand the directory a trailing slash.
+    return Cas::RootNamespace{canonicalDiskPath(shadow_table_dir)};
+}
+
+
+std::optional<ContentAddressedMetadataStorage::Route>
+ContentAddressedMetadataStorage::route(const Cas::PartFilePath & p) const
+{
+    Route r;
+    if (!p.backup_name.empty())
+    {
+        r.ns = shadowNamespace(p.shadow_table_dir);
+        r.ref = p.part_name;
+        r.file = p.file;
+        return r;
+    }
+    if (p.part_name == Cas::kDetachedDirName)
+    {
+        /// The parser reports detached paths with part_name == "detached" and the real detached
+        /// part dir as the first component of `file`. Detached parts share the table namespace and
+        /// INTO the table's OWN archive namespace: each detached part is a ref keyed by
+        /// `detached/<part>` (vs a live `<part>`), so the re-split here keeps the table namespace
+        /// and prepends the `detached/` ref prefix. An empty `p.file` (the bare `<table>/detached`
+        /// container dir) yields an empty ref → the filtered-container listing path.
+        r.ns = liveNamespace(p.table_uuid);
+        auto [part, file] = splitFirstComponent(p.file);
+        r.ref = part.empty() ? "" : std::string(Cas::kDetachedRefPrefix) + part;
+        r.file = file;
+        return r;
+    }
+    if (p.part_name == Cas::kMovingDirName)
+    {
+        /// L1 (MOVE-to-CA fix): re-split exactly like detached, folding onto a `moving/`-PREFIXED
+        /// ref (kMovingRefPrefix) -- NOT the part's final ref directly. Publishing the clone under
+        /// the final ref before the mover's swap would break move crash-atomicity: a crash between
+        /// the clone publication and swapClonedPart would leave a committed LIVE ref that never went
+        /// through the swap, and moving/'s own startup cleanup couldn't distinguish that premature
+        /// ref from a real live part. The staging ref keeps the pre-swap clone un-live; the mover's
+        /// rename does a real ref repoint moving/<part> -> <part> (the same committed-ref-repoint
+        /// path merge-result/delete_tmp renames already use). An empty p.file (the bare
+        /// <table>/moving container dir) yields an empty ref, same convention as detached.
+        r.ns = liveNamespace(p.table_uuid);
+        auto [part, file] = splitFirstComponent(p.file);
+        r.ref = part.empty() ? "" : std::string(Cas::kMovingRefPrefix) + part;
+        r.file = file;
+        return r;
+    }
+    r.ns = liveNamespace(p.table_uuid);
+    r.ref = p.part_name;
+    r.file = p.file;
+    return r;
+}
+
+std::vector<std::string> ContentAddressedMetadataStorage::detachedRefNames(const Cas::RootNamespace & ns) const
+{
+    std::vector<std::string> refs;
+    for (const auto & [ref, _] : store()->listRefs(ns))
+        if (ref.starts_with(Cas::kDetachedRefPrefix))
+            refs.push_back(ref);
+    return refs;
+}
+
+std::vector<std::string> ContentAddressedMetadataStorage::movingRefNames(const Cas::RootNamespace & ns) const
+{
+    std::vector<std::string> refs;
+    for (const auto & [ref, _] : store()->listRefs(ns))
+        if (ref.starts_with(Cas::kMovingRefPrefix))
+            refs.push_back(ref);
+    return refs;
+}
+
+/// ==== read surface ====
+
+bool ContentAddressedMetadataStorage::existsFile(const std::string & path) const
+{
+    /// Probe gate (rev.7 §1): real while live, throws while uncertain (incl. a null/unstarted pool),
+    /// truthfully absent once Vanished.
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return false;
+    if (!Cas::isPartFilePath(path))
+    {
+        if (auto tf = Cas::parseTableFilePath(path))
+        {
+            const auto life = readableNamespaceFilesLife(liveNamespace(tf->table_uuid));
+            return life && store()->getNamespaceFile(*life, tf->tail).has_value();
+        }
+        /// A loose mountpoint object is a plain object at roots/<server_root_id>/<path>.
+        /// Use a HEAD-based existence check (directory-safe), NOT a body read: the traversal in
+        /// system.remote_data_paths probes existsFile on directory-shaped pool paths (e.g. `store`), and a
+        /// body read (getMountpointObject) throws "Is a directory". A directory is not a file.
+        return store()->mountpointObjectExists(serverPrefix() + "/" + path);
+    }
+
+    auto p = Cas::parsePartFilePath(path);
+    if (!p || p->file.empty())
+        return false;
+    auto r = route(*p);
+    if (!r || r->file.empty())
+        return false;
+
+    /// Per-part files flow through the ordinary content path like any other file; no ForceFresh
+    /// special case is needed.
+    /// Safe to serve a CACHED view here: every committed-ref write that could have moved this entry
+    /// (`repointRef`/`promoteBuild`) erases the cached view on success, so a stale hit is impossible
+    /// by construction, not by freshness policy.
+    auto view = partAccess()->getView(r->refKey(), Cas::Freshness::CachedForLoad);
+    return view && view->findFile(r->file);
+}
+
+ContentAddressedMetadataStorage::DirRoute ContentAddressedMetadataStorage::classifyDirectory(const std::string & path) const
+{
+    DirRoute dr;
+
+    /// FREEZE shadow namespace — routed BEFORE the live branches (a shadow table dir also
+    /// satisfies parseTableUuid).
+    if (Cas::isShadowPath(path))
+    {
+        if (auto p = Cas::parsePartFilePath(path); p && !p->backup_name.empty() && p->file.empty())
+        {
+            dr.shape = DirShape::ShadowPart;
+            dr.p = std::move(p);
+            return dr;
+        }
+        if (Cas::endsWithTableUuidPair(path))
+        {
+            dr.shape = DirShape::ShadowTable;
+            return dr;
+        }
+        dr.shape = DirShape::ShadowIntermediate;
+        return dr;
+    }
+
+    /// The Atomic `store/<u3>` shard dir (see listDirectory): route to the generic existence signal
+    /// before parseTableUuid/parseTableFilePath misclaim it as a non-Atomic table.
+    if (Cas::isAtomicShardDir(path))
+    {
+        dr.shape = DirShape::AtomicShard;
+        return dr;
+    }
+
+    if (auto uuid = Cas::parseTableUuid(path))
+    {
+        dr.shape = DirShape::TableDir;
+        dr.uuid = std::move(uuid);
+        return dr;
+    }
+
+    if (auto p = Cas::parsePartFilePath(path))
+    {
+        auto r = route(*p);
+        /// The detached CONTAINER dir <table>/detached.
+        if (r && r->ref.empty() && p->part_name == Cas::kDetachedDirName)
+        {
+            dr.shape = DirShape::DetachedContainer;
+            dr.p = std::move(p);
+            dr.r = std::move(r);
+            return dr;
+        }
+        /// The moving CONTAINER dir <table>/moving (MOVE-to-CA fix): the mover's crash-cleanup
+        /// (MergeTreeData.cpp, MOVING_DIR_NAME) existsDirectory/removeRecursive's this bare path
+        /// at every table load to reclaim a staging ref left behind by an interrupted move.
+        if (r && r->ref.empty() && p->part_name == Cas::kMovingDirName)
+        {
+            dr.shape = DirShape::MovingContainer;
+            dr.p = std::move(p);
+            dr.r = std::move(r);
+            return dr;
+        }
+        /// A part dir (live, detached, or shadow).
+        if (r && !r->ref.empty() && r->file.empty())
+        {
+            dr.shape = DirShape::PartDir;
+            dr.p = std::move(p);
+            dr.r = std::move(r);
+            return dr;
+        }
+        /// A projection dir.
+        if (r && !r->ref.empty())
+        {
+            if (auto prefix = Cas::PartFolderView::projectionDirPrefix(r->file))
+            {
+                dr.shape = DirShape::ProjectionDir;
+                dr.p = std::move(p);
+                dr.r = std::move(r);
+                dr.projection_prefix = std::move(prefix);
+                return dr;
+            }
+        }
+        /// No sub-shape matched: fall through, identical to today's post-`if (p)` continuation.
+    }
+
+    /// A table-level SUBDIRECTORY (deduplication_logs/...).
+    if (auto tf = Cas::parseTableFilePath(path))
+    {
+        dr.shape = DirShape::TableSubdir;
+        dr.tf = std::move(tf);
+        return dr;
+    }
+
+    /// A generic INTERMEDIATE live-tree directory (disk root, `store`, ...).
+    dr.shape = DirShape::GenericIntermediate;
+    return dr;
+}
+
+bool ContentAddressedMetadataStorage::existsDirectory(const std::string & path) const
+{
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return false;
+    const DirRoute dr = classifyDirectory(path);
+    switch (dr.shape)
+    {
+        case DirShape::ShadowPart:
+            return partAccess()->existsRef(Route{shadowNamespace(dr.p->shadow_table_dir), dr.p->part_name, ""}.refKey(),
+                                          Cas::Freshness::CachedForLoad);
+        case DirShape::ShadowTable:
+            return store()->hasAnyRefWithPrefix(shadowNamespace(path), "");
+        case DirShape::ShadowIntermediate:
+        {
+            /// Intermediate dir (shadow/<bk>, shadow/<bk>/store, ...): exists iff SOME shadow namespace
+            /// under this path still has a LIVE ref. A raw object LIST of the mirrored subtree would count
+            /// tombstoned-but-not-yet-GC'd shard/manifest objects — CA removal is tombstone + deferred GC
+            /// (`removeRecursive`/`dropNamespace` only tombstone; `Cas::Gc` physically deletes later) — so a
+            /// just-`UNFREEZE`d backup dir would spuriously "exist" until a GC round runs. Instead
+            /// enumerate the namespaces exactly as `removeRecursive` does (`listNamespaces(scope)`) and
+            /// consult the tombstone-aware `listRefs` (as the `endsWithTableUuidPair` case above does), so
+            /// existence is consistent with the ref-level signal and independent of GC timing.
+            const std::string canonical = canonicalDiskPath(path);
+            const std::string scope = canonical.empty() ? "shadow/" : canonical + "/";
+            const Cas::NamespaceListing listing = store()->listNamespaces(scope);
+            for (const auto & ns : listing.namespaces)
+                if (store()->hasAnyRefWithPrefix(Cas::RootNamespace{ns}, ""))
+                    return true;
+            /// A key this scope's enumeration could not attribute leaves emptiness UNPROVEN, and the
+            /// fail-close answer for an existence probe is "present": reporting absent is what would let
+            /// a caller treat the subtree as gone. Answering present is bounded -- an already-unfrozen
+            /// directory keeps showing up until the key is cleared -- and it never claims absence that
+            /// was not established.
+            ///
+            /// The answer alone would leave an operator with a directory that will not go away and
+            /// nothing naming the key that holds it, so the key and the refusal are LOGGED here too --
+            /// the other three consumers of this enumeration each surface the skip to a human, and a
+            /// boolean is not that. Rate-limited because this is an existence probe on a browse path:
+            /// `LogSeriesLimiter` keys on the LOGGER NAME, so one message per window prints regardless of
+            /// which key it was about.
+            if (!listing.skipped.empty())
+            {
+                LogSeriesLimiter log(getLogger("CasShadowScopeLifelessKey"), /*allowed_count=*/1, /*interval_s=*/60);
+                LOG_WARNING(log,
+                    "existsDirectory('{}'): {} key(s) under this scope name no namespace life, so it "
+                    "cannot be proven empty and is reported as PRESENT. First such key: '{}' ({}). Run "
+                    "`cas-fsck` to enumerate them all.",
+                    path, listing.skipped.size(), listing.skipped.front().key, listing.skipped.front().reason);
+                return true;
+            }
+            return false;
+        }
+        case DirShape::AtomicShard:
+            return liveTreeDirHasChildren(path);
+        case DirShape::TableDir:
+            /// A table directory exists iff its logical namespace still has foreground removal work
+            /// outstanding, or has never proven completion: present while `Creating`, for every `Live`
+            /// row (even zero parts and zero namespace files -- an empty live table is still a table),
+            /// and while `Removing` before its terminal `remove_namespace` transaction is durable;
+            /// absent only once no catalog row exists at all, or the terminal is durably proven. This is
+            /// deliberately NOT "has a committed ref": a table that removed its last part, or that never
+            /// wrote one, must stay present until an actual namespace-drop admits and completes removal
+            /// -- otherwise `DROP TABLE` on such a table would silently skip physical cleanup and leak
+            /// its catalog row forever.
+            return store()->namespaceStillLogicallyPresent(liveNamespace(*dr.uuid));
+        case DirShape::DetachedContainer:
+            /// Exists iff it has at least one reference.
+            return store()->hasAnyRefWithPrefix(dr.r->ns, Cas::kDetachedRefPrefix);
+        case DirShape::MovingContainer:
+            /// Exists iff it has at least one staging ref (MOVE-to-CA fix, mirrors DetachedContainer).
+            return store()->hasAnyRefWithPrefix(dr.r->ns, Cas::kMovingRefPrefix);
+        case DirShape::PartDir:
+            /// Exists iff its ref is present.
+            return partAccess()->existsRef(dr.r->refKey(), Cas::Freshness::CachedForLoad);
+        case DirShape::ProjectionDir:
+        {
+            /// At least one tree entry (or mutable file) under its prefix.
+            auto view = partAccess()->getView(dr.r->refKey(), Cas::Freshness::CachedForLoad);
+            return view && view->hasDirectory(*dr.projection_prefix);
+        }
+        case DirShape::TableSubdir:
+        {
+            /// At least one verbatim file under it.
+            const auto life = readableNamespaceFilesLife(liveNamespace(dr.tf->table_uuid));
+            if (!life)
+                return false;
+            const std::string prefix = dr.tf->tail + "/";
+            for (const auto & name : store()->listNamespaceFiles(*life))
+                if (name.starts_with(prefix))
+                    return true;
+            return false;
+        }
+        case DirShape::GenericIntermediate:
+            /// Exists iff a server-root-scoped mirrored LIST finds any object. Keeps `cd`/existence
+            /// consistent with listDirectory so `clickhouse-disks` traversal behaves like a normal disk.
+            return liveTreeDirHasChildren(path);
+    }
+    return liveTreeDirHasChildren(path);   /// unreachable
+}
+
+bool ContentAddressedMetadataStorage::existsFileOrDirectory(const std::string & path) const
+{
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return false;
+    if (Cas::isPartFilePath(path))
+    {
+        auto p = Cas::parsePartFilePath(path);
+        auto r = p ? route(*p) : std::nullopt;
+        if (r && !r->ref.empty() && !r->file.empty())
+        {
+            auto view = partAccess()->getView(r->refKey(), Cas::Freshness::CachedForLoad);
+            if (!view)
+                return false;
+            return view->hasFile(r->file) || view->hasDirectory(r->file + "/");
+        }
+    }
+    return existsFile(path) || existsDirectory(path);
+}
+
+uint64_t ContentAddressedMetadataStorage::getFileSize(const std::string & path) const
+{
+    /// ContentRead: a size query resolves a specific file; on a Vanished disk it fails loud with the typed
+    /// error rather than silent-absent (never let a reader mistake erased backing for "file not there").
+    checkOpAdmitted(CasOpClass::ContentRead);
+    if (!Cas::isPartFilePath(path))
+    {
+        if (auto bytes = tryGetInManifestBytes(path))   /// verbatim table-level file
+            return bytes->size();
+        if (auto bytes = store()->getMountpointObject(serverPrefix() + "/" + path))
+            return bytes->size();
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: no object for {}", path);
+    }
+
+    auto p = Cas::parsePartFilePath(path);
+    if (!p || p->file.empty())
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: not a part file path: {}", path);
+    auto r = route(*p);
+    if (!r || r->file.empty())
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: not a part file path: {}", path);
+
+    auto view = partAccess()->getView(r->refKey(), Cas::Freshness::CachedForLoad);
+    if (!view)
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: no ref for {}", path);
+    if (auto size = view->fileSize(r->file))
+        return *size;
+    throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: file {} not in manifest of {}", r->file, path);
+}
+
+Poco::Timestamp ContentAddressedMetadataStorage::getLastModified(const std::string & path) const
+{
+    /// ContentRead: resolves a specific part's stamp; loud typed error on a Vanished disk.
+    checkOpAdmitted(CasOpClass::ContentRead);
+    /// Timestamps are DERIVED for content addressing: the part's publish wall-clock, stamped by
+    /// the transaction into the typed `RefPayload.published_at_ms` field (epoch milliseconds).
+    /// Every shape (part dir, detached part dir, projection dir, part file) reports its part's
+    /// stamp; a part published without a stamp (published_at_ms == 0) reports the epoch (harmless:
+    /// stamps only feed cleanup TTLs and system tables).
+    auto resolve_stamp = [&](const Route & r) -> Poco::Timestamp
+    {
+        auto resolved = partAccess()->resolve(r.refKey(), Cas::Freshness::CachedForLoad);
+        if (!resolved)
+            throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: no ref for {}", path);
+        if (resolved->published_at_ms == 0)
+            return Poco::Timestamp(0);
+        /// published_at_ms is epoch milliseconds; Poco::Timestamp::fromEpochTime takes seconds.
+        return Poco::Timestamp::fromEpochTime(static_cast<time_t>(resolved->published_at_ms / 1000));
+    };
+
+    if (auto p = Cas::parsePartFilePath(path))
+    {
+        auto r = route(*p);
+        if (r && !r->ref.empty())
+            return resolve_stamp(*r);
+    }
+    /// Table-level / generic verbatim files: no per-object mtime is kept — epoch.
+    if (existsFile(path))
+        return Poco::Timestamp(0);
+    throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: no object for {}", path);
+}
+
+std::vector<std::string> ContentAddressedMetadataStorage::listDirectory(const std::string & path) const
+{
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return {};
+    const DirRoute dr = classifyDirectory(path);
+    switch (dr.shape)
+    {
+        case DirShape::ShadowPart:
+        {
+            /// Shadow PART dir: the frozen part's file names (first components).
+            auto view = partAccess()->getView(Route{shadowNamespace(dr.p->shadow_table_dir), dr.p->part_name, ""}.refKey(),
+                                             Cas::Freshness::CachedForLoad);
+            return view ? view->listChildren("") : std::vector<std::string>{};
+        }
+        case DirShape::ShadowTable:
+        {
+            /// Shadow TABLE dir: the frozen part names.
+            std::vector<std::string> result;
+            for (const auto & [ref, _] : store()->listRefs(shadowNamespace(path)))
+                result.push_back(ref);
+            return result;
+        }
+        case DirShape::ShadowIntermediate:
+        {
+            /// Enumerate children via a scoped LIST of the mirrored subtree. A
+            /// mirrored LIST naturally surfaces intermediate path segments AND `@cas@`-suffixed
+            /// table dirs; strip the trailing `@cas@` for the logical view. Loose LIST is fine: the
+            /// existing listRefs re-check filters out dropped-but-registered archives so they don't
+            /// appear as false children.
+            const std::string canonical = canonicalDiskPath(path);
+            const std::string scope = canonical.empty() ? "shadow/" : canonical + "/";
+            std::unordered_set<std::string> result;
+            for (const auto & child : store()->listMirroredChildren(scope))
+                result.emplace(stripCasArchiveSuffix(child));
+            return toVector(std::move(result));
+        }
+        case DirShape::AtomicShard:
+            /// A pure intermediate dir whose only child is the uuid-anchored table dir. Its path
+            /// shape collides with the non-Atomic `data/<db>` fallback of both parseTableUuid and
+            /// parseTableFilePath, so it MUST be routed to the generic mirrored LIST BEFORE those
+            /// branches claim it (see classifyDirectory).
+            return listLiveTreeChildren(path);
+        case DirShape::TableDir:
+        {
+            /// Part names (live and `detached/<part>` references) plus table-level verbatim
+            /// file names; addFirstComponent collapses both to their first path segment (live part
+            /// names and the single `detached` subdir, exactly like a nested verbatim file).
+            const auto ns = liveNamespace(*dr.uuid);
+            std::unordered_set<std::string> result;
+            for (const auto & [ref, _] : store()->listRefs(ns))
+                addFirstComponent(result, ref);
+            /// A dropped table lists empty for its namespace files too (its refs are already gone
+            /// via the ref state); only surface verbatim file names while the table is not removed.
+            if (const auto life = readableNamespaceFilesLife(ns))
+                for (const auto & name : store()->listNamespaceFiles(*life))
+                    addFirstComponent(result, name);
+            /// EMPTY-PROOF RULE (Task 9, spec §1 [B3]): an empty table root is exactly what a
+            /// silently-erased backing looks like -- authorize the empty answer only against an
+            /// authoritative, uncached `_pool_meta` positive (see the helper).
+            if (result.empty())
+                confirmPoolIdentityForEmptyEnumeration(path);
+            return toVector(std::move(result));
+        }
+        case DirShape::DetachedContainer:
+        {
+            /// Detached part names (prefix stripped; never files).
+            std::vector<std::string> result;
+            for (const auto & ref : detachedRefNames(dr.r->ns))
+                result.push_back(ref.substr(Cas::kDetachedRefPrefix.size()));
+            /// EMPTY-PROOF RULE (Task 9, spec §1 [B3]): same for an empty detached container root.
+            if (result.empty())
+                confirmPoolIdentityForEmptyEnumeration(path);
+            return result;
+        }
+        case DirShape::MovingContainer:
+        {
+            /// Staging part names (prefix stripped), mirrors DetachedContainer.
+            std::vector<std::string> result;
+            for (const auto & ref : movingRefNames(dr.r->ns))
+                result.push_back(ref.substr(Cas::kMovingRefPrefix.size()));
+            return result;
+        }
+        case DirShape::PartDir:
+        {
+            /// A part dir (live, detached part, shadow handled separately): logical file names,
+            /// nested keys collapsed to their first component (projections surface as ONE
+            /// <proj>.proj entry).
+            auto view = partAccess()->getView(dr.r->refKey(), Cas::Freshness::CachedForLoad);
+            return view ? view->listChildren("") : std::vector<std::string>{};
+        }
+        case DirShape::ProjectionDir:
+        {
+            /// Inner names with the <proj>.proj/ prefix stripped.
+            auto view = partAccess()->getView(dr.r->refKey(), Cas::Freshness::CachedForLoad);
+            return view ? view->listChildren(*dr.projection_prefix) : std::vector<std::string>{};
+        }
+        case DirShape::TableSubdir:
+        {
+            /// Verbatim files under <subdir>/, first-component collapsed.
+            std::unordered_set<std::string> result;
+            if (const auto life = readableNamespaceFilesLife(liveNamespace(dr.tf->table_uuid)))
+                for (const auto & name : store()->listNamespaceFiles(*life))
+                    if (name.starts_with(dr.tf->tail + "/"))
+                        addFirstComponent(result, name.substr(dr.tf->tail.size() + 1));
+            return toVector(std::move(result));
+        }
+        case DirShape::GenericIntermediate:
+            /// The disk root "", `store`, or any loose-file container above a table dir: a
+            /// server-root-scoped mirrored LIST. (`store/<u3>` is handled by AtomicShard above,
+            /// since its non-Atomic-table ambiguity would otherwise misroute it here too late,
+            /// after parseTableUuid/parseTableFilePath have already claimed it.)
+            return listLiveTreeChildren(path);
+    }
+    return listLiveTreeChildren(path);   /// unreachable
+}
+
+DirectoryIteratorPtr ContentAddressedMetadataStorage::iterateDirectory(const std::string & path) const
+{
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return std::make_unique<StaticDirectoryIterator>(std::vector<fs::path>{});
+    /// Mirror MetadataStorageFromPlainObjectStorage: iterateDirectory includes the path.
+    auto names = listDirectory(path);
+    std::vector<fs::path> fs_paths;
+    fs_paths.reserve(names.size());
+    for (const auto & child : names)
+        fs_paths.push_back(fs::path(path) / child);
+    return std::make_unique<StaticDirectoryIterator>(std::move(fs_paths));
+}
+
+bool ContentAddressedMetadataStorage::isDirectoryEmpty(const std::string & path) const
+{
+    /// A Vanished disk reports every directory empty too (truth): the ref-unlink removal path then
+    /// proceeds, letting a vanished-disk table's DROP complete rather than throwing CANNOT_RMDIR.
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return true;
+    /// A part directory's files are virtual (derived from the tree): report it EMPTY so
+    /// DiskObjectStorage::removeDirectory proceeds straight to the ref-unlink instead of throwing
+    /// CANNOT_RMDIR per removal. The same applies to a projection subdirectory. The detached
+    /// CONTAINER and TABLE dirs keep the listing-based emptiness (DROP TABLE's non-empty guard).
+    if (auto p = Cas::parsePartFilePath(path))
+    {
+        auto r = route(*p);
+        if (r && !r->ref.empty() && r->file.empty())
+            return true;
+        if (r && !r->ref.empty() && Cas::PartFolderView::projectionDirPrefix(r->file))
+            return true;
+    }
+    return !iterateDirectory(path)->isValid();
+}
+
+StoredObjects ContentAddressedMetadataStorage::getStorageObjects(const std::string & path) const
+{
+    /// ContentRead: resolves an object; loud typed error on a Vanished disk (never silent-empty).
+    checkOpAdmitted(CasOpClass::ContentRead);
+    /// In-manifest bytes (mutable per-part files, inline entries, verbatim namespace files) have
+    /// no object of their own: DiskObjectStorage::prepareRead serves them via tryGetInManifestBytes
+    /// BEFORE asking for storage objects. The sized empty-key placeholder below keeps size-only
+    /// consumers working and makes any bypassing reader fail LOUDLY (never silently wrong bytes).
+    if (auto bytes = tryGetInManifestBytes(path))
+        return {StoredObject("", path, bytes->size())};
+
+    if (!Cas::isPartFilePath(path))
+    {
+        if (Cas::parseTableFilePath(path))
+            throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+                "ContentAddressed: table-level verbatim file is in-manifest, not a storage object: {}", path);
+        /// A loose mountpoint object: a real plain object at roots/<server_root_id>/<path>. The
+        /// StoredObject key must be the PHYSICAL path (physicalKey-adjusted for Local backends).
+        /// Probe with a HEAD (directory-safe), not a body read: `system.remote_data_paths`
+        /// may reach here on a directory-shaped pool path and a GET would throw "Is a directory".
+        const std::string pool_key = store()->layout().mountpointObjectKey(serverPrefix() + "/" + path);
+        if (store()->mountpointObjectExists(serverPrefix() + "/" + path))
+            return {StoredObject(physicalKey(pool_key), path, getFileSize(path))};
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: no object for {}", path);
+    }
+
+    auto p = Cas::parsePartFilePath(path);
+    if (!p || p->file.empty())
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: not a part file path: {}", path);
+    auto r = route(*p);
+    if (!r || r->file.empty())
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: not a part file path: {}", path);
+
+    /// ONE snapshot for both the facade lookup and the pool `locate` below, so the two can never
+    /// straddle two different mount generations (see `poolAccess()`).
+    const auto snap = poolAccess();
+    auto view = snap.part_access->getView(r->refKey(), Cas::Freshness::CachedForLoad);
+    if (!view)
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: no ref for {}", path);
+    if (const auto * entry = view->findFile(r->file))
+    {
+        const auto location = snap.pool->locate(*entry);
+        /// StoredObject carries no range (the recorded upstream delta) — the PAYLOAD length is the
+        /// size (what every size consumer wants); the header offset is applied by
+        /// getBlobViewPlan's view window, the only byte-reading path.
+        return {StoredObject(location.key, path, location.length)};
+    }
+    throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: file {} not in manifest of {}", r->file, path);
+}
+
+std::optional<StoredObjects> ContentAddressedMetadataStorage::getStorageObjectsIfExist(const std::string & path) const
+{
+    /// A Vanished disk answers absent (truth). Probe first so the non-part `getStorageObjects` fallback
+    /// below (ContentRead) is never reached on a Vanished disk.
+    if (checkOpAdmitted(CasOpClass::Probe) == CasOpAdmission::TruthAbsent)
+        return std::nullopt;
+    /// Non-part shapes (verbatim table files, loose mountpoint objects) are rare paths — the
+    /// generic two-step is fine for them.
+    if (!Cas::isPartFilePath(path))
+    {
+        if (existsFile(path))
+            return getStorageObjects(path);
+        return std::nullopt;
+    }
+    auto p = Cas::parsePartFilePath(path);
+    if (!p || p->file.empty())
+        return std::nullopt;
+    auto r = route(*p);
+    if (!r || r->file.empty())
+        return std::nullopt;
+
+    /// ONE snapshot for both the facade lookup and the pool `locate` below (see `poolAccess()`).
+    const auto snap = poolAccess();
+    auto view = snap.part_access->getView(r->refKey(), Cas::Freshness::CachedForLoad);
+    if (!view)
+        return std::nullopt;
+    const auto * entry = view->findFile(r->file);
+    if (!entry)
+        return std::nullopt;
+    if (entry->placement == Cas::EntryPlacement::Inline)
+        return StoredObjects{StoredObject("", path, entry->size())};
+    const auto location = snap.pool->locate(*entry);
+    return StoredObjects{StoredObject(location.key, path, location.length)};
+}
+
+std::optional<String> ContentAddressedMetadataStorage::tryGetInManifestBytes(const std::string & path) const
+{
+    /// Speculative in-manifest probe: `DiskObjectStorage::prepareRead`/`getStorageObjects` call it before
+    /// falling back to a real storage-object lookup, and both treat "not in-manifest" (`std::nullopt`) as a
+    /// normal outcome. But this is a ContentRead: a disk in a terminal/uncertain/unstarted lifecycle must
+    /// PROPAGATE the typed 668 rather than convert it into a silent-absent `std::nullopt` -- so the gate
+    /// REPLACES the old catch-all that swallowed `poolAccess()`'s `INVALID_STATE` (which had hidden an
+    /// erased/replaced/transient backing behind a FILE_DOESNT_EXIST-shaped answer).
+    checkOpAdmitted(CasOpClass::ContentRead);
+    const PoolAccessSnapshot snap = poolAccess();
+
+    if (!Cas::isPartFilePath(path))
+    {
+        if (auto tf = Cas::parseTableFilePath(path))
+        {
+            const auto life = readableNamespaceFilesLife(liveNamespace(tf->table_uuid));
+            return life ? snap.pool->getNamespaceFile(*life, tf->tail) : std::nullopt;
+        }
+        return std::nullopt;   /// loose files are plain objects, not in-manifest bytes
+    }
+
+    auto p = Cas::parsePartFilePath(path);
+    if (!p || p->file.empty())
+        return std::nullopt;
+    auto r = route(*p);
+    if (!r || r->file.empty())
+        return std::nullopt;
+
+    auto view = snap.part_access->getView(r->refKey(), Cas::Freshness::CachedForLoad);
+    if (!view)
+        return std::nullopt;
+    return view->inlineBytes(r->file);
+}
+
+bool ContentAddressedMetadataStorage::prepareInManifestRead(
+    const std::string & path, const ReadSettings & settings, ReadPipeline & pipeline) const
+{
+    /// In-manifest bytes (mutable per-part files, inline entries, verbatim namespace files):
+    /// served from memory — there is no object to read.
+    auto bytes = tryGetInManifestBytes(path);
+    if (!bytes)
+        return false;
+
+    const auto size = bytes->size();
+    auto creator = [path, data = std::move(*bytes)](
+        const StoredObject &, const ReadSettings &, bool, bool) -> std::unique_ptr<ReadBufferFromFileBase>
+    {
+        return std::make_unique<ReadBufferFromOwnMemoryFile>(path, data);
+    };
+    pipeline.setSource(std::move(creator), {StoredObject("", path, size)}, settings);
+    return true;
+}
+
+std::optional<ContentAddressedMetadataStorage::BlobViewPlan> ContentAddressedMetadataStorage::getBlobViewPlan(
+    const std::string & path) const
+{
+    /// ContentRead: resolves a blob-backed path to its physical window; loud typed error on a Vanished
+    /// disk rather than a silent `std::nullopt` (which a reader would take as "not blob-backed").
+    checkOpAdmitted(CasOpClass::ContentRead);
+    if (!Cas::isPartFilePath(path))
+        return std::nullopt;
+    auto p = Cas::parsePartFilePath(path);
+    if (!p || p->file.empty())
+        return std::nullopt;
+    auto r = route(*p);
+    if (!r || r->file.empty())
+        return std::nullopt;
+    /// ONE snapshot for both the facade lookup and the pool `locate` below, instead of the previous
+    /// `partAccess()` then `store()` pair (each an independent `pointer_mutex` acquisition) -- see
+    /// `poolAccess()`.
+    const auto snap = poolAccess();
+    auto view = snap.part_access->getView(r->refKey(), Cas::Freshness::CachedForLoad);
+    if (!view)
+        return std::nullopt;
+    if (const auto * entry = view->findFile(r->file))
+    {
+        const auto location = snap.pool->locate(*entry);
+        BlobViewPlan plan;
+        /// bytes_size is the readable extent of THIS file's window, NOT the whole blob: a
+        /// right-bounded read stops at payload_end, and a shared blob's bytes beyond it belong
+        /// to other files. The caches key on the physical blob key, so payload ranges are
+        /// shared between every part that references the same blob.
+        plan.object = StoredObject(physicalKey(location.key), path, location.offset + location.length);
+        plan.payload_offset = location.offset;
+        plan.payload_end = location.offset + location.length;
+        return plan;
+    }
+    return std::nullopt;
+}
+
+std::unique_ptr<ReadBufferFromFileBase> ContentAddressedMetadataStorage::readBlobPayload(
+    const Cas::BlobLocation & location, const std::string & path, const ReadSettings & settings) const
+{
+    /// ContentRead: the actual byte read; loud typed error on a Vanished disk instead of a raw backend
+    /// "no such key" from the erased object.
+    checkOpAdmitted(CasOpClass::ContentRead);
+    auto impl = object_storage->readObject(
+        StoredObject(physicalKey(location.key), path, location.offset + location.length), settings);
+    return std::make_unique<ReadBufferFromFileView>(
+        std::move(impl), path, location.offset, location.offset + location.length);
+}
+
+/// ==== `IContentAddressedExchange` ====
+
+bool ContentAddressedMetadataStorage::ownsNamespace(const String & other_server_root_id, const String & root_namespace) const
+{
+    /// Routing for the relink confirm (spec §wire-protocol). `pool_uuid` says which POOL a token refers
+    /// to and is compared by the caller; every server root writing into that pool shares it, so the
+    /// namespace's owner is decided here. `liveNamespace` builds live and detached namespaces as
+    /// `<server_root_id>/<mirrored table dir>`, so ownership is exactly "rooted at MY server root".
+    /// The strict prefix (not a bare equality, not `starts_with(server_root_id)`) is what keeps
+    /// `srv1` from claiming `srv10/...`, and it deliberately does not match a pool-global shadow
+    /// namespace: a FREEZE tree belongs to no single mount and is never a relink source.
+    ///
+    /// Factory-class: no `store()`, no gate, no I/O, no throw. A misrouted question must come back as
+    /// an unproven answer, never as an error.
+    if (other_server_root_id.empty() || other_server_root_id != server_root_id)
+        return false;
+    return root_namespace.starts_with(server_root_id + "/");
+}
+
+CasConfirmAnswer ContentAddressedMetadataStorage::confirmExactRef(
+    const String & root_namespace, const String & ref_name, const String & manifest_ref_text) const
+{
+    /// Gate 1 of the relink confirm: a thin forward to the ledger, whose declaration
+    /// (`CasRefLedger::confirmExactRef`) carries the six-rule snapshot and the zero-I/O contract. This
+    /// layer adds exactly two things: the token text is decoded here, and the disk's own lifecycle is
+    /// answered as `Unknown` instead of as an exception.
+    const auto manifest_ref = Cas::tryParseManifestRef(manifest_ref_text);
+    if (!manifest_ref)
+    {
+        LOG_DEBUG(getLogger("ContentAddressedMetadataStorage"),
+            "Relink confirm for ref '{}' in namespace '{}' is unanswerable: manifest reference '{}' is not "
+            "the canonical epoch:build:ordinal form", ref_name, root_namespace, manifest_ref_text);
+        return CasConfirmAnswer::Unknown;
+    }
+
+    Cas::ConfirmAnswer answer = Cas::ConfirmAnswer::Unknown;
+    try
+    {
+        /// ContentRead: the confirm reads this disk's committed view. A disk that never started, was shut
+        /// down, is transiently not live, or has reached a terminal lifecycle has no committed view to
+        /// speak for -- and `checkOpAdmitted` says so by throwing.
+        checkOpAdmitted(CasOpClass::ContentRead);
+        answer = store()->confirmExactRef(Cas::RootNamespace{root_namespace}, ref_name, *manifest_ref);
+    }
+    catch (const Exception & e)
+    {
+        /// This is NOT a fallback path: `Unknown` is the typed refusal this primitive is built around,
+        /// not an alternate behavior substituted for a failed one. Nothing consequential happens on it --
+        /// the receiver aborts its prepared relink and retries later -- so swallowing the lifecycle
+        /// refusal costs a retry and can never authorize anything. Only `Yes` authorizes, and no `catch`
+        /// can produce a `Yes`.
+        LOG_DEBUG(getLogger("ContentAddressedMetadataStorage"),
+            "Relink confirm for ref '{}' in namespace '{}' is unanswerable on disk '{}': {}",
+            ref_name, root_namespace, disk_name, e.message());
+        return CasConfirmAnswer::Unknown;
+    }
+
+    switch (answer)
+    {
+        case Cas::ConfirmAnswer::Yes:
+            return CasConfirmAnswer::Yes;
+        case Cas::ConfirmAnswer::No:
+            return CasConfirmAnswer::No;
+        case Cas::ConfirmAnswer::Unknown:
+            return CasConfirmAnswer::Unknown;
+    }
+}
+
+std::optional<IContentAddressedExchange::RelinkOffer>
+ContentAddressedMetadataStorage::getRelinkOffer(const String & part_path) const
+{
+    /// Sender side: the committed part's encoded `PartManifest` body — the opaque payload the
+    /// receiver decodes — and the confirm token for it. Resolve the part path to its (ns, ref) exactly
+    /// as the read surface does (route), resolve the committed ref to its ManifestId, read the
+    /// immutable manifest, and re-encode it canonically. nullopt when the path is not a committed
+    /// content-addressed part here (no ref => no relink offer; the sender streams bytes). A live ref to
+    /// a missing/corrupt manifest throws (INV-NO-DANGLE surfaced, never substituted) — the same
+    /// fail-loud contract as partAccess()->getView.
+    /// ContentRead: reading a committed manifest; loud typed error on a Vanished disk.
+    checkOpAdmitted(CasOpClass::ContentRead);
+    auto p = Cas::parsePartFilePath(part_path);
+    if (!p)
+        return std::nullopt;
+    auto r = route(*p);
+    if (!r || r->ref.empty())
+        return std::nullopt;
+
+    auto view = partAccess()->getView(r->refKey(), Cas::Freshness::ForceFresh);
+    if (!view)
+        return std::nullopt;
+
+    /// The token names the manifest THIS view resolved, so the offer and the question the receiver will
+    /// ask are about one and the same object by construction. `manifestId` is the journal identity the
+    /// ledger compares in gate 1, and it is already proven to agree with the body: `readManifest`
+    /// enforces `refMatchesBody`/`manifestNamespaceMatches` and throws `CORRUPTED_DATA` otherwise, so
+    /// there is no disagreement left for this function to discover or to quietly turn into a byte fetch.
+    ///
+    /// `ref_name` is what this mount publishes the part under and `part_name` is what gate 0 looks up
+    /// in the parts set; they coincide for every offer the sender can actually make, because it offers
+    /// only a live part. A staging path (`detached/`, `moving/`) reports the reserved directory name as
+    /// `part_name`, so a token minted for one selects no part at all — `Unknown`, which is the safe
+    /// direction — rather than selecting the wrong one.
+    const auto token = encodeCasRelinkSourceToken(CasRelinkSourceToken{
+        .pool_uuid = pool_uuid,
+        .server_root_id = server_root_id,
+        .root_namespace = r->ns.string(),
+        .ref_name = r->ref,
+        .part_name = p->part_name,
+        .manifest_ref_text = Cas::manifestRefDebugString(view->manifestId().ref)});
+    if (!token)
+        return std::nullopt;
+
+    return RelinkOffer{.manifest_bytes = Cas::encodePartManifest(*view->manifest()), .confirm_token = *token};
+}
+
+namespace
+{
+
+/// The exchange's view of one durable-but-unpromoted relink: a `Cas::PreparedPartWrite` with the two
+/// content-addressed-free verbs `DataPartsExchange` is allowed to know about.
+///
+/// It also OWNS a `shared_ptr` snapshot of the part-folder facade, and that is load-bearing rather than
+/// tidy: `PreparedPartWrite` holds its owner as a raw pointer, and this handle deliberately outlives
+/// the call that made it -- it spans an interserver round trip -- so a concurrent `shutdown` resetting
+/// the disk's facade would dangle that pointer. The snapshot is declared BEFORE the write, so the write
+/// (and any abort its destructor runs) is destroyed first, while the facade is still alive.
+class PreparedRelinkOverPartWrite : public ICaPreparedRelink
+{
+public:
+    PreparedRelinkOverPartWrite(std::shared_ptr<Cas::CachedPartFolderAccess> access_, Cas::PreparedPartWrite write_,
+                                String ref_name_)
+        : access(std::move(access_)), write(std::move(write_)), ref_name(std::move(ref_name_))
+    {
+    }
+
+    CaRelinkPromote promote() override
+    {
+        try
+        {
+            write.promote();
+            return CaRelinkPromote::Committed;
+        }
+        catch (const Exception & e)
+        {
+            /// UNCERTAINTY IS CHECKED FIRST, and it outranks the error code. A `NETWORK_ERROR` out of
+            /// the promote means one of two entirely different things: the promote was rejected before
+            /// its ref-log append (nothing committed), or the append itself was attempted and did not
+            /// resolve -- in which case the promotion PUT may have landed and the ref may be live. The
+            /// error code cannot tell them apart, which is why the transaction records the distinction
+            /// as it happens. Reporting the second case as a mechanism fallback would have the receiver
+            /// fetch the bytes and publish a second time over a relink that already committed.
+            if (write.commitIsUnresolved())
+            {
+                LOG_INFO(getLogger("ContentAddressedMetadataStorage"),
+                    "Relink of part {} could not be resolved: the promotion append may or may not have "
+                    "committed ({}); the caller must retry the whole fetch later, NOT fetch the bytes",
+                    ref_name, e.message());
+                return CaRelinkPromote::Unresolved;
+            }
+            /// The same retryable class the staging half classifies: a body-absent precommit, a
+            /// precommit binding that is no longer the live owner, or a ref conflict. `promote` has
+            /// already abandoned the build on its way out, so the `+1` is released and the sender's
+            /// bytes are a sound recovery. Anything else propagates -- it is not a known-safe
+            /// mechanism failure, and the receiver must not silently turn it into a byte fetch.
+            if (e.code() != ErrorCodes::ABORTED && e.code() != ErrorCodes::NETWORK_ERROR)
+                throw;
+            LOG_INFO(getLogger("ContentAddressedMetadataStorage"),
+                "Relink of part {} could not be promoted (body-absent precommit, precommit not the live "
+                "owner, or a ref conflict): {}; the caller may fetch the bytes from the same source",
+                ref_name, e.message());
+            return CaRelinkPromote::MechanismFallbackAllowed;
+        }
+    }
+
+    void abort() noexcept override
+    {
+        /// Not defensive noise: a `promote` that FAILED discharges the duty itself (its catch abandons
+        /// the build), so the scope guard that always runs finds a terminal handle on exactly that path.
+        if (write.isTerminal())
+            return;
+        try
+        {
+            write.abort();
+        }
+        catch (...)
+        {
+            /// The removal append did not land. `PreparedPartWrite` stays non-terminal, so its own
+            /// destructor retries it; beyond that the durable backstop is the ref lane's wedge, exactly
+            /// as for every other abandon path.
+            tryLogCurrentException(getLogger("ContentAddressedMetadataStorage"),
+                fmt::format("aborting the prepared relink of part {}", ref_name));
+        }
+    }
+
+private:
+    std::shared_ptr<Cas::CachedPartFolderAccess> access;
+    Cas::PreparedPartWrite write;
+    String ref_name;
+};
+
+}
+
+/// TRUST MODEL: adopting a part from a peer-supplied manifest is exactly as trusted as an ordinary
+/// ReplicatedMergeTree interserver part fetch. The interserver HTTP channel — not a per-blob ACL — is
+/// the trust boundary: a malicious or MITM peer on that channel can already serve arbitrary part bytes
+/// that the receiver adopts, in both the byte-streaming and the relink path. Table-level RBAC never
+/// defended against a hostile peer, so relink-by-manifest adds no new trust surface. (See the retracted
+/// umbrella "RBAC bypass" finding.)
+CaRelinkPrepare ContentAddressedMetadataStorage::prepareAdoptFromManifest(
+    const String & part_path, const String & manifest_bytes,
+    std::unique_ptr<ICaPreparedRelink> & out)
+{
+    checkNotReadOnly("prepareAdoptFromManifest (interserver relink receiver)");
+    /// Write class: publishing a receiver-local ref; throws typed on a Vanished disk, 668 while uncertain.
+    checkOpAdmitted(CasOpClass::Write);
+
+    /// Receiver side. Sender identity is non-authoritative: we ignore the decoded ManifestRef,
+    /// root_namespace_id and payload_digest, and use ONLY the entries. We run a normal LOCAL build over
+    /// the SHARED-pool blobs — adopted by hash via adoptEvidence, NO blob body transferred — then stage a
+    /// FRESH receiver-local ManifestId in the receiver namespace and `precommitAdd` it.
+    ///
+    /// The promote does NOT happen here, and that split is the fix for the commit-before-release gap
+    /// this function used to carry (codex-6). The sender's relink response is fire-and-forget: it
+    /// releases the source part when `processQuery` returns, so if this `+1` were not yet durable while
+    /// the source's now-`Outdated` part was collected, the receiver would commit a manifest whose blobs
+    /// are gone. Publishing FIRST and asking SECOND closes THAT window: any removal of the source
+    /// binding is appended strictly after this `+1` is durable in the ref log.
+    ///
+    /// It does NOT establish that every subsequent GC fold SEES the `+1`, and the ordering must not be
+    /// read as if it did: `CaRelinkConfirmCore.tla` config `_sab_holeylist` shows a fold that misses it
+    /// (BACKLOG `{#list-as-journal-dataloss-2026-07-25}`), which is why the caller-side taxonomy states
+    /// plainly that a confirmed relink is not proven dangle-free (`DataPartsExchange.cpp`, "What a
+    /// `yes` does NOT prove"). Durable-before-asking is necessary, not sufficient.
+    ///
+    /// Promotion trusts the adopted leaves via the durable manifest edge (no per-file HEAD/loadMeta
+    /// probe); a genuinely-absent adopted blob is an invariant violation caught by fsck, not here — the
+    /// ordinary ReplicatedMergeTree interserver trust.
+    out.reset();
+
+    /// The RECEIVER's own (namespace, ref) for the target path — never the sender's `root_namespace_id`,
+    /// which is foreign to this server's path-mirroring identity. Routing rather than composing is what
+    /// gives B66b its detached target for free: `TABLE/detached/DIR` folds onto `detached/DIR` in
+    /// the table's OWN namespace, and a live target onto `DIR`, through the same `route` the reads use.
+    /// A path that is not a part DIRECTORY here is a caller error, not a fallback: a byte fetch to the
+    /// same place would be just as wrong, so it must not be quietly substituted. Shadow (FREEZE) paths
+    /// are rejected for the same reason — a backup namespace is never a fetch target.
+    auto p = Cas::parsePartFilePath(part_path);
+    auto r = p ? route(*p) : std::nullopt;
+    if (!p || !r || !p->backup_name.empty() || r->ref.empty() || !r->file.empty())
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "Relink target '{}' does not address a content-addressed part directory of a live table", part_path);
+
+    Cas::PartManifest decoded;
+    try
+    {
+        decoded = Cas::decodePartManifest(manifest_bytes);
+    }
+    catch (const Exception & e)
+    {
+        if (e.code() != ErrorCodes::CORRUPTED_DATA)
+            throw;
+        LOG_INFO(getLogger("ContentAddressedMetadataStorage"), "Relink of part {} not possible: transferred manifest failed to decode ({}); "
+            "caller falls back to a byte fetch", part_path, e.message());
+        return CaRelinkPrepare::MechanismFallbackAllowed;
+    }
+
+    auto access = partAccess();
+    try
+    {
+        out = std::make_unique<PreparedRelinkOverPartWrite>(
+            access, access->prepareEntries(r->refKey(), decoded.entries, Cas::ProvenanceOp::Attach), r->ref);
+        return CaRelinkPrepare::Prepared;
+    }
+    catch (const Exception & e)
+    {
+        /// `ABORTED` or `NETWORK_ERROR` means a body-absent precommit, a precommit binding that is no
+        /// longer the live owner, a ref conflict, or — since the transient-classifier round — this node's
+        /// own mount fence refusing the work (`throwCasTransientUnavailable`): all retryable, and the
+        /// sender still has the part, so the caller may fetch its bytes. `prepareEntries` abandons its own
+        /// build before propagating, so nothing is staged and no `+1` is left behind. The fence case needs
+        /// no special handling and stays fail-close by construction: the byte fetch it falls back to writes
+        /// through the SAME fenced disk and is refused in turn, so the fallback cannot smuggle a write past
+        /// a lost incarnation. Any other error propagates — an unclassified local failure is not evidence
+        /// that a byte fetch would do better.
+        if (e.code() != ErrorCodes::ABORTED && e.code() != ErrorCodes::NETWORK_ERROR)
+            throw;
+        LOG_INFO(getLogger("ContentAddressedMetadataStorage"), "Relink of part {} deferred (body-absent precommit, "
+            "precommit not the live owner, or a ref conflict): {}; caller falls back to a byte fetch", part_path, e.message());
+        return CaRelinkPrepare::MechanismFallbackAllowed;
+    }
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h
new file mode 100644
index 000000000000..8c461f5f4f6d
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h
@@ -0,0 +1,774 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/IMetadataStorage.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Interpreters/Context_fwd.h>
+#include <base/defines.h>
+#include <atomic>
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <unordered_set>
+
+namespace Poco::Util { class AbstractConfiguration; }
+
+namespace DB
+{
+class IDisk;
+using DiskPtr = std::shared_ptr<IDisk>;
+}
+
+namespace DB::Cas
+{
+
+/// Selects where a content-addressed blob is staged before it is published.
+///
+/// `Local` is the default and preserves the existing local scratch-file path byte for byte; it does
+/// not run the conditional-copy probe. `S3` is opt-in and can stream large blobs to an object-store
+/// staging key. That path is usable only after the mount-time probe has demonstrated write-once
+/// conditional copy semantics. If the backend does not enforce those semantics, callers must stay
+/// on `Local`: an unconditional copy could overwrite a live content-addressed blob.
+enum class StagingBackend
+{
+    Local,
+    S3,
+};
+
+}
+
+namespace DB
+{
+
+/// The SIX operation classes the central `ContentAddressedMetadataStorage::checkOpAdmitted` gate keys on
+/// (rev.7 spec §1). Every public metadata/transaction entry declares its class so a single place decides
+/// what happens per pool-lifecycle condition:
+///   - `Factory`     — I/O-free construction / capability / introspection (`createTransaction`, the
+///                     `getType`/`getPath`/capability getters, `gcHealth`, `lifecycleSnapshot`). NEVER
+///                     gated: it works in every state. Such call sites do not invoke `checkOpAdmitted`.
+///   - `Probe`       — existence / enumeration (`existsFile`/`existsDirectory`/`listDirectory`/
+///                     `iterateDirectory`/`isDirectoryEmpty`/`getStorageObjectsIfExist`). Answers the
+///                     truth: real while live, throws while uncertain, absent/empty once `Vanished`.
+///   - `ContentRead` — resolving/serving bytes or per-file metadata (`getStorageObjects`/`getFileSize`/
+///                     `getLastModified`/`getBlobViewPlan`/`readBlobPayload`/`getRelinkOffer`/
+///                     `tryGetInManifestBytes`/`prepareInManifestRead`). Never silent-absent on `Vanished`
+///                     — a loud typed error instead.
+///   - `Write`       — create / write / rename, INCLUDING the previously-no-op sites and a publishing
+///                     `commit`. Throws typed on `Vanished`.
+///   - `Remove`      — ref/file removal (`removeRecursive`/`removeDirectory`/`unlinkFile`, and an
+///                     empty/pure-remove `commit`). No-op SUCCESS on `Vanished` so a vanished-disk table's
+///                     `DROP` completes.
+///   - `Admin`       — `store()`-reaching admin + GC round entry points.
+enum class CasOpClass : uint8_t
+{
+    Factory,
+    Probe,
+    ContentRead,
+    Write,
+    Remove,
+    Admin,
+};
+
+/// The disposition `checkOpAdmitted` returns for the classes that have a truthful short-circuit answer in
+/// the terminal `Vanished` state (Probe / Remove). `Proceed` means run the operation normally against the
+/// live pool; `TruthAbsent` means answer the truth WITHOUT touching the pool (absent/empty for a Probe,
+/// no-op success for a Remove). Every other outcome is a throw, so a caller only ever sees these two.
+enum class CasOpAdmission : uint8_t
+{
+    Proceed,
+    TruthAbsent,
+};
+
+/// A non-gated lifecycle snapshot of one content-addressed disk for `system.cas_mounts`
+/// (rev.7 spec §7, [C5]-visibility). It is a Factory-class read (§1): I/O-free, no `store()`/`poolAccess`,
+/// truthful in EVERY state — including a not-live / vanished pool the store()-class surface refuses, and a
+/// null pool (before `startup` / after `shutdown`). This is what keeps a disappearing disk VISIBLE to the
+/// operator instead of silently missing from the table.
+///   - `lifecycle`      — one of `live` / `not_live` / `identity_lost` / `vanished` (a live pool), or
+///                        `constructing` / `shutdown` (no pool published).
+///   - `reason`         — the ENUM-CLEAN sub-state word: `replaced` / `forgotten` for a
+///                        `vanished` pool, empty otherwise. Kept a small closed vocabulary so a downstream
+///                        `lifecycle || '(' || reason || ')'` yields e.g. exactly `vanished(forgotten)` —
+///                        the [D5] free text lives in `detail`, never here.
+///   - `detail`         — the full [D5] reason text naming the actual failure (the replaced
+///                        diagnosis, the timestamped `FORGET` message, or the identity-loss message); spec §1
+///                        requires it appear verbatim in the snapshot. Empty while `live` and for a null pool.
+///   - `since`          — wall-clock second the current non-`live` state was entered; 0 while `live`/no pool.
+///   - `pool_id`        — last-known pool UUID (empty before the first `startup`); the disk stays
+///                        introspectable under its identity even once the pool is gone.
+///   - `server_root_id` — this server's node-local root id owning the mount slot.
+struct CasLifecycleSnapshot
+{
+    String lifecycle;
+    String reason;
+    String detail;
+    time_t since = 0;
+    String pool_id;
+    String server_root_id;
+};
+
+/// Adapts ClickHouse's `IMetadataStorage` path-based interface to the content-addressed pool.
+///
+/// The class owns the pool and its cached part-folder facade for the lifetime of an opened disk. It
+/// parses disk paths, maps them to pool namespaces and references, and translates manifest entries
+/// into `StoredObjects` or in-memory read sources. Transaction and GC entry points are exposed here
+/// because disk lifecycle and system-query code own those operations; the CAS protocol itself stays
+/// in `Cas::Pool`, `Cas::PartWriteTxn`, and `Cas::Gc`.
+///
+/// Namespace mapping:
+///   live part      SERVER_ID/TABLE_UUID            ref = PART_DIR
+///   detached part  SERVER_ID/TABLE_UUID            ref = detached/DETACHED_PART_DIR
+///   FREEZE shadow  the LITERAL shadow table dir     ref = PART_DIR
+///                  (shadow/BACKUP/store/U3/UUID or shadow/BACKUP/data/DB/TBL — bijective with
+///                  the disk path for both Atomic and non-Atomic layouts, so the shadow tree
+///                  enumerates from `Pool::listNamespaces("shadow/...")`)
+///   generic files  SERVER_ID/_disk                  verbatim namespace files (access probes)
+///
+/// Small per-part files (`uuid.txt`, `metadata_version.txt`, `txn_version.txt`, `checksums.txt`, ...)
+/// are inline-placement manifest tree entries, not sidecar objects. Their bytes are served through
+/// `DiskObjectStorage::prepareRead`'s CA branch via `tryGetInManifestBytes`; `getStorageObjects` returns a
+/// sized placeholder with an EMPTY remote key for them (any consumer bypassing the prepareRead branch
+/// fails loudly, never reads wrong bytes).
+class ContentAddressedMetadataStorage final : public IMetadataStorage, public IContentAddressedExchange
+{
+public:
+    /// Constructs an unopened storage adapter. `settings_` carries every tunable that used to be a
+    /// positional parameter (see `ContentAddressedSettings`) -- it is the single source of defaults, so
+    /// the constructor itself declares none. `server_root_id`/`scratch_path` (the local-scratch
+    /// directory used when a write buffer must spill before hashing and upload, independent of the
+    /// object-storage key prefix) are read from `settings_` rather than taken as their own parameters. A
+    /// non-null `context_` enables the background GC scheduler on the disk-factory path; tests may pass
+    /// null to disable system-log integration and scheduling. `disk_name_` falls back to
+    /// `storage_path_prefix_` when empty, exactly as before this constructor collapsed.
+    ContentAddressedMetadataStorage(
+        ObjectStoragePtr object_storage_,
+        String storage_path_prefix_,
+        String server_id_,
+        String disk_name_,
+        ContextPtr context_,
+        const ContentAddressedSettings & settings_);
+
+    /// Parses a `staging_backend` value (`local` | `s3`). Throws `BAD_ARGUMENTS` for an unrecognized
+    /// value rather than silently selecting a backend.
+    static Cas::StagingBackend parseStagingBackend(const std::string & value);
+
+    /// Reads `staging_backend` from `config`, defaulting to `local`, and parses it. Kept only as a
+    /// thin wrapper around the string-taking overload for callers that still hold a config reference.
+    static Cas::StagingBackend parseStagingBackend(const Poco::Util::AbstractConfiguration & config, const std::string & config_prefix);
+
+    /// Parses a `part_folder_validate` value (`always` | `never` | `age <seconds>`). The `age` form
+    /// accepts only a non-negative integer number of seconds; malformed input and unknown modes throw
+    /// `BAD_ARGUMENTS` instead of silently selecting a policy.
+    static Cas::PartFolderValidate parsePartFolderValidate(const std::string & value);
+
+    /// Reads `part_folder_validate` from `config`, defaulting to `always`, and parses it. Kept only as
+    /// a thin wrapper around the string-taking overload for callers that still hold a config reference.
+    static Cas::PartFolderValidate parsePartFolderValidate(const Poco::Util::AbstractConfiguration & config, const std::string & config_prefix);
+
+    /// Returns the content-addressed metadata storage backing `disk`, or nullptr if `disk` is not
+    /// content-addressed. Plain (non-object-storage) disks do not implement `getMetadataStorage` at
+    /// all and throw `NOT_IMPLEMENTED`; that is treated as "not content-addressed" rather than
+    /// propagated. Any other exception from `getMetadataStorage` is rethrown. Centralizes the
+    /// detection lambda duplicated across `InterpreterSystemQuery` and
+    /// `StorageSystemContentAddressedMounts`; callers there have not yet been migrated to it.
+    static ContentAddressedMetadataStorage * tryFromDisk(const DiskPtr & disk);
+
+    /// Runs one synchronous GC round for tests and diagnostics. If the scheduler is not running,
+    /// this lazily creates one so repeated calls retain the same lease-observation history.
+    void runOneGcRoundForTest();
+
+    /// Runs one synchronous GC round on the caller's thread and emits Start and Finish rows to
+    /// `system.cas_gc_log`. Throws `BAD_ARGUMENTS` when GC is disabled
+    /// by read-only mode or configuration.
+    Cas::RoundReport runGarbageCollectionRoundNow();
+
+    /// Coalesce an administrative liveness hint into the existing periodic GC worker. If this disk has
+    /// no running scheduler, the normal operator-visible retry path remains unchanged.
+    void requestGcRoundSoon();
+
+    /// Rebuilds the GC baseline for the `SYSTEM` disaster-recovery command. Each invocation uses a
+    /// fresh GC identity because `rebuildBaseline` performs its own lease check. A refused rebuild
+    /// (`report.performed == false`) writes nothing; the `SYSTEM` interpreter surfaces the refusal.
+    /// Throws `BAD_ARGUMENTS` when GC is disabled by read-only mode or configuration.
+    Cas::RebuildReport runGcRebuildNow(bool force) const;
+
+    /// The `SYSTEM CAS FSCK` handler: a read-only, independent reachability audit.
+    ///
+    /// FSCK scans the LIVE running pool directly: Admin class -- it routes through
+    /// `checkOpAdmitted(CasOpClass::Admin)` (refuses on a transient / `IdentityLost` / `Vanished` pool,
+    /// consistent with `SYSTEM CAS GC RUN`), because an FSCK of a not-live disk is
+    /// meaningless -- the operator has the snapshot / FORGET path. The scan tolerates concurrent writers:
+    /// its ref-walk findings (missing manifest, dangling blob) are revalidated against a FRESH
+    /// authoritative read before being reported, so a legitimate concurrent republish/drop + GC delete
+    /// never surfaces as a phantom. Held under `lifecycle_mutex` for the whole scan so a concurrent
+    /// lifecycle-control verb (FORGET / GC STOP / GC START) cannot race it.
+    Cas::FsckReport runFsckNow(bool detail) const;
+
+    /// Returns per-disk GC health for `system.cas_mounts`. Returns nullopt when this
+    /// disk has no scheduler because GC is disabled, the disk is read-only, or startup has not run.
+    /// Holds `gc_scheduler_mutex` for the entire call so a concurrent system-table query cannot
+    /// observe a scheduler while `shutdown` destroys it.
+    std::optional<Cas::CasGcScheduler::GcHealth> gcHealth() const;
+
+    /// The non-gated lifecycle snapshot for `system.cas_mounts` (spec §7, Factory class):
+    /// I/O-free, reachable in EVERY state — a live pool (forwards `Pool::lifecycleSnapshot`), a terminal
+    /// pool the store()-class surface refuses, and a null pool (before `startup`/after `shutdown`, reported
+    /// as `constructing`/`shutdown`). Never calls `store()`/`poolAccess()`, never touches the backend, so
+    /// the very disk that vanished stays visible. Takes only a brief `pointer_mutex` snapshot of the pool
+    /// pointer; the identity fields are read from immutable-after-startup storage members.
+    CasLifecycleSnapshot lifecycleSnapshot() const;
+
+    MetadataStorageType getType() const override { return MetadataStorageType::CAS; }
+    const std::string & getPath() const override { return storage_path_full; }
+    bool supportsChmod() const override { return false; }
+    bool supportsStat() const override { return false; }
+    bool isReadOnly() const override { return read_only; }
+    bool isContentAddressed() const override { return true; }
+
+    /// Fail-close gate shared by every mutating entry point (transactions, GC round, GC rebuild,
+    /// pool-member decommission): an observe-only (`<readonly>`) disk must reject them all.
+    void checkNotReadOnly(std::string_view what) const;
+
+    /// THE central six-class operation gate (rev.7 spec §1), consulted at EVERY public metadata/
+    /// transaction entry (see the `CasOpClass` doc above and the method->class inventory at the top of
+    /// the .cpp). Given `op`'s class it inspects the pool lifecycle ONCE and decides:
+    ///   - `Live`                            -> `Proceed` for every class.
+    ///   - null pool (the storage-level Constructing/ShutDown lifecycle -- before `startup`/after
+    ///     `shutdown`) -> throws "not started" for EVERY class, `Probe` included: a storage that has
+    ///     never published a pool (or torn one down) has no benign "absent" answer to give.
+    ///   - `TransientNotLive` / `IdentityLost` -> throws 668 for every class but `Factory` (uncertain
+    ///     backing; the sub-state distinction is surfaced in `system.cas_mounts`, not the
+    ///     op error).
+    ///   - `Vanished*` -> `Probe`/`Remove` answer `TruthAbsent` (absent-empty / no-op success);
+    ///     `ContentRead`/`Write`/`Admin` throw the typed per-reason [D5] message (via
+    ///     `Pool::throwIfLifecycleTerminal`, the single home of those strings).
+    /// `Factory` is never passed here. Public because `ContentAddressedTransaction` funnels its own
+    /// mutating entries through it.
+    CasOpAdmission checkOpAdmitted(CasOpClass op) const;
+
+    /// Content-addressed transactions are eager staging overlays: each mutating disk-transaction
+    /// method reaches the metadata transaction immediately rather than entering the FIFO queue.
+    bool transactionIsStagingOverlay() const override { return true; }
+    bool supportsAtomicFileWrites() const override { return true; }
+    bool supportsTransactionalMutableFiles() const override { return true; }
+    bool areBlobPathsRandom() const override { return false; }
+    uint32_t getHardlinkCount(const std::string &) const override { return 0; }
+
+    /// Creates a write transaction bound to this storage. Throws `READONLY` before allocating one
+    /// when the disk was opened read-only.
+    MetadataTransactionPtr createTransaction() override;
+
+    /// Opens the pool, validates its format and starts the optional GC scheduler. Read-only disks
+    /// skip write probes and GC; failures in startup propagate. Runs exactly once, single-threaded,
+    /// strictly before this object is exposed to any other thread (no other method can be called
+    /// concurrently with it) -- TSA_NO_THREAD_SAFETY_ANALYSIS is deliberate here, not a bypass of a
+    /// real risk: pointer_mutex/gc_scheduler_mutex exist to guard concurrent access AFTER startup,
+    /// which is definitionally impossible during it.
+    void startup() TSA_NO_THREAD_SAFETY_ANALYSIS override;
+    /// Stops the GC scheduler before releasing the part-folder facade and pool. Their destruction is
+    /// synchronized with the accessors and synchronous GC entry points.
+    void shutdown() override;
+
+    /// `SYSTEM CAS FORGET` handler (Task 10, spec §5): the operator force-Vanish. Drives the
+    /// live pool to `Vanished(forgotten)` node-locally via `Pool::forgetDisk`'s fence-first protocol, and
+    /// stops + joins the GC scheduler as part of it. Unlike the store()-class verbs, this must work on a
+    /// NOT-live disk (a stuck transient / `IdentityLost` pool) — that is its purpose — so it reaches the
+    /// pool DIRECTLY, never through `poolAccess()`/`checkOpAdmitted` (which refuse a not-live disk); it is a
+    /// lifecycle verb, like the Factory class. FORGET is an operator ASSERTION, not an erasure proof: the
+    /// resulting [D5] error message (which carries the decommission timestamp) says so. The disk stays
+    /// registered; the six-class gate then answers the truth (Probe/Remove truth-absent, reads throw the
+    /// [D5] message). Idempotent; a disk with no published pool is a no-op. Serialized by `lifecycle_mutex`
+    /// (against FSCK / GC STOP / GC START) and `gc_scheduler_mutex` (against a synchronous round).
+    void forgetDisk() TSA_NO_THREAD_SAFETY_ANALYSIS;
+
+    /// `SYSTEM CAS GC STOP` handler (Task 11, spec §6): stops ONLY the background GC scheduler.
+    /// The disk stays fully usable -- reads/writes are unaffected; this is granular operator control of the
+    /// GC pacer alone, NOT a lifecycle transition. Unlike `forgetDisk` this is
+    /// STOP-IN-PLACE: the scheduler object is RETAINED in the member (not detached/destroyed), so `gcHealth`
+    /// keeps reading its (now stopped) state truthfully and a later `gcStart` re-enters the SAME instance
+    /// with its `gc_id` + lease-observation history preserved. `stop()`
+    /// joins the worker+heartbeat threads and clears the in-process leadership hint. Idempotent (a second
+    /// STOP is a no-op); a no-op success when no scheduler exists (GC disabled / read-only / not started).
+    /// Works on a not-live/Vanished disk too -- stopping GC on a sick disk is legitimate operator action, so
+    /// this does NOT consult `checkOpAdmitted`. Serialized by `lifecycle_mutex` and `gc_scheduler_mutex`.
+    void gcStop() TSA_NO_THREAD_SAFETY_ANALYSIS;
+
+    /// `SYSTEM CAS GC START` handler (Task 11, spec §6): restarts the background GC scheduler
+    /// stopped by `gcStop`, re-entering the SAME instance (its `start()` is re-enterable after a join).
+    /// Leadership is NOT auto-restored -- the scheduler re-acquires the durable `gc/state` lease through the
+    /// next round's normal acquisition. Idempotent (a no-op on a running scheduler). Unlike `gcStop`, it goes
+    /// through the uniform GC gate (`checkOpAdmitted(Admin)`): it refuses on a transient / `IdentityLost` /
+    /// `Vanished` pool with the typed error (668 / [D5]) and on a not-mounted disk -- restarting GC on a
+    /// decommissioned/uncertain pool is meaningless and would only spin failing rounds. Serialized by
+    /// `lifecycle_mutex` and `gc_scheduler_mutex`.
+    void gcStart() TSA_NO_THREAD_SAFETY_ANALYSIS;
+
+    /// Test-only fault-injection hook. When set, `startup` invokes it right before it publishes
+    /// `cas_store`/`part_access`/`gc_scheduler`/`pool_uuid`/`conditional_copy_supported` -- everything
+    /// up to that point (opening the pool, building the part-folder facade, running the capability
+    /// probe, starting the GC scheduler) has already happened into locals, so throwing here proves a
+    /// late startup failure publishes nothing and a retry can still succeed. Left empty (a no-op) in
+    /// production.
+    std::function<void()> startup_fault_injection_for_test;
+
+    /// Tests whether a path is represented by an inline manifest entry, namespace file, or loose
+    /// mountpoint object.
+    bool existsFile(const std::string & path) const override;
+    /// Tests whether a path names a virtual part, table, shadow, or mirrored live-tree directory.
+    bool existsDirectory(const std::string & path) const override;
+    /// Tests both file and directory interpretations of a path.
+    bool existsFileOrDirectory(const std::string & path) const override;
+    /// Returns the logical payload size, excluding a blob envelope.
+    uint64_t getFileSize(const std::string & path) const override;
+    /// Returns the part publication time; other existing files use epoch time because their mtime is
+    /// not retained by the content-addressed namespace.
+    Poco::Timestamp getLastModified(const std::string & path) const override;
+    /// Lists logical children of a virtual or mirrored directory.
+    std::vector<std::string> listDirectory(const std::string & path) const override;
+    /// Iterates over `listDirectory` results with each child joined to `path`.
+    DirectoryIteratorPtr iterateDirectory(const std::string & path) const override;
+    /// Reports virtual part and projection directories as empty so removal unlinks their ref; table
+    /// and container directories use their listing.
+    bool isDirectoryEmpty(const std::string & path) const override;
+    /// Maps a logical path to its storage object. Inline entries return a sized empty-key placeholder
+    /// and must be served by the CA read branch.
+    StoredObjects getStorageObjects(const std::string & path) const override;
+    /// Performs one manifest lookup for part files instead of the inherited `existsFile` plus
+    /// `getStorageObjects` sequence.
+    std::optional<StoredObjects> getStorageObjectsIfExist(const std::string & path) const override;
+
+    /// ==== `IContentAddressedExchange` (interserver relinking facade) ====
+    const String & getPoolUUID() const override { return pool_uuid; }
+    /// Routing predicate for the confirm action: this mount owns a namespace iff the namespace is rooted
+    /// at ITS `server_root_id` (`liveNamespace` builds every live/detached namespace as
+    /// `<server_root_id>/<mirrored table dir>`). Factory-class: I/O-free, ungated, never throws.
+    bool ownsNamespace(const String & other_server_root_id, const String & root_namespace) const override;
+    /// Gate 1 of the relink confirm, forwarded to the pool's ref ledger. Answers `Unknown` for an
+    /// unparsable token and for a disk that has not started or has reached a terminal lifecycle.
+    CasConfirmAnswer confirmExactRef(const String & root_namespace, const String & ref_name,
+                                     const String & manifest_ref_text) const override;
+    /// Returns the canonical encoded manifest for a committed part plus the confirm token minted from
+    /// that same resolution, or nullopt when the path is not a committed CA part or the token cannot be
+    /// minted. Missing or corrupt committed state propagates as an exception.
+    std::optional<RelinkOffer> getRelinkOffer(const String & part_path) const override;
+    /// Stages a peer-supplied manifest into this server's namespace without transferring blob bodies and
+    /// hands back the durable-but-unpromoted handle. The receiver's `part_path` is routed exactly as any
+    /// other part path, so a live target and a `detached/` one (B66b) differ only in the ref the router
+    /// derives. Answers `MechanismFallbackAllowed` for a decode failure or a retryable staging failure so
+    /// the caller can byte-fetch instead; read-only disks throw `READONLY`.
+    CaRelinkPrepare prepareAdoptFromManifest(
+        const String & part_path, const String & manifest_bytes,
+        std::unique_ptr<ICaPreparedRelink> & out) override;
+
+    /// ==== wiring-internal surface (the transaction + the disk's prepareRead CA branch) ====
+
+    /// Returns a shared-ownership snapshot of the opened pool. Throws `INVALID_STATE` when no pool is
+    /// published (before the first `startup`, or after `shutdown`). A thin wrapper over `poolAccess()`.
+    Cas::PoolPtr store() const;
+    /// Returns a shared-ownership snapshot of the cached part-folder facade. Throws `INVALID_STATE`
+    /// under the same not-started condition as `store()`. Committed part-folder reads and mutations
+    /// go through this facade so cache validation remains centralized. Returning a `shared_ptr`
+    /// snapshot (not a reference) means the returned handle keeps the facade alive via its own
+    /// refcount even if `shutdown` concurrently resets the member -- unlike a reference, which would
+    /// dangle the instant `shutdown`'s reset runs. A thin wrapper over `poolAccess()`.
+    std::shared_ptr<Cas::CachedPartFolderAccess> partAccess() const;
+    const std::string & serverRootId() const { return server_root_id; }
+    const std::string & scratchPath() const { return local_scratch_path; }
+    /// Returns the configured staging backend. `Local` is the behavior-preserving default; callers
+    /// must also check `conditionalCopySupported` before using S3 promotion.
+    Cas::StagingBackend stagingBackend() const { return staging_backend; }
+    /// Returns the mount-time conditional-copy capability result. It starts false and becomes true
+    /// only after the backend proves write-once copy semantics, so S3 promotion fails closed.
+    bool conditionalCopySupported() const { return conditional_copy_supported; }
+    /// Returns the underlying object storage for an S3 staging writer. It is meaningful only when
+    /// `stagingBackend` is `S3` and `conditionalCopySupported` is true.
+    const ObjectStoragePtr & objectStorage() const { return object_storage; }
+    /// Returns the physical prefix for this pool's writer-owned staging area. It is the same
+    /// `pool_prefix/staging/server_root_id` subtree used by the capability probe; callers append a
+    /// unique leaf and must not use the probe object itself.
+    String stagingKeyPrefix() const;
+
+    /// Bytes that live INSIDE pool metadata rather than as their own object: an Inline-placement
+    /// manifest tree entry, or a verbatim namespace file. nullopt = the path is blob-backed (a real
+    /// storage object).
+    std::optional<String> tryGetInManifestBytes(const std::string & path) const;
+
+    /// The CA read entry called by `DiskObjectStorage::prepareRead` before the generic
+    /// storage-objects path: serves in-manifest bytes (mutable per-part files, inline entries,
+    /// verbatim namespace files) from memory. Returns false when the path is not in-manifest.
+    /// Declared on `IContentAddressedExchange` (the narrow seam `prepareRead` casts to); `BlobViewPlan`
+    /// is likewise inherited from there.
+    bool prepareInManifestRead(const std::string & path, const ReadSettings & settings, ReadPipeline & pipeline) const override;
+
+    /// Resolves a blob-backed path to its physical object and payload window. Returns nullopt for
+    /// in-manifest, loose, directory, or otherwise unresolved paths.
+    std::optional<BlobViewPlan> getBlobViewPlan(const std::string & path) const override;
+
+    /// Creates a seekable reader over one blob payload, excluding its envelope. Transactions use
+    /// this for read-your-writes; committed reads use `getBlobViewPlan` and the normal pipeline.
+    std::unique_ptr<ReadBufferFromFileBase> readBlobPayload(
+        const Cas::BlobLocation & location, const std::string & path, const ReadSettings & settings) const;
+
+    /// Maps a live table UUID to its pool namespace. Detached parts share that namespace and use
+    /// `detached/`-prefixed references rather than a sibling namespace.
+    Cas::RootNamespace liveNamespace(const std::string & table_uuid) const;
+    /// Canonicalizes a literal shadow-table directory into the pool namespace used by freeze and
+    /// unfreeze paths. A trailing slash is ignored.
+    static Cas::RootNamespace shadowNamespace(const std::string & shadow_table_dir);
+
+    /// The LIFE under which `ns`'s table-level namespace files — `format_version.txt` and the other
+    /// verbatim files — must be read, or `nullopt` when there are none to read.
+    ///
+    /// It answers two things at once because they are one question. A dropped-and-not-recreated table
+    /// (ref-table lifecycle durably `Removed`) is GONE for readers: its files must read as absent even
+    /// while GC has not yet physically reclaimed them (namespace removal is deferred to GC), mirroring
+    /// how its parts already vanish via the ref state. A never-born namespace is likewise empty. And a
+    /// readable namespace's files live under ITS OWN incarnation (Stage B Task 4b), never under a
+    /// previous life's — which is why the readable answer is a life rather than a `true`: a reader that
+    /// has no life cannot form a key at all, so a previous life's surviving objects are unreachable by
+    /// construction rather than by remembering to check something.
+    std::optional<Cas::NamespaceLifeId> readableNamespaceFilesLife(const Cas::RootNamespace & ns) const;
+
+    /// Returns the root prefix for mirrored live-tree objects. The persistent layout identity is
+    /// `server_root_id`; `ServerUUID` remains only the mount-owner token.
+    std::string serverPrefix() const;
+
+    /// Enumerate the children of a GENERIC intermediate live-tree directory (the disk root "",
+    /// `store`, the `store/<u3>` shard dir, or any loose-file container above a table dir) via a
+    /// server-root-scoped mirrored S3 LIST of `roots/<server_root_id>/<path>/`. `@cas@`-suffixed table-dir
+    /// segments are surfaced under their logical (unsuffixed) name. This is what makes top-down
+    /// `clickhouse-disks` traversal of the live tree behave like a normal disk; concrete
+    /// `store/<u3>/<uuid>/<part>/<file>` navigation is still served by the exact-shape branches.
+    std::vector<std::string> listLiveTreeChildren(const std::string & path) const;
+    /// Tests whether the server-root-scoped mirrored subtree has at least one child. The disk root is
+    /// always considered present.
+    bool liveTreeDirHasChildren(const std::string & path) const;
+
+    /// Resolves one parsed path to its namespace, reference, and in-tree file. Detached paths are
+    /// re-split here so their references remain in the table namespace with a `detached/` prefix;
+    /// shadow paths map to a namespace derived from the literal shadow directory.
+    struct Route
+    {
+        Cas::RootNamespace ns{""};
+        /// empty => the path is the namespace's container dir. For a detached part this is
+        /// `detached/<part>` (a ref inside the table namespace, not a separate namespace).
+        std::string ref;
+        std::string file;   /// empty => the path is the part dir itself
+
+        /// The (ns, ref) identity subset — what the part-folder access layer keys on.
+        Cas::PartRefKey refKey() const { return {ns, ref}; }
+    };
+    /// Converts a parsed path into the namespace/reference/file tuple used by the part-folder
+    /// facade. Returns nullopt only when the parsed path cannot be routed.
+    std::optional<Route> route(const Cas::PartFilePath & p) const;
+
+    /// Returns full `detached/<part>` reference names in a namespace.
+    std::vector<std::string> detachedRefNames(const Cas::RootNamespace & ns) const;
+
+    /// Returns full `moving/<part>` staging-reference names in a namespace. Move recovery enumerates
+    /// these names and removes entries left by an interrupted move.
+    std::vector<std::string> movingRefNames(const Cas::RootNamespace & ns) const;
+
+    /// `existsDirectory` and `listDirectory` use one fixed dispatch order to route a path through
+    /// (shadow -> atomic-shard -> table-uuid -> part -> subdir -> generic), previously implemented
+    /// twice and kept in sync by hand. `classifyDirectory` (private, below) computes it once; both
+    /// callers then switch on the resulting shape. `DirShape` and `DirRoute` remain public only so
+    /// `classifyDirectoryForTest` can expose the classification to wiring tests; the logic stays
+    /// private.
+    enum class DirShape
+    {
+        ShadowPart,
+        ShadowTable,
+        ShadowIntermediate,
+        AtomicShard,
+        TableDir,
+        DetachedContainer,
+        MovingContainer,
+        PartDir,
+        ProjectionDir,
+        TableSubdir,
+        GenericIntermediate,
+    };
+
+    struct DirRoute
+    {
+        /// Defaulted so a future classifyDirectory return path that forgets to set it fails safe
+        /// (a defined shape) instead of switching on an indeterminate enum (UB). Matches the
+        /// existing unreachable-fallthrough choice at the bottom of existsDirectory/listDirectory.
+        DirShape shape = DirShape::GenericIntermediate;
+        std::optional<Cas::PartFilePath> p;
+        std::optional<Route> r;
+        std::optional<std::string> uuid;
+        std::optional<Cas::TableFilePath> tf;
+        std::optional<std::string> projection_prefix;
+    };
+
+    /// Test-only accessor exposing the private directory classification so wiring tests can pin the
+    /// dispatch order directly.
+    DirRoute classifyDirectoryForTest(const std::string & path) const { return classifyDirectory(path); }
+
+    /// Test seams for the EMPTY-PROOF RULE (Task 9, spec §1 [B3]). The counter is bumped on every
+    /// authoritative pool-identity probe the empty-proof issues, so a test can assert it fires EXACTLY
+    /// once per EMPTY `TableDir`/`DetachedContainer` enumeration and NEVER on the non-empty hot path,
+    /// a deeper part-dir, or a terminal (`Vanished`) pool. `setEmptyProofProbeOverrideForTest` replaces
+    /// the real backend probe so a test can inject a transport/permission fault (`Indeterminate`/
+    /// `AccessDenied`) deterministically -- the storage builds its own `ObjectStorageBackend` internally,
+    /// so a backend decorator cannot reach it otherwise. Both are inert in production.
+    uint64_t emptyProofProbeCountForTest() const { return empty_proof_probe_count_for_test.load(); }
+    void resetEmptyProofProbeCountForTest() { empty_proof_probe_count_for_test.store(0); }
+    void setEmptyProofProbeOverrideForTest(std::function<Cas::SentinelProbeResult()> fn)
+    {
+        empty_proof_probe_override_for_test = std::move(fn);
+    }
+
+    /// Test-only seam (inert in production): invoked at each manual GC verb's FORGET-race juncture so a test
+    /// can deterministically interleave a concurrent FORGET. For the round verbs
+    /// (`runOneGcRoundForTest`/`runGarbageCollectionRoundNow`) it fires PRE-lock, in the admission->lock
+    /// TOCTOU window (I-1: the call is admitted while `Live`, parks here until FORGET drives the pool
+    /// `Vanished`, then hits the under-lock re-check). For `runGcRebuildNow` it fires WHILE the rebuild HOLDS
+    /// `gc_scheduler_mutex` (I-2: the in-flight window a concurrent FORGET must serialize behind). Empty by
+    /// default; production installs none.
+    void setGcVerbAdmitWindowHookForTest(std::function<void()> fn) { gc_verb_admit_window_hook_for_test = std::move(fn); }
+
+    /// Test-only fault-injection/hook seam for `ContentAddressedTransaction::publishStaging`'s
+    /// promote/repoint call, keyed by the full `(ns, ref)` routed identity via `PartRefKey::cacheKey()`
+    /// (mirrors `CasRefLedger::setRefPreCarveHookForTest`'s no-op-in-production shape) -- a bare ref
+    /// name would misfire across a future multi-namespace fixture where two namespaces coincidentally
+    /// share a ref name. `armPromoteFailureForTest` makes the NEXT `promoteBuild`/`repointRef` call for
+    /// `key` throw instead of committing, modeling a transient promote-time backend failure.
+    /// `setAfterPromoteHookForTest` installs a one-shot callback run synchronously immediately after a
+    /// successful promote/repoint for `key` (before the caller's own post-commit bookkeeping), modeling
+    /// a concurrent writer racing in right after this transaction's confirm -- e.g. repointing the same
+    /// ref to a different manifest so a later rollback's `dropRefIfMatches` must see it changed.
+    void armPromoteFailureForTest(const Cas::PartRefKey & key) { promote_failure_refs_for_test.insert(key.cacheKey()); }
+    bool shouldFailPromoteForTest(const Cas::PartRefKey & key) const { return promote_failure_refs_for_test.contains(key.cacheKey()); }
+    void setAfterPromoteHookForTest(const Cas::PartRefKey & key, std::function<void()> hook)
+    {
+        after_promote_hooks_for_test[key.cacheKey()] = std::move(hook);
+    }
+    /// Invokes and removes `key`'s one-shot hook, if any registered. A no-op when none is installed
+    /// (the production hot path never installs one).
+    void runAfterPromoteHookForTest(const Cas::PartRefKey & key)
+    {
+        auto it = after_promote_hooks_for_test.find(key.cacheKey());
+        if (it == after_promote_hooks_for_test.end())
+            return;
+        auto hook = std::move(it->second);
+        after_promote_hooks_for_test.erase(it);
+        hook();
+    }
+
+private:
+    const ObjectStoragePtr object_storage;
+    const std::string storage_path_prefix;
+    const std::string storage_path_full;
+    const std::string server_id;
+    const std::string server_root_id;
+    const std::string disk_name;
+    const std::string local_scratch_path;
+    const ContextPtr context;
+
+    const bool gc_enabled;
+    const std::chrono::seconds gc_interval;
+    const uint64_t deduplication_cache_bytes;            /// P1 known-present cache byte cap (0=off)
+    const uint64_t deduplication_head_first_min_bytes;   /// P2 HEAD-before-PUT size threshold (0=off)
+    const uint64_t gc_snapshot_generations_to_keep;  /// Number of GC snapshots retained (0 means keep all).
+    const uint64_t gc_shards;                    /// Blob-hash-prefix reducer shard count, fixed at pool creation.
+    const uint64_t manifest_sweep_list_budget_keys;
+    const uint64_t manifest_sweep_delete_budget_keys;
+    const uint64_t gc_round_graduation_budget;
+    const uint64_t gc_round_redelete_budget;
+    const uint64_t gc_round_sweep_namespace_budget;
+    const uint64_t gc_round_sweep_recovery_op_budget;
+    const uint64_t gc_round_ref_cleanup_budget;
+    const uint64_t gc_round_prefix_wholesale_budget;
+    const uint64_t gc_round_handoff_prefix_wholesale_budget;
+    const uint64_t gc_round_outcome_entry_budget;
+    /// GCS single-PUT budget for conditional writes (generation-token stores only): threaded into
+    /// the ObjectStorageBackend construction site in startup(). Irrelevant on ETag stores (AWS et al).
+    const uint64_t gcs_max_conditional_put_bytes;
+    /// Part-folder view cache settings. `cas_part_folder_cache_bytes == 0` disables retention.
+    const uint64_t cas_part_folder_cache_bytes;
+    const uint64_t cas_part_folder_cache_max_entries;
+    const uint64_t cas_part_folder_cache_max_entry_bytes;
+    /// Byte bound for the manifest decode cache in `Cas::Pool`. Zero disables decode caching.
+    const uint64_t manifest_decode_cache_bytes;
+    /// Bounded pool size for GC's per-hash freshness-metadata writes.
+    const uint64_t gc_meta_pool_size;
+    /// Configured staging backend; `Local` preserves the existing write path.
+    const Cas::StagingBackend staging_backend;
+    /// Blob content-hash function passed to `Cas::PoolConfig`.
+    const Cas::BlobHashAlgo blob_hash_algo;
+    /// Whether `blob_hash_algo` may be admitted into the pool's persisted `algos_used` set.
+    const bool blob_hash_allow_new;
+    /// Per-disk `<skip_access_check>` policy passed to `Cas::PoolConfig`.
+    const bool skip_access_check;
+    /// Policy controlling when retained part-folder views revalidate their manifest body.
+    const Cas::PartFolderValidate part_folder_validate;
+    /// Set by the mount-time conditional-copy capability probe — not const because the result is
+    /// unavailable until startup.
+    /// Defaults to false (fail-close): assumed unsupported until the probe proves otherwise.
+    bool conditional_copy_supported = false;
+
+    /// A single coherent snapshot of the pool and its cached part-folder facade, taken under ONE
+    /// `pointer_mutex` acquisition (see `poolAccess()`) so no caller can observe `pool` from one mount
+    /// generation and `part_access` from another -- the two used to be fetched by two separate calls
+    /// to `store()`/`partAccess()` at some call sites, each taking its own `pointer_mutex` snapshot.
+    struct PoolAccessSnapshot
+    {
+        Cas::PoolPtr pool;
+        std::shared_ptr<Cas::CachedPartFolderAccess> part_access;
+    };
+
+    /// Set by startup (Pool::open is fail-closed; empty store == not started). shared_ptr so
+    /// store()/partAccess() can return a by-value snapshot under `pointer_mutex` (see below) instead
+    /// of a reference that could dangle across a concurrent `shutdown` reset.
+    Cas::PoolPtr cas_store TSA_GUARDED_BY(pointer_mutex);
+    /// The part-folder access facade: the normal path
+    /// for committed part/projection reads and committed part-ref mutations. Constructed in
+    /// startup right after Pool::open; reset in shutdown before cas_store. shared_ptr for the same
+    /// snapshot-safety reason as cas_store.
+    std::shared_ptr<Cas::CachedPartFolderAccess> part_access TSA_GUARDED_BY(pointer_mutex);
+    String pool_uuid;
+    /// shared_ptr so `runGarbageCollectionRoundNow`/`runOneGcRoundForTest` can take a snapshot under
+    /// `pointer_mutex`, release it, and run the (long) round via the snapshot -- never holding
+    /// `pointer_mutex` itself for the round's duration, so `gcHealth`/`store`/`partAccess` never
+    /// block behind an in-flight round.
+    std::shared_ptr<Cas::CasGcScheduler> gc_scheduler TSA_GUARDED_BY(pointer_mutex);
+    /// Outermost lock, taken by the lifecycle-control verbs: the FSCK handler, `forgetDisk`, and
+    /// `gcStop`/`gcStart`. Serializes them against each other. Lock order when nested locks are needed:
+    /// `lifecycle_mutex` -> `gc_scheduler_mutex` -> `pointer_mutex`, never the reverse.
+    mutable std::mutex lifecycle_mutex;
+    /// Serializes ONE synchronous GC round at a time and makes `shutdown` wait for an in-flight round
+    /// to finish cleanly (clean GC completion has priority over fast shutdown) -- held for the WHOLE
+    /// round. Deliberately NOT the same mutex as `pointer_mutex` below: this one can be held for a
+    /// long time, so nothing that only needs a brief pointer snapshot may share it.
+    mutable std::mutex gc_scheduler_mutex;
+    bool shutdown_called TSA_GUARDED_BY(gc_scheduler_mutex) = false;
+    /// Guards ONLY reads/writes of `cas_store`/`part_access`/`gc_scheduler` themselves
+    /// (snapshot, create-if-absent, reset) -- always held briefly. Lock ordering when more than one of
+    /// these is needed (the round entry points, `shutdown`, and -- outermost -- `lifecycle_mutex`):
+    /// `lifecycle_mutex` first (if held at all), then `gc_scheduler_mutex`, then `pointer_mutex`
+    /// nested inside, never the reverse.
+    mutable std::mutex pointer_mutex;
+    /// Derived from object_storage->isReadOnly() at startup (the disk's <readonly> config). When set:
+    /// the probe is skipped, no watermark, no GC scheduler, and the mutating surface fails closed.
+    bool read_only = false;
+    /// Joined in front of core keys for DIRECT object_storage reads. The Emulated (Local) backend
+    /// maps bare pool keys under getCommonKeyPrefix; Native passes keys through - this member
+    /// mirrors that rule so readBlobPayload reads exactly where the backend wrote ("" for Native).
+    String physical_key_prefix;
+
+    /// Adds the local-backend common prefix to a pool key when direct object-storage I/O requires
+    /// the physical key; native backends use the key unchanged.
+    String physicalKey(const String & key) const
+    {
+        if (physical_key_prefix.empty())
+            return key;
+        if (physical_key_prefix.back() == '/')
+            return physical_key_prefix + key;
+        return physical_key_prefix + "/" + key;
+    }
+
+    /// The one place that takes a `{pool, facade}` snapshot under a SINGLE `pointer_mutex`
+    /// acquisition. Throws `INVALID_STATE` (via `throwStorageNotStarted`) when no pool is published
+    /// (before the first `startup` or after `shutdown` -- the storage-level Constructing/ShutDown
+    /// lifecycle), then refuses a terminal pool via `throwIfLifecycleTerminal`. `store()`/`partAccess()`
+    /// are thin wrappers over this; every other caller that needs BOTH the pool and the facade for one
+    /// logical operation must call this ONCE and use both fields from the same snapshot, rather than
+    /// calling `store()` and `partAccess()` separately -- otherwise it could straddle a startup/shutdown
+    /// that changes `cas_store`/`part_access`.
+    PoolAccessSnapshot poolAccess() const;
+
+    /// Builds and throws the `INVALID_STATE` "disk is not started" exception `poolAccess()`, the gate,
+    /// and the synchronous GC round entry points throw when no pool is published -- the storage-level
+    /// Constructing (before `startup`) / ShutDown (after `shutdown`) lifecycle. `pool_uuid` (written once
+    /// at the end of a successful `startup`, never reset) distinguishes the two in the message, exactly
+    /// as `lifecycleSnapshot()` reports `constructing`/`shutdown`. A normal, operator-facing refusal,
+    /// never a `LOGICAL_ERROR` (which would abort under debug/ASan builds).
+    [[noreturn]] void throwStorageNotStarted() const;
+
+    /// EMPTY-PROOF RULE (rev.7 spec §1 [B3]): called by `listDirectory` when a `TableDir`/
+    /// `DetachedContainer` enumeration on a NON-terminal (Live or read-only) pool is about to answer
+    /// empty. "Empty at a table root" is exactly what a silently-erased backing looks like, and a
+    /// read-only pool has no lease/observer to detect that erasure any other way (MergeTree skips both
+    /// directory creation and the `format_version.txt` write on a read-only disk) -- enumeration is its
+    /// ONLY line of defense against ATTACHing an empty table over an erased pool. So before answering
+    /// empty, this confirms the pool identity object (`_pool_meta`) with an AUTHORITATIVE, UNCACHED
+    /// probe: `Present` authorizes the empty answer (the pool is genuinely there and genuinely empty);
+    /// `KeyAbsent`/`ContainerAbsent` throw the typed 668 "backing may be erased"; `AccessDenied`/
+    /// `Indeterminate` throw the typed transient 668 (fail-closed, retryable). NEVER reached on a
+    /// `Vanished` pool -- `checkOpAdmitted`'s `Probe`->`TruthAbsent` short-circuit answers truth-empty
+    /// before any classification runs, so the terminal path never pays the probe. Cost: one extra backend
+    /// HEAD per EMPTY table-root enumeration only (attach/load-time); the non-empty hot path is untouched.
+    void confirmPoolIdentityForEmptyEnumeration(const std::string & path) const;
+
+    /// A pool opened as a standalone, UNPUBLISHED view: never touches `cas_store`/`part_access`/
+    /// `gc_scheduler` -- the caller owns it entirely and drops it when done.
+    struct PoolView
+    {
+        Cas::PoolPtr pool;
+        /// The direct-object-storage key prefix for this view's backend (see `physicalKey`'s own
+        /// doc comment): populated for the Emulated (Local) backend, empty ("") for Native. Returned
+        /// rather than written to the `physical_key_prefix` member so this helper stays side-effect-free
+        /// and callable from a `const` method (`runFsckNow`).
+        String physical_key_prefix;
+        /// The resolved (bucket-relative, trailing-slash-trimmed) pool prefix passed into
+        /// `Cas::PoolConfig::pool_prefix` -- `startup()`'s S3-staging capability probe below needs the
+        /// SAME resolved value to build its own probe key, so it is returned here rather than
+        /// recomputed a second time.
+        String pool_prefix;
+    };
+
+    /// Builds the backend + `Cas::PoolConfig` and opens a pool exactly as `startup()` does. A read-only
+    /// (`<readonly>`) disk opens with no write probe, no background watermark, and no GC scheduler. Never
+    /// touches `cas_store`/`part_access`/`gc_scheduler`/`physical_key_prefix`/`pool_uuid`/
+    /// `conditional_copy_supported`; `startup()` applies its own result to those members itself, in its
+    /// single publish step.
+    PoolView openPoolView() const;
+
+    /// Classifies `path`'s directory shape by running the fixed dispatch order once (shadow ->
+    /// atomic-shard -> table-uuid -> part -> subdir -> generic), including the part-branch
+    /// fall-through when no sub-shape matches. Pure path classification — consults no lifecycle
+    /// state (e.g. `readableNamespaceFilesLife`); `existsDirectory`/`listDirectory` apply that gate
+    /// themselves in their per-shape arms, exactly as before this refactor.
+    DirRoute classifyDirectory(const std::string & path) const;
+
+    /// Build the GC round sink: the std::function the scheduler calls per Start/Finish. Captures the
+    /// ContextPtr, converts the POD GcRoundLogRecord into a ContentAddressedGarbageCollectionLogElement,
+    /// and appends it to the SystemLog (best-effort). Returns an empty sink when context is null.
+    Cas::GcRoundLogger makeGcRoundLogger() const;
+
+    /// Builds the per-event CAS audit sink: the `std::function` the pool calls on every
+    /// content-addressed decision. Captures the ContextPtr, converts the decoupled Core POD
+    /// `Cas::CasEvent` into a ContentAddressedLogElement, and appends it to the SystemLog
+    /// (best-effort). Returns an empty sink when context is null (unit tests).
+    Cas::CasEventSink makeCasEventSink() const;
+
+    /// Backing state for the `*ForTest` promote fault-injection/hook seam declared above. Empty in
+    /// production (no test ever arms them); consulted only by `ContentAddressedTransaction::publishStaging`.
+    std::unordered_set<std::string> promote_failure_refs_for_test;
+    std::unordered_map<std::string, std::function<void()>> after_promote_hooks_for_test;
+
+    /// Backing state for the EMPTY-PROOF RULE `*ForTest` seams (Task 9), declared above. The counter is
+    /// bumped on every empty-proof probe; the override, when set, replaces the real backend probe. Both
+    /// are inert in production (no test ever sets the override; the counter is write-only there).
+    mutable std::atomic<uint64_t> empty_proof_probe_count_for_test{0};
+    std::function<Cas::SentinelProbeResult()> empty_proof_probe_override_for_test;
+
+    /// Backing state for the `setGcVerbAdmitWindowHookForTest` seam declared above (the I-1/I-2 admission
+    /// TOCTOU tests). Empty in production; a `const` GC verb reads it and calls the const-qualified
+    /// `std::function::operator()`, so it needs no `mutable`.
+    std::function<void()> gc_verb_admit_window_hook_for_test;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.cpp
new file mode 100644
index 000000000000..a4112fd57cb3
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.cpp
@@ -0,0 +1,214 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h>
+
+#include <Core/BaseSettings.h>
+#include <Core/BaseSettingsFwdMacrosImpl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Poco/Util/AbstractConfiguration.h>
+#include <Common/Exception.h>
+#include <filesystem>
+#include <set>
+
+namespace fs = std::filesystem;
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int NO_ELEMENTS_IN_CONFIG;
+}
+
+/// The `cas` disk block is shared with the generic object-storage/disk layer, whose
+/// keys are consumed elsewhere (`ObjectStorageFactory`, `S3Settings`, `MetadataStorageFactory`'s
+/// `getObjectKeyCompatiblePrefix`, `IDisk`, `DiskFromAST` for the inline SQL `disk(...)` form,
+/// `RegisterDiskObjectStorage`'s fake-transaction gate) and must be skipped here rather than rejected
+/// as unknown. As of 2026-07-21 this list covers ALL in-repo CAS disk configs and inline `disk(...)`
+/// definitions, enumerated from four sources, each filtered down to the keys that are direct
+/// children of an actual `metadata_type=cas` disk block (the raw `rg -o` output below
+/// also contains disk-name and policy/volume wrapper tags, which are not config keys at all):
+///   1) `rg -o "<([a-z_0-9]+)>" -r '$1' utils/ca-soak/configs/storage_conf*.xml utils/ca-soak/configs/storage_overrides*.xml`
+///   2) every CAS integration-test disk config, both the `storage_conf.xml` bodies and the
+///      per-node `server_root_id_node*.xml` overrides (`git grep -l content_addressed
+///      tests/integration | grep -E '\.xml$'`) -- and every integration `test.py`
+///      (`git ls-files 'tests/integration/*/test.py' | xargs grep -l
+///      'metadata_type.*content_addressed'`) confirmed EMPTY: no integration test builds a CAS disk
+///      via inline SQL `disk(...)`, only via these XML configs.
+///   3) every CAS disk config under `tests/config/config.d/cas_*.xml` (the
+///      stateless-lane XML configs) -- these are the only place in the tree with a LOCAL
+///      `object_storage_type` CAS disk, so they are the only source of the generic `path` key below.
+///   4) every inline `disk(...)` SQL construct across ALL `cas_*`
+///      stateless tests, `04278`-`04300` (pre-dating this settings struct) through `05002`-`05015`
+///      (current) -- these supply `name` (read by `DiskFromAST` to derive the ad-hoc disk's name)
+///      and `use_fake_transaction` (validated generically in `RegisterDiskObjectStorage.cpp` against
+///      EVERY metadata type that needs a real transaction, not a CAS-specific check -- exercised by
+///      `05015_cas_reject_fake_transaction` deliberately setting it to assert the REJECTION, which
+///      needs the key to reach that check rather than being rejected earlier as unknown). The
+///      `04278`-`04300` range turned up no keys beyond what `05002`-`05015` already required.
+/// Any new CAS config FAMILY -- a new XML config directory or a new inline-`disk()` test pattern --
+/// added to the tree needs the same four-way scan repeated against it and this note updated.
+/// `skip_access_check` is deliberately NOT in this set: it is registered as a CAS setting
+/// below (the same config key also has meaning to `IDisk::startupImpl`, which drops it before
+/// `metadata_storage->startup()` runs, but that does not make it foreign here).
+static const std::set<std::string> non_cas_keys = {
+    "type", "object_storage_type", "metadata_type", "path", "name", "use_fake_transaction",
+    "endpoint", "access_key_id", "secret_access_key", "region", "use_environment_credentials",
+    "readonly", "expect_continue_min_bytes", "http_client", "key_compatibility_prefix",
+};
+
+/// Config-key convention: the disk block already scopes every key to this disk, so no
+/// key below carries a redundant `cas_`/`ca_` prefix (e.g. `part_folder_cache_bytes`, not
+/// `cas_part_folder_cache_bytes`).
+#define LIST_OF_CONTENT_ADDRESSED_SETTINGS(DECLARE, ALIAS) \
+    DECLARE(String, scratch_path, "", "Server-local scratch dir for the write-buffer spill; a relative value is anchored to the server data path", 0) \
+    DECLARE(Bool,   gc_enabled, true, "Run the background GC scheduler on this disk", 0) \
+    DECLARE(UInt64, gc_interval_sec, 60, "Seconds between background GC rounds (>= 1)", 0) \
+    DECLARE(String, blob_hash, "cityhash128", "Pool blob content-hash function (cityhash128 | xxh3-128 | sha256); fixed at pool creation", 0) \
+    DECLARE(Bool,   blob_hash_allow_new, false, "Explicit opt-in to admit a NEW hash algo into an existing pool's algos_used", 0) \
+    DECLARE(Bool,   skip_access_check, false, "Skip the boot-time capability probe (start now, fix later)", 0) \
+    DECLARE(UInt64, deduplication_cache_bytes, 64ULL << 20, "Byte budget of the blob presence cache (0 disables)", 0) \
+    DECLARE(UInt64, deduplication_head_first_min_bytes, 1ULL << 20, "Minimum blob size to try a HEAD before uploading the body", 0) \
+    DECLARE(UInt64, gc_snapshot_generations_to_keep, 3, "GC snapshot generations retained", 0) \
+    DECLARE(UInt64, gc_shards, 1, "Blob-hash-prefix reducer shards (>= 1); creation-time only", 0) \
+    DECLARE(UInt64, manifest_sweep_list_budget_keys, 1000, "Orphan-manifest sweep LIST budget per round", 0) \
+    DECLARE(UInt64, manifest_sweep_delete_budget_keys, 100, "Orphan-manifest sweep DELETE budget per round", 0) \
+    DECLARE(UInt64, gc_round_graduation_budget, 5000, "Blob graduation (condemned -> delete_pending) cohort cap per round (0 = unbounded)", 0) \
+    DECLARE(UInt64, gc_round_redelete_budget, 5000, "Blob redelete (exact-token delete of a prior delete_pending row) cohort cap per round (0 = unbounded)", 0) \
+    DECLARE(UInt64, gc_round_sweep_namespace_budget, 20, "Orphan-manifest sweep: distinct namespaces per page whose protection view may be built (0 = unbounded)", 0) \
+    DECLARE(UInt64, gc_round_sweep_recovery_op_budget, 5000, "Orphan-manifest sweep: committed-tail ref-log GET/decode ops the recovery walk may spend per round (0 = unbounded)", 0) \
+    DECLARE(UInt64, gc_round_ref_cleanup_budget, 5000, "Ref-object cleanup (covered log/snapshot deletes) cap per round (0 = unbounded)", 0) \
+    DECLARE(UInt64, gc_round_prefix_wholesale_budget, 20000, "Generation-prefix wholesale delete (prune only) object cap per round (0 = unbounded)", 0) \
+    DECLARE(UInt64, gc_round_handoff_prefix_wholesale_budget, 5000, "Post-CAS hand-off generation-prefix reclaim object cap per round, reserved separately from gc_round_prefix_wholesale_budget so a prune-heavy round cannot starve the one-shot hand-off (0 = unbounded)", 0) \
+    DECLARE(UInt64, gc_round_outcome_entry_budget, 5000, "GcOutcomes per-round entry cap across the redelete/spared audit log (0 = unbounded)", 0) \
+    DECLARE(String, server_root_id, "", "REQUIRED explicit layout subtree identity; macros expand as in the s3 endpoint", 0) \
+    DECLARE(UInt64, gcs_max_conditional_put_bytes, 1ULL << 30, "GCS single-PUT budget for conditional writes (generation-token stores only)", 0) \
+    DECLARE(UInt64, part_folder_cache_bytes, 64ULL << 20, "Part-folder view cache byte budget (0 disables retention)", 0) \
+    DECLARE(UInt64, part_folder_cache_max_entries, 10000, "Part-folder view cache entry cap", 0) \
+    DECLARE(UInt64, part_folder_cache_max_entry_bytes, 16ULL << 20, "Oversized part-folder views bypass retention above this size", 0) \
+    DECLARE(String, part_folder_validate, "always", "ForceFresh body re-proof policy (always | never | age <seconds>)", 0) \
+    DECLARE(UInt64, manifest_decode_cache_bytes, 128ULL << 20, "Manifest DECODE cache byte budget (0 disables)", 0) \
+    DECLARE(UInt64, gc_meta_pool_size, 16, "Bounded pool size for GC per-hash freshness-meta writes", 0) \
+    DECLARE(String, staging_backend, "local", "Blob staging backend (local | s3); s3 is opt-in", 0) \
+
+DECLARE_SETTINGS_TRAITS(ContentAddressedSettingsTraits, LIST_OF_CONTENT_ADDRESSED_SETTINGS, CONTENT_ADDRESSED_SETTINGS_SUPPORTED_TYPES)
+
+struct ContentAddressedSettingsImpl : public BaseSettings<ContentAddressedSettingsTraits>
+{
+    /// Parsed by `validate` from the corresponding string setting; cached here (rather than
+    /// re-parsed on every access) because the public header only forward-declares
+    /// `Cas::StagingBackend` / `Cas::PartFolderValidate` and cannot store them by value.
+    Cas::BlobHashAlgo blob_hash_algo_cached = Cas::BlobHashAlgo::CityHash128;
+    Cas::StagingBackend staging_backend_cached = Cas::StagingBackend::Local;
+    Cas::PartFolderValidate part_folder_validate_cached{};
+};
+
+IMPLEMENT_SETTINGS_TRAITS_CUSTOM_IMPL(ContentAddressedSettingsTraits, LIST_OF_CONTENT_ADDRESSED_SETTINGS, ContentAddressedSettings, ContentAddressedSetting)
+
+ContentAddressedSettings::ContentAddressedSettings() : impl(std::make_unique<ContentAddressedSettingsImpl>())
+{
+}
+
+ContentAddressedSettings::~ContentAddressedSettings() = default;
+
+CONTENT_ADDRESSED_SETTINGS_SUPPORTED_TYPES(ContentAddressedSettings, IMPLEMENT_SETTING_SUBSCRIPT_OPERATOR)
+
+ContentAddressedSettings::ContentAddressedSettings(const ContentAddressedSettings & settings)
+    : impl(std::make_unique<ContentAddressedSettingsImpl>(*settings.impl))
+{
+}
+
+void ContentAddressedSettings::loadFromConfig(
+    const Poco::Util::AbstractConfiguration & config,
+    const std::string & config_prefix,
+    const std::string & scratch_path_anchor_if_relative,
+    const std::string & default_scratch_path,
+    const MacroExpander & expand_macros)
+{
+    Poco::Util::AbstractConfiguration::Keys config_keys;
+    config.keys(config_prefix, config_keys);
+
+    for (const std::string & key : config_keys)
+    {
+        if (non_cas_keys.contains(key))
+            continue;
+        impl->set(key, config.getString(config_prefix + "." + key));
+    }
+
+    auto & settings = *this;
+
+    /// Server-local scratch dir for the write-buffer spill. Mirrors how other metadata storages
+    /// compute their local working dir: a real filesystem path, NEVER the object-storage key
+    /// prefix. A configured RELATIVE scratch path is anchored to `scratch_path_anchor_if_relative`
+    /// (the caller-provided server data path), NOT the process CWD (which varies by launch method)
+    /// and NOT `default_scratch_path` -- that default is itself a per-disk subdirectory of the
+    /// server data path (`.../disks/<name>/cas_scratch/`), so anchoring a relative override to it
+    /// instead of to the server data path directly would silently nest the override two levels
+    /// deeper than intended (review finding: this is exactly the pre-existing factory's anchor,
+    /// which callers already depend on in shipped configs). The default is already absolute; only
+    /// an explicit relative override needs anchoring, and only to the server-data-path anchor.
+    if (settings[ContentAddressedSetting::scratch_path].changed)
+    {
+        if (fs::path(settings[ContentAddressedSetting::scratch_path].value).is_relative())
+            settings[ContentAddressedSetting::scratch_path] = (fs::path(scratch_path_anchor_if_relative) / settings[ContentAddressedSetting::scratch_path].value).string();
+    }
+    else
+    {
+        settings[ContentAddressedSetting::scratch_path] = default_scratch_path;
+    }
+
+    /// Phase 0 (mount safety): macros expand here exactly as in the s3 `endpoint`
+    /// (`ObjectStorageFactory`): on a multi-replica stand every replica mounts ONE shared pool
+    /// (same endpoint) and must own a DISTINCT subtree, so the natural single-template config is
+    /// `<server_root_id>{replica}</server_root_id>`. An unknown macro throws (fail closed, via the
+    /// caller-supplied `expand_macros`). Gated on `.changed`: assigning unconditionally would mark the
+    /// field changed even when the key was ABSENT from config, defeating `validate`'s `.changed` check
+    /// below (the ABSENT-vs-invalid `NO_ELEMENTS_IN_CONFIG`-vs-`BAD_ARGUMENTS` distinction) -- a missing
+    /// key must reach `validate` still unchanged, not silently expanded-in-place to the same empty string.
+    if (settings[ContentAddressedSetting::server_root_id].changed)
+        settings[ContentAddressedSetting::server_root_id] = expand_macros(settings[ContentAddressedSetting::server_root_id].value);
+
+    validate();
+}
+
+void ContentAddressedSettings::validate()
+{
+    auto & settings = *this;
+
+    if (settings[ContentAddressedSetting::gc_interval_sec] == 0 || settings[ContentAddressedSetting::gc_shards] == 0)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "content_addressed disk: gc_interval_sec and gc_shards must be >= 1 (got {}, {})",
+            settings[ContentAddressedSetting::gc_interval_sec].value, settings[ContentAddressedSetting::gc_shards].value);
+
+    /// The layout subtree identity is explicit and REQUIRED — no default, so an ABSENT key throws a
+    /// typed `NO_ELEMENTS_IN_CONFIG` (mirroring the `metadata_type` check in `MetadataStorageFactory`),
+    /// distinct from a PRESENT-but-invalid value, which falls through to `validateServerRootId`'s
+    /// `BAD_ARGUMENTS` below. `.changed` is exactly "the config had this key" (or a caller set it
+    /// explicitly via the subscript operator); an unset field never reaches here as anything but empty.
+    if (!settings[ContentAddressedSetting::server_root_id].changed)
+        throw Exception(ErrorCodes::NO_ELEMENTS_IN_CONFIG,
+            "Expected `server_root_id` in config for a content-addressed disk");
+
+    Cas::validateServerRootId(settings[ContentAddressedSetting::server_root_id].value);
+
+    impl->blob_hash_algo_cached = Cas::parseBlobHashAlgo(settings[ContentAddressedSetting::blob_hash].value);
+    impl->staging_backend_cached = ContentAddressedMetadataStorage::parseStagingBackend(settings[ContentAddressedSetting::staging_backend].value);
+    impl->part_folder_validate_cached = ContentAddressedMetadataStorage::parsePartFolderValidate(settings[ContentAddressedSetting::part_folder_validate].value);
+}
+
+Cas::BlobHashAlgo ContentAddressedSettings::blobHashAlgo() const
+{
+    return impl->blob_hash_algo_cached;
+}
+
+Cas::StagingBackend ContentAddressedSettings::stagingBackend() const
+{
+    return impl->staging_backend_cached;
+}
+
+Cas::PartFolderValidate ContentAddressedSettings::partFolderValidate() const
+{
+    return impl->part_folder_validate_cached;
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h
new file mode 100644
index 000000000000..09f189c0d9e2
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h
@@ -0,0 +1,89 @@
+#pragma once
+
+#include <Core/BaseSettingsFwdMacros.h>
+#include <Core/SettingsFields.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <functional>
+#include <memory>
+#include <string>
+
+namespace Poco { namespace Util { class AbstractConfiguration; } } // NOLINT(cppcoreguidelines-virtual-class-destructor)
+
+namespace DB::Cas
+{
+/// Forward declared to keep this header light: the full definitions live in
+/// `ContentAddressedMetadataStorage.h` (`StagingBackend`) and `Parts/PartFolderAccess.h`
+/// (`PartFolderValidate`), which are heavy and — in `ContentAddressedMetadataStorage.h`'s
+/// case — will itself include this header once the metadata storage is rewired onto it.
+/// Both are legal opaque declarations: `StagingBackend` fixes no explicit underlying type
+/// (matching its definition, which leaves it as the implicit `int`), and `PartFolderValidate`
+/// is only ever used here as an incomplete-type function return, never stored by value.
+enum class StagingBackend;
+struct PartFolderValidate;
+}
+
+namespace DB
+{
+struct ContentAddressedSettingsImpl;
+
+/// Resolves `{macro}` placeholders in a config value, e.g. `server_root_id`. Kept as a
+/// type-erased callback (rather than a `Context`/`Macros` reference) so this header stays free
+/// of `Interpreters/Context` — settings loading has no business knowing about the query context.
+using MacroExpander = std::function<std::string(const std::string &)>;
+
+#define CONTENT_ADDRESSED_SETTINGS_SUPPORTED_TYPES(CLASS_NAME, M) \
+    M(CLASS_NAME, String) \
+    M(CLASS_NAME, Bool) \
+    M(CLASS_NAME, UInt64)
+
+CONTENT_ADDRESSED_SETTINGS_SUPPORTED_TYPES(ContentAddressedSettings, DECLARE_SETTING_TRAIT)
+
+/// Declarative settings for the `cas` disk metadata storage, mirroring the
+/// `FileCacheSettings` pimpl/traits shape (`Core/BaseSettings.h`). Replaces the ~25 inline
+/// `config.getX` calls that used to live in `MetadataStorageFactory.cpp`'s
+/// `registerContentAddressedMetadataStorage` lambda; that lambda's key names, defaults, and
+/// per-key rationale are the authoritative source for `LIST_OF_CONTENT_ADDRESSED_SETTINGS`.
+struct ContentAddressedSettings
+{
+    ContentAddressedSettings();
+    ContentAddressedSettings(const ContentAddressedSettings &);
+    ~ContentAddressedSettings();
+
+    CONTENT_ADDRESSED_SETTINGS_SUPPORTED_TYPES(ContentAddressedSettings, DECLARE_SETTING_SUBSCRIPT_OPERATOR)
+
+    /// Loads every key under `config_prefix`, rejecting unknown non-object-storage keys (fail
+    /// closed, mirrors `FileCacheSettings::loadFromConfig`'s `non_cache_keys` skip-set). A missing
+    /// `scratch_path` defaults to `default_scratch_path`; a relative OVERRIDE is anchored to
+    /// `scratch_path_anchor_if_relative` instead (never the process CWD, and never
+    /// `default_scratch_path` -- that default is a per-disk subdirectory, e.g.
+    /// `<server-data-path>/disks/<name>/cas_scratch/`, and anchoring a relative override to it would
+    /// silently nest the override two levels deeper than the server data path the operator meant;
+    /// mirrors `FileCacheSettings::loadFromConfig`'s `cache_path_prefix_if_relative` /
+    /// `default_cache_path` split). `server_root_id` is passed through `expand_macros` before
+    /// `validate` runs. Ends by calling `validate`.
+    void loadFromConfig(
+        const Poco::Util::AbstractConfiguration & config,
+        const std::string & config_prefix,
+        const std::string & scratch_path_anchor_if_relative,
+        const std::string & default_scratch_path,
+        const MacroExpander & expand_macros);
+
+    /// Fail-closed checks: `gc_interval_sec` and `gc_shards` must both be >= 1; `server_root_id` must
+    /// be present (an ABSENT key throws a typed `NO_ELEMENTS_IN_CONFIG`, distinct from a
+    /// PRESENT-but-invalid value, which throws `Cas::validateServerRootId`'s `BAD_ARGUMENTS`); and the
+    /// three enum-valued string settings (`blob_hash`, `staging_backend`, `part_folder_validate`) must
+    /// parse. The parsed enum values are cached for the typed accessors below.
+    void validate();
+
+    /// Typed accessors for the enum-valued string settings, parsed and cached by `validate`.
+    Cas::BlobHashAlgo blobHashAlgo() const;
+    Cas::StagingBackend stagingBackend() const;
+    Cas::PartFolderValidate partFolderValidate() const;
+
+private:
+    /// The parsed enum values live inside `impl` (defined in the .cpp, where the forward-declared
+    /// `Cas::StagingBackend` / `Cas::PartFolderValidate` types are complete), not as members here.
+    std::unique_ptr<ContentAddressedSettingsImpl> impl;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.cpp
new file mode 100644
index 000000000000..de00857ce488
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.cpp
@@ -0,0 +1,1966 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <IO/ReadBufferFromFile.h>
+#include <IO/ReadBufferFromFileView.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/WriteBufferFromFile.h>
+#include <IO/copyData.h>
+#include <Disks/IO/WriteBufferWithFinalizeCallback.h>
+#include <Disks/IDiskTransaction.h>
+#include <Common/thread_local_rng.h>
+#include <Common/config_version.h>
+#include <algorithm>
+#include <filesystem>
+#include <unordered_set>
+#include <Common/Exception.h>
+#include <Common/HashTable/Hash.h>
+#include <Common/getRandomASCIIString.h>
+#include <Common/logger_useful.h>
+#include <Common/ProfileEvents.h>
+#include <Common/ThreadPool.h>
+#include <Common/setThreadName.h>
+#include <Common/threadPoolCallbackRunner.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
+#include <base/hex.h>
+#include <base/scope_guard.h>
+#include <ctime>
+#include <map>
+#include <span>
+#include <vector>
+
+namespace ProfileEvents
+{
+    extern const Event CASBlobUploadFanoutBatches;
+    extern const Event CASBlobUploadFanoutTasks;
+}
+
+namespace fs = std::filesystem;
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int ABORTED;
+    extern const int FILE_DOESNT_EXIST;
+    extern const int LOGICAL_ERROR;
+    extern const int NOT_IMPLEMENTED;
+}
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+
+bool hasSuffix(std::string_view s, std::string_view suffix)
+{
+    return s.size() >= suffix.size() && s.substr(s.size() - suffix.size()) == suffix;
+}
+
+}
+
+bool partFileMustStayBlob(std::string_view file_name)
+{
+    if (file_name == "primary.idx")
+        return true;
+    for (std::string_view suffix : {".bin", ".mrk", ".mrk2", ".mrk3", ".cmrk", ".cmrk2", ".cmrk3"})
+        if (hasSuffix(file_name, suffix))
+            return true;
+    return false;
+}
+
+}
+
+namespace DB
+{
+
+namespace
+{
+
+[[noreturn]] void notYet(const char * op)
+{
+    /// These operations are part of the generic disk-transaction interface but have no
+    /// content-addressed equivalent or are not wired for this storage yet. Keep the message
+    /// self-explanatory because it is visible to operators.
+    throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+        "The operation '{}' is not implemented for a content-addressed disk: it belongs to the "
+        "generic disk-transaction surface that the content-addressed write path does not use. "
+        "Hitting it usually means the disk is wrapped by a layer that bypasses the "
+        "content-addressed write path.", op);
+}
+
+/// Inline candidates above this size spill to a blob instead of riding the tree object — a tuning
+/// knob (could become a disk setting later). Keeps the tree object bounded against an unexpectedly
+/// large eager file.
+constexpr size_t INLINE_CAP = 1024 * 1024;   /// 1 MiB
+
+}
+
+ContentAddressedTransaction::ContentAddressedTransaction(ContentAddressedMetadataStorage & metadata_storage_)
+    : metadata_storage(metadata_storage_)
+{
+}
+
+ContentAddressedTransaction::~ContentAddressedTransaction()
+{
+    /// Always clean up pending staging (whether committed or not). On the success path
+    /// cleanupPendingTempFiles was already called at the end of commit(); this call is the defensive
+    /// backstop for aborted/exception-unwound transactions whose publishStaging never ran.
+    cleanupPendingTempFiles();
+
+    /// An uncommitted transaction's uploads become min_active-spared debris: abandon every
+    /// still-open PartWriteTxn so its build_seq is retired. This replaces the former pin machinery.
+    if (committed)
+        return;
+
+    /// No refs are published before `commit`; moving a part from a temporary to a final path is only
+    /// a re-key in this overlay. An abandoned transaction therefore has no early-published ref to
+    /// compensate for; it only needs to abandon still-open builds below.
+    for (auto & [key, st] : parts)
+    {
+        if (!st.build)
+            continue;
+        try
+        {
+            st.build->abandon();
+        }
+        catch (...)
+        {
+            /// A destructor must not throw. But a failed abandon can leave a LIVE-epoch precommit
+            /// binding that neither GC nor the (prior-epoch-scoped) stale-precommit sweep reclaims
+            /// until this mount remounts -- that must be diagnosable, not silently swallowed.
+            tryLogCurrentException(getLogger("ContentAddressedTransaction"),
+                                   "abandoning a build during transaction destruction "
+                                   "(a live precommit binding may persist until remount)");
+        }
+    }
+}
+
+ContentAddressedTransaction::PartStaging &
+ContentAddressedTransaction::stagingFor(const ContentAddressedMetadataStorage::Route & r)
+{
+    return parts[{r.ns.string(), r.ref}];
+}
+
+Cas::PartWriteTxn & ContentAddressedTransaction::buildFor(
+    const ContentAddressedMetadataStorage::Route & r, PartStaging & st)
+{
+    if (!st.build)
+        st.build = metadata_storage.store()->beginPartWrite(
+            Cas::PartWriteInfo{.intended_ref = r.ns.string() + "/" + r.ref,
+                           .intended_namespace = r.ns, .op = Cas::ProvenanceOp::Insert});
+    return *st.build;
+}
+
+ContentAddressedTransaction::PartStaging * ContentAddressedTransaction::findStaging(
+    const ContentAddressedMetadataStorage::Route & r)
+{
+    auto it = parts.find({r.ns.string(), r.ref});
+    return it == parts.end() ? nullptr : &it->second;
+}
+
+void ContentAddressedTransaction::cleanupPendingTempFiles() noexcept
+{
+    for (auto & [key, st] : parts)
+    {
+        for (const auto & pb : st.pending_blobs)
+        {
+            if (pb.backend == Cas::StagingBackend::Local)
+            {
+                std::error_code ec;
+                std::filesystem::remove(pb.staging_key, ec);
+            }
+            else if (committed)
+            {
+                /// A successful commit deletes the S3 staging object
+                /// HERE — `committed` is only ever true when EVERY part's `publishStaging` ran to
+                /// completion (commit() sets it right before this call), which means every referenced
+                /// pending blob was already promoted (`PartWriteTxn::putBlob` → `promoteStaged`/`resurrect`)
+                /// or, for an orphaned pending blob (its entry removed by `unlinkFile`/`replaceFile`
+                /// before commit), was never going to be promoted at all — either way the staging object
+                /// is no longer needed as a resurrect source, so it is safe to reclaim now.
+                ///
+                /// An ABORTED/exception-unwound transaction (`committed == false`, including a partial
+                /// multi-part commit failure where an EARLIER part's blobs were already promoted) leaves
+                /// its S3 staging objects in place — `staging_key` is a remote object-storage key, never a
+                /// bare `fs::remove` target, and is reclaimed by the mount-lease-scoped sweeper
+                /// (`Cas::sweepOwnMountStaging`), never here. This mirrors the local path's own asymmetry:
+                /// `Local` staging is a private per-transaction scratch file removed unconditionally on
+                /// both commit and abort (nobody else can ever read it), whereas an `S3` staging object
+                /// is the sanctioned resurrect source for the promote gate and must outlive an aborted
+                /// transaction so a later attempt (or the sweeper) can still account for it.
+                try
+                {
+                    metadata_storage.objectStorage()->removeObjectIfExists(StoredObject(pb.staging_key));
+                }
+                catch (...) // NOLINT(bugprone-empty-catch)
+                {
+                    /// Best-effort (noexcept context): a stubborn delete just leaves debris for the
+                    /// mount-lease sweeper to reclaim on a later mount.
+                }
+            }
+            /// else: an S3-mode pending blob of an ABORTED transaction — intentionally left in place
+            /// (see above); the mount-lease sweeper (`Cas::sweepOwnMountStaging`) is its reclaimer.
+        }
+        st.pending_blobs.clear();
+    }
+}
+
+const ContentAddressedTransaction::PartStaging::PendingBlob *
+ContentAddressedTransaction::findPendingBlob(const PartStaging & st, const Cas::BlobRef & ref) const
+{
+    /// Locate a pending blob by ref. Returns nullptr when the blob has already been uploaded
+    /// (post-precommit, pending_blobs is cleared) or was never staged as pending.
+    for (const auto & pb : st.pending_blobs)
+        if (pb.ref == ref)
+            return &pb;
+    return nullptr;
+}
+
+void ContentAddressedTransaction::adoptStagedBlob(
+    const PartStaging::PendingBlob * pb, const Cas::ManifestEntry & entry,
+    PartStaging & dst_st, Cas::PartWriteTxn & dst_build, bool copy_pending)
+{
+    if (pb)
+    {
+        /// Pending blob (not yet uploaded): record a tokenless dependency without any pool operation.
+        /// If copy_pending, push a copy of the pb record into dst_st so publishStaging uploads it
+        /// for the dst part too (hardlink = copy semantics). If !copy_pending, the record is already
+        /// in dst_st (moved by caller) — skip the push.
+        if (copy_pending)
+            dst_st.pending_blobs.push_back(*pb);
+        dst_build.recordPendingBlobDep(entry.ref, entry.blob_size);
+    }
+    else
+    {
+        /// Uploaded / committed: record a tokenless W-EVIDENCE dep — no pool HEAD/GET before precommit.
+        /// §4 manifest-trust: the publish gate (promote) TRUSTS this committed-source adopted leaf via the
+        /// durable manifest edge — it does NOT observe/resurrect it. Only tokened / pending-upload leaves
+        /// are resurrected (by putBlob, before promote); a genuinely-absent adopted blob is an fsck finding.
+        dst_build.adoptEvidence(entry);
+    }
+}
+
+std::optional<ContentAddressedMetadataStorage::Route>
+ContentAddressedTransaction::routeOf(const std::string & path) const
+{
+    auto p = Cas::parsePartFilePath(path);
+    if (!p)
+        return std::nullopt;
+    return metadata_storage.route(*p);
+}
+
+void ContentAddressedTransaction::uploadPendingBlobs(PartStaging & st)
+{
+    /// Build the set of blob hashes actually referenced by the staged manifest. Only Blob
+    /// entries represent pending content uploads — Inline are not pending blobs. A pending_blob whose
+    /// hash is NOT in this set had its entry removed by unlinkFile/replaceFile and must not be uploaded
+    /// (it is an orphan). Its temp file is still cleaned by cleanupPendingTempFiles at commit end.
+    std::unordered_set<Cas::BlobRef, Cas::BlobRefHash> referenced_hashes;
+    for (const auto & entry : st.entries)
+        if (entry.placement == Cas::EntryPlacement::Blob)
+            referenced_hashes.insert(entry.ref);
+
+    /// Build one upload request per referenced pending blob. Duplicate refs (staged-hardlink copies push
+    /// a copy of the record) are collapsed by `fanOutBlobUploads`' grouping, which SUBSUMES the former
+    /// duplicate-membership filter here — the fan-out launches one task per unique ref and merges one
+    /// dep. The upload primitive differs by staging backend exactly as before:
+    ///   - `Cas::StagingBackend::Local`: `open` re-reads the local staged temp file and streams
+    ///     it into a write-once `putIfAbsentStream` create; the local-staging path remains byte-for-byte
+    ///     compatible with its previous behavior.
+    ///   - `Cas::StagingBackend::S3`: the bytes already live in an S3 staging object (`pb.staging_key`);
+    ///     `server_side_copy_from` drives a WRITE-ONCE conditional SERVER-SIDE COPY (and an unconditional
+    ///     resurrect copy FROM the staging object for a condemned incarnation). No local read-back —
+    ///     `open` is left unset.
+    std::vector<Cas::BlobUploadRequest> requests;
+    requests.reserve(st.pending_blobs.size());
+    for (const auto & pb : st.pending_blobs)
+    {
+        if (!referenced_hashes.contains(pb.ref))
+            continue;   /// The entry was removed by unlinkFile/replaceFile; skip this orphan.
+        Cas::BlobSource source;
+        source.size = pb.size;
+        if (pb.backend == Cas::StagingBackend::S3)
+        {
+            source.server_side_copy_from = pb.staging_key;
+        }
+        else
+        {
+            const std::string staging_key = pb.staging_key;
+            source.open = [staging_key]() -> std::unique_ptr<ReadBuffer>
+            {
+                return std::make_unique<ReadBufferFromFile>(staging_key);
+            };
+        }
+        /// `declared_size` mirrors `source.size` (both are `pb.size`); the fan-out fail-closes if they
+        /// ever diverge, so build them together from the one authority.
+        requests.push_back(Cas::BlobUploadRequest{pb.ref, std::move(source), pb.size});
+    }
+
+    if (requests.empty())
+        return;
+
+    /// Fan out the uploads on the server-wide pool (spec §1). The fan-out enforces
+    /// one-task-per-unique-ref grouping, the merge-nothing failure contract, and merges every result
+    /// into `st.build`'s dep set on THIS (the owning writer) thread after the join.
+    Cas::fanOutBlobUploads(*st.build, requests, Cas::blobUploadPool());
+}
+
+void ContentAddressedTransaction::publishStaging(const Cas::RootNamespace & ns, const std::string & ref, PartStaging & st,
+                                                 std::optional<Cas::CommitOutcome> & out_slot)
+{
+    if (st.published)
+        return;   /// this staging was already published earlier in this commit loop — never re-publish
+
+    if (!st.build && st.entries.empty() && st.content_removed.empty())
+    {
+        /// Nothing staged for this ref this transaction -- a touched-but-empty PartStaging (e.g. the
+        /// harmless residue of a removeDirectory that already superseded this staging's marks,
+        /// content_removed cleared to empty). Benign no-op; `out_slot` stays `std::nullopt`.
+        st.published = true;
+        return;
+    }
+
+    /// For committed-ref standalone writes and removal marks, `st.entries` holds only the
+    /// CHANGED/ADDED entries (see stageBlobPartFile / the inline writeFile path) — never the whole
+    /// part once the ref already exists; `st.content_removed` holds paths a same-transaction
+    /// unlinkFile staged for removal (§6). Carry every OTHER committed entry forward (minus any
+    /// content_removed path) and republish once via the repoint path, rather than letting the
+    /// PartWriteTxn path below replace the manifest with just the delta. That would either reject a
+    /// genuine content change or silently drop untouched files. This handles both sub-cases the
+    /// interface allows: entries staged
+    /// WITH a PartWriteTxn (this transaction uploaded new content) and WITHOUT one (a former mutable
+    /// per-part file that is now an ordinary tree entry, or a marks-only removal with no writes).
+    if (!st.entries.empty() || !st.content_removed.empty())
+    {
+        if (auto view = metadata_storage.partAccess()->getView({ns, ref}, Cas::Freshness::ForceFresh))
+        {
+            if (st.build)
+            {
+                /// EDGE-BEFORE-OBSERVE is still load-
+                /// bearing here: a fresh blob's hash must be durably NAMED by a live precommit's
+                /// manifest body before `putBlob` makes its first backend observation. `repointRef`
+                /// below promotes through its OWN internal build (`adoptEvidence`, no `putBlob`) — it
+                /// protects entries whose content ALREADY exists (the carried-forward ones, and this
+                /// transaction's uploads once they land), but cannot itself protect a brand-new upload
+                /// made mid-repoint. So THIS build stages+precommits a SCRATCH manifest over
+                /// `st.entries` (BEFORE it is merged/moved below) — exactly the same closure the normal
+                /// (non-repoint) path further down establishes with `st.build->stageManifest(st.entries)`
+                /// + `precommitAdd`, which already names every hash this transaction is about to upload
+                /// — purely to hold that edge across the upload loop. Once `repointRef`'s own promote
+                /// makes the real (merged) manifest live, this scratch precommit is abandoned; it never
+                /// gets promoted. A marks-only removal never enters this sub-block (`st.build` is null).
+                const Cas::ManifestId scratch_id = st.build->stageManifest(st.entries);
+                st.build->precommitAdd(ns, ref, scratch_id);
+                uploadPendingBlobs(st);
+            }
+
+            std::vector<Cas::ManifestEntry> merged;
+            for (const auto & e : view->manifest()->entries)
+                if (!st.content_removed.contains(e.path)
+                    && std::none_of(st.entries.begin(), st.entries.end(),
+                                     [&](const Cas::ManifestEntry & s) { return s.path == e.path; }))
+                    merged.push_back(e);
+            for (auto & s : st.entries)
+                merged.push_back(std::move(s));
+
+            /// Test-only fault seam (Task 3 TDD): simulate a promote-time backend failure for `ref`
+            /// right before the durable repoint call. A no-op in production (nothing ever arms it).
+            if (metadata_storage.shouldFailPromoteForTest({ns, ref}))
+                throw Exception(ErrorCodes::ABORTED,
+                    "ContentAddressedTransaction: test-injected promote failure for {}/{}", ns.string(), ref);
+
+            /// Capture the exact `CommitOutcome` IMMEDIATELY -- into the caller-provided slot, before
+            /// the scratch-build `abandon()` below (which can itself throw), and before the test-only
+            /// after-promote hook (which can run arbitrary test code) -- so a later throw in either
+            /// cannot lose it (Task 2/3's publish-before-any-throwable-post-commit-work ordering).
+            const Cas::CommitOutcome oc = metadata_storage.partAccess()->repointRef({ns, ref}, std::move(merged), Cas::ProvenanceOp::Other);
+            out_slot = oc;   /// always created=false here: this block only runs once `view` already resolved
+            /// Test-only: models a concurrent writer racing in right after this transaction's own
+            /// confirm (e.g. repointing `ref` again). A no-op in production.
+            metadata_storage.runAfterPromoteHookForTest({ns, ref});
+            if (st.build)
+            {
+                st.build->abandon();   /// scratch precommit's protecting job is done; the real manifest is live
+                st.build.reset();      /// never re-abandon this build from the destructor
+            }
+            st.published = true;
+            return;
+        }
+    }
+
+    if (!st.build)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "ContentAddressedTransaction: staged entries or removal marks for {}/{} without a Build", ns.string(), ref);
+
+    /// Write path (rev. 15): stage the part manifest body (mints a ManifestId), precommitAdd a
+    /// build-intent owner (closure now protected by reachability), upload the pending blobs, then
+    /// promote — an atomic owner move that revalidates every non-tokened blob fail-closed.
+    ///
+    /// ORDERING IS LOAD-BEARING (EDGE-BEFORE-OBSERVE):
+    /// precommitAdd's durable closure names EVERY blob hash BEFORE putBlob makes the first backend
+    /// observation. This is what lets promote skip re-validating tokened leaves (a condemnation in the
+    /// putBlob→promote window cannot graduate — the next fold sees the edge). Moving putBlob before
+    /// precommitAdd would adopt an incarnation with no protecting edge and
+    /// trips the EDGE-BEFORE-OBSERVE fail-closed throw in PartWriteTxn::observeAndAdmit; the TLA+ order
+    /// sabotage (Gate A) is the formal guard.
+    const Cas::ManifestId id = st.build->stageManifest(st.entries);
+    st.build->precommitAdd(ns, ref, id);
+    uploadPendingBlobs(st);
+
+    /// Test-only fault seam (Task 3 TDD): simulate a promote-time backend failure for `ref` right
+    /// before the durable promote call. A no-op in production (nothing ever arms it).
+    if (metadata_storage.shouldFailPromoteForTest({ns, ref}))
+        throw Exception(ErrorCodes::ABORTED,
+            "ContentAddressedTransaction: test-injected promote failure for {}/{}", ns.string(), ref);
+
+    /// The exact, in-lane-derived `created` from `promoteBuild` replaces the racy pre-check this used
+    /// to be (`existsRef` before promote, which a concurrent writer could invalidate in the window
+    /// before the promote's own append confirms). Captured into `out_slot` IMMEDIATELY -- before the
+    /// test-only after-promote hook below (which can run arbitrary test code) or `st.build.reset()`
+    /// -- so a later throw there cannot lose it.
+    const Cas::CommitOutcome oc = metadata_storage.partAccess()->promoteBuild(*st.build, {ns, ref}, st.build->buildId(), id);
+    out_slot = oc;
+    /// Test-only: models a concurrent writer racing in right after this transaction's own confirm. A
+    /// no-op in production.
+    metadata_storage.runAfterPromoteHookForTest({ns, ref});
+    st.build.reset();   /// the build is consumed (promoted); never re-abandon it from the destructor
+    st.published = true;
+}
+
+void ContentAddressedTransaction::commit(const TransactionCommitOptionsVariant &)
+{
+    if (failed)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "retrying a failed content-addressed transaction is not supported");
+
+    /// Operation gate (rev.7 §1). A commit with staged parts to publish is a `Write` (throws the typed
+    /// Vanished [D5] refusal on a Vanished disk); a commit with nothing to publish is the DROP/rename path -- its ref mutations
+    /// already applied immediately (`removeRecursive`/`dropNamespace`/`moveDirectory`), so it is a `Remove`
+    /// that no-op-succeeds on a Vanished disk, which is what lets a vanished-disk table's DROP finish.
+    /// Both throw 668 while the backing is uncertain (transient / IdentityLost).
+    const CasOpClass commit_class = parts.empty() ? CasOpClass::Remove : CasOpClass::Write;
+    if (metadata_storage.checkOpAdmitted(commit_class) == CasOpAdmission::TruthAbsent)
+    {
+        /// Vanished + nothing to publish: complete as a no-op so the DROP/rename finishes. There are no
+        /// staged parts to publish and no local staging to keep; run the same idempotent epilogue a
+        /// normal empty commit runs.
+        committed = true;
+        cleanupPendingTempFiles();
+        force_fresh_validated_refs.clear();
+        return;
+    }
+
+    /// Publish each staged part. [TXN-ONE-PIPELINE] This is the ONLY place a ref becomes durable — the
+    /// tmp->final rename is a pure overlay re-key. Commit
+    /// atomicity: there is no multi-ref atomic publish, so a publish that throws after
+    /// earlier parts already published would leave a PARTIAL commit — some refs durably visible while
+    /// the transaction reports failure, diverging the durable pool from the disk layer's all-or-nothing
+    /// expectation. Track the refs THIS commit creates and, on any exception, best-effort unpublish
+    /// them before rethrowing. A partial commit is NOT a protocol violation (each publish/dropRef is
+    /// individually gate-checked and journalled; the leftover uploads are GC-reclaimable debris) — this
+    /// restores the wiring-layer transaction contract, not a CAS invariant.
+    ///
+    /// Fail-closed (CLAUDE.md): only refs that were ABSENT before we published them are rolled back. A
+    /// ref that already existed is pre-existing data this commit must never destroy on its error path.
+    /// Publishing over a live ref does not occur in the MergeTree write path (unique part names), but
+    /// the rollback must not assume it. updateRefPublishedAt mutations (autocommit one-shots on a
+    /// COMMITTED part) are individually durable by design and are deliberately NOT rolled back.
+    ///
+    /// Task 3: the rollback keys on the EXACT manifest each `publishStaging` call committed
+    /// (`Cas::CommitOutcome`), not merely on "this part's (ns, ref) name" -- an unconditional `dropRef`
+    /// would clobber a DIFFERENT writer's repoint of the same ref name that lands in the window between
+    /// this part's publish and a later part's failure (see `CasCommitRollback.RepointByOtherWriterSurvivesRollback`).
+    /// `part_outcomes` is snapshotted and preallocated up front (one allocation, index-addressed, no
+    /// per-part growth) so `publishStaging` can write `part_outcomes[i]` with a no-throw slot write --
+    /// the precondition for the precise per-part rollback below. Parts are published SERIALLY by the loop
+    /// that follows; only the blob uploads within each part fan out (`fanOutBlobUploads`). The snapshot
+    /// preserves `parts`' own iteration order (the map's (ns, ref) sort order); there is no dependency
+    /// between parts that would require a different order. Concurrent cross-part publication is future
+    /// scope and is NOT done here.
+    struct IndexedPart { Cas::RootNamespace ns; std::string ref; PartStaging * st; };
+    std::vector<IndexedPart> ordered;
+    ordered.reserve(parts.size());
+    for (auto & [key, st] : parts)
+        ordered.push_back({Cas::RootNamespace{key.first}, key.second, &st});
+
+    std::vector<std::optional<Cas::CommitOutcome>> part_outcomes;
+    part_outcomes.assign(ordered.size(), std::nullopt);
+
+    try
+    {
+        for (size_t i = 0; i < ordered.size(); ++i)
+            publishStaging(ordered[i].ns, ordered[i].ref, *ordered[i].st, part_outcomes[i]);
+    }
+    catch (...)
+    {
+        failed = true;
+        /// Compensating rollback. Best-effort: a ref we cannot unpublish becomes unreferenced debris
+        /// (GC-reclaimed); never mask the original failure with a rollback failure. Only a slot whose
+        /// outcome `created` is true names a ref THIS commit made durable for the first time; a
+        /// repoint of an already-committed ref (created=false) is pre-existing data and is never
+        /// dropped. `dropRefIfMatches` additionally guards against a concurrent repoint of the SAME ref
+        /// since this call's own publish: it removes the ref only if it still names the exact
+        /// `manifest_ref` this commit bound, leaving a newer binding untouched.
+        for (const auto & oc : part_outcomes)
+            if (oc && oc->created)
+                metadata_storage.partAccess()->dropRefIfMatches({oc->ns, oc->ref}, oc->manifest_ref);
+        throw;
+    }
+    committed = true;
+    /// All pending blobs have been uploaded in publishStaging; remove their staging resources now.
+    cleanupPendingTempFiles();
+    /// This transaction's unlinkFile ForceFresh-proof memoization is scoped to this transaction only;
+    /// clear it alongside the other per-transaction state resets above.
+    force_fresh_validated_refs.clear();
+}
+
+TransactionCommitOutcomeVariant ContentAddressedTransaction::tryCommit(const TransactionCommitOptionsVariant & options)
+{
+    if (!std::holds_alternative<NoCommitOptions>(options))
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "ContentAddressed transaction supports only tryCommit without options");
+    commit(options);
+    return true;
+}
+
+ObjectStorageKey ContentAddressedTransaction::generateObjectKeyForPath(const std::string &)
+{
+    notYet("generateObjectKeyForPath");
+}
+
+StoredObjects ContentAddressedTransaction::getSubmittedForRemovalBlobs()
+{
+    /// CA never hands shared backing objects to the disk layer for removal — reclamation is the
+    /// GC's. Empty unconditionally because shared content-addressed objects are never owned by the
+    /// disk-layer removal list.
+    return {};
+}
+
+const Cas::ManifestEntry * ContentAddressedTransaction::findStagedEntry(
+    const ContentAddressedMetadataStorage::Route & r) const
+{
+    auto it = parts.find({r.ns.string(), r.ref});
+    if (it == parts.end())
+        return nullptr;
+    auto eit = std::find_if(it->second.entries.begin(), it->second.entries.end(),
+        [&](const Cas::ManifestEntry & e) { return e.path == r.file; });
+    return eit == it->second.entries.end() ? nullptr : &*eit;
+}
+
+std::optional<StoredObjects> ContentAddressedTransaction::tryGetInFlightStorageObjects(const std::string & path) const
+{
+    /// Read-your-writes: a projection spill-and-merge reads back its own staged blocks before
+    /// the parent part's single commit. Staged content blobs may still be pending (not yet uploaded).
+    auto r = const_cast<ContentAddressedTransaction *>(this)->routeOf(path);
+    if (!r || r->file.empty())
+        return {};
+    auto it = parts.find({r->ns.string(), r->ref});
+    if (it == parts.end())
+        return {};
+    if (const auto * entry = findStagedEntry(*r))
+    {
+        if (entry->placement == Cas::EntryPlacement::Blob)
+        {
+            /// A pending blob has not been uploaded yet — its storage object does not exist in
+            /// the pool. Return empty so the caller falls back to tryReadFileInFlight (local temp read).
+            if (findPendingBlob(it->second, entry->ref))
+                return {};
+            const auto location = metadata_storage.store()->locate(*entry);
+            return StoredObjects{StoredObject(location.key, path, location.length)};
+        }
+        /// An Inline entry carries its bytes in `inline_bytes`; `size()` (not `blob_size`, which is 0
+        /// for an inline entry carried forward from a decoded source manifest — createHardLink) reports
+        /// the real inline byte count, so an in-flight read of a carried-forward inline sidecar (e.g. a
+        /// MATERIALIZE-PROJECTION projection marks file) resolves to its real size, matching the
+        /// committed getStorageObjects path.
+        return StoredObjects{StoredObject("", path, entry->size())};
+    }
+    return {};
+}
+
+std::unique_ptr<ReadBufferFromFileBase> ContentAddressedTransaction::tryReadFileInFlight(
+    const std::string & path, const ReadSettings & settings, std::optional<size_t> /*read_hint*/) const
+{
+    auto r = const_cast<ContentAddressedTransaction *>(this)->routeOf(path);
+    if (!r || r->file.empty())
+        return nullptr;
+    auto it = parts.find({r->ns.string(), r->ref});
+    if (it == parts.end())
+        return nullptr;
+    if (const auto * entry = findStagedEntry(*r))
+    {
+        if (entry->placement == Cas::EntryPlacement::Inline)
+            return std::make_unique<ReadBufferFromOwnMemoryFile>(path, entry->inline_bytes);
+        if (entry->placement == Cas::EntryPlacement::Blob)
+        {
+            /// A pending blob has not been uploaded yet — serve reads from the staging area (the
+            /// same bytes that will be promoted to the pool in publishStaging post-precommit): a local
+            /// temp file for `Cas::StagingBackend::Local`, or the S3 staging object for `Cas::StagingBackend::S3`
+            /// (`staging_key` is a remote object key there, never a
+            /// local path, so `ReadBufferFromFile` would misinterpret it as a filesystem path).
+            if (const auto * pb = findPendingBlob(it->second, entry->ref))
+            {
+                if (pb->backend == Cas::StagingBackend::S3)
+                {
+                    /// The staging object holds `[header][payload]`
+                    /// (the fixed-length `blob_header_len` CABL envelope, so the promote can stay a
+                    /// verbatim server-side copy). Read-your-writes must serve the PAYLOAD ONLY — wrap the
+                    /// object read in a `ReadBufferFromFileView` windowed to `[header_len, header_len+size)`
+                    /// so position 0 is the payload start, else the reader would see 256 bytes of header
+                    /// prepended to the payload (corruption). The LOCAL staging temp file holds the payload
+                    /// verbatim (no header), so its path is unchanged.
+                    const uint64_t header_len = metadata_storage.store()->poolMeta().blob_header_len;
+                    const uint64_t payload_end = header_len + pb->size;
+                    auto impl = metadata_storage.objectStorage()->readObject(
+                        StoredObject(pb->staging_key, path, payload_end), settings);
+                    return std::make_unique<ReadBufferFromFileView>(
+                        std::move(impl), path, header_len, payload_end);
+                }
+                return std::make_unique<ReadBufferFromFile>(pb->staging_key);
+            }
+            return metadata_storage.readBlobPayload(metadata_storage.store()->locate(*entry), path, settings);
+        }
+    }
+    return nullptr;
+}
+
+std::optional<uint64_t> ContentAddressedTransaction::tryGetInFlightFileSize(const std::string & path) const
+{
+    auto r = const_cast<ContentAddressedTransaction *>(this)->routeOf(path);
+    if (!r || r->file.empty())
+        return {};
+    auto it = parts.find({r->ns.string(), r->ref});
+    if (it == parts.end())
+        return {};
+    if (const auto * entry = findStagedEntry(*r))
+        /// `size()` (not `blob_size` directly, which is 0 for an inline entry carried forward via
+        /// createHardLink from a decoded source manifest). Without this, an in-flight size query for a
+        /// carried-forward inline sidecar returns 0 — the 02941 MATERIALIZE-PROJECTION "Empty marks
+        /// file: 0, must be: 144" corruption on a same-session read.
+        return entry->size();
+    return {};
+}
+
+bool ContentAddressedTransaction::hasInFlightDirectory(const std::string & path) const
+{
+    /// The directory overlay is true iff at least one staged file lives under `path` for `path`'s
+    /// part - what makes a carried-forward projection dir visible to loadProjections.
+    auto r = const_cast<ContentAddressedTransaction *>(this)->routeOf(path);
+    /// INNER directories only: the overlay exists for staged projection dirs
+    /// The overlay is used by `loadProjections` during finalize. The PART DIR ITSELF answers FALSE - a
+    /// dedup-rejected temporary part still holds its uncommitted transaction at destruction, and
+    /// an overlay "exists" for the bare part dir sends removeIfNeeded into remove(), whose
+    /// bare-disk check then logs the "part to remove doesn't exist" warning.
+    if (!r || r->ref.empty() || r->file.empty())
+        return false;
+    auto it = parts.find({r->ns.string(), r->ref});
+    if (it == parts.end())
+        return false;
+    const std::string prefix = r->file + "/";
+    for (const auto & entry : it->second.entries)
+        if (entry.path.starts_with(prefix))
+            return true;
+    return false;
+}
+
+std::vector<std::string> ContentAddressedTransaction::listInFlightDirectory(const std::string & path) const
+{
+    /// Immediate-child names staged directly under `path` (one level) - loadProjections'
+    /// withPartFormatFromDisk iterates a staged projection dir to find its mark file.
+    auto r = const_cast<ContentAddressedTransaction *>(this)->routeOf(path);
+    std::vector<std::string> result;
+    if (!r || r->ref.empty())
+        return result;
+    auto it = parts.find({r->ns.string(), r->ref});
+    if (it == parts.end())
+        return result;
+    const std::string prefix = r->file.empty() ? "" : r->file + "/";
+    std::set<std::string> names;
+    auto add = [&](const std::string & name)
+    {
+        if (!name.starts_with(prefix) || name.size() <= prefix.size())
+            return;
+        const auto rest = name.substr(prefix.size());
+        const auto slash = rest.find('/');
+        names.insert(slash == std::string::npos ? rest : rest.substr(0, slash));
+    };
+    for (const auto & entry : it->second.entries)
+        add(entry.path);
+    return {names.begin(), names.end()};
+}
+
+void ContentAddressedTransaction::createMetadataFile(const std::string &, const StoredObjects &)
+{
+    notYet("createMetadataFile");
+}
+
+void ContentAddressedTransaction::stageBlobPartFile(
+    const ContentAddressedMetadataStorage::Route & route,
+    const Cas::BlobRef & ref, size_t size, const std::string & staging_key, Cas::StagingBackend backend)
+{
+    /// Do not upload here. Record the pending blob (uploaded post-precommit in publishStaging)
+    /// and a tokenless dependency; putBlob later overwrites it with the tokened dependency.
+    /// The staging bytes are kept (the transaction owns them) — a local temp file for
+    /// `Cas::StagingBackend::Local`, or an S3 staging object for `Cas::StagingBackend::S3`.
+    auto & st = stagingFor(route);
+    st.pending_blobs.push_back({ref, staging_key, size, backend});
+    buildFor(route, st).recordPendingBlobDep(ref, size);
+
+    Cas::ManifestEntry entry;
+    entry.path = route.file;
+    entry.placement = Cas::EntryPlacement::Blob;
+    entry.ref = ref;
+    entry.blob_size = size;
+    std::erase_if(st.entries, [&](const Cas::ManifestEntry & e) { return e.path == entry.path; });
+    st.entries.push_back(std::move(entry));
+}
+
+std::string ContentAddressedTransaction::buildS3StagingBlobHeader(
+    const ContentAddressedMetadataStorage::Route & route) const
+{
+    /// Mirror `PartWriteTxn::uploadFromSource`'s `buildHeader` (minus the dropped `logical_size`/`logical_hash`
+    /// fields and minus `build_id`, which is not known until commit and is diagnostic-only). A FRESH
+    /// `incarnation_tag` per staging object keeps the incarnation zone unique; the header is padded to
+    /// the pool's fixed `blob_header_len` so the payload starts at a constant offset.
+    const Cas::PoolPtr & store = metadata_storage.store();
+    const Cas::PoolMeta & meta = store->poolMeta();
+    const Cas::PoolConfig & cfg = store->poolConfig();
+
+    Cas::EnvelopeHeader header;
+    header.kind = Cas::ObjectKind::Blob;
+    header.incarnation_tag = (static_cast<UInt128>(thread_local_rng()) << 64) | thread_local_rng();
+    header.build_id = 0;   /// not known at stream time; diagnostic-only (not read by GC/read paths)
+    /// ch = the real ClickHouse VERSION_INTEGER (diagnostic-only; consistent with `PartWriteTxn::buildHeader`).
+    /// The v3 envelope drops hash_algo/domain_id/writer_version, so forensics ride on ch + bld.
+    header.provenance = Cas::Provenance{
+        /*created_at_ms*/ 0, cfg.server_id, VERSION_INTEGER, Cas::ProvenanceOp::Other};
+    header.intended_ref = route.ns.string() + "/" + route.ref;
+    /// The v3 codec pads to the pool's fixed header length and TRUNCATES a too-long intended_ref
+    /// internally (it is diagnostic-only), so the old drop-and-retry is gone — one encode call.
+    return Cas::encodeEnvelopeHeader(header, static_cast<uint32_t>(meta.blob_header_len));
+}
+
+std::unique_ptr<WriteBufferFromFileBase> ContentAddressedTransaction::tryCreateWriteBuffer(
+    const std::shared_ptr<IDiskTransaction> & owner,
+    const std::string & path, size_t buf_size, WriteMode mode,
+    const WriteSettings & settings, bool autocommit)
+{
+    /// This transaction owns the write because the blob key is known only after hashing the payload.
+    /// Append is serviceable (read-modify-rewrite) only for a non-part / table-level verbatim file
+    /// (handled inside writeFile). A part file is a content blob or a whole-rewritten inline entry, so
+    /// append on a part-file path is unsupported.
+    if (mode == WriteMode::Append && Cas::isPartFilePath(path))
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED, "Disk does not support WriteMode::Append for content part files");
+
+    /// Autocommit cannot work for a CONTENT BLOB part file (column data/marks, primary.idx): a part's
+    /// blobs are always written together as one build, whose manifest + ref publish only when commit()
+    /// runs. A small INLINE-eligible part file IS autocommittable (a standalone one-shot write): the write
+    /// lands as an ordinary manifest entry and, if the ref is already committed, `publishStaging`'s repoint
+    /// branch carries the rest of the part forward and republishes once (the transactional-INSERT
+    /// creation-CSN fill-in / removal-TID rewrite / rollback path). Verbatim / table-level files (not part
+    /// files) are durable on finalize regardless of `autocommit`.
+    if (autocommit && Cas::isPartFilePath(path))
+    {
+        auto p = Cas::parsePartFilePath(path);
+        if (!p || p->file.empty() || Cas::partFileMustStayBlob(p->file))
+            throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+                "Autocommit writes are not supported for content part files on a content-addressed disk");
+
+        auto inner = writeFile(path, buf_size, mode, settings);
+        auto commit_callback = [owner](size_t) mutable { owner->commit(); };
+        return std::make_unique<WriteBufferWithFinalizeCallback>(
+            std::move(inner), std::move(commit_callback), path, /*create_blob_if_empty=*/true);
+    }
+
+    /// Non-autocommit (or verbatim autocommit): pin the owning disk transaction for the returned buffer's
+    /// lifetime. The CA write buffers capture a bare `this` in their deferred finalize / pin-blob callbacks;
+    /// MergedBlockOutputStream may finalize them LATER (another thread, or after the part storage /
+    /// transaction would otherwise be torn down on async-insert / cancel / exception-unwind). Holding
+    /// `owner` (which owns this ContentAddressedTransaction by shared_ptr) keeps that `this` valid until the
+    /// buffer — and so this callback — is destroyed after finalize (the lifetime guarantee now
+    /// expressed generically via `owner`). No cycle: the transaction does not hold the buffer.
+    auto inner = writeFile(path, buf_size, mode, settings);
+    auto keep_alive_callback = [owner](size_t) mutable {};
+    return std::make_unique<WriteBufferWithFinalizeCallback>(
+        std::move(inner), std::move(keep_alive_callback), path, /*create_blob_if_empty=*/true);
+}
+
+std::unique_ptr<WriteBufferFromFileBase> ContentAddressedTransaction::writeFile(
+    const std::string & path, size_t buf_size, WriteMode mode, const WriteSettings & settings)
+{
+    /// Write gate (rev.7 §1): the single chokepoint every write buffer (both direct and via
+    /// `tryCreateWriteBuffer`) is created through -- refuse on a Vanished (typed [D5]) or transient/
+    /// IdentityLost (668) disk before staging any content or opening a verbatim buffer.
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+    /// Non-part files are VERBATIM namespace files, durable on finalize (no commit involvement -
+    /// the disk layer's autocommit contract for them rides exactly this). Append is serviced by
+    /// read-modify-rewrite: the existing bytes are carried forward (the MVCC mutation-entry CSN
+    /// append depends on this). The `carried` prefix below is read ONCE here, at buffer-open time, and
+    /// frozen into the write callback; `casPutObject`'s CAS loop (invoked from the callback via
+    /// `putNamespaceFile`/`putMountpointObject`) only re-reads the TOKEN on conflict, not this base
+    /// content — see the single-appender invariant documented at `CasPlainObjects::casPutObject`. Safe
+    /// only because the sole production appender (the mutation-entry CSN write) never has a second
+    /// concurrent appender on the same key.
+    if (!Cas::isPartFilePath(path))
+    {
+        if (auto tf = Cas::parseTableFilePath(path))
+        {
+            /// The LIFE is resolved once, here at buffer-open time, and captured by value below, so a
+            /// finalize that runs later writes to the incarnation this open was admitted under -- never
+            /// into whatever life the namespace name happens to denote when the callback fires.
+            const Cas::NamespaceLifeId life
+                = metadata_storage.store()->namespaceLife(metadata_storage.liveNamespace(tf->table_uuid));
+            const std::string name = tf->tail;
+            std::string prefix_bytes;
+            if (mode == WriteMode::Append)
+                if (auto existing = metadata_storage.store()->getNamespaceFile(life, name))
+                    prefix_bytes = std::move(*existing);
+            return std::make_unique<Cas::CaInlineWriteBuffer>(
+                [this, life, name, carried = std::move(prefix_bytes)](std::string bytes)
+                {
+                    metadata_storage.store()->putNamespaceFile(life, name, carried + bytes);
+                });
+        }
+        /// A loose disk file, including the startup write probe, is a plain mountpoint object.
+        const std::string key = metadata_storage.serverRootId() + "/" + path;
+        std::string prefix_bytes;
+        if (mode == WriteMode::Append)
+            if (auto existing = metadata_storage.store()->getMountpointObject(key))
+                prefix_bytes = std::move(*existing);
+        return std::make_unique<Cas::CaInlineWriteBuffer>(
+            [this, key, carried = std::move(prefix_bytes)](std::string bytes)
+            {
+                metadata_storage.store()->putMountpointObject(key, carried + bytes);
+            });
+    }
+
+    auto p = Cas::parsePartFilePath(path);
+    auto r = p ? metadata_storage.route(*p) : std::nullopt;
+    if (!r || r->file.empty())
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "ContentAddressedTransaction::writeFile: not a part file path: {}", path);
+
+    /// The former
+    /// mutable-per-part-file branch (uuid.txt/metadata_version.txt/txn_version.txt staging directly
+    /// into a separate mutable payload) is DELETED here — these three names fall through to the
+    /// ordinary content path below like any other tree file. There is no filename left to special-case:
+    /// `kMutablePerPartFiles`/`isMutablePerPartFile` predicate itself is gone too — there is no
+    /// filename left to special-case. During part build these files land in the initial manifest with
+    /// every other staged file; a standalone write on an already-committed part repoints.
+
+    /// A CONTENT part file that must stay a blob (per-column data/marks, primary.idx): spill + hash,
+    /// then stage the blob as PENDING (precommit-first). The blob is NOT uploaded/promoted here;
+    /// `publishStaging` uploads it (Local) or promotes the S3 staging object post-precommit.
+    /// recordPendingBlobDep (inside stageBlobPartFile) records a tokenless dependency without any
+    /// pool operation at staging time.
+    if (Cas::partFileMustStayBlob(r->file))
+    {
+        /// S3-native staging:
+        /// when this disk opted in (`staging_backend=s3`) AND the mount-time capability probe
+        /// a capability probe proved the object storage enforces write-once conditional copy, stream directly
+        /// to a fresh per-mount S3 staging object while hashing — no local-disk round trip. Otherwise
+        /// (the OFF BY DEFAULT global constraint, or a probe fail-close) fall through to the existing,
+        /// byte-for-byte-unchanged local-temp-file path below.
+        /// Hash with this pool's node-local write algorithm rather than a hardcoded city hash;
+        /// `PoolMeta` no longer records a single pool-wide algorithm --
+        /// mixed-algo pools track `algos_used`; `writeAlgo()` is the write-mint accessor now).
+        const auto hash_algo = metadata_storage.store()->writeAlgo();
+        /// `hash_hex` is rendered by the streaming write buffer at `hash_algo`'s own width —
+        /// parse it back at that SAME width via `Cas::codecFor(hash_algo)` (never a pool-wide
+        /// `DigestCodec`, which no longer exists) into a full `BlobRef` pair.
+
+        if (metadata_storage.stagingBackend() == Cas::StagingBackend::S3 && metadata_storage.conditionalCopySupported())
+        {
+            const std::string staging_key = metadata_storage.stagingKeyPrefix() + "/" + getRandomASCIIString(32) + ".tmp";
+            auto object_sink = metadata_storage.objectStorage()->writeObject(StoredObject(staging_key), WriteMode::Rewrite);
+            /// Build the fixed-length CABL envelope header now (before
+            /// the payload is streamed) so the staging object holds `[header][payload]` and the promote
+            /// stays a verbatim server-side copy. The header carries a FRESH `incarnation_tag`; `build_id`
+            /// is left 0 (not known at stream time — diagnostic-only, not read by GC/read paths). The
+            /// buffer writes this header first, UNHASHED and excluded from the reported size, so the
+            /// content key stays the pool's hash of `payload` and `blob_size` stays the payload size.
+            std::string envelope_header = buildS3StagingBlobHeader(*r);
+            /// rev.7 [C2]: capture the fence generation now, re-checked immediately before the durable
+            /// `sink->finalize()` in `finalizeImpl` (the streaming upload becomes durable there).
+            const Cas::PoolPtr pool = metadata_storage.store();
+            const uint64_t admitted_generation = pool->fenceGeneration();
+            return std::make_unique<Cas::CaContentWriteBuffer>(
+                std::move(object_sink),
+                staging_key,
+                std::move(envelope_header),
+                hash_algo,
+                buf_size,
+                settings.use_adaptive_write_buffer,
+                settings.adaptive_write_buffer_initial_size,
+                [this, route = *r, hash_algo](const std::string & hash_hex, size_t size, const std::string & key)
+                {
+                    const Cas::BlobRef ref{hash_algo, Cas::codecFor(hash_algo).fromHex(hash_hex)};
+                    stageBlobPartFile(route, ref, size, key, Cas::StagingBackend::S3);
+                },
+                [pool, admitted_generation] { pool->checkFenceOrThrow(admitted_generation); });
+        }
+
+        return std::make_unique<Cas::CaContentWriteBuffer>(
+            metadata_storage.scratchPath(),
+            hash_algo,
+            buf_size,
+            settings.use_adaptive_write_buffer,
+            settings.adaptive_write_buffer_initial_size,
+            [this, route = *r, hash_algo](const std::string & hash_hex, size_t size, const std::string & temp_path)
+            {
+                const Cas::BlobRef ref{hash_algo, Cas::codecFor(hash_algo).fromHex(hash_hex)};
+                stageBlobPartFile(route, ref, size, temp_path, Cas::StagingBackend::Local);
+            });
+    }
+
+    /// Inline candidate (small eager metadata): buffer in memory, decide at finalize. <= INLINE_CAP
+    /// rides the single tree object as an Inline entry (one-GET part open); an oversized
+    /// candidate spills to a blob (the safety net).
+    return std::make_unique<Cas::CaInlineWriteBuffer>(
+        [this, route = *r](std::string bytes)
+        {
+            /// Mint via the one write hash function, `Cas::poolContentHash` (algorithm, payload) -> BlobRef
+            /// (`CasPartWriteTxn.h`) -- the SAME mint the streaming blob path's callers use, so an inline file
+            /// and a standalone blob of identical content get the same ref (same content hash identity)
+            /// under EVERY algo, including sha256.
+            const auto hash_algo = metadata_storage.store()->writeAlgo();
+            const Cas::BlobRef ref = Cas::poolContentHash(hash_algo, bytes);
+            if (bytes.size() <= INLINE_CAP)
+            {
+                auto & st = stagingFor(route);
+                /// An inline (no-blob) entry still requires a PartWriteTxn. `publishStaging` stages the
+                /// manifest body, precommits, and promotes the ref even for a part with NO blob uploads;
+                /// it asserts `st.build != nullptr` whenever `st.entries` is non-empty. Without this, a
+                /// part whose files are ALL inline (a tiny/empty merge output, every file <= INLINE_CAP)
+                /// reaches `publishStaging` with entries but no PartWriteTxn -> LOGICAL_ERROR "staged entries
+                /// without a PartWriteTxn" -> a logical-error exception under abort_on_logical_error
+                /// since the inline-files feature). The blob path already establishes the PartWriteTxn via
+                /// `buildFor`; the inline path must do the same.
+                buildFor(route, st);
+                Cas::ManifestEntry entry;
+                entry.path = route.file;
+                entry.placement = Cas::EntryPlacement::Inline;
+                entry.ref = ref;   /// content hash identity (same for inline and blob of same content)
+                /// `blob_size` stays 0 (its default) for an Inline entry — matching decode, which never
+                /// fills it for Inline. `entry.size()` is the logical size, derived from `inline_bytes`.
+                entry.inline_bytes = std::move(bytes);
+                std::erase_if(st.entries, [&](const Cas::ManifestEntry & e) { return e.path == entry.path; });
+                st.entries.push_back(std::move(entry));
+            }
+            else
+            {
+                /// Safety fallback: an unexpectedly large candidate spills to a blob (preserves the
+                /// invariant that big files are not held inline). Write the buffered bytes to a unique
+                /// local temp file (same scratchPath + random-name scheme as CaContentWriteBuffer), then
+                /// stage exactly like a streaming blob.
+                std::filesystem::create_directories(metadata_storage.scratchPath());
+                const std::string temp_path =
+                    metadata_storage.scratchPath() + "/inline_overflow_" + getRandomASCIIString(32) + ".tmp";
+                {
+                    WriteBufferFromFile tmp(temp_path);
+                    tmp.write(bytes.data(), bytes.size());
+                    tmp.finalize();
+                }
+                /// Until stageBlobPartFile takes ownership, WE own the temp file — mirror the blob path,
+                /// where CaContentWriteBuffer's dtor removes it unless the callback succeeded. If
+                /// stageBlobPartFile throws, drop the orphan instead of leaking it into scratch. This
+                /// fallback is always `Cas::StagingBackend::Local` — an oversized inline candidate is rare
+                /// enough (a safety net, not a hot path) that S3-staging mode does not cover it.
+                bool staged = false;
+                SCOPE_EXIT({ if (!staged) { std::error_code ec; std::filesystem::remove(temp_path, ec); } });
+                stageBlobPartFile(route, ref, bytes.size(), temp_path, Cas::StagingBackend::Local);
+                staged = true;
+            }
+        });
+}
+
+void ContentAddressedTransaction::createDirectory(const std::string &)
+{
+    /// Object storage has no real directories (mirrors the plain-rewritable transaction) -- but this is a
+    /// Write: the gate makes it throw typed on a Vanished disk / 668 while uncertain, so a mutation is
+    /// never silently accepted against an erased or unreachable backing (rev.7 §1 previously-no-op site).
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+}
+
+void ContentAddressedTransaction::createDirectoryRecursive(const std::string &)
+{
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+}
+
+void ContentAddressedTransaction::removeDirectory(const std::string & path)
+{
+    /// CONTRACT: `removeDirectory`/`moveDirectory` mutate durable refs at CALL TIME, not at commit —
+    /// this is the everything-immediate model, not a missed "defer to commit" opportunity. `renameParts`
+    /// is the actual commit point; anything that goes wrong after one of these calls is undone by a
+    /// COMPENSATING operation over already-committed state, the same way upstream MergeTree's own
+    /// `rollbackPartsToTemporaryState` and outdated-part cleanup run over committed disk state rather
+    /// than an in-memory intent log. Recording these as staged intents and applying them at commit would
+    /// duplicate that compensation machinery for no correctness gain.
+    ///
+    /// Remove gate (rev.7 §1): a Vanished disk answers no-op success (nothing to remove -- truth), so the
+    /// enclosing DROP completes; a transient / IdentityLost disk throws 668 (the DROP re-queues).
+    if (metadata_storage.checkOpAdmitted(CasOpClass::Remove) == CasOpAdmission::TruthAbsent)
+        return;
+
+    /// The MergeTree fast-removal path unlinks a part's files one by one (no-ops here) and then
+    /// calls removeDirectory(<part>) - the SINGLE authoritative point at which the part's ref must
+    /// be unlinked. Part dirs route to dropRef; anything else is a no-op (object
+    /// storage has no real directories; tables/detached/shadow are removed via removeRecursive).
+    if (auto r = routeOf(path); r && !r->ref.empty() && r->file.empty())
+    {
+        metadata_storage.partAccess()->dropRefIfPresent(r->refKey());
+        /// This transaction's staged removal marks for the same ref (content_removed, populated by
+        /// unlinkFile's per-file unlinks that the MergeTree fast-removal path issues right before this
+        /// call) are superseded by the whole-part ref-drop just performed above — discard them so
+        /// publishStaging's committed-ref repoint branch never chases an already-dropped ref, and the
+        /// dominant removal path pays zero repoints (one ref-drop only).
+        if (auto * st = findStaging(*r))
+        {
+            st->content_removed.clear();
+            st->entries.clear();
+            if (st->build)
+            {
+                st->build->abandon();
+                st->build.reset();
+            }
+        }
+        return;
+    }
+}
+
+void ContentAddressedTransaction::removeRecursive(const std::string & path, const ShouldRemoveObjectsPredicate & /*should_remove_objects*/)
+{
+    /// Removal = pointer-unlink + deferred GC: only refs and verbatim files go; the shared
+    /// blobs/trees are reclaimed by Cas::Gc once unreachable. The predicate gates backing-object
+    /// deletion, which CA always defers, so it is intentionally ignored here.
+
+    /// Remove gate (rev.7 §1): a Vanished disk answers no-op success (nothing to remove), so a
+    /// vanished-disk table's DROP -- which reaches here via `removeSharedRecursive` -- completes; a
+    /// transient / IdentityLost disk throws 668 (the DROP re-queues, drains after recovery/FORGET).
+    if (metadata_storage.checkOpAdmitted(CasOpClass::Remove) == CasOpAdmission::TruthAbsent)
+        return;
+
+    /// FREEZE shadow shapes first (a shadow table dir also satisfies parseTableUuid).
+    if (Cas::isShadowPath(path))
+    {
+        if (auto p = Cas::parsePartFilePath(path); p && !p->backup_name.empty() && p->file.empty())
+        {
+            const auto ns = ContentAddressedMetadataStorage::shadowNamespace(p->shadow_table_dir);
+            metadata_storage.partAccess()->dropRefIfPresent({ns, p->part_name});
+            return;
+        }
+        if (Cas::endsWithTableUuidPair(path))
+        {
+            metadata_storage.partAccess()->dropNamespace(ContentAddressedMetadataStorage::shadowNamespace(path));
+            return;
+        }
+        /// Backup root / intermediate dir (SYSTEM UNFREEZE WITH NAME): drop every shadow
+        /// namespace under it. Canonicalize because callers hand trailing-slash dirs.
+        std::string prefix = path;
+        while (!prefix.empty() && prefix.back() == '/')
+            prefix.pop_back();
+        /// RECORD AND CONTINUE, and the reason is the shape of the alternative. Refusing the DROP would
+        /// make the obstacle permanent, blocked by the one operation that could have cleared it. The GC
+        /// round does not clear it either. For a ref-family key that is a test's claim rather than this
+        /// comment's: `CasRefGc.UnIncarnatedRefKeyAbortsRefFoldingWithoutWedgingTheRound` runs a round
+        /// over such a key and holds that the round deletes nothing and the key survives it. For a
+        /// `_files`-family key the round never gets the chance: `Cas::Gc`'s fold only LISTs
+        /// `casRefsPrefix()`, never `rootsPrefix()`, so such a key sits outside anything a round scans.
+        /// Continuing drops every namespace the enumeration DID name; the offending key stays as
+        /// reported debris. It cannot hide a namespace that has any well-formed key of its own, because
+        /// attribution is per key.
+        const Cas::NamespaceListing listing = metadata_storage.store()->listNamespaces(prefix + "/");
+        for (const Cas::UnattributableNamespaceKey & bad : listing.skipped)
+            LOG_ERROR(getLogger("ContentAddressedTransaction"),
+                "removeRecursive('{}'): key '{}' names no namespace life and was left in place ({}). "
+                "Every namespace this enumeration did name is still dropped; run `cas-fsck` to enumerate "
+                "such keys.", path, bad.key, bad.reason);
+        for (const auto & ns : listing.namespaces)
+            metadata_storage.partAccess()->dropNamespace(Cas::RootNamespace{ns});
+        return;
+    }
+
+    /// Table dir: the table's namespace (live + folded-in detached refs) and every verbatim
+    /// file go in one dropNamespace.
+    if (auto uuid = Cas::parseTableUuid(path))
+    {
+        metadata_storage.partAccess()->dropNamespace(metadata_storage.liveNamespace(*uuid));
+        return;
+    }
+
+    if (auto p = Cas::parsePartFilePath(path))
+    {
+        auto r = metadata_storage.route(*p);
+        /// The detached CONTAINER dir (DROP DETACHED / table-detach): drop all detached refs.
+        if (r && r->ref.empty() && p->part_name == Cas::kDetachedDirName)
+        {
+            for (const auto & ref : metadata_storage.detachedRefNames(r->ns))
+                metadata_storage.partAccess()->dropRefIfPresent({r->ns, ref});
+            return;
+        }
+        /// The moving CONTAINER dir (MOVE-to-CA fix, mirrors detached): the mover's crash-cleanup
+        /// (MergeTreeData.cpp, MOVING_DIR_NAME) calls this at table load to reclaim every staging
+        /// ref an interrupted move left behind.
+        if (r && r->ref.empty() && p->part_name == Cas::kMovingDirName)
+        {
+            for (const auto & ref : metadata_storage.movingRefNames(r->ns))
+                metadata_storage.partAccess()->dropRefIfPresent({r->ns, ref});
+            return;
+        }
+        /// A single part dir (live or detached): drop its ref.
+        if (r && !r->ref.empty() && r->file.empty())
+        {
+            metadata_storage.partAccess()->dropRefIfPresent(r->refKey());
+            return;
+        }
+        /// A projection subdir: virtual (nested in the parent tree) - removal is a no-op; the
+        /// blobs go when the part's ref does.
+        if (r && !r->ref.empty())
+            return;
+    }
+
+    /// Table-level SUBDIRECTORY (deduplication_logs/): remove every verbatim file under it.
+    if (auto tf = Cas::parseTableFilePath(path))
+    {
+        /// The READABLE resolution, which is the non-creating one (`namespaceFilesLifeIfReadable` answers
+        /// an uncataloged namespace from a catalog-only lookup and writes nothing): a removal must never
+        /// birth the namespace it is removing from. No life means there is nothing here to remove.
+        const auto life = metadata_storage.readableNamespaceFilesLife(
+            metadata_storage.liveNamespace(tf->table_uuid));
+        if (!life)
+            return;
+        const std::string prefix = tf->tail + "/";
+        for (const auto & name : metadata_storage.store()->listNamespaceFiles(*life))
+            if (name.starts_with(prefix))
+                metadata_storage.store()->removeNamespaceFile(*life, name);
+        return;
+    }
+}
+
+void ContentAddressedTransaction::createHardLink(const std::string & path_from, const std::string & path_to)
+{
+    /// Write gate (rev.7 §1).
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+    auto src = routeOf(path_from);
+    auto dst = routeOf(path_to);
+    if (!src || src->file.empty() || !dst || dst->file.empty())
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "ContentAddressed: createHardLink requires two part-file paths: {} -> {}", path_from, path_to);
+
+    auto & dst_st = stagingFor(*dst);
+
+    /// Content file. Prefer an entry staged earlier in THIS transaction (the destination PartWriteTxn
+    /// re-observes the blob via cold reuse — its own dependency); else carry forward from the
+    /// COMMITTED source part (adoptFromTree: tokenless evidence pinned by the witnessed live
+    /// source tree, W-EVIDENCE).
+    Cas::ManifestEntry entry;
+    if (auto * src_st = findStaging(*src))
+    {
+        auto it = std::find_if(src_st->entries.begin(), src_st->entries.end(),
+            [&](const Cas::ManifestEntry & e) { return e.path == src->file; });
+        if (it != src_st->entries.end())
+        {
+            entry = *it;
+            if (entry.placement == Cas::EntryPlacement::Blob)
+            {
+                /// Unified adopt dispatch. copy_pending=(&dst_st != src_st) so the pending
+                /// blob record is copied into dst_st only when the destination is a different part
+                /// (hardlink = copy semantics; same-part is a self-ref that shouldn't duplicate the record).
+                const auto * pb = findPendingBlob(*src_st, entry.ref);
+                adoptStagedBlob(pb, entry, dst_st, buildFor(*dst, dst_st), /*copy_pending=*/(&dst_st != src_st));
+            }
+            else if (entry.placement != Cas::EntryPlacement::Inline)
+                throw Exception(ErrorCodes::LOGICAL_ERROR,
+                    "ContentAddressed: staged hardlink of unsupported placement for {}", path_from);
+            entry.path = dst->file;
+            std::erase_if(dst_st.entries, [&](const Cas::ManifestEntry & e) { return e.path == entry.path; });
+            dst_st.entries.push_back(std::move(entry));
+            return;
+        }
+    }
+
+    /// Carry forward from the COMMITTED source part: read the source manifest, find the named entry,
+    /// record a TOKENLESS W-EVIDENCE dep for its blob (no HEAD before precommit; promote re-proves it).
+    /// ForceFresh getView == resolveRef(allow_stale=false) + readManifestShared, so this is the same
+    /// request pattern as before, now instrumented via the facade.
+    auto view = metadata_storage.partAccess()->getView(src->refKey(), Cas::Freshness::ForceFresh);
+    if (!view)
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+            "ContentAddressed: createHardLink source part missing: {}", path_from);
+    const auto * src_entry = view->findFile(src->file);
+    if (!src_entry)
+        throw Exception(ErrorCodes::FILE_DOESNT_EXIST,
+            "ContentAddressed: createHardLink source file missing in manifest: {}", path_from);
+    buildFor(*dst, dst_st).adoptEvidence(*src_entry);
+    entry = *src_entry;
+    entry.path = dst->file;
+    std::erase_if(dst_st.entries, [&](const Cas::ManifestEntry & e) { return e.path == entry.path; });
+    dst_st.entries.push_back(std::move(entry));
+}
+
+void ContentAddressedTransaction::setLastModified(const std::string &, const Poco::Timestamp &)
+{
+    /// Timestamps are derived for content addressing (the publish stamp), so accept and ignore them -- but
+    /// gate as a Write (previously-no-op site, rev.7 §1): never silently accept it on a Vanished/uncertain
+    /// disk.
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+}
+
+void ContentAddressedTransaction::chmod(const String &, mode_t)
+{
+    notYet("chmod");
+}
+
+void ContentAddressedTransaction::setReadOnly(const std::string &)
+{
+    /// Read-only flags have no content-addressed representation — accept and ignore them -- but gate as a
+    /// Write (previously-no-op site, rev.7 §1).
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+}
+
+void ContentAddressedTransaction::moveDirectory(const std::string & path_from, const std::string & path_to)
+{
+    /// Write gate (rev.7 §1): mutates durable refs immediately -- throw before touching them on a
+    /// Vanished/uncertain disk.
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+    /// Same call-time-durability-plus-compensation contract as `removeDirectory` above: this mutates
+    /// durable refs immediately rather than staging an intent for commit; see the contract note there.
+    auto src_p = Cas::parsePartFilePath(path_from);
+    auto dst_p = Cas::parsePartFilePath(path_to);
+    auto src = src_p ? metadata_storage.route(*src_p) : std::nullopt;
+    auto dst = dst_p ? metadata_storage.route(*dst_p) : std::nullopt;
+
+    /// RENAME TABLE / cross-engine move: both endpoints are TABLE dirs. Republish every ref (live
+    /// and folded-in `detached/`-prefixed refs) plus every verbatim file under the new table
+    /// identity, then drop the old namespace (the blobs/trees are content-addressed and untouched).
+    ///
+    /// There is no native cross-namespace atomicity (object storage has no directory rename, unlike a
+    /// non-CAS disk where RENAME TABLE is a single atomic directory rename). This is a best-effort
+    /// multi-op move, but it is RE-DRIVABLE/IDEMPOTENT: `republishRef` no-ops when the source ref is
+    /// already gone (resolveRef miss after a prior drive moved it), `putNamespaceFile` is
+    /// last-writer-wins (re-putting identical bytes is a no-op), and `dropNamespace` of an
+    /// already-empty/absent namespace is a no-op. So a mid-loop throw leaves the table SPLIT across the
+    /// two namespaces, but re-driving the SAME rename completes it. There is no in-call compensation;
+    /// true atomicity would need a durable move-journal (deliberately out of scope — it would touch the
+    /// tested GC/journal layer). On partial failure we log loudly so the split state is diagnosable.
+    if (auto src_table = Cas::parseTableUuid(path_from))
+    {
+        if (auto dst_table = Cas::parseTableUuid(path_to))
+        {
+            const auto from_ns = metadata_storage.liveNamespace(*src_table);
+            const auto to_ns = metadata_storage.liveNamespace(*dst_table);
+            try
+            {
+                for (const auto & [ref, _] : metadata_storage.store()->listRefs(from_ns))
+                    metadata_storage.partAccess()->republishRef({from_ns, ref}, {to_ns, ref});
+                /// Asymmetric by necessity: the SOURCE is read, so it resolves readably and contributes
+                /// nothing when it has no life; the DESTINATION is written, so it resolves the minting way
+                /// and is born here if the rename is what first creates it. Resolved only once the source
+                /// actually has files to move, so a rename of a table with none does not mint a life for a
+                /// destination nothing is written to.
+                const auto from_life = metadata_storage.readableNamespaceFilesLife(from_ns);
+                const std::vector<String> file_names
+                    = from_life ? metadata_storage.store()->listNamespaceFiles(*from_life) : std::vector<String>{};
+                if (!file_names.empty())
+                {
+                    const Cas::NamespaceLifeId to_life = metadata_storage.store()->namespaceLife(to_ns);
+                    for (const auto & name : file_names)
+                        if (auto bytes = metadata_storage.store()->getNamespaceFile(*from_life, name))
+                            metadata_storage.store()->putNamespaceFile(to_life, name, *bytes);
+                }
+                metadata_storage.partAccess()->dropNamespace(from_ns);
+            }
+            catch (...)
+            {
+                LOG_ERROR(getLogger("ContentAddressedTransaction"),
+                    "RENAME TABLE move was only partially applied: the table is SPLIT across namespaces "
+                    "'{}' and '{}'. The move is idempotent — retrying the same RENAME re-drives it to "
+                    "completion (already-moved refs/files are no-ops). Underlying error: {}",
+                    from_ns.string(), to_ns.string(), getCurrentExceptionMessage(/*with_stacktrace=*/false));
+                throw;
+            }
+            return;
+        }
+    }
+
+    if (!src || !dst)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "ContentAddressed: moveDirectory cannot classify {} -> {}", path_from, path_to);
+
+    /// Projection MATERIALIZE/merge renames the staged <proj>_<n>.tmp_proj subdir to <proj>.proj
+    /// inside the SAME staged part: re-key the staged entry-name prefixes so the published tree
+    /// carries the final keys.
+    if (!src->file.empty() && !dst->file.empty()
+        && src->ns.string() == dst->ns.string() && src->ref == dst->ref
+        && src->file.ends_with(".tmp_proj") && dst->file.ends_with(".proj"))
+    {
+        if (auto * st = findStaging(*src))
+        {
+            const std::string old_prefix = src->file + "/";
+            const std::string new_prefix = dst->file + "/";
+            for (auto & entry : st->entries)
+                if (entry.path.starts_with(old_prefix))
+                    entry.path = new_prefix + entry.path.substr(old_prefix.size());
+            return;
+        }
+    }
+
+    /// Every remaining shape is a PART-DIR move: (ns, ref) -> (ns', ref') with empty files. This
+    /// uniformly covers tmp->final (staged), committed renames (delete_tmp_, merge results),
+    /// DETACH (live -> detached ns), detached renames (attaching_/deleting_), and ATTACH
+    /// (detached -> live ns) - in the new layout they are all the same two moves: re-key any
+    /// staging, then move any committed ref.
+    if (!src->ref.empty() && src->file.empty() && !dst->ref.empty() && dst->file.empty())
+    {
+        const std::pair<std::string, std::string> src_key{src->ns.string(), src->ref};
+        const std::pair<std::string, std::string> dst_key{dst->ns.string(), dst->ref};
+        if (src_key == dst_key)
+            return;
+
+        /// Re-key a STAGED source into the destination. A move carries the
+        /// SOURCE's content to the destination — the POSIX `rename` semantic the rest of MergeTree
+        /// assumes, and exactly what `moveFile` does (`dst[file] = src_bytes`). On the happy path the
+        /// destination staging is freshly-created/empty so there is no collision at all; this only
+        /// matters if some future op-order stages the same mutable file under BOTH keys.
+        bool had_staged_source = false;
+        if (auto src_it = parts.find(src_key); src_it != parts.end())
+        {
+            had_staged_source = true;
+            PartStaging & dst_st = parts[dst_key];
+            PartStaging & src_st = src_it->second;
+            for (auto & entry : src_st.entries)
+            {
+                /// A genuine collision (both src and dst independently
+                /// staged DIFFERING bytes for the SAME path) is a fail-loud LOGICAL_ERROR rather than a
+                /// silent lost-update — the same defensive rule this loop used to apply only to the
+                /// three legacy mutable names now applies uniformly to every entry (that scoping was
+                /// itself a leftover of the mutable-file/entry split; there is only one kind of staged
+                /// file left). Identical bytes are a benign idempotent re-key; distinct paths are the
+                /// ordinary source-wins merge (a genuine collision is not expected in normal operation
+                /// — only some future op-order re-keying the same file under both stagings).
+                if (const auto existing = std::find_if(dst_st.entries.begin(), dst_st.entries.end(),
+                        [&](const Cas::ManifestEntry & e) { return e.path == entry.path; });
+                    existing != dst_st.entries.end() && !(*existing == entry))
+                    throw Exception(ErrorCodes::LOGICAL_ERROR,
+                        "ContentAddressed: moveDirectory file collision on '{}' ({} -> {}): "
+                        "source and destination staged different bytes for the same file",
+                        entry.path, src->ref, dst->ref);
+                std::erase_if(dst_st.entries, [&](const Cas::ManifestEntry & e) { return e.path == entry.path; });
+                dst_st.entries.push_back(std::move(entry));
+            }
+            /// Carry any staged removal marks forward too — a re-key of the
+            /// staging key must not silently drop them.
+            for (const auto & file : src_st.content_removed)
+                dst_st.content_removed.insert(file);
+            /// Move pending blobs from src to dst — they will be uploaded in dst's publishStaging.
+            for (auto & pb : src_st.pending_blobs)
+                dst_st.pending_blobs.push_back(std::move(pb));
+            src_st.pending_blobs.clear();
+            if (!dst_st.build)
+            {
+                dst_st.build = std::move(src_st.build);
+            }
+            else if (src_st.build)
+            {
+                /// Two Builds for one destination part: keep the destination's; the source build's
+                /// deps ride the staged entries (re-observed by the destination build at adopt
+                /// time is unnecessary - entries staged via putBlob/adopt carry deps in the SOURCE
+                /// build... merge conservatively by abandoning nothing and re-observing):
+                for (const auto & entry : dst_st.entries)
+                    if (entry.placement == Cas::EntryPlacement::Blob)
+                    {
+                        /// Unified adopt dispatch. Pending blob records were already moved
+                        /// to dst_st.pending_blobs above (MOVE semantics), so copy_pending=false.
+                        adoptStagedBlob(findPendingBlob(dst_st, entry.ref), entry, dst_st, *dst_st.build, /*copy_pending=*/false);
+                    }
+                src_st.build->abandon();
+            }
+            parts.erase(src_it);
+
+            /// A freshly-written part finalized tmp->final is re-keyed in the
+            /// overlay above (entries/marks/pending blobs/build moved src->dst). The durable publish
+            /// happens only in this transaction's commit (the existing publishStaging loop), not in
+            /// this method. `MergeTree` calls that commit from `Transaction::renameParts` while off
+            /// the `data_parts` lock and before the
+            /// Keeper multi. No early-published ref to compensate on abort within this method
+            /// (see ~ContentAddressedTransaction).
+        }
+
+        if (had_staged_source)
+        {
+            /// A nested text-index sub-storage (MergeTask/MutateTask createTemporaryTextIndexStorage)
+            /// may have DURABLY published a committed scratch ref at THIS part's own path holding only
+            /// `<part>/text_index_tmp/` files. That ref is not ours and is not staged; drop it now so the
+            /// overlay we publish in commit() is the authoritative manifest. Independent of our publish
+            /// timing (it targets an already-committed foreign ref), so it stays a call-time drop.
+            metadata_storage.partAccess()->dropRefIfPresent(src->refKey());
+            return;
+        }
+
+        /// Move any COMMITTED source ref (a merge/mutation result rename, DETACH, ATTACH, a
+        /// delete_tmp_ rename, an early-committed child ref being renamed away). Absent = a pure
+        /// staged/tmp move - nothing durable to touch.
+        metadata_storage.partAccess()->republishRef(src->refKey(), dst->refKey());
+        return;
+    }
+
+    throw Exception(ErrorCodes::LOGICAL_ERROR,
+        "ContentAddressed: moveDirectory from {} to {} has an unsupported shape", path_from, path_to);
+}
+
+void ContentAddressedTransaction::moveFile(const std::string & path_from, const std::string & path_to)
+{
+    /// Write gate (rev.7 §1). (`replaceFile` delegates here, so its own gate below is defense in depth.)
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+    /// Verbatim table-level files and loose mountpoint files: physically move the object (the
+    /// mutation entry tmp_mutation_N.txt -> mutation_N.txt rename; already durable from its finalize).
+    if (!Cas::isPartFilePath(path_from) && !Cas::isPartFilePath(path_to))
+    {
+        auto move_table_verbatim = [&](const Cas::TableFilePath & src_tf,
+                                       const Cas::TableFilePath & dst_tf)
+        {
+            const Cas::RootNamespace src_ns = metadata_storage.liveNamespace(src_tf.table_uuid);
+            const Cas::RootNamespace dst_ns = metadata_storage.liveNamespace(dst_tf.table_uuid);
+            if (src_ns.string() == dst_ns.string() && src_tf.tail == dst_tf.tail)
+                return;
+            /// A verbatim rename is emulated as get(src) -> put(dst) -> remove(src) because object
+            /// storage has no atomic rename. SINGLE-WRITER CONTRACT: only the owning server renames its
+            /// own table-level verbatim files (mutation entries), so there is no concurrent writer to
+            /// race the blind put(dst) against — the put's last-writer-wins is safe under that contract.
+            /// Idempotent re-drive: if the source is already gone but the destination is present, a
+            /// previous drive completed this move — treat as done (matches a re-driven FS rename, which
+            /// is an ENOENT-tolerant no-op) instead of throwing FILE_DOESNT_EXIST. An unrelated
+            /// pre-existing destination can never reach this branch: destination names derive
+            /// deterministically from source names, and the SINGLE-WRITER contract means only this
+            /// move's own prior drive can have produced it.
+            /// The source resolves readably (a move reads it) and the destination the minting way (a move
+            /// writes it). A source namespace with no readable life has no file to move, which is the same
+            /// outcome as an absent object and takes the identical already-moved / genuinely-missing split
+            /// below -- so absence of a life is not a separate error path.
+            const auto src_life = metadata_storage.readableNamespaceFilesLife(src_ns);
+            const auto src_bytes = src_life
+                ? metadata_storage.store()->getNamespaceFile(*src_life, src_tf.tail)
+                : std::nullopt;
+            if (!src_bytes)
+            {
+                const auto dst_probe = metadata_storage.readableNamespaceFilesLife(dst_ns);
+                if (dst_probe && metadata_storage.store()->getNamespaceFile(*dst_probe, dst_tf.tail))
+                    return;
+                throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: moveFile source missing: {}", path_from);
+            }
+            metadata_storage.store()->putNamespaceFile(
+                metadata_storage.store()->namespaceLife(dst_ns), dst_tf.tail, *src_bytes);
+            metadata_storage.store()->removeNamespaceFile(*src_life, src_tf.tail);
+        };
+        auto src_tf = Cas::parseTableFilePath(path_from);
+        auto dst_tf = Cas::parseTableFilePath(path_to);
+        if (src_tf && dst_tf)
+        {
+            move_table_verbatim(*src_tf, *dst_tf);
+            return;
+        }
+        /// Loose mountpoint files (rare): read + put + remove plain objects. The same single-writer
+        /// contract + idempotent re-drive as the table-verbatim branch above.
+        const std::string src_key = metadata_storage.serverRootId() + "/" + path_from;
+        const std::string dst_key = metadata_storage.serverRootId() + "/" + path_to;
+        if (src_key == dst_key)
+            return;
+        auto bytes = metadata_storage.store()->getMountpointObject(src_key);
+        if (!bytes)
+        {
+            if (metadata_storage.store()->getMountpointObject(dst_key))
+                return;
+            throw Exception(ErrorCodes::FILE_DOESNT_EXIST, "ContentAddressed: moveFile source missing: {}", path_from);
+        }
+        metadata_storage.store()->putMountpointObject(dst_key, *bytes);
+        metadata_storage.store()->removeMountpointObject(src_key);
+        return;
+    }
+
+    auto src = routeOf(path_from);
+    auto dst = routeOf(path_to);
+    /// A part-DIRECTORY rename reaching moveFile (PartsTemporaryRename::rollBackAll undoes an
+    /// attach via moveFile): delegate to moveDirectory, which owns directory shapes.
+    if (src && dst && !src->ref.empty() && src->file.empty() && !dst->ref.empty() && dst->file.empty())
+    {
+        moveDirectory(path_from, path_to);
+        return;
+    }
+    if (!src || src->file.empty() || !dst || dst->file.empty())
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "ContentAddressed: moveFile requires two part-file paths: {} -> {}", path_from, path_to);
+
+    auto & src_st = stagingFor(*src);
+    auto & dst_st = stagingFor(*dst);
+
+    /// Staged content entry re-keys in place (cross-part included; dependencies follow the entries).
+    /// canonical policy: SOURCE-wins — a move/rename carries the source's content to the destination,
+    /// overwriting any prior dest bytes (the POSIX `rename` semantic, and what the atomic-write
+    /// `.tmp -> final` rename requires). moveDirectory's staged merge is aligned to this same policy.
+    auto it = std::find_if(src_st.entries.begin(), src_st.entries.end(),
+        [&](const Cas::ManifestEntry & e) { return e.path == src->file; });
+    if (it != src_st.entries.end())
+    {
+        auto entry = std::move(*it);
+        src_st.entries.erase(it);
+        entry.path = dst->file;
+        if (&src_st != &dst_st && entry.placement == Cas::EntryPlacement::Blob)
+        {
+            /// Unified adopt dispatch. MOVE semantics — physically move the pending blob
+            /// record from src_st to dst_st FIRST (so dst_st owns the upload), then call adoptStagedBlob
+            /// with copy_pending=false (the record is already in dst_st; no additional copy needed).
+            auto pb_it = std::find_if(src_st.pending_blobs.begin(), src_st.pending_blobs.end(),
+                [&](const PartStaging::PendingBlob & pb) { return pb.ref == entry.ref; });
+            if (pb_it != src_st.pending_blobs.end())
+            {
+                dst_st.pending_blobs.push_back(std::move(*pb_it));
+                src_st.pending_blobs.erase(pb_it);
+            }
+            adoptStagedBlob(findPendingBlob(dst_st, entry.ref), entry, dst_st, buildFor(*dst, dst_st), /*copy_pending=*/false);
+        }
+        std::erase_if(dst_st.entries, [&](const Cas::ManifestEntry & e) { return e.path == entry.path; });
+        dst_st.entries.push_back(std::move(entry));
+        return;
+    }
+    /// Source not staged in this transaction: this would cover a standalone one-shot rename of a
+    /// committed `txn_version.txt` file. Atomic-write storages (including CA) bypass that rename:
+    /// `VersionMetadataOnDisk::storeInfoToDataPartStorage` writes `txn_version.txt` directly, with no
+    /// `.tmp` + `replaceFile` dance. This branch therefore has no live caller and is retained only as
+    /// a fail-loud guard for an unsupported mutation shape.
+    throw Exception(ErrorCodes::LOGICAL_ERROR, "ContentAddressed: moveFile source not staged: {}", path_from);
+}
+
+void ContentAddressedTransaction::replaceFile(const std::string & path_from, const std::string & path_to)
+{
+    /// Write gate (rev.7 §1): refuse before dropping staged destination state on a Vanished/uncertain disk.
+    metadata_storage.checkOpAdmitted(CasOpClass::Write);
+    /// replaceFile = moveFile that overwrites the destination. Drop any staged destination state
+    /// first, then delegate (the verbatim branch's putNamespaceFile already overwrites).
+    if (auto dst = routeOf(path_to); dst && !dst->file.empty())
+    {
+        auto & dst_st = stagingFor(*dst);
+        /// A matching pending_blobs record (if any) is left in place — its temp file is cleaned by
+        /// cleanupPendingTempFiles at commit end, and the orphaned record is filtered out of the
+        /// publish upload by the staged-tree-hash check in publishStaging. We do NOT purge it
+        /// eagerly because the same hash may still be referenced by another staged entry.
+        std::erase_if(dst_st.entries, [&](const Cas::ManifestEntry & e) { return e.path == dst->file; });
+    }
+    moveFile(path_from, path_to);
+}
+
+void ContentAddressedTransaction::unlinkFile(const std::string & path, bool if_exists, bool /*should_remove_objects*/)
+{
+    /// Part file. Two sub-cases:
+    ///   1. A file STAGED in this transaction (content entry or legacy mutable bytes): drop the
+    ///      staged state so it never reaches the published tree.
+    ///   2. A COMMITTED CONTENT file (not staged here): stage a REMOVAL MARK
+    ///      (`content_removed`). The mark is resolved at publish (`publishStaging`): a repoint
+    ///      republishes the manifest minus the removed paths, UNLESS this same transaction also
+    ///      drops the whole part directory (`removeDirectory`), in which case the mark is
+    ///      superseded — see `removeDirectory` below.
+    ///
+    /// This is a load-bearing invariant; do not "fix" it with a blanket fail-closed assert:
+    /// On a content-addressed disk a committed part is ONE atomic ref (its manifest tree); the removal
+    /// UNIT is the whole-part ref-drop done by `removeDirectory(<part>)`, NOT per-file unlinks. The
+    /// MergeTree fast-removal path (IMergeTreeDataPart::remove) unlinks EVERY part file one-by-one and
+    /// THEN calls `removeDirectory` — so a batched per-file unlink storm immediately followed by a
+    /// ref-drop in the SAME transaction must cost exactly one ref-drop and zero repoints, not one
+    /// repoint per unlinked file. `removeDirectory` clears any marks staged here for the same ref
+    /// before the transaction publishes, which is what makes the storm-then-drop shape free. A lone
+    /// surgical unlink NOT followed by a ref-drop in the same transaction (ATTACH's
+    /// `removeVersionMetadata`, a future backfill/repair delete) resolves to one repoint-remove —
+    /// this closes the file's former fail-open (a committed content file could never actually be
+    /// deleted on its own; this behavior now closes that earlier fail-open.
+    ///
+    /// Remove gate (rev.7 §1): a Vanished disk answers no-op success; a transient / IdentityLost disk
+    /// throws 668.
+    if (metadata_storage.checkOpAdmitted(CasOpClass::Remove) == CasOpAdmission::TruthAbsent)
+        return;
+    if (auto r = routeOf(path); r && !r->file.empty())
+    {
+        auto & st = stagingFor(*r);
+        const bool staged_here = std::any_of(st.entries.begin(), st.entries.end(),
+                           [&](const Cas::ManifestEntry & e) { return e.path == r->file; });
+        /// A matching pending_blobs record (if any) is left in place — its temp file is cleaned by
+        /// cleanupPendingTempFiles at commit end, and the orphaned record is filtered out of the
+        /// publish upload by the staged-tree-hash check in publishStaging. We do NOT purge it
+        /// eagerly because the same hash may still be referenced by another staged entry.
+        std::erase_if(st.entries, [&](const Cas::ManifestEntry & e) { return e.path == r->file; });
+        if (!staged_here)
+        {
+            /// One mandatory body-HEAD per (transaction, ref), not per file: the MergeTree fast-removal
+            /// path unlinks every file of the part through THIS transaction right before removeDirectory.
+            /// The first unlink re-proves the body ForceFresh; the rest of the burst reuses that proof.
+            const String memo_key = r->refKey().cacheKey();
+            const bool already_proven = force_fresh_validated_refs.contains(memo_key);
+            const auto view = metadata_storage.partAccess()->getView(
+                r->refKey(), already_proven ? Cas::Freshness::CachedForLoad : Cas::Freshness::ForceFresh);
+            if (view && !already_proven)
+                force_fresh_validated_refs.insert(memo_key);
+            if (!view || !view->hasFile(r->file))
+            {
+                if (if_exists)
+                    return;
+                throw Exception(
+                    ErrorCodes::FILE_DOESNT_EXIST,
+                    "ContentAddressed: unlinkFile target does not exist: {}",
+                    path);
+            }
+            st.content_removed.insert(r->file);
+        }
+        return;
+    }
+
+    /// Verbatim table-level / loose mountpoint file: reclaim the object NOW (GC never scans them;
+    /// a pruned mutation entry would otherwise leak until DROP.
+    if (auto tf = Cas::parseTableFilePath(path))
+    {
+        /// Readable resolution, i.e. the non-creating one: an unlink must not birth a namespace -- and
+        /// `unlinkFile(..., if_exists = true)` is called from cleanup paths whose whole contract is to be
+        /// a no-op. No life means no such file, exactly the absent case the branch below already handles.
+        const auto life = metadata_storage.readableNamespaceFilesLife(
+            metadata_storage.liveNamespace(tf->table_uuid));
+        if (!life || !metadata_storage.store()->getNamespaceFile(*life, tf->tail))
+        {
+            if (if_exists)
+                return;
+            throw Exception(
+                ErrorCodes::FILE_DOESNT_EXIST,
+                "ContentAddressed: unlinkFile target does not exist: {}",
+                path);
+        }
+        metadata_storage.store()->removeNamespaceFile(*life, tf->tail);
+        return;
+    }
+    /// Loose mountpoint file: exact-token delete of the plain object.
+    const String key = metadata_storage.serverRootId() + "/" + path;
+    if (!metadata_storage.store()->getMountpointObject(key))
+    {
+        if (if_exists)
+            return;
+        throw Exception(
+            ErrorCodes::FILE_DOESNT_EXIST,
+            "ContentAddressed: unlinkFile target does not exist: {}",
+            path);
+    }
+    metadata_storage.store()->removeMountpointObject(key);
+}
+
+void ContentAddressedTransaction::truncateFile(const std::string &, size_t)
+{
+    throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+        "truncateFile is not supported on a content-addressed disk (blobs are immutable; "
+        "whole-file rewrites replace the staged entry instead)");
+}
+
+}
+
+namespace DB::Cas
+{
+
+namespace
+{
+/// Ceiling for a CAS write-buffer allocation. An extreme `max_compress_block_size` (or any other
+/// out-of-range buffer-size setting -- fuzzed or misconfigured) flows into `writeFile`'s `buf_size` /
+/// `adaptive_write_buffer_initial_size` and, unclamped, reaches `Memory::alloc` where the allocator's
+/// `checkSize` (>= 0x8000000000000000) fires a `LOGICAL_ERROR` and aborts the server in
+/// debug/sanitizer builds. The ordinary MergeTree writers clamp compress-block sizes to 256 MiB
+/// (`MergeTreeWriterSettings::MAX_COMPRESS_BLOCK_SIZE`) for exactly this reason; the CAS write path
+/// received the value unclamped. Mirror that ceiling here, at the allocation site, so no caller can
+/// pass an absurd size to the allocator. 256 MiB is duplicated (not #included) to keep the Disks layer
+/// free of a Storages/MergeTree dependency; the regression guard is
+/// `04070_no_crash_extreme_compress_block_size` run on a content-addressed storage policy.
+constexpr size_t kMaxCasWriteBufferBytes = 256ULL * 1024 * 1024;
+
+size_t clampCasWriteBufferSize(size_t size)
+{
+    return std::min(size, kMaxCasWriteBufferBytes);
+}
+}
+
+void fanOutBlobUploads(
+    PartWriteTxn & build,
+    std::span<const BlobUploadRequest> requests,
+    ThreadPool & pool,
+    const BlobUploadFanoutHooksForTest * hooks)
+{
+    /// Group by unique ref. Staged-hardlink copies push a DUPLICATE pending-blob record for one BlobRef,
+    /// and the fan-out must launch exactly ONE task per unique ref (spec §1 "One task per unique ref").
+    /// An ordered map gives a DETERMINISTIC dispatch order (ascending `BlobRef`), which fixes the "first
+    /// error" of the merge-nothing contract to a stable task so a failure is reproducible.
+    std::map<BlobRef, BlobUploadRequest> grouped;
+    for (const auto & req : requests)
+    {
+        /// Fail-close: the fan-out groups and conflict-checks on `declared_size`, while `source.size` is
+        /// the per-attempt streaming byte authority. A wiring bug that let them diverge would group on
+        /// one value while streaming another — reject it rather than upload a wrong-length body.
+        if (req.declared_size != req.source.size)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "fanOutBlobUploads: request for {} declares size {} but its source is sized {} -- the "
+                "grouping key and the streaming byte authority must agree",
+                blobIdOf(req.ref), req.declared_size, req.source.size);
+        const auto [it, inserted] = grouped.try_emplace(req.ref, req);
+        if (!inserted && it->second.declared_size != req.declared_size)
+            throw Exception(ErrorCodes::LOGICAL_ERROR,
+                "fanOutBlobUploads: conflicting declared sizes for {} ({} vs {}) -- staged-hardlink "
+                "copies of one ref must agree on size (one task, one dep per unique ref)",
+                blobIdOf(req.ref), it->second.declared_size, req.declared_size);
+    }
+
+    if (grouped.empty())
+        return;
+
+    ProfileEvents::increment(ProfileEvents::CASBlobUploadFanoutBatches);
+
+    /// One result slot per unique ref, pre-sized so element addresses are STABLE: each task writes ONLY
+    /// its own slot (no data race on the vector) and the vector never reallocates. Declared BEFORE the
+    /// runner and the drain guard so it OUTLIVES them — the drain guard joins every scheduled task before
+    /// `results` is destroyed on EVERY path, including a throw raised during the dispatch loop below (the
+    /// B90 lesson, `threadPoolCallbackRunner.h:68`). Tasks capture only owning/value state: a stable slot
+    /// pointer, the request by value (its source is captured by value too), and the txn pointer whose
+    /// pointee outlives the runner.
+    std::vector<BlobUploadResult> results(grouped.size());
+
+    {
+        /// `pool` is disjoint from the S3 writer pool an upload may itself use, and the calling thread
+        /// only submits and joins — it never occupies a pool slot — so a size-1 pool degenerates to a
+        /// correct serial run and can never deadlock. `ThreadName::UNKNOWN`: stage-1 must not add a
+        /// CAS-specific `ThreadName` to the shared enum (`setThreadName.h` is outside the allowed file
+        /// set), and UNKNOWN is the no-op name; it also stays clear of the pool-thread self-join
+        /// assertions in `CasPool`. The query `ThreadGroup` is still propagated per task by the runner.
+        using RunnerTask = ThreadPoolCallbackRunnerLocal<void>::Task;
+        ThreadPoolCallbackRunnerLocal<void> runner(pool, ThreadName::UNKNOWN);
+        const PartWriteTxn * txn = &build;   /// `uploadBlobDetached` is const + build-neutral: safe off-thread
+        /// We track scheduled tasks in OUR OWN vector (via `enqueueAndGiveOwnership`) rather than the
+        /// runner's `enqueueAndKeepTrack`: that helper schedules a task and only THEN appends its handle
+        /// to an UNRESERVED tracking vector, so a `bad_alloc` at the append would leave a
+        /// scheduled-but-untracked task the runner's destructor cannot join — it would run later against
+        /// the already-destroyed `results`/txn (a use-after-free; codex stage-1 review, Critical).
+        /// PRE-RESERVING `handles` to the exact task count makes the append after each schedule a
+        /// no-throw operation, so a task is NEVER scheduled without being tracked in the SAME expression.
+        std::vector<std::shared_ptr<RunnerTask>> handles;
+        handles.reserve(grouped.size());
+        /// Drain on EVERY path: the runner's destructor only joins tasks IT owns (we use
+        /// `enqueueAndGiveOwnership`, so its own set stays empty), so WE must join every scheduled task
+        /// before `results` and the upload sources are destroyed — including when the dispatch loop
+        /// throws (an `on_dispatch`/`after_enqueue` seam, or a scheduling failure mid-loop). Declared
+        /// AFTER `handles`/`runner` so it runs FIRST on scope exit, joining while both are still alive;
+        /// `waitForAllToFinish` only waits (never throws), so it is safe on the unwinding path (the B90
+        /// lesson, `threadPoolCallbackRunner.h:68`).
+        SCOPE_EXIT_SAFE({ ThreadPoolCallbackRunnerLocal<void>::waitForAllToFinish(handles); });
+        size_t idx = 0;
+        for (const auto & [ref, req] : grouped)
+        {
+            BlobUploadResult * slot = &results[idx++];
+            BlobUploadRequest task_req = req;
+            const BlobUploadFanoutHooksForTest * task_hooks = hooks;
+            if (hooks && hooks->on_dispatch)
+                hooks->on_dispatch(ref);   /// may throw ⇒ the drain guard joins already-scheduled tasks
+            /// Schedule and track in ONE no-throw step: `enqueueAndGiveOwnership` returns the handle (the
+            /// task is now runnable) and the pre-reserved `emplace_back` records it without allocating, so
+            /// there is no window in which a scheduled task is untracked.
+            handles.emplace_back(runner.enqueueAndGiveOwnership([slot, txn, req_by_value = std::move(task_req), task_hooks]
+            {
+                if (task_hooks && task_hooks->in_task)
+                    task_hooks->in_task(req_by_value.ref);
+                *slot = txn->uploadBlobDetached(req_by_value);
+            }));
+            if (task_hooks && task_hooks->after_enqueue)
+                task_hooks->after_enqueue(ref);   /// task already tracked ⇒ the drain guard joins it too
+            ProfileEvents::increment(ProfileEvents::CASBlobUploadFanoutTasks);
+        }
+        /// Drain ALL tasks, then rethrow the FIRST (ascending-ref dispatch order) that failed. A rethrow
+        /// bypasses the merge below, so NOTHING is merged: `build` stays at its pre-fan-out pending-dep
+        /// state (merge-nothing). On success this clears `handles`, so the drain guard above then waits an
+        /// empty set; on any throw it leaves them and the guard joins the survivors (already all done).
+        ThreadPoolCallbackRunnerLocal<void>::waitForAllToFinishAndRethrowFirstError(handles);
+    }
+
+    /// Every task succeeded (else we rethrew above): fold all results into `build` on this (the owning
+    /// writer) thread, all-or-nothing (`mergeBlobUploadResults` prevalidates, then build-and-swaps).
+    build.mergeBlobUploadResults(results);
+}
+
+
+CaContentWriteBuffer::CaContentWriteBuffer(
+    std::string temp_dir,
+    Cas::BlobHashAlgo hash_algo,
+    size_t buf_size,
+    bool use_adaptive_buffer_size,
+    size_t adaptive_buffer_initial_size,
+    OnFinalized on_finalized_)
+    : WriteBufferFromFileBase(clampCasWriteBufferSize(use_adaptive_buffer_size ? adaptive_buffer_initial_size : buf_size), nullptr, 0)
+    , on_finalized(std::move(on_finalized_))
+{
+    fs::create_directories(temp_dir);
+    temp_path = temp_dir + "/" + getRandomASCIIString(32) + ".tmp";
+
+    /// The spill buffer is a SECOND per-stream buffer; thread the adaptive flag into it too so a
+    /// wide part keeps its footprint small. Its IO is a local temp file, not the remote stream.
+    sink = std::make_unique<WriteBufferFromFile>(
+        temp_path,
+        clampCasWriteBufferSize(buf_size),
+        /*flags=*/-1,
+        /*throttler=*/nullptr,
+        /*mode=*/0666,
+        /*existing_memory=*/nullptr,
+        /*alignment=*/0,
+        use_adaptive_buffer_size,
+        clampCasWriteBufferSize(adaptive_buffer_initial_size));
+    hashing = Cas::makeBlobHashingWriteBuffer(hash_algo, *sink);
+}
+
+CaContentWriteBuffer::CaContentWriteBuffer(
+    std::unique_ptr<WriteBufferFromFileBase> object_store_sink,
+    std::string object_key,
+    std::string envelope_header,
+    Cas::BlobHashAlgo hash_algo,
+    size_t buf_size,
+    bool use_adaptive_buffer_size,
+    size_t adaptive_buffer_initial_size,
+    OnFinalized on_finalized_,
+    std::function<void()> check_fence_before_finalize_)
+    : WriteBufferFromFileBase(clampCasWriteBufferSize(use_adaptive_buffer_size ? adaptive_buffer_initial_size : buf_size), nullptr, 0)
+    , on_finalized(std::move(on_finalized_))
+    , temp_path(std::move(object_key))
+    , is_s3_staging(true)
+    , sink(std::move(object_store_sink))
+    , check_fence_before_finalize(std::move(check_fence_before_finalize_))
+{
+    /// The sink is ALREADY opened against the staging object by the caller (writeFile) — this
+    /// constructor wraps it in the hashing chain, exactly like the local-temp-file mode.
+    ///
+    /// Write the CABL envelope header to the sink first, directly —
+    /// bypassing `hashing` (so it is excluded from the content hash) and this outer buffer's `count()`
+    /// (so the reported size is the payload only). The staging object therefore holds `[header][payload]`
+    /// and the promote stays a verbatim server-side copy. Only the payload the caller subsequently writes
+    /// through THIS buffer flows through `hashing`. The header write precedes any payload write, so the
+    /// on-object byte order is header-then-payload.
+    if (!envelope_header.empty())
+        sink->write(envelope_header.data(), envelope_header.size());
+
+    /// The adaptive-sizing params only affect THIS outer buffer (mirroring the Local ctor above); the
+    /// sink's own buffering was decided by the caller when it opened the object-store write.
+    hashing = Cas::makeBlobHashingWriteBuffer(hash_algo, *sink);
+}
+
+CaContentWriteBuffer::~CaContentWriteBuffer()
+{
+    /// Best-effort cleanup if finalize was never reached (exception unwind / cancel).
+    cancel();
+    /// If on_finalized ran successfully the transaction (Local mode) or a later promote
+    /// path (S3 mode) owns the staged bytes and cleans them up. Do not remove them here. S3-mode
+    /// staging objects are never removed by this class at all (see cancelImpl / removeTempFile).
+    if (!temp_ownership_transferred && !is_s3_staging)
+        removeTempFile();
+}
+
+void CaContentWriteBuffer::nextImpl()
+{
+    if (!offset())
+        return;
+    hashing->write(working_buffer.begin(), offset());
+}
+
+void CaContentWriteBuffer::finalizeImpl()
+{
+    next();
+    const size_t size = count();
+
+    /// getHashHex flushes the chain and returns the streaming digest (the pool's selected algo) of
+    /// everything written, as 32 lowercase hex chars.
+    const std::string hash_hex = hashing->getHashHex();
+
+    hashing->finalize();
+
+    /// rev.7 [C2]: re-check the fence-generation admission IMMEDIATELY before the durable backend call
+    /// (S3 mode's `sink->finalize()` completes the staging object -- Local mode never sets this
+    /// callback). A fence trip or re-arm since construction aborts here with the typed transient error,
+    /// before the upload becomes durable.
+    if (check_fence_before_finalize)
+        check_fence_before_finalize();
+
+    sink->finalize();
+
+    /// On successful finalize, ownership of temp_path (Local: the local temp path; S3: the
+    /// staging object key) transfers to the caller (the transaction uploads/promotes it and cleans
+    /// up). cancel() still removes/cancels it.
+    if (on_finalized)
+    {
+        on_finalized(hash_hex, size, temp_path);
+        temp_ownership_transferred = true;
+    }
+}
+
+void CaContentWriteBuffer::cancelImpl() noexcept
+{
+    if (hashing)
+        hashing->cancel();
+    if (sink)
+        sink->cancel();
+    /// S3 mode: `temp_path` is a remote object key, not a path on this filesystem — do NOT attempt
+    /// to delete the (possibly partially-written) staging object here. Cancelling `sink` above is
+    /// enough to make sure no partial finalize happens; reclaiming an orphaned staging object is the
+    /// mount-lease sweeper's job.
+    if (!is_s3_staging)
+        removeTempFile();
+}
+
+void CaContentWriteBuffer::removeTempFile() noexcept
+{
+    std::error_code ec;
+    fs::remove(temp_path, ec);
+}
+
+void CaContentWriteBuffer::sync()
+{
+    next();
+    hashing->next();
+    sink->sync();
+}
+
+std::string CaContentWriteBuffer::getFileName() const
+{
+    return temp_path;
+}
+
+CaInlineWriteBuffer::CaInlineWriteBuffer(OnInlined on_inlined_)
+    : WriteBufferFromFileBase(DBMS_DEFAULT_BUFFER_SIZE, nullptr, 0)
+    , on_inlined(std::move(on_inlined_))
+{
+}
+
+CaInlineWriteBuffer::~CaInlineWriteBuffer()
+{
+    cancel();
+}
+
+void CaInlineWriteBuffer::nextImpl()
+{
+    if (!offset())
+        return;
+    accumulated.append(working_buffer.begin(), offset());
+}
+
+void CaInlineWriteBuffer::finalizeImpl()
+{
+    next();
+    if (on_inlined)
+        on_inlined(std::move(accumulated));
+}
+
+void CaInlineWriteBuffer::sync()
+{
+    next();
+}
+
+std::string CaInlineWriteBuffer::getFileName() const
+{
+    return "ca_inline";
+}
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h
new file mode 100644
index 000000000000..90fc00c01352
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h
@@ -0,0 +1,420 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/IMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h>
+#include <Disks/WriteMode.h>
+#include <Common/ThreadPool_fwd.h>
+#include <span>
+#include <Core/Defines.h>
+#include <IO/WriteBufferFromFile.h>
+#include <IO/WriteBufferFromFileBase.h>
+#include <IO/WriteSettings.h>
+#include <functional>
+#include <map>
+#include <memory>
+#include <optional>
+#include <set>
+#include <string>
+#include <string_view>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+
+namespace DB
+{
+
+/// Owns the metadata-side overlay for one object-storage transaction. Part files are accumulated by
+/// routed namespace/ref, then published as manifest trees and refs by `commit`; verbatim namespace
+/// and mountpoint files are written immediately when their buffers finalize. A part uses one
+/// `Cas::PartWriteTxn`, created lazily before the first staged dependency, so the durable manifest
+/// edge is established before the pool observes or uploads a new blob.
+///
+/// Content blobs are first represented by local or S3 staging objects and are uploaded only during
+/// publication. Inline entries remain in the manifest tree when they fit `INLINE_CAP`. The
+/// destructor removes private local staging, leaves aborted S3 staging for the mount-lease sweeper,
+/// and abandons any open part builds; it never publishes an uncommitted ref.
+class ContentAddressedTransaction : public IMetadataTransaction
+{
+public:
+    /// Borrows the metadata storage for the transaction lifetime; staged builds and resources are
+    /// owned by this transaction and finalized or abandoned by `commit`/destruction.
+    explicit ContentAddressedTransaction(ContentAddressedMetadataStorage & metadata_storage_);
+
+    bool supportsChmod() const override { return false; }
+
+    /// Publishes every staged part. Publication is ordered so each new blob is named by a durable
+    /// precommit edge first; if a later part fails, only refs created by this call are compensated.
+    void commit(const TransactionCommitOptionsVariant & options) override;
+
+    /// Accepts only `NoCommitOptions`, delegates to `commit`, and returns its successful outcome.
+    TransactionCommitOutcomeVariant tryCommit(const TransactionCommitOptionsVariant & options) override;
+
+    /// Unsupported because content-addressed keys are generated from payload hashes, not paths.
+    ObjectStorageKey generateObjectKeyForPath(const std::string & path) override;
+    /// Returns no removal list: shared CAS objects are reclaimed only by garbage collection.
+    StoredObjects getSubmittedForRemovalBlobs() override;
+
+    /// Resolves a staged entry to its committed-style storage description. Pending blobs return no
+    /// object because their bytes still reside in staging and must be read through the overlay path.
+    std::optional<StoredObjects> tryGetInFlightStorageObjects(const std::string & path) const override;
+    /// Opens an inline entry, pending local/S3 staging object, or already uploaded blob for a
+    /// read-your-writes operation; returns null when the path is outside this transaction's overlay.
+    std::unique_ptr<ReadBufferFromFileBase> tryReadFileInFlight(
+        const std::string & path, const ReadSettings & settings, std::optional<size_t> read_hint) const override;
+    /// Returns the logical size of a staged entry, including inline bytes and pending blob payloads.
+    std::optional<uint64_t> tryGetInFlightFileSize(const std::string & path) const override;
+    /// Reports only inner staged directories. The bare part directory intentionally remains absent
+    /// so cleanup of a temporary, dedup-rejected part does not treat it as a real directory.
+    bool hasInFlightDirectory(const std::string & path) const override;
+    /// Lists immediate child names visible in the staged directory overlay.
+    std::vector<std::string> listInFlightDirectory(const std::string & path) const override;
+
+    /// This legacy object-list operation has no content-addressed equivalent and throws.
+    void createMetadataFile(const std::string & path, const StoredObjects & objects) override;
+
+    /// Creates the buffer used by the disk transaction for this path. The returned wrapper keeps
+    /// `owner` alive until deferred finalization, because its callback captures this transaction;
+    /// it also applies the content-addressed append and autocommit rules before selecting a blob,
+    /// inline, or verbatim-file buffer. Part blobs cannot be published independently of their
+    /// manifest, while an inline-eligible standalone part file may commit through the repoint path.
+    std::unique_ptr<WriteBufferFromFileBase> tryCreateWriteBuffer(
+        const std::shared_ptr<IDiskTransaction> & owner,
+        const std::string & path, size_t buf_size, WriteMode mode,
+        const WriteSettings & settings, bool autocommit) override;
+
+    /// Creates the inner buffer for a content-addressed path. Part blobs are hashed while being
+    /// staged, small metadata files are accumulated in memory and classified at finalize, and
+    /// verbatim namespace or mountpoint files are read-modify-written when append is requested.
+    std::unique_ptr<WriteBufferFromFileBase> writeFile(
+        const std::string & path,
+        size_t buf_size,
+        WriteMode mode,
+        const WriteSettings & settings);
+
+    /// Directory creation is a metadata no-op because object storage has no directory objects.
+    void createDirectory(const std::string & path) override;
+    /// Recursive directory creation is likewise a no-op; entries create their own prefixes.
+    void createDirectoryRecursive(const std::string & path) override;
+    /// Drops a part ref, or does nothing for a non-part directory. A whole-part drop supersedes any
+    /// per-file removal marks staged earlier in this transaction.
+    void removeDirectory(const std::string & path) override;
+    /// Removes refs, namespace files, and shadow objects while leaving shared CAS objects to GC.
+    void removeRecursive(const std::string & path, const ShouldRemoveObjectsPredicate & should_remove_objects) override;
+    /// Copies an entry between parts, preserving pending staging ownership and tokenless evidence.
+    void createHardLink(const std::string & path_from, const std::string & path_to) override;
+    /// These filesystem metadata operations are unsupported because CAS metadata is immutable and
+    /// object storage exposes neither POSIX timestamps nor mode bits.
+    void setLastModified(const std::string & path, const Poco::Timestamp & timestamp) override;
+    void chmod(const String & path, mode_t mode) override;
+    void setReadOnly(const std::string & path) override;
+    /// Re-keys or merges staged part entries without publishing early; non-part paths use the
+    /// corresponding object-storage copy/remove semantics.
+    void moveDirectory(const std::string & path_from, const std::string & path_to) override;
+    /// Moves a staged entry and its pending ownership, or copies/removes a verbatim object as needed.
+    void moveFile(const std::string & path_from, const std::string & path_to) override;
+    /// Replaces the destination while preserving the source's content-addressed ownership rules.
+    void replaceFile(const std::string & path_from, const std::string & path_to) override;
+    /// Unlinks a staged entry or records a removal mark for an already committed manifest; shared
+    /// blobs are never deleted directly.
+    void unlinkFile(const std::string & path, bool if_exists, bool should_remove_objects) override;
+    /// Truncation is not representable without rewriting the content and is unsupported.
+    void truncateFile(const std::string & path, size_t size) override;
+
+    /// Abandons open builds and cleans owned staging without publishing an uncommitted ref.
+    ~ContentAddressedTransaction() override;
+
+protected:
+    ContentAddressedMetadataStorage & metadata_storage;
+
+private:
+    /// State for one routed part. The build is opened lazily at the first staged write so every
+    /// manifest dependency is recorded by the same `Cas::PartWriteTxn`; pending blobs remain in
+    /// staging until publication.
+    struct PartStaging
+    {
+        /// Created lazily when a staged blob, inline entry, or adopted entry first needs publication.
+        Cas::PartWriteTxnPtr build;
+        std::vector<Cas::ManifestEntry> entries;   /// staged manifest entries (uploads + adoptions)
+        /// Paths removed from a committed part's manifest by this transaction. Publication carries
+        /// forward all other committed entries. If `removeDirectory` drops the whole ref in the same
+        /// transaction, that ref-drop supersedes these marks and clears them.
+        std::set<std::string> content_removed;
+        bool published = false;                    /// set by publishStaging during commit(); the commit
+                                                   /// loop is idempotent (never re-publishes a staging).
+
+        /// `staging_key` is either a private local temp path or an S3 staging object key returned by
+        /// `CaContentWriteBuffer` at finalize. The backend selects cleanup and read-your-writes:
+        /// local files are removed by this transaction, whereas aborted S3 objects must remain
+        /// available to the mount-lease sweeper and to recovery of the promote source.
+        struct PendingBlob { Cas::BlobRef ref; std::string staging_key; uint64_t size = 0; Cas::StagingBackend backend = Cas::StagingBackend::Local; };
+        std::vector<PendingBlob> pending_blobs;    /// Staged blobs uploaded after the manifest edge is precommitted.
+    };
+
+    /// Keyed by (namespace string, ref name) — the routed identity, so live/detached/shadow
+    /// stagings never collide.
+    std::map<std::pair<std::string, std::string>, PartStaging> parts;
+    bool committed = false;
+    bool failed = false;
+
+    /// Memoizes, per (this transaction, ref), whether `unlinkFile` has already re-proven a committed
+    /// ref's manifest body `ForceFresh`. The MergeTree fast-removal path unlinks every file of a part
+    /// through ONE transaction right before `removeDirectory`: the first unlink's `ForceFresh` view
+    /// proves the body once; the rest of the burst reuse that proof (`Cas::Freshness::CachedForLoad`)
+    /// instead of paying one manifest-body HEAD per file. Cleared in `commit()`'s epilogue.
+    std::unordered_set<String> force_fresh_validated_refs;
+
+    /// Stage a CONTENT part file as a blob: record the pending upload + a tokenless dependency
+    /// and add/replace its manifest entry. Shared by the streaming-blob path
+    /// (Local or S3-staging, `backend` says which) and the always-Local inline-cap fallback.
+    void stageBlobPartFile(const ContentAddressedMetadataStorage::Route & route,
+                           const Cas::BlobRef & ref, size_t size, const std::string & staging_key,
+                           Cas::StagingBackend backend);
+
+    /// Builds the fixed-length CABL envelope header for a staging blob, with a fresh `incarnation_tag`,
+    /// so the S3 staging object holds `[header][payload]` and the promote
+    /// stays a verbatim server-side copy. `build_id` is left 0 (not known at stream time; diagnostic-only).
+    std::string buildS3StagingBlobHeader(const ContentAddressedMetadataStorage::Route & route) const;
+
+    /// Returns (and, when necessary, creates) the staging state for a routed namespace/ref.
+    PartStaging & stagingFor(const ContentAddressedMetadataStorage::Route & r);
+    /// Finds existing staging state without creating an entry; returns nullptr when untouched.
+    PartStaging * findStaging(const ContentAddressedMetadataStorage::Route & r);
+    /// Finds the staged manifest entry for a routed file without consulting committed storage.
+    const Cas::ManifestEntry * findStagedEntry(const ContentAddressedMetadataStorage::Route & r) const;
+    /// Returns the pending (staged but not yet uploaded) blob for `ref`, or nullptr when it has
+    /// already been uploaded or was never staged.
+    const PartStaging::PendingBlob * findPendingBlob(const PartStaging & st, const Cas::BlobRef & ref) const;
+    /// Returns the part build, creating it lazily with the routed ref as its intended destination.
+    Cas::PartWriteTxn & buildFor(const ContentAddressedMetadataStorage::Route & r, PartStaging & st);
+    /// Parses a disk path and maps a part-file path to its namespace/ref/file route.
+    std::optional<ContentAddressedMetadataStorage::Route> routeOf(const std::string & path) const;
+
+    /// Removes local staging files after commit or abort. On a successful commit it also removes
+    /// S3 staging objects; aborted S3 objects are intentionally retained for lease-scoped cleanup.
+    void cleanupPendingTempFiles() noexcept;
+
+    /// Uploads through `st.build` only the pending blobs still referenced by `st.entries`. An entry
+    /// removed by `unlinkFile` or `replaceFile` is skipped, but its staging resource is still cleaned
+    /// by `cleanupPendingTempFiles`. Used for both new refs and committed-ref repoints.
+    void uploadPendingBlobs(PartStaging & st);
+
+    /// Adopts a manifest entry into another part while preserving its storage state. For a pending
+    /// blob, `copy_pending` controls whether the staging record is copied (hardlink semantics) or
+    /// has already been moved by the caller; either way the destination records a dependency. For
+    /// an uploaded or committed blob, the destination records tokenless evidence and does not
+    /// perform a pool read before precommit.
+    ///
+    /// `pb != nullptr` (pending, not yet uploaded):
+    ///   - `copy_pending=true`  → push a copy of *pb into dst_st.pending_blobs (hardlink semantics:
+    ///     both src and dst upload independently; src's copy is left in place by the caller).
+    ///   - `copy_pending=false` → the pb record is already in dst_st (moved or already there);
+    ///     just record the dep without any additional push.
+    ///   In both cases: dst_build.recordPendingBlobDep(entry.file_hash, entry.file_size).
+    ///
+    void adoptStagedBlob(const PartStaging::PendingBlob * pb, const Cas::ManifestEntry & entry,
+                         PartStaging & dst_st, Cas::PartWriteTxn & dst_build, bool copy_pending);
+
+    /// Publishes one staged part, either by promoting a newly staged manifest or by repointing an
+    /// existing ref after carrying its unchanged entries forward. It is idempotent within the commit
+    /// loop and marks the staging as published. Writes the exact `Cas::CommitOutcome` into `out_slot`
+    /// the INSTANT `promoteBuild`/`repointRef` confirms -- before any further throwable work (the
+    /// scratch-build abandon, or a test hook) -- so `commit` can roll back precisely with
+    /// `dropRefIfMatches` even when this call later throws. `out_slot` is left `std::nullopt` when this
+    /// staging had nothing to publish or was already published earlier in this commit loop; `commit`
+    /// preallocates one slot per part so this write is a no-throw, index-addressed slot write that never
+    /// grows a container -- it exists for the serial publish loop's rollback bookkeeping. Parts are
+    /// published serially (`commit`'s loop); only the blob uploads WITHIN a part fan out. Concurrent
+    /// cross-part publication is future scope and is NOT done here.
+    void publishStaging(const Cas::RootNamespace & ns, const std::string & ref, PartStaging & st,
+                        std::optional<Cas::CommitOutcome> & out_slot);
+};
+
+}
+
+namespace DB::Cas
+{
+
+/// Part files that must NOT be inlined into the tree: per-column data (`.bin`) and marks (`.mrk*`/
+/// `.cmrk*`) — inlining them would force a full-part fetch and destroy column-read selectivity — plus
+/// `primary.idx`, which can be large (a size-threshold inlining of small primary.idx is a follow-up).
+/// Everything else (the small eager metadata files) is an inline candidate, subject to INLINE_CAP.
+bool partFileMustStayBlob(std::string_view file_name);
+
+/// Test-only fault-injection seams for the intra-part blob-upload fan-out. Inert in production
+/// (`ContentAddressedTransaction::uploadPendingBlobs` passes `nullptr`).
+struct BlobUploadFanoutHooksForTest
+{
+    /// Invoked ONCE per DISPATCHED task, on the calling (dispatch) thread, immediately before the task
+    /// is enqueued, with the unique `BlobRef` the task will upload. Lets a test count tasks (proving the
+    /// one-task-per-unique-ref grouping) and inject a throw mid-dispatch (to prove the runner's scope
+    /// destructor drains every already-scheduled task before the stack unwinds).
+    std::function<void(const BlobRef &)> on_dispatch;
+    /// Invoked at the TOP of each pool task (on the pool thread) BEFORE `uploadBlobDetached`, with the
+    /// task's `BlobRef`. Lets a test rendezvous tasks on a latch (concurrent dedup-cache insertion, pool
+    /// saturation) or fail a specific task deterministically, all without a sleep.
+    std::function<void(const BlobRef &)> in_task;
+    /// Invoked on the DISPATCH thread immediately AFTER a task has been scheduled AND recorded in the
+    /// fan-out's own tracking vector, with that task's `BlobRef`. A throw here exercises the invariant
+    /// that no scheduled task is ever left untracked: because tracking publication is a no-throw append
+    /// into a pre-reserved vector, the task is already tracked when this fires, so the drain-on-every-path
+    /// guard joins it before the captured `results` storage is destroyed.
+    std::function<void(const BlobRef &)> after_enqueue;
+};
+
+/// Fan out a part's pending blob uploads across `pool` and merge the results into `build`
+/// all-or-nothing (spec §1 "Parallel blob upload within a part"). It:
+///   - groups `requests` by `BlobRef` (staged-hardlink copies push duplicate records for one ref), so
+///     it launches EXACTLY ONE `uploadBlobDetached` task per unique ref and merges exactly one dep;
+///   - rejects, before any task runs, a request whose `declared_size` disagrees with its `source.size`
+///     (a wiring bug) and duplicate records for one ref that declare conflicting sizes (a staging bug),
+///     both as `LOGICAL_ERROR`;
+///   - runs each task's `uploadBlobDetached` (a `const`, build-neutral primitive) on `pool` while the
+///     calling thread only submits and joins -- never occupying a pool slot -- so a size-1 pool
+///     degenerates to a correct serial run and can never deadlock;
+///   - honours the MERGE-NOTHING failure contract: the join always drains every task (including on a
+///     throw raised during the dispatch loop, because a scope-exit drain guard joins every
+///     already-scheduled task on the unwinding path); if ANY task threw, NOTHING is merged (`build`
+///     stays byte-for-byte at its pre-fan-out state) and the FIRST task error in ascending-`BlobRef`
+///     dispatch order is rethrown.
+/// The query `ThreadGroup` is propagated to each task the `ThreadPoolCallbackRunnerLocal` way.
+void fanOutBlobUploads(
+    PartWriteTxn & build,
+    std::span<const BlobUploadRequest> requests,
+    ThreadPool & pool,
+    const BlobUploadFanoutHooksForTest * hooks = nullptr);
+
+/// Writes a CONTENT part file while computing its content hash. The blob key is only known
+/// once all bytes are written, so the buffer spills to a unique local temp file while hashing with
+/// `hash_algo` (`Cas::makeBlobHashingWriteBuffer` — the pool's selectable blob-hash
+/// function the wiring defines; the core never re-hashes payloads). `CityHash128` stays the thin
+/// `HashingWriteBuffer` adapter (byte-for-byte unchanged); `XXH3_128` hashes with xxh3 instead. On
+/// finalize it hands (hash_hex, size, temp_path)
+/// to the owning transaction; the transaction owns the staging resource after finalize and uploads it
+/// post-precommit, so finalizeImpl no longer removes it. cancelImpl and the destructor (on error
+/// paths) still remove it.
+///
+/// In S3 staging mode:
+/// a SECOND constructor streams directly to an already-opened object-store sink (an S3 staging
+/// object) while hashing, instead of spilling to a local temp file — see its own doc comment below.
+/// The local-temp-file constructor above is UNCHANGED byte-for-byte; this is an independent mode
+/// selected only by which constructor the caller uses.
+class CaContentWriteBuffer : public WriteBufferFromFileBase
+{
+public:
+    using OnFinalized = std::function<void(const std::string & hash_hex, size_t size, const std::string & temp_path)>;
+
+    /// Local-staging mode (today's default; BYTE-FOR-BYTE unchanged behavior). Buffer sizing mirrors
+    /// the plain object-storage backends: with adaptive sizing on, the working buffer STARTS small
+    /// and grows (what min_columns_to_activate_adaptive_write_buffer toggles — a wide part keeps its
+    /// per-INSERT footprint small).
+    CaContentWriteBuffer(
+        std::string temp_dir,
+        Cas::BlobHashAlgo hash_algo,
+        size_t buf_size,
+        bool use_adaptive_buffer_size,
+        size_t adaptive_buffer_initial_size,
+        OnFinalized on_finalized_);
+
+    /// S3-native staging mode: `object_store_sink` is an ALREADY-OPENED write buffer over the staging
+    /// object at `object_key` (e.g. `object_storage->writeObject(StoredObject(object_key), ...)`).
+    ///
+    /// `envelope_header` is the fixed-length (`blob_header_len`) CABL envelope header the transaction
+    /// built for this staging blob. It is written to the sink FIRST —
+    /// UNHASHED and NOT counted in the reported size — so the staging object holds `[header][payload]`
+    /// and the promote can stay a verbatim server-side copy. Excluding the header from the hash is
+    /// CRITICAL: the content key must be the pool's selected hash of `payload` alone (else the random
+    /// `incarnation_tag` in the header would make every blob's key unique ⇒ zero dedup), and the reported
+    /// blob size must be the
+    /// PAYLOAD size (else the manifest `blob_size` would be payload+`blob_header_len`). Only the PAYLOAD
+    /// bytes written through THIS buffer flow through `hashing` and `count()`.
+    ///
+    /// Bytes are hashed while streaming into `object_store_sink`; on finalize `on_finalized` receives
+    /// `object_key` as its third argument (in place of a local temp path) and `getFileName()` returns
+    /// it too. `cancelImpl` only cancels `object_store_sink` — it never attempts to delete the
+    /// (possibly partially-written) staging object; reclaiming an orphaned staging object after a
+    /// cancelled write belongs to the mount-lease sweeper, not this buffer.
+    ///
+    /// `check_fence_before_finalize_` is the rev.7 [C2] fence-generation admission for this durable write
+    /// (the ONLY durable backend effect of the Local-mode constructor above is a private scratch file, so
+    /// it takes none): it is invoked immediately before the durable `sink->finalize()` call in
+    /// `finalizeImpl`, aborting with the typed transient error on a fence trip or re-arm since admission. It
+    /// defaults empty so the existing direct-construction unit tests (`gtest_cas_s3_staging.cpp`), which
+    /// exercise the buffer mechanics without a real `CasMountRuntime`, are unaffected.
+    CaContentWriteBuffer(
+        std::unique_ptr<WriteBufferFromFileBase> object_store_sink,
+        std::string object_key,
+        std::string envelope_header,
+        Cas::BlobHashAlgo hash_algo,
+        size_t buf_size,
+        bool use_adaptive_buffer_size,
+        size_t adaptive_buffer_initial_size,
+        OnFinalized on_finalized_,
+        std::function<void()> check_fence_before_finalize_ = {});
+
+    ~CaContentWriteBuffer() override;
+
+    void sync() override;
+    std::string getFileName() const override;
+
+private:
+    /// Feeds bytes to the hashing/staging sink while preserving the base write-buffer contract.
+    void nextImpl() override;
+    /// Finalizes the sink, computes the content hash, and transfers the staging resource through
+    /// `on_finalized`; after that callback succeeds, the transaction owns cleanup.
+    void finalizeImpl() override;
+    /// Cancels the sink and removes local staging. S3 staging is left for lease-scoped reclamation.
+    void cancelImpl() noexcept override;
+    /// Removes the local staging path when ownership has not been transferred to the transaction.
+    void removeTempFile() noexcept;
+
+    OnFinalized on_finalized;
+    /// Local mode: the local temp file path (removed by removeTempFile). S3 mode: the staging
+    /// object's key (never fs::remove'd — see is_s3_staging below).
+    std::string temp_path;
+    /// Selects the S3-staging semantics in cancelImpl/the destructor (skip local-file cleanup,
+    /// since `temp_path` is a remote key, not a path on this filesystem). false (the default,
+    /// local-temp-file constructor) is the pre-existing, byte-for-byte-unchanged behavior.
+    bool is_s3_staging = false;
+    /// The spill sink: a local WriteBufferFromFile (Local mode) or the caller-supplied object-store
+    /// sink (S3 mode). Either way it is a SECOND per-stream buffer wrapped by `hashing` below.
+    std::unique_ptr<WriteBufferFromFileBase> sink;
+    /// Built via `Cas::makeBlobHashingWriteBuffer(hash_algo, *sink)`:
+    /// `CityHash128` is a thin adapter over the pre-existing `HashingWriteBuffer` convention (byte-for-byte
+    /// unchanged); `XXH3_128` hashes with the pool's selected algo instead.
+    std::unique_ptr<Cas::IBlobHashingWriteBuffer> hashing;
+    bool temp_ownership_transferred = false;   /// Set after successful `on_finalized`; the destructor skips local cleanup.
+
+    /// rev.7 [C2]: fence-generation re-check invoked immediately before `sink->finalize()` in `finalizeImpl`
+    /// (populated only by the S3-staging constructor; a no-op empty `std::function` for Local mode).
+    std::function<void()> check_fence_before_finalize;
+};
+
+/// Write buffer for bytes that live INSIDE pool metadata (a small inline part file staged into the
+/// manifest tree, or a verbatim namespace file PUT on finalize). Accumulates in memory (the bytes are
+/// tiny) and hands them to the callback at finalize; the callback decides where they go and whether
+/// they are durable immediately (verbatim) or at commit (staged into the part's manifest entries).
+class CaInlineWriteBuffer : public WriteBufferFromFileBase
+{
+public:
+    using OnInlined = std::function<void(std::string bytes)>;
+
+    explicit CaInlineWriteBuffer(OnInlined on_inlined_);
+    ~CaInlineWriteBuffer() override;
+
+    void sync() override;
+    std::string getFileName() const override;
+
+private:
+    /// Appends bytes to the in-memory payload under the base write-buffer contract.
+    void nextImpl() override;
+    /// Hands the complete inline payload to the callback; the callback decides whether it is staged
+    /// in a manifest or written immediately as a verbatim file.
+    void finalizeImpl() override;
+
+    OnInlined on_inlined;
+    std::string accumulated;
+};
+
+}
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/README.md b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/README.md
new file mode 100644
index 000000000000..5c11d6ccdad7
--- /dev/null
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/README.md
@@ -0,0 +1,198 @@
+# Content-Addressed (CAS) metadata storage
+
+This directory implements the `cas` metadata storage for
+`DiskObjectStorage`: a "git for `MergeTree`" content-addressed pool over
+S3-family object storage. Instead of per-part metadata files pointing at
+randomly-named remote objects, every unique payload is stored **once** in a
+shared pool under a key derived from its **content hash**, a part is an
+immutable **manifest** listing its files, and a per-table **ref table** maps
+part names to manifests. Replicas that share one pool deduplicate bytes
+structurally: fetching a part can *relink* the sender's manifest entries by
+hash instead of copying data. This replaces zero-copy replication's shared
+mutable state with immutable, hash-addressed objects plus a small CAS
+(compare-and-swap) protocol.
+
+## The data model in five objects
+
+- **Blob** — the unit of payload. Keyed by its content digest
+  (`CasLayout::blobKey`); an envelope wraps the payload
+  (`Formats/CasBlobEnvelopeFormat`). Blobs are immutable and shared: the pool
+  is the only index, and identity is always established by (re-)hashing the
+  bytes — there are no trust-the-checksum shortcuts. A freshness sidecar
+  (`blobMetaKey` = `blobKey` + `.meta`) carries GC state per hash.
+- **Manifest** — an immutable description of one part: the full file tree with
+  per-file blob references or inline bytes. Small per-part files
+  (`uuid.txt`, `checksums.txt`, ...) live *inside* the manifest as inline tree
+  entries, not as separate objects. Keyed under
+  `cas/manifests/<ns>/<epoch-hex>-<seq-hex>/<ordinal>.zst`.
+- **Ref table** — the mutable naming layer, one namespace per table
+  (`SERVER_ID/TABLE_UUID`), keyed under one LIFE of that namespace: an
+  append-only transaction log plus periodic snapshots (`cas/ns/stream/<life_id>/...`), with mutable
+  checkpoints and namespace files under `cas/ns/state/<life_id>/...`. The catalog resolves each opaque
+  physical ID to its logical namespace life (see `NamespaceLifeId`); stream objects are replayed
+  into an in-memory table mapping
+  ref names (part directory names) to manifests. All mutations go through a
+  precommit/promote two-step so a manifest always has an owner while visible.
+- **Server root** — one per server (`server_root_id` in the disk config): a
+  single-writer mount slot with lease + writer-epoch fencing
+  (`gc/server-roots/<srid>/{owner,epoch,mount}`). A fenced (expired or stolen)
+  writer can never mutate the pool again with stale state.
+- **GC records** — round-based garbage collection state under `gc/...`:
+  leader lease/state, per-generation source-edge runs (which manifests
+  reference which blobs), fold seals, and outcome logs. GC computes blob
+  in-degrees from the edges, *condemns* zero-in-degree blobs, and later
+  deletes them with **exact-token** conditional deletes, so a concurrent
+  writer re-uploading the same content always wins (resurrection is a fresh
+  re-upload — never a read of a condemned object).
+
+Everything persisted is a text format with a versioned header line; see
+`Formats/README.md` for the registry, key map, and evolution rules.
+
+## Lifecycle walkthroughs
+
+- **Write**: `ContentAddressedTransaction` buffers or spills file writes
+  (`scratch_path`, or opt-in S3 staging), hashes the payload, and hands it to
+  `Pool/CasPartWriteTxn`: upload-or-adopt each blob (dedup by hash; a HEAD
+  proves presence before reuse), stage the manifest, `precommitAdd` the ref,
+  then promote it to committed in the ref log.
+- **Read**: disk path → `Parts/PartPathParser` (namespace + part + file) →
+  ref table resolves the manifest → `Pool/CasManifestReader` locates the
+  entry → blob `StoredObject`s or in-manifest inline bytes.
+- **Fetch between replicas** (`ContentAddressedExchange`): the sender ships
+  its manifest bytes; the receiver rebuilds the part by *adopting* the listed
+  blobs by hash in its own namespace (no payload transfer) and publishes a
+  fresh manifest. Any decode or promote failure just falls back to the
+  ordinary byte fetch.
+- **GC round** (`Gc/CasGc`, paced by `Gc/CasGcScheduler`): take/renew the
+  leader lease, collect source edges, fold per-shard in-degrees, condemn new
+  zero-in-degree blobs, exact-token-delete blobs condemned earlier whose
+  state did not change, and seal the round with one CAS on `gc/state`.
+
+## Source layout
+
+The tree is **layered**: entry points at the top level, implementation in
+per-subsystem subdirectories with a strict one-direction include rule.
+
+```
+Primitives → Formats → Backend → Pool → Gc → Tools ≈ Parts → facade (top level)
+```
+
+- **`Primitives/`** — the vocabulary, zero outward dependencies: `CasBlobDigest`
+  (`BlobHashAlgo` + `BlobDigest` + `DigestCodec` + `BlobRef` — blob identity),
+  `CasTypes.h` (the other identity types: `RootNamespace`, `Token`,
+  `ManifestId`, `RefTxnId`), `CasNamespaceLifeId` (`NamespaceLifeId` — one LIFE of
+  a namespace's ref layer, the pair every ref key is built from),
+  `CasBlobHashingWriteBuffer` (streaming
+  hash-and-passthrough machinery), `CasXxh3Streamer` (the isolated vendored
+  xxHash wrapper), `CasCodecUtil` (identifier/hex codec helpers), `CasEvent`
+  (audit-event POD + sink).
+- **`Formats/`** — everything persisted: bytes **and** keys. The per-object
+  text/format files (`CasFormat`, `CasTextFormat`, `CasPartManifestFormat`,
+  `CasRefLogFormat`, …) plus `CasLayout` (the object-key schema). See
+  `Formats/README.md` for the format registry.
+- **`Backend/`** — the token-aware storage seam: `CasBackend` (the contract:
+  get/put/`putIfAbsent`/`casPut`/`deleteExact` with CAS tokens),
+  `CasObjectStorageBackend`, `CasInMemoryBackend`, `CasInstrumentedBackend`,
+  `CasRequestControl` (single-attempt conditional writes, explicit
+  state-aware retries), `CasProbe` (mount-time capability probe).
+- **`Pool/`** — the pool engine: `CasPool` (composition root), `CasPartWriteTxn`
+  (one-part write transaction), `CasRefLedger` + `CasRefProtocol` (ref-table
+  log/snapshot/replay + intake), `CasServerRoot` (mount-claim protocol +
+  single-writer slot + staging sweeper), `CasPoolMeta`, `CasBlobMeta`,
+  `CasManifestReader`, `CasPlainObjects` (the `roots/...` verbatim
+  passthrough), `CasMountRuntime` (fence state shared with lanes).
+- **`Gc/`** — garbage collection: `CasGcScheduler` (pacing thread), `CasGc`
+  (the round engine), `CasGcShardPlan` (sharding math), `CasBlobInDegree`,
+  `CasOrphanManifestSweep`.
+- **`Tools/`** — operator verbs (`clickhouse-disks`): `CasFsck`,
+  `CasDecommission`, `CasInspect`.
+- **`Parts/`** — part semantics over the pool: `PartPathParser` (the
+  ClickHouse-path classifier), `PartFolderAccess` (`PartRefKey` + `Freshness`
+  + `PartFolderValidate` + `PartFolderView` + `CachedPartFolderAccess`).
+- **Top level (facade)** — the entry points: `ContentAddressedMetadataStorage`
+  (the `IMetadataStorage` facade), `ContentAddressedTransaction` (the
+  `IMetadataTransaction`, including the write buffers), `ContentAddressedExchange`
+  (the replication seam).
+
+## Include-direction rule
+
+A file may include only its **own layer** and layers to its **left** in the
+order above. `Tools` and `Parts` are siblings with no edges between them. This
+is enforced by convention (README rule) — there is no CI check.
+
+**Named exceptions** (deliberate):
+
+- The staging sweeper (in `Pool/CasServerRoot`) and `probeConditionalCopy`
+  bypass `Backend` and reach straight into `IObjectStorage`.
+- `Backend` may read `Formats` traits (the provider-metadata mirror).
+
+## Configuration
+
+A CAS disk is an `object_storage` disk with `metadata_type` =
+`cas`. Minimal example (see
+`tests/config/config.d/cas_storage_policy_for_merge_tree_by_default.xml`
+and its `_s3_` sibling for the lane configs used in CI):
+
+```xml
+<disks>
+    <cas>
+        <type>object_storage</type>
+        <object_storage_type>s3</object_storage_type> <!-- or local -->
+        <metadata_type>cas</metadata_type>
+        <!-- Required, validated identity of the layout subtree this server owns.
+             Must be unique per server sharing the pool. -->
+        <server_root_id>replica-1</server_root_id>
+        <path>cas_pool/</path>
+        <!-- Real server-local scratch dir for the write-buffer spill. -->
+        <scratch_path>cas_scratch/</scratch_path>
+        <gc_enabled>1</gc_enabled>
+        <gc_interval_sec>60</gc_interval_sec>
+    </cas>
+</disks>
+```
+
+`<readonly>1</readonly>` opens the disk in observe-only mode: no mount-slot
+claim, no capability probe, no writes — the mode `clickhouse-disks` tools and
+post-mortem inspection use. The full knob set (staging backend, cache sizes,
+GC sharding, hash algorithm, request budgets) is parsed in
+`MetadataStorageFactory.cpp`; each knob is documented at its parse site.
+
+## Operations and observability
+
+- `clickhouse-disks` verbs (all require the disk opened read-only): `fsck`
+  (independent reachability audit of refs → manifests → blobs), `cas-inspect`
+  (decode one pool object by its raw key to JSON), `cas-gc-dryrun` (preview the
+  next GC round's deletes), `cas-gc-rebuild` (disaster-recovery rebuild of the
+  `gc/state` baseline), `cas-drop-member` (decommission a dead pool member).
+- `system.cas_log` — one row per CAS protocol event
+  (uploads, adopts, promotes, condemns, deletes, mount-slot writes, ...);
+  the primary audit trail when investigating pool state.
+- The GC and writer paths also emit `ProfileEvents` counters (grep
+  `ProfileEvents.cpp` for `Cas`).
+
+## Testing
+
+- **Unit tests** (`unit_tests_dbms`): every CAS suite name starts with `Cas`, so
+  `--gtest_filter='Cas*'` runs the whole set — including parameterized suites,
+  whose instantiation prefixes are `Cas`-prefixed too so the `<Inst>/<Suite>`
+  spelling still matches. `utils/cas-gate/generate_cas_suites.sh` fails loud on a
+  CAS suite that does not match, so a new suite cannot silently sit outside the
+  filter; `utils/cas-gate/run_cas_gate_per_suite.sh` runs them one process per
+  suite, so an abort cannot hide the suites after it.
+- **Stateless lanes**: the functional-test jobs "`cas storage`"
+  (local object storage) and "`cas s3 storage`" run the whole
+  stateless suite with `MergeTree` defaulting to a CAS disk. Tests that
+  legitimately cannot run there carry the `no-cas-storage` tag.
+- **Soak / chaos**: `utils/ca-soak/` — multi-replica docker-compose
+  harnesses (fault proxies, GC sharding variants, AWS S3/GCS backends) and
+  adversarial scenarios.
+
+## Reading order
+
+To understand a request end to end, read in this order:
+
+1. `ContentAddressedMetadataStorage` — the facade / routing.
+2. `Parts/PartFolderAccess` (`PartRefKey` → the folder view / cache).
+3. `Pool/CasPool` — the pool composition root and `open` protocol.
+4. `Pool/CasPartWriteTxn` — one-part write transaction.
+5. `Gc/CasGc` — the GC round engine.
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/IMetadataStorage.h b/src/Disks/DiskObjectStorage/MetadataStorages/IMetadataStorage.h
index e82735548a4b..5b1a6ef6d1b0 100644
--- a/src/Disks/DiskObjectStorage/MetadataStorages/IMetadataStorage.h
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/IMetadataStorage.h
@@ -27,6 +27,8 @@ namespace ErrorCodes
     extern const int NOT_IMPLEMENTED;
 }
 
+struct IDiskTransaction;
+
 /// Tries to provide some "transactions" interface, which allow
 /// to execute (commit) operations simultaneously. We don't provide
 /// any snapshot isolation here, so no read operations in transactions
@@ -115,6 +117,16 @@ class IMetadataTransaction : private boost::noncopyable
         throwNotImplemented();
     }
 
+    /// [TXN-ONE-PIPELINE] Optional per-metadata write buffer. Returns a ready-to-use buffer when the
+    /// metadata implementation owns its write mechanism (e.g. a content-addressed hash-on-write buffer
+    /// whose blob key is known only after the last byte). `owner` is the disk transaction that must be
+    /// kept alive for the returned buffer's lifetime and, when `autocommit`, committed from the finalize
+    /// callback. Default nullptr: the caller uses the generic streaming write path unchanged.
+    virtual std::unique_ptr<WriteBufferFromFileBase> tryCreateWriteBuffer(
+        const std::shared_ptr<IDiskTransaction> & /*owner*/,
+        const std::string & /*path*/, size_t /*buf_size*/, WriteMode /*mode*/,
+        const WriteSettings & /*settings*/, bool /*autocommit*/) { return nullptr; }
+
     /// Metadata related methods
 
     /// Generate blob name for passed absolute local path.
@@ -141,6 +153,22 @@ class IMetadataTransaction : private boost::noncopyable
         throwNotImplemented();
     }
 
+    /// In-flight read-your-writes for a part being assembled by THIS transaction (B59). A CA part-build
+    /// transaction stages blobs (uploaded) + mutable bytes before the single commit; these let a reader
+    /// that holds the transaction resolve those staged files before they are committed. Default: no
+    /// in-flight visibility (the committed metadata path is authoritative).
+    virtual std::optional<StoredObjects> tryGetInFlightStorageObjects(const std::string & /*path*/) const { return {}; }
+    virtual std::unique_ptr<ReadBufferFromFileBase> tryReadFileInFlight(
+        const std::string & /*path*/, const ReadSettings & /*settings*/, std::optional<size_t> /*read_hint*/) const { return nullptr; }
+    virtual std::optional<uint64_t> tryGetInFlightFileSize(const std::string & /*path*/) const { return {}; }
+    /// Directory-granularity counterpart of the file trio: true iff this transaction has STAGED at least one
+    /// file under `path` for `path`'s part. Used so a carried-forward projection dir is visible to
+    /// loadProjections during finalize. Default: no in-flight directory visibility.
+    virtual bool hasInFlightDirectory(const std::string & /*path*/) const { return false; }
+    /// Immediate-child names staged directly under `path` (one level). Used so loadProjections'
+    /// withPartFormatFromDisk can iterate a staged projection dir to find its mark file. Default: empty.
+    virtual std::vector<std::string> listInFlightDirectory(const std::string & /*path*/) const { return {}; }
+
     virtual ~IMetadataTransaction() = default;
 
 protected:
@@ -289,6 +317,23 @@ class IMetadataStorage : private boost::noncopyable
         return false;
     }
 
+    /// Returns true if the metadata storage is content-addressed, i.e. blob keys are derived
+    /// from content hashes and are only known after all bytes have been written. Such a storage
+    /// cannot use the up-front-key streaming write path of `DiskObjectStorageTransaction`; the
+    /// disk transaction delegates writes to the metadata transaction's content-addressed buffer.
+    virtual bool isContentAddressed() const { return false; }
+
+    /// [TXN-ONE-PIPELINE] True when a transaction from this storage stages every mutation into a
+    /// transaction-private overlay at call time (eager) rather than queuing effects for FIFO replay in
+    /// commit. When true, DiskObjectStorageTransaction routes every mutating method straight to the
+    /// metadata transaction and keeps its own operations_to_execute queue empty. Default false
+    /// (ordinary object storage).
+    virtual bool transactionIsStagingOverlay() const { return false; }
+
+    /// True when a file write through this metadata storage publishes atomically, i.e. no partial
+    /// content is ever observable under the file's final name (see `IDataPartStorage::supportsAtomicFileWrites`).
+    virtual bool supportsAtomicFileWrites() const { return false; }
+
     using BlobsToRemove = std::unordered_map<StoredObject, LocationSet>;
     virtual BlobsToRemove getBlobsToRemove(const ClusterConfigurationPtr & /*cluster*/, int64_t /*max_count*/) { return {}; }
     virtual int64_t recordAsRemoved(const StoredObjects & /*blobs*/) { return 0; }
@@ -325,6 +370,12 @@ class IMetadataStorage : private boost::noncopyable
     /// True if write with Append mode supported.
     virtual bool supportWritingWithAppend() const { return false; }
 
+    /// True iff this metadata storage can persist the per-part mutable transaction file (txn_version.txt)
+    /// under MVCC. Distinct from supportWritingWithAppend: transactions rewrite txn_version.txt (tmp +
+    /// replaceFile), they never WriteMode::Append, so append-capability is the wrong proxy. A
+    /// content-addressed disk supports the mutable txn file via its per-ref sidecar.
+    virtual bool supportsTransactionalMutableFiles() const { return false; }
+
 protected:
     [[noreturn]] static void throwNotImplemented()
     {
diff --git a/src/Disks/DiskObjectStorage/MetadataStorages/MetadataStorageFactory.cpp b/src/Disks/DiskObjectStorage/MetadataStorages/MetadataStorageFactory.cpp
index 2b2d6a284a4e..9bb1b1fb355d 100644
--- a/src/Disks/DiskObjectStorage/MetadataStorages/MetadataStorageFactory.cpp
+++ b/src/Disks/DiskObjectStorage/MetadataStorages/MetadataStorageFactory.cpp
@@ -7,8 +7,11 @@
 #endif
 #include <Disks/DiskObjectStorage/MetadataStorages/Plain/MetadataStorageFromPlainObjectStorage.h>
 #include <Disks/DiskObjectStorage/MetadataStorages/PlainRewritable/MetadataStorageFromPlainRewritableObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h>
 #include <Disks/DiskObjectStorage/MetadataStorages/Web/MetadataStorageFromStaticFilesWebServer.h>
 #include <Disks/DiskLocal.h>
+#include <Core/ServerUUID.h>
 #include <Interpreters/Context.h>
 
 
@@ -21,6 +24,12 @@ namespace ErrorCodes
     extern const int UNKNOWN_ELEMENT_IN_CONFIG;
     extern const int INVALID_CONFIG_PARAMETER;
     extern const int LOGICAL_ERROR;
+    extern const int NOT_IMPLEMENTED;
+}
+
+namespace ContentAddressedSetting
+{
+    extern const ContentAddressedSettingsString scratch_path;
 }
 
 namespace
@@ -205,6 +214,35 @@ static void registerPlainRewritableMetadataStorage(MetadataStorageFactory & fact
     });
 }
 
+static void registerContentAddressedMetadataStorage(MetadataStorageFactory & factory)
+{
+    factory.registerMetadataStorageType("cas", [](
+        const std::string & name,
+        const Poco::Util::AbstractConfiguration & config,
+        const std::string & config_prefix,
+        const ClusterConfigurationPtr & cluster,
+        const ObjectStorageRouterPtr & object_storages) -> MetadataStoragePtr
+    {
+        checkSingleLocation(cluster);
+
+        const auto local_object_storage = object_storages->takePointingTo(cluster->getLocalLocation());
+        std::string key_compatibility_prefix = getObjectKeyCompatiblePrefix(local_object_storage, config, config_prefix);
+
+        auto global_context = Context::getGlobalContextInstance();
+        ContentAddressedSettings settings;
+        settings.loadFromConfig(
+            config, config_prefix,
+            /*scratch_path_anchor_if_relative=*/ global_context->getPath(),
+            /*default_scratch_path=*/ fs::path(global_context->getPath()) / "disks" / name / "cas_scratch" / "",
+            [&](const std::string & s) { return global_context->getMacros()->expand(s); });
+        fs::create_directories(settings[ContentAddressedSetting::scratch_path].value);
+
+        return std::make_shared<ContentAddressedMetadataStorage>(
+            local_object_storage, key_compatibility_prefix, toString(ServerUUID::get()),
+            name, global_context, settings);
+    });
+}
+
 static void registerMetadataStorageFromStaticFilesWebServer(MetadataStorageFactory & factory)
 {
     factory.registerMetadataStorageType("web", [](
@@ -230,6 +268,7 @@ void registerMetadataStorages()
     registerMetadataStorageFromDisk(factory);
     registerPlainMetadataStorage(factory);
     registerPlainRewritableMetadataStorage(factory);
+    registerContentAddressedMetadataStorage(factory);
     registerMetadataStorageFromStaticFilesWebServer(factory);
 #if CLICKHOUSE_CLOUD
     registerMetadataStorageFromKeeper(factory);
diff --git a/src/Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h b/src/Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h
index 41b16b95e50b..ff77c27915f6 100644
--- a/src/Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h
+++ b/src/Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h
@@ -161,6 +161,23 @@ using ObjectKeysWithMetadata = std::vector<ObjectKeyWithMetadata>;
 class IObjectStorageIterator;
 using ObjectStorageIteratorPtr = std::shared_ptr<IObjectStorageIterator>;
 
+/// Outcome of a token-conditional single-object removal (content-addressed disks).
+enum class ConditionalRemoveOutcome : uint8_t { Removed, TokenMismatch, NotFound };
+struct ConditionalRemoveResult
+{
+    ConditionalRemoveOutcome outcome = ConditionalRemoveOutcome::NotFound;
+    bool created_delete_marker = false;   /// backend reported a versioning delete marker
+};
+
+/// Outcome of a write-once conditional server-side copy (content-addressed disks): `created == true`
+/// means this call won the race and created `object_to`; `created == false` means the destination
+/// already existed (the precondition was rejected) and `dest_etag` is left empty.
+struct ConditionalCopyResult
+{
+    bool created = false;
+    String dest_etag;
+};
+
 /// Base class for all object storages which implement some subset of ordinary filesystem operations.
 ///
 /// Examples of object storages are S3, Azure Blob Storage, HDFS.
@@ -267,6 +284,15 @@ class IObjectStorage
     /// Remove objects on path if exists
     virtual void removeObjectsIfExist(const StoredObjects & object) = 0;
 
+    /// Remove `object` ONLY if its current entity tag equals `etag`. Backends without enforced
+    /// conditional removal MUST NOT override this: the content-addressed capability probe relies on the
+    /// default to fail closed. Supported: S3 (DeleteObject If-Match, GA 2025-09).
+    virtual ConditionalRemoveResult removeObjectIfTokenMatches(const StoredObject & /*object*/, const std::string & /*etag*/)
+    {
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "Conditional (token-exact) object removal is not implemented for {} object storage", getName());
+    }
+
     /// Copy object with different attributes if required
     virtual void copyObject( /// NOLINT
         const StoredObject & object_from,
@@ -275,6 +301,36 @@ class IObjectStorage
         const WriteSettings & write_settings,
         std::optional<ObjectAttributes> object_to_attributes = {}) = 0;
 
+    /// Copy `object_from` to `object_to` WRITE-ONCE: the copy is conditional on `object_to` not
+    /// already existing (`If-None-Match: *`). Returns `created=true` with the destination ETag if
+    /// this call created the object, or `created=false` (empty `dest_etag`) if the destination
+    /// already existed — that is the expected "lost the race" signal, not an error. Any other
+    /// failure propagates as an exception.
+    ///
+    /// Backends without an enforced, native (server-side) conditional copy MUST NOT override this:
+    /// the content-addressed write-once staging promote relies on the default to fail closed rather
+    /// than silently falling back to an unconditional overwrite. Supported: S3 (native `CopyObject`
+    /// / `CompleteMultipartUpload` with `If-None-Match`).
+    virtual ConditionalCopyResult copyObjectConditional(
+        const StoredObject & /*object_from*/,
+        const StoredObject & /*object_to*/,
+        const ReadSettings & /*read_settings*/,
+        const WriteSettings & /*write_settings*/,
+        std::optional<ObjectAttributes> /*object_to_attributes*/)
+    {
+        throw Exception(ErrorCodes::NOT_IMPLEMENTED,
+            "Conditional (write-once) object copy is not implemented for {} object storage", getName());
+    }
+
+    ConditionalCopyResult copyObjectConditional(
+        const StoredObject & object_from,
+        const StoredObject & object_to,
+        const ReadSettings & read_settings,
+        const WriteSettings & write_settings)
+    {
+        return copyObjectConditional(object_from, object_to, read_settings, write_settings, {});
+    }
+
     /// Copy object to another instance of object storage
     /// by default just read the object from source object storage and write
     /// to destination through buffers.
@@ -323,6 +379,22 @@ class IObjectStorage
 
     virtual bool supportParallelWrite() const { return false; }
 
+    /// True when the incarnation tokens this storage returns from writes/HEADs are GCS generation
+    /// numbers riding the ETag plumbing (http_client = gcs_hmac / gcp_oauth conditional dialect).
+    /// Consumers (the CAS backend) stamp TokenType::Generation and route conditional writes
+    /// through the single-PUT path (GCS enforces no preconditions on CompleteMultipartUpload).
+    virtual bool conditionalOpsUseGenerationTokens() const { return false; }
+
+    /// Whether the underlying bucket has object versioning enabled; nullopt when unknown or not
+    /// applicable. Used by the CAS capability probe to fail closed on GCS: on a versioned bucket
+    /// a token-exact DELETE archives a noncurrent generation instead of reclaiming storage.
+    virtual std::optional<bool> isBucketVersioningEnabled() const { return std::nullopt; }
+
+    /// True when this object storage can execute writes under the given retry profile.
+    /// A caller that sets a non-Default profile on WriteSettings MUST check this first and
+    /// fail closed if unsupported (the profile is advisory only to backends that opt in).
+    virtual bool supportsRetryProfile(ObjectStorageRetryProfile profile) const { return profile == ObjectStorageRetryProfile::Default; }
+
     virtual ReadSettings patchSettings(const ReadSettings & read_settings) const;
 
     virtual WriteSettings patchSettings(const WriteSettings & write_settings) const;
diff --git a/src/Disks/DiskObjectStorage/RegisterDiskObjectStorage.cpp b/src/Disks/DiskObjectStorage/RegisterDiskObjectStorage.cpp
index c662bf71facc..304ca5daab4b 100644
--- a/src/Disks/DiskObjectStorage/RegisterDiskObjectStorage.cpp
+++ b/src/Disks/DiskObjectStorage/RegisterDiskObjectStorage.cpp
@@ -6,12 +6,19 @@
 #include <Disks/ReadOnlyDiskWrapper.h>
 #include <Disks/DiskFactory.h>
 #include <Disks/IDisk.h>
+#include <Common/Exception.h>
+#include <base/EnumReflection.h>
 
 #include <fmt/ranges.h>
 
 namespace DB
 {
 
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+
 void registerObjectStorages();
 void registerMetadataStorages();
 void registerDiskObjectStorage(DiskFactory & factory, bool global_skip_access_check);
@@ -77,7 +84,21 @@ void registerDiskObjectStorage(DiskFactory & factory, bool global_skip_access_ch
         LOG_DEBUG(getLogger("registerDiskObjectStorage"), "Metadata type hint: {}", compatibility_metadata_type_hint);
         auto metadata_storage = MetadataStorageFactory::instance().create(name, config, config_prefix, cluster, object_storages, compatibility_metadata_type_hint);
 
-        bool use_fake_transaction = config.getBool(config_prefix + ".use_fake_transaction", metadata_storage->getType() != MetadataStorageType::Keeper);
+        /// Content-addressed metadata (like Keeper) requires real, deferred disk transactions: a part's
+        /// file->blob mappings are accumulated across the whole part write and the manifest + ref are
+        /// published atomically when the transaction commits. A fake (per-file autocommit) transaction
+        /// would write each file independently with no commit point for the manifest/ref publish.
+        const auto metadata_type = metadata_storage->getType();
+        const bool needs_real_transaction = metadata_type == MetadataStorageType::Keeper
+            || metadata_type == MetadataStorageType::CAS;
+        /// An explicit `use_fake_transaction=true` on a metadata type that requires deferred
+        /// transactions would silently break the atomic manifest/ref publish (per-file autocommit,
+        /// no commit point). Reject it instead of honoring it.
+        if (needs_real_transaction && config.getBool(config_prefix + ".use_fake_transaction", false))
+            throw Exception(ErrorCodes::BAD_ARGUMENTS,
+                "Disk '{}': `use_fake_transaction` cannot be enabled for metadata type '{}'",
+                name, magic_enum::enum_name(metadata_type));
+        bool use_fake_transaction = config.getBool(config_prefix + ".use_fake_transaction", !needs_real_transaction);
         DiskPtr disk = std::make_shared<DiskObjectStorage>(
             name,
             std::move(cluster),
diff --git a/src/Disks/DiskType.cpp b/src/Disks/DiskType.cpp
index bf4506b4cbf6..8aef6db87ee0 100644
--- a/src/Disks/DiskType.cpp
+++ b/src/Disks/DiskType.cpp
@@ -19,6 +19,8 @@ MetadataStorageType metadataTypeFromString(const String & type)
         return MetadataStorageType::Plain;
     if (check_type == "plain_rewritable")
         return MetadataStorageType::PlainRewritable;
+    if (check_type == "cas")
+        return MetadataStorageType::CAS;
     if (check_type == "web")
         return MetadataStorageType::StaticWeb;
     if (check_type == "keeper")
diff --git a/src/Disks/DiskType.h b/src/Disks/DiskType.h
index 726557d5575d..7a612d2336de 100644
--- a/src/Disks/DiskType.h
+++ b/src/Disks/DiskType.h
@@ -32,6 +32,7 @@ enum class MetadataStorageType : uint8_t
     Keeper,
     Plain,
     PlainRewritable,
+    CAS,
     StaticWeb,
     Memory,
 };
diff --git a/src/Disks/IDisk.h b/src/Disks/IDisk.h
index 478795f523f1..b79a3171bde7 100644
--- a/src/Disks/IDisk.h
+++ b/src/Disks/IDisk.h
@@ -472,6 +472,13 @@ class IDisk : public Space
     /// If the disk is plain object storage.
     virtual bool isPlain() const { return false; }
 
+    /// If the disk is a content-addressed object-storage pool (`metadata_type = cas`).
+    /// A clean predicate so callers do not have to reach through `getDataSourceDescription`.
+    virtual bool isContentAddressed() const { return false; }
+
+    /// True when a file write on this disk publishes atomically (see `IDataPartStorage::supportsAtomicFileWrites`).
+    virtual bool supportsAtomicFileWrites() const { return false; }
+
     virtual bool isWriteOnce() const { return false; }
 
     virtual bool supportsHardLinks() const { return true; }
diff --git a/src/Disks/ReadOnlyDiskWrapper.h b/src/Disks/ReadOnlyDiskWrapper.h
index e75dd8623d24..a0c28d1c0a18 100644
--- a/src/Disks/ReadOnlyDiskWrapper.h
+++ b/src/Disks/ReadOnlyDiskWrapper.h
@@ -85,6 +85,11 @@ class ReadOnlyDiskWrapper : public IDisk
     NameSet getCacheLayersNames() const override { return delegate->getCacheLayersNames(); }
 
     MetadataStoragePtr getMetadataStorage() override { return delegate->getMetadataStorage(); }
+    /// Forwarded alongside getMetadataStorage: callers that gate on this predicate before reaching
+    /// for the metadata storage (ContentAddressedMetadataStorage::tryFromDisk and friends) must see
+    /// the delegate's answer through the wrapper, or a wrapped content-addressed disk silently
+    /// drops out of the CAS introspection paths.
+    bool isContentAddressed() const override { return delegate->isContentAddressed(); }
 
     std::unordered_map<String, String> getSerializedMetadata(const std::vector<String> & file_paths) const override { return delegate->getSerializedMetadata(file_paths); }
 
diff --git a/src/IO/ReadPipeline.cpp b/src/IO/ReadPipeline.cpp
index fa2e1f075110..1d7a1be480a6 100644
--- a/src/IO/ReadPipeline.cpp
+++ b/src/IO/ReadPipeline.cpp
@@ -10,6 +10,7 @@
 #include <IO/ReadBufferFromEmptyFile.h>
 #include <IO/ReadBufferFromEncryptedFile.h>
 #include <IO/ReadBufferFromFileBase.h>
+#include <IO/ReadBufferFromFileView.h>
 #include <IO/ReadBufferFromFileDecorator.h>
 #include <IO/ReadBufferFromString.h>
 #include <IO/ReaderExecutor.h>
@@ -157,6 +158,17 @@ void ReadPipeline::needDecryption(String path, size_t buffer_size, KeyFinderFunc
         .key_finder = std::move(key_finder)});
 }
 
+void ReadPipeline::needFileView(String file_name, size_t left_bound, size_t right_bound)
+{
+    if (right_bound < left_bound)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "ReadPipeline: file view right bound ({}) is below the left bound ({})", right_bound, left_bound);
+    file_view = FileViewStage{
+        .file_name = std::move(file_name),
+        .left_bound = left_bound,
+        .right_bound = right_bound};
+}
+
 std::unique_ptr<ReadBufferFromFileBase> ReadPipeline::build() const
 {
     if (!source)
@@ -182,7 +194,8 @@ std::unique_ptr<ReadBufferFromFileBase> ReadPipeline::build() const
 
     impl = wrapMemoryCache(std::move(impl));   // Stage 4
     impl = wrapAsyncPrefetch(std::move(impl)); // Stage 5
-    impl = wrapDecryption(std::move(impl));    // Stage 6 (encryption)
+    impl = wrapFileView(std::move(impl));      // Stage 6 (byte window)
+    impl = wrapDecryption(std::move(impl));    // Stage 7 (encryption)
 
     return impl;
 }
@@ -193,14 +206,15 @@ std::unique_ptr<ReadBufferFromFileBase> ReadPipeline::tryBuildReaderExecutor() c
     if (!settings.use_reader_executor)
         return nullptr;
 
-    /// The executor does not implement caches, decryption, async prefetch, or the
-    /// distributed cache, so fall back rather than silently drop a configured stage.
+    /// The executor does not implement caches, decryption, async prefetch, the
+    /// distributed cache, or a file_view byte window, so fall back rather than
+    /// silently drop a configured stage.
     if (distributed_cache || memory_cache || !filesystem_caches.empty()
-        || !decryption_stages.empty() || async_prefetch)
+        || !decryption_stages.empty() || async_prefetch || file_view)
     {
         LOG_DEBUG(log,
             "use_reader_executor: falling back to the legacy read path "
-            "(caches/decryption not yet supported by the executor)");
+            "(caches/decryption/file_view not yet supported by the executor)");
         return nullptr;
     }
 
@@ -673,6 +687,19 @@ std::unique_ptr<ReadBufferFromFileBase> ReadPipeline::wrapAsyncPrefetch(std::uni
         async_prefetch->prefetches_log);
 }
 
+std::unique_ptr<ReadBufferFromFileBase> ReadPipeline::wrapFileView(std::unique_ptr<ReadBufferFromFileBase> impl) const
+{
+    /// -- Stage 6: File view --
+    /// The view translates the consumer's positions/right bounds by `left_bound` and forwards
+    /// them down the chain, so `MergeTreeReaderStream::adjustRightMark` bounds reach the
+    /// object-storage reader and its range requests stay drainable (connection-pool friendly).
+    if (!file_view)
+        return impl;
+
+    return std::make_unique<ReadBufferFromFileView>(
+        std::move(impl), file_view->file_name, file_view->left_bound, file_view->right_bound);
+}
+
 std::unique_ptr<ReadBufferFromFileBase> ReadPipeline::wrapDecryption(std::unique_ptr<ReadBufferFromFileBase> impl) const
 {
     /// -- Stage 6: Decryption (may have multiple layers for double encryption) --
@@ -733,6 +760,8 @@ String ReadPipeline::describe() const
         append("MemoryCache");
     if (async_prefetch)
         append("AsyncPrefetch");
+    if (file_view)
+        append("FileView");
     if (!decryption_stages.empty())
         append("Decrypt");
 
diff --git a/src/IO/ReadPipeline.h b/src/IO/ReadPipeline.h
index 61b5fa506351..586212be4598 100644
--- a/src/IO/ReadPipeline.h
+++ b/src/IO/ReadPipeline.h
@@ -47,7 +47,8 @@ using FilesystemReadPrefetchesLogPtr = std::shared_ptr<FilesystemReadPrefetchesL
 ///   4. DistributedCache   -- ReadBufferFromDistributedCache (with fallback to Gather)
 ///   5. MemoryCache        -- CachedInMemoryReadBufferFromFile
 ///   6. AsyncPrefetch      -- AsynchronousBoundedReadBuffer
-///   7. Encryption         -- ReadBufferFromEncryptedFile (may have multiple layers)
+///   7. FileView           -- ReadBufferFromFileView (a byte window over the chain)
+///   8. Encryption         -- ReadBufferFromEncryptedFile (may have multiple layers)
 class ReadPipeline
 {
 public:
@@ -161,6 +162,15 @@ class ReadPipeline
     /// read from the encryption header. It must return the decryption key.
     void needDecryption(String path, size_t buffer_size, KeyFinderFunc key_finder);
 
+    /// -- File view stage --
+    /// Exposes ONLY the byte window [left_bound, right_bound) of the underlying chain as a
+    /// standalone file named `file_name` (ReadBufferFromFileView). Used by content-addressed
+    /// blob reads, where a logical file is a payload window inside a shared blob (the blob's
+    /// envelope header occupies [0, left_bound)). Sits outside async prefetch — the window's
+    /// seeks and right bounds are translated and forwarded down the standard chain — but
+    /// inside decryption, which operates on logical-file bytes.
+    void needFileView(String file_name, size_t left_bound, size_t right_bound);
+
     /// -- Build the final ReadBuffer chain --
     /// Uses the ReadSettings stored in the source stage.
     std::unique_ptr<ReadBufferFromFileBase> build() const;
@@ -215,6 +225,13 @@ class ReadPipeline
         KeyFinderFunc key_finder;
     };
 
+    struct FileViewStage
+    {
+        String file_name;
+        size_t left_bound = 0;
+        size_t right_bound = 0;
+    };
+
 
     struct DistributedCacheStage
     {
@@ -228,6 +245,7 @@ class ReadPipeline
     std::optional<DistributedCacheStage> distributed_cache;
     std::optional<AsyncPrefetchStage> async_prefetch;
     VectorWithMemoryTracking<DecryptionStage> decryption_stages;
+    std::optional<FileViewStage> file_view;
 
     LoggerPtr log = getLogger("ReadPipeline");
 
@@ -246,6 +264,7 @@ class ReadPipeline
     std::unique_ptr<ReadBufferFromFileBase> buildSingleObjectStage(const std::string & query_id) const;
     std::unique_ptr<ReadBufferFromFileBase> wrapMemoryCache(std::unique_ptr<ReadBufferFromFileBase> impl) const;
     std::unique_ptr<ReadBufferFromFileBase> wrapAsyncPrefetch(std::unique_ptr<ReadBufferFromFileBase> impl) const;
+    std::unique_ptr<ReadBufferFromFileBase> wrapFileView(std::unique_ptr<ReadBufferFromFileBase> impl) const;
     std::unique_ptr<ReadBufferFromFileBase> wrapDecryption(std::unique_ptr<ReadBufferFromFileBase> impl) const;
 };
 
diff --git a/src/IO/WriteBufferFromFileBase.h b/src/IO/WriteBufferFromFileBase.h
index 47dd4f5ed7ae..b60e951d8edf 100644
--- a/src/IO/WriteBufferFromFileBase.h
+++ b/src/IO/WriteBufferFromFileBase.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <optional>
 #include <string>
 
 #include <IO/WriteBuffer.h>
@@ -15,6 +16,12 @@ class WriteBufferFromFileBase : public BufferWithOwnMemory<WriteBuffer>
 
     void sync() override = 0;
     virtual std::string getFileName() const = 0;
+
+    /// The object-storage ETag/token the write produced, if any (e.g. the S3 PutObject /
+    /// CompleteMultipartUpload response ETag). Empty for backends that do not return a write-time
+    /// ETag (local files, etc.). Valid only after a successful finalize(). Lets content-addressed
+    /// callers record the just-written incarnation's token WITHOUT a follow-up HEAD.
+    virtual std::optional<std::string> getResultObjectETag() const { return {}; }
 };
 
 }
diff --git a/src/IO/WriteBufferFromFileDecorator.h b/src/IO/WriteBufferFromFileDecorator.h
index 07f843986bb0..cc05743642f5 100644
--- a/src/IO/WriteBufferFromFileDecorator.h
+++ b/src/IO/WriteBufferFromFileDecorator.h
@@ -19,6 +19,15 @@ class WriteBufferFromFileDecorator : public WriteBufferFromFileBase
 
     void preFinalize() override;
 
+    /// Forward the wrapped buffer's write-time ETag (if it is a file buffer that produced one), so a
+    /// decorated S3 buffer still lets content-addressed callers skip the post-write HEAD.
+    std::optional<std::string> getResultObjectETag() const override
+    {
+        if (const auto * file_buf = dynamic_cast<const WriteBufferFromFileBase *>(impl.get()))
+            return file_buf->getResultObjectETag();
+        return {};
+    }
+
     const WriteBuffer & getImpl() const { return *impl; }
 
 protected:
diff --git a/src/IO/WriteSettings.h b/src/IO/WriteSettings.h
index a3a3cf204f7f..591f803e6400 100644
--- a/src/IO/WriteSettings.h
+++ b/src/IO/WriteSettings.h
@@ -4,9 +4,22 @@
 #include <Common/Scheduler/ResourceLink.h>
 #include <IO/DistributedCacheSettings.h>
 
+#include <optional>
+
 namespace DB
 {
 
+/// Per-write retry-behavior selector, resolved by the object storage that executes the write.
+/// SingleAttempt: exactly one HTTP attempt, no SDK-transparent retries — for conditional writes
+/// whose retry loop lives above the storage client (it must resolve an uncertain PUT before
+/// reissuing). Backends without a SingleAttempt implementation report it via
+/// IObjectStorage::supportsRetryProfile; writers must fail closed rather than fall through.
+enum class ObjectStorageRetryProfile : uint8_t
+{
+    Default,
+    SingleAttempt,
+};
+
 /// Settings to be passed to IDisk::writeFile()
 struct WriteSettings
 {
@@ -23,6 +36,12 @@ struct WriteSettings
     size_t filesystem_cache_reserve_space_wait_lock_timeout_milliseconds = 1000;
 
     bool s3_allow_parallel_part_upload = true;
+    /// Overrides S3RequestSetting::check_objects_after_upload for this write (nullopt = no
+    /// override). Writers of CAS-MUTABLE keys (content-addressed shard manifests) set `false`:
+    /// such a key is legitimately replaced by a concurrent conditional PUT between this upload and
+    /// the check's HEAD, so the size comparison false-positives ("it's a bug in S3") under normal
+    /// contention. Integrity for those keys is the conditional PUT outcome + token, not a recheck.
+    std::optional<bool> s3_check_objects_after_upload_override;
     bool azure_allow_parallel_part_upload = true;
 
     bool use_adaptive_write_buffer = false;
@@ -36,6 +55,26 @@ struct WriteSettings
     std::string object_storage_write_if_none_match; /// Supported only for S3-like object storages.
     std::string object_storage_write_if_match;     /// Supported only for S3-like object storages.
 
+    /// A conditional write on a generation-token store (GCS) must never take the multipart path:
+    /// GCS enforces no preconditions on CompleteMultipartUpload (measured 2026-07-03). When set,
+    /// WriteBufferFromS3 throws instead of starting a multipart upload.
+    bool s3_force_single_part_upload = false;
+    /// Companion cap: raises max_single_part_upload_size / min_upload_part_size in the request
+    /// settings so bodies up to this size stay in ONE part (RAM-buffered). 0 = no override.
+    size_t s3_single_part_upload_max_bytes_override = 0;
+
+    /// Overrides S3RequestSetting::max_unexpected_write_error_retries (default 4) for this write.
+    /// WriteBufferFromS3::makeSinglepartUpload/completeMultipartUpload run their OWN retry loop above
+    /// the S3 client that reissues the identical request (WITH its If-None-Match/If-Match condition)
+    /// on a NO_SUCH_KEY response — a second retry-affecting layer a client-level override
+    /// (a client-level profile override) does not reach. A CAS conditional write sets this to 1 for
+    /// exactly one attempt at this layer too (RFC cas-s3-timeout-retry-control). 0 = no override.
+    size_t s3_max_unexpected_write_error_retries_override = 0;
+
+    /// Selects the retry profile the object storage should execute this write under; see
+    /// ObjectStorageRetryProfile.
+    ObjectStorageRetryProfile object_storage_retry_profile = ObjectStorageRetryProfile::Default;
+
     bool operator==(const WriteSettings & other) const = default;
 };
 
diff --git a/src/Interpreters/MergeTreeTransaction/VersionMetadataOnDisk.cpp b/src/Interpreters/MergeTreeTransaction/VersionMetadataOnDisk.cpp
index eb7b9df9f056..ab7061854ad0 100644
--- a/src/Interpreters/MergeTreeTransaction/VersionMetadataOnDisk.cpp
+++ b/src/Interpreters/MergeTreeTransaction/VersionMetadataOnDisk.cpp
@@ -326,6 +326,18 @@ void VersionMetadataOnDisk::storeInfoToDataPartStorage(
     static constexpr auto filename = TXN_VERSION_METADATA_FILE_NAME;
     static constexpr auto tmp_filename = TMP_TXN_VERSION_METADATA_FILE_NAME;
 
+    if (data_part_storage.supportsAtomicFileWrites())
+    {
+        /// Single atomic write: storages that publish file writes atomically do not need
+        /// the tmp+replace dance (which exists only for partial-local-write crash safety).
+        auto write_settings = storage.getContext()->getWriteSettings();
+        auto buf = data_part_storage.writeFile(filename, 256, write_settings);
+        new_info.writeToBuffer(*buf, /*one_line=*/false);
+        buf->finalize();
+        buf->sync();
+        return;
+    }
+
     try
     {
         {
diff --git a/src/Storages/StorageMergeTree.cpp b/src/Storages/StorageMergeTree.cpp
index b58b4e0dd1f6..1143c40938dd 100644
--- a/src/Storages/StorageMergeTree.cpp
+++ b/src/Storages/StorageMergeTree.cpp
@@ -13,6 +13,7 @@
 #include <Databases/DatabasesCommon.h>
 #include <Databases/IDatabase.h>
 #include <Disks/supportWritingWithAppend.h>
+#include <Disks/DiskObjectStorage/DiskObjectStorage.h>
 #include <IO/SharedThreadPools.h>
 #include <IO/copyData.h>
 #include <Interpreters/ClusterProxy/SelectStreamFactory.h>
@@ -176,11 +177,15 @@ static bool supportTransaction(const Disks & disks, LoggerPtr log)
 {
     for (const auto & disk : disks)
     {
-        if (!supportWritingWithAppend(disk))
-        {
-            LOG_DEBUG(log, "Disk {} does not support writing with append", disk->getName());
-            return false;
-        }
+        if (supportWritingWithAppend(disk))
+            continue;
+        /// A content-addressed disk does not support append, but persists the per-part mutable
+        /// transaction file (txn_version.txt) via its per-ref sidecar, which is all MVCC needs.
+        if (auto * obj = dynamic_cast<DiskObjectStorage *>(disk.get());
+            obj && obj->getMetadataStorage()->supportsTransactionalMutableFiles())
+            continue;
+        LOG_DEBUG(log, "Disk {} does not support transactions", disk->getName());
+        return false;
     }
     return true;
 }
diff --git a/src/Storages/StorageReplicatedMergeTree.cpp b/src/Storages/StorageReplicatedMergeTree.cpp
index ad52354eb849..017f742edd5d 100644
--- a/src/Storages/StorageReplicatedMergeTree.cpp
+++ b/src/Storages/StorageReplicatedMergeTree.cpp
@@ -464,6 +464,18 @@ StorageReplicatedMergeTree::StorageReplicatedMergeTree(
     {
         if (disk->getDataSourceDescription().metadata_type == MetadataStorageType::Keeper)
             throw Exception(ErrorCodes::BAD_ARGUMENTS, "ReplicatedMergeTree doesn't work with 's3_with_keeper' disk type");
+
+        /// B33 (lifted, CAS replication 2b + Phase 3.2): ReplicatedMergeTree on a content-addressed disk
+        /// is allowed. INSERT/SELECT/merge/mutation and fetch-by-relink (the CA analogue of zero-copy
+        /// replication) route through the working whole-part CA transaction / the relink path. The
+        /// replication-queue CLONE paths (queue-driven REPLACE/MOVE/ATTACH PARTITION FROM, the
+        /// cloneAndLoadDataPart-on-the-queue path) were audited in Phase 3.2: they reach the SAME
+        /// whole-part ContentAddressedTransaction the non-replicated stack uses (see
+        /// `MergeTreeData::checkAlterPartitionIsPossible`, reached here by dynamic dispatch — the
+        /// Phase 3.2 fail-closed override in this class was a pure delegation and was deleted by the
+        /// tail de-patch), NOT the per-file-autocommit B21 mode, so they are now permitted. The
+        /// zero-copy lockSharedData/unlockSharedData calls these reach are safe no-ops on CA (they
+        /// early-return on !supportZeroCopyReplication, which CA is).
     }
 
     initializeDirectoriesAndFormatVersion(relative_data_path_, LoadingStrictnessLevel::ATTACH <= mode, date_column_name);

From 1d92894f880da584d0560b6d09de2411893e4e7e Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:35 +0200
Subject: [PATCH 22/30] CAS integration: system logs and introspection

The CAS system logs (definitions, SystemLog registration, Context
getters), the CAS mounts system table, and the per-disk GC-health
asynchronous metrics.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/Common/SystemLogBase.cpp                  |   2 +
 src/Common/SystemLogBase.h                    |   2 +
 .../ContentAddressedGarbageCollectionLog.cpp  | 114 ++++++++
 .../ContentAddressedGarbageCollectionLog.h    |  63 ++++
 src/Interpreters/ContentAddressedLog.cpp      |  73 +++++
 src/Interpreters/ContentAddressedLog.h        |  47 +++
 src/Interpreters/Context.cpp                  |  16 +
 src/Interpreters/Context.h                    |   4 +
 .../ServerAsynchronousMetrics.cpp             |  33 +++
 src/Interpreters/SystemLog.cpp                |   2 +
 src/Interpreters/SystemLog.h                  |   2 +
 .../StorageSystemContentAddressedMounts.cpp   | 273 ++++++++++++++++++
 .../StorageSystemContentAddressedMounts.h     |  38 +++
 src/Storages/System/attachSystemTables.cpp    |   4 +
 14 files changed, 673 insertions(+)
 create mode 100644 src/Interpreters/ContentAddressedGarbageCollectionLog.cpp
 create mode 100644 src/Interpreters/ContentAddressedGarbageCollectionLog.h
 create mode 100644 src/Interpreters/ContentAddressedLog.cpp
 create mode 100644 src/Interpreters/ContentAddressedLog.h
 create mode 100644 src/Storages/System/StorageSystemContentAddressedMounts.cpp
 create mode 100644 src/Storages/System/StorageSystemContentAddressedMounts.h

diff --git a/src/Common/SystemLogBase.cpp b/src/Common/SystemLogBase.cpp
index 2ee953ff7ec6..8adc1a7465ea 100644
--- a/src/Common/SystemLogBase.cpp
+++ b/src/Common/SystemLogBase.cpp
@@ -6,6 +6,8 @@
 #include <Interpreters/TransposedMetricLog.h>
 #include <Interpreters/OpenTelemetrySpanLog.h>
 #include <Interpreters/PartLog.h>
+#include <Interpreters/ContentAddressedGarbageCollectionLog.h>
+#include <Interpreters/ContentAddressedLog.h>
 #include <Interpreters/BackgroundSchedulePoolLog.h>
 #include <Interpreters/QueryMetricLog.h>
 #include <Interpreters/QueryLog.h>
diff --git a/src/Common/SystemLogBase.h b/src/Common/SystemLogBase.h
index d8db456a29dd..c5eeafa162f2 100644
--- a/src/Common/SystemLogBase.h
+++ b/src/Common/SystemLogBase.h
@@ -17,6 +17,8 @@
     M(CrashLogElement) \
     M(OpenTelemetrySpanLogElement) \
     M(PartLogElement) \
+    M(ContentAddressedGarbageCollectionLogElement) \
+    M(ContentAddressedLogElement) \
     M(BackgroundSchedulePoolLogElement) \
     M(QueryLogElement) \
     M(QueryThreadLogElement) \
diff --git a/src/Interpreters/ContentAddressedGarbageCollectionLog.cpp b/src/Interpreters/ContentAddressedGarbageCollectionLog.cpp
new file mode 100644
index 000000000000..fde269c4c999
--- /dev/null
+++ b/src/Interpreters/ContentAddressedGarbageCollectionLog.cpp
@@ -0,0 +1,114 @@
+#include <Interpreters/ContentAddressedGarbageCollectionLog.h>
+#include <base/getFQDNOrHostName.h>
+#include <DataTypes/DataTypeDate.h>
+#include <DataTypes/DataTypeDateTime.h>
+#include <DataTypes/DataTypeDateTime64.h>
+#include <DataTypes/DataTypeEnum.h>
+#include <DataTypes/DataTypeLowCardinality.h>
+#include <DataTypes/DataTypeMap.h>
+#include <DataTypes/DataTypeString.h>
+#include <DataTypes/DataTypesNumber.h>
+#include <Common/DateLUTImpl.h>
+
+namespace DB
+{
+
+ColumnsDescription ContentAddressedGarbageCollectionLogElement::getColumnsDescription()
+{
+    auto type_enum = std::make_shared<DataTypeEnum8>(DataTypeEnum8::Values{
+        {"Start", static_cast<Int8>(START)}, {"Finish", static_cast<Int8>(FINISH)},
+        {"Phase", static_cast<Int8>(PHASE)}});
+    auto outcome_enum = std::make_shared<DataTypeEnum8>(DataTypeEnum8::Values{
+        {"Unknown", static_cast<Int8>(UNKNOWN)}, {"Success", static_cast<Int8>(SUCCESS)},
+        {"NotALeader", static_cast<Int8>(NOT_A_LEADER)}, {"Error", static_cast<Int8>(FAILED)},
+        {"Deferred", static_cast<Int8>(DEFERRED)}});
+    auto trigger_enum = std::make_shared<DataTypeEnum8>(DataTypeEnum8::Values{
+        {"Scheduled", static_cast<Int8>(SCHEDULED)}, {"Manual", static_cast<Int8>(MANUAL)}});
+    auto lc_string = std::make_shared<DataTypeLowCardinality>(std::make_shared<DataTypeString>());
+
+    return ColumnsDescription
+    {
+        {"hostname", lc_string, "Host name of the server executing the round."},
+        {"event_date", std::make_shared<DataTypeDate>(), "Event date."},
+        {"event_time", std::make_shared<DataTypeDateTime>(), "Event time."},
+        {"event_time_microseconds", std::make_shared<DataTypeDateTime64>(6), "Event time with microseconds."},
+        {"event_type", type_enum, "Start or Finish of a GC round, or one Phase of it."},
+        {"disk_name", lc_string, "Content-addressed disk the round ran on."},
+        {"server_root_id", lc_string, "Identifies the mount whose GC scheduler ran this round. Distinguishes concurrent mounters of the same shared pool; join on this column when correlating rounds against `system.cas_mounts`."},
+        {"gc_id", std::make_shared<DataTypeString>(), "GC scheduler instance id (which mounter)."},
+        {"trigger", trigger_enum, "Scheduled (background tick) or Manual (SYSTEM command)."},
+        {"round", std::make_shared<DataTypeUInt64>(), "GC round number (0 on Start)."},
+        {"outcome", outcome_enum, "Unknown (Start) / Success (led, folded, and completed) / NotALeader (another replica holds the GC lease) / Deferred (led but took the skip-unchanged fast path -- no fold ran) / Error (the round threw)."},
+        {"candidates_marked", std::make_shared<DataTypeUInt64>(), "Objects retired (marked) this round."},
+        {"objects_deleted", std::make_shared<DataTypeUInt64>(), "Objects physically deleted this round."},
+        {"objects_absent", std::make_shared<DataTypeUInt64>(), "Retire candidates found already absent."},
+        {"objects_replaced", std::make_shared<DataTypeUInt64>(), "412-saves (a resurrection won the race)."},
+        {"objects_spared", std::make_shared<DataTypeUInt64>(), "Candidates spared (in-degree > 0 at recheck)."},
+        {"manifests_deleted", std::make_shared<DataTypeUInt64>(), "Owner-removed manifest bodies physically deleted this round (counted separately from blob deletes, B11)."},
+        {"entries_condemned", std::make_shared<DataTypeUInt64>(), "Retired entries newly condemned this round (retired-cursor pipeline stage 1)."},
+        {"entries_graduated", std::make_shared<DataTypeUInt64>(), "Retired entries newly floor-passed and republished delete_pending this round (stage 2; deleted the NEXT round)."},
+        {"entries_redeleted", std::make_shared<DataTypeUInt64>(), "Pending exact-token blob deletes executed this round (stage 3)."},
+        {"fence_outs", std::make_shared<DataTypeUInt64>(), "Expired mounts fenced out by this round's heartbeat floor."},
+        {"anomalies", std::make_shared<DataTypeUInt64>(), "Fold clamps surfaced (and survived) this round; steady >0 warrants a look at the round log details."},
+        {"duration_ms", std::make_shared<DataTypeUInt64>(), "Round wall-clock duration (Finish)."},
+        {"error", std::make_shared<DataTypeString>(), "Exception text when outcome = Error."},
+        {"ProfileEvents", std::make_shared<DataTypeMap>(lc_string, std::make_shared<DataTypeUInt64>()),
+            "On a Start/Finish row: the per-round ProfileEvents delta (the Cas* counters and S3 events for this round). On a Phase row: THAT PHASE's delta, so `GROUP BY phase` over `ProfileEvents['S3ListObjects']` attributes the round's LIST budget to the phase that spent it. Empty on the `meta_pool_wait` row by construction — that phase's work runs on other threads (read its `phase_metrics` instead)."},
+        {"round_id", std::make_shared<DataTypeString>(),
+            "Correlator for every row of one round attempt (its Start, each Phase, and its Finish). Minted per attempt; unlike `round` it exists even for a round that never committed and for a round that never led. Group by this column to reconstruct one round."},
+        {"phase", lc_string,
+            "The GC phase this row describes (empty on Start/Finish), in execution order: lease, pre_fold_ref_drain, heartbeat_floor, defer_decision, parent_seal_read, fold_ref_group, fold_seal_read, fold_ref_intake, fold_reduce, fold_seal_write, pending_deletes, meta_pool_wait, round_commit, handoff_reclaim, manifest_deletes, namespace_cleanup, ref_object_cleanup, orphan_sweep. A round that defers, or that never acquires the lease, emits only the phases it reached."},
+        {"phase_duration_microseconds", std::make_shared<DataTypeUInt64>(),
+            "Wall-clock duration of this phase in microseconds (Phase rows only). Microseconds because several phases are routinely sub-millisecond and the point is to see when they are not. Phase durations do not sum to the round's `duration_ms`: the round also does untimed bookkeeping between phases."},
+        {"phase_metrics", std::make_shared<DataTypeMap>(lc_string, std::make_shared<DataTypeUInt64>()),
+            "Phase-specific semantic counts a phase computes for itself and no ProfileEvent can supply (Phase rows only) — for example `changed_shards` on defer_decision, `logs_accounted`/`logs_applied` on fold_ref_intake, `transactions_unapplied` on fold_reduce, `jobs_scheduled`/`jobs_completed` on meta_pool_wait. The verb counts ride the `ProfileEvents` column of the same row."},
+    };
+}
+
+void ContentAddressedGarbageCollectionLogElement::appendToBlock(MutableColumns & columns) const
+{
+    size_t i = 0;
+    columns[i++]->insert(getFQDNOrHostName());
+    columns[i++]->insert(DateLUT::instance().toDayNum(event_time).toUnderType());
+    columns[i++]->insert(event_time);
+    columns[i++]->insert(event_time_microseconds);
+    columns[i++]->insert(static_cast<Int8>(event_type));
+    columns[i++]->insert(disk_name);
+    columns[i++]->insert(srid);
+    columns[i++]->insert(gc_id);
+    columns[i++]->insert(static_cast<Int8>(trigger));
+    columns[i++]->insert(round);
+    columns[i++]->insert(static_cast<Int8>(outcome));
+    columns[i++]->insert(candidates_marked);
+    columns[i++]->insert(objects_deleted);
+    columns[i++]->insert(objects_absent);
+    columns[i++]->insert(objects_replaced);
+    columns[i++]->insert(objects_spared);
+    columns[i++]->insert(manifests_deleted);
+    columns[i++]->insert(entries_condemned);
+    columns[i++]->insert(entries_graduated);
+    columns[i++]->insert(entries_redeleted);
+    columns[i++]->insert(fence_outs);
+    columns[i++]->insert(anomalies);
+    columns[i++]->insert(duration_ms);
+    columns[i++]->insert(error);
+    {
+        Map map;
+        map.reserve(profile_events.size());
+        for (const auto & [k, v] : profile_events)
+            map.push_back(Tuple{k, v});
+        columns[i++]->insert(map);
+    }
+    columns[i++]->insert(round_id);
+    columns[i++]->insert(phase);
+    columns[i++]->insert(phase_duration_microseconds);
+    {
+        Map map;
+        map.reserve(phase_metrics.size());
+        for (const auto & [k, v] : phase_metrics)
+            map.push_back(Tuple{k, v});
+        columns[i++]->insert(map);
+    }
+}
+
+}
diff --git a/src/Interpreters/ContentAddressedGarbageCollectionLog.h b/src/Interpreters/ContentAddressedGarbageCollectionLog.h
new file mode 100644
index 000000000000..9cbdbd3525f6
--- /dev/null
+++ b/src/Interpreters/ContentAddressedGarbageCollectionLog.h
@@ -0,0 +1,63 @@
+#pragma once
+#include <Interpreters/SystemLog.h>
+#include <Core/NamesAndTypes.h>
+#include <Core/NamesAndAliases.h>
+#include <Storages/ColumnsDescription.h>
+
+namespace DB
+{
+
+struct ContentAddressedGarbageCollectionLogElement
+{
+    /// `PHASE`: one row per GC phase, emitted between the round's `START` and `FINISH` and correlated
+    /// with them by `round_id`.
+    enum EventType : int8_t { START = 1, FINISH = 2, PHASE = 3 };
+    /// `DEFERRED`: the round acquired the GC lease and took the skip-unchanged fast path -- no fold, no
+    /// pre-CAS deletes, no `gc/state` CAS. Kept distinct from `SUCCESS` so a query against this table can
+    /// tell a round that genuinely folded and found nothing apart from one that never folded at all.
+    enum Outcome   : int8_t { UNKNOWN = 1, SUCCESS = 2, NOT_A_LEADER = 3, FAILED = 4, DEFERRED = 5 };
+    enum Trigger   : int8_t { SCHEDULED = 1, MANUAL = 2 };
+
+    time_t event_time = 0;
+    Decimal64 event_time_microseconds = 0;
+
+    EventType event_type = START;
+    String disk_name;
+    String srid;         /// server_root_id of the mount whose GC scheduler ran this round
+    String gc_id;
+    Trigger trigger = SCHEDULED;
+
+    UInt64 round = 0;
+    Outcome outcome = UNKNOWN;      /// UNKNOWN on START; set to SUCCESS/NOT_A_LEADER/FAILED on FINISH
+    UInt64 candidates_marked = 0;
+    UInt64 objects_deleted = 0;
+    UInt64 objects_absent = 0;
+    UInt64 objects_replaced = 0;
+    UInt64 objects_spared = 0;
+    UInt64 manifests_deleted = 0;   /// owner-removed manifest bodies deleted (B11 — distinct from blob deletes)
+    UInt64 entries_condemned = 0;   /// retired-cursor pipeline: entries newly condemned this round
+    UInt64 entries_graduated = 0;   /// retired-cursor pipeline: entries newly round-passed (delete_pending) this round
+    UInt64 entries_redeleted = 0;   /// retired-cursor pipeline: pending exact-token blob deletes executed this round
+    UInt64 fence_outs = 0;          /// expired mounts fenced-out by the round's heartbeat floor
+    UInt64 anomalies = 0;           /// fold clamps surfaced this round
+    UInt64 duration_ms = 0;
+    String error;
+    std::map<String, UInt64> profile_events;   /// per-round delta (FINISH); per-phase delta (PHASE)
+
+    String round_id;                           /// correlator for every row of one round attempt
+    String phase;                              /// empty on START/FINISH
+    UInt64 phase_duration_microseconds = 0;              /// PHASE rows only
+    std::map<String, UInt64> phase_metrics;    /// PHASE rows only
+
+    static std::string name() { return "ContentAddressedGarbageCollectionLog"; }
+    static ColumnsDescription getColumnsDescription();
+    static NamesAndAliases getNamesAndAliases() { return {}; }
+    void appendToBlock(MutableColumns & columns) const;
+};
+
+class ContentAddressedGarbageCollectionLog : public SystemLog<ContentAddressedGarbageCollectionLogElement>
+{
+    using SystemLog<ContentAddressedGarbageCollectionLogElement>::SystemLog;
+};
+
+}
diff --git a/src/Interpreters/ContentAddressedLog.cpp b/src/Interpreters/ContentAddressedLog.cpp
new file mode 100644
index 000000000000..9ae6506ea326
--- /dev/null
+++ b/src/Interpreters/ContentAddressedLog.cpp
@@ -0,0 +1,73 @@
+#include <Interpreters/ContentAddressedLog.h>
+#include <base/getFQDNOrHostName.h>
+#include <DataTypes/DataTypeDate.h>
+#include <DataTypes/DataTypeDateTime.h>
+#include <DataTypes/DataTypeDateTime64.h>
+#include <DataTypes/DataTypeLowCardinality.h>
+#include <DataTypes/DataTypeMap.h>
+#include <DataTypes/DataTypeString.h>
+#include <DataTypes/DataTypesNumber.h>
+#include <Common/DateLUTImpl.h>
+
+namespace DB
+{
+
+ColumnsDescription ContentAddressedLogElement::getColumnsDescription()
+{
+    auto lc_string = std::make_shared<DataTypeLowCardinality>(std::make_shared<DataTypeString>());
+    return ColumnsDescription
+    {
+        {"hostname", lc_string, "Host name of the server that emitted the event."},
+        {"event_date", std::make_shared<DataTypeDate>(), "Event date."},
+        {"event_time", std::make_shared<DataTypeDateTime>(), "Event time."},
+        {"event_time_microseconds", std::make_shared<DataTypeDateTime64>(6), "Event time with microseconds."},
+        {"event_type", lc_string, "The CA decision/event (blob_put, blob_reuse_adopt, root_remove, indegree_zero, gc_retire_decision, gc_recheck_verdict, blob_delete, dangling_access, corrupt_dangle, ...)."},
+        {"disk_name", lc_string, "Content-addressed disk / pool the event belongs to."},
+        {"namespace", std::make_shared<DataTypeString>(), "roots/<namespace> (server/table), empty if N/A."},
+        {"ref_name", std::make_shared<DataTypeString>(), "Part name / ref the event concerns, empty if N/A."},
+        {"object_kind", lc_string, "none/blob/manifest/root/snapshot."},
+        {"object_hash", std::make_shared<DataTypeString>(), "Content hash (lowercase hex) of the object, empty if N/A."},
+        {"token", std::make_shared<DataTypeString>(), "Incarnation token (ETag) involved, empty if N/A."},
+        {"round", std::make_shared<DataTypeUInt64>(), "GC round (0 if N/A)."},
+        {"generation", std::make_shared<DataTypeUInt64>(), "GC snapshot generation (0 if N/A)."},
+        {"at_version", std::make_shared<DataTypeUInt64>(), "Manifest shard_version of the driving journal record (0 if N/A)."},
+        {"outcome", lc_string, "Decision outcome (ok/adopt/resurrect/deleted/replaced/spared/absent/zeroed/skipped/...)."},
+        {"reason", lc_string, "Human-readable WHY of the decision (the rationale) -- templated across rows, so LowCardinality."},
+        {"thread_id", std::make_shared<DataTypeUInt64>(), "OS thread that emitted the event."},
+        {"query_id", std::make_shared<DataTypeString>(), "Query id for correlation with system.query_log (empty if N/A)."},
+        {"detail", std::make_shared<DataTypeMap>(lc_string, std::make_shared<DataTypeString>()),
+            "Structured event-specific facts (e.g. condemn_round, superseded_token, code, site)."},
+    };
+}
+
+void ContentAddressedLogElement::appendToBlock(MutableColumns & columns) const
+{
+    size_t i = 0;
+    columns[i++]->insert(getFQDNOrHostName());
+    columns[i++]->insert(DateLUT::instance().toDayNum(event_time).toUnderType());
+    columns[i++]->insert(event_time);
+    columns[i++]->insert(event_time_microseconds);
+    columns[i++]->insert(event_type);
+    columns[i++]->insert(disk_name);
+    columns[i++]->insert(namespace_);
+    columns[i++]->insert(ref_name);
+    columns[i++]->insert(object_kind);
+    columns[i++]->insert(object_hash);
+    columns[i++]->insert(token);
+    columns[i++]->insert(round);
+    columns[i++]->insert(gen);
+    columns[i++]->insert(at_version);
+    columns[i++]->insert(outcome);
+    columns[i++]->insert(reason);
+    columns[i++]->insert(thread_id);
+    columns[i++]->insert(query_id);
+    {
+        Map map;
+        map.reserve(detail.size());
+        for (const auto & [k, v] : detail)
+            map.push_back(Tuple{k, v});
+        columns[i++]->insert(map);
+    }
+}
+
+}
diff --git a/src/Interpreters/ContentAddressedLog.h b/src/Interpreters/ContentAddressedLog.h
new file mode 100644
index 000000000000..84c7ae3c251c
--- /dev/null
+++ b/src/Interpreters/ContentAddressedLog.h
@@ -0,0 +1,47 @@
+#pragma once
+#include <Interpreters/SystemLog.h>
+#include <Core/NamesAndTypes.h>
+#include <Core/NamesAndAliases.h>
+#include <Storages/ColumnsDescription.h>
+#include <map>
+
+namespace DB
+{
+
+/// One row per content-addressed (CA) decision/event (B170). The decoupled Core POD `Cas::CasEvent`
+/// is mapped to this element by `ContentAddressedMetadataStorage::makeCasEventSink` and forwarded to
+/// the SystemLog. Optional (off by default); enabled for soak/CI. The set is exhaustive enough to
+/// reconstruct an entity's whole lifetime; `reason`/`detail` carry each decision's rationale.
+struct ContentAddressedLogElement
+{
+    time_t event_time = 0;
+    Decimal64 event_time_microseconds = 0;
+
+    String event_type;            /// Cas::CasEventType name (snake_case), LowCardinality in the table
+    String disk_name;
+    String namespace_;
+    String ref_name;
+    String object_kind;           /// none/blob/manifest/root/snap
+    String object_hash;
+    String token;
+    UInt64 round = 0;
+    UInt64 gen = 0;
+    UInt64 at_version = 0;
+    String outcome;
+    String reason;
+    UInt64 thread_id = 0;
+    String query_id;
+    std::map<String, String> detail;
+
+    static std::string name() { return "ContentAddressedLog"; }
+    static ColumnsDescription getColumnsDescription();
+    static NamesAndAliases getNamesAndAliases() { return {}; }
+    void appendToBlock(MutableColumns & columns) const;
+};
+
+class ContentAddressedLog : public SystemLog<ContentAddressedLogElement>
+{
+    using SystemLog<ContentAddressedLogElement>::SystemLog;
+};
+
+}
diff --git a/src/Interpreters/Context.cpp b/src/Interpreters/Context.cpp
index 1edce5d06b45..a55da287815b 100644
--- a/src/Interpreters/Context.cpp
+++ b/src/Interpreters/Context.cpp
@@ -6238,6 +6238,22 @@ std::shared_ptr<PartLog> Context::getPartLog() const
     return shared->system_logs->part_log;
 }
 
+std::shared_ptr<ContentAddressedGarbageCollectionLog> Context::getContentAddressedGarbageCollectionLog() const
+{
+    SharedLockGuard lock(shared->mutex);
+    if (!shared->system_logs)
+        return {};
+    return shared->system_logs->cas_gc_log;
+}
+
+std::shared_ptr<ContentAddressedLog> Context::getContentAddressedLog() const
+{
+    SharedLockGuard lock(shared->mutex);
+    if (!shared->system_logs)
+        return {};
+    return shared->system_logs->cas_log;
+}
+
 std::shared_ptr<BackgroundSchedulePoolLog> Context::getBackgroundSchedulePoolLog() const
 {
     SharedLockGuard lock(shared->mutex);
diff --git a/src/Interpreters/Context.h b/src/Interpreters/Context.h
index e939d24e0e86..2ee2713caa70 100644
--- a/src/Interpreters/Context.h
+++ b/src/Interpreters/Context.h
@@ -128,6 +128,8 @@ class QueryMetricLog;
 class QueryThreadLog;
 class QueryViewsLog;
 class PartLog;
+class ContentAddressedGarbageCollectionLog;
+class ContentAddressedLog;
 class BackgroundSchedulePoolLog;
 class TextLog;
 class TraceLog;
@@ -1619,6 +1621,8 @@ class Context: public ContextData, public std::enable_shared_from_this<Context>
     /// Returns an object used to log operations with parts if it possible.
     /// Provide table name to make required checks.
     std::shared_ptr<PartLog> getPartLog() const;
+    std::shared_ptr<ContentAddressedGarbageCollectionLog> getContentAddressedGarbageCollectionLog() const;
+    std::shared_ptr<ContentAddressedLog> getContentAddressedLog() const;
 
     std::shared_ptr<BackgroundSchedulePoolLog> getBackgroundSchedulePoolLog() const;
 
diff --git a/src/Interpreters/ServerAsynchronousMetrics.cpp b/src/Interpreters/ServerAsynchronousMetrics.cpp
index 5f49f439e684..3447e9a70bea 100644
--- a/src/Interpreters/ServerAsynchronousMetrics.cpp
+++ b/src/Interpreters/ServerAsynchronousMetrics.cpp
@@ -11,6 +11,8 @@
 
 #include <Databases/IDatabase.h>
 
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+
 #include <IO/UncompressedCache.h>
 #include <IO/MMappedFileCache.h>
 #include <Common/PageCache.h>
@@ -366,6 +368,37 @@ void ServerAsynchronousMetrics::updateImpl(TimePoint update_time, TimePoint curr
                 }
             }
 #endif
+
+            /// Per-disk CAS GC health, for Prometheus scraping. `tryFromDisk` returns nullptr for a
+            /// disk whose metadata storage is not content-addressed (the common case); `gcHealth()`
+            /// returns nullopt for a content-addressed disk whose GC scheduler has not started yet
+            /// (still opening, read-only, or GC disabled by configuration) -- both are skipped
+            /// silently, same as the DiskUsed_/DiskTotal_ metrics above skip disks that don't report
+            /// space. This runs on every asynchronous-metrics tick for every configured disk and must
+            /// never throw.
+            try
+            {
+                if (auto * ca_storage = ContentAddressedMetadataStorage::tryFromDisk(disk))
+                {
+                    if (auto health = ca_storage->gcHealth())
+                    {
+                        new_values[fmt::format("CASGCIsLeader_{}", name)] = { health->is_leader ? 1 : 0,
+                            "Whether this server currently holds the content-addressed garbage-collection lease for the disk (1) or not (0, e.g. another replica is leading)." };
+                        new_values[fmt::format("CASGCPendingReclaim_{}", name)] = { health->pending_reclaim,
+                            "Cumulative content-addressed objects condemned minus objects physically deleted by this process while it has held the GC lease on the disk. A persistently growing value indicates GC is not keeping up with reclaim." };
+                        new_values[fmt::format("CASGCLastSuccessAgeSeconds_{}", name)] = { health->last_success_age_seconds,
+                            "Seconds since this process last completed a successful content-addressed GC round as leader on the disk (0 if it has never led one)." };
+                        new_values[fmt::format("CASGCWedgedNamespaces_{}", name)] = { health->wedged_namespace_count,
+                            "Number of content-addressed namespaces on the disk currently stuck behind a wedged reference lane, unable to make GC progress." };
+                    }
+                }
+            }
+            catch (...) // NOLINT(bugprone-empty-catch)
+            {
+                /// Sampled on every server tick for every disk; a transient failure here (e.g. a
+                /// store health query hiccup) must never break the rest of asynchronous-metrics
+                /// collection.
+            }
         }
     }
 
diff --git a/src/Interpreters/SystemLog.cpp b/src/Interpreters/SystemLog.cpp
index 00c6f99595cd..a4ce6371abf6 100644
--- a/src/Interpreters/SystemLog.cpp
+++ b/src/Interpreters/SystemLog.cpp
@@ -31,6 +31,8 @@
 #include <Interpreters/TransposedMetricLog.h>
 #include <Interpreters/OpenTelemetrySpanLog.h>
 #include <Interpreters/PartLog.h>
+#include <Interpreters/ContentAddressedGarbageCollectionLog.h>
+#include <Interpreters/ContentAddressedLog.h>
 #include <Interpreters/BackgroundSchedulePoolLog.h>
 #include <Interpreters/PredicateStatisticsLog.h>
 #include <Interpreters/ProcessorsProfileLog.h>
diff --git a/src/Interpreters/SystemLog.h b/src/Interpreters/SystemLog.h
index dcf52c250fe2..684c1f3f4525 100644
--- a/src/Interpreters/SystemLog.h
+++ b/src/Interpreters/SystemLog.h
@@ -17,6 +17,8 @@
     M(QueryLog,              query_log,            "Contains information about executed queries, for example, start time, duration of processing, error messages.") \
     M(QueryThreadLog,        query_thread_log,     "Contains information about threads that execute queries, for example, thread name, thread start time, duration of query processing.") \
     M(PartLog,               part_log,             "This table contains information about events that occurred with data parts in the MergeTree family tables, such as adding or merging data.") \
+    M(ContentAddressedGarbageCollectionLog, cas_gc_log, "Per-round records of the content-addressed (CA) MergeTree garbage collector: a Start and a Finish row per GC round, with counts of objects marked/deleted, duration, outcome, and per-round ProfileEvents.") \
+    M(ContentAddressedLog, cas_log, "Per-event content-addressed (CA) MergeTree audit log: one row per blob/ref/GC decision (put, reuse, retire, delete, root add/remove, in-degree-zero, fence, lease, ...) plus errors (dangling access, fail-closed). Enabled by default while the CA disk feature is experimental (see config.xml); it is the primary forensic instrument for a CA issue and costs nothing when no CA disk is configured.") \
     M(BackgroundSchedulePoolLog, background_schedule_pool_log, "Contains history of background schedule pool task executions.") \
     M(TraceLog,              trace_log,            "Contains stack traces collected by the sampling query profiler.") \
     M(CrashLog,              crash_log,            "Contains information about stack traces for fatal errors. The table does not exist in the database by default, it is created only when fatal errors occur.") \
diff --git a/src/Storages/System/StorageSystemContentAddressedMounts.cpp b/src/Storages/System/StorageSystemContentAddressedMounts.cpp
new file mode 100644
index 000000000000..2a1b82a01ee7
--- /dev/null
+++ b/src/Storages/System/StorageSystemContentAddressedMounts.cpp
@@ -0,0 +1,273 @@
+#include <Storages/System/StorageSystemContentAddressedMounts.h>
+#include <DataTypes/DataTypesNumber.h>
+
+#include <Columns/ColumnNullable.h>
+#include <Columns/ColumnString.h>
+#include <Columns/ColumnsNumber.h>
+#include <Columns/ColumnsDateTime.h>
+#include <DataTypes/DataTypeDateTime.h>
+#include <DataTypes/DataTypeDateTime64.h>
+#include <DataTypes/DataTypeLowCardinality.h>
+#include <DataTypes/DataTypeNullable.h>
+#include <DataTypes/DataTypeString.h>
+#include <DataTypes/DataTypeUUID.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Processors/Sources/SourceFromSingleChunk.h>
+#include <QueryPipeline/Pipe.h>
+#include <Interpreters/Context.h>
+#include <Common/assert_cast.h>
+#include <Common/Exception.h>
+#include <Common/logger_useful.h>
+
+#include <chrono>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int INVALID_STATE;
+}
+
+StorageSystemContentAddressedMounts::StorageSystemContentAddressedMounts(const StorageID & table_id_)
+    : StorageWithCommonVirtualColumns(table_id_)
+{
+    StorageInMemoryMetadata storage_metadata;
+    storage_metadata.setColumns(ColumnsDescription(
+    {
+        {"disk", std::make_shared<DataTypeString>(), "Name of the content-addressed disk."},
+        {"server_root_id", std::make_shared<DataTypeString>(), "Server root id owning the mount slot."},
+        {"server_uuid", std::make_shared<DataTypeUUID>(), "UUID of the server incarnation holding the lease."},
+        {"hostname", std::make_shared<DataTypeString>(), "Hostname recorded in the lease body."},
+        {"process_id", std::make_shared<DataTypeUInt64>(), "Process id recorded in the lease body."},
+        {"writer_epoch", std::make_shared<DataTypeUInt64>(), "Fenced writer epoch of the incarnation."},
+        {"renewal_sequence", std::make_shared<DataTypeUInt64>(), "Lease renewal sequence number."},
+        {"started_at", std::make_shared<DataTypeDateTime64>(3), "Time when the lease started."},
+        {"expires_at", std::make_shared<DataTypeDateTime64>(3), "Time when the lease expires."},
+        {"min_active_build_sequence", std::make_shared<DataTypeUInt64>(), "Oldest in-flight build sequence (UINT64_MAX means the mount said farewell)."},
+        {"gc_fenced", std::make_shared<DataTypeUInt8>(), "1 if GC fenced this slot out (terminal)."},
+        {"state", std::make_shared<DataTypeString>(), "Mount slot state: live, expired, terminated, fenced or corrupt."},
+        {"is_leader", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeUInt8>()), "1 if this server's GC scheduler holds this disk's leadership lease. NULL on rows describing other servers' mounts."},
+        {"pending_reclaim", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeInt64>()), "Cumulative condemned-minus-deleted backlog observed by this process's GC on this disk. NULL on rows describing other servers' mounts."},
+        {"last_success_age_seconds", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeUInt64>()), "Seconds since this disk's GC last led a round (0 if it never led). NULL on rows describing other servers' mounts."},
+        {"wedged_namespace_count", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeUInt64>()), "Ref-append lanes currently wedged on this disk. NULL on rows describing other servers' mounts."},
+        {"lifecycle", std::make_shared<DataTypeString>(), "This server's content-addressed pool lifecycle for the disk (non-gated snapshot, always populated so a not-live disk stays visible): live, not_live, identity_lost, vanished, constructing (never started) or shutdown (torn down)."},
+        {"lifecycle_reason", std::make_shared<DataTypeString>(), "The enum-clean sub-state word for a vanished disk: replaced or forgotten. Empty for every other lifecycle (so lifecycle || '(' || lifecycle_reason || ')' reads e.g. vanished(forgotten))."},
+        {"lifecycle_detail", std::make_shared<DataTypeString>(), "The full typed reason text naming the actual cause when not live: the vanish diagnosis (data root replaced by a foreign pool / decommissioned by SYSTEM CAS FORGET at <time>) or the identity-loss message. Empty when live."},
+        {"lifecycle_since", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeDateTime>()), "When this server entered the current non-live lifecycle state. NULL when live (or the state is not backed by a live pool)."},
+    }));
+    storage_metadata.setVirtuals(createVirtuals());
+    setInMemoryMetadata(storage_metadata);
+}
+
+VirtualColumnsDescription StorageSystemContentAddressedMounts::createVirtuals()
+{
+    VirtualColumnsDescription desc;
+    desc.addEphemeral("_table", std::make_shared<DataTypeLowCardinality>(std::make_shared<DataTypeString>()), "", VirtualsMaterializationPlace::Plan);
+    desc.addEphemeral("_database", std::make_shared<DataTypeLowCardinality>(std::make_shared<DataTypeString>()), "", VirtualsMaterializationPlace::Plan);
+    return desc;
+}
+
+Pipe StorageSystemContentAddressedMounts::read(
+    const Names & column_names,
+    const StorageSnapshotPtr & storage_snapshot,
+    SelectQueryInfo & /*query_info*/,
+    ContextPtr context,
+    QueryProcessingStage::Enum /*processed_stage*/,
+    const size_t /*max_block_size*/,
+    const size_t /*num_streams*/)
+{
+    storage_snapshot->check(column_names);
+
+    MutableColumnPtr col_disk = ColumnString::create();
+    MutableColumnPtr col_srid = ColumnString::create();
+    MutableColumnPtr col_uuid = ColumnUUID::create();
+    MutableColumnPtr col_host = ColumnString::create();
+    MutableColumnPtr col_pid = ColumnUInt64::create();
+    MutableColumnPtr col_epoch = ColumnUInt64::create();
+    MutableColumnPtr col_seq = ColumnUInt64::create();
+    MutableColumnPtr col_started = ColumnDateTime64::create(0, 3);
+    MutableColumnPtr col_expires = ColumnDateTime64::create(0, 3);
+    MutableColumnPtr col_min_active = ColumnUInt64::create();
+    MutableColumnPtr col_fenced = ColumnUInt8::create();
+    MutableColumnPtr col_state = ColumnString::create();
+    MutableColumnPtr col_is_leader = ColumnNullable::create(ColumnUInt8::create(), ColumnUInt8::create());
+    MutableColumnPtr col_pending = ColumnNullable::create(ColumnInt64::create(), ColumnUInt8::create());
+    MutableColumnPtr col_last_success = ColumnNullable::create(ColumnUInt64::create(), ColumnUInt8::create());
+    MutableColumnPtr col_wedged = ColumnNullable::create(ColumnUInt64::create(), ColumnUInt8::create());
+    MutableColumnPtr col_lifecycle = ColumnString::create();
+    MutableColumnPtr col_lifecycle_reason = ColumnString::create();
+    MutableColumnPtr col_lifecycle_detail = ColumnString::create();
+    MutableColumnPtr col_lifecycle_since = ColumnNullable::create(ColumnDateTime::create(), ColumnUInt8::create());
+
+    const uint64_t now_ms = static_cast<uint64_t>(
+        std::chrono::duration_cast<std::chrono::milliseconds>(
+            std::chrono::system_clock::now().time_since_epoch()).count());
+
+    /// The three lifecycle columns are the same for every row of a given disk (one snapshot per disk).
+    /// A lambda keeps them in lockstep with the row-emitting arms below so no arm can forget one.
+    const auto appendLifecycle = [&](const CasLifecycleSnapshot & snap)
+    {
+        col_lifecycle->insert(snap.lifecycle);
+        col_lifecycle_reason->insert(snap.reason);
+        col_lifecycle_detail->insert(snap.detail);
+        if (snap.since != 0)
+            col_lifecycle_since->insert(static_cast<UInt64>(snap.since));
+        else
+            col_lifecycle_since->insertDefault();   /// NULL while live / no backing pool
+    };
+
+    for (const auto & [disk_name, disk] : context->getDisksMap())
+    {
+        auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk);
+        if (!ca)
+            continue;
+
+        /// The NON-GATED lifecycle snapshot (spec §7, Factory class): I/O-free, truthful in EVERY state.
+        /// This is what makes a not-live / stopped / vanished / never-started disk VISIBLE — the
+        /// store()/listMounts path below may refuse or return nothing, but the disk still gets a row
+        /// carrying its lifecycle truth, instead of silently vanishing from the table (the old behavior,
+        /// where a store() refusal or an empty vanished-pool listing dropped the very disk under investigation).
+        const CasLifecycleSnapshot snap = ca->lifecycleSnapshot();
+
+        bool emitted_row = false;
+
+        /// store() -> poolAccess() throws INVALID_STATE both when the disk is NOT MOUNTED (never started /
+        /// shut down / unmounted) AND when the pool is TERMINAL (Vanished/IdentityLost, via
+        /// throwIfLifecycleTerminal) -- exactly the states that must still produce a visible row. Only a
+        /// Live or TransientNotLive pool returns a live handle and reaches the mount-listing path below; a
+        /// terminal or unmounted disk falls through to the synthesized snapshot row. Catch ONLY that typed
+        /// refusal: any OTHER exception is a genuine fault, so let it escape rather than fold a real bug
+        /// into a benign-looking synthesized row.
+        Cas::PoolPtr store;
+        try
+        {
+            store = ca->store();
+        }
+        catch (const Exception & e)
+        {
+            if (e.code() != ErrorCodes::INVALID_STATE)
+                throw;
+            store = nullptr;
+        }
+
+        if (store)
+        {
+            const uint64_t skew_margin_ms = static_cast<uint64_t>(store->poolConfig().mount_lease_ttl_ms.count()) / 2;
+            std::vector<Cas::MountInfo> mounts;
+            bool list_ok = true;
+            try
+            {
+                mounts = Cas::listMounts(store->backend(), store->layout(), now_ms, skew_margin_ms);
+            }
+            catch (...)
+            {
+                /// A transient backend error listing THIS disk's mounts must not blind the operator to the
+                /// other disks' rows, nor drop this disk entirely: fall through to the snapshot-only row so
+                /// the lifecycle is still reported (the whole reason the table is non-gated).
+                tryLogCurrentException(getLogger("StorageSystemContentAddressedMounts"),
+                                       "listing mounts for disk '" + disk_name + "'");
+                list_ok = false;
+            }
+
+            if (list_ok)
+            {
+                const auto health = ca->gcHealth();
+                const String & local_srid = store->poolConfig().server_root_id;
+                for (const auto & m : mounts)
+                {
+                    col_disk->insert(disk_name);
+                    col_srid->insert(m.srid);
+                    assert_cast<ColumnUUID &>(*col_uuid).insertValue(UUID(m.lease.server_uuid));
+                    col_host->insert(m.lease.hostname);
+                    col_pid->insert(m.lease.pid);
+                    col_epoch->insert(m.lease.writer_epoch);
+                    col_seq->insert(m.lease.seq);
+                    assert_cast<ColumnDateTime64 &>(*col_started).insertValue(static_cast<Decimal64>(m.lease.started_at_ms));
+                    assert_cast<ColumnDateTime64 &>(*col_expires).insertValue(static_cast<Decimal64>(m.lease.expires_at_ms));
+                    col_min_active->insert(m.lease.min_active);
+                    col_fenced->insert(static_cast<UInt8>(m.lease.gc_fenced));
+                    col_state->insert(m.state);
+
+                    /// GC health is a process-local fact about THIS server's scheduler. Stamping it onto
+                    /// peer rows misreads as "peer B is GC leader" during incidents — NULL there instead.
+                    const bool is_local_row = (m.srid == local_srid);
+                    if (is_local_row && health)
+                    {
+                        col_is_leader->insert(static_cast<UInt8>(health->is_leader));
+                        col_pending->insert(health->pending_reclaim);
+                        col_last_success->insert(health->last_success_age_seconds);
+                        col_wedged->insert(health->wedged_namespace_count);
+                    }
+                    else
+                    {
+                        col_is_leader->insertDefault();
+                        col_pending->insertDefault();
+                        col_last_success->insertDefault();
+                        col_wedged->insertDefault();
+                    }
+
+                    /// The lifecycle snapshot describes THIS server's pool condition for the disk; it is the
+                    /// same for every mount slot listed, peer rows included.
+                    appendLifecycle(snap);
+                    emitted_row = true;
+                }
+            }
+        }
+
+        /// Ensure the disk is VISIBLE even when no mount row was emitted: not mounted, store() refused,
+        /// listMounts failed, or a vanished (replaced/forgotten) pool with no slots left. Synthesize ONE row from the
+        /// non-gated snapshot, with every live/lease and GC-health column defaulted (0) / NULL.
+        if (!emitted_row)
+        {
+            col_disk->insert(disk_name);
+            col_srid->insert(snap.server_root_id);
+            col_uuid->insertDefault();
+            col_host->insertDefault();
+            col_pid->insertDefault();
+            col_epoch->insertDefault();
+            col_seq->insertDefault();
+            col_started->insertDefault();
+            col_expires->insertDefault();
+            col_min_active->insertDefault();
+            col_fenced->insertDefault();
+            col_state->insertDefault();
+            col_is_leader->insertDefault();
+            col_pending->insertDefault();
+            col_last_success->insertDefault();
+            col_wedged->insertDefault();
+            appendLifecycle(snap);
+        }
+    }
+
+    Columns res_columns;
+    res_columns.emplace_back(std::move(col_disk));
+    res_columns.emplace_back(std::move(col_srid));
+    res_columns.emplace_back(std::move(col_uuid));
+    res_columns.emplace_back(std::move(col_host));
+    res_columns.emplace_back(std::move(col_pid));
+    res_columns.emplace_back(std::move(col_epoch));
+    res_columns.emplace_back(std::move(col_seq));
+    res_columns.emplace_back(std::move(col_started));
+    res_columns.emplace_back(std::move(col_expires));
+    res_columns.emplace_back(std::move(col_min_active));
+    res_columns.emplace_back(std::move(col_fenced));
+    res_columns.emplace_back(std::move(col_state));
+    res_columns.emplace_back(std::move(col_is_leader));
+    res_columns.emplace_back(std::move(col_pending));
+    res_columns.emplace_back(std::move(col_last_success));
+    res_columns.emplace_back(std::move(col_wedged));
+    res_columns.emplace_back(std::move(col_lifecycle));
+    res_columns.emplace_back(std::move(col_lifecycle_reason));
+    res_columns.emplace_back(std::move(col_lifecycle_detail));
+    res_columns.emplace_back(std::move(col_lifecycle_since));
+
+    UInt64 num_rows = res_columns.at(0)->size();
+    Chunk chunk(std::move(res_columns), num_rows);
+
+    return Pipe(std::make_shared<SourceFromSingleChunk>(std::make_shared<const Block>(storage_snapshot->metadata->getSampleBlock()), std::move(chunk)));
+}
+
+}
diff --git a/src/Storages/System/StorageSystemContentAddressedMounts.h b/src/Storages/System/StorageSystemContentAddressedMounts.h
new file mode 100644
index 000000000000..ccc9e40bfc58
--- /dev/null
+++ b/src/Storages/System/StorageSystemContentAddressedMounts.h
@@ -0,0 +1,38 @@
+#pragma once
+
+#include <Storages/StorageWithCommonVirtualColumns.h>
+
+
+namespace DB
+{
+
+class Context;
+
+/// system.cas_mounts: one row per CAS mount slot (`gc/server-roots/<server_root_id>/mount`)
+/// visible from every content-addressed disk — live lease state for operators (who holds which
+/// slot, renewal health, fenced/terminated). Read-only, one LIST + one GET per slot per disk.
+/// The `is_leader` column is per-disk and supersedes the retired process-global `CasGcIsLeader` metric.
+class StorageSystemContentAddressedMounts final : public StorageWithCommonVirtualColumns
+{
+public:
+    explicit StorageSystemContentAddressedMounts(const StorageID & table_id_);
+
+    std::string getName() const override { return "SystemContentAddressedMounts"; }
+
+    static VirtualColumnsDescription createVirtuals();
+
+    using StorageWithCommonVirtualColumns::read;
+
+    Pipe read(
+        const Names & column_names,
+        const StorageSnapshotPtr & storage_snapshot,
+        SelectQueryInfo & query_info,
+        ContextPtr context,
+        QueryProcessingStage::Enum processed_stage,
+        size_t max_block_size,
+        size_t num_streams) override;
+
+    bool isSystemStorage() const override { return true; }
+};
+
+}
diff --git a/src/Storages/System/attachSystemTables.cpp b/src/Storages/System/attachSystemTables.cpp
index ab37f168d51e..d9730da40495 100644
--- a/src/Storages/System/attachSystemTables.cpp
+++ b/src/Storages/System/attachSystemTables.cpp
@@ -78,6 +78,7 @@
 #include <Storages/System/StorageSystemDDLWorkerQueue.h>
 #include <Storages/System/StorageSystemLicenses.h>
 #include <Storages/System/StorageSystemTimeZones.h>
+#include <Storages/System/StorageSystemContentAddressedMounts.h>
 #include <Storages/System/StorageSystemDisks.h>
 #include <Storages/System/StorageSystemDiskTypes.h>
 #include <Storages/System/StorageSystemStoragePolicies.h>
@@ -246,6 +247,9 @@ void attachSystemTablesServer(ContextPtr context, IDatabase & system_database, b
     attachNoDescription<StorageSystemPartsColumns>(context, system_database, "parts_columns", "Contains a list of columns of all currently existing parts of all MergeTree tables. Each column is represented by a single row.");
     attachNoDescription<StorageSystemProjectionPartsColumns>(context, system_database, "projection_parts_columns", "Contains a list of columns of all currently existing projection parts of all MergeTree tables. Each column is represented by a single row.");
     attachNoDescription<StorageSystemDisks>(context, system_database, "disks", "Contains information about disks defined in the server configuration.");
+    attachNoDescription<StorageSystemContentAddressedMounts>(context, system_database, "cas_mounts",
+        "One row per content-addressed (CAS) mount slot: live lease state per server root id — who holds "
+        "which slot, renewal health, fenced/terminated state.");
     attachNoDescription<StorageSystemStoragePolicies>(context, system_database, "storage_policies", "Contains information about storage policies and volumes defined in the server configuration.");
     attach<StorageSystemProcesses>(context, system_database, "processes", "Contains a list of currently executing processes (queries) with their progress.");
     attach<StorageSystemMetrics>(context, system_database, "metrics", "Contains metrics which can be calculated instantly, or have a current value. For example, the number of simultaneously processed queries or the current replica delay. This table is always up to date.");

From 4fb813a9988111f0bc76cb6a590e1d9fddee6d20 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:36 +0200
Subject: [PATCH 23/30] CAS integration: SYSTEM commands

SYSTEM CONTENT ADDRESSED GARBAGE COLLECTION / GC REBUILD / DROP POOL
MEMBER: grammar, AST, interpreter handlers, access checks, and the parser
round-trip test. Includes the magic_enum range widening required once
ASTSystemQuery::Type outgrew the default range.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/Interpreters/InterpreterSystemQuery.cpp | 535 +++++++++++++++++++-
 src/Interpreters/InterpreterSystemQuery.h   |   8 +
 src/Parsers/ASTSystemQuery.cpp              |  40 ++
 src/Parsers/ASTSystemQuery.h                |  22 +
 src/Parsers/ParserSystemQuery.cpp           |  64 +++
 src/Parsers/tests/gtest_Parser.cpp          |  44 ++
 6 files changed, 701 insertions(+), 12 deletions(-)

diff --git a/src/Interpreters/InterpreterSystemQuery.cpp b/src/Interpreters/InterpreterSystemQuery.cpp
index 8220a1a82db6..9ad1bc8dc8c8 100644
--- a/src/Interpreters/InterpreterSystemQuery.cpp
+++ b/src/Interpreters/InterpreterSystemQuery.cpp
@@ -21,6 +21,14 @@
 #include <Databases/enableAllExperimentalSettings.h>
 #include <Disks/DiskObjectStorage/DiskObjectStorage.h>
 #include <Disks/DiskObjectStorage/MetadataStorages/IMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h>
+/// Direct, though `ContentAddressedMetadataStorage.h` above would also pull it in: this TU renders
+/// `FsckReport`'s hard findings into the SQL row, and `CasFsck.h`'s `kFsckHardFindings` tripwire is what
+/// breaks THIS build when a finding is added. Depending on another header's include list for that would
+/// make the coverage silently removable.
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/IDisk.h>
 #include <Formats/FormatSchemaInfo.h>
 #include <Functions/UserDefined/ExternalUserDefinedExecutableFunctionsLoader.h>
 #include <Interpreters/ActionLocksManager.h>
@@ -71,6 +79,7 @@
 #include <Storages/MergeTree/MergeTreeSettings.h>
 #include <Storages/StorageMaterializedView.h>
 #include <Storages/StorageReplicatedMergeTree.h>
+#include <Storages/StorageTableProxy.h>
 #include <Storages/StorageURL.h>
 #include <base/coverage.h>
 #include <Common/CoverageCollection.h>
@@ -94,6 +103,7 @@
 #include <Common/typeid_cast.h>
 #include <Common/SystemAllocatedMemoryHolder.h>
 #include <base/sleep.h>
+#include <fmt/ranges.h>
 
 #include "config.h"
 
@@ -264,6 +274,18 @@ AccessType getRequiredAccessType(StorageActionBlockType action_type)
 
 constexpr std::string_view table_is_not_replicated = "Table {} is not replicated";
 
+/// A table in a database created with `lazy_load_tables = 1` stays wrapped in a `StorageTableProxy`
+/// until its first access, so a `dynamic_cast` to the real engine (e.g. `StorageReplicatedMergeTree`)
+/// fails and a `SYSTEM` verb that targets one specific named table misreports it as not replicated.
+/// Materialize the proxy before such a cast; generic query paths already materialize on read by
+/// design and must not go through this helper.
+StoragePtr unwrapTableProxy(const StoragePtr & storage)
+{
+    if (const auto * proxy = dynamic_cast<const StorageTableProxy *>(storage.get()))
+        return proxy->getNested();
+    return storage;
+}
+
 }
 
 /// Implements SYSTEM [START|STOP] <something action from ActionLocks>
@@ -996,6 +1018,100 @@ BlockIO InterpreterSystemQuery::execute()
 
             break;
         }
+        case Type::CAS_GC_RUN:
+        {
+            /// A manual GC RUN executes REGARDLESS of SYSTEM CAS GC STOP: STOP pauses only the
+            /// background PACER, not the GC engine, so an explicit operator round still runs (explicit intent
+            /// wins). A round that acquires the lease sets the disk's in-process is_leader=true, which its
+            /// introspection can surface transiently even while the background scheduler stays stopped —
+            /// until a peer mounter steals the lease or GC START resumes pacing. This is truthful (the round
+            /// DID lead) and harmless (no background thread acts on it while stopped).
+            getContext()->checkAccess(AccessType::SYSTEM_CAS_GC_RUN);
+            result = runContentAddressedGcRun(query.disk);
+            break;
+        }
+        case Type::CAS_GC_REBUILD:
+        {
+            getContext()->checkAccess(AccessType::SYSTEM_CAS_GC_REBUILD);
+            result = runContentAddressedGcRebuild(query.disk, query.cas_gc_rebuild_force);
+            break;
+        }
+        case Type::CAS_FSCK:
+        {
+            getContext()->checkAccess(AccessType::SYSTEM_CAS_FSCK);
+            result = runContentAddressedFsck(query.disk);
+            break;
+        }
+        case Type::CAS_FORGET:
+        {
+            getContext()->checkAccess(AccessType::SYSTEM_CAS_FORGET);
+            contentAddressedForget(query.disk);
+            break;
+        }
+        case Type::CAS_GC_STOP:
+        {
+            getContext()->checkAccess(AccessType::SYSTEM_CAS_GC_STOP);
+            contentAddressedGcStop(query.disk);
+            break;
+        }
+        case Type::CAS_GC_START:
+        {
+            getContext()->checkAccess(AccessType::SYSTEM_CAS_GC_START);
+            contentAddressedGcStart(query.disk);
+            break;
+        }
+        case Type::CAS_DROP_POOL_MEMBER:
+        {
+            getContext()->checkAccess(AccessType::SYSTEM_CAS_DROP_POOL_MEMBER);
+
+            auto disk = getContext()->getDisk(query.disk);
+            auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk);
+            if (!ca)
+                throw Exception(ErrorCodes::BAD_ARGUMENTS,
+                    "SYSTEM CAS DROP POOL MEMBER: disk '{}' is not a content-addressed disk", query.disk);
+            ca->checkNotReadOnly("SYSTEM CAS DROP POOL MEMBER");
+
+            const auto & host_store = ca->store();
+            const auto report = Cas::decommissionPoolMember(
+                host_store->poolBackendPtr(), host_store->poolConfig(), query.replica, {},
+                [ca] { ca->requestGcRoundSoon(); });
+
+            /// One-row summary result set (precedent: SYNC_FILESYSTEM_CACHE's MutableColumns/
+            /// SourceFromSingleChunk construction above).
+            ColumnsDescription columns{NamesAndTypesList{
+                {"server_root_id", std::make_shared<DataTypeString>()},
+                {"namespaces_removed", std::make_shared<DataTypeUInt64>()},
+                {"namespaces_already_removed", std::make_shared<DataTypeUInt64>()},
+                {"committed_refs_removed", std::make_shared<DataTypeUInt64>()},
+                {"precommits_removed", std::make_shared<DataTypeUInt64>()},
+                {"manifest_debris_removed", std::make_shared<DataTypeUInt64>()},
+                {"staging_objects_removed", std::make_shared<DataTypeUInt64>()},
+                {"mountpoint_objects_removed", std::make_shared<DataTypeUInt64>()},
+                {"slot_removed", std::make_shared<DataTypeUInt8>()},
+                {"warnings", std::make_shared<DataTypeString>()},
+            }};
+            Block sample_block;
+            for (const auto & column : columns)
+                sample_block.insert({column.type->createColumn(), column.type, column.name});
+
+            MutableColumns res_columns = sample_block.cloneEmptyColumns();
+            size_t i = 0;
+            res_columns[i++]->insert(report.srid);
+            res_columns[i++]->insert(report.namespaces_removed);
+            res_columns[i++]->insert(report.namespaces_already_removed);
+            res_columns[i++]->insert(report.committed_refs_removed);
+            res_columns[i++]->insert(report.precommits_removed);
+            res_columns[i++]->insert(report.manifest_debris_removed);
+            res_columns[i++]->insert(report.staging_objects_removed);
+            res_columns[i++]->insert(report.mountpoint_objects_removed);
+            res_columns[i++]->insert(static_cast<UInt8>(report.slot_removed));
+            res_columns[i++]->insert(fmt::format("{}", fmt::join(report.warnings, "; ")));
+
+            size_t num_rows = res_columns[0]->size();
+            auto source = std::make_shared<SourceFromSingleChunk>(std::make_shared<const Block>(std::move(sample_block)), Chunk(std::move(res_columns), num_rows));
+            result.pipeline = QueryPipeline(std::move(source));
+            break;
+        }
         case Type::RESTART_DISK:
         {
             restartDisk(query.disk);
@@ -1259,7 +1375,7 @@ void InterpreterSystemQuery::restoreReplica()
 {
     getContext()->checkAccess(AccessType::SYSTEM_RESTORE_REPLICA, table_id);
 
-    const StoragePtr table_ptr = DatabaseCatalog::instance().getTable(table_id, getContext());
+    const StoragePtr table_ptr = unwrapTableProxy(DatabaseCatalog::instance().getTable(table_id, getContext()));
 
     auto * const table_replicated_ptr = dynamic_cast<StorageReplicatedMergeTree *>(table_ptr.get());
 
@@ -1334,7 +1450,9 @@ StoragePtr InterpreterSystemQuery::doRestartReplica(const StorageID & replica, C
         return nullptr;
     }
 
-    if (!dynamic_cast<const StorageReplicatedMergeTree *>(table.get()))
+    /// Only the type check needs the materialized (unwrapped) storage; the possibly-still-proxied
+    /// `table` is what actually stays registered in `database` and is what gets locked/detached below.
+    if (!dynamic_cast<const StorageReplicatedMergeTree *>(unwrapTableProxy(table).get()))
     {
         if (throw_on_error)
             throw Exception(ErrorCodes::BAD_ARGUMENTS, table_is_not_replicated.data(), replica.getNameForLogs());
@@ -1543,7 +1661,7 @@ void InterpreterSystemQuery::dropReplica(ASTSystemQuery & query)
     if (!table_id.empty())
     {
         getContext()->checkAccess(AccessType::SYSTEM_DROP_REPLICA, table_id);
-        StoragePtr table = DatabaseCatalog::instance().getTable(table_id, getContext());
+        StoragePtr table = unwrapTableProxy(DatabaseCatalog::instance().getTable(table_id, getContext()));
 
         if (!dropStorageReplica(query.replica, table))
             throw Exception(ErrorCodes::BAD_ARGUMENTS, table_is_not_replicated.data(), table_id.getNameForLogs());
@@ -2021,7 +2139,7 @@ bool InterpreterSystemQuery::trySyncReplica(StoragePtr table, SyncReplicaMode sy
             break;
     }
 
-    if (auto * storage_replicated = dynamic_cast<StorageReplicatedMergeTree *>(table.get()))
+    if (auto * storage_replicated = dynamic_cast<StorageReplicatedMergeTree *>(unwrapTableProxy(table).get()))
     {
         auto log = getLogger("InterpreterSystemQuery");
         LOG_TRACE(log, "Synchronizing entries in replica's queue with table's log and waiting for current last entry to be processed");
@@ -2067,7 +2185,7 @@ void InterpreterSystemQuery::syncReplica(ASTSystemQuery & query)
 void InterpreterSystemQuery::waitLoadingParts()
 {
     getContext()->checkAccess(AccessType::SYSTEM_WAIT_LOADING_PARTS, table_id);
-    StoragePtr table = DatabaseCatalog::instance().getTable(table_id, getContext());
+    StoragePtr table = unwrapTableProxy(DatabaseCatalog::instance().getTable(table_id, getContext()));
 
     if (auto * merge_tree = dynamic_cast<MergeTreeData *>(table.get()))
     {
@@ -2139,11 +2257,12 @@ namespace
 
 MergeTreeData & getMergeTreeWithManualSelector(const StoragePtr & table, const StorageID & table_id, const char * action)
 {
-    auto * merge_tree = dynamic_cast<MergeTreeData *>(table.get());
+    const StoragePtr unwrapped = unwrapTableProxy(table);
+    auto * merge_tree = dynamic_cast<MergeTreeData *>(unwrapped.get());
     if (!merge_tree)
         throw Exception(ErrorCodes::BAD_ARGUMENTS,
             "Command {} is supported only for MergeTree-family tables, but got: {}",
-            action, table->getName());
+            action, unwrapped->getName());
 
     const auto algorithm = (*merge_tree->getSettings())[MergeTreeSetting::merge_selector_algorithm].value;
     if (algorithm != MergeSelectorAlgorithm::MANUAL)
@@ -2207,6 +2326,363 @@ void InterpreterSystemQuery::syncMerges()
     throw DB::Exception(DB::ErrorCodes::TIMEOUT_EXCEEDED, "SYNC MERGES {}: command timed out. See the 'max_execution_time' setting", table_id.getNameForLogs());
 }
 
+namespace
+{
+
+/// One-row-per-disk result-set builders for the CAS GC verbs, mirroring the SYSTEM CAS
+/// DROP POOL MEMBER precedent (ColumnsDescription + MutableColumns + SourceFromSingleChunk; see also
+/// SYNC_FILESYSTEM_CACHE above).
+ColumnsDescription contentAddressedGcRoundColumns()
+{
+    return ColumnsDescription{NamesAndTypesList{
+        {"disk", std::make_shared<DataTypeString>()},
+        {"acquired_lease", std::make_shared<DataTypeUInt8>()},
+        {"deferred", std::make_shared<DataTypeUInt8>()},
+        {"round", std::make_shared<DataTypeUInt64>()},
+        {"candidates_marked", std::make_shared<DataTypeUInt64>()},
+        {"objects_deleted", std::make_shared<DataTypeUInt64>()},
+        {"objects_absent", std::make_shared<DataTypeUInt64>()},
+        {"objects_replaced", std::make_shared<DataTypeUInt64>()},
+        {"objects_spared", std::make_shared<DataTypeUInt64>()},
+        {"manifests_deleted", std::make_shared<DataTypeUInt64>()},
+        {"entries_condemned", std::make_shared<DataTypeUInt64>()},
+        {"entries_graduated", std::make_shared<DataTypeUInt64>()},
+        {"entries_redeleted", std::make_shared<DataTypeUInt64>()},
+        {"fence_outs", std::make_shared<DataTypeUInt64>()},
+        {"anomalies", std::make_shared<DataTypeUInt64>()},
+        /// Task 7: the retire pipeline's REMAINING (not this-round-delta) sizes, read from the gc/state
+        /// this round's single CAS just published -- see `Cas::RoundReport`'s field comments. Zero on a
+        /// non-authoritative row (!acquired_lease or deferred), same as every other counter above.
+        {"pending_candidates", std::make_shared<DataTypeUInt64>()},
+        {"pending_condemned", std::make_shared<DataTypeUInt64>()},
+        {"pending_retired", std::make_shared<DataTypeUInt64>()},
+    }};
+}
+
+void appendContentAddressedGcRoundRow(MutableColumns & res_columns, const String & disk_name, const Cas::RoundReport & rep)
+{
+    size_t i = 0;
+    res_columns[i++]->insert(disk_name);
+    res_columns[i++]->insert(static_cast<UInt8>(rep.acquired_lease));
+    res_columns[i++]->insert(static_cast<UInt8>(rep.deferred));
+    res_columns[i++]->insert(rep.round);
+    res_columns[i++]->insert(rep.candidates);
+    res_columns[i++]->insert(rep.deleted);
+    res_columns[i++]->insert(rep.absent);
+    res_columns[i++]->insert(rep.replaced);
+    res_columns[i++]->insert(rep.spared);
+    res_columns[i++]->insert(rep.manifests_deleted);
+    res_columns[i++]->insert(rep.condemned);
+    res_columns[i++]->insert(rep.graduated);
+    res_columns[i++]->insert(rep.redeleted);
+    res_columns[i++]->insert(rep.fence_outs);
+    res_columns[i++]->insert(rep.anomalies.size());
+    res_columns[i++]->insert(rep.pending_candidates);
+    res_columns[i++]->insert(rep.pending_condemned);
+    res_columns[i++]->insert(rep.pending_retired);
+}
+
+ColumnsDescription contentAddressedGcRebuildColumns()
+{
+    return ColumnsDescription{NamesAndTypesList{
+        {"disk", std::make_shared<DataTypeString>()},
+        {"performed", std::make_shared<DataTypeUInt8>()},
+        {"round", std::make_shared<DataTypeUInt64>()},
+        {"generation", std::make_shared<DataTypeUInt64>()},
+        {"namespaces", std::make_shared<DataTypeUInt64>()},
+        {"shards", std::make_shared<DataTypeUInt64>()},
+        {"committed_refs", std::make_shared<DataTypeUInt64>()},
+        {"live_precommits", std::make_shared<DataTypeUInt64>()},
+        {"unowned_alive_manifests", std::make_shared<DataTypeUInt64>()},
+        {"edges", std::make_shared<DataTypeUInt64>()},
+        {"clamped_shards", std::make_shared<DataTypeUInt64>()},
+        /// 1 => the rebuild found no fold seal at all and carried NO durable hold forward, having
+        /// concluded from enumeration alone that the pool never sealed a baseline. On a pool that has
+        /// ever completed a GC round this means the object listing lied.
+        {"virgin_by_enumeration", std::make_shared<DataTypeUInt8>()},
+        /// Which generation's fold seal the rebuild carried holds from; 0 when it carried none.
+        {"adopted_seal_generation", std::make_shared<DataTypeUInt64>()},
+    }};
+}
+
+void appendContentAddressedGcRebuildRow(MutableColumns & res_columns, const String & disk_name, const Cas::RebuildReport & rep)
+{
+    size_t i = 0;
+    res_columns[i++]->insert(disk_name);
+    res_columns[i++]->insert(static_cast<UInt8>(rep.performed));
+    res_columns[i++]->insert(rep.round);
+    res_columns[i++]->insert(rep.generation);
+    res_columns[i++]->insert(rep.namespaces);
+    res_columns[i++]->insert(rep.shards);
+    res_columns[i++]->insert(rep.committed_refs);
+    res_columns[i++]->insert(rep.live_precommits);
+    res_columns[i++]->insert(rep.unowned_alive_manifests);
+    res_columns[i++]->insert(rep.edges);
+    res_columns[i++]->insert(rep.clamped_shards);
+    res_columns[i++]->insert(static_cast<UInt8>(rep.virgin_by_enumeration));
+    res_columns[i++]->insert(rep.adopted_seal_generation);
+}
+
+/// SYSTEM CAS FSCK's one-row-per-disk summary. Named UInt64 columns only, no DETAIL
+/// keyword (YAGNI -- the offline `clickhouse-disks cas-fsck --detail` applet already covers per-object
+/// listing). Field order/names mirror `Cas::FsckReport`; the row was a deliberate SUBSET of it until
+/// 2026-07-29, and is no longer one where findings are concerned -- see the rule stated at
+/// `stale_edge` below. `CommandFsck.cpp`'s `formatFsckSummary` line carries the same fields.
+ColumnsDescription contentAddressedFsckColumns()
+{
+    return ColumnsDescription{NamesAndTypesList{
+        {"disk", std::make_shared<DataTypeString>()},
+        {"reachable", std::make_shared<DataTypeUInt64>()},
+        {"dangling", std::make_shared<DataTypeUInt64>()},
+        {"unreachable", std::make_shared<DataTypeUInt64>()},
+        {"pending_gc", std::make_shared<DataTypeUInt64>()},
+        {"awaiting_gc", std::make_shared<DataTypeUInt64>()},
+        {"unaccounted", std::make_shared<DataTypeUInt64>()},
+        /// EVERY TERM OF `FsckReport::clean` APPEARS HERE. This row is the only view of a report a SQL
+        /// consumer ever gets, so a hard finding the row omits is a finding no query can see — the same
+        /// shape that hid `corrupted_runs` from the text summary for months. The row was a deliberate
+        /// subset until 2026-07-29 and `stale_edge`/`corrupted_runs` were invisible from SQL while
+        /// `clickhouse-disks cas-fsck` surfaced them; then
+        /// `lifeless_keys` was added to `clean` in 2026-07-30 and missed here too. Every time, the rule
+        /// was written in prose, and every time the prose did not hold.
+        ///
+        /// So it no longer lives only in prose: `kFsckHardFindings` (`CasFsck.h`) is the list `clean` is
+        /// computed from, and the `static_assert` beside it breaks the build in THIS translation unit when
+        /// a term is added. Read that assert's message before bumping its count -- it names this site as
+        /// one of the three that owes an update, and says that two of the three have no test that can fail
+        /// on their behalf. WHICH two is in the comment above the assert, not in the message; this site is
+        /// one of them.
+        ///
+        /// `stale_edge` is nonzero only in `detail` mode and this row is built from a summary scan, so it
+        /// reads 0 here always — present because "absent" and "zero" are different facts to a consumer,
+        /// and a column that appears the day the scan gains detail is a schema change nobody asked for.
+        {"stale_edge", std::make_shared<DataTypeUInt64>()},
+        {"corrupted_runs", std::make_shared<DataTypeUInt64>()},
+        /// The ref-stream verdicts (spec §7). `chain_broken` is a HARD finding — it belongs on the row
+        /// for the same reason `dangling` does. `unchecked` is its honest companion: namespaces the audit
+        /// could not prove either way, so a zero here is what makes the other zeros mean something.
+        {"chain_broken", std::make_shared<DataTypeUInt64>()},
+        {"unchecked", std::make_shared<DataTypeUInt64>()},
+        /// A malformed/non-canonical namespace-tree key, or an ambiguous/unreadable catalog
+        /// incarnation — a term of `clean`.
+        {"lifeless_keys", std::make_shared<DataTypeUInt64>()},
+        /// A COMPLETE, canonical namespace-life key whose life is absent from a catalog cut taken after
+        /// the listing: janitor-pending debris, NOT a term of `clean` (see `FsckClass::JanitorPending`).
+        {"namespace_janitor_pending", std::make_shared<DataTypeUInt64>()},
+        {"namespace_janitor_pending_bytes", std::make_shared<DataTypeUInt64>()},
+        {"namespace_janitor_pending_lives", std::make_shared<DataTypeUInt64>()},
+        {"ref_records_walked", std::make_shared<DataTypeUInt64>()},
+        {"physical_bytes", std::make_shared<DataTypeUInt64>()},
+        {"referenced_logical_bytes", std::make_shared<DataTypeUInt64>()},
+        {"distinct_blobs", std::make_shared<DataTypeUInt64>()},
+        {"total_blob_refs", std::make_shared<DataTypeUInt64>()},
+    }};
+}
+
+void appendContentAddressedFsckRow(MutableColumns & res_columns, const String & disk_name, const Cas::FsckReport & rep)
+{
+    size_t i = 0;
+    res_columns[i++]->insert(disk_name);
+    res_columns[i++]->insert(rep.reachable);
+    res_columns[i++]->insert(rep.dangling);
+    res_columns[i++]->insert(rep.unreachable);
+    res_columns[i++]->insert(rep.pending_gc);
+    res_columns[i++]->insert(rep.awaiting_gc);
+    res_columns[i++]->insert(rep.unaccounted);
+    res_columns[i++]->insert(rep.stale_edge);
+    res_columns[i++]->insert(rep.corrupted_runs);
+    res_columns[i++]->insert(rep.chain_broken);
+    res_columns[i++]->insert(rep.unchecked);
+    res_columns[i++]->insert(rep.lifeless_keys);
+    res_columns[i++]->insert(rep.namespace_janitor_pending);
+    res_columns[i++]->insert(rep.namespace_janitor_pending_bytes);
+    res_columns[i++]->insert(rep.namespace_janitor_pending_lives);
+    res_columns[i++]->insert(rep.ref_records_walked);
+    res_columns[i++]->insert(rep.physical_bytes);
+    res_columns[i++]->insert(rep.referenced_logical_bytes);
+    res_columns[i++]->insert(rep.distinct_blobs);
+    res_columns[i++]->insert(rep.total_blob_refs);
+}
+
+}
+
+BlockIO InterpreterSystemQuery::runContentAddressedGcRun(const String & disk_name)
+{
+    ColumnsDescription columns = contentAddressedGcRoundColumns();
+    Block sample_block;
+    for (const auto & column : columns)
+        sample_block.insert({column.type->createColumn(), column.type, column.name});
+    MutableColumns res_columns = sample_block.cloneEmptyColumns();
+
+    if (!disk_name.empty())
+    {
+        auto disk = getContext()->getDisk(disk_name);
+        auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk);
+        if (!ca)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "Disk '{}' is not a content-addressed disk", disk_name);
+        appendContentAddressedGcRoundRow(res_columns, disk_name, ca->runGarbageCollectionRoundNow());   /// synchronous, one round
+    }
+    else
+    {
+        size_t ran = 0;
+        for (const auto & [name, disk] : getContext()->getDisksMap())
+        {
+            if (auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk))
+            {
+                appendContentAddressedGcRoundRow(res_columns, name, ca->runGarbageCollectionRoundNow());   /// synchronous, one round
+                ++ran;
+            }
+        }
+        if (ran == 0)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "No content-addressed disks are configured on this node");
+    }
+
+    size_t num_rows = res_columns[0]->size();
+    auto source = std::make_shared<SourceFromSingleChunk>(std::make_shared<const Block>(std::move(sample_block)), Chunk(std::move(res_columns), num_rows));
+    BlockIO result;
+    result.pipeline = QueryPipeline(std::move(source));
+    return result;
+}
+
+BlockIO InterpreterSystemQuery::runContentAddressedGcRebuild(const String & disk_name, bool force)
+{
+    /// REBUILD requires an EXPLICIT disk (E1): the destructive baseline rebuild must never fan out
+    /// across every content-addressed disk on the node. The parser enforces this syntactically; this is
+    /// the fail-closed backstop for a directly-constructed AST.
+    if (disk_name.empty())
+        throw Exception(ErrorCodes::BAD_ARGUMENTS,
+            "SYSTEM CAS GC REBUILD requires an explicit disk name");
+
+    auto disk = getContext()->getDisk(disk_name);
+    auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk);
+    if (!ca)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Disk '{}' is not a content-addressed disk", disk_name);
+
+    Cas::RebuildReport rep = ca->runGcRebuildNow(force);   /// synchronous, one rebuild
+    if (!rep.performed)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "CAS GC rebuild refused: {}", rep.refusal);
+    LOG_INFO(log,
+        "CAS GC rebuild on disk '{}' completed: round={} generation={} namespaces={} shards={} "
+        "committed_refs={} live_precommits={} unowned_alive_manifests={} edges={} clamped_shards={} "
+        "virgin_by_enumeration={} adopted_seal_generation={}",
+        disk_name, rep.round, rep.generation, rep.namespaces, rep.shards, rep.committed_refs,
+        rep.live_precommits, rep.unowned_alive_manifests, rep.edges, rep.clamped_shards,
+        rep.virgin_by_enumeration, rep.adopted_seal_generation);
+
+    ColumnsDescription columns = contentAddressedGcRebuildColumns();
+    Block sample_block;
+    for (const auto & column : columns)
+        sample_block.insert({column.type->createColumn(), column.type, column.name});
+    MutableColumns res_columns = sample_block.cloneEmptyColumns();
+    appendContentAddressedGcRebuildRow(res_columns, disk_name, rep);
+
+    size_t num_rows = res_columns[0]->size();
+    auto source = std::make_shared<SourceFromSingleChunk>(std::make_shared<const Block>(std::move(sample_block)), Chunk(std::move(res_columns), num_rows));
+    BlockIO result;
+    result.pipeline = QueryPipeline(std::move(source));
+    return result;
+}
+
+BlockIO InterpreterSystemQuery::runContentAddressedFsck(const String & disk_name)
+{
+    /// FSCK runs on a RUNNING disk (rev.8): the scan is read-only and revalidates every ref-walk finding
+    /// against a fresh authoritative read, so it needs no quiesce. The disk is REQUIRED, enforced by the
+    /// parser -- no fan-out form.
+    if (disk_name.empty())
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "SYSTEM CAS FSCK requires an explicit disk name");
+
+    auto disk = getContext()->getDisk(disk_name);
+    auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk);
+    if (!ca)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Disk '{}' is not a content-addressed disk", disk_name);
+
+    const Cas::FsckReport rep = ca->runFsckNow(/* detail= */ false);   /// summary only (no DETAIL keyword yet)
+
+    ColumnsDescription columns = contentAddressedFsckColumns();
+    Block sample_block;
+    for (const auto & column : columns)
+        sample_block.insert({column.type->createColumn(), column.type, column.name});
+    MutableColumns res_columns = sample_block.cloneEmptyColumns();
+    appendContentAddressedFsckRow(res_columns, disk_name, rep);
+
+    size_t num_rows = res_columns[0]->size();
+    auto source = std::make_shared<SourceFromSingleChunk>(std::make_shared<const Block>(std::move(sample_block)), Chunk(std::move(res_columns), num_rows));
+    BlockIO result;
+    result.pipeline = QueryPipeline(std::move(source));
+    return result;
+}
+
+void InterpreterSystemQuery::contentAddressedForget(const String & disk_name)
+{
+    /// The operator "fire-marshal" verb (spec §5): a force-Vanish that decommissions a content-addressed
+    /// disk NODE-LOCALLY. Unlike the store()-class verbs, FORGET must work on a disk that is NOT live --
+    /// that is its whole purpose (a stuck transient/IdentityLost pool, an operator-asserted decommission) --
+    /// so it does NOT go through `checkOpAdmitted`/`store()` (which refuse a not-live disk). It is a
+    /// lifecycle verb like the Factory class: it reaches the pool directly and drives it to
+    /// `Vanished(forgotten)`. FORGET is an operator ASSERTION, not an erasure proof; the resulting [D5]
+    /// error message says so.
+    if (disk_name.empty())
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "SYSTEM CAS FORGET requires an explicit disk name");
+
+    auto disk = getContext()->getDisk(disk_name);                       /// UNKNOWN_DISK on a bad name
+    auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk);
+    if (!ca)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Disk '{}' is not a content-addressed disk", disk_name);
+
+    ca->forgetDisk();
+    LOG_WARNING(log,
+        "SYSTEM CAS FORGET decommissioned content-addressed disk '{}' (node-local; erasure "
+        "NOT verified). The disk stays registered and answers store-class access with a typed error; a "
+        "server restart re-registers the name.",
+        disk_name);
+}
+
+void InterpreterSystemQuery::contentAddressedGcStop(const String & disk_name)
+{
+    /// SYSTEM CAS GC STOP (spec §6): stop ONLY the background GC scheduler on this disk. The
+    /// disk stays fully usable -- reads and writes are unaffected; this is granular operator control of GC
+    /// alone (e.g. to pause reclamation during an incident), not a lifecycle transition. STOP-IN-PLACE: the
+    /// scheduler object is retained so a later GC START restarts the SAME instance (its gc_id and lease
+    /// observation history preserved). Idempotent; works even on a not-live/Vanished disk (stopping GC on a
+    /// sick disk is legitimate). The disk is REQUIRED -- there is no fan-out form.
+    if (disk_name.empty())
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "SYSTEM CAS GC STOP requires an explicit disk name");
+
+    auto disk = getContext()->getDisk(disk_name);                       /// UNKNOWN_DISK on a bad name
+    auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk);
+    if (!ca)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Disk '{}' is not a content-addressed disk", disk_name);
+
+    ca->gcStop();
+    LOG_INFO(log,
+        "SYSTEM CAS GC STOP: stopped the background garbage-collection scheduler on "
+        "content-addressed disk '{}' (the disk stays fully usable; SYSTEM CAS GC START "
+        "resumes it).",
+        disk_name);
+}
+
+void InterpreterSystemQuery::contentAddressedGcStart(const String & disk_name)
+{
+    /// SYSTEM CAS GC START (spec §6): restart the background GC scheduler stopped by GC STOP.
+    /// It re-enters the SAME scheduler instance; leadership is NOT auto-restored -- the scheduler re-acquires
+    /// the durable `gc/state` lease through the next round's normal acquisition. Idempotent (a no-op on a
+    /// running scheduler). Refuses on a decommissioned/uncertain pool (typed error) -- restarting GC there
+    /// would only spin failing rounds. The disk is REQUIRED -- there is no fan-out form.
+    if (disk_name.empty())
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "SYSTEM CAS GC START requires an explicit disk name");
+
+    auto disk = getContext()->getDisk(disk_name);                       /// UNKNOWN_DISK on a bad name
+    auto * ca = ContentAddressedMetadataStorage::tryFromDisk(disk);
+    if (!ca)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Disk '{}' is not a content-addressed disk", disk_name);
+
+    ca->gcStart();
+    LOG_INFO(log,
+        "SYSTEM CAS GC START: resumed the background garbage-collection scheduler on "
+        "content-addressed disk '{}'.",
+        disk_name);
+}
+
 void InterpreterSystemQuery::loadPrimaryKeys()
 {
     loadOrUnloadPrimaryKeysImpl(true);
@@ -2222,7 +2698,7 @@ void InterpreterSystemQuery::loadOrUnloadPrimaryKeysImpl(bool load)
     if (!table_id.empty())
     {
         getContext()->checkAccess(load ? AccessType::SYSTEM_LOAD_PRIMARY_KEY : AccessType::SYSTEM_UNLOAD_PRIMARY_KEY, table_id.database_name, table_id.table_name);
-        StoragePtr table = DatabaseCatalog::instance().getTable(table_id, getContext());
+        StoragePtr table = unwrapTableProxy(DatabaseCatalog::instance().getTable(table_id, getContext()));
 
         if (auto * merge_tree = dynamic_cast<MergeTreeData *>(table.get()))
         {
@@ -2368,7 +2844,7 @@ void InterpreterSystemQuery::flushDistributed(ASTSystemQuery & query)
     if (query.query_settings)
         settings_changes = query.query_settings->as<ASTSetQuery>()->changes;
 
-    if (auto * storage_distributed = dynamic_cast<StorageDistributed *>(DatabaseCatalog::instance().getTable(table_id, getContext()).get()))
+    if (auto * storage_distributed = dynamic_cast<StorageDistributed *>(unwrapTableProxy(DatabaseCatalog::instance().getTable(table_id, getContext())).get()))
         storage_distributed->flushClusterNodesAllData(getContext(), settings_changes);
     else
         throw Exception(ErrorCodes::BAD_ARGUMENTS, "Table {} is not distributed", table_id.getNameForLogs());
@@ -2382,7 +2858,7 @@ void InterpreterSystemQuery::flushObjectStorageQueue(ASTSystemQuery & query)
     if (query.queue_path.empty())
         throw Exception(ErrorCodes::BAD_ARGUMENTS, "PATH must be specified for SYSTEM FLUSH OBJECT STORAGE QUEUE");
 
-    auto table = DatabaseCatalog::instance().getTable(table_id, context);
+    auto table = unwrapTableProxy(DatabaseCatalog::instance().getTable(table_id, context));
     auto * queue = dynamic_cast<StorageObjectStorageQueue *>(table.get());
     if (!queue)
         throw Exception(ErrorCodes::BAD_ARGUMENTS,
@@ -2409,7 +2885,7 @@ void InterpreterSystemQuery::prewarmMarkCache()
 
     getContext()->checkAccess(AccessType::SYSTEM_PREWARM_MARK_CACHE, table_id);
 
-    auto table_ptr = DatabaseCatalog::instance().getTable(table_id, getContext());
+    auto table_ptr = unwrapTableProxy(DatabaseCatalog::instance().getTable(table_id, getContext()));
     auto * merge_tree = dynamic_cast<MergeTreeData *>(table_ptr.get());
     if (!merge_tree)
         throw Exception(ErrorCodes::BAD_ARGUMENTS, "Command PREWARM MARK CACHE is supported only for MergeTree table, but got: {}", table_ptr->getName());
@@ -2433,7 +2909,7 @@ void InterpreterSystemQuery::prewarmPrimaryIndexCache()
 
     getContext()->checkAccess(AccessType::SYSTEM_PREWARM_PRIMARY_INDEX_CACHE, table_id);
 
-    auto table_ptr = DatabaseCatalog::instance().getTable(table_id, getContext());
+    auto table_ptr = unwrapTableProxy(DatabaseCatalog::instance().getTable(table_id, getContext()));
     auto * merge_tree = dynamic_cast<MergeTreeData *>(table_ptr.get());
     if (!merge_tree)
         throw Exception(ErrorCodes::BAD_ARGUMENTS, "Command PREWARM PRIMARY INDEX CACHE is supported only for MergeTree table, but got: {}", table_ptr->getName());
@@ -2765,6 +3241,41 @@ AccessRightsElements InterpreterSystemQuery::getRequiredAccessForDDLOnCluster()
             required_access.emplace_back(AccessType::SYSTEM_WAIT_BLOBS_CLEANUP);
             break;
         }
+        case Type::CAS_GC_RUN:
+        {
+            required_access.emplace_back(AccessType::SYSTEM_CAS_GC_RUN);
+            break;
+        }
+        case Type::CAS_GC_REBUILD:
+        {
+            required_access.emplace_back(AccessType::SYSTEM_CAS_GC_REBUILD);
+            break;
+        }
+        case Type::CAS_DROP_POOL_MEMBER:
+        {
+            required_access.emplace_back(AccessType::SYSTEM_CAS_DROP_POOL_MEMBER);
+            break;
+        }
+        case Type::CAS_FSCK:
+        {
+            required_access.emplace_back(AccessType::SYSTEM_CAS_FSCK);
+            break;
+        }
+        case Type::CAS_FORGET:
+        {
+            required_access.emplace_back(AccessType::SYSTEM_CAS_FORGET);
+            break;
+        }
+        case Type::CAS_GC_STOP:
+        {
+            required_access.emplace_back(AccessType::SYSTEM_CAS_GC_STOP);
+            break;
+        }
+        case Type::CAS_GC_START:
+        {
+            required_access.emplace_back(AccessType::SYSTEM_CAS_GC_START);
+            break;
+        }
         case Type::UNFREEZE:
         {
             required_access.emplace_back(AccessType::SYSTEM_UNFREEZE);
diff --git a/src/Interpreters/InterpreterSystemQuery.h b/src/Interpreters/InterpreterSystemQuery.h
index ab772cbd8801..f56625f43b37 100644
--- a/src/Interpreters/InterpreterSystemQuery.h
+++ b/src/Interpreters/InterpreterSystemQuery.h
@@ -74,6 +74,14 @@ class InterpreterSystemQuery : public IInterpreter, WithMutableContext
     void scheduleMerge(ASTSystemQuery & query);
     void syncMerges();
 
+    BlockIO runContentAddressedGcRun(const String & disk_name);
+    BlockIO runContentAddressedGcRebuild(const String & disk_name, bool force);
+
+    BlockIO runContentAddressedFsck(const String & disk_name);
+    void contentAddressedForget(const String & disk_name);
+    void contentAddressedGcStop(const String & disk_name);
+    void contentAddressedGcStart(const String & disk_name);
+
     void loadPrimaryKeys();
     void unloadPrimaryKeys();
     void loadOrUnloadPrimaryKeysImpl(bool load);
diff --git a/src/Parsers/ASTSystemQuery.cpp b/src/Parsers/ASTSystemQuery.cpp
index c4ee97348899..3b029948ee42 100644
--- a/src/Parsers/ASTSystemQuery.cpp
+++ b/src/Parsers/ASTSystemQuery.cpp
@@ -153,6 +153,9 @@ void ASTSystemQuery::formatImpl(WriteBuffer & ostr, const FormatSettings & setti
         Type::CLEAR_DISTRIBUTED_CACHE,
         Type::SYNC_FILESYSTEM_CACHE,
         Type::CLEAR_QUERY_CACHE,
+        /// The grammar parses `<srid> FROM DISK <disk>` before `ON CLUSTER` (ParserSystemQuery.cpp),
+        /// so the round-trip format must print it last too.
+        Type::CAS_DROP_POOL_MEMBER,
     };
 
     if (!queries_with_on_cluster_at_end.contains(type) && !cluster.empty())
@@ -259,13 +262,50 @@ void ASTSystemQuery::formatImpl(WriteBuffer & ostr, const FormatSettings & setti
 
             break;
         }
+        case Type::CAS_GC_REBUILD:
+        {
+            /// FORCE precedes the required disk name: SYSTEM CAS GC REBUILD
+            /// [FORCE] <disk>.
+            if (cas_gc_rebuild_force)
+                print_keyword(" FORCE");
+            if (!disk.empty())
+            {
+                ostr << ' ';
+                print_identifier(disk);
+            }
+            break;
+        }
+        case Type::CAS_DROP_POOL_MEMBER:
+        {
+            /// SYSTEM CAS DROP POOL MEMBER <srid> FROM DISK <disk> -- both required, both
+            /// quoted string literals (unlike the sibling CAS_* commands' bare identifier
+            /// disk target: an srid is an opaque server-root path, not necessarily identifier-shaped).
+            ostr << ' ' << quoteString(replica);
+            print_keyword(" FROM DISK ") << quoteString(disk);
+            break;
+        }
+        case Type::CAS_FSCK:
+        case Type::CAS_FORGET:
+        case Type::CAS_GC_STOP:
+        case Type::CAS_GC_START:
+        {
+            /// SYSTEM CAS FSCK/FORGET/GC STOP/GC START <disk> -- the disk is REQUIRED
+            /// (unlike GC RUN's optional disk): each scan/decommission/scheduler-control verb targets
+            /// exactly one disk, never a fan-out.
+            ostr << ' ';
+            print_identifier(disk);
+            break;
+        }
         case Type::RELOAD_DICTIONARY:
         case Type::RELOAD_MODEL:
         case Type::RELOAD_FUNCTION:
+        case Type::CAS_GC_RUN:
         case Type::RESTART_DISK:
         case Type::WAIT_BLOBS_CLEANUP:
         case Type::CLEAR_DISK_METADATA_CACHE:
         {
+            /// RELOAD DICTIONARY prints its database/table target, RELOAD MODEL/FUNCTION their
+            /// identifier target; CAS GC RUN's disk is optional.
             if (table)
             {
                 ostr << ' ';
diff --git a/src/Parsers/ASTSystemQuery.h b/src/Parsers/ASTSystemQuery.h
index adf76d898d42..365b7046fa5f 100644
--- a/src/Parsers/ASTSystemQuery.h
+++ b/src/Parsers/ASTSystemQuery.h
@@ -4,6 +4,7 @@
 #include <Parsers/IAST.h>
 #include <Parsers/SyncReplicaMode.h>
 #include <Server/ServerType.h>
+#include <base/EnumReflection.h>
 
 #include "config.h"
 
@@ -146,6 +147,13 @@ class ASTSystemQuery : public IAST, public ASTQueryWithOnCluster
         INSTRUMENT_ADD,
         INSTRUMENT_REMOVE,
         RESET_DDL_WORKER,
+        CAS_GC_RUN,
+        CAS_GC_REBUILD,
+        CAS_DROP_POOL_MEMBER,
+        CAS_FSCK,
+        CAS_FORGET,
+        CAS_GC_STOP,
+        CAS_GC_START,
         END
     };
 
@@ -176,6 +184,9 @@ class ASTSystemQuery : public IAST, public ASTQueryWithOnCluster
     String storage_policy;
     String volume;
     String disk;
+    /// SYSTEM CAS GC REBUILD FORCE [<disk>] — the raw baseline-rebuild disaster
+    /// recovery command's optional FORCE keyword (bypass the "healthy state" refusal).
+    bool cas_gc_rebuild_force = false;
     UInt64 seconds{};
     UInt64 untracked_memory_size{};
 
@@ -265,3 +276,14 @@ class ASTSystemQuery : public IAST, public ASTQueryWithOnCluster
 
 
 }
+
+/// Type has grown past the default magic_enum range [-128, 127] (130+ entries and counting, one per
+/// SYSTEM sub-command): both the type-detection loop in ParserSystemQuery.cpp (which iterates
+/// magic_enum::enum_values<Type>() to try every command's auto-derived keyword phrase) and
+/// ASTSystemQuery::typeToString's index table silently drop/OOB-index any value past the range without
+/// this. Matches the GeometryColumnType / Coordination::OpNum precedent for a large enum.
+template <> struct magic_enum::customize::enum_range<DB::ASTSystemQuery::Type>
+{
+    static constexpr int min = 0;
+    static constexpr int max = 255;
+};
diff --git a/src/Parsers/ParserSystemQuery.cpp b/src/Parsers/ParserSystemQuery.cpp
index 07b79465ea04..4b15505fd9e4 100644
--- a/src/Parsers/ParserSystemQuery.cpp
+++ b/src/Parsers/ParserSystemQuery.cpp
@@ -457,6 +457,70 @@ bool ParserSystemQuery::parseImpl(IParser::Pos & pos, ASTPtr & node, Expected &
                 return false;
             break;
         }
+        case Type::CAS_GC_RUN:
+        {
+            /// SYSTEM CAS GC RUN [<disk>] [ON CLUSTER cluster]. The disk is OPTIONAL.
+            /// When omitted, the empty disk means "all content-addressed disks on this node".
+            /// First try the full target form (which also handles ON CLUSTER); if no disk follows,
+            /// fall back to parsing just the optional ON CLUSTER clause and leave the disk empty.
+            auto saved_pos = pos;
+            Expected target_expected = expected;
+            if (!parseQueryWithOnClusterAndTarget(res, pos, target_expected, SystemQueryTargetType::Disk))
+            {
+                pos = saved_pos;
+                res->disk.clear();
+                if (!parseQueryWithOnCluster(res, pos, expected))
+                    return false;
+            }
+            break;
+        }
+        case Type::CAS_GC_REBUILD:
+        {
+            /// SYSTEM CAS GC REBUILD [FORCE] <disk> [ON CLUSTER cluster]. Unlike the
+            /// per-round GC RUN command, REBUILD requires an EXPLICIT disk: the destructive
+            /// baseline rebuild must never fan out across every content-addressed disk from a bare
+            /// command. parseQueryWithOnClusterAndTarget requires the target, so omitting the disk is a
+            /// syntax error.
+            res->cas_gc_rebuild_force = ParserKeyword{Keyword::FORCE}.ignore(pos, expected);
+            if (!parseQueryWithOnClusterAndTarget(res, pos, expected, SystemQueryTargetType::Disk))
+                return false;
+            break;
+        }
+        case Type::CAS_FSCK:
+        case Type::CAS_FORGET:
+        case Type::CAS_GC_STOP:
+        case Type::CAS_GC_START:
+        {
+            /// SYSTEM CAS FSCK/FORGET/GC STOP/GC START <disk> [ON CLUSTER cluster].
+            /// Unlike GC RUN, the disk is REQUIRED -- mirrors CAS_GC_REBUILD (minus the FORCE
+            /// keyword): parseQueryWithOnClusterAndTarget requires the target, so omitting the disk is a
+            /// syntax error rather than a silent fan-out across every content-addressed disk.
+            if (!parseQueryWithOnClusterAndTarget(res, pos, expected, SystemQueryTargetType::Disk))
+                return false;
+            break;
+        }
+        case Type::CAS_DROP_POOL_MEMBER:
+        {
+            /// SYSTEM CAS DROP POOL MEMBER <srid> FROM DISK <disk> [ON CLUSTER cluster].
+            /// Both the srid and the disk name are REQUIRED quoted string literals -- an srid is an
+            /// opaque server-root path (may contain '/'), not the bare identifier the sibling
+            /// CAS_* commands' disk TARGET accepts, so this does not go through
+            /// parseQueryWithOnClusterAndTarget.
+            ASTPtr ast;
+            if (!ParserStringLiteral{}.parse(pos, ast, expected))
+                return false;
+            res->replica = ast->as<ASTLiteral &>().value.safeGet<String>();
+            if (!ParserKeyword{Keyword::FROM}.ignore(pos, expected))
+                return false;
+            if (!ParserKeyword{Keyword::DISK}.ignore(pos, expected))
+                return false;
+            if (!ParserStringLiteral{}.parse(pos, ast, expected))
+                return false;
+            res->disk = ast->as<ASTLiteral &>().value.safeGet<String>();
+            if (!parseQueryWithOnCluster(res, pos, expected))
+                return false;
+            break;
+        }
         /// FLUSH DISTRIBUTED requires table
         /// START/STOP DISTRIBUTED SENDS does not require table
         case Type::STOP_DISTRIBUTED_SENDS:
diff --git a/src/Parsers/tests/gtest_Parser.cpp b/src/Parsers/tests/gtest_Parser.cpp
index c62fd894e875..b7ec22da4420 100644
--- a/src/Parsers/tests/gtest_Parser.cpp
+++ b/src/Parsers/tests/gtest_Parser.cpp
@@ -9,6 +9,7 @@
 #include <Parsers/ParserOptimizeQuery.h>
 #include <Parsers/ParserRenameQuery.h>
 #include <Parsers/ParserAttachAccessEntity.h>
+#include <Parsers/ParserSystemQuery.h>
 #include <Parsers/ParserQuery.h>
 #include <Parsers/ParserQueryWithOutput.h>
 #include <Parsers/ASTQueryWithOutput.h>
@@ -477,6 +478,49 @@ INSTANTIATE_TEST_SUITE_P(ParserRenameQuery, ParserTest,
         }
 })));
 
+// SYSTEM CAS DROP POOL MEMBER: srid and disk are both required quoted string literals
+// (an srid is an opaque server-root path, not identifier-shaped); ON CLUSTER round-trips as a bare
+// identifier (ASTQueryWithOnCluster::formatOnCluster uses backQuoteIfNeed, no quoting needed for a
+// plain name), even though the parser also accepts a quoted string literal for it on input.
+INSTANTIATE_TEST_SUITE_P(ParserSystemQuery, ParserTest,
+    ::testing::Combine(
+        ::testing::Values(std::make_shared<ParserSystemQuery>()),
+        ::testing::ValuesIn(std::initializer_list<ParserTestCase>{
+        {
+            "SYSTEM CAS DROP POOL MEMBER 'srv1' FROM DISK 'disk1'",
+            "SYSTEM CAS DROP POOL MEMBER 'srv1' FROM DISK 'disk1'"
+        },
+        {
+            "SYSTEM CAS DROP POOL MEMBER 'srv1' FROM DISK 'disk1' ON CLUSTER my_cluster",
+            "SYSTEM CAS DROP POOL MEMBER 'srv1' FROM DISK 'disk1' ON CLUSTER my_cluster"
+        },
+        {
+            "SYSTEM CAS DROP POOL MEMBER 'srv1'",   // missing FROM DISK
+            nullptr
+        },
+        {
+            "SYSTEM CAS DROP POOL MEMBER FROM DISK 'disk1'",   // missing srid
+            nullptr
+        },
+        {
+            "SYSTEM CAS GC RUN",
+            "SYSTEM CAS GC RUN"
+        },
+        {
+            "SYSTEM CAS GC RUN disk1",
+            "SYSTEM CAS GC RUN disk1"
+        },
+        {
+            /// CAS_GC_RUN goes through the shared parseQueryWithOnClusterAndTarget
+            /// helper (like RESTART_DISK / WAIT_BLOBS_CLEANUP / CLEAR_DISK_METADATA_CACHE), whose
+            /// round-trip format always normalizes to "ON CLUSTER cluster target" -- unlike
+            /// CAS_DROP_POOL_MEMBER, which has its own dedicated grammar and prints
+            /// ON CLUSTER last.
+            "SYSTEM CAS GC RUN disk1 ON CLUSTER my_cluster",
+            "SYSTEM CAS GC RUN ON CLUSTER my_cluster disk1"
+        },
+})));
+
 static constexpr size_t kDummyMaxQuerySize = 256 * 1024;
 static constexpr size_t kDummyMaxParserDepth = 256;
 static constexpr size_t kDummyMaxParserBacktracks = 1000000;

From 9fd2b915c97888569568877062818cbd3683ba16 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:36 +0200
Subject: [PATCH 24/30] CAS integration: registries, fetch-by-relink, entry
 points

ProfileEvents/CurrentMetrics/AccessType/FailPoint/ServerSettings entries,
build wiring, the fetch-by-relink replication protocol extension in
DataPartsExchange (same-pool fetch publishes a local ref over shared
blobs; gated by pool_uuid, non-CA fetches unchanged), the clickhouse-disks
CA commands (inspect, fsck, gc-dryrun, gc-rebuild, drop-member), and
server/local entry-point wiring.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 programs/disks/CMakeLists.txt                |   5 +
 programs/disks/CommandCaDropMember.cpp       |  75 +++
 programs/disks/CommandCaGcDryRun.cpp         |  56 ++
 programs/disks/CommandCaGcRebuild.cpp        |  85 +++
 programs/disks/CommandCaInspect.cpp          |  77 +++
 programs/disks/CommandFsck.cpp               | 188 ++++++
 programs/disks/ICommand.h                    |   5 +
 programs/local/LocalServer.cpp               |  12 +
 programs/server/Server.cpp                   |   5 +
 programs/server/config.xml                   |  29 +
 src/Access/Common/AccessType.h               |   7 +
 src/CMakeLists.txt                           |  11 +
 src/Common/CurrentMetrics.cpp                |  10 +
 src/Common/FailPoint.cpp                     |   5 +-
 src/Common/ProfileEvents.cpp                 | 159 +++++
 src/Common/setThreadName.h                   |   6 +
 src/Core/ServerSettings.cpp                  |   5 +
 src/Storages/MergeTree/DataPartsExchange.cpp | 655 ++++++++++++++++++-
 src/Storages/MergeTree/DataPartsExchange.h   |  77 ++-
 19 files changed, 1465 insertions(+), 7 deletions(-)
 create mode 100644 programs/disks/CommandCaDropMember.cpp
 create mode 100644 programs/disks/CommandCaGcDryRun.cpp
 create mode 100644 programs/disks/CommandCaGcRebuild.cpp
 create mode 100644 programs/disks/CommandCaInspect.cpp
 create mode 100644 programs/disks/CommandFsck.cpp

diff --git a/programs/disks/CMakeLists.txt b/programs/disks/CMakeLists.txt
index 53971ee492e6..6a9ae9c33e65 100644
--- a/programs/disks/CMakeLists.txt
+++ b/programs/disks/CMakeLists.txt
@@ -18,6 +18,11 @@ set (CLICKHOUSE_DISKS_SOURCES
     CommandSed.cpp
     CommandHelp.cpp
     CommandTouch.cpp
+    CommandFsck.cpp
+    CommandCaGcDryRun.cpp
+    CommandCaGcRebuild.cpp
+    CommandCaInspect.cpp
+    CommandCaDropMember.cpp
     CommandGetCurrentDiskAndPath.cpp
     Utils.cpp)
 
diff --git a/programs/disks/CommandCaDropMember.cpp b/programs/disks/CommandCaDropMember.cpp
new file mode 100644
index 000000000000..f95693c2a02f
--- /dev/null
+++ b/programs/disks/CommandCaDropMember.cpp
@@ -0,0 +1,75 @@
+#include <Disks/DiskObjectStorage/DiskObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h>
+#include <Common/Exception.h>
+#include <ICommand.h>
+
+#include <iostream>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+
+/// Operator-driven decommission of a DEAD pool member's namespaces, debris, staging, roots objects
+/// and mount slot (`Cas::decommissionPoolMember`, design 2026-07-13-cas-pool-member-decommission
+/// §core). Refuses a live member internally; this command only opens the CA disk read-only and
+/// forwards the pool handle -- the admin claim itself happens inside `decommissionPoolMember`.
+class CommandCaDropMember final : public ICommand
+{
+public:
+    CommandCaDropMember() : ICommand("CommandCaDropMember")
+    {
+        command_name = "cas-drop-member";
+        description = "Decommission a DEAD pool member: erase its namespaces, debris, staging, roots "
+                      "objects and mount slot. Refuses a live member. Open the CA disk read-only "
+                      "(the admin claim is made internally).";
+        options_description.add_options()("member", po::value<String>(), "server_root_id of the dead member");
+        positional_options_description.add("member", 1);
+    }
+
+    void executeImpl(const CommandLineOptions & options, DisksClient & client) override
+    {
+        const String srid = getValueFromCommandLineOptionsThrow<String>(options, "member");
+        auto disk = client.getCurrentDiskWithPath().getDisk();
+
+        auto * dos = dynamic_cast<DiskObjectStorage *>(disk.get());
+        if (!dos)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-drop-member: '{}' is not an object-storage disk", disk->getName());
+
+        auto * ca = dynamic_cast<ContentAddressedMetadataStorage *>(dos->getMetadataStorage().get());
+        if (!ca)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-drop-member: disk '{}' is not content-addressed", disk->getName());
+
+        if (!ca->isReadOnly())
+            throw Exception(ErrorCodes::BAD_ARGUMENTS,
+                "cas-drop-member: open the CA disk read-only (a writable open would claim this tool's "
+                "own server_root_id; the decommission claim happens internally)");
+
+        const auto host_store = ca->store();
+        const auto report = Cas::decommissionPoolMember(
+            host_store->poolBackendPtr(), host_store->poolConfig(), srid);
+
+        std::cout << "server_root_id=" << report.srid << "\n"
+                  << "namespaces_removed=" << report.namespaces_removed << "\n"
+                  << "namespaces_already_removed=" << report.namespaces_already_removed << "\n"
+                  << "committed_refs_removed=" << report.committed_refs_removed << "\n"
+                  << "precommits_removed=" << report.precommits_removed << "\n"
+                  << "manifest_debris_removed=" << report.manifest_debris_removed << "\n"
+                  << "staging_objects_removed=" << report.staging_objects_removed << "\n"
+                  << "mountpoint_objects_removed=" << report.mountpoint_objects_removed << "\n"
+                  << "slot_removed=" << (report.slot_removed ? "true" : "false") << "\n";
+        for (const auto & w : report.warnings)
+            std::cout << "warning=" << w << "\n";
+    }
+};
+
+CommandPtr makeCommandCaDropMember()
+{
+    return std::make_shared<DB::CommandCaDropMember>();
+}
+
+}
diff --git a/programs/disks/CommandCaGcDryRun.cpp b/programs/disks/CommandCaGcDryRun.cpp
new file mode 100644
index 000000000000..2e4ba6a7849e
--- /dev/null
+++ b/programs/disks/CommandCaGcDryRun.cpp
@@ -0,0 +1,56 @@
+#include <Disks/DiskObjectStorage/DiskObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Common/Exception.h>
+#include <ICommand.h>
+
+#include <iostream>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+
+class CommandCaGcDryRun final : public ICommand
+{
+public:
+    CommandCaGcDryRun() : ICommand("CommandCaGcDryRun")
+    {
+        command_name = "cas-gc-dryrun";
+        description = "Preview the next GC round's deletes for a content-addressed pool (read-only, no deletes).";
+    }
+
+    void executeImpl(const CommandLineOptions &, DisksClient & client) override
+    {
+        auto disk = client.getCurrentDiskWithPath().getDisk();
+
+        auto * dos = dynamic_cast<DiskObjectStorage *>(disk.get());
+        if (!dos)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-gc-dryrun: '{}' is not an object-storage disk", disk->getName());
+
+        auto * ca = dynamic_cast<ContentAddressedMetadataStorage *>(dos->getMetadataStorage().get());
+        if (!ca)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-gc-dryrun: disk '{}' is not content-addressed", disk->getName());
+
+        if (!ca->isReadOnly())
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-gc-dryrun: open the CA disk read-only");
+
+        /// A non-leader, read-only Gc handle: previewDeletes never acquires the lease or writes.
+        Cas::Gc gc(ca->store(), UInt128(1));
+        const auto preview = gc.previewDeletes();
+
+        std::cout << "preview_deletes=" << preview.size() << "\n";
+        for (const auto & p : preview)
+            std::cout << p.reason << "\t" << p.key << "\t" << p.size << "\n";
+    }
+};
+
+CommandPtr makeCommandCaGcDryRun()
+{
+    return std::make_shared<DB::CommandCaGcDryRun>();
+}
+
+}
diff --git a/programs/disks/CommandCaGcRebuild.cpp b/programs/disks/CommandCaGcRebuild.cpp
new file mode 100644
index 000000000000..5e488296ac4e
--- /dev/null
+++ b/programs/disks/CommandCaGcRebuild.cpp
@@ -0,0 +1,85 @@
+#include <Disks/DiskObjectStorage/DiskObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Common/Exception.h>
+#include <Common/thread_local_rng.h>
+#include <ICommand.h>
+
+#include <iostream>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+
+/// The gc/state disaster-recovery command (spec 2026-07-03): recomputes the in-degree baseline from
+/// raw owner state and CASes a fresh gc/state when the guard has refused every regular round (a lost
+/// gc/state over trimmed journal history — see docs/superpowers/cas/04-gc-protocol.md#gc-rebuild).
+///
+/// REQUIRES a read-only-opened disk, same as fsck/cas-gc-dryrun: this tool must never claim the live
+/// server's mount (a second live mounter racing the real GC's lease/writes is exactly the split-brain
+/// class the protocol is designed to prevent). Unlike fsck/cas-gc-dryrun, rebuildBaseline DOES write
+/// (a single gc/state CAS) — that write is a deliberate, explicit, operator-invoked exception to
+/// "read-only means no writes", gated on the SAME `isReadOnly()` check so it can only run against a
+/// disk configured with <readonly>true</readonly> (i.e. never against the disk a live server has
+/// mounted for read-write traffic).
+class CommandCaGcRebuild final : public ICommand
+{
+public:
+    CommandCaGcRebuild() : ICommand("CommandCaGcRebuild")
+    {
+        command_name = "cas-gc-rebuild";
+        description = "Disaster recovery: rebuild a content-addressed pool's gc/state baseline from raw owner "
+                      "state after the GC guard has refused every round (see CORRUPTED_DATA in the gc log). "
+                      "Requires a read-only-opened disk; never run against a disk a live server has mounted.";
+        options_description.add_options()("force", "bypass the \"healthy state\" refusal (rebuild even though gc/state and every referenced artifact look fine)");
+    }
+
+    void executeImpl(const CommandLineOptions & options, DisksClient & client) override
+    {
+        const bool force = options.contains("force");
+        auto disk = client.getCurrentDiskWithPath().getDisk();
+
+        auto * dos = dynamic_cast<DiskObjectStorage *>(disk.get());
+        if (!dos)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-gc-rebuild: '{}' is not an object-storage disk", disk->getName());
+
+        auto * ca = dynamic_cast<ContentAddressedMetadataStorage *>(dos->getMetadataStorage().get());
+        if (!ca)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-gc-rebuild: disk '{}' is not content-addressed", disk->getName());
+
+        if (!ca->isReadOnly())
+            throw Exception(
+                ErrorCodes::BAD_ARGUMENTS,
+                "cas-gc-rebuild: open the CA disk read-only (<readonly>true</readonly>) — this tool must never "
+                "claim the live server's mount");
+
+        /// gc_id uniqueness across instances is the Gc caller obligation (a random u128 per invocation);
+        /// this is a one-shot command, so a fresh mint per run is exactly right (no stable-instance
+        /// requirement here — rebuildBaseline does its own lease acquire/steal check internally).
+        const UInt128 gc_id = (static_cast<UInt128>(thread_local_rng()) << 64) | thread_local_rng();
+        Cas::Gc gc(ca->store(), gc_id);
+        const Cas::RebuildReport rep = gc.rebuildBaseline(force);
+
+        std::cout << "performed=" << (rep.performed ? 1 : 0) << " round=" << rep.round << " generation=" << rep.generation
+                  << " namespaces=" << rep.namespaces << " shards=" << rep.shards << " committed_refs=" << rep.committed_refs
+                  << " live_precommits=" << rep.live_precommits << " unowned_alive_manifests=" << rep.unowned_alive_manifests
+                  << " edges=" << rep.edges << " clamped_shards=" << rep.clamped_shards << "\n";
+
+        if (!rep.performed)
+        {
+            std::cout << "refusal=" << rep.refusal << "\n";
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-gc-rebuild: refused: {}", rep.refusal);
+        }
+    }
+};
+
+CommandPtr makeCommandCaGcRebuild()
+{
+    return std::make_shared<DB::CommandCaGcRebuild>();
+}
+
+}
diff --git a/programs/disks/CommandCaInspect.cpp b/programs/disks/CommandCaInspect.cpp
new file mode 100644
index 000000000000..b4a3a24dbb89
--- /dev/null
+++ b/programs/disks/CommandCaInspect.cpp
@@ -0,0 +1,77 @@
+#include <Disks/DiskObjectStorage/DiskObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Common/Exception.h>
+#include <ICommand.h>
+
+#include <iostream>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+
+/// Read-only "decode any object" command: takes the RAW object-storage key (e.g. as printed by
+/// `cas-gc-dryrun` or `fsck`) rather than a ClickHouse-relative path, GETs its bytes straight from
+/// the pool's backend, and dispatches to `Cas::caInspectToJson` (the same free function the unit
+/// tests exercise directly against encoder output). Never writes; safe to run against a live pool.
+class CommandCaInspect final : public ICommand
+{
+public:
+    CommandCaInspect() : ICommand("CommandCaInspect")
+    {
+        command_name = "cas-inspect";
+        description = "Decode a content-addressed pool object (by its raw object-storage key) to JSON (read-only).";
+        options_description.add_options()("key", po::value<String>(), "the raw object-storage key to decode (mandatory, positional)");
+        positional_options_description.add("key", 1);
+    }
+
+    void executeImpl(const CommandLineOptions & options, DisksClient & client) override
+    {
+        const String key = getValueFromCommandLineOptionsThrow<String>(options, "key");
+
+        auto disk = client.getCurrentDiskWithPath().getDisk();
+
+        auto * dos = dynamic_cast<DiskObjectStorage *>(disk.get());
+        if (!dos)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-inspect: '{}' is not an object-storage disk", disk->getName());
+
+        auto * ca = dynamic_cast<ContentAddressedMetadataStorage *>(dos->getMetadataStorage().get());
+        if (!ca)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-inspect: disk '{}' is not content-addressed", disk->getName());
+
+        if (!ca->isReadOnly())
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-inspect: open the CA disk read-only");
+
+        const auto got = ca->store()->backend().get(key);
+        if (!got)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-inspect: key '{}' does not exist", key);
+
+        const Cas::Layout & layout = ca->store()->layout();
+        std::optional<Cas::NamespaceLifeId> resolved_life;
+        std::optional<Cas::NamespaceLifePhysicalId> life_id;
+        if (const auto parsed = layout.parseRefObjectKey(key))
+            life_id = parsed->life_id;
+        else if (const auto parsed_ckpt = layout.parseRefCkptKey(key))
+            life_id = *parsed_ckpt;
+        if (life_id)
+        {
+            const Cas::CasRefCatalog::Snapshot cut = Cas::CasRefCatalog::read(ca->store()->backend(), layout);
+            resolved_life = cut.life_index.resolve(*life_id);
+        }
+
+        std::cout << Cas::caInspectToJson(layout, key, got->bytes, resolved_life) << "\n";
+    }
+};
+
+CommandPtr makeCommandCaInspect()
+{
+    return std::make_shared<DB::CommandCaInspect>();
+}
+
+}
diff --git a/programs/disks/CommandFsck.cpp b/programs/disks/CommandFsck.cpp
new file mode 100644
index 000000000000..5727f7d69c11
--- /dev/null
+++ b/programs/disks/CommandFsck.cpp
@@ -0,0 +1,188 @@
+#include <Disks/DiskObjectStorage/DiskObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Common/Exception.h>
+#include <ICommand.h>
+
+#include <chrono>
+#include <iostream>
+#include <optional>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+
+class CommandFsck final : public ICommand
+{
+public:
+    CommandFsck() : ICommand("CommandFsck")
+    {
+        command_name = "cas-fsck";
+        description = "Independently verify content-addressed pool reachability (read-only). "
+                      "Exits nonzero if any reachable object is missing (dangling).";
+        options_description.add_options()("detail", "list per-object rows (class, key, size, reachable_from)")(
+            "timeout", po::value<UInt64>(), "abort the scan after N seconds with a clear error instead of hanging (default 600; 0 = unbounded)")(
+            "namespace", po::value<String>(), "scope the scan to namespaces with this prefix (skips the pool-wide "
+                                               "physical/pipeline classification; still reports the scoped namespaces' "
+                                               "dangling refs and orphan-manifest debris as unreachable)")(
+            "partial", "on --timeout, print the counts accumulated so far flagged partial=1 instead of aborting empty-handed");
+    }
+
+    void executeImpl(const CommandLineOptions & options, DisksClient & client) override
+    {
+        const bool detail = options.contains("detail");
+        const UInt64 timeout_sec = getValueFromCommandLineOptionsWithDefault<UInt64>(options, "timeout", 600);
+        const String namespace_prefix = options.contains("namespace") ? options["namespace"].as<String>() : "";
+        const bool partial = options.contains("partial");
+        auto disk = client.getCurrentDiskWithPath().getDisk();
+
+        auto * dos = dynamic_cast<DiskObjectStorage *>(disk.get());
+        if (!dos)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-fsck: '{}' is not an object-storage disk", disk->getName());
+
+        auto * ca = dynamic_cast<ContentAddressedMetadataStorage *>(dos->getMetadataStorage().get());
+        if (!ca)
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "cas-fsck: disk '{}' is not content-addressed", disk->getName());
+
+        if (!ca->isReadOnly())
+            throw Exception(
+                ErrorCodes::BAD_ARGUMENTS,
+                "cas-fsck: open the CA disk read-only (<readonly>true</readonly>) so inspection never probes/schedules a live pool");
+
+        /// Progress to stderr so a long scan is visibly working (the reachable=… summary stays on
+        /// stdout, machine-parseable). The deadline bounds a slow-but-progressing scan with a clear
+        /// error; for a single LIST page stuck in S3-client retries, lower the disk's S3 retry budget.
+        Cas::FsckProgress on_progress = [](std::string_view phase, uint64_t objects, uint64_t pages)
+        {
+            std::cerr << "cas-fsck: " << phase << " — " << objects << " objects, " << pages << " pages\n";
+        };
+        std::optional<std::chrono::steady_clock::time_point> deadline;
+        if (timeout_sec > 0)
+            deadline = std::chrono::steady_clock::now() + std::chrono::seconds(timeout_sec);
+
+        const Cas::FsckReport report = Cas::runFsck(*ca->store(), detail, on_progress, deadline, partial, namespace_prefix);
+
+        /// Built by `Cas::formatFsckSummary` rather than here, so the line is reachable from a unit test.
+        /// It was assembled inline until 2026-07-26, and in that time `corrupted_runs` was added to the
+        /// report and to `clean()` without ever being rendered — a hard finding no run could report.
+        std::cout << Cas::formatFsckSummary(report) << "\n";
+
+        /// De-alarm the pipeline classes for humans: on an active pool a nonzero pending/awaiting
+        /// count is the ack-floor deletion pipeline working as designed, not a leak. `stale_edge` is
+        /// deliberately NOT part of this sentence: those blobs look exactly like an `AwaitingGc`
+        /// backlog but will never drain, and being swept into "expected, no action needed" is what
+        /// hid them.
+        if (report.pending_gc + report.awaiting_gc > 0)
+            std::cout << "note: " << report.pending_gc + report.awaiting_gc
+                      << " unreferenced object(s) are inside the normal GC deletion pipeline "
+                         "(condemn -> graduate -> exact-token delete takes ~2-3 rounds) — expected, no action needed\n";
+        if (report.stale_edge > 0)
+            std::cout << "note: " << report.stale_edge
+                      << " unreferenced object(s) carry ONLY source edges naming manifests that no longer "
+                         "exist: their in-degree can never reach zero, so the incremental GC will never "
+                         "reclaim them — NOT expected, investigate (a rebuild of the in-degree state is the "
+                         "only way to clear them)\n";
+        /// `unchecked` is not a finding and does not exit nonzero — it says the audit could not PROVE
+        /// those namespaces either way, which is a statement about coverage. Saying so out loud is the
+        /// whole point: a silent verdict of "no complaints" would read as a clean bill of health.
+        if (report.unchecked > 0)
+            std::cout << "note: " << report.unchecked
+                      << " namespace(s) could NOT be proved either way (an unprovable epoch crossing, an "
+                         "unreadable record, or a namespace the scan could not examine) — this run says "
+                         "nothing about them; the per-namespace reason is listed as an `unchecked` row "
+                         "under --detail\n";
+        if (report.unaccounted > 0)
+            std::cout << "note: " << report.unaccounted
+                      << " object(s) are outside the current GC view — normal only as a transient "
+                         "(created+dropped between GC rounds); re-run cas-fsck after the next round and "
+                         "investigate any that persist\n";
+        /// Not a finding: a canonical namespace-life key whose life is absent from the catalog is the
+        /// protocol-produced interval between a fenced GC exact-deleting a `Removing` row and the
+        /// perpetual namespace janitor reaching it on a later bounded page (its own deletes are
+        /// suppressed for the whole of Stage A). Persistent non-convergence is a leak/liveness question
+        /// for `CASGCNamespaceCleanupLeaks` and the `namespace_cleanup` GC-log phase, not this scan.
+        if (report.namespace_janitor_pending > 0)
+            std::cout << "note: " << report.namespace_janitor_pending
+                      << " namespace-life object(s) (" << report.namespace_janitor_pending_bytes
+                      << " byte(s) across " << report.namespace_janitor_pending_lives
+                      << " life/lives) are janitor-pending — their catalog row is already gone, and the "
+                         "perpetual namespace janitor is the sole intended reclaimer, but its deletes can "
+                         "be deferred (e.g. a destructive-round suppression policy) — not corruption; "
+                         "investigate only if the same objects persist across many completed janitor "
+                         "cycles (listed as `janitor-pending` rows under --detail)\n";
+
+        if (detail)
+        {
+            for (const auto & o : report.objects)
+            {
+                const char * c = "unreachable"; // NOLINT(clang-analyzer-deadcode.DeadStores) - defensive fallback if the enum grows
+                switch (o.cls)
+                {
+                    case Cas::FsckClass::Reachable:   c = "reachable"; break;
+                    case Cas::FsckClass::Dangling:    c = "dangling"; break;
+                    case Cas::FsckClass::Unreachable: c = "unreachable"; break;
+                    case Cas::FsckClass::PendingGc:   c = "pending-gc"; break;
+                    case Cas::FsckClass::AwaitingGc:  c = "awaiting-gc"; break;
+                    case Cas::FsckClass::Unaccounted: c = "unaccounted"; break;
+                    case Cas::FsckClass::StaleEdge:   c = "stale-edge"; break;
+                    case Cas::FsckClass::CorruptedRun: c = "corrupted-run"; break;
+                    case Cas::FsckClass::ChainBroken: c = "chain-broken"; break;
+                    case Cas::FsckClass::Unchecked:   c = "unchecked"; break;
+                    case Cas::FsckClass::LifelessKey: c = "lifeless-key"; break;
+                    case Cas::FsckClass::JanitorPending: c = "janitor-pending"; break;
+                }
+                std::cout << c << "\t" << o.key << "\t" << o.size;
+                for (const auto & r : o.reachable_from)
+                    std::cout << "\t" << r;
+                std::cout << "\n";
+            }
+        }
+
+        if (report.dangling > 0)
+            throw Exception(
+                ErrorCodes::BAD_ARGUMENTS, "cas-fsck: {} reachable object(s) MISSING (INV-NO-LOSS violation)", report.dangling);
+        /// A hole in a ref stream is loss of a different kind: the records above it are unreachable, so
+        /// the table's own history is truncated wherever recovery next reads it. Fatal in the summary AND
+        /// in the exit code (spec §7) — a verdict only a `--detail` reader would notice is a verdict no
+        /// automation acts on.
+        if (report.chain_broken > 0)
+            throw Exception(
+                ErrorCodes::BAD_ARGUMENTS,
+                "cas-fsck: {} namespace(s) have a HOLE in their ref-log stream — an id is absent below a "
+                "durable id of the same epoch, which contiguity (INV-1) makes impossible without a lost "
+                "record; every transaction above the hole is unreachable (positions are listed as "
+                "`chain-broken` rows under --detail)", report.chain_broken);
+        /// A term of `clean()`, and until 2026-07-26 the only one that neither printed nor exited
+        /// nonzero — so a corrupt run was invisible twice over. A seal-checksum mismatch is not debris:
+        /// `fold`/`zeroInDegree`/`previewDeletes` all fail closed on the same run, so GC cannot make
+        /// progress past it, and the audit deliberately continues only so ONE pass enumerates them all.
+        if (report.corrupted_runs > 0)
+            throw Exception(
+                ErrorCodes::BAD_ARGUMENTS,
+                "cas-fsck: {} GC source-edge run(s) failed their whole-file seal checksum — the deletion-"
+                "deriving consumers fail closed on these, so GC cannot advance past them (run keys are "
+                "listed as `corrupted-run` rows under --detail)", report.corrupted_runs);
+        /// A key the `Layout` parsers refuse (no current writer can produce it), or a catalog
+        /// incarnation that is ambiguous or unreadable, is corruption nothing clears on its own: the
+        /// namespace enumeration now reports it instead of aborting, which is what makes an exit code
+        /// the only signal automation can act on. A COMPLETE, canonical namespace-life key whose life is
+        /// simply absent from the catalog is NOT counted here — see the `janitor-pending` note above.
+        if (report.lifeless_keys > 0)
+            throw Exception(
+                ErrorCodes::BAD_ARGUMENTS,
+                "cas-fsck: {} key(s) under this pool name are malformed or unresolvable — no current "
+                "writer could have produced them, or their catalog incarnation is ambiguous/unreadable "
+                "(the keys are listed as `lifeless-key` rows under --detail)", report.lifeless_keys);
+    }
+};
+
+CommandPtr makeCommandFsck()
+{
+    return std::make_shared<DB::CommandFsck>();
+}
+
+}
diff --git a/programs/disks/ICommand.h b/programs/disks/ICommand.h
index b0293d01b3bb..12ef5f214a09 100644
--- a/programs/disks/ICommand.h
+++ b/programs/disks/ICommand.h
@@ -133,6 +133,11 @@ DB::CommandPtr makeCommandSwitchDisk();
 DB::CommandPtr makeCommandGetCurrentDiskAndPath();
 DB::CommandPtr makeCommandHelp(const DisksApp & disks_app);
 DB::CommandPtr makeCommandTouch();
+DB::CommandPtr makeCommandFsck();
+DB::CommandPtr makeCommandCaGcDryRun();
+DB::CommandPtr makeCommandCaGcRebuild();
+DB::CommandPtr makeCommandCaInspect();
+DB::CommandPtr makeCommandCaDropMember();
 DB::CommandPtr makeCommandReadChecksums();
 #if CLICKHOUSE_CLOUD
 DB::CommandPtr makeCommandPackedIO();
diff --git a/programs/local/LocalServer.cpp b/programs/local/LocalServer.cpp
index b5b2c9ecb065..d79979cfdcb3 100644
--- a/programs/local/LocalServer.cpp
+++ b/programs/local/LocalServer.cpp
@@ -64,6 +64,7 @@
 #include <Storages/registerStorages.h>
 #include <Dictionaries/registerDictionaries.h>
 #include <Disks/registerDisks.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
 #include <Formats/registerFormats.h>
 #include <boost/program_options/options_description.hpp>
 #include <base/argsToConfig.h>
@@ -199,6 +200,7 @@ namespace ServerSetting
     extern const ServerSettingsUInt64 max_format_parsing_thread_pool_size;
     extern const ServerSettingsUInt64 max_format_parsing_thread_pool_free_size;
     extern const ServerSettingsUInt64 format_parsing_thread_pool_queue_size;
+    extern const ServerSettingsUInt64 cas_blob_upload_pool_size;
     extern const ServerSettingsUInt64 page_cache_history_window_ms;
     extern const ServerSettingsString page_cache_policy;
     extern const ServerSettingsDouble page_cache_size_ratio;
@@ -419,6 +421,11 @@ void LocalServer::initialize(Poco::Util::Application & self)
         server_settings[ServerSetting::max_format_parsing_thread_pool_size],
         server_settings[ServerSetting::max_format_parsing_thread_pool_free_size],
         server_settings[ServerSetting::format_parsing_thread_pool_queue_size]);
+
+    /// See the explanation near the same line in Server.cpp: `uploadPendingBlobs` reaches this
+    /// pool unconditionally once a `cas` disk commits a part, so every entry point
+    /// that can run a CA INSERT must initialize it, not only `clickhouse-server`.
+    DB::Cas::initializeBlobUploadPool(server_settings[ServerSetting::cas_blob_upload_pool_size]);
 }
 
 
@@ -895,6 +902,11 @@ void LocalServer::cleanup()
 
         client_context.reset();
 
+        /// Joins any outstanding blob-upload fan-out tasks before the context they reference
+        /// is torn down. Idempotent and noexcept, so safe even if never initialized (e.g. no
+        /// `cas` disk was ever used).
+        DB::Cas::shutdownBlobUploadPool();
+
         if (global_context)
         {
             global_context->shutdown();
diff --git a/programs/server/Server.cpp b/programs/server/Server.cpp
index 9260a3c9d2dc..77a608df4b87 100644
--- a/programs/server/Server.cpp
+++ b/programs/server/Server.cpp
@@ -103,6 +103,7 @@
 #include <Databases/registerDatabases.h>
 #include <Dictionaries/registerDictionaries.h>
 #include <Disks/registerDisks.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
 #include <Common/Scheduler/Nodes/registerSchedulerNodes.h>
 #include <Common/Scheduler/Workload/IWorkloadEntityStorage.h>
 #include <Coordination/KeeperContext.h>
@@ -394,6 +395,7 @@ namespace ServerSetting
     extern const ServerSettingsUInt64 max_format_parsing_thread_pool_size;
     extern const ServerSettingsUInt64 max_format_parsing_thread_pool_free_size;
     extern const ServerSettingsUInt64 format_parsing_thread_pool_queue_size;
+    extern const ServerSettingsUInt64 cas_blob_upload_pool_size;
     extern const ServerSettingsUInt64 page_cache_history_window_ms;
     extern const ServerSettingsString page_cache_policy;
     extern const ServerSettingsDouble page_cache_size_ratio;
@@ -1495,6 +1497,7 @@ try
         Stopwatch watch;
         LOG_INFO(log, "Waiting for background threads");
         DB::StaticThreadPool::shutdownAll();
+        DB::Cas::shutdownBlobUploadPool();
         GlobalThreadPool::instance().shutdown();
         LOG_INFO(log, "Background threads finished in {} ms", watch.elapsedMilliseconds());
     });
@@ -1716,6 +1719,8 @@ try
         server_settings[ServerSetting::max_format_parsing_thread_pool_free_size],
         server_settings[ServerSetting::format_parsing_thread_pool_queue_size]);
 
+    DB::Cas::initializeBlobUploadPool(server_settings[ServerSetting::cas_blob_upload_pool_size]);
+
     std::string path_str = getCanonicalPath(String(server_settings[ServerSetting::path]), original_working_directory);
     fs::path path = path_str;
 
diff --git a/programs/server/config.xml b/programs/server/config.xml
index 233aafc84edf..919c3b9881ba 100644
--- a/programs/server/config.xml
+++ b/programs/server/config.xml
@@ -1195,6 +1195,23 @@
     -->
 
     <!-- Query log. Used only for queries with setting log_queries = 1. -->
+    <!-- Per-event audit log of the content-addressed (CAS) storage layer. Enabled by default while
+         the feature is experimental (events are emitted only by content-addressed disks; it costs
+         nothing otherwise). Remove the section to disable. -->
+    <cas_log>
+        <database>system</database>
+        <table>cas_log</table>
+        <partition_by>toYYYYMM(event_date)</partition_by>
+        <flush_interval_milliseconds>7500</flush_interval_milliseconds>
+        <max_size_rows>1048576</max_size_rows>
+        <reserved_size_rows>8192</reserved_size_rows>
+        <buffer_size_rows_flush_threshold>524288</buffer_size_rows_flush_threshold>
+        <flush_on_crash>false</flush_on_crash>
+        <!-- example of a retention policy; disabled by default like every other log's TTL
+        <ttl>event_date + INTERVAL 7 DAY DELETE</ttl>
+        -->
+    </cas_log>
+
     <query_log>
         <!-- What table to insert data. If table is not exist, it will be created.
              When query log structure is changed after system update,
@@ -1300,6 +1317,18 @@
         <flush_on_crash>false</flush_on_crash>
     </part_log>
 
+    <!-- Per-round records of the content-addressed (CA) MergeTree garbage collector. -->
+    <cas_gc_log>
+        <database>system</database>
+        <table>cas_gc_log</table>
+        <partition_by>toYYYYMM(event_date)</partition_by>
+        <flush_interval_milliseconds>7500</flush_interval_milliseconds>
+        <max_size_rows>1048576</max_size_rows>
+        <reserved_size_rows>8192</reserved_size_rows>
+        <buffer_size_rows_flush_threshold>524288</buffer_size_rows_flush_threshold>
+        <flush_on_crash>false</flush_on_crash>
+    </cas_gc_log>
+
     <!-- Contains information about all background tasks that are executed via various background pools.
          See also:
          - background_schedule_pool_size
diff --git a/src/Access/Common/AccessType.h b/src/Access/Common/AccessType.h
index 38766a2ed170..2812990d77e7 100644
--- a/src/Access/Common/AccessType.h
+++ b/src/Access/Common/AccessType.h
@@ -348,6 +348,13 @@ enum class AccessType : uint8_t
     M(SYSTEM_RECONNECT_ZOOKEEPER, "SYSTEM RECONNECT ZOOKEEPER, RECONNECT ZOOKEEPER", GLOBAL, SYSTEM) \
     M(SYSTEM_RELOAD, "", GROUP, SYSTEM) \
     M(SYSTEM_RESTART_DISK, "SYSTEM RESTART DISK", GLOBAL, SYSTEM) \
+    M(SYSTEM_CAS_GC_RUN, "SYSTEM CAS GC RUN", GLOBAL, SYSTEM) \
+    M(SYSTEM_CAS_GC_REBUILD, "SYSTEM CAS GC REBUILD", GLOBAL, SYSTEM) \
+    M(SYSTEM_CAS_DROP_POOL_MEMBER, "SYSTEM CAS DROP POOL MEMBER", GLOBAL, SYSTEM) \
+    M(SYSTEM_CAS_FSCK, "SYSTEM CAS FSCK", GLOBAL, SYSTEM) \
+    M(SYSTEM_CAS_FORGET, "SYSTEM CAS FORGET", GLOBAL, SYSTEM) \
+    M(SYSTEM_CAS_GC_STOP, "SYSTEM CAS GC STOP", GLOBAL, SYSTEM) \
+    M(SYSTEM_CAS_GC_START, "SYSTEM CAS GC START", GLOBAL, SYSTEM) \
     M(SYSTEM_WAIT_BLOBS_CLEANUP, "SYSTEM WAIT BLOBS CLEANUP", GLOBAL, SYSTEM) \
     M(SYSTEM_MERGES, "SYSTEM STOP MERGES, SYSTEM START MERGES, STOP MERGES, START MERGES", TABLE, SYSTEM) \
     M(SYSTEM_TTL_MERGES, "SYSTEM STOP TTL MERGES, SYSTEM START TTL MERGES, STOP TTL MERGES, START TTL MERGES", TABLE, SYSTEM) \
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 7a9602cdbb29..076004d912f5 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -132,6 +132,17 @@ add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/Local)
 add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/Plain)
 add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/PlainRewritable)
 add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/Web)
+add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/ContentAddressed)
+add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives)
+add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats)
+add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend)
+add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool)
+add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc)
+add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools)
+add_headers_and_sources(dbms Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts)
+if (ENABLE_BENCHMARKS)
+    add_subdirectory(Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/benchmarks)
+endif()
 if (TARGET ch_contrib::sqlite)
     add_headers_and_sources(dbms Databases/SQLite)
 endif()
diff --git a/src/Common/CurrentMetrics.cpp b/src/Common/CurrentMetrics.cpp
index cdb399e249ce..0f911b0860be 100644
--- a/src/Common/CurrentMetrics.cpp
+++ b/src/Common/CurrentMetrics.cpp
@@ -230,6 +230,16 @@
     M(DiskPlainRewritableS3DirectoryMapSize, "Number of local-to-remote path entries in the 'plain_rewritable' in-memory map for S3ObjectStorage.") \
     M(DiskPlainRewritableS3FileCount, "Number of file entries in the 'plain_rewritable' in-memory map for S3ObjectStorage.") \
     \
+    M(CASPartFolderCacheBytes, "Estimated bytes retained by the CA part-folder view cache") \
+    M(CASPartFolderCacheEntries, "Entries retained by the CA part-folder view cache") \
+    M(CASManifestDecodeCacheBytes, "Bytes retained by the CA manifest decode cache") \
+    M(CASManifestDecodeCacheEntries, "Entries retained by the CA manifest decode cache") \
+    M(CASDeduplicationCacheBytes, "Bytes retained by the CA blob presence (deduplication) cache") \
+    M(CASDeduplicationCacheEntries, "Entries retained by the CA blob presence (deduplication) cache") \
+    M(CASBlobUploadPoolThreads, "Number of threads in the CA blob upload thread pool.") \
+    M(CASBlobUploadPoolThreadsActive, "Number of threads in the CA blob upload thread pool running a task.") \
+    M(CASBlobUploadPoolThreadsScheduled, "Number of queued or active jobs in the CA blob upload thread pool.") \
+    \
     M(MergeTreeFetchPartitionThreads, "Number of threads for ALTER TABLE FETCH PARTITION") \
     M(MergeTreeFetchPartitionThreadsActive, "Number of threads for ALTER TABLE FETCH PARTITION fetching part") \
     M(MergeTreeFetchPartitionThreadsScheduled, "Number of queued or active part fetches in ALTER TABLE FETCH PARTITION") \
diff --git a/src/Common/FailPoint.cpp b/src/Common/FailPoint.cpp
index 71b6b731d871..f71b159bf513 100644
--- a/src/Common/FailPoint.cpp
+++ b/src/Common/FailPoint.cpp
@@ -159,6 +159,7 @@ static struct InitFiu
     ONCE(smt_commit_exception_before_op) \
     ONCE(disk_object_storage_fail_commit_metadata_transaction) \
     ONCE(disk_object_storage_fail_precommit_metadata_transaction) \
+    REGULAR(part_storage_fail_commit_transaction) \
     ONCE(write_file_operation_fail_on_read) \
     REGULAR(slowdown_parallel_replicas_local_plan_read) \
     ONCE(iceberg_writes_cleanup) \
@@ -217,7 +218,9 @@ static struct InitFiu
     PAUSEABLE(transaction_after_commit_pause) \
     REGULAR(mt_mutate_task_can_skip_conversion_to_nullable_force_null_column_desc) \
     REGULAR(tcp_handler_fail_connection_setup) \
-    REGULAR(distributed_plan_status_check_reenqueue_fault)
+    REGULAR(distributed_plan_status_check_reenqueue_fault) \
+    REGULAR(cas_relink_receiver_force_mechanism_failure) \
+    PAUSEABLE_ONCE(cas_relink_receiver_pause_before_confirm)
 
 namespace FailPoints
 {
diff --git a/src/Common/ProfileEvents.cpp b/src/Common/ProfileEvents.cpp
index 7f35ef869e92..9bda5dcf5c78 100644
--- a/src/Common/ProfileEvents.cpp
+++ b/src/Common/ProfileEvents.cpp
@@ -754,6 +754,164 @@ The server successfully detected this situation and will download merged part fr
     M(S3CopyObject, "Number of S3 API CopyObject calls.", ValueType::Number) \
     M(S3ListObjects, "Number of S3 API ListObjects calls.", ValueType::Number) \
     M(S3HeadObject,  "Number of S3 API HeadObject calls.", ValueType::Number) \
+    /* CA per-namespace S3 op instrumentation (B168 P0). 80 = 8 namespaces × 10 ops; attributes the */ \
+    /* S3 op-count (PUTs/HEADs/404s/412s/LISTs) by CA operation type via the InstrumentedBackend seam. */ \
+    M(CASBlobPut,        "Number of CAS blob PUT requests. Grows with blob uploads and indicates storage write traffic.", ValueType::Number) \
+    M(CASBlobPutDeduplicated,   "Number of CAS blob deduplicating PUT requests. Grows when uploads reuse existing content.", ValueType::Number) \
+    M(CASBlobOverwrite,  "Number of CAS blob overwrite requests. Growing values indicate repeated replacement writes.", ValueType::Number) \
+    M(CASBlobCompareSwap,        "Number of CAS blob compare-and-swap requests. Grows with concurrent metadata updates.", ValueType::Number) \
+    M(CASBlobCompareSwapConflict,"Number of CAS blob compare-and-swap conflicts. A non-zero value indicates concurrent update contention.", ValueType::Number) \
+    M(CASBlobHead,       "Number of CAS blob HEAD requests. Grows with blob existence checks and read validation.", ValueType::Number) \
+    M(CASBlobHeadMiss,   "Number of CAS blob HEAD requests that found no object. A non-zero value indicates absent blobs or stale references.", ValueType::Number) \
+    M(CASBlobDeduplicationCacheHit, "Number of CAS blob presence-cache hits. Growing values indicate effective deduplication and cache reuse.", ValueType::Number) \
+    M(CASBlobHeadFirst,  "Number of CAS blob uploads that try a HEAD before sending the body. Growing values indicate deduplication checks are active.", ValueType::Number) \
+    M(CASBlobBodyPutAvoided, "Number of CAS blob body PUTs avoided after a HEAD found the object present. Growing values indicate saved upload traffic.", ValueType::Number) \
+    M(CASBlobUploadFanoutBatches, "Number of CAS intra-part blob-upload fan-out invocations (one per part publish that had at least one unique blob to upload). Compare with CASBlobUploadFanoutTasks for the average number of unique blobs uploaded per part.", ValueType::Number) \
+    M(CASBlobUploadFanoutTasks, "Number of CAS blob-upload tasks dispatched to the fan-out pool (one per unique blob ref across all part publishes). Duplicate references within a part collapse to one task.", ValueType::Number) \
+    M(CASRefBatchFlushes, "Number of committed CAS ref-log batch flushes. Compare with CASRefBatchedMutations to assess batching efficiency.", ValueType::Number) \
+    M(CASRefBatchedMutations, "Number of CAS ref mutations committed through the per-namespace batching queue. Growth indicates reference-write activity.", ValueType::Number) \
+    M(CASRefBatchScopeCuts, "Number of CAS ref batches cut short by scope limits. Growing values indicate smaller batches and more write overhead.", ValueType::Number) \
+    M(CASRefQueueWaitMicroseconds, "Total time CAS ref writers spent queued, in microseconds. A rising value indicates ref-write contention or backend latency.", ValueType::Microseconds) \
+    M(CASRefRecoveryRestarts, "Number of CAS ref-table recovery retries after a snapshot or log vanished during reading. A non-zero value indicates concurrent cleanup or backend inconsistency.", ValueType::Number) \
+    M(CASRefRecoveryRetries, "Number of CAS ref-table recovery attempts retried after a transient object-store error before the table's load fails. A non-zero value indicates transient object-store disruption during table startup.", ValueType::Number) \
+    M(CASRefAppendWedged, "Number of CAS ref-log append lanes that exhausted retries after an uncertain PUT. A non-zero value indicates ref-log progress may be stalled.", ValueType::Number) \
+    M(CASRefAppendPreAttemptRefused, "Number of CAS ref-log append chunks refused BEFORE any request was sent (the mount fence or the operation deadline rejected while zero attempts had been made). Nothing can be durable, so the lane is deliberately NOT wedged and the caller retries. Counted separately from `CASRefAppendWedged` so a falling wedge count can be read as the availability fix working rather than as nothing happening.", ValueType::Number) \
+    M(CASRefAppendUnwedged, "Number of CAS ref-log append lanes recovered after an uncertain PUT was later observed durable. A non-zero value indicates transient write uncertainty.", ValueType::Number) \
+    M(CASRefAppendDefiniteFailure, "Number of CAS ref-log appends rejected with certainty. A non-zero value indicates invalid requests or backend rejection requiring investigation.", ValueType::Number) \
+    M(CASRefAppendSealRejected, "Number of CAS ref-log transactions conclusively rejected by a successor's epoch seal occupying the id they derived. This is the protocol working -- the writer was deposed and its operation was never acknowledged -- but a lane that keeps counting here is a writer that has lost its mount and does not yet know it.", ValueType::Number) \
+    M(CASRefAppendOccupantUnreadable, "Number of CAS ref-log appends that met a DIFFERENT object at the id they derived and could not read it to tell a successor's epoch seal from a breach of mount write-exclusivity. The decision is deferred to the next attempt, which re-derives the same id. Sustained growth means a real breach may be going unreported: the loud interference path is only reached once the occupant can be read.", ValueType::Number) \
+    M(CASRefNeedsRecovery, "Number of CAS ref append lanes moved to `NeedsRecovery` because a known-durable transaction could not be installed. Such a lane refuses writes, snapshots, and confirmation until durable replay completes.", ValueType::Number) \
+    M(CASRefSweepDeferred, "Number of stale-precommit sweeps deferred after a read-only failure. A non-zero value indicates cleanup is waiting for a later trigger.", ValueType::Number) \
+    M(CASRefSweepRearmed, "Number of failed or partial stale-precommit sweeps scheduled for retry. Growing values indicate persistent cleanup or backend errors.", ValueType::Number) \
+    M(CASRefStalePrecommitsReclaimed, "Number of stale precommit bindings removed by CAS ref cleanup. Growth indicates superseded writer state is being reclaimed.", ValueType::Number) \
+    M(CASRefRollbackBestEffortDropFailed, "Number of rollback cleanup drops that hit a backend failure. A non-zero value means refs may remain live and GC may be delayed.", ValueType::Number) \
+    M(CASRefTableEvictions, "Number of CAS ref-cache table evictions caused by the memory budget. Growing values indicate a small cache and more recovery I/O.", ValueType::Number) \
+    M(CASRefGlobalListPages, "Number of CAS ref LIST pages fetched during GC. Growing values indicate more refs or smaller backend pages to scan.", ValueType::Number) \
+    M(CASRefLogBodyGets, "Number of CAS ref-log bodies read and decoded during GC. Growth indicates more reference history to process.", ValueType::Number) \
+    M(CASRefManifestBodyFoldGets, "Number of manifest bodies read while GC follows reference edges. High values indicate cache misses or many referenced manifests.", ValueType::Number) \
+    M(CASRefEmittedEdges, "Number of reachability edges emitted while GC folds CAS reference history. Growth indicates more reference relationships to process.", ValueType::Number) \
+    M(CASRefCleanupObjectsDeleted, "Number of old CAS ref logs and snapshots deleted after safe coverage was confirmed. Growth indicates cleanup progress.", ValueType::Number) \
+    M(CASRefSnapshotPutBytes, "Total bytes written to CAS ref-table snapshots. A high value indicates frequent or large snapshot publication.", ValueType::Bytes) \
+    M(CASRefSnapshotTailLogs, "Number of CAS ref-log entries compacted into published snapshots. Growth indicates snapshot maintenance work.", ValueType::Number) \
+    M(CASRefSnapshotPublishDispatched, "Number of background CAS ref-table snapshot publications started. High values indicate frequent threshold or read-triggered publishing.", ValueType::Number) \
+    M(CASRefMaterializeInPlace, "Number of CAS ref-table COW container materializations that folded the overlay into a uniquely-owned base in place (the production flush fast path, O(overlay)). Compare with CASRefMaterializeCopy: a high ratio confirms the fast path is firing in production.", ValueType::Number) \
+    M(CASRefMaterializeCopy, "Number of CAS ref-table COW container materializations that had to build a fresh base because a copy still shared it (the O(table size) slow path). A persistently high value versus CASRefMaterializeInPlace means scratch copies are outliving the state-install point.", ValueType::Number) \
+    M(CASRefSnapshotPublishBackoff, "Number of CAS ref-table snapshot publications put into backoff after a non-committed result. A non-zero value indicates write failure or uncertainty.", ValueType::Number) \
+    M(CASRefCheckpointPublished, "Number of CAS ref `_ckpt` checkpoint objects durably updated by a token compare-and-swap. Growth tracks snapshot publication and namespace creation.", ValueType::Number) \
+    M(CASRefCheckpointIdenticalSkip, "Number of CAS ref `_ckpt` updates that needed no write because the merged body already matched the stored one. A high ratio against CASRefCheckpointPublished is the intended steady state.", ValueType::Number) \
+    M(CASRefCheckpointNotAdvanced, "Number of CAS ref-table snapshot publications whose `_ckpt` checkpoint could not be advanced (fenced out, contended, or failed). The snapshot body is durable but is not yet cleanup-authoritative; a non-zero value means recovery keeps replaying from an older base.", ValueType::Number) \
+    M(CASGCClampSuppressedPasses, "Number of CAS GC passes that deferred shard graduation and deletion because reachability was uncertain. In the current stage this is EVERY folding round by construction, not an anomaly: the namespace universe is not yet knowable, so the round's destructive gate is shut unconditionally and this counter tracks folding rounds rather than trouble. What does discriminate is the fold seal's hold set and the tables_held column of the fold_ref_intake phase row -- read those, not this, to tell a healthy suppressed round from a namespace that stopped.", ValueType::Number) \
+    M(CASGCDeadPrecommitSkipped, "Number of CAS GC edges skipped for precommits proven dead below the durable watermark. Growing values indicate stale or incomplete ref history.", ValueType::Number) \
+    M(CASBlobGet,        "Number of CAS blob GET requests. Grows with blob reads and indicates read traffic.", ValueType::Number) \
+    M(CASBlobGetStream,  "Number of streaming CAS blob GET requests. Grows with large or streamed blob reads.", ValueType::Number) \
+    M(CASBlobDelete,     "Number of CAS blob DELETE requests. Grows with cleanup and GC deletion activity.", ValueType::Number) \
+    M(CASBlobList,       "Number of CAS blob LIST requests. Growing values indicate more object enumeration.", ValueType::Number) \
+    M(CASRefRepoint, "Number of CAS committed-reference repoints that republish a manifest. Growth indicates standalone part updates or removals.", ValueType::Number) \
+    M(CASManifestPut,        "Number of CAS manifest PUT requests. Grows with manifest creation and indicates metadata write traffic.", ValueType::Number) \
+    M(CASManifestPutDeduplicated,   "Number of CAS deduplicating manifest PUT requests. Grows when existing manifests are reused.", ValueType::Number) \
+    M(CASManifestOverwrite,  "Number of CAS manifest overwrite requests. Growing values indicate repeated manifest replacement.", ValueType::Number) \
+    M(CASManifestCompareSwap,        "Number of CAS manifest compare-and-swap requests. Grows with concurrent manifest updates.", ValueType::Number) \
+    M(CASManifestCompareSwapConflict,"Number of CAS manifest compare-and-swap conflicts. A non-zero value indicates concurrent update contention.", ValueType::Number) \
+    M(CASManifestHead,       "Number of CAS manifest HEAD requests. Grows with manifest existence checks and validation.", ValueType::Number) \
+    M(CASManifestHeadMiss,   "Number of CAS manifest HEAD requests that found no object. A non-zero value indicates missing or stale manifests.", ValueType::Number) \
+    M(CASManifestGet,        "Number of CAS manifest GET requests. Grows with manifest reads and indicates metadata read traffic.", ValueType::Number) \
+    M(CASManifestGetStream,  "Number of streaming CAS manifest GET requests. Grows with large or streamed manifest reads.", ValueType::Number) \
+    M(CASManifestDelete,     "Number of CAS manifest DELETE requests. Grows with cleanup and GC deletion activity.", ValueType::Number) \
+    M(CASManifestList,       "Number of CAS manifest LIST requests. Growing values indicate more manifest enumeration.", ValueType::Number) \
+    M(CASRootPut,        "Number of CAS root PUT requests. Grows with root creation and indicates metadata write traffic.", ValueType::Number) \
+    M(CASRootPutDeduplicated,   "Number of CAS deduplicating root PUT requests. Grows when existing roots are reused.", ValueType::Number) \
+    M(CASRootOverwrite,  "Number of CAS root overwrite requests. Growing values indicate repeated root replacement.", ValueType::Number) \
+    M(CASRootCompareSwap,        "Number of CAS root compare-and-swap requests. Grows with concurrent root updates.", ValueType::Number) \
+    M(CASRootCompareSwapConflict,"Number of CAS root compare-and-swap conflicts. A non-zero value indicates concurrent update contention.", ValueType::Number) \
+    M(CASRootHead,       "Number of CAS root HEAD requests. Grows with root existence checks and validation.", ValueType::Number) \
+    M(CASRootHeadMiss,   "Number of CAS root HEAD requests that found no object. A non-zero value indicates missing or stale roots.", ValueType::Number) \
+    M(CASRootGet,        "Number of CAS root GET requests. Grows with root reads and indicates metadata read traffic.", ValueType::Number) \
+    M(CASRootGetStream,  "Number of streaming CAS root GET requests. Grows with large or streamed root reads.", ValueType::Number) \
+    M(CASRootDelete,     "Number of CAS root DELETE requests. Grows with cleanup activity.", ValueType::Number) \
+    M(CASRootList,       "Number of CAS root LIST requests. Growing values indicate more root enumeration.", ValueType::Number) \
+    M(CASGCPut,          "Number of CAS GC PUT requests. Grows with GC metadata and object-management writes.", ValueType::Number) \
+    M(CASGCPutDeduplicated,     "Number of deduplicating CAS GC PUT requests. Growth indicates GC reused existing objects.", ValueType::Number) \
+    M(CASGCOverwrite,    "Number of CAS GC overwrite requests. Growing values indicate repeated GC metadata replacement.", ValueType::Number) \
+    M(CASGCCompareSwap,          "Number of CAS GC compare-and-swap requests. Grows with GC state updates.", ValueType::Number) \
+    M(CASGCRetireReplaced, "Number of CAS GC re-condemnations after a resurrected object replaced a retired incarnation. Growth indicates object-generation churn.", ValueType::Number) \
+    M(CASGCCompareSwapConflict,  "Number of CAS GC compare-and-swap conflicts. A non-zero value indicates concurrent GC or metadata updates.", ValueType::Number) \
+    M(CASGCHead,         "Number of CAS GC HEAD requests. Grows with object existence checks during collection.", ValueType::Number) \
+    M(CASGCHeadMiss,     "Number of CAS GC HEAD requests that found no object. A non-zero value indicates already-removed or missing objects.", ValueType::Number) \
+    M(CASGCGet,          "Number of CAS GC GET requests. Grows with collection reads and recovery work.", ValueType::Number) \
+    M(CASGCGetStream,    "Number of streaming CAS GC GET requests. Grows with large collection or recovery reads.", ValueType::Number) \
+    M(CASGCDelete,       "Number of CAS GC DELETE requests. Grows with successful cleanup attempts.", ValueType::Number) \
+    M(CASGCList,         "Number of CAS GC LIST requests. Growing values indicate more collection enumeration.", ValueType::Number) \
+    M(CASServerPut,      "Number of CAS server-object PUT requests. Grows with server metadata writes.", ValueType::Number) \
+    M(CASServerPutDeduplicated, "Number of deduplicating CAS server-object PUT requests. Growth indicates reused server objects.", ValueType::Number) \
+    M(CASServerOverwrite,"Number of CAS server-object overwrite requests. Growing values indicate repeated replacement writes.", ValueType::Number) \
+    M(CASServerCompareSwap,      "Number of CAS server-object compare-and-swap requests. Grows with server metadata updates.", ValueType::Number) \
+    M(CASServerCompareSwapConflict,"Number of CAS server-object compare-and-swap conflicts. A non-zero value indicates concurrent updates.", ValueType::Number) \
+    M(CASServerHead,     "Number of CAS server-object HEAD requests. Grows with existence checks and validation.", ValueType::Number) \
+    M(CASServerHeadMiss, "Number of CAS server-object HEAD requests that found no object. A non-zero value indicates missing server state.", ValueType::Number) \
+    M(CASServerGet,      "Number of CAS server-object GET requests. Grows with server metadata reads.", ValueType::Number) \
+    M(CASServerGetStream,"Number of streaming CAS server-object GET requests. Grows with large metadata reads.", ValueType::Number) \
+    M(CASServerDelete,   "Number of CAS server-object DELETE requests. Grows with server-state cleanup.", ValueType::Number) \
+    M(CASServerList,     "Number of CAS server-object LIST requests. Growing values indicate more server-state enumeration.", ValueType::Number) \
+    M(CASOtherPut,       "Number of CAS other-object PUT requests. Grows with writes to uncategorized CAS objects.", ValueType::Number) \
+    M(CASOtherPutDeduplicated,  "Number of deduplicating CAS other-object PUT requests. Growth indicates reused uncategorized objects.", ValueType::Number) \
+    M(CASOtherOverwrite, "Number of CAS other-object overwrite requests. Growing values indicate repeated replacement writes.", ValueType::Number) \
+    M(CASOtherCompareSwap,       "Number of CAS other-object compare-and-swap requests. Grows with metadata updates.", ValueType::Number) \
+    M(CASOtherCompareSwapConflict,"Number of CAS other-object compare-and-swap conflicts. A non-zero value indicates concurrent updates.", ValueType::Number) \
+    M(CASOtherHead,      "Number of CAS other-object HEAD requests. Grows with existence checks and validation.", ValueType::Number) \
+    M(CASOtherHeadMiss,  "Number of CAS other-object HEAD requests that found no object. A non-zero value indicates missing state.", ValueType::Number) \
+    M(CASOtherGet,       "Number of CAS other-object GET requests. Grows with reads of uncategorized CAS objects.", ValueType::Number) \
+    M(CASOtherGetStream, "Number of streaming CAS other-object GET requests. Grows with large reads.", ValueType::Number) \
+    M(CASOtherDelete,    "Number of CAS other-object DELETE requests. Grows with cleanup activity.", ValueType::Number) \
+    M(CASOtherList,      "Number of CAS other-object LIST requests. Growing values indicate more object enumeration.", ValueType::Number) \
+    M(CASGCRetiredCondemned,   "Number of CAS GC entries newly condemned into the retired set. Growth indicates objects becoming eligible for later cleanup.", ValueType::Number) \
+    M(CASGCRetiredSpared,      "Number of CAS GC retired entries spared after references returned. Growing values indicate reference churn or resurrection.", ValueType::Number) \
+    M(CASGCRetiredSparedByReref, "Number of CAS GC delete_pending entries spared because a fresh deduplicating adopt re-referenced them after graduation (an ordinary observe/condemn race, not a fail-closed abort). Subset of CASGCRetiredSpared.", ValueType::Number) \
+    M(CASGCRetiredGraduated,   "Number of CAS GC retired entries that passed the safety floor and became pending deletion. Growth indicates cleanup progress.", ValueType::Number) \
+    M(CASGCRetiredRedeleted,   "Number of CAS GC pending deletes executed with an exact object token. Growth indicates physical cleanup activity.", ValueType::Number) \
+    M(CASGCUnmatchedRemoveDeltas, "Number of CAS GC in-degree removal deltas that matched no existing source edge. The in-degree model is a set, not a counter, so this is a per-key no-op by design and never causes a false deletion — but a persistent nonzero rate means removal deltas are reaching the reducer without their matching activation, which is a correctness signal.", ValueType::Number) \
+    M(CASGCCondemnMarkerUnconfirmedCarry, "Number of CAS GC retirements delayed because a durable condemn marker could not be confirmed. A non-zero value indicates marker write or read failures and safely postpone deletion.", ValueType::Number) \
+    M(CASGCHeartbeatFenceOuts, "Number of CAS GC operations fenced out for expired mounts. A non-zero value indicates stale mounts or heartbeat delays.", ValueType::Number) \
+    M(CASGCMetaWriteAnomaly, "Number of CAS GC metadata operations that failed on the bounded metadata pool. A non-zero value indicates backend or pool pressure and may delay metadata convergence.", ValueType::Number) \
+    M(CASMetaPut,          "Number of conditional CAS blob-metadata create attempts, including lost races. Growth indicates blob creation, adoption, or write contention.", ValueType::Number) \
+    M(CASMetaCompareSwap,          "Number of conditional CAS blob-metadata rewrites, including lost races. Growing values indicate metadata updates or contention.", ValueType::Number) \
+    M(CASMetaDelete,       "Number of exact-token CAS blob-metadata delete attempts. Growth indicates GC or cleanup activity.", ValueType::Number) \
+    M(CASMetaCreateClean,  "Number of clean metadata records created after fresh blob commits. Growth indicates new blob uploads.", ValueType::Number) \
+    M(CASMetaAdoptBackfill, "Number of clean metadata records backfilled during blob adoption. A non-zero value indicates missing or legacy metadata.", ValueType::Number) \
+    M(CASMetaResurrectClean, "Number of blob metadata records reset to Clean during resurrection. A non-zero value indicates blob resurrection activity.", ValueType::Number) \
+    M(CASGCMetaOps, "Number of per-hash metadata operations executed by CAS GC. Growing values indicate more GC candidates or metadata work.", ValueType::Number) \
+    M(CASGCEnumerationPages, "Number of CAS GC LIST pages fetched while enumerating the object universe. Growing values indicate a larger universe or more frequent scans.", ValueType::Number) \
+    M(CASGCRefWalkPlansBuilt, "Number of complete catalog-authoritative CAS ref walk plans constructed by ordinary GC and rebuild. A regular or rebuilding invocation that reaches the post-LIST catalog cut increments this exactly once, including a round that later defers.", ValueType::Number) \
+    M(CASGCUnmatchedAdoptedParentLives, "Number of adopted-parent CAS ref-life rows dropped because the post-LIST catalog cut has no matching physical life. Each occurrence is inert for planning and suppression and is logged with its exact physical life id; a persistent nonzero rate indicates old generation state is outliving catalog removal.", ValueType::Number) \
+    M(CASGCStuckRemovals, "Number of adopted CAS GC rounds that observed a Removing namespace at or beyond the diagnostic age threshold without terminal cleanup evidence. Incremented and warned every such round; diagnostic only, with no effect on folding, suppression, appends, or deletion.", ValueType::Number) \
+    M(CASGCNamespaceCleanupLeaks, "Number of dead-life namespace objects whose reclamation the perpetual janitor could not confirm because HEAD or exact-delete failed. Each occurrence is logged with the exact key and remains leak-only: it neither suppresses destructive GC nor blocks catalog lifecycle progress.", ValueType::Number) \
+    M(CASGCUnappliedFoldedTransactions, "Number of ref transactions a GC round folded and merged but whose blob deltas never reached a shard reducer. Always 0 on a healthy round; a nonzero value fails the round closed, because the round would otherwise advance its fold cursor past a transaction it never applied.", ValueType::Number) \
+    M(CASGCRebuildVirginByEnumeration, "Number of times SYSTEM CAS GC REBUILD concluded FROM ENUMERATION ALONE that a pool had never sealed a baseline, and so carried no durable holds forward. The verdict rests on three pieces of evidence -- a wide LIST of the gc/gen prefix empty, a narrow probe of gc/gen/1/ empty, and gc/state absent -- and on no point read, because the fold seal key needs an attempt component that is a lease sequence number and has no arithmetic successor to probe. An enumeration that hid every seal INCLUDING generation 1 on a lived-in pool would therefore read as virgin here. Expected to be 0 on any pool that has ever completed a GC round; a nonzero value on such a pool means the rebuild was granted a clean slate it could not prove.", ValueType::Number) \
+    M(CASPartFolderViewHits, "Number of validated hits in the CAS part-folder view cache. Higher values indicate effective cache reuse.", ValueType::Number) \
+    M(CASPartFolderViewValidationMismatches, "Number of part-folder view validation mismatches that required rebuilding. A non-zero value indicates concurrent manifest changes or stale views.", ValueType::Number) \
+    M(CASPartFolderViewMisses, "Number of CAS part-folder view builds with no retained entry. Growing values indicate low cache reuse or folder churn.", ValueType::Number) \
+    M(CASPartFolderViewOversizedBypasses, "Number of part-folder views not retained because they exceeded the entry size limit. A non-zero value indicates large folders reduce cache effectiveness.", ValueType::Number) \
+    M(CASPartFolderViewInvalidations, "Number of part-folder view cache erases after writes, ref drops, or namespace drops. Growth indicates metadata changes.", ValueType::Number) \
+    M(CASPartFolderManifestGets, "Number of part-manifest body GET requests used to build or validate folder views. High values indicate cache misses or validation work.", ValueType::Number) \
+    M(CASConditionalWriteAttempts, "Number of CAS conditional-write HTTP attempts made without transparent SDK retries. Growth indicates CAS metadata or reference updates.", ValueType::Number) \
+    M(CASConditionalWriteCommitted, "Number of CAS conditional writes completed successfully. Growth indicates successful conditional-update activity.", ValueType::Number) \
+    M(CASConditionalWriteDefiniteFailure, "Number of CAS conditional writes rejected with certainty before applying. A non-zero value indicates invalid requests, oversized entities, or access denial.", ValueType::Number) \
+    M(CASConditionalWriteUnresolved, "Number of CAS conditional writes with an unknown outcome after conflict, timeout, connection loss, or server error. A non-zero value indicates backend instability or state requiring resolution.", ValueType::Number) \
+    M(CASConditionalWriteFenceLostPostWrite, "Number of CAS writes that succeeded but lost the final mount-fence check. A non-zero value indicates late responses after the mount lifecycle changed.", ValueType::Number) \
+    M(CASMountLeaseLost, "Counts CAS mount-lease terminal losses: the mount slot object vanished (backing store deleted under a live mount), was superseded by a newer incarnation, advanced past the held token under the keeper's own (uuid, epoch) (state uncertain: an ambiguous landed renewal or epoch-state loss), or was taken by a foreign server. The keeper stopped renewing and latched its write fence to lost; non-zero values mean a mount was lost — investigate via system.cas_log MountConflict rows.", ValueType::Number) \
+    M(CASMountReleaseSkippedForeignOccupant, "Counts CAS mount releases that found a FOREIGN occupant in the slot on a runtime that had already observed its own deposition (write fence latched, renewal stopped). This is the EXPECTED end state of a failover: the farewell is skipped, the successor's slot is left byte-for-byte untouched, and teardown continues. Steady non-zero values on a cluster that is not failing over are worth investigating; a value that tracks failovers is normal.", ValueType::Number) \
+    M(CASMountExclusivityViolation, "Counts CAS mount releases where a runtime that still BELIEVED IT OWNED the mount (no deposition ever observed) found a FOREIGN occupant in the slot. This is the single-writer guarantee being broken, not a failover: the release refuses, the slot is left untouched, and the write fence is latched so the runtime stops trusting itself. This must always be zero.", ValueType::Number) \
+    M(CASRemountHeldTransient, "Counts CAS remount attempts that could not decide the pool's identity and were held transient: the `_pool_meta` probe was inconclusive or its body could not be decoded (a partially written object, an unreadable store, or a pool whose format this build no longer reads). The mount stays fenced closed and the remount loop retries; a value that keeps growing means the pool will never remount without operator action -- read the accompanying warning for the probe's own reason.", ValueType::Number) \
+    M(CASIdentityLost, "Counts CAS pools that entered the IdentityLost lifecycle state: the pool sentinels (_pool_meta and the owner anchor) were observed authoritatively absent (both KeyAbsent). IdentityLost is a fail-loud TERMINAL state -- store-class access fails loud from this point and this pool's background threads (remount + GC) self-exit; there is no observer. Recover by restart or SYSTEM CAS FORGET; a matching-sentinel restore does not auto-revive.", ValueType::Number) \
+    M(CASDataRootVanished, "Counts CAS pools that entered a terminal Vanished lifecycle state (replaced / forgotten): the pool's data root was proven foreign (pool_id mismatch) or was decommissioned by SYSTEM CAS FORGET. The disk stays registered but store-class access fails loud with a typed error naming the sub-state; restart re-registers the name.", ValueType::Number) \
+    M(CASDeduplicationCacheHits, "Number of CAS presence-cache lookups that found a known-present hash. Growth indicates cache reuse and more HEAD-first deduplication checks.", ValueType::Number) \
+    M(CASDeduplicationCacheMisses, "Number of CAS presence-cache lookups without an entry. Growing values indicate a cold cache or increasing new-content uploads.", ValueType::Number) \
+    M(CASRefRecoveryEpochSealed, "Number of CAS ref epoch-seal transactions MINTED by a recovery compare-and-swap walk, one per dead writer epoch it closed. Each seal occupies the exact log key a dying predecessor's in-flight PUT would take, so growth tracks epoch transitions over touched namespaces (including burned, never-written epochs), not anomalies.", ValueType::Number) \
+    M(CASRefRecoveryEpochSealAdopted, "Number of dead CAS ref writer epochs a recovery compare-and-swap walk found ALREADY closed by a concurrent recoverer's seal and adopted. A peer that got there first is the designed outcome, not contention to alarm on.", ValueType::Number) \
+    M(CASRefRecoveryStragglerAdopted, "Number of straggler ref-log transactions a recovery compare-and-swap walk met at the slot it tried to seal and adopted, re-sealing at the new T+1. Non-zero means writes from a dying epoch were still materializing when recovery ran.", ValueType::Number) \
+    M(CASRefRecoveryCancelled, "Number of CAS ref-table recovery attempts abandoned because a self-remount requested cancellation before re-arming the mount fence. Non-zero means remounts are overlapping recoveries; nothing is written or installed on this path.", ValueType::Number) \
+    M(CASRefRecoveryStreamHole, "Number of times CAS ref-table recovery found a 404 BELOW a durable same-epoch witness -- a hole in a stream INV-1 makes dense. Restarted while the restart budget lasts (a racing cleanup is the innocent explanation), then reported as corruption. Any sustained non-zero value is data loss, not noise.", ValueType::Number) \
+    M(CASPartFolderValidateSkipped, "Number of CAS part-folder validation HEADs skipped by policy or a fresh retained view. High values reduce reads but can delay detecting external changes.", ValueType::Number) \
+    M(CASBlobAdoptTrusted, "Number of CAS blob adoptions trusted through a durable manifest edge without per-file probes. Growth indicates manifest-based relinking.", ValueType::Number) \
     M(S3GetObjectTagging, "Number of S3 API GetObjectTagging calls.", ValueType::Number) \
     M(S3CreateMultipartUpload, "Number of S3 API CreateMultipartUpload calls.", ValueType::Number) \
     M(S3UploadPartCopy, "Number of S3 API UploadPartCopy calls.", ValueType::Number) \
@@ -762,6 +920,7 @@ The server successfully detected this situation and will download merged part fr
     M(S3CompleteMultipartUpload, "Number of S3 API CompleteMultipartUpload calls.", ValueType::Number) \
     M(S3PutObject, "Number of S3 API PutObject calls.", ValueType::Number) \
     M(S3GetObject, "Number of S3 API GetObject calls.", ValueType::Number) \
+    M(S3SingleAttemptRetryConsultations, "Number of AWS SDK retry consultations refused by the single-attempt S3 retry profile. Non-zero means the SDK attempted a transparent retry on a write that must make exactly one HTTP attempt.", ValueType::Number) \
     \
     M(DiskS3DeleteObjects, "Number of DiskS3 API DeleteObject(s) calls.", ValueType::Number) \
     M(DiskS3CopyObject, "Number of DiskS3 API CopyObject calls.", ValueType::Number) \
diff --git a/src/Common/setThreadName.h b/src/Common/setThreadName.h
index 437b5c7f184c..2be0ff212c2c 100644
--- a/src/Common/setThreadName.h
+++ b/src/Common/setThreadName.h
@@ -32,6 +32,12 @@ namespace DB
     M(BACKUP_COORDINATION_INTERNAL, "BackupCoordInt") \
     M(BACKUP_WORKER, "BackupWorker") \
     M(CACHE_DICTIONARY_UPDATE_QUEUE, "UpdQueue") \
+    M(CAS_ANOMALY_DIAG, "CasAnomalyDiag") \
+    M(CAS_GC_HEARTBEAT, "CasGcHeartbeat") \
+    M(CAS_GC_SCHEDULER, "CasGcSched") \
+    M(CAS_LEASE_KEEPER, "CasLeaseKeeper") \
+    M(CAS_REF_SNAPSHOT_PUBLISH, "CasRefSnapPub") \
+    M(CAS_REMOUNT, "CasRemount") \
     M(CGROUP_MEMORY_OBSERVER, "CgrpMemUsgObsr") \
     M(CLICKHOUSE_WATCH, "ClickHouseWatch") \
     M(CLUSTER_DISCOVERY, "ClusterDiscover") \
diff --git a/src/Core/ServerSettings.cpp b/src/Core/ServerSettings.cpp
index 1f6eb7e54653..589d450b6d6e 100644
--- a/src/Core/ServerSettings.cpp
+++ b/src/Core/ServerSettings.cpp
@@ -148,6 +148,11 @@ namespace
     DECLARE(UInt64, max_format_parsing_thread_pool_size, 100, R"(
     Maximum total number of threads to use for parsing input.
     )", 0) \
+    DECLARE(UInt64, cas_blob_upload_pool_size, 16, R"(
+    ClickHouse uses threads from this dedicated server-wide pool to upload blobs in parallel when
+    committing a content-addressed (CAS) part. `cas_blob_upload_pool_size` limits the
+    maximum number of threads in the pool. Zero is rejected: the pool must have at least one thread.
+    )", 0) \
     DECLARE(UInt64, max_format_parsing_thread_pool_free_size, 0, R"(
     Maximum number of idle standby threads to keep in the thread pool for parsing input.
     )", 0) \
diff --git a/src/Storages/MergeTree/DataPartsExchange.cpp b/src/Storages/MergeTree/DataPartsExchange.cpp
index 690b40b6662d..00f27191db7b 100644
--- a/src/Storages/MergeTree/DataPartsExchange.cpp
+++ b/src/Storages/MergeTree/DataPartsExchange.cpp
@@ -5,6 +5,9 @@
 #include <Disks/IO/createReadBufferFromFileBase.h>
 #include <Disks/SingleDiskVolume.h>
 #include <Disks/createVolume.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/IMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h>
+#include <Storages/MergeTree/MergeTreeDataPartBuilder.h>
 #include <Formats/NativeWriter.h>
 #include <IO/HTTPCommon.h>
 #include <IO/ReadWriteBufferFromHTTP.h>
@@ -44,6 +47,15 @@ namespace CurrentMetrics
 namespace DB
 {
 
+namespace FailPoints
+{
+    /// CAS fetch-by-relink, receiver side. Both exist because the two exits they drive are properties of
+    /// the sender/receiver PAIR and of the interval between the receiver's publish and its confirm — and
+    /// neither is reachable from configuration, so an integration test cannot produce them any other way.
+    extern const char cas_relink_receiver_force_mechanism_failure[];
+    extern const char cas_relink_receiver_pause_before_confirm[];
+}
+
 namespace MergeTreeSetting
 {
     extern const MergeTreeSettingsBool allow_remote_fs_zero_copy_replication;
@@ -60,6 +72,7 @@ namespace ErrorCodes
     extern const int CHECKSUM_DOESNT_MATCH;
     extern const int INSECURE_PATH;
     extern const int LOGICAL_ERROR;
+    extern const int NETWORK_ERROR;
     extern const int S3_ERROR;
     extern const int ZERO_COPY_REPLICATION_ERROR;
 }
@@ -78,12 +91,78 @@ constexpr auto REPLICATION_PROTOCOL_VERSION_WITH_PARTS_ZERO_COPY = 6;
 constexpr auto REPLICATION_PROTOCOL_VERSION_WITH_PARTS_PROJECTION = 7;
 constexpr auto REPLICATION_PROTOCOL_VERSION_WITH_METADATA_VERSION = 8;
 constexpr auto REPLICATION_PROTOCOL_VERSION_WITH_COLUMNS_SUBSTREAMS = 9;
+/// CAS replication 2b: fetch-by-relink. The receiver advertises its content-addressed pool identity
+/// (`cas_pool_uuid`) and, if it matches the sender's own pool, the sender sends only the
+/// part's content id (`part_id`) + the mutable header — no file bytes — and the receiver "fetches" by
+/// publishing its own ref to the blobs already present in the shared pool (the CA analogue of the
+/// zero-copy metadata-only fetch). Everything is gated behind a matching pool_uuid, so a non-CA fetch
+/// is byte-for-byte unchanged.
+/// Kept although nothing gates on it any more: the offer gate moved to `..._WITH_CA_CONFIRM` below, but
+/// 10 is a version peers still advertise, and deleting the record of what it meant would leave the next
+/// reader unable to tell what an incoming 10 promises (a relink it will NOT confirm).
+[[maybe_unused]] constexpr auto REPLICATION_PROTOCOL_VERSION_WITH_CA_RELINK = 10;
+/// CAS replication, publish-then-confirm (spec §wire-protocol). A relink offer is now accompanied by a
+/// source token, and the endpoint answers a second, part-less request that asks whether that token is
+/// still exactly what the sender's ref names. A server advertising this version serves the confirm
+/// action; a receiver advertising it must confirm before it promotes.
+constexpr auto REPLICATION_PROTOCOL_VERSION_WITH_CA_CONFIRM = 11;
 
 std::string getEndpointId(const std::string & node_id)
 {
     return "DataPartsExchange:" + node_id;
 }
 
+/// CAS replication 2b. The receiver advertises its target pool's identity under this request param so
+/// the sender can decide whether a fetch-by-relink (same pool) is possible.
+constexpr auto CA_POOL_UUID_PARAM = "cas_pool_uuid";
+/// Set on the response when the sender chose the relink path; the receiver then reads the relink payload
+/// (the opaque encoded PartManifest body — self-contained, see part_manifest_v2 below) instead of the
+/// byte stream.
+constexpr auto CA_RELINK_COOKIE = "cas_relink";
+/// All-tree task 7: the manifest is now self-contained (uuid.txt/metadata_version.txt are ordinary
+/// manifest entries, task 6), so the wire payload dropped its trailing metadata_version field (the
+/// manifest bytes are now the ONLY field). Bumped from `part_manifest_v1` so a mixed-build pair (old
+/// sender, new receiver) does not try to parse the old two-field payload under the new one-field shape
+/// — the receiver rejects a cookie value it does not recognize and falls back to a byte fetch instead
+/// of desyncing on the wire format.
+constexpr auto CA_RELINK_COOKIE_VALUE = "part_manifest_v2";
+
+/// CAS fetch-by-relink, publish-then-confirm (spec §wire-protocol). Three names make up the second
+/// request of the handshake.
+///
+/// The request parameter both selects the confirm action and carries its only argument: the opaque
+/// source token the sender minted for the offer. There is no separate action flag, because an action
+/// without its token is not a question anyone can answer, and a token without the action would have to
+/// be ignored — one name cannot be half-present.
+constexpr auto CA_CONFIRM_ACTION_PARAM = "cas_confirm";
+/// Response cookie on the relink offer: the token, opaque to the receiver, echoed back verbatim.
+constexpr auto CA_CONFIRM_TOKEN_COOKIE = "cas_source_token";
+/// Response cookie on the confirm: the answer.
+constexpr auto CA_CONFIRM_ANSWER_COOKIE = "cas_confirm_answer";
+/// The ONLY value that authorizes the receiver to promote.
+constexpr auto CA_CONFIRM_ANSWER_PROVEN = "yes";
+/// Everything else: the source did not prove the binding. The wire vocabulary is deliberately BINARY
+/// even though `CasConfirmAnswer` has three values. `No` and `Unknown` are one outcome for every caller
+/// (see `CasConfirmAnswer`): gate 1 evaluates the mount fence LAST, so a mount that has already lost
+/// its fence — and can no longer speak for the namespace at all — still answers `No` for a token that
+/// does not match its last-known row. Putting `no` on the wire as a distinct value would invite a
+/// receiver to act on it as knowledge, and it is not knowledge. The distinction is diagnostic only, so
+/// it is logged on the sender, where the gate that produced it can be named, and never transmitted.
+/// An ABSENT cookie reads as unproven too, which is what makes an older peer and a failed request the
+/// same safe outcome as a refusal.
+constexpr auto CA_CONFIRM_ANSWER_UNPROVEN = "unproven";
+
+/// Resolve a disk to the content-addressed exchange facade, or nullptr if the disk is not CA. The
+/// cast targets the purpose-built INTERFACE (IContentAddressedExchange), never the concrete
+/// metadata-storage class (M-W design section 4). Used by both the relink sender (the part's
+/// disk) and the relink receiver (the target disk).
+IContentAddressedExchange * tryGetContentAddressedExchange(const DiskPtr & disk)
+{
+    if (!disk || !disk->isContentAddressed())
+        return nullptr;
+    return dynamic_cast<IContentAddressedExchange *>(disk->getMetadataStorage().get());
+}
+
 /// Simple functor for tracking fetch progress in system.replicated_fetches table.
 struct ReplicatedFetchReadCallback
 {
@@ -130,8 +209,108 @@ std::string Service::getId(const std::string & node_id) const
     return getEndpointId(node_id);
 }
 
+CasConfirmAnswer Service::resolveContentAddressedConfirm(
+    const String & pool_uuid,
+    const String & server_root_id,
+    const String & root_namespace,
+    const String & ref_name,
+    const String & part_name,
+    const String & manifest_ref_text) const
+{
+    /// CAS fetch-by-relink, publish-then-confirm (spec §confirm-primitive). The receiver's own `+1` is
+    /// already durable when this runs; a `Yes` is what authorizes it to promote a part whose blobs are
+    /// protected only by THIS server's committed binding of that exact manifest. Every field below comes
+    /// from a remote peer, so nothing here is trusted beyond being used as a lookup key.
+    if (pool_uuid.empty() || server_root_id.empty() || root_namespace.empty() || ref_name.empty() || part_name.empty())
+        return CasConfirmAnswer::Unknown;
+
+    /// Routing. A pool UUID identifies the shared pool, not the mount: every server root writing into it
+    /// reports the same one, so the namespace's owner decides which instance may answer. EXACTLY one
+    /// match is required — zero means this table has no such disk, several mean the question is
+    /// ambiguous, and both are `Unknown` rather than a guess.
+    const IContentAddressedExchange * matched = nullptr;
+    DiskPtr matched_disk;
+    for (const auto & disk : data.getDisks())
+    {
+        const auto * ca_meta = tryGetContentAddressedExchange(disk);
+        if (!ca_meta || ca_meta->getPoolUUID() != pool_uuid || !ca_meta->ownsNamespace(server_root_id, root_namespace))
+            continue;
+        if (matched)
+            return CasConfirmAnswer::Unknown;
+        matched = ca_meta;
+        matched_disk = disk;
+    }
+    if (!matched)
+        return CasConfirmAnswer::Unknown;
+
+    /// Gate 0 — the part-anchored fast filter. It is an AVAILABILITY filter and never a proof (spec
+    /// §confirm-primitive, demoted in rev.5): `rollbackDeletingParts` puts a part back to `Outdated`
+    /// after a failed filesystem removal, and the in-memory part path is deliberately not updated by a
+    /// `delete_tmp_*` rename, so an `Active`/`Outdated` part object authorizes nothing. What it buys is
+    /// a cheap `No` that costs no ledger work; every `Yes` is earned by gate 1 alone.
+    ///
+    /// `Deleting` is excluded by the state filter, an unknown name yields no part at all, and a part of
+    /// this name living on ANOTHER disk is rejected explicitly — `MOVE ... TO DISK` leaves a same-name
+    /// `Active` part behind on the destination disk, and only the instance the token routed to may be
+    /// the one the confirm is about. The parts set is read under its own lock, which
+    /// `getPartIfExists` takes and releases, and the part reference is dropped before any ledger lock.
+    {
+        const auto part_info = MergeTreePartInfo::tryParsePartName(part_name, data.format_version);
+        if (!part_info)
+            return CasConfirmAnswer::Unknown;
+        const auto part = data.getPartIfExists(
+            *part_info, {MergeTreeDataPartState::Active, MergeTreeDataPartState::Outdated});
+        if (!part || part->getDataPartStorage().getDiskName() != matched_disk->getName())
+            return CasConfirmAnswer::No;
+    }
+
+    /// Gate 1 — authoritative, and the only source of a `Yes`.
+    return matched->confirmExactRef(root_namespace, ref_name, manifest_ref_text);
+}
+
+void Service::answerContentAddressedConfirm(const String & token_text, HTTPServerResponse & response) const
+{
+    /// The confirm action's whole handler. It reads no part parameter, sends no body, and touches no
+    /// send metric: the request asks a question about a binding, it does not transfer anything.
+    const auto token = decodeCasRelinkSourceToken(token_text);
+    if (!token)
+    {
+        /// The raw text is NOT logged: it is unvalidated peer bytes, and a decoded token is the only
+        /// form this server has established is free of the control characters that forge log lines.
+        LOG_DEBUG(log, "Relink confirm is unproven: the source token ({} bytes) is not one this server minted",
+            token_text.size());
+        response.addCookie({CA_CONFIRM_ANSWER_COOKIE, CA_CONFIRM_ANSWER_UNPROVEN});
+        return;
+    }
+
+    const CasConfirmAnswer answer = resolveContentAddressedConfirm(
+        token->pool_uuid, token->server_root_id, token->root_namespace,
+        token->ref_name, token->part_name, token->manifest_ref_text);
+
+    /// The `No`/`Unknown` distinction stays here, on the node that computed it and can name the binding
+    /// that produced it. It is triage information, not an authorization, and the wire carries only the
+    /// authorization (`CA_CONFIRM_ANSWER_UNPROVEN`).
+    if (answer != CasConfirmAnswer::Yes)
+        LOG_DEBUG(log, "Relink confirm is unproven ({}) for ref '{}' (part {}, manifest {}) in namespace '{}'",
+            answer == CasConfirmAnswer::No ? "no" : "unknown",
+            token->ref_name, token->part_name, token->manifest_ref_text, token->root_namespace);
+
+    response.addCookie({CA_CONFIRM_ANSWER_COOKIE,
+        answer == CasConfirmAnswer::Yes ? CA_CONFIRM_ANSWER_PROVEN : CA_CONFIRM_ANSWER_UNPROVEN});
+}
+
 void Service::processQuery(const HTMLForm & params, ReadBufferPtr body, WriteBuffer & out, HTTPServerResponse & response)
 {
+    /// CAS fetch-by-relink, publish-then-confirm (spec §wire-protocol): the second request of the
+    /// handshake, dispatched before `part` is required because a confirm carries none — the part name
+    /// is inside the token. Authentication parity with the fetch is inherent: the shared handler
+    /// authenticates before it dispatches to any endpoint.
+    if (const String confirm_token = params.get(CA_CONFIRM_ACTION_PARAM, ""); !confirm_token.empty())
+    {
+        answerContentAddressedConfirm(confirm_token, response);
+        return;
+    }
+
     // nothing to read from body
     body.reset();
 
@@ -145,7 +324,7 @@ void Service::processQuery(const HTMLForm & params, ReadBufferPtr body, WriteBuf
     MergeTreePartInfo::fromPartName(part_name, data.format_version);
 
     /// We pretend to work as older server version, to be sure that client will correctly process our version
-    response.addCookie({"server_protocol_version", toString(std::min(client_protocol_version, REPLICATION_PROTOCOL_VERSION_WITH_COLUMNS_SUBSTREAMS))});
+    response.addCookie({"server_protocol_version", toString(std::min(client_protocol_version, REPLICATION_PROTOCOL_VERSION_WITH_CA_CONFIRM))});
 
     LOG_TRACE(log, "Sending part {}", part_name);
 
@@ -209,6 +388,51 @@ void Service::processQuery(const HTMLForm & params, ReadBufferPtr body, WriteBuf
             writeBinary(projections.size(), out);
         }
 
+        /// CAS replication 2b — fetch-by-relink (spec §4). If the part is on a content-addressed disk and
+        /// the receiver advertised a `cas_pool_uuid` equal to THIS server's own pool_uuid
+        /// (same shared pool), send only the part's content id + the mutable header — no file bytes — so
+        /// the receiver can "fetch" by publishing its own ref to the blobs already in the shared pool.
+        /// Strictly gated on a matching pool_uuid: a non-CA part, a CA part on a different pool, or a
+        /// receiver without the capability all fall through to the unchanged byte path below.
+        ///
+        /// The gate is `..._WITH_CA_CONFIRM`, not `..._WITH_CA_RELINK`: a receiver is offered a relink
+        /// only once it advertises that it will confirm the offer before promoting it. A receiver that
+        /// still advertises `..._WITH_CA_RELINK` gets the bytes — mixed versions degrade to bytes, never
+        /// to an unconfirmed relink. This gate and the version the client advertises
+        /// (`fetchSelectedPart`) are one change in two places; separated in either order they either
+        /// disable relink outright or hand an unconfirmed relink to a receiver that claimed it confirms.
+        if (client_protocol_version >= REPLICATION_PROTOCOL_VERSION_WITH_CA_CONFIRM
+            && part->getDataPartStorage().isContentAddressed())
+        {
+            const String receiver_pool_uuid = parse<String>(params.get(CA_POOL_UUID_PARAM, ""));
+            DiskPtr part_disk = data.getStoragePolicy()->tryGetDiskByName(part->getDataPartStorage().getDiskName());
+            auto * ca_meta = tryGetContentAddressedExchange(part_disk);
+            if (ca_meta && !receiver_pool_uuid.empty() && receiver_pool_uuid == ca_meta->getPoolUUID())
+            {
+                auto offer = ca_meta->getRelinkOffer(part->getDataPartStorage().getRelativePath());
+                if (offer)
+                {
+                    LOG_DEBUG(log, "Sending part {} by relink (content-addressed, shared pool {}), manifest payload {} bytes",
+                        part_name, receiver_pool_uuid, offer->manifest_bytes.size());
+                    response.addCookie({CA_RELINK_COOKIE, CA_RELINK_COOKIE_VALUE});
+                    /// The source token for the confirm request the receiver makes before it promotes
+                    /// (spec §wire-protocol). It always accompanies the offer, and its ABSENCE is what
+                    /// tells a confirm-capable receiver that this sender predates the handshake.
+                    response.addCookie({CA_CONFIRM_TOKEN_COOKIE, offer->confirm_token});
+                    /// The relink payload (B7 part_manifest_v2, all-tree task 7): the opaque encoded
+                    /// PartManifest body (the receiver decodes it, ignores the sender identity, and
+                    /// stages its OWN local manifest over the shared-pool blobs; the legacy part_id wire
+                    /// field carries it). Self-contained: uuid.txt/metadata_version.txt are ordinary
+                    /// manifest entries now (task 6), so no separate mutable-header field is sent.
+                    writeStringBinary(offer->manifest_bytes, out);
+                    data.addLastSentPart(part->info);
+                    return;
+                }
+                /// No offer (no committed ref for this part here, or no mintable token) — fall through
+                /// to the byte path.
+            }
+        }
+
         if ((*data_settings)[MergeTreeSetting::allow_remote_fs_zero_copy_replication] &&
             client_protocol_version >= REPLICATION_PROTOCOL_VERSION_WITH_PARTS_ZERO_COPY)
         {
@@ -421,7 +645,8 @@ std::pair<MergeTreeData::MutableDataPartPtr, scope_guard> Fetcher::fetchSelected
     const String & tmp_prefix_,
     std::optional<CurrentlySubmergingEmergingTagger> * tagger_ptr,
     bool try_zero_copy,
-    DiskPtr disk)
+    DiskPtr disk,
+    bool allow_ca_relink)
 {
     if (blocker.isCancelled())
         throw Exception(ErrorCodes::ABORTED, "Fetching of part was cancelled");
@@ -454,13 +679,51 @@ std::pair<MergeTreeData::MutableDataPartPtr, scope_guard> Fetcher::fetchSelected
     {
         {"endpoint",                endpoint_id},
         {"part",                    part_name},
-        {"client_protocol_version", toString(REPLICATION_PROTOCOL_VERSION_WITH_COLUMNS_SUBSTREAMS)},
+        /// Advertising `..._WITH_CA_CONFIRM` is a PROMISE, not a capability list: this receiver will
+        /// confirm a relink offer against its source before it promotes (`relinkPartToDisk`). It is the
+        /// pair of the sender-side offer gate on the same constant, and the two cannot be separated —
+        /// see the comment there.
+        {"client_protocol_version", toString(REPLICATION_PROTOCOL_VERSION_WITH_CA_CONFIRM)},
         {"compress",                "false"}
     });
 
     if (disk)
         LOG_TRACE(log, "Will fetch to disk {} with type {}", disk->getName(), disk->getDataSourceDescription().toString());
 
+    /// CAS replication 2b — fetch-by-relink (spec §4). Advertise this replica's target content-addressed
+    /// pool identity so a same-pool sender can relink instead of streaming bytes. The target disk is the
+    /// provided one if it is CA, else the first CA disk among the table's disks. A non-CA fetch adds
+    /// nothing here and is byte-for-byte unchanged.
+    /// Gated on `allow_ca_relink` alone (B66b). That flag is the RECURSION BRAKE and nothing else: not
+    /// advertising is what makes the sender stream bytes, so every same-sender byte re-request below
+    /// clears it, and a persistent relink-mechanism failure therefore costs exactly one relink attempt.
+    /// The gate used to be `try_zero_copy && !to_detached`, and BOTH halves were accidents of that same
+    /// brake — `try_zero_copy` because the fallback re-requests with it false, and `!to_detached`
+    /// because the relink path staged at the ACTIVE part path and ignored `to_detached`. `to_detached`
+    /// is now a parameter of `relinkPartToDisk` (it stages under the `detached/` parent), and
+    /// `try_zero_copy` goes back to meaning real zero-copy only.
+    String advertised_pool_uuid;
+    if (allow_ca_relink)
+    {
+        if (auto * ca_meta = tryGetContentAddressedExchange(disk))
+        {
+            advertised_pool_uuid = ca_meta->getPoolUUID();
+            uri.addQueryParameter(CA_POOL_UUID_PARAM, advertised_pool_uuid);
+        }
+        else if (!disk)
+        {
+            for (const auto & data_disk : data.getDisks())
+            {
+                if (auto * ca_disk_meta = tryGetContentAddressedExchange(data_disk))
+                {
+                    advertised_pool_uuid = ca_disk_meta->getPoolUUID();
+                    uri.addQueryParameter(CA_POOL_UUID_PARAM, advertised_pool_uuid);
+                    break;
+                }
+            }
+        }
+    }
+
     Strings capability;
     if (try_zero_copy && (*data_settings)[MergeTreeSetting::allow_remote_fs_zero_copy_replication])
     {
@@ -615,6 +878,78 @@ std::pair<MergeTreeData::MutableDataPartPtr, scope_guard> Fetcher::fetchSelected
     if (server_protocol_version >= REPLICATION_PROTOCOL_VERSION_WITH_PARTS_PROJECTION)
         readBinary(projections, *in);
 
+    /// CAS replication 2b — fetch-by-relink (spec §4; B7 part_manifest_v2, all-tree task 7). The sender
+    /// chose to relink: it sent only the part's encoded PartManifest body, no file bytes. Build the part
+    /// by staging this server's OWN local manifest over the blobs already in the shared pool (adopt-by-hash
+    /// -> revalidate -> promote inside adoptPartFromManifest) — self-contained since task 6 routed
+    /// uuid.txt/metadata_version.txt through the content path, so there is no separate mutable header to
+    /// reconstruct. If the relink is not possible (blob missing/condemned — a transient or a
+    /// genuinely-different pool the cheap pre-filter let through, or a mixed-build pair offering an
+    /// unrecognized cookie value), fall back to a normal byte fetch by re-requesting WITHOUT relink.
+    String ca_relink = parse<String>(in->getResponseCookie(CA_RELINK_COOKIE, ""));
+    if (!ca_relink.empty())
+    {
+        /// Re-request without the relink capability: pass the SAME (CA) disk but disable zero-copy/relink
+        /// so the sender streams bytes; on CA the downloaded files content-address and dedup.
+        ///
+        /// THE RECURSION BRAKE (B66b). `allow_ca_relink=false` is what bounds this: the re-request does
+        /// not advertise the pool identity, so the sender cannot offer relink again, so this lambda
+        /// cannot be reached a second time for the same fetch. Before relink had its own capability the
+        /// brake was implicit in `try_zero_copy=false`; with the two decoupled it has to be spelled out,
+        /// and it must be spelled out at EVERY same-sender fallback — a relink failure that is a
+        /// property of the pair reproduces on every attempt, so without the brake the fallback re-offers
+        /// and recurses without bound. The failures it actually bounds are the ones that leave the CA
+        /// disk resolved and matching: a mixed build offering an unrecognized cookie value, a sender that
+        /// predates the confirm handshake, an undecodable manifest, a local ref conflict. (The
+        /// reservation-outside-the-pool exit below is bounded twice over — it re-requests with the
+        /// non-CA disk it resolved, which cannot advertise anything either way — so do not read that one
+        /// as evidence that the brake is redundant.)
+        auto fall_back_to_byte_fetch = [&]
+        {
+            temporary_directory_lock = {};
+            return fetchSelectedPart(
+                metadata_snapshot, context, part_name, zookeeper_name, replica_path, host, port, timeouts,
+                user, password, interserver_scheme, throttler, to_detached, tmp_prefix, nullptr, false, disk,
+                /*allow_ca_relink=*/ false);
+        };
+
+        if (ca_relink != CA_RELINK_COOKIE_VALUE)
+        {
+            /// Mixed-build cluster (rolling upgrade): this receiver build does not recognize the sender's
+            /// relink wire format. Bail out before reading anything else off the stream rather than
+            /// misparsing an incompatible payload shape.
+            LOG_INFO(log, "Part {} was offered by relink with cookie '{}' (this build expects '{}'); "
+                "falling back to a byte fetch", part_name, ca_relink, CA_RELINK_COOKIE_VALUE);
+            return fall_back_to_byte_fetch();
+        }
+
+        auto * chosen_ca = tryGetContentAddressedExchange(disk);
+        if (!chosen_ca || chosen_ca->getPoolUUID() != advertised_pool_uuid)
+        {
+            LOG_INFO(log, "Part {} was offered by relink for content-addressed pool '{}', but reservation landed "
+                "outside the advertised pool on disk {} (chosen pool: '{}'); falling back to a byte fetch",
+                part_name, advertised_pool_uuid, disk->getName(), chosen_ca ? chosen_ca->getPoolUUID() : "<none>");
+            return fall_back_to_byte_fetch();
+        }
+
+        String sender_manifest_bytes;
+        readStringBinary(sender_manifest_bytes, *in);
+        assertEOF(*in);
+
+        /// Publish-then-confirm (spec §core-idea) happens inside `relinkPartToDisk`, including the second
+        /// interserver request; the token cookie is the sender's offer identity and is opaque here. A
+        /// `nullptr` means the mechanism cannot work but the sender still has the part, so the byte
+        /// re-request below is sound; a THROW means the source did not prove the binding, and the whole
+        /// point of it being a throw is that this fallback must NOT run for it.
+        auto relinked = relinkPartToDisk(part_name, tmp_prefix, disk, to_detached, sender_manifest_bytes,
+            in->getResponseCookie(CA_CONFIRM_TOKEN_COOKIE, ""), uri, creds, timeouts, read_settings);
+        if (relinked)
+            return std::make_pair(std::move(relinked), std::move(temporary_directory_lock));
+
+        LOG_INFO(log, "Relink of part {} is not possible on this pair; falling back to a byte fetch", part_name);
+        return fall_back_to_byte_fetch();
+    }
+
     if (!remote_fs_metadata.empty())
     {
         if (!try_zero_copy)
@@ -656,7 +991,10 @@ std::pair<MergeTreeData::MutableDataPartPtr, scope_guard> Fetcher::fetchSelected
 
             temporary_directory_lock = {};
 
-            /// Try again but without zero-copy
+            /// Try again but without zero-copy. `allow_ca_relink=false` for the same reason as the relink
+            /// branch's fallback above: this is a same-sender byte re-request, and it must not re-open a
+            /// capability the failed attempt is not evidence about. It also preserves the behaviour this
+            /// call had while relink rode on `try_zero_copy` — the flag it already passes as false.
             return fetchSelectedPart(
                 metadata_snapshot,
                 context,
@@ -666,7 +1004,8 @@ std::pair<MergeTreeData::MutableDataPartPtr, scope_guard> Fetcher::fetchSelected
                 host,
                 port,
                 timeouts,
-                user, password, interserver_scheme, throttler, to_detached, tmp_prefix, nullptr, false, disk);
+                user, password, interserver_scheme, throttler, to_detached, tmp_prefix, nullptr, false, disk,
+                /*allow_ca_relink=*/ false);
         }
     }
 
@@ -949,6 +1288,312 @@ MergeTreeData::MutableDataPartPtr Fetcher::downloadPartToDisk(
     return new_data_part;
 }
 
+/// The receiver's half of publish-then-confirm, and its complete failure taxonomy (spec
+/// §failure-taxonomy). Every exit of `relinkPartToDisk` is one of these seven rows; the last two columns
+/// are the questions a reviewer has to be able to answer without reading the control flow, because a
+/// part-exchange path that gets them wrong either loses a part or commits it twice.
+///
+/// 1. THE SOURCE SENT NO TOKEN (a peer that predates the handshake).
+///    `+1`: never staged. Action: return `nullptr`, the caller byte-fetches from the same sender.
+///    Lose a part? No -- the sender still has it and streams it.
+///    Double-promote? No -- nothing was staged, so there is nothing to promote.
+///
+/// 2. `prepareAdoptFromManifest` -> `MechanismFallbackAllowed` (manifest decode failure, or the
+///    retryable staging class: body-absent precommit / precommit no longer the live owner / ref
+///    conflict).
+///    `+1`: NOTHING IS PUBLISHED, and that -- not "never staged" -- is what makes the byte fallback
+///    sound here. A precommit whose ref-log append came back `Unresolved` may in fact be durable, so
+///    `prepareEntries`' own `abandon` queues the exact removal for it (`PartWriteTxn::precommitAdd`
+///    records the intent BEFORE the append precisely so that removal is never skipped) and leaves the
+///    manifest body for GC rather than deleting it. A precommit is not a committed ref: a later byte
+///    fetch publishes the same ref name over it without conflict, and a removal that could not be
+///    appended at all leaks retained blobs -- it never double-publishes.
+///    Action: return `nullptr`, the caller byte-fetches. Lose a part? No, as row 1.
+///    Double-promote? No -- no handle exists, and nothing was committed.
+///
+/// 3. THE CONFIRM DID NOT PROVE THE SOURCE: an `unproven` answer, an absent answer cookie, a transport
+///    failure, a timeout. All one outcome, deliberately (`CasConfirmAnswer`: only `yes` authorizes).
+///    `+1`: durable, then released by `abort`. Action: THROW a locally generated retry-later
+///    `NETWORK_ERROR` naming the source and the part -- never `nullptr`, because a byte re-request goes
+///    back to the very source whose state is in doubt.
+///    Lose a part? No -- the queue stores the exception, backs off, and re-executes the entry, which
+///    recomputes the source and the covering-part discovery. The fetch is postponed, not dropped.
+///    Double-promote? No -- `abort` appends the exact precommit removal and no committed ref exists.
+///
+/// 4. CONFIRM `yes`, `promote` -> `Committed`.
+///    `+1`: committed. Action: return the relinked part; the usual `tmp-fetch_<part>` re-key follows.
+///    Lose a part? No. Double-promote? No -- `promote` is the handle's single terminal operation, the
+///    handle is released immediately after it, and a second call is rejected rather than re-driving a
+///    finished transaction.
+///
+/// 5. CONFIRM `yes`, `promote` -> `MechanismFallbackAllowed` (a local ref conflict; the source proved
+///    its side, this receiver could not commit its own). The promote was rejected BEFORE its ref-log
+///    append, so "nothing was committed" is proven, not assumed -- see row 5b for the case where it is
+///    not.
+///    `+1`: released -- a failed `promote` abandons its build on the way out.
+///    Action: return `nullptr`, the caller byte-fetches. Lose a part? No, as row 1.
+///    Double-promote? No -- the byte fetch starts from a clean slate.
+///
+/// 5b. CONFIRM `yes`, `promote` -> `Unresolved` (the promotion's ref-log append was attempted and came
+///    back without a verdict; the receiver's ref MAY be committed).
+///    `+1`: still owed -- the handle attempts its abandon, which is REJECTED by the state machine if
+///    the promote in fact landed (a promoted binding is no longer a precommit), so no committed ref is
+///    ever undone here.
+///    Action: THROW the retry-later `NETWORK_ERROR`, as row 3 -- returning `nullptr` is the one thing
+///    that must not happen, because a byte fetch would publish the part a SECOND time over a relink
+///    that may already be committed.
+///    Lose a part? No -- retry-later, as row 3. Double-promote? No -- nothing is published on this exit.
+///
+/// 6. ANY OTHER EXCEPTION (an unclassified local error, or a `promote` failure outside the known
+///    retryable class).
+///    `+1`: durable if one was staged, then released by the scope guard. Action: propagate.
+///    Lose a part? No -- retry-later, exactly as row 3. Double-promote? No -- the scope guard runs
+///    `abort` before the exception leaves the function, and the handle's own destructor is the backstop
+///    if that abort's append fails.
+///
+/// The asymmetry between rows 2/5 and row 3 is the entire point of the typed boundary. A byte
+/// re-request goes back to the SAME sender, so it is a sound recovery exactly when the doubt is about
+/// the MECHANISM and the sender is known to still hold the part -- and never when the doubt is about
+/// the source itself. `adoptPartFromManifest` used to collapse the two by catching every `Exception`
+/// and returning `false`.
+///
+/// B66b — WHAT CHANGES WHEN THE TARGET IS `detached/`. Every row above still holds, and the two columns
+/// that matter are unchanged in every one of them, but two rows hold for a DIFFERENT reason and that
+/// difference is worth stating rather than rediscovering:
+///
+/// - Row 3 (and row 6, which recovers the same way) argues "no part is lost" from the replication queue:
+///   it stores the exception, backs off, and re-executes the entry. Two of the three detached callers
+///   have no queue entry -- `FETCH PARTITION`/`FETCH PART ... FROM` are user DDL -- so the retry-later
+///   error surfaces to the user, who re-issues the statement. Nothing is lost either way, and for a
+///   stronger reason than in the active case: a detached fetch is not replication, so no replicated
+///   state was ever expecting the part. (The third, `executeClonePartFromShard`, IS a queue entry and
+///   recovers exactly as the active path does.)
+/// - Row 4's "no double-promote" is about the relink's own terminal operation and is unaffected. What
+///   the CALLER then does with the part differs: `renameTo(detached/<part>, true)` rather than
+///   `renameTempPartAndReplace`. Both are ref repoints within one namespace on a content-addressed disk
+///   (`detached/` is a ref-name prefix, not a namespace), and the detached one keeps its existing
+///   collision behaviour -- an existing `detached/<part>` is displaced. That is the pre-existing
+///   semantic of a detached BYTE fetch, deliberately left alone: relink must not change what a fetch
+///   into `detached/` means, only how the bytes get there.
+///
+/// The staged ref itself is `detached/tmp-fetch_<part>` rather than `tmp-fetch_<part>`, which is what
+/// keeps a failed detached relink from ever being visible as a live part: the abandoned precommit and
+/// the abandoned staging directory both live in the detached ref space.
+///
+/// What a `yes` does NOT prove: `CaRelinkConfirmCore.tla` config `_sab_holeylist` shows that with every
+/// confirm rule intact and one incomplete listing page permitted, `ConfirmedRelinkNeverDangles` still
+/// breaks (BACKLOG `{#list-as-journal-dataloss-2026-07-25}`). A confirmed relink is therefore NOT proven
+/// dangle-free; a `yes` means only "the source still holds exactly this manifest right now", which is
+/// what closes the codex-6 handoff window and nothing more.
+MergeTreeData::MutableDataPartPtr Fetcher::relinkPartToDisk(
+    const String & part_name,
+    const String & tmp_prefix,
+    DiskPtr disk,
+    bool to_detached,
+    const String & sender_manifest_bytes,
+    const String & source_token,
+    const Poco::URI & fetch_uri,
+    const Poco::Net::HTTPBasicCredentials & credentials,
+    const ConnectionTimeouts & timeouts,
+    const ReadSettings & read_settings)
+{
+    auto * ca_meta = tryGetContentAddressedExchange(disk);
+    if (!ca_meta)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "relinkPartToDisk called for a non-content-addressed disk {}", disk->getName());
+
+    if (tmp_prefix.empty()
+        || part_name.empty()
+        || std::string::npos != tmp_prefix.find_first_of("/.")
+        || std::string::npos != part_name.find_first_of("/."))
+        throw Exception(ErrorCodes::LOGICAL_ERROR, "`tmp_prefix` and `part_name` cannot be empty or contain '.' or '/' characters.");
+
+    /// Taxonomy row 1 — the capability gate, and it comes FIRST so a pre-confirm sender costs nothing.
+    /// An absent token is how such a sender identifies itself: it offers relink exactly as before and
+    /// simply has no token cookie to attach. There is no version number to consult here and that is
+    /// deliberate — the sender's advertised version says what it can serve, while the token's presence
+    /// says what it actually did for THIS offer, and only the latter can be confirmed. A relink that
+    /// cannot be confirmed is never promoted, so the bytes are fetched instead.
+    if (source_token.empty())
+    {
+        LOG_INFO(log, "Part {} was offered by relink without a source token, so the offer cannot be confirmed "
+            "(the sender predates the publish-then-confirm handshake); falling back to a byte fetch", part_name);
+        return nullptr;
+    }
+
+    /// Test-only. Forces the "mechanism failed, the sender still has the part" exit (the ACTION of
+    /// taxonomy rows 2 and 5) on EVERY attempt, which is precisely the shape the recursion brake has to
+    /// bound: a persistent property of this sender/receiver pair, so the byte re-request re-offers and
+    /// re-fails unless it clears `allow_ca_relink`. It fires AFTER the token gate and BEFORE
+    /// `prepareAdoptFromManifest`, so nothing is staged and no `+1` has to be released — the failpoint
+    /// injects the exit, never a half-finished transaction.
+    fiu_do_on(FailPoints::cas_relink_receiver_force_mechanism_failure,
+    {
+        LOG_INFO(log, "Failpoint cas_relink_receiver_force_mechanism_failure: abandoning the relink of part {} "
+            "before anything is staged", part_name);
+        return nullptr;
+    });
+
+    /// Stage under the tmp-fetch dir OF THE TARGET PARENT — the table dir, or `TABLE/detached` when
+    /// the caller asked for a detached fetch (B66b). The parent is composed exactly as
+    /// `downloadPartToDisk` composes it, so the two fetch paths put a part in the same place and the
+    /// caller's finalization is unchanged: `renameTempPartAndReplace`'s moveDirectory(tmp-fetch_<part>
+    /// -> <part>) for the active path, `renameTo(detached/<part>)` for the detached one. Both are ref
+    /// repoints within one namespace on a content-addressed disk (`detached/` is a ref-name prefix, not
+    /// a namespace), so a relinked part re-keys exactly as a byte-fetched one does.
+    ///
+    /// The ref name is NOT built here: the disk-relative path is handed to the CA exchange whole and its
+    /// router folds `TABLE/detached/DIR` onto the `detached/DIR` ref, the same routing every other
+    /// read and write of a detached part goes through. This side has no business knowing that prefix,
+    /// and the sender's half of the offer (`getRelinkOffer`) is already addressed by path too.
+    const String part_dir = tmp_prefix + part_name;
+    const String part_relative_path
+        = data.getRelativeDataPath() + String(to_detached ? MergeTreeData::DETACHED_DIR_NAME : "");
+    const String part_path = fs::path(part_relative_path) / part_dir;
+
+    LOG_DEBUG(log, "Relinking part {} (staged as {}) onto content-addressed disk {} from a {}-byte transferred manifest.",
+        part_name, part_path, disk->getName(), sender_manifest_bytes.size());
+
+    /// T1 — PUBLISH. Adopt-from-manifest and precommit, stopping short of the promote (B7
+    /// part_manifest_v2, all-tree task 7): the receiver decodes the transferred body and stages its OWN
+    /// local manifest over the shared-pool blobs (adopt-by-hash). Self-contained:
+    /// uuid.txt/metadata_version.txt are ordinary entries in the transferred manifest (task 6), so there
+    /// is no sidecar to reconstruct. Trust boundary is the interserver channel, as for a normal part
+    /// fetch — see `prepareAdoptFromManifest`.
+    ///
+    /// The order is the whole protocol. This `+1` must be DURABLE before the source is asked anything,
+    /// because the question "do you still hold it?" only excludes a later removal if the receiver's own
+    /// reference is already in the ref log when that removal is appended (spec §correctness). Asking
+    /// first and publishing after would prove nothing about the interval in between. What it does NOT
+    /// establish is that every subsequent GC fold OBSERVES that reference -- see "What a `yes` does NOT
+    /// prove" above; ordering is necessary here, not sufficient.
+    std::unique_ptr<ICaPreparedRelink> prepared;
+    if (ca_meta->prepareAdoptFromManifest(part_path, sender_manifest_bytes, prepared)
+        == CaRelinkPrepare::MechanismFallbackAllowed)
+        return nullptr;                                     /// taxonomy row 2
+    if (!prepared)
+        throw Exception(ErrorCodes::LOGICAL_ERROR,
+            "Relink of part {} reported a prepared write but produced no handle", part_name);
+
+    /// Belt-and-braces over the handle's own destructor: the durable `+1` is released on EVERY exit that
+    /// is not a completed promote, including an exception. `abort` is the non-throwing form by contract —
+    /// this runs while the retry-later error of row 3 is already in flight.
+    SCOPE_EXIT({
+        if (prepared)
+            prepared->abort();
+    });
+
+    /// Test-only, and this is the ONE seam worth injecting on the whole path: it opens the window the
+    /// protocol exists to make safe. The receiver's `+1` is durable and its release is armed, and the
+    /// source has not been asked anything yet, so a test that holds the fetch here can do to the source
+    /// exactly what codex-6 described — merge the part away, run GC to fixpoint — and then observe both
+    /// halves of the contract: the source's blobs survive the round (this receiver's binding protects
+    /// them) and the confirm that follows refuses to authorize a promote (the binding it named is gone).
+    FailPointInjection::pauseFailPoint(FailPoints::cas_relink_receiver_pause_before_confirm);
+
+    /// T2 — CONFIRM. One read-only interserver question, aimed at the endpoint copied out of the fetch
+    /// URI so it reaches exactly the table and replica that made the offer. Only the literal
+    /// `CA_CONFIRM_ANSWER_PROVEN` cookie authorizes a promote: an `unproven` answer, an absent cookie
+    /// (any peer that does not implement the action) and a failed request are ONE outcome, and it is not
+    /// knowledge about the source — see `CasConfirmAnswer` on why `no` is never put on the wire.
+    bool source_proved_the_binding = false;
+    try
+    {
+        Poco::URI confirm_uri;
+        confirm_uri.setScheme(fetch_uri.getScheme());
+        confirm_uri.setHost(fetch_uri.getHost());
+        confirm_uri.setPort(fetch_uri.getPort());
+        Poco::URI::QueryParameters confirm_params;
+        for (const auto & fetch_param : fetch_uri.getQueryParameters())
+            if (fetch_param.first == "endpoint")
+                confirm_params.push_back(fetch_param);
+        confirm_params.emplace_back(CA_CONFIRM_ACTION_PARAM, source_token);
+        confirm_params.emplace_back("compress", "false");
+        confirm_uri.setQueryParameters(confirm_params);
+
+        /// `read_settings` is the caller's, which already caps HTTP retries at one: the queue owns the
+        /// retry policy for a fetch, and a silently retried confirm would widen the window it measures.
+        auto confirm_in = BuilderRWBufferFromHTTP(confirm_uri)
+                              .withConnectionGroup(HTTPConnectionGroupType::HTTP)
+                              .withBypassProxy(true)
+                              .withMethod(Poco::Net::HTTPRequest::HTTP_POST)
+                              .withTimeouts(timeouts)
+                              .withSettings(read_settings)
+                              .withDelayInit(false)
+                              .create(credentials);
+        /// The confirm answer is a cookie and the response body is empty by construction. Requiring EOF
+        /// before reading the answer means a response carrying anything at all — a misrouted reply, a
+        /// desynchronized peer — is unproven rather than half-parsed.
+        assertEOF(*confirm_in);
+        source_proved_the_binding
+            = confirm_in->getResponseCookie(CA_CONFIRM_ANSWER_COOKIE, "") == CA_CONFIRM_ANSWER_PROVEN;
+    }
+    catch (...)
+    {
+        /// Not a fallback: a confirm that could not be delivered is the same "not proven" as a refusal,
+        /// and it takes the same path out. Logged rather than propagated so the error the caller sees is
+        /// the one that names the relink — but logged in full, because the reason (refused, timed out,
+        /// 500) exists nowhere else. `information`, not `error`: a peer restarting mid-fetch is ordinary,
+        /// and the throw below is what makes the failure loud.
+        tryLogCurrentException(log, fmt::format("while confirming the relink offer for part {} with {}",
+            part_name, fetch_uri.getHost()), LogsLevel::information);
+        source_proved_the_binding = false;
+    }
+
+    if (!source_proved_the_binding)
+    {
+        /// Taxonomy row 3. Locally generated on purpose — nothing here is the source's error to report —
+        /// and thrown rather than returned, because the one recovery that is NOT sound after this is a
+        /// byte re-request to the same source. `NETWORK_ERROR` puts it in the retry-later class, so the
+        /// queue stores it, backs off, and re-selects on re-execution.
+        throw Exception(ErrorCodes::NETWORK_ERROR,
+            "Source {} did not prove it still holds the manifest it offered for part {} by relink; "
+            "the relink is abandoned and the fetch will be retried later",
+            fetch_uri.getHost(), part_name);
+    }
+
+    /// T3 — PROMOTE. Only now, and only because the source proved the binding at T2 > T1.
+    switch (prepared->promote())
+    {
+        case CaRelinkPromote::Committed:
+            break;
+        case CaRelinkPromote::MechanismFallbackAllowed:
+            return nullptr;                                 /// taxonomy row 5
+        case CaRelinkPromote::Unresolved:
+            /// The promotion append may have landed, so this is the ONE promote outcome that is not row
+            /// 5: returning `nullptr` would send the caller to fetch the bytes and publish the part a
+            /// second time over a relink that may already be committed. Thrown in the retry-later class
+            /// instead, exactly as an unproven confirm is (row 3) -- the queue stores it, backs off, and
+            /// re-executes, by which time the ref lane has resolved the ambiguity one way or the other.
+            throw Exception(ErrorCodes::NETWORK_ERROR,
+                "Relink of part {} from {} could not be resolved: the promotion may or may not have "
+                "committed, so the bytes must NOT be fetched; the fetch will be retried later",
+                part_name, fetch_uri.getHost());
+    }
+    /// The single terminal operation is done, so the handle owes nothing; releasing it here also disarms
+    /// the scope guard for the part-building code below.
+    prepared.reset();
+
+    auto volume = std::make_shared<SingleDiskVolume>("volume_" + part_name, disk);
+
+    MergeTreeData::MutableDataPartPtr new_data_part;
+    MergeTreeDataPartBuilder builder(data, part_name, volume, part_relative_path, part_dir, getReadSettings());
+    /// Read the part format from the now-published manifest (type + storage type), exactly as the byte
+    /// fetch does — authoritative over the transferred `part_type` header (kept for protocol symmetry).
+    new_data_part = builder.withPartFormatFromDisk().build();
+
+    new_data_part->version->setAndStoreCreationTID(Tx::NonTransactionalTID, nullptr);
+    new_data_part->is_temp = true;
+    /// The blobs are shared in the pool; a discarded temporary relink part must NOT reclaim them (another
+    /// replica's ref keeps them alive). Same policy a zero-copy-fetched temporary part uses.
+    new_data_part->remove_tmp_policy = IMergeTreeDataPart::BlobsRemovalPolicyForTemporaryParts::PRESERVE_BLOBS;
+    new_data_part->modification_time = time(nullptr);
+    new_data_part->loadColumnsChecksumsIndexes(true, false);
+
+    LOG_DEBUG(log, "Relink of part {} onto disk {} finished (no bytes transferred).", part_name, disk->getName());
+    return new_data_part;
+}
+
 }
 
 }
diff --git a/src/Storages/MergeTree/DataPartsExchange.h b/src/Storages/MergeTree/DataPartsExchange.h
index 6e79d67d5714..5bec506eac21 100644
--- a/src/Storages/MergeTree/DataPartsExchange.h
+++ b/src/Storages/MergeTree/DataPartsExchange.h
@@ -18,11 +18,22 @@ namespace zkutil
     using ZooKeeperPtr = std::shared_ptr<ZooKeeper>;
 }
 
+/// Only the content-addressed relink's confirm request needs these, and only as parameter types, so
+/// they are declared rather than included — this header is pulled in by the whole replication tree.
+namespace Poco { class URI; }
+namespace Poco::Net { class HTTPBasicCredentials; }
+
 namespace DB
 {
 
 class StorageReplicatedMergeTree;
 class ReadWriteBufferFromHTTP;
+struct ReadSettings;
+
+/// Declared by `ContentAddressedExchange.h` (the narrow content-addressed seam). Opaque-enum-declared
+/// here so this header stays free of content-addressed includes; the definition must keep the same
+/// underlying type.
+enum class CasConfirmAnswer : uint8_t;
 
 namespace DataPartsExchange
 {
@@ -41,6 +52,27 @@ class Service final : public InterserverIOEndpoint
     void processQuery(const HTMLForm & params, ReadBufferPtr body, WriteBuffer & out, HTTPServerResponse & response) override;
 
 private:
+    /// CAS fetch-by-relink, publish-then-confirm: answer one relink confirm token — "is `manifest_ref_text`
+    /// still exactly what `ref_name` names here?" — for a receiver that has already made its own `+1`
+    /// durable and may promote only on `Yes`. Everything content-addressed is behind
+    /// `IContentAddressedExchange`; what has to live here is what only the storage can see: which of this
+    /// table's disks is entitled to answer (`ownsNamespace` under a matching pool UUID, exactly one match
+    /// or `Unknown`), and gate 0, the part-anchored filter over this table's parts set. Never throws, and
+    /// `No` is not knowledge — see `CasConfirmAnswer`.
+    /// The confirm action's handler: decode the peer's token, resolve it, and set the answer cookie.
+    /// Exactly two answers cross the wire — proven, and not proven — because only `Yes` authorizes
+    /// anything and `No` is not knowledge (see `CasConfirmAnswer`). Never throws: an unparsable token
+    /// is one more unproven answer, not an error the receiver would have to classify.
+    void answerContentAddressedConfirm(const String & token_text, HTTPServerResponse & response) const;
+
+    CasConfirmAnswer resolveContentAddressedConfirm(
+        const String & pool_uuid,
+        const String & server_root_id,
+        const String & root_namespace,
+        const String & ref_name,
+        const String & part_name,
+        const String & manifest_ref_text) const;
+
     MergeTreeData::DataPartPtr findPart(const String & name);
 
     MergeTreeData::DataPart::Checksums sendPartFromDisk(
@@ -81,7 +113,20 @@ class Fetcher final : private boost::noncopyable
         const String & tmp_prefix_ = "",
         std::optional<CurrentlySubmergingEmergingTagger> * tagger_ptr = nullptr,
         bool try_zero_copy = true,
-        DiskPtr dest_disk = nullptr);
+        DiskPtr dest_disk = nullptr,
+        /// CAS fetch-by-relink (spec §B66b): may this request advertise its content-addressed pool
+        /// identity, i.e. may the sender answer with a relink offer instead of the part's bytes?
+        ///
+        /// It is a capability of its own rather than a rider on `try_zero_copy`, and it carries the
+        /// RECURSION BRAKE. Relink used to be gated on `try_zero_copy` purely because the byte-fetch
+        /// fallback re-requests with `try_zero_copy=false`, so the brake came for free; with the two
+        /// decoupled, every same-sender byte re-request must clear THIS flag explicitly or a
+        /// persistent relink-mechanism failure re-offers, re-fails and re-requests without bound.
+        ///
+        /// It defaults to `true`, and that default is what makes a manual `FETCH PARTITION`/`FETCH
+        /// PART` (which passes `try_fetch_shared=false`, so `try_zero_copy` is already false) relink,
+        /// into `detached/` as well as into the active part path.
+        bool allow_ca_relink = true);
 
     /// You need to stop the data transfer.
     ActionBlocker blocker;
@@ -111,6 +156,36 @@ class Fetcher final : private boost::noncopyable
         ThrottlerPtr throttler,
         bool sync);
 
+    /// CAS replication 2b — fetch-by-relink (spec §4), publish-then-confirm (spec §core-idea). Build a
+    /// part WITHOUT downloading any bytes by publishing this server's own ref to the blobs already in the
+    /// shared content-addressed pool. Stages the ref under the tmp-fetch dir of the target parent — the
+    /// table dir, or `detached/` when `to_detached` (B66b) — so the caller's finalization re-keys it to
+    /// the final part name, exactly as for a byte-fetched part: `renameTempPartAndReplace` for the
+    /// active path, `renameTo(detached/<part>)` for the detached one. Then it ASKS THE SOURCE whether it
+    /// still holds exactly the manifest it offered, and only then promotes and loads the part.
+    /// Self-contained (all-tree task 7): the transferred manifest alone is enough to rebuild the part —
+    /// no separate uuid/metadata_version wire fields to reconstruct as a sidecar.
+    ///
+    /// The whole failure taxonomy lives at the definition; the two outcomes a CALLER must distinguish:
+    /// `nullptr` means relink cannot work here and the source still has the part, so a byte re-request to
+    /// the SAME source is sound; a THROW means the source could not prove it still holds the manifest,
+    /// and the one recovery that is not sound is asking that same source for the bytes.
+    ///
+    /// `source_token`, `fetch_uri` and the connection parameters are what the confirm request is built
+    /// from: the token is the sender's opaque offer identity, and the request is aimed at the endpoint
+    /// COPIED out of the fetch URI so it cannot reach a different table or replica than the offer did.
+    MergeTreeData::MutableDataPartPtr relinkPartToDisk(
+        const String & part_name,
+        const String & tmp_prefix,
+        DiskPtr disk,
+        bool to_detached,
+        const String & sender_manifest_bytes,
+        const String & source_token,
+        const Poco::URI & fetch_uri,
+        const Poco::Net::HTTPBasicCredentials & credentials,
+        const ConnectionTimeouts & timeouts,
+        const ReadSettings & read_settings);
+
     MergeTreeData::MutableDataPartPtr downloadPartToDiskRemoteMeta(
        const String & part_name,
        const String & replica_path,

From 07dd887aa327b5cb65542435982a70801959ccdc Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:38 +0200
Subject: [PATCH 25/30] CAS tests: unit (gtest)

The full CAS gtest battery: per-layer unit tests, protocol state-machine
tests, wiring/assembly tests, and the shared test helpers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 src/Disks/tests/cas_format_test_battery.h     |  111 +
 src/Disks/tests/cas_sweep_test_support.h      |   36 +
 src/Disks/tests/cas_test_helpers.h            | 1994 +++++++
 src/Disks/tests/gtest_ca_dedup_cache.cpp      |  203 +
 src/Disks/tests/gtest_ca_transaction.cpp      |  754 +++
 src/Disks/tests/gtest_ca_wiring.cpp           | 2963 +++++++++
 src/Disks/tests/gtest_cas_b140_dangle.cpp     |  127 +
 src/Disks/tests/gtest_cas_backend.cpp         | 1049 ++++
 .../tests/gtest_cas_backend_contract.cpp      |  301 +
 .../tests/gtest_cas_backend_generation.cpp    |  132 +
 src/Disks/tests/gtest_cas_backend_listing.cpp |   43 +
 src/Disks/tests/gtest_cas_blob_digest.cpp     |  266 +
 .../tests/gtest_cas_blob_envelope_format.cpp  |  174 +
 src/Disks/tests/gtest_cas_blob_hasher.cpp     |  182 +
 src/Disks/tests/gtest_cas_blob_indegree.cpp   | 1000 ++++
 src/Disks/tests/gtest_cas_blob_meta.cpp       |  212 +
 .../tests/gtest_cas_blob_meta_format.cpp      |   49 +
 src/Disks/tests/gtest_cas_blob_ref.cpp        |   34 +
 .../tests/gtest_cas_blob_upload_pool.cpp      |  144 +
 .../tests/gtest_cas_blob_upload_pool_env.cpp  |   48 +
 .../tests/gtest_cas_bootstrap_ordering.cpp    |  443 ++
 .../tests/gtest_cas_confirm_exact_ref.cpp     | 1011 ++++
 src/Disks/tests/gtest_cas_decommission.cpp    | 1366 +++++
 .../gtest_cas_decommission_catalog_duties.cpp |  330 +
 src/Disks/tests/gtest_cas_empty_proof.cpp     |  281 +
 src/Disks/tests/gtest_cas_encoding_pins.cpp   |  110 +
 src/Disks/tests/gtest_cas_envelope.cpp        |   55 +
 .../tests/gtest_cas_event_dispatcher.cpp      |  194 +
 src/Disks/tests/gtest_cas_event_log.cpp       |  240 +
 .../tests/gtest_cas_fence_generation.cpp      |  471 ++
 src/Disks/tests/gtest_cas_fold_seal_codec.cpp |   41 +
 .../tests/gtest_cas_fold_seal_format.cpp      |  327 +
 src/Disks/tests/gtest_cas_forget.cpp          |  600 ++
 src/Disks/tests/gtest_cas_format.cpp          |   85 +
 src/Disks/tests/gtest_cas_format_battery.cpp  |   23 +
 src/Disks/tests/gtest_cas_fsck.cpp            | 1584 +++++
 src/Disks/tests/gtest_cas_gc_ack_floor.cpp    | 1262 ++++
 .../tests/gtest_cas_gc_arithmetic_intake.cpp  |  595 ++
 src/Disks/tests/gtest_cas_gc_attempt.cpp      |  189 +
 src/Disks/tests/gtest_cas_gc_bounded_walk.cpp |  570 ++
 src/Disks/tests/gtest_cas_gc_fold.cpp         |  624 ++
 .../tests/gtest_cas_gc_frontier_gate.cpp      | 3481 +++++++++++
 src/Disks/tests/gtest_cas_gc_hold_grammar.cpp | 1590 +++++
 src/Disks/tests/gtest_cas_gc_leak.cpp         |  545 ++
 src/Disks/tests/gtest_cas_gc_log.cpp          |  489 ++
 .../gtest_cas_gc_maintenance_state_format.cpp |  202 +
 .../tests/gtest_cas_gc_outcomes_format.cpp    |  101 +
 src/Disks/tests/gtest_cas_gc_rebuild.cpp      |  681 +++
 src/Disks/tests/gtest_cas_gc_resume.cpp       |  184 +
 src/Disks/tests/gtest_cas_gc_round.cpp        | 2000 +++++++
 src/Disks/tests/gtest_cas_gc_round_defer.cpp  |  640 ++
 .../tests/gtest_cas_gc_shard_incarnation.cpp  |  521 ++
 src/Disks/tests/gtest_cas_gc_shard_plan.cpp   |  637 ++
 src/Disks/tests/gtest_cas_gc_source_edge.cpp  |   84 +
 src/Disks/tests/gtest_cas_gc_state_format.cpp |  179 +
 src/Disks/tests/gtest_cas_gc_stop_start.cpp   |  492 ++
 .../tests/gtest_cas_gc_undercount_repro.cpp   |  416 ++
 src/Disks/tests/gtest_cas_heartbeat.cpp       |  760 +++
 .../tests/gtest_cas_holey_list_detector.cpp   |  306 +
 src/Disks/tests/gtest_cas_ids.cpp             |   41 +
 .../tests/gtest_cas_inline_placement.cpp      |   28 +
 src/Disks/tests/gtest_cas_inspect.cpp         |  222 +
 src/Disks/tests/gtest_cas_json_writer.cpp     |  214 +
 src/Disks/tests/gtest_cas_layout.cpp          |  382 ++
 .../tests/gtest_cas_lifecycle_condition.cpp   |  259 +
 .../tests/gtest_cas_lifecycle_snapshot.cpp    |  236 +
 .../tests/gtest_cas_list_liar_end_to_end.cpp  |  624 ++
 src/Disks/tests/gtest_cas_manifest_id.cpp     |   86 +
 src/Disks/tests/gtest_cas_mount.cpp           | 1602 +++++
 ...est_cas_namespace_file_request_profile.cpp |  558 ++
 .../tests/gtest_cas_namespace_janitor.cpp     |  607 ++
 .../tests/gtest_cas_namespace_life_id.cpp     |  429 ++
 .../tests/gtest_cas_ns_creation_lifecycle.cpp |  530 ++
 .../tests/gtest_cas_ns_file_incarnation.cpp   |  279 +
 .../tests/gtest_cas_ns_file_read_contract.cpp |  247 +
 src/Disks/tests/gtest_cas_observability.cpp   |  355 ++
 src/Disks/tests/gtest_cas_operation_gate.cpp  |  436 ++
 .../tests/gtest_cas_orphan_manifest_sweep.cpp |  678 +++
 .../tests/gtest_cas_orphan_nomination.cpp     |  319 +
 src/Disks/tests/gtest_cas_parallel_commit.cpp |  308 +
 .../tests/gtest_cas_part_folder_access.cpp    | 1291 ++++
 .../tests/gtest_cas_part_folder_view.cpp      |  105 +
 .../tests/gtest_cas_part_manifest_format.cpp  |  472 ++
 src/Disks/tests/gtest_cas_part_write.cpp      | 2517 ++++++++
 .../gtest_cas_part_write_root_dangle.cpp      |  256 +
 src/Disks/tests/gtest_cas_pluggable_hash.cpp  |  939 +++
 src/Disks/tests/gtest_cas_pool.cpp            | 2079 +++++++
 src/Disks/tests/gtest_cas_probe.cpp           |  309 +
 .../tests/gtest_cas_promote_republish.cpp     |  402 ++
 .../tests/gtest_cas_protocol_scenarios.cpp    |  622 ++
 .../gtest_cas_rebuild_condemn_nothing.cpp     |  636 ++
 .../tests/gtest_cas_record_stream_format.cpp  |  255 +
 .../tests/gtest_cas_recovery_grounding.cpp    |  701 +++
 .../tests/gtest_cas_recovery_streaming.cpp    |  647 ++
 src/Disks/tests/gtest_cas_ref_carve.cpp       |  366 ++
 src/Disks/tests/gtest_cas_ref_catalog.cpp     | 1717 ++++++
 .../gtest_cas_ref_catalog_birth_wiring.cpp    |  512 ++
 .../tests/gtest_cas_ref_chunk_preparation.cpp |  276 +
 .../tests/gtest_cas_ref_chunked_flush.cpp     |  919 +++
 src/Disks/tests/gtest_cas_ref_ckpt.cpp        | 1270 ++++
 src/Disks/tests/gtest_cas_ref_ckpt_join.cpp   |  548 ++
 .../tests/gtest_cas_ref_contiguous_alloc.cpp  |  640 ++
 .../tests/gtest_cas_ref_cow_manifest_set.cpp  |  392 ++
 src/Disks/tests/gtest_cas_ref_cow_map.cpp     |  516 ++
 .../tests/gtest_cas_ref_decode_bounds.cpp     |  136 +
 .../tests/gtest_cas_ref_epoch_seal_format.cpp |  491 ++
 src/Disks/tests/gtest_cas_ref_gc.cpp          |  987 +++
 .../tests/gtest_cas_ref_install_safety.cpp    |  959 +++
 src/Disks/tests/gtest_cas_ref_intake.cpp      |  260 +
 .../gtest_cas_ref_lane_exception_safety.cpp   |  217 +
 src/Disks/tests/gtest_cas_ref_log_format.cpp  |  790 +++
 .../tests/gtest_cas_ref_read_contract.cpp     |  256 +
 .../tests/gtest_cas_ref_recovery_cas_walk.cpp | 2010 +++++++
 .../tests/gtest_cas_ref_snapshot_format.cpp   |  436 ++
 ...test_cas_ref_snapshot_publish_ordering.cpp |  501 ++
 .../tests/gtest_cas_ref_statemachine.cpp      | 1445 +++++
 .../gtest_cas_ref_wedge_every_attempt.cpp     | 1476 +++++
 src/Disks/tests/gtest_cas_ref_writer.cpp      | 5299 +++++++++++++++++
 src/Disks/tests/gtest_cas_repoint.cpp         |  107 +
 src/Disks/tests/gtest_cas_request_control.cpp |  833 +++
 .../tests/gtest_cas_retirement_sweep.cpp      |  420 ++
 src/Disks/tests/gtest_cas_s3_staging.cpp      |  773 +++
 src/Disks/tests/gtest_cas_sentinel_probe.cpp  |  264 +
 .../tests/gtest_cas_server_root_format.cpp    |   94 +
 src/Disks/tests/gtest_cas_settings.cpp        |  146 +
 src/Disks/tests/gtest_cas_slot_occupy.cpp     |  334 ++
 .../gtest_cas_sweep_deletion_premise.cpp      |  433 ++
 src/Disks/tests/gtest_cas_text_format.cpp     |  292 +
 .../tests/gtest_cas_truncate_reclaim.cpp      |  282 +
 .../tests/gtest_cas_txn_apply_ledger.cpp      |  159 +
 src/Disks/tests/gtest_cas_upload_detached.cpp |  640 ++
 src/Disks/tests/gtest_cas_upload_fanout.cpp   |  940 +++
 src/Disks/tests/gtest_cas_wire_vocab.cpp      |   53 +
 src/Disks/tests/gtest_cas_writer_duties.cpp   |  545 ++
 134 files changed, 81936 insertions(+)
 create mode 100644 src/Disks/tests/cas_format_test_battery.h
 create mode 100644 src/Disks/tests/cas_sweep_test_support.h
 create mode 100644 src/Disks/tests/cas_test_helpers.h
 create mode 100644 src/Disks/tests/gtest_ca_dedup_cache.cpp
 create mode 100644 src/Disks/tests/gtest_ca_transaction.cpp
 create mode 100644 src/Disks/tests/gtest_ca_wiring.cpp
 create mode 100644 src/Disks/tests/gtest_cas_b140_dangle.cpp
 create mode 100644 src/Disks/tests/gtest_cas_backend.cpp
 create mode 100644 src/Disks/tests/gtest_cas_backend_contract.cpp
 create mode 100644 src/Disks/tests/gtest_cas_backend_generation.cpp
 create mode 100644 src/Disks/tests/gtest_cas_backend_listing.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_digest.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_envelope_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_hasher.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_indegree.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_meta.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_meta_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_ref.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_upload_pool.cpp
 create mode 100644 src/Disks/tests/gtest_cas_blob_upload_pool_env.cpp
 create mode 100644 src/Disks/tests/gtest_cas_bootstrap_ordering.cpp
 create mode 100644 src/Disks/tests/gtest_cas_confirm_exact_ref.cpp
 create mode 100644 src/Disks/tests/gtest_cas_decommission.cpp
 create mode 100644 src/Disks/tests/gtest_cas_decommission_catalog_duties.cpp
 create mode 100644 src/Disks/tests/gtest_cas_empty_proof.cpp
 create mode 100644 src/Disks/tests/gtest_cas_encoding_pins.cpp
 create mode 100644 src/Disks/tests/gtest_cas_envelope.cpp
 create mode 100644 src/Disks/tests/gtest_cas_event_dispatcher.cpp
 create mode 100644 src/Disks/tests/gtest_cas_event_log.cpp
 create mode 100644 src/Disks/tests/gtest_cas_fence_generation.cpp
 create mode 100644 src/Disks/tests/gtest_cas_fold_seal_codec.cpp
 create mode 100644 src/Disks/tests/gtest_cas_fold_seal_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_forget.cpp
 create mode 100644 src/Disks/tests/gtest_cas_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_format_battery.cpp
 create mode 100644 src/Disks/tests/gtest_cas_fsck.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_ack_floor.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_arithmetic_intake.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_attempt.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_bounded_walk.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_fold.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_frontier_gate.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_hold_grammar.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_leak.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_log.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_maintenance_state_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_outcomes_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_rebuild.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_resume.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_round.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_round_defer.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_shard_incarnation.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_shard_plan.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_source_edge.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_state_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_stop_start.cpp
 create mode 100644 src/Disks/tests/gtest_cas_gc_undercount_repro.cpp
 create mode 100644 src/Disks/tests/gtest_cas_heartbeat.cpp
 create mode 100644 src/Disks/tests/gtest_cas_holey_list_detector.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ids.cpp
 create mode 100644 src/Disks/tests/gtest_cas_inline_placement.cpp
 create mode 100644 src/Disks/tests/gtest_cas_inspect.cpp
 create mode 100644 src/Disks/tests/gtest_cas_json_writer.cpp
 create mode 100644 src/Disks/tests/gtest_cas_layout.cpp
 create mode 100644 src/Disks/tests/gtest_cas_lifecycle_condition.cpp
 create mode 100644 src/Disks/tests/gtest_cas_lifecycle_snapshot.cpp
 create mode 100644 src/Disks/tests/gtest_cas_list_liar_end_to_end.cpp
 create mode 100644 src/Disks/tests/gtest_cas_manifest_id.cpp
 create mode 100644 src/Disks/tests/gtest_cas_mount.cpp
 create mode 100644 src/Disks/tests/gtest_cas_namespace_file_request_profile.cpp
 create mode 100644 src/Disks/tests/gtest_cas_namespace_janitor.cpp
 create mode 100644 src/Disks/tests/gtest_cas_namespace_life_id.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ns_creation_lifecycle.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ns_file_incarnation.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ns_file_read_contract.cpp
 create mode 100644 src/Disks/tests/gtest_cas_observability.cpp
 create mode 100644 src/Disks/tests/gtest_cas_operation_gate.cpp
 create mode 100644 src/Disks/tests/gtest_cas_orphan_manifest_sweep.cpp
 create mode 100644 src/Disks/tests/gtest_cas_orphan_nomination.cpp
 create mode 100644 src/Disks/tests/gtest_cas_parallel_commit.cpp
 create mode 100644 src/Disks/tests/gtest_cas_part_folder_access.cpp
 create mode 100644 src/Disks/tests/gtest_cas_part_folder_view.cpp
 create mode 100644 src/Disks/tests/gtest_cas_part_manifest_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_part_write.cpp
 create mode 100644 src/Disks/tests/gtest_cas_part_write_root_dangle.cpp
 create mode 100644 src/Disks/tests/gtest_cas_pluggable_hash.cpp
 create mode 100644 src/Disks/tests/gtest_cas_pool.cpp
 create mode 100644 src/Disks/tests/gtest_cas_probe.cpp
 create mode 100644 src/Disks/tests/gtest_cas_promote_republish.cpp
 create mode 100644 src/Disks/tests/gtest_cas_protocol_scenarios.cpp
 create mode 100644 src/Disks/tests/gtest_cas_rebuild_condemn_nothing.cpp
 create mode 100644 src/Disks/tests/gtest_cas_record_stream_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_recovery_grounding.cpp
 create mode 100644 src/Disks/tests/gtest_cas_recovery_streaming.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_carve.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_catalog.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_catalog_birth_wiring.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_chunk_preparation.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_chunked_flush.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_ckpt.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_ckpt_join.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_contiguous_alloc.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_cow_manifest_set.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_cow_map.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_decode_bounds.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_epoch_seal_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_gc.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_install_safety.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_intake.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_lane_exception_safety.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_log_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_read_contract.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_recovery_cas_walk.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_snapshot_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_snapshot_publish_ordering.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_statemachine.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_wedge_every_attempt.cpp
 create mode 100644 src/Disks/tests/gtest_cas_ref_writer.cpp
 create mode 100644 src/Disks/tests/gtest_cas_repoint.cpp
 create mode 100644 src/Disks/tests/gtest_cas_request_control.cpp
 create mode 100644 src/Disks/tests/gtest_cas_retirement_sweep.cpp
 create mode 100644 src/Disks/tests/gtest_cas_s3_staging.cpp
 create mode 100644 src/Disks/tests/gtest_cas_sentinel_probe.cpp
 create mode 100644 src/Disks/tests/gtest_cas_server_root_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_settings.cpp
 create mode 100644 src/Disks/tests/gtest_cas_slot_occupy.cpp
 create mode 100644 src/Disks/tests/gtest_cas_sweep_deletion_premise.cpp
 create mode 100644 src/Disks/tests/gtest_cas_text_format.cpp
 create mode 100644 src/Disks/tests/gtest_cas_truncate_reclaim.cpp
 create mode 100644 src/Disks/tests/gtest_cas_txn_apply_ledger.cpp
 create mode 100644 src/Disks/tests/gtest_cas_upload_detached.cpp
 create mode 100644 src/Disks/tests/gtest_cas_upload_fanout.cpp
 create mode 100644 src/Disks/tests/gtest_cas_wire_vocab.cpp
 create mode 100644 src/Disks/tests/gtest_cas_writer_duties.cpp

diff --git a/src/Disks/tests/cas_format_test_battery.h b/src/Disks/tests/cas_format_test_battery.h
new file mode 100644
index 000000000000..173bfcc5c4df
--- /dev/null
+++ b/src/Disks/tests/cas_format_test_battery.h
@@ -0,0 +1,111 @@
+#pragma once
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Common/Exception.h>
+#include <fmt/format.h>
+#include <functional>
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int UNKNOWN_FORMAT_VERSION;
+}
+
+/// The shape-level failure-mode battery every v3 format registers with (spec §testing): one call
+/// exercises decode-of-encode, golden text, truncation at line boundaries and inside line 1,
+/// the v+1 gate, wrong-type, and leading garbage. Key-level rules (tolerant/strict/critical/
+/// duplicate) are unit-tested once on JsonObjectReader — the battery stays format-agnostic.
+
+struct FormatBatteryCase
+{
+    DB::Cas::FormatId id;
+    std::function<String()> encode;
+    std::function<void(std::string_view)> decode;
+    String golden;
+    /// Optional format-specific construction for the unsupported-version sample. Fixed-size formats
+    /// use this to preserve their physical envelope while growing a version field across a digit
+    /// boundary; ordinary line-oriented formats use the default textual replacement below.
+    std::function<String(std::string_view)> make_future_version = {};
+};
+
+/// Canonical object headers track the current compatibility generation. The type remains an
+/// explicit test literal at every call site, so a registry/type mismatch cannot be hidden by a
+/// self-derived expectation.
+inline String currentFormatHeader(std::string_view type)
+{
+    return fmt::format("{{\"type\":\"{}\",\"v\":{}}}\n", type, DB::Cas::currentCompatibilityVersion());
+}
+
+namespace cas_battery_detail
+{
+template <typename F>
+void expectCode(int code, F && f, const String & context)
+{
+    try
+    {
+        f();
+        FAIL() << context << ": expected exception " << code;
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), code) << context << ": " << e.message();
+    }
+}
+}
+
+inline void runFormatBattery(const FormatBatteryCase & c)
+{
+    using namespace DB::Cas;
+    namespace ec = DB::ErrorCodes;
+    const FormatTraits & t = traitsFor(c.id);
+
+    const String stored = c.encode();
+    c.decode(stored); /// round-trip: must not throw
+
+    /// Work on the canonical text (identical to `stored` for raw formats).
+    const String text = openObject(c.id, stored);
+    ASSERT_TRUE(text.starts_with("{\"type\":\"")) << t.type;
+
+    if (!c.golden.empty())
+        EXPECT_EQ(text, c.golden) << "golden text drifted for " << t.type;
+    if (looksZstd(stored) && !c.golden.empty())
+        EXPECT_EQ(stored, sealObject(c.id, c.golden)) << "pinned compressed arm drifted for " << t.type;
+
+    /// Truncation at every line boundary (drop the terminator too) fails closed.
+    for (size_t i = 0; i < text.size(); ++i)
+        if (text[i] == '\n')
+            cas_battery_detail::expectCode(ec::CORRUPTED_DATA,
+                [&] { c.decode(text.substr(0, i)); }, fmt::format("{}: cut at line boundary {}", t.type, i));
+
+    /// Truncation inside line 1.
+    const size_t line1 = text.find('\n');
+    ASSERT_NE(line1, String::npos);
+    for (size_t i = 1; i < line1; i += 3)
+        cas_battery_detail::expectCode(ec::CORRUPTED_DATA,
+            [&] { c.decode(text.substr(0, i)); }, fmt::format("{}: cut inside header at {}", t.type, i));
+
+    /// v+1 gate.
+    const String v_now = fmt::format("\"v\":{}", currentCompatibilityVersion());
+    const String v_next = fmt::format("\"v\":{}", currentCompatibilityVersion() + 1);
+    String future;
+    if (c.make_future_version)
+        future = c.make_future_version(text);
+    else
+    {
+        future = text;
+        future.replace(future.find(v_now), v_now.size(), v_next);
+    }
+    cas_battery_detail::expectCode(ec::UNKNOWN_FORMAT_VERSION, [&] { c.decode(future); },
+        fmt::format("{}: v+1", t.type));
+
+    /// Wrong type: another VALID registered type in the header.
+    const std::string_view other = (t.id == FormatId::PoolMeta) ? "cas_owner" : "cas_pool_meta";
+    String mistyped = text;
+    mistyped.replace(mistyped.find(t.type), t.type.size(), String(other));
+    cas_battery_detail::expectCode(ec::CORRUPTED_DATA, [&] { c.decode(mistyped); },
+        fmt::format("{}: wrong type", t.type));
+
+    /// Leading garbage.
+    cas_battery_detail::expectCode(ec::CORRUPTED_DATA, [&] { c.decode("X" + text); },
+        fmt::format("{}: garbage byte", t.type));
+}
diff --git a/src/Disks/tests/cas_sweep_test_support.h b/src/Disks/tests/cas_sweep_test_support.h
new file mode 100644
index 000000000000..c1b5466a1cb1
--- /dev/null
+++ b/src/Disks/tests/cas_sweep_test_support.h
@@ -0,0 +1,36 @@
+#pragma once
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <base/types.h>
+
+namespace DB::Cas::tests
+{
+
+/// TEST-ONLY variant of the cursor page: plans a page via the production `planManifestCursorPage` and
+/// then exact-token-deletes every nomination immediately, with no source-edge retirement and no
+/// `gc/state` adoption of the retirement. Production deletion always goes through `Gc::fold`'s
+/// orphan_sweep phase instead, which adopts the retirements in the same round CAS before deleting —
+/// this shortcut recreates the accounting hole that path exists to close, so it must never be reached
+/// from a production translation unit.
+inline ManifestSweepResult sweepManifestCursorPageForTest(
+    Pool & store,
+    const String & cursor,
+    uint64_t list_budget,
+    uint64_t delete_budget,
+    GcRoundWorkBudget * work_budget = nullptr)
+{
+    ManifestSweepResult result = planManifestCursorPage(
+        store, cursor, list_budget, delete_budget, /*catalog_recovery_authoritative=*/true, work_budget);
+    for (const ManifestSweepResult::Nomination & nomination : result.nominations)
+    {
+        const DeleteOutcome outcome = store.backend().deleteExact(nomination.key, nomination.token);
+        if (classifyDeleteOutcome(outcome) == DeleteClass::Deleted)
+            ++result.deleted;
+        else
+            ++result.skipped;
+    }
+    return result;
+}
+
+}
diff --git a/src/Disks/tests/cas_test_helpers.h b/src/Disks/tests/cas_test_helpers.h
new file mode 100644
index 000000000000..9947c5899edb
--- /dev/null
+++ b/src/Disks/tests/cas_test_helpers.h
@@ -0,0 +1,1994 @@
+#pragma once
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/ObjectStorages/IObjectStorage.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
+#include <Common/Exception.h>
+#include <Common/SipHash.h>
+#include <Common/thread_local_rng.h>
+
+#include <base/hex.h>
+#include <city.h>
+
+#include <gtest/gtest.h>
+#include <IO/HashingReadBuffer.h>
+#include <IO/ReadBufferFromMemory.h>
+/// For `ChunkFaultBackend`'s `DefiniteFailure` mode, which needs a real S3-classified error, and for
+/// the ambiguity it raises otherwise.
+#include <IO/S3Common.h>
+#include <Poco/Exception.h>
+
+#include <algorithm>
+#include <atomic>
+#include <condition_variable>
+#include <filesystem>
+#include <map>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <string>
+#include <unistd.h>
+#include <vector>
+
+/// Per-TU extern declarations for the `ContentAddressedSetting` entries this header's helpers use --
+/// the established pattern for `BaseSettings`-derived classes in this codebase (see e.g.
+/// `RegisterDiskCache.cpp`'s `namespace FileCacheSetting` block): the entries are DEFINED once in
+/// `ContentAddressedSettings.cpp`, and each consumer TU declares only the ones it references.
+namespace DB::ContentAddressedSetting
+{
+    extern const ContentAddressedSettingsString server_root_id;
+    extern const ContentAddressedSettingsString scratch_path;
+}
+
+/// Same per-TU pattern for the error codes this header's fault backends raise (`ChunkFaultBackend`'s
+/// non-S3 build of the `Definite` mode).
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+
+namespace DB::Cas::tests
+{
+
+/// Bring up the server-wide blob upload pool (stage-1 §1) if it is not already up, so any test that
+/// drives a `ContentAddressedTransaction` commit -- whose `uploadPendingBlobs` fans out on this pool --
+/// finds it initialized. ROBUST (init-if-not-initialized, NOT `call_once`): the raw-lifecycle suite in
+/// `gtest_cas_blob_upload_pool.cpp` deliberately shuts the pool down, so a `call_once` helper would fail
+/// to bring it back for a later test. A global test-event listener (`gtest_cas_blob_upload_pool_env.cpp`)
+/// calls this before every test, which is what makes it robust to test ordering.
+inline void ensureBlobUploadPoolForTest(size_t size = 8)
+{
+    if (!DB::Cas::blobUploadPoolInitializedForTest())
+        DB::Cas::initializeBlobUploadPool(size);
+}
+
+
+/// Minimal `ContentAddressedSettings` for a direct-construction gtest fixture: sets only
+/// `server_root_id` and `scratch_path` (the two values every positional-ctor call site used to pass
+/// explicitly) and validates, so the cached enum-valued accessors (`stagingBackend`, `blobHashAlgo`,
+/// `partFolderValidate`) are populated from their (default) string settings exactly as the disk-factory
+/// path would populate them. Callers that need a non-default setting (e.g. `staging_backend=s3`) apply
+/// the override via `settings[ContentAddressedSetting::x] = value;` and re-run `settings.validate()`
+/// themselves before constructing.
+inline DB::ContentAddressedSettings makeSettingsForTest(const std::string & server_root_id, const std::filesystem::path & scratch_path)
+{
+    DB::ContentAddressedSettings settings;
+    settings[DB::ContentAddressedSetting::server_root_id] = server_root_id;
+    settings[DB::ContentAddressedSetting::scratch_path] = scratch_path.string();
+    settings.validate();
+    return settings;
+}
+
+/// Run `fn`, expect a DB::Exception with EXACTLY `expected_code` (CORRUPTED_DATA-vs-NOT_IMPLEMENTED
+/// is part of the fail-closed contract: an unknown future format must be NOT_IMPLEMENTED, never
+/// misreported as corruption).
+template <typename F>
+void expectThrowsCode(int expected_code, F && fn)
+{
+    try
+    {
+        fn();
+        FAIL() << "expected DB::Exception";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), expected_code);
+    }
+}
+
+/// Build a `LocalObjectStorage` rooted at a fresh, unique temporary directory (one per call).
+///
+/// Used by the unit tests that exercise the `Cas::Backend` seam against a real on-disk object storage
+/// (the `EmulatedSingleProcess` adapter mode and the capability probe). For `LocalObjectStorage` the
+/// object key IS the local path verbatim, so the unique root keeps every test instance isolated even
+/// under the parallel gtest runner.
+inline DB::ObjectStoragePtr makeLocalObjectStorageForTest()
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_unit_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    return std::make_shared<DB::LocalObjectStorage>(std::move(settings));
+}
+
+/// Anchor a key under an object storage's own root, for the `Mode::Native` tests.
+///
+/// `Mode::Native` uses a key VERBATIM as the physical `LocalObjectStorage` path — no root-prefix
+/// mapping the way `EmulatedSingleProcess`'s `emuPath` does (`LocalObjectStorage::writeObject`/
+/// `readObject` pass `object.remote_path` straight through). A bare relative key like `"some/key"`
+/// would therefore resolve relative to the TEST PROCESS's working directory rather than the
+/// backend's own unique temp root, leaking a real file on disk that outlives the run and that a
+/// later run then observes as pre-existing state. Worse for an assertion of ABSENCE: it answers
+/// "absent" for a reason that has nothing to do with the property under test.
+inline String nativeKeyUnder(const DB::ObjectStoragePtr & storage, const String & suffix)
+{
+    String root = storage->getCommonKeyPrefix();
+    while (!root.empty() && root.back() == '/')
+        root.pop_back();
+    return root + "/" + suffix;
+}
+
+/// ---- on-storage write fixtures (shared by the Pool read/lifecycle/build tests, Tasks 9-13) ----
+///
+/// These produce objects through the SAME codecs the Pool reads — the documented on-storage
+/// interface, not white-box pokes — so a test asserts a real round trip across the format boundary.
+
+/// CityHash128 of bytes, composed into the canonical lowercase-hex id.
+inline String hexOf(const String & bytes)
+{
+    return getHexUIntLowercase(CityHash_v1_0_2::CityHash128(bytes.data(), bytes.size()));
+}
+
+/// The POOL-WIDE streaming content hash (the production `HashingWriteBuffer` convention: chunked
+/// CityHash128, block = DBMS_DEFAULT_HASHING_BLOCK_SIZE). Tests that exercise the copy-forward
+/// VERIFICATION path must mint blob ids with THIS — the plain `idOf`/`u128Of` below are a
+/// test-local convention (fine everywhere hashes are opaque; refused by the verifier).
+inline String streamingHexOf(const String & payload)
+{
+    DB::ReadBufferFromMemory in(payload.data(), payload.size());
+    DB::HashingReadBuffer hashing(in);
+    hashing.ignoreAll();
+    return getHexUIntLowercase(hashing.getHash());
+}
+
+/// The content id of `bytes` as a UInt128 — definitionally consistent with `idOf` (parses the same hex).
+inline DB::UInt128 u128Of(const String & bytes)
+{
+    return DB::Cas::hexToU128(hexOf(bytes));
+}
+
+/// The content id of `bytes` as a `BlobRef` (CityHash128 — every test pool's default write algo).
+inline DB::Cas::BlobRef idOf(const String & bytes)
+{
+    return DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(bytes))};
+}
+
+/// Write a Blob object: a fixed-length (blob_header_len) envelope followed by the raw payload, keyed
+/// by content. Mirrors what PartWriteTxn::putBlob will emit (Task 11).
+inline DB::Cas::BlobRef writeBlobRaw(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const String & payload,
+    uint64_t blob_header_len, [[maybe_unused]] const DB::UInt128 & domain_id)
+{
+    const DB::Cas::BlobRef id = idOf(payload);
+
+    /// v3 envelope: domain_id/hash_algo dropped (identity is the content key); the `domain_id` param
+    /// is kept for call-site compatibility but no longer stamped.
+    DB::Cas::EnvelopeHeader header;
+    header.kind = DB::Cas::ObjectKind::Blob;
+    header.incarnation_tag = DB::UInt128(0x1234);
+    header.build_id = DB::UInt128(0x5678);
+
+    const String head = DB::Cas::encodeEnvelopeHeader(header, static_cast<uint32_t>(blob_header_len));
+    backend.putIfAbsent(layout.blobKey(id), head + payload);
+    return id;
+}
+
+/// Forward declaration: `appendOwnerEvent` (below) calls `registerNamespaceRaw`, which after Task 4
+/// is a no-op (LIST-based discovery needs no explicit registration) defined further down.
+inline void registerNamespaceRaw(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns);
+
+/// Write a part-manifest body object directly via the manifest codec, exactly as PartWriteTxn::stageManifest
+/// emits it. Returns the ManifestId. Used by GC fold/retire/fsck tests to stage owner targets.
+inline DB::Cas::ManifestId writeManifestRaw(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+    const DB::Cas::RootNamespace & ns, const DB::Cas::ManifestRef & ref,
+    const std::vector<DB::Cas::ManifestEntry> & entries)
+{
+    const DB::Cas::ManifestId id{ns, ref};
+    DB::Cas::PartManifest body;
+    body.ref = ref;
+    body.root_namespace_id = ns;
+    body.entries = entries;
+    body.payload_digest = DB::Cas::computePayloadDigest(body);
+    backend.putIfAbsent(layout.manifestKey(id),
+        DB::Cas::sealObject(DB::Cas::FormatId::PartManifest, DB::Cas::encodePartManifest(body)));
+    return id;
+}
+
+/// A blob ManifestEntry referencing `hash` at `path` (size 1, the GC fold counts edges, not bytes).
+inline DB::Cas::ManifestEntry blobEntryFor(const String & path, const DB::UInt128 & hash, uint64_t size = 1)
+{
+    DB::Cas::ManifestEntry e;
+    e.path = path;
+    e.placement = DB::Cas::EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)};
+    e.blob_size = size;
+    return e;
+}
+
+/// Forward declarations of the ref snapshot+log raw fixtures defined further down (they emit the
+/// snapshot+log objects GC and recovery actually read); the seeding wrappers below emit through them.
+inline void writeRefLogTxnRaw(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RefLogTxn & txn);
+namespace fixture
+{
+    inline DB::Cas::NamespaceLifeId fixtureLife(const DB::Cas::RootNamespace & ns);
+}
+inline void publishRecoverableCkptForSemanticWrapper(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::Cas::RefTxnId & txn_id);
+inline DB::Cas::RefOp namespaceBirthOp();
+inline std::vector<DB::Cas::RefOp> publishCommittedOps(
+    const String & ref_name, const DB::Cas::ManifestRef & manifest_ref);
+
+/// One `owner_transition` op built from an optional old/new `RefOwnerBinding` (removal = old set / new
+/// unset; add-precommit = new set / old unset; promote = both set naming the SAME manifest).
+inline DB::Cas::RefOp ownerTransitionOp(
+    std::optional<DB::Cas::RefOwnerBinding> old_binding, std::optional<DB::Cas::RefOwnerBinding> new_binding)
+{
+    DB::Cas::RefOp op;
+    op.kind = DB::Cas::RefOpKind::OwnerTransition;
+    op.old_binding = std::move(old_binding);
+    op.new_binding = std::move(new_binding);
+    return op;
+}
+
+/// Seed ONE ref-log transaction directly into a table's `_log/` stream -- the snapshot+log replacement
+/// for the removed mutable-shard `appendOwnerEvent`. LIST the table's ref prefix, find the greatest
+/// existing log/snapshot `ref_sequence` (and whether ANY log or snapshot exists at all), prepend a
+/// `namespace_birth` op iff the table has none yet, allocate `txn_id = {writer_epoch=1, greatest+1}`,
+/// and write `RefLogTxn{ns, txn_id, ops}` (no `prev_epoch_seal` -- this fixture never crosses an
+/// epoch transition) via `writeRefLogTxnRaw`. Returns the allocated `ref_sequence`.
+/// `ops` must form a REPLAY-VALID transaction: `fsck`/recovery replay them through the same state
+/// machine the writer uses, and the GC edge extractor reads their manifest edges. The bytes are real
+/// wire-format (the same codec `Pool`'s recovery reads) -- never hand-rolled.
+inline uint64_t appendRefLogSeed(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+    const DB::Cas::RootNamespace & ns, std::vector<DB::Cas::RefOp> ops)
+{
+    /// Stage B (Task 4-C): resolve to whichever life is ALREADY on record (real production birth or the
+    /// sentinel), exactly as `writeRefLogTxnRaw` below now does -- otherwise this scan can miss a REAL
+    /// incarnation's existing log/snap objects, wrongly conclude the table has none, and prepend a second
+    /// `namespaceBirthOp` on top of a namespace that already has one.
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(backend, layout, ns).value_or(fixture::fixtureLife(ns));
+    const String prefix = layout.namespaceStreamPrefix(life);
+    uint64_t greatest_seq = 0;
+    bool any_log_or_snap = false;
+    String cursor;
+    while (true)
+    {
+        const DB::Cas::ListPage page = backend.list(prefix, cursor, /*limit=*/1000);
+        for (const DB::Cas::ListedKey & lk : page.keys)
+        {
+            const auto parsed = layout.parseRefObjectKey(lk.key);
+            if (!parsed)
+                continue;
+            if (parsed->kind == DB::Cas::RefObjectKind::Log || parsed->kind == DB::Cas::RefObjectKind::Snap)
+            {
+                any_log_or_snap = true;
+                greatest_seq = std::max(greatest_seq, parsed->txn_id.ref_sequence);
+            }
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+
+    if (!any_log_or_snap)
+        ops.insert(ops.begin(), namespaceBirthOp());
+
+    DB::Cas::RefLogTxn txn;
+    txn.ns = ns.string();
+    txn.txn_id = DB::Cas::RefTxnId{/*writer_epoch=*/1, /*ref_sequence=*/greatest_seq + 1};
+    txn.ops = std::move(ops);
+    writeRefLogTxnRaw(backend, layout, txn);
+    return txn.txn_id.ref_sequence;
+}
+
+/// Append ONE `owner_transition` op as a standalone ref-log transaction. `shard` is ignored (the
+/// immutable ref model has no per-shard journal); it stays in the signature so existing shard-passing
+/// callers compile unchanged. Returns the allocated `ref_sequence`.
+inline uint64_t appendOwnerEvent(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+    const DB::Cas::RootNamespace & ns, uint64_t /*shard*/,
+    std::optional<DB::Cas::RefOwnerBinding> old_binding,
+    std::optional<DB::Cas::RefOwnerBinding> new_binding)
+{
+    return appendRefLogSeed(backend, layout, ns, {ownerTransitionOp(std::move(old_binding), std::move(new_binding))});
+}
+
+/// Publish a committed ref over `ref_name` (no old unless `old_ref` set). Emits a REPLAY-VALID
+/// transaction: an optional owner-removal of the old committed binding, then add-precommit + promote of
+/// the new manifest (spec §State Transitions has no direct "add committed" shape). Edges: -1(old)+1(new)
+/// or +1(new). Returns the allocated `ref_sequence`.
+inline uint64_t publishCommittedTransition(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const String & ref_name, std::optional<DB::Cas::ManifestRef> old_ref, const DB::Cas::ManifestRef & new_ref,
+    uint64_t /*shard*/ = 0)
+{
+    std::vector<DB::Cas::RefOp> ops;
+    if (old_ref)
+        ops.push_back(ownerTransitionOp(
+            DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Committed, ref_name, *old_ref}, std::nullopt));
+    const std::vector<DB::Cas::RefOp> commit_ops = publishCommittedOps(ref_name, new_ref);
+    ops.insert(ops.end(), commit_ops.begin(), commit_ops.end());
+    const uint64_t sequence = appendRefLogSeed(backend, layout, ns, std::move(ops));
+    publishRecoverableCkptForSemanticWrapper(backend, layout, ns, RefTxnId{1, sequence});
+    return sequence;
+}
+
+/// Drop a committed ref (old committed / new none). Edge -1. Returns the allocated `ref_sequence`.
+inline uint64_t dropRefTransition(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const String & ref_name, const DB::Cas::ManifestRef & old_ref, uint64_t /*shard*/ = 0)
+{
+    const uint64_t sequence = appendRefLogSeed(backend, layout, ns,
+        {ownerTransitionOp(DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Committed, ref_name, old_ref}, std::nullopt)});
+    publishRecoverableCkptForSemanticWrapper(backend, layout, ns, RefTxnId{1, sequence});
+    return sequence;
+}
+
+/// Add a precommit binding (optional owner-removal of a stale committed manifest, then add-precommit of
+/// the new manifest). Edge -1(old)+1(new) or +1(new). `build_id` is dropped (RefLog bindings carry no
+/// build_id; build identity lives in `manifest_ref`). Returns the allocated `ref_sequence`.
+inline uint64_t addPrecommitTransition(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::UInt128 & /*build_id*/, const String & final_ref_name, std::optional<DB::Cas::ManifestRef> old_ref,
+    const DB::Cas::ManifestRef & new_ref, uint64_t /*shard*/ = 0)
+{
+    std::vector<DB::Cas::RefOp> ops;
+    if (old_ref)
+        ops.push_back(ownerTransitionOp(
+            DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Committed, final_ref_name, *old_ref}, std::nullopt));
+    ops.push_back(ownerTransitionOp(
+        std::nullopt, DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Precommit, final_ref_name, new_ref}));
+    const uint64_t sequence = appendRefLogSeed(backend, layout, ns, std::move(ops));
+    publishRecoverableCkptForSemanticWrapper(backend, layout, ns, RefTxnId{1, sequence});
+    return sequence;
+}
+
+/// Promote a precommit to committed at the SAME manifest_ref (old=Precommit, new=Committed). No edge
+/// (net-zero owner move). `build_id` is dropped. Returns the allocated `ref_sequence`.
+inline uint64_t promoteTransition(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::UInt128 & /*build_id*/, const String & final_ref_name, const DB::Cas::ManifestRef & ref,
+    uint64_t /*shard*/ = 0)
+{
+    const uint64_t sequence = appendRefLogSeed(backend, layout, ns,
+        {ownerTransitionOp(
+            DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Precommit, final_ref_name, ref},
+            DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Committed, final_ref_name, ref})});
+    publishRecoverableCkptForSemanticWrapper(backend, layout, ns, RefTxnId{1, sequence});
+    return sequence;
+}
+
+/// Exact-token delete of a manifest body (HEAD then deleteExact). No-op when absent.
+inline void deleteManifestBody(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::ManifestId & id)
+{
+    const String key = layout.manifestKey(id);
+    const DB::Cas::HeadResult h = backend.head(key);
+    if (h.exists)
+        backend.deleteExact(key, h.token);
+}
+
+/// Formerly wrote the namespace into `gc/registry`. Real write helpers now admit the authoritative
+/// catalog row themselves, so this legacy fixture hook has no independent registration work.
+inline void registerNamespaceRaw(
+    DB::Cas::Backend & /*backend*/, const DB::Cas::Layout & /*layout*/, const DB::Cas::RootNamespace & /*ns*/)
+{
+    /// No-op: Task 4 deleted the registry; `cas/ref_catalog` is now the discovery authority.
+}
+
+/// Encode a CAGS document carrying only {round} — everything else defaulted. Callers that only care
+/// about this field (e.g. `injectRetire`) use this shorthand.
+inline String encodeMinimalGcState(uint64_t round)
+{
+    DB::Cas::GcState state;
+    state.round = round;
+    return DB::Cas::encodeGcState(state);
+}
+
+/// Inject condemned bookkeeping + gc/state directly (bypassing a real GC round) so a test can seed the
+/// GC ledger's condemned state at an arbitrary round. Retired-in-snapshot: the condemned entries are
+/// seeded the way a real round leaves them — as `kCondemned` sentinel rows inside an adopted fold seal's
+/// shard run (there is no separate retired-list object). A synthetic +edge/-edge pair nets each blob to
+/// in-degree 0 and a `seed_head` replays the captured token/size so the fold mints the `kCondemned` row.
+/// Also sets {round} on gc/state. Entries carry a `condemn_round` (default 0 → uses `round`); callers
+/// pass fresh (non-pending) condemns. An empty `entries` set just advances {round}.
+inline void injectRetire(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+    uint64_t round, uint64_t shard, std::vector<DB::Cas::RetiredEntry> entries)
+{
+    DB::Cas::GcState gc_state;
+    const DB::Cas::HeadResult head = backend.head(layout.gcStateKey());
+    if (head.exists)
+        gc_state = DB::Cas::decodeGcState(backend.get(layout.gcStateKey())->bytes);
+    gc_state.round = round;
+
+    if (!entries.empty())
+    {
+        const uint64_t generation = 1;
+        const uint64_t attempt = 1;
+        uint64_t condemn_round = round;
+        std::unordered_map<DB::Cas::BlobRef, DB::Cas::HeadResult, DB::Cas::BlobRefHash> seeded;
+        std::vector<DB::Cas::BlobDelta> synth;
+        synth.reserve(entries.size() * 2);
+        for (const DB::Cas::RetiredEntry & e : entries)
+        {
+            if (e.condemn_round)
+                condemn_round = e.condemn_round;
+            seeded.emplace(e.ref, DB::Cas::HeadResult{.exists = true, .size = e.size, .token = e.token, .attributes = {}});
+            synth.push_back(DB::Cas::BlobDelta{.ref = e.ref, .source_id = DB::UInt128{1}, .remove = false});
+            synth.push_back(DB::Cas::BlobDelta{.ref = e.ref, .source_id = DB::UInt128{1}, .remove = true});
+        }
+        const auto seed_head = [&seeded](const DB::Cas::BlobRef & h) -> std::optional<DB::Cas::HeadResult>
+        {
+            const auto it = seeded.find(h);
+            return it == seeded.end() ? std::nullopt : std::optional<DB::Cas::HeadResult>(it->second);
+        };
+        std::vector<DB::Cas::RunRef> out;
+        DB::Cas::foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, generation, attempt,
+            shard, std::move(synth), out, /*current_round*/0, condemn_round, seed_head,
+            /*peek_head*/{}, /*confirm_condemned_marker*/{},
+            /*out_retired*/nullptr, /*suppress_destructive*/false);
+
+        DB::Cas::CasFoldSeal seal;
+        seal.generation = generation;
+        for (DB::Cas::RunRef & r : out)
+            seal.blob_target_runs.push_back(std::move(r));
+        /// Totality over gc_shards so a later real round's graduation/carry reads it zero-I/O.
+        const uint64_t gc_shards = gc_state.gc_shards ? gc_state.gc_shards : 1;
+        for (uint64_t s = 0; s < gc_shards; ++s)
+            seal.condemned_summary[s] = DB::Cas::CondemnedSummary{};
+        DB::Cas::CondemnedSummary cs;
+        cs.condemned_total = entries.size();
+        cs.oldest_nonpending_condemn_round = condemn_round;
+        seal.condemned_summary[shard] = cs;
+        backend.putIfAbsent(layout.foldSealKey(generation, attempt), DB::Cas::encodeFoldSeal(seal));
+
+        gc_state.snap_generation = generation;
+        gc_state.snap_attempt = attempt;
+    }
+
+    const String state = DB::Cas::encodeGcState(gc_state);
+    if (!head.exists)
+        backend.putIfAbsent(layout.gcStateKey(), state);
+    else
+        backend.putOverwrite(layout.gcStateKey(), state, head.token);
+}
+
+/// Adopt a fold seal carrying a given per-gc-shard `condemned_summary` (retired-in-snapshot T4) and point
+/// gc/state at it (snap_generation / snap_attempt / gc_shards), bypassing a real GC round. If a seal
+/// already exists at (generation, attempt) it is overwritten with the new summary (its other fields are
+/// preserved); otherwise a fresh minimal seal is created. Read-modify-CAS on gc/state preserves the lease.
+/// Used by graduationDue tests to drive the zero-I/O signal directly off a controlled seal.
+inline void injectCondemnedSummarySeal(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+    uint64_t generation, uint64_t attempt, uint64_t gc_shards,
+    const std::map<uint64_t, DB::Cas::CondemnedSummary> & summary)
+{
+    const String seal_key = layout.foldSealKey(generation, attempt);
+    DB::Cas::CasFoldSeal seal;
+    const auto existing = backend.get(seal_key);
+    if (existing)
+        seal = DB::Cas::decodeFoldSeal(existing->bytes);
+    else
+        seal.parent_generation = generation ? generation - 1 : 0;
+    seal.generation = generation;
+    seal.condemned_summary = summary;
+    const String seal_bytes = DB::Cas::encodeFoldSeal(seal);
+    if (existing)
+        backend.putOverwrite(seal_key, seal_bytes, existing->token);
+    else
+        backend.putIfAbsent(seal_key, seal_bytes);
+
+    DB::Cas::GcState gc_state;
+    const DB::Cas::HeadResult head = backend.head(layout.gcStateKey());
+    if (head.exists)
+        gc_state = DB::Cas::decodeGcState(backend.get(layout.gcStateKey())->bytes);
+    gc_state.gc_shards = gc_shards;
+    gc_state.snap_generation = generation;
+    gc_state.snap_attempt = attempt;
+    const String state = DB::Cas::encodeGcState(gc_state);
+    if (!head.exists)
+        backend.putIfAbsent(layout.gcStateKey(), state);
+    else
+        backend.putOverwrite(layout.gcStateKey(), state, head.token);
+}
+
+/// Whether blob `hash` is absent from the backend (its exact-token content object is gone).
+inline bool blobAbsent(DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::UInt128 & hash)
+{
+    return !backend.head(layout.blobKey(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)})).exists;
+}
+
+/// ONE round that is allowed to RECLAIM -- the name is the point, so that grepping for the tests whose
+/// subject is reclamation finds exactly them.
+///
+/// The policy is spelled out rather than defaulted so that grepping this name finds every test whose
+/// subject is reclamation, and so that a future change to the default cannot silently change what those
+/// tests mean. It says the same thing the production default says (`UniversePolicy`); a test whose
+/// subject is a SUPPRESSOR passes `StageA_Suppressed` explicitly instead.
+inline DB::Cas::RoundReport runRegularRoundReclaiming(DB::Cas::Gc & gc)
+{
+    return gc.runRegularRound({}, /*allow_steal*/true, DB::Cas::UniversePolicy::Authoritative);
+}
+
+/// Reclaim loop (the canonical retired-cursor pipeline driver): run regular rounds, renewing the store's
+/// own heartbeat after each round (`renewWatermarkOnce` — keeps the lease + build-watermark floor
+/// current; unrelated to graduation, which paces on GC rounds alone). A blob condemned at round K is
+/// deleted by round K+2 (condemn at K -> graduate to delete_pending at K+1, unconditionally -> physical
+/// delete at K+2). Returns true as soon as the blob became absent. Reclamation is the whole point of the
+/// loop, so every round it drives is an authoritative one.
+inline bool runRoundsUntilAbsent(
+    const DB::Cas::PoolPtr & store, DB::Cas::Gc & gc, DB::Cas::Backend & backend,
+    const DB::Cas::Layout & layout, const DB::UInt128 & hash, int max_rounds = 8)
+{
+    for (int i = 0; i < max_rounds; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+        if (blobAbsent(backend, layout, hash))
+            return true;
+    }
+    return blobAbsent(backend, layout, hash);
+}
+
+/// The CURRENT condemned entries for `shard`, read from the adopted fold seal's `blob_target_runs`
+/// (retired-in-snapshot T4): the round no longer writes a separate retired-list object — condemned
+/// entries RIDE the source-edge run as `kCondemned` sentinel rows at the zero-sentinel key. This reads
+/// the seal at (snap_generation, snap_attempt), opens every run for `shard`, and reconstructs the
+/// `RetiredEntry` shape (hash from the run key, the rest from the decoded `CondemnedRow`). Empty when
+/// gc/state / the seal / the runs are absent. Used by ack-floor tests to assert pending/condemn state.
+inline std::vector<DB::Cas::RetiredEntry> currentRetiredSet(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, uint64_t shard)
+{
+    const auto st = backend.get(layout.gcStateKey());
+    if (!st)
+        return {};
+    const DB::Cas::GcState gc_state = DB::Cas::decodeGcState(st->bytes);
+    if (gc_state.snap_generation == 0)
+        return {};
+    const auto seal_bytes = backend.get(layout.foldSealKey(gc_state.snap_generation, gc_state.snap_attempt));
+    if (!seal_bytes)
+        return {};
+    const DB::Cas::CasFoldSeal seal = DB::Cas::decodeFoldSeal(seal_bytes->bytes);
+
+    std::vector<DB::Cas::RetiredEntry> out;
+    for (const DB::Cas::RunRef & run : seal.blob_target_runs)
+    {
+        if (run.shard != shard)
+            continue;
+        auto r = DB::Cas::openSourceEdgeRun(backend, run.key);
+        String k;
+        String p;
+        while (r.next(k, p))
+        {
+            if (p.empty() || p[0] != DB::Cas::kCondemned)
+                continue;
+            DB::Cas::BlobRef ref;
+            DB::UInt128 source_id{};
+            DB::Cas::SourceEdgeKeyCodec::parse(k, ref, source_id);   // throws CORRUPTED_DATA on malformed (fail-closed)
+            const DB::Cas::CondemnedRow row = DB::Cas::decodeCondemnedRow(p);
+            out.push_back(DB::Cas::RetiredEntry{
+                .kind = DB::Cas::ObjectKind::Blob,
+                .ref = ref,
+                .token = row.token,
+                .size = row.size,
+                .condemn_round = row.condemn_round,
+                .delete_pending = row.delete_pending,
+                .marker_confirmed = row.marker_confirmed});
+        }
+    }
+    return out;
+}
+
+/// True iff ANY gc-shard's adopted-seal run still holds a `kCondemned` row — the ack-floor deletion
+/// pipeline is in flight while this is true (retired-in-snapshot T4 replacement for the old
+/// "iterate gc/state.retired_refs" probe). `gc_shards` is read from gc/state when 0 is passed.
+inline bool anyCondemnedInSeal(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, uint64_t gc_shards = 0)
+{
+    const auto st = backend.get(layout.gcStateKey());
+    if (!st)
+        return false;
+    const DB::Cas::GcState gc_state = DB::Cas::decodeGcState(st->bytes);
+    const uint64_t shards = gc_shards ? gc_shards : gc_state.gc_shards;
+    for (uint64_t shard = 0; shard < shards; ++shard)
+        if (!currentRetiredSet(backend, layout, shard).empty())
+            return true;
+    return false;
+}
+
+/// Displace a blob's incarnation out-of-band (as a racing writer would): GET it, mint a fresh
+/// incarnation_tag in its envelope header (preserving header_len + payload), putOverwrite against the
+/// current token, and return the NEW token. Used to drive the W-REVALIDATE adopt branch (current token
+/// differs from the writer's stale observation).
+inline DB::Cas::Token displaceObjectToken(
+    DB::Cas::Backend & backend, const String & key, DB::Cas::ObjectKind kind)
+{
+    const auto got = backend.get(key);
+    if (!got)
+        throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS, "displaceObjectToken: object {} absent", key);
+
+    DB::Cas::EnvelopeHeader header =
+        DB::Cas::decodeEnvelopeHeader(got->bytes, got->bytes.size(), kind);
+    /// A fresh, distinct incarnation_tag forces a distinct body so the displaced token differs.
+    header.incarnation_tag = header.incarnation_tag + DB::UInt128(1);
+    /// Re-encode at the SAME header length the object was decoded with (the v3 pad target).
+    const String new_head = DB::Cas::encodeEnvelopeHeader(header, header.header_len);
+    const String body = new_head + got->bytes.substr(header.header_len);
+
+    return backend.putOverwrite(key, body, got->token).token;
+}
+
+inline DB::Cas::Token displaceBlobToken(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::BlobRef & id)
+{
+    return displaceObjectToken(backend, layout.blobKey(id), DB::Cas::ObjectKind::Blob);
+}
+
+/// ---- GC-core (Phase 1d) test helpers over the part-manifest model ----
+
+/// Open a Pool over `backend`.
+///
+/// `gc_fold_max_defer_rounds` defaults to the PoolConfig default (8) -- unchanged behaviour for every
+/// existing caller. A test that drives MANY consecutive genuinely-idle `runRegularRound` calls and
+/// asserts each one performs a full fold (round/generation advance, trim/sweep/retention) -- exactly
+/// what Phase-4 Lever A (spec 2026-07-06-cas-gc-round-skip-unchanged) is designed to skip -- passes 0
+/// here to force fold-every-round (shouldDeferRound's liveness bound: rounds_since_last_fold(0) >= 0
+/// is always true).
+inline DB::Cas::PoolPtr openPoolForTest(
+    std::shared_ptr<DB::Cas::InMemoryBackend> backend, uint64_t gc_fold_max_defer_rounds = 8)
+{
+    return DB::Cas::Pool::open(std::move(backend),
+        DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                            .gc_fold_max_defer_rounds = gc_fold_max_defer_rounds});
+}
+
+/// Seed the mandatory control objects for an already-existing pool so a subsequent `Pool::open`
+/// VALIDATES a restart instead of bootstrapping a fresh one. Recovery/replay tests seed ref-log,
+/// snapshot, manifest, or gc-state residue directly into a bare backend; in production such residue
+/// only ever exists inside a pool whose FIRST open already minted `_pool_meta` and explicitly
+/// initialized `cas/ref_catalog`. Task 7's zero-write bootstrap check (spec §2 [C4][D2],
+/// `probePoolBootstrapResidual`) REFUSES to bootstrap over residual data, so a raw restart fixture must
+/// establish both mandatory objects itself rather than rely on a production fallback.
+///
+/// Idempotent: `createOrValidate` validates an existing `_pool_meta`. The catalog initializer is
+/// deliberately narrower: it accepts only a canonical EMPTY conflict, so raw recovery fixtures that
+/// have already populated their catalog must mandatory-read and validate it instead of re-running a
+/// new-pool initializer. The default `blob_header_len`/`blob_hash_algo` match `PoolConfig`'s defaults,
+/// so a later `Pool::open` with a default config validates cleanly.
+inline void seedPoolMetaForRestart(
+    DB::Cas::Backend & backend, const String & pool_prefix = "p", uint64_t gc_shards = 1)
+{
+    const DB::Cas::Layout layout(pool_prefix);
+    DB::Cas::PoolMeta::createOrValidate(
+        backend, layout, /*blob_header_len=*/256, gc_shards,
+        DB::Cas::BlobHashAlgo::CityHash128, /*allow_new=*/false, /*allow_mint=*/true);
+    if (!backend.get(layout.refCatalogKey()))
+        DB::Cas::CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    else
+        (void)DB::Cas::CasRefCatalog::read(backend, layout);
+}
+
+/// Write a blob object (envelope + payload) addressed by `hash`, so a HEAD returns a token. The bytes
+/// are arbitrary (GC never reads them); the hash is what the manifest entry references.
+inline void writeBlobBody(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::UInt128 & hash,
+    uint64_t blob_header_len = 256)
+{
+    DB::Cas::EnvelopeHeader header;
+    header.kind = DB::Cas::ObjectKind::Blob;
+    header.incarnation_tag = DB::UInt128(0x1234);
+    header.build_id = DB::UInt128(0x5678);
+    const String head = DB::Cas::encodeEnvelopeHeader(header, static_cast<uint32_t>(blob_header_len));
+    backend.putIfAbsent(layout.blobKey(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)}), head + String("x"));
+}
+
+/// Write a raw blob body (payload written verbatim, no envelope) — the raw-body-refinement shape
+/// (Phase B): the meta descriptor (via the ops layer below) carries all state, the body carries none.
+inline void writeRawBlobBody(DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+                             const DB::UInt128 & hash, const String & payload)
+{
+    backend.casPut(layout.blobKey(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)}), payload, std::nullopt);
+}
+
+/// These `UInt128`-hash meta-op wrappers are the pre-mixed-algo 128-bit-only test convenience surface:
+/// every existing caller operates on a 128-bit (`cityHash128`) test pool, so the ref is built at
+/// `CityHash128` here. The shared `.meta` API (Phase 3 T3) is `BlobRef`-keyed directly and derives its
+/// own codec internally — no codec is threaded from here anymore.
+inline DB::Cas::BlobRef legacyMetaTestRef(const DB::UInt128 & hash)
+{
+    return DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)};
+}
+
+/// Create a Clean meta descriptor for `hash` directly in a test backend. This setup helper deliberately
+/// stays usable before a `Pool` is open; production writes use `putMetaIfAbsent` through the controller.
+inline void writeMetaClean(DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+                           const DB::UInt128 & hash, uint64_t size)
+{
+    const DB::Cas::BlobRef ref = legacyMetaTestRef(hash);
+    backend.putIfAbsent(layout.blobMetaKey(ref), DB::Cas::encodeBlobMeta(
+        DB::Cas::BlobMeta{.state = DB::Cas::MetaState::Clean, .condemn_round = 0, .size = size}));
+}
+
+/// Transition an existing meta descriptor to Condemned at `condemn_round`, via a read-modify-CAS on
+/// its current token (asserts the meta exists — a direct test setup helper, not production code).
+inline void condemnMeta(DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+                        const DB::UInt128 & hash, uint64_t condemn_round)
+{
+    const DB::Cas::BlobRef ref = legacyMetaTestRef(hash);
+    const auto lm = DB::Cas::loadMeta(backend, layout, ref);
+    ASSERT_TRUE(lm.has_value());
+    DB::Cas::BlobMeta c = lm->meta;
+    c.state = DB::Cas::MetaState::Condemned;
+    c.condemn_round = condemn_round;
+    backend.putOverwrite(layout.blobMetaKey(ref), DB::Cas::encodeBlobMeta(c), lm->etag);
+}
+
+/// Load the meta descriptor for `hash` via the shared ops layer (nullopt = absent).
+inline std::optional<DB::Cas::LoadedMeta> loadMetaForTest(DB::Cas::Backend & backend,
+                                                          const DB::Cas::Layout & layout, const DB::UInt128 & hash)
+{
+    return DB::Cas::loadMeta(backend, layout, legacyMetaTestRef(hash));
+}
+
+/// The latest GC generation (snap_generation pointer in gc/state), or 0 when absent.
+inline uint64_t currentGenerationOf(DB::Cas::Backend & backend, const DB::Cas::Layout & layout)
+{
+    const auto got = backend.get(layout.gcStateKey());
+    if (!got)
+        return 0;
+    return DB::Cas::decodeGcState(got->bytes).snap_generation;
+}
+
+/// The adopted attempt (snap_attempt pointer in gc/state), or 0 when absent.
+inline uint64_t currentAttemptOf(DB::Cas::Backend & backend, const DB::Cas::Layout & layout)
+{
+    const auto got = backend.get(layout.gcStateKey());
+    if (!got)
+        return 0;
+    return DB::Cas::decodeGcState(got->bytes).snap_attempt;
+}
+
+/// The current seal's `blob_target_runs` filtered to `shard` (2026-07-02 T0: consumers resolve runs
+/// through seal refs, not by key construction). Scans downward from the current generation for the most
+/// recent existing fold seal (mirrors `foldCursorOf`'s reasoning); absent => empty.
+inline std::vector<DB::Cas::RunRef> runsForShard(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, uint64_t shard)
+{
+    const uint64_t gen = currentGenerationOf(backend, layout);
+    const uint64_t attempt = currentAttemptOf(backend, layout);
+    for (uint64_t g = gen; ; --g)
+    {
+        if (const auto got = backend.get(layout.foldSealKey(g, attempt)))
+        {
+            const DB::Cas::CasFoldSeal seal = DB::Cas::decodeFoldSeal(got->bytes);
+            std::vector<DB::Cas::RunRef> out;
+            for (const DB::Cas::RunRef & r : seal.blob_target_runs)
+                if (r.shard == shard)
+                    out.push_back(r);
+            return out;
+        }
+        if (g == 0)
+            return {};
+    }
+}
+
+/// Stream the sealed in-degree run segments `runs` and count the active source edges (`kEdgeActive`
+/// rows) for `ref`. Test-side replacement for the deleted per-blob point query `inDegreeInGeneration`
+/// (codecs-v3 phase 5: a `cas_run` is a sequential NDJSON stream with no random access, so a blob's
+/// in-degree is recomputed by a full stream-and-count rather than a seek). A condemned / zero-marker
+/// row is not an active edge, so it contributes 0 — matching the old point query's semantics.
+inline int64_t inDegreeInRuns(
+    DB::Cas::Backend & backend, const std::vector<DB::Cas::RunRef> & runs, const DB::Cas::BlobRef & ref)
+{
+    int64_t degree = 0;
+    for (const DB::Cas::RunRef & run : runs)
+    {
+        auto r = DB::Cas::openSourceEdgeRun(backend, run.key);
+        String k;
+        String p;
+        while (r.next(k, p))
+        {
+            if (p.empty() || p[0] != DB::Cas::kEdgeActive)
+                continue;
+            DB::Cas::BlobRef row_ref;
+            DB::UInt128 source_id{};
+            DB::Cas::SourceEdgeKeyCodec::parse(k, row_ref, source_id);   // throws CORRUPTED_DATA on malformed (fail-closed)
+            if (row_ref == ref)
+                ++degree;
+        }
+    }
+    return degree;
+}
+
+/// The in-degree of a blob in the current GC generation's sealed run (0 when absent/zeroed).
+inline int64_t inDegreeOf(DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::UInt128 & hash)
+{
+    return inDegreeInRuns(backend, runsForShard(backend, layout, /*shard*/0), legacyMetaTestRef(hash));
+}
+
+/// The single named entry point for the nonproduction CA shapes this test tree constructs directly,
+/// rather than through the production birth/write paths. Every raw fixture below is one of three
+/// deliberate divergences from what production can ever produce, gathered here under one name so a
+/// future change to any of them has exactly one place to change, not every call site that needs it:
+///   1. `fixtureLife` returns a DETERMINISTIC namespace-derived life identity, never a fresh random
+///      mint the way a real birth (`CasRefCatalog::createNamespace`) would -- opaque and catalog-born
+///      in production, but every raw fixture below needs to derive the SAME identity a namespace's
+///      catalog entry will carry before that entry exists, so its writes and a later read agree on
+///      where to look.
+///   2. `admitLive` reaches `Live` with NO `_ckpt` at all. Production only ever reaches `Live` through
+///      `completeCreation`, which publishes `_ckpt` FIRST; this shape is kept deliberately, because
+///      recovery and failure tests need to exercise a `Live` or `Removing` row missing that authority.
+///   3. `writeRefLogRaw` writes ref-log bytes directly at the resolved fixture identity, bypassing the
+///      writer's own birth/append lane entirely -- exercising the on-storage object shape a real writer
+///      would emit without driving a real writer to produce it.
+namespace fixture
+{
+    /// The deterministic identity a raw fixture uses for a namespace before any catalog entry exists:
+    /// a stable hash of the namespace name, so two fixture writes against the same namespace (and a
+    /// later read) always agree on where to look, without needing a catalog entry to agree through.
+    /// Production incarnations are always catalog-minted (`CasRefCatalog::createNamespace`); this is
+    /// deliberately not that, and every raw fixture below depends on it staying stable byte-for-byte.
+    inline DB::Cas::NamespaceLifeId fixtureLife(const DB::Cas::RootNamespace & ns)
+    {
+        UInt128 fixture_incarnation = sipHash128(ns.string().data(), ns.string().size());
+        if (fixture_incarnation == 0)
+            fixture_incarnation = 1;
+        return DB::Cas::NamespaceLifeId::fromCatalogEntry(ns, fixture_incarnation);
+    }
+}
+
+/// Resolve the opaque life id that keys this namespace's single fold-coverage row.
+inline UInt128 catalogLifeIdForTest(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns)
+{
+    const std::optional<DB::Cas::NamespaceLifeId> life =
+        DB::Cas::CasRefCatalog::lifeIfCataloged(backend, layout, ns);
+    chassert(life.has_value());
+    return life->incarnation;
+}
+
+/// Seed the ADOPTED fold seal's catalog-life coverage row for `ns` and point `gc/state` at it, bypassing a
+/// real round. This is the durable fact the sweep's §6 deletion premise reads
+/// (`CasOrphanManifestSweep.cpp`): `cursor` is the namespace's `last_folded_ref_id`, and a manifest of
+/// an epoch-`E` build is deletable only once that cursor sits in an epoch STRICTLY above `E`.
+/// `hold`, when set, makes the row classification 4 — the strict grammar `encodeFoldSeal` enforces in
+/// both directions, so a hold and a non-4 classification cannot be seeded together.
+///
+/// SHARP EDGE, HANDLED HERE SO NO CALLER HAS TO KNOW IT: a fold seal must carry a `condemned_summary`
+/// entry for EVERY shard in `0..gc_shards-1`. A later real round adopts this object as its PARENT and
+/// throws `CORRUPTED_DATA` — "parent fold seal (generation G, attempt A) lacks a condemned_summary
+/// entry for gc-shard N — the seal is not total over gc_shards" — on a seal that is missing one. The
+/// symptom is nowhere near the cause: the round fails at fold time, or (if it fails before taking the
+/// lease) merely reports `acquired_lease == false`, so a test that seeds a partial seal looks like a
+/// leadership problem. This helper fills the map from `gc/state`'s own `gc_shards`, so seeding a
+/// coverage row is safe to combine with real rounds.
+///
+/// One thing it does NOT do: create `gc/state` in a state a first-ever `Gc` round can take the lease
+/// over. `acquireOrRenewLease` only creates-and-owns when `gc/state` is ABSENT, so seeding before the
+/// first round makes that round back off. Seed AFTER the first round (passing that round's
+/// `currentGenerationOf`/`currentAttemptOf`) when a test drives real rounds.
+inline void seedFoldCursorForTest(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    DB::Cas::RefTxnId cursor, std::optional<DB::Cas::RefHold> hold = std::nullopt,
+    uint64_t generation = 1, uint64_t attempt = 1)
+{
+    DB::Cas::NamespaceLifeId life = fixture::fixtureLife(ns);
+    const DB::Cas::CasRefCatalog::Snapshot catalog_cut = DB::Cas::CasRefCatalog::read(backend, layout);
+    const auto catalog_it = std::find_if(
+        catalog_cut.catalog.entries.begin(), catalog_cut.catalog.entries.end(),
+        [&](const DB::Cas::CatalogEntry & entry) { return entry.ns.string() == ns.string(); });
+    if (catalog_it == catalog_cut.catalog.entries.end())
+    {
+        DB::Cas::CatalogEntry entry;
+        entry.ns = ns;
+        entry.state = DB::Cas::NsState::Live;
+        entry.incarnation = fixture::fixtureLife(ns).incarnation;
+        DB::Cas::CasRefCatalog::casAdmitEntry(backend, layout, 1, entry);
+        life = DB::Cas::NamespaceLifeId::fromCatalogEntry(ns, entry.incarnation);
+    }
+    else
+    {
+        life = DB::Cas::NamespaceLifeId::fromCatalogEntry(ns, catalog_it->incarnation);
+    }
+
+    const String seal_key = layout.foldSealKey(generation, attempt);
+    DB::Cas::CasFoldSeal seal;
+    const auto existing = backend.get(seal_key);
+    if (existing)
+        seal = DB::Cas::decodeFoldSeal(existing->bytes);
+    seal.generation = generation;
+
+    DB::Cas::RefCoverage cov;
+    cov.classification = hold ? 4 : 2;
+    cov.last_folded_ref_id = cursor;
+    cov.hold = hold;
+    seal.ref_lives[life.incarnation].coverage = cov;
+
+    DB::Cas::GcState gc_state;
+    const DB::Cas::HeadResult head = backend.head(layout.gcStateKey());
+    if (head.exists)
+        gc_state = DB::Cas::decodeGcState(backend.get(layout.gcStateKey())->bytes);
+
+    /// Totality over `gc_shards` — see the doc comment's SHARP EDGE note for what throws without it.
+    const uint64_t gc_shards = gc_state.gc_shards ? gc_state.gc_shards : 1;
+    for (uint64_t s = 0; s < gc_shards; ++s)
+        seal.condemned_summary.emplace(s, DB::Cas::CondemnedSummary{});
+
+    const String seal_bytes = DB::Cas::encodeFoldSeal(seal);
+    if (existing)
+        backend.putOverwrite(seal_key, seal_bytes, existing->token);
+    else
+        backend.putIfAbsent(seal_key, seal_bytes);
+
+    gc_state.snap_generation = generation;
+    gc_state.snap_attempt = attempt;
+    const String state = DB::Cas::encodeGcState(gc_state);
+    if (!head.exists)
+        backend.putIfAbsent(layout.gcStateKey(), state);
+    else
+        backend.putOverwrite(layout.gcStateKey(), state, head.token);
+}
+
+/// The folded cursor sealed for (ns, shard) by the latest fold seal, or 0 when absent. After a COMPLETE
+/// round the gc/state generation pointer is the recheck's COMPLETION generation (G+2 for a round started
+/// at G), but the fold seal is written at the FOLD generation (G+1) — recheck writes a completion seal,
+/// not a fold seal. So scan downward from the current generation for the most recent existing fold seal.
+inline uint64_t foldCursorOf(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns, uint64_t shard)
+{
+    chassert(shard == 0);
+    const std::optional<DB::Cas::NamespaceLifeId> life =
+        DB::Cas::CasRefCatalog::lifeIfCataloged(backend, layout, ns);
+    if (!life)
+        return 0;
+    const uint64_t gen = currentGenerationOf(backend, layout);
+    const uint64_t attempt = currentAttemptOf(backend, layout);
+    for (uint64_t g = gen; ; --g)
+    {
+        if (const auto got = backend.get(layout.foldSealKey(g, attempt)))
+        {
+            const DB::Cas::CasFoldSeal seal = DB::Cas::decodeFoldSeal(got->bytes);
+            const auto it = seal.ref_lives.find(life->incarnation);
+            /// Snapshot+log ref model: the per-table durable cursor is `last_folded_ref_id` (a RefTxnId).
+            /// Seeds allocate `writer_epoch = 1`, so the `ref_sequence` is the monotone cursor the seeding
+            /// wrappers return and tests compare against.
+            return it != seal.ref_lives.end() ? it->second.coverage.last_folded_ref_id.ref_sequence : 0;
+        }
+        if (g == 0)
+            return 0;
+    }
+}
+
+/// Set a server root's durable floor (so orphan-sweep eligibility can be driven). After the ack-floor
+/// merge the floor rides the mount lease body (`mountKey`), so this seeds a MountLease carrying
+/// `{writer_epoch, min_active}` — exactly what `prefixEligible` reads.
+inline void setWatermarkMinActive(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const String & server_root_id,
+    uint64_t writer_epoch, uint64_t min_active)
+{
+    DB::Cas::MountLease m;
+    m.server_uuid = DB::UInt128(0);
+    m.writer_epoch = writer_epoch;
+    m.min_active = min_active;
+    m.seq = 1;
+    const String key = layout.mountKey(server_root_id);
+    const DB::Cas::HeadResult h = backend.head(key);
+    if (h.exists)
+        backend.putOverwrite(key, DB::Cas::encodeMountLease(m), h.token);
+    else
+        backend.putIfAbsent(key, DB::Cas::encodeMountLease(m));
+}
+
+/// ---- Task 10 ref snapshot+log raw fixtures ----
+/// Mirror the pre-Task-10 `appendOwnerEvent`/`publishRaw` helpers above, but for the new snapshot+log
+/// object layout: write a ref-object body directly via the SAME codecs `Pool`'s recovery reads,
+/// bypassing the writer's own append lane entirely. Used to seed pre-existing table state before a
+/// fresh `Pool` ever touches the namespace (recovery tests), and to control exact keys/bytes
+/// (restart-on-vanish tests).
+
+/// Writes `snapshot` at `_snap/<snapshot_id>.proto` (create-if-absent). Keys at whichever life the
+/// namespace's catalog entry ALREADY names (a prior real birth's random incarnation, or the sentinel
+/// if none exists yet -- see `writeRefLogTxnRaw`'s identical note); does NOT itself admit an entry, so
+/// a namespace this helper is the ONLY writer for stays exactly as invisible to the catalog as it was
+/// before Task 4-C (unchanged from this helper's own pre-existing scope).
+inline void writeRefSnapshotRaw(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RefTableSnapshot & snapshot)
+{
+    const DB::Cas::RootNamespace ns{snapshot.ns};
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(backend, layout, ns).value_or(fixture::fixtureLife(ns));
+    const String key = layout.refSnapshotKey(life, snapshot.snapshot_id);
+    backend.putIfAbsent(key, DB::Cas::sealObject(DB::Cas::FormatId::RefSnapshot, DB::Cas::encodeRefTableSnapshot(snapshot)));
+}
+
+/// Admits `ns` into the catalog as a `Live` entry, IDEMPOTENTLY (a no-op once `ns` already carries
+/// any entry, of any state -- a test that drove one there itself through the real catalog API is left
+/// alone). Pinned to the deterministic `fixture::fixtureLife` incarnation, NOT
+/// `CasRefCatalog::createNamespace`'s fresh-random mint: every raw fixture below keys its ref-log/
+/// snapshot objects at that SAME derived id, so a randomly minted incarnation would not match them and
+/// the fold's own R10 incarnation filter (`{#r10-groupref-alias}`) would drop every one of their keys
+/// as belonging to a dead life.
+///
+/// All ten raw-write helpers place ref-log bytes at states production's real birth path structurally
+/// cannot produce (INV-1 holes, out-of-order ids, a table with no `_ckpt` -- see the Task 4-B map), so
+/// they can never route through `createNamespace` and mint a real incarnation of their own.
+///
+/// TWO DIVERGENCES from what `createNamespace`/`completeCreation` would produce, both deliberate and
+/// both left as-is rather than "fixed":
+///   1. the incarnation is a deterministic namespace-derived fixture id, not a fresh random mint;
+///   2. this entry reaches `Live` with NO `_ckpt` at all, whereas production only ever reaches `Live`
+///      through `completeCreation`, which publishes `_ckpt` FIRST (INV-4). Several fixtures exist
+///      SPECIFICALLY to build a table with no `_ckpt`, but they must exercise that corruption directly:
+///      lifecycle-authoritative recovery correctly rejects a `Live` or `Removing` row without a
+///      readable `life_epoch`. Ordinary fixtures use `casAdmitRecoverableEntry` below instead.
+inline void casAdmitEntry(DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns)
+{
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    for (const CatalogEntry & entry : snap.catalog.entries)
+        if (entry.ns.string() == ns.string())
+            return;   /// already admitted -- by an earlier raw write to the same namespace, or by the
+                      /// test itself
+    CatalogEntry entry;
+    entry.ns = ns;
+    entry.state = NsState::Live;
+    entry.incarnation = fixture::fixtureLife(ns).incarnation;
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, entry);
+}
+
+namespace fixture
+{
+    /// The admit-Live-without-`_ckpt` pattern (divergence 2 above), reachable through the seam.
+    inline void admitLive(DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns)
+    {
+        casAdmitEntry(backend, layout, ns);
+    }
+}
+
+/// Write the checkpoint frontier that makes a raw `Live` fixture a normal recoverable life. Raw logs
+/// intentionally do not synthesize `_ckpt`: many tests need the missing-checkpoint corruption shape.
+/// A test that invokes lifecycle-authoritative recovery therefore has to state its exact frontier here.
+inline void writeRecoverableCkptForRawFixture(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::Cas::RefCkpt & ckpt)
+{
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(backend, layout);
+    const auto it = std::find_if(
+        catalog_cut.catalog.entries.begin(), catalog_cut.catalog.entries.end(),
+        [&] (const CatalogEntry & entry) { return entry.ns == ns; });
+    if (it == catalog_cut.catalog.entries.end())
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' has no catalog entry", ns.string());
+
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(it->ns, it->incarnation);
+    const PutResult put = backend.putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(ckpt));
+    if (put.outcome != PutOutcome::Done)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' could not publish its checkpoint", ns.string());
+}
+
+/// Advance an existing recoverable raw fixture's exact checkpoint frontier. This intentionally never
+/// creates a missing `_ckpt` or repairs an invalid one: those are distinct raw corruption fixtures.
+inline void advanceRecoverableCkptForRawFixture(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::Cas::RefTxnId & through)
+{
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(backend, layout);
+    const auto it = std::find_if(
+        catalog_cut.catalog.entries.begin(), catalog_cut.catalog.entries.end(),
+        [&] (const CatalogEntry & entry) { return entry.ns == ns; });
+    if (it == catalog_cut.catalog.entries.end())
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' has no catalog entry", ns.string());
+
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(it->ns, it->incarnation);
+    const std::optional<CkptSample> sample = readCkpt(backend, layout, life);
+    if (!sample)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' has no checkpoint to advance", ns.string());
+
+    chooseRecoveryGrounding(*it, sample->ckpt);
+    if (!sample->ckpt.committed_through || through <= *sample->ckpt.committed_through)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' cannot advance its checkpoint monotonically", ns.string());
+
+    RefCkpt advanced = sample->ckpt;
+    advanced.committed_through = through;
+    if (backend.casPut(layout.refCkptKey(life), encodeRefCkpt(advanced), sample->token).outcome != CasOutcome::Committed)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' could not advance its checkpoint", ns.string());
+}
+
+/// Replace an existing recoverable raw fixture checkpoint with the caller's complete next state.
+/// Unlike `advanceRecoverableCkptForRawFixture`, this does not preserve any field implicitly: callers
+/// that model a snapshot or epoch-seal change must name the entire authoritative checkpoint. Missing or
+/// invalid current checkpoints stay corruption fixtures and are never repaired here.
+inline void replaceRecoverableCkptForRawFixture(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::Cas::RefCkpt & next)
+{
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(backend, layout);
+    const auto it = std::find_if(
+        catalog_cut.catalog.entries.begin(), catalog_cut.catalog.entries.end(),
+        [&] (const CatalogEntry & entry) { return entry.ns == ns; });
+    if (it == catalog_cut.catalog.entries.end())
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' has no catalog entry", ns.string());
+
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(it->ns, it->incarnation);
+    const std::optional<CkptSample> existing = readCkpt(backend, layout, life);
+    if (!existing)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' has no checkpoint to replace", ns.string());
+
+    chooseRecoveryGrounding(*it, existing->ckpt);
+    chooseRecoveryGrounding(*it, next);
+    if (next.life_epoch != existing->ckpt.life_epoch)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' cannot replace its checkpoint with a different life epoch", ns.string());
+    if (existing->ckpt.committed_through
+        && (!next.committed_through || *next.committed_through < *existing->ckpt.committed_through))
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' cannot regress its checkpoint frontier", ns.string());
+
+    if (backend.casPut(layout.refCkptKey(life), encodeRefCkpt(next), existing->token).outcome != CasOutcome::Committed)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "raw recovery fixture for namespace '{}' could not replace its checkpoint", ns.string());
+}
+
+/// Publish the checkpoint authority a semantic fixture wrapper owes immediately after its durable raw
+/// log transaction. Raw writers deliberately do not call this: missing, stale, and malformed `_ckpt`
+/// fixtures are meaningful corruption inputs. A semantic wrapper creates the first valid authority or
+/// advances the existing exact checkpoint without discarding its snapshot or epoch-seal fields.
+inline void publishRecoverableCkptForSemanticWrapper(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::Cas::RefTxnId & txn_id)
+{
+    const std::optional<NamespaceLifeId> life = CasRefCatalog::lifeIfCataloged(backend, layout, ns);
+    if (!life)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "semantic ref fixture for namespace '{}' was not admitted", ns.string());
+
+    if (!readCkpt(backend, layout, *life))
+    {
+        const PutResult put = backend.putIfAbsent(layout.refCkptKey(*life), encodeRefCkpt(RefCkpt{
+            .life_epoch = 1,
+            .committed_through = txn_id,
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = std::nullopt,
+        }));
+        if (put.outcome == PutOutcome::Done)
+            return;
+    }
+
+    advanceRecoverableCkptForRawFixture(backend, layout, ns, txn_id);
+}
+
+/// Admit an otherwise empty `Live` fixture together with the immutable checkpoint authority that a
+/// production-created life already has. This is deliberately a SEPARATE helper from `casAdmitEntry`:
+/// raw fixtures that exercise a missing or corrupt `_ckpt` must keep constructing that invalid shape
+/// explicitly. The empty frontier is valid because no raw log has been published yet; a fixture that
+/// seeds logs instead has to name its own exact `committed_through` through
+/// `writeRecoverableCkptForRawFixture`.
+inline void casAdmitRecoverableEntry(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    uint64_t life_epoch = 1)
+{
+    casAdmitEntry(backend, layout, ns);
+
+    const std::optional<NamespaceLifeId> life = CasRefCatalog::lifeIfCataloged(backend, layout, ns);
+    if (!life)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+            "recoverable raw fixture for namespace '{}' was not admitted", ns.string());
+
+    if (backend.head(layout.refCkptKey(*life)).exists)
+        return;
+
+    writeRecoverableCkptForRawFixture(backend, layout, ns, RefCkpt{
+        .life_epoch = life_epoch,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+}
+
+/// Recover from the caller's catalog cut, reading `_ckpt` exactly once for the row in that same cut.
+/// Keeping the cut an argument forces raw-fixture consumers to make the immutable authority visible;
+/// this helper never resolves the namespace or re-reads the catalog on their behalf.
+inline DB::Cas::RecoveredRefTable recoverRefTableDetailedAtCatalogCutForTest(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const CasRefCatalog::Snapshot & catalog_cut,
+    const DB::Cas::RootNamespace & ns)
+{
+    std::optional<CatalogEntry> catalog_entry;
+    const auto it = std::find_if(
+        catalog_cut.catalog.entries.begin(), catalog_cut.catalog.entries.end(),
+        [&] (const CatalogEntry & entry) { return entry.ns == ns; });
+    if (it != catalog_cut.catalog.entries.end())
+        catalog_entry = *it;
+
+    std::optional<RefCkpt> ckpt;
+    if (catalog_entry)
+    {
+        const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(catalog_entry->ns, catalog_entry->incarnation);
+        if (const std::optional<CkptSample> sample = readCkpt(backend, layout, life))
+            ckpt = sample->ckpt;
+    }
+
+    return recoverRefTableDetailedFromAuthority(backend, layout, catalog_entry, ckpt);
+}
+
+/// Writes `txn` at `_log/<txn_id>` (create-if-absent). Admits `txn.ns` into the catalog first
+/// (`casAdmitEntry`, above) -- the fold's universe is catalog-authoritative (Task 4-C), so a raw
+/// fixture that skipped this would be invisible to GC/rebuild/fsck no matter what it wrote to `_log`.
+///
+/// KEYS AT THE NAMESPACE'S CURRENT CATALOG LIFE, NOT UNCONDITIONALLY AT THE SENTINEL: a test that
+/// mixes a REAL birth (`beginPartWrite`/`precommitAdd`, which mints a real random incarnation via
+/// `CasRefLedger::resolveNamespaceLife`) with a raw follow-up write to the SAME namespace (a
+/// repoint/removal simulation, say) needs this write to land where the real content already lives, not
+/// at an unrelated sentinel prefix the fold never reads for that namespace. `casAdmitEntry` above is a
+/// no-op once any entry exists, so resolving the catalog life here yields whichever life is ALREADY on
+/// record -- the real one if a real birth landed first, the fixture identity if this call is what
+/// admitted it (via `casAdmitEntry`, moments ago, in this same function).
+inline void writeRefLogTxnRaw(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RefLogTxn & txn)
+{
+    const DB::Cas::RootNamespace ns{txn.ns};
+    casAdmitEntry(backend, layout, ns);
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(backend, layout, ns).value_or(fixture::fixtureLife(ns));
+    const String key = layout.refLogKey(life, txn.txn_id);
+    backend.putIfAbsent(key, DB::Cas::sealObject(DB::Cas::FormatId::RefLog, DB::Cas::encodeRefLogTxn(txn)));
+}
+
+namespace fixture
+{
+    /// The raw ref-log write pattern (divergence 3 above), reachable through the seam.
+    inline void writeRefLogRaw(DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RefLogTxn & txn)
+    {
+        writeRefLogTxnRaw(backend, layout, txn);
+    }
+}
+
+/// A `Live` snapshot naming exactly `committed` (already-sorted-by-ref_name input expected) with no
+/// precommits — the common recovery-fixture shape.
+inline DB::Cas::RefTableSnapshot minimalLiveSnapshot(
+    const String & ns, DB::Cas::RefTxnId snapshot_id, std::vector<DB::Cas::RefCommittedRow> committed = {})
+{
+    DB::Cas::RefTableSnapshot s;
+    s.ns = ns;
+    s.snapshot_id = snapshot_id;
+    s.committed = std::move(committed);
+    return s;
+}
+
+/// One committed row naming `ref_name` -> `manifest_ref` with `published_at_ms` left at its default
+/// (0, unset) — for tests that don't care about the publish stamp.
+inline DB::Cas::RefCommittedRow committedRow(const String & ref_name, const DB::Cas::ManifestRef & manifest_ref)
+{
+    DB::Cas::RefCommittedRow row;
+    row.ref_name = ref_name;
+    row.manifest_ref = manifest_ref;
+    return row;
+}
+
+/// A `namespace_birth` op — the first op any never-born table's first transaction needs.
+inline DB::Cas::RefOp namespaceBirthOp()
+{
+    DB::Cas::RefOp op;
+    op.kind = DB::Cas::RefOpKind::NamespaceBirth;
+    return op;
+}
+
+/// An `epoch_seal` op — the record that CLOSES an epoch (INV-2). A seal transaction carries exactly
+/// this op and nothing else (grammar enforced by the codec in both directions). The next epoch's first
+/// transaction names the seal it consumed in `prev_epoch_seal`, and that back-chain is what lets a fold
+/// cross epochs without trusting a listing.
+inline DB::Cas::RefOp epochSealOp()
+{
+    DB::Cas::RefOp op;
+    op.kind = DB::Cas::RefOpKind::EpochSeal;
+    return op;
+}
+
+/// Write ONE ref-log transaction at an EXACT id. `appendRefLogSeed` and the wrappers above ALLOCATE
+/// ids arithmetically inside writer epoch 1, so anything that needs a chosen id — a gap, an
+/// out-of-order arrival, or an epoch CROSSING — writes through here instead. The bytes go through the
+/// real codec, so every grammar rule the fold's decoder enforces is enforced here too.
+inline void writeTxnAt(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::Cas::RefTxnId & id, std::vector<DB::Cas::RefOp> ops,
+    std::optional<DB::Cas::RefTxnId> prev_epoch_seal = std::nullopt)
+{
+    DB::Cas::RefLogTxn txn;
+    txn.ns = ns.string();
+    txn.txn_id = id;
+    txn.ops = std::move(ops);
+    txn.prev_epoch_seal = prev_epoch_seal;
+    writeRefLogTxnRaw(backend, layout, txn);
+}
+
+/// Close an epoch at exactly `id`.
+inline void writeSealAt(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::Cas::RefTxnId & id, std::optional<DB::Cas::RefTxnId> prev_epoch_seal = std::nullopt)
+{
+    writeTxnAt(backend, layout, ns, id, {epochSealOp()}, prev_epoch_seal);
+}
+
+/// Publish `ref_name` -> a fresh manifest pinning `blob`, as ONE transaction at exactly `id`
+/// (add-precommit + promote, the only shape that reaches a committed owner). `birth` prepends the
+/// `namespace_birth` op the table's first transaction owes; `prev_epoch_seal` is required on sequence 1
+/// of every epoch above the namespace's genesis. The manifest's prefix is
+/// `{id.writer_epoch, build_sequence}`, so a caller controlling `build_sequence` also controls whether
+/// the orphan sweep's watermark considers that manifest eligible.
+inline void publishAt(
+    DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const DB::Cas::RootNamespace & ns,
+    const DB::Cas::RefTxnId & id, const String & ref_name, uint64_t build_sequence, const DB::UInt128 & blob,
+    bool birth = false, std::optional<DB::Cas::RefTxnId> prev_epoch_seal = std::nullopt)
+{
+    const DB::Cas::ManifestRef mref{.writer_epoch = id.writer_epoch, .build_sequence = build_sequence,
+                                    .manifest_ordinal = 1};
+    writeBlobBody(backend, layout, blob);
+    writeManifestRaw(backend, layout, ns, mref, {blobEntryFor("data.bin", blob)});
+
+    std::vector<DB::Cas::RefOp> ops;
+    if (birth)
+        ops.push_back(namespaceBirthOp());
+    for (const DB::Cas::RefOp & op : publishCommittedOps(ref_name, mref))
+        ops.push_back(op);
+    writeTxnAt(backend, layout, ns, id, std::move(ops), prev_epoch_seal);
+}
+
+/// The two ops a fixture transaction needs to go straight from nothing to a committed ref (spec
+/// §State Transitions has no direct "add committed" shape — only precommit -> promote): an
+/// `owner_transition` add-precommit followed by an `owner_transition` promote of the SAME
+/// (ref_name, manifest_ref). Legal as the tail of one transaction whose earlier ops (if any) left the
+/// table `Live` (prepend `namespaceBirthOp()` for a never-born table).
+inline std::vector<DB::Cas::RefOp> publishCommittedOps(const String & ref_name, const DB::Cas::ManifestRef & manifest_ref)
+{
+    DB::Cas::RefOp add;
+    add.kind = DB::Cas::RefOpKind::OwnerTransition;
+    add.new_binding = DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Precommit, ref_name, manifest_ref};
+
+    DB::Cas::RefOp promote;
+    promote.kind = DB::Cas::RefOpKind::OwnerTransition;
+    promote.old_binding = DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Precommit, ref_name, manifest_ref};
+    promote.new_binding = DB::Cas::RefOwnerBinding{DB::Cas::RefOwnerKind::Committed, ref_name, manifest_ref};
+
+    return {add, promote};
+}
+
+/// Counts head/get/putIfAbsent per key for op-count assertions (Pillar B / A1 tests).
+class CountingBackend : public DB::Cas::InMemoryBackend
+{
+public:
+    /// Unhide the base convenience overloads (omitted Range/ObjectMeta/expected-token forms): the
+    /// overrides below would otherwise shadow them for callers holding a concrete backend type.
+    using DB::Cas::Backend::get;
+    using DB::Cas::Backend::getStream;
+    using DB::Cas::Backend::putIfAbsent;
+    using DB::Cas::Backend::putIfAbsentStream;
+    using DB::Cas::Backend::putOverwrite;
+    using DB::Cas::Backend::casPut;
+
+    DB::Cas::HeadResult head(const String & key) override
+    {
+        {
+            std::lock_guard lock(count_mutex);
+            ++head_counts[key];
+            ++head_total;
+        }
+        return InMemoryBackend::head(key);
+    }
+
+    std::optional<DB::Cas::GetResult> get(const String & key, DB::Cas::Range range) override
+    {
+        {
+            std::lock_guard lock(count_mutex);
+            ++get_counts[key];
+            ++get_total;
+            /// Record the request-size shape per key so streaming-memory gates (Task 3/4) can assert
+            /// the resident-memory bound at the seam: a whole-object read (range.whole()) is a
+            /// violation for a run object; a ranged read tracks its MAX window length per key.
+            if (range.whole())
+                ++whole_get_counts[key];
+            else
+            {
+                const uint64_t len = range.length.has_value() ? *range.length : 0;
+                uint64_t & mx = max_ranged_get_len[key];
+                mx = std::max(mx, len);
+            }
+        }
+        return InMemoryBackend::get(key, range);
+    }
+
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & key, DB::Cas::Range range) override
+    {
+        {
+            std::lock_guard lock(count_mutex);
+            ++get_stream_counts[key];
+            ++get_stream_total;
+        }
+        return InMemoryBackend::getStream(key, range);
+    }
+
+    DB::Cas::ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        {
+            std::lock_guard lock(count_mutex);
+            ++list_counts[prefix];
+            ++list_total;
+        }
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+
+    DB::Cas::PutResult putIfAbsent(const String & key, const String & bytes, const DB::Cas::ObjectMeta & meta) override
+    {
+        {
+            std::lock_guard lock(count_mutex);
+            ++put_counts[key];
+            ++put_total;
+        }
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+
+
+    /// Counted separately from `putIfAbsent` and `casPut`, for the same reason those two are separate: a
+    /// replacement conditioned on an expected token is its own op with its own cost. The namespace-file
+    /// request-profile goldens tell the create path from the replace path on exactly this counter.
+    DB::Cas::PutResult putOverwrite(const String & key, const String & bytes, const DB::Cas::Token & expected,
+                                    const DB::Cas::ObjectMeta & meta) override
+    {
+        {
+            std::lock_guard lock(count_mutex);
+            ++put_overwrite_counts[key];
+            ++put_overwrite_total;
+        }
+        return InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+    }
+
+    /// Counted separately from `putIfAbsent`: a token-CAS is a DIFFERENT op with a different cost, and
+    /// the `_ckpt` no-op contract ("identical merged body issues no write") is asserted on exactly this
+    /// counter -- a create-if-absent count would not see the replace path at all.
+    DB::Cas::CasResult casPut(const String & key, const String & bytes,
+                              const std::optional<DB::Cas::Token> & expected, const DB::Cas::ObjectMeta & meta) override
+    {
+        {
+            std::lock_guard lock(count_mutex);
+            ++cas_put_counts[key];
+            ++cas_put_total;
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+    /// Every ATTEMPTED delete is counted, whatever the backend answers. The destructive gate's tests
+    /// assert that a suppressed round issues NONE, and an attempt that came back `NotFound` is still an
+    /// attempt -- counting only successful ones would let a gate that leaks deletes over already-absent
+    /// keys read as green.
+    DB::Cas::DeleteOutcome deleteExact(const String & key, const DB::Cas::Token & token) override
+    {
+        {
+            std::lock_guard lock(count_mutex);
+            ++delete_counts[key];
+            ++delete_total;
+        }
+        return InMemoryBackend::deleteExact(key, token);
+
+    }
+
+    uint64_t headCount(const String & key) const { return lookup(head_counts, key); }
+    uint64_t casPutCount(const String & key) const { return lookup(cas_put_counts, key); }
+    uint64_t putOverwriteCount(const String & key) const { return lookup(put_overwrite_counts, key); }
+    uint64_t getCount(const String & key) const { return lookup(get_counts, key); }
+    uint64_t putCount(const String & key) const { return lookup(put_counts, key); }
+    uint64_t deleteCount(const String & key) const { return lookup(delete_counts, key); }
+    uint64_t deleteTotal() const { std::lock_guard lock(count_mutex); return delete_total; }
+    /// Attempted deletes against any key whose path CONTAINS `substr` — the per-site assertion the
+    /// destructive-gate tests make ("the generation prune deleted nothing", "the sweep deleted nothing").
+    uint64_t deleteCountForKeysContaining(const String & substr) const
+    {
+        std::lock_guard lock(count_mutex);
+        uint64_t total = 0;
+        for (const auto & [key, n] : delete_counts)
+            if (key.find(substr) != String::npos)
+                total += n;
+        return total;
+    }
+    /// Every key this backend was ever asked to delete, in sorted order — so a failing zero-delete
+    /// assertion names the sites that leaked instead of just reporting a count.
+    std::vector<String> deletedKeys() const
+    {
+        std::lock_guard lock(count_mutex);
+        std::vector<String> keys;
+        keys.reserve(delete_counts.size());
+        for (const auto & [key, n] : delete_counts)
+            keys.push_back(key);
+        return keys;
+    }
+    uint64_t getStreamCount(const String & key) const { return lookup(get_stream_counts, key); }
+    uint64_t listCount(const String & prefix) const { return lookup(list_counts, prefix); }
+    /// The max ranged-get window length observed for `key` (0 if only whole-object gets, or none).
+    uint64_t maxRangedGetLen(const String & key) const { return lookup(max_ranged_get_len, key); }
+    /// How many whole-object gets (range.whole()) hit `key` — nonzero flags a resident-memory
+    /// violation for a run/seal object that a streaming caller must never read whole.
+    uint64_t wholeGetCount(const String & key) const { return lookup(whole_get_counts, key); }
+    /// Every key any counted operation was issued against, plus every LIST prefix, sorted and
+    /// de-duplicated. A request-profile gate asserts the SET, not only the totals, so a new request the
+    /// profile does not allow names its own key in the failure instead of moving an anonymous counter.
+    std::vector<String> touchedKeys() const
+    {
+        std::lock_guard lock(count_mutex);
+        std::vector<String> keys;
+        for (const std::map<String, uint64_t> * m :
+             {&head_counts, &get_counts, &put_counts, &put_overwrite_counts, &cas_put_counts,
+              &get_stream_counts, &list_counts, &delete_counts})
+            for (const auto & [key, n] : *m)
+                keys.push_back(key);
+        std::sort(keys.begin(), keys.end());
+        keys.erase(std::unique(keys.begin(), keys.end()), keys.end());
+        return keys;
+    }
+
+    uint64_t headTotal() const { std::lock_guard lock(count_mutex); return head_total; }
+    uint64_t getTotal() const { std::lock_guard lock(count_mutex); return get_total; }
+    uint64_t putTotal() const { std::lock_guard lock(count_mutex); return put_total; }
+    uint64_t putOverwriteTotal() const { std::lock_guard lock(count_mutex); return put_overwrite_total; }
+    uint64_t casPutTotal() const { std::lock_guard lock(count_mutex); return cas_put_total; }
+    uint64_t getStreamTotal() const { std::lock_guard lock(count_mutex); return get_stream_total; }
+    uint64_t listTotal() const { std::lock_guard lock(count_mutex); return list_total; }
+
+    /// The total number of get + getStream + putIfAbsent operations against any key whose path
+    /// CONTAINS `substr` (T0 idle-round gate: zero run I/O touches every `.../blob_target/...` key).
+    uint64_t ioCountForKeysContaining(const String & substr) const
+    {
+        std::lock_guard lock(count_mutex);
+        uint64_t total = 0;
+        for (const auto & [key, n] : get_counts)
+            if (key.find(substr) != String::npos) total += n;
+        for (const auto & [key, n] : get_stream_counts)
+            if (key.find(substr) != String::npos) total += n;
+        for (const auto & [key, n] : put_counts)
+            if (key.find(substr) != String::npos) total += n;
+        return total;
+    }
+
+    void resetCounts()
+    {
+        std::lock_guard lock(count_mutex);
+        head_counts.clear();
+        get_counts.clear();
+        put_counts.clear();
+        put_overwrite_counts.clear();
+        cas_put_counts.clear();
+        get_stream_counts.clear();
+        list_counts.clear();
+        delete_counts.clear();
+        max_ranged_get_len.clear();
+        whole_get_counts.clear();
+        head_total = get_total = put_total = cas_put_total = get_stream_total = list_total = delete_total = 0;
+        put_overwrite_total = 0;
+
+    }
+
+private:
+    uint64_t lookup(const std::map<String, uint64_t> & m, const String & key) const
+    {
+        std::lock_guard lock(count_mutex);
+        const auto it = m.find(key);
+        return it == m.end() ? 0 : it->second;
+    }
+
+    mutable std::mutex count_mutex;
+    std::map<String, uint64_t> head_counts;
+    std::map<String, uint64_t> get_counts;
+    std::map<String, uint64_t> put_counts;
+    std::map<String, uint64_t> put_overwrite_counts;
+    std::map<String, uint64_t> cas_put_counts;
+    std::map<String, uint64_t> get_stream_counts;
+    std::map<String, uint64_t> list_counts;
+    std::map<String, uint64_t> delete_counts;
+    std::map<String, uint64_t> max_ranged_get_len;
+    std::map<String, uint64_t> whole_get_counts;
+    uint64_t head_total = 0;
+    uint64_t get_total = 0;
+    uint64_t put_total = 0;
+    uint64_t put_overwrite_total = 0;
+    uint64_t cas_put_total = 0;
+    uint64_t get_stream_total = 0;
+    uint64_t list_total = 0;
+    uint64_t delete_total = 0;
+};
+
+/// A backend whose LIST permanently omits every key under a chosen prefix while those keys stay fully
+/// readable by exact key -- the lying-store shape observed in production (`0x1430c`/`0x1430d`), and the
+/// premise of every arithmetic-walk test: a record a listing never mentions is still THERE, so a walk
+/// that computes the id finds it and a walk that enumerates does not.
+///
+/// PERMANENT (not nth-call) omission is deliberate: a lying store need not ever recover the key, and the
+/// arithmetic walk that finds it anyway is the property under test -- these fixtures are about the walk,
+/// not about any one `list` call.
+///
+/// Erasing keys from a page cannot disturb pagination: `ListPage::next_cursor` is computed by the base
+/// backend before the erase, so the next page still resumes strictly after the last key it returned.
+///
+/// Templated on the base so a suite that also needs request COUNTS composes it over `CountingBackend`
+/// without a second copy of the hiding rule (which is a rule about what the store may legally do, and
+/// must therefore read the same everywhere it is modelled).
+template <typename Base>
+class HintHoleBackendOn : public Base
+{
+public:
+    /// Hide every key under `prefix` from LIST -- a whole namespace, including objects a later publish
+    /// adds.
+    void hidePrefix(const String & prefix)
+    {
+        std::lock_guard lock(hide_mutex);
+        hidden_prefixes.push_back(prefix);
+    }
+
+    /// Hide exactly one key. Call AFTER seeding: a fixture that allocates ids by listing would
+    /// otherwise allocate over a hidden record.
+    void hide(const String & key)
+    {
+        std::lock_guard lock(hide_mutex);
+        hidden_keys.insert(key);
+    }
+
+    /// Make the store's enumeration omit EXACTLY `keys` and nothing else -- the whole omission set in
+    /// one call, replacing whatever was hidden before.
+    ///
+    /// This is the RustFS defect reproduced as an interface: every one of these keys stays durable and
+    /// honestly served by `get` / `head` / `putIfAbsent` / `casPut` / `deleteExact`, and only
+    /// enumeration pretends they are not there. Stating the omission as a SET is what lets a test say
+    /// the thing the defect report says -- "ids 3 and 4 are invisible while the LATER id 5 is visible"
+    /// -- in one line, instead of assembling it from repeated single-key calls whose combined effect a
+    /// reader has to reconstruct.
+    ///
+    /// A setter rather than an adder: the omission set is the store's declared behaviour for the rest
+    /// of the test, so a second call REPLACES it (pass `{}` to stop lying, same as `revealAll`).
+    void setListOmissions(std::vector<String> keys)
+    {
+        std::lock_guard lock(hide_mutex);
+        hidden_keys.clear();
+        hidden_prefixes.clear();
+        hidden_keys.insert(keys.begin(), keys.end());
+    }
+
+    /// How many LIST pages actually had a key erased. Every test that hides a key asserts this, so a
+    /// mistyped key cannot let the test pass vacuously -- the hole has to have been SERVED.
+    size_t holesServed() const
+    {
+        std::lock_guard lock(hide_mutex);
+        return served;
+    }
+
+    /// The store stops lying: everything hidden is listed again.
+    void revealAll()
+    {
+        std::lock_guard lock(hide_mutex);
+        hidden_keys.clear();
+        hidden_prefixes.clear();
+    }
+
+    DB::Cas::ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        DB::Cas::ListPage page = Base::list(prefix, cursor, limit);
+        std::lock_guard lock(hide_mutex);
+        if (hidden_keys.empty() && hidden_prefixes.empty())
+            return page;
+        const size_t before = page.keys.size();
+        std::erase_if(page.keys, [&](const DB::Cas::ListedKey & k)
+        {
+            if (hidden_keys.contains(k.key))
+                return true;
+            for (const String & hidden : hidden_prefixes)
+                if (k.key.starts_with(hidden))
+                    return true;
+            return false;
+        });
+        if (page.keys.size() != before)
+            ++served;
+        return page;
+    }
+
+private:
+    mutable std::mutex hide_mutex;
+    std::set<String> hidden_keys;
+    std::vector<String> hidden_prefixes;
+    size_t served = 0;
+};
+
+/// The plain form, over a bare `InMemoryBackend`.
+using HintHoleBackend = HintHoleBackendOn<DB::Cas::InMemoryBackend>;
+
+/// Stand in for the self-remount that `Pool::reportImpossibleInterference` schedules. That reaction
+/// trips the local write fence closed AND schedules a remount; a unit-test Pool runs no background
+/// remount (`background_watermark` is off by design there), so without this the fence stays closed and
+/// every later mutation is refused at the gate -- which is a test-harness artifact, not the production
+/// behaviour. Re-arming directly is the smallest faithful stand-in: it restores writability without the
+/// claim machinery and without discarding the cached ref runtimes, so a test can observe what happens
+/// AFTER the reaction. It bumps the fence GENERATION, exactly as a real re-arm does.
+inline void rearmMountFenceAfterAnomalyForTest(const DB::Cas::PoolPtr & store)
+{
+    store->armMountFence(DB::UInt128{0, 1}, store->liveWriterEpoch(), store->bootMsNow() + 600000);
+}
+
+/// Delegates the FIRST matching `putIfAbsent` to `CountingBackend` -- so the write actually LANDS --
+/// and only THEN throws an ambiguous exception, modelling "our own PUT committed but its response was
+/// lost". Every later call behaves normally, so a caller that retries the SAME (key, bytes) meets its
+/// OWN earlier write as the occupant: the exact input the every-attempt rule's adoption arm adjudicates
+/// (`slotOccupy` reports `Occupied` with bytes equal to the attempt's own).
+///
+/// `key_substr` empty means "the first putIfAbsent of any key"; set it to scope the fault to one key
+/// family when the caller drives a whole Pool (whose bootstrap PUTs would otherwise consume the fault).
+///
+/// Shared rather than TU-local because two suites need exactly this shape: `gtest_cas_slot_occupy.cpp`
+/// pins the primitive's same-call resolve, and `gtest_cas_ref_wedge_every_attempt.cpp` drives the
+/// writer's wedge adoption through it.
+class LandedButAckLostOnceBackend : public CountingBackend
+{
+public:
+    using CountingBackend::putIfAbsent;
+    using CountingBackend::get;
+    String key_substr;
+    bool fired = false;
+    /// Also lose the caller's IMMEDIATE resolve read of the same key, once. Needed only by a caller
+    /// whose conditional-write layer resolves before reissuing (`putIfAbsentControlled`): without it
+    /// that resolve proves the object durable inside the very same attempt and reports `Committed`, so
+    /// no wedge over a DURABLE object can ever form. `slotOccupy` needs no such thing -- it has no
+    /// retry loop -- which is why this defaults off and this file's original caller is unaffected.
+    bool lose_resolve_read = false;
+
+    DB::Cas::PutResult putIfAbsent(const String & key, const String & bytes, const DB::Cas::ObjectMeta & meta) override
+    {
+        if (!fired && (key_substr.empty() || key.find(key_substr) != String::npos))
+        {
+            fired = true;
+            CountingBackend::putIfAbsent(key, bytes, meta);   /// the write LANDS
+            if (lose_resolve_read)
+                fail_get_once_key = key;
+            throw Poco::TimeoutException("LandedButAckLostOnceBackend: simulated lost PUT response");
+        }
+        return CountingBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    std::optional<DB::Cas::GetResult> get(const String & key, DB::Cas::Range range) override
+    {
+        if (!fail_get_once_key.empty() && key == fail_get_once_key)
+        {
+            fail_get_once_key.clear();
+            throw Poco::TimeoutException(
+                "LandedButAckLostOnceBackend: simulated lost GET (read response never arrived)");
+        }
+        return CountingBackend::get(key, range);
+    }
+
+private:
+    String fail_get_once_key;
+};
+
+/// A `CountingBackend` that can fault selected PUTs by key substring (skip the first `fault_skip`
+/// matches, then fault the next `fault_count`), and can latch a matching PUT mid-flight. Same class of
+/// seam as the wedge tests in `gtest_cas_ref_writer.cpp` use
+/// (`fault_key_substr`/`corrupt_key_substr`/`armPutBlock`), narrowed to what the ref-lane tests need.
+/// Shared (rather than TU-local) because the chunk-boundary tests and the post-durable install-safety
+/// tests need exactly the same seam.
+class ChunkFaultBackend : public CountingBackend
+{
+public:
+    using CountingBackend::putIfAbsent;
+    using CountingBackend::get;
+
+    /// Unresolved       -> a lost-response ambiguity, NOTHING landed; with a single-attempt budget this
+    ///                     wedges the lane and a later resolve proves the key ABSENT.
+    /// LandedThenLost   -> our OWN exact bytes land and only the acknowledgement is lost, AND the
+    ///                     controller's immediate resolve-before-reissue GET is lost too. Both legs are
+    ///                     required to wedge over a DURABLE object: the resolve happens inside the same
+    ///                     attempt, so a readable key would prove `Committed` there and no wedge would
+    ///                     ever form. Real-world shape: the write succeeded server-side, the connection
+    ///                     dropped, and the verification read hit the same transient outage. With a
+    ///                     single-attempt budget the lane then wedges over an object that IS durable, so
+    ///                     the NEXT flush's `resolveByExactGet` reports `Committed` and drives the
+    ///                     wedge-RESOLUTION install (spec §A1 site 2) -- the only mode that reaches it.
+    /// Definite         -> an S3-classified malformed request -> `CasWriteOutcome::DefiniteFailure`.
+    /// ForeignConflict  -> a DIFFERENT object lands at the key, then the response is lost -> the
+    ///                     controller's resolve-before-reissue GET observes foreign bytes and throws
+    ///                     CORRUPTED_DATA straight out of the PUT (a proven conflict).
+    enum class Mode { None, Unresolved, LandedThenLost, Definite, ForeignConflict };
+
+    /// Fault matching is single-threaded during a flush (one leader per table PUTs `_log/`), so these
+    /// need no lock; set them before driving the flush.
+    String fault_substr;
+    Mode mode = Mode::None;
+    int fault_skip = 0;
+    int fault_count = 0;
+    /// One-shot: the next `get` of exactly this key throws, then it is cleared. Armed by
+    /// `Mode::LandedThenLost` (see above); settable directly for a bare lost-read fault.
+    String fail_get_once_key;
+
+    std::optional<DB::Cas::GetResult> get(const String & key, DB::Cas::Range range) override
+    {
+        if (!fail_get_once_key.empty() && key == fail_get_once_key)
+        {
+            fail_get_once_key.clear();
+            throw Poco::TimeoutException("ChunkFaultBackend: simulated lost GET (read response never arrived)");
+        }
+        return CountingBackend::get(key, range);
+    }
+
+    DB::Cas::PutResult putIfAbsent(const String & key, const String & bytes, const DB::Cas::ObjectMeta & meta) override
+    {
+        if (mode != Mode::None && !fault_substr.empty() && key.find(fault_substr) != String::npos)
+        {
+            if (fault_skip > 0)
+            {
+                --fault_skip;
+            }
+            else if (fault_count > 0)
+            {
+                --fault_count;
+                switch (mode)
+                {
+                    case Mode::Unresolved:
+                        throw Poco::TimeoutException("ChunkFaultBackend: simulated ambiguous _log PUT (response lost)");
+                    case Mode::LandedThenLost:
+                        /// The write SUCCEEDS -- byte-for-byte what the caller asked for, through the
+                        /// counting path so the object is indistinguishable from a normal PUT -- and only
+                        /// the acknowledgement is lost. The controller's resolve-before-reissue GET is
+                        /// armed to fail ONCE for this key as well, or it would prove the object durable
+                        /// inside this very attempt and the lane would never wedge; the wedge-resolution
+                        /// GET a flush later then reads it normally.
+                        CountingBackend::putIfAbsent(key, bytes, meta);
+                        fail_get_once_key = key;
+                        throw Poco::TimeoutException("ChunkFaultBackend: object landed; response lost");
+                    case Mode::Definite:
+#if USE_AWS_S3
+                        throw DB::S3Exception("ChunkFaultBackend: simulated malformed request",
+                                              Aws::S3::S3Errors::UNKNOWN, "MalformedXML");
+#else
+                        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+                            "ChunkFaultBackend: DefiniteFailure requires S3 error classification (USE_AWS_S3 off)");
+#endif
+                    case Mode::ForeignConflict:
+                        /// A foreign writer lands DIFFERENT bytes at this exact key; then our response is
+                        /// lost, so resolve-before-reissue GETs foreign bytes -> CORRUPTED_DATA.
+                        CountingBackend::putIfAbsent(key, bytes + String("\x01_FOREIGN_DIFFERENT"));
+                        throw Poco::TimeoutException("ChunkFaultBackend: foreign different object landed; response lost");
+                    case Mode::None:
+                        break;
+                }
+            }
+        }
+        {
+            std::unique_lock lk(block_mutex);
+            if (block_armed && !block_substr.empty() && key.find(block_substr) != String::npos)
+            {
+                block_entered = true;
+                block_cv.notify_all();
+                /// Bounded (20s) so a wiring bug bounds the wait rather than hanging the whole suite.
+                block_cv.wait_for(lk, std::chrono::seconds(20), [&] { return !block_armed; });
+            }
+        }
+        return CountingBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    void armBlock(const String & substr)
+    {
+        std::lock_guard lk(block_mutex);
+        block_substr = substr;
+        block_armed = true;
+        block_entered = false;
+    }
+    void awaitBlockEntered()
+    {
+        std::unique_lock lk(block_mutex);
+        /// Bounded (20s): if the latched publisher never reaches its PUT, fail LOUDLY rather than hang.
+        /// The assertion is load-bearing -- without it a wiring regression that never parks the publisher
+        /// would let `SnapshotPublisherLatchedAcrossChunks` pass VACUOUSLY (its final re-fire assertion
+        /// can still hold via a direct, non-coalesced dispatch).
+        block_cv.wait_for(lk, std::chrono::seconds(20), [&] { return block_entered; });
+        ASSERT_TRUE(block_entered) << "latched publisher never entered its blocked PUT within 20s -- "
+                                      "coalescing was not exercised";
+    }
+    void releaseBlock()
+    {
+        {
+            std::lock_guard lk(block_mutex);
+            block_armed = false;
+        }
+        block_cv.notify_all();
+    }
+
+private:
+    std::mutex block_mutex;
+    std::condition_variable block_cv;
+    String block_substr;
+    bool block_armed = false;
+    bool block_entered = false;
+};
+
+/// Fault decorator for the condemn-marker gate tests (codex-review triage 2026-07-17 §3.4): while
+/// armed, every conditional-write attempt against a blob `.meta` key throws. The request controller
+/// exhausts its budget and reports `Unresolved`, so `writeCondemnedMeta` returns false while the round
+/// still commits the unconfirmed retired entry. Every other write passes through. Armed by default;
+/// disarm (`fail_meta_writes = false`) to model the backend healing.
+class MetaWriteFaultBackend : public DB::Cas::InMemoryBackend
+{
+public:
+    /// Unhide the base convenience overloads (omitted Range/ObjectMeta/expected-token forms): the
+    /// overrides below would otherwise shadow them for callers holding a concrete backend type.
+    using DB::Cas::Backend::get;
+    using DB::Cas::Backend::getStream;
+    using DB::Cas::Backend::putIfAbsent;
+    using DB::Cas::Backend::putIfAbsentStream;
+    using DB::Cas::Backend::putOverwrite;
+    using DB::Cas::Backend::casPut;
+
+    DB::Cas::PutResult putIfAbsent(
+        const String & key, const String & bytes, const DB::Cas::ObjectMeta & meta) override
+    {
+        if (fail_meta_writes.load() && key.ends_with(".meta"))
+            throw std::runtime_error("injected fault: blob meta write lost");
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    DB::Cas::PutResult putOverwrite(
+        const String & key, const String & bytes, const DB::Cas::Token & expected,
+        const DB::Cas::ObjectMeta & meta) override
+    {
+        if (fail_meta_writes.load() && key.ends_with(".meta"))
+            throw std::runtime_error("injected fault: blob meta write lost");
+        return InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+    }
+
+    DB::Cas::CasResult casPut(const String & key, const String & bytes,
+                              const std::optional<DB::Cas::Token> & expected,
+                              const DB::Cas::ObjectMeta & meta) override
+    {
+        if (fail_meta_writes.load() && key.ends_with(".meta"))
+            throw std::runtime_error("injected fault: blob meta write lost");
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+
+    std::atomic<bool> fail_meta_writes{true};
+};
+
+}
diff --git a/src/Disks/tests/gtest_ca_dedup_cache.cpp b/src/Disks/tests/gtest_ca_dedup_cache.cpp
new file mode 100644
index 000000000000..003dd3d14f58
--- /dev/null
+++ b/src/Disks/tests/gtest_ca_dedup_cache.cpp
@@ -0,0 +1,203 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/ProfileEvents.h>
+
+namespace ProfileEvents
+{
+extern const Event CASDeduplicationCacheHits;
+extern const Event CASDeduplicationCacheMisses;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace
+{
+
+/// A transparent delegating Backend that counts the two ops P1/P2 trade off against each other:
+/// `head` (the cheap probe HEAD-before-PUT issues) and `putIfAbsentStream` (the body upload a present
+/// HEAD avoids). Everything else is forwarded verbatim so the wrapped backend behaves exactly as a bare
+/// InMemoryBackend would.
+class CountingBackend final : public Backend
+{
+public:
+    explicit CountingBackend(BackendPtr inner_) : inner(std::move(inner_)) {}
+
+    size_t heads = 0;
+    size_t stream_puts = 0;
+
+    HeadResult head(const String & k) override { ++heads; return inner->head(k); }
+    WriteSinkPtr putIfAbsentStream(const String & k, const ObjectMeta & meta) override
+    {
+        ++stream_puts;
+        return inner->putIfAbsentStream(k, meta);
+    }
+
+    std::optional<GetResult> get(const String & k, Range r) override { return inner->get(k, r); }
+    std::optional<GetStreamResult> getStream(const String & k, Range r) override { return inner->getStream(k, r); }
+    ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+    PutResult putIfAbsent(const String & k, const String & b, const ObjectMeta & m) override { return inner->putIfAbsent(k, b, m); }
+    PutResult putOverwrite(const String & k, const String & b, const Token & e, const ObjectMeta & m) override { return inner->putOverwrite(k, b, e, m); }
+    CasResult casPut(const String & k, const String & b, const std::optional<Token> & e, const ObjectMeta & m) override { return inner->casPut(k, b, e, m); }
+    DeleteOutcome deleteExact(const String & k, const Token & t) override { return inner->deleteExact(k, t); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+
+private:
+    BackendPtr inner;
+};
+
+PoolConfig cfg(uint64_t cache_bytes, uint64_t head_first_min_bytes)
+{
+    PoolConfig c{.pool_prefix = "p", .server_root_id = "test"};
+    c.deduplication_cache_bytes = cache_bytes;
+    c.deduplication_head_first_min_bytes = head_first_min_bytes;
+    return c;
+}
+
+}
+
+/// Task 2: the cache itself — add then contains.
+TEST(CASDeduplicationCache, AddThenContains)
+{
+    auto s = Pool::open(std::make_shared<InMemoryBackend>(), cfg(64ULL << 20, 1ULL << 20));
+    const DB::UInt128 h = u128Of("x");
+    EXPECT_FALSE(s->dedupCacheContains(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(h)}));
+    s->dedupCacheAdd(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(h)});
+    EXPECT_TRUE(s->dedupCacheContains(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(h)}));
+}
+
+/// Task 2: deduplication_cache_bytes == 0 disables the cache — add is a no-op, contains is always false.
+TEST(CASDeduplicationCache, DisabledNeverContains)
+{
+    auto s = Pool::open(std::make_shared<InMemoryBackend>(), cfg(/*cache_bytes*/ 0, 1ULL << 20));
+    const DB::UInt128 h = u128Of("x");
+    s->dedupCacheAdd(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(h)});
+    EXPECT_FALSE(s->dedupCacheContains(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(h)}));
+}
+
+/// Task 2: the cache is bounded by bytes — at 64 B/entry a 256 B ceiling holds ~4 entries, so the
+/// earliest-added hash is evicted while a recently-added one survives.
+TEST(CASDeduplicationCache, BoundedByBytes)
+{
+    auto s = Pool::open(std::make_shared<InMemoryBackend>(), cfg(/*cache_bytes*/ 256, 1ULL << 20));
+    const DB::UInt128 first = u128Of("k0");
+    s->dedupCacheAdd(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(first)});
+    for (int i = 1; i < 100; ++i)
+        s->dedupCacheAdd(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("k" + std::to_string(i)))});
+    EXPECT_FALSE(s->dedupCacheContains(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(first)}));            /// evicted long ago
+    EXPECT_TRUE(s->dedupCacheContains(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("k99"))}));     /// most recent survives
+}
+
+/// Task 5 (P1): a cache hit takes the HEAD-first path and skips the body PUT entirely.
+/// (Counters are reset right before each measured putBlob — Pool::open's probe/watermark and
+/// beginPartWrite's heartbeat issue their own backend ops that are irrelevant to the trade-off under test.)
+TEST(CASDeduplicationCache, HitTakesHeadFirstNoBodyPut)
+{
+    auto counting = std::make_shared<CountingBackend>(std::make_shared<InMemoryBackend>());
+    auto s = Pool::open(counting, cfg(64ULL << 20, 1ULL << 20));
+
+    /// First writer: small body, cold cache, below the P2 size threshold ⇒ a normal body PUT.
+    auto b1 = s->beginPartWrite({});
+    counting->stream_puts = 0;
+    b1->putBlob(idOf("dup"), BlobSource::fromString("dup"));
+    EXPECT_EQ(counting->stream_puts, 1u);
+
+    /// Second writer of the same content: the cache now says present ⇒ HEAD-first, no second body PUT.
+    /// The head-first hit ADOPTS an existing incarnation, so it must run under a durable precommit edge
+    /// (EDGE-BEFORE-OBSERVE: stageManifest -> precommitAdd before putBlob). Counters are reset AFTER that
+    /// ceremony so only the measured putBlob is counted.
+    PartWriteInfo info2;
+    info2.intended_ref = "srv/tbl/ref2";
+    auto b2 = s->beginPartWrite(info2);
+    ManifestEntry e2;
+    e2.path = "data.bin";
+    e2.placement = EntryPlacement::Blob;
+    e2.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("dup"))};
+
+    e2.blob_size = 3;
+    const ManifestId id2 = b2->stageManifest({e2});
+    b2->precommitAdd(RootNamespace{"srv/tbl"}, "ref2", id2);
+    counting->stream_puts = 0;
+    b2->putBlob(idOf("dup"), BlobSource::fromString("dup"));
+    EXPECT_EQ(counting->stream_puts, 0u);                  /// body PUT avoided
+}
+
+/// Task 3 (Round-B §0.3 introspection): the raw dedup_cache presence-lookup counters increment
+/// independently of what putBlob does with the answer. First lookup of a fresh hash misses (nothing
+/// cached yet); the identical second writer's lookup hits (the first writer's dedupCacheAdd populated
+/// the entry). putBlob's HEAD-first branch re-checks dedupCacheContains a second time purely to
+/// attribute CASBlobBodyPutAvoided to the cache (CasPartWriteTxn.cpp), so a genuine hit can bump
+/// CASDeduplicationCacheHits twice for one putBlob call -- hence GE, not EQ, on the hit delta below.
+TEST(CASDeduplicationCache, HitMissCountersIncrement)
+{
+    using ProfileEvents::global_counters;
+    auto counting = std::make_shared<CountingBackend>(std::make_shared<InMemoryBackend>());
+    auto s = Pool::open(counting, cfg(64ULL << 20, 1ULL << 20));
+
+    const auto miss_before = global_counters[ProfileEvents::CASDeduplicationCacheMisses].load();
+    const auto hits_before = global_counters[ProfileEvents::CASDeduplicationCacheHits].load();
+
+    /// First writer: cold cache, small body below the P2 size threshold -> the lookup misses.
+    auto b1 = s->beginPartWrite({});
+    b1->putBlob(idOf("dup"), BlobSource::fromString("dup"));
+    EXPECT_EQ(global_counters[ProfileEvents::CASDeduplicationCacheMisses].load() - miss_before, 1);
+
+    /// Second writer, identical content: the cache now holds the hash -> the lookup hits. The
+    /// head-first hit ADOPTS an existing incarnation, so it must run under a durable precommit edge
+    /// (EDGE-BEFORE-OBSERVE: stageManifest -> precommitAdd before putBlob), mirroring
+    /// HitTakesHeadFirstNoBodyPut above.
+    PartWriteInfo info2;
+    info2.intended_ref = "srv/tbl/ref2";
+    auto b2 = s->beginPartWrite(info2);
+    ManifestEntry e2;
+    e2.path = "data.bin";
+    e2.placement = EntryPlacement::Blob;
+    e2.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("dup"))};
+    e2.blob_size = 3;
+    const ManifestId id2 = b2->stageManifest({e2});
+    b2->precommitAdd(RootNamespace{"srv/tbl"}, "ref2", id2);
+    b2->putBlob(idOf("dup"), BlobSource::fromString("dup"));
+
+    EXPECT_EQ(global_counters[ProfileEvents::CASDeduplicationCacheMisses].load() - miss_before, 1);
+    EXPECT_GE(global_counters[ProfileEvents::CASDeduplicationCacheHits].load() - hits_before, 1);
+}
+
+/// Task 5 (P1 safety): a STALE cache hit (hash marked present but absent in the store) must not cause a
+/// dangle — the mandatory HEAD sees 404 and the writer falls through to a real body PUT.
+TEST(CASDeduplicationCache, StaleHitFallsThroughToPut)
+{
+    auto counting = std::make_shared<CountingBackend>(std::make_shared<InMemoryBackend>());
+    auto s = Pool::open(counting, cfg(64ULL << 20, 1ULL << 20));
+
+    /// Poison the cache: claim "stale" is present though nothing was ever uploaded.
+    s->dedupCacheAdd(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("stale"))});
+
+    auto b = s->beginPartWrite({});
+    counting->heads = 0;
+    counting->stream_puts = 0;
+    auto ref = b->putBlob(idOf("stale"), BlobSource::fromString("stale"));
+    EXPECT_EQ(ref.size, 5u);
+    EXPECT_GE(counting->heads, 1u);                        /// the safety HEAD ran
+    EXPECT_EQ(counting->stream_puts, 1u);                  /// and the body was actually uploaded
+    EXPECT_TRUE(counting->head(s->layout().blobKey(ref.ref)).exists);
+}
+
+/// Task 5 (P2): on a cold cache, a body at/above deduplication_head_first_min_bytes still probes HEAD-first
+/// (here the size trigger fires for a tiny body because the threshold is set to 1). The miss falls
+/// through to a real PUT.
+TEST(CASDeduplicationCache, LargeBlobMissTakesHeadFirst)
+{
+    auto counting = std::make_shared<CountingBackend>(std::make_shared<InMemoryBackend>());
+    auto s = Pool::open(counting, cfg(64ULL << 20, /*head_first_min_bytes*/ 1));
+
+    auto b = s->beginPartWrite({});
+    counting->heads = 0;
+    counting->stream_puts = 0;
+    b->putBlob(idOf("big"), BlobSource::fromString("big"));
+    EXPECT_EQ(counting->heads, 1u);                        /// P2 probed before the PUT
+    EXPECT_EQ(counting->stream_puts, 1u);                  /// cold miss ⇒ body uploaded
+}
diff --git a/src/Disks/tests/gtest_ca_transaction.cpp b/src/Disks/tests/gtest_ca_transaction.cpp
new file mode 100644
index 000000000000..d65e63f49401
--- /dev/null
+++ b/src/Disks/tests/gtest_ca_transaction.cpp
@@ -0,0 +1,754 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/ProfileEvents.h>
+#include <IO/ReadHelpers.h>
+#include <IO/ReadSettings.h>
+#include <filesystem>
+
+namespace ProfileEvents
+{
+extern const Event CASRefRepoint;
+extern const Event CASManifestHead;
+}
+
+namespace DB::ErrorCodes
+{
+extern const int NOT_IMPLEMENTED;
+extern const int INVALID_STATE;
+}
+
+/// [TXN-ONE-PIPELINE] CA publish-at-commit lock-scope tests.
+/// Proves that a freshly-written part's FINAL manifest ref is published only by commit(); the
+/// tmp->final rename (moveDirectory) is a pure re-key of the transaction-private overlay and
+/// publishes nothing. This inverts the former B151 publish-at-rename behavior.
+
+namespace
+{
+
+/// Constructs the storage but deliberately does NOT call `startup()` -- used by tests that need to
+/// control when/how startup runs (e.g. injecting a late fault before the atomic publish step).
+std::shared_ptr<DB::ContentAddressedMetadataStorage> makeUnstartedTxStorage()
+{
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_tx_lockscope_scratch");
+    return std::make_shared<DB::ContentAddressedMetadataStorage>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+}
+
+std::shared_ptr<DB::ContentAddressedMetadataStorage> openTxStorage()
+{
+    auto storage = makeUnstartedTxStorage();
+    storage->startup();
+    return storage;
+}
+
+void writeFileTx(DB::IMetadataTransaction & tx, const std::string & path, const std::string & bytes)
+{
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(tx);
+    auto buf = ca_tx.writeFile(path, 65536, DB::WriteMode::Rewrite, {});
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+}
+
+/// Match a manifest entry by its basename (the canonical `path` is the full part-relative path).
+const DB::Cas::ManifestEntry * findByName(const std::vector<DB::Cas::ManifestEntry> & entries, const std::string & name)
+{
+    for (const auto & e : entries)
+    {
+        const auto slash = e.path.find_last_of('/');
+        const std::string base = slash == std::string::npos ? e.path : e.path.substr(slash + 1);
+        if (base == name)
+            return &e;
+    }
+    return nullptr;
+}
+
+}
+
+/// Regression for STID 0883 on the CAS write path: an extreme `max_compress_block_size` (the exact
+/// 2^63-1 the `04070_no_crash_extreme_compress_block_size` stateless test sets) flows into
+/// `writeFile`'s `buf_size` and, unclamped, reaches `Memory::alloc` -- where the allocator's
+/// `checkSize` (>= 0x8000000000000000) fires a `LOGICAL_ERROR` and aborts the server. The ordinary
+/// MergeTree writers clamp compress-block sizes to 256 MiB; the CAS write buffer must do the same at
+/// its own allocation site. Building the buffer with the extreme size must NOT throw/abort, and the
+/// resulting buffer must be clamped -- never allocated at the extreme size.
+TEST(CASContentWriteBuffer, ExtremeBufferSizeIsClampedNotPassedToAllocator)
+{
+    const auto scratch = std::filesystem::temp_directory_path() / "ca_extreme_bufsize_scratch";
+
+    constexpr size_t extreme = 0x7FFFFFFFFFFFFFFFULL;   /// 2^63 - 1; unclamped this crashes the allocator
+    constexpr size_t max_clamped = 256ULL * 1024 * 1024;
+
+    std::unique_ptr<DB::Cas::CaContentWriteBuffer> buf;
+    ASSERT_NO_THROW(
+        buf = std::make_unique<DB::Cas::CaContentWriteBuffer>(
+            scratch.string(),
+            DB::Cas::BlobHashAlgo::CityHash128,
+            /*buf_size=*/extreme,
+            /*use_adaptive_buffer_size=*/false,
+            /*adaptive_buffer_initial_size=*/extreme,
+            [](const std::string &, size_t, const std::string &) {}));
+    ASSERT_TRUE(buf);
+    EXPECT_LE(buf->internalBuffer().size(), max_clamped)
+        << "the CAS write buffer must be clamped, never allocated at the extreme compress-block size";
+
+    buf.reset();
+    std::filesystem::remove_all(scratch);
+}
+
+/// [TXN-ONE-PIPELINE] A freshly-written part is published by commit(), NOT at the tmp->final rename.
+/// moveDirectory only re-keys the transaction overlay; the durable ref appears at commit().
+TEST(CASTransactionLockScope, PublishHappensAtCommitNotRename)
+{
+    auto storage = openTxStorage();
+    auto tx = storage->createTransaction();
+    writeFileTx(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_1_1_0/data.bin", "content-A");
+
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_1_1_0"));
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+
+    tx->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_1_1_0", "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+
+    /// Re-key only: the final ref is NOT durable yet.
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+
+    tx->commit(DB::NoCommitOptions{});
+
+    /// Published by commit().
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"), 9u);
+}
+
+TEST(CASTransactionOps, TruncateFileIsNotSupported)
+{
+    auto storage = openTxStorage();
+    auto tx = storage->createTransaction();
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NOT_IMPLEMENTED,
+        [&] { ca_tx.truncateFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", 0); });
+}
+
+/// [TXN-ONE-PIPELINE] An abandoned transaction (destructed without commit) never published, so the
+/// final ref is simply absent — no early-published ref to drop.
+TEST(CASTransactionLockScope, AbandonedPartLeavesNoRef)
+{
+    auto storage = openTxStorage();
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_3_3_0/data.bin", "abandoned");
+        tx->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_3_3_0", "a11/a11a11a1-1111-4111-8111-111111111111/all_3_3_0");
+        EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_3_3_0"));   /// not published at the rename
+        /// tx goes out of scope WITHOUT commit().
+    }
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_3_3_0"));
+}
+
+/// [TXN-ONE-PIPELINE] commit() publishes the re-keyed part.
+TEST(CASTransactionLockScope, RefPublishedByCommit)
+{
+    auto storage = openTxStorage();
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_4_4_0/data.bin", "kept");
+        tx->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_4_4_0", "a11/a11a11a1-1111-4111-8111-111111111111/all_4_4_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_4_4_0"));
+}
+
+/// A committed-ref rename (no staged source) must NOT spuriously publish — it goes via republishRef.
+TEST(CASTransactionLockScope, CommittedRefMoveDoesNotSpuriouslyPublish)
+{
+    auto storage = openTxStorage();
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_2_2_0/data.bin", "payload");
+        tx->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_2_2_0", "a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0"));
+    {
+        auto tx = storage->createTransaction();
+        tx->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0", "a11/a11a11a1-1111-4111-8111-111111111111/delete_tmp_all_2_2_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/delete_tmp_all_2_2_0"));
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0"));
+}
+
+/// [TXN-ONE-PIPELINE] B183 migration gate: a scratch ref durably published at the part's own (tmp)
+/// BUILD path by a nested sub-storage must be dropped on the staged-source tmp->final finalize, and
+/// commit() must publish the AUTHORITATIVE staged manifest (not the scratch content). This mirrors
+/// `createTemporaryTextIndexStorage`, which publishes scratch under the new_data_part's STILL-TMP
+/// relative path (`MergeTask.cpp` uses `getDataPartStorage().getRelativePath()`) — i.e. the SOURCE of
+/// the tmp->final rename, which is exactly what `moveDirectory`'s `dropRefIfPresent(src->refKey())`
+/// drops. (The plan's destination-path scenario would not reproduce this: `publishStaging`'s
+/// repoint-merge would carry the scratch file forward.)
+TEST(CASTransactionLockScope, StagedFinalizeDropsForeignScratchRef)
+{
+    auto storage = openTxStorage();
+
+    /// A SEPARATE transaction (the nested text-index sub-storage) durably publishes a committed ref at
+    /// the tmp BUILD path holding only a scratch file under `text_index_tmp/`.
+    {
+        auto scratch_tx = storage->createTransaction();
+        writeFileTx(*scratch_tx, "a77/a77a77a7-7777-4777-8777-777777777777/tmp_merge_all_1_1_0/text_index_tmp/scratch.bin", "scratch");
+        scratch_tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsDirectory("a77/a77a77a7-7777-4777-8777-777777777777/tmp_merge_all_1_1_0"));
+
+    /// The real part build: stage the authoritative data.bin under the SAME tmp path, then finalize
+    /// tmp->final. The staged-source finalize drops the foreign scratch ref at the tmp path.
+    auto tx = storage->createTransaction();
+    writeFileTx(*tx, "a77/a77a77a7-7777-4777-8777-777777777777/tmp_merge_all_1_1_0/data.bin", std::string(50000, 'D'));
+    tx->moveDirectory("a77/a77a77a7-7777-4777-8777-777777777777/tmp_merge_all_1_1_0", "a77/a77a77a7-7777-4777-8777-777777777777/all_1_1_0");
+    tx->commit(DB::NoCommitOptions{});
+
+    /// The published manifest is the authoritative one (has data.bin), not the scratch ref.
+    const auto ns = storage->liveNamespace("a77a77a7-7777-4777-8777-777777777777");
+    const auto resolved = storage->store()->resolveRef(ns, "all_1_1_0");
+    ASSERT_TRUE(resolved.has_value());
+    const auto manifest = storage->store()->readManifest(resolved->manifest_id);
+    EXPECT_TRUE(findByName(manifest.entries, "data.bin"));
+    EXPECT_FALSE(findByName(manifest.entries, "scratch.bin"));
+
+    /// The foreign scratch ref at the tmp build path is gone (dropped, not carried forward).
+    EXPECT_FALSE(storage->existsDirectory("a77/a77a77a7-7777-4777-8777-777777777777/tmp_merge_all_1_1_0"));
+}
+
+/// [TXN-ONE-PIPELINE] After a tmp->final re-key, a read THROUGH the open transaction resolves the
+/// staged content under the FINAL path (read-your-writes), before commit(); the inner-directory
+/// overlay is likewise re-keyed and answers under the final path. The staged file lives under an
+/// inner projection dir because the directory overlay tracks INNER dirs only — the part dir itself
+/// answers `hasInFlightDirectory`=false by contract (removeIfNeeded clean early-return; see
+/// `CASWiringInFlight`), so asserting the bare part dir would contradict that invariant.
+TEST(CASTransactionLockScope, ReadYourWritesAfterReKey)
+{
+    auto storage = openTxStorage();
+    auto tx = storage->createTransaction();
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+
+    writeFileTx(*tx, "a33/a33a33a3-3333-4333-8333-333333333333/tmp_insert_all_1_1_0/p.proj/checksums.txt", "the-checksums");
+    tx->moveDirectory("a33/a33a33a3-3333-4333-8333-333333333333/tmp_insert_all_1_1_0", "a33/a33a33a3-3333-4333-8333-333333333333/all_1_1_0");
+
+    /// The overlay answers the final path before commit (read-your-writes).
+    auto buf = ca_tx.tryReadFileInFlight("a33/a33a33a3-3333-4333-8333-333333333333/all_1_1_0/p.proj/checksums.txt", DB::ReadSettings{}, std::nullopt);
+    ASSERT_NE(buf, nullptr);
+    std::string got;
+    DB::readStringUntilEOF(got, *buf);
+    EXPECT_EQ(got, "the-checksums");
+    /// The inner-directory overlay is re-keyed too and resolves under the final path.
+    EXPECT_TRUE(ca_tx.hasInFlightDirectory("a33/a33a33a3-3333-4333-8333-333333333333/all_1_1_0/p.proj"));
+}
+
+/// [TXN-ONE-PIPELINE] Program order in the overlay: create -> delete -> create leaves the file PRESENT
+/// (no delayed delete fires after the later create); delete of a staged file makes it absent to reads.
+TEST(CASTransactionLockScope, OverlayProgramOrder)
+{
+    auto storage = openTxStorage();
+    auto tx = storage->createTransaction();
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+
+    writeFileTx(*tx, "a55/a55a55a5-5555-4555-8555-555555555555/tmp_insert_all_1_1_0/a.txt", "v1");
+    ca_tx.unlinkFile("a55/a55a55a5-5555-4555-8555-555555555555/tmp_insert_all_1_1_0/a.txt", /*if_exists=*/false, /*should_remove_objects=*/true);
+    EXPECT_EQ(ca_tx.tryReadFileInFlight("a55/a55a55a5-5555-4555-8555-555555555555/tmp_insert_all_1_1_0/a.txt", DB::ReadSettings{}, std::nullopt), nullptr);
+
+    writeFileTx(*tx, "a55/a55a55a5-5555-4555-8555-555555555555/tmp_insert_all_1_1_0/a.txt", "v2");
+    tx->moveDirectory("a55/a55a55a5-5555-4555-8555-555555555555/tmp_insert_all_1_1_0", "a55/a55a55a5-5555-4555-8555-555555555555/all_1_1_0");
+    tx->commit(DB::NoCommitOptions{});
+
+    ASSERT_TRUE(storage->existsFile("a55/a55a55a5-5555-4555-8555-555555555555/all_1_1_0/a.txt"));
+    EXPECT_EQ(storage->getFileSize("a55/a55a55a5-5555-4555-8555-555555555555/all_1_1_0/a.txt"), 2u);
+}
+
+/// [02941 root-cause] A carried-forward projection sidecar (createHardLink from a COMMITTED source part
+/// into a mutated tmp part) must be readable through the transaction's in-flight read path BOTH at the
+/// tmp build path (loadProjections runs here during MutateTask finalize) AND after the tmp->final re-key.
+/// This is the exact sequence MATERIALIZE PROJECTION drives on a part that already has the projection.
+/// If the in-flight read returns empty, the mutated part's in-memory projection sub-part loads with 0
+/// marks (the 02941 "Empty marks file: 0, must be: 144" corruption on a same-session projection SELECT).
+TEST(CASTransactionLockScope, InFlightReadCarriedForwardProjectionSidecar)
+{
+    auto storage = openTxStorage();
+
+    /// 1. Commit a source part with a small INLINE projection sidecar (marks-like) + a blob.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b01/b01b01b0-0101-4101-8101-010101010101/tmp_insert_all_1_1_0/data.bin", "the-main-data-bytes");
+        writeFileTx(*tx, "b01/b01b01b0-0101-4101-8101-010101010101/tmp_insert_all_1_1_0/aaaa.proj/data.cmrk4", "PROJMARKS9");
+        tx->moveDirectory("b01/b01b01b0-0101-4101-8101-010101010101/tmp_insert_all_1_1_0", "b01/b01b01b0-0101-4101-8101-010101010101/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsFile("b01/b01b01b0-0101-4101-8101-010101010101/all_1_1_0/aaaa.proj/data.cmrk4"));
+
+    /// 2. Mutation: build a new tmp part + carry the projection sidecar forward via createHardLink.
+    auto tx = storage->createTransaction();
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+    writeFileTx(*tx, "b01/b01b01b0-0101-4101-8101-010101010101/tmp_mut_all_1_1_0_2/data.bin", "mutated-main-data");
+    ca_tx.createHardLink("b01/b01b01b0-0101-4101-8101-010101010101/all_1_1_0/aaaa.proj/data.cmrk4",
+                         "b01/b01b01b0-0101-4101-8101-010101010101/tmp_mut_all_1_1_0_2/aaaa.proj/data.cmrk4");
+
+    /// 2a. loadProjections timing: read the carried sidecar in-flight at the TMP build path (pre-re-key).
+    {
+        auto buf = ca_tx.tryReadFileInFlight("b01/b01b01b0-0101-4101-8101-010101010101/tmp_mut_all_1_1_0_2/aaaa.proj/data.cmrk4", DB::ReadSettings{}, std::nullopt);
+        ASSERT_NE(buf, nullptr) << "carried-forward projection sidecar not readable in-flight at the tmp path";
+        std::string got; DB::readStringUntilEOF(got, *buf);
+        EXPECT_EQ(got, "PROJMARKS9");
+        EXPECT_EQ(ca_tx.tryGetInFlightFileSize("b01/b01b01b0-0101-4101-8101-010101010101/tmp_mut_all_1_1_0_2/aaaa.proj/data.cmrk4"),
+                  std::optional<uint64_t>(10));
+    }
+
+    /// 2b. After the tmp->final re-key (Phase 1), the sidecar must still resolve at the final path.
+    tx->moveDirectory("b01/b01b01b0-0101-4101-8101-010101010101/tmp_mut_all_1_1_0_2", "b01/b01b01b0-0101-4101-8101-010101010101/all_1_1_0_2");
+    {
+        auto buf = ca_tx.tryReadFileInFlight("b01/b01b01b0-0101-4101-8101-010101010101/all_1_1_0_2/aaaa.proj/data.cmrk4", DB::ReadSettings{}, std::nullopt);
+        ASSERT_NE(buf, nullptr) << "carried-forward projection sidecar not readable in-flight at the final path after re-key";
+        std::string got; DB::readStringUntilEOF(got, *buf);
+        EXPECT_EQ(got, "PROJMARKS9");
+    }
+
+    /// 3. And after commit it is durable + correct.
+    tx->commit(DB::NoCommitOptions{});
+    EXPECT_EQ(storage->getFileSize("b01/b01b01b0-0101-4101-8101-010101010101/all_1_1_0_2/aaaa.proj/data.cmrk4"), 10u);
+}
+
+/// [TXN-ONE-PIPELINE] Audit 5: on a commit, only refs this commit CREATED are eligible for rollback;
+/// a repoint of an already-existing ref is NEVER dropped as compensation. `publishStaging` writes a
+/// `CommitOutcome` with `created=false` for the repoint path (a committed ref exists), so `commit`'s
+/// rollback loop skips this slot and the pre-existing part survives with its content carried forward.
+TEST(CASTransactionLockScope, CommitRollbackSparesPreexistingRef)
+{
+    auto storage = openTxStorage();
+
+    /// Pre-existing committed part.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "a88/a88a88a8-8888-4888-8888-888888888888/tmp_insert_all_1_1_0/data.bin", "orig");
+        tx->moveDirectory("a88/a88a88a8-8888-4888-8888-888888888888/tmp_insert_all_1_1_0", "a88/a88a88a8-8888-4888-8888-888888888888/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsDirectory("a88/a88a88a8-8888-4888-8888-888888888888/all_1_1_0"));
+
+    /// A standalone write on the committed part repoints the EXISTING ref. Even if a later part in the
+    /// same commit were to fail, the existing ref must survive: `publishStaging` writes `created=false`
+    /// for this slot, so `commit`'s rollback loop skips it and it is never dropped on the error path.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "a88/a88a88a8-8888-4888-8888-888888888888/all_1_1_0/metadata_version.txt", "1");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    EXPECT_TRUE(storage->existsDirectory("a88/a88a88a8-8888-4888-8888-888888888888/all_1_1_0"));
+    EXPECT_TRUE(storage->existsFile("a88/a88a88a8-8888-4888-8888-888888888888/all_1_1_0/data.bin"));   /// original content carried forward
+}
+
+/// Plan 2d: a small eager metadata file (checksums.txt) is staged INLINE — it rides the single tree
+/// object (one-GET part open) — while per-column data (data.bin) stays a standalone Blob (preserving
+/// column-read selectivity). The inlined file is still readable through the normal read path.
+TEST(CASTransactionInlining, EagerFileInlinedDataBinBlobbed)
+{
+    auto storage = openTxStorage();
+    auto tx = storage->createTransaction();
+    writeFileTx(*tx, "a99/a99a99a9-9999-4999-8999-999999999999/tmp_insert_all_1_1_0/checksums.txt", "the-checksums");
+    writeFileTx(*tx, "a99/a99a99a9-9999-4999-8999-999999999999/tmp_insert_all_1_1_0/data.bin", std::string(50000, 'D'));
+    tx->moveDirectory("a99/a99a99a9-9999-4999-8999-999999999999/tmp_insert_all_1_1_0", "a99/a99a99a9-9999-4999-8999-999999999999/all_1_1_0");
+    tx->commit(DB::NoCommitOptions{});
+
+    /// Resolve the published part to its manifest and inspect placements (the Pool read API, as in
+    /// gtest_cas_pool.cpp: resolveRef -> readManifest).
+    const auto ns = storage->liveNamespace("a99a99a9-9999-4999-8999-999999999999");
+    const auto resolved = storage->store()->resolveRef(ns, "all_1_1_0");
+    ASSERT_TRUE(resolved.has_value());
+    const DB::Cas::PartManifest manifest = storage->store()->readManifest(resolved->manifest_id);
+    const auto & entries = manifest.entries;
+
+    const auto * checksums = findByName(entries, "checksums.txt");
+    const auto * databin   = findByName(entries, "data.bin");
+    ASSERT_TRUE(checksums && databin);
+    EXPECT_EQ(checksums->placement, DB::Cas::EntryPlacement::Inline);
+    EXPECT_EQ(checksums->inline_bytes, "the-checksums");
+    EXPECT_EQ(databin->placement, DB::Cas::EntryPlacement::Blob);
+
+    /// And the inlined file is still readable through the normal read path.
+    EXPECT_EQ(storage->getFileSize("a99/a99a99a9-9999-4999-8999-999999999999/all_1_1_0/checksums.txt"), 13u);
+}
+
+/// all-tree-part-files Task 4: a standalone
+/// write of ONE file onto an ALREADY-COMMITTED part must carry every other file of that part forward
+/// (a repoint, Task 3), never replace the manifest with just the touched file.
+TEST(CASTransactionRepoint, StandaloneWriteOnCommittedPartRepoints)
+{
+    auto storage = openTxStorage();
+
+    /// 1. Write a part (checksums.txt inline + data.bin blob) through a normal transaction; commit.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b02/b02b02b0-0202-4202-8202-020202020202/tmp_insert_all_1_1_0/checksums.txt", "old-checksums");
+        writeFileTx(*tx, "b02/b02b02b0-0202-4202-8202-020202020202/tmp_insert_all_1_1_0/data.bin", "the-data-bytes");
+        tx->moveDirectory("b02/b02b02b0-0202-4202-8202-020202020202/tmp_insert_all_1_1_0", "b02/b02b02b0-0202-4202-8202-020202020202/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsFile("b02/b02b02b0-0202-4202-8202-020202020202/all_1_1_0/checksums.txt"));
+    ASSERT_TRUE(storage->existsFile("b02/b02b02b0-0202-4202-8202-020202020202/all_1_1_0/data.bin"));
+
+    const uint64_t repoints_before = ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load();
+
+    /// 2. New transaction: standalone write of checksums.txt onto the ALREADY-COMMITTED part.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b02/b02b02b0-0202-4202-8202-020202020202/all_1_1_0/checksums.txt", "new-checksums-longer");
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// 3. The new content is served, the untouched file is carried forward unchanged, exactly one
+    /// repoint fired, and an independent fsck reachability walk finds nothing dangling.
+    EXPECT_EQ(storage->getFileSize("b02/b02b02b0-0202-4202-8202-020202020202/all_1_1_0/checksums.txt"), 20u);
+    EXPECT_EQ(storage->getFileSize("b02/b02b02b0-0202-4202-8202-020202020202/all_1_1_0/data.bin"), 14u)
+        << "carry-forward: the untouched file must survive a standalone write on the same part";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load(), repoints_before + 1);
+
+    const auto rep = DB::Cas::runFsck(*storage->store(), /*detail*/false);
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// Task 9 coverage gap (closed here, folded in from the T8 review): ONE uncommitted transaction that
+/// BOTH writes a file and unlinks a DIFFERENT file of the SAME already-committed part must resolve to
+/// exactly ONE repoint carrying the write, the removal, AND every untouched file forward together --
+/// not two independent repoints, and not a lost update from one staged change clobbering the other.
+/// `publishStaging`'s Task 4/8 merge already handles `st.entries` and `st.content_removed` together
+/// (both conditions can be true on the same staging); this pins that the combined shape actually works
+/// end to end through the real transaction, not just through each half in isolation.
+TEST(CASTransactionRepoint, CombinedWriteAndUnlinkSameTxnRepointsOnce)
+{
+    auto storage = openTxStorage();
+
+    /// 1. Commit a part with three files.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b03/b03b03b0-0303-4303-8303-030303030303/tmp_insert_all_1_1_0/checksums.txt", "old-checksums");
+        writeFileTx(*tx, "b03/b03b03b0-0303-4303-8303-030303030303/tmp_insert_all_1_1_0/data.bin", "the-data-bytes");
+        writeFileTx(*tx, "b03/b03b03b0-0303-4303-8303-030303030303/tmp_insert_all_1_1_0/txn_version.txt",
+            "creation_tid: (1,1,00000000-0000-0000-0000-000000000000)");
+        tx->moveDirectory("b03/b03b03b0-0303-4303-8303-030303030303/tmp_insert_all_1_1_0", "b03/b03b03b0-0303-4303-8303-030303030303/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsFile("b03/b03b03b0-0303-4303-8303-030303030303/all_1_1_0/txn_version.txt"));
+
+    const uint64_t repoints_before = ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load();
+
+    /// 2. ONE transaction: write checksums.txt (new bytes) AND unlink txn_version.txt (a DIFFERENT
+    /// file of the same part) -- must resolve to exactly one repoint carrying both changes plus the
+    /// untouched data.bin.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b03/b03b03b0-0303-4303-8303-030303030303/all_1_1_0/checksums.txt", "new-checksums-longer");
+        tx->unlinkFile("b03/b03b03b0-0303-4303-8303-030303030303/all_1_1_0/txn_version.txt", /*if_exists=*/false, /*should_remove_objects=*/true);
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// 3. The written file is updated, the unlinked file is honestly gone, the untouched file survives
+    /// (carry-forward), exactly ONE repoint fired (not two, not zero), and fsck finds nothing dangling.
+    EXPECT_EQ(storage->getFileSize("b03/b03b03b0-0303-4303-8303-030303030303/all_1_1_0/checksums.txt"), 20u);
+    EXPECT_FALSE(storage->existsFile("b03/b03b03b0-0303-4303-8303-030303030303/all_1_1_0/txn_version.txt"));
+    EXPECT_EQ(storage->getFileSize("b03/b03b03b0-0303-4303-8303-030303030303/all_1_1_0/data.bin"), 14u)
+        << "carry-forward: the untouched file must survive a combined write+unlink on the same part";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load(), repoints_before + 1)
+        << "one uncommitted transaction combining a write and an unlink must resolve to exactly one repoint";
+
+    const auto rep = DB::Cas::runFsck(*storage->store(), /*detail*/false);
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// all-tree-part-files Task 6: the mutable-
+/// per-part-file branch is deleted from `writeFile` -- uuid.txt/metadata_version.txt/txn_version.txt
+/// now flow down the ordinary content path, landing in the manifest like any other file.
+TEST(CASTransactionAllTree, BuildTimeSidecarsLandInManifest)
+{
+    auto storage = openTxStorage();
+    auto tx = storage->createTransaction();
+    writeFileTx(*tx, "b04/b04b04b0-0404-4404-8404-040404040404/tmp_insert_all_1_1_0/uuid.txt", "part-uuid-bytes");
+    writeFileTx(*tx, "b04/b04b04b0-0404-4404-8404-040404040404/tmp_insert_all_1_1_0/metadata_version.txt", "3");
+    writeFileTx(*tx, "b04/b04b04b0-0404-4404-8404-040404040404/tmp_insert_all_1_1_0/txn_version.txt", "creation_tid: (1,1,00000000-0000-0000-0000-000000000000)");
+    writeFileTx(*tx, "b04/b04b04b0-0404-4404-8404-040404040404/tmp_insert_all_1_1_0/data.bin", "the-data-bytes");
+    tx->moveDirectory("b04/b04b04b0-0404-4404-8404-040404040404/tmp_insert_all_1_1_0", "b04/b04b04b0-0404-4404-8404-040404040404/all_1_1_0");
+    tx->commit(DB::NoCommitOptions{});
+
+    const auto ns = storage->liveNamespace("b04b04b0-0404-4404-8404-040404040404");
+    const auto resolved = storage->store()->resolveRef(ns, "all_1_1_0");
+    ASSERT_TRUE(resolved.has_value());
+
+    const DB::Cas::PartManifest manifest = storage->store()->readManifest(resolved->manifest_id);
+    const auto & entries = manifest.entries;
+    const auto * uuid_entry = findByName(entries, "uuid.txt");
+    const auto * meta_version_entry = findByName(entries, "metadata_version.txt");
+    const auto * txn_version_entry = findByName(entries, "txn_version.txt");
+    ASSERT_TRUE(uuid_entry && meta_version_entry && txn_version_entry)
+        << "all three sidecar files must land in the manifest as ordinary tree entries";
+    EXPECT_EQ(uuid_entry->placement, DB::Cas::EntryPlacement::Inline);
+    EXPECT_EQ(meta_version_entry->placement, DB::Cas::EntryPlacement::Inline);
+    EXPECT_EQ(txn_version_entry->placement, DB::Cas::EntryPlacement::Inline);
+    EXPECT_EQ(meta_version_entry->inline_bytes, "3");
+
+    /// And they are readable through the normal read path — Task 9 deleted the ForceFresh special
+    /// case these reads used to go through; they now resolve purely via the manifest view like any
+    /// other entry (existsFile / getFileSize / tryGetInManifestBytes).
+    EXPECT_TRUE(storage->existsFile("b04/b04b04b0-0404-4404-8404-040404040404/all_1_1_0/metadata_version.txt"));
+    EXPECT_EQ(storage->getFileSize("b04/b04b04b0-0404-4404-8404-040404040404/all_1_1_0/metadata_version.txt"), 1u);
+}
+
+/// A standalone one-shot write of txn_version.txt onto an ALREADY-COMMITTED part (the MVCC creation-
+/// CSN fill-in / removal-TID rewrite shape) must repoint (Task 4), never orphan the rest of the part.
+TEST(CASTransactionAllTree, CommittedTxnVersionStoreRepoints)
+{
+    auto storage = openTxStorage();
+
+    /// 1. Commit a part WITHOUT txn_version.txt.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b05/b05b05b0-0505-4505-8505-050505050505/tmp_insert_all_1_1_0/checksums.txt", "cs-bytes");
+        writeFileTx(*tx, "b05/b05b05b0-0505-4505-8505-050505050505/tmp_insert_all_1_1_0/data.bin", "the-data-bytes");
+        tx->moveDirectory("b05/b05b05b0-0505-4505-8505-050505050505/tmp_insert_all_1_1_0", "b05/b05b05b0-0505-4505-8505-050505050505/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_FALSE(storage->existsFile("b05/b05b05b0-0505-4505-8505-050505050505/all_1_1_0/txn_version.txt"));
+
+    const uint64_t repoints_before = ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load();
+
+    /// 2. A single-op transaction writes ONLY txn_version.txt onto the already-committed part (mirrors
+    /// the MVCC one-shot autocommit shape: no other file touched in this transaction).
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b05/b05b05b0-0505-4505-8505-050505050505/all_1_1_0/txn_version.txt", "creation_tid: (2,2,00000000-0000-0000-0000-000000000000)");
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// 3. Exactly one repoint; the new file is served; the original files are intact (carry-forward).
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load(), repoints_before + 1);
+    EXPECT_TRUE(storage->existsFile("b05/b05b05b0-0505-4505-8505-050505050505/all_1_1_0/txn_version.txt"));
+    EXPECT_EQ(storage->getFileSize("b05/b05b05b0-0505-4505-8505-050505050505/all_1_1_0/txn_version.txt"), 56u);
+    EXPECT_EQ(storage->getFileSize("b05/b05b05b0-0505-4505-8505-050505050505/all_1_1_0/checksums.txt"), 8u);
+    EXPECT_EQ(storage->getFileSize("b05/b05b05b0-0505-4505-8505-050505050505/all_1_1_0/data.bin"), 14u);
+
+    const auto rep = DB::Cas::runFsck(*storage->store(), /*detail*/false);
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// all-tree-part-files Task 8 (B123 evolution):
+/// a lone surgical unlink of ONE committed content file (not followed by a whole-part removal in the
+/// same transaction — the ATTACH `removeVersionMetadata` shape) must actually delete the file via a
+/// repoint-remove, closing the pre-Task-8 fail-open (unlinkFile of a committed content file used to be
+/// an unconditional no-op).
+TEST(CASTransactionRemove, SurgicalUnlinkRepoints)
+{
+    auto storage = openTxStorage();
+
+    /// 1. Commit a part with txn_version.txt among its files.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b06/b06b06b0-0606-4606-8606-060606060606/tmp_insert_all_1_1_0/checksums.txt", "cs-bytes");
+        writeFileTx(*tx, "b06/b06b06b0-0606-4606-8606-060606060606/tmp_insert_all_1_1_0/data.bin", "the-data-bytes");
+        writeFileTx(*tx, "b06/b06b06b0-0606-4606-8606-060606060606/tmp_insert_all_1_1_0/txn_version.txt",
+            "creation_tid: (1,1,00000000-0000-0000-0000-000000000000)");
+        tx->moveDirectory("b06/b06b06b0-0606-4606-8606-060606060606/tmp_insert_all_1_1_0", "b06/b06b06b0-0606-4606-8606-060606060606/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsFile("b06/b06b06b0-0606-4606-8606-060606060606/all_1_1_0/txn_version.txt"));
+
+    const uint64_t repoints_before = ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load();
+
+    /// 2. A single-op transaction unlinks ONLY txn_version.txt on the already-committed part (mirrors
+    /// ATTACH's removeVersionMetadata: no dir-drop in the same transaction).
+    {
+        auto tx = storage->createTransaction();
+        tx->unlinkFile("b06/b06b06b0-0606-4606-8606-060606060606/all_1_1_0/txn_version.txt", /*if_exists=*/false, /*should_remove_objects=*/true);
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// 3. The file is honestly gone, the untouched files survive (carry-forward), exactly one repoint
+    /// fired, and an independent fsck reachability walk finds nothing dangling.
+    EXPECT_FALSE(storage->existsFile("b06/b06b06b0-0606-4606-8606-060606060606/all_1_1_0/txn_version.txt"));
+    EXPECT_EQ(storage->getFileSize("b06/b06b06b0-0606-4606-8606-060606060606/all_1_1_0/checksums.txt"), 8u);
+    EXPECT_EQ(storage->getFileSize("b06/b06b06b0-0606-4606-8606-060606060606/all_1_1_0/data.bin"), 14u);
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load(), repoints_before + 1);
+
+    const auto rep = DB::Cas::runFsck(*storage->store(), /*detail*/false);
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// all-tree-part-files Task 8 (B123 evolution, spec §6): the DOMINANT CA removal path — the MergeTree
+/// fast-removal shape that unlinks every part file one by one and THEN calls removeDirectory — must
+/// stay exactly one ref-drop and pay ZERO repoints. The per-file removal marks staged by the unlink
+/// storm are superseded by the ref-drop, not individually repointed.
+TEST(CASTransactionRemove, UnlinkStormThenDirDropIsOneRefDrop)
+{
+    auto storage = openTxStorage();
+
+    /// 1. Commit a part with three files.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b07/b07b07b0-0707-4707-8707-070707070707/tmp_insert_all_1_1_0/checksums.txt", "cs-bytes");
+        writeFileTx(*tx, "b07/b07b07b0-0707-4707-8707-070707070707/tmp_insert_all_1_1_0/data.bin", "the-data-bytes");
+        writeFileTx(*tx, "b07/b07b07b0-0707-4707-8707-070707070707/tmp_insert_all_1_1_0/txn_version.txt",
+            "creation_tid: (1,1,00000000-0000-0000-0000-000000000000)");
+        tx->moveDirectory("b07/b07b07b0-0707-4707-8707-070707070707/tmp_insert_all_1_1_0", "b07/b07b07b0-0707-4707-8707-070707070707/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsDirectory("b07/b07b07b0-0707-4707-8707-070707070707/all_1_1_0"));
+
+    const uint64_t repoints_before = ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load();
+
+    /// 2. The MergeTree fast-removal shape (IMergeTreeDataPart::remove, B123): unlink every file
+    /// one-by-one, THEN removeDirectory the part — all in one transaction.
+    {
+        auto tx = storage->createTransaction();
+        tx->unlinkFile("b07/b07b07b0-0707-4707-8707-070707070707/all_1_1_0/checksums.txt", /*if_exists=*/false, /*should_remove_objects=*/true);
+        tx->unlinkFile("b07/b07b07b0-0707-4707-8707-070707070707/all_1_1_0/data.bin", /*if_exists=*/false, /*should_remove_objects=*/true);
+        tx->unlinkFile("b07/b07b07b0-0707-4707-8707-070707070707/all_1_1_0/txn_version.txt", /*if_exists=*/false, /*should_remove_objects=*/true);
+        tx->removeDirectory("b07/b07b07b0-0707-4707-8707-070707070707/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// 3. The whole part is gone via the single ref-drop; the storm of marks never repointed anything.
+    EXPECT_FALSE(storage->existsDirectory("b07/b07b07b0-0707-4707-8707-070707070707/all_1_1_0"));
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load(), repoints_before)
+        << "unlink-storm-then-dir-drop must supersede the marks, not repoint per file";
+
+    const auto rep = DB::Cas::runFsck(*storage->store(), /*detail*/false);
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// Task 22 (URF plan phase 7): the MergeTree fast-removal shape's per-file ForceFresh proof is
+/// memoized per (transaction, ref) in `unlinkFile` — the first unlink's `ForceFresh` `getView` re-proves
+/// the manifest body with one HEAD; the rest of the burst reuses that proof via `CachedForLoad`. This
+/// pins the HEAD-count side of `UnlinkStormThenDirDropIsOneRefDrop` above (which already pins the
+/// repoint count): before this memoization, N unlinks of the same part paid N manifest-body HEADs; now
+/// the whole storm-then-drop transaction pays exactly one.
+TEST(CASTransactionRemove, UnlinkStormMemoizesOneForceFreshHead)
+{
+    auto storage = openTxStorage();
+
+    /// 1. Commit a part with three files.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b09/b09b09b0-0909-4909-8909-090909090909/tmp_insert_all_1_1_0/checksums.txt", "cs-bytes");
+        writeFileTx(*tx, "b09/b09b09b0-0909-4909-8909-090909090909/tmp_insert_all_1_1_0/data.bin", "the-data-bytes");
+        writeFileTx(*tx, "b09/b09b09b0-0909-4909-8909-090909090909/tmp_insert_all_1_1_0/txn_version.txt",
+            "creation_tid: (1,1,00000000-0000-0000-0000-000000000000)");
+        tx->moveDirectory("b09/b09b09b0-0909-4909-8909-090909090909/tmp_insert_all_1_1_0", "b09/b09b09b0-0909-4909-8909-090909090909/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->existsDirectory("b09/b09b09b0-0909-4909-8909-090909090909/all_1_1_0"));
+
+    const uint64_t heads_before = ProfileEvents::global_counters[ProfileEvents::CASManifestHead].load();
+
+    /// 2. The MergeTree fast-removal shape: unlink every file one-by-one, THEN removeDirectory — all
+    /// in ONE transaction (mirrors UnlinkStormThenDirDropIsOneRefDrop above).
+    {
+        auto tx = storage->createTransaction();
+        tx->unlinkFile("b09/b09b09b0-0909-4909-8909-090909090909/all_1_1_0/checksums.txt", /*if_exists=*/false, /*should_remove_objects=*/true);
+        tx->unlinkFile("b09/b09b09b0-0909-4909-8909-090909090909/all_1_1_0/data.bin", /*if_exists=*/false, /*should_remove_objects=*/true);
+        tx->unlinkFile("b09/b09b09b0-0909-4909-8909-090909090909/all_1_1_0/txn_version.txt", /*if_exists=*/false, /*should_remove_objects=*/true);
+        tx->removeDirectory("b09/b09b09b0-0909-4909-8909-090909090909/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// 3. The whole part is gone, and the three-file unlink storm paid exactly ONE manifest-body HEAD
+    /// (the first unlink's ForceFresh proof) — not three. removeDirectory clears the staged removal
+    /// marks, so publishStaging's own (unmemoized) ForceFresh getView never fires for this ref either
+    /// (see UnlinkStormThenDirDropIsOneRefDrop's zero-repoints assertion above).
+    EXPECT_FALSE(storage->existsDirectory("b09/b09b09b0-0909-4909-8909-090909090909/all_1_1_0"));
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASManifestHead].load(), heads_before + 1)
+        << "unlink-storm-then-dir-drop must pay exactly one ForceFresh manifest-body HEAD, not one per file";
+}
+
+/// A create-then-remove of a new part in one transaction must discard both the manifest entries
+/// and the in-flight build, so commit() leaves no ref and no live precommit behind.
+TEST(CASTransactionRemove, CreateThenDirDropDoesNotPublish)
+{
+    auto storage = openTxStorage();
+    auto tx = storage->createTransaction();
+    writeFileTx(*tx, "b08/b08b08b0-0808-4808-8808-080808080808/all_1_1_0/data.bin", "created-then-removed");
+    tx->removeDirectory("b08/b08b08b0-0808-4808-8808-080808080808/all_1_1_0");
+    tx->commit(DB::NoCommitOptions{});
+
+    EXPECT_FALSE(storage->existsDirectory("b08/b08b08b0-0808-4808-8808-080808080808/all_1_1_0"));
+}
+
+/// [Task 3] `startup()` publishes `cas_store`/`part_access`/`gc_scheduler` (and sets
+/// `pool_uuid`/`conditional_copy_supported`) atomically as its LAST action. Everything
+/// before that point -- opening the pool, building the part-folder facade, the capability probe,
+/// starting the GC scheduler -- happens into locals first, so a throw anywhere along the way (here
+/// simulated via `startup_fault_injection_for_test`, injected right before the publish step) must
+/// leave nothing published: `store()` still refuses (null pool -- the Constructing lifecycle,
+/// `INVALID_STATE` "not started") even though `Pool::open` and everything else already succeeded.
+/// Clearing the hook and retrying `startup()` must then succeed cleanly.
+TEST(CASTransactionLifecycle, StartupFailureLatePublishesNothing)
+{
+    auto storage = makeUnstartedTxStorage();
+
+    storage->startup_fault_injection_for_test = [] { throw std::runtime_error("injected late-startup failure"); };
+    EXPECT_ANY_THROW(storage->startup());
+    /// Nothing was published by the failed attempt: store() must still refuse (null pool, not started).
+    EXPECT_ANY_THROW(storage->store());
+
+    storage->startup_fault_injection_for_test = {};
+    EXPECT_NO_THROW(storage->startup());
+    EXPECT_NO_THROW(storage->store());
+}
+
+/// [Task 4] The storage-lifecycle gate: `store()` (and every other caller of `poolAccess()`) must
+/// refuse with `INVALID_STATE` -- an operational condition, not a programming invariant -- whenever
+/// no pool is published (the null-pool ShutDown lifecycle after `shutdown`). This is a deliberate
+/// behavior change from the previous `LOGICAL_ERROR "accessed before startup"`, which would abort a
+/// debug/sanitizer build on a mis-sequenced access instead of surfacing a catchable, operator-actionable
+/// error.
+TEST(CASTransactionWiring, OperationsRefuseWithoutPublishedPool)
+{
+    auto storage = openTxStorage();
+    storage->shutdown();   /// terminal path resets cas_store to null (the ShutDown storage lifecycle)
+
+    try
+    {
+        storage->store();
+        FAIL() << "store() must refuse once the disk has no published pool";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::INVALID_STATE) << e.message();
+    }
+}
+
+/// (rev.8, Task 15) The Dormant/UNMOUNT lifecycle rollback flips the transitional benign-absent probe
+/// behavior to fail-loud on a NULL pool. A storage with no published pool (here: after `shutdown()`, the
+/// null-pool ShutDown storage lifecycle) refuses the ENTIRE surface -- including the read-only
+/// existence/enumeration probes that the old (now-deleted) `DormantDiskAnswersExistenceProbesAsAbsent`
+/// asserted answered benign-absent. This is spec §1's null-pool fail-loud contract: every op class,
+/// `Probe` included, throws `INVALID_STATE` ("not started"); a genuinely `Vanished` POOL is the only
+/// state that answers truth-absent, and a null pool is not that. (Behavior change documented in the
+/// Task 15 report: generic all-disk existence sweeps during server shutdown now see a throw here, not a
+/// benign absent; the shutdown window is the deliberate cost of never lying about a not-started disk.)
+TEST(CASLifecycle, ShutdownDiskProbesFailLoud)
+{
+    auto storage = openTxStorage();
+    storage->shutdown();   /// null pool -- the ShutDown storage lifecycle
+
+    const std::string file = "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin";
+    const std::string part_dir = "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0";
+
+    /// Every read-only probe now THROWS (not started), where the transitional Dormant path answered benign.
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { storage->existsDirectory("store"); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { storage->existsFile(file); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { storage->existsFileOrDirectory(part_dir); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { storage->isDirectoryEmpty("store"); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { (void)storage->listDirectory("store"); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { (void)storage->iterateDirectory("store"); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { (void)storage->getStorageObjectsIfExist(file); });
+
+    /// The content/size surface stays fail-close too (unchanged from the transitional behavior).
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { (void)storage->getFileSize(file); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { (void)storage->getStorageObjects(file); });
+}
diff --git a/src/Disks/tests/gtest_ca_wiring.cpp b/src/Disks/tests/gtest_ca_wiring.cpp
new file mode 100644
index 000000000000..cf9ee1e73dac
--- /dev/null
+++ b/src/Disks/tests/gtest_ca_wiring.cpp
@@ -0,0 +1,2963 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskLocal.h>
+#include <Common/ErrorCodes.h>
+#include <filesystem>
+
+namespace DB::ErrorCodes
+{
+    extern const int NOT_IMPLEMENTED;
+}
+
+/// M-W wiring tier (design 2026-06-11 section 7 tier 3): the ClickHouse-facing translation layer
+/// tested through its own seams. Task 1: PartPathParser — the path-classification rows plus the
+/// shadow/detached/mutable rows the later tasks route on.
+
+using namespace DB::Cas;
+
+TEST(CASPartPathParser, ParsePartFilePathAtomic)
+{
+    auto file = parsePartFilePath("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/columns.txt");
+    ASSERT_TRUE(file.has_value());
+    EXPECT_EQ(file->table_uuid, "a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(file->part_name, "all_1_1_0");
+    EXPECT_EQ(file->file, "columns.txt");
+    EXPECT_TRUE(file->backup_name.empty());
+    EXPECT_TRUE(file->shadow_table_dir.empty());
+
+    auto part_dir = parsePartFilePath("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/"); // trailing slash, no file
+    ASSERT_TRUE(part_dir.has_value());
+    EXPECT_EQ(part_dir->part_name, "all_1_1_0");
+    EXPECT_TRUE(part_dir->file.empty());
+
+    EXPECT_FALSE(parsePartFilePath("a11/a11a11a1-1111-4111-8111-111111111111").has_value()); // table dir, not a part
+    EXPECT_FALSE(parsePartFilePath("123").has_value());        // shallower
+
+    // The real-server shape carries a leading store/; the uuid-pair anchor makes it equivalent.
+    auto atomic = parsePartFilePath("store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin");
+    ASSERT_TRUE(atomic.has_value());
+    EXPECT_EQ(atomic->table_uuid, "a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(atomic->part_name, "all_1_1_0");
+    EXPECT_EQ(atomic->file, "data.bin");
+}
+
+TEST(CASPartPathParser, ThreeCharDatabaseSharingTablePrefixDoesNotFalseAnchorAsAtomic)
+{
+    // T12: a non-Atomic 3-char database directory whose table directory happens to start with the
+    // SAME 3 characters (db "abc", table "abcxyz") used to satisfy the old loose Atomic-anchor shape
+    // check (`prefix.size() == 3 && uuid.compare(0, 3, prefix) == 0`), false-anchoring "abc" as a
+    // UUID hash-prefix and "abcxyz" as the table UUID -- even though neither looks anything like a
+    // real UUID. The anchor now additionally requires the prefix to be lowercase-hex and the
+    // candidate to have the exact 36-char dashed UUID shape, so this path falls through to the
+    // non-Atomic fallback split instead (folding the whole leading path into table_uuid, exactly like
+    // ParsePartFilePathNonAtomic's "data/memory_01069/mt" case).
+    auto d = parsePartFilePath("data/abc/abcxyz/1_1_1_0/x.bin");
+    ASSERT_TRUE(d.has_value());
+    EXPECT_EQ(d->table_uuid, "data/abc/abcxyz");
+    EXPECT_EQ(d->part_name, "1_1_1_0");
+    EXPECT_EQ(d->file, "x.bin");
+}
+
+TEST(CASPartPathParser, RealHexPrefixUuidPairStillAnchorsAsAtomic)
+{
+    // Positive control for the tightened anchor: a REAL Atomic on-disk shape --
+    // store/<uuid[:3]>/<uuid> with the UUID correctly 36-char dashed and genuinely sharing its first
+    // 3 characters with the prefix -- still anchors exactly as before.
+    auto a = parsePartFilePath("store/abc/abc12345-1234-5678-9abc-def012345678/all_1_1_0/x.bin");
+    ASSERT_TRUE(a.has_value());
+    EXPECT_EQ(a->table_uuid, "abc12345-1234-5678-9abc-def012345678");
+    EXPECT_EQ(a->part_name, "all_1_1_0");
+    EXPECT_EQ(a->file, "x.bin");
+}
+
+TEST(CASPartPathParser, ParsePartFilePathProjectionSubPath)
+{
+    // A projection file keeps its FULL in-part relative path as the file (the tree entry name).
+    auto proj = parsePartFilePath("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj/data.bin");
+    ASSERT_TRUE(proj.has_value());
+    EXPECT_EQ(proj->part_name, "all_1_1_0");
+    EXPECT_EQ(proj->file, "p.proj/data.bin");
+}
+
+TEST(CASPartPathParser, ParsePartFilePathNonAtomic)
+{
+    // Non-Atomic (Ordinary/Memory/Lazy) layout: data/<db>/<table>/<part>/<file> — no uuid anchor;
+    // the part dir is recognized by its block-range suffix (B40).
+    auto file = parsePartFilePath("data/memory_01069/mt/all_1_1_0/data.cmrk4");
+    ASSERT_TRUE(file.has_value());
+    EXPECT_EQ(file->table_uuid, "data/memory_01069/mt");
+    EXPECT_EQ(file->part_name, "all_1_1_0");
+    EXPECT_EQ(file->file, "data.cmrk4");
+
+    // Temporary/operation prefixes keep the suffix and stay part dirs.
+    auto tmp = parsePartFilePath("data/memory_01069/mt/tmp_insert_all_1_1_0/data.cmrk4");
+    ASSERT_TRUE(tmp.has_value());
+    EXPECT_EQ(tmp->part_name, "tmp_insert_all_1_1_0");
+
+    // Mutation-level form <partition>_<min>_<max>_<level>_<mutation>.
+    auto mut = parsePartFilePath("data/db/tbl/20200101_1_1_0_5/data.bin");
+    ASSERT_TRUE(mut.has_value());
+    EXPECT_EQ(mut->part_name, "20200101_1_1_0_5");
+
+    // A non-Atomic table-level file is NOT a part file.
+    EXPECT_FALSE(isPartFilePath("data/memory_01069/mt/format_version.txt"));
+    auto tf = parseTableFilePath("data/memory_01069/mt/format_version.txt");
+    ASSERT_TRUE(tf.has_value());
+    EXPECT_EQ(tf->table_uuid, "data/memory_01069/mt");
+    EXPECT_EQ(tf->tail, "format_version.txt");
+
+    EXPECT_EQ(parseTableUuid("data/memory_01069/mt"), std::optional<std::string>("data/memory_01069/mt"));
+
+    // Generic disk-root files classify as nothing (verbatim passthrough).
+    EXPECT_FALSE(isPartFilePath("clickhouse_access_check_xyz"));
+    EXPECT_FALSE(parseTableFilePath("clickhouse_access_check_xyz").has_value());
+    EXPECT_FALSE(parseTableUuid("clickhouse_access_check_xyz").has_value());
+}
+
+TEST(CASPartPathParser, ParseTableUuid)
+{
+    EXPECT_EQ(parseTableUuid("a11/a11a11a1-1111-4111-8111-111111111111/"), std::optional<std::string>("a11a11a1-1111-4111-8111-111111111111"));
+    EXPECT_EQ(parseTableUuid("a11/a11a11a1-1111-4111-8111-111111111111"), std::optional<std::string>("a11a11a1-1111-4111-8111-111111111111"));
+    EXPECT_FALSE(parseTableUuid("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0").has_value()); // part dir, not table dir
+
+    EXPECT_TRUE(endsWithTableUuidPair("store/a11/a11a11a1-1111-4111-8111-111111111111"));
+    EXPECT_FALSE(endsWithTableUuidPair("store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_FALSE(endsWithTableUuidPair("shadow/bk1/store"));
+}
+
+TEST(CASPartPathParser, ParseTableFilePathNested)
+{
+    // The reserved deduplication_logs/ subdir is a table-level namespace, never a part dir.
+    EXPECT_FALSE(isPartFilePath("a11/a11a11a1-1111-4111-8111-111111111111/deduplication_logs/deduplication_log_1.txt"));
+    auto tf = parseTableFilePath("a11/a11a11a1-1111-4111-8111-111111111111/deduplication_logs/deduplication_log_1.txt");
+    ASSERT_TRUE(tf.has_value());
+    EXPECT_EQ(tf->table_uuid, "a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(tf->tail, "deduplication_logs/deduplication_log_1.txt");
+
+    auto flat = parseTableFilePath("a11/a11a11a1-1111-4111-8111-111111111111/format_version.txt");
+    ASSERT_TRUE(flat.has_value());
+    EXPECT_EQ(flat->tail, "format_version.txt");
+
+    EXPECT_FALSE(parseTableFilePath("a11/a11a11a1-1111-4111-8111-111111111111").has_value());
+    EXPECT_FALSE(parseTableFilePath("a11/a11a11a1-1111-4111-8111-111111111111/").has_value());
+
+    EXPECT_TRUE(isPartFilePath("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+}
+
+TEST(CASPartPathParser, ShadowFreezePaths)
+{
+    EXPECT_TRUE(isShadowPath("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_TRUE(isShadowPath("/shadow/bk1"));
+    EXPECT_FALSE(isShadowPath("store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_FALSE(isShadowPath("shadowy/bk1"));
+
+    auto s = parsePartFilePath("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin");
+    ASSERT_TRUE(s.has_value());
+    EXPECT_EQ(s->table_uuid, "a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(s->part_name, "all_1_1_0");
+    EXPECT_EQ(s->file, "data.bin");
+    EXPECT_EQ(s->backup_name, "bk1");
+    EXPECT_EQ(s->shadow_table_dir, "shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111");
+}
+
+TEST(CASPartPathParser, DetachedPathsReportTheSharedDetachedComponent)
+{
+    // The PoC contract (B36): "detached" parses as the part_name; the real detached part dir is
+    // the first component of `file`. The transaction/read routing re-splits on this shape.
+    auto d = parsePartFilePath("a11/a11a11a1-1111-4111-8111-111111111111/detached/attaching_all_0_0_0/metadata_version.txt");
+    ASSERT_TRUE(d.has_value());
+    EXPECT_EQ(d->table_uuid, "a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(d->part_name, std::string(kDetachedDirName));
+    EXPECT_EQ(d->file, "attaching_all_0_0_0/metadata_version.txt");
+}
+
+TEST(CASPartPathParser, MovingPathsReportTheSharedMovingComponent)
+{
+    // Atomic layout: "moving" lands on part_idx for free (it is the component right after the
+    // table <uuid>, same mechanism as "detached" -- no parser change needed here, only route()).
+    auto d = parsePartFilePath("a11/a11a11a1-1111-4111-8111-111111111111/moving/all_1_1_0/data.bin");
+    ASSERT_TRUE(d.has_value());
+    EXPECT_EQ(d->table_uuid, "a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(d->part_name, std::string(kMovingDirName));
+    EXPECT_EQ(d->file, "all_1_1_0/data.bin");
+}
+
+TEST(CASPartPathParser, MovingPathsNonAtomicFoldIntoTheTableNamespace)
+{
+    // Mirrors DetachedPathsNonAtomicFoldIntoTheTableNamespace (U#6): without an explicit anchor
+    // the right-to-left part-dir scan would anchor on the INNER real part dir and fold "moving"
+    // into a spurious table_uuid ("data/<db>/<table>/moving"), diverging from the table's real
+    // namespace -- the identical bug class the detached anchor was added to prevent.
+    auto d = parsePartFilePath("data/db/tbl/moving/all_1_1_0/data.bin");
+    ASSERT_TRUE(d.has_value());
+    EXPECT_EQ(d->table_uuid, "data/db/tbl");
+    EXPECT_EQ(d->part_name, std::string(kMovingDirName));
+    EXPECT_EQ(d->file, "all_1_1_0/data.bin");
+
+    // The bare non-Atomic moving CONTAINER dir folds to part_name == "moving" with an empty
+    // file, exactly like the Atomic container.
+    auto c = parsePartFilePath("data/db/tbl/moving");
+    ASSERT_TRUE(c.has_value());
+    EXPECT_EQ(c->table_uuid, "data/db/tbl");
+    EXPECT_EQ(c->part_name, std::string(kMovingDirName));
+    EXPECT_TRUE(c->file.empty());
+}
+
+TEST(CASPartPathParser, DetachedPathsNonAtomicFoldIntoTheTableNamespace)
+{
+    // U#6: the Ordinary/non-Atomic detached form data/<db>/<table>/detached/<part>/<file> must fold
+    // into the table's OWN namespace with part_name == "detached" (mirroring the Atomic form), so
+    // route() keys the detached/<part> ref off it. The right-to-left part-dir scan would otherwise
+    // anchor on the INNER part dir and fold `detached` into a spurious table_uuid
+    // ("data/<db>/<table>/detached") that DROP TABLE never cleans — a permanently orphaned live ref.
+    auto d = parsePartFilePath("data/db/tbl/detached/attaching_all_0_0_0/metadata_version.txt");
+    ASSERT_TRUE(d.has_value());
+    EXPECT_EQ(d->table_uuid, "data/db/tbl");
+    EXPECT_EQ(d->part_name, std::string(kDetachedDirName));
+    EXPECT_EQ(d->file, "attaching_all_0_0_0/metadata_version.txt");
+
+    // The bare non-Atomic detached CONTAINER dir folds to part_name == "detached" with an empty file,
+    // exactly like the Atomic container, so route()'s empty-ref branch is reached for both layouts.
+    auto c = parsePartFilePath("data/db/tbl/detached");
+    ASSERT_TRUE(c.has_value());
+    EXPECT_EQ(c->table_uuid, "data/db/tbl");
+    EXPECT_EQ(c->part_name, std::string(kDetachedDirName));
+    EXPECT_TRUE(c->file.empty());
+}
+
+TEST(CASPartPathParser, DetachedNamedTableIsKnownAmbiguityFoldedAsReservedDir)
+{
+    // ACCEPTED LIMITATION (see the anchor-site comment in findPartDirComponent): a non-Atomic
+    // database or TABLE literally named "detached" is structurally indistinguishable, from the path
+    // string alone, from the reserved detached subdir of a table one level up — so it gets folded
+    // as the reserved dir, not as a table name. This test PINS that known, deliberately-accepted
+    // behavior (backlogged by the stabilization campaign) so any future change to it is a conscious
+    // one, not an accidental regression.
+    auto d = parsePartFilePath("data/db/detached/all_1_1_0/data.bin");
+    ASSERT_TRUE(d.has_value());
+    EXPECT_EQ(d->table_uuid, "data/db");
+    EXPECT_EQ(d->part_name, std::string(kDetachedDirName));
+    EXPECT_EQ(d->file, "all_1_1_0/data.bin");
+
+    // Consequently the table dir itself is unrecognized: it looks like a detached container instead.
+    EXPECT_FALSE(parseTableUuid("data/db/detached").has_value());
+}
+
+TEST(CASPartPathParser, RawPathSplitMemoizedAcrossClassifiers)
+{
+    // The CA read path runs isPartFilePath then parsePartFilePath on the SAME raw path several times
+    // per logical file-open (existsFile -> getFileSize -> getStorageObjects). The split is a pure
+    // function of the path, so all of those must split the path exactly ONCE (B1).
+    resetSplitCacheForTest();
+    const std::string path = "store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/columns.txt";
+    EXPECT_TRUE(isPartFilePath(path));
+    ASSERT_TRUE(parsePartFilePath(path).has_value());
+    ASSERT_TRUE(parsePartFilePath(path).has_value());
+    EXPECT_EQ(splitCacheMissesForTest(), 1u) << "the same raw path must be split only once";
+
+    // A distinct raw path is a fresh split (miss #2); repeats of it reuse the memo.
+    const std::string other = "store/a22/a22a22a2-2222-4222-8222-222222222222/all_1_1_0/data.bin";
+    EXPECT_TRUE(isPartFilePath(other));
+    EXPECT_TRUE(isPartFilePath(other));
+    EXPECT_EQ(splitCacheMissesForTest(), 2u);
+
+    // Correctness is unchanged: the memoized parse yields the same fields the direct parse would.
+    const auto parsed = parsePartFilePath(path);
+    ASSERT_TRUE(parsed.has_value());
+    EXPECT_EQ(parsed->table_uuid, "a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(parsed->part_name, "all_1_1_0");
+    EXPECT_EQ(parsed->file, "columns.txt");
+}
+
+TEST(CASPartPathParser, SplitCacheEvictionStaysCorrect)
+{
+    // The split cache is a small fixed-capacity FIFO ring, NOT an LRU/MRU: a hit never promotes its
+    // slot, so a path seen recently can still be evicted by unrelated churn through the same thread.
+    // That is only ever a cache-EFFECTIVENESS tradeoff, never a correctness one: pin that once enough
+    // distinct paths evict the first path's cached split, re-parsing it still yields the exact right
+    // result (a forced re-split / cache miss on the re-parse is expected and fine here — the
+    // assertion is correctness under eviction, not hit rate).
+    resetSplitCacheForTest();
+    const std::string first = "store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/columns.txt";
+    ASSERT_TRUE(parsePartFilePath(first).has_value());
+
+    // 8 more distinct paths churn through the ring (capacity 8), evicting `first`'s slot.
+    const std::vector<std::string> table_dirs = {
+        "",
+        "a11/a11a11a1-1111-4111-8111-111111111111",
+        "a22/a22a22a2-2222-4222-8222-222222222222",
+        "a33/a33a33a3-3333-4333-8333-333333333333",
+        "a44/a44a44a4-4444-4444-8444-444444444444",
+        "a55/a55a55a5-5555-4555-8555-555555555555",
+        "a66/a66a66a6-6666-4666-8666-666666666666",
+        "a77/a77a77a7-7777-4777-8777-777777777777",
+        "a88/a88a88a8-8888-4888-8888-888888888888",
+        "a99/a99a99a9-9999-4999-8999-999999999999",
+    };
+    for (int i = 2; i <= 9; ++i)
+    {
+        const std::string path = "store/" + table_dirs[i] + "/all_1_1_0/columns.txt";
+        ASSERT_TRUE(parsePartFilePath(path).has_value());
+    }
+
+    const size_t misses_before_reparse = splitCacheMissesForTest();
+    const auto reparsed = parsePartFilePath(first);
+    ASSERT_TRUE(reparsed.has_value());
+    EXPECT_EQ(reparsed->table_uuid, "a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(reparsed->part_name, "all_1_1_0");
+    EXPECT_EQ(reparsed->file, "columns.txt");
+    // Confirms the re-parse really was a forced re-split (the slot was evicted), not a lucky hit.
+    EXPECT_EQ(splitCacheMissesForTest(), misses_before_reparse + 1);
+}
+
+/// ==== M-W Task 2: the read side over Cas::Pool ====
+/// Fixture: publish parts through the CORE API, then read through the IMetadataStorage surface of
+/// the rewritten ContentAddressedMetadataStorage (real ctor over a Local object storage; the
+/// backend self-selects EmulatedSingleProcess token semantics).
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/Plain/MetadataStorageFromPlainObjectStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <IO/ReadHelpers.h>
+#include <IO/ReadPipeline.h>
+
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace DB::ErrorCodes
+{
+    extern const int FILE_DOESNT_EXIST;
+}
+
+namespace
+{
+
+DB::Cas::ManifestEntry wiringBlobEntry(const String & path, const String & payload)
+{
+    DB::Cas::ManifestEntry e;
+    e.path = path;
+    e.placement = DB::Cas::EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    e.blob_size = payload.size();
+    return e;
+}
+
+/// All-tree-part-files Task 6/9: the small per-part files (uuid.txt, metadata_version.txt, ...) are
+/// ordinary Inline-placement manifest entries now — this is the low-level PartWriteTxn-API equivalent of
+/// what `ContentAddressedTransaction::writeFile`'s inline candidate path stages in production.
+DB::Cas::ManifestEntry wiringInlineEntry(const String & path, const String & bytes)
+{
+    DB::Cas::ManifestEntry e;
+    e.path = path;
+    e.placement = DB::Cas::EntryPlacement::Inline;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(bytes))};
+
+    e.blob_size = bytes.size();
+    e.inline_bytes = bytes;
+    return e;
+}
+
+/// The one table identity these tests use, as a namespace LIFE: namespace files are life-keyed
+/// (directive §2), resolved from the CATALOG exactly as the disk's own write path resolves it. Naming
+/// the Stage-A sentinel here instead would put the fixture's files under a prefix the disk no longer
+/// reads (Task 4b), so `existsFile`/`listDirectory` below would report them absent -- the fixture and
+/// the code under test must agree on the life, and the only way to guarantee that is to ask the same
+/// resolver.
+DB::Cas::NamespaceLifeId wiringLife(DB::ContentAddressedMetadataStorage & storage)
+{
+    return storage.store()->namespaceLife(
+        storage.liveNamespace("a11a11a1-1111-4111-8111-111111111111"));
+}
+
+std::shared_ptr<DB::ContentAddressedMetadataStorage> openWiringStorage()
+{
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_wiring_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+/// One part with a content blob, a projection file, and the small per-part files (uuid.txt,
+/// metadata_version.txt — ordinary Inline entries now, all-tree-part-files Task 6/9), published
+/// through the real PartWriteTxn into `ns` under `ref`.
+void publishWiredPart(
+    DB::ContentAddressedMetadataStorage & storage, const DB::Cas::RootNamespace & ns, const String & ref)
+{
+    /// Port off the removed PartWriteTxn::putTree/publish API onto the part-manifest write flow
+    /// (beginPartWrite → stageManifest → precommitAdd → putBlob → promote). The wiring sets the owning
+    /// namespace EXPLICITLY (intended_namespace) — faithful to ContentAddressedTransaction — so a
+    /// `detached/<part>` ref (which itself contains '/') is staged in the TABLE namespace, not in a
+    /// spurious `<ns>/detached` namespace. intended_ref stays as "ns/ref" diagnostic forensics.
+    DB::Cas::PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    info.intended_namespace = ns;
+    auto build = storage.store()->beginPartWrite(info);
+
+    /// Strictly ascending canonical path order (PartFolderView's binary-search precondition):
+    /// data.bin < metadata_version.txt < p.proj/data.bin < uuid.txt.
+    const auto id = build->stageManifest(
+        {wiringBlobEntry("data.bin", "payload-A"), wiringInlineEntry("metadata_version.txt", "5"),
+         wiringBlobEntry("p.proj/data.bin", "payload-B"), wiringInlineEntry("uuid.txt", "u-123")});
+    build->precommitAdd(ns, ref, id);
+    build->putBlob(idOf("payload-A"), DB::Cas::BlobSource::fromString("payload-A"));
+    build->putBlob(idOf("payload-B"), DB::Cas::BlobSource::fromString("payload-B"));
+    build->promote(ns, ref, build->buildId(), id);
+
+    /// promote stamps published_at_ms with nowMs(); the read assertions want a FIXED stamp, so pin it
+    /// through the set_published_at path (no journal record for anything but the stamp itself).
+    storage.store()->updateRefPublishedAt(ns, ref,
+        [](DB::Cas::RefPublishedAtUpdate & r) { r.published_at_ms = 1700000000ULL * 1000; });   /// epoch ms; getLastModified /1000
+}
+
+}
+
+/// `supportsAtomicFileWrites` (all-tree task 5): the CA metadata storage publishes a file write in
+/// one shot, so `VersionMetadataOnDisk::storeInfoToDataPartStorage` can skip the tmp+replace dance.
+/// A plain (non-content-addressed) metadata storage keeps the base-class default of `false`.
+TEST(CASWiringCapability, SupportsAtomicFileWrites)
+{
+    auto ca_storage = openWiringStorage();
+    EXPECT_TRUE(ca_storage->supportsAtomicFileWrites());
+
+    auto plain_storage = std::make_shared<DB::MetadataStorageFromPlainObjectStorage>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), "", /*object_metadata_cache_size=*/0);
+    EXPECT_FALSE(plain_storage->supportsAtomicFileWrites());
+}
+
+TEST(CASWiringRead, ResolvesPublishedPart)
+{
+    auto storage = openWiringStorage();
+    publishWiredPart(*storage, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0");
+
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/missing.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"), 9u);
+
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111"));
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_9"));
+    EXPECT_FALSE(storage->existsDirectory("a22/a22a22a2-2222-4222-8222-222222222222"));
+
+    /// Part dir listing: nested keys collapse to their first component; the publish stamp
+    /// (published_at_ms typed field) never surfaces as a dir entry — every staged file does.
+    auto names = storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    std::sort(names.begin(), names.end());
+    EXPECT_EQ(names, (std::vector<std::string>{"data.bin", "metadata_version.txt", "p.proj", "uuid.txt"}));
+
+    auto parts = storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111");
+    EXPECT_EQ(parts, (std::vector<std::string>{"all_1_1_0"}));
+
+    /// The part dir reports EMPTY (virtual files; B45) so removeDirectory goes straight to the
+    /// ref-unlink; the table dir keeps listing-based emptiness.
+    EXPECT_TRUE(storage->isDirectoryEmpty("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_FALSE(storage->isDirectoryEmpty("a11/a11a11a1-1111-4111-8111-111111111111"));
+
+    /// Blob-backed file: a real key, PAYLOAD-sized (the envelope header is a read-path concern).
+    auto objects = storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin");
+    ASSERT_EQ(objects.size(), 1u);
+    EXPECT_FALSE(objects[0].remote_path.empty());
+    EXPECT_EQ(objects[0].bytes_size, 9u);
+
+    /// Small Inline entry: bytes live in the shard manifest, not as their own object.
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt"), 5u);
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt"), std::optional<String>("u-123"));
+    auto mobj = storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt");
+    ASSERT_EQ(mobj.size(), 1u);
+    EXPECT_TRUE(mobj[0].remote_path.empty());   /// sized placeholder; bytes ride prepareInManifestRead
+
+    /// The typed publish stamp (published_at_ms epoch ms) backs getLastModified for the part dir and its files.
+    EXPECT_EQ(storage->getLastModified("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0").epochTime(), 1700000000);
+    EXPECT_EQ(storage->getLastModified("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin").epochTime(), 1700000000);
+}
+
+TEST(CASWiringRead, BlobViewPlanRidesTheStandardPipeline)
+{
+    /// The committed read path (B116): an in-manifest file is served from memory via
+    /// prepareInManifestRead; a blob-backed file translates to its physical blob object +
+    /// payload window (getBlobViewPlan) and rides the STANDARD object-storage pipeline,
+    /// bounded by the FileView stage — composed here the way DiskObjectStorage::prepareRead
+    /// composes it.
+    auto object_storage = DB::Cas::tests::makeLocalObjectStorageForTest();
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_wiring_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        object_storage, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    publishWiredPart(*storage, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0");
+
+    /// In-manifest file: memory source, no blob plan.
+    DB::ReadPipeline manifest_pipeline;
+    ASSERT_TRUE(storage->prepareInManifestRead("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt", DB::ReadSettings{}, manifest_pipeline));
+    String manifest_bytes;
+    {
+        auto buf = manifest_pipeline.build();
+        DB::readStringUntilEOF(manifest_bytes, *buf);
+    }
+    EXPECT_EQ(manifest_bytes, "u-123");
+    /// Not a `getBlobViewPlan` call on the in-manifest path here (all-tree Task 6/9: uuid.txt is now
+    /// a real Inline manifest entry): `getBlobViewPlan`'s only production caller
+    /// (`DiskObjectStorage::prepareRead`) never reaches it once `prepareInManifestRead` returns true
+    /// above — `getBlobViewPlan`'s precondition is "confirmed not in-manifest-servable," which calling
+    /// it directly on an Inline path violates. Pre-Task-9 this assertion passed only by coincidence
+    /// (uuid.txt was not a manifest entry at all, so `findFile` returned not-found, not because
+    /// `getBlobViewPlan` gracefully handles an Inline entry it does find).
+
+    /// Blob-backed file: a real physical key and a payload-sized window whose extent equals
+    /// the object's readable size (a right-bounded read never overshoots the window).
+    const std::string path = "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin";
+    auto plan = storage->getBlobViewPlan(path);
+    ASSERT_TRUE(plan.has_value());
+    EXPECT_FALSE(plan->object.remote_path.empty());
+    EXPECT_EQ(plan->object.local_path, path);
+    EXPECT_EQ(plan->payload_end - plan->payload_offset, 9u);
+    EXPECT_EQ(plan->object.bytes_size, plan->payload_end);
+    EXPECT_FALSE(storage->prepareInManifestRead(path, DB::ReadSettings{}, manifest_pipeline = {}));
+
+    auto make_pipeline = [&]
+    {
+        DB::ReadPipeline pipeline;
+        pipeline.setSource(object_storage, {plan->object}, DB::ReadSettings{});
+        pipeline.needGather();
+        pipeline.needFileView(path, plan->payload_offset, plan->payload_end);
+        return pipeline;
+    };
+    EXPECT_EQ(make_pipeline().describe(), "Source(ObjectStorage) -> Gather -> FileView");
+
+    {
+        auto buf = make_pipeline().build();
+        EXPECT_EQ(buf->getFileName(), path);
+        EXPECT_EQ(buf->tryGetFileSize(), std::optional<size_t>(9));
+        String bytes;
+        DB::readStringUntilEOF(bytes, *buf);
+        EXPECT_EQ(bytes, "payload-A");
+    }
+
+    /// Right-bounded read through the view (the MergeTreeReaderStream::adjustRightMark shape):
+    /// the bound is window-relative and forwarded down the chain.
+    {
+        auto buf = make_pipeline().build();
+        buf->setReadUntilPosition(7);
+        String head(7, '\0');
+        buf->readStrict(head.data(), 7);
+        EXPECT_EQ(head, "payload");
+        EXPECT_TRUE(buf->eof());
+        buf->setReadUntilEnd();
+        String tail;
+        DB::readStringUntilEOF(tail, *buf);
+        EXPECT_EQ(tail, "-A");
+    }
+
+    /// Seek inside the window.
+    {
+        auto buf = make_pipeline().build();
+        buf->seek(8, SEEK_SET);
+        String last;
+        DB::readStringUntilEOF(last, *buf);
+        EXPECT_EQ(last, "A");
+    }
+}
+
+TEST(CASWiringRead, ProjectionDirectory)
+{
+    auto storage = openWiringStorage();
+    publishWiredPart(*storage, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0");
+
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj"));
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/q.proj"));
+    EXPECT_EQ(storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj"), (std::vector<std::string>{"data.bin"}));
+    EXPECT_TRUE(storage->isDirectoryEmpty("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj"));   /// B60
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj/data.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj/data.bin"), 9u);
+}
+
+TEST(CASWiringRead, DetachedFoldedIntoTableNamespace)
+{
+    auto storage = openWiringStorage();
+    /// B181: a detached part is a `detached/`-prefixed ref INSIDE the table's own archive namespace,
+    /// not a separate sibling namespace. Publish it that way through the core, and ALSO a live part
+    /// that shares the same base name to prove the live↔detached collision is impossible (the ref
+    /// names `all_1_1_0` and `detached/all_1_1_0` differ — one namespace, no re-split needed).
+    publishWiredPart(*storage, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "detached/broken_all_1_1_0");
+    publishWiredPart(*storage, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "broken_all_1_1_0");
+
+    /// The TABLE dir collapses the `detached/<part>` refs to the single `detached` subdir entry
+    /// alongside the live part name.
+    auto top = storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111");
+    std::sort(top.begin(), top.end());
+    EXPECT_EQ(top, (std::vector<std::string>{"broken_all_1_1_0", "detached"}));
+
+    /// The detached CONTAINER lists the detached part DIRECTORY names (B36's intent), prefix-stripped
+    /// — and NOT the live part of the same base name.
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached"));
+    EXPECT_EQ(storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached"), (std::vector<std::string>{"broken_all_1_1_0"}));
+    /// A single detached part dir + its files (the detached part is its own `detached/`-prefixed ref).
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached/broken_all_1_1_0"));
+    auto names = storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached/broken_all_1_1_0");
+    std::sort(names.begin(), names.end());
+    EXPECT_EQ(names, (std::vector<std::string>{"data.bin", "metadata_version.txt", "p.proj", "uuid.txt"}));
+    /// The B62 shape: a detached part's mutable file resolves through the `detached/`-prefixed ref.
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/detached/broken_all_1_1_0/metadata_version.txt"));
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/detached/broken_all_1_1_0/metadata_version.txt"),
+              std::optional<String>("5"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/detached/broken_all_1_1_0/data.bin"));
+}
+
+TEST(CASWiringRoute, DetachedFoldsIntoTableNamespaceWithPrefixedRef)
+{
+    /// B181: a detached part file routes to the table's OWN archive namespace under a
+    /// `detached/`-prefixed ref — NOT a separate sibling namespace.
+    auto storage = openWiringStorage();
+    auto p = parsePartFilePath("store/a11/a11a11a1-1111-4111-8111-111111111111/detached/broken_all_1_1_0/data.bin");
+    ASSERT_TRUE(p.has_value());
+    auto r = storage->route(*p);
+    ASSERT_TRUE(r.has_value());
+    EXPECT_EQ(r->ns.string(), storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string());
+    EXPECT_EQ(r->ref, "detached/broken_all_1_1_0");
+    EXPECT_EQ(r->file, "data.bin");
+
+    /// The detached CONTAINER dir routes to the table ns with an empty ref (filtered listing).
+    auto pc = parsePartFilePath("store/a11/a11a11a1-1111-4111-8111-111111111111/detached/broken_all_1_1_0");
+    ASSERT_TRUE(pc.has_value());
+    auto rc = storage->route(*pc);
+    ASSERT_TRUE(rc.has_value());
+    EXPECT_EQ(rc->ns.string(), storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string());
+    EXPECT_EQ(rc->ref, "detached/broken_all_1_1_0");
+    EXPECT_TRUE(rc->file.empty());
+}
+
+TEST(CASWiringRoute, MovingFoldsOntoAPrefixedStagingRef)
+{
+    /// L1 (MOVE-to-CA fix): the mover clones a part under TABLE/moving/<part>/ before the
+    /// atomic rename into place. Mirroring `detached`, a moved part resolves onto a
+    /// `moving/`-PREFIXED staging ref -- NOT the part's final live ref directly. Publishing under
+    /// the final ref before the mover's swap would break move crash-atomicity (a crash between the
+    /// clone commit and the swap would leave a committed live ref that never went through the
+    /// swap). The staging ref keeps the pre-swap clone un-live; the mover's rename does a real ref
+    /// repoint moving/<part> -> <part>.
+    auto storage = openWiringStorage();
+    auto p = parsePartFilePath("store/a11/a11a11a1-1111-4111-8111-111111111111/moving/all_1_1_0/data.bin");
+    ASSERT_TRUE(p.has_value());
+    EXPECT_EQ(p->part_name, std::string(kMovingDirName));
+    EXPECT_EQ(p->file, "all_1_1_0/data.bin");
+
+    auto r = storage->route(*p);
+    ASSERT_TRUE(r.has_value());
+    EXPECT_EQ(r->ns.string(), storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string());
+    EXPECT_EQ(r->ref, "moving/all_1_1_0");
+    EXPECT_EQ(r->file, "data.bin");
+
+    /// The bare moving CONTAINER dir TABLE/moving routes to the table ns with an empty ref.
+    auto pc = parsePartFilePath("store/a11/a11a11a1-1111-4111-8111-111111111111/moving");
+    ASSERT_TRUE(pc.has_value());
+    auto rc = storage->route(*pc);
+    ASSERT_TRUE(rc.has_value());
+    EXPECT_EQ(rc->ns.string(), storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string());
+    EXPECT_TRUE(rc->ref.empty());
+    EXPECT_TRUE(rc->file.empty());
+}
+
+TEST(CASWiringRead, ShadowFreezeTree)
+{
+    auto storage = openWiringStorage();
+    publishWiredPart(*storage, DB::ContentAddressedMetadataStorage::shadowNamespace("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0");
+
+    /// Intermediate dirs derive from the registered shadow namespaces.
+    EXPECT_TRUE(storage->existsDirectory("shadow/bk1"));
+    EXPECT_TRUE(storage->existsDirectory("shadow/bk1/store"));
+    EXPECT_FALSE(storage->existsDirectory("shadow/bk2"));
+    EXPECT_EQ(storage->listDirectory("shadow"), (std::vector<std::string>{"bk1"}));
+    EXPECT_EQ(storage->listDirectory("shadow/bk1"), (std::vector<std::string>{"store"}));
+    EXPECT_EQ(storage->listDirectory("shadow/bk1/store"), (std::vector<std::string>{"a11"}));
+    EXPECT_EQ(storage->listDirectory("shadow/bk1/store/a11"), (std::vector<std::string>{"a11a11a1-1111-4111-8111-111111111111"}));
+    /// Shadow TABLE dir (strict uuid-pair anchor) and PART dir.
+    EXPECT_TRUE(storage->existsDirectory("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111"));
+    EXPECT_EQ(storage->listDirectory("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111"), (std::vector<std::string>{"all_1_1_0"}));
+    EXPECT_TRUE(storage->existsDirectory("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    auto names = storage->listDirectory("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    std::sort(names.begin(), names.end());
+    EXPECT_EQ(names, (std::vector<std::string>{"data.bin", "metadata_version.txt", "p.proj", "uuid.txt"}));
+    EXPECT_TRUE(storage->existsFile("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_EQ(storage->getFileSize("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"), 9u);
+}
+
+TEST(CASWiringRead, VerbatimNamespaceFiles)
+{
+    auto storage = openWiringStorage();
+    EXPECT_TRUE(storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string().starts_with("test/"))
+        << storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string();
+    EXPECT_NE(storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string().find("/store/a11/a11a11a1-1111-4111-8111-111111111111@cas@"), std::string::npos)
+        << storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string();
+
+    publishWiredPart(*storage, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0");
+    storage->store()->putNamespaceFile(wiringLife(*storage), "format_version.txt", "1\n");
+    storage->store()->putNamespaceFile(
+        wiringLife(*storage), "deduplication_logs/deduplication_log_1.txt", "log-bytes");
+    /// Loose disk-root files are plain mountpoint objects (design §5.2), not namespace files.
+    storage->store()->putMountpointObject(storage->serverRootId() + "/" + "clickhouse_access_check_xyz", "ok");
+
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/format_version.txt"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/format_version.txt"), 2u);
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/format_version.txt"), std::optional<String>("1\n"));
+
+    /// Table dir listing merges part names + verbatim file first components.
+    auto names = storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111");
+    std::sort(names.begin(), names.end());
+    EXPECT_EQ(names, (std::vector<std::string>{"all_1_1_0", "deduplication_logs", "format_version.txt"}));
+
+    /// The reserved table-level subdir.
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/deduplication_logs"));
+    EXPECT_EQ(storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/deduplication_logs"),
+              (std::vector<std::string>{"deduplication_log_1.txt"}));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/deduplication_logs/deduplication_log_1.txt"));
+
+    /// Loose disk-root files are plain objects — existsFile checks the mountpoint object, not a namespace file.
+    EXPECT_TRUE(storage->existsFile("clickhouse_access_check_xyz"));
+    /// Loose files are real objects — tryGetInManifestBytes returns nullopt (not in-manifest bytes).
+    EXPECT_EQ(storage->tryGetInManifestBytes("clickhouse_access_check_xyz"), std::nullopt);
+    EXPECT_EQ(storage->getFileSize("clickhouse_access_check_xyz"), 2u);
+    EXPECT_FALSE(storage->existsFile("clickhouse_access_check_other"));
+}
+
+/// `DirShape::TableDir`'s `existsDirectory` used to answer "has at least one committed part", so an
+/// Atomic table that only ever wrote its namespace-level `format_version.txt` (no part published yet)
+/// reported its own root as absent. `existsDirectory` is the precheck `MergeTreeData::dropAllData`
+/// uses to decide whether `removeRecursive`/`dropNamespace` needs to run at all -- a false negative
+/// here means `DROP TABLE` on such a table never admits removal, leaking a `Live` catalog row forever.
+TEST(CASWiringRead, TableRootExistsWithNamespaceFilesButNoCommittedRef)
+{
+    auto storage = openWiringStorage();
+    storage->store()->putNamespaceFile(wiringLife(*storage), "format_version.txt", "1\n");
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111"));
+}
+
+/// Same defect, non-Atomic fallback shape (`parseTableUuid` folds the whole leading path into the
+/// "uuid"): a files-only table under `data/<db>/<table>` must be present too.
+TEST(CASWiringRead, TableRootExistsWithNamespaceFilesButNoCommittedRefNonAtomic)
+{
+    auto storage = openWiringStorage();
+    const auto ns = storage->liveNamespace("data/memory_01069/mt");
+    storage->store()->putNamespaceFile(storage->store()->namespaceLife(ns), "format_version.txt", "1\n");
+    EXPECT_TRUE(storage->existsDirectory("data/memory_01069/mt"));
+}
+
+/// A cataloged `Live` life with ZERO refs and ZERO namespace files -- not just zero refs -- must still
+/// report present. `namespaceLife` is the write-side resolution that mints a `Live` catalog row on
+/// first touch; calling it alone (no ref, no namespace file written afterward) is the minimal way to
+/// reach this state, and it prevents a future regression from "catalog OR files" back to "files only".
+TEST(CASWiringRead, EmptyCatalogedLiveTableRootExists)
+{
+    auto storage = openWiringStorage();
+    (void)storage->store()->namespaceLife(storage->liveNamespace("a55a55a5-5555-4555-8555-555555555555"));
+    EXPECT_TRUE(storage->existsDirectory("a55/a55a55a5-5555-4555-8555-555555555555"));
+}
+
+/// C4: the fixed dispatch order is the invariant. Pins the two ambiguous early guards that make the
+/// order load-bearing: store/<u3> (AtomicShard, ambiguous with the non-Atomic table fallback) and a
+/// shadow table dir (which also satisfies parseTableUuid). existsDirectory/listDirectory must agree.
+TEST(CASWiringRoute, DirShapeDispatchOrderIsStable)
+{
+    auto storage = openWiringStorage();
+    publishWiredPart(*storage, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0");
+    publishWiredPart(*storage, DB::ContentAddressedMetadataStorage::shadowNamespace("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0");
+
+    using DS = DB::ContentAddressedMetadataStorage::DirShape;
+    EXPECT_EQ(storage->classifyDirectoryForTest("store/uui").shape,             DS::AtomicShard);
+    EXPECT_EQ(storage->classifyDirectoryForTest("a11/a11a11a1-1111-4111-8111-111111111111").shape,            DS::TableDir);
+    EXPECT_EQ(storage->classifyDirectoryForTest("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0").shape,  DS::PartDir);
+    EXPECT_EQ(storage->classifyDirectoryForTest("a11/a11a11a1-1111-4111-8111-111111111111/detached").shape,   DS::DetachedContainer);
+    EXPECT_EQ(storage->classifyDirectoryForTest("a11/a11a11a1-1111-4111-8111-111111111111/moving").shape,    DS::MovingContainer);
+    EXPECT_EQ(storage->classifyDirectoryForTest("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111").shape, DS::ShadowTable);
+    EXPECT_EQ(storage->classifyDirectoryForTest("shadow/bk1").shape,            DS::ShadowIntermediate);
+    EXPECT_EQ(storage->classifyDirectoryForTest("a11/a11a11a1-1111-4111-8111-111111111111/deduplication_logs").shape, DS::TableSubdir);
+    EXPECT_EQ(storage->classifyDirectoryForTest("store").shape,                 DS::GenericIntermediate);
+}
+
+/// ==== M-W Task 3: the write path through IMetadataTransaction ====
+
+
+namespace
+{
+
+void writeThroughTransaction(DB::IMetadataTransaction & tx, const String & path, const String & bytes)
+{
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(tx);
+    auto buf = ca_tx.writeFile(path, 65536, DB::WriteMode::Rewrite, {});
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+}
+
+}
+
+TEST(CASWiringWrite, ContentRoundTripThroughTransaction)
+{
+    auto storage = openWiringStorage();
+
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "content-A");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/checksums.txt", "sums");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt", "u-42");
+    /// Nothing visible before commit.
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    tx->commit(DB::NoCommitOptions{});
+
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"), 9u);
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt"), std::optional<String>("u-42"));
+    auto names = storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    std::sort(names.begin(), names.end());
+    EXPECT_EQ(names, (std::vector<std::string>{"checksums.txt", "data.bin", "uuid.txt"}));
+    /// The publish stamp was added automatically and is filtered from listings.
+    EXPECT_GT(storage->getLastModified("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0").epochTime(), 1700000000);
+}
+
+TEST(CASWiringWrite, InlineOnlyPartPublishesWithoutBuildCrash)
+{
+    /// Regression (CRASH-CA-S3 "staged entries without a PartWriteTxn"): a part whose files are ALL inline
+    /// — no `partFileMustStayBlob` file (`.bin`/`.mrk*`/`primary.idx`), e.g. an EMPTY merge output that
+    /// writes only `checksums.txt`/`count.txt` and no `data.bin` — staged manifest entries via the
+    /// inline write path, which did NOT establish a PartWriteTxn (only the blob path did, via `buildFor`). So
+    /// `publishStaging` reached its `st.build != nullptr` invariant with entries but no PartWriteTxn and threw
+    /// LOGICAL_ERROR — a SERVER CRASH under `abort_on_logical_error`. Writing only inline metadata files
+    /// to a fresh part and committing must SUCCEED and publish the part. (Bug pre-existed the inline-files
+    /// feature; fix: the inline path now calls `buildFor` like the blob path.)
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/checksums.txt", "sums");   // inline (no blob)
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/count.txt", "0");          // inline (no blob)
+    EXPECT_NO_THROW(tx->commit(DB::NoCommitOptions{}));
+
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/checksums.txt"), std::optional<String>("sums"));
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/count.txt"), std::optional<String>("0"));
+    auto names = storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    std::sort(names.begin(), names.end());
+    EXPECT_EQ(names, (std::vector<std::string>{"checksums.txt", "count.txt"}));
+}
+
+TEST(CASWiringWrite, IdenticalContentDedupsToOneBlob)
+{
+    auto storage = openWiringStorage();
+
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "same-bytes");
+    tx->commit(DB::NoCommitOptions{});
+    auto tx2 = storage->createTransaction();
+    writeThroughTransaction(*tx2, "a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/data.bin", "same-bytes");
+    tx2->commit(DB::NoCommitOptions{});
+
+    /// Identical content => the SAME blob object (the key is the content hash).
+    auto a = storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin");
+    auto b = storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/data.bin");
+    ASSERT_EQ(a.size(), 1u);
+    ASSERT_EQ(b.size(), 1u);
+    EXPECT_EQ(a[0].remote_path, b[0].remote_path);
+}
+
+TEST(CASWiringWrite, UncommittedTransactionPublishesNothing)
+{
+    auto storage = openWiringStorage();
+    {
+        auto tx = storage->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "doomed");
+        /// destroyed without commit => PartWriteTxn abandoned (uploads are heartbeat-gated debris)
+    }
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+}
+
+TEST(CASWiringWrite, MutableOnlyUpdateOnCommittedPart)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "content-A");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/txn_version.txt", "v1");
+    tx->commit(DB::NoCommitOptions{});
+
+    /// The MVCC autocommit one-shot shape: a fresh transaction rewriting ONLY a mutable file of a
+    /// COMMITTED part goes through updateRefPublishedAt (no tree rebuild, no journal record).
+    auto tx2 = storage->createTransaction();
+    writeThroughTransaction(*tx2, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/txn_version.txt", "v2");
+    tx2->commit(DB::NoCommitOptions{});
+
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/txn_version.txt"), std::optional<String>("v2"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));   /// the tree is untouched
+}
+
+TEST(CASWiringWrite, VerbatimFilesDurableOnFinalizeAndAppendable)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    /// Verbatim files are durable on FINALIZE, with no commit (the disk layer's autocommit
+    /// contract for table-level files).
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/mutation_5.txt", "commands\n");
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/mutation_5.txt"));
+
+    /// Append = read-modify-rewrite (the MVCC mutation-entry CSN append).
+    {
+        auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+        auto buf = ca_tx.writeFile("a11/a11a11a1-1111-4111-8111-111111111111/mutation_5.txt", 65536, DB::WriteMode::Append, {});
+        buf->write("csn 42\n", 7);
+        buf->finalize();
+    }
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/mutation_5.txt"),
+              std::optional<String>("commands\ncsn 42\n"));
+}
+
+/// ==== M-W Tasks 5-7: carry-forward, renames, removals, detached/ATTACH/FREEZE ====
+
+TEST(CASWiringOps, HardLinkCarriesForwardWithoutReupload)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "shared-payload");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt", "u-1");
+    tx->commit(DB::NoCommitOptions{});
+
+    /// A mutation/merge carries unchanged files into the new part by hardlink.
+    auto tx2 = storage->createTransaction();
+    tx2->createHardLink("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_5/data.bin");
+    tx2->createHardLink("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/uuid.txt", "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_5/uuid.txt");
+    tx2->commit(DB::NoCommitOptions{});
+
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_5/data.bin"));
+    EXPECT_EQ(storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin")[0].remote_path,
+              storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_5/data.bin")[0].remote_path);
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_5/uuid.txt"), std::optional<String>("u-1"));
+}
+
+TEST(CASWiringOps, TmpToFinalRenamePublishesUnderFinalName)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_1_1_0/data.bin", "fresh");
+    tx->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_1_1_0", "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    tx->commit(DB::NoCommitOptions{});
+
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_insert_all_1_1_0"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+}
+
+TEST(CASWiringOps, CommittedPartRenameMovesTheRef)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "bytes");
+    tx->commit(DB::NoCommitOptions{});
+
+    /// MergeTree renames a part to delete_tmp_<part> before removing it.
+    auto tx2 = storage->createTransaction();
+    tx2->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0", "a11/a11a11a1-1111-4111-8111-111111111111/delete_tmp_all_1_1_0");
+    tx2->commit(DB::NoCommitOptions{});
+
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/delete_tmp_all_1_1_0"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/delete_tmp_all_1_1_0/data.bin"));
+}
+
+TEST(CASWiringOps, ProjectionTmpRenameRekeysStagedEntries)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "main");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p_1.tmp_proj/data.bin", "proj");
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+    ca_tx.moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p_1.tmp_proj", "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj");
+    tx->commit(DB::NoCommitOptions{});
+
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj/data.bin"));
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p_1.tmp_proj/data.bin"));
+    EXPECT_EQ(storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/p.proj"), (std::vector<std::string>{"data.bin"}));
+}
+
+TEST(CASWiringOps, DetachAttachRoundTrip)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "detachable");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/metadata_version.txt", "3");
+    tx->commit(DB::NoCommitOptions{});
+
+    /// DETACH: a committed part moves into the detached namespace - pure ref ops.
+    auto tx2 = storage->createTransaction();
+    tx2->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0", "a11/a11a11a1-1111-4111-8111-111111111111/detached/all_1_1_0");
+    tx2->commit(DB::NoCommitOptions{});
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached/all_1_1_0"));
+    EXPECT_EQ(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/detached/all_1_1_0/metadata_version.txt"),
+              std::optional<String>("3"));
+
+    /// ATTACH: stage-rename within detached, then publish back into the live namespace.
+    auto tx3 = storage->createTransaction();
+    tx3->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached/all_1_1_0", "a11/a11a11a1-1111-4111-8111-111111111111/detached/attaching_all_1_1_0");
+    tx3->commit(DB::NoCommitOptions{});
+    auto tx4 = storage->createTransaction();
+    tx4->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached/attaching_all_1_1_0", "a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0");
+    tx4->commit(DB::NoCommitOptions{});
+
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/data.bin"));
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached/attaching_all_1_1_0"));
+    EXPECT_EQ(storage->listDirectory("a11/a11a11a1-1111-4111-8111-111111111111/detached"), (std::vector<std::string>{}));
+}
+
+TEST(CASWiringOps, RemovalsDropRefsAndNamespaces)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "gone-soon");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/data.bin", "stays");
+    tx->commit(DB::NoCommitOptions{});
+    storage->store()->putNamespaceFile(wiringLife(*storage), "format_version.txt", "1\n");
+
+    /// The fast-removal path (all-tree Task 8, B123 evolution): per-file unlinks stage removal marks
+    /// (`content_removed`) but nothing durable changes until commit; removeDirectory(<part>) drops the
+    /// ref and supersedes any marks staged for it in the SAME transaction — still exactly one ref-drop,
+    /// zero repoints. `existsFile` below stays true because this whole sequence is one uncommitted
+    /// transaction (`tx2`), not because the unlink was a no-op.
+    auto tx2 = storage->createTransaction();
+    tx2->unlinkFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", false, false);
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));   /// still committed
+    tx2->removeDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0"));
+
+    /// DROP TABLE: removeRecursive on the table dir drops the live + detached namespaces.
+    tx2->removeRecursive("a11/a11a11a1-1111-4111-8111-111111111111", {});
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111"));
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/format_version.txt"));
+}
+
+/// The negative test that forbids hooking last-part removal as table-drop admission: removing a
+/// table's ONLY part is indistinguishable, from that call alone, from a merge, a TTL cleanup, or a
+/// `TRUNCATE` that leaves the table usable. The root must stay present, and a fresh part must still be
+/// publishable into it.
+TEST(CASWiringOps, LastRefRemovalIsNotNamespaceRemoval)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a66/a66a66a6-6666-4666-8666-666666666666/all_1_1_0/data.bin", "only-part");
+    tx->commit(DB::NoCommitOptions{});
+    storage->store()->putNamespaceFile(
+        storage->store()->namespaceLife(storage->liveNamespace("a66a66a6-6666-4666-8666-666666666666")),
+        "format_version.txt", "1\n");
+
+    auto tx2 = storage->createTransaction();
+    tx2->removeDirectory("a66/a66a66a6-6666-4666-8666-666666666666/all_1_1_0");
+    EXPECT_FALSE(storage->existsDirectory("a66/a66a66a6-6666-4666-8666-666666666666/all_1_1_0"));
+    EXPECT_TRUE(storage->existsDirectory("a66/a66a66a6-6666-4666-8666-666666666666"))
+        << "removing the table's last part must not be treated as DROP TABLE admission";
+
+    auto tx3 = storage->createTransaction();
+    writeThroughTransaction(*tx3, "a66/a66a66a6-6666-4666-8666-666666666666/all_2_2_0/data.bin", "new-part");
+    tx3->commit(DB::NoCommitOptions{});
+    EXPECT_TRUE(storage->existsDirectory("a66/a66a66a6-6666-4666-8666-666666666666/all_2_2_0"))
+        << "the namespace never transitioned to Removing, so a fresh part publishes normally";
+}
+
+/// A files-only table root (no part ever published) becomes logically absent IMMEDIATELY once
+/// `removeRecursive` durably completes the removal -- no GC round required. This is the same-call
+/// synchronous half of the fix: `DROP TABLE ... SYNC` must not depend on GC latency to observe removal.
+TEST(CASWiringOps, FilesOnlyTableRootRemovalIsImmediatelyAbsentWithoutGc)
+{
+    auto storage = openWiringStorage();
+    const auto ns = storage->liveNamespace("a77a77a7-7777-4777-8777-777777777777");
+    storage->store()->putNamespaceFile(storage->store()->namespaceLife(ns), "format_version.txt", "1\n");
+    EXPECT_TRUE(storage->existsDirectory("a77/a77a77a7-7777-4777-8777-777777777777"));
+
+    auto tx = storage->createTransaction();
+    tx->removeRecursive("a77/a77a77a7-7777-4777-8777-777777777777", {});
+    EXPECT_FALSE(storage->existsDirectory("a77/a77a77a7-7777-4777-8777-777777777777"))
+        << "the terminal remove_namespace transaction is durable synchronously";
+    EXPECT_FALSE(storage->existsFile("a77/a77a77a7-7777-4777-8777-777777777777/format_version.txt"));
+}
+
+/// REMOVED (all-tree-part-files Task 6):
+/// `MutableTmpMoveOnCommittedPart` exercised `VersionMetadataOnDisk`'s OLD atomic-write dance —
+/// autocommit `txn_version.txt.tmp`, then a standalone one-shot `moveFile(.tmp -> txn_version.txt)`
+/// — via `ContentAddressedTransaction::moveFile` directly. That dance no longer exists in production:
+/// Task 5's `supportsAtomicFileWrites` short-circuit makes `VersionMetadataOnDisk::storeInfoToData-
+/// PartStorage` write `txn_version.txt` directly in one shot on a CA disk, with no `.tmp` file and no
+/// rename ever produced. Task 9 completed the cleanup this comment used to defer: `moveFile`'s legacy
+/// "rename FROM a committed mutable-per-part-file, source not staged in this transaction" branch is
+/// now DELETED (it had been provably unreachable since Task 5, and rebuilding it against `entries`
+/// would only add unused surface for a dead path). Coverage that remains valid: Task 5's own
+/// capability test proves no `.tmp` file is ever created; `CASTransactionAllTree.CommittedTxnVersion-
+/// StoreRepoints` (`gtest_ca_transaction.cpp`) proves the real, live path — a standalone write of
+/// `txn_version.txt` directly onto an already-committed part — repoints correctly.
+
+TEST(CASWiringOps, VerbatimMoveAndUnlink)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_mutation_5.txt", "cmds");
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+    ca_tx.moveFile("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mutation_5.txt", "a11/a11a11a1-1111-4111-8111-111111111111/mutation_5.txt");
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/mutation_5.txt"));
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mutation_5.txt"));
+    ca_tx.unlinkFile("a11/a11a11a1-1111-4111-8111-111111111111/mutation_5.txt", false, false);
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/mutation_5.txt"));
+}
+
+TEST(CASWiringOps, UnlinkHonorsIfExistsForPartFiles)
+{
+    auto storage = openWiringStorage();
+    const String path = "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin";
+
+    auto missing_tx = storage->createTransaction();
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::FILE_DOESNT_EXIST,
+        [&] { missing_tx->unlinkFile(path, /*if_exists=*/false, /*should_remove_objects=*/true); });
+
+    auto ignored_tx = storage->createTransaction();
+    EXPECT_NO_THROW(ignored_tx->unlinkFile(path, /*if_exists=*/true, /*should_remove_objects=*/true));
+    EXPECT_NO_THROW(ignored_tx->commit(DB::NoCommitOptions{}));
+    EXPECT_FALSE(storage->existsFile(path));
+
+    auto create_tx = storage->createTransaction();
+    writeThroughTransaction(*create_tx, path, "payload");
+    create_tx->commit(DB::NoCommitOptions{});
+    ASSERT_TRUE(storage->existsFile(path));
+
+    auto existing_tx = storage->createTransaction();
+    EXPECT_NO_THROW(existing_tx->unlinkFile(path, /*if_exists=*/false, /*should_remove_objects=*/true));
+    EXPECT_NO_THROW(existing_tx->commit(DB::NoCommitOptions{}));
+    EXPECT_FALSE(storage->existsFile(path));
+}
+
+TEST(CASWiringOps, TableRenameMovesRefsFilesAndDetached)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "live");
+    tx->commit(DB::NoCommitOptions{});
+    auto tx2 = storage->createTransaction();
+    tx2->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0", "a11/a11a11a1-1111-4111-8111-111111111111/detached/all_1_1_0");   /// one detached part
+    tx2->commit(DB::NoCommitOptions{});
+    auto tx3 = storage->createTransaction();
+    writeThroughTransaction(*tx3, "a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/data.bin", "live2");
+    tx3->commit(DB::NoCommitOptions{});
+    storage->store()->putNamespaceFile(wiringLife(*storage), "format_version.txt", "1\n");
+
+    auto tx4 = storage->createTransaction();
+    tx4->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111", "a22/a22a22a2-2222-4222-8222-222222222222");
+    tx4->commit(DB::NoCommitOptions{});
+
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111"));
+    EXPECT_TRUE(storage->existsDirectory("a22/a22a22a2-2222-4222-8222-222222222222"));
+    EXPECT_TRUE(storage->existsFile("a22/a22a22a2-2222-4222-8222-222222222222/all_2_2_0/data.bin"));
+    EXPECT_TRUE(storage->existsFile("a22/a22a22a2-2222-4222-8222-222222222222/format_version.txt"));
+    EXPECT_TRUE(storage->existsDirectory("a22/a22a22a2-2222-4222-8222-222222222222/detached/all_1_1_0"));
+}
+
+/// B126: RENAME TABLE move_namespace is idempotent — re-driving the SAME rename after it completed is a
+/// clean no-op (the source namespace is already gone), so a partial-failure re-drive is safe.
+TEST(CASWiringOps, TableRenameIsIdempotentOnRedrive)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "live");
+    tx->commit(DB::NoCommitOptions{});
+    storage->store()->putNamespaceFile(wiringLife(*storage), "format_version.txt", "1\n");
+
+    auto tx2 = storage->createTransaction();
+    tx2->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111", "a22/a22a22a2-2222-4222-8222-222222222222");
+    tx2->commit(DB::NoCommitOptions{});
+
+    /// Re-drive the identical rename: a11a11a1-1111-4111-8111-111111111111 is empty/gone, so every step no-ops; must not throw.
+    auto tx3 = storage->createTransaction();
+    EXPECT_NO_THROW(tx3->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111", "a22/a22a22a2-2222-4222-8222-222222222222"));
+    tx3->commit(DB::NoCommitOptions{});
+
+    EXPECT_TRUE(storage->existsFile("a22/a22a22a2-2222-4222-8222-222222222222/all_1_1_0/data.bin"));
+    EXPECT_TRUE(storage->existsFile("a22/a22a22a2-2222-4222-8222-222222222222/format_version.txt"));
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111"));
+}
+
+/// B123: a verbatim-file move (get->put->remove, no native rename) is idempotent on re-drive — once the
+/// source is gone but the destination is present, a re-driven move is a no-op, not a FILE_DOESNT_EXIST.
+TEST(CASWiringOps, VerbatimMoveIsIdempotentOnRedrive)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_mutation_7.txt", "cmds");
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+    ca_tx.moveFile("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mutation_7.txt", "a11/a11a11a1-1111-4111-8111-111111111111/mutation_7.txt");
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/mutation_7.txt"));
+    /// Re-drive: source gone, destination present → no-op (no throw).
+    EXPECT_NO_THROW(ca_tx.moveFile("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mutation_7.txt", "a11/a11a11a1-1111-4111-8111-111111111111/mutation_7.txt"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/mutation_7.txt"));
+    /// Both source and destination absent → genuine missing source still throws.
+    EXPECT_ANY_THROW(ca_tx.moveFile("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mutation_8.txt", "a11/a11a11a1-1111-4111-8111-111111111111/mutation_8.txt"));
+}
+
+/// B124: moveDirectory's staged-merge is source-wins, and a genuine collision (the same mutable file
+/// staged under BOTH the source and destination part keys with DIFFERING bytes) fails loud instead of
+/// silently dropping a just-written file. Identical bytes are a benign idempotent re-key.
+///
+/// The "fails loud" collision throws LOGICAL_ERROR, which aborts the whole process in debug/sanitizer
+/// builds (Exception.cpp's handle_error_code) instead of behaving like a catchable exception -- so
+/// EXPECT_ANY_THROW only makes sense in a plain release build. CASWiringOpsDeathTest below proves the
+/// SAME collision positively aborts under debug/sanitizer builds instead (same pattern as the existing
+/// CASBlobDigestDeathTest precedent).
+TEST(CASWiringOps, MoveDirectoryMutableCollisionPolicy)
+{
+#ifndef DEBUG_OR_SANITIZER_BUILD
+    /// Differing bytes → fail loud.
+    {
+        auto storage = openWiringStorage();
+        auto tx = storage->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_x/txn_version.txt", "A");
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_9/txn_version.txt", "B");
+        auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+        EXPECT_ANY_THROW(ca_tx.moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_x", "a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_9"));
+    }
+#endif
+    /// Identical bytes → benign, no throw (source-wins, idempotent). Both parts carry real content so
+    /// the eager publish-at-rename builds a proper ref (a mutable-only staging would instead hit
+    /// updateRefPublishedAt on a not-yet-committed ref — unrelated to the collision policy under test).
+    /// data.bin must ALSO match now: all-tree Task 9 generalized the differing-bytes collision check
+    /// from the legacy mutable-file names to every entry, so a differing data.bin would (correctly)
+    /// throw too and defeat this block's "benign" premise.
+    {
+        auto storage = openWiringStorage();
+        auto tx = storage->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_y/data.bin", "d1");
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_y/txn_version.txt", "SAME");
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_8_8_8/data.bin", "d1");
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_8_8_8/txn_version.txt", "SAME");
+        auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+        EXPECT_NO_THROW(ca_tx.moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_y", "a11/a11a11a1-1111-4111-8111-111111111111/all_8_8_8"));
+    }
+}
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+/// Debug/sanitizer-build counterpart to MoveDirectoryMutableCollisionPolicy's "differing bytes → fail
+/// loud" case: LOGICAL_ERROR aborts the process here instead of throwing a catchable exception, so the
+/// check must be a death test (same pattern as CASBlobDigestDeathTest in gtest_cas_blob_digest.cpp).
+TEST(CASWiringOpsDeathTest, MoveDirectoryMutableCollisionPolicyAborts)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_x/txn_version.txt", "A");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_9/txn_version.txt", "B");
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+    EXPECT_DEATH({ ca_tx.moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_x", "a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_9"); }, "");
+}
+#endif
+
+/// D3 review pin: moveDirectory's staged-merge collision code has four (src build?, dst build?)
+/// combinations. This one — destination already holds a staged PartWriteTxn, source has none — proved
+/// confusable when the plan's author sketched a fix: a naive rewrite of the four-way branch can fall
+/// through to `src_st.build->abandon()` on a null build. The merge must be a pure no-op on the
+/// destination's build in this combination — no abandon, no adopt — while everything else (any
+/// removal marks carried from the source) still merges in and the destination's own content
+/// publishes exactly as staged.
+///
+/// T9-review fix (all-tree-part-files): the ORIGINAL construction staged the source via a
+/// `txn_version.txt` WRITE, relying on the pre-Task-6 "the mutable-file write path never calls
+/// buildFor" fact to keep `src_st.build` null. Since Task 6/9, `writeFile`'s inline-candidate path
+/// (which `txn_version.txt` now takes — it is an ordinary tree entry, not a mutable sidecar file)
+/// unconditionally calls `buildFor` for ANY inline entry, so the source silently acquired a REAL
+/// PartWriteTxn and this test drifted onto the *other* merge branch (`else if (src_st.build)`) without
+/// failing — both branches produce the same externally-visible result (assertions passed either
+/// way), so the drift was invisible. Fixed by staging the source via `unlinkFile` instead of a
+/// write: Task 8's removal-mark staging (`content_removed`) is the one remaining staging shape that
+/// genuinely never calls `buildFor` (`publishStaging`'s own `!st.build && ...` guard depends on
+/// this), so `parts[src_key]` exists but `src_st.build` stays null again, restoring the test's
+/// documented precondition.
+///
+/// Made RED-able (the review's ask): `PartWriteTxn::abandon()` unconditionally emits a `BuildAbort`
+/// `CasEvent` (`CasPartWriteTxn.cpp`) — this only happens if the buggy `else if (src_st.build)` branch
+/// runs `src_st.build->abandon()`. Registering an event sink (`Cas::Pool::setEventSink`, the same
+/// public test hook `gtest_cas_event_log.cpp` uses) and asserting no `BuildAbort` event fires is a
+/// genuine behavioral discriminator between the two merge branches — not just "assertions pass
+/// either way" — so a future regression that gives the source a PartWriteTxn again fails this test loudly.
+TEST(CASWiringOps, MoveDirectoryOntoExistingDestinationBuildSurvives)
+{
+    std::vector<DB::Cas::CasEvent> events;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto storage = openWiringStorage();
+    storage->store()->setEventSink([&](const DB::Cas::CasEvent & e) { events.push_back(e); });
+
+    /// unlinkFile now honors if_exists=false (triage #24, 8fc0c964a5b): the target must be real. Commit
+    /// it in its own transaction first so the removal below targets a genuinely-committed file, not a
+    /// never-existed path.
+    auto setup_tx = storage->createTransaction();
+    writeThroughTransaction(*setup_tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_z/txn_version.txt", "creation_tid: (7,7,00000000-0000-0000-0000-000000000000)");
+    setup_tx->commit(DB::NoCommitOptions{});
+
+    auto tx = storage->createTransaction();
+    /// Destination already has a real blob upload staged -> a live PartWriteTxn.
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_7_7_7/data.bin", "dst-content");
+    /// Source is staged with ONLY a removal mark (Task 8's content_removed staging) -> parts[src_key]
+    /// exists, but src_st.build stays null (unlinkFile never calls buildFor).
+    tx->unlinkFile("a11/a11a11a1-1111-4111-8111-111111111111/tmp_z/txn_version.txt", /*if_exists=*/false, /*should_remove_objects=*/true);
+
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+    EXPECT_NO_THROW(ca_tx.moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_z", "a11/a11a11a1-1111-4111-8111-111111111111/all_7_7_7"));
+
+    /// The discriminator: no BuildAbort event means src_st.build->abandon() was never called during
+    /// the re-key merge, i.e. the intended neither-branch no-op merge ran, not the two-builds
+    /// merge-and-abandon branch. Checked right after the re-key so it stays scoped to the merge.
+    EXPECT_FALSE(std::any_of(events.begin(), events.end(),
+        [](const DB::Cas::CasEvent & e) { return e.type == DB::Cas::CasEventType::BuildAbort; }))
+        << "src_st.build->abandon() fired — the source unexpectedly has a real PartWriteTxn again";
+
+    /// [TXN-ONE-PIPELINE] the re-key does not publish; the destination's build is materialized only at
+    /// commit(). The destination's own build then publishes its own content untouched; the source's
+    /// removal mark names a path never committed anywhere, so it is a harmless no-op once merged into
+    /// the destination's (first-time-published) staging.
+    tx->commit(DB::NoCommitOptions{});
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_7_7_7"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_7_7_7/data.bin"), 11u);   /// "dst-content"
+    EXPECT_FALSE(storage->tryGetInManifestBytes("a11/a11a11a1-1111-4111-8111-111111111111/all_7_7_7/txn_version.txt").has_value());
+
+    storage->store()->setEventSink(nullptr);
+}
+
+TEST(CASWiringOps, FreezeViaHardLinksIntoShadow)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "frozen-bytes");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/metadata_version.txt", "7");
+    tx->commit(DB::NoCommitOptions{});
+
+    /// FREEZE clones a committed part file-by-file into the shadow tree via hardlinks; the staged
+    /// shadow part publishes at commit (pool-global - any replica reads the backup).
+    auto tx2 = storage->createTransaction();
+    tx2->createHardLink("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin");
+    tx2->createHardLink("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/metadata_version.txt",
+                        "shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/metadata_version.txt");
+    tx2->commit(DB::NoCommitOptions{});
+
+    EXPECT_TRUE(storage->existsDirectory("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_TRUE(storage->existsFile("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_EQ(storage->tryGetInManifestBytes("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/metadata_version.txt"),
+              std::optional<String>("7"));
+
+    /// UNFREEZE: removeRecursive of the backup root drops every shadow namespace under it.
+    auto tx3 = storage->createTransaction();
+    tx3->removeRecursive("shadow/bk1", {});
+    EXPECT_FALSE(storage->existsDirectory("shadow/bk1/store/a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_FALSE(storage->existsDirectory("shadow/bk1"));
+}
+
+/// ==== M-W Task 8: in-flight read-your-writes (B59) ====
+
+TEST(CASWiringInFlight, StagedFilesVisibleBeforeCommit)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/p.proj/data.bin", "proj-bytes");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/uuid.txt", "u-9");
+
+    /// B188 precommit-first: content blobs are PENDING (staged locally, not yet uploaded). So
+    /// tryGetInFlightStorageObjects returns {} — the pool object does not exist yet. The caller
+    /// (DataPartStorageOnDiskFull::prepareRead) falls back to tryGetInFlightFileSize to get the size
+    /// and then serves the content via tryReadFileInFlight (local temp file). File sizes and directory
+    /// overlay still work because they are driven by the staged tree entry, not the pool.
+    auto objects = tx->tryGetInFlightStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/p.proj/data.bin");
+    EXPECT_FALSE(objects.has_value());
+    EXPECT_EQ(tx->tryGetInFlightFileSize("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/p.proj/data.bin"), std::optional<uint64_t>(10));
+    EXPECT_EQ(tx->tryGetInFlightFileSize("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/uuid.txt"), std::optional<uint64_t>(3));
+    EXPECT_FALSE(tx->tryGetInFlightFileSize("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/missing.bin").has_value());
+
+    /// Bytes read back: a pending blob from the local temp file (B188); staged mutable bytes from memory.
+    {
+        auto buf = tx->tryReadFileInFlight("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/p.proj/data.bin", {}, std::nullopt);
+        ASSERT_TRUE(buf);
+        String read;
+        readStringUntilEOF(read, *buf);
+        EXPECT_EQ(read, "proj-bytes");   /// B188: served from local temp file (pending upload)
+    }
+    {
+        auto buf = tx->tryReadFileInFlight("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/uuid.txt", {}, std::nullopt);
+        ASSERT_TRUE(buf);
+        String read;
+        readStringUntilEOF(read, *buf);
+        EXPECT_EQ(read, "u-9");
+    }
+
+    /// The directory overlay answers for INNER dirs only (the PoC contract): the part dir itself
+    /// is FALSE so a rejected temporary part's removeIfNeeded takes the clean early-return path.
+    EXPECT_FALSE(tx->hasInFlightDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0"));
+    EXPECT_TRUE(tx->hasInFlightDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/p.proj"));
+    EXPECT_FALSE(tx->hasInFlightDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/q.proj"));
+    auto top = tx->listInFlightDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0");
+    EXPECT_EQ(top, (std::vector<std::string>{"p.proj", "uuid.txt"}));
+    EXPECT_EQ(tx->listInFlightDirectory("a11/a11a11a1-1111-4111-8111-111111111111/tmp_mut_all_1_1_0/p.proj"),
+              (std::vector<std::string>{"data.bin"}));
+}
+
+namespace DB::ErrorCodes
+{
+    extern const int NOT_IMPLEMENTED;
+}
+
+/// ==== M-W Task 10: the GC scheduler end-to-end through the wiring ====
+
+TEST(CASWiringGc, DroppedPartIsReclaimedByRounds)
+{
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "reclaim-me");
+    tx->commit(DB::NoCommitOptions{});
+
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+    const auto blob_key = storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin")[0].remote_path;
+
+    auto tx2 = storage->createTransaction();
+    tx2->removeDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");   /// dropRef - the part is unreachable now
+
+    /// Round 1 folds the drop and retires+deletes the part MANIFEST; the freed blob is retired+deleted
+    /// by a FOLLOWING round (next-round reclamation, M-C3). The steal needs one extra observation
+    /// window between rounds (the pacing scheduler is stable across these calls - each call after the
+    /// first re-acquires via renewal).
+    storage->runOneGcRoundForTest();
+    storage->runOneGcRoundForTest();
+    storage->runOneGcRoundForTest();
+
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    /// The relink offer (B7 part_manifest_v2): a reclaimed part is no longer a committed CA part here,
+    /// so getRelinkOffer offers NOTHING and the sender streams bytes — the documented fallback.
+    EXPECT_FALSE(exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0").has_value());
+
+    /// A fresh identical write re-CREATES the content at the same key and reads back fine.
+    auto tx3 = storage->createTransaction();
+    writeThroughTransaction(*tx3, "a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_0/data.bin", "reclaim-me");
+    tx3->commit(DB::NoCommitOptions{});
+    EXPECT_EQ(storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_0/data.bin")[0].remote_path, blob_key);
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_0/data.bin"));
+}
+
+/// B199 (real-path displacement reclamation, ported off the tree model to part manifests): re-writing
+/// the SAME part path with DISTINCT content publishes a NEW part ManifestId over the ref (a true-removal
+/// of the old owner manifest + an activation of the new one in the single ordered journal — no shared
+/// content-addressed identity between the two parts). GC must reclaim the displaced (manifestA) unique
+/// blobs while never losing the live (manifestB) closure.
+///
+/// NOTE (port): the original repro pre-deleted treeA's TREE OBJECT before the fold to exercise the
+/// tree-era inline-closure 404 path (the precommit `Add` carried treeA's closure INLINE so the fold
+/// recorded edges without a `readTree`). That mechanism is gone: a part manifest carries its OWN blob
+/// edges and the fold reads the ONE removal-target body to release them (a missing removal body clamps
+/// + records an anomaly, never guesses). So this port drives the genuine manifest displacement WITHOUT
+/// the out-of-band pre-delete twist — the reclamation contract (no leak / no loss) is what survives.
+///
+/// PORT (rev. 15 displacement shape): a part is a single-owner ManifestId and `promote` is a PURE OWNER
+/// MOVE (precommit→committed). Re-publishing over a LIVE committed ref does NOT emit a removal of the
+/// displaced owner (the displaced manifest is not named in any event), so its blobs would never get a
+/// -1 — there is no in-place "republish-over-committed". The genuine displacement that DOES journal a
+/// true-removal is the real MergeTree pattern: DROP the old part (dropRef appends old→none, leaving the
+/// old body present for the fold to read the -1 edges), THEN publish the new part. GC folds manifestA's
+/// removal, retires its now-zero-in-degree blobs, and the recheck cleanup deletes the owner-removed
+/// body. We do NOT pre-delete manifestA's body — only GC deletes an owner-removed body, after sealing
+/// its decrements.
+TEST(CASWiringGc, DisplacedTreeBlobsReclaimedThroughRealPath)
+{
+    auto storage = openWiringStorage();
+
+    /// Commit manifestA with unique content (data-A / mark-A), through the real precommit-first transaction.
+    {
+        auto tx = storage->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0/data.bin", "data-A");
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0/data.cmrk3", "mark-A");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    const auto resolved_a = storage->store()->resolveRef(storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_0_0_0");
+    ASSERT_TRUE(resolved_a.has_value());
+    const DB::Cas::ManifestId manifest_a = resolved_a->manifest_id;
+
+    /// DISPLACE (true-removal repoint): drop the old part so dropRef journals manifestA's removal
+    /// (old=committed(manifestA)→new=none) — this leaves manifestA's body PRESENT for the fold to read
+    /// its -1 edges. Then re-write the SAME part path with DISTINCT content (data-B / mark-B), which
+    /// publishes a NEW part ManifestId over the (now free) ref. Confirm the displacement is real: the
+    /// ref resolves to a DIFFERENT manifest.
+    {
+        auto tx = storage->createTransaction();
+        tx->removeDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    {
+        auto tx = storage->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0/data.bin", "data-B");
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0/data.cmrk3", "mark-B");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    const auto resolved_b = storage->store()->resolveRef(storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_0_0_0");
+    ASSERT_TRUE(resolved_b.has_value());
+    ASSERT_FALSE(manifest_a == resolved_b->manifest_id)
+        << "the second write must displace the ref to a distinct part manifest (last-op-wins)";
+
+    /// Drive GC to a fixpoint. Displacement reclamation needs the next-round cascade (manifestA's
+    /// removal folds, its blobs hit zero in-degree, a following round retires+deletes them); give a
+    /// generous bound so the displaced closure fully drains.
+    for (int i = 0; i < 8; ++i)
+        storage->runOneGcRoundForTest();
+
+    const DB::Cas::FsckReport after = DB::Cas::runFsck(*storage->store(), /*detail=*/false);
+    EXPECT_EQ(after.dangling, 0u) << "displacement must never lose a reachable object (manifestB stays live)";
+    EXPECT_GT(after.reachable, 0u) << "the live ref points at manifestB; manifestB's closure is reachable";
+    /// The REAL path: `runOneGcRoundForTest` drives the production scheduler, so the displaced closure
+    /// is not merely recognized as unreachable but actually reclaimed -- recognition alone would leave
+    /// every displacement leaking one part's unique blobs forever.
+    EXPECT_EQ(after.unreachable, 0u)
+        << "manifestA's unique blobs (data-A / mark-A) must be RECLAIMED once the displacement folds, "
+        << "not just recognized as unreachable; unreachable=" << after.unreachable;
+}
+
+/// ==== M-W Task 11 / B7: the DataPartsExchange facade (manifest relink, part_manifest_v2) ====
+
+/// Publish-then-confirm (Task 14) split the receiver's adoption into `prepare` + `promote`, with the
+/// interserver confirm interposed between them. The confirm belongs to `Fetcher`, not to the storage, so
+/// the tests below that only care about the ADOPTION drive both halves back to back through this helper
+/// -- which is exactly what `publishEntries` does for the atomic callers. `false` is the
+/// `MechanismFallbackAllowed` outcome of either half: nothing published, the caller byte-fetches.
+namespace
+{
+
+/// The RECEIVER's disk-relative staging path for every relink test below: the tmp-fetch dir of the
+/// receiving table (a22...), which is a DIFFERENT table from the sender's (a11...) -- that is what makes
+/// the "the sender's namespace id is ignored" assertions meaningful. `prepareAdoptFromManifest` is
+/// addressed by path, exactly like `getRelinkOffer`, so the ref name is the router's business.
+constexpr auto kReceiverTmpFetchPath = "a22/a22a22a2-2222-4222-8222-222222222222/tmp-fetch_all_1_1_0";
+
+bool adoptPartFromManifestAndPromote(DB::IContentAddressedExchange & exchange, const String & part_path,
+                                     const String & manifest_bytes)
+{
+    std::unique_ptr<DB::ICaPreparedRelink> prepared;
+    if (exchange.prepareAdoptFromManifest(part_path, manifest_bytes, prepared)
+        == DB::CaRelinkPrepare::MechanismFallbackAllowed)
+        return false;
+    EXPECT_NE(prepared, nullptr) << "a Prepared outcome must carry the handle that owes the terminal operation";
+    return prepared->promote() == DB::CaRelinkPromote::Committed;
+}
+
+}
+
+/// B7 sender side: getRelinkOffer returns the COMMITTED part's encoded PartManifest body — the
+/// opaque payload the receiver decodes. The bytes must decode to the same entries the part was
+/// published with; an absent part offers nothing (the sender streams bytes — the documented fallback).
+/// Task 13 adds the second half of the offer: the confirm token, which must name the SAME manifest the
+/// body carries. That equality is the offer's whole safety property — a token naming anything else
+/// would have the receiver confirm a manifest whose entries it never adopted.
+TEST(CASWiringExchange, GetRelinkOfferReturnsBodyAndTokenForCommittedPart)
+{
+    auto storage = openWiringStorage();
+    /// Publish a real committed part (data.bin + a projection blob + mutable per-part files).
+    publishWiredPart(*storage, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0");
+
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+    EXPECT_FALSE(exchange->getPoolUUID().empty());
+
+    auto offer = exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    ASSERT_TRUE(offer.has_value());
+    EXPECT_FALSE(offer->manifest_bytes.empty());
+
+    /// The transferred body decodes to the SAME entries the part names — the blob entries AND the
+    /// per-part files (uuid.txt/metadata_version.txt are ordinary tree entries now, all-tree Task 6/9).
+    /// The sender's ManifestRef/namespace/digest are present but non-authoritative downstream.
+    const DB::Cas::PartManifest decoded = DB::Cas::decodePartManifest(offer->manifest_bytes);
+    ASSERT_EQ(decoded.entries.size(), 4u);
+    EXPECT_EQ(decoded.entries[0].path, "data.bin");
+    EXPECT_EQ(decoded.entries[0].ref.digest.toU128(), u128Of("payload-A"));
+    EXPECT_EQ(decoded.entries[2].path, "p.proj/data.bin");
+    EXPECT_EQ(decoded.entries[2].ref.digest.toU128(), u128Of("payload-B"));
+
+    /// The token: it decodes, it names this mount and this pool, and it names the manifest that the
+    /// body just decoded to.
+    const auto token = DB::decodeCasRelinkSourceToken(offer->confirm_token);
+    ASSERT_TRUE(token.has_value()) << "the sender minted a token its own decoder rejects: " << offer->confirm_token;
+    EXPECT_EQ(token->pool_uuid, exchange->getPoolUUID());
+    EXPECT_EQ(token->root_namespace, storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111").string());
+    EXPECT_EQ(token->ref_name, "all_1_1_0");
+    EXPECT_EQ(token->part_name, "all_1_1_0");
+    EXPECT_EQ(token->manifest_ref_text, DB::Cas::manifestRefDebugString(decoded.ref));
+    EXPECT_TRUE(exchange->ownsNamespace(token->server_root_id, token->root_namespace))
+        << "the minted token must route back to the mount that minted it";
+
+    /// An absent part is not a committed CA part here -> no offer.
+    EXPECT_FALSE(exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_9_9_9").has_value());
+}
+
+/// B7 receiver side (the core): take a COMMITTED part's transferred manifest bytes and adopt them into
+/// a DIFFERENT table namespace WITHOUT moving any blob body (blobs are shared by hash in the pool).
+/// The receiver stages its OWN fresh local manifest, precommitAdd + promote it, and reports success.
+/// Asserts: success; the adopted ref is live + loadable; the receiver's ManifestId differs from the
+/// sender's (no shared identity); the ref lives in the RECEIVER namespace (no cross-namespace adoption);
+/// and NO blob body was uploaded by the receiver (the put-counter stays flat across adopt).
+TEST(CASWiringExchange, AdoptPartFromManifestPublishesFreshLocalManifest)
+{
+    auto storage = openWiringStorage();
+    const auto sender_ns = storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111");
+    publishWiredPart(*storage, sender_ns, "all_1_1_0");
+
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+
+    auto offer = exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    ASSERT_TRUE(offer.has_value());
+    const String & bytes = offer->manifest_bytes;
+    const DB::Cas::ManifestId sender_id =
+        storage->store()->resolveRef(sender_ns, "all_1_1_0")->manifest_id;
+
+    /// Count blob PUTs over the adopt: a manifest relink must NOT upload any blob body (the blobs are
+    /// already in the shared pool, adopted by hash). We assert via the blob keys' presence/incarnation:
+    /// the receiver never overwrites or re-creates the blobs — their head tokens are unchanged.
+    const auto data_key = storage->store()->layout().blobKey(idOf("payload-A"));
+    const auto proj_key = storage->store()->layout().blobKey(idOf("payload-B"));
+    const auto data_tok_before = storage->store()->backend().head(data_key).token;
+    const auto proj_tok_before = storage->store()->backend().head(proj_key).token;
+
+    /// Adopt into a DIFFERENT table (a22a22a2-2222-4222-8222-222222222222). The transferred body's root_namespace_id is the sender's
+    /// (a11a11a1-1111-4111-8111-111111111111) — the receiver must IGNORE it and use a22a22a2-2222-4222-8222-222222222222.
+    const bool ok = adoptPartFromManifestAndPromote(*exchange, kReceiverTmpFetchPath, bytes);
+    EXPECT_TRUE(ok);
+
+    /// The adopted ref is live in the RECEIVER namespace and loadable.
+    const auto receiver_ns = storage->liveNamespace("a22a22a2-2222-4222-8222-222222222222");
+    auto receiver_resolved = storage->store()->resolveRef(receiver_ns, "tmp-fetch_all_1_1_0");
+    ASSERT_TRUE(receiver_resolved.has_value());
+    const DB::Cas::PartManifest receiver_manifest =
+        storage->store()->readManifest(receiver_resolved->manifest_id);
+    ASSERT_EQ(receiver_manifest.entries.size(), 4u);
+    EXPECT_EQ(receiver_manifest.entries[0].ref.digest.toU128(), u128Of("payload-A"));
+
+    /// FRESH receiver-local identity: a DIFFERENT ManifestId from the sender's, in the RECEIVER namespace.
+    EXPECT_FALSE(sender_id == receiver_resolved->manifest_id)
+        << "the receiver must mint its OWN manifest id, not share the sender's";
+    EXPECT_EQ(receiver_resolved->manifest_id.root_namespace.string(), receiver_ns.string())
+        << "the adopted manifest must live in the receiver namespace (derived from table_uuid), not the sender's";
+    EXPECT_FALSE(receiver_ns.string() == sender_ns.string());
+
+    /// NO blob body was uploaded: the shared blobs' incarnations are untouched by the adopt.
+    EXPECT_EQ(storage->store()->backend().head(data_key).token, data_tok_before)
+        << "adopt-from-manifest must not re-upload a blob already in the shared pool";
+    EXPECT_EQ(storage->store()->backend().head(proj_key).token, proj_tok_before);
+}
+
+/// B7 fail-closed: if a referenced blob is absent/condemned in the pool, adoptPartFromManifest must
+/// promote-abort and return FALSE (NOT throw) so the caller byte-fetches — exactly where the old pin
+/// protocol fell back. Nothing is published (no dangling ref).
+TEST(CASWiringExchange, AdoptFailsClosedAndFallsBackOnCondemnedBlob)
+{
+    /// §4 manifest-trust (test name is legacy — adopt no longer fails closed on a raced pool blob):
+    /// adoptPartFromManifest runs the receiver's local promote, which TRUSTS the committed-source adopted
+    /// leaves via the durable manifest edge — no per-file HEAD/loadMeta probe on the pool blobs. So even if
+    /// a pool blob raced to absent, adopt SUCCEEDS and publishes the receiver ref. This matches ordinary
+    /// ReplicatedMergeTree interserver trust: the sender served the manifest from a LIVE part whose refs pin
+    /// the blobs at in-degree >= 1, so this scenario cannot arise on the real fetch path; a genuinely-absent
+    /// adopted blob is an fsck finding, not an adopt-time abort.
+    auto storage = openWiringStorage();
+    const auto sender_ns = storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111");
+    publishWiredPart(*storage, sender_ns, "all_1_1_0");
+
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+    auto offer = exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    ASSERT_TRUE(offer.has_value());
+    const String & bytes = offer->manifest_bytes;
+
+    /// Artificially delete a referenced pool blob — the live-sender invariant excludes this on the real
+    /// path; §4 promote does not re-probe it, so adopt trusts the manifest edge and publishes.
+    const auto data_key = storage->store()->layout().blobKey(idOf("payload-A"));
+    const auto h = storage->store()->backend().head(data_key);
+    ASSERT_TRUE(h.exists);
+    ASSERT_EQ(storage->store()->backend().deleteExact(data_key, h.token).kind,
+              DB::Cas::DeleteOutcome::Kind::Deleted);
+
+    /// §4: promote trusts the adopted leaves — no re-probe — so adopt SUCCEEDS (returns true) and publishes.
+    const bool ok = adoptPartFromManifestAndPromote(*exchange, kReceiverTmpFetchPath, bytes);
+    EXPECT_TRUE(ok) << "§4: adopt trusts the manifest edge; a raced pool blob is not re-probed at promote";
+
+    /// The receiver ref publishes (the D4 trade-off), and the deleted pool blob surfaces via fsck's
+    /// reachable-but-absent scan (the backstop — INV-NO-DANGLE-via-fsck).
+    EXPECT_TRUE(storage->store()->resolveRef(storage->liveNamespace("a22a22a2-2222-4222-8222-222222222222"), "tmp-fetch_all_1_1_0").has_value());
+    const DB::Cas::FsckReport rep = DB::Cas::runFsck(*storage->store(), /*detail=*/true);
+    EXPECT_GE(rep.dangling, 1u) << "§4 D4 backstop: the deleted pool blob must surface as an fsck dangling "
+                                   "finding (dangling=" << rep.dangling << ")";
+}
+
+/// All-tree task 7/9: relink self-containment. Task 6 routes uuid.txt/metadata_version.txt through
+/// the content path, so a committed part's manifest ENTRIES already carry these files — the receiver
+/// no longer needs a mutable_files sidecar to reconstruct them. Task 9 completed the cleanup:
+/// `adoptPartFromManifest` no longer even HAS a sidecar parameter (Fetcher::relinkPartToDisk's call
+/// site simply dropped the argument). This publishes a part whose per-part files are ordinary
+/// manifest entries and adopts it, mirroring the post-task-9 call site exactly.
+TEST(CASWiringExchange, AdoptPartFromManifestSelfContainedWithoutMutableFilesSidecar)
+{
+    auto storage = openWiringStorage();
+    const auto sender_ns = storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111");
+
+    DB::Cas::PartWriteInfo info;
+    info.intended_ref = sender_ns.string() + "/all_1_1_0";
+    info.intended_namespace = sender_ns;
+    auto build = storage->store()->beginPartWrite(info);
+    const auto id = build->stageManifest(
+        {wiringBlobEntry("data.bin", "payload-A"),
+         wiringBlobEntry("uuid.txt", "payload-uuid"),
+         wiringBlobEntry("metadata_version.txt", "payload-mv")});
+    build->precommitAdd(sender_ns, "all_1_1_0", id);
+    build->putBlob(idOf("payload-A"), DB::Cas::BlobSource::fromString("payload-A"));
+    build->putBlob(idOf("payload-uuid"), DB::Cas::BlobSource::fromString("payload-uuid"));
+    build->putBlob(idOf("payload-mv"), DB::Cas::BlobSource::fromString("payload-mv"));
+    build->promote(sender_ns, "all_1_1_0", build->buildId(), id);
+
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+    auto offer = exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    ASSERT_TRUE(offer.has_value());
+    const String & bytes = offer->manifest_bytes;
+    const DB::Cas::PartManifest decoded = DB::Cas::decodePartManifest(bytes);
+    ASSERT_EQ(decoded.entries.size(), 3u) << "uuid.txt/metadata_version.txt travel as ordinary entries";
+
+    /// No sidecar parameter to pass anymore — exactly what Fetcher::relinkPartToDisk's call looks like
+    /// now that the manifest is self-contained (no reconstruction from a wire-transferred header).
+    const bool ok = adoptPartFromManifestAndPromote(*exchange, kReceiverTmpFetchPath, bytes);
+    EXPECT_TRUE(ok);
+
+    const auto receiver_ns = storage->liveNamespace("a22a22a2-2222-4222-8222-222222222222");
+    auto resolved = storage->store()->resolveRef(receiver_ns, "tmp-fetch_all_1_1_0");
+    ASSERT_TRUE(resolved.has_value());
+
+    const DB::Cas::PartManifest receiver_manifest = storage->store()->readManifest(resolved->manifest_id);
+    ASSERT_EQ(receiver_manifest.entries.size(), 3u);
+    bool has_uuid_entry = false;
+    bool has_metadata_version_entry = false;
+    for (const auto & entry : receiver_manifest.entries)
+    {
+        if (entry.path == "uuid.txt")
+            has_uuid_entry = true;
+        if (entry.path == "metadata_version.txt")
+            has_metadata_version_entry = true;
+    }
+    EXPECT_TRUE(has_uuid_entry) << "uuid.txt must read back as an ordinary content entry, not mutable_files";
+    EXPECT_TRUE(has_metadata_version_entry)
+        << "metadata_version.txt must read back as an ordinary content entry, not mutable_files";
+}
+
+/// Publish-then-confirm, receiver half (Task 14): `prepare` must make the receiver's `+1` DURABLE while
+/// publishing NOTHING. That combination is the protocol -- the durable `+1` is what a later `yes` is
+/// worth anything against, and the absent committed ref is what makes an unproven source cost nothing.
+TEST(CASWiringExchange, PrepareAdoptIsDurableButPublishesNothingUntilPromote)
+{
+    auto storage = openWiringStorage();
+    const auto sender_ns = storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111");
+    publishWiredPart(*storage, sender_ns, "all_1_1_0");
+
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+    auto offer = exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    ASSERT_TRUE(offer.has_value());
+
+    const auto receiver_ns = storage->liveNamespace("a22a22a2-2222-4222-8222-222222222222");
+    std::unique_ptr<DB::ICaPreparedRelink> prepared;
+    ASSERT_EQ(exchange->prepareAdoptFromManifest(kReceiverTmpFetchPath, offer->manifest_bytes, prepared),
+              DB::CaRelinkPrepare::Prepared);
+    ASSERT_NE(prepared, nullptr);
+
+    EXPECT_FALSE(storage->store()->resolveRef(receiver_ns, "tmp-fetch_all_1_1_0").has_value())
+        << "prepare must not commit the ref -- the source has not been asked anything yet";
+    EXPECT_EQ(storage->store()->livePrecommitsForTest(receiver_ns).size(), 1u)
+        << "prepare must leave the receiver's +1 durable, or a later confirm proves nothing";
+
+    EXPECT_EQ(prepared->promote(), DB::CaRelinkPromote::Committed);
+    EXPECT_TRUE(storage->store()->resolveRef(receiver_ns, "tmp-fetch_all_1_1_0").has_value());
+    EXPECT_TRUE(storage->store()->livePrecommitsForTest(receiver_ns).empty())
+        << "promote moves the binding out of the precommit view";
+}
+
+/// The unproven-source branch of the taxonomy (row 3), at the storage seam: `abort` releases the durable
+/// `+1` and publishes nothing. A leaked same-epoch precommit is reclaimed by nothing -- not the
+/// prior-epoch stale sweep, not GC -- so this removal is the ONLY thing standing between an unproven
+/// confirm and permanently retained blobs.
+TEST(CASWiringExchange, AbortedPrepareReleasesThePrecommitAndPublishesNothing)
+{
+    auto storage = openWiringStorage();
+    const auto sender_ns = storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111");
+    publishWiredPart(*storage, sender_ns, "all_1_1_0");
+
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+    auto offer = exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    ASSERT_TRUE(offer.has_value());
+
+    const auto receiver_ns = storage->liveNamespace("a22a22a2-2222-4222-8222-222222222222");
+    std::unique_ptr<DB::ICaPreparedRelink> prepared;
+    ASSERT_EQ(exchange->prepareAdoptFromManifest(kReceiverTmpFetchPath, offer->manifest_bytes, prepared),
+              DB::CaRelinkPrepare::Prepared);
+    ASSERT_NE(prepared, nullptr);
+    ASSERT_EQ(storage->store()->livePrecommitsForTest(receiver_ns).size(), 1u);
+
+    prepared->abort();
+    EXPECT_TRUE(storage->store()->livePrecommitsForTest(receiver_ns).empty())
+        << "abort must append the exact precommit removal, not merely drop the transaction";
+    EXPECT_FALSE(storage->store()->resolveRef(receiver_ns, "tmp-fetch_all_1_1_0").has_value())
+        << "an aborted relink must leave no committed ref behind";
+
+    /// A second `abort` -- what the scope guard does after an explicit one -- must be a silent no-op
+    /// rather than an error, and destruction of an aborted handle must not re-drive anything.
+    prepared->abort();
+    prepared.reset();
+    EXPECT_TRUE(storage->store()->livePrecommitsForTest(receiver_ns).empty());
+}
+
+/// The `MechanismFallbackAllowed` branch (taxonomy row 2): an undecodable manifest is a mechanism
+/// failure, not a source failure -- the sender still has the part, so the receiver byte-fetches. Nothing
+/// may be staged, because there is no handle to abort it with.
+TEST(CASWiringExchange, PrepareAdoptOfAnUndecodableManifestAllowsTheByteFallback)
+{
+    auto storage = openWiringStorage();
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+
+    const auto receiver_ns = storage->liveNamespace("a22a22a2-2222-4222-8222-222222222222");
+    std::unique_ptr<DB::ICaPreparedRelink> prepared;
+    EXPECT_EQ(exchange->prepareAdoptFromManifest(kReceiverTmpFetchPath, "not a manifest at all", prepared),
+              DB::CaRelinkPrepare::MechanismFallbackAllowed);
+    EXPECT_EQ(prepared, nullptr) << "no handle may be returned when nothing was staged";
+    EXPECT_TRUE(storage->store()->livePrecommitsForTest(receiver_ns).empty());
+    EXPECT_FALSE(storage->store()->resolveRef(receiver_ns, "tmp-fetch_all_1_1_0").has_value());
+}
+
+/// B66b: a relink whose TARGET is a DETACHED part dir -- what `FETCH PARTITION ... TO detached` now
+/// does instead of streaming bytes. Nothing about the detached case is special-cased on the receiver:
+/// `Fetcher::relinkPartToDisk` hands over the staging path under the `detached/` parent and the router
+/// folds it onto a `detached/`-prefixed ref in the table's OWN namespace, exactly as every other read
+/// and write of a detached part is routed.
+///
+/// The load-bearing assertion is the NEGATIVE one. A detached fetch must publish a detached ref and
+/// nothing else: a live ref of the same name would make an un-attached part visible to the table, which
+/// is the one way a detached target could differ from the active one in a way that matters.
+TEST(CASWiringExchange, AdoptIntoADetachedTargetPublishesADetachedRefAndNoLiveRef)
+{
+    auto storage = openWiringStorage();
+    const auto sender_ns = storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111");
+    publishWiredPart(*storage, sender_ns, "all_1_1_0");
+
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+    auto offer = exchange->getRelinkOffer("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0");
+    ASSERT_TRUE(offer.has_value());
+
+    /// The receiver's staging path under the detached parent -- the path `relinkPartToDisk` composes
+    /// with `to_detached`, and the same one `downloadPartToDisk` would have written bytes into.
+    const String detached_tmp_path
+        = "a22/a22a22a2-2222-4222-8222-222222222222/detached/tmp-fetch_all_1_1_0";
+    EXPECT_TRUE(adoptPartFromManifestAndPromote(*exchange, detached_tmp_path, offer->manifest_bytes));
+
+    const auto receiver_ns = storage->liveNamespace("a22a22a2-2222-4222-8222-222222222222");
+    EXPECT_TRUE(storage->store()->resolveRef(receiver_ns, "detached/tmp-fetch_all_1_1_0").has_value())
+        << "the detached target must publish the `detached/`-prefixed ref in the table's own namespace";
+    EXPECT_FALSE(storage->store()->resolveRef(receiver_ns, "tmp-fetch_all_1_1_0").has_value())
+        << "a detached fetch must NOT publish a live ref of the same name";
+
+    /// The adopted part reads back through the ordinary path surface, blobs and per-part files alike --
+    /// no bytes were transferred for any of them.
+    EXPECT_TRUE(storage->existsFile(detached_tmp_path + "/data.bin"));
+    EXPECT_TRUE(storage->existsFile(detached_tmp_path + "/p.proj/data.bin"));
+    EXPECT_TRUE(storage->existsFile(detached_tmp_path + "/uuid.txt"));
+
+    /// Finalization, unchanged by this task: `IMergeTreeDataPart::renameTo(detached/<part>)` is a
+    /// moveDirectory of the staged dir to its final detached name, which on a content-addressed disk is
+    /// a ref repoint WITHIN the same namespace -- the same shape the active path's
+    /// `renameTempPartAndReplace` uses, and the reason the relinked detached part needs no new
+    /// finalization of its own.
+    {
+        auto tx = storage->createTransaction();
+        tx->moveDirectory(detached_tmp_path, "a22/a22a22a2-2222-4222-8222-222222222222/detached/all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    EXPECT_TRUE(storage->existsFile(
+        "a22/a22a22a2-2222-4222-8222-222222222222/detached/all_1_1_0/data.bin"));
+    EXPECT_FALSE(storage->store()->resolveRef(receiver_ns, "detached/tmp-fetch_all_1_1_0").has_value());
+    EXPECT_EQ(storage->detachedRefNames(receiver_ns), (std::vector<std::string>{"detached/all_1_1_0"}));
+    EXPECT_FALSE(storage->store()->resolveRef(receiver_ns, "all_1_1_0").has_value())
+        << "the detached finalization must stay inside the `detached/` ref space";
+}
+
+/// A relink target that is not a part DIRECTORY is a caller bug, and it must be loud rather than
+/// answered with `MechanismFallbackAllowed`: the byte fetch that a fallback invites would write to the
+/// same wrong place. The table dir stands in for the whole class (a file inside a part, a FREEZE shadow
+/// path, a bare `detached` container) -- all of them route to something that is not a part ref.
+///
+/// The refusal throws LOGICAL_ERROR, which aborts the whole process in debug/sanitizer builds
+/// (Exception.cpp's handle_error_code) instead of behaving like a catchable exception -- so the
+/// EXPECT_THROW form only makes sense in a plain release build, and CASWiringExchangeDeathTest below
+/// proves the SAME refusals positively abort under debug/sanitizer builds instead (same pattern as
+/// CASWiringOpsDeathTest above).
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASWiringExchange, PrepareAdoptRefusesATargetThatIsNotAPartDirectory)
+{
+    auto storage = openWiringStorage();
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+
+    std::unique_ptr<DB::ICaPreparedRelink> prepared;
+    EXPECT_THROW(exchange->prepareAdoptFromManifest(
+                     "a22/a22a22a2-2222-4222-8222-222222222222", std::string{}, prepared),
+                 DB::Exception);
+    EXPECT_THROW(exchange->prepareAdoptFromManifest(
+                     "a22/a22a22a2-2222-4222-8222-222222222222/tmp-fetch_all_1_1_0/data.bin",
+                     std::string{}, prepared),
+                 DB::Exception);
+    EXPECT_THROW(exchange->prepareAdoptFromManifest(
+                     "shadow/bk1/store/a22/a22a22a2-2222-4222-8222-222222222222/all_1_1_0",
+                     std::string{}, prepared),
+                 DB::Exception);
+    EXPECT_EQ(prepared, nullptr);
+}
+#else
+TEST(CASWiringExchangeDeathTest, PrepareAdoptRefusesATargetThatIsNotAPartDirectoryAborts)
+{
+    auto storage = openWiringStorage();
+    auto * exchange = dynamic_cast<DB::IContentAddressedExchange *>(storage.get());
+    ASSERT_NE(exchange, nullptr);
+
+    std::unique_ptr<DB::ICaPreparedRelink> prepared;
+    EXPECT_DEATH(exchange->prepareAdoptFromManifest(
+                     "a22/a22a22a2-2222-4222-8222-222222222222", std::string{}, prepared),
+                 "does not address a content-addressed part directory");
+    EXPECT_DEATH(exchange->prepareAdoptFromManifest(
+                     "a22/a22a22a2-2222-4222-8222-222222222222/tmp-fetch_all_1_1_0/data.bin",
+                     std::string{}, prepared),
+                 "does not address a content-addressed part directory");
+    EXPECT_DEATH(exchange->prepareAdoptFromManifest(
+                     "shadow/bk1/store/a22/a22a22a2-2222-4222-8222-222222222222/all_1_1_0",
+                     std::string{}, prepared),
+                 "does not address a content-addressed part directory");
+    EXPECT_EQ(prepared, nullptr);
+}
+#endif
+
+/// ==== Commit atomicity (B122): a publish failing mid-loop must not leave a PARTIAL commit ====
+
+#include <Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.h>
+#include <Common/Exception.h>
+#include <algorithm>
+#include <set>
+
+namespace DB::ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+    extern const int CORRUPTED_DATA;
+    extern const int READONLY;
+}
+
+namespace
+{
+
+/// A LocalObjectStorage whose writeObject can be armed to throw — the single seam needed to drive a
+/// backend write failure at a chosen point. The hook runs BEFORE the write is created; throwing from
+/// it fails the put exactly as a real backend error would. Everything else delegates to the base.
+class FaultyLocalObjectStorage : public DB::LocalObjectStorage
+{
+public:
+    using DB::LocalObjectStorage::LocalObjectStorage;
+
+    std::function<void(const std::string &)> on_write;
+
+    std::unique_ptr<DB::WriteBufferFromFileBase> writeObject(
+        const DB::StoredObject & object,
+        DB::WriteMode mode,
+        std::optional<DB::ObjectAttributes> attributes,
+        size_t buf_size,
+        const DB::WriteSettings & write_settings) override
+    {
+        if (on_write)
+            on_write(object.remote_path);
+        return DB::LocalObjectStorage::writeObject(object, mode, attributes, buf_size, write_settings);
+    }
+};
+
+/// True for a per-part manifest BODY object (<...>/cas/manifests/<namespace...>/<epoch-seq>/<NNNNNN>.zst)
+/// — the FIRST durable object `publishStaging` writes for a part (via `PartWriteTxn::stageManifest`). Since Task B
+/// (chaos-tolerance-report) that write rides the CAS request controller: a transient fault is retried
+/// (budgeted attempts + resolve-before-reissue), so an injected fault must be PERSISTENT to fail the
+/// publish — the controller exhausts its budget and `stageManifest` throws ABORTED out of `publishStaging`.
+/// Exactly one body per part (retries re-PUT the same per-part key), so counting FIRST attempts isolates
+/// part publishes one-for-one. Ref-log txns (`cas/ns/stream/.../_log/...`), tree blobs (`blobs/`), GC state
+/// (`gc/`) and verbatim files are excluded.
+///
+/// The suffix is taken from `storedSuffix(FormatId::PartManifest)` (the registered v3 stored suffix, now
+/// `.zst`) rather than hard-coded: codecs-v3 phase-3 made the part manifest an Always-compressed text
+/// object, changing the body key from the pre-v3 `.proto` to `.zst`. The old hard-coded
+/// `.ends_with(".proto")` stopped matching after that cutover, so the fault never fired and this
+/// (test-local) predicate silently no-op'd — the same failure mode this comment already recorded for the
+/// earlier `RootShardManifest` removal (commit `318291fe5e5`, whose all-digits key stopped matching).
+/// Sourcing the suffix from the format registry keeps the predicate correct across future
+/// compression-policy changes.
+bool isPartManifestBodyPath(const std::string & path)
+{
+    return path.find("/cas/manifests/") != std::string::npos
+        && path.ends_with(DB::Cas::storedSuffix(DB::Cas::FormatId::PartManifest));
+}
+
+std::shared_ptr<FaultyLocalObjectStorage> makeFaultyStorageForTest()
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("ca_b122_" + unique)).string();
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+    return std::make_shared<FaultyLocalObjectStorage>(DB::LocalObjectStorageSettings("test", root, /*read_only_=*/false));
+}
+
+}
+
+TEST(CASWiringWrite, PartialCommitRollsBackPublishedParts)
+{
+    auto faulty = makeFaultyStorageForTest();
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_b122_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        faulty, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    /// The manifest-body PUT rides the CAS request controller, whose inter-attempt backoff would
+    /// otherwise serve the REAL capped-exponential sleeps (~56s at the default budget) while the
+    /// persistent injected fault exhausts the whole attempt budget. Neutralize only the sleeps — the
+    /// retry/exhaustion/rollback semantics under test are unchanged.
+    storage->store()->setCasRetrySleepForTest([](uint64_t) {});
+
+    /// Two parts in ONE transaction, published sequentially at commit (the staging map orders all_1_1_0
+    /// before all_2_2_0). writeThroughTransaction only STAGES to local temp files here — the pool writes
+    /// (manifest bodies, blob uploads, ref-log promotes) all happen later, inside commit's publishStaging.
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "content-A");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/data.bin", "content-B");
+
+    /// Fail the SECOND part's manifest-body write (all_2_2_0's stageManifest) — by then all_1_1_0 has
+    /// fully published (its manifest body + blob + promoted ref). A pre-B122 commit() would leave
+    /// all_1_1_0 durably visible: a partial commit. PERSISTENT (`>= 2`, not one-shot): the manifest
+    /// body PUT rides the CAS request controller (Task B), which absorbs a transient fault by design —
+    /// only a fault that outlasts the whole attempt budget fails the publish (as ABORTED).
+    /// CORRUPTED_DATA (not LOGICAL_ERROR): `handle_error_code` (Exception.cpp) aborts the whole
+    /// process for LOGICAL_ERROR under debug/sanitizer builds, since that code means "an internal
+    /// invariant broke" there -- but this is a simulated BACKEND write failure, not an invariant
+    /// violation, so it must stay a catchable exception. CORRUPTED_DATA keeps the exact same
+    /// `isDeterministicLocalFailure` classification LOGICAL_ERROR had (CasRequestControl.cpp), so the
+    /// controller's retry/exhaustion behavior under test is unchanged.
+    int manifest_writes = 0;
+    faulty->on_write = [&](const std::string & path)
+    {
+        if (isPartManifestBodyPath(path) && ++manifest_writes >= 2)
+            throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA, "injected publish failure (B122)");
+    };
+
+    EXPECT_THROW(tx->commit(DB::NoCommitOptions{}), DB::Exception);
+
+    /// All-or-nothing: the part that DID publish must have been rolled back (commit's compensating
+    /// `dropRefIfMatches`, keyed on the exact `CommitOutcome` `all_1_1_0`'s own publish produced).
+    /// Disarm first so the read-back assertions run clean — the rollback itself only writes ref-log
+    /// ops, never a manifest body, so it does not re-trip the count-2 fault.
+    faulty->on_write = nullptr;
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0"));
+}
+
+TEST(CASWiringReadOnly, ObserveOnlyOpenReadsButRejectsWrites)
+{
+    /// 1. Writable storage publishes a part into a fixed root.
+    const auto root = (std::filesystem::temp_directory_path()
+                       / ("ca_ro_" + std::to_string(::getpid()))).string();
+    std::error_code ec; std::filesystem::remove_all(root, ec); std::filesystem::create_directories(root, ec);
+    auto writable_os = std::make_shared<DB::LocalObjectStorage>(
+        DB::LocalObjectStorageSettings("test", root, /*read_only_=*/false));
+    {
+        auto w_settings = DB::Cas::tests::makeSettingsForTest(
+            "test", std::filesystem::temp_directory_path() / "ca_ro_scratch");
+        auto w = std::make_shared<DB::ContentAddressedMetadataStorage>(
+            writable_os, "pool", "srv1", "", nullptr, w_settings);
+        w->startup();
+        auto tx = w->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "ro-bytes");
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// 2. Read-only object storage over the SAME root => observe-only metadata storage.
+    auto ro_os = std::make_shared<DB::LocalObjectStorage>(
+        DB::LocalObjectStorageSettings("test", root, /*read_only_=*/true));
+    /// Same `server_root_id` as the writer: live namespaces are rooted by configured layout identity, so an
+    /// observe-only mount reads the same server-root's data — the WORM scenario.
+    auto ro_settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_ro_scratch2");
+    auto ro = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        ro_os, "pool", "srv1", "", nullptr, ro_settings);
+    ro->startup();   /// must NOT throw (probe skipped — a probe write would fail on a read-only os)
+
+    EXPECT_TRUE(ro->isReadOnly());
+    /// Reads work:
+    EXPECT_TRUE(ro->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_EQ(ro->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"), 8u);
+    /// Writes fail closed:
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::READONLY,
+        [&] { ro->createTransaction(); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::READONLY,
+        [&]
+        {
+            std::unique_ptr<DB::ICaPreparedRelink> prepared;
+            ro->prepareAdoptFromManifest("a11/a11a11a1-1111-4111-8111-111111111111/tmp-fetch", std::string{}, prepared);
+        });
+}
+
+TEST(CASWiringRead, UnsetPublishedAtMsReturnsEpoch)
+{
+    /// A ref published without a stamp (published_at_ms == 0, the default) must return the epoch
+    /// (Poco::Timestamp(0)) rather than throwing: stamps only feed cleanup TTLs and system tables,
+    /// so a missing stamp is harmless.
+    auto storage = openWiringStorage();
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "x");
+    tx->commit(DB::NoCommitOptions{});
+
+    /// Ensure published_at_ms is unset (the default is 0).
+    storage->store()->updateRefPublishedAt(storage->liveNamespace("a11a11a1-1111-4111-8111-111111111111"), "all_1_1_0",
+        [](DB::Cas::RefPublishedAtUpdate & r) { r.published_at_ms = 0; });
+
+    EXPECT_EQ(storage->getLastModified("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0").epochTime(), 0);
+}
+
+/// ==== B188 precommit-first order invariant (Task 6) ====
+///
+/// A RecordingLocalObjectStorage records the four IObjectStorage methods the CA emulated-mode backend
+/// uses on the commit path — writeObject (PUT), exists + getObjectMetadata (the HEAD), and readObject
+/// (the GET) — as (op_name, logical_key). "Logical" means the bare pool key (without the emu_root
+/// prefix) — the same string the Layout functions produce, so the `/blobs/`, `/trees/`, and opaque
+/// ref-stream (`/cas/ns/stream/<life_id>/`) substring tests are unambiguous.
+///
+/// After commit the test asserts: the FIRST write that appends the create-precommit owner event (the
+/// first durable CAS to the target ROOT SHARD's key — owner_kind == Precommit; the converged rev. 15
+/// model has NO `_precommits` namespace, the precommit binding lives in the target shard's journal)
+/// happened before ALL ops (read OR write) on keys containing "/blobs/" or "/trees/". The precommit
+/// owner record is what pins the in-flight build-root closure so GC cannot reclaim the not-yet-uploaded
+/// content objects; therefore every pool op touching a content blob or the manifest tree must be AFTER
+/// the precommit owner record is durably written. The READ gating is the heart of the B188 fix: the
+/// original bug was an EAGER HEAD on a content blob during staging, before any precommit protection
+/// existed — a write-only assertion would not catch its reintroduction.
+
+namespace
+{
+
+/// Records the four IObjectStorage methods the CA emulated-mode backend uses on the commit path
+/// (writeObject/exists/getObjectMetadata/readObject). listObjects/copyObject are deliberately NOT
+/// overridden — they are not on the commit path the order invariant gates.
+class RecordingLocalObjectStorage final : public DB::LocalObjectStorage
+{
+public:
+    using DB::LocalObjectStorage::LocalObjectStorage;
+
+    struct Record
+    {
+        std::string op;    /// "writeObject" | "exists" | "getObjectMetadata" | "readObject"
+        std::string key;   /// logical (emu_root stripped)
+    };
+
+    /// Append-only; mutable so the const read methods (exists/readObject/tryGetObjectMetadata) can
+    /// record. No mutex — these tests are single-threaded.
+    mutable std::vector<Record> ops;
+
+    /// Strip the common-key-prefix (emu_root) to recover the logical key. The emu_root is returned by
+    /// getCommonKeyPrefix() and always ends with a path separator in LocalObjectStorage.
+    std::string toLogical(const std::string & physical) const
+    {
+        const std::string root = getCommonKeyPrefix();
+        std::string logical;
+        if (!root.empty() && physical.starts_with(root))
+            logical = physical.substr(root.size());
+        else
+            logical = physical;
+        /// Strip any leading slash left after prefix removal.
+        if (!logical.empty() && logical.front() == '/')
+            logical = logical.substr(1);
+        return logical;
+    }
+
+    std::unique_ptr<DB::WriteBufferFromFileBase> writeObject(
+        const DB::StoredObject & object,
+        DB::WriteMode mode,
+        std::optional<DB::ObjectAttributes> attributes,
+        size_t buf_size,
+        const DB::WriteSettings & write_settings) override
+    {
+        ops.push_back({"writeObject", toLogical(object.remote_path)});
+        return DB::LocalObjectStorage::writeObject(object, mode, attributes, buf_size, write_settings);
+    }
+
+    /// Backs the CA backend's `head` (emuExists) and gates its `get` (emuExists before emuRead).
+    bool exists(const DB::StoredObject & object) const override
+    {
+        ops.push_back({"exists", toLogical(object.remote_path)});
+        return DB::LocalObjectStorage::exists(object);
+    }
+
+    /// Backs the CA backend's `head` size/attributes lookup (emuPath stat).
+    std::optional<DB::ObjectMetadata> tryGetObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        ops.push_back({"getObjectMetadata", toLogical(path)});
+        return DB::LocalObjectStorage::tryGetObjectMetadata(path, with_tags);
+    }
+
+    /// Backs the CA backend's `get` body read (readObjectRanged).
+    std::unique_ptr<DB::ReadBufferFromFileBase> readObject(
+        const DB::StoredObject & object,
+        const DB::ReadSettings & read_settings,
+        std::optional<size_t> read_hint,
+        bool use_external_buffer,
+        bool restrict_seek) const override
+    {
+        ops.push_back({"readObject", toLogical(object.remote_path)});
+        return DB::LocalObjectStorage::readObject(object, read_settings, read_hint, use_external_buffer, restrict_seek);
+    }
+};
+
+std::shared_ptr<RecordingLocalObjectStorage> makeRecordingStorageForTest(const std::string & tag)
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("ca_b188_" + tag + "_" + unique)).string();
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+    return std::make_shared<RecordingLocalObjectStorage>(
+        DB::LocalObjectStorageSettings("test", root, /*read_only_=*/false));
+}
+
+/// True for a durable ref-object write key under `/cas/ns/stream/`. In the snapshot+log ref model the
+/// writer's first durable ref write on the precommit path is an immutable transaction-log object
+/// (`<...>/cas/ns/stream/<life_id>/_log/<txn-id>.zst`); a published table snapshot is
+/// `<...>/_snap/<txn-id>.zst`. The predicate anchors on whichever durable ref write comes first. It
+/// excludes blobs (`/blobs/`), part-manifests (`/cas/manifests/...`), GC state (`/gc/`), and verbatim
+/// files (`/_files/...`).
+bool isRefWriteKey(const std::string & key)
+{
+    if (key.find("/cas/ns/stream/") == std::string::npos)
+        return false;
+    return key.find("/_log/") != std::string::npos || key.find("/_snap/") != std::string::npos;
+}
+
+/// Index of the first writeObject that durably appends the create-precommit ref transaction — i.e. the
+/// first durable write (writeObject) of a ref-object key (a `_log/<txn-id>` object in the snapshot+log
+/// model). Anchors on the WRITE, not on any op: recovery READS the ref prefix before the durable write,
+/// so an any-op scan would anchor on that READ rather than the durable write. Returns -1 if no ref write
+/// was recorded.
+int firstPrecommitWriteIdx(const std::vector<RecordingLocalObjectStorage::Record> & log)
+{
+    for (int i = 0; i < static_cast<int>(log.size()); ++i)
+        if (log[i].op == "writeObject" && isRefWriteKey(log[i].key))
+            return i;
+    return -1;
+}
+
+}
+
+/// B188: every pool op (read OR write) on /blobs/ or /trees/ must come AFTER the first write that
+/// appends the create-precommit owner event (the first root-shard CAS) — including HEAD
+/// (exists/getObjectMetadata) and GET (readObject), since the
+/// exact bug was an eager HEAD on a content blob during staging. The transaction writes a fresh
+/// content file (pending blob) AND adopts an existing committed blob via hardlink — both paths must
+/// satisfy the invariant.
+TEST(CASWiringPrecommitOrder, NoContentPoolOpBeforePrecommit)
+{
+    auto recording = makeRecordingStorageForTest("order");
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_b188_order_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        recording, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+
+    /// Phase 1: publish a committed source part — this gives us a committed blob to adopt in Phase 2.
+    {
+        auto tx = storage->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0/data.bin", "source-blob");
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// Phase 2: a new transaction that BOTH writes a fresh content blob (all_1_1_0/data.bin, pending)
+    /// AND carries forward that PENDING blob via hardlink into a second fresh part (all_2_2_0/extra.bin,
+    /// the cross-part pending-source adopt path). We clear the op log after Phase 1 so only Phase 2's
+    /// ops are analysed.
+    recording->ops.clear();
+
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "fresh-content");
+    /// Adopt by hardlinking a PENDING blob (the file just written above) into a SECOND fresh part
+    /// (all_2_2_0). This is the B188-relevant adopt: the cross-part pending-source branch copies the
+    /// PendingBlob into the dst build (NO eager pool op — the blob is not durable yet, so a HEAD/GET on
+    /// it before precommit would be the exact bug). We deliberately do NOT adopt from the committed
+    /// source part here: adoptFromTree(committed source) legitimately READS that source's
+    /// already-durable, ref-pinned tree during staging — a foreign-tree read that is NOT a B188
+    /// violation (the invariant is about THIS build's own not-yet-uploaded content, never a committed
+    /// object owned by a live part). Gating it would be a false positive; see the committed-source
+    /// adopt coverage in CASWiringOps.HardLinkCarriesForwardWithoutReupload.
+    tx->createHardLink("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/extra.bin");
+    tx->commit(DB::NoCommitOptions{});
+
+    const auto & log = recording->ops;
+
+    /// The content objects THIS transaction publishes are exactly the BLOB keys it WRITES under
+    /// /blobs/ (the fresh/pending content blobs). The B188 invariant is that the build must not touch
+    /// ITS OWN not-yet-protected content before precommit. NOTE (rev. 15 manifest model): the staged
+    /// part-manifest body (`/_manifests/...`) is the precommit's EVIDENCE and is therefore written
+    /// BEFORE precommitAdd by design (stageManifest → precommitAdd → putBlob → promote) — it is NOT a
+    /// gated content object. Only the content BLOBS must wait for the precommit. Reads of foreign
+    /// committed objects (another part's blob) are legitimate and must not be gated — so we restrict
+    /// the gate to the set of /blobs/ keys this transaction itself wrote.
+    std::set<std::string> own_content_keys;
+    for (const auto & r : log)
+        if (r.op == "writeObject" && r.key.find("/blobs/") != std::string::npos)
+            own_content_keys.insert(r.key);
+
+    /// Anchor on the first precommit WRITE (the durable casPut), not on any precommit-key op.
+    const int first_precommit_idx = firstPrecommitWriteIdx(log);
+    ASSERT_GE(first_precommit_idx, 0)
+        << "No create-precommit owner write (root-shard CAS) was recorded — precommit step did not fire";
+
+    /// Every op (read OR write) on one of THIS build's own content blobs must have an index AFTER
+    /// first_precommit_idx. This gates HEAD (exists/getObjectMetadata) and GET (readObject), not just
+    /// PUT (writeObject) — an eager HEAD/GET on the build's own pending blob before precommit is the
+    /// exact B188 regression this guards against.
+    for (int i = 0; i < static_cast<int>(log.size()); ++i)
+    {
+        if (!own_content_keys.contains(log[i].key))
+            continue;
+        EXPECT_GT(i, first_precommit_idx)
+            << "Own-content pool op '" << log[i].op << "' on '" << log[i].key << "' at index " << i
+            << " came BEFORE the first precommit write at index " << first_precommit_idx
+            << " — violates B188 precommit-first invariant (no HEAD/GET/PUT on this build's content before precommit)";
+    }
+
+    /// Sanity: both parts are readable after commit, with the SAME underlying blob (content identity).
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/extra.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"), 13u);   /// "fresh-content"
+    EXPECT_EQ(storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin")[0].remote_path,
+              storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_2_2_0/extra.bin")[0].remote_path);
+
+    /// Confirm at least one blob WRITE and one staged-manifest WRITE were recorded (both the upload
+    /// path and the manifest-evidence path were exercised), so the gate above actually had content
+    /// keys to check and the precommit anchored on a real build.
+    const bool has_blob_write = std::any_of(log.begin(), log.end(),
+        [](const RecordingLocalObjectStorage::Record & r)
+        { return r.op == "writeObject" && r.key.find("/blobs/") != std::string::npos; });
+    const bool has_tree_write = std::any_of(log.begin(), log.end(),
+        [](const RecordingLocalObjectStorage::Record & r)
+        { return r.op == "writeObject" && r.key.find("/cas/manifests/") != std::string::npos; });
+    EXPECT_TRUE(has_blob_write) << "No /blobs/ write recorded — fresh blob path not exercised";
+    EXPECT_TRUE(has_tree_write) << "No /cas/manifests/ write recorded — manifest staging path not exercised";
+    EXPECT_FALSE(own_content_keys.empty()) << "No own content keys collected — gate would be vacuous";
+}
+
+/// B188 committed-source adopt (the LITERAL bug path): when createHardLink carries forward a blob
+/// from a COMMITTED source part (the source is NOT staged in this transaction), it takes the
+/// adoptFromTree -> adoptEvidence branch — a TOKENLESS W-EVIDENCE dep with NO eager HEAD on the
+/// adopted blob. The regression this guards is reverting adoptEvidence to a reuseBlob(false) (or any
+/// observeAndAdmit) that HEADs the adopted blob during staging, before any precommit protection
+/// exists. The own-content gate in NoContentPoolOpBeforePrecommit CANNOT catch this: the adopted blob
+/// is FOREIGN (owned by the live source part, never written by this transaction), so it is absent from
+/// own_content_keys. This test asserts a TARGETED invariant on that exact foreign blob key: no
+/// exists/getObjectMetadata/readObject/writeObject on it before first_precommit_idx.
+///
+/// adoptFromTree legitimately READS the source TREE during staging (to find the entry) — that is fine
+/// and is NOT asserted here; the assertion is scoped to the adopted BLOB key alone.
+TEST(CASWiringPrecommitOrder, CommittedSourceAdoptNoHeadBeforePrecommit)
+{
+    auto recording = makeRecordingStorageForTest("committed_adopt");
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_b188_committed_adopt_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        recording, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+
+    /// Phase 1: commit a source part with a content blob. Capture the source blob's logical key from
+    /// the recorded /blobs/ write (the SAME key derivation the recorder uses, so the substring/index
+    /// comparisons in Phase 2 line up exactly).
+    recording->ops.clear();
+    {
+        auto tx = storage->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0/data.bin", "committed-source-blob");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    std::string source_blob_key;
+    for (const auto & r : recording->ops)
+    {
+        if (r.op == "writeObject" && r.key.find("/blobs/") != std::string::npos)
+        {
+            source_blob_key = r.key;
+            break;
+        }
+    }
+    ASSERT_FALSE(source_blob_key.empty())
+        << "Phase 1 recorded no /blobs/ write — could not capture the committed-source blob key";
+
+    /// Phase 2: a FRESH transaction that hardlinks the COMMITTED source blob into a NEW part. The
+    /// source part (all_0_0_0) is not staged here, so createHardLink takes the committed-source branch
+    /// (adoptFromTree -> adoptEvidence). Clear the log so only Phase 2's ops are analysed.
+    recording->ops.clear();
+    {
+        auto tx = storage->createTransaction();
+        tx->createHardLink("a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0/data.bin", "a11/a11a11a1-1111-4111-8111-111111111111/all_5_5_0/data.bin");
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    const auto & log = recording->ops;
+
+    /// Anchor on the first precommit WRITE (the durable casPut), not on any precommit-key op.
+    const int first_precommit_idx = firstPrecommitWriteIdx(log);
+    ASSERT_GE(first_precommit_idx, 0)
+        << "No create-precommit owner write (root-shard CAS) was recorded — precommit step did not fire";
+
+    /// TARGETED assertion: the adopted (foreign, committed) blob key must NOT be touched by ANY op
+    /// (HEAD via exists/getObjectMetadata, GET via readObject, or PUT via writeObject) before the
+    /// precommit write. With the bug reintroduced, adoptEvidence -> reuseBlob -> observeAndAdmit would
+    /// HEAD this exact key during staging at an index < first_precommit_idx, failing here.
+    bool adopted_blob_touched_before_precommit = false;
+    for (int i = 0; i < first_precommit_idx; ++i)
+    {
+        if (log[i].key == source_blob_key)
+        {
+            adopted_blob_touched_before_precommit = true;
+            ADD_FAILURE()
+                << "Adopted committed-source blob op '" << log[i].op << "' on '" << log[i].key
+                << "' at index " << i << " came BEFORE the first precommit write at index "
+                << first_precommit_idx << " — violates B188 (committed-source adopt must not HEAD/GET/"
+                << "PUT the adopted blob before precommit; expected a tokenless adoptEvidence dep)";
+        }
+    }
+    EXPECT_FALSE(adopted_blob_touched_before_precommit);
+
+    /// The committed-source adopt also must NOT re-upload the blob at all (content carried forward by
+    /// reference): no writeObject on the source blob key in Phase 2.
+    const bool reuploaded = std::any_of(log.begin(), log.end(),
+        [&](const RecordingLocalObjectStorage::Record & r)
+        { return r.op == "writeObject" && r.key == source_blob_key; });
+    EXPECT_FALSE(reuploaded) << "Committed-source adopt re-uploaded the blob — should carry by reference";
+
+    /// Sanity: the new part reads back and shares the source blob object.
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_5_5_0/data.bin"));
+    EXPECT_EQ(storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_0_0_0/data.bin")[0].remote_path,
+              storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_5_5_0/data.bin")[0].remote_path);
+}
+
+/// B188 pending-blob hardlink (Task 6 Test 2): within a SINGLE transaction, write a content file
+/// into part X (pending blob, not yet uploaded), then createHardLink that SAME file into part Y
+/// (the cross-part pending-source branch: `&dst_st != src_st`, copies the PendingBlob record so
+/// publishStaging uploads it for the dst part too). After commit both parts must read back the
+/// identical content.
+TEST(CASWiringPending, HardlinkOfPendingBlobCommitsAndReadsBack)
+{
+    auto storage = openWiringStorage();
+
+    auto tx = storage->createTransaction();
+
+    /// Write fresh content into part X — the blob is PENDING (not uploaded yet, temp-file only).
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_10_10_0/data.bin", "pending-payload");
+
+    /// Before commit, hardlink part X's file into part Y. At this point:
+    ///   - src_st = staging for all_10_10_0 (exists: contains the pending blob)
+    ///   - dst_st = staging for all_11_11_0 (created fresh here)
+    ///   - &dst_st != src_st => PendingBlob is COPIED into dst_st.pending_blobs
+    ///   - Both builds get recordPendingBlobDep (tokenless dep — no pool op until post-precommit)
+    tx->createHardLink("a11/a11a11a1-1111-4111-8111-111111111111/all_10_10_0/data.bin", "a11/a11a11a1-1111-4111-8111-111111111111/all_11_11_0/data.bin");
+
+    /// Nothing visible yet (B188: no uploads before precommit).
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_10_10_0/data.bin"));
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_11_11_0/data.bin"));
+
+    tx->commit(DB::NoCommitOptions{});
+
+    /// Both parts must be visible and carry the same content.
+    ASSERT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_10_10_0/data.bin"));
+    ASSERT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_11_11_0/data.bin"));
+
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_10_10_0/data.bin"), 15u);   /// "pending-payload"
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_11_11_0/data.bin"), 15u);
+
+    /// Both parts must point to the SAME underlying blob object (content-addressed identity).
+    auto objs_x = storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_10_10_0/data.bin");
+    auto objs_y = storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_11_11_0/data.bin");
+    ASSERT_EQ(objs_x.size(), 1u);
+    ASSERT_EQ(objs_y.size(), 1u);
+    EXPECT_EQ(objs_x[0].remote_path, objs_y[0].remote_path)
+        << "Hardlinked pending blob must map to the SAME pool object in both parts";
+}
+
+/// ==== B190 Task 4: precommit-first for republishRef and committed-source createHardLink ====
+///
+/// B190-A: republishRef (called by moveDirectory for a COMMITTED part rename — RENAME TABLE, DETACH,
+/// ATTACH, delete_tmp_ rename) must carry the source part's BLOBS forward by TOKENLESS W-EVIDENCE
+/// (adoptEvidence), NOT by HEAD/GET/PUT on the source blob before precommit. In the rev. 15 manifest
+/// model republishRef legitimately READS the FOREIGN source MANIFEST body (to copy its entries into a
+/// fresh dst manifest) during staging — that is the manifest-era analog of the old adoptFromTree
+/// source-tree read and is NOT a violation (see CommittedSourceAdoptNoHeadBeforePrecommit). The
+/// invariant that survives: the source BLOB key must not be touched before the first precommit write.
+TEST(CASWiringPrecommitOrder, RepublishRefNoTreeHeadBeforePrecommit)
+{
+    auto recording = makeRecordingStorageForTest("republish");
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_b190_republish_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        recording, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+
+    /// Phase 1: commit a source part. Capture its BLOB key from the /blobs/ write (republishRef must
+    /// carry this blob by reference, never touching it before precommit).
+    recording->ops.clear();
+    {
+        auto tx = storage->createTransaction();
+        writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "republish-source");
+        tx->commit(DB::NoCommitOptions{});
+    }
+    std::string source_blob_key;
+    for (const auto & r : recording->ops)
+    {
+        if (r.op == "writeObject" && r.key.find("/blobs/") != std::string::npos)
+        {
+            source_blob_key = r.key;
+            break;
+        }
+    }
+    ASSERT_FALSE(source_blob_key.empty())
+        << "Phase 1 recorded no /blobs/ write — could not capture the source blob key";
+
+    /// Phase 2: a COMMITTED rename (delete_tmp_ pattern) that triggers republishRef. Clear the log
+    /// so only Phase 2's ops are analysed.
+    recording->ops.clear();
+    {
+        auto tx = storage->createTransaction();
+        tx->moveDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0", "a11/a11a11a1-1111-4111-8111-111111111111/delete_tmp_all_1_1_0");
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    const auto & log = recording->ops;
+
+    const int first_precommit_idx = firstPrecommitWriteIdx(log);
+    ASSERT_GE(first_precommit_idx, 0)
+        << "No create-precommit owner write (root-shard CAS) was recorded — precommit step did not fire";
+
+    /// The source BLOB key must NOT be accessed (HEAD via exists/getObjectMetadata, GET via readObject,
+    /// or PUT via writeObject) before the precommit write. With an eager adopt-by-HEAD on the source
+    /// blob (the regression), observeAndAdmit HEADs the blob key at an index < first_precommit_idx,
+    /// failing here. A tokenless adoptEvidence dep touches nothing.
+    bool blob_touched_before_precommit = false;
+    for (int i = 0; i < first_precommit_idx; ++i)
+    {
+        if (log[i].key == source_blob_key)
+        {
+            blob_touched_before_precommit = true;
+            ADD_FAILURE()
+                << "republishRef blob op '" << log[i].op << "' on '" << log[i].key
+                << "' at index " << i << " came BEFORE the first precommit write at index "
+                << first_precommit_idx << " — violates B190 precommit-first: republishRef must not "
+                << "HEAD/GET/PUT the source blob before precommit (use tokenless adoptEvidence)";
+        }
+    }
+    EXPECT_FALSE(blob_touched_before_precommit);
+
+    /// Sanity: the renamed part is visible under the new name and NOT under the old name.
+    EXPECT_FALSE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0"));
+    EXPECT_TRUE(storage->existsDirectory("a11/a11a11a1-1111-4111-8111-111111111111/delete_tmp_all_1_1_0"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/delete_tmp_all_1_1_0/data.bin"));
+}
+
+/// B190-B: the adoptStagedBlob helper unifies the 6 inline pending/uploaded adopt blocks from
+/// createHardLink / moveFile / moveDirectory. The observable invariant: after refactoring, ALL
+/// six sites still produce the same result as before — pending blobs are copied (hardlink) or
+/// moved (moveFile/moveDirectory), and uploaded blobs are adopted by tokenless evidence. This test
+/// exercises the non-trivial CROSS-PART pending path (createHardLink copies; moveFile moves) and
+/// verifies both a copy and a move of the SAME pending source produce the correct committed state.
+TEST(CASWiringPrecommitOrder, AdoptStagedBlobHelperUnifiesSixSites)
+{
+    /// Use a recording storage so we can verify no pre-precommit pool ops on own content.
+    auto recording = makeRecordingStorageForTest("adopt_helper");
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_b190_adopt_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        recording, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+
+    recording->ops.clear();
+
+    /// One transaction: write a pending blob into part A, hardlink (COPY pending) into part B,
+    /// and moveFile (MOVE pending) of a DIFFERENT pending blob from part A into part C.
+    auto tx = storage->createTransaction();
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_A_A_0/data.bin", "blob-for-copy");
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_A_A_0/extra.bin", "blob-for-move");
+
+    /// createHardLink = COPY semantics: both src and dst should see the blob after commit.
+    tx->createHardLink("a11/a11a11a1-1111-4111-8111-111111111111/all_A_A_0/data.bin", "a11/a11a11a1-1111-4111-8111-111111111111/all_B_B_0/data.bin");
+
+    /// moveFile cross-part = MOVE semantics: src loses the blob, dst gains it.
+    {
+        auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+        ca_tx.moveFile("a11/a11a11a1-1111-4111-8111-111111111111/all_A_A_0/extra.bin", "a11/a11a11a1-1111-4111-8111-111111111111/all_C_C_0/extra.bin");
+    }
+
+    tx->commit(DB::NoCommitOptions{});
+
+    const auto & log = recording->ops;
+    const int first_precommit_idx = firstPrecommitWriteIdx(log);
+    ASSERT_GE(first_precommit_idx, 0)
+        << "No create-precommit owner write (root-shard CAS) was recorded — precommit step did not fire";
+
+    /// Collect own content keys (the content BLOBS this transaction wrote). The staged part-manifest
+    /// body (`/_manifests/...`) is the precommit's evidence and is written before precommit by design,
+    /// so it is NOT gated content — only /blobs/ are.
+    std::set<std::string> own_content_keys;
+    for (const auto & r : log)
+        if (r.op == "writeObject" && r.key.find("/blobs/") != std::string::npos)
+            own_content_keys.insert(r.key);
+
+    /// No own-content pool op before precommit (B188 invariant extends to all adopt sites).
+    for (int i = 0; i < static_cast<int>(log.size()); ++i)
+    {
+        if (!own_content_keys.contains(log[i].key))
+            continue;
+        EXPECT_GT(i, first_precommit_idx)
+            << "Own-content op '" << log[i].op << "' on '" << log[i].key << "' at index " << i
+            << " before precommit at " << first_precommit_idx;
+    }
+
+    /// COPY semantics: both A and B see the copied blob.
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_A_A_0/data.bin"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_B_B_0/data.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_A_A_0/data.bin"), 13u);   /// "blob-for-copy" (13 bytes)
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_B_B_0/data.bin"), 13u);
+    EXPECT_EQ(storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_A_A_0/data.bin")[0].remote_path,
+              storage->getStorageObjects("a11/a11a11a1-1111-4111-8111-111111111111/all_B_B_0/data.bin")[0].remote_path)
+        << "COPY (hardlink): both parts must share the same blob object";
+
+    /// MOVE semantics: A loses extra.bin, C gains it.
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_A_A_0/extra.bin"));
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_C_C_0/extra.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_C_C_0/extra.bin"), 13u);   /// "blob-for-move" (13 bytes)
+}
+
+/// ==== B189: orphaned pending blob must NOT be uploaded after unlinkFile / replaceFile ====
+///
+/// When a file is written (pending blob X) and then unlinked (or replaced) within the same
+/// transaction, X's tree entry is removed — so X is NOT referenced by the staged tree. Before the
+/// B189 fix, publishStaging iterated pending_blobs unconditionally and uploaded X anyway (a wasted
+/// PUT of an unreferenced blob). After the fix, publishStaging builds the set of blob hashes
+/// referenced by the staged tree entries and uploads ONLY those — orphaned blobs are skipped.
+///
+/// The test uses RecordingLocalObjectStorage to capture every writeObject call. After commit it
+/// checks that the orphaned blob's pool key received NO writeObject, while a kept blob (written and
+/// NOT removed in the same transaction) IS uploaded.
+TEST(CASWiringOps, OrphanedPendingBlobNotUploadedAfterUnlink)
+{
+    auto recording = makeRecordingStorageForTest("b189_unlink");
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_b189_unlink_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        recording, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+
+    recording->ops.clear();
+
+    auto tx = storage->createTransaction();
+
+    /// Write blob X — this will be unlinked (orphaned) before commit.
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/orphan.bin", "orphan-bytes");
+
+    /// Write blob Y — this is kept (its tree entry survives to the staged tree).
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/kept.bin", "kept-bytes");
+
+    /// Unlink blob X — removes its tree entry; the pending_blobs record remains but is now orphaned.
+    tx->unlinkFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/orphan.bin", false, false);
+
+    /// Sanity: the unlinked file is no longer staged (in-flight should not report it).
+    EXPECT_FALSE(tx->tryGetInFlightFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/orphan.bin").has_value());
+    EXPECT_EQ(tx->tryGetInFlightFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/kept.bin"), std::optional<uint64_t>(10));
+
+    tx->commit(DB::NoCommitOptions{});
+
+    const auto & log = recording->ops;
+
+    /// Collect blob BODY keys written by this transaction (only /blobs/ writeObjects). Exclude the
+    /// per-hash `.meta` freshness descriptor sibling (`blobMetaKey` = body key + `.meta`, spec
+    /// §meta-protocols v3): it lives under the same /blobs/ prefix but is NOT a blob upload, so it must
+    /// not inflate the body-upload count. `putBlob` writes exactly one such `.meta` per body.
+    std::vector<std::string> blob_writes;
+    for (const auto & r : log)
+        if (r.op == "writeObject" && r.key.find("/blobs/") != std::string::npos && !r.key.ends_with(".meta"))
+            blob_writes.push_back(r.key);
+
+    /// Exactly ONE blob must have been uploaded (the kept one). The orphaned blob's pool key must
+    /// NOT appear in any writeObject — B189: orphan is filtered out of the publish upload.
+    EXPECT_EQ(blob_writes.size(), 1u)
+        << "Expected exactly 1 blob upload (the kept blob); got " << blob_writes.size()
+        << ". If 2, the orphaned pending blob was uploaded — B189 regression.";
+
+    /// The kept file is visible after commit; the orphaned file is not.
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/kept.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/kept.bin"), 10u);   /// "kept-bytes"
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/orphan.bin"));
+}
+
+/// B189 companion: the same orphan-filter applies when the tree entry is removed by replaceFile
+/// (the destination entry erased before the move). Write blob X to dst, then replaceFile src->dst
+/// (erases X's entry, moves src's entry to dst). The orphaned X must not be uploaded.
+TEST(CASWiringOps, OrphanedPendingBlobNotUploadedAfterReplace)
+{
+    auto recording = makeRecordingStorageForTest("b189_replace");
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_b189_replace_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        recording, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+
+    recording->ops.clear();
+
+    auto tx = storage->createTransaction();
+
+    /// Write blob X into the destination slot — it will be erased by replaceFile.
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin", "original-bytes");
+
+    /// Write blob Y into the source slot — it will replace the destination.
+    writeThroughTransaction(*tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/new.bin", "replacement-bytes");
+
+    /// replaceFile: erases the dst entry (X orphaned), then moves src->dst.
+    {
+        auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+        ca_tx.replaceFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/new.bin", "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin");
+    }
+
+    tx->commit(DB::NoCommitOptions{});
+
+    const auto & log = recording->ops;
+
+    /// Exactly ONE blob must have been uploaded (the replacement blob Y). Exclude the per-hash `.meta`
+    /// freshness descriptor sibling (see the AfterUnlink test) — it is not a blob body upload.
+    std::vector<std::string> blob_writes;
+    for (const auto & r : log)
+        if (r.op == "writeObject" && r.key.find("/blobs/") != std::string::npos && !r.key.ends_with(".meta"))
+            blob_writes.push_back(r.key);
+
+    EXPECT_EQ(blob_writes.size(), 1u)
+        << "Expected exactly 1 blob upload (the replacement blob); got " << blob_writes.size()
+        << ". If 2, the orphaned original blob was uploaded — B189 regression.";
+
+    /// After commit the destination slot carries the replacement content.
+    EXPECT_TRUE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"));
+    EXPECT_EQ(storage->getFileSize("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin"), 17u);   /// "replacement-bytes"
+    EXPECT_FALSE(storage->existsFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/new.bin"));
+}
+
+/// ==== Promote tokened-leaf edge-protection (spec 2026-07-09-cas-writer-gc-simplification, Phase A) ====
+///
+/// A fast GC can PREMATURELY condemn a blob a writer just putBlob'd, in the tiny putBlob->promote window
+/// (the precommit->blob edge is not yet folded, so GC reads in-degree 0). Under EDGE-BEFORE-OBSERVE the
+/// precommit closure named the blob BEFORE putBlob observed it, so the condemnation cannot graduate to a
+/// delete (the next fold sees the edge, d >= 1, spared) — it is doomed, not the blob. promote therefore
+/// does NOT re-validate or resurrect a TOKENED leaf; it commits with the blob's token UNCHANGED. The only
+/// blob-side abort promote still performs is the owner-liveness check (a reclaimed precommit) — which runs
+/// BEFORE any blob work and touches nothing.
+///
+/// These tests drive the REAL writer sequence (stageManifest -> precommitAdd -> putBlob -> promote) against
+/// a raw in-memory Pool (no background GC → deterministic), and condemn the blob's CURRENT token by seeding
+/// gc/state + the per-hash freshness meta the way a real GC condemn does (see `seedCondemnBlobToken` below).
+
+namespace DB::ErrorCodes
+{
+    extern const int ABORTED;
+    extern const int NETWORK_ERROR;
+}
+
+namespace
+{
+
+DB::Cas::PoolPtr openResurrectStore(std::shared_ptr<DB::Cas::InMemoryBackend> & out_backend)
+{
+    out_backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    return DB::Cas::Pool::open(
+        out_backend, DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// Condemn (kind=Blob, hash, token) by seeding gc/state + a per-shard retired set (the durable GC ledger
+/// shape — RetiredEntry, exact-token delete, unchanged by this task) AND condemning the per-hash freshness
+/// meta, which is what the writer's condemned decision ACTUALLY point-reads (spec §meta-protocols v3).
+/// Bumps the round so the retirement is a fresh one; leaves the object itself in place (condemn, NOT delete).
+void seedCondemnBlobToken(DB::Cas::Pool & store, const DB::UInt128 & hash,
+                          [[maybe_unused]] const DB::Cas::Token & token, [[maybe_unused]] uint64_t size)
+{
+    using namespace DB::Cas;
+    Backend & b = store.backend();
+    const Layout & layout = store.layout();
+
+    GcState state;
+    const HeadResult head = b.head(layout.gcStateKey());
+    if (head.exists)
+    {
+        const auto got = b.get(layout.gcStateKey());
+        state = decodeGcState(got->bytes);
+    }
+    state.round += 1;
+
+    /// Retired-in-snapshot: there is no separate retired-list object to seed — condemned state rides the
+    /// GC snapshot runs, which this writer-side edge-protection test does not exercise. The writer's
+    /// condemned decision point-reads the per-hash freshness meta (condemned below), so bumping the round
+    /// and condemning the meta is enough.
+    if (head.exists)
+        b.putOverwrite(layout.gcStateKey(), encodeGcState(state), head.token);
+    else
+        b.putIfAbsent(layout.gcStateKey(), encodeGcState(state));
+
+    /// The writer's fresh upload (putBlob) already wrote a Clean meta for `hash` (Task 3), so this is a
+    /// plain Clean -> Condemned CAS — exactly what GC's real condemn path does.
+    DB::Cas::tests::condemnMeta(b, layout, hash, state.round);
+}
+
+}
+
+/// A blob condemned in the putBlob->promote window is EDGE-PROTECTED (spec
+/// 2026-07-09-cas-writer-gc-simplification, Phase A): the precommit closure naming the blob was durable
+/// BEFORE putBlob observed it, so a condemnation in this window cannot graduate to a delete (the next fold
+/// sees the edge, d >= 1, spared). promote therefore does NOT re-check or resurrect a TOKENED leaf — it
+/// commits leaving the blob's token UNCHANGED (no resurrect PUT). The premature condemn is doomed on its own.
+TEST(CASWiringResurrect, PromoteIgnoresCondemnedTokenedBlobEdgeProtected)
+{
+    using namespace DB::Cas;
+    std::shared_ptr<InMemoryBackend> backend;
+    auto store = openResurrectStore(backend);
+    const RootNamespace ns{"test/tbl"};
+    const String ref = "all_1_1_0";
+    const String P = "resurrect-me";
+
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = store->beginPartWrite(info);
+
+    const ManifestId id = build->stageManifest({wiringBlobEntry("data.bin", P)});
+    build->precommitAdd(ns, ref, id);
+    build->putBlob(idOf(P), BlobSource::fromString(P));
+
+    /// Condemn the freshly-uploaded blob's CURRENT token (GC condemning the not-yet-folded fresh incarnation).
+    const String blob_key = store->layout().blobKey(idOf(P));
+    const HeadResult h1 = store->backend().head(blob_key);
+    ASSERT_TRUE(h1.exists);
+    const Token t0 = h1.token;
+    seedCondemnBlobToken(*store, u128Of(P), t0, h1.size);
+    {
+        const auto lm = DB::Cas::tests::loadMetaForTest(store->backend(), store->layout(), u128Of(P));
+        ASSERT_TRUE(lm.has_value() && lm->meta.state == MetaState::Condemned)
+            << "precondition: the putBlob'd token must be condemned before promote";
+    }
+
+    /// promote must NOT abort AND must NOT touch the tokened leaf — it is edge-protected.
+    EXPECT_NO_THROW(build->promote(ns, ref, build->buildId(), id));
+
+    /// The ref is committed and the blob's token is UNCHANGED — no resurrect PUT ran (tokened leaves are
+    /// not re-validated: EDGE-BEFORE-OBSERVE guarantees the condemnation is doomed, not the blob).
+    EXPECT_TRUE(store->resolveRef(ns, ref).has_value()) << "the ref must resolve after promote";
+    const HeadResult h2 = store->backend().head(blob_key);
+    ASSERT_TRUE(h2.exists);
+    EXPECT_EQ(h2.token, t0)
+        << "tokened leaf is edge-protected: promote must not re-upload it (token unchanged)";
+}
+
+/// promote is a PURE owner MOVE (Δ=0 blob delta) — sound ONLY while this build's precommit is STILL the
+/// live owner of the ref (`WPromote owner==bld` / INV_NO_DANGLE): a Δ=0 move over a ref with no live
+/// precommit edge would republish a committed manifest onto to-be-deleted blobs. So when the precommit
+/// binding is absent from the ref-table state, promote MUST fail closed with ABORTED — at the owner-liveness
+/// check in the append closure, which runs BEFORE any blob revalidation, so NO consequential PUT / resurrect
+/// happens (a condemned leaf is left untouched, exactly as on the success path).
+///
+/// This drives that guard the DETERMINISTIC way: a promote whose precommit was NEVER added (so the binding
+/// is simply absent). The original "precommit added, then REMOVED out from under a still-live build" shape is
+/// NOT reachable by any deterministic single-threaded in-runtime actor: `PartWriteTxn::abandon` marks the build
+/// not-alive (`requireAlive` → LOGICAL_ERROR) and `Pool::dropNamespace` cancels the build (`requireAlive` →
+/// ABORTED) — BOTH trip `requireAlive` at promote's first line, before this closure ever runs. Only a narrow
+/// promote-vs-dropNamespace RACE (dropNamespace clears the binding in the window between promote's
+/// `requireAlive` and its append closure) reaches the closure guard, which is therefore a defensive backstop
+/// (a candidate for a later dead-code review — out of scope here). The previous version of this test faked
+/// the removal with an out-of-band `appendOwnerEvent` the single-leader runtime never observes — an
+/// unreachable state that surfaced as a CORRUPTED_DATA ref-log collision, not the intended ABORTED.
+TEST(CASWiringResurrect, PromoteWithoutLivePrecommitAbortsWithoutResurrect)
+{
+    using namespace DB::Cas;
+    std::shared_ptr<InMemoryBackend> backend;
+    auto store = openResurrectStore(backend);
+    const RootNamespace ns{"test/tbl"};
+    const String ref = "all_2_2_0";
+    const String P = "abandoned-me";
+
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = store->beginPartWrite(info);
+
+    /// Stage the manifest and upload the (fresh) blob, but DO NOT precommitAdd — so the precommit owner
+    /// binding is absent from the ref-table state when promote runs. A fresh putBlob needs no precommit
+    /// (only the ADOPT path requires the durable edge); it records the tokened leaf t0.
+    const ManifestId id = build->stageManifest({wiringBlobEntry("data.bin", P)});
+    build->putBlob(idOf(P), BlobSource::fromString(P));
+
+    const String blob_key = store->layout().blobKey(idOf(P));
+    const HeadResult h1 = store->backend().head(blob_key);
+    ASSERT_TRUE(h1.exists);
+    /// Condemn the leaf so that, WERE the blob gate reached, promote would resurrect it — proving the abort
+    /// happens strictly BEFORE any blob work.
+    seedCondemnBlobToken(*store, u128Of(P), h1.token, h1.size);
+    {
+        const auto lm = DB::Cas::tests::loadMetaForTest(store->backend(), store->layout(), u128Of(P));
+        ASSERT_TRUE(lm.has_value() && lm->meta.state == MetaState::Condemned);
+    }
+
+    /// promote aborts at the owner-liveness check (NETWORK_ERROR, fix #37 phase 2), before the blob gate.
+    try
+    {
+        build->promote(ns, ref, build->buildId(), id);
+        FAIL() << "expected promote to abort: the precommit is not the live owner of the ref";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR);
+    }
+
+    /// No blob work ran before the abort: the leaf's token is UNCHANGED (still the condemned one) and its
+    /// meta is still Condemned — the owner check aborts before any PUT / resurrect.
+    const HeadResult h2 = store->backend().head(blob_key);
+    ASSERT_TRUE(h2.exists);
+    EXPECT_EQ(h2.token, h1.token)
+        << "the aborting path must perform no PUT — the tokened leaf is untouched";
+    const auto lm_after = DB::Cas::tests::loadMetaForTest(store->backend(), store->layout(), u128Of(P));
+    EXPECT_TRUE(lm_after.has_value() && lm_after->meta.state == MetaState::Condemned)
+        << "no re-upload/resurrect before the owner check — the token is still the condemned one";
+}
+
+/// tryFromDisk must be exception-free for a plain local disk: it runs on every
+/// asynchronous-metrics tick for every configured disk, and probing via
+/// `getMetadataStorage`'s NOT_IMPLEMENTED throw pollutes `system.errors` (the Exception
+/// constructor counts the error even when the throw is caught) — a steady +N/s stream on a
+/// pure-local server, caught as a stray-error failure by strict-error tests
+/// (`test_cancel_backup`'s NoTrashChecker, Altinity PR#2073).
+TEST(CASWiring, TryFromDiskOnLocalDiskIsExceptionFreeAndCountsNoError)
+{
+    auto tmp = std::filesystem::temp_directory_path() / "ca_wiring_tryfromdisk_test";
+    std::filesystem::create_directories(tmp);
+    const DB::DiskPtr local = std::make_shared<DB::DiskLocal>("tryfromdisk_local", tmp.string());
+
+    const auto before = DB::ErrorCodes::values[DB::ErrorCodes::NOT_IMPLEMENTED].get().local.count;
+    auto * ca = DB::ContentAddressedMetadataStorage::tryFromDisk(local);
+    const auto after = DB::ErrorCodes::values[DB::ErrorCodes::NOT_IMPLEMENTED].get().local.count;
+
+    EXPECT_EQ(ca, nullptr);
+    EXPECT_EQ(after, before)
+        << "tryFromDisk on a non-content-addressed disk must not construct (and thereby count) "
+           "a NOT_IMPLEMENTED exception — it runs per disk on every asynchronous-metrics tick";
+    std::filesystem::remove_all(tmp);
+}
diff --git a/src/Disks/tests/gtest_cas_b140_dangle.cpp b/src/Disks/tests/gtest_cas_b140_dangle.cpp
new file mode 100644
index 000000000000..35adecc330e2
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_b140_dangle.cpp
@@ -0,0 +1,127 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <string>
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace
+{
+
+/// The dangle is about the SINGLE snap shard's in-degree, and one cursor_key covers both refs.
+PoolPtr openTestPool(std::shared_ptr<InMemoryBackend> & out_backend)
+{
+    out_backend = std::make_shared<InMemoryBackend>();
+    return Pool::open(out_backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+size_t runGcToFixpoint(Gc & gc, size_t max_rounds = 64)
+{
+    size_t rounds = 0;
+    for (; rounds < max_rounds; ++rounds)
+    {
+        RoundReport rep;
+        try
+        {
+            rep = gc.runRegularRound();
+        }
+        catch (const DB::Exception &)
+        {
+            /// The fail-closed coherence guard refused this round (CORRUPTED_DATA): no delete
+            /// happened, the live blob is safe. Stop — re-running would just throw again.
+            break;
+        }
+        if (!rep.acquired_lease)
+            continue;
+        if (rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0)
+            break;
+    }
+    return rounds;
+}
+
+}
+
+/// B140-DANGLE — the soak's INV-NO-LOSS finding, ported to the root-local part-manifest model.
+///
+/// THE PROPERTY (unchanged across the redesign): a content-shared / deduplicated blob `B` referenced
+/// by TWO live parts must NEVER be deleted when only ONE of those refs is dropped. In the old tree
+/// model the loss arose from a `GcSnap` cursor-skip under-count (the committed `folded_cursor` ran
+/// ahead of the snap's edges, so the second live part's edge was never folded). That white-box
+/// failure mode is structurally IMPOSSIBLE in the manifest model: there is no separate snap; per-blob
+/// in-degree is derived by folding the ONE ordered `RootOwnerEvent` journal, the fold cursor lives in
+/// the `CasFoldSeal` (one durable unit with the sealed deltas, never diverging), and each part's blob
+/// edges come from reading its OWN manifest body at fold time. So this is now a black-box no-loss
+/// oracle: two live refs share `B`, drop one, GC to a fixpoint, assert `B` survives (`dangling == 0`)
+/// because the surviving ref's manifest still contributes its +1 edge — `B`'s in-degree never reaches 0.
+TEST(CASGCDangle, SharedBlobSurvivesDropOfOneOfTwoLiveRefs)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// rb_live -> manifest { data.bin: B }. B is uploaded here.
+    {
+        PartWriteInfo info;
+        info.intended_ref = ns.string() + "/rb_live";
+        auto build = s->beginPartWrite(info);
+        build->putBlob(idOf("B"), BlobSource::fromString("B"));
+        ManifestEntry e;
+        e.path = "data.bin";
+        e.placement = EntryPlacement::Blob;
+        e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("B"))};
+
+        e.blob_size = std::string("B").size();
+        const ManifestId id = build->stageManifest({e});
+        build->precommitAdd(ns, "rb_live", id);
+        build->promote(ns, "rb_live", build->buildId(), id);
+        s->renewWatermarkOnce();
+    }
+
+    /// rb_cur -> a DISTINCT manifest { other.bin: B } that REUSES the same shared blob B (tokenless
+    /// adopt — the soak's cross-node `adopt`). Still live.
+    {
+        PartWriteInfo info;
+        info.intended_ref = ns.string() + "/rb_cur";
+        auto build = s->beginPartWrite(info);
+        ManifestEntry e;
+        e.path = "other.bin";
+        e.placement = EntryPlacement::Blob;
+        e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("B"))};
+
+        e.blob_size = std::string("B").size();
+        build->adoptEvidence(e);   /// tokenless dep (no HEAD) — the cross-node adopt
+        const ManifestId id = build->stageManifest({e});
+        build->precommitAdd(ns, "rb_cur", id);
+        build->promote(ns, "rb_cur", build->buildId(), id);
+        s->renewWatermarkOnce();
+    }
+
+    /// Drop rb_live: its manifest's -1 on B lands, but rb_cur's manifest still contributes +1, so B's
+    /// in-degree stays >= 1 and B is never a zero-in-degree candidate.
+    s->dropRef(ns, "rb_live");
+    s->renewWatermarkOnce();
+
+    Gc gc(s, hexToU128("00000000000000000000000000000001"));
+    const size_t rounds = runGcToFixpoint(gc);
+
+    /// rb_cur is still LIVE and still resolves through a present manifest — its blob B must survive.
+    ASSERT_TRUE(s->resolveRef(ns, "rb_cur").has_value());
+
+    const FsckReport rep = runFsck(*s, /*detail=*/true);
+
+    /// THE DANGLE ASSERTION: GC must NEVER delete a blob a live ref references.
+    EXPECT_EQ(rep.dangling, 0u)
+        << "B140-dangle: GC deleted shared blob B still referenced by the live ref rb_cur "
+        << "after " << rounds << " rounds (dangling=" << rep.dangling << ", reachable=" << rep.reachable
+        << ", B_present=" << b->head(s->layout().blobKey(idOf("B"))).exists << ").";
+    EXPECT_TRUE(b->head(s->layout().blobKey(idOf("B"))).exists)
+        << "shared blob B must remain present while rb_cur references it";
+}
diff --git a/src/Disks/tests/gtest_cas_backend.cpp b/src/Disks/tests/gtest_cas_backend.cpp
new file mode 100644
index 000000000000..2538017dc398
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_backend.cpp
@@ -0,0 +1,1049 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h>
+#include <Common/ProfileEvents.h>
+#include <IO/ReadHelpers.h>
+#include <IO/WriteHelpers.h>
+
+#if USE_AWS_S3
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/StoredObject.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Disks/WriteMode.h>
+#include <IO/ReadBufferFromFileBase.h>
+#include <IO/ReadSettings.h>
+#include <IO/S3Common.h>
+#include <chrono>
+#include <filesystem>
+#include <map>
+#include <mutex>
+#endif
+
+using namespace DB::Cas;
+
+/// Minimal concrete implementation that overrides every pure virtual with trivial defaults.
+/// Purpose: verify the interface compiles, is overridable, and result-type defaults are sane.
+struct NullBackend final : Backend
+{
+    std::optional<GetResult> get(const String & /*key*/, Range /*range*/) override
+    {
+        return std::nullopt;
+    }
+
+    std::optional<GetStreamResult> getStream(const String & /*key*/, Range /*range*/) override
+    {
+        return std::nullopt;
+    }
+
+    HeadResult head(const String & /*key*/) override
+    {
+        return HeadResult{};
+    }
+
+    PutResult putIfAbsent(const String & /*key*/, const String & /*bytes*/, const ObjectMeta & /*meta*/) override
+    {
+        return {PutOutcome::Done, {}};
+    }
+
+    WriteSinkPtr putIfAbsentStream(const String & /*key*/, const ObjectMeta & /*meta*/) override
+    {
+        return nullptr;   /// trivial default — streaming behavior is pinned by the CASBackendContract suite
+    }
+
+    PutResult putOverwrite(const String & /*key*/, const String & /*bytes*/, const Token & /*expected*/, const ObjectMeta & /*meta*/) override
+    {
+        return {PutOutcome::PreconditionFailed, {}};
+    }
+
+    CasResult casPut(const String & /*key*/, const String & /*bytes*/, const std::optional<Token> & /*expected*/, const ObjectMeta & /*meta*/) override
+    {
+        return {CasOutcome::Conflict, {}};
+    }
+
+    DeleteOutcome deleteExact(const String & /*key*/, const Token & /*token*/) override
+    {
+        return DeleteOutcome{};
+    }
+
+    ListPage list(const String & /*prefix*/, const String & /*cursor*/, size_t /*limit*/) override
+    {
+        return ListPage{};
+    }
+
+    bool supportsListTokens() const override { return false; }
+};
+
+TEST(CASBackend, NullBackendShapeAndDefaults)
+{
+    NullBackend b;
+    // Use the base-class reference so virtual dispatch uses base-class default args.
+    Backend & ref = b;
+
+    // get returns absent
+    EXPECT_FALSE(ref.get("k").has_value());
+
+    // head returns non-existent
+    HeadResult h = b.head("k");
+    EXPECT_FALSE(h.exists);
+    EXPECT_EQ(h.size, 0u);
+    EXPECT_TRUE(h.token.empty());
+
+    // putIfAbsent returns Done
+    EXPECT_EQ(ref.putIfAbsent("k", "v").outcome, PutOutcome::Done);
+
+    // putOverwrite returns PreconditionFailed
+    EXPECT_EQ(ref.putOverwrite("k", "v", Token{}).outcome, PutOutcome::PreconditionFailed);
+
+    // casPut returns Conflict
+    EXPECT_EQ(ref.casPut("k", "v", std::nullopt).outcome, CasOutcome::Conflict);
+
+    // deleteExact default kind is NotFound
+    DeleteOutcome d = b.deleteExact("k", Token{});
+    EXPECT_EQ(d.kind, DeleteOutcome::Kind::NotFound);
+    EXPECT_FALSE(d.created_delete_marker);
+
+    // list returns empty page
+    ListPage page = b.list("p/", "", 10);
+    EXPECT_TRUE(page.keys.empty());
+    EXPECT_TRUE(page.next_cursor.empty());
+
+    // Range::whole() helper
+    EXPECT_TRUE(Range{}.whole());
+    Range r1; r1.offset = 1;
+    EXPECT_FALSE(r1.whole());
+    Range r2; r2.length = 5u;
+    EXPECT_FALSE(r2.whole());
+}
+
+// =====================================================================
+// Task 3: CasInMemoryBackend — enforcing token semantics
+// =====================================================================
+
+TEST(CASInMemory, PutIfAbsentAndGet)
+{
+    InMemoryBackend b;
+    const auto put = b.putIfAbsent("k", "v1");
+    const Token t1 = put.token;
+    EXPECT_EQ(put.outcome, PutOutcome::Done);
+    EXPECT_FALSE(t1.empty());
+    EXPECT_EQ(b.putIfAbsent("k", "clobber").outcome, PutOutcome::PreconditionFailed);
+    auto g = b.get("k");
+    ASSERT_TRUE(g.has_value());
+    EXPECT_EQ(g->bytes, "v1");
+    EXPECT_EQ(g->token, t1);
+    EXPECT_FALSE(b.get("absent").has_value());
+}
+
+TEST(CASInMemory, OverwriteIsTokenExactAndMintsFreshToken)
+{
+    InMemoryBackend b;
+    const Token t1 = b.putIfAbsent("k", "v1").token;
+    EXPECT_EQ(b.putOverwrite("k", "v2", Token{"wrong", TokenType::Emulated}).outcome, PutOutcome::PreconditionFailed);
+    EXPECT_EQ(b.get("k")->bytes, "v1");                       // untouched on mismatch
+    const auto overwrite = b.putOverwrite("k", "v2", t1);
+    EXPECT_EQ(overwrite.outcome, PutOutcome::Done);
+    EXPECT_NE(overwrite.token, t1);                           // tokens never repeat
+    EXPECT_EQ(b.get("k")->bytes, "v2");
+}
+
+TEST(CASInMemory, CasPutCreateAndSwap)
+{
+    InMemoryBackend b;
+    const auto create = b.casPut("m", "s1", std::nullopt);
+    const Token t1 = create.token;
+    EXPECT_EQ(create.outcome, CasOutcome::Committed);                             // create-if-absent
+    EXPECT_EQ(b.casPut("m", "s1x", std::nullopt).outcome, CasOutcome::Conflict);  // exists now
+    EXPECT_EQ(b.casPut("m", "s2", Token{"stale", TokenType::Emulated}).outcome, CasOutcome::Conflict);
+    EXPECT_EQ(b.get("m")->bytes, "s1");
+    EXPECT_EQ(b.casPut("m", "s2", t1).outcome, CasOutcome::Committed);
+    EXPECT_EQ(b.get("m")->bytes, "s2");
+}
+
+TEST(CASInMemory, DeleteExactEnforced)
+{
+    InMemoryBackend b;
+    const Token t1 = b.putIfAbsent("k", "v1").token;
+    auto d1 = b.deleteExact("k", Token{"wrong", TokenType::Emulated});
+    EXPECT_EQ(d1.kind, DeleteOutcome::Kind::TokenMismatch);
+    EXPECT_TRUE(b.get("k").has_value());                      // SURVIVES wrong-token delete
+    auto d2 = b.deleteExact("k", t1);
+    EXPECT_EQ(d2.kind, DeleteOutcome::Kind::Deleted);
+    EXPECT_FALSE(d2.created_delete_marker);
+    EXPECT_FALSE(b.get("k").has_value());
+    EXPECT_EQ(b.deleteExact("k", t1).kind, DeleteOutcome::Kind::NotFound);
+}
+
+TEST(CASInMemory, RangeGetAndHeadAndList)
+{
+    InMemoryBackend b;
+    b.putIfAbsent("p/a", "0123456789");
+    b.putIfAbsent("p/b", "xy");
+    b.putIfAbsent("q/c", "z");
+    EXPECT_EQ(b.get("p/a", Range{.offset = 2, .length = 3})->bytes, "234");
+    auto h = b.head("p/a");
+    EXPECT_TRUE(h.exists);
+    EXPECT_EQ(h.size, 10u);
+    auto page = b.list("p/", "", 10);
+    ASSERT_EQ(page.keys.size(), 2u);                          // sorted, prefix-scoped
+    EXPECT_EQ(page.keys[0].key, "p/a");
+    EXPECT_EQ(page.keys[1].key, "p/b");
+    EXPECT_TRUE(page.next_cursor.empty());
+    auto page1 = b.list("p/", "", 1);                         // pagination
+    EXPECT_EQ(page1.keys.size(), 1u);
+    EXPECT_EQ(page1.keys[0].key, "p/a");
+    EXPECT_EQ(page1.next_cursor, "p/a");
+    EXPECT_FALSE(page1.next_cursor.empty());
+    auto page2 = b.list("p/", page1.next_cursor, 1);
+    EXPECT_EQ(page2.keys[0].key, "p/b");
+}
+
+// =====================================================================
+// Task 4: CasInMemoryBackend — fault injection and probe-test modes
+// =====================================================================
+
+TEST(CASInMemoryFaults, HeldDeleteLandsLater)
+{
+    InMemoryBackend b;
+    const Token t1 = b.putIfAbsent("k", "v1").token;
+    b.setHoldDeletes(true);
+    auto d = b.deleteExact("k", t1);                  // message "sent", not landed
+    EXPECT_EQ(d.kind, DeleteOutcome::Kind::Deleted);  // caller sees the send accepted
+    EXPECT_TRUE(b.get("k").has_value());              // ... but nothing landed yet
+    ASSERT_EQ(b.pendingDeletes(), 1u);
+    // the object is resurrected before the zombie lands:
+    b.putOverwrite("k", "v1'", t1);
+    auto landed = b.landPendingDelete(0);             // the zombie lands NOW
+    EXPECT_EQ(landed.kind, DeleteOutcome::Kind::TokenMismatch);   // 412 — INV-NO-RETURN in miniature
+    EXPECT_EQ(b.get("k")->bytes, "v1'");
+}
+
+TEST(CASInMemoryFaults, InjectedCasConflictFiresOnce)
+{
+    InMemoryBackend b;
+    const Token t1 = b.casPut("m", "s1", std::nullopt).token;
+    b.failNextCasPut("m");
+    EXPECT_EQ(b.casPut("m", "s2", t1).outcome, CasOutcome::Conflict);     // injected
+    EXPECT_EQ(b.get("m")->bytes, "s1");
+    EXPECT_EQ(b.casPut("m", "s2", t1).outcome, CasOutcome::Committed);    // next attempt is real
+}
+
+TEST(CASInMemoryFaults, NonEnforcingModeMimicsBadBackend)
+{
+    InMemoryBackend b;
+    b.setEnforceTokens(false);                        // MinIO-OSS-shaped backend
+    b.putIfAbsent("k", "v1");
+    auto d = b.deleteExact("k", Token{"totally-wrong", TokenType::Emulated});
+    EXPECT_EQ(d.kind, DeleteOutcome::Kind::Deleted);  // silently deletes anyway — the dangerous behavior
+    EXPECT_FALSE(b.get("k").has_value());
+}
+
+TEST(CASInMemoryFaults, VersioningMarkerMode)
+{
+    InMemoryBackend b;
+    b.setSimulateDeleteMarkers(true);
+    const Token t1 = b.putIfAbsent("k", "v1").token;
+    EXPECT_TRUE(b.deleteExact("k", t1).created_delete_marker);    // probe must reject this pool
+}
+
+TEST(CASInMemoryBackend, RoundTripsUserMetadata)
+{
+    DB::Cas::InMemoryBackend backend;
+    const DB::Cas::ObjectMeta meta{{"cas_owner", "ab:7:42"}};
+    ASSERT_EQ(backend.putIfAbsent("k/key", "body", meta).outcome, DB::Cas::PutOutcome::Done);
+
+    const auto hr = backend.head("k/key");
+    ASSERT_TRUE(hr.exists);
+    ASSERT_EQ(hr.attributes.at("cas_owner"), "ab:7:42");
+
+    const auto gr = backend.get("k/key");
+    ASSERT_TRUE(gr.has_value());
+    ASSERT_EQ(gr->attributes.at("cas_owner"), "ab:7:42");
+}
+
+// =====================================================================
+// getStream seam (forward-only reads of write-once objects)
+// =====================================================================
+
+TEST(CASBackendStream, StreamsBodyWindow)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    backend->putIfAbsent("k", "0123456789");
+    auto got = backend->getStream("k", DB::Cas::Range{.offset = 2, .length = 5});
+    ASSERT_TRUE(got.has_value());
+    String out;
+    DB::readStringUntilEOF(out, *got->stream);
+    EXPECT_EQ(out, "23456");
+    EXPECT_FALSE(got->token.empty());
+    EXPECT_FALSE(backend->getStream("absent").has_value());
+}
+
+// =====================================================================
+// B168 P0: InstrumentedBackend per-namespace/op ProfileEvents
+// =====================================================================
+
+namespace ProfileEvents
+{
+extern const Event CASBlobPut;
+extern const Event CASBlobPutDeduplicated;
+extern const Event CASBlobHead;
+extern const Event CASBlobHeadMiss;
+extern const Event CASGCCompareSwap;
+}
+
+TEST(CASInstrumentedBackend, ClassifierAndPerNamespaceOpEvents)
+{
+    /// Namespace classification by substring.
+    EXPECT_EQ(classifyCasNs("pool/blobs/ab/abcdef"), CasNs::Blob);
+    EXPECT_EQ(classifyCasNs("pool/gc/registry"), CasNs::Gc);   /// gc/ prefix covers GC state (state, retired sets, etc.)
+    EXPECT_EQ(classifyCasNs("pool/roots/default/_files/x"), CasNs::Root);
+    EXPECT_EQ(classifyCasNs("pool/gc/state"), CasNs::Gc);
+    /// D3: the old per-server-control key shapes (`_watermark`, `_precommits/<n>`) have no producer
+    /// anymore -- control state now lives under `/gc/server-roots/...` (classifies as Gc). A key of
+    /// this legacy shape, if it ever showed up, would fall through to the generic /roots/ rule.
+    EXPECT_EQ(classifyCasNs("pool/roots/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/_watermark"), CasNs::Root);
+    EXPECT_EQ(classifyCasNs("pool/roots/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/_precommits/3"), CasNs::Root);
+    EXPECT_EQ(classifyCasNs("pool/_pool_meta"), CasNs::Other);
+    /// Final opaque-life layout: both immutable streams and point/path-addressed state remain Root
+    /// instrumentation, while part manifests remain Manifest. None may fall into Other (the
+    /// 2026-07-03 operator-stand CREATE storm misread as CASOtherHeadMiss=102 because of this).
+    EXPECT_EQ(classifyCasNs("pool/cas/ns/stream/00000000000000000000000000000017/_log/1-1.zst"), CasNs::Root);
+    EXPECT_EQ(classifyCasNs("pool/cas/ns/state/00000000000000000000000000000017/_ckpt.zst"), CasNs::Root);
+    EXPECT_EQ(classifyCasNs("pool/cas/ns/state/00000000000000000000000000000017/_files/format_version.txt"), CasNs::Root);
+    EXPECT_EQ(classifyCasNs("pool/cas/manifests/0/srv/store/d18/uuid@cas@/24/1/000001.proto"), CasNs::Manifest);
+
+    auto inner = std::make_shared<InMemoryBackend>();
+    InstrumentedBackend b(inner);
+
+    using ProfileEvents::global_counters;
+    const auto blob_put_before   = global_counters[ProfileEvents::CASBlobPut].load();
+    const auto blob_dedup_before = global_counters[ProfileEvents::CASBlobPutDeduplicated].load();
+    const auto blob_head_before  = global_counters[ProfileEvents::CASBlobHead].load();
+    const auto blob_miss_before  = global_counters[ProfileEvents::CASBlobHeadMiss].load();
+    const auto gc_cas_before     = global_counters[ProfileEvents::CASGCCompareSwap].load();
+
+    const String blob_key = "pool/blobs/ab/abcdef0123456789";
+
+    /// First put of a blob ⇒ Put.
+    EXPECT_EQ(b.putIfAbsent(blob_key, "payload").outcome, PutOutcome::Done);
+    /// Second put of the same key ⇒ PutDeduplicated (content already exists).
+    EXPECT_EQ(b.putIfAbsent(blob_key, "payload").outcome, PutOutcome::PreconditionFailed);
+    /// head of an absent blob key ⇒ HeadMiss (the 404 signal).
+    EXPECT_FALSE(b.head("pool/blobs/zz/absent").exists);
+    /// head of the present blob key ⇒ Head.
+    EXPECT_TRUE(b.head(blob_key).exists);
+    /// casPut create on a gc key ⇒ Gc Cas.
+    EXPECT_EQ(b.casPut("pool/gc/state", "g1", std::nullopt).outcome, CasOutcome::Committed);
+    /// Streaming put to a fresh blob key, then finalize ⇒ Put.
+    {
+        auto sink = b.putIfAbsentStream("pool/blobs/cd/cafebabe");
+        ASSERT_TRUE(sink != nullptr);
+        DB::writeString(String("streamed"), sink->buffer());
+        EXPECT_EQ(sink->finalize().outcome, PutOutcome::Done);
+    }
+
+    /// Under coverage builds ProfileEvents propagate into a thread-local subtree that does not reach
+    /// `global_counters`; deltas read 0 there only (see gtest_unique_key_index_cache).
+#if !WITH_COVERAGE
+    EXPECT_EQ(global_counters[ProfileEvents::CASBlobPut].load()      - blob_put_before,   2u);
+    EXPECT_EQ(global_counters[ProfileEvents::CASBlobPutDeduplicated].load() - blob_dedup_before, 1u);
+    EXPECT_EQ(global_counters[ProfileEvents::CASBlobHead].load()     - blob_head_before,  1u);
+    EXPECT_EQ(global_counters[ProfileEvents::CASBlobHeadMiss].load() - blob_miss_before,  1u);
+    EXPECT_EQ(global_counters[ProfileEvents::CASGCCompareSwap].load()        - gc_cas_before,     1u);
+#else
+    (void)blob_put_before; (void)blob_dedup_before; (void)blob_head_before;
+    (void)blob_miss_before; (void)gc_cas_before;
+#endif
+}
+
+// =====================================================================
+// M-C2 Task 2: typed S3 precondition signal
+// =====================================================================
+
+#if USE_AWS_S3
+
+/// The Native conditional-PUT path discriminates a lost precondition by the canonical S3 error code
+/// string ("PreconditionFailed", "NoSuchKey", ...) that `S3Exception` carries from the response XML
+/// `<Code>` — a 412 is UNMODELED for the AWS SDK (the enum value is UNKNOWN), so the name is the only
+/// machine-readable signal.
+TEST(CASS3Signal, S3ExceptionCarriesCanonicalErrorName)
+{
+    DB::S3Exception e("412 from backend", Aws::S3::S3Errors::UNKNOWN, "PreconditionFailed");
+    EXPECT_EQ(e.getExceptionName(), "PreconditionFailed");
+    DB::S3Exception bare("no name attached", Aws::S3::S3Errors::UNKNOWN);
+    EXPECT_TRUE(bare.getExceptionName().empty());
+}
+
+namespace
+{
+
+/// WriteBuffer stub whose finalize throws a configured S3Exception — drives the classifier directly.
+class ThrowOnFinalizeBuffer final : public DB::WriteBuffer
+{
+public:
+    ThrowOnFinalizeBuffer() : DB::WriteBuffer(nullptr, 0) {}
+
+    explicit ThrowOnFinalizeBuffer(DB::S3Exception e) : DB::WriteBuffer(nullptr, 0), to_throw(std::move(e)) {}
+
+private:
+    void nextImpl() override {}
+
+    void finalizeImpl() override
+    {
+        if (to_throw)
+            throw *to_throw; /// NOLINT(cert-err09-cpp,cert-err60-cpp,cert-err61-cpp,misc-throw-by-value-catch-by-reference) -- the mock stores the configured exception to throw later, so it cannot be an anonymous temporary
+    }
+
+    std::optional<DB::S3Exception> to_throw;
+};
+
+}
+
+/// detail::finalizeConditionalWrite maps a lost precondition to an OUTCOME by exact-matching the
+/// canonical S3 error name (plus the modeled NO_SUCH_KEY enum, which WriteBufferFromS3 surfaces
+/// nameless on retry exhaustion) and rethrows anything else.
+TEST(CASS3Signal, FinalizeClassifierMapsPreconditionLossExactly)
+{
+    using DB::Cas::detail::finalizeConditionalWrite;
+
+    auto classify = [](DB::S3Exception e)
+    {
+        ThrowOnFinalizeBuffer buf(std::move(e));
+        return finalizeConditionalWrite(buf);
+    };
+
+    EXPECT_EQ(classify(DB::S3Exception("412", Aws::S3::S3Errors::UNKNOWN, "PreconditionFailed")),
+              PutOutcome::PreconditionFailed);
+    EXPECT_EQ(classify(DB::S3Exception("404 gone under If-Match", Aws::S3::S3Errors::UNKNOWN, "NoSuchKey")),
+              PutOutcome::PreconditionFailed);
+    EXPECT_EQ(classify(DB::S3Exception("retries exhausted, no name attached", Aws::S3::S3Errors::NO_SUCH_KEY)),
+              PutOutcome::PreconditionFailed);
+
+    ThrowOnFinalizeBuffer unrelated(DB::S3Exception("503", Aws::S3::S3Errors::UNKNOWN, "SlowDown"));
+    EXPECT_THROW(finalizeConditionalWrite(unrelated), DB::S3Exception);
+
+    ThrowOnFinalizeBuffer clean;
+    EXPECT_EQ(finalizeConditionalWrite(clean), PutOutcome::Done);
+}
+
+namespace
+{
+
+/// A `LocalObjectStorage` that round-trips user metadata in-process. The production
+/// `LocalObjectStorage` deliberately drops the `attributes` argument of `writeObject` and never
+/// populates `ObjectMetadata::attributes` (local files carry no `x-amz-meta-*`), so it cannot stand
+/// in for S3/RustFS when verifying the metadata threading. This test-only subclass records the
+/// attributes passed on write, keyed by physical path, and injects them back on metadata reads —
+/// exactly what a real object store does for `x-amz-meta-*`. It exercises the `EmulatedSingleProcess`
+/// `ObjectStorageBackend` threading (`putIfAbsent` → `writeObject` attributes → `head` attributes)
+/// without a live S3 backend; the real S3/RustFS round trip is verified empirically out-of-band.
+class AttributePreservingLocalObjectStorage final : public DB::LocalObjectStorage
+{
+public:
+    using DB::LocalObjectStorage::LocalObjectStorage;
+
+    std::unique_ptr<DB::WriteBufferFromFileBase> writeObject(
+        const DB::StoredObject & object,
+        DB::WriteMode mode,
+        std::optional<DB::ObjectAttributes> attributes,
+        size_t buf_size,
+        const DB::WriteSettings & write_settings) override
+    {
+        if (attributes.has_value())
+        {
+            std::lock_guard lock(mutex);
+            saved_attributes[object.remote_path] = *attributes;
+        }
+        return DB::LocalObjectStorage::writeObject(object, mode, attributes, buf_size, write_settings);
+    }
+
+    std::optional<DB::ObjectMetadata> tryGetObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        auto metadata = DB::LocalObjectStorage::tryGetObjectMetadata(path, with_tags);
+        if (metadata)
+            inject(path, *metadata);
+        return metadata;
+    }
+
+    DB::ObjectMetadata getObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        auto metadata = DB::LocalObjectStorage::getObjectMetadata(path, with_tags);
+        inject(path, metadata);
+        return metadata;
+    }
+
+private:
+    void inject(const std::string & path, DB::ObjectMetadata & metadata) const
+    {
+        std::lock_guard lock(mutex);
+        if (auto it = saved_attributes.find(path); it != saved_attributes.end())
+            metadata.attributes = it->second;
+    }
+
+    mutable std::mutex mutex;
+    mutable std::map<std::string, DB::ObjectAttributes> saved_attributes;
+};
+
+DB::ObjectStoragePtr makeAttributePreservingStorageForTest()
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_meta_unit_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    return std::make_shared<AttributePreservingLocalObjectStorage>(std::move(settings));
+}
+
+}
+
+/// The `EmulatedSingleProcess` `ObjectStorageBackend` must thread user metadata through to the
+/// underlying object storage's `writeObject` attributes on `putIfAbsent` and read it back into
+/// `HeadResult::attributes` on `head`. Verified here over an attribute-preserving object storage
+/// (the production `LocalObjectStorage` drops attributes); the live S3/RustFS round trip is verified
+/// empirically out-of-band.
+TEST(CASObjectStorageBackend, EmulatedRoundTripsUserMetadata)
+{
+    ObjectStorageBackend backend(makeAttributePreservingStorageForTest(), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+    const DB::Cas::ObjectMeta meta{{"cas_owner", "ab:7:42"}};
+    ASSERT_EQ(backend.putIfAbsent("k/key", "body", meta).outcome, DB::Cas::PutOutcome::Done);
+
+    const auto hr = backend.head("k/key");
+    ASSERT_TRUE(hr.exists);
+    ASSERT_EQ(hr.attributes.at("cas_owner"), "ab:7:42");
+}
+
+namespace
+{
+
+/// A `LocalObjectStorage` whose `readObject` throws `S3Exception(NO_SUCH_KEY)` for a configured
+/// physical key, while `tryGetObjectMetadata` still reports that key as PRESENT.
+/// This simulates the HEAD→GET race window: the HEAD succeeds, then the object is deleted before
+/// the GET arrives.
+class NativeReadThrowsNoSuchKeyObjectStorage final : public DB::LocalObjectStorage
+{
+public:
+    using DB::LocalObjectStorage::LocalObjectStorage;
+
+    void setThrowOnRead(const std::string & path)
+    {
+        throw_on_read_path = path;
+    }
+
+    std::unique_ptr<DB::ReadBufferFromFileBase> readObject(
+        const DB::StoredObject & object,
+        const DB::ReadSettings & read_settings,
+        std::optional<size_t> read_hint,
+        bool use_external_buffer,
+        bool restrict_seek) const override
+    {
+        if (object.remote_path == throw_on_read_path)
+            throw DB::S3Exception(
+                "NoSuchKey: The specified key does not exist.",
+                Aws::S3::S3Errors::NO_SUCH_KEY);
+
+        return DB::LocalObjectStorage::readObject(object, read_settings, read_hint, use_external_buffer, restrict_seek);
+    }
+
+private:
+    std::string throw_on_read_path;
+};
+
+struct ThrowOnReadFixture
+{
+    DB::ObjectStoragePtr storage;
+    /// Anchored under `storage`'s own root, because `Mode::Native` hands the key to the object storage
+    /// verbatim and this one is a real filesystem.
+    std::string key;
+};
+
+ThrowOnReadFixture makeThrowOnReadStorageForTest(const std::string & key_suffix)
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_midget_unit_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    auto storage = std::make_shared<NativeReadThrowsNoSuchKeyObjectStorage>(std::move(settings));
+    const std::string key = DB::Cas::tests::nativeKeyUnder(storage, key_suffix);
+
+    /// Write the object so tryGetObjectMetadata reports it present (HEAD succeeds).
+    {
+        auto buf = storage->writeObject(DB::StoredObject(key), DB::WriteMode::Rewrite, std::nullopt);
+        buf->write("content", 7);
+        buf->finalize();
+    }
+
+    /// Now configure: future readObject calls for this key will throw NO_SUCH_KEY.
+    storage->setThrowOnRead(key);
+    return {std::move(storage), key};
+}
+
+}
+
+/// `ObjectStorageBackend::get` in `Native` mode: when `tryGetObjectMetadata` (`nativeHead`) reports the
+/// key PRESENT but `readObject` throws `S3Exception(NO_SUCH_KEY)` — simulating a deletion in the
+/// HEAD→GET window — `get` MUST return `std::nullopt` rather than letting the raw exception escape.
+TEST(CASObjectStorageBackend, NativeModeGetReturnsNulloptOnMidGetNoSuchKey)
+{
+    /// The Native mode backend uses the key verbatim as the physical path (no emu_root prefix), so the
+    /// logical key IS the physical one the fixture wrote and armed.
+    const auto fixture = makeThrowOnReadStorageForTest("pool/blobs/ab/abcdef0123456789abcdef0123456789");
+
+    ObjectStorageBackend backend(fixture.storage, ObjectStorageBackend::Mode::Native);
+
+    /// `get` HEADs before it reads and answers nullopt for an absent key, so without this the nullopt
+    /// below would be satisfied by an object the fixture failed to place — the mid-GET race would go
+    /// untested and the case would still pass.
+    Backend & iface = backend;
+    ASSERT_TRUE(iface.head(fixture.key).exists);
+
+    /// HEAD reports the key present; readObject then throws NO_SUCH_KEY.
+    /// Contract: get must return std::nullopt, not propagate the S3Exception.
+    /// Call through the base-class interface so the default `Range{}` arg is available.
+    const auto result = iface.get(fixture.key);
+    EXPECT_FALSE(result.has_value());
+}
+
+/// A ranged `get` over a real `LocalObjectStorage` returns exactly the requested window, with the
+/// same clamping the old read-whole-then-substr path had: a window whose offset is at or past EOF
+/// yields an empty result. The only-the-window I/O property (no whole-object read) is enforced by
+/// the `readObjectRanged` rewrite and cross-checked by the request-size gate in a later task.
+TEST(CASObjectStorageBackend, RangedGetReadsOnlyTheWindow)
+{
+    auto backend = std::make_shared<ObjectStorageBackend>(
+        tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+    const String payload = String(300000, 'a') + String(300000, 'b') + String(300000, 'c');
+    backend->putIfAbsent("p/obj", payload);
+
+    const auto mid = backend->get("p/obj", DB::Cas::Range{.offset = 300000, .length = 300000});
+    ASSERT_TRUE(mid.has_value());
+    EXPECT_EQ(mid->bytes, String(300000, 'b'));
+
+    const auto tail = backend->get("p/obj", DB::Cas::Range{.offset = 600000, .length = std::nullopt});
+    ASSERT_TRUE(tail.has_value());
+    EXPECT_EQ(tail->bytes, String(300000, 'c'));
+
+    const auto past = backend->get("p/obj", DB::Cas::Range{.offset = 1000000, .length = 10});
+    ASSERT_TRUE(past.has_value());
+    EXPECT_TRUE(past->bytes.empty());
+}
+
+/// codex-review-triage §3.18, finding 19c: the `EmulatedSingleProcess` adapter used to mint tokens
+/// from a plain in-process counter (`emu_seq`), NOT actually seeded from the underlying object's etag
+/// despite the class comment's claim. After a process restart (modeled here as a fresh
+/// `ObjectStorageBackend` instance over the SAME storage) the counter restarts at 0 and can re-mint a
+/// value that TEXTUALLY collides with a token persisted before the restart (e.g. a GC condemned-delete
+/// token queued for replay), even though the two values name completely different incarnations of the
+/// key. `deleteExact` must never let a stale, pre-restart token match a freshly recreated object.
+TEST(CASObjectStorageBackend, EmuTokenSurvivesProcessRestartAcrossRecreate)
+{
+    auto storage = tests::makeLocalObjectStorageForTest();
+
+    auto backend1 = std::make_shared<ObjectStorageBackend>(storage, ObjectStorageBackend::Mode::EmulatedSingleProcess);
+    /// A throwaway prior mutation on a DIFFERENT key: with the old counter this advances backend1's
+    /// process-wide op counter to 1, so "k/restart"'s own mint below lands on 2 — chosen so it collides
+    /// with backend2's post-restart recreate mint further down (also its SECOND op; see there).
+    ASSERT_EQ(backend1->putIfAbsent("k/other", "junk").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend1->putIfAbsent("k/restart", "v1").outcome, PutOutcome::Done);
+    const Token stale_token = backend1->head("k/restart").token;
+
+    /// Simulate a process restart: a brand-new `ObjectStorageBackend` instance (fresh emu state) over
+    /// the SAME underlying storage — exactly what happens when the CAS process restarts.
+    auto backend2 = std::make_shared<ObjectStorageBackend>(storage, ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+    /// Delete and recreate the key through the NEW instance — a fresh incarnation with a fresh mtime.
+    /// This is backend2's first-ever op (op 1) then a delete (no mint) then the recreate (op 2) — the
+    /// same op-index as `stale_token` above under the old counter, so the two textually collide there.
+    const Token current = backend2->head("k/restart").token;
+    ASSERT_EQ(backend2->deleteExact("k/restart", current).kind, DeleteOutcome::Kind::Deleted);
+    ASSERT_EQ(backend2->putIfAbsent("k/restart", "v2-after-restart").outcome, PutOutcome::Done);
+
+    /// The pre-restart token must NEVER match the post-restart incarnation, however coincidentally a
+    /// process-local counter would have re-minted the identical textual value.
+    const auto stale_delete = backend2->deleteExact("k/restart", stale_token);
+    EXPECT_EQ(stale_delete.kind, DeleteOutcome::Kind::TokenMismatch);
+
+    /// The live (post-restart) incarnation must be untouched by the rejected stale delete.
+    EXPECT_TRUE(backend2->head("k/restart").exists);
+}
+
+/// codex-review-triage §3.18, finding №18: `list`'s `EmulatedSingleProcess` branch minted its per-key
+/// token via `tokenForList`, which always stamps `native_token_type` (ETag) REGARDLESS of `mode` --
+/// while `head`/`get` mint `TokenType::Emulated`. `Token::operator==` compares type AND value, so a
+/// list-derived token could never satisfy an emulated `deleteExact`/`putOverwrite` expectation: a
+/// fail-safe leak (never a wrong delete), but every consumer of listed tokens (GC namespace cleanup,
+/// `deletePrefixWholesale`, orphan sweep, decommission drain) always saw `TokenMismatch` against a
+/// LOCAL pool. `list` must surface the SAME (type, value) as `head` for the same key.
+TEST(CASObjectStorageBackend, EmulatedListTokenMatchesHeadToken)
+{
+    auto backend = std::make_shared<ObjectStorageBackend>(
+        tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+    ASSERT_EQ(backend->putIfAbsent("k/listed", "body").outcome, PutOutcome::Done);
+
+    const Token head_token = backend->head("k/listed").token;
+    ASSERT_EQ(head_token.type, TokenType::Emulated);
+
+    const ListPage page = backend->list("k/", "", /*limit=*/10);
+    ASSERT_EQ(page.keys.size(), 1u);
+    ASSERT_TRUE(page.keys.front().token.has_value());
+    EXPECT_EQ(*page.keys.front().token, head_token);
+}
+
+namespace
+{
+
+/// A `LocalObjectStorage` whose reported etag never changes -- simulating a filesystem/clock whose
+/// mtime resolution is too coarse to separate two writes issued back-to-back (the "same mtime
+/// quantum" hazard flagged for the etag-seeded emu token: two DIFFERENT incarnations must still mint
+/// DIFFERENT tokens even when the storage's own etag does not advance between them).
+class FixedEtagLocalObjectStorage final : public DB::LocalObjectStorage
+{
+public:
+    using DB::LocalObjectStorage::LocalObjectStorage;
+
+    std::optional<DB::ObjectMetadata> tryGetObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        auto metadata = DB::LocalObjectStorage::tryGetObjectMetadata(path, with_tags);
+        if (metadata)
+            metadata->etag = "same-quantum";
+        return metadata;
+    }
+};
+
+DB::ObjectStoragePtr makeFixedEtagStorageForTest()
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_fixed_etag_unit_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    return std::make_shared<FixedEtagLocalObjectStorage>(std::move(settings));
+}
+
+}
+
+/// The mtime-resolution guard (codex-review-triage §3.18, 19c step 4): two writes to the same key
+/// whose underlying etag does not advance between them (stubbed here to model a coarse clock) must
+/// still mint DISTINCT emulated tokens, and a stale token from the first incarnation must not match
+/// the second.
+TEST(CASObjectStorageBackend, EmuTokenDisambiguatesSameEtagRewrite)
+{
+    ObjectStorageBackend backend(makeFixedEtagStorageForTest(), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+    const auto put1 = backend.putIfAbsent("k/tick", "v1");
+    ASSERT_EQ(put1.outcome, PutOutcome::Done);
+    const auto put2 = backend.putOverwrite("k/tick", "v2", put1.token);
+    ASSERT_EQ(put2.outcome, PutOutcome::Done);
+
+    EXPECT_NE(put1.token.value, put2.token.value);
+    EXPECT_EQ(put1.token.type, TokenType::Emulated);
+    EXPECT_EQ(put2.token.type, TokenType::Emulated);
+
+    /// A stale delete using the FIRST incarnation's token must not match the live (second) one.
+    EXPECT_EQ(backend.deleteExact("k/tick", put1.token).kind, DeleteOutcome::Kind::TokenMismatch);
+    EXPECT_TRUE(backend.head("k/tick").exists);
+}
+
+namespace
+{
+
+/// A `LocalObjectStorage` that always reports a caller-supplied, fixed NUMERIC etag string — lets a
+/// test pin `emuMintToken`'s etag input to a precise, controlled nanosecond value (an old timestamp
+/// vs. one close to "now") regardless of the real filesystem clock. Used to test the
+/// `emu_token_state` erase-on-delete bound (codex-review-triage §3.18, Important #1): the entry
+/// must be erased only when the deleted incarnation's own etag is comfortably in the past.
+class FixedNumericEtagLocalObjectStorage final : public DB::LocalObjectStorage
+{
+public:
+    FixedNumericEtagLocalObjectStorage(DB::LocalObjectStorageSettings settings, String etag_)
+        : DB::LocalObjectStorage(std::move(settings)), etag(std::move(etag_))
+    {
+    }
+
+    std::optional<DB::ObjectMetadata> tryGetObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        auto metadata = DB::LocalObjectStorage::tryGetObjectMetadata(path, with_tags);
+        if (metadata)
+            metadata->etag = etag;
+        return metadata;
+    }
+
+private:
+    String etag;
+};
+
+DB::ObjectStoragePtr makeFixedNumericEtagStorageForTest(const String & etag)
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_fixed_numeric_etag_unit_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    return std::make_shared<FixedNumericEtagLocalObjectStorage>(std::move(settings), etag);
+}
+
+class ClockEtagLocalObjectStorage final : public DB::LocalObjectStorage
+{
+public:
+    ClockEtagLocalObjectStorage(DB::LocalObjectStorageSettings settings, std::shared_ptr<std::atomic<uint64_t>> now_ns_)
+        : DB::LocalObjectStorage(std::move(settings)), now_ns(std::move(now_ns_))
+    {
+    }
+
+    std::optional<DB::ObjectMetadata> tryGetObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        auto metadata = DB::LocalObjectStorage::tryGetObjectMetadata(path, with_tags);
+        if (metadata)
+            metadata->etag = std::to_string(now_ns->load());
+        return metadata;
+    }
+
+private:
+    std::shared_ptr<std::atomic<uint64_t>> now_ns;
+};
+
+DB::ObjectStoragePtr makeClockEtagStorageForTest(const std::shared_ptr<std::atomic<uint64_t>> & now_ns)
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_clock_etag_unit_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    return std::make_shared<ClockEtagLocalObjectStorage>(std::move(settings), now_ns);
+}
+
+}
+
+/// codex-review-triage §3.18, Important #1: `emu_token_state` must be BOUNDED, not grow for the
+/// lifetime of the backend instance. `deleteExact` erases a key's entry only when its last-minted
+/// etag is comfortably (>= 2s) in the past — recent enough to still collide with an immediate
+/// same-process recreate must be RETAINED (the mtime-quantum guard stays intact).
+TEST(CASObjectStorageBackend, DeleteExactErasesEmuTokenStateOnlyWhenEtagIsComfortablyOld)
+{
+    /// An etag far in the past (nanoseconds since epoch, ~2001): delete must erase the entry, so an
+    /// immediate recreate reporting the SAME fixed etag is treated as a brand-new incarnation (bare
+    /// etag, no disambiguator) rather than a same-quantum tie with the just-consumed delete token.
+    {
+        const String old_etag = "1000000000000000000";
+        ObjectStorageBackend backend(makeFixedNumericEtagStorageForTest(old_etag), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+        const auto put1 = backend.putIfAbsent("k/old", "v1");
+        ASSERT_EQ(put1.outcome, PutOutcome::Done);
+        ASSERT_EQ(put1.token.value, old_etag);
+        ASSERT_EQ(backend.deleteExact("k/old", put1.token).kind, DeleteOutcome::Kind::Deleted);
+
+        const auto put2 = backend.putIfAbsent("k/old", "v2");
+        ASSERT_EQ(put2.outcome, PutOutcome::Done);
+        EXPECT_EQ(put2.token.value, old_etag) << "entry should have been erased on delete (etag comfortably old), "
+                                                  "so the recreate mints the bare etag, not a disambiguated one";
+    }
+
+    /// An etag within the safety margin of "now": delete must RETAIN the entry, so the same
+    /// immediate-recreate scenario still gets disambiguated -- the guard this bound must not break.
+    {
+        const auto now_ns = std::chrono::duration_cast<std::chrono::nanoseconds>(
+            std::chrono::system_clock::now().time_since_epoch()).count();
+        const String recent_etag = std::to_string(now_ns);
+        ObjectStorageBackend backend(makeFixedNumericEtagStorageForTest(recent_etag), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+        const auto put1 = backend.putIfAbsent("k/fresh", "v1");
+        ASSERT_EQ(put1.outcome, PutOutcome::Done);
+        ASSERT_EQ(put1.token.value, recent_etag);
+        ASSERT_EQ(backend.deleteExact("k/fresh", put1.token).kind, DeleteOutcome::Kind::Deleted);
+
+        const auto put2 = backend.putIfAbsent("k/fresh", "v2");
+        ASSERT_EQ(put2.outcome, PutOutcome::Done);
+        EXPECT_EQ(put2.token.value, recent_etag + "#1") << "entry should have been RETAINED on delete (etag recent), "
+                                                            "so the recreate is disambiguated against it";
+    }
+}
+
+TEST(CASObjectStorageBackend, EmuTokenStateEventuallyPrunesDistinctShortLivedKeys)
+{
+    constexpr uint64_t start_ns = 1'700'000'000'000'000'000ULL;
+    constexpr uint64_t step_ns = 100'000'000ULL;
+    constexpr size_t key_count = 128;
+    constexpr size_t expected_recent_key_bound = 24;
+
+    auto now_ns = std::make_shared<std::atomic<uint64_t>>(start_ns);
+    ObjectStorageBackend backend(makeClockEtagStorageForTest(now_ns), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+    for (size_t i = 0; i < key_count; ++i)
+    {
+        const uint64_t current_ns = start_ns + i * step_ns;
+        now_ns->store(current_ns);
+        backend.setEmuNowNsForTest(current_ns);
+
+        const String key = "k/short-lived-" + std::to_string(i);
+        const auto put = backend.putIfAbsent(key, "body");
+        ASSERT_EQ(put.outcome, PutOutcome::Done);
+        ASSERT_EQ(backend.deleteExact(key, put.token).kind, DeleteOutcome::Kind::Deleted);
+    }
+
+    const uint64_t sweep_ns = start_ns + key_count * step_ns + 2'000'000'000ULL;
+    now_ns->store(sweep_ns);
+    backend.setEmuNowNsForTest(sweep_ns);
+    const auto trigger = backend.putIfAbsent("k/sweep-trigger", "body");
+    ASSERT_EQ(trigger.outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.deleteExact("k/sweep-trigger", trigger.token).kind, DeleteOutcome::Kind::Deleted);
+
+    EXPECT_LE(backend.emuTokenStateSizeForTest(), expected_recent_key_bound)
+        << "token state should track only the bounded recent-key window, not all " << key_count << " deleted keys";
+}
+
+namespace
+{
+
+/// A `LocalObjectStorage` that counts `writeObject`/`removeObjectIfTokenMatches` calls -- used to
+/// prove that a wrong-dialect expected token is rejected LOCALLY, before anything reaches the wire.
+class CallCountingObjectStorage final : public DB::LocalObjectStorage
+{
+public:
+    using DB::LocalObjectStorage::LocalObjectStorage;
+
+    std::unique_ptr<DB::WriteBufferFromFileBase> writeObject(
+        const DB::StoredObject & object,
+        DB::WriteMode mode,
+        std::optional<DB::ObjectAttributes> attributes,
+        size_t buf_size,
+        const DB::WriteSettings & write_settings) override
+    {
+        ++write_calls;
+        return DB::LocalObjectStorage::writeObject(object, mode, attributes, buf_size, write_settings);
+    }
+
+    DB::ConditionalRemoveResult removeObjectIfTokenMatches(const DB::StoredObject & object, const std::string & etag) override
+    {
+        ++remove_if_matches_calls;
+        return DB::LocalObjectStorage::removeObjectIfTokenMatches(object, etag);
+    }
+
+    std::atomic<int> write_calls{0};
+    std::atomic<int> remove_if_matches_calls{0};
+};
+
+DB::ObjectStoragePtr makeCallCountingStorageForTest()
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_call_counting_unit_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    return std::make_shared<CallCountingObjectStorage>(std::move(settings));
+}
+
+}
+
+/// codex-review-triage §3.18, finding №19: Native-mode conditional mutations forward only
+/// `Token::value` to the wire (`object_storage_write_if_match` / `removeObjectIfTokenMatches`),
+/// blind to `Token::type`. A wrong-dialect token whose VALUE happens to equal the live incarnation's
+/// must be rejected LOCALLY -- before any wire call is made -- never merely rely on the remote
+/// backend to reject a foreign-dialect value it was never designed to compare.
+TEST(CASObjectStorageBackend, NativeRejectsWrongDialectTokenBeforeTouchingTheWire)
+{
+    auto storage = std::static_pointer_cast<CallCountingObjectStorage>(makeCallCountingStorageForTest());
+    ObjectStorageBackend backend(storage, ObjectStorageBackend::Mode::Native);
+
+    ASSERT_EQ(backend.putIfAbsent("k/dialect", "v1").outcome, PutOutcome::Done);
+    const Token live = backend.head("k/dialect").token;
+    ASSERT_EQ(live.type, TokenType::ETag);
+
+    storage->write_calls = 0;
+    storage->remove_if_matches_calls = 0;
+
+    /// Same wire VALUE, wrong dialect TYPE (Emulated instead of this backend's native ETag dialect).
+    const Token wrong_type_token{live.value, TokenType::Emulated};
+
+    EXPECT_EQ(backend.putOverwrite("k/dialect", "v2", wrong_type_token).outcome, PutOutcome::PreconditionFailed);
+    EXPECT_EQ(backend.casPut("k/dialect", "v2", wrong_type_token).outcome, CasOutcome::Conflict);
+    EXPECT_EQ(backend.deleteExact("k/dialect", wrong_type_token).kind, DeleteOutcome::Kind::TokenMismatch);
+
+    EXPECT_EQ(storage->write_calls.load(), 0);
+    EXPECT_EQ(storage->remove_if_matches_calls.load(), 0);
+
+    /// The live incarnation must be untouched by all three rejected attempts.
+    EXPECT_EQ(backend.head("k/dialect").token, live);
+}
+
+/// §1 (opt round-B): the fold/point GETs read tiny bodies but a default `ReadBufferFromS3` preallocates
+/// ~1 MiB. `casSizedReadSettings` shrinks the buffer to the known body size + slack, capped at the
+/// caller's default — never larger than before, regardless of the reported size.
+TEST(CASSizedReadSettings, CapsToKnownSizePlusSlackButNeverAboveBase)
+{
+    DB::ReadSettings base;
+    base.remote_fs_settings.buffer_size = 1ULL << 20;   /// 1 MiB default
+    base.local_fs_settings.buffer_size = 1ULL << 20;
+
+    /// A ~3.7 KB fold body: buffer shrinks to size + slack, far below the 1 MiB default.
+    const auto small = DB::Cas::casSizedReadSettings(base, 3700);
+    EXPECT_EQ(small.remote_fs_settings.buffer_size, 3700 + DB::Cas::CAS_FOLD_READ_SLACK_BYTES);
+    EXPECT_EQ(small.local_fs_settings.buffer_size, 3700 + DB::Cas::CAS_FOLD_READ_SLACK_BYTES);
+
+    /// A body larger than the default is capped AT the default (never grown).
+    const auto big = DB::Cas::casSizedReadSettings(base, 8ULL << 20);
+    EXPECT_EQ(big.remote_fs_settings.buffer_size, 1ULL << 20);
+
+    /// Unknown size (0) = leave the base untouched (the metadata-fetch fallback path).
+    const auto unknown = DB::Cas::casSizedReadSettings(base, 0);
+    EXPECT_EQ(unknown.remote_fs_settings.buffer_size, 1ULL << 20);
+}
+
+/// The CountingBackend request-shape recorders that the streaming-memory gates (Task 3/4) consume:
+/// per-key/total getStream counts, the max ranged-get window per key, and the whole-object get flag.
+TEST(CountingBackendShape, RecordsGetStreamAndRangeShape)
+{
+    DB::Cas::tests::CountingBackend backend;
+    backend.putIfAbsent("k", String(1000, 'x'));
+
+    /// A whole-object get flags the resident-memory violation; a ranged get tracks the max window.
+    backend.get("k");
+    backend.get("k", DB::Cas::Range{.offset = 0, .length = 100});
+    backend.get("k", DB::Cas::Range{.offset = 10, .length = 400});
+    EXPECT_EQ(backend.wholeGetCount("k"), 1u);
+    EXPECT_EQ(backend.maxRangedGetLen("k"), 400u);
+
+    /// getStream counters (per-key and total).
+    backend.getStream("k", DB::Cas::Range{.offset = 2, .length = 5});
+    backend.getStream("k");
+    backend.getStream("absent");
+    EXPECT_EQ(backend.getStreamCount("k"), 2u);
+    EXPECT_EQ(backend.getStreamTotal(), 3u);
+
+    backend.resetCounts();
+    EXPECT_EQ(backend.wholeGetCount("k"), 0u);
+    EXPECT_EQ(backend.maxRangedGetLen("k"), 0u);
+    EXPECT_EQ(backend.getStreamTotal(), 0u);
+}
+
+#endif
diff --git a/src/Disks/tests/gtest_cas_backend_contract.cpp b/src/Disks/tests/gtest_cas_backend_contract.cpp
new file mode 100644
index 000000000000..ee665ac5067e
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_backend_contract.cpp
@@ -0,0 +1,301 @@
+#include <gtest/gtest.h>
+#include <IO/ReadBufferFromString.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <functional>
+#include <memory>
+
+using namespace DB::Cas;
+
+/// Parameterized contract suite: every case creates a fresh backend from the factory,
+/// then exercises the Backend seam generically (no InMemoryBackend-specific calls).
+/// Fault-injection-only features are excluded — those are InMemory-specific tests.
+class CASBackendContract : public ::testing::TestWithParam<std::function<BackendPtr()>>
+{
+};
+
+TEST_P(CASBackendContract, PutIfAbsentAndGet)
+{
+    auto b = GetParam()();
+    const auto put = b->putIfAbsent("k", "v1");
+    const Token t1 = put.token;
+    EXPECT_EQ(put.outcome, PutOutcome::Done);
+    EXPECT_FALSE(t1.empty());
+    EXPECT_EQ(b->putIfAbsent("k", "clobber").outcome, PutOutcome::PreconditionFailed);
+    auto g = b->get("k");
+    ASSERT_TRUE(g.has_value());
+    EXPECT_EQ(g->bytes, "v1");
+    EXPECT_EQ(g->token, t1);
+    EXPECT_FALSE(b->get("absent").has_value());
+}
+
+TEST_P(CASBackendContract, OverwriteIsTokenExactAndMintsFreshToken)
+{
+    auto b = GetParam()();
+    const Token t1 = b->putIfAbsent("k", "v1").token;
+    EXPECT_EQ(b->putOverwrite("k", "v2", Token{"wrong", TokenType::Emulated}).outcome, PutOutcome::PreconditionFailed);
+    EXPECT_EQ(b->get("k")->bytes, "v1");                       // untouched on mismatch
+    const auto overwrite = b->putOverwrite("k", "v2", t1);
+    EXPECT_EQ(overwrite.outcome, PutOutcome::Done);
+    EXPECT_NE(overwrite.token, t1);                            // tokens never repeat
+    EXPECT_EQ(b->get("k")->bytes, "v2");
+}
+
+TEST_P(CASBackendContract, CasPutCreateAndSwap)
+{
+    auto b = GetParam()();
+    const auto create = b->casPut("m", "s1", std::nullopt);
+    const Token t1 = create.token;
+    EXPECT_EQ(create.outcome, CasOutcome::Committed);                              // create-if-absent
+    EXPECT_EQ(b->casPut("m", "s1x", std::nullopt).outcome, CasOutcome::Conflict);  // exists now
+    EXPECT_EQ(b->casPut("m", "s2", Token{"stale", TokenType::Emulated}).outcome, CasOutcome::Conflict);
+    EXPECT_EQ(b->get("m")->bytes, "s1");
+    EXPECT_EQ(b->casPut("m", "s2", t1).outcome, CasOutcome::Committed);
+    EXPECT_EQ(b->get("m")->bytes, "s2");
+}
+
+TEST_P(CASBackendContract, DeleteExactnessAndSurvival)
+{
+    auto b = GetParam()();
+    const Token t1 = b->putIfAbsent("k", "v1").token;
+    auto d1 = b->deleteExact("k", Token{"wrong", TokenType::Emulated});
+    EXPECT_EQ(d1.kind, DeleteOutcome::Kind::TokenMismatch);
+    EXPECT_TRUE(b->get("k").has_value());                      // SURVIVES wrong-token delete
+    auto d2 = b->deleteExact("k", t1);
+    EXPECT_EQ(d2.kind, DeleteOutcome::Kind::Deleted);
+    EXPECT_FALSE(d2.created_delete_marker);
+    EXPECT_FALSE(b->get("k").has_value());
+}
+
+TEST_P(CASBackendContract, DeleteNotFound)
+{
+    auto b = GetParam()();
+    const Token t1 = b->putIfAbsent("k", "v1").token;
+    b->deleteExact("k", t1);
+    EXPECT_EQ(b->deleteExact("k", t1).kind, DeleteOutcome::Kind::NotFound);
+}
+
+TEST_P(CASBackendContract, RangeGet)
+{
+    auto b = GetParam()();
+    b->putIfAbsent("k", "0123456789");
+    Range r;
+    r.offset = 2;
+    r.length = 3u;
+    EXPECT_EQ(b->get("k", r)->bytes, "234");
+}
+
+TEST_P(CASBackendContract, Head)
+{
+    auto b = GetParam()();
+    b->putIfAbsent("k", "hello");
+    auto h = b->head("k");
+    EXPECT_TRUE(h.exists);
+    EXPECT_EQ(h.size, 5u);
+    EXPECT_FALSE(h.token.empty());
+    auto h2 = b->head("missing");
+    EXPECT_FALSE(h2.exists);
+}
+
+TEST_P(CASBackendContract, ListPagination)
+{
+    auto b = GetParam()();
+    b->putIfAbsent("p/a", "0123456789");
+    b->putIfAbsent("p/b", "xy");
+    b->putIfAbsent("q/c", "z");
+    auto page = b->list("p/", "", 10);
+    ASSERT_EQ(page.keys.size(), 2u);                          // sorted, prefix-scoped
+    EXPECT_EQ(page.keys[0].key, "p/a");
+    EXPECT_EQ(page.keys[1].key, "p/b");
+    EXPECT_TRUE(page.next_cursor.empty());
+    auto page1 = b->list("p/", "", 1);                        // pagination
+    EXPECT_EQ(page1.keys.size(), 1u);
+    EXPECT_EQ(page1.keys[0].key, "p/a");
+    EXPECT_EQ(page1.next_cursor, "p/a");
+    EXPECT_FALSE(page1.next_cursor.empty());
+    auto page2 = b->list("p/", page1.next_cursor, 1);
+    EXPECT_EQ(page2.keys[0].key, "p/b");
+}
+
+TEST_P(CASBackendContract, ReadAfterWrite)
+{
+    auto b = GetParam()();
+    const Token t1 = b->putIfAbsent("rw", "payload").token;
+    auto g = b->get("rw");
+    ASSERT_TRUE(g.has_value());
+    EXPECT_EQ(g->bytes, "payload");
+    EXPECT_EQ(g->token, t1);
+    auto h = b->head("rw");
+    EXPECT_TRUE(h.exists);
+    EXPECT_EQ(h.token, t1);
+}
+
+/// After an object is created then deleted (key absent again), BOTH conditional updates against a stale
+/// token must be rejected with the object still absent — a token-conditional update can never resurrect a
+/// missing key. For the Native S3 adapter this pins the 404-on-If-Match -> PreconditionFailed/Conflict
+/// mapping; for every backend it pins that absence is not a write opportunity for a stale token.
+TEST_P(CASBackendContract, OverwriteAndCasOnMissingKey)
+{
+    auto b = GetParam()();
+    const Token t1 = b->putIfAbsent("k", "v1").token;
+    EXPECT_EQ(b->deleteExact("k", t1).kind, DeleteOutcome::Kind::Deleted);
+    ASSERT_FALSE(b->get("k").has_value());                     // key is absent
+
+    EXPECT_EQ(b->putOverwrite("k", "v2", t1).outcome, PutOutcome::PreconditionFailed);
+    EXPECT_FALSE(b->get("k").has_value());                     // still absent
+
+    EXPECT_EQ(b->casPut("k", "v2", t1).outcome, CasOutcome::Conflict);
+    EXPECT_FALSE(b->get("k").has_value());                     // still absent
+}
+
+TEST_P(CASBackendContract, StreamPutRoundTrip)
+{
+    auto b = GetParam()();
+    auto sink = b->putIfAbsentStream("k/stream1");
+    sink->buffer().write("hello ", 6);
+    sink->buffer().write("world", 5);
+    const auto res = sink->finalize();
+    const Token tok = res.token;
+    ASSERT_EQ(res.outcome, PutOutcome::Done);
+    ASSERT_FALSE(tok.empty());
+    auto got = b->get("k/stream1");
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, "hello world");
+    EXPECT_EQ(got->token, tok);
+}
+
+TEST_P(CASBackendContract, StreamPutPreconditionAtFinalize)
+{
+    auto b = GetParam()();
+    const auto first_put = b->putIfAbsent("k/stream2", "original");
+    const Token first = first_put.token;
+    ASSERT_EQ(first_put.outcome, PutOutcome::Done);
+    auto sink = b->putIfAbsentStream("k/stream2");
+    sink->buffer().write("loser", 5);
+    ASSERT_EQ(sink->finalize().outcome, PutOutcome::PreconditionFailed);
+    auto got = b->get("k/stream2");
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, "original");    /// the failed conditional write left the object unmodified
+    EXPECT_EQ(got->token, first);
+}
+
+TEST_P(CASBackendContract, StreamPutCancelLeavesNothing)
+{
+    auto b = GetParam()();
+    {
+        auto sink = b->putIfAbsentStream("k/stream3");
+        sink->buffer().write("partial", 7);
+        sink->cancel();
+    }
+    EXPECT_FALSE(b->head("k/stream3").exists);
+}
+
+TEST_P(CASBackendContract, StreamPutDestructionWithoutFinalizeLeavesNothing)
+{
+    auto b = GetParam()();
+    {
+        auto sink = b->putIfAbsentStream("k/stream4");
+        sink->buffer().write("partial", 7);
+        /// no finalize, no cancel — destructor must behave as cancel (never publish)
+    }
+    EXPECT_FALSE(b->head("k/stream4").exists);
+}
+
+TEST_P(CASBackendContract, StreamPutEmptyBody)
+{
+    auto b = GetParam()();
+    auto sink = b->putIfAbsentStream("k/stream_empty");
+    const auto res = sink->finalize();
+    const Token tok = res.token;
+    ASSERT_EQ(res.outcome, PutOutcome::Done);
+    ASSERT_FALSE(tok.empty());
+    auto got = b->get("k/stream_empty");
+    ASSERT_TRUE(got.has_value());
+    EXPECT_TRUE(got->bytes.empty());
+    EXPECT_EQ(got->token, tok);
+    auto h = b->head("k/stream_empty");
+    EXPECT_TRUE(h.exists);
+    EXPECT_EQ(h.size, 0u);
+}
+
+/// ~1 MB written in chunks: exercises buffer growth in the memory-buffered sinks and, for the
+/// future Native sink, the real streaming path.
+TEST_P(CASBackendContract, StreamPutLargeBody)
+{
+    auto b = GetParam()();
+    String chunk(4096, '\0');
+    for (size_t i = 0; i < chunk.size(); ++i)
+        chunk[i] = static_cast<char>('a' + i % 26);
+
+    String expected;
+    auto sink = b->putIfAbsentStream("k/stream_large");
+    for (size_t written = 0; written < (1 << 20); written += chunk.size())
+    {
+        sink->buffer().write(chunk.data(), chunk.size());
+        expected += chunk;
+    }
+    const auto res = sink->finalize();
+    const Token tok = res.token;
+    ASSERT_EQ(res.outcome, PutOutcome::Done);
+
+    auto got = b->get("k/stream_large");
+    ASSERT_TRUE(got.has_value());
+    ASSERT_EQ(got->bytes.size(), expected.size());
+    EXPECT_EQ(got->bytes, expected);
+    EXPECT_EQ(got->token, tok);
+}
+
+INSTANTIATE_TEST_SUITE_P(CASInMemory, CASBackendContract,
+    ::testing::Values(+[]() -> BackendPtr { return std::make_shared<InMemoryBackend>(); }));
+
+INSTANTIATE_TEST_SUITE_P(CASLocal, CASBackendContract,
+    ::testing::Values(+[]() -> BackendPtr
+    {
+        return std::make_shared<ObjectStorageBackend>(
+            DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+    }));
+
+/// The unconditional resurrect counts while streaming and aborts WITHOUT publishing when the reader
+/// yields a different byte count than declared. With no precondition on the write, this is the last
+/// line of defence against a source truncated after hashing: a post-write check would fire only after
+/// the short body had displaced the condemned incarnation.
+TEST_P(CASBackendContract, ResurrectWrongSizePublishesNothing)
+{
+    auto b = GetParam()();
+    const auto created = b->putIfAbsent("k/res_short", "condemned-body");
+    ASSERT_EQ(created.outcome, PutOutcome::Done);
+
+    DB::ReadBufferFromOwnString in{String("short")};
+    EXPECT_THROW(b->resurrect(in, /*payload_size=*/1000, "k/res_short", String("HDR")), DB::Exception);
+
+    const auto got = b->get("k/res_short");
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, "condemned-body") << "a size-mismatched resurrect must publish nothing";
+    EXPECT_EQ(got->token, created.token);
+}
+
+/// The resurrect works on EVERY mode of every backend -- the emulated (local object storage) mode
+/// included. The former conditional putOverwrite supported local disks, and losing that would make a
+/// condemned blob unrepairable on a local content-addressed disk.
+TEST_P(CASBackendContract, ResurrectReplacesBodyAndMintsFreshToken)
+{
+    auto b = GetParam()();
+    const auto created = b->putIfAbsent("k/res_ok", "condemned-body");
+    ASSERT_EQ(created.outcome, PutOutcome::Done);
+
+    const String payload = "resurrected-payload";
+    DB::ReadBufferFromOwnString in{payload};
+    const Token fresh = b->resurrect(in, payload.size(), "k/res_ok", String("HDR"));
+    EXPECT_FALSE(fresh.empty());
+    EXPECT_NE(fresh, created.token);
+
+    const auto got = b->get("k/res_ok");
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, "HDR" + payload);
+
+    /// INV-NO-RETURN: the queued exact-token delete of the condemned incarnation misses the fresh one.
+    EXPECT_EQ(b->deleteExact("k/res_ok", created.token).kind, DeleteOutcome::Kind::TokenMismatch);
+    EXPECT_TRUE(b->head("k/res_ok").exists);
+}
diff --git a/src/Disks/tests/gtest_cas_backend_generation.cpp b/src/Disks/tests/gtest_cas_backend_generation.cpp
new file mode 100644
index 000000000000..c4476718195a
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_backend_generation.cpp
@@ -0,0 +1,132 @@
+#include <gtest/gtest.h>
+#include <IO/ReadBufferFromString.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+using namespace DB::Cas;
+
+/// Every Token{...} the backend mints must carry native_token_type instead of a hardcoded
+/// TokenType::ETag (Task 5). Mode::Native over a LocalObjectStorage has no write-time ETag, so
+/// putIfAbsent's PutResult falls back to a HEAD internally — that HEAD is also a stamping site,
+/// so the assertion below exercises both the direct-etag and the HEAD-fallback mint paths.
+TEST(CASBackendGeneration, StampedTokenTypeFollowsNativeKind)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+    b->setNativeTokenTypeForTest(TokenType::Generation);
+
+    const auto put = b->putIfAbsent("p/gen/tok", "v1");
+    EXPECT_EQ(put.token.type, TokenType::Generation);
+
+    const auto hr = b->head("p/gen/tok");
+    ASSERT_TRUE(hr.exists);
+    EXPECT_EQ(hr.token.type, TokenType::Generation);
+}
+
+/// checkPoolPreconditions on a Native, generation-dialect (GCS) backend consults
+/// isBucketVersioningEnabled. LocalObjectStorage does not override that method, so it inherits the
+/// IObjectStorage base default, which returns nullopt (the check is inconclusive). Per the hook's
+/// documented behaviour, an inconclusive check must NOT fail closed — only a CONFIRMED `true` throws.
+TEST(CASBackendGeneration, CheckPoolPreconditionsProceedsOnUnknownVersioning)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+    b->setNativeTokenTypeForTest(TokenType::Generation);
+
+    EXPECT_NO_THROW(b->checkPoolPreconditions());
+}
+
+/// The ETag-dialect (AWS-compatible) backend never consults bucket versioning at all — the check is
+/// a silent no-op for any backend that is not Native + TokenType::Generation.
+TEST(CASBackendGeneration, CheckPoolPreconditionsNoOpOnEtagDialect)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+    ASSERT_EQ(b->nativeTokenType(), TokenType::ETag);
+
+    EXPECT_NO_THROW(b->checkPoolPreconditions());
+}
+
+/// GCS enforces NO preconditions on CompleteMultipartUpload (measured 2026-07-03), so a conditional
+/// write on a generation-token store must never take the multipart path. conditionalWriteSettings
+/// must force the single-PUT path (and raise the single-part cap to conditional_single_put_cap) when
+/// the backend's native token kind is Generation, and stay a no-op otherwise (ETag dialect).
+TEST(CASBackendGeneration, ListTokensDisabledOnGenerationStores)
+{
+    /// XML LIST bodies carry MD5-style ETags that the dialect cannot rewrite to generations; a
+    /// list-derived token on a generation store is a poisoned If-Match (live GC on GCS died there).
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+    EXPECT_TRUE(b->supportsListTokens());
+    b->setNativeTokenTypeForTest(TokenType::Generation);
+    EXPECT_FALSE(b->supportsListTokens());
+    b->setNativeTokenTypeForTest(TokenType::ETag);
+    EXPECT_TRUE(b->supportsListTokens());
+}
+
+TEST(CASBackendGeneration, ConditionalWriteSettingsForceSinglePutOnGenerationStores)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native,
+        /*conditional_single_put_cap=*/123);
+    b->setNativeTokenTypeForTest(TokenType::Generation);
+    const auto ws = b->conditionalWriteSettingsForTest();
+    EXPECT_TRUE(ws.s3_force_single_part_upload);
+    EXPECT_EQ(ws.s3_single_part_upload_max_bytes_override, 123u);
+
+    b->setNativeTokenTypeForTest(TokenType::ETag);
+    const auto ws2 = b->conditionalWriteSettingsForTest();
+    EXPECT_FALSE(ws2.s3_force_single_part_upload);
+    EXPECT_EQ(ws2.s3_single_part_upload_max_bytes_override, 0u);
+}
+
+/// C1: the three token-policy helpers are the single source of truth for how a Native-mode backend
+/// mints a HEAD/PUT token, gates a LIST token, and compares tokens. Characterizes the behavior the
+/// scattered call sites have today so the consolidation stays byte-for-byte behavior-preserving.
+TEST(CASBackendGeneration, TokenPolicyHelpersAreConsistentWithDialect)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+
+    /// ETag dialect: head/put tokens carry ETag; list surfaces the same-typed token for a non-empty etag.
+    ASSERT_EQ(b->nativeTokenType(), TokenType::ETag);
+    EXPECT_EQ(b->tokenForHead("abc").type, TokenType::ETag);
+    EXPECT_EQ(b->tokenForHead("abc"), (Token{"abc", TokenType::ETag}));
+    ASSERT_TRUE(b->tokenForList("abc").has_value());
+    EXPECT_EQ(*b->tokenForList("abc"), b->tokenForHead("abc"));   /// list token == head token (same etag)
+    EXPECT_FALSE(b->tokenForList("").has_value());                /// empty etag => no list token
+
+    /// Generation dialect (GCS): head token flips to Generation; list tokens are disabled wholesale
+    /// (poisoned If-Match), so tokenForList is always nullopt regardless of the etag.
+    b->setNativeTokenTypeForTest(TokenType::Generation);
+    EXPECT_EQ(b->tokenForHead("g1").type, TokenType::Generation);
+    EXPECT_FALSE(b->tokenForList("g1").has_value());
+
+    /// tokenMatches is exact identity (value AND type) — a same-value/different-type token never matches.
+    EXPECT_TRUE(ObjectStorageBackend::tokenMatches(Token{"x", TokenType::ETag}, Token{"x", TokenType::ETag}));
+    EXPECT_FALSE(ObjectStorageBackend::tokenMatches(Token{"x", TokenType::ETag}, Token{"x", TokenType::Emulated}));
+}
+
+/// The single-PUT cap binds CONDITIONAL writes, because that is where GCS drops the precondition. A
+/// resurrect carries no precondition, so it must not be capped -- this is the regression test for a
+/// ceiling that used to apply here and no longer does. A future consistency-minded refactor that
+/// routes the resurrect through conditionalWriteSettings would silently reintroduce it; this fails
+/// then.
+TEST(CASBackendGeneration, ResurrectIsNotBoundByTheSinglePutCap)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native,
+        /*conditional_single_put_cap=*/16);
+    b->setNativeTokenTypeForTest(TokenType::Generation);
+
+    ASSERT_EQ(b->putIfAbsent("p/gen/res", "original").outcome, PutOutcome::Done);
+
+    const String payload(1024, 'x');   /// far above the 16-byte cap
+    DB::ReadBufferFromOwnString in{payload};
+    const Token tok = b->resurrect(in, payload.size(), "p/gen/res", String("HDR"));
+    EXPECT_FALSE(tok.empty());
+
+    auto got = b->get("p/gen/res");
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, "HDR" + payload);
+}
diff --git a/src/Disks/tests/gtest_cas_backend_listing.cpp b/src/Disks/tests/gtest_cas_backend_listing.cpp
new file mode 100644
index 000000000000..591dfdba65db
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_backend_listing.cpp
@@ -0,0 +1,43 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+
+#include <algorithm>
+#include <vector>
+
+using namespace DB::Cas;
+
+TEST(CASBackendListing, ForEachWalksEveryPageOnce)
+{
+    InMemoryBackend b;
+    for (int i = 0; i < 2500; ++i)
+        b.putIfAbsent("p/" + std::to_string(1000000 + i), "v");
+    b.putIfAbsent("q/other", "v");   /// out of prefix — must not be visited
+
+    std::vector<String> seen;
+    forEachListedKey(b, "p/", [&](const ListedKey & k) { seen.push_back(k.key); }, /*page_limit=*/1000);
+    EXPECT_EQ(seen.size(), 2500u);                                  /// paged (3 pages), no key dropped/duplicated
+    EXPECT_TRUE(std::is_sorted(seen.begin(), seen.end()));
+}
+
+TEST(CASBackendListing, ForEachEmptyPrefixVisitsNothing)
+{
+    InMemoryBackend b;
+    b.putIfAbsent("q/other", "v");
+
+    size_t visits = 0;
+    forEachListedKey(b, "p/", [&](const ListedKey &) { ++visits; });
+    EXPECT_EQ(visits, 0u);
+}
+
+TEST(CASBackendListing, ClassifyMapsEveryDeleteKind)
+{
+    EXPECT_EQ(classifyDeleteOutcome({DeleteOutcome::Kind::Deleted, false}),       DeleteClass::Deleted);
+    EXPECT_EQ(classifyDeleteOutcome({DeleteOutcome::Kind::NotFound, false}),      DeleteClass::Absent);
+    EXPECT_EQ(classifyDeleteOutcome({DeleteOutcome::Kind::TokenMismatch, false}), DeleteClass::Replaced);
+
+    EXPECT_EQ(deleteClassName(DeleteClass::Deleted),  "deleted");
+    EXPECT_EQ(deleteClassName(DeleteClass::Absent),   "absent");
+    EXPECT_EQ(deleteClassName(DeleteClass::Replaced), "replaced");
+}
diff --git a/src/Disks/tests/gtest_cas_blob_digest.cpp b/src/Disks/tests/gtest_cas_blob_digest.cpp
new file mode 100644
index 000000000000..9a87f08ccda2
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_digest.cpp
@@ -0,0 +1,266 @@
+#include <gtest/gtest.h>
+
+/// CAS pluggable-blob-hash Phase 2, Task 1: `BlobDigest` (the pool-scoped variable-length content digest, ADDITIVE-ONLY -- no
+/// existing `UInt128 blob_hash` field is migrated in this task) + the ONE `PoolMeta`-scoped
+/// `DigestCodec` all digest<->hex/bytes conversion must route through.
+///
+/// THE KEY GATE (`ShardOfBitIdenticalToOldHighBitsOver200RandomValues` below): `DigestCodec`'s
+/// `shardOf` (an explicit big-endian read of the first 8 digest bytes) must be bit-identical to
+/// today's `static_cast<uint64_t>(blob_hash >> 64)` (`CasGcShardPlan.h`'s `blobShard`) for every
+/// 128-bit digest -- otherwise an existing cityHash128/xxh3-128 pool would silently reshard on
+/// upgrade. This is load-bearing: it is what makes Phase 2 safe to land under running pools.
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include "cas_test_helpers.h"
+
+#include <base/defines.h>
+
+#include <cstdint>
+#include <random>
+#include <unordered_map>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+extern const int BAD_ARGUMENTS;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+UInt128 randomU128(std::mt19937_64 & rng)
+{
+    const UInt128 hi = rng();
+    const UInt128 lo = rng();
+    return (hi << 64) | lo;
+}
+
+}
+
+/// ---- THE KEY GATE ----
+
+TEST(CASBlobDigest, ShardOfBitIdenticalToOldHighBitsOver200RandomValues)
+{
+    std::mt19937_64 rng(0xC0FFEE); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    const DigestCodec codec16(/*blob_hash_len*/ 16);
+
+    for (int i = 0; i < 200; ++i)
+    {
+        const UInt128 v = randomU128(rng);
+        const uint64_t old_high64 = static_cast<uint64_t>(v >> 64);
+        const uint64_t got = codec16.shardOf(BlobDigest::fromU128(v));
+        EXPECT_EQ(got, old_high64) << "mismatch for random UInt128 iteration " << i;
+    }
+
+    /// Edge cases: all-zero and all-one high halves.
+    EXPECT_EQ(codec16.shardOf(BlobDigest::fromU128(UInt128(0))), 0u);
+    const UInt128 all_ones = ~UInt128(0);
+    EXPECT_EQ(codec16.shardOf(BlobDigest::fromU128(all_ones)), static_cast<uint64_t>(all_ones >> 64));
+}
+
+/// The same gate, but via `Cas::codecFor` (`CasBlobRef.h`), the ONE way production code obtains a
+/// codec (Phase 3 T4 deleted the pool-scoped `DigestCodec(PoolMeta)` constructor -- a mixed-algo
+/// pool has no single width; the codec is selected per-algo, never per-pool).
+TEST(CASBlobDigest, ShardOfViaPoolMetaConstructedCodecMatchesOldBlobShard)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+    const PoolMeta pm = PoolMeta::createOrValidate(*backend, layout, /*blob_header_len*/ 256, BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+    ASSERT_EQ(pm.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::CityHash128)}));
+    const DigestCodec codec = codecFor(BlobHashAlgo::CityHash128);
+
+    std::mt19937_64 rng(12345); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    for (int i = 0; i < 200; ++i)
+    {
+        const UInt128 v = randomU128(rng);
+        EXPECT_EQ(codec.shardOf(BlobDigest::fromU128(v)), static_cast<uint64_t>(v >> 64));
+        /// `blobShard` (`CasGcShardPlan.h`) additionally takes `% gc_shards`; at `gc_shards == 1`
+        /// every hash routes to shard 0, so this only pins the trivial single-shard case -- the
+        /// bit-identical pre-mod value is already pinned by the assertion above.
+        EXPECT_EQ(blobShard(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(v)}, /*gc_shards*/ 1), 0u);
+    }
+}
+
+/// ---- round-trip ----
+
+TEST(CASBlobDigest, HexRoundTripLen16)
+{
+    const DigestCodec codec(16);
+    std::mt19937_64 rng(1); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    for (int i = 0; i < 50; ++i)
+    {
+        const BlobDigest d = BlobDigest::fromU128(randomU128(rng));
+        const String hex = codec.toHex(d);
+        EXPECT_EQ(hex.size(), 32u);
+        EXPECT_EQ(codec.fromHex(hex), d);
+    }
+}
+
+TEST(CASBlobDigest, HexRoundTripLen32)
+{
+    const DigestCodec codec(32);
+    BlobDigest d;
+    for (size_t i = 0; i < d.bytes.size(); ++i)
+        d.bytes[i] = static_cast<uint8_t>(i * 7 + 1);
+
+    const String hex = codec.toHex(d);
+    EXPECT_EQ(hex.size(), 64u);
+    EXPECT_EQ(codec.fromHex(hex), d);
+}
+
+TEST(CASBlobDigest, BytesBERoundTripLen16)
+{
+    const DigestCodec codec(16);
+    std::mt19937_64 rng(2); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    for (int i = 0; i < 50; ++i)
+    {
+        const BlobDigest d = BlobDigest::fromU128(randomU128(rng));
+        const String bytes = codec.toBytesBE(d);
+        EXPECT_EQ(bytes.size(), 16u);
+        EXPECT_EQ(codec.fromBytesBE(bytes), d);
+    }
+}
+
+TEST(CASBlobDigest, BytesBERoundTripLen32)
+{
+    const DigestCodec codec(32);
+    BlobDigest d;
+    for (size_t i = 0; i < d.bytes.size(); ++i)
+        d.bytes[i] = static_cast<uint8_t>(255 - i);
+
+    const String bytes = codec.toBytesBE(d);
+    EXPECT_EQ(bytes.size(), 32u);
+    EXPECT_EQ(codec.fromBytesBE(bytes), d);
+}
+
+/// `toBytesBE` at len16 must produce exactly the `u128ToBytesBE` bytes for the 16-byte prefix --
+/// same byte order, so a 128-bit pool's on-wire bytes stay unchanged when a later task migrates a
+/// field from `UInt128` to `BlobDigest`.
+TEST(CASBlobDigest, BytesBEAgreesWithU128ToBytesBEAtLen16)
+{
+    const DigestCodec codec(16);
+    std::mt19937_64 rng(3); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    for (int i = 0; i < 20; ++i)
+    {
+        const UInt128 v = randomU128(rng);
+        EXPECT_EQ(codec.toBytesBE(BlobDigest::fromU128(v)), u128ToBytesBE(v));
+    }
+}
+
+/// ---- width rejection ----
+
+TEST(CASBlobDigest, FromHexRejectsWrongWidth)
+{
+    const DigestCodec codec16(16);
+    const DigestCodec codec32(32);
+
+    /// A 16-byte codec must reject a 64-hex (32-byte) string.
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&] { codec16.fromHex(std::string(64, 'a')); });
+    /// A 32-byte codec must reject a 32-hex (16-byte) string.
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&] { codec32.fromHex(std::string(32, 'a')); });
+    /// Any non-hex character is rejected too.
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&] { codec16.fromHex(std::string(31, 'a') + "z"); });
+}
+
+TEST(CASBlobDigest, FromBytesBERejectsWrongWidth)
+{
+    const DigestCodec codec16(16);
+    const DigestCodec codec32(32);
+
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&] { codec16.fromBytesBE(std::string(32, '\0')); });
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&] { codec32.fromBytesBE(std::string(16, '\0')); });
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&] { codec16.fromBytesBE(std::string(15, '\0')); });
+}
+
+/// ---- UInt128 conversion ----
+
+TEST(CASBlobDigest, U128RoundTrip)
+{
+    std::mt19937_64 rng(4); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    for (int i = 0; i < 200; ++i)
+    {
+        const UInt128 v = randomU128(rng);
+        EXPECT_EQ(BlobDigest::fromU128(v).toU128(), v);
+    }
+    EXPECT_EQ(BlobDigest::fromU128(UInt128(0)).toU128(), UInt128(0));
+}
+
+TEST(CASBlobDigest, FromU128LeavesTailZero)
+{
+    std::mt19937_64 rng(5); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    const UInt128 v = randomU128(rng);
+    const BlobDigest d = BlobDigest::fromU128(v);
+    for (size_t i = 16; i < d.bytes.size(); ++i)
+        EXPECT_EQ(d.bytes[i], 0u) << "tail byte " << i << " must be zero for a 128-bit-pool digest";
+}
+
+/// ---- hasher / container use ----
+
+TEST(CASBlobDigest, UsableAsUnorderedMapKey)
+{
+    std::mt19937_64 rng(6); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    std::unordered_map<BlobDigest, int, BlobDigestHash> m;
+    std::vector<BlobDigest> digests;
+    for (int i = 0; i < 20; ++i)
+    {
+        const BlobDigest d = BlobDigest::fromU128(randomU128(rng));
+        digests.push_back(d);
+        m[d] = i;
+    }
+    for (int i = 0; i < 20; ++i)
+        EXPECT_EQ(m.at(digests[static_cast<size_t>(i)]), i);
+}
+
+/// ---- PoolMeta::algos_used records the creating algo (Phase 3 T4 -- the width itself is no longer
+/// pool state at all: `blobHashLenFor(algo)`/`codecFor(algo)` derive it per-algo, never per-pool) ----
+
+TEST(CASBlobDigest, PoolMetaRecordsCreatingAlgoAndWidthDerivesFromIt)
+{
+    {
+        auto backend = std::make_shared<InMemoryBackend>();
+        const Layout layout("p1");
+        const PoolMeta pm = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+        EXPECT_EQ(pm.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::CityHash128)}));
+        EXPECT_EQ(blobHashLenFor(BlobHashAlgo::CityHash128), 16u);
+    }
+    {
+        auto backend = std::make_shared<InMemoryBackend>();
+        const Layout layout("p2");
+        const PoolMeta pm = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::XXH3_128, /*allow_new*/ false, /*allow_mint*/ true);
+        EXPECT_EQ(pm.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::XXH3_128)}));
+        EXPECT_EQ(blobHashLenFor(BlobHashAlgo::XXH3_128), 16u);
+    }
+    {
+        auto backend = std::make_shared<InMemoryBackend>();
+        const Layout layout("p3");
+        const PoolMeta pm = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::Sha256, /*allow_new*/ false, /*allow_mint*/ true);
+        EXPECT_EQ(pm.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::Sha256)}));
+        EXPECT_EQ(blobHashLenFor(BlobHashAlgo::Sha256), 32u);
+
+        /// Reopen (decode path) must re-derive the same recorded algo.
+        const PoolMeta reopened = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::Sha256);
+        EXPECT_EQ(reopened.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::Sha256)}));
+    }
+}
+
+/// ---- zero-tail len-drift guard (debug/sanitizer builds only: chassert aborts the process) ----
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASBlobDigestDeathTest, ZeroTailChassertFiresOnNonZeroTailAtLen16)
+{
+    const DigestCodec codec16(16);
+    BlobDigest d = BlobDigest::fromU128(UInt128(1));
+    d.bytes[16] = 0x42; /// corrupt a tail byte beyond the pool's 16-byte width
+
+    EXPECT_DEATH({ (void)codec16.toHex(d); }, "");
+    EXPECT_DEATH({ (void)codec16.toBytesBE(d); }, "");
+}
+#endif
diff --git a/src/Disks/tests/gtest_cas_blob_envelope_format.cpp b/src/Disks/tests/gtest_cas_blob_envelope_format.cpp
new file mode 100644
index 000000000000..1119a5be0a64
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_envelope_format.cpp
@@ -0,0 +1,174 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+
+#include <stdexcept>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes { extern const int CORRUPTED_DATA; extern const int UNKNOWN_FORMAT_VERSION; }
+
+namespace
+{
+EnvelopeHeader sampleHeader(const String & ref)
+{
+    EnvelopeHeader h;
+    h.kind = ObjectKind::Blob;
+    h.incarnation_tag = hexToU128("0102030405060708090a0b0c0d0e0f10");
+    h.build_id = hexToU128("1112131415161718191a1b1c1d1e1f20");
+    h.provenance = Provenance{1752537600123ULL, hexToU128("2122232425262728292a2b2c2d2e2f30"), 26006001u, ProvenanceOp::Merge};
+    h.intended_ref = ref;
+    return h;
+}
+constexpr uint32_t L = 256;
+
+/// The envelope has a fixed physical length. At generation 9 there is no unsupported one-digit
+/// version, so replacing `9` with `10` must consume one byte from the space pad rather than silently
+/// turning the 256-byte fixture into a different wire shape.
+String blobEnvelopeWithFutureVersion(std::string_view text)
+{
+    const String v_now = fmt::format("\"v\":{}", currentCompatibilityVersion());
+    const String v_next = fmt::format("\"v\":{}", currentCompatibilityVersion() + 1);
+    String future(text);
+    const size_t version_pos = future.find(v_now);
+    if (version_pos == String::npos || v_next.size() < v_now.size())
+        throw std::logic_error("blob-envelope future-version fixture cannot locate the current version");
+
+    future.replace(version_pos, v_now.size(), v_next);
+    const size_t growth = v_next.size() - v_now.size();
+    const size_t newline_pos = future.find('\n');
+    if (newline_pos == String::npos || newline_pos < growth
+        || future.substr(newline_pos - growth, growth) != String(growth, ' '))
+        throw std::logic_error("blob-envelope future-version fixture has insufficient padding");
+    future.erase(newline_pos - growth, growth);
+    return future;
+}
+}
+
+TEST(CASBlobEnvelopeFormat, FixedLengthAndPadZone)
+{
+    EnvelopeHeader h = sampleHeader("t-abc/all_1_2_0");
+    const String head = encodeEnvelopeHeader(h, L);
+    ASSERT_EQ(head.size(), L);                       /// exactly blob_header_len
+    EXPECT_EQ(head[L - 1], '\n');                     /// terminator at byte 255
+    const String json = fmt::format(R"({{"type":"cas_blob","v":{},)", currentCompatibilityVersion()) +
+                        "\"tag\":\"0102030405060708090a0b0c0d0e0f10\","
+                        "\"bld\":\"1112131415161718191a1b1c1d1e1f20\",\"ts\":1752537600123,"
+                        "\"by\":\"2122232425262728292a2b2c2d2e2f30\",\"op\":\"merge\",\"ch\":26006001,"
+                        "\"ref\":\"t-abc/all_1_2_0\"}";
+    ASSERT_LT(json.size(), L);
+    EXPECT_EQ(head.substr(0, json.size()), json);                        /// '/' UNescaped (local escaper)
+    EXPECT_EQ(head.substr(json.size(), (L - 1) - json.size()), String((L - 1) - json.size(), ' ')); /// pad = spaces
+    /// round-trip
+    const EnvelopeHeader back = decodeEnvelopeHeader(head, head.size(), ObjectKind::Blob);
+    EXPECT_EQ(back.incarnation_tag, h.incarnation_tag);
+    EXPECT_EQ(back.build_id, h.build_id);
+    ASSERT_TRUE(back.provenance.has_value());
+    EXPECT_EQ(back.provenance->created_at_ms, 1752537600123ULL);
+    EXPECT_EQ(back.provenance->ch_version, 26006001u);
+    EXPECT_EQ(back.provenance->op, ProvenanceOp::Merge);
+    ASSERT_TRUE(back.intended_ref.has_value());
+    EXPECT_EQ(*back.intended_ref, "t-abc/all_1_2_0");
+    EXPECT_EQ(back.header_len, L);
+    EXPECT_EQ(payloadOffset(back), L);
+}
+
+TEST(CASBlobEnvelopeFormat, RefTruncatedToExactBudget)
+{
+    /// A 200-char ref cannot fit; it is truncated so the header is EXACTLY 256 bytes and the pad holds.
+    EnvelopeHeader h = sampleHeader(String(200, 'a'));
+    const String head = encodeEnvelopeHeader(h, L);
+    ASSERT_EQ(head.size(), L);
+    EXPECT_EQ(head[L - 1], '\n');
+    const EnvelopeHeader back = decodeEnvelopeHeader(head, head.size(), ObjectKind::Blob);
+    ASSERT_TRUE(back.intended_ref.has_value());
+    /// Budget is deterministic. Compute json_len for the SAME header with an empty ref; each extra 'a'
+    /// is one escaped byte, so the truncated 'a' count is exactly (L-1) - json_len(empty ref).
+    EnvelopeHeader probe = sampleHeader("");
+    const String empty_ref_head = encodeEnvelopeHeader(probe, L);
+    const size_t json_len_empty = empty_ref_head.find_last_not_of(' ', (L - 1) - 1) + 1;
+    const size_t budget = (L - 1) - json_len_empty;
+    EXPECT_EQ(back.intended_ref->size(), budget) << "ref truncated to the exact byte budget";
+    for (char c : *back.intended_ref)
+        EXPECT_EQ(c, 'a');
+}
+
+TEST(CASBlobEnvelopeFormat, PadZoneSmugglingFailsClosed)
+{
+    EnvelopeHeader h = sampleHeader("r");
+    const String head = encodeEnvelopeHeader(h, L);
+    const size_t json_len = head.find_last_not_of(' ', (L - 1) - 1) + 1; /// first pad byte index = json_len
+    ASSERT_LT(json_len, L - 1);
+    /// A non-space byte smuggled into the pad zone -> CORRUPTED_DATA.
+    String smuggled = head;
+    smuggled[json_len + 1] = 'x';
+    EXPECT_THROW(decodeEnvelopeHeader(smuggled, smuggled.size(), ObjectKind::Blob), DB::Exception);
+    /// Byte 255 not '\n' -> CORRUPTED_DATA.
+    String no_nl = head;
+    no_nl[L - 1] = ' ';
+    EXPECT_THROW(decodeEnvelopeHeader(no_nl, no_nl.size(), ObjectKind::Blob), DB::Exception);
+}
+
+TEST(CASBlobEnvelopeFormat, GatesAndCriticalKey)
+{
+    /// wrong type -> CORRUPTED_DATA; future v -> UNKNOWN_FORMAT_VERSION.
+    EnvelopeHeader h = sampleHeader("r");
+    const String head = encodeEnvelopeHeader(h, L);
+    String wrong_type = head;
+    wrong_type.replace(wrong_type.find("cas_blob"), 8, "cas_xxxx");
+    EXPECT_THROW(decodeEnvelopeHeader(wrong_type, wrong_type.size(), ObjectKind::Blob), DB::Exception);
+    const String current_version = fmt::format("\"v\":{}", currentCompatibilityVersion());
+    String future = blobEnvelopeWithFutureVersion(head);
+    cas_battery_detail::expectCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION,
+        [&] { decodeEnvelopeHeader(future, future.size(), ObjectKind::Blob); }, "future blob-envelope version");
+
+    String out_of_range = head;
+    const size_t out_of_range_version_at = out_of_range.find(current_version);
+    ASSERT_NE(out_of_range_version_at, String::npos);
+    out_of_range.replace(out_of_range_version_at, current_version.size(), "\"v\":4294967299");
+    try
+    {
+        decodeEnvelopeHeader(out_of_range, out_of_range.size(), ObjectKind::Blob);
+        FAIL() << "expected CORRUPTED_DATA";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+    }
+    /// an unknown `!`-critical key fails closed.
+    EnvelopeHeader hc = sampleHeader("r");
+    hc.emit_unknown_critical_key = true;
+    const String crit = encodeEnvelopeHeader(hc, L);
+    EXPECT_THROW(decodeEnvelopeHeader(crit, crit.size(), ObjectKind::Blob), DB::Exception);
+}
+
+TEST(CASBlobEnvelopeFormat, RefEscaperAlphabetPinned)
+{
+    /// Pins the LOCAL escaper's alphabet (§ref-escaper): " and \ escape, control chars -> \uXXXX,
+    /// '/' passes VERBATIM. Goes RED if anyone "unifies" this with writeStringValue/FormatSettings —
+    /// the 256-byte budget arithmetic depends on this alphabet being codec-owned and frozen.
+    EnvelopeHeader h = sampleHeader(String("a/b\"c\\d") + '\x01' + "e");
+    const String head = encodeEnvelopeHeader(h, L);
+    const String expected_ref_json = R"("a/b\"c\\d\u0001e")";
+    EXPECT_NE(head.find("\"ref\":" + expected_ref_json), String::npos)
+        << "escaper alphabet drifted: '/' must be verbatim, quote/backslash escaped, control -> \\uXXXX";
+}
+
+TEST(CASFormatBattery, BlobEnvelope)
+{
+    /// The golden is CONSTRUCTED from the hand-pinned json literal (same one FixedLengthAndPadZone
+    /// asserts) + the derived pad — NOT self-computed via encodeEnvelopeHeader, which would compare
+    /// the encoder to itself and pin nothing.
+    const String json = fmt::format(R"({{"type":"cas_blob","v":{},)", currentCompatibilityVersion()) +
+                        "\"tag\":\"0102030405060708090a0b0c0d0e0f10\","
+                        "\"bld\":\"1112131415161718191a1b1c1d1e1f20\",\"ts\":1752537600123,"
+                        "\"by\":\"2122232425262728292a2b2c2d2e2f30\",\"op\":\"merge\",\"ch\":26006001,"
+                        "\"ref\":\"t-abc/all_1_2_0\"}";
+    const String golden = json + String((L - 1) - json.size(), ' ') + '\n';
+    runFormatBattery(FormatBatteryCase{
+        .id = FormatId::Blob,
+        .encode = [&] { EnvelopeHeader e = sampleHeader("t-abc/all_1_2_0"); return sealObject(FormatId::Blob, encodeEnvelopeHeader(e, L)); },
+        .decode = [](std::string_view s) { decodeEnvelopeHeader(String(openObject(FormatId::Blob, s)), s.size(), ObjectKind::Blob); },
+        .golden = golden,
+        .make_future_version = blobEnvelopeWithFutureVersion});
+}
diff --git a/src/Disks/tests/gtest_cas_blob_hasher.cpp b/src/Disks/tests/gtest_cas_blob_hasher.cpp
new file mode 100644
index 000000000000..0aa1b3ff35a4
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_hasher.cpp
@@ -0,0 +1,182 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h>
+/// `CasXxh3Streamer.h` is the isolated xxHash wrapper (a system header): it gives us `Cas::xxh3_128_oneshot`
+/// as an independent one-shot reference without pulling raw xxHash symbols (or their warnings) into
+/// this test — see the header's own comment for the lz4-shadowing / `-Werror` reasons.
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasXxh3Streamer.h>
+#include <IO/HashingWriteBuffer.h>
+#include <IO/WriteBufferFromString.h>
+#include <base/extended_types.h>
+#include <base/hex.h>
+
+#include <algorithm>
+#include <string>
+
+using namespace DB;
+using namespace DB::Cas;
+
+namespace
+{
+
+/// A deterministic, non-repeating-byte payload (not all-zero / all-same, so a byte-order or
+/// endianness bug in either hash path would not accidentally cancel out).
+std::string makePayload(size_t size)
+{
+    std::string s;
+    s.reserve(size);
+    for (size_t i = 0; i < size; ++i)
+        s.push_back(static_cast<char>('a' + (i % 23)));
+    return s;
+}
+
+}
+
+TEST(CASBlobHasher, Xxh3StreamingMatchesOneShotAndBlobHashHexOneShot)
+{
+    const std::string payload = makePayload(10000);
+
+    std::string sink_data;
+    std::string streaming_hex;
+    {
+        WriteBufferFromString sink(sink_data);
+        auto hashing = makeBlobHashingWriteBuffer(BlobHashAlgo::XXH3_128, sink);
+
+        /// Feed the payload through several `write()` chunks to exercise the streaming state across
+        /// multiple `nextImpl` flushes, not just a single call.
+        size_t offset = 0;
+        constexpr size_t chunk = 777;
+        while (offset < payload.size())
+        {
+            const size_t n = std::min(chunk, payload.size() - offset);
+            hashing->write(payload.data() + offset, n);
+            offset += n;
+        }
+
+        streaming_hex = hashing->getHashHex();
+        hashing->finalize();
+        sink.finalize();
+    }
+
+    /// The passthrough forwarded every byte unchanged.
+    EXPECT_EQ(sink_data, payload);
+    EXPECT_EQ(streaming_hex.size(), 32u);
+
+    /// xxh3 streaming == xxh3 one-shot (unlike cityHash128, xxh3's streaming digest is defined to
+    /// agree with the one-shot digest -- see `ImplXXH3_128` in `Functions/FunctionsHashing.h`).
+    UInt64 os_low = 0;
+    UInt64 os_high = 0;
+    Cas::xxh3_128_oneshot(payload.data(), payload.size(), os_low, os_high);
+    const std::string one_shot_hex = getHexUIntLowercase(UInt128{os_low, os_high});
+    EXPECT_EQ(streaming_hex, one_shot_hex);
+
+    /// The one-shot re-hash helper must agree with both.
+    EXPECT_EQ(blobHashHexOneShot(BlobHashAlgo::XXH3_128, payload), one_shot_hex);
+}
+
+TEST(CASBlobHasher, CityHash128ByteIdenticalToHashingWriteBuffer)
+{
+    /// Cover payloads both under and over one `DBMS_DEFAULT_HASHING_BLOCK_SIZE` (2048 B) hash block,
+    /// plus exactly at the boundary, since the chunked convention only matters once a payload spans
+    /// more than one block.
+    for (const size_t size : {size_t(100), size_t(2000), size_t(2048), size_t(5000)})
+    {
+        SCOPED_TRACE(size);
+        const std::string payload = makePayload(size);
+
+        /// Reference: today's convention, `HashingWriteBuffer` used directly.
+        std::string ref_sink_data;
+        std::string ref_hex;
+        {
+            WriteBufferFromString ref_sink(ref_sink_data);
+            HashingWriteBuffer ref_hashing(ref_sink);
+            ref_hashing.write(payload.data(), payload.size());
+            ref_hex = getHexUIntLowercase(ref_hashing.getHash());
+            ref_hashing.finalize();
+            ref_sink.finalize();
+        }
+
+        /// The selectable factory, defaulted to CityHash128 -- must be byte-identical.
+        std::string sink_data;
+        std::string hex;
+        {
+            WriteBufferFromString sink(sink_data);
+            auto hashing = makeBlobHashingWriteBuffer(BlobHashAlgo::CityHash128, sink);
+            hashing->write(payload.data(), payload.size());
+            hex = hashing->getHashHex();
+            hashing->finalize();
+            sink.finalize();
+        }
+
+        EXPECT_EQ(hex, ref_hex);
+        EXPECT_EQ(hex.size(), 32u);
+        EXPECT_EQ(sink_data, ref_sink_data);
+        EXPECT_EQ(sink_data, payload);
+
+        /// The one-shot re-hash helper must agree too.
+        EXPECT_EQ(blobHashHexOneShot(BlobHashAlgo::CityHash128, payload), ref_hex);
+    }
+}
+
+TEST(CASBlobHasher, AlgoNameAndParseRoundTrip)
+{
+    EXPECT_EQ(blobHashAlgoName(BlobHashAlgo::CityHash128), "ch128");
+    EXPECT_EQ(blobHashAlgoName(BlobHashAlgo::XXH3_128), "xxh3");
+    EXPECT_EQ(blobHashAlgoName(BlobHashAlgo::Sha256), "sha256");
+
+    EXPECT_EQ(parseBlobHashAlgo("cityhash128"), BlobHashAlgo::CityHash128);
+    EXPECT_EQ(parseBlobHashAlgo("xxh3-128"), BlobHashAlgo::XXH3_128);
+    /// Parses even though it is rejected downstream (config-layer rejection is a later task).
+    EXPECT_EQ(parseBlobHashAlgo("sha256"), BlobHashAlgo::Sha256);
+
+    EXPECT_THROW(parseBlobHashAlgo("bogus"), DB::Exception);
+    EXPECT_THROW(parseBlobHashAlgo("cityHash128"), DB::Exception); // case-sensitive
+    EXPECT_THROW(parseBlobHashAlgo(""), DB::Exception);
+}
+
+TEST(CASBlobHasher, Sha256OneShotGoldenVectors)
+{
+    /// NIST/FIPS 180-2 test vectors, the standard SHA-256 sanity check.
+    EXPECT_EQ(blobHashHexOneShot(BlobHashAlgo::Sha256, "abc"),
+        "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
+    EXPECT_EQ(blobHashHexOneShot(BlobHashAlgo::Sha256, ""),
+        "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855");
+}
+
+TEST(CASBlobHasher, Sha256StreamingMatchesOneShotAndIsPassthrough)
+{
+    /// Bigger than `DBMS_DEFAULT_HASHING_BLOCK_SIZE` (2048 B) so the payload chunks through several
+    /// `nextImpl` flushes, not just a single call.
+    const std::string payload = makePayload(200 * 1024);
+
+    std::string sink_data;
+    std::string streaming_hex;
+    {
+        WriteBufferFromString sink(sink_data);
+        auto hashing = makeBlobHashingWriteBuffer(BlobHashAlgo::Sha256, sink);
+
+        /// Feed the payload through several `write()` chunks to exercise the streaming EVP digest
+        /// across multiple `nextImpl` flushes.
+        size_t offset = 0;
+        constexpr size_t chunk = 4096;
+        while (offset < payload.size())
+        {
+            const size_t n = std::min(chunk, payload.size() - offset);
+            hashing->write(payload.data() + offset, n);
+            offset += n;
+        }
+
+        streaming_hex = hashing->getHashHex();
+        hashing->finalize();
+        sink.finalize();
+    }
+
+    /// The passthrough forwarded every byte unchanged.
+    EXPECT_EQ(sink_data, payload);
+    EXPECT_EQ(streaming_hex.size(), 64u);
+
+    /// SHA-256 streaming == SHA-256 one-shot (like xxh3, unlike cityHash128 -- SHA-256 has no
+    /// chunked convention to preserve).
+    const std::string one_shot_hex = blobHashHexOneShot(BlobHashAlgo::Sha256, payload);
+    EXPECT_EQ(streaming_hex, one_shot_hex);
+}
diff --git a/src/Disks/tests/gtest_cas_blob_indegree.cpp b/src/Disks/tests/gtest_cas_blob_indegree.cpp
new file mode 100644
index 000000000000..e47b7351f063
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_indegree.cpp
@@ -0,0 +1,1000 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <IO/WriteBufferFromString.h>
+#include <Common/Exception.h>
+
+namespace DB::ErrorCodes { extern const int CORRUPTED_DATA; extern const int NOT_IMPLEMENTED; }
+
+using namespace DB::Cas;
+
+namespace
+{
+UInt128 b(uint64_t n) { return UInt128(n); }
+UInt128 s(uint64_t n) { return UInt128(n); }   // source-edge id
+/// A `BlobRef` (CityHash128) for the same literal `n` — every existing test's `BlobDelta.ref` /
+/// `BlobCandidate.ref` / `inDegreeInRuns` argument is a `BlobRef` as of Phase 3 T3.
+BlobRef bh(uint64_t n) { return BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(n))}; }
+
+/// Scale thresholds for the "the run genuinely spans several blocks" sanity assertions below. These are
+/// NOT format constants — the SourceEdge run is a plain NDJSON stream (`CasRecordStreamFormat`) with no
+/// block framing of its own — they only pin the same byte-size scale the (now-deleted, codecs-v3 phase 6)
+/// `CasRunFile` block codec used, so the multi-block-sized fixtures below stay meaningfully large.
+/// (Previously read straight off `CasRunFile.h`'s own `kRunTargetBlockSize`/`kRunHardCapBlockSize`; this
+/// file's `#include` of that header looked removable when `CasRunFile` was deleted in the phase-6 cutover,
+/// but these two thresholds turned out to be the only remaining users — hence the local, explicitly-legacy
+/// copies here instead of a dangling include. Values unchanged.)
+constexpr uint32_t kLegacyBlockSize = 256u * 1024u;
+constexpr uint32_t kLegacyHardCapBlockSize = 1024u * 1024u;
+}
+
+TEST(CASBlobInDegree, FoldStartsFromEmptyPriorGeneration)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// Generation 1 from empty prior: two distinct edges on b1 and one on b2.
+    /// Edge (b1,s1), (b1,s2), (b2,s1) => indeg(b1)=2, indeg(b2)=1.
+    std::vector<BlobDelta> deltas{
+        {bh(1), s(1), false},
+        {bh(1), s(2), false},
+        {bh(2), s(1), false},
+    };
+    std::vector<RunRef> runs;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, /*new*/1, /*attempt*/0, /*shard*/0, deltas, runs);
+    ASSERT_FALSE(runs.empty());
+
+    const auto zero = zeroInDegree(backend, runs);
+    EXPECT_TRUE(zero.empty());   /// nothing at zero yet
+}
+
+TEST(CASBlobInDegree, PlusMinusCancelToZeroDetectsCandidate)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// Gen 1: activate edge (b1,s1) and (b2,s1).
+    std::vector<RunRef> runs1;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, /*attempt*/0, 0,
+        {{bh(1), s(1), false}, {bh(2), s(1), false}}, runs1);
+
+    /// Generation 2 merges prior gen-1 run (resolved via runs1 refs) with removal of (b1,s1): indeg(b1)=0, indeg(b2)=1.
+    std::vector<RunRef> runs2;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1, /*new*/2, /*attempt*/0, 0,
+        {{bh(1), s(1), true}}, runs2);
+
+    const auto zero = zeroInDegree(backend, runs2);
+    ASSERT_EQ(zero.size(), 1u);
+    EXPECT_EQ(zero[0].ref, bh(1));
+}
+
+TEST(CASBlobInDegree, RunsAreByteDeterministic)
+{
+    InMemoryBackend a;
+    InMemoryBackend b2;
+    Layout layout{"pool"};
+    std::vector<RunRef> ra;
+    std::vector<RunRef> rb;
+    /// Same deltas in a DIFFERENT input order must produce the same sealed run bytes (sorted by key).
+    foldDeltasIntoGeneration(a,  layout, /*prior_runs*/{}, 1, /*attempt*/0, 0,
+        {{bh(3), s(1), false}, {bh(1), s(1), false}, {bh(2), s(1), false}}, ra);
+    foldDeltasIntoGeneration(b2, layout, /*prior_runs*/{}, 1, /*attempt*/0, 0,
+        {{bh(1), s(1), false}, {bh(2), s(1), false}, {bh(3), s(1), false}}, rb);
+    const auto ga = a.get(layout.blobTargetRunKey(1, /*attempt*/0, 0, 0));
+    const auto gb = b2.get(layout.blobTargetRunKey(1, /*attempt*/0, 0, 0));
+    ASSERT_TRUE(ga.has_value());
+    ASSERT_TRUE(gb.has_value());
+    EXPECT_EQ(ga->bytes, gb->bytes);
+    ASSERT_EQ(ra.size(), 1u);
+    ASSERT_EQ(rb.size(), 1u);
+    EXPECT_EQ(ra[0].checksum, rb[0].checksum);
+}
+
+TEST(CASBlobInDegree, SameEdgeActivatedTwiceCountsOnce)
+{
+    /// Idempotency: activating the same (blob_hash, source_id) twice must not double-count.
+    /// The source-edge set is a SET, not a counter — re-adding the same edge is a no-op.
+    /// indeg(b1) must be 1 after both activations, not 2.
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+    std::vector<BlobDelta> deltas{
+        {bh(1), s(1), false},   // activate (b1,s1)
+        {bh(1), s(1), false},   // same edge again — must deduplicate
+    };
+    std::vector<RunRef> runs;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, /*attempt*/0, 0, deltas, runs);
+    ASSERT_FALSE(runs.empty());
+
+    const int64_t deg = DB::Cas::tests::inDegreeInRuns(backend, runs, bh(1));
+    EXPECT_EQ(deg, 1);   /// deduplicated, not 2
+
+    const auto zero = zeroInDegree(backend, runs);
+    EXPECT_TRUE(zero.empty());   /// b1 still has an active edge
+}
+
+TEST(CASBlobInDegree, FoldDeltaByteEqualReplayAdopts)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+    std::vector<BlobDelta> deltas{{bh(1), s(1), false}};
+    std::vector<RunRef> runs1;
+    std::vector<RunRef> runs2;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, /*attempt*/7, /*shard*/0, deltas, runs1);
+    /// Same inputs, same attempt => byte-identical run already present => adopt, no throw.
+    EXPECT_NO_THROW(foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, /*attempt*/7, /*shard*/0, deltas, runs2));
+    EXPECT_EQ(runs1, runs2);
+}
+
+TEST(CASBlobInDegree, FoldDeltaDivergentBytesThrowsCorrupted)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+    /// Pre-occupy the run key (attempt 7) with junk, then fold => divergent => CORRUPTED_DATA.
+    backend.putIfAbsent(layout.blobTargetRunKey(1, /*attempt*/7, /*shard*/0, /*seq*/0), "not-a-valid-run");
+    std::vector<BlobDelta> deltas{{bh(1), s(1), false}};
+    std::vector<RunRef> runs;
+    EXPECT_THROW(foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, /*attempt*/7, /*shard*/0, deltas, runs),
+                 DB::Exception);
+}
+
+/// ==== two-cursor settlement merge (retired-in-snapshot T3, spec §2.1/§3) ====
+///
+/// The retired input is no longer a separate `prior_retired` vector — the prior generation's `kCondemned`
+/// rows RIDE the source-edge run at the zero-sentinel key. These helpers build such a prior run directly
+/// (via the sorted-NDJSON `SourceEdgeRunWriter`, codecs-v3 phase 5) and decode a run for assertions.
+
+namespace
+{
+
+/// A `kCondemned` sentinel record for `h` at the zero source_id, carrying the condemned incarnation.
+SourceEdgeRecord condemnedRec(UInt128 h, const CondemnedRow & row)
+{
+    return SourceEdgeRecord{.ref = BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h)},
+                            .source_id = UInt128{0}, .marker = kCondemned,
+                            .delete_pending = row.delete_pending, .token = row.token,
+                            .size = row.size, .condemn_round = row.condemn_round};
+}
+
+/// An active-edge record (`kEdgeActive`) for `h` at source `sid`.
+SourceEdgeRecord edgeRec(UInt128 h, UInt128 sid)
+{
+    return SourceEdgeRecord{.ref = BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h)},
+                            .source_id = sid, .marker = kEdgeActive};
+}
+
+/// head_blob / peek_head stub: present with a fixed token/size.
+std::function<std::optional<HeadResult>(const BlobRef &)> headPresent(const String & tok, uint64_t size)
+{
+    return [tok, size](const BlobRef &) -> std::optional<HeadResult>
+    {
+        HeadResult hr;
+        hr.exists = true;
+        hr.size = size;
+        hr.token = Token{.value = tok, .type = TokenType::Emulated};
+        return hr;
+    };
+}
+
+/// A `CondemnedRow` mirroring the old `entry(hash, condemn_round)` fixture (token "t", size 1).
+CondemnedRow condemnedRowFor(uint64_t condemn_round, const String & tok = "t",
+                             bool delete_pending = false, uint64_t size = 1)
+{
+    return CondemnedRow{.delete_pending = delete_pending,
+                        .token = Token{.value = tok, .type = TokenType::Emulated},
+                        .size = size, .condemn_round = condemn_round};
+}
+
+/// Build a source-edge run (`kSourceEdgeKeySchema128`) carrying the given `kCondemned` sentinel rows
+/// and surviving edges, write it under `blobTargetRunKey(gen, attempt, shard, 0)`, and return its
+/// `RunRef`. Rows are emitted in (blob_hash, source_id) order (sentinels at source_id 0 sort first
+/// per blob).
+RunRef writeSourceEdgeRun(InMemoryBackend & backend, const Layout & layout,
+                          uint64_t gen, uint64_t attempt, uint64_t shard,
+                          const std::vector<std::pair<UInt128, CondemnedRow>> & condemned,
+                          const std::vector<std::pair<UInt128, UInt128>> & edges = {})
+{
+    std::vector<SourceEdgeRecord> recs;
+    for (const auto & [h, row] : condemned)
+        recs.push_back(condemnedRec(h, row));
+    for (const auto & [h, sid] : edges)
+        recs.push_back(edgeRec(h, sid));
+    /// The writer requires non-decreasing (ref, source_id) order (sentinels at source_id 0 sort first
+    /// per blob, exactly reproducing the old raw-key order).
+    std::stable_sort(recs.begin(), recs.end(), [](const SourceEdgeRecord & a, const SourceEdgeRecord & bb)
+    {
+        if (a.ref < bb.ref)
+            return true;
+        if (bb.ref < a.ref)
+            return false;
+        return a.source_id < bb.source_id;
+    });
+
+    DB::WriteBufferFromOwnString out;
+    SourceEdgeRunWriter writer(out);
+    for (const auto & rec : recs)
+        writer.append(rec);
+    writer.finish();
+    out.finalize();
+
+    const String bytes = out.str();
+    const String key = layout.blobTargetRunKey(gen, attempt, shard, 0);
+    backend.putIfAbsent(key, bytes);
+    return RunRef{.key = key, .checksum = sourceEdgeRunChecksum(bytes), .shard = shard, .generation = gen};
+}
+
+struct DecodedRun
+{
+    std::vector<std::pair<UInt128, CondemnedRow>> condemned;   /// (blob_hash, row)
+    std::vector<UInt128> zero_markers;                         /// blob hashes with a zero-transition marker
+    std::vector<std::pair<UInt128, UInt128>> edges;            /// (blob_hash, source_id)
+};
+
+DecodedRun decodeRun(InMemoryBackend & backend, const RunRef & run)
+{
+    DecodedRun d;
+    auto r = openSourceEdgeRun(backend, run.key);
+    /// Every run this test helper decodes is CityHash128 (16-byte), so `.toU128()` is a
+    /// provably-exact round trip.
+    String k;
+    String p;
+    while (r.next(k, p))
+    {
+        BlobRef bh_ref;
+        UInt128 sid;
+        SourceEdgeKeyCodec::parse(k, bh_ref, sid);   // throws CORRUPTED_DATA on a malformed key (fail-closed)
+        const UInt128 bh = bh_ref.digest.toU128();
+        EXPECT_FALSE(p.empty());
+        if (p.empty())
+            continue;
+        if (p[0] == kCondemned)
+            d.condemned.emplace_back(bh, decodeCondemnedRow(p));
+        else if (p[0] == kZeroMarker)
+            d.zero_markers.push_back(bh);
+        else if (p[0] == kEdgeActive)
+            d.edges.emplace_back(bh, sid);
+        else
+            ADD_FAILURE() << "unknown run row type";
+    }
+    return d;
+}
+
+}
+
+/// Per-consumer whole-file seal-checksum RED tests (codecs-v3 phase 5, Task 6): a run whose ROWS are
+/// well-formed (so `cursor.advance()` never aborts first) but whose `RunRef.checksum` disagrees with the
+/// stored bytes must fail closed at each deletion-deriving consumer BEFORE any decision is produced. The
+/// stored bytes are the valid run; only the seal checksum handed to the consumer is wrong.
+TEST(CASBlobInDegree, FoldSealChecksumMismatchFailsClosed)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+    const RunRef good = writeSourceEdgeRun(backend, layout, /*gen*/1, /*attempt*/0, /*shard*/0,
+                                           /*condemned*/{}, /*edges*/{{b(1), s(1)}});
+    RunRef bad = good;
+    bad.checksum = good.checksum + 1;   /// rows still parse; only the seal disagrees
+    std::vector<RunRef> prior{bad};
+    std::vector<RunRef> out;
+    /// A delta on a DIFFERENT blob forces the two-cursor merge to stream the prior run to completion, so
+    /// the end-of-segment verifyAgainst fires (not a row-invariant abort).
+    EXPECT_THROW(
+        foldDeltasIntoGeneration(backend, layout, prior, /*new*/2, /*attempt*/0, /*shard*/0,
+                                 std::vector<BlobDelta>{{bh(2), s(1), false}}, out),
+        DB::Exception);
+}
+
+TEST(CASBlobInDegree, ZeroInDegreeSealChecksumMismatchFailsClosed)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+    const RunRef good = writeSourceEdgeRun(backend, layout, /*gen*/1, /*attempt*/0, /*shard*/0,
+                                           /*condemned*/{}, /*edges*/{{b(1), s(1)}});
+    RunRef bad = good;
+    bad.checksum = good.checksum + 1;
+    std::vector<RunRef> runs{bad};
+    EXPECT_THROW(zeroInDegree(backend, runs), DB::Exception);
+}
+
+TEST(CASThreeCursorMerge, FloorBoundary)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// Gen 1's run holds one unrelated surviving edge (b9) plus the carried kCondemned rows for A=b1
+    /// (condemned round 2) and B=b2 (round 3); neither A nor B has any edge (in-degree 0 by definition).
+    /// current_round = 3: strictly-below graduates, at-the-current-round stays.
+    const RunRef gen1 = writeSourceEdgeRun(backend, layout, /*gen*/1, 0, 0,
+        {{b(1), condemnedRowFor(2)}, {b(2), condemnedRowFor(3)}}, {{b(9), s(1)}});
+
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{gen1}, 2, 0, 0, {}, runs2,
+        /*current_round*/3, /*condemn_round*/4, /*head_blob*/{}, /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr);
+
+    /// Two-phase graduation: the floor-passed entry is REPUBLISHED pending (still in the list);
+    /// its physical delete belongs to the NEXT pass.
+    ASSERT_EQ(rmr.graduated.size(), 1u);
+    EXPECT_EQ(rmr.graduated[0].ref, bh(1));
+    EXPECT_TRUE(rmr.graduated[0].delete_pending);
+    ASSERT_EQ(rmr.still_retired.size(), 2u);
+    EXPECT_EQ(rmr.still_retired[0].ref, bh(1));
+    EXPECT_TRUE(rmr.still_retired[0].delete_pending);
+    EXPECT_EQ(rmr.still_retired[1].ref, bh(2));
+    EXPECT_FALSE(rmr.still_retired[1].delete_pending);
+    EXPECT_EQ(rmr.still_retired[1].condemn_round, 3u);   /// carried unchanged, not re-stamped
+    EXPECT_TRUE(rmr.spared.empty());
+    EXPECT_TRUE(rmr.redelete.empty());
+
+    /// still_retired mirrors exactly the kCondemned rows written into the output run, in order.
+    const DecodedRun out = decodeRun(backend, runs2[0]);
+    ASSERT_EQ(out.condemned.size(), 2u);
+    EXPECT_EQ(out.condemned[0].first, b(1));
+    EXPECT_TRUE(out.condemned[0].second.delete_pending);
+    EXPECT_EQ(out.condemned[1].first, b(2));
+    EXPECT_FALSE(out.condemned[1].second.delete_pending);
+    EXPECT_TRUE(out.zero_markers.empty());
+}
+
+TEST(CASThreeCursorMerge, PendingRedeletesAndDrops)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// A row the PRIOR pass published as delete_pending (carried on gen 1's run): this pass hands it to
+    /// `redelete` (executed pre-CAS by the caller) and drops it from the output run.
+    const RunRef gen1 = writeSourceEdgeRun(backend, layout, /*gen*/1, 0, 0,
+        {{b(1), condemnedRowFor(1, "t", /*delete_pending*/true)}});
+
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{gen1}, 2, 0, 0, {}, runs2,
+        /*current_round*/9, /*condemn_round*/9, /*head_blob*/{}, /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr);
+
+    ASSERT_EQ(rmr.redelete.size(), 1u);
+    EXPECT_EQ(rmr.redelete[0].ref, bh(1));
+    EXPECT_TRUE(rmr.still_retired.empty());
+    EXPECT_TRUE(rmr.graduated.empty());
+    EXPECT_TRUE(rmr.spared.empty());
+
+    /// The redeleted blob leaves the run entirely (no sentinel carried, no zero marker — untouched).
+    const DecodedRun out = decodeRun(backend, runs2[0]);
+    EXPECT_TRUE(out.condemned.empty());
+    EXPECT_TRUE(out.zero_markers.empty());
+}
+
+TEST(CASThreeCursorMerge, RecoverySpares)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// A (=b1) is retired at round 1 and would long since have graduated (current_round = 5) — but this
+    /// pass's delta adds an edge to it: recovery WINS over graduation, the entry is dropped as spared.
+    const RunRef gen1 = writeSourceEdgeRun(backend, layout, /*gen*/1, 0, 0, {{b(1), condemnedRowFor(1)}});
+
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{gen1}, 2, 0, 0, {{bh(1), s(1), false}}, runs2,
+        /*current_round*/5, /*condemn_round*/6, /*head_blob*/{}, /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr);
+
+    ASSERT_EQ(rmr.spared.size(), 1u);
+    EXPECT_EQ(rmr.spared[0].ref, bh(1));
+    EXPECT_TRUE(rmr.graduated.empty());
+    EXPECT_TRUE(rmr.still_retired.empty());
+
+    /// b1 recovered its edge: the output run carries the surviving edge and no sentinel for it.
+    const DecodedRun out = decodeRun(backend, runs2[0]);
+    EXPECT_TRUE(out.condemned.empty());
+    ASSERT_EQ(out.edges.size(), 1u);
+    EXPECT_EQ(out.edges[0].first, b(1));
+}
+
+TEST(CASThreeCursorMerge, NewCandidateCondemned)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// Gen 1: C (=b3) has one edge. Gen 2 removes it => transition to zero, not retired =>
+    /// condemned with the head-captured token at THIS pass's condemn_round.
+    std::vector<RunRef> runs1;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, 0, 0, {{bh(3), s(1), false}}, runs1);
+
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1, 2, 0, 0, {{bh(3), s(1), true}}, runs2,
+        /*current_round*/0, /*condemn_round*/7, headPresent("t9", 42), /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr);
+
+    ASSERT_EQ(rmr.still_retired.size(), 1u);
+    EXPECT_EQ(rmr.still_retired[0].ref, bh(3));
+    EXPECT_EQ(rmr.still_retired[0].token.value, "t9");
+    EXPECT_EQ(rmr.still_retired[0].size, 42u);
+    EXPECT_EQ(rmr.still_retired[0].condemn_round, 7u);
+    EXPECT_TRUE(rmr.graduated.empty());
+    EXPECT_TRUE(rmr.spared.empty());
+
+    /// The fresh condemn is emitted as a kCondemned row (not a zero marker) into the output run.
+    const DecodedRun out = decodeRun(backend, runs2[0]);
+    ASSERT_EQ(out.condemned.size(), 1u);
+    EXPECT_EQ(out.condemned[0].first, b(3));
+    EXPECT_EQ(out.condemned[0].second.token.value, "t9");
+    EXPECT_TRUE(out.zero_markers.empty());
+}
+
+TEST(CASThreeCursorMerge, AbsentBlobNotCondemned)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// Same transition-to-zero as above, but the blob object is already gone at condemn time:
+    /// nothing to delete later, so no entry is minted — a plain zero marker is emitted instead.
+    std::vector<RunRef> runs1;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, 0, 0, {{bh(3), s(1), false}}, runs1);
+
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1, 2, 0, 0, {{bh(3), s(1), true}}, runs2,
+        /*current_round*/0, /*condemn_round*/7,
+        [](const BlobRef &) -> std::optional<HeadResult> { return std::nullopt; }, /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr);
+
+    EXPECT_TRUE(rmr.still_retired.empty());
+    EXPECT_TRUE(rmr.graduated.empty());
+    EXPECT_TRUE(rmr.spared.empty());
+
+    const DecodedRun out = decodeRun(backend, runs2[0]);
+    EXPECT_TRUE(out.condemned.empty());
+    ASSERT_EQ(out.zero_markers.size(), 1u);
+    EXPECT_EQ(out.zero_markers[0], b(3));
+}
+
+TEST(CASThreeCursorMerge, SnapshotEdgesUnperturbedByRetired)
+{
+    /// Retired-in-snapshot changes the byte-invariant: the retired machinery now WRITES kCondemned
+    /// sentinel rows into the run, so a retired-engaged run is no longer byte-identical to a plain one.
+    /// The preserved invariant (spec §2.1) is narrower: the retired machinery touches ONLY the sentinel
+    /// namespace — the surviving EDGE rows are byte-identical to a plain fold of the same deltas.
+    InMemoryBackend plain;
+    InMemoryBackend engaged;
+    Layout layout{"pool"};
+
+    std::vector<RunRef> r1;
+    foldDeltasIntoGeneration(plain, layout, /*prior_runs*/{}, 1, 0, 0,
+        {{bh(1), s(1), false}, {bh(2), s(1), false}, {bh(2), s(2), true}}, r1);
+
+    /// Engaged: the SAME deltas, but the prior run carries retired rows for b1 (which the delta re-edges
+    /// => spared) and b5 (no edge => graduates past the floor).
+    const RunRef prior = writeSourceEdgeRun(engaged, layout, /*gen*/1, 0, 0,
+        {{b(1), condemnedRowFor(1)}, {b(5), condemnedRowFor(2)}});
+    std::vector<RunRef> r2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(engaged, layout, /*prior_runs*/{prior}, 2, 0, 0,
+        {{bh(1), s(1), false}, {bh(2), s(1), false}, {bh(2), s(2), true}}, r2,
+        /*current_round*/9, /*condemn_round*/3, headPresent("t", 1), /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr);
+
+    const DecodedRun plain_run = decodeRun(plain, r1[0]);
+    const DecodedRun engaged_run = decodeRun(engaged, r2[0]);
+    EXPECT_EQ(plain_run.edges, engaged_run.edges);   /// edge rows byte-identical
+    EXPECT_TRUE(plain_run.condemned.empty());
+    /// The engaged run carries only the retired sentinel(s) on top: b1 spared (no row), b5 graduated.
+    ASSERT_EQ(engaged_run.condemned.size(), 1u);
+    EXPECT_EQ(engaged_run.condemned[0].first, b(5));
+    EXPECT_TRUE(engaged_run.condemned[0].second.delete_pending);
+}
+
+TEST(CASTwoCursorMerge, CarriedSentinelIsNotATouch)
+{
+    /// Gen 1 condemns b (a real +edge/-edge net-to-zero with head_blob present) -> a kCondemned row. Gen 2
+    /// has NO deltas at all: the carried row must (a) survive byte-identically, (b) emit no zero marker,
+    /// (c) never call peek_head (a carried sentinel is not a touch).
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// Gen 1: (b,s1) added then removed => net-to-zero => fresh condemn at round 5 (token "tok", size 7).
+    std::vector<RunRef> runs1;
+    RetiredMergeResult rmr1;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, 0, 0,
+        {{bh(2), s(1), false}, {bh(2), s(1), true}}, runs1,
+        /*current_round*/0, /*condemn_round*/5, headPresent("tok", 7), /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr1);
+    ASSERT_EQ(rmr1.still_retired.size(), 1u);
+    {
+        const DecodedRun g1 = decodeRun(backend, runs1[0]);
+        ASSERT_EQ(g1.condemned.size(), 1u);
+        EXPECT_EQ(g1.condemned[0].first, b(2));
+        EXPECT_TRUE(g1.zero_markers.empty());   /// a condemned blob emits kCondemned, never a zero marker
+    }
+
+    /// Gen 2: empty deltas, current_round 1 (< 5 => b carries, does not graduate). peek_head must NOT fire.
+    size_t peek_calls = 0;
+    auto peek = [&](const BlobRef &) -> std::optional<HeadResult> { ++peek_calls; return {}; };
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr2;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1, 2, 0, 0, {}, runs2,
+        /*current_round*/1, /*condemn_round*/6, /*head_blob*/{}, peek, /*confirm_condemned_marker*/{}, &rmr2);
+
+    EXPECT_EQ(peek_calls, 0u);
+    ASSERT_EQ(rmr2.still_retired.size(), 1u);
+    EXPECT_EQ(rmr2.still_retired[0].ref, bh(2));
+    EXPECT_EQ(rmr2.still_retired[0].condemn_round, 5u);   /// carried unchanged
+    EXPECT_TRUE(rmr2.graduated.empty());
+
+    const DecodedRun g2 = decodeRun(backend, runs2[0]);
+    ASSERT_EQ(g2.condemned.size(), 1u);
+    EXPECT_EQ(g2.condemned[0].first, b(2));
+    EXPECT_EQ(g2.condemned[0].second.token.value, "tok");
+    EXPECT_EQ(g2.condemned[0].second.size, 7u);
+    EXPECT_TRUE(g2.zero_markers.empty());
+}
+
+TEST(CASTwoCursorMerge, MalformedRunFailsClosed)
+{
+    Layout layout{"pool"};
+
+    /// (1) An active edge at the reserved sentinel source_id 0 -> the merge cursor fails closed.
+    {
+        InMemoryBackend backend;
+        DB::WriteBufferFromOwnString out;
+        SourceEdgeRunWriter writer(out);
+        writer.append(edgeRec(1, UInt128{0}));   // edge at sentinel key
+        writer.finish();
+        out.finalize();
+        const String bytes = out.str();
+        const RunRef bad{.key = layout.blobTargetRunKey(1, 0, 0, 0),
+                         .checksum = sourceEdgeRunChecksum(bytes), .shard = 0, .generation = 1};
+        backend.putIfAbsent(bad.key, bytes);
+
+        std::vector<RunRef> runs2;
+        EXPECT_THROW(foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{bad}, 2, 0, 0, {}, runs2),
+                     DB::Exception);
+    }
+
+    /// (2) Two sentinel rows for one blob -> duplicate sentinel -> the merge cursor fails closed.
+    {
+        InMemoryBackend backend;
+        DB::WriteBufferFromOwnString out;
+        SourceEdgeRunWriter writer(out);
+        /// Same (b,0) key twice (equal keys are allowed by the writer) — two condemned sentinels for b1.
+        writer.append(condemnedRec(1, condemnedRowFor(1)));
+        writer.append(condemnedRec(1, condemnedRowFor(2)));
+        writer.finish();
+        out.finalize();
+        const String bytes = out.str();
+        const RunRef bad{.key = layout.blobTargetRunKey(1, 0, 0, 0),
+                         .checksum = sourceEdgeRunChecksum(bytes), .shard = 0, .generation = 1};
+        backend.putIfAbsent(bad.key, bytes);
+
+        std::vector<RunRef> runs2;
+        EXPECT_THROW(foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{bad}, 2, 0, 0, {}, runs2),
+                     DB::Exception);
+    }
+}
+
+/// A prior run spanning several blocks folds correctly with the streaming prior cursor AND the backend
+/// sees only block-bounded ranged/stream requests for it — never a whole-object get of the prior run
+/// key. Byte-reproducibility of the merged output is the load-bearing canary (the merge logic is
+/// unchanged; only the prior cursor's byte source moved from materialize-whole to stream).
+TEST(CASBlobInDegree, FoldStreamsPriorRunBlockBounded)
+{
+    using DB::Cas::tests::CountingBackend;
+    CountingBackend backend;
+    /// InMemory oracle: the SAME two folds against a plain backend must yield byte-identical runs —
+    /// the streaming cursor changes I/O shape, not bytes.
+    InMemoryBackend oracle;
+    Layout layout{"pool"};
+
+    /// Gen 1 from empty prior: enough edges that the SourceEdge run spills across many 256KB blocks.
+    /// Each record is 4 + 32(key) + 4 + 1(payload) = 41 bytes, so ~20000 edges is ~820KB => several
+    /// blocks under the default block_size, exercising the multi-block streaming path in the fold.
+    std::vector<BlobDelta> gen1;
+    gen1.reserve(20000);
+    for (uint64_t i = 0; i < 20000; ++i)
+        gen1.push_back({bh(i), s(1), false});
+
+    std::vector<RunRef> runs1_c;
+    std::vector<RunRef> runs1_o;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, 0, 0, gen1, runs1_c);
+    foldDeltasIntoGeneration(oracle, layout, /*prior_runs*/{}, 1, 0, 0, gen1, runs1_o);
+
+    const String gen1_run_key = layout.blobTargetRunKey(1, 0, 0, 0);
+    const auto gen1_run = backend.get(gen1_run_key);
+    ASSERT_TRUE(gen1_run.has_value());
+    const String gen1_run_bytes = gen1_run->bytes;
+    /// Sanity: the prior run really spans several blocks (else the block-bounded assertions are
+    /// vacuous). Blocks seal at kLegacyBlockSize (256KB); ~820KB is 3-4 blocks.
+    ASSERT_GT(gen1_run_bytes.size(), static_cast<size_t>(kLegacyBlockSize) * 3);
+
+    /// Reset counters and fold gen 2 with a small delta: remove one edge and add another. The prior
+    /// gen-1 run must be consumed via the streaming cursor (head + tail get + body getStream + per-seq
+    /// head probe), NEVER a whole-object get.
+    backend.resetCounts();
+    std::vector<BlobDelta> gen2{{bh(0), s(1), true}, {bh(19999), s(2), false}};
+    std::vector<RunRef> runs2_c;
+    std::vector<RunRef> runs2_o;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1_c, 2, 0, 0, gen2, runs2_c);
+    foldDeltasIntoGeneration(oracle, layout, /*prior_runs*/runs1_o, 2, 0, 0, gen2, runs2_o);
+
+    /// Byte-reproducibility canary: streaming and materialized folds produce identical output bytes.
+    const String gen2_run_key = layout.blobTargetRunKey(2, 0, 0, 0);
+    const auto gen2_c = backend.get(gen2_run_key);
+    const auto gen2_o = oracle.get(gen2_run_key);
+    ASSERT_TRUE(gen2_c.has_value());
+    ASSERT_TRUE(gen2_o.has_value());
+    EXPECT_EQ(gen2_c->bytes, gen2_o->bytes);
+    ASSERT_EQ(runs2_c.size(), 1u);
+    ASSERT_EQ(runs2_o.size(), 1u);
+    EXPECT_EQ(runs2_c[0].checksum, runs2_o[0].checksum);
+
+    /// The core assertion: no whole-object get of the prior run key — every read carried a Range or a
+    /// stream (the resident-memory proof at the seam).
+    EXPECT_EQ(backend.wholeGetCount(gen1_run_key), 0u);
+    /// The cursor opened the prior run's segment via the streaming reader (head + tail get + getStream).
+    EXPECT_GE(backend.getStreamCount(gen1_run_key), 1u);
+    /// Every ranged-get window on the prior run stays within one block + the footer allowance. This
+    /// bound is strict here because the prior run's footer fits inside the fixed tail probe (only very
+    /// large runs — ~13k blocks — spill the footer past the probe and add one exact-footer get; a note
+    /// for that regime lives in the streaming reader's open comment).
+    EXPECT_LE(backend.maxRangedGetLen(gen1_run_key),
+              static_cast<uint64_t>(kLegacyHardCapBlockSize) + 64u * 1024u);
+    /// Streaming open touches the prior run's tail probe (and at most one exact-footer get); it is never
+    /// re-materialized whole.
+    EXPECT_LE(backend.getCount(gen1_run_key), 2u);
+}
+
+/// The preview consumer `zeroInDegree` streams a multi-block run instead of materializing it whole: the
+/// backend sees only block-bounded ranged/stream requests for the run key (never a whole-object get), and
+/// the candidate set equals the pre-change (borrowed-mode) result. Byte-parity against an InMemory oracle
+/// is the load-bearing canary — the scan logic is unchanged; only the byte source moved to the stream.
+TEST(CASBlobInDegree, ZeroInDegreeStreamsBlockBounded)
+{
+    using DB::Cas::tests::CountingBackend;
+    CountingBackend backend;
+    InMemoryBackend oracle;
+    Layout layout{"pool"};
+
+    /// Gen 1 from empty prior: ~20000 active edges spill the SourceEdge run across several 256KB blocks.
+    std::vector<BlobDelta> gen1;
+    gen1.reserve(20000);
+    for (uint64_t i = 0; i < 20000; ++i)
+        gen1.push_back({bh(i), s(1), false});
+
+    std::vector<RunRef> runs1_c;
+    std::vector<RunRef> runs1_o;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, 0, 0, gen1, runs1_c);
+    foldDeltasIntoGeneration(oracle, layout, /*prior_runs*/{}, 1, 0, 0, gen1, runs1_o);
+
+    /// Gen 2 removes every edge on two of the blobs => two zero-transition markers in the gen-2 run,
+    /// which is itself multi-block (the surviving-edge rows still span blocks).
+    std::vector<BlobDelta> gen2{{bh(0), s(1), true}, {bh(19999), s(1), true}};
+    std::vector<RunRef> runs2_c;
+    std::vector<RunRef> runs2_o;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1_c, 2, 0, 0, gen2, runs2_c);
+    foldDeltasIntoGeneration(oracle, layout, /*prior_runs*/runs1_o, 2, 0, 0, gen2, runs2_o);
+
+    const String gen2_run_key = layout.blobTargetRunKey(2, 0, 0, 0);
+    const auto gen2_run = backend.get(gen2_run_key);
+    ASSERT_TRUE(gen2_run.has_value());
+    /// Sanity: the run genuinely spans several blocks (else the block-bounded assertions are vacuous).
+    ASSERT_GT(gen2_run->bytes.size(), static_cast<size_t>(kLegacyBlockSize) * 3);
+
+    backend.resetCounts();
+    const auto zero_c = zeroInDegree(backend, runs2_c);
+    const auto zero_o = zeroInDegree(oracle, runs2_o);
+
+    /// Equivalence with the borrowed-mode (InMemory oracle) result: same candidates, in the same order.
+    ASSERT_EQ(zero_c.size(), zero_o.size());
+    ASSERT_EQ(zero_c.size(), 2u);
+    for (size_t i = 0; i < zero_c.size(); ++i)
+        EXPECT_EQ(zero_c[i].ref, zero_o[i].ref);
+
+    /// The core assertion: no whole-object get of the run key — every read carried a Range or a stream.
+    EXPECT_EQ(backend.wholeGetCount(gen2_run_key), 0u);
+    /// The scan opened the run via the streaming reader (head + tail get + getStream).
+    EXPECT_GE(backend.getStreamCount(gen2_run_key), 1u);
+    /// Every ranged-get window stays within one block + the footer allowance (the seam memory bound).
+    EXPECT_LE(backend.maxRangedGetLen(gen2_run_key),
+              static_cast<uint64_t>(kLegacyHardCapBlockSize) + 64u * 1024u);
+    /// Streaming open touches the tail probe (and at most one exact-footer get); never re-materialized whole.
+    EXPECT_LE(backend.getCount(gen2_run_key), 2u);
+}
+
+/// ==== kCondemned row codec + typed source-edge open (retired-in-snapshot T2, spec §2.1) ====
+
+TEST(CASCondemnedRow, RoundTripAllTokenTypes)
+{
+    for (auto type : {DB::Cas::TokenType::ETag, DB::Cas::TokenType::Generation, DB::Cas::TokenType::Emulated})
+    {
+        DB::Cas::CondemnedRow row;
+        row.delete_pending = (type == DB::Cas::TokenType::Generation);
+        row.marker_confirmed = (type == DB::Cas::TokenType::Emulated);
+        row.token = DB::Cas::Token{.value = "etag-abc-123", .type = type};
+        row.size = 4096;
+        row.condemn_round = 7;
+        const auto bytes = DB::Cas::encodeCondemnedRow(row);
+        ASSERT_EQ(bytes[0], DB::Cas::kCondemned);
+        EXPECT_EQ(DB::Cas::decodeCondemnedRow(bytes), row);
+    }
+}
+
+TEST(CASCondemnedRow, UnknownFlagBitsFailClosed)
+{
+    DB::Cas::CondemnedRow row;
+    row.token = DB::Cas::Token{.value = "t", .type = DB::Cas::TokenType::ETag};
+    auto bytes = DB::Cas::encodeCondemnedRow(row);
+    bytes[1] = 4;   // flags byte: only bits 0 (delete_pending) and 1 (marker_confirmed) are defined
+    EXPECT_THROW(DB::Cas::decodeCondemnedRow(bytes), DB::Exception);
+}
+
+TEST(CASCondemnedRow, UnknownTokenTypeFailsClosed)
+{
+    DB::Cas::CondemnedRow row;
+    row.token = DB::Cas::Token{.value = "t", .type = DB::Cas::TokenType::ETag};
+    auto bytes = DB::Cas::encodeCondemnedRow(row);
+    bytes[2] = 99;   // token_type byte (offset: [0]=0x02 [1]=flags [2]=token_type)
+    EXPECT_THROW(DB::Cas::decodeCondemnedRow(bytes), DB::Exception);
+}
+
+TEST(CASCondemnedRow, TruncatedPayloadFailsClosed)
+{
+    DB::Cas::CondemnedRow row;
+    row.token = DB::Cas::Token{.value = "0123456789", .type = DB::Cas::TokenType::ETag};
+    auto bytes = DB::Cas::encodeCondemnedRow(row);
+    bytes.resize(bytes.size() - 3);   // token bytes shorter than declared token_len
+    EXPECT_THROW(DB::Cas::decodeCondemnedRow(bytes), DB::Exception);
+}
+
+TEST(CASSourceEdgeRun, SourceEdgeIdZeroIsReserved)
+{
+    /// The zero source_id is the sentinel namespace; producers fail closed on a zero hash
+    /// (probability 2^-128 — the check documents the reservation).
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            DB::Cas::assertValidSourceEdgeId(UInt128{0});
+        },
+        "source_id 0 is the reserved sentinel key");
+    EXPECT_NO_THROW(DB::Cas::assertValidSourceEdgeId(UInt128{1}));
+}
+
+/// ==== schema 3 key codec (Phase 3 T3, mixed-algo pools) ====
+
+TEST(CASSourceEdgeKeySchema3, MixedWidthKeysOrderAlgoFirst)
+{
+    const BlobDigest d16 = BlobDigest::fromU128((UInt128(0xFFFFFFFFFFFFFFFFULL) << 64) | 0xFFULL);
+    BlobDigest d32{};                                    /// sha256 digest starting 0x00,0x01 — small bytes
+    d32.bytes[1] = 0x01;
+    const BlobRef ch{BlobHashAlgo::CityHash128, d16};    /// algo=1, digest all-FF prefix
+    const BlobRef sh{BlobHashAlgo::Sha256, d32};         /// algo=3, tiny digest
+    const String k_ch = SourceEdgeKeyCodec::key(ch, UInt128(7));   /// 33 bytes
+    const String k_sh = SourceEdgeKeyCodec::key(sh, UInt128(7));   /// 49 bytes
+    EXPECT_EQ(k_ch.size(), 33u);
+    EXPECT_EQ(k_sh.size(), 49u);
+    /// algo byte decides BEFORE any digest byte can: ch128(1) < sha256(3) even though the ch128
+    /// digest bytes are all 0xFF and the sha256 digest bytes are almost all zero.
+    EXPECT_LT(k_ch, k_sh);
+    /// sentinel-first inside one blob group:
+    EXPECT_LT(SourceEdgeKeyCodec::key(ch, UInt128(0)), k_ch);
+}
+
+TEST(CASSourceEdgeKeySchema3, ParseFailsClosed)
+{
+    BlobRef r; UInt128 sid;
+    String k = SourceEdgeKeyCodec::key(BlobRef{BlobHashAlgo::XXH3_128, BlobDigest::fromU128(UInt128(5))}, UInt128(9));
+    SourceEdgeKeyCodec::parse(k, r, sid);
+    EXPECT_EQ(r.algo, BlobHashAlgo::XXH3_128);
+    EXPECT_EQ(r.digest.toU128(), UInt128(5));
+    EXPECT_EQ(sid, UInt128(9));
+    k[0] = static_cast<char>(99);                        /// unknown algo byte
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NOT_IMPLEMENTED, [&]{ SourceEdgeKeyCodec::parse(k, r, sid); });
+    k[0] = static_cast<char>(1);                         /// known algo, wrong length (33 expected, this is 33 — truncate)
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]{ SourceEdgeKeyCodec::parse(std::string_view(k).substr(0, 20), r, sid); });
+}
+
+TEST(CASBlobInDegree, TwoAlgoFoldSettlesBothInOneShardRun)
+{
+    /// Step 3 (Phase 3 T3): extend the fold with deltas for ch128:X and sha256:Y in ONE shard run —
+    /// both settle (edges present, condemn on removal works per ref), mixed rows in one run, no
+    /// algo loop.
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    const BlobRef ch_x{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(11))};
+    BlobDigest sha_y{};
+    sha_y.bytes[0] = 0xAB;
+    const BlobRef sha_y_ref{BlobHashAlgo::Sha256, sha_y};
+
+    std::vector<RunRef> runs1;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, 1, /*attempt*/0, 0,
+        {{ch_x, s(1), false}, {sha_y_ref, s(1), false}}, runs1);
+    ASSERT_FALSE(runs1.empty());
+
+    EXPECT_EQ(DB::Cas::tests::inDegreeInRuns(backend, runs1, ch_x), 1);
+    EXPECT_EQ(DB::Cas::tests::inDegreeInRuns(backend, runs1, sha_y_ref), 1);
+    EXPECT_TRUE(zeroInDegree(backend, runs1).empty());
+
+    /// Remove both edges in gen 2: each transitions to zero independently, condemned per its own ref.
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1, 2, /*attempt*/0, 0,
+        {{ch_x, s(1), true}, {sha_y_ref, s(1), true}}, runs2,
+        /*current_round*/0, /*condemn_round*/1, headPresent("t", 1), /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr);
+
+    ASSERT_EQ(rmr.still_retired.size(), 2u);
+    std::vector<BlobRef> condemned_refs{rmr.still_retired[0].ref, rmr.still_retired[1].ref};
+    EXPECT_NE(std::find(condemned_refs.begin(), condemned_refs.end(), ch_x), condemned_refs.end());
+    EXPECT_NE(std::find(condemned_refs.begin(), condemned_refs.end(), sha_y_ref), condemned_refs.end());
+    EXPECT_EQ(DB::Cas::tests::inDegreeInRuns(backend, runs2, ch_x), 0);
+    EXPECT_EQ(DB::Cas::tests::inDegreeInRuns(backend, runs2, sha_y_ref), 0);
+}
+
+/// [UNMATCHED-MINUS-ONE] pin. In-degree is a SET of source edges applied last-wins per
+/// (ref, ManifestId, path) key -- NOT a counter. A removal delta whose matching activation was
+/// never folded (reachable today via a false-404 at the activation fold plus a dead-build skip)
+/// must therefore be a per-key NO-OP: it marks an already-absent edge absent and cannot strip a
+/// sibling manifest's edge for the SAME blob. The whole "that interleaving is harmless" argument in
+/// the publish-confirm design rests on this; if the model ever regresses to counter arithmetic this
+/// test goes red and premature deletion becomes reachable again.
+TEST(CASBlobInDegree, UnmatchedRemovalIsAPerKeyNoOpAndSparesSiblingEdges)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// Generation 1: blob b1 is referenced by TWO distinct sources (two manifests).
+    std::vector<RunRef> runs1;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, /*new_generation*/1, /*attempt*/0, /*shard*/0,
+        {{bh(1), s(1), false}, {bh(1), s(2), false}}, runs1);
+
+    /// Generation 2: fold a removal for a THIRD source that never had an activation folded.
+    std::vector<RunRef> runs2;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1, /*new_generation*/2, /*attempt*/0, /*shard*/0,
+        {{bh(1), s(99), true}}, runs2);
+
+    /// Both original edges survive: the unmatched removal touched only its own (absent) key.
+    const DecodedRun out = decodeRun(backend, runs2[0]);
+    ASSERT_EQ(out.edges.size(), 2u) << "an unmatched removal must not strip sibling edges";
+    /// And the blob is NOT a deletion candidate.
+    const auto zero = zeroInDegree(backend, runs2);
+    EXPECT_TRUE(zero.empty()) << "b1 still has two live source edges";
+}
+
+/// The silence in `UnmatchedRemovalIsAPerKeyNoOpAndSparesSiblingEdges` above is exactly what let a whole
+/// class of GC defects survive months of soak runs undetected — the fold's per-key no-op left no trace.
+/// This test pins the COUNTING surface added on top: `RetiredMergeResult::unmatched_removes` /
+/// `unmatched_remove_example` must report the unmatched remove precisely (one hit, naming the right blob
+/// and source id), while the byte-level no-op behaviour (asserted above) is unchanged.
+TEST(CASBlobInDegree, UnmatchedRemovalIsCountedWithAnExample)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    /// Generation 1: blob b1 is referenced by TWO distinct sources (two manifests), same fixture as the
+    /// no-op test above.
+    std::vector<RunRef> runs1;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, /*new_generation*/1, /*attempt*/0, /*shard*/0,
+        {{bh(1), s(1), false}, {bh(1), s(2), false}}, runs1);
+
+    /// Generation 2: fold a removal for a THIRD source that never had an activation folded.
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/runs1, /*new_generation*/2, /*attempt*/0, /*shard*/0,
+        {{bh(1), s(99), true}}, runs2,
+        /*current_round*/0, /*condemn_round*/0, /*head_blob*/{}, /*peek_head*/{}, /*confirm_condemned_marker*/{}, &rmr);
+
+    /// The run is byte-identical to the no-op test's outcome for the blob's OTHER edges: both survive.
+    const DecodedRun out = decodeRun(backend, runs2[0]);
+    ASSERT_EQ(out.edges.size(), 2u) << "the counting surface must not perturb the no-op fold outcome";
+    EXPECT_EQ(out.edges[0].first, b(1));
+    EXPECT_EQ(out.edges[1].first, b(1));
+    std::vector<UInt128> surviving_sources{out.edges[0].second, out.edges[1].second};
+    EXPECT_NE(std::find(surviving_sources.begin(), surviving_sources.end(), s(1)), surviving_sources.end());
+    EXPECT_NE(std::find(surviving_sources.begin(), surviving_sources.end(), s(2)), surviving_sources.end());
+
+    /// The counting surface reports exactly the one unmatched remove, naming the right blob and source id.
+    EXPECT_EQ(rmr.unmatched_removes, 1u);
+    ASSERT_TRUE(rmr.unmatched_remove_example.has_value());
+    EXPECT_EQ(rmr.unmatched_remove_example->ref, bh(1));
+    EXPECT_EQ(rmr.unmatched_remove_example->source_id, s(99));
+}
+
+namespace
+{
+/// N distinct condemned rows for blobs b(1)..b(n), same shape `condemnedRowFor` produces, varying
+/// only the token so distinct rows are trivially distinguishable in a failure message.
+std::vector<std::pair<UInt128, CondemnedRow>> condemnedCohort(uint64_t n, uint64_t condemn_round, bool delete_pending)
+{
+    std::vector<std::pair<UInt128, CondemnedRow>> rows;
+    for (uint64_t i = 1; i <= n; ++i)
+        rows.push_back({b(i), condemnedRowFor(condemn_round, "t" + std::to_string(i), delete_pending)});
+    return rows;
+}
+}
+
+/// The redelete cohort is capped at `GcRoundWorkBudget::max_redeletes` per call. Excess
+/// entries stay in `still_retired`, still `delete_pending`, to be redeleted by a later round — the
+/// durable pipeline never loses one to the cap.
+TEST(CASThreeCursorMerge, RedeleteBudgetCapsCohortAndCarriesExcess)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+    const RunRef gen1 = writeSourceEdgeRun(backend, layout, 1, 0, 0, condemnedCohort(10, 1, /*delete_pending*/true));
+
+    GcRoundWorkBudget budget;
+    budget.max_redeletes = 3;
+
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{gen1}, 2, 0, 0, {}, runs2,
+        /*current_round*/9, /*condemn_round*/9, /*head_blob*/{}, /*peek_head*/{}, /*confirm_condemned_marker*/{},
+        &rmr, /*suppress_destructive*/false, /*out_applied_by_txn_ordinal*/nullptr,
+        /*source_retirements*/{}, &budget);
+
+    EXPECT_EQ(rmr.redelete.size(), 3u);
+    EXPECT_EQ(budget.redeletes_used, 3u);
+    EXPECT_TRUE(rmr.graduated.empty());
+    EXPECT_TRUE(rmr.spared.empty());
+    ASSERT_EQ(rmr.still_retired.size(), 7u);
+    for (const RetiredEntry & e : rmr.still_retired)
+        EXPECT_TRUE(e.delete_pending) << "carried entries stay delete_pending, unexecuted this round";
+}
+
+/// Mirror test for the graduation cap: entries past `max_graduations` carry unchanged (still
+/// condemned, NOT yet delete_pending) rather than being force-graduated; the floor re-evaluates them
+/// next round.
+TEST(CASThreeCursorMerge, GraduationBudgetCapsCohortAndCarriesExcess)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+    const RunRef gen1 = writeSourceEdgeRun(backend, layout, 1, 0, 0, condemnedCohort(10, /*condemn_round*/1, /*delete_pending*/false));
+
+    GcRoundWorkBudget budget;
+    budget.max_graduations = 3;
+
+    std::vector<RunRef> runs2;
+    RetiredMergeResult rmr;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{gen1}, 2, 0, 0, {}, runs2,
+        /*current_round*/5, /*condemn_round*/6, /*head_blob*/{}, /*peek_head*/{}, /*confirm_condemned_marker*/{},
+        &rmr, /*suppress_destructive*/false, /*out_applied_by_txn_ordinal*/nullptr,
+        /*source_retirements*/{}, &budget);
+
+    EXPECT_EQ(rmr.graduated.size(), 3u);
+    EXPECT_EQ(budget.graduations_used, 3u);
+    ASSERT_EQ(rmr.still_retired.size(), 10u);
+    size_t pending_count = 0;
+    size_t carried_count = 0;
+    for (const RetiredEntry & e : rmr.still_retired)
+        e.delete_pending ? ++pending_count : ++carried_count;
+    EXPECT_EQ(pending_count, 3u)  << "only the graduated 3 are republished delete_pending";
+    EXPECT_EQ(carried_count, 7u) << "the rest carry unchanged, still eligible next round";
+}
+
+/// The mandatory convergence proof: a cohort well past the per-round cap fully drains over
+/// ceil(N / cap) rounds, feeding each round's output run back as the next round's prior — the exact
+/// shape a real GC round repeats every pass.
+TEST(CASThreeCursorMerge, RedeleteBudgetDrainsCohortToFixpointOverRounds)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+    std::vector<RunRef> priors{writeSourceEdgeRun(backend, layout, 1, 0, 0, condemnedCohort(10, 1, /*delete_pending*/true))};
+
+    uint64_t total_redeleted = 0;
+    uint64_t rounds = 0;
+    while (rounds < 10)
+    {
+        GcRoundWorkBudget budget;
+        budget.max_redeletes = 3;
+        std::vector<RunRef> out_runs;
+        RetiredMergeResult rmr;
+        foldDeltasIntoGeneration(backend, layout, priors, 2 + rounds, 0, 0, {}, out_runs,
+            /*current_round*/100, /*condemn_round*/100, /*head_blob*/{}, /*peek_head*/{}, /*confirm_condemned_marker*/{},
+            &rmr, /*suppress_destructive*/false, /*out_applied_by_txn_ordinal*/nullptr,
+            /*source_retirements*/{}, &budget);
+        total_redeleted += rmr.redelete.size();
+        ++rounds;
+        if (rmr.still_retired.empty())
+            break;
+        ASSERT_FALSE(out_runs.empty());
+        priors = out_runs;
+    }
+    EXPECT_EQ(total_redeleted, 10u) << "no entry lost to the cap across the whole drain";
+    EXPECT_EQ(rounds, 4u) << "ceil(10 / 3) rounds to fully drain";
+}
diff --git a/src/Disks/tests/gtest_cas_blob_meta.cpp b/src/Disks/tests/gtest_cas_blob_meta.cpp
new file mode 100644
index 000000000000..e7094d36d754
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_meta.cpp
@@ -0,0 +1,212 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include "cas_test_helpers.h"
+
+#include <Poco/Exception.h>
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+/// Codec tests (round-trip both states, fail-closed decode) moved to gtest_cas_blob_meta_format.cpp
+/// with the v3 text cutover; the lifecycle + inspect tests below stay — they exercise the Core ops
+/// and CasInspect against the stable encode/decode signatures and must pass unchanged.
+
+TEST(CASBlobMeta, PutIfAbsentThenCasTransitions)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const BlobRef ref{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("hash-a"))};
+    const BlobMeta clean{.state = MetaState::Clean, .size = 10};
+
+    const CasOverwriteResult created = putMetaIfAbsent(*store, ref, clean);
+    EXPECT_EQ(created.outcome, CasOverwriteOutcome::Committed);
+
+    const CasOverwriteResult dup = putMetaIfAbsent(*store, ref, clean);
+    EXPECT_EQ(dup.outcome, CasOverwriteOutcome::Committed);   /// exact-byte resolution adopts the existing marker
+
+    const auto lm = loadMeta(*backend, store->layout(), ref);
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(lm->meta.state, MetaState::Clean);
+
+    const CasOverwriteResult condemned = casMeta(*store, ref, lm->etag,
+        BlobMeta{.state = MetaState::Condemned, .condemn_round = 5, .size = 10});
+    EXPECT_EQ(condemned.outcome, CasOverwriteOutcome::Committed);
+
+    const CasOverwriteResult stale = casMeta(*store, ref, lm->etag,   /// stale token loses
+        BlobMeta{.state = MetaState::Clean});
+    EXPECT_EQ(stale.outcome, CasOverwriteOutcome::Conflict);
+}
+
+TEST(CASBlobMeta, DeleteMetaExactMatchesEtag)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const BlobRef ref{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("hash-b"))};
+    putMetaIfAbsent(*store, ref, BlobMeta{.state = MetaState::Condemned});
+    const auto lm = loadMeta(*backend, store->layout(), ref);
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(deleteMetaExact(*backend, store->layout(), ref, lm->etag).kind, DeleteOutcome::Kind::Deleted);
+    EXPECT_FALSE(loadMeta(*backend, store->layout(), ref).has_value());
+}
+
+/// Phase 3 T3 (mixed-algo pools, was CAS pluggable-blob-hash Phase 2 Task 5 crux Test 2): the `.meta`
+/// API round-trips a 32-byte (`sha256`-width) `BlobRef` key — the meta object lands under a 64-hex
+/// key, exercising the SAME `putMetaIfAbsent`/`loadMeta`/`casMeta`/`deleteMetaExact` surface PartWriteTxn/Gc
+/// use, just at a wider algo. Writes use the `Pool`'s controller; reads and exact deletion retain their
+/// direct `Backend`/`Layout` surface.
+TEST(CASBlobMeta, PutLoadCasDeleteRoundTripAtWidth32)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+
+    /// A distinguishable 32-byte digest (not merely a 16-byte value zero-tailed): every byte set.
+    BlobDigest h;
+    for (size_t i = 0; i < h.bytes.size(); ++i)
+        h.bytes[i] = static_cast<uint8_t>(i + 1);
+    const BlobRef ref{BlobHashAlgo::Sha256, h};
+    const String hex = codecFor(BlobHashAlgo::Sha256).toHex(h);
+    EXPECT_EQ(hex.size(), 64u) << "a 32-byte digest renders 64 hex chars";
+
+    const CasOverwriteResult created = putMetaIfAbsent(*store, ref,
+        BlobMeta{.state = MetaState::Clean, .size = 555});
+    ASSERT_EQ(created.outcome, CasOverwriteOutcome::Committed);
+    EXPECT_TRUE(backend->head(layout.blobMetaKey(ref)).exists)
+        << "the meta object must land under the 64-hex key, not a truncated 32-hex one";
+
+    const auto lm = loadMeta(*backend, layout, ref);
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(lm->meta.state, MetaState::Clean);
+    EXPECT_EQ(lm->meta.size, 555u);
+
+    const CasOverwriteResult condemned = casMeta(*store, ref, lm->etag,
+        BlobMeta{.state = MetaState::Condemned, .condemn_round = 7, .size = 555});
+    ASSERT_EQ(condemned.outcome, CasOverwriteOutcome::Committed);
+    const auto lm2 = loadMeta(*backend, layout, ref);
+    ASSERT_TRUE(lm2.has_value());
+    EXPECT_EQ(lm2->meta.state, MetaState::Condemned);
+
+    EXPECT_EQ(deleteMetaExact(*backend, layout, ref, lm2->etag).kind, DeleteOutcome::Kind::Deleted);
+    EXPECT_FALSE(loadMeta(*backend, layout, ref).has_value());
+}
+
+namespace
+{
+
+class ControlledMetaWriteFaultBackend : public InMemoryBackend
+{
+public:
+    bool throw_next_create = false;
+    bool throw_next_overwrite = false;
+    uint64_t create_attempts = 0;
+    uint64_t overwrite_attempts = 0;
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        ++create_attempts;
+        if (throw_next_create)
+        {
+            throw_next_create = false;
+            throw Poco::TimeoutException("scripted meta create ambiguity");
+        }
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        ++overwrite_attempts;
+        if (throw_next_overwrite)
+        {
+            throw_next_overwrite = false;
+            throw Poco::TimeoutException("scripted meta overwrite ambiguity");
+        }
+        return InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+    }
+};
+
+}
+
+TEST(CASBlobMeta, WritesUsePoolRequestController)
+{
+    auto backend = std::make_shared<ControlledMetaWriteFaultBackend>();
+    auto store = openPoolForTest(backend);
+    store->setCasRetrySleepForTest([](uint64_t) {});
+    backend->create_attempts = 0;
+    backend->overwrite_attempts = 0;
+
+    const BlobRef ref{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("hash-controlled"))};
+    backend->throw_next_create = true;
+    EXPECT_EQ(
+        putMetaIfAbsent(*store, ref, BlobMeta{.state = MetaState::Clean, .size = 10}).outcome,
+        CasOverwriteOutcome::Committed);
+    EXPECT_EQ(backend->create_attempts, 2u);
+
+    const auto clean = loadMeta(*backend, store->layout(), ref);
+    ASSERT_TRUE(clean.has_value());
+    backend->throw_next_overwrite = true;
+    EXPECT_EQ(
+        casMeta(*store, ref, clean->etag, BlobMeta{.state = MetaState::Condemned, .condemn_round = 1, .size = 10}).outcome,
+        CasOverwriteOutcome::Committed);
+    EXPECT_EQ(backend->overwrite_attempts, 2u);
+}
+
+/// Phase 3 T3 (was Phase 2 Task 5 crux Test 2, dedup half): the dedup-cache set is `BlobRef`-keyed and
+/// admits a 32-byte digest without truncation/collision against its 16-byte zero-tailed sibling — even
+/// under a DIFFERENT algo (the whole point of the pair identity).
+TEST(CASBlobMeta, DeduplicationCacheAdmitsWidth32Digest)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    PoolConfig cfg{.pool_prefix = "p", .server_root_id = "test", .deduplication_cache_bytes = 64ULL << 20};
+    auto store = Pool::open(backend, cfg);
+
+    BlobDigest wide;
+    for (size_t i = 0; i < wide.bytes.size(); ++i)
+        wide.bytes[i] = static_cast<uint8_t>(i + 1);
+    /// The 16-byte prefix of `wide`, zero-tailed — a DIFFERENT logical identity at width 16.
+    BlobDigest narrow;
+    for (size_t i = 0; i < 16; ++i)
+        narrow.bytes[i] = wide.bytes[i];
+
+    const BlobRef wide_ref{BlobHashAlgo::Sha256, wide};
+    const BlobRef narrow_ref{BlobHashAlgo::CityHash128, narrow};
+    EXPECT_FALSE(store->dedupCacheContains(wide_ref));
+    EXPECT_FALSE(store->dedupCacheContains(narrow_ref));
+    store->dedupCacheAdd(wide_ref);
+    EXPECT_TRUE(store->dedupCacheContains(wide_ref));
+    EXPECT_FALSE(store->dedupCacheContains(narrow_ref)) << "a 32-byte digest must not collide with its zero-tailed 16-byte prefix";
+}
+
+/// `cas-inspect` dispatch (CasInspect.cpp): a `.meta` key must decode as a BlobMeta, NOT fall through
+/// to the `blobs/` envelope branch (the `.meta` key shares the `blobsPrefix()` prefix with a body key).
+TEST(CASBlobMeta, InspectRendersCondemnedMeta)
+{
+    const Layout layout("p");
+    const BlobRef ref{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("hash-inspect"))};
+    const String key = layout.blobMetaKey(ref);
+    const BlobMeta m{.version = 1, .state = MetaState::Condemned, .condemn_round = 9, .size = 123};
+
+    const String json = caInspectToJson(layout, key, encodeBlobMeta(m));
+    EXPECT_NE(json.find("\"object\":\"blob_meta\""), String::npos);
+    EXPECT_NE(json.find("\"condemned\""), String::npos);
+    EXPECT_NE(json.find("\"condemn_round\":9"), String::npos);
+    EXPECT_NE(json.find("\"size\":123"), String::npos);
+}
+
+TEST(CASBlobMeta, InspectRendersCleanMeta)
+{
+    const Layout layout("p");
+    const BlobRef ref{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("hash-inspect-clean"))};
+    const String key = layout.blobMetaKey(ref);
+    const BlobMeta m{.version = 1, .state = MetaState::Clean, .condemn_round = 0, .size = 7};
+
+    const String json = caInspectToJson(layout, key, encodeBlobMeta(m));
+    EXPECT_NE(json.find("\"clean\""), String::npos);
+}
diff --git a/src/Disks/tests/gtest_cas_blob_meta_format.cpp b/src/Disks/tests/gtest_cas_blob_meta_format.cpp
new file mode 100644
index 000000000000..4aae227d0968
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_meta_format.cpp
@@ -0,0 +1,49 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobMetaFormat.h>
+#include <IO/ReadBufferFromMemory.h>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes { extern const int CORRUPTED_DATA; }
+
+TEST(CASFormatBattery, BlobMeta)
+{
+    BlobMeta m;
+    m.state = MetaState::Clean;
+    m.condemn_round = 0;
+    m.size = 12345;
+    runFormatBattery(FormatBatteryCase{
+        .id = FormatId::BlobMeta,
+        .encode = [&] { return sealObject(FormatId::BlobMeta, encodeBlobMeta(m)); },
+        .decode = [](std::string_view s) { decodeBlobMeta(std::string(openObject(FormatId::BlobMeta, s))); },
+        .golden = "{\"type\":\"cas_blob_meta\",\"v\":9}\n"
+                  "{\"st\":\"clean\",\"cr\":\"0\",\"sz\":\"12345\"}\n"});
+}
+
+TEST(CASBlobMetaFormat, CondemnedRoundTripAllFields)
+{
+    BlobMeta m;
+    m.state = MetaState::Condemned;
+    m.condemn_round = 7;
+    m.size = 4096;
+    const BlobMeta back = decodeBlobMeta(encodeBlobMeta(m));
+    EXPECT_EQ(back.state, MetaState::Condemned);
+    EXPECT_EQ(back.condemn_round, 7u);
+    EXPECT_EQ(back.size, 4096u);
+    EXPECT_EQ(encodeBlobMeta(m),
+        "{\"type\":\"cas_blob_meta\",\"v\":9}\n{\"st\":\"condemned\",\"cr\":\"7\",\"sz\":\"4096\"}\n");
+}
+
+TEST(CASBlobMetaFormat, FailsClosedOnUnknownStateAndTruncation)
+{
+    /// Unknown state word -> CORRUPTED_DATA (mirrors the old `state > Condemned` reject).
+    /// `v:3` is deliberate and must NOT follow a future `G_BUILD` bump: any version <= G_BUILD passes
+    /// the header gate, which is the point — the BODY is what has to fail here.
+    const String bad_state = "{\"type\":\"cas_blob_meta\",\"v\":3}\n{\"st\":\"zombie\",\"cr\":\"0\",\"sz\":\"0\"}\n";
+    EXPECT_THROW(decodeBlobMeta(bad_state), DB::Exception);
+    /// Missing state key -> CORRUPTED_DATA.
+    const String no_state = "{\"type\":\"cas_blob_meta\",\"v\":3}\n{\"cr\":\"0\",\"sz\":\"0\"}\n";
+    EXPECT_THROW(decodeBlobMeta(no_state), DB::Exception);
+    /// Truncated (header only) -> CORRUPTED_DATA.
+    EXPECT_THROW(decodeBlobMeta("{\"type\":\"cas_blob_meta\",\"v\":3}\n"), DB::Exception);
+}
diff --git a/src/Disks/tests/gtest_cas_blob_ref.cpp b/src/Disks/tests/gtest_cas_blob_ref.cpp
new file mode 100644
index 000000000000..a75b5b968502
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_ref.cpp
@@ -0,0 +1,34 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <unordered_set>
+
+using namespace DB::Cas;
+
+TEST(CASBlobRef, SameDigestDifferentAlgoAreDistinct)
+{
+    const BlobDigest d = BlobDigest::fromU128(UInt128(0xDEADBEEF));
+    const BlobRef a{BlobHashAlgo::CityHash128, d};
+    const BlobRef b{BlobHashAlgo::XXH3_128, d};
+    EXPECT_NE(a, b);
+    EXPECT_LT(a, b);                                    /// algo=1 < algo=2
+    std::unordered_set<BlobRef, BlobRefHash> s{a, b};
+    EXPECT_EQ(s.size(), 2u);
+}
+
+TEST(CASBlobRef, OrderIsAlgoThenDigest)
+{
+    const BlobRef small_algo_big_digest{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(0) - 1)};
+    const BlobRef big_algo_small_digest{BlobHashAlgo::Sha256, BlobDigest::fromU128(UInt128(1))};
+    EXPECT_LT(small_algo_big_digest, big_algo_small_digest);   /// algo decides first
+}
+
+TEST(CASBlobRef, HexAndIdRenderAtAlgoWidth)
+{
+    BlobRef r16{BlobHashAlgo::XXH3_128, BlobDigest::fromU128(UInt128(0xAB))};
+    EXPECT_EQ(blobHexOf(r16).size(), 32u);
+    EXPECT_EQ(blobIdOf(r16).substr(0, 5), "xxh3:");
+    BlobRef r32{BlobHashAlgo::Sha256, {}};
+    for (size_t i = 0; i < 32; ++i) r32.digest.bytes[i] = static_cast<uint8_t>(i);
+    EXPECT_EQ(blobHexOf(r32).size(), 64u);
+    EXPECT_EQ(blobIdOf(r32).substr(0, 7), "sha256:");
+}
diff --git a/src/Disks/tests/gtest_cas_blob_upload_pool.cpp b/src/Disks/tests/gtest_cas_blob_upload_pool.cpp
new file mode 100644
index 000000000000..3e451810f744
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_upload_pool.cpp
@@ -0,0 +1,144 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
+#include <Common/Exception.h>
+#include <Common/ThreadPool.h>
+
+#include <atomic>
+#include <mutex>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+extern const int BAD_ARGUMENTS;
+extern const int LOGICAL_ERROR;
+}
+
+namespace
+{
+
+/// Mirrors `gtest_cas_part_manifest_format.cpp`'s inlined assertion helper rather than pulling in
+/// `Disks/tests/cas_test_helpers.h` for one tiny check.
+template <typename F>
+void expectThrowsCode(int expected_code, F && fn)
+{
+    try
+    {
+        fn();
+        FAIL() << "expected DB::Exception";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), expected_code);
+    }
+}
+
+/// The pattern stage-1 T5's fan-out fixtures reuse: lazily bring the server-wide pool up on first
+/// need. Deliberately NOT torn down between calls -- `initializeBlobUploadPool` is once-only for
+/// the lifetime of the binary via this helper, matching how the real server wires it once at
+/// startup. Tests that need to exercise the raw init/shutdown lifecycle contract itself (this file)
+/// call `initializeBlobUploadPool`/`shutdownBlobUploadPool` directly instead of through this helper.
+void ensureBlobUploadPoolForTest(size_t size)
+{
+    static std::once_flag once;
+    std::call_once(once, [size] { initializeBlobUploadPool(size); });
+}
+
+}
+
+/// `blobUploadPool()` on an uninitialized pool throws `LOGICAL_ERROR`, which aborts the whole
+/// process in debug/sanitizer builds instead of behaving like a catchable exception (see
+/// `handle_error_code` in `Common/Exception.cpp`) -- `CASBlobUploadPoolDeathTest` below proves the
+/// abort positively in those builds instead, following `gtest_cas_gc_state_format.cpp`'s pattern.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASBlobUploadPool, GetterThrowsBeforeInit)
+{
+    shutdownBlobUploadPool();
+    EXPECT_FALSE(blobUploadPoolInitializedForTest());
+    expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [] { blobUploadPool(); });
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASBlobUploadPoolDeathTest, GetterAbortsBeforeInit)
+{
+    shutdownBlobUploadPool();
+    ASSERT_FALSE(blobUploadPoolInitializedForTest());
+    EXPECT_DEATH({ (void)blobUploadPool(); }, "");
+}
+#endif
+
+TEST(CASBlobUploadPool, InitZeroRejected)
+{
+    shutdownBlobUploadPool();
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [] { initializeBlobUploadPool(0); });
+    /// A rejected init must not leave the pool half-initialized.
+    EXPECT_FALSE(blobUploadPoolInitializedForTest());
+    shutdownBlobUploadPool();
+}
+
+TEST(CASBlobUploadPool, InitThenGetWorks)
+{
+    shutdownBlobUploadPool();
+    initializeBlobUploadPool(4);
+    EXPECT_TRUE(blobUploadPoolInitializedForTest());
+
+    std::atomic<int> ran{0};
+    blobUploadPool().scheduleOrThrowOnError([&ran] { ++ran; });
+    blobUploadPool().wait();
+    EXPECT_EQ(ran.load(), 1);
+
+    shutdownBlobUploadPool();
+}
+
+/// Same debug/sanitizer-abort caveat as `GetterThrowsBeforeInit` above: the second
+/// `initializeBlobUploadPool` call throws `LOGICAL_ERROR`, which aborts under
+/// `DEBUG_OR_SANITIZER_BUILD`.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASBlobUploadPool, DoubleInitThrows)
+{
+    shutdownBlobUploadPool();
+    initializeBlobUploadPool(2);
+    expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [] { initializeBlobUploadPool(2); });
+    shutdownBlobUploadPool();
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASBlobUploadPoolDeathTest, DoubleInitAborts)
+{
+    shutdownBlobUploadPool();
+    initializeBlobUploadPool(2);
+    EXPECT_DEATH({ (void)initializeBlobUploadPool(2); }, "");
+    shutdownBlobUploadPool();
+}
+#endif
+
+TEST(CASBlobUploadPool, ShutdownIdempotent)
+{
+    /// Idempotent even when never initialized.
+    shutdownBlobUploadPool();
+    shutdownBlobUploadPool();
+    EXPECT_FALSE(blobUploadPoolInitializedForTest());
+
+    initializeBlobUploadPool(3);
+    shutdownBlobUploadPool();
+    /// Idempotent after a real init + shutdown too.
+    shutdownBlobUploadPool();
+    EXPECT_FALSE(blobUploadPoolInitializedForTest());
+}
+
+TEST(CASBlobUploadPool, EnsureForTestHelperLazilyInitializes)
+{
+    shutdownBlobUploadPool();
+    EXPECT_FALSE(blobUploadPoolInitializedForTest());
+
+    ensureBlobUploadPoolForTest(4);
+    EXPECT_TRUE(blobUploadPoolInitializedForTest());
+
+    /// Idempotent: a pool already up must not throw on a repeated call.
+    ensureBlobUploadPoolForTest(4);
+    EXPECT_TRUE(blobUploadPoolInitializedForTest());
+
+    shutdownBlobUploadPool();
+}
diff --git a/src/Disks/tests/gtest_cas_blob_upload_pool_env.cpp b/src/Disks/tests/gtest_cas_blob_upload_pool_env.cpp
new file mode 100644
index 000000000000..6f1ed8c0171e
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_blob_upload_pool_env.cpp
@@ -0,0 +1,48 @@
+#include <gtest/gtest.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+/// Stage-1 §1: `ContentAddressedTransaction::uploadPendingBlobs` fans out on the server-wide blob
+/// upload pool, whose getter is fail-loud (throws `LOGICAL_ERROR` -- an ABORT under a sanitizer build --
+/// if the pool was never initialized). Any CA test that commits a transaction with a pending blob would
+/// therefore abort the whole `unit_tests_dbms` process if the pool happened to be down.
+///
+/// This listener brings the pool up before EVERY test, so the pool is always initialized at the start of
+/// a test body regardless of link/run order. It is deliberately a before-each hook (not a one-shot
+/// `Environment::SetUp`): the raw-lifecycle suite in `gtest_cas_blob_upload_pool.cpp` shuts the pool
+/// down inside its own bodies, and those tests explicitly re-establish whatever pool state they assert
+/// on as their FIRST action, so re-ensuring the pool before them is harmless.
+///
+/// It ALSO shuts the pool down once, in `OnTestProgramEnd` (the last gtest event, fired from inside
+/// `RUN_ALL_TESTS` before `gtest_main`'s exit `SCOPE_EXIT`). This is mandatory, not cosmetic: the blob
+/// upload pool is a `ThreadFromGlobalPool`-backed `ThreadPool`, so its idle workers occupy GlobalThreadPool
+/// std::threads. `gtest_main` shuts the GlobalThreadPool down at process exit by JOINING those std::threads
+/// -- but a lingering blob-pool worker never returns until the blob pool itself is destroyed, so leaving
+/// the pool up at exit deadlocks the whole binary (main joins a std::thread that is running a blob-pool
+/// worker that waits for the blob pool to shut down). Draining it here, before `RUN_ALL_TESTS` returns,
+/// releases those std::threads first.
+namespace
+{
+
+class BlobUploadPoolEnsuringListener : public ::testing::EmptyTestEventListener
+{
+public:
+    void OnTestStart(const ::testing::TestInfo &) override
+    {
+        DB::Cas::tests::ensureBlobUploadPoolForTest();
+    }
+
+    void OnTestProgramEnd(const ::testing::UnitTest &) override
+    {
+        /// Release the pool's GlobalThreadPool-backed workers BEFORE `gtest_main` joins the GlobalThreadPool
+        /// at exit (see the class comment) -- otherwise the binary deadlocks at exit. Idempotent.
+        DB::Cas::shutdownBlobUploadPool();
+    }
+};
+
+const bool registered_blob_upload_pool_listener = []
+{
+    ::testing::UnitTest::GetInstance()->listeners().Append(new BlobUploadPoolEnsuringListener);
+    return true;
+}();
+
+}
diff --git a/src/Disks/tests/gtest_cas_bootstrap_ordering.cpp b/src/Disks/tests/gtest_cas_bootstrap_ordering.cpp
new file mode 100644
index 000000000000..aa469021de77
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_bootstrap_ordering.cpp
@@ -0,0 +1,443 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+#include <Common/Exception.h>
+
+#include <mutex>
+#include <optional>
+#include <string>
+#include <vector>
+
+/// Task 7 (spec §2 "Startup [C4], ordered vs the capability probe [D2]"): the writable `Pool::open`
+/// bootstrap sequence is (0) a ZERO-WRITE residual check FIRST — before any probe write — that ignores
+/// structurally-valid `_probe/` debris; (1) only then the mutating `_probe/` capability battery; (2) then
+/// `PoolMeta::createOrValidate`, which may mint a missing `_pool_meta` only over a genuinely empty prefix.
+/// A missing `_pool_meta` over residual (non-`_probe`) data fails startup loud with ZERO writes — closing
+/// the "restart poisons a partially-erased pool" hole. These are black-box tests over `Pool::open`,
+/// asserting behavior AND ordering via an op-recording backend (they fail on the pre-Task-7 open, which
+/// bootstraps a fresh identity unconditionally and performs no residual LIST before the battery).
+
+namespace DB::ErrorCodes
+{
+extern const int INVALID_STATE;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+const String kPrefix = "p";
+const String kSrid = "test";
+const String kPoolMetaKey = "p/_pool_meta";
+/// A well-formed per-mount probe uid: exactly 32 lowercase hex chars (`u128ToHex`'s shape).
+const String kProbeUid = "0123456789abcdef0123456789abcdef";
+const String kProbeUid2 = "fedcba9876543210fedcba9876543210";
+
+/// Records the ORDER of backend operations so a test can assert that the residual LIST precedes the first
+/// write, and that a fail path performs zero writes. Delegates every operation to `InMemoryBackend`
+/// unchanged; `Pool::open` wraps this in its `InstrumentedBackend`, which forwards every op here.
+class RecordingBackend final : public InMemoryBackend
+{
+public:
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    enum class Op : uint8_t { List, PutIfAbsent, PutOverwrite, CasPut, Delete };
+    struct Entry
+    {
+        Op op;
+        String key;   /// the LIST prefix, or the written key
+    };
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        record(Op::List, prefix);
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        record(Op::PutIfAbsent, key);
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        record(Op::PutOverwrite, key);
+        return InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+    }
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected, const ObjectMeta & meta) override
+    {
+        record(Op::CasPut, key);
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        record(Op::Delete, key);
+        return InMemoryBackend::deleteExact(key, token);
+    }
+    /// The bootstrap path (battery + createOrValidate + mount protocol) issues only whole-String writes,
+    /// never a streaming create, so recording the four write ops above captures every write `open` can do.
+
+    static bool isWrite(Op op)
+    {
+        return op == Op::PutIfAbsent || op == Op::PutOverwrite || op == Op::CasPut || op == Op::Delete;
+    }
+
+    void clearLog()
+    {
+        std::lock_guard l(mutex_);
+        log_.clear();
+    }
+    std::vector<Entry> snapshot() const
+    {
+        std::lock_guard l(mutex_);
+        return log_;
+    }
+    size_t writeCount() const
+    {
+        std::lock_guard l(mutex_);
+        size_t n = 0;
+        for (const auto & e : log_)
+            if (isWrite(e.op))
+                ++n;
+        return n;
+    }
+
+private:
+    void record(Op op, const String & key)
+    {
+        std::lock_guard l(mutex_);
+        log_.push_back({op, key});
+    }
+    mutable std::mutex mutex_;
+    std::vector<Entry> log_;
+};
+
+/// Models a stale LIST result for `cas/ref_catalog`: the object was listed, then disappeared before
+/// the exact validation GET. The bootstrap must treat this as residual, never as a new-pool proof.
+class CatalogMissingAfterListBackend final : public InMemoryBackend
+{
+public:
+    using Backend::get;
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (key == Layout{kPrefix}.refCatalogKey())
+            return std::nullopt;
+        return InMemoryBackend::get(key, range);
+    }
+};
+
+PoolConfig makeConfig()
+{
+    PoolConfig cfg;
+    cfg.pool_prefix = kPrefix;
+    cfg.server_root_id = kSrid;
+    cfg.wait_sleep_fn = [](uint64_t) {};   /// never block a synchronous test on an open/teardown wait
+    return cfg;
+}
+
+template <typename F>
+void expectThrowsCodeContaining(int expected_code, const String & needle, F && fn);
+
+void expectCatalogResidueRefusesWithoutPoolMeta(const String & bytes, const String & extra_key = {})
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    const Layout layout{kPrefix};
+    ASSERT_EQ(backend->putIfAbsent(layout.refCatalogKey(), bytes).outcome, PutOutcome::Done);
+    if (!extra_key.empty())
+        ASSERT_EQ(backend->putIfAbsent(extra_key, "residual").outcome, PutOutcome::Done);
+    backend->clearLog();
+
+    try
+    {
+        Pool::open(backend, makeConfig());
+        FAIL() << "expected residual catalog bootstrap refusal";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::INVALID_STATE);
+    }
+    EXPECT_EQ(backend->writeCount(), 0u);
+    EXPECT_FALSE(backend->head(layout.poolMetaKey()).exists);
+}
+
+/// Index of the first op matching `pred`, if any.
+template <typename Pred>
+std::optional<size_t> firstIndex(const std::vector<RecordingBackend::Entry> & log, Pred && pred)
+{
+    for (size_t i = 0; i < log.size(); ++i)
+        if (pred(log[i]))
+            return i;
+    return std::nullopt;
+}
+
+/// Assert `fn` throws a DB::Exception with `expected_code` AND a message containing `needle`.
+template <typename F>
+void expectThrowsCodeContaining(int expected_code, const String & needle, F && fn)
+{
+    try
+    {
+        fn();
+        FAIL() << "expected a DB::Exception, none thrown";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), expected_code);
+        EXPECT_NE(e.message().find(needle), String::npos)
+            << "message did not contain '" << needle << "': " << e.message();
+    }
+}
+
+}
+
+/// (a) Empty prefix → open succeeds, `_pool_meta` is created, AND the op-log proves the residual LIST of
+/// the pool prefix happened BEFORE any write (the ordering [D2] mandates: no probe write may precede the
+/// emptiness proof).
+TEST(CASBootstrapOrdering, EmptyPrefixOpensAndListsBeforeAnyWrite)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    backend->clearLog();
+
+    PoolPtr store = Pool::open(backend, makeConfig());
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::Live);
+    EXPECT_TRUE(backend->get(kPoolMetaKey).has_value()) << "_pool_meta must be created on a fresh empty prefix";
+
+    const auto log = backend->snapshot();
+    const auto residual_list = firstIndex(log, [](const RecordingBackend::Entry & e)
+        { return e.op == RecordingBackend::Op::List && e.key == kPrefix + "/"; });
+    const auto first_write = firstIndex(log, [](const RecordingBackend::Entry & e)
+        { return RecordingBackend::isWrite(e.op); });
+
+    ASSERT_TRUE(residual_list.has_value()) << "the zero-write residual LIST of '" << kPrefix << "/' must run";
+    ASSERT_TRUE(first_write.has_value()) << "a fresh open must eventually write (battery/meta/mount)";
+    EXPECT_LT(*residual_list, *first_write) << "the residual LIST must precede every write";
+}
+
+/// The residue an incomplete erase would have left behind: a real ref-log object key, built through
+/// `Layout` so it carries the life segment every ref key has. The residual check is LIST-based and
+/// never parses it, but seeding a shape this build cannot write would make the comment below a lie.
+namespace
+{
+String residualRefLogKey()
+{
+    return Layout{"p"}.refLogKey(DB::Cas::tests::fixture::fixtureLife(RootNamespace{"test%2Fabcd"}), RefTxnId{1, 1});
+}
+}
+
+/// (b) A prefix holding `cas/ns/stream/…` residue but NO `_pool_meta` → open fails typed (INVALID_STATE),
+/// and ZERO writes hit the backend (the mutating battery must NOT have run — the residual check throws
+/// first).
+TEST(CASBootstrapOrdering, ResidualWithoutMetaFailsTypedWithZeroWrites)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    /// Seed residue an incomplete erase would have left behind (a ref-log object), with no `_pool_meta`.
+    ASSERT_EQ(backend->putIfAbsent(residualRefLogKey(), "x").outcome,
+              PutOutcome::Done);
+    backend->clearLog();
+
+    expectThrowsCodeContaining(DB::ErrorCodes::INVALID_STATE, "refusing to bootstrap over residual data",
+                               [&] { Pool::open(backend, makeConfig()); });
+
+    EXPECT_EQ(backend->writeCount(), 0u) << "the fail path must perform zero writes (battery never ran)";
+    EXPECT_FALSE(backend->get(kPoolMetaKey).has_value()) << "a fresh _pool_meta must NOT have been minted";
+}
+
+/// (c) A prefix containing ONLY stale, structurally-valid `_probe/<hex>/…` debris (a crash-mid-battery
+/// leftover) → treated as empty → open succeeds and bootstraps a fresh pool. The debris-skip is what makes
+/// a normal restart-after-crash recover instead of wedging.
+TEST(CASBootstrapOrdering, StaleProbeDebrisOnlyIsTreatedAsEmpty)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    ASSERT_EQ(backend->putIfAbsent("p/_probe/" + kProbeUid + "/token", "probe-v1").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent("p/_probe/" + kProbeUid + "/cas", "cas-s1").outcome, PutOutcome::Done);
+    backend->clearLog();
+
+    PoolPtr store;
+    ASSERT_NO_THROW(store = Pool::open(backend, makeConfig()));
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::Live);
+    EXPECT_TRUE(backend->get(kPoolMetaKey).has_value()) << "_pool_meta must be created over a probe-only prefix";
+}
+
+TEST(CASBootstrapOrdering, CanonicalEmptyCatalogOnlyIsTheSoleRetryablePreMetaResidue)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    const Layout layout{kPrefix};
+    ASSERT_EQ(backend->putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(RefCatalog{})).outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent(kPrefix + "/_probe/" + kProbeUid + "/token", "probe-v1").outcome, PutOutcome::Done);
+    backend->clearLog();
+
+    PoolPtr store;
+    ASSERT_NO_THROW(store = Pool::open(backend, makeConfig()));
+    EXPECT_TRUE(backend->head(layout.poolMetaKey()).exists);
+}
+
+TEST(CASBootstrapOrdering, MalformedCatalogOnlyResidueRefusesWithoutPoolMeta)
+{
+    expectCatalogResidueRefusesWithoutPoolMeta("not a catalog");
+}
+
+TEST(CASBootstrapOrdering, NoncanonicalCatalogOnlyResidueRefusesWithoutPoolMeta)
+{
+    String noncanonical = encodeRefCatalog(RefCatalog{});
+    noncanonical.insert(noncanonical.find('\n') - 1, ",\"noncanonical\":0");
+    ASSERT_TRUE(decodeRefCatalog(noncanonical).entries.empty()) << "fixture must be decodable but noncanonical";
+    expectCatalogResidueRefusesWithoutPoolMeta(noncanonical);
+}
+
+TEST(CASBootstrapOrdering, NonemptyCatalogOnlyResidueRefusesWithoutPoolMeta)
+{
+    const RefCatalog nonempty{.entries = {CatalogEntry{
+        .ns = RootNamespace{"test/nonempty"}, .state = NsState::Live, .incarnation = UInt128{1}, .creator = std::nullopt}}};
+    expectCatalogResidueRefusesWithoutPoolMeta(encodeRefCatalog(nonempty));
+}
+
+TEST(CASBootstrapOrdering, CatalogWithAnyOtherCasResidueRefusesWithoutPoolMeta)
+{
+    const String canonical_empty = encodeRefCatalog(RefCatalog{});
+    const Layout layout{kPrefix};
+    const std::vector<String> residuals{
+        layout.ownerKey("test"), layout.epochKey("test"), layout.mountKey("test"),
+        layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(RootNamespace{"test/ns"}), RefTxnId{1, 1}),
+        layout.manifestKey(ManifestId{RootNamespace{"test/ns"}, ManifestRef{1, 1, 1}}),
+        layout.serverRootDataPrefix("test") + "residual", kPrefix + "/unknown"};
+    for (const String & residual : residuals)
+        expectCatalogResidueRefusesWithoutPoolMeta(canonical_empty, residual);
+}
+
+TEST(CASBootstrapOrdering, ListedCatalogMissingAtExactGetRefusesWithoutPoolMeta)
+{
+    auto backend = std::make_shared<CatalogMissingAfterListBackend>();
+    const Layout layout{kPrefix};
+    ASSERT_EQ(backend->putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(RefCatalog{})).outcome, PutOutcome::Done);
+
+    expectThrowsCodeContaining(DB::ErrorCodes::INVALID_STATE, "refusing to bootstrap over residual data",
+                               [&] { Pool::open(backend, makeConfig()); });
+    EXPECT_FALSE(backend->head(layout.poolMetaKey()).exists);
+}
+
+/// (d) An existing healthy pool (meta present + data) → reopen is unchanged: the pool identity is
+/// PRESERVED (the residual check sees `_pool_meta` present → the normal validate path; `_pool_meta` is
+/// never re-minted).
+TEST(CASBootstrapOrdering, HealthyPoolReopenPreservesIdentity)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+
+    UInt128 pool_id_first;
+    {
+        PoolPtr store = Pool::open(backend, makeConfig());
+        pool_id_first = store->poolMeta().pool_id;
+    }   /// clean teardown: drained farewell, so the reopen reclaims immediately
+
+    PoolPtr store2 = Pool::open(backend, makeConfig());
+    EXPECT_EQ(store2->lifecycle(), PoolLifecycle::Live);
+    EXPECT_EQ(store2->poolMeta().pool_id, pool_id_first)
+        << "a healthy reopen must NOT re-mint _pool_meta — the pool identity must be preserved";
+}
+
+/// (e) [D2] concurrent-opener case: debris from a SECOND concurrent fresh opener's in-flight battery (a
+/// distinct probe uid) is skipped by the SAME structural rule as (c). Two openers racing over one shared
+/// pool prefix must not make each other's zero-write residual check fail.
+TEST(CASBootstrapOrdering, ConcurrentOpenerProbeDebrisIsAlsoSkipped)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    /// This mount's own crashed battery AND a concurrent opener's in-flight battery.
+    ASSERT_EQ(backend->putIfAbsent("p/_probe/" + kProbeUid + "/token", "probe-v1").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent("p/_probe/" + kProbeUid2 + "/token", "probe-v1").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent("p/_probe/" + kProbeUid2 + "/cas", "cas-s1").outcome, PutOutcome::Done);
+    backend->clearLog();
+
+    PoolPtr store;
+    ASSERT_NO_THROW(store = Pool::open(backend, makeConfig()));
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::Live);
+}
+
+/// (f) The reserved subtree boundary: only objects strictly under `<prefix>/_probe/` are ignorable
+/// debris. A SIBLING look-alike that merely starts with `_probe` but is NOT under the `_probe/` subtree
+/// (here `_probelike/…`) is genuine residual — the trailing `/` in the reserved prefix keeps it out — so
+/// bootstrap fails closed over it. (Any object literally under `_probe/`, whatever its leaf shape, is
+/// ephemeral capability-probe scratch a content-addressed pool never uses for durable state.)
+TEST(CASBootstrapOrdering, ProbeSiblingLookalikeIsResidualNotDebris)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    ASSERT_EQ(backend->putIfAbsent("p/_probelike/token", "x").outcome, PutOutcome::Done);
+    backend->clearLog();
+
+    expectThrowsCodeContaining(DB::ErrorCodes::INVALID_STATE, "refusing to bootstrap over residual data",
+                               [&] { Pool::open(backend, makeConfig()); });
+    EXPECT_EQ(backend->writeCount(), 0u);
+    EXPECT_FALSE(backend->get(kPoolMetaKey).has_value());
+}
+
+/// (g) An OBSERVE / read-only open over a partially-erased pool (residual data, `_pool_meta` deleted)
+/// must NOT mint a fresh `_pool_meta` — there is no truly-read-only backend, so a mint here is a real
+/// write that would poison the next writable mount's residual check. It fails closed (typed INVALID_STATE)
+/// with ZERO writes. The read-only path skips the residual check, so the fail-closed gate lives in
+/// `createOrValidate` (`allow_mint=false`).
+TEST(CASBootstrapOrdering, ReadOnlyOverResidualWithoutMetaFailsClosedNoMint)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    ASSERT_EQ(backend->putIfAbsent(residualRefLogKey(), "x").outcome,
+              PutOutcome::Done);
+    backend->clearLog();
+
+    PoolConfig cfg = makeConfig();
+    cfg.read_only = true;
+    expectThrowsCodeContaining(DB::ErrorCodes::INVALID_STATE, "refusing to mint outside the verified bootstrap path",
+                               [&] { Pool::open(backend, cfg); });
+
+    EXPECT_EQ(backend->writeCount(), 0u) << "an observe open must never write (least of all mint _pool_meta)";
+    EXPECT_FALSE(backend->get(kPoolMetaKey).has_value());
+}
+
+/// (h) An observe / read-only open over a HEALTHY pool (meta present) is unchanged: it validates the
+/// existing `_pool_meta` and succeeds, preserving the pool identity. `allow_mint=false` is never consulted
+/// on the validate path.
+TEST(CASBootstrapOrdering, ReadOnlyOverHealthyPoolSucceedsUnchanged)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    UInt128 pool_id_first;
+    {
+        PoolPtr store = Pool::open(backend, makeConfig());   /// writable: creates _pool_meta
+        pool_id_first = store->poolMeta().pool_id;
+    }
+
+    PoolConfig cfg = makeConfig();
+    cfg.read_only = true;
+    PoolPtr ro;
+    ASSERT_NO_THROW(ro = Pool::open(backend, cfg));
+    ASSERT_TRUE(ro);
+    EXPECT_EQ(ro->poolMeta().pool_id, pool_id_first) << "an observe open over a healthy pool must not re-mint";
+}
+
+/// (i) `openForDecommission` over a pool whose `_pool_meta` is absent but whose owner anchor survives (a
+/// partial erase) must NOT bootstrap a fresh identity — it fails closed (typed INVALID_STATE) with no
+/// mint. Decommission operates on an existing member; a missing meta is a broken state, not a bootstrap.
+TEST(CASBootstrapOrdering, DecommissionWithAbsentMetaFailsClosedNoMint)
+{
+    auto backend = std::make_shared<RecordingBackend>();
+    {
+        PoolPtr store = Pool::open(backend, makeConfig());   /// establishes owner anchor + _pool_meta
+    }
+    /// Delete only `_pool_meta`, leaving the owner anchor (and other control objects) behind.
+    {
+        const auto h = backend->head(kPoolMetaKey);
+        ASSERT_TRUE(h.exists);
+        ASSERT_EQ(backend->deleteExact(kPoolMetaKey, h.token).kind, DeleteOutcome::Kind::Deleted);
+    }
+    backend->clearLog();
+
+    expectThrowsCodeContaining(DB::ErrorCodes::INVALID_STATE, "refusing to mint outside the verified bootstrap path",
+                               [&] { Pool::openForDecommission(backend, makeConfig(), kSrid); });
+
+    EXPECT_EQ(backend->writeCount(), 0u) << "decommission must not mint a fresh _pool_meta";
+    EXPECT_FALSE(backend->get(kPoolMetaKey).has_value());
+}
diff --git a/src/Disks/tests/gtest_cas_confirm_exact_ref.cpp b/src/Disks/tests/gtest_cas_confirm_exact_ref.cpp
new file mode 100644
index 000000000000..2ba4c79d3f91
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_confirm_exact_ref.cpp
@@ -0,0 +1,1011 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedExchange.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/MemoryTracker.h>
+
+#include <atomic>
+#include <cctype>
+#include <chrono>
+#include <condition_variable>
+#include <exception>
+#include <filesystem>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <string_view>
+#include <thread>
+#include <vector>
+
+/// Task 10 (spec §confirm-primitive, "Gate 1 -- exact-token identity under a lane snapshot"): the
+/// ledger-side half of the publish-then-confirm relink handoff.
+///
+/// `confirmExactRef` is a GATE, and the only direction in which it may fail is `Unknown`. A `Yes`
+/// authorizes a remote receiver to promote a manifest it staged from this writer's blobs, so a `Yes`
+/// produced from a stale, lagging or partially-recovered view is a live-blob-deletion bug, not a
+/// missed optimization. Every test below therefore pins one of the six snapshot rules by constructing
+/// the exact state in which a naive "look the row up and compare" implementation would answer `Yes`
+/// (or `No`) and asserting `Unknown` instead.
+///
+/// Two properties are contract, not detail, and are asserted as such:
+///   - ZERO object-store I/O. A cold, evicted or recovering table answers `Unknown`; it must not
+///     recover from storage to answer, and it must not even MATERIALIZE a runtime -- a read-only
+///     interserver query must never be able to make this writer do work.
+///   - The snapshot spans BOTH lane mutexes, so an append admitted concurrently is ordered strictly
+///     after it: there is no window in which the confirm says `Yes` while a removal of that ref is
+///     already admitted.
+///
+/// The suite name is prefixed `Cas` so it is covered by the `Cas*` unit-test gate filter.
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int MEMORY_LIMIT_EXCEEDED;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::CountingBackend;
+
+namespace
+{
+
+/// A `CountingBackend` with two recovery-side seams: a one-shot NON-transient exact GET failure (which
+/// leaves the namespace runtime resident but UNRECOVERED, because recovery fails closed), and a
+/// blocking exact GET (which parks a caller inside `ensureRefTableRecovered` with
+/// `recovery_in_progress` set). The failure is deliberately `CORRUPTED_DATA`:
+/// `isTransientRecoveryError` does not list it, so recovery fails fast instead of burning its retry
+/// budget.
+class RecoveryLatchBackend : public CountingBackend
+{
+public:
+    using CountingBackend::get;
+    using CountingBackend::getStream;
+    using CountingBackend::putIfAbsent;
+    using CountingBackend::putIfAbsentStream;
+    using CountingBackend::putOverwrite;
+    using CountingBackend::casPut;
+
+    /// Set before the driving call; consumed by the first matching recovery GET.
+    String fail_get_once_key;
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (!fail_get_once_key.empty() && key == fail_get_once_key)
+        {
+            fail_get_once_key.clear();
+            throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+                "RecoveryLatchBackend: simulated non-transient exact GET failure");
+        }
+        {
+            std::unique_lock lk(m);
+            if (!block_key.empty() && key == block_key)
+            {
+                entered = true;
+                cv.notify_all();
+                /// Bounded (20s) so a wiring bug bounds the wait instead of hanging the suite.
+                cv.wait_for(lk, std::chrono::seconds(20), [&] { return block_key.empty(); });
+            }
+        }
+        return CountingBackend::get(key, range);
+    }
+
+    void armBlockedGet(const String & key)
+    {
+        std::lock_guard lk(m);
+        block_key = key;
+        entered = false;
+    }
+    void awaitBlockedGet()
+    {
+        std::unique_lock lk(m);
+        cv.wait_for(lk, std::chrono::seconds(20), [&] { return entered; });
+        ASSERT_TRUE(entered) << "the recovery GET never parked -- the in-progress window was not exercised";
+    }
+    void releaseBlockedGet()
+    {
+        {
+            std::lock_guard lk(m);
+            block_key.clear();
+        }
+        cv.notify_all();
+    }
+
+private:
+    std::mutex m;
+    std::condition_variable cv;
+    String block_key;
+    bool entered = false;
+};
+
+/// Parks the append lane's leader in the pre-carve window -- BEFORE it takes `ref_queue_mutex`, before
+/// any `PUT`, and therefore with the table's apply-state still `Clean`. That is what isolates the
+/// quiescence rule: the only thing wrong with the table while parked is that an append is in flight.
+struct LeaderLatch
+{
+    std::mutex m;
+    std::condition_variable cv;
+    bool entered = false;
+    bool released = false;
+
+    void arm(const PoolPtr & store)
+    {
+        store->setRefPreCarveHookForTest([this]
+        {
+            std::unique_lock lk(m);
+            if (entered)
+                return;   /// only the FIRST carve parks; retries proceed straight through
+            entered = true;
+            cv.notify_all();
+            /// Bounded (20s): a staging bug must bound the wait, not block the whole suite.
+            cv.wait_for(lk, std::chrono::seconds(20), [this] { return released; });
+        });
+    }
+    void awaitEntered()
+    {
+        std::unique_lock lk(m);
+        cv.wait_for(lk, std::chrono::seconds(20), [this] { return entered; });
+        ASSERT_TRUE(entered) << "the append lane's leader never reached the pre-carve window";
+    }
+    void release()
+    {
+        {
+            std::lock_guard lk(m);
+            released = true;
+        }
+        cv.notify_all();
+    }
+};
+
+/// Rendezvous for the co-batching pre-carve hook of the chunked-flush case (same shape as
+/// `gtest_cas_ref_chunked_flush.cpp`'s `CaseSync`).
+struct CaseSync
+{
+    std::mutex m;
+    std::condition_variable cv;
+    bool entered = false;
+};
+
+/// `num_pairs` add-then-remove precommit op pairs for distinct refs, each naming a distinct manifest.
+/// Every pair is undone immediately, so the LIVE state stays ~empty and validating thousands of ops
+/// stays linear -- it is the OP COUNT, not the resident state, that drives the chunk split under test.
+std::vector<RefOp> precommitAddRemovePairs(const String & prefix, size_t num_pairs, uint64_t manifest_epoch)
+{
+    std::vector<RefOp> ops;
+    ops.reserve(num_pairs * 2);
+    for (size_t i = 0; i < num_pairs; ++i)
+    {
+        const String ref = prefix + std::to_string(i);
+        const ManifestRef manifest{manifest_epoch, i + 1, 1};
+        RefOp add;
+        add.kind = RefOpKind::OwnerTransition;
+        add.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref, manifest};
+        ops.push_back(std::move(add));
+        RefOp remove;
+        remove.kind = RefOpKind::OwnerTransition;
+        remove.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref, manifest};
+        ops.push_back(std::move(remove));
+    }
+    return ops;
+}
+
+PoolPtr openPool(const BackendPtr & backend)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+PoolPtr openPoolWithConfig(const BackendPtr & backend, PoolConfig config)
+{
+    config.pool_prefix = "p";
+    config.server_root_id = "test";
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, std::move(config));
+}
+
+/// A legal blob-free part: an empty-entry manifest is enough to drive a real precommit+promote pair
+/// through the append lane and leave a committed ref behind. Returns the committed `ManifestId`.
+ManifestId publishEmptyPart(const PoolPtr & s, const RootNamespace & ns, const String & ref,
+                            bool allow_repoint = false)
+{
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id, allow_repoint);
+    return id;
+}
+
+/// Every request class `CountingBackend` observes, summed. The zero-I/O contract is asserted against
+/// this total, so a confirm that quietly grew a HEAD or a GET fails the test rather than the review.
+uint64_t backendRequests(const CountingBackend & b)
+{
+    return b.headTotal() + b.getTotal() + b.getStreamTotal() + b.putTotal() + b.listTotal();
+}
+
+/// One-shot throwing probe in the post-durable install region -- the only way to reach `NeedsRecovery`
+/// transition now that §A1 made every install region allocation-free. Copied in shape from
+/// `gtest_cas_ref_install_safety.cpp`: the exception is built OUTSIDE the region (constructing one
+/// inside would allocate and trip `DENY_ALLOCATIONS_IN_SCOPE`), and it is `MEMORY_LIMIT_EXCEEDED`
+/// rather than `LOGICAL_ERROR`, which aborts at construction in debug builds.
+void armOneShotInstallFailure(const PoolPtr & store)
+{
+    auto planned = std::make_exception_ptr(DB::Exception(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+        "simulated allocation failure inside the post-durable install region"));
+    auto fired = std::make_shared<std::atomic<bool>>(false);
+    store->setInstallRegionProbeForTest([planned, fired]
+    {
+        if (fired->exchange(true))
+            return;
+        ALLOW_ALLOCATIONS_IN_SCOPE;
+        std::rethrow_exception(planned);
+    });
+}
+
+}
+
+
+/// Rule 5, the affirmative case: a warm, quiescent, `Ready`, fenced table whose committed row for
+/// the ref names EXACTLY the asked-about manifest answers `Yes`. Its two negatives share the test
+/// because they are the same rule read the other way: a different `ManifestRef` under the right name,
+/// and a name that has no committed row at all, are both `No` -- a PROOF of the negative, not an
+/// ambiguity.
+TEST(CASConfirmExactRef, QuiescentExactMatchIsYes)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_yes"};
+
+    const ManifestId id = publishEmptyPart(store, ns, "x");
+
+    EXPECT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::Yes);
+
+    /// Same name, a manifest this ref never named.
+    ManifestRef other = id.ref;
+    ++other.manifest_ordinal;
+    EXPECT_EQ(store->confirmExactRef(ns, "x", other), ConfirmAnswer::No);
+
+    /// A name with no committed row at all -- on a warm table that is knowledge, not ambiguity.
+    EXPECT_EQ(store->confirmExactRef(ns, "no_such_ref", id.ref), ConfirmAnswer::No);
+}
+
+
+/// Rule 5, the repoint case (spec §testing "repointed live part"): the part is still live and the ref
+/// name still resolves, but it now names a DIFFERENT manifest. The old token must be `No` -- this is
+/// the case gate 0 cannot see at all, because the part object is `Active` throughout.
+TEST(CASConfirmExactRef, RepointedRefIsNo)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_repoint"};
+
+    const ManifestId first = publishEmptyPart(store, ns, "x");
+    ASSERT_EQ(store->confirmExactRef(ns, "x", first.ref), ConfirmAnswer::Yes);
+
+    const ManifestId second = publishEmptyPart(store, ns, "x", /*allow_repoint=*/true);
+    ASSERT_NE(first.ref, second.ref) << "a repoint must mint a fresh ManifestRef (the ABA barrier)";
+
+    EXPECT_EQ(store->confirmExactRef(ns, "x", first.ref), ConfirmAnswer::No);
+    EXPECT_EQ(store->confirmExactRef(ns, "x", second.ref), ConfirmAnswer::Yes);
+}
+
+
+/// Rule 5, the drop-and-recreate case: the ref name is removed and then published again. The name
+/// resolves again, so only EXACT `ManifestRef` equality separates the new binding from the old one --
+/// mint-tightening (spec §A3) is what guarantees the two can never collide.
+TEST(CASConfirmExactRef, DroppedAndRecreatedRefIsNo)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_recreate"};
+
+    const ManifestId first = publishEmptyPart(store, ns, "x");
+    ASSERT_EQ(store->confirmExactRef(ns, "x", first.ref), ConfirmAnswer::Yes);
+
+    store->dropRef(ns, "x");
+    EXPECT_EQ(store->confirmExactRef(ns, "x", first.ref), ConfirmAnswer::No)
+        << "a dropped ref cannot authorize anything";
+
+    const ManifestId second = publishEmptyPart(store, ns, "x");
+    ASSERT_NE(first.ref, second.ref);
+    EXPECT_EQ(store->confirmExactRef(ns, "x", first.ref), ConfirmAnswer::No);
+    EXPECT_EQ(store->confirmExactRef(ns, "x", second.ref), ConfirmAnswer::Yes);
+}
+
+
+/// Rule 2, the cold case: a namespace this mount has never touched has no resident runtime, so the
+/// answer is `Unknown` -- and producing it must cost ZERO object-store requests AND must not create a
+/// runtime. Materializing one here would let a remote caller populate this writer's table cache with
+/// unrecovered entries by asking about namespaces that do not exist.
+TEST(CASConfirmExactRef, ColdTableIsUnknownWithZeroBackendRequests)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace warm{"srv1/confirm_cold_warm"};
+    const RootNamespace cold{"srv1/confirm_cold_never_touched"};
+
+    const ManifestId id = publishEmptyPart(store, warm, "x");
+
+    const size_t cached_before = store->refTablesCachedCountForTest();
+    backend->resetCounts();
+
+    EXPECT_EQ(store->confirmExactRef(cold, "x", id.ref), ConfirmAnswer::Unknown);
+
+    EXPECT_EQ(backendRequests(*backend), 0u)
+        << "a cold table must answer Unknown without recovering from storage";
+    EXPECT_EQ(store->refTablesCachedCountForTest(), cached_before)
+        << "confirmExactRef must find the runtime, never create one";
+}
+
+
+/// Rule 2, the evicted case: a table that WAS warm and was dropped by the whole-table cache budget is
+/// indistinguishable from a cold one here -- the runtime is gone, so the committed view is gone with
+/// it, and re-reading it would be object-store I/O.
+TEST(CASConfirmExactRef, EvictedTableIsUnknownWithZeroBackendRequests)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolWithConfig(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .ref_table_cache_bytes = 1});
+    const RootNamespace ns_a{"srv1/confirm_evict_a"};
+    const RootNamespace ns_b{"srv1/confirm_evict_b"};
+
+    const ManifestId id_a = publishEmptyPart(store, ns_a, "x");
+    ASSERT_EQ(store->confirmExactRef(ns_a, "x", id_a.ref), ConfirmAnswer::Yes);
+
+    /// A 1-byte budget is below one table's weight, so touching another table evicts the idle one.
+    publishEmptyPart(store, ns_b, "y");
+    ASSERT_FALSE(store->refTableCachedForTest(ns_a)) << "ns_a must have been evicted";
+
+    backend->resetCounts();
+    EXPECT_EQ(store->confirmExactRef(ns_a, "x", id_a.ref), ConfirmAnswer::Unknown);
+    EXPECT_EQ(backendRequests(*backend), 0u)
+        << "an evicted table must answer Unknown without re-recovering";
+    EXPECT_FALSE(store->refTableCachedForTest(ns_a))
+        << "the confirm must not have re-recovered the evicted table as a side effect";
+}
+
+
+/// Rule 2, the resident-but-unrecovered case: recovery failed closed, so a runtime EXISTS in the cache
+/// with an empty, meaningless `state`. A naive lookup reads that empty state and answers `No`; the
+/// correct answer is `Unknown`, because nothing about the durable table is known here.
+TEST(CASConfirmExactRef, UnrecoveredResidentTableIsUnknownWithZeroBackendRequests)
+{
+    auto backend = std::make_shared<RecoveryLatchBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_unrecovered"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns);
+
+    const size_t cached_before = store->refTablesCachedCountForTest();
+    backend->fail_get_once_key = store->layout().refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_THROW(store->resolveRef(ns, "x"), DB::Exception);
+
+    ASSERT_EQ(store->refTablesCachedCountForTest(), cached_before + 1u)
+        << "the failed recovery must still leave a runtime resident";
+    ASSERT_FALSE(store->refTableCachedForTest(ns)) << "that runtime must be UNRECOVERED";
+
+    backend->resetCounts();
+    EXPECT_EQ(store->confirmExactRef(ns, "x", ManifestRef{1, 1, 1}), ConfirmAnswer::Unknown);
+    EXPECT_EQ(backendRequests(*backend), 0u)
+        << "an unrecovered table must answer Unknown without driving recovery";
+    EXPECT_FALSE(store->refTableCachedForTest(ns))
+        << "the confirm must not have recovered the table as a side effect";
+}
+
+
+/// Rule 2, the recovering case: another caller is INSIDE `ensureRefTableRecovered`, parked on its
+/// exact `_ckpt` GET. The runtime is resident, `recovery_in_progress` is set, and the state is still
+/// empty. Waiting for that recovery would be exactly the "recover from storage to answer" this
+/// primitive refuses.
+TEST(CASConfirmExactRef, RecoveryInProgressIsUnknownWithZeroBackendRequests)
+{
+    auto backend = std::make_shared<RecoveryLatchBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_recovering"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns);
+    backend->armBlockedGet(store->layout().refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns)));
+    std::exception_ptr recovery_error;
+    std::thread recoverer([&]
+    {
+        try
+        {
+            store->resolveRef(ns, "x");
+        }
+        catch (...)
+        {
+            recovery_error = std::current_exception();
+        }
+    });
+    backend->awaitBlockedGet();
+
+    backend->resetCounts();
+    /// The parked exact GET is outside `state_mutex` (up to the 20s block bound), so this
+    /// call must NOT be a blocking acquire of that mutex: waiting would make the confirm pay for
+    /// somebody else's recovery while holding pool-wide append admission. The elapsed bound is what
+    /// pins that -- it is an order of magnitude below the park, so it cannot pass by luck.
+    const auto started = std::chrono::steady_clock::now();
+    const ConfirmAnswer answer = store->confirmExactRef(ns, "x", ManifestRef{1, 1, 1});
+    const auto elapsed = std::chrono::steady_clock::now() - started;
+    const uint64_t requests = backendRequests(*backend);
+
+    backend->releaseBlockedGet();
+    recoverer.join();
+
+    if (recovery_error)
+    {
+        try
+        {
+            std::rethrow_exception(recovery_error);
+        }
+        catch (const std::exception & e)
+        {
+            FAIL() << "the driving recovery unexpectedly failed: " << e.what();
+        }
+        catch (...)
+        {
+            FAIL() << "the driving recovery unexpectedly failed with a non-standard exception";
+        }
+    }
+
+    EXPECT_EQ(answer, ConfirmAnswer::Unknown);
+    EXPECT_EQ(requests, 0u)
+        << "a recovering table must answer Unknown without issuing (or waiting on) any request";
+    EXPECT_LT(elapsed, std::chrono::seconds(5))
+        << "the confirm waited for the in-progress recovery instead of answering Unknown";
+}
+
+
+/// Rule 3, the in-flight case: an append is admitted and its leader is parked in the pre-carve window.
+/// Nothing is durable yet and the committed row still matches EXACTLY -- which is precisely why a
+/// naive implementation answers `Yes` here, and precisely why that is the TOCTOU this design closes.
+/// The apply-state is still `Clean` at this point, so rule 3 is what produces the `Unknown`, not
+/// rule 4.
+TEST(CASConfirmExactRef, InFlightAppendIsUnknown)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_inflight"};
+
+    const ManifestId id = publishEmptyPart(store, ns, "x");
+    ASSERT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::Yes);
+
+    LeaderLatch latch;
+    latch.arm(store);
+    std::thread dropper([&] { store->dropRef(ns, "x"); });
+    latch.awaitEntered();
+
+    /// Sampled while parked, asserted after the join: a failed assertion here must not skip the
+    /// release, or the still-joinable `dropper` would terminate the whole suite instead of failing one
+    /// test.
+    const bool leader_active = store->refLeaderActiveForTest(ns);
+    const RefLaneState apply_state = store->laneStateForTest(ns);
+    const ConfirmAnswer while_in_flight = store->confirmExactRef(ns, "x", id.ref);
+
+    latch.release();
+    dropper.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    EXPECT_TRUE(leader_active);
+    EXPECT_EQ(apply_state, RefLaneState::Ready)
+        << "the pre-carve window is before any PUT, so rule 4 must not be what answers here";
+    EXPECT_EQ(while_in_flight, ConfirmAnswer::Unknown)
+        << "an admitted append makes the whole table's committed view provisional";
+
+    EXPECT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::No);
+}
+
+
+/// Rule 3, mid-tenure (spec §testing "mid-tenure chunked flush", `CarvePhaseForTest::ChunkReseed`):
+/// one leader tenure commits MULTIPLE durable transactions, so at a chunk boundary the table is
+/// PARTIALLY durable. `leader_active` covers the whole tenure, so this is already `Unknown` -- a wider
+/// unknown window under load, never a hole. The confirm is issued on the leader's own thread, which is
+/// safe because the boundary holds neither lane mutex.
+TEST(CASConfirmExactRef, MidTenureChunkBoundaryIsUnknown)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_mid_tenure"};
+
+    const ManifestId id = publishEmptyPart(store, ns, "seed");
+    ASSERT_EQ(store->confirmExactRef(ns, "seed", id.ref), ConfirmAnswer::Yes);
+
+    std::atomic<int> boundaries{0};
+    std::atomic<int> unknown_at_boundary{0};
+    std::atomic<int> requests_at_boundary{0};
+    store->setCarveHookForTest([&](CasRefLedger::CarvePhaseForTest phase)
+    {
+        if (phase != CasRefLedger::CarvePhaseForTest::ChunkReseed)
+            return;
+        boundaries.fetch_add(1);
+        const uint64_t before = backendRequests(*backend);
+        if (store->confirmExactRef(ns, "seed", id.ref) == ConfirmAnswer::Unknown)
+            unknown_at_boundary.fetch_add(1);
+        requests_at_boundary.fetch_add(static_cast<int>(backendRequests(*backend) - before));
+    });
+
+    /// Two co-batched items of 3000 ops each (1500 precommit add/remove pairs). One item may not
+    /// exceed the 5000-op `ref_txn_max_ops` cap on its own -- that fails the item outright -- so the
+    /// chunk boundary has to come from a BATCH: 6000 ops carved into one tenure split into two
+    /// transactions, firing exactly one boundary. The pre-carve hook parks the first caller until the
+    /// second is queued, which is what makes the co-batching deterministic.
+    auto sync = std::make_shared<CaseSync>();
+    store->setRefPreCarveHookForTest([sync, store, ns]
+    {
+        std::unique_lock lk(sync->m);
+        if (sync->entered)
+            return;
+        sync->entered = true;
+        sync->cv.notify_all();
+        sync->cv.wait_for(lk, std::chrono::seconds(20),
+                          [&] { return store->refQueuePendingForTest(ns) >= 2; });
+    });
+
+    auto append = [&store, &ns](const String & prefix, uint64_t manifest_epoch)
+    {
+        std::vector<RefOp> item_ops = precommitAddRemovePairs(prefix, 1500, manifest_epoch);
+        store->appendRefOps(ns, MutationScope::ref(prefix),
+                            [ops = std::move(item_ops)](const RefTableState &) { return ops; },
+                            RootMutationOrigin::Writer, RootMutationKind::Publish);
+    };
+    std::thread a([&] { append("aaa_", 900000001); });
+    {
+        std::unique_lock lk(sync->m);
+        sync->cv.wait_for(lk, std::chrono::seconds(20), [&] { return sync->entered; });
+    }
+    std::thread b([&] { append("bbb_", 900000002); });
+    /// The parked leader re-evaluates its predicate only when notified, so the queue depth is polled
+    /// here and the leader released explicitly once both items are admitted.
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(20);
+    while (store->refQueuePendingForTest(ns) < 2 && std::chrono::steady_clock::now() < deadline)
+        std::this_thread::yield();
+    sync->cv.notify_all();
+    a.join();
+    b.join();
+    store->setRefPreCarveHookForTest(nullptr);
+    store->setCarveHookForTest(nullptr);
+
+    ASSERT_GE(boundaries.load(), 1) << "the flush did not chunk -- the mid-tenure window was not exercised";
+    EXPECT_EQ(unknown_at_boundary.load(), boundaries.load())
+        << "a mid-tenure, partially-durable table must never confirm";
+    EXPECT_EQ(requests_at_boundary.load(), 0) << "the mid-tenure confirm must still be I/O-free";
+
+    /// The tenure is over: the seed ref is untouched by it and confirms again.
+    EXPECT_EQ(store->confirmExactRef(ns, "seed", id.ref), ConfirmAnswer::Yes);
+}
+
+
+/// Rule 3, the wedge case: the lane holds one conditional `PUT` whose outcome is unknown, so the table
+/// may be MISSING a durable transaction -- possibly the very removal being asked about. The committed
+/// row still matches exactly, so only the wedge can produce the refusal.
+TEST(CASConfirmExactRef, WedgedLaneIsUnknown)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_wedge"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns);
+
+    const ManifestId id = publishEmptyPart(store, ns, "x");
+    ASSERT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::Yes);
+
+    store->forceWedgeForTest(ns, /*writer_epoch=*/1, /*ref_sequence=*/9999,
+                             store->layout().refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{1, 9999}), "synthetic");
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+
+    backend->resetCounts();
+    EXPECT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::Unknown);
+    EXPECT_EQ(backendRequests(*backend), 0u)
+        << "a wedged lane must answer Unknown without trying to resolve the wedge";
+}
+
+
+/// `NeedsRecovery` is table-scoped, so confirmation refuses even a row that still looks perfect.
+TEST(CASConfirmExactRef, NeedsRecoveryIsUnknown)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_poison"};
+
+    const ManifestId keep = publishEmptyPart(store, ns, "keep");
+    ASSERT_EQ(store->confirmExactRef(ns, "keep", keep.ref), ConfirmAnswer::Yes);
+
+    armOneShotInstallFailure(store);
+    EXPECT_THROW(publishEmptyPart(store, ns, "other"), DB::Exception);
+    store->setInstallRegionProbeForTest(nullptr);
+
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    ASSERT_FALSE(store->refLaneWedgedForTest(ns));
+    ASSERT_FALSE(store->refLeaderActiveForTest(ns));
+
+    EXPECT_EQ(store->confirmExactRef(ns, "keep", keep.ref), ConfirmAnswer::Unknown)
+        << "a table that may be missing a durable transaction cannot confirm ANY of its rows";
+}
+
+
+/// Rule 6, checked LAST: the committed row matches exactly, the lane is quiescent and clean -- but this
+/// node no longer holds the mount incarnation, so it is no longer the namespace's single writer and
+/// cannot speak for the durable table at all. Another writer may already have repointed the ref.
+TEST(CASConfirmExactRef, LostMountFenceIsUnknown)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_fence"};
+
+    const ManifestId id = publishEmptyPart(store, ns, "x");
+    ASSERT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::Yes);
+
+    store->tripMountLost();
+
+    ASSERT_TRUE(store->refTableCachedForTest(ns))
+        << "the table must still be resident, so it is the FENCE that refuses, not residency";
+    backend->resetCounts();
+    EXPECT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::Unknown);
+    EXPECT_EQ(backendRequests(*backend), 0u);
+
+    /// The fence is checked LAST, so it gates only the `Yes`: a token that does not match the committed
+    /// row is still reported as `No` under a lost fence. That is deliberate and harmless -- `No` and
+    /// `Unknown` are the same outcome for the caller (both `SourceProofFailed`) -- and pinning it here
+    /// keeps a future reordering of the rules from changing the answer silently.
+    ManifestRef other = id.ref;
+    ++other.manifest_ordinal;
+    EXPECT_EQ(store->confirmExactRef(ns, "x", other), ConfirmAnswer::No);
+}
+
+
+/// The two-mutex snapshot race (spec §testing, "an append admitted concurrently is ordered strictly
+/// after the snapshot"). The confirm holds `ref_queue_mutex` across the whole snapshot, and admission
+/// (`pending.push_back`) takes that same mutex, so every append is either entirely before the snapshot
+/// (and visible as a pending item -> `Unknown`) or entirely after it. What must NOT exist is a window
+/// in which the removal is admitted and the confirm still says `Yes`.
+///
+/// The three phases are driven deterministically rather than hammered: before admission -> `Yes`;
+/// from admission until the transaction is durable -> `Unknown`; after -> `No`. `Yes` is never
+/// observable once the removal has been admitted.
+TEST(CASConfirmExactRef, ConcurrentAppendIsOrderedAfterTheSnapshot)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/confirm_race"};
+
+    const ManifestId id = publishEmptyPart(store, ns, "x");
+    ASSERT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::Yes) << "phase 1: before admission";
+
+    LeaderLatch latch;
+    latch.arm(store);
+    std::thread dropper([&] { store->dropRef(ns, "x"); });
+    latch.awaitEntered();
+
+    /// Phase 2: admitted, nothing durable. Sampled repeatedly so a single lucky interleaving cannot
+    /// pass for the invariant, and TALLIED rather than asserted -- an assertion here would skip the
+    /// release below and terminate the suite on the still-joinable `dropper`.
+    int not_unknown = 0;
+    int saw_yes = 0;
+    for (int i = 0; i < 64; ++i)
+    {
+        const ConfirmAnswer a = store->confirmExactRef(ns, "x", id.ref);
+        if (a != ConfirmAnswer::Unknown)
+            ++not_unknown;
+        if (a == ConfirmAnswer::Yes)
+            ++saw_yes;
+    }
+
+    latch.release();
+    dropper.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    EXPECT_EQ(saw_yes, 0) << "phase 2: an admitted removal must never leave a Yes visible";
+    EXPECT_EQ(not_unknown, 0) << "phase 2: an admitted append makes the committed view provisional";
+
+    /// Phase 3: durable and applied.
+    EXPECT_EQ(store->confirmExactRef(ns, "x", id.ref), ConfirmAnswer::No) << "phase 3: after the removal";
+}
+
+
+/// ===========================================================================================
+/// Task 11: the EXCHANGE-level confirm -- `IContentAddressedExchange::ownsNamespace` (routing) and
+/// `::confirmExactRef` (the storage forward of gate 1, plus the token text and the disk lifecycle).
+///
+/// Gate 0 is not exercised here on purpose: it reads a `StorageReplicatedMergeTree` parts set, so
+/// `Deleting`, absent, other-disk and the `MOVE ... TO DISK` same-name case are integration-level and
+/// belong to the Task 16 pytest battery.
+/// ===========================================================================================
+
+namespace
+{
+
+/// A storage adapter over its own private local object storage. `startup` is the caller's business:
+/// several tests below assert behavior BEFORE it and AFTER `shutdown`.
+std::shared_ptr<DB::ContentAddressedMetadataStorage> makeExchangeStorage(const std::string & server_root_id)
+{
+    auto settings = tests::makeSettingsForTest(
+        server_root_id, std::filesystem::temp_directory_path() / "ca_confirm_exchange_scratch");
+    return std::make_shared<DB::ContentAddressedMetadataStorage>(
+        tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+}
+
+const std::string kExchangeTableDir = "e11/e11e11e1-0808-4808-8808-080808080808";
+const std::string kExchangePartName = "all_1_1_0";
+const std::string kExchangePartDir = kExchangeTableDir + "/" + kExchangePartName;
+
+/// Commit one real part through the ordinary transaction path, so the committed binding under test is
+/// produced exactly the way an INSERT produces it.
+void commitExchangePart(DB::ContentAddressedMetadataStorage & storage)
+{
+    auto tx = storage.createTransaction();
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+    auto buf = ca_tx.writeFile(kExchangeTableDir + "/tmp_insert_" + kExchangePartName + "/data.bin",
+                               65536, DB::WriteMode::Rewrite, {});
+    const std::string bytes = "content-of-the-part";
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+    tx->moveDirectory(kExchangeTableDir + "/tmp_insert_" + kExchangePartName, kExchangePartDir);
+    tx->commit(DB::NoCommitOptions{});
+}
+
+/// The token the sender mints for the committed part, decoded back into its fields. Read through the
+/// real offer path rather than reconstructed, so the tests below exercise exactly what goes on the wire.
+DB::CasRelinkSourceToken readSourceToken(const DB::ContentAddressedMetadataStorage & storage)
+{
+    const auto offer = storage.getRelinkOffer(kExchangePartDir);
+    EXPECT_TRUE(offer.has_value()) << "the committed part must offer a manifest to relink";
+    if (!offer)
+        return {};
+    const auto token = DB::decodeCasRelinkSourceToken(offer->confirm_token);
+    EXPECT_TRUE(token.has_value()) << "the sender minted a token its own decoder rejects";
+    return token.value_or(DB::CasRelinkSourceToken{});
+}
+
+}
+
+
+/// The canonical text form of a `ManifestRef` becomes wire input with Task 11: it is what the confirm
+/// token carries and what `confirmExactRef` compares. It is therefore parsed as untrusted input --
+/// exactly three decimal fields, nothing consumed partially, no sign, no padding -- and the parser is
+/// pinned as the exact inverse of the renderer, because a mismatch between the two would silently turn
+/// every confirm into an `Unknown` (or, far worse, make two different manifests compare equal).
+TEST(CASConfirmExactRef, ManifestRefTextRoundTripsAndRejectsMalformedTokens)
+{
+    for (const ManifestRef & ref : {ManifestRef{1, 1, 1}, ManifestRef{7, 42, 999999},
+                                    ManifestRef{18446744073709551615ULL, 18446744073709551615ULL, 123}})
+    {
+        const String text = manifestRefDebugString(ref);
+        const auto parsed = tryParseManifestRef(text);
+        ASSERT_TRUE(parsed.has_value()) << "the renderer produced text its own parser rejects: " << text;
+        EXPECT_EQ(*parsed, ref) << text;
+    }
+
+    EXPECT_EQ(manifestRefDebugString(ManifestRef{7, 42, 3}), "7:42:3") << "the canonical form is epoch:build:ordinal";
+
+    for (const std::string_view malformed : {
+             "", "1", "1:2", "1:2:3:4", ":2:3", "1::3", "1:2:", "1:2:x", "x:2:3", " 1:2:3", "1:2:3 ",
+             "1: 2:3", "+1:2:3", "-1:2:3", "1:2:3\n", "0x1:2:3",
+             /// `0` is the reserved invalid ordinal and is never emitted; `1000000` is past the six-digit
+             /// filename range, so neither can name a real manifest.
+             "1:2:0", "1:2:1000000",
+             /// One past `uint64` / `uint32` -- `from_chars` reports overflow rather than truncating.
+             "18446744073709551616:1:1", "1:18446744073709551616:1", "1:1:4294967296"})
+    {
+        EXPECT_FALSE(tryParseManifestRef(malformed).has_value())
+            << "accepted a malformed manifest reference: '" << malformed << "'";
+    }
+}
+
+
+/// Task 13, the confirm token's wire codec (spec §wire-protocol). The token is minted by the sender,
+/// stored nowhere, and handed back by an untrusted peer, so the only property that matters is that
+/// decode is the exact inverse of encode: the fields the sender meant are the fields that route and
+/// compare. A codec that merged two fields, or that let a separator through unescaped, would let a
+/// peer aim a confirm at a namespace the sender never named.
+TEST(CASConfirmExactRef, SourceTokenRoundTripsThroughItsWireForm)
+{
+    const auto round_trip = [](const DB::CasRelinkSourceToken & token, const char * what)
+    {
+        const auto text = DB::encodeCasRelinkSourceToken(token);
+        ASSERT_TRUE(text.has_value()) << what;
+        /// The wire form is cookie-safe and URL-safe by construction: only the RFC 3986 unreserved set,
+        /// the escape character, and the field separator ever appear in it.
+        for (const char ch : *text)
+            EXPECT_TRUE(std::isalnum(static_cast<unsigned char>(ch))
+                        || std::string_view("-._~%|").find(ch) != std::string_view::npos)
+                << "the wire form leaked an unsafe character '" << ch << "' from " << what << ": " << *text;
+
+        const auto decoded = DB::decodeCasRelinkSourceToken(*text);
+        ASSERT_TRUE(decoded.has_value()) << what << ": " << *text;
+        EXPECT_EQ(decoded->pool_uuid, token.pool_uuid) << what;
+        EXPECT_EQ(decoded->server_root_id, token.server_root_id) << what;
+        EXPECT_EQ(decoded->root_namespace, token.root_namespace) << what;
+        EXPECT_EQ(decoded->ref_name, token.ref_name) << what;
+        EXPECT_EQ(decoded->part_name, token.part_name) << what;
+        EXPECT_EQ(decoded->manifest_ref_text, token.manifest_ref_text) << what;
+    };
+
+    round_trip({"abcdef0123456789", "srv1", "srv1/store/abc/abcdef@cas@", "all_1_1_0", "all_1_1_0", "1:1:1"},
+               "the ordinary shape");
+    /// The characters that make a naive codec wrong: the separator itself, the escape character, the
+    /// cookie-forbidden set, and the `/`+`@` a namespace and a detached ref carry as a matter of course.
+    round_trip({"p|o%o=l", "srv 1;x,y", "srv 1;x,y/store/abc/abcdef@cas@", "detached/broken_all_1_1_0",
+                "all_1_1_0", "18446744073709551615:18446744073709551615:999999"},
+               "the hostile shape");
+    /// A field at the cap must survive; one past it must not (asserted below).
+    round_trip({String(256, 'a'), "srv1", "srv1/store/abc/abcdef@cas@", "all_1_1_0", "all_1_1_0", "1:1:1"},
+               "a field at the length cap");
+}
+
+/// Everything a peer can hand back that is not a token this sender minted. None of these may decode:
+/// a decoded-but-wrong token routes a confirm somewhere, and "somewhere" is exactly what routing exists
+/// to prevent. Refusing costs a byte fetch and nothing else.
+TEST(CASConfirmExactRef, SourceTokenRejectsMalformedAndOverlongInput)
+{
+    const DB::CasRelinkSourceToken good{"pool", "srv1", "srv1/store/abc/abcdef@cas@", "all_1_1_0", "all_1_1_0", "1:1:1"};
+    const String text = DB::encodeCasRelinkSourceToken(good).value();
+
+    /// Every literal below is a SEVEN-segment token (version + six fields) unless it is testing the
+    /// segment count itself, so each case fails for the reason it names and not because it is short.
+    for (const std::string_view malformed : {
+             /// Empty, no version, the wrong version, and versions that merely start or end right.
+             "", "|a|b|c|d|e|f", "car0|a|b|c|d|e|f", "car|a|b|c|d|e|f", "car11|a|b|c|d|e|f",
+             /// Too few and too many fields -- a shape that is one field off must not shift the rest.
+             "car1|a|b|c|d|e", "car1|a|b|c|d|e|f|g", "car1", "car1|",
+             /// An empty field: a token with a hole in it routes somewhere it was not meant to.
+             "car1||b|c|d|e|f", "car1|a|b|c|d|e|",
+             /// Malformed escapes: truncated, non-hex, and a lone escape character.
+             "car1|%|b|c|d|e|f", "car1|%4|b|c|d|e|f", "car1|%zz|b|c|d|e|f", "car1|a%|b|c|d|e|f",
+             /// Unescaped bytes outside the unreserved set: the decoder is the encoder's inverse, so
+             /// anything the encoder would have escaped is not a token, however readable it looks.
+             "car1|a/b|c|d|e|f|g", "car1|a b|c|d|e|f|g", "car1|a@b|c|d|e|f|g", "car1|1:1:1|b|c|d|e|f",
+             /// A control character smuggled in as an escape -- the classic forged-log-line vector.
+             "car1|a%00b|c|d|e|f|g", "car1|a%0Ab|c|d|e|f|g"})
+    {
+        EXPECT_FALSE(DB::decodeCasRelinkSourceToken(malformed).has_value())
+            << "accepted a malformed source token: '" << malformed << "'";
+    }
+
+    /// Over-long: refused in BOTH directions, so an over-long field can neither be minted nor accepted.
+    DB::CasRelinkSourceToken too_long = good;
+    too_long.ref_name = String(257, 'a');
+    EXPECT_FALSE(DB::encodeCasRelinkSourceToken(too_long).has_value());
+    EXPECT_FALSE(DB::decodeCasRelinkSourceToken(
+        "car1|pool|srv1|ns|" + String(257, 'a') + "|all_1_1_0|1%3A1%3A1").has_value());
+    /// A field whose ENCODED form blows the whole-token cap (every byte escapes to three).
+    DB::CasRelinkSourceToken all_escaped = good;
+    all_escaped.root_namespace = String(200, ' ');
+    all_escaped.ref_name = String(200, ' ');
+    EXPECT_FALSE(DB::encodeCasRelinkSourceToken(all_escaped).has_value());
+    EXPECT_FALSE(DB::decodeCasRelinkSourceToken(String(2000, 'a')).has_value());
+
+    /// An empty field is refused on the way out too, not only on the way in.
+    DB::CasRelinkSourceToken empty_field = good;
+    empty_field.part_name.clear();
+    EXPECT_FALSE(DB::encodeCasRelinkSourceToken(empty_field).has_value());
+
+    /// The control-character refusal is symmetric: the sender cannot mint one either.
+    DB::CasRelinkSourceToken control = good;
+    control.server_root_id = "srv\n1";
+    EXPECT_FALSE(DB::encodeCasRelinkSourceToken(control).has_value());
+
+    /// Sanity: the good token itself decodes, so the rejections above are about the input and not about
+    /// a codec that refuses everything.
+    EXPECT_TRUE(DB::decodeCasRelinkSourceToken(text).has_value());
+}
+
+
+/// Routing (spec §wire-protocol). A pool UUID is shared by every server root writing into the pool, so
+/// it cannot select the mount entitled to answer for a namespace; `ownsNamespace` is what does. It is a
+/// pure string question about the mount's own identity: no pool, no I/O, no lifecycle -- asserted here
+/// by answering the same before `startup`, while live, and after `shutdown`. A routing predicate that
+/// could throw would turn a misrouted question into an error instead of an unproven answer.
+TEST(CASConfirmExactRef, OwnsNamespaceSelectsTheMountByServerRootInEveryLifecycleState)
+{
+    auto storage = makeExchangeStorage("srv1");
+
+    const auto assert_routing = [&](const char * phase)
+    {
+        /// Live and detached namespaces are `<server_root_id>/<mirrored table dir>` (`liveNamespace`).
+        EXPECT_TRUE(storage->ownsNamespace("srv1", "srv1/store/abc/abcdef@cas@")) << phase;
+        EXPECT_TRUE(storage->ownsNamespace("srv1", storage->liveNamespace("abcdef").string())) << phase;
+
+        /// A different server root's namespace, and this namespace asked about under a different server
+        /// root: the same pool, a different owner. Both must miss, or a confirm could be answered by a
+        /// mount that never wrote the ref.
+        EXPECT_FALSE(storage->ownsNamespace("srv2", "srv1/store/abc/abcdef@cas@")) << phase;
+        EXPECT_FALSE(storage->ownsNamespace("srv1", "srv2/store/abc/abcdef@cas@")) << phase;
+
+        /// The prefix trap: a bare `starts_with(server_root_id)` would let `srv1` claim `srv10`.
+        EXPECT_FALSE(storage->ownsNamespace("srv1", "srv10/store/abc/abcdef@cas@")) << phase;
+        EXPECT_FALSE(storage->ownsNamespace("srv10", "srv10/store/abc/abcdef@cas@")) << phase;
+
+        /// The server root itself is not a namespace, and a pool-global FREEZE tree belongs to no mount.
+        EXPECT_FALSE(storage->ownsNamespace("srv1", "srv1")) << phase;
+        EXPECT_FALSE(storage->ownsNamespace("srv1", "shadow/backup/store/abc/abcdef")) << phase;
+
+        /// Empty fields are never a match -- an absent token field must not route anywhere.
+        EXPECT_FALSE(storage->ownsNamespace("", "")) << phase;
+        EXPECT_FALSE(storage->ownsNamespace("", "srv1/store/abc/abcdef@cas@")) << phase;
+        EXPECT_FALSE(storage->ownsNamespace("srv1", "")) << phase;
+    };
+
+    assert_routing("before startup");
+    storage->startup();
+    assert_routing("while live");
+    storage->shutdown();
+    assert_routing("after shutdown");
+}
+
+
+/// The storage forward of gate 1, driven end to end: a part committed through the ordinary transaction
+/// path, and a token read out of the very manifest body the sender puts on the wire. The three
+/// non-`Yes` cases pin the two halves this layer adds on top of the ledger -- the token text is decoded
+/// here, and a namespace this mount holds no resident runtime for is an ambiguity, not a `No`.
+TEST(CASConfirmExactRef, StorageConfirmAnswersForTheCommittedBinding)
+{
+    auto storage = makeExchangeStorage("test");
+    storage->startup();
+    commitExchangePart(*storage);
+
+    const DB::CasRelinkSourceToken token = readSourceToken(*storage);
+    ASSERT_TRUE(storage->ownsNamespace("test", token.root_namespace))
+        << "the sender must route its own committed namespace to itself: " << token.root_namespace;
+
+    EXPECT_EQ(storage->confirmExactRef(token.root_namespace, kExchangePartName, token.manifest_ref_text),
+              DB::CasConfirmAnswer::Yes);
+
+    /// A manifest this ref never named, and a ref name that was never committed: both are knowledge on
+    /// a warm table, and both are `No` -- which the caller must still treat as "not proven".
+    const auto other_ref = tryParseManifestRef(token.manifest_ref_text);
+    ASSERT_TRUE(other_ref.has_value());
+    EXPECT_EQ(storage->confirmExactRef(token.root_namespace, kExchangePartName,
+                                       manifestRefDebugString(ManifestRef{other_ref->writer_epoch,
+                                                                          other_ref->build_sequence,
+                                                                          other_ref->manifest_ordinal + 1})),
+              DB::CasConfirmAnswer::No);
+    EXPECT_EQ(storage->confirmExactRef(token.root_namespace, "all_9_9_9", token.manifest_ref_text),
+              DB::CasConfirmAnswer::No);
+
+    /// A namespace with no resident runtime: the ledger will not recover one to answer, so this is an
+    /// ambiguity. It must not read as "the ref does not exist".
+    EXPECT_EQ(storage->confirmExactRef("test/store/zzz/zzzzzz@cas@", kExchangePartName, token.manifest_ref_text),
+              DB::CasConfirmAnswer::Unknown);
+
+    /// An unparsable token: the question cannot be understood, so it cannot be answered `No`.
+    EXPECT_EQ(storage->confirmExactRef(token.root_namespace, kExchangePartName, "not-a-manifest-ref"),
+              DB::CasConfirmAnswer::Unknown);
+    EXPECT_EQ(storage->confirmExactRef(token.root_namespace, kExchangePartName, ""),
+              DB::CasConfirmAnswer::Unknown);
+
+    storage->shutdown();
+}
+
+
+/// The disk's own lifecycle is an answer, not an exception. The confirm is served on an interserver
+/// request, and the caller has a durable precommit waiting on it: a thrown `INVALID_STATE` would have
+/// to be classified by the HTTP layer, whereas `Unknown` is already the taxonomy's "not proven". Only
+/// `Yes` authorizes, and no lifecycle state can produce one.
+TEST(CASConfirmExactRef, StorageConfirmIsUnknownWhenTheDiskCannotSpeakForItsView)
+{
+    auto storage = makeExchangeStorage("test");
+
+    /// Never started: no pool has ever been published, so there is no committed view at all.
+    EXPECT_EQ(storage->confirmExactRef("test/store/abc/abcdef@cas@", kExchangePartName, "1:1:1"),
+              DB::CasConfirmAnswer::Unknown);
+
+    storage->startup();
+    commitExchangePart(*storage);
+    const DB::CasRelinkSourceToken token = readSourceToken(*storage);
+    ASSERT_EQ(storage->confirmExactRef(token.root_namespace, kExchangePartName, token.manifest_ref_text),
+              DB::CasConfirmAnswer::Yes);
+
+    /// Shut down: the same question, the same token, and the same table -- but this process no longer
+    /// speaks for the namespace.
+    storage->shutdown();
+    EXPECT_EQ(storage->confirmExactRef(token.root_namespace, kExchangePartName, token.manifest_ref_text),
+              DB::CasConfirmAnswer::Unknown);
+}
diff --git a/src/Disks/tests/gtest_cas_decommission.cpp b/src/Disks/tests/gtest_cas_decommission.cpp
new file mode 100644
index 000000000000..138d69b0f9b6
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_decommission.cpp
@@ -0,0 +1,1366 @@
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <algorithm>
+#include <atomic>
+#include <limits>
+#include <stdexcept>
+#include <tuple>
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+
+using namespace DB;
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+/// Open a store for the VICTIM srid over `backend` (the pool's future dead member).
+PoolPtr openVictim(std::shared_ptr<InMemoryBackend> backend)
+{
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "victim"});
+}
+
+void drainCompletedNamespaceRemovals(const std::shared_ptr<InMemoryBackend> & backend)
+{
+    PoolConfig config{
+        .pool_prefix = "p",
+        .server_root_id = "gc",
+        .gc_fold_threshold = 1,
+        .gc_fold_max_defer_rounds = 0};
+    auto store = Pool::open(backend, config);
+    Gc gc(store, UInt128{991});
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+}
+
+/// Fails `deleteExact` for one or two designated keys -- either by throwing (a transient backend
+/// hiccup) or by returning a synthetic `TokenMismatch` (a "listed but raced" outcome) -- delegating
+/// every other key to the base `InMemoryBackend` untouched. Drives the drain phases' per-object
+/// fail-close path (`deleteListedPrefix`/`sweepNamespace`, `CasDecommission.cpp`/
+/// `CasOrphanManifestSweep.cpp`): a failure on one listed object must record a warning and let the rest
+/// of the sweep proceed, never abort the whole phase.
+///
+class FailingDeleteBackend : public InMemoryBackend
+{
+public:
+    void failWithThrow(const String & key) { throw_key = key; }
+    void failWithTokenMismatch(const String & key) { mismatch_key = key; }
+    /// Clears every injected failure -- the resume half of a fail-then-retry test (Task 4).
+    void disarm() { throw_key.clear(); mismatch_key.clear(); }
+
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        if (key == throw_key)
+            throw std::runtime_error("injected transient delete failure for " + key);
+        if (key == mismatch_key)
+            return DeleteOutcome{.kind = DeleteOutcome::Kind::TokenMismatch};
+        return InMemoryBackend::deleteExact(key, token);
+    }
+
+private:
+    String throw_key;
+    String mismatch_key;
+};
+
+/// Replaces the durable catalog immediately after returning the first armed catalog read. This
+/// distinguishes the immutable cut validated before decommission impersonation from a later mount
+/// safety observation without assuming those two decisions share one GET.
+class CatalogChangesAfterFirstReadBackend : public InMemoryBackend
+{
+public:
+    using Backend::get;
+
+    void armCatalogReplacement(
+        const String & key, RefCatalog replacement_, size_t completed_reads_before_replacement = 0)
+    {
+        catalog_key = key;
+        replacement = std::move(replacement_);
+        reads_to_skip = completed_reads_before_replacement;
+        armed = true;
+    }
+
+    bool fired() const { return replacement_fired; }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        auto got = InMemoryBackend::get(key, range);
+        if (!armed || replacement_fired || key != catalog_key)
+            return got;
+        if (reads_to_skip > 0)
+        {
+            --reads_to_skip;
+            return got;
+        }
+        if (!got)
+            throw std::runtime_error("catalog replacement fixture: catalog is absent");
+
+        replacement_fired = true;
+        const PutResult put = InMemoryBackend::putOverwrite(
+            key, encodeRefCatalog(replacement), got->token, {});
+        if (put.outcome != PutOutcome::Done)
+            throw std::runtime_error("catalog replacement fixture: rewrite conflicted");
+        return got;
+    }
+
+private:
+    String catalog_key;
+    RefCatalog replacement;
+    size_t reads_to_skip = 0;
+    bool armed = false;
+    bool replacement_fired = false;
+};
+
+std::vector<std::tuple<String, String, Token>> snapshotPrefixObjects(
+    InMemoryBackend & backend, const String & prefix)
+{
+    std::vector<std::tuple<String, String, Token>> objects;
+    String cursor;
+    while (true)
+    {
+        const ListPage page = backend.list(prefix, cursor, 1000);
+        for (const ListedKey & listed : page.keys)
+        {
+            const auto got = backend.get(listed.key);
+            if (!got)
+                throw std::runtime_error("prefix snapshot fixture: listed object disappeared");
+            objects.emplace_back(listed.key, got->bytes, got->token);
+        }
+        if (page.next_cursor.empty())
+            return objects;
+        cursor = page.next_cursor;
+    }
+}
+
+/// Installs a same-UUID successor deterministically in the retirement tail's read/delete window.
+/// Once armed, the backend recognizes the admin's clean farewell `putOverwrite`. On the next read of
+/// either mutable control object it first captures the value that read observed, then bumps `epoch`
+/// and reclaims `mount` with fresh tokens before returning the captured result. Thus the caller holds
+/// exactly the stale token it would have obtained immediately before a concurrent restart reclaimed
+/// the slot, without threads or sleeps.
+class SuccessorReclaimAfterFarewellBackend : public InMemoryBackend
+{
+public:
+    using Backend::get;
+    using Backend::putOverwrite;
+
+    void armForSuccessorReclaim() { armed = true; }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        std::optional<GetResult> result = InMemoryBackend::get(key, range);
+        if (farewell_seen && !successor_injected && (key == mount_key || key == epoch_key))
+            injectSuccessor();
+        return result;
+    }
+
+    PutResult putOverwrite(
+        const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        const PutResult result = InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+        if (armed && key == mount_key && result.outcome == PutOutcome::Done)
+        {
+            const MountLease mount = decodeMountLease(bytes);
+            if (mount.min_active == std::numeric_limits<uint64_t>::max())
+                farewell_seen = true;
+        }
+        return result;
+    }
+
+    bool successorInjected() const { return successor_injected; }
+    const Token & successorMountToken() const { return successor_mount_token; }
+    const Token & successorEpochToken() const { return successor_epoch_token; }
+    const String & successorMountBytes() const { return successor_mount_bytes; }
+    const String & successorEpochBytes() const { return successor_epoch_bytes; }
+
+private:
+    void injectSuccessor()
+    {
+        const auto epoch = InMemoryBackend::get(epoch_key, {});
+        const auto mount = InMemoryBackend::get(mount_key, {});
+        if (!epoch || !mount)
+            throw std::runtime_error("successor-reclaim fixture: control object disappeared before reclaim");
+
+        ServerEpoch epoch_value = decodeServerEpoch(epoch->bytes);
+        const uint64_t successor_writer_epoch = epoch_value.next_writer_epoch;
+        ++epoch_value.next_writer_epoch;
+        successor_epoch_bytes = encodeServerEpoch(epoch_value);
+        const CasResult epoch_put = InMemoryBackend::casPut(
+            epoch_key, successor_epoch_bytes, std::optional<Token>{epoch->token}, {});
+        if (epoch_put.outcome != CasOutcome::Committed)
+            throw std::runtime_error("successor-reclaim fixture: epoch bump conflicted");
+        successor_epoch_token = epoch_put.token;
+
+        MountLease mount_value = decodeMountLease(mount->bytes);
+        mount_value.writer_epoch = successor_writer_epoch;
+        ++mount_value.seq;
+        ++mount_value.started_at_ms;
+        mount_value.expires_at_ms = mount_value.started_at_ms + 30'000;
+        mount_value.min_active = 0;
+        mount_value.gc_fenced = false;
+        successor_mount_bytes = encodeMountLease(mount_value);
+        const PutResult mount_put = InMemoryBackend::putOverwrite(
+            mount_key, successor_mount_bytes, mount->token, {});
+        if (mount_put.outcome != PutOutcome::Done)
+            throw std::runtime_error("successor-reclaim fixture: mount reclaim conflicted");
+        successor_mount_token = mount_put.token;
+        successor_injected = true;
+    }
+
+    inline static const String mount_key = "p/gc/server-roots/victim/mount";
+    inline static const String epoch_key = "p/gc/server-roots/victim/epoch";
+    bool armed = false;
+    bool farewell_seen = false;
+    bool successor_injected = false;
+    Token successor_mount_token;
+    Token successor_epoch_token;
+    String successor_mount_bytes;
+    String successor_epoch_bytes;
+};
+
+/// Recreates the mutable slot objects immediately after decommission successfully deletes `epoch`.
+/// This models a same-UUID successor starting in the final retirement window: `owner` remains the
+/// unchanged identity anchor, while the successor legitimately creates a fresh `epoch` and `mount`.
+class SuccessorReclaimAfterEpochDeleteBackend : public InMemoryBackend
+{
+public:
+    using Backend::get;
+    using Backend::putOverwrite;
+
+    void armForSuccessorReclaim() { armed = true; }
+
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        const DeleteOutcome result = InMemoryBackend::deleteExact(key, token);
+        if (armed && !successor_injected && key == epoch_key
+            && classifyDeleteOutcome(result) == DeleteClass::Deleted)
+        {
+            injectSuccessor();
+        }
+        return result;
+    }
+
+    bool successorInjected() const { return successor_injected; }
+    uint64_t ownerRewriteAttempts() const { return owner_rewrite_attempts; }
+    const Token & successorMountToken() const { return successor_mount_token; }
+    const Token & successorEpochToken() const { return successor_epoch_token; }
+    const String & successorMountBytes() const { return successor_mount_bytes; }
+    const String & successorEpochBytes() const { return successor_epoch_bytes; }
+
+private:
+    PutResult putOverwrite(
+        const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        if (key == owner_key)
+            ++owner_rewrite_attempts;
+        return InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+    }
+
+    void injectSuccessor()
+    {
+        successor_epoch_bytes = encodeServerEpoch(ServerEpoch{.next_writer_epoch = 102});
+        const PutResult epoch_put = InMemoryBackend::putIfAbsent(epoch_key, successor_epoch_bytes, {});
+        if (epoch_put.outcome != PutOutcome::Done)
+            throw std::runtime_error("late-successor fixture: epoch recreation conflicted");
+        successor_epoch_token = epoch_put.token;
+
+        successor_mount_bytes = encodeMountLease(MountLease{
+            .server_uuid = UInt128(0x1234),
+            .writer_epoch = 101,
+            .hostname = "successor",
+            .pid = 42,
+            .started_at_ms = 1'000,
+            .seq = 1,
+            .expires_at_ms = 31'000,
+            .min_active = 0,
+        });
+        const PutResult mount_put = InMemoryBackend::putIfAbsent(mount_key, successor_mount_bytes, {});
+        if (mount_put.outcome != PutOutcome::Done)
+            throw std::runtime_error("late-successor fixture: mount recreation conflicted");
+        successor_mount_token = mount_put.token;
+        successor_injected = true;
+    }
+
+    inline static const String mount_key = "p/gc/server-roots/victim/mount";
+    inline static const String epoch_key = "p/gc/server-roots/victim/epoch";
+    inline static const String owner_key = "p/gc/server-roots/victim/owner";
+    bool armed = false;
+    bool successor_injected = false;
+    uint64_t owner_rewrite_attempts = 0;
+    Token successor_mount_token;
+    Token successor_epoch_token;
+    String successor_mount_bytes;
+    String successor_epoch_bytes;
+};
+
+/// Rewrites the owner anchor after decommission reads it but before its conditional tombstone write.
+/// Returning the captured result gives decommission a stale owner token, deterministically modeling
+/// the successor race without threads or sleeps.
+class SuccessorOwnerRewriteBeforeTombstoneBackend : public InMemoryBackend
+{
+public:
+    using Backend::get;
+
+    void armForSuccessorRewrite() { armed = true; }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        std::optional<GetResult> result = InMemoryBackend::get(key, range);
+        if (armed && epoch_deleted && !successor_injected && key == owner_key && result)
+        {
+            successor_owner_bytes = encodeOwner(OwnerObject{
+                .server_uuid = decodeOwner(result->bytes).server_uuid,
+                .retired_at_ms = std::nullopt,
+            });
+            const PutResult put = InMemoryBackend::putOverwrite(
+                owner_key, successor_owner_bytes, result->token, {});
+            if (put.outcome != PutOutcome::Done)
+                throw std::runtime_error("owner-successor fixture: owner rewrite conflicted");
+            successor_owner_token = put.token;
+            successor_injected = true;
+        }
+        return result;
+    }
+
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        const DeleteOutcome result = InMemoryBackend::deleteExact(key, token);
+        if (armed && key == epoch_key && classifyDeleteOutcome(result) == DeleteClass::Deleted)
+            epoch_deleted = true;
+        return result;
+    }
+
+    bool successorInjected() const { return successor_injected; }
+    const Token & successorOwnerToken() const { return successor_owner_token; }
+    const String & successorOwnerBytes() const { return successor_owner_bytes; }
+
+private:
+    inline static const String epoch_key = "p/gc/server-roots/victim/epoch";
+    inline static const String owner_key = "p/gc/server-roots/victim/owner";
+    bool armed = false;
+    bool epoch_deleted = false;
+    bool successor_injected = false;
+    Token successor_owner_token;
+    String successor_owner_bytes;
+};
+
+/// Models an "ambiguous success" on the final owner tombstone write: the conditional overwrite
+/// actually lands (InMemoryBackend applies it), but the response is then lost (a transient
+/// exception is thrown on the SAME call, exactly as a real SDK timeout after a landed write would
+/// look). Before the fix, decommission caught any exception here and reported failure
+/// unconditionally; the controlled overwrite must resolve this via a GET (the current bytes match
+/// what was intended) and report Committed instead.
+class AmbiguousOwnerTombstoneBackend : public InMemoryBackend
+{
+public:
+    using Backend::putOverwrite;
+
+    void armForAmbiguousTombstone() { armed = true; }
+
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        const PutResult result = InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+        if (armed && !fired && key == owner_key && result.outcome == PutOutcome::Done)
+        {
+            fired = true;
+            throw std::runtime_error("ambiguous-tombstone fixture: response lost after the write landed");
+        }
+        return result;
+    }
+
+private:
+    inline static const String owner_key = "p/gc/server-roots/victim/owner";
+    bool armed = false;
+    bool fired = false;
+};
+
+/// Seed one victim table with `committed` committed refs and `precommits` dangling precommit bindings,
+/// via the raw ref-log seeding helpers (fixture idiom of e.g. `gtest_cas_gc_fold.cpp`: `writeManifestRaw`
+/// + `publishCommittedTransition`/`addPrecommitTransition` against `victim`'s own backend/layout) -- this
+/// fixture only needs the ref-table SHAPE `dropNamespace` erases, not a real build. Precommit bindings
+/// are seeded at an artificially high `writer_epoch` so the writer's own stale-precommit sweep (armed
+/// unconditionally by this table's recovery, unrelated to decommission -- spec §Clean Up Old Precommits)
+/// never reclaims them, in its OWN separate transaction, ahead of `dropNamespace`'s removal.
+void makeTableWithRefs(Pool & victim, const String & ns_str, uint64_t committed, uint64_t precommits)
+{
+    const RootNamespace ns(ns_str);
+    Backend & backend = victim.backend();
+    const Layout & layout = victim.layout();
+
+    /// Final physical ids are pool-wide. The generic raw-write helper intentionally uses one shared
+    /// transition sentinel, so this multi-namespace fixture admits a distinct deterministic test life
+    /// before invoking it; the helper then resolves and preserves that existing catalog identity.
+    const CasRefCatalog::Snapshot catalog = CasRefCatalog::read(backend, layout);
+    const auto existing = std::find_if(catalog.catalog.entries.begin(), catalog.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns.string() == ns.string(); });
+    if (existing == catalog.catalog.entries.end())
+    {
+        static std::atomic<uint64_t> next_test_life{1000};
+        CatalogEntry entry;
+        entry.ns = ns;
+        entry.state = NsState::Live;
+        entry.incarnation = UInt128{next_test_life.fetch_add(1)};
+        CasRefCatalog::casAdmitEntry(backend, layout, 1, entry);
+    }
+
+    uint64_t last_ref_sequence = 0;
+    for (uint64_t i = 0; i < committed; ++i)
+    {
+        const ManifestRef ref{.writer_epoch = 1, .build_sequence = i + 1, .manifest_ordinal = 1};
+        writeManifestRaw(backend, layout, ns, ref, {});
+        last_ref_sequence = publishCommittedTransition(backend, layout, ns, "committed_" + std::to_string(i), std::nullopt, ref);
+    }
+    for (uint64_t i = 0; i < precommits; ++i)
+    {
+        const ManifestRef ref{.writer_epoch = 999999, .build_sequence = i + 1, .manifest_ordinal = 1};
+        writeManifestRaw(backend, layout, ns, ref, {});
+        last_ref_sequence = addPrecommitTransition(backend, layout, ns, UInt128(1), "precommit_" + std::to_string(i), std::nullopt, ref);
+    }
+
+    /// Semantic transition helpers already publish `_ckpt`; replace their final checkpoint through
+    /// the exact token-CAS fixture helper to make this fixture's complete intended state explicit.
+    replaceRecoverableCkptForRawFixture(backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = last_ref_sequence ? std::optional<RefTxnId>{RefTxnId{1, last_ref_sequence}} : std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    /// Self-checking: `listRefs` must observe exactly `committed` committed refs before returning.
+    ASSERT_EQ(victim.listRefs(ns).size(), committed);
+}
+
+/// Pre-precommit manifest debris: a staged manifest body under `ns_str`, at the store's own
+/// `writer_epoch`, named by NO owner event -- a build the writer staged and never finished (fixture
+/// idiom of `gtest_cas_orphan_manifest_sweep.cpp`'s `EligibleAndUnownedIsDeleted`). `build_sequence = 99`
+/// is picked well clear of `makeTableWithRefs`'s own committed/precommit build sequences so it can never
+/// collide with a real owned manifest key. Returns the seeded body's `ManifestId` so a caller can target
+/// it (e.g. its exact object key) for further fixture setup.
+ManifestId seedOrphanManifestBody(Pool & victim, const String & ns_str)
+{
+    const RootNamespace ns(ns_str);
+    const ManifestRef ref{.writer_epoch = victim.writerEpoch(), .build_sequence = 99, .manifest_ordinal = 1};
+    const ManifestId id = writeManifestRaw(victim.backend(), victim.layout(), ns, ref, {});
+    /// EXPECT, not ASSERT: this function returns a value now, and ASSERT_* expands to a bare `return;`
+    /// -- invalid in a non-void function.
+    EXPECT_TRUE(victim.backend().head(victim.layout().manifestKey(id)).exists);
+    return id;
+}
+
+/// THE MANIFEST-DEBRIS DRAIN NO LONGER DELETES, AND THE FIXTURES BELOW SAY SO RATHER THAN WORKING
+/// AROUND IT. The drain goes through `sweepNamespace`, which is subject to the §6 deletion premise: a
+/// manifest of an epoch-`E` build is deletable only once the namespace's sealed fold cursor sits in an
+/// epoch STRICTLY above `E`. Every object in these fixtures -- the table's ref stream, the debris, and
+/// the removal transaction decommission itself appends -- lives in ONE writer epoch, and a single-epoch
+/// pool cannot satisfy that: any cursor high enough to clear the debris's epoch also sits above the
+/// removal record, which would strip the tail-removal protection off the table's real manifests. The
+/// two facts are mutually exclusive here, so there is no honest seeding that restores the deletions;
+/// the tests assert retention, and the drain's reclaim path returns with registers R2/R3 (Stage B).
+
+}
+
+TEST(CASDecommission, RefusesLiveMember)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto victim = openVictim(backend);   /// keeps its mount lease unexpired — the member is alive
+
+    expectThrowsCode(ErrorCodes::ABORTED, [&]
+    {
+        Pool::openForDecommission(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    });
+}
+
+TEST(CASDecommission, ClaimsDeadMemberAndBumpsEpoch)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    uint64_t victim_epoch = 0;
+    {
+        auto victim = openVictim(backend);
+        victim_epoch = victim->writerEpoch();
+    }   /// graceful close: lease stamped already-expired + farewell — the slot is claimable
+
+    auto admin = Pool::openForDecommission(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    ASSERT_TRUE(admin != nullptr);
+    EXPECT_GT(admin->writerEpoch(), victim_epoch);
+    /// The admin store IS the victim server root now (impersonation).
+    EXPECT_EQ(admin->poolConfig().server_root_id, "victim");
+}
+
+TEST(CASDecommission, AlwaysRenewsAdminClaimEvenWhenHostDiskIsObserveOnly)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    {
+        auto victim = openVictim(backend);
+    }   /// graceful close: lease stamped already-expired + farewell — the slot is claimable
+
+    /// The calling (host) disk may be observe-only, i.e. its own PoolConfig carries
+    /// background_watermark = false. The decommission admin claim must renew its lease
+    /// regardless -- a long drain must not expire midway just because the host mount doesn't
+    /// run a background renewer for its OWN mount.
+    auto admin = Pool::openForDecommission(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin", .background_watermark = false}, "victim");
+    ASSERT_TRUE(admin != nullptr);
+    EXPECT_TRUE(admin->poolConfig().background_watermark);
+}
+
+TEST(CASDecommission, RefusesUnknownMember)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    expectThrowsCode(ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        Pool::openForDecommission(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "never_existed");
+    });
+}
+
+TEST(CASDecommission, SecondConcurrentDecommissionRefused)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    { auto victim = openVictim(backend); }
+
+    auto first = Pool::openForDecommission(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    expectThrowsCode(ErrorCodes::ABORTED, [&]
+    {
+        Pool::openForDecommission(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin2"}, "victim");
+    });
+}
+
+TEST(CASDecommission, DuplicateLifeIdRefusesBeforeAnyNamespaceOrSlotMutation)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    { auto victim = openVictim(backend); }
+    const Layout layout("p");
+    RefCatalog catalog;
+    catalog.entries = {
+        CatalogEntry{.ns = RootNamespace{"victim/a"}, .state = NsState::Live, .incarnation = UInt128{77}},
+        CatalogEntry{
+            .ns = RootNamespace{"victim/b"},
+            .state = NsState::Removing,
+            .incarnation = UInt128{77},
+            .removal_started_round = 1},
+    };
+    const auto empty_catalog = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(empty_catalog);
+    ASSERT_EQ(backend->putOverwrite(
+        layout.refCatalogKey(), encodeRefCatalog(catalog), empty_catalog->token).outcome, PutOutcome::Done);
+    const auto owner_before = backend->get(layout.ownerKey("victim"));
+    const auto epoch_before = backend->get(layout.epochKey("victim"));
+    const auto mount_before = backend->get(layout.mountKey("victim"));
+    ASSERT_TRUE(owner_before);
+    ASSERT_TRUE(epoch_before);
+    ASSERT_TRUE(mount_before);
+
+    EXPECT_THROW(decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim"), DB::Exception);
+    const auto owner_after = backend->get(layout.ownerKey("victim"));
+    const auto epoch_after = backend->get(layout.epochKey("victim"));
+    const auto mount_after = backend->get(layout.mountKey("victim"));
+    ASSERT_TRUE(owner_after);
+    ASSERT_TRUE(epoch_after);
+    ASSERT_TRUE(mount_after);
+    EXPECT_EQ(owner_after->bytes, owner_before->bytes);
+    EXPECT_EQ(owner_after->token, owner_before->token);
+    EXPECT_EQ(epoch_after->bytes, epoch_before->bytes);
+    EXPECT_EQ(epoch_after->token, epoch_before->token);
+    EXPECT_EQ(mount_after->bytes, mount_before->bytes);
+    EXPECT_EQ(mount_after->token, mount_before->token);
+}
+
+TEST(CASDecommission, CatalogCutIsValidatedBeforeImpersonationAndReusedForSelection)
+{
+    auto backend = std::make_shared<CatalogChangesAfterFirstReadBackend>();
+    { auto victim = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "victim"}); }
+    const Layout layout("p");
+
+    RefCatalog ambiguous;
+    ambiguous.entries = {
+        CatalogEntry{.ns = RootNamespace{"other/a"}, .state = NsState::Live, .incarnation = UInt128{77}},
+        CatalogEntry{
+            .ns = RootNamespace{"other/b"},
+            .state = NsState::Removing,
+            .incarnation = UInt128{77},
+            .removal_started_round = 1},
+    };
+
+    const auto owner_before = backend->get(layout.ownerKey("victim"));
+    const auto epoch_before = backend->get(layout.epochKey("victim"));
+    const auto mount_before = backend->get(layout.mountKey("victim"));
+    ASSERT_TRUE(owner_before);
+    ASSERT_TRUE(epoch_before);
+    ASSERT_TRUE(mount_before);
+    backend->armCatalogReplacement(layout.refCatalogKey(), std::move(ambiguous));
+
+    EXPECT_THROW(decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim"), DB::Exception);
+    ASSERT_TRUE(backend->fired());
+
+    const auto owner_after = backend->get(layout.ownerKey("victim"));
+    const auto epoch_after = backend->get(layout.epochKey("victim"));
+    const auto mount_after = backend->get(layout.mountKey("victim"));
+    ASSERT_TRUE(owner_after);
+    ASSERT_TRUE(epoch_after);
+    ASSERT_TRUE(mount_after);
+    EXPECT_EQ(owner_after->bytes, owner_before->bytes);
+    EXPECT_EQ(owner_after->token, owner_before->token);
+    EXPECT_EQ(epoch_after->bytes, epoch_before->bytes);
+    EXPECT_EQ(epoch_after->token, epoch_before->token);
+    EXPECT_EQ(mount_after->bytes, mount_before->bytes);
+    EXPECT_EQ(mount_after->token, mount_before->token);
+}
+
+TEST(CASDecommission, NamespaceSelectionUsesThePreImpersonationCut)
+{
+    auto backend = std::make_shared<CatalogChangesAfterFirstReadBackend>();
+    { auto victim = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "victim"}); }
+    const Layout layout("p");
+
+    RefCatalog later;
+    later.entries = {
+        CatalogEntry{.ns = RootNamespace{"victim/late"}, .state = NsState::Live, .incarnation = UInt128{88}},
+    };
+    backend->armCatalogReplacement(layout.refCatalogKey(), std::move(later));
+
+    const DecommissionReport report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    ASSERT_TRUE(backend->fired());
+    EXPECT_EQ(report.namespaces_removed, 0u)
+        << "a namespace visible only to mount safety's later observation is outside the validated cut";
+    EXPECT_EQ(report.namespaces_already_removed, 0u);
+}
+
+TEST(CASDecommission, SameNameRebirthAfterTheCutIsRefusedWithoutTouchingTheNewLife)
+{
+    auto backend = std::make_shared<CatalogChangesAfterFirstReadBackend>();
+    { auto victim = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "victim"}); }
+    const Layout layout("p");
+    const RootNamespace ns{"victim/same"};
+    const NamespaceLifeId old_life = NamespaceLifeId::fromCatalogEntry(ns, UInt128{70});
+    const NamespaceLifeId new_life = NamespaceLifeId::fromCatalogEntry(ns, UInt128{71});
+
+    RefCatalog old_catalog;
+    old_catalog.entries = {
+        CatalogEntry{.ns = ns, .state = NsState::Live, .incarnation = old_life.incarnation},
+    };
+    const auto empty_catalog = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(empty_catalog);
+    ASSERT_EQ(backend->putOverwrite(
+        layout.refCatalogKey(), encodeRefCatalog(old_catalog), empty_catalog->token).outcome,
+        PutOutcome::Done);
+
+    RefLogTxn new_birth;
+    new_birth.ns = ns.string();
+    new_birth.txn_id = RefTxnId{1, 1};
+    new_birth.ops = {namespaceBirthOp()};
+    ASSERT_EQ(backend->putIfAbsent(
+        layout.refLogKey(new_life, new_birth.txn_id),
+        sealObject(FormatId::RefLog, encodeRefLogTxn(new_birth))).outcome,
+        PutOutcome::Done);
+    RefLogTxn new_seal;
+    new_seal.ns = ns.string();
+    new_seal.txn_id = RefTxnId{1, 2};
+    new_seal.ops = {epochSealOp()};
+    ASSERT_EQ(backend->putIfAbsent(
+        layout.refLogKey(new_life, new_seal.txn_id),
+        sealObject(FormatId::RefLog, encodeRefLogTxn(new_seal))).outcome,
+        PutOutcome::Done);
+    const auto new_life_before = snapshotPrefixObjects(*backend, layout.namespaceStreamPrefix(new_life));
+
+    RefCatalog replacement;
+    replacement.entries = {
+        CatalogEntry{.ns = ns, .state = NsState::Live, .incarnation = new_life.incarnation},
+    };
+    /// Read 1 captures the immutable selection cut. Read 2 is mount safety; replace immediately
+    /// after returning that old observation, so the name-only call is the first consumer of the
+    /// same-name new incarnation.
+    backend->armCatalogReplacement(
+        layout.refCatalogKey(), std::move(replacement), /*completed_reads_before_replacement=*/1);
+
+    String refusal;
+    try
+    {
+        (void)decommissionPoolMember(
+            backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    }
+    catch (const DB::Exception & e)
+    {
+        refusal = e.message();
+    }
+    EXPECT_NE(refusal.find("changed incarnation after the validated catalog cut"), String::npos)
+        << refusal;
+    ASSERT_TRUE(backend->fired());
+    EXPECT_EQ(snapshotPrefixObjects(*backend, layout.namespaceStreamPrefix(new_life)), new_life_before)
+        << "decommission must not append a removal transaction to the post-cut incarnation";
+}
+
+TEST(CASDecommission, VictimNameMatchesOneCanonicalPathComponent)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    { auto victim = openVictim(backend); }
+
+    const RootNamespace neighbor_ns{"victim2/db/t1"};
+    {
+        auto neighbor = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "victim2"});
+        makeTableWithRefs(*neighbor, neighbor_ns.string(), /*committed=*/1, /*precommits=*/0);
+    }
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    EXPECT_EQ(report.namespaces_removed, 0u);
+
+    auto neighbor = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "victim2"});
+    EXPECT_EQ(neighbor->listRefs(neighbor_ns).size(), 1u)
+        << "decommissioning victim must not select victim2 by raw string prefix";
+}
+
+TEST(CASDecommission, ErasesAllVictimNamespaces)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    {
+        auto victim = openVictim(backend);
+        /// Two tables: ns "victim/db/t1" with 2 committed refs, ns "victim/db/t2" with 1 committed
+        /// ref + 1 stale precommit (fixture idiom of gtest_cas_ref_writer.cpp).
+        makeTableWithRefs(*victim, "victim/db/t1", /*committed=*/2, /*precommits=*/0);
+        makeTableWithRefs(*victim, "victim/db/t2", /*committed=*/1, /*precommits=*/1);
+    }
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_EQ(report.srid, "victim");
+    EXPECT_EQ(report.namespaces_removed, 2u);
+    EXPECT_EQ(report.namespaces_already_removed, 0u);
+    EXPECT_EQ(report.committed_refs_removed, 3u);
+    EXPECT_EQ(report.precommits_removed, 1u);
+    EXPECT_EQ(report.edge_deltas_emitted, 4u);
+
+    /// Terminal publication is writer work; exact catalog-row deletion remains GC work. The first
+    /// command therefore keeps the slot as an ownership anchor, and a retry may retire it only after
+    /// GC's next invocation drains the completed `Removing` rows.
+    EXPECT_FALSE(report.warnings.empty());
+    EXPECT_FALSE(report.slot_removed);
+    drainCompletedNamespaceRemovals(backend);
+    const auto retired = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin2"}, "victim");
+    EXPECT_TRUE(retired.warnings.empty());
+    EXPECT_TRUE(retired.slot_removed);
+}
+
+/// Task 2 review finding 1: `makeTableWithRefs`'s precommit seed uses an artificially high
+/// `writer_epoch` (999999) specifically to dodge the writer's OWN stale-precommit sweep -- which
+/// means it never exercised the path a REAL victim precommit takes. A genuine writer stamps
+/// `manifest_ref.writer_epoch` from its OWN `liveWriterEpoch()` at precommit time
+/// (`PartWriteTxn::precommitAdd`, CasPool.cpp:2087), i.e. the victim's era -- always LOWER than the admin
+/// mount's freshly-minted epoch (`openForDecommission` always bumps strictly higher). `appendRefOps`
+/// hoists `maybeSweepStalePrecommits` at its top (CasPool.cpp:1716), so without the
+/// `skip_stale_precommit_sweep` fix that sweep would reclaim this realistic-epoch precommit in its
+/// OWN transaction before `dropNamespace`'s removal transaction ever counts it, leaving
+/// `precommits_removed` at 0 for exactly the case that matters.
+TEST(CASDecommission, CountsRealisticEpochPrecommit)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    uint64_t victim_epoch = 0;
+    {
+        auto victim = openVictim(backend);
+        victim_epoch = victim->writerEpoch();
+        makeTableWithRefs(*victim, "victim/db/t1", /*committed=*/1, /*precommits=*/0);
+
+        const RootNamespace ns("victim/db/t1");
+        /// `build_sequence = 2`: distinct from `makeTableWithRefs`'s committed ref (`build_sequence = 1`)
+        /// -- a REAL build's `ManifestRef` is unique per build, and a colliding one would trip the ref
+        /// state machine's "manifest already has a conflicting owner" guard.
+        const ManifestRef ref{.writer_epoch = victim_epoch, .build_sequence = 2, .manifest_ordinal = 1};
+        writeManifestRaw(victim->backend(), victim->layout(), ns, ref, {});
+        addPrecommitTransition(victim->backend(), victim->layout(), ns, UInt128(1), "precommit_0", std::nullopt, ref);
+    }
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_EQ(report.namespaces_removed, 1u);
+    EXPECT_EQ(report.committed_refs_removed, 1u);
+    EXPECT_EQ(report.precommits_removed, 1u);
+    EXPECT_EQ(report.edge_deltas_emitted, 2u);
+}
+
+/// Task 2 review finding 2: the `member_decommission` begin/namespace_removed/end events
+/// (CasDecommission.cpp) had no assertion at all. Wire a capturing sink (the `gtest_cas_event_log.cpp`
+/// idiom) into `decommissionPoolMember` and check the emitted sequence and its per-namespace detail.
+TEST(CASDecommission, EmitsMemberDecommissionEvents)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    {
+        auto victim = openVictim(backend);
+        makeTableWithRefs(*victim, "victim/db/t1", /*committed=*/1, /*precommits=*/0);
+    }
+
+    std::vector<CasEvent> seen;
+    (void)decommissionPoolMember(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim",
+        [&](const CasEvent & e) { seen.push_back(e); });
+
+    std::vector<CasEvent> member_events;
+    for (const auto & e : seen)
+        if (e.type == CasEventType::MemberDecommission)
+            member_events.push_back(e);
+
+    ASSERT_EQ(member_events.size(), 3u);
+    EXPECT_EQ(member_events[0].outcome, "begin");
+    EXPECT_EQ(member_events[1].outcome, "namespace_removed");
+    EXPECT_EQ(member_events[1].detail.at("namespace"), "victim/db/t1");
+    EXPECT_EQ(member_events[1].detail.at("committed"), "1");
+    EXPECT_EQ(member_events[1].detail.at("precommits"), "0");
+    EXPECT_EQ(member_events[2].outcome, "end");
+    EXPECT_EQ(member_events[2].detail.at("namespaces_removed"), "1");
+}
+
+/// Task 3: the manifest-debris / staging / roots drain phases fill their three `DecommissionReport`
+/// counters and leave nothing of the victim behind under `staging/` or `roots/`.
+TEST(CASDecommission, DrainsDebrisStagingAndRoots)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    {
+        auto victim = openVictim(backend);
+        makeTableWithRefs(*victim, "victim/db/t1", 1, 0);
+        seedOrphanManifestBody(*victim, "victim/db/t1");
+    }
+    /// Foreign staging + mountpoint objects, written raw (no writer machinery needed): the victim's
+    /// writers are fenced by the claim before decommission ever gets here, so these are ordinary debris,
+    /// not a live in-flight write.
+    backend->putIfAbsent("p/staging/victim/upload1.tmp", "x");
+    backend->putIfAbsent("p/staging/victim/upload2.tmp", "x");
+    backend->putIfAbsent("p/roots/victim/clickhouse_access_check_abc", "x");
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    /// The staging and mountpoint phases are unchanged and still drain completely. The manifest-debris
+    /// phase retains under the §6 premise (see the note on the helpers above) and reports why, which is
+    /// what keeps the slot; `RetainsDebrisWhoseEpochSealIsUnconsumed` is that outcome's own test.
+    EXPECT_EQ(report.manifest_debris_removed, 0u);
+    EXPECT_EQ(report.staging_objects_removed, 2u);
+    EXPECT_EQ(report.mountpoint_objects_removed, 1u);
+    EXPECT_FALSE(report.warnings.empty())
+        << "the retained debris is reported, so the incomplete drain is visible";
+
+    /// Nothing of the victim remains under staging/ or roots/ (scoped LISTs are empty). Those two phases
+    /// run to completion even though the debris phase retained -- the drain is per-phase, not all-or-nothing.
+    EXPECT_TRUE(backend->list("p/staging/victim/", "", 10).keys.empty());
+    EXPECT_TRUE(backend->list("p/roots/victim/", "", 10).keys.empty());
+}
+
+/// The §6 deletion premise applies to the decommission drain too, and this pins what that COSTS. With no
+/// sealed fold cursor for the victim's namespace — the state of a pool whose GC has not folded past the
+/// victim's closed epoch — the drain cannot show the debris is unreferenced, so it RETAINS it, says why
+/// in `warnings`, and therefore keeps the slot for a later re-run. Delay, not damage: the objects are
+/// untouched and a re-run after GC catches up drains them (`DrainsDebrisStagingAndRoots`).
+///
+/// This is the visible edge of a real Stage-A limitation, not a test-only artifact: debris under a
+/// namespace GC never folds — the pure pre-precommit orphan, whose whole point is that no ref record was
+/// ever appended for it — has no cursor to consume any seal, so the premise retains it indefinitely.
+/// Reclaiming it needs the sweep's own rework (registers R2/R3, Stage B), which is why the premise ships
+/// as the safety floor and not as the reclaim policy.
+TEST(CASDecommission, RetainsDebrisWhoseEpochSealIsUnconsumed)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    String debris_key;
+    {
+        auto victim = openVictim(backend);
+        makeTableWithRefs(*victim, "victim/db/t1", 1, 0);
+        const ManifestId debris_id = seedOrphanManifestBody(*victim, "victim/db/t1");
+        debris_key = victim->layout().manifestKey(debris_id);
+        /// Deliberately NO `seedFoldedPastVictimEpoch` here — that absence is the subject.
+    }
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_EQ(report.manifest_debris_removed, 0u);
+    EXPECT_TRUE(backend->head(debris_key).exists)
+        << "the body is retained untouched, not deleted and not corrupted";
+    ASSERT_FALSE(report.warnings.empty())
+        << "a retained manifest is a visible decision -- the operator must be able to see why the drain "
+           "did not complete";
+    bool named = false;
+    for (const String & w : report.warnings)
+        if (w.find(debris_key) != String::npos && w.find("seal") != String::npos)
+            named = true;
+    EXPECT_TRUE(named) << "the warning names the object and the premise that retained it";
+    EXPECT_FALSE(report.slot_removed)
+        << "an incomplete drain keeps the slot as the resume anchor, exactly as a per-key failure does";
+}
+
+/// Task 3 fail-close nuance (spec §core "Fail-close"): a per-object failure in the staging/roots drain
+/// -- a thrown exception (a transient hiccup) or a `TokenMismatch` outcome (a "listed but raced" miss)
+/// -- must record a warning and let the rest of the sweep proceed, never abort the whole phase or the
+/// whole command. One staging object throws, the roots object comes back `TokenMismatch`; the OTHER
+/// staging object must still be deleted and counted.
+TEST(CASDecommission, PerObjectFailureWarnsAndContinuesDrain)
+{
+    auto backend = std::make_shared<FailingDeleteBackend>();
+    {
+        auto victim = openVictim(backend);
+        makeTableWithRefs(*victim, "victim/db/t1", 1, 0);
+    }
+    backend->putIfAbsent("p/staging/victim/upload_ok.tmp", "x");
+    backend->putIfAbsent("p/staging/victim/upload_throws.tmp", "x");
+    backend->putIfAbsent("p/roots/victim/clickhouse_access_check_abc", "x");
+    backend->failWithThrow("p/staging/victim/upload_throws.tmp");
+    backend->failWithTokenMismatch("p/roots/victim/clickhouse_access_check_abc");
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_EQ(report.staging_objects_removed, 1u)
+        << "the OTHER staging object must still be deleted despite the injected failure on its sibling";
+    EXPECT_EQ(report.mountpoint_objects_removed, 0u);
+    EXPECT_EQ(report.warnings.size(), 2u)
+        << "one warning for the thrown exception, one for the TokenMismatch outcome";
+
+    EXPECT_FALSE(backend->head("p/staging/victim/upload_ok.tmp").exists)
+        << "the healthy staging object was actually deleted, not merely skipped";
+    EXPECT_TRUE(backend->head("p/staging/victim/upload_throws.tmp").exists)
+        << "the failing object is left behind (untouched) so a re-run can retry it";
+    EXPECT_TRUE(backend->head("p/roots/victim/clickhouse_access_check_abc").exists)
+        << "TokenMismatch means nothing was actually deleted -- the object survives";
+}
+
+/// Opaque physical debris carries no logical owner and therefore cannot widen or redirect
+/// decommission's catalog-derived victim set. Task 5's ownership-tree janitor owns that debris.
+TEST(CASDecommission, LifelessPhysicalKeyCannotRedirectCatalogOwnedDecommission)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    String lifeless;
+    {
+        auto victim = openVictim(backend);
+        makeTableWithRefs(*victim, "victim/db/t1", 1, 0);
+        /// Hand-built: no helper can mint the un-incarnated shape any more.
+        lifeless = victim->layout().casRefsPrefix() + String("victim/db/t1/_log/")
+            + renderRefTxnId(RefTxnId{1, 1}) + ".zst";
+        ASSERT_EQ(backend->putIfAbsent(lifeless, "garbage").outcome, PutOutcome::Done);
+    }
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    EXPECT_EQ(report.namespaces_removed, 1u);
+    EXPECT_TRUE(backend->head(lifeless).exists)
+        << "decommission must neither adopt nor delete an unowned physical life key";
+}
+
+/// The manifest-debris drain honors the same tolerate-and-continue contract as
+/// `deleteListedPrefix`: a per-key `deleteExact` failure becomes a warning, while the namespace
+/// erasure and subsequent staging drain continue. Protection reads now use opaque physical life
+/// prefixes, so a logical-name substring can no longer target an otherwise unlisted namespace.
+TEST(CASDecommission, ManifestDebrisDeleteFailureWarnsAndContinues)
+{
+    auto backend = std::make_shared<FailingDeleteBackend>();
+    String debris_key;
+    {
+        auto victim = openVictim(backend);
+        makeTableWithRefs(*victim, "victim/db/t1", 1, 0);
+        const ManifestId debris_id = seedOrphanManifestBody(*victim, "victim/db/t1");
+        debris_key = victim->layout().manifestKey(debris_id);
+    }
+    backend->failWithThrow(debris_key);
+    backend->putIfAbsent("p/staging/victim/upload_ok.tmp", "x");
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_EQ(report.namespaces_removed, 1u)
+        << "victim/db/t1's namespace erasure (Task 2) is untouched by either injected failure";
+    EXPECT_EQ(report.manifest_debris_removed, 0u);
+    EXPECT_EQ(report.warnings.size(), 1u)
+        << "the thrown per-key delete must keep the retirement tail fail-closed";
+    EXPECT_EQ(report.staging_objects_removed, 1u)
+        << "the staging phase still ran to completion after the manifest-debris phase's failures -- "
+           "the whole command did not abort";
+
+    EXPECT_TRUE(backend->head(debris_key).exists)
+        << "the failing object is left behind (untouched) so a re-run can retry it";
+}
+
+/// GC owns the completed catalog-row deletion. Once it drains the row, a clean decommission retry
+/// removes the mutable slot objects and tombstones the owner anchor.
+TEST(CASDecommission, RemovesMutableSlotAndRefusesTombstonedRerun)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    {
+        auto victim = openVictim(backend);
+        makeTableWithRefs(*victim, "victim/db/t1", 1, 0);
+    }
+    const auto pending = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    EXPECT_FALSE(pending.slot_removed);
+    EXPECT_FALSE(pending.warnings.empty());
+    drainCompletedNamespaceRemovals(backend);
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin2"}, "victim");
+    EXPECT_TRUE(report.slot_removed);
+    EXPECT_TRUE(report.warnings.empty());
+    EXPECT_FALSE(backend->get("p/gc/server-roots/victim/mount").has_value());
+    const auto owner = backend->get("p/gc/server-roots/victim/owner");
+    ASSERT_TRUE(owner.has_value());
+    EXPECT_TRUE(decodeOwner(owner->bytes).retired_at_ms.has_value());
+    EXPECT_FALSE(backend->get("p/gc/server-roots/victim/epoch").has_value());
+
+    expectThrowsCode(ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        decommissionPoolMember(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "a2"}, "victim");
+    });
+}
+
+/// Triage #9: a successor may reclaim the same UUID immediately after the decommission admin writes
+/// its farewell. The retirement tail must use the farewell/claimed-epoch tokens captured around that
+/// release, delete `mount` first, and stop on its `TokenMismatch`; re-reading current tokens would
+/// delete the live successor's control objects and falsely report the slot removed.
+TEST(CASDecommission, SuccessorReclaimFencesSlotRetirementTail)
+{
+    auto backend = std::make_shared<SuccessorReclaimAfterFarewellBackend>();
+    { auto victim = openVictim(backend); }
+    backend->armForSuccessorReclaim();
+
+    std::vector<CasEvent> seen;
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim",
+        [&](const CasEvent & event) { seen.push_back(event); });
+
+    ASSERT_TRUE(backend->successorInjected());
+    EXPECT_FALSE(report.slot_removed);
+    ASSERT_EQ(report.warnings.size(), 1u);
+    EXPECT_NE(report.warnings.front().find("p/gc/server-roots/victim/mount"), String::npos);
+    EXPECT_NE(report.warnings.front().find("replaced"), String::npos);
+
+    const auto mount = backend->get("p/gc/server-roots/victim/mount");
+    ASSERT_TRUE(mount.has_value());
+    EXPECT_EQ(mount->token, backend->successorMountToken());
+    EXPECT_EQ(mount->bytes, backend->successorMountBytes());
+
+    const auto epoch = backend->get("p/gc/server-roots/victim/epoch");
+    ASSERT_TRUE(epoch.has_value());
+    EXPECT_EQ(epoch->token, backend->successorEpochToken());
+    EXPECT_EQ(epoch->bytes, backend->successorEpochBytes());
+    EXPECT_TRUE(backend->get("p/gc/server-roots/victim/owner").has_value());
+
+    ASSERT_FALSE(seen.empty());
+    EXPECT_EQ(seen.back().outcome, "end");
+    EXPECT_EQ(seen.back().detail.at("slot_removed"), "0");
+}
+
+/// A successor can also restart after both stale mutable objects were deleted but before `owner` is
+/// retired. Mere presence of either freshly recreated mutable object must stop owner retirement.
+TEST(CASDecommission, SuccessorReclaimAfterEpochDeleteKeepsOwnerAnchor)
+{
+    auto backend = std::make_shared<SuccessorReclaimAfterEpochDeleteBackend>();
+    { auto victim = openVictim(backend); }
+
+    const String owner_key = "p/gc/server-roots/victim/owner";
+    const auto original_owner = backend->get(owner_key);
+    ASSERT_TRUE(original_owner.has_value());
+    backend->armForSuccessorReclaim();
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    ASSERT_TRUE(backend->successorInjected());
+    EXPECT_FALSE(report.slot_removed);
+    EXPECT_FALSE(report.warnings.empty());
+    EXPECT_EQ(backend->ownerRewriteAttempts(), 0u);
+
+    const auto owner = backend->get(owner_key);
+    ASSERT_TRUE(owner.has_value());
+    EXPECT_EQ(owner->token, original_owner->token);
+    EXPECT_EQ(owner->bytes, original_owner->bytes);
+
+    const auto mount = backend->get("p/gc/server-roots/victim/mount");
+    ASSERT_TRUE(mount.has_value());
+    EXPECT_EQ(mount->token, backend->successorMountToken());
+    EXPECT_EQ(mount->bytes, backend->successorMountBytes());
+
+    const auto epoch = backend->get("p/gc/server-roots/victim/epoch");
+    ASSERT_TRUE(epoch.has_value());
+    EXPECT_EQ(epoch->token, backend->successorEpochToken());
+    EXPECT_EQ(epoch->bytes, backend->successorEpochBytes());
+}
+
+/// Triage #9 control: absent a successor interleaving, the fenced tail removes both mutable control
+/// objects, tombstones the owner anchor, and preserves the existing successful `slot_removed=1` result.
+TEST(CASDecommission, FencedSlotRetirementTailRetiresUncontendedSlot)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    { auto victim = openVictim(backend); }
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_TRUE(report.warnings.empty());
+    EXPECT_TRUE(report.slot_removed);
+    EXPECT_FALSE(backend->get("p/gc/server-roots/victim/mount").has_value());
+    EXPECT_FALSE(backend->get("p/gc/server-roots/victim/epoch").has_value());
+    const auto owner = backend->get("p/gc/server-roots/victim/owner");
+    ASSERT_TRUE(owner.has_value());
+    EXPECT_TRUE(decodeOwner(owner->bytes).retired_at_ms.has_value());
+}
+
+TEST(CASDecommission, SuccessfulDecommissionLeavesTombstonedOwnerAnchor)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    { auto victim = openVictim(backend); }
+
+    const String owner_key = "p/gc/server-roots/victim/owner";
+    const auto before = backend->get(owner_key);
+    ASSERT_TRUE(before.has_value());
+    EXPECT_FALSE(decodeOwner(before->bytes).retired_at_ms.has_value());
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_TRUE(report.warnings.empty());
+    EXPECT_TRUE(report.slot_removed);
+    const auto after = backend->get(owner_key);
+    ASSERT_TRUE(after.has_value());
+    EXPECT_NE(after->token, before->token);
+    EXPECT_EQ(decodeOwner(after->bytes).server_uuid, decodeOwner(before->bytes).server_uuid);
+    EXPECT_TRUE(decodeOwner(after->bytes).retired_at_ms.has_value());
+}
+
+TEST(CASDecommission, SuccessorOwnerRewriteWinsBeforeTombstone)
+{
+    auto backend = std::make_shared<SuccessorOwnerRewriteBeforeTombstoneBackend>();
+    { auto victim = openVictim(backend); }
+    backend->armForSuccessorRewrite();
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    ASSERT_TRUE(backend->successorInjected());
+    EXPECT_FALSE(report.slot_removed);
+    ASSERT_EQ(report.warnings.size(), 1u);
+    EXPECT_NE(report.warnings.front().find("successor reclaimed"), String::npos);
+
+    const auto owner = backend->get("p/gc/server-roots/victim/owner");
+    ASSERT_TRUE(owner.has_value());
+    EXPECT_EQ(owner->token, backend->successorOwnerToken());
+    EXPECT_EQ(owner->bytes, backend->successorOwnerBytes());
+    EXPECT_FALSE(decodeOwner(owner->bytes).retired_at_ms.has_value());
+}
+
+/// Final whole-branch review finding (Important):
+/// a transient exception on the owner tombstone write must not be reported as a hard failure when the
+/// write actually landed -- the controlled overwrite resolves this via GET (current bytes already
+/// match the intended tombstone) instead of the old bare putOverwrite's "any exception = failure".
+TEST(CASDecommission, OwnerTombstoneAmbiguousSuccessResolvesToCommitted)
+{
+    auto backend = std::make_shared<AmbiguousOwnerTombstoneBackend>();
+    { auto victim = openVictim(backend); }
+    backend->armForAmbiguousTombstone();
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_TRUE(report.slot_removed) << "the ambiguous write actually landed and must resolve to Committed";
+    EXPECT_TRUE(report.warnings.empty());
+
+    const auto owner = backend->get("p/gc/server-roots/victim/owner");
+    ASSERT_TRUE(owner.has_value());
+    EXPECT_TRUE(decodeOwner(owner->bytes).retired_at_ms.has_value());
+}
+
+/// Delegates every op to `inner`, except `deleteExact`: while `armed`, any key starting with
+/// `fail_prefix` throws an injected transient failure instead of deleting -- models a real backend
+/// transiently failing to delete under one whole prefix. `disarm()` clears the failure (the resume
+/// half of `FailedDrainKeepsSlotThenResumes`). Forwards every pure-virtual `Backend` member (the
+/// `CasBackend.h` list) to `inner` untouched.
+class FailDeletesUnderPrefixBackend : public Backend
+{
+public:
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    FailDeletesUnderPrefixBackend(std::shared_ptr<InMemoryBackend> inner_, String fail_prefix_)
+        : inner(std::move(inner_)), fail_prefix(std::move(fail_prefix_))
+    {
+    }
+
+    void disarm() { armed = false; }
+
+    std::optional<GetResult> get(const String & key, Range range) override { return inner->get(key, range); }
+    std::optional<GetStreamResult> getStream(const String & key, Range range) override { return inner->getStream(key, range); }
+    HeadResult head(const String & key) override { return inner->head(key); }
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        return inner->putIfAbsent(key, bytes, meta);
+    }
+    WriteSinkPtr putIfAbsentStream(const String & key, const ObjectMeta & meta) override
+    {
+        return inner->putIfAbsentStream(key, meta);
+    }
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        return inner->putOverwrite(key, bytes, expected, meta);
+    }
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected, const ObjectMeta & meta) override
+    {
+        return inner->casPut(key, bytes, expected, meta);
+    }
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        if (armed && key.starts_with(fail_prefix))
+            throw Exception(ErrorCodes::S3_ERROR, "injected transient delete failure for {}", key);
+        return inner->deleteExact(key, token);
+    }
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override { return inner->list(prefix, cursor, limit); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+
+private:
+    std::shared_ptr<InMemoryBackend> inner;
+    String fail_prefix;
+    bool armed = true;
+};
+
+/// Task 4 fail-close: a drain failure under the roots prefix keeps the slot terminated-but-present
+/// (`report.slot_removed == false`, the mount object survives as the resume anchor). Once the fault is
+/// cleared, a re-run finishes the job: the already-erased namespace is counted as
+/// `namespaces_already_removed`, the leftover roots object is finally swept, and the slot is removed.
+TEST(CASDecommission, FailedDrainKeepsSlotThenResumes)
+{
+    auto inner = std::make_shared<InMemoryBackend>();
+    {
+        auto victim = Pool::open(inner, PoolConfig{.pool_prefix = "p", .server_root_id = "victim"});
+        makeTableWithRefs(*victim, "victim/db/t1", 1, 0);
+    }
+    inner->putIfAbsent("p/roots/victim/loose_file", "x");
+
+    auto failing = std::make_shared<FailDeletesUnderPrefixBackend>(inner, "p/roots/victim/");
+    const auto first = decommissionPoolMember(
+        failing, PoolConfig{.pool_prefix = "p", .server_root_id = "a1"}, "victim");
+    EXPECT_FALSE(first.warnings.empty());
+    EXPECT_FALSE(first.slot_removed);
+    EXPECT_TRUE(inner->get("p/gc/server-roots/victim/mount").has_value())
+        << "slot kept -- resume anchor";
+
+    failing->disarm();
+    const auto second = decommissionPoolMember(
+        failing, PoolConfig{.pool_prefix = "p", .server_root_id = "a2"}, "victim");
+    EXPECT_FALSE(second.warnings.empty());
+    EXPECT_FALSE(second.slot_removed);
+    EXPECT_EQ(second.namespaces_already_removed, 1u);
+    EXPECT_EQ(second.mountpoint_objects_removed, 1u);
+
+    drainCompletedNamespaceRemovals(inner);
+    const auto third = decommissionPoolMember(
+        failing, PoolConfig{.pool_prefix = "p", .server_root_id = "a3"}, "victim");
+    EXPECT_TRUE(third.warnings.empty());
+    EXPECT_TRUE(third.slot_removed);
+}
+
+/// Task 4 fail-close, manifest-debris variant (review follow-up: the plan's own example only exercises
+/// a roots-phase failure). A per-key `deleteExact` throw inside the manifest-debris drain must ALSO
+/// keep the slot: `report.slot_removed == false`, the mount object survives, and once the injected
+/// failure is cleared a re-run drains the leftover debris and removes the slot.
+TEST(CASDecommission, ManifestDebrisFailureKeepsSlotThenResumes)
+{
+    auto backend = std::make_shared<FailingDeleteBackend>();
+    String debris_key;
+    {
+        auto victim = openVictim(backend);
+        makeTableWithRefs(*victim, "victim/db/t1", 1, 0);
+        const ManifestId debris_id = seedOrphanManifestBody(*victim, "victim/db/t1");
+        debris_key = victim->layout().manifestKey(debris_id);
+    }
+    backend->failWithThrow(debris_key);
+
+    const auto first = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "a1"}, "victim");
+    EXPECT_FALSE(first.warnings.empty());
+    EXPECT_FALSE(first.slot_removed);
+    EXPECT_EQ(first.manifest_debris_removed, 0u);
+    EXPECT_TRUE(backend->get("p/gc/server-roots/victim/mount").has_value())
+        << "slot kept -- resume anchor";
+    EXPECT_TRUE(backend->head(debris_key).exists)
+        << "the failing object is left behind (untouched) so a re-run can retry it";
+
+    /// COVERAGE LOST HERE, DELIBERATELY NAMED. Before the §6 premise, clearing the injected failure let
+    /// a re-run drain the debris and retire the slot, which is what proved the per-key fail-close path
+    /// RESUMES rather than merely refuses. Under the premise the sweep never reaches `deleteExact` for
+    /// this body at all (single-epoch pool -- see the note on the helpers above), so disarming changes
+    /// nothing and the resume half of this test is no longer expressible. What survives is the half that
+    /// still has a mechanism: the slot stays kept across the re-run, and the object stays untouched.
+    /// The resume assertion comes back with the drain's reclaim path (registers R2/R3, Stage B).
+    backend->disarm();
+    const auto second = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "a2"}, "victim");
+    EXPECT_EQ(second.namespaces_already_removed, 1u);
+    EXPECT_EQ(second.manifest_debris_removed, 0u);
+    EXPECT_FALSE(second.slot_removed);
+    EXPECT_TRUE(backend->head(debris_key).exists);
+    EXPECT_TRUE(backend->get("p/gc/server-roots/victim/mount").has_value())
+        << "the slot is still the resume anchor -- nothing was retired against unreclaimed debris";
+}
+
+/// Task 5 (Task-1 carry-forward, escalated by review): preserve recovery from the legacy partial
+/// hand-cleanup shape where owner and epoch are absent but the mount lease remains. Triage #9 changed
+/// new retirements to delete `mountKey`/`epochKey` and tombstone `ownerKey`, so the current tail no
+/// longer creates this shape, but `openForDecommission`'s owner-anchor-absent +
+/// mount-lease-present fallback ("partial hand-cleanup: adopt from the lease", `CasPool.cpp`) remains
+/// compatibility-critical for slots left by older binaries or manual repair.
+///
+/// `claimOwnerOrThrow` (`CasServerRoot.cpp`) gates the owner-absent path a SECOND, stricter way: the
+/// same catalog cut must name no `Creating`, `Live` or `Removing` namespace under this canonical root,
+/// and the name-bearing `cas/manifests/<srid>/` and `roots/<srid>/` families must be empty. Opaque
+/// stream/state debris cannot be attributed to a server root and is deliberately inert. This test
+/// therefore uses a victim with NO namespaces at all: identity persisted
+/// (mount/owner/epoch exist from a real graceful close), data subtree genuinely empty -- the exact
+/// precondition the fallback is designed for. Simulate the crash directly: claim the slot once (exactly
+/// `decommissionPoolMember`'s own first step), let it close gracefully (the mount-lease keeper's
+/// farewell stamp, same as a real `admin.reset()`), then manually strike `epochKey`+`ownerKey`, leaving
+/// `mountKey`. A `decommissionPoolMember` re-run must resolve identity via the mount-lease fallback and
+/// finish retiring the slot; a further re-run then sees the tombstone and refuses to resume it.
+TEST(CASDecommission, MidRetirementCrashResumesViaMountLeaseFallback)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    { auto victim = openVictim(backend); }   /// identity only -- no namespace, so the subtree stays empty
+
+    const Layout layout("p");
+    /// Claim the slot once, exactly as `decommissionPoolMember`'s own first step would -- this (re)writes
+    /// fresh epoch/owner/mount control objects. Closing gracefully (scope exit) stamps the mount lease's
+    /// farewell, matching what a real slot retirement's `admin.reset()` does right before its delete loop.
+    {
+        auto admin = Pool::openForDecommission(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "chk"}, "victim");
+    }
+
+    /// Manually strike epoch + owner, leaving the mount -- the legacy partial hand-cleanup shape.
+    for (const String & key : {layout.epochKey("victim"), layout.ownerKey("victim")})
+    {
+        const auto head = backend->head(key);
+        ASSERT_TRUE(head.exists);
+        backend->deleteExact(key, head.token);
+    }
+    ASSERT_FALSE(backend->get(layout.epochKey("victim")).has_value());
+    ASSERT_FALSE(backend->get(layout.ownerKey("victim")).has_value());
+    ASSERT_TRUE(backend->get(layout.mountKey("victim")).has_value())
+        << "the mount lease must survive -- it is the resume anchor the fallback reads";
+
+    const auto report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "a2"}, "victim");
+
+    EXPECT_TRUE(report.warnings.empty());
+    EXPECT_EQ(report.namespaces_removed, 0u);
+    EXPECT_TRUE(report.slot_removed);
+    EXPECT_FALSE(backend->get(layout.epochKey("victim")).has_value());
+    const auto owner = backend->get(layout.ownerKey("victim"));
+    ASSERT_TRUE(owner.has_value());
+    EXPECT_TRUE(decodeOwner(owner->bytes).retired_at_ms.has_value());
+    EXPECT_FALSE(backend->get(layout.mountKey("victim")).has_value());
+
+    expectThrowsCode(ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        decommissionPoolMember(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "a3"}, "victim");
+    });
+}
diff --git a/src/Disks/tests/gtest_cas_decommission_catalog_duties.cpp b/src/Disks/tests/gtest_cas_decommission_catalog_duties.cpp
new file mode 100644
index 000000000000..a674f13667b6
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_decommission_catalog_duties.cpp
@@ -0,0 +1,330 @@
+#include "cas_test_helpers.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasDecommission.h>
+
+#include <algorithm>
+#include <atomic>
+#include <optional>
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+
+using namespace DB;
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+PoolPtr openVictim(const std::shared_ptr<InMemoryBackend> & backend)
+{
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "victim"});
+}
+
+CatalogEntry catalogEntry(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const RefCatalog catalog = CasRefCatalog::read(backend, layout).catalog;
+    const auto it = std::find_if(catalog.entries.begin(), catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    if (it == catalog.entries.end())
+        throw Exception(ErrorCodes::CORRUPTED_DATA, "fixture catalog entry '{}' is absent", ns.string());
+    return *it;
+}
+
+void makeRemoving(Backend & backend, const Layout & layout, const CatalogEntry & live)
+{
+    CasRefCatalog::casUpdate(backend, layout, [&](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        const auto it = std::find(next.entries.begin(), next.entries.end(), live);
+        if (it == next.entries.end())
+            throw Exception(ErrorCodes::CORRUPTED_DATA, "fixture exact Live row changed");
+        it->state = NsState::Removing;
+        it->removal_started_round = 0;
+        return next;
+    });
+}
+
+bool slotObjectExists(Backend & backend, const String & leaf)
+{
+    return backend.head("p/gc/server-roots/victim/" + leaf).exists;
+}
+
+class AddVictimEntryDuringRootDrainBackend final : public InMemoryBackend
+{
+public:
+    void arm() { armed = true; }
+    bool fired() const { return added; }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = InMemoryBackend::list(prefix, cursor, limit);
+        if (armed && !added && prefix == "p/roots/victim/" && cursor.empty())
+        {
+            added = true;
+            CasRefCatalog::casAdmitEntry(
+                *this, Layout("p"), 1,
+                CatalogEntry{
+                    .ns = RootNamespace("victim/db/late"),
+                    .state = NsState::Live,
+                    .incarnation = UInt128{707}});
+        }
+        return page;
+    }
+
+private:
+    bool armed = false;
+    bool added = false;
+};
+
+/// Admits the late catalog entry between the retirement tail's two exact catalog reads
+/// (`retirement_catalog_cut`, then `fresh_retirement_catalog`), never before. The mountpoint drain's
+/// `list("p/roots/victim/", ...)` is the last LIST call in `decommissionPoolMember` before either
+/// read, so it orders the two `get("p/cas/ref_catalog")` calls that follow it: the first is
+/// `retirement_catalog_cut`, the second is `fresh_retirement_catalog`. Mutating on the second call
+/// makes that read observe a catalog the first read did not.
+class MutateCatalogBetweenRetirementReadsBackend final : public InMemoryBackend
+{
+public:
+    void arm() { armed = true; }
+    bool fired() const { return added; }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = InMemoryBackend::list(prefix, cursor, limit);
+        if (armed && !past_mountpoint_drain && prefix == "p/roots/victim/" && cursor.empty())
+            past_mountpoint_drain = true;
+        return page;
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (armed && past_mountpoint_drain && !added && key == "p/cas/ref_catalog")
+        {
+            if (!seen_retirement_catalog_cut)
+                seen_retirement_catalog_cut = true;
+            else
+            {
+                added = true;
+                CasRefCatalog::casAdmitEntry(
+                    *this, Layout("p"), 1,
+                    CatalogEntry{
+                        .ns = RootNamespace("victim/db/late"),
+                        .state = NsState::Live,
+                        .incarnation = UInt128{707}});
+            }
+        }
+        return InMemoryBackend::get(key, range);
+    }
+
+private:
+    bool armed = false;
+    bool past_mountpoint_drain = false;
+    bool seen_retirement_catalog_cut = false;
+    bool added = false;
+};
+
+TEST(CASDecommissionCatalogDuties, RemovingWithoutCheckpointIsCorruptionAndKeepsSlot)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    {
+        auto victim = openVictim(backend);
+        const CatalogEntry live{
+            .ns = RootNamespace("victim/db/missing_ckpt"),
+            .state = NsState::Live,
+            .incarnation = UInt128{701}};
+        CasRefCatalog::casAdmitEntry(
+            *backend, victim->layout(), victim->poolConfig().gc_shards,
+            live);
+        makeRemoving(*backend, victim->layout(), live);
+    }
+
+    expectThrowsCode(ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        (void)decommissionPoolMember(
+            backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+    });
+
+    EXPECT_TRUE(slotObjectExists(*backend, "owner"));
+    EXPECT_TRUE(slotObjectExists(*backend, "epoch"));
+    EXPECT_TRUE(slotObjectExists(*backend, "mount"));
+    EXPECT_EQ(catalogEntry(*backend, Layout("p"), RootNamespace("victim/db/missing_ckpt")).state,
+        NsState::Removing);
+}
+
+TEST(CASDecommissionCatalogDuties, RemovingWithCheckpointResumesTerminalAndKeepsSlotForGc)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const RootNamespace ns("victim/db/pending_terminal");
+    std::optional<NamespaceLifeId> life;
+    {
+        auto victim = openVictim(backend);
+        life = victim->namespaceLife(ns);
+        const CatalogEntry live = catalogEntry(*backend, victim->layout(), ns);
+        makeRemoving(*backend, victim->layout(), live);
+        ASSERT_TRUE(backend->head(victim->layout().refCkptKey(*life)).exists);
+        ASSERT_TRUE(backend->list(victim->layout().namespaceStreamPrefix(*life), "", 100).keys.empty());
+    }
+
+    std::atomic<uint64_t> wake_requests{0};
+    const DecommissionReport report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim", {},
+        [&] { wake_requests.fetch_add(1); });
+
+    EXPECT_EQ(report.namespaces_already_removed, 1u);
+    EXPECT_EQ(wake_requests.load(), 1u);
+    EXPECT_FALSE(report.slot_removed);
+    EXPECT_FALSE(report.warnings.empty());
+    EXPECT_TRUE(slotObjectExists(*backend, "owner"));
+
+    const ListPage stream = backend->list(Layout("p").namespaceStreamPrefix(*life), "", 100);
+    ASSERT_EQ(stream.keys.size(), 1u);
+    const auto parsed = Layout("p").parseRefObjectKey(stream.keys.front().key);
+    ASSERT_TRUE(parsed);
+    const auto body = backend->get(stream.keys.front().key);
+    ASSERT_TRUE(body);
+    const RefLogTxn terminal = decodeRefLogTxn(
+        openObject(FormatId::RefLog, body->bytes), ns.string(), parsed->txn_id);
+    ASSERT_EQ(terminal.ops.size(), 2u);
+    EXPECT_EQ(terminal.ops.front().kind, RefOpKind::NamespaceBirth);
+    EXPECT_EQ(terminal.ops.back().kind, RefOpKind::RemoveNamespace);
+}
+
+TEST(CASDecommissionCatalogDuties, PartialRemovalProgressStillWakesGcWhenLaterNamespaceFails)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const RootNamespace progressed_ns("victim/db/a_progressed");
+    const RootNamespace broken_ns("victim/db/z_missing_ckpt");
+    std::optional<NamespaceLifeId> progressed_life;
+    {
+        auto victim = openVictim(backend);
+        progressed_life = victim->namespaceLife(progressed_ns);
+        const CatalogEntry progressed_live = catalogEntry(*backend, victim->layout(), progressed_ns);
+        makeRemoving(*backend, victim->layout(), progressed_live);
+
+        const CatalogEntry broken_live{
+            .ns = broken_ns,
+            .state = NsState::Live,
+            .incarnation = UInt128{713}};
+        CasRefCatalog::casAdmitEntry(
+            *backend, victim->layout(), victim->poolConfig().gc_shards, broken_live);
+        makeRemoving(*backend, victim->layout(), broken_live);
+        ASSERT_FALSE(backend->head(victim->layout().refCkptKey(
+            NamespaceLifeId::fromCatalogEntry(broken_ns, broken_live.incarnation))).exists);
+    }
+
+    std::atomic<uint64_t> wake_requests{0};
+    expectThrowsCode(ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        (void)decommissionPoolMember(
+            backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim", {},
+            [&] { wake_requests.fetch_add(1); });
+    });
+
+    EXPECT_EQ(wake_requests.load(), 1u)
+        << "progress already made for an earlier life must wake GC even when a later life fails closed";
+    EXPECT_TRUE(slotObjectExists(*backend, "owner"));
+    const ListPage progressed_stream
+        = backend->list(Layout("p").namespaceStreamPrefix(*progressed_life), "", 100);
+    ASSERT_EQ(progressed_stream.keys.size(), 1u);
+}
+
+TEST(CASDecommissionCatalogDuties, VictimEntryAppearingBeforeTheOwnershipCutKeepsSlot)
+{
+    auto backend = std::make_shared<AddVictimEntryDuringRootDrainBackend>();
+    { auto victim = openVictim(backend); }
+    backend->arm();
+
+    const DecommissionReport report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_TRUE(backend->fired());
+    EXPECT_FALSE(report.slot_removed);
+    ASSERT_FALSE(report.warnings.empty());
+    EXPECT_NE(report.warnings.front().find("pool member decommission underway: 1 namespace(s)"), String::npos)
+        << report.warnings.front();
+    EXPECT_TRUE(slotObjectExists(*backend, "owner"));
+    EXPECT_EQ(catalogEntry(*backend, Layout("p"), RootNamespace("victim/db/late")).state, NsState::Live);
+}
+
+TEST(CASDecommissionCatalogDuties, CatalogTokenMovedBetweenOwnershipCutAndRetirementKeepsSlot)
+{
+    auto backend = std::make_shared<MutateCatalogBetweenRetirementReadsBackend>();
+    { auto victim = openVictim(backend); }
+    backend->arm();
+
+    const DecommissionReport report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_TRUE(backend->fired());
+    EXPECT_FALSE(report.slot_removed);
+    ASSERT_FALSE(report.warnings.empty());
+    EXPECT_NE(report.warnings.front().find("catalog changed after the victim ownership check"), String::npos)
+        << report.warnings.front();
+    EXPECT_TRUE(slotObjectExists(*backend, "owner"));
+    EXPECT_EQ(catalogEntry(*backend, Layout("p"), RootNamespace("victim/db/late")).state, NsState::Live);
+}
+
+TEST(CASDecommissionCatalogDuties, FoldedTerminalRemainsGcOwnedAndOnlyRequestsAnotherRound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const RootNamespace ns("victim/db/folded_terminal");
+    std::optional<NamespaceLifeId> life;
+    std::vector<String> stream_before;
+    {
+        PoolConfig config{
+            .pool_prefix = "p",
+            .server_root_id = "victim",
+            .gc_fold_threshold = 1,
+            .gc_fold_max_defer_rounds = 0};
+        auto victim = Pool::open(backend, config);
+        life = victim->namespaceLife(ns);
+        victim->putNamespaceFile(*life, "format_version.txt", "1\n");
+        victim->dropNamespace(ns);
+
+        Gc gc(victim, UInt128{811});
+        ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+        ASSERT_EQ(catalogEntry(*backend, victim->layout(), ns).state, NsState::Removing);
+        for (const ListedKey & key : backend->list(victim->layout().namespaceStreamPrefix(*life), "", 100).keys)
+            stream_before.push_back(key.key);
+        ASSERT_FALSE(stream_before.empty());
+    }
+
+    std::atomic<uint64_t> wake_requests{0};
+    const DecommissionReport report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim", {},
+        [&] { wake_requests.fetch_add(1); });
+
+    EXPECT_EQ(wake_requests.load(), 1u);
+    EXPECT_EQ(report.namespaces_already_removed, 1u);
+    EXPECT_FALSE(report.slot_removed);
+    EXPECT_EQ(catalogEntry(*backend, Layout("p"), ns).state, NsState::Removing);
+    std::vector<String> stream_after;
+    for (const ListedKey & key : backend->list(Layout("p").namespaceStreamPrefix(*life), "", 100).keys)
+        stream_after.push_back(key.key);
+    EXPECT_EQ(stream_after, stream_before)
+        << "decommission must not append a second terminal or become a catalog deletion driver";
+}
+
+TEST(CASDecommissionCatalogDuties, OpaqueLifeDebrisWithoutCatalogOwnershipDoesNotBlockRetirement)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    { auto victim = openVictim(backend); }
+    const Layout layout("p");
+    const NamespaceLifeId dead_life
+        = NamespaceLifeId::fromCatalogEntry(RootNamespace("historical/name"), UInt128{709});
+    const String debris_key = layout.refCkptKey(dead_life);
+    ASSERT_EQ(backend->putIfAbsent(debris_key, "debris").outcome, PutOutcome::Done);
+
+    const DecommissionReport report = decommissionPoolMember(
+        backend, PoolConfig{.pool_prefix = "p", .server_root_id = "admin"}, "victim");
+
+    EXPECT_TRUE(report.warnings.empty());
+    EXPECT_TRUE(report.slot_removed);
+    EXPECT_TRUE(backend->head(debris_key).exists);
+}
+
+}
diff --git a/src/Disks/tests/gtest_cas_empty_proof.cpp b/src/Disks/tests/gtest_cas_empty_proof.cpp
new file mode 100644
index 000000000000..4754786e7624
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_empty_proof.cpp
@@ -0,0 +1,281 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <filesystem>
+#include <functional>
+#include <memory>
+#include <optional>
+#include <string>
+
+/// Task 9 (rev.7 spec §1 "empty-proof rule" [B3]): the last silent-empty-load killer. On a PRE-TERMINAL
+/// (Live) or READ-ONLY pool, an enumeration about to answer EMPTY at a table root must first CONFIRM the
+/// pool identity object (`_pool_meta`) exists with an AUTHORITATIVE, UNCACHED probe -- because "empty" at a
+/// table root is exactly what a silently-erased backing looks like, and a read-only pool has no
+/// keeper/lease/observer to catch that erasure any other way. These tests build a real
+/// `ContentAddressedMetadataStorage` over a Local object storage (the gtest_cas_operation_gate.cpp harness)
+/// and exercise the rule across the six cells the brief enumerates.
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int INVALID_STATE;
+extern const int NETWORK_ERROR;
+}
+
+using namespace DB;
+using DB::Cas::PoolLifecycle;
+using DB::Cas::ProbeOutcome;
+using DB::Cas::SentinelProbeResult;
+
+namespace
+{
+
+/// A committed (non-empty) table dir + part reused across the tests (the exact shape
+/// gtest_ca_transaction.cpp / gtest_cas_operation_gate.cpp use).
+const std::string kTableDir = "g80/g80g80g8-0808-4808-8808-080808080808";
+const std::string kPartDir = kTableDir + "/all_1_1_0";
+/// A DIFFERENT, never-committed-to table dir: genuinely empty for every test, distinct uuid so a
+/// commit to kTableDir can never make it non-empty.
+const std::string kEmptyTableDir = "g99/g99g99g9-0909-4909-8909-090909090909";
+
+std::shared_ptr<ContentAddressedMetadataStorage> openStorage()
+{
+    auto settings = Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_empty_proof_scratch");
+    auto storage = std::make_shared<ContentAddressedMetadataStorage>(
+        Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+/// Commit one real part into `kTableDir`, leaving that table dir non-empty (tmp -> final rename -> commit).
+void commitOnePart(ContentAddressedMetadataStorage & storage)
+{
+    auto tx = storage.createTransaction();
+    auto & ca_tx = dynamic_cast<ContentAddressedTransaction &>(*tx);
+    auto buf = ca_tx.writeFile(kTableDir + "/tmp_insert_all_1_1_0/data.bin", 65536, WriteMode::Rewrite, {});
+    const std::string bytes = "content-of-the-part";
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+    tx->moveDirectory(kTableDir + "/tmp_insert_all_1_1_0", kPartDir);
+    tx->commit(NoCommitOptions{});
+}
+
+/// A read-only mount over a backing a writable mount already bootstrapped (`_pool_meta` present). The
+/// writable mount minted the pool identity then shut down; the read-only mount validates `_pool_meta`,
+/// takes NO lease and runs NO erasure observer (it stays `Live` forever) -- exactly the state in which
+/// enumeration is the ONLY line of defense against a later erasure. Returns {ro storage, backing root}.
+struct ReadOnlyMount
+{
+    std::shared_ptr<ContentAddressedMetadataStorage> ro;
+    std::string root;
+};
+
+/// Delete ONLY the physical `_pool_meta` object under `root`, leaving the container directory and every
+/// other object intact — so a subsequent authoritative `probeSentinel` verdicts `KeyAbsent` (the identity
+/// key is gone while the container is alive), NOT `ContainerAbsent` (which a whole-root `remove_all` yields).
+/// This models the realistic "someone rm'd just the identity object" / partial-erase shape. Returns whether
+/// exactly one `_pool_meta` file was found and removed, so the test can guard against a vacuous pass.
+bool deleteOnlyPoolMetaUnder(const std::string & root)
+{
+    size_t removed = 0;
+    for (const auto & entry : std::filesystem::recursive_directory_iterator(root))
+    {
+        if (entry.is_regular_file() && entry.path().filename() == "_pool_meta")
+        {
+            std::filesystem::remove(entry.path());
+            ++removed;
+        }
+    }
+    return removed == 1;
+}
+
+/// The message thrown by `fn`, or a failure if it did not throw a `DB::Exception`.
+std::string messageOf(const std::function<void()> & fn)
+{
+    try
+    {
+        fn();
+    }
+    catch (const Exception & e)
+    {
+        return std::string(e.message());
+    }
+    ADD_FAILURE() << "expected a DB::Exception";
+    return {};
+}
+
+ReadOnlyMount openReadOnlyOverBootstrappedBacking()
+{
+    auto settings = Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_empty_proof_ro_scratch");
+
+    /// (1) A writable mount bootstraps `_pool_meta` over a fresh backing, then shuts down.
+    auto rw_os = Cas::tests::makeLocalObjectStorageForTest();
+    const std::string root = rw_os->getCommonKeyPrefix();
+    {
+        auto w = std::make_shared<ContentAddressedMetadataStorage>(
+            rw_os, "pool", "srv1", "", nullptr, settings);
+        w->startup();
+        w->shutdown();
+    }
+
+    /// (2) A read-only mount over the SAME backing validates `_pool_meta` and mounts `Live` (no lease,
+    /// no watermark, no observer -- read-only opens never enter the lifecycle machinery).
+    DB::LocalObjectStorageSettings ro_settings("test", root, /*read_only_=*/true);
+    auto ro_os = std::make_shared<DB::LocalObjectStorage>(std::move(ro_settings));
+    auto ro = std::make_shared<ContentAddressedMetadataStorage>(
+        ro_os, "pool", "srv1", "", nullptr, settings);
+    ro->startup();
+    return {std::move(ro), root};
+}
+
+}
+
+/// (a) THE RO-ATTACH silent-empty killer: a read-only pool whose whole backing was erased must throw,
+/// never answer empty. Both mandatory authorities disappear; table enumeration observes the missing
+/// `cas/ref_catalog` first, so `CORRUPTED_DATA` takes precedence over the later `_pool_meta` empty-proof
+/// check. The pool-meta-only companion below keeps the typed 668 contract pinned separately.
+TEST(CASEmptyProof, ReadOnlyOverErasedBackingThrowsInsteadOfEmpty)
+{
+    auto mount = openReadOnlyOverBootstrappedBacking();
+
+    /// Baseline while the backing is intact: the empty table root answers empty truthfully (`_pool_meta`
+    /// present authorizes it), issuing exactly one confirming probe.
+    mount.ro->resetEmptyProofProbeCountForTest();
+    EXPECT_TRUE(mount.ro->listDirectory(kEmptyTableDir).empty());
+    EXPECT_EQ(mount.ro->emptyProofProbeCountForTest(), 1u);
+
+    /// Erase the backing out from under the (still Live) read-only mount: `_pool_meta` and everything.
+    std::filesystem::remove_all(mount.root);
+
+    /// Now the SAME empty listing must refuse on the first missing mandatory control it observes.
+    Cas::tests::expectThrowsCode(ErrorCodes::CORRUPTED_DATA, [&] { mount.ro->listDirectory(kEmptyTableDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::CORRUPTED_DATA, [&] { mount.ro->iterateDirectory(kEmptyTableDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::CORRUPTED_DATA, [&] { mount.ro->isDirectoryEmpty(kEmptyTableDir); });
+}
+
+/// (a2, acceptance matrix — T9 review's KeyAbsent-specific real-backend follow-up) Test (a) erases the
+/// WHOLE backing (`remove_all(root)`), so its probe verdicts `ContainerAbsent`. This test deletes ONLY the
+/// `_pool_meta` object against the REAL Local backend — the container directory and every other object stay
+/// intact — so the authoritative probe verdicts `KeyAbsent` instead. Both flavours must reach the SAME
+/// "backing may be erased" refusal (distinct from the transient "transport or permission fault" one), so a
+/// targeted deletion of just the identity object (a partial erase) is caught exactly like a whole-root wipe.
+TEST(CASEmptyProof, ReadOnlyWithOnlyPoolMetaDeletedThrowsErasedFlavoredOnKeyAbsent)
+{
+    auto mount = openReadOnlyOverBootstrappedBacking();
+
+    /// Baseline while the backing is intact: the empty table root answers empty truthfully with one probe.
+    mount.ro->resetEmptyProofProbeCountForTest();
+    EXPECT_TRUE(mount.ro->listDirectory(kEmptyTableDir).empty());
+    EXPECT_EQ(mount.ro->emptyProofProbeCountForTest(), 1u);
+
+    /// Delete ONLY `_pool_meta` (container + every sibling object intact) → the probe verdicts KeyAbsent.
+    ASSERT_TRUE(deleteOnlyPoolMetaUnder(mount.root))
+        << "expected exactly one _pool_meta object to remove; otherwise this test is vacuous";
+
+    /// The KeyAbsent miss reaches the erased-flavored typed 668, NOT the transient one, and never answers empty.
+    const std::string msg = messageOf([&] { mount.ro->listDirectory(kEmptyTableDir); });
+    EXPECT_NE(msg.find("pool identity object absent"), std::string::npos) << msg;
+    EXPECT_NE(msg.find("the backing may be erased"), std::string::npos) << msg;
+    EXPECT_EQ(msg.find("transport or permission fault"), std::string::npos)
+        << "a KeyAbsent miss must give the erased message, not the transient/retry one: " << msg;
+
+    /// The other enumeration entry points refuse identically.
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { mount.ro->iterateDirectory(kEmptyTableDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { mount.ro->isDirectoryEmpty(kEmptyTableDir); });
+}
+
+/// (b) A Live pool over a genuinely-empty table dir with `_pool_meta` present answers empty AND issues
+/// EXACTLY ONE uncached sentinel probe -- and it happens on the empty (`isDirectoryEmpty` == true) path.
+TEST(CASEmptyProof, LiveEmptyTableDirAnswersEmptyWithExactlyOneProbe)
+{
+    auto storage = openStorage();
+
+    storage->resetEmptyProofProbeCountForTest();
+    EXPECT_TRUE(storage->isDirectoryEmpty(kEmptyTableDir));
+    EXPECT_EQ(storage->emptyProofProbeCountForTest(), 1u)
+        << "the empty table-root answer must confirm the pool identity with exactly one probe";
+
+    /// listDirectory / iterateDirectory each independently issue exactly one confirming probe too.
+    storage->resetEmptyProofProbeCountForTest();
+    EXPECT_TRUE(storage->listDirectory(kEmptyTableDir).empty());
+    EXPECT_EQ(storage->emptyProofProbeCountForTest(), 1u);
+}
+
+/// (c) The zero-cost hot path: a NON-empty table dir issues NO probe at all.
+TEST(CASEmptyProof, LiveNonEmptyTableDirIssuesNoProbe)
+{
+    auto storage = openStorage();
+    commitOnePart(*storage);
+
+    storage->resetEmptyProofProbeCountForTest();
+    EXPECT_FALSE(storage->listDirectory(kTableDir).empty());
+    EXPECT_FALSE(storage->isDirectoryEmpty(kTableDir));
+    EXPECT_EQ(storage->emptyProofProbeCountForTest(), 0u)
+        << "the non-empty hot path must never touch the empty-proof probe";
+}
+
+/// (d) A Vanished pool answers truth-empty WITHOUT any probe: `checkOpAdmitted`'s Probe -> TruthAbsent
+/// short-circuit answers before classification, so the terminal path never pays the empty-proof.
+TEST(CASEmptyProof, VanishedPoolAnswersTruthEmptyWithoutProbe)
+{
+    auto storage = openStorage();
+    commitOnePart(*storage);
+    auto pool = storage->store();   /// captured while Live
+    pool->setLifecycleForTest(PoolLifecycle::VanishedForgotten);
+
+    storage->resetEmptyProofProbeCountForTest();
+    EXPECT_TRUE(storage->listDirectory(kTableDir).empty());
+    EXPECT_TRUE(storage->isDirectoryEmpty(kTableDir));
+    EXPECT_FALSE(storage->iterateDirectory(kTableDir)->isValid());
+    EXPECT_EQ(storage->emptyProofProbeCountForTest(), 0u)
+        << "a Vanished pool answers truth-empty directly -- the gate short-circuits before the empty-proof";
+}
+
+/// (e) Scope discipline: a deeper (non-root) part-dir enumeration that answers empty is NOT gated.
+TEST(CASEmptyProof, DeeperPartDirEmptyAnswerIsNotGated)
+{
+    auto storage = openStorage();
+
+    /// A never-committed part dir under a table root: classifies as PartDir, answers empty, no probe.
+    const std::string absent_part_dir = kEmptyTableDir + "/all_9_9_0";
+    storage->resetEmptyProofProbeCountForTest();
+    EXPECT_TRUE(storage->listDirectory(absent_part_dir).empty());
+    EXPECT_EQ(storage->emptyProofProbeCountForTest(), 0u)
+        << "only the TableDir/DetachedContainer roots are gated -- deeper part-dirs are not";
+}
+
+/// (f) A probe that cannot establish absence (transport/permission fault) throws the typed TRANSIENT
+/// refusal, never an empty answer. Unproven absence is unavailability, so the refusal carries the
+/// upstream-retryable class -- unlike the `KeyAbsent`/`ContainerAbsent` arm, where absence IS proven and
+/// the 668 stands. The fault is injected through the empty-proof override seam.
+TEST(CASEmptyProof, IndeterminateProbeThrowsTransientNeverEmpty)
+{
+    auto storage = openStorage();
+    storage->setEmptyProofProbeOverrideForTest(
+        [] { return SentinelProbeResult{ProbeOutcome::Indeterminate, std::nullopt}; });
+
+    storage->resetEmptyProofProbeCountForTest();
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->listDirectory(kEmptyTableDir); });
+    EXPECT_EQ(storage->emptyProofProbeCountForTest(), 1u);
+
+    /// The transient message names the fault (a retryable condition), distinct from the erased message.
+    std::string msg;
+    try
+    {
+        storage->listDirectory(kEmptyTableDir);
+    }
+    catch (const Exception & e)
+    {
+        msg = std::string(e.message());
+    }
+    EXPECT_NE(msg.find("transport or permission fault"), std::string::npos) << msg;
+    EXPECT_NE(msg.find("TRANSIENT"), std::string::npos) << msg;
+}
diff --git a/src/Disks/tests/gtest_cas_encoding_pins.cpp b/src/Disks/tests/gtest_cas_encoding_pins.cpp
new file mode 100644
index 000000000000..f4c3a913fda7
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_encoding_pins.cpp
@@ -0,0 +1,110 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <IO/WriteBufferFromString.h>
+#include <fmt/format.h>
+
+using namespace DB;
+using namespace DB::Cas;
+
+/// These literals pin the CANONICAL BYTES of the CAS text encoders as of the commit that
+/// introduced this file. The CasJsonWriter migration (2026-07-20 spec) must keep every one of
+/// them green UNMODIFIED: canonical text is byte-compared on retries and deterministic adoption,
+/// and the incremental ref budget counters assume these exact sizes. Never edit an expected
+/// string here to make a test pass — that means the encoder's bytes drifted, which is the bug.
+
+TEST(CASEncodingPins, RefLogTxnAllOpKinds)
+{
+    RefLogTxn txn;
+    txn.ns = "roots/pin";
+    txn.txn_id = RefTxnId{7, 9};
+
+    RefOp birth;
+    birth.kind = RefOpKind::NamespaceBirth;
+    txn.ops.push_back(birth);
+
+    RefOp transition;
+    transition.kind = RefOpKind::OwnerTransition;
+    transition.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "20260101_0_1_1_1", ManifestRef{1, 2, 3}};
+    transition.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "20260101_0_1_1_1", ManifestRef{1, 2, 3}};
+    txn.ops.push_back(transition);
+
+    RefOp set_published_at;
+    set_published_at.kind = RefOpKind::SetPublishedAt;
+    /// NOTE the split literals: "\x01" "e" (else the hex escape would swallow the 'e') and
+    /// "\xA8" "f" (else it would swallow the 'f'). `checkCanonicalRefName` forbids '\\' and NUL but
+    /// not quote/newline/control bytes/U+2028, so `ref_name` -- the only free-form string `RefOp`
+    /// still carries now that `payload` is gone -- exercises quote, newline, a bare control byte,
+    /// and the three-byte U+2028 sequence. Backslash escaping is pinned separately, over an
+    /// unrestricted string, by `gtest_cas_json_writer.cpp`'s `CASJsonWriterEscaping` suite.
+    set_published_at.ref_name = String("20260101_0_1_1_1\"c\nd") + "\x01" "e" + "\xE2\x80\xA8" "f";
+    set_published_at.expected_manifest_ref = ManifestRef{1, 2, 3};
+    set_published_at.published_at_ms = 1234;
+    txn.ops.push_back(set_published_at);
+
+    RefOp removal;
+    removal.kind = RefOpKind::RemoveNamespace;
+    txn.ops.push_back(removal);
+
+    const String expected = fmt::format("{{\"type\":\"cas_ref_log\",\"v\":{}}}\n", currentCompatibilityVersion()) +
+        "{\"ns\":\"roots/pin\",\"we\":\"7\",\"rs\":\"9\"}\n"
+        "{\"op\":\"namespace_birth\"}\n"
+        "{\"op\":\"owner_transition\",\"obk\":\"precommit\",\"orn\":\"20260101_0_1_1_1\","
+        "\"ome\":\"1\",\"omb\":\"2\",\"omo\":3,\"nbk\":\"committed\",\"nrn\":\"20260101_0_1_1_1\","
+        "\"nme\":\"1\",\"nmb\":\"2\",\"nmo\":3}\n"
+        "{\"op\":\"set_published_at\",\"rn\":\"20260101_0_1_1_1\\\"c\\nd\\u0001e\\u2028f\","
+        "\"me\":\"1\",\"mb\":\"2\",\"mo\":3,\"ts\":1234}\n"
+        "{\"op\":\"remove_namespace\"}\n"
+        "{\"n\":4}\n";
+    EXPECT_EQ(encodeRefLogTxn(txn), expected);
+}
+
+TEST(CASEncodingPins, RefSnapshotLive)
+{
+    RefTableSnapshot snap;
+    snap.ns = "roots/pin";
+    snap.snapshot_id = RefTxnId{7, 9};
+
+    RefCommittedRow row;
+    row.ref_name = "20260101_0_1_1_1";
+    row.manifest_ref = ManifestRef{1, 2, 3};
+    row.published_at_ms = 5;
+    snap.committed.push_back(row);
+
+    snap.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "20260102_0_2_2_2", ManifestRef{4, 5, 6}});
+
+    const String expected = fmt::format("{{\"type\":\"cas_ref_snap\",\"v\":{}}}\n", currentCompatibilityVersion()) +
+        "{\"ns\":\"roots/pin\",\"we\":\"7\",\"rs\":\"9\",\"lc\":\"live\"}\n"
+        "{\"k\":\"c\",\"rn\":\"20260101_0_1_1_1\",\"me\":\"1\",\"mb\":\"2\",\"mo\":3,\"ts\":5}\n"
+        "{\"k\":\"p\",\"rn\":\"20260102_0_2_2_2\",\"me\":\"4\",\"mb\":\"5\",\"mo\":6}\n"
+        "{\"n\":2}\n";
+    EXPECT_EQ(encodeRefTableSnapshot(snap), expected);
+}
+
+TEST(CASEncodingPins, SourceEdgeRunLines)
+{
+    WriteBufferFromOwnString out;
+    SourceEdgeRunWriter writer(out);
+
+    SourceEdgeRecord active;
+    active.ref = BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(2))};
+    active.source_id = UInt128(5);
+    active.marker = kEdgeActive;
+    writer.append(active);
+
+    writer.finish();
+    out.finalize();
+
+    /// The exact "b" rendering (algo byte + digest hex) is pinned as a whole line; the point is
+    /// that Task 8's line-scratch rewrite must reproduce it byte-for-byte.
+    const String text = out.str();
+    const String header = fmt::format("{{\"type\":\"cas_run\",\"v\":{},\"kind\":\"source_edge\"}}\n", currentCompatibilityVersion());
+    const String expected_record =
+        "{\"b\":\"0100000000000000000000000000000002\",\"s\":\"00000000000000000000000000000005\",\"m\":\"edge\"}\n";
+    const String trailer = "{\"n\":1}\n";
+    /// There is exactly one record, so the whole buffer must be byte-identical to header + record + trailer.
+    const String expected_full = header + expected_record + trailer;
+    EXPECT_EQ(text, expected_full) << text;
+}
diff --git a/src/Disks/tests/gtest_cas_envelope.cpp b/src/Disks/tests/gtest_cas_envelope.cpp
new file mode 100644
index 000000000000..7dac7ddf6eed
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_envelope.cpp
@@ -0,0 +1,55 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <string>
+
+using namespace DB;
+using namespace DB::Cas;
+
+/// The v3 blob-envelope shape (256-byte JSON header + payload). Full round-trip / gate / pad-zone /
+/// budget / critical-key coverage lives in gtest_cas_blob_envelope_format.cpp; these two keep the
+/// cases that file does not exercise: a header with NO provenance/ref, and the incarnation-zone
+/// independence of the payload.
+
+TEST(CASEnvelope, BlobRoundTripNoExtensions)
+{
+    const std::string payload = "hello payload";
+    EnvelopeHeader h;
+    h.kind = ObjectKind::Blob;
+    h.incarnation_tag = 0x22;
+    h.build_id = 0x33;
+    const std::string obj = encodeEnvelopeHeader(h, 256) + payload;
+
+    const EnvelopeHeader d = decodeEnvelopeHeader(obj, obj.size(), ObjectKind::Blob);
+    EXPECT_EQ(d.kind, ObjectKind::Blob);
+    EXPECT_EQ(d.compatibility_version, G_BUILD);
+    EXPECT_FALSE(d.provenance.has_value());    /// none set -> the ts/by/op/ch keys are absent
+    EXPECT_FALSE(d.intended_ref.has_value());  /// none set -> the ref key is omitted
+    EXPECT_EQ(d.header_len, 256u);
+    /// payload starts right after the fixed-length header.
+    EXPECT_EQ(obj.substr(payloadOffset(d)), payload);
+}
+
+TEST(CASEnvelope, IncarnationZoneDoesNotAffectPayload)
+{
+    /// Two objects with the SAME payload but DIFFERENT incarnation_tag/build_id encode to different
+    /// header bytes, yet both carry the same payload at the same fixed offset — the incarnation zone
+    /// never affects the payload. Identity is the content key, not any header field.
+    const std::string payload = "same content";
+    EnvelopeHeader a;
+    a.kind = ObjectKind::Blob;
+    a.incarnation_tag = 0xAAAA;
+    a.build_id = 0xBBBB;
+    EnvelopeHeader b = a;
+    b.incarnation_tag = 0xCCCC;
+    b.build_id = 0xDDDD;
+
+    const std::string ha = encodeEnvelopeHeader(a, 256);
+    const std::string hb = encodeEnvelopeHeader(b, 256);
+    EXPECT_NE(ha, hb);   /// headers differ (incarnation zone)
+
+    const EnvelopeHeader da = decodeEnvelopeHeader(ha + payload, ha.size() + payload.size(), ObjectKind::Blob);
+    const EnvelopeHeader db = decodeEnvelopeHeader(hb + payload, hb.size() + payload.size(), ObjectKind::Blob);
+    EXPECT_EQ((ha + payload).substr(payloadOffset(da)), payload);
+    EXPECT_EQ((hb + payload).substr(payloadOffset(db)), payload);
+}
diff --git a/src/Disks/tests/gtest_cas_event_dispatcher.cpp b/src/Disks/tests/gtest_cas_event_dispatcher.cpp
new file mode 100644
index 000000000000..010d55d90455
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_event_dispatcher.cpp
@@ -0,0 +1,194 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasEventDispatcher.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <atomic>
+#include <chrono>
+#include <future>
+#include <latch>
+#include <mutex>
+#include <set>
+#include <string>
+#include <thread>
+#include <vector>
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace
+{
+
+/// A single-blob part: upload one blob, stage a one-entry manifest naming it, precommit + promote the
+/// ref. Mirrors `publishOneBlobPart` in `gtest_cas_event_log.cpp` so a committed ref exists for
+/// `resolveRef` to resolve and emit against.
+void publishOneBlobPart(const PoolPtr & s, const String & ns, const String & ref, const String & payload)
+{
+    const RootNamespace nsr{ns};
+    PartWriteInfo info;
+    info.intended_ref = ns + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    ManifestEntry e;
+    e.path = "data.bin";
+    e.placement = EntryPlacement::Blob;
+    e.ref = BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of(payload))};
+    e.blob_size = payload.size();
+    const ManifestId id = build->stageManifest({e});
+    build->precommitAdd(nsr, ref, id);
+    build->promote(nsr, ref, build->buildId(), id);
+}
+
+}
+
+/// Concurrent emitters must be serialized: N threads emit M events each into a sink that appends to a
+/// DELIBERATELY UNGUARDED vector. If the dispatcher did not serialize delivery the concurrent
+/// `push_back`s would tear the vector (and TSan on that lane would flag the data race); serialized
+/// delivery makes the unguarded append correct. The count/uniqueness assertions catch dropped or
+/// duplicated events on any lane.
+TEST(CASEventDispatcher, SerializesConcurrentEmitters)
+{
+    EventDispatcher disp;
+    std::vector<CasEvent> seen;   /// unguarded on purpose -- the dispatcher is the only serialization
+    disp.setSink([&](CasEvent e) { seen.push_back(std::move(e)); });
+
+    constexpr int N = 8;     /// emitter threads
+    constexpr int M = 250;   /// emits per thread
+    std::latch start{N};     /// release all emitters together to maximize contention on the dispatcher
+    std::vector<std::thread> threads;
+    threads.reserve(N);
+    for (int t = 0; t < N; ++t)
+        threads.emplace_back([&, t]
+        {
+            start.arrive_and_wait();
+            for (int m = 0; m < M; ++m)
+            {
+                CasEvent e;
+                e.type = CasEventType::BlobPut;
+                e.object_hash = std::to_string(t * M + m);
+                disp.emit(std::move(e));
+            }
+        });
+    for (auto & th : threads)
+        th.join();
+
+    ASSERT_EQ(seen.size(), static_cast<size_t>(N * M));
+    std::set<String> ids;
+    for (const auto & e : seen)
+        ids.insert(e.object_hash);
+    EXPECT_EQ(ids.size(), static_cast<size_t>(N * M)) << "every emitted event delivered exactly once";
+}
+
+/// A sink that emits again from inside its own delivery must not deadlock. The drain-loop design
+/// never holds the dispatcher mutex across the sink call, so the reentrant `emit` acquires the mutex,
+/// finds a drain already running, enqueues, and returns; the running loop delivers it after the
+/// current sink returns. Delivery is synchronous on the emitting thread, so no timed wait is needed:
+/// `emit` returns only after the whole queue (including the reentrant event) has drained.
+TEST(CASEventDispatcher, ReentrantSinkDoesNotDeadlock)
+{
+    EventDispatcher disp;
+    std::vector<CasEventType> delivered;
+    std::atomic<bool> reentered_once{false};
+    disp.setSink([&](CasEvent e)
+    {
+        delivered.push_back(e.type);
+        if (e.type == CasEventType::BlobPut && !reentered_once.exchange(true))
+        {
+            CasEvent second;
+            second.type = CasEventType::BlobDelete;
+            disp.emit(std::move(second));
+        }
+    });
+
+    CasEvent first;
+    first.type = CasEventType::BlobPut;
+    disp.emit(std::move(first));
+
+    ASSERT_EQ(delivered.size(), 2u) << "both the original and the reentrant event must be delivered";
+    EXPECT_EQ(delivered[0], CasEventType::BlobPut);
+    EXPECT_EQ(delivered[1], CasEventType::BlobDelete)
+        << "the reentrant event is drained AFTER the current sink returns, not recursively";
+}
+
+/// Test 17: a ledger emission must fire OUTSIDE the ledger lock. Install a sink that, on delivery of
+/// a `RefResolve` event, re-enters a ledger read (`resolveRef`) that itself takes `state_mutex`; then
+/// drive a real emitting `resolveRef` on a worker thread while a second thread emits upload-style
+/// events concurrently. If `resolveRef` emitted while holding `state_mutex` (the pre-fix defect), the
+/// worker would re-lock `state_mutex` on the same thread from inside the sink and self-deadlock. The
+/// restructured emit (after the lock scope) lets the reentrant read take the lock freshly, and the
+/// dispatcher serializes the concurrent upload emissions.
+TEST(CASEventDispatcher, LedgerEmissionOutsideLocks)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    std::vector<CasEvent> seen;   /// declared before the Pool so it outlives any late background emit
+    std::mutex seen_mutex;
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+
+    const RootNamespace ns{"srv1/tbl"};
+    const String ref = "all_0_0_0";
+    publishOneBlobPart(s, ns.string(), ref, "the-resolvable-payload");
+
+    std::atomic<bool> reentered{false};
+    s->setEventSink([&](CasEvent e)
+    {
+        {
+            std::lock_guard<std::mutex> g(seen_mutex);
+            seen.push_back(e);
+        }
+        /// Re-enter a ledger read that takes `state_mutex`, exactly once (`Deferred` => this read
+        /// itself emits nothing, so there is no unbounded emit recursion). Under the pre-fix code the
+        /// outer `resolveRef` still holds `state_mutex` here, so this call self-deadlocks.
+        if (e.type == CasEventType::RefResolve && !reentered.exchange(true))
+            (void)s->resolveRef(ns, ref, false, ResolveAudit::Deferred);
+    });
+
+    std::promise<void> resolve_done;
+    auto resolve_future = resolve_done.get_future();
+    std::thread resolver([&]
+    {
+        (void)s->resolveRef(ns, ref);   /// ResolveAudit::Emit (default) -> emits RefResolve -> drives the sink
+        resolve_done.set_value();
+    });
+
+    /// A second thread emits upload-task-style events concurrently with the resolve, so the dispatcher's
+    /// serialization is exercised alongside the reentrancy path.
+    std::thread uploader([&]
+    {
+        for (int i = 0; i < 32; ++i)
+        {
+            CasEvent up;
+            up.type = CasEventType::BlobPut;
+            up.object_hash = "up-" + std::to_string(i);
+            up.reason = "concurrent upload-task emission";
+            s->emitEvent(std::move(up));
+        }
+    });
+
+    /// Bounded wait: the resolve is two in-memory map lookups plus queue drains -- microseconds of
+    /// real work. 10 seconds is many orders of magnitude above that and only elapses if the
+    /// emit-under-lock defect self-deadlocks the worker on `state_mutex`.
+    const auto status = resolve_future.wait_for(std::chrono::seconds(10));
+    ASSERT_EQ(status, std::future_status::ready)
+        << "resolveRef with a re-entrant sink did not complete: emission is happening under state_mutex";
+    resolver.join();
+    uploader.join();
+
+    EXPECT_TRUE(reentered.load()) << "the reentrant ledger read must have run";
+    std::lock_guard<std::mutex> g(seen_mutex);
+    size_t resolves = 0;
+    size_t uploads = 0;
+    for (const auto & e : seen)
+    {
+        if (e.type == CasEventType::RefResolve)
+            ++resolves;
+        else if (e.type == CasEventType::BlobPut)
+            ++uploads;
+    }
+    EXPECT_GE(resolves, 1u) << "the driving resolve emitted its RefResolve";
+    EXPECT_EQ(uploads, 32u) << "every concurrent upload emission was delivered exactly once";
+}
diff --git a/src/Disks/tests/gtest_cas_event_log.cpp b/src/Disks/tests/gtest_cas_event_log.cpp
new file mode 100644
index 000000000000..3cba6eb671de
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_event_log.cpp
@@ -0,0 +1,240 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Interpreters/ContentAddressedLog.h>
+#include <DataTypes/DataTypeLowCardinality.h>
+#include <Common/typeid_cast.h>
+#include <mutex>
+#include <vector>
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+/// Round-B opt §6: `reason` is templated rationale (a handful of distinct strings repeated across
+/// every row), unlike `object_hash`/`token` which are genuinely per-row varied -- it belongs alongside
+/// the log's other LowCardinality columns (event_type/object_kind/outcome), not as a full String.
+TEST(CASContentAddressedLog, ReasonColumnIsLowCardinality)
+{
+    const auto columns = DB::ContentAddressedLogElement::getColumnsDescription();
+    const auto & reason_col = columns.get("reason");
+    EXPECT_TRUE(typeid_cast<const DB::DataTypeLowCardinality *>(reason_col.type.get()))
+        << "reason column must be LowCardinality(String) (Round-B opt §6)";
+}
+TEST(CASEvent, ConstructAndCopyAndName)
+{
+    CasEvent e;
+    e.type = CasEventType::BlobDelete;
+    e.object_kind = CasEventObjectKind::Blob;
+    e.object_hash = "abcd";
+    e.token = "tok";
+    e.round = 7; e.gen = 3;
+    e.reason = "in-degree 0 after strip";
+    e.detail["freed"] = "10";
+    CasEvent c = e;
+    EXPECT_EQ(c.type, CasEventType::BlobDelete);
+    EXPECT_EQ(c.object_hash, "abcd");
+    EXPECT_EQ(c.detail.at("freed"), "10");
+    EXPECT_EQ(toString(CasEventType::BlobDelete), "blob_delete");
+    EXPECT_EQ(toString(CasEventType::IndegZero), "indegree_zero");
+    EXPECT_EQ(toString(CasEventType::GcRecheckVerdict), "gc_recheck_verdict");
+    EXPECT_EQ(toString(CasEventObjectKind::Manifest), "manifest");
+}
+
+TEST(CASEvent, PoolEmitsToSink)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    std::vector<CasEvent> seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    s->setEventSink([&](const CasEvent & e){ seen.push_back(e); });
+    CasEvent e;
+    e.type = CasEventType::BlobPut;
+    e.object_hash = "h";
+    s->emitEvent(std::move(e));
+    ASSERT_EQ(seen.size(), 1u);
+    EXPECT_EQ(seen[0].type, CasEventType::BlobPut);
+    /// null sink => no-op (no crash, no row); a fresh event, not the one already moved above.
+    s->setEventSink(nullptr);
+    CasEvent e2;
+    e2.type = CasEventType::BlobPut;
+    s->emitEvent(std::move(e2));
+    EXPECT_EQ(seen.size(), 1u);
+}
+
+/// Round-B opt §6: `emitEvent` takes the event BY VALUE (moved-through, not `const &`), so a
+/// caller's local is genuinely moved-from -- not merely copied via a const reference -- by the time
+/// the sink runs. Mirrors `makeCasEventSink`'s own move-out-of-the-by-value-event idiom (a small test
+/// double stands in for the `ContentAddressedLogElement` it would normally build).
+TEST(CASEvent, EmitEventMovesSourceIntoSink)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    String captured_reason;
+    std::map<String, String> captured_detail;
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    s->setEventSink([&](CasEvent ev)
+    {
+        captured_reason = std::move(ev.reason);
+        captured_detail = std::move(ev.detail);
+    });
+    CasEvent e;
+    e.type = CasEventType::BlobPut;
+    e.reason = "sentinel-reason";
+    e.detail["k"] = "v";
+    s->emitEvent(std::move(e));
+    EXPECT_EQ(captured_reason, "sentinel-reason");
+    EXPECT_EQ(captured_detail.at("k"), "v");
+    /// the source event must be MOVED-FROM after emit, not merely aliased/copied through -- reading
+    /// `e` here is the whole point of the test, not an oversight.
+    EXPECT_TRUE(e.reason.empty()); // NOLINT(bugprone-use-after-move, hicpp-invalid-access-moved)
+    EXPECT_TRUE(e.detail.empty()); // NOLINT(bugprone-use-after-move, hicpp-invalid-access-moved)
+}
+
+namespace
+{
+
+/// A single-blob part: upload one blob, stage a one-entry manifest naming it, precommit + promote the
+/// ref. Returns the blob's object_hash (lowercase hex) so the test can filter the captured rows by it.
+String publishOneBlobPart(const PoolPtr & s, const String & ns, const String & ref, const String & payload)
+{
+    const RootNamespace nsr{ns};
+    PartWriteInfo info;
+    info.intended_ref = ns + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    ManifestEntry e;
+    e.path = "data.bin";
+    e.placement = EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    e.blob_size = payload.size();
+    const ManifestId id = build->stageManifest({e});
+    build->precommitAdd(nsr, ref, id);
+    build->promote(nsr, ref, build->buildId(), id);
+    /// Phase 3 (mixed-algo pools): every blob-content-hash event render is `blobIdOf(ref)`
+    /// ("<algoName>:<hex>"), never a bare hex -- the prime directive that a digest never appears
+    /// without its algo.
+    return DB::Cas::blobIdOf(e.ref);
+}
+
+/// Whether the CURRENT retired list (any gc-shard) still holds an entry (ack-floor pipeline in flight).
+bool anyRetiredPending(const PoolPtr & s)
+{
+    /// Retired-in-snapshot (T4): condemned state rides the adopted fold seal's kCondemned rows, not a
+    /// separate retired list — reconstruct the in-flight set from the seal.
+    return DB::Cas::tests::anyCondemnedInSeal(s->backend(), s->layout());
+}
+
+/// Drive regular GC to a fixpoint over the ACK-FLOOR round (renew the store's mount ack after each round;
+/// stay alive while any work counter is nonzero OR an in-flight retired entry remains).
+void runGcToFixpoint(const PoolPtr & s, Gc & gc, size_t max_rounds = 64)
+{
+    for (size_t r = 0; r < max_rounds; ++r)
+    {
+        const RoundReport rep = DB::Cas::tests::runRegularRoundReclaiming(gc);
+        if (!rep.acquired_lease)
+            continue;
+        s->renewWatermarkOnce();
+        const bool no_work = rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0;
+        if (no_work && !anyRetiredPending(s))
+            break;
+    }
+}
+
+bool hasType(const std::vector<CasEvent> & events, CasEventType t)
+{
+    for (const auto & e : events)
+        if (e.type == t)
+            return true;
+    return false;
+}
+
+}
+
+/// B170 Task 4 acceptance: drive a full publish -> drop -> GC-to-delete lifecycle through a capturing
+/// sink and assert (a) the taxonomy of events is emitted, (b) EVERY event carries a non-empty reason,
+/// (c) filtering by a deleted blob's object_hash reconstructs its edge/retire/delete chain in order.
+TEST(CASEvent, LifecycleReconstructionFromRows)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    /// Declared BEFORE the Pool so they OUTLIVE it: the Pool's background retired-view syncer can emit
+    /// (e.g. a view-advance event) right up to the Pool's destructor, and a sink capturing locals that
+    /// die first is a use-after-scope (found by ASan 2026-07-09; the production sink captures the Context
+    /// shared_ptr by value and is immune).
+    std::vector<CasEvent> events;
+    std::mutex events_mutex;
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+
+    s->setEventSink([&](const CasEvent & e)
+    {
+        std::lock_guard lock(events_mutex);
+        events.push_back(e);
+    });
+
+    const RootNamespace ns{"srv1/tbl"};
+    const String ref = "all_0_0_0";
+    const String payload = "the-doomed-blob-payload";
+
+    /// publish -> the blob's whole closure is born and a ref names it.
+    const String blob_hash = publishOneBlobPart(s, ns.string(), ref, payload);
+
+    /// drop the ref and advance the watermark so the now-unreferenced closure is collectable.
+    s->dropRef(ns, ref);
+    s->renewWatermarkOnce();
+
+    /// GC reclaims the tree and the blob to a fixpoint.
+    Gc gc(s, u128Of("gc-event-log"));
+    runGcToFixpoint(s, gc);
+
+    /// The blob must actually be gone (the delete fired).
+    ASSERT_FALSE(b->head(s->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of(payload))})).exists)
+        << "GC must have deleted the now-unreferenced blob";
+
+    /// (a) the expected taxonomy was emitted across the lifecycle (manifest model: no standalone trees).
+    EXPECT_TRUE(hasType(events, CasEventType::BlobPut));
+    EXPECT_TRUE(hasType(events, CasEventType::RootAdd))
+        << "a fold must have recorded the manifest owner's blob edge (+1)";
+    EXPECT_TRUE(hasType(events, CasEventType::RefDrop));
+    EXPECT_TRUE(hasType(events, CasEventType::IndegZero));
+    EXPECT_TRUE(hasType(events, CasEventType::GcRetireObserve)
+        || hasType(events, CasEventType::GcRetireDecision)
+        || hasType(events, CasEventType::GcRecheckVerdict))
+        << "a GC retire/recheck transition must be recorded";
+    EXPECT_TRUE(hasType(events, CasEventType::BlobDelete) || hasType(events, CasEventType::ManifestDelete))
+        << "the single content-delete site must emit a delete row";
+
+    /// (b) completeness mandate: every emitted event has a non-empty reason (the human WHY).
+    for (const auto & e : events)
+        EXPECT_FALSE(e.reason.empty())
+            << "event " << toString(e.type) << " (" << e.object_hash << ") has an empty reason";
+
+    /// (c) lifecycle reconstruction: filtering by the deleted blob's object_hash yields, in time
+    /// order, at least its in-degree-zero -> retire-observe -> delete chain — its whole story.
+    std::vector<CasEventType> chain;
+    for (const auto & e : events)
+        if (e.object_hash == blob_hash)
+            chain.push_back(e.type);
+
+    ASSERT_FALSE(chain.empty()) << "no rows reference the deleted blob " << blob_hash;
+
+    /// The decisive ordering: the blob's in-degree hit 0 BEFORE GC observed/condemned it, which was
+    /// BEFORE it was deleted. Find the first index of each and assert the order.
+    auto firstIndexOf = [&](CasEventType t) -> int
+    {
+        for (size_t i = 0; i < chain.size(); ++i)
+            if (chain[i] == t)
+                return static_cast<int>(i);
+        return -1;
+    };
+    const int i_indeg = firstIndexOf(CasEventType::IndegZero);
+    const int i_observe = firstIndexOf(CasEventType::GcRetireObserve);
+    const int i_delete = firstIndexOf(CasEventType::BlobDelete);
+    ASSERT_GE(i_indeg, 0) << "the blob's indegree_zero must be in its chain";
+    ASSERT_GE(i_observe, 0) << "the blob's gc_retire_observe must be in its chain";
+    ASSERT_GE(i_delete, 0) << "the blob's blob_delete must be in its chain";
+    EXPECT_LT(i_indeg, i_observe) << "in-degree hit 0 before GC observed it";
+    EXPECT_LT(i_observe, i_delete) << "GC observed it before deleting it";
+}
diff --git a/src/Disks/tests/gtest_cas_fence_generation.cpp b/src/Disks/tests/gtest_cas_fence_generation.cpp
new file mode 100644
index 000000000000..74404f68c203
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_fence_generation.cpp
@@ -0,0 +1,471 @@
+#include <gtest/gtest.h>
+#include "cas_test_helpers.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasMountRuntime.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+
+#include <functional>
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <utility>
+
+namespace DB::ErrorCodes
+{
+    extern const int NETWORK_ERROR;
+}
+
+/// Task 4 (spec §1 "Gate lifetime [C2]"): every durable-effect path on the plain-object surface
+/// (`CasPlainObjects::casPutObject`/`casRemoveObject`), the S3-native staging-buffer finalize
+/// (`Cas::CaContentWriteBuffer`), and the part-write condemned-displacement raw writes capture the mount
+/// runtime's fence generation at admission and re-check it -- and `mayMutate()` -- immediately before their
+/// durable backend call, throwing the typed transient error (`NETWORK_ERROR` -- the upstream-retryable
+/// class every CA write-plane transient uses) on a mismatch instead of letting a stale-incarnation write
+/// land.
+///
+/// These tests drive a real `Cas::Pool` over `InMemoryBackend` (the "Emulated"-style in-memory
+/// backend) via `Pool::open`, exactly like `gtest_cas_mount.cpp`/`gtest_cas_s3_staging.cpp` -- the
+/// fence is tripped/observed through `Pool`'s public forwarders (`tripMountLost`, `mayMutate`,
+/// `fenceGeneration`, `checkFenceOrThrow`).
+
+using namespace DB::Cas;
+
+namespace
+{
+
+/// A backend whose `head()` call can trigger an injected side-effect exactly once -- deterministically
+/// simulates a fence trip landing BETWEEN a durable-effect operation's admission and its durable
+/// backend call, with no real concurrency at all (mirrors the injected-fault shape of
+/// `TransportFaultBackend` in gtest_cas_sentinel_probe.cpp, but fires a callback instead of throwing).
+class TripOnHeadBackend final : public InMemoryBackend
+{
+public:
+    HeadResult head(const String & key) override
+    {
+        if (trigger)
+            std::exchange(trigger, {})();
+        return InMemoryBackend::head(key);
+    }
+
+    std::function<void()> trigger;
+};
+
+/// Same idea as `TripOnHeadBackend`, but fires on the SECOND `head()` call and forces a first-attempt
+/// `PreconditionFailed` so the retry loop actually reaches a second iteration -- proves the fence
+/// re-check runs on EVERY conditional-retry iteration, not just the admission-time first attempt.
+class TripOnSecondHeadBackend final : public InMemoryBackend
+{
+public:
+    using Backend::putIfAbsent;
+
+    HeadResult head(const String & key) override
+    {
+        ++head_calls;
+        if (head_calls == 2 && trigger)
+            std::exchange(trigger, {})();
+        return InMemoryBackend::head(key);
+    }
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (fail_first_put)
+        {
+            fail_first_put = false;
+            return PutResult{.outcome = PutOutcome::PreconditionFailed, .token = {}};
+        }
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    int head_calls = 0;
+    /// Default false: `Pool::open`'s own capability probe issues `putIfAbsent` calls before the test
+    /// gets to arm this, and those must succeed normally. The test flips this to `true` only right
+    /// before driving the write it actually targets.
+    bool fail_first_put = false;
+    std::function<void()> trigger;
+};
+
+/// A minimal in-memory `WriteBufferFromFileBase` standing in for an object-store sink, trimmed to just
+/// what these tests observe (whether `finalizeImpl` ran) -- mirrors `FakeStagingSink` in
+/// gtest_cas_s3_staging.cpp (not reusable from here: that one lives in that file's own anonymous
+/// namespace).
+class RecordingSink final : public DB::WriteBufferFromFileBase
+{
+public:
+    explicit RecordingSink(std::string key_)
+        : DB::WriteBufferFromFileBase(/*buf_size=*/8192, nullptr, 0), key(std::move(key_))
+    {
+    }
+
+    void sync() override {}
+    std::string getFileName() const override { return key; }
+    bool wasFinalizedForTest() const { return did_finalize; }
+
+protected:
+    void nextImpl() override
+    {
+        if (offset())
+            written.append(working_buffer.begin(), offset());
+    }
+
+    void finalizeImpl() override
+    {
+        next();
+        did_finalize = true;
+    }
+
+    void cancelImpl() noexcept override { cancelled = true; }
+
+private:
+    std::string key;
+    std::string written;
+    bool did_finalize = false;
+    bool cancelled = false;
+};
+
+PoolPtr openTestPool(BackendPtr backend)
+{
+    return Pool::open(std::move(backend), PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// I2 (backlog {#c2-resurrect-putoverwrite-fence-check}): a backend that (1) trips an injected side-effect
+/// on the FIRST head() -- deterministically landing a fence trip BETWEEN `streamIfAbsent`'s conditional
+/// create (which returns Occupied over a present condemned blob WITHOUT a head -- see
+/// `conditionalCreateControlled`) and `uploadFromSource`'s condemned-displacement decision (whose first
+/// head is the `head(key)` right before the raw `resurrect`/`putOverwrite` write) -- and (2) records
+/// those two raw displacement calls, so a test proves the [C2] fence check aborted BEFORE any durable
+/// displacement landed. Mirrors `TripOnHeadBackend` above and `RecordingStagingBackend` in
+/// gtest_cas_s3_staging.cpp (neither reusable across their anonymous namespaces).
+class TripOnHeadDisplacementBackend final : public InMemoryBackend
+{
+public:
+    using Backend::putOverwrite;
+
+    HeadResult head(const String & key) override
+    {
+        if (trigger)
+            std::exchange(trigger, {})();
+        return InMemoryBackend::head(key);
+    }
+
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        ++put_overwrite_calls;
+        return InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+    }
+
+    Token resurrect(DB::ReadBuffer & payload, uint64_t payload_size, const String & blob_key, const String & fresh_header) override
+    {
+        ++resurrect_staged_calls;
+        return InMemoryBackend::resurrect(payload, payload_size, blob_key, fresh_header);
+    }
+
+    std::function<void()> trigger;
+    int put_overwrite_calls = 0;
+    int resurrect_staged_calls = 0;
+};
+
+/// A `BlobSource` that promotes via the S3 server-side-copy path (no local `open`) -- mirrors
+/// `serverSideCopySource` in gtest_cas_s3_staging.cpp.
+BlobSource serverSideCopySource(const std::string & staging_key, uint64_t size)
+{
+    BlobSource source;
+    source.size = size;
+    source.server_side_copy_from = staging_key;
+    return source;
+}
+
+TEST(CASFenceGeneration, RearmPublishesTheNewGenerationBeforeOpeningTheFence)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openTestPool(backend);
+    const RootNamespace ns{"srv1/rearm-publication-order"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+    ASSERT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+
+    store->tripMountLost();
+    const uint64_t dead_generation = store->fenceGeneration();
+    bool admitted_in_interposition = false;
+    store->setArmMountFenceInterpositionHookForTest([&]
+    {
+        EXPECT_EQ(store->fenceGeneration(), dead_generation + 1)
+            << "the fresh generation must be visible before the fence can become live";
+        try
+        {
+            (void)store->namespaceLife(ns);
+            admitted_in_interposition = true;
+        }
+        catch (const DB::Exception & e)
+        {
+            EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR);
+        }
+        EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u)
+            << "no runtime may be published in the re-arm interposition";
+    });
+
+    store->armMountFence(DB::UInt128{0, 1}, store->writerEpoch(), store->bootMsNow() + 600000);
+    store->setArmMountFenceInterpositionHookForTest(nullptr);
+
+    EXPECT_FALSE(admitted_in_interposition);
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+    EXPECT_NO_THROW((void)store->namespaceLife(ns));
+    EXPECT_EQ(store->refTableRuntimeAdmittedFenceGenerationForTest(ns), store->fenceGeneration());
+}
+
+}
+
+/// (a) `casPutObject` (reached via `Pool::putNamespaceFile`) with the fence tripped BETWEEN admission
+/// and the durable PUT: the typed transient refusal, and the object is never actually written.
+TEST(CASFenceGeneration, PlainObjectPutAbortsWhenFenceTripsBetweenAdmissionAndDurableCall)
+{
+    auto backend = std::make_shared<TripOnHeadBackend>();
+    auto store = openTestPool(backend);
+    ASSERT_TRUE(store->mayMutate());
+
+    const RootNamespace ns{"test/ns"};
+    backend->trigger = [&] { store->tripMountLost(); };
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        store->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "somefile", "hello");
+    });
+
+    /// No durable write ever landed -- assert via the Emulated backend listing.
+    EXPECT_TRUE(store->listNamespaceFiles(DB::Cas::tests::fixture::fixtureLife(ns)).empty());
+}
+
+/// `casRemoveObject`'s delete sibling, same shape: the fence trips between admission and the durable
+/// delete, so the victim object survives untouched.
+TEST(CASFenceGeneration, PlainObjectRemoveAbortsWhenFenceTripsBetweenAdmissionAndDurableCall)
+{
+    auto backend = std::make_shared<TripOnHeadBackend>();
+    auto store = openTestPool(backend);
+    const RootNamespace ns{"test/ns"};
+
+    /// Seed the victim BEFORE arming the trigger -- the seeding write itself must not trip the fence.
+    store->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "victim", "still here");
+    ASSERT_TRUE(store->mayMutate());
+
+    backend->trigger = [&] { store->tripMountLost(); };
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        store->removeNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "victim");
+    });
+
+    /// The durable delete never ran -- the object survives (reads are not fence-gated by this task).
+    const auto still_there = store->getNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "victim");
+    ASSERT_TRUE(still_there.has_value());
+    EXPECT_EQ(*still_there, "still here");
+}
+
+/// The fence re-check must run before EVERY conditional-retry iteration, not just the first attempt
+/// (spec wording, verbatim): a synthetic `PreconditionFailed` forces a second loop iteration, and the
+/// fence trips on the SECOND `head()` call. If the check ran only once, at admission, this write would
+/// incorrectly succeed on the retry.
+TEST(CASFenceGeneration, PlainObjectPutRechecksFenceOnEveryRetryIterationNotJustFirst)
+{
+    auto backend = std::make_shared<TripOnSecondHeadBackend>();
+    auto store = openTestPool(backend);
+    ASSERT_TRUE(store->mayMutate());
+
+    const RootNamespace ns{"test/ns"};
+    backend->head_calls = 0;   /// reset past whatever `Pool::open`'s own probe/mount claim already did
+    backend->fail_first_put = true;
+    backend->trigger = [&] { store->tripMountLost(); };
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        store->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "somefile", "hello");
+    });
+
+    EXPECT_EQ(backend->head_calls, 2);
+    EXPECT_TRUE(store->listNamespaceFiles(DB::Cas::tests::fixture::fixtureLife(ns)).empty());
+}
+
+/// (b) The S3-native staging-buffer finalize: the fence trips AFTER the buffer is constructed
+/// (admission) but BEFORE `finalize()` reaches the durable `sink->finalize()` call -- same typed abort,
+/// and the sink is never actually finalized (`on_finalized` never fires either, so the transaction
+/// never learns of a promote-worthy hash/size for bytes that were never durable).
+TEST(CASFenceGeneration, S3StagingFinalizeAbortsWhenFenceTripsBeforeDurableCall)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openTestPool(backend);
+    ASSERT_TRUE(store->mayMutate());
+
+    const std::string staging_key = "staging/mount1/racer.tmp";
+    auto * sink_ptr = new RecordingSink(staging_key);
+    std::unique_ptr<DB::WriteBufferFromFileBase> sink(sink_ptr);
+
+    bool on_finalized_called = false;
+    const uint64_t admitted_generation = store->fenceGeneration();
+
+    auto buf = std::make_unique<CaContentWriteBuffer>(
+        std::move(sink),
+        staging_key,
+        /*envelope_header=*/std::string(),
+        BlobHashAlgo::CityHash128,
+        /*buf_size=*/8192,
+        /*use_adaptive_buffer_size=*/false,
+        /*adaptive_buffer_initial_size=*/0,
+        [&](const std::string &, size_t, const std::string &) { on_finalized_called = true; },
+        [store, admitted_generation] { store->checkFenceOrThrow(admitted_generation); });
+
+    const std::string payload = "some bytes that must never become durable";
+    buf->write(payload.data(), payload.size());
+
+    /// The race this test targets: admission already captured `admitted_generation` above, and now the
+    /// fence trips before `finalize()` runs.
+    store->tripMountLost();
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { buf->finalize(); });
+
+    EXPECT_FALSE(on_finalized_called);
+    EXPECT_FALSE(sink_ptr->wasFinalizedForTest());
+}
+
+/// (d) Happy path unchanged: an ordinary plain-object write/read/remove, and an ordinary S3-staging
+/// finalize, both succeed exactly as before when the fence stays live throughout.
+TEST(CASFenceGeneration, HappyPathPlainObjectWriteReadRemoveUnaffected)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openTestPool(backend);
+    const RootNamespace ns{"test/ns"};
+
+    store->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "a", "hello");
+    const auto got = store->getNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "a");
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(*got, "hello");
+
+    store->removeNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "a");
+    EXPECT_FALSE(store->getNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "a").has_value());
+}
+
+TEST(CASFenceGeneration, HappyPathS3StagingFinalizeUnaffected)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openTestPool(backend);
+
+    const std::string staging_key = "staging/mount1/happy.tmp";
+    auto * sink_ptr = new RecordingSink(staging_key);
+    std::unique_ptr<DB::WriteBufferFromFileBase> sink(sink_ptr);
+
+    bool on_finalized_called = false;
+    const uint64_t admitted_generation = store->fenceGeneration();
+
+    auto buf = std::make_unique<CaContentWriteBuffer>(
+        std::move(sink),
+        staging_key,
+        /*envelope_header=*/std::string(),
+        BlobHashAlgo::CityHash128,
+        /*buf_size=*/8192,
+        /*use_adaptive_buffer_size=*/false,
+        /*adaptive_buffer_initial_size=*/0,
+        [&](const std::string &, size_t, const std::string &) { on_finalized_called = true; },
+        [store, admitted_generation] { store->checkFenceOrThrow(admitted_generation); });
+
+    const std::string payload = "unaffected happy path bytes";
+    buf->write(payload.data(), payload.size());
+    buf->finalize();
+
+    EXPECT_TRUE(on_finalized_called);
+    EXPECT_TRUE(sink_ptr->wasFinalizedForTest());
+}
+
+/// (I2, backlog {#c2-resurrect-putoverwrite-fence-check}) The condemned-displacement `putOverwrite` branch
+/// of `PartWriteTxn::uploadFromSource` -- a RAW backend write outside the request controller's fence gate,
+/// the last durable write left uncovered by Task 4. Seed a PRESENT, CONDEMNED blob, then trip the fence on
+/// the head() that precedes the displacement decision: the [C2] `checkFenceOrThrow` must abort with the
+/// typed transient refusal BEFORE `putOverwrite`, leaving the condemned incarnation untouched (no
+/// stale-incarnation displacement).
+TEST(CASFenceGeneration, CondemnedPutOverwriteAbortsWhenFenceTripsBeforeDurableCall)
+{
+    auto backend = std::make_shared<TripOnHeadDisplacementBackend>();
+    auto store = openTestPool(backend);
+    ASSERT_TRUE(store->mayMutate());
+
+    const std::string payload = "condemned-overwrite-victim";
+    const DB::UInt128 hash = DB::Cas::tests::u128Of(payload);
+    const BlobRef id = DB::Cas::tests::idOf(payload);
+    const std::string blob_key = store->layout().blobKey(id);
+
+    /// A PRESENT, CONDEMNED incarnation (raw body + Clean meta flipped to Condemned) -- the only state that
+    /// routes `uploadFromSource` into the streaming `putOverwrite` displacement branch.
+    const uint64_t header_len = store->poolMeta().blob_header_len;
+    std::string raw_body(header_len, '\0');
+    raw_body += payload;
+    DB::Cas::tests::writeRawBlobBody(*backend, store->layout(), hash, raw_body);
+    DB::Cas::tests::writeMetaClean(*backend, store->layout(), hash, payload.size());
+    DB::Cas::tests::condemnMeta(*backend, store->layout(), hash, /*condemn_round=*/1);
+    const DB::Cas::Token condemned_token = backend->head(blob_key).token;
+    ASSERT_FALSE(condemned_token.empty());
+
+    auto build = store->beginPartWrite({});
+    /// Reset counters past the setup writes (`condemnMeta` itself putOverwrites the meta key) and arm AFTER
+    /// beginPartWrite (whose W-HEARTBEAT would otherwise consume the one-shot trigger). The first head()
+    /// after this is the `head(key)` at uploadFromSource's condemned-displacement decision.
+    backend->put_overwrite_calls = 0;
+    backend->resurrect_staged_calls = 0;
+    backend->trigger = [&] { store->tripMountLost(); };
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        build->putBlob(id, BlobSource::fromString(payload));
+    });
+
+    /// No displacement write landed: no putOverwrite at all, and the condemned incarnation survives with its
+    /// original token.
+    EXPECT_EQ(backend->put_overwrite_calls, 0) << "the fence check must abort before the raw putOverwrite";
+    const DB::Cas::HeadResult after = backend->head(blob_key);
+    ASSERT_TRUE(after.exists);
+    EXPECT_EQ(after.token, condemned_token) << "the condemned blob must be untouched (INV: never a stale displacement)";
+}
+
+/// (I2, backlog {#c2-resurrect-putoverwrite-fence-check}) The S3-native-staging sibling: the condemned
+/// displacement `resurrect` branch (`BlobSource::server_side_copy_from` set) is the same RAW,
+/// controller-uncoupled backend write. Same setup + fence trip -> the [C2] `checkFenceOrThrow` aborts with
+/// the typed transient refusal BEFORE `resurrect`, so no unconditional re-upload displaces
+/// the condemned incarnation.
+TEST(CASFenceGeneration, CondemnedResurrectStagedAbortsWhenFenceTripsBeforeDurableCall)
+{
+    auto backend = std::make_shared<TripOnHeadDisplacementBackend>();
+    auto store = openTestPool(backend);
+    ASSERT_TRUE(store->mayMutate());
+
+    const std::string payload(300, 'c');
+    const DB::UInt128 hash = DB::Cas::tests::u128Of(payload);
+    const BlobRef id = DB::Cas::tests::idOf(payload);
+    const std::string blob_key = store->layout().blobKey(id);
+    const std::string staging_key = "p/staging/mount1/ccc.tmp";
+
+    /// The staging object holds `[header][payload]`; the condemned blob is a plausible prior incarnation.
+    DB::Cas::EnvelopeHeader staging_h;
+    staging_h.kind = DB::Cas::ObjectKind::Blob;
+    staging_h.incarnation_tag = DB::UInt128(0xC0FFEE);
+    const std::string staging_header = DB::Cas::encodeEnvelopeHeader(
+        staging_h, static_cast<uint32_t>(store->poolMeta().blob_header_len));
+    const std::string staging_bytes = staging_header + payload;
+    backend->putIfAbsent(staging_key, staging_bytes);
+    backend->putIfAbsent(blob_key, staging_bytes);
+    DB::Cas::tests::writeMetaClean(*backend, store->layout(), hash, payload.size());
+    DB::Cas::tests::condemnMeta(*backend, store->layout(), hash, /*condemn_round=*/5);
+    const DB::Cas::Token condemned_token = backend->head(blob_key).token;
+    ASSERT_FALSE(condemned_token.empty());
+
+    auto build = store->beginPartWrite({});
+    backend->put_overwrite_calls = 0;
+    backend->resurrect_staged_calls = 0;
+    backend->trigger = [&] { store->tripMountLost(); };
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        build->putBlob(id, serverSideCopySource(staging_key, payload.size()));
+    });
+
+    /// No unconditional server-side copy displaced the condemned incarnation.
+    EXPECT_EQ(backend->resurrect_staged_calls, 0) << "the fence check must abort before the raw resurrect";
+    const DB::Cas::HeadResult after = backend->head(blob_key);
+    ASSERT_TRUE(after.exists);
+    EXPECT_EQ(after.token, condemned_token) << "the condemned blob must be untouched (INV: never a stale displacement)";
+}
diff --git a/src/Disks/tests/gtest_cas_fold_seal_codec.cpp b/src/Disks/tests/gtest_cas_fold_seal_codec.cpp
new file mode 100644
index 000000000000..feee48a70069
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_fold_seal_codec.cpp
@@ -0,0 +1,41 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+
+using namespace DB::Cas;
+
+/// The GC-reclaim tests that used to live here (`AbandonedPrecommitOrphansManifestUntilFix`,
+/// `ReclaimIsIdempotentAndSelfTerminating`, `SkipPreservedForLivePrecommitAndForNoPrecommit`,
+/// `DoubleRemovalOfReclaimedPrecommitIsIdempotent`) were removed with the snapshot+log ref model.
+/// They asserted that GC reclaims an abandoned precommit once the mount watermark proves it dead, and
+/// that the token-diff Skip optimization self-terminates. Per spec §Responsibility Boundary, reclaiming
+/// an abandoned precommit is now the WRITER's job (it appends the exact `owner_transition` removal), and
+/// the token-diff Skip machinery (`computeDiscoverDecisions`/`discoverDecisionsForTest`) no longer exists
+/// -- the "did it change" signal is simply logs above the durable cursor. There is no GC-side reclaim to
+/// assert, so these tests are obsolete rather than adaptable.
+///
+/// The live-precommit watermark fields (`has_live_precommit`/`min_live_precommit_*`) that fed that
+/// removed reclaim were deleted from `RefCoverage` with it (T13). The still-meaningful fold-seal
+/// assertion is the round-trip of `last_folded_ref_id` -- the per-table durable ref cursor that replaced
+/// them in the same struct under the snapshot+log ref model.
+TEST(CASFoldSealCodec, RefLifeCoverageRoundTripsLastFoldedRefId)
+{
+    CasFoldSeal seal;
+    seal.generation = 3;
+    seal.parent_generation = 2;
+    RefCoverage cov;
+    cov.classification = 1;
+    cov.last_folded_ref_id = RefTxnId{4, 11};
+    constexpr UInt128 life_id{1};
+    seal.ref_lives[life_id].coverage = cov;
+
+    const CasFoldSeal back = decodeFoldSeal(encodeFoldSeal(seal));
+    const RefCoverage & r = back.ref_lives.at(life_id).coverage;
+    EXPECT_EQ(r.last_folded_ref_id, (RefTxnId{4, 11}));
+
+    /// Default (nothing folded) round-trips as {0,0}.
+    CasFoldSeal empty_seal;
+    constexpr UInt128 empty_life_id{2};
+    empty_seal.ref_lives[empty_life_id].coverage = RefCoverage{};
+    const CasFoldSeal e_back = decodeFoldSeal(encodeFoldSeal(empty_seal));
+    EXPECT_EQ(e_back.ref_lives.at(empty_life_id).coverage.last_folded_ref_id, (RefTxnId{}));
+}
diff --git a/src/Disks/tests/gtest_cas_fold_seal_format.cpp b/src/Disks/tests/gtest_cas_fold_seal_format.cpp
new file mode 100644
index 000000000000..afe6331a4ed8
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_fold_seal_format.cpp
@@ -0,0 +1,327 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <limits>
+#include <algorithm>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes { extern const int CORRUPTED_DATA; extern const int LOGICAL_ERROR; }
+
+namespace
+{
+CasFoldSeal sampleFoldSeal()
+{
+    CasFoldSeal seal;
+    seal.generation = 7;
+    seal.parent_generation = 6;
+    seal.ref_lives[UInt128{1}].coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{3, 4}};
+    seal.ref_lives[UInt128{2}].coverage = RefCoverage{.classification = 1};
+    seal.blob_target_runs.push_back(RunRef{.key = "gc/gen/7/blob_target/0/0", .checksum = UInt128(0xABCDEF)});
+    return seal;
+}
+
+void eraseRequiredField(String & encoded, std::string_view field)
+{
+    const size_t pos = encoded.find(field);
+    ASSERT_NE(pos, String::npos);
+    encoded.erase(pos, field.size());
+}
+}
+
+TEST(CASFormatBattery, FoldSeal)
+{
+    CasFoldSeal seal;
+    seal.generation = 5;
+    seal.parent_generation = 4;
+    seal.ref_lives[UInt128{1}].coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{7, 11}};
+    seal.blob_target_runs.push_back(RunRef{.key = "r0", .checksum = UInt128(0x0f), .shard = 0, .generation = 5});
+    seal.condemned_summary[0] = CondemnedSummary{.condemned_total = 3, .pending_total = 1,
+                                                 .oldest_nonpending_condemn_round = 4};
+    runFormatBattery({FormatId::FoldSeal,
+        [&] { return sealObject(FormatId::FoldSeal, encodeFoldSeal(seal)); },
+        [](std::string_view s) { decodeFoldSeal(std::string(openObject(FormatId::FoldSeal, s))); },
+        currentFormatHeader("cas_fold_seal") +
+        "{\"g\":\"5\",\"pg\":\"4\"}\n"
+        "{\"k\":\"rfl\",\"life\":\"00000000000000000000000000000001\",\"cls\":2,\"lfe\":\"7\",\"lfs\":\"11\"}\n"
+        "{\"k\":\"btr\",\"key\":\"r0\",\"ck\":\"0000000000000000000000000000000f\",\"shard\":0,\"gen\":\"5\"}\n"
+        "{\"k\":\"cnd\",\"shard\":0,\"ct\":3,\"pt\":1,\"ocr\":\"4\"}\n"
+        "{\"n\":3}\n"});
+}
+
+TEST(CASFoldSealFormat, RoundTripsAllFields)
+{
+    const CasFoldSeal in = sampleFoldSeal();
+    const CasFoldSeal out = decodeFoldSeal(encodeFoldSeal(in));
+
+    EXPECT_EQ(out.generation, in.generation);
+    EXPECT_EQ(out.parent_generation, in.parent_generation);
+    ASSERT_EQ(out.ref_lives.size(), in.ref_lives.size());
+    EXPECT_EQ(out.ref_lives.at(UInt128{1}).coverage.classification, 2);
+    EXPECT_EQ(out.ref_lives.at(UInt128{1}).coverage.last_folded_ref_id, (RefTxnId{3, 4}));
+    ASSERT_EQ(out.blob_target_runs.size(), 1u);
+    EXPECT_EQ(out.blob_target_runs[0].key, "gc/gen/7/blob_target/0/0");
+    EXPECT_EQ(out.blob_target_runs[0].checksum, UInt128(0xABCDEF));
+    EXPECT_EQ(out, in);
+}
+
+TEST(CASFoldSealFormat, AuthoritativeDecodeRejectsTwoBlobTargetRunsForOneShard)
+{
+    const Layout layout("p");
+    CasFoldSeal seal;
+    seal.generation = 7;
+    seal.parent_generation = 6;
+    seal.blob_target_runs = {
+        RunRef{.key = layout.blobTargetRunKey(7, 1, 0, 0), .checksum = UInt128{1}, .shard = 0, .generation = 7},
+        RunRef{.key = layout.blobTargetRunKey(7, 2, 0, 0), .checksum = UInt128{2}, .shard = 0, .generation = 7},
+    };
+    seal.condemned_summary[0] = CondemnedSummary{};
+
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(encodeFoldSeal(seal), layout, /*gc_shards=*/1); },
+        "duplicate blob-target shard");
+}
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASFoldSealFormatDeathTest, ProducerValidationRejectsMalformedSealBeforePut)
+{
+    const Layout layout("p");
+    CasFoldSeal seal;
+    seal.blob_target_runs = {
+        RunRef{.key = layout.blobTargetRunKey(7, 1, 0, 0), .checksum = UInt128{1}, .shard = 0, .generation = 7},
+        RunRef{.key = layout.blobTargetRunKey(7, 2, 0, 0), .checksum = UInt128{2}, .shard = 0, .generation = 7}};
+    seal.condemned_summary[0] = CondemnedSummary{};
+    EXPECT_DEATH({ validateFoldSealForWrite(seal, layout, 1); }, "duplicate blob-target shard");
+}
+#else
+TEST(CASFoldSealFormat, ProducerValidationRejectsMalformedSealBeforePut)
+{
+    const Layout layout("p");
+    CasFoldSeal seal;
+    seal.blob_target_runs = {
+        RunRef{.key = layout.blobTargetRunKey(7, 1, 0, 0), .checksum = UInt128{1}, .shard = 0, .generation = 7},
+        RunRef{.key = layout.blobTargetRunKey(7, 2, 0, 0), .checksum = UInt128{2}, .shard = 0, .generation = 7}};
+    seal.condemned_summary[0] = CondemnedSummary{};
+    cas_battery_detail::expectCode(DB::ErrorCodes::LOGICAL_ERROR,
+        [&] { validateFoldSealForWrite(seal, layout, 1); }, "duplicate blob-target shard");
+}
+#endif
+
+TEST(CASFoldSealFormat, AuthoritativeDecodeRequiresEveryBlobTargetAndSummaryField)
+{
+    const Layout layout("p");
+    CasFoldSeal seal;
+    seal.generation = 7;
+    seal.parent_generation = 6;
+    seal.blob_target_runs.push_back(RunRef{
+        .key = layout.blobTargetRunKey(7, 1, 0, 0),
+        .checksum = UInt128{1},
+        .shard = 0,
+        .generation = 7});
+    seal.condemned_summary[0] = CondemnedSummary{};
+    const String valid = encodeFoldSeal(seal);
+
+    for (const std::string_view field : {
+        R"(,"key":"p/gc/gen/7/attempt/1/blob_target/0/0")",
+        R"(,"ck":"00000000000000000000000000000001")",
+        R"(,"gen":"7")",
+        ",\"ct\":0",
+        ",\"pt\":0",
+        R"(,"ocr":"18446744073709551615")"})
+    {
+        String malformed = valid;
+        eraseRequiredField(malformed, field);
+        cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+            [&] { decodeFoldSeal(malformed, layout, 1); }, "missing");
+    }
+
+    /// `shard` occurs once on each row; remove each occurrence independently.
+    String missing_btr_shard = valid;
+    eraseRequiredField(missing_btr_shard, ",\"shard\":0");
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(missing_btr_shard, layout, 1); }, "missing");
+
+    String missing_cnd_shard = valid;
+    const size_t first_shard = missing_cnd_shard.find(",\"shard\":0");
+    ASSERT_NE(first_shard, String::npos);
+    const size_t second_shard = missing_cnd_shard.find(",\"shard\":0", first_shard + 1);
+    ASSERT_NE(second_shard, String::npos);
+    missing_cnd_shard.erase(second_shard, std::string_view(",\"shard\":0").size());
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(missing_cnd_shard, layout, 1); }, "missing");
+}
+
+TEST(CASFoldSealFormat, AuthoritativeDecodeRejectsNoncanonicalRowsAndIncompleteSummaryDomain)
+{
+    const Layout layout("p");
+    CasFoldSeal seal;
+    seal.generation = 7;
+    seal.parent_generation = 6;
+    seal.blob_target_runs.push_back(RunRef{
+        .key = layout.blobTargetRunKey(7, 1, 1, 0),
+        .checksum = UInt128{1},
+        .shard = 1,
+        .generation = 7});
+    seal.condemned_summary[0] = CondemnedSummary{};
+
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(encodeFoldSeal(seal), layout, 1); }, "outside");
+
+    seal.blob_target_runs[0].shard = 0;
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(encodeFoldSeal(seal), layout, 1); }, "not canonical");
+
+    seal.blob_target_runs.clear();
+    seal.condemned_summary.clear();
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(encodeFoldSeal(seal), layout, 1); }, "exactly 1");
+
+    seal.condemned_summary[0] = CondemnedSummary{};
+    seal.condemned_summary[1] = CondemnedSummary{};
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(encodeFoldSeal(seal), layout, 1); }, "exactly 1");
+}
+
+TEST(CASFoldSealFormat, AuthoritativeDecodeRejectsContradictorySummaryCounts)
+{
+    const Layout layout("p");
+    CasFoldSeal seal;
+    seal.condemned_summary[0] = CondemnedSummary{
+        .condemned_total = 1,
+        .pending_total = 2,
+        .oldest_nonpending_condemn_round = 3};
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(encodeFoldSeal(seal), layout, 1); }, "greater than");
+
+    seal.condemned_summary[0] = CondemnedSummary{
+        .condemned_total = 2,
+        .pending_total = 1,
+        .oldest_nonpending_condemn_round = std::numeric_limits<uint64_t>::max()};
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(encodeFoldSeal(seal), layout, 1); }, "real oldest");
+}
+
+TEST(CASFoldSealFormat, RejectsUnexpectedGeneration)
+{
+    CasFoldSeal seal;
+    seal.generation = 5;
+    const String encoded = encodeFoldSeal(seal);
+
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeFoldSeal(encoded, /*expected_generation=*/6); }, "unexpected generation");
+    EXPECT_EQ(decodeFoldSeal(encoded, /*expected_generation=*/5).generation, 5);
+    EXPECT_EQ(decodeFoldSeal(encoded).generation, 5);
+}
+
+TEST(CASFoldSeal, EncodingIsByteDeterministic)
+{
+    const CasFoldSeal in = sampleFoldSeal();
+    EXPECT_EQ(encodeFoldSeal(in), encodeFoldSeal(in));
+}
+
+TEST(CASFoldSealFormat, TextIsByteDeterministic)
+{
+    CasFoldSeal a;
+    a.generation = 5;
+    a.parent_generation = 4;
+    a.blob_target_runs = {RunRef{"z", UInt128(2), 1, 5}, RunRef{"a", UInt128(1), 0, 5}};
+    CasFoldSeal b = a;
+    std::reverse(b.blob_target_runs.begin(), b.blob_target_runs.end());   /// same set, different order
+    EXPECT_EQ(encodeFoldSeal(a), encodeFoldSeal(b));   /// encoder must sort runs by key
+}
+
+TEST(CASFoldSeal, RejectsEmptyAndBadMagic)
+{
+    EXPECT_ANY_THROW(decodeFoldSeal(""));
+    EXPECT_ANY_THROW(decodeFoldSeal("not-a-seal"));
+}
+
+TEST(CASFoldSeal, CoverageRecordsEveryCatalogLife)
+{
+    CasFoldSeal in = sampleFoldSeal();
+    in.ref_lives[UInt128{3}].coverage = RefCoverage{.classification = 0};
+    const CasFoldSeal out = decodeFoldSeal(encodeFoldSeal(in));
+    EXPECT_TRUE(out.ref_lives.contains(UInt128{3}));
+    EXPECT_EQ(out.ref_lives.size(), 3u);
+}
+
+TEST(CASFoldSeal, FoldSealCondemnedSummaryRoundTrips)
+{
+    /// A seal carrying a non-empty condemned_summary over 2 shards (one a zero entry) round-trips and
+    /// compares equal, and the UINT64_MAX "none" sentinel survives.
+    CasFoldSeal s;
+    s.generation = 9;
+    s.parent_generation = 8;
+    s.ref_lives[UInt128{1}].coverage = RefCoverage{.classification = 2};
+    s.blob_target_runs.push_back(RunRef{.key = "gc/gen/9/blob_target/0/0", .checksum = UInt128(0x77),
+                                        .shard = 0, .generation = 9});
+    s.condemned_summary[0] = CondemnedSummary{.condemned_total = 3, .pending_total = 1,
+                                              .oldest_nonpending_condemn_round = 5};
+    s.condemned_summary[1] = CondemnedSummary{};   /// explicit zero entry (totality over gc_shards)
+
+    const CasFoldSeal out = decodeFoldSeal(encodeFoldSeal(s));
+    EXPECT_EQ(out, s);
+    ASSERT_EQ(out.condemned_summary.size(), 2u);
+    EXPECT_EQ(out.condemned_summary.at(0).condemned_total, 3u);
+    EXPECT_EQ(out.condemned_summary.at(0).pending_total, 1u);
+    EXPECT_EQ(out.condemned_summary.at(0).oldest_nonpending_condemn_round, 5u);
+    EXPECT_EQ(out.condemned_summary.at(1).oldest_nonpending_condemn_round,
+              std::numeric_limits<uint64_t>::max());   /// UINT64_MAX sentinel survives
+
+    EXPECT_TRUE(decodeFoldSeal(encodeFoldSeal(CasFoldSeal{})).condemned_summary.empty());
+}
+
+/// Mutation caught: restoring separate `cov` and `nsc` rows, dropping the cleanup evidence, or
+/// serializing the row under a logical namespace changes these literal generation-8 bytes.
+TEST(CASFoldSealFormat, UnifiedRefLifeRowRoundTripsCoverageHoldAndCleanupEvidence)
+{
+    CasFoldSeal seal;
+    seal.generation = 8;
+    seal.parent_generation = 7;
+    const UInt128 life_id{0x1234};
+    seal.ref_lives.emplace(life_id, RefLifeFoldState{
+        .coverage = RefCoverage{
+            .classification = 4,
+            .last_folded_ref_id = RefTxnId{3, 4},
+            .hold = RefHold{
+                .reason = HoldReason::ManifestBodyMissing,
+                .offending_position = RefTxnId{5, 6},
+                .retry_count = 7,
+                .next_retry_round = 8}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{9, 10}}});
+
+    const String expected = currentFormatHeader("cas_fold_seal") +
+        "{\"g\":\"8\",\"pg\":\"7\"}\n"
+        "{\"k\":\"rfl\",\"life\":\"00000000000000000000000000001234\",\"cls\":4,"
+        "\"lfe\":\"3\",\"lfs\":\"4\",\"hr\":\"manifest_body_missing\",\"hpe\":\"5\","
+        "\"hps\":\"6\",\"hrc\":7,\"hnr\":\"8\",\"rte\":\"9\",\"rts\":\"10\"}\n"
+        "{\"n\":1}\n";
+
+    EXPECT_EQ(encodeFoldSeal(seal), expected);
+    EXPECT_EQ(decodeFoldSeal(expected), seal);
+}
+
+/// Mutation caught: accepting the generation-6 split coverage collection would leave a second
+/// namespace-keyed source of lifecycle work in a generation-7 process.
+TEST(CASFoldSealFormat, UnifiedCodecRejectsLegacyCoverageRecord)
+{
+    const String old =
+        "{\"type\":\"cas_fold_seal\",\"v\":7}\n"
+        "{\"g\":\"8\",\"pg\":\"7\"}\n"
+        "{\"k\":\"cov\",\"key\":\"name/0\",\"cls\":2,\"lfe\":\"3\",\"lfs\":\"4\"}\n"
+        "{\"n\":1}\n";
+    cas_battery_detail::expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeFoldSeal(old); }, "legacy coverage");
+}
+
+/// Mutation caught: accepting the generation-6 cleanup-item state would restore the independent
+/// marker-driven `Pending`/`Completed` handshake.
+TEST(CASFoldSealFormat, UnifiedCodecRejectsLegacyNamespaceCleanupRecord)
+{
+    const String old =
+        "{\"type\":\"cas_fold_seal\",\"v\":7}\n"
+        "{\"g\":\"8\",\"pg\":\"7\"}\n"
+        "{\"k\":\"nsc\",\"ns\":\"name\",\"rte\":\"3\",\"rts\":\"4\",\"st\":\"completed\"}\n"
+        "{\"n\":1}\n";
+    cas_battery_detail::expectCode(
+        DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeFoldSeal(old); }, "legacy namespace cleanup");
+}
diff --git a/src/Disks/tests/gtest_cas_forget.cpp b/src/Disks/tests/gtest_cas_forget.cpp
new file mode 100644
index 000000000000..f92b2f67ac4f
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_forget.cpp
@@ -0,0 +1,600 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <filesystem>
+#include <functional>
+#include <future>
+#include <limits>
+#include <memory>
+#include <mutex>
+#include <stdexcept>
+#include <string>
+#include <thread>
+
+/// Task 10 (rev.7 spec §5): `SYSTEM CAS FORGET` — the operator force-Vanish. FORGET drives a
+/// content-addressed pool to `Vanished(forgotten)` with the fence-first protocol: (1) publish terminal
+/// intent, (2) trip the local fence, (3+4) stop the GC scheduler, (5) join keeper/remount, drain, retire
+/// the keeper WITHOUT an unearned clean farewell, (6) publish `Vanished(forgotten)` with the [D5] message
+/// carrying the decommission timestamp. These tests exercise the Pool-level protocol body (`Pool::forgetDisk`)
+/// and the end-to-end verb through a real `ContentAddressedMetadataStorage` (the six-class gate wired to the
+/// new state). Harness patterns follow gtest_cas_lifecycle_condition.cpp and gtest_cas_operation_gate.cpp.
+
+namespace DB::ErrorCodes
+{
+extern const int INVALID_STATE;
+}
+
+using namespace DB;
+using DB::Cas::PoolLifecycle;
+using DB::Cas::tests::CountingBackend;
+
+namespace
+{
+
+const String kSrid = "test";
+
+/// A test-authored [D5] reason with a RECOGNIZABLE timestamp — the Pool-level tests assert this exact
+/// string flows through `enterVanished` into the `throwIfLifecycleTerminal` message (the timestamp
+/// threading the metadata storage does in production). It keeps the two [D5] substrings the gate relies on.
+const String kForgetReason =
+    "decommissioned by SYSTEM CAS FORGET at 2099-01-02 03:04:05 UTC — erasure was NOT "
+    "verified; if this was a mistake the data may be intact (restart re-registers the name)";
+
+/// Delete an existing key exactly (its current token comes from the same GET). Mirrors
+/// gtest_cas_lifecycle_condition.cpp — used to drive a live pool into `IdentityLost`.
+void deleteKeyExact(DB::Cas::Backend & backend, const String & key)
+{
+    const auto got = backend.get(key);
+    ASSERT_TRUE(got.has_value()) << "expected '" << key << "' to exist before deletion";
+    if (got)
+        backend.deleteExact(key, got->token);
+}
+
+/// GC's fence-out applied directly to the mount lease (preserve the body, set `gc_fenced`, bump `seq`) —
+/// a subsequent `tryRemountOnce` verdicts `Recover` and reclaims a FRESH incarnation immediately (no
+/// lease-expiry wait), reaching `armMountFence`. Mirrors gtest_cas_lifecycle_condition.cpp's helper.
+void fenceOutMount(DB::Cas::Backend & backend, const String & mount_key)
+{
+    const auto got = backend.get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    DB::Cas::MountLease m = DB::Cas::decodeMountLease(got->bytes);
+    m.gc_fenced = true;
+    m.seq += 1;
+    ASSERT_EQ(backend.putOverwrite(mount_key, DB::Cas::encodeMountLease(m), got->token).outcome,
+              DB::Cas::PutOutcome::Done);
+}
+
+/// A Backend decorator whose head/get/list throw an untyped transport error while `fail` is armed — so a
+/// self-remount attempt verdicts `StayTransient` (fast, no lease-expiry wait) and the remount loop keeps
+/// spinning. Starts DISARMED so `Pool::open` succeeds. Mirrors gtest_cas_lifecycle_condition.cpp's decorator.
+class ToggleableTransportFaultBackend final : public DB::Cas::InMemoryBackend
+{
+public:
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    DB::Cas::HeadResult head(const String & key) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::head(key);
+    }
+    std::optional<DB::Cas::GetResult> get(const String & key, DB::Cas::Range range) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::get(key, range);
+    }
+    DB::Cas::ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+
+    std::atomic<bool> fail{false};
+};
+
+/// The message thrown by `fn`, or a failure if it did not throw a `DB::Exception`.
+std::string messageOf(const std::function<void()> & fn)
+{
+    try
+    {
+        fn();
+    }
+    catch (const Exception & e)
+    {
+        return std::string(e.message());
+    }
+    ADD_FAILURE() << "expected a DB::Exception";
+    return {};
+}
+
+/// A live table dir + committed part reused by the end-to-end gate test (the shape
+/// gtest_cas_operation_gate.cpp uses).
+const std::string kTableDir = "gg0/gg0gg0g0-0808-4808-8808-080808080808";
+const std::string kPartDir = kTableDir + "/all_1_1_0";
+const std::string kPartFile = kPartDir + "/data.bin";
+
+std::shared_ptr<ContentAddressedMetadataStorage> openForgetStorage()
+{
+    auto settings = Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_forget_scratch");
+    auto storage = std::make_shared<ContentAddressedMetadataStorage>(
+        Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+void commitOnePart(ContentAddressedMetadataStorage & storage)
+{
+    auto tx = storage.createTransaction();
+    auto & ca_tx = dynamic_cast<ContentAddressedTransaction &>(*tx);
+    auto buf = ca_tx.writeFile(kTableDir + "/tmp_insert_all_1_1_0/data.bin", 65536, WriteMode::Rewrite, {});
+    const std::string bytes = "content-of-the-part";
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+    tx->moveDirectory(kTableDir + "/tmp_insert_all_1_1_0", kPartDir);
+    tx->commit(NoCommitOptions{});
+}
+
+/// Deterministically interleave a real FORGET into the admission->lock window of a manual GC verb (the
+/// I-1/I-2 admission TOCTOU), with BOUNDED condition-variable waits and never a sleep. The sequence pinned:
+///   M (this thread, running `gc_verb`): passes the verb's pre-lock admission gate while `Live`, then the
+///     installed seam signals `admitted` and blocks until `forget_done`, then M resumes to acquire
+///     `gc_scheduler_mutex` and hit the under-lock re-check.
+///   F (the FORGET thread): waits for `admitted`, runs the REAL `forgetDisk` (acquiring lifecycle +
+///     gc_scheduler mutexes while M holds NEITHER -- M is parked in the seam BEFORE the lock), settling the
+///     pool `Vanished(forgotten)`, then signals `forget_done`.
+/// Returns the exception message `gc_verb` threw (via `messageOf`), so the caller asserts the typed [D5]
+/// refusal. The 30s bounds trip ONLY on a genuine deadlock regression, never in the happy path.
+std::string raceForgetIntoGcVerbWindow(ContentAddressedMetadataStorage & storage,
+                                       const std::function<void()> & gc_verb)
+{
+    std::mutex m;
+    std::condition_variable cv;
+    bool admitted = false;
+    bool forget_done = false;
+
+    storage.setGcVerbAdmitWindowHookForTest([&]
+    {
+        {
+            std::lock_guard lk(m);
+            admitted = true;
+        }
+        cv.notify_all();
+        std::unique_lock lk(m);
+        EXPECT_TRUE(cv.wait_for(lk, std::chrono::seconds(30), [&] { return forget_done; }))
+            << "the concurrent FORGET must complete within the bound (else the interleave deadlocked)";
+    });
+
+    std::thread forgetter([&]
+    {
+        {
+            std::unique_lock lk(m);
+            EXPECT_TRUE(cv.wait_for(lk, std::chrono::seconds(30), [&] { return admitted; }))
+                << "the GC verb must reach the admission->lock window before FORGET runs";
+        }
+        storage.forgetDisk();
+        {
+            std::lock_guard lk(m);
+            forget_done = true;
+        }
+        cv.notify_all();
+    });
+
+    const std::string msg = messageOf(gc_verb);
+    forgetter.join();
+    storage.setGcVerbAdmitWindowHookForTest({});   /// clear the seam (references this frame's locals)
+    return msg;
+}
+
+}
+
+/// (a) FORGET on a LIVE pool: the local fence is tripped, the injected GC-stop step runs, the pool settles
+/// `Vanished(forgotten)`, and store-class access fails loud with the timestamped [D5] message.
+TEST(CASForget, ForgetOnLivePoolTripsFenceAndVanishes)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::Live);
+    ASSERT_TRUE(store->mayMutate());
+
+    bool gc_stopped = false;
+    store->forgetDisk([&] { gc_stopped = true; }, kForgetReason);
+
+    /// Step 3/4 ran (the GC-stop callback was invoked from inside the protocol).
+    EXPECT_TRUE(gc_stopped);
+    /// Terminal truth, fence tripped.
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+    EXPECT_TRUE(store->isVanished());
+    EXPECT_FALSE(store->mayMutate());
+
+    /// The [D5] message carries the operator's FORGET timestamp (threaded through the reason) and still
+    /// names the sub-state ("erasure was NOT verified").
+    const std::string msg = messageOf([&] { store->throwIfLifecycleTerminal(); });
+    EXPECT_NE(msg.find("SYSTEM CAS FORGET at "), std::string::npos) << msg;
+    EXPECT_NE(msg.find("2099-01-02 03:04:05 UTC"), std::string::npos) << msg;
+    EXPECT_NE(msg.find("erasure was NOT verified"), std::string::npos) << msg;
+}
+
+/// (a') FORGET stops AND joins a real `CasGcScheduler`'s worker + heartbeat threads (the injected GC-stop
+/// step). A long interval keeps any round from firing during the test window, so this isolates the
+/// thread-lifecycle: `start()` spawns the two workers, FORGET's callback `stop()`s + joins them, and the
+/// test completing (no hang) plus a clean `isQuiescent()` proves the join.
+TEST(CASForget, ForgetStopsAndJoinsRealGcScheduler)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    Cas::CasGcScheduler sched(store, std::chrono::seconds(3600), "CasForgetTest", "forget-disk");
+    sched.start();
+
+    bool gc_joined = false;
+    store->forgetDisk([&] { sched.stop(); gc_joined = true; }, kForgetReason);
+
+    EXPECT_TRUE(gc_joined);
+    /// What this proves is the JOIN: `stop()` returned, so the worker + heartbeat threads are joined and
+    /// the test could not have hung; `isQuiescent()` confirms no round is in flight. NOTE: the callback is
+    /// only `sched.stop()`, which does NOT itself clear the in-process `i_am_leader` hint — the
+    /// metadata-storage handler clears leadership by DESTROYING the scheduler (see
+    /// `ContentAddressedMetadataStorage::forgetDisk`), so asserting `is_leader == false` here would be
+    /// vacuous (this 3600s scheduler never led) or, after a real round, wrong.
+    EXPECT_TRUE(sched.isQuiescent()) << "no GC round may be in flight after FORGET joined the scheduler";
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+}
+
+/// (c) Double FORGET is idempotent: the second call is a no-op (the pool is already `Vanished(forgotten)`),
+/// so it never re-runs the protocol — the GC-stop callback is NOT invoked again, and the first reason wins.
+TEST(CASForget, DoubleForgetIsIdempotent)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    int gc_stops = 0;
+    store->forgetDisk([&] { ++gc_stops; }, kForgetReason);
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+    ASSERT_EQ(gc_stops, 1);
+
+    /// A second FORGET with a DIFFERENT reason must change nothing (first terminal transition wins) and
+    /// must NOT re-enter the teardown (idempotent short-circuit on `isVanished()`).
+    store->forgetDisk([&] { ++gc_stops; }, "a different reason that must be ignored");
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+    EXPECT_EQ(gc_stops, 1) << "the idempotent second FORGET must not re-run the protocol";
+
+    const std::string msg = messageOf([&] { store->throwIfLifecycleTerminal(); });
+    EXPECT_NE(msg.find("2099-01-02 03:04:05 UTC"), std::string::npos)
+        << "the first FORGET's reason must win: " << msg;
+}
+
+/// (d) FORGET on an `IdentityLost` pool → `Vanished(forgotten)` — the escape hatch. `IdentityLost` is
+/// non-absorbing and has no benign answer, so FORGET is the operator's way out.
+TEST(CASForget, ForgetOnIdentityLostPoolVanishesForgotten)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    /// Delete both pool sentinels while other objects remain, then drive the identity gate: the pool enters
+    /// `IdentityLost` (never `Vanished`) — exactly gtest_cas_lifecycle_condition.cpp scenario (a).
+    deleteKeyExact(*backend, store->layout().poolMetaKey());
+    deleteKeyExact(*backend, store->layout().ownerKey(kSrid));
+    EXPECT_FALSE(store->tryRemountOnce());
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::IdentityLost);
+    ASSERT_FALSE(store->isVanished());
+
+    bool gc_stopped = false;
+    store->forgetDisk([&] { gc_stopped = true; }, kForgetReason);
+
+    EXPECT_TRUE(gc_stopped);
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+    EXPECT_TRUE(store->isVanished());
+}
+
+/// (a'') The clean-farewell is EARNED, never unconditional: on a drained pool FORGET stamps the mount lease
+/// with the terminated sentinel (`min_active == UINT64_MAX`) so a same-server restart reclaims immediately,
+/// but with an UNSETTLED (wedged) ref lane it must NOT — the lease is left to expire by observation.
+TEST(CASForget, ForgetCleanFarewellGatedOnDrain)
+{
+    using DB::Cas::decodeMountLease;
+    constexpr uint64_t kTerminated = std::numeric_limits<uint64_t>::max();
+
+    /// Drained pool → clean farewell written (lease stamped terminated).
+    {
+        auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+        auto store = DB::Cas::tests::openPoolForTest(backend);
+        const String mount_key = store->layout().mountKey(kSrid);
+        ASSERT_NE(decodeMountLease(backend->get(mount_key)->bytes).min_active, kTerminated);   /// baseline
+
+        store->forgetDisk([] {}, kForgetReason);
+        ASSERT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+
+        const auto got = backend->get(mount_key);
+        ASSERT_TRUE(got.has_value());
+        EXPECT_EQ(decodeMountLease(got->bytes).min_active, kTerminated)
+            << "a drained FORGET earns the clean-release farewell";
+    }
+
+    /// Unsettled (wedged) ref lane → NO clean farewell (the drain cannot certify a clean death).
+    {
+        auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+        auto store = DB::Cas::tests::openPoolForTest(backend);
+        const String mount_key = store->layout().mountKey(kSrid);
+
+        const DB::Cas::RootNamespace ns{"test/forget_wedge"};
+        store->forceWedgeForTest(ns, /*writer_epoch*/ 1, /*ref_sequence*/ 1, "bogus/_log/key", "bogus-bytes");
+        ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+
+        store->forgetDisk([] {}, kForgetReason);
+        ASSERT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+
+        const auto got = backend->get(mount_key);
+        ASSERT_TRUE(got.has_value()) << "the lease object must still be present (expiry by observation)";
+        EXPECT_NE(decodeMountLease(got->bytes).min_active, kTerminated)
+            << "an unearned clean farewell must NOT be written when the ref lanes did not drain";
+    }
+}
+
+/// (b1) BOUNDED COMPLETION: FORGET racing an ACTIVE self-remount thread joins it without deadlock. Here the
+/// faulting backend keeps every attempt at `StayTransient` (it never reaches `armMountFence`), so this
+/// isolates the join/no-deadlock property; the fence re-arm path is covered by (b2) below. Uses a
+/// `std::future` timeout wait (never a sleep) — the timeout only fires on a genuine deadlock regression.
+TEST(CASForget, ForgetRacingActiveRemountThreadCompletesBounded)
+{
+    auto backend = std::make_shared<ToggleableTransportFaultBackend>();
+    /// `background_watermark = true` so `scheduleRemount` actually spawns a recovery thread (mirrors
+    /// gtest_cas_pool.cpp's ShutdownGuardRefusesToArmRemount setup).
+    auto store = DB::Cas::Pool::open(backend,
+        DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test", .background_watermark = true});
+
+    /// Arm the fault so every remount attempt verdicts `StayTransient` fast (no lease-expiry wait), then
+    /// trip the fence and spawn the recovery thread — it now loops `tryRemountOnce` against the fault.
+    backend->fail.store(true);
+    store->tripMountLost();
+    ASSERT_TRUE(store->scheduleRemountForTest()) << "the recovery thread must be armed and running";
+
+    /// FORGET from ANOTHER thread must join the active remount thread and finish in bounded time.
+    std::promise<void> done;
+    auto fut = done.get_future();
+    std::thread forgetter([&]
+    {
+        store->forgetDisk([] {}, kForgetReason);
+        done.set_value();
+    });
+    EXPECT_EQ(fut.wait_for(std::chrono::seconds(30)), std::future_status::ready)
+        << "FORGET must not deadlock against an in-flight self-remount";
+    forgetter.join();
+
+    /// Disarm before ~Pool so its residual teardown is not fighting the injected fault.
+    backend->fail.store(false);
+
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+    EXPECT_FALSE(store->mayMutate()) << "the fence must stay latched even if a raced reclaim re-armed it";
+}
+
+/// (b2) FENCE RE-LATCH REGRESSION GUARD (the fix's raison d'être): a self-remount that reaches
+/// `armMountFence` re-arms the local fence (`lost=false`) after FORGET has already tripped it. FORGET's
+/// SECOND `tripMountLost` — placed AFTER the remount thread is joined — must override it.
+///
+/// (b1)'s fault keeps every attempt at `StayTransient`, so it can NOT catch removal of that second trip. To
+/// make EXACTLY ONE reclaim reach `armMountFence` inside FORGET's window, deterministically and without a
+/// sleep, we drive a REAL `tryRemountOnce` from FORGET's own GC-stop step (invoked at spec §5 step 3/4,
+/// strictly AFTER the fence trip): the mount is fenced-out so the reclaim succeeds fast and re-arms the
+/// fence, and `tryRemountOnce`'s step-0 gate checks `isVanished()` — still false in this window — so it does
+/// NOT bail. The re-arm therefore lands after trip#1 and before trip#2, exactly the interval trip#2 guards.
+/// Verified to go RED when trip#2 is removed (see task-10-report.md — test_task10b_reddemo.log).
+TEST(CASForget, ForgetReLatchesFenceAfterAReclaimReachesArmMountFence)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    /// Make the current mount claimable so a self-remount SUCCEEDS fast and reaches `armMountFence`.
+    fenceOutMount(*backend, store->layout().mountKey(kSrid));
+
+    bool reclaimed = false;
+    store->forgetDisk([&] { reclaimed = store->tryRemountOnce(); }, kForgetReason);
+
+    /// Guard against a vacuous pass: if the injected reclaim did not actually succeed (reach
+    /// `armMountFence`), there is no re-arm for trip#2 to override and the test proves nothing.
+    ASSERT_TRUE(reclaimed) << "the injected reclaim must reach armMountFence, else this guard is vacuous";
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten);
+    EXPECT_FALSE(store->mayMutate())
+        << "FORGET's post-join fence re-latch (trip#2) must override the fence the reclaim re-armed";
+}
+
+/// (b3) PROMOTION-GUARD REGRESSION (spec §9 rev.8 item 7): with the erasure-proof excised, the natural
+/// `Vanished(replaced)` verdict is the ONLY remaining mid-FORGET natural-terminal race. A `tryRemountOnce`
+/// in flight during FORGET — one that passed step 0's `isVanished()` gate before FORGET published its intent
+/// — must NOT settle `Vanished(replaced)` and mislabel the operator-visible reason; FORGET's
+/// `Vanished(forgotten)` must win. We drive a REAL `tryRemountOnce` from FORGET's own GC-stop step (spec §5
+/// step 3/4, strictly AFTER the step-1 intent publish, BEFORE the step-6 settle), against a FOREIGN
+/// `_pool_meta` (the `Replaced` verdict), and assert the guard bailed.
+TEST(CASForget, ForgetIntentBlocksNaturalReplacedPromotion)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    /// Make the identity gate verdict `Replaced`: overwrite `_pool_meta` with a FOREIGN pool_id (present,
+    /// mismatched identity) — exactly gtest_cas_lifecycle_condition.cpp scenario (b).
+    const String meta_key = store->layout().poolMetaKey();
+    const auto got = backend->get(meta_key);
+    ASSERT_TRUE(got.has_value());
+    DB::Cas::PoolMeta foreign = DB::Cas::decodePoolMeta(got->bytes);
+    foreign.pool_id = foreign.pool_id + DB::UInt128(1);
+    ASSERT_EQ(backend->putOverwrite(meta_key, DB::Cas::encodePoolMeta(foreign), got->token).outcome,
+              DB::Cas::PutOutcome::Done);
+
+    /// The in-flight gate (run from the GC-stop callback) reaches the `Replaced` verdict but must BAIL on the
+    /// already-published intent rather than settle `Vanished(replaced)`.
+    bool replaced_settled_midforget = false;
+    store->forgetDisk(
+        [&]
+        {
+            store->tryRemountOnce();
+            replaced_settled_midforget = (store->lifecycle() == PoolLifecycle::VanishedReplaced);
+        },
+        kForgetReason);
+
+    EXPECT_FALSE(replaced_settled_midforget)
+        << "a mid-FORGET Replaced verdict must NOT settle — the intent guard bails before enterVanished(Replaced)";
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::VanishedForgotten)
+        << "FORGET's Vanished(forgotten) must win (first terminal STATE transition)";
+    const std::string msg = messageOf([&] { store->throwIfLifecycleTerminal(); });
+    EXPECT_NE(msg.find("erasure was NOT verified"), std::string::npos) << msg;
+    EXPECT_EQ(msg.find("foreign pool"), std::string::npos)
+        << "the reason must NOT be the mislabeled Replaced text: " << msg;
+}
+
+/// (e) End-to-end through the verb entry `ContentAddressedMetadataStorage::forgetDisk` and the six-class
+/// gate: after FORGET, a Probe answers truth-absent, a Remove no-ops, and a content read throws the [D5]
+/// message with the REAL decommission timestamp produced by the handler.
+TEST(CASForget, ForgetEndToEndGatesTruthWithTimestampedMessage)
+{
+    auto storage = openForgetStorage();
+    commitOnePart(*storage);
+    ASSERT_TRUE(storage->existsFile(kPartFile));   /// Live baseline
+
+    storage->forgetDisk();
+
+    /// Probe → truth-absent (no throw): the committed part reads absent on a forgotten disk.
+    EXPECT_FALSE(storage->existsFile(kPartFile));
+    EXPECT_FALSE(storage->existsDirectory(kPartDir));
+
+    /// Remove → no-op success (this is what lets a forgotten-disk table's DROP complete).
+    EXPECT_NO_THROW({
+        auto tx = storage->createTransaction();
+        tx->removeRecursive(kTableDir, /*should_remove_objects=*/nullptr);
+        tx->commit(NoCommitOptions{});
+    });
+
+    /// Content read → the typed [D5] message, with the handler's real UTC timestamp.
+    const std::string msg = messageOf([&] { storage->getFileSize(kPartFile); });
+    EXPECT_NE(msg.find("SYSTEM CAS FORGET at "), std::string::npos) << msg;
+    EXPECT_NE(msg.find(" UTC"), std::string::npos) << msg;
+    EXPECT_NE(msg.find("erasure was NOT verified"), std::string::npos) << msg;
+}
+
+/// (I-1 regression) A manual `SYSTEM CAS GC RUN` admitted while `Live` but that acquires
+/// `gc_scheduler_mutex` strictly AFTER a concurrent FORGET completes must NOT resurrect a `CasGcScheduler`
+/// on the now-`Vanished` pool: the under-lock admission re-check refuses with the typed [D5] message. The
+/// interleave is deterministic (bounded cv waits, no sleep) — the GC-verb seam parks the RUN in the
+/// admission→lock window while the FORGET thread drives the real teardown. The lasting-damage observable is
+/// `gcHealth()` staying empty: a resurrected scheduler (the pre-fix behavior) would make it non-empty.
+/// Verified RED against the pre-fix ordering (see task-17-report.md).
+TEST(CASForget, GcRunAdmittedWhileLiveRefusesAfterConcurrentForget)
+{
+    auto storage = openForgetStorage();
+    /// Capture the pool while Live (store() is fail-closed once Vanished) to assert its terminal state after.
+    auto pool = storage->store();
+    ASSERT_EQ(pool->lifecycle(), PoolLifecycle::Live);
+    ASSERT_FALSE(storage->gcHealth().has_value())
+        << "no scheduler exists before the first GC round (unit-test null context creates none at startup)";
+
+    const std::string msg = raceForgetIntoGcVerbWindow(
+        *storage, [&] { storage->runGarbageCollectionRoundNow(); });
+
+    /// The refusal is the typed FORGET [D5] message (an under-lock admission throw), not a round-internal
+    /// error and not a silently-run round.
+    EXPECT_NE(msg.find("erasure was NOT verified"), std::string::npos) << msg;
+    /// The I-1 lasting-damage observable: NO scheduler was created on the decommissioned pool.
+    EXPECT_FALSE(storage->gcHealth().has_value())
+        << "a GC RUN refused post-FORGET must NOT resurrect the scheduler on a Vanished pool";
+    EXPECT_EQ(pool->lifecycle(), PoolLifecycle::VanishedForgotten);
+}
+
+/// (I-2 regression) A `SYSTEM CAS GC REBUILD` holds `gc_scheduler_mutex` for its whole
+/// duration, so a concurrent FORGET must SERIALIZE behind it — FORGET cannot report the disk decommissioned
+/// while the rebuild is still issuing durable `gc/`-plane writes. Deterministic (bounded cv waits + a bounded
+/// negative future poll anchored by a positive control, never a sleep-to-fix-a-race): the in-lock seam parks
+/// the rebuild WHILE it holds the mutex; a FORGET launched in that window must NOT complete until the rebuild
+/// releases the lock. The pre-fix `runGcRebuildNow` took NO lock, so an in-flight rebuild was invisible to
+/// FORGET and FORGET would complete immediately. Verified RED against the pre-fix code (see task-17-report.md).
+TEST(CASForget, GcRebuildInFlightSerializesForget)
+{
+    auto storage = openForgetStorage();
+    auto pool = storage->store();   /// captured while Live
+    ASSERT_EQ(pool->lifecycle(), PoolLifecycle::Live);
+
+    std::mutex m;
+    std::condition_variable cv;
+    bool rebuild_holds_lock = false;
+    bool may_release = false;
+
+    /// In-lock seam: fires WHILE the rebuild holds `gc_scheduler_mutex`. It parks there (bounded) until the
+    /// coordinator has verified FORGET is blocked, then lets the rebuild finish and release the lock.
+    storage->setGcVerbAdmitWindowHookForTest([&]
+    {
+        {
+            std::lock_guard lk(m);
+            rebuild_holds_lock = true;
+        }
+        cv.notify_all();
+        std::unique_lock lk(m);
+        EXPECT_TRUE(cv.wait_for(lk, std::chrono::seconds(30), [&] { return may_release; }))
+            << "the coordinator must release the in-flight rebuild within the bound";
+    });
+
+    /// The rebuild runs on its own thread; it holds the lock through the seam above.
+    std::promise<void> rebuild_done_p;
+    auto rebuild_done = rebuild_done_p.get_future();
+    std::thread rebuilder([&]
+    {
+        /// On release the pool may already be Vanished (RED path: FORGET ran unserialized) — `store()` then
+        /// throws; swallow it, the assertions below carry the verdict.
+        try { storage->runGcRebuildNow(/*force=*/false); } catch (...) {} // NOLINT(bugprone-empty-catch)
+        rebuild_done_p.set_value();
+    });
+
+    /// Wait until the rebuild is genuinely in flight (holding the lock).
+    {
+        std::unique_lock lk(m);
+        ASSERT_TRUE(cv.wait_for(lk, std::chrono::seconds(30), [&] { return rebuild_holds_lock; }))
+            << "the rebuild must reach its in-lock seam";
+    }
+
+    /// Launch FORGET while the rebuild holds the lock. With the fix it BLOCKS on `gc_scheduler_mutex`;
+    /// without the fix (pre-fix rebuild took no lock) it runs straight through.
+    std::promise<void> forget_done_p;
+    auto forget_done = forget_done_p.get_future();
+    std::thread forgetter([&] { storage->forgetDisk(); forget_done_p.set_value(); });
+
+    /// The discriminator: FORGET must NOT complete while the rebuild holds the lock. This bounded negative
+    /// observation is anchored by the positive control below (FORGET DOES complete once the lock releases),
+    /// so the window's meaning is real, not a race hidden behind a sleep.
+    EXPECT_EQ(forget_done.wait_for(std::chrono::seconds(2)), std::future_status::timeout)
+        << "FORGET must serialize behind an in-flight GC rebuild (the rebuild holds gc_scheduler_mutex)";
+
+    /// Release the in-flight rebuild; it finishes and drops the lock, and FORGET can now proceed.
+    {
+        std::lock_guard lk(m);
+        may_release = true;
+    }
+    cv.notify_all();
+
+    ASSERT_EQ(rebuild_done.wait_for(std::chrono::seconds(30)), std::future_status::ready)
+        << "the rebuild must complete after release";
+    ASSERT_EQ(forget_done.wait_for(std::chrono::seconds(30)), std::future_status::ready)
+        << "once the rebuild releases the lock, the serialized FORGET completes (positive control)";
+
+    rebuilder.join();
+    forgetter.join();
+    storage->setGcVerbAdmitWindowHookForTest({});
+
+    EXPECT_EQ(pool->lifecycle(), PoolLifecycle::VanishedForgotten)
+        << "FORGET settles the pool Vanished(forgotten) once it is no longer serialized behind the rebuild";
+}
diff --git a/src/Disks/tests/gtest_cas_format.cpp b/src/Disks/tests/gtest_cas_format.cpp
new file mode 100644
index 000000000000..de1b3e5109b0
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_format.cpp
@@ -0,0 +1,85 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Common/Exception.h>
+
+namespace DB::ErrorCodes
+{
+    extern const int UNKNOWN_FORMAT_VERSION;
+    extern const int LOGICAL_ERROR;
+}
+
+using namespace DB::Cas;
+
+TEST(CASFormat, ChangePointsExistForEveryClass)
+{
+    /// Every class that existed from the start has a non-empty, gen-1 baseline.
+    for (auto id : {FormatId::Blob,
+                    FormatId::GcState,
+                    FormatId::PoolMeta, FormatId::Roster,
+                    FormatId::GcOutcomes,
+                    FormatId::PartManifest, FormatId::RunFile,
+                    FormatId::FoldSeal})
+    {
+        auto cps = changePoints(id);
+        ASSERT_FALSE(cps.empty());
+        EXPECT_EQ(cps.front().generation, 1u);
+        EXPECT_EQ(cps.front().min_reader, 1u);
+    }
+}
+
+/// A class BORN after generation 1 begins its history at its birth generation, not at 1. `RefCkpt`
+/// (spec INV-4) was introduced at generation 4: there is no such thing as a generation-1 `_ckpt`, and a
+/// `{1, 1}` baseline would assert that a generation-1 reader could read one. Its history then gained a
+/// three later breaking entries: generation 5 re-keyed it under `<ns>/<incarnation>/`, generation 6
+/// moved it to opaque life-owned state, and generation 9 added the exact committed frontier. Neither
+/// change touches the gen-1 baseline. Pinned because the decision is
+/// invisible otherwise — nothing consults `changePoints` at decode time yet, so a wrong entry here
+/// would sit unnoticed until the day a per-class reader floor is wired and starts admitting objects it
+/// should refuse.
+TEST(CASFormat, ChangePointsOfAClassBornAfterGenerationOneStartAtItsBirth)
+{
+    const auto cps = changePoints(FormatId::RefCkpt);
+    ASSERT_EQ(cps.size(), 4u);
+    EXPECT_EQ(cps.front().generation, kContiguousRefStreamsGeneration);
+    EXPECT_EQ(cps.front().min_reader, kContiguousRefStreamsGeneration);
+    EXPECT_GT(cps.front().generation, 1u) << "the point of this test is that it is NOT the gen-1 baseline";
+    EXPECT_EQ(cps[1].generation, kNamespaceLifeKeyedGeneration);
+    EXPECT_EQ(cps[1].min_reader, kNamespaceLifeKeyedGeneration);
+    EXPECT_EQ(cps[2].generation, kOpaqueNamespaceLifeLayoutGeneration);
+    EXPECT_EQ(cps[2].min_reader, kOpaqueNamespaceLifeLayoutGeneration);
+    EXPECT_EQ(cps.back().generation, kCommittedRefFrontierGeneration);
+    EXPECT_EQ(cps.back().min_reader, kCommittedRefFrontierGeneration);
+}
+
+TEST(CASFormat, PoolMetaTracksTheRecreateOnlyRecoveryFrontierGeneration)
+{
+    const auto cps = changePoints(FormatId::PoolMeta);
+    ASSERT_EQ(cps.size(), 3u);
+    EXPECT_EQ(cps.back().generation, kCommittedRefFrontierGeneration);
+    EXPECT_EQ(cps.back().min_reader, kCommittedRefFrontierGeneration);
+}
+
+TEST(CASFormat, CurrentVersionsAreGBuild)
+{
+    EXPECT_EQ(currentWriterVersion(), G_BUILD);
+    EXPECT_EQ(currentCompatibilityVersion(), G_BUILD);
+}
+
+TEST(CASFormat, CheckCompatibilityPassesWhenKnown)
+{
+    EXPECT_NO_THROW(checkCompatibility(1u, "manifest"));
+    EXPECT_NO_THROW(checkCompatibility(G_BUILD, "manifest"));
+}
+
+TEST(CASFormat, CheckCompatibilityFailsClosedOnFuture)
+{
+    try
+    {
+        checkCompatibility(G_BUILD + 1, "manifest");
+        FAIL() << "expected UNKNOWN_FORMAT_VERSION";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::UNKNOWN_FORMAT_VERSION);
+    }
+}
diff --git a/src/Disks/tests/gtest_cas_format_battery.cpp b/src/Disks/tests/gtest_cas_format_battery.cpp
new file mode 100644
index 000000000000..f6ba9bcdbcd0
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_format_battery.cpp
@@ -0,0 +1,23 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+
+using namespace DB::Cas;
+
+/// The real cas_pool_meta case replaces the phase-1 toy proving instance. Every other control-plane
+/// format registers its own battery row in its own gtest_cas_<object>_format.cpp file (Tasks 3-6).
+
+TEST(CASFormatBattery, PoolMeta)
+{
+    PoolMeta pm;
+    pm.pool_id = hexToU128("00112233445566778899aabbccddeeff");
+    pm.blob_header_len = 256;
+    pm.min_reader_generation = 3;
+    pm.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128)};
+    runFormatBattery(FormatBatteryCase{
+        .id = FormatId::PoolMeta,
+        .encode = [&] { return sealObject(FormatId::PoolMeta, encodePoolMeta(pm)); },
+        .decode = [](std::string_view s) { decodePoolMeta(std::string(openObject(FormatId::PoolMeta, s))); },
+        .golden = currentFormatHeader("cas_pool_meta") +
+                  "{\"pid\":\"00112233445566778899aabbccddeeff\",\"hln\":256,\"gcs\":1,\"mrg\":3,\"alg\":\"ch128\"}\n"});
+}
diff --git a/src/Disks/tests/gtest_cas_fsck.cpp b/src/Disks/tests/gtest_cas_fsck.cpp
new file mode 100644
index 000000000000..71abb917eaa2
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_fsck.cpp
@@ -0,0 +1,1584 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include "cas_test_helpers.h"
+
+#include <algorithm>
+#include <filesystem>
+#include <functional>
+#include <mutex>
+#include <optional>
+#include <string_view>
+
+namespace DB::ErrorCodes
+{
+extern const int INVALID_STATE;
+extern const int NETWORK_ERROR;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+constexpr uint64_t kWriterEpoch = 7;
+const String kServerRoot = "00";
+ManifestRef ref(uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = kWriterEpoch, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+
+/// B207 race-simulation harness: `InMemoryBackend` is documented "not final: tests subclass it to
+/// distort single behaviors". `runFsck`'s ref-walk and its physical blob listing (`listAll` over
+/// `layout.blobsPrefix()`) are two separate calls to `Backend::list` minutes apart in production; here
+/// we fire an injected mutation the FIRST time `list` is called against the armed prefix — i.e.
+/// strictly AFTER the ref-walk has captured its (now stale) `reachable_blobs`/`blob_labels` view, and
+/// strictly BEFORE the HEAD-confirm loop sees the physical listing. That reproduces the race
+/// deterministically, without any real timing.
+class RepublishOnListBackend : public InMemoryBackend
+{
+public:
+    void armOnFirstList(String prefix, std::function<void()> mutation)
+    {
+        std::lock_guard<std::mutex> lock(arm_mutex);
+        armed_prefix = std::move(prefix);
+        pending_mutation = std::move(mutation);
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        std::function<void()> to_run;
+        {
+            std::lock_guard<std::mutex> lock(arm_mutex);
+            if (pending_mutation && prefix == armed_prefix)
+            {
+                to_run = std::move(pending_mutation);
+                pending_mutation = nullptr;
+            }
+        }
+        if (to_run)
+            to_run();
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+
+private:
+    std::mutex arm_mutex;
+    String armed_prefix;
+    std::function<void()> pending_mutation;
+};
+
+/// Companion to `RepublishOnListBackend` for the MANIFEST phantom-dangle race: the ref-walk's
+/// per-namespace recovery captures each committed `(ref -> manifest)` minutes before the per-ref
+/// `backend.get(mkey)` that confirms the manifest body. This backend fires an injected mutation the
+/// FIRST time `get` is called for the armed manifest key — strictly AFTER the walk captured its (now
+/// stale) row and AT the GET that would otherwise read the manifest — reproducing "ref republished/
+/// dropped + old manifest legitimately GC-deleted" deterministically, with no real timing.
+class MutateOnFirstGetBackend : public InMemoryBackend
+{
+public:
+    void armOnFirstGet(String key, std::function<void()> mutation)
+    {
+        std::lock_guard<std::mutex> lock(arm_mutex);
+        armed_key = std::move(key);
+        pending_mutation = std::move(mutation);
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        std::function<void()> to_run;
+        {
+            std::lock_guard<std::mutex> lock(arm_mutex);
+            if (pending_mutation && key == armed_key)
+            {
+                to_run = std::move(pending_mutation);
+                pending_mutation = nullptr;
+            }
+        }
+        if (to_run)
+            to_run();
+        return InMemoryBackend::get(key, range);
+    }
+
+private:
+    std::mutex arm_mutex;
+    String armed_key;
+    std::function<void()> pending_mutation;
+};
+
+enum class FsckListingMode : uint8_t
+{
+    Full,
+    Empty,
+    Partial,
+    Reordered,
+};
+
+/// Distort only one namespace stream LIST after fixture deposition. Exact GET/HEAD and every other
+/// prefix retain ordinary backend semantics, so the test varies the hint and nothing authoritative.
+class FsckListingBackend : public InMemoryBackend
+{
+public:
+    void distort(String prefix_, FsckListingMode mode_)
+    {
+        prefix = std::move(prefix_);
+        mode = mode_;
+    }
+
+    ListPage list(const String & listed_prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = InMemoryBackend::list(listed_prefix, cursor, limit);
+        if (listed_prefix != prefix)
+            return page;
+        if (mode == FsckListingMode::Empty)
+            page.keys.clear();
+        else if (mode == FsckListingMode::Partial && !page.keys.empty())
+            page.keys.erase(page.keys.begin());
+        else if (mode == FsckListingMode::Reordered)
+            std::reverse(page.keys.begin(), page.keys.end());
+        return page;
+    }
+
+private:
+    String prefix;
+    FsckListingMode mode = FsckListingMode::Full;
+};
+
+/// Fail one exact GET without disturbing LIST or any other object read. This keeps the checkpoint
+/// authority stable while proving that fsck distinguishes a transport failure from durable corruption.
+class FailExactGetBackend : public InMemoryBackend
+{
+public:
+    void fail(String key_)
+    {
+        key = std::move(key_);
+    }
+
+    std::optional<GetResult> get(const String & requested_key, Range range) override
+    {
+        if (requested_key == key)
+            throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR, "injected exact GET failure");
+        return InMemoryBackend::get(requested_key, range);
+    }
+
+private:
+    String key;
+};
+
+/// Publish the exact `_ckpt` authority an ordinary Live test life would have after its first committed
+/// record. Raw ref-log helpers deliberately do not do this: several protocol tests need malformed or
+/// pre-creation states. Fsck tests that exercise a recoverable Live life must make the durable authority
+/// explicit instead of accidentally borrowing the legacy LIST-only recovery rule.
+void writeFsckCheckpoint(Backend & backend, const Layout & layout, const RootNamespace & ns, RefTxnId committed_through)
+{
+    const CasRefCatalog::Snapshot cut = CasRefCatalog::read(backend, layout);
+    const auto it = std::find_if(cut.catalog.entries.begin(), cut.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    ASSERT_NE(it, cut.catalog.entries.end());
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(it->ns, it->incarnation);
+    const String key = layout.refCkptKey(life);
+    const String body = encodeRefCkpt(RefCkpt{
+        .life_epoch = committed_through.writer_epoch,
+        .committed_through = committed_through,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt});
+    const HeadResult current = backend.head(key);
+    const PutResult put = current.exists
+        ? backend.putOverwrite(key, body, current.token)
+        : backend.putIfAbsent(key, body);
+    ASSERT_EQ(put.outcome, PutOutcome::Done);
+}
+
+void writeFsckCheckpointWithBase(
+    Backend & backend, const Layout & layout, const RootNamespace & ns, RefTxnId base,
+    std::optional<RefTxnId> last_epoch_seal = std::nullopt)
+{
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(backend, layout, ns);
+    ASSERT_EQ(backend.putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = base,
+        .checkpoint_snapshot_id = base,
+        .last_epoch_seal = last_epoch_seal})).outcome, PutOutcome::Done);
+}
+
+void expectCheckpointBaseVerdict(
+    const FsckReport & report, const String & exact_base_key, FsckClass expected_class,
+    std::string_view expected_reason)
+{
+    EXPECT_EQ(report.chain_broken, expected_class == FsckClass::ChainBroken ? 1u : 0u);
+    EXPECT_EQ(report.unchecked, expected_class == FsckClass::Unchecked ? 1u : 0u);
+    EXPECT_EQ(report.clean(), expected_class != FsckClass::ChainBroken);
+
+    size_t matching = 0;
+    for (const FsckObject & object : report.objects)
+    {
+        if (object.cls != expected_class || object.key != exact_base_key)
+            continue;
+        ++matching;
+        ASSERT_EQ(object.reachable_from.size(), 1u);
+        EXPECT_NE(object.reachable_from.front().find(expected_reason), String::npos);
+    }
+    EXPECT_EQ(matching, 1u) << "the checkpoint-base verdict must identify its exact named base and cause";
+}
+
+/// Test-only external catalog writer. It changes the namespace's current logical life after fsck took
+/// its catalog cut, precisely the competing-cut mutation that fsck must not splice into its verdict.
+void replaceCatalogLife(Backend & backend, const Layout & layout, const RootNamespace & ns, UInt128 incarnation)
+{
+    CasRefCatalog::Snapshot current = CasRefCatalog::read(backend, layout);
+    const auto it = std::find_if(current.catalog.entries.begin(), current.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    ASSERT_NE(it, current.catalog.entries.end());
+    it->incarnation = incarnation;
+    it->state = NsState::Live;
+    it->creator.reset();
+    it->removal_started_round.reset();
+    ASSERT_TRUE(current.token.has_value());
+    ASSERT_EQ(backend.putOverwrite(layout.refCatalogKey(), encodeRefCatalog(current.catalog), *current.token).outcome,
+        PutOutcome::Done);
+}
+
+FsckReport runFsckWithListingMode(FsckListingMode mode, std::string_view suffix)
+{
+    auto backend = std::make_shared<FsckListingBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/listing_" + String(suffix) + "@cas@"};
+    const ManifestRef r1 = ref(1, 0xD1);
+    const ManifestRef r2 = ref(2, 0xD2);
+    const DB::UInt128 h1 = u128Of("fsck-listing-old-" + String(suffix));
+    const DB::UInt128 h2 = u128Of("fsck-listing-new-" + String(suffix));
+    writeBlobBody(*backend, layout, h1);
+    writeBlobBody(*backend, layout, h2);
+    writeManifestRaw(*backend, layout, ns, r1, {blobEntryFor("a", h1)});
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("a", h2)});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r1);
+    const uint64_t frontier = publishCommittedTransition(*backend, layout, ns, "tbl", r1, r2);
+    writeFsckCheckpoint(*backend, layout, ns, RefTxnId{1, frontier});
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    backend->distort(layout.namespaceStreamPrefix(life), mode);
+    return runFsck(*store, /*detail=*/true);
+}
+
+void expectListingIndependentFsck(const FsckReport & report)
+{
+    EXPECT_EQ(report.chain_broken, 0u);
+    EXPECT_EQ(report.unchecked, 0u);
+    EXPECT_EQ(report.dangling, 0u);
+    EXPECT_EQ(report.ref_records_walked, 2u);
+    EXPECT_EQ(report.reachable, 1u);
+}
+
+struct FsckAuthorityVerdict
+{
+    bool clean = false;
+    uint64_t hard_findings = 0;
+    uint64_t chain_broken = 0;
+    uint64_t unchecked = 0;
+    uint64_t ref_records_walked = 0;
+    uint64_t reachable = 0;
+    uint64_t dangling = 0;
+
+    bool operator==(const FsckAuthorityVerdict &) const = default;
+};
+
+FsckAuthorityVerdict authorityVerdict(const FsckReport & report)
+{
+    uint64_t hard_findings = 0;
+    for (const FsckHardFinding & finding : kFsckHardFindings)
+        hard_findings += report.*(finding.value);
+    return FsckAuthorityVerdict{
+        .clean = report.clean(),
+        .hard_findings = hard_findings,
+        .chain_broken = report.chain_broken,
+        .unchecked = report.unchecked,
+        .ref_records_walked = report.ref_records_walked,
+        .reachable = report.reachable,
+        .dangling = report.dangling,
+    };
+}
+
+FsckReport runCheckpointBaseFsckWithListingMode(
+    FsckListingMode mode, std::string_view suffix, bool corrupt_exact_base)
+{
+    auto backend = std::make_shared<FsckListingBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/listing_base_" + String(suffix) + "@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+
+    const RefTxnId base{1, 1};
+    const RefLogTxn birth{
+        .ns = ns.string(), .txn_id = base, .ops = {namespaceBirthOp()}, .prev_epoch_seal = std::nullopt};
+    fixture::writeRefLogRaw(*backend, layout, birth);
+    RefTableState base_state;
+    applyRefLogTxn(base_state, birth);
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(base_state, ns.string()));
+    writeFsckCheckpointWithBase(*backend, layout, ns, base);
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    if (corrupt_exact_base)
+    {
+        const String base_snapshot_key = layout.refSnapshotKey(life, base);
+        const HeadResult head = backend->head(base_snapshot_key);
+        EXPECT_TRUE(head.exists);
+        if (head.exists)
+            EXPECT_EQ(backend->deleteExact(base_snapshot_key, head.token).kind, DeleteOutcome::Kind::Deleted);
+    }
+    else
+    {
+        /// This newer pair is deliberately outside `_ckpt.committed_through`. It is inert garbage:
+        /// changing whether LIST happens to reveal it must not add or remove an fsck finding.
+        const RefTxnId unadopted{1, 2};
+        const RefOwnerBinding listed_binding{RefOwnerKind::Precommit, "listed", ref(9, 0xE9)};
+        const RefLogTxn listed_log{
+            .ns = ns.string(),
+            .txn_id = unadopted,
+            .ops = {ownerTransitionOp(std::nullopt, listed_binding)},
+            .prev_epoch_seal = std::nullopt};
+        fixture::writeRefLogRaw(*backend, layout, listed_log);
+        RefTableState listed_state = base_state;
+        applyRefLogTxn(listed_state, listed_log);
+        RefTableSnapshot unadopted_snapshot = snapshotOf(listed_state, ns.string());
+        unadopted_snapshot.precommits.push_back(
+            RefOwnerBinding{RefOwnerKind::Precommit, "unlisted", ref(10, 0xEA)});
+        writeRefSnapshotRaw(*backend, layout, unadopted_snapshot);
+    }
+
+    backend->distort(layout.namespaceStreamPrefix(life), mode);
+    return runFsck(*store, /*detail=*/true);
+}
+}
+
+/// A committed ref whose manifest body is present and whose blobs exist => clean.
+TEST(CASFsck, CleanManifestPoolHasNoDangling)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, sequence});
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// A committed ref naming a MISSING manifest body is an ERROR (Dangling).
+TEST(CASFsck, OwnerVisibleMissingManifestBodyIsError)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    const uint64_t sequence = publishCommittedTransition(
+        *backend, store->layout(), ns, "tbl", std::nullopt, r);  // no body written
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, sequence});
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_FALSE(rep.clean());
+    EXPECT_GE(rep.dangling, 1u);
+}
+
+/// A committed ref whose blob body is missing is an ERROR (Dangling).
+TEST(CASFsck, ReachableBlobMissingIsError)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});  // no blob body
+    const uint64_t sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, sequence});
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_FALSE(rep.clean());
+    EXPECT_GE(rep.dangling, 1u);
+}
+
+/// fsck RECORDS AND CONTINUES over a key that names no namespace life. It is the forensic tool an
+/// operator reaches for after something has already gone wrong, so one bad key must not make it report
+/// NOTHING -- including about the healthy namespaces it would never reach. The finding is hard (an
+/// un-incarnated key is corruption behind the format bump) and counted once per key, not once per sweep.
+TEST(CASFsck, LifelessKeyIsRecordedAndTheHealthyNamespaceIsStillReported)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, sequence});
+
+    /// Hand-built: no helper can mint the un-incarnated shape any more.
+    const String lifeless = store->layout().casRefsPrefix() + ns.string() + "/_log/"
+        + renderRefTxnId(RefTxnId{1, 1}) + ".zst";
+    ASSERT_EQ(backend->putIfAbsent(lifeless, "garbage").outcome, PutOutcome::Done);
+
+    FsckReport rep;
+    ASSERT_NO_THROW(rep = runFsck(*store, /*detail*/true))
+        << "the audit must not be taken out by the damage it exists to report";
+
+    /// The finding, named, and counted ONCE even though several sweeps enumerate namespaces.
+    EXPECT_EQ(rep.lifeless_keys, 1u);
+    EXPECT_FALSE(rep.clean());
+    bool saw = false;
+    for (const FsckObject & o : rep.objects)
+        if (o.cls == FsckClass::LifelessKey)
+        {
+            saw = true;
+            EXPECT_EQ(o.key, lifeless);
+        }
+    EXPECT_TRUE(saw) << "a counted finding with no row is a number nobody can act on";
+
+    /// And the healthy namespace was still reached: its committed ref resolved to a present manifest and
+    /// a present blob, which only a sweep that ran can report.
+    EXPECT_GE(rep.reachable, 1u);
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// A COMPLETE, canonical namespace-life key (a real `_files` write under a real admitted life) whose
+/// catalog row is then removed entirely -- exactly what a fenced GC's exact-CAS row deletion leaves
+/// behind, before the perpetual namespace janitor's next page reaches it -- must classify as
+/// `janitor_pending`, a SOFT finding, never `lifeless_keys`. The report stays clean.
+TEST(CASFsck, CanonicalDeadLifeResidueIsJanitorPendingNotHardFinding)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const NamespaceLifeId life = store->namespaceLife(ns);
+    store->putNamespaceFile(life, "format_version.txt", "1\n");
+
+    /// Simulate a fenced GC's exact-CAS catalog-row deletion: the row is gone, the life-owned physical
+    /// object above survives it (the janitor's own job, not GC's own round). `casUpdate` deliberately
+    /// refuses to add or delete rows (there is no generic catalog remove-by-name API -- deletion is
+    /// only `deleteCompletedRemoving`/`cancelStalledCreating`, both requiring the full fenced-GC
+    /// protocol this fixture is not driving), so inject the post-deletion catalog snapshot directly,
+    /// mirroring `DuplicateLifeIdIsReportedWhileAnUnrelatedUniqueNamespaceStillProgresses` below.
+    {
+        CasRefCatalog::Snapshot snapshot = CasRefCatalog::read(*backend, store->layout());
+        const auto it = std::find_if(snapshot.catalog.entries.begin(), snapshot.catalog.entries.end(),
+            [&](const CatalogEntry & entry) { return entry.ns == ns; });
+        ASSERT_NE(it, snapshot.catalog.entries.end());
+        snapshot.catalog.entries.erase(it);
+        const auto catalog_head = backend->head(store->layout().refCatalogKey());
+        ASSERT_TRUE(catalog_head.exists);
+        ASSERT_EQ(backend->putOverwrite(store->layout().refCatalogKey(), encodeRefCatalog(snapshot.catalog),
+            catalog_head.token).outcome, PutOutcome::Done);
+    }
+
+    FsckReport rep;
+    ASSERT_NO_THROW(rep = runFsck(*store, /*detail*/true))
+        << "janitor-pending residue must never abort the scan";
+
+    EXPECT_EQ(rep.lifeless_keys, 0u);
+    EXPECT_GE(rep.namespace_janitor_pending, 1u);
+    EXPECT_EQ(rep.namespace_janitor_pending_lives, 1u);
+    EXPECT_TRUE(rep.clean()) << "janitor-pending residue is not a hard finding";
+    bool saw = false;
+    for (const FsckObject & o : rep.objects)
+        if (o.cls == FsckClass::JanitorPending)
+            saw = true;
+    EXPECT_TRUE(saw) << "a counted soft finding with no row is a number nobody can act on";
+}
+
+/// The observe-then-cut race: a life admitted between fsck's namespace-tree LIST and the catalog cut
+/// it takes AFTER that listing must NOT be misread as residue. Mirrors
+/// `CASNamespaceJanitor.PostListCatalogCutProtectsConcurrentCreationWithOneGet` -- the same ordering,
+/// the same reason: creation admits `Creating` before writing any life-owned object, so a life visible
+/// only in the LATER cut cannot have raced this listing.
+namespace
+{
+class AdmitLifeAfterNamespaceListingBackend : public InMemoryBackend
+{
+public:
+    explicit AdmitLifeAfterNamespaceListingBackend(NamespaceLifeId life_) : protected_life(std::move(life_)) {}
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = InMemoryBackend::list(prefix, cursor, limit);
+        if (!published && prefix.ends_with("/cas/ns/"))
+        {
+            published = true;
+            CasRefCatalog::casAdmitEntry(*this, Layout("p"), /*gc_shards*/1,
+                CatalogEntry{.ns = protected_life.ns, .state = NsState::Live,
+                    .incarnation = protected_life.incarnation});
+        }
+        return page;
+    }
+
+private:
+    NamespaceLifeId protected_life;
+    bool published = false;
+};
+}
+
+TEST(CASFsck, LifeAdmittedBetweenNamespaceListingAndLaterCutIsNotResidue)
+{
+    const RootNamespace ns{"00/late@cas@"};
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(ns, UInt128{909});
+    auto backend = std::make_shared<AdmitLifeAfterNamespaceListingBackend>(life);
+    auto store = openPoolForTest(backend);
+    /// The physical object exists before the listing runs, exactly as a legitimate late admission would
+    /// leave it: written only after `casAdmitEntry` above, but here pre-seeded since the injected
+    /// backend admits the CATALOG row, not the physical file, on the list callback.
+    ASSERT_EQ(backend->putIfAbsent(store->layout().namespaceFilesPrefix(life) + "format_version.txt", "1\n").outcome,
+        PutOutcome::Done);
+
+    FsckReport rep;
+    ASSERT_NO_THROW(rep = runFsck(*store, /*detail*/true));
+    EXPECT_EQ(rep.namespace_janitor_pending, 0u)
+        << "a life visible in the post-listing cut must not be misclassified as residue";
+    EXPECT_EQ(rep.lifeless_keys, 0u);
+    EXPECT_TRUE(rep.clean());
+}
+
+/// Malformed or non-canonical namespace-tree shapes must stay HARD findings even after the
+/// janitor-pending split: a dirty `_files` relative name (the parser-asymmetry fix), a zero life id, an
+/// uppercase life id, and an unrecognized kind directory all name no current writer's grammar.
+TEST(CASFsck, MalformedNamespaceTreeShapesStayHardFindings)
+{
+    const Layout layout("p");
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"00/bb@cas@"}, UInt128{909});
+    const struct { String key; String description; } cases[] = {
+        {layout.namespaceFilesPrefix(life) + "../escape", "dirty _files relative name"},
+        {"p/cas/ns/state/" + String(32, '0') + "/_files/format_version.txt", "zero life id"},
+        {"p/cas/ns/state/112233445566778899AABBCCDDEEFF01/_files/format_version.txt", "uppercase life id"},
+        {"p/cas/ns/stream/" + renderIncarnation(UInt128{909}) + "/_unknown_kind/x.zst", "unknown kind directory"},
+    };
+    for (const auto & c : cases)
+    {
+        auto backend = std::make_shared<InMemoryBackend>();
+        auto store = openPoolForTest(backend);
+        ASSERT_EQ(backend->putIfAbsent(c.key, "garbage").outcome, PutOutcome::Done) << c.description;
+
+        FsckReport rep;
+        ASSERT_NO_THROW(rep = runFsck(*store, /*detail*/true)) << c.description;
+        EXPECT_GE(rep.lifeless_keys, 1u) << c.description;
+        EXPECT_EQ(rep.namespace_janitor_pending, 0u) << c.description;
+        EXPECT_FALSE(rep.clean()) << c.description;
+    }
+}
+
+/// Mutation caught: calling the destructive consumer's global `throwIfAmbiguous` from fsck aborts
+/// before the unique row is audited. The read-only tool reports the ambiguous physical id and
+/// continues through an unrelated unique namespace.
+TEST(CASFsck, DuplicateLifeIdIsReportedWhileAnUnrelatedUniqueNamespaceStillProgresses)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace unique_ns{"00/unique@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeManifestRaw(*backend, layout, unique_ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t sequence = publishCommittedTransition(*backend, layout, unique_ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, layout, unique_ns, RefTxnId{1, sequence});
+
+    CasRefCatalog::Snapshot snapshot = CasRefCatalog::read(*backend, layout);
+    snapshot.catalog.entries.push_back(CatalogEntry{
+        .ns = RootNamespace{"bad/a"}, .state = NsState::Live, .incarnation = UInt128{777}});
+    snapshot.catalog.entries.push_back(CatalogEntry{
+        .ns = RootNamespace{"bad/b"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{777},
+        .removal_started_round = 1});
+    std::sort(snapshot.catalog.entries.begin(), snapshot.catalog.entries.end(),
+        [](const CatalogEntry & lhs, const CatalogEntry & rhs) { return lhs.ns.string() < rhs.ns.string(); });
+    const auto catalog_head = backend->head(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_head.exists);
+    ASSERT_EQ(backend->putOverwrite(layout.refCatalogKey(), encodeRefCatalog(snapshot.catalog), catalog_head.token).outcome,
+        PutOutcome::Done);
+
+    FsckReport report;
+    ASSERT_NO_THROW(report = runFsck(*store, /*detail=*/true));
+    EXPECT_GE(report.lifeless_keys, 1u);
+    EXPECT_GE(report.reachable, 1u) << "the unrelated unique namespace must still be audited";
+    EXPECT_EQ(report.dangling, 0u);
+}
+
+/// A physical namespace-life key whose life id is ambiguous in the POST-LISTING cut (two catalog rows
+/// share one incarnation) must be recorded as a `lifeless_keys` finding and must NOT abort the scan:
+/// `CatalogLifeIndex::resolve` throws `CORRUPTED_DATA` on a duplicate, and the janitor-pending
+/// classification loop must catch it exactly like every other catalog-authority failure in this scan.
+/// Mirrors `DuplicateLifeIdIsReportedWhileAnUnrelatedUniqueNamespaceStillProgresses`, but that fixture
+/// has no physical object under the duplicated life id, so it never drives a candidate into the new
+/// post-listing loop at all -- this is the case that actually exercises it.
+TEST(CASFsck, AmbiguousLifeUnderAPhysicalKeyIsRecordedNotAborted)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace unique_ns{"00/unique@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeManifestRaw(*backend, layout, unique_ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t sequence = publishCommittedTransition(*backend, layout, unique_ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, layout, unique_ns, RefTxnId{1, sequence});
+
+    const NamespaceLifeId duplicated_life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"bad/a"}, UInt128{777});
+    ASSERT_EQ(backend->putIfAbsent(layout.namespaceFilesPrefix(duplicated_life) + "format_version.txt", "1\n").outcome,
+        PutOutcome::Done);
+
+    CasRefCatalog::Snapshot snapshot = CasRefCatalog::read(*backend, layout);
+    snapshot.catalog.entries.push_back(CatalogEntry{
+        .ns = RootNamespace{"bad/a"}, .state = NsState::Live, .incarnation = UInt128{777}});
+    snapshot.catalog.entries.push_back(CatalogEntry{
+        .ns = RootNamespace{"bad/b"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{777},
+        .removal_started_round = 1});
+    std::sort(snapshot.catalog.entries.begin(), snapshot.catalog.entries.end(),
+        [](const CatalogEntry & lhs, const CatalogEntry & rhs) { return lhs.ns.string() < rhs.ns.string(); });
+    const auto catalog_head = backend->head(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_head.exists);
+    ASSERT_EQ(backend->putOverwrite(layout.refCatalogKey(), encodeRefCatalog(snapshot.catalog), catalog_head.token).outcome,
+        PutOutcome::Done);
+
+    FsckReport report;
+    ASSERT_NO_THROW(report = runFsck(*store, /*detail=*/true))
+        << "an ambiguous life under a physical key must be a recorded finding, never an abort";
+    EXPECT_GE(report.lifeless_keys, 1u);
+    EXPECT_GE(report.reachable, 1u) << "the unrelated unique namespace must still be audited";
+    EXPECT_EQ(report.dangling, 0u);
+}
+
+/// Fsck's namespace universe is catalog-authoritative. Admit `ns`, publish one real ref-log record,
+/// then hide its whole stream prefix from LIST. Fsck must still retain the namespace from the catalog
+/// cut and use the checkpoint-anchored arithmetic walk to read the record.
+///
+/// Proves `ref_records_walked`, not `dangling`/`clean()`: `checkRefStream` (which this proves runs) has
+/// its own `_ckpt`-anchored arithmetic walk and so is reachable here. The distinct
+/// `manifestStillReferenced` recheck now receives the same frozen catalog row and exact `_ckpt` authority;
+/// its competing-cut regression is pinned separately by `MissingManifestRecheckStaysOnInitialCatalogCut`.
+TEST(CASFsck, CatalogLiveNamespaceHiddenFromListIsStillWalked)
+{
+    auto backend = std::make_shared<HintHoleBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/hidden_from_list@cas@"};
+
+    fixture::admitLive(*backend, layout, ns);
+    const uint64_t sequence = appendRefLogSeed(
+        *backend, layout, ns, {});   // one real record: a birth-only ref-log transaction
+
+    /// `casAdmitEntry` never publishes a `_ckpt` (by its own design), and the write above used
+    /// `appendRefLogSeed`'s hardcoded writer_epoch 1. `checkRefStream`'s own walk needs SOME anchor -- a
+    /// `_ckpt.life_epoch`, a listed snapshot, or a listed log -- to know where to start reading, and this
+    /// test is about to hide every listed one. Without an anchor the walk sees nothing at all and
+    /// correctly treats the namespace as never-born, the same "nothing to probe" trap the I4 replacement
+    /// controls hit and were restructured around (fold-before-hide). fsck has no "fold" step to run
+    /// first, so the anchor is published directly, by exact key, before the hide -- the exact-key GET
+    /// this enables is unaffected by list-hiding either way.
+    writeFsckCheckpoint(*backend, layout, ns, RefTxnId{1, sequence});
+    const NamespaceLifeId life = fixture::fixtureLife(ns);
+
+    backend->hidePrefix(layout.namespaceStreamPrefix(life));
+
+    FsckReport rep;
+    ASSERT_NO_THROW(rep = runFsck(*store, /*detail*/true));
+    EXPECT_GT(backend->holesServed(), 0u)
+        << "the hide must actually have been exercised by the stream LIST, or this test passes vacuously";
+    EXPECT_GE(rep.ref_records_walked, 1u)
+        << "the namespace must be discovered and its stream actually read even when LIST omits every one "
+           "of its keys, or the catalog-authoritative universe supplement did not run";
+}
+
+TEST(CASFsckAuthority, FullListingDoesNotDefineStreamGeometry)
+{
+    expectListingIndependentFsck(runFsckWithListingMode(FsckListingMode::Full, "full"));
+}
+
+TEST(CASFsckAuthority, EmptyListingDoesNotDefineStreamGeometry)
+{
+    expectListingIndependentFsck(runFsckWithListingMode(FsckListingMode::Empty, "empty"));
+}
+
+TEST(CASFsckAuthority, PartialListingDoesNotDefineStreamGeometry)
+{
+    expectListingIndependentFsck(runFsckWithListingMode(FsckListingMode::Partial, "partial"));
+}
+
+TEST(CASFsckAuthority, ReorderedListingDoesNotDefineStreamGeometry)
+{
+    expectListingIndependentFsck(runFsckWithListingMode(FsckListingMode::Reordered, "reordered"));
+}
+
+/// Stream LIST is not fsck authority. The same exact catalog + `_ckpt` + checkpoint-base triple must
+/// yield the same result when LIST is complete, empty, partial, or reordered. A newer unadopted log and
+/// snapshot are inert garbage, while damage to the exact checkpoint base remains a hard finding under
+/// every listing. Mutation caught: the old LIST-derived snapshot oracle makes only listings that reveal
+/// the unadopted pair non-clean.
+TEST(CASFsckAuthority, StreamListingDoesNotChangeCheckpointBaseVerdict)
+{
+    const std::array modes{
+        FsckListingMode::Full,
+        FsckListingMode::Empty,
+        FsckListingMode::Partial,
+        FsckListingMode::Reordered,
+    };
+
+    std::optional<FsckAuthorityVerdict> clean_reference;
+    std::optional<FsckAuthorityVerdict> corrupt_reference;
+    for (size_t i = 0; i < modes.size(); ++i)
+    {
+        const FsckAuthorityVerdict clean = authorityVerdict(runCheckpointBaseFsckWithListingMode(
+            modes[i], "clean_" + std::to_string(i), /*corrupt_exact_base=*/false));
+        if (!clean_reference)
+            clean_reference = clean;
+        EXPECT_EQ(clean, *clean_reference);
+        EXPECT_TRUE(clean.clean);
+        EXPECT_EQ(clean.hard_findings, 0u);
+
+        const FsckAuthorityVerdict corrupt = authorityVerdict(runCheckpointBaseFsckWithListingMode(
+            modes[i], "corrupt_" + std::to_string(i), /*corrupt_exact_base=*/true));
+        if (!corrupt_reference)
+            corrupt_reference = corrupt;
+        EXPECT_EQ(corrupt, *corrupt_reference);
+        EXPECT_FALSE(corrupt.clean);
+        EXPECT_EQ(corrupt.chain_broken, 1u);
+        EXPECT_EQ(corrupt.unchecked, 0u);
+        EXPECT_EQ(corrupt.hard_findings, 1u);
+    }
+}
+
+/// A durable but unfrontiered F+1 is not part of this fsck cut. Mutation caught: probing one position
+/// beyond `_ckpt.committed_through` walks the visible record and changes both coverage and reachability.
+TEST(CASFsckAuthority, VisibleFPlusOneDoesNotAffectVerdict)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/visible_f_plus_one@cas@"};
+    const ManifestRef committed_ref = ref(1, 0xE1);
+    const ManifestRef unfrontiered_ref = ref(2, 0xE2);
+    const DB::UInt128 committed_blob = u128Of("fsck-frontier-committed");
+    writeBlobBody(*backend, layout, committed_blob);
+    writeManifestRaw(*backend, layout, ns, committed_ref, {blobEntryFor("a", committed_blob)});
+    const uint64_t frontier = publishCommittedTransition(
+        *backend, layout, ns, "tbl", std::nullopt, committed_ref);
+    writeFsckCheckpoint(*backend, layout, ns, RefTxnId{1, frontier});
+
+    /// The object is durable and visible, but `_ckpt` is deliberately NOT advanced to it. The semantic
+    /// convenience wrapper advances `_ckpt`, so deposit this unfrontiered F+1 as the raw transaction
+    /// shape that a stopped writer can leave behind. Its missing manifest would become a false dangle if
+    /// either fsck leg adopted F+1.
+    std::vector<RefOp> unfrontiered_ops;
+    unfrontiered_ops.push_back(ownerTransitionOp(
+        RefOwnerBinding{RefOwnerKind::Committed, "tbl", committed_ref}, std::nullopt));
+    const std::vector<RefOp> commit_ops = publishCommittedOps("tbl", unfrontiered_ref);
+    unfrontiered_ops.insert(unfrontiered_ops.end(), commit_ops.begin(), commit_ops.end());
+    ASSERT_EQ(appendRefLogSeed(*backend, layout, ns, std::move(unfrontiered_ops)), frontier + 1);
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_EQ(report.chain_broken, 0u);
+    EXPECT_EQ(report.unchecked, 0u);
+    EXPECT_EQ(report.ref_records_walked, 1u);
+    EXPECT_EQ(report.dangling, 0u);
+    EXPECT_EQ(report.reachable, 1u);
+}
+
+/// INV-2 materializes every burned global epoch, including an empty one, as a sequence-1 seal. A
+/// direct `{1,2}` -> `{7,1}` chain that omits `{2,1}` is therefore data loss, not a legal sparse epoch
+/// transition. Mutation caught: accepting the later head as a shortcut blesses the missing seal.
+TEST(CASFsckAuthority, MissingBurnedEpochSealIsChainBroken)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/skipped_writer_epoch@cas@"};
+
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 1}, .ops = {namespaceBirthOp()},
+        .prev_epoch_seal = std::nullopt});
+    RefOp seal;
+    seal.kind = RefOpKind::EpochSeal;
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 2}, .ops = {seal},
+        .prev_epoch_seal = std::nullopt});
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    /// The codec now rejects this skip. Deposit its old on-disk corruption shape by changing only the
+    /// fixed-width epoch token of an otherwise encodable body, so fsck still proves that a missing
+    /// intermediate epoch is reported rather than treated as a sparse legal transition.
+    String skipped_bytes = encodeRefLogTxn(RefLogTxn{
+        .ns = ns.string(), .txn_id = RefTxnId{7, 1}, .ops = {}, .prev_epoch_seal = RefTxnId{6, 1}});
+    const String old_epoch_token = R"("!pse":"6")";
+    const auto old_epoch = skipped_bytes.find(old_epoch_token);
+    ASSERT_NE(old_epoch, String::npos);
+    skipped_bytes.replace(old_epoch, old_epoch_token.size(), R"("!pse":"1")");
+    ASSERT_EQ(backend->putIfAbsent(layout.refLogKey(life, RefTxnId{7, 1}),
+        sealObject(FormatId::RefLog, skipped_bytes)).outcome, PutOutcome::Done);
+
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{7, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{6, 1}})).outcome, PutOutcome::Done);
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_EQ(report.chain_broken, 1u);
+    EXPECT_EQ(report.unchecked, 0u);
+    EXPECT_EQ(report.ref_records_walked, 2u);
+}
+
+/// The checkpoint base is the inclusive frontier, so there is no replay tail in which another hole
+/// could satisfy this test. The missing same-id log itself makes the stable exact authority corrupt.
+/// Mutation caught: mapping every `readCheckpointSnapshotBase` failure to `Unchecked` leaves `clean` true.
+TEST(CASFsckAuthority, MissingCheckpointBaseLogIsChainBroken)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/missing_checkpoint_base_log@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+
+    const RefTxnId base{1, 1};
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    writeFsckCheckpointWithBase(*backend, layout, ns, base);
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_EQ(report.ref_records_walked, 0u);
+    expectCheckpointBaseVerdict(
+        report, layout.refSnapshotKey(life, base), FsckClass::ChainBroken, "has no matching log");
+}
+
+/// A present, valid non-seal base log rules out a stream hole; only its checkpoint-named same-id
+/// snapshot is absent. Stable exact absence is damage, not lost diagnostic coverage.
+TEST(CASFsckAuthority, MissingCheckpointBaseSnapshotIsChainBroken)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/missing_checkpoint_base_snapshot@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+
+    const RefTxnId base{1, 1};
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(), .txn_id = base, .ops = {namespaceBirthOp()}, .prev_epoch_seal = std::nullopt});
+    writeFsckCheckpointWithBase(*backend, layout, ns, base);
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_EQ(report.ref_records_walked, 0u);
+    expectCheckpointBaseVerdict(
+        report, layout.refSnapshotKey(life, base), FsckClass::ChainBroken,
+        "is absent under the supplied immutable lifecycle authority");
+}
+
+/// `_ckpt.checkpoint_snapshot_id` names a state snapshot, never an `EpochSeal`. The forged base is an
+/// OLDER seal, deliberately different from `last_epoch_seal`, so comparing checkpoint metadata cannot
+/// expose it: the stream audit must exact-read the base log and reject it before recovery can bless the
+/// same-id snapshot.
+TEST(CASFsckAuthority, CheckpointSnapshotAtOlderEpochSealIsChainBroken)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint_base_seal@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+
+    const RefLogTxn birth{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 1}, .ops = {namespaceBirthOp()},
+        .prev_epoch_seal = std::nullopt};
+    fixture::writeRefLogRaw(*backend, layout, birth);
+    RefOp seal;
+    seal.kind = RefOpKind::EpochSeal;
+    const RefLogTxn seal_txn{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 2}, .ops = {seal},
+        .prev_epoch_seal = std::nullopt};
+    fixture::writeRefLogRaw(*backend, layout, seal_txn);
+    RefOp later_seal;
+    later_seal.kind = RefOpKind::EpochSeal;
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(), .txn_id = RefTxnId{2, 1}, .ops = {later_seal},
+        .prev_epoch_seal = RefTxnId{1, 2}});
+
+    RefTableState through_seal;
+    applyRefLogTxn(through_seal, birth);
+    applyRefLogTxn(through_seal, seal_txn);
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(through_seal, ns.string()));
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = RefTxnId{1, 2},
+        .last_epoch_seal = RefTxnId{2, 1}})).outcome, PutOutcome::Done);
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_EQ(report.ref_records_walked, 0u)
+        << "the seal is rejected as the checkpoint base, not walked as a normal replay record";
+    expectCheckpointBaseVerdict(
+        report, layout.refSnapshotKey(life, RefTxnId{1, 2}), FsckClass::ChainBroken,
+        "names an EpochSeal, not a snapshot base");
+}
+
+/// An unstable transport failure while exact-reading the same valid checkpoint base proves neither
+/// presence nor absence. It remains the honest third answer and must not become a hard finding.
+TEST(CASFsckAuthority, CheckpointBaseTransportFailureIsUnchecked)
+{
+    auto backend = std::make_shared<FailExactGetBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint_base_transport@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+
+    const RefTxnId base{1, 1};
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(), .txn_id = base, .ops = {namespaceBirthOp()}, .prev_epoch_seal = std::nullopt});
+    RefTableState state;
+    applyRefLogTxn(state, RefLogTxn{
+        .ns = ns.string(), .txn_id = base, .ops = {namespaceBirthOp()}, .prev_epoch_seal = std::nullopt});
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(state, ns.string()));
+    writeFsckCheckpointWithBase(*backend, layout, ns, base);
+    backend->fail(layout.refLogKey(life, base));
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_EQ(report.ref_records_walked, 0u);
+    expectCheckpointBaseVerdict(
+        report, layout.refSnapshotKey(life, base), FsckClass::Unchecked, "injected exact GET failure");
+}
+
+/// The sampled checkpoint is immutable input, but cleanup may advance `_ckpt` after that sample and
+/// retire its old base before fsck exact-reads it. The miss is then authority instability, not evidence
+/// that either durable checkpoint incarnation was internally corrupt.
+/// Mutation caught: classifying `CORRUPTED_DATA` without rechecking the sampled checkpoint token.
+TEST(CASFsckAuthority, CheckpointBaseVanishingAfterAuthorityAdvanceIsUnchecked)
+{
+    auto backend = std::make_shared<MutateOnFirstGetBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint_base_advanced@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+
+    const RefTxnId old_base{1, 1};
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    writeFsckCheckpointWithBase(*backend, layout, ns, old_base);
+    backend->armOnFirstGet(layout.refLogKey(life, old_base), [&]
+    {
+        const String ckpt_key = layout.refCkptKey(life);
+        const HeadResult head = backend->head(ckpt_key);
+        ASSERT_TRUE(head.exists);
+        ASSERT_EQ(backend->putOverwrite(ckpt_key, encodeRefCkpt(RefCkpt{
+            .life_epoch = 1,
+            .committed_through = std::nullopt,
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = std::nullopt}), head.token).outcome, PutOutcome::Done);
+    });
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_EQ(report.ref_records_walked, 0u);
+    expectCheckpointBaseVerdict(
+        report, layout.refSnapshotKey(life, old_base), FsckClass::Unchecked,
+        "checkpoint authority changed while validating its snapshot base");
+}
+
+/// A Live catalog row without `_ckpt` is not a recoverable table, even when a listing happens to show a
+/// complete ref log. Mutation caught: replacing the authority-taking recovery with the old LIST replay
+/// makes this audit look clean and silently blesses a life whose durable frontier is unknown.
+TEST(CASFsck, LiveNamespaceWithoutCheckpointIsUnchecked)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/live_without_checkpoint@cas@"};
+    const ManifestRef r = ref(1, 0xC1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    /// `publishCommittedTransition` correctly advances `_ckpt`; this test instead deposits the raw
+    /// missing-checkpoint corruption shape that fsck must refuse to recover.
+    std::vector<RefOp> ops = publishCommittedOps("tbl", r);
+    appendRefLogSeed(*backend, store->layout(), ns, std::move(ops));
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_GE(report.unchecked, 1u)
+        << "a Live life with no exact checkpoint cannot be recovered from a convenient LIST";
+    EXPECT_EQ(report.reachable, 0u)
+        << "fsck must not consume refs after the mandatory recovery authority was absent";
+}
+
+/// fsck's missing-manifest recheck runs after its primary walk. If it re-resolves the name from a second
+/// catalog cut, a concurrent rebirth can make the old durable owner disappear from the recheck and hide
+/// a real dangle. The initial cut's row and exact checkpoint must remain the sole authority throughout
+/// the whole fsck call.
+///
+/// Mutation caught: re-resolve `ns` from `manifestStillReferenced`. The first manifest GET changes the
+/// catalog to a fresh, empty life; the second resolution then sees no owner and suppresses the dangle. A
+/// recovery from the original cut continues to see the original owner and reports it.
+TEST(CASFsck, MissingManifestRecheckStaysOnInitialCatalogCut)
+{
+    auto backend = std::make_shared<MutateOnFirstGetBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/frozen_fsck_cut@cas@"};
+    const ManifestRef r = ref(1, 0xC2);
+    const uint64_t sequence = publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r);
+    /// `publishCommittedTransition`'s raw fixture log writer uses its documented epoch 1.
+    writeFsckCheckpoint(*backend, layout, ns, RefTxnId{1, sequence});
+
+    backend->armOnFirstGet(layout.manifestKey(ManifestId{ns, r}), [&]
+    {
+        replaceCatalogLife(*backend, layout, ns, UInt128{0xC3});
+    });
+
+    const FsckReport report = runFsck(*store, /*detail=*/true);
+    EXPECT_GE(report.dangling, 1u)
+        << "the initial catalog-cut owner still names the absent manifest despite a later rebirth";
+}
+
+/// A pre-precommit body in an eligible prefix (no owner) is INFO (Unreachable), not an error.
+TEST(CASFsck, ReclaimablePrePrecommitBodyIsInfo)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    /// Seed a birth-only ref log through the fixture helper, which also admits the catalog row, while
+    /// leaving NO committed owner; the manifest body below is orphan debris.
+    const uint64_t sequence = appendRefLogSeed(*backend, store->layout(), ns, {});
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, sequence});
+    const ManifestRef r = ref(5, 0xAB);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});   // body, no owner
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);   // eligible
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_TRUE(rep.clean());            // not an error
+    EXPECT_GE(rep.unreachable, 1u);      // counted as info/unreachable
+}
+
+/// Pipeline classification (2026-07-02): a condemned-but-present blob is PendingGc — an EXPECTED
+/// pipeline state (deletion is scheduled), never the suspicious "unreachable" lump beta testers
+/// read as a leak. clean() is unaffected.
+TEST(CASFsck, CondemnedBlobClassifiesPendingGc)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t publish_sequence = publishCommittedTransition(
+        *backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, publish_sequence});
+    Gc gc(store, hexToU128("00000000000000000000000000000001"));
+    gc.runRegularRound();
+    const uint64_t drop_sequence = dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, drop_sequence});
+    gc.runRegularRound();   /// -1 folds => zero => condemned into the retired list; blob still present
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.pending_gc, 1u);
+    EXPECT_EQ(rep.unaccounted, 0u);
+    bool saw = false;
+    for (const FsckObject & o : rep.objects)
+        if (o.cls == FsckClass::PendingGc)
+        {
+            saw = true;
+            ASSERT_FALSE(o.reachable_from.empty());
+            EXPECT_NE(o.reachable_from[0].find("condemned at round"), String::npos);
+        }
+    EXPECT_TRUE(saw);
+}
+
+/// A drop whose -1 has NOT folded yet: the blob's edges are still in the GC snapshot => AwaitingGc
+/// (expected), not Unaccounted.
+TEST(CASFsck, DroppedButUnfoldedBlobClassifiesAwaitingGc)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t publish_sequence = publishCommittedTransition(
+        *backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, publish_sequence});
+    Gc gc(store, hexToU128("00000000000000000000000000000001"));
+    gc.runRegularRound();                                        /// +1 folded into the snapshot
+    const uint64_t drop_sequence = dropRefTransition(
+        *backend, store->layout(), ns, "tbl", r);  /// -1 NOT folded (no round)
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, drop_sequence});
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.awaiting_gc, 1u);
+    EXPECT_EQ(rep.unaccounted, 0u);
+}
+
+/// Stale-edge cross-check, NEGATIVE side: the residual edge's source manifest body is still PRESENT in
+/// the pool, so its removal still has a `-1` to fold (and the orphan sweep still has a body to reclaim).
+/// That is a genuine mid-pipeline backlog and must keep the `AwaitingGc` verdict — the new check may
+/// never turn an ordinary unfolded drop into a hard finding.
+TEST(CASFsck, UnfoldedDropWithPresentSourceManifestStaysAwaitingGc)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t publish_sequence = publishCommittedTransition(
+        *backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, publish_sequence});
+    Gc gc(store, hexToU128("00000000000000000000000000000001"));
+    gc.runRegularRound();                                        /// +1 folded into the snapshot
+    const uint64_t drop_sequence = dropRefTransition(
+        *backend, store->layout(), ns, "tbl", r);  /// -1 NOT folded; the BODY survives
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, drop_sequence});
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.awaiting_gc, 1u);
+    EXPECT_EQ(rep.stale_edge, 0u);
+    bool saw = false;
+    for (const FsckObject & o : rep.objects)
+        if (o.cls == FsckClass::AwaitingGc)
+            saw = true;
+    EXPECT_TRUE(saw);
+}
+
+/// Stale-edge cross-check, POSITIVE side: the blob's only residual `+1` names a manifest that no longer
+/// exists anywhere in the pool, so no `-1` is left to fold — the in-degree stays at 1 for every future
+/// round and the incremental GC can never nominate the blob. It must NOT be labeled `AwaitingGc`
+/// ("expected, no action needed", the sentence that hid 56 permanently retained blobs); it is the hard
+/// `StaleEdge` finding and the report is not `clean()`.
+TEST(CASFsck, ResidualEdgeNamingAnAbsentManifestClassifiesStaleEdge)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    const ManifestId id = writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t publish_sequence = publishCommittedTransition(
+        *backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, publish_sequence});
+    Gc gc(store, hexToU128("00000000000000000000000000000001"));
+    gc.runRegularRound();                                        /// +1 folded into the snapshot
+    const uint64_t drop_sequence = dropRefTransition(
+        *backend, store->layout(), ns, "tbl", r);  /// the owner is gone ...
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, drop_sequence});
+    deleteManifestBody(*backend, store->layout(), id);           /// ... and so is the body, un-folded
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_EQ(rep.stale_edge, 1u);
+    EXPECT_EQ(rep.awaiting_gc, 0u);
+    EXPECT_EQ(rep.dangling, 0u) << "no committed ref names the manifest any more — this is not a dangle";
+    EXPECT_FALSE(rep.clean());
+    bool saw = false;
+    for (const FsckObject & o : rep.objects)
+        if (o.cls == FsckClass::StaleEdge)
+        {
+            saw = true;
+            ASSERT_FALSE(o.reachable_from.empty());
+            EXPECT_NE(o.reachable_from[0].find("no longer exist"), String::npos);
+        }
+    EXPECT_TRUE(saw);
+}
+
+/// GC never ran on the pool: nothing is classifiable through the GC view — everything unreferenced
+/// is AwaitingGc ("GC has not run yet"), never a false Unaccounted alarm.
+TEST(CASFsck, GcNeverRanClassifiesAwaitingGc)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(5));   /// present, never referenced, no gc/state
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.awaiting_gc, 1u);
+    EXPECT_EQ(rep.unaccounted, 0u);
+}
+
+/// A blob outside the WHOLE GC view on a pool where GC runs: Unaccounted — expected only as a
+/// transient (fast create+drop between rounds); persistent occurrences violate INV-2.
+TEST(CASFsck, ForeignBlobClassifiesUnaccounted)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, sequence});
+    Gc gc(store, hexToU128("00000000000000000000000000000001"));
+    gc.runRegularRound();
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(0xF0F0));   /// never referenced anywhere
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.unaccounted, 1u);
+    EXPECT_EQ(rep.pending_gc, 0u);
+}
+
+/// A `.meta` descriptor whose body is missing is ADVISORY (meta_without_body), NOT a hard finding:
+/// GC deletes the body FIRST and drops the `.meta` afterwards on a bounded, error-suppressed advisory
+/// pool that may drop the op, so a single raw LIST legitimately observes a body-less `.meta` mid-
+/// graduation and no finite grace makes a persistent one hard evidence. It is still counted/reported;
+/// it must NOT be a `dangling` (nothing referenced it) and NOT one of the present-but-unreferenced blob
+/// pipeline classes (the `.meta` key is excluded from body classification entirely). `clean()` stays TRUE.
+TEST(CASFsck, MetaWithoutBodyIsAdvisoryNotHard)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const DB::UInt128 h = u128Of("meta-without-body");
+    writeMetaClean(*backend, store->layout(), h, /*size*/ 10);   /// meta only, no body written
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_GE(rep.meta_without_body, 1u);   // still counted and reported in the full report
+    EXPECT_EQ(rep.dangling, 0u);
+    EXPECT_EQ(rep.unreachable, 0u);
+    EXPECT_EQ(rep.pending_gc, 0u);
+    EXPECT_EQ(rep.awaiting_gc, 0u);
+    EXPECT_EQ(rep.unaccounted, 0u);
+    EXPECT_TRUE(rep.clean());   // meta_without_body is advisory — excluded from clean()
+}
+
+/// A body with no `.meta` sibling is a BENIGN not-yet-adopted (or crashed-birth) artifact — NOT a
+/// dangle, and it must still classify through the ordinary present-but-unreferenced pipeline.
+TEST(CASFsck, BodyWithoutMetaIsBenign)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const DB::UInt128 h = u128Of("body-without-meta");
+    writeBlobBody(*backend, store->layout(), h);   /// body only, no meta written
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_GE(rep.body_without_meta, 1u);
+    EXPECT_EQ(rep.dangling, 0u);
+    EXPECT_EQ(rep.meta_without_body, 0u);
+    EXPECT_TRUE(rep.clean());
+}
+
+/// A scan whose deadline is already in the past: partial_on_deadline=false keeps the old
+/// throw-on-timeout contract; partial_on_deadline=true returns the accumulated lower-bound counts
+/// instead of failing empty-handed (the 2026-07-05 campaign lost 5 verdicts to this).
+TEST(CASFsckPartial, DeadlineReturnsAccumulatedCountsInsteadOfThrowing)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, sequence});
+
+    const auto past = std::chrono::steady_clock::now() - std::chrono::seconds(1);
+    /// partial_on_deadline=false keeps the old contract:
+    EXPECT_THROW(DB::Cas::runFsck(*store, /*detail=*/false, {}, past), DB::Exception);
+    /// partial_on_deadline=true returns a flagged report:
+    const auto report = DB::Cas::runFsck(*store, false, {}, past, /*partial_on_deadline=*/true);
+    EXPECT_TRUE(report.partial);
+    EXPECT_FALSE(report.partial_reason.empty());
+}
+
+/// A `namespace_prefix` scopes the scan to only the matching namespaces' refs (dangling-only): no
+/// pool-wide unreachable/pending/awaiting/unaccounted classification, since that needs the whole pool.
+TEST(CASFsckScoped, NamespacePrefixChecksOnlyMatchingRefsDanglingOnly)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+
+    const RootNamespace ns_a{"nsa"};
+    const ManifestRef r_a = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns_a, r_a, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t sequence_a = publishCommittedTransition(
+        *backend, store->layout(), ns_a, "tbl", std::nullopt, r_a);
+    writeFsckCheckpoint(*backend, store->layout(), ns_a, RefTxnId{1, sequence_a});
+
+    const RootNamespace ns_b{"nsb"};
+    const ManifestRef r_b = ref(1, 0xB1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(2));
+    writeManifestRaw(*backend, store->layout(), ns_b, r_b, {blobEntryFor("b", DB::UInt128(2))});
+    const uint64_t sequence_b = publishCommittedTransition(
+        *backend, store->layout(), ns_b, "tbl", std::nullopt, r_b);
+    writeFsckCheckpoint(*backend, store->layout(), ns_b, RefTxnId{1, sequence_b});
+
+    const auto scoped = DB::Cas::runFsck(*store, false, {}, {}, false, /*namespace_prefix=*/"nsa");
+    EXPECT_EQ(scoped.dangling, 0u);
+    EXPECT_GT(scoped.reachable, 0u);
+    /// Scoped mode skips only the POOL-WIDE physical/pipeline classification; the manifest-debris
+    /// pass stays active for the scoped namespaces, so `unreachable` here counts THEIR orphan
+    /// manifest bodies — zero in this clean setup, legitimately nonzero on a churned pool.
+    EXPECT_EQ(scoped.unreachable, 0u);
+    EXPECT_EQ(scoped.pending_gc + scoped.awaiting_gc + scoped.unaccounted, 0u);
+}
+
+/// B207: the ref-walk and the HEAD-confirm run minutes apart with no snapshot. A ref that gets
+/// RE-PUBLISHED to a different manifest in that window, combined with a legitimate GC delete of the
+/// blob it used to name, must NOT surface as a phantom `dangling` — only a CURRENT ref over an absent
+/// object is a real dangle.
+TEST(CASFsck, PhantomDanglingFromRepublishedRefIsReresolvedAway)
+{
+    auto backend = std::make_shared<RepublishOnListBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xA1);
+    const ManifestRef r2 = ref(2, 0xA2);
+    const DB::UInt128 h1 = u128Of("b207-phantom-old");
+    const DB::UInt128 h2 = u128Of("b207-phantom-new");
+
+    writeBlobBody(*backend, store->layout(), h1);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", h1)});
+    const uint64_t initial_sequence = publishCommittedTransition(
+        *backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, initial_sequence});
+
+    /// Fires strictly between the ref-walk (which captures ref "tbl" -> r1, blob h1, as reachable) and
+    /// the HEAD-confirm's physical listing — exactly the window B207 is about.
+    backend->armOnFirstList(store->layout().blobsPrefix(), [&]
+    {
+        writeBlobBody(*backend, store->layout(), h2);
+        writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", h2)});
+        const uint64_t repoint_sequence = publishCommittedTransition(
+            *backend, store->layout(), ns, "tbl", r1, r2);   /// re-publish
+        writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, repoint_sequence});
+
+        const String old_key = store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h1)});
+        const HeadResult head = backend->head(old_key);
+        ASSERT_TRUE(head.exists);
+        backend->deleteExact(old_key, head.token);   /// legitimate GC delete of the now-unreferenced blob
+    });
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_EQ(rep.dangling, 0u);
+    EXPECT_TRUE(rep.clean());
+}
+
+/// Same race, but the ref is DROPPED (not re-published) in the window between the walk and the
+/// HEAD-confirm — also must not surface as a phantom dangle.
+TEST(CASFsck, PhantomDanglingFromDroppedRefIsReresolvedAway)
+{
+    auto backend = std::make_shared<RepublishOnListBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xA1);
+    const DB::UInt128 h1 = u128Of("b207-phantom-dropped");
+
+    writeBlobBody(*backend, store->layout(), h1);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", h1)});
+    const uint64_t initial_sequence = publishCommittedTransition(
+        *backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, initial_sequence});
+
+    backend->armOnFirstList(store->layout().blobsPrefix(), [&]
+    {
+        const uint64_t drop_sequence = dropRefTransition(
+            *backend, store->layout(), ns, "tbl", r1);   /// ref dropped since the walk
+        writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, drop_sequence});
+
+        const String old_key = store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h1)});
+        const HeadResult head = backend->head(old_key);
+        ASSERT_TRUE(head.exists);
+        backend->deleteExact(old_key, head.token);   /// legitimate GC delete after the drop folds
+    });
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_EQ(rep.dangling, 0u);
+    EXPECT_TRUE(rep.clean());
+}
+
+/// Companion: the fix must never HIDE a real loss. A blob that a CURRENT ref still names, but whose
+/// object is genuinely gone (an operator error, a storage-layer bug — NOT a legitimate GC delete),
+/// stays `dangling` after the re-resolve.
+TEST(CASFsck, RealDanglingStillCaughtAfterReresolve)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    const DB::UInt128 h = u128Of("b207-real-dangle");
+
+    writeBlobBody(*backend, store->layout(), h);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", h)});
+    const uint64_t sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, sequence});
+
+    const String key = store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h)});
+    const HeadResult head = backend->head(key);
+    ASSERT_TRUE(head.exists);
+    backend->deleteExact(key, head.token);   /// genuine loss — the ref is UNCHANGED, still names this blob
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_EQ(rep.dangling, 1u);
+    EXPECT_FALSE(rep.clean());
+}
+
+/// The MANIFEST analogue of the blob phantom-dangle. The ref-walk captures "tbl" -> r1's manifest, then
+/// the ref is RE-PUBLISHED to a different manifest r2 and the OLD r1 manifest body is legitimately
+/// GC-deleted before the per-ref body GET. The missing OLD manifest must be revalidated away — a fresh
+/// re-resolve shows the CURRENT ref no longer names it — never surfacing as a phantom `dangling`.
+TEST(CASFsck, PhantomDanglingManifestFromRepublishedRefIsReresolvedAway)
+{
+    auto backend = std::make_shared<MutateOnFirstGetBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xA1);
+    const ManifestRef r2 = ref(2, 0xA2);
+    const DB::UInt128 h1 = u128Of("phantom-manifest-old");
+    const DB::UInt128 h2 = u128Of("phantom-manifest-new");
+
+    writeBlobBody(*backend, store->layout(), h1);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", h1)});
+    const uint64_t initial_sequence = publishCommittedTransition(
+        *backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, initial_sequence});
+
+    const String m1_key = store->layout().manifestKey(ManifestId{ns, r1});
+    /// Fires strictly between the ref-walk (captures "tbl" -> r1) and the per-ref GET of r1's manifest:
+    /// re-publish "tbl" to r2 and legitimately GC-delete the now-superseded r1 manifest body.
+    backend->armOnFirstGet(m1_key, [&]
+    {
+        writeBlobBody(*backend, store->layout(), h2);
+        writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", h2)});
+        const uint64_t repoint_sequence = publishCommittedTransition(
+            *backend, store->layout(), ns, "tbl", r1, r2);   /// re-publish
+        writeFsckCheckpoint(*backend, store->layout(), ns, RefTxnId{1, repoint_sequence});
+
+        const HeadResult head = backend->head(m1_key);
+        ASSERT_TRUE(head.exists);
+        backend->deleteExact(m1_key, head.token);   /// legitimate GC delete of the superseded manifest
+    });
+
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_EQ(rep.dangling, 0u);
+    EXPECT_TRUE(rep.clean());
+}
+
+namespace
+{
+/// Build a real `ContentAddressedMetadataStorage` over Local object storage and start it (Mounted) --
+/// the same harness gtest_cas_operation_gate.cpp uses. Each call gets an isolated pool root.
+std::shared_ptr<DB::ContentAddressedMetadataStorage> openRunningStorageForTest()
+{
+    auto settings = makeSettingsForTest("test", std::filesystem::temp_directory_path() / "ca_fsck_running_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+/// Commit one real part (tmp -> final rename -> commit) so a RUNNING FSCK has live committed content.
+void commitOneRunningPart(DB::ContentAddressedMetadataStorage & storage)
+{
+    const std::string table_dir = "g80/g80g80g8-0808-4808-8808-080808080808";
+    auto tx = storage.createTransaction();
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+    auto buf = ca_tx.writeFile(table_dir + "/tmp_insert_all_1_1_0/data.bin", 65536, DB::WriteMode::Rewrite, {});
+    const std::string bytes = "content-of-the-part";
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+    tx->moveDirectory(table_dir + "/tmp_insert_all_1_1_0", table_dir + "/all_1_1_0");
+    tx->commit(DB::NoCommitOptions{});
+}
+}
+
+/// (rev.8) FSCK runs on a RUNNING disk: scanning a live pool with one committed part succeeds and reports
+/// its content (the one-row summary the SQL verb renders from this report).
+TEST(CASFsckRunning, FsckOnMountedDiskSucceeds)
+{
+    auto storage = openRunningStorageForTest();
+    commitOneRunningPart(*storage);
+
+    FsckReport rep;
+    EXPECT_NO_THROW(rep = storage->runFsckNow(/*detail=*/false));
+    EXPECT_TRUE(rep.clean());
+    EXPECT_GE(rep.distinct_blobs, 1u) << "the running scan must see the live committed part's blob";
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// (rev.8) FSCK is Admin-class: on a not-live pool (a lease blip / IdentityLost) it refuses before
+/// scanning, exactly like the GC entry points -- an FSCK of a disk whose data root may be gone or replaced
+/// is meaningless (the operator has the snapshot / FORGET path). The two states refuse in DIFFERENT
+/// classes, and the pairing is the point: a lease blip is transient unavailability (upstream-retryable),
+/// an identity loss is terminal (668).
+TEST(CASFsckRunning, FsckOnNotLiveDiskRefusesTransientRetryableAndIdentityLostTerminal)
+{
+    for (const auto & [lc, code] : {std::pair{PoolLifecycle::TransientNotLive, DB::ErrorCodes::NETWORK_ERROR},
+                                    std::pair{PoolLifecycle::IdentityLost, DB::ErrorCodes::INVALID_STATE}})
+    {
+        auto storage = openRunningStorageForTest();
+        storage->store()->setLifecycleForTest(lc);   /// one force from Live; no later store() call
+        expectThrowsCode(code, [&] { storage->runFsckNow(/*detail=*/false); });
+    }
+}
+
+/// The summary line is the ONLY thing most consumers ever read: the soak harness parses it, an operator
+/// eyeballs it, and `exit_code` gates CI on it. So a field that `clean()` treats as a hard finding but the
+/// summary omits is invisible in practice, however faithfully it is counted -- which is exactly what
+/// happened to `corrupted_runs`: counted since the seal check landed, part of `clean()`, rendered in
+/// `--detail` rows, and absent from the summary, so no run has ever reported one.
+///
+/// This test ITERATES `kFsckHardFindings` -- the list `clean()` is computed from -- and never names a
+/// finding itself, so a term added to that list and not rendered fails HERE. It used to claim exactly
+/// that while its body was a hand-listed set of five names, and the claim was false: `lifeless_keys` was
+/// added to `clean()` and nothing failed anywhere, which is how it reached the SQL row's absence too.
+/// `formatFsckSummary` exists to be testable at all: the line used to be built inline in
+/// `CommandFsck::executeImpl`, where nothing could reach it.
+///
+/// A per-finding DISTINCT value is what makes this more than a substring sweep: it catches a formatter
+/// that prints the right names against the wrong counters.
+TEST(CASFsckSummary, EveryHardFindingAppearsOnTheSummaryLine)
+{
+    FsckReport rep;
+    uint64_t value = 11;
+    for (const FsckHardFinding & finding : kFsckHardFindings)
+    {
+        rep.*finding.value = value;
+        value += 11;
+    }
+
+    const String line = formatFsckSummary(rep);
+
+    value = 11;
+    for (const FsckHardFinding & finding : kFsckHardFindings)
+    {
+        const String token = String(finding.name) + "=" + std::to_string(value);
+        EXPECT_NE(line.find(token), String::npos)
+            << "hard finding '" << finding.name << "' is missing from the summary line (expected `"
+            << token << "`); the line was: " << line;
+        value += 11;
+    }
+
+    /// A report carrying these values is NOT clean; the line must not be mistakable for a clean one.
+    EXPECT_FALSE(rep.clean());
+}
+
+/// A zero must be PRINTED, not omitted. The harness's `stale_edge_verdict` fails closed on an absent key
+/// precisely because "field missing" and "field zero" are different facts, and a formatter that skips
+/// zeros would turn every clean pool into an unparseable one.
+TEST(CASFsckSummary, ZeroValuedHardFindingsAreStillPrinted)
+{
+    const String line = formatFsckSummary(FsckReport{});
+    /// Iterated for the same reason the test above is: a new hard finding printed only when nonzero is a
+    /// finding the harness's fail-closed-on-absence consumers would read as missing.
+    for (const FsckHardFinding & finding : kFsckHardFindings)
+        EXPECT_NE(line.find(String(finding.name) + "=0"), String::npos)
+            << "hard finding '" << finding.name << "' prints no zero; the line was: " << line;
+    EXPECT_EQ(line.find("partial="), String::npos) << "a non-partial report must not claim partial: " << line;
+}
+
+/// A partial scan is a lower bound over the visited subset, so the flag and its reason must travel WITH
+/// the counts -- a consumer that sees the numbers but not `partial=1` reads a truncated walk as the pool
+/// truth.
+TEST(CASFsckSummary, PartialFlagAndReasonTravelWithTheCounts)
+{
+    FsckReport rep;
+    rep.partial = true;
+    rep.partial_reason = "deadline exceeded after 180s";
+    const String line = formatFsckSummary(rep);
+    EXPECT_NE(line.find("partial=1"), String::npos) << line;
+    EXPECT_NE(line.find("reason='deadline exceeded after 180s'"), String::npos) << line;
+}
diff --git a/src/Disks/tests/gtest_cas_gc_ack_floor.cpp b/src/Disks/tests/gtest_cas_gc_ack_floor.cpp
new file mode 100644
index 000000000000..10c745b30bc7
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_ack_floor.cpp
@@ -0,0 +1,1262 @@
+#include <gtest/gtest.h>
+
+#include <optional>
+#include <set>
+#include <vector>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Common/ProfileEvents.h>
+#include "cas_test_helpers.h"
+
+namespace ProfileEvents
+{
+extern const Event CASMetaDelete;
+extern const Event CASGCCondemnMarkerUnconfirmedCarry;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+ManifestRef ref(const String &, uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+bool blobExists(InMemoryBackend & b, const Layout & layout, const UInt128 & hash)
+{
+    return b.head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)})).exists;
+}
+
+/// The current retired entry for `hash` (dereferenced through gc/state.retired_refs, shard 0), or nullopt.
+std::optional<RetiredEntry> currentEntryFor(Backend & backend, const Layout & layout, const UInt128 & hash)
+{
+    for (const RetiredEntry & e : currentRetiredSet(backend, layout, /*shard*/0))
+        if (e.ref == BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)})
+            return e;
+    return std::nullopt;
+}
+
+/// Decorator reproducing the rustfs quirk (observed 2026-07-11): a conditional exact-token delete against
+/// an object that is ALREADY absent can answer HTTP 412 (precondition failed), which this backend layer
+/// maps to `TokenMismatch` -- not the 404-shaped `NotFound` an in-memory backend naturally returns. For
+/// keys marked via `quirkOnAbsent`, `deleteExact` forces exactly that answer whenever the underlying
+/// object is gone, letting a test drive the GC redelete site through the disambiguation path
+/// backend-agnostically (without guessing at real rustfs HTTP mappings).
+class TokenMismatchOnAbsentBackend : public InMemoryBackend
+{
+public:
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        if (quirk_keys.contains(key) && !InMemoryBackend::head(key).exists)
+        {
+            DeleteOutcome d;
+            d.kind = DeleteOutcome::Kind::TokenMismatch;
+            return d;
+        }
+        return InMemoryBackend::deleteExact(key, token);
+    }
+
+    void quirkOnAbsent(const String & key) { quirk_keys.insert(key); }
+
+private:
+    std::set<String> quirk_keys;
+};
+
+class CkptReplacementConflictBackend : public InMemoryBackend
+{
+public:
+    CasResult casPut(
+        const String & key, const String & bytes, const std::optional<Token> & expected,
+        const ObjectMeta & meta) override
+    {
+        if (conflict_once && key == watched_key)
+        {
+            conflict_once = false;
+            return CasResult{CasOutcome::Conflict, {}};
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+
+    String watched_key;
+    bool conflict_once = false;
+};
+}
+
+TEST(CASSemanticRefFixture, WrapperCreatesInitialRecoverableCheckpoint)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/semantic-create@cas@"};
+    const ManifestRef manifest = ref("srv-a:1", 1, 0xAB);
+
+    const uint64_t sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, manifest);
+    const RefTxnId expected_id{manifest.writer_epoch, sequence};
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns);
+    const auto ckpt = readCkpt(*backend, store->layout(), life);
+
+    ASSERT_TRUE(ckpt.has_value());
+    EXPECT_EQ(ckpt->ckpt.life_epoch, 1);
+    EXPECT_EQ(ckpt->ckpt.committed_through, expected_id);
+    EXPECT_FALSE(ckpt->ckpt.checkpoint_snapshot_id.has_value());
+    EXPECT_FALSE(ckpt->ckpt.last_epoch_seal.has_value());
+}
+
+TEST(CASSemanticRefFixture, WrapperAdvancesCheckpointWithoutDiscardingSnapshot)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/semantic-advance@cas@"};
+    const ManifestRef manifest = ref("srv-a:1", 1, 0xAC);
+
+    const uint64_t publish_sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, manifest);
+    const RefTxnId publish_id{manifest.writer_epoch, publish_sequence};
+    writeRefSnapshotRaw(*backend, store->layout(), minimalLiveSnapshot(ns.string(), publish_id, {committedRow("tbl", manifest)}));
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns);
+    const auto before_drop = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(before_drop.has_value());
+    RefCkpt with_snapshot = before_drop->ckpt;
+    with_snapshot.checkpoint_snapshot_id = publish_id;
+    ASSERT_EQ(backend->casPut(
+        store->layout().refCkptKey(life), encodeRefCkpt(with_snapshot), before_drop->token).outcome,
+        CasOutcome::Committed);
+
+    const uint64_t drop_sequence = dropRefTransition(*backend, store->layout(), ns, "tbl", manifest);
+    const RefTxnId drop_id{manifest.writer_epoch, drop_sequence};
+    const auto ckpt = readCkpt(*backend, store->layout(), life);
+
+    ASSERT_TRUE(ckpt.has_value());
+    EXPECT_EQ(ckpt->ckpt.committed_through, drop_id);
+    EXPECT_EQ(ckpt->ckpt.checkpoint_snapshot_id, publish_id);
+    EXPECT_FALSE(ckpt->ckpt.last_epoch_seal.has_value());
+}
+
+TEST(CASSemanticRefFixture, CheckpointAdvanceRejectsNonMonotoneAndInvalidState)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/semantic-refusal@cas@"};
+    const ManifestRef manifest = ref("srv-a:1", 1, 0xAD);
+
+    const uint64_t sequence = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, manifest);
+    const RefTxnId id{manifest.writer_epoch, sequence};
+    EXPECT_THROW(advanceRecoverableCkptForRawFixture(*backend, store->layout(), ns, id), DB::Exception);
+
+    const RootNamespace invalid_ns{"00/semantic-invalid@cas@"};
+    fixture::admitLive(*backend, store->layout(), invalid_ns);
+    const NamespaceLifeId invalid_life = *CasRefCatalog::lifeIfCataloged(*backend, store->layout(), invalid_ns);
+    const String invalid_key = store->layout().refCkptKey(invalid_life);
+    ASSERT_EQ(backend->putIfAbsent(invalid_key, "not a checkpoint").outcome, PutOutcome::Done);
+    EXPECT_THROW(advanceRecoverableCkptForRawFixture(*backend, store->layout(), invalid_ns, id), DB::Exception);
+    EXPECT_EQ(backend->get(invalid_key)->bytes, "not a checkpoint");
+}
+
+TEST(CASRawRefFixture, RawLogWriteDoesNotCreateCheckpoint)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/raw-no-ckpt@cas@"};
+    const RefTxnId id{1, 1};
+
+    fixture::writeRefLogRaw(*backend, store->layout(), RefLogTxn{
+        .ns = ns.string(),
+        .txn_id = id,
+        .ops = {namespaceBirthOp()},
+        .prev_epoch_seal = std::nullopt,
+    });
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns);
+    EXPECT_FALSE(readCkpt(*backend, store->layout(), life).has_value());
+}
+
+TEST(CASRawRefFixture, ReplaceRecoverableCheckpointWritesTheSuppliedFullState)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/replace-ckpt@cas@"};
+    const ManifestRef manifest = ref("srv-a:1", 1, 0xAE);
+    const RefTxnId first_id{manifest.writer_epoch,
+        publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, manifest)};
+    const RefTxnId seal_id{first_id.writer_epoch, first_id.ref_sequence + 1};
+    writeRefSnapshotRaw(*backend, store->layout(), minimalLiveSnapshot(ns.string(), first_id, {committedRow("tbl", manifest)}));
+    writeSealAt(*backend, store->layout(), ns, seal_id);
+
+    const RefCkpt next{
+        .life_epoch = 1,
+        .committed_through = seal_id,
+        .checkpoint_snapshot_id = first_id,
+        .last_epoch_seal = seal_id,
+    };
+    replaceRecoverableCkptForRawFixture(*backend, store->layout(), ns, next);
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns);
+    const auto replaced = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(replaced.has_value());
+    EXPECT_EQ(replaced->ckpt.life_epoch, next.life_epoch);
+    EXPECT_EQ(replaced->ckpt.committed_through, next.committed_through);
+    EXPECT_EQ(replaced->ckpt.checkpoint_snapshot_id, next.checkpoint_snapshot_id);
+    EXPECT_EQ(replaced->ckpt.last_epoch_seal, next.last_epoch_seal);
+}
+
+TEST(CASRawRefFixture, ReplaceRecoverableCheckpointRejectsStaleRegressiveAndWrongLife)
+{
+    auto backend = std::make_shared<CkptReplacementConflictBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/replace-ckpt-refusal@cas@"};
+    const ManifestRef manifest = ref("srv-a:1", 1, 0xAF);
+    const RefTxnId id{manifest.writer_epoch,
+        publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, manifest)};
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns);
+    const auto existing = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(existing.has_value());
+
+    RefCkpt wrong_life = existing->ckpt;
+    wrong_life.life_epoch = *wrong_life.life_epoch + 1;
+    wrong_life.committed_through = RefTxnId{*wrong_life.life_epoch, 1};
+    EXPECT_THROW(replaceRecoverableCkptForRawFixture(*backend, store->layout(), ns, wrong_life), DB::Exception);
+
+    RefCkpt regressive = existing->ckpt;
+    regressive.committed_through = std::nullopt;
+    EXPECT_THROW(replaceRecoverableCkptForRawFixture(*backend, store->layout(), ns, regressive), DB::Exception);
+
+    backend->watched_key = store->layout().refCkptKey(life);
+    backend->conflict_once = true;
+    EXPECT_THROW(replaceRecoverableCkptForRawFixture(*backend, store->layout(), ns, existing->ckpt), DB::Exception);
+    EXPECT_EQ(readCkpt(*backend, store->layout(), life)->ckpt.committed_through, id);
+}
+
+/// The owner-removed manifest body is deleted only after a full round (its decrement is sealed — #11).
+TEST(CASGCRetire, ManifestBodyDeletedAfterDecrementsSealed)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    runRegularRoundReclaiming(gc);
+    EXPECT_FALSE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+}
+
+/// A publish racing the pass (in-degree restored) is SPARED, not deleted (#14).
+TEST(CASGCRecheck, PublishRacingFenceSparesBlob)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref("srv-a:1", 1, 0xA1);
+    const ManifestRef r2 = ref("srv-a:1", 2, 0xA2);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    // Repoint the ref from r1 to r2 (both reference blob 1) in the same window before the next round
+    // folds. ONE repoint event {old=committed(r1), new=committed(r2)} — the -1 (r1's body) and +1
+    // (r2's body) net to in-degree 1, so blob 1 is re-pinned and must be SPARED. (Not a separate drop
+    // THEN repoint — that would double-count the -1 on r1's body and over-delete.)
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", r1, r2);
+    gc.runRegularRound();   // net in-degree 1 => spared
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+}
+
+/// A genuinely unreferenced blob is deleted with its exact token (the single content-delete site). The
+/// delete is not one-round-after-drop: the entry condemns, graduates the round AFTER the condemning
+/// round (round-paced, unconditional), then the NEXT pass executes the exact-token delete.
+TEST(CASGCRecheck, UnreferencedBlobDeletedExactToken)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    // The drop's -1 condemns blob 1; the retired-cursor pipeline (condemn -> graduate -> delete) reclaims it.
+    EXPECT_TRUE(runRoundsUntilAbsent(store, gc, *backend, store->layout(), DB::UInt128(1)));
+    EXPECT_FALSE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+}
+
+/// Task 5 (spec 2026-07-09 §raw-body-refinement, v3): GC writes the writer's freshness meta ALONGSIDE
+/// the unchanged ledger retire (RetiredEntry, body token) — the meta is the writer/promote gate's
+/// point-read signal (Task 3/4), not a replacement for the ledger or the exact-token body delete.
+TEST(CASGCRetire, CondemnWritesMetaCondemned)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();   /// +1 folds; blob referenced (`writeBlobBody` never wrote a meta itself)
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    gc.runRegularRound();   /// -1 folds => in-degree 0 => condemned THIS round
+
+    const auto lm = loadMetaForTest(*backend, store->layout(), DB::UInt128(1));
+    ASSERT_TRUE(lm.has_value()) << "GC must write the freshness meta at condemn time (Task 5)";
+    EXPECT_EQ(lm->meta.state, MetaState::Condemned);
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1))) << "condemned, NOT yet deleted";
+}
+
+/// Task 5: the round's exact-token body delete drops the meta alongside it (advisory, no tombstone —
+/// an absent meta reads exactly like a Clean one for the writer's point-read gate).
+TEST(CASGCRetire, DeleteRemovesBodyAndMeta)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    /// §0 introspection: the meta drop below rides `deleteMetaExact` (`CASMetaDelete` choke point).
+    const auto delete_before = ProfileEvents::global_counters[ProfileEvents::CASMetaDelete].load();
+    // condemn -> graduate (round-paced) -> delete (the retired-cursor pipeline).
+    ASSERT_TRUE(runRoundsUntilAbsent(store, gc, *backend, store->layout(), DB::UInt128(1)));
+
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1))) << "body gone via exact-token delete";
+    EXPECT_FALSE(loadMetaForTest(*backend, store->layout(), DB::UInt128(1)).has_value())
+        << "the meta must be dropped alongside the exact-token body delete (Task 5)";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMetaDelete].load() - delete_before, 1);
+}
+
+/// GC freshness meta is ADD-ONLY (spec 2026-07-11 deposed-leader `clearSparedMeta` fix): an entry whose
+/// in-degree recovers before graduation is SPARED (unchanged ledger behavior) but GC must NEVER flip its
+/// meta `Condemned -> Clean` on the spare. A deposed leader that cleared-then-lost the round would leave a
+/// stray-`Clean` over a still-condemned body; a writer reading `Clean` would reuse the exact condemned
+/// token, which a stale exact-token redelete then deletes (INV_NO_LOSS live-blob loss).
+/// The spare leaves the meta `Condemned`;
+/// ONLY a writer that displaces the body with a fresh incarnation token publishes `Clean`.
+TEST(CASGCRetire, SpareLeavesMetaCondemned)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    /// A content-addressed body + Clean meta via a real fresh upload, so a later writer dedup-attempt
+    /// resolves to THIS exact hash (GC condemns it; the writer resurrects it).
+    const String payload = "spare-add-only-payload";
+    const UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+    {
+        auto seed = store->beginPartWrite({});
+        seed->putBlob(id, BlobSource::fromString(payload));
+    }
+    const Token t_seed = backend->head(store->layout().blobKey(id)).token;
+
+    const ManifestRef r1 = ref("srv-a:1", 1, 0xA1);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", hash)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+    gc.runRegularRound();   /// +1 folds
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r1);
+    gc.runRegularRound();   /// -1 folds => in-degree 0 => condemned; meta flipped Condemned
+    ASSERT_TRUE(currentEntryFor(*backend, store->layout(), hash).has_value());
+    {
+        const auto lm = loadMetaForTest(*backend, store->layout(), hash);
+        ASSERT_TRUE(lm.has_value());
+        ASSERT_EQ(lm->meta.state, MetaState::Condemned);
+    }
+
+    /// Re-reference the SAME blob (same body/token — never re-uploaded) via a fresh ref before
+    /// graduation. The pass merge nets in-degree back to 1: the prior retired entry is SPARED
+    /// (recovery wins, even past the floor) -- not the resurrect-supersede path (the token never changed).
+    const ManifestRef r2 = ref("srv-a:1", 2, 0xA2);
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", hash)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_detached", std::nullopt, r2);
+    gc.runRegularRound();   /// +1 folds => spared
+
+    EXPECT_FALSE(currentEntryFor(*backend, store->layout(), hash).has_value())
+        << "the spared entry drops from the retired set";
+    EXPECT_TRUE(blobExists(*backend, store->layout(), hash));
+    EXPECT_EQ(backend->head(store->layout().blobKey(id)).token, t_seed)
+        << "spare does not touch the body — the incarnation token is unchanged";
+
+    /// ADD-ONLY: the spare must NOT clear the meta back to Clean (that is the deposed-leader hole).
+    {
+        const auto lm = loadMetaForTest(*backend, store->layout(), hash);
+        ASSERT_TRUE(lm.has_value());
+        EXPECT_EQ(lm->meta.state, MetaState::Condemned)
+            << "GC freshness meta is add-only: a spare leaves the meta Condemned (never -> Clean)";
+    }
+
+    /// Only a WRITER re-publishes Clean, and only by displacing the body with a fresh incarnation token:
+    /// a dedup-attempt on the condemned hash resurrects (uploadFromSource) — the body token CHANGES and
+    /// the meta flips to Clean WITH that token change.
+    auto build = store->beginPartWrite({});
+    auto ref_w = build->putBlob(id, BlobSource::fromString(payload));
+    EXPECT_EQ(ref_w.ref, id);
+    const Token t_resurrect = backend->head(store->layout().blobKey(id)).token;
+    EXPECT_NE(t_resurrect, t_seed) << "resurrect displaces the body with a fresh incarnation token";
+    const auto lm_after = loadMetaForTest(*backend, store->layout(), hash);
+    ASSERT_TRUE(lm_after.has_value());
+    EXPECT_EQ(lm_after->meta.state, MetaState::Clean)
+        << "the writer's resurrect path is the SOLE Condemned -> Clean transition";
+}
+
+/// Two-leader stale-redelete regression — the executable form of the deposed-leader spec §2. A stale
+/// leader's pre-CAS exact-token redelete `deleteExact(h, t1)` must never delete a live reuse. With the
+/// buggy clear-on-spare, a spare publishes `Clean`; a writer reads `Clean` and REUSES `t1`; the stale
+/// `deleteExact(t1)` then deletes the LIVE body (INV_NO_LOSS). Add-only meta closes it: the spare leaves
+/// `Condemned`, the writer resurrects to `t2`, and the stale `deleteExact(t1)` is a `TokenMismatch` no-op.
+///
+/// Interleaving fidelity (APPROXIMATED): the deposed leader's destructive side effect is its pre-CAS
+/// exact-token `deleteExact(h, t1)`. We reproduce it deterministically by CAPTURING `t1` at condemn time
+/// (exactly the token a paused leader's `delete_pending` snapshot holds) and firing that exact
+/// `deleteExact` AFTER the surviving leader's spare and the writer's resurrect — the faithful destructive
+/// op, without a mid-round CAS-interrupt seam on the delete path (which the backend does not expose).
+TEST(CASGCRetire, StaleRedeleteAfterSpareDoesNotDeleteLiveReuse)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const String payload = "two-leader-stale-redelete-payload";
+    const UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+    const String blob_key = store->layout().blobKey(id);
+    {
+        auto seed = store->beginPartWrite({});
+        seed->putBlob(id, BlobSource::fromString(payload));
+    }
+
+    const ManifestRef r1 = ref("srv-a:1", 1, 0xA1);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", hash)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+    gc.runRegularRound();   /// +1 folds
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r1);
+    gc.runRegularRound();   /// -1 => in-degree 0 => condemned at t1
+
+    /// The OLD leader L1's planned pre-CAS delete uses the EXACT token it observed at condemn: capture t1.
+    const auto condemned_entry = currentEntryFor(*backend, store->layout(), hash);
+    ASSERT_TRUE(condemned_entry.has_value());
+    const Token t1 = condemned_entry->token;
+    ASSERT_EQ(backend->head(blob_key).token, t1);
+
+    /// A NEW leader L2 folds a +1 that recovered h's in-degree and adopts a SPARE for h.
+    const ManifestRef r2 = ref("srv-a:1", 2, 0xA2);
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", hash)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_live", std::nullopt, r2);
+    gc.runRegularRound();   /// +1 => spared
+
+    /// Add-only: the spare left the meta Condemned (the stale-redelete guard depends on it).
+    {
+        const auto lm = loadMetaForTest(*backend, store->layout(), hash);
+        ASSERT_TRUE(lm.has_value());
+        EXPECT_EQ(lm->meta.state, MetaState::Condemned)
+            << "add-only: a spare must not clear the meta (the writer must still see the hash condemned)";
+    }
+
+    /// A writer dedup-hits h. It point-reads Condemned and RESURRECTS to a fresh token t2
+    /// (uploadFromSource) — it never reuses t1.
+    {
+        auto build = store->beginPartWrite({});
+        build->putBlob(id, BlobSource::fromString(payload));
+    }
+    const Token t2 = backend->head(blob_key).token;
+    EXPECT_NE(t2, t1) << "the writer resurrected to a fresh incarnation, not a reuse of t1";
+
+    /// L1 resumes and executes its stale pre-CAS exact-token redelete `deleteExact(h, t1)`: it must be a
+    /// TokenMismatch no-op (the live body is now t2), NEVER a Deleted of the live reuse.
+    const DeleteOutcome stale = backend->deleteExact(blob_key, t1);
+    EXPECT_EQ(stale.kind, DeleteOutcome::Kind::TokenMismatch)
+        << "the stale exact-token redelete must miss the live reuse (add-only closes INV_NO_LOSS)";
+
+    /// The live body under t2 survives, stays reachable via the committed r2, and fsck sees no dangle.
+    const HeadResult hr = backend->head(blob_key);
+    ASSERT_TRUE(hr.exists);
+    EXPECT_EQ(hr.token, t2);
+    replaceRecoverableCkptForRawFixture(
+        *backend, store->layout(), ns,
+        RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 3},
+                .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+    EXPECT_EQ(runFsck(*store, /*detail=*/false).dangling, 0u)
+        << "no live reference dangles: the stale redelete did not delete the reused body";
+}
+
+/// Copy-forward aftermath, republished arm (spec 2026-07-02-cas-copy-forward-condemned-evidence.md):
+/// after a condemned incarnation (hash, t0) is displaced by a verified copy-forward (fresh token t1)
+/// and the republished part's +1 lands, the listed (hash, t0) entry settles WITHOUT touching the new
+/// incarnation: its exact-token delete is a mismatch no-op and the entry drops; the blob survives at t1.
+TEST(CASGCRetire, CopyForwardedBlobSurvivesWhenRepublished)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref("srv-a:1", 1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r1);
+    gc.runRegularRound();   /// -1 folds => in-degree 0 => entry (1, t0) condemned
+    ASSERT_TRUE(currentEntryFor(*backend, store->layout(), DB::UInt128(1)).has_value());
+
+    /// The raw equivalent of a writer resurrect (PartWriteTxn::uploadFromSource): displace EXACTLY t0 with the
+    /// same verified bytes under a fresh token t1, then republish a part referencing the blob (the
+    /// promoted dst ref of a republishRef move).
+    const String blob_key = store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(1))});
+    const Token t0 = backend->head(blob_key).token;
+    const auto res = backend->putOverwrite(blob_key, backend->get(blob_key)->bytes, t0);
+    ASSERT_EQ(res.outcome, PutOutcome::Done);
+    const ManifestRef r2 = ref("srv-a:1", 2, 0xA2);
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_detached", std::nullopt, r2);
+
+    /// The +1 folds => spared; the (1, t0) entry drops; the t1 incarnation is never deleted.
+    for (int i = 0; i < 4; ++i)
+        gc.runRegularRound();
+    EXPECT_FALSE(currentEntryFor(*backend, store->layout(), DB::UInt128(1)).has_value());
+    const HeadResult hr = backend->head(blob_key);
+    ASSERT_TRUE(hr.exists);
+    EXPECT_EQ(hr.token, res.token);
+}
+
+/// Copy-forward aftermath, stale-entry arm: a listed (hash, t0) entry whose incarnation was
+/// displaced (token now t1) with NO accompanying owner events. The entry graduates and its
+/// exact-token delete MISMATCHES — a no-op, the entry drops, the t1 incarnation is NEVER
+/// wrong-token-deleted (no wedge, no unsafe delete). This is a RAW-displacement model, stronger
+/// than the real flow: in real `republishRef` the dst precommit + body are durable BEFORE the
+/// promote pre-pass runs (reachability-before-content, B188), so an abandoned real copy-forward
+/// is fully reclaimed by the pipeline (+1 spare -> reclaim -1 -> transition to zero -> fresh
+/// (hash, t1) entry -> exact delete). The raw shape pins the GC-side invariant in isolation.
+TEST(CASGCRetire, AbandonedCopyForwardDropsEntryWithoutWrongTokenDelete)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref("srv-a:1", 1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r1);
+    runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(currentEntryFor(*backend, store->layout(), DB::UInt128(1)).has_value());
+
+    const String blob_key = store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(1))});
+    const Token t0 = backend->head(blob_key).token;
+    const auto res = backend->putOverwrite(blob_key, backend->get(blob_key)->bytes, t0);
+    ASSERT_EQ(res.outcome, PutOutcome::Done);
+
+    /// No events land at all (raw displacement). Drive rounds with the store's ack kept current so
+    /// the (1, t0) entry graduates; its exact-token delete mismatches t1 and the entry drops.
+    for (int i = 0; i < 6; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+    }
+    EXPECT_FALSE(currentEntryFor(*backend, store->layout(), DB::UInt128(1)).has_value())
+        << "the stale (hash, t0) entry must settle (mismatch redelete drops it), not wedge the list";
+    const HeadResult hr = backend->head(blob_key);
+    ASSERT_TRUE(hr.exists) << "the fresh incarnation must never be deleted under the stale token";
+    EXPECT_EQ(hr.token, res.token);
+}
+
+/// A completed round adopts the SAME attempt its fold minted (the round's single gc/state CAS commits the
+/// fold's (snap_generation, snap_attempt) together). Completion seals are a retired concept, so the durable
+/// index of the adopted round is the FOLD seal at (snap_generation, snap_attempt). Across rounds each
+/// `runRegularRound` re-acquires the lease (bumping `lease.seq`), so a later round mints a FRESH attempt.
+TEST(CASGCRecheck, CompletionInheritsFoldAttempt)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+
+    gc.runRegularRound();          // round 1: one pass, single CAS commits (snap_generation, snap_attempt)
+    const auto after_round1 = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    // The round adopted the attempt of THIS round's fold: snap_attempt == the lease.seq that folded it.
+    EXPECT_EQ(after_round1.snap_attempt, after_round1.lease.seq);
+    EXPECT_GT(after_round1.snap_generation, 0u);
+    // The fold seal is durable under the adopted (snap_generation, snap_attempt) pair (no completion seal).
+    EXPECT_TRUE(backend->head(store->layout()
+        .foldSealKey(after_round1.snap_generation, after_round1.snap_attempt)).exists);
+
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    gc.runRegularRound();          // round 2: re-acquire (bump lease.seq) -> fresh attempt at its fold
+    const auto after_round2 = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    EXPECT_EQ(after_round2.snap_attempt, after_round2.lease.seq);
+    EXPECT_GT(after_round2.snap_attempt, after_round1.snap_attempt);   // per-round monotone attempt
+    EXPECT_GT(after_round2.snap_generation, after_round1.snap_generation);
+    EXPECT_TRUE(backend->head(store->layout()
+        .foldSealKey(after_round2.snap_generation, after_round2.snap_attempt)).exists);
+}
+
+/// ---- round-paced graduation suite (spec 2026-07-02 + Task-9 amendment; re-keyed off acks in v3 Task 6) ----
+
+/// A regular round performs NO writes to the ref objects: ref state is writer-owned (immutable
+/// `_log`/`_snap`), and GC only reads it (plus deletes covered objects via ref-object cleanup, which
+/// needs a covering snapshot -- none exists here). So a no-op round adds and removes NO ref object.
+TEST(CASGCAckFloor, NoOpRoundDoesNotMutateRefShards)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt,
+        ManifestRef{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1});
+    Gc gc(store, kGc);
+    gc.runRegularRound();   // first round folds the publish
+
+    const auto listRefKeys = [&]
+    {
+        std::set<String> keys;
+        String cursor;
+        for (;;)
+        {
+            const ListPage page = backend->list(store->layout().namespaceStreamPrefix(fixture::fixtureLife(ns)), cursor, 1000);
+            for (const ListedKey & lk : page.keys)
+                keys.insert(lk.key);
+            if (page.next_cursor.empty())
+                break;
+            cursor = page.next_cursor;
+        }
+        return keys;
+    };
+
+    const std::set<String> before = listRefKeys();
+    ASSERT_FALSE(before.empty()) << "the publish must have written at least one ref object";
+
+    gc.runRegularRound();   // a second, no-op round must not add or remove any ref object
+    const std::set<String> after = listRefKeys();
+    EXPECT_EQ(before, after) << "a no-op GC round must not mutate the table's ref objects";
+    // The registry object is gone (Task 4); the fence never existed to write it.
+    EXPECT_FALSE(backend->get("p/gc/registry").has_value());
+}
+
+/// The canonical pipeline: a blob condemned at round K stays present after the condemning round; the
+/// VERY NEXT round graduates it (round-paced, unconditional — condemn_round < current_round the first
+/// round current_round exceeds it) and publishes it delete_pending — the blob still exists; the round
+/// AFTER THAT executes the exact-token delete and the blob becomes absent. This pins the critical
+/// off-by-one: current_round MUST equal state.round + 1 (the SAME basis condemn_round is stamped at),
+/// so an entry graduates exactly one round after it was condemned — never the same round, never never.
+TEST(CASGCAckFloor, CondemnThenGraduatesNextRoundThenDeletes)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+
+    runRegularRoundReclaiming(gc);                 // round 1: folds the +1; blob referenced
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+
+    // The condemning round: the -1 drops in-degree to 0; the blob is condemned into the current retired
+    // list but NOT deleted. The entry is present and NOT yet pending. report.condemned counts it.
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        EXPECT_EQ(rep.condemned, 1u);        // one blob condemned this round
+        EXPECT_EQ(rep.graduated, 0u);        // must NOT graduate the same round it was condemned
+        EXPECT_EQ(rep.redeleted, 0u);        // nothing pending to delete yet
+        EXPECT_TRUE(blobExists(*backend, store->layout(), blob));
+        const auto e = currentEntryFor(*backend, store->layout(), blob);
+        ASSERT_TRUE(e.has_value());
+        EXPECT_FALSE(e->delete_pending);   // condemned, not yet graduated
+    }
+
+    // The VERY NEXT round graduates it deterministically (no ack/heartbeat dependency).
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        EXPECT_EQ(rep.graduated, 1u);
+        EXPECT_EQ(rep.redeleted, 0u);   // the delete lands on the NEXT pass, not this one
+        const auto e = currentEntryFor(*backend, store->layout(), blob);
+        ASSERT_TRUE(e.has_value());
+        EXPECT_TRUE(e->delete_pending);
+        EXPECT_TRUE(blobExists(*backend, store->layout(), blob));   // pending: still present this pass
+    }
+
+    // The pass AFTER the pending publish executes the exact-token delete; the blob becomes absent and the
+    // entry is dropped from the current retired list. report.redeleted counts the executed pending delete.
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        EXPECT_EQ(rep.redeleted, 1u);        // the pending delete executed this round
+        EXPECT_FALSE(blobExists(*backend, store->layout(), blob));
+        EXPECT_FALSE(currentEntryFor(*backend, store->layout(), blob).has_value());
+    }
+}
+
+/// End-to-end through the real round driver (`Gc::runRegularRound`) rather than
+/// `foldDeltasIntoGeneration` directly: a cohort well past `gc_round_redelete_budget` still drains
+/// completely, but no single round's `redeleted` count exceeds the cap — the same convergence the
+/// merge-level `CASThreeCursorMerge` budget tests pin, proven through the production entry point.
+TEST(CASGCAckFloor, RedeleteBudgetCapsRoundDrainAndConverges)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    constexpr uint64_t kCap = 5;
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_round_redelete_budget = kCap});
+    const RootNamespace ns{"00/aa@cas@"};
+
+    constexpr uint64_t kCohort = 20;
+    std::vector<UInt128> blobs;
+    std::vector<ManifestRef> refs;
+    for (uint64_t i = 1; i <= kCohort; ++i)
+    {
+        const UInt128 blob(i);
+        const ManifestRef r = ref("srv-a:1", i, static_cast<uint32_t>(i));
+        blobs.push_back(blob);
+        refs.push_back(r);
+        writeBlobBody(*backend, store->layout(), blob);
+        writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+        publishCommittedTransition(*backend, store->layout(), ns, "tbl" + std::to_string(i), std::nullopt, r);
+    }
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);   // round 1: folds every publish, every blob referenced
+
+    for (uint64_t i = 1; i <= kCohort; ++i)
+        dropRefTransition(*backend, store->layout(), ns, "tbl" + std::to_string(i), refs[i - 1]);
+
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);   // condemning round
+        EXPECT_EQ(rep.condemned, kCohort);
+        EXPECT_EQ(rep.graduated, 0u);
+        EXPECT_EQ(rep.redeleted, 0u);
+    }
+    {
+        // Graduation has no budget configured in this test (default 0 = unbounded) — the whole
+        // cohort graduates together, isolating the redelete cap as the only thing under test.
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        EXPECT_EQ(rep.graduated, kCohort);
+        EXPECT_EQ(rep.redeleted, 0u);
+    }
+
+    uint64_t total_redeleted = 0;
+    uint64_t rounds = 0;
+    while (total_redeleted < kCohort && rounds < 10)
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        EXPECT_LE(rep.redeleted, kCap) << "a round must never redelete past gc_round_redelete_budget";
+        total_redeleted += rep.redeleted;
+        ++rounds;
+    }
+    EXPECT_EQ(total_redeleted, kCohort) << "no entry lost to the cap across the whole drain";
+    EXPECT_EQ(rounds, kCohort / kCap) << "ceil(20 / 5) rounds to fully drain the cohort";
+    for (const UInt128 & blob : blobs)
+        EXPECT_FALSE(blobExists(*backend, store->layout(), blob));
+}
+
+/// A publish re-referencing the condemned blob before graduation is folded and SPARES the entry: the entry
+/// is dropped (recovery wins even past graduation) and the blob survives.
+TEST(CASGCAckFloor, PublishBeforeGraduationSpares)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref("srv-a:1", 1, 0xA1);
+    const ManifestRef r2 = ref("srv-a:1", 2, 0xA2);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+
+    gc.runRegularRound();
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r1);
+    gc.runRegularRound();   // condemns blob 1 (in-degree 0)
+    store->renewWatermarkOnce();
+    ASSERT_TRUE(currentEntryFor(*backend, store->layout(), blob).has_value());
+
+    // Re-publish a committed ref pointing at the same blob BEFORE it graduates: the next pass folds the +1,
+    // the merge sees in-degree 1, and the entry is spared (dropped from the retired list).
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r2);
+    gc.runRegularRound();
+    store->renewWatermarkOnce();
+    EXPECT_FALSE(currentEntryFor(*backend, store->layout(), blob).has_value());   // spared: entry dropped
+    EXPECT_TRUE(blobExists(*backend, store->layout(), blob));
+
+    // Keep running: the re-referenced blob must never be deleted.
+    for (int i = 0; i < 4; ++i)
+    {
+        gc.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(blobExists(*backend, store->layout(), blob));
+}
+
+/// A dead mount is fenced out by the round's heartbeat step: gc_fenced is set on its body (a
+/// token-guarded rewrite that bumps seq). The fence is pure liveness (re-arms the write fence so a
+/// resumed sleeper can never mutate again); reclaim itself no longer depends on any mount's heartbeat —
+/// graduation paces on GC rounds. The fenced mount's own subsequent renew then fails closed, because the
+/// fence invalidated the token it held.
+///
+/// Rev.6 §token-stability observation (Task 9): the fence-out no longer trusts a bare wall-clock stamp
+/// (`expires_at_ms`) against the GC's own clock — it fences ONLY once GC has watched a mount's write
+/// token hold unchanged for the full threshold on its OWN monotonic clock. That takes (at least) two
+/// `computeHeartbeatFloor` calls spanning the threshold, so this test drives the GC leader's own
+/// (persistent) `mono_ms_fn` across two rounds: round 1 seeds the observation for both mounts; the
+/// STORE's own mount is then renewed (as a live leader would) before round 2 crosses the threshold —
+/// srid2, never renewed again after its one-shot claim, is the one that gets fenced.
+TEST(CASGCAckFloor, ExpiredMountFencedOutAndExcluded)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    std::vector<CasEvent> events;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+
+    // srid2's keeper claims ONE lease via `start()` and is never renewed again — tests never enable
+    // the background renewal thread (`background_watermark` defaults to false), so this alone models a
+    // crashed process: a body that is live-shaped (not terminated, not fenced) but whose write token
+    // never changes again.
+    const String srid2 = "stale-server";
+    MountLeaseKeeper srid2_keeper(backend, layout, srid2, DB::UInt128(0x2222), /*writer_epoch=*/1,
+        std::chrono::milliseconds(100), [] { return 1000u; }, [] { return 0u; });
+    srid2_keeper.start();
+    ASSERT_FALSE(decodeMountLease(backend->get(layout.mountKey(srid2))->bytes).gc_fenced);
+
+    // The fence-out threshold on the GC leader's OWN monotonic clock — mirrors the production formula
+    // in `Gc::runRegularRound` (ttl + 5% drift allowance + one round's worth of renewal slack).
+    const uint64_t ttl_ms = static_cast<uint64_t>(store->poolConfig().mount_lease_ttl_ms.count());
+    const uint64_t threshold_ms = ttl_ms + ttl_ms / 20
+        + static_cast<uint64_t>(store->poolConfig().mount_renew_period.count());
+
+    uint64_t gc_now = 1'000'000;   // audit-only wall clock; never gates the fence decision
+    uint64_t gc_mono = 0;
+    Gc gc(store, kGc, [&] { return gc_now; }, [&] { return gc_mono; });
+
+    // Capture the emitted events so we can assert the round emits exactly one GcFenceOut row for srid2.
+    store->setEventSink([&](const CasEvent & e) { events.push_back(e); });
+
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(7);
+    writeBlobBody(*backend, layout, blob);
+    writeManifestRaw(*backend, layout, ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r);
+
+    // Round 1 (mono 0): first sight of both mounts — observation starts, nothing fenced yet.
+    const RoundReport rep1 = gc.runRegularRound();
+    EXPECT_EQ(rep1.fence_outs, 0u);
+
+    // The store's OWN mount renews between rounds (as a live leader would); srid2 never does.
+    store->renewWatermarkOnce();
+    gc_mono = threshold_ms;
+
+    // Round 2 (mono == threshold): srid2's original token has held stable for the full threshold —
+    // fenced. The store's own (just-renewed) mount restarts its observation and stays live.
+    const RoundReport rep = gc.runRegularRound();
+
+    EXPECT_EQ(rep.fence_outs, 1u);   // exactly one dead mount fenced-out this round
+    const MountLease fenced = decodeMountLease(backend->get(layout.mountKey(srid2))->bytes);
+    EXPECT_TRUE(fenced.gc_fenced);
+
+    // Exactly one GcFenceOut audit row was emitted, naming srid2 in its detail.
+    size_t fence_out_rows = 0;
+    for (const CasEvent & e : events)
+        if (e.type == CasEventType::GcFenceOut)
+        {
+            ++fence_out_rows;
+            EXPECT_EQ(e.outcome, "fenced");
+            EXPECT_FALSE(e.reason.empty());
+            const auto it = e.detail.find("server_root_id");
+            ASSERT_NE(it, e.detail.end());
+            EXPECT_EQ(it->second, srid2);
+        }
+    EXPECT_EQ(fence_out_rows, 1u);
+
+    // srid2's writer comes back and tries to renew: its held token was invalidated by the fence rewrite,
+    // so renewOnce fails closed. (It renews on its own clock; liveness is irrelevant — the token guard
+    // trips regardless.)
+    EXPECT_THROW(srid2_keeper.renewOnce(), DB::Exception);
+
+    // The fence-out is pure liveness cleanup: reclaim proceeds through the normal (round-paced) pipeline
+    // regardless of srid2's fate — fencing one stale mount must never wedge the reclaim pipeline.
+    dropRefTransition(*backend, layout, ns, "tbl", r);
+    EXPECT_TRUE(runRoundsUntilAbsent(store, gc, *backend, layout, blob));
+}
+
+/// fix-round F6 (author-review: `Gc`'s own `mono_ms_fn` used to default to the RAW static `Pool::
+/// bootMs()`, bypassing the Pool's own injectable `config.boot_ms_fn` -- a time-controlled test can
+/// desync the mount side's fake clock from the GC side's real one). This mirrors
+/// `ExpiredMountFencedOutAndExcluded` exactly, except: the Pool is opened with an injected
+/// `boot_ms_fn` driving a FAKE clock that barely advances in real time, and `Gc` is constructed WITHOUT
+/// an explicit `mono_ms_fn` -- exercising the DEFAULT under test. If the default still read the real
+/// wall clock, this round would see essentially zero elapsed mono time and never cross the fence-out
+/// threshold; the fix makes it default to `store->bootMsNow()`, which tracks the SAME fake clock.
+TEST(CASGCAckFloor, DefaultMonoClockTracksPoolsInjectedBootClockNotWallClock)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    uint64_t fake_boot = 0;
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+        .boot_ms_fn = [&] { return fake_boot; }});
+    const Layout & layout = store->layout();
+
+    // A stale mount, exactly as `ExpiredMountFencedOutAndExcluded`: one claim, never renewed again.
+    const String srid2 = "stale-server";
+    MountLeaseKeeper srid2_keeper(backend, layout, srid2, DB::UInt128(0x2222), /*writer_epoch=*/1,
+        std::chrono::milliseconds(100), [] { return 1000u; }, [&] { return fake_boot; });
+    srid2_keeper.start();
+    ASSERT_FALSE(decodeMountLease(backend->get(layout.mountKey(srid2))->bytes).gc_fenced);
+
+    const uint64_t ttl_ms = static_cast<uint64_t>(store->poolConfig().mount_lease_ttl_ms.count());
+    const uint64_t threshold_ms = ttl_ms + ttl_ms / 20
+        + static_cast<uint64_t>(store->poolConfig().mount_renew_period.count());
+
+    // `Gc` constructed with only `now_ms_fn` -- `mono_ms_fn` is left at its DEFAULT (the fix under test).
+    Gc gc(store, kGc, [] { return 1'000'000u; });
+
+    const RoundReport rep1 = gc.runRegularRound();
+    EXPECT_EQ(rep1.fence_outs, 0u);
+
+    store->renewWatermarkOnce();
+    fake_boot = threshold_ms;   // advance the FAKE clock only; this test runs in well under a millisecond
+
+    const RoundReport rep2 = gc.runRegularRound();
+    EXPECT_EQ(rep2.fence_outs, 1u)
+        << "Gc's default mono_ms_fn must track the Pool's injected boot clock, not the real wall clock";
+    EXPECT_TRUE(decodeMountLease(backend->get(layout.mountKey(srid2))->bytes).gc_fenced);
+}
+
+/// deleteExact against a blob the writer RECREATED (fresh incarnation, different token) between the pending
+/// publish and the deleting pass lands TokenMismatch — a terminal-OK outcome recorded as a replace: the
+/// fresh incarnation is a live object and survives. report.replaced counts it.
+TEST(CASGCAckFloor, RecreatedBlobDeleteIsTokenMismatchOk)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    const BlobRef blob_id{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob)};
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    runRegularRoundReclaiming(gc);   // condemn (captures the ORIGINAL token)
+    store->renewWatermarkOnce();
+
+    // Drive rounds until the entry is delete_pending (the token it holds is the original observation).
+    bool pending = false;
+    for (int i = 0; i < 6 && !pending; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+        const auto e = currentEntryFor(*backend, store->layout(), blob);
+        pending = e && e->delete_pending;
+    }
+    ASSERT_TRUE(pending);
+
+    // The writer recreates the blob with a FRESH incarnation before the deleting pass: the current token no
+    // longer matches the pending entry's captured token.
+    displaceBlobToken(*backend, store->layout(), blob_id);
+
+    // The deleting pass issues deleteExact(entry.token) → TokenMismatch → Replaced. The fresh incarnation
+    // survives; the entry is dropped.
+    const RoundReport rep = runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    EXPECT_EQ(rep.replaced, 1u);
+    EXPECT_TRUE(blobExists(*backend, store->layout(), blob));   // the recreated incarnation is live
+    EXPECT_FALSE(currentEntryFor(*backend, store->layout(), blob).has_value());
+}
+
+/// Idempotent replay of a crashed round: a fresh Gc instance (new lease seq = new attempt) re-runs a round
+/// and completes; a delete that already landed under a prior pass replays onto NotFound (Absent outcome)
+/// and the round still completes. We model the crash-after-delete-before-CAS replay by manually deleting
+/// the pending blob (its exact token) BEFORE the deleting pass, then asserting the pass reports the delete
+/// as absent (report.absent == 1) and completes (round advances).
+TEST(CASGCAckFloor, ResumeAfterCrashBetweenRetiredPutAndStateCas)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    const BlobRef blob_id{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob)};
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    // A fresh Gc per round (each acquires the lease, bumping lease.seq = a fresh attempt) — the replay
+    // property: no wedging, each round completes under its own fresh attempt.
+    {
+        Gc gc(store, kGc);
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+    }
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+
+    RetiredEntry pending_entry;
+    bool pending = false;
+    for (int i = 0; i < 6 && !pending; ++i)
+    {
+        Gc gc(store, kGc);
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+        const auto e = currentEntryFor(*backend, store->layout(), blob);
+        if (e && e->delete_pending)
+        {
+            pending = true;
+            pending_entry = *e;
+        }
+    }
+    ASSERT_TRUE(pending);
+
+    // Simulate a crashed deleting pass that DID land the exact-token delete but crashed before the gc/state
+    // CAS. The next (fresh-attempt) pass replays the delete → the object is already gone → NotFound → the
+    // pass records Absent and completes.
+    ASSERT_EQ(backend->deleteExact(store->layout().blobKey(blob_id), pending_entry.token).kind,
+              DeleteOutcome::Kind::Deleted);
+
+    const uint64_t round_before = decodeGcState(backend->get(store->layout().gcStateKey())->bytes).round;
+    Gc gc2(store, kGc);
+    const RoundReport rep = runRegularRoundReclaiming(gc2);
+    store->renewWatermarkOnce();
+    EXPECT_EQ(rep.absent, 1u);   // the replayed delete found the object already gone
+    const uint64_t round_after = decodeGcState(backend->get(store->layout().gcStateKey())->bytes).round;
+    EXPECT_GT(round_after, round_before);   // the round completed (no wedge)
+    EXPECT_FALSE(currentEntryFor(*backend, store->layout(), blob).has_value());
+}
+
+/// Backend-agnostic regression for the rustfs 412-on-absent quirk: a conditional exact-token delete
+/// against an object that is ALREADY absent answers `TokenMismatch`, not `NotFound`, on this backend
+/// (`TokenMismatchOnAbsentBackend` reproduces it deterministically). The redelete site must disambiguate
+/// via a follow-up HEAD: the object is truly gone, so the outcome must settle as Absent (never Replaced)
+/// and the `.meta` cleanup (gated on Deleted/NotFound) must still run.
+TEST(CASGCAckFloor, TokenMismatchOnAbsentBlobSettlesAsAbsentAndDropsMeta)
+{
+    auto backend = std::make_shared<TokenMismatchOnAbsentBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    const BlobRef blob_id{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob)};
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    runRegularRoundReclaiming(gc);   // condemn
+    store->renewWatermarkOnce();
+
+    // Drive rounds until the entry is delete_pending, capturing its exact condemn-time token.
+    RetiredEntry pending_entry;
+    bool pending = false;
+    for (int i = 0; i < 6 && !pending; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+        const auto e = currentEntryFor(*backend, store->layout(), blob);
+        if (e && e->delete_pending)
+        {
+            pending = true;
+            pending_entry = *e;
+        }
+    }
+    ASSERT_TRUE(pending);
+    {
+        const auto lm = loadMetaForTest(*backend, store->layout(), blob);
+        ASSERT_TRUE(lm.has_value()) << "the blob must still carry its Condemned freshness meta pre-delete";
+        ASSERT_EQ(lm->meta.state, MetaState::Condemned);
+    }
+
+    // The object is genuinely gone already (as if a prior crashed pass landed the delete); confirm that,
+    // then arm the quirk so the NEXT conditional delete against this now-absent key answers TokenMismatch
+    // instead of NotFound (the rustfs 412-on-absent behavior).
+    const String blob_key = store->layout().blobKey(blob_id);
+    ASSERT_EQ(backend->deleteExact(blob_key, pending_entry.token).kind, DeleteOutcome::Kind::Deleted);
+    ASSERT_FALSE(backend->head(blob_key).exists);
+    backend->quirkOnAbsent(blob_key);
+
+    // The deleting pass replays deleteExact(entry.token): the backend answers TokenMismatch (quirk), but
+    // the follow-up HEAD shows the object absent, so the fix disambiguates the outcome to Absent and still
+    // runs the `.meta` cleanup.
+    const RoundReport rep = runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    EXPECT_EQ(rep.absent, 1u) << "the 412-on-absent quirk must settle as Absent, not Replaced";
+    EXPECT_EQ(rep.replaced, 0u);
+    EXPECT_FALSE(currentEntryFor(*backend, store->layout(), blob).has_value());
+    EXPECT_FALSE(loadMetaForTest(*backend, store->layout(), blob).has_value())
+        << ".meta cleanup (gated on Deleted/NotFound) must still run on the disambiguated Absent outcome";
+}
+
+/// ---- condemn-marker gate suite ----
+///
+/// The per-hash condemn marker is LOAD-BEARING for the delete edge: the writer's adopt gate point-reads
+/// the meta and an ABSENT meta reads as Clean, so a blob whose condemn-marker write was swallowed can be
+/// same-token adopted by a writer landing in the [discovery-LIST, deleteExact] window — invisible to the
+/// graduating fold — and the exact-token redelete then deletes a body under a live committed edge
+/// (dangling manifest). Graduation to `delete_pending` therefore requires CONFIRMED durable `Condemned`
+/// evidence for the entry; absent evidence CARRIES the entry to the next round (fail-safe delay, never a
+/// fail-open delete) and retries the marker so a healed backend restores liveness.
+
+/// A condemned entry whose marker write was swallowed must be CARRIED round after round — never
+/// graduated, never deleted — until durable `Condemned` evidence exists. Once the backend heals, the
+/// carry-time marker retry lands and the normal two-phase pipeline reclaims the blob (delay, not a leak).
+TEST(CASGCCondemnMarker, SwallowedMarkerWriteCarriesEntryInsteadOfDeleting)
+{
+    auto backend = std::make_shared<MetaWriteFaultBackend>();
+    auto store = openPoolForTest(backend);
+    store->setCasRetrySleepForTest([](uint64_t) {});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+
+    runRegularRoundReclaiming(gc);   // +1 folds; blob referenced
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    runRegularRoundReclaiming(gc);   // the condemning round; the controlled marker write exhausts as Unresolved
+    ASSERT_FALSE(loadMetaForTest(*backend, store->layout(), blob).has_value())
+        << "precondition: the injected fault must have lost the condemn-marker write";
+    ASSERT_TRUE(currentEntryFor(*backend, store->layout(), blob).has_value())
+        << "precondition: the retired entry must have been committed despite the lost marker";
+
+    /// Rounds keep coming while the marker stays unwritable: without durable Condemned evidence the
+    /// entry must be CARRIED — a writer reading the absent meta may have adopted this exact token.
+    const auto carries_before =
+        ProfileEvents::global_counters[ProfileEvents::CASGCCondemnMarkerUnconfirmedCarry].load();
+    for (int i = 0; i < 4; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+        EXPECT_TRUE(blobExists(*backend, store->layout(), blob))
+            << "round " << i << " after condemn: deleted without a durable condemn marker";
+    }
+    const auto e = currentEntryFor(*backend, store->layout(), blob);
+    ASSERT_TRUE(e.has_value()) << "the entry must remain retired (carried), not dropped";
+    EXPECT_FALSE(e->delete_pending) << "graduation must be refused without a confirmed marker";
+    EXPECT_FALSE(e->marker_confirmed);
+    EXPECT_GE(ProfileEvents::global_counters[ProfileEvents::CASGCCondemnMarkerUnconfirmedCarry].load()
+                  - carries_before, 4u)
+        << "every refused graduation must count one unconfirmed carry";
+
+    /// Heal the backend: the carry-time retry publishes the marker, the entry confirms + graduates, and
+    /// the pipeline reclaims the blob and drops the meta.
+    backend->fail_meta_writes.store(false);
+    EXPECT_TRUE(runRoundsUntilAbsent(store, gc, *backend, store->layout(), blob));
+    EXPECT_FALSE(currentEntryFor(*backend, store->layout(), blob).has_value());
+    EXPECT_FALSE(loadMetaForTest(*backend, store->layout(), blob).has_value());
+}
+
+/// The healthy-path counterpart: with the condemn-time marker write landing normally, the gate must not
+/// change the canonical schedule — condemned at round K, graduated (delete_pending) at K+1, deleted at
+/// K+2 — and the durable Condemned marker exists from the condemning round on.
+TEST(CASGCCondemnMarker, DurableMarkerKeepsCanonicalGraduationSchedule)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+
+    runRegularRoundReclaiming(gc);
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);   // condemn round K
+        EXPECT_EQ(rep.condemned, 1u);
+        const auto lm = loadMetaForTest(*backend, store->layout(), blob);
+        ASSERT_TRUE(lm.has_value());
+        EXPECT_EQ(lm->meta.state, MetaState::Condemned);
+    }
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);   // K+1: confirmed marker => graduates on schedule
+        EXPECT_EQ(rep.graduated, 1u);
+        const auto e = currentEntryFor(*backend, store->layout(), blob);
+        ASSERT_TRUE(e.has_value());
+        EXPECT_TRUE(e->delete_pending);
+        EXPECT_TRUE(e->marker_confirmed) << "a delete_pending row must carry the confirmation bit";
+        EXPECT_TRUE(blobExists(*backend, store->layout(), blob));
+    }
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);   // K+2: the pending delete executes
+        EXPECT_EQ(rep.redeleted, 1u);
+        EXPECT_FALSE(blobExists(*backend, store->layout(), blob));
+    }
+}
+
+/// The leader-restart path: `condemn_markers_confirmed` is a process-local registry on `Gc`, lost on a
+/// GC leader restart. Same idiom as the crash-replay tests above (`Gc gc2(store, kGc)` -- a fresh `Gc`
+/// object under the SAME identity models a process restart that resumes its own lease, not a steal by a
+/// different owner). The fresh instance must still confirm graduation via the ONE synchronous `loadMeta`
+/// re-check: the durable `Condemned` meta observed now is sufficient evidence on its own, with no
+/// in-process (hash, token) confirmation available at all. This proves the fallback branch -- not just
+/// the in-process registry -- authorizes the delete.
+TEST(CASGCCondemnMarker, LoadMetaFallbackConfirmsGraduationAfterLeaderRestart)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    {
+        /// The first (soon-to-be-gone) leader: seeds the blob, condemns it, and lets the marker write
+        /// land on the healthy backend. Its `condemn_markers_confirmed` registry dies with it.
+        Gc gc(store, kGc);
+        runRegularRoundReclaiming(gc);
+        dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+        const RoundReport rep = runRegularRoundReclaiming(gc);   // condemn round
+        EXPECT_EQ(rep.condemned, 1u);
+        store->renewWatermarkOnce();
+    }
+    const auto lm = loadMetaForTest(*backend, store->layout(), blob);
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(lm->meta.state, MetaState::Condemned)
+        << "precondition: the durable marker must be on disk before the simulated restart";
+
+    /// A brand-new `Gc` object under the SAME identity -- an empty `condemn_markers_confirmed`, exactly
+    /// as after a process restart that resumes its own lease. It never observed the condemn round above,
+    /// so the in-process confirmation path (`condemnMarkerConfirmedInProcess`) has nothing to return true
+    /// for; only the `loadMeta` fallback can authorize graduation.
+    Gc gc2(store, kGc);
+    const RoundReport rep = runRegularRoundReclaiming(gc2);
+    EXPECT_EQ(rep.graduated, 1u)
+        << "the loadMeta fallback (leader-restart path) must authorize graduation from durable evidence "
+           "alone";
+    const auto e = currentEntryFor(*backend, store->layout(), blob);
+    ASSERT_TRUE(e.has_value());
+    EXPECT_TRUE(e->delete_pending);
+    EXPECT_TRUE(e->marker_confirmed) << "a delete_pending row confirmed via loadMeta still carries the bit";
+    EXPECT_TRUE(blobExists(*backend, store->layout(), blob));
+}
diff --git a/src/Disks/tests/gtest_cas_gc_arithmetic_intake.cpp b/src/Disks/tests/gtest_cas_gc_arithmetic_intake.cpp
new file mode 100644
index 000000000000..870bcdc20295
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_arithmetic_intake.cpp
@@ -0,0 +1,595 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+
+#include <mutex>
+#include <set>
+
+/// ARITHMETIC REF INTAKE (spec 2026-07-27 "ref chain complete cut" §5).
+///
+/// The GC fold used to walk the ids the round's LIST returned. That made the LIST a source of TRUTH
+/// about which records exist, and an object store that omits a durable key from a listing -- observed
+/// in production as the `0x1430c`/`0x1430d` shape -- silently skipped those records' owner edges and
+/// then sealed a cursor ABOVE them, so their blobs looked unreferenced forever after.
+///
+/// Under INV-1 (per-namespace contiguous ids) the ids within one `(namespace, writer_epoch)` are dense
+/// `1..T`, so the next record's id is COMPUTABLE: `cursor + 1`. The fold therefore steps by arithmetic
+/// and reads each expected id by EXACT key (the per-record GET was always owed -- the round read every
+/// record's body anyway). The listing is demoted to a HINT with two jobs: it says which namespaces
+/// exist, and it supplies the witnesses that make an absent expected-next decidable:
+///
+///   * absent at `expected`, no listed id above it  => the namespace's frontier this round (normal end)
+///   * absent at `expected`, a listed id above it   => IMPOSSIBLE under contiguity: the store is lying
+///                                                     or a durable record was lost. Hold the namespace
+///                                                     (classification 4), cursor unmoved.
+///
+/// Epochs are crossed ONLY by consuming the `EpochSeal` that closes an epoch (INV-2). The seal folds as
+/// an applied no-op (probe B2: `produced=false`), and the next epoch's start is `{E', 1}` -- reached
+/// through the `prev_epoch_seal` back-chain, never guessed from the hint, so an epoch the hint omits
+/// entirely is still walked.
+///
+/// These tests drive REAL rounds over an in-memory pool whose LIST omits keys that are genuinely
+/// present (`HintHoleBackend`) -- the production shape, reproduced.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+
+/// The lying store is `HintHoleBackend` from `cas_test_helpers.h`: LIST permanently omits keys that
+/// stay readable by exact key, so these tests exercise the intake's arithmetic walk rather than any
+/// one `list` call.
+
+/// The `RefCoverage` the newest fold seal recorded for `ns`'s opaque catalog life, or nullopt when the
+/// seal has no entry for it. Scans downward from the adopted generation for the most recent fold seal,
+/// mirroring `foldCursorOf`'s reasoning (a completed round's `gc/state` points at the recheck generation).
+std::optional<RefCoverage> coverageOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const uint64_t gen = currentGenerationOf(backend, layout);
+    const uint64_t attempt = currentAttemptOf(backend, layout);
+    const UInt128 life_id = catalogLifeIdForTest(backend, layout, ns);
+    for (uint64_t g = gen; ; --g)
+    {
+        if (const auto got = backend.get(layout.foldSealKey(g, attempt)))
+        {
+            const CasFoldSeal seal = decodeFoldSeal(got->bytes);
+            const auto it = seal.ref_lives.find(life_id);
+            if (it == seal.ref_lives.end())
+                return std::nullopt;
+            return it->second.coverage;
+        }
+        if (g == 0)
+            return std::nullopt;
+    }
+}
+
+RefTxnId cursorOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const auto cov = coverageOf(backend, layout, ns);
+    return cov ? cov->last_folded_ref_id : RefTxnId{};
+}
+
+uint8_t classificationOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const auto cov = coverageOf(backend, layout, ns);
+    return cov ? cov->classification : 0;
+}
+
+/// The `fold_ref_intake` phase metrics of the round `sched` runs -- the only place probe B1's two
+/// numbers are observable.
+std::map<String, UInt64> runRoundAndReadIntakeMetrics(const PoolPtr & store)
+{
+    std::vector<GcRoundLogRecord> rows;
+    CasGcScheduler sched(store, std::chrono::seconds(1), "test::gc", "ca",
+                         [&](const GcRoundLogRecord & r) { rows.push_back(r); });
+    EXPECT_TRUE(sched.runOneRoundNow(GcRoundLogRecord::Trigger::Manual).acquired_lease);
+    for (const GcRoundLogRecord & r : rows)
+        if (r.event_type == GcRoundLogRecord::EventType::Phase && r.phase == "fold_ref_intake")
+            return r.phase_metrics;
+    return {};
+}
+
+}
+
+/// ===================== THE BLOCKER, AS A UNIT TEST =====================
+///
+/// Five records, all durable and all readable by exact key; the hint omits the two in the MIDDLE.
+/// Arithmetic intake never consults the hint for what to read next, so the omission is a non-event:
+/// every record folds, every blob keeps its owner edge, and the cursor lands on the true tail.
+///
+/// Under listing-driven intake this test fails on the blobs, not on the cursor: the cursor still
+/// reached `{1, 5}` (the last LISTED id) while the two hidden records' edges were never folded -- the
+/// exact damage shape the production blocker caused, since a cursor sealed above an unfolded record can
+/// never be re-read.
+TEST(CASGCArithmeticIntake, HintOmittingMiddleRecordsFoldsThroughUnnoticed)
+{
+    auto backend = std::make_shared<HintHoleBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    for (uint64_t i = 1; i <= 5; ++i)
+        publishAt(*backend, layout, ns, RefTxnId{1, i}, "ref_" + std::to_string(i), i,
+                  DB::UInt128(i), /*birth=*/i == 1);
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 5},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    backend->hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 3}));
+    backend->hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 4}));
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    ASSERT_GT(backend->holesServed(), 0u) << "the hint hole was never actually served";
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 5}));
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 2) << "a folded namespace is `changed`";
+    for (uint64_t i = 1; i <= 5; ++i)
+        EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(i)), 1)
+            << "blob " << i << " lost its owner edge: its record was skipped because the hint omitted it";
+}
+
+/// A clean, hole-free namespace still ends its walk exactly where it should: at the first absent id,
+/// with no witness above it and therefore no hold.
+TEST(CASGCArithmeticIntake, WalkEndsAtFrontierWithoutHold)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// Fold every round: the default defer window would skip the second round entirely and leave the
+    /// first round's seal in place, so the assertion below would read a stale coverage record.
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    for (uint64_t i = 1; i <= 3; ++i)
+        publishAt(*backend, layout, ns, RefTxnId{1, i}, "ref_" + std::to_string(i), i,
+                  DB::UInt128(i), /*birth=*/i == 1);
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 3},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 3}));
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 2);
+
+    /// A second round over an unchanged namespace pays exactly one exact GET, finds the same frontier,
+    /// and neither advances nor holds.
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 3}));
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 1) << "an unchanged namespace is `carried`";
+}
+
+/// ===================== EPOCHS ARE CROSSED ONLY BY CONSUMING A SEAL =====================
+///
+/// `{1,1} {1,2} seal{1,3} | {2,1} {2,2}`: the seal is applied as a table no-op, counted applied, and
+/// the walk continues at `{2, 1}` -- whose `prev_epoch_seal` names the seal just consumed, which is what
+/// makes the crossing provable rather than guessed.
+TEST(CASGCArithmeticIntake, SealCrossesEpochAndIsAppliedAsNoOp)
+{
+    auto backend = std::make_shared<HintHoleBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    writeSealAt(*backend, layout, ns, RefTxnId{1, 3});
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "ref_3", 3, DB::UInt128(3),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 3});
+    publishAt(*backend, layout, ns, RefTxnId{2, 2}, "ref_4", 4, DB::UInt128(4));
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 3},
+    });
+
+    /// The hint hides the seal AND the new epoch's first record: neither the epoch boundary nor its
+    /// start may depend on the listing.
+    backend->hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 3}));
+    backend->hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{2, 1}));
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    ASSERT_GT(backend->holesServed(), 0u);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{2, 2}));
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 2);
+    for (uint64_t i = 1; i <= 4; ++i)
+        EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(i)), 1) << "blob " << i;
+}
+
+/// Two chained seals in ONE round, the middle epoch entirely EMPTY (its seal is its only record, at
+/// sequence 1, carrying `prev_epoch_seal`). The hint omits that whole epoch, so the only way to reach it
+/// is the back-chain: the record the hint DOES show names the seal that must be consumed first.
+TEST(CASGCArithmeticIntake, ChainedEmptyEpochSealsBothConsumedInOneRound)
+{
+    auto backend = std::make_shared<HintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    writeSealAt(*backend, layout, ns, RefTxnId{1, 2});
+    writeSealAt(*backend, layout, ns, RefTxnId{2, 1}, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    publishAt(*backend, layout, ns, RefTxnId{3, 1}, "ref_2", 2, DB::UInt128(2),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{2, 1});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{3, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{2, 1},
+    });
+
+    backend->hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{2, 1}));
+
+    const auto intake = runRoundAndReadIntakeMetrics(store);
+    ASSERT_FALSE(intake.empty()) << "no fold_ref_intake row";
+    ASSERT_GT(backend->holesServed(), 0u);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{3, 1}));
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 1);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(2)), 1);
+
+    /// THE ASSERTION THAT MAKES THIS TEST ABOUT THE BACK-CHAIN. Every check above is also satisfied by
+    /// an id-ordered walk over the listed keys — which is why this case passed before the change. Two
+    /// crossings can only happen by consuming `seal{1,2}` and then `seal{2,1}`, and `{2,1}` is reachable
+    /// only through `{3,1}`'s `prev_epoch_seal`, since the hint never mentions it.
+    EXPECT_EQ(intake.at("epoch_crossings"), 2u)
+        << "the walk must cross TWICE, through a hidden epoch it can only reach by the seal chain";
+    EXPECT_EQ(intake.at("logs_accounted"), intake.at("logs_applied"));
+    EXPECT_EQ(intake.at("logs_applied"), 4u) << "two records and two seals, all applied";
+    /// The checkpoint names the complete, authoritative frontier, so the bounded walk has no absent
+    /// probes to perform. The hidden epoch remains reachable only through the seal chain.
+    EXPECT_EQ(intake.at("absent_probes"), 0u);
+}
+
+/// A round that ends ON a seal (nothing above it yet) leaves the cursor there. The NEXT round must
+/// still cross into the epoch that appears later -- the cursor sitting on a closed epoch's seal is the
+/// ordinary steady state after a writer-epoch change, not a wedge.
+TEST(CASGCArithmeticIntake, CursorRestingOnSealCrossesInALaterRound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    writeSealAt(*backend, layout, ns, RefTxnId{1, 2});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    ASSERT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 2})) << "the round consumed the seal";
+
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "ref_2", 2, DB::UInt128(2),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    advanceRecoverableCkptForRawFixture(*backend, layout, ns, RefTxnId{2, 1});
+
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{2, 1}));
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 2);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(2)), 1);
+}
+
+/// ===================== IMPOSSIBLE SHAPES HOLD THE NAMESPACE =====================
+///
+/// `{1,3}` is genuinely absent while `{1,4}` is present AND listed. Contiguity says that cannot happen,
+/// so whatever sits behind the gap may be an acked `+1`: the namespace is held at classification 4 with
+/// its cursor UNMOVED, rather than sealing past the gap.
+///
+/// Listing-driven intake folded `{1,4}` and sealed the cursor at it -- permanently, since a record below
+/// the cursor is never re-read.
+TEST(CASGCArithmeticIntake, GapBelowWitnessHoldsNamespaceAtClassificationFour)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    /// {1,3} is never written -- the record that vanished.
+    publishAt(*backend, layout, ns, RefTxnId{1, 4}, "ref_4", 4, DB::UInt128(4));
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 4},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 2}))
+        << "the cursor must not advance past a gap";
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 4);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(4)), 0)
+        << "the record above the gap was not folded";
+}
+
+/// A record of a LATER epoch reachable while the current epoch's seal was never consumed: the crossing
+/// has no proof (the later epoch's `prev_epoch_seal` names a seal this cursor never reached), so the
+/// namespace holds instead of jumping the boundary.
+TEST(CASGCArithmeticIntake, UnconsumedSealCrossingHoldsNamespace)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    /// Epoch 1's missing `{1,2}` seal is the exact position epoch 2 claims to chain from. With no
+    /// same-epoch witness above it, the later epoch is the only witness and the crossing must hold.
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "ref_2", 2, DB::UInt128(2),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 1}));
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 4);
+    const auto coverage = coverageOf(*backend, layout, ns);
+    ASSERT_TRUE(coverage && coverage->hold.has_value());
+    EXPECT_EQ(coverage->hold->reason, HoldReason::UnconsumedSealCrossing);
+    EXPECT_EQ(coverage->hold->offending_position, (RefTxnId{1, 2}));
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(2)), 0);
+}
+
+/// The back-chain proves the IDENTITY of the position an epoch chains from; it does not, by itself,
+/// prove that position is a SEAL. Here a writer names an ordinary record (`{1,2}`, epoch 1's last
+/// record, never sealed) as `{2,1}`'s `prev_epoch_seal`. Identity matches, so a chain-only check would
+/// grant the crossing and declare epoch 1 closed while its writer may still be appending -- any later
+/// `{1,k}` would then land permanently below the cursor, which is exactly the damage the seal exists to
+/// prevent. The walk applied that record itself this round, so it knows its kind for free: refuse.
+TEST(CASGCArithmeticIntake, CrossingFromANonSealRecordIsRefusedEvenWhenTheChainMatches)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    /// `{1,2}` is an ordinary published record, NOT an `EpochSeal` -- and epoch 2 chains to it anyway.
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "ref_3", 3, DB::UInt128(3),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 2}))
+        << "epoch 1 was never sealed, so the cursor may not leave it";
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 4);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 1) << "epoch 1's records still fold";
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(2)), 1);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(3)), 0)
+        << "the record beyond the unsealed boundary must NOT be folded";
+}
+
+/// The crossing reads the epoch-start record to prove the chain, and the walk then reads it again to
+/// fold it. A record that answers the first read and not the second would make the next iteration
+/// re-derive the SAME crossing from the same unchanged cursor and resolve to the same position -- an
+/// infinite spin inside one namespace's walk. The strict-progress guard turns that into a hold.
+///
+/// The fixture is the only shape that reaches it: a key that alternates present/absent across reads, so
+/// `crossFromSeal` keeps succeeding while the walk's own GET keeps failing.
+TEST(CASGCArithmeticIntake, EpochStartThatAnswersOnlyEveryOtherReadHoldsInsteadOfSpinning)
+{
+    /// Answers `flaky` on odd-numbered reads and 404s on even ones. Nothing else is disturbed.
+    class AlternatingGetBackend : public InMemoryBackend
+    {
+    public:
+        using DB::Cas::Backend::get;
+        String flaky;
+        size_t reads = 0;
+
+        std::optional<GetResult> get(const String & key, Range range) override
+        {
+            if (key == flaky && ++reads % 2 == 0)
+                return std::nullopt;
+            return InMemoryBackend::get(key, range);
+        }
+    };
+
+    auto backend = std::make_shared<AlternatingGetBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    writeSealAt(*backend, layout, ns, RefTxnId{1, 2});
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "ref_2", 2, DB::UInt128(2),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    /// A THIRD epoch is what keeps the unstable position from reading as a frontier: without a witness
+    /// strictly above it, an absent `{2,1}` is just "the namespace ends here" and the walk stops
+    /// normally. With `{3,1}` listed, the walk must keep trying to cross -- and the chain from `{3,1}`
+    /// leads back to `{2,1}` every time, which is the spin.
+    publishAt(*backend, layout, ns, RefTxnId{3, 1}, "ref_3", 3, DB::UInt128(3),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{2, 1});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{3, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{2, 1},
+    });
+
+    /// Arm only after seeding, so the fixture's own writes are undisturbed and the read counter starts
+    /// at the round's first read of this key (`crossFromSeal`'s, which must succeed).
+    backend->flaky = layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{2, 1});
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);   /// must RETURN -- the spin is the failure mode
+    ASSERT_GE(backend->reads, 3u)
+        << "the crossing must have re-proved the same position after the walk failed to read it; "
+           "fewer reads means the guard was never reached";
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 2}))
+        << "the cursor stops on the seal it consumed and never enters the unstable epoch";
+    EXPECT_EQ(classificationOf(*backend, layout, ns), 4);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(2)), 0);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(3)), 0)
+        << "nothing above the unstable position may be folded either";
+}
+
+/// ===================== A PER-NAMESPACE FAILURE IS NOT A ROUND FAILURE =====================
+///
+/// Spec §5 narrows the whole-round abort to a key that cannot be attributed to any namespace. An
+/// undecodable BODY belongs to exactly one namespace, so it clamps that namespace and nothing else:
+/// `ns_a` holds at its last good record while `ns_b` folds and seals normally in the same round.
+TEST(CASGCArithmeticIntake, CorruptBodyClampsOneNamespaceWhileAnotherFolds)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns_a{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns_a);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    const RootNamespace ns_b{"00/bb@cas@"};
+
+    publishAt(*backend, layout, ns_a, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    backend->putIfAbsent(layout.refLogKey(fixture::fixtureLife(ns_a), RefTxnId{1, 2}), "this is not a cas_ref_log object");
+    writeRecoverableCkptForRawFixture(*backend, layout, ns_a, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    publishAt(*backend, layout, ns_b, RefTxnId{1, 1}, "ref_1", 11, DB::UInt128(11), /*birth=*/true);
+    publishAt(*backend, layout, ns_b, RefTxnId{1, 2}, "ref_2", 12, DB::UInt128(12));
+    writeSealAt(*backend, layout, ns_b, RefTxnId{1, 3});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns_b, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 3},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 3},
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns_a), (RefTxnId{1, 1}));
+    EXPECT_EQ(classificationOf(*backend, layout, ns_a), 4);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns_b), (RefTxnId{1, 3}))
+        << "a sibling namespace's corrupt body must not stop this one";
+    EXPECT_EQ(classificationOf(*backend, layout, ns_b), 2);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(11)), 1);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(12)), 1);
+}
+
+/// ===================== PROBE B1 OVER AN ARITHMETIC CUT =====================
+///
+/// `logs_accounted` is recomputed from the SEALED cut -- the arithmetic distance the round's cursors
+/// claim to cover -- and compared with the count the walk incremented once per applied record. The two
+/// can only differ if a cursor moved over a position nothing applied, which is precisely the damage
+/// listing-driven intake used to do silently. The identity must survive both a hint hole (positions
+/// applied that the listing never mentioned) and a seal crossing (an applied no-op, and a cut that
+/// spans two epochs).
+TEST(CASGCArithmeticIntake, B1IdentityHoldsOverAHoleyCutThatCrossesASeal)
+{
+    auto backend = std::make_shared<HintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    writeSealAt(*backend, layout, ns, RefTxnId{1, 3});
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "ref_3", 3, DB::UInt128(3),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 3});
+    publishAt(*backend, layout, ns, RefTxnId{2, 2}, "ref_4", 4, DB::UInt128(4));
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 3},
+    });
+
+    backend->hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 2}));
+
+    const auto intake = runRoundAndReadIntakeMetrics(store);
+    ASSERT_FALSE(intake.empty()) << "no fold_ref_intake row";
+    ASSERT_GT(backend->holesServed(), 0u);
+
+    EXPECT_EQ(intake.at("logs_accounted"), intake.at("logs_applied"));
+    EXPECT_EQ(intake.at("logs_applied"), 5u)
+        << "four records and the seal: the seal is APPLIED, as a no-op";
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{2, 2}));
+}
+
+/// A namespace the hint omits ENTIRELY still folds through the checkpoint's authoritative frontier.
+/// Every log key remains readable by exact key, and the cursor reaches `{1,3}` despite the empty hint.
+/// When the hint reappears, it changes neither the cursor nor the owner edges.
+TEST(CASGCArithmeticIntake, WhollyOmittedNamespaceFoldsThroughAuthoritativeCheckpoint)
+{
+    auto backend = std::make_shared<HintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    for (uint64_t i = 1; i <= 3; ++i)
+        publishAt(*backend, layout, ns, RefTxnId{1, i}, "ref_" + std::to_string(i), i,
+                  DB::UInt128(i), /*birth=*/i == 1);
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 3},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    for (uint64_t i = 1; i <= 3; ++i)
+        backend->hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, i}));
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    const auto hidden_cov = coverageOf(*backend, layout, ns);
+    ASSERT_TRUE(hidden_cov.has_value())
+        << "the namespace is `Live` in the catalog, so it stays in the universe even fully hidden";
+    EXPECT_EQ(hidden_cov->classification, 2) << "the checkpoint's frontier is folded by exact key";
+    EXPECT_EQ(hidden_cov->last_folded_ref_id, (RefTxnId{1, 3}));
+
+    /// The store stops lying: the already folded namespace reappears.
+    backend->revealAll();
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 3}));
+    for (uint64_t i = 1; i <= 3; ++i)
+        EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(i)), 1) << "blob " << i;
+}
diff --git a/src/Disks/tests/gtest_cas_gc_attempt.cpp b/src/Disks/tests/gtest_cas_gc_attempt.cpp
new file mode 100644
index 000000000000..b919298ae24e
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_attempt.cpp
@@ -0,0 +1,189 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace DB::ErrorCodes
+{
+extern const int ABORTED;
+}
+
+/// Unit-level GC-CONCURRENT-LEADER-LEAK regression (the original bug the attempt-scoped-generation fix
+/// closes), ported to the one-pass ack-floor round.
+///
+/// The historical wedge: two GC leaders fold the same generation. A DEPOSED leader writes its
+/// `fold_seal(G_f)` to a FINAL `gc/gen/<G_f>/fold_seal` key just before its lease-guarded `gc/state` CAS
+/// fails (lease lost mid-round). That orphaned write-once seal then poisons every future round: each
+/// honest round recomputes `G_f`, hits the orphan's divergent bytes, throws "concurrent leader"
+/// (`ABORTED`) forever — GC wedged, nothing reclaimed.
+///
+/// The fix: every per-round `gc/gen` artifact is ATTEMPT-scoped (keyed by the folding leader's
+/// `lease.seq`). A deposed leader writes its fold seal under its OWN attempt `a1`, which the failed
+/// `gc/state` CAS never adopts — so it is pure unadopted debris, invisible to every reader resolving
+/// only the adopted `(snap_generation, snap_attempt)`. The next honest round renews the lease (a fresh
+/// `lease.seq`), folds under a DIFFERENT attempt, never collides, and drains.
+///
+/// In the ONE-PASS round there is a SINGLE `gc/state` CAS per round (fold/publish/deletes all precede it),
+/// so the deposition point is simply that single round-commit CAS. This test denies it once (leaving the
+/// deposed fold seal under `a1`), then runs an honest GC to a fixpoint and asserts it drains the
+/// now-unreachable blob to zero without wedging.
+
+namespace
+{
+
+const UInt128 kGcA = hexToU128("00000000000000000000000000000001");
+
+ManifestRef ref(const String &, uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+
+/// Whether a blob's body object is present in the backend (HEADs the object key directly).
+bool blobExists(InMemoryBackend & b, const Layout & layout, const UInt128 & hash)
+{
+    return b.head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)})).exists;
+}
+
+/// Whether the CURRENT retired list (any gc-shard) still holds an entry — the ack-floor deletion pipeline
+/// is in flight while this is true.
+bool anyRetiredPending(const PoolPtr & s)
+{
+    /// Retired-in-snapshot (T4): condemned state rides the adopted fold seal's kCondemned rows, not a
+    /// separate retired list — reconstruct the in-flight set from the seal.
+    return anyCondemnedInSeal(s->backend(), s->layout());
+}
+
+/// Drive regular GC to a fixpoint over the ACK-FLOOR round (advancing the store's own mount ack after each
+/// round so the floor follows the committed round; stay alive while any work counter is nonzero OR the
+/// current retired list still holds an in-flight entry).
+size_t runGcToFixpoint(const PoolPtr & s, Gc & gc, size_t max_rounds = 64)
+{
+    size_t rounds = 0;
+    for (; rounds < max_rounds; ++rounds)
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        if (!rep.acquired_lease)
+            continue;
+        s->renewWatermarkOnce();
+        const bool no_work = rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0;
+        if (no_work && !anyRetiredPending(s))
+            break;
+    }
+    return rounds;
+}
+
+/// A backend that throws ONCE on the SINGLE round-commit `gc/state` CAS — the casPut that advances
+/// snap_generation (the one-pass round has exactly one such CAS; the lease-acquire CAS does not advance
+/// snap_generation, so "advances snap_generation" uniquely picks the round commit).
+class InterruptRoundCasBackend : public InMemoryBackend
+{
+public:
+    explicit InterruptRoundCasBackend(String gc_state_key_) : gc_state_key(std::move(gc_state_key_)) {}
+
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        if (arm_interrupt && key == gc_state_key)
+        {
+            const auto stored = get(key);
+            const uint64_t stored_gen = stored ? decodeGcState(stored->bytes).snap_generation : 0;
+            const uint64_t next_gen = decodeGcState(bytes).snap_generation;
+            if (next_gen > stored_gen)
+            {
+                arm_interrupt = false;   /// one-shot: only depose the first round-commit CAS
+                throw DB::Exception(DB::ErrorCodes::ABORTED,
+                    "test-injected: round-commit gc/state CAS denied (leader deposed mid-round; lease lost)");
+            }
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+
+    bool arm_interrupt = false;
+
+private:
+    String gc_state_key;
+};
+
+}
+
+/// A leader whose round-commit CAS is denied (lease lost mid-round) leaves its fold seal ONLY under its
+/// own attempt `a1`; it never occupies the adopted attempt, so a subsequent honest round is not wedged
+/// and drains the now-unreachable blob to zero.
+TEST(CASGCAttempt, DeposedFoldAttemptDoesNotWedge)
+{
+    auto backend = std::make_shared<InterruptRoundCasBackend>(/*gc_state_key*/ "p/gc/state");
+    auto store = openPoolForTest(backend);
+    ASSERT_EQ(store->layout().gcStateKey(), "p/gc/state");   // guard the injected key against layout drift
+
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGcA);
+
+    // Round 1 (honest): fold the +1 so the blob is pinned in the in-degree generation, and adopt the
+    // first (snap_generation, snap_attempt).
+    runRegularRoundReclaiming(gc);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1) << "blob pinned by the committed ref";
+    const auto after_fold = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    ASSERT_EQ(after_fold.snap_attempt, after_fold.lease.seq);
+    ASSERT_GT(after_fold.snap_generation, 0u);
+
+    // Drop the only ref and advance the watermark floor so the now-orphaned blob is not spared in-flight.
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    store->renewWatermarkOnce();
+
+    // Round 2 (DEPOSED): the round folds the -1 and writes its fold seal under its own attempt `a1`, then
+    // its single round-commit CAS is DENIED (lease lost mid-round). The round must throw and must NOT
+    // advance the adopted (snap_generation, snap_attempt).
+    backend->arm_interrupt = true;
+    EXPECT_ANY_THROW(runRegularRoundReclaiming(gc));   // ABORTED: round-commit CAS denied
+    backend->arm_interrupt = false;
+
+    const auto after_deposed = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    EXPECT_EQ(after_deposed.snap_generation, after_fold.snap_generation)
+        << "the denied round-commit CAS must NOT advance the adopted generation";
+    EXPECT_EQ(after_deposed.snap_attempt, after_fold.snap_attempt)
+        << "the denied round-commit CAS must NOT advance the adopted attempt";
+
+    // The deposed leader DID write its fold seal under its OWN attempt `a1` (= the lease.seq it renewed
+    // for round 2, which is strictly past the still-adopted attempt) at its fold generation `G_f`
+    // (= snap_generation + 1; fold mints the next generation, the round-commit CAS adopts it). That orphan
+    // is pure debris: it is under an attempt that gc/state never adopted, so no reader resolving
+    // (snap_generation, snap_attempt) can see it. On a PRE-FIX tree this seal would instead sit at the
+    // FINAL `gc/gen/<G_f>/fold_seal` key and wedge every future round's fold at the same G_f.
+    const uint64_t a1 = after_fold.lease.seq + 1;       // round 2 renewed the lease => seq bumped once
+    const uint64_t g_f = after_fold.snap_generation + 1;  // the generation the deposed fold minted
+    EXPECT_NE(a1, after_deposed.snap_attempt) << "the deposed attempt must differ from the adopted one";
+    EXPECT_TRUE(backend->head(store->layout().foldSealKey(g_f, a1)).exists)
+        << "the deposed leader's fold seal is durable under its own (unadopted) attempt a1";
+    EXPECT_FALSE(backend->head(store->layout().foldSealKey(g_f, after_deposed.snap_attempt)).exists)
+        << "no fold seal exists under the still-adopted attempt at the deposed fold generation (orphan is invisible)";
+
+    // An HONEST GC to a fixpoint (CAS now allowed). The KEY property: with attempt-scoping this SUCCEEDS —
+    // the next honest fold mints a FRESH attempt (a different lease.seq), never collides with the deposed
+    // seal under a1, and drains the unreachable blob. On a pre-fix (final-key) tree, the next fold would
+    // adopt-collide with the deposed final-key seal's divergent bytes and throw forever (GC wedged).
+    EXPECT_NO_THROW(runGcToFixpoint(store, gc));
+
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "the dropped blob must be reclaimed (GC drained past the deposed attempt)";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0) << "no stranded positive in-degree";
+    EXPECT_EQ(runFsck(*store, /*detail=*/false).unreachable, 0u)
+        << "INV-NO-LEAK: the deposed fold attempt did not wedge GC; the pool fully drained";
+
+    // GC advanced past the deposed attempt: the adopted (snap_generation, snap_attempt) moved on, and the
+    // adopted attempt is a fresh one (never the deposed a1).
+    const auto after_drain = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    EXPECT_GT(after_drain.snap_generation, after_fold.snap_generation) << "completion advanced the generation";
+    EXPECT_NE(after_drain.snap_attempt, a1) << "the drained round never adopted the deposed attempt a1";
+}
diff --git a/src/Disks/tests/gtest_cas_gc_bounded_walk.cpp b/src/Disks/tests/gtest_cas_gc_bounded_walk.cpp
new file mode 100644
index 000000000000..e9041dd7b223
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_bounded_walk.cpp
@@ -0,0 +1,570 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+
+/// THE BOUNDED FOLD WALK.
+///
+/// Arithmetic ref intake reads the next record by exact key -- `cursor + 1` -- and stops when that read
+/// comes back absent. That is exact and immune to a lying listing, and it is also, on its own, a walk
+/// with no last record: a namespace whose writer keeps appending never produces the absent read, so a
+/// round's duration stopped being `backlog / walker_rate` and became `backlog / (walker - writer)`. It
+/// diverges the moment a writer keeps up. Measured on a hot pool: ZERO completed GC rounds in 42
+/// minutes, and with them nothing that paces on rounds -- fold seal, cursors, the sampled store-quality
+/// detector, ref-object cleanup -- ever ran again.
+///
+/// The bound is `_ckpt.committed_through`, snapshotted once per namespace before the walk and never
+/// re-read within the round, so the work is finite and fixed before the round began however fast the
+/// writer appends. It is the AUTHORITY ceiling too -- a record above it is durable but is not logical
+/// history yet -- so ONE comparison both terminates the round and refuses to fold uncommitted work.
+///
+/// IT DOES NOT BOUND WHAT THE ROUND READS, and that is not an oversight. The read at `cursor + 1`
+/// produces the frontier proof, an unproven namespace suppresses all
+/// destructive work, and suppression stops the ref-object cleanup that would have drained the listing --
+/// so a namespace that stops being read can never become provable again by any route. "Skip the quiet
+/// namespace entirely" therefore is not a cheaper version of this design, it is a GC that permanently
+/// reclaims nothing; the saving is one `GET` and that `GET` is the proof.
+///
+/// So the properties these tests pin are:
+///   * a round folds through its round-start committed frontier and no further, whatever lands meanwhile;
+///   * a namespace whose tail did not move folds NOTHING and a matching CTE proves its carried cursor;
+///   * a round where no tail moved is skipped outright by the existing defer machinery;
+///   * a raw record ABOVE the committed frontier neither extends the fold nor suppresses destruction;
+///   * a manifest edge fold costs one GET and never a HEAD;
+///   * a namespace that folded nothing still keeps its sealed coverage row.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+
+/// Composed over `CountingBackend` because these tests assert REQUEST COUNTS: "folds nothing and reads
+/// once" is the claim, and only a counting backend can check it.
+using CountingHintHoleBackend = DB::Cas::tests::HintHoleBackendOn<DB::Cas::tests::CountingBackend>;
+
+/// The `RefCoverage` the newest fold seal recorded for `ns`'s opaque catalog life. Scans downward from
+/// the adopted generation for the most recent fold seal, mirroring `foldCursorOf`'s reasoning (a
+/// completed round's `gc/state` points at the recheck generation, which writes a completion seal rather
+/// than a fold seal).
+std::optional<RefCoverage> coverageOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const uint64_t gen = currentGenerationOf(backend, layout);
+    const uint64_t attempt = currentAttemptOf(backend, layout);
+    const UInt128 life_id = catalogLifeIdForTest(backend, layout, ns);
+    for (uint64_t g = gen; ; --g)
+    {
+        if (const auto got = backend.get(layout.foldSealKey(g, attempt)))
+        {
+            const CasFoldSeal seal = decodeFoldSeal(got->bytes);
+            const auto it = seal.ref_lives.find(life_id);
+            if (it == seal.ref_lives.end())
+                return std::nullopt;
+            return it->second.coverage;
+        }
+        if (g == 0)
+            return std::nullopt;
+    }
+}
+
+RefTxnId cursorOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const auto cov = coverageOf(backend, layout, ns);
+    return cov ? cov->last_folded_ref_id : RefTxnId{};
+}
+
+/// A phase metric, or 0 when the row does not carry it. Reading it this way rather than through
+/// `std::map::at` is deliberate: against the unbounded walk these columns do not exist yet, and a
+/// missing column should fail the assertion that names it, not abort the test with an exception.
+UInt64 metric(const std::map<String, UInt64> & row, const String & name)
+{
+    const auto it = row.find(name);
+    return it == row.end() ? 0 : it->second;
+}
+
+/// Every key the backend was asked to delete, so a failing zero-delete assertion names the site that
+/// leaked instead of only reporting a count.
+String deletedKeysMessage(const CountingBackend & backend)
+{
+    String out;
+    for (const String & key : backend.deletedKeys())
+        out += "\n    " + key;
+    return out.empty() ? String{" (none)"} : out;
+}
+
+/// Every `_log/` GET this round issued against `ns` over ids `first..last` -- the "read once, fold
+/// nothing" claim, made against the store rather than against a counter the fold keeps about itself.
+uint64_t refLogGetsFor(const CountingBackend & backend, const Layout & layout, const RootNamespace & ns,
+                       uint64_t first, uint64_t last, uint64_t epoch = 1)
+{
+    uint64_t total = 0;
+    for (uint64_t i = first; i <= last; ++i)
+        total += backend.getCount(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{epoch, i}));
+    return total;
+}
+
+/// One round driven directly on `Gc`, capturing the `fold_ref_intake` phase row. Driving `Gc` rather
+/// than `CasGcScheduler` is what lets a caller choose the universe policy, which the suppression test
+/// needs. An EMPTY row means the round deferred and folded nothing at all.
+std::map<String, UInt64> runRoundCapturingIntake(Gc & gc, UniversePolicy policy = UniversePolicy::kDefault)
+{
+    std::map<String, UInt64> intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = gc.runRegularRound({}, /*allow_steal*/true, policy);
+    gc.setPhaseSink({});
+    EXPECT_TRUE(report.acquired_lease) << "the round must have run at all";
+    return intake;
+}
+
+/// A store whose writer keeps pace with the walker EXACTLY: every time the fold reads the newest record
+/// by exact key, one more record lands above it.
+///
+/// This is a mid-round appender expressed as a synchronous hook rather than as a thread, and the
+/// determinism is the point. The property under test is "the round stops at the tail it froze, however
+/// much arrives afterwards", and a thread can only make appends arrive at times the scheduler chooses --
+/// including, on an unlucky run, entirely after the walk has gone past. The hook reproduces the WORST
+/// case (writer rate == walker rate, the rate at which the unbounded walk provably never terminates) on
+/// every run, and `max_appends` bounds it so that the UNPATCHED walk still finishes and can be measured
+/// rather than hanging the suite.
+class ChasingWriterBackend : public CountingBackend
+{
+public:
+    using CountingBackend::get;
+
+    /// Start appending above `published_through` (writer epoch 1) whenever the tail is read, up to
+    /// `max_appends` further records.
+    void arm(const Layout * layout_, const RootNamespace & ns_, uint64_t published_through, uint64_t max_appends)
+    {
+        layout = layout_;
+        ns = ns_;
+        published = published_through;
+        limit = published_through + max_appends;
+    }
+
+    /// Stop appending; the tail stands still from here on.
+    void disarm() { layout = nullptr; }
+
+    uint64_t publishedThrough() const { return published; }
+
+    std::optional<DB::Cas::GetResult> get(const String & key, DB::Cas::Range range) override
+    {
+        auto result = CountingBackend::get(key, range);
+        if (!layout || appending || published >= limit)
+            return result;
+        if (key != layout->refLogKey(fixture::fixtureLife(ns), RefTxnId{1, published}))
+            return result;
+
+        /// The walk just consumed the tail; the writer answers with the next record. Guarded against
+        /// re-entry because publishing issues backend calls of its own.
+        appending = true;
+        const uint64_t next = published + 1;
+        publishAt(*this, *layout, ns, RefTxnId{1, next}, "ref_" + std::to_string(next), next, DB::UInt128(next));
+        published = next;
+        appending = false;
+        return result;
+    }
+
+private:
+    const Layout * layout = nullptr;
+    RootNamespace ns{};
+    uint64_t published = 0;
+    uint64_t limit = 0;
+    bool appending = false;
+};
+
+/// Publish ids `first .. last` of `ns` in writer epoch 1, each pinning its own blob.
+void publishRange(Backend & backend, const Layout & layout, const RootNamespace & ns, uint64_t first, uint64_t last)
+{
+    for (uint64_t i = first; i <= last; ++i)
+        publishAt(backend, layout, ns, RefTxnId{1, i}, "ref_" + std::to_string(i), i, DB::UInt128(i),
+                  /*birth=*/i == 1);
+}
+
+}
+
+/// ===================== (a) THE ROUND FOLDS THROUGH THE TAIL IT FROZE =====================
+///
+/// `planted` records are durable when the round starts. While it walks, the writer keeps pace exactly --
+/// every record the fold reads is answered with another one above it. The round must fold through the
+/// tail it saw at round start and stop, leaving the stragglers to the round that lists them.
+///
+/// Against the unbounded walk this fails on the cursor: it chases the appends and seals a cursor far
+/// above the round-start tail. On a real pool nothing bounds that chase at all; the appender here stops
+/// after `appended_mid_round` so the unpatched behaviour is measurable rather than a hang.
+TEST(CASGCBoundedWalk, ARoundFoldsThroughItsRoundStartTailAndLeavesTheStragglers)
+{
+    const uint64_t planted = 6;
+    const uint64_t appended_mid_round = 40;
+
+    auto backend = std::make_shared<ChasingWriterBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/hot@cas@"};
+
+    publishRange(*backend, layout, ns, 1, planted);
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, planted},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    backend->arm(&layout, ns, planted, appended_mid_round);
+
+    Gc gc(store, kGc);
+    const std::map<String, UInt64> intake = runRoundCapturingIntake(gc);
+
+    ASSERT_GT(backend->publishedThrough(), planted)
+        << "the mid-round appender never fired, so this test proves nothing about a moving tail";
+
+    const auto cov = coverageOf(*backend, layout, ns);
+    ASSERT_TRUE(cov.has_value()) << "the round must seal a coverage row for the namespace it walked";
+    EXPECT_EQ(cov->last_folded_ref_id, (RefTxnId{1, planted}))
+        << "the walk must fold through the round-start tail and no further -- it chased the writer";
+    EXPECT_FALSE(cov->hold.has_value()) << "reaching the committed frontier is not a hold";
+    EXPECT_NE(cov->classification, 4) << "reaching the committed frontier is not a clamp";
+    EXPECT_EQ(metric(intake, "tails_advanced"), 1u);
+    EXPECT_EQ(metric(intake, "logs_applied"), planted) << "exactly the round-start backlog was folded";
+
+    /// The stragglers are not lost: the next round's listing has a higher tail and folds through it.
+    backend->disarm();
+    const uint64_t total = backend->publishedThrough();
+    ASSERT_GT(total, planted);
+    advanceRecoverableCkptForRawFixture(*backend, layout, ns, RefTxnId{1, total});
+    const std::map<String, UInt64> second = runRoundCapturingIntake(gc);
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, total}))
+        << "the records that landed mid-round are folded by the round that lists them";
+    EXPECT_EQ(metric(second, "tails_advanced"), 1u);
+}
+
+/// ===================== (b) A CTE-AUTHORIZED UNCHANGED NAMESPACE FOLDS NOTHING =====================
+///
+/// Two namespaces; only one gets a new record. The unchanged one must fold NOTHING. Its CTE already
+/// authorizes the sealed cursor as a frontier, so an exact probe at `cursor + 1` would be redundant.
+TEST(CASGCBoundedWalk, ACTEAuthorizedUnchangedNamespaceFoldsNothingWithoutAProbe)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace moved{"00/moved@cas@"};
+    const RootNamespace still{"00/still@cas@"};
+
+    publishRange(*backend, layout, moved, 1, 2);
+    publishRange(*backend, layout, still, 1, 2);
+    writeRecoverableCkptForRawFixture(*backend, layout, moved, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    writeRecoverableCkptForRawFixture(*backend, layout, still, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    runRoundCapturingIntake(gc);
+    ASSERT_EQ(cursorOf(*backend, layout, still), (RefTxnId{1, 2})) << "the seeding round must fold both";
+    ASSERT_EQ(cursorOf(*backend, layout, moved), (RefTxnId{1, 2}));
+
+    /// Only `moved` advances.
+    publishAt(*backend, layout, moved, RefTxnId{1, 3}, "ref_3", 3, DB::UInt128(0x33));
+    advanceRecoverableCkptForRawFixture(*backend, layout, moved, RefTxnId{1, 3});
+
+    backend->resetCounts();
+    const std::map<String, UInt64> intake = runRoundCapturingIntake(gc);
+
+    EXPECT_EQ(refLogGetsFor(*backend, layout, still, 1, 8), 0u)
+        << "the CTE already proves the unchanged namespace's sealed frontier";
+    EXPECT_EQ(backend->getCount(layout.refLogKey(fixture::fixtureLife(still), RefTxnId{1, 3})), 0u)
+        << "a valid CTE needs no successor probe";
+    EXPECT_EQ(metric(intake, "tails_unchanged"), 1u);
+    EXPECT_EQ(metric(intake, "tails_advanced"), 1u);
+    EXPECT_EQ(metric(intake, "logs_applied"), 1u) << "only the one new record was folded, pool-wide";
+    EXPECT_EQ(cursorOf(*backend, layout, moved), (RefTxnId{1, 3})) << "the advanced namespace still folds";
+}
+
+/// ===================== (c) A ROUND WHERE NO TAIL MOVED IS SKIPPED OUTRIGHT =====================
+///
+/// The round-level skip is the EXISTING defer machinery, whose signal is already exactly this
+/// comparison: `RefScanSummary::changed_shards` counts the namespaces whose greatest listed log sits
+/// above their sealed cursor. So a round in which no tail moved folds nothing at all -- no intake phase,
+/// no per-namespace walk, not even the probes -- and an append un-defers it.
+TEST(CASGCBoundedWalk, ARoundWhereNoTailMovedIsDeferredAndAnAppendUnDefersIt)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);   /// the DEFAULT defer window: this test is about the skip
+    const Layout & layout = store->layout();
+    const RootNamespace a{"00/aa@cas@"};
+    const RootNamespace b{"00/bb@cas@"};
+
+    publishRange(*backend, layout, a, 1, 2);
+    publishRange(*backend, layout, b, 1, 2);
+    writeRecoverableCkptForRawFixture(*backend, layout, a, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    writeRecoverableCkptForRawFixture(*backend, layout, b, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_FALSE(runRoundCapturingIntake(gc).empty()) << "the seeding round must actually fold";
+    ASSERT_EQ(cursorOf(*backend, layout, a), (RefTxnId{1, 2}));
+    ASSERT_EQ(cursorOf(*backend, layout, b), (RefTxnId{1, 2}));
+
+    backend->resetCounts();
+    EXPECT_TRUE(runRoundCapturingIntake(gc).empty())
+        << "no tail moved, so the round has no fold to run";
+    EXPECT_EQ(refLogGetsFor(*backend, layout, a, 1, 8), 0u) << "a deferred round reads no ref log at all";
+    EXPECT_EQ(refLogGetsFor(*backend, layout, b, 1, 8), 0u);
+
+    /// One append, in `a` only.
+    publishAt(*backend, layout, a, RefTxnId{1, 3}, "ref_3", 3, DB::UInt128(0xaa3));
+    advanceRecoverableCkptForRawFixture(*backend, layout, a, RefTxnId{1, 3});
+
+    backend->resetCounts();
+    const std::map<String, UInt64> woken = runRoundCapturingIntake(gc);
+    ASSERT_FALSE(woken.empty()) << "an append must un-defer the round";
+    EXPECT_EQ(metric(woken, "tails_advanced"), 1u) << "the appended-to namespace is walked again";
+    EXPECT_EQ(metric(woken, "tails_unchanged"), 1u) << "and only that one";
+    EXPECT_EQ(cursorOf(*backend, layout, a), (RefTxnId{1, 3}));
+    EXPECT_EQ(refLogGetsFor(*backend, layout, b, 1, 8), 0u)
+        << "the still-quiet namespace's CTE proves its carried frontier without a probe";
+}
+
+/// ============ (d) A RAW RECORD ABOVE THE COMMITTED FRONTIER PROVES AND SUPPRESSES NOTHING ============
+///
+/// This is the safety argument. It is stated under an explicit `Authoritative` policy so that the claim
+/// is about the frontier terms and not about which policy the caller happened to pass.
+///
+/// The CTE fixes the namespace's committed frontier at `{1,3}`. A raw record ABOVE that frontier was
+/// never committed, so it is neither a reason to extend the fold nor a reason to suppress destruction:
+/// LIST may observe it, but it cannot manufacture a later authoritative frontier. The dropped blob is
+/// therefore reclaimable after the normal condemn/graduation pipeline.
+TEST(CASGCBoundedWalk, ARawRecordBeyondTheCommittedFrontierCannotSuppressDestruction)
+{
+    auto backend = std::make_shared<ChasingWriterBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/hot@cas@"};
+    const DB::UInt128 blob(0xd00d);
+
+    /// Publish a blob and drop it, both within the round-start listing: its folded in-degree returns to
+    /// zero, so a round with a complete frontier would condemn it.
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "kept", 1, DB::UInt128(0x1), /*birth=*/true);
+    const ManifestRef doomed{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 1};
+    writeBlobBody(*backend, layout, blob);
+    writeManifestRaw(*backend, layout, ns, doomed, {blobEntryFor("data.bin", blob)});
+    writeTxnAt(*backend, layout, ns, RefTxnId{1, 2}, publishCommittedOps("doomed", doomed));
+    dropRefTransition(*backend, layout, ns, "doomed", doomed);
+
+    /// One record lands mid-round, above the committed frontier.
+    backend->arm(&layout, ns, /*published_through*/ 3, /*max_appends*/ 1);
+
+    /// From HERE the deletes are the ROUND's. Opening the pool runs a capability probe that writes and
+    /// deletes its own `_probe/` keys, and counting those against the round would make this assertion
+    /// fail on debris that has nothing to do with the destructive gate.
+    backend->resetCounts();
+
+    Gc gc(store, kGc);
+    const std::map<String, UInt64> intake = runRoundCapturingIntake(gc, UniversePolicy::Authoritative);
+
+    ASSERT_EQ(backend->publishedThrough(), 4u) << "the mid-round appender never fired";
+    ASSERT_EQ(metric(intake, "tails_advanced"), 1u) << "the namespace must have been walked";
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 3})) << "and folded exactly its committed frontier";
+    EXPECT_EQ(metric(intake, "frontier_proven"), 1u)
+        << "the CTE, not the raw record beyond it, fixes the namespace frontier";
+    EXPECT_EQ(metric(intake, "frontier_namespaces"), 1u) << "it is still in the round's universe";
+    EXPECT_EQ(backend->deleteTotal(), 1u)
+        << "the committed frontier permits the round's immediate manifest cleanup. Deleted:"
+        << deletedKeysMessage(*backend);
+    EXPECT_TRUE(backend->head(layout.blobKey(legacyMetaTestRef(blob))).exists);
+
+    /// The raw F+1 record remains outside the CTE; it cannot defer the normal destructive pipeline.
+    backend->disarm();
+    EXPECT_TRUE(runRoundsUntilAbsent(store, gc, *backend, layout, blob, /*max_rounds*/ 8))
+        << "the committed frontier must permit reclamation despite the raw record above it";
+}
+
+/// A store that hides a namespace's records from every LIST does not lose them: the namespace goes
+/// QUIET, and a quiet namespace is exactly the shape the exact-key probe at `cursor + 1` exists for. It
+/// has no listed tail at all, and no bound is taken from a listing anyway -- bounding a namespace by a
+/// tail the liar refuses to admit to would hand it the omission it was hoping for.
+TEST(CASGCBoundedWalk, AListHiddenTailIsCaughtAndFoldedByTheQuietProbePath)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/liar@cas@"};
+
+    publishRange(*backend, layout, ns, 1, 2);
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    Gc gc(store, kGc);
+    runRoundCapturingIntake(gc);
+    ASSERT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 2}));
+
+    /// A third record lands and the store stops listing the namespace at the same moment: its listed
+    /// tail is now nothing at all, while `{1, 3}` is durable and readable by exact key.
+    publishAt(*backend, layout, ns, RefTxnId{1, 3}, "ref_3", 3, DB::UInt128(0x1a3));
+    advanceRecoverableCkptForRawFixture(*backend, layout, ns, RefTxnId{1, 3});
+    backend->hidePrefix(layout.namespaceStreamPrefix(fixture::fixtureLife(ns)));
+
+    const std::map<String, UInt64> intake = runRoundCapturingIntake(gc);
+    EXPECT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, 3}))
+        << "the exact-key probe sees what LIST omits, so the hidden record is folded, not lost";
+    EXPECT_EQ(metric(intake, "unhinted_quiet_walked"), 1u) << "it is the quiet path that caught it";
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(0x1a3)), 1)
+        << "the hidden record's owner edge must be folded, or its blob looks unreferenced";
+}
+
+/// ===================== (e) ONE ROUND TRIP PER MANIFEST EDGE =====================
+///
+/// The edge fold used to pay HEAD-then-GET: two serial round trips per manifest edge, on every folded
+/// log, where the GET alone already answers "is it there".
+TEST(CASGCBoundedWalk, ManifestEdgeFoldsPayAGetAndNeverAHead)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const uint64_t records = 3;
+    publishRange(*backend, layout, ns, 1, records);
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, records},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    backend->resetCounts();
+    Gc gc(store, kGc);
+    runRoundCapturingIntake(gc);
+    ASSERT_EQ(cursorOf(*backend, layout, ns), (RefTxnId{1, records})) << "the round must have folded them";
+
+    uint64_t manifest_heads = 0;
+    uint64_t manifest_gets = 0;
+    for (uint64_t i = 1; i <= records; ++i)
+    {
+        const ManifestId id{ns, ManifestRef{.writer_epoch = 1, .build_sequence = i, .manifest_ordinal = 1}};
+        manifest_heads += backend->headCount(layout.manifestKey(id));
+        manifest_gets += backend->getCount(layout.manifestKey(id));
+    }
+    EXPECT_EQ(manifest_heads, 0u) << "the fold must not HEAD a manifest body it is about to GET";
+    EXPECT_GT(manifest_gets, 0u) << "the bodies were read, so the counters really are watching these keys";
+}
+
+/// An absent manifest body still takes the record-and-continue path -- the GET's own absence is the
+/// signal the HEAD used to carry -- and still raises the fold barrier without ever HEADing the key.
+TEST(CASGCBoundedWalk, AnAbsentManifestBodyStillHoldsWithoutAHead)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishRange(*backend, layout, ns, 1, 3);
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 3},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    const ManifestId gone{ns, ManifestRef{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 1}};
+    deleteManifestBody(*backend, layout, gone);
+
+    backend->resetCounts();
+    Gc gc(store, kGc);
+    runRoundCapturingIntake(gc);
+
+    const auto cov = coverageOf(*backend, layout, ns);
+    ASSERT_TRUE(cov.has_value());
+    EXPECT_EQ(cov->last_folded_ref_id, (RefTxnId{1, 1}))
+        << "the cursor stays BELOW the log whose manifest body is missing";
+    ASSERT_TRUE(cov->hold.has_value()) << "an absent committed manifest body raises the fold barrier";
+    EXPECT_EQ(cov->hold->reason, HoldReason::ManifestBodyMissing);
+    EXPECT_EQ(cov->hold->offending_position, (RefTxnId{1, 2}));
+    EXPECT_EQ(cov->classification, 4);
+    EXPECT_EQ(backend->headCount(layout.manifestKey(gone)), 0u)
+        << "absence is decided by the GET, so the missing body costs no HEAD either";
+}
+
+/// ===================== (f) A NAMESPACE THAT FOLDED NOTHING KEEPS ITS COVERAGE ROW ===================
+///
+/// The fold seal writes a row only for the namespaces the intake loop visits, so any future shortcut
+/// that stops visiting a namespace with nothing to fold would DROP its cursor -- and a dropped cursor is
+/// not a lost optimisation, it is a re-fold from `{0, 0}`: every owner edge counted a second time, every
+/// blob's in-degree inflated, and the eventual correction mass-condemning live data. This pins the row
+/// against exactly that.
+TEST(CASGCBoundedWalk, ANamespaceThatFoldedNothingKeepsItsSealedCursor)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace quiet{"00/quiet@cas@"};
+    const RootNamespace moved{"00/moved@cas@"};
+
+    publishRange(*backend, layout, quiet, 1, 3);
+    publishRange(*backend, layout, moved, 1, 1);
+    writeRecoverableCkptForRawFixture(*backend, layout, quiet, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 3},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    writeRecoverableCkptForRawFixture(*backend, layout, moved, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    runRoundCapturingIntake(gc);
+    const auto before = coverageOf(*backend, layout, quiet);
+    ASSERT_TRUE(before.has_value());
+    ASSERT_EQ(before->last_folded_ref_id, (RefTxnId{1, 3}));
+    ASSERT_FALSE(before->hold.has_value());
+
+    /// A second round in which `quiet` folds nothing and `moved` does, so the round really does write a
+    /// new seal that could have dropped the row.
+    publishAt(*backend, layout, moved, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(0x22));
+    advanceRecoverableCkptForRawFixture(*backend, layout, moved, RefTxnId{1, 2});
+    const std::map<String, UInt64> intake = runRoundCapturingIntake(gc);
+    ASSERT_EQ(metric(intake, "tails_unchanged"), 1u) << "the fixture must actually exercise the quiet case";
+
+    const auto after = coverageOf(*backend, layout, quiet);
+    ASSERT_TRUE(after.has_value())
+        << "the coverage row was DROPPED -- the next round would re-fold this namespace from {0,0}";
+    /// The CURSOR and the HOLD are what the next round trusts, and both ride unchanged.
+    /// `classification` legitimately moves from 2 ("this round folded records") to 1 ("unchanged"),
+    /// because that is what the round did — it is the one field that may differ, so it is the one field
+    /// asserted loosely.
+    EXPECT_EQ(after->last_folded_ref_id, before->last_folded_ref_id)
+        << "a namespace that folded nothing must keep the cursor it had";
+    EXPECT_EQ(after->hold, before->hold);
+    EXPECT_NE(after->classification, 4) << "folding nothing is not a clamp";
+    EXPECT_EQ(metric(intake, "frontier_namespaces"), 2u)
+        << "it stays in the round's universe, so its proof is still owed";
+}
diff --git a/src/Disks/tests/gtest_cas_gc_fold.cpp b/src/Disks/tests/gtest_cas_gc_fold.cpp
new file mode 100644
index 000000000000..c764aa0ac0b7
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_fold.cpp
@@ -0,0 +1,624 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include "cas_test_helpers.h"
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+
+namespace
+{
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+ManifestRef ref(const String &, uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+}
+
+/// Committed new_manifest => +1 per blob entry (BlobInDegreeMatchesActiveManifests).
+/// After a fold, gc/state records snap_attempt == the folding leader's lease.seq, and the fold seal
+/// lives under (snap_generation, snap_attempt).
+TEST(CASGCFold, FoldAdoptsAttemptEqualsLeaseSeq)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+
+    const auto st = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    EXPECT_EQ(st.snap_attempt, st.lease.seq);
+    EXPECT_GT(st.snap_generation, 0u);
+    /// The one-pass round's fold seal is durable under (snap_generation, snap_attempt) — the adopted
+    /// attempt locates it (a seal under any other attempt would be unadopted debris).
+    EXPECT_TRUE(backend->head(store->layout().foldSealKey(st.snap_generation, st.snap_attempt)).exists);
+}
+
+TEST(CASGCFold, CommittedAddEmitsPlusOnePerBlob)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r,
+        {blobEntryFor("a", DB::UInt128(1)), blobEntryFor("b", DB::UInt128(2))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(2)), 1);
+}
+
+/// Owner removal => -1 per blob entry; in-degree returns to 0.
+TEST(CASGCFold, RemovalEmitsMinusOne)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    gc.runRegularRound();
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+}
+
+/// Precommit with a PRESENT, valid body => +1.
+TEST(CASGCFold, PrecommitBodyPresentEmitsPlusOne)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    addPrecommitTransition(*backend, store->layout(), ns, DB::UInt128(7), "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+}
+
+/// Precommit whose body is ABSENT => NO delta (control #4); the 404 must NOT throw.
+TEST(CASGCFold, PrecommitMissingBodyEmitsNoDelta)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    addPrecommitTransition(*backend, store->layout(), ns, DB::UInt128(7), "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    EXPECT_NO_THROW(gc.runRegularRound());
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+}
+
+/// FOLD BARRIER (control #23): a LIVE precommit binding whose body is missing does NOT advance the
+/// durable fold cursor past its activation event; when the body appears the cursor advances.
+TEST(CASGCFold, FoldBarrierHaltsCursorAtLiveMissingBodyPrecommit)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const uint64_t v = addPrecommitTransition(*backend, store->layout(), ns, DB::UInt128(7), "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    EXPECT_NO_THROW(gc.runRegularRound());
+    EXPECT_LT(foldCursorOf(*backend, store->layout(), ns, 0), v);   // barrier: halted at the activation
+
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    gc.runRegularRound();
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    EXPECT_GE(foldCursorOf(*backend, store->layout(), ns, 0), v);   // barrier lifted by activation
+}
+
+/// Promote of an already-activated precommit is a PURE OWNER MOVE: NO delta, body not condemned.
+TEST(CASGCFold, PromoteOfActivatedPrecommitEmitsNoDelta)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    addPrecommitTransition(*backend, store->layout(), ns, DB::UInt128(7), "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    promoteTransition(*backend, store->layout(), ns, DB::UInt128(7), "tbl", r);
+    gc.runRegularRound();
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);   // unchanged, still pinned
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);   // not condemned
+}
+
+/// Committed add naming a MISSING body (404) => clamp + anomaly, never a guessed +1, never a throw.
+TEST(CASGCFold, CommittedMissingBodyClampsCursorAndRecordsAnomaly)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const uint64_t v = publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);  // no body
+    Gc gc(store, kGc);
+    RoundReport report;
+    EXPECT_NO_THROW(report = gc.runRegularRound());
+    EXPECT_TRUE(report.hasAnomaly(ns, /*shard*/0));
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+    EXPECT_LT(foldCursorOf(*backend, store->layout(), ns, 0), v);
+}
+
+/// A body whose self-ref disagrees (PRESENT but INVALID) => hard fail closed (controls #19/#20).
+TEST(CASGCFold, RefMismatchFailsClosed)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    PartManifest bad;
+    bad.ref = ref("srv-a:1", 1, 0xBB);   // != r
+    bad.root_namespace_id = ns;
+    bad.entries = {blobEntryFor("a", DB::UInt128(1))};
+    bad.payload_digest = computePayloadDigest(bad);
+    backend->putIfAbsent(store->layout().manifestKey(ManifestId{ns, r}), encodePartManifest(bad));
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]{ gc.runRegularRound(); });
+}
+
+/// Owner-removal whose OLD committed body is gone at removal-fold => clamp + anomaly, no partial -1.
+TEST(CASGCFold, RemovalWithMissingOldBodyClampsAndRecordsAnomaly)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();   // +1; blob 1 in-degree 1
+
+    const uint64_t removal_version = dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    deleteManifestBody(*backend, store->layout(), ManifestId{ns, r});   // body gone before its decrement
+
+    RoundReport report;
+    EXPECT_NO_THROW(report = gc.runRegularRound());
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);   // unchanged: no silent -1
+    EXPECT_TRUE(report.hasAnomaly(ns, /*shard*/0));
+    EXPECT_LT(foldCursorOf(*backend, store->layout(), ns, 0), removal_version);
+}
+
+/// (The two `CASGCFold.IncarnationMismatchRestartsFoldAtZero*` tests were removed with the snapshot+log
+/// ref model: they injected a stale per-shard fold cursor beyond the live mutable shard's version and
+/// asserted the fold RESET the cursor to 0 on an incarnation mismatch. There is no mutable per-shard
+/// cursor to stale-reset anymore -- the durable cursor is a strictly-increasing `RefTxnId`, and a
+/// recreated namespace uses a GREATER `writer_epoch`, so the ABA hazard is impossible by construction.
+/// The ref-model equivalent -- `remove_namespace` then a later `namespace_birth` with a greater id folds
+/// normally -- is covered by `gtest_cas_gc_shard_incarnation.cpp` and `gtest_cas_ref_gc.cpp`.)
+
+/// T0 (2026-07-02 snapshot-streaming): an idle round — no journal changes, no retired entries — touches
+/// ZERO run objects. After one populated round, reset the counters and run a no-op round; the fold must
+/// carry the parent generation's `RunRef` verbatim into the new fold_seal (same key, same checksum, same
+/// generation) and NOT read or write any `.../blob_target/...` object.
+TEST(CASGCFold, EmptyDeltaShardCarriesParentRunRef)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    /// gc_fold_max_defer_rounds=0 forces fold-every-round: this test exercises the pure-ref-carry FOLD
+    /// path on an idle round; without it the round would DEFER (re-adopt the sealed generation) and never
+    /// mint the carried generation this test inspects.
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();   // round 1: folds the +1, seals the gen-1 blob_target run
+
+    const auto st1 = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    const auto parent_seal = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(st1.snap_generation, st1.snap_attempt))->bytes);
+    ASSERT_EQ(parent_seal.blob_target_runs.size(), 1u);
+    const RunRef parent_ref = parent_seal.blob_target_runs.front();
+
+    backend->resetCounts();
+    gc.runRegularRound();   // round 2: no changes => pure ref-carry, zero run I/O
+
+    EXPECT_EQ(backend->ioCountForKeysContaining("/blob_target/"), 0u)
+        << "idle round must not GET/getStream/PUT any blob_target run object";
+
+    const auto st2 = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    EXPECT_GT(st2.snap_generation, st1.snap_generation);
+    const auto new_seal = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(st2.snap_generation, st2.snap_attempt))->bytes);
+    ASSERT_EQ(new_seal.blob_target_runs.size(), 1u);
+    const RunRef carried = new_seal.blob_target_runs.front();
+    EXPECT_EQ(carried.key, parent_ref.key) << "carried ref points at the PARENT generation's run key";
+    EXPECT_EQ(carried.checksum, parent_ref.checksum);
+    EXPECT_EQ(carried.shard, 0u);
+    EXPECT_EQ(carried.generation, st1.snap_generation)
+        << "the carried ref names the generation whose key namespace physically holds the object";
+}
+
+/// The round AFTER a ref-carry, with a real delta, folds THROUGH the carried ref: the new generation's
+/// run is produced from the OLD-generation run (resolved via the carried ref, not by key construction)
+/// merged with the delta, and the resulting in-degree is correct.
+TEST(CASGCFold, FoldResolvesThroughCarriedRef)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref("srv-a:1", 1, 0xAA);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();   // gen 1: blob 1 in-degree 1
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    gc.runRegularRound();   // gen 2: no delta => carries the gen-1 ref
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1)
+        << "in-degree resolves through the carried parent ref";
+
+    // A real delta on the NEXT round must fold through the carried ref and drop blob 1 to zero.
+    const ManifestRef r2 = ref("srv-a:2", 2, 0xBB);
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", r1, r2);
+
+    gc.runRegularRound();   // gen 3: -1 on blob 1 (old owner dropped), +1 on blob 2
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0)
+        << "fold through the carried ref applied the -1 correctly";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(2)), 1);
+}
+
+/// previewDeletes resolves runs through the current seal's refs, not by key construction. After a
+/// pure ref-carry round the current seal's `blob_target_runs` point at an OLDER generation's key; the
+/// preview must open that physical object via the ref and report the correct in-degree — here blob 1 is
+/// still referenced, so its carried-ref-resolved in-degree is 1 and it is NOT surfaced as a candidate.
+/// (A carried ref that the preview failed to resolve would mis-open the run and either throw or spuriously
+/// surface the still-referenced blob.)
+TEST(CASGCFold, PreviewResolvesCarriedRef)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// gc_fold_max_defer_rounds=0 forces the idle second round to FOLD (pure ref-carry) rather than
+    /// DEFER, so the current seal's `blob_target_runs` point at the parent generation's key (the carried
+    /// ref this test resolves through).
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();   // gen 1: blob referenced, in-degree 1
+    const auto st1 = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+
+    gc.runRegularRound();   // gen 2: no delta, no retired => pure ref-carry (ref points back at gen 1)
+    const auto st2 = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    ASSERT_GT(st2.snap_generation, st1.snap_generation);
+    const auto seal2 = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(st2.snap_generation, st2.snap_attempt))->bytes);
+    ASSERT_EQ(seal2.blob_target_runs.size(), 1u);
+    ASSERT_EQ(seal2.blob_target_runs.front().generation, st1.snap_generation)
+        << "the current seal's ref physically lives at the parent generation (carried, not reconstructed)";
+
+    // The preview resolves the carried ref (a gen-1 physical key) and computes in-degree 1 => blob 1 is
+    // not a delete candidate. Resolution-by-ref is the property under test.
+    const auto preview = gc.previewDeletes();
+    for (const auto & e : preview)
+        EXPECT_NE(e.ref, (DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(blob)})) << "still-referenced blob must not be surfaced (carried ref resolved to in-degree 1)";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), blob), 1)
+        << "in-degree through the carried parent ref is 1";
+}
+
+/// Per-consumer whole-file seal-checksum RED tests (codecs-v3 phase 5, Task 6) at the seal-driven
+/// consumers. Setup: fold one referenced blob into a sealed generation, then corrupt the persisted
+/// seal's blob_target_runs[0].checksum (the stored run bytes stay valid), so the abort comes from the
+/// seal-checksum verify, not a row invariant.
+namespace
+{
+String corruptSealedRunChecksum(InMemoryBackend & backend, const Layout & layout, const GcState & st)
+{
+    const String sk = layout.foldSealKey(st.snap_generation, st.snap_attempt);
+    const auto existing = backend.get(sk);
+    auto seal = decodeFoldSeal(existing->bytes);
+    if (seal.blob_target_runs.empty())
+        return {};
+    const String run_key = seal.blob_target_runs.front().key;
+    seal.blob_target_runs.front().checksum = seal.blob_target_runs.front().checksum + 1;
+    backend.putOverwrite(sk, encodeFoldSeal(seal), existing->token);
+    return run_key;
+}
+}
+
+TEST(CASGCFold, PreviewDeletesSealChecksumMismatchFailsClosed)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();   // seals gen-1 with one blob_target run
+    const auto st = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    ASSERT_FALSE(corruptSealedRunChecksum(*backend, store->layout(), st).empty());
+
+    // A deletion preview must never be derived from an unverified run: fail closed.
+    Gc gc2(store, kGc);   // fresh read of the corrupted seal
+    EXPECT_THROW(gc2.previewDeletes(), DB::Exception);
+}
+
+TEST(CASGCFold, FsckSealChecksumMismatchCataloguedAndAuditCompletes)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref("srv-a:1", 1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    replaceRecoverableCkptForRawFixture(
+        *backend, store->layout(), ns,
+        RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    const auto st = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+
+    /// A present-but-unreferenced blob (written AFTER the round so GC never touches it) is what makes
+    /// fsck enter its GC-pipeline classification path (guarded by a non-empty unreferenced set), which
+    /// is where it streams + seal-checksum-verifies the snapshot runs.
+    writeBlobBody(*backend, store->layout(), DB::UInt128(2));
+
+    const String bad_run_key = corruptSealedRunChecksum(*backend, store->layout(), st);
+    ASSERT_FALSE(bad_run_key.empty());
+
+    // fsck is a read-only auditor: it must CATALOGUE the corrupt run and COMPLETE, not abort the scan.
+    FsckReport report;
+    EXPECT_NO_THROW(report = runFsck(*store, /*detail*/ true));
+    EXPECT_GE(report.corrupted_runs, 1u);
+    bool catalogued = false;
+    for (const auto & o : report.objects)
+        if (o.cls == FsckClass::CorruptedRun && o.key == bad_run_key)
+            catalogued = true;
+    EXPECT_TRUE(catalogued) << "the corrupt run must be catalogued with its key";
+}
+
+/// A mid-log clamp must be RECOVERABLE (spec §Step 3 transaction atomicity). A single log carrying two
+/// ops -- [drop committed A (a `-1` whose body is present at removal-fold), add precommit B (whose body is
+/// transiently absent)] -- clamps on B. The `-1` on A must NOT be merged into the round's owner-removed
+/// cleanup, because the post-CAS body delete would then reclaim A's body while A's edge stays unfolded
+/// behind the clamp; the next re-fold of that same log would then find A's body missing and clamp forever
+/// (a permanent pool-wide destructive freeze). With per-log staging, A's body survives the clamp round and
+/// the log folds cleanly once B's body reappears.
+TEST(CASGCFold, MidLogClampPreservesEarlierRemovalBodyAndRecovers)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef a = ref("srv-a:1", 1, 0xAA);
+    const ManifestRef b = ref("srv-a:2", 2, 0xBB);
+
+    /// Round 0: commit A (references blob 1). A's body is present and folds a +1.
+    writeManifestRaw(*backend, store->layout(), ns, a, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "r1", std::nullopt, a);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    /// ONE log with two ops: drop committed A (`-1`, body present), then add precommit B (`+1`, body
+    /// staged then removed => a transient 404 clamps the log after A's `-1` already folded).
+    writeManifestRaw(*backend, store->layout(), ns, b, {blobEntryFor("b", DB::UInt128(2))});
+    deleteManifestBody(*backend, store->layout(), ManifestId{ns, b});   // B's body absent => clamp
+    const uint64_t log_seq = appendRefLogSeed(*backend, store->layout(), ns,
+        {ownerTransitionOp(RefOwnerBinding{RefOwnerKind::Committed, "r1", a}, std::nullopt),
+         ownerTransitionOp(std::nullopt, RefOwnerBinding{RefOwnerKind::Precommit, "r2", b})});
+    advanceRecoverableCkptForRawFixture(*backend, store->layout(), ns, RefTxnId{1, log_seq});
+
+    const RoundReport clamp_report = gc.runRegularRound();
+    EXPECT_TRUE(clamp_report.hasAnomaly(ns, /*shard*/0)) << "the missing B body must clamp this log";
+    EXPECT_LT(foldCursorOf(*backend, store->layout(), ns, 0), log_seq) << "the clamp halts the cursor below the log";
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, a})).exists)
+        << "A's body must survive the clamp round: its `-1` was staged, not merged, so no post-CAS delete "
+           "reclaimed it -- otherwise the re-fold would clamp on A's missing body forever";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1) << "A's `-1` was not adopted (clamp)";
+
+    /// The transient 404 heals: B's body reappears. The next round re-folds the SAME log cleanly.
+    writeManifestRaw(*backend, store->layout(), ns, b, {blobEntryFor("b", DB::UInt128(2))});
+    const RoundReport clean_report = gc.runRegularRound();
+    EXPECT_FALSE(clean_report.hasAnomaly(ns, /*shard*/0)) << "with both bodies present the log folds; no clamp";
+    EXPECT_GE(foldCursorOf(*backend, store->layout(), ns, 0), log_seq) << "the cursor advanced past the recovered log";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0) << "A's `-1` applied";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(2)), 1) << "B's `+1` applied";
+}
+
+/// A `+1` precommit whose body is PERMANENTLY absent and whose build is below the durable watermark floor
+/// (provably dead -- the exact fact the orphan sweep uses to reclaim the body) must be SKIPPED, not held on
+/// the fold barrier forever. Without a terminal rule this table clamps every round with no resolution (a
+/// late-predecessor precommit whose body was already reclaimed). The watermark is seeded so the precommit's
+/// build is dead; the fold must advance the cursor past the log and record no clamp anomaly.
+TEST(CASGCFold, DeadPrecommitWithMissingBodyIsSkippedNotClampedForever)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    /// The namespace's server-root prefix is "srv"; seed its watermark floor so build_sequence 5 is retired.
+    const RootNamespace ns{"srv/tbl"};
+    setWatermarkMinActive(*backend, store->layout(), "srv", /*writer_epoch*/1, /*min_active*/10);
+
+    /// A precommit naming a build (writer_epoch 1, build_sequence 5) whose body is never written.
+    const ManifestRef dead = ManifestRef{.writer_epoch = 1, .build_sequence = 5, .manifest_ordinal = 1};
+    const uint64_t log_seq =
+        addPrecommitTransition(*backend, store->layout(), ns, DB::UInt128(7), "r1", std::nullopt, dead);
+
+    Gc gc(store, kGc);
+    const RoundReport report = gc.runRegularRound();
+    EXPECT_FALSE(report.hasAnomaly(ns, /*shard*/0))
+        << "a provably-dead precommit's missing body is skipped, not clamped";
+    EXPECT_GE(foldCursorOf(*backend, store->layout(), ns, 0), log_seq)
+        << "the fold advanced past the log instead of holding the barrier forever";
+
+    /// A second identical round stays clean (terminal resolution, not a recurring clamp).
+    const RoundReport report2 = gc.runRegularRound();
+    EXPECT_FALSE(report2.hasAnomaly(ns, /*shard*/0)) << "the resolution is terminal: no recurring clamp";
+}
+
+/// A10: a single clamp anomaly must suppress ALL destructive actions in the round — the merge-side
+/// deletes AND the post-CAS ref/namespace cleanup — from ONE decision, not two independent recomputes
+/// of !report.anomalies.empty() that a future edit could desync (over-delete class). This pins that a
+/// clamped round reclaims nothing.
+TEST(CASGCFold, SingleAnomalySuppressesEveryDestructiveActionInTheRound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef a = ref("srv-a:1", 1, 0xAA);
+    const ManifestRef b = ref("srv-a:2", 2, 0xBB);
+
+    /// Round 0: commit A (references blob 1); its body folds a +1.
+    writeManifestRaw(*backend, store->layout(), ns, a, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "r1", std::nullopt, a);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    ASSERT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    /// One log: drop committed A (`-1`, body present) then add precommit B whose body is absent -> the
+    /// missing B body clamps the log AFTER A's `-1` folded.
+    writeManifestRaw(*backend, store->layout(), ns, b, {blobEntryFor("b", DB::UInt128(2))});
+    deleteManifestBody(*backend, store->layout(), ManifestId{ns, b});
+    const uint64_t log_seq = appendRefLogSeed(*backend, store->layout(), ns,
+        {ownerTransitionOp(RefOwnerBinding{RefOwnerKind::Committed, "r1", a}, std::nullopt),
+         ownerTransitionOp(std::nullopt, RefOwnerBinding{RefOwnerKind::Precommit, "r2", b})});
+    advanceRecoverableCkptForRawFixture(*backend, store->layout(), ns, RefTxnId{1, log_seq});
+
+    const RoundReport rep = gc.runRegularRound();
+    ASSERT_TRUE(rep.hasAnomaly(ns, /*shard*/0)) << "the missing B body must clamp this round";
+    /// The clamp suppresses the WHOLE destructive pipeline this round: no deletes, no redeletes, and
+    /// A's `-1` stays unadopted (its body must survive, else the re-fold clamps on it forever).
+    EXPECT_EQ(rep.deleted, 0u);
+    EXPECT_EQ(rep.redeleted, 0u);
+    EXPECT_EQ(rep.graduated, 0u);
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, a})).exists);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+}
+
+/// A10 follow-up: the round-side destructive gates -- the perpetual dead-life janitor AND
+/// `cleanupRefObjects`' covered ref-object deletion -- must ALSO honor the round's ONE
+/// `suppress_destructive` decision, not just fold()'s merge-side reducers pinned above. A clamp anomaly in
+/// one namespace must suppress destructive cleanup POOL-WIDE: dead-life physical debris must not be
+/// swept, and an unrelated live
+/// table's snapshot-covered ref-log must not be deleted, in the SAME clamped round. A clean round
+/// afterward proves the setup really was cleanup-eligible, not vacuously untouched.
+TEST(CASGCFold, RoundSideAnomalySuppressesRefLogCleanupWhileRemovalDebrisStaysJanitorWork)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    Gc gc(store, kGc);
+
+    /// Namespace 1: the clamp trigger (same construction as
+    /// SingleAnomalySuppressesEveryDestructiveActionInTheRound above).
+    const RootNamespace ns_clamp{"00/aa@cas@"};
+    const ManifestRef a = ref("srv-a:1", 1, 0xAA);
+    const ManifestRef b = ref("srv-a:2", 2, 0xBB);
+    writeManifestRaw(*backend, layout, ns_clamp, a, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns_clamp, "r1", std::nullopt, a);
+    runRegularRoundReclaiming(gc);   /// folds A cleanly; establishes the baseline before the clamp
+
+    /// Namespace 2: a namespace mid-removal with physical manifest and verbatim-file debris. Generation
+    /// 7 has no lifecycle-specific cleanup pass: terminal folding records evidence, while these bytes
+    /// remain inert work for the perpetual janitor and orphan-manifest sweep.
+    const RootNamespace ns_removed{"00/cc@cas@"};
+    RefOp remove_op;
+    remove_op.kind = RefOpKind::RemoveNamespace;
+    const uint64_t removal_log_seq = appendRefLogSeed(*backend, layout, ns_removed, {remove_op});
+    writeRecoverableCkptForRawFixture(
+        *backend, layout, ns_removed,
+        RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, removal_log_seq},
+                .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+    /// Keyed at the life the CATALOG names for this namespace (`appendRefLogSeed` admitted it above),
+    /// which is the physical life that owns the eventual janitor work. Spelling the sentinel here instead
+    /// would plant debris under the wrong life and make the retention assertion vacuous.
+    const String debris_key
+        = layout.namespaceFilesPrefix(CasRefCatalog::lifeIfCataloged(*backend, layout, ns_removed).value())
+        + "leftover_verbatim_file";
+    backend->putIfAbsent(debris_key, "debris");
+    const ManifestRef removed_body = ref("srv-r:1", 1, 0xEE);
+    writeManifestRaw(*backend, layout, ns_removed, removed_body, {blobEntryFor("r", DB::UInt128(9))});
+    const String debris_manifest_key = layout.manifestKey(ManifestId{ns_removed, removed_body});
+
+    /// Namespace 3: a live table with an exact checkpoint-named recovery triple -- exactly what a
+    /// clamp-free round's `cleanupRefObjects` may clean below that base.
+    const RootNamespace ns_covered{"00/dd@cas@"};
+    const ManifestRef c1 = ref("srv-c:1", 1, 0xCC);
+    const ManifestRef c2 = ref("srv-c:2", 2, 0xDD);
+    writeManifestRaw(*backend, layout, ns_covered, c1, {blobEntryFor("c", DB::UInt128(3))});
+    writeManifestRaw(*backend, layout, ns_covered, c2, {blobEntryFor("d", DB::UInt128(4))});
+    const uint64_t cv1 = publishCommittedTransition(*backend, layout, ns_covered, "t1", std::nullopt, c1);
+    const uint64_t cv2 = publishCommittedTransition(*backend, layout, ns_covered, "t2", std::nullopt, c2);
+    writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns_covered.string(), RefTxnId{1, cv2},
+        {committedRow("t1", c1), committedRow("t2", c2)}));
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns_covered, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, cv2},
+        .checkpoint_snapshot_id = RefTxnId{1, cv2},
+        .last_epoch_seal = std::nullopt,
+    });
+    const String covered_log_key = layout.refLogKey(fixture::fixtureLife(ns_covered), RefTxnId{1, cv1});
+    ASSERT_TRUE(backend->head(covered_log_key).exists);
+
+    /// Trigger the clamp in ns_clamp: drop committed A, add precommit B whose body is absent.
+    writeManifestRaw(*backend, layout, ns_clamp, b, {blobEntryFor("b", DB::UInt128(2))});
+    deleteManifestBody(*backend, layout, ManifestId{ns_clamp, b});
+    const uint64_t clamp_log_seq = appendRefLogSeed(*backend, layout, ns_clamp,
+        {ownerTransitionOp(RefOwnerBinding{RefOwnerKind::Committed, "r1", a}, std::nullopt),
+         ownerTransitionOp(std::nullopt, RefOwnerBinding{RefOwnerKind::Precommit, "r2", b})});
+    advanceRecoverableCkptForRawFixture(*backend, layout, ns_clamp, RefTxnId{1, clamp_log_seq});
+
+    const RoundReport rep = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(rep.hasAnomaly(ns_clamp, /*shard*/0)) << "the missing B body must clamp this round";
+    EXPECT_EQ(rep.deleted, 0u);
+    EXPECT_EQ(rep.redeleted, 0u);
+    EXPECT_EQ(rep.graduated, 0u);
+
+    /// Removal folding never performs lifecycle-specific physical cleanup, with or without a clamp.
+    EXPECT_TRUE(backend->head(debris_manifest_key).exists)
+        << "removed manifest debris remains ordinary orphan-sweep work";
+    EXPECT_TRUE(backend->head(debris_key).exists)
+        << "removed verbatim-file debris remains ordinary janitor work";
+
+    /// `cleanupRefObjects` must not have deleted anything anywhere this round.
+    EXPECT_TRUE(backend->head(covered_log_key).exists)
+        << "a clamp anywhere in the round must suppress ref-log cleanup pool-wide, even for an unrelated live table";
+
+    /// Heal the clamp and run a clean round. Ordinary ref-log cleanup resumes, while removal debris
+    /// remains physically untouched by the lifecycle path.
+    writeManifestRaw(*backend, layout, ns_clamp, b, {blobEntryFor("b", DB::UInt128(2))});
+    const RoundReport clean_rep = runRegularRoundReclaiming(gc);
+    EXPECT_FALSE(clean_rep.hasAnomaly(ns_clamp, /*shard*/0));
+    EXPECT_TRUE(backend->head(debris_manifest_key).exists)
+        << "a clamp-free fold still performs no lifecycle-specific manifest deletion";
+    EXPECT_TRUE(backend->head(debris_key).exists)
+        << "a clamp-free fold still performs no lifecycle-specific verbatim-file deletion";
+    EXPECT_FALSE(backend->head(covered_log_key).exists) << "a clamp-free round cleans the covered ref-log";
+}
diff --git a/src/Disks/tests/gtest_cas_gc_frontier_gate.cpp b/src/Disks/tests/gtest_cas_gc_frontier_gate.cpp
new file mode 100644
index 000000000000..56a793157b81
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_frontier_gate.cpp
@@ -0,0 +1,3481 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CatalogLifecycleReconciler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Common/ProfileEvents.h>
+#include "cas_test_helpers.h"
+
+#include <Poco/StreamChannel.h>
+
+#include <algorithm>
+#include <condition_variable>
+#include <mutex>
+#include <set>
+#include <sstream>
+#include <stdexcept>
+#include <thread>
+
+namespace DB::ErrorCodes
+{
+    extern const int NETWORK_ERROR;
+    extern const int CORRUPTED_DATA;
+}
+
+/// THE DESTRUCTIVE-ROUND FRONTIER PROOF (spec 2026-07-27 "ref chain complete cut" §5).
+///
+/// Reachability is a property of the WHOLE POOL. A blob is unreferenced only if no namespace anywhere
+/// owns an edge to it, so a round that deletes one is asserting something about every namespace at
+/// once -- including the ones it never looked at. Task 7 made the per-namespace half of that assertion
+/// cheap and exact: one `GET` at the cursor's arithmetic successor, absent means end-of-stream. Task 8
+/// made a namespace that could NOT be walked say so durably. What neither can supply is the SET those
+/// proofs have to cover, and that is what this task is about.
+///
+/// So the gate has three terms, and a round destroys only when all three are clear:
+///
+///     suppress_destructive = any anomaly this round
+///                          OR any hold the seal carries
+///                          OR the frontier is incomplete
+///
+/// The second term is STRUCTURAL. Every hold recorded today also records an anomaly, so the first term
+/// happens to imply it -- but the invariant is the hold SET, not that coincidence, and the gate reads
+/// the seal directly so that a future change to anomaly recording cannot quietly open it.
+///
+/// The third term is the SET, and only the catalog supplies it. The scenario that makes that so is the
+/// one these tests open with: a hidden acked `+1` in a namespace no listing mentions and no sealed cursor
+/// names, while a visible `-1` elsewhere drives the shared blob's OBSERVABLE in-degree to zero. Every
+/// proof the round holds comes back clean and the blob is still owned. It survives because the round's
+/// universe is the catalog's `Live`/`Removing` set, so that namespace is a member the round owes a proof
+/// for and cannot supply one -- neither the listing's silence nor the missing cursor can shrink the set.
+///
+/// The tests here come in two shapes. Ones whose subject is the OPEN gate run on the production path,
+/// with no policy argument at all. Ones whose subject is a SUPPRESSOR either pass `StageA_Suppressed`
+/// explicitly or arrange the suppressing condition on the pool, and assert every delete family inert PER
+/// FAMILY -- an aggregate zero can hide one family running while another did not.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace ProfileEvents
+{
+extern const Event CASGCRefWalkPlansBuilt;
+extern const Event CASGCUnmatchedAdoptedParentLives;
+extern const Event CASGCNamespaceCleanupLeaks;
+}
+
+namespace
+{
+
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+
+/// The lying store, shared from `cas_test_helpers.h`: every key is served by exact GET while the
+/// selected ones are HIDDEN from every LIST. That is the only way to build the cross-namespace
+/// scenario -- the hidden namespace's records stay durable and readable, so a round that KNOWS to
+/// look for them finds them, while a round that only enumerates never learns they exist. Composed
+/// over `CountingBackend` because these tests also assert request counts.
+using CountingHintHoleBackend = DB::Cas::tests::HintHoleBackendOn<DB::Cas::tests::CountingBackend>;
+
+class DrainRaceBackend final : public CountingBackend
+{
+public:
+    using CountingBackend::casPut;
+    using CountingBackend::get;
+    using CountingBackend::putIfAbsent;
+
+    void blockNextCatalogCas(const String & key)
+    {
+        std::lock_guard lock(control_mutex);
+        catalog_key = key;
+        block_next_catalog_cas = true;
+    }
+
+    void loseNextCatalogCasResponse(const String & key)
+    {
+        std::lock_guard lock(control_mutex);
+        catalog_key = key;
+        lose_next_catalog_cas_response = true;
+    }
+
+    void conflictNextCatalogCas(const String & key)
+    {
+        std::lock_guard lock(control_mutex);
+        catalog_key = key;
+        conflict_next_catalog_cas = true;
+    }
+
+    void waitForBlockedCatalogCas()
+    {
+        std::unique_lock lock(control_mutex);
+        control_cv.wait(lock, [&] { return catalog_cas_blocked; });
+    }
+
+    void releaseBlockedCatalogCas()
+    {
+        std::lock_guard lock(control_mutex);
+        release_catalog_cas = true;
+        control_cv.notify_all();
+    }
+
+    void clearJournal()
+    {
+        std::lock_guard lock(journal_mutex);
+        journal.clear();
+    }
+
+    std::vector<String> journalSnapshot() const
+    {
+        std::lock_guard lock(journal_mutex);
+        return journal;
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        record("get " + key);
+        return CountingBackend::get(key, range);
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        record("list " + prefix);
+        return CountingBackend::list(prefix, cursor, limit);
+    }
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        record("put_begin " + key);
+        const PutResult result = CountingBackend::putIfAbsent(key, bytes, meta);
+        record("put_end " + key);
+        return result;
+    }
+
+    CasResult casPut(
+        const String & key, const String & bytes, const std::optional<Token> & expected,
+        const ObjectMeta & meta) override
+    {
+        record("cas_begin " + key);
+        bool lose_response = false;
+        bool force_conflict = false;
+        {
+            std::unique_lock lock(control_mutex);
+            if (key == catalog_key && block_next_catalog_cas)
+            {
+                block_next_catalog_cas = false;
+                catalog_cas_blocked = true;
+                control_cv.notify_all();
+                control_cv.wait(lock, [&] { return release_catalog_cas; });
+            }
+            if (key == catalog_key && lose_next_catalog_cas_response)
+            {
+                lose_next_catalog_cas_response = false;
+                lose_response = true;
+            }
+            if (key == catalog_key && conflict_next_catalog_cas)
+            {
+                conflict_next_catalog_cas = false;
+                force_conflict = true;
+            }
+        }
+        if (force_conflict)
+        {
+            record("cas_forced_conflict " + key);
+            return {.outcome = CasOutcome::Conflict, .token = {}};
+        }
+        const CasResult result = CountingBackend::casPut(key, bytes, expected, meta);
+        record("cas_end " + key);
+        if (lose_response && result.outcome == CasOutcome::Committed)
+        {
+            record("cas_response_lost " + key);
+            throw std::runtime_error("injected lost catalog CAS response");
+        }
+        return result;
+    }
+
+private:
+    void record(String entry) const
+    {
+        std::lock_guard lock(journal_mutex);
+        journal.push_back(std::move(entry));
+    }
+
+    mutable std::mutex journal_mutex;
+    mutable std::vector<String> journal;
+    std::mutex control_mutex;
+    std::condition_variable control_cv;
+    String catalog_key;
+    bool block_next_catalog_cas = false;
+    bool catalog_cas_blocked = false;
+    bool release_catalog_cas = false;
+    bool lose_next_catalog_cas_response = false;
+    bool conflict_next_catalog_cas = false;
+};
+
+class PostFoldUnreadableTerminalBackend final : public CountingBackend
+{
+public:
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = CountingBackend::list(prefix, cursor, limit);
+        if (prefix.ends_with("/cas/ns/"))
+            for (ListedKey & listed : page.keys)
+                listed.token.reset();
+        return page;
+    }
+
+    HeadResult head(const String & key) override
+    {
+        if (key == unreadable_key)
+            throw std::runtime_error("injected post-fold terminal read failure for " + key);
+        return CountingBackend::head(key);
+    }
+
+    void makeUnreadable(String key)
+    {
+        unreadable_key = std::move(key);
+    }
+
+    bool existsIgnoringFault(const String & key)
+    {
+        return CountingBackend::head(key).exists;
+    }
+
+private:
+    String unreadable_key;
+};
+
+class ScopedCasGcLogCapture
+{
+public:
+    ScopedCasGcLogCapture()
+        : logger(getLogger("CasGc"))
+        , channel(new Poco::StreamChannel(stream))
+        , old_channel(logger->getChannel())
+        , old_level(logger->getLevel())
+    {
+        logger->setChannel(channel.get());
+        logger->setLevel("warning");
+    }
+
+    ~ScopedCasGcLogCapture()
+    {
+        logger->setChannel(old_channel);
+        logger->setLevel(old_level);
+    }
+
+    String captured() const
+    {
+        return stream.str();
+    }
+
+private:
+    LoggerPtr logger;
+    std::ostringstream stream; // STYLE_CHECK_ALLOW_STD_STRING_STREAM
+    Poco::AutoPtr<Poco::StreamChannel> channel;
+    Poco::Channel * old_channel;
+    int old_level;
+};
+
+struct CompletedRemovingFixture
+{
+    RootNamespace ns;
+    UInt128 life_id{};
+    String checkpoint_key;
+    String checkpoint_bytes;
+};
+
+CompletedRemovingFixture seedCompletedRemoving(
+    DrainRaceBackend & backend, const PoolPtr & store, const UInt128 & lease_owner)
+{
+    const Layout & layout = store->layout();
+    CompletedRemovingFixture fixture{
+        .ns = RootNamespace{"00/drain-race@cas@"},
+        .life_id = UInt128{177},
+        .checkpoint_key = {},
+        .checkpoint_bytes = {}};
+    CasRefCatalog::casAdmitEntry(backend, layout, store->poolConfig().gc_shards, CatalogEntry{
+        .ns = fixture.ns, .state = NsState::Live, .incarnation = fixture.life_id});
+    fixture.checkpoint_key = layout.refCkptKey(
+        NamespaceLifeId::fromCatalogEntry(fixture.ns, fixture.life_id));
+    fixture.checkpoint_bytes = encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    backend.putIfAbsent(fixture.checkpoint_key, fixture.checkpoint_bytes);
+    EXPECT_TRUE(store->namespaceFilesLifeIfReadable(fixture.ns));
+    CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        next.entries[0].state = NsState::Removing;
+        next.entries[0].removal_started_round = 1;
+        return next;
+    });
+
+    CasFoldSeal parent;
+    parent.generation = 1;
+    parent.ref_lives.emplace(fixture.life_id, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 1}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 1}}});
+    for (uint64_t shard = 0; shard < store->poolConfig().gc_shards; ++shard)
+        parent.condemned_summary.emplace(shard, CondemnedSummary{});
+    backend.putIfAbsent(layout.foldSealKey(1, 1), encodeFoldSeal(parent));
+
+    GcState state;
+    state.round = 1;
+    state.gc_shards = store->poolConfig().gc_shards;
+    state.snap_generation = 1;
+    state.snap_attempt = 1;
+    state.lease = GcLease{.owner = lease_owner, .seq = 1};
+    backend.putIfAbsent(layout.gcStateKey(), encodeGcState(state));
+
+    return fixture;
+}
+
+void seedCompletedRemovingBatch(
+    DrainRaceBackend & backend, const PoolPtr & store, const UInt128 & lease_owner, size_t count)
+{
+    const Layout & layout = store->layout();
+    std::vector<CatalogEntry> entries;
+    entries.reserve(count);
+    for (size_t i = 0; i < count; ++i)
+    {
+        CatalogEntry entry{
+            .ns = RootNamespace{fmt::format("00/drain-batch-{}@cas@", i)},
+            .state = NsState::Live,
+            .incarnation = UInt128{200 + i}};
+        CasRefCatalog::casAdmitEntry(backend, layout, store->poolConfig().gc_shards, entry);
+        entries.push_back(std::move(entry));
+    }
+    CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        for (CatalogEntry & entry : next.entries)
+        {
+            entry.state = NsState::Removing;
+            entry.removal_started_round = 1;
+        }
+        return next;
+    });
+
+    CasFoldSeal parent;
+    parent.generation = 1;
+    for (const CatalogEntry & entry : entries)
+        parent.ref_lives.emplace(entry.incarnation, RefLifeFoldState{
+            .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 1}},
+            .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 1}}});
+    for (uint64_t shard = 0; shard < store->poolConfig().gc_shards; ++shard)
+        parent.condemned_summary.emplace(shard, CondemnedSummary{});
+    ASSERT_EQ(backend.putIfAbsent(layout.foldSealKey(1, 1), encodeFoldSeal(parent)).outcome,
+        PutOutcome::Done);
+
+    GcState state;
+    state.round = 1;
+    state.gc_shards = store->poolConfig().gc_shards;
+    state.snap_generation = 1;
+    state.snap_attempt = 1;
+    state.lease = GcLease{.owner = lease_owner, .seq = 1};
+    ASSERT_EQ(backend.putIfAbsent(layout.gcStateKey(), encodeGcState(state)).outcome, PutOutcome::Done);
+}
+
+enum class CompetingCatalogOutcome : uint8_t
+{
+    Absent,
+    Replacement,
+};
+
+class CASGCCompletedRemovalFenceRace : public testing::TestWithParam<CompetingCatalogOutcome>
+{
+};
+
+void transferGcLease(DrainRaceBackend & backend, const Layout & layout, const UInt128 & new_owner)
+{
+    const auto got = backend.get(layout.gcStateKey());
+    ASSERT_TRUE(got);
+    GcState state = decodeGcState(got->bytes);
+    state.lease.owner = new_owner;
+    ++state.lease.seq;
+    ASSERT_EQ(backend.casPut(layout.gcStateKey(), encodeGcState(state), got->token).outcome,
+        CasOutcome::Committed);
+}
+
+size_t findJournalAfter(const std::vector<String> & journal, const String & entry, size_t after)
+{
+    const auto it = std::find(journal.begin() + static_cast<ptrdiff_t>(after), journal.end(), entry);
+    return it == journal.end() ? journal.size() : static_cast<size_t>(it - journal.begin());
+}
+
+/// A pool whose GC frontier-probe budget is set explicitly. Everything else matches `openPoolForTest`.
+PoolPtr openPoolWithProbeBudget(std::shared_ptr<InMemoryBackend> backend, uint64_t budget)
+{
+    return Pool::open(std::move(backend),
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .gc_frontier_probe_budget = budget, .gc_fold_max_defer_rounds = 0});
+}
+
+/// Publish `ref_name` in `ns` pinning `blob`, allocating the next ref-log id. Writes the blob body and
+/// the manifest body too, so the published edge is one GC can actually fold.
+ManifestRef publish(Backend & backend, const Layout & layout, const RootNamespace & ns,
+                    const String & ref_name, uint64_t build_sequence, const DB::UInt128 & blob)
+{
+    const ManifestRef mref{.writer_epoch = 1, .build_sequence = build_sequence, .manifest_ordinal = 1};
+    writeBlobBody(backend, layout, blob);
+    writeManifestRaw(backend, layout, ns, mref, {blobEntryFor("data.bin", blob)});
+    publishCommittedTransition(backend, layout, ns, ref_name, std::nullopt, mref);
+    return mref;
+}
+
+/// The blob key for a raw hash, as the tests spell it.
+String blobKeyOf(const Layout & layout, const DB::UInt128 & hash)
+{
+    return layout.blobKey(legacyMetaTestRef(hash));
+}
+
+/// The sealed fold cursor for `ns` as a full `RefTxnId`. Every seed here allocates `writer_epoch = 1`,
+/// which is what `foldCursorOf` (returning the sequence alone) assumes too.
+RefTxnId sealedCursorOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    return RefTxnId{1, foldCursorOf(backend, layout, ns, /*shard*/ 0)};
+}
+
+/// Drive `rounds` GC rounds under the given policy, renewing the store's watermark between them the way
+/// the production scheduler does.
+void drive(const PoolPtr & store, Gc & gc, int rounds, UniversePolicy policy)
+{
+    for (int i = 0; i < rounds; ++i)
+    {
+        gc.runRegularRound({}, /*allow_steal*/true, policy);
+        store->renewWatermarkOnce();
+    }
+}
+
+/// Every key the backend was asked to delete, rendered for a failing assertion's message.
+String deletedKeysMessage(const CountingBackend & backend)
+{
+    String out;
+    for (const String & key : backend.deletedKeys())
+        out += "\n    " + key;
+    return out.empty() ? String{" (none)"} : out;
+}
+
+/// The gate's own verdict for one round, READ OFF THE PHASE ROWS rather than recomputed in the test. A
+/// test that re-derived `frontier_complete` from the tally would agree with a wrong formula just as
+/// readily as with the right one.
+struct GateVerdict
+{
+    bool saw_fold = false;
+    bool frontier_complete = false;
+    bool suppress_destructive = false;
+    uint64_t frontier_namespaces = 0;
+    uint64_t frontier_proven = 0;
+    uint64_t frontier_unprobed_budget = 0;
+    uint64_t catalog_entries = 0;
+    bool catalog_proved_empty = false;
+};
+
+GateVerdict runRoundCapturingGate(const PoolPtr & store, Gc & gc, UniversePolicy policy)
+{
+    GateVerdict verdict;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        const auto value = [&](const char * name) -> std::optional<UInt64>
+        {
+            const auto it = rec.metrics.find(name);
+            return it == rec.metrics.end() ? std::nullopt : std::optional<UInt64>{it->second};
+        };
+        if (rec.phase == "fold_reduce")
+        {
+            if (const auto complete = value("frontier_complete"))
+            {
+                verdict.saw_fold = true;
+                verdict.frontier_complete = *complete != 0;
+            }
+            if (const auto suppress = value("suppress_destructive"))
+                verdict.suppress_destructive = *suppress != 0;
+        }
+        else if (rec.phase == "fold_ref_intake")
+        {
+            if (const auto total = value("frontier_namespaces"))
+                verdict.frontier_namespaces = *total;
+            if (const auto proven = value("frontier_proven"))
+                verdict.frontier_proven = *proven;
+            if (const auto unprobed = value("frontier_unprobed_budget"))
+                verdict.frontier_unprobed_budget = *unprobed;
+            if (const auto entries = value("catalog_entries"))
+                verdict.catalog_entries = *entries;
+            if (const auto proved_empty = value("catalog_proved_empty"))
+                verdict.catalog_proved_empty = *proved_empty != 0;
+        }
+    });
+    gc.runRegularRound({}, /*allow_steal*/true, policy);
+    gc.setPhaseSink({});
+    store->renewWatermarkOnce();
+    return verdict;
+}
+
+/// Every delete family a round can reach, asserted PER FAMILY: an aggregate zero can hide one family
+/// running while another did not.
+void expectEveryDeleteFamilyInert(const CountingBackend & backend, const char * where)
+{
+    EXPECT_EQ(backend.deleteCountForKeysContaining("/blobs/"), 0u) << where << ": blob delete";
+    EXPECT_EQ(backend.deleteCountForKeysContaining("/cas/manifests/"), 0u)
+        << where << ": manifest-body delete";
+    EXPECT_EQ(backend.deleteCountForKeysContaining("/gc/gen/"), 0u)
+        << where << ": generation prune and hand-off reclaim";
+    EXPECT_EQ(backend.deleteCountForKeysContaining("/cas/ns/stream/"), 0u)
+        << where << ": covered-log / superseded-snapshot cleanup";
+    EXPECT_EQ(backend.deleteTotal(), 0u)
+        << where << ": a family not named above also ran. Deleted:" << deletedKeysMessage(backend);
+}
+
+}
+
+/// ===================== A HIDDEN `+1` IN AN UNKNOWN NAMESPACE =====================
+///
+/// Two namespaces share one blob. `visible` publishes it and then drops it, so the round observes
+/// `+1` then `-1` and reads the blob's in-degree as zero. `hidden` also owns it -- durably, acked,
+/// readable by exact key -- but is absent from the round's LIST hint. Its own publish still carries a
+/// real checkpoint, so the arithmetic walk finds and folds its `+1` by exact key regardless of what the
+/// LIST omits.
+///
+/// The three arms below are the whole argument: the exact-key probe finds `hidden`'s edge and saves the
+/// blob on a complete frontier; the blob still drains once `hidden` also honestly folds its own removal;
+/// and a namespace inside the universe with a sealed cursor has its hidden `+1` found by the exact-key
+/// probe the same way.
+
+namespace
+{
+/// Build the shared-blob scenario. `hidden` owns `blob` and is hidden from every LIST; `visible`
+/// publishes and drops it. Returns the pool.
+PoolPtr buildCrossNamespaceScenario(const std::shared_ptr<CountingHintHoleBackend> & backend,
+                                    const RootNamespace & hidden, const RootNamespace & visible,
+                                    const DB::UInt128 & blob, bool fold_hidden_first)
+{
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    if (fold_hidden_first)
+    {
+        /// Give the hidden namespace a sealed cursor, WITHOUT folding the edge under test. It publishes
+        /// an unrelated blob and one round folds that; from then on the namespace is in the universe via
+        /// its cursor even after the hint stops naming it.
+        ///
+        /// The unrelated blob is what makes this arm mean anything: if the shared blob's `+1` had
+        /// already been folded by the seeding round, the blob would survive on the DURABLE in-degree and
+        /// the test would pass whether or not the round probes anything. Publishing it only AFTER the
+        /// seal puts it strictly above the cursor, so the probe is the one and only thing that can find
+        /// it.
+        publish(*backend, layout, hidden, "seed_ref", 7, DB::UInt128(0x5eed));
+        Gc seed(store, kGc);
+        seed.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+
+    publish(*backend, layout, hidden, "kept_ref", 1, blob);
+    backend->hidePrefix(layout.namespaceStreamPrefix(fixture::fixtureLife(hidden)));
+
+    const ManifestRef dropped = publish(*backend, layout, visible, "dropped_ref", 2, blob);
+    dropRefTransition(*backend, layout, visible, "dropped_ref", dropped);
+    return store;
+}
+}
+
+/// Rounds on the PRODUCTION path -- no policy argument -- because that is what the claim is about.
+/// `hidden`'s own publish left it a real checkpoint, so the arithmetic walk's exact-key probe finds and
+/// folds its `+1` no matter what the LIST hides: the blob survives on its own complete, proven frontier,
+/// not on the round declining to touch anything.
+TEST(CASGCFrontierGate, AHiddenEdgeIsFoundByTheExactKeyProbeAndSavesTheBlobOnACompleteFrontier)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    const RootNamespace hidden{"00/hidden@cas@"};
+    const RootNamespace visible{"00/visible@cas@"};
+    const DB::UInt128 blob(0x5ade);
+
+    auto store = buildCrossNamespaceScenario(backend, hidden, visible, blob, /*fold_hidden_first=*/false);
+    const Layout & layout = store->layout();
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+    GateVerdict verdict;
+    for (int i = 0; i < 5; ++i)
+    {
+        const GateVerdict round = runRoundCapturingGate(store, gc, UniversePolicy::kDefault);
+        if (round.saw_fold)
+            verdict = round;
+        store->renewWatermarkOnce();
+    }
+
+    ASSERT_TRUE(verdict.saw_fold) << "no round folded, so none published a gate verdict";
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "the blob a hidden namespace still owns must survive";
+    EXPECT_TRUE(verdict.frontier_complete)
+        << "the exact-key probe reads at `cursor + 1` and a LIST hole cannot hide an exact key, so the "
+           "catalog-named hidden namespace IS provable — if this is false the blob above survived on "
+           "suppression instead of on its own in-degree, which proves nothing about the edge";
+    EXPECT_FALSE(verdict.suppress_destructive);
+}
+
+/// The arm above asserts "nothing was deleted", which does not on its own distinguish the gate correctly
+/// refusing from the round simply never deleting anything at all. This
+/// is the positive control: `hidden` is genuinely folded through its OWN drop of the same
+/// blob (an honest exact-key read of a record the LIST still hides -- the arithmetic-intake mechanism
+/// this whole file is about), so its frontier is REALLY proven, not merely declared so, and the blob is
+/// REALLY unreferenced by both namespaces. The round drains it -- the zero-deletion arm above would pass
+/// identically if the round were simply incapable of ever deleting anything.
+TEST(CASGCFrontierGate, TheSameBlobDrainsOnceHiddenGenuinelyProvesItsOwnFrontier)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    const RootNamespace hidden{"00/hidden@cas@"};
+    const RootNamespace visible{"00/visible@cas@"};
+    const DB::UInt128 blob(0x5ade);
+
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    Gc gc(store, kGc);
+
+    /// `hidden`'s BIRTH is folded (and its cursor SEALED) with everything still listed -- a namespace
+    /// with no `_ckpt` (the raw-fixture admission this file's helper uses never publishes one) has no
+    /// genesis signal EXCEPT a sealed cursor or a visible LIST, so a real fold first is what makes an
+    /// arithmetic (cursor-relative) genesis available at all for what follows.
+    const ManifestRef kept = publish(*backend, layout, hidden, "kept_ref", 1, blob);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    store->renewWatermarkOnce();
+
+    /// NOW `hidden` drops its own reference (written while still fully listed, so the raw fixture's own
+    /// LIST -- finding the greatest existing log id, to derive the next one -- sees the truth), and
+    /// ONLY THEN does its whole prefix vanish from every subsequent LIST. With a sealed cursor already
+    /// in hand the walk's genesis is arithmetic (`cursor + 1`), so this drop is found and folded by
+    /// exact key alone -- the arithmetic-intake mechanism this whole file is about, exercised honestly
+    /// rather than declared past by fiat.
+    dropRefTransition(*backend, layout, hidden, "kept_ref", kept);
+    backend->hidePrefix(layout.namespaceStreamPrefix(fixture::fixtureLife(hidden)));
+
+    const ManifestRef dropped = publish(*backend, layout, visible, "dropped_ref", 2, blob);
+    dropRefTransition(*backend, layout, visible, "dropped_ref", dropped);
+
+    drive(store, gc, /*rounds*/ 5, UniversePolicy::Authoritative);
+
+    EXPECT_FALSE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "both namespaces genuinely proved their frontier and the blob is genuinely unreferenced -- "
+           "the round must still be able to reclaim it";
+}
+
+/// AND THE PER-NAMESPACE LOGIC IS WHAT SAVES IT. Identical to the arm above except that the hidden
+/// namespace was folded once first, so it carries a sealed cursor and is therefore IN the universe even
+/// though the hint has gone silent about it. The round probes its expected-next by exact key, finds the
+/// record the listing hid, folds the `+1`, and the blob is never condemned.
+TEST(CASGCFrontierGate, AKnownNamespaceIsProbedByExactKeyAndItsHiddenEdgeSavesTheBlob)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    const RootNamespace hidden{"00/hidden@cas@"};
+    const RootNamespace visible{"00/visible@cas@"};
+    const DB::UInt128 blob(0x5ade);
+
+    auto store = buildCrossNamespaceScenario(backend, hidden, visible, blob, /*fold_hidden_first=*/true);
+    const Layout & layout = store->layout();
+
+    Gc gc(store, kGc);
+    drive(store, gc, /*rounds*/ 5, UniversePolicy::Authoritative);
+
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "the cursor kept the namespace in the universe, so its frontier was probed and its edge folded";
+}
+
+/// ===================== THE GATE FORMULA, TERM BY TERM =====================
+///
+/// The healthy case first, because every suppressor arm below is only meaningful against it: a pool with
+/// nothing hidden, nothing held, no anomaly, and every namespace walked to an honest end-of-stream OPENS
+/// the gate and reclaims. The two booleans are read off the fold's own rows, so a formula that computed
+/// them differently would fail here rather than agree with the test.
+TEST(CASGCFrontierGate, AHealthyCatalogRoundOpensTheGateAndReclaims)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    const DB::UInt128 blob(0xdead);
+
+    const ManifestRef mref = publish(*backend, layout, ns, "ref_1", 1, blob);
+    dropRefTransition(*backend, layout, ns, "ref_1", mref);
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+    const GateVerdict verdict = runRoundCapturingGate(store, gc, UniversePolicy::kDefault);
+
+    ASSERT_TRUE(verdict.saw_fold) << "the round did not fold, so it published no gate verdict";
+    EXPECT_TRUE(verdict.frontier_complete)
+        << "every namespace in a healthy catalog universe reached a proven frontier";
+    EXPECT_FALSE(verdict.suppress_destructive)
+        << "no anomaly, no hold, a complete frontier -- the gate has nothing left to refuse on";
+    EXPECT_GT(verdict.frontier_namespaces, 0u)
+        << "a zero-namespace universe would satisfy the equality vacuously; this pool must not be one";
+    EXPECT_EQ(verdict.frontier_proven, verdict.frontier_namespaces);
+
+    /// And the gate being open is worth something: the condemned blob actually drains.
+    EXPECT_TRUE(runRoundsUntilAbsent(store, gc, *backend, layout, blob))
+        << "an unsuppressed round must reclaim a blob no ref owns any more";
+}
+
+/// ===================== EVERY DESTRUCTIVE SITE, INDIVIDUALLY =====================
+///
+/// The inventory as an assertion. The pool below has real work waiting at every gated site: a
+/// graduated blob to delete, an owner-removed manifest body to delete, aged generations to prune and
+/// hand off, ref logs and snapshots covered by a durable snapshot, and a removed namespace with a
+/// Pending cleanup item. A suppressed round issues ZERO deletes against ALL of them, and the per-site
+/// assertions name which one leaked if any does.
+
+namespace
+{
+/// A pool with destructive work pending at every site, plus a few completed rounds so generations have
+/// aged past the retention floor. Returns the hash of a blob whose in-degree has dropped to zero.
+DB::UInt128 buildPoolWithWorkAtEverySite(const std::shared_ptr<CountingBackend> & backend,
+                                         const PoolPtr & store, Gc & gc)
+{
+    const Layout & layout = store->layout();
+    const RootNamespace live{"00/live@cas@"};
+    const RootNamespace doomed{"00/doomed@cas@"};
+    const DB::UInt128 blob(0xfeed);
+
+    /// A long-lived namespace that keeps publishing, so snapshots and covered logs accumulate and
+    /// generations keep advancing past the retention floor.
+    for (uint64_t i = 1; i <= 4; ++i)
+    {
+        publish(*backend, layout, live, "ref_" + std::to_string(i), i, DB::UInt128(0x1000 + i));
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+    }
+
+    /// The condemnable blob: published in `doomed`, then dropped. Its manifest body becomes
+    /// owner-removed cleanup work at the same time.
+    const ManifestRef mref = publish(*backend, layout, doomed, "doomed_ref", 9, blob);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, layout, doomed, "doomed_ref", mref);
+    return blob;
+}
+}
+
+/// (3a) THE NEGATIVE-POLICY SEAM, and the per-site inventory at the same time. A caller that supplies no
+/// universe suppresses on that term ALONE: this pool has no anomaly, no hold, and a frontier every
+/// per-namespace probe proves -- the control at the end of the test is what says so, since the identical
+/// pool drains on the production path.
+TEST(CASGCFrontierGate, EveryInventoriedDestructiveSiteIsInertUnderSuppression)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    Gc gc(store, kGc);
+    const DB::UInt128 blob = buildPoolWithWorkAtEverySite(backend, store, gc);
+
+    /// From here on the rounds supply NO universe: every site has work queued and every site must
+    /// decline it.
+    backend->resetCounts();
+    const GateVerdict verdict = runRoundCapturingGate(store, gc, UniversePolicy::StageA_Suppressed);
+    for (int i = 0; i < 5; ++i)
+        runRoundCapturingGate(store, gc, UniversePolicy::StageA_Suppressed);
+
+    ASSERT_TRUE(verdict.saw_fold) << "the round did not fold, so it published no gate verdict";
+    EXPECT_FALSE(verdict.frontier_complete)
+        << "with no universe supplied the frontier can never be complete, whatever the probes proved";
+    EXPECT_TRUE(verdict.suppress_destructive);
+    expectEveryDeleteFamilyInert(*backend, "no universe supplied");
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists);
+
+    /// The control: the identical pool DOES reclaim at those sites on the production path, so the zeros
+    /// above are the gate at work and not an empty work queue -- and it is also what makes the "on that
+    /// term alone" claim above true rather than assumed.
+    drive(store, gc, /*rounds*/ 4, UniversePolicy::kDefault);
+    EXPECT_GT(backend->deleteTotal(), 0u)
+        << "the work queue was real -- a round with a universe drains it";
+    EXPECT_FALSE(backend->head(blobKeyOf(layout, blob)).exists);
+}
+
+/// (1) ONE ANOMALY. A namespace whose `_ckpt` is present but undecodable records the "no usable
+/// checkpoint" anomaly, and the round declines every site on that. It leaves the frontier incomplete too,
+/// so what this arm pins is "an anomaly suppresses", not "only the anomaly does" -- which is why every
+/// assertion below is about inertness and not about which term fired.
+TEST(CASGCFrontierGate, AnUndecodableCheckpointAnomalySuppressesEveryDeleteFamily)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    Gc gc(store, kGc);
+    const DB::UInt128 blob = buildPoolWithWorkAtEverySite(backend, store, gc);
+
+    const RootNamespace damaged{"00/damaged@cas@"};
+    publish(*backend, layout, damaged, "damaged_ref", 42, DB::UInt128(0xda43));
+    /// Resolved through the catalog, not minted from the namespace name: the corruption has to land on
+    /// the very object the round's own life resolution will read, or the round folds normally and this
+    /// test measures nothing.
+    const std::optional<NamespaceLifeId> damaged_life =
+        CasRefCatalog::lifeIfCataloged(*backend, layout, damaged);
+    ASSERT_TRUE(damaged_life.has_value()) << "the publish must have left a catalog entry to resolve";
+    const std::optional<CkptSample> damaged_ckpt = readCkpt(*backend, layout, *damaged_life);
+    ASSERT_TRUE(damaged_ckpt.has_value()) << "the publish must have left a `_ckpt` to damage";
+    ASSERT_EQ(backend->casPut(layout.refCkptKey(*damaged_life), "not a checkpoint",
+                              damaged_ckpt->token).outcome, CasOutcome::Committed);
+
+    backend->resetCounts();
+    std::vector<size_t> anomaly_counts;
+    for (int i = 0; i < 6; ++i)
+    {
+        anomaly_counts.push_back(gc.runRegularRound().anomalies.size());
+        store->renewWatermarkOnce();
+    }
+
+    EXPECT_GT(anomaly_counts.front(), 0u)
+        << "the undecodable `_ckpt` must be RECORDED, not silently absorbed -- a silent exit would make "
+           "this test pass for the wrong reason";
+    expectEveryDeleteFamilyInert(*backend, "one anomaly");
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists);
+}
+
+/// (2) ONE CARRIED HOLD. The gate's second term reads the SEAL, not this round's anomaly list, so the
+/// round that matters here is a LATER one: the hold was detected earlier, rides forward because its
+/// offending position is still unresolved, and must suppress on its own.
+TEST(CASGCFrontierGate, ACarriedHoldSuppressesEveryDeleteFamily)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    Gc gc(store, kGc);
+    const DB::UInt128 blob = buildPoolWithWorkAtEverySite(backend, store, gc);
+
+    /// A committed gap: the checkpoint says `{1,2}` is committed while only `{1,1}` was ever written, so
+    /// the walk reads `{1,2}` absent BELOW its authority ceiling and holds there. Nothing repairs it, so
+    /// every later round re-detects the same position and carries the same hold.
+    const RootNamespace gapped{"00/gapped@cas@"};
+    publish(*backend, layout, gapped, "gapped_ref", 44, DB::UInt128(0x6a9));
+    replaceRecoverableCkptForRawFixture(*backend, layout, gapped, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    std::map<String, UInt64> intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    gc.runRegularRound();
+    gc.setPhaseSink({});
+    store->renewWatermarkOnce();
+    ASSERT_FALSE(intake.empty());
+    ASSERT_GT(intake.at("tables_held"), 0u)
+        << "the gap must be HELD, or the later rounds carry nothing and this test proves nothing";
+
+    backend->resetCounts();
+    for (int i = 0; i < 5; ++i)
+    {
+        gc.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+    expectEveryDeleteFamilyInert(*backend, "one carried hold");
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists);
+}
+
+/// (3c) THE PROBE BUDGET. A namespace with a sealed cursor, no `_ckpt` and no listing left can be proven
+/// only by a successor probe; a zero budget denies it one, so it counts toward the universe and not toward
+/// the proofs, and the equality fails.
+TEST(CASGCFrontierGate, AnExhaustedProbeBudgetSuppressesEveryDeleteFamily)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolWithProbeBudget(backend, /*budget*/ 0);
+    const Layout & layout = store->layout();
+
+    Gc gc(store, kGc);
+    const DB::UInt128 blob = buildPoolWithWorkAtEverySite(backend, store, gc);
+
+    /// The budget is spent only on a namespace the round knows about and can reach NO other way. Three
+    /// conditions, and all three are load-bearing: a SEALED cursor (an unhinted namespace with no cursor
+    /// is a no-genesis shape the budget never reaches), NO listing (a hinted target is walked for free),
+    /// and NO readable `_ckpt` (a recoverable checkpoint proves the frontier without spending a probe --
+    /// which is why publishing and hiding alone leaves the namespace provable and measures nothing).
+    const RootNamespace quiet{"00/quiet@cas@"};
+    publish(*backend, layout, quiet, "quiet_ref", 43, DB::UInt128(0x9a1e));
+    runRoundCapturingGate(store, gc, UniversePolicy::StageA_Suppressed);
+    ASSERT_NE(sealedCursorOf(*backend, layout, quiet), (RefTxnId{}))
+        << "without a sealed cursor the namespace never becomes a budget-spending probe target";
+
+    const std::optional<NamespaceLifeId> quiet_life =
+        CasRefCatalog::lifeIfCataloged(*backend, layout, quiet);
+    ASSERT_TRUE(quiet_life.has_value());
+    const std::optional<CkptSample> quiet_ckpt = readCkpt(*backend, layout, *quiet_life);
+    ASSERT_TRUE(quiet_ckpt.has_value()) << "there must be a `_ckpt` to remove";
+    ASSERT_EQ(backend->deleteExact(layout.refCkptKey(*quiet_life), quiet_ckpt->token).kind,
+              DeleteOutcome::Kind::Deleted);
+    backend->hidePrefix(layout.namespaceStreamPrefix(*quiet_life));
+
+    backend->resetCounts();
+    const GateVerdict verdict = runRoundCapturingGate(store, gc, UniversePolicy::kDefault);
+    for (int i = 0; i < 5; ++i)
+        runRoundCapturingGate(store, gc, UniversePolicy::kDefault);
+
+    ASSERT_TRUE(verdict.saw_fold);
+    EXPECT_GT(verdict.frontier_unprobed_budget, 0u)
+        << "this arm must suppress on the BUDGET term; a zero here means some other suppressor fired and "
+           "the test would pass without ever exhausting a budget";
+    EXPECT_LT(verdict.frontier_proven, verdict.frontier_namespaces)
+        << "the unprobed namespace must count toward the universe and not toward the proofs";
+    EXPECT_FALSE(verdict.frontier_complete);
+    EXPECT_TRUE(verdict.suppress_destructive);
+    expectEveryDeleteFamilyInert(*backend, "exhausted probe budget");
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists);
+}
+
+/// `frontier_proven == frontier_namespaces` is `0 == 0` -- vacuously TRUE -- on an empty universe, which
+/// is not by itself a proof of anything: a fresh pool, a damaged catalog, and a genuinely emptied pool
+/// all produce the same zeros. The gate's non-vacuity term therefore has TWO ways to be satisfied:
+/// `frontier_namespaces > 0` (an ordinary nonempty pool, everything proven), or the round's own hot-scan
+/// catalog cut positively proving the universe empty (present, token-bearing, decoded, zero rows of
+/// every lifecycle state). The next two tests are that positive/negative pair.
+TEST(CASGCFrontierGate, ADecodedTokenBearingEmptyCatalogCompletesTheFrontierAndDrainsRetiredWork)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const DB::UInt128 blob(0xbead);
+
+    /// Built to make `frontier_namespaces` GENUINELY zero by every source that feeds it -- no catalog
+    /// entry (this pool never admitted a namespace), no sealed cursor, no ref-log hint -- while a
+    /// condemned blob with a real, present body and in-degree 0 sits queued exactly the way a real
+    /// round leaves one (`injectRetire`).
+    writeBlobBody(*backend, layout, blob);
+    const BlobRef blob_ref = legacyMetaTestRef(blob);
+    const Token blob_token = backend->head(layout.blobKey(blob_ref)).token;
+    injectRetire(*backend, layout, /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = blob_ref, .token = blob_token, .size = 0}});
+    store->renewWatermarkOnce();
+
+    ASSERT_TRUE(CasRefCatalog::read(*backend, layout).catalog.entries.empty())
+        << "the scenario needs a genuinely, provably empty catalog, or this test measures nothing";
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+
+    /// `injectRetire` seeds the condemned entry WITHOUT the durable per-hash `Condemned` meta a real
+    /// condemn round writes (`fold`'s own `scheduleCondemnMarkerWrite` side effect) -- so this fixture's
+    /// round cadence is longer than the textbook condemn->graduate->delete: the lease is UNCLAIMED
+    /// (`injectRetire` writes `gc/state` directly, never through a real acquire), so the first round
+    /// only arms `acquireOrRenewLease`'s two-tick steal-safety window; the graduation gate's own
+    /// `confirm_condemned_marker` then fails its first sighting, schedules the meta write, and CARRIES
+    /// the entry (not yet delete_pending) while `meta_pool_wait` lands it durably by that round's end;
+    /// only the round after that sees the durable meta and actually graduates; and the physical delete
+    /// is the round after THAT. MEASURED (phase-sink instrumentation, not a guess): round 1 arms
+    /// (`saw_fold == false`), round 2 is the first to fold with the gate open while the blob is still
+    /// present (the carry round), round 3 graduates, round 4 executes the delete -- four rounds exactly.
+    /// Bound the drive at that plus one (5): enough slack for the fixture's own cadence to be measured
+    /// without hand-counting rounds against this gate, but tight enough that a real regression in the
+    /// confirm/retry cadence still fails loudly instead of silently absorbing into a generous loop.
+    ASSERT_TRUE(backend->head(layout.blobKey(blob_ref)).exists)
+        << "the scenario starts with the condemned blob present, or the loop below measures nothing";
+
+    constexpr int kMaxRounds = 5;   /// measured cadence (4) + 1; see the comment above
+    /// Observe the TWO-PHASE PIPELINE explicitly: an open, unsuppressed gate while the blob was still
+    /// present (the graduate side), and only a STRICTLY LATER round removing it (the delete side).
+    /// Asserting only the final state and the last verdict would pass just as readily if the blob
+    /// vanished by some other path entirely.
+    int round_gate_opened_while_present = -1;   /// the graduate side: FIRST round that folded, unsuppressed,
+                                                 /// with the blob still present
+    int round_blob_vanished = -1;               /// the delete side
+    GateVerdict last;
+    int rounds_run = 0;
+    for (int i = 0; i < kMaxRounds && backend->head(layout.blobKey(blob_ref)).exists; ++i)
+    {
+        last = runRoundCapturingGate(store, gc, UniversePolicy::Authoritative);
+        ++rounds_run;
+        const bool still_present = backend->head(layout.blobKey(blob_ref)).exists;
+        if (round_gate_opened_while_present < 0 && last.saw_fold && !last.suppress_destructive && still_present)
+            round_gate_opened_while_present = rounds_run;
+        if (round_blob_vanished < 0 && !still_present)
+            round_blob_vanished = rounds_run;
+    }
+
+    ASSERT_GT(rounds_run, 0) << "the loop must actually run, or every assertion below is vacuous";
+    ASSERT_LE(rounds_run, kMaxRounds)
+        << "the drain took more than the measured cadence -- this is a real regression in the "
+           "confirm/retry pacing, not something to hide by bumping the bound; re-derive the cadence";
+    ASSERT_TRUE(last.saw_fold);
+    EXPECT_EQ(last.frontier_namespaces, 0u);
+    EXPECT_EQ(last.frontier_proven, 0u);
+    EXPECT_TRUE(last.catalog_proved_empty)
+        << "the catalog cut itself must be the proof, not the bare 0==0 equality";
+    EXPECT_TRUE(last.frontier_complete);
+    EXPECT_FALSE(last.suppress_destructive);
+    ASSERT_GT(round_gate_opened_while_present, 0)
+        << "the gate must have opened (folded, unsuppressed) at least one round BEFORE the blob was "
+           "gone -- the graduate side of the two-phase pipeline -- not just at the round that deleted it";
+    ASSERT_GT(round_blob_vanished, round_gate_opened_while_present)
+        << "the delete must be a round STRICTLY LATER than the one that opened the gate, never the same "
+           "round -- a round that both graduates and deletes in one step would hide the two-phase split";
+    EXPECT_FALSE(backend->head(layout.blobKey(blob_ref)).exists)
+        << "a proved-empty universe is a COMPLETE frontier, not a suppressed one -- the condemned blob "
+           "must drain through the ordinary two-phase pipeline instead of leaking forever";
+}
+
+/// The negative half of the pair. A `Creating` row is a birth in progress (spec §3: no publication can
+/// exist under it, so `live_incarnation`/`walk_plan.lives()` exclude it -- see the R10 comment above the
+/// intake loop), not an empty universe -- but it produces the SAME `frontier_namespaces ==
+/// frontier_proven == 0` a genuinely empty catalog does. Only `catalog_cut_proved_empty` tells them
+/// apart, because it reads `entries` (every lifecycle state), not the frontier counters.
+TEST(CASGCFrontierGate, AZeroWalkableFrontierWithACreatingCatalogRowIsNotProvedEmpty)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const DB::UInt128 blob(0xbead);
+
+    writeBlobBody(*backend, layout, blob);
+    const BlobRef blob_ref = legacyMetaTestRef(blob);
+    const Token blob_token = backend->head(layout.blobKey(blob_ref)).token;
+    injectRetire(*backend, layout, /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = blob_ref, .token = blob_token, .size = 0}});
+    store->renewWatermarkOnce();
+
+    const RootNamespace stalled{"00/stalled@cas@"};
+    CatalogEntry entry;
+    entry.ns = stalled;
+    entry.state = NsState::Creating;
+    entry.incarnation = hexToU128("00000000000000000000000000000042");
+    entry.creator = CreatorFence{
+        .server_root_id = "test-stalled-creator", .writer_epoch = 1, .fence_generation = 1};
+    CasRefCatalog::casAdmitEntry(*backend, layout, /*gc_shards*/ 1, entry);
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+    /// `injectRetire` leaves `gc/state`'s lease unclaimed (owner 0); the first round only arms the
+    /// steal-safety window (see `ADecodedTokenBearingEmptyCatalogCompletesTheFrontierAndDrainsRetiredWork`)
+    /// and folds nothing, so it is spent here rather than counted among the assertions below.
+    const GateVerdict warm_up = runRoundCapturingGate(store, gc, UniversePolicy::Authoritative);
+    EXPECT_FALSE(warm_up.saw_fold);
+    for (int i = 0; i < 6; ++i)
+    {
+        const GateVerdict v = runRoundCapturingGate(store, gc, UniversePolicy::Authoritative);
+        ASSERT_TRUE(v.saw_fold);
+        EXPECT_EQ(v.frontier_namespaces, 0u);
+        EXPECT_EQ(v.frontier_proven, 0u);
+        EXPECT_FALSE(v.catalog_proved_empty)
+            << "a Creating-only catalog is a birth in progress, not proof of an empty universe";
+        EXPECT_FALSE(v.frontier_complete);
+        EXPECT_TRUE(v.suppress_destructive);
+    }
+    expectEveryDeleteFamilyInert(*backend, "Creating-only catalog");
+    EXPECT_TRUE(backend->head(layout.blobKey(blob_ref)).exists);
+}
+
+/// The bootstrap-only absent-as-empty representation (`initializeEmptyForNewPool`) must never leak into
+/// the operational round: an absent mandatory catalog is corruption, never an empty authority set.
+TEST(CASGCFrontierGate, AnAbsentCatalogNeverReadsAsAnEmptyUniverse)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    const Token catalog_token = backend->head(layout.refCatalogKey()).token;
+    ASSERT_EQ(backend->deleteExact(layout.refCatalogKey(), catalog_token).kind, DeleteOutcome::Kind::Deleted);
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+    bool saw_fold = false;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec) { if (rec.phase == "fold_reduce") saw_fold = true; });
+    try
+    {
+        gc.runRegularRound({}, /*allow_steal*/true, UniversePolicy::Authoritative);
+        FAIL() << "expected the missing mandatory catalog to throw before any fold gate verdict";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+    }
+    gc.setPhaseSink({});
+    EXPECT_FALSE(saw_fold) << "an absent catalog must abort before the round computes any gate verdict";
+    EXPECT_EQ(backend->deleteTotal(), 0u) << "no destructive work may run on an unauthorized round";
+}
+
+/// A malformed, truncated, wrong-typed, count-mismatched, or future-versioned catalog must not decode
+/// into a `Snapshot` at all -- these are the replacement guards for R11's original "damaged catalog"
+/// concern, now that the empty case has a positive proof to keep separate from a broken one.
+/// One table-driven test: each row installs a different broken body at the mandatory key and expects
+/// decode failure before any destructive work.
+///
+/// NOT covered here: a header version BELOW `RefCatalog`'s own birth generation. `checkCompatibility`
+/// today only rejects a version ABOVE `G_BUILD`; a version below a type's birth floor decodes as if it
+/// were legal, and `decodeRefCatalog` discards the parsed header entirely, so "decoded successfully"
+/// does not yet imply "legal version for this type". That gap is not load-bearing for THIS proof: the
+/// proof is token-present + a full structural decode (type, complete records, matching count trailer,
+/// no trailing bytes) + zero entries, and a well-formed-but-out-of-protocol empty catalog is already an
+/// accepted residual under the trusted-store model (the token proves byte identity, not history) --
+/// closing the version floor would only shrink that residual, not remove it. Tracked separately as
+/// `[cas-format-version-floor]` in `BACKLOG.md`; deliberately out of scope for this gate.
+TEST(CASGCFrontierGate, AMalformedCatalogNeverDecodesIntoAnEmptyProof)
+{
+    /// A one-entry catalog's canonical bytes, the base every mutation below starts from.
+    RefCatalog one_entry;
+    CatalogEntry entry;
+    entry.ns = RootNamespace{"00/malformed-base@cas@"};
+    entry.state = NsState::Live;
+    entry.incarnation = hexToU128("00000000000000000000000000000099");
+    one_entry.entries.push_back(entry);
+    const String base = encodeRefCatalog(one_entry);
+    const String empty_base = encodeRefCatalog(RefCatalog{});
+
+    const String type_needle = fmt::format("\"type\":\"{}\"", traitsFor(FormatId::RefCatalog).type);
+    ASSERT_NE(empty_base.find(type_needle), String::npos);
+    const String version_needle = fmt::format("\"v\":{}", currentCompatibilityVersion());
+    ASSERT_NE(empty_base.find(version_needle), String::npos);
+    ASSERT_NE(base.find("\"n\":1"), String::npos);
+
+    const auto replaceOnce = [](const String & haystack, const String & needle, const String & replacement) -> String
+    {
+        const auto pos = haystack.find(needle);
+        EXPECT_NE(pos, String::npos) << "expected to find '" << needle << "'";
+        String out = haystack;
+        out.replace(pos, needle.size(), replacement);
+        return out;
+    };
+
+    struct Case { const char * name; String bytes; };
+    const std::vector<Case> cases = {
+        {"wrong-type", replaceOnce(empty_base, type_needle, "\"type\":\"cas_ref_ckpt\"")},
+        /// The one version case the CURRENT (unmodified) gate actually enforces: a version ABOVE
+        /// `G_BUILD` is refused by `checkCompatibility` before decode proceeds.
+        {"future-version", replaceOnce(empty_base, version_needle, "\"v\":999999")},
+        {"trailer-count-mismatch", replaceOnce(base, "\"n\":1", "\"n\":2")},
+        /// The trailer line entirely gone: decode's post-entry loop expects another line and hits EOF.
+        {"missing-trailer", base.substr(0, base.rfind("{\"n\":1}\n"))},
+        /// The trailer present but its own line has no terminator: EOF strictly inside a line.
+        {"truncated-mid-line", base.substr(0, base.size() - 2)},
+    };
+
+    for (const Case & c : cases)
+    {
+        auto backend = std::make_shared<CountingBackend>();
+        auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+        const Layout & layout = store->layout();
+        const Token bootstrap_token = backend->head(layout.refCatalogKey()).token;
+        ASSERT_EQ(backend->casPut(layout.refCatalogKey(), c.bytes, bootstrap_token).outcome,
+            CasOutcome::Committed) << c.name;
+
+        Gc gc(store, kGc);
+        backend->resetCounts();
+        bool saw_fold = false;
+        gc.setPhaseSink([&](const GcPhaseRecord & rec) { if (rec.phase == "fold_reduce") saw_fold = true; });
+        EXPECT_THROW(
+            gc.runRegularRound({}, /*allow_steal*/true, UniversePolicy::Authoritative), DB::Exception)
+            << c.name;
+        gc.setPhaseSink({});
+        EXPECT_FALSE(saw_fold) << c.name << ": a broken catalog must abort before any fold gate verdict";
+        EXPECT_EQ(backend->deleteTotal(), 0u) << c.name << ": no destructive work may run on it";
+    }
+}
+
+/// `StageA_Suppressed` refuses outright regardless of what the catalog proves -- a proved-empty cut
+/// satisfies the frontier term but is not the only term the gate reads.
+TEST(CASGCFrontierGate, AProvedEmptyCatalogUnderStageASuppressedStaysSuppressed)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const DB::UInt128 blob(0xbead);
+
+    writeBlobBody(*backend, layout, blob);
+    const BlobRef blob_ref = legacyMetaTestRef(blob);
+    const Token blob_token = backend->head(layout.blobKey(blob_ref)).token;
+    injectRetire(*backend, layout, /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = blob_ref, .token = blob_token, .size = 0}});
+    store->renewWatermarkOnce();
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+    /// `injectRetire` leaves `gc/state`'s lease unclaimed (owner 0); the first round only arms the
+    /// steal-safety window (see `ADecodedTokenBearingEmptyCatalogCompletesTheFrontierAndDrainsRetiredWork`)
+    /// and folds nothing, independent of policy -- lease acquisition precedes the destructive gate.
+    const GateVerdict warm_up = runRoundCapturingGate(store, gc, UniversePolicy::StageA_Suppressed);
+    EXPECT_FALSE(warm_up.saw_fold);
+    for (int i = 0; i < 6; ++i)
+    {
+        const GateVerdict v = runRoundCapturingGate(store, gc, UniversePolicy::StageA_Suppressed);
+        ASSERT_TRUE(v.saw_fold);
+        EXPECT_TRUE(v.catalog_proved_empty)
+            << "the catalog cut is still genuinely empty -- the fact does not depend on policy";
+        EXPECT_FALSE(v.frontier_complete)
+            << "StageA_Suppressed refuses outright no matter what the catalog cut proves";
+        EXPECT_TRUE(v.suppress_destructive);
+    }
+    expectEveryDeleteFamilyInert(*backend, "StageA_Suppressed over a proved-empty catalog");
+    EXPECT_TRUE(backend->head(layout.blobKey(blob_ref)).exists);
+}
+
+/// THE BIRTH-AFTER-EMPTY-CUT BLOB RACE. The proved-empty exception's soundness rests on one hard fact:
+/// under this pool's protocol every live or live-precommit edge requires an exact `Live` catalog row
+/// (INV-3), so a catalog cut with zero rows proves no namespace ANYWHERE holds one -- AT THAT INSTANT.
+/// A namespace born strictly after the cut is invisible to the round that took it; safety for blob
+/// CONTENT then rests entirely on the condemned-marker/resurrection protocol (EDGE-BEFORE-OBSERVE:
+/// `ContentAddressedTransaction.cpp` persists the precommit edge before observing/uploading the pool
+/// blob), never on the frontier proof, which by construction cannot see a birth postdating its own cut.
+/// This test pins that: a real writer, through the production `createNamespace` lifecycle
+/// (`precommitAdd` on a namespace that has never existed), lands a precommit edge to an
+/// ALREADY-CONDEMNED blob strictly after round R's catalog cut but strictly before round R executes the
+/// pending delete that cut licensed.
+TEST(CASGCFrontierGate, ANamespaceBornAfterTheEmptyCutResurrectsTheCondemnedBlobInstead)
+{
+    ensureBlobUploadPoolForTest();
+
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace doomed{"00/doomed@cas@"};
+
+    const String payload = "empty-cut-birth-race-payload";
+    const DB::UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+    const String key = layout.blobKey(id);
+    String raw_body(store->poolMeta().blob_header_len, '\0');
+    raw_body += payload;
+    writeRawBlobBody(*backend, layout, hash, raw_body);
+
+    const ManifestRef mref{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1};
+    writeManifestRaw(*backend, layout, doomed, mref, {blobEntryFor("data.bin", hash)});
+    publishCommittedTransition(*backend, layout, doomed, "ref_1", std::nullopt, mref);
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);                                 /// folds the +1 edge
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, layout, doomed, "ref_1", mref);
+    runRegularRoundReclaiming(gc);                                 /// condemns: durable Condemned meta
+    store->renewWatermarkOnce();
+    const Token condemned_token = backend->head(key).token;
+    const auto condemned_meta = loadMetaForTest(*backend, layout, hash);
+    ASSERT_TRUE(condemned_meta.has_value());
+    ASSERT_EQ(condemned_meta->meta.state, MetaState::Condemned)
+        << "the delete round R is about to execute must be backed by durable Condemned evidence";
+    runRegularRoundReclaiming(gc);                                 /// graduates: publishes delete_pending
+    store->renewWatermarkOnce();
+
+    /// Remove `doomed` entirely -- the ONLY way the catalog can become genuinely, provably empty. A raw
+    /// `RemoveNamespace` op plus the Removing-state CAS mirrors
+    /// `CleanupEvidenceLeavesRemovedNamespaceCheckpointForJanitor`'s own recipe exactly.
+    RefOp remove_op;
+    remove_op.kind = RefOpKind::RemoveNamespace;
+    const uint64_t remove_seq = appendRefLogSeed(*backend, layout, doomed, {remove_op});
+    publishRecoverableCkptForSemanticWrapper(*backend, layout, doomed, RefTxnId{1, remove_seq});
+    CasRefCatalog::casUpdate(*backend, layout, [&](const RefCatalog & current) -> RefCatalog
+    {
+        RefCatalog next = current;
+        const auto it = std::find_if(next.entries.begin(), next.entries.end(),
+            [&](const CatalogEntry & e) { return e.ns == doomed; });
+        EXPECT_NE(it, next.entries.end());
+        it->state = NsState::Removing;
+        it->removal_started_round = 1;
+        return next;
+    });
+
+    /// A SUPPRESSED round folds `doomed` through its removal terminal and records `cleanup_evidence` in
+    /// the seal, WITHOUT executing the delete_pending the graduate round above published -- suppressed
+    /// rounds carry pending deletes forward untouched. `StageA_Suppressed` here is the test's OWN
+    /// control over timing, not the scenario under test: it exists only to keep blob X's delete pending
+    /// until round R below, rather than letting it drain the ordinary way while `doomed` is still Live.
+    gc.runRegularRound({}, /*allow_steal*/true, UniversePolicy::StageA_Suppressed);
+    store->renewWatermarkOnce();
+    EXPECT_TRUE(backend->head(key).exists) << "the pending delete must still be carried, not yet run";
+
+    /// Round R: its pre-fold drain (`drainCompletedRemoving`) reads the round just above's
+    /// `cleanup_evidence` and drops `doomed`'s catalog row BEFORE this round's own hot-scan `GET` --
+    /// so round R's catalog cut is the first one that is genuinely, provably empty. The hook fires the
+    /// instant that cut is taken and races a real namespace birth into the window before round R's own
+    /// pre-CAS delete phase runs.
+    bool hook_fired = false;
+    Token fresh_token{};
+    gc.setPostHotScanCatalogReadHookForTest([&]()
+    {
+        hook_fired = true;
+        ASSERT_TRUE(CasRefCatalog::read(*backend, layout).catalog.entries.empty())
+            << "the race must land inside the window where the cut itself is already empty";
+
+        const RootNamespace newborn{"00/newborn@cas@"};
+        auto build = store->beginPartWrite(
+            PartWriteInfo{.intended_ref = newborn.string() + "/ref_1", .intended_namespace = newborn});
+        const ManifestId new_id = build->stageManifest({blobEntryFor("data.bin", hash)});
+        build->precommitAdd(newborn, "ref_1", new_id);            /// mints `newborn` via real createNamespace
+        const PutBlobResult uploaded = build->putBlob(id, BlobSource::fromString(payload));
+        EXPECT_EQ(uploaded.ref, id);
+        fresh_token = backend->head(key).token;
+        EXPECT_NE(fresh_token, condemned_token)
+            << "the writer must have observed Condemned and resurrected -- a fresh token, not an adopt "
+               "of the dying incarnation";
+    });
+
+    const GateVerdict verdict = runRoundCapturingGate(store, gc, UniversePolicy::Authoritative);
+    ASSERT_TRUE(hook_fired) << "the race hook never fired -- this test proves nothing about the race";
+    ASSERT_TRUE(verdict.saw_fold);
+    EXPECT_TRUE(verdict.catalog_proved_empty);
+    EXPECT_TRUE(verdict.frontier_complete);
+    EXPECT_FALSE(verdict.suppress_destructive);
+
+    EXPECT_TRUE(backend->head(key).exists)
+        << "the resurrected incarnation must survive round R's delete";
+    EXPECT_EQ(backend->head(key).token, fresh_token) << "and it is still the writer's incarnation";
+    EXPECT_EQ(backend->deleteExact(key, condemned_token).kind, DeleteOutcome::Kind::TokenMismatch)
+        << "the condemned token can never remove the fresh object (INV_NO_LOSS)";
+
+    /// A later round's own fresh catalog cut names `newborn`, folds its `+1`, and the blob's frontier is
+    /// intact going forward.
+    const GateVerdict later = runRoundCapturingGate(store, gc, UniversePolicy::Authoritative);
+    ASSERT_TRUE(later.saw_fold);
+    EXPECT_EQ(later.frontier_namespaces, 1u);
+    EXPECT_EQ(later.frontier_proven, 1u);
+    EXPECT_TRUE(backend->head(key).exists) << "the newly folded owner keeps the blob alive";
+}
+
+/// The generation prune's cursor must not move on a suppressed round either. It is a monotone
+/// high-water mark that the wholesale prune never revisits, so a cursor that advanced past a generation
+/// this round declined to delete would strand that generation's whole prefix with no reclaimer left.
+TEST(CASGCFrontierGate, ASuppressedRoundDoesNotAdvanceTheGenerationPruneCursor)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    Gc gc(store, kGc);
+    for (uint64_t i = 1; i <= 6; ++i)
+    {
+        publish(*backend, layout, ns, "ref_" + std::to_string(i), i, DB::UInt128(0x2000 + i));
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+    }
+    const uint64_t pruned_through_before =
+        decodeGcState(backend->get(layout.gcStateKey())->bytes).snap_pruned_through;
+
+    for (uint64_t i = 7; i <= 10; ++i)
+    {
+        publish(*backend, layout, ns, "ref_" + std::to_string(i), i, DB::UInt128(0x2000 + i));
+        gc.runRegularRound({}, /*allow_steal*/true, UniversePolicy::StageA_Suppressed);
+        store->renewWatermarkOnce();
+    }
+
+    EXPECT_EQ(decodeGcState(backend->get(layout.gcStateKey())->bytes).snap_pruned_through,
+              pruned_through_before)
+        << "the retention cursor is a high-water mark; it may not pass a generation nothing deleted";
+}
+
+/// THE HAND-OFF RECLAIM, WHICH THE INVENTORY TEST ABOVE CANNOT REACH. This site only fires for a
+/// generation the wholesale prune SKIPPED while a live ref still pinned it (so the retention cursor
+/// moved past it and will never revisit it) and which a later round's ref then moves off. Building that
+/// takes a deliberately idle shard and a retention cursor driven past it, which is why it gets its own
+/// test rather than riding on the inventory pool.
+///
+/// It is reachable under suppression precisely because FOLDING still happens on a suppressed round: the
+/// ref moves off the old generation exactly as it would otherwise, and only the reclaim is withheld.
+TEST(CASGCFrontierGate, TheHandOffReclaimIsInertUnderSuppression)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .gc_snapshot_generations_to_keep = 1, .gc_fold_max_defer_rounds = 0});
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r1 = publish(*backend, layout, ns, "tbl", 1, DB::UInt128(0xa1));
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    const uint64_t old_gen = decodeGcState(backend->get(layout.gcStateKey())->bytes).snap_generation;
+    const String old_prefix = layout.gcGenPrefix(old_gen);
+    ASSERT_FALSE(backend->list(old_prefix, "", 1000).keys.empty());
+
+    /// Idle-carry the ref until the retention cursor is strictly PAST its generation. Until then an
+    /// ordinary prune could still reclaim it and the hand-off would not be the load-bearing path.
+    for (int i = 0; i < 6; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+    }
+    ASSERT_GT(decodeGcState(backend->get(layout.gcStateKey())->bytes).snap_pruned_through, old_gen)
+        << "the generation must be behind the retention cursor before the hand-off is exercised";
+    ASSERT_FALSE(backend->list(old_prefix, "", 1000).keys.empty())
+        << "and still retained, because a live ref pins it";
+
+    /// A real delta moves the shard's run off the old generation. This is the round the hand-off would
+    /// reclaim it on -- and it supplies no universe.
+    const ManifestRef r2{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 1};
+    writeBlobBody(*backend, layout, DB::UInt128(0xb2));
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("data.bin", DB::UInt128(0xb2))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", r1, r2);
+
+    backend->resetCounts();
+    gc.runRegularRound({}, /*allow_steal*/true, UniversePolicy::StageA_Suppressed);
+
+    EXPECT_EQ(backend->deleteCountForKeysContaining("/gc/gen/"), 0u)
+        << "a suppressed round hands nothing off. Deleted:" << deletedKeysMessage(*backend);
+    EXPECT_FALSE(backend->list(old_prefix, "", 1000).keys.empty())
+        << "the superseded generation's prefix survives a suppressed round intact";
+
+    /// AND THE OPPORTUNITY IS CONSUMED, NOT DEFERRED -- the one place in this task where the gate
+    /// costs something permanent, so it is asserted here rather than left to be discovered later.
+    ///
+    /// The hand-off is a one-shot DIFFERENCE: it compares the PARENT seal's runs against the new
+    /// seal's, and the suppressed round above already folded the delta, so the next round's parent
+    /// seal no longer mentions the old generation. Nothing revisits it -- the retention cursor is
+    /// already past it and the prune never goes back. The prefix is left to `fsck`, which is exactly
+    /// the outcome the site's own doc comment already records for a crash in the same window ("the
+    /// cursor already advanced, so a plain retry will NOT re-attempt it; fsck is the backstop").
+    /// Bounded (one small run per shard per occurrence) and not a correctness problem.
+    ///
+    /// The hand-off itself is not going untested: `CASGCRetention.HandOffDeletesSupersededRef` drives
+    /// the same transition on an authoritative round and asserts the prefix IS reclaimed.
+    runRegularRoundReclaiming(gc);
+    EXPECT_FALSE(backend->list(old_prefix, "", 1000).keys.empty())
+        << "the hand-off is a one-shot difference: the suppressed round consumed it, so the prefix is "
+           "now fsck's problem rather than a later round's";
+}
+
+/// THE ORPHAN-MANIFEST SWEEP, which the inventory pool above also cannot reach: it only deletes bodies
+/// that no ref names AND whose build is provably dead by the durable watermark floor, so it needs a
+/// pool seeded with exactly that -- orphan bodies and a floor above them.
+///
+/// It is gated with its CURSOR, not just its deletes. The cursor paces a cold-prefix enumeration and
+/// nothing revisits a range it passed, so advancing it on a round that swept nothing would silently
+/// skip that range forever. A suppressed round therefore declines the whole pass.
+TEST(CASGCFrontierGate, TheOrphanManifestSweepAndItsCursorAreInertUnderSuppression)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "gc-runner",
+                   .manifest_sweep_list_budget_keys = 1, .manifest_sweep_delete_budget_keys = 1,
+                   .gc_fold_max_defer_rounds = 0});
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"test/aa@cas@"};
+    /// The control arm below needs a recoverable catalog life whose frontier is exactly the carried
+    /// cursor. An empty non-seal transaction is a valid genesis that recovers to an empty table while
+    /// leaving the manifest epoch below the cursor's epoch.
+    fixture::admitLive(*backend, layout, ns);
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(),
+        .txn_id = RefTxnId{6, 1},
+        .ops = {},
+        .prev_epoch_seal = std::nullopt,
+    });
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 6,
+        .committed_through = RefTxnId{6, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    /// Two manifest bodies no ref ever named, under a build the durable floor has already passed.
+    const ManifestRef r1{.writer_epoch = 5, .build_sequence = 0xCA01, .manifest_ordinal = 1};
+    const ManifestRef r2{.writer_epoch = 5, .build_sequence = 0xCA02, .manifest_ordinal = 1};
+    writeManifestRaw(*backend, layout, ns, r1, {blobEntryFor("a", DB::UInt128(0xa1))});
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("b", DB::UInt128(0xb2))});
+    setWatermarkMinActive(*backend, layout, "test", r1.writer_epoch, /*min_active*/ 0xCA03);
+    /// The §6 deletion premise is a second precondition on the CONTROL arm below: a manifest of an
+    /// epoch-`E` build is deletable only once the namespace's sealed fold cursor sits in an epoch
+    /// strictly above `E`. Sealing that cursor here is what keeps this test about the GATE — without it
+    /// the control arm would stop deleting for the premise's reason, and a removed gate would no longer
+    /// show up as a difference between the two arms. A real round rewrites this row with the same cursor
+    /// (the namespace is known, quiet and unheld, so the walk probes `cursor+1`, finds the frontier and
+    /// carries the cursor), so the seeded fact survives every round below.
+    seedFoldCursorForTest(*backend, layout, ns, RefTxnId{r1.writer_epoch + 1, 1});
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+    for (int i = 0; i < 4; ++i)
+    {
+        gc.runRegularRound({}, /*allow_steal*/true, UniversePolicy::StageA_Suppressed);
+        store->renewWatermarkOnce();
+    }
+
+    EXPECT_EQ(backend->deleteCountForKeysContaining("/cas/manifests/"), 0u)
+        << "a suppressed round sweeps nothing. Deleted:" << deletedKeysMessage(*backend);
+    EXPECT_TRUE(backend->head(layout.manifestKey(ManifestId{ns, r1})).exists);
+    EXPECT_TRUE(backend->head(layout.manifestKey(ManifestId{ns, r2})).exists);
+    EXPECT_TRUE(decodeGcState(backend->get(layout.gcStateKey())->bytes).manifest_sweep_cursor.empty())
+        << "the sweep cursor must not advance over a range the round declined to sweep -- nothing "
+           "revisits it";
+
+    /// The control: the same orphans ARE swept once the universe is authoritative.
+    for (int i = 0; i < 4; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+    }
+    EXPECT_FALSE(backend->head(layout.manifestKey(ManifestId{ns, r1})).exists);
+    EXPECT_FALSE(backend->head(layout.manifestKey(ManifestId{ns, r2})).exists);
+}
+
+
+/// ===================== QUIET NAMESPACES AND THE PROBE BUDGET =====================
+
+/// THE TALLY ARITHMETIC, at a PARTIAL budget — the case neither 0 nor the default reaches.
+///
+/// `frontier_namespaces` is the denominator an operator reads as "the round's universe", and the
+/// integration test reads it too. A valid checkpoint at every quiet namespace's carried cursor is
+/// authoritative independently of LIST and the probe budget: all three lives are proven without
+/// successor probes, so the budget leaves no namespace unprobed.
+TEST(CASGCFrontierGate, APartialProbeBudgetPublishesATallyThatMatchesTheSealedSet)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolWithProbeBudget(backend, /*budget*/ 1);
+    const Layout & layout = store->layout();
+    const RootNamespace a{"00/quiet_a@cas@"};
+    const RootNamespace b{"00/quiet_b@cas@"};
+    const RootNamespace c{"00/quiet_c@cas@"};
+
+    for (const RootNamespace & ns : {a, b, c})
+        publish(*backend, layout, ns, "ref_1", 1, DB::UInt128(0x300 + ns.string().size()));
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    for (const RootNamespace & ns : {a, b, c})
+        ASSERT_NE(sealedCursorOf(*backend, layout, ns), (RefTxnId{})) << ns.string();
+
+    /// All three go unhinted at once. Their valid checkpoint frontiers still prove their carried
+    /// cursors, so this does not consume the successor-probe budget.
+    for (const RootNamespace & ns : {a, b, c})
+        backend->hidePrefix(layout.namespaceStreamPrefix(fixture::fixtureLife(ns)));
+
+    std::map<String, UInt64> intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+
+    ASSERT_FALSE(intake.empty()) << "the intake phase must have emitted its row";
+    EXPECT_EQ(intake["unhinted_quiet_walked"], 3u)
+        << "a valid checkpoint frontier makes every quiet life eligible without a successor probe";
+    EXPECT_EQ(intake["frontier_unprobed_budget"], 0u)
+        << "the CTE authority, not the probe budget, decides these quiet lives";
+    EXPECT_EQ(intake["frontier_proven"], 3u)
+        << "each carried cursor equals its valid checkpoint frontier";
+    EXPECT_EQ(intake["frontier_namespaces"], 3u)
+        << "the denominator is the complete authoritative set of sealed quiet lives";
+    EXPECT_EQ(intake["frontier_proven"], intake["frontier_namespaces"])
+        << "a valid CTE frontier remains authoritative even when LIST omits every namespace";
+
+    /// And the seal really does carry all three rows — the denominator's claim, checked against the
+    /// object it describes rather than against another counter.
+    for (const RootNamespace & ns : {a, b, c})
+    {
+        EXPECT_NE(sealedCursorOf(*backend, layout, ns), (RefTxnId{}))
+            << "every namespace in the tally must have a sealed cursor: " << ns.string();
+        const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+        const auto checkpoint = readCkpt(*backend, layout, life);
+        ASSERT_TRUE(checkpoint.has_value());
+        EXPECT_EQ(checkpoint->ckpt.committed_through, (RefTxnId{1, 1}))
+            << "LIST omission and the probe budget do not alter a valid CTE";
+    }
+}
+
+/// A checkpoint boundary already equal to the carried cursor proves a quiet catalog life complete;
+/// GC must not manufacture a successor `GET` merely because its LIST is empty.
+TEST(CASGCFrontierGate, AQuietKnownNamespaceAtItsCheckpointFrontierCostsNoSuccessorGet)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace quiet{"00/quiet@cas@"};
+
+    publish(*backend, layout, quiet, "ref_1", 1, DB::UInt128(0x11));
+    replaceRecoverableCkptForRawFixture(*backend, layout, quiet, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+
+    const RefTxnId sealed = sealedCursorOf(*backend, layout, quiet);
+    ASSERT_NE(sealed, (RefTxnId{})) << "the seeding round must have sealed a cursor to carry";
+
+    /// Now the store stops listing the namespace entirely.
+    backend->hidePrefix(layout.namespaceStreamPrefix(fixture::fixtureLife(quiet)));
+    backend->resetCounts();
+    std::map<String, UInt64> intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+
+    const String expected_next =
+        layout.refLogKey(fixture::fixtureLife(quiet), RefTxnId{sealed.writer_epoch, sealed.ref_sequence + 1});
+    EXPECT_EQ(backend->getCount(expected_next), 0u)
+        << "the inclusive checkpoint boundary proves this quiet life without a successor probe";
+    EXPECT_TRUE(report.anomalies.empty());
+    ASSERT_FALSE(intake.empty());
+    EXPECT_EQ(intake["frontier_proven"], intake["frontier_namespaces"])
+        << "the inherited cursor already at the checkpoint boundary is destructive-eligible";
+}
+
+/// A checkpoint must never retreat below a sealed cursor. Its inclusive frontier can prove a cursor
+/// already at that point, but cannot explain one that has advanced beyond it.
+TEST(CASGCFrontierGate, CheckpointFrontierBehindAnInheritedCursorFailsClosed)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint-behind-inherited-cursor@cas@"};
+
+    fixture::admitLive(*backend, layout, ns);
+    publish(*backend, layout, ns, "first", 1, DB::UInt128(0xfb));
+    publish(*backend, layout, ns, "second", 2, DB::UInt128(0xfc));
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 2}));
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    const String checkpoint_key = layout.refCkptKey(life);
+    const HeadResult checkpoint_head = backend->head(checkpoint_key);
+    ASSERT_TRUE(checkpoint_head.exists);
+    ASSERT_EQ(backend->putOverwrite(checkpoint_key, encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    }), checkpoint_head.token).outcome, PutOutcome::Done);
+
+    std::map<String, UInt64> intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+
+    EXPECT_FALSE(report.anomalies.empty());
+    ASSERT_FALSE(intake.empty());
+    EXPECT_LT(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// A carried `EpochSeal` may have its authoritative successor in the next epoch. The arithmetic
+/// successor in the sealed epoch is absent by design, so the exact checkpoint frontier must nominate
+/// the shared seal-chain crossing before that absence is classified as a same-epoch gap.
+TEST(CASGCFrontierGate, CheckpointFrontierCrossesAnInheritedEpochSeal)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint-inherited-seal-crossing@cas@"};
+    const DB::UInt128 crossed_blob(0xfd);
+
+    fixture::admitLive(*backend, layout, ns);
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "birth", 1, DB::UInt128(0xfe), /*birth=*/true);
+    writeSealAt(*backend, layout, ns, RefTxnId{1, 2});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 2}));
+
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "crossed", 2, crossed_blob,
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    const String checkpoint_key = layout.refCkptKey(life);
+    const HeadResult checkpoint_head = backend->head(checkpoint_key);
+    ASSERT_TRUE(checkpoint_head.exists);
+    ASSERT_EQ(backend->putOverwrite(checkpoint_key, encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    }), checkpoint_head.token).outcome, PutOutcome::Done);
+
+    std::map<String, UInt64> intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+
+    EXPECT_TRUE(report.anomalies.empty());
+    EXPECT_GT(inDegreeOf(*backend, layout, crossed_blob), 0);
+    ASSERT_FALSE(intake.empty());
+    EXPECT_EQ(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// The exact checkpoint successor must chain to the seal just consumed. Merely being in the next epoch
+/// is insufficient: an incorrect predecessor would skip an unclosed history segment forever.
+TEST(CASGCFrontierGate, CheckpointFrontierRejectsWrongPredecessorAfterFreshEpochSeal)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint-wrong-fresh-seal-predecessor@cas@"};
+
+    fixture::admitLive(*backend, layout, ns);
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "birth", 1, DB::UInt128(0xff), /*birth=*/true);
+    writeSealAt(*backend, layout, ns, RefTxnId{1, 2});
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "wrong_predecessor", 2, DB::UInt128(0x100),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 1});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    });
+
+    std::map<String, UInt64> intake;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+
+    EXPECT_FALSE(report.anomalies.empty());
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 2}));
+    ASSERT_FALSE(intake.empty());
+    EXPECT_LT(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// A namespace that was WRONGLY quiet -- the hint hid a record that is durably there -- is walked this
+/// round, not next: the probe finds the record and the walk continues from it.
+TEST(CASGCFrontierGate, AWronglyQuietNamespaceIsWalkedTheSameRound)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace quiet{"00/quiet@cas@"};
+    const DB::UInt128 late_blob(0x77);
+
+    publish(*backend, layout, quiet, "ref_1", 1, DB::UInt128(0x11));
+    replaceRecoverableCkptForRawFixture(*backend, layout, quiet, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    const RefTxnId sealed_before = sealedCursorOf(*backend, layout, quiet);
+
+    /// A second publish lands, and the store hides the namespace from every LIST at the same moment.
+    publish(*backend, layout, quiet, "ref_2", 2, late_blob);
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, quiet);
+    const String checkpoint_key = layout.refCkptKey(life);
+    const HeadResult checkpoint_head = backend->head(checkpoint_key);
+    ASSERT_TRUE(checkpoint_head.exists);
+    ASSERT_EQ(backend->putOverwrite(checkpoint_key, encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    }), checkpoint_head.token).outcome, PutOutcome::Done);
+    backend->hidePrefix(layout.namespaceStreamPrefix(fixture::fixtureLife(quiet)));
+
+    runRegularRoundReclaiming(gc);
+
+    EXPECT_LT(sealed_before, sealedCursorOf(*backend, layout, quiet))
+        << "the probe found the hidden record, so the walk folded it and the cursor advanced";
+    EXPECT_GT(inDegreeOf(*backend, layout, late_blob), 0)
+        << "the hidden publish's edge folded this round -- the hint never mentioned it";
+}
+
+/// The catalog life is grounded by its exact decoded `_ckpt`, not by the round's listing or a later
+/// absent probe. A durable `F+1` is physically present but not committed history, so this fold may apply
+/// only `F`; in particular it must not read `F+2`. Reaching `F` still proves the checkpoint-bounded
+/// cut, so the physical successor cannot suppress otherwise eligible destructive work.
+TEST(CASGCFrontierGate, CheckpointFrontierBoundsOrdinaryFoldBeforeDurableSuccessor)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint-bounds-fold@cas@"};
+    const DB::UInt128 committed_blob(0xf1);
+    const DB::UInt128 beyond_frontier_blob(0xf2);
+
+    fixture::admitLive(*backend, layout, ns);
+    publish(*backend, layout, ns, "committed", 1, committed_blob);
+    const ManifestRef uncommitted{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 1};
+    writeBlobBody(*backend, layout, beyond_frontier_blob);
+    writeManifestRaw(*backend, layout, ns, uncommitted, {blobEntryFor("data.bin", beyond_frontier_blob)});
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(),
+        .txn_id = RefTxnId{1, 2},
+        .ops = publishCommittedOps("durable_but_uncommitted", uncommitted),
+        .prev_epoch_seal = std::nullopt,
+    });
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    std::map<String, UInt64> intake;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    backend->resetCounts();
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(report.acquired_lease);
+    gc.setPhaseSink({});
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 1}));
+    EXPECT_EQ(inDegreeOf(*backend, layout, beyond_frontier_blob), 0)
+        << "a durable log above `_ckpt.committed_through` is not foldable history";
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{1, 2})), 0u)
+        << "the checkpoint frontier stops the walk before `F+1`";
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{1, 3})), 0u)
+        << "a 404 above `F+1` must not authorize the destructive frontier";
+    EXPECT_TRUE(report.anomalies.empty());
+    ASSERT_FALSE(intake.empty());
+    EXPECT_EQ(intake["frontier_proven"], intake["frontier_namespaces"])
+        << "the consumed checkpoint frontier, not the physical successor, authorizes this cut";
+}
+
+/// With no physical successor at all, consuming the exact inclusive checkpoint frontier proves this
+/// catalog life complete. This is the control for the same bounded-cut proof exercised with a durable
+/// uncommitted successor above.
+TEST(CASGCFrontierGate, ConsumedCheckpointFrontierProvesOrdinaryLifeWithoutSuccessor)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint-complete-fold@cas@"};
+
+    fixture::admitLive(*backend, layout, ns);
+    publish(*backend, layout, ns, "committed", 1, DB::UInt128(0xf3));
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    std::map<String, UInt64> intake;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    backend->resetCounts();
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(report.acquired_lease);
+    gc.setPhaseSink({});
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 1}));
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{1, 2})), 0u)
+        << "the checkpoint boundary proves the cut without a post-frontier 404";
+    EXPECT_TRUE(report.anomalies.empty());
+    ASSERT_FALSE(intake.empty());
+    EXPECT_EQ(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// The same cut remains complete when the durable uncommitted successor is hidden from every LIST.
+/// Exact reads still serve that successor, but the checkpoint ceiling must leave it untouched and must
+/// not let the list omission suppress the checkpoint-bounded destructive path.
+TEST(CASGCFrontierGate, CheckpointFrontierProvesLifeWithHiddenDurableSuccessor)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/checkpoint-hidden-successor@cas@"};
+    const DB::UInt128 beyond_frontier_blob(0xf4);
+
+    fixture::admitLive(*backend, layout, ns);
+    publish(*backend, layout, ns, "committed", 1, DB::UInt128(0xf5));
+    const ManifestRef uncommitted{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 1};
+    writeBlobBody(*backend, layout, beyond_frontier_blob);
+    writeManifestRaw(*backend, layout, ns, uncommitted, {blobEntryFor("data.bin", beyond_frontier_blob)});
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(),
+        .txn_id = RefTxnId{1, 2},
+        .ops = publishCommittedOps("hidden_durable_but_uncommitted", uncommitted),
+        .prev_epoch_seal = std::nullopt,
+    });
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    backend->hide(layout.refLogKey(life, RefTxnId{1, 2}));
+
+    std::map<String, UInt64> intake;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    backend->resetCounts();
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(report.acquired_lease);
+    gc.setPhaseSink({});
+
+    EXPECT_GT(backend->holesServed(), 0u) << "the F+1 log must really be hidden from LIST";
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 1}));
+    EXPECT_EQ(inDegreeOf(*backend, layout, beyond_frontier_blob), 0);
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{1, 2})), 0u)
+        << "the hidden durable successor is outside the checkpoint cut";
+    EXPECT_TRUE(report.anomalies.empty());
+    ASSERT_FALSE(intake.empty());
+    EXPECT_EQ(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// The checkpoint's inclusive endpoint is itself a durable witness. If that exact log is absent,
+/// the namespace is corrupt rather than complete; a 404 at the endpoint must not authorize cleanup.
+TEST(CASGCFrontierGate, MissingCommittedCheckpointLogHoldsInsteadOfProvingTheFrontier)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/missing-committed-checkpoint-log@cas@"};
+
+    fixture::admitLive(*backend, layout, ns);
+    publish(*backend, layout, ns, "first", 1, DB::UInt128(0xf6));
+    publish(*backend, layout, ns, "missing_but_committed", 2, DB::UInt128(0xf7));
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    const String missing_key = layout.refLogKey(life, RefTxnId{1, 2});
+    const HeadResult missing_head = backend->head(missing_key);
+    ASSERT_TRUE(missing_head.exists);
+    ASSERT_EQ(backend->deleteExact(missing_key, missing_head.token).kind, DeleteOutcome::Kind::Deleted);
+
+    std::map<String, UInt64> intake;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(report.acquired_lease);
+    gc.setPhaseSink({});
+
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 1}));
+    EXPECT_FALSE(report.anomalies.empty()) << "the missing committed checkpoint record is corruption";
+    ASSERT_FALSE(intake.empty());
+    EXPECT_LT(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// A checkpoint may name a durable record that the round's LIST omitted. Exact GETs must still fold
+/// that committed record; the frozen list tail is only a scheduling hint, never a history boundary.
+TEST(CASGCFrontierGate, HiddenCommittedCheckpointLogIsFoldedThroughTheAuthorityCeiling)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/hidden-committed-checkpoint-log@cas@"};
+    const DB::UInt128 hidden_blob(0xf8);
+
+    fixture::admitLive(*backend, layout, ns);
+    publish(*backend, layout, ns, "first", 1, DB::UInt128(0xf9));
+    publish(*backend, layout, ns, "hidden_but_committed", 2, hidden_blob);
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    backend->hide(layout.refLogKey(life, RefTxnId{1, 2}));
+
+    std::map<String, UInt64> intake;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(report.acquired_lease);
+    gc.setPhaseSink({});
+
+    EXPECT_GT(backend->holesServed(), 0u) << "the committed endpoint must really be omitted from LIST";
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 2}));
+    EXPECT_GT(inDegreeOf(*backend, layout, hidden_blob), 0);
+    EXPECT_TRUE(report.anomalies.empty());
+    ASSERT_FALSE(intake.empty());
+    EXPECT_EQ(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// A valid checkpoint with no committed record is an authoritative empty history. It is complete for
+/// a never-folded life without probing a fabricated first transaction.
+TEST(CASGCFrontierGate, EmptyCheckpointFrontierProvesAnUnfoldedLife)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/empty-checkpoint-frontier@cas@"};
+
+    casAdmitRecoverableEntry(*backend, layout, ns);
+
+    std::map<String, UInt64> intake;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(report.acquired_lease);
+    gc.setPhaseSink({});
+
+    EXPECT_TRUE(report.anomalies.empty());
+    ASSERT_FALSE(intake.empty());
+    EXPECT_EQ(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// Empty history cannot explain an inherited cursor. An operator-corrupted checkpoint that erases its
+/// own committed boundary must clamp the life rather than silently authorize destruction.
+TEST(CASGCFrontierGate, EmptyCheckpointFrontierRejectsAnInheritedCursor)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/empty-checkpoint-after-cursor@cas@"};
+
+    fixture::admitLive(*backend, layout, ns);
+    publish(*backend, layout, ns, "first", 1, DB::UInt128(0xfa));
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 1}));
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    const String checkpoint_key = layout.refCkptKey(life);
+    const HeadResult checkpoint_head = backend->head(checkpoint_key);
+    ASSERT_TRUE(checkpoint_head.exists);
+    ASSERT_EQ(backend->putOverwrite(checkpoint_key, encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    }), checkpoint_head.token).outcome, PutOutcome::Done);
+
+    std::map<String, UInt64> intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(report.acquired_lease);
+    gc.setPhaseSink({});
+
+    EXPECT_FALSE(report.anomalies.empty()) << "an empty checkpoint cannot explain a nonzero cursor";
+    ASSERT_FALSE(intake.empty());
+    EXPECT_LT(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// A catalog `Live` life without its exact checkpoint cannot derive either its genesis or a frontier
+/// from the ref LIST. Even a durable listed first log must be retained until the authority is repaired.
+TEST(CASGCFrontierGate, CatalogLifeWithoutCheckpointDefersWithoutUsingListedFrontier)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/missing-checkpoint-fold@cas@"};
+    const DB::UInt128 blob(0xc7);
+
+    fixture::admitLive(*backend, layout, ns);
+    const ManifestRef manifest{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1};
+    writeBlobBody(*backend, layout, blob);
+    writeManifestRaw(*backend, layout, ns, manifest, {blobEntryFor("data.bin", blob)});
+    appendRefLogSeed(*backend, layout, ns, publishCommittedOps("must_remain_unfolded", manifest));
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    ASSERT_FALSE(readCkpt(*backend, layout, life).has_value());
+
+    std::map<String, UInt64> intake;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            intake = rec.metrics;
+    });
+    backend->resetCounts();
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    gc.setPhaseSink({});
+
+    EXPECT_EQ(foldCursorOf(*backend, layout, ns, /*shard=*/0), 0u);
+    EXPECT_EQ(inDegreeOf(*backend, layout, blob), 0)
+        << "a missing checkpoint must defer rather than fold the listed log";
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{1, 1})), 0u)
+        << "the listed log is not authority for a checkpoint-less catalog life";
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{1, 2})), 0u)
+        << "the next 404 is not authority for a checkpoint-less catalog life";
+    EXPECT_EQ(backend->deleteTotal(), 0u) << deletedKeysMessage(*backend);
+    ASSERT_FALSE(intake.empty());
+    EXPECT_LT(intake["frontier_proven"], intake["frontier_namespaces"]);
+}
+
+/// A valid checkpoint frontier proves a quiet unhinted life without spending the successor-probe budget.
+/// A zero budget therefore cannot suppress unrelated destructive work merely because this life is absent
+/// from LIST.
+TEST(CASGCFrontierGate, AnExhaustedProbeBudgetSealsCursorsAndDeletesNothing)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolWithProbeBudget(backend, /*budget*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace quiet{"00/quiet@cas@"};
+    const RootNamespace busy{"00/busy@cas@"};
+    const DB::UInt128 blob(0xbeef);
+
+    publish(*backend, layout, quiet, "quiet_ref", 1, DB::UInt128(0x11));
+    const ManifestRef mref = publish(*backend, layout, busy, "busy_ref", 2, blob);
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    const RefTxnId quiet_cursor = sealedCursorOf(*backend, layout, quiet);
+    ASSERT_NE(quiet_cursor, (RefTxnId{}));
+
+    /// The quiet namespace goes unhinted and the budget is zero. Its CTE still proves the carried
+    /// cursor, while the busy namespace drops its ref and may proceed through reclamation.
+    backend->hidePrefix(layout.namespaceStreamPrefix(fixture::fixtureLife(quiet)));
+    dropRefTransition(*backend, layout, busy, "busy_ref", mref);
+
+    backend->resetCounts();
+    drive(store, gc, /*rounds*/ 5, UniversePolicy::Authoritative);
+
+    EXPECT_GT(backend->deleteTotal(), 0u)
+        << "the quiet life's checkpoint authority leaves unrelated deletion eligible";
+    EXPECT_FALSE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "the busy life's removal remains reclaimable despite the quiet LIST omission";
+    EXPECT_EQ(sealedCursorOf(*backend, layout, quiet), quiet_cursor)
+        << "the unprobed namespace's cursor rides verbatim -- it is never dropped";
+    const NamespaceLifeId quiet_life = *CasRefCatalog::lifeIfCataloged(*backend, layout, quiet);
+    const auto quiet_checkpoint = readCkpt(*backend, layout, quiet_life);
+    ASSERT_TRUE(quiet_checkpoint.has_value());
+    EXPECT_EQ(quiet_checkpoint->ckpt.committed_through, quiet_cursor)
+        << "the quiet life's valid CTE is unaffected by LIST omission and a zero probe budget";
+    EXPECT_GT(decodeGcState(backend->get(layout.gcStateKey())->bytes).round, 1u)
+        << "the round still commits; only its destructive half is withheld";
+}
+
+/// ===================== A COMMITTED GAP IS REDETECTED UNTIL REPAIRED =====================
+///
+/// A hold's committed checkpoint frontier remains a durable witness of its own gap. Hiding the later
+/// log from LIST cannot make that gap quiet: every retry exact-reads the missing position, redetects the
+/// hold, and suppresses destructive work until an operator repairs the record stream.
+TEST(CASGCFrontierGate, ACommittedGapIsRedetectedAndSuppressesEveryRound)
+{
+    auto backend = std::make_shared<CountingHintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace held{"00/held@cas@"};
+    const RootNamespace busy{"00/busy@cas@"};
+    const DB::UInt128 blob(0xbeef);
+
+    /// {1,3} never existed while {1,4} is durable and listed.
+    publish(*backend, layout, held, "ref_1", 1, DB::UInt128(0x21));
+    publish(*backend, layout, held, "ref_2", 2, DB::UInt128(0x22));
+    const ManifestRef orphan_ref{.writer_epoch = 1, .build_sequence = 4, .manifest_ordinal = 1};
+    writeBlobBody(*backend, layout, DB::UInt128(0x24));
+    writeManifestRaw(*backend, layout, held, orphan_ref, {blobEntryFor("data.bin", DB::UInt128(0x24))});
+    RefLogTxn txn;
+    txn.ns = held.string();
+    txn.txn_id = RefTxnId{1, 4};
+    txn.ops = publishCommittedOps("ref_4", orphan_ref);
+    fixture::writeRefLogRaw(*backend, layout, txn);
+    replaceRecoverableCkptForRawFixture(*backend, layout, held, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 4},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    std::map<String, UInt64> first_intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            first_intake = rec.metrics;
+    });
+    const RoundReport first_round = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+    store->renewWatermarkOnce();
+    ASSERT_EQ(sealedCursorOf(*backend, layout, held), (RefTxnId{1, 2}))
+        << "round 1 must stop below the gap and hold there";
+    ASSERT_FALSE(first_intake.empty());
+    EXPECT_GT(first_intake["tables_clamped"], 0u);
+    EXPECT_GT(first_intake["tables_held"], 0u);
+    EXPECT_FALSE(first_round.anomalies.empty());
+
+    const NamespaceLifeId held_life = *CasRefCatalog::lifeIfCataloged(*backend, layout, held);
+    const auto held_checkpoint = readCkpt(*backend, layout, held_life);
+    ASSERT_TRUE(held_checkpoint.has_value());
+    EXPECT_EQ(held_checkpoint->ckpt.committed_through, (RefTxnId{1, 4}));
+
+    /// Hiding `{1,4}` from LIST does not hide the committed CTE frontier. The next round exact-reads
+    /// the missing `{1,3}`, re-detects the gap, and seals a fresh hold.
+    backend->hidePrefix(layout.refLogKey(fixture::fixtureLife(held), RefTxnId{1, 4}));
+
+    /// Meanwhile a blob elsewhere becomes condemnable, so the round has real destructive work to decline.
+    const ManifestRef mref = publish(*backend, layout, busy, "busy_ref", 9, blob);
+    dropRefTransition(*backend, layout, busy, "busy_ref", mref);
+
+    backend->resetCounts();
+    std::map<String, UInt64> second_intake;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            second_intake = rec.metrics;
+    });
+    const RoundReport second_round = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+    store->renewWatermarkOnce();
+
+    ASSERT_FALSE(second_intake.empty());
+    EXPECT_GT(second_intake["tables_clamped"], 0u)
+        << "the committed `{1,4}` frontier is a durable witness that re-detects the missing `{1,3}`";
+    EXPECT_GT(second_intake["tables_held"], 0u)
+        << "the fresh clamp preserves the unresolved hold in the next sealed coverage";
+    EXPECT_FALSE(second_round.anomalies.empty());
+
+    drive(store, gc, /*rounds*/ 4, UniversePolicy::Authoritative);
+
+    EXPECT_EQ(backend->deleteTotal(), 0u)
+        << "the re-detected committed gap suppresses each round's destructive work. "
+           "Deleted:" << deletedKeysMessage(*backend);
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists);
+    EXPECT_EQ(sealedCursorOf(*backend, layout, held), (RefTxnId{1, 2}))
+        << "the committed gap remains unresolved and the cursor cannot advance through it";
+    const auto final_checkpoint = readCkpt(*backend, layout, held_life);
+    ASSERT_TRUE(final_checkpoint.has_value());
+    EXPECT_EQ(final_checkpoint->ckpt.committed_through, (RefTxnId{1, 4}));
+}
+
+/// ===================== THE TEMPORAL LEMMA, ALL THREE ARMS =====================
+///
+/// The gate says WHEN a round may destroy. These say that even a round which may destroy cannot
+/// destroy a blob some edge still owns, over the three interleavings that matter.
+
+/// ARM (a): a `+1` that lands after this round's probes and is followed by the SAME round's
+/// condemnation. Round pacing makes it safe on its own: an entry condemned at round K cannot graduate
+/// before K+1 and cannot be deleted before K+2, so the round that condemns never deletes.
+TEST(CASGCFrontierGate, ABlobCondemnedThisRoundIsNeverDeletedThisRound)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    const DB::UInt128 blob(0xc04d);
+
+    const ManifestRef mref = publish(*backend, layout, ns, "ref_1", 1, blob);
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+
+    dropRefTransition(*backend, layout, ns, "ref_1", mref);
+    backend->resetCounts();
+    runRegularRoundReclaiming(gc);
+
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "the condemning round must not also delete";
+    EXPECT_EQ(backend->deleteCount(blobKeyOf(layout, blob)), 0u)
+        << "not merely still present -- the delete was never attempted";
+}
+
+/// ARM (c) of the temporal lemma is the delete-site in-degree re-read, and it is NORMATIVE (spec §5,
+/// third arm): an edge folded AFTER the condemnation but BEFORE the delete pass spares the blob
+/// outright, `indeg > 0` winning over `delete_pending` past the floor. The other two arms bound WHEN
+/// and WHAT a delete may remove; only this one asks whether the blob is still referenced at the moment
+/// the pass decides.
+TEST(CASGCFrontierGate, ALateEdgeSparesADeletePendingBlobAtTheDeleteSite)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    const DB::UInt128 blob(0x1a7e);
+
+    const ManifestRef mref = publish(*backend, layout, ns, "ref_1", 1, blob);
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+
+    /// Condemn it, then graduate it to delete_pending.
+    dropRefTransition(*backend, layout, ns, "ref_1", mref);
+    runRegularRoundReclaiming(gc);            /// condemn
+    store->renewWatermarkOnce();
+    runRegularRoundReclaiming(gc);            /// graduate: delete_pending published
+    store->renewWatermarkOnce();
+
+    /// A new owner appears BEFORE the delete pass. The pass recomputes the in-degree from the merge it
+    /// just ran and finds it nonzero.
+    const ManifestRef revived{.writer_epoch = 1, .build_sequence = 42, .manifest_ordinal = 1};
+    writeManifestRaw(*backend, layout, ns, revived, {blobEntryFor("data.bin", blob)});
+    publishCommittedTransition(*backend, layout, ns, "revived_ref", std::nullopt, revived);
+
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "the delete-site in-degree re-read spares a blob a fresh edge re-referenced";
+    EXPECT_GT(inDegreeOf(*backend, layout, blob), 0);
+}
+
+/// ARM (b): a TOKENED adoption of an already-delete-pending blob. The writer's admit gate reads the
+/// `Condemned` meta, refuses to adopt the dying incarnation, and rematerializes from its own source as
+/// a FRESH incarnation -- so the delayed exact-token delete the previous round published finds a
+/// different token and removes nothing. The blob's identity is preserved by re-upload, never by
+/// reviving the condemned object.
+TEST(CASGCFrontierGate, AResurrectedIncarnationSurvivesTheDelayedStaleTokenDelete)
+{
+    ensureBlobUploadPoolForTest();
+
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    /// A REAL content-addressed blob, so the writer path below addresses exactly the object GC condemns.
+    const String payload = "frontier-gate-resurrect-payload";
+    const DB::UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+    const String key = layout.blobKey(id);
+    String raw_body(store->poolMeta().blob_header_len, '\0');
+    raw_body += payload;
+    writeRawBlobBody(*backend, layout, hash, raw_body);
+
+    /// Publish and drop it so GC condemns and then graduates it to delete_pending.
+    const ManifestRef mref{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1};
+    writeManifestRaw(*backend, layout, ns, mref, {blobEntryFor("data.bin", hash)});
+    publishCommittedTransition(*backend, layout, ns, "ref_1", std::nullopt, mref);
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, layout, ns, "ref_1", mref);
+    runRegularRoundReclaiming(gc);            /// condemn: writes the durable Condemned meta
+    store->renewWatermarkOnce();
+    runRegularRoundReclaiming(gc);            /// graduate: publishes delete_pending against THIS token
+    store->renewWatermarkOnce();
+
+    const Token condemned_token = backend->head(key).token;
+    const auto condemned_meta = loadMetaForTest(*backend, layout, hash);
+    ASSERT_TRUE(condemned_meta.has_value());
+    ASSERT_EQ(condemned_meta->meta.state, MetaState::Condemned)
+        << "the delete GC is about to execute must be backed by durable Condemned evidence";
+
+    /// A writer now adopts the blob through the REAL admit gate. It point-reads the Condemned meta,
+    /// refuses to adopt the dying incarnation, and rematerializes from its OWN source bytes -- never by
+    /// reading the condemned object. The key ends up holding a DIFFERENT incarnation.
+    auto build = store->beginPartWrite({});
+    const PutBlobResult uploaded = build->putBlob(id, BlobSource::fromString(payload));
+    EXPECT_EQ(uploaded.ref, id);
+    const Token fresh_token = backend->head(key).token;
+    ASSERT_NE(fresh_token, condemned_token) << "a resurrect must displace the condemned incarnation";
+
+    /// GC's delayed delete still names the OLD token. It cannot touch the new object.
+    drive(store, gc, /*rounds*/ 2, UniversePolicy::Authoritative);
+
+    ASSERT_TRUE(backend->head(key).exists)
+        << "the resurrected incarnation survives the delete published against its predecessor";
+    EXPECT_EQ(backend->head(key).token, fresh_token) << "and it is still the writer's incarnation";
+    EXPECT_EQ(backend->deleteExact(key, condemned_token).kind, DeleteOutcome::Kind::TokenMismatch)
+        << "the condemned token can never remove the fresh object (INV-NO-RETURN)";
+}
+
+/// ARM (c): a TOKENLESS relink -- the receiver adopts by evidence, holding no token at all. Safety
+/// then rests entirely on ORDER, so the operation journal has to show it: the receiver's `+1` is
+/// durable BEFORE the source releases its own committed edge, and no point in the schedule leaves the
+/// blob with zero durable owners.
+TEST(CASGCFrontierGate, ATokenlessRelinkMakesTheReceiverEdgeDurableBeforeTheSourceReleases)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace source{"00/source@cas@"};
+    const RootNamespace receiver{"00/receiver@cas@"};
+    const DB::UInt128 blob(0x8e11);
+
+    const ManifestRef source_ref = publish(*backend, layout, source, "part_1", 1, blob);
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    ASSERT_GT(inDegreeOf(*backend, layout, blob), 0);
+
+    /// The relink, in the only order the protocol permits: the receiver's manifest body and its
+    /// committed edge first (tokenless -- it never HEADs the blob), and only afterwards the source's
+    /// removal. Between the two writes the blob has TWO durable owners; it never has zero.
+    const ManifestRef receiver_ref{.writer_epoch = 1, .build_sequence = 5, .manifest_ordinal = 1};
+    writeManifestRaw(*backend, layout, receiver, receiver_ref, {blobEntryFor("data.bin", blob)});
+    publishCommittedTransition(*backend, layout, receiver, "part_1", std::nullopt, receiver_ref);
+
+    /// The round that observes ONLY the receiver's `+1` -- the exact midpoint of the schedule.
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    EXPECT_GE(inDegreeOf(*backend, layout, blob), 2)
+        << "at the midpoint both owners are durable; the handoff never dips to zero";
+
+    dropRefTransition(*backend, layout, source, "part_1", source_ref);
+    drive(store, gc, /*rounds*/ 4, UniversePolicy::Authoritative);
+
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "the source released its edge only after the receiver's was durable, so nothing may collect it";
+    EXPECT_EQ(inDegreeOf(*backend, layout, blob), 1)
+        << "the receiver is the sole remaining owner";
+}
+
+/// ===================== CLEANUP RANGES ARE COMPUTED, NOT ENUMERATED =====================
+///
+/// `planRefCleanup` is pure, so the boundary arithmetic is pinned directly rather than inferred from a
+/// round's side effects. Its sole coverage authority is the checkpoint-named base; a listed snapshot
+/// is merely a physical observation until the same-id triple has been validated.
+
+TEST(CASGCFrontierGateCleanupRange, CoveredLogsStopAtTheMinimumOfCheckpointAndCursor)
+{
+    RefTableListing listing;
+    listing.logs = {{1, 1}, {1, 2}, {1, 3}, {1, 4}, {1, 5}};
+    listing.snapshots = {{1, 5}};
+
+    /// No checkpoint means no recovery base at all. A snapshot PUT that has not reached the `_ckpt`
+    /// CAS must retain every listed object.
+    const RefCleanupPlan without = planRefCleanup(listing, RefTxnId{1, 4});
+    EXPECT_TRUE(without.deletable_logs.empty());
+    EXPECT_TRUE(without.deletable_snapshots.empty());
+
+    /// A checkpoint BELOW the cursor tightens it to {1,2}. Its exact `_log` witness must survive,
+    /// so cleanup may remove only the strictly older entry.
+    const RefCleanupPlan with = planRefCleanup(listing, RefTxnId{1, 4}, RefTxnId{1, 2});
+    EXPECT_EQ(with.deletable_logs, (std::vector<RefTxnId>{{1, 1}}))
+        << "the checkpoint witness and everything above it must survive";
+
+    /// Once validation has established a later checkpoint base, its earlier covered history is
+    /// reclaimable even if the hot fold cursor has not yet reached that base.
+    const RefCleanupPlan ahead = planRefCleanup(listing, RefTxnId{1, 4}, RefTxnId{1, 9});
+    EXPECT_EQ(ahead.deletable_logs, (std::vector<RefTxnId>{{1, 1}, {1, 2}, {1, 3}, {1, 4}}));
+    EXPECT_EQ(ahead.deletable_snapshots, (std::vector<RefTxnId>{{1, 5}}));
+}
+
+TEST(CASGCFrontierGateCleanupRange, ASnapshotAtTheCheckpointSurvivesAndOnlyStrictlyOlderOnesGo)
+{
+    RefTableListing listing;
+    listing.logs = {{1, 1}, {1, 2}, {1, 3}};
+    listing.snapshots = {{1, 1}, {1, 2}, {1, 3}};
+
+    /// A LIST-only newest snapshot is never a cleanup boundary.
+    const RefCleanupPlan without = planRefCleanup(listing, RefTxnId{1, 3});
+    EXPECT_TRUE(without.deletable_snapshots.empty());
+
+    /// With the checkpoint AT {1,2}, only {1,1} is strictly below it. The snapshot the checkpoint names
+    /// is the one a recovering reader samples, so it must survive its own cleanup.
+    const RefCleanupPlan with = planRefCleanup(listing, RefTxnId{1, 3}, RefTxnId{1, 2});
+    EXPECT_EQ(with.deletable_snapshots, (std::vector<RefTxnId>{{1, 1}}));
+
+    /// The oldest checkpoint deletes nothing at all.
+    const RefCleanupPlan oldest = planRefCleanup(listing, RefTxnId{1, 3}, RefTxnId{1, 1});
+    EXPECT_TRUE(oldest.deletable_snapshots.empty());
+}
+
+/// Cleanup shares recovery's validator rather than inferring its own authority from a LIST. The
+/// missing-base case is the no-checkpoint range above; the three physical triple failures below must
+/// each reject exactly the checkpoint-named candidate.
+TEST(CASGCFrontierGateCleanupRange, CheckpointBaseValidatorRejectsMissingLogSnapshotAndSeal)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout{"p"};
+    const RefTxnId base{1, 1};
+    const RefCkpt checkpoint{
+        .life_epoch = 1,
+        .committed_through = base,
+        .checkpoint_snapshot_id = base,
+        .last_epoch_seal = std::nullopt};
+    CasRefCatalog::initializeEmptyForNewPool(*backend, layout);
+
+    {
+        const RootNamespace ns{"00/cleanup-missing-base-log@cas@"};
+        fixture::admitLive(*backend, layout, ns);
+        const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, layout, ns).value();
+        writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), base));
+        EXPECT_THROW((void)readCheckpointSnapshotBase(*backend, layout, life, checkpoint), DB::Exception);
+    }
+    {
+        const RootNamespace ns{"00/cleanup-missing-base-snapshot@cas@"};
+        fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+            .ns = ns.string(), .txn_id = base, .ops = {namespaceBirthOp()}, .prev_epoch_seal = std::nullopt});
+        const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, layout, ns).value();
+        EXPECT_THROW((void)readCheckpointSnapshotBase(*backend, layout, life, checkpoint), DB::Exception);
+    }
+    {
+        const RootNamespace ns{"00/cleanup-seal-is-not-base@cas@"};
+        writeSealAt(*backend, layout, ns, base);
+        const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, layout, ns).value();
+        writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), base));
+        EXPECT_THROW((void)readCheckpointSnapshotBase(*backend, layout, life, checkpoint), DB::Exception);
+    }
+}
+
+TEST(CASGCFrontierGateCleanupRange, LaterEpochBaseWithoutItsContextualBacklinkCannotLicenseDeletion)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout{"p"};
+    CasRefCatalog::initializeEmptyForNewPool(*backend, layout);
+    const RefTxnId seal_id{1, 2};
+    const RefTxnId base_id{2, 1};
+
+    const auto expect_no_deletion_authority = [&](const RootNamespace & ns, std::optional<RefTxnId> backlink)
+    {
+        fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+            .ns = ns.string(), .txn_id = RefTxnId{1, 1}, .ops = {namespaceBirthOp()},
+            .prev_epoch_seal = std::nullopt});
+        writeSealAt(*backend, layout, ns, seal_id);
+        fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+            .ns = ns.string(), .txn_id = base_id, .ops = {}, .prev_epoch_seal = backlink});
+        writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), base_id));
+        const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, layout, ns).value();
+
+        std::optional<RefTxnId> validated_base;
+        try
+        {
+            (void)readCheckpointSnapshotBase(*backend, layout, life, RefCkpt{
+                .life_epoch = 1,
+                .committed_through = base_id,
+                .checkpoint_snapshot_id = base_id,
+                .last_epoch_seal = seal_id});
+            validated_base = base_id;
+        }
+        catch (const DB::Exception &) // NOLINT(bugprone-empty-catch): failure to validate is the tested case -- `validated_base` deliberately stays nullopt
+        {
+        }
+
+        RefTableListing listing;
+        listing.logs = {{1, 1}, seal_id, base_id};
+        listing.snapshots = {{1, 1}, base_id};
+        const RefCleanupPlan plan = planRefCleanup(listing, base_id, validated_base);
+        EXPECT_TRUE(plan.deletable_logs.empty());
+        EXPECT_TRUE(plan.deletable_snapshots.empty());
+    };
+
+    expect_no_deletion_authority(RootNamespace{"00/cleanup-base-missing-backlink@cas@"}, std::nullopt);
+    expect_no_deletion_authority(RootNamespace{"00/cleanup-base-wrong-backlink@cas@"}, RefTxnId{1, 99});
+}
+
+/// Folding a namespace terminal records evidence but performs no lifecycle-specific physical cleanup.
+/// The checkpoint is inert debris for the perpetual janitor, and no `_cleanup` marker is published.
+TEST(CASGCFrontierGate, CleanupEvidenceLeavesRemovedNamespaceCheckpointForJanitor)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace removed{"00/removed@cas@"};
+    const RefOp birth_op = namespaceBirthOp();
+    RefOp remove_op;
+    remove_op.kind = RefOpKind::RemoveNamespace;
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = removed.string(), .txn_id = RefTxnId{1, 1}, .ops = {birth_op}, .prev_epoch_seal = std::nullopt});
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = removed.string(), .txn_id = RefTxnId{1, 2}, .ops = {remove_op}, .prev_epoch_seal = std::nullopt});
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, layout, removed).value();
+    CasRefCatalog::casUpdate(*backend, layout, [&](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        const auto it = std::find_if(next.entries.begin(), next.entries.end(), [&](const CatalogEntry & entry)
+        {
+            return entry.ns == removed;
+        });
+        EXPECT_NE(it, next.entries.end());
+        it->state = NsState::Removing;
+        it->removal_started_round = 1;
+        return next;
+    });
+    const String ckpt_key = layout.refCkptKey(life);
+    backend->putIfAbsent(ckpt_key, encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    }));
+
+    /// The removal evidence must arise from a replay-valid terminal lifecycle, rather than merely
+    /// from a raw terminal record that the recovery state machine refuses.
+    const RecoveredRefTable recovered = recoverRefTableDetailedAtCatalogCutForTest(
+        *backend, layout, CasRefCatalog::read(*backend, layout), removed);
+    EXPECT_EQ(recovered.state.getLifecycle(), RefLifecycle::Removed);
+    EXPECT_EQ(recovered.state.getRemoveTxnId(), (RefTxnId{1, 2}));
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    const GcState st = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const CasFoldSeal seal = decodeFoldSeal(
+        backend->get(layout.foldSealKey(st.snap_generation, st.snap_attempt))->bytes);
+    const auto row_it = seal.ref_lives.find(life.incarnation);
+    ASSERT_NE(row_it, seal.ref_lives.end());
+    ASSERT_TRUE(row_it->second.cleanup_evidence.has_value());
+    EXPECT_EQ(row_it->second.cleanup_evidence->remove_txn_id, (RefTxnId{1, 2}));
+    EXPECT_TRUE(backend->head(ckpt_key).exists);
+    for (const String & key : backend->touchedKeys())
+        EXPECT_EQ(key.find("/_cleanup/"), String::npos) << key;
+
+    /// Round 2 drops `removed`'s catalog row (the pre-fold drain, using round 1's `cleanup_evidence`),
+    /// which makes THIS round's own hot-scan catalog cut genuinely, provably empty -- so its destructive
+    /// gate opens for the first time (`catalog_cut_proved_empty`), and the namespace janitor -- a
+    /// separate `namespace_cleanup` phase the SAME round call also runs -- reclaims the now-orphaned
+    /// checkpoint. Reclaiming a removed namespace's `_ckpt` once the pool empties is exactly the
+    /// standstill this gate exists to fix, so the janitor running here is the fix working, not a
+    /// regression. What this test still pins is the DISCRIMINATION the title promises: the FOLD stage
+    /// itself performs no lifecycle-specific physical cleanup (asserted above, unchanged), and the
+    /// janitor is attributed the delete via its OWN phase counters -- never inferred from end-state
+    /// absence, which would not distinguish "the janitor did it" from "something else did".
+    std::map<String, UInt64> janitor_metrics;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "namespace_cleanup")
+            janitor_metrics = rec.metrics;
+    });
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    gc.setPhaseSink({});
+
+    EXPECT_TRUE(CasRefCatalog::read(*backend, layout).catalog.entries.empty());
+    EXPECT_FALSE(CasRefCatalog::lifeIfCataloged(*backend, layout, removed));
+    ASSERT_FALSE(janitor_metrics.empty()) << "the namespace_cleanup phase must have run this round";
+    EXPECT_GE(janitor_metrics.at("janitor_deleted"), 1u)
+        << "the janitor's OWN counter must show the delete -- now that the proved-empty gate has "
+           "opened, not because some other site happened to remove the key";
+    EXPECT_FALSE(backend->head(ckpt_key).exists);
+    EXPECT_EQ(backend->deleteCount(ckpt_key), 1);
+}
+
+/// Once a terminal has folded, a later physical read failure is janitor debt, not lifecycle evidence
+/// loss. Removing this per-key leak handling would either make the signal disappear or let one dead
+/// object prevent the janitor from considering the rest of its page.
+TEST(CASGCFrontierGate, PostFoldUnreadableTerminalIsCountedWithoutSuppressingProgress)
+{
+    auto backend = std::make_shared<PostFoldUnreadableTerminalBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace removed{"00/post-fold-unreadable@cas@"};
+    const RootNamespace progressing{"00/post-fold-progress@cas@"};
+
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = removed.string(), .txn_id = RefTxnId{1, 1}, .ops = {namespaceBirthOp()},
+        .prev_epoch_seal = std::nullopt});
+    RefOp remove_op;
+    remove_op.kind = RefOpKind::RemoveNamespace;
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = removed.string(), .txn_id = RefTxnId{1, 2}, .ops = {remove_op},
+        .prev_epoch_seal = std::nullopt});
+    const NamespaceLifeId removed_life = CasRefCatalog::lifeIfCataloged(*backend, layout, removed).value();
+    CasRefCatalog::casUpdate(*backend, layout, [&](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        const auto it = std::find_if(next.entries.begin(), next.entries.end(), [&](const CatalogEntry & entry)
+        {
+            return entry.ns == removed;
+        });
+        if (it == next.entries.end())
+            throw std::runtime_error("test fixture lost removing catalog row");
+        it->state = NsState::Removing;
+        it->removal_started_round = 1;
+        return next;
+    });
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(removed_life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    })).outcome, PutOutcome::Done);
+
+    const DB::UInt128 blob(0xfeed);
+    const ManifestRef manifest = publish(*backend, layout, progressing, "victim", 1, blob);
+    const ManifestId manifest_id{progressing, manifest};
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    const GcState folded_state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const CasFoldSeal folded_seal = decodeFoldSeal(
+        backend->get(layout.foldSealKey(folded_state.snap_generation, folded_state.snap_attempt))->bytes);
+    const auto folded_row = folded_seal.ref_lives.find(removed_life.incarnation);
+    ASSERT_NE(folded_row, folded_seal.ref_lives.end());
+    ASSERT_TRUE(folded_row->second.cleanup_evidence.has_value());
+
+    dropRefTransition(*backend, layout, progressing, "victim", manifest);
+    const String terminal_key = layout.refLogKey(removed_life, RefTxnId{1, 2});
+    const String later_dead_residue = layout.refLogKey(removed_life, RefTxnId{1, 3});
+    ASSERT_EQ(backend->putIfAbsent(later_dead_residue, "dead residue after the folded terminal").outcome,
+        PutOutcome::Done);
+    backend->makeUnreadable(terminal_key);
+
+    std::map<String, UInt64> namespace_cleanup;
+    const uint64_t leaks_before
+        = ProfileEvents::global_counters[ProfileEvents::CASGCNamespaceCleanupLeaks].load();
+    gc.setPhaseSink([&](const GcPhaseRecord & record)
+    {
+        if (record.phase == "namespace_cleanup")
+            namespace_cleanup = record.metrics;
+    });
+    ScopedCasGcLogCapture log_capture;
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+
+    ASSERT_TRUE(report.acquired_lease);
+    EXPECT_FALSE(CasRefCatalog::lifeIfCataloged(*backend, layout, removed))
+        << "post-fold physical cleanup cannot gate catalog removal";
+    EXPECT_TRUE(CasRefCatalog::lifeIfCataloged(*backend, layout, progressing));
+    EXPECT_EQ(report.manifests_deleted, 1u)
+        << "the janitor leak cannot promote itself into pool-wide destructive suppression";
+    EXPECT_FALSE(backend->head(layout.manifestKey(manifest_id)).exists);
+    EXPECT_TRUE(backend->existsIgnoringFault(terminal_key));
+    EXPECT_FALSE(backend->existsIgnoringFault(later_dead_residue))
+        << "one unreadable key cannot stop the perpetual janitor from deciding the rest of its page";
+    ASSERT_FALSE(namespace_cleanup.empty());
+    EXPECT_EQ(namespace_cleanup["leaked"], 1u);
+    EXPECT_EQ(
+        ProfileEvents::global_counters[ProfileEvents::CASGCNamespaceCleanupLeaks].load() - leaks_before,
+        1u);
+    const String captured = log_capture.captured();
+    EXPECT_NE(captured.find(terminal_key), String::npos);
+    EXPECT_NE(captured.find("leak"), String::npos);
+}
+
+TEST(CASGCFrontierGate, UnmatchedAdoptedParentLifeDoesNotSuppressAuthoritativeDeletion)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/unmatched-parent@cas@"};
+    const DB::UInt128 blob(0xcafe);
+    const ManifestRef mref = publish(*backend, layout, ns, "victim", 1, blob);
+    const ManifestId manifest_id{ns, mref};
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_TRUE(backend->head(layout.manifestKey(manifest_id)).exists);
+
+    const GcState before = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const String parent_seal_key = layout.foldSealKey(before.snap_generation, before.snap_attempt);
+    const auto parent_object = backend->get(parent_seal_key);
+    ASSERT_TRUE(parent_object);
+    CasFoldSeal parent = decodeFoldSeal(parent_object->bytes, before.snap_generation);
+    const UInt128 unmatched_life = hexToU128("fedcba98765432100123456789abcdef");
+    ASSERT_FALSE(parent.ref_lives.contains(unmatched_life));
+    parent.ref_lives.emplace(unmatched_life, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{9, 9}}});
+    ASSERT_EQ(
+        backend->putOverwrite(parent_seal_key, encodeFoldSeal(parent), parent_object->token).outcome,
+        PutOutcome::Done);
+
+    dropRefTransition(*backend, layout, ns, "victim", mref);
+    const uint64_t events_before =
+        ProfileEvents::global_counters[ProfileEvents::CASGCUnmatchedAdoptedParentLives].load();
+    const RoundReport report = runRegularRoundReclaiming(gc);
+
+    ASSERT_TRUE(report.acquired_lease);
+    EXPECT_EQ(
+        ProfileEvents::global_counters[ProfileEvents::CASGCUnmatchedAdoptedParentLives].load() - events_before,
+        1u);
+    EXPECT_EQ(report.manifests_deleted, 1u)
+        << "an unmatched adopted-parent row is observed and dropped, not promoted to pool-wide suppression";
+    EXPECT_FALSE(backend->head(layout.manifestKey(manifest_id)).exists)
+        << "the valid manifest candidate must be physically deleted by the same authoritative round";
+
+    const GcState after = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const CasFoldSeal successor = decodeFoldSeal(
+        backend->get(layout.foldSealKey(after.snap_generation, after.snap_attempt))->bytes,
+        after.snap_generation);
+    EXPECT_FALSE(successor.ref_lives.contains(unmatched_life));
+}
+
+TEST(CASCatalogLifecycleReconciler, EmptyCatalogReturnsAuthoritativeCompleteCut)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    ASSERT_TRUE(CasRefCatalog::initializeEmptyForNewPool(*backend, layout).catalog.entries.empty());
+
+    CasFoldSeal parent;
+    CatalogLifecycleReconciler reconciler(
+        *backend,
+        layout,
+        parent,
+        /*admitted_generation=*/1,
+        [](uint64_t)
+        {
+            return CasRefCatalog::LeaderFenceStatus::Held;
+        });
+    const CatalogLifecycleReconcileResult result = reconciler.reconcile();
+
+    EXPECT_EQ(result.authority_status, AuthorityStatus::Authoritative);
+    EXPECT_EQ(result.catalog_resolution, CatalogResolution::DrainComplete);
+    ASSERT_TRUE(result.final_catalog_cut);
+    EXPECT_TRUE(result.final_catalog_cut->catalog.entries.empty());
+    EXPECT_TRUE(result.retired_lives.empty());
+    EXPECT_EQ(result.deleted, 0);
+}
+
+TEST(CASCatalogLifecycleReconciler, DeletesEligibleRowsFromReturnedResolutionCuts)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    constexpr size_t deletes = 3;
+    seedCompletedRemovingBatch(*backend, store, kGc, deletes);
+    const auto parent_object = backend->get(layout.foldSealKey(1, 1));
+    ASSERT_TRUE(parent_object);
+    const CasFoldSeal parent = decodeFoldSeal(parent_object->bytes);
+    backend->clearJournal();
+    backend->resetCounts();
+
+    CatalogLifecycleReconciler reconciler(
+        *backend,
+        layout,
+        parent,
+        /*admitted_generation=*/1,
+        [](uint64_t)
+        {
+            return CasRefCatalog::LeaderFenceStatus::Held;
+        });
+    const CatalogLifecycleReconcileResult result = reconciler.reconcile();
+
+    EXPECT_EQ(result.authority_status, AuthorityStatus::Authoritative);
+    EXPECT_EQ(result.catalog_resolution, CatalogResolution::DrainComplete);
+    EXPECT_EQ(result.deleted, deletes);
+    ASSERT_EQ(result.retired_lives.size(), deletes);
+    ASSERT_TRUE(result.final_catalog_cut);
+    EXPECT_TRUE(result.final_catalog_cut->catalog.entries.empty());
+    const std::vector<String> journal = backend->journalSnapshot();
+    const String catalog_get = "get " + layout.refCatalogKey();
+    EXPECT_EQ(std::count(journal.begin(), journal.end(), catalog_get), deletes + 1);
+}
+
+TEST(CASCatalogLifecycleReconciler, ReturnsRetiredLifeWhenAuthorityMovesAfterResolution)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const CompletedRemovingFixture fixture = seedCompletedRemoving(*backend, store, kGc);
+    const auto parent_object = backend->get(layout.foldSealKey(1, 1));
+    ASSERT_TRUE(parent_object);
+    const CasFoldSeal parent = decodeFoldSeal(parent_object->bytes);
+    size_t fence_checks = 0;
+
+    CatalogLifecycleReconciler reconciler(
+        *backend,
+        layout,
+        parent,
+        /*admitted_generation=*/1,
+        [&fence_checks](uint64_t)
+        {
+            ++fence_checks;
+            return fence_checks == 2
+                ? CasRefCatalog::LeaderFenceStatus::Moved
+                : CasRefCatalog::LeaderFenceStatus::Held;
+        });
+    const CatalogLifecycleReconcileResult result = reconciler.reconcile();
+
+    EXPECT_EQ(result.authority_status, AuthorityStatus::FencedOut);
+    EXPECT_EQ(result.catalog_resolution, CatalogResolution::ExactRowAbsent);
+    ASSERT_EQ(result.retired_lives.size(), 1);
+    EXPECT_EQ(result.retired_lives.front(),
+        NamespaceLifeId::fromCatalogEntry(fixture.ns, fixture.life_id));
+    EXPECT_EQ(result.deleted, 0);
+    EXPECT_FALSE(result.final_catalog_cut);
+}
+
+TEST(CASCatalogLifecycleReconciler, InitialFenceLossReportsEligibleRowStillPresent)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const CompletedRemovingFixture fixture = seedCompletedRemoving(*backend, store, kGc);
+    const auto parent_object = backend->get(layout.foldSealKey(1, 1));
+    ASSERT_TRUE(parent_object);
+    const CasFoldSeal parent = decodeFoldSeal(parent_object->bytes);
+    backend->resetCounts();
+
+    CatalogLifecycleReconciler reconciler(
+        *backend,
+        layout,
+        parent,
+        /*admitted_generation=*/1,
+        [](uint64_t)
+        {
+            return CasRefCatalog::LeaderFenceStatus::Moved;
+        });
+    const CatalogLifecycleReconcileResult result = reconciler.reconcile();
+
+    EXPECT_EQ(result.authority_status, AuthorityStatus::FencedOut);
+    EXPECT_EQ(result.catalog_resolution, CatalogResolution::ExactRowStillPresent);
+    EXPECT_TRUE(result.retired_lives.empty());
+    EXPECT_EQ(result.deleted, 0);
+    EXPECT_FALSE(result.final_catalog_cut);
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), 2)
+        << "the initial selection and mandatory erase-resolution cuts are the only catalog reads";
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), 0);
+    EXPECT_EQ(CasRefCatalog::lifeIfCataloged(*backend, layout, fixture.ns),
+        NamespaceLifeId::fromCatalogEntry(fixture.ns, fixture.life_id));
+}
+
+TEST(CASCatalogLifecycleReconciler, RetriesFromTheMandatoryConflictResolutionCut)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    seedCompletedRemoving(*backend, store, kGc);
+    const auto parent_object = backend->get(layout.foldSealKey(1, 1));
+    ASSERT_TRUE(parent_object);
+    const CasFoldSeal parent = decodeFoldSeal(parent_object->bytes);
+    backend->clearJournal();
+    backend->resetCounts();
+    backend->conflictNextCatalogCas(layout.refCatalogKey());
+
+    CatalogLifecycleReconciler reconciler(
+        *backend,
+        layout,
+        parent,
+        /*admitted_generation=*/1,
+        [](uint64_t)
+        {
+            return CasRefCatalog::LeaderFenceStatus::Held;
+        });
+    const CatalogLifecycleReconcileResult result = reconciler.reconcile();
+
+    EXPECT_EQ(result.authority_status, AuthorityStatus::Authoritative);
+    EXPECT_EQ(result.catalog_resolution, CatalogResolution::DrainComplete);
+    EXPECT_EQ(result.deleted, 1);
+    const std::vector<String> journal = backend->journalSnapshot();
+    const String catalog_get = "get " + layout.refCatalogKey();
+    EXPECT_EQ(std::count(journal.begin(), journal.end(), catalog_get), 3)
+        << "the token-conflict retry must reuse its mandatory resolution cut";
+}
+
+TEST(CASCatalogLifecycleReconciler, PropagatesAuthorityFailureBeforeEraseCas)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    seedCompletedRemoving(*backend, store, kGc);
+    const auto parent_object = backend->get(layout.foldSealKey(1, 1));
+    ASSERT_TRUE(parent_object);
+    const CasFoldSeal parent = decodeFoldSeal(parent_object->bytes);
+    size_t fence_checks = 0;
+
+    CatalogLifecycleReconciler reconciler(
+        *backend,
+        layout,
+        parent,
+        /*admitted_generation=*/1,
+        [&fence_checks](uint64_t)
+        {
+            if (++fence_checks == 2)
+                throw std::runtime_error("injected reconciler authority failure before CAS");
+            return CasRefCatalog::LeaderFenceStatus::Held;
+        });
+    try
+    {
+        (void)reconciler.reconcile();
+        FAIL() << "the authority exception must propagate";
+    }
+    catch (const std::runtime_error & e)
+    {
+        EXPECT_STREQ(e.what(), "injected reconciler authority failure before CAS");
+    }
+}
+
+TEST(CASCatalogLifecycleReconciler, PropagatesAuthorityFailureAfterMandatoryResolution)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const CompletedRemovingFixture fixture = seedCompletedRemoving(*backend, store, kGc);
+    const auto parent_object = backend->get(layout.foldSealKey(1, 1));
+    ASSERT_TRUE(parent_object);
+    const CasFoldSeal parent = decodeFoldSeal(parent_object->bytes);
+    size_t fence_checks = 0;
+
+    CatalogLifecycleReconciler reconciler(
+        *backend,
+        layout,
+        parent,
+        /*admitted_generation=*/1,
+        [&fence_checks](uint64_t)
+        {
+            if (++fence_checks == 3)
+                throw std::runtime_error("injected reconciler authority failure after resolution");
+            return CasRefCatalog::LeaderFenceStatus::Held;
+        });
+    try
+    {
+        (void)reconciler.reconcile();
+        FAIL() << "the authority exception must propagate";
+    }
+    catch (const std::runtime_error & e)
+    {
+        EXPECT_STREQ(e.what(), "injected reconciler authority failure after resolution");
+        EXPECT_FALSE(CasRefCatalog::lifeIfCataloged(*backend, layout, fixture.ns));
+    }
+}
+
+TEST(CASGCFrontierGate, HealthyRebuildUsesTheCatalogLifecycleReconciler)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const CompletedRemovingFixture fixture = seedCompletedRemoving(*backend, store, kGc);
+    const uint64_t catalog_cas_before = backend->casPutCount(layout.refCatalogKey());
+
+    Gc gc(store, kGc);
+    const RebuildReport result = gc.rebuildBaseline(/*force=*/true);
+
+    EXPECT_TRUE(result.performed);
+    EXPECT_FALSE(CasRefCatalog::lifeIfCataloged(*backend, layout, fixture.ns));
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), catalog_cas_before + 1);
+}
+
+TEST(CASGCFrontierGate, DamagedStateRebuildDoesNotDeleteCompletedRemovingRows)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/damaged-rebuild-removing@cas@"};
+    CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards, CatalogEntry{
+        .ns = ns, .state = NsState::Live, .incarnation = UInt128{901}});
+    CasRefCatalog::casUpdate(*backend, layout, [](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        next.entries.front().state = NsState::Removing;
+        next.entries.front().removal_started_round = 1;
+        return next;
+    });
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    const uint64_t catalog_cas_before = backend->casPutCount(layout.refCatalogKey());
+
+    Gc gc(store, kGc);
+    const RebuildReport result = gc.rebuildBaseline(/*force=*/false);
+
+    EXPECT_TRUE(result.performed);
+    EXPECT_TRUE(CasRefCatalog::lifeIfCataloged(*backend, layout, ns));
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), catalog_cas_before);
+}
+
+TEST(CASGCFrontierGate, DeferredRoundDrainsCompletedRemovingBeforeReturning)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/100);
+    const Layout & layout = store->layout();
+    const RootNamespace removed{"00/deferred-removed@cas@"};
+    const UInt128 life_id{77};
+    CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards, CatalogEntry{
+        .ns = removed, .state = NsState::Live, .incarnation = life_id});
+    CasRefCatalog::casUpdate(*backend, layout, [&](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        next.entries[0].state = NsState::Removing;
+        next.entries[0].removal_started_round = 1;
+        return next;
+    });
+
+    CasFoldSeal parent;
+    parent.generation = 1;
+    parent.ref_lives.emplace(life_id, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 1}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 1}}});
+    for (uint64_t shard = 0; shard < store->poolConfig().gc_shards; ++shard)
+        parent.condemned_summary.emplace(shard, CondemnedSummary{});
+    ASSERT_EQ(backend->putIfAbsent(layout.foldSealKey(1, 1), encodeFoldSeal(parent)).outcome, PutOutcome::Done);
+    GcState state;
+    state.round = 1;
+    state.gc_shards = store->poolConfig().gc_shards;
+    state.snap_generation = 1;
+    state.snap_attempt = 1;
+    state.lease = GcLease{.owner = kGc, .seq = 1};
+    ASSERT_EQ(backend->putIfAbsent(layout.gcStateKey(), encodeGcState(state)).outcome, PutOutcome::Done);
+
+    const String ckpt_key = layout.refCkptKey(NamespaceLifeId::fromCatalogEntry(removed, life_id));
+    ASSERT_EQ(backend->putIfAbsent(ckpt_key, "inert checkpoint debris").outcome, PutOutcome::Done);
+    const uint64_t catalog_cas_before = backend->casPutCount(layout.refCatalogKey());
+
+    Gc gc(store, kGc);
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(report.acquired_lease);
+    EXPECT_TRUE(report.deferred);
+    EXPECT_TRUE(CasRefCatalog::read(*backend, layout).catalog.entries.empty());
+    EXPECT_FALSE(CasRefCatalog::lifeIfCataloged(*backend, layout, removed));
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), catalog_cas_before + 1);
+    EXPECT_TRUE(backend->head(ckpt_key).exists);
+    EXPECT_EQ(backend->deleteCount(ckpt_key), 0);
+}
+
+TEST(CASGCFrontierGate, StaleIssuedCatalogCasLosesAfterNewLeaderHelpsBeforeListing)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const UInt128 leader_b = hexToU128("00000000000000000000000000000002");
+    const CompletedRemovingFixture fixture = seedCompletedRemoving(*backend, store, kGc);
+    backend->clearJournal();
+    backend->blockNextCatalogCas(layout.refCatalogKey());
+
+    std::exception_ptr leader_a_failure;
+    std::thread leader_a([&]
+    {
+        try
+        {
+            Gc gc_a(store, kGc);
+            (void)runRegularRoundReclaiming(gc_a);
+        }
+        catch (...)
+        {
+            leader_a_failure = std::current_exception();
+        }
+    });
+    backend->waitForBlockedCatalogCas();
+
+    transferGcLease(*backend, layout, leader_b);
+    RoundReport report_b;
+    std::exception_ptr leader_b_failure;
+    /// `fixture.ns` is Removing with a durable `_ckpt`, same shape as
+    /// `CleanupEvidenceLeavesRemovedNamespaceCheckpointForJanitor`: once leader_b's round drops its
+    /// catalog row, the resulting cut is genuinely, provably empty, the destructive gate opens, and the
+    /// namespace janitor -- a separate `namespace_cleanup` phase within this SAME round -- reclaims the
+    /// checkpoint. Captured so the assertions below can attribute the delete to the janitor rather than
+    /// assume survival.
+    std::map<String, UInt64> janitor_metrics_b;
+    try
+    {
+        Gc gc_b(store, leader_b);
+        gc_b.setPhaseSink([&](const GcPhaseRecord & rec)
+        {
+            if (rec.phase == "namespace_cleanup")
+                janitor_metrics_b = rec.metrics;
+        });
+        report_b = runRegularRoundReclaiming(gc_b);
+        gc_b.setPhaseSink({});
+    }
+    catch (...)
+    {
+        leader_b_failure = std::current_exception();
+    }
+
+    const std::vector<String> before_a_release = backend->journalSnapshot();
+    backend->releaseBlockedCatalogCas();
+    leader_a.join();
+
+    ASSERT_FALSE(leader_b_failure);
+    ASSERT_TRUE(report_b.acquired_lease);
+    ASSERT_FALSE(report_b.deferred);
+    ASSERT_TRUE(CasRefCatalog::read(*backend, layout).catalog.entries.empty());
+
+    const size_t catalog_cas_end = findJournalAfter(before_a_release, "cas_end " + layout.refCatalogKey(), 0);
+    ASSERT_LT(catalog_cas_end, before_a_release.size());
+    const size_t conclusive_rescan = findJournalAfter(
+        before_a_release, "get " + layout.refCatalogKey(), catalog_cas_end + 1);
+    ASSERT_LT(conclusive_rescan, before_a_release.size());
+    const size_t stream_list = findJournalAfter(
+        before_a_release, "list " + layout.casRefsPrefix(), conclusive_rescan + 1);
+    ASSERT_LT(stream_list, before_a_release.size());
+    const size_t fresh_catalog_cut = findJournalAfter(
+        before_a_release, "get " + layout.refCatalogKey(), stream_list + 1);
+    ASSERT_LT(fresh_catalog_cut, before_a_release.size());
+    const GcState adopted = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const String successor_seal_key = layout.foldSealKey(adopted.snap_generation, adopted.snap_attempt);
+    const size_t successor_seal_put = findJournalAfter(
+        before_a_release, "put_end " + successor_seal_key, fresh_catalog_cut + 1);
+    ASSERT_LT(successor_seal_put, before_a_release.size());
+    const size_t successor_adoption = findJournalAfter(
+        before_a_release, "cas_end " + layout.gcStateKey(), successor_seal_put + 1);
+    ASSERT_LT(successor_adoption, before_a_release.size());
+    EXPECT_LT(catalog_cas_end, conclusive_rescan);
+    EXPECT_LT(conclusive_rescan, stream_list);
+    EXPECT_LT(stream_list, fresh_catalog_cut);
+    /// The invariant this ordering must still prove: the fold's OWN walk plan is built from the single
+    /// hot-scan cut, taken immediately after the ref-object LIST, with no earlier catalog read sneaking
+    /// into that construction. `fresh_catalog_cut` is defined as the FIRST catalog `get` after
+    /// `stream_list` (the `findJournalAfter` search above), so that already holds by construction --
+    /// the walk plan physically cannot have consumed an earlier one.
+    ///
+    /// What this test used to also assert -- no SECOND catalog read anywhere before the seal PUT -- is
+    /// no longer the right claim once the destructive gate can open on a proved-empty cut: other
+    /// destructive families this SAME round now also runs (the orphan-manifest sweep, the namespace
+    /// janitor) take their OWN separate catalog cuts by design, each after its own candidate listing,
+    /// to resolve authority against a fresh read rather than the fold's frozen one -- exactly the shape
+    /// measured here (`list p/cas/manifests/` immediately followed by a second `get
+    /// p/cas/ref_catalog`, before the seal PUT, from the orphan sweep). That is expected, not redundant,
+    /// so it is not asserted against; the fold's own single-cut plan construction is what remains pinned.
+    EXPECT_LT(fresh_catalog_cut, successor_seal_put);
+    EXPECT_LT(successor_seal_put, successor_adoption);
+
+    ASSERT_TRUE(leader_a_failure);
+    EXPECT_FALSE(CasRefCatalog::lifeIfCataloged(*backend, layout, fixture.ns));
+    /// Same discrimination as `CleanupEvidenceLeavesRemovedNamespaceCheckpointForJanitor`: leader_b's
+    /// round both drops `fixture.ns`'s catalog row AND, because the resulting cut is genuinely,
+    /// provably empty, opens the destructive gate -- so the namespace janitor reclaims the checkpoint
+    /// in this SAME round. Attribute the delete to the janitor's own counter rather than assume either
+    /// survival (the old expectation) or absence (which an unchecked dereference here cannot
+    /// distinguish from "never existed").
+    ASSERT_FALSE(janitor_metrics_b.empty()) << "the namespace_cleanup phase must have run this round";
+    EXPECT_GE(janitor_metrics_b.at("janitor_deleted"), 1u)
+        << "the janitor's OWN counter must show the delete, now that the proved-empty gate has opened";
+    EXPECT_FALSE(backend->get(fixture.checkpoint_key).has_value());
+    EXPECT_EQ(backend->deleteCount(fixture.checkpoint_key), 1);
+}
+
+TEST(CASGCFrontierGate, LostCatalogCasResponseIsResolvedBeforeListing)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const CompletedRemovingFixture fixture = seedCompletedRemoving(*backend, store, kGc);
+    backend->clearJournal();
+    backend->loseNextCatalogCasResponse(layout.refCatalogKey());
+
+    /// Same shape as `StaleIssuedCatalogCasLosesAfterNewLeaderHelpsBeforeListing`: `fixture.ns` is
+    /// Removing with a durable `_ckpt`, so this round both drops its catalog row and, because the
+    /// resulting cut is genuinely, provably empty, opens the destructive gate -- the namespace janitor
+    /// (a separate `namespace_cleanup` phase within this SAME round) reclaims the checkpoint. Captured
+    /// so the assertions below attribute the delete to the janitor's own counter.
+    std::map<String, UInt64> janitor_metrics;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "namespace_cleanup")
+            janitor_metrics = rec.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+    ASSERT_TRUE(report.acquired_lease);
+    ASSERT_FALSE(report.deferred);
+
+    const std::vector<String> journal = backend->journalSnapshot();
+    const size_t response_lost = findJournalAfter(
+        journal, "cas_response_lost " + layout.refCatalogKey(), 0);
+    ASSERT_LT(response_lost, journal.size());
+    const size_t conclusive_rescan = findJournalAfter(
+        journal, "get " + layout.refCatalogKey(), response_lost + 1);
+    ASSERT_LT(conclusive_rescan, journal.size());
+    const size_t stream_list = findJournalAfter(
+        journal, "list " + layout.casRefsPrefix(), conclusive_rescan + 1);
+    ASSERT_LT(stream_list, journal.size());
+    const size_t fresh_catalog_cut = findJournalAfter(
+        journal, "get " + layout.refCatalogKey(), stream_list + 1);
+    ASSERT_LT(fresh_catalog_cut, journal.size());
+    EXPECT_LT(response_lost, conclusive_rescan);
+    EXPECT_LT(conclusive_rescan, stream_list);
+    EXPECT_LT(stream_list, fresh_catalog_cut);
+
+    EXPECT_FALSE(CasRefCatalog::lifeIfCataloged(*backend, layout, fixture.ns));
+    /// See the discrimination comment in `CleanupEvidenceLeavesRemovedNamespaceCheckpointForJanitor`:
+    /// attribute the delete to the janitor's own counter, never to end-state absence alone, and never
+    /// assume survival -- both would be indistinguishable from a bug on this exact line (the old
+    /// unchecked `->bytes` here is what aborted the whole binary once the janitor started reclaiming).
+    ASSERT_FALSE(janitor_metrics.empty()) << "the namespace_cleanup phase must have run this round";
+    EXPECT_GE(janitor_metrics.at("janitor_deleted"), 1u)
+        << "the janitor's OWN counter must show the delete, now that the proved-empty gate has opened";
+    EXPECT_FALSE(backend->get(fixture.checkpoint_key).has_value());
+    EXPECT_EQ(backend->deleteCount(fixture.checkpoint_key), 1);
+}
+
+/// A stale leader may learn from its mandatory resolution read that the old life is gone, and must
+/// invalidate that exact runtime, but loss of the leader fence remains the control outcome. It must
+/// abort before the hot LIST and cannot build or publish any successor generation.
+TEST_P(CASGCCompletedRemovalFenceRace, FencedLeaderStopsAfterWinnerRemovesOrReplacesLife)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const UInt128 leader_b = hexToU128("00000000000000000000000000000002");
+    const CompletedRemovingFixture fixture = seedCompletedRemoving(*backend, store, kGc);
+    const NamespaceLifeId predecessor_life
+        = NamespaceLifeId::fromCatalogEntry(fixture.ns, fixture.life_id);
+    ASSERT_TRUE(store->refTableRecoveredForTest(fixture.ns))
+        << "the fixture must retain a resident predecessor runtime before removal";
+    ASSERT_EQ(store->refTableLifeForTest(fixture.ns), predecessor_life);
+    const uint64_t predecessor_runtime = store->refTableRuntimeIdentityForTest(fixture.ns);
+    ASSERT_NE(predecessor_runtime, 0u);
+    backend->clearJournal();
+    backend->blockNextCatalogCas(layout.refCatalogKey());
+
+    std::exception_ptr leader_a_failure;
+    std::thread leader_a([&]
+    {
+        try
+        {
+            Gc gc_a(store, kGc);
+            (void)runRegularRoundReclaiming(gc_a);
+        }
+        catch (...)
+        {
+            leader_a_failure = std::current_exception();
+        }
+    });
+    backend->waitForBlockedCatalogCas();
+
+    transferGcLease(*backend, layout, leader_b);
+    const CasRefCatalog::Snapshot observed = CasRefCatalog::read(*backend, layout);
+    RefCatalog winner_catalog;
+    if (GetParam() == CompetingCatalogOutcome::Replacement)
+    {
+        winner_catalog.entries.push_back(CatalogEntry{
+            .ns = fixture.ns,
+            .state = NsState::Live,
+            .incarnation = UInt128{178}});
+        /// Mirror production's publish-then-flip order: the successor life needs a readable `_ckpt`
+        /// before its catalog row can read `Live`, or `chooseRecoveryGrounding` rejects it.
+        const NamespaceLifeId successor_life = NamespaceLifeId::fromCatalogEntry(fixture.ns, UInt128{178});
+        backend->putIfAbsent(layout.refCkptKey(successor_life), encodeRefCkpt(RefCkpt{
+            .life_epoch = 1,
+            .committed_through = std::nullopt,
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = std::nullopt,
+        }));
+    }
+    ASSERT_EQ(backend->casPut(
+        layout.refCatalogKey(), encodeRefCatalog(winner_catalog), observed.token).outcome,
+        CasOutcome::Committed);
+
+    backend->clearJournal();
+    const uint64_t plans_before  /// NOLINT(clang-analyzer-deadcode.DeadStores)
+        = ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load();
+    backend->releaseBlockedCatalogCas();
+    leader_a.join();
+
+    const std::vector<String> journal = backend->journalSnapshot();
+    ASSERT_TRUE(leader_a_failure);
+    try
+    {
+        std::rethrow_exception(leader_a_failure);
+        FAIL() << "expected DB::Exception";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR);
+        EXPECT_NE(e.message().find("pre-fold drain lost authority"), String::npos) << e.message();
+    }
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load() - plans_before, 0u);
+    EXPECT_EQ(findJournalAfter(journal, "list " + layout.casRefsPrefix(), 0), journal.size());
+    EXPECT_EQ(findJournalAfter(journal, "cas_begin " + layout.gcStateKey(), 0), journal.size());
+    EXPECT_FALSE(std::any_of(journal.begin(), journal.end(), [](const String & entry)
+    {
+        return entry.starts_with("put_begin ") && entry.ends_with("/fold_seal");
+    }));
+    EXPECT_LT(findJournalAfter(journal, "get " + layout.refCatalogKey(), 0), journal.size())
+        << "the stale leader must still complete mandatory erase resolution";
+
+    (void)store->namespaceLife(fixture.ns);
+    EXPECT_NE(store->refTableRuntimeIdentityForTest(fixture.ns), 0u);
+    ASSERT_TRUE(store->refTableLifeForTest(fixture.ns));
+    EXPECT_NE(store->refTableLifeForTest(fixture.ns), predecessor_life)
+        << "the next name-based resolution must not retain the retired predecessor life";
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    CASWinnerShape,
+    CASGCCompletedRemovalFenceRace,
+    testing::Values(CompetingCatalogOutcome::Absent, CompetingCatalogOutcome::Replacement),
+    [](const testing::TestParamInfo<CompetingCatalogOutcome> & parameter)
+    {
+        return parameter.param == CompetingCatalogOutcome::Absent ? "Absent" : "Replacement";
+    });
+
+/// One initial full catalog read selects the first row; each successful erase's mandatory resolution
+/// read becomes the next selection snapshot. Therefore N uncontended deletes cost N+1 reads before
+/// the hot LIST. The round then takes one post-LIST walk-plan cut and, later in the separate
+/// `namespace_cleanup` phase, one post-page janitor cut.
+TEST(CASGCFrontierGate, CompletedRemovalDrainUsesNPlusOneCatalogReads)
+{
+    auto backend = std::make_shared<DrainRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    constexpr size_t deletes = 3;
+    seedCompletedRemovingBatch(*backend, store, kGc, deletes);
+    backend->clearJournal();
+    backend->resetCounts();
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    const std::vector<String> journal = backend->journalSnapshot();
+    const size_t stream_list = findJournalAfter(journal, "list " + layout.casRefsPrefix(), 0);
+    ASSERT_LT(stream_list, journal.size());
+    const String catalog_get = "get " + layout.refCatalogKey();
+    EXPECT_EQ(std::count(journal.begin(), journal.begin() + static_cast<ptrdiff_t>(stream_list), catalog_get),
+        deletes + 1);
+    const size_t walk_plan_cut = findJournalAfter(journal, catalog_get, stream_list);
+    ASSERT_LT(walk_plan_cut, journal.size());
+    /// Between the hot walk-plan cut and the janitor's own page, the orphan-manifest sweep -- ANOTHER
+    /// destructive family the now-open gate also unlocks (the batch drain above empties the catalog, so
+    /// this round's frontier is proved empty and the sweep's own `!suppress_destructive` gate opens
+    /// too) -- lists its own manifest candidates and takes its OWN separate catalog cut to resolve
+    /// authority, exactly as the janitor does. Located explicitly so the final read count below states
+    /// what it counts rather than drifting silently the next time a family is unlocked.
+    const size_t orphan_sweep_list = findJournalAfter(journal, "list " + layout.casManifestsPrefix(), walk_plan_cut);
+    ASSERT_LT(orphan_sweep_list, journal.size());
+    const size_t orphan_sweep_cut = findJournalAfter(journal, catalog_get, orphan_sweep_list);
+    ASSERT_LT(orphan_sweep_cut, journal.size());
+    const size_t janitor_list
+        = findJournalAfter(journal, "list " + layout.namespaceRootPrefix(), orphan_sweep_cut);
+    ASSERT_LT(janitor_list, journal.size());
+    const size_t janitor_cut = findJournalAfter(journal, catalog_get, janitor_list);
+    ASSERT_LT(janitor_cut, journal.size());
+    EXPECT_EQ(findJournalAfter(journal, catalog_get, janitor_cut + 1), journal.size())
+        << "one hot walk-plan cut, one orphan-sweep cut, and one janitor page cut are the only "
+           "post-drain catalog reads";
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamRootPrefix()), 1u);
+    EXPECT_EQ(backend->listCount(layout.namespaceRootPrefix()), 1u);
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), deletes + 4)
+        << "N+1 drain reads, one post-hot-LIST walk-plan cut, one orphan-manifest-sweep cut (now that "
+           "the proved-empty gate has opened, unlocking that destructive family too), and one separate "
+           "post-janitor-page cut";
+}
diff --git a/src/Disks/tests/gtest_cas_gc_hold_grammar.cpp b/src/Disks/tests/gtest_cas_gc_hold_grammar.cpp
new file mode 100644
index 000000000000..19a63bc6842a
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_hold_grammar.cpp
@@ -0,0 +1,1590 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasByteBudget.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Common/ProfileEvents.h>
+#include <base/defines.h>
+#include "cas_test_helpers.h"
+
+#include <algorithm>
+#include <cstring>
+#include <functional>
+#include <limits>
+#include <mutex>
+#include <set>
+#include <utility>
+#include <vector>
+
+/// DURABLE HOLDS (spec 2026-07-27 "ref chain complete cut" §5).
+///
+/// A namespace whose ref-log walk meets an IMPOSSIBLE shape stops there, and that stop has to survive
+/// the round. Before this task the stop was a single bit — `classification == 4` — and everything that
+/// explained it (what went wrong, and exactly WHERE) lived in a log line and an in-memory anomaly, both
+/// gone by the next round. That is not enough for three separate reasons:
+///
+///   * the next round could not RETRY the exact position, so a hold only survived while the round's
+///     hint happened to keep mentioning the namespace;
+///   * the hold could be cleared by an ABSENT — precisely the observation a lying store produces, and
+///     precisely the shape that made the hold necessary in the first place;
+///   * REBUILD rewrote coverage from owner state and silently dropped every hold, handing back a
+///     baseline that looked proven when it was not.
+///
+/// So the hold is now DURABLE and STRICTLY GRAMMARED: `{reason, offending_position, retry_count,
+/// next_retry_round}` present if and only if `classification == 4`, rejected in both directions
+/// otherwise. It rides the seal across rounds — including rounds whose hint omits the namespace
+/// entirely — and across REBUILD, and it clears by exactly ONE event: the fold resolving the offending
+/// position and that result being adopted in `gc/state`.
+///
+/// The carried hold is also a WITNESS, and a better one than the listing: it is durable proof that the
+/// walk once reached that position, so an absent below it is a gap rather than a frontier no matter
+/// what the hint says this round. That is what makes "retry the exact offending position" work for a
+/// hold that sits above an epoch boundary.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int LIMIT_EXCEEDED;
+extern const int LOGICAL_ERROR;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASGCRebuildVirginByEnumeration;
+}
+
+namespace
+{
+
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+
+/// ===================== FIXTURES =====================
+
+/// A backend that hides keys from every LIST while serving them by exact key (the observed lying-store
+/// shape) AND counts reads. The hold tests need both: the hint has to go quiet while the exact GET the
+/// hold forces stays observable.
+class HintHoleCountingBackend : public CountingBackend
+{
+public:
+    void hide(const String & key)
+    {
+        std::lock_guard lock(m);
+        hidden.insert(key);
+    }
+
+    size_t holesServed() const
+    {
+        std::lock_guard lock(m);
+        return served;
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = CountingBackend::list(prefix, cursor, limit);
+        std::lock_guard lock(m);
+        if (hidden.empty())
+            return page;
+        const size_t before = page.keys.size();
+        std::erase_if(page.keys, [&](const ListedKey & k) { return hidden.contains(k.key); });
+        if (page.keys.size() != before)
+            ++served;
+        return page;
+    }
+
+private:
+    mutable std::mutex m;
+    std::set<String> hidden;
+    size_t served = 0;
+};
+
+/// Write the namespace's `_ckpt` naming `checkpoint` as its snapshot base, through the real codec — the
+/// fold's second witness source is a decode of exactly these bytes, so a hand-rolled body would prove
+/// nothing about the object the writers actually publish.
+void writeCkptAt(
+    Backend & backend, const Layout & layout, const RootNamespace & ns, const RefTxnId & checkpoint)
+{
+    writeRecoverableCkptForRawFixture(backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = checkpoint,
+        .checkpoint_snapshot_id = checkpoint,
+        .last_epoch_seal = std::nullopt,
+    });
+}
+
+/// Establish only the immutable recovery frontier for a raw-log fixture. Unlike `writeCkptAt`, this
+/// does not claim a snapshot exists: rebuild tests need to replay the log through this exact position.
+void writeCommittedCkptAt(
+    Backend & backend, const Layout & layout, const RootNamespace & ns, const RefTxnId & committed_through)
+{
+    writeRecoverableCkptForRawFixture(backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = committed_through,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+}
+
+/// The newest fold seal, scanning downward from the adopted generation (a completed round's gc/state
+/// points at the recheck generation).
+std::optional<CasFoldSeal> newestSeal(Backend & backend, const Layout & layout)
+{
+    const uint64_t gen = currentGenerationOf(backend, layout);
+    const uint64_t attempt = currentAttemptOf(backend, layout);
+    for (uint64_t g = gen; ; --g)
+    {
+        if (const auto got = backend.get(layout.foldSealKey(g, attempt)))
+            return decodeFoldSeal(got->bytes);
+        if (g == 0)
+            return std::nullopt;
+    }
+}
+
+std::optional<RefCoverage> coverageOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const auto seal = newestSeal(backend, layout);
+    if (!seal)
+        return std::nullopt;
+    const auto it = seal->ref_lives.find(catalogLifeIdForTest(backend, layout, ns));
+    if (it == seal->ref_lives.end())
+        return std::nullopt;
+    return it->second.coverage;
+}
+
+/// The cursor `ns` was sealed at, or `{0, 0}` when the round sealed NO row for it at all. It never
+/// dereferences a disengaged optional: a test that aborts the process takes every test after it in the
+/// binary down with it, and "there is no coverage row" is exactly the shape a regression in the hold
+/// carry produces — so it has to read as a failed expectation, not as a crash that hides the rest.
+RefTxnId sealedCursorOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const auto cov = coverageOf(backend, layout, ns);
+    EXPECT_TRUE(cov.has_value()) << "no coverage row for " << ns.string();
+    return cov ? cov->last_folded_ref_id : RefTxnId{};
+}
+
+/// The coverage row a round MUST have sealed for `ns`, held. Fails the test rather than returning an
+/// empty optional, so every caller below reads a real hold.
+RefHold holdOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const auto cov = coverageOf(backend, layout, ns);
+    EXPECT_TRUE(cov.has_value()) << "no coverage row for " << ns.string();
+    if (!cov)
+        return RefHold{};
+    EXPECT_EQ(cov->classification, 4) << "a held namespace is classification 4";
+    EXPECT_TRUE(cov->hold.has_value()) << "classification 4 without a hold is the forbidden shape";
+    return cov->hold ? *cov->hold : RefHold{};
+}
+
+UInt128 fixtureLifeId(std::string_view key)
+{
+    return key.ends_with("/1") ? UInt128{2} : UInt128{1};
+}
+
+RefCoverage & fixtureCoverage(CasFoldSeal & seal, std::string_view key)
+{
+    return seal.ref_lives[fixtureLifeId(key)].coverage;
+}
+
+const RefCoverage & fixtureCoverage(const CasFoldSeal & seal, std::string_view key)
+{
+    return seal.ref_lives.at(fixtureLifeId(key)).coverage;
+}
+
+/// A seal carrying exactly one held coverage row with every numeric at its maximum and a coverage key
+/// that needs escaping — the widest row the per-row line budget has to survive. The caller supplies the
+/// key, which is what the line-cap tests below grow byte by byte.
+CasFoldSeal maximalHoldSeal(const String & map_key)
+{
+    CasFoldSeal seal;
+    seal.generation = std::numeric_limits<uint64_t>::max();
+    seal.parent_generation = std::numeric_limits<uint64_t>::max();
+    RefCoverage cov;
+    cov.classification = 4;
+    cov.last_folded_ref_id = RefTxnId{std::numeric_limits<uint64_t>::max(),
+                                      std::numeric_limits<uint64_t>::max()};
+    cov.hold = RefHold{.reason = HoldReason::UnconsumedSealCrossing,   /// the longest reason word
+                       .offending_position = RefTxnId{std::numeric_limits<uint64_t>::max(),
+                                                      std::numeric_limits<uint64_t>::max()},
+                       .retry_count = std::numeric_limits<uint32_t>::max(),
+                       .next_retry_round = std::numeric_limits<uint64_t>::max()};
+    fixtureCoverage(seal, map_key) = cov;
+    return seal;
+}
+
+/// The `cov` line of an encoded seal (line 3: header, meta, then the single record).
+String covLineOf(const String & encoded)
+{
+    size_t begin = encoded.find('\n') + 1;   /// past the header
+    begin = encoded.find('\n', begin) + 1;   /// past the meta line
+    return encoded.substr(begin, encoded.find('\n', begin) - begin);
+}
+
+/// A one-row seal whose coverage is ordinary and CLEAN: folded through its cursor, nothing held.
+CasFoldSeal cleanSeal(const String & map_key)
+{
+    CasFoldSeal seal;
+    seal.generation = 3;
+    seal.parent_generation = 2;
+    RefCoverage cov;
+    cov.classification = 2;
+    cov.last_folded_ref_id = RefTxnId{4, 5};
+    fixtureCoverage(seal, map_key) = cov;
+    return seal;
+}
+
+/// A one-row seal whose coverage is HELD at an exact position — the row every erasure shape below is
+/// trying to make disappear.
+CasFoldSeal heldSeal(const String & map_key)
+{
+    CasFoldSeal seal = cleanSeal(map_key);
+    RefCoverage & cov = fixtureCoverage(seal, map_key);
+    cov.classification = 4;
+    cov.hold = RefHold{.reason = HoldReason::GapBelowWitness, .offending_position = RefTxnId{4, 6},
+                       .retry_count = 7, .next_retry_round = 99};
+    return seal;
+}
+
+/// The header and meta lines (1 and 2) of an encoded seal, terminators included.
+String headerAndMetaOf(const String & encoded)
+{
+    const size_t past_meta = encoded.find('\n', encoded.find('\n') + 1) + 1;
+    EXPECT_NE(past_meta, 0u);
+    return encoded.substr(0, past_meta);
+}
+
+/// Assemble a raw seal object from `records` (one record per element, no terminators), on `prototype`'s
+/// header and meta lines, closed by the trailer count those records imply. This is the ONLY way to put a
+/// repeated record key on the wire: `CasFoldSeal` stores keyed maps, so a duplicate is not a value any
+/// producer can hold — it is a shape a forged, truncated, or mis-merged object has.
+String sealTextWith(const String & prototype, const std::vector<String> & records)
+{
+    String text = headerAndMetaOf(prototype);
+    for (const String & record : records)
+        text += record + "\n";
+    return text + "{\"n\":" + std::to_string(records.size()) + "}\n";
+}
+
+/// Replace the coverage row's `cls` value with `raw`, VERBATIM. The point is to write integers no
+/// `RefCoverage` can hold: the field is a byte in the struct, so a wide value exists only on the wire,
+/// which is exactly where a reader has to catch it. `cls` is never the last field of a `cov` record, so
+/// the value always ends at a comma.
+String withRawClassification(const String & encoded, std::string_view raw)
+{
+    const size_t at = encoded.find("\"cls\":");
+    EXPECT_NE(at, String::npos);
+    const size_t begin = at + strlen("\"cls\":");
+    const size_t end = encoded.find(',', begin);
+    EXPECT_NE(end, String::npos);
+    return encoded.substr(0, begin) + String{raw} + encoded.substr(end);
+}
+
+/// Replace the FIRST occurrence of `field` with `replacement` (both are whole `"key":value` fragments),
+/// so a test states the exact wire shape it is feeding the decoder.
+String withField(const String & encoded, const String & field, const String & replacement)
+{
+    const size_t at = encoded.find(field);
+    EXPECT_NE(at, String::npos) << "the encoder does not emit " << field;
+    return encoded.substr(0, at) + replacement + encoded.substr(at + field.size());
+}
+
+/// Every coverage row the ENCODER must refuse, each paired with why producing it would be a bug in our
+/// own fold rather than corruption arriving from a store. Shared by the two builds' assertions below so
+/// the release expectation and the sanitizer death expectation can never drift apart.
+std::vector<std::pair<const char *, CasFoldSeal>> illFormedSealsTheEncoderMustRefuse()
+{
+    std::vector<std::pair<const char *, CasFoldSeal>> out;
+
+    /// The pairing, both ways round.
+    CasFoldSeal hold_on_folded = heldSeal("ns/0");
+    fixtureCoverage(hold_on_folded, "ns/0").classification = 2;
+    out.emplace_back("a hold on a folded (2) row claims a stop that did not happen", hold_on_folded);
+
+    CasFoldSeal clamped_without_hold = heldSeal("ns/0");
+    fixtureCoverage(clamped_without_hold, "ns/0").hold.reset();
+    out.emplace_back("a clamped (4) row with no hold is indistinguishable from a clean cursor once "
+                     "durable", clamped_without_hold);
+
+    /// The closed set. 3 is the dangerous one: it passes the sweep's `== 4` and `== 0` refusals and
+    /// reaches the deletion premise, which is a refusal written in terms of the set.
+    CasFoldSeal classification_three = cleanSeal("ns/0");
+    fixtureCoverage(classification_three, "ns/0").classification = 3;
+    out.emplace_back("classification 3 is not one of {0,1,2,4} and passes every refusal stated in terms "
+                     "of them", classification_three);
+
+    CasFoldSeal classification_max = cleanSeal("ns/0");
+    fixtureCoverage(classification_max, "ns/0").classification = 255;
+    out.emplace_back("classification 255 is not one of {0,1,2,4}", classification_max);
+
+    /// The self-erasing hold, and its half-zero sibling.
+    CasFoldSeal hold_at_zero = heldSeal("ns/0");
+    fixtureCoverage(hold_at_zero, "ns/0").hold->offending_position = RefTxnId{};
+    out.emplace_back("a hold at {0,0} is cleared by the first record the next round folds", hold_at_zero);
+
+    CasFoldSeal hold_zero_sequence = heldSeal("ns/0");
+    fixtureCoverage(hold_zero_sequence, "ns/0").hold->offending_position = RefTxnId{7, 0};
+    out.emplace_back("a hold position with a zero component is not a renderable id", hold_zero_sequence);
+
+    return out;
+}
+
+}
+
+/// ===================== THE SHARED BYTE ARITHMETIC =====================
+///
+/// Two caps, two predicates, one place they are computed. Stage B's catalog reuses THESE functions for
+/// its additive "does one more entry still fit" question, so their boundary behaviour is pinned here
+/// rather than re-derived per format: a cap is the largest PERMITTED value (equality fits), and every
+/// sum saturates, because a wrapped sum answers "fits" for an object that does not — turning an
+/// overflow into a durable object nothing can read.
+TEST(CASGCHoldGrammarBudget, BothPredicatesAcceptEqualityAndRefuseOneMore)
+{
+    static_assert(fitsLineCap(64, 64));
+    static_assert(!fitsLineCap(65, 64));
+    static_assert(fitsObjectCap(40, 24, 64));
+    static_assert(!fitsObjectCap(40, 25, 64));
+
+    EXPECT_TRUE(fitsLineCap(64, 64));
+    EXPECT_FALSE(fitsLineCap(65, 64));
+    EXPECT_TRUE(fitsObjectCap(64, 0, 64));
+    EXPECT_FALSE(fitsObjectCap(64, 1, 64));
+
+    /// A cap of 0 means the format declares none (a streamed object never materialized whole).
+    EXPECT_TRUE(fitsLineCap(std::numeric_limits<uint64_t>::max(), 0));
+    EXPECT_TRUE(fitsObjectCap(std::numeric_limits<uint64_t>::max(), 1, 0));
+}
+
+TEST(CASGCHoldGrammarBudget, SumsSaturateInsteadOfWrapping)
+{
+    constexpr uint64_t kMax = std::numeric_limits<uint64_t>::max();
+    static_assert(addByteBudget(kMax, 1) == kMax);
+    static_assert(addByteBudget(kMax, kMax) == kMax);
+    static_assert(addByteBudget(3, 4) == 7);
+
+    /// The predicate that matters: a reservation that would wrap must REFUSE, not report a tiny sum.
+    EXPECT_FALSE(fitsObjectCap(kMax, 2, 256 * 1024 * 1024));
+}
+
+/// ===================== THE STRICT CLASSIFICATION-4 GRAMMAR =====================
+
+TEST(CASGCHoldGrammar, EveryHoldReasonRoundTrips)
+{
+    for (const HoldReason reason : {HoldReason::GapBelowWitness, HoldReason::UnconsumedSealCrossing,
+                                    HoldReason::WitnessDisappeared, HoldReason::BodyUndecodable,
+                                    HoldReason::ManifestBodyMissing, HoldReason::CheckpointUndecodable})
+    {
+        CasFoldSeal seal;
+        seal.generation = 3;
+        seal.parent_generation = 2;
+        RefCoverage cov;
+        cov.classification = 4;
+        cov.last_folded_ref_id = RefTxnId{4, 5};
+        cov.hold = RefHold{.reason = reason, .offending_position = RefTxnId{4, 6},
+                           .retry_count = 7, .next_retry_round = 99};
+        fixtureCoverage(seal, "ns/0") = cov;
+
+        const CasFoldSeal back = decodeFoldSeal(encodeFoldSeal(seal));
+        EXPECT_EQ(back, seal) << "hold reason " << static_cast<int>(reason);
+        ASSERT_TRUE(fixtureCoverage(back, "ns/0").hold.has_value());
+        EXPECT_EQ(fixtureCoverage(back, "ns/0").hold->reason, reason);
+    }
+}
+
+/// THE ENCODER'S HALF OF THE GRAMMAR, in one place. Every shape here is OUR OWN fold handing the codec
+/// a row it must never make durable, so the refusal is `LOGICAL_ERROR` — the code `encodeGcState` raises
+/// for the same category of impossible input — and not the `CORRUPTED_DATA` reserved for bytes that
+/// arrived from a store. Under a debug or sanitizer build that code ABORTS at construction
+/// (`handle_error_code`), so the same table is asserted as a death expectation there; the contract
+/// ("these bytes are never produced") is what both forms pin.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASGCHoldGrammar, TheEncoderRefusesEveryIllFormedCoverageRow)
+{
+    for (const auto & entry : illFormedSealsTheEncoderMustRefuse())
+    {
+        SCOPED_TRACE(entry.first);
+        expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&] { encodeFoldSeal(entry.second); });
+    }
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASGCHoldGrammarDeathTest, TheEncoderRefusesEveryIllFormedCoverageRow)
+{
+    for (const auto & entry : illFormedSealsTheEncoderMustRefuse())
+    {
+        SCOPED_TRACE(entry.first);
+        EXPECT_DEATH({ (void)encodeFoldSeal(entry.second); }, "");
+    }
+}
+#endif
+
+TEST(CASGCHoldGrammar, AHoldOnAnyOtherClassificationIsRefusedByTheDecoder)
+{
+    CasFoldSeal seal;
+    seal.generation = 1;
+    RefCoverage cov;
+
+    /// Bytes some other producer wrote. Built by demoting a legitimate held row's classification, so the
+    /// hold fields are exactly the ones the encoder emits.
+    cov.classification = 4;
+    cov.hold = RefHold{.reason = HoldReason::GapBelowWitness, .offending_position = RefTxnId{1, 2},
+                       .retry_count = 0, .next_retry_round = 1};
+    fixtureCoverage(seal, "ns/0") = cov;
+    String text = encodeFoldSeal(seal);
+    const size_t at = text.find("\"cls\":4");
+    ASSERT_NE(at, String::npos);
+    text[at + 6] = '2';
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeFoldSeal(text); });
+}
+
+TEST(CASGCHoldGrammar, ClassificationFourWithoutAHoldIsRefusedByTheDecoder)
+{
+    CasFoldSeal seal;
+    seal.generation = 1;
+    RefCoverage cov;
+    cov.classification = 4;
+    cov.last_folded_ref_id = RefTxnId{1, 1};
+
+    /// Every single hold field is REQUIRED: dropping any one of them is corruption, not a default.
+    cov.hold = RefHold{.reason = HoldReason::BodyUndecodable, .offending_position = RefTxnId{1, 2},
+                       .retry_count = 3, .next_retry_round = 4};
+    fixtureCoverage(seal, "ns/0") = cov;
+    const String whole = encodeFoldSeal(seal);
+    for (const String & field : {String(R"("hr":"body_undecodable")"), String(R"("hpe":"1")"),
+                                 String(R"("hps":"2")"), String(R"("hrc":3)"), String(R"("hnr":"4")")})
+    {
+        SCOPED_TRACE("without " + field);
+        const size_t at = whole.find(field);
+        ASSERT_NE(at, String::npos) << "the encoder does not emit " << field;
+        String without = whole;
+        without.erase(at - 1, field.size() + 1);   /// the field and the ',' before it
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeFoldSeal(without); });
+    }
+}
+
+TEST(CASGCHoldGrammar, DuplicateHoldKeyIsCorruptedData)
+{
+    CasFoldSeal seal;
+    seal.generation = 1;
+    RefCoverage cov;
+    cov.classification = 4;
+    cov.hold = RefHold{.reason = HoldReason::GapBelowWitness, .offending_position = RefTxnId{1, 2},
+                       .retry_count = 0, .next_retry_round = 5};
+    fixtureCoverage(seal, "ns/0") = cov;
+
+    const String whole = encodeFoldSeal(seal);
+    const String field = R"("hr":"gap_below_witness")";
+    const size_t at = whole.find(field);
+    ASSERT_NE(at, String::npos);
+    /// The same key twice, with a DIFFERENT value: last-wins would silently rewrite the reason.
+    String doubled = whole;
+    doubled.insert(at, R"("hr":"witness_disappeared",)");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeFoldSeal(doubled); });
+}
+
+TEST(CASGCHoldGrammar, UnknownHoldReasonWordIsCorruptedData)
+{
+    CasFoldSeal seal;
+    seal.generation = 1;
+    RefCoverage cov;
+    cov.classification = 4;
+    cov.hold = RefHold{.reason = HoldReason::GapBelowWitness, .offending_position = RefTxnId{1, 2},
+                       .retry_count = 0, .next_retry_round = 5};
+    fixtureCoverage(seal, "ns/0") = cov;
+
+    String text = encodeFoldSeal(seal);
+    const size_t at = text.find("gap_below_witness");
+    ASSERT_NE(at, String::npos);
+    text.replace(at, strlen("gap_below_witness"), "gap_below_witnesX");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeFoldSeal(text); });
+}
+
+/// ===================== THE THREE WAYS A SEAL CAN ERASE A HOLD =====================
+///
+/// The three shapes below are one finding, and it is about what a fold seal is FOR. The hold is the only
+/// durable record that a namespace stopped and where; everything downstream reads the seal and nothing
+/// re-derives the stop. So a seal that decodes into "no hold here" is not a lossy read, it is a licence
+/// to delete: the sweep's §6 refusals are stated as `classification == 4` / `== 0` / `hold.has_value()`,
+/// and a row that slips past all three reaches an irreversible delete of a manifest the fold never
+/// accounted for. Each shape gets past a DIFFERENT one of the decoder's checks, which is why they are
+/// pinned separately rather than as one "malformed seal" case.
+
+/// (1) The classification the reader never sees. `cls` is narrowed to a byte, so an integer on the wire
+/// is truncated first and validated (if at all) afterwards: 258 becomes 2, "everything through the
+/// cursor was folded". The value has to be judged WIDE, before the narrowing, or the wire can buy
+/// coverage that no fold ever performed.
+TEST(CASGCHoldGrammar, AClassificationOutsideTheGrammarIsCorruptedData)
+{
+    const String clean = encodeFoldSeal(cleanSeal("ns/0"));
+    ASSERT_EQ(fixtureCoverage(decodeFoldSeal(clean), "ns/0").classification, 2)
+        << "the unmodified row is the one every case below deviates from";
+
+    /// In-range bytes that are simply not classifications. 3 is the one the sweep's refusals miss.
+    for (const std::string_view raw : {"3", "5", "6", "255"})
+    {
+        SCOPED_TRACE(String{"cls="} + String{raw});
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                         [&] { decodeFoldSeal(withRawClassification(clean, raw)); });
+    }
+
+    /// Wide integers whose LOW BYTE lands inside the grammar: 258 -> 2 (fully folded), 256 -> 0
+    /// (absent), 260 -> 4 (clamped). Each would decode as a row the fold never wrote.
+    for (const std::string_view raw : {"256", "258", "260", "18446744073709551615"})
+    {
+        SCOPED_TRACE(String{"cls="} + String{raw});
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                         [&] { decodeFoldSeal(withRawClassification(clean, raw)); });
+    }
+}
+
+/// And the field itself is required: an absent `cls` reads as 0, which is not "nothing was said about
+/// this namespace" but the positive claim "no round folded it".
+TEST(CASGCHoldGrammar, ACoverageRowWithoutAClassificationIsCorruptedData)
+{
+    const String clean = encodeFoldSeal(cleanSeal("ns/0"));
+    const size_t at = clean.find("\"cls\":2,");
+    ASSERT_NE(at, String::npos);
+    String without = clean;
+    without.erase(at, strlen("\"cls\":2,"));
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeFoldSeal(without); });
+}
+
+/// (2) The hold that clears itself. `{0,0}` passes the completeness check — every field is present — and
+/// then the carry rule drops it on the next round, because a hold rides forward only while the walk
+/// stops BELOW its position and nothing is below zero. The namespace advances with no record that it was
+/// ever held. A zero in EITHER component is the same defect, and is additionally unnameable: the sweep
+/// renders the position when it reports what it retained, and `renderRefTxnId` refuses a zero component.
+TEST(CASGCHoldGrammar, AHoldWhoseOffendingPositionHasAZeroComponentIsCorruptedData)
+{
+    const String held = encodeFoldSeal(heldSeal("ns/0"));
+    ASSERT_TRUE(fixtureCoverage(decodeFoldSeal(held), "ns/0").hold.has_value());
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        decodeFoldSeal(withField(withField(held, R"("hpe":"4")", R"("hpe":"0")"),
+                                 R"("hps":"6")", R"("hps":"0")"));
+    });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(withField(held, R"("hpe":"4")", R"("hpe":"0")")); });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(withField(held, R"("hps":"6")", R"("hps":"0")")); });
+}
+
+/// (3) The duplicate row. Two `cov` records for the same (namespace, shard) — held first, clean second —
+/// used to be accepted with last-wins, so a single appended line erased a hold without touching the one
+/// that recorded it. There is exactly one row per key, and a second one is corruption.
+TEST(CASGCHoldGrammar, ASecondCoverageRowForTheSameKeyIsCorruptedData)
+{
+    const String held_line = covLineOf(encodeFoldSeal(heldSeal("ns/0")));
+    const String clean_line = covLineOf(encodeFoldSeal(cleanSeal("ns/0")));
+    const String other_clean_line = covLineOf(encodeFoldSeal(cleanSeal("ns/1")));
+    const String prototype = encodeFoldSeal(heldSeal("ns/0"));
+
+    /// The CONTROL first: the same two-record assembly with DIFFERENT keys decodes, so the refusal below
+    /// is about the repeated key and not about the way these bytes are forged.
+    const CasFoldSeal two_keys = decodeFoldSeal(sealTextWith(prototype, {held_line, other_clean_line}));
+    ASSERT_EQ(two_keys.ref_lives.size(), 2u);
+    ASSERT_TRUE(fixtureCoverage(two_keys, "ns/0").hold.has_value());
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(sealTextWith(prototype, {held_line, clean_line})); });
+    /// Order does not redeem it: a clean row followed by a held one is the same broken object.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(sealTextWith(prototype, {clean_line, held_line})); });
+    /// Nor does repeating the identical row.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(sealTextWith(prototype, {held_line, held_line})); });
+
+    /// The ENCODER needs no matching check, and this is why: the seal stores keyed maps, so a second row
+    /// for a key is not a value any producer can construct — assigning it replaces the first.
+    CasFoldSeal seal = heldSeal("ns/0");
+    fixtureCoverage(seal, "ns/0") = fixtureCoverage(cleanSeal("ns/0"), "ns/0");
+    EXPECT_EQ(seal.ref_lives.size(), 1u);
+}
+
+/// The same one-record-per-key rule applies to `cnd`: a repeated row rewrites a shard's condemned
+/// totals, which graduation paces on.
+TEST(CASGCHoldGrammar, ASecondCondemnedSummaryRecordIsCorruptedData)
+{
+    CasFoldSeal seal = cleanSeal("ns/0");
+    seal.condemned_summary[0] = CondemnedSummary{.condemned_total = 5, .pending_total = 1,
+                                                 .oldest_nonpending_condemn_round = 3};
+    const String encoded = encodeFoldSeal(seal);
+
+    /// Lines 3..4 are `rfl`, `cnd` in the encoder's fixed order.
+    std::vector<String> lines;
+    for (size_t begin = headerAndMetaOf(encoded).size(); begin < encoded.size();)
+    {
+        const size_t end = encoded.find('\n', begin);
+        ASSERT_NE(end, String::npos);
+        lines.push_back(encoded.substr(begin, end - begin));
+        begin = end + 1;
+    }
+    ASSERT_EQ(lines.size(), 3u) << "rfl, cnd and the trailer";
+    const String ref_life_line = lines[0];
+    const String cnd_line = lines[1];
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(sealTextWith(encoded, {ref_life_line, cnd_line, cnd_line})); });
+    /// The unduplicated assembly is the control.
+    const std::vector<String> one_of_each{ref_life_line, cnd_line};
+    EXPECT_NO_THROW(decodeFoldSeal(sealTextWith(encoded, one_of_each)));
+}
+
+/// Unified cleanup evidence still requires a canonical nonzero removal transaction id. Decoding
+/// foreign bytes must fail this read, never the process.
+TEST(CASGCHoldGrammar, CleanupEvidenceWithAZeroRemovalIdIsCorruptedData)
+{
+    CasFoldSeal seal = cleanSeal("ns/0");
+    seal.ref_lives.at(fixtureLifeId("ns/0")).cleanup_evidence =
+        RefCleanupEvidence{.remove_txn_id = RefTxnId{2, 3}};
+    const String encoded = encodeFoldSeal(seal);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(withField(encoded, R"("rte":"2")", R"("rte":"0")")); });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(withField(encoded, R"("rts":"3")", R"("rts":"0")")); });
+    /// Omitted entirely is the same thing: the fields default to zero.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { decodeFoldSeal(withField(encoded, R"("rte":"2",)", "")); });
+}
+
+/// The OBJECT cap bounds the whole seal. Nothing on the fold-seal READ path enforces it (the seal
+/// is read raw, never through `openObject`), so an oversized PUT would leave a durable seal that no
+/// later round can decode — unrecoverable. The gate therefore sits before the bytes are handed out, and
+/// equality is still accepted: the cap is the largest permitted size, not the first forbidden one.
+TEST(CASGCHoldGrammar, ObjectCapAcceptsEqualityAndRefusesOneMoreByte)
+{
+    const uint64_t object_cap = foldSealCaps().object_cap;
+    ASSERT_EQ(object_cap, 256u * 1024 * 1024);
+
+    EXPECT_NO_THROW(checkFoldSealObjectBytes(object_cap - 1));
+    EXPECT_NO_THROW(checkFoldSealObjectBytes(object_cap));
+    expectThrowsCode(DB::ErrorCodes::LIMIT_EXCEEDED, [&] { checkFoldSealObjectBytes(object_cap + 1); });
+
+    /// An ordinary seal is nowhere near it, so the gate costs a comparison and changes nothing.
+    EXPECT_NO_THROW(encodeFoldSeal(maximalHoldSeal("ns/0")));
+}
+
+/// ===================== HOLDS ARE CREATED WITH AN EXACT POSITION =====================
+
+TEST(CASGCHoldGrammar, GapBelowWitnessNamesTheExactAbsentPosition)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    /// {1,3} never existed; {1,4} is durable AND listed, so the gap is impossible under contiguity.
+    publishAt(*backend, layout, ns, RefTxnId{1, 4}, "ref_4", 4, DB::UInt128(4));
+    writeCommittedCkptAt(*backend, layout, ns, RefTxnId{1, 4});
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const RefHold hold = holdOf(*backend, layout, ns);
+    EXPECT_EQ(hold.reason, HoldReason::GapBelowWitness);
+    EXPECT_EQ(hold.offending_position, (RefTxnId{1, 3}));
+    EXPECT_EQ(hold.retry_count, 0u) << "the round that creates a hold has retried nothing yet";
+    EXPECT_GT(hold.next_retry_round, 0u);
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 2}));
+}
+
+TEST(CASGCHoldGrammar, UnconsumedSealCrossingNamesTheAbsentPosition)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    /// Epoch 1 ends at {1,1} with NO seal, and epoch 2 chains to a seal at {1,3} that this cursor
+    /// never consumed (and that does not exist). The nearest witness above the absent {1,2} therefore
+    /// sits in another epoch, and the crossing has nothing to prove itself from.
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "ref_2", 2, DB::UInt128(2),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 3});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 3},
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const RefHold hold = holdOf(*backend, layout, ns);
+    EXPECT_EQ(hold.reason, HoldReason::UnconsumedSealCrossing);
+    EXPECT_EQ(hold.offending_position, (RefTxnId{1, 2})) << "the hold names the position that read absent";
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 1}));
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(2)), 0)
+        << "nothing beyond the unproven boundary may fold";
+}
+
+TEST(CASGCHoldGrammar, UndecodableBodyNamesTheRecordItCouldNotRead)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    backend->putIfAbsent(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 2}), "this is not a cas_ref_log object");
+    writeCommittedCkptAt(*backend, layout, ns, RefTxnId{1, 2});
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const RefHold hold = holdOf(*backend, layout, ns);
+    EXPECT_EQ(hold.reason, HoldReason::BodyUndecodable);
+    EXPECT_EQ(hold.offending_position, (RefTxnId{1, 2}));
+}
+
+/// The fold barrier is a hold too, and it is the ONE hold whose ordinary cause is benign: a writer that
+/// has appended its precommit record but not yet finished uploading the manifest body. It gets the same
+/// durable treatment as the corruption shapes because it stops the namespace the same way — and because
+/// a barrier that is durably named is one an operator can distinguish from a wedge.
+TEST(CASGCHoldGrammar, MissingManifestBodyBarrierIsADurableHold)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    deleteManifestBody(*backend, layout,
+                       ManifestId{ns, ManifestRef{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 1}});
+    writeCommittedCkptAt(*backend, layout, ns, RefTxnId{1, 2});
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const RefHold hold = holdOf(*backend, layout, ns);
+    EXPECT_EQ(hold.reason, HoldReason::ManifestBodyMissing);
+    EXPECT_EQ(hold.offending_position, (RefTxnId{1, 2})) << "the hold names the LOG whose edges could not fold";
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 1}));
+}
+
+/// An above-cursor record that answered one GET and then stopped answering is CORRUPTION, not a
+/// frontier: nothing may legitimately remove an object above the fold cursor. It is the one hold shape
+/// that no amount of waiting can clear, and naming it durably is what stops a later round from reading
+/// the same namespace as quiet and granting it a frontier proof.
+TEST(CASGCHoldGrammar, AWitnessThatStopsAnsweringIsWitnessDisappeared)
+{
+    /// Answers on odd-numbered reads and 404s on even ones: `crossFromSeal` proves the position, and
+    /// the walk's own GET of it then fails.
+    class AlternatingGetBackend : public InMemoryBackend
+    {
+    public:
+        using DB::Cas::Backend::get;
+        String flaky;
+        size_t reads = 0;
+
+        std::optional<GetResult> get(const String & key, Range range) override
+        {
+            if (key == flaky && ++reads % 2 == 0)
+                return std::nullopt;
+            return InMemoryBackend::get(key, range);
+        }
+    };
+
+    auto backend = std::make_shared<AlternatingGetBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    writeSealAt(*backend, layout, ns, RefTxnId{1, 2});
+    publishAt(*backend, layout, ns, RefTxnId{2, 1}, "ref_2", 2, DB::UInt128(2),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    /// A third epoch keeps the unstable position from reading as a frontier.
+    publishAt(*backend, layout, ns, RefTxnId{3, 1}, "ref_3", 3, DB::UInt128(3),
+              /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{2, 1});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{3, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{2, 1},
+    });
+    backend->flaky = layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{2, 1});
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const RefHold hold = holdOf(*backend, layout, ns);
+    EXPECT_EQ(hold.reason, HoldReason::WitnessDisappeared);
+    /// The walk crossed into epoch 2 on the record's first answer and then could not read it: the hold
+    /// names {2,1}, the position that stopped being readable, and the cursor stays on the seal below it.
+    EXPECT_EQ(hold.offending_position, (RefTxnId{2, 1}));
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 2}));
+}
+
+/// ===================== THE SECOND WITNESS: `_ckpt.checkpoint` =====================
+///
+/// A listing is a SNAPSHOT: a record that became durable after the enumeration is invisible to that
+/// round's probes, so an absent expected-next reads as a frontier when it is really a gap. The
+/// namespace's own durable checkpoint decides the same question without asking the listing anything —
+/// and this pair of pools is the proof, because they differ in nothing else.
+TEST(CASGCHoldGrammar, CheckpointWitnessHoldsAGapTheHintIsSilentAbout)
+{
+    const RootNamespace ns{"00/aa@cas@"};
+    /// Stage B (Task 4-C): no pin needed here -- `publishAt` below (draining into `writeRefLogTxnRaw`)
+    /// admits `ns` into the catalog itself, once per pool, inside each nested block's own `seed` call.
+    const auto seed = [&](HintHoleCountingBackend & backend, const Layout & layout)
+    {
+        publishAt(backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+        publishAt(backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+        /// {1,3} is missing and {1,4}, though durable, is invisible to every LIST.
+        publishAt(backend, layout, ns, RefTxnId{1, 4}, "ref_4", 4, DB::UInt128(4));
+        backend.hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 4}));
+    };
+
+    /// Hint-only: nothing above {1,2} is visible, so the walk honestly reads a frontier and does not hold.
+    {
+        auto backend = std::make_shared<HintHoleCountingBackend>();
+        auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+        seed(*backend, store->layout());
+        Gc gc(store, kGc);
+        ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+        ASSERT_GT(backend->holesServed(), 0u);
+        const auto cov = coverageOf(*backend, store->layout(), ns);
+        ASSERT_TRUE(cov.has_value());
+        EXPECT_FALSE(cov->hold.has_value()) << "without a witness an absent IS the frontier";
+    }
+
+    /// Same pool, same hint, plus the checkpoint: the gap becomes decidable and holds at the same
+    /// position, with the same reason, as if the hint had shown the witness itself.
+    ///
+    /// The `_ckpt` object is hidden from every LIST as well, so the two pools' listings are byte-for-byte
+    /// the same and the only difference between them is an object reachable by EXACT KEY alone. That is
+    /// what makes this a proof of hint-INDEPENDENCE rather than of a richer hint.
+    {
+        auto backend = std::make_shared<HintHoleCountingBackend>();
+        auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+        const Layout & layout = store->layout();
+        seed(*backend, layout);
+        writeCkptAt(*backend, layout, ns, RefTxnId{1, 4});
+        backend->hide(layout.refCkptKey(fixture::fixtureLife(ns)));
+
+        Gc gc(store, kGc);
+        ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+        const RefHold hold = holdOf(*backend, layout, ns);
+        EXPECT_EQ(hold.reason, HoldReason::GapBelowWitness);
+        EXPECT_EQ(hold.offending_position, (RefTxnId{1, 3}));
+    }
+}
+
+/// The namespace whose second witness matters MOST: one the hint has stopped mentioning entirely, kept
+/// in the round's universe by nothing but its CARRIED HOLD. Its checkpoint is why
+/// `readCheckpointWitnesses` takes the parent cursors as well as the hint — the hold alone witnesses only
+/// the position it stopped at, so a gap ABOVE that position, once the hold resolves, has no witness left.
+TEST(CASGCHoldGrammar, CheckpointWitnessReachesAHeldNamespaceTheHintNoLongerNames)
+{
+    const RootNamespace ns{"00/aa@cas@"};
+    /// Stage B (Task 4-C): no pin needed -- `publishAt` inside `seedPool` (draining into
+    /// `writeRefLogTxnRaw`) admits `ns` into each nested block's own pool.
+
+    /// Round 1 in both pools: held at {1,3} by a gap below the listed witness {1,4}. Then the hint goes
+    /// silent about every one of the namespace's objects, {1,3} becomes readable (so the hold resolves and
+    /// the walk runs on), and a durable-but-unlisted {1,6} leaves a fresh gap at {1,5}.
+    const auto seedPool = [&](HintHoleCountingBackend & backend, const Layout & layout, Gc & gc)
+    {
+        publishAt(backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+        publishAt(backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+        publishAt(backend, layout, ns, RefTxnId{1, 4}, "ref_4", 4, DB::UInt128(4));
+        writeCommittedCkptAt(backend, layout, ns, RefTxnId{1, 4});
+        EXPECT_TRUE(gc.runRegularRound().acquired_lease);
+        EXPECT_EQ(holdOf(backend, layout, ns).offending_position, (RefTxnId{1, 3}));
+
+        publishAt(backend, layout, ns, RefTxnId{1, 3}, "ref_3", 3, DB::UInt128(3));
+        publishAt(backend, layout, ns, RefTxnId{1, 6}, "ref_6", 6, DB::UInt128(6));
+        for (const RefTxnId & id : {RefTxnId{1, 1}, RefTxnId{1, 2}, RefTxnId{1, 3}, RefTxnId{1, 4},
+                                    RefTxnId{1, 6}})
+            backend.hide(layout.refLogKey(fixture::fixtureLife(ns), id));
+    };
+
+    /// Hold-witness only: it witnesses {1,3}, which the walk has now passed, so the absent {1,5} above it
+    /// is an honest frontier and the namespace comes out clean.
+    {
+        auto backend = std::make_shared<HintHoleCountingBackend>();
+        auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+        Gc gc(store, kGc);
+        seedPool(*backend, store->layout(), gc);
+
+        ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+        const auto cov = coverageOf(*backend, store->layout(), ns);
+        ASSERT_TRUE(cov.has_value());
+        EXPECT_FALSE(cov->hold.has_value()) << "a resolved hold witnesses nothing above itself";
+        EXPECT_EQ(cov->last_folded_ref_id, (RefTxnId{1, 4}));
+    }
+
+    /// Same pool, plus the checkpoint — read by exact key for a namespace THIS round's hint never names.
+    {
+        auto backend = std::make_shared<HintHoleCountingBackend>();
+        auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+        const Layout & layout = store->layout();
+        Gc gc(store, kGc);
+        seedPool(*backend, layout, gc);
+        advanceRecoverableCkptForRawFixture(*backend, layout, ns, RefTxnId{1, 6});
+        backend->hide(layout.refCkptKey(fixture::fixtureLife(ns)));
+
+        ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+        const RefHold hold = holdOf(*backend, layout, ns);
+        EXPECT_EQ(hold.reason, HoldReason::GapBelowWitness);
+        EXPECT_EQ(hold.offending_position, (RefTxnId{1, 5}));
+        EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 4}));
+    }
+}
+
+/// The second witness can also be UNREADABLE, and that is a different answer from absent. An absent
+/// `_ckpt` says "this namespace published no checkpoint" and honestly contributes no witness; a present
+/// one that will not decode says "this namespace HAS a checkpoint and we cannot read it", which no walk
+/// may treat as no witness.
+///
+/// It is still ONE NAMESPACE'S object. The fold used to fail the whole round closed on it — every
+/// namespace's cursor, seal and cleanup stopped, every round, on one unreadable 4 KiB object, and the
+/// exception named neither the namespace nor the key. The rule is the one §5 states for every other
+/// per-namespace failure: hold the namespace that owns the object, fold everything else.
+TEST(CASGCHoldGrammar, AnUndecodableCheckpointHoldsOnlyItsOwnNamespace)
+{
+    const RootNamespace bad{"00/aa@cas@"};
+    /// Stage B (Task 4-C): no pin needed -- `publishAt(..., birth=true)` below (draining into
+    /// `writeRefLogTxnRaw`) admits `bad` into the catalog itself, pinned to the same sentinel this
+    /// test's own `fixture::fixtureLife(bad)` key computations already assume.
+    const RootNamespace good{"00/bb@cas@"};
+
+    auto backend = std::make_shared<HintHoleCountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    Gc gc(store, kGc);
+
+    publishAt(*backend, layout, bad, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, bad, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    publishAt(*backend, layout, good, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(11), /*birth=*/true);
+    writeCkptAt(*backend, layout, bad, RefTxnId{1, 2});
+    writeCkptAt(*backend, layout, good, RefTxnId{1, 1});
+
+    /// Round 1 is the BASELINE both namespaces are measured against: each folds its whole stream and
+    /// seals a cursor, and neither holds.
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    ASSERT_EQ(sealedCursorOf(*backend, layout, bad), (RefTxnId{1, 2}));
+    ASSERT_EQ(sealedCursorOf(*backend, layout, good), (RefTxnId{1, 1}));
+    ASSERT_FALSE(coverageOf(*backend, layout, bad)->hold.has_value());
+
+    /// Corrupt EXACTLY ONE OBJECT: the first namespace's `_ckpt` body. Nothing else in the pool changes,
+    /// so everything the next round does differently is attributable to this one object.
+    const String bad_ckpt_key = layout.refCkptKey(fixture::fixtureLife(bad));
+    const HeadResult ckpt_head = backend->head(bad_ckpt_key);
+    ASSERT_TRUE(ckpt_head.exists);
+    ASSERT_EQ(backend->putOverwrite(bad_ckpt_key, "this is not a cas_ref_ckpt", ckpt_head.token).outcome,
+              PutOutcome::Done);
+
+    /// Work only a round that COMPLETES can fold.
+    publishAt(*backend, layout, good, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(12));
+    const String good_ckpt_key = layout.refCkptKey(fixture::fixtureLife(good));
+    const HeadResult good_ckpt_head = backend->head(good_ckpt_key);
+    ASSERT_TRUE(good_ckpt_head.exists);
+    ASSERT_EQ(backend->putOverwrite(good_ckpt_key, encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = RefTxnId{1, 2},
+        .last_epoch_seal = std::nullopt,
+    }), good_ckpt_head.token).outcome, PutOutcome::Done);
+
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    /// The namespace that owns the object is held, at the position its walk would have read next, and
+    /// its coverage row rides UNCHANGED — the cursor may not move while the hold stands.
+    const RefHold hold = holdOf(*backend, layout, bad);
+    EXPECT_EQ(hold.reason, HoldReason::CheckpointUndecodable);
+    EXPECT_EQ(hold.offending_position, (RefTxnId{1, 3}));
+    EXPECT_EQ(sealedCursorOf(*backend, layout, bad), (RefTxnId{1, 2}));
+
+    /// The other namespace folded its new record. This is the whole point of the finding: one corrupt
+    /// object must not stop the pool.
+    const auto good_cov = coverageOf(*backend, layout, good);
+    ASSERT_TRUE(good_cov.has_value());
+    EXPECT_FALSE(good_cov->hold.has_value()) << "the corrupt object belongs to the OTHER namespace";
+    EXPECT_EQ(good_cov->last_folded_ref_id, (RefTxnId{1, 2}));
+    EXPECT_EQ(good_cov->classification, 2);
+
+    /// And nothing was destroyed for the held namespace: a hold shuts the round's destructive gate, so
+    /// its ref objects — including the ones a cleanup range computed WITHOUT the unreadable checkpoint
+    /// would have widened onto — are all still there.
+    for (const RefTxnId & id : {RefTxnId{1, 1}, RefTxnId{1, 2}})
+        EXPECT_TRUE(backend->head(layout.refLogKey(fixture::fixtureLife(bad), id)).exists)
+            << "ref log " << renderRefTxnId(id) << " of the held namespace was deleted";
+}
+
+/// THE OTHER ARM OF THE SAME RULE, and the one that must NOT mint a hold.
+///
+/// A namespace can carry an undecodable `_ckpt` and offer the walk NO POSITION TO READ: never folded
+/// (no sealed cursor) and no listed log. Two ways to get there, both real. A writer publishes the
+/// object around its namespace's birth, so a `_ckpt` that lands before the birth log is durable is
+/// exactly this shape. And `parseRefCkptKey` deliberately resolves anything of the form
+/// `<something>/_ckpt`, so a key with a stray segment names the checkpoint of a table that has no logs
+/// and no snapshots and never will (`CasLayout.h`, "the phantom table it names ... the fold does
+/// nothing for it") — which is precisely the object that used to halt GC for the entire pool.
+///
+/// NO HOLD IS MINTED, and that is a positive design choice rather than a shortfall. A hold is not just
+/// a stop flag: its `offending_position` is read by every later round as a DURABLE WITNESS that some
+/// round once reached that position, which turns an absent below it into a gap rather than a frontier.
+/// The walk here reached nothing, so any position would be invented — `{0, 0}` is rejected outright by
+/// both codecs, and any canonical value would plant a permanent false witness under a namespace whose
+/// records legitimately do not exist. The anomaly carries it instead, which is enough because it shuts
+/// the same round-wide destructive gate a hold would, and because everything the checkpoint gates is a
+/// no-op for a namespace the walk cannot even start on: nothing to fold, and an empty delete plan.
+TEST(CASGCHoldGrammar, AnUndecodableCheckpointWithNoWalkPositionRecordsAnAnomalyAndMintsNoHold)
+{
+    const RootNamespace phantom{"00/aa@cas@"};
+    const RootNamespace good{"00/bb@cas@"};
+
+    auto backend = std::make_shared<HintHoleCountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    Gc gc(store, kGc);
+
+    /// Stage B (Task 4-C): `phantom` gets no birth and no other production touch -- it is meant to
+    /// have no logs and no snapshots, ever. But `discoverUniverse` is now catalog-authoritative, so a
+    /// namespace absent from the catalog is invisible to the walk (R10 treats it as foreign-prefix-inert),
+    /// and this test's whole premise -- that GC still surfaces an anomaly for an uncataloged `_ckpt` --
+    /// would be silently defeated. Admitting it here (still with no `_ckpt` of its own) is what keeps
+    /// `phantom` reachable by `readCheckpointWitnesses` without giving it the birth this test deliberately
+    /// withholds.
+    fixture::admitLive(*backend, layout, phantom);
+
+    /// A lone `_ckpt` with an undecodable body, and NOTHING else under that namespace.
+    backend->putIfAbsent(layout.refCkptKey(fixture::fixtureLife(phantom)), "this is not a cas_ref_ckpt");
+    publishAt(*backend, layout, good, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(11), /*birth=*/true);
+    writeCommittedCkptAt(*backend, layout, good, RefTxnId{1, 1});
+
+    const RoundReport report = gc.runRegularRound();
+    ASSERT_TRUE(report.acquired_lease);
+
+    /// The anomaly is the whole carrier here: it is what shuts the round's destructive gate, and the
+    /// gate is what keeps `cleanupRefObjects` from computing this namespace's delete range from an
+    /// ABSENT checkpoint — which is the WIDEST reading, not the safest.
+    /// Located by namespace and shard, then CHECKED ON ITS REASON. Asserting only that "some anomaly
+    /// exists for this namespace" is a pin any future unrelated anomaly would satisfy, and this test
+    /// would then stop testing anything; the reason check is what keeps it pinned to this arm. Note it
+    /// is also stricter than searching BY reason would be — it requires the FIRST anomaly recorded for
+    /// this namespace to be this one, not merely that one of them somewhere is.
+    const auto anomaly = std::find_if(report.anomalies.begin(), report.anomalies.end(),
+        [&](const RoundAnomaly & a) { return a.ns.string() == phantom.string() && a.shard == 0; });
+    ASSERT_NE(anomaly, report.anomalies.end())
+        << "an unreadable `_ckpt` must be surfaced even when there is no walk to stop";
+    EXPECT_NE(anomaly->reason.find("_ckpt"), String::npos)
+        << "the anomaly must say WHAT stopped the namespace, not merely that something did";
+
+    const auto cov = coverageOf(*backend, layout, phantom);
+    ASSERT_TRUE(cov.has_value());
+    EXPECT_FALSE(cov->hold.has_value()) << "a hold here could only name a position no round ever read";
+    EXPECT_EQ(cov->classification, 1) << "nothing was folded, so the row is `unchanged`";
+    EXPECT_EQ(cov->last_folded_ref_id, (RefTxnId{}));
+
+    /// Same isolation as the held arm: the pool keeps working.
+    const auto good_cov = coverageOf(*backend, layout, good);
+    ASSERT_TRUE(good_cov.has_value());
+    EXPECT_FALSE(good_cov->hold.has_value());
+    EXPECT_EQ(good_cov->last_folded_ref_id, (RefTxnId{1, 1}));
+
+    /// The unreadable object itself is never deleted as debris — repairing it is the operator's move,
+    /// and GC removing it would erase the only evidence of what stopped the namespace.
+    EXPECT_TRUE(backend->head(layout.refCkptKey(fixture::fixtureLife(phantom))).exists);
+}
+
+/// ===================== THE HOLD IS DURABLE =====================
+
+namespace
+{
+
+/// Seed a namespace held at {1,3} by a gap below the listed witness {1,4}, then make the hint forget
+/// the namespace exists. Returns the round-1 hold.
+RefHold seedHeldThenUnhinted(
+    const std::shared_ptr<HintHoleCountingBackend> & backend, const PoolPtr & store,
+    const RootNamespace & ns, Gc & gc)
+{
+    const Layout & layout = store->layout();
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    publishAt(*backend, layout, ns, RefTxnId{1, 4}, "ref_4", 4, DB::UInt128(4));
+    writeCommittedCkptAt(*backend, layout, ns, RefTxnId{1, 4});
+
+    EXPECT_TRUE(gc.runRegularRound().acquired_lease);
+    const RefHold hold = holdOf(*backend, layout, ns);
+
+    /// Every one of the namespace's objects vanishes from every LIST while staying readable by key:
+    /// the round that follows has no hint entry for this namespace at all.
+    for (const RefTxnId & id : {RefTxnId{1, 1}, RefTxnId{1, 2}, RefTxnId{1, 4}})
+        backend->hide(layout.refLogKey(fixture::fixtureLife(ns), id));
+    return hold;
+}
+
+}
+
+TEST(CASGCHoldGrammar, HoldRidesARoundWhoseHintOmitsTheNamespace)
+{
+    auto backend = std::make_shared<HintHoleCountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const RootNamespace ns{"00/aa@cas@"};
+    Gc gc(store, kGc);
+    const RefHold first = seedHeldThenUnhinted(backend, store, ns, gc);
+
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    ASSERT_GT(backend->holesServed(), 0u);
+
+    const RefHold second = holdOf(*backend, store->layout(), ns);
+    EXPECT_EQ(second.reason, first.reason) << "a quiet hint must not rewrite why the namespace is held";
+    EXPECT_EQ(second.offending_position, first.offending_position);
+    EXPECT_EQ(sealedCursorOf(*backend, store->layout(), ns), (RefTxnId{1, 2}))
+        << "the cursor may not advance while the hold stands";
+    /// The one field that moves, and the reason it exists: it counts the rounds that retried and failed.
+    EXPECT_EQ(second.retry_count, first.retry_count + 1);
+}
+
+TEST(CASGCHoldGrammar, HoldForcesAnExactRetryOfItsOffendingPositionWhenUnhinted)
+{
+    auto backend = std::make_shared<HintHoleCountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    Gc gc(store, kGc);
+    seedHeldThenUnhinted(backend, store, ns, gc);
+
+    const String offending = store->layout().refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 3});
+    const uint64_t before = backend->getCount(offending);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    EXPECT_GT(backend->getCount(offending), before)
+        << "a carried hold must read its offending position by EXACT key; the hint cannot be asked, "
+           "because the hint no longer mentions the namespace at all";
+}
+
+/// The clearing rule, stated as a test: an absent proves nothing. The round below observes the
+/// offending position absent AGAIN, with no witness anywhere — exactly the observation a lying store
+/// produces — and the hold survives it. Only the record actually appearing, being folded, and the
+/// result reaching `gc/state` clears it.
+TEST(CASGCHoldGrammar, HoldClearsOnlyByFoldingThroughTheOffendingPosition)
+{
+    auto backend = std::make_shared<HintHoleCountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    Gc gc(store, kGc);
+    seedHeldThenUnhinted(backend, store, ns, gc);
+
+    /// Round 2: another absent, no witness. NOT a clearance.
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    EXPECT_EQ(holdOf(*backend, layout, ns).offending_position, (RefTxnId{1, 3}));
+
+    /// The record appears at last (still invisible to every LIST — the hold is the only thing that
+    /// knows to look there).
+    publishAt(*backend, layout, ns, RefTxnId{1, 3}, "ref_3", 3, DB::UInt128(3));
+    backend->hide(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 3}));
+
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    const auto cov = coverageOf(*backend, layout, ns);
+    ASSERT_TRUE(cov.has_value());
+    EXPECT_FALSE(cov->hold.has_value()) << "folding through the offending position is what clears a hold";
+    EXPECT_EQ(cov->classification, 2);
+    EXPECT_EQ(cov->last_folded_ref_id, (RefTxnId{1, 4})) << "the walk resumed past the resolved gap";
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(4)), 1)
+        << "the record above the gap finally contributed its owner edge";
+}
+
+/// ===================== REBUILD =====================
+
+namespace
+{
+
+/// Rewrite the fold seal at an EXACT `(generation, attempt)`, applying `mutate` to it. Needed where
+/// the seal under test is not the adopted one — a step-down test plants its hold in a generation the
+/// pool has already moved past.
+void mutateSealAt(Backend & backend, const Layout & layout, uint64_t generation, uint64_t attempt,
+                  const std::function<void(CasFoldSeal &)> & mutate)
+{
+    const String key = layout.foldSealKey(generation, attempt);
+    CasFoldSeal seal = decodeFoldSeal(backend.get(key)->bytes);
+    mutate(seal);
+    backend.putOverwrite(key, encodeFoldSeal(seal), backend.head(key).token);
+}
+
+/// Rewrite the adopted fold seal, applying `mutate` to it. Used to plant a hold that the rebuild must
+/// then carry: planting it directly (rather than by holding a real round) keeps the REBUILD tests about
+/// the carry, not about how the hold arose.
+void mutateAdoptedSeal(Backend & backend, const Layout & layout, const std::function<void(CasFoldSeal &)> & mutate)
+{
+    const GcState st = decodeGcState(backend.get(layout.gcStateKey())->bytes);
+    const String key = layout.foldSealKey(st.snap_generation, st.snap_attempt);
+    CasFoldSeal seal = decodeFoldSeal(backend.get(key)->bytes);
+    mutate(seal);
+    backend.putOverwrite(key, encodeFoldSeal(seal), backend.head(key).token);
+}
+
+RefHold plantedHold()
+{
+    return RefHold{.reason = HoldReason::WitnessDisappeared, .offending_position = RefTxnId{4, 9},
+                   .retry_count = 17, .next_retry_round = 23};
+}
+
+}
+
+/// A rebuild carries a hold only for the matching catalog life. A historical row whose id is absent
+/// from the rebuild cut is dropped and cannot mint output work.
+TEST(CASGCHoldGrammar, RebuildCarriesMatchingHoldAndDropsAbsentLife)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    writeCommittedCkptAt(*backend, layout, ns, RefTxnId{1, 1});
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const UInt128 life_id = catalogLifeIdForTest(*backend, layout, ns);
+    constexpr UInt128 absent_life_id{0xfeed};
+    mutateAdoptedSeal(*backend, layout, [&](CasFoldSeal & seal)
+    {
+        RefCoverage & cov = seal.ref_lives.at(life_id).coverage;
+        cov.classification = 4;
+        cov.hold = plantedHold();
+        RefCoverage gone;
+        gone.classification = 4;
+        gone.last_folded_ref_id = RefTxnId{2, 2};
+        gone.hold = RefHold{.reason = HoldReason::GapBelowWitness, .offending_position = RefTxnId{2, 3},
+                            .retry_count = 1, .next_retry_round = 2};
+        seal.ref_lives[absent_life_id].coverage = gone;
+    });
+
+    const RebuildReport rep = gc.rebuildBaseline(/*force=*/true);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+
+    const auto rebuilt = newestSeal(*backend, layout);
+    ASSERT_TRUE(rebuilt.has_value());
+    const auto rediscovered = rebuilt->ref_lives.find(life_id);
+    ASSERT_NE(rediscovered, rebuilt->ref_lives.end());
+    EXPECT_EQ(rediscovered->second.coverage.classification, 4);
+    ASSERT_TRUE(rediscovered->second.coverage.hold.has_value());
+    EXPECT_EQ(*rediscovered->second.coverage.hold, plantedHold());
+    EXPECT_FALSE(rebuilt->ref_lives.contains(absent_life_id));
+}
+
+/// AN ORDINARY CRASH IS NOT A CORRUPT POOL. A round writes its runs during the reduce phase and its
+/// fold seal only at phase 10/18, so a crash in between leaves the newest generation existing WITHOUT
+/// a seal — the commonest shape there is. If discovery stopped at the listing's maximum it would find
+/// no seal there, conclude it could enumerate nothing, and refuse — telling the operator to recreate a
+/// pool whose holds are sitting readable one generation down.
+///
+/// So discovery steps DOWN through the generations the listing itself reported until one carries a
+/// seal. That spends no trust the maximum had not already been given. What it does NOT weaken is the
+/// refusal above the maximum: that one stays terminal, because a seal found there is the listing
+/// caught lying, not merely being incomplete about seals.
+TEST(CASGCHoldGrammar, RebuildStepsDownPastACrashedNewestGenerationToTheSealBelowIt)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    writeCommittedCkptAt(*backend, layout, ns, RefTxnId{1, 1});
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    const GcState after_first = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const uint64_t older_generation = after_first.snap_generation;
+    const uint64_t older_attempt = after_first.snap_attempt;
+    const UInt128 life_id = catalogLifeIdForTest(*backend, layout, ns);
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    advanceRecoverableCkptForRawFixture(*backend, layout, ns, RefTxnId{1, 2});
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    const GcState after_second = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    ASSERT_GT(after_second.snap_generation, older_generation) << "the fixture needs two generations";
+
+    /// The older generation is the one holding the pool's durable hold.
+    mutateSealAt(*backend, layout, older_generation, older_attempt, [&](CasFoldSeal & seal)
+    {
+        RefCoverage & cov = seal.ref_lives.at(life_id).coverage;
+        cov.classification = 4;
+        cov.hold = plantedHold();
+    });
+
+    /// THE CRASH: the newest generation's run objects are there, its seal never got written. Then
+    /// `gc/state` is lost, which is this path's whole premise.
+    const String newest_seal = layout.foldSealKey(after_second.snap_generation, after_second.snap_attempt);
+    const HeadResult seal_head = backend->head(newest_seal);
+    ASSERT_TRUE(seal_head.exists);
+    ASSERT_EQ(backend->deleteExact(newest_seal, seal_head.token).kind, DeleteOutcome::Kind::Deleted);
+    ASSERT_FALSE(backend->list(layout.gcGenPrefix(after_second.snap_generation), "", 1).keys.empty())
+        << "the crashed generation must still hold objects, or it is not the shape being modelled";
+    const HeadResult sh = backend->head(layout.gcStateKey());
+    ASSERT_EQ(backend->deleteExact(layout.gcStateKey(), sh.token).kind, DeleteOutcome::Kind::Deleted);
+
+    Gc gc2(store, hexToU128("0000000000000000000000000000000c"));
+    const RebuildReport rep = gc2.rebuildBaseline(/*force=*/false);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_FALSE(rep.virgin_by_enumeration) << "a pool with a readable seal is not virgin";
+    EXPECT_EQ(rep.adopted_seal_generation, older_generation)
+        << "the report must name WHICH generation the holds came from, so a step-down is visible";
+
+    const auto rebuilt = newestSeal(*backend, layout);
+    ASSERT_TRUE(rebuilt.has_value());
+    const auto it = rebuilt->ref_lives.find(life_id);
+    ASSERT_NE(it, rebuilt->ref_lives.end());
+    ASSERT_TRUE(it->second.coverage.hold.has_value())
+        << "a crash between the run writes and the seal write turned into 'recreate the pool', and the "
+           "hold readable one generation down was thrown away with it";
+    EXPECT_EQ(*it->second.coverage.hold, plantedHold());
+}
+
+/// With no readable prior seal there is nothing to carry, and the holds it may have contained are
+/// unknowable. The rebuild refuses rather than blessing a baseline whose provenance it cannot state —
+/// a pool-wide hold is not representable (there is no offending position anyone could ever fold
+/// through), so the honest answer is the refusal, and the recovery path is pool recreation.
+TEST(CASGCHoldGrammar, RebuildRefusesWithAMissingPriorSeal)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const GcState st = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    ASSERT_GT(st.snap_generation, 0u);
+    const String seal_key = layout.foldSealKey(st.snap_generation, st.snap_attempt);
+    const HeadResult sh = backend->head(seal_key);
+    ASSERT_TRUE(sh.exists);
+    ASSERT_EQ(backend->deleteExact(seal_key, sh.token).kind, DeleteOutcome::Kind::Deleted);
+
+    /// FORCE does not buy past it either: force means "rebuild deliberately", never "drop the holds".
+    for (const bool force : {false, true})
+    {
+        SCOPED_TRACE(force ? "force" : "plain");
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { gc.rebuildBaseline(force); });
+    }
+
+    const GcState after = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    EXPECT_EQ(after.snap_generation, st.snap_generation) << "a refused rebuild adopts nothing";
+}
+
+TEST(CASGCHoldGrammar, RebuildRefusesWithAnUndecodablePriorSeal)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const GcState st = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const String seal_key = layout.foldSealKey(st.snap_generation, st.snap_attempt);
+    backend->putOverwrite(seal_key, "{\"type\":\"cas_fold_seal\",\"v\":4}\nthis is not a seal body\n",
+                          backend->head(seal_key).token);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { gc.rebuildBaseline(/*force=*/true); });
+}
+
+/// LOSING THE POINTER IS NOT WEAKER THAN LOSING THE SEAL. `gc/state` names the adopted seal, and it is
+/// the seal that carries the holds — so if the refusal only covered an unreadable seal, the *lesser*
+/// corruption (the pointer is gone, every seal intact) would be treated more permissively than the
+/// greater one, and the rebuild would write a baseline with no hold in it at all.
+///
+/// That matters because holds are not re-derivable by the next walk. `WitnessDisappeared` names a
+/// record that is *gone*: the next round reads a clean frontier and would hand the namespace exactly
+/// the frontier proof the hold exists to deny. Same for any hold whose only witness was the checkpoint
+/// or the hold itself.
+///
+/// So with no adopted baseline named, the rebuild finds the newest fold seal OBJECT by enumeration and
+/// carries its holds. This keeps the pool's disaster recovery intact — losing `gc/state` on a
+/// lived-in pool is the scenario `REBUILD` exists for — while making it impossible to write a
+/// hold-free baseline over a pool that had holds.
+TEST(CASGCHoldGrammar, RebuildWithLostStateStillCarriesHoldsFromTheNewestSeal)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    writeCommittedCkptAt(*backend, layout, ns, RefTxnId{1, 1});
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    const UInt128 life_id = catalogLifeIdForTest(*backend, layout, ns);
+    mutateAdoptedSeal(*backend, layout, [&](CasFoldSeal & seal)
+    {
+        RefCoverage & cov = seal.ref_lives.at(life_id).coverage;
+        cov.classification = 4;
+        cov.hold = plantedHold();
+    });
+
+    /// The pointer vanishes; every seal object survives.
+    const HeadResult sh = backend->head(layout.gcStateKey());
+    ASSERT_TRUE(sh.exists);
+    ASSERT_EQ(backend->deleteExact(layout.gcStateKey(), sh.token).kind, DeleteOutcome::Kind::Deleted);
+
+    Gc gc2(store, hexToU128("00000000000000000000000000000009"));
+    const RebuildReport rep = gc2.rebuildBaseline(/*force=*/false);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+
+    const auto rebuilt = newestSeal(*backend, layout);
+    ASSERT_TRUE(rebuilt.has_value());
+    const auto it = rebuilt->ref_lives.find(life_id);
+    ASSERT_NE(it, rebuilt->ref_lives.end());
+    ASSERT_TRUE(it->second.coverage.hold.has_value())
+        << "the rebuild blessed a baseline with no hold in it, having read no seal at all";
+    EXPECT_EQ(*it->second.coverage.hold, plantedHold());
+}
+
+/// ...and when that newest seal cannot be read either, there is nothing left to carry and no way to
+/// know what was lost, so the rebuild refuses exactly as it does for an unreadable adopted seal.
+TEST(CASGCHoldGrammar, RebuildRefusesWhenTheNewestSealIsUnreadableAndTheStateIsLost)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const GcState st = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const String seal_key = layout.foldSealKey(st.snap_generation, st.snap_attempt);
+    backend->putOverwrite(seal_key, "{\"type\":\"cas_fold_seal\",\"v\":4}\nthis is not a seal body\n",
+                          backend->head(seal_key).token);
+    const HeadResult sh = backend->head(layout.gcStateKey());
+    ASSERT_EQ(backend->deleteExact(layout.gcStateKey(), sh.token).kind, DeleteOutcome::Kind::Deleted);
+
+    Gc gc2(store, hexToU128("0000000000000000000000000000000a"));
+    for (const bool force : {false, true})
+    {
+        SCOPED_TRACE(force ? "force" : "plain");
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { gc2.rebuildBaseline(force); });
+    }
+}
+
+/// NEWEST-NESS IS NOT READ OFF A LISTING. Taking the newest seal from the pool-wide enumeration would
+/// put the same hole one layer up: a listing that omits the true newest seal hands back an OLDER one,
+/// and every hold detected since that older seal is silently lost. Two narrow single-generation probes
+/// above the listing's maximum ask whether it lied.
+///
+/// And when it did lie, the answer is REFUSAL, not adoption of the newer seal. A store that misreports
+/// its own enumeration DURING DISASTER RECOVERY does not get a second guess: adopting whatever the
+/// second query happened to return would move the same trust one query along and prove nothing.
+///
+/// The fixture is the production shape rather than a contrivance: the broad `gc/gen/` enumeration
+/// omits the newest generation's objects while a listing scoped to that generation still returns
+/// them — the same class of lie the arithmetic ref walk was built for one layer down.
+TEST(CASGCHoldGrammar, RebuildRefusesWhenANarrowProbeFindsASealAboveTheListingMaximum)
+{
+    /// Omits keys from ONE enumeration prefix only. Every other query — including a listing scoped to
+    /// the generation itself — answers truthfully.
+    class BroadListHoleBackend : public InMemoryBackend
+    {
+    public:
+        String hide_under_prefix;
+        String hidden_key_infix;
+        size_t holes_served = 0;
+
+        ListPage list(const String & prefix, const String & cursor, size_t limit) override
+        {
+            ListPage page = InMemoryBackend::list(prefix, cursor, limit);
+            if (prefix != hide_under_prefix)
+                return page;
+            const size_t before = page.keys.size();
+            std::erase_if(page.keys,
+                          [&](const ListedKey & k) { return k.key.find(hidden_key_infix) != String::npos; });
+            if (page.keys.size() != before)
+                ++holes_served;
+            return page;
+        }
+    };
+
+    auto backend = std::make_shared<BroadListHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_2", 2, DB::UInt128(2));
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const GcState st = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    ASSERT_GT(st.snap_generation, 1u) << "the fixture needs a newer generation to hide";
+    const UInt128 life_id = catalogLifeIdForTest(*backend, layout, ns);
+    mutateAdoptedSeal(*backend, layout, [&](CasFoldSeal & seal)
+    {
+        RefCoverage & cov = seal.ref_lives.at(life_id).coverage;
+        cov.classification = 4;
+        cov.hold = plantedHold();
+    });
+
+    /// The pool-wide enumeration loses the newest generation entirely; the pointer to it is deleted.
+    const String gen_prefix = layout.gcGenPrefix(0);
+    backend->hide_under_prefix = gen_prefix.substr(0, gen_prefix.size() - 2);   /// ".../gc/gen/"
+    backend->hidden_key_infix = layout.gcGenPrefix(st.snap_generation);
+    const HeadResult sh = backend->head(layout.gcStateKey());
+    ASSERT_EQ(backend->deleteExact(layout.gcStateKey(), sh.token).kind, DeleteOutcome::Kind::Deleted);
+
+    Gc gc2(store, hexToU128("0000000000000000000000000000000b"));
+    for (const bool force : {false, true})
+    {
+        SCOPED_TRACE(force ? "force" : "plain");
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { gc2.rebuildBaseline(force); });
+    }
+    ASSERT_GT(backend->holes_served, 0u) << "the broad listing never actually lied";
+
+    /// Nothing was adopted: the refusal fires before the lease, so the pool is exactly as it was.
+    EXPECT_FALSE(backend->head(layout.gcStateKey()).exists)
+        << "a refused rebuild must not mint a baseline, nor a bootstrap body";
+}
+
+/// The virgin verdict, pinned so the refusal can never grow to swallow a fresh pool — and pinned as
+/// what it actually is. It rests on THREE pieces of enumeration evidence (wide LIST empty, narrow
+/// generation-1 probe empty, no `gc/state`) and on no point read at all, so it is COUNTED: an operator
+/// reading a disaster-recovery run needs to see that the clean slate came from enumeration rather than
+/// from proof. `CASGCRebuildVirginByEnumeration` on a pool that has ever completed a round means the
+/// enumeration lied.
+TEST(CASGCHoldGrammar, RebuildProceedsOnAPoolThatNeverSealedABaselineAndCountsTheVerdict)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    /// No round has run, so there is no `gc/state` and no seal — only owner state to rebuild from.
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_1", 1, DB::UInt128(1), /*birth=*/true);
+    writeCommittedCkptAt(*backend, layout, ns, RefTxnId{1, 1});
+    ASSERT_FALSE(backend->head(layout.gcStateKey()).exists);
+
+    using ProfileEvents::global_counters;
+    const auto virgin_before = global_counters[ProfileEvents::CASGCRebuildVirginByEnumeration].load();
+
+    Gc gc(store, kGc);
+    const RebuildReport rep = gc.rebuildBaseline(/*force=*/false);
+    EXPECT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_GT(global_counters[ProfileEvents::CASGCRebuildVirginByEnumeration].load(), virgin_before)
+        << "a clean slate granted from enumeration alone must be visible to whoever reads the run";
+}
diff --git a/src/Disks/tests/gtest_cas_gc_leak.cpp b/src/Disks/tests/gtest_cas_gc_leak.cpp
new file mode 100644
index 000000000000..a02deb07d0bc
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_leak.cpp
@@ -0,0 +1,545 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <iostream>
+#include <string>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+extern const int FILE_DOESNT_EXIST;
+extern const int ABORTED;
+}
+
+/// NO-LEAK property suite (C++ verification of the R0 INV-NO-LEAK invariant for the root-local
+/// part-manifest model). Every dropped/abandoned closure must be FULLY reclaimed: after GC reaches a
+/// fixpoint, NO blob or manifest object may remain for the reclaimed part, and the in-degree generation
+/// must hold no stranded positive counter for a now-unreferenced blob.
+///
+/// The model has changed since the tree/snap era: a part is one immutable single-owner `ManifestId`
+/// (only blobs stay content-addressed; manifests are NEVER shared across instances — backlog item B7).
+/// The leak scenarios below therefore drive the REAL write flow (`stageManifest -> precommitAdd ->
+/// putBlob -> promote`) and the real drop/abandon paths, then assert the reclaimed closure leaves no
+/// debris. The old "adopt-by-tree relink" leak cases (B7) are REMOVED: there is no shared content id,
+/// no subtree placement, `getPartTreeId` returns nullopt and `adoptPart` throws `NOT_IMPLEMENTED`; the
+/// byte-stream-fallback relink is an ordinary publish covered by the no-leak displacement repros below.
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+using DB::Cas::tests::inDegreeOf;
+using DB::Cas::tests::publishCommittedTransition;
+
+namespace
+{
+
+PoolPtr openTestPool(std::shared_ptr<InMemoryBackend> & out_backend)
+{
+    out_backend = std::make_shared<InMemoryBackend>();
+    return Pool::open(out_backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// Whether the CURRENT retired list (any gc-shard) still holds an entry — the ack-floor deletion pipeline
+/// (condemn -> graduate -> delete) is in flight while this is true.
+bool anyRetiredPending(const PoolPtr & s)
+{
+    /// Retired-in-snapshot (T4): condemned state rides the adopted fold seal's kCondemned rows, not a
+    /// separate retired list — reconstruct the in-flight set from the seal.
+    return DB::Cas::tests::anyCondemnedInSeal(s->backend(), s->layout());
+}
+
+/// Drive regular GC to a fixpoint. A condemned blob is not deleted in the round that folds its removal:
+/// it condemns, then graduates the round after (round-paced, unconditional), then the NEXT pass deletes
+/// it. The loop renews the store's own heartbeat after each round (`renewWatermarkOnce`, unrelated to
+/// graduation timing but keeping the build-watermark floor and lease current) and stays alive while ANY
+/// work counter is nonzero OR the current retired list still holds an in-flight entry.
+size_t runGcToFixpoint(const PoolPtr & s, Gc & gc, size_t max_rounds = 64)
+{
+    size_t rounds = 0;
+    for (; rounds < max_rounds; ++rounds)
+    {
+        const RoundReport rep = DB::Cas::tests::runRegularRoundReclaiming(gc);
+        if (!rep.acquired_lease)
+            continue;
+        s->renewWatermarkOnce();
+        const bool no_work = rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0;
+        if (no_work && !anyRetiredPending(s))
+            break;
+    }
+    return rounds;
+}
+
+/// A `ManifestEntry` for a Blob leaf at `path` referencing `payload`'s content hash.
+ManifestEntry blobEntry(const String & path, const String & payload)
+{
+    ManifestEntry e;
+    e.path = path;
+    e.placement = EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    e.blob_size = payload.size();
+    return e;
+}
+
+/// Publish ONE ref naming a two-blob part through the REAL writer transaction sequence — the exact order
+/// the wiring drives (EDGE-BEFORE-OBSERVE): `beginPartWrite -> stageManifest(entries) -> precommitAdd ->
+/// putBlob(each body) -> promote`. The durable precommit closure names every blob hash before putBlob
+/// makes the first backend observation. Returns the published `ManifestId` so a caller can later HEAD
+/// its body / assert reclaim.
+ManifestId publishTwoBlobPart(
+    const PoolPtr & s, const RootNamespace & ns, const String & ref,
+    const String & payload_a, const String & payload_b)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", payload_a),
+                                                blobEntry("data.cmrk3", payload_b)});
+    build->precommitAdd(ns, ref, id);
+    build->putBlob(idOf(payload_a), BlobSource::fromString(payload_a));
+    build->putBlob(idOf(payload_b), BlobSource::fromString(payload_b));
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+/// Publish ONE ref naming a single-blob part through the real writer sequence. Returns its ManifestId.
+ManifestId publishOneBlobPart(
+    const PoolPtr & s, const RootNamespace & ns, const String & ref, const String & payload)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", payload)});
+    build->precommitAdd(ns, ref, id);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+/// The semantic `publishCommittedTransition` wrapper advances its same-life checkpoint. Keep this
+/// assertion explicit so this leak fixture cannot silently depend on a historical checkpoint lag.
+void assertSemanticTransitionCheckpoint(
+    Backend & backend, const Layout & layout, const RootNamespace & ns, const RefTxnId & committed_through)
+{
+    const auto life = CasRefCatalog::lifeIfCataloged(backend, layout, ns);
+    ASSERT_TRUE(life);
+    const String key = layout.refCkptKey(*life);
+    const auto before = backend.get(key);
+    ASSERT_TRUE(before);
+
+    RefCkpt ckpt = decodeRefCkpt(before->bytes);
+    ASSERT_TRUE(ckpt.committed_through);
+    EXPECT_EQ(*ckpt.committed_through, committed_through);
+}
+
+/// Whether a blob's body object is present in the backend (HEADs blobKey directly — the GC retire path
+/// HEADs the object key, never the Pool's manifest decode cache).
+bool blobPresent(const std::shared_ptr<InMemoryBackend> & b, const Layout & layout, const String & payload)
+{
+    return b->head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of(payload))})).exists;
+}
+
+/// Whether a manifest body object is present in the backend.
+bool manifestPresent(const std::shared_ptr<InMemoryBackend> & b, const Layout & layout, const ManifestId & id)
+{
+    return b->head(layout.manifestKey(id)).exists;
+}
+
+/// Stage partB's full closure (its two distinct blob bodies + its manifest body) through the REAL
+/// writer primitives WITHOUT publishing an owner — `beginPartWrite -> putBlob(each) -> stageManifest`. The
+/// bytes are durable in the backend but no journal owner names them yet; the caller installs partB as
+/// the new owner via a REPOINT (see displaceAndGc). Returns partB's ManifestId.
+ManifestId stagePartBClosure(
+    const PoolPtr & s, const RootNamespace & ns, const String & ref,
+    const String & payload_a, const String & payload_b)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    build->putBlob(idOf(payload_a), BlobSource::fromString(payload_a));
+    build->putBlob(idOf(payload_b), BlobSource::fromString(payload_b));
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", payload_a),
+                                                blobEntry("data.cmrk3", payload_b)});
+    /// No precommitAdd / promote: the repoint below installs partB committed in ONE owner-move event.
+    return id;
+}
+
+/// Reproduce displacement on the SAME (s, ns, ref) and run GC to a fixpoint. partB's distinct blobs
+/// displace partA's via a REPOINT of the ref (one RootOwnerEvent old={Committed,ref,partA}/
+/// new={Committed,ref,partB}) — the real production shape of last-owner-wins, NOT a body delete.
+///
+/// Crucially the test does NOT delete partA's manifest body. In the part-manifest model a true removal
+/// (the repoint's -1) is derived by GC READING partA's body at removal-fold time; only GC may delete a
+/// committed owner's body, and only AFTER the -1 is sealed (recheck cleanup, control #11). So GC folds
+/// the repoint: -1 for partA's blobs (body present), +1 for partB's blobs, retires + deletes partA's
+/// now-zero-in-degree blobs, and recheck cleanup deletes partA's owner-removed body. Returns the fsck
+/// report so the caller can assert the no-leak end state (partA's blobs AND body gone, unreachable==0).
+FsckReport displaceAndGc(
+    const PoolPtr & s, const std::shared_ptr<InMemoryBackend> & b,
+    const RootNamespace & ns, const String & ref, const ManifestId & part_a)
+{
+    /// Stage partB's full closure (blobs + body present), then repoint the ref from partA to partB.
+    const ManifestId part_b = stagePartBClosure(s, ns, ref, "data-B", "mark-B");
+
+    EXPECT_TRUE(b->head(s->layout().manifestKey(part_a)).exists)
+        << "partA manifest body must still be present so GC can read its -1 edges at removal-fold";
+
+    /// REPOINT: old={Committed,ref,partA} / new={Committed,ref,partB} in the single ordered journal.
+    const uint64_t repoint_sequence = publishCommittedTransition(*b, s->layout(), ns, ref, part_a.ref, part_b.ref);
+    assertSemanticTransitionCheckpoint(*b, s->layout(), ns, RefTxnId{1, repoint_sequence});
+
+    /// The repoint dropped partA's owner; advance the watermark floor so partA's now-orphaned blobs are
+    /// not spared as in-flight, then run GC to a fixpoint.
+    s->renewWatermarkOnce();
+    Gc gc(s, hexToU128("00000000000000000000000000000001"));
+    runGcToFixpoint(s, gc);
+    return runFsck(*s, /*detail=*/false);
+}
+
+}
+
+/// NO-LEAK (S1, fold interleaved): partA is published and folded ONCE (its body present, +1 per blob),
+/// then partB REPOINTS the ref away from partA (partA's body stays present so GC reads its -1 edges at
+/// removal-fold; only GC deletes the owner-removed body, after the -1 is sealed). GC must reclaim partA's
+/// blobs to a fixpoint: no blob/manifest object remains for partA and the in-degree generation holds no
+/// stranded positive counter for partA's blobs.
+TEST(CASGCLeak, DisplacedPartBlobsReclaimedFoldBetween)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"test/tbl"};
+    const String ref = "all_0_0_0";
+
+    const ManifestId part_a = publishTwoBlobPart(s, ns, ref, "data-A", "mark-A");
+
+    /// A GC fold runs HERE, before any displacement — partA's body is present, so the fold records +1 for
+    /// each of partA's blobs into the durable in-degree generation.
+    {
+        Gc gc(s, hexToU128("00000000000000000000000000000001"));
+        runGcToFixpoint(s, gc);
+    }
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("data-A")), 1) << "partA's data blob is pinned (+1)";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("mark-A")), 1) << "partA's mark blob is pinned (+1)";
+
+    const FsckReport after = displaceAndGc(s, b, ns, ref, part_a);
+
+    EXPECT_EQ(after.dangling, 0u) << "S1 INV-NO-LOSS: displacement must never lose a reachable object";
+    EXPECT_GT(after.reachable, 0u) << "S1: the live ref points at partB; partB's closure is reachable";
+    EXPECT_EQ(after.unreachable, 0u)
+        << "S1 INV-NO-LEAK: an interleaved fold recorded partA's edges; the removal -1 + retire must "
+           "reclaim partA's blobs (unreachable=" << after.unreachable << ")";
+
+    /// Backend-level no-debris: partA's blobs and body object are gone; the in-degree counters are 0.
+    EXPECT_FALSE(blobPresent(b, s->layout(), "data-A")) << "S1: partA data blob object must be deleted";
+    EXPECT_FALSE(blobPresent(b, s->layout(), "mark-A")) << "S1: partA mark blob object must be deleted";
+    EXPECT_FALSE(manifestPresent(b, s->layout(), part_a)) << "S1: partA manifest body must be gone";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("data-A")), 0) << "S1: no stranded positive in-degree";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("mark-A")), 0) << "S1: no stranded positive in-degree";
+}
+
+/// NO-LEAK (S2, NO fold interleaved — the decisive worst case): partA is published, then IMMEDIATELY
+/// repointed to partB before ANY GC fold runs. The single fold therefore folds partA's activation (+1)
+/// and its removal (-1, read from partA's still-present body) in one pass; the retire reclaims partA's
+/// blobs and recheck cleanup deletes partA's owner-removed body. No debris may remain.
+TEST(CASGCLeak, DisplacedPartBlobsReclaimedNoFoldBetween)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"test/tbl"};
+    const String ref = "all_0_0_0";
+
+    const ManifestId part_a = publishTwoBlobPart(s, ns, ref, "data-A", "mark-A");
+
+    const FsckReport after = displaceAndGc(s, b, ns, ref, part_a);
+
+    EXPECT_EQ(after.dangling, 0u) << "S2 INV-NO-LOSS: displacement must never lose a reachable object";
+    EXPECT_GT(after.reachable, 0u) << "S2: the live ref points at partB; partB's closure is reachable";
+    EXPECT_EQ(after.unreachable, 0u)
+        << "S2 INV-NO-LEAK: partA's blobs must be reclaimed even with no interleaved fold — the recorded "
+           "owner edges drive the removal -1 + retire (unreachable=" << after.unreachable << ")";
+
+    EXPECT_FALSE(blobPresent(b, s->layout(), "data-A")) << "S2: partA data blob object must be deleted";
+    EXPECT_FALSE(blobPresent(b, s->layout(), "mark-A")) << "S2: partA mark blob object must be deleted";
+    EXPECT_FALSE(manifestPresent(b, s->layout(), part_a)) << "S2: partA manifest body must be gone";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("data-A")), 0) << "S2: no stranded positive in-degree";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("mark-A")), 0) << "S2: no stranded positive in-degree";
+}
+
+/// NO-LEAK (drop): a fully-committed part is published, folded (+1 per blob), then its ref is dropped.
+/// GC must reclaim the WHOLE closure — both blobs and the manifest body — leaving no debris and no
+/// stranded positive in-degree.
+TEST(CASGCLeak, DroppedPartFullyReclaimed)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"test/tbl"};
+    const String ref = "all_1_1_0";
+
+    const ManifestId id = publishTwoBlobPart(s, ns, ref, "drop-data", "drop-mark");
+    {
+        Gc gc(s, hexToU128("00000000000000000000000000000002"));
+        runGcToFixpoint(s, gc);
+    }
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("drop-data")), 1);
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("drop-mark")), 1);
+
+    s->dropRef(ns, ref);
+    s->renewWatermarkOnce();   /// advance the floor so the now-unreferenced closure is not spared
+
+    Gc gc(s, hexToU128("00000000000000000000000000000002"));
+    runGcToFixpoint(s, gc);
+
+    const FsckReport after = runFsck(*s, /*detail=*/false);
+    EXPECT_EQ(after.dangling, 0u) << "drop INV-NO-LOSS: nothing reachable was lost";
+    EXPECT_EQ(after.unreachable, 0u)
+        << "drop INV-NO-LEAK: the dropped closure's blobs + body must be fully reclaimed "
+           "(unreachable=" << after.unreachable << ")";
+    EXPECT_FALSE(blobPresent(b, s->layout(), "drop-data")) << "dropped data blob must be deleted";
+    EXPECT_FALSE(blobPresent(b, s->layout(), "drop-mark")) << "dropped mark blob must be deleted";
+    EXPECT_FALSE(manifestPresent(b, s->layout(), id)) << "dropped manifest body must be gone";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("drop-data")), 0) << "no stranded positive in-degree";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of("drop-mark")), 0) << "no stranded positive in-degree";
+}
+
+/// NO-LEAK (resurrect-reupload): a blob incarnation A is published, dropped, and condemned by ONE GC
+/// round (retired, NOT yet deleted — it is still mid-pipeline). A fresh build then dedup-hits the SAME
+/// content hash: `putBlob` HEADs A, sees it condemned via the per-hash freshness meta point-read, and —
+/// per INV-1 (revival-from-source) — re-uploads a DISTINCT incarnation B at the same content-addressed key
+/// (fresh `incarnation_tag`, never a GET of the dying object A). B is referenced by a second ref, then
+/// that ref is dropped too. GC must fold B's own activation/removal exactly like any other incarnation
+/// and reclaim it to a fixpoint: no blob object may remain for the content hash and the in-degree
+/// generation must hold no stranded positive counter.
+///
+/// This reproduces RESURRECT-REUPLOAD-ORPHAN: if GC's bookkeeping keys off the content hash rather than
+/// the (hash, token) incarnation identity, it may treat the hash as "already handled" from A's retire
+/// cycle and never open a fresh condemn cycle for B once B's in-degree drops to zero — B then orphans
+/// forever (unreachable > 0, its body never deleted).
+TEST(CASGCLeak, ResurrectReplacedIncarnationReclaimed)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"test/tbl"};
+    const String P = "resurrect-payload";
+
+    /// 1. Publish ref r1 -> token A referenced; capture A.
+    publishOneBlobPart(s, ns, "r1", P);
+    const HeadResult hA = b->head(s->layout().blobKey(idOf(P)));
+    ASSERT_TRUE(hA.exists);
+
+    /// 2. Drop r1 -> A dereferenced.
+    s->dropRef(ns, "r1");
+    s->renewWatermarkOnce();   /// advance the floor so A is not spared as in-flight
+
+    /// 3. ONE GC round: A transitions to in-degree 0 and is condemned (retired), NOT yet deleted.
+    Gc gc(s, hexToU128("00000000000000000000000000000004"));
+    DB::Cas::tests::runRegularRoundReclaiming(gc);
+    {
+        const auto lm = DB::Cas::tests::loadMetaForTest(*b, s->layout(), u128Of(P));
+        ASSERT_TRUE(lm.has_value() && lm->meta.state == MetaState::Condemned)
+            << "precondition: token A must be condemned before the resurrect";
+    }
+    ASSERT_TRUE(blobPresent(b, s->layout(), P)) << "A not yet deleted (still in the pipeline)";
+
+    /// 4. RESURRECT: a fresh build dedup-hits P; putBlob sees A condemned -> re-uploads a DISTINCT
+    /// incarnation B at the same content-addressed key (INV-1 revival-from-source).
+    publishOneBlobPart(s, ns, "r2", P);
+    const HeadResult hB = b->head(s->layout().blobKey(idOf(P)));
+    ASSERT_TRUE(hB.exists);
+    ASSERT_NE(hB.token.value, hA.token.value) << "resurrect must mint a new incarnation token B";
+
+    /// 5. Drop r2 -> B dereferenced.
+    s->dropRef(ns, "r2");
+    s->renewWatermarkOnce();
+
+    /// 6. Run GC to fixpoint. The replaced incarnation B MUST be reclaimed.
+    runGcToFixpoint(s, gc);
+
+    const FsckReport after = runFsck(*s, /*detail=*/false);
+    EXPECT_EQ(after.dangling, 0u) << "resurrect INV-NO-LOSS: nothing reachable was lost";
+    EXPECT_EQ(after.unreachable, 0u)
+        << "resurrect INV-NO-LEAK: the resurrect-replaced incarnation B must not orphan "
+           "(unreachable=" << after.unreachable << ")";
+    EXPECT_FALSE(blobPresent(b, s->layout(), P)) << "B's object must be deleted";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of(P)), 0) << "no stranded positive in-degree";
+}
+
+/// IDEMPOTENCY of the RESURRECT-REUPLOAD-ORPHAN fold: drives the exact same condemn-A / resurrect-B /
+/// drop-B / reclaim sequence as `ResurrectReplacedIncarnationReclaimed` above, then keeps running the
+/// regular round PAST the fixpoint. The re-condemn that reclaims the resurrect-replaced incarnation B
+/// must fire exactly once: extra rounds on an already-reclaimed content hash must be no-ops (no
+/// re-condemn churn, no duplicate retired entry) and must never manufacture fresh fsck debris.
+TEST(CASGCLeak, ResurrectReplacedReclaimIsIdempotent)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"test/tbl"};
+    const String P = "resurrect-payload-idem";
+
+    /// 1. Publish ref r1 -> token A referenced, then drop it.
+    publishOneBlobPart(s, ns, "r1", P);
+    s->dropRef(ns, "r1");
+    s->renewWatermarkOnce();   /// advance the floor so A is not spared as in-flight
+
+    /// 2. ONE GC round: A transitions to in-degree 0 and is condemned (retired), NOT yet deleted.
+    Gc gc(s, hexToU128("00000000000000000000000000000005"));
+    DB::Cas::tests::runRegularRoundReclaiming(gc);
+
+    /// 3. RESURRECT: r2 dedup-hits P while A is condemned -> mints a fresh incarnation B.
+    publishOneBlobPart(s, ns, "r2", P);
+    s->dropRef(ns, "r2");
+    s->renewWatermarkOnce();
+
+    /// 4. Reclaim B to a fixpoint (the RESURRECT-REUPLOAD-ORPHAN fold under test).
+    runGcToFixpoint(s, gc);
+    ASSERT_FALSE(blobPresent(b, s->layout(), P)) << "B must be reclaimed before the idempotency check";
+
+    /// 5. Extra rounds past the fixpoint: nothing is left to do for this hash. The fold must not
+    /// re-condemn it (that would be the churn/duplicate-entry bug) and must not resurrect any debris.
+    for (int round = 0; round < 3; ++round)
+    {
+        const RoundReport r = DB::Cas::tests::runRegularRoundReclaiming(gc);
+        if (r.acquired_lease)
+            EXPECT_EQ(r.condemned, 0u) << "no re-condemn of an already-reclaimed hash on extra round " << round;
+        s->renewWatermarkOnce();
+    }
+
+    EXPECT_FALSE(blobPresent(b, s->layout(), P)) << "stays deleted across extra rounds";
+    EXPECT_EQ(inDegreeOf(*b, s->layout(), u128Of(P)), 0) << "no stranded positive in-degree";
+
+    const FsckReport after = runFsck(*s, /*detail=*/false);
+    EXPECT_EQ(after.unreachable, 0u) << "re-condemn churn must not manufacture a fresh unreachable object";
+    EXPECT_EQ(after.dangling, 0u) << "idempotent extra rounds must never lose a reachable object";
+}
+
+/// WRITER-SIDE half of the RESURRECT-REUPLOAD-ORPHAN fold: after the round that folds the resurrect-
+/// replaced incarnation B's dereference re-condemns B, a fresh writer dedup-hitting the SAME content hash
+/// must see B as condemned via the per-hash freshness meta point-read — never as an adoptable live token.
+/// If GC's bookkeeping instead kept treating B as adopt-eligible (the pre-fix bug), a concurrent writer's
+/// `putBlob` would adopt the being-reclaimed B rather than resurrect a fresh incarnation, racing the
+/// delete pipeline.
+///
+/// Depending on round timing, by the time the meta is checked B may be (a) still present and visibly
+/// condemned, or (b) already physically deleted by the delete pipeline (meta dropped alongside it) — BOTH
+/// outcomes prove B is never adoptable. The assertion only fails on the pre-fix shape: B present and NOT
+/// condemned.
+TEST(CASGCLeak, ResurrectReplacedTokenIsCondemnedInMeta)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"test/tbl"};
+    Gc gc(s, hexToU128("00000000000000000000000000000006"));
+    const String P = "resurrect-payload-view";
+
+    /// 1. Publish ref r1 -> token A referenced; capture A, then drop it and condemn via ONE GC round.
+    publishOneBlobPart(s, ns, "r1", P);
+    const HeadResult hA = b->head(s->layout().blobKey(idOf(P)));
+    ASSERT_TRUE(hA.exists);
+    s->dropRef(ns, "r1");
+    s->renewWatermarkOnce();   /// advance the floor so A is not spared as in-flight
+    gc.runRegularRound();
+
+    /// 2. RESURRECT: r2 dedup-hits P while A is condemned -> mints a fresh incarnation B.
+    publishOneBlobPart(s, ns, "r2", P);
+    const HeadResult hB = b->head(s->layout().blobKey(idOf(P)));
+    ASSERT_TRUE(hB.exists);
+    ASSERT_NE(hB.token.value, hA.token.value) << "resurrect must mint a distinct incarnation";
+    s->dropRef(ns, "r2");
+    s->renewWatermarkOnce();
+
+    /// 3. The round that folds B's dereference re-condemns token B.
+    gc.runRegularRound();
+
+    const auto lm = DB::Cas::tests::loadMetaForTest(*b, s->layout(), u128Of(P));
+    EXPECT_TRUE((lm.has_value() && lm->meta.state == MetaState::Condemned) || !blobPresent(b, s->layout(), P))
+        << "the replaced incarnation B must be visible as condemned (or already reclaimed) so a "
+           "dedup-hitting writer resurrects, not adopts";
+}
+
+/// (The NO-LEAK-on-abandon test `CASGCLeak.AbandonedPrecommitReclaimsOwnBlobs` was removed with the
+/// snapshot+log ref model: it asserted GC AUTOMATICALLY reclaims a crashed build's abandoned precommit and
+/// collects its own unique blob. Per spec §Responsibility Boundary that reclaim is now the WRITER's job
+/// (it appends the exact `owner_transition` removal on recovery); GC never scans for or removes precommit
+/// bindings. The writer-side abandon/recovery cleanup is exercised by the writer tests.)
+
+/// REUSE-vs-GC race (no-LOSS half of the no-leak family): a build ADOPTS a committed blob B by tokenless
+/// evidence (B present, not yet condemned), the committed ref pinning B is DROPPED, GC retires+deletes B
+/// AND completes the round, and only THEN does the build try to publish a manifest naming B.
+///
+/// The promote gate re-observes the loss (it re-HEADs every blob leaf and fails closed on a deleted dep,
+/// throwing a retryable ABORTED) — it must NEVER silently commit a dangling ref. The assertion is the
+/// no-LOSS guarantee: `dangling==0`. (A tokenless adopt has no body to re-upload, so a real caller would
+/// re-derive B from source on retry; here we only confirm the gate fails closed.)
+TEST(CASReuseGcRace, ReuseOfBlobDeletedBeforePublish)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"test/tbl"};
+    const String B = "shared-blob-payload";
+    const String U = "build2-unique-blob";
+
+    /// build1: commit part_1 -> manifest -> blob B.
+    publishOneBlobPart(s, ns, "part_1", B);
+
+    /// build2: adopt B by tokenless evidence (no HEAD) and upload its OWN unique blob U. It does NOT yet
+    /// stage a manifest or precommit — the scenario is that GC deletes B BEFORE build2 publishes a manifest
+    /// naming it. (Staging+precommitting BEFORE the drop would make the precommit's activating +1 PIN B —
+    /// B would never reach in-degree 0 and GC could not delete it, so the race could not be reproduced.)
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/part_2";
+    auto build2 = s->beginPartWrite(info);
+
+    ManifestEntry eb;
+    eb.path = "data.bin";
+    eb.placement = EntryPlacement::Blob;
+    eb.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(B))};
+
+    eb.blob_size = B.size();
+    build2->adoptEvidence(eb);                                   /// tokenless dep (no HEAD)
+    build2->putBlob(idOf(U), BlobSource::fromString(U));         /// build2's own unique, protected blob
+
+    /// Drop the committed pin on B and advance the watermark so B (owned by the finished build1) is not
+    /// spared. No owner names B now (build2 has not staged/precommitted), so GC folds B to in-degree 0
+    /// once part_1 is dropped.
+    s->dropRef(ns, "part_1");
+    s->renewWatermarkOnce();
+
+    /// GC reclaims build1's manifest and the now-unreferenced B to a fixpoint, completing the rounds.
+    {
+        Gc gc(s, u128Of("gc-reuse-race"));
+        runGcToFixpoint(s, gc);
+    }
+    ASSERT_FALSE(blobPresent(b, s->layout(), B))
+        << "GC must have deleted the now-unreferenced reused blob B";
+
+    /// Only NOW does build2 publish a manifest naming the (just-deleted) B: stage the body + precommit.
+    const ManifestId id2 = build2->stageManifest({eb, blobEntry("uniq.bin", U)});
+    build2->precommitAdd(ns, "part_2", id2);
+
+    /// build2 promotes part_2 -> id2 -> {B, U}. §4 manifest-trust: B is a committed-source adopted leaf,
+    /// so the promote gate TRUSTS it (no HEAD/loadMeta probe) and commits — it does NOT re-observe the
+    /// deleted B. This is the accepted D4 trade-off. On the real reuse/relink path B CANNOT be deleted
+    /// while build2's precommit edge is live: precommitAdd durably appends the Precommit OwnerTransition
+    /// (CasPartWriteTxn.cpp precommitAdd) BEFORE promote, and promote re-proves that edge live (WPromote
+    /// owner==bld) BEFORE it trusts the leaf — so B has in-degree >= 1 and GC (the sole deleter) cannot
+    /// collect it. This test injects the loss DIRECTLY (raw GC-to-fixpoint after dropping EVERY owner,
+    /// with build2 not yet precommitted), which the live-precommit invariant excludes. So the dangle is
+    /// not prevented at promote under §4 — it is DETECTED by fsck (the backstop).
+    EXPECT_NO_THROW(build2->promote(ns, "part_2", build2->buildId(), id2));
+
+    /// THE BACKSTOP (INV-NO-DANGLE-via-fsck): fsck's reachable-but-absent scan reports the committed-yet-
+    /// deleted B as dangling. This is where an absent adopted blob surfaces under §4 — not at the promote
+    /// gate. Detection moved, it did not disappear.
+    const FsckReport rep = runFsck(*s, /*detail=*/true);
+    EXPECT_GE(rep.dangling, 1u)
+        << "§4 D4 backstop: promote trusts the adopted leaf and commits; the deleted B must surface as an "
+           "fsck dangling finding (dangling=" << rep.dangling << ", reachable=" << rep.reachable << ")";
+}
diff --git a/src/Disks/tests/gtest_cas_gc_log.cpp b/src/Disks/tests/gtest_cas_gc_log.cpp
new file mode 100644
index 000000000000..23aefcfb31d0
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_log.cpp
@@ -0,0 +1,489 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <Common/Exception.h>
+
+#include <string>
+#include <vector>
+
+/// Unit coverage for the CA GC scheduler's logging sink (the source of
+/// `system.cas_gc_log`). The scheduler emits a Start + Finish
+/// `GcRoundLogRecord` per round through the injected `GcRoundLogger`; here we capture the records in
+/// a vector and assert their shape over a real (in-memory) Pool driven through a dropped-then-
+/// collectable object — the same Pool/Backend fixture the B140 reclaim test uses.
+///
+/// NOTE on ProfileEvents: `runOneRoundNow` runs on THIS (bare gtest) thread, which has no attached
+/// `ThreadStatus`, so the scheduler's `CurrentThread::isInitialized()` guard skips per-round
+/// ProfileEvents capture. The `profile_events` map is therefore EXPECTED to be empty here and this
+/// test does NOT assert it non-empty (the on-server paths are attached; the functional/soak coverage
+/// asserts non-empty there).
+
+namespace DB::ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+using Rec = DB::Cas::GcRoundLogRecord;
+
+namespace
+{
+
+/// Publish one part `ref` with a single content blob whose payload is `payload`. Returns the manifest id.
+ManifestId publishPart(const PoolPtr & s, const String & ns, const String & ref, const String & payload)
+{
+    const RootNamespace nsr{ns};
+    PartWriteInfo info;
+    info.intended_ref = ns + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+
+    ManifestEntry e;
+    e.path = "data.bin";
+    e.placement = EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    e.blob_size = payload.size();
+
+    const ManifestId id = build->stageManifest({e});
+    build->precommitAdd(nsr, ref, id);
+    build->promote(nsr, ref, build->buildId(), id);
+    return id;
+}
+
+}
+
+namespace
+{
+/// One round emits a Start, then one Phase row per GC phase it reached, then a Finish. Tests that care
+/// only about the round-outcome rows filter the phase rows out through this.
+std::vector<Rec> roundRowsOnly(const std::vector<Rec> & rows)
+{
+    std::vector<Rec> out;
+    for (const Rec & r : rows)
+        if (r.event_type != Rec::EventType::Phase)
+            out.push_back(r);
+    return out;
+}
+}
+
+/// The happy path: a marking round (candidates_marked > 0). Each `runOneRoundNow` must emit exactly one
+/// Start, then its phase rows, then one Finish, with `disk_name`/`gc_id` set and `duration_ms`
+/// populated on the Finish.
+///
+/// It drives the PRODUCTION scheduler, so it covers both halves of the pipeline: a MARKING round
+/// (candidates condemned, nothing deleted) and, some rounds later once the mount's ack floor graduates
+/// them, a DELETING round whose Finish carries the count through. The ordering is asserted, because a
+/// deletion reported before its marking would mean the row is not describing the round it names.
+TEST(CASGCLog, EmitsStartFinishWithCounts)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// gc_fold_max_defer_rounds=0: this test drives up to 16 consecutive rounds through the scheduler
+    /// (no direct Gc handle to override per-instance) expecting each to fold; force fold-every-round
+    /// (Phase-4 Lever A would otherwise defer once the pool quiesces, stalling the mark-then-delete
+    /// pipeline within the round budget).
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// Publish a part, then drop it so its blob/tree become collectable.
+    publishPart(store, ns.string(), "all_0_0_0", "hello-cas-gc-log");
+    store->dropRef(ns, "all_0_0_0");
+    /// Advance the durable watermark floor past the build's seq so the build-watermark guard no
+    /// longer spares the now-dropped objects (the background renewer is off in this test).
+    store->renewWatermarkOnce();
+
+    std::vector<Rec> rows;
+    DB::Cas::CasGcScheduler sched(
+        store, std::chrono::seconds(1), "test::gc", "ca",
+        [&](const Rec & r) { rows.push_back(r); });
+
+    /// Drive rounds until we observe both a marking round and a deletion round. Under the ack-floor
+    /// pipeline a candidate is marked (condemned) in one round and physically deleted a few rounds later,
+    /// once the mount's ack floor graduates it — so advance the store's own mount ack after each round
+    /// (renewWatermarkOnce runs the beat) and give the pipeline a generous round budget. Each
+    /// runOneRoundNow call appends a Start, the round's phase rows, and a Finish.
+    bool saw_marked = false;
+    bool saw_deleted = false;
+    size_t marking_finish_idx = 0;
+    size_t deleting_finish_idx = 0;
+    uint64_t total_deleted = 0;
+    constexpr size_t max_rounds = 16;
+    for (size_t round = 0; round < max_rounds; ++round)
+    {
+        const size_t before = rows.size();
+        sched.runOneRoundNow(Rec::Trigger::Manual);
+        store->renewWatermarkOnce();
+
+        /// Each call emits exactly one Start (first) and one Finish (last), with the round's phase rows
+        /// in between.
+        ASSERT_GE(rows.size(), before + 2u) << "each round must emit at least a Start and a Finish";
+        ASSERT_EQ(rows[before].event_type, Rec::EventType::Start);
+        ASSERT_EQ(rows.back().event_type, Rec::EventType::Finish);
+        for (size_t i = before + 1; i + 1 < rows.size(); ++i)
+            ASSERT_EQ(rows[i].event_type, Rec::EventType::Phase)
+                << "only Phase rows may sit between a round's Start and Finish";
+
+        const size_t finish_idx = rows.size() - 1;
+        const Rec & fin = rows[finish_idx];
+        if (!saw_marked && fin.candidates_marked > 0)
+        {
+            saw_marked = true;
+            marking_finish_idx = finish_idx;
+        }
+        if (!saw_deleted && fin.objects_deleted > 0)
+        {
+            saw_deleted = true;
+            deleting_finish_idx = finish_idx;
+        }
+        total_deleted += fin.objects_deleted;
+    }
+
+    ASSERT_TRUE(saw_marked) << "expected a round that marked at least one candidate";
+    EXPECT_GT(rows[marking_finish_idx].candidates_marked, 0u);
+    EXPECT_GT(rows[marking_finish_idx].entries_condemned, 0u);
+    ASSERT_TRUE(saw_deleted) << "expected a round that physically deleted at least one object";
+    EXPECT_GE(deleting_finish_idx, marking_finish_idx)
+        << "an object cannot be reported deleted before the round that condemned it";
+    EXPECT_GT(total_deleted, 0u)
+        << "the deleted count must reach the Finish row, not stop inside the round";
+
+    /// Identity + timing fields are set on every record.
+    for (const Rec & r : rows)
+    {
+        EXPECT_EQ(r.disk_name, "ca");
+        EXPECT_FALSE(r.gc_id.empty());
+        EXPECT_EQ(r.trigger, Rec::Trigger::Manual);
+    }
+    /// The round-outcome rows alternate Start, Finish, Start, Finish, ... once the phase rows are
+    /// filtered out; `duration_ms` is meaningful on each Finish (populated unconditionally there).
+    const std::vector<Rec> round_rows = roundRowsOnly(rows);
+    ASSERT_EQ(round_rows.size() % 2, 0u);
+    for (size_t i = 0; i < round_rows.size(); ++i)
+        EXPECT_EQ(round_rows[i].event_type,
+                  i % 2 == 0 ? Rec::EventType::Start : Rec::EventType::Finish);
+}
+
+namespace
+{
+
+/// A backend that throws on `list`, the first thing the GC round does (namespace discovery via the
+/// roots registry / listing). Used to drive the Aborted-Finish path: the round throws, the scheduler
+/// emits an Aborted Finish with the exception text, and `runOneRoundNow` rethrows.
+class ThrowingBackend : public InMemoryBackend
+{
+public:
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (arm)
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS, "injected backend list failure");
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (arm)
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS, "injected backend get failure");
+        return InMemoryBackend::get(key, range);
+    }
+
+    HeadResult head(const String & key) override
+    {
+        if (arm)
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS, "injected backend head failure");
+        return InMemoryBackend::head(key);
+    }
+
+    /// Armed only after Pool::open, so opening (which reads/initialises gc state) succeeds.
+    std::atomic<bool> arm{false};
+};
+
+}
+
+/// A7-HIGH-fix: the manual `SYSTEM ... GC` path (runOneRoundNow) reuses ONE stable Gc instance across
+/// calls (A7 — the lease's observation-window steal protocol compares consecutive observations of the
+/// SAME observer), but it must be OBSERVE-ONLY with respect to STEALING: the protocol's safety argument
+/// requires the two observations that flag an incumbent "frozen" to be spaced by real wall time (>= the
+/// heartbeat cadence H) so a live incumbent gets a chance to pulse in between — a guarantee only the
+/// background loop's own interval-paced ticks provide. Two manual calls have no such guarantee (they
+/// can land microseconds apart in a real query), so a manual round must NEVER execute the steal CAS,
+/// no matter how many times it re-observes the same frozen tuple. Dead-incumbent recovery stays the
+/// loop's job (bounded ~2*interval; covered by the CASGCLease loop-driven steal tests in
+/// gtest_cas_gc_round.cpp, e.g. StealAfterObservedNonRenewalBumpsEpoch / FailoverStealOnceHeartbeatStops).
+/// Deterministic: "time" is the order of runRegularRound calls; no sleep, no clock, no threads.
+TEST(CASGCSchedulerSteal, ManualRoundNeverStealsEvenADeadIncumbent)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+
+    /// A foreign incumbent takes the lease and then DIES (never renews, never heartbeats).
+    const UInt128 kIncumbent = hexToU128("00000000000000000000000000000abc");
+    Gc incumbent(store, kIncumbent);
+    ASSERT_TRUE(incumbent.runRegularRound().acquired_lease);
+
+    DB::Cas::CasGcScheduler sched(store, std::chrono::seconds(1), "test::gc", "ca");
+
+    /// obs #1: records the incumbent's (owner, seq, hb=absent).
+    EXPECT_FALSE(sched.runOneRoundNow(Rec::Trigger::Manual).acquired_lease);
+    /// obs #2 and #3: the same frozen (owner, seq, hb) observed repeatedly would be steal-eligible on
+    /// the loop path (see the Core-level test this mirrors), but the manual path keeps backing off.
+    EXPECT_FALSE(sched.runOneRoundNow(Rec::Trigger::Manual).acquired_lease);
+    EXPECT_FALSE(sched.runOneRoundNow(Rec::Trigger::Manual).acquired_lease);
+}
+
+/// Negative-control companion to the test above (reviewer-requested): with the incumbent visibly alive
+/// (its heartbeat advancing between the manual round's observations, exactly like
+/// CASGCLease.HeartbeatBlocksFalseStealOfAliveLeader at the Core level), the manual round must still
+/// correctly back off — confirming the new observe-only branch didn't regress the PRE-EXISTING
+/// incumbent_renewed/hb_alive liveness detection (this test would already pass on the protocol's own
+/// terms even without the A7-HIGH-fix; it pins that the fix didn't break it).
+TEST(CASGCSchedulerSteal, ManualRoundNeverStealsALiveHeartbeatingIncumbent)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+
+    const UInt128 kIncumbent = hexToU128("00000000000000000000000000000abc");
+    Gc incumbent(store, kIncumbent);
+    ASSERT_TRUE(incumbent.runRegularRound().acquired_lease);
+
+    DB::Cas::CasGcScheduler sched(store, std::chrono::seconds(1), "test::gc", "ca");
+
+    /// obs #1: records (owner=incumbent, seq, hb=absent).
+    EXPECT_FALSE(sched.runOneRoundNow(Rec::Trigger::Manual).acquired_lease);
+    Gc::pulseHeartbeat(*store, kIncumbent);   /// the incumbent is alive and pulsing (hb 0->1)
+    /// obs #2: hb advanced since obs #1 => alive => no steal (never reaches the observe-only branch).
+    EXPECT_FALSE(sched.runOneRoundNow(Rec::Trigger::Manual).acquired_lease);
+    Gc::pulseHeartbeat(*store, kIncumbent);   /// hb 1->2
+    EXPECT_FALSE(sched.runOneRoundNow(Rec::Trigger::Manual).acquired_lease);
+}
+
+/// A round whose backend throws must produce a Finish with `outcome == Aborted` and a non-empty
+/// `error`, and `runOneRoundNow` must rethrow the exception (the round failure is observable, not
+/// swallowed — the logging sink itself is best-effort, but the round error propagates).
+TEST(CASGCLog, AbortedFinishOnThrowingRound)
+{
+    auto backend = std::make_shared<ThrowingBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+
+    std::vector<Rec> rows;
+    DB::Cas::CasGcScheduler sched(
+        store, std::chrono::seconds(1), "test::gc", "ca",
+        [&](const Rec & r) { rows.push_back(r); });
+
+    backend->arm.store(true);
+
+    EXPECT_THROW(sched.runOneRoundNow(Rec::Trigger::Manual), DB::Exception);
+
+    /// A throwing round still emits a Start and an Aborted Finish. It also emits the phase row of the
+    /// phase it died in -- the timer is RAII, so it fires during unwinding, which is exactly the forensic
+    /// record a failed round needs. That is also why `round_id`, not `round`, is the correlator: this
+    /// round has no round number at all.
+    const std::vector<Rec> round_rows = roundRowsOnly(rows);
+    ASSERT_EQ(round_rows.size(), 2u) << "a throwing round still emits a Start and a (Aborted) Finish";
+    EXPECT_EQ(round_rows[0].event_type, Rec::EventType::Start);
+    EXPECT_EQ(round_rows[1].event_type, Rec::EventType::Finish);
+    EXPECT_EQ(round_rows[1].outcome, Rec::Outcome::Failed);
+    EXPECT_FALSE(round_rows[1].error.empty()) << "a failed Finish must carry the exception text";
+    EXPECT_EQ(round_rows[1].disk_name, "ca");
+    EXPECT_FALSE(round_rows[1].gc_id.empty());
+    EXPECT_FALSE(round_rows[1].round_id.empty());
+    for (const Rec & r : rows)
+        EXPECT_EQ(r.round_id, round_rows[0].round_id)
+            << "every row of a FAILED round must still correlate through round_id";
+}
+
+/// Every row of one round -- its Start, each of its Phase rows, and its Finish -- carries the SAME
+/// non-empty `round_id`, and two rounds carry DIFFERENT ones. That is the property the column exists
+/// for: `round` is 0 on Start, is only known after the round's single `gc/state` CAS, and is absent on a
+/// round that never led, so it cannot serve as the correlator.
+TEST(CASGCLog, EveryRowOfARoundSharesOneRoundId)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"srv1/tbl"};
+    publishPart(store, ns.string(), "all_0_0_0", "hello-round-id");
+    store->dropRef(ns, "all_0_0_0");
+    store->renewWatermarkOnce();
+
+    std::vector<Rec> rows;
+    DB::Cas::CasGcScheduler sched(
+        store, std::chrono::seconds(1), "test::gc", "ca",
+        [&](const Rec & r) { rows.push_back(r); });
+
+    sched.runOneRoundNow(Rec::Trigger::Manual);
+    const size_t after_first = rows.size();
+    ASSERT_GE(after_first, 2u);
+    const String first_id = rows.front().round_id;
+    EXPECT_FALSE(first_id.empty());
+    for (size_t i = 0; i < after_first; ++i)
+        EXPECT_EQ(rows[i].round_id, first_id) << "row " << i << " of the first round has a different round_id";
+
+    store->renewWatermarkOnce();
+    sched.runOneRoundNow(Rec::Trigger::Manual);
+    ASSERT_GT(rows.size(), after_first);
+    const String second_id = rows[after_first].round_id;
+    EXPECT_FALSE(second_id.empty());
+    EXPECT_NE(second_id, first_id) << "two rounds must not share a round_id";
+    for (size_t i = after_first; i < rows.size(); ++i)
+        EXPECT_EQ(rows[i].round_id, second_id);
+}
+
+namespace
+{
+/// The phase names of one round, in emission order.
+std::vector<String> phaseNames(const std::vector<Rec> & rows, size_t from)
+{
+    std::vector<String> out;
+    for (size_t i = from; i < rows.size(); ++i)
+        if (rows[i].event_type == Rec::EventType::Phase)
+            out.push_back(rows[i].phase);
+    return out;
+}
+
+/// The `phase_metrics` of the named phase of one round. Fails the caller's expectation if absent.
+std::map<String, UInt64> metricsOf(const std::vector<Rec> & rows, size_t from, const String & phase)
+{
+    for (size_t i = from; i < rows.size(); ++i)
+        if (rows[i].event_type == Rec::EventType::Phase && rows[i].phase == phase)
+            return rows[i].phase_metrics;
+    return {};
+}
+}
+
+/// A FOLDING round emits every phase, in execution order, and each phase's row carries the semantic
+/// counts only that phase can compute. This is the test that would catch an instrumentation site
+/// silently dropping out of the round -- a phase that stops emitting reads exactly like a phase that
+/// costs nothing, which is the failure mode this whole change exists to prevent.
+///
+/// ProfileEvents are deliberately NOT asserted: `runOneRoundNow` runs on the bare gtest thread, which
+/// has no attached `ThreadStatus`, so per-phase capture degrades to an empty map exactly as the
+/// round-level capture already does (see the note at the top of this file).
+TEST(CASGCLog, FoldingRoundEmitsEveryPhaseInOrder)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"srv1/tbl"};
+    publishPart(store, ns.string(), "all_0_0_0", "hello-cas-gc-phases");
+    store->dropRef(ns, "all_0_0_0");
+    store->renewWatermarkOnce();
+
+    std::vector<Rec> rows;
+    DB::Cas::CasGcScheduler sched(
+        store, std::chrono::seconds(1), "test::gc", "ca",
+        [&](const Rec & r) { rows.push_back(r); });
+
+    ASSERT_TRUE(sched.runOneRoundNow(Rec::Trigger::Manual).acquired_lease);
+
+    const std::vector<String> expected = {
+        "lease", "pre_fold_ref_drain", "heartbeat_floor", "defer_decision", "parent_seal_read",
+        "fold_ref_group", "fold_seal_read", "fold_ref_intake",
+        "fold_reduce", "fold_seal_write",
+        "pending_deletes", "meta_pool_wait", "round_commit", "handoff_reclaim",
+        "manifest_deletes", "namespace_cleanup", "ref_object_cleanup", "orphan_sweep"};
+    EXPECT_EQ(phaseNames(rows, 0), expected);
+
+    /// Every phase row is a Phase row of THIS round and carries a duration field (0 is a legitimate
+    /// microsecond reading for a phase that did nothing, so only the shape is asserted).
+    for (const Rec & r : rows)
+        if (r.event_type == Rec::EventType::Phase)
+        {
+            EXPECT_EQ(r.round_id, rows.front().round_id);
+            EXPECT_FALSE(r.phase.empty());
+            EXPECT_TRUE(r.error.empty());
+        }
+
+    /// The defer decision reports the signal it decided on, and the two fold-seal reads it paid for.
+    const auto defer = metricsOf(rows, 0, "defer_decision");
+    EXPECT_EQ(defer.at("deferred"), 0u) << "this round folded, so it cannot report itself deferred";
+    EXPECT_EQ(defer.at("fold_seal_reads"), 2u);
+    EXPECT_GT(defer.at("namespaces_seen"), 0u);
+
+    const auto ref_group = metricsOf(rows, 0, "fold_ref_group");
+    EXPECT_EQ(ref_group.at("ref_folding_aborted"), 0u);
+    EXPECT_GT(ref_group.at("ref_keys_listed"), 0u);
+
+    /// Probe B1's identity, as an OBSERVABLE property of the table rather than an assumption in a
+    /// comment: the round sealed coverage over exactly the logs it folded.
+    const auto intake = metricsOf(rows, 0, "fold_ref_intake");
+    EXPECT_EQ(intake.at("logs_accounted"), intake.at("logs_applied"));
+    EXPECT_GT(intake.at("logs_applied"), 0u);
+    EXPECT_GT(intake.at("deltas_emitted"), 0u);
+
+    /// Probe B2's verdict. Nonzero would have thrown, so the row can only ever read 0 on a round that
+    /// reached its Finish -- which is the point: the column is the round's own attestation.
+    EXPECT_EQ(metricsOf(rows, 0, "fold_reduce").at("transactions_unapplied"), 0u);
+
+    /// The honest gap: the meta pool's work runs on other threads, so this row's ProfileEvents delta is
+    /// empty by construction and these two counts are its ONLY signal. They must be real numbers.
+    const auto meta = metricsOf(rows, 0, "meta_pool_wait");
+    EXPECT_GT(meta.at("jobs_scheduled"), 0u) << "this round condemns, so it schedules condemn-marker writes";
+    EXPECT_EQ(meta.at("jobs_completed"), meta.at("jobs_scheduled"))
+        << "every scheduled job must have finished by the time the wait returns";
+}
+
+/// A round that never leads emits ONLY the phase it reached. `round` does not exist for such a round,
+/// so `round_id` is the only thing tying its rows together -- which is why it is the correlator.
+TEST(CASGCLog, NotALeaderRoundEmitsOnlyTheLeasePhase)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+
+    /// A foreign incumbent holds the lease, so the scheduler's round backs off immediately.
+    Gc incumbent(store, hexToU128("00000000000000000000000000000abc"));
+    ASSERT_TRUE(incumbent.runRegularRound().acquired_lease);
+
+    std::vector<Rec> rows;
+    DB::Cas::CasGcScheduler sched(
+        store, std::chrono::seconds(1), "test::gc", "ca",
+        [&](const Rec & r) { rows.push_back(r); });
+    EXPECT_FALSE(sched.runOneRoundNow(Rec::Trigger::Manual).acquired_lease);
+
+    EXPECT_EQ(phaseNames(rows, 0), (std::vector<String>{"lease"}));
+    EXPECT_EQ(metricsOf(rows, 0, "lease").at("acquired"), 0u);
+    ASSERT_EQ(rows.size(), 3u);
+    EXPECT_EQ(rows.back().outcome, Rec::Outcome::NotALeader);
+    for (const Rec & r : rows)
+        EXPECT_EQ(r.round_id, rows.front().round_id);
+}
+
+/// B3: the scheduler exposes per-disk GC health for system.cas_mounts (the process-
+/// global CurrentMetrics gauges were clobbered with >= 2 CAS disks). Drive one leader round and
+/// assert the health snapshot reflects leadership, the pending-reclaim backlog and a fresh success.
+TEST(CASGCHealth, ReflectsLeadershipAndPendingReclaim)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"srv1/tbl"};
+    publishPart(store, ns.string(), "all_0_0_0", "hello-cas-gc-health");
+    store->dropRef(ns, "all_0_0_0");
+    store->renewWatermarkOnce();
+
+    DB::Cas::CasGcScheduler sched(store, std::chrono::seconds(1), "test::gc", "ca", {});
+
+    const auto h0 = sched.gcHealth();
+    EXPECT_FALSE(h0.is_leader);
+    EXPECT_FALSE(h0.ever_succeeded);
+    EXPECT_EQ(h0.pending_reclaim, 0);
+    EXPECT_EQ(h0.wedged_namespace_count, 0u);
+
+    const RoundReport rep = sched.runOneRoundNow(Rec::Trigger::Manual);
+    ASSERT_TRUE(rep.acquired_lease);
+
+    const auto h1 = sched.gcHealth();
+    EXPECT_TRUE(h1.is_leader);
+    EXPECT_TRUE(h1.ever_succeeded);
+    EXPECT_EQ(h1.pending_reclaim,
+              static_cast<Int64>(rep.condemned) - static_cast<Int64>(rep.redeleted));
+    EXPECT_EQ(h1.wedged_namespace_count, 0u);
+    EXPECT_LT(h1.last_success_age_seconds, 60u);
+}
diff --git a/src/Disks/tests/gtest_cas_gc_maintenance_state_format.cpp b/src/Disks/tests/gtest_cas_gc_maintenance_state_format.cpp
new file mode 100644
index 000000000000..158040be38c9
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_maintenance_state_format.cpp
@@ -0,0 +1,202 @@
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcMaintenanceStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h>
+#include <fmt/format.h>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LIMIT_EXCEEDED;
+    extern const int UNKNOWN_FORMAT_VERSION;
+}
+
+namespace
+{
+class FailingMaintenanceReadBackend : public InMemoryBackend
+{
+public:
+    std::optional<GetResult> get(const String &, Range) override
+    {
+        throw std::runtime_error("injected maintenance read failure");
+    }
+};
+}
+
+TEST(CASGCMaintenanceStateFormat, RegistryLayoutAndCanonicalCodec)
+{
+    EXPECT_EQ(static_cast<uint16_t>(FormatId::GcMaintenanceState), 25);
+    const auto points = changePoints(FormatId::GcMaintenanceState);
+    ASSERT_EQ(points.size(), 1u);
+    EXPECT_EQ(points[0].generation, 7);
+    EXPECT_EQ(points[0].min_reader, 7);
+    const FormatTraits & traits = traitsFor(FormatId::GcMaintenanceState);
+    EXPECT_EQ(traits.type, "cas_gc_maintenance_state");
+    EXPECT_EQ(traits.family, TextFamily::Control);
+    EXPECT_EQ(traits.strictness, KeyStrictness::Strict);
+    EXPECT_EQ(traits.compression, CompressionPolicy::Never);
+    EXPECT_EQ(traits.object_cap, 512 * 1024);
+    EXPECT_EQ(traits.line_cap, 512 * 1024);
+    EXPECT_EQ(storedSuffix(FormatId::GcMaintenanceState), "");
+    EXPECT_EQ(traitsForType("cas_gc_maintenance_state"), &traits);
+
+    const Layout layout("p");
+    EXPECT_EQ(layout.gcMaintenanceStateKey(), "p/gc/maintenance_state");
+    EXPECT_NE(layout.gcMaintenanceStateKey(), layout.gcStateKey());
+    EXPECT_NE(layout.gcMaintenanceStateKey(), layout.gcHbKey());
+
+    const GcMaintenanceState empty;
+    EXPECT_EQ(encodeGcMaintenanceState(empty), fmt::format(
+        "{{\"type\":\"cas_gc_maintenance_state\",\"v\":{}}}\n{{\"cur\":\"\"}}\n", currentCompatibilityVersion()));
+    const GcMaintenanceState state{.janitor_cursor = R"(cas/ns/a/"quoted"\\next)"};
+    EXPECT_EQ(decodeGcMaintenanceState(encodeGcMaintenanceState(state)), state);
+}
+
+TEST(CASGCMaintenanceStateFormat, RejectsMalformedAndBoundsCursor)
+{
+    const auto bad = [](std::string_view body)
+    {
+        return "{\"type\":\"cas_gc_maintenance_state\",\"v\":7}\n" + String(body);
+    };
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeGcMaintenanceState(bad("{}\n")); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeGcMaintenanceState(bad("{\"cur\":\"a\",\"cur\":\"b\"}\n")); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeGcMaintenanceState(bad("{\"cur\":\"a\",\"extra\":1}\n")); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeGcMaintenanceState(bad("{\"cur\":\"a\"}\nx")); });
+
+    const GcMaintenanceState at_limit{.janitor_cursor = String(kMaxGcMaintenanceCursorBytes, 'x')};
+    EXPECT_EQ(decodeGcMaintenanceState(encodeGcMaintenanceState(at_limit)), at_limit);
+    const GcMaintenanceState over_limit{.janitor_cursor = String(kMaxGcMaintenanceCursorBytes + 1, 'x')};
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LIMIT_EXCEEDED,
+        [&] { (void)encodeGcMaintenanceState(over_limit); });
+    const String raw = "{\"type\":\"cas_gc_maintenance_state\",\"v\":7}\n{\"cur\":\"" + over_limit.janitor_cursor + "\"}\n";
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeGcMaintenanceState(raw); });
+    String oversized = R"({"type":"cas_gc_maintenance_state","v":7,"pad":")";
+    oversized.append(448 * 1024, 'x');
+    oversized += "\"}\n{\"cur\":\"";
+    oversized.append(kMaxGcMaintenanceCursorBytes, 'y');
+    oversized += "\"}\n";
+    ASSERT_GT(oversized.size(), traitsFor(FormatId::GcMaintenanceState).object_cap);
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeGcMaintenanceState(oversized); });
+}
+
+TEST(CASGCMaintenanceState, ReadsAndCasWithoutAdoptingConflicts)
+{
+    InMemoryBackend backend;
+    const Layout layout("p");
+    const String key = layout.gcMaintenanceStateKey();
+    const GcMaintenanceReadResult absent = readGcMaintenanceState(backend, layout);
+    EXPECT_EQ(absent.status, GcMaintenanceReadStatus::Absent);
+    EXPECT_FALSE(absent.state);
+    EXPECT_FALSE(absent.token);
+
+    const GcMaintenanceState first{.janitor_cursor = "cas/ns/first"};
+    const GcMaintenanceCasResult created = casGcMaintenanceState(backend, layout, std::nullopt, first);
+    EXPECT_EQ(created.outcome, GcMaintenanceCasOutcome::Committed);
+    const GcMaintenanceReadResult valid = readGcMaintenanceState(backend, layout);
+    ASSERT_EQ(valid.status, GcMaintenanceReadStatus::Valid);
+    ASSERT_TRUE(valid.token);
+    ASSERT_TRUE(valid.state);
+    EXPECT_EQ(*valid.state, first);
+
+    const GcMaintenanceCasResult advanced = casGcMaintenanceState(backend, layout, valid.token,
+        GcMaintenanceState{.janitor_cursor = "cas/ns/advanced"});
+    ASSERT_EQ(advanced.outcome, GcMaintenanceCasOutcome::Committed);
+    const auto advanced_body = backend.get(key);
+    ASSERT_TRUE(advanced_body);
+
+    ASSERT_EQ(backend.casPut(key, encodeGcMaintenanceState({.janitor_cursor = "winner"}), advanced_body->token).outcome,
+        CasOutcome::Committed);
+    const GcMaintenanceCasResult conflict = casGcMaintenanceState(backend, layout, valid.token,
+        GcMaintenanceState{.janitor_cursor = "loser"});
+    EXPECT_EQ(conflict.outcome, GcMaintenanceCasOutcome::Conflict);
+    EXPECT_EQ(decodeGcMaintenanceState(backend.get(key)->bytes).janitor_cursor, "winner");
+}
+
+TEST(CASGCMaintenanceState, ClassifiesCorruptionAndResetsOnlyExactToken)
+{
+    InMemoryBackend backend;
+    const Layout layout("p");
+    const String key = layout.gcMaintenanceStateKey();
+    ASSERT_EQ(backend.putIfAbsent(key, "malformed").outcome, PutOutcome::Done);
+    const GcMaintenanceReadResult corrupt = readGcMaintenanceState(backend, layout);
+    ASSERT_EQ(corrupt.status, GcMaintenanceReadStatus::Corrupt);
+    ASSERT_TRUE(corrupt.token);
+    EXPECT_FALSE(corrupt.state);
+    EXPECT_FALSE(corrupt.diagnostic.empty());
+    ASSERT_EQ(casGcMaintenanceState(backend, layout, corrupt.token, {}).outcome, GcMaintenanceCasOutcome::Committed);
+    EXPECT_EQ(decodeGcMaintenanceState(backend.get(key)->bytes), GcMaintenanceState{});
+}
+
+TEST(CASGCMaintenanceState, UsesExactlyOneReadOrCasAttempt)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("p");
+    const String key = layout.gcMaintenanceStateKey();
+    EXPECT_EQ(readGcMaintenanceState(backend, layout).status, GcMaintenanceReadStatus::Absent);
+    EXPECT_EQ(backend.getCount(key), 1u);
+
+    backend.resetCounts();
+    ASSERT_EQ(casGcMaintenanceState(backend, layout, std::nullopt, {}).outcome,
+        GcMaintenanceCasOutcome::Committed);
+    EXPECT_EQ(backend.casPutCount(key), 1u);
+    EXPECT_EQ(backend.getCount(key), 0u);
+
+    backend.resetCounts();
+    EXPECT_EQ(casGcMaintenanceState(backend, layout, std::nullopt,
+        GcMaintenanceState{.janitor_cursor = "loser"}).outcome, GcMaintenanceCasOutcome::Conflict);
+    EXPECT_EQ(backend.casPutCount(key), 1u);
+    EXPECT_EQ(backend.getCount(key), 0u);
+
+    const auto current = backend.get(key);
+    ASSERT_TRUE(current);
+    ASSERT_EQ(backend.casPut(key, encodeGcMaintenanceState({.janitor_cursor = "winner"}), current->token).outcome,
+        CasOutcome::Committed);
+    backend.resetCounts();
+    EXPECT_EQ(casGcMaintenanceState(backend, layout, current->token,
+        GcMaintenanceState{.janitor_cursor = "stale"}).outcome, GcMaintenanceCasOutcome::Conflict);
+    EXPECT_EQ(backend.casPutCount(key), 1u);
+    EXPECT_EQ(backend.getCount(key), 0u);
+    EXPECT_EQ(decodeGcMaintenanceState(backend.InMemoryBackend::get(key)->bytes).janitor_cursor, "winner");
+}
+
+TEST(CASGCMaintenanceState, FutureVersionPropagatesInsteadOfResetting)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("p");
+    const String key = layout.gcMaintenanceStateKey();
+    ASSERT_EQ(backend.putIfAbsent(key, fmt::format(
+        "{{\"type\":\"cas_gc_maintenance_state\",\"v\":{}}}\n{{\"cur\":\"\"}}\n", currentCompatibilityVersion() + 1)).outcome,
+        PutOutcome::Done);
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION,
+        [&] { (void)readGcMaintenanceState(backend, layout); });
+    EXPECT_EQ(backend.casPutCount(key), 0u);
+
+    FailingMaintenanceReadBackend failing;
+    EXPECT_THROW((void)readGcMaintenanceState(failing, layout), std::runtime_error);
+}
+
+TEST(CASGCMaintenanceState, LosingCorruptResetPreservesConcurrentWinner)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("p");
+    const String key = layout.gcMaintenanceStateKey();
+    ASSERT_EQ(backend.putIfAbsent(key, "corrupt").outcome, PutOutcome::Done);
+    const auto corrupt = readGcMaintenanceState(backend, layout);
+    ASSERT_EQ(corrupt.status, GcMaintenanceReadStatus::Corrupt);
+    ASSERT_TRUE(corrupt.token);
+    ASSERT_EQ(backend.casPut(key, encodeGcMaintenanceState({.janitor_cursor = "winner"}), corrupt.token).outcome,
+        CasOutcome::Committed);
+    backend.resetCounts();
+    EXPECT_EQ(casGcMaintenanceState(backend, layout, corrupt.token, {}).outcome,
+        GcMaintenanceCasOutcome::Conflict);
+    EXPECT_EQ(backend.casPutCount(key), 1u);
+    EXPECT_EQ(backend.getCount(key), 0u);
+    EXPECT_EQ(decodeGcMaintenanceState(backend.InMemoryBackend::get(key)->bytes).janitor_cursor, "winner");
+}
diff --git a/src/Disks/tests/gtest_cas_gc_outcomes_format.cpp b/src/Disks/tests/gtest_cas_gc_outcomes_format.cpp
new file mode 100644
index 000000000000..e6530f131299
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_outcomes_format.cpp
@@ -0,0 +1,101 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcOutcomesFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/Exception.h>
+
+using namespace DB::Cas;
+
+namespace
+{
+
+/// Same tiny inline copy as `gtest_cas_part_manifest_format.cpp`'s `expectThrowsCode`: stays clear
+/// of `Disks/tests/cas_test_helpers.h`, which would drag in the whole CAS backend/store machinery
+/// this file otherwise has no need for.
+template <typename F>
+void expectThrowsCode(int expected_code, F && fn)
+{
+    try
+    {
+        fn();
+        FAIL() << "expected DB::Exception";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), expected_code);
+    }
+}
+
+}
+
+TEST(CASFormatBattery, GcOutcomes)
+{
+    OutcomeLog log;
+    OutcomeEntry e;
+    e.kind = ObjectKind::Blob;
+    e.ref = BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128("00112233445566778899aabbccddeeff"))};
+    e.token = Token{"e-1", TokenType::ETag};
+    e.outcome = OutcomeKind::Deleted;
+    log.entries.push_back(e);
+    runFormatBattery({FormatId::GcOutcomes,
+        [&] { return sealObject(FormatId::GcOutcomes, encodeOutcomeLog(log)); },
+        [](std::string_view d) { decodeOutcomeLog(std::string(openObject(FormatId::GcOutcomes, d))); },
+        currentFormatHeader("cas_gc_outcomes") +
+        "{\"k\":\"blob\",\"ha\":\"ch128\",\"h\":\"00112233445566778899aabbccddeeff\","
+        "\"tt\":\"etag\",\"tv\":\"e-1\",\"oc\":\"deleted\"}\n{\"n\":1}\n"});
+}
+
+TEST(CASGCOutcomesFormat, EmptyRoundTrips)
+{
+    EXPECT_EQ(decodeOutcomeLog(encodeOutcomeLog(OutcomeLog{})).entries.size(), 0u);
+}
+
+TEST(CASGCOutcomesFormat, MultiEntryRoundTripAllOutcomes)
+{
+    OutcomeLog log;
+    log.entries.push_back({ObjectKind::Blob, BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128("aa00000000000000000000000000000a"))},
+                           Token{"etag-1", TokenType::ETag}, OutcomeKind::Deleted});
+    log.entries.push_back({ObjectKind::Blob, BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128("bb00000000000000000000000000000b"))},
+                           Token{"7", TokenType::Emulated}, OutcomeKind::Spared});
+    log.entries.push_back({ObjectKind::Blob, BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128("cc00000000000000000000000000000c"))},
+                           Token{"8", TokenType::Emulated}, OutcomeKind::Replaced});
+    log.entries.push_back({ObjectKind::Blob, BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128("dd00000000000000000000000000000d"))},
+                           Token{"9", TokenType::Emulated}, OutcomeKind::Absent});
+    const String text = encodeOutcomeLog(log);
+    const OutcomeLog d = decodeOutcomeLog(text);
+    ASSERT_EQ(d.entries.size(), 4u);
+    EXPECT_EQ(d.entries[0].ref, (BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128("aa00000000000000000000000000000a"))}));
+    EXPECT_EQ(d.entries[0].outcome, OutcomeKind::Deleted);
+    EXPECT_EQ(d.entries[1].outcome, OutcomeKind::Spared);
+    EXPECT_EQ(d.entries[2].outcome, OutcomeKind::Replaced);
+    EXPECT_EQ(d.entries[3].outcome, OutcomeKind::Absent);
+    EXPECT_EQ(d.entries[0].token.value, "etag-1");
+    EXPECT_EQ(d.entries[0].token.type, TokenType::ETag);
+    EXPECT_EQ(d.entries[3].token.value, "9");
+    /// Insertion order + byte-stable text (the encoder is a pure function of the log).
+    EXPECT_EQ(encodeOutcomeLog(d), text);
+}
+
+TEST(CASGCOutcomesFormat, GarbageAndUnknownWordsFailClosed)
+{
+    EXPECT_THROW(decodeOutcomeLog(String("")), DB::Exception);
+    EXPECT_THROW(decodeOutcomeLog(String("not a cas object\n")), DB::Exception);
+    /// A record with an unknown outcome word fails closed.
+    const String bad = "{\"type\":\"cas_gc_outcomes\",\"v\":3}\n"
+                       "{\"k\":\"blob\",\"ha\":\"ch128\",\"h\":\"00112233445566778899aabbccddeeff\","
+                       "\"tt\":\"etag\",\"tv\":\"x\",\"oc\":\"bogus\"}\n{\"n\":1}\n";
+    EXPECT_THROW(decodeOutcomeLog(bad), DB::Exception);
+    /// A trailer count mismatch fails closed.
+    const String miscount = "{\"type\":\"cas_gc_outcomes\",\"v\":3}\n{\"n\":5}\n";
+    EXPECT_THROW(decodeOutcomeLog(miscount), DB::Exception);
+}
+
+TEST(CASGCOutcomesFormat, DigestWidthMismatchFailsClosedWithCorruptedData)
+{
+    /// `ch128` (CityHash128) digests are 16 bytes = 32 hex chars; here the "h" field is truncated
+    /// to 30 hex chars. Must surface as CORRUPTED_DATA (malformed serialized input), not
+    /// `fromHex`'s BAD_ARGUMENTS.
+    const String bad = "{\"type\":\"cas_gc_outcomes\",\"v\":3}\n"
+                       "{\"k\":\"blob\",\"ha\":\"ch128\",\"h\":\"00112233445566778899aabbccddee\","
+                       "\"tt\":\"etag\",\"tv\":\"x\",\"oc\":\"deleted\"}\n{\"n\":1}\n";
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeOutcomeLog(bad); });
+}
diff --git a/src/Disks/tests/gtest_cas_gc_rebuild.cpp b/src/Disks/tests/gtest_cas_gc_rebuild.cpp
new file mode 100644
index 000000000000..aa896dbf68df
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_rebuild.cpp
@@ -0,0 +1,681 @@
+#include <gtest/gtest.h>
+
+#include <optional>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Common/ProfileEvents.h>
+#include "cas_test_helpers.h"
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace ProfileEvents
+{
+extern const Event CASGCRefWalkPlansBuilt;
+}
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+}
+
+namespace
+{
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+ManifestRef ref(uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+}
+
+/// (`CASGCBaselineGuard.FreshStateOverTrimmedJournalsFailsClosed` was removed with the snapshot+log ref
+/// model. It asserted that a fresh GC over a MUTABLE shard journal whose folded history had been TRIMMED
+/// must refuse, lest it fold only the surviving tails and mass-delete live data. Immutable `_log`/`_snap`
+/// objects are never trimmed in place: a fresh GC always reconstructs the FULL ref state via the recovery
+/// equation (newest snapshot + later log tail), so the "trimmed history" hazard cannot arise. The
+/// vanished-`gc/state` disaster-recovery path is covered by `CASGCRebuild.RecoversLostStateAndConverges`,
+/// and the corrupt-bookkeeping guard by `CASGCBaselineGuard.AbsentAdoptedSealFailsClosed`.)
+
+/// A genuinely fresh pool (journals start at version 1) passes the guard — rounds run as today.
+TEST(CASGCBaselineGuard, GenuinelyFreshPoolIsUnaffected)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    EXPECT_NO_THROW(gc.runRegularRound());
+    EXPECT_NO_THROW(gc.runRegularRound());
+}
+
+/// (б) audit: snap_generation > 0 whose adopted fold seal is ABSENT must be CORRUPTED_DATA,
+/// never silently treated as an empty baseline.
+TEST(CASGCBaselineGuard, AbsentAdoptedSealFailsClosed)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+
+    /// Corrupt (б): delete the adopted fold seal out from under a healthy gc/state.
+    const GcState st = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    ASSERT_GT(st.snap_generation, 0u);
+    const String seal_key = store->layout().foldSealKey(st.snap_generation, st.snap_attempt);
+    const HeadResult sh = backend->head(seal_key);
+    ASSERT_TRUE(sh.exists);
+    ASSERT_EQ(backend->deleteExact(seal_key, sh.token).kind, DeleteOutcome::Kind::Deleted);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { gc.runRegularRound(); });
+}
+
+/// (а): lose gc/state on a lived-in pool -> guard blocks rounds -> rebuild -> rounds converge: the
+/// live blob intact, the round minted strictly above the last one seen, and the REBUILT baseline is a
+/// working one — a ref dropped AFTER it is still reclaimed by ordinary rounds.
+///
+/// A blob dropped BEFORE the rebuild is a different matter, and this test pins it: the rebuild derives
+/// edges from owner state, so a blob no owner names gets no row at all, and a rebuild CONDEMNS NOTHING
+/// (spec §7 — the condemnation that used to catch this case was the r5-finding-4 data-loss vector).
+/// Such a blob is retained until register R4's build/upload registry can enumerate it safely. That is
+/// the NAMED Stage-A residual, and it is asserted here rather than left to be discovered.
+TEST(CASGCRebuild, RecoversLostStateAndConverges)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// gc_fold_max_defer_rounds=0: this test drives MANY consecutive rounds via runRoundsUntilAbsent
+    /// expecting every one to fold (Phase-4 Lever A would otherwise defer once the pool quiesces,
+    /// stalling the reclaim loop below the 8-round budget); force fold-every-round.
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef live_r = ref(1, 0xA1);
+    const ManifestRef dead_r = ref(2, 0xA2);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeBlobBody(*backend, store->layout(), DB::UInt128(2));
+    writeManifestRaw(*backend, store->layout(), ns, live_r, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, store->layout(), ns, dead_r, {blobEntryFor("b", DB::UInt128(2))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_live", std::nullopt, live_r);
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_dead", std::nullopt, dead_r);
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    dropRefTransition(*backend, store->layout(), ns, "tbl_dead", dead_r);
+    runRegularRoundReclaiming(gc);   /// -1 folds; eager trim cuts the journal
+    store->renewWatermarkOnce();   /// renews the lease + build-watermark floor
+
+    /// Capture the round reached before gc/state is destroyed (the rebuild must mint strictly above it).
+    const auto pre_rebuild_got = backend->get(store->layout().gcStateKey());
+    ASSERT_TRUE(pre_rebuild_got.has_value());
+    const uint64_t pre_rebuild_round = decodeGcState(pre_rebuild_got->bytes).round;
+    ASSERT_EQ(backend->deleteExact(store->layout().gcStateKey(), pre_rebuild_got->token).kind, DeleteOutcome::Kind::Deleted);
+
+    Gc gc2(store, hexToU128("00000000000000000000000000000003"));
+    /// A fresh GC over the orphaned generation artifacts fails closed: re-folding from a fresh gc/state
+    /// collides with a leftover run object (divergent bytes) — the disaster is surfaced, never silently
+    /// double-applied. That first round also re-mints a superficially-healthy gc/state (snap_generation 0),
+    /// so the recovery is a DELIBERATE force-rebuild (the auto-rebuild correctly refuses to discard a
+    /// state that "looks healthy" without the operator's force).
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { runRegularRoundReclaiming(gc2); });
+
+    const RebuildReport rep = gc2.rebuildBaseline(/*force*/ true);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_EQ(rep.committed_refs, 1u);
+    EXPECT_EQ(rep.namespaces, 1u);
+
+    /// Round strictly above the fence/state/generation numbers seen so far.
+    EXPECT_GT(rep.round, pre_rebuild_round);
+
+    /// The live blob is intact, and blob 2 — dropped BEFORE the rebuild, so invisible to a baseline
+    /// derived from owner state — is RETAINED. Retention, not loss: the named residual above.
+    for (int i = 0; i < 4; ++i)
+    {
+        runRegularRoundReclaiming(gc2);
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(backend->head(store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(1))})).exists);
+    EXPECT_TRUE(backend->head(store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(2))})).exists)
+        << "a rebuild condemns nothing, so a pre-rebuild drop is retained — never reclaimed by a "
+           "substitute pass, and never lost";
+
+    /// The rebuilt baseline is a WORKING one: a ref published over it and then dropped still folds to
+    /// zero and is reclaimed by ordinary rounds. Without this the test would prove only that the
+    /// pipeline stopped deleting.
+    const ManifestRef post_r = ref(3, 0xA3);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(3));
+    writeManifestRaw(*backend, store->layout(), ns, post_r, {blobEntryFor("c", DB::UInt128(3))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_post", std::nullopt, post_r);
+    runRegularRoundReclaiming(gc2);
+    dropRefTransition(*backend, store->layout(), ns, "tbl_post", post_r);
+    EXPECT_TRUE(runRoundsUntilAbsent(store, gc2, *backend, store->layout(), DB::UInt128(3)))
+        << "the rebuilt baseline must still reclaim what it can actually see";
+}
+
+/// (б): a run object named by a healthy state is lost -> the regular round fails closed -> the
+/// PLAIN rebuild (no FORCE) recovers, and rounds converge afterwards.
+TEST(CASGCRebuild, RecoversLostGenerationArtifact)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+
+    /// Lose one snapshot run object out from under the healthy state.
+    const GcState st = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    const auto seal = decodeFoldSeal(backend->get(store->layout().foldSealKey(st.snap_generation, st.snap_attempt))->bytes);
+    ASSERT_FALSE(seal.blob_target_runs.empty());
+    const String run_key = seal.blob_target_runs.front().key;
+    const HeadResult rh = backend->head(run_key);
+    ASSERT_TRUE(rh.exists);
+    ASSERT_EQ(backend->deleteExact(run_key, rh.token).kind, DeleteOutcome::Kind::Deleted);
+
+    /// A pure ref-carry round would not read the lost run; land a REAL delta so the fold's
+    /// three-cursor merge must stream the prior run — and fails closed on its absence.
+    const ManifestRef r2 = ref(2, 0xB7);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(3));
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("c", DB::UInt128(3))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl2", std::nullopt, r2);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { gc.runRegularRound(); });
+
+    const RebuildReport rep = gc.rebuildBaseline(/*force*/ false);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_NO_THROW(gc.runRegularRound());
+    EXPECT_TRUE(backend->head(store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(1))})).exists);
+}
+
+/// FORCE: a healthy state refuses the plain rebuild; FORCE rebuilds; rounds run clean after.
+TEST(CASGCRebuild, HealthyStateRequiresForce)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+
+    const RebuildReport refused = gc.rebuildBaseline(/*force*/ false);
+    EXPECT_FALSE(refused.performed);
+    EXPECT_NE(refused.refusal.find("FORCE"), String::npos);
+
+    backend->resetCounts();
+    const uint64_t plans_before
+        = ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load();
+    const RebuildReport forced = gc.rebuildBaseline(/*force*/ true);
+    ASSERT_TRUE(forced.performed) << forced.refusal;
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load() - plans_before, 1u)
+        << "healthy FORCE REBUILD must share the same one-shot authoritative plan builder";
+    EXPECT_EQ(backend->getCount(store->layout().refCatalogKey()), 2u)
+        << "healthy FORCE REBUILD may read the catalog for the conclusive drain and the one post-LIST cut only";
+    EXPECT_EQ(backend->listCount(store->layout().namespaceStreamRootPrefix()), 1u);
+    EXPECT_NO_THROW(gc.runRegularRound());
+}
+
+/// The post-LIST catalog cut and its exact `_ckpt` are REBUILD's authority. A visible later log is
+/// not admitted merely because a LIST would find it: it may be a durable-but-unfrontiered writer
+/// attempt, and folding its missing manifest would turn a safe rebuild into a false refusal.
+///
+/// This catches a regression back to the legacy `recoverRefTable` overload, whose full LIST-derived
+/// replay consumes the second transaction and therefore refuses on `unfrontiered`'s missing body.
+TEST(CASGCRebuild, FrozenCheckpointFrontierExcludesVisibleUnfrontieredTail)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/rebuild-frozen-frontier@cas@"};
+    const UInt128 life_id{0xF001};
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(ns, life_id);
+    CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards,
+        CatalogEntry{.ns = ns, .state = NsState::Live, .incarnation = life_id});
+
+    const ManifestRef admitted = ref(1, 0xA1);
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeManifestRaw(*backend, layout, ns, admitted, {blobEntryFor("a", DB::UInt128(1))});
+
+    std::vector<RefOp> first_ops{namespaceBirthOp()};
+    const auto admitted_ops = publishCommittedOps("admitted", admitted);
+    first_ops.insert(first_ops.end(), admitted_ops.begin(), admitted_ops.end());
+    fixture::writeRefLogRaw(*backend, layout,
+        RefLogTxn{.ns = ns.string(), .txn_id = RefTxnId{1, 1}, .ops = std::move(first_ops), .prev_epoch_seal = std::nullopt});
+
+    /// This record is real and listable, but the writer never published it through `_ckpt`.
+    const ManifestRef unfrontiered = ref(2, 0xB2);
+    fixture::writeRefLogRaw(*backend, layout,
+        RefLogTxn{.ns = ns.string(), .txn_id = RefTxnId{1, 2}, .ops = publishCommittedOps("unfrontiered", unfrontiered),
+                  .prev_epoch_seal = std::nullopt});
+    ASSERT_TRUE(backend->head(layout.refLogKey(life, RefTxnId{1, 2})).exists);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    Gc gc(store, kGc);
+    const RebuildReport report = gc.rebuildBaseline(/*force=*/false);
+
+    ASSERT_TRUE(report.performed) << report.refusal;
+    EXPECT_EQ(report.committed_refs, 1u);
+}
+
+/// A catalog-admitted life without its exact checkpoint has no bounded recovery frontier. REBUILD
+/// must refuse rather than falling back to a list-derived history and publishing a baseline it cannot
+/// prove complete.
+TEST(CASGCRebuild, LiveCatalogLifeWithoutCheckpointFailsClosed)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/rebuild-missing-checkpoint@cas@"};
+    const UInt128 life_id{0xF002};
+    CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards,
+        CatalogEntry{.ns = ns, .state = NsState::Live, .incarnation = life_id});
+
+    const ManifestRef admitted = ref(1, 0xA2);
+    writeBlobBody(*backend, layout, DB::UInt128(2));
+    writeManifestRaw(*backend, layout, ns, admitted, {blobEntryFor("a", DB::UInt128(2))});
+    std::vector<RefOp> ops{namespaceBirthOp()};
+    const auto admitted_ops = publishCommittedOps("admitted", admitted);
+    ops.insert(ops.end(), admitted_ops.begin(), admitted_ops.end());
+    fixture::writeRefLogRaw(*backend, layout,
+        RefLogTxn{.ns = ns.string(), .txn_id = RefTxnId{1, 1}, .ops = std::move(ops), .prev_epoch_seal = std::nullopt});
+
+    Gc gc(store, kGc);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)gc.rebuildBaseline(/*force=*/false); });
+
+    const auto state = backend->get(layout.gcStateKey());
+    ASSERT_TRUE(state);
+    EXPECT_EQ(decodeGcState(state->bytes).snap_generation, 0u)
+        << "a rejected recovery must not adopt a new baseline";
+}
+
+/// A syntactically valid snapshot at an OLDER `EpochSeal` id must not let REBUILD synthesize a
+/// baseline. The forged base differs from `last_epoch_seal`, so metadata equality cannot reject it;
+/// REBUILD must use the retained same-id log witness.
+TEST(CASGCRebuild, CheckpointSnapshotAtOlderEpochSealFailsClosed)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/rebuild-checkpoint-base-seal@cas@"};
+    const UInt128 life_id{0xF003};
+    CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards,
+        CatalogEntry{.ns = ns, .state = NsState::Live, .incarnation = life_id});
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(ns, life_id);
+
+    const RefLogTxn birth{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 1}, .ops = {namespaceBirthOp()},
+        .prev_epoch_seal = std::nullopt};
+    fixture::writeRefLogRaw(*backend, layout, birth);
+    RefOp seal;
+    seal.kind = RefOpKind::EpochSeal;
+    const RefLogTxn seal_txn{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 2}, .ops = {seal},
+        .prev_epoch_seal = std::nullopt};
+    fixture::writeRefLogRaw(*backend, layout, seal_txn);
+    RefOp later_seal;
+    later_seal.kind = RefOpKind::EpochSeal;
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(), .txn_id = RefTxnId{2, 1}, .ops = {later_seal},
+        .prev_epoch_seal = RefTxnId{1, 2}});
+    RefTableState through_seal;
+    applyRefLogTxn(through_seal, birth);
+    applyRefLogTxn(through_seal, seal_txn);
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(through_seal, ns.string()));
+
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = RefTxnId{1, 2},
+        .last_epoch_seal = RefTxnId{2, 1}})).outcome, PutOutcome::Done);
+
+    Gc gc(store, kGc);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)gc.rebuildBaseline(/*force=*/false); });
+
+    const auto state = backend->get(layout.gcStateKey());
+    ASSERT_TRUE(state);
+    EXPECT_EQ(decodeGcState(state->bytes).snap_generation, 0u)
+        << "a rejected checkpoint base must not publish a REBUILD baseline";
+}
+
+TEST(CASGCRebuild, DamagedGenerationZeroStatePerformsNoCatalogDrainMutation)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/removing-without-parent@cas@"};
+    const UInt128 life_id{91};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, CatalogEntry{
+        .ns = ns, .state = NsState::Live, .incarnation = life_id});
+    CasRefCatalog::casUpdate(*backend, layout, [](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        next.entries[0].state = NsState::Removing;
+        next.entries[0].removal_started_round = 1;
+        return next;
+    });
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(NamespaceLifeId::fromCatalogEntry(ns, life_id)), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    const uint64_t catalog_cas_before = backend->casPutCount(layout.refCatalogKey());
+    const uint64_t plans_before
+        = ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load();
+
+    Gc gc(store, kGc);
+    const RebuildReport report = gc.rebuildBaseline(/*force*/ false);
+    ASSERT_TRUE(report.performed) << report.refusal;
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load() - plans_before, 1u);
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), catalog_cas_before);
+    const CasRefCatalog::Snapshot catalog = CasRefCatalog::read(*backend, layout);
+    ASSERT_EQ(catalog.catalog.entries.size(), 1u);
+    EXPECT_EQ(catalog.catalog.entries[0].state, NsState::Removing);
+    EXPECT_EQ(catalog.catalog.entries[0].incarnation, life_id);
+}
+
+/// Refusal: a committed owner with a MISSING manifest body is data loss — the rebuild refuses,
+/// names the owner, and writes nothing (gc/state stays absent).
+TEST(CASGCRebuild, MissingCommittedManifestRefuses)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef a = ref(1, 0xA1);
+    const ManifestRef b = ref(2, 0xB2);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, a, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, store->layout(), ns, b, {blobEntryFor("b", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_a", std::nullopt, a);
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_b", std::nullopt, b);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    gc.runRegularRound();   /// trim
+
+    /// Disaster pair: gc/state lost AND tbl_b's manifest body lost.
+    const HeadResult st = backend->head(store->layout().gcStateKey());
+    backend->deleteExact(store->layout().gcStateKey(), st.token);
+    const String mkey = store->layout().manifestKey(ManifestId{ns, b});
+    const HeadResult mh = backend->head(mkey);
+    ASSERT_TRUE(mh.exists);
+    backend->deleteExact(mkey, mh.token);
+
+    Gc gc2(store, hexToU128("00000000000000000000000000000004"));
+    const RebuildReport rep = gc2.rebuildBaseline(/*force*/ false);
+    EXPECT_FALSE(rep.performed);
+    EXPECT_NE(rep.refusal.find("tbl_b"), String::npos) << rep.refusal;
+    /// The lease acquire minted a gen-0 bootstrap body (that is the acquire's contract, not the
+    /// rebuild's); the rebuild's own contract is that NO baseline was blessed by the refusal.
+    const auto post = backend->get(store->layout().gcStateKey());
+    ASSERT_TRUE(post.has_value());
+    const GcState post_state = decodeGcState(post->bytes);
+    EXPECT_EQ(post_state.snap_generation, 0u) << "a refused rebuild must not adopt a baseline";
+}
+
+/// A live precommit with a durable body contributes edges (no clamp); the rebuilt baseline
+/// protects its blob from condemnation.
+TEST(CASGCRebuild, LivePrecommitEdgesIncluded)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef pre = ref(7, 0xC1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(9));
+    writeManifestRaw(*backend, store->layout(), ns, pre, {blobEntryFor("p", DB::UInt128(9))});
+    addPrecommitTransition(
+        *backend, store->layout(), ns, /*build_id*/ DB::UInt128(0x77), "part_pre", std::nullopt, pre);
+
+    /// No round before the rebuild: the journal still carries the create-precommit event (a round's
+    /// eager trim would cut it — the trimmed-but-live case is the next test). gc/state absent =>
+    /// the plain rebuild is allowed.
+    Gc gc2(store, hexToU128("00000000000000000000000000000005"));
+    const RebuildReport rep = gc2.rebuildBaseline(/*force*/ false);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_EQ(rep.live_precommits, 1u);
+    EXPECT_EQ(rep.clamped_shards, 0u);
+
+    /// The precommit's blob is edge-protected: rounds never reclaim it while the precommit lives.
+    for (int i = 0; i < 4; ++i)
+    {
+        gc2.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(backend->head(store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(9))})).exists);
+}
+
+/// O(budget) attempt iteration: a tiny edge budget forces multi-batch folding; the rebuilt
+/// baseline still protects every committed blob (same convergence as the single-batch path).
+TEST(CASGCRebuild, BatchedRebuildProtectsAllRefs)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    constexpr uint64_t gc_shards = 2;
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_shards = gc_shards});
+    const RootNamespace ns{"00/aa@cas@"};
+    constexpr uint64_t edge_budget = 2;
+    constexpr uint64_t refs_per_shard = edge_budget + 1;
+    std::vector<UInt128> blobs;
+    for (uint64_t shard = 0; shard < gc_shards; ++shard)
+    {
+        for (uint64_t i = 0; i < refs_per_shard; ++i)
+        {
+            const uint64_t sequence = blobs.size() + 1;
+            const UInt128 blob = (UInt128{shard + gc_shards * i} << 64) | UInt128{sequence};
+            ASSERT_EQ(blobShard(legacyMetaTestRef(blob), gc_shards), shard);
+            blobs.push_back(blob);
+            writeBlobBody(*backend, store->layout(), blob);
+            const ManifestRef r = ref(sequence, 0xA0 + sequence);
+            writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("f", blob)});
+            publishCommittedTransition(
+                *backend, store->layout(), ns, "tbl_" + std::to_string(sequence), std::nullopt, r);
+        }
+    }
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    gc.runRegularRound();
+    const HeadResult st = backend->head(store->layout().gcStateKey());
+    backend->deleteExact(store->layout().gcStateKey(), st.token);
+
+    Gc gc2(store, hexToU128("00000000000000000000000000000006"));
+    /// Every shard has `edge_budget + 1` live edges, so each independently crosses the flush budget;
+    /// a test with only a pool-wide excess would not prove multiple batches for every non-empty shard.
+    gc2.setRebuildEdgeBudgetForTest(edge_budget);
+    const RebuildReport rep = gc2.rebuildBaseline(/*force*/ false);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_EQ(rep.committed_refs, blobs.size());
+
+    /// Multiple rebuild flushes still converge to one authoritative row domain: no more than one
+    /// canonical seq-0 `btr` per shard and exactly one `cnd` per shard. These are the cardinalities the
+    /// catalog admission reservation over-covers independently of catalog-entry count.
+    const GcState rebuilt_state = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    const CasFoldSeal rebuilt_seal = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(
+            rebuilt_state.snap_generation, rebuilt_state.snap_attempt))->bytes,
+        store->layout(), gc_shards);
+    ASSERT_EQ(rebuilt_seal.condemned_summary.size(), gc_shards);
+    bool run_seen[gc_shards] = {false, false};
+    ASSERT_EQ(rebuilt_seal.blob_target_runs.size(), gc_shards);
+    for (const RunRef & run : rebuilt_seal.blob_target_runs)
+    {
+        ASSERT_LT(run.shard, gc_shards);
+        EXPECT_FALSE(run_seen[run.shard]);
+        run_seen[run.shard] = true;
+        const auto parsed = store->layout().parseBlobTargetRunKey(run.key);
+        ASSERT_TRUE(parsed.has_value());
+        EXPECT_EQ(parsed->shard, run.shard);
+        EXPECT_EQ(parsed->generation, run.generation);
+        EXPECT_EQ(parsed->seq, 0u);
+    }
+    EXPECT_TRUE(run_seen[0]);
+    EXPECT_TRUE(run_seen[1]);
+
+    for (int i = 0; i < 5; ++i)
+    {
+        gc2.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+    for (const UInt128 blob : blobs)
+        EXPECT_TRUE(backend->head(store->layout().blobKey(legacyMetaTestRef(blob))).exists)
+            << "blob " << u128ToHex(blob);
+}
+
+/// Trimmed-but-live (design delta 2): the precommit's journal evidence is gone (trim), the build
+/// is NOT provably dead (a live build holds min_active down) — the unowned-alive sweep must
+/// over-protect the manifest's edges.
+TEST(CASGCRebuild, UnownedAliveManifestOverProtected)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    /// A LIVE build pins min_active at its build_seq, so higher build sequences are not provably dead.
+    auto live_build = store->beginPartWrite({});
+    store->renewWatermarkOnce();
+
+    /// An unowned manifest from build_seq 7 (no journal events at all — the trimmed shape).
+    const ManifestRef pre = ref(7, 0xC1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(9));
+    writeManifestRaw(*backend, store->layout(), ns, pre, {blobEntryFor("p", DB::UInt128(9))});
+    /// The namespace must be discoverable: give it one committed ref on another manifest.
+    const ManifestRef anchor = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, anchor, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, anchor);
+
+    Gc gc(store, hexToU128("00000000000000000000000000000007"));
+    const RebuildReport rep = gc.rebuildBaseline(/*force*/ false);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_EQ(rep.unowned_alive_manifests, 1u);
+
+    /// Over-protected: rounds never reclaim the unowned-alive manifest's blob.
+    for (int i = 0; i < 4; ++i)
+    {
+        gc.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(backend->head(store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(9))})).exists);
+}
+
+/// Task 4 (SYSTEM CAS GC REBUILD): a rebuild refuses when ANOTHER Gc instance holds
+/// the lease, even under FORCE (FORCE bypasses the "healthy state" refusal, not the lease). Gc A's
+/// runRegularRound freshly acquires/renews the lease; Gc B (a different gc_id) must see it as live.
+TEST(CASGCRebuild, LeaseConflictRefuses)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc_a(store, kGc);
+    gc_a.runRegularRound();   /// Gc A acquires/renews the lease.
+
+    Gc gc_b(store, hexToU128("00000000000000000000000000000002"));
+    const RebuildReport rep = gc_b.rebuildBaseline(/*force*/ true);
+    EXPECT_FALSE(rep.performed);
+    EXPECT_NE(rep.refusal.find("lease"), String::npos) << rep.refusal;
+    EXPECT_NE(rep.refusal.find("leader"), String::npos) << rep.refusal;
+}
+
+/// A rebuild CONDEMNS NOTHING (spec §7). The zero-edge condemnation that used to live here — a
+/// `blobs/` LIST whose every unreached body was condemned into the rebuilt run — was the
+/// r5-finding-4 data-loss vector: the rebuild's own traversal is listing-driven, so a hidden
+/// durable owner made this pass condemn acked data. Its removal, the NAMED residual it leaves
+/// (manifest-less orphans are retained until register R4's build/upload registry), and the
+/// hold-carry that had to survive the removal are all covered by
+/// `gtest_cas_rebuild_condemn_nothing.cpp`.
+
+/// CLAMP SUPPRESSION regression (2026-07-03 night soak: 31 dangling blobs). A committed +1 for
+/// blob X lands on a shard whose fold cursor is CLAMPED (behind a bodiless precommit — the fold
+/// barrier), while X's only FOLDED edge (a committed ref on ANOTHER shard) drops. Without
+/// suppression the pipeline condemns, graduates and DELETES X while its landed +1 sits unfolded
+/// behind the clamp; the clamp release then folds the +1 into a DANGLING reference (the model's
+/// SabotageSkipChangedShard, realized). With suppression a clamped pass neither graduates nor
+/// redeletes; X survives until the clamp clears, after which the +1 folds and X is SPARED.
+TEST(CASGCClampSuppression, LandedEdgeBehindClampNeverDeleted)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    std::vector<CasEvent> seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    /// Folded baseline: blob X referenced by committed tbl_a (manifest m1) on shard 1.
+    const ManifestRef m1 = ref(1, 0xA1);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, m1, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_a", std::nullopt, m1, /*shard*/1);
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    store->renewWatermarkOnce();
+
+    /// The CLAMP on shard 0: a bodiless precommit (fold barrier — its manifest body never written).
+    const ManifestRef pre = ref(9, 0xEE);
+    addPrecommitTransition(*backend, store->layout(), ns, /*build_id*/ DB::UInt128(0x99), "part_pre",
+                           std::nullopt, pre, /*shard*/0);
+
+    /// BEHIND the clamp: a committed +1 for X (manifest m2, tbl_b) on shard 0 — landed, unfoldable
+    /// until the barrier clears. Then tbl_a drops on shard 1 — X's only FOLDED edge disappears.
+    const ManifestRef m2 = ref(2, 0xB2);
+    writeManifestRaw(*backend, store->layout(), ns, m2, {blobEntryFor("b", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_b", std::nullopt, m2, /*shard*/0);
+    dropRefTransition(*backend, store->layout(), ns, "tbl_a", m1, /*shard*/1);
+
+    /// Rounds with acks current: X reaches folded in-degree 0 and is condemned, but every pass is
+    /// CLAMPED (the bodiless precommit persists), so nothing may graduate or delete.
+    /// Observability (2026-07-03): every clamp emits a gc_fold_clamp event with the reason.
+    store->setEventSink([&](const CasEvent & e){ if (e.type == CasEventType::GcFoldClamp) seen.push_back(e); });
+    const String blob_key = store->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(1))});
+    for (int i = 0; i < 6; ++i)
+    {
+        gc.runRegularRound();
+        store->renewWatermarkOnce();
+        ASSERT_TRUE(backend->head(blob_key).exists)
+            << "round " << i << ": X was deleted while its landed +1 sat unfolded behind the clamp";
+    }
+
+    ASSERT_FALSE(seen.empty()) << "each clamped pass must emit a gc_fold_clamp event";
+    EXPECT_NE(seen.front().reason.find("fold barrier"), String::npos);
+    /// Snapshot+log ref model: the clamp is per-table (one ref-log stream per namespace, no ref shards),
+    /// so the event names the clamped `log` and the `resolved_through` cursor rather than a shard number.
+    EXPECT_TRUE(seen.front().detail.contains("log"))
+        << "clamp event must name the clamped log id";
+    EXPECT_TRUE(seen.front().detail.contains("resolved_through"))
+        << "clamp event must name the cursor it resolved through";
+    store->setEventSink(nullptr);
+
+    /// Release the clamp: the precommit's body lands (the build finished staging). The next rounds
+    /// fold through the barrier, m2's +1 lands, and X is SPARED (entry dropped, blob intact).
+    writeManifestRaw(*backend, store->layout(), ns, pre, {blobEntryFor("p", DB::UInt128(1))});
+    for (int i = 0; i < 4; ++i)
+    {
+        gc.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(backend->head(blob_key).exists);
+    /// And the pipeline is unwedged: a genuinely-unreferenced blob still gets reclaimed.
+    const ManifestRef m3 = ref(3, 0xC3);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(5));
+    writeManifestRaw(*backend, store->layout(), ns, m3, {blobEntryFor("c", DB::UInt128(5))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl_c", std::nullopt, m3, /*shard*/1);
+    gc.runRegularRound();
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, store->layout(), ns, "tbl_c", m3, /*shard*/1);
+    EXPECT_TRUE(runRoundsUntilAbsent(store, gc, *backend, store->layout(), DB::UInt128(5)));
+}
diff --git a/src/Disks/tests/gtest_cas_gc_resume.cpp b/src/Disks/tests/gtest_cas_gc_resume.cpp
new file mode 100644
index 000000000000..6c707bdd60cd
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_resume.cpp
@@ -0,0 +1,184 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace DB::ErrorCodes
+{
+extern const int ABORTED;
+}
+
+namespace
+{
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+ManifestRef ref(uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+bool blobExists(InMemoryBackend & b, const Layout & layout, const UInt128 & hash)
+{
+    return b.head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)})).exists;
+}
+
+/// Whether the CURRENT retired list (any gc-shard) still holds an entry.
+bool anyRetiredPending(const PoolPtr & s)
+{
+    /// Retired-in-snapshot (T4): condemned state rides the adopted fold seal's kCondemned rows, not a
+    /// separate retired list — reconstruct the in-flight set from the seal.
+    return anyCondemnedInSeal(s->backend(), s->layout());
+}
+
+/// Drive regular GC to a fixpoint over the ACK-FLOOR round (renew the store's mount ack after each round;
+/// stay alive while any work counter is nonzero OR an in-flight retired entry remains).
+size_t runGcToFixpoint(const PoolPtr & s, Gc & gc, size_t max_rounds = 64)
+{
+    size_t rounds = 0;
+    for (; rounds < max_rounds; ++rounds)
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        if (!rep.acquired_lease)
+            continue;
+        s->renewWatermarkOnce();
+        const bool no_work = rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0;
+        if (no_work && !anyRetiredPending(s))
+            break;
+    }
+    return rounds;
+}
+
+/// A backend that denies ONCE the SINGLE round-commit `gc/state` CAS — the casPut that advances
+/// snap_generation (the one-pass round has exactly one such CAS; the lease-acquire CAS does not advance
+/// snap_generation). A denied round leaves only never-adopted attempt-scoped debris (fold seal / retired
+/// list under an attempt gc/state never adopted); a fresh-attempt rerun is idempotent.
+class InterruptRoundCasBackend : public InMemoryBackend
+{
+public:
+    explicit InterruptRoundCasBackend(String gc_state_key_) : gc_state_key(std::move(gc_state_key_)) {}
+
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        if (arm_interrupt && key == gc_state_key)
+        {
+            const auto stored = get(key);
+            const uint64_t stored_gen = stored ? decodeGcState(stored->bytes).snap_generation : 0;
+            const uint64_t next_gen = decodeGcState(bytes).snap_generation;
+            if (next_gen > stored_gen)
+            {
+                arm_interrupt = false;   /// one-shot: only depose the first round-commit CAS
+                throw DB::Exception(DB::ErrorCodes::ABORTED,
+                    "test-injected: round-commit gc/state CAS denied (leader deposed mid-round)");
+            }
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+
+    bool arm_interrupt = false;
+
+private:
+    String gc_state_key;
+};
+}
+
+/// (`CASGCRound.TrimDropsFoldedOwnerEvents` was removed with the snapshot+log ref model: it asserted GC
+/// trims folded owner events out of a MUTABLE shard journal in place. Immutable `_log` objects are never
+/// trimmed in place; the new-model equivalent -- ref-object cleanup deletes a covered `_log`/`_snap` key
+/// once BOTH the durable cursor AND a checkpoint-named validated recovery triple cover it -- is exercised in
+/// `gtest_cas_ref_gc.cpp` (`RefObjectCleanupRetainsCheckpointNamedTriple`).)
+
+/// A crashed round leaves only never-adopted attempt-scoped debris (there is no resume machinery in the
+/// one-pass round). A fresh Gc simply re-runs the round under a fresh attempt and the deletion pipeline
+/// converges idempotently — a delete that already landed replays onto NotFound.
+TEST(CASGCReplay, FreshAttemptRerunCompletes)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+
+    // Drive the ack-floor pipeline to a fixpoint: the blob condemns, graduates, then is deleted. Every
+    // step is exact-token / write-once, so a replay is idempotent (the unit oracle for the crash-replay rule).
+    runGcToFixpoint(store, gc);
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+
+    // Re-running again is a clean no-op (idempotent): the blob stays gone, no throw.
+    EXPECT_NO_THROW(runRegularRoundReclaiming(gc));
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+}
+
+/// Crash-replay idempotence: a round is deposed at its SINGLE round-commit CAS (lease lost mid-round),
+/// leaving only never-adopted attempt-scoped debris (a fold seal + retired list under an attempt gc/state
+/// never adopted). A SECOND leader (different id => a fresh lease.seq, hence a fresh attempt) re-runs the
+/// round from scratch and completes: no wedge, no CORRUPTED_DATA, and the prior-round artifacts under the
+/// old (unadopted) attempt are simply unreferenced. The pool drains to a fixpoint.
+TEST(CASGCReplay, DeposedRoundRerunsUnderFreshAttempt)
+{
+    auto backend = std::make_shared<InterruptRoundCasBackend>(/*gc_state_key*/ "p/gc/state");
+    auto store = openPoolForTest(backend);
+    ASSERT_EQ(store->layout().gcStateKey(), "p/gc/state");   // guard the injected key against layout drift
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    // First leader folds + adopts the first (snap_generation, snap_attempt).
+    Gc gc1(store, hexToU128("00000000000000000000000000000001"));
+    runRegularRoundReclaiming(gc1);
+    store->renewWatermarkOnce();
+    const auto after_fold = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    ASSERT_EQ(after_fold.snap_attempt, after_fold.lease.seq);
+    ASSERT_GT(after_fold.snap_generation, 0u);
+
+    // Drop the only ref, then drive the round whose single commit CAS is DENIED (leader deposed mid-round).
+    // The round folded under a FRESH attempt and published its fold seal + retired list under that attempt,
+    // but the commit never adopted them — pure unadopted debris. gc/state is unchanged.
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    backend->arm_interrupt = true;
+    EXPECT_THROW(runRegularRoundReclaiming(gc1), DB::Exception);
+    backend->arm_interrupt = false;
+
+    const auto after_interrupt = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    EXPECT_EQ(after_interrupt.snap_generation, after_fold.snap_generation)
+        << "the denied round-commit CAS must NOT advance the adopted generation";
+    EXPECT_EQ(after_interrupt.snap_attempt, after_fold.snap_attempt)
+        << "the denied round-commit CAS must NOT advance the adopted attempt";
+    // The deposed round's fold seal is durable under its OWN (unadopted) attempt — unreferenced by gc/state.
+    const uint64_t deposed_attempt = after_fold.lease.seq + 1;   // round 2 renewed the lease once
+    const uint64_t deposed_gen = after_fold.snap_generation + 1;
+    EXPECT_TRUE(backend->head(store->layout().foldSealKey(deposed_gen, deposed_attempt)).exists)
+        << "the deposed round's fold seal is durable under its own unadopted attempt (harmless debris)";
+
+    // A DIFFERENT leader takes over. The lease steal protocol observes the stalled lease twice before
+    // stealing: the first round only observes and defers; the second steals and re-runs the round from
+    // scratch under its own fresh attempt.
+    Gc gc2(store, hexToU128("00000000000000000000000000000002"));
+    EXPECT_NO_THROW(runRegularRoundReclaiming(gc2));   // observe-and-defer (lease not yet provably stalled)
+    store->renewWatermarkOnce();
+
+    // From here gc2 owns the lease; drive it to a fixpoint. It must drain the unreachable blob WITHOUT
+    // wedging on the deposed attempt's debris (attempt-scoping keeps that debris invisible).
+    EXPECT_NO_THROW(runGcToFixpoint(store, gc2));
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+
+    const auto after_drain = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    EXPECT_GT(after_drain.snap_generation, after_fold.snap_generation) << "the round completed under gc2";
+    EXPECT_NE(after_drain.snap_attempt, deposed_attempt) << "the drained round never adopted the deposed attempt";
+
+    // A further round is a clean no-op.
+    EXPECT_NO_THROW(runRegularRoundReclaiming(gc2));
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+}
diff --git a/src/Disks/tests/gtest_cas_gc_round.cpp b/src/Disks/tests/gtest_cas_gc_round.cpp
new file mode 100644
index 000000000000..b9ed3278a95a
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_round.cpp
@@ -0,0 +1,2000 @@
+#include <gtest/gtest.h>
+
+#include <optional>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Common/ProfileEvents.h>
+#include "cas_test_helpers.h"
+
+namespace DB::ErrorCodes
+{
+extern const int BAD_ARGUMENTS;
+extern const int CORRUPTED_DATA;
+extern const int ABORTED;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASGCMetaOps;
+extern const Event CASGCEnumerationPages;
+extern const Event CASMountExclusivityViolation;
+extern const Event CASGCRetiredSpared;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+/// ROUND-LEVEL end-to-end GC tests over the root-local part-manifest model (one-pass ack-floor round:
+/// heartbeat floor -> fold with the three-cursor merge -> pre-CAS exact-token deletes -> single CAS -> trim).
+///
+/// This file is the survivor of the old snap/cascade-based `gtest_cas_gc_round.cpp`. The per-STEP
+/// behaviours it used to cover have moved to the dedicated GC-core suites and are intentionally NOT
+/// re-tested here:
+///   - fold edge dispatch (committed/precommit/promote/removal +/-1, 404 clamp/anomaly, fold barrier,
+///     ref-mismatch fail-closed) -> gtest_cas_gc_fold.cpp
+///   - condemn/graduate/delete + spare (manifest body deferred delete, publish racing the pass is spared,
+///     unreferenced blob exact-token delete) -> gtest_cas_gc_ack_floor.cpp
+///   - trim of folded owner events + idempotent crash replay -> gtest_cas_gc_resume.cpp
+/// What remains here is what those step suites do NOT cover: the LEASE/leadership protocol (the round's
+/// only stateful concurrency), the cursor-key codec, and the multi-round END-TO-END reclaim scenarios
+/// driven to fixpoint (publish->drop->reclaim, multi-ref sharing, spare-on-recheck race, idempotent
+/// fixpoint, split-brain duplicate-work-only). Every kept test keeps STRONG no-loss / no-dangle / no-leak
+/// assertions. No test sleeps or reads a clock — "time" is the order of `runRegularRound` calls.
+
+namespace
+{
+
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+const UInt128 kGcA = hexToU128("0000000000000000000000000000000a");
+const UInt128 kGcB = hexToU128("0000000000000000000000000000000b");
+const UInt128 kGcC = hexToU128("0000000000000000000000000000000c");
+
+ManifestRef ref(uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+
+bool blobExists(InMemoryBackend & b, const Layout & layout, const UInt128 & hash)
+{
+    return b.head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)})).exists;
+}
+
+bool manifestExists(InMemoryBackend & b, const Layout & layout, const ManifestId & id)
+{
+    return b.head(layout.manifestKey(id)).exists;
+}
+
+PoolPtr openTestPool(std::shared_ptr<InMemoryBackend> & out_backend)
+{
+    out_backend = std::make_shared<InMemoryBackend>();
+    return Pool::open(out_backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+PoolPtr openTestPoolWithConfig(std::shared_ptr<InMemoryBackend> & out_backend, PoolConfig config)
+{
+    out_backend = std::make_shared<InMemoryBackend>();
+    return Pool::open(out_backend, std::move(config));
+}
+
+/// Fault decorator for triage #5's regression test (`CASGCRetention.LosingRoundNeverDestroysParentSealGeneration`
+/// below): the `fail_at_call`-th `casPut` against `faulted_key` returns `Conflict` instead of committing —
+/// deterministically and single-threaded reproducing "this round's own gc/state CAS lost the race to a
+/// concurrent leader," which is the only condition under which the pre-CAS wholesale prune's choice of
+/// `referenced_generations` is externally observable (a round whose own CAS SUCCEEDS reclaims the same
+/// generation moments later via the existing, unrelated post-CAS hand-off delete regardless of this fix,
+/// so faulting the CAS is required, not optional, to pin the production call site). A call count, not a
+/// one-shot arm flag: `Gc::acquireOrRenewLease` issues its OWN earlier `casPut` on the very same gc/state
+/// key to renew the lease BEFORE a round folds — that renewal must SUCCEED (so the round actually reaches
+/// the fold/prune it's meant to exercise), and only the round's LATER, final round-commit `casPut` must
+/// be the one that loses. `fail_at_call` is 1-indexed and lets the test target that specific call exactly,
+/// computed from `calls_to_faulted_key` observed so far rather than hardcoded.
+class GcStateCasFaultBackend : public InMemoryBackend
+{
+public:
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    CasResult casPut(const String & key, const String & bytes,
+                     const std::optional<Token> & expected, const ObjectMeta & meta) override
+    {
+        if (key == faulted_key)
+        {
+            ++calls_to_faulted_key;
+            if (fail_at_call != 0 && calls_to_faulted_key == fail_at_call)
+                return CasResult{CasOutcome::Conflict, {}};
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+
+    String faulted_key;
+    size_t calls_to_faulted_key = 0;
+    size_t fail_at_call = 0;   /// 0 = never fault; else fault exactly the Nth casPut to `faulted_key`
+};
+
+GcState readState(InMemoryBackend & b, const Pool & s)
+{
+    const auto got = b.get(s.layout().gcStateKey());
+    if (!got)
+    {
+        ADD_FAILURE() << "gc/state absent";
+        return {};
+    }
+    return decodeGcState(got->bytes);
+}
+
+/// Whether ANY gc-shard's adopted-seal run still holds a `kCondemned` row (retired-in-snapshot T4: the
+/// retired state rides the snapshot run, not a separate retired-list object) — the ack-floor deletion
+/// pipeline is still in flight while this is true.
+bool anyRetiredPending(InMemoryBackend & b, const Pool & s)
+{
+    return anyCondemnedInSeal(b, s.layout());
+}
+
+/// Drive a Gc to fixpoint over the round-paced retired-cursor pipeline: run rounds, renewing the store's
+/// own heartbeat after each (`renewWatermarkOnce` — keeps the lease + build-watermark floor current;
+/// graduation itself paces on rounds alone). A condemned blob traverses the multi-round condemn ->
+/// graduate -> delete pipeline, so "fixpoint" is reached only when a round did NO work AND the current
+/// retired list is empty (nothing still in flight). Returns the number of rounds that held the lease and
+/// did work. Bounded so a non-converging core fails downstream assertions rather than hanging.
+size_t driveToFixpoint(InMemoryBackend & backend, const PoolPtr & store, Gc & gc)
+{
+    size_t working_rounds = 0;
+    for (size_t r = 0; r < 64; ++r)
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        if (!rep.acquired_lease)
+            continue;
+        store->renewWatermarkOnce();
+        const bool no_work = rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0;
+        if (no_work && !anyRetiredPending(backend, *store))
+            break;
+        if (!no_work)
+            ++working_rounds;
+    }
+    return working_rounds;
+}
+
+/// A full key -> token snapshot of the backend, for the previewDeletes write-free invariant: any
+/// put/casPut/overwrite mints a fresh token (or adds a key) and any delete removes one, so an unchanged
+/// map across a call proves it performed NO writes.
+std::map<String, String> snapshotKeyTokens(InMemoryBackend & b)
+{
+    std::map<String, String> out;
+    String cursor;
+    while (true)
+    {
+        const ListPage page = b.list("", cursor, 100000);
+        for (const ListedKey & k : page.keys)
+            out[k.key] = k.token ? k.token->value : String{};
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    return out;
+}
+
+}
+
+/// ---- LEASE / leadership protocol (the round's only stateful concurrency) ----
+///
+/// The lease steal window is observation-based and deterministic (see CasGc.h): a contender becomes
+/// steal-eligible when it observes the SAME (owner, seq) across two of its own consecutive round
+/// attempts. The new model keeps gc/state {round, snap_generation, lease}, so these tests
+/// are model-agnostic and were ported verbatim from the pre-redesign suite.
+
+TEST(CASGCLease, FreshPoolAcquiresAndRenews)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc(s, kGc);
+
+    EXPECT_TRUE(gc.runRegularRound().acquired_lease);
+    const GcState st1 = readState(*b, *s);
+    EXPECT_EQ(st1.lease.owner, kGc);
+    const uint64_t seq1 = st1.lease.seq;
+    EXPECT_GE(seq1, 1u);
+
+    EXPECT_TRUE(gc.runRegularRound().acquired_lease);            /// renew
+    const GcState st2 = readState(*b, *s);
+    EXPECT_EQ(st2.lease.owner, kGc);
+    EXPECT_GT(st2.lease.seq, seq1);                              /// seq strictly advanced
+}
+
+TEST(CASGCLease, ContenderBacksOffWhileIncumbentRenews)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// first sight: record observation
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);           /// incumbent renews (seq advances)
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// gc2 sees a NEW seq => incumbent alive
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// alive again - never steals while renewing
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcA);
+}
+
+TEST(CASGCLease, StealAfterObservedNonRenewalAdvancesLease)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);
+    const GcState st0 = readState(*b, *s);
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// observation recorded; gc1 then DIES
+    EXPECT_TRUE(gc2.runRegularRound().acquired_lease);           /// same (owner, seq) observed twice => steal
+    const GcState st = readState(*b, *s);
+    EXPECT_EQ(st.lease.owner, kGcB);
+    EXPECT_GT(st.lease.seq, st0.lease.seq);
+}
+
+TEST(CASGCLease, HeartbeatBlocksFalseStealOfAliveLeader)
+{
+    /// B160: a slow-but-alive incumbent whose lease.seq is frozen for its (long) round must NOT be
+    /// stolen from, because its advisory heartbeat keeps advancing.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);          /// gc1 leads (seq frozen for its round)
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);         /// gc2 observes (gc/hb absent yet)
+
+    Gc::pulseHeartbeat(*s, kGcA);                              /// gc1 mid-round but heartbeating (hb 0->1)
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);         /// hb advanced => alive => NO steal
+    Gc::pulseHeartbeat(*s, kGcA);                              /// hb 1->2
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);         /// still no steal while heartbeating
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcA);            /// gc1 still owns the lease
+}
+
+/// A7-HIGH-fix follow-up (residual timing window): an allow_steal=false observation of a foreign
+/// incumbent (the manual `SYSTEM ... GC` path) must NOT arm the frozen-tuple comparison that the loop's
+/// very next (allow_steal=true) call uses to decide whether to steal. Without this, a manual round's
+/// observation at time t, immediately followed by an unluckily-timed scheduled tick at t+epsilon (no
+/// real chance for a live incumbent to heartbeat in between), would see the SAME (owner, seq, hb) twice
+/// and steal a LIVE leader — exactly the hazard the allow_steal gate alone does not close, since it only
+/// stops the MANUAL call itself from executing the steal CAS, not from contaminating the shared `Gc`
+/// instance's observation state that the next allow_steal=true call reads.
+TEST(CASGCLease, ManualObservationNeverArmsTheLoopsStealDecision)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);   /// plays the scheduler's ONE shared Gc, observed by both manual and loop calls
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);           /// gc1 leads; never renews, never heartbeats
+
+    /// Manual observation "at t": allow_steal=false. Would normally be obs #1, but must NOT record it.
+    EXPECT_FALSE(gc2.runRegularRound({}, /*allow_steal=*/false).acquired_lease);
+
+    /// Loop-path call "immediately after" (allow_steal=true, the default): with the fix, gc2's
+    /// last_seen_* is UNTOUCHED by the manual call above, so this is still effectively obs #1 (first
+    /// sight) => must NOT steal. Pre-fix (manual observations armed the state), this would see the same
+    /// frozen tuple as "twice observed" and steal gc1's still-live lease.
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcA);               /// gc1 keeps the lease
+
+    /// The loop still recovers a genuinely dead incumbent across its OWN two spaced observations: the
+    /// call above was the loop's real obs #1 (now armed, since allow_steal=true); this one is obs #2 of
+    /// the same still-frozen tuple => steal-eligible => steals. Recovery is delayed, not disabled.
+    EXPECT_TRUE(gc2.runRegularRound().acquired_lease);
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcB);
+}
+
+/// P3-B1 (2026-07-11 mid-switch soak wedge): CasGcScheduler used to flip its `i_am_leader` flag (which
+/// gates the heartbeat thread's pulses) only AFTER `runRegularRound` RETURNS, while the lease is
+/// acquired INSIDE the round, before the (potentially long) fold. A brand-new leader's FIRST round
+/// therefore ran the whole fold with no heartbeat cover: a follower observing the frozen (owner, seq)
+/// across two of its own ticks steals deterministically once that first round outlasts ~2 ticks -
+/// mutual-steal livelock under a slow fold. The fix moves the "start heartbeating" action to the
+/// INSTANT the lease is acquired (`Gc::runRegularRound`'s new `on_lease_acquired` hook), fired before
+/// the fold begins. These two tests pin the protocol both ways at the `Gc` level (the scheduler itself
+/// only wires `i_am_leader.store(true, ...)` + one `pulseHeartbeat` call into that hook - a thread-pacing
+/// wire-up not practically unit-testable without sleeps; verified by code review + the full gtest run).
+
+TEST(CASGCLease, WithoutAcquireTimePulseFirstRoundStealsDeterministically)
+{
+    /// RED-before-the-fix scenario: gc1 acquires the lease and (simulating a long first round) never
+    /// pulses `gc/hb` and never renews - exactly what happened before `on_lease_acquired` existed.
+    /// gc2's SECOND observation of the same frozen (owner, seq, hb) steals, per the documented protocol.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);    /// gc1 becomes leader; NO pulse follows (the bug)
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);   /// obs #1: records (owner=A, seq, hb=absent)
+    EXPECT_TRUE(gc2.runRegularRound().acquired_lease);    /// obs #2: unchanged => steal-eligible => STEALS
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcB);
+}
+
+TEST(CASGCLease, AcquireTimePulseProtectsNewLeadersFirstRound)
+{
+    /// GREEN-after-the-fix scenario: with the fix, `i_am_leader` flips true and the FIRST pulse fires
+    /// the instant gc1 acquires the lease - before B's first observation even happens - and the
+    /// (separately-threaded, out of scope here) `heartbeatLoop` keeps landing further pulses on its own
+    /// cadence for as long as `i_am_leader` stays true, i.e. for the whole duration of gc1's first round.
+    /// The net effect proven here is the one that matters: SOME pulse lands between B's two
+    /// observations (not just before both, and not only after both), so B's second observation sees hb
+    /// advanced relative to its first and backs off instead of stealing - exactly what never happened
+    /// pre-fix, when `i_am_leader` (and hence every pulse) was gated on the round having already returned.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);    /// gc1 becomes leader (still mid-fold, seq frozen)
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);   /// obs #1: records (owner=A, seq, hb=absent)
+    Gc::pulseHeartbeat(*s, kGcA);                          /// a heartbeatLoop tick lands mid-round (hb 0->1)
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);   /// obs #2: hb advanced since obs #1 => alive => NO steal
+
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcA);       /// gc1 keeps the lease through its whole first round
+}
+
+TEST(CASGCLease, StaleOwnerHeartbeatDoesNotEnableFalseSteal)
+{
+    /// A deposed leader's heartbeat thread keeps pulsing until its next round notices the lost lease
+    /// (`i_am_leader` is only reset there), and `pulseHeartbeat` stamps `owner = self` while a losing
+    /// CAS write silently vanishes — so a zombie old leader can keep `gc/hb.owner` pointing at ITSELF
+    /// even while the live new leader is pulsing too. The liveness gate must therefore treat ANY
+    /// movement of the observed (owner, hb_seq) pair between a follower's two ticks as "someone is
+    /// alive": comparing hb_seq is only meaningful against the SAME remembered hb owner. The old
+    /// predicate compared `hb.owner` with the LEASE owner instead, so a zombie-owned hb read as
+    /// "not the leader's heartbeat" on both ticks and a live, pulsing new leader got its lease stolen.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+    Gc gc3(s, kGcC);
+
+    /// gc1 leads, beats, then dies mid-round; gc2 legitimately steals the lease.
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);
+    Gc::pulseHeartbeat(*s, kGcA);
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);   /// obs #1 of gc1's frozen tuple
+    EXPECT_TRUE(gc2.runRegularRound().acquired_lease);    /// obs #2: frozen lease + frozen hb => steal
+    ASSERT_EQ(readState(*b, *s).lease.owner, kGcB);
+
+    /// gc2 is now mid-long-round (lease tuple frozen) and PULSING — but gc1's zombie heartbeat
+    /// thread interleaves after every gc2 pulse, so the follower gc3 only ever OBSERVES gc1-owned
+    /// heartbeats. The pair keeps moving, which is proof of life.
+    Gc::pulseHeartbeat(*s, kGcB);
+    Gc::pulseHeartbeat(*s, kGcA);                          /// zombie masks gc2's pulse
+    EXPECT_FALSE(gc3.runRegularRound().acquired_lease);   /// obs #1: records (hb owner=A, seq)
+    Gc::pulseHeartbeat(*s, kGcB);
+    Gc::pulseHeartbeat(*s, kGcA);                          /// zombie masks again
+    EXPECT_FALSE(gc3.runRegularRound().acquired_lease);   /// obs #2: hb pair MOVED => alive => NO steal
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcB);       /// the live leader keeps its lease
+
+    /// Liveness is preserved: once everything genuinely freezes (gc2 dead, zombie gone), the next
+    /// tick completes the window — obs #2 above already re-armed on the now-frozen (lease, hb) pair.
+    EXPECT_TRUE(gc3.runRegularRound().acquired_lease);    /// still frozen a full tick later => steal
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcC);
+}
+
+TEST(CASGCLease, FailoverStealOnceHeartbeatStops)
+{
+    /// B160: once the incumbent stops heartbeating (it died), a follower observing the now-frozen
+    /// heartbeat steals — automatic failover is preserved.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);         /// obs #1
+    Gc::pulseHeartbeat(*s, kGcA);                              /// one last pulse (hb 0->1)
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);         /// hb advanced => no steal; records hb=1
+    /// gc1 now DEAD: no renew, no further pulse. hb stays at 1 == gc2's last observation.
+    EXPECT_TRUE(gc2.runRegularRound().acquired_lease);          /// hb frozen + seq frozen => STEAL
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcB);
+}
+
+TEST(CASGCLease, DeadIncumbentThenRevivedIncumbentWinsRace)
+{
+    /// A stalled incumbent that revives and renews BEFORE the contender's second look resets the
+    /// contender's window: gc2's second observation sees a NEW seq => NOT steal-eligible => backs off.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// obs #1
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);           /// gc1 revives and renews
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// new seq seen => window resets
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcA);
+}
+
+TEST(CASGCLease, ConcurrentStealLosesCas)
+{
+    /// The CAS-race horn: gc2 is steal-eligible and goes for the CAS, but gc/state moved under it
+    /// (injected one-shot conflict). It must back off (never acquired=true off a lost CAS) and the
+    /// owner on storage must be unperturbed. The injected conflict left the object unchanged, so gc2's
+    /// NEXT round is steal-eligible again and succeeds.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);
+    const GcState st0 = readState(*b, *s);
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// obs #1; gc1 stalls now
+    b->failNextCasPut(s->layout().gcStateKey());                 /// inject: gc2's steal CAS conflicts
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// steal attempt loses the CAS => back off
+    const GcState st1 = readState(*b, *s);
+    EXPECT_EQ(st1.lease.owner, kGcA);                            /// unchanged
+    EXPECT_EQ(st1.lease.seq, st0.lease.seq);                     /// nothing clobbered
+    EXPECT_TRUE(gc2.runRegularRound().acquired_lease);           /// still steal-eligible => succeeds now
+    EXPECT_EQ(readState(*b, *s).lease.owner, kGcB);
+}
+
+TEST(CASGCLease, CreateConflictReReadsWithinTheBound)
+{
+    /// The create-Conflict branch: a fresh pool where the create-if-absent CAS conflicts (one-shot).
+    /// The contender re-reads and falls through within its bounded (2) CAS attempts — the re-read still
+    /// finds the key absent, so the second attempt creates and acquires.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc(s, hexToU128("0000000000000000000000000000000c"));
+
+    b->failNextCasPut(s->layout().gcStateKey());
+    EXPECT_TRUE(gc.runRegularRound().acquired_lease);
+    const GcState st = readState(*b, *s);
+    EXPECT_EQ(st.lease.owner, hexToU128("0000000000000000000000000000000c"));
+    EXPECT_EQ(st.lease.seq, 1u);
+}
+
+TEST(CASGCLease, CtorFailsClosedOnBadArguments)
+{
+    /// Guards: a null store and gc_id == 0 (reserved for "lease never held") are caller bugs.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&] { Gc(nullptr, kGc); });
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&] { Gc(s, DB::UInt128(0)); });
+}
+
+TEST(CASGCLease, IncumbentRenewConflictRetriesOnceAndAcquires)
+{
+    /// The incumbent's own renew CAS conflicts (one-shot). Re-read sees our own ownership => the renew
+    /// is retried ONCE within the bounded (2) CAS attempts => acquired. Never acquired=true without a
+    /// Committed CAS — storage must carry the seq the SECOND (committed) attempt wrote.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc(s, hexToU128("0000000000000000000000000000000d"));
+
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);            /// create: seq 1
+    b->failNextCasPut(s->layout().gcStateKey());                 /// inject: the renew CAS conflicts
+    EXPECT_TRUE(gc.runRegularRound().acquired_lease);            /// re-read (still us) => retried once
+    const GcState st = readState(*b, *s);
+    EXPECT_EQ(st.lease.owner, hexToU128("0000000000000000000000000000000d"));
+    EXPECT_EQ(st.lease.seq, 2u);                                 /// the committed retry's seq
+}
+
+TEST(CASGCLease, VanishedStateAfterObservationFailsClosed)
+{
+    /// gc/state is never legally deleted - absent AFTER a recorded observation proves an out-of-model
+    /// deletion. Recreating a default state would reset round/cursors; the lease protocol
+    /// must fail closed (CORRUPTED_DATA) instead.
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    Gc gc1(s, kGcA);
+    Gc gc2(s, kGcB);
+
+    ASSERT_TRUE(gc1.runRegularRound().acquired_lease);
+    EXPECT_FALSE(gc2.runRegularRound().acquired_lease);          /// gc2 records an observation
+
+    const auto head = b->head(s->layout().gcStateKey());         /// out-of-model wipe (raw delete)
+    ASSERT_TRUE(head.exists);
+    ASSERT_EQ(b->deleteExact(s->layout().gcStateKey(), head.token).kind, DeleteOutcome::Kind::Deleted);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { gc2.runRegularRound(); });
+}
+
+/// ---- END-TO-END round scenarios driven to fixpoint (the headline value of this file) ----
+
+/// publish -> drop -> GC-to-fixpoint reclaim: a committed ref names a blob; after the ref is dropped,
+/// the round protocol collects the blob (exact-token delete) AND the owner-removed manifest body, and a
+/// further round is a clean no-op. The strongest no-loss/no-leak oracle: while the ref is live the blob
+/// is NEVER touched; once dropped, BOTH the blob and the manifest are gone and nothing dangles.
+TEST(CASGCRound, PublishDropReclaimsBlobAndManifestToFixpoint)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    const ManifestId id{ns, r};
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);
+    /// While live: the blob's in-degree is 1 and NOTHING is collected (no-loss).
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+    EXPECT_TRUE(manifestExists(*backend, store->layout(), id));
+
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    driveToFixpoint(*backend, store, gc);
+    /// After drop + fixpoint: the blob's only edge is gone, the blob is collected, the owner-removed
+    /// manifest body is collected, and the in-degree generation reflects zero (no-leak / no-dangle).
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+    EXPECT_FALSE(manifestExists(*backend, store->layout(), id));
+
+    /// Idempotent: re-running to fixpoint changes nothing and never throws.
+    EXPECT_NO_THROW(driveToFixpoint(*backend, store, gc));
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+}
+
+/// retired-in-snapshot T4: after a round condemns one blob, the ADOPTED fold seal's per-shard
+/// condemned_summary reflects it (condemned_total == 1, pending_total == 0) — distilled zero-I/O from the
+/// kCondemned rows the fold sealed into the snapshot run.
+TEST(CASGCRound, CondemnRoundSealSummaryCountsCondemned)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();                 /// folds the +1
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);   /// the -1 condemns it
+
+    /// Drive rounds until the blob shows up condemned in the adopted-seal run; capture that seal.
+    bool condemned = false;
+    CasFoldSeal seal;
+    for (int i = 0; i < 6 && !condemned; ++i)
+    {
+        gc.runRegularRound();
+        store->renewWatermarkOnce();
+        const GcState st = readState(*backend, *store);
+        seal = decodeFoldSeal(
+            backend->get(store->layout().foldSealKey(st.snap_generation, st.snap_attempt))->bytes);
+        for (const RetiredEntry & e : currentRetiredSet(*backend, store->layout(), /*shard*/0))
+            if (e.ref == DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(DB::UInt128(1))})
+                condemned = true;
+    }
+    ASSERT_TRUE(condemned) << "blob never condemned into the snapshot run";
+    ASSERT_TRUE(seal.condemned_summary.contains(0)) << "seal summary must be total over gc_shards";
+    EXPECT_EQ(seal.condemned_summary.at(0).condemned_total, 1u);
+    EXPECT_EQ(seal.condemned_summary.at(0).pending_total, 0u)
+        << "a freshly condemned entry is not yet delete_pending";
+    EXPECT_LT(seal.condemned_summary.at(0).oldest_nonpending_condemn_round,
+              std::numeric_limits<uint64_t>::max())
+        << "a non-pending condemned entry records its condemn round";
+}
+
+/// retired-in-snapshot T5: `previewDeletes` streams the adopted seal's `kCondemned` rows and reports each
+/// with the STORED condemn-time token — `awaiting_graduation` while newly condemned, then `delete_pending`
+/// once graduated, and NOTHING once the exact-token redelete has removed the blob. The preview performs no
+/// HEAD on the condemned rows (the token is durable in-run) and is WRITE-FREE throughout (spec §5 req 1).
+TEST(CASGCRound, PreviewReportsCondemnedRowsAndIsWriteFree)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    const UInt128 blob = DB::UInt128(1);
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);                 /// round 1: folds the +1; blob referenced
+    EXPECT_TRUE(gc.previewDeletes().empty()) << "a live-referenced blob is never previewed for deletion";
+
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    runRegularRoundReclaiming(gc);                 /// condemning round: -1 => in-degree 0 => kCondemned row (not pending)
+
+    /// Write-free contract: a full key->token snapshot must be identical across the previewDeletes call.
+    const auto before = snapshotKeyTokens(*backend);
+    const std::vector<Gc::PreviewEntry> awaiting = gc.previewDeletes();
+    const auto after = snapshotKeyTokens(*backend);
+    EXPECT_EQ(before, after) << "previewDeletes must perform NO writes (put/casPut/overwrite/delete)";
+
+    ASSERT_EQ(awaiting.size(), 1u) << "exactly the one condemned blob is previewed";
+    EXPECT_EQ(awaiting[0].ref, (DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(blob)}));
+    EXPECT_EQ(awaiting[0].key, store->layout().blobKey(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(blob)}));
+    EXPECT_EQ(awaiting[0].reason, "awaiting_graduation");
+    EXPECT_FALSE(awaiting[0].token.value.empty()) << "must carry the stored condemn-time token";
+    EXPECT_GT(awaiting[0].condemn_round, 0u) << "must carry the stored condemn round";
+
+    runRegularRoundReclaiming(gc);                 /// graduation round: entry becomes delete_pending (blob still present)
+    const std::vector<Gc::PreviewEntry> pending = gc.previewDeletes();
+    ASSERT_EQ(pending.size(), 1u);
+    EXPECT_EQ(pending[0].ref, (DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(blob)}));
+    EXPECT_EQ(pending[0].reason, "delete_pending");
+    EXPECT_FALSE(pending[0].token.value.empty());
+
+    runRegularRoundReclaiming(gc);                 /// redelete round: exact-token delete; entry dropped; blob gone
+    EXPECT_FALSE(blobExists(*backend, store->layout(), blob));
+    EXPECT_TRUE(gc.previewDeletes().empty()) << "nothing to preview once the blob is redeleted";
+}
+
+/// A fully idle fold pure-carries every shard's authoritative rows verbatim. The parent is first made
+/// non-vacuous with one live blob in each of two shards; the forced no-delta successor must preserve
+/// both `btr` rows and the total `cnd` domain byte-for-byte.
+TEST(CASGCRound, PureCarryRoundPreservesAuthoritativeShardRowsVerbatim)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .gc_shards = 2, .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+
+    Gc gc(store, kGc);
+
+    const UInt128 shard0_blob{1};
+    const UInt128 shard1_blob = (UInt128{1} << 64) | UInt128{1};
+    ASSERT_EQ(blobShard(legacyMetaTestRef(shard0_blob), 2), 0u);
+    ASSERT_EQ(blobShard(legacyMetaTestRef(shard1_blob), 2), 1u);
+
+    const ManifestRef r0 = ref(1, 0xAA);
+    const ManifestRef r1 = ref(2, 0xBB);
+    writeBlobBody(*backend, store->layout(), shard0_blob);
+    writeBlobBody(*backend, store->layout(), shard1_blob);
+    writeManifestRaw(*backend, store->layout(), ns, r0, {blobEntryFor("a", shard0_blob)});
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("b", shard1_blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl0", std::nullopt, r0);
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl1", std::nullopt, r1);
+    gc.runRegularRound();
+    const GcState st1 = readState(*backend, *store);
+    const CasFoldSeal seal1 = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(st1.snap_generation, st1.snap_attempt))->bytes,
+        store->layout(), /*gc_shards=*/2);
+
+    /// No state changes after the parent. The zero defer bound forces an actual fold rather than DEFER,
+    /// so every shard takes the production pure-carry path.
+    gc.runRegularRound();
+    const GcState st2 = readState(*backend, *store);
+    const CasFoldSeal seal2 = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(st2.snap_generation, st2.snap_attempt))->bytes,
+        store->layout(), /*gc_shards=*/2);
+
+    /// TOTALITY: both seals carry a summary entry for every gc-shard.
+    ASSERT_EQ(seal1.condemned_summary.size(), 2u);
+    ASSERT_EQ(seal2.condemned_summary.size(), 2u);
+    EXPECT_TRUE(seal1.condemned_summary.contains(0) && seal1.condemned_summary.contains(1));
+    EXPECT_TRUE(seal2.condemned_summary.contains(0) && seal2.condemned_summary.contains(1));
+
+    /// Capacity reserves one widest `btr` row per shard. Pin the production pure-carry seal to the
+    /// authoritative grammar that makes that bound sufficient: at most one in-range canonical seq-0
+    /// run per shard, beside exactly one `cnd` row for every shard.
+    bool run_seen[2] = {false, false};
+    ASSERT_EQ(seal1.blob_target_runs.size(), 2u);
+    ASSERT_EQ(seal2.blob_target_runs.size(), 2u);
+    for (const RunRef & run : seal2.blob_target_runs)
+    {
+        ASSERT_LT(run.shard, 2u);
+        EXPECT_FALSE(run_seen[run.shard]);
+        run_seen[run.shard] = true;
+        const auto parsed = store->layout().parseBlobTargetRunKey(run.key);
+        ASSERT_TRUE(parsed.has_value());
+        EXPECT_EQ(parsed->shard, run.shard);
+        EXPECT_EQ(parsed->generation, run.generation);
+        EXPECT_EQ(parsed->seq, 0u);
+    }
+    EXPECT_TRUE(run_seen[0]);
+    EXPECT_TRUE(run_seen[1]);
+    for (uint64_t shard = 0; shard < 2; ++shard)
+    {
+        const auto parent_run = std::find_if(
+            seal1.blob_target_runs.begin(), seal1.blob_target_runs.end(),
+            [shard](const RunRef & run) { return run.shard == shard; });
+        const auto carried_run = std::find_if(
+            seal2.blob_target_runs.begin(), seal2.blob_target_runs.end(),
+            [shard](const RunRef & run) { return run.shard == shard; });
+        ASSERT_NE(parent_run, seal1.blob_target_runs.end());
+        ASSERT_NE(carried_run, seal2.blob_target_runs.end());
+        EXPECT_EQ(*carried_run, *parent_run);
+    }
+
+    /// VERBATIM CARRY: nothing was ever condemned, so every shard's summary is the zero entry, carried
+    /// unchanged from parent to child across the fully idle fold.
+    for (uint64_t shard = 0; shard < 2; ++shard)
+    {
+        EXPECT_EQ(seal2.condemned_summary.at(shard), seal1.condemned_summary.at(shard))
+            << "shard " << shard << " summary must be carried verbatim from the parent seal";
+        EXPECT_EQ(seal2.condemned_summary.at(shard).condemned_total, 0u);
+    }
+}
+
+/// Attempt-scoping (B2): a fold seal planted under a NON-adopted attempt at the adopted generation
+/// must be INVISIBLE to every reader. A deposed leader writes its fold seal under its own (unadopted)
+/// `lease.seq`; that artifact lives at `foldSealKey(snap_generation, snap_attempt + k)` and no decision
+/// path may resolve it. `previewDeletes` reads the in-degree generation strictly at the adopted
+/// `(snap_generation, snap_attempt)`, so the decoy must not change its output and must not throw. This
+/// is the implementation-level complement to the TLA+ `INV_ONLY_ADOPTED_VIEWABLE` gate.
+TEST(CASGCRound, NonAdoptedAttemptSealIgnored)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();
+    const GcState st = readState(*backend, *store);
+    ASSERT_GT(st.snap_generation, 0u);
+
+    /// Control preview BEFORE the decoy (previewDeletes is write-free, so the result is deterministic).
+    const auto control = gc.previewDeletes();
+
+    /// Plant a decoy fold seal under a DIFFERENT attempt at the SAME generation (a deposed leader's
+    /// unadopted artifact). It must be invisible to the adopted-attempt readers.
+    backend->putIfAbsent(store->layout().foldSealKey(st.snap_generation, st.snap_attempt + 999),
+                         "decoy-seal-bytes");
+
+    /// No reader resolves the non-adopted attempt: no throw, and the preview is unchanged by the decoy.
+    std::vector<Gc::PreviewEntry> after;
+    EXPECT_NO_THROW(after = gc.previewDeletes());
+    EXPECT_EQ(after.size(), control.size())
+        << "a non-adopted attempt's fold seal must not influence previewDeletes";
+
+    /// A further full round must still proceed without throwing and without the decoy wedging it.
+    EXPECT_NO_THROW(gc.runRegularRound());
+}
+
+/// B11: the round summary must count manifest-body (tree) deletes separately from blob deletes. A drop
+/// that reclaims one manifest body must report manifests_deleted >= 1 in the RoundReport of the
+/// reclaiming round, while blobs and manifests remain separately countable.
+TEST(CASGCRound, RoundSummaryCountsManifestBodyDeletes)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xCC);
+    const ManifestId id{ns, r};
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(3));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(3))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);   /// fold the publish; no delete yet
+
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+
+    /// §0 introspection: both counters are captured BEFORE the condemn+delete pipeline below, which
+    /// drives the round's meta pool (condemn/spare/delete) and its own orphan-sweep cursor pass.
+    const auto meta_ops_before = ProfileEvents::global_counters[ProfileEvents::CASGCMetaOps].load();
+    const auto pages_before = ProfileEvents::global_counters[ProfileEvents::CASGCEnumerationPages].load();
+
+    /// Ack-floor drift: the owner-removed manifest body is deleted in the CONDEMNING round (post-CAS,
+    /// after its -1 is adopted), while the blob's exact-token delete happens a few rounds later once the
+    /// ack floor graduates its retired entry. So the two deletes fall in DIFFERENT reports now — accumulate
+    /// across the pipeline (renewing the ack each round so the floor advances) and assert both were counted.
+    uint64_t total_manifests_deleted = 0;
+    uint64_t total_blob_deleted = 0;
+    for (size_t i = 0; i < 64; ++i)
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        if (!rep.acquired_lease)
+            continue;
+        store->renewWatermarkOnce();
+        total_manifests_deleted += rep.manifests_deleted;
+        total_blob_deleted += rep.deleted;
+        if (total_manifests_deleted > 0 && total_blob_deleted > 0)
+            break;
+    }
+
+    /// B11: the manifest-body delete must be counted separately from the blob delete.
+    EXPECT_GE(total_manifests_deleted, 1u)
+        << "round summary must count the owner-removed manifest body delete (B11 — manifests_deleted)";
+    /// Blobs and manifests are separately countable: the blob delete (deleted >= 1) is independent.
+    EXPECT_GE(total_blob_deleted, 1u)
+        << "the blob exact-token delete must still be counted in deleted";
+    /// The manifest body is gone and the blob is gone — no-leak / no-dangle.
+    EXPECT_FALSE(manifestExists(*backend, store->layout(), id));
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(3)));
+
+    /// §0 introspection: the exact-token blob delete above scheduled at least one per-hash freshness-meta
+    /// op on the round's bounded meta pool, and every round ran its own orphan-manifest-sweep cursor pass
+    /// (default `manifest_sweep_list_budget_keys` is nonzero), fetching at least one LIST page directly.
+    EXPECT_GE(ProfileEvents::global_counters[ProfileEvents::CASGCMetaOps].load() - meta_ops_before, 1);
+    EXPECT_GE(ProfileEvents::global_counters[ProfileEvents::CASGCEnumerationPages].load() - pages_before, 1);
+}
+
+/// Manifest-body cleanup (post-CAS `manifest_deletes` phase) has no cap: the ref-log intake cursor that
+/// discovers each owner-removed manifest commits in the SAME round's CAS that produces `mf_cleanup`, so an
+/// entry a cap declined would never be re-derived by this pipeline -- a bounded burst would become a
+/// permanent leak. Five tables' manifests are all owner-removed in one fold; the round must delete all
+/// five bodies in the same round, with nothing left un-deleted.
+TEST(CASGCRound, ManifestCleanupDrainsEntireRoundWithNoSkips)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "gc-runner",
+        .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    constexpr int kManifests = 5;
+
+    std::vector<ManifestId> ids;
+    for (int i = 0; i < kManifests; ++i)
+    {
+        const ManifestRef r = ref(1, 0xD0 + i);
+        writeBlobBody(*backend, store->layout(), DB::UInt128(100 + i));
+        writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(100 + i))});
+        publishCommittedTransition(*backend, store->layout(), ns, "tbl" + std::to_string(i), std::nullopt, r);
+        ids.push_back(ManifestId{ns, r});
+    }
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);   /// fold all five +1s; no manifest owner-removed yet
+    for (const ManifestId & id : ids)
+        ASSERT_TRUE(manifestExists(*backend, store->layout(), id));
+
+    /// Remove all five owners in one window; the next fold's intake sees all five `-1` edges together.
+    for (int i = 0; i < kManifests; ++i)
+        dropRefTransition(*backend, store->layout(), ns, "tbl" + std::to_string(i), ref(1, 0xD0 + i));
+
+    const RoundReport rep = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(rep.acquired_lease);
+    EXPECT_EQ(rep.manifests_deleted, kManifests)
+        << "manifest_deletes must drain the entire mf_cleanup vector in one round, not cap it";
+
+    for (const ManifestId & id : ids)
+        EXPECT_FALSE(manifestExists(*backend, store->layout(), id))
+            << "an unbudgeted cleanup must leave nothing surviving the round it was discovered in";
+}
+
+/// §0 introspection follow-up: `CASGCEnumerationPages` must not depend on the orphan-manifest sweep alone
+/// (`manifest_sweep_list_budget_keys` zeroed below disables that pass entirely). The mandatory per-round
+/// `cas/ns/stream/` scan -- `listRefPrefix`'s pre-fold DEFER signal and the fold share its result --
+/// must still land at least one page each round.
+TEST(CASGCRound, EnumerationPagesCountedEvenWithSweepBudgetZeroed)
+{
+    std::shared_ptr<InMemoryBackend> backend;
+    PoolConfig config;
+    config.pool_prefix = "p";
+    config.server_root_id = "test";
+    config.manifest_sweep_list_budget_keys = 0;   /// disables the orphan sweep's own LIST entirely
+    config.gc_fold_max_defer_rounds = 0;          /// force fold-every-round (Phase-4 Lever A would defer)
+    auto store = openTestPoolWithConfig(backend, config);
+
+    Gc gc(store, kGc);
+    const auto pages_before = ProfileEvents::global_counters[ProfileEvents::CASGCEnumerationPages].load();
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    EXPECT_GE(ProfileEvents::global_counters[ProfileEvents::CASGCEnumerationPages].load() - pages_before, 1)
+        << "the round's own cas/ns/stream/ enumeration must count pages independent of "
+           "the orphan sweep";
+}
+
+/// M1 REGRESSION (cross-round fold cursor must survive independent of trim): a folded-but-untrimmed owner
+/// event must NOT be re-folded by the next round. With eager trim the folded event is removed so the bug
+/// (sealedCursorOf resetting to 0 after a completed round, because snap_generation points at the COMPLETION
+/// generation whose fold_seal lives at the parent) is MASKED. Disable trim to expose it: the publish event
+/// stays in the journal, so a round that re-folds from 0 emits a SECOND +1 and drives the blob's in-degree
+/// to 2 (a silent over-pin => leak). The fix carries the per-shard fold cursor into the completion seal so
+/// the next round recovers the exact cursor. Asserts in-degree stays EXACTLY 1 across >= 2 re-folds.
+TEST(CASGCRound, FoldCursorSurvivesAcrossRoundsWithoutTrim)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.setTrimEnabledForTest(false);   /// keep the folded publish event in the journal across rounds
+
+    /// Round 1 folds the +1 edge: in-degree 1, blob pinned.
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    /// Several more rounds. The publish event is STILL in the journal (trim off). Each round must
+    /// recover the exact sealed cursor and re-fold NOTHING for this shard — in-degree stays exactly 1.
+    for (int round = 0; round < 3; ++round)
+    {
+        ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+        EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1)
+            << "round " << round << ": a folded-but-untrimmed event was re-folded => blob in-degree double-counted";
+    }
+
+    /// No-loss throughout: the live blob and its owner body are intact.
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+    EXPECT_TRUE(manifestExists(*backend, store->layout(), ManifestId{ns, r}));
+}
+
+/// Multi-ref sharing (INV-NO-LOSS): one blob referenced by TWO committed refs is spared until BOTH
+/// drop. Dropping the first ref must NOT collect the blob (the second ref still pins it); only after the
+/// second ref drops does the round collect it.
+TEST(CASGCRound, SharedBlobSparedUntilBothRefsDrop)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xA1);
+    const ManifestRef r2 = ref(2, 0xA2);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    /// Two distinct manifests at two distinct refs, BOTH referencing the same shared blob 1.
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl1", std::nullopt, r1);
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl2", std::nullopt, r2);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 2);   /// two source edges
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+
+    /// Drop the FIRST ref: in-degree falls to 1, blob STILL pinned by tbl2 (spared).
+    dropRefTransition(*backend, store->layout(), ns, "tbl1", r1);
+    driveToFixpoint(*backend, store, gc);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "shared blob must survive while a second ref still names it";
+
+    /// Drop the SECOND ref: in-degree reaches 0, blob is finally collected.
+    dropRefTransition(*backend, store->layout(), ns, "tbl2", r2);
+    driveToFixpoint(*backend, store, gc);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+}
+
+/// `gc_round_outcome_entry_budget` bounds only the `GcOutcomes` AUDIT row per spared decision, never the
+/// decision itself. Five blobs are condemned (owner dropped, indegree 0, durable retired rows), then --
+/// BEFORE graduation -- a fresh manifest re-references all five (the `CASThreeCursorMerge.RecoverySpares`
+/// shape, scaled up and driven through the real round path): recovery wins unconditionally for every one
+/// of them. `CASGCRetiredSpared` and blob survival prove all five decisions happened regardless of the
+/// budget, but with a budget of 2, only 2 of the 5 get a row in the round's `GcOutcomes` log, so
+/// `RoundReport::spared` (tallied from that log) reports 2, not 5.
+TEST(CASGCRound, OutcomeEntryBudgetCapsSparedLogRowsWithoutRecondemning)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .gc_round_outcome_entry_budget = 2,
+        .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xC1);
+    const ManifestRef r2 = ref(2, 0xC2);
+    constexpr int kBlobs = 5;
+
+    std::vector<ManifestEntry> entries;
+    for (int i = 0; i < kBlobs; ++i)
+    {
+        writeBlobBody(*backend, store->layout(), DB::UInt128(i + 1));
+        entries.push_back(blobEntryFor("p" + std::to_string(i), DB::UInt128(i + 1)));
+    }
+    writeManifestRaw(*backend, store->layout(), ns, r1, entries);
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);
+    for (int i = 0; i < kBlobs; ++i)
+        ASSERT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(i + 1)), 1);
+
+    /// Drop the only ref: one round later all five blobs are condemned (indegree 0, durable retired rows).
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r1);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    for (int i = 0; i < kBlobs; ++i)
+        ASSERT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(i + 1)), 0);
+    store->renewWatermarkOnce();
+
+    /// BEFORE graduation, a fresh manifest re-references all five: the NEXT fold recomputes indegree 1 for
+    /// every one of them -- recovery wins over graduation for every entry, unconditionally.
+    writeManifestRaw(*backend, store->layout(), ns, r2, entries);
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl2", std::nullopt, r2);
+
+    const auto spared_events_before = ProfileEvents::global_counters[ProfileEvents::CASGCRetiredSpared].load();
+    const RoundReport rep = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(rep.acquired_lease);
+    const uint64_t total_spared_reported = rep.spared;
+
+    /// THE LOAD-BEARING ASSERTION: the audit log under-reports (capped at the budget) while every
+    /// decision it under-reports still happened correctly.
+    EXPECT_EQ(total_spared_reported, 2u)
+        << "GcOutcomes rows must be capped at gc_round_outcome_entry_budget, not one per spared entry";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASGCRetiredSpared].load() - spared_events_before, kBlobs)
+        << "every spared decision must still happen even when its audit row is capped";
+    for (int i = 0; i < kBlobs; ++i)
+    {
+        EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(i + 1)))
+            << "blob " << i << " must survive -- the cap must never re-condemn a spared entry";
+        EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(i + 1)), 1);
+    }
+}
+
+/// Spare-during-the-pass, multi-blob discrimination: a drop condemns two blobs; in the SAME window
+/// between rounds (before the next pass folds), one of them is re-referenced under a fresh ref. The pass
+/// folds the racing publish and SPARES the re-referenced blob (recovery wins in the pass merge, dropping
+/// its retired entry), while the genuinely-unreferenced blob proceeds through the condemn -> graduate ->
+/// delete pipeline. The discriminating assertion: at fixpoint, one is spared (kept) and the other gone.
+TEST(CASGCRound, RepublishDuringFenceWindowSparesOnlyReReferencedBlob)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xB1);
+    const ManifestRef r2 = ref(2, 0xB2);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));   /// kept (will be re-referenced)
+    writeBlobBody(*backend, store->layout(), DB::UInt128(2));   /// genuinely dropped
+    writeManifestRaw(*backend, store->layout(), ns, r1,
+        {blobEntryFor("a", DB::UInt128(1)), blobEntryFor("b", DB::UInt128(2))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(2)), 1);
+
+    /// Repoint the ref from r1 to r2 between rounds: ONE event {old=committed(r1), new=committed(r2)}.
+    /// The -1 (r1's body: blobs 1 AND 2) and +1 (r2's body: blob 1 only) net to in-degree 1 for blob 1
+    /// (re-referenced => SPARED in the pass merge) and 0 for blob 2 (genuinely unreferenced => condemned,
+    /// then reclaimed by the ack-floor pipeline). (A separate drop THEN repoint would double-count the -1
+    /// on r1's blobs and drive blob 2 to -1 — an undercount the in-degree fold fails closed on.)
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", r1, r2);
+
+    driveToFixpoint(*backend, store, gc);
+    /// Blob 1 is re-referenced (net in-degree 1) => SPARED; blob 2 is genuinely unreferenced => GONE.
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "the racing republish must spare blob 1";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(2)))
+        << "the genuinely-unreferenced blob 2 must be collected";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(2)), 0);
+}
+
+/// Idempotent fixpoint: once a pool is quiescent (all live refs folded, nothing to collect), repeated
+/// rounds are pure no-ops — no blob is collected, no manifest disappears, the in-degree generation is
+/// stable, and no round throws. The split-brain-safety bedrock: every step is idempotent.
+TEST(CASGCRound, IdempotentRerunAtFixpointIsNoOp)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    const ManifestId id{ns, r};
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);
+    const uint64_t gen0 = currentGenerationOf(*backend, store->layout());
+
+    /// At quiescence: a fresh round does NO work (no candidates/deletes/spares) and changes nothing.
+    const RoundReport quiescent = gc.runRegularRound();
+    EXPECT_TRUE(quiescent.acquired_lease);
+    EXPECT_EQ(quiescent.candidates, 0u);
+    EXPECT_EQ(quiescent.deleted, 0u);
+    EXPECT_EQ(quiescent.spared, 0u);
+
+    EXPECT_NO_THROW(driveToFixpoint(*backend, store, gc));
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));   /// no-loss
+    EXPECT_TRUE(manifestExists(*backend, store->layout(), id));          /// no-loss
+    /// The CONTENT no-op invariant: the live blob's durable in-degree is unchanged (still pinned). The
+    /// generation POINTER advances every round by design (each fold seals a fresh generation for durable
+    /// cursor coverage, and recheck seals the completion generation), even when no edges change — so the
+    /// quiescence guarantee is "no candidates/deletes/spares + nothing lost", NOT a frozen generation.
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    EXPECT_GE(currentGenerationOf(*backend, store->layout()), gen0)
+        << "the generation pointer is monotone; a quiescent round never moves it backward";
+}
+
+/// Split-brain: two leaders racing the same pool only DUPLICATE WORK, never double-delete or lose data.
+/// gc1 leads and folds the live publish; the ref is then dropped; gc2 steals the lease (stale leader)
+/// and both contend to collect the now-unreferenced blob. The exact-token delete is the only destructive
+/// authority, so the blob is removed exactly once and a losing/duplicate attempt is a harmless 404/412 —
+/// no exception escapes, and the blob ends up gone exactly once with no dangling owner.
+TEST(CASGCRound, SplitBrainLeadersOnlyDuplicateWork)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    const ManifestId id{ns, r};
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc1(store, kGcA);
+    Gc gc2(store, kGcB);
+
+    /// gc1 leads; fold the publish edge.
+    ASSERT_TRUE(runRegularRoundReclaiming(gc1).acquired_lease);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    /// The ref is dropped; gc1 stalls. gc2 observes the frozen lease twice and STEALS (new epoch).
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    EXPECT_FALSE(runRegularRoundReclaiming(gc2).acquired_lease);   /// obs #1
+    ASSERT_TRUE(runRegularRoundReclaiming(gc2).acquired_lease);    /// obs #2 => steal
+
+    /// Both leaders now drive rounds. The blob is collected exactly once; duplicate attempts are
+    /// harmless. No round throws.
+    EXPECT_NO_THROW(driveToFixpoint(*backend, store, gc2));
+    EXPECT_NO_THROW(driveToFixpoint(*backend, store, gc1));   /// the revived stale leader backs off / duplicates harmlessly
+
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "the dropped blob must be collected exactly once across both leaders";
+    EXPECT_FALSE(manifestExists(*backend, store->layout(), id));
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+}
+
+/// (`CASGCRound.TrimOnlyBelowSealedCoverage` and the B12 lazy/batched-trim tests
+/// `LazyTrimSkipsSmallJournalAndKeepsTokenStable`, `LazyTrimCompactsAtThresholdOrSoftLimit`,
+/// `MaintenanceTrimCompactsEverythingOnce` were removed with the snapshot+log ref model. They asserted
+/// GC compacts a MUTABLE shard journal in place (INV-JOURNAL-COVERAGE / `gc_trim_min_events` gates).
+/// Immutable `_log` objects are never trimmed in place: covered `_log`/`_snap` keys are DELETED by
+/// ref-object cleanup once BOTH the durable cursor AND a checkpoint-named validated recovery triple
+/// cover them -- exercised in `gtest_cas_ref_gc.cpp` (`RefObjectCleanupRetainsCheckpointNamedTriple`).)
+
+/// ---- INTENTIONALLY NOT PORTED (covered elsewhere or obsolete in the manifest model) ----
+///
+/// The removed snap/cascade/tree cases and where their behaviour now lives:
+///   - CasGcFold.{FreshUploadsAreNeverCandidates, DropZeroesTreeButChildStaysPinned,
+///     RepublishSameRefIsLastOpWins, ExpansionIsOncePerTree, IncrementalSecondFoldOnlyNewRecords,
+///     DurableSnapBeforeCursorAdvance, ForeignDivergentGenerationIsProbedPast,
+///     GenerationProbeRecoversAfterLostCursorCas, SnapShardsOtherThanOneIsNotImplemented,
+///     AbsentTree*, NoChurnRound*} — fold-step behaviour now in gtest_cas_gc_fold.cpp
+///     (CommittedAdd/Removal/Precommit/FoldBarrier/Clamp+anomaly/RefMismatch).
+///   - CasGcCorruptCommittedTree.MissingTreeOfLiveRefDoesNotHaltGc — now
+///     CASGCFold.CommittedMissingBodyClampsCursorAndRecordsAnomaly.
+///   - CASGCRetire.{Observes*, AbsentCandidate*, DeletedCandidate*, DeleteTimePrune*, BlobOnlyPrune*,
+///     RetireForgets*, RetireSetsDurable*, Diverged*, BlobHeaderUnderflow*, RetireUsesFoldCommitted*,
+///     RetireReplayAdoptsOwnCrashedAttempt} — retire-step behaviour now split between
+///     gtest_cas_gc_ack_floor.cpp and the retire-view suite.
+///   - CASGCRecheck.{SparedWhenPublishRacesTheFence, ReplacedWhenResurrectionWins, AbsentWhenAlreadyGone}
+///     — now CASGCRecheck.{PublishRacingFenceSparesBlob, UnreferencedBlobDeletedExactToken}.
+///   - CasGcFence.* / CasGcDiscovery.UsesRegistryNotList — the fence machinery is retired; the equivalent
+///     no-op-round-does-not-mutate-ref-shards property is gtest_cas_gc_ack_floor.cpp::
+///     CASGCAckFloor.NoOpRoundDoesNotMutateRefShards (+ helper registerNamespaceRaw discovery is
+///     exercised by every fold test).
+///   - CasGcCascade.* — the cascade/closure model is REMOVED; in-degree is per-blob, so a shared
+///     child surviving one parent's deletion is now CASGCRound.SharedBlobSparedUntilBothRefsDrop above,
+///     and "never cascades on replaced" is CASGCRound.RepublishDuringFenceWindowSparesOnlyReReferencedBlob.
+///   - CasGcTrim.* — now gtest_cas_gc_resume.cpp::CASGCRound.TrimDropsFoldedOwnerEvents.
+///   - CasGcResume.{CompletesRoundAfterCrashBeforeFencePersist, AdoptsOutcomesAfterCrashBeforeCascadePersist}
+///     — now gtest_cas_gc_resume.cpp::CasGcResume.ResumeFromDurableFoldSealCompletesRound.
+///   - CasGcScenario.ZombieDeleteAfterResurrectIs412 — relied on the snap/tree publish path + held
+///     in-flight deletes; the in-degree-spare equivalent is RepublishDuringFenceWindowSparesOnly...
+///     above (exact-token delete is the sole authority; a zombie carrying a stale token 412s).
+///   - CASGCRound.PreviewDeletesIsWriteFreeAndSubsetOfUnreachable — previewDeletes survives, but it is
+///     covered by the fsck/preview suite; not duplicated here.
+///   - CasGcWatermark.LiveBuildPrecommitHonoredAcrossGcRounds /
+///     CASGCRetire.ReclaimsAbandonedPrecommitWhenFloorPasses — precommit removal is now the WRITER's job
+///     (an exact `owner_transition` on abandon, or a fenced successor's stale-precommit sweep); GC no
+///     longer reclaims abandoned precommits. Exercised by the orphan-manifest-sweep / build-root suites.
+
+/// B9 snap-generation retention, reimplemented over the run/generation model: after a generation is
+/// adopted the GC prunes the per-generation seal/run/cleanup objects of generations at or below the
+/// retention floor (snap_generation - gc_snapshot_generations_to_keep), advancing snap_pruned_through. This
+/// test drives enough rounds to accumulate several generations, then asserts that everything at or below
+/// the floor is GONE while the last `keep` generations (and the live current one) remain.
+TEST(CASGCSnapRetention, PrunesOldGenerationsKeepingLastThree)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// keep the default 3 generations; one root shard so cursor keys are "ns/0".
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_snapshot_generations_to_keep = 3, .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    /// Several quiescent rounds, each advancing the generation pointer (fold + completion). Enough to
+    /// push generations below the floor.
+    for (int i = 0; i < 8; ++i)
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    const GcState st = readState(*backend, *store);
+    const uint64_t keep = 3;
+    ASSERT_GT(st.snap_generation, keep);
+    const uint64_t floor = st.snap_generation - keep;
+
+    /// snap_pruned_through reached the floor (bounded burst is large enough for this generation count).
+    EXPECT_EQ(st.snap_pruned_through, floor)
+        << "retention cursor must reach the floor (snap_generation - keep)";
+
+    /// Every generation at or below the floor is fully gone (fold seal absent).
+    for (uint64_t g = 1; g <= floor; ++g)
+    {
+        EXPECT_FALSE(backend->head(store->layout().foldSealKey(g, st.snap_attempt)).exists)
+            << "fold seal of pruned generation " << g << " must be gone";
+        EXPECT_FALSE(backend->head(store->layout().blobTargetRunKey(g, st.snap_attempt, /*shard*/0, /*seq*/0)).exists)
+            << "blob-target run of pruned generation " << g << " must be gone";
+    }
+
+    /// The fold seal at the current generation survives (the live in-degree view).
+    EXPECT_TRUE(backend->head(store->layout().foldSealKey(st.snap_generation, st.snap_attempt)).exists)
+        << "the current generation's seal must NOT be pruned";
+
+    /// No-loss: the live blob and owner body are intact throughout retention pruning.
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+    EXPECT_TRUE(manifestExists(*backend, store->layout(), ManifestId{ns, r}));
+}
+
+/// Task 9 (wholesale generation-retention): a generation may hold artifacts under MULTIPLE attempts
+/// (each round mints a fresh `lease.seq`, and a deposed leader may have written debris under its own
+/// unadopted attempt). When a generation ages past the retention floor it must be reclaimed WHOLESALE
+/// — every attempt's artifacts (incl. the attempt-scoped `retired/` and `outcomes/` sets that now live
+/// under `gc/gen/<g>/attempt/<a>/`), not just the final adopted attempt's. This test plants a retired
+/// set AND a decoy fold seal under a NON-adopted attempt at an old generation, ages that generation out,
+/// and asserts the whole `gc/gen/<g>/` subtree is gone (the per-key single-attempt prune leaked it).
+TEST(CASGCSnapRetention, WholesalePruneReclaimsAllAttemptsIncludingRetiredOutcomes)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_snapshot_generations_to_keep = 3, .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    /// One round to establish the first completed generation and learn its adopted attempt (derive both
+    /// from gc/state — never hardcode a generation; the round folds and completes, so it is > 1).
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    const GcState st1 = readState(*backend, *store);
+    ASSERT_GT(st1.snap_generation, 0u);
+    const uint64_t old_gen = st1.snap_generation;
+    const uint64_t adopted_attempt_g1 = st1.snap_attempt;
+
+    /// Plant debris under a NON-adopted attempt of generation 1: a retired set, an outcomes log, a fold
+    /// seal, and a blob-target run — exactly the families a deposed leader would have written before its
+    /// CAS failed. The per-key single-attempt prune (keyed on the FINAL snap_attempt) never touches them.
+    const uint64_t decoy_attempt = adopted_attempt_g1 + 777;
+    const String decoy_outcomes = store->layout().outcomesKey(old_gen, decoy_attempt, /*round*/0, /*shard*/0);
+    const String decoy_seal = store->layout().foldSealKey(old_gen, decoy_attempt);
+    const String decoy_run = store->layout().blobTargetRunKey(old_gen, decoy_attempt, /*shard*/0, /*seq*/0);
+    backend->putIfAbsent(decoy_outcomes, "decoy-outcomes");
+    backend->putIfAbsent(decoy_seal, "decoy-seal");
+    backend->putIfAbsent(decoy_run, "decoy-run");
+
+    /// Drop the ref so the next fold writes a FRESH run under a newer generation and the adopted seal's
+    /// blob_target ref moves OFF `old_gen`. Under T0 reference-parent carry, a still-referenced generation
+    /// is deliberately retained (its run is live), so `old_gen` can only age out once nothing references
+    /// its run anymore — which the drop guarantees.
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+
+    /// Age generation 1 well past the retention floor (keep=3): several more quiescent rounds.
+    for (int i = 0; i < 8; ++i)
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    const GcState st = readState(*backend, *store);
+    ASSERT_GT(st.snap_generation, old_gen + 3) << "generation 1 must be below the retention floor";
+
+    /// The ENTIRE gc/gen/<old_gen>/ subtree — across ALL attempts — must be reclaimed.
+    EXPECT_FALSE(backend->head(decoy_outcomes).exists) << "non-adopted outcomes log leaked past retention";
+    EXPECT_FALSE(backend->head(decoy_seal).exists) << "non-adopted fold seal leaked past retention";
+    EXPECT_FALSE(backend->head(decoy_run).exists) << "non-adopted blob-target run leaked past retention";
+
+    /// Nothing remains under the old generation prefix at all.
+    const ListPage residue = backend->list(store->layout().gcGenPrefix(old_gen), "", 1000);
+    EXPECT_TRUE(residue.keys.empty()) << "old generation prefix must be fully reclaimed; left "
+                                      << residue.keys.size() << " objects";
+
+    /// The drop was necessary to move the seal's blob_target ref off `old_gen` so it could age out (under
+    /// T0 a still-referenced generation is deliberately retained — see the comment above). The blob is
+    /// condemned by the drop and, being round-paced, graduates and is physically deleted well within the
+    /// 8 quiescent rounds above — retire drain is not the property under test here (generation retention
+    /// is), so this only asserts the reclaim pipeline is not itself broken by the retention plumbing.
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+    /// The owner-removed manifest body IS reclaimed by the part-manifest cleanup pass over the aging rounds.
+    EXPECT_FALSE(manifestExists(*backend, store->layout(), ManifestId{ns, r}));
+}
+
+/// `deletePrefixWholesale`'s callers now draw from the round's shared
+/// object-count budget instead of `UINT64_MAX`, and `snap_pruned_through` must advance only past a
+/// FULLY drained generation -- never past one the budget cut short, or its undeleted remainder would
+/// be stranded behind a cursor this loop never revisits. A tiny budget (2 objects/round) against a
+/// generation carrying far more debris than that forces multiple rounds to fully drain it; the
+/// invariant under test is that AT EVERY ROUND, `snap_pruned_through >= old_gen` implies the old
+/// generation's prefix is already empty -- the cursor never claims completion it has not earned.
+TEST(CASGCSnapRetention, PruneRespectsPrefixWholesaleBudgetAndNeverStrandsAPartialGeneration)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .gc_snapshot_generations_to_keep = 3,
+        .gc_round_prefix_wholesale_budget = 2,
+        .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    const GcState st1 = readState(*backend, *store);
+    const uint64_t old_gen = st1.snap_generation;
+
+    /// Ten extra debris objects under generation 1's prefix -- far more than the 2-object round budget
+    /// can wholesale-delete in a single pass, regardless of whatever real fold artifacts already live
+    /// there.
+    for (int i = 0; i < 10; ++i)
+        backend->putIfAbsent(store->layout().gcGenPrefix(old_gen) + "debris" + std::to_string(i), "x");
+
+    /// Move the ref off `old_gen`'s run (as `WholesalePruneReclaimsAllAttemptsIncludingRetiredOutcomes`
+    /// does) so the WHOLESALE RETENTION PRUNE -- not the one-shot post-CAS hand-off -- is what
+    /// eventually processes this generation once the cursor reaches it.
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+
+    size_t previous_residue = backend->list(store->layout().gcGenPrefix(old_gen), "", 1000).keys.size();
+    std::optional<int> drain_start_round;   /// first round the residue count actually DROPS
+    std::optional<int> drain_done_round;    /// first round the residue reaches zero
+    for (int i = 0; i < 40 && !drain_done_round; ++i)
+    {
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+        const GcState st = readState(*backend, *store);
+        const ListPage residue = backend->list(store->layout().gcGenPrefix(old_gen), "", 1000);
+
+        if (st.snap_pruned_through >= old_gen)
+            EXPECT_TRUE(residue.keys.empty())
+                << "round " << i << ": snap_pruned_through (" << st.snap_pruned_through
+                << ") claims generation " << old_gen << " is behind it, but " << residue.keys.size()
+                << " object(s) remain -- the cursor advanced past a partially-drained prefix";
+
+        if (!drain_start_round && residue.keys.size() < previous_residue)
+            drain_start_round = i;
+        if (residue.keys.empty())
+            drain_done_round = i;
+        previous_residue = residue.keys.size();
+    }
+
+    ASSERT_TRUE(drain_start_round.has_value()) << "the round loop never even started draining the debris";
+    ASSERT_TRUE(drain_done_round.has_value())
+        << "the budget-limited generation must eventually fully drain within a generous round bound";
+    /// THE LOAD-BEARING ASSERTION: with a 2-object budget against 10+ debris objects, draining cannot
+    /// finish the SAME round it starts -- it must take several rounds. An unbounded
+    /// `deletePrefixWholesale` call (the mutation this pins) drains everything the round it starts,
+    /// collapsing this gap to zero.
+    EXPECT_GT(*drain_done_round, *drain_start_round)
+        << "draining finished the same round it started -- the per-round budget is not load-bearing";
+}
+
+/// Reclaim-VIA-RETENTION of a non-adopted current-generation attempt orphan (KISS prune model). A
+/// deposed leader can write its fold seal under an attempt that lost CAS #1 to a higher-seq adopter —
+/// debris at the FOLD generation under a NON-adopted attempt. There is NO per-round current-generation
+/// attempt-sweep anymore (it cost a per-round LIST for a rare collision); the wholesale
+/// generation-retention prune is the SOLE reclaimer. So such an orphan is NOT reclaimed within one
+/// round; instead it waits until its generation ages past `keep` and the wholesale prefix-delete
+/// reclaims the whole `gc/gen/<g>/` subtree — every attempt at once, including this orphan. This test
+/// plants the orphan at a fold generation, ages that generation out, and asserts retention reclaims it.
+TEST(CASGCSnapRetention, ReclaimsNonAdoptedCurrentGenAttemptViaRetention)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// keep=3 retention floor (matches WholesalePruneReclaimsAllAttemptsIncludingRetiredOutcomes).
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_snapshot_generations_to_keep = 3, .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    /// Drive a couple of rounds so snap_attempt is comfortably above 0 (a low orphan seq exists below it).
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    const GcState st = readState(*backend, *store);
+    ASSERT_GT(st.snap_attempt, 0u) << "need snap_attempt > 0 so a strictly-older orphan attempt exists";
+
+    /// Plant a deposed competitor's debris at the next round's FOLD generation under an attempt strictly
+    /// older than that round's adopted attempt — exactly the orphan the old per-round sweep targeted.
+    const uint64_t orphan_gen = st.snap_generation + 1;
+    const uint64_t orphan_attempt = st.snap_attempt - 1;
+    const String orphan_seal = store->layout().foldSealKey(orphan_gen, orphan_attempt);
+    const String orphan_run = store->layout().blobTargetRunKey(orphan_gen, orphan_attempt, 0, 0);
+    backend->putIfAbsent(orphan_seal, "orphan-seal");
+    backend->putIfAbsent(orphan_run, "orphan-run");
+
+    /// One more round folds into `orphan_gen` and completes. The orphan must SURVIVE this round — there
+    /// is no current-generation sweep; retention has not yet reached `orphan_gen`.
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    EXPECT_TRUE(backend->head(orphan_seal).exists)
+        << "orphan must survive its own round — there is no per-round current-gen sweep";
+
+    /// Age `orphan_gen` well past the retention floor (keep=3): several more quiescent rounds. The
+    /// wholesale generation-retention prune then reclaims the WHOLE `gc/gen/<orphan_gen>/` subtree,
+    /// including this non-adopted attempt's debris.
+    for (int i = 0; i < 8; ++i)
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    const GcState st_after = readState(*backend, *store);
+    ASSERT_GT(st_after.snap_generation, orphan_gen + 3) << "orphan_gen must be below the retention floor";
+
+    EXPECT_FALSE(backend->head(orphan_seal).exists)
+        << "non-adopted attempt orphan must be reclaimed by wholesale retention once its generation ages out";
+    EXPECT_FALSE(backend->head(orphan_run).exists)
+        << "the whole orphan subtree must be reclaimed by wholesale retention";
+
+    /// No-loss: the live data is intact throughout.
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+    EXPECT_TRUE(manifestExists(*backend, store->layout(), ManifestId{ns, r}));
+}
+
+/// ---- Task 7 (2026-07-02 snapshot-streaming): ref-aware retention + post-CAS hand-off delete ----
+
+/// Retention must NOT reclaim a generation whose run the live seal still references, EVEN once the
+/// retention cursor (`snap_pruned_through`) has advanced past that generation. With `keep=1` and a live
+/// ref that idle-carries across generations, `pruneSupersededGenerations` SKIPS gen-1's prefix every
+/// round while advancing the cursor over it. The gen-1 run object (physically holding the seal's ref)
+/// must survive, and folding/in-degree resolution THROUGH the carried ref must keep working.
+TEST(CASGCRetention, PruneRetainsLiveReferencedRun)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// keep=1: the retention floor is aggressive so the cursor reaches gen-1's neighbourhood fast.
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_snapshot_generations_to_keep = 1, .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);   // gen 1: the blob's run is sealed under gen-1's key namespace
+    const GcState st1 = readState(*backend, *store);
+    const uint64_t ref_gen = st1.snap_generation;
+
+    /// The gen-1 seal's ref names gen-1's physical run key — capture it so we can assert the OBJECT
+    /// (not just the generation number) survives retention.
+    const auto seal1 = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(st1.snap_generation, st1.snap_attempt))->bytes);
+    ASSERT_EQ(seal1.blob_target_runs.size(), 1u);
+    const String referenced_run_key = seal1.blob_target_runs.front().key;
+    ASSERT_EQ(seal1.blob_target_runs.front().generation, ref_gen);
+    ASSERT_TRUE(backend->head(referenced_run_key).exists);
+
+    /// Several idle rounds: no delta, no retired => pure ref-carry. Each round advances the generation
+    /// and, once adopted_generation > keep, drives the retention prune forward. gen-1 is referenced every
+    /// round, so it is SKIPPED (retained) even as `snap_pruned_through` climbs past it.
+    for (int i = 0; i < 6; ++i)
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    const GcState st = readState(*backend, *store);
+    /// The cursor has advanced strictly past the referenced generation (the retention prune SKIPPED it
+    /// but still moved the high-water cursor forward) — this is the exact window Task 7 guards.
+    ASSERT_GT(st.snap_pruned_through, ref_gen)
+        << "the retention cursor must have advanced past the still-referenced generation";
+
+    /// The referenced run object is STILL ALIVE despite the cursor passing its generation.
+    EXPECT_TRUE(backend->head(referenced_run_key).exists)
+        << "a run referenced by the live seal must be retained even after the cursor passes its generation";
+
+    /// The current seal still references that same physical gen-1 object (carried, not reconstructed),
+    /// and in-degree resolution THROUGH the carried ref still works.
+    const auto seal_now = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(st.snap_generation, st.snap_attempt))->bytes);
+    ASSERT_EQ(seal_now.blob_target_runs.size(), 1u);
+    EXPECT_EQ(seal_now.blob_target_runs.front().key, referenced_run_key);
+    EXPECT_EQ(seal_now.blob_target_runs.front().generation, ref_gen);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1)
+        << "folding still resolves in-degree through the retained, carried parent ref";
+
+    /// No-loss end-to-end: the live blob is intact.
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+}
+
+/// When a later delta finally REPLACES the carried ref with a fresh run, the superseded old-generation
+/// run — whose generation the retention cursor already passed while it was retained — is reclaimed by the
+/// post-CAS HAND-OFF delete in `runRegularRound` (the wholesale prune never revisits a generation behind
+/// its cursor, so the ordinary prune would leak it). The whole `gc/gen/<old>/` prefix must be gone.
+TEST(CASGCRetention, HandOffDeletesSupersededRef)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_snapshot_generations_to_keep = 1, .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xAA);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+
+    Gc gc(store, kGc);
+    runRegularRoundReclaiming(gc);   // gen 1: run sealed under gen-1
+    const GcState st1 = readState(*backend, *store);
+    const uint64_t old_gen = st1.snap_generation;
+    const String old_prefix = store->layout().gcGenPrefix(old_gen);
+    ASSERT_FALSE(backend->list(old_prefix, "", 1000).keys.empty()) << "gen-1 prefix must be populated";
+
+    /// Idle-carry the gen-1 ref until the retention cursor has advanced strictly PAST gen-1. Until it
+    /// does, a normal prune could still reclaim gen-1 when the ref moves — the hand-off is only load-
+    /// bearing once gen-1 is BEHIND the cursor.
+    for (int i = 0; i < 6; ++i)
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_GT(readState(*backend, *store).snap_pruned_through, old_gen)
+        << "gen-1 must be behind the retention cursor before the hand-off is exercised";
+    /// gen-1 is retained (referenced) even though the cursor passed it.
+    ASSERT_FALSE(backend->list(old_prefix, "", 1000).keys.empty())
+        << "the referenced gen-1 prefix must still exist before the ref moves off it";
+
+    /// A real delta: swap the ref to a new manifest naming a different blob. The next fold writes a FRESH
+    /// run under the new generation and the seal's shard-0 ref moves OFF gen-1.
+    const ManifestRef r2 = ref(2, 0xBB);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(2));
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", r1, r2);
+
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);   // folds through the carried ref; ref leaves gen-1
+
+    /// The seal no longer references gen-1 ...
+    const GcState st_after = readState(*backend, *store);
+    const auto seal_after = decodeFoldSeal(
+        backend->get(store->layout().foldSealKey(st_after.snap_generation, st_after.snap_attempt))->bytes);
+    for (const RunRef & rr : seal_after.blob_target_runs)
+        EXPECT_NE(rr.generation, old_gen) << "the live seal must have moved its ref off gen-1";
+
+    /// ... and the post-CAS hand-off delete reclaimed gen-1's WHOLE prefix (not just the single run
+    /// object): seal, attempt subtree, run — all gone. The ordinary prune would have leaked it because its
+    /// cursor is already past gen-1.
+    const ListPage residue = backend->list(old_prefix, "", 1000);
+    EXPECT_TRUE(residue.keys.empty())
+        << "the superseded gen-1 prefix must be hand-off deleted; left " << residue.keys.size() << " objects";
+
+    /// The now-referenced blob 2 is intact; folding through the fresh run resolves it.
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(2)));
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(2)), 1);
+}
+
+/// The post-CAS hand-off draws from its OWN reserve, so a prune that spends its ENTIRE (separate, tiny)
+/// budget in a round can never leave the hand-off with zero. Combines the two existing shapes: a
+/// debris-heavy generation only the ordinary PRUNE ever touches (like
+/// `PruneRespectsPrefixWholesaleBudgetAndNeverStrandsAPartialGeneration`, mid-drain over several rounds on
+/// a starvation-small prune budget), running CONCURRENTLY with an idle-carried ref that finally moves off
+/// its generation (like `HandOffDeletesSupersededRef`) in one of those very same mid-drain rounds.
+TEST(CASGCRetention, HandoffOwnBudgetSurvivesAPruneHeavyRound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// `gc_shards = 2` with the "keep" and "debris" blobs routed to DIFFERENT shards is load-bearing: with
+    /// the default single shard, ANY delta anywhere rewrites the pool's one shared run object every round,
+    /// which would drag the "keep" ref's physical run forward the moment the debris table is touched --
+    /// destroying the idle-carry this test depends on. Two independent shards keep debris activity from
+    /// disturbing the kept ref's generation at all until its ref is explicitly moved.
+    /// `keep=5` (not the more aggressive `keep=1` other hand-off tests use) is ALSO load-bearing: the
+    /// debris generation must still be numerically AHEAD of the cursor at the moment its own drop folds,
+    /// or that fold's post-CAS hand-off phase -- not the ordinary prune -- would claim it (the same
+    /// one-round "parent-seal protects, then hand-off claims" shape `HandOffDeletesSupersededRef` relies
+    /// on, which this test must deliberately avoid for the debris generation).
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .gc_snapshot_generations_to_keep = 5,
+        .gc_shards = 2,
+        .gc_round_prefix_wholesale_budget = 2,             /// prune: starvation-small, shared by nothing else
+        .gc_round_handoff_prefix_wholesale_budget = 5,      /// hand-off: its own separate reserve
+        .gc_fold_max_defer_rounds = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    Gc gc(store, kGc);
+    /// `blobShard` uses only the digest's high 64 bits, so a small integer's `UInt128` (high bits zero)
+    /// always routes to shard 0 regardless of `gc_shards` -- these two differ in the high half so they
+    /// land in different shards of a 2-shard pool.
+    const DB::UInt128 blob_keep_1 = hexToU128("00000000000000010000000000000000");
+    const DB::UInt128 blob_keep_2 = hexToU128("00000000000000010000000000000001");
+    const DB::UInt128 blob_debris = hexToU128("00000000000000020000000000000000");
+
+    /// The HAND-OFF generation: ns "keep" idle-carries this ref for several rounds until the cursor has
+    /// advanced strictly past it (referenced generations are skipped for free -- no budget spent).
+    const ManifestRef r_keep_1 = ref(1, 0xE1);
+    writeBlobBody(*backend, store->layout(), blob_keep_1);
+    writeManifestRaw(*backend, store->layout(), ns, r_keep_1, {blobEntryFor("a", blob_keep_1)});
+    publishCommittedTransition(*backend, store->layout(), ns, "keep", std::nullopt, r_keep_1);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    const uint64_t handoff_gen = readState(*backend, *store).snap_generation;
+    const String handoff_prefix = store->layout().gcGenPrefix(handoff_gen);
+    ASSERT_FALSE(backend->list(handoff_prefix, "", 1000).keys.empty());
+
+    for (int i = 0; i < 20; ++i)
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_GT(readState(*backend, *store).snap_pruned_through, handoff_gen)
+        << "the hand-off generation must be behind the cursor before this test is meaningful";
+    ASSERT_FALSE(backend->list(handoff_prefix, "", 1000).keys.empty())
+        << "still referenced -- must survive despite the cursor having passed it";
+
+    /// The PRUNE-DEBRIS generation: a second table, on the OTHER shard, unreferenced from the start,
+    /// carrying far more debris than the tiny prune budget can drain in one round. Minted well AHEAD of
+    /// the current cursor (see the `keep=5` note above), so its own drop-fold is NOT immediately
+    /// hand-off-eligible.
+    const ManifestRef r_debris = ref(1, 0xE2);
+    writeBlobBody(*backend, store->layout(), blob_debris);
+    writeManifestRaw(*backend, store->layout(), ns, r_debris, {blobEntryFor("b", blob_debris)});
+    publishCommittedTransition(*backend, store->layout(), ns, "debris", std::nullopt, r_debris);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    const uint64_t debris_gen = readState(*backend, *store).snap_generation;
+    ASSERT_GT(debris_gen, readState(*backend, *store).snap_pruned_through)
+        << "the debris generation must still be ahead of the cursor when its drop folds, or the hand-off "
+           "(not the prune) would claim it";
+    for (int i = 0; i < 10; ++i)
+        backend->putIfAbsent(store->layout().gcGenPrefix(debris_gen) + "debris" + std::to_string(i), "x");
+    dropRefTransition(*backend, store->layout(), ns, "debris", r_debris);
+
+    /// Drive rounds until the debris generation is MID-DRAIN (prune has started but not yet finished it --
+    /// the round-budget of 2 against 10+ objects guarantees several such rounds exist).
+    bool mid_drain = false;
+    for (int i = 0; i < 20 && !mid_drain; ++i)
+    {
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+        const size_t residue = backend->list(store->layout().gcGenPrefix(debris_gen), "", 1000).keys.size();
+        mid_drain = residue > 0 && residue < 10;
+    }
+    ASSERT_TRUE(mid_drain) << "the debris generation never reached a partially-drained state to test against";
+    ASSERT_FALSE(backend->list(handoff_prefix, "", 1000).keys.empty())
+        << "the hand-off generation must still be intact (untouched) going into the contended round";
+
+    /// NOW, in a round where the prune is busy mid-drain on the debris generation (spending its entire
+    /// small budget there), move the kept ref off the hand-off generation -- a fresh manifest replaces it.
+    const ManifestRef r_keep_2 = ref(2, 0xE3);
+    writeBlobBody(*backend, store->layout(), blob_keep_2);
+    writeManifestRaw(*backend, store->layout(), ns, r_keep_2, {blobEntryFor("a", blob_keep_2)});
+    publishCommittedTransition(*backend, store->layout(), ns, "keep", r_keep_1, r_keep_2);
+    const size_t debris_residue_before = backend->list(store->layout().gcGenPrefix(debris_gen), "", 1000).keys.size();
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    /// THE LOAD-BEARING ASSERTIONS: the prune spent its whole (separate) budget on the debris generation
+    /// this very round (proving the two really contended for I/O in the same round) ...
+    const size_t debris_residue_after = backend->list(store->layout().gcGenPrefix(debris_gen), "", 1000).keys.size();
+    EXPECT_EQ(debris_residue_before - debris_residue_after, 2u)
+        << "the prune must have spent its entire per-round budget on the debris generation this round";
+    /// ... and the hand-off, drawing from its OWN reserve, still fully reclaimed the generation the ref
+    /// just moved off -- zero, not starved to zero by the prune's consumption.
+    EXPECT_TRUE(backend->list(handoff_prefix, "", 1000).keys.empty())
+        << "the hand-off must not be starved by a prune-heavy round that exhausted a SEPARATE budget";
+}
+
+/// triage #5, driven through the REAL call site (`Gc::runRegularRound`, not a test seam): a losing
+/// leader's pre-CAS wholesale generation-retention prune must never destroy a generation the PARENT
+/// (currently-adopted, pre-fold) seal still references, even when the round's own PROPOSED seal has
+/// already moved off it and the round's own `gc/state` CAS then loses. `GcStateCasFaultBackend` makes
+/// this round's own round-commit CAS return `Conflict` — deterministically and single-threaded standing
+/// in for a concurrent leader winning first — which is the only condition under which the fix is
+/// externally observable: a round whose own CAS SUCCEEDS reclaims the same generation moments later via
+/// the existing (unrelated, unchanged) post-CAS hand-off delete regardless of this fix, so a plain
+/// successful round cannot tell bug from fix apart.
+TEST(CASGCRetention, LosingRoundNeverDestroysParentSealGeneration)
+{
+    auto backend = std::make_shared<GcStateCasFaultBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_snapshot_generations_to_keep = 1});
+    const Layout & layout = store->layout();
+    backend->faulted_key = layout.gcStateKey();
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xAA);
+
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeManifestRaw(*backend, layout, ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r1);
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);   // round 1: gen=1 adopted, referencing blob 1's run.
+    const GcState st1 = readState(*backend, *store);
+    const uint64_t g_parent = st1.snap_generation;
+
+    const auto seal1 = decodeFoldSeal(backend->get(layout.foldSealKey(st1.snap_generation, st1.snap_attempt))->bytes);
+    ASSERT_EQ(seal1.blob_target_runs.size(), 1u);
+    const String parent_run_key = seal1.blob_target_runs.front().key;
+    const String parent_gen_prefix = layout.gcGenPrefix(g_parent);
+    ASSERT_FALSE(backend->list(parent_gen_prefix, "", 1000).keys.empty());
+
+    /// A real delta: swap the ref to a new manifest naming a different blob. The next fold will move
+    /// shard 0's run OFF `g_parent` onto a fresh generation.
+    const ManifestRef r2 = ref(2, 0xBB);
+    writeBlobBody(*backend, layout, DB::UInt128(2));
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", r1, r2);
+
+    /// Arm the fault for the NEXT round's SECOND casPut on gc/state, not its first: the first is
+    /// `acquireOrRenewLease`'s own lease-renewal CAS (must SUCCEED, so the round actually folds), and the
+    /// second is the round's final round-commit CAS (the one that must LOSE, exactly as if a concurrent
+    /// leader had already committed a different seal first).
+    const size_t calls_before = backend->calls_to_faulted_key;
+    backend->fail_at_call = calls_before + 2;
+    bool threw_aborted = false;
+    try
+    {
+        gc.runRegularRound();
+    }
+    catch (const DB::Exception & e)
+    {
+        threw_aborted = (e.code() == DB::ErrorCodes::ABORTED);
+        if (!threw_aborted)
+            throw;
+    }
+    ASSERT_TRUE(threw_aborted) << "the losing round's own gc/state CAS must fail and propagate ABORTED";
+    EXPECT_EQ(backend->calls_to_faulted_key, calls_before + 2)
+        << "the round must have made exactly the expected two gc/state casPut attempts (renew + commit)";
+
+    /// GREEN evidence: the losing round's pre-CAS prune must NOT have destroyed `g_parent` — it is still
+    /// exactly what the (unreplaced, still-adopted) parent seal references.
+    EXPECT_FALSE(backend->list(parent_gen_prefix, "", 1000).keys.empty())
+        << "a losing round must never destroy the generation the still-adopted parent seal references";
+    EXPECT_TRUE(backend->head(parent_run_key).exists)
+        << "the parent seal's exact run object must survive a losing round's pre-CAS prune";
+
+    /// GC is NOT wedged: gc/state is unchanged (the CAS never committed) and the original blob still
+    /// resolves cleanly through the surviving parent run — no `CORRUPTED_DATA` from a dangling reference.
+    EXPECT_EQ(readState(*backend, *store).snap_generation, g_parent);
+    EXPECT_TRUE(blobExists(*backend, layout, DB::UInt128(1)));
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 1);
+
+    /// A subsequent round (fault already disarmed) must succeed normally, AND must still reclaim the
+    /// losing round's own abandoned attempt debris — a generation referenced by NEITHER the parent nor
+    /// the new proposed seal — proving the fix does not turn pruning off altogether.
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    const uint64_t g_after = readState(*backend, *store).snap_generation;
+    ASSERT_GT(g_after, g_parent);
+    for (uint64_t g = g_parent + 1; g < g_after; ++g)
+        EXPECT_TRUE(backend->list(layout.gcGenPrefix(g), "", 1000).keys.empty())
+            << "generation " << g << " (the losing round's own abandoned attempt debris, referenced by "
+               "neither the parent nor the new proposed seal) must still be reclaimed on a successful "
+               "round — the fix must not disable pruning";
+
+    EXPECT_TRUE(blobExists(*backend, layout, DB::UInt128(2)));
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(2)), 1);
+}
+
+/// keep == 0 is the forensics "keep ALL" mode: NO generation is pruned, snap_pruned_through stays 0.
+TEST(CASGCSnapRetention, KeepZeroPrunesNothing)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_snapshot_generations_to_keep = 0});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    for (int i = 0; i < 6; ++i)
+        ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const GcState st = readState(*backend, *store);
+    EXPECT_EQ(st.snap_pruned_through, 0u) << "keep==0 must prune nothing";
+
+    /// Every seal from generation 1 up to the current one remains. Each generation was sealed under the
+    /// attempt of the round that produced it (attempt == that round's lease.seq, which bumps every round),
+    /// so a historical generation's seal lives under an earlier attempt than the final snap_attempt — scan
+    /// all attempts up to snap_attempt and require the seal to survive under one of them.
+    for (uint64_t g = 1; g <= st.snap_generation; ++g)
+    {
+        bool seal_present = false;
+        for (uint64_t a = 0; a <= st.snap_attempt && !seal_present; ++a)
+            seal_present = backend->head(store->layout().foldSealKey(g, a)).exists;
+        EXPECT_TRUE(seal_present) << "keep==0: seal of generation " << g << " must remain";
+    }
+}
+
+TEST(CASGCRound, OrphanManifestCursorSweepDeletesAndPersistsCursor)
+{
+    std::shared_ptr<InMemoryBackend> backend;
+    PoolConfig config;
+    config.pool_prefix = "p";
+    /// The GC runner owns a different mount from the synthetic `test` watermark below. This keeps the
+    /// cursor-sweep assertions in the parent process without replacing its live keeper incarnation.
+    config.server_root_id = "gc-runner";
+    config.manifest_sweep_list_budget_keys = 1;
+    config.manifest_sweep_delete_budget_keys = 1;
+    /// This test drives MANY consecutive rounds expecting each to sweep + persist the cursor; force
+    /// fold-every-round (Phase-4 Lever A would otherwise defer once the pool quiesces).
+    config.gc_fold_max_defer_rounds = 0;
+    auto store = openTestPoolWithConfig(backend, config);
+
+    const RootNamespace ns{"test/aa@cas@"};
+    registerNamespaceRaw(*backend, store->layout(), ns);
+    const ManifestRef r1 = ref(5, 0xCA01);
+    const ManifestRef r2 = ref(5, 0xCA02);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    setWatermarkMinActive(*backend, store->layout(), "test", r1.writer_epoch, /*min_active*/6);
+
+    /// The §6 deletion premise is a second precondition on every sweep deletion: a manifest of an
+    /// epoch-`E` build is deletable only once the namespace's sealed fold cursor sits in an epoch
+    /// STRICTLY above `E`. The debris above is epoch 1, so the namespace's own ref log has to cross
+    /// into epoch 2 and the ROUND has to fold that crossing.
+    ///
+    /// The crossing is written record by record and folded by the round's own arithmetic intake, which
+    /// makes this the composition of the whole chain in one test: a real `EpochSeal` is minted at
+    /// `{1,2}`, `RefTableState::apply` consumes it as INV-2's chain link when the epoch-2 record names
+    /// it in `prev_epoch_seal`, the walk CROSSES on that back-chain, the round seals a cursor in epoch
+    /// 2, and the premise then admits a deletion for the crossed epoch. Nothing here is seeded: an
+    /// injected cursor would prove only that the premise reads a number, not that the number can be
+    /// produced.
+    ///
+    /// The live publications use build sequences ABOVE the watermark's `min_active`, so the only
+    /// sweep-ELIGIBLE manifests in the namespace remain the two debris bodies -- the premise, not the
+    /// watermark, is what this test varies.
+    publishAt(*backend, store->layout(), ns, RefTxnId{1, 1}, "tbl", /*build_sequence=*/7,
+              DB::UInt128(0xB10B1), /*birth=*/true);
+    writeRecoverableCkptForRawFixture(*backend, store->layout(), ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    EXPECT_TRUE(manifestExists(*backend, store->layout(), ManifestId{ns, r1}))
+        << "a cursor still INSIDE epoch 1 proves nothing about epoch 1's closing seal: the premise retains";
+    EXPECT_TRUE(manifestExists(*backend, store->layout(), ManifestId{ns, r2}));
+
+    /// The round above already persisted a mid-circuit cursor while deleting nothing, which is the
+    /// cursor half of this test's subject: the sweep examined a key, retained it, and durably recorded
+    /// where it got to.
+    EXPECT_FALSE(readState(*backend, *store).manifest_sweep_cursor.empty())
+        << "the sweep persisted the cursor it examined to, even having deleted nothing";
+
+    /// Close epoch 1 and open epoch 2 over the seal it consumed.
+    writeSealAt(*backend, store->layout(), ns, RefTxnId{1, 2});
+    publishAt(*backend, store->layout(), ns, RefTxnId{2, 1}, "tbl2", /*build_sequence=*/7,
+              DB::UInt128(0xB10B2), /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    const std::optional<NamespaceLifeId> life = CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns);
+    ASSERT_TRUE(life.has_value());
+    const String ckpt_key = store->layout().refCkptKey(*life);
+    const auto old_ckpt = backend->get(ckpt_key);
+    ASSERT_TRUE(old_ckpt.has_value());
+    ASSERT_EQ(backend->putOverwrite(ckpt_key, encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    }), old_ckpt->token).outcome, PutOutcome::Done);
+
+    /// The list budget is one key per round, so reclaiming both debris bodies takes a circuit.
+    for (int round = 0; round < 12; ++round)
+    {
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease) << "round " << round;
+        if (!manifestExists(*backend, store->layout(), ManifestId{ns, r1})
+            && !manifestExists(*backend, store->layout(), ManifestId{ns, r2}))
+            break;
+    }
+    EXPECT_FALSE(manifestExists(*backend, store->layout(), ManifestId{ns, r1}));
+    EXPECT_FALSE(manifestExists(*backend, store->layout(), ManifestId{ns, r2}));
+
+    /// What ADMITTED those deletions, stated rather than inferred: the round folded the crossing itself
+    /// and sealed a cursor in the epoch above the debris. Without this the two expectations above would
+    /// still pass if the premise ever stopped consulting the cursor at all.
+    {
+        const GcState st = readState(*backend, *store);
+        const CasFoldSeal seal = decodeFoldSeal(
+            backend->get(store->layout().foldSealKey(st.snap_generation, st.snap_attempt))->bytes);
+        const auto it = seal.ref_lives.find(catalogLifeIdForTest(*backend, store->layout(), ns));
+        ASSERT_NE(it, seal.ref_lives.end()) << "the round must have sealed a coverage row";
+        EXPECT_FALSE(it->second.coverage.hold.has_value()) << "a held namespace can never reach the premise";
+        EXPECT_EQ(it->second.coverage.last_folded_ref_id, (RefTxnId{2, 1}))
+            << "the cursor must sit in the epoch ABOVE the debris, reached by folding the seal at {1,2}";
+    }
+
+    /// Replacing a live Pool's own mount with a synthetic foreign watermark must make release fail
+    /// CLOSED. This was an `EXPECT_DEATH` pinning a `LOGICAL_ERROR` abort; the abort was the defect
+    /// (it fires from `~Pool`, defeating `finishTeardown`'s own catch by aborting at exception
+    /// construction, and it fires in ASan builds on any deposed writer's shutdown). What it was really
+    /// protecting is asserted directly now: the runtime never had a failed renewal, so it still
+    /// believed it owned the mount, which makes this the exclusivity-violation arm — refuse, leave the
+    /// occupant byte-for-byte untouched, and SURVIVE the teardown.
+    std::shared_ptr<InMemoryBackend> foreign_backend;
+    PoolConfig foreign_config = config;
+    foreign_config.server_root_id = "test";
+    auto invalid_store = openTestPoolWithConfig(foreign_backend, std::move(foreign_config));
+    const String foreign_mount_key = invalid_store->layout().mountKey("test");
+    setWatermarkMinActive(*foreign_backend, invalid_store->layout(), "test", r1.writer_epoch, /*min_active*/6);
+    const auto occupant_before = foreign_backend->get(foreign_mount_key);
+    ASSERT_TRUE(occupant_before.has_value());
+    const uint64_t violations_before
+        = ProfileEvents::global_counters[ProfileEvents::CASMountExclusivityViolation].load();
+
+    invalid_store.reset();   /// must not abort, must not terminate
+
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMountExclusivityViolation].load(),
+              violations_before + 1)
+        << "a runtime that never observed a deposition must report the foreign occupant as a broken "
+           "single-writer guarantee";
+    const auto occupant_after = foreign_backend->get(foreign_mount_key);
+    ASSERT_TRUE(occupant_after.has_value()) << "the release must never delete another incarnation's lease";
+    EXPECT_EQ(occupant_after->bytes, occupant_before->bytes)
+        << "the release must leave the slot byte-for-byte untouched, never stamp our farewell over it";
+}
+
+/// Source-edge idempotency: re-folding the same blob activation does not double-count.
+/// A blob activated twice from the SAME source edge (same ManifestId + path) has in-degree 1, not 2.
+TEST(CASGCRound, FoldManifestEdgesEmitsOnePlusEdgePerBlob)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);
+
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1)
+        << "a single published manifest must contribute exactly one source edge per blob";
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "the blob must still exist (in-degree > 0)";
+}
+
+/// Re-fold of a removal is idempotent: the fold barrier + source-edge set model ensure that
+/// folding the same removal twice (the H1b scenario) does NOT drive the in-degree below zero.
+TEST(CASGCRound, ReFoldOfRemovalIsIdempotent)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    /// Drop the ref and run to fixpoint. The blob should be reclaimed.
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    EXPECT_NO_THROW(driveToFixpoint(*backend, store, gc))
+        << "re-fold of a removal must be idempotent (source-edge set, never underflows)";
+
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "the blob must be reclaimed after the only reference is dropped";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+}
+
+/// Two distinct manifests referencing the same blob contribute TWO independent source edges.
+/// Dropping one manifest leaves the other's edge intact (in-degree stays 1, blob is spared).
+TEST(CASGCRound, TwoManifestsTwoSourceEdgesDropOneSpares)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r1 = ref(1, 0xAA);
+    const ManifestRef r2 = ref(2, 0xBB);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl1", std::nullopt, r1);
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl2", std::nullopt, r2);
+
+    Gc gc(store, kGc);
+    driveToFixpoint(*backend, store, gc);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 2)
+        << "two distinct manifests referencing the same blob must each contribute one source edge";
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)));
+
+    /// Drop one of the two references; the other still pins the blob.
+    dropRefTransition(*backend, store->layout(), ns, "tbl1", r1);
+    driveToFixpoint(*backend, store, gc);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1)
+        << "after dropping one of two references the in-degree must be 1";
+    EXPECT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "the blob must survive — the second reference still pins it";
+}
diff --git a/src/Disks/tests/gtest_cas_gc_round_defer.cpp b/src/Disks/tests/gtest_cas_gc_round_defer.cpp
new file mode 100644
index 000000000000..a33816a8bac2
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_round_defer.cpp
@@ -0,0 +1,640 @@
+#include <gtest/gtest.h>
+
+#include <set>
+#include <limits>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Common/ProfileEvents.h>
+#include "cas_test_helpers.h"
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace ProfileEvents
+{
+extern const Event CASGCRefWalkPlansBuilt;
+}
+
+namespace
+{
+const UInt128 kGc = UInt128(0xAB);
+}
+
+TEST(CASGCRoundDefer, PredicateTruthTable)
+{
+    /// threshold=1 (default): defer ONLY when zero shards changed AND no graduation due AND within bound.
+    EXPECT_TRUE (shouldDeferRound(/*changed*/0, /*grad_due*/false, /*since*/0, /*threshold*/1, /*max*/8));
+    EXPECT_FALSE(shouldDeferRound(1, false, 0, 1, 8));   // a shard changed => fold
+    EXPECT_FALSE(shouldDeferRound(0, true,  0, 1, 8));   // graduation due => force fold
+    EXPECT_FALSE(shouldDeferRound(0, false, 8, 1, 8));   // defer bound reached => force fold
+
+    /// threshold=3 (batching): defer while accumulated changed shards < threshold, no grad, within bound.
+    EXPECT_TRUE (shouldDeferRound(2, false, 0, 3, 8));
+    EXPECT_FALSE(shouldDeferRound(3, false, 0, 3, 8));   // reached threshold => fold
+    EXPECT_FALSE(shouldDeferRound(2, true,  0, 3, 8));   // graduation due => force fold regardless of size
+    EXPECT_FALSE(shouldDeferRound(2, false, 8, 3, 8));   // bound reached => force fold
+}
+
+/// graduationDue (retired-in-snapshot T4): read ZERO-I/O from the adopted seal's condemned_summary. An
+/// entry whose oldest non-pending condemn round crosses current_round forces it true; a delete_pending
+/// entry forces it true regardless of the round; otherwise false.
+TEST(CASGCRoundDefer, GraduationDueDetectsDuePendingAndRoundCrossing)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+
+    /// Adopt a seal whose shard-0 summary holds one condemned-but-not-yet-graduated entry (round 2).
+    injectCondemnedSummarySeal(*backend, layout, /*generation*/1, /*attempt*/1, /*gc_shards*/1,
+        {{0, CondemnedSummary{.condemned_total = 1, .pending_total = 0,
+                              .oldest_nonpending_condemn_round = 2}}});
+
+    Gc gc(store, kGc);
+    const GcState state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+
+    EXPECT_FALSE(gc.graduationDueForTest(state, /*current_round=*/2))
+        << "oldest non-pending condemn round (2) is not < current_round (2); not yet due to graduate";
+    EXPECT_TRUE(gc.graduationDueForTest(state, /*current_round=*/3))
+        << "oldest non-pending condemn round (2) < current_round (3) => due to graduate";
+
+    /// Re-adopt a seal whose summary entry is delete_pending: due regardless of the round.
+    injectCondemnedSummarySeal(*backend, layout, /*generation*/1, /*attempt*/1, /*gc_shards*/1,
+        {{0, CondemnedSummary{.condemned_total = 1, .pending_total = 1,
+                              .oldest_nonpending_condemn_round = std::numeric_limits<uint64_t>::max()}}});
+    const GcState state_pending = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+
+    EXPECT_TRUE(gc.graduationDueForTest(state_pending, /*current_round=*/0))
+        << "a delete_pending entry must force graduationDue true regardless of current_round";
+}
+
+/// graduationDue fail-closed: when the adopted seal OBJECT is deleted out from under gc/state, the signal
+/// must be TRUE (forces the fold so the round's own fail-closed path surfaces the corrupt bookkeeping),
+/// never a silent defer.
+TEST(CASGCRoundDefer, GraduationDueFailsClosedWhenSealMissing)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+
+    injectCondemnedSummarySeal(*backend, layout, /*generation*/1, /*attempt*/1, /*gc_shards*/1,
+        {{0, CondemnedSummary{}}});
+    const GcState state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+
+    /// Delete the adopted seal object (corrupt destructive bookkeeping).
+    const String seal_key = layout.foldSealKey(state.snap_generation, state.snap_attempt);
+    const HeadResult h = backend->head(seal_key);
+    ASSERT_TRUE(h.exists);
+    ASSERT_EQ(backend->deleteExact(seal_key, h.token).kind, DeleteOutcome::Kind::Deleted);
+
+    Gc gc(store, kGc);
+    EXPECT_TRUE(gc.graduationDueForTest(state, /*current_round=*/5))
+        << "a missing adopted seal must fail-closed to a forced fold";
+}
+
+/// graduationDue is FALSE on a TOTAL all-zero summary: nothing condemned in any shard => nothing due.
+TEST(CASGCRoundDefer, GraduationDueFalseOnAllZeroSummary)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_shards = 2});
+    const Layout & layout = store->layout();
+
+    injectCondemnedSummarySeal(*backend, layout, /*generation*/1, /*attempt*/1, /*gc_shards*/2,
+        {{0, CondemnedSummary{}}, {1, CondemnedSummary{}}});
+    const GcState state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+
+    Gc gc(store, kGc);
+    EXPECT_FALSE(gc.graduationDueForTest(state, /*current_round=*/9))
+        << "an all-zero total summary means nothing is due to graduate";
+
+    /// Fail-closed if the summary is NOT total over gc_shards (shard 1 missing).
+    injectCondemnedSummarySeal(*backend, layout, /*generation*/1, /*attempt*/1, /*gc_shards*/2,
+        {{0, CondemnedSummary{}}});
+    const GcState partial = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    EXPECT_TRUE(gc.graduationDueForTest(partial, /*current_round=*/9))
+        << "a summary not total over gc_shards is corrupt => fail-closed force-fold";
+}
+
+/// `listRefPrefix`'s `changed_shards`: with the fold seal covering shard s at its current token, a quiescent pool reports
+/// 0; after one publish to a ref in shard s, it reports 1.
+TEST(CASGCRoundDefer, ChangedShardCountIsZeroWhenQuiescent)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 0xAA};
+
+    writeBlobBody(*backend, layout, UInt128(1));
+    writeManifestRaw(*backend, layout, ns, r1, {blobEntryFor("a", UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r1);
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);   /// fold; the round's own trim then rewrites the
+                                                         /// shard (compacting the just-folded event), so
+                                                         /// its sealed token is the PRE-trim snapshot.
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);   /// a second, work-free round: nothing left to
+                                                         /// trim, so THIS round's fold seal finally
+                                                         /// captures the shard's actual current token.
+
+    const GcState quiescent_state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    EXPECT_EQ(gc.listRefPrefixForTest(quiescent_state).changed_shards, 0u)
+        << "a quiescent shard (listed token == sealed token) must not count as changed";
+
+    /// Publish a second ref into the SAME shard: its LISTED token now differs from what
+    /// `quiescent_state`'s adopted fold seal recorded.
+    const ManifestRef r2{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 0xBB};
+    writeBlobBody(*backend, layout, UInt128(2));
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("b", UInt128(2))});
+    publishCommittedTransition(*backend, layout, ns, "tbl2", std::nullopt, r2);
+
+    EXPECT_EQ(gc.listRefPrefixForTest(quiescent_state).changed_shards, 1u)
+        << "one shard whose token advanced since the sealed generation must count as changed";
+}
+
+/// Mutation caught: widening the hot LIST from `cas/ns/stream/` to `cas/ns/` would offer `_ckpt` and
+/// `_files` state objects to the fold. The backend-observed result set must contain both immutable
+/// stream kinds and neither state kind.
+TEST(CASGCRoundDefer, HotEnumerationOffersLogsAndSnapshotsButNeverCheckpointOrFiles)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"name-must-not-appear"}, UInt128{0x123});
+    const RefTxnId id{1, 1};
+    const String log_key = layout.refLogKey(life, id);
+    const String snap_key = layout.refSnapshotKey(life, id);
+    const String ckpt_key = layout.refCkptKey(life);
+    const String file_key = layout.namespaceFileKey(life, "f");
+    ASSERT_EQ(backend->putIfAbsent(log_key, "log").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent(snap_key, "snap").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent(ckpt_key, "ckpt").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent(file_key, "file").outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    Gc gc(store, kGc);
+    const RefScanSummary scan = gc.listRefPrefixForTest(GcState{});
+    const std::set<String> offered(scan.keys.begin(), scan.keys.end());
+    EXPECT_EQ(offered, (std::set<String>{log_key, snap_key}));
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamRootPrefix()), 1u);
+    EXPECT_EQ(backend->listCount(layout.namespaceRootPrefix()), 0u);
+    EXPECT_EQ(backend->listCount(layout.namespaceStateRootPrefix()), 0u);
+}
+
+/// The authoritative cut follows the completed hot LIST. A listed life absent from that later cut is
+/// inert dead-life debris: it is not admitted and does not defer the round or read the body.
+TEST(CASGCRoundDefer, ListedLifeAbsentFromThePostListCatalogCutIsInertDebris)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const NamespaceLifeId unknown = NamespaceLifeId::fromCatalogEntry(RootNamespace{"cannot-authorize"}, UInt128{0x456});
+    const String log_key = layout.refLogKey(unknown, RefTxnId{1, 1});
+    ASSERT_EQ(backend->putIfAbsent(log_key, "not-read-on-defer").outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    Gc gc(store, kGc);
+    const RoundReport report = gc.runRegularRound({}, /*allow_steal=*/true, UniversePolicy::Authoritative);
+    EXPECT_FALSE(report.deferred);
+    EXPECT_EQ(backend->getCount(log_key), 0u)
+        << "inert means the body is never read: the life is absent from the authoritative cut, so no "
+           "admission and no fold intake can touch it";
+    /// The debris IS reclaimed in this round, and that is the janitor's designed job, not the fold's:
+    /// a life id absent from the catalog cut is a dead life, and the namespace janitor deletes its
+    /// objects by exact token behind the same fence. A round over a proved-empty catalog completes its
+    /// frontier, so nothing suppresses that reclaim any more -- the object is dropped without ever
+    /// being read or admitted, which is exactly what "inert debris" means here.
+    EXPECT_EQ(backend->deleteCount(log_key), 1u);
+}
+
+/// The post-LIST cut classifies every immutable stream kind, not only logs. A snapshot belonging to a
+/// life absent from that later cut is inert debris and its body is not read.
+TEST(CASGCRoundDefer, SnapshotLifeAbsentFromThePostListCatalogCutIsInertDebris)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const NamespaceLifeId unknown = NamespaceLifeId::fromCatalogEntry(RootNamespace{"cannot-authorize"}, UInt128{0x457});
+    const String snapshot_key = layout.refSnapshotKey(unknown, RefTxnId{1, 1});
+    ASSERT_EQ(backend->putIfAbsent(snapshot_key, "not-read-on-defer").outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    Gc gc(store, kGc);
+    const RoundReport report = gc.runRegularRound({}, /*allow_steal=*/true, UniversePolicy::Authoritative);
+    EXPECT_FALSE(report.deferred);
+    EXPECT_EQ(backend->getCount(snapshot_key), 0u)
+        << "inert means the body is never read, whatever immutable stream kind it is";
+    /// As for the log above: the dead life's snapshot is reclaimed by the janitor by exact token,
+    /// never read and never admitted.
+    EXPECT_EQ(backend->deleteCount(snapshot_key), 1u);
+}
+
+/// ---- Task 4: the DEFER short-circuit wired into runRegularRound ----
+
+/// Idle round re-adopts: after a settled round, a subsequent round with zero changed shards and no
+/// graduation due sets report.deferred=true and performs dramatically less generation-run I/O than a
+/// real fold round (no `blob_target` run object touched at all -- the fold never runs). Snap
+/// generation/attempt are untouched (the snapshot is not rebuilt).
+///
+/// SETTLING NOTE: immutable `_log` objects are never trimmed in place (unlike the legacy mutable shard
+/// journal, whose fold-then-trim token rewrite forced a second settling round), so the pool quiesces the
+/// round AFTER the folding round -- the very next round defers.
+TEST(CASGCRoundDefer, IdleRoundDefersAndReadsNoGeneration)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 0xAA};
+    writeBlobBody(*backend, store->layout(), UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+    const RoundReport fold_rep = gc.runRegularRound();   /// round 1: folds the +1 (no trim-lag, quiesces at once)
+    ASSERT_FALSE(fold_rep.deferred);
+    const uint64_t fold_round_gets = backend->getTotal();
+    EXPECT_GT(fold_round_gets, 0u) << "sanity: a real fold round performs some GETs";
+
+    const auto st_before = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+
+    backend->resetCounts();
+    const RoundReport rep = gc.runRegularRound();   /// round 2: genuinely quiesced now => must defer
+    const uint64_t defer_round_gets = backend->getTotal();
+
+    EXPECT_TRUE(rep.deferred) << "a settled idle round must re-adopt the sealed generation, not fold";
+    /// A deferred round mints no new round (CasGc.cpp:runRegularRound's defer branch), so the honest
+    /// `report.round` is the round that was ALREADY adopted before this round started -- the same round
+    /// the preceding fold round committed. Guards against the bug where the defer path returned WITHOUT
+    /// ever assigning `report.round`, leaving it at its zero-initialized default and making every
+    /// deferred round print `CA GC round 0` regardless of how far GC had actually progressed.
+    EXPECT_NE(rep.round, 0u) << "a deferred round must report a truthful, nonzero round number";
+    EXPECT_EQ(rep.round, fold_rep.round)
+        << "a deferred round re-adopts the already-committed round, not a fabricated new one";
+
+    const auto st_after = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+    EXPECT_EQ(st_after.snap_generation, st_before.snap_generation)
+        << "a deferred round must not mint a new generation (snapshot rebuild elided)";
+    EXPECT_EQ(st_after.snap_attempt, st_before.snap_attempt);
+
+    /// SECONDARY (not over-fit to "exactly 0 gets" -- the decision itself pays a bounded retired-list +
+    /// discovery-LIST cost that may share the same get counter): the deferred round touches NO
+    /// blob_target run object at all (fold never runs, so foldDeltasIntoGeneration never executes), and
+    /// its total get volume sits far below a genuine fold round's.
+    EXPECT_EQ(backend->ioCountForKeysContaining("/blob_target/"), 0u)
+        << "a deferred round must never GET/getStream/PUT any blob_target run object";
+    EXPECT_LT(defer_round_gets, fold_round_gets)
+        << "a deferred round's read volume must sit far below a real fold round's";
+}
+
+/// Every ordinary round constructs one complete catalog-authoritative walk plan after the hot LIST,
+/// before deciding DEFER. A fold consumes that exact frozen plan; it must not build another one.
+TEST(CASGCRoundDefer, FoldAndDeferEachBuildExactlyOneCompletePostListWalkPlan)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/one-walk-plan@cas@"};
+    const ManifestRef ref{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1};
+    writeBlobBody(*backend, layout, UInt128{1});
+    writeManifestRaw(*backend, layout, ns, ref, {blobEntryFor("a", UInt128{1})});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, ref);
+
+    Gc gc(store, kGc);
+    std::vector<GcPhaseRecord> phases;
+    gc.setPhaseSink([&](const GcPhaseRecord & phase) { phases.push_back(phase); });
+
+    backend->resetCounts();
+    const uint64_t fold_builds_before
+        = ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load();
+    ASSERT_FALSE(gc.runRegularRound().deferred);
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load() - fold_builds_before, 1u);
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamRootPrefix()), 1u)
+        << "the hot walk must enumerate the stream tree exactly once";
+    EXPECT_EQ(backend->listCount(layout.namespaceRootPrefix()), 1u)
+        << "the bounded janitor page is a distinct ownership-tree enumeration";
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), 3u)
+        << "generation zero has no drain read: one cut builds the hot walk plan, one follows the janitor "
+           "page, and `planManifestCursorPage` takes its own";
+    const auto fold_decision = std::find_if(phases.begin(), phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "defer_decision";
+    });
+    ASSERT_NE(fold_decision, phases.end());
+    EXPECT_EQ(fold_decision->metrics.at("walk_plan_builds"), 1u);
+    EXPECT_EQ(fold_decision->metrics.at("walk_plan_rows"), 1u);
+    const auto fold_cleanup = std::find_if(phases.begin(), phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "namespace_cleanup";
+    });
+    ASSERT_NE(fold_cleanup, phases.end());
+    EXPECT_EQ(fold_cleanup->metrics.at("janitor_pages"), 1u);
+
+    phases.clear();
+    backend->resetCounts();
+    const uint64_t defer_builds_before
+        = ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load();
+    ASSERT_TRUE(gc.runRegularRound().deferred);
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load() - defer_builds_before, 1u);
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamRootPrefix()), 1u)
+        << "a deferred round still builds exactly one complete hot walk plan";
+    EXPECT_EQ(backend->listCount(layout.namespaceRootPrefix()), 1u)
+        << "the janitor remains one separately paced ownership-tree page";
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), 3u)
+        << "one adopted-parent drain cut, one post-hot-LIST cut, and one post-janitor-page cut";
+    const auto defer_decision = std::find_if(phases.begin(), phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "defer_decision";
+    });
+    ASSERT_NE(defer_decision, phases.end());
+    EXPECT_EQ(defer_decision->metrics.at("walk_plan_builds"), 1u);
+    EXPECT_EQ(defer_decision->metrics.at("walk_plan_rows"), 1u);
+    EXPECT_EQ(defer_decision->metrics.at("walk_plan_dropped_parent_rows"), 0u);
+    EXPECT_EQ(defer_decision->metrics.at("walk_plan_dropped_listed_lives"), 0u);
+    EXPECT_EQ(defer_decision->metrics.at("walk_plan_dropped_tails"), 0u);
+    const auto defer_cleanup = std::find_if(phases.begin(), phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "namespace_cleanup";
+    });
+    ASSERT_NE(defer_cleanup, phases.end());
+    EXPECT_EQ(defer_cleanup->metrics.at("janitor_pages"), 1u);
+}
+
+/// A maintenance cursor can be left between pages while the correctness state is already quiescent.
+/// The next acquired round may DEFER its fold, but it has no authoritative destructive verdict. It
+/// must therefore inspect exactly one janitor page without deleting OR advancing past it; the bounded
+/// forced fold then retries the same page under its computed global gate and reclaims the debris.
+TEST(CASGCRoundDefer, DeferredRoundRetriesPartialJanitorPageAtForcedFoldWithoutPublishingSuccessor)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/1);
+    const Layout & layout = store->layout();
+    const NamespaceLifeId dead_a
+        = NamespaceLifeId::fromCatalogEntry(RootNamespace{"dead/a"}, UInt128{0xDA});
+    const NamespaceLifeId dead_b
+        = NamespaceLifeId::fromCatalogEntry(RootNamespace{"dead/b"}, UInt128{0xDB});
+    const String key_a = layout.refCkptKey(dead_a);
+    const String key_b = layout.refCkptKey(dead_b);
+    ASSERT_EQ(backend->putIfAbsent(key_a, "dead-a").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent(key_b, "dead-b").outcome, PutOutcome::Done);
+
+    /// Establish real opaque backend progress rather than fabricating a cursor value. One key remains
+    /// after this page and the durable cursor must be non-empty.
+    const NamespaceJanitorResult first_page
+        = NamespaceJanitor(*backend, layout, 1).runOnePage(false, [] { return true; });
+    ASSERT_EQ(first_page.pages, 1u);
+    ASSERT_EQ(first_page.deleted, 1u);
+    const GcMaintenanceReadResult partial = readGcMaintenanceState(*backend, layout);
+    ASSERT_EQ(partial.status, GcMaintenanceReadStatus::Valid);
+    ASSERT_TRUE(partial.state);
+    ASSERT_FALSE(partial.state->janitor_cursor.empty());
+    ASSERT_EQ(static_cast<uint64_t>(backend->head(key_a).exists) + static_cast<uint64_t>(backend->head(key_b).exists), 1u);
+
+    /// Give the forced fold a nonempty, fully proved authoritative universe. The R11 floor correctly
+    /// refuses to open the destructive gate for an empty 0-of-0 universe even in the test-only policy.
+    const RootNamespace live_namespace{"live/frontier@cas@"};
+    fixture::admitLive(*backend, layout, live_namespace);
+    ASSERT_EQ(backend->putIfAbsent(
+        layout.refCkptKey(fixture::fixtureLife(live_namespace)),
+        encodeRefCkpt(RefCkpt{
+            .life_epoch = std::optional<uint64_t>{1},
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = std::nullopt})).outcome,
+        PutOutcome::Done);
+
+    backend->resetCounts();
+    std::vector<GcPhaseRecord> phases;
+    Gc gc(store, kGc);
+    gc.setPhaseSink([&](const GcPhaseRecord & phase) { phases.push_back(phase); });
+    const uint64_t plans_before
+        = ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load();
+
+    const RoundReport report = gc.runRegularRound();
+
+    ASSERT_TRUE(report.acquired_lease);
+    ASSERT_TRUE(report.deferred);
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASGCRefWalkPlansBuilt].load() - plans_before, 1u)
+        << "DEFER still constructs its one immutable hot walk plan, never a second janitor-derived plan";
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamRootPrefix()), 1u);
+    EXPECT_EQ(backend->listCount(layout.namespaceRootPrefix()), 1u)
+        << "the deferred round must inspect exactly one separately paced janitor page";
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), 2u)
+        << "generation zero pays one hot walk-plan cut and one post-janitor-page cut";
+    const auto cleanup = std::find_if(phases.begin(), phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "namespace_cleanup";
+    });
+    ASSERT_NE(cleanup, phases.end());
+    EXPECT_EQ(cleanup->metrics.at("janitor_pages"), 1u);
+    EXPECT_GE(cleanup->metrics.at("janitor_keys"), 1u);
+    EXPECT_EQ(cleanup->metrics.at("janitor_deleted"), 0u);
+
+    const GcMaintenanceReadResult deferred_progress = readGcMaintenanceState(*backend, layout);
+    ASSERT_EQ(deferred_progress.status, GcMaintenanceReadStatus::Valid);
+    ASSERT_TRUE(deferred_progress.state);
+    EXPECT_EQ(deferred_progress.state->janitor_cursor, partial.state->janitor_cursor)
+        << "a suppressed DEFER page is undecided and must remain selected for the authoritative fold";
+    EXPECT_EQ(static_cast<uint64_t>(backend->head(key_a).exists) + static_cast<uint64_t>(backend->head(key_b).exists), 1u);
+
+    const auto gc_state = backend->get(layout.gcStateKey());
+    ASSERT_TRUE(gc_state);
+    const GcState state = decodeGcState(gc_state->bytes);
+    EXPECT_EQ(state.snap_generation, 0u);
+    EXPECT_EQ(state.snap_attempt, 0u);
+    EXPECT_FALSE(backend->head(layout.foldSealKey(1, 1)).exists)
+        << "maintenance on DEFER must not publish a fold successor";
+
+    backend->resetCounts();
+    phases.clear();
+    const RoundReport folded = gc.runRegularRound({}, true, UniversePolicy::Authoritative);
+    ASSERT_TRUE(folded.acquired_lease);
+    ASSERT_FALSE(folded.deferred)
+        << "gc_fold_max_defer_rounds=1 forces the round immediately following one DEFER to fold";
+    EXPECT_EQ(backend->listCount(layout.namespaceRootPrefix()), 1u)
+        << "the authoritative fold must run the janitor exactly once, not once per call site";
+    const auto folded_cleanup = std::find_if(phases.begin(), phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "namespace_cleanup";
+    });
+    ASSERT_NE(folded_cleanup, phases.end());
+    EXPECT_EQ(folded_cleanup->metrics.at("janitor_pages"), 1u);
+    EXPECT_GE(folded_cleanup->metrics.at("janitor_keys"), 1u);
+    EXPECT_EQ(folded_cleanup->metrics.at("janitor_deleted"), 1u);
+    EXPECT_EQ(static_cast<uint64_t>(backend->head(key_a).exists) + static_cast<uint64_t>(backend->head(key_b).exists), 0u)
+        << "the fold must retry and delete the exact page that DEFER left undecided";
+    const GcMaintenanceReadResult completed = readGcMaintenanceState(*backend, layout);
+    ASSERT_EQ(completed.status, GcMaintenanceReadStatus::Valid);
+    ASSERT_TRUE(completed.state);
+    EXPECT_TRUE(completed.state->janitor_cursor.empty());
+}
+
+/// The same idle-defer property under a sharded blob-target GC (gc_shards=2): graduationDue's loop
+/// over state.retired_refs and `listRefPrefix`'s discovery must both settle to "nothing due" once
+/// quiesced, regardless of how many gc-shards partition the retired bookkeeping.
+TEST(CASGCRoundDefer, IdleRoundDefersUnderShardedGc)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .gc_shards = 2});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 0xAA};
+    writeBlobBody(*backend, store->layout(), UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    ASSERT_FALSE(gc.runRegularRound().deferred);   /// round 1: folds the publish
+
+    /// Immutable `_log` objects are never trimmed in place, so there is no fold-then-trim token-rewrite
+    /// lag: the pool quiesces after the folding round, and the very next round defers.
+    const RoundReport rep = gc.runRegularRound();   /// round 2: quiesced
+    EXPECT_TRUE(rep.deferred) << "idle pool under gc_shards=2 must defer once settled";
+}
+
+/// The +1 guard (mirror of the 2026-06-27 leak): a blob condemned + published delete_pending, then
+/// re-referenced WHILE it is pending, must NOT be over-deleted -- the due graduation forces a fold
+/// (never a defer) that sees the +1 and spares the blob.
+TEST(CASGCRoundDefer, DueGraduationForcesFoldAndSparesReReferencedBlob)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const UInt128 blob(1);
+    const ManifestRef r1{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 0xAA};
+    writeBlobBody(*backend, store->layout(), blob);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+
+    runRegularRoundReclaiming(gc);                 /// folds the +1; blob referenced
+    store->renewWatermarkOnce();
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r1);   /// the -1 condemns it
+
+    runRegularRoundReclaiming(gc);                 /// the condemning round
+    store->renewWatermarkOnce();
+
+    /// Drive rounds until the entry graduates (published delete_pending) -- mirrors
+    /// CASGCAckFloor.CondemnThenDeleteNextRoundAfterAcks. It is still PRESENT at that pass, and the
+    /// ack floor is by construction already past its condemn_round (that is what graduated it).
+    bool saw_pending = false;
+    for (int i = 0; i < 6 && !saw_pending; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+        for (const RetiredEntry & e : currentRetiredSet(*backend, store->layout(), /*shard*/0))
+            if (e.ref == DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(blob)} && e.delete_pending)
+                saw_pending = true;
+    }
+    ASSERT_TRUE(saw_pending) << "entry never reached delete_pending";
+    ASSERT_FALSE(blobAbsent(*backend, store->layout(), blob)) << "pending: still present this pass";
+
+    /// While B sits delete_pending, a NEW manifest re-references it -- a genuine +1 racing the
+    /// already-published pending delete.
+    const ManifestRef r2{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 0xBB};
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("b", blob)});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl2", std::nullopt, r2);
+
+    /// The next pass would otherwise execute B's pending exact-token delete; graduationDue must force
+    /// a FOLD (never a DEFER) so the +1 is folded in and the blob is spared, not deleted.
+    const RoundReport rep = runRegularRoundReclaiming(gc);
+    EXPECT_FALSE(rep.deferred) << "a due graduation must force a fold, never defer";
+    EXPECT_FALSE(blobAbsent(*backend, store->layout(), blob)) << "the re-referenced blob must survive";
+
+    const FsckReport fsck = runFsck(*store, /*detail*/true);
+    EXPECT_EQ(fsck.dangling, 0u);
+}
+
+/// Companion to the test above: it proves `graduationDue` is the SOLE fold trigger at the assertion
+/// round. `DueGraduationForcesFoldAndSparesReReferencedBlob` opens its store at the DEFAULT
+/// `gc_fold_threshold` (1), so at its assertion round the +1 re-reference ALSO makes
+/// `changed_shards (>= 1) >= fold_threshold (1)` true -- that branch of `shouldDeferRound` would force
+/// the very same fold even if `graduationDue` were deleted or hard-wired false. Here `gc_fold_threshold`
+/// and `gc_fold_max_defer_rounds` are both set to 1000, so neither the changed-shards branch (one
+/// changed shard is nowhere near 1000) nor the liveness-bound branch (this is round 1) can fire --
+/// `graduationDue` is the ONLY thing in `shouldDeferRound` that can force this round's fold, making
+/// `EXPECT_FALSE(rep.deferred)` below load-bearing for `graduationDue` specifically.
+TEST(CASGCRoundDefer, DueGraduationIsSoleFoldTriggerAtHighThreshold)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .gc_fold_threshold = 1000, .gc_fold_max_defer_rounds = 1000});
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    const UInt128 blob(1);
+
+    Gc gc(store, kGc);
+    /// Warm-up round on the still-empty pool: `gc/state` does not exist yet, so lease acquisition takes
+    /// the create-fresh path and succeeds immediately (`gc_id` becomes the owner in storage). This
+    /// matters because the `injectCondemnedSummarySeal` seeding below writes `gc/state` directly, and a fresh `Gc`
+    /// object's FIRST-EVER `acquireOrRenewLease` call against a PRE-EXISTING lease it has never observed
+    /// refuses to steal it (two-observation safety against stealing from a live incumbent) -- it would
+    /// return `acquired_lease=false` and the round would bail out BEFORE the fold-decision code, making
+    /// `EXPECT_FALSE(rep.deferred)` below vacuously true regardless of `graduationDue`. Running this
+    /// warm-up round FIRST makes `gc_id` the observed incumbent, so the assertion round's lease RENEWAL
+    /// (not a steal) succeeds unconditionally and the round actually reaches the decision it's testing.
+    gc.runRegularRound();
+
+    writeBlobBody(*backend, layout, blob);
+
+    /// Seed the adopted fold seal's condemned_summary with B already `delete_pending` (pending_total = 1),
+    /// mirroring `CASGCRoundDefer.GraduationDueDetectsDuePendingAndRoundCrossing`. Retired-in-snapshot
+    /// (T4): graduationDue reads this summary ZERO-I/O off the adopted seal — a delete_pending entry forces
+    /// it true regardless of the round. At `gc_fold_threshold = 1000` a real condemn -> graduate pipeline of
+    /// `runRegularRound` calls is not usable to set this up: every round before graduation would ITSELF
+    /// defer (nothing due yet, and changed_shards never nears 1000), so the due-pending summary is injected
+    /// directly instead of driven through real rounds.
+    injectCondemnedSummarySeal(*backend, layout, /*generation*/1, /*attempt*/1, /*gc_shards*/1,
+        {{0, CondemnedSummary{.condemned_total = 1, .pending_total = 1,
+                              .oldest_nonpending_condemn_round = std::numeric_limits<uint64_t>::max()}}});
+
+    /// The +1: a fresh manifest re-references B while it sits `delete_pending` -- one changed shard,
+    /// far below the threshold of 1000.
+    const ManifestRef r{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 0xBB};
+    writeManifestRaw(*backend, layout, ns, r, {blobEntryFor("a", blob)});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r);
+
+    const RoundReport rep = gc.runRegularRound();
+
+    /// DISCRIMINATING (load-bearing): with graduationDue intact, the due delete_pending entry forces
+    /// the fold. If graduationDue were broken/hard-wired false, changed_shards (1) < threshold (1000)
+    /// and the defer bound (1000) is nowhere near reached, so `shouldDeferRound` would return true and
+    /// this round would DEFER instead.
+    EXPECT_FALSE(rep.deferred) << "a due graduation must be the SOLE fold trigger at a high fold threshold";
+    EXPECT_FALSE(blobAbsent(*backend, layout, blob)) << "the re-referenced blob must survive the forced fold";
+
+    const FsckReport fsck = runFsck(*store, /*detail*/true);
+    EXPECT_EQ(fsck.dangling, 0u);
+}
+
+/// Bounded deferral: with a large fold_threshold and a small standing delta (one shard changed,
+/// forever, since deferring never resolves it), at most gc_fold_max_defer_rounds consecutive rounds
+/// defer, then one round forces a fold (the liveness bound).
+TEST(CASGCRoundDefer, BoundedDeferralForcesFoldWithinWindow)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .gc_fold_threshold = 100, .gc_fold_max_defer_rounds = 3});
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 0xAA};
+    writeBlobBody(*backend, store->layout(), UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    for (int i = 0; i < 3; ++i)
+    {
+        const RoundReport rep = gc.runRegularRound();
+        EXPECT_TRUE(rep.deferred) << "round " << (i + 1) << " is within the defer bound";
+    }
+    const RoundReport rep4 = gc.runRegularRound();
+    EXPECT_FALSE(rep4.deferred) << "the 4th round hits the defer bound and must force-fold";
+}
diff --git a/src/Disks/tests/gtest_cas_gc_shard_incarnation.cpp b/src/Disks/tests/gtest_cas_gc_shard_incarnation.cpp
new file mode 100644
index 000000000000..a487a51a8532
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_shard_incarnation.cpp
@@ -0,0 +1,521 @@
+#include <gtest/gtest.h>
+
+#include <algorithm>
+#include <vector>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+
+namespace DB::ErrorCodes
+{
+extern const int ABORTED;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+using DB::Cas::tests::injectRetire;
+
+namespace
+{
+
+PoolPtr makePoolWithShards(std::shared_ptr<InMemoryBackend> & out_backend, uint64_t gc_shards = 1)
+{
+    out_backend = std::make_shared<InMemoryBackend>();
+    return Pool::open(out_backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_shards = gc_shards});
+}
+
+ManifestRef testRef(uint64_t seq)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = 1};
+}
+
+}
+
+/// Review I5: `discoverUniverse` is catalog-authoritative (Task 4-C), and this test used to survive
+/// the switch from LIST-based discovery unchanged -- `publishCommittedTransition` admits a catalog
+/// entry as its own side effect, so LIST-based and catalog-based discovery were indistinguishable to
+/// it. Pins the three shapes that actually distinguish the two sources directly:
+///   (a) a `Live` catalog entry with ZERO ref objects IS in the universe -- the catalog alone decides;
+///   (b) a `Creating` entry is EXCLUDED -- spec §3, no publication can exist yet;
+///   (c) a namespace with ref OBJECTS but NO catalog entry is EXCLUDED -- the C1 shape: the catalog is
+///       the authority, so its absence is authoritative too, however much debris LIST would still find.
+TEST(CASGCShardIncarnation, DiscoveryEqualsPresentShards)
+{
+    for (const uint64_t gc_shards : {1u, 4u})
+    {
+        std::shared_ptr<InMemoryBackend> backend;
+        auto store = makePoolWithShards(backend, gc_shards);
+        Gc gc(store, hexToU128("0000000000000000000000000000000a"));
+        const Layout & layout = store->layout();
+
+        const RootNamespace ns_live_empty{"srv1/tblLiveEmpty"};
+        const RootNamespace ns_creating{"srv1/tblCreating"};
+        const RootNamespace ns_uncataloged{"srv1/tblUncataloged"};
+
+        /// (a) Admitted Live, nothing else ever written under it.
+        fixture::admitLive(*backend, layout, ns_live_empty);
+
+        /// (b) A genuinely Creating entry, admitted directly (step 1 alone -- never completed to Live).
+        CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards, CatalogEntry{.ns = ns_creating, .state = NsState::Creating,
+            .incarnation = UInt128(1), .creator = CreatorFence{.server_root_id = "test", .writer_epoch = 1, .fence_generation = 1}});
+
+        /// (c) Ref objects present, but the catalog was never told (or has since forgotten): write
+        /// through the real path, which self-admits, then strip the entry back out to simulate "the
+        /// catalog does not name it" without touching the ref objects it left behind.
+        writeManifestRaw(*backend, layout, ns_uncataloged, testRef(1), {});
+        publishCommittedTransition(*backend, layout, ns_uncataloged, "part_1", std::nullopt, testRef(1), /*shard=*/0);
+        {
+            CasRefCatalog::Snapshot snap = CasRefCatalog::read(*backend, layout);
+            std::erase_if(snap.catalog.entries, [&](const CatalogEntry & e) { return e.ns.string() == ns_uncataloged.string(); });
+            const HeadResult h = backend->head(layout.refCatalogKey());
+            ASSERT_TRUE(h.exists);
+            ASSERT_EQ(backend->putOverwrite(layout.refCatalogKey(), encodeRefCatalog(snap.catalog), h.token).outcome,
+                       PutOutcome::Done);
+        }
+
+        const auto universe = gc.discoverUniverseForTest();
+
+        /// Stage B (Task 4-C): the universe is catalog-authoritative now, so it is a life per namespace
+        /// (there are no numeric shards to destructure -- see `NamespaceLifeId`), never a
+        /// `(namespace, shard)` pair.
+        bool found_live_empty = false;
+        for (const NamespaceLifeId & life : universe)
+        {
+            if (life.ns.string() == ns_live_empty.string())
+                found_live_empty = true;
+            EXPECT_NE(life.ns.string(), ns_creating.string()) << "a Creating entry must never be discovered";
+            EXPECT_NE(life.ns.string(), ns_uncataloged.string())
+                << "ref objects with no catalog entry must not be discovered, however much debris LIST would find";
+        }
+        EXPECT_TRUE(found_live_empty) << "a Live catalog entry with zero ref objects must still be discovered";
+
+        /// Confirm (b) really is still Creating (not merely absent from a differently-shaped universe).
+        const CasRefCatalog::Snapshot final_snap = CasRefCatalog::read(*backend, layout);
+        const auto creating_it = std::find_if(final_snap.catalog.entries.begin(), final_snap.catalog.entries.end(),
+            [&](const CatalogEntry & e) { return e.ns.string() == ns_creating.string(); });
+        ASSERT_NE(creating_it, final_snap.catalog.entries.end());
+        EXPECT_EQ(creating_it->state, NsState::Creating);
+    }
+}
+
+/// Catalog ambiguity stops destructive GC and REBUILD before either can derive authority from a
+/// first row. No attempted delete is allowed on the rejected regular round.
+TEST(CASGCShardIncarnation, DuplicateLifeIdStopsDestructiveRoundAndRebuild)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_shards = 1});
+    const Layout & layout = store->layout();
+    RefCatalog catalog;
+    catalog.entries = {
+        CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Live, .incarnation = UInt128{77}},
+        CatalogEntry{
+            .ns = RootNamespace{"b"},
+            .state = NsState::Removing,
+            .incarnation = UInt128{77},
+            .removal_started_round = 1},
+    };
+    const auto empty_catalog = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(empty_catalog);
+    ASSERT_EQ(backend->putOverwrite(
+        layout.refCatalogKey(), encodeRefCatalog(catalog), empty_catalog->token).outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    Gc gc(store, hexToU128("0000000000000000000000000000000a"));
+    EXPECT_THROW(gc.runRegularRound({}, /*allow_steal=*/true, UniversePolicy::Authoritative), DB::Exception);
+    EXPECT_EQ(backend->deleteTotal(), 0u);
+    EXPECT_THROW(gc.rebuildBaseline(/*force=*/true), DB::Exception);
+}
+
+/// A physical life id carries no reversible logical namespace component. Once the catalog moves a
+/// logical name to a new life, the former stream is opaque debris: it cannot redirect GC to that name
+/// or contribute an edge to the current-life fold. The separately paced janitor may reclaim its
+/// unowned physical objects after that fold.
+TEST(CASGCShardIncarnation, DeadLifeStreamIsOpaqueInertDebris)
+{
+    std::shared_ptr<InMemoryBackend> backend;
+    auto store = makePoolWithShards(backend, /*gc_shards=*/1);
+    Gc gc(store, hexToU128("0000000000000000000000000000000a"));
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/tblIncarnationSwap"};
+
+    CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards, CatalogEntry{.ns = ns, .state = NsState::Live,
+        .incarnation = UInt128(11), .creator = std::nullopt});   // Live forbids a creator fence
+    const ManifestRef dead_ref = testRef(1);
+    writeBlobBody(*backend, layout, UInt128(11));
+    writeManifestRaw(*backend, layout, ns, dead_ref, {blobEntryFor("dead", UInt128(11))});
+    std::vector<RefOp> dead_ops{namespaceBirthOp()};
+    const auto dead_committed_ops = publishCommittedOps("part_dead", dead_ref);
+    dead_ops.insert(dead_ops.end(), dead_committed_ops.begin(), dead_committed_ops.end());
+    appendRefLogSeed(*backend, layout, ns, std::move(dead_ops));   // real but unacknowledged record at incarnation 11
+    const NamespaceLifeId dead_life = NamespaceLifeId::fromCatalogEntry(ns, UInt128(11));
+
+    {
+        CasRefCatalog::Snapshot snap = CasRefCatalog::read(*backend, layout);
+        const auto it = std::find_if(snap.catalog.entries.begin(), snap.catalog.entries.end(),
+            [&](const CatalogEntry & e) { return e.ns.string() == ns.string(); });
+        ASSERT_NE(it, snap.catalog.entries.end());
+        it->incarnation = UInt128(22);   // "recreated" -- same name, different (empty) key space
+        const HeadResult h = backend->head(layout.refCatalogKey());
+        ASSERT_TRUE(h.exists);
+        ASSERT_EQ(backend->putOverwrite(layout.refCatalogKey(), encodeRefCatalog(snap.catalog), h.token).outcome,
+                   PutOutcome::Done);
+    }
+
+    const NamespaceLifeId current_life = NamespaceLifeId::fromCatalogEntry(ns, UInt128(22));
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(current_life), encodeRefCkpt(RefCkpt{
+        .life_epoch = std::optional<uint64_t>{1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    const ManifestRef current_ref = testRef(2);
+    writeBlobBody(*backend, layout, UInt128(22));
+    writeManifestRaw(*backend, layout, ns, current_ref, {blobEntryFor("current", UInt128(22))});
+    std::vector<RefOp> current_ops{namespaceBirthOp()};
+    const auto committed_ops = publishCommittedOps("part_current", current_ref);
+    current_ops.insert(current_ops.end(), committed_ops.begin(), committed_ops.end());
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 1}, .ops = std::move(current_ops), .prev_epoch_seal = std::nullopt});
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = std::optional<uint64_t>{1},
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt});
+
+    const RoundReport report = gc.runRegularRound({}, /*allow_steal=*/true, UniversePolicy::Authoritative);
+    EXPECT_TRUE(report.anomalies.empty());
+    EXPECT_FALSE(report.deferred);
+    EXPECT_EQ(inDegreeOf(*backend, layout, UInt128(22)), 1);
+    EXPECT_EQ(inDegreeOf(*backend, layout, UInt128(11)), 0)
+        << "the unmatched old life must not contribute its unacknowledged edge to the current-life fold";
+}
+
+/// Checkpoints live in the state tree and are read by exact key from the catalog cut. They are never
+/// discovered through the hot stream LIST, so hiding one from LIST must not affect the round.
+TEST(CASGCShardIncarnation, CurrentLifeCheckpointIsReadByExactKeyOutsideHotList)
+{
+    auto backend = std::make_shared<HintHoleBackendOn<CountingBackend>>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test", .gc_shards = 1,
+        .gc_fold_max_defer_rounds = 0});
+    Gc gc(store, hexToU128("0000000000000000000000000000000a"));
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/tblOrdinaryRebirth"};
+
+    CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards, CatalogEntry{.ns = ns, .state = NsState::Live,
+        .incarnation = UInt128(11), .creator = std::nullopt});
+
+    CatalogEntry after_rebirth{.ns = ns, .state = NsState::Live, .incarnation = UInt128(22), .creator = std::nullopt};
+    {
+        CasRefCatalog::Snapshot snap = CasRefCatalog::read(*backend, layout);
+        const auto it = std::find_if(snap.catalog.entries.begin(), snap.catalog.entries.end(),
+            [&](const CatalogEntry & e) { return e.ns.string() == ns.string(); });
+        ASSERT_NE(it, snap.catalog.entries.end());
+        *it = after_rebirth;   // "recreated" -- same name, new (current) incarnation 22
+        const HeadResult h = backend->head(layout.refCatalogKey());
+        ASSERT_TRUE(h.exists);
+        ASSERT_EQ(backend->putOverwrite(layout.refCatalogKey(), encodeRefCatalog(snap.catalog), h.token).outcome,
+                   PutOutcome::Done);
+    }
+
+    /// The successor's own genesis `_ckpt`, published for the current physical life. Hiding it from
+    /// LIST must be irrelevant because the walk obtains state only through exact GETs.
+    const NamespaceLifeId current_life = NamespaceLifeId::fromCatalogEntry(ns, UInt128(22));
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(current_life),
+        encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .checkpoint_snapshot_id = std::nullopt,
+                              .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    backend->hide(layout.refCkptKey(current_life));
+    backend->resetCounts();
+    std::vector<GcPhaseRecord> phases;
+    gc.setPhaseSink([&](const GcPhaseRecord & phase) { phases.push_back(phase); });
+
+    const RoundReport report = gc.runRegularRound({}, /*allow_steal=*/true, UniversePolicy::Authoritative);
+    EXPECT_FALSE(report.deferred) << "the forced catalog-only fold must reach checkpoint intake";
+    EXPECT_GT(backend->getCount(layout.refCkptKey(current_life)), 0u)
+        << "the catalog-derived current life must drive an exact checkpoint GET";
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamRootPrefix()), 1u)
+        << "the round must build exactly one hot stream plan";
+    EXPECT_EQ(backend->listCount(layout.namespaceStateRootPrefix()), 0u)
+        << "checkpoint state must never receive its own hot LIST";
+    EXPECT_EQ(backend->listCount(layout.namespaceRootPrefix()), 1u)
+        << "the only broader LIST is the separately paced janitor page";
+    EXPECT_EQ(backend->holesServed(), 1u)
+        << "the hidden checkpoint is omitted only from the janitor's broad page, never from the hot stream LIST";
+    EXPECT_TRUE(backend->head(layout.refCkptKey(current_life)).exists)
+        << "the post-page catalog cut retains the current life even when LIST omitted its checkpoint";
+    const auto cleanup = std::find_if(phases.begin(), phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "namespace_cleanup";
+    });
+    ASSERT_NE(cleanup, phases.end());
+    EXPECT_EQ(cleanup->metrics.at("janitor_pages"), 1u);
+    EXPECT_EQ(cleanup->metrics.at("janitor_deleted"), 0u);
+    bool saw_anomaly_for_ns = false;
+    for (const RoundAnomaly & a : report.anomalies)
+        if (a.ns.string() == ns.string())
+            saw_anomaly_for_ns = true;
+    EXPECT_FALSE(saw_anomaly_for_ns)
+        << "the current life's `_ckpt` is real and readable by its exact catalog-derived key";
+}
+
+/// A stream life absent from the immutable catalog cut cannot be attributed to any logical namespace.
+/// It remains inert debris rather than producing a made-up name or a round anomaly.
+TEST(CASGCShardIncarnation, UncatalogedStreamLifeDefersWithoutInventingNamespace)
+{
+    std::shared_ptr<InMemoryBackend> backend;
+    auto store = makePoolWithShards(backend, /*gc_shards=*/1);
+    Gc gc(store, hexToU128("0000000000000000000000000000000a"));
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/tblForgotten"};
+
+    writeManifestRaw(*backend, layout, ns, testRef(1), {});
+    publishCommittedTransition(*backend, layout, ns, "part_1", std::nullopt, testRef(1), /*shard=*/0);
+    const NamespaceLifeId forgotten_life = store->namespaceLife(ns);
+
+    {
+        CasRefCatalog::Snapshot snap = CasRefCatalog::read(*backend, layout);
+        std::erase_if(snap.catalog.entries, [&](const CatalogEntry & e) { return e.ns.string() == ns.string(); });
+        const HeadResult h = backend->head(layout.refCatalogKey());
+        ASSERT_TRUE(h.exists);
+        ASSERT_EQ(backend->putOverwrite(layout.refCatalogKey(), encodeRefCatalog(snap.catalog), h.token).outcome,
+                   PutOutcome::Done);
+    }
+
+    const RoundReport report = gc.runRegularRound({}, /*allow_steal=*/true, UniversePolicy::Authoritative);
+    EXPECT_TRUE(report.anomalies.empty());
+    EXPECT_FALSE(backend->list(layout.namespaceStreamPrefix(forgotten_life), "", 100).keys.empty());
+}
+
+/// State-tree objects are point-addressed only. A stalled creator's checkpoint and an unowned opaque
+/// checkpoint are both outside the hot stream scan and cannot manufacture logical namespace anomalies.
+TEST(CASGCShardIncarnation, StateCheckpointsOutsideCatalogAreInertToHotWalk)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_shards = 1});
+    Gc gc(store, hexToU128("0000000000000000000000000000000a"));
+    const Layout & layout = store->layout();
+    const RootNamespace creating_ns{"srv1/tblStalledBirth"};
+    const RootNamespace unrelated_gone_ns{"srv1/tblGenuinelyGone"};
+
+    /// Step 1 of createNamespace: insert the Creating entry with a live creator fence.
+    CasRefCatalog::casAdmitEntry(*backend, layout, store->poolConfig().gc_shards, CatalogEntry{.ns = creating_ns, .state = NsState::Creating,
+        .incarnation = UInt128(33),
+        .creator = CreatorFence{.server_root_id = "test", .writer_epoch = 1, .fence_generation = 1}});
+    /// Step 2, without step 3: publish the genesis `_ckpt` directly, at the SAME incarnation the
+    /// Creating entry names -- exactly what `completeCreation` durably leaves behind if the creator
+    /// crashes between its own steps 2 and 3.
+    const NamespaceLifeId creating_life = NamespaceLifeId::fromCatalogEntry(creating_ns, UInt128(33));
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(creating_life),
+        encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .checkpoint_snapshot_id = std::nullopt,
+                              .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    /// Opaque state debris with no corresponding catalog entry.
+    const NamespaceLifeId gone_life = NamespaceLifeId::fromCatalogEntry(unrelated_gone_ns, UInt128(44));
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(gone_life),
+        encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .checkpoint_snapshot_id = std::nullopt,
+                              .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    /// Add one fully current stream so the round performs a fold rather than stopping at an empty
+    /// walk. Catalog and checkpoint admission keep this traffic out of the janitor's dead-life set,
+    /// isolating the one deliberately unowned checkpoint below.
+    const RootNamespace ordinary_ns{"srv1/tblOrdinaryTraffic"};
+    fixture::admitLive(*backend, layout, ordinary_ns);
+    const NamespaceLifeId ordinary_life = fixture::fixtureLife(ordinary_ns);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(ordinary_life),
+        encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .checkpoint_snapshot_id = std::nullopt,
+                              .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    appendRefLogSeed(*backend, layout, ordinary_ns, {});
+
+    backend->resetCounts();
+    std::vector<GcPhaseRecord> phases;
+    gc.setPhaseSink([&](const GcPhaseRecord & phase) { phases.push_back(phase); });
+
+    const RoundReport report = gc.runRegularRound({}, /*allow_steal=*/true, UniversePolicy::Authoritative);
+    bool saw_stalled_birth_anomaly = false;
+    bool saw_genuinely_gone_anomaly = false;
+    for (const RoundAnomaly & a : report.anomalies)
+    {
+        if (a.ns.string() == creating_ns.string())
+            saw_stalled_birth_anomaly = true;
+        if (a.ns.string() == unrelated_gone_ns.string())
+            saw_genuinely_gone_anomaly = true;
+    }
+    EXPECT_FALSE(saw_stalled_birth_anomaly);
+    EXPECT_FALSE(saw_genuinely_gone_anomaly);
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamRootPrefix()), 1u)
+        << "all hot intake must consume one immutable stream listing";
+    EXPECT_EQ(backend->listCount(layout.namespaceStateRootPrefix()), 0u)
+        << "state checkpoints are never a hot discovery source";
+    EXPECT_EQ(backend->listCount(layout.namespaceRootPrefix()), 1u)
+        << "the only ownership-tree listing belongs to the independently paced janitor";
+    EXPECT_GT(backend->getCount(layout.refCkptKey(ordinary_life)), 0u)
+        << "the cataloged Live life is read by its exact checkpoint key";
+    EXPECT_EQ(backend->getCount(layout.refCkptKey(creating_life)), 0u)
+        << "Creating is retained by the janitor cut but excluded from hot checkpoint intake";
+    EXPECT_EQ(backend->getCount(layout.refCkptKey(gone_life)), 0u)
+        << "uncataloged state debris is classified by the janitor page, never exact-read by the hot walk";
+    EXPECT_TRUE(backend->head(layout.refCkptKey(creating_life)).exists);
+    EXPECT_TRUE(backend->head(layout.refCkptKey(ordinary_life)).exists);
+    EXPECT_FALSE(backend->head(layout.refCkptKey(gone_life)).exists)
+        << "catalog absence is inert to the hot walk but authorizes the later janitor exact-token delete";
+    const auto cleanup = std::find_if(phases.begin(), phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "namespace_cleanup";
+    });
+    ASSERT_NE(cleanup, phases.end());
+    EXPECT_EQ(cleanup->metrics.at("janitor_pages"), 1u);
+    EXPECT_EQ(cleanup->metrics.at("janitor_deleted"), 1u);
+}
+
+/// `listNamespaces` projects the authoritative catalog; physical streams never contribute names.
+TEST(CASGCShardIncarnation, ListNamespacesFromCatalog)
+{
+    for (const uint64_t gc_shards : {1u, 4u})
+    {
+        std::shared_ptr<InMemoryBackend> backend;
+        auto store = makePoolWithShards(backend, gc_shards);
+
+        const RootNamespace ns_a{"srv1/tblA"};
+
+        EXPECT_TRUE(store->listNamespaces("").namespaces.empty());
+
+        /// The real writer path admits the catalog row for namespace A.
+        writeManifestRaw(*backend, store->layout(), ns_a, testRef(1), {});
+        publishCommittedTransition(*backend, store->layout(), ns_a, "part_1", std::nullopt, testRef(1), /*shard=*/0);
+
+        const auto nss = store->listNamespaces("").namespaces;
+        ASSERT_EQ(nss.size(), 1u);
+        EXPECT_EQ(nss[0], "srv1/tblA");
+
+        /// Prefix filter: no match.
+        EXPECT_TRUE(store->listNamespaces("srv2/").namespaces.empty());
+        /// Prefix filter: match.
+        const auto filtered = store->listNamespaces("srv1/").namespaces;
+        ASSERT_EQ(filtered.size(), 1u);
+        EXPECT_EQ(filtered[0], "srv1/tblA");
+    }
+}
+
+/// Task 5: THM-NO-RETURN create-race. A NEWBORN ref-shard is born fenced to the current GC round
+/// (self-floor: `fence_round` self-floors to `currentGcRound()` on the create-if-absent branch).
+///
+/// Scenario (registry-free create-race):
+///   1. Open a Pool (gc/state absent).
+///   2. Write blob b1's body directly to the backend (present, not yet condemned).
+///   3. Inject gc/state at round 1 with b1 condemned (its current token in the retired set).
+///      b1's body is still PRESENT — this simulates GC having fenced+retired b1 but not yet
+///      deleted it (the retired-but-body-present window).
+///   4. A writer for NEWBORN ns B calls `precommitAdd` → reads `currentGcRound() = 1` →
+///      the NEWBORN shard is born with `fence_round = 1` (self-floor).
+///   5. `promote` binds the condemned-but-present tokenless leaf AS IS (spec
+///      2026-07-09-cas-writer-gc-simplification D5: there is no writer-side view refresh at promote
+///      any more). This is safe because the precommit closure's edge is journal-durable BEFORE
+///      promote returns (EDGE-BEFORE-OBSERVE): the NEXT GC fold sees net in-degree >= 1 for b1 and
+///      SPARES the entry, regardless of when it would otherwise graduate — the condemnation is
+///      doomed, never the blob. INV-NO-DANGLE holds (dangling=0 in fsck).
+///
+/// Both gc_shards=1 and gc_shards>1 are exercised. The self-floor and promote gate are independent
+/// of the blob-hash-prefix sharding axis (fence_round lives in the ROOT shard).
+TEST(CASGCShardIncarnation, NewbornPrecommitProtectsDedupBlobAgainstConcurrentDrop)
+{
+    for (const uint64_t gc_shards : {1u, 4u})
+    {
+        std::shared_ptr<InMemoryBackend> backend;
+        auto store = makePoolWithShards(backend, gc_shards);
+        const RootNamespace ns_b{"srv1/tblB"};
+
+        /// --- Phase 1: Write b1's body directly (before any GC). ---
+        /// Mint b1 under the POOL streaming-hash id (via a throwaway build's putBlob) so the in-closure
+        /// copy-forward verifier accepts its payload — the plain CityHash test id would be refused.
+        const String b1_payload = "shared-blob-b1";
+        const String b1_hex = streamingHexOf(b1_payload);
+        const BlobRef b1_ref{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128(b1_hex))};
+        {
+            auto seed = store->beginPartWrite({});
+            seed->putBlob(b1_ref, BlobSource::fromString(b1_payload));
+        }
+        const String b1_key = store->layout().blobKey(b1_ref);
+        ASSERT_TRUE(backend->head(b1_key).exists)
+            << "b1 body must be present after the seed putBlob";
+        const Token b1_token = backend->head(b1_key).token;
+
+        /// --- Phase 2: Inject gc/state at round 1 with b1 CONDEMNED (body still present). ---
+        /// This simulates GC having advanced to round 1 and retired b1 (condemned token recorded
+        /// in the retired set) but not yet deleted b1's body object.
+        injectRetire(*backend, store->layout(), /*round*/ 1, /*shard*/ 0,
+            {RetiredEntry{.kind = ObjectKind::Blob, .ref = b1_ref,
+                          .token = b1_token, .size = static_cast<uint64_t>(b1_payload.size())}});
+
+        /// Sanity: currentGcRound() reads gc/state fresh and returns 1.
+        ASSERT_EQ(store->currentGcRound(), 1u)
+            << "currentGcRound() must return the injected round";
+
+        /// --- Phase 3: Writer for NEWBORN ns B — b1 condemned but body present ---
+        PartWriteInfo info_b;
+        info_b.intended_ref = ns_b.string() + "/part_b1";
+        auto build_b = store->beginPartWrite(info_b);
+
+        /// Adopt b1 by tokenless evidence (simulating the dedup case: the writer observed b1
+        /// present BEFORE the GC round — no HEAD here, just evidence).
+        ManifestEntry dep_b1;
+        dep_b1.path = "data.bin";
+        dep_b1.placement = EntryPlacement::Blob;
+        dep_b1.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hexToU128(b1_hex))};
+
+        dep_b1.blob_size = b1_payload.size();
+        build_b->adoptEvidence(dep_b1);
+
+        const ManifestId id_b = build_b->stageManifest({dep_b1});
+
+        /// precommitAdd: NEWBORN shard does not exist yet. Reads currentGcRound() = 1 → stamps
+        /// fence_round = 1 (self-floor). An existing shard would keep its old fence_round.
+        build_b->precommitAdd(ns_b, "part_b1", id_b);
+
+        /// --- Phase 4: promote — the safety assertion (Phase-A contract) ---
+        /// Spec 2026-07-09-cas-writer-gc-simplification D5: there is NO writer-side view refresh at
+        /// promote any more — the K3 gate binds the condemned-but-present token AS IS. This is SAFE
+        /// because the precommit closure's edge has been journal-durable since precommitAdd (BEFORE
+        /// promote returns), so the NEXT GC fold sees net in-degree >= 1 for b1 and SPARES the entry
+        /// (EDGE-BEFORE-OBSERVE) regardless of round-paced graduation timing — the condemnation is
+        /// doomed, never the blob. (No GC round runs in this test at all; the argument is what makes
+        /// deferring the round safe, not something this test drives to completion.)
+        /// The former behavior (self-floor-forced refresh → in-closure copy-forward → fresh incarnation)
+        /// was TLA+-Gate-A-verified redundant; the shard's fence_round stamp itself (THM-NO-RETURN birth
+        /// floor) remains and is asserted by the sibling shard-incarnation tests.
+        EXPECT_NO_THROW(build_b->promote(ns_b, "part_b1", build_b->buildId(), id_b))
+            << "gc_shards=" << gc_shards << ": promote must commit — the durable edge protects the "
+               "condemned-but-present tokenless leaf without any refresh or copy-forward";
+        EXPECT_TRUE(store->resolveRef(ns_b, "part_b1").has_value())
+            << "gc_shards=" << gc_shards << ": the ref must commit";
+        /// The condemned token is bound UNCHANGED — no displacement happens (and none is needed).
+        EXPECT_EQ(backend->head(b1_key).token, b1_token)
+            << "gc_shards=" << gc_shards << ": no copy-forward under the Phase-A contract — the token "
+               "stays; the folded edge will spare it at the next fold (no round runs here to delete it)";
+
+        /// INV-NO-DANGLE: the body is present and no GC round ever runs in this test to fold the
+        /// precommit/committed edge; a real deployment's next fold would see net in-degree >= 1 and
+        /// spare the entry. A regression that let the delete pipeline race a live durable edge would
+        /// produce dangling=1 here.
+        const FsckReport rep = runFsck(*store, /*detail=*/false);
+        EXPECT_EQ(rep.dangling, 0u)
+            << "gc_shards=" << gc_shards << ": INV-NO-DANGLE violated — a committed ref names a "
+               "missing blob (dangling=" << rep.dangling << ", reachable=" << rep.reachable << ")";
+    }
+}
+
+/// The five shard-OBJECT-reclaim tests that used to follow (`DroppedShardObjectIsReclaimed`,
+/// `IdleButLiveShardNotReclaimed`, `RecreateAfterReclaimFoldsFromZero`, `ActivatedPrecommitBlocksShardReclaim`,
+/// `ReviveRacesReclaimAborts`) were removed with the snapshot+log ref model. They asserted GC reclaims /
+/// token-guards a MUTABLE per-namespace ref-shard object at `rootShardKey(ns, shard)`. There is no such
+/// mutable object anymore: a namespace's ref state is its immutable `_log`/`_snap` objects, physical
+/// reclamation belongs to the perpetual namespace janitor, and ABA safety is structural -- a recreated
+/// namespace uses a different opaque life id. The still-meaningful reincarnation case (a terminal old
+/// life followed by a new life folds without inheriting the old cursor) is covered by
+/// `gtest_cas_ref_gc.cpp`; lifecycle completion itself requires only folded terminal evidence and the
+/// exact catalog-row mutation.
diff --git a/src/Disks/tests/gtest_cas_gc_shard_plan.cpp b/src/Disks/tests/gtest_cas_gc_shard_plan.cpp
new file mode 100644
index 000000000000..c45cd3ff772d
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_shard_plan.cpp
@@ -0,0 +1,637 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include "cas_test_helpers.h"
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+TEST(CASGCShardConfig, DefaultIsSingleShard)
+{
+    PoolConfig cfg;
+    EXPECT_EQ(cfg.gc_shards, 1u);
+    EXPECT_EQ(cfg.manifest_sweep_list_budget_keys, 1000u);
+    EXPECT_EQ(cfg.manifest_sweep_delete_budget_keys, 100u);
+}
+
+TEST(CASGCShardConfig, GcStateRoundTripPreservesShardCount)
+{
+    GcState s;
+    s.gc_shards = 4;
+    s.round = 7;
+    const GcState d = decodeGcState(encodeGcState(s));
+    EXPECT_EQ(d.gc_shards, 4u);
+    EXPECT_EQ(d.round, 7u);
+}
+
+/// ---- blobShard tests (Phase 4, Task 3) ----
+
+TEST(CASGCShardScatter, DeterministicAndStable)
+{
+    /// A fixed hash — the same bytes every run. blobShard must return the same value twice,
+    /// must be strictly less than gc_shards=4, and must be 0 when gc_shards=1.
+    const UInt128 h = hexToU128("0102030405060708090a0b0c0d0e0f10");
+    const BlobRef hd{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h)};
+
+    const uint64_t s4a = blobShard(hd, 4);
+    const uint64_t s4b = blobShard(hd, 4);
+
+    EXPECT_EQ(s4a, s4b) << "blobShard must be deterministic";
+    EXPECT_LT(s4a, 4u) << "blobShard result must be < gc_shards";
+    EXPECT_EQ(blobShard(hd, 1), 0u) << "gc_shards==1 must route every hash to shard 0";
+}
+
+TEST(CASGCShardScatter, DisjointCoverageOverManyHashes)
+{
+    /// Over 4096 spread-out hashes with gc_shards=4: every result in [0,4) and every shard
+    /// gets at least one hash (no dead shard).
+    constexpr uint64_t kNumHashes = 4096;
+    constexpr uint64_t kShards = 4;
+
+    std::vector<bool> seen(kShards, false);
+    for (uint64_t i = 0; i < kNumHashes; ++i)
+    {
+        /// Spread: use i in the high and low halves to avoid clustering.
+        const UInt128 h = (static_cast<UInt128>(i * 0x9e3779b97f4a7c15ULL) << 64)
+                        | static_cast<UInt128>(i * 0x6c62272e07bb0142ULL);
+        const uint64_t s = blobShard(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h)}, kShards);
+        ASSERT_LT(s, kShards) << "blobShard out of range at i=" << i;
+        seen[s] = true;
+    }
+
+    for (uint64_t s = 0; s < kShards; ++s)
+        EXPECT_TRUE(seen[s]) << "shard " << s << " received no hashes (dead shard)";
+}
+
+/// ---- ShardReducer tests (Phase 4, Task 4) ----
+
+/// Build two blob hashes that route to DIFFERENT shards under gc_shards=2.
+/// Returns {hash_for_shard0, hash_for_shard1}.
+static std::pair<BlobRef, BlobRef> makeTwoShardHashes()
+{
+    /// Scan pairs (i, j): find hash_a -> shard 0, hash_b -> shard 1 under gc_shards=2.
+    /// We construct candidates by setting the high 64 bits and leaving the low 64 bits zero
+    /// so blobShard = high64 % 2.  i=0 => shard 0, i=1 => shard 1.
+    const UInt128 h0 = static_cast<UInt128>(0ULL) << 64;   /// high64=0 => shard 0
+    const UInt128 h1 = static_cast<UInt128>(1ULL) << 64;   /// high64=1 => shard 1
+    return {BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h0)},
+            BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h1)}};
+}
+
+/// `ShardReducer::reduce` merges deltas into the correct per-shard in-degree run.
+///
+/// Scenario: scatter (+1 b1, +1 b1, -1 b1, +1 b2) across two shards.
+///   - b1 routes to shard 0; net = +2 - 1 = 1; in-degree after reduce = 1.
+///   - b2 routes to shard 1; net = +1; in-degree after reduce = 1.
+///   - Each reducer touches ONLY its own shard's key space.
+TEST(CASGCShardReducer, MergesDeltasToInDegree)
+{
+    const auto [b1, b2] = makeTwoShardHashes();
+    ASSERT_EQ(blobShard(b1, 2), 0u) << "b1 must route to shard 0";
+    ASSERT_EQ(blobShard(b2, 2), 1u) << "b2 must route to shard 1";
+
+    /// Construct source-edge deltas directly (the production fold produces these via
+    /// `foldManifestEdges`, bucketed by `blobShard`):
+    /// b1 shard=0: source 1 activates, source 2 activates, source 1 removes => 1 active edge
+    /// b2 shard=1: source 3 activates => 1 active edge
+    std::vector<std::vector<BlobDelta>> buckets(2);
+    buckets[0] = {
+        BlobDelta{.ref = b1, .source_id = UInt128(1), .remove = false},
+        BlobDelta{.ref = b1, .source_id = UInt128(2), .remove = false},
+        BlobDelta{.ref = b1, .source_id = UInt128(1), .remove = true},
+    };
+    buckets[1] = {
+        BlobDelta{.ref = b2, .source_id = UInt128(3), .remove = false},
+    };
+
+    /// Verify bucket net effects (source 2 survives for b1; source 3 survives for b2).
+    ASSERT_EQ(buckets.size(), 2u);
+    {
+        int64_t net_b1 = 0;
+        for (const auto & d : buckets[0])
+            if (d.ref == b1)
+                net_b1 += d.remove ? -1 : +1;
+        EXPECT_EQ(net_b1, 1) << "shard-0 bucket net delta for b1 must be +1";
+    }
+    {
+        int64_t net_b2 = 0;
+        for (const auto & d : buckets[1])
+            if (d.ref == b2)
+                net_b2 += d.remove ? -1 : +1;
+        EXPECT_EQ(net_b2, 1) << "shard-1 bucket net delta for b2 must be +1";
+    }
+
+    /// Reduce: each reducer merges its shard's deltas into generation 1 (prior = 0 = fresh).
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+
+    ShardReducer r0(0, 2);
+    ShardReducer r1(1, 2);
+
+    EXPECT_TRUE(r0.owns(b1)) << "r0 must own b1";
+    EXPECT_FALSE(r0.owns(b2)) << "r0 must not own b2";
+    EXPECT_TRUE(r1.owns(b2)) << "r1 must own b2";
+    EXPECT_FALSE(r1.owns(b1)) << "r1 must not own b1";
+
+    const auto runs0 = r0.reduce(*backend, layout, /*prior_runs=*/{}, /*new_generation=*/1, /*attempt=*/0,
+                                 std::move(buckets[0]));
+    const auto runs1 = r1.reduce(*backend, layout, /*prior_runs=*/{}, /*new_generation=*/1, /*attempt=*/0,
+                                 std::move(buckets[1]));
+
+    ASSERT_EQ(runs0.size(), 1u) << "shard-0 reduce must produce exactly one RunRef";
+    ASSERT_EQ(runs1.size(), 1u) << "shard-1 reduce must produce exactly one RunRef";
+
+    /// The keys must be distinct (disjoint shard namespaces).
+    EXPECT_NE(runs0[0].key, runs1[0].key) << "shard-0 and shard-1 run keys must be distinct";
+
+    /// Read back in-degree from the sealed runs (resolved via each reduce's returned refs).
+    const int64_t indeg_b1 = inDegreeInRuns(*backend, runs0, b1);
+    const int64_t indeg_b2 = inDegreeInRuns(*backend, runs1, b2);
+    EXPECT_EQ(indeg_b1, 1) << "b1 in-degree after reduce must be 1";
+    EXPECT_EQ(indeg_b2, 1) << "b2 in-degree after reduce must be 1";
+
+    /// Cross-shard reads: shard-0's run must not contain b2; shard-1's run must not contain b1.
+    EXPECT_EQ(inDegreeInRuns(*backend, runs0, b2), 0)
+        << "shard-0 run must not mention b2";
+    EXPECT_EQ(inDegreeInRuns(*backend, runs1, b1), 0)
+        << "shard-1 run must not mention b1";
+}
+
+/// `ShardReducer::owns` partitions the blob hash space: for any hash, exactly ONE reducer among
+/// {r0, r1} owns it (union == all, intersection == empty).
+TEST(CASGCShardReducer, TwoReducersCoverDisjointShards)
+{
+    constexpr uint64_t kNumHashes = 4096;
+    constexpr uint64_t kGcShards = 2;
+
+    ShardReducer r0(0, kGcShards);
+    ShardReducer r1(1, kGcShards);
+
+    for (uint64_t i = 0; i < kNumHashes; ++i)
+    {
+        const UInt128 h = (static_cast<UInt128>(i * 0x9e3779b97f4a7c15ULL) << 64)
+                        | static_cast<UInt128>(i * 0x6c62272e07bb0142ULL);
+        const BlobRef href{BlobHashAlgo::CityHash128, BlobDigest::fromU128(h)};
+        const bool o0 = r0.owns(href);
+        const bool o1 = r1.owns(href);
+
+        /// Exactly one of the two reducers must own every hash.
+        ASSERT_TRUE(o0 || o1)
+            << "hash " << i << " is owned by neither shard (gap in coverage)";
+        ASSERT_FALSE(o0 && o1)
+            << "hash " << i << " is owned by BOTH shards (overlap in coverage)";
+    }
+}
+
+/// ---- manifestCleanupShard tests (Phase 4, Task 5) ----
+
+/// Two `ManifestId`s with the SAME `ManifestRef` but DIFFERENT namespaces must be unequal (proving
+/// qualified identity), and `manifestCleanupShard` must depend on the namespace — not just the ref.
+///
+/// Phase 0 `SabotageKeyByRefNotId`: if routing used only the `ManifestRef`, two namespaces sharing
+/// the same ref would land on the same worker, merging cleanup work that belongs to distinct objects.
+TEST(CASGCShardCleanup, RoutesByQualifiedManifestIdNotRef)
+{
+    /// Shared ManifestRef: identical across both ManifestIds.
+    const ManifestRef shared_ref{
+        .writer_epoch = 1,
+        .build_sequence = 7,
+        .manifest_ordinal = 1,
+    };
+
+    const ManifestId id_a{RootNamespace("ns_alpha"), shared_ref};
+    const ManifestId id_b{RootNamespace("ns_beta"), shared_ref};
+
+    /// The two ids are unequal (different namespace => different qualified identity).
+    EXPECT_NE(id_a, id_b) << "ManifestIds with different namespaces must be unequal";
+
+    /// Both results must be in range.
+    constexpr uint64_t kShards = 4;
+    const uint64_t shard_a = manifestCleanupShard(id_a, kShards);
+    const uint64_t shard_b = manifestCleanupShard(id_b, kShards);
+    EXPECT_LT(shard_a, kShards) << "shard for id_a must be < gc_shards";
+    EXPECT_LT(shard_b, kShards) << "shard for id_b must be < gc_shards";
+
+    /// Deterministic: same id always routes to the same shard.
+    EXPECT_EQ(manifestCleanupShard(id_a, kShards), shard_a) << "manifestCleanupShard must be deterministic";
+    EXPECT_EQ(manifestCleanupShard(id_b, kShards), shard_b) << "manifestCleanupShard must be deterministic";
+
+    /// Single-shard equivalence: gc_shards==1 routes everything to shard 0.
+    EXPECT_EQ(manifestCleanupShard(id_a, 1), 0u) << "gc_shards==1 must route to shard 0";
+    EXPECT_EQ(manifestCleanupShard(id_b, 1), 0u) << "gc_shards==1 must route to shard 0";
+
+    /// KEY ASSERTION: routing depends on the namespace, not the ref alone.
+    /// Scan namespace-pair candidates (varying only the namespace string) until we find two that
+    /// route to different shards under gc_shards=8. This directly demonstrates that
+    /// `manifestCleanupShard` is NOT a function of `ManifestRef` alone.
+    bool found_namespace_split = false;
+    for (uint64_t i = 0; i < 256 && !found_namespace_split; ++i)
+    {
+        const ManifestId probe_a{RootNamespace("namespace_probe_" + std::to_string(i)), shared_ref};
+        for (uint64_t j = i + 1; j < 256 && !found_namespace_split; ++j)
+        {
+            const ManifestId probe_b{RootNamespace("namespace_probe_" + std::to_string(j)), shared_ref};
+            if (manifestCleanupShard(probe_a, 8) != manifestCleanupShard(probe_b, 8))
+                found_namespace_split = true;
+        }
+    }
+    EXPECT_TRUE(found_namespace_split)
+        << "could not find two namespace variants of the same ManifestRef that route to different "
+           "shards — routing is not namespace-sensitive (SabotageKeyByRefNotId hazard)";
+}
+
+/// Over many `ManifestId`s with `gc_shards=4`: every owner shard is covered, and each id lands in
+/// exactly one shard (total, disjoint coverage).
+TEST(CASGCShardCleanup, DisjointWorkerCoverage)
+{
+    constexpr uint64_t kNumIds = 4096;
+    constexpr uint64_t kShards = 4;
+
+    std::vector<bool> seen(kShards, false);
+    for (uint64_t i = 0; i < kNumIds; ++i)
+    {
+        /// Vary both namespace and ManifestRef fields to spread the distribution.
+        const ManifestId id{
+            RootNamespace("ns_" + std::to_string(i % 16)),
+            ManifestRef{
+                .writer_epoch = 1 + i / 16,
+                .build_sequence = i,
+                .manifest_ordinal = static_cast<uint32_t>(i % kMaxManifestOrdinal + 1),
+            },
+        };
+
+        const uint64_t s = manifestCleanupShard(id, kShards);
+        ASSERT_LT(s, kShards) << "manifestCleanupShard out of range at i=" << i;
+        seen[s] = true;
+    }
+
+    for (uint64_t s = 0; s < kShards; ++s)
+        EXPECT_TRUE(seen[s]) << "owner shard " << s << " received no ManifestIds (dead shard)";
+}
+
+/// The sharded fold (gc_shards > 1) partitions a flat `BlobDelta` stream by `blobShard` and folds
+/// each bucket via its own `ShardReducer`, exactly as `Gc::fold` does. This test replicates that
+/// partition-and-reduce step over `gc_shards = 2` and asserts each blob's in-degree lands in its
+/// owning shard's run and nowhere else. (The full two-replica round is covered by Task 8.)
+TEST(CASGCShardCoordinator, ShardedFoldRoutesDeltasToOwningShards)
+{
+    constexpr uint64_t kGcShards = 2;
+    const auto [b0, b1] = makeTwoShardHashes();
+    ASSERT_EQ(blobShard(b0, kGcShards), 0u);
+    ASSERT_EQ(blobShard(b1, kGcShards), 1u);
+
+    /// A flat delta stream as produced by `foldManifestEdges`: b0 net +1 (two +1, one -1), b1 net +1.
+    std::vector<BlobDelta> deltas{
+        BlobDelta{.ref = b0, .source_id = UInt128(1), .remove = false},
+        BlobDelta{.ref = b1, .source_id = UInt128(2), .remove = false},
+        BlobDelta{.ref = b0, .source_id = UInt128(3), .remove = false},
+        BlobDelta{.ref = b0, .source_id = UInt128(1), .remove = true},
+    };
+
+    /// Partition by blobShard — the exact step the sharded fold runs before reducing.
+    std::vector<std::vector<BlobDelta>> buckets(kGcShards);
+    for (BlobDelta & d : deltas)
+        buckets[blobShard(d.ref, kGcShards)].push_back(d);
+
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+
+    std::vector<std::vector<RunRef>> shard_runs(kGcShards);
+    for (uint64_t shard = 0; shard < kGcShards; ++shard)
+    {
+        ShardReducer reducer{shard, kGcShards};
+        shard_runs[shard] = reducer.reduce(*backend, layout, /*prior_runs=*/{}, /*new_generation=*/1, /*attempt=*/0,
+                                           std::move(buckets[shard]));
+    }
+
+    EXPECT_EQ(inDegreeInRuns(*backend, shard_runs[0], b0), 1)
+        << "b0 must fold into shard-0 with in-degree 1";
+    EXPECT_EQ(inDegreeInRuns(*backend, shard_runs[1], b1), 1)
+        << "b1 must fold into shard-1 with in-degree 1";
+    EXPECT_EQ(inDegreeInRuns(*backend, shard_runs[1], b0), 0)
+        << "b0 must NOT appear in shard-1's run";
+    EXPECT_EQ(inDegreeInRuns(*backend, shard_runs[0], b1), 0)
+        << "b1 must NOT appear in shard-0's run";
+}
+
+/// ---- Phase 4, Task 7: single-shard equivalence ----
+///
+/// Prove that the sharded partition+reduce path (gc_shards=2, all blobs routing to shard 0) produces
+/// the SAME per-blob in-degrees as the single-shard (gc_shards=1, Phase 1d) fold over an IDENTICAL
+/// journal. This is approach (a) from the spec: choose blob hashes whose high64 % 2 == 0 so shard 1's
+/// bucket is always empty; the sharded path's shard-0 reducer and the single-shard path both call
+/// `foldDeltasIntoGeneration` with the same delta stream (one routing into shard 0 of 2, the other
+/// into shard 0 of 1).
+///
+/// NOTE ON SEAL-BYTE EQUALITY: byte-for-byte equality of the `CasFoldSeal` is NOT asserted here. The
+/// fold seal records the `blobTargetRunKey(gen, shard, seq)` path, which embeds the shard number. The
+/// single-shard path writes `blobTargetRunKey(g, 0, 0)` for gc_shards=1, while the sharded path writes
+/// `blobTargetRunKey(g, 0, 0)` for the shard-0 run AND `blobTargetRunKey(g, 1, 0)` for the (empty)
+/// shard-1 run. The per-blob in-degree (the load-bearing property — it drives the spare/delete
+/// decision) is identical; the seal's key-set legitimately differs by shard count.
+TEST(CASGCShardEquivalence, SingleShardMatchesPhase1dInDegree)
+{
+    /// Build three blob hashes that ALL route to shard 0 under gc_shards=2 (high64 % 2 == 0).
+    /// high64=0 => shard 0, high64=2 => shard 0, high64=4 => shard 0.
+    const UInt128 hA = static_cast<UInt128>(0ULL) << 64;   /// high64=0, routes to shard 0
+    const UInt128 hB = static_cast<UInt128>(2ULL) << 64;   /// high64=2, routes to shard 0
+    const UInt128 hC = static_cast<UInt128>(4ULL) << 64;   /// high64=4, routes to shard 0
+
+    const BlobRef refA{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hA)};
+    const BlobRef refB{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hB)};
+    const BlobRef refC{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hC)};
+    ASSERT_EQ(blobShard(refA, 2), 0u) << "hA must route to shard 0 under gc_shards=2";
+    ASSERT_EQ(blobShard(refB, 2), 0u) << "hB must route to shard 0 under gc_shards=2";
+    ASSERT_EQ(blobShard(refC, 2), 0u) << "hC must route to shard 0 under gc_shards=2";
+    ASSERT_EQ(blobShard(refA, 1), 0u) << "hA must route to shard 0 under gc_shards=1";
+    ASSERT_EQ(blobShard(refB, 1), 0u) << "hB must route to shard 0 under gc_shards=1";
+    ASSERT_EQ(blobShard(refC, 1), 0u) << "hC must route to shard 0 under gc_shards=1";
+
+    /// Construct the journal: hA gets net +2 (published twice), hB gets net +1, hC gets net 0 (publish
+    /// then drop => transitions to zero). This exercises all three outcomes (>1, =1, =0) for the
+    /// equivalence proof.
+    ///
+    /// Note: net +2 is unrealistic for production (two DISTINCT manifests can share a blob, each
+    /// contributing +1 independently) but is valid for the fold math test. It directly verifies that
+    /// accumulators sum correctly under both paths.
+    const RootNamespace ns{"ns-equiv"};
+    const ManifestRef rA1{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = static_cast<uint32_t>(0x1)};
+    const ManifestRef rA2{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = static_cast<uint32_t>(0x2)};
+    const ManifestRef rB{.writer_epoch = 1, .build_sequence = 3, .manifest_ordinal = static_cast<uint32_t>(0x3)};
+    const ManifestRef rC{.writer_epoch = 1, .build_sequence = 4, .manifest_ordinal = static_cast<uint32_t>(0x4)};
+
+    /// Helper lambda that sets up a fresh backend + store with the shared scripted journal, runs one GC
+    /// round with the given gc_shards, and returns the per-blob in-degrees in the sealed generation.
+    /// Returns {indeg_A, indeg_B, indeg_C}.
+    auto runJournalAndGetInDegrees = [&](uint64_t gc_shards) -> std::tuple<int64_t, int64_t, int64_t>
+    {
+        auto backend = std::make_shared<InMemoryBackend>();
+        const Layout layout("p");
+        /// Raw journal fixtures model an already-created pool and therefore establish both mandatory
+        /// controls before writing residual data.
+        seedPoolMetaForRestart(*backend);
+
+        /// Write blob bodies so HEAD returns a token (GC retires zero-in-degree blobs only if present).
+        writeBlobBody(*backend, layout, hA);
+        writeBlobBody(*backend, layout, hB);
+        writeBlobBody(*backend, layout, hC);
+
+        /// Write manifests: rA1 references hA once; rA2 also references hA once; rB references hB;
+        /// rC references hC. Each publication contributes +1 per referenced blob.
+        writeManifestRaw(*backend, layout, ns, rA1, {blobEntryFor("a", hA)});
+        writeManifestRaw(*backend, layout, ns, rA2, {blobEntryFor("a", hA)});
+        writeManifestRaw(*backend, layout, ns, rB,  {blobEntryFor("b", hB)});
+        writeManifestRaw(*backend, layout, ns, rC,  {blobEntryFor("c", hC)});
+
+        /// Publish all four refs (tbl1=rA1, tbl2=rA2, tbl3=rB, tbl4=rC).
+        publishCommittedTransition(*backend, layout, ns, "tbl1", std::nullopt, rA1);
+        publishCommittedTransition(*backend, layout, ns, "tbl2", std::nullopt, rA2);
+        publishCommittedTransition(*backend, layout, ns, "tbl3", std::nullopt, rB);
+        publishCommittedTransition(*backend, layout, ns, "tbl4", std::nullopt, rC);
+        /// Drop tbl4 (hC net = 0): rC removed from the live set.
+        dropRefTransition(*backend, layout, ns, "tbl4", rC);
+
+        /// Open a store with the given `gc_shards` over the pre-seeded restart state.
+        auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .gc_shards = gc_shards});
+        const UInt128 gc_id = UInt128(0xDEADBEEF42ULL);
+        Gc gc(store, gc_id);
+        EXPECT_TRUE(gc.runRegularRound().acquired_lease);
+
+        /// The fold seal for new_generation (== snap_generation after fold) holds the in-degree runs.
+        /// After runRegularRound the snap_generation points at the COMPLETION generation; the fold
+        /// generation is snap_generation - 1 for the first full round. Use inDegreeOf (which reads
+        /// currentGenerationOf = completion generation) for the final in-degrees.
+        const std::vector<RunRef> shard0 = runsForShard(*backend, layout, /*shard=*/0);
+        const int64_t iA = inDegreeInRuns(*backend, shard0, refA);
+        const int64_t iB = inDegreeInRuns(*backend, shard0, refB);
+        const int64_t iC = inDegreeInRuns(*backend, shard0, refC);
+        return {iA, iB, iC};
+    };
+
+    const auto [a1, b1_indeg, c1] = runJournalAndGetInDegrees(/*gc_shards=*/1);
+    const auto [a2, b2_indeg, c2] = runJournalAndGetInDegrees(/*gc_shards=*/2);
+
+    /// The in-degree values must match exactly between the two runs.
+    EXPECT_EQ(a1, a2)
+        << "hA in-degree must match: gc_shards=1 gives " << a1 << ", gc_shards=2 gives " << a2;
+    EXPECT_EQ(b1_indeg, b2_indeg)
+        << "hB in-degree must match: gc_shards=1 gives " << b1_indeg << ", gc_shards=2 gives " << b2_indeg;
+    EXPECT_EQ(c1, c2)
+        << "hC in-degree must match: gc_shards=1 gives " << c1 << ", gc_shards=2 gives " << c2;
+
+    /// Cross-check the known correct values (derivable from the scripted journal).
+    /// hA: +1 (tbl1/rA1) + 1 (tbl2/rA2) = 2.
+    EXPECT_EQ(a1, 2) << "hA in-degree must be 2 (two distinct live refs both citing hA)";
+    /// hB: +1 (tbl3/rB) = 1.
+    EXPECT_EQ(b1_indeg, 1) << "hB in-degree must be 1";
+    /// hC: +1 (tbl4/rC publish) - 1 (tbl4 drop) = 0.
+    EXPECT_EQ(c1, 0) << "hC in-degree must be 0 (publish then drop; net zero)";
+}
+
+/// ---- Phase 4, Task 8: two-replica disjoint-shard concurrency ----
+///
+/// With gc_shards=2 over a shared `InMemoryBackend`:
+///   (a) DISJOINTNESS: a shard-0 reducer's product covers only hashes routing to shard 0; shard-1
+///       covers only hashes routing to shard 1 (`owns` check).
+///   (b) PER-SHARD RUNS: each reducer writes its own write-once blob-target run; the runs for the two
+///       shards are disjoint object keys and durably present after each `ShardReducer::reduce`.
+///   (c) MERGED IN-DEGREE: the merged in-degrees across both shards equal the expected edge multiset,
+///       and each blob is absent from the other shard's run (cross-shard disjointness).
+///
+/// Interleaving: driven entirely from the test thread (no threads, no sleeps). The two reducers are
+/// constructed and called sequentially from the test thread. This proves the protocol is correct even
+/// when reducer work interleaves arbitrarily — the key-space disjointness is static.
+TEST(CASGCShardTwoReplica, DisjointShardsConcurrentPerShardRuns)
+{
+    constexpr uint64_t kGcShards = 2;
+    constexpr uint64_t kNewGen = 1;
+    constexpr uint64_t kAttempt = 0;
+
+    /// b0 routes to shard 0, b1 routes to shard 1 (from makeTwoShardHashes).
+    const auto [b0, b1] = makeTwoShardHashes();
+    ASSERT_EQ(blobShard(b0, kGcShards), 0u) << "b0 must route to shard 0";
+    ASSERT_EQ(blobShard(b1, kGcShards), 1u) << "b1 must route to shard 1";
+
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+
+    /// (a) DISJOINTNESS — verify `owns` predicate before any reduce.
+    ShardReducer r0(0, kGcShards);
+    ShardReducer r1(1, kGcShards);
+
+    EXPECT_TRUE(r0.owns(b0))  << "shard-0 reducer must own b0";
+    EXPECT_FALSE(r0.owns(b1)) << "shard-0 reducer must NOT own b1";
+    EXPECT_TRUE(r1.owns(b1))  << "shard-1 reducer must own b1";
+    EXPECT_FALSE(r1.owns(b1) && r0.owns(b1)) << "no hash may be owned by both reducers";
+
+    /// Construct disjoint delta streams: b0 gets net +2 in shard 0; b1 gets net +1 in shard 1.
+    /// In production these buckets are produced by `foldManifestEdges` and partitioned by `blobShard`
+    /// (two distinct manifests both referencing b0 contribute two source edges; one manifest
+    /// referencing b1 contributes one source edge).
+    std::vector<BlobDelta> bucket0 = {
+        BlobDelta{.ref = b0, .source_id = UInt128(1), .remove = false},
+        BlobDelta{.ref = b0, .source_id = UInt128(2), .remove = false},
+    };
+    std::vector<BlobDelta> bucket1 = {
+        BlobDelta{.ref = b1, .source_id = UInt128(3), .remove = false},
+    };
+
+    /// (b) PER-SHARD RUNS — drive both reducers.
+    ///
+    /// Run shard-0 reducer (simulates the shard-0 replica's work).
+    const auto runs0 = r0.reduce(*backend, layout, /*prior_runs=*/{}, kNewGen, kAttempt, std::move(bucket0));
+    ASSERT_FALSE(runs0.empty()) << "shard-0 reducer must produce at least one RunRef";
+
+    /// Run shard-1 reducer (simulates the shard-1 replica's work, interleaved from the test thread).
+    const auto runs1 = r1.reduce(*backend, layout, /*prior_runs=*/{}, kNewGen, kAttempt, std::move(bucket1));
+    ASSERT_FALSE(runs1.empty()) << "shard-1 reducer must produce at least one RunRef";
+
+    /// The blob-target runs for both shards are durably present (the reducer's write-once `putIfAbsent`),
+    /// at disjoint object keys.
+    EXPECT_TRUE(backend->head(layout.blobTargetRunKey(kNewGen, kAttempt, /*shard=*/0, /*seq=*/0)).exists)
+        << "shard-0 blob-target run must be durably written by r0.reduce";
+    EXPECT_TRUE(backend->head(layout.blobTargetRunKey(kNewGen, kAttempt, /*shard=*/1, /*seq=*/0)).exists)
+        << "shard-1 blob-target run must be durably written by r1.reduce";
+
+    /// (c) MERGED IN-DEGREE — the merged in-degrees across both shards equal the expected edge multiset.
+    EXPECT_EQ(inDegreeInRuns(*backend, runs0, b0), 2)
+        << "b0 in-degree must be 2 in shard-0 run";
+    EXPECT_EQ(inDegreeInRuns(*backend, runs1, b1), 1)
+        << "b1 in-degree must be 1 in shard-1 run";
+    /// Cross-shard: each blob must be absent from the other shard's run.
+    EXPECT_EQ(inDegreeInRuns(*backend, runs0, b1), 0)
+        << "b1 must NOT appear in shard-0's run (cross-shard disjointness)";
+    EXPECT_EQ(inDegreeInRuns(*backend, runs1, b0), 0)
+        << "b0 must NOT appear in shard-1's run (cross-shard disjointness)";
+}
+
+/// ---- Phase 4 regression: gc_shards>1 retire-drain (High #1) ----
+///
+/// A FULL round-protocol regression that drives publish -> drop -> reclaim end-to-end under
+/// `gc_shards = 2` with a droppable blob owned by a NON-zero shard. The fold/`ShardReducer` write one
+/// in-degree run PER shard, so a zero-in-degree blob owned by shard 1..N is only ever retired (and
+/// then exact-token deleted) if `retire`/`previewDeletes` scan EVERY blob-target shard. Before
+/// `5f5fa5f7906` both hardcoded shard 0: a shard-1 candidate was never scanned, never retired, and
+/// leaked forever. After the fix both shards are drained.
+///
+/// The test plants TWO droppable blobs in the SAME round — one owned by shard 0, one owned by shard 1
+/// (verified via `blobShard(hash, 2)`) — and asserts BOTH are reclaimed. The shard-0 blob proves the
+/// round works at all; the shard-1 blob is the regression's teeth (it would leak pre-fix while shard-0
+/// still drained, so a single-blob test could pass even with the bug).
+///
+/// HOW IT WOULD LEAK PRE-FIX: under the old shard-0-only `retire`, the round folds the drop (shard-1
+/// blob's in-degree -> 0 in shard 1's run) but `retire` only reads shard 0's in-degree run and only
+/// writes shard 0's retired set, so the shard-1 zero-in-degree blob is never proposed for retirement.
+/// `previewDeletes` (also shard-0-only pre-fix) never lists it, the recheck never spares-or-deletes it,
+/// and `blobExists(b1)` stays true at fixpoint. The shard-0 blob would still be reclaimed — which is
+/// exactly why the existing in-degree-equivalence tests (all blobs route to shard 0) did not catch it.
+TEST(CASGCShardRetireDrain, ReclaimsDroppableBlobOwnedByNonZeroShard)
+{
+    constexpr uint64_t kGcShards = 2;
+
+    /// Two blob hashes routing to DIFFERENT shards under gc_shards=2. blobShard = high64 % 2.
+    /// high64=0 => shard 0; high64=1 => shard 1.
+    const UInt128 blob_shard0 = (static_cast<UInt128>(0ULL) << 64) | static_cast<UInt128>(7ULL); /// high64=0 => shard 0
+    const UInt128 blob_shard1 = (static_cast<UInt128>(1ULL) << 64) | static_cast<UInt128>(7ULL); /// high64=1 => shard 1
+    ASSERT_EQ(blobShard(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob_shard0)}, kGcShards), 0u) << "blob_shard0 must route to shard 0";
+    ASSERT_EQ(blobShard(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob_shard1)}, kGcShards), 1u) << "blob_shard1 must route to shard 1 (regression teeth)";
+
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                                                 .gc_shards = kGcShards});
+    const Layout & layout = store->layout();
+
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r0{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = static_cast<uint32_t>(0xA0)};
+    const ManifestRef r1{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = static_cast<uint32_t>(0xA1)};
+    const ManifestId id0{ns, r0};
+    const ManifestId id1{ns, r1};
+
+    /// Local blobExists (the round-level helper is file-local to gtest_cas_gc_round.cpp).
+    auto blobExists = [&](const UInt128 & hash)
+    {
+        return backend->head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)})).exists;
+    };
+    auto manifestExists = [&](const ManifestId & id)
+    {
+        return backend->head(layout.manifestKey(id)).exists;
+    };
+    /// Whether ANY gc-shard still holds an in-flight condemned entry (the ack-floor deletion pipeline is
+    /// in flight while this is true). Retired-in-snapshot (T4): reconstructed from the adopted fold seal's
+    /// kCondemned rows across all shards, not a separate retired list.
+    auto anyRetiredPending = [&]
+    {
+        return anyCondemnedInSeal(*backend, layout);
+    };
+    /// Drive to a fixpoint over the ACK-FLOOR round: advance the store's mount ack each round (so the floor
+    /// follows the committed round) and stay alive while any work counter is nonzero OR an in-flight
+    /// retired entry remains in ANY shard.
+    auto driveToFixpoint = [&](Gc & gc)
+    {
+        for (size_t r = 0; r < 64; ++r)
+        {
+            const RoundReport rep = runRegularRoundReclaiming(gc);
+            if (!rep.acquired_lease)
+                continue;
+            store->renewWatermarkOnce();
+            const bool no_work = rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+                && rep.replaced == 0 && rep.spared == 0;
+            if (no_work && !anyRetiredPending())
+                break;
+        }
+    };
+
+    /// Publish: ref r0 names the shard-0 blob, ref r1 names the shard-1 blob (distinct refs => distinct
+    /// edges, each contributing +1 to its blob's in-degree in its OWNING shard's run).
+    writeBlobBody(*backend, layout, blob_shard0);
+    writeBlobBody(*backend, layout, blob_shard1);
+    writeManifestRaw(*backend, layout, ns, r0, {blobEntryFor("a", blob_shard0)});
+    writeManifestRaw(*backend, layout, ns, r1, {blobEntryFor("b", blob_shard1)});
+    publishCommittedTransition(*backend, layout, ns, "tbl0", std::nullopt, r0);
+    publishCommittedTransition(*backend, layout, ns, "tbl1", std::nullopt, r1);
+
+    const UInt128 gc_id = UInt128(0xDEADBEEF42ULL);
+    Gc gc(store, gc_id);
+    driveToFixpoint(gc);
+
+    /// While both refs are live: each blob's in-degree is 1 in its OWNING shard's run, and nothing is
+    /// collected (no-loss). Derive generation/attempt from gc/state — never hardcode.
+    const GcState live = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    ASSERT_GT(live.snap_generation, 0u);
+    ASSERT_EQ(live.gc_shards, kGcShards) << "the pool must be running with gc_shards=2";
+    EXPECT_EQ(inDegreeInRuns(*backend, runsForShard(*backend, layout, /*shard=*/0), BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob_shard0)}), 1)
+        << "shard-0 blob in-degree must be 1 while live";
+    EXPECT_EQ(inDegreeInRuns(*backend, runsForShard(*backend, layout, /*shard=*/1), BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob_shard1)}), 1)
+        << "shard-1 blob in-degree must be 1 while live";
+    EXPECT_TRUE(blobExists(blob_shard0));
+    EXPECT_TRUE(blobExists(blob_shard1));
+
+    /// Drop BOTH refs: each blob's only edge goes away (in-degree -> 0 in its owning shard's run).
+    dropRefTransition(*backend, layout, ns, "tbl0", r0);
+    dropRefTransition(*backend, layout, ns, "tbl1", r1);
+    driveToFixpoint(gc);
+
+    /// After drop + fixpoint: BOTH blobs are retired and exact-token deleted, and BOTH owner-removed
+    /// manifest bodies are collected. The shard-1 blob is the regression's teeth — pre-`5f5fa5f` it
+    /// would still exist here because retire/previewDeletes never scanned shard 1.
+    EXPECT_EQ(inDegreeInRuns(*backend, runsForShard(*backend, layout, /*shard=*/0), BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob_shard0)}), 0)
+        << "shard-0 blob in-degree must be 0 after drop";
+    EXPECT_EQ(inDegreeInRuns(*backend, runsForShard(*backend, layout, /*shard=*/1), BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(blob_shard1)}), 0)
+        << "shard-1 blob in-degree must be 0 after drop";
+    EXPECT_FALSE(blobExists(blob_shard0)) << "shard-0 droppable blob must be reclaimed";
+    EXPECT_FALSE(blobExists(blob_shard1))
+        << "shard-1 droppable blob must be reclaimed (High #1: retire must scan ALL shards, not just shard 0)";
+    EXPECT_FALSE(manifestExists(id0)) << "shard-0 owner-removed manifest body must be reclaimed";
+    EXPECT_FALSE(manifestExists(id1)) << "shard-1 owner-removed manifest body must be reclaimed";
+
+    /// Idempotent: another fixpoint changes nothing and never throws.
+    EXPECT_NO_THROW(driveToFixpoint(gc));
+    EXPECT_FALSE(blobExists(blob_shard0));
+    EXPECT_FALSE(blobExists(blob_shard1));
+}
diff --git a/src/Disks/tests/gtest_cas_gc_source_edge.cpp b/src/Disks/tests/gtest_cas_gc_source_edge.cpp
new file mode 100644
index 000000000000..5fffe7ad2a97
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_source_edge.cpp
@@ -0,0 +1,84 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/Exception.h>
+
+using namespace DB::Cas;
+
+TEST(CASSourceEdge, IdIsDeterministicAndPathSensitive)
+{
+    const ManifestId id{RootNamespace{"00/aa@cas@"}, ManifestRef{.writer_epoch = 1, .build_sequence = 15, .manifest_ordinal = 1}};
+    EXPECT_EQ(sourceEdgeId(id, "a.bin"), sourceEdgeId(id, "a.bin"));           // deterministic
+    EXPECT_NE(sourceEdgeId(id, "a.bin"), sourceEdgeId(id, "b.bin"));           // path-sensitive
+    const ManifestId id2{id.root_namespace, ManifestRef{.writer_epoch = 1, .build_sequence = 31, .manifest_ordinal = 1}};
+    EXPECT_NE(sourceEdgeId(id, "a.bin"), sourceEdgeId(id2, "a.bin"));          // ref-sensitive
+}
+
+TEST(CASSourceEdge, RunKeyRoundTripsAndOrdersByBlobThenSource)
+{
+    const BlobRef b1{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(1))};
+    const BlobRef b2{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(2))};
+    const UInt128 s1(10);
+    const UInt128 s2(20);
+
+    BlobRef gb;
+    UInt128 gs;
+    SourceEdgeKeyCodec::parse(SourceEdgeKeyCodec::key(b1, s1), gb, gs);
+    EXPECT_EQ(gb, b1);
+    EXPECT_EQ(gs, s1);
+    EXPECT_LT(SourceEdgeKeyCodec::key(b1, s2), SourceEdgeKeyCodec::key(b2, s1));   // ref is the primary sort
+    EXPECT_LT(SourceEdgeKeyCodec::key(b1, s1), SourceEdgeKeyCodec::key(b1, s2));   // source_id is the secondary sort
+}
+
+TEST(CASSourceEdge, KeyCodecSha256RoundTripAndRejectsBadSizes)
+{
+    /// sha256 (32-byte digest) round trip: key is 1 + 32 + 16 = 49 bytes, parse recovers the full ref.
+    BlobDigest d32{};
+    for (size_t i = 0; i < d32.bytes.size(); ++i)
+        d32.bytes[i] = static_cast<uint8_t>(i + 1);
+    const BlobRef sha_ref{BlobHashAlgo::Sha256, d32};
+    const UInt128 sid(0xABCDu);
+    const String key32 = SourceEdgeKeyCodec::key(sha_ref, sid);
+    ASSERT_EQ(key32.size(), 49u);
+    BlobRef gb;
+    UInt128 gs;
+    SourceEdgeKeyCodec::parse(key32, gb, gs);
+    EXPECT_EQ(gb, sha_ref);
+    EXPECT_EQ(gs, sid);
+
+    /// ch128 (16-byte digest): key is 1 + 16 + 16 = 33 bytes.
+    const BlobRef ch_ref{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(0x0102030405060708ULL))};
+    const String key16 = SourceEdgeKeyCodec::key(ch_ref, sid);
+    ASSERT_EQ(key16.size(), 33u);
+    EXPECT_EQ(key16.substr(1), String(reinterpret_cast<const char *>(ch_ref.digest.bytes.data()), 16) + u128ToBytesBE(sid));
+
+    /// Fail-close: a wrong-size key throws CORRUPTED_DATA, never a silent false. `key16` truncated by
+    /// one byte still declares algo=ch128 (33-byte width expected) but is only 32 bytes.
+    EXPECT_THROW(SourceEdgeKeyCodec::parse(key16.substr(0, key16.size() - 1), gb, gs), DB::Exception);
+    EXPECT_THROW(SourceEdgeKeyCodec::parse(String(20, '\0'), gb, gs), DB::Exception);
+
+    /// Unknown algo byte -> NOT_IMPLEMENTED (fail closed).
+    String bad_key = key32;
+    bad_key[0] = static_cast<char>(99);
+    EXPECT_THROW(SourceEdgeKeyCodec::parse(bad_key, gb, gs), DB::Exception);
+}
+
+TEST(CASSourceEdge, KeyOrderSentinelFirstAtLen32)
+{
+    /// At sha256 width, the sentinel (source_id 0) sorts before any nonzero source_id for the same
+    /// digest, and digest magnitude order is preserved (big-endian raw-byte lexicographic order ==
+    /// numeric magnitude order for a width-homogeneous run — the consult's load-bearing fact).
+    BlobDigest d{};
+    d.bytes[0] = 0x10;
+    const BlobRef ref{BlobHashAlgo::Sha256, d};
+    EXPECT_LT(SourceEdgeKeyCodec::key(ref, UInt128(0)), SourceEdgeKeyCodec::key(ref, UInt128(1)));
+
+    BlobDigest d_small{};
+    d_small.bytes[0] = 0x01;
+    BlobDigest d_large{};
+    d_large.bytes[0] = 0x02;
+    const BlobRef ref_small{BlobHashAlgo::Sha256, d_small};
+    const BlobRef ref_large{BlobHashAlgo::Sha256, d_large};
+    EXPECT_LT(SourceEdgeKeyCodec::key(ref_small, UInt128(5)), SourceEdgeKeyCodec::key(ref_large, UInt128(5)));
+}
diff --git a/src/Disks/tests/gtest_cas_gc_state_format.cpp b/src/Disks/tests/gtest_cas_gc_state_format.cpp
new file mode 100644
index 000000000000..aa661429f813
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_state_format.cpp
@@ -0,0 +1,179 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+}
+
+TEST(CASFormatBattery, GcState)
+{
+    GcState s;
+    s.round = 4;
+    s.gc_shards = 1;
+    s.snap_generation = 9;
+    s.snap_pruned_through = 7;
+    s.snap_attempt = 3;
+    s.manifest_sweep_cursor = "";
+    s.lease = GcLease{UInt128(1), 12};
+    runFormatBattery({FormatId::GcState,
+        [&] { return sealObject(FormatId::GcState, encodeGcState(s)); },
+        [](std::string_view d) { decodeGcState(std::string(openObject(FormatId::GcState, d))); },
+        currentFormatHeader("cas_gc_state") +
+        "{\"rnd\":\"4\",\"gcs\":1,\"sg\":\"9\",\"spt\":\"7\",\"sa\":\"3\",\"msc\":\"\","
+        "\"lo\":\"00000000000000000000000000000001\",\"ls\":\"12\"}\n"});
+}
+
+TEST(CASFormatBattery, GcHeartbeat)
+{
+    GcHeartbeat hb{UInt128(1), 1741};
+    runFormatBattery({FormatId::GcHeartbeat,
+        [&] { return sealObject(FormatId::GcHeartbeat, encodeGcHeartbeat(hb)); },
+        [](std::string_view d) { decodeGcHeartbeat(std::string(openObject(FormatId::GcHeartbeat, d))); },
+        currentFormatHeader("cas_gc_hb") +
+        "{\"by\":\"00000000000000000000000000000001\",\"seq\":\"1741\"}\n"});
+}
+
+/// ---------- field round-trips (migrated from gtest_cas_gc_formats.cpp, re-pointed at the text codec) ----------
+
+TEST(CASGCStateFormat, RoundTripsCoreFields)
+{
+    GcState s;
+    s.round = 7;
+    s.gc_shards = 1;
+    s.snap_generation = 12;
+    s.lease.owner = hexToU128("00000000000000000000000000000005");
+    s.lease.seq = 5;
+    auto d = decodeGcState(encodeGcState(s));
+    EXPECT_EQ(d.round, 7u);
+    EXPECT_EQ(d.gc_shards, 1u);
+    EXPECT_EQ(d.snap_generation, 12u);
+    EXPECT_EQ(d.lease.owner, hexToU128("00000000000000000000000000000005"));
+    EXPECT_EQ(d.lease.seq, 5u);
+}
+
+TEST(CASGCStateFormat, SnapPrunedThroughAndAttemptAndCursorRoundTrip)
+{
+    GcState s;
+    s.gc_shards = 2;
+    s.snap_generation = 42;
+    s.snap_pruned_through = 38;
+    s.snap_attempt = 7;
+    s.manifest_sweep_cursor = "p/cas/manifests/server/store/abc/table@cas@/writer/42/aa/id";
+    auto d = decodeGcState(encodeGcState(s));
+    EXPECT_EQ(d.snap_pruned_through, 38u);
+    EXPECT_EQ(d.snap_attempt, 7u);
+    EXPECT_EQ(d.manifest_sweep_cursor, s.manifest_sweep_cursor);
+}
+
+TEST(CASGCStateFormat, DefaultsRoundTrip)
+{
+    GcState s;   /// gc_shards defaults to 1
+    EXPECT_EQ(s.gc_shards, 1u);
+    auto d = decodeGcState(encodeGcState(s));
+    EXPECT_EQ(d.round, 0u);
+    EXPECT_EQ(d.snap_attempt, 0u);
+    EXPECT_TRUE(d.manifest_sweep_cursor.empty());
+    EXPECT_EQ(d.lease.owner, UInt128{});
+}
+
+TEST(CASGCStateFormat, RejectsZeroGcShards)
+{
+    /// `v:3` is deliberate and must NOT follow a future `G_BUILD` bump: any version <= G_BUILD passes
+    /// the header gate, which is the point — the BODY is what has to fail here.
+    const String bad = "{\"type\":\"cas_gc_state\",\"v\":3}\n"
+                       "{\"rnd\":\"0\",\"gcs\":0,\"sg\":\"0\",\"spt\":\"0\",\"sa\":\"0\",\"msc\":\"\","
+                       "\"lo\":\"00000000000000000000000000000000\",\"ls\":\"0\"}\n";
+    EXPECT_THROW(decodeGcState(bad), DB::Exception);
+}
+
+#ifndef DEBUG_OR_SANITIZER_BUILD
+/// encodeGcState(gc_shards=0) throws LOGICAL_ERROR, which aborts the whole process in debug/sanitizer
+/// builds instead of behaving like a catchable exception -- CASGCStateFormatDeathTest below proves the
+/// abort positively in those builds instead.
+TEST(CASGCStateFormat, RejectsZeroGcShardsOnEncode)
+{
+    GcState state;
+    state.gc_shards = 0;
+
+    try
+    {
+        encodeGcState(state);
+        FAIL() << "expected exception code " << DB::ErrorCodes::LOGICAL_ERROR;
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::LOGICAL_ERROR);
+    }
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASGCStateFormatDeathTest, RejectsZeroGcShardsOnEncodeAborts)
+{
+    GcState state;
+    state.gc_shards = 0;
+    EXPECT_DEATH({ (void)encodeGcState(state); }, "");
+}
+#endif
+
+TEST(CASGCStateFormat, RejectsAbsentGcShards)
+{
+    /// An absent gcs key must fail closed (the writer always emits it) rather than silently defaulting
+    /// to the struct's gc_shards = 1 — a missing shard count means a corrupt object, not "use the floor".
+    /// `v:3` is deliberate and must NOT follow a future `G_BUILD` bump: any version <= G_BUILD passes
+    /// the header gate, which is the point — the BODY is what has to fail here.
+    const String bad = "{\"type\":\"cas_gc_state\",\"v\":3}\n"
+                       "{\"rnd\":\"0\",\"sg\":\"0\",\"spt\":\"0\",\"sa\":\"0\",\"msc\":\"\","
+                       "\"lo\":\"00000000000000000000000000000000\",\"ls\":\"0\"}\n";
+    EXPECT_THROW(decodeGcState(bad), DB::Exception);
+}
+
+TEST(CASGCStateFormat, GarbageFailsClosed)
+{
+    EXPECT_THROW(decodeGcState(String("")), DB::Exception);
+    EXPECT_THROW(decodeGcState(String("not a cas object\n")), DB::Exception);
+}
+
+TEST(CASGCHeartbeatFormat, RoundTripAndBoundaries)
+{
+    GcHeartbeat hb;
+    hb.owner = hexToU128("0123456789abcdeffedcba9876543210");
+    hb.hb_seq = 12345;
+    GcHeartbeat d = decodeGcHeartbeat(encodeGcHeartbeat(hb));
+    EXPECT_EQ(d.owner, hb.owner);
+    EXPECT_EQ(d.hb_seq, 12345u);
+
+    GcHeartbeat z;
+    z.owner = hexToU128("ffffffffffffffffffffffffffffffff");
+    z.hb_seq = 0;
+    EXPECT_EQ(decodeGcHeartbeat(encodeGcHeartbeat(z)).owner, z.owner);
+    EXPECT_THROW(decodeGcHeartbeat(String("short")), DB::Exception);
+}
+
+TEST(CASGCHeartbeatFormat, RejectsMissingIdentityFields)
+{
+    /// `v:3` is deliberate and must NOT follow a future `G_BUILD` bump: any version <= G_BUILD passes
+    /// the header gate, which is the point — the BODY is what has to fail here.
+    const String header = "{\"type\":\"cas_gc_hb\",\"v\":3}\n";
+
+    const auto expectCorrupted = [](const String & data)
+    {
+        try
+        {
+            decodeGcHeartbeat(data);
+            FAIL() << "expected CORRUPTED_DATA";
+        }
+        catch (const DB::Exception & e)
+        {
+            EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+        }
+    };
+
+    expectCorrupted(header + "{\"seq\":\"1741\"}\n");
+    expectCorrupted(header + "{\"by\":\"00000000000000000000000000000001\"}\n");
+}
diff --git a/src/Disks/tests/gtest_cas_gc_stop_start.cpp b/src/Disks/tests/gtest_cas_gc_stop_start.cpp
new file mode 100644
index 000000000000..fa4c864cc8de
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_stop_start.cpp
@@ -0,0 +1,492 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <cstdint>
+#include <filesystem>
+#include <future>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <string>
+#include <unistd.h>
+#include <vector>
+
+/// Task 11 (rev.7 spec §6): `SYSTEM CAS GC STOP` / `GC START` -- granular operator control
+/// of ONLY the background GC scheduler. STOP is STOP-IN-PLACE: it joins the worker + heartbeat threads and
+/// clears the in-process leadership hint, but RETAINS the scheduler object so a later START restarts the
+/// SAME instance (its `gc_id` + lease-observation history preserved). The disk stays fully usable (reads/
+/// writes unaffected) while GC is stopped. START refuses on a decommissioned/uncertain pool (typed 668).
+///
+/// These tests exercise the scheduler-level behavior directly (`CasGcScheduler::stop`/`start`) and the
+/// end-to-end verbs through a real `ContentAddressedMetadataStorage`. Harness patterns follow
+/// gtest_cas_forget.cpp and gtest_cas_gc_log.cpp.
+
+namespace DB::ErrorCodes
+{
+extern const int INVALID_STATE;
+}
+
+using namespace DB;
+using DB::Cas::CasGcScheduler;
+using DB::Cas::GcRoundLogRecord;
+using DB::Cas::InMemoryBackend;
+using DB::Cas::PoolLifecycle;
+using DB::Cas::RoundReport;
+using DB::Cas::tests::openPoolForTest;
+
+namespace
+{
+
+/// A live table dir + committed part reused by the "reads/writes unaffected while stopped" test (the shape
+/// gtest_cas_forget.cpp / gtest_cas_operation_gate.cpp use).
+const std::string kTableDir = "gg0/gg0gg0g0-0808-4808-8808-080808080808";
+const std::string kPartDir = kTableDir + "/all_1_1_0";
+const std::string kPartFile = kPartDir + "/data.bin";
+
+/// The Pool-level `server_root_id` `openPoolForTest` mints (mirrors gtest_cas_lifecycle_condition.cpp).
+const std::string kSrid = "test";
+
+/// GC's fence-out applied directly to the mount lease (preserve the body, set `gc_fenced`, bump `seq`) so a
+/// subsequent `tryRemountOnce` verdicts `Recover` and reclaims a FRESH incarnation immediately (no
+/// lease-expiry wait), driving a transient-not-live pool back to `Live`. Mirrors
+/// gtest_cas_lifecycle_condition.cpp's helper — used by the operator-STOP-persistence test below.
+void fenceOutMount(DB::Cas::Backend & backend, const String & mount_key)
+{
+    const auto got = backend.get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    DB::Cas::MountLease m = DB::Cas::decodeMountLease(got->bytes);
+    m.gc_fenced = true;
+    m.seq += 1;
+    ASSERT_EQ(backend.putOverwrite(mount_key, DB::Cas::encodeMountLease(m), got->token).outcome,
+              DB::Cas::PutOutcome::Done);
+}
+
+/// A real `ContentAddressedMetadataStorage` over a fresh, unique local object storage. `context == nullptr`
+/// (a unit-test mount), so `startup()` creates NO GC scheduler -- the GC entry points, and `gcStart`, create
+/// one lazily. GC is enabled by default (`gc_enabled == true`, `gc_interval_sec == 60`), so no background
+/// round fires during the sub-second test window. Mirrors gtest_cas_forget.cpp's `openForgetStorage`.
+std::shared_ptr<ContentAddressedMetadataStorage> openGcStorage()
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto scratch = std::filesystem::temp_directory_path()
+        / ("ca_gc_stopstart_scratch_" + std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1)));
+    auto settings = Cas::tests::makeSettingsForTest("test", scratch);
+    auto storage = std::make_shared<ContentAddressedMetadataStorage>(
+        Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+void commitOnePart(ContentAddressedMetadataStorage & storage)
+{
+    auto tx = storage.createTransaction();
+    auto & ca_tx = dynamic_cast<ContentAddressedTransaction &>(*tx);
+    auto buf = ca_tx.writeFile(kTableDir + "/tmp_insert_all_1_1_0/data.bin", 65536, WriteMode::Rewrite, {});
+    const std::string bytes = "content-of-the-part";
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+    tx->moveDirectory(kTableDir + "/tmp_insert_all_1_1_0", kPartDir);
+    tx->commit(NoCommitOptions{});
+}
+
+/// A thread-safe sink for the scheduler's per-round log records, with a condition variable so a test can
+/// WAIT (never sleep) for a background round to land. `waitForSuccessFinish` blocks until a Finish record
+/// with `outcome == Success` (the round acquired/renewed the GC lease) appears at index >= `from`, or the
+/// timeout trips (only on a genuine hang/regression -- the round is sub-millisecond on an in-memory pool).
+class RoundLogSink
+{
+public:
+    Cas::GcRoundLogger logger()
+    {
+        return [this](const GcRoundLogRecord & r)
+        {
+            std::lock_guard lock(mutex);
+            records.push_back(r);
+            cv.notify_all();
+        };
+    }
+
+    /// Index one past the current end of the record log -- the "from" watermark for a subsequent wait.
+    size_t mark()
+    {
+        std::lock_guard lock(mutex);
+        return records.size();
+    }
+
+    /// The first Success Finish record at index >= `from`, waiting up to `timeout`. Returns nullopt on
+    /// timeout so the caller asserts with a clear message rather than hanging.
+    std::optional<GcRoundLogRecord> waitForSuccessFinish(size_t from, std::chrono::milliseconds timeout)
+    {
+        std::unique_lock lock(mutex);
+        const bool ok = cv.wait_for(lock, timeout, [&]
+        {
+            for (size_t i = from; i < records.size(); ++i)
+                if (records[i].event_type == GcRoundLogRecord::EventType::Finish
+                    && records[i].outcome == GcRoundLogRecord::Outcome::Success)
+                    return true;
+            return false;
+        });
+        if (!ok)
+            return std::nullopt;
+        for (size_t i = from; i < records.size(); ++i)
+            if (records[i].event_type == GcRoundLogRecord::EventType::Finish
+                && records[i].outcome == GcRoundLogRecord::Outcome::Success)
+                return records[i];
+        return std::nullopt;
+    }
+
+private:
+    std::mutex mutex;
+    std::condition_variable cv;
+    std::vector<GcRoundLogRecord> records;
+};
+
+/// A generous wait bound for a background round to land -- trips only on a real deadlock/regression.
+constexpr std::chrono::milliseconds kRoundWait{60000};
+
+/// Bound for the [C1] self-exit observation: comfortably above the 1s pacing interval (so a slow CI box
+/// still sees the loop tick + observe the terminal state) yet short enough that the RED demo (self-exit
+/// removed) fails fast rather than hanging for `kRoundWait`.
+constexpr std::chrono::milliseconds kSelfExitWait{15000};
+
+/// A bounded OBSERVATION window (not a sleep-to-fix-a-race) for the "stays stopped across recovery" test:
+/// comfortably above the 1s pacing interval so a running scheduler would have filled it with several
+/// rounds, yet short enough to keep the negative assertion cheap. Its meaning is anchored by a positive
+/// control (an explicit START right after DOES produce a round through the same sink).
+constexpr std::chrono::milliseconds kStayStoppedWindow{3000};
+
+}
+
+/// (C1) A NATURAL terminal transition (`VanishedReplaced`, or here `VanishedForgotten` forced via the test
+/// seam) is never accompanied by a `stop()` on this scheduler — only `~Pool`/FORGET join it. The scheduler's
+/// OWN loops must observe the terminal lifecycle at their next tick and self-exit,
+/// so the pacing loop stops spamming Failed rounds (the G2 zombie) and the steal-capable loop can never
+/// fold/condemn a foreign pool's prefix. Drive it while RUNNING, then vanish it, then prove BOTH loops
+/// self-exit (bounded cv wait, no sleep) and that no further round-log rows appear.
+TEST(CASGCStopStart, SchedulerSelfExitsOnNaturalVanished)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+
+    RoundLogSink sink;
+    /// 1s interval: the loop ticks ~1s; the cv wait below (never a sleep) synchronizes on real records.
+    CasGcScheduler sched(store, std::chrono::seconds(1), "CasGcSelfExitTest", "ca-disk", sink.logger());
+    sched.start();
+
+    /// Prove the loop is genuinely RUNNING first: a background round must land and acquire the lease.
+    ASSERT_TRUE(sink.waitForSuccessFinish(/*from=*/0, kRoundWait).has_value())
+        << "the scheduler must be pacing rounds before we drive it terminal";
+
+    /// A natural terminal transition (forced here via the seam; in production `VanishedReplaced` and
+    /// `IdentityLost` arrive identically, WITHOUT anyone calling stop() on this scheduler).
+    store->setLifecycleForTest(PoolLifecycle::VanishedForgotten);
+
+    ASSERT_TRUE(sched.waitForTerminalSelfExitForTest(kSelfExitWait))
+        << "both the pacing and heartbeat loops must self-exit once the pool is Vanished";
+
+    /// No further round-log rows appear after the self-exit: both loops have returned, so capture the
+    /// count, reap them with stop() (a hang/double-terminate here would fail the test), and assert stable.
+    const size_t count_at_exit = sink.mark();
+    sched.stop();
+    EXPECT_EQ(sink.mark(), count_at_exit) << "a self-exited pacing loop must emit no further round records";
+    EXPECT_FALSE(sched.gcHealth().is_leader);
+}
+
+/// (C1, rev.8 §9 item 8) `IdentityLost` is now a fail-loud TERMINAL state, so the scheduler must self-exit
+/// there exactly as it does on `Vanished` — a scheduler ticking against a half-erased pool is a pure zombie
+/// (eternal `CORRUPTED_DATA` retries against the vanished `gc/state`). Prove BOTH loops self-exit and that no
+/// further round-log rows appear, and that leadership is cleared.
+TEST(CASGCStopStart, SchedulerSelfExitsOnIdentityLost)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+
+    RoundLogSink sink;
+    CasGcScheduler sched(store, std::chrono::seconds(1), "CasGcIdentityLostTest", "ca-disk", sink.logger());
+    sched.start();
+
+    ASSERT_TRUE(sink.waitForSuccessFinish(/*from=*/0, kRoundWait).has_value());
+
+    store->setLifecycleForTest(PoolLifecycle::IdentityLost);
+
+    ASSERT_TRUE(sched.waitForTerminalSelfExitForTest(kSelfExitWait))
+        << "IdentityLost is terminal (rev.8): both the pacing and heartbeat loops must self-exit";
+
+    const size_t count_at_exit = sink.mark();
+    sched.stop();
+    EXPECT_EQ(sink.mark(), count_at_exit) << "a self-exited pacing loop must emit no further round records";
+    EXPECT_FALSE(sched.gcHealth().is_leader) << "a self-exited scheduler must report it no longer leads";
+}
+
+/// (C1 cleanup hygiene) After BOTH loops self-exit on a terminal transition, `stop()` must cleanly reap
+/// the already-finished (joinable) threads, a second `stop()` is a safe no-op, and destruction (scope exit
+/// → ~CasGcScheduler → stop()) runs clean — the ThreadFromGlobalPool join/reset contract holds for a
+/// self-exited thread exactly as for a stop()-signalled one.
+TEST(CASGCStopStart, StopAndDestroyCleanAfterSelfExit)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    {
+        RoundLogSink sink;
+        CasGcScheduler sched(store, std::chrono::seconds(1), "CasGcSelfExitCleanupTest", "ca-disk", sink.logger());
+        sched.start();
+        ASSERT_TRUE(sink.waitForSuccessFinish(/*from=*/0, kRoundWait).has_value());
+
+        store->setLifecycleForTest(PoolLifecycle::VanishedReplaced);
+        ASSERT_TRUE(sched.waitForTerminalSelfExitForTest(kSelfExitWait));
+
+        EXPECT_NO_THROW(sched.stop()) << "stop() must cleanly join the self-exited threads";
+        EXPECT_NO_THROW(sched.stop()) << "a second stop() after self-exit is a safe no-op";
+        /// Destruction at scope exit runs stop() a third time — also clean (test completing proves it).
+    }
+    SUCCEED();
+}
+
+/// (a + e) STOP joins the worker + heartbeat threads and clears the in-process leadership hint. The T10
+/// lesson: make the assertion REAL -- acquire leadership via a manual round FIRST, so `is_leader` is
+/// genuinely true before STOP for the clear to prove anything (otherwise `EXPECT_FALSE` would be vacuous).
+TEST(CASGCStopStart, StopJoinsWorkersAndClearsLeadershipHint)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+
+    /// A long interval keeps any BACKGROUND round from firing; the manual round below is what leads.
+    CasGcScheduler sched(store, std::chrono::seconds(3600), "CasGcStopStartTest", "ca-disk");
+    sched.start();
+
+    /// Acquire REAL leadership: a manual round on a free lease acquires it.
+    const RoundReport rep = sched.runOneRoundNow();
+    ASSERT_TRUE(rep.acquired_lease) << "a manual round on a fresh pool must acquire the free GC lease";
+    ASSERT_TRUE(sched.gcHealth().is_leader) << "leadership must be true BEFORE stop for the clear to prove anything";
+    ASSERT_TRUE(sched.isQuiescent()) << "the manual round completed; nothing is in flight";
+
+    sched.stop();   /// joins loop + heartbeat threads (the test completing without hanging proves the join)
+
+    EXPECT_TRUE(sched.isQuiescent()) << "no GC round may be in flight after stop joined the workers";
+    EXPECT_FALSE(sched.gcHealth().is_leader)
+        << "stop must clear the in-process leadership hint (the disk no longer leads GC)";
+}
+
+/// (b) START after STOP restarts the SAME scheduler: background rounds resume, they carry the SAME gc_id
+/// (identity preserved across the restart), and leadership is re-entered via the next round's NORMAL
+/// acquisition (is_leader becomes true only after the restarted background round re-acquires the lease).
+/// Deterministic and sleep-free: a condition variable fed by the round logger waits for each background
+/// Finish. This also exercises `start()`'s post-join re-entrancy -- a bug there would hang the wait.
+TEST(CASGCStopStart, StartAfterStopResumesBackgroundRoundsWithSameGcId)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+
+    RoundLogSink sink;
+    /// 1s interval: the background loop's first round fires ~1s after start(); the cv wait (not a sleep)
+    /// synchronizes on the actual Finish record.
+    CasGcScheduler sched(store, std::chrono::seconds(1), "CasGcStopStartTest", "ca-disk", sink.logger());
+
+    /// First run: background rounds start and one acquires the lease.
+    sched.start();
+    const auto first = sink.waitForSuccessFinish(/*from=*/0, kRoundWait);
+    ASSERT_TRUE(first.has_value()) << "the background scheduler must run a round and acquire the lease after start()";
+    EXPECT_TRUE(sched.gcHealth().is_leader) << "leadership is held after the first background round";
+    const std::string gc_id_before = first->gc_id;
+    EXPECT_FALSE(gc_id_before.empty());
+
+    /// Stop: leadership hint cleared, threads joined.
+    sched.stop();
+    EXPECT_FALSE(sched.gcHealth().is_leader) << "stop clears the leadership hint";
+    const size_t after_stop = sink.mark();
+
+    /// Restart the SAME instance: a NEW background round must land, re-acquiring the lease, and it must
+    /// carry the SAME gc_id (proving the instance -- and its lease observer -- survived the restart).
+    sched.start();
+    const auto second = sink.waitForSuccessFinish(/*from=*/after_stop, kRoundWait);
+    ASSERT_TRUE(second.has_value()) << "background rounds must resume after START (start() is re-enterable post-join)";
+    EXPECT_EQ(second->gc_id, gc_id_before) << "the restarted scheduler must preserve its gc_id (same instance)";
+    EXPECT_TRUE(sched.gcHealth().is_leader)
+        << "leadership is re-entered via the restarted round's normal lease acquisition";
+
+    sched.stop();
+}
+
+/// (c) STOP and START are both idempotent: a second STOP on an already-stopped scheduler is a safe no-op,
+/// and a second START on a running one is a no-op that leaves it running (a manual round still works).
+TEST(CASGCStopStart, StopAndStartAreIdempotent)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    CasGcScheduler sched(store, std::chrono::seconds(3600), "CasGcStopStartTest", "ca-disk");
+
+    sched.start();
+    EXPECT_NO_THROW(sched.start()) << "a second START on a running scheduler is a no-op";
+
+    sched.stop();
+    EXPECT_NO_THROW(sched.stop()) << "a second STOP on a stopped scheduler is a safe no-op";
+    EXPECT_TRUE(sched.isQuiescent());
+    EXPECT_FALSE(sched.gcHealth().is_leader);
+
+    /// After the double-stop, START still restarts the same instance and it runs a round.
+    sched.start();
+    const RoundReport rep = sched.runOneRoundNow();
+    EXPECT_TRUE(rep.acquired_lease) << "the restarted scheduler still runs rounds after idempotent stop/start";
+    sched.stop();
+}
+
+/// (d) START refuses on a Vanished disk with the typed 668 (`INVALID_STATE`) error -- restarting GC on a
+/// decommissioned pool is meaningless and would only spin failing rounds -- while STOP on the SAME
+/// Vanished disk (with a live scheduler present) SUCCEEDS: stopping the reclaimer on a sick disk is a
+/// legitimate operator action, so STOP never consults the operation gate.
+TEST(CASGCStopStart, StartRefusesOnVanishedButStopSucceeds)
+{
+    /// START on a Vanished disk -> typed 668. No scheduler needed: the gate refuses before touching it.
+    {
+        auto storage = openGcStorage();
+        auto pool = storage->store();                       /// captured while Live (store() throws once Vanished)
+        pool->setLifecycleForTest(PoolLifecycle::VanishedForgotten);
+        Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->gcStart(); });
+    }
+
+    /// STOP on a Vanished disk WITH a live scheduler -> succeeds.
+    {
+        auto storage = openGcStorage();
+        storage->gcStart();                                 /// Live: lazily creates + starts a scheduler
+        ASSERT_TRUE(storage->gcHealth().has_value()) << "gcStart must have created a scheduler on a Live disk";
+
+        auto pool = storage->store();
+        pool->setLifecycleForTest(PoolLifecycle::VanishedForgotten);
+
+        EXPECT_NO_THROW(storage->gcStop()) << "stopping GC on a Vanished disk is legitimate operator action";
+    }
+}
+
+/// (f) The disk stays fully usable while its GC scheduler is stopped: a store()-path write + read succeed
+/// after `gcStop`. STOP controls ONLY the GC pacer, not the disk's data plane.
+TEST(CASGCStopStart, DiskReadsWritesUnaffectedWhileGcStopped)
+{
+    auto storage = openGcStorage();
+    storage->gcStart();   /// create + start the scheduler
+    storage->gcStop();    /// stop it in place (scheduler retained, threads joined)
+
+    /// A write (commit a part) and a read (existsFile) both succeed with GC stopped.
+    EXPECT_NO_THROW(commitOnePart(*storage));
+    EXPECT_TRUE(storage->existsFile(kPartFile)) << "reads/writes must be unaffected while the GC scheduler is stopped";
+
+    /// And START brings the scheduler back (idempotent, re-enterable) without disturbing the data.
+    EXPECT_NO_THROW(storage->gcStart());
+    EXPECT_TRUE(storage->existsFile(kPartFile));
+    storage->gcStop();
+}
+
+/// (T11 M3, acceptance matrix) Two threads hammering `gcStop`/`gcStart` on the SAME storage concurrently.
+/// The verbs serialize on `lifecycle_mutex` (then `gc_scheduler_mutex`, always in that order — so there is
+/// no lock-order inversion and hence no deadlock), so each call is atomic: the barrage interleaves in any
+/// order but never tears the retained scheduler pointer or its worker-thread set. We bound each worker with
+/// a `std::future` timeout (never a sleep) so a deadlock regression fails FAST instead of hanging the suite,
+/// and — since the final serialized call determines the resting state — a single quiet STOP then START at
+/// the end lands the object in a well-defined, usable state (last call wins). ASan/TSan running this proves
+/// the racing start()/stop() thread spawns+joins never race the shared members.
+TEST(CASGCStopStart, ConcurrentStopStartFromTwoThreadsStaysConsistent)
+{
+    auto storage = openGcStorage();
+
+    /// 200 iterations each, opposite phase, so the two threads spend the whole run contending on the
+    /// lifecycle mutex with one about to START while the other is about to STOP.
+    constexpr int kIters = 200;
+    auto worker = [&](bool start_first)
+    {
+        for (int i = 0; i < kIters; ++i)
+        {
+            if (start_first) { storage->gcStart(); storage->gcStop(); }
+            else             { storage->gcStop();  storage->gcStart(); }
+        }
+    };
+
+    auto a = std::async(std::launch::async, worker, true);
+    auto b = std::async(std::launch::async, worker, false);
+    ASSERT_EQ(a.wait_for(std::chrono::seconds(60)), std::future_status::ready)
+        << "two-thread GC stop/start must not deadlock (both verbs lock lifecycle_mutex then gc_scheduler_mutex)";
+    ASSERT_EQ(b.wait_for(std::chrono::seconds(60)), std::future_status::ready)
+        << "two-thread GC stop/start must not deadlock";
+    a.get();
+    b.get();
+
+    /// No torn state: a scheduler exists (both workers created/re-entered one) and its health snapshot is
+    /// coherently queryable rather than reading a half-published pointer.
+    ASSERT_TRUE(storage->gcHealth().has_value()) << "the scheduler must exist and report coherent health after the barrage";
+
+    /// Last call wins: once contention ends, one serialized STOP lands it stopped (leadership cleared,
+    /// quiescent), and one serialized START lands it running again — each observed deterministically.
+    storage->gcStop();
+    ASSERT_TRUE(storage->gcHealth().has_value());
+    EXPECT_FALSE(storage->gcHealth()->is_leader) << "a final serialized STOP clears leadership -- last call wins";
+
+    storage->gcStart();
+    EXPECT_TRUE(storage->gcHealth().has_value()) << "a final serialized START leaves the scheduler present";
+
+    /// The data plane is unharmed by the whole barrage: a write + read still succeed.
+    EXPECT_NO_THROW(commitOnePart(*storage));
+    EXPECT_TRUE(storage->existsFile(kPartFile));
+    storage->gcStop();
+}
+
+/// (T11 cannot-verify, acceptance matrix) Operator intent PERSISTS across a transient recovery: after the
+/// operator STOPs GC, the disk loses its mount lease (transient-not-live) and self-remounts back to Live —
+/// and NOTHING restarts the GC scheduler. Recovery is a Pool-internal operation with no reference to the
+/// scheduler; only an explicit START (`SYSTEM CAS GC START`) resumes it. We prove the scheduler
+/// was genuinely running+leading first, STOP it, drive a real transient→Live recovery on the pool, then show
+/// it stays stopped across a bounded observation window (a running 1s-paced scheduler would have produced
+/// several rounds), and finally that an explicit START — the ONLY resumption path — brings rounds back on the
+/// SAME instance (`gc_id` preserved). The positive control makes the negative meaningful: the sink IS live.
+TEST(CASGCStopStart, OperatorStopPersistsAcrossTransientRecovery)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+
+    RoundLogSink sink;
+    /// 1s interval so a RUNNING scheduler would pace rounds within the observation window below.
+    CasGcScheduler sched(store, std::chrono::seconds(1), "CasGcStopPersistTest", "ca-disk", sink.logger());
+
+    /// The operator has GC running and leading.
+    sched.start();
+    ASSERT_TRUE(sink.waitForSuccessFinish(/*from=*/0, kRoundWait).has_value())
+        << "the scheduler must be pacing rounds and leading before the operator stops it";
+    ASSERT_TRUE(sched.gcHealth().is_leader);
+
+    /// The operator STOPs GC (stop-in-place: threads joined, leadership hint cleared).
+    sched.stop();
+    ASSERT_FALSE(sched.gcHealth().is_leader);
+    const size_t after_stop = sink.mark();
+
+    /// The disk now suffers a transient mount-lease loss and self-remounts back to Live (a fresh
+    /// incarnation), WITHOUT any operator action — exactly the recovery §4 describes.
+    store->tripMountLost();
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::TransientNotLive);
+    fenceOutMount(*backend, store->layout().mountKey(kSrid));
+    ASSERT_TRUE(store->tryRemountOnce()) << "the self-remount must reclaim a fresh incarnation";
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::Live) << "the pool must auto-recover to Live";
+
+    /// The operator's STOP persists: recovery restarted NOTHING. The scheduler is still not leading and
+    /// still quiescent, and NO background round appears across a window a running scheduler would have
+    /// filled many times over.
+    EXPECT_FALSE(sched.gcHealth().is_leader);
+    EXPECT_TRUE(sched.isQuiescent());
+    EXPECT_FALSE(sink.waitForSuccessFinish(after_stop, kStayStoppedWindow).has_value())
+        << "a self-remount recovery must NOT restart an operator-STOPped GC scheduler";
+
+    /// Positive control: only an explicit START resumes rounds, on the SAME instance (gc_id preserved).
+    /// This also proves the sink WOULD have caught a round, so the negative above is meaningful.
+    sched.start();
+    const auto resumed = sink.waitForSuccessFinish(after_stop, kRoundWait);
+    ASSERT_TRUE(resumed.has_value()) << "an explicit START must resume background rounds after the recovery";
+    EXPECT_TRUE(sched.gcHealth().is_leader);
+    sched.stop();
+}
diff --git a/src/Disks/tests/gtest_cas_gc_undercount_repro.cpp b/src/Disks/tests/gtest_cas_gc_undercount_repro.cpp
new file mode 100644
index 000000000000..430e2faa9293
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_gc_undercount_repro.cpp
@@ -0,0 +1,416 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+
+#include <functional>
+
+/// Regression suite for the soak S04 / S04b undercount failures:
+///   Code: 246 CORRUPTED_DATA: CAS blob in-degree: merged in-degree -1 < 0 for a blob ...
+///
+/// H1 (DeposedFoldAdopt) and H1b (FenceWindowReRemoval) guard against the fence-window re-fold
+/// undercount. Fixed STRUCTURALLY by replacing the persisted integer in-degree with an idempotent
+/// source-edge SET: re-folding a fence-window removal across generations is a set-difference no-op,
+/// so the underflow cannot occur (NOT by patching the sealed cursor — that approach was rejected).
+///
+/// H2 (DuplicateRemovalIdempotent) guards against the duplicate-remove undercount that existed when
+/// in-degree was a persisted integer: two events both carrying `old=committed(r1)` subtracted -1
+/// twice from a blob's count, driving it to -1. Same fix — the second removal of an already-absent
+/// edge is a no-op. (Formerly staged the second event as a `{old=committed(r1),
+/// new=committed(r2)}` "repoint" -- a single op naming DIFFERENT manifests in its old/new bindings.
+/// Post-classifier (see Pool/CasRefProtocol.cpp's `classifyOwnerTransitionShape`) that single-op shape
+/// is not representable at all: `manifestEdgesOfTxn` now throws `CORRUPTED_DATA` on it, same as the
+/// state machine always has. The ACTUALLY representable duplicate-removal hazard -- two SEPARATE
+/// remove-committed events both naming `old=committed(r1)`, which the GC fold extracts blindly without
+/// replaying the state machine -- is what this test exercises instead.)
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int ABORTED;
+}
+
+namespace
+{
+
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+const UInt128 kGcA = hexToU128("0000000000000000000000000000000a");
+const UInt128 kGcB = hexToU128("0000000000000000000000000000000b");
+
+ManifestRef ref(uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+
+bool blobExists(InMemoryBackend & b, const Layout & layout, const UInt128 & hash)
+{
+    return b.head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)})).exists;
+}
+
+/// A committed `RefOwnerBinding` for a raw `owner_transition` op. The raw appender is now
+/// `tests::appendOwnerEvent`, which writes ONE `owner_transition` ref-log transaction via
+/// `writeRefLogTxnRaw` at the next `RefTxnId` -- the GC fold EXTRACTS edges from each log
+/// (`manifestEdgesOfTxn`) and never replays them through the state machine, so a SHAPE-legal
+/// `old_binding` (an exact `remove committed` op) that no longer names the table's current committed
+/// owner is still folded -- it is not caught until (and unless) the full state machine replays the
+/// log. That is exactly the "duplicate removal of an already-removed committed ref" hazard H2 below
+/// exercises: two SEPARATE remove-committed events for the same `(ref_name, manifest_ref)`, each
+/// individually shape-legal (`classifyOwnerTransitionShape` accepts every one), but the second is a
+/// stale repeat the idempotent source-edge set must absorb rather than double-subtract.
+RefOwnerBinding committed(const String & ref_name, const ManifestRef & r)
+{
+    return RefOwnerBinding{RefOwnerKind::Committed, ref_name, r};
+}
+
+}
+
+/// ============================ H2: DUPLICATE COMMITTED REMOVAL IS IDEMPOTENT (REGRESSION GUARD) ========
+///
+/// Two SEPARATE journal events both carry the EXACT same `old = committed(r1)` removal:
+///   v2: DROP r1              {old=committed(r1), new=none}  => removes r1's source-edge to {1,2}
+///   v3: DUPLICATE DROP r1    {old=committed(r1), new=none}  => removes r1's source-edge to {1,2} AGAIN
+///
+/// Each event is individually SHAPE-legal (`classifyOwnerTransitionShape` accepts a bare
+/// `old=Committed, new=none` removal unconditionally; it has no state to check that the removal is
+/// still live). The GC fold extracts edges from each log directly, without replaying the state machine
+/// (which alone would notice the second removal names an owner that is no longer bound), so both
+/// events fold. Under the OLD integer in-degree model this drove blob 2 to prior(1) + (-1) + (-1) = -1
+/// and threw CORRUPTED_DATA. Under the FIXED idempotent source-edge SET model the second removal of
+/// r1's edge to a blob is a no-op: each source edge is present or absent, and removing an
+/// already-absent edge is silent.
+///
+/// Correct post-fix behaviour: GC must NOT throw, and both blobs -- owned only by r1, which has no
+/// live owner after the (idempotent) drop -- become collectible (in-degree 0, keys gone).
+TEST(CASGCUndercount, H2DuplicateCommittedRemovalIsIdempotentNoUnderflow)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r1 = ref(1, 0xB1);
+
+    /// r1 pins blobs {1,2}.
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeBlobBody(*backend, store->layout(), DB::UInt128(2));
+    writeManifestRaw(*backend, store->layout(), ns, r1,
+        {blobEntryFor("a", DB::UInt128(1)), blobEntryFor("b", DB::UInt128(2))});
+
+    /// v1: publish r1 (owner: none -> committed(r1)). Fold it so blobs 1 and 2 are each pinned at 1.
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+    ASSERT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(2)), 1);
+
+    /// Stage TWO distinct transactions, each carrying the SAME removal event for r1, in ONE fold window
+    /// (r1's body is NOT deleted until recheck, so both events are resolved at fold time):
+    ///   v2: DROP r1           {old=committed(r1), new=none}
+    ///   v3: DUPLICATE DROP r1 {old=committed(r1), new=none}
+    /// The second removal of r1's edges is a no-op under the idempotent set model.
+    appendOwnerEvent(*backend, store->layout(), ns, 0, committed("tbl", r1), std::nullopt);
+    const uint64_t duplicate_removal_sequence
+        = appendOwnerEvent(*backend, store->layout(), ns, 0, committed("tbl", r1), std::nullopt);
+    advanceRecoverableCkptForRawFixture(
+        *backend, store->layout(), ns, RefTxnId{1, duplicate_removal_sequence});
+
+    /// Drive GC to fixpoint (advancing the mount ack each round so the ack floor graduates the condemned
+    /// blobs): must complete without throwing and collect both blobs.
+    ASSERT_NO_THROW({
+        for (int i = 0; i < 12; ++i)
+        {
+            runRegularRoundReclaiming(gc);
+            store->renewWatermarkOnce();
+        }
+    }) << "H2 regression: duplicate removal of an already-removed committed ref must NOT underflow; "
+       << "the idempotent edge set absorbs the duplicate removal";
+
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0)
+        << "blob 1 is unreferenced (r1 dropped) and must be collected";
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "blob 1 must be physically removed from the store";
+
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(2)), 0)
+        << "blob 2 is unreferenced (r1 dropped) and must be collected";
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(2)))
+        << "blob 2 must be physically removed from the store";
+}
+
+/// ============================ H1: CURSOR RE-FOLD UNDER ABORT ============================
+///
+/// Hypothesis H1: a removal `-1` is folded, but the SINGLE round-commit CAS that would durably advance the
+/// cursor past that removal LOSES to a concurrent leader (ABORTED). The cursor is NOT advanced, so a later
+/// honest round RE-FOLDS the same removal against a parent generation whose in-degree for that blob has
+/// already reached 0 => -1.
+///
+/// We reproduce the deposed-round-commit injection from gtest_cas_gc_attempt.cpp
+/// (DeposedFoldAttemptDoesNotWedge): deny the SINGLE round-commit gc/state CAS (the one that advances
+/// snap_generation) of the round that folds the drop's -1. The deposed round left only never-adopted
+/// attempt-scoped debris, so the retry re-folds the -1 against the still-adopted parent (in-degree 1),
+/// producing a clean 0 — never a double-applied -1.
+class InterruptRoundCasBackend : public InMemoryBackend
+{
+public:
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        if (arm_interrupt && key == gc_state_key)
+        {
+            const auto stored = get(key);
+            const uint64_t stored_gen = stored ? decodeGcState(stored->bytes).snap_generation : 0;
+            const uint64_t next_gen = decodeGcState(bytes).snap_generation;
+            if (next_gen > stored_gen)
+            {
+                arm_interrupt = false;
+                throw DB::Exception(DB::ErrorCodes::ABORTED,
+                    "test-injected: round-commit gc/state CAS denied (leader deposed mid-round; lease lost)");
+            }
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+
+    bool arm_interrupt = false;
+    String gc_state_key = "p/gc/state";
+};
+
+TEST(CASGCUndercount, H1DrainAfterDeposedRemovalFoldDoesNotUnderflow)
+{
+    auto backend = std::make_shared<InterruptRoundCasBackend>();
+    auto store = openPoolForTest(backend);
+    ASSERT_EQ(store->layout().gcStateKey(), "p/gc/state");
+
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    /// Round 1 (honest): fold +1, pin blob 1.
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    store->renewWatermarkOnce();
+    ASSERT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    /// Drop the only ref and advance the watermark floor.
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    store->renewWatermarkOnce();
+
+    /// Round 2 (DEPOSED): fold the -1, then the round-commit CAS is denied (ABORTED). The adopted
+    /// (snap_generation, snap_attempt) must NOT advance.
+    backend->arm_interrupt = true;
+    EXPECT_ANY_THROW(runRegularRoundReclaiming(gc));
+    backend->arm_interrupt = false;
+
+    /// Honest drive to fixpoint (advancing the mount ack each round). H1 predicts the re-fold of the -1
+    /// underflows; the current code predicts a clean drain. Capture whichever happens.
+    bool threw_undercount = false;
+    try
+    {
+        for (int i = 0; i < 32; ++i)
+        {
+            runRegularRoundReclaiming(gc);
+            store->renewWatermarkOnce();
+        }
+    }
+    catch (const DB::Exception & e)
+    {
+        threw_undercount = (e.code() == DB::ErrorCodes::CORRUPTED_DATA
+            && e.message().find("merged in-degree -1 < 0") != String::npos);
+        if (!threw_undercount)
+            throw;
+        std::cerr << "H1 captured exception: " << e.message() << "\n";
+    }
+
+    if (threw_undercount)
+    {
+        FAIL() << "H1 REPRODUCED: the deposed removal fold underflowed on re-fold";
+    }
+    else
+    {
+        /// H1 did NOT reproduce with a single deposed round: the drain is clean.
+        EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+            << "H1-not-reproduced: the pool drained cleanly (single deposed round is idempotent)";
+        EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+    }
+}
+
+/// ==================== H1b: A CONCURRENT-DROP REMOVAL IS FOLDED ONCE (IDEMPOTENCE) ====================
+///
+/// The idempotence claim that survives the redesign, without the (retired) fence-window framing: a removal
+/// that lands AFTER a round's fold sealed its cursor but BEFORE that round's single commit CAS must be
+/// folded EXACTLY ONCE by a later round — never re-folded to drive the blob in-degree below zero.
+///
+/// In the one-pass round there is a single gc/state CAS (fold -> publish -> commit). We inject the drop
+/// just before that commit CAS lands, so the event (v2) is above the fold's sealed cursor (v1) this round.
+/// The committed round adopts the fold seal at cursor v1; the next round folds (v1, v2] as an ordinary -1
+/// against the still-live parent (blob 1 at in-degree 1 => 0). The source-edge SET model makes a re-fold
+/// of the same removal a set-difference no-op, so the in-degree never underflows.
+class DropAtCommitBackend : public InMemoryBackend
+{
+public:
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        /// The one-pass round has a SINGLE gc/state CAS that advances snap_generation. Fire the injected
+        /// drop ONCE, just before that CAS commits — so the drop event is above this round's sealed cursor.
+        if (arm_drop && key == gc_state_key)
+        {
+            const auto stored = get(key);
+            if (stored)
+            {
+                const GcState prev = decodeGcState(stored->bytes);
+                const GcState next = decodeGcState(bytes);
+                if (next.snap_generation > prev.snap_generation)
+                {
+                    arm_drop = false;
+                    if (on_commit)
+                        on_commit();
+                }
+            }
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+
+    bool arm_drop = false;
+    String gc_state_key = "p/gc/state";
+    std::function<void()> on_commit;
+};
+
+TEST(CASGCUndercount, H1bFenceWindowRemovalReFoldedNextRoundUnderflows)
+{
+    auto backend = std::make_shared<DropAtCommitBackend>();
+    /// gc_fold_max_defer_rounds=0 forces fold-every-round: the injected drop fires from `on_commit`,
+    /// which only runs on the round-commit CAS that ADVANCES snap_generation. With immutable logs an idle
+    /// round DEFERS (never advancing the generation), so a default store would never fire the injection --
+    /// forcing a fold each round keeps the fence-window injection point (and its re-fold) reachable.
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    ASSERT_EQ(store->layout().gcStateKey(), "p/gc/state");
+
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(1, 0xAA);
+    writeBlobBody(*backend, store->layout(), DB::UInt128(1));
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    /// Round 1 (honest): fold +1, pin blob 1 at in-degree 1. Cursor sealed at v1.
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    store->renewWatermarkOnce();
+    ASSERT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 1);
+
+    /// Round 2: just before the round-commit CAS lands (after the fold sealed its cursor at v1), inject the
+    /// DROP as v2. The fold this round saw only up to v1 (no change), so the sealed cursor stays v1; the
+    /// drop event v2 is above it and survives trim. The NEXT round folds (v1, v2] => -1 on blob 1 against
+    /// the still-live parent (in-degree 1 => 0). It must NOT be re-folded a second time.
+    backend->arm_drop = true;
+    backend->on_commit = [&]
+    {
+        dropRefTransition(*backend, store->layout(), ns, "tbl", r);
+    };
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    backend->arm_drop = false;
+
+    /// CORRECT behaviour: the concurrently-dropped blob is reclaimed exactly once and GC stays quiescent —
+    /// the removal folds ONCE (idempotent source-edge set), NEVER driving the in-degree below zero. Advance
+    /// the mount ack each round so the ack floor graduates and deletes the condemned blob.
+    EXPECT_NO_THROW({
+        for (int i = 0; i < 12; ++i)
+        {
+            runRegularRoundReclaiming(gc);
+            store->renewWatermarkOnce();
+        }
+    }) << "undercount: a concurrent-drop removal was re-folded and drove the blob in-degree < 0";
+
+    EXPECT_FALSE(blobExists(*backend, store->layout(), DB::UInt128(1)))
+        << "the concurrently-dropped blob must be reclaimed";
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+}
+
+/// ============== UNRECOGNIZED owner_transition SHAPE ABORTS THE ROUND, NEVER DELETES =================
+///
+/// A decodable ref log whose `owner_transition` op is SHAPE-illegal (here: neither `old_binding` nor
+/// `new_binding` set) is exactly what `classifyOwnerTransitionShape` (Pool/CasRefProtocol.cpp) throws
+/// `CORRUPTED_DATA` on. `writeRefLogTxnRaw` -- the same codec real writers use -- never checks op-shape
+/// legality at encode/decode time, so this body is perfectly decodable; only `manifestEdgesOfTxn`'s
+/// shape classification rejects it, at GC fold time.
+///
+/// `Gc::fold` extracts edges inside the SAME try-block as `decodeRefLogTxn`
+/// (Gc/CasGc.cpp), so the throw gets the identical "ref log body invalid: ref folding aborted this
+/// round" treatment as an undecodable body: no cursor advance for ANY table (not just the corrupt
+/// one), no ref delta lands, and the recorded anomaly drives `suppress_destructive`, which gates OFF
+/// every graduated/pending blob delete for the WHOLE round -- including a blob in a namespace the
+/// corrupt log never touched.
+TEST(CASGCUndercount, UnrecognizedOwnerTransitionShapeAbortsRoundNeverDeletes)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const RootNamespace corrupt_ns{"00/bb@cas@"};
+    const ManifestRef r1 = ref(1, 0xC1);
+
+    writeBlobBody(*backend, store->layout(), DB::UInt128(9));
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(9))});
+
+    /// Publish r1 (pins blob 9) and drop it again -- an ordinary, LEGAL removal that, absent
+    /// corruption, condemns blob 9 and (over a few more rounds, matching H1/H2 above) physically
+    /// deletes it.
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r1);
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    store->renewWatermarkOnce();
+    ASSERT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(9)), 1);
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r1);
+
+    /// Drive rounds until blob 9 first reaches in-degree 0 (condemned) -- still physically present:
+    /// deletion is two-phase (a later round graduates it to `delete_pending`, a later round still
+    /// executes the delete), so a freshly condemned blob is never deleted in the same round.
+    bool condemned = false;
+    for (int i = 0; i < 12 && !condemned; ++i)
+    {
+        ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+        store->renewWatermarkOnce();
+        condemned = (inDegreeOf(*backend, store->layout(), DB::UInt128(9)) == 0);
+    }
+    ASSERT_TRUE(condemned) << "setup: blob 9 must reach in-degree 0 (condemned) before injecting corruption";
+    ASSERT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(9)))
+        << "setup: a freshly condemned blob must still be physically present";
+
+    const uint64_t cursor_before = foldCursorOf(*backend, store->layout(), ns, /*shard*/0);
+
+    /// A LEGAL, foldable log in `ns` ITSELF, staged AFTER capturing `cursor_before`. Without it, `ns`
+    /// has nothing new to fold, so the "ns cursor did not advance" assertion below is vacuous -- it
+    /// would pass even if the abort were per-table rather than round-wide. A duplicate remove-committed
+    /// of r1 is shape-legal and foldable (idempotent on the source-edge set, so it does not disturb blob
+    /// 9's already-condemned state), so absent the round-wide abort, folding `ns` WOULD advance its
+    /// cursor past this log -- making the pin below load-bearing.
+    appendOwnerEvent(*backend, store->layout(), ns, 0, committed("tbl", r1), std::nullopt);
+
+    /// A decodable but SHAPE-illegal owner_transition (neither binding) in an UNRELATED table.
+    appendRefLogSeed(*backend, store->layout(), corrupt_ns, {ownerTransitionOp(std::nullopt, std::nullopt)});
+
+    /// Drive MANY more rounds with the corrupt log present. Absent corruption blob 9 -- already
+    /// condemned -- would graduate and be physically deleted within a handful more rounds (exactly
+    /// what H1/H2 above demonstrate for an equivalent drop). Every round here must instead: not throw,
+    /// leave every table's cursor exactly where it was (including `ns`, which the corrupt log never
+    /// touched -- ref-folding abort is round-wide, never per-table), record the anomaly, and never
+    /// physically delete blob 9.
+    for (int i = 0; i < 20; ++i)
+    {
+        RoundReport rep;
+        ASSERT_NO_THROW(rep = runRegularRoundReclaiming(gc))
+            << "round " << i << ": an unrecognized owner_transition shape must abort ref folding, "
+               "never throw out of the round";
+        store->renewWatermarkOnce();
+        EXPECT_FALSE(rep.anomalies.empty())
+            << "round " << i << ": the round must record the unrecognized-shape anomaly";
+        EXPECT_EQ(foldCursorOf(*backend, store->layout(), ns, /*shard*/0), cursor_before)
+            << "round " << i << ": ns's cursor must not advance on a round whose ref folding aborted";
+        ASSERT_TRUE(blobExists(*backend, store->layout(), DB::UInt128(9)))
+            << "round " << i << ": a previously-eligible (condemned) blob must NOT be deleted while "
+               "ref folding is aborted -- destructive work is suppressed for the whole round";
+    }
+}
diff --git a/src/Disks/tests/gtest_cas_heartbeat.cpp b/src/Disks/tests/gtest_cas_heartbeat.cpp
new file mode 100644
index 000000000000..1b26b2a9a635
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_heartbeat.cpp
@@ -0,0 +1,760 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <base/scope_guard.h>
+
+#include <atomic>
+#include <limits>
+#include <thread>
+
+namespace DB::ErrorCodes
+{
+    extern const int NETWORK_ERROR;
+    extern const int ABORTED;
+}
+
+using namespace DB::Cas;
+
+
+/// MountLeaseKeeper behavior: the per-server mount lease and the merged build-watermark floor ride the
+/// SAME slot, renewed by one beat. The keeper anchors durably before return, adopts a slot already
+/// written by `claimMount` (same uuid+epoch), re-reads the callback on each renew and bumps `seq`,
+/// stamps the farewell sentinel (`min_active = UINT64_MAX`, `expires_at_ms <= now`) on `stop`, and
+/// fails closed on any foreign touch (`renewOnce` throws).
+
+namespace
+{
+/// The normal steady-state flow: `claimMount` writes the live (uuid, epoch) mount, THEN the keeper
+/// adopts it. Seed that claim so `start` adopts instead of self-tripping the double-start guard.
+void seedOwnClaim(Backend & b, const Layout & l, const String & srid, UInt128 uuid, uint64_t epoch,
+                  uint64_t now_ms, uint64_t ttl_ms)
+{
+    ASSERT_EQ(claimMount(b, l, srid, uuid, epoch, now_ms, ttl_ms).kind, MountClaimResult::Claimed);
+}
+
+/// Fix #37 phase 1: `shouldFenceOnTransientRenewFailure` is `protected` on `MountLeaseKeeper` (it is an
+/// internal decision hook, not part of the public keeper API) -- promote it to `public` here so these
+/// tests can drive it directly, without needing a real background thread.
+class TestableMountLeaseKeeper : public MountLeaseKeeper
+{
+public:
+    using MountLeaseKeeper::MountLeaseKeeper;
+    using MountLeaseKeeper::shouldFenceOnTransientRenewFailure;
+    using MountLeaseKeeper::onRenewSucceeded;
+};
+}
+
+TEST(CASHeartbeat, AnchorCarriesFloor)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    uint64_t min_active_now = 5;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [&] { return min_active_now; });
+    keeper.start();
+
+    auto hr = backend->head(layout.mountKey(srid));
+    ASSERT_TRUE(hr.exists);
+    auto m = decodeMountLease(backend->get(layout.mountKey(srid))->bytes);
+    EXPECT_EQ(m.writer_epoch, 9u);
+    EXPECT_EQ(m.min_active, 5u);
+    EXPECT_EQ(m.seq, 1u);
+    EXPECT_FALSE(m.gc_fenced);
+}
+
+TEST(CASHeartbeat, RenewRereadsCallbackAndBumpsSeq)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    uint64_t min_active_now = 5;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [&] { return min_active_now; });
+    keeper.start();
+
+    /// The dynamic field moves; the renewal re-reads it off the callback and bumps seq.
+    now_ms = 1500;
+    min_active_now = 8;
+    keeper.renewOnce();
+
+    auto m = decodeMountLease(backend->get(layout.mountKey(srid))->bytes);
+    EXPECT_EQ(m.min_active, 8u);
+    EXPECT_EQ(m.seq, 2u);
+    EXPECT_EQ(m.expires_at_ms, 1500u + 100u);
+}
+
+TEST(CASHeartbeat, StopStampsExpiredAndFarewellSentinel)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [] { return uint64_t{5}; });
+    keeper.start();
+
+    now_ms = 2000;
+    keeper.stop();
+
+    auto m = decodeMountLease(backend->get(layout.mountKey(srid))->bytes);
+    /// Terminal body stamps the lease already-expired (so a same-server reopen reclaims immediately)
+    /// AND folds the watermark farewell into it (min_active = UINT64_MAX).
+    EXPECT_LE(m.expires_at_ms, now_ms);
+    EXPECT_EQ(m.min_active, std::numeric_limits<uint64_t>::max());
+}
+
+/// Phase A (spec rev.4 2026-07-24): a confirmed renewal mismatch whose re-read shows OUR OWN
+/// (uuid, epoch), unfenced, is state UNCERTAINTY (an ambiguous landed renewal of ours, or a
+/// same-pair twin after epoch-state loss) — fail closed via fence + self-remount, never an
+/// exception that aborts debug/ASan builds at construction.
+TEST(CASHeartbeat, SameEpochUnfencedTouchIsUncertainNotFatal)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [] { return uint64_t{5}; });
+    keeper.start();
+
+    /// The slot advances past our held token under our own pair (the ambiguous-landed-renewal shape).
+    const HeadResult h = backend->head(layout.mountKey(srid));
+    ASSERT_TRUE(h.exists);
+    MountLease advanced;
+    advanced.server_uuid = uuid;
+    advanced.writer_epoch = 9;
+    advanced.seq = 99;
+    backend->putOverwrite(layout.mountKey(srid), encodeMountLease(advanced), h.token);
+
+    try
+    {
+        keeper.renewOnce();
+        FAIL() << "renewOnce must throw on a confirmed mismatch";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::ABORTED) << e.message();
+        EXPECT_NE(e.message().find("state uncertain"), String::npos) << e.message();
+        /// Forensics must ride in the message: the observed seq and our local seq.
+        EXPECT_NE(e.message().find("seq=99"), String::npos) << e.message();
+        /// The local-seq fragment specifically -- not just any "seq=99" substring (which the
+        /// OBSERVED holder's own describeMountHolder text could also satisfy on its own).
+        EXPECT_NE(e.message().find("vs our seq="), String::npos) << e.message();
+    }
+}
+
+/// A body under our own uuid but a NEWER writer_epoch is proven supersession — a normal fencing
+/// outcome (the TLA model's localLost), fail closed but never an abort.
+TEST(CASHeartbeat, SupersededTouchIsFailClosedNotFatal)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [] { return uint64_t{5}; });
+    keeper.start();
+
+    const HeadResult h = backend->head(layout.mountKey(srid));
+    ASSERT_TRUE(h.exists);
+    MountLease successor;
+    successor.server_uuid = uuid;
+    successor.writer_epoch = 10;
+    successor.seq = 1;
+    backend->putOverwrite(layout.mountKey(srid), encodeMountLease(successor), h.token);
+
+    try
+    {
+        keeper.renewOnce();
+        FAIL() << "renewOnce must throw on a confirmed mismatch";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::ABORTED) << e.message();
+        EXPECT_NE(e.message().find("superseded by a newer incarnation"), String::npos) << e.message();
+    }
+}
+
+/// A foreign server holding our mount slot must FAIL CLOSED — and must not take the process with it.
+///
+/// This test used to be `ForeignUuidTouchStillDies`, an `EXPECT_DEATH` that pinned the abort. The abort
+/// was the defect: the arm raised `LOGICAL_ERROR`, which aborts at CONSTRUCTION in debug/ASan builds,
+/// and it does so on the keeper's BACKGROUND thread — so an environment-reachable condition (clear the
+/// prefix, recreate under a different server id, and the survivor's next renewal lands there; see
+/// `CASRefContiguousAlloc.SurvivingWriterIsFencedByTheRecreatedPoolsMount`, which drives exactly that)
+/// took the whole server down, and took the ASan gate down with it.
+///
+/// What must NOT change is the outcome, which is what this test now pins: `renewOnce` throws, the throw
+/// carries the foreign holder's identity, and it is classified `ABORTED` — the same mount-lost class the
+/// sibling fencing arms use, which the background loop turns into a latched write fence. The
+/// `abort_on_logical_error` arming is deliberately kept: with it ON, a `LOGICAL_ERROR` would still abort,
+/// so reaching the `EXPECT_THROW` at all is the proof that this condition is no longer classified as one.
+TEST(CASHeartbeat, ForeignUuidTouchFailsClosedWithoutAborting)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [] { return uint64_t{5}; });
+    keeper.start();
+
+    const HeadResult h = backend->head(layout.mountKey(srid));
+    ASSERT_TRUE(h.exists);
+    MountLease foreign;
+    foreign.server_uuid = UInt128(0x9999);
+    foreign.writer_epoch = 1;
+    foreign.seq = 1;
+    backend->putOverwrite(layout.mountKey(srid), encodeMountLease(foreign), h.token);
+
+    /// Restored on every exit: this flag is process-global and every later test in this binary would
+    /// inherit it.
+    const bool armed_before = DB::abort_on_logical_error.load(std::memory_order_relaxed);
+    DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+    SCOPE_EXIT({ DB::abort_on_logical_error.store(armed_before, std::memory_order_relaxed); });
+
+    String message;
+    int code = 0;
+    try
+    {
+        keeper.renewOnce();
+        FAIL() << "a foreign holder must fail the renewal closed, not be silently taken over";
+    }
+    catch (const DB::Exception & e)
+    {
+        message = e.message();
+        code = e.code();
+    }
+    EXPECT_NE(message.find("held by a foreign server"), String::npos) << message;
+    EXPECT_EQ(code, DB::ErrorCodes::ABORTED)
+        << "the mount-lost class the background loop latches the write fence on -- and, critically, not "
+           "LOGICAL_ERROR, which would abort this keeper thread and the whole process with it";
+}
+
+/// Mount-slot writer audit (the P1 "foreign writer" instrument): every mount-slot WRITE and every
+/// OBSERVED foreign/conflicting body becomes an event, carrying the conflicting body's identity —
+/// the payload the chronic "touched by a foreign writer" collisions need to be diagnosable.
+TEST(CASMountAudit, ClaimReleaseAndForeignConflictEmitEvents)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    std::vector<CasEvent> seen;
+    CasEventSink sink = [&](const CasEvent & e) { seen.push_back(e); };
+
+    const uint64_t now_ms = 1'000'000;
+    /// mint for uuid 1 -> one mount_claim
+    ASSERT_EQ(claimMount(*backend, layout, "a", UInt128{1}, 1, now_ms, /*ttl_ms=*/10'000, {}, sink).kind,
+              MountClaimResult::Claimed);
+    ASSERT_EQ(seen.size(), 1u);
+    EXPECT_EQ(seen[0].type, CasEventType::MountClaim);
+    EXPECT_EQ(seen[0].detail.at("server_root_id"), "a");
+    EXPECT_EQ(seen[0].detail.at("branch"), "mint");
+
+    /// a FOREIGN uuid claiming a live slot -> mount_conflict carrying the current holder's identity
+    seen.clear();
+    (void)claimMount(*backend, layout, "a", UInt128{2}, 1, now_ms, /*ttl_ms=*/10'000, {}, sink);
+    ASSERT_FALSE(seen.empty());
+    EXPECT_EQ(seen.back().type, CasEventType::MountConflict);
+    EXPECT_EQ(seen.back().detail.at("server_root_id"), "a");
+    /// The conflict must carry the ORIGINAL holder's identity (uuid 1, the minter) — not the
+    /// foreign claimer's (uuid 2).
+    EXPECT_EQ(seen.back().detail.at("holder_uuid"), u128ToHex(UInt128{1}));
+    EXPECT_NE(seen.back().detail.at("holder_uuid"), u128ToHex(UInt128{2}));
+}
+
+/// The MountLeaseKeeper wiring: `start` adopting an already-claimed slot emits mount_claim, `stop`
+/// (the farewell write) emits mount_release.
+TEST(CASMountAudit, KeeperAdoptEmitsClaimAndTerminateEmitsRelease)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+
+    std::vector<CasEvent> seen;
+    CasEventSink sink = [&](const CasEvent & e) { seen.push_back(e); };
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [] { return uint64_t{5}; }, sink);
+    keeper.start();
+
+    ASSERT_EQ(seen.size(), 1u);
+    EXPECT_EQ(seen[0].type, CasEventType::MountClaim);
+    EXPECT_EQ(seen[0].detail.at("branch"), "adopt");
+
+    seen.clear();
+    now_ms = 2000;
+    keeper.stop();
+
+    ASSERT_EQ(seen.size(), 1u);
+    EXPECT_EQ(seen[0].type, CasEventType::MountRelease);
+    EXPECT_EQ(seen[0].detail.at("branch"), "farewell");
+}
+
+/// Keeper-level foreign-conflict refusal: the mount slot is already held by a FOREIGN uuid (X) when
+/// a keeper for a DIFFERENT uuid (Y) tries to claim it. This must fail closed and — since the
+/// mount-audit sink is not yet installed at first-open — name X in the exception's message text
+/// (the only identity carrier in err.log at that point). MountConflict payload coverage is above.
+TEST(CASMountAudit, KeeperForeignConflictRefusesAndNamesHolder)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid_x(0x1111);
+    const UInt128 uuid_y(0x2222);
+    uint64_t now_ms = 1000;
+
+    /// Foreign holder X claims the slot first.
+    ASSERT_EQ(claimMount(*backend, layout, srid, uuid_x, /*our_epoch=*/1, now_ms, /*ttl_ms=*/100).kind,
+              MountClaimResult::Claimed);
+
+    MountLeaseKeeper keeper(backend, layout, srid, uuid_y, /*writer_epoch=*/1, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [] { return uint64_t{5}; });
+
+    /// The enriched refusal message must name the OBSERVED holder (X), not the caller (Y).
+    const String holder_uuid = u128ToHex(uuid_x);
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            keeper.start();
+        },
+        holder_uuid);
+}
+
+/// `Pool::open` can fail before/inside `doStart` (e.g. a foreign-conflict refusal, see
+/// `KeeperForeignConflictRefusesAndNamesHolder` above) — the keeper is destroyed without ever having
+/// claimed anything. Teardown must not throw "release before start"; there is nothing to release. A
+/// stop AFTER a successful start still performs the farewell (covered by
+/// `StopStampsExpiredAndFarewellSentinel` above); a genuinely-started DOUBLE terminate stays loud.
+TEST(CASHeartbeat, StopBeforeStartIsQuietNoOp)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    uint64_t now_ms = 1000;
+    MountLeaseKeeper keeper(backend, layout, "a", UInt128{1}, /*writer_epoch=*/1, std::chrono::milliseconds(10'000),
+                            [&] { return now_ms; }, [] { return uint64_t{0}; });
+    /// start() never called.
+    EXPECT_NO_THROW(keeper.stop());
+    EXPECT_NO_THROW(keeper.stop());
+}
+
+/// "A fence costs an epoch" at the keeper layer: the GC fenced our fresh lease before we adopted it
+/// (the lease expired mid-open — e.g. a slow first beat). This must fail closed with a TYPED,
+/// recoverable `MountFencedException`, distinct from the generic "touched by a foreign writer"
+/// `LOGICAL_ERROR` — the open path (Task 4) tells "re-open with a fresh epoch" apart from "fail hard"
+/// by this code, not by parsing message text.
+TEST(CASMountAudit, KeeperAdoptRefusesFencedSelfWithTypedError)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+
+    /// mint (uuid, epoch 9), then fence it in place (what computeHeartbeatFloor does on expiry):
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+    {
+        auto got = backend->get(layout.mountKey(srid));
+        MountLease fenced = decodeMountLease(got->bytes);
+        fenced.gc_fenced = true;
+        fenced.seq += 1;
+        ASSERT_EQ(backend->putOverwrite(layout.mountKey(srid), encodeMountLease(fenced), got->token).outcome,
+                  PutOutcome::Done);
+    }
+
+    std::vector<CasEvent> seen;
+    CasEventSink sink = [&](const CasEvent & e) { seen.push_back(e); };
+    /// A keeper for the SAME (uuid, epoch) tries to adopt the now-fenced slot.
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [] { return uint64_t{5}; }, sink);
+
+    bool threw = false;
+    try
+    {
+        keeper.start();
+    }
+    catch (const MountFencedException & e)
+    {
+        threw = true;
+        EXPECT_NE(e.message().find("fenced by GC"), String::npos) << e.message();
+        EXPECT_EQ(e.message().find("foreign writer"), String::npos) << e.message();
+    }
+    EXPECT_TRUE(threw);
+
+    ASSERT_FALSE(seen.empty());
+    EXPECT_EQ(seen.back().type, CasEventType::MountConflict);
+    EXPECT_EQ(seen.back().detail.at("branch"), "fenced_by_gc");
+}
+
+/// A renew mismatch is classified by BODY, not blamed on "a foreign writer" by default: the GC can
+/// fence our OWN (uuid, epoch) mount slot after our lease expires (a late renewal beat racing the
+/// GC's fence-out). The keeper must re-read and recognize this as its OWN incarnation being fenced —
+/// a recoverable `MountFencedException`, not the generic single-writer-violation text.
+TEST(CASHeartbeat, RenewOverFencedOwnSlotIsClassifiedNotForeign)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/100);
+
+    std::vector<CasEvent> seen;
+    CasEventSink sink = [&](const CasEvent & e) { seen.push_back(e); };
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(100),
+                            [&] { return now_ms; }, [] { return uint64_t{5}; }, sink);
+    keeper.start();
+    seen.clear();
+
+    /// Mid-run: the GC fences our own (uuid, epoch) mount slot in place (as `computeHeartbeatFloor`
+    /// does on an expired lease), preserving the whole body — a token-guarded putOverwrite, exactly
+    /// as the GC's own fence-out does it.
+    {
+        const auto got = backend->get(layout.mountKey(srid));
+        MountLease fenced = decodeMountLease(got->bytes);
+        fenced.gc_fenced = true;
+        fenced.seq += 1;
+        ASSERT_EQ(backend->putOverwrite(layout.mountKey(srid), encodeMountLease(fenced), got->token).outcome,
+                  PutOutcome::Done);
+    }
+
+    /// The renewal must classify the fence honestly — not "foreign writer":
+    try
+    {
+        keeper.renewOnce();
+        FAIL() << "renewOnce over a fenced slot must throw";
+    }
+    catch (const MountFencedException & e)
+    {
+        EXPECT_TRUE(e.message().find("fenced by GC") != String::npos);
+        EXPECT_TRUE(e.message().find("foreign writer") == String::npos);
+    }
+    /// and the capture sink saw mount_conflict branch=fenced_by_gc with the fenced body's identity.
+    ASSERT_FALSE(seen.empty());
+    EXPECT_EQ(seen.back().type, CasEventType::MountConflict);
+    EXPECT_EQ(seen.back().detail.at("branch"), "fenced_by_gc");
+    EXPECT_EQ(seen.back().detail.at("holder_uuid"), u128ToHex(uuid));
+}
+
+/// Fix #37 phase 1: a TRANSIENT renewal failure (the background loop's `renewOnce` threw, but NOT via a
+/// confirmed `onRenewMismatch`) must not fence while the last confirmed lease still has more than
+/// `lease_safety_margin` left before it would expire -- the mount-lease protocol guarantees no other
+/// writer can claim the slot before that deadline, so riding it out is safe.
+TEST(CASHeartbeat, TransientRetryStaysWithinLeaseDeadline)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/1000);
+
+    TestableMountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9,
+                                     std::chrono::milliseconds(1000), [&] { return now_ms; },
+                                     [] { return uint64_t{0}; }, CasEventSink{},
+                                     /*lease_safety_margin=*/std::chrono::milliseconds(100));
+    keeper.start();   /// claim() anchors confirmed_deadline_ms = 1000 (now) + 1000 (ttl) = 2000
+
+    /// Well before the deadline's safety margin (2000 - 100 = 1900): must NOT fence.
+    now_ms = 1500;
+    EXPECT_FALSE(keeper.shouldFenceOnTransientRenewFailure());
+
+    /// At/after the safety-margin boundary: must fence.
+    now_ms = 1900;
+    EXPECT_TRUE(keeper.shouldFenceOnTransientRenewFailure());
+    now_ms = 2000;
+    EXPECT_TRUE(keeper.shouldFenceOnTransientRenewFailure());
+}
+
+/// A successful renew extends the confirmed deadline -- the boundary that WOULD have tripped against
+/// the OLD deadline no longer does against the refreshed one. `confirmed_deadline_ms` is refreshed by
+/// `onRenewSucceeded` (the hook the real background loop calls after a successful beat -- see
+/// `CasPool.cpp`'s note that unit tests drive `renewOnce` directly and never through the loop, so this
+/// test calls the promoted `onRenewSucceeded` itself to model exactly what one successful real beat
+/// does), not by a bare `renewOnce` call in isolation.
+TEST(CASHeartbeat, SuccessfulRenewExtendsTransientRetryDeadline)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/1000);
+
+    TestableMountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9,
+                                     std::chrono::milliseconds(1000), [&] { return now_ms; },
+                                     [] { return uint64_t{0}; }, CasEventSink{},
+                                     /*lease_safety_margin=*/std::chrono::milliseconds(100));
+    keeper.start();   /// confirmed_deadline_ms = 2000
+
+    now_ms = 1900;
+    ASSERT_TRUE(keeper.shouldFenceOnTransientRenewFailure()) << "sanity: 1900 trips the OLD deadline";
+
+    /// A renew at now_ms=1900 succeeds; onRenewSucceeded (as the background loop would call it)
+    /// refreshes confirmed_deadline_ms to 1900 + 1000 = 2900.
+    keeper.renewOnce();
+    keeper.onRenewSucceeded();
+    EXPECT_FALSE(keeper.shouldFenceOnTransientRenewFailure())
+        << "the refreshed deadline (2900, margin 100) must not trip at now_ms=1900 any more";
+}
+
+/// Fence-not-rescue round follow-up #1: the redo site in `CasPool.cpp`'s `mountWritable` (the
+/// claim-consumed-the-TTL branch) calls `renewOnce` DIRECTLY -- never through
+/// `onRenewSucceeded` (that hook is only ever invoked by `backgroundLoop`). Before this fix, only
+/// `onRenewSucceeded` refreshed `confirmed_deadline_ms`, so a direct `renewOnce` call left the wall
+/// deadline stale at the pre-redo anchor. A successful renewal must refresh the deadline regardless
+/// of who called `renewOnce` -- background loop or a direct caller alike.
+TEST(CASHeartbeat, DirectRenewOnceRefreshesConfirmedDeadlineWithoutOnRenewSucceeded)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/1000);
+
+    TestableMountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9,
+                                     std::chrono::milliseconds(1000), [&] { return now_ms; },
+                                     [] { return uint64_t{0}; }, CasEventSink{},
+                                     /*lease_safety_margin=*/std::chrono::milliseconds(100));
+    keeper.start();   /// confirmed_deadline_ms = 2000
+
+    now_ms = 1900;
+    ASSERT_TRUE(keeper.shouldFenceOnTransientRenewFailure()) << "sanity: 1900 trips the OLD deadline";
+
+    /// A DIRECT renewOnce -- exactly what the redo site calls, with `onRenewSucceeded` never invoked
+    /// anywhere near it -- must ALSO refresh confirmed_deadline_ms to 1900 + 1000 = 2900.
+    keeper.renewOnce();
+    EXPECT_FALSE(keeper.shouldFenceOnTransientRenewFailure())
+        << "a direct renewOnce (with no onRenewSucceeded call at all) must refresh the confirmed "
+           "deadline too -- the redo's wall anchor must not go stale";
+}
+
+namespace
+{
+/// Wraps an `InMemoryBackend` so `putOverwrite` throws a TRANSIENT (non-mismatch) exception for the
+/// first `fault_count` calls, then delegates normally. Models a `putOverwrite` that fails before any
+/// outcome is observed (timeout / 5xx / connection reset) -- exactly the case fix #37 phase 1 targets,
+/// as opposed to a `PreconditionFailed` (a CONFIRMED, backend-observed mismatch).
+class TransientPutOverwriteFaultBackend final : public InMemoryBackend
+{
+public:
+    int fault_count = 0;
+
+    PutResult putOverwrite(const String & k, const String & b, const Token & e, const ObjectMeta & m) override
+    {
+        if (fault_count > 0)
+        {
+            --fault_count;
+            throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR, "injected transient putOverwrite fault");
+        }
+        return InMemoryBackend::putOverwrite(k, b, e, m);
+    }
+};
+}
+
+/// Real background thread: two transient faults, then the third beat lands. The loop must NOT stop and
+/// must NOT fence (on_lost never fires) -- it just keeps retrying at the normal period.
+TEST(CASHeartbeat, BackgroundLoopRetriesTransientFailureWithoutFencingOrStopping)
+{
+    auto backend = std::make_shared<TransientPutOverwriteFaultBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/30000);
+
+    std::atomic<bool> lost{false};
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(30000),
+                            [&] { return now_ms; }, [] { return uint64_t{0}; }, CasEventSink{},
+                            std::chrono::milliseconds(2000));
+    keeper.setFenceCallbacks([](uint64_t) {}, [&] { lost = true; });
+    keeper.start();   /// the adopt-path putOverwrite must land BEFORE the faults are armed below.
+
+    /// Arm the faults only for the BACKGROUND renewals under test -- `start`'s own adopt-path
+    /// putOverwrite above must not be faulted, or it throws straight out of this test body instead of
+    /// exercising the loop's transient-retry path.
+    backend->fault_count = 2;
+    keeper.startBackground(std::chrono::milliseconds(20));
+
+    /// Bounded poll (not a blind sleep): waits for the REAL background thread to land a renewal past
+    /// the two faults. Generous 5s timeout; a background-thread test cannot be made synchronous without
+    /// a dedicated test seam this codebase does not have (see gtest_cas_pool.cpp's preference for
+    /// synchronous renewOnce-driven tests elsewhere -- not applicable here, since the loop-continuation
+    /// behavior under test only exists inside backgroundLoop itself).
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(5);
+    uint64_t seq = 1;
+    while (std::chrono::steady_clock::now() < deadline)
+    {
+        seq = decodeMountLease(backend->get(layout.mountKey(srid))->bytes).seq;
+        if (seq >= 2)
+            break;
+        std::this_thread::sleep_for(std::chrono::milliseconds(5));
+    }
+    keeper.stopBackground();
+
+    EXPECT_GE(seq, 2u) << "background loop never recovered from the transient faults";
+    EXPECT_FALSE(lost.load()) << "a transient putOverwrite failure must not trip the fence";
+}
+
+/// A CONFIRMED mismatch (a same-(uuid, epoch) unfenced touch lands on the slot -- state
+/// uncertainty, spec rev.4) must fence immediately, even with the deadline nowhere near expiry --
+/// the other half of fix #37 phase 1's distinction. Phase A repartition: this shape is no longer
+/// fatal (it throws `ABORTED`, not `LOGICAL_ERROR`), so the loop must recover via `on_lost` instead
+/// of dying -- observe the fence the same way `BackgroundLoopRetriesTransientFailureWithoutFencingOrStopping`
+/// observes non-fencing.
+TEST(CASHeartbeat, BackgroundLoopFencesImmediatelyOnConfirmedMismatch)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/30000);
+
+    std::atomic<bool> lost{false};
+    MountLeaseKeeper keeper(
+        backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(30000),
+        [&] { return now_ms; }, [] { return uint64_t{0}; }, CasEventSink{}, std::chrono::milliseconds(2000));
+    keeper.setFenceCallbacks([](uint64_t) {}, [&] { lost = true; });
+    keeper.start();
+
+    /// A same-(uuid, epoch) unfenced touch overwrites the slot BEFORE the first background beat.
+    const HeadResult h = backend->head(layout.mountKey(srid));
+    MountLease advanced;
+    advanced.server_uuid = uuid;
+    advanced.writer_epoch = 9;
+    advanced.seq = 99;
+    ASSERT_EQ(backend->putOverwrite(layout.mountKey(srid), encodeMountLease(advanced), h.token).outcome,
+              PutOutcome::Done);
+
+    keeper.startBackground(std::chrono::milliseconds(20));
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(5);
+    while (!lost.load() && std::chrono::steady_clock::now() < deadline)
+        std::this_thread::sleep_for(std::chrono::milliseconds(5));
+    keeper.stopBackground();
+
+    EXPECT_TRUE(lost.load()) << "background loop must fence immediately on a confirmed same-epoch mismatch";
+}
+
+namespace
+{
+/// The landed-but-unacked case: the putOverwrite APPLIES to the in-memory state, THEN throws a
+/// transient exception — the exact CI shape (a client-side timeout whose PUT landed server-side).
+/// The existing TransientPutOverwriteFaultBackend throws BEFORE applying and cannot model this.
+class ApplyThenThrowPutOverwriteFaultBackend final : public InMemoryBackend
+{
+public:
+    int fault_count = 0;
+
+    PutResult putOverwrite(const String & k, const String & b, const Token & e, const ObjectMeta & m) override
+    {
+        if (fault_count > 0)
+        {
+            --fault_count;
+            InMemoryBackend::putOverwrite(k, b, e, m);   /// the write LANDS...
+            throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR,
+                "injected ambiguous fault: applied, ack lost");   /// ...the ack does not.
+        }
+        return InMemoryBackend::putOverwrite(k, b, e, m);
+    }
+};
+}
+
+/// End-to-end reproduction of the CI crash (Altinity PR#2073, asan CAS-s3 stateless): beat 1's
+/// renewal lands but its ack is lost (transient -> the loop retries, deadline permitting); beat 2
+/// renews with the now-stale token, gets a CONFIRMED mismatch, re-reads our own advanced body ->
+/// the Phase A uncertain branch -> the loop stops, on_lost fires (fence latches; in production the
+/// Pool self-remounts from there). No process death anywhere.
+TEST(CASHeartbeat, BackgroundLoopSurvivesAmbiguousLandedRenewal)
+{
+    auto backend = std::make_shared<ApplyThenThrowPutOverwriteFaultBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/30000);
+
+    std::atomic<bool> lost{false};
+    MountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9, std::chrono::milliseconds(30000),
+                            [&] { return now_ms; }, [] { return uint64_t{0}; }, CasEventSink{},
+                            std::chrono::milliseconds(2000));
+    keeper.setFenceCallbacks([](uint64_t) {}, [&] { lost = true; });
+    keeper.start();   /// the adopt-path putOverwrite must land unfaulted.
+
+    backend->fault_count = 1;   /// beat 1: lands + throws (ambiguous); beat 2: confirmed mismatch.
+    keeper.startBackground(std::chrono::milliseconds(20));
+
+    /// Bounded poll for on_lost (never a blind sleep): the deadline is generous; the loop needs
+    /// two ~20ms beats. abort_on_logical_error stays ON to prove no branch constructs one.
+    DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(5);
+    while (!lost.load() && std::chrono::steady_clock::now() < deadline)
+        std::this_thread::sleep_for(std::chrono::milliseconds(5));
+    /// Restore the default before returning: unlike the EXPECT_DEATH-wrapped uses above (which flip
+    /// this flag only inside a forked child), this test sets it in the actual test process — leaving
+    /// it ON would make every LOGICAL_ERROR raised by any LATER test in this binary abort instead of
+    /// throwing, regardless of that test's own intent.
+    DB::abort_on_logical_error.store(false, std::memory_order_relaxed);
+    EXPECT_TRUE(lost.load()) << "the confirmed mismatch after an ambiguous landed renewal must "
+                                "latch the fence via on_lost (and must not abort the process)";
+    keeper.stopBackground();
+}
+
+/// Phase B: the confirmed-lease deadline anchors at the ATTEMPT-START instant, not the response
+/// instant — a slow ack must not extend the local fence past what the durable body authorizes.
+TEST(CASHeartbeat, RenewDeadlineAnchorsAtAttemptStartNotResponseTime)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const String srid = "test";
+    const UInt128 uuid(0x1234);
+    uint64_t now_ms = 1000;
+    seedOwnClaim(*backend, layout, srid, uuid, /*epoch=*/9, now_ms, /*ttl_ms=*/1000);
+
+    TestableMountLeaseKeeper keeper(backend, layout, srid, uuid, /*writer_epoch=*/9,
+                                    std::chrono::milliseconds(1000),
+                                    [&] { return now_ms; }, [] { return uint64_t{0}; }, CasEventSink{},
+                                    std::chrono::milliseconds(100));
+    keeper.start();   /// claim at now=1000 -> anchored confirmed deadline 2000
+
+    /// Beat at now=1500; the "ack" (onRenewSucceeded) arrives late, at now=2400 — after the
+    /// durable expiry stamped by THIS beat's payload (1500+1000=2500 durable; anchor 1500).
+    now_ms = 1500;
+    keeper.renewOnce();
+    now_ms = 2400;
+    keeper.onRenewSucceeded();
+
+    /// Anchored: deadline = 1500 + 1000 = 2500. At now=2401 with margin 100 the boundary check is
+    /// 2401 + 100 >= 2500 -> must fence. (Response-time behavior — the bug — would give
+    /// 2400 + 1000 = 3400 and NOT fence.)
+    now_ms = 2401;
+    EXPECT_TRUE(keeper.shouldFenceOnTransientRenewFailure())
+        << "a late ack must not extend the confirmed deadline past attempt-start + TTL";
+}
diff --git a/src/Disks/tests/gtest_cas_holey_list_detector.cpp b/src/Disks/tests/gtest_cas_holey_list_detector.cpp
new file mode 100644
index 000000000000..6c1728d3ba31
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_holey_list_detector.cpp
@@ -0,0 +1,306 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/ProfileEvents.h>
+
+#include <algorithm>
+#include <mutex>
+#include <set>
+#include <string>
+#include <vector>
+
+/// SKIPPED-TRANSACTION suite. The defect class: a GC round's fold cursor advances past a ref
+/// transaction the round never applied. Once the cursor is sealed above a record, that record can
+/// never be folded again, so BOTH directions of the damage are permanent:
+///
+///   - RETENTION: a skipped `-1` leaves a residual `+1`, so the blob is never reclaimed (a leak);
+///   - DELETION:  a skipped `+1` hides a live owner, so GC deletes a blob a committed manifest still
+///     references (data loss).
+///
+/// The suspected MECHANISM (a `LIST` page that omits a durable key) is UNCONFIRMED — a holey page was
+/// never directly observed, it survives by elimination, and `CaRelinkConfirmCore.tla` `_sab_holeylist`
+/// proves the mechanism is SUFFICIENT, not that it is what happened. These tests therefore use the
+/// holey listing only as the cheapest way to make the EFFECT executable; nothing here may key on how
+/// the hole was produced.
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace
+{
+
+/// A backend that drops ONE chosen key from ONE chosen `list` call, while leaving exact `get`/`head`
+/// of that key working. This is the minimal realisation of "the store returned an incomplete answer":
+/// the record is durable and readable, it is simply absent from one enumeration. The mechanism is
+/// deliberately NOT modelled (no page split, no cursor games) — the arithmetic intake under test must
+/// not depend on how the hole was produced.
+///
+/// WHICH call is explicit and load-bearing. A GC round enumerates the ref prefix ONCE, in
+/// `Gc::listRefPrefix`, and the fold regroups that same enumeration -- so `nth = 0` is the walk whose
+/// hole the fold would have to survive, and it is the one every test here arms. `nth` counts, from the
+/// moment `omitFromNthListCall` is called, only those `list` calls that WOULD have returned the key — so
+/// unrelated prefix enumerations do not shift it.
+/// Arm the sabotage AFTER every seeding write: the writer's own sequence allocation lists the
+/// namespace prefix and would otherwise consume a qualifying call.
+///
+/// Erasing a key from the page never disturbs pagination: `ListPage::next_cursor` is the LAST key the
+/// underlying backend returned and is computed before the erase, so the next page still resumes
+/// strictly after it.
+class HoleyListBackend : public InMemoryBackend
+{
+public:
+    /// Omit `key` from the `nth` (0-based) subsequent qualifying `list` call. Resets the counter.
+    void omitFromNthListCall(const String & key, size_t nth)
+    {
+        std::lock_guard lock(m);
+        omitted = key;
+        target_call = nth;
+        seen_calls = 0;
+        served = false;
+    }
+
+    /// Whether the hole was actually served. Every test asserts this, so a mis-typed key or a
+    /// miscounted `nth` cannot let a test pass vacuously.
+    bool holeServed() const
+    {
+        std::lock_guard lock(m);
+        return served;
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = InMemoryBackend::list(prefix, cursor, limit);
+        std::lock_guard lock(m);
+        if (omitted.empty())
+            return page;
+        auto it = std::find_if(page.keys.begin(), page.keys.end(),
+                               [&](const ListedKey & k) { return k.key == omitted; });
+        if (it == page.keys.end())
+            return page;              /// not a qualifying call — do not count it
+        if (seen_calls++ != target_call)
+            return page;
+        page.keys.erase(it);
+        served = true;
+        omitted.clear();              /// one hole only
+        return page;
+    }
+
+private:
+    mutable std::mutex m;
+    String omitted;
+    size_t target_call = 0;
+    size_t seen_calls = 0;
+    bool served = false;
+};
+
+PoolPtr openHoleyPool(std::shared_ptr<HoleyListBackend> & out_backend)
+{
+    out_backend = std::make_shared<HoleyListBackend>();
+    return Pool::open(out_backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// A `ManifestEntry` for a Blob leaf at `path` referencing `payload`'s content hash.
+ManifestEntry blobEntry(const String & path, const String & payload)
+{
+    ManifestEntry e;
+    e.path = path;
+    e.placement = EntryPlacement::Blob;
+    e.ref = BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of(payload))};
+    e.blob_size = payload.size();
+    return e;
+}
+
+/// Publish one single-blob part through the REAL writer sequence and return its `ManifestId`.
+ManifestId publishOneBlobPart(const PoolPtr & s, const RootNamespace & ns, const String & ref,
+                              const String & payload)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", payload)});
+    build->precommitAdd(ns, ref, id);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+bool blobPresent(const std::shared_ptr<HoleyListBackend> & b, const Layout & layout, const String & payload)
+{
+    return b->head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128,
+                                          BlobDigest::fromU128(u128Of(payload))})).exists;
+}
+
+/// Every ref object key of one namespace. Used to identify WHICH objects a publish appended, rather
+/// than guessing a sequence number.
+std::set<String> listRefKeys(Backend & b, const Layout & layout, const RootNamespace & ns)
+{
+    /// Stage B (Task 4-C): `ns` is born through the REAL append lane here, so its objects sit at a
+    /// real catalog-minted incarnation, not the Stage-A sentinel.
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(b, layout, ns).value();
+    std::set<String> keys;
+    forEachListedKey(b, layout.namespaceStreamPrefix(life), [&](const ListedKey & k) { keys.insert(k.key); });
+    return keys;
+}
+
+/// The keys present in `after` and not in `before`.
+std::vector<String> addedKeys(const std::set<String> & before, const std::set<String> & after)
+{
+    std::vector<String> added;
+    std::set_difference(after.begin(), after.end(), before.begin(), before.end(),
+                        std::back_inserter(added));
+    return added;
+}
+
+/// Among `candidates`, the ONE ref-log key whose transaction emits an edge of sign `change` naming
+/// `manifest_id`.
+///
+/// Selecting the key by DECODING is load-bearing. One logical publish appends SEVERAL ref-log
+/// transactions (`precommitAdd`, then `promote`), and it is the `precommitAdd` that carries the `+1`
+/// activation — a promote is an owner move at the same `manifest_ref` and emits no edge at all. Picking
+/// "the greatest new key" would therefore omit the wrong object and the sabotage would be a no-op that
+/// still let the test pass.
+String refLogKeyEmittingEdge(Backend & b, const Layout & layout, const RootNamespace & ns,
+                             const std::vector<String> & candidates, const ManifestId & manifest_id,
+                             int change)
+{
+    std::vector<String> hits;
+    for (const String & key : candidates)
+    {
+        const auto parsed = layout.parseRefObjectKey(key);
+        if (!parsed || parsed->kind != RefObjectKind::Log)
+            continue;
+        const auto got = b.get(key);
+        if (!got)
+            continue;
+        const RefLogTxn txn =
+            decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), parsed->txn_id);
+        for (const RefManifestEdge & e : manifestEdgesOfTxn(txn))
+            if (e.change == change && e.manifest_id == manifest_id)
+            {
+                hits.push_back(key);
+                break;
+            }
+    }
+    EXPECT_EQ(hits.size(), 1u) << "expected exactly one ref log emitting a " << change
+                               << " edge for the manifest, found " << hits.size();
+    return hits.empty() ? String{} : hits.front();
+}
+
+void runRounds(const PoolPtr & s, Gc & gc, int rounds)
+{
+    for (int i = 0; i < rounds; ++i)
+    {
+        DB::Cas::tests::runRegularRoundReclaiming(gc);
+        s->renewWatermarkOnce();
+    }
+}
+
+}
+
+/// RETENTION DIRECTION (the RCA's primary reproduction). A ref-log record omitted from a listing used to
+/// sort at or below the cursor forever, so restoring the listing could not recover it: the blob's `-1`
+/// never folded and the blob was retained permanently. Under arithmetic intake the omitted record is
+/// reached by exact key on the very round that was lied to, so the removal folds and the blob dies on
+/// the normal schedule — no abort, and no waiting for the store to become honest again.
+TEST(CASHoleyListDetector, OmittedRemoveRecordIsSkippedForever)
+{
+    std::shared_ptr<HoleyListBackend> b;
+    auto s = openHoleyPool(b);
+    const Layout & layout = s->layout();
+    const RootNamespace ns{"test/tbl"};
+    const String payload = "holey-payload";
+
+    /// A: publish the part (its `+1` edges). Folded by the rounds below.
+    const ManifestId part = publishOneBlobPart(s, ns, "part_a", payload);
+    Gc gc(s, hexToU128("00000000000000000000000000000001"));
+    runRounds(s, gc, 2);
+    ASSERT_TRUE(blobPresent(b, layout, payload));
+
+    /// R: drop the ref (the `-1`). Through the REAL writer API, never the raw ref-log helper: the raw
+    /// helper allocates a sequence by listing, which collides with the ledger's own in-memory sequence
+    /// as soon as the same namespace is written through the writer again (`part_h` below).
+    const std::set<String> before_drop = listRefKeys(*b, layout, ns);
+    s->dropRef(ns, "part_a");
+    const std::set<String> after_drop = listRefKeys(*b, layout, ns);
+    const String remove_key =
+        refLogKeyEmittingEdge(*b, layout, ns, addedKeys(before_drop, after_drop), part, -1);
+    ASSERT_FALSE(remove_key.empty());
+
+    /// H: a later, unrelated record so the cursor has a reason to advance past R even when R is not
+    /// returned.
+    publishOneBlobPart(s, ns, "part_h", "harmless-payload");
+    s->renewWatermarkOnce();   /// advance the floor so the dropped closure is not spared as in-flight
+
+    /// nth = 0: the round's own enumeration of the ref prefix — the one the fold regroups, and the only
+    /// walk whose hole the intake has to survive. Armed LAST, after every seeding write, so no
+    /// writer-side namespace listing consumes a qualifying call.
+    b->omitFromNthListCall(remove_key, /*nth=*/0);
+
+    runRounds(s, gc, 1);
+    ASSERT_TRUE(b->holeServed()) << "the sabotage never fired — the omitted key was never listed";
+
+    /// Drive to the reclaim. The point is that the FIRST of these rounds — the one served the hole —
+    /// already folded the removal; the rest are the condemn/graduate/delete pacing.
+    runRounds(s, gc, 12);
+
+    EXPECT_FALSE(blobPresent(b, layout, payload))
+        << "the removal was hidden from one enumeration and never folded — the cursor advanced past a "
+           "record the round never applied, which is the skipped-transaction defect itself";
+}
+
+/// DELETION DIRECTION (the mirror safety test from the RCA). Two owners share ONE deduplicated blob.
+/// The SECOND owner's `+1` is omitted from one listing while the FIRST owner's `-1` folds normally,
+/// so GC sees zero edges for a blob a live manifest still references. THIS MUST NEVER DELETE THE BLOB.
+TEST(CASHoleyListDetector, OmittedActivationNeverPermitsDeletingALiveBlob)
+{
+    std::shared_ptr<HoleyListBackend> b;
+    auto s = openHoleyPool(b);
+    const Layout & layout = s->layout();
+    const RootNamespace ns{"test/tbl"};
+    const String payload = "shared-payload";
+
+    /// M1 owns the token. Fold it so its `+1` is durable in the in-degree generation.
+    const ManifestId m1 = publishOneBlobPart(s, ns, "part_1", payload);
+    Gc gc(s, hexToU128("00000000000000000000000000000001"));
+    runRounds(s, gc, 2);
+    ASSERT_TRUE(blobPresent(b, layout, payload));
+    ASSERT_TRUE(b->head(layout.manifestKey(m1)).exists)
+        << "M1's body must still be present so its `-1` edges are readable at removal-fold";
+
+    /// M2 adopts the SAME deduplicated blob (`putBlob` of an identical payload dedups). Learn WHICH
+    /// ref-log object carries M2's ACTIVATION by diffing the namespace's ref prefix around the publish
+    /// and decoding the new objects — do NOT guess a sequence number and do not append a probe
+    /// transaction (that would perturb the very stream under test).
+    const std::set<String> before = listRefKeys(*b, layout, ns);
+    const ManifestId m2 = publishOneBlobPart(s, ns, "part_2", payload);
+    const std::set<String> after = listRefKeys(*b, layout, ns);
+    const String m2_key = refLogKeyEmittingEdge(*b, layout, ns, addedKeys(before, after), m2, +1);
+    ASSERT_FALSE(m2_key.empty());
+
+    /// M1's removal folds normally. Through the REAL writer API (see the retention test's note).
+    s->dropRef(ns, "part_1");
+    s->renewWatermarkOnce();   /// advance the floor so the removed closure is not spared as in-flight
+
+    /// nth = 0: the round's own walk (see the note in the retention test). Armed LAST so the writer's
+    /// own namespace listings cannot shift the count.
+    b->omitFromNthListCall(m2_key, /*nth=*/0);
+
+    runRounds(s, gc, 12);   /// condemn -> graduate -> delete needs several rounds
+    /// The anti-vacuity check, and it is the RIGHT one now: a run that merely happened not to delete the
+    /// blob must not pass for the wrong reason, and what makes this run non-trivial is that the hole was
+    /// actually SERVED to the enumeration the fold works from. (It used to be "and the detector fired",
+    /// which was only ever a proxy for that — and is now a property of a different, sampled mechanism,
+    /// pinned in `CASRetirementSweep`.)
+    ASSERT_TRUE(b->holeServed()) << "the sabotage never fired — the omitted key was never listed";
+
+    EXPECT_TRUE(blobPresent(b, layout, payload))
+        << "GC deleted a blob that manifest " << manifestRefDebugString(m2.ref)
+        << " still references — the skipped-transaction DATA-LOSS class, reproduced";
+}
diff --git a/src/Disks/tests/gtest_cas_ids.cpp b/src/Disks/tests/gtest_cas_ids.cpp
new file mode 100644
index 000000000000..046696124bd3
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ids.cpp
@@ -0,0 +1,41 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <unordered_set>
+
+using namespace DB::Cas;
+
+TEST(CASIds, StrongTypingAndContainers)
+{
+    /// Test the strong-typed-string class `RootNamespace`.
+    /// (`BlobId` was deleted in the mixed-algo-pools refactor; `TreeId` was part of the
+    /// standalone-tree layer excised in the rev. 15 `PartManifest` redesign.)
+    RootNamespace ns1{"srv1"};
+    RootNamespace ns2{"srv1"};
+    RootNamespace ns3{"srv2"};
+    EXPECT_EQ(ns1, ns2);
+    EXPECT_NE(ns1, ns3);
+    std::unordered_set<RootNamespace> s{ns1, ns3};
+    EXPECT_EQ(s.size(), 2u);
+}
+
+TEST(CASIds, HexU128RoundTrip)
+{
+    // UInt128 is a global typedef (wide::integer<128,unsigned>), not in DB:: namespace.
+    const UInt128 v = (UInt128(0x0123456789abcdefULL) << 64) | 0xfedcba9876543210ULL;
+    const auto hex = u128ToHex(v);
+    EXPECT_EQ(hex.size(), 32u);
+    EXPECT_EQ(hexToU128(hex), v);
+    EXPECT_THROW(hexToU128("zz"), DB::Exception);          // not hex
+    EXPECT_THROW(hexToU128("0123"), DB::Exception);        // wrong length
+}
+
+TEST(CASToken, Basics)
+{
+    Token a{"etag-1", TokenType::ETag};
+    Token b{"etag-1", TokenType::ETag};
+    Token c{"etag-2", TokenType::ETag};
+    EXPECT_EQ(a, b);
+    EXPECT_NE(a, c);
+    EXPECT_TRUE(Token{}.empty());
+    EXPECT_FALSE(a.empty());
+}
diff --git a/src/Disks/tests/gtest_cas_inline_placement.cpp b/src/Disks/tests/gtest_cas_inline_placement.cpp
new file mode 100644
index 000000000000..5b4000636fa0
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_inline_placement.cpp
@@ -0,0 +1,28 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+
+using DB::Cas::partFileMustStayBlob;
+
+TEST(CASInlinePlacement, ColumnAndMarkFilesStayBlob)
+{
+    EXPECT_TRUE(partFileMustStayBlob("data.bin"));
+    EXPECT_TRUE(partFileMustStayBlob("data.mrk"));
+    EXPECT_TRUE(partFileMustStayBlob("data.mrk2"));
+    EXPECT_TRUE(partFileMustStayBlob("data.mrk3"));
+    EXPECT_TRUE(partFileMustStayBlob("data.cmrk"));
+    EXPECT_TRUE(partFileMustStayBlob("data.cmrk2"));
+    EXPECT_TRUE(partFileMustStayBlob("data.cmrk3"));
+    EXPECT_TRUE(partFileMustStayBlob("primary.idx"));   // potentially large; stays blob (follow-up tuning)
+}
+
+TEST(CASInlinePlacement, EagerMetadataFilesAreInlineCandidates)
+{
+    EXPECT_FALSE(partFileMustStayBlob("checksums.txt"));
+    EXPECT_FALSE(partFileMustStayBlob("columns.txt"));
+    EXPECT_FALSE(partFileMustStayBlob("count.txt"));
+    EXPECT_FALSE(partFileMustStayBlob("serialization.json"));
+    EXPECT_FALSE(partFileMustStayBlob("metadata_version.txt"));
+    EXPECT_FALSE(partFileMustStayBlob("partition.dat"));
+    EXPECT_FALSE(partFileMustStayBlob("minmax_date.idx"));
+    EXPECT_FALSE(partFileMustStayBlob("default_compression_codec.txt"));
+}
diff --git a/src/Disks/tests/gtest_cas_inspect.cpp b/src/Disks/tests/gtest_cas_inspect.cpp
new file mode 100644
index 000000000000..d807bec3f931
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_inspect.cpp
@@ -0,0 +1,222 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <IO/WriteBufferFromString.h>
+#include "cas_test_helpers.h"
+
+using namespace DB::Cas;
+
+namespace
+{
+
+ManifestRef manifestRef(uint64_t epoch, uint64_t seq, uint32_t ordinal)
+{
+    return ManifestRef{epoch, seq, ordinal};
+}
+
+BlobRef bh(uint64_t n)
+{
+    return BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(n))};
+}
+
+}
+
+/// Stage-1 T12 (spec §4 "RefOp payload removal"): `cas-inspect` renders the renamed `SetPublishedAt`
+/// op kind, and neither the ref-log nor the ref-snapshot rendering carries a `payload_size` key --
+/// `RefOp`/`RefCommittedRow` no longer have a `payload` field to size.
+
+TEST(CASInspect, RendersSetPublishedAtOpWithNoPayloadSizeKey)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/db/tbl"};
+    const RefTxnId id{7, 9};
+
+    RefLogTxn txn;
+    txn.ns = ns.string();
+    txn.txn_id = id;
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "all_1_1_0";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    op.published_at_ms = 42;
+    txn.ops.push_back(op);
+
+    const String key = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), id);
+    const String bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(txn));
+
+    const String json = caInspectToJson(layout, key, bytes, DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_NE(json.find(R"("kind":"SetPublishedAt")"), String::npos) << json;
+    EXPECT_EQ(json.find("payload"), String::npos) << json;
+}
+
+/// Task-1 review finding M5: `cas inspect` renders the new `EpochSeal` op kind and the txn-level
+/// `prev_epoch_seal` chain field, needed to debug INV-2 seal chains without a raw byte dump.
+TEST(CASInspect, RendersEpochSealTxnWithPrevEpochSeal)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/db/tbl"};
+    const RefTxnId id{3, 1};
+
+    RefLogTxn txn;
+    txn.ns = ns.string();
+    txn.txn_id = id;
+    txn.prev_epoch_seal = RefTxnId{2, 9};
+    RefOp op;
+    op.kind = RefOpKind::EpochSeal;
+    txn.ops.push_back(op);
+
+    const String key = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), id);
+    const String bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(txn));
+
+    const String json = caInspectToJson(layout, key, bytes, DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_NE(json.find(R"("kind":"EpochSeal")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("prev_epoch_seal":{"writer_epoch":2,"ref_sequence":9})"), String::npos) << json;
+}
+
+TEST(CASInspect, RendersCommittedRowWithNoPayloadSizeKey)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/db/tbl"};
+    const RefTxnId id{7, 9};
+
+    RefTableSnapshot snap;
+    snap.ns = ns.string();
+    snap.snapshot_id = id;
+    RefCommittedRow row;
+    row.ref_name = "all_1_1_0";
+    row.manifest_ref = manifestRef(1, 1, 1);
+    row.published_at_ms = 42;
+    snap.committed.push_back(row);
+
+    const String key = layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), id);
+    const String bytes = sealObject(FormatId::RefSnapshot, encodeRefTableSnapshot(snap));
+
+    const String json = caInspectToJson(layout, key, bytes, DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_EQ(json.find("payload"), String::npos) << json;
+    EXPECT_EQ(json.find("lifecycle"), String::npos) << json;
+    EXPECT_EQ(json.find("remove_txn_id"), String::npos) << json;
+    EXPECT_NE(json.find(R"("published_at_ms":42)"), String::npos) << json;
+}
+
+/// A blob-target source-edge run segment (`Layout::blobTargetRunKey`) is the ground truth for every
+/// in-degree question; `cas-inspect` decodes it with the typed `SourceEdgeRunView` reader (not by hand)
+/// and must distinguish an active edge from a condemned sentinel row, decoding the latter's fields.
+TEST(CASInspect, RendersBlobTargetRunEdgeAndCondemnedRows)
+{
+    const Layout layout("p");
+
+    /// `SourceEdgeRunWriter::append` requires non-decreasing `(ref, source_id)` order; the condemned
+    /// sentinel sorts first for its blob (source_id 0), and `bh(1) < bh(2)`, so appending in this
+    /// order already satisfies it.
+    SourceEdgeRecord condemned_rec;
+    condemned_rec.ref = bh(1);
+    condemned_rec.source_id = UInt128{0};
+    condemned_rec.marker = kCondemned;
+    condemned_rec.delete_pending = true;
+    condemned_rec.token = Token{.value = "etag-1", .type = TokenType::Emulated};
+    condemned_rec.size = 123;
+    condemned_rec.condemn_round = 7;
+    condemned_rec.marker_confirmed = true;
+
+    SourceEdgeRecord edge_rec;
+    edge_rec.ref = bh(2);
+    edge_rec.source_id = UInt128(9);
+    edge_rec.marker = kEdgeActive;
+
+    DB::WriteBufferFromOwnString out;
+    SourceEdgeRunWriter writer(out);
+    writer.append(condemned_rec);
+    writer.append(edge_rec);
+    writer.finish();
+    out.finalize();
+    const String bytes = out.str();
+
+    const String key = layout.blobTargetRunKey(/*generation*/2, /*attempt*/0, /*shard*/0, /*seq*/0);
+
+    const String json = caInspectToJson(layout, key, bytes);
+    EXPECT_NE(json.find(R"("object":"blob_target_run")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("generation":2)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("kind":"edge")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("kind":"condemned")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("delete_pending":true)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("condemn_round":7)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("value":"etag-1")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("rows":2)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("distinct_blobs":2)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("edges":1)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("condemned":1)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("zero_markers":0)"), String::npos) << json;
+}
+
+/// Stage A task 5 (spec INV-4): the `_ckpt` renders as its own object kind. It is point-addressed in
+/// `cas/ns/state/` with no transaction id, so it has a separate dispatch from stream objects and once
+/// fell through to
+/// `BAD_ARGUMENTS` for it -- and it is precisely the object an operator reaches for when asking "what
+/// is recovery's base" or "why is cleanup not reclaiming anything".
+TEST(CASInspect, RendersRefCkptWithEveryFieldPresent)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/db/tbl"};
+
+    const RefCkpt ckpt{.life_epoch = std::optional<uint64_t>{7},
+                       .committed_through = RefTxnId{7, 9},
+                       .checkpoint_snapshot_id = RefTxnId{7, 9},
+                       .last_epoch_seal = RefTxnId{6, 4}};
+
+    const String json = caInspectToJson(
+        layout, layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns)), encodeRefCkpt(ckpt),
+        DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_NE(json.find(R"("object":"ref_ckpt")"), String::npos) << json;
+    /// The namespace comes from the KEY: a `_ckpt` body does not name it.
+    EXPECT_NE(json.find(R"("namespace":"srv1/db/tbl")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("life_epoch":7)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("committed_through":{"writer_epoch":7,"ref_sequence":9})"), String::npos) << json;
+    EXPECT_NE(json.find(R"("writer_epoch":7,"ref_sequence":9)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("writer_epoch":6,"ref_sequence":4)"), String::npos) << json;
+}
+
+/// The absences are the interesting readings, so they render as explicit `null`s rather than missing
+/// keys: no checkpoint means recovery has no base AND nothing is deletable, which is a very different
+/// report from "the key is there and I could not tell you what is in it".
+TEST(CASInspect, RendersRefCkptAbsencesAsExplicitNulls)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/db/fresh"};
+
+    const String json = caInspectToJson(
+        layout, layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns)), encodeRefCkpt(RefCkpt{}),
+        DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_NE(json.find(R"("object":"ref_ckpt")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("life_epoch":null)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("checkpoint_snapshot_id":null)"), String::npos) << json;
+    EXPECT_NE(json.find(R"("last_epoch_seal":null)"), String::npos) << json;
+}
+
+/// A listed physical id cannot supply a namespace. Inspect must receive the unique catalog join, and
+/// a different logical spelling at the same id is rejected by the decoded object's own namespace.
+TEST(CASInspect, RefObjectRequiresTheExactCatalogResolution)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/db/tbl"};
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(ns, UInt128{91});
+    RefLogTxn txn;
+    txn.ns = ns.string();
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp birth;
+    birth.kind = RefOpKind::NamespaceBirth;
+    txn.ops = {birth};
+    const String key = layout.refLogKey(life, txn.txn_id);
+    const String bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(txn));
+
+    EXPECT_THROW(caInspectToJson(layout, key, bytes), DB::Exception);
+    EXPECT_THROW(caInspectToJson(
+        layout, key, bytes, NamespaceLifeId::fromCatalogEntry(RootNamespace{"redirected"}, life.incarnation)),
+        DB::Exception);
+    EXPECT_NO_THROW(caInspectToJson(layout, key, bytes, life));
+}
diff --git a/src/Disks/tests/gtest_cas_json_writer.cpp b/src/Disks/tests/gtest_cas_json_writer.cpp
new file mode 100644
index 000000000000..f04b89255818
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_json_writer.cpp
@@ -0,0 +1,214 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobDigest.h>
+#include <IO/WriteBufferFromString.h>
+#include <IO/WriteHelpers.h>
+#include <Formats/FormatSettings.h>
+#include <random>
+
+using namespace DB;
+using namespace DB::Cas;
+
+TEST(CASJsonWriter, KeyValueSequenceMatchesCanonicalShape)
+{
+    CasJsonWriter w;
+    bool first = true;
+    w.key("we", first);
+    w.u64StringValue(7);
+    w.key("mo", first);
+    w.u64Number(3);
+    w.key("ok", first);
+    w.boolValue(true);
+    w.key("o", "me", first);
+    w.u64StringValue(1);
+    w.closeObject(first);
+    w.newline();
+    EXPECT_EQ(std::move(w).take(), "{\"we\":\"7\",\"mo\":3,\"ok\":true,\"ome\":\"1\"}\n");
+}
+
+TEST(CASJsonWriter, EmptyObjectAndClear)
+{
+    CasJsonWriter w;
+    bool first = true;
+    w.closeObject(first);
+    EXPECT_EQ(w.view(), "{}");
+    w.clear();
+    EXPECT_EQ(w.size(), 0u);
+}
+
+TEST(CASJsonWriter, Hex128MatchesU128ToHex)
+{
+    const UInt128 v = (UInt128(0x0123456789abcdefULL) << 64) | UInt128(0xfedcba9876543210ULL);
+    CasJsonWriter w;
+    w.hex128Value(v);
+    EXPECT_EQ(std::move(w).take(), "\"" + u128ToHex(v) + "\"");
+}
+
+TEST(CASJsonWriter, U64Extremes)
+{
+    CasJsonWriter w;
+    w.u64Number(0);
+    w.appendChar(' ');
+    w.u64Number(UINT64_MAX);
+    EXPECT_EQ(std::move(w).take(), "0 18446744073709551615");
+}
+
+namespace
+{
+String referenceJson(std::string_view s)
+{
+    DB::FormatSettings settings;
+    settings.json.escape_forward_slashes = false;   /// the pinned CAS canon
+    DB::WriteBufferFromOwnString out;
+    DB::writeJSONString(s, out, settings);
+    out.finalize();
+    return out.str();
+}
+
+String writerJson(std::string_view s)
+{
+    DB::Cas::CasJsonWriter w;
+    w.stringValue(s);
+    return std::move(w).take();
+}
+}
+
+TEST(CASJsonWriterEscaping, TargetedCorpusMatchesWriteJSONString)
+{
+    const std::vector<String> corpus = {
+        "",
+        "plain_safe_ref_name_20260101_0_1_1_1",
+        "roots/pin",                                    /// '/' must stay UNESCAPED
+        "quote\"inside", "back\\slash", "both\\\"x",
+        String("\b\f\n\r\t"),
+        String(1, '\0'), String("a") + '\0' + "b",
+        String("\x01\x02\x03\x1e\x1f"),
+        "\xE2\x80\xA8", "\xE2\x80\xA9",                 /// U+2028 / U+2029 ->   /
+        "x\xE2\x80\xA8" "y", // NOLINT(bugprone-suspicious-missing-comma): deliberate adjacent-literal concatenation, testing a U+2028 sequence split across two source literals
+        "\xE2",                                          /// truncated lead byte at end
+        "\xE2\x80",                                      /// truncated pair at end
+        "\xE2\x21\x21",                                  /// 0xE2 + non-continuation bytes
+        "\xE2\x80\x21",
+        "\xE2\xE2\x80\xA8",                              /// lead byte immediately before a real sequence
+        "\xC3\xA9\xF0\x9F\x98\x80",                      /// ordinary multi-byte UTF-8 passes through
+        "\xff\xfe invalid utf8 \x80",
+        String(1000, 'a'),                               /// long safe run (vector path)
+        String(1000, '"'),                               /// special-dense
+    };
+    for (const String & s : corpus)
+        EXPECT_EQ(writerJson(s), referenceJson(s)) << "input bytes: " << s.size();
+}
+
+TEST(CASJsonWriterEscaping, FuzzMatchesWriteJSONString)
+{
+    std::mt19937 rng(20260720); // NOLINT(cert-msc32-c, cert-msc51-cpp)
+    for (int iter = 0; iter < 5000; ++iter)
+    {
+        const size_t len = rng() % 200;
+        String s(len, '\0');
+        const int mode = iter % 3;
+        for (auto & c : s)
+        {
+            if (mode == 0)
+                c = static_cast<char>(rng() % 256);                     /// full byte range
+            else if (mode == 1)
+                c = static_cast<char>('a' + rng() % 26);                /// safe-only
+            else
+            {
+                static constexpr char specials[] = {'"', '\\', '\n', '\x01', '\xE2', '\x80', '\xA8', 'z'};
+                c = specials[rng() % (sizeof(specials))];               /// special-dense
+            }
+        }
+        ASSERT_EQ(writerJson(s), referenceJson(s)) << "iter " << iter;
+    }
+}
+
+/// ---- CasJsonWriter overloads of the shared vocabulary (Task 4) ----
+///
+/// The production WriteBuffer vocabulary was retired in Task 9 (CasJsonWriter is now the only CAS
+/// text writer). `reference_vocab` below is a verbatim copy of the retired implementation, kept
+/// test-local so these differential tests keep an independent oracle instead of comparing
+/// CasJsonWriter against itself.
+namespace reference_vocab
+{
+namespace
+{
+/// Verbatim copy of the retired WriteBuffer-based CAS vocabulary (CasTextFormat.cpp pre-CasJsonWriter),
+/// kept as the differential reference. jsonWriteSettings is inlined: escape_forward_slashes=false.
+const DB::FormatSettings & settings()
+{
+    static const DB::FormatSettings s = []
+    {
+        DB::FormatSettings fs;
+        fs.json.escape_forward_slashes = false;
+        return fs;
+    }();
+    return s;
+}
+
+void writeKey(DB::WriteBuffer & out, std::string_view key, bool & first)
+{
+    DB::writeChar(first ? '{' : ',', out);
+    first = false;
+    DB::writeChar('"', out);
+    out.write(key.data(), key.size());
+    DB::writeChar('"', out);
+    DB::writeChar(':', out);
+}
+
+void writeStringValue(DB::WriteBuffer & out, std::string_view s) { DB::writeJSONString(s, out, settings()); }
+
+void writeHex128Value(DB::WriteBuffer & out, const UInt128 & v)
+{
+    DB::writeChar('"', out);
+    const String hex = DB::Cas::u128ToHex(v);
+    out.write(hex.data(), hex.size());
+    DB::writeChar('"', out);
+}
+
+void writeU64StringValue(DB::WriteBuffer & out, uint64_t v)
+{
+    DB::writeChar('"', out);
+    DB::writeIntText(v, out);
+    DB::writeChar('"', out);
+}
+
+void writeBoolValue(DB::WriteBuffer & out, bool v) { writeCString(v ? "true" : "false", out); }
+
+void closeObject(DB::WriteBuffer & out, bool & first)
+{
+    if (first)
+        DB::writeChar('{', out);
+    first = false;
+    DB::writeChar('}', out);
+}
+}
+}
+
+TEST(CASJsonWriterVocab, MatchesReferenceVocabulary)
+{
+    using namespace DB::Cas;
+    const UInt128 h = (UInt128(0xdeadbeefULL) << 64) | UInt128(42);
+
+    DB::WriteBufferFromOwnString ref;
+    CasJsonWriter w;
+    bool rf = true;
+    bool wf = true;
+
+    reference_vocab::writeKey(ref, "a", rf);           writeKey(w, "a", wf);
+    reference_vocab::writeStringValue(ref, "x/\"y");   writeStringValue(w, "x/\"y");
+    reference_vocab::writeKey(ref, "h", rf);           writeKey(w, "h", wf);
+    reference_vocab::writeHex128Value(ref, h);         writeHex128Value(w, h);
+    reference_vocab::writeKey(ref, "u", rf);           writeKey(w, "u", wf);
+    reference_vocab::writeU64StringValue(ref, UINT64_MAX); writeU64StringValue(w, UINT64_MAX);
+    reference_vocab::writeKey(ref, "b", rf);           writeKey(w, "b", wf);
+    reference_vocab::writeBoolValue(ref, false);       writeBoolValue(w, false);
+    reference_vocab::writeKey(ref, "n", rf);           writeKey(w, "n", wf);
+    DB::writeIntText(uint64_t(12345), ref); writeIntText(uint64_t(12345), w);
+    reference_vocab::closeObject(ref, rf);             closeObject(w, wf);
+    DB::writeChar('\n', ref);         writeChar('\n', w);
+    ref.finalize();
+    EXPECT_EQ(std::move(w).take(), ref.str());
+}
diff --git a/src/Disks/tests/gtest_cas_layout.cpp b/src/Disks/tests/gtest_cas_layout.cpp
new file mode 100644
index 000000000000..1006e87188a2
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_layout.cpp
@@ -0,0 +1,382 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartPathParser.h>
+#include "cas_test_helpers.h"
+
+using namespace DB::Cas;
+
+namespace
+{
+/// A `BlobRef` at `algo` whose first bytes are `0x00, 0xaa, 0xbb` (the rest zero) -- for key-shape
+/// tests that need a stable, recognizable hex prefix. `Layout` no longer captures an algo (Phase 3
+/// T2/T3): every blob key is built from a `BlobRef` alone, so key-shape tests construct one directly.
+BlobRef prefixedRef(BlobHashAlgo algo)
+{
+    BlobDigest d{};
+    d.bytes[0] = 0x00; d.bytes[1] = 0xaa; d.bytes[2] = 0xbb;
+    return BlobRef{algo, d};
+}
+}
+
+TEST(CASLayout, KeyShapes)
+{
+    /// Per design §10 EVERY algo carries an explicit path segment: `blobs/ch128/...`, not the legacy
+    /// `blobs/...`.
+    Layout l{"p"};
+    const BlobRef ref = prefixedRef(BlobHashAlgo::CityHash128);
+    const String hex = codecFor(BlobHashAlgo::CityHash128).toHex(ref.digest);
+    EXPECT_EQ(l.blobKey(ref), "p/blobs/ch128/" + hex.substr(0, 2) + "/" + hex);
+    EXPECT_EQ(l.gcStateKey(), "p/gc/state");
+    EXPECT_EQ(l.outcomesKey(4, 42, 7, 1), "p/gc/gen/4/attempt/42/outcomes/7/1.zst");
+    EXPECT_EQ(l.poolMetaKey(), "p/_pool_meta");
+}
+
+TEST(CASLayout, BlobKeyCarriesAlgoSegment)
+{
+    /// Every algo gets its own segment (design §3/§10), so two algos can never collide in the key
+    /// space even after a config change on a fresh pool. `Layout` itself carries no algo anymore --
+    /// the segment comes from the `BlobRef` passed to `blobKey`/`blobMetaKey`.
+    const Layout l("p");
+
+    const BlobRef ch128_ref = prefixedRef(BlobHashAlgo::CityHash128);
+    const String ch128_hex = codecFor(BlobHashAlgo::CityHash128).toHex(ch128_ref.digest);
+    EXPECT_EQ(l.blobKey(ch128_ref), "p/blobs/ch128/" + ch128_hex.substr(0, 2) + "/" + ch128_hex);
+    EXPECT_EQ(l.blobMetaKey(ch128_ref), l.blobKey(ch128_ref) + ".meta");
+
+    const BlobRef xxh3_ref = prefixedRef(BlobHashAlgo::XXH3_128);
+    const String xxh3_hex = codecFor(BlobHashAlgo::XXH3_128).toHex(xxh3_ref.digest);
+    EXPECT_EQ(l.blobKey(xxh3_ref), "p/blobs/xxh3/" + xxh3_hex.substr(0, 2) + "/" + xxh3_hex);
+    EXPECT_EQ(l.blobMetaKey(xxh3_ref), l.blobKey(xxh3_ref) + ".meta");
+
+    const BlobRef sha256_ref = prefixedRef(BlobHashAlgo::Sha256);
+    const String sha256_hex = codecFor(BlobHashAlgo::Sha256).toHex(sha256_ref.digest);
+    EXPECT_EQ(l.blobKey(sha256_ref), "p/blobs/sha256/" + sha256_hex.substr(0, 2) + "/" + sha256_hex);
+
+    /// Trees/manifests/refs are UNCHANGED -- only blob-body keys gain the algo segment.
+    EXPECT_EQ(l.blobsPrefix(), "p/blobs/");
+}
+
+TEST(CASLayout, RootNamespaceKeys)
+{
+    Layout l("p");
+    RootNamespace ns{"srv1/3f2e-uuid"};
+    const NamespaceLifeId ns_id = DB::Cas::tests::fixture::fixtureLife(ns);
+    EXPECT_EQ(l.namespaceStreamPrefix(ns_id),
+        "p/cas/ns/stream/" + renderIncarnation(ns_id.incarnation) + "/");
+    EXPECT_EQ(l.namespaceFileKey(ns_id, "format_version.txt"),
+        "p/cas/ns/state/" + renderIncarnation(ns_id.incarnation) + "/_files/format_version.txt");
+    EXPECT_EQ(l.namespaceFilesPrefix(ns_id),
+        "p/cas/ns/state/" + renderIncarnation(ns_id.incarnation) + "/_files/");
+}
+
+TEST(CASLayout, OpaqueLifeIdSeparatesStreamFromState)
+{
+    /// This catches a builder that accidentally puts the logical namespace back into a life-owned
+    /// key. The two different names deliberately share one physical id: object identity is the id,
+    /// while the name remains catalog-only.
+    Layout l("p");
+    const UInt128 life_id = UInt128(0x1234);
+    const NamespaceLifeId first = NamespaceLifeId::fromCatalogEntry(RootNamespace{"root/first"}, life_id);
+    const NamespaceLifeId second = NamespaceLifeId::fromCatalogEntry(RootNamespace{"root/second"}, life_id);
+    const RefTxnId txn{7, 9};
+
+    EXPECT_EQ(l.namespaceStreamPrefix(first), "p/cas/ns/stream/00000000000000000000000000001234/");
+    EXPECT_EQ(l.namespaceStatePrefix(first), "p/cas/ns/state/00000000000000000000000000001234/");
+    EXPECT_EQ(l.refLogKey(first, txn), "p/cas/ns/stream/00000000000000000000000000001234/_log/0000000000000007-0000000000000009.zst");
+    EXPECT_EQ(l.refSnapshotKey(first, txn), "p/cas/ns/stream/00000000000000000000000000001234/_snap/0000000000000007-0000000000000009.zst");
+    EXPECT_EQ(l.refCkptKey(first), "p/cas/ns/state/00000000000000000000000000001234/_ckpt");
+    EXPECT_EQ(l.namespaceFileKey(first, "nested/file"), "p/cas/ns/state/00000000000000000000000000001234/_files/nested/file");
+
+    EXPECT_EQ(l.refLogKey(second, txn), l.refLogKey(first, txn));
+    EXPECT_EQ(l.namespaceFileKey(second, "nested/file"), l.namespaceFileKey(first, "nested/file"));
+}
+
+TEST(CASLayout, RelocatedRefAndManifestKeys)
+{
+    Layout l("p");
+    const RootNamespace ns{"srid/store/ab/uuid@cas@"};
+    const NamespaceLifeId ns_id = DB::Cas::tests::fixture::fixtureLife(ns);
+    EXPECT_EQ(l.namespaceStreamPrefix(ns_id),
+        "p/cas/ns/stream/" + renderIncarnation(ns_id.incarnation) + "/");
+    EXPECT_EQ(l.casRefsPrefix(), "p/cas/ns/stream/");
+    /// All manifests of a namespace: cas/manifests/<ns>/ (replaces roots/<ns>/_manifests/).
+    EXPECT_EQ(l.manifestNamespacePrefix(ns), "p/cas/manifests/srid/store/ab/uuid@cas@/");
+
+    /// manifestKey: canonical hex build directory, under cas/manifests/<ns>/ (no /_manifests/ infix).
+    ManifestId id;
+    id.root_namespace = ns;
+    id.ref.writer_epoch = 1;
+    id.ref.build_sequence = 1042;
+    id.ref.manifest_ordinal = 1;
+    const String key = l.manifestKey(id);
+    EXPECT_EQ(key, "p/cas/manifests/srid/store/ab/uuid@cas@/"
+        "0000000000000001-0000000000000412/000001.zst");
+    EXPECT_EQ(key.find("/_manifests/"), String::npos) << key;
+}
+
+TEST(CASLayout, RootNamespaceValidation)
+{
+    Layout l("p");
+    /// Opaque physical life keys deliberately do not inspect the logical namespace. Namespace-bearing
+    /// families such as manifests remain responsible for validating it.
+    EXPECT_THROW(l.manifestNamespacePrefix(RootNamespace{""}), DB::Exception);
+    EXPECT_THROW(l.manifestNamespacePrefix(RootNamespace{"/lead"}), DB::Exception);
+    EXPECT_THROW(l.manifestNamespacePrefix(RootNamespace{"trail/"}), DB::Exception);
+    /// File names may be NESTED relative paths (M-W T2: deduplication_logs/...); only unclean
+    /// shapes are rejected (empty, leading/trailing '/', empty segments, '..' escapes).
+    const NamespaceLifeId ok_id = DB::Cas::tests::fixture::fixtureLife(RootNamespace{"ok"});
+    EXPECT_NO_THROW(l.namespaceFileKey(ok_id, "a/b"));
+    EXPECT_THROW(l.namespaceFileKey(ok_id, ""), DB::Exception);
+    EXPECT_THROW(l.namespaceFileKey(ok_id, "/lead"), DB::Exception);
+    EXPECT_THROW(l.namespaceFileKey(ok_id, "trail/"), DB::Exception);
+    EXPECT_THROW(l.namespaceFileKey(ok_id, "a//b"), DB::Exception);
+    EXPECT_THROW(l.namespaceFileKey(ok_id, "../up"), DB::Exception);
+    EXPECT_THROW(l.namespaceFileKey(ok_id, "a/../b"), DB::Exception);
+
+    EXPECT_THROW(l.manifestNamespacePrefix(RootNamespace{"a//b"}), DB::Exception);
+    EXPECT_THROW(l.manifestNamespacePrefix(RootNamespace{"srv1/_files/x"}), DB::Exception);
+    EXPECT_NO_THROW(l.manifestNamespacePrefix(RootNamespace{"my_files/tbl"}));
+}
+
+TEST(CASLayout, GenerationAndRootsKeys)
+{
+    Layout l("p");
+    /// rev. 15: gc/snap is gone; generations carry write-once seals + blob-target / cleanup runs.
+    /// rev. 16: every per-round artifact is attempt-scoped under gc/gen/<gen>/attempt/<attempt>/.
+    EXPECT_EQ(l.foldSealKey(12, 0), "p/gc/gen/12/attempt/0/fold_seal");
+    EXPECT_EQ(l.blobTargetRunKey(12, 0, 0, 0), "p/gc/gen/12/attempt/0/blob_target/0/0");
+    EXPECT_EQ(l.namespaceRootPrefix(), "p/cas/ns/");
+    EXPECT_EQ(l.rootsPrefix(), "p/roots/");
+}
+
+TEST(CASLayout, AttemptScopedGenKeys)
+{
+    DB::Cas::Layout layout("p");
+    EXPECT_EQ(layout.foldSealKey(4, 42), "p/gc/gen/4/attempt/42/fold_seal");
+    EXPECT_EQ(layout.blobTargetRunKey(4, 42, 3, 0), "p/gc/gen/4/attempt/42/blob_target/3/0");
+    EXPECT_EQ(layout.outcomesKey(5, 42, 7, 3), "p/gc/gen/5/attempt/42/outcomes/7/3.zst");
+    EXPECT_EQ(layout.gcGenPrefix(4), "p/gc/gen/4/");
+    EXPECT_EQ(layout.gcGenAttemptPrefix(4, 42), "p/gc/gen/4/attempt/42/");
+}
+
+TEST(CASLayout, RegistryDeletedGcDiscoveryViaList)
+{
+    /// Task 4: the namespace registry (`gc/registry`) is deleted; discovery authority moved to LIST.
+    /// The `_registry` namespace segment is not reserved (it was only reserved while the registry lived
+    /// under `roots/_registry`, which was already relocated to `gc/registry` before being deleted).
+    Layout l("p");
+    EXPECT_NO_THROW(l.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(RootNamespace{"a/_registry@cas@"})));
+    /// Opaque stream keys are independent of namespace-segment reservations.
+    EXPECT_NO_THROW(l.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(RootNamespace{"a/_files"})));
+}
+
+TEST(CASLayout, CasArchiveSuffixConstant)
+{
+    EXPECT_EQ(DB::Cas::kCasArchiveSuffix, "@cas@");
+}
+
+TEST(CASVfsPaths, MirroredArchiveNamespace)
+{
+    using DB::Cas::mirroredArchiveNamespace;
+    /// Atomic: bare uuid -> store/<u3>/<uuid>@cas@
+    EXPECT_EQ(mirroredArchiveNamespace("3f2a0000-0000-0000-0000-000000000001"),
+              "store/3f2/3f2a0000-0000-0000-0000-000000000001@cas@");
+    /// Non-Atomic: a full data/db/tbl path is used verbatim, @cas@ appended to the last segment.
+    EXPECT_EQ(mirroredArchiveNamespace("data/mydb/events"),
+              "data/mydb/events@cas@");
+}
+
+TEST(CASLayout, ManifestKeyShape)
+{
+    Layout l("p");
+    ManifestId id;
+    id.root_namespace = RootNamespace("srv-a/3f2e-uuid@cas@");
+    id.ref.writer_epoch = 7;
+    id.ref.build_sequence = 1042;
+    id.ref.manifest_ordinal = 1;
+    const String key = l.manifestKey(id);
+    EXPECT_EQ(key,
+        "p/cas/manifests/srv-a/3f2e-uuid@cas@/"
+        "0000000000000007-0000000000000412/000001.zst");
+}
+
+TEST(CASLayout, ManifestsSegmentReserved)
+{
+    Layout l("p");
+    ManifestId bad;
+    bad.root_namespace = RootNamespace("srv-a/_manifests/x");
+    EXPECT_THROW(l.manifestKey(bad), DB::Exception);
+    /// Opaque life prefixes ignore the logical spelling; manifests still enforce the reservation.
+    EXPECT_NO_THROW(l.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(RootNamespace{"srv-a/_manifests/tbl"})));
+    EXPECT_NO_THROW(l.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(RootNamespace{"my_manifests/tbl"})));
+}
+
+TEST(CASLayout, ManifestKeyHexRoundTrip)
+{
+    Layout l("p");
+    ManifestId id;
+    id.root_namespace = RootNamespace("srv-a/3f2e-uuid@cas@");
+    id.ref.writer_epoch = 7;
+    id.ref.build_sequence = 0x8e;
+    id.ref.manifest_ordinal = 42;
+    const String key = l.manifestKey(id);
+    EXPECT_EQ(key,
+        "p/cas/manifests/srv-a/3f2e-uuid@cas@/"
+        "0000000000000007-000000000000008e/000042.zst");
+
+    const auto parsed = l.parseManifestKey(key);
+    ASSERT_TRUE(parsed.has_value());
+    EXPECT_EQ(parsed->root_namespace, id.root_namespace);
+    EXPECT_EQ(parsed->ref, id.ref);
+
+    /// The old two-directory decimal shape (`<writer_epoch>/<build_sequence>/<ordinal>.zst`) is no
+    /// longer canonical: the segment right before the file is a plain decimal number, not two
+    /// fixed-width hex fields joined by '-', so `parseRefTxnId` rejects it.
+    EXPECT_FALSE(l.parseManifestKey("p/cas/manifests/srv-a/3f2e-uuid@cas@/7/142/000042.zst").has_value());
+    /// Foreign prefix, missing build segment, non-registered-suffix file, and out-of-range ordinal
+    /// are all rejected.
+    EXPECT_FALSE(l.parseManifestKey("p/cas/refs/srv-a/3f2e-uuid@cas@/"
+        "0000000000000007-000000000000008e/000042.zst").has_value());
+    EXPECT_FALSE(l.parseManifestKey("p/cas/manifests/0000000000000007-000000000000008e/000042.zst").has_value());
+    EXPECT_FALSE(l.parseManifestKey("p/cas/manifests/srv-a/3f2e-uuid@cas@/"
+        "0000000000000007-000000000000008e/000042.bin").has_value());
+    EXPECT_FALSE(l.parseManifestKey("p/cas/manifests/srv-a/3f2e-uuid@cas@/"
+        "0000000000000007-000000000000008e/000000.zst").has_value());
+    EXPECT_FALSE(l.parseManifestKey("p/cas/manifests/srv-a/3f2e-uuid@cas@/"
+        "0000000000000007-000000000000008E/000042.zst").has_value());   /// uppercase hex
+}
+
+TEST(CASLayout, RefObjectKeyRoundTrips)
+{
+    Layout l("p");
+    const RootNamespace ns{"srv1/tbl@cas@"};
+    const NamespaceLifeId ns_id = DB::Cas::tests::fixture::fixtureLife(ns);
+    const RefTxnId id{7, 0x8e};
+    const String life = "p/cas/ns/stream/" + renderIncarnation(ns_id.incarnation) + "/";
+
+    const String log_key = l.refLogKey(ns_id, id);
+    EXPECT_EQ(log_key, life + "_log/0000000000000007-000000000000008e.zst");
+    const auto parsed_log = l.parseRefObjectKey(log_key);
+    ASSERT_TRUE(parsed_log.has_value());
+    EXPECT_EQ(parsed_log->life_id, ns_id.incarnation);
+    EXPECT_EQ(parsed_log->kind, RefObjectKind::Log);
+    EXPECT_EQ(parsed_log->txn_id, id);
+
+    const String snap_key = l.refSnapshotKey(ns_id, id);
+    EXPECT_EQ(snap_key, life + "_snap/0000000000000007-000000000000008e.zst");
+    const auto parsed_snap = l.parseRefObjectKey(snap_key);
+    ASSERT_TRUE(parsed_snap.has_value());
+    EXPECT_EQ(parsed_snap->life_id, ns_id.incarnation);
+    EXPECT_EQ(parsed_snap->kind, RefObjectKind::Snap);
+    EXPECT_EQ(parsed_snap->txn_id, id);
+
+}
+
+TEST(CASLayout, RefObjectKeyLexicalOrder)
+{
+    Layout l("p");
+    const NamespaceLifeId ns_id = DB::Cas::tests::fixture::fixtureLife(RootNamespace{"srv1/tbl@cas@"});
+    const RefTxnId id{7, 0x8e};
+    EXPECT_LT(l.refLogKey(ns_id, id), l.refSnapshotKey(ns_id, id));
+}
+
+TEST(CASLayout, ParseRefObjectKeyRejections)
+{
+    Layout l("p");
+    const RootNamespace ns{"srv1/tbl@cas@"};
+    const NamespaceLifeId ns_id = DB::Cas::tests::fixture::fixtureLife(ns);
+    const RefTxnId id{7, 0x8e};
+    const String log_key = l.refLogKey(ns_id, id);
+    const String snap_key = l.refSnapshotKey(ns_id, id);
+
+    /// Foreign top-level prefix.
+    EXPECT_FALSE(l.parseRefObjectKey("p/cas/manifests/srv1/tbl@cas@/_log/" + renderRefTxnId(id)).has_value());
+    /// Unknown kind directory (also covers the removed numeric-shard ref-key shape, which has no kind dir).
+    EXPECT_FALSE(l.parseRefObjectKey("p/cas/ns/stream/00000000000000000000000000000001/_bogus/" + renderRefTxnId(id)).has_value());
+    EXPECT_FALSE(l.parseRefObjectKey(l.namespaceStreamPrefix(ns_id) + "3").has_value());
+    /// Uppercase hex and a short id are non-canonical RefTxnId renders. The id is judged BEFORE the
+    /// life segment, so these stay "not ours" rather than becoming an incarnation refusal.
+    EXPECT_FALSE(l.parseRefObjectKey(l.namespaceStreamPrefix(ns_id) + "_log/"
+        "0000000000000007-000000000000008E").has_value());
+    EXPECT_FALSE(l.parseRefObjectKey(l.namespaceStreamPrefix(ns_id) + "_log/7-8e").has_value());
+    /// `_snap` without its stored suffix, and WITH a stray one, are both rejected. The suffix is taken
+    /// from the registry rather than spelled out: it was `.proto` when this test was written and is
+    /// `.zst` today, and stripping the wrong number of characters would have tested nothing.
+    const String snap_suffix{storedSuffix(FormatId::RefSnapshot)};
+    EXPECT_FALSE(l.parseRefObjectKey(snap_key.substr(0, snap_key.size() - snap_suffix.size())).has_value());
+    EXPECT_FALSE(l.parseRefObjectKey(log_key + ".proto").has_value());
+    /// Trailing garbage after the id.
+    EXPECT_FALSE(l.parseRefObjectKey(log_key + "/extra").has_value());
+    EXPECT_FALSE(l.parseRefObjectKey(snap_key + "/extra").has_value());
+    /// Missing namespace segment entirely.
+    EXPECT_FALSE(l.parseRefObjectKey("p/cas/ns/stream/_log/" + renderRefTxnId(id)).has_value());
+    /// The `_ckpt` (spec INV-4) has no kind directory and no transaction id, so the id-bearing parser
+    /// must not claim it. Every sweep over the ref prefix has to consult `parseRefCkptKey` as well --
+    /// `groupRefKeys` treats a key neither parser recognizes as corruption that aborts ref folding.
+    EXPECT_FALSE(l.parseRefObjectKey(l.refCkptKey(ns_id)).has_value());
+}
+
+/// Stage A task 5 (spec INV-4): `refCkptKey` and `parseRefCkptKey` are inverses, and the `_ckpt`
+/// parser is exactly as strict as its id-bearing sibling -- it claims OUR checkpoint keys and nothing
+/// else. A key that is not one of ours at all still yields `std::nullopt` rather than an exception,
+/// for the same reason `parseRefObjectKey` does: classifying an untrusted listed key is an ordinary
+/// "is this ours" question. Refusal is reserved for a key that IS ours but names no life --
+/// `gtest_cas_ref_namespace_id.cpp` owns that half.
+TEST(CASLayout, RefCkptKeyRoundTripsAndRejectsEverythingElse)
+{
+    Layout l("p");
+    const RootNamespace ns{"srv1/tbl@cas@"};
+    const NamespaceLifeId ns_id = DB::Cas::tests::fixture::fixtureLife(ns);
+    const RefTxnId id{7, 0x8e};
+
+    /// The state prefix plus the bare leaf, with no compression suffix (the format is raw), so the key
+    /// is exactly `cas/ns/state/<life_id>/_ckpt`.
+    EXPECT_EQ(l.refCkptKey(ns_id), l.namespaceStatePrefix(ns_id) + "_ckpt");
+    EXPECT_EQ(l.parseRefCkptKey(l.refCkptKey(ns_id)), ns_id.incarnation);
+    const NamespaceLifeId deep = DB::Cas::tests::fixture::fixtureLife(RootNamespace{"a/b/c"});
+    EXPECT_EQ(l.parseRefCkptKey(l.refCkptKey(deep)), deep.incarnation);
+
+    /// Foreign pool prefix.
+    EXPECT_FALSE(l.parseRefCkptKey("q/cas/ns/state/00000000000000000000000000000001/_ckpt").has_value());
+    /// The two id-bearing kinds are not checkpoints.
+    EXPECT_FALSE(l.parseRefCkptKey(l.refLogKey(ns_id, id)).has_value());
+    EXPECT_FALSE(l.parseRefCkptKey(l.refSnapshotKey(ns_id, id)).has_value());
+    /// A suffix the registry does not put there, and trailing garbage.
+    EXPECT_FALSE(l.parseRefCkptKey(l.refCkptKey(ns_id) + ".zst").has_value());
+    EXPECT_FALSE(l.parseRefCkptKey(l.refCkptKey(ns_id) + "/extra").has_value());
+    /// A near-miss leaf name.
+    EXPECT_FALSE(l.parseRefCkptKey(l.namespaceStatePrefix(ns_id) + "_ckp").has_value());
+    EXPECT_FALSE(l.parseRefCkptKey(l.namespaceStatePrefix(ns_id) + "_ckpt2").has_value());
+    /// Missing namespace segment entirely.
+    EXPECT_FALSE(l.parseRefCkptKey("p/cas/ns/state/_ckpt").has_value());
+    /// The mirror of the rejection above: `_ckpt` is not a canonical `RefTxnId` render, so a key that
+    /// puts it inside a kind directory is claimed by NEITHER parser.
+    EXPECT_FALSE(l.parseRefObjectKey(l.namespaceStreamPrefix(ns_id) + "_log/_ckpt").has_value());
+    /// The same key used to be READ by this parser as the checkpoint of a phantom namespace named
+    /// `srv1/tbl@cas@/<inc>/_log`, because a namespace is an OPAQUE multi-segment string and nothing
+    /// distinguished a deeper real namespace from a shallower one with a stray segment. The life
+    /// segment closes that: `_log` is not a canonical incarnation, so the key is now REFUSED instead
+    /// of quietly naming a table that cannot exist.
+    EXPECT_FALSE(l.parseRefCkptKey(l.namespaceStatePrefix(ns_id) + "_log/_ckpt").has_value());
+}
+
+/// C3: blobKey/parseBlobKey are inverses; pins the grammar before relocating the definitions
+/// from CasPartWriteTxn.cpp to CasLayout.cpp (relocation must not change a single byte of output).
+TEST(CASLayout, BlobKeyRoundTripsThroughParse)
+{
+    DB::Cas::Layout layout("pool0");
+    const DB::Cas::BlobRef ref{DB::Cas::BlobHashAlgo::XXH3_128,
+                               DB::Cas::codecFor(DB::Cas::BlobHashAlgo::XXH3_128).fromHex(std::string(32, 'a'))};
+    const String body = layout.blobKey(ref);
+    const String meta = layout.blobMetaKey(ref);
+    EXPECT_EQ(meta, body + ".meta");
+
+    auto parsed_body = layout.parseBlobKey(body);
+    auto parsed_meta = layout.parseBlobKey(meta);   /// body and .meta parse to the SAME BlobRef
+    ASSERT_TRUE(parsed_body.has_value());
+    ASSERT_TRUE(parsed_meta.has_value());
+    EXPECT_EQ(*parsed_body, ref);
+    EXPECT_EQ(*parsed_meta, ref);
+    EXPECT_FALSE(layout.parseBlobKey("pool0/blobs/unknown-algo/aa/aa00").has_value());  /// foreign => nullopt
+}
diff --git a/src/Disks/tests/gtest_cas_lifecycle_condition.cpp b/src/Disks/tests/gtest_cas_lifecycle_condition.cpp
new file mode 100644
index 000000000000..9ab0a9bf4ee4
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_lifecycle_condition.cpp
@@ -0,0 +1,259 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <algorithm>
+#include <atomic>
+#include <memory>
+#include <stdexcept>
+#include <string>
+
+/// Task 5 (spec §§1-3): the pool lifecycle condition + the identity gate at step 0 of `tryRemountOnce`.
+/// These tests open a real writable `Pool` over the in-memory ("Emulated"-style) backend, manipulate the
+/// pool sentinels behind the pool's back, then drive the gate through the synchronous `tryRemountOnce`
+/// seam and assert the resulting lifecycle condition + the store()-class refusal. They follow
+/// gtest_cas_sentinel_probe.cpp's harness patterns; the op counter is `tests::CountingBackend`.
+
+namespace DB::ErrorCodes
+{
+extern const int INVALID_STATE;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::CountingBackend;
+
+namespace
+{
+
+const String kSrid = "test";
+
+/// Delete an existing key exactly (its current token comes from the same GET). Returns the deleted body
+/// so a test can restore it verbatim later (scenario d).
+String deleteKeyReturningBody(Backend & backend, const String & key)
+{
+    const auto got = backend.get(key);
+    EXPECT_TRUE(got.has_value()) << "expected '" << key << "' to exist before deletion";
+    if (!got)
+        return {};
+    backend.deleteExact(key, got->token);
+    return got->bytes;
+}
+
+/// GC's fence-out applied directly to the mount lease: preserve the body, set `gc_fenced`, bump `seq`
+/// (token-guarded). A subsequent `tryRemountOnce` whose identity gate verdicts `Recover` then reclaims a
+/// fresh incarnation and returns true. Mirrors gtest_cas_pool.cpp's `fenceOutMount`.
+void fenceOutMount(Backend & backend, const String & mount_key)
+{
+    const auto got = backend.get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    MountLease m = decodeMountLease(got->bytes);
+    m.gc_fenced = true;
+    m.seq += 1;
+    ASSERT_EQ(backend.putOverwrite(mount_key, encodeMountLease(m), got->token).outcome, PutOutcome::Done);
+}
+
+/// A Backend decorator whose head/get/list throw an untyped transport error while `fail` is armed. Starts
+/// DISARMED so `Pool::open` succeeds; a test arms it only to make the identity probe inconclusive. Mirrors
+/// gtest_cas_sentinel_probe.cpp's `TransportFaultBackend`, but toggleable AFTER open.
+class ToggleableTransportFaultBackend final : public InMemoryBackend
+{
+public:
+    /// Unhide the base convenience overloads, matching every other Backend subclass in this suite.
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    HeadResult head(const String & key) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::head(key);
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::get(key, range);
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+
+    std::atomic<bool> fail{false};
+};
+
+}
+
+/// (a) `_pool_meta` + the owner anchor authoritatively absent → the gate enters `IdentityLost` (never
+/// `Vanished`) and store()-class access fails loud. rev.8: `IdentityLost` is a fail-loud TERMINAL state —
+/// `isVanished()` still reads false (it is a distinct terminal), but a direct gate re-probe refuses without
+/// ever claiming/allocating/writing (the thread-exit behavior of the background observer is covered by
+/// `RemountThreadSelfExitsOnceIdentityLost` below).
+TEST(CASLifecycleCondition, SentinelsDeletedEntersIdentityLostTerminal)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::Live);
+
+    const String meta_key = store->layout().poolMetaKey();
+    const String owner_key = store->layout().ownerKey(kSrid);
+
+    /// Both sentinels gone (other objects may or may not remain — rev.8 does not distinguish).
+    deleteKeyReturningBody(*backend, meta_key);
+    deleteKeyReturningBody(*backend, owner_key);
+
+    /// Even from `Live` (no fence trip), a direct remount attempt transitions through `TransientNotLive`
+    /// and enters `IdentityLost` at step 0 — WITHOUT reaching `claimOwnerOrThrow`.
+    EXPECT_FALSE(store->tryRemountOnce());
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::IdentityLost);
+    EXPECT_FALSE(store->isVanished()) << "IdentityLost is a distinct terminal, not a Vanished state";
+
+    /// store()-class access now fails loud with the typed lifecycle error.
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->throwIfLifecycleTerminal(); });
+
+    /// A direct gate re-probe still refuses without mutating: it probes the sentinels authoritatively and
+    /// performs ZERO writes (never claims/allocates/mounts on a terminal pool).
+    backend->resetCounts();
+    EXPECT_FALSE(store->tryRemountOnce());
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::IdentityLost);
+    EXPECT_EQ(backend->putTotal(), 0u) << "a terminal-IdentityLost gate probe must never claim, allocate, or write";
+    EXPECT_GE(backend->headCount(meta_key), 1u) << "the gate still probes _pool_meta authoritatively";
+}
+
+/// (a2) rev.8 thread-exit: `IdentityLost` is terminal, so the background self-remount thread must self-exit
+/// — mirroring how a `Vanished` pool refuses to arm one. With `background_watermark = true`, `scheduleRemount`
+/// must REFUSE to arm a recovery thread once the pool is `IdentityLost` (`remountTerminal()` covers it),
+/// exactly as it refuses on a published `Vanished` intent.
+TEST(CASLifecycleCondition, RemountThreadSelfExitsOnceIdentityLost)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// `background_watermark = true` so `scheduleRemount` actually arms a recovery thread in production mode
+    /// (mirrors gtest_cas_pool.cpp's ShutdownGuardRefusesToArmRemount setup).
+    auto store = DB::Cas::Pool::open(backend,
+        DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test", .background_watermark = true});
+
+    /// Drive the pool terminal (IdentityLost) synchronously first — a direct gate call, no thread spawned.
+    deleteKeyReturningBody(*backend, store->layout().poolMetaKey());
+    deleteKeyReturningBody(*backend, store->layout().ownerKey(kSrid));
+    EXPECT_FALSE(store->tryRemountOnce());
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::IdentityLost);
+
+    /// The keeper's on-lost callback (or any `scheduleRemount`) must now refuse: no observer runs on a
+    /// terminal pool.
+    EXPECT_FALSE(store->scheduleRemountForTest())
+        << "an IdentityLost pool is terminal (rev.8) — scheduleRemount must not arm a recovery thread";
+}
+
+/// (b) `_pool_meta` present but its `pool_id` is foreign → `Vanished(replaced)` immediately.
+TEST(CASLifecycleCondition, PoolMetaForeignPoolIdEntersVanishedReplacedImmediately)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::Live);
+
+    /// Overwrite `_pool_meta` with a FOREIGN pool_id (identity replaced); the object stays present.
+    const String meta_key = store->layout().poolMetaKey();
+    const auto got = backend->get(meta_key);
+    ASSERT_TRUE(got.has_value());
+    PoolMeta foreign = decodePoolMeta(got->bytes);
+    foreign.pool_id = foreign.pool_id + DB::UInt128(1);
+    ASSERT_EQ(backend->putOverwrite(meta_key, encodePoolMeta(foreign), got->token).outcome, PutOutcome::Done);
+
+    EXPECT_FALSE(store->tryRemountOnce());
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::VanishedReplaced);
+    EXPECT_TRUE(store->isVanished());
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->throwIfLifecycleTerminal(); });
+}
+
+/// (c) [B6] trap: `_pool_meta` present, pool_id + blob_header_len match, but `algos_used` differs → NOT a
+/// replacement (`algos_used` is legally mutable); the existing recovery proceeds and the pool returns to
+/// `Live`.
+TEST(CASLifecycleCondition, PoolMetaAlgosUsedDifferIsNotReplacementRecoveryProceeds)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    const String meta_key = store->layout().poolMetaKey();
+    const auto got = backend->get(meta_key);
+    ASSERT_TRUE(got.has_value());
+    PoolMeta mutated = decodePoolMeta(got->bytes);
+    /// pool_id + blob_header_len UNCHANGED; only `algos_used` gains a member (a mutable field, [B6]).
+    const auto extra = static_cast<uint8_t>(BlobHashAlgo::XXH3_128);
+    ASSERT_FALSE(std::binary_search(mutated.algos_used.begin(), mutated.algos_used.end(), extra));
+    mutated.algos_used.push_back(extra);
+    std::sort(mutated.algos_used.begin(), mutated.algos_used.end());
+    ASSERT_EQ(backend->putOverwrite(meta_key, encodePoolMeta(mutated), got->token).outcome, PutOutcome::Done);
+
+    /// Fence out the mount so the (correctly non-replacement) recovery cleanly reclaims a fresh incarnation.
+    fenceOutMount(*backend, store->layout().mountKey(kSrid));
+
+    /// A differing `algos_used` must NOT read as a foreign pool: the gate verdicts `Recover`, recovery
+    /// completes, and the pool is `Live` — never `Vanished`.
+    EXPECT_TRUE(store->tryRemountOnce());
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::Live);
+    EXPECT_FALSE(store->isVanished());
+}
+
+/// (d) [D3] no auto-revival: from `IdentityLost`, restoring both sentinels with matching identity does NOT
+/// bring the disk back — the observer stays fail-loud; only a restart recovers.
+TEST(CASLifecycleCondition, IdentityLostDoesNotAutoReviveWhenSentinelsRestored)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    const String meta_key = store->layout().poolMetaKey();
+    const String owner_key = store->layout().ownerKey(kSrid);
+
+    const String meta_body = deleteKeyReturningBody(*backend, meta_key);
+    const String owner_body = deleteKeyReturningBody(*backend, owner_key);
+
+    EXPECT_FALSE(store->tryRemountOnce());
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::IdentityLost);
+
+    /// Restore both sentinels verbatim (a backup restore with matching identity).
+    ASSERT_EQ(backend->putIfAbsent(meta_key, meta_body).outcome, PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent(owner_key, owner_body).outcome, PutOutcome::Done);
+
+    /// The gate now sees Present+match, but the state is `IdentityLost`, so it stays fail-loud.
+    EXPECT_FALSE(store->tryRemountOnce());
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::IdentityLost);
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->throwIfLifecycleTerminal(); });
+}
+
+/// (e) Transport error from the probe → the pool stays `TransientNotLive` (recoverable); absence is never
+/// proven, so no terminal transition fires and store()-class access does NOT throw the terminal lifecycle
+/// error (the transient class stays fence-gated until Task 8).
+TEST(CASLifecycleCondition, ProbeTransportErrorStaysTransientAndRetries)
+{
+    auto backend = std::make_shared<ToggleableTransportFaultBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::Live);
+
+    /// Arm the transport fault: the identity probe's head/get/list now throw → Indeterminate.
+    backend->fail.store(true);
+
+    EXPECT_FALSE(store->tryRemountOnce());
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::TransientNotLive);
+    EXPECT_FALSE(store->isVanished());
+    EXPECT_NO_THROW(store->throwIfLifecycleTerminal());
+
+    /// A second attempt with the fault still armed remains transient (retries continue).
+    EXPECT_FALSE(store->tryRemountOnce());
+    EXPECT_EQ(store->lifecycle(), PoolLifecycle::TransientNotLive);
+
+    /// Disarm before teardown so `~Pool()`'s clean-farewell write is not fighting the injected fault.
+    backend->fail.store(false);
+}
diff --git a/src/Disks/tests/gtest_cas_lifecycle_snapshot.cpp b/src/Disks/tests/gtest_cas_lifecycle_snapshot.cpp
new file mode 100644
index 000000000000..154c365a2a6d
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_lifecycle_snapshot.cpp
@@ -0,0 +1,236 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <filesystem>
+#include <functional>
+#include <memory>
+#include <string>
+
+/// Task 12 (rev.7 spec §7, [C5]-visibility): the NON-GATED lifecycle snapshot backing
+/// `system.cas_mounts`. A Factory-class read (spec §1): I/O-free, no `store()`/`poolAccess`,
+/// truthful in EVERY state — so a not-live / stopped / vanished / never-started disk stays VISIBLE to the
+/// operator instead of silently missing from the table. These tests exercise the accessor directly (the
+/// SQL-level assertions land in Task 14): `ContentAddressedMetadataStorage::lifecycleSnapshot` at the
+/// storage level, and `Pool::lifecycleSnapshot` at the pool level (including the zero-backend-op proof).
+/// Harness patterns follow gtest_cas_operation_gate.cpp / gtest_cas_forget.cpp.
+
+namespace DB::ErrorCodes
+{
+extern const int INVALID_STATE;
+}
+
+using namespace DB;
+using DB::Cas::PoolLifecycle;
+using DB::Cas::tests::CountingBackend;
+
+namespace
+{
+
+const std::string kSrid = "test";
+
+/// A live table dir + committed part reused by the storage-level tests (the shape
+/// gtest_cas_operation_gate.cpp / gtest_cas_forget.cpp use).
+const std::string kTableDir = "sn0/sn0sn0s0-0808-4808-8808-080808080808";
+const std::string kPartDir = kTableDir + "/all_1_1_0";
+const std::string kPartFile = kPartDir + "/data.bin";
+
+std::shared_ptr<ContentAddressedMetadataStorage> openSnapshotStorage()
+{
+    auto settings = Cas::tests::makeSettingsForTest(
+        kSrid, std::filesystem::temp_directory_path() / "ca_snapshot_scratch");
+    auto storage = std::make_shared<ContentAddressedMetadataStorage>(
+        Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+void commitOnePart(ContentAddressedMetadataStorage & storage)
+{
+    auto tx = storage.createTransaction();
+    auto & ca_tx = dynamic_cast<ContentAddressedTransaction &>(*tx);
+    auto buf = ca_tx.writeFile(kTableDir + "/tmp_insert_all_1_1_0/data.bin", 65536, WriteMode::Rewrite, {});
+    const std::string bytes = "content-of-the-part";
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+    tx->moveDirectory(kTableDir + "/tmp_insert_all_1_1_0", kPartDir);
+    tx->commit(NoCommitOptions{});
+}
+
+/// Delete an existing key exactly (its current token comes from the same GET) — used to drive a live pool
+/// into a NATURAL `IdentityLost`. Mirrors gtest_cas_forget.cpp / gtest_cas_lifecycle_condition.cpp.
+void deleteKeyExact(DB::Cas::Backend & backend, const String & key)
+{
+    const auto got = backend.get(key);
+    ASSERT_TRUE(got.has_value()) << "expected '" << key << "' to exist before deletion";
+    if (got)
+        backend.deleteExact(key, got->token);
+}
+
+}
+
+/// (a) Live: the snapshot reads `live` with no reason and no `since`, and always carries the disk's
+/// last-known identity (pool_id + server_root_id).
+TEST(CASLifecycleSnapshot, LiveIsTruthfulWithIdentity)
+{
+    auto storage = openSnapshotStorage();
+    commitOnePart(*storage);
+
+    const CasLifecycleSnapshot snap = storage->lifecycleSnapshot();
+    EXPECT_EQ(snap.lifecycle, "live");
+    EXPECT_TRUE(snap.reason.empty()) << snap.reason;
+    EXPECT_TRUE(snap.detail.empty()) << snap.detail;
+    EXPECT_EQ(snap.since, 0) << "a live pool has no lifecycle `since`";
+    EXPECT_EQ(snap.server_root_id, storage->serverRootId());
+    EXPECT_FALSE(snap.pool_id.empty()) << "a started disk knows its pool identity";
+    EXPECT_EQ(snap.pool_id, storage->getPoolUUID());
+}
+
+/// (b) IdentityLost (forced from Live on the captured handle, the gate-test idiom): the snapshot names the
+/// non-auto-recovering `identity_lost` state with the [D5] detail present and `since` set. The enum-clean
+/// `reason` word is empty here — it carries only the `vanished` sub-state, and `identity_lost` is already
+/// fully named by the `lifecycle` column.
+TEST(CASLifecycleSnapshot, IdentityLostHasDetailAndSince)
+{
+    auto storage = openSnapshotStorage();
+    commitOnePart(*storage);
+    auto pool = storage->store();   /// captured while Live (store() is fail-closed on a terminal pool)
+
+    pool->setLifecycleForTest(PoolLifecycle::IdentityLost);
+
+    const CasLifecycleSnapshot snap = storage->lifecycleSnapshot();
+    EXPECT_EQ(snap.lifecycle, "identity_lost");
+    EXPECT_TRUE(snap.reason.empty()) << "reason is the vanish sub-state word only: " << snap.reason;
+    EXPECT_NE(snap.detail.find("identity lost"), std::string::npos) << snap.detail;
+    EXPECT_NE(snap.since, 0) << "a not-live state carries the wall-clock instant it was entered";
+    /// Identity survives a terminal state — the disk stays introspectable under it.
+    EXPECT_EQ(snap.pool_id, storage->getPoolUUID());
+}
+
+/// (c) VanishedForgotten via the REAL verb (`storage->forgetDisk()`): the snapshot reads `vanished` with the
+/// enum-clean `reason` word `forgotten` (so Task 14's `lifecycle || '(' || lifecycle_reason || ')'` reads
+/// EXACTLY `vanished(forgotten)`), the [D5] `detail` carrying the operator's decommission timestamp, `since`
+/// set, and the identity still present.
+TEST(CASLifecycleSnapshot, VanishedForgottenIsEnumCleanWithTimestampedDetail)
+{
+    auto storage = openSnapshotStorage();
+    commitOnePart(*storage);
+    const String pool_id_before = storage->getPoolUUID();
+
+    storage->forgetDisk();
+
+    const CasLifecycleSnapshot snap = storage->lifecycleSnapshot();
+    EXPECT_EQ(snap.lifecycle, "vanished");
+    EXPECT_EQ(snap.reason, "forgotten");
+    /// Task 14's teardown check depends on this exact concatenation.
+    EXPECT_EQ(snap.lifecycle + "(" + snap.reason + ")", "vanished(forgotten)");
+    EXPECT_NE(snap.detail.find("SYSTEM CAS FORGET at "), std::string::npos) << snap.detail;
+    EXPECT_NE(snap.detail.find("erasure was NOT verified"), std::string::npos) << snap.detail;
+    EXPECT_NE(snap.since, 0);
+    /// The disk stays registered and introspectable under its identity after FORGET.
+    EXPECT_EQ(snap.pool_id, pool_id_before);
+    EXPECT_EQ(snap.server_root_id, storage->serverRootId());
+}
+
+/// (d) A null pool never crashes the accessor and reports the storage-level lifecycle: `constructing`
+/// before the first startup, `shutdown` after teardown. reason/since stay empty/0 (no terminal cause).
+TEST(CASLifecycleSnapshot, NullPoolReportsConstructingThenShutdown)
+{
+    auto settings = Cas::tests::makeSettingsForTest(
+        kSrid, std::filesystem::temp_directory_path() / "ca_snapshot_null_scratch");
+    auto storage = std::make_shared<ContentAddressedMetadataStorage>(
+        Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+
+    /// Constructed but never started: no pool published.
+    const CasLifecycleSnapshot before = storage->lifecycleSnapshot();
+    EXPECT_EQ(before.lifecycle, "constructing");
+    EXPECT_TRUE(before.reason.empty());
+    EXPECT_TRUE(before.detail.empty());
+    EXPECT_EQ(before.since, 0);
+    EXPECT_TRUE(before.pool_id.empty()) << "no identity before startup";
+    EXPECT_EQ(before.server_root_id, kSrid) << "the identity is known from config even pre-startup";
+
+    storage->startup();
+    ASSERT_EQ(storage->lifecycleSnapshot().lifecycle, "live");
+
+    storage->shutdown();
+    const CasLifecycleSnapshot after = storage->lifecycleSnapshot();
+    EXPECT_EQ(after.lifecycle, "shutdown") << "a torn-down disk is distinguishable from a never-started one";
+    EXPECT_FALSE(after.pool_id.empty()) << "the last-known identity survives shutdown";
+}
+
+/// (e) The accessor is I/O-free (spec §1 Factory class): NO backend op runs, in any lifecycle state. Proven
+/// against a `CountingBackend` — the totals recorded after open do not move across snapshot reads, whether
+/// the pool is Live or forced terminal.
+TEST(CASLifecycleSnapshot, PerformsZeroBackendOps)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    const uint64_t head0 = backend->headTotal();
+    const uint64_t get0 = backend->getTotal();
+    const uint64_t put0 = backend->putTotal();
+    const uint64_t getstream0 = backend->getStreamTotal();
+    const uint64_t list0 = backend->listTotal();
+
+    const auto assertNoIo = [&](const char * where)
+    {
+        EXPECT_EQ(backend->headTotal(), head0) << where;
+        EXPECT_EQ(backend->getTotal(), get0) << where;
+        EXPECT_EQ(backend->putTotal(), put0) << where;
+        EXPECT_EQ(backend->getStreamTotal(), getstream0) << where;
+        EXPECT_EQ(backend->listTotal(), list0) << where;
+    };
+
+    /// Live snapshot: zero I/O.
+    (void)store->lifecycleSnapshot();
+    assertNoIo("live snapshot must not touch the backend");
+
+    /// Forced terminal snapshot (the very state the store()-class surface refuses): still zero I/O.
+    store->setLifecycleForTest(PoolLifecycle::VanishedReplaced);
+    const DB::Cas::Pool::LifecycleSnapshot vanished = store->lifecycleSnapshot();
+    assertNoIo("a vanished-pool snapshot must not touch the backend");
+    EXPECT_EQ(vanished.lifecycle, PoolLifecycle::VanishedReplaced);
+}
+
+/// (f) A NATURAL transition (not the forced setter) captures the detail + `since`, and the snapshot's detail
+/// is EXACTLY the [D5] text `throwIfLifecycleTerminal` throws (minus the pool-name prefix) — the spec §1
+/// "same reason strings in the snapshot and the error" guarantee, so the two can never drift.
+TEST(CASLifecycleSnapshot, NaturalIdentityLostMatchesThrowDetail)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+
+    /// Delete both pool sentinels while other objects remain, then drive the identity gate → IdentityLost
+    /// (never Vanished), exactly gtest_cas_lifecycle_condition.cpp scenario (a).
+    deleteKeyExact(*backend, store->layout().poolMetaKey());
+    deleteKeyExact(*backend, store->layout().ownerKey(kSrid));
+    EXPECT_FALSE(store->tryRemountOnce());
+    ASSERT_EQ(store->lifecycle(), PoolLifecycle::IdentityLost);
+
+    const DB::Cas::Pool::LifecycleSnapshot snap = store->lifecycleSnapshot();
+    EXPECT_EQ(snap.lifecycle, PoolLifecycle::IdentityLost);
+    EXPECT_NE(snap.since, 0) << "the natural enterIdentityLost transition stamps the wall-clock `since`";
+    EXPECT_FALSE(snap.detail.empty());
+
+    /// The snapshot detail is the SAME [D5] text the typed error surfaces: the throw is
+    /// "content-addressed pool '<srid>' <detail>", so the error message must contain the snapshot detail.
+    std::string thrown;
+    try
+    {
+        store->throwIfLifecycleTerminal();
+        ADD_FAILURE() << "IdentityLost must throw from throwIfLifecycleTerminal";
+    }
+    catch (const Exception & e)
+    {
+        thrown = std::string(e.message());
+    }
+    EXPECT_NE(thrown.find(snap.detail), std::string::npos)
+        << "snapshot detail and the typed error must not drift\n  detail: " << snap.detail
+        << "\n  thrown: " << thrown;
+}
diff --git a/src/Disks/tests/gtest_cas_list_liar_end_to_end.cpp b/src/Disks/tests/gtest_cas_list_liar_end_to_end.cpp
new file mode 100644
index 000000000000..1b48718663cb
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_list_liar_end_to_end.cpp
@@ -0,0 +1,624 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include "cas_test_helpers.h"
+
+#include <map>
+#include <string>
+#include <vector>
+
+/// THE 2026-07-25 RELEASE BLOCKER, AS A PERMANENT REGRESSION.
+///
+/// The defect the object store actually exhibited (`reports/2026-07-26-list-incompleteness-proof/`):
+/// objects that were durable, acked, and readable by exact key were OMITTED from enumeration, while a
+/// LATER key under the same prefix was listed. Nothing was lost and nothing was corrupt -- the store
+/// simply under-reported what it held.
+///
+/// Every CAS reader that treated a listing as a CENSUS then drew a false conclusion from it, and the
+/// two that mattered drew ruinous ones. The GC fold walked the ids the listing returned, so it skipped
+/// the omitted records' owner edges AND sealed a cursor above them -- and nothing ever re-reads below a
+/// sealed cursor, so those edges were lost permanently: blobs that were still referenced looked
+/// unreferenced forever after. Recovery replayed the listing, so a table came back missing an ACKED
+/// transaction while looking perfectly healthy.
+///
+/// The answer is that a listing is a HINT and arithmetic is the census. Ids are dense `1..T`
+/// within `(namespace, writer_epoch)` (INV-1), so the next record's id is COMPUTABLE and every record
+/// is read by EXACT KEY. A hidden-but-durable contiguous id is then a NON-EVENT -- the walk finds it
+/// anyway -- while a genuinely absent expected id is a durable HOLD, never a silent skip.
+///
+/// This file is that claim stated end to end, against a store that lies exactly the way the real one
+/// did. `setListOmissions` names the omitted keys; `get`/`head`/`putIfAbsent`/`casPut`/`deleteExact`
+/// keep serving them honestly. Each test below asserts the lie changed NOTHING -- not the folded
+/// edges, not the cursor, not the recovered table, not fsck's verdict -- and the arms that are about
+/// reclamation additionally assert that reclamation still happens, so "nothing was deleted" can never
+/// pass for "the lie was harmless".
+///
+/// The unit-level statements about the walk itself live in `gtest_cas_gc_arithmetic_intake.cpp`, and
+/// the destructive gate's own inventory lives in `gtest_cas_gc_frontier_gate.cpp`. This file is the
+/// INTEGRATION of the two: real rounds, real recovery, real fsck, one lying store.
+///
+/// The suite name is prefixed `Cas` so the `Cas*` unit-test gate filter covers it.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+
+/// The lying store: LIST omits the named keys, everything else serves them honestly. Composed over
+/// `CountingBackend` so the arms whose subject is reclamation can assert on DELETES rather than only on
+/// what survived.
+using LiarBackend = HintHoleBackendOn<CountingBackend>;
+
+String blobKeyOf(const Layout & layout, const DB::UInt128 & hash)
+{
+    return layout.blobKey(legacyMetaTestRef(hash));
+}
+
+/// The sealed fold cursor for `ns` as a full `RefTxnId`. Every fixture here writes ids inside writer
+/// epoch 1, which is the assumption `foldCursorOf` (returning the sequence alone) already makes.
+RefTxnId sealedCursorOf(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    return RefTxnId{1, foldCursorOf(backend, layout, ns, /*shard*/ 0)};
+}
+
+/// Drop the committed ref `ref_name` (currently naming `old_ref`) as ONE transaction at EXACTLY `id`.
+/// The `dropRefTransition` helper allocates its id by LISTING, which a fixture that hides keys must
+/// never do -- it would allocate over a hidden record. Every id in this file is therefore chosen.
+void dropAt(Backend & backend, const Layout & layout, const RootNamespace & ns, const RefTxnId & id,
+            const String & ref_name, const ManifestRef & old_ref)
+{
+    writeTxnAt(backend, layout, ns, id,
+               {ownerTransitionOp(RefOwnerBinding{RefOwnerKind::Committed, ref_name, old_ref}, std::nullopt)});
+}
+
+/// The manifest `publishAt` mints for a given (id, build_sequence) -- needed to drop that ref later.
+ManifestRef publishedManifest(const RefTxnId & id, uint64_t build_sequence)
+{
+    return ManifestRef{.writer_epoch = id.writer_epoch, .build_sequence = build_sequence, .manifest_ordinal = 1};
+}
+
+/// One round plus everything a verdict in this file is allowed to rest on: the report (which carries
+/// the anomaly list), the two intake phase rows (which carry the hold count and the one remaining
+/// whole-round ref abort), and the gate's own verdict off `fold_reduce` (read, not recomputed, so a test
+/// cannot agree with a wrong formula just as readily as with the right one).
+struct RoundEvidence
+{
+    RoundReport report;
+    std::map<String, UInt64> intake;   /// `fold_ref_intake`
+    std::map<String, UInt64> group;    /// `fold_ref_group`
+    bool saw_fold = false;
+    bool frontier_complete = false;
+    bool suppress_destructive = false;
+};
+
+RoundEvidence runRoundCapturing(Gc & gc, UniversePolicy policy)
+{
+    RoundEvidence evidence;
+    gc.setPhaseSink([&](const GcPhaseRecord & rec)
+    {
+        if (rec.phase == "fold_ref_intake")
+            evidence.intake = rec.metrics;
+        else if (rec.phase == "fold_ref_group")
+            evidence.group = rec.metrics;
+        else if (rec.phase == "fold_reduce")
+        {
+            evidence.saw_fold = true;
+            if (const auto it = rec.metrics.find("frontier_complete"); it != rec.metrics.end())
+                evidence.frontier_complete = it->second != 0;
+            if (const auto it = rec.metrics.find("suppress_destructive"); it != rec.metrics.end())
+                evidence.suppress_destructive = it->second != 0;
+        }
+    });
+    evidence.report = gc.runRegularRound({}, /*allow_steal*/ true, policy);
+    gc.setPhaseSink({});
+    return evidence;
+}
+
+/// "ZERO ANOMALIES", spelled out once so every test means the same thing by it: the round recorded no
+/// anomaly, sealed no hold, and did not abort ref folding. A lie the walk absorbs must be invisible in
+/// all three -- a hold in particular would be a WRONG (if safe) answer, since it would suppress the
+/// round's destructive half over records that were durable all along.
+void expectNoAnomalies(const RoundEvidence & evidence, const char * where)
+{
+    EXPECT_TRUE(evidence.report.anomalies.empty())
+        << where << ": the round recorded " << evidence.report.anomalies.size()
+        << " anomaly/anomalies; a hidden-but-durable contiguous id is a NON-EVENT";
+    ASSERT_FALSE(evidence.intake.empty()) << where << ": no `fold_ref_intake` row was emitted";
+    EXPECT_EQ(evidence.intake.at("tables_held"), 0u)
+        << where << ": a namespace was HELD -- the walk mistook an omitted-but-durable record for a gap";
+    EXPECT_EQ(evidence.intake.at("ref_folding_aborted"), 0u) << where;
+    ASSERT_FALSE(evidence.group.empty()) << where << ": no `fold_ref_group` row was emitted";
+    EXPECT_EQ(evidence.group.at("ref_folding_aborted"), 0u) << where;
+}
+
+/// The pool's view of a table, rendered so a failing comparison prints something a human can read.
+std::map<String, String> refsOf(const PoolPtr & store, const RootNamespace & ns)
+{
+    std::map<String, String> out;
+    for (const auto & [ref_name, resolved] : store->listRefs(ns))
+        out[ref_name] = std::to_string(resolved.manifest_id.ref.writer_epoch) + "/"
+            + std::to_string(resolved.manifest_id.ref.build_sequence) + "/"
+            + std::to_string(resolved.manifest_id.ref.manifest_ordinal);
+    return out;
+}
+
+/// THE STREAM UNDER TEST, written identically into any backend: five ordinary publishes at
+/// `{1,1}..{1,5}`, each pinning its own blob, plus the `_ckpt` a recovering reader starts from. Shared
+/// so the oracle arms can seed a lying store and an honest one from the SAME code and compare outcomes
+/// rather than compare against a hand-written expectation.
+void seedFiveRecordStream(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    seedPoolMetaForRestart(backend, layout.poolPrefix());
+    for (uint64_t i = 1; i <= 5; ++i)
+        publishAt(backend, layout, ns, RefTxnId{1, i}, "ref_" + std::to_string(i), i,
+                  DB::UInt128(i), /*birth=*/i == 1);
+    writeRecoverableCkptForRawFixture(backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 5},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+}
+
+/// The exact defect shape: ids 3 and 4 invisible while the LATER id 5 is visible.
+std::vector<String> hiddenMiddleOf(const Layout & layout, const RootNamespace & ns)
+{
+    return {layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 3}),
+            layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 4})};
+}
+
+PoolConfig recoveryPoolConfig()
+{
+    PoolConfig config;
+    config.pool_prefix = "p";
+    config.server_root_id = "test";
+    config.server_id = DB::UInt128(1);
+    /// No background publication: a threshold-triggered snapshot would move the base under the
+    /// comparison these tests make about what recovery reconstructed.
+    config.snapshot_log_count_threshold = 1ULL << 40;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    return config;
+}
+
+PoolPtr openRecoveryPool(const std::shared_ptr<LiarBackend> & backend)
+{
+    seedPoolMetaForRestart(*backend, "p");
+    return Pool::open(backend, recoveryPoolConfig());
+}
+
+}
+
+/// ===================== THE BLOCKER, FULL PIPELINE =====================
+///
+/// Five durable records; the store lists 1, 2 and 5 and pretends 3 and 4 do not exist. Arithmetic
+/// intake never asks the listing what to read next, so all five fold, every blob keeps its owner edge,
+/// and the cursor lands on the true tail.
+///
+/// Under listing-driven intake this fails on the BLOBS, not on the cursor: the cursor still reaches
+/// `{1,5}` (the last listed id) while records 3 and 4 were never folded -- and since nothing re-reads
+/// below a sealed cursor, their edges are gone for good. That is the production damage, exactly.
+TEST(CASListLiarEndToEnd, TheHiddenMiddleOfTheStreamFoldsThroughUnnoticed)
+{
+    auto backend = std::make_shared<LiarBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/blocker@cas@"};
+
+    seedFiveRecordStream(*backend, layout, ns);
+    backend->setListOmissions(hiddenMiddleOf(layout, ns));
+
+    Gc gc(store, kGc);
+    const RoundEvidence evidence = runRoundCapturing(gc, UniversePolicy::kDefault);
+    ASSERT_TRUE(evidence.report.acquired_lease);
+    ASSERT_GT(backend->holesServed(), 0u)
+        << "the omission was never actually served -- the test would pass vacuously";
+
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 5}))
+        << "the walk must reach the true tail of the stream";
+    for (uint64_t i = 1; i <= 5; ++i)
+        EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(i)), 1)
+            << "blob " << i << " lost its owner edge: its record was skipped because the store hid it";
+
+    expectNoAnomalies(evidence, "hidden middle");
+    EXPECT_EQ(evidence.intake.at("logs_applied"), 5u) << "all five records are APPLIED, not three";
+    EXPECT_EQ(evidence.intake.at("logs_accounted"), evidence.intake.at("logs_applied"))
+        << "probe B1: the arithmetic cut the cursors claim must equal what the walk applied";
+}
+
+/// RECOVERY, UNDER THE SAME LIE, AGAINST AN HONEST ORACLE. The comparison is against a second pool
+/// seeded by the SAME code over a store that does not lie -- not against a hand-written expectation,
+/// which could encode the same mistake the code makes.
+TEST(CASListLiarEndToEnd, RecoveryUnderTheSameLieReconstructsExactlyTheTruth)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"00/recover@cas@"};
+
+    auto honest_backend = std::make_shared<LiarBackend>();
+    seedFiveRecordStream(*honest_backend, layout, ns);
+    auto honest = openRecoveryPool(honest_backend);
+    const std::map<String, String> truth = refsOf(honest, ns);
+
+    auto lying_backend = std::make_shared<LiarBackend>();
+    seedFiveRecordStream(*lying_backend, layout, ns);
+    lying_backend->setListOmissions(hiddenMiddleOf(layout, ns));
+    auto lying = openRecoveryPool(lying_backend);
+    const std::map<String, String> recovered = refsOf(lying, ns);
+
+    ASSERT_GT(lying_backend->holesServed(), 0u)
+        << "the omission was never actually served -- the test would pass vacuously";
+    EXPECT_EQ(truth.size(), 5u) << "the oracle itself must see all five published refs";
+    EXPECT_EQ(recovered, truth)
+        << "a table recovered under an omitting listing must be byte-identical to the truth; the "
+           "blocker's recovery came back missing an ACKED transaction and looked healthy";
+}
+
+/// ===================== THE DATA-LOSS ARM =====================
+///
+/// A blob with two owners. The `+1` that publishes the SECOND owner rides the hidden id; the `-1` that
+/// releases the first is visible and lands above it. The arithmetic fold reads both, so the blob's
+/// in-degree is 1 and it is never condemned.
+///
+/// Listing-driven intake folds the visible `-1`, never folds the hidden `+1`, and seals the cursor
+/// above it: the blob's in-degree reads zero while a live ref still names it, and the round deletes
+/// data that is referenced. That is the data loss, and it is why this arm asserts the blob was never
+/// even offered for deletion rather than merely that it is still present.
+TEST(CASListLiarEndToEnd, AHiddenPlusOneKeepsItsBlobWhenAVisibleMinusOneLandsLater)
+{
+    auto backend = std::make_shared<LiarBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/dataloss@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    const DB::UInt128 shared(0x5ade);
+
+    /// `ref_a` and `ref_b` both pin `shared`; `ref_b`'s publish is the record the store will hide.
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_a", 1, shared, /*birth=*/true);
+    publishAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_b", 2, shared);
+    dropAt(*backend, layout, ns, RefTxnId{1, 3}, "ref_a", publishedManifest(RefTxnId{1, 1}, 1));
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 3},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    backend->setListOmissions({layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 2})});
+
+    Gc gc(store, kGc);
+    const RoundEvidence first = runRoundCapturing(gc, UniversePolicy::Authoritative);
+    ASSERT_TRUE(first.report.acquired_lease);
+    ASSERT_GT(backend->holesServed(), 0u)
+        << "the omission was never actually served -- the test would pass vacuously";
+    expectNoAnomalies(first, "hidden +1");
+
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 3}));
+    EXPECT_EQ(inDegreeOf(*backend, layout, shared), 1)
+        << "the hidden publish's `+1` must be folded: `ref_b` still owns this blob";
+
+    /// Rounds that are ALLOWED to reclaim, and would, if the in-degree were wrong.
+    for (int i = 0; i < 5; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, shared)).exists)
+        << "a blob a live ref still names was DELETED -- the hidden `+1` was never folded";
+    EXPECT_EQ(backend->deleteCount(blobKeyOf(layout, shared)), 0u)
+        << "not merely still present: the delete was never even attempted";
+}
+
+/// ===================== THE LEAK ARM =====================
+///
+/// The mirror image, and the reason the arm above is not the whole story. Here the hidden record
+/// carries the `-1` that releases the blob's last owner, and a visible record lands above it. The
+/// arithmetic fold reads the `-1`, so the in-degree reaches zero and the blob is actually reclaimed.
+///
+/// Listing-driven intake skips the `-1` and seals the cursor above it, so the blob keeps a phantom
+/// owner forever: not data loss, but an object no incremental round can ever reclaim. Asserting the
+/// blob DOES go away is also what stops the data-loss arm above from being satisfiable by a fold that
+/// simply never deletes anything.
+TEST(CASListLiarEndToEnd, AHiddenMinusOneIsStillFoldedSoTheBlobIsActuallyReclaimed)
+{
+    auto backend = std::make_shared<LiarBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/leak@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    const DB::UInt128 released(0xdea1);
+    const DB::UInt128 unrelated(0xb00c);
+
+    publishAt(*backend, layout, ns, RefTxnId{1, 1}, "ref_a", 1, released, /*birth=*/true);
+    dropAt(*backend, layout, ns, RefTxnId{1, 2}, "ref_a", publishedManifest(RefTxnId{1, 1}, 1));
+    /// A VISIBLE record above the hidden one. Without it the hidden id would be the stream's tail, and
+    /// a listing-driven walk would merely stop below it -- deferring the `-1` rather than sealing past
+    /// it, which is not the permanent damage this arm is about.
+    publishAt(*backend, layout, ns, RefTxnId{1, 3}, "ref_c", 3, unrelated);
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 3},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    backend->setListOmissions({layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 2})});
+
+    Gc gc(store, kGc);
+    const RoundEvidence condemning = runRoundCapturing(gc, UniversePolicy::Authoritative);
+    ASSERT_TRUE(condemning.report.acquired_lease);
+    ASSERT_GT(backend->holesServed(), 0u)
+        << "the omission was never actually served -- the test would pass vacuously";
+    expectNoAnomalies(condemning, "hidden -1");
+
+    EXPECT_EQ(sealedCursorOf(*backend, layout, ns), (RefTxnId{1, 3}));
+    EXPECT_EQ(inDegreeOf(*backend, layout, released), 0)
+        << "the hidden `-1` must be folded: nothing owns this blob any more";
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, released)).exists)
+        << "round pacing: the round that CONDEMNS never also deletes";
+
+    store->renewWatermarkOnce();
+    EXPECT_TRUE(runRoundsUntilAbsent(store, gc, *backend, layout, released))
+        << "the blob was never reclaimed -- the hidden `-1` left it pinned by a phantom owner";
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, unrelated)).exists)
+        << "and the still-owned blob is untouched";
+}
+
+/// ===================== THE CROSS-NAMESPACE SHOT =====================
+///
+/// The shape that is not about walking a single namespace: it is about a namespace the store hides in
+/// its ENTIRETY, never just one record inside it.
+///
+/// Two namespaces share a blob. `visible` publishes it and then drops it, so the round observes `+1`
+/// then `-1` and reads the blob's in-degree as zero. `hidden` also owns it -- durably, acked, readable
+/// by exact key -- but the store omits its ENTIRE ref stream, so no listing mentions it. `hidden`'s own
+/// publish still leaves it a real `_ckpt`, and a `_ckpt` is read by exact key, so the arithmetic walk's
+/// first probe finds and folds `hidden`'s `+1` regardless of what the listing omits: the blob survives
+/// on its own complete, folded frontier.
+///
+/// This is NOT a duplicate of `gtest_cas_gc_frontier_gate.cpp`'s twin: that file's backend hides only a
+/// hint prefix, while this one is the end-to-end LIST-liar backend from this file's own header -- the
+/// distinct thing this test proves is that arithmetic intake reads a record the backend actively hides
+/// from every enumeration, in the full pipeline (real pool, real recovery-shaped checkpoints), not that
+/// the gate's universe/count terms hold. `gtest_cas_gc_frontier_gate.cpp` owns those terms: its
+/// (3a)/(3b)/(3c) suppressor arms are what pin `universe_authoritative`, the empty-universe floor, and
+/// the probe budget -- terms this fixture cannot exercise, because grounding both namespaces here makes
+/// `frontier_namespaces > 0` and `universe_authoritative` true unconditionally.
+
+namespace
+{
+/// Build the shared-blob scenario and return the manifest `visible` will drop. Both namespaces are
+/// grounded with a real `_ckpt` reflecting what was actually published (`writeRecoverableCkptForRawFixture`,
+/// the idiom every other test in this file uses): otherwise neither namespace has a usable checkpoint at
+/// all, the round suppresses on that anomaly alone, and the scenario proves nothing about `hidden`
+/// specifically.
+ManifestRef buildKillShot(const std::shared_ptr<LiarBackend> & backend, const Layout & layout,
+                          const RootNamespace & hidden, const RootNamespace & visible,
+                          const DB::UInt128 & blob)
+{
+    publishAt(*backend, layout, hidden, RefTxnId{1, 1}, "kept_ref", 1, blob, /*birth=*/true);
+    writeRecoverableCkptForRawFixture(*backend, layout, hidden, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    publishAt(*backend, layout, visible, RefTxnId{1, 1}, "dropped_ref", 2, blob, /*birth=*/true);
+    const ManifestRef dropped = publishedManifest(RefTxnId{1, 1}, 2);
+    dropAt(*backend, layout, visible, RefTxnId{1, 2}, "dropped_ref", dropped);
+    writeRecoverableCkptForRawFixture(*backend, layout, visible, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    /// The whole of `hidden`'s ref stream goes invisible -- the namespace itself is what the listing
+    /// stops mentioning, not a record inside it. Its `_ckpt` stays readable by exact key, which is what
+    /// lets the arithmetic walk find and fold its birth despite the omission.
+    backend->setListOmissions({layout.refLogKey(fixture::fixtureLife(hidden), RefTxnId{1, 1}),
+                               layout.refCkptKey(fixture::fixtureLife(hidden))});
+    return dropped;
+}
+}
+
+/// Rounds on the PRODUCTION path -- no policy argument anywhere -- because that is the posture the
+/// claim is about: the arithmetic walk's exact-key probe reaches `hidden`'s birth despite the store
+/// hiding its whole stream from every listing, so the frontier it proves is complete and the blob
+/// survives on its own folded in-degree, not on a caller declining to supply a universe.
+TEST(CASListLiarEndToEnd, AHiddenNamespacesBirthIsFoundByExactKeyAndSavesTheBlobOnACompleteFrontier)
+{
+    auto backend = std::make_shared<LiarBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace hidden{"00/hidden@cas@"};
+    const RootNamespace visible{"00/visible@cas@"};
+    const DB::UInt128 blob(0x5ade);
+
+    buildKillShot(backend, layout, hidden, visible, blob);
+
+    Gc gc(store, kGc);
+    backend->resetCounts();
+    RoundEvidence evidence;
+    for (int i = 0; i < 5; ++i)
+    {
+        const RoundEvidence round = runRoundCapturing(gc, UniversePolicy::kDefault);
+        if (round.saw_fold)
+            evidence = round;
+        store->renewWatermarkOnce();
+    }
+
+    ASSERT_TRUE(evidence.saw_fold) << "no round folded, so none published a gate verdict";
+    ASSERT_GT(backend->holesServed(), 0u)
+        << "the omission was never actually served -- the test would pass vacuously";
+    EXPECT_TRUE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "the blob a hidden namespace still owns must survive";
+    EXPECT_EQ(backend->deleteCount(blobKeyOf(layout, blob)), 0u)
+        << "not merely still present: the blob must never even be offered for deletion";
+    EXPECT_TRUE(evidence.frontier_complete)
+        << "`hidden`'s own `_ckpt` is read by exact key, so its frontier is provable despite the "
+           "listing omission -- if this is false the blob above survived on suppression instead of on "
+           "its own in-degree, which proves nothing about the edge";
+    EXPECT_FALSE(evidence.suppress_destructive);
+}
+
+/// The arm above asserts "nothing was deleted", which on its own does not distinguish the gate correctly
+/// refusing from the round simply never deleting anything. Positive control:
+/// `hidden` drops its OWN reference too (still by exact key, still hidden from every listing), so its
+/// frontier is REALLY proven by the arithmetic-intake exact-key probe -- never declared so by fiat --
+/// and the blob is REALLY unreferenced by both namespaces. The round drains it.
+TEST(CASListLiarEndToEnd, TheSameBlobDrainsOnceHiddenGenuinelyProvesItsOwnFrontier)
+{
+    auto backend = std::make_shared<LiarBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace hidden{"00/hidden@cas@"};
+    const RootNamespace visible{"00/visible@cas@"};
+    const DB::UInt128 blob(0x5ade);
+
+    publishAt(*backend, layout, hidden, RefTxnId{1, 1}, "kept_ref", 1, blob, /*birth=*/true);
+    const ManifestRef kept = publishedManifest(RefTxnId{1, 1}, 1);
+    writeRecoverableCkptForRawFixture(*backend, layout, hidden, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    Gc gc(store, kGc);
+
+    /// `hidden`'s birth is folded (and its cursor SEALED) while everything is still listed. Its
+    /// checkpoint proves that exact initial frontier; the real fold then makes the arithmetic
+    /// (cursor-relative) genesis available for what follows.
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    store->renewWatermarkOnce();
+
+    /// NOW `hidden` drops its own reference, and ONLY THEN does its whole prefix vanish from LIST. With
+    /// a sealed cursor already in hand, the walk's genesis for `hidden` is arithmetic (`cursor + 1`), so
+    /// this drop is found and folded by exact key alone -- the arithmetic-intake mechanism this whole
+    /// file is about, exercised honestly rather than declared past by fiat.
+    dropAt(*backend, layout, hidden, RefTxnId{1, 2}, "kept_ref", kept);
+    advanceRecoverableCkptForRawFixture(*backend, layout, hidden, RefTxnId{1, 2});
+    backend->hidePrefix(layout.namespaceStreamPrefix(fixture::fixtureLife(hidden)));
+
+    publishAt(*backend, layout, visible, RefTxnId{1, 1}, "dropped_ref", 2, blob, /*birth=*/true);
+    const ManifestRef dropped = publishedManifest(RefTxnId{1, 1}, 2);
+    dropAt(*backend, layout, visible, RefTxnId{1, 2}, "dropped_ref", dropped);
+    writeRecoverableCkptForRawFixture(*backend, layout, visible, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    for (int i = 0; i < 5; ++i)
+    {
+        runRegularRoundReclaiming(gc);
+        store->renewWatermarkOnce();
+    }
+
+    ASSERT_GT(backend->holesServed(), 0u)
+        << "the omission was never actually served -- the test would pass vacuously";
+    EXPECT_FALSE(backend->head(blobKeyOf(layout, blob)).exists)
+        << "both namespaces genuinely proved their frontier and the blob is genuinely unreferenced -- "
+           "the round must still be able to reclaim it";
+}
+
+/// ===================== FSCK =====================
+///
+/// fsck runs two checkpoint-grounded passes over a namespace's ref stream.
+///
+///   * `checkRefStream` walks arithmetically by exact key. An omitted-but-durable record is a
+///     non-event to it, exactly as it is to the GC fold. That is the pass the arms below pin.
+///   * the reachability pass uses the same catalog row and exact `_ckpt` to recover the ref table
+///     without stream enumeration.
+///
+/// Both arms are written against an HONEST TWIN seeded by the same code, not against hand-written
+/// expectations: the claim is "identical to the truth", and a pass that quietly examined fewer records
+/// would satisfy a hand-written "clean" just as well.
+
+TEST(CASListLiarEndToEnd, FsckArithmeticStreamAuditIsUnmovedByAHiddenMiddle)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"00/fsck@cas@"};
+
+    auto honest_backend = std::make_shared<LiarBackend>();
+    seedFiveRecordStream(*honest_backend, layout, ns);
+    auto honest = openRecoveryPool(honest_backend);
+    const FsckReport truth = runFsck(*honest, /*detail=*/true);
+
+    auto lying_backend = std::make_shared<LiarBackend>();
+    seedFiveRecordStream(*lying_backend, layout, ns);
+    lying_backend->setListOmissions(hiddenMiddleOf(layout, ns));
+    auto lying = openRecoveryPool(lying_backend);
+    const FsckReport under_lie = runFsck(*lying, /*detail=*/true);
+
+    ASSERT_GT(lying_backend->holesServed(), 0u)
+        << "the omission was never actually served -- the test would pass vacuously";
+
+    EXPECT_TRUE(truth.clean()) << "the oracle itself must be clean, or the comparison means nothing";
+    EXPECT_GT(truth.ref_records_walked, 0u);
+    EXPECT_GT(truth.reachable, 0u)
+        << "the honest oracle must recover at least one live object, or reachability equality is vacuous";
+
+    /// The arithmetic pass: a hidden record is a non-event, and no finding is manufactured out of it.
+    EXPECT_TRUE(under_lie.clean())
+        << "fsck must not manufacture a finding out of an omitted-but-durable record";
+    EXPECT_EQ(under_lie.chain_broken, 0u)
+        << "a record the listing hid is NOT a broken chain: the walk reads it by exact key";
+    EXPECT_EQ(under_lie.dangling, 0u);
+    EXPECT_EQ(under_lie.ref_records_walked, truth.ref_records_walked)
+        << "the arithmetic walk must read the SAME number of records under the lie";
+
+    EXPECT_EQ(under_lie.unchecked, 0u)
+        << "an omitted durable record must not turn a healthy checkpoint-bounded namespace unchecked";
+    EXPECT_EQ(under_lie.reachable, truth.reachable)
+        << "the reachability recovery must observe the same exact committed frontier under the lie";
+}
+
+/// A hidden tail record is the silent variant of the historical residual: a LIST-driven replay could
+/// return a plausible but short table. Checkpoint-bounded recovery must produce the honest table even
+/// though the list omission is served.
+TEST(CASListLiarEndToEnd, FsckReachabilityRecoveryMatchesTruthUnderAHiddenTailTransaction)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"00/fsck_tail@cas@"};
+    /// Stage B (Task 4-C): no pin needed -- `seedFiveRecordStream` below calls `publishAt` (draining
+    /// into `writeRefLogTxnRaw`), which admits `ns` into each of the two independent backends' own
+    /// catalogs itself.
+
+    auto honest_backend = std::make_shared<LiarBackend>();
+    seedFiveRecordStream(*honest_backend, layout, ns);
+    auto honest = openRecoveryPool(honest_backend);
+    const FsckReport truth = runFsck(*honest, /*detail=*/true);
+
+    auto lying_backend = std::make_shared<LiarBackend>();
+    seedFiveRecordStream(*lying_backend, layout, ns);
+    lying_backend->setListOmissions({layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 5})});
+    auto lying = openRecoveryPool(lying_backend);
+    const FsckReport under_lie = runFsck(*lying, /*detail=*/true);
+
+    ASSERT_GT(lying_backend->holesServed(), 0u)
+        << "the omission was never actually served -- the test would pass vacuously";
+    EXPECT_GT(truth.reachable, 0u)
+        << "the honest oracle must recover at least one live object, or reachability equality is vacuous";
+
+    /// The arithmetic pass is unmoved here too: it probes `{1,5}` by exact key and finds it.
+    EXPECT_EQ(under_lie.chain_broken, 0u);
+    EXPECT_EQ(under_lie.ref_records_walked, truth.ref_records_walked)
+        << "the arithmetic walk reads the hidden tail by exact key, so it counts the same records";
+
+    EXPECT_EQ(under_lie.unchecked, 0u)
+        << "a LIST omission must not make a checkpoint-bounded namespace unchecked";
+    EXPECT_EQ(under_lie.reachable, truth.reachable)
+        << "the exact committed frontier must include the hidden tail transaction";
+}
diff --git a/src/Disks/tests/gtest_cas_manifest_id.cpp b/src/Disks/tests/gtest_cas_manifest_id.cpp
new file mode 100644
index 000000000000..8be89457de93
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_manifest_id.cpp
@@ -0,0 +1,86 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <map>
+#include <set>
+#include <unordered_map>
+#include <unordered_set>
+
+using namespace DB::Cas;
+
+namespace
+{
+
+ManifestRef ref(uint64_t w, uint64_t seq, uint64_t m)
+{
+    return ManifestRef{w, seq, static_cast<uint32_t>(m)};
+}
+
+ManifestId id(const char * ns, uint64_t w, uint64_t seq, uint64_t m)
+{
+    return ManifestId{RootNamespace(ns), ref(w, seq, m)};
+}
+
+}
+
+TEST(CASManifestId, RefEqualityAndOrdering)
+{
+    EXPECT_EQ(ref(1, 2, 3), ref(1, 2, 3));
+    EXPECT_NE(ref(1, 2, 3), ref(1, 2, 4));
+    /// Strict total order: distinct by manifest_ordinal, then build_sequence, then writer_epoch.
+    EXPECT_LT(ref(1, 2, 3), ref(1, 2, 4));
+    EXPECT_LT(ref(1, 2, 9), ref(1, 3, 0));
+    EXPECT_LT(ref(1, 9, 9), ref(2, 0, 0));
+    EXPECT_FALSE(ref(1, 2, 3) < ref(1, 2, 3));
+}
+
+TEST(CASManifestId, IdIsNamespaceQualified)
+{
+    /// Same ref tuple, different namespace => DIFFERENT ids (the SabotageKeyByRefNotId guard).
+    EXPECT_NE(id("nsA", 1, 1, 1), id("nsB", 1, 1, 1));
+    EXPECT_EQ(id("nsA", 1, 1, 1), id("nsA", 1, 1, 1));
+    /// Ordering separates by namespace first.
+    EXPECT_LT(id("nsA", 9, 9, 9), id("nsB", 0, 0, 0));
+}
+
+TEST(CASManifestId, UsableAsMapAndSetKey)
+{
+    std::set<ManifestId> s;
+    s.insert(id("nsA", 1, 1, 1));
+    s.insert(id("nsB", 1, 1, 1));   /// distinct namespace -> distinct key
+    s.insert(id("nsA", 1, 1, 1));   /// duplicate -> no growth
+    EXPECT_EQ(s.size(), 2u);
+
+    std::map<ManifestRef, int> m;
+    m[ref(1, 1, 1)] = 10;
+    m[ref(1, 1, 2)] = 20;
+    EXPECT_EQ(m.size(), 2u);
+    EXPECT_EQ(m[ref(1, 1, 1)], 10);
+}
+
+TEST(CASManifestId, UsableInUnorderedContainers)
+{
+    /// std::hash<ManifestRef> / std::hash<ManifestId> let the read-path cache (Phase 1c) and GC use
+    /// unordered_map/set. Equal values => equal hash; distinct values => (overwhelmingly) distinct.
+    std::unordered_set<ManifestId> s;
+    s.insert(id("nsA", 1, 1, 1));
+    s.insert(id("nsB", 1, 1, 1));   /// distinct namespace -> distinct key
+    s.insert(id("nsA", 1, 1, 1));   /// duplicate -> no growth
+    EXPECT_EQ(s.size(), 2u);
+
+    std::unordered_map<ManifestRef, int> m;
+    m[ref(1, 1, 1)] = 10;
+    m[ref(1, 1, 1)] = 11;           /// same key overwrites
+    m[ref(1, 1, 2)] = 20;
+    EXPECT_EQ(m.size(), 2u);
+    EXPECT_EQ(m.at(ref(1, 1, 1)), 11);
+
+    EXPECT_EQ(std::hash<ManifestId>{}(id("nsA", 1, 1, 1)), std::hash<ManifestId>{}(id("nsA", 1, 1, 1)));
+}
+
+TEST(CASManifestId, ManifestOrdinalFileName)
+{
+    EXPECT_EQ(manifestOrdinalFileName(1), "000001.zst");
+    EXPECT_EQ(manifestOrdinalFileName(999999), "999999.zst");
+    EXPECT_THROW(manifestOrdinalFileName(0), DB::Exception);
+    EXPECT_THROW(manifestOrdinalFileName(1000000), DB::Exception);
+}
diff --git a/src/Disks/tests/gtest_cas_mount.cpp b/src/Disks/tests/gtest_cas_mount.cpp
new file mode 100644
index 000000000000..0d61f91206e8
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_mount.cpp
@@ -0,0 +1,1602 @@
+#include <gtest/gtest.h>
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Common/ProfileEvents.h>
+
+#include <chrono>
+#include <limits>
+#include <map>
+#include <string>
+
+namespace DB::ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+    extern const int CORRUPTED_DATA;
+    extern const int FILE_DOESNT_EXIST;
+    extern const int LOGICAL_ERROR;
+}
+
+namespace ProfileEvents
+{
+    extern const Event CASMountLeaseLost;
+    extern const Event CASMountExclusivityViolation;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+const ObserveRefCatalog & emptyCatalogObservation()
+{
+    static const ObserveRefCatalog observe = [] { return RefCatalog{}; };
+    return observe;
+}
+
+RefCatalog catalogOwning(const String & ns, NsState state)
+{
+    CatalogEntry entry{.ns = RootNamespace{ns}, .state = state, .incarnation = UInt128{42}};
+    if (state == NsState::Creating)
+        entry.creator = CreatorFence{.server_root_id = "root/x", .writer_epoch = 1, .fence_generation = 1};
+    return RefCatalog{.entries = {std::move(entry)}};
+}
+
+class OwnerConflictRevealsManifestBackend : public InMemoryBackend
+{
+public:
+    using InMemoryBackend::putIfAbsent;
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (!fired && key == "p/gc/server-roots/root/x/owner")
+        {
+            fired = true;
+            InMemoryBackend::putIfAbsent("p/cas/manifests/root/x/table/debris", "x");
+            return {PutOutcome::PreconditionFailed, {}};
+        }
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    bool fired = false;
+};
+
+class EpochConflictRevealsManifestBackend : public InMemoryBackend
+{
+public:
+    using InMemoryBackend::casPut;
+
+    CasResult casPut(
+        const String & key, const String & bytes, const std::optional<Token> & expected,
+        const ObjectMeta & meta) override
+    {
+        if (!fired && key == "p/gc/server-roots/root/x/epoch")
+        {
+            fired = true;
+            /// Install the competing allocator's winning epoch before revealing owned work. The
+            /// retry must not accept that now-present epoch without rechecking the entire emptiness
+            /// bundle that authorized the original absent-epoch attempt.
+            const CasResult winner = InMemoryBackend::casPut(
+                key, encodeServerEpoch(ServerEpoch{.next_writer_epoch = 2}), expected, meta);
+            winner_installed = winner.outcome == CasOutcome::Committed;
+            InMemoryBackend::putIfAbsent("p/cas/manifests/root/x/table/debris", "x");
+            return {CasOutcome::Conflict, {}};
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+
+    bool fired = false;
+    bool winner_installed = false;
+};
+
+}
+
+TEST(CASServerRootId, ValidationAcceptsCleanPathsRejectsBad)
+{
+    EXPECT_NO_THROW(validateServerRootId("replica-a"));
+    EXPECT_NO_THROW(validateServerRootId("shard-01/replica-a"));
+    EXPECT_THROW(validateServerRootId(""), DB::Exception);
+    EXPECT_THROW(validateServerRootId("/replica"), DB::Exception);
+    EXPECT_THROW(validateServerRootId("replica/"), DB::Exception);
+    EXPECT_THROW(validateServerRootId("a//b"), DB::Exception);
+    EXPECT_THROW(validateServerRootId("a/../b"), DB::Exception);
+    EXPECT_THROW(validateServerRootId("a/_files/b"), DB::Exception);
+}
+
+TEST(CASServerRoot, KeysAndCodecsRoundTrip)
+{
+    Layout layout("p");
+
+    /// Layout keys under gc/server-roots/<srid>/.
+    EXPECT_EQ(layout.serverRootPrefix("replica-a"), "p/gc/server-roots/replica-a/");
+    EXPECT_EQ(layout.ownerKey("replica-a"), "p/gc/server-roots/replica-a/owner");
+    EXPECT_EQ(layout.epochKey("replica-a"), "p/gc/server-roots/replica-a/epoch");
+    EXPECT_EQ(layout.mountKey("replica-a"), "p/gc/server-roots/replica-a/mount");
+
+    /// Owner round-trip.
+    {
+        OwnerObject o;
+        o.server_uuid = (UInt128(0x0123456789abcdefULL) << 64) | UInt128(0xfedcba9876543210ULL);
+        const OwnerObject back = decodeOwner(encodeOwner(o));
+        EXPECT_EQ(back.server_uuid, o.server_uuid);
+    }
+
+    /// ServerEpoch round-trip.
+    {
+        ServerEpoch e;
+        e.next_writer_epoch = 4242;
+        const ServerEpoch back = decodeServerEpoch(encodeServerEpoch(e));
+        EXPECT_EQ(back.next_writer_epoch, e.next_writer_epoch);
+    }
+
+    /// MountLease round-trip.
+    {
+        MountLease m;
+        m.server_uuid = (UInt128(0xdeadbeefcafef00dULL) << 64) | UInt128(0x0011223344556677ULL);
+        m.writer_epoch = 7;
+        m.hostname = "host-1.example.com";
+        m.pid = 12345;
+        m.started_at_ms = 1700000000000ULL;
+        m.seq = 99;
+        m.expires_at_ms = 1700000030000ULL;
+        const MountLease back = decodeMountLease(encodeMountLease(m));
+        EXPECT_EQ(back.server_uuid, m.server_uuid);
+        EXPECT_EQ(back.writer_epoch, m.writer_epoch);
+        EXPECT_EQ(back.hostname, m.hostname);
+        EXPECT_EQ(back.pid, m.pid);
+        EXPECT_EQ(back.started_at_ms, m.started_at_ms);
+        EXPECT_EQ(back.seq, m.seq);
+        EXPECT_EQ(back.expires_at_ms, m.expires_at_ms);
+    }
+
+    /// Fail-closed decode on garbage bytes.
+    EXPECT_THROW(decodeOwner("not-a-proto-with-magic"), DB::Exception);
+    EXPECT_THROW(decodeServerEpoch(""), DB::Exception);
+    EXPECT_THROW(decodeMountLease(""), DB::Exception);
+}
+
+TEST(CASServerRootClaim, OwnerStickyAndForeignFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    EXPECT_NO_THROW(claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation()));     // fresh empty root → claim
+    EXPECT_NO_THROW(claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation()));     // same uuid → ok
+    EXPECT_THROW(claimOwnerOrThrow(*b, l, "r", UInt128(2), emptyCatalogObservation()), DB::Exception);  // foreign → fail closed
+}
+
+TEST(CASServerRootClaim, TombstonedSameOwnerFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    b->putIfAbsent(l.ownerKey("r"), encodeOwner(OwnerObject{
+        .server_uuid = UInt128(1),
+        .retired_at_ms = 1752537600000ULL,
+    }));
+
+    try
+    {
+        claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation());
+        FAIL() << "expected a tombstoned owner claim to fail closed";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+        EXPECT_NE(e.message().find("decommissioned"), String::npos) << e.message();
+        EXPECT_EQ(e.message().find("owned by a different server"), String::npos) << e.message();
+    }
+}
+
+TEST(CASServerRootEpoch, AllocatorIsMonotoneAndSurvivesMountConcept)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("r");
+    claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation());
+    const uint64_t e1 = allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation());
+    const uint64_t e2 = allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation());
+    EXPECT_GE(e1, 1u);                                             // 0 is a reserved sentinel
+    EXPECT_GT(e2, e1);                                             // strictly increasing
+
+    /// Deleting the (separate) mount object must NOT reset the epoch. No mount has been written in
+    /// Task 4, so deleteExact of a non-existent mount is a NotFound no-op that touches nothing.
+    const auto del = b->deleteExact(l.mountKey("r"), b->head(l.mountKey("r")).token);
+    EXPECT_EQ(del.kind, DeleteOutcome::Kind::NotFound);
+    EXPECT_GT(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), e2);
+}
+
+/// Phase C (spec rev.4): an ABSENT epoch object over a PRESENT mount object means durable epoch
+/// state was lost while a mount is live/recent — re-minting epoch 1 there is how a same-(uuid,
+/// epoch) twin is born. Refuse.
+TEST(CASMount, EpochRemintOverExistingMountRefuses)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation());
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), /*our_epoch=*/1, /*now_ms=*/1000, /*ttl_ms=*/30000).kind,
+              MountClaimResult::Claimed);
+    /// The epoch object is ABSENT (never created in this sequence) while the mount exists:
+    EXPECT_THROW(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), DB::Exception);   /// CORRUPTED_DATA
+}
+
+TEST(CASMount, EpochRemintAuthoritativeAbsenceMints)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation());
+    EXPECT_EQ(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), 1u);   /// fresh root: both control objects absent
+    EXPECT_EQ(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), 2u);   /// epoch present now: normal CAS bump, no probe
+}
+
+/// The probe outcome gates the mint: anything short of authoritative KeyAbsent fails closed.
+TEST(CASMount, EpochRemintIndeterminateProbeFailsClosed)
+{
+    class IndeterminateProbeBackend final : public InMemoryBackend
+    {
+    public:
+        SentinelProbeResult probeSentinelRaw(const String &) override
+        {
+            return {.outcome = ProbeOutcome::Indeterminate, .body = std::nullopt};
+        }
+    };
+    auto b = std::make_shared<IndeterminateProbeBackend>();
+    Layout l("p");
+    claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation());
+    EXPECT_THROW(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), DB::Exception);
+}
+
+/// Decommission over a TERMINAL (expired/fenced) mount with a lost epoch object proceeds and mints
+/// an epoch DISTINCT from the surviving mount's — the same-pair state is unrepresentable.
+TEST(CASMount, DecommissionRemintOverTerminalMountMintsDistinctEpoch)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation());
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), /*our_epoch=*/3, /*now_ms=*/1000, /*ttl_ms=*/100).kind,
+              MountClaimResult::Claimed);
+    /// now_ms=5000: the ttl_ms=100 lease above is long expired -> terminal.
+    EXPECT_EQ(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::DecommissionRecovery, /*now_ms=*/5000, emptyCatalogObservation()), 4u);
+}
+
+/// Decommission over a LIVE mount with a lost epoch refuses — the blind bypass would recreate the
+/// forbidden pair (codex round-3 finding 1) and defeat CASDecommission.RefusesLiveMember.
+TEST(CASMount, DecommissionRemintOverLiveMountRefuses)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation());
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), /*our_epoch=*/1, /*now_ms=*/1000, /*ttl_ms=*/30000).kind,
+              MountClaimResult::Claimed);
+    EXPECT_THROW(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::DecommissionRecovery, /*now_ms=*/2000, emptyCatalogObservation()),
+                 DB::Exception);   /// ABORTED: live member
+}
+
+/// The steady-state path (epoch object PRESENT) must never pay the probe — pins the zero
+/// normal-path cost the spec claims.
+TEST(CASMount, EpochBumpWithPresentEpochIssuesNoProbe)
+{
+    class ProbeCountingBackend final : public InMemoryBackend
+    {
+    public:
+        int probes = 0;
+        SentinelProbeResult probeSentinelRaw(const String & k) override
+        {
+            ++probes;
+            return InMemoryBackend::probeSentinelRaw(k);
+        }
+    };
+    auto b = std::make_shared<ProbeCountingBackend>();
+    Layout l("p");
+    claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation());
+    EXPECT_EQ(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), 1u);   /// bootstrap: ONE probe (absent-epoch branch)
+    const int probes_after_bootstrap = b->probes;
+    EXPECT_EQ(allocateWriterEpoch(*b, l, "r", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), 2u);   /// epoch present: normal CAS bump...
+    EXPECT_EQ(b->probes, probes_after_bootstrap) << "...must not probe the mount key";
+}
+
+TEST(CASServerRootClaim, MissingOwnerOverNonEmptyRootIsCorrupted)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    /// Simulate existing data without an owner (identity lost): plant a key under roots/<srid>/.
+    b->putIfAbsent(l.serverRootDataPrefix("r") + "some-data", "x");
+    EXPECT_THROW(claimOwnerOrThrow(*b, l, "r", UInt128(1), emptyCatalogObservation()), DB::Exception);
+}
+
+TEST(CASServerRootSafety, EveryCatalogLifecycleStateBlocksOwnerAndEpochRecreation)
+{
+    const Layout layout("p");
+    for (const NsState state : {NsState::Creating, NsState::Live, NsState::Removing})
+    {
+        RefCatalog catalog = catalogOwning("root/x/table", state);
+        const ObserveRefCatalog observe = [catalog] { return catalog; };
+
+        InMemoryBackend owner_backend;
+        EXPECT_THROW(claimOwnerOrThrow(owner_backend, layout, "root/x", UInt128{1}, observe), DB::Exception);
+        EXPECT_FALSE(owner_backend.head(layout.ownerKey("root/x")).exists);
+
+        InMemoryBackend epoch_backend;
+        EXPECT_THROW(allocateWriterEpoch(
+            epoch_backend, layout, "root/x", EpochMintPolicy::NormalMount, 0, observe), DB::Exception);
+        EXPECT_FALSE(epoch_backend.head(layout.epochKey("root/x")).exists);
+    }
+}
+
+TEST(CASServerRootSafety, OwnershipUsesAPathComponentBoundary)
+{
+    InMemoryBackend backend;
+    const Layout layout("p");
+    EXPECT_TRUE(serverRootSubtreeEmpty(
+        backend, layout, "root/x", catalogOwning("root/xy/table", NsState::Live)));
+    EXPECT_FALSE(serverRootSubtreeEmpty(
+        backend, layout, "root/x", catalogOwning("root/x/table", NsState::Live)));
+}
+
+TEST(CASServerRootSafety, OpaqueStreamAndStateDebrisAloneDoesNotBlockRecreation)
+{
+    InMemoryBackend backend;
+    const Layout layout("p");
+    const NamespaceLifeId dead = NamespaceLifeId::fromCatalogEntry(RootNamespace{"unowned"}, UInt128{99});
+    ASSERT_EQ(backend.putIfAbsent(layout.refLogKey(dead, RefTxnId{1, 1}), "debris").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(layout.refCkptKey(dead), "debris").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(layout.namespaceFileKey(dead, "f"), "debris").outcome, PutOutcome::Done);
+
+    EXPECT_NO_THROW(claimOwnerOrThrow(backend, layout, "root/x", UInt128{1}, emptyCatalogObservation()));
+    EXPECT_EQ(allocateWriterEpoch(
+        backend, layout, "root/x", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), 1u);
+}
+
+TEST(CASServerRootSafety, ManifestAndLooseRootDebrisStillBlockRecreation)
+{
+    const Layout layout("p");
+    for (const String & key : {
+             layout.casManifestsServerPrefix("root/x") + "table/debris",
+             layout.serverRootDataPrefix("root/x") + "loose"})
+    {
+        InMemoryBackend backend;
+        ASSERT_EQ(backend.putIfAbsent(key, "x").outcome, PutOutcome::Done);
+        EXPECT_THROW(claimOwnerOrThrow(
+            backend, layout, "root/x", UInt128{1}, emptyCatalogObservation()), DB::Exception);
+        EXPECT_THROW(allocateWriterEpoch(
+            backend, layout, "root/x", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), DB::Exception);
+    }
+}
+
+TEST(CASServerRootSafety, UnreadableCatalogNeverFallsBackToPhysicalGuesses)
+{
+    InMemoryBackend backend;
+    const Layout layout("p");
+    const ObserveRefCatalog unreadable = []() -> RefCatalog
+    {
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA, "injected unreadable catalog");
+    };
+    EXPECT_THROW(claimOwnerOrThrow(backend, layout, "root/x", UInt128{1}, unreadable), DB::Exception);
+    EXPECT_THROW(allocateWriterEpoch(
+        backend, layout, "root/x", EpochMintPolicy::NormalMount, 0, unreadable), DB::Exception);
+    EXPECT_FALSE(backend.head(layout.ownerKey("root/x")).exists);
+    EXPECT_FALSE(backend.head(layout.epochKey("root/x")).exists);
+}
+
+TEST(CASServerRootSafety, OwnerConflictRecomputesTheWholeEmptinessBundle)
+{
+    OwnerConflictRevealsManifestBackend backend;
+    const Layout layout("p");
+    EXPECT_THROW(claimOwnerOrThrow(
+        backend, layout, "root/x", UInt128{1}, emptyCatalogObservation()), DB::Exception);
+    EXPECT_TRUE(backend.fired);
+    EXPECT_FALSE(backend.head(layout.ownerKey("root/x")).exists);
+}
+
+TEST(CASServerRootSafety, EpochConflictRecomputesTheWholeEmptinessBundle)
+{
+    EpochConflictRevealsManifestBackend backend;
+    const Layout layout("p");
+    EXPECT_THROW(allocateWriterEpoch(
+        backend, layout, "root/x", EpochMintPolicy::NormalMount, 0, emptyCatalogObservation()), DB::Exception);
+    EXPECT_TRUE(backend.fired);
+    ASSERT_TRUE(backend.winner_installed);
+    const auto epoch = backend.get(layout.epochKey("root/x"));
+    ASSERT_TRUE(epoch.has_value());
+    EXPECT_EQ(decodeServerEpoch(epoch->bytes).next_writer_epoch, 2u)
+        << "the rejected allocator must not consume an epoch from the conflict winner";
+}
+
+TEST(CASMountLease, AbsentClaimThenRenewBumpsSeq)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    uint64_t now = 1000;
+    auto r = claimMount(*b, l, "r", UInt128(1), /*epoch*/ 7, now, /*ttl*/ 100);
+    EXPECT_EQ(r.kind, MountClaimResult::Claimed);
+    MountLeaseKeeper k(b, l, "r", UInt128(1), 7, std::chrono::milliseconds(100), [&] { return now; },
+                       [] { return uint64_t{0}; });
+    k.start();
+    EXPECT_EQ(decodeMountLease(b->get(l.mountKey("r"))->bytes).seq, 1u);
+    k.renewOnce();
+    EXPECT_EQ(decodeMountLease(b->get(l.mountKey("r"))->bytes).seq, 2u);
+}
+
+/// STID 3982-3b48: `rm -rf` of the pool dir under a live mount deletes the mount slot object out from
+/// under a running keeper. The next background renewal must fail closed (stop renewing, latch the
+/// write fence to lost) WITHOUT constructing a `LOGICAL_ERROR` -- that aborts debug/ASan builds at
+/// exception construction, and there is no foreign writer here to fail closed against, only an
+/// environmental condition.
+TEST(CASMountLease, VanishedBackingStoreStopsRenewalWithoutLogicalError)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    uint64_t now = 1000;
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), /*epoch*/ 7, now, /*ttl*/ 100).kind, MountClaimResult::Claimed);
+    MountLeaseKeeper k(b, l, "r", UInt128(1), 7, std::chrono::milliseconds(100), [&] { return now; },
+                       [] { return uint64_t{0}; });
+    k.start();
+
+    const String mount_key = l.mountKey("r");
+    const auto lost_before = ProfileEvents::global_counters[ProfileEvents::CASMountLeaseLost].load();  /// NOLINT(clang-analyzer-deadcode.DeadStores)
+
+    /// Simulate `rm -rf` of the backing store: the mount slot object is gone, but the keeper still
+    /// holds a (now stale) token for it.
+    ASSERT_EQ(b->deleteExact(mount_key, b->head(mount_key).token).kind, DeleteOutcome::Kind::Deleted);
+
+    try
+    {
+        k.renewOnce();
+        FAIL() << "renew against a vanished mount object must throw";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::FILE_DOESNT_EXIST) << e.message();
+        EXPECT_NE(e.code(), DB::ErrorCodes::LOGICAL_ERROR);
+    }
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMountLeaseLost].load(), lost_before + 1);
+}
+
+/// STID 3982-3b48 (part 1b): the terminal/clean-release counterpart to the renewal fix above. When
+/// the backing store vanishes (`rm -rf` of the pool dir), the renewal side already stops non-fatally
+/// (see the previous test); teardown then runs the terminal release (`stop()` -> `terminate()`),
+/// which used to unconditionally throw `LOGICAL_ERROR` once the token-guarded farewell PUT observed
+/// an absent object. The desired end state of a release ("no live lease object") is already true, so
+/// this must be a no-op, never a `LOGICAL_ERROR` (which aborts debug/ASan builds).
+///
+/// Driven WITHOUT a prior failed renew, so the count is deterministic: this is the only place along
+/// this path that increments `CASMountLeaseLost`, so we expect exactly +1 (not +2, since renewal was
+/// never invoked here).
+TEST(CASMountLease, TerminateAfterVanishedBackingStoreIsNoOpRelease)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    uint64_t now = 1000;
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), /*epoch*/ 7, now, /*ttl*/ 100).kind, MountClaimResult::Claimed);
+    MountLeaseKeeper k(b, l, "r", UInt128(1), 7, std::chrono::milliseconds(100), [&] { return now; },
+                       [] { return uint64_t{0}; });
+    k.start();
+
+    const String mount_key = l.mountKey("r");
+    const auto lost_before = ProfileEvents::global_counters[ProfileEvents::CASMountLeaseLost].load();
+
+    /// Simulate `rm -rf` of the backing store: the mount slot object is gone before we ever attempt
+    /// a renewal, so `terminate()`'s token-guarded farewell PUT is the first thing to observe it.
+    ASSERT_EQ(b->deleteExact(mount_key, b->head(mount_key).token).kind, DeleteOutcome::Kind::Deleted);
+
+    EXPECT_NO_THROW(k.stop())
+        << "clean release against a vanished store must be a no-op, not a LOGICAL_ERROR abort";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMountLeaseLost].load(), lost_before + 1);
+}
+
+/// rev.6: a bare `claimMount` (no `proven_dead_token`) NEVER reclaims a same-uuid, different-epoch
+/// lease off a wall-clock-looking-expired stamp — only `claimMountAwaitingExpiry`'s observation loop
+/// can turn that into a reclaim. Renamed from `...ExpiredReclaims` to describe the corrected behavior.
+TEST(CASMountLease, SameUuidLiveFailsForeignFailsExpiredStillLiveDoubleStart)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    claimMount(*b, l, "r", UInt128(1), 7, /*now*/ 1000, /*ttl*/ 100);    // A live until 1100
+    // same uuid, lease still live → double-start guard:
+    EXPECT_EQ(claimMount(*b, l, "r", UInt128(1), 8, 1050, 100).kind, MountClaimResult::LiveDoubleStart);
+    // foreign uuid, even after expiry → fail closed:
+    EXPECT_EQ(claimMount(*b, l, "r", UInt128(2), 1, 1200, 100).kind, MountClaimResult::ForeignOwner);
+    // same uuid, even after the stamp LOOKS expired on our wall clock → still LiveDoubleStart: no
+    // proven_dead_token was supplied, so there is no certificate of death to reclaim on.
+    EXPECT_EQ(claimMount(*b, l, "r", UInt128(1), 9, 1200, 100).kind, MountClaimResult::LiveDoubleStart);
+}
+
+TEST(CASMountMessage, DoubleStartTextHasIdentityAndRemediation)
+{
+    MountLease m;
+    m.server_uuid = (UInt128(0xdeadbeefcafef00dULL) << 64) | UInt128(0x0011223344556677ULL);
+    m.writer_epoch = 7;
+    m.hostname = "host-9.example.com";
+    m.pid = 4242;
+    m.seq = 13;
+    m.expires_at_ms = 1700000030000ULL;
+
+    const std::string msg = mountDoubleStartMessage("replica-a", m);
+
+    /// Identity / existing-holder fields.
+    EXPECT_NE(msg.find("server_root_id"), std::string::npos);
+    EXPECT_NE(msg.find("'replica-a'"), std::string::npos);
+    EXPECT_NE(msg.find("hostname=host-9.example.com"), std::string::npos);
+    EXPECT_NE(msg.find("pid=4242"), std::string::npos);
+    EXPECT_NE(msg.find("last_seq=13"), std::string::npos);
+    EXPECT_NE(msg.find("expires_at_ms=1700000030000"), std::string::npos);
+    /// New wait-aware remediation (this server already waited; the lease kept being renewed).
+    EXPECT_NE(msg.find("waited"), std::string::npos);
+    EXPECT_NE(msg.find("unique"), std::string::npos);
+    EXPECT_NE(msg.find("reclaim the mount on restart"), std::string::npos);
+    EXPECT_NE(msg.find("uuid file"), std::string::npos);
+    /// Clock-skew caveat + manual mount-object delete escape hatch.
+    EXPECT_NE(msg.find("CLOCK SKEW"), std::string::npos);
+    EXPECT_NE(msg.find("NTP"), std::string::npos);
+    EXPECT_NE(msg.find("manually delete the mount"), std::string::npos);
+    EXPECT_NE(msg.find("gc/server-roots/replica-a/mount"), std::string::npos);
+}
+
+/// rev.6: a stamped `expires_at_ms` that already looks past-due on our wall clock must NOT shortcut
+/// the observation wait — the old "instant, zero-sleep" reclaim this test name described was exactly
+/// the cross-node wall-clock trust rev.6 removes. Renamed to describe the CORRECTED behavior: the
+/// wall-clock-looking-expired stamp buys nothing, the full threshold is still observed.
+TEST(CASMountAwaitExpiry, PastExpiryStillPaysTheFullObservationThreshold)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    /// A prior incarnation (uuid=1, epoch=7) claimed a lease live until 1100.
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), 7, /*now*/ 1000, /*ttl*/ 100).kind, MountClaimResult::Claimed);
+
+    uint64_t wall = 1200;                // already past 1100 on wall clock — irrelevant to the decision
+    uint64_t mono = 0;
+    int sleeps = 0;
+    auto now_fn = [&] { return wall; };
+    auto mono_fn = [&] { return mono; };
+    auto sleep_fn = [&](uint64_t ms) { wall += ms; mono += ms; ++sleeps; };
+
+    const auto r = claimMountAwaitingExpiry(
+        *b, l, "r", UInt128(1), /*our_epoch*/ 8, now_fn, mono_fn, /*ttl*/ 100, /*poll*/ 25, sleep_fn);
+    EXPECT_EQ(r.kind, MountClaimResult::Claimed);
+    EXPECT_GT(sleeps, 0);                                    // NOT instant — no wall-clock trust
+    EXPECT_GE(mono, 100 + 100 / 20 + 25);                     // full observation threshold paid
+    EXPECT_EQ(decodeMountLease(b->get(l.mountKey("r"))->bytes).writer_epoch, 8u);   // reclaimed as us
+}
+
+TEST(CASMountAwaitExpiry, FutureExpiryReclaimsAfterClockAdvances)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), 7, /*now*/ 1000, /*ttl*/ 100).kind, MountClaimResult::Claimed);
+
+    uint64_t wall = 1000;                // lease looks live until 1100, holder does NOT renew
+    uint64_t mono = 0;
+    auto now_fn = [&] { return wall; };
+    auto mono_fn = [&] { return mono; };
+    auto sleep_fn = [&](uint64_t ms) { wall += ms; mono += ms; };
+
+    const auto r = claimMountAwaitingExpiry(
+        *b, l, "r", UInt128(1), /*our_epoch*/ 8, now_fn, mono_fn, /*ttl*/ 100, /*poll*/ 50, sleep_fn);
+    EXPECT_EQ(r.kind, MountClaimResult::Claimed);
+    const auto body = decodeMountLease(b->get(l.mountKey("r"))->bytes);
+    EXPECT_EQ(body.writer_epoch, 8u);
+    EXPECT_EQ(body.seq, 2u);                                         // reclaim continues seq (prev 1 + 1)
+}
+
+/// rev.6: a genuinely live twin now times out via BOUNDED OBSERVATION RESTARTS (its every renewal
+/// bumps the write-token, forcing a restart each poll), never via a wall-clock deadline.
+TEST(CASMountAwaitExpiry, LiveRenewingTwinTimesOutAsDoubleStart)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), 7, /*now*/ 1000, /*ttl*/ 100).kind, MountClaimResult::Claimed);
+
+    uint64_t wall = 1000;
+    uint64_t mono = 0;
+    auto now_fn = [&] { return wall; };
+    auto mono_fn = [&] { return mono; };
+    /// Each poll: both clocks advance AND the live holder (uuid=1, epoch=7) renews its own lease —
+    /// the observed write-token changes on EVERY poll, forcing a restart every time.
+    auto sleep_fn = [&](uint64_t ms)
+    {
+        wall += ms;
+        mono += ms;
+        ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), 7, wall, 100).kind, MountClaimResult::Claimed);
+    };
+
+    const auto r = claimMountAwaitingExpiry(
+        *b, l, "r", UInt128(1), /*our_epoch*/ 8, now_fn, mono_fn, /*ttl*/ 100, /*poll*/ 20, sleep_fn);
+    EXPECT_EQ(r.kind, MountClaimResult::LiveDoubleStart);
+    EXPECT_EQ(decodeMountLease(b->get(l.mountKey("r"))->bytes).writer_epoch, 7u);   // still the holder's
+}
+
+namespace
+{
+/// fix-round F5 harness: makes the mount key vanish to EVERY `get()`, unconditionally, while the real
+/// underlying object stays put -- forcing `claimMount`'s own internal GET to take the absent-slot race
+/// branch every call (its `putIfAbsent` then fails against the real, still-present object, returning
+/// `LiveDoubleStart` with no token -- fix-round F8 leaves `.token` unset on exactly this branch, since
+/// no re-read was done). That in turn forces `claimMountAwaitingExpiry`'s F8 fallback re-GET, which
+/// ALSO sees the slot as vanished -- deterministically reproducing "the slot vanished between
+/// claimMount's own GET and ours" on EVERY loop iteration, not just a lucky one-shot race.
+class AlwaysVanishesBackend final : public DB::Cas::Backend
+{
+public:
+    explicit AlwaysVanishesBackend(std::shared_ptr<DB::Cas::Backend> inner_) : inner(std::move(inner_)) {}
+    String watched_key;
+
+    std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override
+    {
+        if (k == watched_key)
+            return std::nullopt;
+        return inner->get(k, r);
+    }
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+    DB::Cas::HeadResult head(const String & k) override { return inner->head(k); }
+    DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+    DB::Cas::PutResult putIfAbsent(const String & k, const String & b, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsent(k, b, m); }
+    DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsentStream(k, m); }
+    DB::Cas::PutResult putOverwrite(const String & k, const String & b, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & m) override { return inner->putOverwrite(k, b, e, m); }
+    DB::Cas::CasResult casPut(const String & k, const String & b, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & m) override { return inner->casPut(k, b, e, m); }
+    DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & t) override { return inner->deleteExact(k, t); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+
+private:
+    std::shared_ptr<DB::Cas::Backend> inner;
+};
+}
+
+/// fix-round F5 (author-review: `!got -> continue` in the observation loop, with no sleep and outside
+/// the restart limit, spins `get`/`claimMount`/`put` at backend RTT under persistent slot churn). A
+/// backend that makes the mount slot look vanished to every GET must still terminate (bounded restarts,
+/// not an infinite loop) AND must pace itself (the injected `sleep_fn` must actually fire) rather than
+/// busy-spin.
+TEST(CASMountAwaitExpiry, PersistentSlotVanishPacesAndBoundsRestartsInsteadOfSpinning)
+{
+    auto inner = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    /// A real slot exists underneath (uuid 1, epoch 7) so `claimMount`'s absent-slot `putIfAbsent`
+    /// genuinely fails every time (never accidentally re-mints).
+    ASSERT_EQ(claimMount(*inner, l, "r", UInt128(1), 7, /*now*/ 1000, /*ttl*/ 100).kind, MountClaimResult::Claimed);
+
+    auto vanishing = std::make_shared<AlwaysVanishesBackend>(inner);
+    vanishing->watched_key = l.mountKey("r");
+
+    uint64_t wall = 1000;
+    uint64_t mono = 0;
+    int sleeps = 0;
+    auto now_fn = [&] { return wall; };
+    auto mono_fn = [&] { return mono; };
+    auto sleep_fn = [&](uint64_t ms) { wall += ms; mono += ms; ++sleeps; };
+
+    const auto r = claimMountAwaitingExpiry(
+        *vanishing, l, "r", UInt128(1), /*our_epoch*/ 8, now_fn, mono_fn, /*ttl*/ 100, /*poll*/ 20, sleep_fn);
+    EXPECT_EQ(r.kind, MountClaimResult::LiveDoubleStart) << "must terminate (bounded), not loop forever";
+    EXPECT_GT(sleeps, 0) << "a persistently vanishing slot must still pace via sleep_fn, not busy-spin";
+    /// The real epoch-7 lease is untouched -- every `putIfAbsent` attempt against it genuinely fails
+    /// (the object is still there), so it is never accidentally re-minted over.
+    EXPECT_EQ(decodeMountLease(inner->get(l.mountKey("r"))->bytes).writer_epoch, 7u);
+}
+
+TEST(CASMountAwaitExpiry, ForeignUuidFailsClosedImmediately)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    /// A foreign server (uuid=2) holds the mount.
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(2), 1, /*now*/ 1000, /*ttl*/ 100).kind, MountClaimResult::Claimed);
+
+    uint64_t now = 1000;
+    int sleeps = 0;
+    auto now_fn = [&] { return now; };
+    auto mono_fn = [&] { return uint64_t{0}; };
+    auto sleep_fn = [&](uint64_t ms) { now += ms; ++sleeps; };
+
+    const auto r = claimMountAwaitingExpiry(
+        *b, l, "r", UInt128(1), /*our_epoch*/ 8, now_fn, mono_fn, /*ttl*/ 100, /*poll*/ 25, sleep_fn);
+    EXPECT_EQ(r.kind, MountClaimResult::ForeignOwner);
+    EXPECT_EQ(sleeps, 0);                                            // never waits across UUIDs
+}
+
+/// rev.6: the predecessor's own stamped `expires_at_ms` (however skewed) is NEVER consulted for the
+/// reclaim decision any more — the wait is bounded purely by OUR OWN `ttl_ms`-derived threshold. A
+/// prior incarnation minted with an absurdly large `ttl` (so its own stamp claims aliveness for
+/// ~100000ms) still reclaims within the SAME small threshold as any other case, because that stamp is
+/// never read for timing.
+TEST(CASMountAwaitExpiry, SkewedFarFutureExpiryHasNoEffectOnObservationThreshold)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), 7, /*now*/ 1000, /*ttl*/ 100000).kind, MountClaimResult::Claimed);
+
+    uint64_t wall = 1000;
+    uint64_t mono = 0;
+    auto now_fn = [&] { return wall; };
+    auto mono_fn = [&] { return mono; };
+    auto sleep_fn = [&](uint64_t ms) { wall += ms; mono += ms; };
+
+    const auto r = claimMountAwaitingExpiry(
+        *b, l, "r", UInt128(1), /*our_epoch*/ 8, now_fn, mono_fn, /*ttl*/ 100, /*poll*/ 20, sleep_fn);
+    EXPECT_EQ(r.kind, MountClaimResult::Claimed);
+    EXPECT_LE(mono, 100u + 100u / 20 + 20u + 20u);      // bounded by OUR threshold, not the predecessor's stamp
+    EXPECT_EQ(decodeMountLease(b->get(l.mountKey("r"))->bytes).writer_epoch, 8u);   // reclaimed
+}
+
+TEST(CASMountLease, KeeperStartAdoptsOurOwnClaimNotDoubleStart)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    uint64_t now = 1000;
+    // The normal flow: claimMount writes the live mount under (uuid=1, epoch=7), THEN keeper.start().
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), /*epoch*/ 7, now, /*ttl*/ 100).kind, MountClaimResult::Claimed);
+    MountLeaseKeeper k(b, l, "r", UInt128(1), /*epoch*/ 7, std::chrono::milliseconds(100), [&] { return now; },
+                       [] { return uint64_t{0}; });
+    EXPECT_NO_THROW(k.start());     // adopts our own live (uuid=1,epoch=7) mount — NOT a double-start
+    EXPECT_EQ(decodeMountLease(b->get(l.mountKey("r"))->bytes).writer_epoch, 7u);
+
+    // A keeper for the SAME uuid but a DIFFERENT live epoch must fail closed (superseded/double-start):
+    MountLeaseKeeper k2(b, l, "r", UInt128(1), /*epoch*/ 8, std::chrono::milliseconds(100), [&] { return now; },
+                        [] { return uint64_t{0}; });
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            k2.start();
+        },
+        "held by a different writer_epoch");
+}
+
+TEST(CASMountFence, SupersededWriterRefusedNoS3Read)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "r"});
+
+    /// Permissive default: a Pool that has NOT armed the fence allows mutations.
+    EXPECT_TRUE(store->mayMutate());
+
+    /// Latching loss: once the renewer trips the fence it stays lost (purely local — no S3 read).
+    store->tripMountLost();
+    EXPECT_FALSE(store->mayMutate());
+
+    /// A real mutate entrypoint that funnels through mutateShard now fails closed at the gate, BEFORE
+    /// the mutate lambda runs (so this is the ABORTED gate throw, not a FILE_DOESNT_EXIST from inside).
+    const RootNamespace ns{"srv1/tbl"};
+    EXPECT_THROW(store->dropRef(ns, "any_ref"), DB::Exception);
+}
+
+TEST(CASMountStartup, SecondServerSameRootFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s1 = Pool::open(b, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r"});
+    /// A second server (different uuid) on the SAME server_root_id + same backend → fail closed
+    /// (the owner gate rejects the foreign uuid before any mount/epoch mutation).
+    EXPECT_THROW(
+        Pool::open(b, PoolConfig{
+            .pool_prefix = "p", .server_id = UInt128(2), .server_root_id = "r"}),
+        DB::Exception);
+}
+
+TEST(CASMountStartup, WriterEpochStrictlyIncreasesAcrossReopen)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s1 = Pool::open(b, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r"});
+    const uint64_t e1 = s1->writerEpoch();
+
+    /// Simulate shutdown: the Pool dtor stops the keeper, whose terminate() retires the lease
+    /// (stamps it already-expired). The owner + the durable epoch object stay sticky.
+    s1.reset();
+
+    /// Same server reopen → reclaims the (now-expired, different-epoch) mount and allocates a strictly
+    /// higher durable writer_epoch.
+    auto s2 = Pool::open(b, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r"});
+    const uint64_t e2 = s2->writerEpoch();
+    EXPECT_GT(e2, e1);
+}
+
+TEST(CASMountStartup, FreshWritablePoolBootstrapsAnExplicitEmptyCatalog)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r",
+        .skip_access_check = true});
+
+    const auto catalog = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog.has_value());
+    EXPECT_TRUE(decodeRefCatalog(catalog->bytes).entries.empty());
+}
+
+TEST(CASMountStartup, ExistingPoolWithoutCatalogFailsBeforeSlotMutation)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+    {
+        auto store = Pool::open(backend, PoolConfig{
+            .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r",
+            .skip_access_check = true});
+    }
+
+    /// Old raw fixtures did not persist an empty catalog. Make this an explicit existing-pool
+    /// fixture before removing the mandatory object whose loss the mount must reject.
+    if (!backend->head(layout.refCatalogKey()).exists)
+        ASSERT_EQ(backend->putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(RefCatalog{})).outcome,
+                  PutOutcome::Done);
+    const HeadResult catalog_head = backend->head(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_head.exists);
+    ASSERT_EQ(backend->deleteExact(layout.refCatalogKey(), catalog_head.token).kind,
+              DeleteOutcome::Kind::Deleted);
+
+    const auto owner_before = backend->get(layout.ownerKey("r"));
+    const auto epoch_before = backend->get(layout.epochKey("r"));
+    const auto mount_before = backend->get(layout.mountKey("r"));
+    ASSERT_TRUE(owner_before.has_value());
+    ASSERT_TRUE(epoch_before.has_value());
+    ASSERT_TRUE(mount_before.has_value());
+
+    EXPECT_THROW(Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r",
+        .skip_access_check = true}), DB::Exception);
+
+    const auto owner_after = backend->get(layout.ownerKey("r"));
+    const auto epoch_after = backend->get(layout.epochKey("r"));
+    const auto mount_after = backend->get(layout.mountKey("r"));
+    ASSERT_TRUE(owner_after.has_value());
+    ASSERT_TRUE(epoch_after.has_value());
+    ASSERT_TRUE(mount_after.has_value());
+    EXPECT_EQ(owner_after->bytes, owner_before->bytes);
+    EXPECT_EQ(owner_after->token, owner_before->token);
+    EXPECT_EQ(epoch_after->bytes, epoch_before->bytes);
+    EXPECT_EQ(epoch_after->token, epoch_before->token);
+    EXPECT_EQ(mount_after->bytes, mount_before->bytes);
+    EXPECT_EQ(mount_after->token, mount_before->token);
+}
+
+TEST(CASMountReadOnly, ForeignOwnedPoolOpensWithoutMutation)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+
+    /// Server A claims the pool (writable): owner = uuid(1), a durable epoch + a live mount lease.
+    auto a = Pool::open(b, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r"});
+
+    /// Capture the control objects BEFORE the read-only open so we can prove it mutated nothing.
+    const auto owner_before = b->get(l.ownerKey("r"));
+    const auto mount_before = b->get(l.mountKey("r"));
+    const auto epoch_before = b->get(l.epochKey("r"));
+    ASSERT_TRUE(owner_before.has_value());
+    ASSERT_TRUE(mount_before.has_value());
+    ASSERT_TRUE(epoch_before.has_value());
+
+    /// A READ-ONLY observer with a DIFFERENT server_id on the SAME backend/server_root_id must NOT
+    /// throw — a read-only mount never participates in the owner/epoch/mount protocol, so a pool
+    /// owned by another server_uuid is freely observable.
+    PoolPtr ro;
+    EXPECT_NO_THROW(
+        ro = Pool::open(b, PoolConfig{
+            .pool_prefix = "p", .server_id = UInt128(2), .server_root_id = "r",
+            .read_only = true}));
+    EXPECT_NE(ro, nullptr);
+
+    /// And it mutated nothing: owner still decodes to A's uuid, the mount body is still A's, and the
+    /// raw bytes of owner/epoch/mount are byte-for-byte unchanged (no second owner, no re-claim).
+    const auto owner_after = b->get(l.ownerKey("r"));
+    const auto mount_after = b->get(l.mountKey("r"));
+    const auto epoch_after = b->get(l.epochKey("r"));
+    ASSERT_TRUE(owner_after.has_value());
+    ASSERT_TRUE(mount_after.has_value());
+    ASSERT_TRUE(epoch_after.has_value());
+
+    EXPECT_EQ(decodeOwner(owner_after->bytes).server_uuid, UInt128(1));
+    EXPECT_EQ(decodeMountLease(mount_after->bytes).server_uuid, UInt128(1));
+
+    EXPECT_EQ(owner_after->bytes, owner_before->bytes);
+    EXPECT_EQ(mount_after->bytes, mount_before->bytes);
+    EXPECT_EQ(epoch_after->bytes, epoch_before->bytes);
+}
+
+/// Pool::open must call validateCasRequestBudget itself (not just the free function in isolation —
+/// see gtest_cas_request_control.cpp for that): an inconsistent cas_request_budget must refuse a
+/// writable mount end-to-end (RFC cas-s3-timeout-retry-control §required-timeout-model), never mount
+/// silently with a budget that could let a controlled attempt outlive the lease it is fenced under.
+TEST(CASMountStartup, RefusesWritableOpenWithInconsistentCasRequestBudget)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+
+    /// attempt_timeout_ms + lease_safety_margin_ms == mount_lease_ttl_ms below (30000): not STRICTLY
+    /// less, so this must be rejected.
+    const CasRequestBudget bad_budget{
+        .attempt_timeout_ms = 25000, .operation_deadline_ms = 30000, .max_attempts = 3, .lease_safety_margin_ms = 5000};
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        Pool::open(b, PoolConfig{
+            .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r",
+            .mount_lease_ttl_ms = std::chrono::milliseconds(30000),
+            .cas_request_budget = bad_budget});
+    });
+}
+
+TEST(CASMountStartup, StaleSelfMountReclaimedAfterWait)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+
+    /// Server A opens writable with a SHORT lease TTL and no background renewer (`background_watermark`
+    /// defaults false). The test captures its live mount body, destroys the real Pool cleanly, then
+    /// replays that body to simulate a crashed process whose lease survives but is never renewed.
+    /// This test's short lease TTL is far below the CasRequestBudget defaults (RFC
+    /// cas-s3-timeout-retry-control §required-timeout-model requires attempt_timeout + safety_margin <
+    /// lease TTL), so it also scales down cas_request_budget to fit — the budget itself is not
+    /// exercised here, only Pool::open's validateCasRequestBudget startup gate.
+    const CasRequestBudget tiny_budget{
+        .attempt_timeout_ms = 50, .operation_deadline_ms = 500, .max_attempts = 1, .lease_safety_margin_ms = 50};
+    auto a = Pool::open(b, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(300),
+        .mount_renew_period = std::chrono::milliseconds(100),
+        .cas_request_budget = tiny_budget});
+    ASSERT_NE(a, nullptr);
+    const uint64_t e1 = a->writerEpoch();
+    const String mount_key = a->layout().mountKey("r");
+    const auto stale_mount = b->get(mount_key);
+    ASSERT_TRUE(stale_mount.has_value());
+
+    /// Preserve A's live lease as if its process disappeared without running C++ teardown. Destroying
+    /// the real Pool first keeps the parent process valid; replaying the saved body recreates the exact
+    /// durable stale-lease state that a crashed process would leave behind.
+    a.reset();
+    const auto farewell = b->get(mount_key);
+    ASSERT_TRUE(farewell.has_value());
+    ASSERT_EQ(b->putOverwrite(mount_key, stale_mount->bytes, farewell->token).outcome, PutOutcome::Done);
+
+    /// A restart of the SAME server (same uuid) must NOT abort: it waits out the stale lease (<= ~300ms)
+    /// and reclaims the mount, coming up with a strictly higher durable writer_epoch. The replayed live
+    /// body hides A's clean farewell, so the reclaim is `MountPriorState::UncleanObserved`. Inject a
+    /// fake `boot_ms_fn` + `wait_sleep_fn` (mirroring
+    /// `CASMountOpenWaits.UncleanOpenPaysOnlyTheObservationWindow`) so the observation window resolves
+    /// instantly instead of blocking this test on real time.
+    uint64_t a2_fake_boot = 0;
+    PoolPtr a2;
+    EXPECT_NO_THROW(
+        a2 = Pool::open(b, PoolConfig{
+            .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r",
+            .mount_lease_ttl_ms = std::chrono::milliseconds(300),
+            .mount_renew_period = std::chrono::milliseconds(100),
+            .cas_request_budget = tiny_budget,
+            .boot_ms_fn = [&a2_fake_boot] { return a2_fake_boot; },
+            .wait_sleep_fn = [&a2_fake_boot](uint64_t ms) { a2_fake_boot += ms; }}));
+    ASSERT_NE(a2, nullptr);
+    EXPECT_GT(a2->writerEpoch(), e1);
+
+    /// The original live-object overlap: a first Pool is still alive when a replacement reclaims its
+    /// slot, so the first one's release meets a stranger. This was an `EXPECT_DEATH` pinning a
+    /// `LOGICAL_ERROR` abort — which fires from `~Pool`, defeating `finishTeardown`'s own catch by
+    /// aborting at exception construction. The first Pool never observed a deposition (nothing failed
+    /// its renewal; the slot was reclaimed underneath it), so this is the exclusivity-violation arm:
+    /// refuse, leave the reclaimer's slot untouched, latch the fence, and SURVIVE.
+    auto overlap_backend = std::make_shared<InMemoryBackend>();
+    auto first = Pool::open(overlap_backend, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(300),
+        .mount_renew_period = std::chrono::milliseconds(100),
+        .cas_request_budget = tiny_budget});
+    const String overlap_mount_key = first->layout().mountKey("r");
+
+    uint64_t overlap_fake_boot = 0;
+    auto replacement = Pool::open(overlap_backend, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "r",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(300),
+        .mount_renew_period = std::chrono::milliseconds(100),
+        .cas_request_budget = tiny_budget,
+        .boot_ms_fn = [&overlap_fake_boot] { return overlap_fake_boot; },
+        .wait_sleep_fn = [&overlap_fake_boot](uint64_t ms) { overlap_fake_boot += ms; }});
+    ASSERT_NE(replacement, nullptr);
+
+    const auto reclaimer_slot_before = overlap_backend->get(overlap_mount_key);
+    ASSERT_TRUE(reclaimer_slot_before.has_value());
+    const uint64_t overlap_violations_before
+        = ProfileEvents::global_counters[ProfileEvents::CASMountExclusivityViolation].load();
+
+    first.reset();   /// must not abort, must not terminate
+
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMountExclusivityViolation].load(),
+              overlap_violations_before + 1);
+    const auto reclaimer_slot_after = overlap_backend->get(overlap_mount_key);
+    ASSERT_TRUE(reclaimer_slot_after.has_value());
+    EXPECT_EQ(reclaimer_slot_after->bytes, reclaimer_slot_before->bytes)
+        << "the deposed Pool's release must not retire the reclaimer's lease";
+    EXPECT_TRUE(replacement->mayMutate()) << "and must not disturb the live reclaimer";
+}
+
+TEST(CASMountLease, BodyCarriesFloorAndFence)
+{
+    MountLease m;
+    m.server_uuid = UInt128(0xAB);
+    m.writer_epoch = 7;
+    m.hostname = "h";
+    m.pid = 42;
+    m.started_at_ms = 1000;
+    m.seq = 3;
+    m.expires_at_ms = 2000;
+    m.min_active = 5;
+    m.gc_fenced = true;
+    const MountLease d = decodeMountLease(encodeMountLease(m));
+    EXPECT_EQ(d.min_active, 5u);
+    EXPECT_TRUE(d.gc_fenced);
+    EXPECT_EQ(d.writer_epoch, 7u);
+}
+
+TEST(CASMountLease, RetiredSentinelRoundTrips)
+{
+    MountLease m;
+    m.min_active = std::numeric_limits<uint64_t>::max();
+    EXPECT_EQ(decodeMountLease(encodeMountLease(m)).min_active,
+              std::numeric_limits<uint64_t>::max());
+}
+
+/// ---- Task 7 / Task 9: GC heartbeat classification with token-guarded, observation-based fence-out ----
+
+namespace
+{
+/// A fixed, fake "now" — no real clocks in these tests. Lease timestamps are chosen relative to it.
+/// Rev.6 §token-stability observation removed the wall clock from the fence DECISION; `kNowMs` below
+/// is threaded through only as `computeHeartbeatFloor`'s audit-only `now_ms`.
+constexpr uint64_t kNowMs = 1'000'000;
+/// The fence-out threshold measured on the LEADER's OWN monotonic clock (`mono_now_ms`), independent
+/// of any lease's stamped `expires_at_ms`.
+constexpr uint64_t kStableThresholdMs = 10'000;
+
+/// Seed one mount body under mountKey(srid) via the on-storage codec (`encodeMountLease` +
+/// `putIfAbsent`) — the same interface the keeper writes through.
+MountLease seedMount(
+    Backend & b, const Layout & l, const String & srid,
+    uint64_t expires_at_ms, bool gc_fenced, uint64_t min_active, uint64_t seq = 1)
+{
+    MountLease m;
+    m.server_uuid = UInt128(srid.back());   // distinct per srid; content is irrelevant to the gate
+    m.writer_epoch = 1;
+    m.hostname = "h-" + srid;
+    m.pid = 100;
+    m.started_at_ms = kNowMs;
+    m.seq = seq;
+    m.expires_at_ms = expires_at_ms;
+    m.min_active = min_active;
+    m.gc_fenced = gc_fenced;
+    b.putIfAbsent(l.mountKey(srid), encodeMountLease(m));
+    return m;
+}
+
+/// Simulate a keeper's real renewal between two `computeHeartbeatFloor` calls: a token-guarded
+/// overwrite that bumps `seq` (and so mints a fresh backend token), leaving everything else as-is.
+/// Models the one thing the observation-based fence cares about: the write token changed, so any
+/// in-progress observation of the OLD token must restart.
+void renewMount(Backend & b, const Layout & l, const String & srid)
+{
+    const auto got = b.get(l.mountKey(srid));
+    ASSERT_TRUE(got.has_value());
+    MountLease m = decodeMountLease(got->bytes);
+    m.seq += 1;
+    const PutResult res = b.putOverwrite(l.mountKey(srid), encodeMountLease(m), got->token);
+    ASSERT_EQ(res.outcome, PutOutcome::Done);
+}
+}
+
+TEST(CASHeartbeatFloor, FirstSightNeverFencesEvenIfStampLooksExpired)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+
+    /// A stamp that would have read as long-expired under the old skew-margin comparison — under
+    /// rev.6 observation the stamp is never even consulted for the fence decision.
+    seedMount(*b, l, "s1", /*expires*/ 10, /*fenced*/ false, /*min_active*/ 0);
+
+    MountObservationMap obs;
+    const HeartbeatFloor floor = computeHeartbeatFloor(*b, l, /*now_ms*/ kNowMs, /*mono_now_ms*/ 0,
+                                                         kStableThresholdMs, obs);
+
+    EXPECT_EQ(floor.fenced_now, 0u);
+    EXPECT_EQ(floor.live, 1u);
+    ASSERT_TRUE(obs.contains("s1"));
+    EXPECT_EQ(obs.at("s1").first_seen_mono_ms, 0u);
+}
+
+TEST(CASHeartbeatFloor, StableTokenPastThresholdIsFenced)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    seedMount(*b, l, "s1", /*expires*/ 10, /*fenced*/ false, /*min_active*/ 0);
+
+    MountObservationMap obs;
+    const HeartbeatFloor floor_before = computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ 0, kStableThresholdMs, obs);
+    EXPECT_EQ(floor_before.fenced_now, 0u);
+
+    const MountLease before = decodeMountLease(b->get(l.mountKey("s1"))->bytes);
+
+    /// No renewal in between: the SAME token, observed since mono 0, is now stable for the full
+    /// threshold on the leader's own clock.
+    const HeartbeatFloor floor2 = computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ kStableThresholdMs,
+                                                          kStableThresholdMs, obs);
+
+    EXPECT_EQ(floor2.fenced_now, 1u);
+    EXPECT_EQ(floor2.fenced_srids, std::vector<String>{"s1"});
+    const MountLease fenced = decodeMountLease(b->get(l.mountKey("s1"))->bytes);
+    EXPECT_TRUE(fenced.gc_fenced);
+    EXPECT_EQ(fenced.seq, before.seq + 1);
+}
+
+TEST(CASHeartbeatFloor, RenewalBetweenRoundsRestartsObservation)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    seedMount(*b, l, "s1", /*expires*/ 10, /*fenced*/ false, /*min_active*/ 0);
+
+    MountObservationMap obs;
+    computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ 0, kStableThresholdMs, obs);
+    ASSERT_TRUE(obs.contains("s1"));
+    const Token first_token = obs.at("s1").token;
+
+    renewMount(*b, l, "s1");
+    const Token renewed_token = b->get(l.mountKey("s1"))->token;
+    EXPECT_NE(renewed_token, first_token);
+
+    const HeartbeatFloor floor2 = computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ kStableThresholdMs,
+                                                          kStableThresholdMs, obs);
+
+    EXPECT_EQ(floor2.fenced_now, 0u);
+    ASSERT_TRUE(obs.contains("s1"));
+    EXPECT_EQ(obs.at("s1").token, renewed_token);
+    EXPECT_EQ(obs.at("s1").first_seen_mono_ms, kStableThresholdMs);
+}
+
+/// fix-round F7 (author-review: `Gc::mount_obs` not pruned for srids gone from LIST -> slow unbounded
+/// growth on a long-lived leader, worsened by pool-member decommission). A srid whose `/mount` key is
+/// removed ENTIRELY (not merely fenced/terminated -- those already `obs.erase` themselves mid-loop) is
+/// never visited by a later LIST pass again, so its observation entry must be pruned at end-of-round,
+/// not linger in `obs` forever.
+TEST(CASHeartbeatFloor, UnseenSridPrunedFromObservationMap)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+    seedMount(*b, l, "s1", /*expires*/ 10, /*fenced*/ false, /*min_active*/ 0);
+    seedMount(*b, l, "s2", /*expires*/ 10, /*fenced*/ false, /*min_active*/ 0);
+
+    MountObservationMap obs;
+    computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ 0, kStableThresholdMs, obs);
+    ASSERT_TRUE(obs.contains("s1"));
+    ASSERT_TRUE(obs.contains("s2"));
+
+    /// s2's `/mount` key is removed entirely -- e.g. `SYSTEM CAS DROP POOL MEMBER` -- so
+    /// no future LIST pass will ever visit it again. s1 renews (a live keeper would), so its OWN
+    /// observation restarts and it stays `live` -- isolating this test to the pruning behavior alone,
+    /// not confounding it with s1 also becoming fence-eligible (which would erase its `obs` entry too,
+    /// for an unrelated reason).
+    renewMount(*b, l, "s1");
+    const auto s2_key = l.mountKey("s2");
+    const auto got = b->get(s2_key);
+    ASSERT_TRUE(got.has_value());
+    ASSERT_EQ(b->deleteExact(s2_key, got->token).kind, DeleteOutcome::Kind::Deleted);
+
+    computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ kStableThresholdMs, kStableThresholdMs, obs);
+    EXPECT_TRUE(obs.contains("s1"));
+    EXPECT_FALSE(obs.contains("s2"))
+        << "a srid removed from the LIST entirely must be pruned from obs, not linger forever";
+}
+
+TEST(CASHeartbeatFloor, ClassifiesAndFencesOut)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+
+    /// two live mounts — genuinely renewing between the two rounds below, so their observation never
+    /// stabilizes.
+    seedMount(*b, l, "s1", /*expires*/ kNowMs + 60'000, /*fenced*/ false, /*min_active*/ 0);
+    seedMount(*b, l, "s2", /*expires*/ kNowMs + 60'000, /*fenced*/ false, /*min_active*/ 0);
+    /// dead — no renewal between the two rounds below — must be fenced-out by the second call.
+    seedMount(*b, l, "s3", /*expires*/ kNowMs - 60'000, /*fenced*/ false, /*min_active*/ 0);
+    /// already-fenced — excluded, body byte-identical after both calls (no PUT).
+    seedMount(*b, l, "s4", /*expires*/ kNowMs - 60'000, /*fenced*/ true, /*min_active*/ 0);
+    /// terminated (min_active == UINT64_MAX) with expired-looking timestamps — excluded, not fenced.
+    seedMount(*b, l, "s5", /*expires*/ kNowMs - 60'000, /*fenced*/ false,
+              /*min_active*/ std::numeric_limits<uint64_t>::max());
+
+    MountObservationMap obs;
+
+    /// Round 1 (mono 0): first sight of every non-terminal mount — nothing is fence-eligible yet.
+    const HeartbeatFloor floor_before = computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ 0, kStableThresholdMs, obs);
+    EXPECT_EQ(floor_before.live, 3u);            // s1, s2, s3: observation just started
+    EXPECT_EQ(floor_before.terminated, 1u);      // s5
+    EXPECT_EQ(floor_before.fenced_now, 0u);
+    EXPECT_EQ(floor_before.already_fenced, 1u);  // s4
+
+    /// s1 and s2 renew between rounds (as a live keeper would); s3 does not (it crashed).
+    renewMount(*b, l, "s1");
+    renewMount(*b, l, "s2");
+
+    const auto s3_before = b->get(l.mountKey("s3"));
+    const auto s4_before = b->get(l.mountKey("s4"));
+    ASSERT_TRUE(s3_before.has_value());
+    ASSERT_TRUE(s4_before.has_value());
+
+    /// Round 2 (mono == threshold): s1/s2's renewed tokens restart their observation (still live);
+    /// s3's original token has now held stable for the full threshold -> fenced.
+    const HeartbeatFloor floor2 = computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ kStableThresholdMs,
+                                                          kStableThresholdMs, obs);
+
+    EXPECT_EQ(floor2.live, 2u);            // s1, s2: renewed, observation restarted
+    EXPECT_EQ(floor2.terminated, 1u);      // s5
+    EXPECT_EQ(floor2.fenced_now, 1u);      // s3
+    EXPECT_EQ(floor2.already_fenced, 1u);  // s4
+
+    /// The dead body was fenced: gc_fenced set, seq bumped, the rest of the body preserved.
+    const auto s3_after = b->get(l.mountKey("s3"));
+    ASSERT_TRUE(s3_after.has_value());
+    const MountLease s3_prev = decodeMountLease(s3_before->bytes);
+    const MountLease s3_now = decodeMountLease(s3_after->bytes);
+    EXPECT_TRUE(s3_now.gc_fenced);
+    EXPECT_EQ(s3_now.seq, s3_prev.seq + 1);
+    EXPECT_EQ(s3_now.server_uuid, s3_prev.server_uuid);
+    EXPECT_EQ(s3_now.writer_epoch, s3_prev.writer_epoch);
+    EXPECT_EQ(s3_now.hostname, s3_prev.hostname);
+    EXPECT_EQ(s3_now.expires_at_ms, s3_prev.expires_at_ms);
+
+    /// The already-fenced body was not touched (no PUT) across either call.
+    const auto s4_after = b->get(l.mountKey("s4"));
+    ASSERT_TRUE(s4_after.has_value());
+    EXPECT_EQ(s4_after->bytes, s4_before->bytes);
+}
+
+namespace
+{
+/// A delegating backend whose `putOverwrite` of the target mount key first performs an inner renewal
+/// (a real, token-correct overwrite that pushes expiry far into the future) and THEN delegates — so
+/// the caller's fence-out overwrite lands on a stale token and returns PreconditionFailed. The inner
+/// renewal runs exactly once (`renewed`), modelling a holder that renews concurrently in the window
+/// between the function's GET and its fence-out PUT.
+class RenewOnFenceBackend : public InMemoryBackend
+{
+public:
+    RenewOnFenceBackend(String target_key_, uint64_t renewed_expires_ms_)
+        : target_key(std::move(target_key_)), renewed_expires_ms(renewed_expires_ms_)
+    {
+    }
+
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected,
+                           const ObjectMeta & meta) override
+    {
+        if (key == target_key && !renewed)
+        {
+            renewed = true;
+            /// The holder renews under the real current token: fresh far-future expiry.
+            const auto got = InMemoryBackend::get(key, {});
+            MountLease m = decodeMountLease(got->bytes);
+            m.seq += 1;
+            m.expires_at_ms = renewed_expires_ms;
+            const PutResult renew = InMemoryBackend::putOverwrite(key, encodeMountLease(m), got->token);
+            EXPECT_EQ(renew.outcome, PutOutcome::Done);
+        }
+        return InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+    }
+
+private:
+    String target_key;
+    uint64_t renewed_expires_ms;
+    bool renewed = false;
+};
+}
+
+TEST(CASHeartbeatFloor, FenceOutLosesTokenRaceReclassifiesLive)
+{
+    Layout l("p");
+    auto b = std::make_shared<RenewOnFenceBackend>(
+        l.mountKey("s1"), /*renewed_expires*/ kNowMs + 120'000);
+
+    seedMount(*b, l, "s1", /*expires*/ kNowMs - 60'000, /*fenced*/ false, /*min_active*/ 0);
+
+    MountObservationMap obs;
+    /// Round 1: first sight, observation starts — never reaches the fence-out path (the race
+    /// decorator stays armed for round 2).
+    const HeartbeatFloor floor_before = computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ 0, kStableThresholdMs, obs);
+    EXPECT_EQ(floor_before.fenced_now, 0u);
+
+    /// Round 2: the token has been stable past threshold, so the function attempts the fence-out.
+    /// The decorator renews concurrently under the real token, the PUT hits PreconditionFailed, the
+    /// function re-GETs and reclassifies it as live (observation restarted on the new token) — never
+    /// fenced.
+    const HeartbeatFloor floor2 = computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ kStableThresholdMs,
+                                                          kStableThresholdMs, obs);
+
+    EXPECT_EQ(floor2.fenced_now, 0u);
+    EXPECT_EQ(floor2.live, 1u);
+
+    const auto after = b->get(l.mountKey("s1"));
+    ASSERT_TRUE(after.has_value());
+    EXPECT_FALSE(decodeMountLease(after->bytes).gc_fenced);
+}
+
+TEST(CASHeartbeatFloor, EmptyPrefixYieldsNoLiveMounts)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l("p");
+
+    MountObservationMap obs;
+    const HeartbeatFloor floor = computeHeartbeatFloor(*b, l, kNowMs, /*mono*/ 0, kStableThresholdMs, obs);
+
+    EXPECT_EQ(floor.live, 0u);
+    EXPECT_EQ(floor.terminated, 0u);
+    EXPECT_EQ(floor.fenced_now, 0u);
+    EXPECT_EQ(floor.already_fenced, 0u);
+}
+
+/// ---- Task 1 (Phase 2): `listMounts` — read-only mount-slot enumeration for introspection ----
+
+TEST(CASListMounts, ClassifiesEveryStateReadOnly)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const uint64_t now_ms = 1'000'000;
+    const uint64_t ttl_ms = 10'000;
+
+    /// live: fresh claim for srid "a"
+    ASSERT_EQ(claimMount(*backend, layout, "a", UInt128{1}, /*our_epoch=*/1, now_ms, ttl_ms).kind,
+              MountClaimResult::Claimed);
+    /// expired: claim for "b" whose lease ran out long before now_ms
+    ASSERT_EQ(claimMount(*backend, layout, "b", UInt128{2}, 1, now_ms - 100'000, ttl_ms).kind,
+              MountClaimResult::Claimed);
+    /// corrupt: garbage bytes in "c"'s mount slot
+    backend->putIfAbsent(layout.mountKey("c"), "garbage-not-a-proto", {});
+
+    auto mounts = listMounts(*backend, layout, now_ms, /*skew_margin_ms=*/ttl_ms / 2);
+    ASSERT_EQ(mounts.size(), 3u);
+    std::map<String, String> by_srid;
+    for (const auto & m : mounts)
+        by_srid[m.srid] = m.state;
+    EXPECT_EQ(by_srid["a"], "live");
+    EXPECT_EQ(by_srid["b"], "expired");
+    EXPECT_EQ(by_srid["c"], "corrupt");
+
+    /// READ-ONLY guarantee: "b" is expired but must NOT be fenced by listMounts
+    /// (computeHeartbeatFloor would stamp gc_fenced=true; the introspection view must not).
+    auto again = listMounts(*backend, layout, now_ms, ttl_ms / 2);
+    for (const auto & m : again)
+        if (m.srid == "b")
+        {
+            EXPECT_FALSE(m.lease.gc_fenced);
+            EXPECT_EQ(m.state, "expired");
+        }
+}
+
+/// A `srid` may itself contain `/` (e.g. `shard-01/replica-a` — legal per
+/// `CASServerRootId.ValidationAcceptsCleanPathsRejectsBad`). Slicing the key by the last `/` before
+/// the `/mount` suffix (as opposed to by `serverRootsPrefix()` length) truncates it to `replica-a`.
+TEST(CASListMounts, NestedSridIsNotTruncated)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    const uint64_t now_ms = 1'000'000;
+    const uint64_t ttl_ms = 10'000;
+
+    ASSERT_EQ(claimMount(*backend, layout, "shard-01/replica-a", UInt128{1}, /*our_epoch=*/1, now_ms, ttl_ms).kind,
+              MountClaimResult::Claimed);
+
+    auto mounts = listMounts(*backend, layout, now_ms, /*skew_margin_ms=*/ttl_ms / 2);
+    ASSERT_EQ(mounts.size(), 1u);
+    EXPECT_EQ(mounts[0].srid, "shard-01/replica-a");
+    EXPECT_EQ(mounts[0].state, "live");
+}
+
+/// "A fence costs an epoch": a same-(uuid, epoch) re-claim must NOT refresh a `gc_fenced` body in
+/// place — that would resurrect a fenced incarnation. It is terminal for THIS epoch; only a
+/// DIFFERENT (fresh) epoch may reclaim the slot.
+TEST(CASClaimMount, SameEpochFencedIsNotRefreshable)
+{
+    using namespace DB::Cas;
+    auto backend = std::make_shared<InMemoryBackend>();
+    Layout layout("pool");
+    /// mint for (uuid 1, epoch 1), then fence it in place (what computeHeartbeatFloor does):
+    ASSERT_EQ(claimMount(*backend, layout, "a", DB::UInt128{1}, 1, 1000, 10'000).kind,
+              MountClaimResult::Claimed);
+    {
+        auto got = backend->get(layout.mountKey("a"));
+        MountLease fenced = decodeMountLease(got->bytes);
+        fenced.gc_fenced = true;
+        fenced.seq += 1;
+        ASSERT_EQ(backend->putOverwrite(layout.mountKey("a"), encodeMountLease(fenced), got->token).outcome,
+                  PutOutcome::Done);
+    }
+    /// Same (uuid, epoch) re-claim must NOT refresh a fenced body — a fence costs an epoch:
+    const auto r = claimMount(*backend, layout, "a", DB::UInt128{1}, 1, 2000, 10'000);
+    EXPECT_EQ(r.kind, MountClaimResult::FencedSelf);
+    /// The body on the backend is still the fenced one (no write happened):
+    EXPECT_TRUE(decodeMountLease(backend->get(layout.mountKey("a"))->bytes).gc_fenced);
+    /// A DIFFERENT epoch reclaims immediately (existing branch, unchanged):
+    EXPECT_EQ(claimMount(*backend, layout, "a", DB::UInt128{1}, 2, 2000, 10'000).kind,
+              MountClaimResult::Claimed);
+}
+
+/// ---- rev.6 Task 4: observation-based lease reclaim (no cross-node wall-clock trust) ----
+
+/// A same-uuid, different-epoch lease whose STAMPED `expires_at_ms` looks long expired on OUR wall
+/// clock must NOT be reclaimed by that comparison alone — a clock-skewed or simply late-observing
+/// caller must never trust a bare wall-clock read across incarnations. `claimMount` (without a
+/// `proven_dead_token`) always reports `LiveDoubleStart` for this branch now; only the observation
+/// loop (`claimMountAwaitingExpiry`) may turn it into a reclaim, and only after proving death on ITS
+/// OWN clock.
+TEST(CASMountObservation, ExpiredLookingLeaseIsNotReclaimedByWallClock)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l{"p"};
+    /// Predecessor epoch 7 stamped expires_at_ms = 1000; our wall clock says 999999 (long past).
+    auto first = claimMount(*b, l, "r", UInt128(1), 7, /*now_ms=*/500, /*ttl_ms=*/500);
+    ASSERT_EQ(first.kind, MountClaimResult::Claimed);
+    auto r = claimMount(*b, l, "r", UInt128(1), /*our_epoch=*/8, /*now_ms=*/999999, 500);
+    EXPECT_EQ(r.kind, MountClaimResult::LiveDoubleStart);  /// no wall-clock trust
+}
+
+/// The observation loop reclaims once the write-token has held stable for the FULL rate-bound
+/// threshold (`ttl_ms + ttl_ms/20 + poll_interval_ms`) on its OWN (injected, fake) clock — never
+/// short-circuiting on the wall clock, which this test drives to an irrelevant, already-expired value.
+TEST(CASMountObservation, TokenStableForThresholdThenReclaimed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l{"p"};
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), 7, 500, 500).kind, MountClaimResult::Claimed);
+    uint64_t mono = 0;
+    std::vector<uint64_t> sleeps;
+    auto r = claimMountAwaitingExpiry(*b, l, "r", UInt128(1), 8,
+        []{ return uint64_t{999999}; },                 /// wall clock: irrelevant
+        [&]{ return mono; },                             /// observation clock
+        /*ttl_ms=*/500, /*poll_interval_ms=*/50,
+        [&](uint64_t ms){ sleeps.push_back(ms); mono += ms; });
+    EXPECT_EQ(r.kind, MountClaimResult::Claimed);
+    EXPECT_EQ(r.prior, MountPriorState::UncleanObserved);
+    EXPECT_GE(mono, 500 + 500 / 20 + 50);               /// full threshold actually waited
+}
+
+/// A renewal DURING the observation window (the real holder is still alive) bumps the write-token —
+/// the loop must detect the mismatch and RESTART the observation from the new token, never reclaiming
+/// off a window that started watching a now-superseded token.
+TEST(CASMountObservation, RenewalDuringObservationRestartsIt)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l{"p"};
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), 7, 500, 500).kind, MountClaimResult::Claimed);
+
+    /// The real (still-alive) holder's keeper for epoch 7: `start()` adopts the slot `claimMount` just
+    /// wrote (no seq bump, per the ADOPT RULE), then `renewOnce()` bumps the token mid-observation.
+    uint64_t keeper_wall = 500;
+    MountLeaseKeeper keeper(b, l, "r", UInt128(1), 7, std::chrono::milliseconds(500),
+                             [&] { return keeper_wall; }, [] { return uint64_t{0}; });
+    keeper.start();
+
+    const uint64_t threshold_ms = 500 + 500 / 20 + 50;   /// = 575
+    uint64_t mono = 0;
+    bool renewed = false;
+    int wait_starts = 0;
+    auto r = claimMountAwaitingExpiry(*b, l, "r", UInt128(1), 8,
+        []{ return uint64_t{999999}; },                 /// wall clock: irrelevant
+        [&]{ return mono; },                             /// observation clock
+        /*ttl_ms=*/500, /*poll_interval_ms=*/50,
+        [&](uint64_t ms)
+        {
+            mono += ms;
+            /// Renew once, close to (but before) the first window's threshold would complete —
+            /// almost the whole first window is wasted, forcing a near-full second window.
+            if (!renewed && mono >= threshold_ms - 50)
+            {
+                renewed = true;
+                keeper.renewOnce();
+            }
+        },
+        /*on_wait_start=*/[&](const MountLease &, uint64_t) { ++wait_starts; });
+
+    EXPECT_EQ(r.kind, MountClaimResult::Claimed);
+    EXPECT_EQ(r.prior, MountPriorState::UncleanObserved);
+    EXPECT_EQ(wait_starts, 2);                           /// the renewal forced exactly one restart
+    /// The restart's own window did not begin until at least (threshold - poll) had already elapsed,
+    /// so total elapsed time is well over a single threshold window.
+    EXPECT_GE(mono, (threshold_ms - 50) + threshold_ms);
+}
+
+/// A GC-fenced lease is a terminal, already-threshold-gated certificate of death (the fence-out
+/// itself cost the predecessor an epoch) — the observation loop must reclaim it on the FIRST attempt,
+/// with zero polling/sleeping.
+TEST(CASMountObservation, GcFencedIsReclaimedInstantlyWithPriorFenced)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l{"p"};
+    ASSERT_EQ(claimMount(*b, l, "r", UInt128(1), 7, 1000, 500).kind, MountClaimResult::Claimed);
+
+    /// Fence it manually (what `computeHeartbeatFloor`'s fence-out does): gc_fenced=true, seq+1,
+    /// token-guarded.
+    {
+        auto got = b->get(l.mountKey("r"));
+        ASSERT_TRUE(got.has_value());
+        MountLease fenced = decodeMountLease(got->bytes);
+        fenced.gc_fenced = true;
+        fenced.seq += 1;
+        ASSERT_EQ(b->putOverwrite(l.mountKey("r"), encodeMountLease(fenced), got->token).outcome,
+                  PutOutcome::Done);
+    }
+
+    int sleeps = 0;
+    auto r = claimMountAwaitingExpiry(*b, l, "r", UInt128(1), /*our_epoch=*/8,
+        []{ return uint64_t{999999}; },
+        []{ return uint64_t{0}; },
+        /*ttl_ms=*/500, /*poll_interval_ms=*/50,
+        [&](uint64_t) { ++sleeps; });
+
+    EXPECT_EQ(r.kind, MountClaimResult::Claimed);
+    EXPECT_EQ(r.prior, MountPriorState::Fenced);
+    EXPECT_EQ(sleeps, 0);
+}
+
+/// ---- Stage B Task 3: `isCreatorFenceTerminal` -- the cross-process terminality predicate
+/// `CasRefCatalog::reconcileStaleCreator` gates on. Built from `writer_epoch` plus the SAME two
+/// clock-free certificates `probeNonTerminalMountSlots`/`computeHeartbeatFloor` already use, PLUS a
+/// third certificate available only here: a currently-live DIFFERENT `writer_epoch` at the slot. ----
+
+TEST(CASFenceTerminal, AbsentMountSlotIsNotTerminal)
+{
+    InMemoryBackend b;
+    Layout l{"p"};
+    EXPECT_FALSE(isCreatorFenceTerminal(b, l, "never-mounted", 1))
+        << "absence proves nothing about liveness -- never waved through";
+}
+
+TEST(CASFenceTerminal, UndecodableMountBodyIsNotTerminal)
+{
+    InMemoryBackend b;
+    Layout l{"p"};
+    b.putIfAbsent(l.mountKey("r"), "garbage-not-a-lease", {});
+    EXPECT_FALSE(isCreatorFenceTerminal(b, l, "r", 1))
+        << "an unreadable lease of some other format generation must block, never wave through";
+}
+
+TEST(CASFenceTerminal, GcFencedIsTerminal)
+{
+    InMemoryBackend b;
+    Layout l{"p"};
+    ASSERT_EQ(claimMount(b, l, "r", UInt128(1), /*our_epoch=*/7, 1000, 500).kind, MountClaimResult::Claimed);
+    auto got = b.get(l.mountKey("r"));
+    ASSERT_TRUE(got.has_value());
+    MountLease fenced = decodeMountLease(got->bytes);
+    fenced.gc_fenced = true;
+    ASSERT_EQ(b.putOverwrite(l.mountKey("r"), encodeMountLease(fenced), got->token).outcome, PutOutcome::Done);
+
+    EXPECT_TRUE(isCreatorFenceTerminal(b, l, "r", 7));
+}
+
+TEST(CASFenceTerminal, CleanFarewellIsTerminal)
+{
+    InMemoryBackend b;
+    Layout l{"p"};
+    ASSERT_EQ(claimMount(b, l, "r", UInt128(1), /*our_epoch=*/7, 1000, 500).kind, MountClaimResult::Claimed);
+    auto got = b.get(l.mountKey("r"));
+    ASSERT_TRUE(got.has_value());
+    MountLease retired = decodeMountLease(got->bytes);
+    retired.min_active = std::numeric_limits<uint64_t>::max();
+    ASSERT_EQ(b.putOverwrite(l.mountKey("r"), encodeMountLease(retired), got->token).outcome, PutOutcome::Done);
+
+    EXPECT_TRUE(isCreatorFenceTerminal(b, l, "r", 7));
+}
+
+TEST(CASFenceTerminal, ADifferentLiveWriterEpochIsTerminalForTheOldOne)
+{
+    InMemoryBackend b;
+    Layout l{"p"};
+    /// Slot now held at epoch 8 -- epoch 7's incarnation is superseded regardless of ITS OWN
+    /// certificate (neither fenced nor farewelled).
+    ASSERT_EQ(claimMount(b, l, "r", UInt128(1), /*our_epoch=*/8, 1000, 500).kind, MountClaimResult::Claimed);
+
+    EXPECT_TRUE(isCreatorFenceTerminal(b, l, "r", 7))
+        << "a different epoch is currently live at this slot -- epoch 7 can never reclaim it";
+    EXPECT_FALSE(isCreatorFenceTerminal(b, l, "r", 8))
+        << "epoch 8 IS the current live epoch -- not terminal";
+}
+
+/// A merely EXPIRED lease (wall-clock past `expires_at_ms`, same epoch, no certificate) must NOT be
+/// treated as terminal -- mirrors `claimMount`'s own refusal to trust a bare timestamp comparison.
+TEST(CASFenceTerminal, ExpiredButSameEpochAndUncertifiedIsNotTerminal)
+{
+    InMemoryBackend b;
+    Layout l{"p"};
+    /// A lease whose stamped expiry is already far in the past, same epoch throughout.
+    ASSERT_EQ(claimMount(b, l, "r", UInt128(1), /*our_epoch=*/7, /*now_ms=*/0, /*ttl_ms=*/1).kind,
+              MountClaimResult::Claimed);
+
+    EXPECT_FALSE(isCreatorFenceTerminal(b, l, "r", 7))
+        << "expiry alone is never a certificate of death, exactly like claimMount's own discipline";
+}
diff --git a/src/Disks/tests/gtest_cas_namespace_file_request_profile.cpp b/src/Disks/tests/gtest_cas_namespace_file_request_profile.cpp
new file mode 100644
index 000000000000..bfb62f817ecb
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_namespace_file_request_profile.cpp
@@ -0,0 +1,558 @@
+#include <gtest/gtest.h>
+#include "cas_test_helpers.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <IO/WriteHelpers.h>
+#include <Core/Defines.h>
+
+/// Per-TU declaration of the one setting this file overrides, following the pattern `cas_test_helpers.h`
+/// documents for `server_root_id`/`scratch_path`: defined once in `ContentAddressedSettings.cpp`, declared
+/// by each consumer for what it actually references.
+namespace DB::ContentAddressedSetting
+{
+    extern const ContentAddressedSettingsBool gc_enabled;
+}
+
+#include <filesystem>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <vector>
+
+/// The namespace-file REQUEST PROFILE gate (directive §dedup-performance-constraint) -- for the
+/// `Pool` namespace-file surface, which is the layer that must be read carefully below.
+///
+/// `MergeTreeDeduplicationLog` rotates namespace files on the insert path because the CA disk cannot
+/// append, so every namespace-file operation's request profile is insert latency. The directive's
+/// constraint has five clauses: no catalog request per file operation, no ref-log append, no blob
+/// upload, no folder-manifest rewrite, and unchanged direct-object backend request counts.
+///
+/// WHAT THIS FILE PINS: the last clause, per key. The counts below were READ OFF this tree before any
+/// key change and pasted as literals, which is the whole point of the file -- expectations re-derived
+/// after a change measure the change against itself. Incarnation qualification changes the KEY a
+/// namespace file is stored under, so the keys are derived from `Layout` rather than spelled out; what
+/// must not move is the count per key and the set of keys touched.
+///
+/// TWO KINDS OF GATE LIVE IN THIS FILE, and mistaking one for the other would misread what it proves.
+/// The per-operation COUNTS above are baseline-anchored: they were read off the tree BEFORE the key change
+/// and pasted as literals, so they detect DRIFT from a measured past. The four negatives below are a
+/// FORWARD ALARM: a zero has no baseline to drift from, and the disk-layer cases could not have existed
+/// before the life resolution they measure was put on that path. They are no weaker for it -- they fail
+/// the moment a catalog, ref-log, blob or manifest request appears where none belongs -- but they are not
+/// evidence that anything was "unchanged".
+///
+/// WHERE THE OTHER FOUR CLAUSES ARE FENCED, and why they could not be fenced by the cases above. Every
+/// pool-layer case drives `Pool::putNamespaceFile`/`getNamespaceFile`/`removeNamespaceFile`/
+/// `listNamespaceFiles`, which reach `CasPlainObjects` and have no catalog, ref-log, blob or manifest
+/// path to take -- so at THAT layer the four negatives hold by construction of the call and measuring
+/// them proves nothing. The layer where they can be violated is the DISK operation above, where
+/// `ContentAddressedTransaction::writeFile` resolves the namespace's life; that is exactly where Task 4b
+/// put a life resolution, so that is where a per-operation catalog GET would appear. The two
+/// `CASNamespaceFileDiskProfile` cases at the bottom of this file fence it there, through a recording
+/// `IObjectStorage` (the metadata storage builds its own `Backend` from an `ObjectStoragePtr`, so the
+/// object storage is the injectable seam -- no production surface is widened for the test's benefit).
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+const String kNsString = "test/req_profile@cas@";
+const String kFile = "format_version.txt";
+/// A NESTED relative name, which is what the dedup log actually stores (its segments live in a
+/// table-level subdirectory), so the profile is captured on the shape the constraint is about.
+const String kSegment1 = "deduplication_logs/deduplication_log_1.txt";
+const String kSegment2 = "deduplication_logs/deduplication_log_2.txt";
+
+/// The identity every case below operates under. `fixture::fixtureLife` is the transitional mint Task 6
+/// deletes; what matters to this file is only that ONE life is used throughout, so a count is not
+/// split across two prefixes.
+NamespaceLifeId testLife()
+{
+    return fixture::fixtureLife(RootNamespace{kNsString});
+}
+
+/// A pool over `CountingBackend`, with the counts reset AFTER open: `Pool::open` runs its own
+/// capability probe and mount claim, and those requests belong to no file operation.
+PoolPtr openCountedPool(std::shared_ptr<CountingBackend> & out_backend)
+{
+    out_backend = std::make_shared<CountingBackend>();
+    PoolPtr store = openPoolForTest(out_backend);
+    out_backend->resetCounts();
+    return store;
+}
+
+}
+
+/// CREATE (the key is absent) and REWRITE (the key is present) are different request shapes on the
+/// same call, and the profile pins both: one HEAD to learn the token, then the create-if-absent or the
+/// token-conditioned replacement that HEAD selected.
+TEST(CASNamespaceFileRequestProfile, CreateThenRewrite)
+{
+    std::shared_ptr<CountingBackend> backend;
+    PoolPtr store = openCountedPool(backend);
+    const NamespaceLifeId life = testLife();
+    const String key = store->layout().namespaceFileKey(life, kFile);
+
+    store->putNamespaceFile(life, kFile, "1\n");
+
+    EXPECT_EQ(backend->headCount(key), 1u);
+    EXPECT_EQ(backend->putCount(key), 1u);            /// putIfAbsent -- the key was absent
+    EXPECT_EQ(backend->putOverwriteCount(key), 0u);
+    EXPECT_EQ(backend->getCount(key), 0u);
+    EXPECT_EQ(backend->deleteCount(key), 0u);
+    EXPECT_EQ(backend->listTotal(), 0u);
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+    EXPECT_EQ(backend->touchedKeys(), std::vector<String>{key});
+
+    backend->resetCounts();
+    store->putNamespaceFile(life, kFile, "2\n");
+
+    EXPECT_EQ(backend->headCount(key), 1u);
+    EXPECT_EQ(backend->putOverwriteCount(key), 1u);   /// token-conditioned replacement -- it existed
+    EXPECT_EQ(backend->putCount(key), 0u);
+    EXPECT_EQ(backend->getCount(key), 0u);
+    EXPECT_EQ(backend->deleteCount(key), 0u);
+    EXPECT_EQ(backend->listTotal(), 0u);
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+    EXPECT_EQ(backend->touchedKeys(), std::vector<String>{key});
+}
+
+/// A plain read is one whole-object GET and nothing else.
+TEST(CASNamespaceFileRequestProfile, Read)
+{
+    std::shared_ptr<CountingBackend> backend;
+    PoolPtr store = openCountedPool(backend);
+    const NamespaceLifeId life = testLife();
+    const String key = store->layout().namespaceFileKey(life, kFile);
+
+    store->putNamespaceFile(life, kFile, "1\n");
+    backend->resetCounts();
+
+    EXPECT_EQ(store->getNamespaceFile(life, kFile), String("1\n"));
+
+    EXPECT_EQ(backend->getCount(key), 1u);
+    EXPECT_EQ(backend->wholeGetCount(key), 1u);
+    EXPECT_EQ(backend->headCount(key), 0u);
+    EXPECT_EQ(backend->putCount(key), 0u);
+    EXPECT_EQ(backend->putOverwriteCount(key), 0u);
+    EXPECT_EQ(backend->touchedKeys(), std::vector<String>{key});
+}
+
+/// APPEND on a CA disk is serviced by read-modify-rewrite, and its request shape is the composition of
+/// the two calls that implement it: a GET of the current body, then a whole-body PUT of base+delta.
+/// Driven here as that composition against the same key, which is the shape whose count must not move.
+TEST(CASNamespaceFileRequestProfile, ReadModifyRewriteAppend)
+{
+    std::shared_ptr<CountingBackend> backend;
+    PoolPtr store = openCountedPool(backend);
+    const NamespaceLifeId life = testLife();
+    const String key = store->layout().namespaceFileKey(life, kSegment1);
+
+    store->putNamespaceFile(life, kSegment1, "base");
+    backend->resetCounts();
+
+    const std::optional<String> carried = store->getNamespaceFile(life, kSegment1);
+    ASSERT_TRUE(carried.has_value());
+    store->putNamespaceFile(life, kSegment1, *carried + "-delta");
+
+    EXPECT_EQ(backend->getCount(key), 1u);
+    EXPECT_EQ(backend->headCount(key), 1u);
+    EXPECT_EQ(backend->putOverwriteCount(key), 1u);
+    EXPECT_EQ(backend->putCount(key), 0u);
+    EXPECT_EQ(backend->deleteCount(key), 0u);
+    EXPECT_EQ(backend->listTotal(), 0u);
+    EXPECT_EQ(backend->touchedKeys(), std::vector<String>{key});
+    EXPECT_EQ(store->getNamespaceFile(life, kSegment1), String("base-delta"));
+}
+
+/// REMOVE is exact-token deletion, so it is one HEAD for the token plus one delete against it.
+TEST(CASNamespaceFileRequestProfile, Remove)
+{
+    std::shared_ptr<CountingBackend> backend;
+    PoolPtr store = openCountedPool(backend);
+    const NamespaceLifeId life = testLife();
+    const String key = store->layout().namespaceFileKey(life, kFile);
+
+    store->putNamespaceFile(life, kFile, "1\n");
+    backend->resetCounts();
+
+    store->removeNamespaceFile(life, kFile);
+
+    EXPECT_EQ(backend->headCount(key), 1u);
+    EXPECT_EQ(backend->deleteCount(key), 1u);
+    EXPECT_EQ(backend->getCount(key), 0u);
+    EXPECT_EQ(backend->putCount(key), 0u);
+    EXPECT_EQ(backend->putOverwriteCount(key), 0u);
+    EXPECT_EQ(backend->listTotal(), 0u);
+    EXPECT_EQ(backend->touchedKeys(), std::vector<String>{key});
+    EXPECT_FALSE(store->getNamespaceFile(life, kFile).has_value());
+}
+
+/// ROTATION is the sequence the constraint names: the retiring segment is enumerated, the new segment
+/// is created, and the retired one is removed. One LIST of the files prefix serves the enumeration (a
+/// single page here), and each segment carries its own create or remove shape.
+TEST(CASNamespaceFileRequestProfile, DedupLogRotation)
+{
+    std::shared_ptr<CountingBackend> backend;
+    PoolPtr store = openCountedPool(backend);
+    const NamespaceLifeId life = testLife();
+    const String prefix = store->layout().namespaceFilesPrefix(life);
+    const String old_key = store->layout().namespaceFileKey(life, kSegment1);
+    const String new_key = store->layout().namespaceFileKey(life, kSegment2);
+
+    store->putNamespaceFile(life, kSegment1, "segment-1-records");
+    backend->resetCounts();
+
+    const std::vector<String> before = store->listNamespaceFiles(life);
+    ASSERT_EQ(before, std::vector<String>{kSegment1});
+    store->putNamespaceFile(life, kSegment2, "segment-2-records");
+    store->removeNamespaceFile(life, kSegment1);
+
+    EXPECT_EQ(backend->listCount(prefix), 1u);
+    EXPECT_EQ(backend->listTotal(), 1u);
+    EXPECT_EQ(backend->headCount(new_key), 1u);
+    EXPECT_EQ(backend->putCount(new_key), 1u);
+    EXPECT_EQ(backend->putOverwriteCount(new_key), 0u);
+    EXPECT_EQ(backend->headCount(old_key), 1u);
+    EXPECT_EQ(backend->deleteCount(old_key), 1u);
+    EXPECT_EQ(backend->getTotal(), 0u);              /// rotation reads no body
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+    /// Sorted, and the files prefix is a proper prefix of both segment keys, so it comes first.
+    EXPECT_EQ(backend->touchedKeys(), (std::vector<String>{prefix, old_key, new_key}));
+
+    EXPECT_EQ(store->listNamespaceFiles(life), std::vector<String>{kSegment2});
+}
+
+
+/// ===================== THE FOUR NEGATIVES, AT THE DISK LAYER =====================
+///
+/// Constraint 16's other four clauses: a namespace-file operation performs no catalog request, no
+/// ref-log append, no blob upload and no folder-manifest rewrite. They are asserted here rather than
+/// above because only here is there a life resolution to get wrong.
+///
+/// WHAT MAKES THE CLAIM NON-TRIVIAL. Task 4b's read and write paths resolve a catalog-minted life. That
+/// resolution is per TABLE-OPEN -- `CasRefLedger` caches it on the table's runtime -- so the steady-state
+/// operation pays nothing for it. If it ever became per-operation, `format_version.txt` and every
+/// dedup-log rotation on the insert path would carry a catalog round trip, and nothing else in the suite
+/// would notice. `SteadyStateFileOperationsTouchNoCatalogRefBlobOrManifestKey` is that alarm, and
+/// `TheLifeResolutionIsPaidOncePerTableOpen` is the other half: it shows the birth cost EXISTS and is
+/// paid exactly once, so the steady-state zeros are a real property rather than an artifact of a fixture
+/// that never triggered a resolution at all.
+
+namespace
+{
+
+/// A `LocalObjectStorage` that records every key it is asked about, per operation family. Used to ask
+/// "was any key under these four families touched at all", which is a question about WHICH keys an
+/// operation reaches -- not about counts -- so recording the key set is the whole instrument.
+///
+/// It overrides every method `CasObjectStorageBackend` reaches: a family left un-overridden would be an
+/// unrecorded path, and an assertion of "nothing touched it" would then be silently satisfied by the
+/// gap rather than by the behaviour.
+class RecordingObjectStorage : public DB::LocalObjectStorage
+{
+public:
+    using DB::LocalObjectStorage::LocalObjectStorage;
+
+    bool exists(const DB::StoredObject & object) const override
+    {
+        record(object.remote_path, /*is_write*/ false);
+        return DB::LocalObjectStorage::exists(object);
+    }
+
+    std::unique_ptr<DB::ReadBufferFromFileBase> readObject(
+        const DB::StoredObject & object, const DB::ReadSettings & read_settings,
+        std::optional<size_t> read_hint, bool use_external_buffer,
+        bool restrict_seek) const override
+    {
+        record(object.remote_path, /*is_write*/ false);
+        return DB::LocalObjectStorage::readObject(object, read_settings, read_hint, use_external_buffer, restrict_seek);
+    }
+
+    std::unique_ptr<DB::WriteBufferFromFileBase> writeObject(
+        const DB::StoredObject & object, DB::WriteMode mode,
+        std::optional<DB::ObjectAttributes> attributes,
+        size_t buf_size,
+        const DB::WriteSettings & write_settings) override
+    {
+        record(object.remote_path, /*is_write*/ true);
+        return DB::LocalObjectStorage::writeObject(object, mode, attributes, buf_size, write_settings);
+    }
+
+    void removeObjectIfExists(const DB::StoredObject & object) override
+    {
+        record(object.remote_path, /*is_write*/ true);
+        DB::LocalObjectStorage::removeObjectIfExists(object);
+    }
+
+    void removeObjectsIfExist(const DB::StoredObjects & objects) override
+    {
+        for (const DB::StoredObject & object : objects)
+            record(object.remote_path, /*is_write*/ true);
+        DB::LocalObjectStorage::removeObjectsIfExist(objects);
+    }
+
+    DB::ObjectMetadata getObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        record(path, /*is_write*/ false);
+        return DB::LocalObjectStorage::getObjectMetadata(path, with_tags);
+    }
+
+    std::optional<DB::ObjectMetadata> tryGetObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        record(path, /*is_write*/ false);
+        return DB::LocalObjectStorage::tryGetObjectMetadata(path, with_tags);
+    }
+
+    void listObjects(const std::string & path, DB::RelativePathsWithMetadata & children, size_t max_keys) const override
+    {
+        record(path, /*is_write*/ false);
+        DB::LocalObjectStorage::listObjects(path, children, max_keys);
+    }
+
+    bool existsOrHasAnyChild(const std::string & path) const override
+    {
+        record(path, /*is_write*/ false);
+        return DB::LocalObjectStorage::existsOrHasAnyChild(path);
+    }
+
+    void copyObject(
+        const DB::StoredObject & object_from, const DB::StoredObject & object_to,
+        const DB::ReadSettings & read_settings, const DB::WriteSettings & write_settings,
+        std::optional<DB::ObjectAttributes> object_to_attributes) override
+    {
+        record(object_from.remote_path, /*is_write*/ false);
+        record(object_to.remote_path, /*is_write*/ true);
+        DB::LocalObjectStorage::copyObject(object_from, object_to, read_settings, write_settings, object_to_attributes);
+    }
+
+    /// Every recorded key containing `needle`, in first-touch order, so a failure names the offender.
+    std::vector<String> touchedContaining(std::string_view needle) const
+    {
+        std::lock_guard lock(mutex);
+        std::vector<String> out;
+        for (const String & key : touched)
+            if (key.find(needle) != String::npos)
+                out.push_back(key);
+        return out;
+    }
+
+    std::vector<String> writtenContaining(std::string_view needle) const
+    {
+        std::lock_guard lock(mutex);
+        std::vector<String> out;
+        for (const String & key : written)
+            if (key.find(needle) != String::npos)
+                out.push_back(key);
+        return out;
+    }
+
+    void resetRecords()
+    {
+        std::lock_guard lock(mutex);
+        touched.clear();
+        written.clear();
+    }
+
+private:
+    void record(const std::string & key, bool is_write) const
+    {
+        std::lock_guard lock(mutex);
+        touched.push_back(key);
+        if (is_write)
+            written.push_back(key);
+    }
+
+    mutable std::mutex mutex;
+    mutable std::vector<String> touched;
+    mutable std::vector<String> written;
+};
+
+const std::string kTableUuid = "a11a11a1-1111-4111-8111-111111111111";
+const std::string kTablePath = "a11/a11a11a1-1111-4111-8111-111111111111";
+
+/// The four families the constraint forbids a file operation from touching, as key substrings. Taken
+/// from `Layout` where a helper exists rather than spelled out, so a layout change breaks this by
+/// failing to compile or by moving the substring, not by silently matching nothing.
+struct ForbiddenFamily
+{
+    String needle;
+    String clause;
+};
+
+std::vector<ForbiddenFamily> forbiddenFamilies(const DB::Cas::Layout & layout)
+{
+    return {
+        {layout.refCatalogKey(), "no catalog request"},
+        {layout.casRefsPrefix(), "no ref-log append"},
+        {layout.blobsPrefix(), "no blob upload"},
+        {layout.casManifestsPrefix(), "no folder-manifest rewrite"},
+    };
+}
+
+std::shared_ptr<DB::ContentAddressedMetadataStorage> openRecordingStorage(
+    std::shared_ptr<RecordingObjectStorage> & out_object_storage)
+{
+    static std::atomic<uint64_t> counter{0};
+    const String unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_ns_file_profile_" + unique)).string();
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    out_object_storage = std::make_shared<RecordingObjectStorage>(
+        DB::LocalObjectStorageSettings("test", root, /*read_only_=*/false));
+
+    auto settings = makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / ("cas_ns_file_profile_scratch_" + unique));
+    /// A GC round touches `cas/ref_catalog`, `cas/ns/stream/` and `cas/manifests/`, so with the
+    /// background scheduler enabled these zeros would hold only because the first tick (60s) outlives the
+    /// test. A timer is not a fence.
+    settings[DB::ContentAddressedSetting::gc_enabled] = false;
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        out_object_storage, "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+/// One verbatim namespace file written through the REAL disk write path (the buffer whose finalize
+/// callback reaches `putNamespaceFile`), not through the pool surface.
+void writeVerbatimThroughDisk(   /// ASSERT_* inside -> must return void
+    DB::ContentAddressedMetadataStorage & storage, const std::string & path, const String & bytes,
+    DB::WriteMode mode = DB::WriteMode::Rewrite)
+{
+    /// `tryCreateWriteBuffer` is the interface entry the disk itself uses, so this drives the same
+    /// buffer construction (and the same autocommit-on-finalize contract for verbatim files) that a real
+    /// write does. `owner` is null here: only a part-blob buffer's deferred finalize needs the pin, and
+    /// a verbatim file finalizes inline, inside this call's scope.
+    auto tx = storage.createTransaction();
+    auto buf = tx->tryCreateWriteBuffer(
+        /*owner*/ nullptr, path, DB::DBMS_DEFAULT_BUFFER_SIZE, mode, {}, /*autocommit*/ true);
+    ASSERT_TRUE(buf != nullptr);
+    DB::writeString(bytes, *buf);
+    buf->finalize();
+}
+
+}
+
+/// The steady state: with the table open and its life already resolved, no namespace-file operation --
+/// rewrite, append, read, rotation, remove -- touches a catalog, ref, blob or manifest key.
+TEST(CASNamespaceFileDiskProfile, SteadyStateFileOperationsTouchNoCatalogRefBlobOrManifestKey)
+{
+    std::shared_ptr<RecordingObjectStorage> object_storage;
+    auto storage = openRecordingStorage(object_storage);
+    const DB::Cas::Layout & layout = storage->store()->layout();
+
+    /// Open the table by doing the first file operation, which is what resolves (and here mints) the
+    /// life. Everything measured below happens after it.
+    writeVerbatimThroughDisk(*storage, kTablePath + "/format_version.txt", "1\n");
+    ASSERT_TRUE(storage->existsFile(kTablePath + "/format_version.txt"));
+
+    object_storage->resetRecords();
+
+    /// A whole-file rewrite, the read-modify-rewrite append, a read, and a dedup-log rotation
+    /// (create the new segment, enumerate, drop the retired one) -- the four shapes the constraint names.
+    writeVerbatimThroughDisk(*storage, kTablePath + "/format_version.txt", "2\n");
+    writeVerbatimThroughDisk(*storage, kTablePath + "/deduplication_logs/deduplication_log_1.txt", "a");
+    writeVerbatimThroughDisk(
+        *storage, kTablePath + "/deduplication_logs/deduplication_log_1.txt", "b", DB::WriteMode::Append);
+    EXPECT_EQ(storage->tryGetInManifestBytes(kTablePath + "/deduplication_logs/deduplication_log_1.txt"),
+              std::optional<String>("ab"));
+    writeVerbatimThroughDisk(*storage, kTablePath + "/deduplication_logs/deduplication_log_2.txt", "c");
+    storage->createTransaction()->unlinkFile(
+        kTablePath + "/deduplication_logs/deduplication_log_1.txt", /*if_exists*/ false, /*remove_metadata_only*/ false);
+
+    for (const ForbiddenFamily & family : forbiddenFamilies(layout))
+        EXPECT_EQ(object_storage->touchedContaining(family.needle), std::vector<String>{})
+            << "Constraint 16, '" << family.clause << "': a namespace-file operation reached " << family.needle;
+
+    /// A positive control on the instrument itself: the operations above DID reach the store, so the
+    /// four empty answers are the absence of those families and not a recorder that recorded nothing.
+    EXPECT_FALSE(object_storage->writtenContaining("/_files/").empty())
+        << "the recorder must have seen the file writes themselves";
+}
+
+/// The other half: the life resolution is real and is paid ONCE per table-open. Without this, the zeros
+/// above could be produced by a fixture in which no resolution ever happened.
+TEST(CASNamespaceFileDiskProfile, TheLifeResolutionIsPaidOncePerTableOpen)
+{
+    std::shared_ptr<RecordingObjectStorage> object_storage;
+    auto storage = openRecordingStorage(object_storage);
+    const DB::Cas::Layout & layout = storage->store()->layout();
+
+    /// The FIRST namespace-file operation on a never-opened table resolves the life from the catalog,
+    /// minting the namespace when it names none -- so it DOES reach the catalog. That is the per-open
+    /// cost, and the reason the steady-state case above resets its records after this point.
+    writeVerbatimThroughDisk(*storage, kTablePath + "/format_version.txt", "1\n");
+    EXPECT_FALSE(object_storage->touchedContaining(layout.refCatalogKey()).empty())
+        << "the first file operation must resolve a life, which reaches the catalog";
+
+    object_storage->resetRecords();
+
+    /// The second operation on the SAME open table resolves nothing: the life is cached on the table's
+    /// runtime. This is the assertion that says "per table-open", and it is the one that would fail if a
+    /// future change moved the resolution onto the operation.
+    writeVerbatimThroughDisk(*storage, kTablePath + "/format_version.txt", "2\n");
+    EXPECT_EQ(object_storage->touchedContaining(layout.refCatalogKey()), std::vector<String>{})
+        << "a second file operation must not re-resolve the life";
+}
+
+
+/// THE REMOVAL PATHS MUST NOT CREATE A NAMESPACE — the case that regressed silently in this task's first
+/// round, so it is pinned on the catalog rather than on the file outcome.
+///
+/// Why the file outcome cannot pin it: `unlinkFile`/`removeRecursive` against a never-opened table
+/// answer "absent" both before and after the defect, because a freshly minted namespace has no files
+/// either. The only observable difference is the catalog write, so that is what is asserted. And it
+/// matters twice over: `unlinkFile(..., if_exists = true)` is called from cleanup paths whose contract is
+/// to be a no-op, and the catalog is ONE pool-wide object under a capacity-admission predicate — a
+/// removal that admits an entry per never-created table grows it without bound.
+TEST(CASNamespaceFileDiskProfile, RemovalOnANeverOpenedTableLeavesTheCatalogUntouched)
+{
+    std::shared_ptr<RecordingObjectStorage> object_storage;
+    auto storage = openRecordingStorage(object_storage);
+    const DB::Cas::Layout & layout = storage->store()->layout();
+
+    /// A valid pool already owns its explicit empty mandatory catalog. Nothing has opened this table:
+    /// no namespace file written, no part published, and no ref operation has changed that object.
+    const auto catalog_before = storage->store()->backend().get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_before);
+    EXPECT_TRUE(decodeRefCatalog(catalog_before->bytes).entries.empty());
+    object_storage->resetRecords();
+
+    /// Three removal shapes, all against paths under a table that does not exist.
+    storage->createTransaction()->unlinkFile(
+        kTablePath + "/format_version.txt", /*if_exists*/ true, /*remove_metadata_only*/ false);
+    storage->createTransaction()->removeRecursive(
+        kTablePath + "/deduplication_logs", DB::IMetadataTransaction::ShouldRemoveObjectsPredicate{});
+    /// The table directory ITSELF, which is a different arm from the subdirectory above: it is the one
+    /// that reaches the ref layer's namespace drop and its ref enumeration, rather than only the
+    /// namespace-file resolver.
+    storage->createTransaction()->removeRecursive(
+        kTablePath, DB::IMetadataTransaction::ShouldRemoveObjectsPredicate{});
+    EXPECT_FALSE(storage->existsFile(kTablePath + "/format_version.txt"));
+
+    EXPECT_EQ(object_storage->writtenContaining(layout.refCatalogKey()), std::vector<String>{})
+        << "a removal must not write the catalog: it must not birth the namespace it is removing from";
+    const auto catalog_after_removal = storage->store()->backend().get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_after_removal);
+    EXPECT_EQ(catalog_after_removal->bytes, catalog_before->bytes);
+    EXPECT_EQ(catalog_after_removal->token, catalog_before->token)
+        << "the mandatory catalog must remain byte-for-byte and token-for-token unchanged";
+
+    /// Not vacuous: the SAME operations on the same table after a write do reach the file, so the zeros
+    /// above are the absence of a birth and not the absence of any work.
+    writeVerbatimThroughDisk(*storage, kTablePath + "/format_version.txt", "1\n");
+    ASSERT_TRUE(storage->existsFile(kTablePath + "/format_version.txt"));
+    storage->createTransaction()->unlinkFile(
+        kTablePath + "/format_version.txt", /*if_exists*/ false, /*remove_metadata_only*/ false);
+    EXPECT_FALSE(storage->existsFile(kTablePath + "/format_version.txt"));
+    /// Positive control: the write really did birth the namespace and mutate the same catalog object
+    /// whose stability the removal assertions pin above.
+    const auto catalog_after_birth = storage->store()->backend().get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_after_birth);
+    EXPECT_NE(catalog_after_birth->bytes, catalog_after_removal->bytes);
+    EXPECT_NE(catalog_after_birth->token, catalog_after_removal->token);
+}
diff --git a/src/Disks/tests/gtest_cas_namespace_janitor.cpp b/src/Disks/tests/gtest_cas_namespace_janitor.cpp
new file mode 100644
index 000000000000..7a3455738bc4
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_namespace_janitor.cpp
@@ -0,0 +1,607 @@
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasNamespaceJanitor.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcMaintenanceState.h>
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+class OrderedJanitorBackend : public CountingBackend
+{
+public:
+    using CountingBackend::get;
+    std::vector<String> events;
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (prefix.ends_with("/cas/ns/"))
+            events.push_back("list");
+        return CountingBackend::list(prefix, cursor, limit);
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (key.ends_with("/cas/ref_catalog"))
+            events.push_back("catalog");
+        return CountingBackend::get(key, range);
+    }
+};
+
+class OmitFirstNamespacePageBackend : public CountingBackend
+{
+public:
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (omit && prefix.ends_with("/cas/ns/"))
+        {
+            omit = false;
+            return {};
+        }
+        return CountingBackend::list(prefix, cursor, limit);
+    }
+private:
+    bool omit = true;
+};
+
+class ReplaceBeforeJanitorDeleteBackend : public CountingBackend
+{
+public:
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        if (!replaced)
+        {
+            replaced = true;
+            const auto current = InMemoryBackend::get(key);
+            if (current)
+                (void)InMemoryBackend::casPut(key, "winner", current->token);
+        }
+        return CountingBackend::deleteExact(key, token);
+    }
+private:
+    bool replaced = false;
+};
+
+class TokenlessListBackend : public CountingBackend
+{
+public:
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = CountingBackend::list(prefix, cursor, limit);
+        for (ListedKey & key : page.keys)
+            key.token.reset();
+        return page;
+    }
+
+    bool supportsListTokens() const override { return false; }
+
+    HeadResult head(const String & key) override
+    {
+        HeadResult result = CountingBackend::head(key);
+        if (!replaced && result.exists && key == replace_on_head)
+        {
+            replaced = true;
+            (void)InMemoryBackend::casPut(key, "winner", result.token);
+        }
+        return result;
+    }
+
+    String replace_on_head;
+
+private:
+    bool replaced = false;
+};
+
+class FenceLossDuringHeadBackend : public TokenlessListBackend
+{
+public:
+    HeadResult head(const String & key) override
+    {
+        HeadResult result = TokenlessListBackend::head(key);
+        fence_held = false;
+        return result;
+    }
+
+    bool fence_held = true;
+};
+
+class CatalogAfterListBackend : public CountingBackend
+{
+public:
+    explicit CatalogAfterListBackend(NamespaceLifeId life_) : protected_life(std::move(life_)) {}
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = CountingBackend::list(prefix, cursor, limit);
+        if (!published && prefix.ends_with("/cas/ns/"))
+        {
+            published = true;
+            const String catalog_key = "p/cas/ref_catalog";
+            /// This models a CONCURRENT actor's read, not the janitor's own -- counting it here would
+            /// make `PostListCatalogCutProtectsConcurrentCreationWithOneGet`'s "exactly one get" assertion
+            /// count this simulated actor's read as the janitor's, defeating the point of that assertion.
+            const auto current = InMemoryBackend::get(catalog_key, {}); // NOLINT(bugprone-parent-virtual-call)
+            if (current)
+            {
+                RefCatalog catalog;
+                catalog.entries.push_back(CatalogEntry{.ns = protected_life.ns, .state = NsState::Live,
+                    .incarnation = protected_life.incarnation});
+                (void)InMemoryBackend::casPut(catalog_key, encodeRefCatalog(catalog), current->token);
+            }
+        }
+        return page;
+    }
+private:
+    NamespaceLifeId protected_life;
+    bool published = false;
+};
+
+class RejectCursorBackend : public CountingBackend
+{
+public:
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (prefix.ends_with("/cas/ns/") && !cursor.empty())
+            throw std::runtime_error("backend rejected cursor");
+        return CountingBackend::list(prefix, cursor, limit);
+    }
+};
+
+class FailMaintenancePublicationBackend : public CountingBackend
+{
+public:
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+        const ObjectMeta & meta) override
+    {
+        if (fail_publication && key.ends_with("/gc/maintenance_state"))
+            throw std::runtime_error("maintenance publication failed");
+        return CountingBackend::casPut(key, bytes, expected, meta);
+    }
+    bool fail_publication = false;
+};
+
+void seedCatalog(CountingBackend & backend, const Layout & layout, RefCatalog catalog = {})
+{
+    ASSERT_EQ(backend.putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(catalog)).outcome, PutOutcome::Done);
+}
+
+NamespaceLifeId life(const char * name, uint64_t id)
+{
+    const RootNamespace ns{name};
+    return NamespaceLifeId::fromCatalogEntry(ns, UInt128{id});
+}
+
+}
+
+TEST(CASNamespaceJanitor, DeletesDeadFilesAndCheckpointFromOnePostListCatalogCut)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const auto dead = life("dead", 41);
+    const String file = layout.namespaceFilesPrefix(dead) + "part/data.bin";
+    const String ckpt = layout.refCkptKey(dead);
+    ASSERT_EQ(backend.putIfAbsent(file, "file-bytes").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(ckpt, "ckpt-bytes").outcome, PutOutcome::Done);
+    backend.resetCounts();
+
+    NamespaceJanitor janitor(backend, layout, 100);
+    const NamespaceJanitorResult result = janitor.runOnePage(false, [] { return true; });
+
+    EXPECT_EQ(result.pages, 1u);
+    EXPECT_EQ(result.keys, 2u);
+    EXPECT_EQ(result.deleted, 2u);
+    EXPECT_FALSE(backend.get(file));
+    EXPECT_FALSE(backend.get(ckpt));
+    EXPECT_EQ(backend.listCount(layout.namespaceRootPrefix()), 1u);
+    EXPECT_EQ(backend.getCount(layout.refCatalogKey()), 1u);
+    EXPECT_EQ(readGcMaintenanceState(backend, layout).state, GcMaintenanceState{});
+}
+
+TEST(CASNamespaceJanitor, RetainsEveryCurrentLifecycleAndSuppressesAmbiguousCut)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    RefCatalog catalog;
+    CatalogEntry creating{.ns = RootNamespace{"creating"}, .state = NsState::Creating, .incarnation = UInt128{51},
+        .creator = CreatorFence{.server_root_id = "srv", .writer_epoch = 1, .fence_generation = 1}};
+    CatalogEntry live{.ns = RootNamespace{"live"}, .state = NsState::Live, .incarnation = UInt128{52}};
+    CatalogEntry removing{.ns = RootNamespace{"removing"}, .state = NsState::Removing, .incarnation = UInt128{53},
+        .removal_started_round = 1};
+    catalog.entries = {creating, live, removing};
+    seedCatalog(backend, layout, catalog);
+    for (const auto & entry : catalog.entries)
+        ASSERT_EQ(backend.putIfAbsent(layout.refCkptKey(
+            NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation)), "keep").outcome, PutOutcome::Done);
+
+    NamespaceJanitor janitor(backend, layout, 100);
+    const auto result = janitor.runOnePage(false, [] { return true; });
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_EQ(backend.deleteTotal(), 0u);
+}
+
+TEST(CASNamespaceJanitor, CatalogFirstCreatingRetainsEveryObjectOfTheNewLife)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    const CatalogEntry creating{
+        .ns = RootNamespace{"catalog-first"},
+        .state = NsState::Creating,
+        .incarnation = UInt128{54},
+        .creator = CreatorFence{.server_root_id = "srv", .writer_epoch = 2, .fence_generation = 3}};
+    seedCatalog(backend, layout, RefCatalog{.entries = {creating}});
+
+    /// The production creation order is the point: the catalog row is durable before either object.
+    const NamespaceLifeId creating_life
+        = NamespaceLifeId::fromCatalogEntry(creating.ns, creating.incarnation);
+    const String ckpt = layout.refCkptKey(creating_life);
+    const String file = layout.namespaceFilesPrefix(creating_life) + "data";
+    ASSERT_EQ(backend.putIfAbsent(ckpt, "checkpoint").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(file, "file").outcome, PutOutcome::Done);
+    backend.resetCounts();
+
+    const NamespaceJanitorResult result
+        = NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; });
+
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_EQ(backend.deleteTotal(), 0u);
+    EXPECT_EQ(backend.getCount(layout.refCatalogKey()), 1u);
+    EXPECT_TRUE(backend.get(ckpt));
+    EXPECT_TRUE(backend.get(file));
+}
+
+TEST(CASNamespaceJanitor, CancelledCreatingCheckpointIsReclaimedThroughPublicLifecycle)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    const CatalogEntry creating{
+        .ns = RootNamespace{"cancelled"},
+        .state = NsState::Creating,
+        .incarnation = UInt128{55},
+        .creator = CreatorFence{.server_root_id = "dead-srv", .writer_epoch = 4, .fence_generation = 5}};
+    seedCatalog(backend, layout, RefCatalog{.entries = {creating}});
+    const String ckpt = layout.refCkptKey(
+        NamespaceLifeId::fromCatalogEntry(creating.ns, creating.incarnation));
+    ASSERT_EQ(backend.putIfAbsent(ckpt, "cancelled-checkpoint").outcome, PutOutcome::Done);
+
+    ASSERT_EQ(CasRefCatalog::cancelStalledCreating(
+        backend, layout, creating, [](const CreatorFence &) { return true; },
+        /*admitted_generation=*/7, [](uint64_t) {}),
+        CasRefCatalog::StalledCreatingCancelOutcome::Cancelled);
+    EXPECT_TRUE(CasRefCatalog::read(backend, layout).catalog.entries.empty());
+
+    const NamespaceJanitorResult result
+        = NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; });
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_FALSE(backend.get(ckpt));
+}
+
+TEST(CASNamespaceJanitor, SuppressionAndFenceLossDeleteNothing)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String first = layout.refCkptKey(life("dead-a", 61));
+    const String second = layout.refCkptKey(life("dead-b", 62));
+    ASSERT_EQ(backend.putIfAbsent(first, "first").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(second, "second").outcome, PutOutcome::Done);
+
+    NamespaceJanitor janitor(backend, layout, 1);
+    EXPECT_EQ(janitor.runOnePage(true, [] { return true; }).deleted, 0u);
+    EXPECT_EQ(readGcMaintenanceState(backend, layout).status, GcMaintenanceReadStatus::Absent)
+        << "a globally suppressed page is undecided and must not mint cleanup progress";
+    EXPECT_EQ(backend.putCount(layout.gcMaintenanceStateKey()), 0u);
+    EXPECT_EQ(backend.casPutCount(layout.gcMaintenanceStateKey()), 0u);
+    EXPECT_EQ(janitor.runOnePage(false, [] { return false; }).deleted, 0u);
+    EXPECT_EQ(readGcMaintenanceState(backend, layout).status, GcMaintenanceReadStatus::Absent)
+        << "fence loss must not mint progress past a page whose deletion was not authorized";
+    EXPECT_TRUE(backend.get(first));
+    EXPECT_TRUE(backend.get(second));
+    EXPECT_EQ(backend.deleteTotal(), 0u);
+}
+
+TEST(CASNamespaceJanitor, FenceLossOnRetainedOnlyPageDoesNotAdvanceCursor)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    const CatalogEntry current{
+        .ns = RootNamespace{"current"}, .state = NsState::Live, .incarnation = UInt128{63}};
+    seedCatalog(backend, layout, RefCatalog{.entries = {current}});
+    const NamespaceLifeId current_life
+        = NamespaceLifeId::fromCatalogEntry(current.ns, current.incarnation);
+    const String ckpt = layout.refCkptKey(current_life);
+    const String file = layout.namespaceFilesPrefix(current_life) + "data";
+    ASSERT_EQ(backend.putIfAbsent(ckpt, "checkpoint").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(file, "file").outcome, PutOutcome::Done);
+
+    const NamespaceJanitorResult result
+        = NamespaceJanitor(backend, layout, 1).runOnePage(false, [] { return false; });
+
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_EQ(backend.deleteTotal(), 0u);
+    EXPECT_TRUE(backend.get(ckpt));
+    EXPECT_TRUE(backend.get(file));
+    EXPECT_EQ(readGcMaintenanceState(backend, layout).status, GcMaintenanceReadStatus::Absent)
+        << "a tenure that observes fence loss cannot publish progress even when every object was retained";
+}
+
+TEST(CASNamespaceJanitor, FenceLossAfterLastDeleteRetainsCursorWithoutRollingBackDelete)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String dead = layout.refCkptKey(life("dead-after-delete", 64));
+    ASSERT_EQ(backend.putIfAbsent(dead, "dead").outcome, PutOutcome::Done);
+    uint64_t fence_checks = 0;
+
+    const NamespaceJanitorResult result
+        = NamespaceJanitor(backend, layout, 1).runOnePage(false, [&] { return fence_checks++ == 0; });
+
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_FALSE(backend.get(dead))
+        << "the exact delete completed under the fence and is never rolled back";
+    EXPECT_EQ(fence_checks, 2u)
+        << "the fence must be checked before deletion and again immediately before cursor publication";
+    EXPECT_EQ(readGcMaintenanceState(backend, layout).status, GcMaintenanceReadStatus::Absent)
+        << "losing the fence after the delete keeps this page selected for an idempotent retry";
+}
+
+TEST(CASNamespaceJanitor, CursorResumesThenResetsAtEnd)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const auto dead = life("dead", 71);
+    ASSERT_EQ(backend.putIfAbsent(layout.namespaceFilesPrefix(dead) + "a", "a").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(layout.namespaceFilesPrefix(dead) + "b", "b").outcome, PutOutcome::Done);
+
+    NamespaceJanitor first_process(backend, layout, 1);
+    EXPECT_EQ(first_process.runOnePage(false, [] { return true; }).deleted, 1u);
+    const auto mid = readGcMaintenanceState(backend, layout);
+    ASSERT_EQ(mid.status, GcMaintenanceReadStatus::Valid);
+    ASSERT_TRUE(mid.state);
+    EXPECT_FALSE(mid.state->janitor_cursor.empty());
+    NamespaceJanitor restarted_process(backend, layout, 1);
+    EXPECT_EQ(restarted_process.runOnePage(false, [] { return true; }).deleted, 1u);
+    EXPECT_TRUE(readGcMaintenanceState(backend, layout).state->janitor_cursor.empty());
+}
+
+TEST(CASNamespaceJanitor, TakesOneCatalogCutAfterListingAndContinuesPastMalformedKey)
+{
+    OrderedJanitorBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const auto dead = life("dead", 81);
+    const String valid = layout.namespaceFilesPrefix(dead) + "data";
+    const String malformed = layout.namespaceStreamRootPrefix() + "not-a-life/_log/1-1.zst";
+    const String malformed_state = layout.namespaceStateRootPrefix() + "not-a-life/_ckpt";
+    ASSERT_EQ(backend.putIfAbsent(valid, "v").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(malformed, "bad").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(malformed_state, "bad-state").outcome, PutOutcome::Done);
+    backend.resetCounts();
+    backend.events.clear();
+
+    const auto result = NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; });
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_FALSE(result.anomalies.empty());
+    EXPECT_TRUE(backend.get(malformed));
+    EXPECT_TRUE(backend.get(malformed_state));
+    ASSERT_EQ(backend.events.size(), 2u);
+    EXPECT_EQ(backend.events[0], "list");
+    EXPECT_EQ(backend.events[1], "catalog");
+    EXPECT_EQ(backend.getCount(layout.refCatalogKey()), 1u);
+}
+
+TEST(CASNamespaceJanitor, MalformedKeyIsFinalAndAdvancesCursor)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String first = layout.namespaceStreamRootPrefix() + "bad-a/_log/1-1.zst";
+    const String second = layout.namespaceStreamRootPrefix() + "bad-b/_log/1-1.zst";
+    ASSERT_EQ(backend.putIfAbsent(first, "first").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(second, "second").outcome, PutOutcome::Done);
+
+    const NamespaceJanitorResult result
+        = NamespaceJanitor(backend, layout, 1).runOnePage(false, [] { return true; });
+
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_FALSE(result.anomalies.empty());
+    EXPECT_TRUE(backend.get(first));
+    EXPECT_TRUE(backend.get(second));
+    const GcMaintenanceReadResult progress = readGcMaintenanceState(backend, layout);
+    ASSERT_EQ(progress.status, GcMaintenanceReadStatus::Valid);
+    ASSERT_TRUE(progress.state);
+    EXPECT_FALSE(progress.state->janitor_cursor.empty())
+        << "malformed keys are surfaced and skipped, but do not pin the cleanup cycle";
+}
+
+TEST(CASNamespaceJanitor, DuplicateCurrentLifeSuppressesWholePage)
+{
+    CountingBackend backend;
+    const Layout layout("p");
+    RefCatalog catalog;
+    catalog.entries = {
+        CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Live, .incarnation = UInt128{91}},
+        CatalogEntry{.ns = RootNamespace{"b"}, .state = NsState::Live, .incarnation = UInt128{91}}};
+    seedCatalog(backend, layout, catalog);
+    const String dead_a = layout.refCkptKey(life("dead-a", 92));
+    const String dead_b = layout.refCkptKey(life("dead-b", 93));
+    ASSERT_EQ(backend.putIfAbsent(dead_a, "a").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(dead_b, "b").outcome, PutOutcome::Done);
+    const auto result = NamespaceJanitor(backend, layout, 1).runOnePage(false, [] { return true; });
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_EQ(backend.deleteTotal(), 0u);
+    EXPECT_TRUE(backend.get(dead_a));
+    EXPECT_TRUE(backend.get(dead_b));
+    EXPECT_EQ(readGcMaintenanceState(backend, layout).status, GcMaintenanceReadStatus::Absent)
+        << "an ambiguous catalog cut leaves the selected page undecided for an authoritative retry";
+}
+
+TEST(CASNamespaceJanitor, CorruptProgressResetsWithoutDeletingAndFilesOnlyOmittedCycleRetries)
+{
+    OmitFirstNamespacePageBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String dead = layout.namespaceFilesPrefix(life("dead", 101)) + "only-residue";
+    ASSERT_EQ(backend.putIfAbsent(dead, "bytes").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(layout.gcMaintenanceStateKey(), "corrupt").outcome, PutOutcome::Done);
+    EXPECT_EQ(NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; }).deleted, 0u);
+    EXPECT_TRUE(backend.get(dead));
+    EXPECT_EQ(readGcMaintenanceState(backend, layout).status, GcMaintenanceReadStatus::Valid);
+    EXPECT_EQ(NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; }).deleted, 0u);
+    EXPECT_TRUE(backend.get(dead));
+    EXPECT_EQ(NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; }).deleted, 1u);
+    EXPECT_FALSE(backend.get(dead));
+}
+
+TEST(CASNamespaceJanitor, ExactTokenMismatchRetainsConcurrentReplacement)
+{
+    ReplaceBeforeJanitorDeleteBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String dead = layout.refCkptKey(life("dead-a", 111));
+    const String later = layout.refCkptKey(life("dead-b", 112));
+    ASSERT_EQ(backend.putIfAbsent(dead, "old").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(later, "later").outcome, PutOutcome::Done);
+    const auto result = NamespaceJanitor(backend, layout, 1).runOnePage(false, [] { return true; });
+    EXPECT_EQ(result.deleted, 0u);
+    ASSERT_TRUE(backend.get(dead));
+    EXPECT_EQ(backend.get(dead)->bytes, "winner");
+    EXPECT_TRUE(backend.get(later));
+    const GcMaintenanceReadResult progress = readGcMaintenanceState(backend, layout);
+    ASSERT_EQ(progress.status, GcMaintenanceReadStatus::Valid);
+    ASSERT_TRUE(progress.state);
+    EXPECT_FALSE(progress.state->janitor_cursor.empty())
+        << "an exact-token mismatch retains the rewrite but completes this page's decision";
+}
+
+TEST(CASNamespaceJanitor, TokenlessListHeadsDeadKeysAndRetainsConcurrentReplacement)
+{
+    TokenlessListBackend backend;
+    const Layout layout("p");
+    const CatalogEntry current{
+        .ns = RootNamespace{"current"}, .state = NsState::Live, .incarnation = UInt128{161}};
+    seedCatalog(backend, layout, RefCatalog{.entries = {current}});
+    const String live_key = layout.refCkptKey(NamespaceLifeId::fromCatalogEntry(current.ns, current.incarnation));
+    const String dead_key = layout.refCkptKey(life("dead", 162));
+    const String raced_key = layout.namespaceFilesPrefix(life("raced", 163)) + "data";
+    ASSERT_EQ(backend.putIfAbsent(live_key, "live").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(dead_key, "dead").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(raced_key, "old").outcome, PutOutcome::Done);
+    backend.replace_on_head = raced_key;
+    backend.resetCounts();
+
+    const auto result = NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; });
+
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_TRUE(result.anomalies.empty());
+    EXPECT_TRUE(backend.get(live_key));
+    EXPECT_FALSE(backend.get(dead_key));
+    ASSERT_TRUE(backend.get(raced_key));
+    EXPECT_EQ(backend.get(raced_key)->bytes, "winner");
+    EXPECT_EQ(backend.headCount(live_key), 0u);
+    EXPECT_EQ(backend.headCount(dead_key), 1u);
+    EXPECT_EQ(backend.headCount(raced_key), 1u);
+    EXPECT_EQ(backend.deleteCount(dead_key), 1u);
+    EXPECT_EQ(backend.deleteCount(raced_key), 1u);
+}
+
+TEST(CASNamespaceJanitor, TokenlessListRechecksFenceAfterHeadBeforeDelete)
+{
+    FenceLossDuringHeadBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String dead_key = layout.refCkptKey(life("dead", 164));
+    ASSERT_EQ(backend.putIfAbsent(dead_key, "dead").outcome, PutOutcome::Done);
+    backend.resetCounts();
+
+    const auto result = NamespaceJanitor(backend, layout, 100).runOnePage(
+        false, [&] { return backend.fence_held; });
+
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_EQ(backend.headCount(dead_key), 1u);
+    EXPECT_EQ(backend.deleteCount(dead_key), 0u);
+    EXPECT_TRUE(backend.get(dead_key));
+}
+
+TEST(CASNamespaceJanitor, PostListCatalogCutProtectsConcurrentCreationWithOneGet)
+{
+    const auto created = life("created", 121);
+    CatalogAfterListBackend backend(created);
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String first = layout.refCkptKey(created);
+    const String second = layout.namespaceFilesPrefix(created) + "data";
+    ASSERT_EQ(backend.putIfAbsent(first, "ckpt").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(second, "file").outcome, PutOutcome::Done);
+    backend.resetCounts();
+    const auto result = NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; });
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_EQ(backend.deleteTotal(), 0u);
+    EXPECT_EQ(backend.getCount(layout.refCatalogKey()), 1u);
+    EXPECT_TRUE(backend.get(first));
+    EXPECT_TRUE(backend.get(second));
+}
+
+TEST(CASNamespaceJanitor, BackendRejectedCursorResetsExactlyAndDeletesNothing)
+{
+    RejectCursorBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String dead = layout.refCkptKey(life("dead", 131));
+    ASSERT_EQ(backend.putIfAbsent(dead, "bytes").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.putIfAbsent(layout.gcMaintenanceStateKey(),
+        encodeGcMaintenanceState({.janitor_cursor = "rejected"})).outcome, PutOutcome::Done);
+    EXPECT_THROW(NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; }), std::runtime_error);
+    EXPECT_EQ(backend.deleteTotal(), 0u);
+    EXPECT_TRUE(backend.get(dead));
+    EXPECT_TRUE(readGcMaintenanceState(backend, layout).state->janitor_cursor.empty());
+}
+
+TEST(CASNamespaceJanitor, CursorPublicationFailureIsLeakOnly)
+{
+    FailMaintenancePublicationBackend backend;
+    const Layout layout("p");
+    seedCatalog(backend, layout);
+    const String dead = layout.refCkptKey(life("dead", 141));
+    ASSERT_EQ(backend.putIfAbsent(dead, "bytes").outcome, PutOutcome::Done);
+    backend.fail_publication = true;
+    const auto result = NamespaceJanitor(backend, layout, 100).runOnePage(false, [] { return true; });
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_FALSE(result.anomalies.empty());
+    EXPECT_FALSE(backend.get(dead));
+}
+
+TEST(CASNamespaceJanitorIntegration, RegularGcRoundDeletesDeadNamespaceBytes)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace live_namespace{"00/live@cas@"};
+    fixture::admitLive(*backend, layout, live_namespace);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(fixture::fixtureLife(live_namespace)),
+        encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{1},
+                              .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt})).outcome,
+        PutOutcome::Done);
+    const String dead = layout.refCkptKey(life("dead", 151));
+    ASSERT_EQ(backend->putIfAbsent(dead, "checkpoint").outcome, PutOutcome::Done);
+
+    std::map<String, UInt64> namespace_cleanup;
+    Gc gc(store, UInt128{152});
+    gc.setPhaseSink([&](const GcPhaseRecord & record)
+    {
+        if (record.phase == "namespace_cleanup")
+            namespace_cleanup = record.metrics;
+    });
+    const RoundReport report = runRegularRoundReclaiming(gc);
+    gc.setPhaseSink({});
+
+    ASSERT_TRUE(report.acquired_lease);
+    EXPECT_FALSE(backend->get(dead));
+    ASSERT_FALSE(namespace_cleanup.empty());
+    EXPECT_EQ(namespace_cleanup["janitor_pages"], 1u);
+    EXPECT_GE(namespace_cleanup["janitor_keys"], 1u);
+    EXPECT_EQ(namespace_cleanup["janitor_deleted"], 1u);
+}
diff --git a/src/Disks/tests/gtest_cas_namespace_life_id.cpp b/src/Disks/tests/gtest_cas_namespace_life_id.cpp
new file mode 100644
index 000000000000..9c2156d52aea
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_namespace_life_id.cpp
@@ -0,0 +1,429 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasNamespaceLifeId.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Common/Exception.h>
+/// Explicit rather than relying on a transitive path: `DEBUG_OR_SANITIZER_BUILD` (used below to gate
+/// the `*DeathTest` split) must resolve in THIS translation unit.
+#include <base/defines.h>
+
+#include <concepts>
+#include <type_traits>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+
+namespace
+{
+
+/// The two incarnations of ONE namespace used throughout: distinct, nonzero, and rendering to two
+/// hex segments that differ in the first character, so a key that carried the wrong one is visible
+/// in the failure message rather than hidden in the tail of 32 digits.
+UInt128 incarnationA()
+{
+    return (static_cast<UInt128>(0x1122'3344'5566'7788ULL) << 64) | static_cast<UInt128>(0x99aa'bbcc'ddee'ff01ULL);
+}
+
+UInt128 incarnationB()
+{
+    return (static_cast<UInt128>(0xfedc'ba98'7654'3210ULL) << 64) | static_cast<UInt128>(0x0123'4567'89ab'cdefULL);
+}
+
+const String kNs = "srv1/tbl@cas@";
+const String kHexA = "112233445566778899aabbccddeeff01";
+const String kHexB = "fedcba98765432100123456789abcdef";
+const String kTxn = "0000000000000007-000000000000008e";
+
+/// Asserts that `body` refuses with CORRUPTED_DATA and that the message names `key` -- the refusal is
+/// only useful to an operator if it says which object was rejected (the CI-observability rule).
+template <class F>
+void expectRefusalNaming(F && body, const String & key)
+{
+    try
+    {
+        std::forward<F>(body)();
+        FAIL() << "expected a refusal for key '" << key << "', got none";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA) << "for key '" << key << "'";
+        EXPECT_NE(e.message().find(key), String::npos)
+            << "refusal does not name the offending key '" << key << "'; message: " << e.message();
+    }
+}
+
+/// The compile-time half of spec §9 r9-5 #3, one pair per migrated helper. The NEGATIVE proves the
+/// namespace-only overload is gone; the POSITIVE proves the concept is actually looking at a real
+/// member, so a typo in the requires-clause cannot make the negative pass vacuously. Both halves are
+/// genuinely templated on `L`, so a missing member is a substitution failure rather than a hard error.
+template <class L>
+concept HasNamespaceOnlyRefsNamespacePrefix =
+    requires(const L & l, const RootNamespace & ns) { l.namespaceStreamPrefix(ns); };
+template <class L>
+concept HasIncarnationRefsNamespacePrefix =
+    requires(const L & l, const NamespaceLifeId & id) { l.namespaceStreamPrefix(id); };
+
+template <class L>
+concept HasNamespaceOnlyRefLogKey =
+    requires(const L & l, const RootNamespace & ns, const RefTxnId & id) { l.refLogKey(ns, id); };
+template <class L>
+concept HasIncarnationRefLogKey =
+    requires(const L & l, const NamespaceLifeId & ns_id, const RefTxnId & id) { l.refLogKey(ns_id, id); };
+
+template <class L>
+concept HasNamespaceOnlyRefSnapshotKey =
+    requires(const L & l, const RootNamespace & ns, const RefTxnId & id) { l.refSnapshotKey(ns, id); };
+template <class L>
+concept HasIncarnationRefSnapshotKey =
+    requires(const L & l, const NamespaceLifeId & ns_id, const RefTxnId & id) { l.refSnapshotKey(ns_id, id); };
+
+template <class L>
+concept HasNamespaceOnlyRefCkptKey =
+    requires(const L & l, const RootNamespace & ns) { l.refCkptKey(ns); };
+template <class L>
+concept HasIncarnationRefCkptKey =
+    requires(const L & l, const NamespaceLifeId & id) { l.refCkptKey(id); };
+
+/// The namespace-FILE half of the same pattern (directive §1: "Delete all namespace-only ref and
+/// namespace-file key overloads"), paired the same way.
+template <class L>
+concept HasNamespaceOnlyNamespaceFileKey =
+    requires(const L & l, const RootNamespace & ns, const String & n) { l.namespaceFileKey(ns, n); };
+template <class L>
+concept HasIncarnationNamespaceFileKey =
+    requires(const L & l, const NamespaceLifeId & life, const String & n) { l.namespaceFileKey(life, n); };
+
+template <class L>
+concept HasNamespaceOnlyNamespaceFilesPrefix =
+    requires(const L & l, const RootNamespace & ns) { l.namespaceFilesPrefix(ns); };
+template <class L>
+concept HasIncarnationNamespaceFilesPrefix =
+    requires(const L & l, const NamespaceLifeId & life) { l.namespaceFilesPrefix(life); };
+
+/// The two OUT-OF-SCOPE families (Constraint 12, directive §2 "Keep these unchanged"): loose
+/// mountpoint objects and part manifests keep the identity they have today. Each is paired in the
+/// opposite direction from the migrated helpers -- the POSITIVE is the un-life-scoped overload that
+/// must survive, the NEGATIVE is the life-scoped overload that must never appear.
+template <class L>
+concept HasUnscopedMountpointObjectKey =
+    requires(const L & l, const String & key) { l.mountpointObjectKey(key); };
+template <class L>
+concept HasLifeScopedMountpointObjectKey =
+    requires(const L & l, const NamespaceLifeId & life, const String & key) { l.mountpointObjectKey(life, key); };
+
+template <class L>
+concept HasNamespaceOnlyManifestNamespacePrefix =
+    requires(const L & l, const RootNamespace & ns) { l.manifestNamespacePrefix(ns); };
+template <class L>
+concept HasLifeScopedManifestNamespacePrefix =
+    requires(const L & l, const NamespaceLifeId & life) { l.manifestNamespacePrefix(life); };
+
+}
+
+/// Every ref-layer key names one life by an opaque fixed-width physical id. The logical namespace is
+/// intentionally absent and must be supplied by a catalog join.
+TEST(CASNamespaceLifeId, KeysCarryTheIncarnationSegment)
+{
+    Layout l("p");
+    const NamespaceLifeId id = NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationA());
+    const RefTxnId txn{7, 0x8e};
+    const String life = "p/cas/ns/stream/" + kHexA + "/";
+    const String state = "p/cas/ns/state/" + kHexA + "/";
+
+    EXPECT_EQ(l.namespaceStreamPrefix(id), life);
+    EXPECT_EQ(l.refLogKey(id, txn), life + "_log/" + kTxn + ".zst");
+    EXPECT_EQ(l.refSnapshotKey(id, txn), life + "_snap/" + kTxn + ".zst");
+    EXPECT_EQ(l.refCkptKey(id), state + "_ckpt");
+
+    const auto parsed_log = l.parseRefObjectKey(l.refLogKey(id, txn));
+    ASSERT_TRUE(parsed_log.has_value());
+    EXPECT_EQ(parsed_log->life_id, id.incarnation);
+    EXPECT_EQ(parsed_log->kind, RefObjectKind::Log);
+    EXPECT_EQ(parsed_log->txn_id, txn);
+
+    const auto parsed_snap = l.parseRefObjectKey(l.refSnapshotKey(id, txn));
+    ASSERT_TRUE(parsed_snap.has_value());
+    EXPECT_EQ(parsed_snap->life_id, id.incarnation);
+    EXPECT_EQ(parsed_snap->kind, RefObjectKind::Snap);
+
+    EXPECT_EQ(l.parseRefCkptKey(l.refCkptKey(id)), id.incarnation);
+}
+
+/// The property the type exists for: two lives of the SAME namespace name share no key at all, so a
+/// reborn namespace can neither read nor delete the previous life's objects by name.
+TEST(CASNamespaceLifeId, TwoLivesOfOneNamespaceShareNoKeys)
+{
+    Layout l("p");
+    const RefTxnId txn{7, 0x8e};
+    const NamespaceLifeId first = NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationA());
+    const NamespaceLifeId second = NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationB());
+
+    EXPECT_EQ(first.ns, second.ns);
+    EXPECT_NE(first, second);
+    EXPECT_NE(l.namespaceStreamPrefix(first), l.namespaceStreamPrefix(second));
+    EXPECT_NE(l.refLogKey(first, txn), l.refLogKey(second, txn));
+    EXPECT_NE(l.refCkptKey(first), l.refCkptKey(second));
+
+    /// Neither life's prefix covers the other: a LIST of one enumerates only its own objects.
+    EXPECT_FALSE(l.refLogKey(second, txn).starts_with(l.namespaceStreamPrefix(first)));
+    EXPECT_FALSE(l.refLogKey(first, txn).starts_with(l.namespaceStreamPrefix(second)));
+
+    /// A key spelling the other life parses back to the OTHER id -- the parser reports what the key
+    /// says; it is the catalog, not the parser, that decides which lives are current.
+    const auto parsed = l.parseRefObjectKey(l.refLogKey(second, txn));
+    ASSERT_TRUE(parsed.has_value());
+    EXPECT_EQ(parsed->life_id, second.incarnation);
+    EXPECT_NE(parsed->life_id, first.incarnation);
+}
+
+/// Zero is not a wildcard and not "the namespace itself": it can never be constructed, so it can
+/// never reach a key builder.
+///
+/// Both throws below raise `LOGICAL_ERROR`, which aborts the process in debug/sanitizer builds
+/// instead of behaving like a catchable exception (`Common/Exception.cpp`'s `handle_error_code`) --
+/// `CASNamespaceLifeIdDeathTest.ZeroIncarnationIsUnconstructibleAborts` below proves the abort
+/// positively in those builds instead.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASNamespaceLifeId, ZeroIncarnationIsUnconstructible)
+{
+    EXPECT_THROW(NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, UInt128{0}), DB::Exception);
+    EXPECT_THROW(renderIncarnation(UInt128{0}), DB::Exception);
+    EXPECT_NO_THROW(NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationA()));
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASNamespaceLifeIdDeathTest, ZeroIncarnationIsUnconstructibleAborts)
+{
+    EXPECT_DEATH({ (void)NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, UInt128{0}); }, "incarnation must be nonzero");
+    EXPECT_DEATH({ (void)renderIncarnation(UInt128{0}); }, "incarnation must be nonzero");
+    EXPECT_NO_THROW(NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationA()));
+}
+#endif
+
+/// Generation-5 namespace-bearing keys are outside the generation-6 parser roots altogether. Pool
+/// admission rejects their generation before any listed-key parser is involved.
+TEST(CASNamespaceLifeId, GenerationFiveNamespaceBearingKeysAreOutsideTheFinalGrammar)
+{
+    Layout l("p");
+    const String legacy_log = "p/cas/refs/" + kNs + "/_log/" + kTxn + ".zst";
+    const String legacy_snap = "p/cas/refs/" + kNs + "/_snap/" + kTxn + ".zst";
+    const String legacy_cleanup = "p/cas/refs/" + kNs + "/_cleanup/" + kTxn;
+    const String legacy_ckpt = "p/cas/refs/" + kNs + "/_ckpt";
+    /// A single-segment namespace leaves nothing at all where the incarnation belongs.
+    const String legacy_single_segment = "p/cas/refs/srv1/_log/" + kTxn + ".zst";
+
+    EXPECT_FALSE(l.parseRefObjectKey(legacy_log));
+    EXPECT_FALSE(l.parseRefObjectKey(legacy_snap));
+    EXPECT_FALSE(l.parseRefObjectKey(legacy_cleanup));
+    EXPECT_FALSE(l.parseRefObjectKey(legacy_single_segment));
+    EXPECT_FALSE(l.parseRefCkptKey(legacy_ckpt));
+}
+
+/// An all-zero incarnation segment is well-formed hex naming no life, so it is corruption on the read
+/// side exactly as it is unconstructible on the write side.
+TEST(CASNamespaceLifeId, ParsersRefuseAZeroIncarnation)
+{
+    Layout l("p");
+    const String zeros(32, '0');
+    const String zero_log = "p/cas/ns/stream/" + zeros + "/_log/" + kTxn + ".zst";
+    const String zero_ckpt = "p/cas/ns/state/" + zeros + "/_ckpt";
+
+    expectRefusalNaming([&] { l.parseRefObjectKey(zero_log); }, zero_log);
+    expectRefusalNaming([&] { l.parseRefCkptKey(zero_ckpt); }, zero_ckpt);
+}
+
+/// The incarnation segment has ONE canonical spelling. A key that is nearly right -- wrong width,
+/// upper case, a non-hex digit -- is refused rather than repaired, so two spellings of one life can
+/// never both exist.
+TEST(CASNamespaceLifeId, ParsersRefuseAMalformedIncarnationSegment)
+{
+    Layout l("p");
+    const String upper = "112233445566778899AABBCCDDEEFF01";
+    const String too_short = kHexA.substr(0, 31);
+    const String too_long = kHexA + "0";
+    const String non_hex = kHexA.substr(0, 31) + "z";
+
+    for (const String & bad : {upper, too_short, too_long, non_hex})
+    {
+        const String log_key = "p/cas/ns/stream/" + bad + "/_log/" + kTxn + ".zst";
+        const String ckpt_key = "p/cas/ns/state/" + bad + "/_ckpt";
+        expectRefusalNaming([&] { l.parseRefObjectKey(log_key); }, log_key);
+        expectRefusalNaming([&] { l.parseRefCkptKey(ckpt_key); }, ckpt_key);
+    }
+}
+
+/// The boundary between "corrupt" and "not ours". Refusal is reserved for keys the parser has already
+/// recognized as OUR ref objects; anything else keeps returning `std::nullopt`, because classifying an
+/// untrusted listed key remains an ordinary "is this ours" question and a sweep must be able to walk
+/// past foreign debris without an exception.
+TEST(CASNamespaceLifeId, ForeignAndUnrecognizedKeysStayInert)
+{
+    Layout l("p");
+    const NamespaceLifeId id = NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationA());
+    const RefTxnId txn{7, 0x8e};
+
+    /// Foreign top-level prefix (another pool, another subtree).
+    EXPECT_FALSE(l.parseRefObjectKey("q/cas/refs/" + kNs + "/" + kHexA + "/_log/" + kTxn + ".zst").has_value());
+    EXPECT_FALSE(l.parseRefCkptKey("q/cas/refs/" + kNs + "/" + kHexA + "/_ckpt").has_value());
+    EXPECT_FALSE(l.parseRefObjectKey("p/cas/manifests/" + kNs + "/" + kHexA + "/_log/" + kTxn).has_value());
+    /// An unrecognized kind directory is not one of our ref objects, so its incarnation segment is
+    /// never even reached.
+    EXPECT_FALSE(l.parseRefObjectKey("p/cas/refs/" + kNs + "/_bogus/" + kTxn).has_value());
+    /// A non-canonical transaction id likewise loses the key before the incarnation is judged.
+    EXPECT_FALSE(l.parseRefObjectKey(l.namespaceStreamPrefix(id) + "_log/7-8e").has_value());
+    /// The two parsers stay disjoint: neither claims the other's objects.
+    EXPECT_FALSE(l.parseRefObjectKey(l.refCkptKey(id)).has_value());
+    EXPECT_FALSE(l.parseRefCkptKey(l.refLogKey(id, txn)).has_value());
+    /// No namespace and no incarnation at all.
+    EXPECT_FALSE(l.parseRefObjectKey("p/cas/refs/_log/" + kTxn + ".zst").has_value());
+    EXPECT_FALSE(l.parseRefCkptKey("p/cas/refs/_ckpt").has_value());
+}
+
+/// Namespace files are life-keyed too: `cas/ns/state/<life_id>/_files/<relative-name>`. The
+/// round trip covers a flat name and a NESTED one, because the dedup log's segments live in a
+/// table-level subdirectory and the nested shape is the one on the insert path.
+TEST(CASNamespaceLifeId, NamespaceFileKeysCarryTheIncarnationSegment)
+{
+    Layout l("p");
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationA());
+    const String files = "p/cas/ns/state/" + kHexA + "/_files/";
+
+    EXPECT_EQ(l.namespaceFilesPrefix(life), files);
+    EXPECT_EQ(l.namespaceFileKey(life, "format_version.txt"), files + "format_version.txt");
+    EXPECT_EQ(l.namespaceFileKey(life, "deduplication_logs/deduplication_log_1.txt"),
+              files + "deduplication_logs/deduplication_log_1.txt");
+
+    for (const String & name : {String("format_version.txt"), String("deduplication_logs/deduplication_log_1.txt")})
+    {
+        const auto parsed = l.parseNamespaceFileKey(l.namespaceFileKey(life, name));
+        ASSERT_TRUE(parsed.has_value()) << "for name '" << name << "'";
+        EXPECT_EQ(parsed->life_id, life.incarnation);
+        EXPECT_EQ(parsed->relative_name, name);
+    }
+
+    /// Two lives of one namespace share no file key either, and neither files prefix covers the other.
+    const NamespaceLifeId second = NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationB());
+    EXPECT_NE(l.namespaceFileKey(life, "format_version.txt"), l.namespaceFileKey(second, "format_version.txt"));
+    EXPECT_FALSE(l.namespaceFileKey(second, "format_version.txt").starts_with(l.namespaceFilesPrefix(life)));
+}
+
+/// Generation-5 namespace-bearing file keys are outside the final parser root. Malformed ids under the
+/// final state root are corruption and name the offending key.
+TEST(CASNamespaceLifeId, NamespaceFileParserRefusesLegacyAndMalformedIncarnations)
+{
+    Layout l("p");
+    const String zeros(32, '0');
+    const String legacy = "p/roots/" + kNs + "/_files/format_version.txt";
+    /// A single-segment namespace leaves nothing at all where the incarnation belongs.
+    const String legacy_single_segment = "p/roots/srv1/_files/format_version.txt";
+    const String zero_inc = "p/cas/ns/state/" + zeros + "/_files/format_version.txt";
+
+    EXPECT_FALSE(l.parseNamespaceFileKey(legacy));
+    EXPECT_FALSE(l.parseNamespaceFileKey(legacy_single_segment));
+    expectRefusalNaming([&] { l.parseNamespaceFileKey(zero_inc); }, zero_inc);
+
+    const String upper = "112233445566778899AABBCCDDEEFF01";
+    const String too_short = kHexA.substr(0, 31);
+    const String too_long = kHexA + "0";
+    const String non_hex = kHexA.substr(0, 31) + "z";
+    for (const String & bad : {upper, too_short, too_long, non_hex})
+    {
+        const String key = "p/cas/ns/state/" + bad + "/_files/format_version.txt";
+        expectRefusalNaming([&] { l.parseNamespaceFileKey(key); }, key);
+    }
+}
+
+/// The corrupt/not-ours boundary for file keys, mirroring the ref parsers': refusal is reserved for
+/// keys already identified as OUR namespace files by their reserved `_files` segment. A loose
+/// mountpoint object has no such segment and is a legitimate inhabitant of `roots/`, so it must parse
+/// as `std::nullopt` and never as damage.
+TEST(CASNamespaceLifeId, ForeignAndMountpointKeysStayInertForTheFileParser)
+{
+    Layout l("p");
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationA());
+
+    EXPECT_FALSE(l.parseNamespaceFileKey("q/roots/" + kNs + "/" + kHexA + "/_files/x").has_value());
+    EXPECT_FALSE(l.parseNamespaceFileKey(l.mountpointObjectKey("srv1/clickhouse_access_check_abc")).has_value());
+    EXPECT_FALSE(l.parseNamespaceFileKey("p/cas/refs/" + kNs + "/" + kHexA + "/_files/x").has_value());
+    /// The files prefix itself names no file: there is no relative name after the reserved segment.
+    EXPECT_FALSE(l.parseNamespaceFileKey(l.namespaceFilesPrefix(life)).has_value());
+    /// And the ref parsers do not claim a file key.
+    EXPECT_FALSE(l.parseRefObjectKey(l.namespaceFileKey(life, "x")).has_value());
+    EXPECT_FALSE(l.parseRefCkptKey(l.namespaceFileKey(life, "x")).has_value());
+}
+
+/// Physical file keys use only `life_id`, so changing the logical spelling cannot redirect a key. A
+/// relative name may still contain `_files` and round-trips after the fixed delimiter.
+TEST(CASNamespaceLifeId, PhysicalFileKeysIgnoreLogicalNamespaceSpelling)
+{
+    Layout l("p");
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(RootNamespace{kNs}, incarnationA());
+    const NamespaceLifeId differently_named = NamespaceLifeId::fromCatalogEntry(
+        RootNamespace{"different/_files/spelling"}, incarnationA());
+    EXPECT_EQ(l.namespaceFilesPrefix(life), l.namespaceFilesPrefix(differently_named));
+    const String nested_name = "deduplication_logs/_files/log_1.txt";
+    const auto parsed = l.parseNamespaceFileKey(l.namespaceFileKey(life, nested_name));
+    ASSERT_TRUE(parsed.has_value());
+    EXPECT_EQ(parsed->life_id, life.incarnation);
+    EXPECT_EQ(parsed->relative_name, nested_name);
+}
+
+/// The "cannot compile" half of spec §9 r9-5 #3: after this task there is no way to reach a ref-layer
+/// key from a namespace alone, so dropping the incarnation is a compile error rather than an aliasing
+/// bug. Each helper is asserted twice -- the namespace-only form absent, the incarnation form present.
+TEST(CASNamespaceLifeId, NamespaceOnlyKeyHelpersDoNotExist)
+{
+    static_assert(!HasNamespaceOnlyRefsNamespacePrefix<Layout>);
+    static_assert(HasIncarnationRefsNamespacePrefix<Layout>);
+
+    static_assert(!HasNamespaceOnlyRefLogKey<Layout>);
+    static_assert(HasIncarnationRefLogKey<Layout>);
+
+    static_assert(!HasNamespaceOnlyRefSnapshotKey<Layout>);
+    static_assert(HasIncarnationRefSnapshotKey<Layout>);
+
+    static_assert(!HasNamespaceOnlyRefCkptKey<Layout>);
+    static_assert(HasIncarnationRefCkptKey<Layout>);
+
+    static_assert(!HasNamespaceOnlyNamespaceFileKey<Layout>);
+    static_assert(HasIncarnationNamespaceFileKey<Layout>);
+
+    static_assert(!HasNamespaceOnlyNamespaceFilesPrefix<Layout>);
+    static_assert(HasIncarnationNamespaceFilesPrefix<Layout>);
+
+    SUCCEED();
+}
+
+/// Directive §1's remaining requirements on the type, fenced rather than fixed: the type declares no
+/// conversion operator and no `RootNamespace` constructor takes a `NamespaceLifeId`, so nothing
+/// interconverts in either direction today and only an explicit `.ns` crosses. Without these
+/// assertions a later convenience conversion would land unnoticed, and dropping the incarnation would
+/// become representable again -- which is the property the whole re-keying rests on.
+TEST(CASNamespaceLifeId, NamespaceLifeIdAndRootNamespaceDoNotInterconvert)
+{
+    static_assert(!std::convertible_to<NamespaceLifeId, RootNamespace>);
+    static_assert(!std::constructible_from<RootNamespace, NamespaceLifeId>);
+    static_assert(!std::is_default_constructible_v<NamespaceLifeId>);
+
+    SUCCEED();
+}
+
+/// The out-of-scope fences, and they are POSITIVE on purpose: Constraint 12 keeps loose mountpoint
+/// objects and part manifests on the identity they have today, so this task must NOT have qualified
+/// them. If a negative here fails, someone added a life-scoped overload to a family the amendment
+/// explicitly excluded; if a positive fails, someone removed the un-scoped one those callers use.
+TEST(CASNamespaceLifeId, MountpointObjectsAndManifestsStayUnqualified)
+{
+    static_assert(HasUnscopedMountpointObjectKey<Layout>);
+    static_assert(!HasLifeScopedMountpointObjectKey<Layout>);
+
+    static_assert(HasNamespaceOnlyManifestNamespacePrefix<Layout>);
+    static_assert(!HasLifeScopedManifestNamespacePrefix<Layout>);
+
+    SUCCEED();
+}
diff --git a/src/Disks/tests/gtest_cas_ns_creation_lifecycle.cpp b/src/Disks/tests/gtest_cas_ns_creation_lifecycle.cpp
new file mode 100644
index 000000000000..7acab161e63d
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ns_creation_lifecycle.cpp
@@ -0,0 +1,530 @@
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+/// Explicit rather than relying on a transitive path: `DEBUG_OR_SANITIZER_BUILD` (used below to gate
+/// the `*DeathTest` split) must resolve in THIS translation unit.
+#include <base/defines.h>
+#include <fmt/format.h>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+    extern const int NETWORK_ERROR;
+}
+
+/// Stage B Task 3 (spec §3, the ref-chain catalog's creation lifecycle): the three-conditional-write
+/// sequence that carries a namespace from nothing to `Live` --
+///   1. catalog CAS: insert `{ns, Creating, fresh incarnation, creator}` (`CasRefCatalog::createNamespace`,
+///      built on Task 2's `casAdmitEntry`);
+///   2. `_ckpt` create (`CasRefCatalog::completeCreation`, step 2 -- Stage A's `publishCkpt` unchanged);
+///   3. catalog CAS: `Creating -> Live`, re-presenting the creator's admission GENERATION and
+///      value-CASing the OBSERVED entry (`completeCreation`, step 3 -- the "ZombieGoLive" guard) --
+/// plus stale-`Creating` reconciliation (`CasRefCatalog::reconcileStaleCreator`) and the publication
+/// gate (`checkPublicationAdmittedOrThrow`).
+///
+/// The suite name is prefixed `Cas` so it is covered by the `Cas*` unit-test gate filter.
+
+namespace
+{
+
+/// A fence that never refuses, for tests whose subject is not the fence -- same helper, same intent,
+/// as `gtest_cas_ref_ckpt.cpp`'s identically-named constant (not shared: each `_ckpt`/catalog test file
+/// defines its own copy, matching that file's own precedent).
+const std::function<void(uint64_t)> ALWAYS_ADMITTED = [](uint64_t) {};
+
+/// A deadline far enough out that only the test's own contention decides the outcome -- mirrors
+/// `gtest_cas_ref_ckpt.cpp`'s `generousDeadline`.
+CkptDeadline generousDeadline()
+{
+    return CkptDeadline{[] { return uint64_t{1000}; }, 60000};
+}
+
+CreatorFence creatorFence(const String & srid, uint64_t writer_epoch, uint64_t fence_generation = 1)
+{
+    return CreatorFence{.server_root_id = srid, .writer_epoch = writer_epoch, .fence_generation = fence_generation};
+}
+
+/// A `is_creator_fence_terminal` stub that answers the same fixed verdict for every fence -- for tests
+/// whose subject is not terminality itself (that predicate's own tests live in `gtest_cas_mount.cpp`,
+/// next to `isCreatorFenceTerminal`, the real implementation this stub stands in for).
+std::function<bool(const CreatorFence &)> fixedTerminality(bool terminal)
+{
+    return [terminal](const CreatorFence &) { return terminal; };
+}
+
+const CatalogEntry * findEntryForTest(const RefCatalog & catalog, const RootNamespace & ns)
+{
+    for (const CatalogEntry & e : catalog.entries)
+        if (e.ns.string() == ns.string())
+            return &e;
+    return nullptr;
+}
+
+/// Raw lifecycle tests operate below `Pool::open`, so model an already-bootstrapped pool explicitly.
+class InitializedCatalogBackend : public InMemoryBackend
+{
+public:
+    InitializedCatalogBackend()
+    {
+        CasRefCatalog::initializeEmptyForNewPool(*this, Layout("p"));
+    }
+};
+
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Happy path: all three writes land
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASNsCreationLifecycle, HappyPathReachesLiveWithADurableCkptAndAStableIncarnation)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence creator = creatorFence("srv1", /*writer_epoch=*/5);
+
+    const auto outcome = CasRefCatalog::createNamespace(
+        backend, layout, 1, ns, creator, /*admitted_generation=*/1, ALWAYS_ADMITTED, generousDeadline());
+    EXPECT_EQ(outcome, CasRefCatalog::NamespaceCreationOutcome::Live);
+
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * entry = findEntryForTest(snap.catalog, ns);
+    ASSERT_NE(entry, nullptr);
+    EXPECT_EQ(entry->state, NsState::Live);
+    EXPECT_EQ(entry->creator, std::nullopt) << "creator is forbidden outside Creating (strict grammar)";
+    const UInt128 incarnation = entry->incarnation;
+    EXPECT_NE(incarnation, UInt128(0));
+
+    const std::optional<CkptSample> ckpt = readCkpt(backend, layout, NamespaceLifeId::fromCatalogEntry(entry->ns, incarnation));
+    ASSERT_TRUE(ckpt.has_value()) << "step 2's _ckpt must be durable";
+    EXPECT_EQ(ckpt->ckpt.life_epoch, 5u) << "INV-4's genesis epoch is the creator's writer_epoch";
+
+    /// Re-reading the catalog again must show the SAME incarnation -- nothing mints a second one.
+    EXPECT_EQ(CasRefCatalog::read(backend, layout).catalog.entries.at(0).incarnation, incarnation);
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// `createNamespace` refuses a namespace that already has an entry (Task 2 review's own note: this
+/// is Task 3's job, not `casAdmitEntry`'s duplicate-namespace grammar refusal).
+/// ---------------------------------------------------------------------------------------------
+
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASNsCreationLifecycle, CreateNamespaceRejectsAnAlreadyExistingEntry)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence creator = creatorFence("srv1", 1);
+    ASSERT_EQ(CasRefCatalog::createNamespace(backend, layout, 1, ns, creator, 1, ALWAYS_ADMITTED, generousDeadline()),
+               CasRefCatalog::NamespaceCreationOutcome::Live);
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&]
+    {
+        CasRefCatalog::createNamespace(backend, layout, 1, ns, creatorFence("srv2", 2), 1, ALWAYS_ADMITTED, generousDeadline());
+    });
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASNsCreationLifecycleDeathTest, CreateNamespaceRejectsAnAlreadyExistingEntryAborts)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence creator = creatorFence("srv1", 1);
+    ASSERT_EQ(CasRefCatalog::createNamespace(backend, layout, 1, ns, creator, 1, ALWAYS_ADMITTED, generousDeadline()),
+               CasRefCatalog::NamespaceCreationOutcome::Live);
+
+    EXPECT_DEATH(
+        {
+            CasRefCatalog::createNamespace(backend, layout, 1, ns, creatorFence("srv2", 2), 1, ALWAYS_ADMITTED, generousDeadline());
+        },
+        "already carries a catalog entry");
+}
+#endif
+
+/// ---------------------------------------------------------------------------------------------
+/// `Creating` forbids publication
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASNsCreationLifecycle, CreatingForbidsPublication)
+{
+    RefCatalog catalog;
+    catalog.entries.push_back(CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Creating,
+        .incarnation = UInt128(1), .creator = creatorFence("srv1", 1)});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR,
+        [&] { CasRefCatalog::checkPublicationAdmittedOrThrow(catalog, RootNamespace{"a"}); });
+}
+
+TEST(CASNsCreationLifecycle, LiveAndRemovingAndAbsentAllAdmitPublication)
+{
+    RefCatalog catalog;
+    catalog.entries.push_back(CatalogEntry{.ns = RootNamespace{"live"}, .state = NsState::Live, .incarnation = UInt128(1)});
+    catalog.entries.push_back(CatalogEntry{.ns = RootNamespace{"removing"}, .state = NsState::Removing, .incarnation = UInt128(2)});
+    EXPECT_NO_THROW(CasRefCatalog::checkPublicationAdmittedOrThrow(catalog, RootNamespace{"live"}));
+    EXPECT_NO_THROW(CasRefCatalog::checkPublicationAdmittedOrThrow(catalog, RootNamespace{"removing"}));
+    EXPECT_NO_THROW(CasRefCatalog::checkPublicationAdmittedOrThrow(catalog, RootNamespace{"never-heard-of"}));
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// ZombieGoLive: fenced-out between the `_ckpt` publish and the `Creating -> Live` CAS
+/// ---------------------------------------------------------------------------------------------
+
+/// A fence callback that admits its FIRST call (spent by step 2's `publishCkpt`) and refuses every
+/// call after (spent by step 3's `mutate`) -- deterministically reproducing "fenced out between the
+/// `_ckpt` create and the `Creating -> Live` CAS" without a second thread or fault injection.
+namespace
+{
+std::function<void(uint64_t)> admittedOnceThenFenced()
+{
+    auto calls = std::make_shared<int>(0);
+    return [calls](uint64_t admitted)
+    {
+        if (++*calls > 1)
+            throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR, "fence generation moved since admission ({})", admitted);
+    };
+}
+}
+
+TEST(CASNsCreationLifecycle, FencedOutBetweenTheCkptPublishAndGoLiveRefusesAndLeavesEntryCreating)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence creator = creatorFence("srv1", 5);
+
+    const auto outcome = CasRefCatalog::createNamespace(
+        backend, layout, 1, ns, creator, /*admitted_generation=*/1, admittedOnceThenFenced(), generousDeadline());
+    EXPECT_EQ(outcome, CasRefCatalog::NamespaceCreationOutcome::FencedOut);
+
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * entry = findEntryForTest(snap.catalog, ns);
+    ASSERT_NE(entry, nullptr);
+    EXPECT_EQ(entry->state, NsState::Creating) << "step 3 never ran its CAS -- ZombieGoLive refuses before sending it";
+    ASSERT_TRUE(entry->creator.has_value());
+    EXPECT_EQ(*entry->creator, creator);
+
+    /// Step 2's _ckpt DID land (it is not what the fence check gates) -- CKPT-FAILED-BIRTH-DEBRIS is a
+    /// different mechanism (the OLD `RefOpKind::NamespaceBirth` writer, `Pool/CasRefLedger.cpp`); this
+    /// driver's own `_ckpt` is simply left in place for whichever actor next reconciles this entry.
+    EXPECT_TRUE(readCkpt(backend, layout, NamespaceLifeId::fromCatalogEntry(entry->ns, entry->incarnation)).has_value());
+}
+
+/// Regression (CI PR#2073, `03611_freeze_partition_parallel_verbose` under `amd_tsan, cas s3 storage`):
+/// sibling openers of the SAME namespace race `resolveNamespaceLife`'s "no entry" read the same way
+/// concurrent per-part `ALTER TABLE ... FREEZE` threads race the table's one shadow-store namespace.
+/// The loser's own `createNamespace` read lands AFTER the winner's step 1, observing `Creating` -- that
+/// must send the loser back through the resume loop (`Superseded`), never abort the server.
+TEST(CASNsCreationLifecycle, CreateNamespaceRacingASiblingsStillCreatingEntryReportsSupersededNotAbort)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence winner = creatorFence("srv1", 1);
+
+    /// Leaves the entry in `Creating` without reaching `Live` -- the same shape `resolveNamespaceLife`
+    /// observes when a sibling thread's `casAdmitEntry` has landed but its `completeCreation` has not.
+    const auto winner_outcome = CasRefCatalog::createNamespace(
+        backend, layout, 1, ns, winner, /*admitted_generation=*/1, admittedOnceThenFenced(), generousDeadline());
+    ASSERT_EQ(winner_outcome, CasRefCatalog::NamespaceCreationOutcome::FencedOut);
+    ASSERT_EQ(CasRefCatalog::read(backend, layout).catalog.entries.at(0).state, NsState::Creating);
+
+    /// The loser: a second call, as if a sibling thread's own outer "no entry" read had raced ahead of
+    /// this one. Same fence as the winner (sibling threads of one query share a mount's fence) --
+    /// exercising exactly the case `resolveNamespaceLife`'s "own fence -> completeCreation" branch is
+    /// built to resume, never a `LOGICAL_ERROR` abort.
+    const auto loser_outcome = CasRefCatalog::createNamespace(
+        backend, layout, 1, ns, winner, /*admitted_generation=*/1, ALWAYS_ADMITTED, generousDeadline());
+    EXPECT_EQ(loser_outcome, CasRefCatalog::NamespaceCreationOutcome::Superseded);
+
+    /// Nothing about the winner's own still-`Creating` entry was disturbed by the loser's refused call.
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * entry = findEntryForTest(snap.catalog, ns);
+    ASSERT_NE(entry, nullptr);
+    EXPECT_EQ(entry->state, NsState::Creating);
+    ASSERT_TRUE(entry->creator.has_value());
+    EXPECT_EQ(*entry->creator, winner);
+}
+
+/// Second catch-point of the same CI PR#2073 race, distinct from the test above. That test starts the
+/// winner FIRST, so the loser's own outer pre-check read (`createNamespace`'s `read(...)` before step
+/// 1) already observes `Creating` and takes the fast top-of-function refusal. This test instead lands
+/// the winner's ENTIRE `createNamespace` call inside the window between the loser's pre-check read
+/// (which observes NOTHING) and the loser's own step 1 read -- the shape CI actually hit as an
+/// encode-time `LOGICAL_ERROR` ("entries are not canonically ordered ... no duplicate namespace"), not
+/// the top-of-function one: both openers pass the pre-check, so both proceed to admit a row for the
+/// same namespace, and only `createNamespaceStep1`'s own per-read recheck (not `createNamespace`'s
+/// single upfront read) can catch it.
+TEST(CASNsCreationLifecycle, CreateNamespaceRacingASiblingsFullCreateBetweenPreCheckAndStep1ReportsSupersededNotAbort)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence winner = creatorFence("srv1", 1);
+    const CreatorFence loser = creatorFence("srv1", 2);
+
+    /// Fires exactly once, inside the LOSER's `createNamespace` call, after its pre-check read already
+    /// observed no entry -- synchronously runs the winner's own full `createNamespace` to completion
+    /// (all the way to `Live`) before the loser's step 1 performs its own first read. The production
+    /// call site swaps the hook into a local before invoking it, so the global is already empty by the
+    /// time this body runs: the winner's own nested call, and every later call in the test, run
+    /// hook-free without this body needing to clear it itself.
+    CasRefCatalog::setCreateNamespaceStep1PreReadHookForTest([&]
+    {
+        const auto winner_outcome = CasRefCatalog::createNamespace(
+            backend, layout, 1, ns, winner, /*admitted_generation=*/1, ALWAYS_ADMITTED, generousDeadline());
+        ASSERT_EQ(winner_outcome, CasRefCatalog::NamespaceCreationOutcome::Live);
+    });
+
+    const auto loser_outcome = CasRefCatalog::createNamespace(
+        backend, layout, 1, ns, loser, /*admitted_generation=*/1, ALWAYS_ADMITTED, generousDeadline());
+    EXPECT_EQ(loser_outcome, CasRefCatalog::NamespaceCreationOutcome::Superseded);
+
+    /// Exactly one row for `ns`, owned by the winner, at `Live` -- the loser's refused admission left
+    /// no trace and did not disturb it.
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    size_t rows_for_ns = 0;
+    for (const CatalogEntry & e : snap.catalog.entries)
+        if (e.ns.string() == ns.string())
+            ++rows_for_ns;
+    EXPECT_EQ(rows_for_ns, 1u);
+    const CatalogEntry * entry = findEntryForTest(snap.catalog, ns);
+    ASSERT_NE(entry, nullptr);
+    EXPECT_EQ(entry->state, NsState::Live);
+    EXPECT_FALSE(entry->creator.has_value()) << "Live entries carry no creator fence";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Token-stale: the observed entry no longer matches at the `Creating -> Live` CAS
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASNsCreationLifecycle, EntryStolenByAConcurrentReconcilerRefusesGoLiveAndLeavesTheStolenEntryAlone)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence original_creator = creatorFence("srv1", 5);
+    const CreatorFence thief = creatorFence("srv2", 9);
+
+    /// Write 1 only -- models "crash after write 1": no _ckpt yet, entry still Creating.
+    const CatalogEntry entry{.ns = ns, .state = NsState::Creating, .incarnation = UInt128(42), .creator = original_creator};
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, entry);
+
+    /// `check_fence_or_throw` is the seam this driver calls on EVERY attempt -- once inside step 2's
+    /// `publishCkpt`, once more inside step 3's own `mutate` -- so smuggling a REAL concurrent write
+    /// into it (rather than faking the outcome) has to land on the SECOND call specifically, or the
+    /// steal itself would run twice (and the second run would see its own first result and refuse).
+    /// This reproduces "stolen between the creator's _ckpt publish and its Creating -> Live CAS"
+    /// without a second thread. The steal itself must succeed (asserted), so the mismatch
+    /// `completeCreation` sees below is the entry ACTUALLY changing, not a contrived stub.
+    auto calls = std::make_shared<int>(0);
+    const std::function<void(uint64_t)> steal_before_the_go_live_cas = [&, calls](uint64_t)
+    {
+        if (++*calls == 2)
+            ASSERT_EQ(CasRefCatalog::reconcileStaleCreator(backend, layout, entry, thief, fixedTerminality(true), /*admitted_generation=*/1, ALWAYS_ADMITTED),
+                       CasRefCatalog::ReconcileCreatorOutcome::Reconciled);
+    };
+
+    const auto outcome = CasRefCatalog::completeCreation(
+        backend, layout, entry, /*admitted_generation=*/1, steal_before_the_go_live_cas, generousDeadline());
+    EXPECT_EQ(outcome, CasRefCatalog::NamespaceCreationOutcome::Superseded);
+
+    /// `read`'s `Snapshot` is bound to a name here, not chained through a temporary: a `const
+    /// CatalogEntry *` taken from `.catalog` of an unbound temporary dangles the instant the full
+    /// expression ends, which every other site in this file (and the copy/paste that spread it) got
+    /// wrong until ASan caught it.
+    const CasRefCatalog::Snapshot snap_after = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * after = findEntryForTest(snap_after.catalog, ns);
+    ASSERT_NE(after, nullptr);
+    EXPECT_EQ(after->state, NsState::Creating) << "the ORIGINAL creator's attempt wrote nothing -- only the thief's CAS did";
+    ASSERT_TRUE(after->creator.has_value());
+    EXPECT_EQ(*after->creator, thief) << "the entry is exactly what the thief left it as, untouched by our refused attempt";
+    EXPECT_EQ(after->incarnation, entry.incarnation) << "reconciliation never mints a fresh incarnation";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Both stale at once: fence moved AND the entry was stolen -- refused (fence checked first).
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASNsCreationLifecycle, BothFenceAndEntryStaleRefusesGoLiveViaTheFenceCheckWhichRunsFirst)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence original_creator = creatorFence("srv1", 5);
+    const CreatorFence thief = creatorFence("srv2", 9);
+
+    const CatalogEntry entry{.ns = ns, .state = NsState::Creating, .incarnation = UInt128(42), .creator = original_creator};
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, entry);
+
+    /// Same steal as the test above, landing on the SECOND `check_fence_or_throw` call (step 3's own
+    /// `mutate`, not step 2's `publishCkpt`) -- but this one ALSO throws on that same second call, so
+    /// both axes go stale in the SAME `mutate` invocation. `completeCreation`'s fence check runs before
+    /// its entry check (documented ordering), so this is reported `FencedOut`; the assertions below
+    /// confirm the entry ALSO changed, so the test is not merely re-proving the fence-only case above.
+    auto calls = std::make_shared<int>(0);
+    const std::function<void(uint64_t)> steal_and_fence_before_the_go_live_cas = [&, calls](uint64_t admitted)
+    {
+        if (++*calls == 2)
+        {
+            ASSERT_EQ(CasRefCatalog::reconcileStaleCreator(backend, layout, entry, thief, fixedTerminality(true), /*admitted_generation=*/1, ALWAYS_ADMITTED),
+                       CasRefCatalog::ReconcileCreatorOutcome::Reconciled);
+            throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR, "fence generation moved since admission ({})", admitted);
+        }
+    };
+
+    const auto outcome = CasRefCatalog::completeCreation(
+        backend, layout, entry, /*admitted_generation=*/1, steal_and_fence_before_the_go_live_cas, generousDeadline());
+    EXPECT_EQ(outcome, CasRefCatalog::NamespaceCreationOutcome::FencedOut)
+        << "both checks would refuse; the fence check speaks first by this driver's fixed ordering";
+
+    const CasRefCatalog::Snapshot snap_after = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * after = findEntryForTest(snap_after.catalog, ns);
+    ASSERT_NE(after, nullptr);
+    ASSERT_TRUE(after->creator.has_value());
+    EXPECT_EQ(*after->creator, thief) << "confirms the entry axis really did go stale too, not just the fence";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Stale-`Creating` reconciliation
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASNsCreationLifecycle, ReconcileRefusedWhileTheOriginalCreatorFenceIsStillLive)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CatalogEntry entry{.ns = ns, .state = NsState::Creating, .incarnation = UInt128(7),
+                              .creator = creatorFence("srv1", 5)};
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, entry);
+
+    const auto outcome = CasRefCatalog::reconcileStaleCreator(
+        backend, layout, entry, creatorFence("srv2", 9), fixedTerminality(false), /*admitted_generation=*/1, ALWAYS_ADMITTED);
+    EXPECT_EQ(outcome, CasRefCatalog::ReconcileCreatorOutcome::CreatorFenceStillLive);
+
+    const CasRefCatalog::Snapshot snap_after = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * after = findEntryForTest(snap_after.catalog, ns);
+    ASSERT_NE(after, nullptr);
+    EXPECT_EQ(*after, entry) << "refused -- nothing written";
+}
+
+TEST(CASNsCreationLifecycle, ReconcileSucceedsTokenExactlyAfterTheOriginalCreatorFenceIsTerminalThenResumesToLive)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CreatorFence original_creator = creatorFence("srv1", 5);
+    const CreatorFence new_creator = creatorFence("srv2", 9);
+    const CatalogEntry entry{.ns = ns, .state = NsState::Creating, .incarnation = UInt128(7), .creator = original_creator};
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, entry);   /// "crash after write 1" -- no _ckpt yet
+
+    ASSERT_EQ(CasRefCatalog::reconcileStaleCreator(backend, layout, entry, new_creator, fixedTerminality(true), /*admitted_generation=*/1, ALWAYS_ADMITTED),
+               CasRefCatalog::ReconcileCreatorOutcome::Reconciled);
+
+    CatalogEntry taken_over = entry;
+    taken_over.creator = new_creator;
+    const CasRefCatalog::Snapshot snap_mid = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * mid = findEntryForTest(snap_mid.catalog, ns);
+    ASSERT_NE(mid, nullptr);
+    EXPECT_EQ(*mid, taken_over) << "creator moved to the new actor; state and incarnation unchanged";
+
+    const auto outcome = CasRefCatalog::completeCreation(
+        backend, layout, taken_over, /*admitted_generation=*/1, ALWAYS_ADMITTED, generousDeadline());
+    EXPECT_EQ(outcome, CasRefCatalog::NamespaceCreationOutcome::Live);
+
+    const CasRefCatalog::Snapshot snap_final = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * final_entry = findEntryForTest(snap_final.catalog, ns);
+    ASSERT_NE(final_entry, nullptr);
+    EXPECT_EQ(final_entry->state, NsState::Live);
+    EXPECT_EQ(final_entry->incarnation, entry.incarnation) << "the SAME incarnation throughout -- resumption, not rebirth";
+    const std::optional<CkptSample> ckpt = readCkpt(backend, layout, NamespaceLifeId::fromCatalogEntry(final_entry->ns, final_entry->incarnation));
+    ASSERT_TRUE(ckpt.has_value());
+    EXPECT_EQ(ckpt->ckpt.life_epoch, new_creator.writer_epoch)
+        << "the RESUMING actor's writer_epoch is the genesis epoch that actually landed";
+}
+
+/// "Stale token at reconciliation -> fail closed": a SECOND reconciler racing the first, both reading
+/// the SAME stale `observed` before either writes.
+TEST(CASNsCreationLifecycle, ReconcileFailsClosedWhenTheEntryAlreadyChanged)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const RootNamespace ns{"a"};
+    const CatalogEntry entry{.ns = ns, .state = NsState::Creating, .incarnation = UInt128(7),
+                              .creator = creatorFence("srv1", 5)};
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, entry);
+
+    const CreatorFence first_reconciler = creatorFence("srv2", 9);
+    const CreatorFence second_reconciler = creatorFence("srv3", 11);
+
+    ASSERT_EQ(CasRefCatalog::reconcileStaleCreator(backend, layout, entry, first_reconciler, fixedTerminality(true), /*admitted_generation=*/1, ALWAYS_ADMITTED),
+               CasRefCatalog::ReconcileCreatorOutcome::Reconciled);
+
+    /// The second reconciler still holds the ORIGINAL `entry` it read before either of them wrote --
+    /// token-exactness must refuse it even though the terminality predicate would still say yes.
+    const auto outcome = CasRefCatalog::reconcileStaleCreator(
+        backend, layout, entry, second_reconciler, fixedTerminality(true), /*admitted_generation=*/1, ALWAYS_ADMITTED);
+    EXPECT_EQ(outcome, CasRefCatalog::ReconcileCreatorOutcome::EntryChanged);
+
+    const CasRefCatalog::Snapshot snap_after = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * after = findEntryForTest(snap_after.catalog, ns);
+    ASSERT_NE(after, nullptr);
+    ASSERT_TRUE(after->creator.has_value());
+    EXPECT_EQ(*after->creator, first_reconciler) << "the second reconciler's refused attempt changed nothing";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Preconditions: `completeCreation`/`reconcileStaleCreator` refuse anything but a `Creating` entry
+/// with a creator fence -- a caller bug, not a race, hence `LOGICAL_ERROR`.
+/// ---------------------------------------------------------------------------------------------
+
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASNsCreationLifecycle, CompleteCreationRejectsANonCreatingEntry)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const CatalogEntry live{.ns = RootNamespace{"a"}, .state = NsState::Live, .incarnation = UInt128(1)};
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&]
+    {
+        CasRefCatalog::completeCreation(backend, layout, live, 1, ALWAYS_ADMITTED, generousDeadline());
+    });
+}
+
+TEST(CASNsCreationLifecycle, ReconcileStaleCreatorRejectsANonCreatingEntry)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const CatalogEntry live{.ns = RootNamespace{"a"}, .state = NsState::Live, .incarnation = UInt128(1)};
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&]
+    {
+        CasRefCatalog::reconcileStaleCreator(backend, layout, live, creatorFence("srv2", 2), fixedTerminality(true), /*admitted_generation=*/1, ALWAYS_ADMITTED);
+    });
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASNsCreationLifecycleDeathTest, CompleteCreationRejectsANonCreatingEntryAborts)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const CatalogEntry live{.ns = RootNamespace{"a"}, .state = NsState::Live, .incarnation = UInt128(1)};
+    EXPECT_DEATH(
+        { CasRefCatalog::completeCreation(backend, layout, live, 1, ALWAYS_ADMITTED, generousDeadline()); },
+        "not a Creating entry");
+}
+
+TEST(CASNsCreationLifecycleDeathTest, ReconcileStaleCreatorRejectsANonCreatingEntryAborts)
+{
+    InitializedCatalogBackend backend;
+    Layout layout("p");
+    const CatalogEntry live{.ns = RootNamespace{"a"}, .state = NsState::Live, .incarnation = UInt128(1)};
+    EXPECT_DEATH(
+        {
+            CasRefCatalog::reconcileStaleCreator(backend, layout, live, creatorFence("srv2", 2), fixedTerminality(true), /*admitted_generation=*/1, ALWAYS_ADMITTED);
+        },
+        "not a Creating entry");
+}
+#endif
diff --git a/src/Disks/tests/gtest_cas_ns_file_incarnation.cpp b/src/Disks/tests/gtest_cas_ns_file_incarnation.cpp
new file mode 100644
index 000000000000..901b223cb90f
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ns_file_incarnation.cpp
@@ -0,0 +1,279 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include "cas_test_helpers.h"
+#include <algorithm>
+
+namespace DB::ErrorCodes
+{
+    extern const int UNKNOWN_FORMAT_VERSION;
+}
+
+/// Namespace files are keyed by an opaque LIFE, not by its name: `cas/ns/state/<life_id>/_files/<name>`
+/// (Stage B Task 4b, directive design change 2). This file pins the three properties that re-key exists
+/// to produce, and the one it must NOT produce.
+///
+/// THE HOLE IT CLOSES. Before the re-key, a namespace file lived at a name-keyed prefix shared by every
+/// life of that name. A file the store's LIST omitted therefore survived namespace removal -- nothing
+/// enumerated it, so nothing deleted it -- and then became VISIBLE to the next namespace created under
+/// the same name, because that namespace read the same prefix. Deletion was load-bearing for
+/// correctness, and deletion depends on enumeration, which is the one thing an object store is allowed
+/// to be late about (`HintHoleBackendOn` is that lateness as an interface -- see its doc).
+///
+/// WHY THE KEY IS THE FIX AND THE DELETE IS NOT. After the re-key the old file is at a prefix the new
+/// life cannot name. It is unreachable whether or not it was ever deleted, so a blind LIST costs
+/// STORAGE and nothing else -- the directive's "LIST omission may only leak storage, never visibility,
+/// rebirth or deletion safety". `ColdReaderUsesCatalogCutWhileOldFileSurvivesRemoval` asserts exactly
+/// that split by leaving the old object physically present and byte-intact through the real lifecycle.
+///
+/// WHAT REBIRTH NO LONGER WAITS FOR. Catalog removal depends on folded terminal evidence for the old
+/// opaque life, not on a physical-empty proof. `RebirthDoesNotWaitForFilesToBeEmpty` keeps old `_files`
+/// bytes present while that evidence is adopted; their later reclamation belongs to the perpetual
+/// janitor and is not a precondition for same-name reuse.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+const String kNsString = "00/aa@cas@";
+const String kFile = "format_version.txt";
+
+const UInt128 kGcId = hexToU128("00000000000000000000000000000001");
+
+/// Create a real catalog life and a replay-valid `Live` ref table through the production writer path.
+void publishWithProductionBirth(const PoolPtr & store, const RootNamespace & ns, const String & ref)
+{
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = store->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+}
+
+}
+
+/// THE COUPLED HEADLINE. A real removal reaches a catalog-absent cut even when LIST permanently omits
+/// an old-life file. A cold reader follows that catalog cut rather than the physical residue, while an
+/// already-held exact life remains stale-or-NotFound and can never cross into the successor life.
+TEST(CASNsFileIncarnation, ColdReaderUsesCatalogCutWhileOldFileSurvivesRemoval)
+{
+    auto backend = std::make_shared<HintHoleBackendOn<CountingBackend>>();
+    PoolPtr store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{kNsString};
+    const String old_bytes = "old-life\n";
+    const String successor_bytes = "successor-life\n";
+
+    publishWithProductionBirth(store, ns, "predecessor");
+    const std::optional<NamespaceLifeId> old_life = store->namespaceFilesLifeIfReadable(ns);
+    ASSERT_TRUE(old_life);
+    store->putNamespaceFile(*old_life, kFile, old_bytes);
+    const String old_key = layout.namespaceFileKey(*old_life, kFile);
+    backend->hide(old_key);
+
+    ASSERT_TRUE(backend->head(old_key).exists) << "the lie must be in LIST only -- the object is durable";
+    ASSERT_TRUE(store->listNamespaceFiles(*old_life).empty())
+        << "precondition: enumeration omits the file, so no cleanup pass can ever find it";
+    const size_t holes_before_gc = backend->holesServed();
+
+    store->dropNamespace(ns);
+    ASSERT_TRUE(CasRefCatalog::lifeIfCataloged(*backend, layout, ns));
+
+    Gc gc(store, kGcId);
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred) << "N: the production terminal must fold";
+    ASSERT_TRUE(CasRefCatalog::lifeIfCataloged(*backend, layout, ns))
+        << "the terminal fold alone must not erase its catalog row";
+    (void)runRegularRoundReclaiming(gc);
+    ASSERT_FALSE(CasRefCatalog::lifeIfCataloged(*backend, layout, ns))
+        << "N+1: the pre-fold drain must erase the exact completed Removing row";
+    ASSERT_GT(backend->holesServed(), holes_before_gc)
+        << "the GC janitor must observe the injected LIST hole after the explicit precondition LIST";
+
+    const auto old_head = backend->head(old_key);
+    ASSERT_TRUE(old_head.exists) << "logical removal must not depend on physical empty";
+    const auto old_object = backend->get(old_key);
+    ASSERT_TRUE(old_object);
+    EXPECT_EQ(old_object->bytes, old_bytes);
+
+    PoolPtr cold = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "cold-reader", .gc_fold_max_defer_rounds = 0});
+    EXPECT_FALSE(cold->namespaceFilesLifeIfReadable(ns))
+        << "a fresh reader follows the absent catalog row, not discoverable or exact-key old bytes";
+
+    publishWithProductionBirth(store, ns, "successor");
+    const std::optional<NamespaceLifeId> successor_life = cold->namespaceFilesLifeIfReadable(ns);
+    ASSERT_TRUE(successor_life);
+    ASSERT_NE(successor_life->incarnation, old_life->incarnation);
+    cold->putNamespaceFile(*successor_life, kFile, successor_bytes);
+    EXPECT_EQ(cold->getNamespaceFile(*successor_life, kFile), successor_bytes);
+
+    const std::optional<String> retained_old = store->getNamespaceFile(*old_life, kFile);
+    EXPECT_TRUE(!retained_old || *retained_old == old_bytes)
+        << "an exact predecessor life may be stale or NotFound, but never aliases successor bytes";
+    EXPECT_NE(retained_old, std::optional<String>{successor_bytes});
+
+    for (const String & key : backend->touchedKeys())
+        EXPECT_EQ(key.find("/_cleanup/"), String::npos) << key;
+}
+
+/// The non-minting reader assignment site accepts exactly a catalog `Live` row. `Creating`,
+/// `Removing`, and absence neither install a runtime life nor mutate durable catalog/stream state.
+TEST(CASNsFileIncarnation, FreshReaderAssignsOnlyLiveCatalogLifeWithoutMutation)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    PoolPtr store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace creating{"00/creating@cas@"};
+    const RootNamespace live{"00/live@cas@"};
+    const RootNamespace removing{"00/removing@cas@"};
+    const RootNamespace absent{"00/absent@cas@"};
+
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, CatalogEntry{
+        .ns = creating,
+        .state = NsState::Creating,
+        .incarnation = UInt128{31},
+        .creator = CreatorFence{.server_root_id = "foreign", .writer_epoch = 7, .fence_generation = 1}});
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, CatalogEntry{
+        .ns = live, .state = NsState::Live, .incarnation = UInt128{32}});
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, CatalogEntry{
+        .ns = removing, .state = NsState::Live, .incarnation = UInt128{33}});
+    CasRefCatalog::casUpdate(*backend, layout, [&](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        const auto it = std::find_if(next.entries.begin(), next.entries.end(), [&](const CatalogEntry & entry)
+        {
+            return entry.ns == removing;
+        });
+        chassert(it != next.entries.end());
+        it->state = NsState::Removing;
+        it->removal_started_round = 1;
+        return next;
+    });
+
+    /// Only a `Live` catalog row is readable. Give that exact life the empty checkpoint authority
+    /// that production creation publishes; the other rows deliberately remain raw lifecycle states.
+    writeRecoverableCkptForRawFixture(*backend, layout, live, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    backend->resetCounts();
+    EXPECT_FALSE(store->namespaceFilesLifeIfReadable(creating));
+    EXPECT_FALSE(store->namespaceFilesLifeIfReadable(removing));
+    EXPECT_FALSE(store->namespaceFilesLifeIfReadable(absent));
+    const std::optional<NamespaceLifeId> readable = store->namespaceFilesLifeIfReadable(live);
+    ASSERT_TRUE(readable);
+    EXPECT_EQ(readable->incarnation, UInt128{32});
+
+    EXPECT_FALSE(store->refTableLifeForTest(creating));
+    EXPECT_FALSE(store->refTableLifeForTest(removing));
+    EXPECT_FALSE(store->refTableLifeForTest(absent));
+    ASSERT_TRUE(store->refTableLifeForTest(live));
+    EXPECT_EQ(store->refTableLifeForTest(live)->incarnation, UInt128{32});
+    EXPECT_EQ(backend->putTotal(), 0u);
+    EXPECT_EQ(backend->putOverwriteTotal(), 0u);
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+}
+
+/// A real GC fold records terminal evidence for the previous life while its namespace-file debris
+/// remains physically present. Lifecycle completion is therefore independent of `_files` enumeration;
+/// the perpetual janitor may reclaim the bytes later without participating in the removal proof.
+TEST(CASNsFileIncarnation, RebirthDoesNotWaitForFilesToBeEmpty)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    PoolPtr store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{kNsString};
+
+    /// A removed namespace (a bare `remove_namespace` transaction -- no committed refs, so no
+    /// owner-removal edge confounds this with an unconditional delete path) whose only surviving
+    /// physical objects are namespace files: one flat, one nested in the dedup-log shape.
+    {
+        RefOp remove_op;
+        remove_op.kind = RefOpKind::RemoveNamespace;
+        appendRefLogSeed(*backend, layout, ns, {remove_op});
+    }
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, layout, ns).value();
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+    const String debris_key = layout.namespaceFileKey(life, kFile);
+    backend->putIfAbsent(debris_key, "1\n");
+    backend->putIfAbsent(layout.namespaceFileKey(life, "deduplication_logs/deduplication_log_1.txt"), "records");
+
+    Gc gc(store, kGcId);
+    gc.runRegularRound();
+
+    /// Folding the terminal records positive evidence on the same life row even though files remain.
+    const GcState state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    ASSERT_GT(state.snap_generation, 0u);
+    const CasFoldSeal seal = decodeFoldSeal(
+        backend->get(layout.foldSealKey(state.snap_generation, state.snap_attempt))->bytes);
+    const auto row_it = seal.ref_lives.find(life.incarnation);
+    ASSERT_NE(row_it, seal.ref_lives.end());
+    ASSERT_TRUE(row_it->second.cleanup_evidence.has_value());
+    EXPECT_EQ(row_it->second.cleanup_evidence->remove_txn_id, (RefTxnId{1, 1}));
+    EXPECT_TRUE(backend->head(debris_key).exists) << "cleanup evidence does not gate on physical deletion";
+}
+
+/// An old-format pool carrying unqualified `roots/<ns>/_files/x` keys is REFUSED AT OPEN. It is not
+/// read, not migrated, and not silently re-keyed: the file layer rides Task 4's format bump B, and the
+/// pool-open floor is what makes "there is nothing to migrate" true rather than merely intended.
+///
+/// Asserted at OPEN rather than at the parser on purpose: `Layout` has no unqualified key constructor
+/// at all (a compile-time concept check in `gtest_cas_namespace_life_id.cpp` pins that, and
+/// `parseNamespaceFileKey`'s refusal of a legacy key is pinned there too), so the only reachable
+/// question left is whether a pool that CONTAINS such keys can be opened. It cannot.
+TEST(CASNsFileIncarnation, LegacyUnqualifiedFileKeyIsRefusedAtOpen)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+
+    /// A generation-5 `_pool_meta`: the current encoder's output with its header generation moved back
+    /// one, so every other byte is exactly what that generation really wrote.
+    PoolMeta meta;
+    meta.pool_id = hexToU128("0123456789abcdef0123456789abcdef");
+    meta.blob_header_len = 256;
+    meta.min_reader_generation = kNamespaceLifeKeyedGeneration - 1;
+    meta.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128)};
+    String encoded = encodePoolMeta(meta);
+    const String current_v = "\"v\":" + std::to_string(G_BUILD);
+    const String legacy_v = "\"v\":" + std::to_string(kNamespaceLifeKeyedGeneration);
+    const size_t at = encoded.find(current_v);
+    /// Guard the substitution itself: a silent no-op here would leave a CURRENT-generation pool and the
+    /// test would pass by opening a pool it believes it downgraded.
+    ASSERT_NE(at, String::npos) << "pool-meta header no longer spells its generation as " << current_v;
+    encoded.replace(at, current_v.size(), legacy_v);
+    ASSERT_NE(encoded.find(legacy_v), String::npos);
+    backend->putIfAbsent(layout.poolMetaKey(), encoded);
+
+    /// The legacy artifact this task removes: a namespace file keyed by NAME ONLY, with no incarnation
+    /// segment. Written as raw bytes because no code path in the tree can produce this key any more.
+    backend->putIfAbsent("p/roots/" + kNsString + "/_files/" + kFile, "1\n");
+
+    try
+    {
+        openPoolForTest(backend);
+        FAIL() << "an old-format pool must fail closed at open, naming recreation";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::UNKNOWN_FORMAT_VERSION);
+        EXPECT_NE(e.message().find("recreate"), String::npos)
+            << "the refusal must tell the operator what to do; got: " << e.message();
+    }
+}
diff --git a/src/Disks/tests/gtest_cas_ns_file_read_contract.cpp b/src/Disks/tests/gtest_cas_ns_file_read_contract.cpp
new file mode 100644
index 000000000000..3166230f5ec8
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ns_file_read_contract.cpp
@@ -0,0 +1,247 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Core/Defines.h>
+#include <IO/WriteHelpers.h>
+
+#include <algorithm>
+#include <atomic>
+#include <filesystem>
+#include <memory>
+#include <optional>
+#include <unistd.h>
+
+namespace DB::ContentAddressedSetting
+{
+    extern const ContentAddressedSettingsBool gc_enabled;
+}
+
+namespace DB::ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+    extern const int NETWORK_ERROR;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+const String kTableUuid = "a11a11a1-1111-4111-8111-111111111111";
+const String kTablePath = "a11/a11a11a1-1111-4111-8111-111111111111";
+const String kFile = "format_version.txt";
+const String kFilePath = kTablePath + "/" + kFile;
+const UInt128 kLife2Id = hexToU128("22222222222222222222222222222222");
+
+struct DiskFixture
+{
+    DB::ObjectStoragePtr object_storage;
+    std::shared_ptr<DB::ContentAddressedMetadataStorage> storage;
+};
+
+DiskFixture openDiskFixture()
+{
+    static std::atomic<uint64_t> counter{0};
+    const String unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    auto settings = makeSettingsForTest(
+        "srv1", std::filesystem::temp_directory_path() / ("cas_ns_file_contract_scratch_" + unique));
+    settings[DB::ContentAddressedSetting::gc_enabled] = false;
+
+    DiskFixture fixture;
+    fixture.object_storage = makeLocalObjectStorageForTest();
+    fixture.storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        fixture.object_storage, "pool", "srv1", "", nullptr, settings);
+    fixture.storage->startup();
+    return fixture;
+}
+
+void writeVerbatimThroughDisk(
+    DB::ContentAddressedMetadataStorage & storage, const String & path, const String & bytes)
+{
+    auto transaction = storage.createTransaction();
+    auto & ca_transaction = dynamic_cast<DB::ContentAddressedTransaction &>(*transaction);
+    auto buffer = ca_transaction.writeFile(path, DB::DBMS_DEFAULT_BUFFER_SIZE, DB::WriteMode::Rewrite, {});
+    ASSERT_NE(dynamic_cast<CaInlineWriteBuffer *>(buffer.get()), nullptr);
+    DB::writeString(bytes, *buffer);
+    buffer->finalize();
+}
+
+/// Delete the current catalog life through the production exact-removal authority while retaining all
+/// old physical bytes and the original process's already-resident runtime.
+void deleteCatalogLife(
+    DB::ContentAddressedMetadataStorage & storage, const NamespaceLifeId & life1)
+{
+    Backend & backend = storage.store()->backend();
+    const Layout & layout = storage.store()->layout();
+    CasRefCatalog::casUpdate(backend, layout, [&](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        const auto it = std::find_if(next.entries.begin(), next.entries.end(), [&](const CatalogEntry & entry)
+        {
+            return entry.ns == life1.ns && entry.incarnation == life1.incarnation;
+        });
+        if (it == next.entries.end())
+            throw DB::Exception(
+                DB::ErrorCodes::LOGICAL_ERROR, "Missing fixture catalog life '{}'", life1.ns.string());
+        it->state = NsState::Removing;
+        it->removal_started_round = 1;
+        return next;
+    });
+
+    const CasRefCatalog::Snapshot snapshot = CasRefCatalog::read(backend, layout);
+    const auto it = std::find_if(snapshot.catalog.entries.begin(), snapshot.catalog.entries.end(), [&](const CatalogEntry & entry)
+    {
+        return entry.ns == life1.ns && entry.incarnation == life1.incarnation;
+    });
+    if (it == snapshot.catalog.entries.end())
+        throw DB::Exception(
+            DB::ErrorCodes::LOGICAL_ERROR, "Missing Removing fixture catalog life '{}'", life1.ns.string());
+
+    CasFoldSeal parent;
+    parent.ref_lives.emplace(life1.incarnation, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 1}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 1}}});
+    if (CasRefCatalog::deleteCompletedRemoving(
+            backend, layout, *it, parent, 1,
+            [](uint64_t) { return CasRefCatalog::LeaderFenceStatus::Held; })
+        != CasRefCatalog::CompletedRemovingDeleteOutcome::Deleted)
+        throw DB::Exception(
+            DB::ErrorCodes::LOGICAL_ERROR, "Failed to delete fixture catalog life '{}'", life1.ns.string());
+
+}
+
+NamespaceLifeId admitReplacementLife(
+    DB::ContentAddressedMetadataStorage & storage, const NamespaceLifeId & life1)
+{
+    if (life1.incarnation == kLife2Id)
+        throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "Fixture life ids unexpectedly collide");
+    const NamespaceLifeId life2 = NamespaceLifeId::fromCatalogEntry(life1.ns, kLife2Id);
+    CasRefCatalog::casAdmitEntry(
+        storage.store()->backend(), storage.store()->layout(), storage.store()->poolConfig().gc_shards, CatalogEntry{
+        .ns = life2.ns, .state = NsState::Live, .incarnation = life2.incarnation});
+    return life2;
+}
+
+NamespaceLifeId replaceCatalogLife(
+    DB::ContentAddressedMetadataStorage & storage, const NamespaceLifeId & life1)
+{
+    deleteCatalogLife(storage, life1);
+    return admitReplacementLife(storage, life1);
+}
+
+NamespaceLifeId currentLife(DB::ContentAddressedMetadataStorage & storage)
+{
+    const RootNamespace ns = storage.liveNamespace(kTableUuid);
+    const auto life = storage.store()->namespaceFilesLifeIfReadable(ns);
+    if (!life)
+        throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "Fixture namespace '{}' has no readable life", ns.string());
+    return *life;
+}
+
+}
+
+/// This storage already holds life 1. Reusing its warm runtime after same-name rebirth is a retained
+/// life-handle operation, not a fresh logical-name admission: it may still see predecessor bytes (or
+/// answer absent), but the opaque physical life id makes successor bytes structurally unreachable.
+TEST(CASNamespaceFileReadContract, HeldLifeAfterSameNameRebirthNeverSeesSuccessorBytes)
+{
+    DiskFixture fixture = openDiskFixture();
+    writeVerbatimThroughDisk(*fixture.storage, kFilePath, "life-1\n");
+    const NamespaceLifeId life1 = currentLife(*fixture.storage);
+    const NamespaceLifeId life2 = replaceCatalogLife(*fixture.storage, life1);
+    fixture.storage->store()->putNamespaceFile(life2, kFile, "life-2\n");
+
+    const std::optional<String> held_read = fixture.storage->tryGetInManifestBytes(kFilePath);
+    EXPECT_NE(held_read, std::optional<String>{"life-2\n"});
+    EXPECT_TRUE(!held_read || held_read == std::optional<String>{"life-1\n"});
+    EXPECT_EQ(fixture.storage->store()->getNamespaceFile(life1, kFile), std::optional<String>{"life-1\n"});
+}
+
+/// Mutation caught: capturing only the namespace name and resolving it when the buffer finalizes would
+/// overwrite life 2. The real buffer must retain the exact life admitted when it was opened.
+TEST(CASNamespaceFileReadContract, DelayedInlineFinalizeCannotChangeSuccessorTokenOrBytes)
+{
+    DiskFixture fixture = openDiskFixture();
+    writeVerbatimThroughDisk(*fixture.storage, kFilePath, "life-1-before\n");
+    const NamespaceLifeId life1 = currentLife(*fixture.storage);
+
+    auto delayed_transaction = fixture.storage->createTransaction();
+    auto & ca_transaction = dynamic_cast<DB::ContentAddressedTransaction &>(*delayed_transaction);
+    auto delayed_buffer = ca_transaction.writeFile(
+        kFilePath, DB::DBMS_DEFAULT_BUFFER_SIZE, DB::WriteMode::Rewrite, {});
+    ASSERT_NE(dynamic_cast<CaInlineWriteBuffer *>(delayed_buffer.get()), nullptr);
+    DB::writeString("life-1-delayed\n", *delayed_buffer);
+
+    const NamespaceLifeId life2 = replaceCatalogLife(*fixture.storage, life1);
+    fixture.storage->store()->putNamespaceFile(life2, kFile, "life-2-stable\n");
+    Backend & backend = fixture.storage->store()->backend();
+    const Layout & layout = fixture.storage->store()->layout();
+    const String life1_key = layout.namespaceFileKey(life1, kFile);
+    const String life2_key = layout.namespaceFileKey(life2, kFile);
+    ASSERT_TRUE(std::filesystem::exists(nativeKeyUnder(fixture.object_storage, life2_key)));
+    const HeadResult life2_before = backend.head(life2_key);
+    ASSERT_TRUE(life2_before.exists);
+    const auto life2_body_before = backend.get(life2_key);
+    ASSERT_TRUE(life2_body_before.has_value());
+    ASSERT_EQ(life2_body_before->bytes, "life-2-stable\n");
+
+    bool stale_failure = false;
+    try
+    {
+        delayed_buffer->finalize();
+    }
+    catch (const DB::Exception & e)
+    {
+        stale_failure = true;
+        EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR);
+        EXPECT_NE(e.message().find("retrying later"), String::npos);
+    }
+
+    const HeadResult life2_after = backend.head(life2_key);
+    ASSERT_TRUE(life2_after.exists);
+    EXPECT_EQ(life2_after.token, life2_before.token);
+    const auto life2_body_after = backend.get(life2_key);
+    ASSERT_TRUE(life2_body_after.has_value());
+    EXPECT_EQ(life2_body_after->bytes, "life-2-stable\n");
+
+    if (!stale_failure)
+    {
+        ASSERT_TRUE(std::filesystem::exists(nativeKeyUnder(fixture.object_storage, life1_key)));
+        const auto life1_body = backend.get(life1_key);
+        ASSERT_TRUE(life1_body.has_value());
+        EXPECT_EQ(life1_body->bytes, "life-1-delayed\n");
+    }
+}
+
+/// `listNamespaceFiles` derives its LIST prefix from `layout.namespaceFilesPrefix(life)` -- a physical
+/// life-scoped stream, not the catalog. Listing under a held life must cost exactly one LIST of the
+/// files prefix and nothing else.
+TEST(CASNamespaceFileReadContract, ListThroughHeldLifeIssuesZeroCatalogRequests)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    PoolPtr store = openPoolForTest(backend);
+    const NamespaceLifeId life = fixture::fixtureLife(RootNamespace{"00/ns_file_list_contract@cas@"});
+    const String prefix = store->layout().namespaceFilesPrefix(life);
+
+    store->putNamespaceFile(life, "a.txt", "a\n");
+    store->putNamespaceFile(life, "b.txt", "b\n");
+    backend->resetCounts();
+
+    const std::vector<String> names = store->listNamespaceFiles(life);
+
+    std::vector<String> sorted_names = names;
+    std::sort(sorted_names.begin(), sorted_names.end());
+    EXPECT_EQ(sorted_names, (std::vector<String>{"a.txt", "b.txt"}));
+
+    /// Positive control: the journal recorded exactly one LIST against the namespace-file stream
+    /// prefix and nothing else, so the touched-set assertion below names an absence, not a
+    /// recorder that never saw anything.
+    EXPECT_EQ(backend->listCount(prefix), 1u);
+    EXPECT_EQ(backend->listTotal(), 1u);
+    EXPECT_EQ(backend->touchedKeys(), std::vector<String>{prefix});
+}
diff --git a/src/Disks/tests/gtest_cas_observability.cpp b/src/Disks/tests/gtest_cas_observability.cpp
new file mode 100644
index 000000000000..ada9cc9e572a
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_observability.cpp
@@ -0,0 +1,355 @@
+#include <gtest/gtest.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasInspect.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Common/ProfileEvents.h>
+#include <algorithm>
+#include <memory>
+#include <vector>
+
+namespace ProfileEvents
+{
+extern const Event CASGCRetiredCondemned;
+extern const Event CASGCRetireReplaced;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+using DB::Cas::tests::currentRetiredSet;
+
+namespace
+{
+
+PoolPtr openPool(std::shared_ptr<InMemoryBackend> & b)
+{
+    b = std::make_shared<InMemoryBackend>();
+    return Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// Publish ONE ref naming a single-blob part through the real writer sequence (mirrors
+/// `publishOneBlobPart` in `gtest_cas_gc_leak.cpp`, duplicated here because that helper has internal
+/// linkage in its own translation unit).
+ManifestId publishOneBlobPart(
+    const PoolPtr & s, const RootNamespace & ns, const String & ref, const String & payload)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    DB::Cas::ManifestEntry e;
+    e.path = "data.bin";
+    e.placement = EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    e.blob_size = payload.size();
+    /// Wiring order (EDGE-BEFORE-OBSERVE): stageManifest -> precommitAdd -> putBlob -> promote.
+    const ManifestId id = build->stageManifest({e});
+    build->precommitAdd(ns, ref, id);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+}
+
+/// B170/Task 1 (Part A audit events): `PartWriteTxn::stageManifest` writes a part-manifest body but never
+/// emitted an audit row for it — the log could not answer "when was this manifest written." Verifies
+/// the emitted `ManifestPut` event (exactly once per successful stage).
+TEST(CASObservability, StageManifestEmitsManifestPut)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    std::vector<CasEvent> seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto s = openPool(b);
+    s->setEventSink([&](const CasEvent & e){ seen.push_back(e); });
+
+    const RootNamespace ns{"srv/tbl@cas@"};
+    auto build = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/all_0_0_0", .intended_namespace = ns});
+    ManifestEntry e;
+    e.path = "f";
+    e.placement = EntryPlacement::Inline;
+    e.inline_bytes = "AAA";
+    const ManifestId id = build->stageManifest({e});
+    s->setEventSink(nullptr);
+
+    EXPECT_EQ(std::count_if(seen.begin(), seen.end(),
+        [](const CasEvent & x){ return x.type == CasEventType::ManifestPut; }), 1);
+
+    const auto it = std::find_if(seen.begin(), seen.end(),
+        [](const CasEvent & x){ return x.type == CasEventType::ManifestPut; });
+    ASSERT_NE(it, seen.end());
+    EXPECT_EQ(it->object_kind, CasEventObjectKind::Manifest);
+    EXPECT_EQ(it->object_hash, manifestRefDebugString(id.ref));
+    EXPECT_FALSE(it->token.empty());
+}
+
+/// `PartWriteTxn::abandon` removes a live precommit's owner binding (the correctness-bearing step) but never
+/// audited the removal — the log could not distinguish "never precommitted" from "precommitted then
+/// abandoned." Verifies the emitted `PrecommitRemoved` event (exactly once, only when a precommit was
+/// actually live).
+TEST(CASObservability, AbandonEmitsPrecommitRemoved)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    std::vector<CasEvent> seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto s = openPool(b);
+
+    const RootNamespace ns{"srv/tbl@cas@"};
+    auto build = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/all_0_0_0", .intended_namespace = ns});
+    ManifestEntry e;
+    e.path = "f";
+    e.placement = EntryPlacement::Inline;
+    e.inline_bytes = "AAA";
+    const ManifestId id = build->stageManifest({e});
+    build->precommitAdd(ns, "all_0_0_0", id);
+
+    s->setEventSink([&](const CasEvent & x){ seen.push_back(x); });
+    build->abandon();
+    s->setEventSink(nullptr);
+
+    EXPECT_EQ(std::count_if(seen.begin(), seen.end(),
+        [](const CasEvent & x){ return x.type == CasEventType::PrecommitRemoved; }), 1);
+
+    const auto it = std::find_if(seen.begin(), seen.end(),
+        [](const CasEvent & x){ return x.type == CasEventType::PrecommitRemoved; });
+    ASSERT_NE(it, seen.end());
+    EXPECT_EQ(it->namespace_, ns.string());
+    EXPECT_EQ(it->ref_name, "all_0_0_0");
+    EXPECT_EQ(it->object_kind, CasEventObjectKind::Root);
+    EXPECT_EQ(it->object_hash, manifestRefDebugString(id.ref));
+}
+
+/// A build that never precommitted has nothing to remove: `abandon` must not fabricate a
+/// `PrecommitRemoved` row for a binding that was never live.
+TEST(CASObservability, AbandonWithoutPrecommitEmitsNoPrecommitRemoved)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    std::vector<CasEvent> seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto s = openPool(b);
+
+    const RootNamespace ns{"srv/tbl@cas@"};
+    auto build = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/all_0_0_0", .intended_namespace = ns});
+    ManifestEntry e;
+    e.path = "f";
+    e.placement = EntryPlacement::Inline;
+    e.inline_bytes = "AAA";
+    build->stageManifest({e});   /// staged, never precommitted
+
+    s->setEventSink([&](const CasEvent & x){ seen.push_back(x); });
+    build->abandon();
+    s->setEventSink(nullptr);
+
+    EXPECT_EQ(std::count_if(seen.begin(), seen.end(),
+        [](const CasEvent & x){ return x.type == CasEventType::PrecommitRemoved; }), 0);
+}
+
+/// Task 2 (Part A audit fix, 2026-07-08): the resurrect-supersede branch inside `closeBlob`
+/// (`CasBlobInDegree.cpp`) used to peek the current token via `head_blob` — the FRESH-CONDEMN
+/// observation hook — which double-emitted `blob_retire` alongside `blob_retire_replaced` and
+/// double-counted `CASGCRetiredCondemned` for what is really ONE physical condemnation (the resurrect
+/// replaced a stale retired entry with the current token). Drives the same condemn-A / resurrect-B /
+/// drop-B sequence as `CASGCLeak.ResurrectReplacedIncarnationReclaimed`, then isolates the ONE round
+/// that folds B's create+drop and supersedes A's stale retired entry: that round must emit exactly one
+/// `blob_retire_replaced` (carrying the STALE token A in `detail["superseded_token"]`), ZERO
+/// `blob_retire` for this hash, one `CASGCRetireReplaced` increment, and NO `CASGCRetiredCondemned`
+/// double-count.
+TEST(CASObservability, ResurrectSupersedeEmitsOnlyRetireReplacedWithOldToken)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    std::vector<CasEvent> seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto s = openPool(b);
+    const RootNamespace ns{"test/tbl"};
+    const String P = "resurrect-payload-audit";
+
+    /// 1. Publish ref r1 -> token A referenced; drop it; ONE GC round condemns A (retired, not deleted).
+    publishOneBlobPart(s, ns, "r1", P);
+    const HeadResult hA = b->head(s->layout().blobKey(idOf(P)));
+    ASSERT_TRUE(hA.exists);
+    s->dropRef(ns, "r1");
+    s->renewWatermarkOnce();
+
+    Gc gc(s, hexToU128("000000000000000000000000000000ab"));
+    {
+        const RoundReport rep = gc.runRegularRound();
+        ASSERT_TRUE(rep.acquired_lease);
+    }
+    {
+        const auto lm = DB::Cas::tests::loadMetaForTest(*b, s->layout(), u128Of(P));
+        ASSERT_TRUE(lm.has_value() && lm->meta.state == MetaState::Condemned)
+            << "precondition: token A must be condemned before the resurrect";
+    }
+
+    /// 2. RESURRECT: r2 dedup-hits P while A is condemned -> mints a fresh incarnation B; drop it too.
+    publishOneBlobPart(s, ns, "r2", P);
+    const HeadResult hB = b->head(s->layout().blobKey(idOf(P)));
+    ASSERT_TRUE(hB.exists);
+    ASSERT_NE(hB.token.value, hA.token.value) << "resurrect must mint a new incarnation token B";
+    s->dropRef(ns, "r2");
+    s->renewWatermarkOnce();
+
+    /// 3. The NEXT round folds r2's create+drop in one pass and must SUPERSEDE A's stale retired entry
+    /// with a fresh condemn of B (peek, not the fresh-condemn `head_blob` hook). Capture events + the
+    /// counters for exactly THIS round.
+    using ProfileEvents::global_counters;
+    const auto condemned_before = global_counters[ProfileEvents::CASGCRetiredCondemned].load();
+    const auto replaced_before  = global_counters[ProfileEvents::CASGCRetireReplaced].load();
+
+    s->setEventSink([&](const CasEvent & e){ seen.push_back(e); });
+    const RoundReport rep = gc.runRegularRound();
+    s->setEventSink(nullptr);
+    ASSERT_TRUE(rep.acquired_lease);
+
+    const auto condemned_after = global_counters[ProfileEvents::CASGCRetiredCondemned].load();
+    const auto replaced_after  = global_counters[ProfileEvents::CASGCRetireReplaced].load();
+
+    /// Phase 3 (mixed-algo pools): event `object_hash` renders are `blobIdOf(ref)` ("<algoName>:<hex>"),
+    /// never a bare hex.
+    const String hash_hex = DB::Cas::blobIdOf(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(P))});
+    const auto is_this_blob = [&](const CasEvent & e){ return e.object_hash == hash_hex; };
+
+    EXPECT_EQ(std::count_if(seen.begin(), seen.end(),
+        [&](const CasEvent & e){ return is_this_blob(e) && e.type == CasEventType::BlobRetire; }), 0)
+        << "supersede must not also emit blob_retire (that is the fresh-condemn hook's event)";
+
+    std::vector<CasEvent> replaced_events;
+    std::copy_if(seen.begin(), seen.end(), std::back_inserter(replaced_events),
+        [&](const CasEvent & e){ return is_this_blob(e) && e.type == CasEventType::BlobRetireReplaced; });
+    ASSERT_EQ(replaced_events.size(), 1u) << "exactly one blob_retire_replaced for the supersede";
+    EXPECT_EQ(replaced_events[0].token, hB.token.value) << "the event's own token is the fresh CURRENT token B";
+    ASSERT_TRUE(replaced_events[0].detail.count("superseded_token"));
+    EXPECT_FALSE(replaced_events[0].detail.at("superseded_token").empty());
+    EXPECT_EQ(replaced_events[0].detail.at("superseded_token"), hA.token.value)
+        << "superseded_token must name the STALE token (A) the resurrect replaced";
+
+    EXPECT_EQ(replaced_after - replaced_before, 1u) << "CASGCRetireReplaced increments exactly once";
+    EXPECT_EQ(condemned_after - condemned_before, 0u)
+        << "supersede peek must not fresh-condemn -- CASGCRetiredCondemned must not double-count";
+
+    /// Size-unit regression guard (audit fix, 2026-07-08): `peek_head` used to return the RAW
+    /// `backend.head(...)` size (physical, header-included), while the fresh-condemn hook `head_blob`
+    /// strips the pool's fixed blob header via `retiredLogicalSize` before the size lands in
+    /// `RetiredEntry.size`. That mismatch meant supersede-minted entries and fresh-condemn entries carried
+    /// two different unit conventions in the SAME persisted `RetiredSet`. The superseded entry (now naming
+    /// the fresh token B) must carry the LOGICAL size -- i.e. the payload length, with the pool's blob
+    /// header already stripped -- exactly like a fresh condemn of the same blob would.
+    const std::vector<RetiredEntry> retired = currentRetiredSet(*b, s->layout(), /*shard*/0);
+    const auto it = std::find_if(retired.begin(), retired.end(),
+        [&](const RetiredEntry & e){ return e.kind == ObjectKind::Blob && e.ref == DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(P))}; });
+    ASSERT_NE(it, retired.end()) << "the superseded entry must be present in the current retired set";
+    EXPECT_EQ(it->token.value, hB.token.value) << "the persisted entry names the fresh CURRENT token B";
+    EXPECT_EQ(it->size, P.size())
+        << "supersede must persist the LOGICAL size (payload length, header stripped), matching what "
+           "a fresh condemn of the same blob would carry -- not the raw physical (header-included) size";
+}
+
+/// Task 3 (Part B, `clickhouse-disks cas-inspect`): `caInspectToJson` is a FREE function (no
+/// disk/backend involved) that decodes any CA bucket object at `key` and renders it as JSON, purely
+/// by matching `key` against `Layout`'s prefixes/key-shapes and calling the matching `decode*`.
+/// These tests drive it directly against real encoder output (one per recognized key shape) plus the
+/// unknown-key fail-closed path — the same function the CLI command (`CommandCaInspect.cpp`) calls.
+
+/// The legacy mutable ref-shard object is gone (snapshot+log ref model); inspect now decodes the two
+/// immutable ref objects. A `_snap/<id>.proto` renders as a ref-table snapshot...
+TEST(CASObservability, CaInspectDecodesRefSnapshotToJson)
+{
+    using DB::Cas::tests::committedRow;
+    using DB::Cas::tests::minimalLiveSnapshot;
+    Layout layout("p");
+    const RootNamespace ns{"srv/tbl@cas@"};
+    const RefTxnId snap_id{1, 7};
+    const RefTableSnapshot snap = minimalLiveSnapshot(ns.string(), snap_id,
+        {committedRow("all_0_0_0", ManifestRef{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 1})});
+    const String key = layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), snap_id);
+    const String json = caInspectToJson(
+        layout, key, encodeRefTableSnapshot(snap), DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_NE(json.find(R"("object":"ref_snapshot")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("namespace":"srv/tbl@cas@")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("snapshot_id":{"writer_epoch":1,"ref_sequence":7})"), String::npos) << json;
+    EXPECT_NE(json.find(R"("ref_name":"all_0_0_0")"), String::npos) << json;
+    EXPECT_NE(json.find(R"("precommits":[])"), String::npos) << json;
+    EXPECT_EQ(json.find("\"lifecycle\""), String::npos)
+        << "generation-8 snapshot inspection must not recreate lifecycle state retired from the snapshot DTO";
+}
+
+/// ...and a `_log/<txn-id>` renders as a ref-transaction log.
+TEST(CASObservability, CaInspectDecodesRefLogToJson)
+{
+    Layout layout("p");
+    const RootNamespace ns{"srv/tbl@cas@"};
+    const RefTxnId txn_id{1, 8};
+    RefLogTxn txn;
+    txn.ns = ns.string();
+    txn.txn_id = txn_id;
+    RefOp add;
+    add.kind = RefOpKind::OwnerTransition;
+    add.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "all_0_0_0",
+        ManifestRef{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 1}};
+    txn.ops = {add};
+    const String key = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), txn_id);
+    const String json = caInspectToJson(
+        layout, key, encodeRefLogTxn(txn), DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_NE(json.find("ref_log"), String::npos);
+    EXPECT_NE(json.find("OwnerTransition"), String::npos);
+    EXPECT_NE(json.find("all_0_0_0"), String::npos);
+}
+
+TEST(CASObservability, CaInspectDecodesPartManifestToJson)
+{
+    Layout layout("p");
+    const RootNamespace ns{"srv/tbl@cas@"};
+
+    PartManifest m;
+    m.ref = ManifestRef{.writer_epoch = 1, .build_sequence = 2, .manifest_ordinal = 3};
+    m.root_namespace_id = ns;
+    ManifestEntry e;
+    e.path = "data.bin";
+    e.placement = EntryPlacement::Inline;
+    e.inline_bytes = "hello";
+    m.entries = {e};
+    m.payload_digest = computePayloadDigest(m);
+
+    const ManifestId id{.root_namespace = ns, .ref = m.ref};
+    const String key = layout.manifestKey(id);
+    const String json = caInspectToJson(layout, key, encodePartManifest(m));
+    EXPECT_NE(json.find("\"root_namespace_id\""), String::npos);
+    EXPECT_NE(json.find("data.bin"), String::npos);
+    EXPECT_NE(json.find("\"manifest_ordinal\":3"), String::npos);
+}
+
+TEST(CASObservability, CaInspectDecodesMountLeaseToJson)
+{
+    Layout layout("p");
+    MountLease lease;
+    lease.server_uuid = hexToU128("000000000000000000000000000000ab");
+    lease.writer_epoch = 5;
+    lease.hostname = "host1";
+    lease.pid = 123;
+
+    const String key = layout.mountKey("srid1");
+    const String json = caInspectToJson(layout, key, encodeMountLease(lease));
+    EXPECT_NE(json.find("\"writer_epoch\":5"), String::npos);
+    EXPECT_NE(json.find("host1"), String::npos);
+}
+
+TEST(CASObservability, CaInspectDecodesGcStateToJson)
+{
+    Layout layout("p");
+    GcState state;
+    state.round = 42;
+    state.gc_shards = 4;
+
+    const String key = layout.gcStateKey();
+    const String json = caInspectToJson(layout, key, encodeGcState(state));
+    EXPECT_NE(json.find("\"round\":42"), String::npos);
+    EXPECT_NE(json.find("\"gc_shards\":4"), String::npos);
+}
+
+TEST(CASObservability, CaInspectUnknownKeyThrows)
+{
+    Layout layout("p");
+    EXPECT_THROW(caInspectToJson(layout, "p/not/a/ca/object", "xxxx"), DB::Exception);   /// BAD_ARGUMENTS
+}
diff --git a/src/Disks/tests/gtest_cas_operation_gate.cpp b/src/Disks/tests/gtest_cas_operation_gate.cpp
new file mode 100644
index 000000000000..6d16c3946fae
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_operation_gate.cpp
@@ -0,0 +1,436 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcScheduler.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Storages/MergeTree/checkDataPart.h>
+#include <Common/Exception.h>
+
+#include <chrono>
+#include <filesystem>
+#include <functional>
+#include <memory>
+#include <stdexcept>
+#include <string>
+
+/// Task 8 (rev.7 spec §1): the central six-class operation gate (`checkOpAdmitted`), the `Vanished` truth
+/// semantics, and the [D5] per-reason typed messages. These tests build a real
+/// `ContentAddressedMetadataStorage` over a Local object storage (the same harness as
+/// gtest_ca_transaction.cpp), commit a real part, then force the pool lifecycle condition directly via the
+/// Task-5 setter (`Pool::setLifecycleForTest`) to pin each class × state cell of the spec §1 table and
+/// assert what every public entry does.
+///
+/// NOTE the harness idiom: `store()` itself is fail-closed on a terminal pool (it throws), so a test
+/// captures the `PoolPtr` ONCE while the pool is still `Live` and drives `setLifecycleForTest` on that
+/// captured handle -- the SAME object the metadata storage's `cas_store` points at -- rather than calling
+/// `store()` again after forcing a terminal state.
+
+namespace DB::ErrorCodes
+{
+extern const int INVALID_STATE;
+extern const int NETWORK_ERROR;
+extern const int FILE_DOESNT_EXIST;
+}
+
+using namespace DB;
+using DB::Cas::PoolLifecycle;
+
+namespace
+{
+
+/// A live table dir + part reused across the tests (the exact shape gtest_ca_transaction.cpp uses).
+const std::string kTableDir = "g80/g80g80g8-0808-4808-8808-080808080808";
+const std::string kPartDir = kTableDir + "/all_1_1_0";
+const std::string kPartFile = kPartDir + "/data.bin";
+
+std::shared_ptr<ContentAddressedMetadataStorage> openGateStorage()
+{
+    auto settings = Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_op_gate_scratch");
+    auto storage = std::make_shared<ContentAddressedMetadataStorage>(
+        Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+/// Commit one real part (tmp -> final rename -> commit), leaving `kPartFile` durable and `kPartDir`/
+/// `kTableDir` non-empty. Every op below runs against this committed state.
+void commitOnePart(ContentAddressedMetadataStorage & storage)
+{
+    auto tx = storage.createTransaction();
+    auto & ca_tx = dynamic_cast<ContentAddressedTransaction &>(*tx);
+    auto buf = ca_tx.writeFile(kTableDir + "/tmp_insert_all_1_1_0/data.bin", 65536, WriteMode::Rewrite, {});
+    const std::string bytes = "content-of-the-part";
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+    tx->moveDirectory(kTableDir + "/tmp_insert_all_1_1_0", kPartDir);
+    tx->commit(NoCommitOptions{});
+}
+
+std::string messageOf(const std::function<void()> & fn)
+{
+    try
+    {
+        fn();
+    }
+    catch (const Exception & e)
+    {
+        return std::string(e.message());
+    }
+    ADD_FAILURE() << "expected a DB::Exception";
+    return {};
+}
+
+/// The thrown exception itself, for the tests that assert against an upstream CLASSIFIER rather than
+/// against an error code. NEVER returns a null `exception_ptr`: every consumer feeds the result to a
+/// classifier that rethrows it, and `std::rethrow_exception(nullptr)` is undefined behaviour that takes
+/// the whole binary down instead of failing one test. On the nothing-was-thrown path the failure is
+/// recorded and a SENTINEL is returned -- the test has already failed by then, and the sentinel merely
+/// keeps the assertion that follows harmless.
+std::exception_ptr exceptionOf(const std::function<void()> & fn)
+{
+    try
+    {
+        fn();
+    }
+    catch (...)
+    {
+        return std::current_exception();
+    }
+    ADD_FAILURE() << "expected a DB::Exception, nothing was thrown";
+    return std::make_exception_ptr(std::runtime_error("exceptionOf sentinel: nothing was thrown"));
+}
+
+/// The Pool-level `server_root_id` a test mount uses (mirrors gtest_cas_lifecycle_condition.cpp).
+const std::string kSrid = "test";
+
+/// GC's fence-out applied to the mount lease (preserve the body, set `gc_fenced`, bump `seq`) so a
+/// subsequent `tryRemountOnce` verdicts `Recover` and reclaims a FRESH incarnation immediately, driving a
+/// transient-not-live pool back to `Live` without a lease-expiry wait. Mirrors
+/// gtest_cas_lifecycle_condition.cpp's helper.
+void fenceOutMount(DB::Cas::Backend & backend, const String & mount_key)
+{
+    const auto got = backend.get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    DB::Cas::MountLease m = DB::Cas::decodeMountLease(got->bytes);
+    m.gc_fenced = true;
+    m.seq += 1;
+    ASSERT_EQ(backend.putOverwrite(mount_key, DB::Cas::encodeMountLease(m), got->token).outcome,
+              DB::Cas::PutOutcome::Done);
+}
+
+}
+
+/// (a) Probes on a Vanished disk answer the truth: absent/empty, WITHOUT reaching the pool.
+TEST(CASOperationGate, ProbesOnVanishedAnswerAbsentEmpty)
+{
+    auto storage = openGateStorage();
+    commitOnePart(*storage);
+    auto pool = storage->store();   /// captured while Live
+
+    /// Live baseline: the probes see the committed part.
+    ASSERT_TRUE(storage->existsFile(kPartFile));
+    ASSERT_TRUE(storage->existsDirectory(kPartDir));
+    ASSERT_TRUE(storage->existsFileOrDirectory(kPartFile));
+    ASSERT_FALSE(storage->isDirectoryEmpty(kTableDir));
+    ASSERT_FALSE(storage->listDirectory(kTableDir).empty());
+    ASSERT_TRUE(storage->getStorageObjectsIfExist(kPartFile).has_value());
+
+    pool->setLifecycleForTest(PoolLifecycle::VanishedReplaced);
+
+    EXPECT_FALSE(storage->existsFile(kPartFile));
+    EXPECT_FALSE(storage->existsDirectory(kPartDir));
+    EXPECT_FALSE(storage->existsFileOrDirectory(kPartFile));
+    EXPECT_TRUE(storage->listDirectory(kTableDir).empty());
+    EXPECT_FALSE(storage->iterateDirectory(kTableDir)->isValid());
+    EXPECT_TRUE(storage->isDirectoryEmpty(kTableDir));
+    EXPECT_FALSE(storage->getStorageObjectsIfExist(kPartFile).has_value());
+    /// The offender `liveTreeDirHasChildren` hardcoded-true is now truthful too: the disk root reads absent.
+    EXPECT_FALSE(storage->liveTreeDirHasChildren(""));
+}
+
+/// (b) Removes on a Vanished disk are no-op SUCCESS and never touch the backend: after restoring Live the
+/// part is still there. This is what lets a vanished-disk table's DROP complete.
+TEST(CASOperationGate, RemovesOnVanishedAreNoOpSuccessBackendUntouched)
+{
+    auto storage = openGateStorage();
+    commitOnePart(*storage);
+    auto pool = storage->store();   /// captured while Live
+    ASSERT_TRUE(storage->existsDirectory(kPartDir));
+
+    pool->setLifecycleForTest(PoolLifecycle::VanishedReplaced);
+
+    /// A whole-table removeRecursive + commit (the DROP shape): both no-op-succeed.
+    {
+        auto tx = storage->createTransaction();
+        EXPECT_NO_THROW(tx->removeRecursive(kTableDir, /*should_remove_objects=*/nullptr));
+        EXPECT_NO_THROW(tx->commit(NoCommitOptions{}));   /// empty parts -> Remove -> no-op success
+    }
+    /// A single removeDirectory of the part dir + commit: no-op-succeed.
+    {
+        auto tx = storage->createTransaction();
+        EXPECT_NO_THROW(tx->removeDirectory(kPartDir));
+        EXPECT_NO_THROW(tx->commit(NoCommitOptions{}));
+    }
+
+    /// Truth check: nothing was actually removed. Back on Live the part is intact.
+    pool->setLifecycleForTest(PoolLifecycle::Live);
+    EXPECT_TRUE(storage->existsDirectory(kPartDir)) << "a remove on a Vanished disk must not touch the backend";
+    EXPECT_TRUE(storage->existsFile(kPartFile));
+}
+
+/// (c) A content read on a Vanished disk throws the typed per-reason [D5] message -- the exact substring
+/// names the ACTUAL sub-state (replaced / forgotten), never a wrong diagnosis.
+TEST(CASOperationGate, ContentReadOnVanishedThrowsTypedPerReasonMessage)
+{
+    auto storage = openGateStorage();
+    commitOnePart(*storage);
+    auto pool = storage->store();   /// captured while Live
+
+    pool->setLifecycleForTest(PoolLifecycle::VanishedReplaced);
+    EXPECT_NE(messageOf([&] { storage->getFileSize(kPartFile); }).find("foreign pool"), std::string::npos);
+    EXPECT_NE(messageOf([&] { storage->getStorageObjects(kPartFile); }).find("foreign pool"), std::string::npos);
+
+    pool->setLifecycleForTest(PoolLifecycle::VanishedForgotten);
+    EXPECT_NE(messageOf([&] { storage->getFileSize(kPartFile); }).find("erasure was NOT verified"),
+              std::string::npos);
+}
+
+/// (d1) Every class but Factory refuses on `TransientNotLive` — and the refusal carries the TRANSIENT
+/// class (`NETWORK_ERROR`), not the terminal 668. The split from `IdentityLost` (test d2) is the whole
+/// point: a lease blip is unavailability, an identity loss is damage, and consumers outside CAS act on
+/// the difference. `ReplicatedMergeTreePartCheckThread` declares a part broken and detaches it for any
+/// refusal its `isRetryableException` hatch does not recognise, so coding a blip 668 made healthy parts
+/// look corrupt (BACKLOG {#lease-blip-part-check-collapse}).
+TEST(CASOperationGate, EveryClassThrowsRetryableTransientOnTransientNotLive)
+{
+    auto storage = openGateStorage();
+    commitOnePart(*storage);
+    storage->store()->setLifecycleForTest(PoolLifecycle::TransientNotLive);   /// one force from Live
+
+    /// Probe
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->existsFile(kPartFile); });
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->existsDirectory(kPartDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->listDirectory(kTableDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->isDirectoryEmpty(kTableDir); });
+    /// ContentRead
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->getFileSize(kPartFile); });
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->getStorageObjects(kPartFile); });
+    /// Write (via a transaction)
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] {
+        auto tx = storage->createTransaction();
+        auto & ca_tx = dynamic_cast<ContentAddressedTransaction &>(*tx);
+        ca_tx.writeFile(kTableDir + "/tmp_x/data.bin", 65536, WriteMode::Rewrite, {});
+    });
+    /// Remove (via a transaction)
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] {
+        auto tx = storage->createTransaction();
+        tx->removeRecursive(kTableDir, /*should_remove_objects=*/nullptr);
+    });
+    /// Admin
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->runOneGcRoundForTest(); });
+
+    /// The coarser code buys retryability at the cost of precision, so the MESSAGE carries the whole
+    /// truth: which CA condition, and that it is transient rather than an error of record.
+    const std::string msg = messageOf([&] { storage->getFileSize(kPartFile); });
+    EXPECT_NE(msg.find("mount lease not held"), std::string::npos) << msg;
+    EXPECT_NE(msg.find("TRANSIENT"), std::string::npos) << msg;
+    EXPECT_NE(msg.find("recovers to Live"), std::string::npos) << msg;
+}
+
+/// (d2) `IdentityLost` is TERMINAL — the sentinels are gone, nothing auto-recovers — so it keeps the 668
+/// (`INVALID_STATE`) class and its own richer [D5] diagnosis ("identity lost … restart or FORGET").
+/// Nothing about the transient re-coding may leak here: a terminal state that read as retryable would
+/// make every consumer spin forever on a disk that will never come back.
+TEST(CASOperationGate, EveryClassThrows668OnIdentityLost)
+{
+    auto storage = openGateStorage();
+    commitOnePart(*storage);
+    storage->store()->setLifecycleForTest(PoolLifecycle::IdentityLost);   /// one force from Live
+
+    /// Probe
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->existsFile(kPartFile); });
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->existsDirectory(kPartDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->listDirectory(kTableDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->isDirectoryEmpty(kTableDir); });
+    /// ContentRead
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->getFileSize(kPartFile); });
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->getStorageObjects(kPartFile); });
+    /// Write (via a transaction)
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] {
+        auto tx = storage->createTransaction();
+        auto & ca_tx = dynamic_cast<ContentAddressedTransaction &>(*tx);
+        ca_tx.writeFile(kTableDir + "/tmp_x/data.bin", 65536, WriteMode::Rewrite, {});
+    });
+    /// Remove (via a transaction)
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] {
+        auto tx = storage->createTransaction();
+        tx->removeRecursive(kTableDir, /*should_remove_objects=*/nullptr);
+    });
+    /// Admin
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->runOneGcRoundForTest(); });
+
+    EXPECT_NE(messageOf([&] { storage->getFileSize(kPartFile); }).find("identity lost"), std::string::npos);
+}
+
+/// (d3) The contract the d1/d2 split exists to satisfy, asserted against upstream's OWN predicate instead
+/// of a code number: `isRetryableException` is what `ReplicatedMergeTreePartCheckThread::checkPartImpl`
+/// consults before declaring a part broken. A transient CA refusal must satisfy it (the part stays
+/// queued); a terminal one must not (the disk is genuinely unusable and must surface). Pinning the
+/// predicate rather than `NETWORK_ERROR` keeps this test meaningful if upstream's list ever moves.
+TEST(CASOperationGate, TransientRefusalIsUpstreamRetryableTerminalIsNot)
+{
+    {
+        auto storage = openGateStorage();
+        commitOnePart(*storage);
+        storage->store()->setLifecycleForTest(PoolLifecycle::TransientNotLive);
+        EXPECT_TRUE(isRetryableException(exceptionOf([&] { storage->getFileSize(kPartFile); })))
+            << "a lease blip must not read as part damage to the part-check thread";
+    }
+    {
+        auto storage = openGateStorage();
+        commitOnePart(*storage);
+        storage->store()->setLifecycleForTest(PoolLifecycle::IdentityLost);
+        /// Pin WHICH error is being classified before classifying it: `EXPECT_FALSE` alone passes for any
+        /// non-retryable error, so a future regression that threw something else entirely here -- or threw
+        /// from the wrong site -- would slip through as a pass.
+        Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->getFileSize(kPartFile); });
+        EXPECT_NE(messageOf([&] { storage->getFileSize(kPartFile); }).find("identity lost"), std::string::npos);
+        EXPECT_FALSE(isRetryableException(exceptionOf([&] { storage->getFileSize(kPartFile); })))
+            << "a terminal identity loss must NOT be retried forever as if it were transient";
+    }
+}
+
+/// (e) `createTransaction` (Factory: I/O-free) and the capability/introspection getters construct fine on
+/// a Vanished disk -- so a vanished-disk table's DROP can allocate its removal transaction.
+TEST(CASOperationGate, FactoryClassWorksOnVanished)
+{
+    auto storage = openGateStorage();
+    storage->store()->setLifecycleForTest(PoolLifecycle::VanishedForgotten);   /// one force from Live
+
+    EXPECT_NO_THROW({ auto tx = storage->createTransaction(); (void)tx; });
+    EXPECT_EQ(storage->getType(), MetadataStorageType::CAS);
+    EXPECT_NO_THROW((void)storage->getPath());
+    EXPECT_NO_THROW((void)storage->isContentAddressed());
+}
+
+/// (f) `tryGetInManifestBytes` PROPAGATES the typed refusal — terminal 668 on a `Vanished` disk, the
+/// transient class in a lease gap — rather than converting either into a silent-absent `std::nullopt`
+/// (the narrowed catch). RED before the narrowing.
+TEST(CASOperationGate, TryGetInManifestBytesPropagatesTypedError)
+{
+    auto storage = openGateStorage();
+    commitOnePart(*storage);
+    auto pool = storage->store();   /// captured while Live
+
+    pool->setLifecycleForTest(PoolLifecycle::VanishedReplaced);
+    /// Never FILE_DOESNT_EXIST, never a swallowed nullopt -- the typed INVALID_STATE escapes.
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] {
+        storage->tryGetInManifestBytes(kTableDir + "/format_version.txt");
+    });
+
+    pool->setLifecycleForTest(PoolLifecycle::TransientNotLive);
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] {
+        storage->tryGetInManifestBytes(kTableDir + "/format_version.txt");
+    });
+}
+
+/// (g) (rev.8, Task 15) Null-pool fail-loud: the Dormant/UNMOUNT rollback replaced the transitional
+/// not-Mounted branch (which answered `Probe`->benign-absent) with a null-pool fail-loud. A storage whose
+/// pool is torn down (`shutdown()`) refuses EVERY class, `Probe` included, with `INVALID_STATE`
+/// ("not started") -- there is no benign-absent answer for a not-started disk; only a genuinely `Vanished`
+/// POOL answers truth-absent. Replaces the deleted `DormantDiskKeepsOldBenignAbsent_RemoveAtTask15`.
+TEST(CASOperationGate, NullPoolFailsLoudForEveryClass)
+{
+    auto storage = openGateStorage();
+    commitOnePart(*storage);
+    ASSERT_TRUE(storage->existsDirectory(kPartDir));
+
+    storage->shutdown();   /// null pool -- the ShutDown storage lifecycle
+
+    /// Probes now THROW (not started), NOT the transitional benign-absent answer.
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->existsFile(kPartFile); });
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->existsDirectory(kPartDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { (void)storage->listDirectory(kTableDir); });
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->isDirectoryEmpty(kTableDir); });
+    /// Store-class ops throw the same INVALID_STATE ("not started"), not the typed Vanished message.
+    Cas::tests::expectThrowsCode(ErrorCodes::INVALID_STATE, [&] { storage->getFileSize(kPartFile); });
+}
+
+/// (h) The raw GC round entry points refuse on a not-live pool (Admin class): typed [D5] reason once Vanished.
+TEST(CASOperationGate, GcEntryPointsRefuseOnNotLive)
+{
+    auto storage = openGateStorage();
+    auto pool = storage->store();   /// captured while Live
+
+    pool->setLifecycleForTest(PoolLifecycle::VanishedReplaced);
+    EXPECT_NE(messageOf([&] { storage->runOneGcRoundForTest(); }).find("foreign pool"), std::string::npos);
+
+    pool->setLifecycleForTest(PoolLifecycle::TransientNotLive);
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->runOneGcRoundForTest(); });
+}
+
+/// (i) `CasGcScheduler::isQuiescent` reflects the round-in-flight flag: a round in flight => not quiescent.
+/// (This is the join-completion signal the FORGET / GC-STOP tests rely on.)
+TEST(CASOperationGate, GcSchedulerIsQuiescentReflectsRoundInFlight)
+{
+    auto backend = std::make_shared<Cas::InMemoryBackend>();
+    auto pool = Cas::tests::openPoolForTest(backend);
+    auto scheduler = std::make_shared<Cas::CasGcScheduler>(
+        pool, std::chrono::seconds(3600), "op-gate-test-gc", "disk", Cas::GcRoundLogger{});
+    EXPECT_TRUE(scheduler->isQuiescent());
+    scheduler->setRoundInFlightForTest(true);
+    EXPECT_FALSE(scheduler->isQuiescent()) << "a round in flight must NOT read as GC-quiescent";
+    scheduler->setRoundInFlightForTest(false);
+    EXPECT_TRUE(scheduler->isQuiescent());
+}
+
+/// (j) (acceptance matrix — transient auto-recovery / DROP-drain round-trip) The full §4 recovery arc on ONE
+/// storage: a Remove-class op (the DROP shape) throws the typed transient refusal while the mount lease is
+/// lost, then SUCCEEDS and actually drains once the disk self-remounts back to Live — no operator action,
+/// no restart. Where test (d) forces `TransientNotLive` via the setter to pin the gap, this drives a REAL
+/// transient→Live recovery (`tripMountLost` → fence-out → `tryRemountOnce`) so the throw-then-drain is one
+/// continuous arc on the same pool. Closes the "access throws in the gap, auto-recovers, a Remove re-queues
+/// and drains" matrix row end-to-end (the per-table DROP re-queue itself is the MergeTree caller's job; the
+/// CAS contract is exactly this: refuse in the gap, admit after recovery).
+TEST(CASOperationGate, RemoveThrowsDuringTransientAndDrainsAfterRecovery)
+{
+    auto storage = openGateStorage();
+    commitOnePart(*storage);
+    auto pool = storage->store();   /// captured while Live (store() is fail-closed once not-live)
+    ASSERT_EQ(pool->lifecycle(), PoolLifecycle::Live);
+    ASSERT_TRUE(storage->existsDirectory(kPartDir));   /// Live baseline: the part is present.
+
+    /// The mount lease is transiently lost — the pool goes TransientNotLive.
+    pool->tripMountLost();
+    ASSERT_EQ(pool->lifecycle(), PoolLifecycle::TransientNotLive);
+
+    /// In the gap, EVERY store-class access throws the typed transient refusal — the Remove (DROP shape)
+    /// included, and a content read too. Nothing is answered benign, nothing is silently dropped.
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] {
+        auto tx = storage->createTransaction();
+        tx->removeRecursive(kTableDir, /*should_remove_objects=*/nullptr);
+    });
+    Cas::tests::expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { storage->getFileSize(kPartFile); });
+    const std::string gap_msg = messageOf([&] { storage->getFileSize(kPartFile); });
+    EXPECT_NE(gap_msg.find("mount lease not held"), std::string::npos)
+        << "the gap message must name the transient (auto-recovering) condition: " << gap_msg;
+
+    /// The lease is restored: the disk self-remounts a fresh incarnation and auto-recovers to Live.
+    fenceOutMount(pool->backend(), pool->layout().mountKey(kSrid));
+    ASSERT_TRUE(pool->tryRemountOnce()) << "the self-remount must reclaim a fresh incarnation";
+    ASSERT_EQ(pool->lifecycle(), PoolLifecycle::Live) << "the pool must auto-recover to Live";
+
+    /// After recovery the SAME Remove drains: it commits cleanly and actually removes the part.
+    {
+        auto tx = storage->createTransaction();
+        EXPECT_NO_THROW(tx->removeRecursive(kTableDir, /*should_remove_objects=*/nullptr));
+        EXPECT_NO_THROW(tx->commit(NoCommitOptions{}));
+    }
+    EXPECT_FALSE(storage->existsDirectory(kPartDir))
+        << "the re-queued removal must drain (really remove the part) once the disk recovers to Live";
+    EXPECT_FALSE(storage->existsFile(kPartFile));
+}
diff --git a/src/Disks/tests/gtest_cas_orphan_manifest_sweep.cpp b/src/Disks/tests/gtest_cas_orphan_manifest_sweep.cpp
new file mode 100644
index 000000000000..45f82c1a9e33
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_orphan_manifest_sweep.cpp
@@ -0,0 +1,678 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Common/Exception.h>
+#include "cas_sweep_test_support.h"
+#include "cas_test_helpers.h"
+#include <algorithm>
+#include <limits>
+#include <stdexcept>
+#include <utility>
+#include <vector>
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+constexpr uint64_t kWriterEpoch = 7;
+const String kServerRoot = "00";
+ManifestRef ref(uint64_t seq, uint64_t inst)
+{
+    return ManifestRef{.writer_epoch = kWriterEpoch, .build_sequence = seq, .manifest_ordinal = static_cast<uint32_t>(inst)};
+}
+
+/// The §6 deletion premise (`manifestDeletionPremise`) is a SECOND precondition on every deletion below,
+/// alongside the watermark eligibility these tests are about: a manifest of an epoch-`E` build is
+/// deletable only once the namespace's sealed fold cursor sits in an epoch strictly above `E`. Tests
+/// whose subject is the eligibility or ownership rule therefore have to establish it, or they would
+/// assert a deletion the premise (not the rule under test) prevented. Tests whose subject is RETENTION
+/// deliberately do NOT call this — see `CASSweepDeletionPremise` for the premise's own coverage.
+void seedConsumedSealCursor(InMemoryBackend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    seedFoldCursorForTest(backend, layout, ns, RefTxnId{kWriterEpoch + 1, 1});
+}
+
+/// The catalog cut and `_ckpt` are recovery's sole authority. A fixture that expects a catalog-named
+/// life to be swept must establish the same empty, fully readable recovery state a real completed
+/// creation would have, rather than relying on the retired sentinel fallback.
+void seedEmptyRecoveryAuthority(InMemoryBackend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const CasRefCatalog::Snapshot catalog = CasRefCatalog::read(backend, layout);
+    const auto entry = std::find_if(catalog.catalog.entries.begin(), catalog.catalog.entries.end(),
+        [&](const CatalogEntry & candidate) { return candidate.ns == ns; });
+    ASSERT_NE(entry, catalog.catalog.entries.end());
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry->ns, entry->incarnation);
+    ASSERT_EQ(backend.putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = std::optional<uint64_t>{kWriterEpoch},
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+}
+
+/// Replaces the catalog row immediately before its second read after arming. The legacy orphan path
+/// reads a catalog cut for coverage, then resolves the name again inside recovery; the second read can
+/// splice a successor life into the old coverage decision. An authority-threaded path has no second
+/// catalog read, so this seam must remain dormant.
+class CatalogChangingOnSecondReadBackend : public InMemoryBackend
+{
+public:
+    using Backend::get;
+
+    void arm(const Layout & layout, CatalogEntry predecessor_, CatalogEntry successor_)
+    {
+        catalog_key = layout.refCatalogKey();
+        predecessor = std::move(predecessor_);
+        successor = std::move(successor_);
+        catalog_reads = 0;
+        armed = true;
+    }
+
+    bool didSwitch() const { return did_switch; }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (armed && key == catalog_key && ++catalog_reads == 2)
+        {
+            const auto current = InMemoryBackend::get(key, range);
+            if (!current)
+                throw std::runtime_error("test catalog disappeared");
+            RefCatalog next = decodeRefCatalog(current->bytes);
+            const auto it = std::find(next.entries.begin(), next.entries.end(), predecessor);
+            if (it == next.entries.end())
+                throw std::runtime_error("test predecessor catalog row disappeared");
+            *it = successor;
+            if (InMemoryBackend::casPut(key, encodeRefCatalog(next), current->token).outcome != CasOutcome::Committed)
+                throw std::runtime_error("test catalog replacement conflicted");
+            did_switch = true;
+        }
+        return InMemoryBackend::get(key, range);
+    }
+
+private:
+    String catalog_key;
+    CatalogEntry predecessor;
+    CatalogEntry successor;
+    uint64_t catalog_reads = 0;
+    bool armed = false;
+    bool did_switch = false;
+};
+
+/// Rewrites a listed manifest after the page captured it but before the page takes its lifecycle cut.
+/// The old implementation performed its candidate GET after that cut and would delete this replacement
+/// with its new token. The fixed path may nominate the old observation, but exact-token deletion loses.
+class ReplacingManifestAfterObservationBackend : public InMemoryBackend
+{
+public:
+    using Backend::get;
+
+    void arm(const Layout & layout, String manifest_key_)
+    {
+        catalog_key = layout.refCatalogKey();
+        manifests_prefix = layout.casManifestsPrefix();
+        manifest_key = std::move(manifest_key_);
+        listed_page = false;
+        replaced_manifest = false;
+        armed = true;
+    }
+
+    bool didReplace() const { return replaced_manifest; }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        const ListPage page = InMemoryBackend::list(prefix, cursor, limit);
+        if (armed && prefix == manifests_prefix)
+            listed_page = true;
+        return page;
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        const auto result = InMemoryBackend::get(key, range);
+        if (armed && listed_page && !replaced_manifest && key == catalog_key)
+        {
+            const auto current = InMemoryBackend::get(manifest_key);
+            if (!current)
+                throw std::runtime_error("test manifest disappeared before replacement");
+            if (InMemoryBackend::casPut(manifest_key, current->bytes, current->token).outcome != CasOutcome::Committed)
+                throw std::runtime_error("test manifest replacement conflicted");
+            replaced_manifest = true;
+        }
+        return result;
+    }
+
+private:
+    String catalog_key;
+    String manifests_prefix;
+    String manifest_key;
+    bool armed = false;
+    bool listed_page = false;
+    bool replaced_manifest = false;
+};
+}
+
+/// A staged-but-unowned body in an ELIGIBLE prefix, absent from the owner view, is deleted (#7).
+TEST(CASOrphanManifestSweep, EligibleAndUnownedIsDeleted)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    registerNamespaceRaw(*backend, store->layout(), ns);
+    const ManifestRef r = ref(5, 0xAB);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});   // body, no owner
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, /*min_active*/6);   // 6 > 5 => eligible
+    seedConsumedSealCursor(*backend, store->layout(), ns);
+    seedEmptyRecoveryAuthority(*backend, store->layout(), ns);
+
+    sweepNamespace(*store, ns, BuildPrefix{.writer_epoch = kWriterEpoch, .build_sequence = 5});
+    EXPECT_FALSE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+}
+
+/// The orphan sweep must not turn a forged same-id snapshot at an OLDER `EpochSeal` into an empty owner
+/// view. The base differs from `last_epoch_seal`, so metadata equality cannot catch it; the candidate
+/// remains retained until the checkpoint is repaired.
+TEST(CASOrphanManifestSweep, CheckpointSnapshotAtOlderEpochSealSkipsDeletion)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/sweep-checkpoint-base-seal@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+
+    const RefLogTxn birth{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 1}, .ops = {namespaceBirthOp()},
+        .prev_epoch_seal = std::nullopt};
+    fixture::writeRefLogRaw(*backend, layout, birth);
+    RefOp seal;
+    seal.kind = RefOpKind::EpochSeal;
+    const RefLogTxn seal_txn{
+        .ns = ns.string(), .txn_id = RefTxnId{1, 2}, .ops = {seal},
+        .prev_epoch_seal = std::nullopt};
+    fixture::writeRefLogRaw(*backend, layout, seal_txn);
+    RefOp later_seal;
+    later_seal.kind = RefOpKind::EpochSeal;
+    fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(), .txn_id = RefTxnId{2, 1}, .ops = {later_seal},
+        .prev_epoch_seal = RefTxnId{1, 2}});
+    RefTableState through_seal;
+    applyRefLogTxn(through_seal, birth);
+    applyRefLogTxn(through_seal, seal_txn);
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(through_seal, ns.string()));
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = RefTxnId{1, 2},
+        .last_epoch_seal = RefTxnId{2, 1}})).outcome, PutOutcome::Done);
+
+    const ManifestRef candidate = ref(5, 0xAC);
+    const String candidate_key = layout.manifestKey(ManifestId{ns, candidate});
+    writeManifestRaw(*backend, layout, ns, candidate, {blobEntryFor("a", DB::UInt128(1))});
+    setWatermarkMinActive(*backend, layout, kServerRoot, kWriterEpoch, /*min_active=*/6);
+    seedConsumedSealCursor(*backend, layout, ns);
+
+    std::vector<String> warnings;
+    EXPECT_EQ(sweepNamespace(*store, ns, BuildPrefix{.writer_epoch = kWriterEpoch, .build_sequence = 5}, &warnings), 0u);
+    EXPECT_TRUE(backend->head(candidate_key).exists);
+    ASSERT_FALSE(warnings.empty());
+}
+
+/// A body that IS in the owner view (committed) is NEVER swept (#8).
+TEST(CASOrphanManifestSweep, OwnedBodyIsSkipped)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(5, 0xAB);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);  // now owned
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+
+    sweepNamespace(*store, ns, BuildPrefix{.writer_epoch = kWriterEpoch, .build_sequence = 5});
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+}
+
+/// GC-WEDGE regression (2026-07-10): a COMMITTED ref that has been DROPPED but whose removal `-1` is NOT
+/// yet sealed (transition_version above the sealed fold cursor, which is 0 for this fresh pool) must
+/// SURVIVE the sweep — the GC fold still needs the body to emit the `-1` (delete-after-sealed-decrements).
+/// A promoted build retires its build_seq, so the prefix is watermark-eligible; before the fix the sweep
+/// deleted the body in the dropRef→fold window → the removal-fold then clamped FOREVER on the missing
+/// committed body → pool-wide GC stop. The pending-removal protection now covers COMMITTED (not only
+/// PRECOMMIT) removals.
+///
+/// SINCE THE §6 PREMISE, this shape is held by TWO independent facts: the tail-removal protection this
+/// test is named for, and the premise's rule (1) — the fixture seals no fold cursor, so epoch
+/// `kWriterEpoch`'s closing seal is not consumed either. They cannot be separated HERE: the removal log
+/// sits in a lower epoch than the build, so any cursor high enough to satisfy rule (1) would also sit
+/// above the log and stop the tail scan from reading it at all. The case where the tail-removal
+/// protection is the ONLY thing standing — a removal in a LATER epoch, which is the direction removals
+/// actually cross — is `CASSweepDeletionPremise.AnUnconsumedTailRemovalRetainsItsTarget`.
+TEST(CASOrphanManifestSweep, PendingCommittedRemovalBodyIsSkipped)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(5, 0xAB);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);   // committed owner
+    dropRefTransition(*backend, store->layout(), ns, "tbl", r);   // dropped: pending committed removal, -1 unsealed
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);   // 6 > 5 => prefix eligible
+
+    sweepNamespace(*store, ns, BuildPrefix{.writer_epoch = kWriterEpoch, .build_sequence = 5});
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists)
+        << "a dropped-but-unsealed committed manifest body must survive the sweep (delete-after-sealed-"
+           "decrements) — else the removal-fold clamps forever on the missing body (GC-WEDGE-2026-07-10)";
+}
+
+/// The sweep emits NO blob deltas: the in-degree generation is unchanged.
+TEST(CASOrphanManifestSweep, EmitsNoBlobDeltas)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(5, 0xAB);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+    seedConsumedSealCursor(*backend, store->layout(), ns);
+    seedEmptyRecoveryAuthority(*backend, store->layout(), ns);
+    // The sweep must not advance the in-degree generation: capture it AFTER the fixture's own seal.
+    const uint64_t gen_before = currentGenerationOf(*backend, store->layout());
+
+    sweepNamespace(*store, ns, BuildPrefix{.writer_epoch = kWriterEpoch, .build_sequence = 5});
+    EXPECT_EQ(currentGenerationOf(*backend, store->layout()), gen_before);
+    EXPECT_EQ(inDegreeOf(*backend, store->layout(), DB::UInt128(1)), 0);
+}
+
+TEST(CASOrphanManifestSweep, CursorPageAdvancesAndWrapsWithListBudget)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    registerNamespaceRaw(*backend, store->layout(), ns);
+    const ManifestRef r1 = ref(5, 0xE1);
+    const ManifestRef r2 = ref(5, 0xE2);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, /*min_active*/6);
+
+    const ManifestSweepResult first = sweepManifestCursorPageForTest(*store, "", /*list_budget*/1, /*delete_budget*/0);
+    EXPECT_EQ(first.listed, 1u);
+    EXPECT_FALSE(first.wrapped);
+    EXPECT_FALSE(first.next_cursor.empty());
+
+    const ManifestSweepResult second = sweepManifestCursorPageForTest(*store, first.next_cursor, /*list_budget*/100, /*delete_budget*/0);
+    EXPECT_GE(second.listed, 1u);
+    EXPECT_TRUE(second.wrapped);
+    EXPECT_TRUE(second.next_cursor.empty());
+}
+
+/// A NON-eligible prefix (no watermark fact) deletes NOTHING (#9: frozen-seq is not authority).
+TEST(CASOrphanManifestSweep, NoWatermarkIsNotAuthority)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(5, 0xAB);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    // No setWatermarkMinActive — no durable fact => not eligible.
+    sweepNamespace(*store, ns, BuildPrefix{.writer_epoch = kWriterEpoch, .build_sequence = 5});
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+}
+
+TEST(CASOrphanManifestSweep, CursorPageDeletesEligibleUnownedBody)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    registerNamespaceRaw(*backend, store->layout(), ns);
+    const ManifestRef r = ref(5, 0xAC);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+    seedConsumedSealCursor(*backend, store->layout(), ns);
+    seedEmptyRecoveryAuthority(*backend, store->layout(), ns);
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget*/100, /*delete_budget*/10);
+    EXPECT_GE(result.listed, 1u);
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_FALSE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+}
+
+TEST(CASOrphanManifestSweep, CursorPageRespectsDeleteBudget)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    registerNamespaceRaw(*backend, store->layout(), ns);
+    const ManifestRef r1 = ref(5, 0xAD);
+    const ManifestRef r2 = ref(5, 0xAE);
+    writeManifestRaw(*backend, store->layout(), ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, store->layout(), ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+    seedConsumedSealCursor(*backend, store->layout(), ns);
+    seedEmptyRecoveryAuthority(*backend, store->layout(), ns);
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget*/100, /*delete_budget*/1);
+    EXPECT_EQ(result.deleted, 1u);
+    const bool first_exists = backend->head(store->layout().manifestKey(ManifestId{ns, r1})).exists;
+    const bool second_exists = backend->head(store->layout().manifestKey(ManifestId{ns, r2})).exists;
+    EXPECT_NE(first_exists, second_exists);
+}
+
+/// A physical manifest captured before a catalog cut which omits its name is dead-life debris: a live
+/// creation cannot publish a life-owned object before its catalog row. It therefore has an eventual
+/// page-sweep owner without trying to reconstruct a deleted incarnation from the key.
+TEST(CASOrphanManifestSweep, CursorPageDeletesObservedBodyWhenCatalogOmitsNamespace)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/catalog-absent-debris@cas@"};
+    const ManifestRef r = ref(5, 0xA9);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("debris", DB::UInt128(9))});
+    /// No mount lease/watermark exists: after legal catalog-row deletion there may be no server-root
+    /// state left to supply one. The post-observation absent row is the complete dead-life proof.
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget=*/100, /*delete_budget=*/10);
+
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_FALSE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+}
+
+/// The candidate body and token must be frozen before the later catalog cut. A concurrent same-key
+/// replacement after that observation is a new physical incarnation and must lose the old-token delete.
+TEST(CASOrphanManifestSweep, CursorPageCannotDeleteManifestReplacedAfterObservation)
+{
+    auto backend = std::make_shared<ReplacingManifestAfterObservationBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/replace-after-observation@cas@"};
+    registerNamespaceRaw(*backend, store->layout(), ns);
+    const ManifestRef r = ref(5, 0xAA);
+    const String key = store->layout().manifestKey(ManifestId{ns, r});
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("body", DB::UInt128(10))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+    seedConsumedSealCursor(*backend, store->layout(), ns);
+    seedEmptyRecoveryAuthority(*backend, store->layout(), ns);
+    backend->arm(store->layout(), key);
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget=*/100, /*delete_budget=*/10);
+
+    EXPECT_TRUE(backend->didReplace());
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_TRUE(backend->head(key).exists);
+}
+
+/// Any duplicate current life id makes the catalog-to-physical join ambiguous. The cursor page is
+/// destructive, so the whole cut must be rejected before it can nominate even an unrelated body.
+TEST(CASOrphanManifestSweep, CursorPageRefusesAmbiguousCatalogLifeIndex)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/ambiguous-life@cas@"};
+    registerNamespaceRaw(*backend, store->layout(), ns);
+    const ManifestRef r = ref(5, 0xAB);
+    const String key = store->layout().manifestKey(ManifestId{ns, r});
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("body", DB::UInt128(11))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+    seedConsumedSealCursor(*backend, store->layout(), ns);
+    seedEmptyRecoveryAuthority(*backend, store->layout(), ns);
+
+    const CasRefCatalog::Snapshot before = CasRefCatalog::read(*backend, store->layout());
+    RefCatalog damaged = before.catalog;
+    CatalogEntry duplicate = damaged.entries.front();
+    duplicate.ns = RootNamespace{"00/ambiguous-life-twin@cas@"};
+    damaged.entries.push_back(duplicate);
+    std::sort(damaged.entries.begin(), damaged.entries.end(),
+        [](const CatalogEntry & lhs, const CatalogEntry & rhs) { return lhs.ns.string() < rhs.ns.string(); });
+    ASSERT_EQ(backend->casPut(store->layout().refCatalogKey(), encodeRefCatalog(damaged), before.token).outcome,
+              CasOutcome::Committed);
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { sweepManifestCursorPageForTest(*store, "", /*list_budget=*/100, /*delete_budget=*/10); });
+    EXPECT_TRUE(backend->head(key).exists);
+}
+
+TEST(CASOrphanManifestSweep, CursorPageSkipsOwnedBody)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/aa@cas@"};
+    const ManifestRef r = ref(5, 0xAF);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, store->layout(), ns, "tbl", std::nullopt, r);
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget*/100, /*delete_budget*/10);
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists);
+}
+
+/// A catalog-named life cannot be treated as an empty table merely because its mandatory recovery
+/// checkpoint is missing. The orphan sweep is destructive, so it must retain the body until the
+/// caller can recover from the same frozen catalog row and its exact `_ckpt`.
+TEST(CASOrphanManifestSweep, MissingRequiredCheckpointSuppressesDestructiveDecision)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/authority-required@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);
+    const ManifestRef r = ref(5, 0xB0);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+    seedConsumedSealCursor(*backend, store->layout(), ns);
+
+    const CatalogEntry entry = CasRefCatalog::read(*backend, store->layout()).catalog.entries.front();
+    ASSERT_FALSE(readCkpt(*backend, store->layout(), NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation)));
+
+    sweepNamespace(*store, ns, BuildPrefix{.writer_epoch = kWriterEpoch, .build_sequence = 5});
+
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists)
+        << "without the exact _ckpt required by a Live catalog row, the sweep must retain rather than "
+           "derive an empty owner set";
+}
+
+/// A decoded fold cursor at an `EpochSeal` advances to the next GLOBAL writer epoch. Even when this
+/// namespace was inactive, every intermediate epoch exists as a chained sequence-1 empty seal, so the
+/// exact tail begins at `{E+1, 1}` and must consume each one before reaching a later removal. The
+/// removal's target stays protected while an unrelated eligible body remains deletable; retaining both
+/// would hide a false missing-log failure at the first intermediate seal.
+TEST(CASOrphanManifestSweep, EpochSealFoldCursorCrossesTailByExactDecodedSuccessor)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/seal-cursor-tail@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);
+    const CatalogEntry entry = CasRefCatalog::read(*backend, store->layout()).catalog.entries.front();
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation);
+
+    const ManifestRef removed{.writer_epoch = 1, .build_sequence = 5, .manifest_ordinal = 1};
+    const ManifestRef unowned{.writer_epoch = 1, .build_sequence = 6, .manifest_ordinal = 1};
+    const ManifestRef still_owned{.writer_epoch = 2, .build_sequence = 1, .manifest_ordinal = 1};
+    publishAt(*backend, store->layout(), ns, RefTxnId{1, 1}, "dropped", removed.build_sequence, DB::UInt128(0xA1), /*birth=*/true);
+    writeSealAt(*backend, store->layout(), ns, RefTxnId{1, 2});
+    writeTxnAt(*backend, store->layout(), ns, RefTxnId{2, 1}, publishCommittedOps("still-owned", still_owned), RefTxnId{1, 2});
+    writeSealAt(*backend, store->layout(), ns, RefTxnId{2, 2});
+    writeSealAt(*backend, store->layout(), ns, RefTxnId{3, 1}, RefTxnId{2, 2});
+    writeSealAt(*backend, store->layout(), ns, RefTxnId{4, 1}, RefTxnId{3, 1});
+    writeSealAt(*backend, store->layout(), ns, RefTxnId{5, 1}, RefTxnId{4, 1});
+    writeSealAt(*backend, store->layout(), ns, RefTxnId{6, 1}, RefTxnId{5, 1});
+    for (uint64_t epoch = 3; epoch <= 6; ++epoch)
+        ASSERT_TRUE(backend->head(store->layout().refLogKey(life, RefTxnId{epoch, 1})).exists)
+            << "fixture must deposit every intermediate exact successor in the catalog life";
+    writeTxnAt(*backend, store->layout(), ns, RefTxnId{7, 1},
+        {ownerTransitionOp(RefOwnerBinding{RefOwnerKind::Committed, "dropped", removed}, std::nullopt)},
+        RefTxnId{6, 1});
+    writeSealAt(*backend, store->layout(), ns, RefTxnId{7, 2});
+    writeManifestRaw(*backend, store->layout(), ns, unowned, {blobEntryFor("unowned", DB::UInt128(0xA2))});
+    ASSERT_EQ(backend->putIfAbsent(store->layout().refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{7, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{7, 2}})).outcome, PutOutcome::Done);
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 7);
+    seedFoldCursorForTest(*backend, store->layout(), ns, RefTxnId{2, 2});
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget=*/100, /*delete_budget=*/10);
+
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, removed})).exists)
+        << "the exact successor of the folded epoch seal contains this body's unconsumed -1";
+    EXPECT_FALSE(backend->head(store->layout().manifestKey(ManifestId{ns, unowned})).exists)
+        << "an unrelated eligible body must still drain; retaining it would mask a geometry failure";
+}
+
+/// A cleaned inherited cursor does not let a later epoch backlink skip the mandatory immediately-next
+/// global epoch. `{3,1}` is missing here, so the direct `{7,1} -> {2,2}` link cannot authorize a tail
+/// scan; the whole namespace must fail closed and retain even an otherwise unowned eligible body.
+TEST(CASOrphanManifestSweep, MissingImmediateEpochAfterCleanedCursorCannotBeSkippedByLaterBacklink)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/missing-next-epoch@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);
+    const CatalogEntry entry = CasRefCatalog::read(*backend, store->layout()).catalog.entries.front();
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation);
+
+    const RefTxnId cursor{2, 2};
+    writeSealAt(*backend, store->layout(), ns, cursor);
+    const HeadResult cursor_head = backend->head(store->layout().refLogKey(life, cursor));
+    ASSERT_TRUE(cursor_head.exists);
+    ASSERT_EQ(classifyDeleteOutcome(
+        backend->deleteExact(store->layout().refLogKey(life, cursor), cursor_head.token)), DeleteClass::Deleted);
+
+    const ManifestRef phantom{.writer_epoch = 7, .build_sequence = 1, .manifest_ordinal = 1};
+    /// The codec refuses this skipped predecessor when a writer tries to create it. Inject the malformed
+    /// physical lure explicitly: a reader must still not enumerate forward to it when `{3,1}` is absent.
+    RefLogTxn direct_later_link{
+        .ns = ns.string(),
+        .txn_id = RefTxnId{7, 1},
+        .ops = publishCommittedOps("phantom", phantom),
+        .prev_epoch_seal = RefTxnId{6, 1}};
+    String malformed_later_link = encodeRefLogTxn(direct_later_link);
+    const String encoded_predecessor{R"("!pse":"6")"};
+    const size_t predecessor_pos = malformed_later_link.find(encoded_predecessor);
+    ASSERT_NE(predecessor_pos, String::npos);
+    malformed_later_link.replace(
+        predecessor_pos, encoded_predecessor.size(), R"("!pse":"2")");
+    ASSERT_EQ(backend->putIfAbsent(
+        store->layout().refLogKey(life, RefTxnId{7, 1}), sealObject(FormatId::RefLog, malformed_later_link)).outcome,
+        PutOutcome::Done);
+    writeRefSnapshotRaw(*backend, store->layout(), RefTableSnapshot{
+        .ns = ns.string(),
+        .snapshot_id = RefTxnId{7, 1},
+        .committed = {committedRow("phantom", phantom)},
+        .precommits = {}});
+    ASSERT_EQ(backend->putIfAbsent(store->layout().refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{7, 1},
+        .checkpoint_snapshot_id = RefTxnId{7, 1},
+        .last_epoch_seal = RefTxnId{6, 1}})).outcome, PutOutcome::Done);
+
+    const ManifestRef victim{.writer_epoch = 1, .build_sequence = 5, .manifest_ordinal = 1};
+    writeManifestRaw(*backend, store->layout(), ns, victim, {blobEntryFor("victim", DB::UInt128(0xC1))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+    seedFoldCursorForTest(*backend, store->layout(), ns, cursor);
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget=*/100, /*delete_budget=*/10);
+
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, victim})).exists);
+}
+
+/// Control for the cleaned-cursor path: the exact immediately-next epoch head exists and names the
+/// deleted seal, so the tail is readable. Its `-1` protects the removed body while an unrelated eligible
+/// body proves the namespace was scanned rather than retained wholesale. The checkpoint base is the
+/// following same-epoch transaction: recovery therefore has a retained exact anchor without turning the
+/// deliberately cleaned predecessor seal into part of that anchor's proof.
+TEST(CASOrphanManifestSweep, CleanedCursorCrossesOnlyThroughExactImmediateEpochHead)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/exact-next-epoch@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);
+    const CatalogEntry entry = CasRefCatalog::read(*backend, store->layout()).catalog.entries.front();
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation);
+
+    const RefTxnId cursor{2, 2};
+    writeSealAt(*backend, store->layout(), ns, cursor);
+    const HeadResult cursor_head = backend->head(store->layout().refLogKey(life, cursor));
+    ASSERT_TRUE(cursor_head.exists);
+    ASSERT_EQ(classifyDeleteOutcome(
+        backend->deleteExact(store->layout().refLogKey(life, cursor), cursor_head.token)), DeleteClass::Deleted);
+
+    const ManifestRef removed{.writer_epoch = 1, .build_sequence = 5, .manifest_ordinal = 1};
+    writeTxnAt(*backend, store->layout(), ns, RefTxnId{3, 1},
+        {ownerTransitionOp(RefOwnerBinding{RefOwnerKind::Committed, "removed", removed}, std::nullopt)}, cursor);
+    const ManifestRef absent_anchor{.writer_epoch = 1, .build_sequence = 7, .manifest_ordinal = 1};
+    writeTxnAt(*backend, store->layout(), ns, RefTxnId{3, 2},
+        {ownerTransitionOp(RefOwnerBinding{RefOwnerKind::Committed, "absent-anchor", absent_anchor}, std::nullopt)});
+    writeRefSnapshotRaw(*backend, store->layout(), RefTableSnapshot{
+        .ns = ns.string(), .snapshot_id = RefTxnId{3, 2}, .committed = {}, .precommits = {}});
+    ASSERT_EQ(backend->putIfAbsent(store->layout().refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{3, 2},
+        .checkpoint_snapshot_id = RefTxnId{3, 2},
+        .last_epoch_seal = cursor})).outcome, PutOutcome::Done);
+
+    const ManifestRef unowned{.writer_epoch = 1, .build_sequence = 6, .manifest_ordinal = 1};
+    writeManifestRaw(*backend, store->layout(), ns, removed, {blobEntryFor("removed", DB::UInt128(0xC2))});
+    writeManifestRaw(*backend, store->layout(), ns, unowned, {blobEntryFor("unowned", DB::UInt128(0xC3))});
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 7);
+    seedFoldCursorForTest(*backend, store->layout(), ns, cursor);
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget=*/100, /*delete_budget=*/10);
+
+    EXPECT_EQ(result.deleted, 1u);
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, removed})).exists);
+    EXPECT_FALSE(backend->head(store->layout().manifestKey(ManifestId{ns, unowned})).exists);
+}
+
+/// The catalog row used to obtain coverage and the life used to recover ownership must be ONE frozen
+/// authority cut. A later catalog row for the same name may not make the old life's committed manifest
+/// look orphaned and therefore deletable.
+TEST(CASOrphanManifestSweep, LaterCatalogCutCannotSpliceOwnershipAuthority)
+{
+    auto backend = std::make_shared<CatalogChangingOnSecondReadBackend>();
+    auto store = openPoolForTest(backend);
+    const RootNamespace ns{"00/frozen-catalog-cut@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);
+    const CatalogEntry predecessor = CasRefCatalog::read(*backend, store->layout()).catalog.entries.front();
+
+    const ManifestRef r = ref(5, 0xB1);
+    writeManifestRaw(*backend, store->layout(), ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    ASSERT_EQ(publishCommittedTransition(*backend, store->layout(), ns, "live", std::nullopt, r), 1u);
+    setWatermarkMinActive(*backend, store->layout(), kServerRoot, kWriterEpoch, 6);
+    seedConsumedSealCursor(*backend, store->layout(), ns);
+
+    CatalogEntry successor = predecessor;
+    successor.incarnation = DB::UInt128(0xBEEF);
+    backend->arm(store->layout(), predecessor, successor);
+
+    sweepNamespace(*store, ns, BuildPrefix{.writer_epoch = kWriterEpoch, .build_sequence = 5});
+
+    EXPECT_FALSE(backend->didSwitch())
+        << "the sweep must not resolve a second catalog cut after it starts using the frozen entry";
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(ManifestId{ns, r})).exists)
+        << "the committed predecessor manifest must remain protected by the same frozen authority cut";
+}
+
+/// The LIST-based late-log detector that lived here is RETIRED with the sentinel seal, and it is worth
+/// recording why rather than leaving a hole in this file's coverage story.
+///
+/// It existed because the old seal was a SNAPSHOT at a synthetic `{E-1, UINT64_MAX}` id: that object
+/// occupied no `_log` key, so a dying predecessor's in-flight PUT could still land in the dead epoch and
+/// the only possible response was to notice it afterwards and report it. INV-2's seal is a
+/// TRANSACTION at exactly `{E, T+1}` -- the key that ghost would take -- so the store's own write-once
+/// create refuses it. There is nothing left to detect at that shape: an id above the seal cannot be
+/// minted either, because ids are state-derived and a writer that could derive `{E, T+2}` would have had
+/// to observe the seal first.
+///
+/// `CasEventType::RefLateLogDetected` is retired WITH the detector -- pre-release, so a vocabulary entry
+/// nothing can emit is just dead surface. Soak scenario S38 (`s38_late_put_injection.py`) keeps its
+/// injection and FLIPS its assertion: from "the detection fired" to "the fence held" -- the late PUT's
+/// conditional create must LOSE to the occupied slot, with zero data loss and the namespace folding
+/// normally.
diff --git a/src/Disks/tests/gtest_cas_orphan_nomination.cpp b/src/Disks/tests/gtest_cas_orphan_nomination.cpp
new file mode 100644
index 000000000000..039d113cb134
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_orphan_nomination.cpp
@@ -0,0 +1,319 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+constexpr uint64_t kCandidateEpoch = 1;
+constexpr uint64_t kCandidateBuild = 5;
+const UInt128 kGcId = hexToU128("000000000000000000000000000000d8");
+
+ManifestRef candidateRef()
+{
+    return ManifestRef{.writer_epoch = kCandidateEpoch, .build_sequence = kCandidateBuild, .manifest_ordinal = 1};
+}
+
+bool manifestExists(Backend & backend, const Layout & layout, const ManifestId & id)
+{
+    return backend.head(layout.manifestKey(id)).exists;
+}
+
+bool activeSourceExists(Backend & backend, const Layout & layout, const UInt128 & source_id)
+{
+    const auto state_got = backend.get(layout.gcStateKey());
+    if (!state_got)
+        return false;
+    const GcState state = decodeGcState(state_got->bytes);
+    const auto seal_got = backend.get(layout.foldSealKey(state.snap_generation, state.snap_attempt));
+    if (!seal_got)
+        return false;
+    const CasFoldSeal seal = decodeFoldSeal(seal_got->bytes);
+    for (const RunRef & run : seal.blob_target_runs)
+    {
+        SourceEdgeRunView view = openSourceEdgeRun(backend, run.key);
+        String key;
+        String payload;
+        while (view.next(key, payload))
+        {
+            if (payload.empty() || payload[0] != kEdgeActive)
+                continue;
+            BlobRef ref;
+            UInt128 row_source{};
+            SourceEdgeKeyCodec::parse(key, ref, row_source);
+            if (row_source == source_id)
+                return true;
+        }
+        view.verifyAgainst(run.checksum);
+    }
+    return false;
+}
+
+size_t condemnedCount(Backend & backend, const Layout & layout)
+{
+    size_t count = 0;
+    const GcState state = decodeGcState(backend.get(layout.gcStateKey())->bytes);
+    const CasFoldSeal seal = decodeFoldSeal(
+        backend.get(layout.foldSealKey(state.snap_generation, state.snap_attempt))->bytes);
+    for (const RunRef & run : seal.blob_target_runs)
+    {
+        SourceEdgeRunView view = openSourceEdgeRun(backend, run.key);
+        String key;
+        String payload;
+        while (view.next(key, payload))
+            count += !payload.empty() && payload[0] == kCondemned;
+        view.verifyAgainst(run.checksum);
+    }
+    return count;
+}
+
+class NominationBackend : public InMemoryBackend
+{
+public:
+    using Backend::deleteExact;
+    using Backend::get;
+    using Backend::putOverwrite;
+
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        if (key == watched_manifest_key)
+        {
+            source_absent_when_delete_started = !activeSourceExists(*this, layout, watched_source_id);
+            if (replace_manifest_before_delete)
+            {
+                const auto got = get(key);
+                if (got)
+                    putOverwrite(key, got->bytes, got->token);
+            }
+        }
+        return InMemoryBackend::deleteExact(key, token);
+    }
+
+    Layout layout{"p"};
+    String watched_manifest_key;
+    UInt128 watched_source_id{};
+    bool source_absent_when_delete_started = false;
+    bool replace_manifest_before_delete = false;
+};
+
+struct ReadyFixture
+{
+    std::shared_ptr<NominationBackend> backend;
+    PoolPtr store;
+    std::unique_ptr<Gc> gc;
+    RootNamespace ns{"test/aa@cas@"};
+    ManifestId candidate{ns, candidateRef()};
+    std::vector<BlobRef> blobs;
+};
+
+ReadyFixture makeReadyFixture()
+{
+    ReadyFixture f;
+    f.backend = std::make_shared<NominationBackend>();
+    PoolConfig config;
+    config.pool_prefix = "p";
+    config.server_root_id = "gc-runner";
+    config.manifest_sweep_list_budget_keys = 100;
+    config.manifest_sweep_delete_budget_keys = 100;
+    config.gc_fold_max_defer_rounds = 0;
+    f.store = Pool::open(f.backend, config);
+    f.backend->layout = f.store->layout();
+    f.gc = std::make_unique<Gc>(f.store, kGcId);
+
+    /// Establish a real catalog life and fold its cursor across epoch 1 before introducing the orphan.
+    publishAt(*f.backend, f.store->layout(), f.ns, RefTxnId{1, 1}, "live-a", /*build_sequence=*/7,
+              UInt128(0x7001), /*birth=*/true);
+    EXPECT_TRUE(runRegularRoundReclaiming(*f.gc).acquired_lease);
+    writeSealAt(*f.backend, f.store->layout(), f.ns, RefTxnId{1, 2});
+    publishAt(*f.backend, f.store->layout(), f.ns, RefTxnId{2, 1}, "live-b", /*build_sequence=*/7,
+              UInt128(0x7002), /*birth=*/false, /*prev_epoch_seal=*/RefTxnId{1, 2});
+    /// Raw log helpers intentionally do not manufacture lifecycle authority. This fixture's durable
+    /// frontier includes the predecessor seal and the epoch-2 start, so nomination is exercised rather
+    /// than being (correctly) skipped for a missing `_ckpt`.
+    writeRecoverableCkptForRawFixture(*f.backend, f.store->layout(), f.ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    });
+    EXPECT_TRUE(runRegularRoundReclaiming(*f.gc).acquired_lease);
+    setWatermarkMinActive(*f.backend, f.store->layout(), "test", kCandidateEpoch, /*min_active=*/6);
+
+    std::vector<ManifestEntry> entries;
+    std::vector<BlobDelta> seeded_edges;
+    for (uint64_t i = 0; i < 6; ++i)
+    {
+        const UInt128 digest = UInt128(0x8000 + i);
+        const BlobRef blob = legacyMetaTestRef(digest);
+        f.blobs.push_back(blob);
+        writeBlobBody(*f.backend, f.store->layout(), digest);
+        const String path = "blob-" + std::to_string(i);
+        entries.push_back(blobEntryFor(path, digest));
+        seeded_edges.push_back(BlobDelta{
+            .ref = blob,
+            .source_id = sourceEdgeId(f.candidate, path),
+            .remove = false});
+        if (i < 4)
+            seeded_edges.push_back(BlobDelta{
+                .ref = blob,
+                .source_id = UInt128(0x9000 + i),
+                .remove = false});
+    }
+    writeManifestRaw(*f.backend, f.store->layout(), f.ns, f.candidate.ref, entries);
+
+    /// Seed the exact S42 precondition: the candidate manifest's `+1` edges are already in the adopted
+    /// run, yet the recovered owner view does not name the body. Four blobs also have another source.
+    const auto state_got = f.backend->get(f.store->layout().gcStateKey());
+    EXPECT_TRUE(state_got.has_value());
+    GcState state = decodeGcState(state_got->bytes);
+    const auto parent_got = f.backend->get(
+        f.store->layout().foldSealKey(state.snap_generation, state.snap_attempt));
+    EXPECT_TRUE(parent_got.has_value());
+    CasFoldSeal seal = decodeFoldSeal(parent_got->bytes);
+    const uint64_t new_generation = state.snap_generation + 1;
+    const uint64_t new_attempt = state.snap_attempt + 1000;
+    std::vector<RunRef> runs;
+    RetiredMergeResult retired;
+    foldDeltasIntoGeneration(
+        *f.backend, f.store->layout(), seal.blob_target_runs,
+        new_generation, new_attempt, /*shard=*/0, std::move(seeded_edges), runs,
+        /*current_round=*/state.round, /*condemn_round=*/state.round,
+        {}, {}, {}, &retired, /*suppress_destructive=*/false, nullptr);
+    seal.parent_generation = state.snap_generation;
+    seal.generation = new_generation;
+    seal.blob_target_runs = std::move(runs);
+    seal.condemned_summary[0] = CondemnedSummary{};
+    putDeterministicArtifact(
+        *f.backend, f.store->layout().foldSealKey(new_generation, new_attempt), encodeFoldSeal(seal));
+    state.snap_generation = new_generation;
+    state.snap_attempt = new_attempt;
+    f.backend->putOverwrite(f.store->layout().gcStateKey(), encodeGcState(state), state_got->token);
+
+    f.backend->watched_manifest_key = f.store->layout().manifestKey(f.candidate);
+    f.backend->watched_source_id = sourceEdgeId(f.candidate, "blob-0");
+    return f;
+}
+
+}
+
+/// S42: sweeping an aborted precommit must retire that manifest's exact source edges before deleting
+/// the body. Other sources stay intact, and only the two uniquely-owned blobs enter retirement.
+TEST(CASOrphanNomination, RetiresExactManifestSourcesBeforeDelete)
+{
+    ReadyFixture f = makeReadyFixture();
+
+    /// The nominating round's own `fold_reduce` phase carries probe B1/B2's per-round verdict; capture
+    /// it so the orphan-sourced retirement can be proven accounting-neutral on the real end-to-end path,
+    /// not only on the synthetic `foldDeltasIntoGeneration` call `SourceRetirementIsAccountingNeutral`
+    /// drives below.
+    std::optional<GcPhaseRecord> fold_reduce;
+    f.gc->setPhaseSink([&](const GcPhaseRecord & rec) { if (rec.phase == "fold_reduce") fold_reduce = rec; });
+
+    ASSERT_TRUE(runRegularRoundReclaiming(*f.gc).acquired_lease);
+
+    EXPECT_FALSE(manifestExists(*f.backend, f.store->layout(), f.candidate));
+    EXPECT_TRUE(f.backend->source_absent_when_delete_started)
+        << "the adopted in-degree run must retire the manifest source before exact deletion begins";
+    for (size_t i = 0; i < f.blobs.size(); ++i)
+    {
+        EXPECT_FALSE(activeSourceExists(
+            *f.backend, f.store->layout(), sourceEdgeId(f.candidate, "blob-" + std::to_string(i))));
+        EXPECT_EQ(inDegreeInRuns(*f.backend, runsForShard(*f.backend, f.store->layout(), 0), f.blobs[i]),
+                  i < 4 ? 1 : 0);
+    }
+    EXPECT_EQ(condemnedCount(*f.backend, f.store->layout()), 2u);
+
+    ASSERT_TRUE(fold_reduce.has_value());
+    EXPECT_EQ(fold_reduce->metrics.at("unmatched_removes"), 0u)
+        << "the orphan source retirements are exact removes against a present edge, never an unmatched one";
+    EXPECT_EQ(fold_reduce->metrics.at("transactions_unapplied"), 0u)
+        << "the retirement input rides the reducer alongside ordinary deltas without stranding a "
+           "committed+produced ref transaction unapplied";
+}
+
+/// A nomination must exact-GET and decode the manifest before it can derive any source-edge identity.
+TEST(CASOrphanNomination, CorruptManifestIsRetainedAndSurfaced)
+{
+    ReadyFixture f = makeReadyFixture();
+    const auto got = f.backend->get(f.backend->watched_manifest_key);
+    ASSERT_TRUE(got.has_value());
+    f.backend->putOverwrite(f.backend->watched_manifest_key, "not a sealed manifest", got->token);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { runRegularRoundReclaiming(*f.gc); });
+    EXPECT_TRUE(f.backend->head(f.backend->watched_manifest_key).exists);
+}
+
+/// Manifest identities are immutable. A changed token at the same key is illegal ABA, not an ordinary
+/// exact-delete race that may be silently treated as spared.
+TEST(CASOrphanNomination, TokenAbaIsRetainedAndSurfaced)
+{
+    ReadyFixture f = makeReadyFixture();
+    f.backend->replace_manifest_before_delete = true;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { runRegularRoundReclaiming(*f.gc); });
+    EXPECT_TRUE(f.backend->head(f.backend->watched_manifest_key).exists);
+}
+
+/// Nomination PLANNING itself is gated on `!suppress_destructive`
+/// (`Gc::fold`'s orphan_sweep call site), not merely its eventual delete -- a suppressed pass must
+/// never even LIST candidates. The suppressed universe is selected explicitly, because that is the
+/// subject: a round on the production default would open the gate and sweep.
+TEST(CASOrphanNomination, SuppressedRoundNominatesNothing)
+{
+    ReadyFixture f = makeReadyFixture();
+
+    std::optional<GcPhaseRecord> orphan_sweep;
+    f.gc->setPhaseSink([&](const GcPhaseRecord & rec) { if (rec.phase == "orphan_sweep") orphan_sweep = rec; });
+
+    ASSERT_TRUE(f.gc->runRegularRound({}, /*allow_steal*/true,
+                                     UniversePolicy::StageA_Suppressed).acquired_lease);
+
+    ASSERT_TRUE(orphan_sweep.has_value());
+    EXPECT_EQ(orphan_sweep->metrics.at("suppressed"), 1u);
+    EXPECT_EQ(orphan_sweep->metrics.at("listed"), 0u)
+        << "planning is gated on !suppress_destructive; a suppressed pass must not even LIST candidates";
+    EXPECT_EQ(orphan_sweep->metrics.at("deleted"), 0u);
+    EXPECT_TRUE(manifestExists(*f.backend, f.store->layout(), f.candidate))
+        << "the orphan body must survive a suppressed round";
+}
+
+/// The retirement input is deliberately outside both ref-transaction accounting mechanisms: a
+/// matching edge disappears, an already-absent one stays an idempotent no-op, and neither can alter B2.
+TEST(CASOrphanNomination, SourceRetirementIsAccountingNeutral)
+{
+    InMemoryBackend backend;
+    const Layout layout{"p"};
+    const BlobRef blob = legacyMetaTestRef(UInt128(0xA001));
+    const UInt128 source = UInt128(0xA002);
+    std::vector<RunRef> parent_runs;
+    foldDeltasIntoGeneration(
+        backend, layout, {}, /*new_generation=*/1, /*attempt=*/1, /*shard=*/0,
+        {BlobDelta{.ref = blob, .source_id = source, .remove = false}}, parent_runs);
+
+    std::vector<RunRef> next_runs;
+    RetiredMergeResult retired;
+    std::vector<uint8_t> applied{0x5A};
+    foldDeltasIntoGeneration(
+        backend, layout, parent_runs, /*new_generation=*/2, /*attempt=*/2, /*shard=*/0,
+        {}, next_runs, /*current_round=*/1, /*condemn_round=*/1,
+        {}, {}, {}, &retired, /*suppress_destructive=*/false, &applied,
+        {BlobSourceRetirement{.ref = blob, .source_id = source},
+         BlobSourceRetirement{.ref = blob, .source_id = UInt128(0xA003)}});
+
+    EXPECT_EQ(inDegreeInRuns(backend, next_runs, blob), 0);
+    EXPECT_EQ(retired.unmatched_removes, 0u);
+    EXPECT_EQ(applied, (std::vector<uint8_t>{0x5A}));
+}
diff --git a/src/Disks/tests/gtest_cas_parallel_commit.cpp b/src/Disks/tests/gtest_cas_parallel_commit.cpp
new file mode 100644
index 000000000000..7ae5ab676712
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_parallel_commit.cpp
@@ -0,0 +1,308 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <fmt/format.h>
+#include <gtest/gtest.h>
+#include <atomic>
+#include <filesystem>
+
+/// Task 2 of the CAS parallel-write-path plan (docs/superpowers/sdd): `promoteBuild`/`repointRef`
+/// return an exact, in-lane-derived `Cas::CommitOutcome` instead of `void`/`bool`, and
+/// `dropRefIfMatches` gives a future rollback a conditional drop keyed on that exact outcome instead
+/// of the unsafe-under-concurrency `dropRef` (which removes whatever manifest currently occupies the
+/// ref name). This suite grows across the later parallel-commit tasks; here it only proves the
+/// outcome is exact and that the conditional drop is a true guard -- still single-threaded commit, no
+/// concurrency yet.
+///
+/// Task 3 reworks `ContentAddressedTransaction::commit()`'s rollback to be EXACT (per-part
+/// `Cas::CommitOutcome` slots + `dropRefIfMatches`) while the commit loop stays single-threaded --
+/// correctness-first, before Task 5 adds concurrency. `CasCommitRollback` below drives real
+/// `ContentAddressedTransaction`s (not the bare pool primitives `CaWiringFixture` above exercises)
+/// through the exact `publishStaging` call path production `commit()` uses, so the fault seams
+/// (`armPromoteFailure`/`armAfterPromoteHook`) fire from the real thing.
+
+using namespace DB;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+/// Fixture mirroring `gtest_cas_part_folder_access.cpp`'s `publishPart`/`cacheOn` helpers: a fresh
+/// in-memory pool + a `CachedPartFolderAccess` facade over it, plus the minimal staging helpers this
+/// suite's tests need (stage a simple one-file part without promoting it; stage-and-promote it in one
+/// call; repoint an already-committed ref onto a fresh manifest, modeling a later writer).
+struct CaWiringFixture
+{
+    std::shared_ptr<Cas::InMemoryBackend> backend = std::make_shared<Cas::InMemoryBackend>();
+    Cas::PoolPtr store = openPoolForTest(backend);
+    Cas::CachedPartFolderAccess access{store};
+    Cas::RootNamespace namespace_{"srv/t1"};
+    int content_counter = 0;
+
+    const Cas::RootNamespace & ns() const { return namespace_; }
+    Cas::CachedPartFolderAccess & partAccess() { return access; }
+
+    static Cas::ManifestEntry inlineEntry(const String & path, const String & bytes)
+    {
+        Cas::ManifestEntry e;
+        e.path = path;
+        e.placement = Cas::EntryPlacement::Inline;
+        e.ref = Cas::BlobRef{Cas::BlobHashAlgo::CityHash128, Cas::BlobDigest::fromU128(u128Of(bytes))};
+        e.blob_size = bytes.size();
+        e.inline_bytes = bytes;
+        return e;
+    }
+
+    struct Staged
+    {
+        Cas::PartWriteTxnPtr build;
+        Cas::ManifestId id;
+    };
+
+    /// Stages a fresh build (manifest + precommit) for `key` over `blobs` inline entries, WITHOUT
+    /// promoting it -- the caller drives `promoteBuild` itself so it can observe the exact
+    /// `CommitOutcome` the promote primitive derives.
+    Staged stageSimplePart(const Cas::PartRefKey & key, int blobs) const
+    {
+        std::vector<Cas::ManifestEntry> entries;
+        for (int i = 0; i < blobs; ++i)
+            entries.push_back(inlineEntry(fmt::format("f{}", i), fmt::format("payload-{}-{}", key.ref, i)));
+        auto build = store->beginPartWrite(Cas::PartWriteInfo{
+            .intended_ref = key.ns.string() + "/" + key.ref, .intended_namespace = key.ns, .op = Cas::ProvenanceOp::Insert});
+        const Cas::ManifestId id = build->stageManifest(entries);
+        build->precommitAdd(key.ns, key.ref, id);
+        return {std::move(build), id};
+    }
+
+    /// Stages and promotes one simple part end-to-end, returning the exact `CommitOutcome`.
+    Cas::CommitOutcome commitSimplePart(const Cas::PartRefKey & key, int blobs)
+    {
+        auto staged = stageSimplePart(key, blobs);
+        return access.promoteBuild(*staged.build, key, staged.build->buildId(), staged.id);
+    }
+
+    /// Repoints an already-committed `key` onto a fresh manifest (different content), through the
+    /// public `repointRef` primitive -- models "another writer" rebinding the ref after this
+    /// fixture's own `commitSimplePart`.
+    Cas::CommitOutcome repointToFreshManifest(const Cas::PartRefKey & key)
+    {
+        return access.repointRef(key, {inlineEntry("f0", fmt::format("repoint-{}", ++content_counter))},
+            Cas::ProvenanceOp::Other);
+    }
+};
+
+}
+
+TEST(CASCommitOutcome, PromoteReportsCreatedAndManifest)
+{
+    CaWiringFixture fx;
+    const Cas::PartRefKey key{fx.ns(), "20260101_1_1_0"};
+    auto staged = fx.stageSimplePart(key, /*blobs=*/1);
+
+    const Cas::CommitOutcome oc = fx.partAccess().promoteBuild(*staged.build, key, staged.build->buildId(), staged.id);
+
+    EXPECT_TRUE(oc.created);
+    EXPECT_EQ(oc.ns.string(), key.ns.string());
+    EXPECT_EQ(oc.ref, key.ref);
+    EXPECT_EQ(oc.manifest_ref, staged.id.ref);
+}
+
+TEST(CASCommitOutcome, DropRefIfMatchesRemovesOnlyExact)
+{
+    CaWiringFixture fx;
+    const Cas::PartRefKey key{fx.ns(), "20260101_2_2_0"};
+    const Cas::CommitOutcome oc1 = fx.commitSimplePart(key, /*blobs=*/1);
+    EXPECT_TRUE(oc1.created);
+
+    /// Rebind key -> M2 (a legitimate repoint by "another writer").
+    const Cas::CommitOutcome oc2 = fx.repointToFreshManifest(key);
+    EXPECT_FALSE(oc2.created);
+    ASSERT_NE(oc1.manifest_ref, oc2.manifest_ref);
+
+    /// Conditional drop keyed on the STALE M1 must NOT remove the current M2 binding.
+    EXPECT_FALSE(fx.partAccess().dropRefIfMatches(key, oc1.manifest_ref));
+    EXPECT_TRUE(fx.partAccess().existsRef(key, Cas::Freshness::ForceFresh));
+
+    /// Conditional drop keyed on the CURRENT M2 removes it.
+    EXPECT_TRUE(fx.partAccess().dropRefIfMatches(key, oc2.manifest_ref));
+    EXPECT_FALSE(fx.partAccess().existsRef(key, Cas::Freshness::ForceFresh));
+}
+
+TEST(CASCommitOutcome, DropRefIfMatchesOnAbsentRefIsANoOp)
+{
+    CaWiringFixture fx;
+    const Cas::PartRefKey key{fx.ns(), "20260101_3_3_0"};
+    Cas::ManifestRef bogus;
+    EXPECT_FALSE(fx.partAccess().dropRefIfMatches(key, bogus)) << "no committed ref at all: nothing to match";
+    EXPECT_FALSE(fx.partAccess().existsRef(key, Cas::Freshness::ForceFresh));
+}
+
+/// `repointRef`'s byte-equal candidate is a documented ZERO-pool-mutation no-op (it must not mint a
+/// fresh manifest just to compare it). The returned `CommitOutcome` must still describe reality: the
+/// CURRENTLY committed manifest, unchanged, `created=false`.
+TEST(CASCommitOutcome, RepointRefByteEqualNoOpReportsCurrentManifestNotCreated)
+{
+    CaWiringFixture fx;
+    const Cas::PartRefKey key{fx.ns(), "20260101_4_4_0"};
+    const Cas::CommitOutcome oc1 = fx.commitSimplePart(key, /*blobs=*/1);
+
+    const Cas::CommitOutcome oc_noop = fx.partAccess().repointRef(
+        key, {CaWiringFixture::inlineEntry("f0", fmt::format("payload-{}-0", key.ref))}, Cas::ProvenanceOp::Other);
+    EXPECT_FALSE(oc_noop.created);
+    EXPECT_EQ(oc_noop.manifest_ref, oc1.manifest_ref);
+}
+
+namespace
+{
+
+/// Fixture for the `CasCommitRollback` suite: wraps a real `ContentAddressedMetadataStorage` and
+/// drives ordinary `ContentAddressedTransaction`s through disk paths, so the fault seams under test
+/// (`ContentAddressedMetadataStorage::armPromoteFailureForTest`/`setAfterPromoteHookForTest`, the
+/// minimal test-only hooks this task adds) fire from the SAME `publishStaging` call path production
+/// `commit()` uses -- unlike `CaWiringFixture` above, which pokes the bare pool primitives directly.
+/// Every part in one fixture instance shares ONE fixed table uuid (and therefore one `RootNamespace`),
+/// matching every test's single `fx.ns()`.
+struct CaTxnRollbackFixture
+{
+    static constexpr const char * kTableUuid = "c3c3c3c3-0000-4000-8000-c3c3c3c3c3c3";
+
+    std::shared_ptr<DB::ContentAddressedMetadataStorage> storage;
+    Cas::RootNamespace namespace_;
+    Cas::ManifestRef last_repoint_manifest;
+    int content_counter = 0;
+
+    static std::string tablePrefix()
+    {
+        return std::string(kTableUuid).substr(0, 3) + "/" + kTableUuid;
+    }
+
+    const Cas::RootNamespace & ns() const { return namespace_; }
+    Cas::CachedPartFolderAccess & partAccess() const { return *storage->partAccess(); }
+
+    DB::MetadataTransactionPtr beginTxn() const { return storage->createTransaction(); }
+
+    /// Stages `blobs` small distinct files for `key` under a tmp build dir and re-keys them to the
+    /// final ref name -- the standard MergeTree-insert shape (`gtest_ca_transaction.cpp`'s
+    /// `writeFileTx` + `moveDirectory` idiom) this storage's routing expects; `key.ns` must be `ns()`.
+    void stageInto(const DB::MetadataTransactionPtr & txn, const Cas::PartRefKey & key, int blobs)
+    {
+        auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*txn);
+        const std::string tmp_dir = tablePrefix() + "/tmp_insert_" + key.ref;
+        for (int i = 0; i < blobs; ++i)
+        {
+            auto buf = ca_tx.writeFile(fmt::format("{}/f{}.bin", tmp_dir, i), 65536, DB::WriteMode::Rewrite, {});
+            const std::string bytes = fmt::format("payload-{}-{}", key.ref, i);
+            buf->write(bytes.data(), bytes.size());
+            buf->finalize();
+        }
+        txn->moveDirectory(tmp_dir, tablePrefix() + "/" + key.ref);
+    }
+
+    /// Stages and commits one part end-to-end in its own transaction -- sets up a pre-existing
+    /// committed ref before the transaction under test begins.
+    void commitSimplePart(const Cas::PartRefKey & key, int blobs)
+    {
+        auto txn = beginTxn();
+        stageInto(txn, key, blobs);
+        txn->commit(DB::NoCommitOptions{});
+    }
+
+    /// Repoints an already-committed `key` onto a fresh manifest through the public `repointRef`
+    /// primitive directly -- models "another writer" rebinding the ref concurrently with the
+    /// transaction under test. Records the manifest for `lastRepointManifest()`.
+    void repointToFreshManifest(const Cas::PartRefKey & key)
+    {
+        const std::string bytes = fmt::format("repoint-{}", ++content_counter);
+        Cas::ManifestEntry e;
+        e.path = "f0.bin";
+        e.placement = Cas::EntryPlacement::Inline;
+        e.ref = Cas::BlobRef{Cas::BlobHashAlgo::CityHash128, Cas::BlobDigest::fromU128(u128Of(bytes))};
+        e.blob_size = bytes.size();
+        e.inline_bytes = bytes;
+        const auto oc = partAccess().repointRef(key, {e}, Cas::ProvenanceOp::Other);
+        last_repoint_manifest = oc.manifest_ref;
+    }
+
+    /// The manifest CURRENTLY bound to `key`, or a default-constructed (zero) `ManifestRef` when `key`
+    /// has no committed ref at all.
+    Cas::ManifestRef currentManifest(const Cas::PartRefKey & key) const
+    {
+        auto view = partAccess().getView(key, Cas::Freshness::ForceFresh);
+        return view ? view->manifestId().ref : Cas::ManifestRef{};
+    }
+
+    Cas::ManifestRef lastRepointManifest() const { return last_repoint_manifest; }
+
+    /// Test-only fault seam (see `ContentAddressedMetadataStorage::armPromoteFailureForTest`): the
+    /// NEXT `publishStaging` promote/repoint for `key` (the full `(ns, ref)` routed identity) throws
+    /// instead of committing.
+    void armPromoteFailure(const Cas::PartRefKey & key) const { storage->armPromoteFailureForTest(key); }
+    /// Test-only hook (see `ContentAddressedMetadataStorage::setAfterPromoteHookForTest`): runs once,
+    /// synchronously, immediately after `key`'s promote/repoint confirms.
+    void armAfterPromoteHook(const Cas::PartRefKey & key, std::function<void()> hook) const
+    {
+        storage->setAfterPromoteHookForTest(key, std::move(hook));
+    }
+};
+
+CaTxnRollbackFixture makeCaWiringFixture()
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto scratch = std::filesystem::temp_directory_path()
+        / fmt::format("ca_commit_rollback_scratch_{}_{}", ::getpid(), counter.fetch_add(1));
+    auto settings = DB::Cas::tests::makeSettingsForTest("test", scratch);
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+
+    CaTxnRollbackFixture fx;
+    fx.storage = storage;
+    fx.namespace_ = storage->liveNamespace(CaTxnRollbackFixture::kTableUuid);
+    return fx;
+}
+
+}
+
+/// [TXN-ONE-PIPELINE] Task 3: `commit()` publishes `new_a` (created=true) then fails on `new_b`'s
+/// promote. The rollback must drop the just-created `new_a` (absent afterward) but never touch the
+/// unrelated `pre_existing` ref committed by an EARLIER, already-finished transaction.
+TEST(CASCommitRollback, AbsentBeforeDroppedPreExistingUntouched)
+{
+    auto fx = makeCaWiringFixture();
+    const Cas::PartRefKey pre{fx.ns(), "pre_existing_1_1_0"};
+    fx.commitSimplePart(pre, 1);                                  // a pre-existing ref, must survive
+    // A transaction that commits one NEW part then fails on a second part's promote.
+    auto txn = fx.beginTxn();
+    fx.stageInto(txn, {fx.ns(), "new_a_1_1_0"}, 1);
+    fx.stageInto(txn, {fx.ns(), "new_b_1_1_0"}, 1);
+    fx.armPromoteFailure({fx.ns(), "new_b_1_1_0"});               // fault injection in publishStaging's promote
+    EXPECT_ANY_THROW(txn->commit({}));
+    EXPECT_FALSE(fx.partAccess().existsRef({fx.ns(), "new_a_1_1_0"}, Cas::Freshness::ForceFresh)); // rolled back
+    EXPECT_TRUE (fx.partAccess().existsRef(pre, Cas::Freshness::ForceFresh));                       // untouched
+}
+
+/// [TXN-ONE-PIPELINE] Task 3: T1 (this transaction) promotes `shared` (M1), then a concurrent writer
+/// (modeled by the after-promote hook) repoints it to M2 BEFORE T1's own commit later fails on
+/// `poison`'s promote. Rollback must use `dropRefIfMatches(M1)`: M1 != the now-current M2, so the
+/// conditional drop must leave `shared` bound to M2 untouched.
+///
+/// `commit()` publishes `parts` in the map's own (ns, ref) sort order -- so the "shared" part is named
+/// `a_shared_...` and the "poison" part `z_poison_...` here purely so `'a' < 'z'` makes "shared"
+/// publish (and get repointed by the hook) deterministically BEFORE "poison" fails; this is a test
+/// naming choice, not a production ordering guarantee.
+TEST(CASCommitRollback, RepointByOtherWriterSurvivesRollback)
+{
+    auto fx = makeCaWiringFixture();
+    const Cas::PartRefKey key{fx.ns(), "a_shared_1_1_0"};
+    auto txn = fx.beginTxn();
+    fx.stageInto(txn, key, 1);                                   // T1 will create R -> M1
+    fx.armAfterPromoteHook(key, [&]{ fx.repointToFreshManifest(key); }); // T2 repoints R -> M2 right after T1's promote
+    fx.stageInto(txn, {fx.ns(), "z_poison_1_1_0"}, 1);
+    fx.armPromoteFailure({fx.ns(), "z_poison_1_1_0"});
+    EXPECT_ANY_THROW(txn->commit({}));
+    // T1's rollback used dropRefIfMatches(M1); M2 != M1 so it must survive.
+    EXPECT_TRUE(fx.partAccess().existsRef(key, Cas::Freshness::ForceFresh));
+    EXPECT_EQ(fx.currentManifest(key), fx.lastRepointManifest());
+}
diff --git a/src/Disks/tests/gtest_cas_part_folder_access.cpp b/src/Disks/tests/gtest_cas_part_folder_access.cpp
new file mode 100644
index 000000000000..e3948b39cdd3
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_part_folder_access.cpp
@@ -0,0 +1,1291 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/ProfileEvents.h>
+#include <Poco/Exception.h>
+#include <Poco/Util/XMLConfiguration.h>
+#include <gtest/gtest.h>
+#include <algorithm>
+#include <latch>
+#include <sstream>
+#include <thread>
+#include <type_traits>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+    extern const int FILE_DOESNT_EXIST;
+    extern const int ABORTED;
+    extern const int BAD_ARGUMENTS;
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+    extern const int MEMORY_LIMIT_EXCEEDED;
+    extern const int NETWORK_ERROR;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASRefRollbackBestEffortDropFailed;
+extern const Event CASPartFolderValidateSkipped;
+}
+
+using namespace DB;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+Cas::ManifestEntry inlineEntry(const String & path, const String & bytes)
+{
+    Cas::ManifestEntry e;
+    e.path = path;
+    e.placement = Cas::EntryPlacement::Inline;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(bytes))};
+
+    e.blob_size = bytes.size();
+    e.inline_bytes = bytes;
+    return e;
+}
+
+/// Publish `entries` as committed ref `ns/ref` through the real writer protocol.
+Cas::ManifestId publishPart(const Cas::PoolPtr & store, const Cas::RootNamespace & ns,
+                            const String & ref, std::vector<Cas::ManifestEntry> entries)
+{
+    auto build = store->beginPartWrite(Cas::PartWriteInfo{.intended_ref = ns.string() + "/" + ref,
+                                                  .intended_namespace = ns, .op = Cas::ProvenanceOp::Insert});
+    const Cas::ManifestId id = build->stageManifest(entries);
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+Cas::CachedPartFolderAccess::CacheParams cacheOn()
+{
+    return {.cache_bytes = 64ULL << 20, .max_entries = 10000, .max_entry_bytes = 16ULL << 20,
+            .explain_enabled = true, .validate = {}};
+}
+
+/// Mirrors gtest_cas_s3_staging.cpp's helper of the same shape: the shape a real CAS disk config
+/// has under `storage_configuration.disks.<name>`, so `config_prefix = "disk"` reads exactly like
+/// the disk factory's `config_prefix`. Used to unit-test `parsePartFolderValidate` standalone.
+Poco::AutoPtr<Poco::Util::XMLConfiguration> configWithDiskSection(const std::string & inner_xml)
+{
+    std::istringstream xml_stream( // STYLE_CHECK_ALLOW_STD_STRING_STREAM
+        "<clickhouse><disk>" + inner_xml + "</disk></clickhouse>");
+    return new Poco::Util::XMLConfiguration(xml_stream);
+}
+
+/// Every mutating backend op throws once armed — models a correlated backend outage during the
+/// transaction's compensating rollback (dropRef must append a removal, which mutates the backend).
+class RollbackFaultBackend final : public Cas::InMemoryBackend
+{
+public:
+    std::atomic<bool> armed{false};
+
+    Cas::PutResult putIfAbsent(const String & k, const String & b, const Cas::ObjectMeta & m) override
+    {
+        failIfArmed();
+        return InMemoryBackend::putIfAbsent(k, b, m);
+    }
+
+    Cas::WriteSinkPtr putIfAbsentStream(const String & k, const Cas::ObjectMeta & m) override
+    {
+        failIfArmed();
+        return InMemoryBackend::putIfAbsentStream(k, m);
+    }
+
+    Cas::PutResult putOverwrite(const String & k, const String & b, const Cas::Token & e, const Cas::ObjectMeta & m) override
+    {
+        failIfArmed();
+        return InMemoryBackend::putOverwrite(k, b, e, m);
+    }
+
+    Cas::CasResult casPut(const String & k, const String & b, const std::optional<Cas::Token> & e, const Cas::ObjectMeta & m) override
+    {
+        failIfArmed();
+        return InMemoryBackend::casPut(k, b, e, m);
+    }
+
+    Cas::DeleteOutcome deleteExact(const String & k, const Cas::Token & t) override
+    {
+        failIfArmed();
+        return InMemoryBackend::deleteExact(k, t);
+    }
+
+private:
+    void failIfArmed()
+    {
+        if (armed.load())
+            throw Exception(ErrorCodes::ABORTED, "injected backend outage");
+    }
+};
+
+/// Task 7 (`publishEntries` abandons its build on exception): forces publishEntries's PROMOTE step
+/// specifically -- not the earlier stageManifest/precommitAdd writes -- to observe a proven ref-log
+/// conflict. `skip` lets the FIRST matching '_log/' PUT (precommitAdd's OwnerTransition-to-Precommit)
+/// land normally; the fault then fires on the SECOND (promote's atomic precommit->committed move).
+/// Mirrors `RefWriterTestBackend::corrupt_key_substr` (gtest_cas_ref_writer.cpp, reproduced locally
+/// because that class lives in a different translation unit): landing a DIFFERENT object at the
+/// intended key makes `putIfAbsentControlled`'s resolve-before-reissue observe a proven conflict
+/// (CORRUPTED_DATA) rather than the ambiguous-timeout shape, which would instead wedge the whole
+/// table's append lane.
+class PromoteConflictOnceBackend final : public Cas::InMemoryBackend
+{
+public:
+    String fault_key_substr;
+    int skip = 0;
+    int fault_count = 0;
+    /// Every create ATTEMPTED at a matching key, faulted or not. It is how a test observes that a
+    /// cleanup path ran its ref-log append at all, on a table where that append can no longer succeed.
+    int matching_put_attempts = 0;
+
+    Cas::PutResult putIfAbsent(const String & key, const String & bytes, const Cas::ObjectMeta & meta) override
+    {
+        if (!fault_key_substr.empty() && key.find(fault_key_substr) != String::npos)
+        {
+            ++matching_put_attempts;
+            if (skip > 0)
+                --skip;
+            else if (fault_count > 0)
+            {
+                --fault_count;
+                /// The 3-arg qualified call bypasses virtual dispatch entirely (unlike a 2-arg
+                /// convenience overload, which would re-enter this very override through the vtable).
+                InMemoryBackend::putIfAbsent(key, bytes + String("\x01_FOREIGN_DIFFERENT"), meta);
+                throw Poco::TimeoutException("PromoteConflictOnceBackend: a foreign different object landed; response lost");
+            }
+        }
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+};
+
+/// The same shape as `PromoteConflictOnceBackend`, except its fault is a whitelisted SYNCHRONOUS
+/// REJECTION -- an S3-classified malformed request, which `classifyConditionalWriteResult` proves was
+/// never applied. That distinction is the whole reason this second backend exists: a proven DIFFERENT
+/// OBJECT is a breach of mount write-exclusivity and fences the whole mount closed, so every cleanup
+/// append after it is refused at the gate and becomes unobservable. A definite rejection is an ordinary
+/// failed write -- nothing is fenced, nothing is wedged, the table stays usable -- so the cleanup
+/// appends that follow DO reach the store and can be counted.
+class PromoteDefiniteFailureBackend final : public Cas::InMemoryBackend
+{
+public:
+    String fault_key_substr;
+    int skip = 0;
+    int fault_count = 0;
+    int matching_put_attempts = 0;
+
+    Cas::PutResult putIfAbsent(const String & key, const String & bytes, const Cas::ObjectMeta & meta) override
+    {
+        if (!fault_key_substr.empty() && key.find(fault_key_substr) != String::npos)
+        {
+            ++matching_put_attempts;
+            if (skip > 0)
+                --skip;
+            else if (fault_count > 0)
+            {
+                --fault_count;
+                throw DB::S3Exception("PromoteDefiniteFailureBackend: simulated malformed request",
+                                      Aws::S3::S3Errors::UNKNOWN, "MalformedXML");
+            }
+        }
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+};
+
+}
+
+TEST(CASPartFolderAccess, RetainedHitSkipsManifestHead)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+    const Cas::PartRefKey key{ns, "part_1"};
+    const String manifest_key = layout.manifestKey(id);
+
+    backend->resetCounts();
+    for (int i = 0; i < 5; ++i)
+        ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);
+
+    /// The one-GET goal (spec acceptance 4): ONE body GET, ONE mandatory HEAD (the cold build);
+    /// every subsequent CachedForLoad call is a validated hit — zero manifest ops.
+    EXPECT_EQ(backend->getCount(manifest_key), 1u);
+    EXPECT_EQ(backend->headCount(manifest_key), 1u);
+    EXPECT_TRUE(access.explain(key).retained);
+    EXPECT_EQ(access.explain(key).last_decision,
+              Cas::CachedPartFolderAccess::LastDecision::Hit);
+}
+
+TEST(CASPartFolderAccess, HitPathJournalEmptyAndCheapWhenExplainDisabled)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    /// Retention ON, explain journal OFF (the production default): the hit path must take neither the
+    /// per-disk explain mutex nor write a journal entry (B2).
+    Cas::CachedPartFolderAccess access(store,
+        {.cache_bytes = 64ULL << 20, .max_entries = 10000, .max_entry_bytes = 16ULL << 20,
+         .explain_enabled = false, .validate = {}});
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+    const Cas::PartRefKey key{ns, "part_1"};
+    const String manifest_key = layout.manifestKey(id);
+
+    backend->resetCounts();
+    for (int i = 0; i < 5; ++i)
+        ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);
+
+    /// Same request oracle as RetainedHitSkipsManifestHead — one cold build, then validated hits.
+    EXPECT_EQ(backend->getCount(manifest_key), 1u);
+    EXPECT_EQ(backend->headCount(manifest_key), 1u);
+    /// The journal is never written when disabled.
+    EXPECT_EQ(access.explainJournalSizeForTest(), 0u);
+    /// explain() still reports live retention truthfully, but the decision defaults to Miss (unwritten).
+    EXPECT_TRUE(access.explain(key).retained);
+    EXPECT_EQ(access.explain(key).last_decision,
+              Cas::CachedPartFolderAccess::LastDecision::Miss);
+}
+
+TEST(CASPartFolderAccess, GetViewServesCommittedFolder)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    publishPart(store, ns, "part_1",
+                {inlineEntry("checksums.txt", "cs"), inlineEntry("count.txt", "1"), inlineEntry("txn_version.txt", "v1")});
+
+    Cas::CachedPartFolderAccess access(store);
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    auto view = access.getView(key, Cas::Freshness::CachedForLoad);
+    ASSERT_NE(view, nullptr);
+    EXPECT_NE(view->findFile("checksums.txt"), nullptr);
+    EXPECT_EQ(view->inlineBytes("txn_version.txt"), std::optional<String>("v1"));
+
+    /// Absent ref => nullptr, never an exception, never retained (nothing to retain in Phase 2).
+    EXPECT_EQ(access.getView({ns, "absent"}, Cas::Freshness::CachedForLoad), nullptr);
+    EXPECT_TRUE(access.existsRef(key, Cas::Freshness::CachedForLoad));
+    EXPECT_FALSE(access.existsRef({ns, "absent"}, Cas::Freshness::ForceFresh));
+    ASSERT_TRUE(access.resolve(key, Cas::Freshness::ForceFresh).has_value());
+}
+
+TEST(CASPartFolderAccess, GetViewFailsClosedOnMissingBody)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+
+    /// Physically delete the live manifest body (a protocol violation) — every getView mode must
+    /// surface INV-NO-DANGLE as FILE_DOESNT_EXIST in Phase 2 (there is no retained view to hit).
+    /// Retention is off (the single-arg ctor below), so this is the `always` (default) part_folder_validate
+    /// mode under test regardless — the `never`/`age` skip is proven by the ValidateNever/ValidateAge
+    /// tests further down, which turn retention ON.
+    deleteManifestBody(*backend, layout, id);
+
+    Cas::CachedPartFolderAccess access(store);
+    const Cas::PartRefKey key{ns, "part_1"};
+    for (auto freshness : {Cas::Freshness::CachedForLoad,
+                           Cas::Freshness::ForceFresh,
+                           Cas::Freshness::StrictValidate})
+        expectThrowsCode(ErrorCodes::FILE_DOESNT_EXIST, [&] { access.getView(key, freshness); });
+}
+
+TEST(CASPartFolderAccess, WritePrimitivesRoundTrip)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store);
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    /// promoteBuild: the transaction's terminal publish step, through the facade.
+    auto build = store->beginPartWrite(Cas::PartWriteInfo{.intended_ref = ns.string() + "/part_1",
+                                                  .intended_namespace = ns, .op = Cas::ProvenanceOp::Insert});
+    const Cas::ManifestId id = build->stageManifest({inlineEntry("checksums.txt", "cs")});
+    build->precommitAdd(ns, "part_1", id);
+    access.promoteBuild(*build, key, build->buildId(), id);
+    ASSERT_TRUE(access.existsRef(key, Cas::Freshness::ForceFresh));
+
+    /// dropRefIfPresent: replay-safe (absent ref is success, not failure).
+    access.dropRefIfPresent(key);
+    EXPECT_FALSE(access.existsRef(key, Cas::Freshness::ForceFresh));
+    access.dropRefIfPresent(key);                              /// second drop: no-op, no throw
+    access.dropRefBestEffort(key);                             /// noexcept even when absent
+
+    /// dropNamespace clears the whole namespace.
+    publishPart(store, ns, "part_2", {inlineEntry("checksums.txt", "cs")});
+    access.dropNamespace(ns);
+    EXPECT_FALSE(access.existsRef({ns, "part_2"}, Cas::Freshness::ForceFresh));
+}
+
+TEST(CASPartFolderAccess, RepublishRefMovesCommittedRef)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store);
+    publishPart(store, ns, "src_part", {inlineEntry("checksums.txt", "cs"), inlineEntry("txn_version.txt", "v1")});
+
+    EXPECT_FALSE(access.republishRef({ns, "absent"}, {ns, "dst"}));   /// absent source: nothing written
+
+    ASSERT_TRUE(access.republishRef({ns, "src_part"}, {ns, "dst_part"}));
+    EXPECT_FALSE(access.existsRef({ns, "src_part"}, Cas::Freshness::ForceFresh));
+    auto view = access.getView({ns, "dst_part"}, Cas::Freshness::ForceFresh);
+    ASSERT_NE(view, nullptr);
+    EXPECT_NE(view->findFile("checksums.txt"), nullptr);
+    EXPECT_EQ(view->inlineBytes("txn_version.txt"), std::optional<String>("v1"));   /// carried over
+}
+
+TEST(CASPartFolderAccess, RepublishRefIdempotentRedriveAndConflict)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store);
+
+    /// Re-drive: dst already committed with the SAME content (a prior attempt's promote landed,
+    /// only dropRef(src) was interrupted) -- idempotent-skip: drop src, dst's manifest is untouched
+    /// (all-tree-part-files Task 9: there is no separate mutable payload left to drift/re-sync --
+    /// identical `entries` is the whole idempotency contract now).
+    publishPart(store, ns, "src", {inlineEntry("f", "same")});
+    publishPart(store, ns, "dst", {inlineEntry("f", "same")});
+    const auto dst_id_before = access.resolve({ns, "dst"}, Cas::Freshness::ForceFresh)->manifest_id;
+    ASSERT_TRUE(access.republishRef({ns, "src"}, {ns, "dst"}));
+    EXPECT_FALSE(access.existsRef({ns, "src"}, Cas::Freshness::ForceFresh));
+    auto resolved = access.resolve({ns, "dst"}, Cas::Freshness::ForceFresh);
+    EXPECT_EQ(resolved->manifest_id, dst_id_before) << "idempotent re-drive must not mint a fresh manifest";
+
+    /// Conflict: dst committed with DIFFERENT content — fail closed, src untouched.
+    publishPart(store, ns, "src2", {inlineEntry("f", "one")});
+    publishPart(store, ns, "dst2", {inlineEntry("f", "two")});
+    expectThrowsCode(ErrorCodes::ABORTED, [&] { access.republishRef({ns, "src2"}, {ns, "dst2"}); });
+    EXPECT_TRUE(access.existsRef({ns, "src2"}, Cas::Freshness::ForceFresh));
+}
+
+/// Task 7: `publishEntries`'s `catch (...) { build->abandon(); throw; }` must leave no live-epoch
+/// precommit binding behind when its promote fails -- only `abandon()` removes it (the build
+/// destructor merely retires the build seq; GC never touches a live precommit). Drives the failure
+/// through `republishRef` -> `publishEntries`, with the fault isolated to promote's own ref-log
+/// append (precommitAdd's own append is let through first via `skip`).
+TEST(CASPartFolderAccess, PublishEntriesAbandonsBuildOnPromoteFailure)
+{
+    auto backend = std::make_shared<PromoteConflictOnceBackend>();
+    auto store = Cas::Pool::open(backend, Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const Cas::RootNamespace ns{"srv/t1"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+    Cas::CachedPartFolderAccess access(store);
+
+    publishPart(store, ns, "src", {inlineEntry("f", "same")});
+
+    backend->fault_key_substr = store->layout().namespaceStreamPrefix(fixture::fixtureLife(ns)) + "_log/";
+    backend->skip = 1;         /// let precommitAdd's own ref-log append land normally
+    backend->fault_count = 1;  /// fault exactly promote's ref-log append
+    const int attempts_before = backend->matching_put_attempts;
+
+    /// republishRef(src, dst) drives publishEntries(dst, ...): precommitAdd succeeds, promote's
+    /// appendRefOps observes a proven conflict and throws CORRUPTED_DATA -- publishEntries's catch must
+    /// abandon() the build before rethrowing.
+    expectThrowsCode(ErrorCodes::CORRUPTED_DATA, [&] { access.republishRef({ns, "src"}, {ns, "dst"}); });
+    /// The anomaly fenced this runtime, so a post-fence `ForceFresh` read must refuse rather than
+    /// authorizing its stale generation. The backend assertions below prove directly that `dst` never
+    /// committed and that no append skipped around the damaged slot.
+    expectThrowsCode(ErrorCodes::NETWORK_ERROR,
+        [&] { (void)access.existsRef({ns, "dst"}, Cas::Freshness::ForceFresh); });
+
+    /// EXACTLY two ref-log create attempts reach the store: precommitAdd's own append and the promote's
+    /// faulted one. Both of the cleanup appends that follow -- `promote`'s catch-abandon and the handle
+    /// destructor's backstop -- are refused at the mount-fence gate before they reach the store, because
+    /// proving a different object at our own key now fences the mount closed and schedules a remount
+    /// (review I5: the append site self-heals like the wedge-resolve site instead of leaving this table
+    /// blocked until a manual remount).
+    ///
+    /// COVERAGE NOTE, deliberately explicit: this count no longer DISCRIMINATES whether the catch-abandon
+    /// ran. It used to (four attempts with it, three without), and that only worked because the
+    /// catch-abandon could still reach the store and fail there, making the destructor retry. With the
+    /// fence closed both cleanups are refused identically and unobservably, so the assertion below is a
+    /// shape check, not the regression guard it was. The guard cannot be restored in THIS scenario --
+    /// nothing the cleanup does is observable once the mount is fenced -- and it is not silently
+    /// dropped: the property it protected is stated here, and reclaiming the binding is now the
+    /// scheduled remount's job (a fresh incarnation re-derives the table and the stale-precommit sweep
+    /// reclaims), not this best-effort abandon's.
+    EXPECT_EQ(backend->matching_put_attempts, attempts_before + 2)
+        << "only precommitAdd's append and the promote's faulted one may reach the store; every cleanup "
+           "append after the anomaly is refused at the fence";
+    EXPECT_FALSE(store->mayMutate()) << "the proven conflict must fence this mount closed";
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), 1u)
+        << "and must schedule exactly one remount -- the self-heal that replaces the manual one";
+
+    /// And nothing was written ABOVE the damage: the occupant is the GREATEST log id in the namespace
+    /// (keys render the id in fixed-width hex, so lexical order is id order). An append that carved a
+    /// fresh id to get past the foreign object would sort above it.
+    String greatest_key;
+    size_t foreign_objects = 0;
+    for (String cursor;;)
+    {
+        const Cas::ListPage page = backend->list(backend->fault_key_substr, cursor, 1000);
+        for (const auto & listed : page.keys)
+        {
+            if (listed.key > greatest_key)
+                greatest_key = listed.key;
+            const auto body = backend->get(listed.key);
+            if (body && body->bytes.find("_FOREIGN_DIFFERENT") != String::npos)
+                ++foreign_objects;
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    EXPECT_EQ(foreign_objects, 1u) << "the foreign object must still own the key it took";
+    ASSERT_FALSE(greatest_key.empty());
+    const auto greatest_body = backend->get(greatest_key);
+    ASSERT_TRUE(greatest_body.has_value());
+    EXPECT_NE(greatest_body->bytes.find("_FOREIGN_DIFFERENT"), String::npos)
+        << "the foreign occupant must still be the highest id in this table's stream: a log object above "
+           "it would mean an append carved a fresh id around the damage instead of failing closed";
+}
+
+/// The DISCRIMINATING guard for the same duty, on the path where it can still be observed: a promote
+/// failure that is an ordinary failed write rather than a breach of mount write-exclusivity. Nothing is
+/// fenced and nothing is wedged, so both cleanup appends reach the store and the two worlds separate.
+///
+/// The fault covers TWO appends, and that is the whole construction:
+///   with `promote`'s catch-abandon -- precommitAdd lands (skipped), promote's append is refused,
+///   the catch-abandon's append is refused too, and the handle DESTRUCTOR's backstop retries and lands:
+///   FOUR attempts, and no binding is left behind;
+///   without it -- precommitAdd lands, promote's append is refused, and the destructor's backstop takes
+///   the second fault and is refused: THREE attempts, and the precommit binding LEAKS.
+/// So the count and the end state disagree between the two worlds, which is what makes this a guard
+/// rather than a shape check. `livePrecommitsForTest` is the direct statement of the property --
+/// `publishEntries` must not walk away from a live precommit binding -- and the count is what pins
+/// WHERE the cleanup came from.
+TEST(CASPartFolderAccess, PublishEntriesAbandonsBuildOnARetryablePromoteFailure)
+{
+    auto backend = std::make_shared<PromoteDefiniteFailureBackend>();
+    auto store = Cas::Pool::open(backend, Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const Cas::RootNamespace ns{"srv/t1"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+    Cas::CachedPartFolderAccess access(store);
+
+    publishPart(store, ns, "src", {inlineEntry("f", "same")});
+
+    backend->fault_key_substr = store->layout().namespaceStreamPrefix(fixture::fixtureLife(ns)) + "_log/";
+    backend->skip = 1;         /// let precommitAdd's own ref-log append land normally
+    backend->fault_count = 2;  /// fault promote's append AND the cleanup append that follows it
+    const int attempts_before = backend->matching_put_attempts;
+
+    /// A definite rejection is reported to the caller as a retry-later failure, not as corruption.
+    expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { access.republishRef({ns, "src"}, {ns, "dst"}); });
+    EXPECT_FALSE(access.existsRef({ns, "dst"}, Cas::Freshness::ForceFresh)) << "the failed promote never committed dst";
+
+    EXPECT_TRUE(store->mayMutate()) << "an ordinary failed write must not fence the mount";
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), 0u) << "and must not schedule a remount";
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "a definite rejection is proven non-durable: no wedge";
+
+    EXPECT_EQ(backend->matching_put_attempts, attempts_before + 4)
+        << "three attempts means only the destructor backstop ran -- publishEntries stopped abandoning "
+           "the build at the promote site";
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty())
+        << "publishEntries must not walk away from a live precommit binding";
+}
+
+
+/// ==== Task 12: the prepared-part-write handle (spec §relink-handle) ====
+/// `prepareEntries` stops after `precommitAdd`, so the durable-but-unpromoted state -- the window the
+/// relink confirm round-trip has to sit inside -- becomes an OWNED object instead of an interval inside
+/// one call. Every test below pins one half of that ownership contract.
+
+/// Prepare-then-promote must be indistinguishable from today's atomic `publishEntries`, and the state
+/// BETWEEN the two halves must be exactly one live precommit and no committed ref.
+TEST(CASPartFolderAccess, PrepareThenPromoteMatchesPublishEntries)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+
+    const std::vector<Cas::ManifestEntry> entries{inlineEntry("f", "one"), inlineEntry("g", "two")};
+    const Cas::CommitOutcome published = access.publishEntries({ns, "via_publish"}, entries, Cas::ProvenanceOp::Insert);
+
+    auto prepared = access.prepareEntries({ns, "via_prepare"}, entries, Cas::ProvenanceOp::Insert);
+
+    /// The interposition point: the manifest is durable and owned by a LIVE precommit, but nothing is
+    /// committed yet. This is precisely the state the confirm round-trip runs in.
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).contains({"via_prepare", prepared.manifestId().ref}))
+        << "prepareEntries must leave the precommit binding live -- it is the durable `+1`";
+    EXPECT_FALSE(access.existsRef({ns, "via_prepare"}, Cas::Freshness::ForceFresh))
+        << "prepareEntries must not commit the ref";
+
+    const Cas::CommitOutcome promoted = prepared.promote();
+    EXPECT_EQ(promoted.ns.string(), ns.string());
+    EXPECT_EQ(promoted.ref, "via_prepare");
+    EXPECT_EQ(promoted.manifest_ref, prepared.manifestId().ref);
+    EXPECT_TRUE(promoted.created);
+    EXPECT_EQ(promoted.created, published.created) << "the split must reproduce publishEntries's outcome shape";
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty()) << "promote moves the binding out of the precommit view";
+
+    auto view = access.getView({ns, "via_prepare"}, Cas::Freshness::ForceFresh);
+    ASSERT_NE(view, nullptr);
+    EXPECT_EQ(view->inlineBytes("f"), std::optional<String>("one"));
+    EXPECT_EQ(view->inlineBytes("g"), std::optional<String>("two"));
+}
+
+/// Abort is not "drop the handle": it must APPEND the exact precommit removal. An abandoned precommit
+/// that keeps its `+1` is the retention-leak class (`BACKLOG {#unmatched-minus-one-retention-leak}`),
+/// and the stale-precommit sweep is prior-epoch-scoped, so a same-epoch leak is never reclaimed.
+/// Asserted through the ledger's own precommit view rather than inferred from a later `precommitAdd`.
+TEST(CASPartFolderAccess, PrepareThenAbortAppendsThePrecommitRemoval)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+
+    auto prepared = access.prepareEntries({ns, "part_1"}, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+    const Cas::ManifestId id = prepared.manifestId();
+    ASSERT_TRUE(store->livePrecommitsForTest(ns).contains({"part_1", id.ref}));
+
+    prepared.abort();
+
+    EXPECT_FALSE(access.existsRef({ns, "part_1"}, Cas::Freshness::ForceFresh)) << "an aborted prepare commits nothing";
+    EXPECT_FALSE(store->livePrecommitsForTest(ns).contains({"part_1", id.ref}))
+        << "abort must append the EXACT precommit removal; a same-epoch precommit left behind retains its "
+           "blobs forever (the prior-epoch-scoped stale sweep never reclaims it)";
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+    /// The precommit BODY survives (delete-after-sealed-decrements) -- the removal queues GC's `-1`,
+    /// it does not writer-delete the manifest. Mirrors
+    /// `CASPartWriteTxn.AbandonAppendsPrecommitRemovalAndKeepsLivePrecommitBody`.
+    EXPECT_TRUE(backend->head(store->layout().manifestKey(id)).exists);
+}
+
+/// A forgotten terminal must be impossible, not merely discouraged: `~PartWriteTxn` only retires the
+/// build sequence, so the handle's own destructor is the last-resort owner of the precommit removal.
+TEST(CASPartFolderAccess, DestroyingAnUnfinishedPreparedPartWriteAborts)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+
+    std::optional<Cas::ManifestId> id;
+    {
+        auto prepared = access.prepareEntries({ns, "part_1"}, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+        id = prepared.manifestId();
+        ASSERT_TRUE(store->livePrecommitsForTest(ns).contains({"part_1", id->ref}));
+    }   /// neither promoted nor aborted
+
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty())
+        << "destruction without a terminal must still append the precommit removal";
+    EXPECT_FALSE(access.existsRef({ns, "part_1"}, Cas::Freshness::ForceFresh));
+}
+
+/// The terminal flag is explicit and one-shot: a second `promote`/`abort` is a caller bug, not an
+/// idempotent no-op, and must never re-drive the (already dead) transaction.
+///
+/// The rejection throws LOGICAL_ERROR, which aborts the whole process in debug/sanitizer builds
+/// (Exception.cpp's handle_error_code) instead of behaving like a catchable exception -- so the
+/// expectThrowsCode form only makes sense in a plain release build, and the DeathTest variant below
+/// proves the SAME rejections positively abort under debug/sanitizer builds instead (same pattern as
+/// CASWiringOpsDeathTest in gtest_ca_wiring.cpp).
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASPartFolderAccess, PreparedPartWriteRejectsASecondTerminal)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+
+    auto promoted = access.prepareEntries({ns, "promoted"}, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+    EXPECT_FALSE(promoted.isTerminal());
+    promoted.promote();
+    EXPECT_TRUE(promoted.isTerminal());
+    expectThrowsCode(ErrorCodes::LOGICAL_ERROR, [&] { promoted.promote(); });
+    expectThrowsCode(ErrorCodes::LOGICAL_ERROR, [&] { promoted.abort(); });
+
+    auto aborted = access.prepareEntries({ns, "aborted"}, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+    aborted.abort();
+    EXPECT_TRUE(aborted.isTerminal());
+    expectThrowsCode(ErrorCodes::LOGICAL_ERROR, [&] { aborted.abort(); });
+    expectThrowsCode(ErrorCodes::LOGICAL_ERROR, [&] { aborted.promote(); });
+
+    EXPECT_TRUE(access.existsRef({ns, "promoted"}, Cas::Freshness::ForceFresh));
+    EXPECT_FALSE(access.existsRef({ns, "aborted"}, Cas::Freshness::ForceFresh));
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+}
+#else
+TEST(CASPartFolderAccessDeathTest, PreparedPartWriteRejectsASecondTerminalAborts)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+
+    auto promoted = access.prepareEntries({ns, "promoted"}, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+    promoted.promote();
+    EXPECT_TRUE(promoted.isTerminal());
+    EXPECT_DEATH(promoted.promote(), "owes exactly one terminal operation");
+    EXPECT_DEATH(promoted.abort(), "owes exactly one terminal operation");
+
+    auto aborted = access.prepareEntries({ns, "aborted"}, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+    aborted.abort();
+    EXPECT_TRUE(aborted.isTerminal());
+    EXPECT_DEATH(aborted.abort(), "owes exactly one terminal operation");
+    EXPECT_DEATH(aborted.promote(), "owes exactly one terminal operation");
+
+    EXPECT_TRUE(access.existsRef({ns, "promoted"}, Cas::Freshness::ForceFresh));
+    EXPECT_FALSE(access.existsRef({ns, "aborted"}, Cas::Freshness::ForceFresh));
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+}
+#endif
+
+/// Move-only, and the move transfers the terminal duty in full: the moved-from handle is already
+/// terminal (its destructor must not re-abort a transaction the destination now owns), while the
+/// destination still owes exactly one terminal.
+TEST(CASPartFolderAccess, PreparedPartWriteMoveTransfersTheTerminalDuty)
+{
+    static_assert(!std::is_copy_constructible_v<Cas::PreparedPartWrite>);
+    static_assert(!std::is_copy_assignable_v<Cas::PreparedPartWrite>);
+
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+
+    auto source = access.prepareEntries({ns, "part_1"}, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+    const Cas::ManifestId id = source.manifestId();
+    {
+        Cas::PreparedPartWrite moved = std::move(source);
+        /// NOLINTNEXTLINE(bugprone-use-after-move,clang-analyzer-cplusplus.Move,hicpp-invalid-access-moved)
+        EXPECT_TRUE(source.isTerminal()) << "a moved-from handle owes nothing";
+#ifndef DEBUG_OR_SANITIZER_BUILD
+        expectThrowsCode(ErrorCodes::LOGICAL_ERROR, [&] { source.abort(); });
+#else
+        /// LOGICAL_ERROR aborts the process in debug/sanitizer builds; EXPECT_DEATH forks, so the
+        /// parent's state (and the rest of this test) is unaffected.
+        EXPECT_DEATH(source.abort(), "owes exactly one terminal operation");
+#endif
+        EXPECT_TRUE(store->livePrecommitsForTest(ns).contains({"part_1", id.ref}))
+            << "the moved-from handle must not have aborted the transaction it handed over";
+        EXPECT_EQ(moved.manifestId().ref, id.ref);
+        moved.promote();
+    }   /// the moved-from handle's destructor also runs here: it must be a no-op, not a second abort
+
+    EXPECT_TRUE(access.existsRef({ns, "part_1"}, Cas::Freshness::ForceFresh));
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+}
+
+TEST(CASPartFolderAccess, ExplainRecordsDecisions)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, {.explain_enabled = true, .validate = {}});
+    publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    access.getView(key, Cas::Freshness::CachedForLoad);
+    EXPECT_EQ(access.explain(key).last_decision,
+              Cas::CachedPartFolderAccess::LastDecision::Miss);       /// cold build
+    EXPECT_FALSE(access.explain(key).retained);                                    /// Phase 3: never
+
+    access.getView(key, Cas::Freshness::ForceFresh);
+    EXPECT_EQ(access.explain(key).last_decision,
+              Cas::CachedPartFolderAccess::LastDecision::ForceFreshRead);
+
+    access.getView(key, Cas::Freshness::StrictValidate);
+    EXPECT_EQ(access.explain(key).last_decision,
+              Cas::CachedPartFolderAccess::LastDecision::StrictBypass);
+
+    access.dropRef(key);
+    EXPECT_EQ(access.explain(key).last_decision,
+              Cas::CachedPartFolderAccess::LastDecision::Invalidated);
+    EXPECT_GT(access.explain(key).estimated_bytes, 0u);
+}
+
+TEST(CASPartFolderAccess, BaselineRequestCountsWithoutRetention)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store);
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+    const Cas::PartRefKey key{ns, "part_1"};
+    const String manifest_key = layout.manifestKey(id);
+
+    backend->resetCounts();
+    constexpr int n = 5;
+    for (int i = 0; i < n; ++i)
+        ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);
+
+    /// The Phase-3 baseline (retention off): one manifest-body GET (the decode cache absorbs the
+    /// rest) but a mandatory manifest HEAD per call. Phase 4's validated hits remove the HEADs;
+    /// this test pins the numbers Phase 4 improves.
+    EXPECT_EQ(backend->getCount(manifest_key), 1u);
+    EXPECT_EQ(backend->headCount(manifest_key), static_cast<uint64_t>(n));
+}
+
+/// ==== Phase 4 (retention) semantics battery: spec §Testing acceptance criteria ====
+
+/// REMOVED (all-tree-part-files Task 9):
+/// `MutableRefreshWithoutManifestRead` and `WriteThroughEraseThenRebuild` proved the cache facade's
+/// `LastDecision::MutableRefresh` fast path -- a cheap re-check that could serve a retained view whose
+/// manifest was unchanged but whose separate mutable payload had drifted, without a manifest re-read.
+/// That whole two-tier freshness model is gone: every per-part file is an ordinary manifest entry now,
+/// so ANY content change is a manifest change (`repointRef`) and the existing manifest-id staleness
+/// check (`getView`'s `cached->manifestId() == resolved->manifest_id` compare) is the only freshness
+/// check left -- there is no cheaper "payload-only" path to test separately. Coverage that remains
+/// valid: `MismatchRebuildAfterRepublish` below proves the cache correctly rebuilds when the manifest
+/// id changes under a retained view (the one case the deleted tests' "erase => cold rebuild" half also
+/// exercised); `gtest_cas_repoint.cpp` (Task 3) proves `repointRef` erases the affected view on success.
+TEST(CASPartFolderAccess, MismatchRebuildAfterRepublish)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+    publishPart(store, ns, "part_1", {inlineEntry("f", "orig")});
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);   /// retained
+
+    /// Drop + republish the SAME ref name with DIFFERENT content through the raw Core protocol (no
+    /// facade => no write-through erase): the retained entry survives with a manifest_id that no
+    /// longer resolves — the next CachedForLoad hits the manifest-changed compare (step 2c).
+    store->dropRef(ns, "part_1");
+    const auto id2 = publishPart(store, ns, "part_1", {inlineEntry("f", "DIFFERENT")});
+    const String manifest_key2 = layout.manifestKey(id2);
+    backend->resetCounts();
+
+    auto view = access.getView(key, Cas::Freshness::CachedForLoad);
+    ASSERT_NE(view, nullptr);
+    EXPECT_NE(view->findFile("f"), nullptr);
+    EXPECT_EQ(view->findFile("f")->inline_bytes, "DIFFERENT");    /// never the stale view
+    EXPECT_EQ(backend->getCount(manifest_key2), 1u);              /// one new manifest GET
+    EXPECT_EQ(access.explain(key).last_decision,
+              Cas::CachedPartFolderAccess::LastDecision::Miss);   /// rebuilt, now retained
+    EXPECT_TRUE(access.explain(key).retained);
+}
+
+TEST(CASPartFolderAccess, ForceFreshFailsClosedWhileRetainedViewExists)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("f", "x")});
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);   /// retained
+    deleteManifestBody(*backend, layout, id);   /// protocol violation: live body vanishes
+
+    /// Write-evidence and strict paths surface INV-NO-DANGLE immediately (mandatory HEAD)...
+    expectThrowsCode(ErrorCodes::FILE_DOESNT_EXIST,
+        [&] { access.getView(key, Cas::Freshness::ForceFresh); });
+    expectThrowsCode(ErrorCodes::FILE_DOESNT_EXIST,
+        [&] { access.getView(key, Cas::Freshness::StrictValidate); });
+
+    /// ...while a validated CachedForLoad hit still serves the immutable decode — the documented
+    /// residual delta (spec §Staleness Equivalence): detection deferred, never for write evidence.
+    EXPECT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);
+}
+
+/// ==== §3 (part_folder_validate): the ForceFresh body re-proof HEAD is configurable ====
+
+TEST(CASPartFolderAccess, ValidateNeverServesRetainedViewWithoutBodyHead)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+
+    auto params = cacheOn();
+    params.validate = {Cas::PartFolderValidate::Mode::Never, 0};
+    Cas::CachedPartFolderAccess access(store, params);
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    /// Prime the retained view (pays the HEAD once).
+    ASSERT_NE(access.getView(key, Cas::Freshness::ForceFresh), nullptr);
+    /// Body vanishes (a protocol violation the net would normally catch)...
+    deleteManifestBody(*backend, layout, id);
+    const auto skips_before = ProfileEvents::global_counters[ProfileEvents::CASPartFolderValidateSkipped].load();
+    /// ...but `never` serves the retained view, no HEAD, no throw.
+    EXPECT_NO_THROW(access.getView(key, Cas::Freshness::ForceFresh));
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASPartFolderValidateSkipped].load() - skips_before, 1);
+}
+
+TEST(CASPartFolderAccess, ValidateAlwaysStillHeadsEveryForceFresh)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+
+    Cas::CachedPartFolderAccess access(store, cacheOn());   /// default = Always
+    const Cas::PartRefKey key{ns, "part_1"};
+    ASSERT_NE(access.getView(key, Cas::Freshness::ForceFresh), nullptr);
+    deleteManifestBody(*backend, layout, id);
+    /// `always` re-proves the body every ForceFresh — the deleted body surfaces as FILE_DOESNT_EXIST.
+    expectThrowsCode(ErrorCodes::FILE_DOESNT_EXIST,
+        [&] { access.getView(key, Cas::Freshness::ForceFresh); });
+}
+
+TEST(CASPartFolderAccess, ValidateAgeSkipsWithinWindowThenHeadsAfter)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+
+    auto params = cacheOn();
+    params.validate = {Cas::PartFolderValidate::Mode::Age, /*age_seconds=*/5};
+    /// An injected clock (spec §3 TDD requirement): the SAME function stamps the retained view's
+    /// validated_at_ms (buildView) and drives the age-window comparison (getView), so the test controls
+    /// both sides of the comparison deterministically -- no real sleep.
+    std::atomic<uint64_t> fake_now_ms{1'000'000};
+    Cas::CachedPartFolderAccess access(store, params, [&] { return fake_now_ms.load(); });
+    const Cas::PartRefKey key{ns, "part_1"};
+    const String manifest_key = layout.manifestKey(id);
+
+    /// Prime the retained view (pays the HEAD once) at fake_now_ms.
+    ASSERT_NE(access.getView(key, Cas::Freshness::ForceFresh), nullptr);
+    const uint64_t heads_after_prime = backend->headCount(manifest_key);
+
+    /// +2s: still inside the 5s window — served from the retained view, no new HEAD.
+    fake_now_ms += 2000;
+    ASSERT_NE(access.getView(key, Cas::Freshness::ForceFresh), nullptr);
+    EXPECT_EQ(backend->headCount(manifest_key), heads_after_prime);
+
+    /// +6s from the ORIGINAL stamp (past the 5s window): re-proves the body via a fresh HEAD.
+    fake_now_ms += 4000;
+    ASSERT_NE(access.getView(key, Cas::Freshness::ForceFresh), nullptr);
+    EXPECT_GT(backend->headCount(manifest_key), heads_after_prime);
+}
+
+/// ==== §3: `parsePartFolderValidate` config parsing, standalone (mirrors CASS3Staging's
+/// parseStagingBackend coverage) -- review finding: std::stoull silently accepted a leading '-'
+/// (unsigned wraparound), so a malformed `age -5` never hit the parser's own fail-closed throw.
+/// These pin the fixed `std::from_chars`-based parsing directly, with no disk/store needed. ====
+
+TEST(CASPartFolderValidateParse, DefaultConfigParsesToAlways)
+{
+    /// No `part_folder_validate` key at all -- the byte-for-byte-pre-§3-behavior default.
+    auto config = configWithDiskSection("<scratch_path>/tmp/whatever</scratch_path>");
+    const auto v = ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk");
+    EXPECT_EQ(v.mode, Cas::PartFolderValidate::Mode::Always);
+}
+
+TEST(CASPartFolderValidateParse, ParsesAlways)
+{
+    auto config = configWithDiskSection("<part_folder_validate>always</part_folder_validate>");
+    const auto v = ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk");
+    EXPECT_EQ(v.mode, Cas::PartFolderValidate::Mode::Always);
+}
+
+TEST(CASPartFolderValidateParse, ParsesNever)
+{
+    auto config = configWithDiskSection("<part_folder_validate>never</part_folder_validate>");
+    const auto v = ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk");
+    EXPECT_EQ(v.mode, Cas::PartFolderValidate::Mode::Never);
+}
+
+TEST(CASPartFolderValidateParse, ParsesPositiveAge)
+{
+    auto config = configWithDiskSection("<part_folder_validate>age 5</part_folder_validate>");
+    const auto v = ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk");
+    EXPECT_EQ(v.mode, Cas::PartFolderValidate::Mode::Age);
+    EXPECT_EQ(v.age_seconds, 5u);
+}
+
+TEST(CASPartFolderValidateParse, AcceptsAgeZeroAsADegenerateButValidWindow)
+{
+    /// `age 0` is accepted, not rejected: it is a well-formed (if degenerate -- effectively an
+    /// almost-always-expired window) configuration, not malformed input. Only genuinely malformed
+    /// suffixes (negative, non-digit, empty, trailing garbage) fail closed below.
+    auto config = configWithDiskSection("<part_folder_validate>age 0</part_folder_validate>");
+    const auto v = ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk");
+    EXPECT_EQ(v.mode, Cas::PartFolderValidate::Mode::Age);
+    EXPECT_EQ(v.age_seconds, 0u);
+}
+
+TEST(CASPartFolderValidateParse, NegativeAgeThrows)
+{
+    /// The bug this regression-guards: std::stoull("-5") used to return 18446744073709551611
+    /// (unsigned wraparound) instead of rejecting the leading '-'.
+    auto config = configWithDiskSection("<part_folder_validate>age -5</part_folder_validate>");
+    expectThrowsCode(ErrorCodes::BAD_ARGUMENTS,
+        [&] { ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk"); });
+}
+
+TEST(CASPartFolderValidateParse, NonDigitAgeThrows)
+{
+    auto config = configWithDiskSection("<part_folder_validate>age abc</part_folder_validate>");
+    expectThrowsCode(ErrorCodes::BAD_ARGUMENTS,
+        [&] { ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk"); });
+}
+
+TEST(CASPartFolderValidateParse, TrailingGarbageAfterAgeThrows)
+{
+    auto config = configWithDiskSection("<part_folder_validate>age 5abc</part_folder_validate>");
+    expectThrowsCode(ErrorCodes::BAD_ARGUMENTS,
+        [&] { ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk"); });
+}
+
+TEST(CASPartFolderValidateParse, EmptyAgeSuffixThrows)
+{
+    auto config = configWithDiskSection("<part_folder_validate>age </part_folder_validate>");
+    expectThrowsCode(ErrorCodes::BAD_ARGUMENTS,
+        [&] { ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk"); });
+}
+
+TEST(CASPartFolderValidateParse, UnknownValueThrows)
+{
+    /// Fail-closed: an unrecognized value must NEVER silently become `never`/`always`.
+    auto config = configWithDiskSection("<part_folder_validate>sometimes</part_folder_validate>");
+    expectThrowsCode(ErrorCodes::BAD_ARGUMENTS,
+        [&] { ContentAddressedMetadataStorage::parsePartFolderValidate(*config, "disk"); });
+}
+
+TEST(CASPartFolderAccess, AbsenceIsNeverRetained)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+    publishPart(store, ns, "part_1", {inlineEntry("f", "x")});
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);   /// retained
+    access.dropRef(key);
+    EXPECT_EQ(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);   /// absent: nullptr, never retained
+
+    /// Re-publish under the SAME ref name: immediately visible, no stale absence remembered.
+    publishPart(store, ns, "part_1", {inlineEntry("f", "y")});
+    auto view = access.getView(key, Cas::Freshness::CachedForLoad);
+    ASSERT_NE(view, nullptr);
+    EXPECT_EQ(view->inlineBytes("f"), std::optional<String>("y"));
+}
+
+/// Task 23 (URF plan phase 7): `getView` emits a `RefResolve` audit event only when the access does
+/// real resolve work -- a warm `CachedForLoad` hit whose retained view already matches the fresh
+/// resolve serves the call with no new information, so it must add no row. `resolveRef` itself defers
+/// the emit on this call path (`ResolveAudit::Deferred`, `CachedPartFolderAccess::resolve`), and
+/// `getView` re-emits the identical event on every OTHER path -- cold builds and `ForceFresh`.
+TEST(CASPartFolderAccess, GetViewEmitsRefResolveOnlyOnRealResolveWork)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    std::vector<Cas::CasEvent> seen;
+    store->setEventSink([&](const Cas::CasEvent & e) { seen.push_back(e); });
+    Cas::CachedPartFolderAccess access(store, cacheOn());   /// retention on, validate == Always (default)
+
+    const auto refResolveCount = [&]
+    {
+        return std::count_if(seen.begin(), seen.end(),
+            [](const Cas::CasEvent & e) { return e.type == Cas::CasEventType::RefResolve; });
+    };
+
+    /// Cold CachedForLoad build: real resolve work -> exactly one RefResolve.
+    ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);
+    EXPECT_EQ(refResolveCount(), 1);
+
+    /// Warm hit: the retained view still matches the fresh resolve, so this call serves the SAME
+    /// manifest with no new information -- before this fix it would emit a SECOND RefResolve
+    /// (resolveRef emitted unconditionally); after the fix it must add none.
+    ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);
+    EXPECT_EQ(refResolveCount(), 1) << "a warm view-cache hit must not add a RefResolve row";
+
+    /// ForceFresh always re-proves the manifest body under the default Always validation policy, so
+    /// this is real resolve work again -> +1.
+    ASSERT_NE(access.getView(key, Cas::Freshness::ForceFresh), nullptr);
+    EXPECT_EQ(refResolveCount(), 2);
+
+    store->setEventSink(nullptr);
+}
+
+TEST(CASPartFolderAccess, OversizedViewServedNotRetained)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    /// max_entry_bytes = 1: every real view (>= the 256-byte fixed overhead alone) is oversized.
+    Cas::CachedPartFolderAccess access(store,
+        Cas::CachedPartFolderAccess::CacheParams{
+            .cache_bytes = 64ULL << 20, .max_entries = 10000, .max_entry_bytes = 1,
+            .explain_enabled = true, .validate = {}});
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("f", "x")});
+    const Cas::PartRefKey key{ns, "part_1"};
+    const String manifest_key = layout.manifestKey(id);
+
+    auto view1 = access.getView(key, Cas::Freshness::CachedForLoad);
+    ASSERT_NE(view1, nullptr);
+    EXPECT_FALSE(access.explain(key).retained);
+    EXPECT_EQ(access.explain(key).last_decision,
+              Cas::CachedPartFolderAccess::LastDecision::OversizedBypass);
+
+    const uint64_t head_before = backend->headCount(manifest_key);
+    auto view2 = access.getView(key, Cas::Freshness::CachedForLoad);
+    ASSERT_NE(view2, nullptr);
+    EXPECT_GT(backend->headCount(manifest_key), head_before);   /// not retained: re-HEADs every call
+    EXPECT_FALSE(access.explain(key).retained);
+}
+
+TEST(CASPartFolderAccess, DisabledModeKeepsBaseline)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    /// CacheParams{} (cache_bytes == 0): the explicit disable switch, same as the single-arg ctor.
+    Cas::CachedPartFolderAccess access(store, Cas::CachedPartFolderAccess::CacheParams{});
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+    const Cas::PartRefKey key{ns, "part_1"};
+    const String manifest_key = layout.manifestKey(id);
+
+    backend->resetCounts();
+    constexpr int n = 5;
+    for (int i = 0; i < n; ++i)
+        ASSERT_NE(access.getView(key, Cas::Freshness::CachedForLoad), nullptr);
+
+    /// Exactly the Phase-3 baseline: bytes=0 restores the no-retention call graph byte-for-byte.
+    EXPECT_EQ(backend->getCount(manifest_key), 1u);
+    EXPECT_EQ(backend->headCount(manifest_key), static_cast<uint64_t>(n));
+    EXPECT_FALSE(access.explain(key).retained);
+}
+
+TEST(CASPartFolderAccess, SingleFlightColdBuild)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::Layout layout("p");
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("f", "x")});
+    const Cas::PartRefKey key{ns, "part_1"};
+    const String manifest_key = layout.manifestKey(id);
+
+    backend->resetCounts();
+    constexpr int k = 8;
+    std::latch start_gate(k);
+    std::vector<std::thread> threads;
+    std::vector<std::shared_ptr<const Cas::PartFolderView>> results(k);
+    for (int i = 0; i < k; ++i)
+        threads.emplace_back([&, i]
+        {
+            start_gate.arrive_and_wait();
+            results[i] = access.getView(key, Cas::Freshness::CachedForLoad);
+        });
+    for (auto & t : threads)
+        t.join();
+
+    for (const auto & r : results)
+        EXPECT_NE(r, nullptr);
+    EXPECT_EQ(backend->getCount(manifest_key), 1u);   /// single-flight: ONE body GET for the burst
+}
+
+TEST(CASPartFolderAccess, DropNamespaceErasesAllViews)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    /// Review C2: deliberately NOT pinned -- `ns` gets a REAL, random catalog incarnation from
+    /// `publishPart` below, which is what this test drives production's namespace-drop/recreate
+    /// terminal snapshot or retirement checkpoint at. Pinning
+    /// it to the sentinel would make production's real-incarnation path untested by the one test that
+    /// exercises it end-to-end (the exact gap C2 named).
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+    publishPart(store, ns, "part_1", {inlineEntry("f", "x")});
+    publishPart(store, ns, "part_2", {inlineEntry("f", "y")});
+    const Cas::PartRefKey key1{ns, "part_1"};
+    const Cas::PartRefKey key2{ns, "part_2"};
+
+    ASSERT_NE(access.getView(key1, Cas::Freshness::CachedForLoad), nullptr);   /// retained
+    ASSERT_NE(access.getView(key2, Cas::Freshness::CachedForLoad), nullptr);   /// retained
+    EXPECT_TRUE(access.explain(key1).retained);
+    EXPECT_TRUE(access.explain(key2).retained);
+
+    access.dropNamespace(ns);
+
+    /// dropNamespace removes the namespace via the ref-log `remove_namespace` transaction AND erases every
+    /// cached view: the dropped entries must not masquerade as "retained", and no stale key1/key2 view may
+    /// be served.
+    EXPECT_FALSE(access.explain(key1).retained);
+    EXPECT_FALSE(access.explain(key2).retained);   /// dropped too, even though never re-touched
+
+    /// A fresh getView on the removed namespace is a COLD MISS (nullptr) -- never a stale hit on the
+    /// dropped manifest. A residual retained entry would instead be served here without ever going through
+    /// validate-on-hit, exactly the masquerade this guards against.
+    EXPECT_EQ(access.getView(key1, Cas::Freshness::CachedForLoad), nullptr);
+    EXPECT_EQ(access.getView(key2, Cas::Freshness::CachedForLoad), nullptr);
+
+}
+
+TEST(CASPartFolderAccess, BestEffortRollbackDropCountsAndSurvivesABackendOutage)
+{
+    auto backend = std::make_shared<RollbackFaultBackend>();
+    auto store = openPoolForTest(backend);
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+
+    const Cas::RootNamespace ns_a{"srv/ta"};
+    const Cas::RootNamespace ns_b{"srv/tb"};
+    publishPart(store, ns_a, "part_a", {inlineEntry("checksums.txt", "cs")});
+    publishPart(store, ns_b, "part_b", {inlineEntry("checksums.txt", "cs")});
+
+    backend->armed = true;
+    /// Sanity: with the backend armed, a real dropRef propagates (so the fault reaches the catch).
+    EXPECT_ANY_THROW(store->dropRef(ns_a, "part_a"));
+
+    using ProfileEvents::global_counters;
+    const auto before = global_counters[ProfileEvents::CASRefRollbackBestEffortDropFailed].load();
+    /// The compensating-rollback path must NOT throw (noexcept) and MUST record the swallowed failure.
+    access.dropRefBestEffort(Cas::PartRefKey{ns_b, "part_b"});
+    const auto after = global_counters[ProfileEvents::CASRefRollbackBestEffortDropFailed].load();
+    EXPECT_EQ(after, before + 1);
+
+    backend->armed = false;   /// let store teardown release its lease cleanly
+}
+
+namespace
+{
+
+/// A pool whose ref lane makes ONE attempt per append. That is what turns a single lost-response fault
+/// into a conclusive `Unresolved`: with retries allowed the controller's resolve-before-reissue would
+/// settle the ambiguity inside the same attempt and the lane would never wedge. Same budget shape, and
+/// the same reason, as `gtest_cas_ref_install_safety.cpp`'s `openPoolSingleAttempt`.
+Cas::PoolPtr openPoolSingleAttempt(const std::shared_ptr<Cas::InMemoryBackend> & backend)
+{
+    Cas::PoolConfig cfg{.pool_prefix = "p", .server_root_id = "test"};
+    Cas::CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;   /// strictly above attempt_timeout_ms: equality is a wall-clock race (validateCasRequestBudget)
+    budget.lease_safety_margin_ms = 100;
+    cfg.cas_request_budget = budget;
+    return Cas::Pool::open(backend, cfg);
+}
+
+}
+
+/// Part B review, MAJOR 3a: a promote whose ref-log append did not resolve MUST NOT be reported as
+/// "nothing was committed".
+///
+/// `PreparedRelinkOverPartWrite::promote` maps a `NETWORK_ERROR` to `MechanismFallbackAllowed`, which
+/// tells the interserver receiver to fetch the part's bytes from the same sender instead. That is sound
+/// only when the promote is PROVEN not to have committed. It is not proven here: the promotion object
+/// landed and only its acknowledgement was lost, so the ref below IS committed while `promote` reports
+/// failure -- and a byte fetch on top of it is a sequential double publication of one logical fetch.
+///
+/// The transaction therefore records the distinction where it is knowable (around its own append)
+/// rather than leaving it to be guessed from an error code, which cannot carry it: the SAME
+/// `NETWORK_ERROR` is raised by a promote rejected before the append (proof of the negative) and by one
+/// whose append never resolved.
+TEST(CASPartFolderAccess, AnUnresolvedPromoteIsNotReportedAsDefinitelyNotCommitted)
+{
+    auto backend = std::make_shared<Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    auto prepared = access.prepareEntries(key, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+    ASSERT_FALSE(prepared.commitIsUnresolved()) << "no promote has been attempted yet";
+
+    /// The promotion's own ref-log object lands; only the acknowledgement, and the controller's
+    /// verifying read, are lost. Scoped to this namespace's ref log so nothing else consumes the fault.
+    backend->fault_substr = store->layout().namespaceStreamPrefix(fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = Cas::tests::ChunkFaultBackend::Mode::LandedThenLost;
+    backend->fault_count = 1;
+    expectThrowsCode(ErrorCodes::NETWORK_ERROR, [&] { prepared.promote(); });
+
+    EXPECT_TRUE(prepared.commitIsUnresolved())
+        << "a promote whose append may have landed must not be classified as a mechanism failure -- the "
+           "receiver would fetch the bytes and publish the same part a second time";
+
+    /// The hazard itself, stated as an assertion: the promote DID commit. Any further append into this
+    /// table resolves the wedge first, which is what makes the committed row visible.
+    backend->mode = Cas::tests::ChunkFaultBackend::Mode::None;
+    access.prepareEntries({ns, "flush_driver"}, {inlineEntry("f", "two")}, Cas::ProvenanceOp::Insert).abort();
+    EXPECT_TRUE(access.existsRef(key, Cas::Freshness::ForceFresh))
+        << "the promotion object landed, so 'the promote failed' says nothing about the ref";
+}
+
+/// Part B review, MAJOR 3b: nothing after a durable commit may throw before the handle records it.
+///
+/// `promoteBuild` used to assemble its `CommitOutcome` -- two `String` copies -- and invalidate the
+/// cached view AFTER the durable append and BEFORE `PreparedPartWrite::promote` set `terminal`. An
+/// allocation failure in that window therefore entered the failed-promote catch with the ref already
+/// committed, where the handle abandons its build and reports the promote as failed. The outcome's
+/// strings are now copied BEFORE the append and the commit is recorded in an allocation-free region
+/// immediately after it, so the window is empty by construction; the probe below fires just past it.
+TEST(CASPartFolderAccess, APostCommitFailureLeavesTheHandleTerminal)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForTest(backend);
+    const Cas::RootNamespace ns{"srv/t1"};
+    Cas::CachedPartFolderAccess access(store, cacheOn());
+    const Cas::PartRefKey key{ns, "part_1"};
+
+    auto prepared = access.prepareEntries(key, {inlineEntry("f", "one")}, Cas::ProvenanceOp::Insert);
+
+    std::vector<Cas::CasEvent> seen;
+    store->setEventSink([&](const Cas::CasEvent & e) { seen.push_back(e); });
+
+    /// `MEMORY_LIMIT_EXCEEDED` -- what a tracked allocation failure actually raises -- and deliberately
+    /// not `LOGICAL_ERROR`, which aborts at construction in debug/sanitizer builds.
+    access.setPostCommitProbeForTest([]
+    {
+        throw Exception(ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+            "simulated allocation failure in the post-commit work of promoteBuild");
+    });
+    expectThrowsCode(ErrorCodes::MEMORY_LIMIT_EXCEEDED, [&] { prepared.promote(); });
+    access.setPostCommitProbeForTest(nullptr);
+    store->setEventSink(nullptr);
+
+    EXPECT_TRUE(prepared.isTerminal())
+        << "the commit is durable, so the handle owes nothing";
+    EXPECT_TRUE(access.existsRef(key, Cas::Freshness::ForceFresh)) << "the promote really did commit";
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+
+    /// The discriminating assertion. `isTerminal` alone is not one: the old code reached the catch,
+    /// abandoned an ALREADY PROMOTED build -- which succeeds, because a promoted build no longer owes a
+    /// precommit removal -- and so ended up terminal too, by accident. What the abandon leaves behind is
+    /// the audit trail of a publish that is reported as thrown away while its ref is committed.
+    const auto build_aborts = std::count_if(seen.begin(), seen.end(),
+        [](const Cas::CasEvent & e) { return e.type == Cas::CasEventType::BuildAbort; });
+    EXPECT_EQ(build_aborts, 0)
+        << "a build whose promote is DURABLE was abandoned by the failed-promote catch: the handle had "
+           "not yet recorded the commit when the post-commit work threw";
+    EXPECT_EQ(std::count_if(seen.begin(), seen.end(),
+        [](const Cas::CasEvent & e) { return e.type == Cas::CasEventType::BuildPublish; }), 1);
+}
+
+/// Part B review, MAJOR 4: move ASSIGNMENT is deleted rather than implemented.
+///
+/// It cannot be implemented correctly. Overwriting a handle that still owes a terminal must first
+/// discharge that duty, and `abandon` appends through the ref lane, so it can FAIL -- which a move
+/// assignment has no way to report. The old implementation overwrote the destination's build even when
+/// `abandonBuildBestEffort` returned false, permanently dropping a cleanup owner: a live-epoch precommit
+/// that no sweep and no GC ever reclaims. Nothing needs the operator (the interserver relink's handle is
+/// move CONSTRUCTED into place), and a contract that cannot be relied on is worse than none.
+TEST(CASPartFolderAccess, PreparedPartWriteIsNotMoveAssignable)
+{
+    EXPECT_FALSE(std::is_move_assignable_v<Cas::PreparedPartWrite>)
+        << "a move assignment cannot discharge a terminal duty that may fail to be discharged";
+    EXPECT_TRUE(std::is_move_constructible_v<Cas::PreparedPartWrite>);
+}
diff --git a/src/Disks/tests/gtest_cas_part_folder_view.cpp b/src/Disks/tests/gtest_cas_part_folder_view.cpp
new file mode 100644
index 000000000000..be9b9e91de91
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_part_folder_view.cpp
@@ -0,0 +1,105 @@
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h>
+#include <gtest/gtest.h>
+
+using namespace DB;
+
+TEST(CASPartRefKey, CacheKeyIsUnambiguous)
+{
+    /// Refs may contain '/' (the `detached/<part>` fold, B181); the '\0' join keeps
+    /// (ns="a", ref="b/c") distinct from (ns="a/b", ref="c").
+    const Cas::PartRefKey k1{Cas::RootNamespace{"a"}, "b/c"};
+    const Cas::PartRefKey k2{Cas::RootNamespace{"a/b"}, "c"};
+    EXPECT_NE(k1.cacheKey(), k2.cacheKey());
+    EXPECT_FALSE(k1 == k2);
+    EXPECT_TRUE((k1 == Cas::PartRefKey{Cas::RootNamespace{"a"}, "b/c"}));
+}
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <algorithm>
+
+namespace
+{
+
+using namespace DB;
+
+std::shared_ptr<const Cas::PartFolderView> makeView()
+{
+    auto manifest = std::make_shared<Cas::PartManifest>();
+    auto add = [&](const char * path, Cas::EntryPlacement placement, const char * bytes, uint64_t blob_size)
+    {
+        Cas::ManifestEntry e;
+        e.path = path;
+        e.placement = placement;
+        e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(UInt128(manifest->entries.size() + 1))};
+
+        e.blob_size = blob_size;
+        e.inline_bytes = bytes;
+        manifest->entries.push_back(e);
+    };
+    /// Canonical (sorted) order — the ctor chasserts it. All-tree-part-files Task 9: `txn_version.txt`
+    /// is an ordinary Inline entry now, not a separate mutable payload.
+    add("checksums.txt", Cas::EntryPlacement::Inline, "cs", 2);
+    add("data.bin", Cas::EntryPlacement::Blob, "", 100);
+    add("p.proj/checksums.txt", Cas::EntryPlacement::Inline, "pc", 2);
+    add("p.proj/data.bin", Cas::EntryPlacement::Blob, "", 50);
+    add("txn_version.txt", Cas::EntryPlacement::Inline, "ver", 3);
+
+    return std::make_shared<const Cas::PartFolderView>(
+        Cas::PartRefKey{Cas::RootNamespace{"srv/t"}, "part_1"},
+        Cas::ManifestId{Cas::RootNamespace{"srv/t"}, Cas::ManifestRef{1, 2, 3}},
+        /*manifest_size=*/1000, manifest,
+        /*validated_at_ms=*/42);
+}
+
+std::vector<String> sorted(std::vector<String> v) { std::sort(v.begin(), v.end()); return v; }
+
+}
+
+TEST(CASPartFolderView, FindFileAndHasFile)
+{
+    auto v = makeView();
+    ASSERT_NE(v->findFile("data.bin"), nullptr);
+    EXPECT_EQ(v->findFile("data.bin")->blob_size, 100u);
+    EXPECT_EQ(v->findFile("absent.bin"), nullptr);
+    EXPECT_TRUE(v->hasFile("p.proj/data.bin"));
+    EXPECT_TRUE(v->hasFile("txn_version.txt"));      /// an ordinary Inline entry
+    EXPECT_FALSE(v->hasFile("p.proj"));              /// a directory, not a file
+}
+
+TEST(CASPartFolderView, ListChildrenCollapsesFirstComponent)
+{
+    auto v = makeView();
+    EXPECT_EQ(sorted(v->listChildren("")),
+              sorted({"checksums.txt", "data.bin", "p.proj", "txn_version.txt"}));
+    EXPECT_EQ(sorted(v->listChildren("p.proj/")), sorted({"checksums.txt", "data.bin"}));
+    EXPECT_TRUE(v->listChildren("q.proj/").empty());
+}
+
+TEST(CASPartFolderView, HasDirectory)
+{
+    auto v = makeView();
+    EXPECT_TRUE(v->hasDirectory("p.proj/"));
+    EXPECT_FALSE(v->hasDirectory("q.proj/"));
+}
+
+TEST(CASPartFolderView, SizesAndBytes)
+{
+    auto v = makeView();
+    EXPECT_EQ(v->fileSize("checksums.txt"), std::optional<uint64_t>(2));   /// inline: bytes size
+    EXPECT_EQ(v->fileSize("data.bin"), std::optional<uint64_t>(100));      /// blob: blob_size
+    EXPECT_EQ(v->fileSize("txn_version.txt"), std::optional<uint64_t>(3)); /// inline: bytes size
+    EXPECT_EQ(v->fileSize("absent"), std::nullopt);
+    EXPECT_EQ(v->inlineBytes("checksums.txt"), std::optional<String>("cs"));
+    EXPECT_EQ(v->inlineBytes("data.bin"), std::nullopt);                   /// blob has no inline bytes
+    EXPECT_EQ(v->inlineBytes("txn_version.txt"), std::optional<String>("ver"));
+    EXPECT_GE(v->estimatedBytes(), 1000u);                                 /// >= manifest_size
+}
+
+TEST(CASPartFolderView, ProjectionDirPrefixRecognizer)
+{
+    using V = Cas::PartFolderView;
+    EXPECT_EQ(V::projectionDirPrefix("p.proj"), std::optional<std::string>("p.proj/"));
+    EXPECT_EQ(V::projectionDirPrefix("a/b.tmp_proj"), std::optional<std::string>("a/b.tmp_proj/"));
+    EXPECT_EQ(V::projectionDirPrefix("data.bin"), std::nullopt);
+    EXPECT_EQ(V::projectionDirPrefix(""), std::nullopt);
+}
diff --git a/src/Disks/tests/gtest_cas_part_manifest_format.cpp b/src/Disks/tests/gtest_cas_part_manifest_format.cpp
new file mode 100644
index 000000000000..5d64b0b91bd9
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_part_manifest_format.cpp
@@ -0,0 +1,472 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/Exception.h>
+#include <utility>
+
+using namespace DB::Cas;
+
+namespace
+{
+
+/// NOT `Disks/tests/cas_test_helpers.h`'s `DB::Cas::tests::expectThrowsCode`: pulling in that header
+/// drags along a large chunk of the CAS backend/store machinery this file has no other need for, so it
+/// stays clear of `cas_test_helpers.h` entirely and inlines its own copy of the same tiny assertion
+/// instead.
+template <typename F>
+void expectThrowsCode(int expected_code, F && fn)
+{
+    try
+    {
+        fn();
+        FAIL() << "expected DB::Exception";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), expected_code);
+    }
+}
+
+/// One Blob + one Inline entry, matching the plan's §text-shape illustration verbatim (codecs-v3
+/// phase 6): deliberately NOT path-sorted on input, so the round trip also exercises canonical
+/// path-order encoding.
+PartManifest sample()
+{
+    PartManifest m;
+    m.ref = ManifestRef{5, 15, 1};
+    m.root_namespace_id = RootNamespace("00/aa@cas@");
+
+    ManifestEntry inl;
+    inl.path = "c/small.txt";
+    inl.placement = EntryPlacement::Inline;
+    inl.inline_bytes = "hello world!";   /// 12 raw bytes, no embedded '\n'
+
+    ManifestEntry blob;
+    blob.path = "a/b.bin";
+    blob.placement = EntryPlacement::Blob;
+    blob.ref = BlobRef{BlobHashAlgo::CityHash128, codecFor(BlobHashAlgo::CityHash128).fromHex("00112233445566778899aabbccddeeff")};
+    blob.blob_size = 4096;
+
+    m.entries = {inl, blob};   /// deliberately out of canonical order
+    /// Set LAST, after all other fields (matches gtest_cas_manifest_codec.cpp's
+    /// makeTwoEntryManifestForOrderTest): decode now recomputes + verifies this, so a placeholder
+    /// value here would make every test that round-trips `sample()` through decode fail closed.
+    m.payload_digest = computePayloadDigest(m);
+    return m;
+}
+
+}
+
+TEST(CASFormatBattery, PartManifest)
+{
+    const PartManifest m = sample();
+    /// Interpolate the REAL digest (never hand-compute a CityHash128 hex by hand) so the golden text
+    /// stays self-consistent with whatever sample() produces, now that decode verifies payload_digest.
+    const String golden =
+        currentFormatHeader("cas_part_manifest") +
+        "{\"me\":\"5\",\"mb\":\"15\",\"mo\":1,\"ns\":\"00/aa@cas@\",\"pd\":\"" + u128ToHex(m.payload_digest) + "\"}\n" // NOLINT(modernize-raw-string-literal): mixes '\"' quoting with '\n' line endings across this concatenated literal; a raw string can't hold the newline as-is.
+        "{\"p\":\"a/b.bin\",\"pm\":\"blob\",\"ha\":\"ch128\",\"h\":\"00112233445566778899aabbccddeeff\",\"sz\":4096}\n"
+        "{\"p\":\"c/small.txt\",\"pm\":\"inline\",\"il\":12}\n"
+        "{\"n\":2}\n"
+        "==> c/small.txt il=12 <==\n"
+        "hello world!\n";
+    runFormatBattery({FormatId::PartManifest,
+        [&] { return sealObject(FormatId::PartManifest, encodePartManifest(m)); },
+        [](std::string_view d) { decodePartManifest(std::string(openObject(FormatId::PartManifest, d))); },
+        golden});
+}
+
+TEST(CASPartManifestFormat, RoundTripDescriptorAndEntries)
+{
+    const PartManifest m = sample();
+    const PartManifest got = decodePartManifest(encodePartManifest(m));
+    EXPECT_EQ(got.ref, m.ref);
+    EXPECT_EQ(got.root_namespace_id, m.root_namespace_id);
+    EXPECT_EQ(got.payload_digest, m.payload_digest);
+    ASSERT_EQ(got.entries.size(), 2u);
+
+    /// canonical path order: "a/b.bin" < "c/small.txt"
+    EXPECT_EQ(got.entries[0].path, "a/b.bin");
+    EXPECT_EQ(got.entries[0].placement, EntryPlacement::Blob);
+    EXPECT_EQ(got.entries[0].ref, m.entries[1].ref);
+    EXPECT_EQ(got.entries[0].blob_size, 4096u);
+
+    EXPECT_EQ(got.entries[1].path, "c/small.txt");
+    EXPECT_EQ(got.entries[1].placement, EntryPlacement::Inline);
+    /// The payload-zone round trip: exact raw bytes recovered from the banner+bytes+'\n' zone.
+    EXPECT_EQ(got.entries[1].inline_bytes, "hello world!");
+}
+
+TEST(CASPartManifestFormat, EmptyEntriesRoundTrips)
+{
+    PartManifest m = sample();
+    m.entries.clear();
+    m.payload_digest = computePayloadDigest(m);   /// recompute: content changed, sample()'s digest is stale
+    const PartManifest got = decodePartManifest(encodePartManifest(m));
+    EXPECT_TRUE(got.entries.empty());
+    EXPECT_EQ(got.ref, m.ref);
+    /// No payload zone at all when there are no Inline entries.
+    EXPECT_FALSE(encodePartManifest(m).contains("==>"));
+}
+
+TEST(CASPartManifestFormat, PlacementWordsRenderAndRejectUnknown)
+{
+    const String text = encodePartManifest(sample());
+    EXPECT_NE(text.find("\"pm\":\"blob\""), String::npos);
+    EXPECT_NE(text.find("\"pm\":\"inline\""), String::npos);
+
+    /// An unknown placement word fails closed.
+    String bad = text;
+    const size_t pos = bad.find(R"("pm":"blob")");
+    ASSERT_NE(pos, String::npos);
+    bad.replace(pos, String(R"("pm":"blob")").size(), R"("pm":"bogus")");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodePartManifest(bad); });
+}
+
+/// Proves the payload zone, not JSON-string escaping: an Inline entry whose bytes contain an
+/// embedded '\n', a NUL byte, and a '"' character round-trip byte-faithfully. If this content were
+/// carried as a JSON string value it would need escaping (or would be flatly invalid for the NUL
+/// byte); the payload zone instead carries it as raw length-delimited bytes.
+TEST(CASPartManifestFormat, InlineBytesWithEmbeddedSpecialCharsRoundTripByteFaithfully)
+{
+    PartManifest m;
+    m.ref = ManifestRef{7, 21, 2};
+    m.root_namespace_id = RootNamespace("00/bb@cas@");
+
+    ManifestEntry e;
+    e.path = "weird.bin";
+    e.placement = EntryPlacement::Inline;
+    e.inline_bytes = "line1\nline2";
+    e.inline_bytes.push_back('\0');
+    e.inline_bytes += "after-nul\"quoted\"end";
+    m.entries = {e};
+    m.payload_digest = computePayloadDigest(m);
+
+    const PartManifest got = decodePartManifest(encodePartManifest(m));
+    ASSERT_EQ(got.entries.size(), 1u);
+    EXPECT_EQ(got.entries[0].inline_bytes, m.entries[0].inline_bytes);
+    EXPECT_EQ(got.entries[0].inline_bytes.size(), e.inline_bytes.size());
+}
+
+TEST(CASPartManifestFormat, ByteDeterminism)
+{
+    const PartManifest m = sample();
+    /// Encode twice -> identical bytes. Also encode a copy with entries pre-shuffled into the other
+    /// order -> still identical, because the encoder sorts canonically.
+    PartManifest m2 = m;
+    std::swap(m2.entries[0], m2.entries[1]);
+    EXPECT_EQ(encodePartManifest(m), encodePartManifest(m));
+    EXPECT_EQ(encodePartManifest(m), encodePartManifest(m2));
+}
+
+TEST(CASPartManifestFormat, MixedAlgoEntriesRoundTrip)
+{
+    PartManifest m;
+    m.ref = ManifestRef{9, 33, 4};
+    m.root_namespace_id = RootNamespace("00/cc@cas@");
+
+    ManifestEntry e16;
+    e16.path = "a/ch128.bin";
+    e16.placement = EntryPlacement::Blob;
+    e16.ref = BlobRef{BlobHashAlgo::CityHash128,
+        codecFor(BlobHashAlgo::CityHash128).fromHex("00112233445566778899aabbccddeeff")};
+    e16.blob_size = 100;
+
+    ManifestEntry e32;
+    e32.path = "b/sha256.bin";
+    e32.placement = EntryPlacement::Blob;
+    e32.ref = BlobRef{BlobHashAlgo::Sha256, codecFor(BlobHashAlgo::Sha256).fromHex(String(64, 'a'))};
+    e32.blob_size = 200;
+
+    m.entries = {e16, e32};
+    m.payload_digest = computePayloadDigest(m);
+
+    const PartManifest got = decodePartManifest(encodePartManifest(m));
+    ASSERT_EQ(got.entries.size(), 2u);
+    EXPECT_EQ(got.entries[0].path, "a/ch128.bin");
+    EXPECT_EQ(got.entries[0].ref, e16.ref);
+    EXPECT_EQ(got.entries[0].blob_size, 100u);
+    EXPECT_EQ(got.entries[1].path, "b/sha256.bin");
+    EXPECT_EQ(got.entries[1].ref, e32.ref);
+    EXPECT_EQ(got.entries[1].blob_size, 200u);
+}
+
+/// Builds a single-Blob-entry manifest whose entry path is exactly `path` -- `encodePartManifest`
+/// itself does not validate path shape (only ordering/duplicates), so this lets the negative cases
+/// below reach `decodePartManifest`'s shape check unobstructed.
+static PartManifest manifestWithSinglePath(std::string_view path)
+{
+    PartManifest m;
+    m.ref = ManifestRef{17, 66, 7};
+    m.root_namespace_id = RootNamespace("00/ff@cas@");
+
+    ManifestEntry e;
+    e.path = String(path);
+    e.placement = EntryPlacement::Blob;
+    e.ref = BlobRef{BlobHashAlgo::CityHash128,
+        codecFor(BlobHashAlgo::CityHash128).fromHex("00112233445566778899aabbccddeeff")};
+    e.blob_size = 10;
+    m.entries = {e};
+    m.payload_digest = computePayloadDigest(m);
+    return m;
+}
+
+/// T11: manifest bytes arrive over the interserver relink channel, so decode enforces the same path
+/// hygiene as CasLayout::checkNamespace -- relative, no empty/'.'/'..' segments, no leading '/'.
+/// `encodePartManifest` does not itself reject these (see `manifestWithSinglePath`), so each case
+/// must fail closed at decode time instead.
+TEST(CASPartManifestFormat, DecodeRejectsMalformedEntryPaths)
+{
+    for (const char * path : {"../evil", "/abs", "", "a//b", "a/./b"})
+    {
+        SCOPED_TRACE(path);
+        const String encoded = encodePartManifest(manifestWithSinglePath(path));
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodePartManifest(encoded); });
+    }
+}
+
+/// Legal projection subdirectories (`<projection>.proj/<file>`) must not be caught by the shape
+/// check above -- it is syntactic only, not a directory-depth restriction.
+TEST(CASPartManifestFormat, DecodeAcceptsLegalProjectionSubdirPath)
+{
+    const PartManifest m = manifestWithSinglePath("proj.proj/data.bin");
+    const PartManifest got = decodePartManifest(encodePartManifest(m));
+    ASSERT_EQ(got.entries.size(), 1u);
+    EXPECT_EQ(got.entries[0].path, "proj.proj/data.bin");
+}
+
+TEST(CASPartManifestFormat, DuplicatePathRejectedOnEncode)
+{
+    PartManifest m = sample();
+    ManifestEntry dup = m.entries[0];   /// same path as an existing entry
+    m.entries.push_back(dup);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodePartManifest(m); });
+}
+
+/// Hand-forge two valid entry-record LINES swapped out of canonical order (no CRC-patching forge
+/// helpers needed - this is a text format, lines carry no per-line checksum). Both entries are Blob
+/// (no payload-zone bytes), so the swap cannot disturb payload-zone alignment - it isolates exactly
+/// the ordering check.
+TEST(CASPartManifestFormat, DecodeRejectsOutOfOrderEntries)
+{
+    PartManifest m;
+    m.ref = ManifestRef{11, 44, 5};
+    m.root_namespace_id = RootNamespace("00/dd@cas@");
+
+    auto mkBlob = [](std::string_view path)
+    {
+        ManifestEntry e;
+        e.path = String(path);
+        e.placement = EntryPlacement::Blob;
+        e.ref = BlobRef{BlobHashAlgo::CityHash128,
+            codecFor(BlobHashAlgo::CityHash128).fromHex("00112233445566778899aabbccddeeff")};
+        e.blob_size = 10;
+        return e;
+    };
+    /// "a/one.bin" and "b/two.bin" are the same length, so swapping their record lines in place
+    /// does not shift any other byte offset in the text.
+    m.entries = {mkBlob("a/one.bin"), mkBlob("b/two.bin"), mkBlob("c/three.bin")};
+    m.payload_digest = computePayloadDigest(m);
+
+    const String text = encodePartManifest(m);
+    const size_t pos_a = text.find(R"("p":"a/one.bin")");
+    const size_t pos_b = text.find(R"("p":"b/two.bin")");
+    ASSERT_NE(pos_a, String::npos);
+    ASSERT_NE(pos_b, String::npos);
+
+    const size_t a_start = text.rfind('\n', pos_a) + 1;
+    const size_t a_end = text.find('\n', pos_a) + 1;
+    const size_t b_start = text.rfind('\n', pos_b) + 1;
+    const size_t b_end = text.find('\n', pos_b) + 1;
+    const String a_line = text.substr(a_start, a_end - a_start);
+    const String b_line = text.substr(b_start, b_end - b_start);
+    ASSERT_EQ(a_line.size(), b_line.size());
+
+    String forged = text;
+    forged.replace(a_start, a_line.size(), b_line);
+    forged.replace(b_start, b_line.size(), a_line);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodePartManifest(forged); });
+}
+
+/// a < b < c in canonical order; forge entry c's path to equal entry a's path. A naive "only check
+/// adjacent pairs" implementation would miss this (c is only ever compared against b, never against
+/// a); requiring strict ascending order against just the immediately-preceding entry still catches
+/// it, because the forged c(=a's path) is no longer greater than b either.
+TEST(CASPartManifestFormat, DecodeRejectsNonAdjacentDuplicatePath)
+{
+    PartManifest m;
+    m.ref = ManifestRef{13, 55, 6};
+    m.root_namespace_id = RootNamespace("00/ee@cas@");
+
+    auto mkBlob = [](std::string_view path)
+    {
+        ManifestEntry e;
+        e.path = String(path);
+        e.placement = EntryPlacement::Blob;
+        e.ref = BlobRef{BlobHashAlgo::CityHash128,
+            codecFor(BlobHashAlgo::CityHash128).fromHex("00112233445566778899aabbccddeeff")};
+        e.blob_size = 10;
+        return e;
+    };
+    m.entries = {mkBlob("aaa/one.bin"), mkBlob("bbb/two.bin"), mkBlob("ccc/three.bin")};
+    m.payload_digest = computePayloadDigest(m);
+
+    String forged = encodePartManifest(m);
+    const String needle = R"("p":"ccc/three.bin")";
+    const size_t pos = forged.find(needle);
+    ASSERT_NE(pos, String::npos);
+    forged.replace(pos, needle.size(), R"("p":"aaa/one.bin")");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodePartManifest(forged); });
+}
+
+TEST(CASPartManifestFormat, UnknownEntryAlgoFailsClosed)
+{
+    String bad = encodePartManifest(sample());
+    const String needle = R"("ha":"ch128")";
+    const size_t pos = bad.find(needle);
+    ASSERT_NE(pos, String::npos);
+    bad.replace(pos, needle.size(), R"("ha":"bogus")");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodePartManifest(bad); });
+}
+
+/// `DigestCodec::fromHex` throws BAD_ARGUMENTS (not CORRUPTED_DATA) on a width mismatch; decode must
+/// check the width itself first so this fails closed with the same code every other decode error
+/// here uses.
+TEST(CASPartManifestFormat, DigestHexWidthMismatchFailsClosedNotBadArguments)
+{
+    String bad = encodePartManifest(sample());
+    const String key = R"("h":")";
+    const size_t key_pos = bad.find(key);
+    ASSERT_NE(key_pos, String::npos);
+    const size_t hex_start = key_pos + key.size();
+    const size_t hex_end = bad.find('"', hex_start);
+    ASSERT_NE(hex_end, String::npos);
+    ASSERT_EQ(hex_end - hex_start, 32u);   /// ch128: 16-byte digest -> 32 hex chars
+    bad.erase(hex_start, 1);               /// drop one hex char -> width mismatch (31 chars)
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodePartManifest(bad); });
+}
+
+/// Pure-function properties of computePayloadDigest, independent of decode-time verification: stable
+/// across calls for identical content, independent of the payload_digest field's own value, and
+/// content-sensitive (changes when real content changes).
+TEST(CASPartManifestFormat, PayloadDigestStableAndContentSensitive)
+{
+    const PartManifest m = sample();
+    PartManifest with_different_stored_digest = m;
+    with_different_stored_digest.payload_digest = UInt128(0x1234);
+    EXPECT_EQ(computePayloadDigest(m), computePayloadDigest(m));
+    EXPECT_EQ(computePayloadDigest(m), computePayloadDigest(with_different_stored_digest));
+
+    /// m.entries[1] is the Blob entry (m.entries[0] is Inline, whose blob_size is unused on the
+    /// wire) - changing its blob_size changes the canonical encoding and therefore the digest.
+    ASSERT_EQ(m.entries[1].placement, EntryPlacement::Blob);
+    PartManifest changed = m;
+    changed.entries[1].blob_size += 1;
+    EXPECT_NE(computePayloadDigest(m), computePayloadDigest(changed));
+}
+
+/// No-smuggling: one extra trailing byte after the last payload-zone segment (or after the trailer,
+/// when there are no Inline entries) must be rejected - exercises the final `!in.eof()` check.
+TEST(CASPartManifestFormat, TrailingByteAfterPayloadZoneFailsClosed)
+{
+    String bad = encodePartManifest(sample());
+    bad += "X";
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodePartManifest(bad); });
+}
+
+/// An Inline entry's record "il" disagrees with what the payload zone's banner+bytes actually
+/// declare (the banner and bytes are left as originally written; only the record line's "il" is
+/// edited). The record's declared `il` is what decode uses both to build the expected banner text
+/// and to know how many bytes to read from the zone, so this must fail closed rather than silently
+/// reading the wrong byte count.
+TEST(CASPartManifestFormat, InlineRecordIlMismatchWithPayloadZoneBannerFailsClosed)
+{
+    String bad = encodePartManifest(sample());
+    const String needle = "\"il\":12";
+    const size_t pos = bad.find(needle);
+    ASSERT_NE(pos, String::npos);
+    bad.replace(pos, needle.size(), "\"il\":13");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodePartManifest(bad); });
+}
+
+/// ==== migrated from gtest_cas_manifest_codec.cpp (deleted in the phase-6 binary->text cutover,
+/// Task 3): these exercise refMatchesBody/manifestNamespaceMatches/findEntry/entryRange, pure
+/// functions carried over verbatim from the retired binary codec (untouched by the wire-shape
+/// migration) — reusing this file's own sample() fixture instead of reintroducing a second one. ====
+
+TEST(CASPartManifestFormat, RefMatchesBodyAcceptsExactRef)
+{
+    const PartManifest m = sample();
+    /// The journal ref equals the body ref -> true.
+    EXPECT_TRUE(refMatchesBody(m.ref, m));
+}
+
+TEST(CASPartManifestFormat, RefMatchesBodyRejectsEachFieldMismatch)
+{
+    const PartManifest m = sample();
+    ManifestRef wrong_writer = m.ref; wrong_writer.writer_epoch = m.ref.writer_epoch + 1;
+    ManifestRef wrong_seq = m.ref;    wrong_seq.build_sequence = m.ref.build_sequence + 1;
+    ManifestRef wrong_inst = m.ref;   wrong_inst.manifest_ordinal = m.ref.manifest_ordinal + 1;
+    EXPECT_FALSE(refMatchesBody(wrong_writer, m));
+    EXPECT_FALSE(refMatchesBody(wrong_seq, m));
+    EXPECT_FALSE(refMatchesBody(wrong_inst, m));
+}
+
+TEST(CASPartManifestFormat, ManifestNamespaceMatchesAcceptsOwningNs)
+{
+    const PartManifest m = sample();
+    EXPECT_TRUE(manifestNamespaceMatches(m.root_namespace_id, m));
+}
+
+TEST(CASPartManifestFormat, ManifestNamespaceMatchesRejectsForeignNs)
+{
+    const PartManifest m = sample();
+    /// sample()'s namespace is "00/aa@cas@" — pick a genuinely foreign one and a strict-prefix one.
+    EXPECT_FALSE(manifestNamespaceMatches(RootNamespace("00/bb@cas@"), m));
+    /// A namespace that is a prefix but not equal is still a mismatch (no loose comparison).
+    EXPECT_FALSE(manifestNamespaceMatches(RootNamespace("00/aa"), m));
+}
+
+TEST(CASPartManifestFormat, FindEntryBinarySearch)
+{
+    std::vector<ManifestEntry> entries;
+    for (const char * p : {"a.txt", "b/inner.txt", "b/z.txt", "c.txt"})
+    {
+        ManifestEntry e;
+        e.path = p;
+        e.placement = EntryPlacement::Inline;
+        e.inline_bytes = "v";
+        entries.push_back(e);
+    }
+    EXPECT_NE(findEntry(entries, "a.txt"), nullptr);
+    EXPECT_EQ(findEntry(entries, "a.txt")->path, "a.txt");
+    EXPECT_NE(findEntry(entries, "c.txt"), nullptr);          /// last element
+    EXPECT_EQ(findEntry(entries, "b"), nullptr);              /// prefix of a path, not a path
+    EXPECT_EQ(findEntry(entries, "zzz"), nullptr);            /// past the end
+    EXPECT_EQ(findEntry({}, "a"), nullptr);                   /// empty
+}
+
+TEST(CASPartManifestFormat, EntryRangeContiguousPrefix)
+{
+    std::vector<ManifestEntry> entries;
+    for (const char * p : {"a.txt", "p.proj/data.bin", "p.proj/x.txt", "q.txt"})
+    {
+        ManifestEntry e;
+        e.path = p;
+        e.placement = EntryPlacement::Inline;
+        e.inline_bytes = "v";
+        entries.push_back(e);
+    }
+    auto [first, last] = entryRange(entries, "p.proj/");
+    ASSERT_EQ(last - first, 2);
+    EXPECT_EQ(first->path, "p.proj/data.bin");
+    EXPECT_EQ((last - 1)->path, "p.proj/x.txt");
+
+    auto [w1, w2] = entryRange(entries, "");                  /// empty prefix = whole span
+    EXPECT_EQ(w2 - w1, 4);
+
+    auto [n1, n2] = entryRange(entries, "zzz/");              /// no match
+    EXPECT_EQ(n1, n2);
+}
diff --git a/src/Disks/tests/gtest_cas_part_write.cpp b/src/Disks/tests/gtest_cas_part_write.cpp
new file mode 100644
index 000000000000..2fe5db215b5c
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_part_write.cpp
@@ -0,0 +1,2517 @@
+#include <gtest/gtest.h>
+#include <algorithm>
+#include <cmath>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <IO/ReadBufferFromString.h>
+#include <IO/HashingReadBuffer.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/WriteBuffer.h>
+#include <IO/WriteBufferFromString.h>
+#include <IO/WriteHelpers.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <Poco/Exception.h>
+
+namespace ProfileEvents
+{
+extern const Event CASMetaPut;
+extern const Event CASMetaCompareSwap;
+extern const Event CASMetaCreateClean;
+extern const Event CASMetaAdoptBackfill;
+extern const Event CASMetaResurrectClean;
+extern const Event CASBlobAdoptTrusted;
+}
+
+namespace DB::ErrorCodes
+{
+extern const int BAD_ARGUMENTS;
+extern const int FILE_DOESNT_EXIST;
+extern const int INVALID_STATE;
+extern const int LOGICAL_ERROR;
+extern const int NOT_IMPLEMENTED;
+extern const int ABORTED;
+extern const int CORRUPTED_DATA;
+extern const int LIMIT_EXCEEDED;
+extern const int NETWORK_ERROR;
+extern const int UNKNOWN_EXCEPTION;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::condemnMeta;
+using DB::Cas::tests::expectThrowsCode;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::injectRetire;
+using DB::Cas::tests::loadMetaForTest;
+using DB::Cas::tests::streamingHexOf;
+using DB::Cas::tests::u128Of;
+using DB::Cas::tests::writeMetaClean;
+using DB::Cas::tests::writeRawBlobBody;
+
+namespace
+{
+
+PoolPtr openPool(const std::shared_ptr<InMemoryBackend> & b)
+{
+    return Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// Start a build whose owning manifest namespace + final ref name are `ns`/`ref` (promote/stageManifest
+/// derive the manifest namespace by splitting PartWriteInfo::intended_ref on the LAST '/').
+PartWriteTxnPtr startBuildFor(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    return s->beginPartWrite(info);
+}
+
+/// A one-entry Blob ManifestEntry for `payload` at `path` (the build's stageManifest entry).
+ManifestEntry blobManifestEntry(const String & path, const String & payload)
+{
+    ManifestEntry e;
+    e.path = path;
+    e.placement = EntryPlacement::Blob;
+    e.ref = BlobRef{BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    e.blob_size = payload.size();
+    return e;
+}
+
+/// The streaming (production-convention) `BlobRef` of `payload` — CityHash128 at the write width.
+BlobRef streamRefOf(const String & payload)
+{
+    return BlobRef{BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hexToU128(streamingHexOf(payload)))};
+}
+
+ManifestEntry blobManifestEntryStreaming(const String & path, const String & payload)
+{
+    ManifestEntry e;
+    e.path = path;
+    e.placement = EntryPlacement::Blob;
+    e.ref = streamRefOf(payload);
+
+    e.blob_size = payload.size();
+    return e;
+}
+
+/// The full single-blob write flow (EDGE-BEFORE-OBSERVE wiring order):
+/// stageManifest(one entry) -> precommitAdd -> putBlob -> promote. Returns the committed ManifestId.
+ManifestId publishOneBlobPart(
+    const PoolPtr & s, const RootNamespace & ns, const String & ref, const String & path, const String & payload)
+{
+    auto build = startBuildFor(s, ns, ref);
+    const ManifestId id = build->stageManifest({blobManifestEntry(path, payload)});
+    build->precommitAdd(ns, ref, id);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+/// A one-shot backend hook (mirrors the WriteCountingBackend delegation pattern in gtest_cas_pool.cpp):
+/// it delegates every op to a wrapped Backend, but the FIRST time head(target_key) is called it fires a
+/// deleteExact(target_key, condemned_token) AFTER computing the (present) HEAD result and BEFORE returning
+/// it — simulating GC's exact-token content delete landing in the writer's HEAD->GET window (B136).
+class HeadThenDeleteOnceBackend final : public DB::Cas::Backend
+{
+public:
+    HeadThenDeleteOnceBackend(BackendPtr inner_, String target_key_, DB::Cas::Token condemned_)
+        : inner(std::move(inner_)), target_key(std::move(target_key_)), condemned(condemned_) {}
+
+    DB::Cas::HeadResult head(const String & k) override
+    {
+        const DB::Cas::HeadResult hr = inner->head(k);
+        if (k == target_key && !fired)
+        {
+            fired = true;
+            /// GC's single content-delete site, landing in the HEAD->GET window.
+            inner->deleteExact(target_key, condemned);
+        }
+        return hr;
+    }
+
+    std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override { return inner->get(k, r); }
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+    DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+    DB::Cas::PutResult putIfAbsent(const String & k, const String & b, const DB::Cas::ObjectMeta & meta) override { return inner->putIfAbsent(k, b, meta); }
+    DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & meta) override { return inner->putIfAbsentStream(k, meta); }
+    DB::Cas::PutResult putOverwrite(const String & k, const String & b, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & meta) override { return inner->putOverwrite(k, b, e, meta); }
+    DB::Cas::Token resurrect(DB::ReadBuffer & payload, uint64_t payload_size, const String & k, const String & fresh_header) override { return inner->resurrect(payload, payload_size, k, fresh_header); }
+    DB::Cas::CasResult casPut(const String & k, const String & b, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & meta) override { return inner->casPut(k, b, e, meta); }
+    DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & t) override { return inner->deleteExact(k, t); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+
+private:
+    BackendPtr inner;
+    String target_key;
+    DB::Cas::Token condemned;
+    bool fired = false;
+};
+
+/// A delegating backend that counts head()/get() calls per key. Lets a test assert the promote gate
+/// performs ZERO per-file probes on a TRUSTED adopted leaf (§4 manifest-trust): no presence HEAD on
+/// the blob key, no loadMeta GET on the blob-meta key.
+class KeyCountingBackend final : public DB::Cas::Backend
+{
+public:
+    explicit KeyCountingBackend(BackendPtr inner_) : inner(std::move(inner_)) {}
+
+    size_t headCountFor(const String & k) const { auto it = head_counts.find(k); return it == head_counts.end() ? 0 : it->second; }
+    size_t getCountFor(const String & k) const { auto it = get_counts.find(k); return it == get_counts.end() ? 0 : it->second; }
+
+    DB::Cas::HeadResult head(const String & k) override { ++head_counts[k]; return inner->head(k); }
+    std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override { ++get_counts[k]; return inner->get(k, r); }
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+    DB::Cas::ListPage list(const String & pfx, const String & c, size_t l) override { return inner->list(pfx, c, l); }
+    DB::Cas::PutResult putIfAbsent(const String & k, const String & b, const DB::Cas::ObjectMeta & meta) override { return inner->putIfAbsent(k, b, meta); }
+    DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & meta) override { return inner->putIfAbsentStream(k, meta); }
+    DB::Cas::PutResult putOverwrite(const String & k, const String & b, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & meta) override { return inner->putOverwrite(k, b, e, meta); }
+    DB::Cas::Token resurrect(DB::ReadBuffer & payload, uint64_t payload_size, const String & k, const String & fresh_header) override { return inner->resurrect(payload, payload_size, k, fresh_header); }
+    DB::Cas::CasResult casPut(const String & k, const String & b, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & meta) override { return inner->casPut(k, b, e, meta); }
+    DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & t) override { return inner->deleteExact(k, t); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+
+private:
+    BackendPtr inner;
+    std::map<String, size_t> head_counts;
+    std::map<String, size_t> get_counts;
+};
+
+}
+
+TEST(CASPartWriteTxn, PutBlobWritesEnvelopeWithFixedHeader)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    auto build = s->beginPartWrite({});
+    auto ref = build->putBlob(idOf("hello world"), BlobSource::fromString("hello world"));
+    EXPECT_EQ(ref.size, 11u);
+
+    auto raw = b->get(s->layout().blobKey(ref.ref));
+    ASSERT_TRUE(raw.has_value());
+    auto h = decodeEnvelopeHeader(raw->bytes, raw->bytes.size(), ObjectKind::Blob);
+    EXPECT_EQ(h.header_len, s->poolMeta().blob_header_len);   /// 256
+    /// `logical_size`/`logical_hash` were dropped 2026-07-11, and `domain_id` in codecs-v3 phase 7
+    /// (the pool id no longer travels in the envelope) — identity is the content key and the payload
+    /// starts at the fixed offset `header_len`.
+    EXPECT_EQ(h.build_id, build->buildId());
+    EXPECT_NE(h.incarnation_tag, UInt128{});
+    EXPECT_EQ(raw->bytes.substr(h.header_len), "hello world");
+}
+
+TEST(CASPartWriteTxn, StageManifestUsesPerBuildOrdinals)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"test/tbl@cas@"};
+
+    auto build = startBuildFor(s, ns, "all_1_1_0");
+    const ManifestId first = build->stageManifest({blobManifestEntry("a.bin", "a")});
+    const ManifestId second = build->stageManifest({blobManifestEntry("b.bin", "b")});
+
+    EXPECT_EQ(first.ref.writer_epoch, s->writerEpoch());
+    EXPECT_EQ(first.ref.build_sequence, build->buildSeq());
+    EXPECT_EQ(first.ref.manifest_ordinal, 1u);
+    EXPECT_EQ(second.ref.writer_epoch, first.ref.writer_epoch);
+    EXPECT_EQ(second.ref.build_sequence, first.ref.build_sequence);
+    EXPECT_EQ(second.ref.manifest_ordinal, 2u);
+    /// Canonical hex build directory (spec §Manifest Identifier): `<epoch-hex>-<build-seq-hex>/`.
+    const String build_segment = renderRefTxnId(RefTxnId{s->writerEpoch(), build->buildSeq()});
+    EXPECT_EQ(s->layout().manifestKey(first), "p/cas/manifests/test/tbl@cas@/" + build_segment + "/000001.zst");
+    EXPECT_EQ(s->layout().manifestKey(second), "p/cas/manifests/test/tbl@cas@/" + build_segment + "/000002.zst");
+
+    auto next_build = startBuildFor(s, ns, "all_2_2_0");
+    const ManifestId next = next_build->stageManifest({blobManifestEntry("c.bin", "c")});
+    EXPECT_EQ(next.ref.writer_epoch, first.ref.writer_epoch);
+    EXPECT_NE(next.ref.build_sequence, first.ref.build_sequence);
+    EXPECT_EQ(next.ref.manifest_ordinal, 1u);
+}
+
+/// B171: the `cas_owner` owner-triple stamping (`PartWriteTxn::ownerMeta`) was DELETED — protection is now
+/// the build-root precommit edge (reachability), not revocable object metadata GC reads per-candidate.
+/// The old `CASPartWriteTxn.BlobCarriesOwnerTripleInMetadata` asserted that stamping; its coverage is replaced
+/// by the build-root precommit/reclaim tests (`CASPartWriteTxnRoot*`, `CASPartWriteTxnRootDangle*`), which prove a
+/// written-but-unreferenced object is protected by a live precommit and collectable once it is abandoned.
+
+TEST(CASPartWriteTxn, PutBlobDedupSecondWriterAdopts)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+
+    /// First writer FRESH-uploads (legal pre-precommit — newborn-debris watermark).
+    auto build_a = s->beginPartWrite({});
+    auto ref_a = build_a->putBlob(idOf("dup"), BlobSource::fromString("dup"));
+    const Token token_a = b->head(s->layout().blobKey(ref_a.ref)).token;
+
+    /// Second writer ADOPTS — the adopt must happen under a durable precommit edge (EDGE-BEFORE-OBSERVE:
+    /// stageManifest -> precommitAdd -> putBlob), so give build_b the wiring order.
+    const RootNamespace ns_b{"srv/tbl"};
+    auto build_b = startBuildFor(s, ns_b, "ref_b");
+    const ManifestId id_b = build_b->stageManifest({blobManifestEntry("data.bin", "dup")});
+    build_b->precommitAdd(ns_b, "ref_b", id_b);
+    auto ref_b = build_b->putBlob(idOf("dup"), BlobSource::fromString("dup"));
+
+    EXPECT_EQ(ref_b.ref, ref_a.ref);
+    /// A's incarnation survives — the second writer adopts, nothing was overwritten.
+    EXPECT_EQ(b->head(s->layout().blobKey(ref_a.ref)).token, token_a);
+}
+
+/// Task 3 (spec §meta-protocols v3): the writer's dedup gate no longer consults the RetireView for the
+/// condemned decision — it point-reads the per-hash freshness meta instead. A fresh (absent -> present)
+/// upload must WRITE that meta as Clean so future point-readers (other writers, GC) can see it.
+TEST(CASPartWriteTxn, PutBlobFreshUploadWritesCleanMeta)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    auto build = s->beginPartWrite({});
+
+    const String payload = "fresh-meta-payload";
+    auto ref = build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    EXPECT_EQ(ref.size, payload.size());
+
+    const auto lm = loadMetaForTest(*b, s->layout(), u128Of(payload));
+    ASSERT_TRUE(lm.has_value()) << "a fresh upload must write a Clean meta descriptor (writer point-read protocol)";
+    EXPECT_EQ(lm->meta.state, MetaState::Clean);
+    EXPECT_EQ(lm->meta.size, payload.size());
+}
+
+/// §0 introspection: a fresh body upload writes the Clean meta exactly once through the
+/// `putMetaIfAbsent` choke point (`CASMetaPut`), tagged with its reason (`CASMetaCreateClean`).
+TEST(CASPartWriteTxnMetaCounters, CreateCleanAndChokePointCountOnFreshBody)
+{
+    /// Fresh body upload writes the Clean meta exactly once: CASMetaPut +1 (choke point)
+    /// and CASMetaCreateClean +1 (reason). Reuse the fixture of the nearest putBlob test.
+    const auto put_before = ProfileEvents::global_counters[ProfileEvents::CASMetaPut].load();
+    const auto reason_before = ProfileEvents::global_counters[ProfileEvents::CASMetaCreateClean].load();
+
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    auto build = s->beginPartWrite({});
+
+    const String payload = "fresh-meta-payload-counters";
+    auto ref = build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    EXPECT_EQ(ref.size, payload.size());
+
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMetaPut].load() - put_before, 1);
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMetaCreateClean].load() - reason_before, 1);
+}
+
+/// §0 introspection: an adopt of a pre-existing body that has NO meta at all (a pre-protocol blob, or a
+/// lost race with a concurrent fresh-uploader's own meta write) backfills a Clean meta through the
+/// `putMetaIfAbsent` choke point (`CASMetaPut`), tagged with its reason (`CASMetaAdoptBackfill`). No
+/// existing test elsewhere in the suite drives this branch: every other pre-seeded raw body in this file
+/// pairs `writeRawBlobBody` with `writeMetaClean`, which skips the `!lm` backfill branch entirely.
+TEST(CASPartWriteTxnMetaCounters, AdoptBackfillCountsChokePointAndReason)
+{
+    const auto put_before = ProfileEvents::global_counters[ProfileEvents::CASMetaPut].load();
+    const auto reason_before = ProfileEvents::global_counters[ProfileEvents::CASMetaAdoptBackfill].load();
+
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+
+    const String payload = "adopt-backfill-payload-counters";
+    const UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+
+    /// Pre-seed a present body big enough that observeAndAdmit's logical-size guard does not underflow —
+    /// deliberately WITHOUT any meta (unlike PutBlobAdoptsWhenMetaCleanNoRetireView), so the adopt reaches
+    /// the `!lm` backfill branch.
+    const uint64_t header_len = s->poolMeta().blob_header_len;
+    String raw_body(header_len, '\0');
+    raw_body += payload;
+    writeRawBlobBody(*b, s->layout(), hash, raw_body);
+
+    /// Adopt must happen under a durable precommit edge (EDGE-BEFORE-OBSERVE).
+    const RootNamespace ns{"srv/tbl"};
+    auto build = startBuildFor(s, ns, "ref_adopt_backfill");
+    const ManifestId manifest_id = build->stageManifest({blobManifestEntry("data.bin", payload)});
+    build->precommitAdd(ns, "ref_adopt_backfill", manifest_id);
+    auto ref = build->putBlob(id, BlobSource::fromString(payload));
+    EXPECT_EQ(ref.ref, id);
+
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMetaPut].load() - put_before, 1);
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMetaAdoptBackfill].load() - reason_before, 1);
+
+    const auto lm = loadMetaForTest(*b, s->layout(), hash);
+    ASSERT_TRUE(lm.has_value()) << "the adopt-backfill must leave a Clean meta for future point-readers";
+    EXPECT_EQ(lm->meta.state, MetaState::Clean);
+}
+
+/// The adopt decision is driven PURELY by the meta point-read — no RetireView is ever seeded in this
+/// test. A pre-existing body plus an independent Clean meta must be adopted (no putOverwrite/re-upload:
+/// the pre-seeded incarnation's token survives untouched), and the meta stays Clean.
+TEST(CASPartWriteTxn, PutBlobAdoptsWhenMetaCleanNoRetireView)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+
+    const String payload = "adopt-meta-payload";
+    const UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+    const String blob_key = s->layout().blobKey(id);
+
+    /// Pre-seed a body big enough that observeAndAdmit's logical-size guard (hr.size - header_len)
+    /// does not underflow, plus an INDEPENDENT Clean meta — deliberately NOT via a real putBlob (so the
+    /// adopt decision below cannot be riding on THIS task's own fresh-upload meta write).
+    const uint64_t header_len = s->poolMeta().blob_header_len;
+    String raw_body(header_len, '\0');
+    raw_body += payload;
+    writeRawBlobBody(*b, s->layout(), hash, raw_body);
+    writeMetaClean(*b, s->layout(), hash, payload.size());
+    const Token t0 = b->head(blob_key).token;
+
+    /// Adopt must happen under a durable precommit edge (EDGE-BEFORE-OBSERVE), mirroring
+    /// PutBlobDedupSecondWriterAdopts above.
+    const RootNamespace ns{"srv/tbl"};
+    auto build = startBuildFor(s, ns, "ref_adopt");
+    const ManifestId manifest_id = build->stageManifest({blobManifestEntry("data.bin", payload)});
+    build->precommitAdd(ns, "ref_adopt", manifest_id);
+    auto ref = build->putBlob(id, BlobSource::fromString(payload));
+
+    EXPECT_EQ(ref.ref, id);
+    /// Adopted: the pre-seeded incarnation survives untouched — no putOverwrite/re-upload happened.
+    EXPECT_EQ(b->head(blob_key).token, t0);
+
+    const auto lm = loadMetaForTest(*b, s->layout(), hash);
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(lm->meta.state, MetaState::Clean) << "an adopt must leave the meta Clean";
+}
+
+/// A4 (negative): observeAndAdmit's EDGE-BEFORE-OBSERVE invariant — adopting an EXISTING incarnation is
+/// safe ONLY under this build's durable precommit closure — was guarded only by chassert(precommitted),
+/// which is compiled out in release. A putBlob that reaches the adopt path with NO precommit (the wiring
+/// order stageManifest -> precommitAdd -> putBlob violated) must fail closed with a real LOGICAL_ERROR,
+/// not silently adopt a blob the newborn-debris watermark does not cover.
+TEST(CASPartWriteTxn, AdoptBeforePrecommitFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+
+    const String payload = "adopt-before-precommit-payload";
+    const UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+
+    /// Pre-seed a present body (padded past the pool header so the logical-size guard does not
+    /// underflow) + an independent Clean meta, so putBlob's upload conflicts on the present object and
+    /// takes the ADOPT branch of observeAndAdmit — mirroring PutBlobAdoptsWhenMetaCleanNoRetireView.
+    const uint64_t header_len = s->poolMeta().blob_header_len;
+    String raw_body(header_len, '\0');
+    raw_body += payload;
+    writeRawBlobBody(*b, s->layout(), hash, raw_body);
+    writeMetaClean(*b, s->layout(), hash, payload.size());
+
+    /// Start a build but DO NOT call precommitAdd: the adopt runs with `precommitted == false`.
+    const RootNamespace ns{"srv/tbl"};
+    auto build = startBuildFor(s, ns, "ref_adopt");
+
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->putBlob(id, BlobSource::fromString(payload));
+        },
+        "EDGE-BEFORE-OBSERVE invariant violated");
+}
+
+/// The resurrect decision (displace a condemned body) is likewise driven PURELY by the meta point-read
+/// — again, no RetireView is seeded. A condemned meta must cause putBlob to displace the body (a fresh
+/// token, the old one never returns — INV-NO-RETURN, unchanged body mechanics) AND flip the meta back
+/// to Clean.
+TEST(CASPartWriteTxn, PutBlobResurrectsWhenMetaCondemned)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+
+    const String payload = "resurrect-meta-payload";
+    const UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+    const String blob_key = s->layout().blobKey(id);
+
+    const uint64_t header_len = s->poolMeta().blob_header_len;
+    String raw_body(header_len, '\0');
+    raw_body += payload;
+    writeRawBlobBody(*b, s->layout(), hash, raw_body);
+    writeMetaClean(*b, s->layout(), hash, payload.size());
+    condemnMeta(*b, s->layout(), hash, /*condemn_round*/ 1);
+    const Token t0 = b->head(blob_key).token;
+
+    /// NO retire-view seeding anywhere: the resurrect must be decided purely from the meta point-read.
+    auto build = s->beginPartWrite({});
+    auto ref = build->putBlob(id, BlobSource::fromString(payload));
+    EXPECT_EQ(ref.ref, id);
+
+    /// Resurrected: the condemned incarnation was displaced by a fresh one.
+    const HeadResult hr = b->head(blob_key);
+    ASSERT_TRUE(hr.exists);
+    EXPECT_NE(hr.token, t0) << "a condemned incarnation must be displaced by a fresh one (resurrect)";
+    EXPECT_EQ(b->deleteExact(blob_key, t0).kind, DeleteOutcome::Kind::TokenMismatch)
+        << "the condemned token must never return (INV-NO-RETURN)";
+
+    const auto lm = loadMetaForTest(*b, s->layout(), hash);
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(lm->meta.state, MetaState::Clean) << "a resurrect must flip the meta back to Clean";
+}
+
+/// §0 introspection: the resurrect (condemned-displacement) meta flip goes through the `casMeta`
+/// choke point (`CASMetaCompareSwap`), tagged with its reason (`CASMetaResurrectClean`).
+TEST(CASPartWriteTxnMetaCounters, ResurrectCountsCasAndReason)
+{
+    const auto cas_before = ProfileEvents::global_counters[ProfileEvents::CASMetaCompareSwap].load();
+    const auto reason_before = ProfileEvents::global_counters[ProfileEvents::CASMetaResurrectClean].load();
+
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+
+    const String payload = "resurrect-meta-payload-counters";
+    const UInt128 hash = u128Of(payload);
+    const BlobRef id = idOf(payload);
+
+    const uint64_t header_len = s->poolMeta().blob_header_len;
+    String raw_body(header_len, '\0');
+    raw_body += payload;
+    writeRawBlobBody(*b, s->layout(), hash, raw_body);
+    writeMetaClean(*b, s->layout(), hash, payload.size());
+    condemnMeta(*b, s->layout(), hash, /*condemn_round*/ 1);
+
+    auto build = s->beginPartWrite({});
+    auto ref = build->putBlob(id, BlobSource::fromString(payload));
+    EXPECT_EQ(ref.ref, id);
+
+    EXPECT_GE(ProfileEvents::global_counters[ProfileEvents::CASMetaCompareSwap].load() - cas_before, 1);
+    EXPECT_GE(ProfileEvents::global_counters[ProfileEvents::CASMetaResurrectClean].load() - reason_before, 1);
+}
+
+TEST(CASPartWriteTxn, PutBlobWrongSizeFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    auto build = s->beginPartWrite({});
+
+    BlobSource lying;
+    lying.size = 11;   /// declares 11 but writes 5
+    lying.open = []() -> std::unique_ptr<DB::ReadBuffer>
+    { return std::make_unique<DB::ReadBufferFromOwnString>(String("short")); };
+
+    const BlobRef id = idOf("does-not-matter");
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->putBlob(id, std::move(lying));
+        },
+        "source streamed");
+    /// The cancelled stream created nothing.
+    EXPECT_FALSE(b->head(s->layout().blobKey(id)).exists);
+}
+
+/// The happy-path upload STREAMS the source directly into the put sink — it does NOT pre-materialize the
+/// whole blob into an in-memory String before the I/O. We assert this by counting `open`
+/// invocations: a single fresh upload must invoke it EXACTLY ONCE (streamed straight into the sink). The
+/// previous implementation buffered the whole blob into a `String source_bytes` first (a full in-memory
+/// copy whose peak grew ~linearly with the blob size — the OOM); that pass would invoke `open`
+/// before the sink write. One invocation here is the streaming-not-materializing guarantee.
+TEST(CASPartWriteTxn, PutBlobStreamsSourceOnceNoFullMaterialization)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    auto build = s->beginPartWrite({});
+
+    const String payload = "streamed-not-materialized";
+    int invocations = 0;
+    BlobSource source;
+    source.size = payload.size();
+    source.open = [&invocations, &payload]() -> std::unique_ptr<DB::ReadBuffer>
+    {
+        ++invocations;
+        return std::make_unique<DB::ReadBufferFromOwnString>(payload);
+    };
+
+    auto ref = build->putBlob(idOf(payload), std::move(source));
+    EXPECT_EQ(ref.size, payload.size());
+    EXPECT_EQ(invocations, 1) << "happy-path upload must stream the source exactly once (no pre-materialization pass)";
+
+    /// And the object really landed with the streamed payload (at the fixed header offset).
+    auto raw = b->get(s->layout().blobKey(ref.ref));
+    ASSERT_TRUE(raw.has_value());
+    auto h = decodeEnvelopeHeader(raw->bytes, raw->bytes.size(), ObjectKind::Blob);
+    EXPECT_EQ(raw->bytes.substr(h.header_len), payload);
+}
+
+/// B190: reuseBlob is removed (it had no production callers post-B188). Its behaviors are now covered by:
+///   - trusted adopted leaf at gate: PromoteTrustsAdoptedLeafNoProbeManifestTrust (CasPartWriteTxn) — §4
+///     manifest-trust: a committed-source adopted leaf publishes with NO per-file probe; a tokened leaf
+///     is edge-protected (Phase A) and never re-observed at the gate.
+///   - absent adopted leaf trusted:  PromoteTrustsAdoptedLeafEvenIfBackendRaced (CasPartWriteTxn) — the D4
+///     trade-off (a genuinely-absent adopted blob is caught by fsck, not the promote gate).
+///   - evidence tokenless vs tokened: DepIsTokenedDiscriminatesPutBlobVsAdopt (CASPartWriteTxnReuseBlob).
+///   - no-dep / staging-bug fail-closed: PromoteCondemnedLeafWithoutDepAbortsFailClosed (CasPartWriteTxn).
+
+TEST(CASPartWriteTxnReuseBlob, DepIsTokenedDiscriminatesPutBlobVsAdopt)
+{
+    /// B156b discriminator unit: putBlob records a TOKENED dep (token observed at upload time),
+    /// adoptEvidence records a TOKENLESS W-EVIDENCE dep (no token; liveness from the source ref).
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+
+    auto build = s->beginPartWrite({});
+
+    /// putBlob'd hash ⇒ tokened.
+    build->putBlob(idOf("written"), BlobSource::fromString("written"));
+    EXPECT_TRUE(build->depIsTokened(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("written"))}));
+
+    /// Adopted hash ⇒ tokenless. adoptEvidence records the dep directly from a resolved ManifestEntry
+    /// (the source manifest's entry); no body needs to be in hand for the dep to be recorded.
+    build->adoptEvidence(blobManifestEntry("f", "adopted"));
+    EXPECT_FALSE(build->depIsTokened(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("adopted"))}));
+
+    /// Unknown hash ⇒ no dep, not tokened.
+    EXPECT_FALSE(build->depIsTokened(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("unknown"))}));
+}
+
+/// B190: ReuseBlobCondemnedThrowsAbortedRetryable is removed (reuseBlob is gone). §4 manifest-trust: a
+/// committed-source adopted leaf is TRUSTED at the promote gate (no HEAD/loadMeta probe), so a condemned
+/// pool blob no longer surfaces at promote — covered by PromoteTrustsAdoptedLeafNoProbeManifestTrust.
+
+TEST(CASPartWriteTxn, PutBlobResurrectVanishedReUploadsHeldBody)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+
+    /// 1. Write payload-X via a throwaway build to create the blob; capture its token t0.
+    BlobRef id;
+    Token t0;
+    {
+        auto s0 = openPool(b);
+        auto build0 = s0->beginPartWrite({});
+        id = build0->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X")).ref;
+        t0 = b->head(s0->layout().blobKey(id)).token;
+    }
+
+    /// 2. Condemn (Blob, hash(X), t0) in the retire view.
+    DB::Cas::Layout layout("p");
+    const String blob_key = layout.blobKey(id);
+    /// v3: the writer's condemned decision is a per-hash meta point-read (not the retire-view). Condemn the
+    /// meta; t0 stays as the body token the delete-hook below fires with.
+    condemnMeta(*b, layout, u128Of("payload-X"), /*condemn_round*/ 1);
+
+    /// 3. Wrap the backend so the NEXT head(blob_key) returns the (present) result and THEN fires
+    ///    deleteExact(blob_key, t0) exactly once — GC's delete in the HEAD->GET window. Open a FRESH
+    ///    Pool over the hook so its retire view (refreshed at open) sees the condemnation.
+    auto hook = std::make_shared<HeadThenDeleteOnceBackend>(b, blob_key, t0);
+    auto s = Pool::open(hook, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    auto build = s->beginPartWrite({});
+
+    /// 4. putBlob with a re-invokable body.
+    ///    BEFORE fix: putIfAbsent -> PreconditionFailed -> observeAndAdmit HEAD (present, condemned)
+    ///                -> resurrect GET (vanished, deleted in the window) -> throws FILE_DOESNT_EXIST.
+    ///    AFTER fix:  condemned dedup → uploadFromSource directly from held body; the object is
+    ///                recreated under a FRESH token. NO GET of the condemned object (INV-1).
+    auto ref = build->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X"));
+    EXPECT_EQ(ref.ref, id);
+
+    /// 5. The blob is present again under a FRESH token, with the same payload; and the condemned token
+    ///    never returns (INV-NO-RETURN).
+    const HeadResult hr = b->head(blob_key);
+    ASSERT_TRUE(hr.exists);
+    EXPECT_NE(hr.token, t0);
+
+    auto raw = b->get(blob_key);
+    ASSERT_TRUE(raw.has_value());
+    auto h = decodeEnvelopeHeader(raw->bytes, raw->bytes.size(), ObjectKind::Blob);
+    EXPECT_EQ(h.header_len, s->poolMeta().blob_header_len);
+    EXPECT_EQ(raw->bytes.substr(h.header_len), "payload-X");
+
+    EXPECT_EQ(b->deleteExact(blob_key, t0).kind, DeleteOutcome::Kind::TokenMismatch);
+
+    /// The freshness meta must be reconciled to Clean too, not left stale at Condemned: the fresh
+    /// re-upload's meta write (writeFreshMetaClean) must find and fix the pre-existing Condemned
+    /// marker via the SAME reload-and-reconcile path a resurrect uses, not silently discard the
+    /// conflict (a stale Condemned marker would otherwise mislead every future point-reader).
+    const auto lm = loadMetaForTest(*b, s->layout(), u128Of("payload-X"));
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(lm->meta.state, MetaState::Clean)
+        << "a fresh re-upload over a stale Condemned marker must reconcile it back to Clean";
+}
+
+/// A persistently-failing freshness-meta write (every attempt of every outer reload-retry) must
+/// surface as a controlled retry-later signal, not silently succeed with the marker left stale
+/// (S22 RCA). The blob body PUT
+/// itself is unaffected (MetaWriteFaultBackend only faults `.meta` keys) -- only the meta write
+/// exhausts, and that exhaustion must reach putBlob's caller as NETWORK_ERROR.
+TEST(CASPartWriteTxn, PutBlobFreshMetaExhaustionThrowsRetryLater)
+{
+    /// Short budget + zero backoff: keep the test fast. Each of writeResurrectMetaClean's 8 outer
+    /// attempts calls putMetaIfAbsent, which itself retries up to max_attempts times internally —
+    /// with max_attempts=1 the controller gives up on the first faulted attempt each time.
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.retry_initial_backoff_ms = 0;
+    auto b = std::make_shared<DB::Cas::tests::MetaWriteFaultBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .cas_request_budget = budget});
+    auto build = s->beginPartWrite({});
+
+    const String payload = "fresh-meta-exhaustion-payload";
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    });
+
+    /// The body itself landed (only .meta writes are faulted) -- confirming the failure is
+    /// specifically the freshness marker, not the blob body.
+    const HeadResult hr = b->head(s->layout().blobKey(idOf(payload)));
+    EXPECT_TRUE(hr.exists) << "the body PUT is unaffected by the meta-only fault";
+}
+
+/// INV-1 (revival-from-source): a condemned blob is NEVER read via GET to revive it.
+/// putBlob on a condemned-dedup hit must re-upload from its OWN source bytes — never calling
+/// backend().get(blob_key). This test counts backend GETs on the blob key and asserts zero.
+TEST(CASPartWriteTxn, PutBlobCondemnedDedupNeverGetsTheDyingObject)
+{
+    /// A delegating backend that counts get() calls on a specific key to assert INV-1.
+    struct GetCountingBackend final : public DB::Cas::Backend
+    {
+        explicit GetCountingBackend(BackendPtr inner_, String watched_key_)
+            : inner(std::move(inner_)), watched_key(std::move(watched_key_)) {}
+        size_t get_count = 0;
+
+        DB::Cas::HeadResult head(const String & k) override { return inner->head(k); }
+        std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override
+        {
+            if (k == watched_key)
+                ++get_count;
+            return inner->get(k, r);
+        }
+        std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+        DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+        DB::Cas::PutResult putIfAbsent(const String & k, const String & bts, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsent(k, bts, m); }
+        DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsentStream(k, m); }
+        DB::Cas::PutResult putOverwrite(const String & k, const String & bts, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & m) override { return inner->putOverwrite(k, bts, e, m); }
+        DB::Cas::Token resurrect(DB::ReadBuffer & payload, uint64_t payload_size, const String & k, const String & fresh_header) override { return inner->resurrect(payload, payload_size, k, fresh_header); }
+        DB::Cas::CasResult casPut(const String & k, const String & bts, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & m) override { return inner->casPut(k, bts, e, m); }
+        DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & tok) override { return inner->deleteExact(k, tok); }
+        bool supportsListTokens() const override { return inner->supportsListTokens(); }
+    private:
+        BackendPtr inner;
+        String watched_key;
+    };
+
+    auto b = std::make_shared<InMemoryBackend>();
+
+    /// 1. Upload blob Y via a throwaway build; capture the token t0.
+    BlobRef id;
+    Token t0;
+    {
+        auto s0 = openPool(b);
+        auto build0 = s0->beginPartWrite({});
+        id = build0->putBlob(idOf("payload-Y"), BlobSource::fromString("payload-Y")).ref;
+        t0 = b->head(s0->layout().blobKey(id)).token;
+    }
+
+    /// 2. Condemn (Blob, hash(Y), t0) in the retire view, then GC-delete the object so it is absent
+    ///    (simulates GC completing the delete before the writer's dedup hit).
+    DB::Cas::Layout layout("p");
+    const String blob_key = layout.blobKey(id);
+    injectRetire(*b, layout, /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("payload-Y"))}, .token = t0, .size = 9}});
+    b->deleteExact(blob_key, t0);
+    ASSERT_FALSE(b->head(blob_key).exists);
+
+    /// 3. Open a fresh Pool over a GET-counting wrapper; the retire view sees the condemnation at open.
+    auto counting = std::make_shared<GetCountingBackend>(b, blob_key);
+    auto s = Pool::open(counting, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    auto build = s->beginPartWrite({});
+
+    /// 4. putBlob Y — the object is absent (was deleted). The dedup-hit (PreconditionFailed) path
+    ///    won't fire (object is gone, so putIfAbsentStream → Done on the first attempt).
+    ///    However, even if a racing re-creation happens between the check and the upload, the
+    ///    condemned branch must NEVER call backend().get(blob_key).
+    auto ref = build->putBlob(idOf("payload-Y"), BlobSource::fromString("payload-Y"));
+    EXPECT_EQ(ref.ref, id);
+    EXPECT_EQ(counting->get_count, 0u) << "INV-1: putBlob must not GET the dying object to revive it";
+
+    const HeadResult hr = b->head(blob_key);
+    ASSERT_TRUE(hr.exists);
+    EXPECT_NE(hr.token, t0) << "a fresh incarnation must have a new token";
+    const auto raw = b->get(blob_key);
+    ASSERT_TRUE(raw.has_value());
+    const auto hdr = decodeEnvelopeHeader(raw->bytes, raw->bytes.size(), ObjectKind::Blob);
+    EXPECT_EQ(raw->bytes.substr(hdr.header_len), "payload-Y");
+}
+
+/// INV-1 variant: blob is PRESENT and condemned (GC hasn't fired the delete yet). putBlob dedup-hits
+/// it via PreconditionFailed, sees condemned token, and must re-upload from source — NEVER GET.
+TEST(CASPartWriteTxn, PutBlobCondemnedDedupPresentNeverGetsTheDyingObject)
+{
+    struct GetCountingBackend final : public DB::Cas::Backend
+    {
+        explicit GetCountingBackend(BackendPtr inner_, String watched_key_)
+            : inner(std::move(inner_)), watched_key(std::move(watched_key_)) {}
+        size_t get_count = 0;
+
+        DB::Cas::HeadResult head(const String & k) override { return inner->head(k); }
+        std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override
+        {
+            if (k == watched_key)
+                ++get_count;
+            return inner->get(k, r);
+        }
+        std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+        DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+        DB::Cas::PutResult putIfAbsent(const String & k, const String & bts, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsent(k, bts, m); }
+        DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsentStream(k, m); }
+        DB::Cas::PutResult putOverwrite(const String & k, const String & bts, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & m) override { return inner->putOverwrite(k, bts, e, m); }
+        DB::Cas::Token resurrect(DB::ReadBuffer & payload, uint64_t payload_size, const String & k, const String & fresh_header) override { return inner->resurrect(payload, payload_size, k, fresh_header); }
+        DB::Cas::CasResult casPut(const String & k, const String & bts, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & m) override { return inner->casPut(k, bts, e, m); }
+        DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & tok) override { return inner->deleteExact(k, tok); }
+        bool supportsListTokens() const override { return inner->supportsListTokens(); }
+    private:
+        BackendPtr inner;
+        String watched_key;
+    };
+
+    auto b = std::make_shared<InMemoryBackend>();
+
+    /// 1. Upload blob Z via a throwaway build; capture the token t0.
+    BlobRef id;
+    Token t0;
+    {
+        auto s0 = openPool(b);
+        auto build0 = s0->beginPartWrite({});
+        id = build0->putBlob(idOf("payload-Z"), BlobSource::fromString("payload-Z")).ref;
+        t0 = b->head(s0->layout().blobKey(id)).token;
+    }
+
+    /// 2. Condemn (Blob, hash(Z), t0) — object still PRESENT (GC condemned but not yet deleted).
+    DB::Cas::Layout layout("p");
+    const String blob_key = layout.blobKey(id);
+    /// v3: condemn via the per-hash meta (the writer's freshness point-read), object still PRESENT.
+    condemnMeta(*b, layout, u128Of("payload-Z"), /*condemn_round*/ 1);
+    ASSERT_TRUE(b->head(blob_key).exists) << "blob must be PRESENT for the condemned-present path";
+
+    /// 3. Open a fresh Pool over a GET-counting wrapper.
+    auto counting = std::make_shared<GetCountingBackend>(b, blob_key);
+    auto s = Pool::open(counting, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    auto build = s->beginPartWrite({});
+
+    /// 4. putBlob Z: putIfAbsentStream → PreconditionFailed (object present) → observeAndAdmit →
+    ///    sees condemned token → must call uploadFromSource (NOT resurrect/GET).
+    auto ref = build->putBlob(idOf("payload-Z"), BlobSource::fromString("payload-Z"));
+    EXPECT_EQ(ref.ref, id);
+    EXPECT_EQ(counting->get_count, 0u) << "INV-1: putBlob must not GET the condemned object";
+
+    const HeadResult hr = b->head(blob_key);
+    ASSERT_TRUE(hr.exists);
+    EXPECT_NE(hr.token, t0) << "condemned incarnation must be displaced by a fresh token";
+    const auto raw = b->get(blob_key);
+    ASSERT_TRUE(raw.has_value());
+    const auto hdr = decodeEnvelopeHeader(raw->bytes, raw->bytes.size(), ObjectKind::Blob);
+    EXPECT_EQ(raw->bytes.substr(hdr.header_len), "payload-Z");
+}
+
+TEST(CASPartWriteTxn, PutBlobVanishDuringRevivalReUploadsNotFatal)
+{
+    /// B190 sibling (INV-3): inside uploadFromSource the post-412 path re-observes via the 3-arg
+    /// observeAndAdmit. If the object is GC-deleted in the window (present at the conditional PUT
+    /// → 412, but gone at the subsequent HEAD), the 3-arg overload throws FILE_DOESNT_EXIST. Before
+    /// the fix that escaped putBlob's ABORTED-only retry catch as a FATAL INSERT failure. putBlob
+    /// HOLDS the source bytes, so a vanish here must RE-UPLOAD from those bytes within the bounded
+    /// retry loop — never fatal. The fix wraps uploadFromSource's two 3-arg observeAndAdmit calls so
+    /// FILE_DOESNT_EXIST becomes the retryable ABORTED putBlob already handles.
+    struct ScriptedVanishBackend final : public DB::Cas::Backend
+    {
+        ScriptedVanishBackend(BackendPtr inner_, String watched_key_)
+            : inner(std::move(inner_)), watched_key(std::move(watched_key_)) {}
+
+        size_t head_absent_budget = 2;       /// first N head(watched) calls report absent
+        size_t finalize_412_budget = 2;      /// first N finalize() on watched return PreconditionFailed
+
+        /// A sink wrapper that forces PreconditionFailed for the scripted budget, else delegates.
+        struct ScriptedSink final : public DB::Cas::WriteSink
+        {
+            ScriptedSink(WriteSinkPtr inner_, bool force_412_)
+                : inner(std::move(inner_)), force_412(force_412_) {}
+            DB::WriteBuffer & buffer() override { return inner->buffer(); }
+            DB::Cas::PutResult finalize() override
+            {
+                if (force_412)
+                {
+                    /// Abandon the underlying upload so the key is never created by it, and report 412.
+                    inner->cancel();
+                    return {DB::Cas::PutOutcome::PreconditionFailed, {}};
+                }
+                return inner->finalize();
+            }
+            void cancel() noexcept override { inner->cancel(); }
+            WriteSinkPtr inner;
+            bool force_412;
+        };
+
+        DB::Cas::HeadResult head(const String & k) override
+        {
+            if (k == watched_key && head_absent_budget > 0)
+            {
+                --head_absent_budget;
+                return DB::Cas::HeadResult{};   /// exists == false
+            }
+            return inner->head(k);
+        }
+        DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & meta) override
+        {
+            const bool force_412 = (k == watched_key && finalize_412_budget > 0);
+            if (force_412)
+                --finalize_412_budget;
+            return std::make_unique<ScriptedSink>(inner->putIfAbsentStream(k, meta), force_412);
+        }
+        std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override { return inner->get(k, r); }
+        std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+        DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+        DB::Cas::PutResult putIfAbsent(const String & k, const String & bts, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsent(k, bts, m); }
+        DB::Cas::PutResult putOverwrite(const String & k, const String & bts, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & m) override { return inner->putOverwrite(k, bts, e, m); }
+        DB::Cas::Token resurrect(DB::ReadBuffer & payload, uint64_t payload_size, const String & k, const String & fresh_header) override { return inner->resurrect(payload, payload_size, k, fresh_header); }
+        DB::Cas::CasResult casPut(const String & k, const String & bts, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & m) override { return inner->casPut(k, bts, e, m); }
+        DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & t) override { return inner->deleteExact(k, t); }
+        bool supportsListTokens() const override { return inner->supportsListTokens(); }
+
+        BackendPtr inner;
+        String watched_key;
+    };
+
+    auto raw = std::make_shared<InMemoryBackend>();
+    DB::Cas::Layout layout("p");
+    const String blob_key = layout.blobKey(idOf("payload-V"));
+
+    /// The blob does NOT need to pre-exist: the scripted backend models the conditional-PUT 412
+    /// (object present at PUT time) independently of the inner store, then reports absent on HEAD.
+    auto scripted = std::make_shared<ScriptedVanishBackend>(raw, blob_key);
+    auto s = Pool::open(scripted, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    /// Wiring order (EDGE-BEFORE-OBSERVE): the revive re-observes via observeAndAdmit, which requires a
+    /// durable precommit edge — stageManifest -> precommitAdd before putBlob.
+    const RootNamespace ns_v{"srv/tbl"};
+    auto build = startBuildFor(s, ns_v, "part_v");
+    const ManifestId id_v = build->stageManifest({blobManifestEntry("data.bin", "payload-V")});
+    build->precommitAdd(ns_v, "part_v", id_v);
+
+    /// putBlob holds "payload-V" as source bytes. The vanish-during-revival must NOT be fatal:
+    ///   BEFORE fix: observeAndAdmit throws FILE_DOESNT_EXIST, escapes putBlob's ABORTED-only catch → fatal.
+    ///   AFTER fix:  wrapped to ABORTED → putBlob retries → re-uploads from held bytes → succeeds.
+    PutBlobResult ref;
+    EXPECT_NO_THROW(ref = build->putBlob(idOf("payload-V"), BlobSource::fromString("payload-V")));
+    EXPECT_EQ(ref.ref, idOf("payload-V"));
+
+    /// The blob is present with a fresh incarnation and the exact payload — re-uploaded from source.
+    const HeadResult hr = raw->head(blob_key);
+    ASSERT_TRUE(hr.exists) << "putBlob must have re-uploaded the vanished blob from its held source bytes";
+    const auto stored = raw->get(blob_key);
+    ASSERT_TRUE(stored.has_value());
+    const auto h = decodeEnvelopeHeader(stored->bytes, stored->bytes.size(), ObjectKind::Blob);
+    EXPECT_EQ(stored->bytes.substr(h.header_len), "payload-V");
+}
+
+TEST(CASPartWriteTxn, PromoteTrustsAdoptedLeafNoProbeManifestTrust)
+{
+    /// §4 manifest-trust: a committed-source adoptEvidence leaf is TRUSTED at the promote gate — the live
+    /// source pins the blob (in-degree >= 1, not condemnable) and this build's precommit edge is durable,
+    /// so promote publishes with NO per-file HEAD (presence) and NO loadMeta GET (the condemned point-read)
+    /// and NO copy-forward. The durable manifest edge is the liveness evidence. `CASBlobAdoptTrusted` counts
+    /// the trusted leaf. A KeyCountingBackend proves zero probes on the blob key and the blob-meta key.
+    auto raw = std::make_shared<InMemoryBackend>();
+    auto counting = std::make_shared<KeyCountingBackend>(raw);
+    auto s = Pool::open(counting, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// A committed-source blob lives in the shared pool (seeded via a throwaway build on the same store).
+    {
+        auto seed = s->beginPartWrite({});
+        seed->putBlob(streamRefOf("payload-TR"), BlobSource::fromString("payload-TR"));
+    }
+    const String blob_key = s->layout().blobKey(streamRefOf("payload-TR"));
+    const String meta_key = s->layout().blobMetaKey(streamRefOf("payload-TR"));
+
+    auto build = startBuildFor(s, ns, "part_1");
+    const ManifestEntry entry = blobManifestEntryStreaming("data.bin", "payload-TR");
+    build->adoptEvidence(entry);
+    const ManifestId id = build->stageManifest({entry});
+    build->precommitAdd(ns, "part_1", id);
+
+    const auto trusted_before = ProfileEvents::global_counters[ProfileEvents::CASBlobAdoptTrusted].load();
+    const size_t head_before = counting->headCountFor(blob_key);
+    const size_t meta_get_before = counting->getCountFor(meta_key);
+
+    build->promote(ns, "part_1", build->buildId(), id);
+
+    EXPECT_TRUE(s->resolveRef(ns, "part_1").has_value());
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASBlobAdoptTrusted].load() - trusted_before, 1);
+    EXPECT_EQ(counting->headCountFor(blob_key) - head_before, 0u) << "trust must not HEAD the adopted blob";
+    EXPECT_EQ(counting->getCountFor(meta_key) - meta_get_before, 0u) << "trust must not loadMeta the adopted blob";
+}
+
+TEST(CASPartWriteTxn, PromoteTrustsAdoptedLeafEvenIfBackendRaced)
+{
+    /// §4 manifest-trust trade-off (D4 relink interserver-trust model): a committed-source adopted leaf is
+    /// published WITHOUT a presence probe. Even if the pool object raced to absent between adopt and
+    /// promote, promote does NOT re-observe it — the ref publishes. A genuinely-absent adopted blob is an
+    /// invariant violation detected by fsck (or an actual body GET on read), not caught at the promote gate.
+    /// This is the deliberate reduction from the pre-§4 "absent adopted leaf => ABORTED at gate".
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// Seed X, adopt it, then delete it out from under the build (a landed GC delete in the adopt->promote
+    /// window). The pre-§4 gate HEADed X, found it absent, and threw ABORTED; §4 trusts the durable edge.
+    {
+        auto seed = s->beginPartWrite({});
+        seed->putBlob(streamRefOf("payload-RACE"), BlobSource::fromString("payload-RACE"));
+    }
+    const String blob_key = s->layout().blobKey(streamRefOf("payload-RACE"));
+    const Token t0 = b->head(blob_key).token;
+
+    auto build = startBuildFor(s, ns, "part_1");
+    const ManifestEntry entry = blobManifestEntryStreaming("data.bin", "payload-RACE");
+    build->adoptEvidence(entry);
+    const ManifestId id = build->stageManifest({entry});
+    build->precommitAdd(ns, "part_1", id);
+
+    ASSERT_EQ(b->deleteExact(blob_key, t0).kind, DeleteOutcome::Kind::Deleted);
+    ASSERT_FALSE(b->head(blob_key).exists);
+
+    EXPECT_NO_THROW(build->promote(ns, "part_1", build->buildId(), id));
+    EXPECT_TRUE(s->resolveRef(ns, "part_1").has_value());
+}
+
+TEST(CASPartWriteTxn, PromoteSwallowsPostDurableEventSinkFailure)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    auto build = startBuildFor(s, ns, "part_1");
+
+    ManifestEntry entry;
+    entry.path = "data.bin";
+    entry.placement = EntryPlacement::Inline;
+    entry.ref = idOf("payload");
+    entry.blob_size = 7;
+    entry.inline_bytes = "payload";
+    const ManifestId id = build->stageManifest({entry});
+    build->precommitAdd(ns, "part_1", id);
+
+    /// UNKNOWN_EXCEPTION (not LOGICAL_ERROR): this simulates an arbitrary observer/sink callback
+    /// failing, not a CAS invariant violation -- LOGICAL_ERROR would abort the whole process under
+    /// debug/sanitizer builds instead of behaving like a catchable exception.
+    s->setEventSink([](const CasEvent & e)
+    {
+        if (e.type == CasEventType::BuildPublish)
+            throw DB::Exception(DB::ErrorCodes::UNKNOWN_EXCEPTION, "injected post-durable event sink failure");
+    });
+
+    EXPECT_NO_THROW(build->promote(ns, "part_1", build->buildId(), id));
+    const auto resolved = s->resolveRef(ns, "part_1");
+    ASSERT_TRUE(resolved);
+    EXPECT_EQ(resolved->manifest_id, id);
+    s->setEventSink(nullptr);
+}
+
+TEST(CASPartWriteTxn, PromoteCondemnedLeafWithoutDepAbortsFailClosed)
+{
+    /// A manifest blob leaf with NO recorded dep (a staging-bug shape: neither putBlob nor adoptEvidence
+    /// recorded it) must fail closed at the promote gate — isTrustedAdopt is false (no tokened dep and no
+    /// committed-source adopt), so §4 never silently publishes it. Under manifest-trust there is NO per-file
+    /// probe, so the fail-closed is a LOGICAL_ERROR decided from the dep set alone — it fires regardless of
+    /// the pool blob's presence/condemnation (here the blob is even condemned, but that is never observed).
+    /// The no-dep shape is reachable through the public build API (stageManifest names the leaf without any
+    /// dep having been recorded), so no test accessor for the private predicate is needed.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// X exists (streaming-keyed) but THIS build records NO dep for it (no putBlob, no adoptEvidence).
+    {
+        auto seed = s->beginPartWrite({});
+        seed->putBlob(streamRefOf("payload-NODEP"), BlobSource::fromString("payload-NODEP"));
+    }
+    const String blob_key = s->layout().blobKey(streamRefOf("payload-NODEP"));
+    const Token t0 = b->head(blob_key).token;
+
+    auto build = startBuildFor(s, ns, "part_1");
+    const ManifestEntry entry = blobManifestEntryStreaming("data.bin", "payload-NODEP");
+    /// NB: NO adoptEvidence(entry) — deps stays empty for this hash, so isTrustedAdopt is false.
+    const ManifestId id = build->stageManifest({entry});
+    build->precommitAdd(ns, "part_1", id);
+
+    /// Condemn X (present) via the meta — under §4 the gate never point-reads it (no probe on a non-trusted
+    /// leaf), so this only confirms the fail-closed does not depend on the leaf being clean.
+    condemnMeta(*b, s->layout(), hexToU128(streamingHexOf("payload-NODEP")), /*condemn_round*/ 1);
+
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->promote(ns, "part_1", build->buildId(), id);
+        },
+        "no tokened and no adopted dep");
+    EXPECT_FALSE(s->resolveRef(ns, "part_1").has_value());
+    /// The pool blob was never touched (no probe, no displacement).
+    EXPECT_EQ(b->head(blob_key).token, t0);
+}
+
+TEST(CASPartWriteTxn, PromoteRevalidatesBlobPresenceFailClosed)
+{
+    /// Port of the old W-TREE-BUILD bottom-up enforcement (PutTreeEnforcesBottomUp): the surviving
+    /// "a committed ref never names a missing dependency" invariant. In the part-manifest model
+    /// stageManifest does not validate its entries' bodies. §4 manifest-trust: the fail-closed authority at
+    /// the promote gate is now the DEP SET, not a backend HEAD — a leaf named by the manifest with NO
+    /// tokened dep (never putBlob'd) and NO adopted dep (never adoptEvidence'd) is a staging bug and fails
+    /// closed with LOGICAL_ERROR (a real write always records a dep for every leaf). No per-file probe.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// Stage + precommit a manifest naming a blob hash that was NEVER uploaded (no dep recorded).
+    auto build = startBuildFor(s, ns, "part_1");
+    const ManifestId mid = build->stageManifest({blobManifestEntry("data.bin", "never-uploaded")});
+    build->precommitAdd(ns, "part_1", mid);
+
+    /// promote must fail closed: the leaf has no tokened and no adopted dep ⇒ LOGICAL_ERROR. No ref committed.
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->promote(ns, "part_1", build->buildId(), mid);
+        },
+        "no tokened and no adopted dep");
+    EXPECT_FALSE(s->resolveRef(ns, "part_1").has_value());
+
+    /// After uploading the blob, a fresh build's promote succeeds — the same manifest content is now
+    /// fully present.
+    auto build2 = startBuildFor(s, ns, "part_1");
+    build2->putBlob(idOf("never-uploaded"), BlobSource::fromString("never-uploaded"));
+    const ManifestId mid2 = build2->stageManifest({blobManifestEntry("data.bin", "never-uploaded")});
+    build2->precommitAdd(ns, "part_1", mid2);
+    EXPECT_NO_THROW(build2->promote(ns, "part_1", build2->buildId(), mid2));
+    EXPECT_TRUE(s->resolveRef(ns, "part_1").has_value());
+}
+
+TEST(CASPartWriteTxn, AdoptEvidenceRecordsTokenlessDep)
+{
+    /// Port of AdoptFromTreeRecordsEvidence. adoptEvidence records a TOKENLESS W-EVIDENCE dep directly
+    /// from a resolved ManifestEntry — a Blob entry is tokenless (depIsTokened false), an Inline entry
+    /// records nothing. §4: whether the dep is a committed-source adopt vs absent (adopted vs no-dep) is
+    /// asserted end-to-end at the promote gate by PromoteTrustsAdoptedLeafNoProbeManifestTrust (positive:
+    /// adopted leaf ⇒ trusted, no probe) and PromoteCondemnedLeafWithoutDepAbortsFailClosed (negative
+    /// control: no dep ⇒ fail closed, LOGICAL_ERROR).
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    auto build = s->beginPartWrite({});
+
+    const ManifestEntry adopted = blobManifestEntry("data.bin", "source-blob");
+    build->adoptEvidence(adopted);
+    EXPECT_FALSE(build->depIsTokened(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("source-blob"))}));
+
+    /// An Inline entry references no standalone object → records nothing (never tokened).
+    ManifestEntry inline_entry;
+    inline_entry.path = "small";
+    inline_entry.placement = EntryPlacement::Inline;
+    inline_entry.inline_bytes = "abc";
+    build->adoptEvidence(inline_entry);
+    EXPECT_FALSE(build->depIsTokened(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("abc"))}));
+}
+
+TEST(CASPartWriteTxn, AbandonRemovesStagedDebrisAndDisables)
+{
+    /// Port of AbandonLeavesDebrisAndDisables to the new abandon semantics (CasPartWriteTxn.cpp abandon):
+    /// abandon best-effort exact-token-DELETEs this build's STAGED manifest debris, leaves blob bodies
+    /// (full GC's job via min_active), and disables the build (further ops throw via requireAlive).
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    auto build = startBuildFor(s, ns, "ref");
+
+    auto blob_ref = build->putBlob(idOf("kept"), BlobSource::fromString("kept"));
+    const ManifestId mid = build->stageManifest({blobManifestEntry("f", "kept")});
+
+    /// The staged manifest body and the blob are present before abandon.
+    EXPECT_TRUE(b->head(s->layout().blobKey(blob_ref.ref)).exists);
+    EXPECT_TRUE(b->head(s->layout().manifestKey(mid)).exists);
+
+    build->abandon();
+
+    /// Blob stays (debris — full GC reclaims it). The staged manifest debris is best-effort cleaned now.
+    EXPECT_TRUE(b->head(s->layout().blobKey(blob_ref.ref)).exists);
+    EXPECT_FALSE(b->head(s->layout().manifestKey(mid)).exists)
+        << "abandon must best-effort delete this build's staged manifest debris";
+
+    /// Further operations throw via requireAlive.
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->putBlob(idOf("after"), BlobSource::fromString("after"));
+        },
+        "has been abandoned");
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->stageManifest({blobManifestEntry("g", "kept")});
+        },
+        "has been abandoned");
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->precommitAdd(ns, "ref", mid);
+        },
+        "has been abandoned");
+}
+
+TEST(CASPartWriteTxn, PublishHappyPathRoundTrip)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    auto build = startBuildFor(s, ns, "part_1");
+
+    auto blob = build->putBlob(idOf("hello world"), BlobSource::fromString("hello world"));
+    EXPECT_EQ(blob.size, 11u);
+
+    const ManifestId id = build->stageManifest({blobManifestEntry("data.bin", "hello world")});
+    build->precommitAdd(ns, "part_1", id);
+    build->promote(ns, "part_1", build->buildId(), id);
+
+    auto r = s->resolveRef(ns, "part_1");
+    ASSERT_TRUE(r.has_value());
+    EXPECT_EQ(r->manifest_id, id);
+
+    /// Read the manifest back and locate its single blob leaf.
+    const PartManifest manifest = s->readManifest(id);
+    ASSERT_EQ(manifest.entries.size(), 1u);
+    const auto * entry = findEntry(manifest.entries, "data.bin");
+    ASSERT_TRUE(entry != nullptr);
+    const auto loc = s->locate(*entry);
+    auto got = b->get(loc.key, Range{loc.offset, loc.length});
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, "hello world");
+}
+
+TEST(CASPartWriteTxn, PromoteCrossNamespaceManifestFailsClosed)
+{
+    /// Port of PublishRequiresTreeInDepSet. The W-DEP-SET "root must be a built/adopted dep" authority
+    /// is gone (the tree object model it guarded is gone); the surviving fail-closed authority that
+    /// refuses an inconsistent commit target is the namespace consistency check in precommitAdd/promote
+    /// (CasPartWriteTxn.cpp): a manifest whose root_namespace != the target namespace is a bug ⇒ LOGICAL_ERROR.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    const RootNamespace other_ns{"srv1/other"};
+
+    auto build = startBuildFor(s, ns, "part_1");
+    build->putBlob(idOf("hello world"), BlobSource::fromString("hello world"));
+    /// The manifest is minted in `ns` (derived from intended_ref). Promoting/precommitting it into a
+    /// DIFFERENT namespace must fail closed.
+    const ManifestId id = build->stageManifest({blobManifestEntry("data.bin", "hello world")});
+
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->precommitAdd(other_ns, "part_1", id);
+        },
+        "precommitAdd: manifest namespace");
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->promote(other_ns, "part_1", build->buildId(), id);
+        },
+        "promote: manifest namespace");
+}
+
+/// (CASPartWriteTxn.PublishOwnThreadConflictRetries was removed with the legacy mutable ref-shard lane: it
+/// injected a Conflict on the promote's shard `casPut` and asserted the shard re-read/retry. The ref model
+/// has no shard CAS -- promote appends a write-once ref-log object via `putIfAbsentControlled`, and an
+/// uncertain create is resolved by exact-key observation, covered by the ref-writer uncertain-result tests
+/// (`gtest_cas_ref_writer.cpp`), not a CAS retry.)
+
+TEST(CASPartWriteTxn, PublishIntoSecondNamespaceSameBlob)
+{
+    /// Port of PublishIntoSecondNamespaceSameTree. A part manifest is single-owner and namespace-qualified
+    /// (precommitAdd/promote enforce id.root_namespace == target_ns), so the SAME ManifestId cannot be
+    /// published into two namespaces — each namespace gets its OWN manifest. The invariant the original
+    /// test protected is preserved at the BLOB plane: the shared blob is uploaded ONCE and adopted by the
+    /// second build (its token is unchanged after the second publish).
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns1{"srv1/tbl"};
+    const RootNamespace ns2{"srv1/tbl/detached"};
+
+    /// First build publishes part_1 in ns1, uploading the blob.
+    auto build1 = startBuildFor(s, ns1, "part_1");
+    auto blob = build1->putBlob(idOf("hello world"), BlobSource::fromString("hello world"));
+    const String blob_key = s->layout().blobKey(blob.ref);
+    const Token blob_token = b->head(blob_key).token;
+    const ManifestId id1 = build1->stageManifest({blobManifestEntry("data.bin", "hello world")});
+    build1->precommitAdd(ns1, "part_1", id1);
+    build1->promote(ns1, "part_1", build1->buildId(), id1);
+
+    /// Second build publishes part_1 in ns2 referencing the SAME blob: putBlob dedup-hits and ADOPTS the
+    /// present incarnation (no re-upload), so the blob token is unchanged. Wiring order
+    /// (EDGE-BEFORE-OBSERVE): stageManifest -> precommitAdd -> putBlob -> promote.
+    auto build2 = startBuildFor(s, ns2, "part_1");
+    const ManifestId id2 = build2->stageManifest({blobManifestEntry("data.bin", "hello world")});
+    build2->precommitAdd(ns2, "part_1", id2);
+    build2->putBlob(idOf("hello world"), BlobSource::fromString("hello world"));
+    build2->promote(ns2, "part_1", build2->buildId(), id2);
+
+    auto r1 = s->resolveRef(ns1, "part_1");
+    auto r2 = s->resolveRef(ns2, "part_1");
+    ASSERT_TRUE(r1.has_value());
+    ASSERT_TRUE(r2.has_value());
+    EXPECT_EQ(r1->manifest_id, id1);
+    EXPECT_EQ(r2->manifest_id, id2);
+
+    /// The blob object was uploaded once: its token is unchanged after both publishes.
+    EXPECT_EQ(b->head(blob_key).token, blob_token);
+}
+
+/// Task 10: refs are no longer sharded (one whole-table cache per namespace, spec §Table State), so
+/// there is no more "same shard" CAS-conflict-retry to force — two builds publishing into the SAME
+/// TABLE now serialize through the append lane's per-namespace batching queue instead (exercised by
+/// gtest_cas_ref_writer.cpp's co-batch/queue tests). What remains a real regression to guard is the
+/// end-to-end outcome: two builds publishing distinct refs into one namespace both land correctly.
+TEST(CASPartWriteTxn, TwoBuildsPublishToSameNamespaceBothLand)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    const String ref1 = "a";
+    const String ref2 = "b";
+
+    auto build_a = startBuildFor(s, ns, ref1);
+    build_a->putBlob(idOf("content-a"), BlobSource::fromString("content-a"));
+    const ManifestId id_a = build_a->stageManifest({blobManifestEntry("data.bin", "content-a")});
+    build_a->precommitAdd(ns, ref1, id_a);
+    build_a->promote(ns, ref1, build_a->buildId(), id_a);
+
+    auto build_b = startBuildFor(s, ns, ref2);
+    build_b->putBlob(idOf("content-b"), BlobSource::fromString("content-b"));
+    const ManifestId id_b = build_b->stageManifest({blobManifestEntry("data.bin", "content-b")});
+    build_b->precommitAdd(ns, ref2, id_b);
+    build_b->promote(ns, ref2, build_b->buildId(), id_b);
+
+    auto r1 = s->resolveRef(ns, ref1);
+    auto r2 = s->resolveRef(ns, ref2);
+    ASSERT_TRUE(r1.has_value());
+    ASSERT_TRUE(r2.has_value());
+    EXPECT_EQ(r1->manifest_id, id_a);
+    EXPECT_EQ(r2->manifest_id, id_b);
+    EXPECT_EQ(s->listRefs(ns).size(), 2u);
+}
+
+TEST(CASPartWriteTxn, FirstPublishMakesNamespaceDiscoverable)
+{
+    /// After Task 4 the registry is deleted; the first publication admits the namespace to the
+    /// authoritative catalog before appending its stream record.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"srv9/fresh"};
+
+    EXPECT_TRUE(s->listNamespaces("").namespaces.empty());
+    publishOneBlobPart(s, ns, "part_1", "f", "reg-payload");
+
+    /// The namespace is now discoverable from the catalog -- no registry write needed.
+    const auto all = s->listNamespaces("").namespaces;
+    ASSERT_EQ(all.size(), 1u);
+    EXPECT_EQ(all[0], "srv9/fresh");
+}
+
+TEST(CASPartWriteTxn, AdoptEvidenceNoBackendOp)
+{
+    /// B188: adoptEvidence records a TOKENLESS W-EVIDENCE dep from an already-resolved ManifestEntry
+    /// WITHOUT any backend call (no HEAD, no GET, no PUT).
+    ///
+    /// Two behavioural assertions:
+    ///   1. No backend op fires during adoptEvidence (counted via a delegating wrapper).
+    ///   2. The recorded dep is tokenless: after adoptEvidence(entry), depIsTokened is false (the
+    ///      W-EVIDENCE dep is what the promote gate later revalidates).
+
+    /// A delegating wrapper that counts every backend access path including the streaming write path.
+    struct LocalCountingBackend final : public Backend
+    {
+        explicit LocalCountingBackend(BackendPtr inner_) : inner(std::move(inner_)) {}
+        size_t heads = 0;
+        size_t stream_puts = 0;
+        size_t gets = 0;
+
+        HeadResult head(const String & k) override { ++heads; return inner->head(k); }
+        WriteSinkPtr putIfAbsentStream(const String & k, const ObjectMeta & meta) override
+        {
+            ++stream_puts;
+            return inner->putIfAbsentStream(k, meta);
+        }
+        std::optional<GetResult> get(const String & k, Range r) override { ++gets; return inner->get(k, r); }
+        std::optional<GetStreamResult> getStream(const String & k, Range r) override { return inner->getStream(k, r); }
+        ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+        PutResult putIfAbsent(const String & k, const String & bts, const ObjectMeta & m) override { return inner->putIfAbsent(k, bts, m); }
+        PutResult putOverwrite(const String & k, const String & bts, const Token & e, const ObjectMeta & m) override { return inner->putOverwrite(k, bts, e, m); }
+        Token resurrect(DB::ReadBuffer & payload, uint64_t payload_size, const String & k, const String & fresh_header) override { return inner->resurrect(payload, payload_size, k, fresh_header); }
+        CasResult casPut(const String & k, const String & bts, const std::optional<Token> & e, const ObjectMeta & m) override { return inner->casPut(k, bts, e, m); }
+        DeleteOutcome deleteExact(const String & k, const Token & t) override { return inner->deleteExact(k, t); }
+        bool supportsListTokens() const override { return inner->supportsListTokens(); }
+    private:
+        BackendPtr inner;
+    };
+
+    auto raw = std::make_shared<InMemoryBackend>();
+    auto counting = std::make_shared<LocalCountingBackend>(raw);
+    auto s = Pool::open(counting, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    auto build = s->beginPartWrite({});
+
+    /// A Blob ManifestEntry. adoptEvidence is called on a hand-crafted entry — that IS the B188 interface.
+    const ManifestEntry entry = blobManifestEntry("b188.bin", "b188-content");
+
+    /// Reset the counters after Pool::open (which may HEAD/GET gc/server-roots etc. during startup).
+    counting->heads = 0;
+    counting->stream_puts = 0;
+    counting->gets = 0;
+
+    /// adoptEvidence — must record the dep WITHOUT touching the backend.
+    EXPECT_NO_THROW(build->adoptEvidence(entry));
+    EXPECT_EQ(counting->heads, 0u) << "adoptEvidence must not HEAD the backend";
+    EXPECT_EQ(counting->stream_puts, 0u) << "adoptEvidence must not PUT to the backend";
+    EXPECT_EQ(counting->gets, 0u) << "adoptEvidence must not GET from the backend";
+
+    /// The dep is recorded — a tokenless W-EVIDENCE dep (depIsTokened false) that the promote gate later
+    /// revalidates; the no-backend-op counts above are the B188 contract's primary guard.
+    EXPECT_FALSE(build->depIsTokened(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("b188-content"))}));
+
+    /// Inline entry: adoptEvidence records nothing (Inline has no standalone object) and no backend op.
+    ManifestEntry inline_entry;
+    inline_entry.path = "small";
+    inline_entry.placement = EntryPlacement::Inline;
+    inline_entry.inline_bytes = "xy";
+    EXPECT_NO_THROW(build->adoptEvidence(inline_entry));
+    EXPECT_EQ(counting->heads, 0u);
+    EXPECT_EQ(counting->stream_puts, 0u);
+    EXPECT_EQ(counting->gets, 0u);
+    EXPECT_FALSE(build->depIsTokened(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of("xy"))}));
+}
+
+TEST(CASPartWriteTxn, ConvergesUnderProductiveGc)
+{
+    /// B167/B171 LIVENESS — the re-upload/condemn livelock, now closed by the build-root precommit edge.
+    ///
+    /// THE BUG (before the fix): a blob H was referenced, dropped, and GC-condemned (everEdged ∧ InDeg=0,
+    /// condemned in the retire view). A NEW build dedup-HITS H by content and must re-upload it from
+    /// source — it re-streams a FRESH incarnation of H. But the productive GC, re-deriving H as a
+    /// zero-in-degree candidate every round, kept RE-CONDEMNING and exact-token-DELETING that fresh
+    /// incarnation in the build's upload→commit window. The build never converged → livelock.
+    ///
+    /// THE FIX (B171): protection is the build-root PRECOMMIT EDGE. PartWriteTxn B precommits its manifest (naming
+    /// H) BEFORE the adversarial loop, so the GC fold lifts H to in-degree ≥ 1 — H is never even a
+    /// zero-in-degree candidate and is SPARED every round until B promotes (the committed ref then pins H).
+    ///
+    /// FORM: full adversarial loop. A real Gc drives complete runRegularRound rounds against the same
+    /// pool while build B holds an active watermark covering H's incarnation. We assert H is SPARED
+    /// every round and that B promotes within a BOUNDED number of GC rounds, after which H reads back.
+    auto b = std::make_shared<InMemoryBackend>();
+    const RootNamespace ns{"srv1/tbl"};
+
+    PoolConfig cfg;
+    cfg.pool_prefix = "p";
+    cfg.server_root_id = "test";
+    cfg.server_id = UInt128(0xAB);
+    cfg.background_watermark = false;
+    const String content = "shared-content";
+
+    /// 1. PartWriteTxn A creates H ("shared-content"), publishes a part referencing it, then drops the ref.
+    ///    Capture H's first incarnation token so we can condemn exactly it.
+    BlobRef h;
+    Token h_token0;
+    {
+        auto s0 = Pool::open(b, cfg);
+        publishOneBlobPart(s0, ns, "part_1", "f", content);
+        h = idOf(content);
+        h_token0 = b->head(s0->layout().blobKey(h)).token;
+        s0->dropRef(ns, "part_1");
+    }
+
+    /// 2. Condemn (Blob, H, h_token0): `injectRetire` seeds the LEDGER (a real round's later settle/spare
+    ///    of h_token0 rides this entry — the adversarial loop below still exercises that), and v3's
+    ///    `condemnMeta` seeds the per-hash META (the writer's condemned decision is now a point-read of
+    ///    it, not the retire-view — Task 3). `publishOneBlobPart` already created H's meta as Clean, so
+    ///    condemnMeta's read-modify-CAS finds it. Together these reproduce exactly what a real GC condemn
+    ///    now writes (Task 5), without driving a full round just to observe H at in-degree 0.
+    DB::Cas::Layout layout("p");
+    injectRetire(*b, layout, /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(content))}, .token = h_token0,
+                      .size = content.size()}});
+    condemnMeta(*b, layout, u128Of(content), /*condemn_round*/ 1);
+
+    /// 3. Open the live Pool and start build B. B dedup-hits the condemned H and re-uploads from
+    ///    source (uploadFromSource via putBlob): a fresh incarnation, a NEW token. B stays ACTIVE for
+    ///    the whole adversarial loop — its build_seq is never retired below.
+    auto s = Pool::open(b, cfg);
+    const String blob_key = s->layout().blobKey(h);
+    auto build_b = startBuildFor(s, ns, "part_2");
+
+    /// B190: use putBlob (holds source bytes). putBlob detects the condemned dedup hit and calls
+    /// uploadFromSource — no GET of dying object.
+    const auto ref_b = build_b->putBlob(h, BlobSource::fromString(content));
+    ASSERT_EQ(ref_b.ref, h);
+
+    const HeadResult after_reupload = b->head(blob_key);
+    ASSERT_TRUE(after_reupload.exists);
+    EXPECT_NE(after_reupload.token, h_token0);   /// a genuinely fresh incarnation
+
+    /// 4. PartWriteTxn B stages its manifest referencing H and PRECOMMITS it (build-root edge). H is now
+    ///    protected by reachability: the GC fold lifts H to in-degree ≥ 1 from the precommit.
+    const ManifestId mid_b = build_b->stageManifest({blobManifestEntry("f", content)});
+    build_b->precommitAdd(ns, "part_2", mid_b);
+
+    /// 5. THE ADVERSARIAL LOOP. A real, productive GC keeps trying to reclaim. It reclaims the now-
+    ///    unreferenced part_1 manifest (build A's, UNprotected) but H stays pinned by B's PRECOMMIT edge
+    ///    (in-degree ≥ 1), so H is never even a zero-in-degree candidate. We drive far more rounds than B
+    ///    needs to promote; H must survive ALL of them. Each round renews B's watermark so the crash
+    ///    detector keeps judging B live.
+    Gc gc(s, hexToU128("00000000000000000000000000000001"));
+    constexpr int MAX_GC_ROUNDS = 8;
+
+    const auto driveRoundAndAssertHSpared = [&](int round_no)
+    {
+        /// A LIVE server renews its watermark continuously. Renew once per GC round so B's watermark seq
+        /// ADVANCES between rounds — that is precisely what distinguishes a live server from a crashed one
+        /// (a frozen B would have its precommit reclaimed; an advancing seq keeps it).
+        s->renewWatermarkOnce();
+        gc.runRegularRound();
+        const HeadResult hr = b->head(blob_key);
+        ASSERT_TRUE(hr.exists) << "H was deleted by GC at round " << round_no
+                               << " despite being pinned by the live build B's precommit (B167 livelock would do this)";
+        const auto raw = b->get(blob_key);
+        ASSERT_TRUE(raw.has_value());
+        const auto hdr = decodeEnvelopeHeader(raw->bytes, raw->bytes.size(), ObjectKind::Blob);
+        EXPECT_EQ(raw->bytes.substr(hdr.header_len), content)
+            << "H's content was lost/corrupted at round " << round_no;
+    };
+
+    /// Phase 1 — the livelock window. H is referenced by NO committed TABLE ref (B has not promoted yet)
+    /// but IS named by B's precommit, so the build-root fold lifts it to in-degree ≥ 1. Drive several
+    /// full rounds; the precommit edge must SPARE H's fresh incarnation every round.
+    int rounds_run = 0;
+    constexpr int PRE_PUBLISH_ROUNDS = 4;
+    for (int i = 0; i < PRE_PUBLISH_ROUNDS; ++i)
+    {
+        driveRoundAndAssertHSpared(++rounds_run);
+        if (::testing::Test::HasFatalFailure())
+            return;
+    }
+
+    /// Phase 2 — converge. With H still alive (spared through the whole window), build B promotes a part
+    /// referencing it. The promote gate sees H present + live (fresh incarnation uploaded above), so it
+    /// commits. This MUST succeed — the build converges in bounded steps.
+    build_b->promote(ns, "part_2", build_b->buildId(), mid_b);
+    const bool published = true;
+
+    /// Phase 3 — keep the GC hammering after promote. H is now pinned by the committed ref's manifest
+    /// edge; the GC must keep sparing it as a genuinely-reachable node.
+    while (rounds_run < MAX_GC_ROUNDS)
+    {
+        driveRoundAndAssertHSpared(++rounds_run);
+        if (::testing::Test::HasFatalFailure())
+            return;
+    }
+
+    /// 6. ASSERT convergence: promote SUCCEEDED within the bounded budget, and H reads back intact.
+    ASSERT_TRUE(published) << "build B never published — the B167 livelock is back";
+    EXPECT_LE(rounds_run, MAX_GC_ROUNDS);
+
+    const auto resolved = s->resolveRef(ns, "part_2");
+    ASSERT_TRUE(resolved.has_value());
+    EXPECT_EQ(resolved->manifest_id, mid_b);
+
+    const PartManifest manifest = s->readManifest(mid_b);
+    ASSERT_EQ(manifest.entries.size(), 1u);
+    const auto * entry = findEntry(manifest.entries, "f");
+    ASSERT_TRUE(entry != nullptr);
+    const auto loc = s->locate(*entry);
+    const auto got = b->get(loc.key, Range{loc.offset, loc.length});
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, content);
+}
+
+/// BUG 1 (WPromote owner==bld): promote is a PURE owner MOVE (Δ=0 — it restores no blob in-degree). The
+/// TLA+ `WPromote` requires the precommit to STILL be the live owner of the ref before the move (`owner[m]
+/// = bld`). If the precommit was removed/reclaimed (an abandon or GC reclaim appended a removal event), a
+/// Δ=0 move would re-publish a committed ref over blobs whose in-degree was already decremented to 0 — GC
+/// then deletes them ⇒ a reachable committed manifest with dangling blobs (INV_NO_DANGLE violation).
+/// promote MUST fail closed (ABORTED) unless the precommit is the current live owner binding of the ref.
+TEST(CASPartWriteTxn, PromoteFailsClosedWhenPrecommitNoLongerLiveOwner)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    auto build = startBuildFor(s, ns, "part_1");
+
+    build->putBlob(idOf("hello world"), BlobSource::fromString("hello world"));
+    const ManifestId id = build->stageManifest({blobManifestEntry("data.bin", "hello world")});
+    build->precommitAdd(ns, "part_1", id);
+
+    /// Make the precommit NO LONGER the live owner: append an exact precommit-removal ref-log
+    /// transaction exactly as an abandon / GC reclaim would (spec §Remove Precommit) -- via the SAME
+    /// public append lane a real abandon/reclaim would use, simulating an external actor this build
+    /// object does not know about (not this build's own `abandon()`, which would also retire it and
+    /// mask the "precommit no longer live" guard behind requireAlive()'s own rejection).
+    s->appendRefOps(ns, MutationScope::ref("part_1"),
+        [&](const RefTableState &) -> std::vector<RefOp>
+        {
+            RefOp op;
+            op.kind = RefOpKind::OwnerTransition;
+            op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "part_1", id.ref};
+            return {op};
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Abandon);
+
+    /// promote must fail closed: the precommit is no longer the live owner, so a Δ=0 move would dangle.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR,
+        [&] { build->promote(ns, "part_1", build->buildId(), id); });
+    /// No ref committed.
+    EXPECT_FALSE(s->resolveRef(ns, "part_1").has_value());
+}
+
+/// BUG 1 happy path: a promote whose precommit is STILL the live owner succeeds (the guard must not
+/// reject the normal commit). Distinct from PublishHappyPathRoundTrip in that it pins the WPromote guard.
+TEST(CASPartWriteTxn, PromoteSucceedsWhenPrecommitIsLiveOwner)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    auto build = startBuildFor(s, ns, "part_1");
+
+    build->putBlob(idOf("hello world"), BlobSource::fromString("hello world"));
+    const ManifestId id = build->stageManifest({blobManifestEntry("data.bin", "hello world")});
+    build->precommitAdd(ns, "part_1", id);
+
+    EXPECT_NO_THROW(build->promote(ns, "part_1", build->buildId(), id));
+    ASSERT_TRUE(s->resolveRef(ns, "part_1").has_value());
+    EXPECT_EQ(s->resolveRef(ns, "part_1")->manifest_id, id);
+}
+
+/// all-tree-part-files Task 2 (TLA+ `WRepoint`):
+/// `promote`'s existing unique-ref guard (BUG 1a) refuses to overwrite a committed ref naming a
+/// DIFFERENT manifest -- correct for an ACCIDENTAL double-publish, but there is no way to perform an
+/// INTENDED repoint (a standalone write/remove on an already-committed part) without it. `allow_repoint`
+/// opts into exactly that: the guard's throw is skipped, and the committed-transition RefOp (old =
+/// the currently-committed manifest, new = the incoming one) is appended in the SAME ref-log record as
+/// the ordinary precommit->committed promotion -- the C++ realization of `WRepoint`'s one-event
+/// old-binding/new-binding shape (Phase 0, task-1 gate). Without the flag, behavior is BYTE-IDENTICAL
+/// to today (BUG 1a still fires).
+TEST(CASPartWriteTxnRepoint, PromoteRepointsCommittedRef)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    /// The sink target must outlive the Pool: `~Pool` emits terminate events into the sink.
+    std::vector<CasEvent> events;
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// Publish ref "part_1" -> M1 through the normal build path.
+    auto build1 = startBuildFor(s, ns, "part_1");
+    build1->putBlob(idOf("m1"), BlobSource::fromString("m1"));
+    const ManifestId m1_id = build1->stageManifest({blobManifestEntry("data.bin", "m1")});
+    build1->precommitAdd(ns, "part_1", m1_id);
+    build1->promote(ns, "part_1", build1->buildId(), m1_id);
+    ASSERT_TRUE(s->resolveRef(ns, "part_1").has_value());
+    EXPECT_EQ(s->resolveRef(ns, "part_1")->manifest_id, m1_id);
+
+    /// A second build stages M2 (one extra entry) onto the SAME ref.
+    auto build2 = startBuildFor(s, ns, "part_1");
+    build2->putBlob(idOf("m2"), BlobSource::fromString("m2"));
+    build2->putBlob(idOf("m2x"), BlobSource::fromString("m2x"));
+    const ManifestId m2_id = build2->stageManifest({blobManifestEntry("data.bin", "m2"), blobManifestEntry("extra.bin", "m2x")});
+    build2->precommitAdd(ns, "part_1", m2_id);
+
+    /// allow_repoint = false (the default) -> NETWORK_ERROR (CAS write-retry-later), existing invariant
+    /// untouched; M1 still resolves.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR,
+        [&] { build2->promote(ns, "part_1", build2->buildId(), m2_id); });
+    EXPECT_EQ(s->resolveRef(ns, "part_1")->manifest_id, m1_id);
+
+    /// The failed no-flag attempt threw BEFORE appendRefOps returned, so build2's precommit is still the
+    /// live owner (no removal was appended) -- the SAME build/manifest can be retried with the flag.
+    s->setEventSink([&](const CasEvent & e) { events.push_back(e); });
+    EXPECT_NO_THROW(build2->promote(ns, "part_1", build2->buildId(), m2_id, /*allow_repoint=*/true));
+    auto resolved = s->resolveRef(ns, "part_1");
+    ASSERT_TRUE(resolved);
+    EXPECT_EQ(resolved->manifest_id.ref, m2_id.ref);
+
+    /// Every effective repoint is loud (spec §4): exactly one RefRepoint event, naming the ref and the
+    /// old manifest it replaced.
+    size_t repoint_events = 0;
+    for (const CasEvent & e : events)
+        if (e.type == CasEventType::RefRepoint)
+        {
+            ++repoint_events;
+            EXPECT_EQ(e.ref_name, "part_1");
+            EXPECT_EQ(e.detail.at("old_manifest"), manifestRefDebugString(m1_id.ref));
+        }
+    EXPECT_EQ(repoint_events, 1u);
+}
+
+/// BUG 2 (WAbandonPrecommit; delete-after-sealed-decrements): once `precommitAdd` has made a manifest a
+/// LIVE precommit owner input, `abandon` must NOT writer-delete its body. The TLA+ `WAbandonPrecommit`
+/// appends a REMOVAL event (`old = precommit(build_id, final_ref, T)`, `new = none`) and NEVER deletes
+/// the body — GC decrements the precommit's blob edges and deletes the body only after the decrement is
+/// sealed. Writer-deleting a live precommit body strands GC's fold barrier (live precommit, missing body
+/// → clamp forever) or loses the activating +1.
+TEST(CASPartWriteTxn, AbandonAppendsPrecommitRemovalAndKeepsLivePrecommitBody)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    auto build = startBuildFor(s, ns, "part_1");
+
+    build->putBlob(idOf("kept"), BlobSource::fromString("kept"));
+    const ManifestId mid = build->stageManifest({blobManifestEntry("data.bin", "kept")});
+    const String manifest_key = s->layout().manifestKey(mid);
+    const UInt128 abandoned_build_id = build->buildId();
+    build->precommitAdd(ns, "part_1", mid);
+
+    /// The precommit manifest body is present before abandon.
+    ASSERT_TRUE(b->head(manifest_key).exists);
+
+    build->abandon();
+
+    /// (a) the LIVE precommit body must SURVIVE abandon (left for GC after the sealed decrement).
+    EXPECT_TRUE(b->head(manifest_key).exists)
+        << "abandon must NOT writer-delete a live precommit body (delete-after-sealed-decrements)";
+
+    /// (b) the exact precommit binding is gone (spec §Remove Precommit: an exact owner_transition
+    /// removal, old=Precommit new=none). Proven black-box: a FRESH precommitAdd for the ref must
+    /// succeed -- if abandon had failed to remove the exact binding, this would instead throw
+    /// CORRUPTED_DATA ("add precommit ... already exists"). The probe manifest must be freshly staged
+    /// BY `rebuild` itself rather than re-precommitting `mid` (which `build`, a different transaction,
+    /// staged): A3 mint-tightening now refuses an unowned `ManifestId` from any transaction other than
+    /// the one that minted it, regardless of whether abandon's removal landed, so re-using `mid` here
+    /// would no longer distinguish the property under test. Content identity is irrelevant to the
+    /// ref-level owner-slot check this proves, so a fresh manifest is just as conclusive a probe.
+    (void)abandoned_build_id;
+    auto rebuild = startBuildFor(s, ns, "part_1");
+    const ManifestId rebuild_mid = rebuild->stageManifest({blobManifestEntry("data.bin", "kept")});
+    EXPECT_NO_THROW(rebuild->precommitAdd(ns, "part_1", rebuild_mid));
+}
+
+/// BUG 2 regression for the never-precommitted path: a manifest that was STAGED but never precommitted is
+/// still best-effort writer-deleted by abandon (pre-precommit debris) — only a LIVE precommit body is
+/// spared. Confirms the fix narrows the skip to the precommitted manifest exactly.
+TEST(CASPartWriteTxn, AbandonStillDeletesNeverPrecommittedStagedDebris)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    auto build = startBuildFor(s, ns, "part_1");
+
+    build->putBlob(idOf("kept"), BlobSource::fromString("kept"));
+    /// Two staged manifests: one becomes the precommit, the other is pure pre-precommit debris.
+    const ManifestId debris = build->stageManifest({blobManifestEntry("debris.bin", "kept")});
+    const ManifestId precommitted = build->stageManifest({blobManifestEntry("data.bin", "kept")});
+    build->precommitAdd(ns, "part_1", precommitted);
+
+    build->abandon();
+
+    /// The never-precommitted debris is best-effort deleted; the live precommit body survives.
+    EXPECT_FALSE(b->head(s->layout().manifestKey(debris)).exists)
+        << "never-precommitted staged debris must still be best-effort deleted by abandon";
+    EXPECT_TRUE(b->head(s->layout().manifestKey(precommitted)).exists)
+        << "the live precommit body must be spared";
+}
+
+/// Task 6 (review finding 2): `abandon()`'s three audit `EventEmitter{*store}.emit(...)` calls are each
+/// wrapped `try { ... } catch (...) { tryLogCurrentException(...); }`, mirroring `promote`'s own
+/// post-durable emit guard -- a throwing sink (e.g. a bad_alloc growing the system-log queue, or a
+/// Context/log-shutdown edge) must never turn an otherwise-successful abandon into a reported failure.
+TEST(CASPartWriteTxn, AbandonSwallowsThrowingEventSink)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl_abandon_sink"};
+    auto build = startBuildFor(s, ns, "part_1");
+
+    build->putBlob(idOf("kept"), BlobSource::fromString("kept"));
+    const ManifestId mid = build->stageManifest({blobManifestEntry("data.bin", "kept")});
+    build->precommitAdd(ns, "part_1", mid);
+
+    /// UNKNOWN_EXCEPTION (not LOGICAL_ERROR): mirrors `PromoteSwallowsPostDurableEventSinkFailure`
+    /// above -- this simulates an arbitrary observer/sink callback failing, not a CAS invariant
+    /// violation. LOGICAL_ERROR would abort the whole process under debug/sanitizer builds instead of
+    /// behaving like a catchable exception.
+    s->setEventSink([](const CasEvent &)
+    {
+        throw DB::Exception(DB::ErrorCodes::UNKNOWN_EXCEPTION, "injected event sink failure");
+    });
+
+    EXPECT_NO_THROW(build->abandon());
+    s->setEventSink(nullptr);
+
+    /// The precommit binding is gone despite the sink failure -- proven black-box exactly like
+    /// `AbandonAppendsPrecommitRemovalAndKeepsLivePrecommitBody` above: a FRESH precommitAdd for the
+    /// ref must succeed (it would instead throw CORRUPTED_DATA "add precommit ... already exists" had
+    /// the throwing sink aborted the removal). The probe manifest is freshly staged BY `rebuild`
+    /// itself, not `mid` (staged by `build`, a different transaction) -- A3 mint-tightening now refuses
+    /// a foreign id unconditionally, so re-using `mid` would no longer isolate the property under test.
+    auto rebuild = startBuildFor(s, ns, "part_1");
+    const ManifestId rebuild_mid = rebuild->stageManifest({blobManifestEntry("data.bin", "kept")});
+    EXPECT_NO_THROW(rebuild->precommitAdd(ns, "part_1", rebuild_mid));
+}
+
+namespace
+{
+
+/// Forces the SINGLE ref-log ('_log/' key) PUT that `abandon()`'s precommit-removal `appendRefOps`
+/// issues to observe a PROVEN conflict instead of a genuine ambiguity. Mirrors
+/// `RefWriterTestBackend::corrupt_key_substr` (gtest_cas_ref_writer.cpp, reproduced locally because
+/// that class lives in a different translation unit): landing a DIFFERENT object at the intended key
+/// makes `putIfAbsentControlled`'s resolve-before-reissue observe a real conflict and throw
+/// CORRUPTED_DATA -- a CONCLUSIVE rejection ("do NOT wedge: the cache is unchanged and nothing of ours
+/// is durable", CasRefLedger.cpp's `commitRefChunk`), unlike a genuinely-ambiguous timeout, which would
+/// instead WEDGE the whole table's append lane (`rt->append_attempt`) until the SAME key resolves durable -- a
+/// state a one-shot fault can never itself clear, since wedge resolution only re-GETs the intended key
+/// and never re-PUTs it (proven by
+/// `CASRefWriterAppendLane.WedgedLaneBlocksSameTableWhileOtherTableProceeds`). A conflict leaves the cached
+/// ref-table state untouched, so the SAME logical retry reaches its append again -- and under INV-1 that
+/// retry re-derives the SAME id from that unchanged state, so it meets the same occupant rather than
+/// carving a fresh id around it.
+class RefLogConflictOnceBackend final : public InMemoryBackend
+{
+public:
+    String corrupt_key_substr;
+    int corrupt_count = 0;
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (corrupt_count > 0 && !corrupt_key_substr.empty() && key.find(corrupt_key_substr) != String::npos)
+        {
+            --corrupt_count;
+            /// The 3-arg qualified call bypasses virtual dispatch entirely (unlike the 2-arg
+            /// convenience overload, which would re-enter this very override through the vtable).
+            InMemoryBackend::putIfAbsent(key, bytes + String("\x01_FOREIGN_DIFFERENT"), meta);
+            throw Poco::TimeoutException("RefLogConflictOnceBackend: a foreign different object landed; response lost");
+        }
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+};
+
+}
+
+/// Task 6 (review finding 2): `alive` now flips to false only AFTER the correctness-bearing precommit
+/// removal's `appendRefOps` succeeds, so a caller that catches an append failure can retry `abandon()`
+/// on the SAME object. Before the fix, `alive = false` ran unconditionally before that append, so a
+/// retry would hit `requireAlive`'s "has been abandoned" LOGICAL_ERROR instead.
+TEST(CASPartWriteTxn, AbandonRetryableAfterAppendFailure)
+{
+    auto b = std::make_shared<RefLogConflictOnceBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"srv1/tbl_abandon_retry"};
+    /// Stage B (Task 4-C): pin `ns`'s real incarnation to the Stage-A sentinel BEFORE the first real
+    /// append, so the corruption injected below (at a key computed from that sentinel) actually lands
+    /// on the path production writes to -- otherwise `precommitAdd` mints an unrelated random
+    /// incarnation and the corruption below misses it entirely.
+    DB::Cas::tests::casAdmitRecoverableEntry(*b, s->layout(), ns, s->liveWriterEpoch());
+    auto build = startBuildFor(s, ns, "part_1");
+
+    build->putBlob(idOf("kept"), BlobSource::fromString("kept"));
+    const ManifestId mid = build->stageManifest({blobManifestEntry("data.bin", "kept")});
+    build->precommitAdd(ns, "part_1", mid);
+
+    b->corrupt_key_substr = s->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    b->corrupt_count = 1;
+
+    /// First abandon(): the precommit-removal appendRefOps' single PUT observes a foreign object at its
+    /// exact key (a proven conflict) -> CORRUPTED_DATA propagates.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { build->abandon(); });
+
+    /// The proven conflict fences the mount closed and schedules a remount (the append site routes
+    /// through the anomaly policy exactly as the wedge-resolve site does). Re-arming only the test
+    /// fence does not replace this runtime, so its immutable admitted generation remains stale and its
+    /// terminal `Faulted` lane remains blocked behind that outer refusal.
+    DB::Cas::tests::rearmMountFenceAfterAnomalyForTest(s);
+
+    /// The retryability under test: the SAME object accepts a second abandon() -- `alive` was not
+    /// flipped by the failed append, so this is not the "has been abandoned" condition the unfixed code
+    /// produced. It reaches immutable-runtime admission and is refused by the stale generation; only a
+    /// real remount may replace that runtime and reach a fresh lane.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { build->abandon(); });
+
+    /// The removal never landed, and nothing was written around the occupant: the precommit binding this
+    /// build owns is still live, exactly where the failed abandon left it. That is the honest end state
+    /// under the fail-closed contract -- the old proof (a fresh `precommitAdd` for the same ref
+    /// succeeding, which showed the binding gone) needed one more successful append on a table that can
+    /// no longer take one.
+    String greatest_key;
+    size_t foreign_objects = 0;
+    for (String cursor;;)
+    {
+        const ListPage page = b->list(b->corrupt_key_substr, cursor, 1000);
+        for (const auto & listed : page.keys)
+        {
+            if (listed.key > greatest_key)
+                greatest_key = listed.key;
+            const auto body = b->get(listed.key);
+            if (body && body->bytes.find("_FOREIGN_DIFFERENT") != String::npos)
+                ++foreign_objects;
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    EXPECT_EQ(foreign_objects, 1u) << "the foreign object must still own the key it took";
+    ASSERT_FALSE(greatest_key.empty());
+    const auto greatest_body = b->get(greatest_key);
+    ASSERT_TRUE(greatest_body.has_value());
+    EXPECT_NE(greatest_body->bytes.find("_FOREIGN_DIFFERENT"), String::npos)
+        << "the foreign occupant must still be the highest id in this table's stream: a log object above "
+           "it would mean an append carved a fresh id around the damage instead of failing closed";
+}
+
+/// ------------------------------------------------------------------------------------------------
+/// OQ7 manifest-cap fail-close (S07): the scenario suite tried to reach `stageManifest`'s encoded-bytes
+/// cap through a wide-column SQL `INSERT`, but the cap sits 3+ orders of magnitude above what dev SQL
+/// can reach in reasonable time (confirmed: even a 20000-column full-scale insert cannot get there). So
+/// this P0 safety path is not scenario-testable and is exercised directly here instead.
+/// ------------------------------------------------------------------------------------------------
+
+namespace
+{
+
+/// Mirrors `CasPartWriteTxn.cpp`'s private `kMaxManifestEncodedBytes` (256 MiB). There is no way to read a
+/// file-local `constexpr` from a different translation unit, so this is kept in sync by hand — if that
+/// cap ever changes, update this one to match.
+constexpr uint64_t kExpectedManifestEncodedCap = 256ULL << 20;
+
+/// The exact encoded size `PartWriteTxn::stageManifest` would compute for a single Blob-placement entry whose
+/// path is `path_len` bytes long, staged under `ns` — measured through the SAME `encodePartManifest`
+/// codec `stageManifest` calls, so this is an exact reproduction rather than a hand-derived estimate.
+/// `ref` and `payload_digest` are fixed-width fields (20 and 16 bytes respectively): their VALUES don't
+/// affect the encoded size, only their presence does, so the zero-valued placeholders here reproduce
+/// the exact same byte count `stageManifest` would produce with its real (non-zero) values.
+size_t manifestEncodedSizeForPathLen(const RootNamespace & ns, size_t path_len)
+{
+    PartManifest probe;
+    probe.root_namespace_id = ns;
+    ManifestEntry e;
+    e.path = String(path_len, 'a');
+    e.placement = EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(UInt128{})};
+
+    e.blob_size = 12345;
+    probe.entries = {std::move(e)};
+    return encodePartManifest(probe).size();
+}
+
+/// Finds the exact boundary: the SMALLEST `path_len` whose single-entry manifest encodes to MORE than
+/// `kExpectedManifestEncodedCap` bytes under `ns`. `path_len - 1` therefore encodes to AT MOST the cap
+/// (the encoding is monotonic in `path_len` — a longer path can only grow the encoded size). Starts
+/// from a linear estimate (the encoding is affine in `path_len`: fixed framing overhead plus a constant
+/// number of bytes per path byte) and walks to the exact crossing, so this stays correct even if the
+/// framing overhead changes, without needing a full binary search over a ~256 MiB range.
+size_t findManifestEncodedCapBoundaryPathLen(const RootNamespace & ns)
+{
+    constexpr size_t probe_lo = 1000;
+    constexpr size_t probe_hi = 2'000'000;
+    const size_t size_lo = manifestEncodedSizeForPathLen(ns, probe_lo);
+    const size_t size_hi = manifestEncodedSizeForPathLen(ns, probe_hi);
+    const double slope = static_cast<double>(size_hi - size_lo) / static_cast<double>(probe_hi - probe_lo);
+    const double intercept = static_cast<double>(size_lo) - slope * static_cast<double>(probe_lo);
+
+    size_t path_len = static_cast<size_t>(std::ceil(
+        (static_cast<double>(kExpectedManifestEncodedCap) - intercept) / slope)) + 1;
+
+    while (manifestEncodedSizeForPathLen(ns, path_len) <= kExpectedManifestEncodedCap)
+        ++path_len;
+    while (path_len > 1 && manifestEncodedSizeForPathLen(ns, path_len - 1) > kExpectedManifestEncodedCap)
+        --path_len;
+    return path_len;
+}
+
+/// A one-entry Blob ManifestEntry with a synthetic `path_len`-byte path (used only to inflate the
+/// encoded manifest size towards the OQ7 cap).
+ManifestEntry wideBlobManifestEntry(size_t path_len)
+{
+    ManifestEntry e;
+    e.path = String(path_len, 'a');
+    e.placement = EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(UInt128{0x42})};
+
+    e.blob_size = 12345;
+    return e;
+}
+
+}
+
+/// Boundary case 1/2: a manifest whose encoded size is the LARGEST that still fits under the cap stages
+/// successfully. Proves the cap enforcement isn't overly conservative — a real just-under-the-limit
+/// manifest is not mistakenly rejected.
+TEST(CASPartWriteTxn, ManifestCapEncodedBytesJustUnderStagesSuccessfully)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    /// A frozen boot_ms_fn (not the shared openPool helper): this test's manifest sits just under the
+    /// 256 MiB cap, so encodePartManifest/sealObject do real, sizeable CPU work before the single
+    /// InMemoryBackend put (which always succeeds deterministically, no faults). Under heavy
+    /// instrumentation (TSan) that encode+seal step alone can take long enough in real wall-clock time
+    /// to cross the mount lease's fence margin (CasMountRuntime::refAppendFenceOk) and the CAS request
+    /// controller's own deadline (both consult the SAME injected clock, CasRefLedger.cpp) before the
+    /// attempt even resolves -- a sanitizer-speed artifact unrelated to what this test verifies. Freezing
+    /// the clock decouples the outcome from real execution speed: the single attempt now succeeds or
+    /// fails purely on the backend's own (deterministic) behavior, on any build.
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .boot_ms_fn = [] { return uint64_t{0}; }});
+    const RootNamespace ns{"srv1/tbl"};
+
+    const size_t path_len_over = findManifestEncodedCapBoundaryPathLen(ns);
+    ASSERT_GT(path_len_over, 1u);
+    const size_t path_len_under = path_len_over - 1;
+    ASSERT_LE(manifestEncodedSizeForPathLen(ns, path_len_under), kExpectedManifestEncodedCap);
+
+    auto build = startBuildFor(s, ns, "wide_part");
+    const ManifestId id = build->stageManifest({wideBlobManifestEntry(path_len_under)});
+    EXPECT_EQ(id.root_namespace, ns);
+    EXPECT_TRUE(b->head(s->layout().manifestKey(id)).exists)
+        << "a just-under-cap manifest must actually be written";
+}
+
+/// Boundary case 2/2: a manifest whose encoded size exceeds the cap by the smallest possible margin
+/// (one more path byte than the passing case above) throws `LIMIT_EXCEEDED` fail-closed, BEFORE the body
+/// write — no manifest object lands in the backend for the rejected attempt.
+TEST(CASPartWriteTxn, ManifestCapEncodedBytesOverThrowsBeforeBodyWrite)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    const size_t path_len_over = findManifestEncodedCapBoundaryPathLen(ns);
+    ASSERT_GT(manifestEncodedSizeForPathLen(ns, path_len_over), kExpectedManifestEncodedCap);
+
+    auto build = startBuildFor(s, ns, "wide_part");
+
+    const size_t keys_before = b->list("", "", 100).keys.size();
+    bool threw = false;
+    try
+    {
+        build->stageManifest({wideBlobManifestEntry(path_len_over)});
+    }
+    catch (const DB::Exception & e)
+    {
+        threw = true;
+        EXPECT_EQ(e.code(), DB::ErrorCodes::LIMIT_EXCEEDED);
+        EXPECT_NE(e.message().find("exceeds cap"), String::npos) << e.message();
+    }
+    EXPECT_TRUE(threw) << "an over-cap manifest must throw, not silently truncate or accept";
+
+    /// Fail-closed BEFORE the body write: the over-cap attempt must not have created ANY new object
+    /// (no partial state, no orphaned blob/manifest debris for a manifest that was never accepted).
+    const size_t keys_after = b->list("", "", 100).keys.size();
+    EXPECT_EQ(keys_before, keys_after)
+        << "stageManifest must fail closed before writing the manifest body, leaving no new objects";
+}
+
+/// spec §9.9 (mixed-algo pools, Phase 3 T2) — the W-DEP-SET cross-satisfaction crux: a manifest with
+/// two entries carrying the SAME digest VALUE under TWO DIFFERENT algos (`ch128:X` / `xxh3:X`). Only
+/// `ch128:X`'s body is ever putBlob'd; `xxh3:X`'s body never lands anywhere. Promote MUST fail closed —
+/// the tokened `ch128:X` dep must NEVER be read as satisfying the non-tokened `xxh3:X` leaf.
+/// This test is RED (wrongly passes / silently promotes) if `PartWriteTxn::deps` (the W-DEP-SET) were keyed on
+/// a bare digest instead of the full `BlobRef` pair: both entries would collapse to the SAME map key
+/// (the digest alone), so `depIsTokened` would report the xxh3 leaf as edge-protected via the ch128
+/// entry's putBlob and promote would skip its revalidation (and hence its absence) entirely.
+TEST(CASPartWriteTxn, WDepSetCrossAlgoSatisfactionFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl"};
+
+    const BlobDigest shared_digest = BlobDigest::fromU128(u128Of("shared-digest-value"));
+
+    ManifestEntry e_ch128;
+    e_ch128.path = "a.bin";
+    e_ch128.placement = EntryPlacement::Blob;
+    e_ch128.ref = BlobRef{BlobHashAlgo::CityHash128, shared_digest};
+    e_ch128.blob_size = 3;
+
+    ManifestEntry e_xxh3;
+    e_xxh3.path = "b.bin";
+    e_xxh3.placement = EntryPlacement::Blob;
+    e_xxh3.ref = BlobRef{BlobHashAlgo::XXH3_128, shared_digest};   /// SAME digest bytes, DIFFERENT algo
+    e_xxh3.blob_size = 3;
+
+    auto build = startBuildFor(s, ns, "part_mixed");
+    const ManifestId id = build->stageManifest({e_ch128, e_xxh3});
+    build->precommitAdd(ns, "part_mixed", id);
+
+    /// Only the ch128 leaf's body is ever uploaded — its BlobId hex is the digest at the ch128 width,
+    /// which addresses EXACTLY `e_ch128`'s object key (`blobs/ch128/...`), a DISTINCT key from
+    /// `e_xxh3`'s (`blobs/xxh3/...`), even though the raw digest bytes are identical.
+    build->putBlob(BlobRef{BlobHashAlgo::CityHash128, shared_digest}, BlobSource::fromString("abc"));
+
+    /// promote must fail closed: the xxh3:X leaf has NO tokened dep and NO adopted dep — never silently
+    /// satisfied by the ch128:X entry's tokened dep (same digest bytes, distinct object key). §4
+    /// manifest-trust: an unsatisfied leaf is caught by the dep set (isTrustedAdopt false, not tokened) and
+    /// fails closed with LOGICAL_ERROR — a staging bug — without any backend probe on the xxh3 key.
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->promote(ns, "part_mixed", build->buildId(), id);
+        },
+        "no tokened and no adopted dep");
+
+    /// No committed ref appears — the promote aborted before installing one.
+    EXPECT_FALSE(s->resolveRef(ns, "part_mixed").has_value());
+}
+
+/// =====================================================================================
+/// Task B (chaos-tolerance-report §Task B): stageManifest's part-manifest conditional PUT rides the
+/// shared CasRequestController — budgeted attempts + resolve-before-reissue — instead of the old
+/// single bare attempt (which a 19s object-store pause killed while every read path survived).
+/// =====================================================================================
+
+namespace
+{
+
+/// Faults the part-manifest body PUT (`/cas/manifests/` keys) with an ambiguous
+/// (Unresolved-classified) timeout a bounded number of times, mirroring RefWriterTestBackend's fault
+/// seam (gtest_cas_ref_writer.cpp). Covers BOTH write primitives — `putIfAbsent` (the controller
+/// path) and `putIfAbsentStream` (the pre-controller path) — with ONE shared `fault_count` and the
+/// same land/plant side effects, so the assertions are flip-proof against either implementation of
+/// the stage write.
+class ManifestPutFaultBackend final : public InMemoryBackend
+{
+public:
+    int fault_count = 0;                 /// remaining ambiguous faults on matching body PUTs
+    bool land_despite_fault = false;     /// the faulted attempt's own write actually lands (response lost)
+    String plant_different_on_fault;     /// a FOREIGN different body lands at the key before the fault
+    int put_attempts = 0;                /// matching body-PUT attempts observed (both primitives)
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (!isManifestBodyKey(key))
+            return InMemoryBackend::putIfAbsent(key, bytes, meta);
+        ++put_attempts;
+        maybeFault(key, bytes);
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    WriteSinkPtr putIfAbsentStream(const String & key, const ObjectMeta & meta) override
+    {
+        if (!isManifestBodyKey(key))
+            return InMemoryBackend::putIfAbsentStream(key, meta);
+
+        /// Counts/faults at finalize — the moment the old single-attempt path observed its timeout.
+        struct CountingOrFaultingSink final : WriteSink
+        {
+            ManifestPutFaultBackend & parent;
+            String key;
+            ObjectMeta meta;
+            DB::WriteBufferFromOwnString buf;
+
+            CountingOrFaultingSink(ManifestPutFaultBackend & parent_, String key_, ObjectMeta meta_)
+                : parent(parent_), key(std::move(key_)), meta(std::move(meta_)) {}
+
+            DB::WriteBuffer & buffer() override { return buf; }
+            PutResult finalize() override
+            {
+                ++parent.put_attempts;
+                const String & bytes = buf.str();
+                parent.maybeFault(key, bytes);
+                return parent.InMemoryBackend::putIfAbsent(key, bytes, meta);
+            }
+            void cancel() noexcept override {}
+        };
+        return std::make_unique<CountingOrFaultingSink>(*this, key, meta);
+    }
+
+private:
+    static bool isManifestBodyKey(const String & key) { return key.find("/cas/manifests/") != String::npos; }
+
+    /// One fault: apply the configured server-side effect, then lose the response.
+    void maybeFault(const String & key, const String & bytes)
+    {
+        if (fault_count <= 0)
+            return;
+        --fault_count;
+        if (!plant_different_on_fault.empty())
+            InMemoryBackend::putIfAbsent(key, plant_different_on_fault, {});
+        else if (land_despite_fault)
+            InMemoryBackend::putIfAbsent(key, bytes, {});
+        throw Poco::TimeoutException("ManifestPutFaultBackend: simulated ambiguous result (response lost)");
+    }
+};
+
+}
+
+/// The Task B core: two consecutive ambiguous timeouts on the part-manifest body PUT (each resolved
+/// to "absent" by the controller's exact-GET), then a clean third attempt. The old single-attempt
+/// path fails the whole stage on the FIRST timeout (the observed 19s-pause INSERT kill); the
+/// controller path must ride its attempt budget and succeed.
+TEST(CASPartWriteTxnStageManifestRetry, AmbiguousTimeoutsThenCommitSucceedsWithinBudget)
+{
+    /// Zero backoff: the retry semantics are under test here, not the (controller-level-tested)
+    /// inter-attempt sleep schedule — keep the suite free of real sleeps.
+    CasRequestBudget budget;
+    budget.retry_initial_backoff_ms = 0;
+    auto b = std::make_shared<ManifestPutFaultBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .cas_request_budget = budget});
+    const RootNamespace ns{"srv/tbl"};
+
+    auto build = startBuildFor(s, ns, "part_retry");
+    b->fault_count = 2;
+    const ManifestId id = build->stageManifest({blobManifestEntry("a.bin", "a")});
+
+    EXPECT_EQ(b->put_attempts, 3) << "two faulted attempts + the committing third";
+    const auto got = b->get(s->layout().manifestKey(id));
+    ASSERT_TRUE(got.has_value()) << "the staged manifest body must be durable";
+    EXPECT_EQ(decodePartManifest(openObject(FormatId::PartManifest, got->bytes)).ref, id.ref);
+}
+
+/// Ambiguous-but-landed: the FIRST attempt's response is lost AFTER the write actually landed
+/// server-side. Resolve-before-reissue's exact-GET observes the identical bytes and reports
+/// Committed — the stage succeeds WITHOUT a reissue (no duplicate PUT of the object), and the
+/// `ManifestPut` audit event carries the landed incarnation's token (from the resolve GET).
+TEST(CASPartWriteTxnStageManifestRetry, AmbiguousLandedWriteResolvesToCommittedWithoutReissue)
+{
+    auto b = std::make_shared<ManifestPutFaultBackend>();
+    /// The sink target must outlive the Pool: `~Pool` emits terminate events into the sink.
+    std::vector<CasEvent> events;
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl"};
+
+    s->setEventSink([&](const CasEvent & e) { events.push_back(e); });
+
+    auto build = startBuildFor(s, ns, "part_landed");
+    b->fault_count = 1;
+    b->land_despite_fault = true;
+    const ManifestId id = build->stageManifest({blobManifestEntry("a.bin", "a")});
+
+    EXPECT_EQ(b->put_attempts, 1) << "a landed ambiguous attempt must be resolved, never reissued";
+    const String key = s->layout().manifestKey(id);
+    ASSERT_TRUE(b->get(key).has_value());
+
+    const auto ev = std::find_if(events.begin(), events.end(),
+                                 [](const CasEvent & e) { return e.type == CasEventType::ManifestPut; });
+    ASSERT_NE(ev, events.end()) << "the stage must still emit its ManifestPut audit event";
+    EXPECT_EQ(ev->token, b->head(key).token.value)
+        << "the audit token must be the landed incarnation's token";
+}
+
+/// A DIFFERENT object at the exact staged key (a foreign body ahead of our ambiguous attempt) is a
+/// proven conflict — the NoManifestIdReuse invariant broke — and must stay the loud CORRUPTED_DATA
+/// class: never a retry signal, never silently adopted.
+TEST(CASPartWriteTxnStageManifestRetry, DifferentObjectAtKeyStaysLoudConflict)
+{
+    auto b = std::make_shared<ManifestPutFaultBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl"};
+
+    auto build = startBuildFor(s, ns, "part_conflict");
+    b->fault_count = 1;
+    b->plant_different_on_fault = "a-foreign-different-manifest-body";
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        build->stageManifest({blobManifestEntry("a.bin", "a")});
+    });
+    EXPECT_EQ(b->put_attempts, 1) << "a proven conflict is never retried";
+}
+
+/// Budget exhaustion: EVERY attempt is ambiguous and nothing ever lands. The controller reports
+/// Unresolved after `max_attempts` and stageManifest maps it to NETWORK_ERROR (fix #37 phase 2) —
+/// the same retryable abort class the ref-log lane's exhausted budget maps to. Nothing was durably
+/// named: the caller re-stages with a fresh ManifestId.
+TEST(CASPartWriteTxnStageManifestRetry, BudgetExhaustionMapsToNetworkError)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 3;
+    budget.retry_initial_backoff_ms = 0;   /// no real sleeps; the backoff schedule has its own tests
+    auto b = std::make_shared<ManifestPutFaultBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .cas_request_budget = budget});
+    const RootNamespace ns{"srv/tbl"};
+
+    auto build = startBuildFor(s, ns, "part_exhausted");
+    b->fault_count = 1000;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        build->stageManifest({blobManifestEntry("a.bin", "a")});
+    });
+    EXPECT_EQ(b->put_attempts, 3) << "attempts must be bounded by the configured budget";
+}
+
+/// =====================================================================================
+/// Task B follow-up (availfix): the two conditional-create paths that still bypassed the request
+/// controller — the BLOB body `putIfAbsentStream` create and `promoteStaged`'s conditional
+/// server-side copy (both issued inside `PartWriteTxn::uploadFromSource`'s streamIfAbsent) — now ride the
+/// same budgeted-attempts machinery. Reissue re-streams from the writer's REPLAYABLE source
+/// (`BlobSource::open` re-reads the staged temp file / re-issues the copy from the intact
+/// staging object — INV-1, never a GET-revive); ambiguity resolves by exact-key OCCUPANCY (the key
+/// embeds the content hash, so any occupant IS the intended content — the same trust model as the
+/// plain 412-adopt path).
+/// =====================================================================================
+
+namespace
+{
+
+/// Faults blob-body conditional creates — BOTH primitives `uploadFromSource` can issue: the streaming
+/// `putIfAbsentStream` (local staging) and `promoteStaged`'s conditional server-side copy (S3-native
+/// staging) — with an ambiguous (Unresolved-classified) timeout a bounded number of times, mirroring
+/// ManifestPutFaultBackend above. Blob META writes (`.meta` keys, plain putIfAbsent) are never faulted.
+class BlobPutFaultBackend final : public InMemoryBackend
+{
+public:
+    int fault_count = 0;                 /// remaining ambiguous faults on matching create attempts
+    bool land_despite_fault = false;     /// the faulted attempt's own write actually lands (response lost)
+    int stream_attempts = 0;             /// blob-body streaming-PUT finalize attempts observed
+    int copy_attempts = 0;               /// promoteStaged conditional-copy attempts observed
+
+    WriteSinkPtr putIfAbsentStream(const String & key, const ObjectMeta & meta) override
+    {
+        if (!isBlobBodyKey(key))
+            return InMemoryBackend::putIfAbsentStream(key, meta);
+
+        /// Counts/faults at finalize — the moment the old single-attempt path observed its timeout.
+        struct CountingOrFaultingSink final : WriteSink
+        {
+            BlobPutFaultBackend & parent;
+            String key;
+            ObjectMeta meta;
+            DB::WriteBufferFromOwnString buf;
+
+            CountingOrFaultingSink(BlobPutFaultBackend & parent_, String key_, ObjectMeta meta_)
+                : parent(parent_), key(std::move(key_)), meta(std::move(meta_)) {}
+
+            DB::WriteBuffer & buffer() override { return buf; }
+            PutResult finalize() override
+            {
+                ++parent.stream_attempts;
+                const String & bytes = buf.str();
+                parent.maybeFault(key, bytes);
+                return parent.InMemoryBackend::putIfAbsent(key, bytes, meta);
+            }
+            void cancel() noexcept override {}
+        };
+        return std::make_unique<CountingOrFaultingSink>(*this, key, meta);
+    }
+
+    PutResult promoteStaged(const String & staging_key, const String & blob_key) override
+    {
+        ++copy_attempts;
+        if (fault_count > 0)
+        {
+            --fault_count;
+            if (land_despite_fault)
+                InMemoryBackend::promoteStaged(staging_key, blob_key);
+            throw Poco::TimeoutException("BlobPutFaultBackend: simulated ambiguous copy (response lost)");
+        }
+        return InMemoryBackend::promoteStaged(staging_key, blob_key);
+    }
+
+private:
+    static bool isBlobBodyKey(const String & key)
+    {
+        return key.find("/blobs/") != String::npos && !key.ends_with(".meta");
+    }
+
+    /// One fault: apply the configured server-side effect, then lose the response.
+    void maybeFault(const String & key, const String & bytes)
+    {
+        if (fault_count <= 0)
+            return;
+        --fault_count;
+        if (land_despite_fault)
+            InMemoryBackend::putIfAbsent(key, bytes, {});
+        throw Poco::TimeoutException("BlobPutFaultBackend: simulated ambiguous result (response lost)");
+    }
+};
+
+/// Zero-backoff store over a BlobPutFaultBackend: the sleep schedule has its own controller-level
+/// tests; these Pool-level tests pin the retry/resolve/abort semantics without real sleeps.
+PoolPtr openBlobFaultPool(const std::shared_ptr<BlobPutFaultBackend> & b, uint32_t max_attempts = CasRequestBudget{}.max_attempts)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = max_attempts;
+    budget.retry_initial_backoff_ms = 0;
+    return Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .cas_request_budget = budget});
+}
+
+/// A replayable BlobSource that COUNTS its own re-streams — pins INV-1's "retry = fresh re-stream
+/// from the writer's own source" (never a GET of the dying/failed object).
+BlobSource countingSource(const String & payload, int & payload_streams)
+{
+    BlobSource source;
+    source.size = payload.size();
+    source.open = [payload, &payload_streams]() -> std::unique_ptr<DB::ReadBuffer>
+    {
+        ++payload_streams;
+        return std::make_unique<DB::ReadBufferFromOwnString>(payload);
+    };
+    return source;
+}
+
+}
+
+/// The core ride: two consecutive ambiguous timeouts on the blob-body streaming PUT (each resolved
+/// "absent" by the controller's occupancy HEAD), then a clean third attempt. The old single-attempt
+/// path failed the whole INSERT on the FIRST timeout (the raw Poco::TimeoutException escaped
+/// putBlob); the controller path rides its budget, RE-STREAMING the payload from the writer's own
+/// replayable source on every attempt.
+TEST(CASPartWriteTxnBlobPutRetry, AmbiguousTimeoutsThenCommitRestreamsFromSource)
+{
+    auto b = std::make_shared<BlobPutFaultBackend>();
+    auto s = openBlobFaultPool(b);
+    const RootNamespace ns{"srv/tbl"};
+    const String payload = "blob-payload-A";
+
+    auto build = startBuildFor(s, ns, "part_blob_retry");
+    const ManifestId id = build->stageManifest({blobManifestEntry("a.bin", payload)});
+    build->precommitAdd(ns, "part_blob_retry", id);
+
+    int payload_streams = 0;
+    b->fault_count = 2;
+    const PutBlobResult res = build->putBlob(idOf(payload), countingSource(payload, payload_streams));
+    EXPECT_EQ(res.size, payload.size());
+
+    EXPECT_EQ(b->stream_attempts, 3) << "two faulted attempts + the committing third";
+    EXPECT_EQ(payload_streams, 3) << "every reissue must RE-STREAM from the writer's own source (INV-1)";
+    EXPECT_TRUE(b->head(s->layout().blobKey(idOf(payload))).exists) << "the blob body must be durable";
+}
+
+/// Ambiguous-but-landed: the FIRST attempt's response is lost AFTER the write actually landed
+/// server-side. The occupancy resolve observes the key present and the existing 412 machinery takes
+/// over — the occupant is ADOPTED (content-addressed identity: any occupant of this key IS the
+/// content), with NO reissue and NO second body upload.
+TEST(CASPartWriteTxnBlobPutRetry, AmbiguousLandedWriteAdoptsOccupantWithoutReupload)
+{
+    auto b = std::make_shared<BlobPutFaultBackend>();
+    /// The sink target must outlive the Pool: `~Pool` emits terminate events into the sink.
+    std::vector<CasEvent> events;
+    auto s = openBlobFaultPool(b);
+    const RootNamespace ns{"srv/tbl"};
+    const String payload = "blob-payload-B";
+
+    s->setEventSink([&](const CasEvent & e) { events.push_back(e); });
+
+    auto build = startBuildFor(s, ns, "part_blob_landed");
+    const ManifestId id = build->stageManifest({blobManifestEntry("a.bin", payload)});
+    build->precommitAdd(ns, "part_blob_landed", id);
+
+    int payload_streams = 0;
+    b->fault_count = 1;
+    b->land_despite_fault = true;
+    const PutBlobResult res = build->putBlob(idOf(payload), countingSource(payload, payload_streams));
+    EXPECT_EQ(res.size, payload.size());
+
+    EXPECT_EQ(b->stream_attempts, 1) << "a landed ambiguous attempt must be resolved, never reissued";
+    EXPECT_EQ(payload_streams, 1);
+
+    const String key = s->layout().blobKey(idOf(payload));
+    const auto adopt = std::find_if(events.begin(), events.end(),
+                                    [](const CasEvent & e) { return e.type == CasEventType::BlobReuseAdopt; });
+    ASSERT_NE(adopt, events.end()) << "the landed occupant must be ADOPTED (the standard dedup leg)";
+    EXPECT_EQ(adopt->token, b->head(key).token.value) << "the adopted token must be the landed incarnation's";
+    EXPECT_EQ(std::count_if(events.begin(), events.end(),
+                            [](const CasEvent & e) { return e.type == CasEventType::BlobPut; }), 0)
+        << "no fresh-upload event: the body was never re-uploaded";
+}
+
+/// Budget exhaustion: EVERY attempt is ambiguous and nothing ever lands. The controller reports the
+/// uncertainty and uploadFromSource maps it to NETWORK_ERROR (fix #37 phase 2) -- the same retryable
+/// abort class stageManifest and the ref-log lane map their exhausted budgets to. Unlike the OLD
+/// ABORTED mapping, putBlob's bounded condemned-churn loop (8 rounds) does NOT re-drive this: it only
+/// catches ABORTED, so a NETWORK_ERROR escapes on the FIRST attempt -- desirable (no point hammering a
+/// lost fence locally 8 times; the caller's own backoff, e.g. the merge queue's, is what should retry).
+TEST(CASPartWriteTxnBlobPutRetry, BudgetExhaustionMapsToNetworkErrorAndEscapesImmediately)
+{
+    auto b = std::make_shared<BlobPutFaultBackend>();
+    auto s = openBlobFaultPool(b, /*max_attempts=*/3);
+    const RootNamespace ns{"srv/tbl"};
+    const String payload = "blob-payload-C";
+
+    auto build = startBuildFor(s, ns, "part_blob_exhausted");
+    const ManifestId id = build->stageManifest({blobManifestEntry("a.bin", payload)});
+    build->precommitAdd(ns, "part_blob_exhausted", id);
+
+    int payload_streams = 0;
+    b->fault_count = 1000000;
+    bool threw = false;
+    try
+    {
+        build->putBlob(idOf(payload), countingSource(payload, payload_streams));
+    }
+    catch (const DB::Exception & e)
+    {
+        threw = true;
+        EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR);
+        EXPECT_NE(e.message().find("UNCERTAIN"), String::npos) << e.message();
+    }
+    EXPECT_TRUE(threw);
+    EXPECT_EQ(b->stream_attempts, 3) << "the 3-attempt controller budget for ONE outer attempt -- "
+                                          "putBlob's outer condemned-churn loop must NOT re-drive a NETWORK_ERROR";
+}
+
+/// promoteStaged conditional copy, ambiguous-but-landed: the copy's response is lost AFTER the
+/// destination was created. The occupancy resolve observes the destination present and the occupant
+/// is adopted — Committed-in-effect WITHOUT a re-copy.
+TEST(CASPartWriteTxnPromoteStagedRetry, AmbiguousCopyLandedAdoptsDestinationWithoutRecopy)
+{
+    auto b = std::make_shared<BlobPutFaultBackend>();
+    /// The sink target must outlive the Pool: `~Pool` emits terminate events into the sink.
+    std::vector<CasEvent> events;
+    auto s = openBlobFaultPool(b);
+    const RootNamespace ns{"srv/tbl"};
+    const String payload = "staged-payload-A";
+    /// The staging object: [pool-fixed-length envelope header][payload], promoted VERBATIM by the copy.
+    const String staging_key = "p/staging/test/blob-a";
+    const String staging_bytes = String(s->poolMeta().blob_header_len, 'h') + payload;
+    ASSERT_EQ(b->putIfAbsent(staging_key, staging_bytes).outcome, PutOutcome::Done);
+
+    s->setEventSink([&](const CasEvent & e) { events.push_back(e); });
+
+    auto build = startBuildFor(s, ns, "part_copy_landed");
+    const ManifestId id = build->stageManifest({blobManifestEntry("a.bin", payload)});
+    build->precommitAdd(ns, "part_copy_landed", id);
+
+    BlobSource source;
+    source.size = payload.size();
+    source.server_side_copy_from = staging_key;
+    b->fault_count = 1;
+    b->land_despite_fault = true;
+    const PutBlobResult res = build->putBlob(idOf(payload), std::move(source));
+    EXPECT_EQ(res.size, payload.size());
+
+    EXPECT_EQ(b->copy_attempts, 1) << "a landed ambiguous copy must be resolved, never re-copied";
+    const String key = s->layout().blobKey(idOf(payload));
+    const auto got = b->get(key);
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, staging_bytes) << "the destination is the staging object's verbatim copy";
+    EXPECT_NE(std::find_if(events.begin(), events.end(),
+                           [](const CasEvent & e) { return e.type == CasEventType::BlobReuseAdopt; }),
+              events.end()) << "the landed destination must be ADOPTED";
+}
+
+/// promoteStaged conditional copy, ambiguous-and-absent: the first copy attempt times out with
+/// nothing landing; the resolve observes the destination absent and the copy is REISSUED from the
+/// (intact, still-staged) source object — the second attempt commits.
+TEST(CASPartWriteTxnPromoteStagedRetry, AmbiguousCopyAbsentReattemptsAndCommits)
+{
+    auto b = std::make_shared<BlobPutFaultBackend>();
+    auto s = openBlobFaultPool(b);
+    const RootNamespace ns{"srv/tbl"};
+    const String payload = "staged-payload-B";
+    const String staging_key = "p/staging/test/blob-b";
+    const String staging_bytes = String(s->poolMeta().blob_header_len, 'h') + payload;
+    ASSERT_EQ(b->putIfAbsent(staging_key, staging_bytes).outcome, PutOutcome::Done);
+
+    auto build = startBuildFor(s, ns, "part_copy_retry");
+    const ManifestId id = build->stageManifest({blobManifestEntry("a.bin", payload)});
+    build->precommitAdd(ns, "part_copy_retry", id);
+
+    BlobSource source;
+    source.size = payload.size();
+    source.server_side_copy_from = staging_key;
+    b->fault_count = 1;
+    const PutBlobResult res = build->putBlob(idOf(payload), std::move(source));
+    EXPECT_EQ(res.size, payload.size());
+
+    EXPECT_EQ(b->copy_attempts, 2) << "the faulted attempt + the committing reissue";
+    const String key = s->layout().blobKey(idOf(payload));
+    const auto got = b->get(key);
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, staging_bytes);
+}
diff --git a/src/Disks/tests/gtest_cas_part_write_root_dangle.cpp b/src/Disks/tests/gtest_cas_part_write_root_dangle.cpp
new file mode 100644
index 000000000000..22d0efe45227
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_part_write_root_dangle.cpp
@@ -0,0 +1,256 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <string>
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace
+{
+
+/// Mirrors the B140 repro.
+PoolPtr openTestPool(std::shared_ptr<InMemoryBackend> & out_backend)
+{
+    out_backend = std::make_shared<InMemoryBackend>();
+    return Pool::open(out_backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+size_t runGcToFixpoint(Gc & gc, size_t max_rounds = 64)
+{
+    size_t rounds = 0;
+    for (; rounds < max_rounds; ++rounds)
+    {
+        RoundReport rep;
+        try
+        {
+            rep = gc.runRegularRound();
+        }
+        catch (const DB::Exception &)
+        {
+            break;
+        }
+        if (!rep.acquired_lease)
+            continue;
+        if (rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0)
+            break;
+    }
+    return rounds;
+}
+
+ManifestEntry blobEntry(const String & name, const String & payload)
+{
+    ManifestEntry e;
+    e.path = name;
+    e.placement = EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    e.blob_size = payload.size();
+    return e;
+}
+
+}
+
+/// B171 build-root / precommit, RED repro of the B140-dangle at unit level driven entirely through the
+/// public PartWriteTxn/Pool/Gc API (no snap injection):
+///
+///   PartWriteTxn A uploads blob P and publishes refA -> t1 -> { data.bin: P }. A is then RELEASED (dtor),
+///   retiring its build_seq so the GC watermark `min_active` advances PAST A. P now carries A's
+///   `cas_owner` and is no longer protected by any in-flight build.
+///
+///   PartWriteTxn B starts and ADOPTS the same blob P via tokenless evidence (adoptEvidence — the cross-node
+///   adopt case), assembles t2 -> { other.bin: P }, and `precommit(t2)` — which publishes a durable
+///   build-root ref so GC's fold lifts the in-degree of P's closure.
+///
+///   refA is dropped + watermark renewed; GC runs to fixpoint. P is protected by B's precommit edge,
+///   so GC must NOT delete it. PartWriteTxn B then publishes refB -> t2 successfully.
+///
+/// THE POSITIVE INVARIANT: the whole flow must succeed AND P must survive, because B's precommit pins
+/// P's closure across A's retire + GC (B171 two-phase commit; `checkAndResolveDeps` proves closure
+/// present at publish time).
+TEST(CASPartWriteTxnRootDangle, SharedBlobSurvivesSourceDropDuringBuild)
+{
+    std::shared_ptr<InMemoryBackend> backend;
+    auto s = openTestPool(backend);
+    const RootNamespace ns{"test/tbl"};
+    const String P = "shared-blob-payload-P";
+
+    /// PartWriteTxn A: upload P, publish refA -> manifest -> { data.bin: P }, then release A so its build_seq
+    /// retires and min_active advances past it.
+    {
+        PartWriteInfo info;
+        info.intended_ref = ns.string() + "/refA";
+        auto a = s->beginPartWrite(info);
+        a->putBlob(idOf(P), BlobSource::fromString(P));
+        const ManifestId id = a->stageManifest({blobEntry("data.bin", P)});
+        a->precommitAdd(ns, "refA", id);
+        a->promote(ns, "refA", a->buildId(), id);
+    }
+    s->renewWatermarkOnce();   /// A is gone; min_active now advances past A's build_seq
+
+    /// PartWriteTxn B: adopt the SAME blob P (cross-node adopt — tokenless evidence via adoptEvidence), assemble
+    /// its manifest, and precommitAdd it. The precommit pins P's closure (fold +1 edge) for the build.
+    PartWriteInfo binfo;
+    binfo.intended_ref = ns.string() + "/refB";
+    auto b = s->beginPartWrite(binfo);
+    const ManifestEntry pe = blobEntry("other.bin", P);
+    b->adoptEvidence(pe);
+    const ManifestId t2 = b->stageManifest({pe});
+    b->precommitAdd(ns, "refB", t2);
+
+    /// The source ref disappears, and the watermark is renewed so the closure looks collectable.
+    s->dropRef(ns, "refA");
+    s->renewWatermarkOnce();
+
+    /// GC to fixpoint. P must survive: the live precommit binding for refB activates a +1 blob edge on
+    /// P during the fold, so P never reaches in-degree 0 (B171 two-phase commit).
+    Gc gc(s, u128Of("gc-b171"));
+    runGcToFixpoint(gc);
+
+    /// PartWriteTxn B commits refB by promoting its precommit. Should succeed end-to-end; if it throws (e.g.
+    /// ABORTED because the blob is gone) that is itself the RED outcome.
+    ASSERT_NO_THROW(b->promote(ns, "refB", b->buildId(), t2))
+        << "B171: PartWriteTxn B's promote must succeed — the precommit should have kept P alive";
+
+    /// The blob B references must still be present (no dangle), and refB must resolve.
+    ASSERT_TRUE(backend->head(s->layout().blobKey(idOf(P))).exists)
+        << "B171-dangle: GC deleted the shared blob P that PartWriteTxn B adopted — its cas_owner was the "
+        << "retired PartWriteTxn A and the stub precommit published no build-root edge, so inDeg(P) hit 0 "
+        << "and the single content-delete site removed it. refB now dangles.";
+    ASSERT_TRUE(s->resolveRef(ns, "refB").has_value())
+        << "B171: refB must resolve to its committed manifest";
+}
+
+/// B171 INV-COMMIT-FAILCLOSED: even if the build-root precommit is PREMATURELY RECLAIMED mid-build
+/// (e.g. a live build whose watermark renewer froze and was falsely judged dead), the real commit must
+/// NEVER publish a table ref over a missing dependency. It must fail closed — abort — never dangle.
+///
+/// Setup mirrors the primary repro: PartWriteTxn A publishes refA -> t1 -> { data.bin: P } then retires; PartWriteTxn
+/// B adopts P, assembles t2 -> { other.bin: P }, and precommits t2 (a real build-root edge now protects
+/// P). We then SIMULATE the premature reclaim by manually dropping the build-root ref (as GC's reclaim
+/// would) AND dropping refA, then renew the watermark and run GC to fixpoint. With P's only protection
+/// (the precommit edge) gone and its owner retired, GC deletes P. PartWriteTxn B's publish must now ABORT
+/// (`checkAndResolveDeps` finds the adopted blob absent and not re-creatable) instead of committing a dangle.
+TEST(CASPartWriteTxnRootDangle, PrematureReclaimCommitFailsClosed)
+{
+    std::shared_ptr<InMemoryBackend> backend;
+    auto s = openTestPool(backend);
+    const RootNamespace ns{"test/tbl"};
+    const String P = "shared-blob-payload-P-reclaim";
+
+    /// PartWriteTxn A: upload P, publish refA -> manifest, retire A so min_active advances past it.
+    {
+        PartWriteInfo info;
+        info.intended_ref = ns.string() + "/refA";
+        auto a = s->beginPartWrite(info);
+        a->putBlob(idOf(P), BlobSource::fromString(P));
+        const ManifestId id = a->stageManifest({blobEntry("data.bin", P)});
+        a->precommitAdd(ns, "refA", id);
+        a->promote(ns, "refA", a->buildId(), id);
+    }
+    s->renewWatermarkOnce();
+
+    /// PartWriteTxn B: adopt P via tokenless evidence, assemble its manifest, precommitAdd it (the precommit
+    /// owner binding for refB now protects P with a +1 fold edge).
+    PartWriteInfo binfo;
+    binfo.intended_ref = ns.string() + "/refB";
+    auto b = s->beginPartWrite(binfo);
+    const ManifestEntry pe2 = blobEntry("other.bin", P);
+    b->adoptEvidence(pe2);
+    const ManifestId t2 = b->stageManifest({pe2});
+    b->precommitAdd(ns, "refB", t2);
+
+    /// SIMULATE a premature reclaim having already collected P: had the precommit binding been wrongly
+    /// reclaimed with no other owner, GC would condemn+delete P's closure. Reproduce that END STATE
+    /// directly by deleting P's blob object. (The durable ref-log stream is owned by the live writer, so a
+    /// RAW removal append would collide with the writer's own `RefTxnId` sequence allocation on the next
+    /// flush; the property under test is the COMMIT gate's fail-closed behavior against a missing
+    /// dependency, not the reclaim mechanics -- so we go straight to the reclaimed state.)
+    {
+        const String pkey = s->layout().blobKey(idOf(P));
+        const HeadResult h = backend->head(pkey);
+        ASSERT_TRUE(h.exists) << "P must be present before the simulated reclaim";
+        ASSERT_EQ(backend->deleteExact(pkey, h.token).kind, DeleteOutcome::Kind::Deleted);
+    }
+    /// Drop the source ref too (the state a real premature reclaim leaves: P unprotected and gone).
+    s->dropRef(ns, "refA");
+    s->renewWatermarkOnce();
+
+    /// The shared blob must be GONE (the premature reclaim collected it).
+    ASSERT_FALSE(backend->head(s->layout().blobKey(idOf(P))).exists)
+        << "premature-reclaim setup invalid: P should have been collected after losing its precommit";
+
+    /// §4 manifest-trust (test name is legacy — B171 INV-COMMIT-FAILCLOSED for an ADOPTED leaf now moves to
+    /// fsck): P is a committed-source adopted leaf, so PartWriteTxn B's promote TRUSTS it (no HEAD/loadMeta probe)
+    /// and COMMITS refB. On the real reuse/relink path this dangle is UNREACHABLE: precommitAdd durably
+    /// appended refB's Precommit OwnerTransition (CasPartWriteTxn.cpp precommitAdd) BEFORE promote, and promote
+    /// re-proves that edge is the LIVE owner (WPromote owner==bld) BEFORE trusting P — so P has in-degree
+    /// >= 1 and GC (the sole deleter) cannot collect it. This test injects the collection DIRECTLY (a raw
+    /// deleteExact while refB's precommit is still live), which the live-precommit invariant excludes. So
+    /// promote SUCCEEDS; the dangle is not prevented at promote but DETECTED by fsck (the backstop).
+    ASSERT_NO_THROW(b->promote(ns, "refB", b->buildId(), t2))
+        << "§4: an adopted leaf is trusted at promote — a missing dependency is not re-observed here";
+
+    /// Trust never fabricates the missing blob (it never touches P); refB IS committed (naming absent P).
+    ASSERT_FALSE(backend->head(s->layout().blobKey(idOf(P))).exists)
+        << "trust never fabricates the missing blob — P stays absent";
+    ASSERT_TRUE(s->resolveRef(ns, "refB").has_value())
+        << "§4: refB commits under trust (the D4 trade-off); the dangle is caught by fsck, below";
+
+    /// THE BACKSTOP (INV-NO-DANGLE-via-fsck): fsck's reachable-but-absent scan reports refB's absent P as
+    /// dangling — this is where the B171 guarantee lives under §4. Detection moved, it did not disappear.
+    const FsckReport rep = runFsck(*s, /*detail=*/true);
+    EXPECT_GE(rep.dangling, 1u)
+        << "§4 D4 backstop: refB committed over the deleted P; fsck must report it dangling (dangling="
+        << rep.dangling << ", reachable=" << rep.reachable << ")";
+}
+
+/// (The GC-reclaim test `CASPartWriteTxnRoot.AbandonedPrecommitReclaimed` -- which asserted GC AUTOMATICALLY
+/// reclaims an abandoned precommit of a judged-dead build and then collects its closure -- was removed
+/// with the snapshot+log ref model. Per spec §Responsibility Boundary, reclaiming an abandoned precommit
+/// is now the WRITER's job (it appends the exact `owner_transition` removal on recovery); GC never scans
+/// for or removes precommit bindings, and there is no mutable shard journal to append a `PrecommitRemove`
+/// into. The `precommitRemovalAppended` shard-journal probe it shared with `LivePrecommitNotReclaimed`
+/// went with it.)
+
+/// B8 CONSERVATISM (liveness-correctness guard): a live in-flight build's precommit binding (and its
+/// pinned blobs) must survive a full GC run, and the build must still be able to promote it. In the
+/// snapshot+log model GC never reclaims a precommit at all, so this is purely a liveness pin: the live
+/// precommit's `+1` fold edge keeps its exclusively-owned blob alive across GC.
+TEST(CASPartWriteTxnRoot, LivePrecommitNotReclaimed)
+{
+    std::shared_ptr<InMemoryBackend> backend;
+    auto s = openTestPool(backend);
+    const RootNamespace ns{"test/tbl"};
+    const String Q = "live-build-blob-payload-Q";
+
+    /// PartWriteTxn B stays ALIVE: upload Q, assemble, precommitAdd — and we DO NOT retire its seq. So
+    /// `min_active <= build_seq` (B is in-flight) and the watermark keeps a live, advancing seq.
+    PartWriteInfo binfo;
+    binfo.intended_ref = ns.string() + "/refLive";
+    auto b = s->beginPartWrite(binfo);
+    b->putBlob(idOf(Q), BlobSource::fromString(Q));
+    const ManifestId t = b->stageManifest({blobEntry("data.bin", Q)});
+    b->precommitAdd(ns, "refLive", t);
+    s->renewWatermarkOnce();
+    ASSERT_LE(s->minActive(), b->buildSeq()) << "precondition: B must be in-flight (min_active <= seq)";
+
+    /// GC to fixpoint while B is live.
+    Gc gc(s, u128Of("gc-b8-live"));
+    runGcToFixpoint(gc);
+
+    /// Q must still be present (the live precommit's +1 edge pins it across GC).
+    ASSERT_TRUE(backend->head(s->layout().blobKey(idOf(Q))).exists)
+        << "B8 conservatism: the live precommit must keep its blob alive across GC";
+
+    /// B can still commit (the precommit is intact).
+    ASSERT_NO_THROW(b->promote(ns, "refLive", b->buildId(), t))
+        << "B8 conservatism: a live build must still be able to promote its untouched precommit";
+}
diff --git a/src/Disks/tests/gtest_cas_pluggable_hash.cpp b/src/Disks/tests/gtest_cas_pluggable_hash.cpp
new file mode 100644
index 000000000000..a4146ddf65c1
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_pluggable_hash.cpp
@@ -0,0 +1,939 @@
+#include <gtest/gtest.h>
+
+/// P1-T2 (CAS pluggable-blob-hash Phase 1):
+/// `PoolMeta` records the pool-wide `blob_hash_algo` and `PoolMeta::createOrValidate` fail-closes on a
+/// disk config that disagrees with an existing pool's recorded algo -- the pool-wide durability
+/// invariant (never silently re-hash an existing pool).
+///
+/// Phase 3 T4 RELAXES that single fail-closed
+/// value into `PoolMeta::algos_used` (sorted, append-only): a config algo already a MEMBER is
+/// accepted with no write (steady state); a non-member is admitted via a CAS-union ONLY when the
+/// disk opts in (`blob_hash_allow_new`), and refused (`BAD_ARGUMENTS`, same as before) otherwise --
+/// a changed config alone must never silently turn a pool mixed. See `AdmissionIsFlagGated` and
+/// `ConcurrentAdmissionUnions` below.
+///
+/// P1-T3a (this file, extended): the pool's `blob_hash_algo` is threaded into the three hash sites
+/// (spec §5/§6) -- `Cas::CaContentWriteBuffer` (streaming blob-body hash),
+/// `PartWriteTxn`'s envelope `hash_algo` field, and (transitively, via `Cas::blobHashHexOneShot`) the
+/// `poolContentHash` content-key mint on the write path. `poolContentHash` itself is a static
+/// helper in `CasPartWriteTxn.cpp` and not directly reachable from a gtest; its production callers already
+/// exercise the default `CityHash128` path, and it delegates to the SAME `Cas::blobHashHexOneShot`
+/// this file tests directly below.
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasBlobHashingWriteBuffer.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasCodecUtil.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_test_helpers.h"
+
+#include <IO/WriteBufferFromFile.h>
+
+
+#include <algorithm>
+#include <filesystem>
+#include <unordered_set>
+#include <utility>
+
+namespace DB::ErrorCodes
+{
+extern const int BAD_ARGUMENTS;
+extern const int UNKNOWN_FORMAT_VERSION;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+
+namespace
+{
+
+/// A deterministic, non-repeating-byte payload spanning several `DBMS_DEFAULT_HASHING_BLOCK_SIZE`
+/// (2048 B) blocks, so a chunked-vs-one-shot divergence (the CityHash128 pitfall documented on
+/// `poolContentHash`) would not accidentally go unnoticed.
+std::string makeMultiBlockPayload(size_t size = 5000)
+{
+    std::string s;
+    s.reserve(size);
+    for (size_t i = 0; i < size; ++i)
+        s.push_back(static_cast<char>('a' + (i % 23)));
+    return s;
+}
+
+/// A blob written at its OWN algo's content key, plus the key it landed at.
+struct SeededBlob
+{
+    BlobRef ref;
+    String key;
+};
+
+/// Write a blob body of `algo` at its content key, reference it from a committed ref, and DROP that
+/// ref — so the blob reaches a folded in-degree of zero and the ORDINARY pipeline condemns it by
+/// transition-to-zero. The caller then runs the rounds that fold the `+1` and the `-1`.
+///
+/// These tests used to seed a blob no manifest ever named and lean on `rebuildBaseline`'s LIST/HEAD
+/// sweep, which was the only path that could condemn such a blob. That sweep is GONE (spec §7: a
+/// rebuild condemns nothing — it was the r5-finding-4 data-loss vector), and a blob nothing names is
+/// now retained by design. What these tests actually guard — that a blob is recognized under its OWN
+/// `<algo>` path segment by the fold's key codec, `previewDeletes`, the exact-token delete and fsck,
+/// rather than silently skipped as foreign — is unaffected, and lives on the PRODUCTION path, which is
+/// where it is now exercised. Reverting either per-algo port still turns these red.
+SeededBlob seedReferencedBlob(Pool & store, Backend & backend, const RootNamespace & ns, BlobHashAlgo algo,
+                              uint64_t build_sequence, size_t payload_size, const String & ref_name)
+{
+    const std::string payload = makeMultiBlockPayload(payload_size);
+    const BlobRef ref{algo, codecFor(algo).fromHex(blobHashHexOneShot(algo, payload))};
+    const String key = store.layout().blobKey(ref);
+
+    EnvelopeHeader header;
+    header.kind = ObjectKind::Blob;
+    header.incarnation_tag = UInt128(0x1234);
+    header.build_id = UInt128(0x5678);
+    backend.putIfAbsent(key, encodeEnvelopeHeader(header, static_cast<uint32_t>(store.poolMeta().blob_header_len)) + payload);
+
+    ManifestEntry entry;
+    entry.path = "data_" + std::to_string(build_sequence) + ".bin";
+    entry.placement = EntryPlacement::Blob;
+    entry.ref = ref;   /// the entry carries the blob's OWN algo, not the pool's write algo
+    entry.blob_size = 1;
+
+    const ManifestRef mref{.writer_epoch = 1, .build_sequence = build_sequence, .manifest_ordinal = 1};
+    writeManifestRaw(backend, store.layout(), ns, mref, {entry});
+    publishCommittedTransition(backend, store.layout(), ns, ref_name, std::nullopt, mref);
+    return SeededBlob{ref, key};
+}
+
+/// Drop the committed ref `seedReferencedBlob` published, so the blob's only edge disappears.
+void dropSeededRef(Pool & store, Backend & backend, const RootNamespace & ns, uint64_t build_sequence,
+                   const String & ref_name)
+{
+    const ManifestRef mref{.writer_epoch = 1, .build_sequence = build_sequence, .manifest_ordinal = 1};
+    dropRefTransition(backend, store.layout(), ns, ref_name, mref);
+}
+
+}
+
+TEST(CASPluggableHash, PoolMetaRoundTripsAlgosUsed)
+{
+    PoolMeta pm;
+    pm.pool_id = u128Of("pool-a");
+    pm.blob_header_len = 256;
+    pm.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128), static_cast<uint8_t>(BlobHashAlgo::XXH3_128)};
+
+    const PoolMeta back = decodePoolMeta(encodePoolMeta(pm));
+    EXPECT_EQ(back.algos_used, pm.algos_used);
+    EXPECT_EQ(back.blob_header_len, 256u);
+}
+
+TEST(CASPluggableHash, CreateOrValidateRecordsConfigAlgoOnFreshPool)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+
+    const PoolMeta pm = PoolMeta::createOrValidate(*backend, layout, /*blob_header_len*/ 256, BlobHashAlgo::XXH3_128, /*allow_new*/ false, /*allow_mint*/ true);
+    EXPECT_EQ(pm.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::XXH3_128)}));
+
+    /// Reopening with the SAME algo is a no-op reopen: the recorded value comes back unchanged.
+    const PoolMeta reopened = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::XXH3_128);
+    EXPECT_EQ(reopened.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::XXH3_128)}));
+    EXPECT_EQ(reopened.pool_id, pm.pool_id);
+}
+
+TEST(CASPluggableHash, CreateOrValidateDefaultsToCityHash128)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+
+    const PoolMeta pm = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+    EXPECT_EQ(pm.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::CityHash128)}));
+}
+
+/// Phase 3 T4 (spec §5, replaces the Phase 1/2 unconditional-fail-close test of the same shape):
+/// admission of a NEW algo is EXPLICIT OPT-IN -- the default reopen with a non-member algo still
+/// fails closed (BAD_ARGUMENTS), but the message now names `<blob_hash_allow_new>` and the pool
+/// is truly extensible with the flag set. See `AdmissionIsFlagGated` below for the full flow.
+TEST(CASPluggableHash, CreateOrValidateFailsClosedOnAlgoMismatchWithoutFlag)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+
+    PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::XXH3_128, /*allow_new*/ false);
+    });
+
+    /// The pool is untouched by the refused reopen: a subsequent open with the ORIGINAL algo still
+    /// succeeds and returns the same pool_id.
+    const PoolMeta reopened = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::CityHash128);
+    EXPECT_EQ(reopened.algos_used, (std::vector<uint8_t>{static_cast<uint8_t>(BlobHashAlgo::CityHash128)}));
+}
+
+/// spec §9.1 at the unit level: admission of a new algo requires the flag; once admitted, membership
+/// alone is the steady-state check (the flag is not needed again for the same algo).
+TEST(CASPluggableHash, AdmissionIsFlagGated)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+    PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+
+    /// without the flag: refuse, pool untouched
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    { PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::Sha256, false); });
+
+    /// with the flag: admitted
+    const PoolMeta admitted = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::Sha256, true);
+    EXPECT_EQ(admitted.algos_used, (std::vector<uint8_t>{1, 3}));
+
+    /// steady state: admitted algo reopens WITHOUT the flag
+    const PoolMeta steady = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::Sha256, false);
+    EXPECT_EQ(steady.algos_used, (std::vector<uint8_t>{1, 3}));
+}
+
+TEST(CASPluggableHash, ConcurrentAdmissionUnions)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+    PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::CityHash128, false, /*allow_mint*/ true);
+    PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::XXH3_128, true);
+    PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::Sha256, true);
+    const PoolMeta final_pm = PoolMeta::createOrValidate(*backend, layout, 256, BlobHashAlgo::CityHash128, false);
+    EXPECT_EQ(final_pm.algos_used, (std::vector<uint8_t>{1, 2, 3}));   /// union, sorted, nothing lost
+}
+
+/// ---- P1-T3a: the pool's blob_hash_algo threaded into the streaming write-buffer hash site ----
+
+/// `Cas::CaContentWriteBuffer`'s LOCAL-staging constructor (the everyday spill-to-temp-file
+/// mode `ContentAddressedTransaction::writeFile` uses), built with `BlobHashAlgo::XXH3_128`, must hash
+/// the streamed payload with xxh3 -- agreeing with the standalone `blobHashHexOneShot` one-shot helper
+/// (the same convention `poolContentHash`'s re-hash uses).
+TEST(CASPluggableHash, ContentWriteBufferLocalModeHashesWithSelectedAlgoXxh3)
+{
+    const std::string payload = makeMultiBlockPayload();
+    const auto temp_dir = (std::filesystem::temp_directory_path() / "cas_pluggable_hash_xxh3_local").string();
+
+    std::string got_hash_hex;
+    size_t got_size = 0;
+    auto buf = std::make_unique<DB::Cas::CaContentWriteBuffer>(
+        temp_dir,
+        BlobHashAlgo::XXH3_128,
+        /*buf_size=*/8192,
+        /*use_adaptive_buffer_size=*/false,
+        /*adaptive_buffer_initial_size=*/0,
+        [&](const std::string & hash_hex, size_t size, const std::string &)
+        {
+            got_hash_hex = hash_hex;
+            got_size = size;
+        });
+
+    /// Write in two chunks so more than one nextImpl flush happens (exercises the streaming state, not
+    /// just a single call).
+    buf->write(payload.data(), 1234);
+    buf->write(payload.data() + 1234, payload.size() - 1234);
+    buf->finalize();
+
+    EXPECT_EQ(got_size, payload.size());
+    EXPECT_EQ(got_hash_hex, blobHashHexOneShot(BlobHashAlgo::XXH3_128, payload));
+    /// A wrong-but-plausible result (e.g. accidentally still hashing with cityHash128) would silently
+    /// produce a DIFFERENT hex string -- pin that the two algos disagree on this payload, so the
+    /// assertion above is actually discriminating.
+    EXPECT_NE(got_hash_hex, blobHashHexOneShot(BlobHashAlgo::CityHash128, payload));
+}
+
+/// The DEFAULT algo (`CityHash128`) through the SAME write buffer must stay byte-for-byte unchanged --
+/// the CAS pluggable-blob-hash invariant (spec §8). Compares against `blobHashHexOneShot`, which
+/// `gtest_cas_blob_hasher.cpp`'s `CityHash128ByteIdenticalToHashingWriteBuffer` already proves is
+/// byte-identical to the pre-existing plain `HashingWriteBuffer` convention.
+TEST(CASPluggableHash, ContentWriteBufferLocalModeCityHash128Unchanged)
+{
+    const std::string payload = makeMultiBlockPayload();
+    const auto temp_dir = (std::filesystem::temp_directory_path() / "cas_pluggable_hash_ch128_local").string();
+
+    std::string got_hash_hex;
+    auto buf = std::make_unique<DB::Cas::CaContentWriteBuffer>(
+        temp_dir,
+        BlobHashAlgo::CityHash128,
+        /*buf_size=*/8192,
+        /*use_adaptive_buffer_size=*/false,
+        /*adaptive_buffer_initial_size=*/0,
+        [&](const std::string & hash_hex, size_t, const std::string &)
+        {
+            got_hash_hex = hash_hex;
+        });
+
+    buf->write(payload.data(), payload.size());
+    buf->finalize();
+
+    EXPECT_EQ(got_hash_hex, blobHashHexOneShot(BlobHashAlgo::CityHash128, payload));
+}
+
+/// (codecs-v3 phase 7) The two former `Pool...StampsEnvelopeHashAlgo...` tests were REMOVED: the v3
+/// blob envelope no longer carries a `hash_algo` field (the algo identity lives in the blob KEY, spec
+/// §blob-envelope). Algo correctness for the write path is covered by the P1-T3b blob-body-PATH-key
+/// tests below (they assert the blob key uses the pool's algo), which is the surviving source of truth.
+
+/// ---- P1-T3b: the pool's blob_hash_algo threaded into blob-body PATH keys (spec §3/§10) ----
+
+/// A blob written and promoted through a live ref on an xxh3-128 pool lands under the
+/// `blobs/xxh3/<shard>/<hex>` path segment (not the bare `blobs/<shard>/<hex>` shape), is readable at
+/// that key, and `runFsck`'s LIST-based discovery (`Layout::blobsPrefix`, deliberately algo-agnostic)
+/// finds it reachable and clean -- proving the GC/fsck key-parse (which takes only the LAST path
+/// component as the hex digest, `CasGc.cpp`/`CasFsck.cpp`) still works with the extra segment.
+TEST(CASPluggableHash, Xxh3BlobLandsUnderAlgoSegmentAndIsDiscoveredCleanByFsck)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .blob_hash_algo = BlobHashAlgo::XXH3_128});
+
+    const RootNamespace ns{"srv1/tbl"};
+    const std::string payload = makeMultiBlockPayload();
+    const BlobRef id{BlobHashAlgo::XXH3_128, codecFor(BlobHashAlgo::XXH3_128).fromHex(blobHashHexOneShot(BlobHashAlgo::XXH3_128, payload))};
+
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/rb";
+    auto build = store->beginPartWrite(info);
+    build->putBlob(id, BlobSource::fromString(payload));
+
+    /// The blob body landed under the algo-segmented path -- readable there, not at the legacy
+    /// no-segment shape.
+    const String blob_key = store->layout().blobKey(id);
+    EXPECT_NE(blob_key.find("/blobs/xxh3/"), String::npos) << blob_key;
+    EXPECT_EQ(blob_key.find("/blobs/ch128/"), String::npos) << blob_key;
+    EXPECT_TRUE(backend->head(blob_key).exists);
+
+    ManifestEntry e;
+    e.path = "data.bin";
+    e.placement = EntryPlacement::Blob;
+    e.ref = id;
+
+    e.blob_size = payload.size();
+    const ManifestId mid = build->stageManifest({e});
+    build->precommitAdd(ns, "rb", mid);
+    build->promote(ns, "rb", build->buildId(), mid);
+    store->renewWatermarkOnce();
+
+    const FsckReport rep = runFsck(*store, /*detail=*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.dangling, 0u);
+    EXPECT_GE(rep.reachable, 1u);
+
+    /// Not merely "clean by omission" (e.g. a bug that silently LISTed nothing): the physical listing
+    /// actually walked the algo-segmented key.
+    const bool found = std::any_of(rep.objects.begin(), rep.objects.end(),
+        [](const FsckObject & o) { return o.key.find("/blobs/xxh3/") != String::npos; });
+    EXPECT_TRUE(found);
+}
+
+/// ============================================================================================
+/// CAS pluggable-blob-hash Phase 2 Task 5 -- THE CRUX (anti-silent-leak regression gate).
+///
+/// Two sites classify a blob by parsing its object-key hex into a hash set: `CasGc.cpp`'s condemn
+/// path (the fold's transition-to-zero, and — until spec §7 removed it — `Gc::rebuildBaseline`'s
+/// LIST/HEAD sweep) and `CasFsck.cpp`'s
+/// present-but-unreferenced classification. Both used to route through the bare, fixed-width
+/// `hexToU128` (32-hex-only) inside a `catch(...) continue` / no-catch-at-all — so a 64-hex `sha256`
+/// key either (a) fell into the "foreign key shape — not ours" catch and was silently treated as
+/// debris (the condemn sweep: the blob is NEVER condemned — a permanent GC leak), or (b) threw
+/// uncaught out of fsck's present-but-unreferenced loop (a hard fsck failure on a live sha256 pool).
+/// Phase 2 Task 5 ports both to the pool-scoped `DigestCodec::fromHex`, which parses a CORRECT-WIDTH
+/// key (16 OR 32 bytes) — a genuinely foreign key shape (e.g. a `.meta` sibling) still falls into
+/// the catch, but a real sha256 blob no longer does.
+///
+/// This test constructs a `sha256`-algo pool DIRECTLY via `PoolConfig` (this bypasses only the
+/// disk-config *factory* guard in `MetadataStorageFactory.cpp`, which Task 6 removes — `Pool::open`
+/// itself has never gated on algo) and writes a blob body straight at its 64-hex content-addressed key
+/// (bypassing `PartWriteTxn::putBlob`, whose OWN internal `logical_hash` stays a fixed 128-bit
+/// representation until a later task — see the Task 5 report), references it, and drops the reference
+/// so the fold condemns it. It then drives BOTH crux sites and asserts the blob is CLASSIFIED, not
+/// silently skipped as foreign.
+///
+/// MUST GO RED if either port is reverted to `hexToU128`: reverting `CasGc.cpp`'s fold leaves
+/// `condemned_total == 0` (never condemned) and `previewDeletes()` empty; reverting `CasFsck.cpp`'s
+/// sites either throws out of `runFsck` or leaves the blob unclassified/absent from `unreachable`.
+TEST(CASPluggableHash, Sha256BlobSeenByCondemnSweepAndFsckNotSilentlySkipped)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .blob_hash_algo = BlobHashAlgo::Sha256, .gc_fold_max_defer_rounds = 0});
+    ASSERT_EQ(blobHashLenFor(store->writeAlgo()), 32u) << "sha256 must derive a 32-byte digest width";
+
+    const DigestCodec codec = codecFor(store->writeAlgo());
+    const std::string payload = makeMultiBlockPayload();
+    const std::string hex = blobHashHexOneShot(BlobHashAlgo::Sha256, payload);
+    ASSERT_EQ(hex.size(), 64u) << "sha256 renders 64 lowercase hex chars";
+    const BlobDigest digest = codec.fromHex(hex);   // round-trip sanity: must not throw at width 32
+
+    /// Reference the blob from a committed ref, then drop that ref: the fold sees `+1` then `-1`, the
+    /// blob transitions to in-degree zero, and the ORDINARY condemn path claims it. Every per-algo
+    /// parse this test guards sits on that path.
+    const RootNamespace ns{"00/aa@cas@"};
+    Gc gc(store, UInt128(1));
+    const SeededBlob seeded = seedReferencedBlob(*store, *backend, ns, BlobHashAlgo::Sha256,
+                                                 /*build_sequence=*/1, /*payload_size=*/5000, "tbl_sha");
+    const BlobRef id = seeded.ref;
+    const String blob_key = seeded.key;
+    EXPECT_NE(blob_key.find("/blobs/sha256/"), String::npos) << blob_key;
+    ASSERT_TRUE(backend->head(blob_key).exists) << "the sha256 blob body must be present before the fold";
+    ASSERT_EQ(codecFor(store->writeAlgo()).fromHex(hex), digest) << "fixture sanity: the seeded digest is ours";
+
+    /// ---- Site 1: the fold's condemn path ----
+    runRegularRoundReclaiming(gc);   /// folds the +1
+    dropSeededRef(*store, *backend, ns, /*build_sequence=*/1, "tbl_sha");
+    runRegularRoundReclaiming(gc);   /// folds the -1: transition to zero => condemned
+
+    const auto state_bytes = backend->get(store->layout().gcStateKey());
+    ASSERT_TRUE(state_bytes.has_value());
+    const GcState state = decodeGcState(state_bytes->bytes);
+    ASSERT_GT(state.snap_generation, 0u);
+    const auto seal_bytes = backend->get(store->layout().foldSealKey(state.snap_generation, state.snap_attempt));
+    ASSERT_TRUE(seal_bytes.has_value());
+    const CasFoldSeal seal = decodeFoldSeal(seal_bytes->bytes);
+    ASSERT_TRUE(seal.condemned_summary.contains(0)) << "the seal's condemned_summary must be total over gc_shards";
+    EXPECT_EQ(seal.condemned_summary.at(0).condemned_total, 1u)
+        << "THE CRUX: the sha256 blob must be condemned by the fold -- a silent-leak regression (a "
+           "reverted CasGc.cpp codec.fromHex port) leaves this at 0";
+
+    /// previewDeletes streams the SAME adopted seal via the run's own SourceEdgeKeyCodec (never pool
+    /// meta) and must report exactly our blob, at its real 32-byte digest.
+    const std::vector<Gc::PreviewEntry> preview = gc.previewDeletes();
+    ASSERT_EQ(preview.size(), 1u) << "THE CRUX: previewDeletes must surface the condemned sha256 blob";
+    EXPECT_EQ(preview[0].ref, id);
+    EXPECT_EQ(preview[0].key, blob_key);
+
+    /// ---- Site 2: fsck's present-but-unreferenced classification ----
+    /// Must complete without throwing (a reverted port either throws BAD_ARGUMENTS out of the
+    /// no-try/catch parse sites, or silently drops the blob from every classified set) and must
+    /// physically account for the blob.
+    FsckReport frep;
+    ASSERT_NO_THROW(frep = runFsck(*store, /*detail=*/true));
+    EXPECT_GE(frep.unreachable, 1u)
+        << "THE CRUX: fsck's physical listing must count the sha256 blob as unreachable-but-present, "
+           "not silently omit it";
+    const auto oit = std::find_if(frep.objects.begin(), frep.objects.end(),
+        [&](const FsckObject & o) { return o.key == blob_key; });
+    ASSERT_NE(oit, frep.objects.end()) << "the sha256 blob must appear in fsck's detailed object list";
+    /// The fold above already condemned it into the GC snapshot, so fsck's GC-pipeline-view
+    /// classification (not the generic Unaccounted bucket -- reachable only by width-correctly pairing
+    /// the fsck-side hash against the run's kCondemned row hash) must recognize it as known-to-GC.
+    EXPECT_EQ(oit->cls, FsckClass::PendingGc)
+        << "THE CRUX: fsck must pair the sha256 blob against the GC snapshot's kCondemned row (a "
+           "silent-leak regression in CasFsck.cpp's unref_hashes/in_run_hashes/retired_by_hash port "
+           "leaves this as the generic Unaccounted bucket instead)";
+}
+
+/// ============================================================================================
+/// CAS pluggable-blob-hash Phase 2 Task 6 -- end-to-end sha256 WRITE path (in-memory; the real
+/// wiring-level integration + soak is Task 7).
+///
+/// Before this task, `PartWriteTxn`'s OWN write-path internals stayed a fixed 128-bit representation
+/// downstream of the mint (`poolContentHash`/`PartWriteTxn::putBlob`'s `logical_hash`, the `deps` map key, the
+/// event-log `object_hash` render, and `objectKey`) -- safe only because the disk-config factory guard
+/// (`MetadataStorageFactory.cpp`) blocked any real sha256 pool from reaching `PartWriteTxn` at all (see the
+/// Task 5 report and the "Task 6+" comments this task removes). Task 6 finishes those sites AND lifts
+/// the guard in the SAME commit. This test drives a REAL `PartWriteTxn` (`putBlob` -> `stageManifest` ->
+/// `precommitAdd` -> `promote`) on a `Sha256` pool and asserts:
+///   1. the blob lands under `blobs/sha256/<64-hex>` and the manifest entry's `blob_hash`, read back via
+///      `decodePartManifest`, is the FULL 32-byte digest (bytes beyond 16 are non-zero for a real sha256
+///      digest, i.e. NOT truncated to `.toU128()`'s low 16 bytes);
+///   2. an inline file and a standalone blob of IDENTICAL content get the SAME 32-byte `file_hash` under
+///      sha256 -- mirroring the (fixed) `ContentAddressedTransaction.cpp` inline-candidate formula
+///      (`blobHashHexOneShot(pool_algo, bytes)` -> pool-scoped `DigestCodec::fromHex`) directly at the
+///      Core level, since exercising the wiring itself is Task 7's job;
+///   3. `runFsck` on the pool is clean (no dangling, no foreign) -- the whole write -> GC -> fsck loop
+///      agrees on the 64-hex key.
+TEST(CASPluggableHash, Sha256BuildWritesFullWidthDigestAndInlineEqualsBlob)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .blob_hash_algo = BlobHashAlgo::Sha256});
+    ASSERT_EQ(blobHashLenFor(store->writeAlgo()), 32u) << "sha256 must derive a 32-byte digest width";
+    const DigestCodec codec = codecFor(store->writeAlgo());
+
+    const RootNamespace ns{"srv1/tbl"};
+    const std::string payload = makeMultiBlockPayload();
+    const std::string hex = blobHashHexOneShot(BlobHashAlgo::Sha256, payload);
+    ASSERT_EQ(hex.size(), 64u) << "sha256 renders 64 lowercase hex chars";
+    const BlobRef id{BlobHashAlgo::Sha256, codec.fromHex(hex)};
+
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/part1";
+    auto build = store->beginPartWrite(info);
+    const PutBlobResult ref = build->putBlob(id, BlobSource::fromString(payload));
+    EXPECT_EQ(ref.size, payload.size());
+
+    /// THE CRUX (blob side): the blob body lands under the sha256-segmented path, addressed by the
+    /// FULL 64-hex key -- `PartWriteTxn::putBlob`'s internal `logical_hash` must not have silently narrowed it
+    /// to a 32-hex (128-bit) key before this task.
+    const String blob_key = store->layout().blobKey(id);
+    EXPECT_NE(blob_key.find("/blobs/sha256/"), String::npos) << blob_key;
+    ASSERT_TRUE(backend->head(blob_key).exists);
+
+    /// Mirror the (fixed) inline-candidate hash site directly: same content, same pool algo, via the
+    /// SAME public formula ContentAddressedTransaction.cpp's writeFile now uses -- NOT the old hardcoded
+    /// CityHash128 (which would produce a DIFFERENT, 128-bit-then-zero-padded value here).
+    const BlobDigest inline_hash = codec.fromHex(blobHashHexOneShot(BlobHashAlgo::Sha256, payload));
+    const BlobDigest blob_hash = codec.fromHex(hex);
+    EXPECT_EQ(inline_hash, blob_hash) << "inline == blob: identical content must hash identically under sha256";
+
+    /// THE CRUX (width): a genuine 32-byte sha256 digest must NOT be zero-padded past byte 16 -- the
+    /// shape `BlobDigest::fromU128` (or a reverted hardcoded-CityHash128 inline site) would produce.
+    const bool tail_nonzero = std::any_of(blob_hash.bytes.begin() + 16, blob_hash.bytes.end(),
+        [](uint8_t b) { return b != 0; });
+    EXPECT_TRUE(tail_nonzero) << "a genuine sha256 digest must not be zero-padded past byte 16";
+
+    ManifestEntry blob_entry;
+    blob_entry.path = "data.bin";
+    blob_entry.placement = EntryPlacement::Blob;
+    blob_entry.ref = BlobRef{BlobHashAlgo::Sha256, blob_hash};
+    blob_entry.blob_size = payload.size();
+
+    ManifestEntry inline_entry;
+    inline_entry.path = "checksums.txt";
+    inline_entry.placement = EntryPlacement::Inline;
+    inline_entry.ref = BlobRef{BlobHashAlgo::Sha256, inline_hash};
+    inline_entry.blob_size = payload.size();
+    inline_entry.inline_bytes = payload;
+
+    const ManifestId mid = build->stageManifest({blob_entry, inline_entry});
+    build->precommitAdd(ns, "part1", mid);
+    build->promote(ns, "part1", build->buildId(), mid);
+    store->renewWatermarkOnce();
+
+    /// Read the committed manifest back -- the on-disk `blob_hash` must be the FULL 32-byte digest, not
+    /// truncated by the manifest codec or by anything upstream of `stageManifest`.
+    const auto manifest_bytes = backend->get(store->layout().manifestKey(mid));
+    ASSERT_TRUE(manifest_bytes.has_value());
+    const PartManifest read_back = decodePartManifest(openObject(FormatId::PartManifest, manifest_bytes->bytes));
+    ASSERT_EQ(read_back.entries.size(), 2u);
+    const auto read_blob_it = std::find_if(read_back.entries.begin(), read_back.entries.end(),
+        [](const ManifestEntry & e) { return e.placement == EntryPlacement::Blob; });
+    ASSERT_NE(read_blob_it, read_back.entries.end());
+    EXPECT_EQ(read_blob_it->ref.digest, blob_hash);
+    const bool read_tail_nonzero = std::any_of(read_blob_it->ref.digest.bytes.begin() + 16,
+        read_blob_it->ref.digest.bytes.end(), [](uint8_t b) { return b != 0; });
+    EXPECT_TRUE(read_tail_nonzero) << "the manifest's on-disk blob_hash must not be truncated either";
+
+    /// The write -> GC -> fsck loop must agree end-to-end on the 64-hex key: clean, no dangling.
+    const FsckReport rep = runFsck(*store, /*detail=*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.dangling, 0u);
+    EXPECT_GE(rep.reachable, 1u);
+}
+
+/// ============================================================================================
+/// CAS mixed-algo pools Phase 3 T5:
+/// path-derived `BlobRef` in the sweep/fsck (`Layout::parseBlobKey`) and per-entry admission
+/// validation at `foldManifestEdges` with refresh-on-miss.
+/// ============================================================================================
+
+/// spec §9.8 -- THE race regression this task exists to close. Each `Pool`'s `admitted_algos` cache
+/// is a MONOTONE snapshot seeded once at `Pool::open` and never re-read on its own; if node A admits
+/// a brand-new algo and publishes a manifest naming it, node B's stale cache must NOT fail the fold
+/// closed forever -- `foldManifestEdges` must refresh `_pool_meta` on the very first miss and accept
+/// once the fresh read proves the algo genuinely admitted. Node B is opened BEFORE node A performs the
+/// admission on purpose: constructing B afterward would seed its cache already-fresh and never
+/// exercise the race the fix targets.
+TEST(CASPluggableHash, StaleAlgoRegistryRefreshOnMiss)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+
+    /// Node B opens FIRST -- its admitted-cache seeds at {ch128} only, before sha256 exists anywhere.
+    auto store_b = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "b",
+                   .blob_hash_algo = BlobHashAlgo::CityHash128});
+    ASSERT_TRUE(store_b->isAlgoAdmitted(BlobHashAlgo::CityHash128));
+    ASSERT_FALSE(store_b->isAlgoAdmitted(BlobHashAlgo::Sha256));
+
+    /// Node A opens SECOND, admits sha256 via the opt-in flag, and publishes a manifest naming a
+    /// sha256 blob through the real PartWriteTxn path (putBlob -> stageManifest -> precommitAdd -> promote).
+    auto store_a = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "a",
+                   .blob_hash_algo = BlobHashAlgo::Sha256, .blob_hash_allow_new = true});
+    ASSERT_TRUE(store_a->isAlgoAdmitted(BlobHashAlgo::Sha256));
+
+    const RootNamespace ns{"srv1/tbl"};
+    const std::string payload = makeMultiBlockPayload();
+    const BlobRef id{BlobHashAlgo::Sha256, codecFor(BlobHashAlgo::Sha256).fromHex(blobHashHexOneShot(BlobHashAlgo::Sha256, payload))};
+
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/part1";
+    auto build = store_a->beginPartWrite(info);
+    build->putBlob(id, BlobSource::fromString(payload));
+
+    ManifestEntry e;
+    e.path = "data.bin";
+    e.placement = EntryPlacement::Blob;
+    e.ref = id;
+    e.blob_size = payload.size();
+    const ManifestId mid = build->stageManifest({e});
+    build->precommitAdd(ns, "part1", mid);
+    build->promote(ns, "part1", build->buildId(), mid);
+    store_a->renewWatermarkOnce();
+
+    /// B's cache is STILL stale here -- it has never re-read `_pool_meta` since open.
+    ASSERT_FALSE(store_b->isAlgoAdmitted(BlobHashAlgo::Sha256));
+
+    /// B folds the committed ref naming the sha256 entry: without refresh-on-miss this throws
+    /// CORRUPTED_DATA ("manifest entry algo sha256 not admitted"); with it, the miss triggers exactly
+    /// one `refreshAdmittedAlgos()` and the fold proceeds.
+    Gc gc(store_b, UInt128(1));
+    const RebuildReport rep = gc.rebuildBaseline(/*force*/ true);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_EQ(rep.committed_refs, 1u);
+    EXPECT_TRUE(store_b->isAlgoAdmitted(BlobHashAlgo::Sha256)) << "the miss must have unioned B's cache";
+}
+
+/// spec §9.4 half: an object whose key names an algo THIS BUILD has never heard of (a genuinely
+/// foreign top-level segment, e.g. planted by a different/future tool) must never be treated as one
+/// of ours -- the GC must skip it (never condemn or delete it) and fsck must classify it into the
+/// generic `Unaccounted` bucket (never throw, never silently drop it from the physical listing).
+/// In the SAME pass, a 2-algo pool's OWN blobs under `blobs/ch128/` and `blobs/sha256/` must both
+/// still be classified normally -- the foreign segment must not make the fold/fsck narrow to one
+/// algo or blind them to the others.
+TEST(CASPluggableHash, ForeignAlgoSegmentIsDebrisNotOurs)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .blob_hash_algo = BlobHashAlgo::CityHash128, .gc_fold_max_defer_rounds = 0});
+    /// Admit sha256 into the SAME pool from a second mount, then pull the union into `store`'s cache.
+    Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test2",
+                   .blob_hash_algo = BlobHashAlgo::Sha256, .blob_hash_allow_new = true});
+    store->refreshAdmittedAlgos();
+    ASSERT_TRUE(store->isAlgoAdmitted(BlobHashAlgo::Sha256));
+
+    /// Two of the pool's OWN blobs -- one per algo -- each referenced by a committed ref that is then
+    /// DROPPED, so the fold condemns both by transition-to-zero.
+    const RootNamespace ns{"00/aa@cas@"};
+    Gc gc(store, UInt128(1));
+    const SeededBlob ch = seedReferencedBlob(*store, *backend, ns, BlobHashAlgo::CityHash128,
+                                             /*build_sequence=*/1, /*payload_size=*/5001, "tbl_ch");
+    const SeededBlob sh = seedReferencedBlob(*store, *backend, ns, BlobHashAlgo::Sha256,
+                                             /*build_sequence=*/2, /*payload_size=*/5002, "tbl_sh");
+    const BlobRef ch_ref = ch.ref;
+    const BlobRef sh_ref = sh.ref;
+    const String ch_key = ch.key;
+    const String sh_key = sh.key;
+
+    /// A FOREIGN object under an algo segment `blobHashAlgoName` never renders ("md5") -- not one of
+    /// ours under any circumstance.
+    const String foreign_key = store->layout().blobsPrefix() + "md5/aa/" + std::string(32, 'a');
+    backend->putIfAbsent(foreign_key, std::string("not a real envelope"));
+
+    runRegularRoundReclaiming(gc);   /// folds both +1s
+    dropSeededRef(*store, *backend, ns, /*build_sequence=*/1, "tbl_ch");
+    dropSeededRef(*store, *backend, ns, /*build_sequence=*/2, "tbl_sh");
+    runRegularRoundReclaiming(gc);   /// folds both -1s: both transition to zero
+
+    /// The fold condemns exactly the two OWN blobs -- never the foreign object.
+    const std::vector<Gc::PreviewEntry> preview = gc.previewDeletes();
+    ASSERT_EQ(preview.size(), 2u);
+    std::unordered_set<BlobRef, BlobRefHash> condemned_refs;
+    for (const auto & p : preview)
+    {
+        condemned_refs.insert(p.ref);
+        EXPECT_NE(p.key, foreign_key);
+    }
+    EXPECT_TRUE(condemned_refs.count(ch_ref));
+    EXPECT_TRUE(condemned_refs.count(sh_ref));
+    EXPECT_TRUE(backend->head(foreign_key).exists) << "the foreign object must never be touched by the fold";
+
+    const FsckReport frep = runFsck(*store, /*detail=*/true);
+    /// The physical listing counts all THREE unreferenced objects (two ours + one foreign).
+    EXPECT_EQ(frep.unreachable, 3u);
+    const auto foreign_obj = std::find_if(frep.objects.begin(), frep.objects.end(),
+        [&](const FsckObject & o) { return o.key == foreign_key; });
+    ASSERT_NE(foreign_obj, frep.objects.end()) << "the foreign object must still appear in the physical listing";
+    /// ... but classified as generic Unaccounted -- it can never pair against the GC snapshot, which
+    /// only ever knows about OUR two algo-segmented refs.
+    EXPECT_EQ(foreign_obj->cls, FsckClass::Unaccounted);
+
+    /// The two OWN blobs are recognized under their OWN algo segment in the SAME pass.
+    const auto ch_obj = std::find_if(frep.objects.begin(), frep.objects.end(),
+        [&](const FsckObject & o) { return o.key == ch_key; });
+    const auto sh_obj = std::find_if(frep.objects.begin(), frep.objects.end(),
+        [&](const FsckObject & o) { return o.key == sh_key; });
+    ASSERT_NE(ch_obj, frep.objects.end());
+    ASSERT_NE(sh_obj, frep.objects.end());
+    EXPECT_EQ(ch_obj->cls, FsckClass::PendingGc);
+    EXPECT_EQ(sh_obj->cls, FsckClass::PendingGc);
+}
+
+/// ============================================================================================
+/// CAS reader-generation gate (`Core/Formats/CasFormat.h`'s `G_BUILD`) was raised to 4 for
+/// per-namespace contiguous ref-log ids (INV-1) and has since moved again, to 5, for Stage B's
+/// namespace-life-keyed ref layer ("format bump B", `kNamespaceLifeKeyedGeneration`) -- this test's
+/// assertions read `G_BUILD` itself rather than a hardcoded generation number for exactly that reason,
+/// so a THIRD bump does not silently make them false. `PoolMeta::createOrValidate`'s open-time
+/// CAS-raise targets `G_BUILD`, and `decodePoolMeta` fail-closes BOTH on a FUTURE
+/// `min_reader_generation` AND on a BACKWARD pool whose header `compatibility_version` is below
+/// `kNamespaceLifeKeyedGeneration` (which, being the LATER of the two historical breaking-change
+/// floors, subsumes `kContiguousRefStreamsGeneration` -- see `CasPoolMetaFormat.cpp`).
+/// ============================================================================================
+
+TEST(CASPluggableHash, ReaderGenerationIsRaisedToGBuild)
+{
+    EXPECT_GE(G_BUILD, kNamespaceLifeKeyedGeneration)
+        << "the reader-generation gate must be at least the namespace-life-keyed floor it enforces";
+
+    /// A freshly opened/created pool records `min_reader_generation == G_BUILD` (the open-time
+    /// CAS-raise, `PoolMeta::createOrValidate`, always targets this build's own floor).
+    {
+        auto backend = std::make_shared<InMemoryBackend>();
+        auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+        EXPECT_EQ(store->poolMeta().min_reader_generation, G_BUILD);
+
+        const auto meta_bytes = backend->get(store->layout().poolMetaKey());
+        ASSERT_TRUE(meta_bytes.has_value());
+        EXPECT_EQ(decodePoolMeta(meta_bytes->bytes).min_reader_generation, G_BUILD);
+    }
+
+    /// FORWARD gate: a pool-meta carrying `min_reader_generation == G_BUILD + 1` (one generation past
+    /// THIS build's floor) still fails closed at open -- the startup gate (`decodePoolMeta`) rejects it
+    /// even though generation 4 is now understood.
+    {
+        auto backend = std::make_shared<InMemoryBackend>();
+        const Layout layout("p");
+        PoolMeta pm = PoolMeta::createOrValidate(*backend, layout, /*blob_header_len*/ 256, BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+        pm.min_reader_generation = G_BUILD + 1;
+        ASSERT_TRUE(backend->casPut(layout.poolMetaKey(), encodePoolMeta(pm), backend->get(layout.poolMetaKey())->token).outcome == CasOutcome::Committed);
+
+        expectThrowsCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION, [&]
+        { Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}); });
+    }
+
+    /// BACKWARD floor: a pool whose header `v` (compatibility_version) is BELOW `G_BUILD` was written
+    /// by an older build this reader can no longer trust -- today that is one generation short of
+    /// `kNamespaceLifeKeyedGeneration`, a pool whose ref-object keys carry no incarnation segment,
+    /// which this build's parsers refuse as corruption rather than read. Craft it at the text layer:
+    /// take a fresh pool-meta and rewrite its line-1 version gate down to `G_BUILD - 1` (an older
+    /// build would have stamped exactly that).
+    {
+        auto backend = std::make_shared<InMemoryBackend>();
+        const Layout layout("p");
+        PoolMeta pm = PoolMeta::createOrValidate(*backend, layout, /*blob_header_len*/ 256, BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+        const String fresh_bytes = encodePoolMeta(pm);
+
+        const String from = "\"v\":" + std::to_string(G_BUILD);
+        const String to = "\"v\":" + std::to_string(G_BUILD - 1);
+        const auto pos = fresh_bytes.find(from);
+        ASSERT_NE(pos, String::npos);   // sanity: a fresh pool stamps the header at the floor
+        String downgraded = fresh_bytes;
+        downgraded.replace(pos, from.size(), to);
+        ASSERT_TRUE(backend->casPut(layout.poolMetaKey(), downgraded, backend->get(layout.poolMetaKey())->token).outcome == CasOutcome::Committed);
+
+        /// `decodePoolMeta`'s backward floor rejects the downgraded bytes directly...
+        expectThrowsCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION, [&] { decodePoolMeta(downgraded); });
+        /// ...and so does a full `Pool::open` (decoding the pool-meta is its first fail-closed step).
+        expectThrowsCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION, [&]
+        { Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}); });
+    }
+}
+
+/// ============================================================================================
+/// CAS mixed-algo pools Phase 3 T6:
+/// cross-cutting cruxes over a pool that genuinely mixes algos end-to-end (reclaim + distinctness).
+/// The no-bare-digest grep gates (design Step 3) are run separately, not as gtest bodies.
+/// ============================================================================================
+
+/// spec §9.3 -- THE reclaim crux. A pool admits BOTH `ch128` and `sha256`; a blob body is
+/// planted directly under EACH algo's segment (mirrors `Sha256BlobSeenByCondemnSweepAndFsckNotSilentlySkipped`'s
+/// fixture, widened to two algos). The fold must condemn BOTH into the SAME baseline
+/// (`previewDeletes` surfaces both refs), and driving the round-paced pipeline to completion (graduate,
+/// then the exact-token delete) must reclaim BOTH bodies -- the backend ends up holding ZERO blob
+/// bytes of EITHER algo, and fsck reports clean.
+///
+/// MUST GO RED if any settlement/graduation/delete path silently narrows to one algo -- e.g. a fold
+/// that only accounts `blobs/ch128/`, a graduation/delete loop that iterates a digest-only set and
+/// coalesces the two algos' entries, or an fsck reachability check that stops after the first algo it
+/// sees.
+TEST(CASPluggableHash, TwoAlgoBlobsBothFullyReclaimed)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .blob_hash_algo = BlobHashAlgo::CityHash128, .gc_fold_max_defer_rounds = 0});
+    /// Admit sha256 into the SAME pool from a second mount, then pull the union into `store`'s cache
+    /// (mirrors `ForeignAlgoSegmentIsDebrisNotOurs`'s admission fixture).
+    Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test2",
+                   .blob_hash_algo = BlobHashAlgo::Sha256, .blob_hash_allow_new = true});
+    store->refreshAdmittedAlgos();
+    ASSERT_TRUE(store->isAlgoAdmitted(BlobHashAlgo::Sha256));
+
+    /// Two of the pool's OWN blobs -- one per algo -- referenced then dropped, so the fold condemns
+    /// both by transition-to-zero.
+    const RootNamespace ns{"00/aa@cas@"};
+    Gc gc(store, UInt128(1));
+    const SeededBlob ch = seedReferencedBlob(*store, *backend, ns, BlobHashAlgo::CityHash128,
+                                             /*build_sequence=*/1, /*payload_size=*/5001, "tbl_ch");
+    const SeededBlob sh = seedReferencedBlob(*store, *backend, ns, BlobHashAlgo::Sha256,
+                                             /*build_sequence=*/2, /*payload_size=*/5002, "tbl_sh");
+    const String ch_key = ch.key;
+    const String sh_key = sh.key;
+    ASSERT_TRUE(backend->head(ch_key).exists);
+    ASSERT_TRUE(backend->head(sh_key).exists);
+
+    runRegularRoundReclaiming(gc);   /// folds both +1s
+    dropSeededRef(*store, *backend, ns, /*build_sequence=*/1, "tbl_ch");
+    dropSeededRef(*store, *backend, ns, /*build_sequence=*/2, "tbl_sh");
+    runRegularRoundReclaiming(gc);   /// folds both -1s: both condemned in the same round
+
+    /// previewDeletes covers BOTH refs from the adopted seal -- never just one algo.
+    {
+        const std::vector<Gc::PreviewEntry> preview = gc.previewDeletes();
+        ASSERT_EQ(preview.size(), 2u);
+        std::unordered_set<BlobRef, BlobRefHash> refs;
+        for (const auto & p : preview)
+            refs.insert(p.ref);
+        EXPECT_TRUE(refs.count(ch.ref));
+        EXPECT_TRUE(refs.count(sh.ref));
+    }
+
+    /// Drive the round-paced pipeline to actual physical deletion: the fold condemned both at its
+    /// round; the VERY NEXT round graduates them (unconditionally, round-paced); the round after that
+    /// executes the exact-token delete for both.
+    {
+        const RoundReport rep1 = runRegularRoundReclaiming(gc);
+        EXPECT_EQ(rep1.graduated, 2u) << "both algos' blobs must graduate together in one round";
+        EXPECT_TRUE(backend->head(ch_key).exists);   // pending: still present this pass
+        EXPECT_TRUE(backend->head(sh_key).exists);
+    }
+    {
+        const RoundReport rep2 = runRegularRoundReclaiming(gc);
+        EXPECT_EQ(rep2.redeleted, 2u) << "both algos' pending deletes must execute together in one round";
+    }
+
+    /// THE CRUX: after graduation the backend holds ZERO blob bodies of EITHER algo.
+    EXPECT_FALSE(backend->head(ch_key).exists) << "the ch128 blob must be physically reclaimed";
+    EXPECT_FALSE(backend->head(sh_key).exists) << "the sha256 blob must be physically reclaimed";
+
+    const FsckReport frep = runFsck(*store, /*detail=*/true);
+    EXPECT_TRUE(frep.clean());
+    EXPECT_EQ(frep.dangling, 0u);
+}
+
+/// spec §9.5 -- same-digest-different-algo end-to-end. `ch128:X` and `xxh3:X` share the SAME 16-byte
+/// digest VALUE but are DISTINCT blob identities (`BlobRef` is the pair): distinct object keys, distinct
+/// `.meta`, distinct bodies, distinct settlement rows (fold both -> distinct in-degree per ref), and
+/// dropping ONE ref's committed manifest reclaims ONLY that algo's blob -- the other stays fully
+/// readable throughout.
+///
+/// MUST GO RED if anything upstream of `BlobRef` ever collapses identity to the bare digest (e.g. a
+/// settlement/meta/condemn site keyed on `BlobDigest` alone) -- the two blobs would alias into one row
+/// and dropping one ref would (wrongly) reclaim or corrupt the other.
+TEST(CASPluggableHash, SameDigestDifferentAlgoDistinctBodiesAndSettlement)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .blob_hash_algo = BlobHashAlgo::CityHash128, .gc_fold_max_defer_rounds = 0});
+    Pool::open(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test2",
+                   .blob_hash_algo = BlobHashAlgo::XXH3_128, .blob_hash_allow_new = true});
+    store->refreshAdmittedAlgos();
+    ASSERT_TRUE(store->isAlgoAdmitted(BlobHashAlgo::XXH3_128));
+
+    /// SAME 16-byte digest VALUE under two different algos -- deliberately NOT derived from either
+    /// body's real content hash: the crux under test is identity distinctness (the pair), not hash
+    /// correctness (already covered by the sha256/xxh3 write-path tests above).
+    const BlobDigest shared_digest = BlobDigest::fromU128(UInt128(0xC0FFEE));
+    const BlobRef ref_ch{BlobHashAlgo::CityHash128, shared_digest};
+    const BlobRef ref_xx{BlobHashAlgo::XXH3_128, shared_digest};
+    /// Distinct content, not merely distinct length: `makeMultiBlockPayload` at two different sizes
+    /// would make the shorter body a byte-for-byte PREFIX of the longer one (same repeating pattern
+    /// from the same phase), which would defeat the "must not contain" assertions below.
+    const std::string body_ch = makeMultiBlockPayload(4001);
+    std::string body_xx = makeMultiBlockPayload(4002);
+    std::reverse(body_xx.begin(), body_xx.end());
+    ASSERT_NE(body_ch, body_xx);
+
+    const RootNamespace ns{"srv1/tbl"};
+
+    PartWriteInfo info_a;
+    info_a.intended_ref = ns.string() + "/part_a";
+    auto build_a = store->beginPartWrite(info_a);
+    build_a->putBlob(ref_ch, BlobSource::fromString(body_ch));
+    ManifestEntry e_a;
+    e_a.path = "a.bin"; e_a.placement = EntryPlacement::Blob; e_a.ref = ref_ch; e_a.blob_size = body_ch.size();
+    const ManifestId mid_a = build_a->stageManifest({e_a});
+    build_a->precommitAdd(ns, "part_a", mid_a);
+    build_a->promote(ns, "part_a", build_a->buildId(), mid_a);
+
+    PartWriteInfo info_b;
+    info_b.intended_ref = ns.string() + "/part_b";
+    auto build_b = store->beginPartWrite(info_b);
+    build_b->putBlob(ref_xx, BlobSource::fromString(body_xx));
+    ManifestEntry e_b;
+    e_b.path = "b.bin"; e_b.placement = EntryPlacement::Blob; e_b.ref = ref_xx; e_b.blob_size = body_xx.size();
+    const ManifestId mid_b = build_b->stageManifest({e_b});
+    build_b->precommitAdd(ns, "part_b", mid_b);
+    build_b->promote(ns, "part_b", build_b->buildId(), mid_b);
+    store->renewWatermarkOnce();
+
+    /// Distinct object keys and distinct bodies despite the SAME digest value.
+    const String key_ch = store->layout().blobKey(ref_ch);
+    const String key_xx = store->layout().blobKey(ref_xx);
+    EXPECT_NE(key_ch, key_xx);
+    const auto raw_ch = backend->get(key_ch);
+    const auto raw_xx = backend->get(key_xx);
+    ASSERT_TRUE(raw_ch.has_value());
+    ASSERT_TRUE(raw_xx.has_value());
+    EXPECT_NE(raw_ch->bytes.find(body_ch), String::npos);
+    EXPECT_NE(raw_xx->bytes.find(body_xx), String::npos);
+    EXPECT_EQ(raw_ch->bytes.find(body_xx), String::npos) << "the ch128 body must not contain the xxh3 payload";
+    EXPECT_EQ(raw_xx->bytes.find(body_ch), String::npos) << "the xxh3 body must not contain the ch128 payload";
+
+    /// Distinct `.meta` objects.
+    const String meta_ch = store->layout().blobMetaKey(ref_ch);
+    const String meta_xx = store->layout().blobMetaKey(ref_xx);
+    EXPECT_NE(meta_ch, meta_xx);
+    EXPECT_TRUE(backend->head(meta_ch).exists);
+    EXPECT_TRUE(backend->head(meta_xx).exists);
+
+    /// Distinct settlement (in-degree per ref, keyed on the FULL `BlobRef` pair -- never the shared
+    /// bare digest, which would alias the two rows into one).
+    Gc gc(store, UInt128(1));
+    runRegularRoundReclaiming(gc);
+    {
+        const GcState st = decodeGcState(backend->get(store->layout().gcStateKey())->bytes);
+        const CasFoldSeal seal = decodeFoldSeal(
+            backend->get(store->layout().foldSealKey(st.snap_generation, st.snap_attempt))->bytes);
+        EXPECT_EQ(inDegreeInRuns(*backend, seal.blob_target_runs, ref_ch), 1);
+        EXPECT_EQ(inDegreeInRuns(*backend, seal.blob_target_runs, ref_xx), 1);
+    }
+
+    /// Dropping ONLY `part_a`'s committed ref condemns+reclaims ONLY `ch128:X`; `xxh3:X` (the SAME
+    /// digest value, a DIFFERENT algo) stays referenced and fully readable throughout.
+    store->dropRef(ns, "part_a");
+    runRegularRoundReclaiming(gc);   // condemns ch128:X (in-degree drops to 0); xxh3:X is untouched (still ref'd)
+    runRegularRoundReclaiming(gc);   // graduates ch128:X
+    runRegularRoundReclaiming(gc);   // executes the exact-token delete for ch128:X
+
+    EXPECT_FALSE(backend->head(key_ch).exists) << "ch128:X must be reclaimed once its ref is dropped";
+    EXPECT_TRUE(backend->head(key_xx).exists)
+        << "THE CRUX: xxh3:X (same digest value, different algo) must remain readable after ch128:X "
+           "is reclaimed -- a digest-only settlement would have condemned/deleted both together";
+    const auto still_readable = backend->get(key_xx);
+    ASSERT_TRUE(still_readable.has_value());
+    EXPECT_NE(still_readable->bytes.find(body_xx), String::npos);
+
+    const FsckReport frep = runFsck(*store, /*detail=*/true);
+    EXPECT_TRUE(frep.clean());
+    EXPECT_EQ(frep.dangling, 0u);
+}
diff --git a/src/Disks/tests/gtest_cas_pool.cpp b/src/Disks/tests/gtest_cas_pool.cpp
new file mode 100644
index 000000000000..53f19ed32738
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_pool.cpp
@@ -0,0 +1,2079 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Poco/Exception.h>
+#include <algorithm>
+#include <atomic>
+#include <condition_variable>
+#include <future>
+#include <limits>
+#include <optional>
+#include <thread>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+extern const int ABORTED;
+extern const int BAD_ARGUMENTS;
+extern const int CORRUPTED_DATA;
+extern const int NOT_IMPLEMENTED;
+extern const int UNKNOWN_FORMAT_VERSION;
+extern const int FILE_DOESNT_EXIST;
+extern const int UNKNOWN_EXCEPTION;
+extern const int NETWORK_ERROR;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASRefRecoveryEpochSealed;
+extern const Event CASMountExclusivityViolation;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::blobEntryFor;
+using DB::Cas::tests::expectThrowsCode;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace
+{
+/// Counts mutating backend calls so a test can assert an open path is write-free.
+class WriteCountingBackend final : public DB::Cas::Backend
+{
+public:
+    explicit WriteCountingBackend(std::shared_ptr<DB::Cas::Backend> inner_) : inner(std::move(inner_)) {}
+    size_t writes = 0;
+
+    std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override { return inner->get(k, r); }
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+    DB::Cas::HeadResult head(const String & k) override { return inner->head(k); }
+    DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+    DB::Cas::PutResult putIfAbsent(const String & k, const String & b, const DB::Cas::ObjectMeta & meta) override { ++writes; return inner->putIfAbsent(k, b, meta); }
+    DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & meta) override { ++writes; return inner->putIfAbsentStream(k, meta); }
+    DB::Cas::PutResult putOverwrite(const String & k, const String & b, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & meta) override { ++writes; return inner->putOverwrite(k, b, e, meta); }
+    DB::Cas::CasResult casPut(const String & k, const String & b, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & meta) override { ++writes; return inner->casPut(k, b, e, meta); }
+    DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & t) override { ++writes; return inner->deleteExact(k, t); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+private:
+    std::shared_ptr<DB::Cas::Backend> inner;
+};
+
+/// Publish one part `ref` through the REAL PartWriteTxn write path: stage a manifest holding a single content
+/// blob whose payload is `payload`, precommit-add into the owning shard, then promote precommit ->
+/// committed. Returns the published ManifestId. This is the canonical write-side fixture for the
+/// read-path tests (the same shape as `publishPart` in gtest_cas_gc_log.cpp). The manifest entry path
+/// is `data.bin` unless `entry_path` overrides it.
+ManifestId publishPart(
+    const PoolPtr & s, const String & ns, const String & ref, const String & payload,
+    const String & entry_path = "data.bin")
+{
+    const RootNamespace nsr{ns};
+    PartWriteInfo info;
+    info.intended_ref = ns + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+
+    ManifestEntry e;
+    e.path = entry_path;
+    e.placement = EntryPlacement::Blob;
+    e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    e.blob_size = payload.size();
+
+    const ManifestId id = build->stageManifest({e});
+    build->precommitAdd(nsr, ref, id);
+    build->promote(nsr, ref, build->buildId(), id);
+    return id;
+}
+
+/// A ManifestRef carrying a unique instance id derived from `tag` (all fields explicit so the
+/// missing-designated-field-initializer warning never fires). The writer/build fields are stable test
+/// constants — the read path keys identity by the full ref, so any consistent choice works here.
+ManifestRef manifestRefFor(const String & tag)
+{
+    uint32_t ordinal = 1;
+    for (char c : tag)
+        ordinal = ordinal * 131 + static_cast<unsigned char>(c);
+    ordinal = ordinal % 999999 + 1;
+    return ManifestRef{
+        .writer_epoch = 1,
+        .build_sequence = 1,
+        .manifest_ordinal = ordinal};
+}
+
+/// Publish a part holding the given manifest entries verbatim through the real PartWriteTxn. Used by read-path
+/// lookup/list tests that want a precise multi-entry manifest. Each Blob entry's body MUST be present at
+/// promote: the promote gate revalidates EVERY blob leaf with a HEAD and fails closed on an absent body.
+/// So write a blob body for each Blob entry (addressed by its hash) and record it as W-EVIDENCE before
+/// staging. Inline entries need no body. Returns the published ManifestId.
+ManifestId publishPartWithEntries(
+    const PoolPtr & s, const String & ns, const String & ref, std::vector<ManifestEntry> entries)
+{
+    const RootNamespace nsr{ns};
+    PartWriteInfo info;
+    info.intended_ref = ns + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    for (const auto & e : entries)
+        if (e.placement == EntryPlacement::Blob)
+        {
+            /// Materialize the blob body so the promote-time HEAD revalidation succeeds, then record the
+            /// tokenless W-EVIDENCE dep (the gate re-observes the current token at promote).
+            DB::Cas::tests::writeBlobBody(s->backend(), s->layout(), e.ref.digest.toU128());
+            build->adoptEvidence(e);
+        }
+    const ManifestId id = build->stageManifest(std::move(entries));
+    build->precommitAdd(nsr, ref, id);
+    build->promote(nsr, ref, build->buildId(), id);
+    return id;
+}
+}
+
+TEST(CASPool, ReadOnlyOpenSkipsProbe)
+{
+    auto shared = std::make_shared<DB::Cas::InMemoryBackend>();
+
+    DB::Cas::PoolConfig cfg;
+    cfg.pool_prefix = "pool";
+    cfg.server_id = DB::UInt128(1);
+    cfg.server_root_id = "test";
+    /// Writable open: creates _pool_meta and runs the probe (which writes+cleans up).
+    DB::Cas::Pool::open(std::make_shared<WriteCountingBackend>(shared), cfg);
+
+    /// Read-only re-open over the SAME data must perform ZERO writes (no probe, meta already present).
+    auto counter = std::make_shared<WriteCountingBackend>(shared);
+    DB::Cas::PoolConfig ro = cfg;
+    ro.read_only = true;
+    auto store = DB::Cas::Pool::open(counter, ro);
+    EXPECT_EQ(counter->writes, 0u);
+    ASSERT_NE(store, nullptr);
+}
+
+namespace
+{
+/// Records whether any MUTATING op touched a `_probe/` key, so a test can assert an open ran (or
+/// skipped) the capability probe. Mirrors WriteCountingBackend above but keys on the probe subtree.
+class ProbeWatchingBackend final : public DB::Cas::Backend
+{
+public:
+    explicit ProbeWatchingBackend(std::shared_ptr<DB::Cas::Backend> inner_) : inner(std::move(inner_)) {}
+    bool probe_touched = false;
+
+    std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override { return inner->get(k, r); }
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+    DB::Cas::HeadResult head(const String & k) override { return inner->head(k); }
+    DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+    DB::Cas::PutResult putIfAbsent(const String & k, const String & b, const DB::Cas::ObjectMeta & m) override { note(k); return inner->putIfAbsent(k, b, m); }
+    DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & m) override { note(k); return inner->putIfAbsentStream(k, m); }
+    DB::Cas::PutResult putOverwrite(const String & k, const String & b, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & m) override { note(k); return inner->putOverwrite(k, b, e, m); }
+    DB::Cas::CasResult casPut(const String & k, const String & b, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & m) override { note(k); return inner->casPut(k, b, e, m); }
+    DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & t) override { note(k); return inner->deleteExact(k, t); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+private:
+    void note(const String & k) { if (k.find("/_probe/") != String::npos) probe_touched = true; }
+    std::shared_ptr<DB::Cas::Backend> inner;
+};
+}
+
+TEST(CASPool, SkipAccessCheckOpenSkipsProbeButStaysWritable)
+{
+    auto shared = std::make_shared<DB::Cas::InMemoryBackend>();
+
+    DB::Cas::PoolConfig cfg;
+    cfg.pool_prefix = "pool";
+    cfg.server_id = DB::UInt128(1);
+    cfg.server_root_id = "srv-1";
+
+    /// Baseline: a normal writable open runs the capability probe (PUT+delete of `_probe/` keys).
+    {
+        auto watch = std::make_shared<ProbeWatchingBackend>(shared);
+        auto s = DB::Cas::Pool::open(watch, cfg);
+        ASSERT_NE(s, nullptr);
+        EXPECT_TRUE(watch->probe_touched) << "the probe must run by default";
+    }
+
+    /// skip_access_check open ("start now, fix later"): NO probe I/O, yet still a WRITABLE mount
+    /// (owner/epoch/mount/watermark bootstrap writes still happen — unlike a read_only open, which is
+    /// a total no-op). Distinct root over the same (now-created) pool.
+    {
+        auto watch = std::make_shared<ProbeWatchingBackend>(shared);
+        DB::Cas::PoolConfig sac = cfg;
+        sac.server_id = DB::UInt128(2);
+        sac.server_root_id = "srv-2";
+        sac.skip_access_check = true;
+        auto s = DB::Cas::Pool::open(watch, sac);
+        ASSERT_NE(s, nullptr);
+        EXPECT_FALSE(watch->probe_touched) << "skip_access_check must perform no probe I/O";
+
+        /// Prove the mount is genuinely WRITABLE, not merely non-null — a read_only open would also
+        /// satisfy the two assertions above. Publish a part through the real PartWriteTxn write path
+        /// (beginPartWrite/putBlob/stageManifest/precommitAdd/promote) and read it back.
+        publishPart(s, "srv-2/tbl", "part_1", "payload-x");
+        const auto r = s->resolveRef(DB::Cas::RootNamespace{"srv-2/tbl"}, "part_1");
+        ASSERT_TRUE(r.has_value()) << "skip_access_check open must accept real writes, not just open";
+    }
+}
+
+namespace
+{
+/// A backend whose checkConditionalWriteSingleAttemptSupport ALWAYS throws — a stand-in for a
+/// Native-mode backend with no working single-attempt client (see
+/// ObjectStorageBackend::checkConditionalWriteSingleAttemptSupport). Pins that skip_access_check does
+/// NOT bypass this gate: the regression this guards is reverting Pool::open's skip_access_check
+/// branch back to the naive "wrap the whole probe" shape, which would silently skip this check too.
+class ThrowingSingleAttemptBackend final : public DB::Cas::Backend
+{
+public:
+    explicit ThrowingSingleAttemptBackend(std::shared_ptr<DB::Cas::Backend> inner_) : inner(std::move(inner_)) {}
+
+    std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override { return inner->get(k, r); }
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+    DB::Cas::HeadResult head(const String & k) override { return inner->head(k); }
+    DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+    DB::Cas::PutResult putIfAbsent(const String & k, const String & b, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsent(k, b, m); }
+    DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsentStream(k, m); }
+    DB::Cas::PutResult putOverwrite(const String & k, const String & b, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & m) override { return inner->putOverwrite(k, b, e, m); }
+    DB::Cas::CasResult casPut(const String & k, const String & b, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & m) override { return inner->casPut(k, b, e, m); }
+    DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & t) override { return inner->deleteExact(k, t); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+    void checkConditionalWriteSingleAttemptSupport() override
+    {
+        throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED, "test: no single-attempt client");
+    }
+private:
+    std::shared_ptr<DB::Cas::Backend> inner;
+};
+}
+
+TEST(CASPool, SkipAccessCheckStillEnforcesSingleAttemptGate)
+{
+    auto backend = std::make_shared<ThrowingSingleAttemptBackend>(std::make_shared<DB::Cas::InMemoryBackend>());
+
+    DB::Cas::PoolConfig cfg;
+    cfg.pool_prefix = "pool";
+    cfg.server_id = DB::UInt128(1);
+    cfg.server_root_id = "test";
+    cfg.skip_access_check = true;
+
+    /// skip_access_check must NOT bypass checkConditionalWriteSingleAttemptSupport (RFC
+    /// cas-s3-timeout-retry-control): a writable open still refuses to mount on a backend that cannot
+    /// prove single-attempt conditional-write support, exactly as it does without skip_access_check.
+    EXPECT_THROW(DB::Cas::Pool::open(backend, cfg), DB::Exception);
+}
+
+TEST(CASPool, MinActiveTracksInFlightBuilds)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    DB::Cas::PoolConfig cfg;
+    cfg.pool_prefix = "pool";
+    cfg.server_id = DB::UInt128(1);
+    cfg.server_root_id = "test";
+    cfg.background_watermark = false;
+    auto store = DB::Cas::Pool::open(backend, cfg);
+
+    ASSERT_EQ(store->minActive(), store->peekNextBuildSeq());   /// no builds: floor == next seq
+    auto b1 = store->beginPartWrite({});                            /// seq 1
+    auto b2 = store->beginPartWrite({});                            /// seq 2
+    ASSERT_EQ(store->minActive(), 1u);
+    b1->abandon();                                              /// finishes seq 1
+    ASSERT_EQ(store->minActive(), 2u);                          /// floor advances
+    b2->abandon();
+    ASSERT_EQ(store->minActive(), store->peekNextBuildSeq());   /// empty again
+}
+
+/// A throwing audit sink must NOT break a storage operation. The single reentrancy-safe event
+/// dispatcher (stage-1 §1, Task 2) CONTAINS sink exceptions ("never throws through"), so an arbitrary
+/// observer/sink callback failing during `beginPartWrite` is swallowed and construction succeeds --
+/// consistent with `CASPartWriteTxn.AbandonSwallowsThrowingEventSink` and
+/// `PromoteSwallowsPostDurableEventSinkFailure`, which already establish that an audit-sink failure
+/// never aborts the operation. Before Task 2 the sink was invoked directly and its exception
+/// propagated out of construction (audit-log backpressure breaking a write); the dispatcher removes
+/// that. The build_seq lifecycle is still exercised: the in-flight build holds the `minActive` GC
+/// floor and is retired on `abandon`.
+TEST(CASPool, BeginPartWriteSwallowsThrowingEventSink)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    DB::Cas::PoolConfig cfg;
+    cfg.pool_prefix = "pool";
+    cfg.server_id = DB::UInt128(1);
+    cfg.server_root_id = "test";
+    cfg.background_watermark = false;
+    auto store = DB::Cas::Pool::open(backend, cfg);
+
+    const uint64_t next_seq = store->peekNextBuildSeq();
+    /// UNKNOWN_EXCEPTION (not LOGICAL_ERROR): this simulates an arbitrary observer/sink callback
+    /// failing, not a CAS invariant violation -- LOGICAL_ERROR would abort the whole process under
+    /// debug/sanitizer builds instead of behaving like a catchable exception.
+    store->setEventSink([](const CasEvent & e)
+    {
+        if (e.type == CasEventType::BuildStart)
+            throw DB::Exception(DB::ErrorCodes::UNKNOWN_EXCEPTION, "injected audit sink failure");
+    });
+
+    PartWriteTxnPtr build;
+    ASSERT_NO_THROW({ build = store->beginPartWrite({}); })
+        << "a throwing audit sink must be contained by the dispatcher, not fail construction";
+    store->setEventSink(nullptr);
+
+    EXPECT_EQ(build->buildSeq(), next_seq);
+    EXPECT_EQ(store->peekNextBuildSeq(), next_seq + 1);
+    EXPECT_EQ(store->minActive(), build->buildSeq());              /// the in-flight build holds the floor
+    build->abandon();
+    EXPECT_EQ(store->minActive(), store->peekNextBuildSeq());      /// retired on abandon
+}
+
+TEST(CASPool, BuildSeqIsStrictlyMonotone)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    DB::Cas::PoolConfig cfg;
+    cfg.pool_prefix = "pool";
+    cfg.server_id = DB::UInt128(1);
+    cfg.server_root_id = "test";
+    cfg.background_watermark = false;
+    auto store = DB::Cas::Pool::open(backend, cfg);
+    auto a = store->beginPartWrite({});
+    auto sa = a->buildSeq();
+    a->abandon();
+    auto b = store->beginPartWrite({});
+    ASSERT_GT(b->buildSeq(), sa);                               /// never reused, never lower
+}
+
+TEST(CASPoolMeta, CreateThenReopen)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout layout("p");
+    PoolMeta created = PoolMeta::createOrValidate(*b, layout, /*blob_header_len*/ 256,
+        BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+    EXPECT_NE(created.pool_id, UInt128{});
+    PoolMeta reopened = PoolMeta::createOrValidate(*b, layout, /*blob_header_len*/ 512);
+    EXPECT_EQ(reopened.pool_id, created.pool_id);     /// pool is authoritative — config ignored on reopen
+    EXPECT_EQ(reopened.blob_header_len, 256u);
+}
+
+TEST(CASPoolMeta, FailClosed)
+{
+    Layout layout("p");
+    /// Garbage bytes are not a valid cas_pool_meta text object => CORRUPTED_DATA at the header line
+    /// (createOrValidate path). The future-version fail-closed (v > G_BUILD => UNKNOWN_FORMAT_VERSION)
+    /// is exercised at the codec level by the battery's per-row v+1 gate.
+    auto b2 = std::make_shared<InMemoryBackend>();
+    b2->putIfAbsent(layout.poolMetaKey(), "garbage");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { PoolMeta::createOrValidate(*b2, layout, 256); });
+}
+
+TEST(CASPoolMeta, RoundTripAndReadability)
+{
+    PoolMeta pm;
+    pm.pool_id = hexToU128("0123456789abcdeffedcba9876543210");
+    pm.blob_header_len = 256;
+    pm.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128)};
+
+    const String encoded = encodePoolMeta(pm);
+    /// v3 text form: a header line + one JSON body object, human-readable (jq/less friendly). No binary
+    /// magic; the object starts with '{' and names its type so a reader can identify it by eye.
+    ASSERT_GE(encoded.size(), 8u);
+    EXPECT_EQ(encoded.front(), '{');
+    EXPECT_NE(encoded.find(String("cas_pool_meta")), String::npos);
+    EXPECT_EQ(encoded.find(String("CAPM")), String::npos);
+
+    PoolMeta decoded = decodePoolMeta(encoded);
+    EXPECT_EQ(decoded.pool_id, pm.pool_id);
+    EXPECT_EQ(decoded.blob_header_len, pm.blob_header_len);
+}
+
+TEST(CASPoolMeta, RejectsBadConstantsAtCreation)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout layout("p");
+
+    /// not 8-aligned (above the floor, so it is the alignment rule that rejects it)
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS,
+        [&] { PoolMeta::createOrValidate(*b, layout, 250); });
+    /// below the v3 envelope floor (240) but 8-aligned: rejected by the floor, not the alignment rule.
+    /// Without the raised floor this pool would pass creation and LOGICAL_ERROR on the first blob write.
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS,
+        [&] { PoolMeta::createOrValidate(*b, layout, 128); });
+    /// well below the floor
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS,
+        [&] { PoolMeta::createOrValidate(*b, layout, 64); });
+    /// above the 16 KiB ceiling
+    expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS,
+        [&] { PoolMeta::createOrValidate(*b, layout, 17 * 1024); });
+
+    /// A creation that fails config validation must not have written anything.
+    EXPECT_FALSE(b->get(layout.poolMetaKey()).has_value());
+}
+
+TEST(CASPoolMeta, RejectsBadConstantsOnDecode)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout layout("p");
+    /// Encode a PoolMeta with blob_header_len=100 (not 8-aligned); decode must reject it as CORRUPTED_DATA.
+    PoolMeta bad_pm;
+    bad_pm.pool_id = hexToU128("00000000000000000000000000000001");
+    bad_pm.blob_header_len = 100;   /// violates 8-alignment invariant
+    b->putIfAbsent(layout.poolMetaKey(), encodePoolMeta(bad_pm));
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { PoolMeta::createOrValidate(*b, layout, 256); });
+}
+
+TEST(CASPoolMeta, DecodeGarbageFails)
+{
+    /// Any non-CAPM framing byte sequence => CORRUPTED_DATA.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [] { decodePoolMeta(String("garbage")); });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [] { decodePoolMeta(String("")); });
+}
+
+TEST(CASPoolMeta, ConcurrentCreateRace)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout layout("p");
+
+    /// A racing creator already wrote a valid foreign pool_id. createOrValidate must NOT overwrite it:
+    /// it re-reads (after losing the create-if-absent CAS, or seeing it present) and returns the
+    /// foreign pool_id, validated like a reopen.
+    const UInt128 foreign = hexToU128("0123456789abcdeffedcba9876543210");
+    PoolMeta foreign_pm;
+    foreign_pm.pool_id = foreign;
+    foreign_pm.blob_header_len = 256;
+    foreign_pm.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128)};
+    b->putIfAbsent(layout.poolMetaKey(), encodePoolMeta(foreign_pm));
+
+    PoolMeta result = PoolMeta::createOrValidate(*b, layout, /*blob_header_len*/ 512);
+    EXPECT_EQ(result.pool_id, foreign);
+    EXPECT_EQ(result.blob_header_len, 256u);     /// the foreign pool's constants win
+}
+
+TEST(CASPoolMeta, CasConflictReReadsWinner)
+{
+    /// The subtlest branch: the initial GET sees ABSENT, so createOrValidate proceeds to the
+    /// create-if-absent casPut — and loses, because a racing creator committed in between. The loser
+    /// must then re-read and return the WINNER's pool identity, not LOGICAL_ERROR. A single-threaded
+    /// `failNextCasPut` alone cannot exercise this: it returns Conflict without leaving the object
+    /// readable, so the re-read would fire the LOGICAL_ERROR guard. We model the real interleaving
+    /// with a backend whose casPut commits the winner's object (via the public putIfAbsent) and THEN
+    /// reports Conflict — exactly what the loser observes.
+    class RacingBackend : public InMemoryBackend
+    {
+    public:
+        String winner_bytes;
+        CasResult casPut(const String & key, const String & bytes,
+            const std::optional<Token> & expected, const ObjectMeta & meta) override
+        {
+            if (!winner_committed)
+            {
+                winner_committed = true;
+                /// The winner lands first; our create-if-absent now necessarily conflicts.
+                putIfAbsent(key, winner_bytes);
+                return {CasOutcome::Conflict, {}};
+            }
+            return InMemoryBackend::casPut(key, bytes, expected, meta);
+        }
+    private:
+        bool winner_committed = false;
+    };
+
+    const UInt128 winner = hexToU128("0123456789abcdeffedcba9876543210");
+    PoolMeta winner_pm;
+    winner_pm.pool_id = winner;
+    winner_pm.blob_header_len = 256;
+    winner_pm.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128)};
+
+    auto b = std::make_shared<RacingBackend>();
+    b->winner_bytes = encodePoolMeta(winner_pm);
+    Layout layout("p");
+
+    /// Our config (512) is what we WOULD have minted, but we lose the race and inherit the winner.
+    PoolMeta result = PoolMeta::createOrValidate(*b, layout, /*blob_header_len*/ 512,
+        BlobHashAlgo::CityHash128, /*allow_new*/ false, /*allow_mint*/ true);
+    EXPECT_EQ(result.pool_id, winner);
+    EXPECT_EQ(result.blob_header_len, 256u);
+}
+
+TEST(CASPool, OpenFailsClosedOnNonEnforcingBackend)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    b->setEnforceTokens(false);
+    expectThrowsCode(DB::ErrorCodes::NOT_IMPLEMENTED,
+        [&] { Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}); });   /// the probe error contract
+}
+
+TEST(CASPool, OpenCreatesPoolMetaAndReopens)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    /// Two CONCURRENT opens over the same POOL: a shared pool is the multi-server model, so each
+    /// mounts a DISTINCT server_root_id (and a distinct server_id) — same-root same-uuid co-mounting
+    /// is correctly fail-closed by the mount-safety protocol. This test only asserts that pool-meta is
+    /// pool-authoritative and shared across opens.
+    auto s1 = Pool::open(b, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "srv-1"});
+    auto s2 = Pool::open(b, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(2), .server_root_id = "srv-2"});
+    EXPECT_EQ(s1->poolMeta().pool_id, s2->poolMeta().pool_id);      /// pool authoritative
+}
+
+TEST(CASPool, OpenWithExplicitConstantsCreatesThem)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .blob_header_len = 512});
+    EXPECT_EQ(s->poolMeta().blob_header_len, 512u);                 /// config applies at creation
+}
+
+TEST(CASPool, VerbatimFilesLifecycle)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+    s->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "format_version.txt", "1\n");
+    s->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "uuid.txt", "abc");
+    EXPECT_EQ(s->getNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "format_version.txt"), String("1\n"));
+    EXPECT_FALSE(s->getNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "absent").has_value());
+    auto names = s->listNamespaceFiles(DB::Cas::tests::fixture::fixtureLife(ns));
+    EXPECT_EQ(names, (std::vector<String>{"format_version.txt", "uuid.txt"}));
+    s->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "uuid.txt", "def");                     /// overwrite allowed (head + putOverwrite)
+    EXPECT_EQ(s->getNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "uuid.txt"), String("def"));
+}
+
+TEST(CASPool, ListNamespaceFilesEmpty)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+    EXPECT_TRUE(s->listNamespaceFiles(DB::Cas::tests::fixture::fixtureLife(ns)).empty());
+}
+
+/// ---------- read side (spec §6): resolveRef / readManifest / findEntry / entryRange / listRefs ----------
+
+/// Phase 1c read path: a published ref resolves to a ManifestId; readManifest returns the immutable
+/// body; locate yields a ranged blob read; an Inline entry has no location. Replaces the old
+/// resolveRef().tree_id / readTree round trip (the tree model is gone — a part is a single ManifestId).
+TEST(CASPool, ResolveReturnsManifestId)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// blob "hello world" + an inline file, published through the real PartWriteTxn write path.
+    const String payload = "hello world";
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/part_1";
+    auto build = s->beginPartWrite(info);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+
+    ManifestEntry blob_entry;
+    blob_entry.path = "data.bin";
+    blob_entry.placement = EntryPlacement::Blob;
+    blob_entry.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload))};
+
+    blob_entry.blob_size = payload.size();
+    ManifestEntry inline_entry;
+    inline_entry.path = "small.txt";
+    inline_entry.placement = EntryPlacement::Inline;
+    inline_entry.inline_bytes = "tiny\n";
+
+    const ManifestId id = build->stageManifest({blob_entry, inline_entry});
+    build->precommitAdd(ns, "part_1", id);
+    build->promote(ns, "part_1", build->buildId(), id);
+
+    auto r = s->resolveRef(ns, "part_1");
+    ASSERT_TRUE(r.has_value());
+    EXPECT_EQ(r->manifest_id, id);                  /// resolve yields the published ManifestId
+
+    auto manifest = s->readManifest(r->manifest_id);
+    ASSERT_EQ(manifest.entries.size(), 2u);
+
+    /// "data.bin" sorts before "small.txt" (canonical path order).
+    const auto * data = findEntry(manifest.entries, "data.bin");
+    ASSERT_TRUE(data != nullptr);
+    auto loc = s->locate(*data);
+    EXPECT_EQ(loc.offset, s->poolMeta().blob_header_len);
+    EXPECT_EQ(loc.length, payload.size());
+
+    auto bytes = b->get(loc.key, Range{loc.offset, loc.length});
+    ASSERT_TRUE(bytes.has_value());
+    EXPECT_EQ(bytes->bytes, payload);               /// ranged read, no header touch
+
+    const auto * small = findEntry(manifest.entries, "small.txt");
+    ASSERT_TRUE(small != nullptr);
+    EXPECT_THROW(s->locate(*small), DB::Exception);  /// Inline has no location
+}
+
+/// readManifest fail-closes on a body whose self-described `ref`/`root_namespace_id` does NOT match the
+/// resolved ManifestId — the ref is addressing the wrong object / a cross-namespace dangle. We stage a
+/// body raw (writeManifestRaw, the on-storage write fixture) at a ManifestId, then resolve through a
+/// committed binding that names a DIFFERENT ManifestRef pointing at the SAME object key — so the head
+/// succeeds, the body decodes, but refMatchesBody fails => CORRUPTED_DATA.
+TEST(CASPool, ReadManifestValidatesBodyAndFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"srv1/tbl"};
+    Layout layout("p");
+
+    /// (1) ref/namespace mismatch: the BODY self-describes namespace `srv1/other`, but it is addressed
+    /// as a manifest of `srv1/tbl` => manifestNamespaceMatches fails => CORRUPTED_DATA. We craft an id
+    /// whose key lives under `srv1/tbl` but whose body carries the foreign namespace.
+    {
+        const ManifestRef ref = manifestRefFor("mismatch-ns");
+        const ManifestId addressed{.root_namespace = ns, .ref = ref};
+        /// Encode a body that claims a DIFFERENT namespace than `addressed.root_namespace`.
+        PartManifest body;
+        body.ref = ref;                                     /// ref matches
+        body.root_namespace_id = RootNamespace{"srv1/other"};  /// namespace does NOT
+        body.entries = {blobEntryFor("f", u128Of("x"), 1)};
+        body.payload_digest = computePayloadDigest(body);
+        b->putIfAbsent(layout.manifestKey(addressed), encodePartManifest(body));
+
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { s->readManifest(addressed); });
+    }
+
+    /// (2) ref mismatch: the body self-describes a DIFFERENT ManifestRef than the id addressing it =>
+    /// refMatchesBody fails => CORRUPTED_DATA.
+    {
+        const ManifestRef addressed_ref = manifestRefFor("addressed-ref");
+        const ManifestRef body_ref = manifestRefFor("body-ref-other");
+        const ManifestId addressed{.root_namespace = ns, .ref = addressed_ref};
+        PartManifest body;
+        body.ref = body_ref;                                /// ref does NOT match `addressed`
+        body.root_namespace_id = ns;                        /// namespace matches
+        body.entries = {blobEntryFor("f", u128Of("y"), 1)};
+        body.payload_digest = computePayloadDigest(body);
+        b->putIfAbsent(layout.manifestKey(addressed), encodePartManifest(body));
+
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { s->readManifest(addressed); });
+    }
+
+    /// (3) a committed ref naming a manifest with NO body present => readManifest throws
+    /// FILE_DOESNT_EXIST (INV-NO-DANGLE surfaced on the read path). resolveRef itself SUCCEEDS — refs
+    /// are pure manifest state. A raw ref-log fixture (not the real PartWriteTxn path, which validates the
+    /// body exists at promote) is the only way to construct this state.
+    {
+        const ManifestRef missing_ref = manifestRefFor("never-staged");
+        DB::Cas::tests::fixture::writeRefLogRaw(*b, layout, RefLogTxn{ns.string(), RefTxnId{1, 1},
+            {DB::Cas::tests::namespaceBirthOp(), DB::Cas::tests::publishCommittedOps("part_dangle", missing_ref)[0],
+             DB::Cas::tests::publishCommittedOps("part_dangle", missing_ref)[1]}, std::nullopt});
+        DB::Cas::tests::writeRecoverableCkptForRawFixture(*b, layout, ns, RefCkpt{
+            .life_epoch = 1,
+            .committed_through = RefTxnId{1, 1},
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = std::nullopt,
+        });
+
+        auto r = s->resolveRef(ns, "part_dangle");
+        ASSERT_TRUE(r.has_value());
+        expectThrowsCode(DB::ErrorCodes::FILE_DOESNT_EXIST, [&] { s->readManifest(r->manifest_id); });
+    }
+}
+
+/// findEntry and entryRange over a decoded part manifest's canonical-path-ordered entries.
+TEST(CASPool, LookupAndListOverManifestEntries)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// A multi-file/multi-directory part: top-level + a projection subdir.
+    std::vector<ManifestEntry> entries;
+    entries.push_back(blobEntryFor("columns.txt", u128Of("cols"), 4));
+    entries.push_back(blobEntryFor("data.bin", u128Of("data"), 8));
+    entries.push_back(blobEntryFor("p.proj/data.bin", u128Of("proj-data"), 6));
+    entries.push_back(blobEntryFor("p.proj/columns.txt", u128Of("proj-cols"), 5));
+    const ManifestId id = publishPartWithEntries(s, ns.string(), "all_1_1_0", entries);
+
+    auto r = s->resolveRef(ns, "all_1_1_0");
+    ASSERT_TRUE(r.has_value());
+    EXPECT_EQ(r->manifest_id, id);
+    auto manifest = s->readManifest(r->manifest_id);
+    ASSERT_EQ(manifest.entries.size(), 4u);
+
+    /// findEntry: exact-path hit + miss.
+    const auto * hit = findEntry(manifest.entries, "data.bin");
+    ASSERT_TRUE(hit != nullptr);
+    EXPECT_EQ(hit->ref.digest.toU128(), u128Of("data"));
+    EXPECT_TRUE(findEntry(manifest.entries, "no_such_file") == nullptr);
+
+    /// entryRange under "p.proj/" yields exactly the two projection files, in canonical order.
+    auto [proj_first, proj_last] = entryRange(manifest.entries, "p.proj/");
+    std::vector<ManifestEntry> proj(proj_first, proj_last);
+    ASSERT_EQ(proj.size(), 2u);
+    EXPECT_EQ(proj[0].path, "p.proj/columns.txt");
+    EXPECT_EQ(proj[1].path, "p.proj/data.bin");
+
+    /// The empty prefix lists everything (all four), still in canonical order.
+    auto [all_first, all_last] = entryRange(manifest.entries, "");
+    std::vector<ManifestEntry> all(all_first, all_last);
+    ASSERT_EQ(all.size(), 4u);
+    EXPECT_EQ(all[0].path, "columns.txt");
+    EXPECT_EQ(all[3].path, "p.proj/data.bin");
+}
+
+/// The Phase 1c manifest decode cache is keyed by (ManifestId, Token). Resolve+read the same ref twice:
+/// the second readManifest must be served from the cache (no second GET of the body). A fresh publish
+/// under a DIFFERENT ref name mints a NEW ManifestId (and a new shard token), so the cache misses and
+/// the body is fetched again. A CountingBackend asserts the body GET count.
+TEST(CASPool, ManifestCacheIsKeyedByIdAndToken)
+{
+    auto b = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"srv1/tbl"};
+    Layout layout("p");
+
+    const ManifestId id1 = publishPart(s, ns.string(), "part_1", "payload-1");
+    const String key1 = layout.manifestKey(id1);
+
+    /// First read: a body GET populates the (id1, token) cache entry.
+    {
+        auto r = s->resolveRef(ns, "part_1");
+        ASSERT_TRUE(r.has_value());
+        auto m = s->readManifest(r->manifest_id);
+        ASSERT_EQ(m.entries.size(), 1u);
+    }
+    const uint64_t gets_after_first = b->getCount(key1);
+    ASSERT_GE(gets_after_first, 1u);               /// the first read DID fetch the body
+
+    /// Second read of the SAME id: the (id, token) cache must serve it — NO additional body GET.
+    {
+        auto r = s->resolveRef(ns, "part_1");
+        ASSERT_TRUE(r.has_value());
+        EXPECT_EQ(r->manifest_id, id1);
+        auto m = s->readManifest(r->manifest_id);
+        ASSERT_EQ(m.entries.size(), 1u);
+    }
+    EXPECT_EQ(b->getCount(key1), gets_after_first)
+        << "second readManifest re-GET the body for the same (ManifestId, Token) — cache miss";
+
+    /// A fresh publish under a DIFFERENT ref name mints a NEW ManifestId: the cache (keyed by id) misses.
+    /// (Promoting a different manifest over the SAME committed ref is a distinct promote-over-committed
+    /// leak that `PartWriteTxn::promote` now forbids — see the CASPromoteRepublish tests.)
+    const ManifestId id2 = publishPart(s, ns.string(), "part_2", "payload-2");
+    EXPECT_FALSE(id2 == id1);                       /// a new publish never reuses a ManifestId
+    const String key2 = layout.manifestKey(id2);
+
+    auto r2 = s->resolveRef(ns, "part_2");
+    ASSERT_TRUE(r2.has_value());
+    EXPECT_EQ(r2->manifest_id, id2);               /// resolve now sees the new manifest
+    auto m2 = s->readManifest(r2->manifest_id);
+    ASSERT_EQ(m2.entries.size(), 1u);
+    EXPECT_GE(b->getCount(key2), 1u)               /// the new id's body WAS fetched (cache miss)
+        << "fresh publish (new ManifestId) should miss the id-keyed manifest cache";
+}
+
+/// Phase 5 (part-folder cache spec): manifest_cache is now a byte-weighted CacheBase LRU instead of a
+/// count-only bound, since decoded manifests carry inline bytes and can each be megabytes.
+TEST(CASPool, ManifestDecodeCacheIsByteBounded)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    const DB::Cas::Layout layout("p");
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    const DB::Cas::RootNamespace ns{"srv/t1"};
+
+    /// 8 manifests x ~1 MiB of inline bytes; a 2 MiB decode-cache bound must hold while every
+    /// read stays correct (evicted decodes just re-GET + re-decode).
+    std::vector<DB::Cas::ManifestId> ids;
+    std::vector<DB::Cas::RefOp> birth_ops{DB::Cas::tests::namespaceBirthOp()};
+    for (int i = 0; i < 8; ++i)
+    {
+        const DB::Cas::ManifestRef ref{.writer_epoch = 1, .build_sequence = static_cast<uint64_t>(i + 1),
+                                       .manifest_ordinal = 1};
+        DB::Cas::ManifestEntry e;
+        e.path = "big.txt";
+        e.placement = DB::Cas::EntryPlacement::Inline;
+        e.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(DB::UInt128(i + 1))};
+
+        e.inline_bytes = String(1 << 20, static_cast<char>('a' + i));
+        e.blob_size = e.inline_bytes.size();
+        ids.push_back(DB::Cas::tests::writeManifestRaw(*backend, layout, ns, ref, {e}));
+
+        const String ref_name = "part_" + std::to_string(i);
+        std::vector<DB::Cas::RefOp> ops = i == 0 ? birth_ops : std::vector<DB::Cas::RefOp>{};
+        const auto committed_ops = DB::Cas::tests::publishCommittedOps(ref_name, ref);
+        ops.insert(ops.end(), committed_ops.begin(), committed_ops.end());
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{1, static_cast<uint64_t>(i + 1)}, ops, std::nullopt});
+    }
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 8},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    DB::Cas::PoolConfig config{.pool_prefix = "p", .server_root_id = "test"};
+    config.manifest_decode_cache_bytes = 2ULL << 20;
+    auto store = DB::Cas::Pool::open(backend, std::move(config));
+
+    uint64_t total_gets = 0;
+    for (int round = 0; round < 2; ++round)
+        for (int i = 0; i < 8; ++i)
+        {
+            auto resolved = store->resolveRef(ns, "part_" + std::to_string(i));
+            ASSERT_TRUE(resolved.has_value());
+            auto m = store->readManifestShared(resolved->manifest_id);
+            ASSERT_EQ(m->entries.size(), 1u);
+            EXPECT_EQ(m->entries[0].inline_bytes[0], static_cast<char>('a' + i));   /// always correct
+        }
+    for (const auto & id : ids)
+        total_gets += backend->getCount(layout.manifestKey(id));
+
+    /// The bound forces re-GETs (16 reads over a 2 MiB window of ~1 MiB decodes cannot all hit),
+    /// proving eviction actually happens...
+    EXPECT_GT(total_gets, 8u);
+    /// ...and the cache reports an in-bound retained size.
+    EXPECT_LE(store->manifestDecodeCacheBytesForTest(), 2ULL << 20);
+}
+
+TEST(CASPool, ResolveDecodeCacheInvalidatesOnWrite)
+{
+    /// B113: resolveRef uses a token-validated shard-manifest decode cache. A write to the shard
+    /// mints a new token, so a subsequent resolve must observe the change (cache must NOT serve a
+    /// stale decoded manifest). Without token invalidation this would still see the dropped ref.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+
+    publishPart(s, ns.string(), "part_1", "payload-1");
+
+    /// First resolve decodes + caches; second is a cache hit — both must see part_1.
+    ASSERT_TRUE(s->resolveRef(ns, "part_1").has_value());
+    ASSERT_TRUE(s->resolveRef(ns, "part_1").has_value());
+
+    /// Write through the Pool (mutateShard => new shard token), removing part_1.
+    s->dropRef(ns, "part_1");
+
+    /// The cache must invalidate on the token change: resolve now reflects the drop.
+    EXPECT_FALSE(s->resolveRef(ns, "part_1").has_value());
+    EXPECT_TRUE(s->listRefs(ns).empty());
+}
+
+TEST(CASPool, ResolveAbsentRefAndAbsentNamespace)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+
+    /// A freshly-opened pool has no shard manifests: an absent shard is an empty manifest, so resolve
+    /// yields nullopt and listRefs is empty (NOT an error).
+    EXPECT_FALSE(s->resolveRef(ns, "anything").has_value());
+    EXPECT_TRUE(s->listRefs(ns).empty());
+}
+
+TEST(CASPool, ListRefsMergesAllShards)
+{
+    /// Task 10: refs are no longer sharded (the snapshot+log protocol caches one coherent table state
+    /// per namespace, not one manifest per shard) -- this now proves listRefs returns every committed
+    /// ref of a table built from a single multi-owner transaction, the closest surviving analogue of
+    /// the old "merges refs spread across shards" contract.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    Layout layout("p");
+    RootNamespace ns{"srv1/tbl"};
+
+    std::vector<RefOp> ops{DB::Cas::tests::namespaceBirthOp()};
+    for (char c = 'a'; c <= 'h'; ++c)
+    {
+        const String ref(1, c);
+        const auto committed_ops = DB::Cas::tests::publishCommittedOps(ref, manifestRefFor("manifest-" + ref));
+        ops.insert(ops.end(), committed_ops.begin(), committed_ops.end());
+    }
+    DB::Cas::tests::fixture::writeRefLogRaw(*b, layout, RefLogTxn{ns.string(), RefTxnId{1, 1}, ops, std::nullopt});
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*b, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    auto refs = s->listRefs(ns);
+    ASSERT_EQ(refs.size(), 8u);
+    for (char c = 'a'; c <= 'h'; ++c)
+    {
+        const String ref(1, c);
+        ASSERT_TRUE(refs.count(ref));
+        EXPECT_EQ(refs.at(ref).manifest_id.ref, manifestRefFor("manifest-" + ref));
+        EXPECT_EQ(refs.at(ref).manifest_id.root_namespace.string(), ns.string());
+    }
+}
+
+/// An empty namespace recovers from its exact `_ckpt` authority and exact successor GET. It performs
+/// ZERO LISTs and ZERO HEADs: recovery no longer enumerates the stream, and it never probes a shard
+/// fan-out. Measure deltas around `listRefs`; `Pool::open` and fixture admission have their own metadata
+/// traffic.
+TEST(CASPool, ListRefsEmptyNamespaceCostsZeroListsAndHeads)
+{
+    auto b = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+    /// EMPTY, but EXISTING and recoverable. A namespace the catalog does not name is answered from the
+    /// catalog and never reaches recovery; that separate shape is measured by the case below.
+    DB::Cas::tests::casAdmitRecoverableEntry(*b, Layout("p"), ns);
+
+    const uint64_t heads_before = b->headTotal();
+    const uint64_t lists_before = b->listTotal();
+
+    auto refs = s->listRefs(ns);
+
+    EXPECT_TRUE(refs.empty());
+    EXPECT_EQ(b->headTotal() - heads_before, 0u)
+        << "empty-namespace listRefs must not HEAD any shard";
+    EXPECT_EQ(b->listTotal() - lists_before, 0u)
+        << "checkpoint-grounded recovery reads exact keys and must not LIST the ref stream";
+}
+
+/// The other shape: a namespace that was never born. A read must not be what brings one into existence,
+/// so the answer comes from the catalog alone -- no recovery, and therefore not even the one LIST the
+/// case above pins.
+TEST(CASPool, ListRefsOnANeverBornNamespaceCostsNoListAndNoHead)
+{
+    auto b = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+
+    const uint64_t heads_before = b->headTotal();
+    const uint64_t lists_before = b->listTotal();
+    const uint64_t gets_before = b->getTotal();
+
+    auto refs = s->listRefs(ns);
+
+    EXPECT_TRUE(refs.empty());
+    EXPECT_EQ(b->listTotal() - lists_before, 0u)
+        << "a never-born namespace has no ref stream to LIST";
+    EXPECT_EQ(b->headTotal() - heads_before, 0u);
+    /// Positive control: the zeros above are the answer coming from the catalog, not from a call that
+    /// did nothing at all.
+    EXPECT_GT(b->getTotal() - gets_before, 0u)
+        << "the answer must come from a catalog read";
+}
+
+/// listRefs must return every committed ref of a table, correctly, regardless of how many refs the
+/// table holds (Task 10: there is no more shard fan-out to discover -- see the comment inside).
+TEST(CASPool, ListRefsReturnsSameContentAsBefore)
+{
+    /// Task 10: there is no more per-shard HEAD fan-out to bound (a warm listRefs costs ZERO requests;
+    /// a cold empty one costs zero LISTs and HEADs, already covered by
+    /// `ListRefsEmptyNamespaceCostsZeroListsAndHeads`) -- this now just proves the returned content is
+    /// correct for a multi-ref table built from a single raw ref-log fixture.
+    auto b = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    Layout layout("p");
+    RootNamespace ns{"srv1/tbl"};
+
+    std::vector<RefOp> ops{DB::Cas::tests::namespaceBirthOp()};
+    for (const String & ref : {String("a"), String("m"), String("z")})
+    {
+        const auto committed_ops = DB::Cas::tests::publishCommittedOps(ref, manifestRefFor("manifest-" + ref));
+        ops.insert(ops.end(), committed_ops.begin(), committed_ops.end());
+    }
+    DB::Cas::tests::fixture::writeRefLogRaw(*b, layout, RefLogTxn{ns.string(), RefTxnId{1, 1}, ops, std::nullopt});
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*b, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    auto refs = s->listRefs(ns);
+
+    ASSERT_EQ(refs.size(), 3u);
+    for (const String & ref : {String("a"), String("m"), String("z")})
+    {
+        ASSERT_TRUE(refs.count(ref));
+        EXPECT_EQ(refs.at(ref).manifest_id.ref, manifestRefFor("manifest-" + ref));
+        EXPECT_EQ(refs.at(ref).manifest_id.root_namespace.string(), ns.string());
+    }
+}
+
+/// A stray key under the namespace's ref-object prefix that does not parse as one of Task 10's
+/// `_log`/`_snap` kinds (a foreign/corrupt object) must not break listRefs — it is skipped
+/// defensively, listRefs still returns the legit refs and never throws.
+TEST(CASPool, ListRefsSkipsForeignKeys)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    Layout layout("p");
+    RootNamespace ns{"srv1/tbl"};
+
+    const String ref = "legit";
+    const ManifestRef mref = manifestRefFor("manifest-" + ref);
+    DB::Cas::tests::fixture::writeRefLogRaw(*b, layout, RefLogTxn{ns.string(), RefTxnId{1, 1},
+        {DB::Cas::tests::namespaceBirthOp(), DB::Cas::tests::publishCommittedOps(ref, mref)[0],
+         DB::Cas::tests::publishCommittedOps(ref, mref)[1]}, std::nullopt});
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*b, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    /// A stray key directly under the namespace's ref-object prefix that is not `_log`/
+    /// `_snap` shaped (also covers the legacy shard-number layout GC/dropNamespace still write).
+    b->putIfAbsent(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "garbage", "not-a-ref-object");
+
+    std::map<String, Resolved> refs;
+    EXPECT_NO_THROW(refs = s->listRefs(ns));
+    ASSERT_EQ(refs.size(), 1u);
+    ASSERT_TRUE(refs.count(ref));
+    EXPECT_EQ(refs.at(ref).manifest_id.ref, mref);
+}
+
+/// readManifest fails CLOSED on a corrupt or kind-mismatched manifest body addressed by a live id.
+TEST(CASPool, ReadManifestFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    Layout layout("p");
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// (1) Garbage bytes at the manifest key => decodePartManifest throws CORRUPTED_DATA.
+    {
+        const ManifestRef ref = manifestRefFor("garbage-body");
+        const ManifestId id{.root_namespace = ns, .ref = ref};
+        b->putIfAbsent(layout.manifestKey(id), "not a valid manifest body");
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { s->readManifest(id); });
+    }
+
+    /// (2) A ref naming a manifest id with NO object present => readManifest throws FILE_DOESNT_EXIST
+    /// (INV-NO-DANGLE), carrying the manifest key.
+    {
+        const ManifestRef ref = manifestRefFor("absent-body");
+        const ManifestId id{.root_namespace = ns, .ref = ref};
+        expectThrowsCode(DB::ErrorCodes::FILE_DOESNT_EXIST, [&] { s->readManifest(id); });
+    }
+}
+
+/// ---------- ref lifecycle: dropRef / updateRefPublishedAt / dropNamespace ----------
+
+TEST(CASPool, DropRefAppendsJournalAtomically)
+{
+    /// Task 10: the OLD shared-journal record assertions are gone (there is no shared mutable journal
+    /// object anymore — dropRef appends its OWN immutable ref-log transaction); the surviving
+    /// behavioral contract is: the drop is atomic (visible to resolveRef only once durable), and
+    /// dropping a missing ref is fail-closed, never a silent no-op.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+
+    publishPart(s, ns.string(), "part_1", "payload-1");
+    ASSERT_TRUE(s->resolveRef(ns, "part_1").has_value());
+
+    s->dropRef(ns, "part_1");
+    EXPECT_FALSE(s->resolveRef(ns, "part_1").has_value());
+    EXPECT_TRUE(s->listRefs(ns).empty());
+
+    /// Dropping a missing ref is fail-closed, never a silent no-op.
+    expectThrowsCode(DB::ErrorCodes::FILE_DOESNT_EXIST, [&] { s->dropRef(ns, "no_such_ref"); });
+}
+
+/// Task 10 renamed this from "...WithoutJournal": updateRefPublishedAt now DOES append an immutable
+/// `set_published_at` ref-log transaction (spec §Update Payload) -- the old journal-free in-place field
+/// mutation had no equivalent once persistence is an append-only log; every change, even timestamp-only,
+/// must be a logged operation to be part of the ordered history. All-tree-part-files Task 9: the
+/// carrier's mutable-file map is gone -- `published_at_ms` is the only field left to mutate. The
+/// surviving contract is the user-visible one: a `published_at_ms` update is observable through
+/// resolveRef and the manifest edge cannot change on this path -- the `RefPublishedAtUpdate` carrier
+/// deliberately has no `manifest_ref` field, so a reachability change is structurally impossible here
+/// (it goes through publish/drop/repoint instead).
+TEST(CASPool, UpdateRefPublishedAtUpdatesPublishedAtMs)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+
+    const ManifestId id = publishPart(s, ns.string(), "part_1", "payload-1");
+    const ManifestRef manifest_ref = id.ref;
+
+    s->updateRefPublishedAt(ns, "part_1", [](RefPublishedAtUpdate & r) { r.published_at_ms = 1; });
+    s->updateRefPublishedAt(ns, "part_1", [](RefPublishedAtUpdate & r) { r.published_at_ms = 7; });
+
+    auto after = s->resolveRef(ns, "part_1");
+    ASSERT_TRUE(after.has_value());
+    EXPECT_EQ(after->published_at_ms, 7u);
+    EXPECT_EQ(after->manifest_id.ref, manifest_ref);
+}
+
+/// Task 11: dropNamespace removes every owner through the ref-log `remove_namespace` transaction and
+/// performs NO physical deletion at all -- verbatim files survive until GC's perpetual janitor
+/// reclaims the dead life. So after the drop every ref resolves away and
+/// `listRefs` is empty, but the verbatim files remain readable.
+TEST(CASPool, DropNamespaceRemovesEveryOwnerButLeavesFilesForGc)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    RootNamespace ns{"srv1/tbl"};
+
+    const std::vector<String> ref_names{"alpha", "bravo", "charlie"};
+    for (const String & name : ref_names)
+        publishPart(s, ns.string(), name, "payload-" + name);
+    for (const String & name : ref_names)
+        ASSERT_TRUE(s->resolveRef(ns, name).has_value());
+
+    s->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "format_version.txt", "1\n");
+    s->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "uuid.txt", "abc");
+
+    s->dropNamespace(ns);
+
+    for (const String & name : ref_names)
+        EXPECT_FALSE(s->resolveRef(ns, name).has_value());
+    EXPECT_TRUE(s->listRefs(ns).empty());
+
+    /// The writer performs NO physical deletion; verbatim files survive until the perpetual janitor
+    /// reclaims the dead life.
+    EXPECT_TRUE(s->getNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "format_version.txt").has_value());
+    EXPECT_TRUE(s->getNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "uuid.txt").has_value());
+
+    /// Repeated drop is idempotent: no throw, no second transaction (nothing left to observe changing).
+    EXPECT_NO_THROW(s->dropNamespace(ns));
+
+    /// Ordinary mutations on a cataloged `Removing` life are rejected with typed retry-later until
+    /// the terminal fold and catalog-only drain complete.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { s->dropRef(ns, "alpha"); });
+}
+
+TEST(CASPool, ListNamespacesFromCatalog)
+{
+    /// `listNamespaces` projects logical names from the authoritative catalog. Physical life keys
+    /// contain no namespace spelling and therefore cannot participate in this enumeration.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+
+    EXPECT_TRUE(s->listNamespaces("").namespaces.empty());   /// fresh pool: empty catalog
+
+    /// The real publication path admits each namespace before writing its stream.
+    DB::Cas::tests::publishCommittedTransition(*b, s->layout(), RootNamespace{"srv1/tbl"},
+        "ref1", std::nullopt, DB::Cas::ManifestRef{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1});
+    DB::Cas::tests::publishCommittedTransition(*b, s->layout(), RootNamespace{"shadow/bk1/tbl"},
+        "ref1", std::nullopt, DB::Cas::ManifestRef{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1});
+    DB::Cas::tests::publishCommittedTransition(*b, s->layout(), RootNamespace{"shadow/bk2/tbl"},
+        "ref1", std::nullopt, DB::Cas::ManifestRef{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1});
+
+    const auto all = s->listNamespaces("").namespaces;
+    EXPECT_EQ(all.size(), 3u);
+    const auto shadows = s->listNamespaces("shadow/").namespaces;
+    ASSERT_EQ(shadows.size(), 2u);
+    /// listNamespaces returns results from an unordered_set; sort for deterministic comparison.
+    auto sorted_shadows = shadows;
+    std::sort(sorted_shadows.begin(), sorted_shadows.end());
+    EXPECT_EQ(sorted_shadows[0], "shadow/bk1/tbl");
+    EXPECT_EQ(sorted_shadows[1], "shadow/bk2/tbl");
+    EXPECT_TRUE(s->listNamespaces("nope/").namespaces.empty());
+}
+
+/// Physical namespace files carry only an opaque life id and cannot mint a logical catalog row.
+TEST(CASPool, ListNamespacesDoesNotMintLogicalNamesFromFileKeys)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"test/tbl@cas@"};
+
+    s->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "format_version.txt", "1\n");
+    /// A second life of the SAME name, written by exact key because no helper mints two lives yet.
+    const NamespaceLifeId other = NamespaceLifeId::fromCatalogEntry(ns, DB::UInt128(0x5eed));
+    ASSERT_EQ(b->putIfAbsent(s->layout().namespaceFileKey(other, "format_version.txt"), "1\n").outcome,
+              PutOutcome::Done);
+
+    const NamespaceListing listing = s->listNamespaces("");
+    EXPECT_TRUE(listing.skipped.empty());
+    EXPECT_TRUE(listing.namespaces.empty());
+}
+
+/// Catalog discovery neither adopts nor reports malformed physical debris. Diagnostic ownership-tree
+/// scans, not ordinary logical enumeration, classify those keys.
+TEST(CASPool, ListNamespacesDoesNotTreatPhysicalDebrisAsCatalogAuthority)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const RootNamespace ns{"test/tbl@cas@"};
+
+    /// One well-formed key per family, so the namespace is attributable either way.
+    DB::Cas::tests::publishCommittedTransition(*b, s->layout(), ns,
+        "ref1", std::nullopt, DB::Cas::ManifestRef{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1});
+    s->putNamespaceFile(DB::Cas::tests::fixture::fixtureLife(ns), "format_version.txt", "1\n");
+
+    /// Hand-built un-incarnated keys: no helper can mint either shape any more.
+    const String lifeless_ref = s->layout().casRefsPrefix() + ns.string() + "/_log/"
+        + renderRefTxnId(RefTxnId{1, 1}) + ".zst";
+    const String lifeless_file = s->layout().rootsPrefix() + ns.string() + "/_files/format_version.txt";
+    ASSERT_EQ(b->putIfAbsent(lifeless_ref, "garbage").outcome, PutOutcome::Done);
+    ASSERT_EQ(b->putIfAbsent(lifeless_file, "garbage").outcome, PutOutcome::Done);
+
+    NamespaceListing listing;
+    ASSERT_NO_THROW(listing = s->listNamespaces(""))
+        << "one un-attributable key must not abort the enumeration for every consumer of it";
+
+    /// The healthy namespace is still listed -- attribution is per key, so a namespace disappears only
+    /// when every key that would name it is unattributable.
+    ASSERT_EQ(listing.namespaces.size(), 1u);
+    EXPECT_EQ(listing.namespaces[0], ns.string());
+
+    EXPECT_TRUE(listing.skipped.empty());
+    EXPECT_TRUE(b->head(lifeless_ref).exists);
+    EXPECT_TRUE(b->head(lifeless_file).exists);
+}
+
+TEST(CASPool, ListMirroredChildren)
+{
+    using namespace DB::Cas;
+    auto b = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    /// Seed two catalog-authoritative shadow archives; physical files alone carry no logical path.
+    DB::Cas::tests::fixture::admitLive(*b, store->layout(), RootNamespace{"shadow/bk1/store/3f2/3f2a-uuid@cas@"});
+    DB::Cas::tests::fixture::admitLive(*b, store->layout(), RootNamespace{"shadow/bk2/store/3f2/3f2a-uuid@cas@"});
+    auto children = store->listMirroredChildren("shadow/");
+    std::sort(children.begin(), children.end());
+    ASSERT_EQ(children.size(), 2u);
+    EXPECT_EQ(children[0], "bk1");
+    EXPECT_EQ(children[1], "bk2");
+}
+
+namespace
+{
+
+/// Delegating backend that fences the mount slot IN PLACE the first time a `get` returns a present
+/// body for the armed key — reproducing the S13 window: the GC's token-guarded fence-out lands
+/// between the keeper adopt's GET and its CAS. The caller's subsequent token-guarded `putOverwrite`
+/// then fails `PreconditionFailed`, the adopt re-reads, sees `gc_fenced`, and throws
+/// `MountFencedException` — which `Pool::open`'s fence-recovery loop must turn into a fresh-epoch
+/// retry rather than a permanent wedge (P3.1 vector C).
+class FenceInAdoptWindowBackend final : public DB::Cas::Backend
+{
+public:
+    explicit FenceInAdoptWindowBackend(std::shared_ptr<DB::Cas::Backend> inner_) : inner(std::move(inner_)) {}
+    String fence_key;   /// empty = fault disarmed; set to the mount key to arm the one-shot fence
+
+    std::optional<DB::Cas::GetResult> get(const String & k, DB::Cas::Range r) override
+    {
+        auto got = inner->get(k, r);
+        if (!fence_key.empty() && k == fence_key && got.has_value())
+        {
+            /// One-shot: fence the slot in place exactly as `computeHeartbeatFloor` does (preserve the
+            /// body, gc_fenced = true, seq + 1, token-guarded against the value we just read), then
+            /// disarm so the retry can adopt cleanly.
+            DB::Cas::MountLease fenced = DB::Cas::decodeMountLease(got->bytes);
+            fenced.gc_fenced = true;
+            fenced.seq += 1;
+            inner->putOverwrite(k, DB::Cas::encodeMountLease(fenced), got->token);
+            fence_key.clear();
+        }
+        return got;
+    }
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & k, DB::Cas::Range r) override { return inner->getStream(k, r); }
+    DB::Cas::HeadResult head(const String & k) override { return inner->head(k); }
+    DB::Cas::ListPage list(const String & p, const String & c, size_t l) override { return inner->list(p, c, l); }
+    DB::Cas::PutResult putIfAbsent(const String & k, const String & b, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsent(k, b, m); }
+    DB::Cas::WriteSinkPtr putIfAbsentStream(const String & k, const DB::Cas::ObjectMeta & m) override { return inner->putIfAbsentStream(k, m); }
+    DB::Cas::PutResult putOverwrite(const String & k, const String & b, const DB::Cas::Token & e, const DB::Cas::ObjectMeta & m) override { return inner->putOverwrite(k, b, e, m); }
+    DB::Cas::CasResult casPut(const String & k, const String & b, const std::optional<DB::Cas::Token> & e, const DB::Cas::ObjectMeta & m) override { return inner->casPut(k, b, e, m); }
+    DB::Cas::DeleteOutcome deleteExact(const String & k, const DB::Cas::Token & t) override { return inner->deleteExact(k, t); }
+    bool supportsListTokens() const override { return inner->supportsListTokens(); }
+
+private:
+    std::shared_ptr<DB::Cas::Backend> inner;
+};
+
+}
+
+TEST(CASPoolMountFence, OpenRecoversFromFenceInAdoptWindowWithFreshEpoch)
+{
+    auto inner = std::make_shared<InMemoryBackend>();
+    auto fencing = std::make_shared<FenceInAdoptWindowBackend>(inner);
+    /// Arm the one-shot fence on the mount slot. Pool::open first claims the mount (fresh mint), then
+    /// the keeper adopts it — the adopt's GET trips the fence, its CAS fails, and open must recover.
+    const DB::Cas::Layout layout("p");
+    fencing->fence_key = layout.mountKey("test");
+
+    /// The retry that recovers from the fence reclaims a same-uuid, different-epoch, `gc_fenced` body
+    /// -> `MountPriorState::Fenced` (a fenced prior is reclaimed on the first attempt, with no
+    /// observation polling -- see `CASMountOpenWaits.FencedPriorReclaimsWithoutAnyWait`). The injected
+    /// `boot_ms_fn`/`wait_sleep_fn` below keep this test off the real clock regardless.
+    uint64_t fake_boot = 0;
+    DB::Cas::PoolPtr store;
+    ASSERT_NO_THROW(
+        store = DB::Cas::Pool::open(fencing,
+            DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                .boot_ms_fn = [&fake_boot] { return fake_boot; },
+                .wait_sleep_fn = [&fake_boot](uint64_t ms) { fake_boot += ms; }}))
+        << "open must recover from a fence in the adopt window, not wedge (exit-49 S13 bug)";
+    ASSERT_TRUE(store);
+
+    /// The final live lease is unfenced and at a HIGHER writer_epoch than the first attempt (a fence
+    /// costs an epoch): the first claim took epoch 1, got fenced, the retry took epoch 2 and mounted.
+    const auto got = inner->get(layout.mountKey("test"));
+    ASSERT_TRUE(got.has_value());
+    const MountLease final_lease = decodeMountLease(got->bytes);
+    EXPECT_FALSE(final_lease.gc_fenced);
+    EXPECT_GT(final_lease.writer_epoch, 1u) << "recovery must draw a fresh writer_epoch";
+    EXPECT_TRUE(fencing->fence_key.empty()) << "the one-shot fence must have fired";
+}
+
+/// Task 12: the write-fence deadline is a CLOCK_BOOTTIME instant (boottime includes VM-suspend time,
+/// so a resumed sleeper sees its fence expired — unlike CLOCK_MONOTONIC, which freezes across suspend).
+/// A CLOCK_MONOTONIC freeze cannot be simulated in a unit test, so we exercise the injected-fn seam: a
+/// fake boot clock that we advance past the ttl must flip mayMutate to false and make a gated mutate
+/// fail closed with ABORTED.
+TEST(CASPool, WriteFenceUsesInjectedBootClock)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    uint64_t fake_boot = 1'000'000;   /// arbitrary boottime origin (ms)
+    auto store = DB::Cas::Pool::open(backend, DB::Cas::PoolConfig{
+        .pool_prefix = "p",
+        .server_root_id = "test",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(30000),
+        .boot_ms_fn = [&] { return fake_boot; },
+    });
+
+    /// Freshly armed at open (deadline = fake_boot + ttl): well within the ttl, mutations are allowed.
+    EXPECT_TRUE(store->mayMutate());
+
+    /// Advance the boot clock just short of the deadline — still armed.
+    fake_boot += 29999;
+    EXPECT_TRUE(store->mayMutate());
+
+    /// Cross the deadline (ttl elapsed with no renew — a resumed sleeper's view). The fence must expire.
+    /// (The "a gated mutate then fails closed with ABORTED" leg used `mutateShardForTest` -- the held
+    /// Phase-E shard lane -- and moves there; here we pin the boot-clock fence flip itself.)
+    fake_boot += 2;   /// now fake_boot = origin + 30001 > origin + 30000
+    EXPECT_FALSE(store->mayMutate());
+}
+
+/// ==== self-remount after GC fence-out (liveness counterpart of the fence-out safety rule) ====
+
+namespace
+{
+
+/// GC's fence-out, applied directly: preserve the body, set gc_fenced, bump seq (token-guarded).
+void fenceOutMount(DB::Cas::Backend & backend, const String & mount_key)
+{
+    const auto got = backend.get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    MountLease m = decodeMountLease(got->bytes);
+    m.gc_fenced = true;
+    m.seq += 1;
+    ASSERT_EQ(backend.putOverwrite(mount_key, encodeMountLease(m), got->token).outcome,
+              DB::Cas::PutOutcome::Done);
+}
+
+}
+
+TEST(CASPoolRemount, FenceOutThenSelfRemountRestoresWrites)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    const String mount_key = store->layout().mountKey("test");
+    const uint64_t epoch_before = decodeMountLease(backend->get(mount_key)->bytes).writer_epoch;
+    EXPECT_EQ(store->liveWriterEpoch(), epoch_before);
+
+    fenceOutMount(*backend, mount_key);
+
+    /// The keeper's next renewal fails closed (foreign touch — never re-mint).
+    EXPECT_THROW(store->renewWatermarkOnce(), DB::Exception);
+
+    /// Self-remount claims a FRESH incarnation: epoch bumped, gc_fenced cleared, writes restored.
+    ASSERT_TRUE(store->tryRemountOnce());
+    const MountLease after = decodeMountLease(backend->get(mount_key)->bytes);
+    EXPECT_EQ(after.writer_epoch, epoch_before + 1);
+    EXPECT_FALSE(after.gc_fenced);
+    EXPECT_EQ(store->liveWriterEpoch(), epoch_before + 1);
+
+    /// The renewal path works again (the new keeper owns the slot). (The follow-on "...and so does a
+    /// ref-shard mutation" check used `mutateShardForTest` -- the held Phase-E shard lane -- and moves
+    /// to Phase E's own tests; the self-remount liveness assertion above is the point of this test.)
+    EXPECT_NO_THROW(store->renewWatermarkOnce());
+}
+
+TEST(CASPoolRemount, OldEpochBuildFailsClosedAfterRemount)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    auto build = store->beginPartWrite({});
+
+    fenceOutMount(*backend, store->layout().mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce());
+
+    /// The build was minted under the superseded incarnation — every further step fails closed.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR,
+        [&] { build->putBlob(DB::Cas::tests::idOf("x"), DB::Cas::BlobSource::fromString("x")); });
+
+    /// A FRESH build under the live incarnation works.
+    auto fresh = store->beginPartWrite({});
+    EXPECT_NO_THROW(fresh->putBlob(DB::Cas::tests::idOf("y"), DB::Cas::BlobSource::fromString("y")));
+}
+
+TEST(CASPoolRemount, ForeignOwnerIsNeverTakenOver)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    const String mount_key = store->layout().mountKey("test");
+
+    /// A genuinely foreign uuid holds the mount (live or not — foreign is terminal for the claim).
+    const auto got = backend->get(mount_key);
+    MountLease foreign = decodeMountLease(got->bytes);
+    foreign.server_uuid = foreign.server_uuid + DB::UInt128(1);
+    foreign.seq += 1;
+    ASSERT_EQ(backend->putOverwrite(mount_key, encodeMountLease(foreign), got->token).outcome,
+              DB::Cas::PutOutcome::Done);
+
+    EXPECT_FALSE(store->tryRemountOnce());
+    /// The foreign body is untouched (no takeover, ever).
+    EXPECT_EQ(decodeMountLease(backend->get(mount_key)->bytes).server_uuid, foreign.server_uuid);
+
+    /// Move the parent fixture to the production-recognized fenced terminal state before explicitly
+    /// destroying its superseded keeper. The unfenced foreign-release guard is covered separately below.
+    fenceOutMount(*backend, mount_key);
+    store.reset();
+
+    /// A foreign owner is never taken over — at remount OR at release. This was an `EXPECT_DEATH`
+    /// pinning a `LOGICAL_ERROR` abort on the release half; the abort fired from `~Pool` and defeated
+    /// `finishTeardown`'s own catch by aborting at exception construction. The runtime never observed a
+    /// deposition (the slot was overwritten out of band), so the release takes the
+    /// exclusivity-violation arm: refuse, leave the foreign occupant untouched, and SURVIVE teardown.
+    auto foreign_backend = std::make_shared<InMemoryBackend>();
+    auto invalid_store = DB::Cas::tests::openPoolForTest(foreign_backend);
+    const String foreign_mount_key = invalid_store->layout().mountKey("test");
+    const auto foreign_got = foreign_backend->get(foreign_mount_key);
+    ASSERT_TRUE(foreign_got.has_value());
+    MountLease foreign_lease = decodeMountLease(foreign_got->bytes);
+    foreign_lease.server_uuid = foreign_lease.server_uuid + DB::UInt128(1);
+    foreign_lease.seq += 1;
+    ASSERT_EQ(
+        foreign_backend->putOverwrite(foreign_mount_key, encodeMountLease(foreign_lease), foreign_got->token).outcome,
+        DB::Cas::PutOutcome::Done);
+    const auto occupant_before = foreign_backend->get(foreign_mount_key);
+    ASSERT_TRUE(occupant_before.has_value());
+
+    EXPECT_FALSE(invalid_store->tryRemountOnce()) << "a foreign owner is never taken over at remount";
+
+    const uint64_t violations_before
+        = ProfileEvents::global_counters[ProfileEvents::CASMountExclusivityViolation].load();
+    invalid_store.reset();   /// must not abort, must not terminate
+
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMountExclusivityViolation].load(),
+              violations_before + 1)
+        << "the release must report the broken single-writer guarantee rather than dying on it";
+    const auto occupant_after = foreign_backend->get(foreign_mount_key);
+    ASSERT_TRUE(occupant_after.has_value()) << "nor is it taken over at release";
+    EXPECT_EQ(occupant_after->bytes, occupant_before->bytes)
+        << "the slot must be left byte-for-byte as the foreign owner wrote it";
+}
+
+TEST(CASPoolRemount, ShutdownGuardRefusesToArmRemount)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// background_watermark = true so scheduleRemount actually arms a recovery thread in production mode
+    /// (the same gate every background thread checks).
+    auto store = DB::Cas::Pool::open(backend,
+        DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test", .background_watermark = true});
+
+    /// Teardown has begun: ~Pool() latches this at its very top, BEFORE its only remount-thread join.
+    store->beginShutdownForTest();
+
+    /// A lease-renewal failure firing DURING teardown re-enters scheduleRemount (the keeper's on_lost
+    /// callback). With the guard it must refuse to spawn; without it, it arms remount_thread AFTER
+    /// ~Pool()'s join — the leftover joinable ThreadFromGlobalPool handle then abort()s the process at
+    /// member destruction (std::terminate). Reading joinable() immediately after the synchronous call is
+    /// race-free: the armed thread never touches the handle.
+    EXPECT_FALSE(store->scheduleRemountForTest())
+        << "scheduleRemount must not arm a recovery thread once teardown has begun";
+}
+
+namespace
+{
+/// A sequenced fake boot clock: the first N `bootMsNow()` calls return the values queued via
+/// `.queue`, in order; every call after the queue drains returns `.steady`. `CasMountRuntime::bootMsNow`
+/// re-invokes `PoolConfig::boot_ms_fn` on EVERY call, with zero memoization -- so a plain call-counter
+/// deterministically distinguishes an early (anchor) reading from a later (response-time) one, with no
+/// real sleep and no threads.
+struct SequencedBootClock
+{
+    std::vector<uint64_t> queue;
+    size_t next = 0;
+    uint64_t steady = 0;
+
+    uint64_t operator()()
+    {
+        if (next < queue.size())
+            return queue[next++];
+        return steady;
+    }
+};
+}
+
+/// Phase B addendum 2 (task 5b review, reviewer's probe): the self-remount arm must anchor at the
+/// claim attempt's pre-I/O instant (`remount_anchor_boot_ms`, captured right after `installKeeper`
+/// and right before `keeperStart()` in `Pool::tryRemountOnce`), never at a later reading taken after
+/// `keeperStart`/`quiesceRefTablesForRemount` have already run.
+///
+/// The two `bootMsNow()` calls of interest, in the ORDER each code version issues them:
+///   - FIXED code: call #1 = the new anchor (`remount_anchor_boot_ms`, before `keeperStart`);
+///     call #2 = `MountLeaseKeeper::prepareRenew`'s own internal boot read inside `keeperStart`'s
+///     `doStart` (feeds only the keeper's OWN internal `confirmed_deadline_ms` -- unrelated to the
+///     Pool-level arm -- so its value is irrelevant to the arm post-fix).
+///   - PRE-FIX code (no anchor line): call #1 = that SAME `prepareRenew` read (now the first boot
+///     call of the attempt, since nothing reads the clock before `keeperStart`); call #2 = the
+///     arm-site's own `mount_runtime.bootMsNow()`, read AFTER `keeperStart` returns -- the stale,
+///     response-time reading this whole fix exists to stop using.
+/// A sequenced clock returning 10000 then 999999 (an inflated, much-later reading) therefore arms
+/// the FIXED code from 10000 and the PRE-FIX code from 999999, regardless of which call site reads
+/// which value -- letting a single deterministic probe (`mayMutate()` at boot == 10000+ttl) tell
+/// them apart with no sleep and no thread. (TDD evidence for both branches is recorded in the task-5
+/// report, not re-asserted here: this test body only encodes the FIXED expectation.)
+TEST(CASPoolRemount, RemountArmAnchorsAtClaimAttemptNotResponseTime)
+{
+    SequencedBootClock clock;
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = DB::Cas::Pool::open(backend, DB::Cas::PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(30'000),
+        .boot_ms_fn = [&] { return clock(); },
+    });
+    ASSERT_TRUE(store);
+
+    /// Trip the fence exactly as every other remount test in this file does.
+    fenceOutMount(*backend, store->layout().mountKey("test"));
+
+    /// Arm the sequence for the upcoming remount attempt: the initial `open` above already drained
+    /// an unrelated number of `bootMsNow()` calls (all served from `.steady = 0` -- irrelevant, since
+    /// nothing probes the resulting arm before this point). Reset the counter so the FIRST call from
+    /// here on is the remount attempt's own call #1.
+    clock.queue = {10000, 999999};
+    clock.next = 0;
+
+    ASSERT_TRUE(store->tryRemountOnce());
+
+    /// Probe at boot == anchor + ttl (10000 + 30000 = 40000): the fixed code armed from the anchor
+    /// (10000), so the fence has JUST expired here -- `mayMutate` must be false. (The pre-fix code
+    /// would still read `mayMutate` as true here, armed from 999999 + 30000 -- see the TDD run in the
+    /// report.)
+    clock.steady = 40000;
+    EXPECT_FALSE(store->mayMutate())
+        << "the remount arm must anchor at the claim attempt's pre-I/O instant, not a later "
+           "response-time reading taken after keeperStart/quiesceRefTablesForRemount";
+}
+
+/// ==== rev.6 Task 5: clean-release drain gates the farewell marker ====
+
+namespace
+{
+/// Forces the FIRST `putIfAbsent` whose key contains `fault_key_substr` to throw an ambiguous
+/// (Unresolved-classified) exception, `fault_count` times -- the minimal one-shot subset of
+/// `RefWriterTestBackend`'s fault injection (gtest_cas_ref_writer.cpp) this file's shutdown test needs
+/// to drive a ref-log append into the `Unresolved`/wedge outcome, with `max_attempts = 1` in the budget
+/// so the single failed attempt exhausts the retry budget immediately.
+class UnresolvedPutBackend final : public DB::Cas::tests::CountingBackend
+{
+public:
+    String fault_key_substr;
+    int fault_count = 0;
+
+    DB::Cas::PutResult putIfAbsent(const String & key, const String & bytes, const DB::Cas::ObjectMeta & meta) override
+    {
+        if (fault_count > 0 && !fault_key_substr.empty() && key.find(fault_key_substr) != String::npos)
+        {
+            --fault_count;
+            throw Poco::TimeoutException("UnresolvedPutBackend: simulated ambiguous result (response lost)");
+        }
+        return DB::Cas::tests::CountingBackend::putIfAbsent(key, bytes, meta);
+    }
+};
+}
+
+TEST(CASPoolShutdown, CleanStopDrainsAndWritesFarewell)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = DB::Cas::Pool::open(backend, DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    publishPart(store, "srv/clean_stop", "x", "payload");
+
+    const String mount_key = store->layout().mountKey("test");
+    store.reset();   /// drives ~Pool(): with no in-flight ref-log PUT, the drain must succeed.
+
+    const auto got = backend->get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    const MountLease lease = decodeMountLease(got->bytes);
+    EXPECT_EQ(lease.min_active, std::numeric_limits<uint64_t>::max())
+        << "a clean drain (no in-flight ref-log PUT) must write the farewell marker";
+}
+
+TEST(CASPoolShutdown, UnresolvedWedgeSkipsFarewell)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;   /// strictly above attempt_timeout_ms: equality is a wall-clock race (validateCasRequestBudget)
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<UnresolvedPutBackend>();
+    auto store = DB::Cas::Pool::open(backend, DB::Cas::PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test", .cas_request_budget = budget});
+    /// By value: `layout` is used after `store.reset()` below, a reference would dangle.
+    const Layout layout = store->layout();
+    const RootNamespace ns{"srv/wedge_shutdown"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so the fault
+    /// injected below (computed from that same sentinel) lands on the key production actually writes
+    /// to -- otherwise the real append mints an unrelated random incarnation and the fault misses.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, layout, ns, store->liveWriterEpoch());
+    publishPart(store, ns.string(), "x", "payload");
+
+    /// Force the ref-log append the drop below performs into the Unresolved/wedge outcome (as in the
+    /// wedge tests in gtest_cas_ref_writer.cpp): the single attempt the budget allows fails ambiguously.
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+
+    const String mount_key = store->layout().mountKey("test");
+    store.reset();   /// drives ~Pool(): the still-wedged lane must skip the farewell marker.
+
+    const auto got = backend->get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    const MountLease lease = decodeMountLease(got->bytes);
+    EXPECT_NE(lease.min_active, std::numeric_limits<uint64_t>::max())
+        << "an unresolved ref-log PUT must skip the clean-release farewell marker";
+    EXPECT_FALSE(lease.gc_fenced);
+
+    /// A successor claimMount on this body must return LiveDoubleStart (unclean path): no certificate of
+    /// death (not fenced, not the clean farewell marker, no proven-dead observation) justifies a
+    /// same-uuid, different-epoch reclaim.
+    const MountClaimResult claim = claimMount(*backend, layout, "test", lease.server_uuid,
+        lease.writer_epoch + 1, /*now_ms=*/1, /*ttl_ms=*/30000);
+    EXPECT_EQ(claim.kind, MountClaimResult::LiveDoubleStart);
+}
+
+/// ==== What a writable mount open may block on ====
+///
+/// Exactly one thing: the token-stability observation window, and only when the predecessor's death
+/// has to be OBSERVED rather than certified. The post-reclaim materialization grace (`T_mat`) that
+/// used to run beside it is retired -- it existed so a straggler conditional `PUT` from the dying
+/// epoch would settle before the successor trusted its recovery LISTINGS, and recovery does not trust
+/// listings any more (it walks arithmetically and fences the straggler with an in-band `EpochSeal`).
+/// These three tests pin the surviving shape from all three directions: observed-dead, certified-dead,
+/// and cleanly departed.
+
+TEST(CASMountOpenWaits, UncleanOpenPaysOnlyTheObservationWindow)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l{"p"};
+    DB::Cas::tests::seedPoolMetaForRestart(*b);
+    /// Predecessor: claim epoch 7, no farewell (simulate crash: just drop the keeper) -- a bare
+    /// `claimMount` plants the lease directly, with no clean-farewell `min_active` marker and no
+    /// `gc_fenced`, so the successor below has no certificate of death until it observes one itself.
+    ASSERT_EQ(claimMount(*b, l, "test", UInt128(1), /*epoch*/ 7, /*now_ms*/ 1000, /*ttl_ms*/ 500).kind,
+              MountClaimResult::Claimed);
+    /// A real predecessor at epoch 7 durably minted it first (`allocateWriterEpoch` always runs
+    /// before the mount claim); seed that durable epoch object here too, or the successor's own
+    /// `allocateWriterEpoch` trips the Phase C guard (epoch absent, mount present -> fail closed).
+    b->putIfAbsent(l.epochKey("test"), encodeServerEpoch(ServerEpoch{.next_writer_epoch = 8}));
+
+    /// A 500ms lease TTL is far below the default `cas_request_budget` (RFC
+    /// cas-s3-timeout-retry-control §required-timeout-model requires attempt_timeout + safety_margin <
+    /// lease TTL), so scale the budget down to fit -- mirrors `CasMountStartup::StaleSelfMountReclaimedAfterWait`.
+    const CasRequestBudget tiny_budget{
+        .attempt_timeout_ms = 50, .operation_deadline_ms = 500, .max_attempts = 1, .lease_safety_margin_ms = 50};
+
+    uint64_t fake_boot = 0;
+    std::vector<uint64_t> waits;
+    PoolPtr store;
+    ASSERT_NO_THROW(
+        store = Pool::open(b, PoolConfig{
+            .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "test",
+            .mount_lease_ttl_ms = std::chrono::milliseconds(500),
+            .mount_renew_period = std::chrono::milliseconds(100),
+            .cas_request_budget = tiny_budget,
+            .boot_ms_fn = [&] { return fake_boot; },
+            .wait_sleep_fn = [&](uint64_t ms) { fake_boot += ms; waits.push_back(ms); },
+        }));
+    ASSERT_TRUE(store);
+
+    /// The token-stability observation window (>= the 500ms ttl) is paid, because this predecessor's
+    /// death was never certified -- only observed.
+    uint64_t total = 0;
+    for (uint64_t w : waits)
+        total += w;
+    EXPECT_GE(total, 500u) << "the observation window must have been paid";
+    /// And NOTHING is paid on top of it. Every recorded wait is a poll of that window, bounded by the
+    /// lease TTL; a wait longer than the whole window can only be a reintroduced grace period.
+    for (uint64_t w : waits)
+        EXPECT_LE(w, 500u)
+            << "an unclean reclaim must not block on any wait beyond the observation poll -- the "
+               "straggler it used to wait out is fenced by the recovery seal instead";
+}
+
+TEST(CASMountOpenWaits, CleanOpenSkipsAllWaits)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    /// Predecessor released cleanly (drain + farewell from Task 5): open, then reset() drives ~Pool(),
+    /// which -- with nothing in flight -- writes the farewell marker (min_active == UINT64_MAX).
+    auto predecessor = Pool::open(b, PoolConfig{
+        .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "test"});
+    predecessor.reset();
+
+    std::vector<uint64_t> waits;
+    PoolPtr successor;
+    ASSERT_NO_THROW(
+        successor = Pool::open(b, PoolConfig{
+            .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "test",
+            .wait_sleep_fn = [&](uint64_t ms) { waits.push_back(ms); },
+        }));
+    ASSERT_TRUE(successor);
+
+    EXPECT_TRUE(waits.empty())
+        << "a clean farewell (Task 5) needs no observation window";
+}
+
+TEST(CASMountOpenWaits, FencedPriorReclaimsWithoutAnyWait)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    Layout l{"p"};
+    DB::Cas::tests::seedPoolMetaForRestart(*b);
+    ASSERT_EQ(claimMount(*b, l, "test", UInt128(1), /*epoch*/ 7, /*now_ms*/ 1000, /*ttl_ms*/ 500).kind,
+              MountClaimResult::Claimed);
+    /// A real predecessor at epoch 7 durably minted it first (`allocateWriterEpoch` always runs
+    /// before the mount claim); seed that durable epoch object here too, or the successor's own
+    /// `allocateWriterEpoch` trips the Phase C guard (epoch absent, mount present -> fail closed).
+    b->putIfAbsent(l.epochKey("test"), encodeServerEpoch(ServerEpoch{.next_writer_epoch = 8}));
+    /// Predecessor lease carries gc_fenced=true: fence it directly, exactly as `computeHeartbeatFloor`'s
+    /// fence-out does (preserve the body, gc_fenced = true, seq + 1, token-guarded).
+    fenceOutMount(*b, l.mountKey("test"));
+
+    /// See UncleanOpenPaysOnlyTheObservationWindow above: a 500ms TTL needs a scaled-down budget too.
+    const CasRequestBudget tiny_budget{
+        .attempt_timeout_ms = 50, .operation_deadline_ms = 500, .max_attempts = 1, .lease_safety_margin_ms = 50};
+
+    std::vector<uint64_t> waits;
+    PoolPtr store;
+    ASSERT_NO_THROW(
+        store = Pool::open(b, PoolConfig{
+            .pool_prefix = "p", .server_id = UInt128(1), .server_root_id = "test",
+            .mount_lease_ttl_ms = std::chrono::milliseconds(500),
+            .cas_request_budget = tiny_budget,
+            .wait_sleep_fn = [&](uint64_t ms) { waits.push_back(ms); },
+        }));
+    ASSERT_TRUE(store);
+
+    /// A GC-fenced prior is a terminal, already-threshold-gated certificate of death -- reclaimed on the
+    /// FIRST attempt, with no observation polling. It is also an UNCLEAN prior, which used to mean it
+    /// paid the materialization grace; nothing is owed now, so this open blocks on nothing at all.
+    EXPECT_TRUE(waits.empty())
+        << "a certified-dead predecessor needs neither the observation window nor any grace period";
+}
+
+namespace
+{
+/// Stalls the CLAIM ITSELF past the lease TTL, and counts what the open writes afterwards.
+///
+/// The mount key is written twice before the write fence arms: once by `claimMount`'s reclaim, then
+/// once by the keeper's adopt -- and the fence's anchor is taken BETWEEN them. So advancing the
+/// injected boot clock on the SECOND write models exactly the thing the Phase B redo exists for: the
+/// claim's own I/O outliving the lease it is about to arm a fence under. (This used to be modelled by
+/// a materialization grace long enough to consume the TTL; that wait is retired, and the guard it
+/// motivated is not -- a stalled socket can still outlive a validated request budget.)
+class StalledMountClaimBackend final : public DB::Cas::InMemoryBackend
+{
+public:
+    String mount_key;
+    std::function<void()> on_second_mount_write;
+    std::atomic<int> mount_writes{0};
+    std::atomic<int> mount_writes_after_stall{0};
+
+    DB::Cas::PutResult putOverwrite(const String & k, const String & b, const DB::Cas::Token & e,
+                                    const DB::Cas::ObjectMeta & m) override
+    {
+        if (k == mount_key)
+        {
+            const int n = ++mount_writes;
+            if (n == 2 && on_second_mount_write)
+                on_second_mount_write();
+            else if (n > 2)
+                ++mount_writes_after_stall;
+        }
+        return InMemoryBackend::putOverwrite(k, b, e, m);
+    }
+};
+}
+
+/// Phase B startup-arm (spec rev.4, codex round-3 finding 2): a claim path that consumed the lease TTL
+/// must force ONE fresh conditional lease write before arming — the fence must never arm from an anchor
+/// that has already expired (a successor could have legally reclaimed meanwhile).
+TEST(CASPool, StartupArmRedoesLeaseWriteWhenTheClaimConsumesTtl)
+{
+    auto backend = std::make_shared<StalledMountClaimBackend>();
+    DB::Cas::Layout layout("pool");
+    DB::Cas::tests::seedPoolMetaForRestart(*backend, "pool");
+    const String srid = "s";
+    const DB::UInt128 uuid(0x42);
+    backend->mount_key = layout.mountKey(srid);
+
+    /// Seed a FENCED, expired predecessor body under a DIFFERENT epoch (7, matching
+    /// `FencedPriorPaysOnlyTmat`'s convention). The durable epoch object seeded a few lines below
+    /// carries `next_writer_epoch = 8`, so THIS pool's own first-allocated `writer_epoch` is 8 --
+    /// non-colliding with the seeded epoch-7 prior by construction. With no collision the first
+    /// (and only) claim attempt reclaims directly with MountPriorState::Fenced, with no silent
+    /// FencedSelf fence-recovery detour to account for -- so the mount key is written exactly twice
+    /// before the arm, which is what the stall hook counts on.
+    {
+        DB::Cas::MountLease prior;
+        prior.server_uuid = uuid;
+        prior.writer_epoch = 7;
+        prior.seq = 7;
+        prior.expires_at_ms = 1;      /// long expired
+        prior.gc_fenced = true;
+        backend->putIfAbsent(layout.mountKey(srid), DB::Cas::encodeMountLease(prior));
+    }
+    /// A real predecessor at epoch 7 durably minted it first (`allocateWriterEpoch` always runs
+    /// before the mount claim); seed that durable epoch object here too, or `Pool::open`'s own
+    /// `allocateWriterEpoch` trips the Phase C guard (epoch absent, mount present -> fail closed).
+    backend->putIfAbsent(layout.epochKey(srid), DB::Cas::encodeServerEpoch(DB::Cas::ServerEpoch{.next_writer_epoch = 8}));
+    uint64_t fake_boot_ms = 10'000;
+    DB::Cas::PoolConfig cfg;
+    cfg.pool_prefix = "pool";
+    cfg.server_id = uuid;
+    cfg.server_root_id = srid;
+    cfg.mount_lease_ttl_ms = std::chrono::milliseconds(30'000);
+    cfg.boot_ms_fn = [&] { return fake_boot_ms; };
+    /// The keeper's adopt write stalls for 40 s of boot clock -- past the 30 s TTL the anchor a few
+    /// microseconds earlier was taken against.
+    backend->on_second_mount_write = [&] { fake_boot_ms += 40'000; };
+
+    auto store = DB::Cas::Pool::open(backend, cfg);
+    ASSERT_NE(store, nullptr);
+
+    ASSERT_EQ(backend->mount_writes.load(), 3)
+        << "the fixture assumes exactly two mount writes before the redo (the reclaim and the keeper's "
+           "adopt, with the fence anchor between them); a different sequence would make the stall land "
+           "somewhere else and this test would stop testing the redo";
+    EXPECT_EQ(backend->mount_writes_after_stall.load(), 1)
+        << "a TTL-consuming claim must be followed by exactly ONE fresh conditional lease write "
+           "(the re-anchoring redo) before the write fence arms";
+}
+
+/// ==== What a self-remount may block on ====
+///
+/// Nothing an operator configures. The remount used to consult `refLanesSettledForRemount` and pay the
+/// materialization grace whenever a ref lane still held an undecided `PUT`; both are retired, because
+/// the undecided `PUT` is settled by the protocol rather than waited out — recovery closes the dead
+/// epoch with an in-band `EpochSeal` written as a conditional create, and the straggler's own create
+/// loses to it. `gtest_cas_retirement_sweep.cpp` proves that conflict directly; these two pin that the
+/// wait is gone from both the drained and the still-wedged path.
+
+TEST(CASRemountWaits, DrainedRemountPaysNoWait)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    uint64_t fake_boot = 1'000'000;
+    std::vector<uint64_t> waits;
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(30000),
+        .boot_ms_fn = [&] { return fake_boot; },
+        .wait_sleep_fn = [&](uint64_t ms) { fake_boot += ms; waits.push_back(ms); },
+    });
+    ASSERT_TRUE(store);
+    EXPECT_TRUE(waits.empty()) << "a fresh mount (no predecessor) pays no wait at open";
+
+    /// Trip the fence: advance the local boot clock past the deadline (as in `WriteFenceUsesInjectedBootClock`
+    /// above) and mark the durable lease `gc_fenced` (the certificate `claimMountAwaitingExpiry` reclaims
+    /// on its FIRST attempt, no observation polling -- avoids a real sleep in this test).
+    fake_boot += 30001;
+    fenceOutMount(*backend, store->layout().mountKey("test"));
+
+    /// No in-flight ref-log PUT at all -- the easy direction.
+    ASSERT_TRUE(store->tryRemountOnce());
+
+    EXPECT_TRUE(waits.empty())
+        << "a drained self-remount must pay no wait";
+}
+
+TEST(CASRemountWaits, UnresolvedWedgeRemountPaysNoWaitEither)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;   /// strictly above attempt_timeout_ms: equality is a wall-clock race (validateCasRequestBudget)
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<UnresolvedPutBackend>();
+    uint64_t fake_boot = 1'000'000;
+    std::vector<uint64_t> waits;
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(30000),
+        .cas_request_budget = budget,
+        .boot_ms_fn = [&] { return fake_boot; },
+        .wait_sleep_fn = [&](uint64_t ms) { fake_boot += ms; waits.push_back(ms); },
+    });
+    ASSERT_TRUE(store);
+    EXPECT_TRUE(waits.empty()) << "a fresh mount (no predecessor) pays no wait at open";
+
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv/remount_wedge"};
+    /// Stage B (Task 4-C): see `CASPoolShutdown.UnresolvedWedgeSkipsFarewell`'s identical comment.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, layout, ns, store->liveWriterEpoch());
+    publishPart(store, ns.string(), "x", "payload");
+
+    /// Force the ref-log append `dropRef` below performs into the Unresolved/wedge outcome (as in
+    /// `CASPoolShutdown.UnresolvedWedgeSkipsFarewell`): the single attempt the budget allows fails
+    /// ambiguously.
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+
+    /// Trip the fence exactly as in `DrainedRemountSkipsGrace` above.
+    fake_boot += 30001;
+    fenceOutMount(*backend, store->layout().mountKey("test"));
+
+    /// THE HARD DIRECTION, and the one the retired wait existed for: a ref lane that still holds an
+    /// UNDECIDED conditional PUT when the fence trips. It used to buy a 30 s grace. It buys nothing now
+    /// -- the remount proceeds straight through, and the undecided PUT is decided by the seal the next
+    /// recovery writes into its slot.
+    ASSERT_TRUE(store->tryRemountOnce());
+
+    EXPECT_TRUE(waits.empty())
+        << "an unresolved ref-lane wedge must not make the remount block: the straggler it describes is "
+           "fenced by the recovery seal, not waited out";
+}
+
+/// Sealing is decided by ARITHMETIC -- `epoch < live_epoch` -- and by nothing else. This test used to
+/// pin the opposite ("a table recovered under a later CLEAN boundary must not seal"), which was the
+/// right rule while a seal was a synthetic SNAPSHOT published only to close an unclean handover: such a
+/// seal after a clean shutdown was pure parasitic cost, so it was gated on the per-epoch unclean flag.
+///
+/// INV-2's seal is not that object. It is the chain link that makes a MISSING epoch detectable across a
+/// transition, and a chain that skips every epoch whose mount happened to shut down cleanly is not a
+/// chain -- the next sequence-1 transaction would have no `prev_epoch_seal` to name, and no reader could
+/// tell "epoch 2 was empty" from "epoch 2's records are gone". So a late-touched table now closes EVERY
+/// dead epoch below the live one, however its predecessors died, and this test pins that plus the two
+/// things that must still be true: the seals land IN-BAND (at log keys, at the slot a straggler would
+/// have taken) and no synthetic seal SNAPSHOT is written anywhere.
+TEST(CASRemountWaits, ALateTouchedTableClosesEveryDeadEpochInBandHoweverItsPredecessorsDied)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;   /// strictly above attempt_timeout_ms: equality is a wall-clock race (validateCasRequestBudget)
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<UnresolvedPutBackend>();
+    uint64_t fake_boot = 1'000'000;
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(30000),
+        .cas_request_budget = budget,
+        .boot_ms_fn = [&] { return fake_boot; },
+        .wait_sleep_fn = [&](uint64_t ms) { fake_boot += ms; },
+    });
+    ASSERT_TRUE(store);
+
+    const Layout & layout = store->layout();
+    const RootNamespace ns1{"srv/table_a"};
+    const RootNamespace ns2{"srv/table_b"};
+    /// Stage B (Task 4-C): `ns1` is pinned because the fault below targets its key by exact sentinel
+    /// match. `ns2` must ALSO be pinned: the epoch-close assertions further down read its ref-log keys
+    /// directly at `DB::Cas::tests::fixture::fixtureLife(ns2)`.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, layout, ns1, store->liveWriterEpoch());
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, layout, ns2, store->liveWriterEpoch());
+    publishPart(store, ns1.string(), "x", "payload-a");
+    /// ns2's epoch-1 data: never touched again by this incarnation until the final check below, well
+    /// after both remounts -- the "table recovered for the first time, late" the fix must not over-seal.
+    /// Distinct content from ns1's part: identical payloads collide on the same blob and race
+    /// `PartWriteTxn::observeAndAdmit`'s newborn-debris watermark, unrelated to what this test is about.
+    publishPart(store, ns2.string(), "y", "payload-b");
+
+    /// Force ns1's ref-log append into the Unresolved/wedge outcome (mirrors
+    /// `UnresolvedWedgeRemountPaysNoWaitEither` above).
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns1)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns1, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns1));
+
+    /// Self-remount #1: UNCLEAN (the wedge above). Epoch 1 -> 2.
+    fake_boot += 30001;
+    fenceOutMount(*backend, store->layout().mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce());
+    ASSERT_EQ(store->liveWriterEpoch(), 2u);
+
+    /// Self-remount #2: CLEAN (no wedge left behind -- `quiesceRefTablesForRemount` already cleared the
+    /// cache). Epoch 2 -> 3.
+    fake_boot += 30001;
+    fenceOutMount(*backend, store->layout().mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce());
+    ASSERT_EQ(store->liveWriterEpoch(), 3u);
+
+    using ProfileEvents::global_counters;
+    const auto sealed_before = global_counters[ProfileEvents::CASRefRecoveryEpochSealed].load();
+
+    /// ns2's FIRST recovery under this incarnation happens now, at epoch 3 -- strictly after both
+    /// remounts. Its only data is at epoch 1, so epochs 1 and 2 are both dead for it.
+    EXPECT_EQ(store->listRefs(ns2).size(), 1u);
+
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefRecoveryEpochSealed].load(), sealed_before + 2)
+        << "both dead epochs must be closed -- the chain link is what a later reader needs to tell an "
+           "EMPTY epoch from a LOST one, and that is independent of how each mount ended";
+    EXPECT_TRUE(backend->get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns2), RefTxnId{1, 2})).has_value())
+        << "epoch 1 closes at the slot right after its last durable id, in-band";
+    EXPECT_TRUE(backend->get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns2), RefTxnId{2, 1})).has_value())
+        << "empty epoch 2 closes at its own sequence 1, chained to the epoch-1 seal";
+    const RefTxnId retired_sentinel_id{2, std::numeric_limits<uint64_t>::max()};
+    EXPECT_FALSE(backend->get(layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns2), retired_sentinel_id)).has_value())
+        << "and NO synthetic seal snapshot is written: that shape is retired";
+}
+
+TEST(CASPool, ReadManifestSharedReturnsSharedDecodeWithoutCopy)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    const DB::Cas::Layout layout("p");
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    const DB::Cas::RootNamespace ns{"srv/t1"};
+    const DB::Cas::ManifestRef ref{.writer_epoch = 1, .build_sequence = 1, .manifest_ordinal = 1};
+    const auto id = DB::Cas::tests::writeManifestRaw(*backend, layout, ns, ref,
+        {DB::Cas::tests::blobEntryFor("data.bin", DB::UInt128(7))});
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{1, 1},
+        {DB::Cas::tests::namespaceBirthOp(), DB::Cas::tests::publishCommittedOps("part_1", ref)[0],
+         DB::Cas::tests::publishCommittedOps("part_1", ref)[1]}, std::nullopt});
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    auto store = DB::Cas::Pool::open(backend,
+        DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const auto resolved = store->resolveRef(ns, "part_1");
+    ASSERT_TRUE(resolved.has_value());
+
+    const String manifest_key = layout.manifestKey(id);
+    backend->resetCounts();
+
+    auto m1 = store->readManifestShared(resolved->manifest_id);
+    auto m2 = store->readManifestShared(resolved->manifest_id);
+    EXPECT_EQ(m1.get(), m2.get());                          /// the SAME shared decode, no copy
+    EXPECT_EQ(backend->getCount(manifest_key), 1u);         /// one body GET
+    EXPECT_EQ(backend->headCount(manifest_key), 2u);        /// mandatory HEAD per call (unchanged)
+    ASSERT_EQ(m1->entries.size(), 1u);
+    EXPECT_EQ(m1->entries[0].path, "data.bin");
+}
+
+/// Coverage gap (Task 13a): restores the get/exists/remove roundtrip for the mount access-check probe
+/// object. The old `CASPool.MountpointObjectRoundTrip` was dropped in the refactor; the wiring test only
+/// exercises `putMountpointObject` + `existsFile`, leaving `getMountpointObject`'s value round-trip and
+/// `removeMountpointObject` unasserted even though both `Pool` methods remain live.
+TEST(CASPool, MountpointObjectRoundTrip)
+{
+    auto b = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = DB::Cas::Pool::open(b, DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const String key = "srv1/clickhouse_access_check_abc";
+    EXPECT_FALSE(store->getMountpointObject(key).has_value());
+    EXPECT_FALSE(store->mountpointObjectExists(key));
+    store->putMountpointObject(key, "probe-bytes");
+    EXPECT_TRUE(store->mountpointObjectExists(key));
+    auto got = store->getMountpointObject(key);
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(*got, "probe-bytes");
+    store->removeMountpointObject(key);
+    EXPECT_FALSE(store->getMountpointObject(key).has_value());
+    EXPECT_FALSE(store->mountpointObjectExists(key));
+}
diff --git a/src/Disks/tests/gtest_cas_probe.cpp b/src/Disks/tests/gtest_cas_probe.cpp
new file mode 100644
index 000000000000..f2362b9a1bec
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_probe.cpp
@@ -0,0 +1,309 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+namespace DB
+{
+namespace ErrorCodes
+{
+    extern const int NOT_IMPLEMENTED;
+}
+}
+
+using namespace DB::Cas;
+
+TEST(CASProbe, PassesOnEnforcingBackend)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    EXPECT_NO_THROW(runCapabilityProbe(*b, "p/.cas_probe"));
+    EXPECT_TRUE(b->list("p/.cas_probe", "", 10).keys.empty());   // probe cleans up after itself
+}
+
+/// AWS S3 answers 400 InvalidArgument to a conditional DELETE with an EMPTY If-Match, and the
+/// probe's exit cleanup used to issue exactly that (deleteExact with the absent HeadResult's empty
+/// token) after step 8 had already deleted the probe keys — two scary AWSClient <Error> log lines
+/// on every real-S3 mount. The cleanup must HEAD-gate the delete instead of firing blindly.
+class EmptyTokenDeleteRecorder : public InMemoryBackend
+{
+public:
+    size_t empty_token_deletes = 0;
+
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        if (token.empty())
+            ++empty_token_deletes;
+        return InMemoryBackend::deleteExact(key, token);
+    }
+};
+
+TEST(CASProbe, CleanupNeverDeletesWithEmptyToken)
+{
+    auto b = std::make_shared<EmptyTokenDeleteRecorder>();
+    EXPECT_NO_THROW(runCapabilityProbe(*b, "p/.cas_probe"));
+    EXPECT_EQ(b->empty_token_deletes, 0u);
+}
+
+TEST(CASProbe, FailsClosedOnNonEnforcingDelete)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    b->setEnforceTokens(false);                                  // the MinIO-OSS failure mode
+    EXPECT_THROW(runCapabilityProbe(*b, "p/.cas_probe"), DB::Exception);
+}
+
+TEST(CASProbe, FailsClosedOnDeleteMarkers)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    b->setSimulateDeleteMarkers(true);                           // versioning enabled on the prefix
+    EXPECT_THROW(runCapabilityProbe(*b, "p/.cas_probe"), DB::Exception);
+}
+
+TEST(CASProbe, PassesOnEmulatedLocal)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+    EXPECT_NO_THROW(runCapabilityProbe(*b, "p/.cas_probe"));
+}
+
+/// B135: two servers mounting the SAME shared CA pool concurrently must not race on the probe keys.
+/// We simulate "a concurrent mounter's probe is in flight" by PRE-SEEDING the fixed-name probe key
+/// `<pool>/_probe/token` over a shared backend, then opening the Pool. With the OLD fixed-key probe
+/// the open's `putIfAbsent("<pool>/_probe/token", …)` returns PreconditionFailed and `Pool::open`
+/// throws NOT_IMPLEMENTED ("putIfAbsent on a fresh key returned PreconditionFailed"). With the
+/// per-mount unique probe prefix `<pool>/_probe/<rand>/token`, the seeded key does not collide and
+/// the open succeeds — exactly the concurrent-shared-pool-mount behaviour we need.
+TEST(CASProbe, ConcurrentMountsDoNotCollide)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+
+    /// Simulate a concurrent mounter whose probe object under the legacy fixed key is still present.
+    ASSERT_EQ(b->putIfAbsent("p/_probe/token", "concurrent-mounter-in-flight").outcome, PutOutcome::Done);
+
+    /// A real (second) mount over the same shared pool must still succeed — its probe runs under a
+    /// fresh per-mount-unique prefix and never touches the seeded fixed key.
+    EXPECT_NO_THROW(Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}));
+
+    /// And two genuinely-concurrent mounts (distinct unique prefixes) both succeed over one backend.
+    EXPECT_NO_THROW(Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}));
+
+    /// The seeded fixed-key artifact is untouched (the probe never collided with it).
+    EXPECT_TRUE(b->get("p/_probe/token").has_value());
+}
+
+/// The probe must consult the backend's store-preconditions hook BEFORE the op battery: a
+/// generation-dialect store on a VERSIONED bucket passes every conditional-op check, but its
+/// token-exact DELETEs archive noncurrent generations instead of reclaiming storage — only the
+/// hook can see that, so a throwing hook must fail the probe closed.
+class PreconditionRefusingBackend : public InMemoryBackend
+{
+public:
+    void checkPoolPreconditions() override
+    {
+        throw DB::Exception(DB::ErrorCodes::NOT_IMPLEMENTED,
+            "test: store precondition violated (e.g. bucket versioning enabled)");
+    }
+};
+
+TEST(CASProbe, FailsClosedOnPoolPreconditions)
+{
+    auto b = std::make_shared<PreconditionRefusingBackend>();
+    EXPECT_THROW(runCapabilityProbe(*b, "p/.cas_probe"), DB::Exception);
+    /// The hook fires FIRST: no probe keys may have been written.
+    EXPECT_TRUE(b->list("p/.cas_probe", "", 10).keys.empty());
+}
+
+/// `Pool::open` wraps the pool backend in `InstrumentedBackend` BEFORE calling `runCapabilityProbe`
+/// (see CasPool.cpp), so the hook must actually fire THROUGH the wrapper on the real mount path —
+/// not just on a raw backend, which `FailsClosedOnPoolPreconditions` above already covers.
+TEST(CASProbe, PoolPreconditionsFireThroughInstrumentedWrapper)
+{
+    auto inner = std::make_shared<PreconditionRefusingBackend>();
+    InstrumentedBackend wrapped(inner);
+    EXPECT_THROW(runCapabilityProbe(wrapped, "p/.cas_probe"), DB::Exception);
+    /// The hook fires FIRST: no probe keys may have been written to the inner backend.
+    EXPECT_TRUE(inner->list("p/.cas_probe", "", 10).keys.empty());
+}
+
+/// RFC cas-s3-timeout-retry-control: a Native-mode mount over an object storage that does not support
+/// the SingleAttempt retry profile must never silently proceed under the disk's default (~500-attempt)
+/// transparent retry policy — see Backend::checkConditionalWriteSingleAttemptSupport.
+/// LocalObjectStorage never supports the profile (IObjectStorage::supportsRetryProfile's default
+/// implementation only answers true for Default), so Native mode over it is exactly the case this must
+/// refuse. EmulatedSingleProcess is exempt: it never claims single-attempt S3 semantics in the first
+/// place (PassesOnEmulatedLocal above).
+TEST(CASProbe, FailsClosedOnUnsupportedSingleAttemptProfile)
+{
+    auto native = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+    EXPECT_THROW(native->checkConditionalWriteSingleAttemptSupport(), DB::Exception);
+
+    auto emulated = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::EmulatedSingleProcess);
+    EXPECT_NO_THROW(emulated->checkConditionalWriteSingleAttemptSupport());
+}
+
+/// The same fail-closed refusal through the actual capability probe (Step 0b) — the real gate a
+/// writable Pool::open goes through, not just the hook in isolation above.
+TEST(CASProbe, MissingSingleAttemptClientFailsCapabilityProbe)
+{
+    auto storage = DB::Cas::tests::makeLocalObjectStorageForTest();
+    /// Native mode passes the key to the object storage verbatim, so the probe prefix must be anchored
+    /// under this storage's own root: a bare prefix lands beside the test process, where an object left
+    /// by another run answers the LIST below and an unrooted LIST answers "no keys" for free.
+    const String probe_prefix = DB::Cas::tests::nativeKeyUnder(storage, "p/.cas_probe");
+
+    auto b = std::make_shared<ObjectStorageBackend>(storage, ObjectStorageBackend::Mode::Native);
+    EXPECT_THROW(runCapabilityProbe(*b, probe_prefix), DB::Exception);
+    /// The hook fires before the op battery: no probe keys may have been written.
+    EXPECT_TRUE(b->list(probe_prefix, "", 10).keys.empty());
+
+    /// The same LIST can see a key that IS under the prefix — otherwise the emptiness above would be
+    /// indistinguishable from a prefix this backend can never enumerate.
+    ASSERT_EQ(b->putIfAbsent(probe_prefix + "/token", "probe-v1").outcome, PutOutcome::Done);
+    EXPECT_FALSE(b->list(probe_prefix, "", 10).keys.empty());
+}
+
+/// Mirrors PoolPreconditionsFireThroughInstrumentedWrapper: the real mount path wraps the backend in
+/// InstrumentedBackend BEFORE calling runCapabilityProbe, so this check must fire through it too.
+TEST(CASProbe, MissingSingleAttemptClientFiresThroughInstrumentedWrapper)
+{
+    auto storage = DB::Cas::tests::makeLocalObjectStorageForTest();
+    auto inner = std::make_shared<ObjectStorageBackend>(storage, ObjectStorageBackend::Mode::Native);
+    InstrumentedBackend wrapped(inner);
+    EXPECT_THROW(runCapabilityProbe(wrapped, DB::Cas::tests::nativeKeyUnder(storage, "p/.cas_probe")), DB::Exception);
+}
+
+namespace
+{
+
+/// Models the exact shape of the trust-flip this suite must catch a regression of
+/// (codex-review-triage §3.18, Critical): like the production `ObjectStorageBackend` in Native mode,
+/// this backend mints and expects tokens under a dialect (`TokenType::ETag`) OTHER than
+/// `TokenType::Emulated`, and rejects a foreign-dialect `expected`/`token` argument LOCALLY --
+/// before the value it carries ever reaches the real conditional-compare beneath the gate (`inner`,
+/// a genuinely enforcing `InMemoryBackend`, standing in for "the wire"). Every gated method counts
+/// how many times it actually delegated to `inner`, so a test can tell "rejected by the dialect
+/// gate" apart from "rejected by the real enforcement" -- the exact distinction `Cas::Probe` exists
+/// to prove, and the one the №19 hardening risked collapsing (see CasProbe.cpp step 3/5c/6).
+class DialectGatedCountingBackend final : public Backend
+{
+public:
+    std::optional<GetResult> get(const String & key, Range range) override { return inner.get(key, range); }
+
+    std::optional<GetStreamResult> getStream(const String & key, Range range) override { return inner.getStream(key, range); }
+
+    HeadResult head(const String & key) override
+    {
+        HeadResult r = inner.head(key);
+        if (r.exists)
+            r.token.type = TokenType::ETag;
+        return r;
+    }
+
+    bool supportsListTokens() const override { return inner.supportsListTokens(); }
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        /// No `expected` token to gate -- matches production (ObjectStorageBackend::putIfAbsent has
+        /// no dialect check either).
+        PutResult r = inner.putIfAbsent(key, bytes, meta);
+        if (r.outcome == PutOutcome::Done)
+            r.token.type = TokenType::ETag;
+        return r;
+    }
+
+    WriteSinkPtr putIfAbsentStream(const String & key, const ObjectMeta & meta) override { return inner.putIfAbsentStream(key, meta); }
+
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        if (expected.type != TokenType::ETag)
+            return {PutOutcome::PreconditionFailed, {}};   /// dialect-gated: never reaches `inner`
+        ++overwrite_reached;
+        PutResult r = inner.putOverwrite(key, bytes, Token{expected.value, TokenType::Emulated}, meta);
+        if (r.outcome == PutOutcome::Done)
+            r.token.type = TokenType::ETag;
+        return r;
+    }
+
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected, const ObjectMeta & meta) override
+    {
+        if (expected.has_value() && expected->type != TokenType::ETag)
+            return {CasOutcome::Conflict, {}};   /// dialect-gated: never reaches `inner`
+        ++casput_reached;
+        std::optional<Token> retyped;
+        if (expected.has_value())
+            retyped = Token{expected->value, TokenType::Emulated};
+        CasResult r = inner.casPut(key, bytes, retyped, meta);
+        if (r.outcome == CasOutcome::Committed)
+            r.token.type = TokenType::ETag;
+        return r;
+    }
+
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        if (token.type != TokenType::ETag)
+        {
+            DeleteOutcome d;
+            d.kind = DeleteOutcome::Kind::TokenMismatch;   /// dialect-gated: never reaches `inner`
+            return d;
+        }
+        ++delete_reached;
+        return inner.deleteExact(key, Token{token.value, TokenType::Emulated});
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage p = inner.list(prefix, cursor, limit);
+        for (auto & k : p.keys)
+            if (k.token)
+                k.token->type = TokenType::ETag;
+        return p;
+    }
+
+    /// Number of times putOverwrite/casPut(with expected)/deleteExact actually delegated to `inner`
+    /// (i.e. reached the real enforcement) rather than being short-circuited by the dialect gate.
+    int overwrite_reached = 0;
+    int casput_reached = 0;
+    int delete_reached = 0;
+
+private:
+    InMemoryBackend inner;
+};
+
+}
+
+/// codex-review-triage §3.18, Critical: `runCapabilityProbe`'s three wrong-token sites (step 3
+/// putOverwrite, step 5c casPut, step 6 deleteExact) must send a token in the LIVE dialect this
+/// backend mints (t1.type / ct1.type / t2.type), not a hardcoded `TokenType::Emulated`. A backend
+/// whose native dialect differs from Emulated -- exactly what `ObjectStorageBackend` mints in Native
+/// mode -- would otherwise reject the old hardcoded tokens LOCALLY via a dialect gate, never
+/// exercising the real conditional enforcement those three steps exist to validate; the probe would
+/// still report success (the outcome enums match either way), so a regression here is invisible
+/// unless something counts whether the real enforcement was ever reached. `DialectGatedCountingBackend`
+/// enforces real (correct) conditional semantics AND gates on dialect exactly like the production
+/// risk, so `runCapabilityProbe` runs to completion (unlike a real Native-mode ObjectStorageBackend
+/// over LocalObjectStorage, which cannot even reach this point -- see
+/// MissingSingleAttemptClientFailsCapabilityProbe and the fact that LocalObjectStorage does not honor
+/// WriteSettings conditions at all); the exact reached-counts below pin down that every wrong-token
+/// site got past the gate: a probe that regressed to the hardcoded-Emulated construction would still
+/// pass (no throw) but under-count here by exactly one at each of the three sites, since the dialect
+/// gate would swallow that one call before `inner` ever saw it.
+TEST(CASProbe, WrongTokenAttemptsReachTheBackendPastTheDialectGate)
+{
+    DialectGatedCountingBackend b;
+    EXPECT_NO_THROW(runCapabilityProbe(b, "p/.cas_probe"));
+
+    /// putOverwrite: step 3 (wrong token) + step 4 (correct token) -- both live-dialect, both gated
+    /// through to `inner`.
+    EXPECT_EQ(b.overwrite_reached, 2);
+    /// casPut: 5a (create), 5b (conflict-on-exists, no expected token to gate), 5c (wrong token,
+    /// live-dialect), 5d (correct token) -- all four reach `inner`.
+    EXPECT_EQ(b.casput_reached, 4);
+    /// deleteExact: step 6 (wrong token, live-dialect) + step 8 (correct token) + step 9 cleanup
+    /// (correct token for cas_key) -- all three reach `inner`.
+    EXPECT_EQ(b.delete_reached, 3);
+}
diff --git a/src/Disks/tests/gtest_cas_promote_republish.cpp b/src/Disks/tests/gtest_cas_promote_republish.cpp
new file mode 100644
index 000000000000..8c6b504535f4
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_promote_republish.cpp
@@ -0,0 +1,402 @@
+#include <gtest/gtest.h>
+#include <filesystem>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPartManifestFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+/// RED/characterization tests for the promote-over-committed leak fix:
+///   BUG 1a (PROMOTE-OVER-COMMITTED-LEAK): `PartWriteTxn::promote` silently overwrites `refs[final_ref_name]`
+///   when it already names a DIFFERENT committed manifest, orphaning the old manifest (leak). The fix
+///   (Task 2) makes this throw `ABORTED` instead.
+///   BUG 1c: `republishRef`'s only idempotency gate is "source absent" -- a re-drive after a crash
+///   between `promote(dst)` and `dropRef(src)` finds dst ALREADY committed with the (same) content it is
+///   about to re-publish, but re-stages+re-promotes anyway, minting a fresh manifest and orphaning the
+///   first attempt's manifest. The fix (Task 3) makes the re-drive idempotent (content-keyed, not
+///   ManifestId-keyed) when dst matches, and fail-closed (`ABORTED`) when dst holds different content.
+///
+/// These tests are EXPECTED TO FAIL pre-fix -- that failure IS the bug reproducing. They must not be
+/// weakened to pass; Tasks 2/3 make them pass.
+
+namespace DB::ErrorCodes
+{
+extern const int ABORTED;
+extern const int NETWORK_ERROR;
+extern const int LOGICAL_ERROR;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+PoolPtr openPool(const std::shared_ptr<InMemoryBackend> & b)
+{
+    return Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// One inline-entry manifest naming `path` with content `bytes` (distinct bytes => distinct content).
+/// EntryPlacement::Inline means `promote`'s blob-leaf revalidation skips it entirely -- no real blob
+/// objects are needed for these tests.
+std::vector<ManifestEntry> inlineEntries(const String & path, const String & bytes)
+{
+    ManifestEntry e;
+    e.path = path;
+    e.placement = EntryPlacement::Inline;
+    e.inline_bytes = bytes;
+    return {e};
+}
+
+/// The full write flow for an INLINE-only manifest: stageManifest -> precommitAdd -> promote. Returns
+/// the committed ManifestId.
+ManifestId publishCommitted(const PoolPtr & s, const RootNamespace & ns, const String & ref,
+                            const std::vector<ManifestEntry> & entries)
+{
+    auto build = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    const ManifestId id = build->stageManifest(entries);
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+/// The ContentAddressedTransaction fixture (mirrors gtest_ca_transaction.cpp's openTxStorage /
+/// writeFileTx): a real disk-layer storage + transaction, used to drive `republishRef` through its
+/// ONLY caller (`ContentAddressedTransaction::moveDirectory`'s committed-source-ref-move branch),
+/// since `republishRef` itself is private.
+std::shared_ptr<DB::ContentAddressedMetadataStorage> openTxStorage()
+{
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "ca_tx_promote_republish_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+    storage->startup();
+    return storage;
+}
+
+void writeFileTx(DB::IMetadataTransaction & tx, const std::string & path, const std::string & bytes)
+{
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(tx);
+    auto buf = ca_tx.writeFile(path, 65536, DB::WriteMode::Rewrite, {});
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+}
+
+}
+
+/// BUG 1a: promoting a DIFFERENT manifest onto an already-committed ref must fail closed (ABORTED),
+/// not silently overwrite (which orphans the old manifest, PROMOTE-OVER-COMMITTED-LEAK).
+/// PRE-FIX: promote() does not throw -- this test FAILS (RED), which IS the leak reproducing.
+TEST(CASPromoteRepublish, PromoteOverDifferentCommittedRefFailsClosed)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl@cas@"};
+    const String ref = "all_0_0_0";
+
+    publishCommitted(s, ns, ref, inlineEntries("f", "AAA"));   // committed T_old
+
+    auto build2 = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    const ManifestId id2 = build2->stageManifest(inlineEntries("f", "BBB"));   // DIFFERENT content
+    build2->precommitAdd(ns, ref, id2);
+
+    try
+    {
+        build2->promote(ns, ref, build2->buildId(), id2);
+        FAIL() << "PRE-FIX: promote silently overwrote a committed ref (PROMOTE-OVER-COMMITTED-LEAK); "
+                  "POST-FIX must throw a CAS write-retry-later NETWORK_ERROR";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR);
+    }
+}
+
+/// Re-promoting the SAME manifest_ref onto its own committed ref must NOT throw (idempotent
+/// re-promote): the fix's guard keys on a DIFFERENT manifest_ref, not merely "ref already committed".
+/// This is expected to pass BOTH pre- and post-fix (it is not part of the bug).
+TEST(CASPromoteRepublish, PromoteSameManifestIsIdempotent)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl@cas@"};
+    const String ref = "all_0_0_0";
+    const ManifestId id = publishCommitted(s, ns, ref, inlineEntries("f", "AAA"));
+
+    /// Re-precommit + re-promote the SAME id onto the same ref: allowed (same manifest_ref).
+    auto build2 = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    build2->precommitAdd(ns, ref, id);
+    EXPECT_NO_THROW(build2->promote(ns, ref, build2->buildId(), id));
+}
+
+/// Sanity companion to BUG 1a: promote over an ABSENT ref (the normal insert path) must succeed
+/// unconditionally -- the fail-close guard must only fire for an EXISTING different committed ref.
+TEST(CASPromoteRepublish, PromoteOverAbsentRefSucceeds)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl@cas@"};
+    const String ref = "all_0_0_0";
+
+    auto build = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    const ManifestId id = build->stageManifest(inlineEntries("f", "AAA"));
+    build->precommitAdd(ns, ref, id);
+    EXPECT_NO_THROW(build->promote(ns, ref, build->buildId(), id));
+}
+
+/// BUG 1c: a `republishRef` re-drive where the destination is ALREADY committed with the SAME content
+/// (the crash-before-`dropRef(src)` state) must be idempotent: skip the re-stage/re-promote, drop src,
+/// and leave dst's manifest UNCHANGED (no fresh manifest minted for identical content).
+///
+/// PRE-FIX: republishRef's only idempotency gate is "source absent" -- it re-stages+re-promotes
+/// unconditionally, minting a FRESH manifest id at dst even though the content is identical, orphaning
+/// the first attempt's manifest. This test asserts dst's ManifestId is UNCHANGED across the re-drive --
+/// PRE-FIX this FAILS (RED: the id changes, proving the orphaning leak).
+TEST(CASPromoteRepublish, RepublishReDriveOverCommittedDstIsIdempotent)
+{
+    auto storage = openTxStorage();
+    const auto ns = storage->liveNamespace("b09b09b0-0909-4909-8909-090909090909");
+    const String src_ref = "all_1_1_0";
+    const String dst_ref = "detached_all_1_1_0";
+    const String src_path = "b09/b09b09b0-0909-4909-8909-090909090909/" + src_ref;
+    const String dst_path = "b09/b09b09b0-0909-4909-8909-090909090909/" + dst_ref;
+
+    /// 1. Publish a committed src part via the normal write flow (tmp -> final rename, B151
+    ///    publish-at-rename), exactly as gtest_ca_transaction.cpp's fixtures do.
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b09/b09b09b0-0909-4909-8909-090909090909/tmp_insert_" + src_ref + "/data.bin", "payload-A");
+        tx->moveDirectory("b09/b09b09b0-0909-4909-8909-090909090909/tmp_insert_" + src_ref, src_path);
+        tx->commit(DB::NoCommitOptions{});
+    }
+    ASSERT_TRUE(storage->store()->resolveRef(ns, src_ref).has_value());
+
+    /// 2. Construct the "crash-before-dropRef(src)" state of a PRIOR republishRef drive by replaying
+    ///    its exact body (resolve src -> adoptEvidence every entry -> stageManifest(same entries) ->
+    ///    precommitAdd -> promote) WITHOUT the trailing dropRef(src). This leaves BOTH src and dst
+    ///    committed, with dst holding the SAME content as src -- precisely the state a re-driven
+    ///    republishRef must handle idempotently (ContentAddressedTransaction.cpp:143-169).
+    const auto resolved_src = storage->store()->resolveRef(ns, src_ref);
+    ASSERT_TRUE(resolved_src.has_value());
+    const PartManifest src_manifest = storage->store()->readManifest(resolved_src->manifest_id);
+    {
+        auto build = storage->store()->beginPartWrite(
+            PartWriteInfo{.intended_ref = ns.string() + "/" + dst_ref, .intended_namespace = ns});
+        for (const auto & entry : src_manifest.entries)
+            build->adoptEvidence(entry);
+        const ManifestId id = build->stageManifest(src_manifest.entries);
+        build->precommitAdd(ns, dst_ref, id);
+        build->promote(ns, dst_ref, build->buildId(), id);
+        /// Deliberately NO dropRef(ns, src_ref) here -- this is the simulated crash.
+    }
+    ASSERT_TRUE(storage->store()->resolveRef(ns, src_ref).has_value())
+        << "src must still be committed (the simulated crash happened before dropRef)";
+    const auto resolved_dst_before = storage->store()->resolveRef(ns, dst_ref);
+    ASSERT_TRUE(resolved_dst_before.has_value());
+    const ManifestId dst_id_before = resolved_dst_before->manifest_id;
+
+    /// 3. RE-DRIVE the same rename through the real transaction path: both endpoints are already
+    ///    committed-ref part paths (not a table-level rename, no staged source in this fresh
+    ///    transaction) -- moveDirectory's "move any COMMITTED source ref" branch calls
+    ///    republishRef(src, dst) for real (the only way to reach the private method).
+    {
+        auto tx = storage->createTransaction();
+        tx->moveDirectory(src_path, dst_path);
+        tx->commit(DB::NoCommitOptions{});
+    }
+
+    /// 4. Idempotency: src dropped, dst unchanged (SAME ManifestId -- no second manifest minted for
+    ///    identical content, so nothing orphaned).
+    EXPECT_FALSE(storage->store()->resolveRef(ns, src_ref).has_value())
+        << "src ref must be dropped by the re-drive";
+    const auto resolved_dst_after = storage->store()->resolveRef(ns, dst_ref);
+    ASSERT_TRUE(resolved_dst_after.has_value());
+    EXPECT_EQ(resolved_dst_after->manifest_id, dst_id_before)
+        << "PRE-FIX: republishRef re-drive mints a FRESH manifest for identical content, orphaning the "
+           "first attempt's manifest (BUG 1c leak). POST-FIX: idempotent no-op, same manifest.";
+    EXPECT_EQ(storage->getFileSize(dst_path + "/data.bin"), 9u);
+}
+
+/// REMOVED (all-tree-part-files Task 9):
+/// `RepublishReDriveResyncsDriftedMutableFiles` proved that `republishRef`'s idempotent-skip path
+/// re-synced dst's `mutable_files` from src's CURRENT resolve when src's mutable payload drifted
+/// between the crashed attempt and the re-drive. That side channel is gone -- `metadata_version.txt`
+/// etc. are ordinary manifest entries now, so a src drift of that kind changes `entries`, and
+/// `republishRef`'s idempotency check (`dst_manifest->entries != src_manifest->entries`) now correctly
+/// treats it as a genuine content conflict (ABORTED) rather than silently resyncing a side payload --
+/// there is no longer a "same content, drifted sidecar" state to re-sync. `RepublishReDriveOver-
+/// CommittedDstIsIdempotent` above remains the live coverage for the idempotent-skip path itself.
+
+/// Companion conflict case: a re-drive where dst is committed to DIFFERENT content than src is a
+/// genuine conflict (an ATTACH-onto-existing-name collision), not a re-drive -- it must fail closed
+/// (ABORTED), never silently drop src (which would lose src's content) nor silently overwrite dst.
+/// This scenario reaches the SAME `promote`-over-different-committed-ref guard as BUG 1a, so pre-fix it
+/// behaves the same way BUG 1a does: no throw (silent overwrite), which is also a leak/data-loss risk.
+TEST(CASPromoteRepublish, RepublishReDriveOverDifferentContentDstFailsClosed)
+{
+    auto storage = openTxStorage();
+    const auto ns = storage->liveNamespace("b0ab0ab0-0a0a-4a0a-8a0a-0a0a0a0a0a0a");
+    const String src_ref = "all_2_2_0";
+    const String dst_ref = "detached_all_2_2_0";
+    const String src_path = "b0a/b0ab0ab0-0a0a-4a0a-8a0a-0a0a0a0a0a0a/" + src_ref;
+    const String dst_path = "b0a/b0ab0ab0-0a0a-4a0a-8a0a-0a0a0a0a0a0a/" + dst_ref;
+
+    /// src committed with content "payload-SRC".
+    {
+        auto tx = storage->createTransaction();
+        writeFileTx(*tx, "b0a/b0ab0ab0-0a0a-4a0a-8a0a-0a0a0a0a0a0a/tmp_insert_" + src_ref + "/data.bin", "payload-SRC");
+        tx->moveDirectory("b0a/b0ab0ab0-0a0a-4a0a-8a0a-0a0a0a0a0a0a/tmp_insert_" + src_ref, src_path);
+        tx->commit(DB::NoCommitOptions{});
+    }
+    /// dst ALREADY committed with genuinely DIFFERENT content (not a re-drive artifact -- a real
+    /// name collision), via a completely independent build.
+    publishCommitted(storage->store(), ns, dst_ref, inlineEntries("data.bin", "different-content"));
+    ASSERT_TRUE(storage->store()->resolveRef(ns, src_ref).has_value());
+    ASSERT_TRUE(storage->store()->resolveRef(ns, dst_ref).has_value());
+
+    try
+    {
+        auto tx = storage->createTransaction();
+        tx->moveDirectory(src_path, dst_path);
+        tx->commit(DB::NoCommitOptions{});
+        FAIL() << "PRE-FIX: republishRef silently overwrote dst's different content "
+                  "(promote-over-committed leak); POST-FIX must throw ABORTED and leave src intact";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::ABORTED);
+    }
+}
+
+/// BUG 2: `abandon` must emit its precommit removal (an exact `owner_transition`) BEFORE retiring the
+/// build_seq, so the build stays active until that removal is durable and no freshness-window consumer
+/// judges the manifest build-dead while an un-removed precommit still names it (GC no longer reclaims
+/// abandoned precommits — the writer removes them itself).
+///
+/// A3 mint-tightening INVERTS this
+/// test's original tail assertion. Before A3, this test's black-box PROOF that `abandon()` had really
+/// removed the exact precommit binding was that a FRESH `precommitAdd` for the SAME (ref_name,
+/// manifest_ref) succeeded -- a still-live binding would instead throw CORRUPTED_DATA ("add precommit
+/// ... already exists"). That proof mechanism no longer works: `rebuild` is a DIFFERENT `PartWriteTxn`
+/// from `build` and never staged `id` itself (`build` did), so `precommitAdd` now refuses it
+/// UNCONDITIONALLY under A3 -- regardless of whether abandon's removal ever landed. Re-owning a
+/// dropped identity from a transaction that did not mint it would let a later relink confirm's exact
+/// `ManifestRef` equality (Part B of the same design) compare true against a token whose blobs may
+/// already be reclaimed -- an ABA the whole publish-confirm design depends on being structurally
+/// impossible. The removal-before-retire property this test used to prove is unaffected by A3 and
+/// stays covered by the TLA+ `WAbandonPrecommit` model; `PrecommitAddRejectsAnIdThisTxnDidNotStage`
+/// below is the dedicated A3 regression pin.
+///
+/// `rebuild->precommitAdd(ns, ref, id)` below throws `LOGICAL_ERROR`, which aborts the whole process in
+/// debug/sanitizer builds instead of behaving like a catchable exception -- `CASPromoteRepublishDeathTest.
+/// AbandonEmitsRemovalBeforeRetireAborts` below proves the abort positively in those builds instead.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASPromoteRepublish, AbandonEmitsRemovalBeforeRetire)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl@cas@"};
+    const String ref = "all_0_0_0";
+    auto build = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    const ManifestId id = build->stageManifest(inlineEntries("f", "AAA"));
+    build->precommitAdd(ns, ref, id);
+    build->abandon();
+
+    /// `rebuild` never staged `id` -- A3 refuses it on that basis alone, before ever reaching the
+    /// ledger-state check that would otherwise distinguish "removed" from "still live".
+    auto rebuild = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    try
+    {
+        rebuild->precommitAdd(ns, ref, id);
+        FAIL() << "A3 mint-tightening: precommitAdd must refuse an id 'rebuild' never staged, even one "
+                  "'build' legitimately staged and precommitted before dropping it via abandon()";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::LOGICAL_ERROR);
+    }
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASPromoteRepublishDeathTest, AbandonEmitsRemovalBeforeRetireAborts)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl@cas@"};
+    const String ref = "all_0_0_0";
+    auto build = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    const ManifestId id = build->stageManifest(inlineEntries("f", "AAA"));
+    build->precommitAdd(ns, ref, id);
+    build->abandon();
+
+    auto rebuild = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    EXPECT_DEATH({ rebuild->precommitAdd(ns, ref, id); }, "");
+}
+#endif
+
+/// A3 mint-tightening's dedicated regression pin: an unowned `ManifestId` may enter ownership ONLY
+/// from the transaction that freshly staged it. Without this, a dropped identity could be re-owned
+/// later, which would make the relink confirm's exact-`ManifestRef` equality an ABA (the same token
+/// could then name a manifest whose blobs were already reclaimed). No production path performs this
+/// transition -- every real caller precommits an id it JUST staged itself, on the SAME `PartWriteTxn`
+/// (`ContentAddressedTransaction.cpp:358,412`, `PartFolderAccess.cpp:352`).
+///
+/// `txn2->precommitAdd(ns, ref, id)` below throws `LOGICAL_ERROR`, which aborts the whole process in
+/// debug/sanitizer builds instead of behaving like a catchable exception -- `CASPromoteRepublishDeathTest.
+/// PrecommitAddRejectsAnIdThisTxnDidNotStageAborts` below proves the abort positively in those builds
+/// instead (it cannot also re-check the post-throw ref-log-tail/resolveRef state this test verifies,
+/// since there IS no post-abort state in a real debug/sanitizer build).
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASPromoteRepublish, PrecommitAddRejectsAnIdThisTxnDidNotStage)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl_mint_tighten@cas@"};
+    const String ref = "all_0_0_0";
+
+    /// txn1 mints `id` and abandons before ever precommitting it -- a genuinely unowned identity (it
+    /// was never even a live precommit), the simplest form A3 must still refuse for a foreign txn.
+    auto txn1 = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    const ManifestId id = txn1->stageManifest(inlineEntries("f", "AAA"));
+    txn1->abandon();
+
+    const size_t tail_before = s->tailSinceSnapshotCountForTest(ns);
+
+    /// txn2 never staged `id` -- only the transaction that minted an id may precommit it.
+    auto txn2 = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    try
+    {
+        txn2->precommitAdd(ns, ref, id);
+        FAIL() << "A3 mint-tightening: precommitAdd must refuse an id staged by a DIFFERENT transaction";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::LOGICAL_ERROR);
+    }
+
+    /// Nothing was appended: the ref-log tail is unchanged and `ref` still has no owner at all.
+    EXPECT_EQ(s->tailSinceSnapshotCountForTest(ns), tail_before);
+    EXPECT_FALSE(s->resolveRef(ns, ref).has_value());
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASPromoteRepublishDeathTest, PrecommitAddRejectsAnIdThisTxnDidNotStageAborts)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv/tbl_mint_tighten@cas@"};
+    const String ref = "all_0_0_0";
+
+    auto txn1 = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    const ManifestId id = txn1->stageManifest(inlineEntries("f", "AAA"));
+    txn1->abandon();
+
+    auto txn2 = s->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref, .intended_namespace = ns});
+    EXPECT_DEATH({ txn2->precommitAdd(ns, ref, id); }, "");
+}
+#endif
diff --git a/src/Disks/tests/gtest_cas_protocol_scenarios.cpp b/src/Disks/tests/gtest_cas_protocol_scenarios.cpp
new file mode 100644
index 000000000000..574f85d743eb
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_protocol_scenarios.cpp
@@ -0,0 +1,622 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+/// Multi-actor protocol scenarios for the root-local part-manifest model (CA GC redesign rev. 15).
+/// Ported from the removed tree/closure model. The single-call `publish(ns, ref, tree, RefPayload{})`
+/// gate is gone; a write is now the four-step flow (EDGE-BEFORE-OBSERVE order):
+///   stageManifest(entries) -> precommitAdd(ns, ref, id) -> putBlob(...) -> promote(ns, ref, build_id, id)
+/// The fail-closed publish gate that those scenarios exercise now lives in TWO places (Phase A of spec
+/// 2026-07-09-cas-writer-gc-simplification):
+///   • putBlob: INV-1 condemned-dedup re-upload from the writer's OWN source bytes (never GETs the
+///     dying object);
+///   • promote: TOKENED leaves (this build putBlob'd them) are EDGE-PROTECTED and NOT re-validated — the
+///     precommit closure named them before putBlob observed them, so a condemnation in the
+///     putBlob→promote window is doomed (the next fold spares it). promote commits with the tokened
+///     blob's token UNCHANGED. Only NON-tokened leaves get the single mandatory presence observation: a
+///     tokenless W-EVIDENCE adopt that is condemned-but-present is displaced by a verified copy-forward
+///     (the committed ref names a FRESH incarnation); absent, or condemned + no-dep, fails closed
+///     (ABORTED). promote refreshes the retire view when the fence is ahead.
+/// These scenarios assert the no-dangle / no-loss / fail-closed protocol properties faithfully on that
+/// flow. The strong safety assertions are preserved.
+///
+/// DELETED (Phase A): `RevalidateAbsentTokenedBlobResurrectsFromSource`. Its premise — a putBlob'd
+/// (tokened) blob body hand-deleted before the gate, then resurrected — is protocol-unreachable under
+/// EDGE-BEFORE-OBSERVE: a tokened leaf under a durable precommit closure cannot be GC-deleted in the
+/// putBlob→promote window, and promote no longer re-validates tokened leaves at all. Deleting a
+/// putBlob'd body out-of-band is corruption, which is `cas-fsck`'s domain, not the promote gate's.
+
+namespace DB::ErrorCodes
+{
+extern const int ABORTED;
+extern const int FILE_DOESNT_EXIST;
+extern const int LOGICAL_ERROR;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::blobEntryFor;
+using DB::Cas::tests::condemnMeta;
+using DB::Cas::tests::displaceBlobToken;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::injectRetire;
+using DB::Cas::tests::loadMetaForTest;
+using DB::Cas::tests::streamingHexOf;
+using DB::Cas::tests::u128Of;
+using DB::Cas::tests::writeBlobRaw;
+
+namespace
+{
+
+PoolPtr openPool(const std::shared_ptr<InMemoryBackend> & b)
+{
+    return Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// A single-blob manifest entry naming `payload` at `path` (the entry the part's manifest carries).
+ManifestEntry blobEntry(const String & path, const String & payload)
+{
+    return blobEntryFor(path, u128Of(payload), payload.size());
+}
+
+/// Start a build whose `intended_ref` is "ns/ref" — REQUIRED: stageManifest derives the manifest's
+/// owning namespace by splitting intended_ref on the LAST '/'. (See PartWriteTxn::manifestNamespace.)
+PartWriteTxnPtr startBuildFor(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    return s->beginPartWrite(info);
+}
+
+/// The full write flow for a part whose only file is `payload` at `path` (blob placement). Uploads the
+/// blob via putBlob, stages the manifest, precommits, then promotes. Returns the committed ManifestId.
+/// Mirrors what the old single-call `publish` did on the tree model.
+ManifestId publishBlobPart(
+    const PoolPtr & s, const RootNamespace & ns, const String & ref, const String & path, const String & payload)
+{
+    auto build = startBuildFor(s, ns, ref);
+    /// Wiring order (EDGE-BEFORE-OBSERVE): stageManifest -> precommitAdd -> putBlob -> promote.
+    const ManifestId id = build->stageManifest({blobEntry(path, payload)});
+    build->precommitAdd(ns, ref, id);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+/// Read the part's blob back through the full read stack (resolveRef → readManifest → findEntry →
+/// locate → ranged GET) and assert it returns `payload`. This is the INV-NO-DANGLE check: every named
+/// object resolves and reads.
+void assertPartReads(
+    const std::shared_ptr<InMemoryBackend> & b, const PoolPtr & s,
+    const RootNamespace & ns, const String & ref, const String & path, const String & payload)
+{
+    auto r = s->resolveRef(ns, ref);
+    ASSERT_TRUE(r.has_value());
+
+    const PartManifest manifest = s->readManifest(r->manifest_id);
+    const auto * entry = findEntry(manifest.entries, path);
+    ASSERT_TRUE(entry != nullptr);
+    auto loc = s->locate(*entry);
+    auto got = b->get(loc.key, Range{loc.offset, loc.length});
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, payload);
+}
+
+}
+
+TEST(CASProtocol, FenceConflictCondemnedTokenedBlobCommitsWithTokenUnchanged)
+{
+    /// EDGE-BEFORE-OBSERVE (spec 2026-07-09-cas-writer-gc-simplification, Phase A): a blob leaf whose
+    /// CURRENT token is condemned at the promote gate, but which THIS build putBlob'd (tokened dep under
+    /// the durable precommit closure), is EDGE-PROTECTED — the condemnation is doomed (the next fold spares
+    /// it) and promote does NOT re-validate or re-upload the tokened leaf. promote COMMITS with the blob's
+    /// token UNCHANGED; the premature condemn is invisible to the client.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// Wiring order: stage + precommit (durable edge) BEFORE putBlob observes X (records token t0).
+    auto build = startBuildFor(s, ns, "part_1");
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", "payload-X")});
+    build->precommitAdd(ns, "part_1", id);
+    build->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X"));
+
+    const String blob_key = s->layout().blobKey(idOf("payload-X"));
+    const Token t0 = b->head(blob_key).token;
+
+    /// GC condemns X at t0 in round 1 and fences the namespace to round 1.
+    injectRetire(*b, s->layout(), /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("payload-X"))}, .token = t0, .size = 9}});
+
+    /// promote: mutateShard refreshes the view (fence_round 1 > view round 0), but the tokened leaf is
+    /// edge-protected — skipped, not re-validated ⇒ commit, token unchanged.
+    build->promote(ns, "part_1", build->buildId(), id);
+
+    /// The ref is committed and reads back; the blob still rides t0 (no re-upload).
+    assertPartReads(b, s, ns, "part_1", "data.bin", "payload-X");
+    EXPECT_EQ(b->head(blob_key).token, t0);
+}
+
+TEST(CASProtocol, RevalidateReObservesStaleTokenKeepsWhenUnchanged)
+{
+    /// A blob dedup-adopted (tokened dep) under the precommit closure; an EMPTY retire set at round 1.
+    /// Under EDGE-BEFORE-OBSERVE the tokened leaf is NOT re-observed at the promote gate at all — it is
+    /// edge-protected — so promote commits in place with the token UNCHANGED (no HEAD, no rewrite).
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// X pre-exists out-of-band; the build dedup-adopts it via putBlob (records the current token t0).
+    writeBlobRaw(*b, s->layout(), "payload-X", s->poolMeta().blob_header_len, s->poolMeta().pool_id);
+    const String blob_key = s->layout().blobKey(idOf("payload-X"));
+    const Token t0 = b->head(blob_key).token;
+
+    /// Wiring order: stage + precommit (durable edge) BEFORE the adopting putBlob.
+    auto build = startBuildFor(s, ns, "part_1");
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", "payload-X")});
+    build->precommitAdd(ns, "part_1", id);
+    build->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X"));   /// dedup → adopts t0
+
+    /// GC advanced the round to 1 with an EMPTY retired set; fence to 1. X is NOT condemned and its
+    /// token is unchanged.
+    injectRetire(*b, s->layout(), /*round*/ 1, /*shard*/ 0, {});
+
+    /// promote: the tokened leaf is edge-protected (not re-observed) ⇒ commit in place (KEEP).
+    build->promote(ns, "part_1", build->buildId(), id);
+
+    assertPartReads(b, s, ns, "part_1", "data.bin", "payload-X");
+    /// No rewrite happened — the tokened leaf was never touched, so its token stays at t0.
+    EXPECT_EQ(b->head(blob_key).token, t0);
+}
+
+TEST(CASProtocol, RevalidateReObservesStaleTokenAdoptsWhenDisplaced)
+{
+    /// A blob displaced out-of-band to a fresh live token t1 before promote. Phase-A contract: the leaf is
+    /// TOKENED (putBlob-adopted), so promote SKIPS it entirely (edge-protected — EDGE-BEFORE-OBSERVE); no
+    /// re-HEAD happens. The commit still rides the displaced object correctly because the manifest names
+    /// the HASH, not a token — this is the black-box "displaced object still reads by content key" check.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    writeBlobRaw(*b, s->layout(), "payload-X", s->poolMeta().blob_header_len, s->poolMeta().pool_id);
+    const String blob_key = s->layout().blobKey(idOf("payload-X"));
+    const Token t0 = b->head(blob_key).token;
+
+    auto build = startBuildFor(s, ns, "part_1");
+    /// Wiring order (EDGE-BEFORE-OBSERVE): stageManifest -> precommitAdd -> putBlob.
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", "payload-X")});
+    build->precommitAdd(ns, "part_1", id);
+    build->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X"));   /// dedup → adopts t0
+
+    /// Another writer displaces X out-of-band ⇒ a new current token t1 (same payload, fresh tag).
+    const Token t1 = displaceBlobToken(*b, s->layout(), idOf("payload-X"));
+    EXPECT_NE(t1, t0);
+
+    /// GC advanced to round 1 with an EMPTY retired set; fence to 1.
+    injectRetire(*b, s->layout(), /*round*/ 1, /*shard*/ 0, {});
+
+    /// promote refreshes ⇒ revalidate X ⇒ HEAD current t1 not condemned ⇒ commit. The dep rides t1.
+    build->promote(ns, "part_1", build->buildId(), id);
+    assertPartReads(b, s, ns, "part_1", "data.bin", "payload-X");
+    EXPECT_EQ(b->head(blob_key).token, t1);
+
+    /// Black-box proof the part reads the t1 incarnation: re-publish the same blob into a SECOND
+    /// namespace with NO new GC injection. The blob is already present at t1; nothing is re-uploaded.
+    publishBlobPart(s, RootNamespace{"srv1/tbl/copy"}, "part_2", "data.bin", "payload-X");
+    EXPECT_EQ(b->head(blob_key).token, t1);
+    assertPartReads(b, s, RootNamespace{"srv1/tbl/copy"}, "part_2", "data.bin", "payload-X");
+
+    /// Independent discriminator that the blob rides t1, not the stale t0: t0 is DEAD. A deleteExact
+    /// against t0 must TokenMismatch (INV-NO-RETURN — t0 was displaced and can never be current again).
+    EXPECT_EQ(b->deleteExact(blob_key, t0).kind, DeleteOutcome::Kind::TokenMismatch);
+}
+
+TEST(CASProtocol, RevalidateAdoptsLiveTokenWhenOnlyPhantomCondemnedAtDifferentToken)
+{
+    /// A blob whose OWN current token t0 is LIVE, but a DIFFERENT phantom token t_other for the same
+    /// hash IS condemned. The build putBlob-adopts t0 (tokened dep), so promote does not re-observe it
+    /// (edge-protected) and commits in place: the blob keeps t0 (no upload, no displacement). The phantom
+    /// condemnation is for a different incarnation and never touches t0.
+    auto b = std::make_shared<InMemoryBackend>();
+    const RootNamespace ns{"srv1/tbl"};
+
+    DB::Cas::Layout layout("p");
+    {
+        auto s0 = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+        writeBlobRaw(*b, s0->layout(), "payload-X", s0->poolMeta().blob_header_len, s0->poolMeta().pool_id);
+    }
+    const String blob_key = layout.blobKey(idOf("payload-X"));
+    const Token t0 = b->head(blob_key).token;
+    const Token t_other{"emulated-phantom", DB::Cas::TokenType::Emulated};
+    ASSERT_NE(t_other, t0);
+
+    injectRetire(*b, layout, /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("payload-X"))}, .token = t_other, .size = 9}});
+    /// Fence to round 1 BEFORE opening the store, so the store's open-time refresh lands the view at
+    /// round 1 already populated.
+
+    auto s = Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});   /// open-time refresh ⇒ view round 1
+    /// Wiring order: stage + precommit (durable edge) BEFORE the adopting putBlob.
+    auto build = startBuildFor(s, ns, "part_1");
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", "payload-X")});
+    build->precommitAdd(ns, "part_1", id);
+    build->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X"));   /// dedup → adopts t0
+
+    /// promote: the tokened leaf is edge-protected (not re-observed) ⇒ commit. Lands. t0 untouched.
+    build->promote(ns, "part_1", build->buildId(), id);
+
+    assertPartReads(b, s, ns, "part_1", "data.bin", "payload-X");
+
+    /// The object was NOT displaced — it STAYS at t0 (no re-upload, only re-validated).
+    EXPECT_EQ(b->head(blob_key).token, t0);
+}
+
+/// (DELETED, Phase A) RevalidateAbsentTokenedBlobResurrectsFromSource — see the file-header note: a
+/// hand-deleted putBlob'd (tokened) body is protocol-unreachable under EDGE-BEFORE-OBSERVE (a tokened
+/// leaf under a durable precommit closure cannot be GC-deleted in the putBlob→promote window, and promote
+/// no longer re-validates tokened leaves). Out-of-band body deletion is `cas-fsck`'s domain.
+
+TEST(CASProtocol, EvidenceHitCondemnedPresentBlobCopiesForwardInClosure)
+{
+    /// W-EVIDENCE (tokenless adopted dep) on a blob X whose hash is condemned-but-PRESENT. §4 manifest-trust
+    /// (test name is legacy — there is no copy-forward any more): a committed-source adopted leaf is TRUSTED
+    /// at the promote gate. The gate does NOT observe X — no HEAD, no meta point-read, no displacement — it
+    /// publishes on the strength of the durable manifest edge (D4 relink trust). So promote SUCCEEDS and X's
+    /// existing incarnation is left EXACTLY as-is: the token is UNCHANGED (never displaced) and the condemned
+    /// meta is NOT flipped (the gate never reads or writes it). A non-tokened leaf is the only leaf promote
+    /// still decides on; here it is trusted (tokened leaves are edge-protected).
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// X pre-exists with token t0; the manifest names it as a tokenless adopted leaf.
+    const String hex = streamingHexOf("payload-X");
+    {
+        auto seed = s->beginPartWrite({});
+        seed->putBlob(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128(hex))}, BlobSource::fromString("payload-X"));
+    }
+    const String blob_key = s->layout().blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128(hex))});
+    const Token t0 = b->head(blob_key).token;
+
+    auto build = startBuildFor(s, ns, "part_1");
+    ManifestEntry entry = blobEntry("data.bin", "payload-X");
+    entry.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hexToU128(hex))};   /// streaming-convention id (matches the minted blob)
+    build->adoptEvidence(entry);   /// tokenless W-EVIDENCE dep on X (no HEAD, no upload)
+    const ManifestId id = build->stageManifest({entry});
+    build->precommitAdd(ns, "part_1", id);
+
+    /// GC condemns X's hash in round 1 via the meta — under §4 the promote gate never reads it.
+    condemnMeta(*b, s->layout(), hexToU128(hex), /*condemn_round*/ 1);
+
+    /// promote: the adopted leaf is trusted ⇒ commit, no probe, no displacement.
+    EXPECT_NO_THROW(build->promote(ns, "part_1", build->buildId(), id));
+
+    /// The ref stands; X rides its ORIGINAL token t0 (trust never displaces a trusted leaf).
+    EXPECT_TRUE(s->resolveRef(ns, "part_1").has_value());
+    EXPECT_EQ(b->head(blob_key).token, t0) << "trust must not displace the adopted blob";
+
+    /// The meta is untouched — still Condemned (the gate never reads or flips it under trust).
+    const auto lm_after = loadMetaForTest(*b, s->layout(), hexToU128(hex));
+    ASSERT_TRUE(lm_after.has_value());
+    EXPECT_EQ(lm_after->meta.state, MetaState::Condemned) << "trust must not flip the meta";
+}
+
+TEST(CASProtocol, WedgedHeartbeatCondemnedTokenedBlobCommitsWithTokenUnchanged)
+{
+    /// A build whose watermark never renews finds its OWN putBlob'd upload condemned by full GC while its
+    /// precommit is STILL the live owner (this setup injects only the retire set + fence, no owner-removal
+    /// — the false-positive-freeze window BEFORE any GC reclaim). The tokened leaf is EDGE-PROTECTED: the
+    /// precommit closure named it before putBlob observed it, so the condemnation is doomed and promote
+    /// does NOT re-validate it — promote COMMITS with the token UNCHANGED, closing the window invisibly.
+    /// The genuine dead-build case (precommit reclaimed ⇒ owner check aborts, NO re-upload) is covered
+    /// separately by CaWiringResurrect.PromoteAbandonedPrecommitAbortsWithoutResurrect.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// Wiring order: stage + precommit (durable edge) BEFORE putBlob observes X.
+    auto build = startBuildFor(s, ns, "part_1");
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", "payload-X")});
+    build->precommitAdd(ns, "part_1", id);
+    build->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X"));
+
+    const String blob_key = s->layout().blobKey(idOf("payload-X"));
+    const Token t0 = b->head(blob_key).token;
+
+    /// Full GC condemned the build's OWN upload.
+    injectRetire(*b, s->layout(), /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("payload-X"))}, .token = t0, .size = 9}});
+
+    /// promote: the tokened leaf is edge-protected — skipped, not re-validated ⇒ commit, token unchanged.
+    build->promote(ns, "part_1", build->buildId(), id);
+    assertPartReads(b, s, ns, "part_1", "data.bin", "payload-X");
+    EXPECT_EQ(b->head(blob_key).token, t0);
+}
+
+TEST(CASProtocol, AbandonLeavesDebrisAndDisables)
+{
+    /// abandon leaves the uploaded blob + staged manifest body as debris (reaped by the orphan sweep);
+    /// no owner transition is touched, and further build ops fail LOGICAL_ERROR (requireAlive).
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    auto build = startBuildFor(s, ns, "part_1");
+    auto blob = build->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X"));
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", "payload-X")});
+
+    build->abandon();
+
+    /// The uploaded blob remains as debris; the staged manifest body is best-effort deleted by abandon.
+    EXPECT_TRUE(b->head(s->layout().blobKey(blob.ref)).exists);
+    EXPECT_FALSE(b->head(s->layout().manifestKey(id)).exists);   /// best-effort cleanup ran
+    EXPECT_TRUE(s->listRefs(ns).empty());
+
+    /// Further build ops ⇒ LOGICAL_ERROR (requireAlive).
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            build->stageManifest({blobEntry("data.bin", "payload-X")});
+        },
+        "PartWriteTxn has been abandoned");
+}
+
+TEST(CASProtocol, DropReattachThroughDetachedNamespace)
+{
+    /// ATTACH choreography (design §4): publish part_1 in ns; re-publish into ns/detached + drop part_1
+    /// from ns; then re-publish part_1 back in ns + drop from detached. The BLOB is never re-uploaded
+    /// (its token is stable throughout); each namespace gets its own single-owner manifest.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    const RootNamespace detached{"srv1/tbl/detached"};
+
+    publishBlobPart(s, ns, "part_1", "data.bin", "payload-X");
+
+    const String blob_key = s->layout().blobKey(idOf("payload-X"));
+    const Token blob_tok = b->head(blob_key).token;
+
+    EXPECT_TRUE(s->listRefs(ns).contains("part_1"));
+    EXPECT_TRUE(s->listRefs(detached).empty());
+
+    /// Move to detached: re-publish into detached (adopting the live blob), drop from ns.
+    publishBlobPart(s, detached, "part_1", "data.bin", "payload-X");
+    s->dropRef(ns, "part_1");
+
+    EXPECT_TRUE(s->listRefs(ns).empty());
+    ASSERT_TRUE(s->listRefs(detached).contains("part_1"));
+    assertPartReads(b, s, detached, "part_1", "data.bin", "payload-X");
+
+    /// Re-attach: re-publish part_1 back in ns, drop from detached.
+    publishBlobPart(s, ns, "part_1", "data.bin", "payload-X");
+    s->dropRef(detached, "part_1");
+
+    ASSERT_TRUE(s->listRefs(ns).contains("part_1"));
+    assertPartReads(b, s, ns, "part_1", "data.bin", "payload-X");
+    EXPECT_TRUE(s->listRefs(detached).empty());
+
+    /// The blob was never re-uploaded (token stable throughout — every publish dedup-adopted it).
+    EXPECT_EQ(b->head(blob_key).token, blob_tok);
+}
+
+TEST(CASProtocol, FreezeIntoShadowNamespace)
+{
+    /// FREEZE survives the table's part lifecycle (design §4): a shadow ref is a reachability root that
+    /// outlives the dropped live ref.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+    const RootNamespace shadow{"shadow/backup1/tbl"};
+
+    publishBlobPart(s, ns, "part_1", "data.bin", "payload-X");
+
+    /// Freeze into the shadow namespace (adopting the live blob), then drop the live ref.
+    publishBlobPart(s, shadow, "part_1", "data.bin", "payload-X");
+    s->dropRef(ns, "part_1");
+
+    EXPECT_TRUE(s->listRefs(ns).empty());
+    /// The shadow ref still resolves and reads after the live ref is gone.
+    assertPartReads(b, s, shadow, "part_1", "data.bin", "payload-X");
+}
+
+TEST(CASProtocol, DisplacedToLiveTokenCommitsAtCurrentIncarnation)
+{
+    /// (Ported from the former ResurrectLosesRace scenario.) A blob displaced to a LIVE t1 (while its old
+    /// t0 is condemned for a now-defunct incarnation) is SAFE to commit: the committed manifest names a
+    /// blob HASH, the live t1 incarnation backs it, and GC's exact-token delete of t0 only TokenMismatches.
+    /// Phase-A contract: the leaf is TOKENED, so promote does not re-HEAD it at all (edge-protected —
+    /// EDGE-BEFORE-OBSERVE); the commit is correct by content addressing, not by revalidation. The old
+    /// conservative ABORTED has no manifest-model analog.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    writeBlobRaw(*b, s->layout(), "payload-X", s->poolMeta().blob_header_len, s->poolMeta().pool_id);
+    const String blob_key = s->layout().blobKey(idOf("payload-X"));
+    const Token t0 = b->head(blob_key).token;
+
+    auto build = startBuildFor(s, ns, "part_1");
+    /// Wiring order (EDGE-BEFORE-OBSERVE): stageManifest -> precommitAdd -> putBlob.
+    const ManifestId id = build->stageManifest({blobEntry("data.bin", "payload-X")});
+    build->precommitAdd(ns, "part_1", id);
+    build->putBlob(idOf("payload-X"), BlobSource::fromString("payload-X"));   /// dedup → adopts t0
+
+    /// Another writer displaces X to t1 (uncondemned) before our gate runs.
+    const Token t1 = displaceBlobToken(*b, s->layout(), idOf("payload-X"));
+    ASSERT_NE(t1, t0);
+
+    /// The view still condemns the OLD t0 at round 1, fenced.
+    injectRetire(*b, s->layout(), /*round*/ 1, /*shard*/ 0,
+        {RetiredEntry{.kind = ObjectKind::Blob, .ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of("payload-X"))}, .token = t0, .size = 9}});
+
+    /// promote: revalidate X ⇒ HEAD current t1 (NOT condemned; only the defunct t0 is) ⇒ commit.
+    build->promote(ns, "part_1", build->buildId(), id);
+
+    /// The blob lives at t1 (the displacing writer's incarnation) and the part reads.
+    EXPECT_EQ(b->head(blob_key).token, t1);
+    assertPartReads(b, s, ns, "part_1", "data.bin", "payload-X");
+
+    /// NO-LOSS / NO-RETURN: t0 is dead — a deleteExact against it TokenMismatches (the GC delete of the
+    /// condemned t0 spares the live t1).
+    EXPECT_EQ(b->deleteExact(blob_key, t0).kind, DeleteOutcome::Kind::TokenMismatch);
+}
+
+TEST(CASProtocol, NewNamespacePublishGatedByShardFenceFloor)
+{
+    /// Regression test (test name is legacy — the fence machinery is gone): build B adopts a blob, the
+    /// ack-floor GC pipeline retires + deletes it, then B publishes into a fresh namespace. §4 manifest-
+    /// trust: B's leaf is a committed-source adopted leaf, so promote TRUSTS it (no HEAD/loadMeta probe) and
+    /// COMMITS. On the real path this dangle is UNREACHABLE — B's precommit edge pins the blob at in-degree
+    /// >= 1 through promote (CasPartWriteTxn.cpp precommitAdd → promote's WPromote owner==bld re-proof precedes the
+    /// trust), so GC cannot delete it; here the test drives GC to delete the blob while B has NOT yet
+    /// precommitted, which the live-precommit invariant excludes. The dangle is DETECTED by fsck's
+    /// reachable-but-absent scan (the backstop), not prevented at promote.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+
+    /// 1. part_1 → a blob in namespace A, through the real PartWriteTxn.
+    const RootNamespace ns_a{"srv1/tbl"};
+    auto build_a = startBuildFor(s, ns_a, "part_1");
+    build_a->putBlob(idOf("floor-payload"), BlobSource::fromString("floor-payload"));
+    const ManifestId id_a = build_a->stageManifest({blobEntry("data.bin", "floor-payload")});
+    build_a->precommitAdd(ns_a, "part_1", id_a);
+    build_a->promote(ns_a, "part_1", build_a->buildId(), id_a);
+    const String blob_key = s->layout().blobKey(idOf("floor-payload"));
+
+    /// 2. build B adopts the blob (tokenless W-EVIDENCE) while the view is still at round 0.
+    auto build_b = startBuildFor(s, RootNamespace{"srv2/new"}, "part_x");
+    build_b->adoptEvidence(blobEntry("data.bin", "floor-payload"));
+
+    /// 3. drop part_1 from A; the ack-floor GC pipeline retires the blob at t0 and deletes it. build_a
+    /// finished, so advancing the watermark floor condemns the blob. Drive rounds advancing the store's
+    /// own mount ack after each (so the floor graduates the condemned entry and the delete lands).
+    s->dropRef(ns_a, "part_1");
+    build_a.reset();
+    s->renewWatermarkOnce();
+    Gc gc(s, hexToU128("00000000000000000000000000000001"));
+    for (size_t r = 0; r < 16; ++r)
+    {
+        const RoundReport rep = DB::Cas::tests::runRegularRoundReclaiming(gc);
+        s->renewWatermarkOnce();
+        if (!b->head(blob_key).exists)
+            break;
+    }
+    /// The blob (unreachable) was deleted at t0.
+    EXPECT_FALSE(b->head(blob_key).exists);
+
+    /// 4. build B publishes into a BRAND-NEW namespace. §4 manifest-trust: the adopted leaf is trusted at
+    /// promote (no probe) ⇒ promote SUCCEEDS and commits a manifest naming the deleted blob (the dangle).
+    const ManifestId id_b = build_b->stageManifest({blobEntry("data.bin", "floor-payload")});
+    build_b->precommitAdd(RootNamespace{"srv2/new"}, "part_x", id_b);
+    EXPECT_NO_THROW(build_b->promote(RootNamespace{"srv2/new"}, "part_x", build_b->buildId(), id_b));
+
+    /// The ref committed over the deleted blob (the D4 trade-off); the backstop is fsck's reachable-but-
+    /// absent scan (INV-NO-DANGLE-via-fsck).
+    EXPECT_TRUE(s->resolveRef(RootNamespace{"srv2/new"}, "part_x").has_value());
+    const FsckReport rep = runFsck(*s, /*detail=*/true);
+    EXPECT_GE(rep.dangling, 1u) << "§4 D4 backstop: part_x committed over the GC-deleted blob; fsck must "
+                                   "report it dangling (dangling=" << rep.dangling << ")";
+}
+
+TEST(CASProtocol, FreshEvidenceDepWithViewHitIsResolvedByGate)
+{
+    /// §4 manifest-trust (test name is legacy — the gate no longer "resolves" a tokenless leaf by observing
+    /// it): a committed-source adopted leaf whose blob is condemned-but-PRESENT is TRUSTED at the promote
+    /// gate. There is NO per-file probe (no HEAD, no meta point-read) and NO copy-forward — the durable
+    /// manifest edge is the liveness evidence (D4 relink trust). promote SUCCEEDS and X keeps its ORIGINAL
+    /// incarnation: the token t0 is UNCHANGED (never displaced). A tokened leaf is edge-protected; only a
+    /// non-tokened leaf is decided here, and a committed-source adopt is trusted.
+    auto b = std::make_shared<InMemoryBackend>();
+
+    DB::Cas::Layout layout("p");
+    const String hex = streamingHexOf("payload-fresh-ev");
+    {
+        auto s0 = openPool(b);
+        auto build0 = s0->beginPartWrite({});
+        build0->putBlob(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128(hex))}, BlobSource::fromString("payload-fresh-ev"));
+    }
+    const String blob_key = layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128(hex))});
+    const Token t0 = b->head(blob_key).token;
+    condemnMeta(*b, layout, hexToU128(hex), /*condemn_round*/ 1);
+
+    auto s = openPool(b);
+
+    const RootNamespace ns{"srv1/tbl"};
+    auto build = startBuildFor(s, ns, "part_1");
+    /// adoptEvidence records a TOKENLESS dep.
+    ManifestEntry entry = blobEntry("data.bin", "payload-fresh-ev");
+    entry.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hexToU128(hex))};   /// streaming-convention id (matches the minted blob)
+    build->adoptEvidence(entry);
+    const ManifestId id = build->stageManifest({entry});
+    build->precommitAdd(ns, "part_1", id);
+
+    /// promote trusts the adopted leaf ⇒ commit, no probe, no displacement.
+    EXPECT_NO_THROW(build->promote(ns, "part_1", build->buildId(), id));
+
+    EXPECT_EQ(b->head(blob_key).token, t0) << "trust must not displace the adopted blob";
+    EXPECT_TRUE(s->resolveRef(ns, "part_1").has_value());
+}
+
+TEST(CASProtocol, AdoptedLeafCarriesRealBlobSize)
+{
+    /// B92 round-trip (re-expressed on the manifest model): an adopted leaf must carry its real
+    /// blob_size, NOT 0. PartWriteTxn A publishes a blob; build B adopts that leaf into a second ref. The
+    /// adopted manifest's entry must report the same non-zero blob_size as the original.
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// PartWriteTxn A: a blob with a real payload so blob_size > 0.
+    const ManifestId id_a = publishBlobPart(s, ns, "ref_a", "data.bin", "payload-B92");
+
+    const PartManifest manifest_a = s->readManifest(id_a);
+    const auto * entry_a = findEntry(manifest_a.entries, "data.bin");
+    ASSERT_TRUE(entry_a != nullptr);
+    const uint64_t size_a = entry_a->blob_size;
+    EXPECT_NE(size_a, 0u) << "ref A blob_size must be non-zero";
+    EXPECT_EQ(size_a, String("payload-B92").size());
+
+    /// PartWriteTxn B: adopt the same leaf, publish as ref_b (no re-upload).
+    auto build_b = startBuildFor(s, ns, "ref_b");
+    ASSERT_TRUE(entry_a != nullptr);
+    build_b->adoptEvidence(*entry_a);
+    const ManifestId id_b = build_b->stageManifest({*entry_a});
+    build_b->precommitAdd(ns, "ref_b", id_b);
+    build_b->promote(ns, "ref_b", build_b->buildId(), id_b);
+
+    /// Resolve ref B: the adopted leaf's blob_size must match ref A (round-trip invariant for B92).
+    const PartManifest manifest_b = s->readManifest(s->resolveRef(ns, "ref_b")->manifest_id);
+    const auto * entry_b = findEntry(manifest_b.entries, "data.bin");
+    ASSERT_TRUE(entry_b != nullptr);
+    EXPECT_NE(entry_b->blob_size, 0u) << "adopted leaf blob_size must not be 0 (B92)";
+    EXPECT_EQ(entry_b->blob_size, size_a) << "adopted-leaf blob_size mismatch (B92 round-trip)";
+}
+
+/// ---- Genuinely-obsolete pure-tree-model scenarios (no manifest analog) ----
+
+TEST(CASProtocol, DISABLED_RevalidateAbsentTreeDepRecreates)
+{
+    GTEST_SKIP() << "Obsolete (tree model). The gate's 'absent tree dep recreated from retained "
+                    "payload' behavior has no manifest analog: a part manifest body is staged ONCE by "
+                    "stageManifest and promote never re-creates it — an absent/invalid body at promote "
+                    "fails closed (ABORTED). The blob-leaf absent-recreate case is covered by putBlob's "
+                    "INV-1 re-upload-from-source path, not by the publish gate.";
+}
+
+TEST(CASProtocol, DISABLED_AdoptTreeOfReclaimedTreeFailsClosedAtAdoptTime)
+{
+    GTEST_SKIP() << "Obsolete (tree model). adoptTree's fail-closed observe-at-adopt-time (one HEAD, "
+                    "FILE_DOESNT_EXIST on an absent detached tree) has no manifest analog: the manifest "
+                    "model's adoptEvidence is deliberately TOKENLESS and performs NO backend call — the "
+                    "no-dangle guarantee for an adopted-but-reclaimed leaf is enforced at the promote "
+                    "gate (unconditional blob revalidation ⇒ ABORTED), covered by "
+                    "NewNamespacePublishGatedByShardFenceFloor and FreshEvidenceDepWithViewHitIsResolvedByGate.";
+}
diff --git a/src/Disks/tests/gtest_cas_rebuild_condemn_nothing.cpp b/src/Disks/tests/gtest_cas_rebuild_condemn_nothing.cpp
new file mode 100644
index 000000000000..afe5ed26fb44
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_rebuild_condemn_nothing.cpp
@@ -0,0 +1,636 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasGcStateFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include "cas_test_helpers.h"
+
+#include <algorithm>
+#include <set>
+#include <stdexcept>
+#include <string>
+#include <vector>
+
+/// REBUILD CONDEMNS NOTHING, AND fsck WALKS STREAMS BY ARITHMETIC (spec 2026-07-27 "ref chain complete
+/// cut" §7).
+///
+/// REBUILD used to end with a LIST of `blobs/` and condemn every listed body its traversal had not
+/// reached. That is the r5-finding-4 data-loss vector: the traversal itself is listing-driven, so a
+/// store that omits a durable ref-log or manifest key from a LIST hides a LIVE owner, and the very same
+/// pass then condemns the blob that owner pins. One lying enumeration, and acked data is scheduled for
+/// deletion. The condemnation is GONE — REBUILD rebuilds cursors and edges and reclaims nothing.
+///
+/// The NAMED residual that removal creates (Stage-A staging contract, register R4): a blob whose
+/// manifest no longer exists anywhere is unreclaimable until the build/upload registry can enumerate
+/// in-flight uploads. No substitute reclamation is added in its place — a quiet one would be the same
+/// vector wearing a different hat (Constraint 3: no fallback).
+///
+/// fsck's half is the other side of the same rule: it may not rest a verdict on a listing either. It
+/// walks each namespace's ref stream by ARITHMETIC from `_ckpt.checkpoint` upward, reading every id by
+/// exact key, and reports one verdict per namespace — `chain-broken` (a 404 below a CONFIRMED durable
+/// same-epoch id: a hole, fatal in the summary AND in the exit code), `unchecked` (could not prove it
+/// either way), or nothing at all. A finding is RECORDED, never thrown: an fsck that dies on the first
+/// bad namespace says nothing about the ones it never reached.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+
+const RootNamespace kNsA{"00/aa@cas@"};
+const RootNamespace kNsB{"00/zz@cas@"};
+
+/// Makes a second REBUILD catalog GET observe a different authority set. The command must take one
+/// immutable cut at entry, so a correct implementation never triggers the mutation.
+class CatalogChangesOnSecondReadBackend : public CountingBackend
+{
+public:
+    using Backend::get;
+
+    void armCatalogMutation(const String & key)
+    {
+        catalog_key = key;
+        catalog_reads = 0;
+        armed = true;
+    }
+
+    size_t catalogReads() const { return catalog_reads; }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        auto got = CountingBackend::get(key, range);
+        if (!armed || key != catalog_key)
+            return got;
+
+        ++catalog_reads;
+        if (catalog_reads != 2)
+            return got;
+        if (!got)
+            throw std::runtime_error("catalog mutation fixture: second catalog read found absence");
+
+        const PutResult put = CountingBackend::putOverwrite(
+            key, encodeRefCatalog(RefCatalog{}), got->token, {});
+        if (put.outcome != PutOutcome::Done)
+            throw std::runtime_error("catalog mutation fixture: catalog rewrite conflicted");
+        return CountingBackend::get(key, range);
+    }
+
+private:
+    String catalog_key;
+    size_t catalog_reads = 0;
+    bool armed = false;
+};
+
+BlobRef blobRefOf(const DB::UInt128 & hash)
+{
+    return BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)};
+}
+
+bool blobPresent(Backend & backend, const Layout & layout, const DB::UInt128 & hash)
+{
+    return backend.head(layout.blobKey(blobRefOf(hash))).exists;
+}
+
+/// Whether ANY run the newest fold seal references carries a `kCondemned` row for `hash`. This is where
+/// a rebuild used to put its zero-edge condemnations, so "nothing was condemned" is checked HERE rather
+/// than by watching for a deletion several rounds later.
+bool condemnedInSealedRuns(Backend & backend, const Layout & layout, const DB::UInt128 & hash)
+{
+    const GcState st = decodeGcState(backend.get(layout.gcStateKey())->bytes);
+    const auto sealed = backend.get(layout.foldSealKey(st.snap_generation, st.snap_attempt));
+    if (!sealed)
+        return false;
+    const CasFoldSeal seal = decodeFoldSeal(sealed->bytes);
+    for (const RunRef & r : seal.blob_target_runs)
+    {
+        auto reader = openSourceEdgeRun(backend, r.key);
+        String k;
+        String p;
+        while (reader.next(k, p))
+        {
+            BlobRef ref;
+            UInt128 sid;
+            SourceEdgeKeyCodec::parse(k, ref, sid);
+            if (p.empty() || p[0] != kCondemned)
+                continue;
+            if (ref.digest.toU128() == hash)
+                return true;
+        }
+    }
+    return false;
+}
+
+/// Publish `ref_name` -> a fresh manifest pinning `blob` at exactly `id`, and return the manifest's key
+/// so a test can hide it from the listing.
+String publishAtReturningManifestKey(
+    Backend & backend, const Layout & layout, const RootNamespace & ns, const RefTxnId & id,
+    const String & ref_name, uint64_t build_sequence, const DB::UInt128 & blob, bool birth = false,
+    std::optional<RefTxnId> prev_epoch_seal = std::nullopt)
+{
+    publishAt(backend, layout, ns, id, ref_name, build_sequence, blob, birth, prev_epoch_seal);
+    return layout.manifestKey(ManifestId{ns, ManifestRef{.writer_epoch = id.writer_epoch,
+                                                         .build_sequence = build_sequence,
+                                                         .manifest_ordinal = 1}});
+}
+
+/// Publish the exact `_ckpt` that makes a raw fixture recoverable. The real writers go through
+/// `publishCkpt`, which merges by semantic maximum and additionally refuses a `life_epoch` below the
+/// durable one; this helper instead makes the fixture's one admissible recovery frontier explicit.
+void writeCkptRaw(Backend & backend, const Layout & layout, const RootNamespace & ns, const RefCkpt & ckpt)
+{
+    writeRecoverableCkptForRawFixture(backend, layout, ns, ckpt);
+}
+
+/// The table state after applying exactly `ids`, through the same builder as recovery — so a snapshot
+/// built from it is what the codec itself would have published.
+RefTableState stateAfter(Backend & backend, const Layout & layout, const RootNamespace & ns,
+                         const std::vector<RefTxnId> & ids)
+{
+    RefReplayBuilder builder(std::nullopt);
+    for (const RefTxnId & id : ids)
+    {
+        const auto got = backend.get(layout.refLogKey(fixture::fixtureLife(ns), id));
+        if (!got)
+            throw std::runtime_error("stateAfter: fixture log " + std::to_string(id.writer_epoch) + "-"
+                                     + std::to_string(id.ref_sequence) + " is missing");
+        builder.applyOne(decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), id),
+                         got->bytes.size());
+    }
+    return std::move(builder).finish().state;
+}
+
+/// Two writer epochs joined by a real seal: `{1,1} {1,2}` then the `{1,3}` seal, then `{2,1}` naming it
+/// as its `prev_epoch_seal` and `{2,2}` after it. The exact-authority walk must handle this crossing.
+void seedSealedTwoEpochStream(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    publishAt(backend, layout, ns, RefTxnId{1, 1}, "ref_a", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(backend, layout, ns, RefTxnId{1, 2}, "ref_b", 2, DB::UInt128(2));
+    writeSealAt(backend, layout, ns, RefTxnId{1, 3});
+    publishAt(backend, layout, ns, RefTxnId{2, 1}, "ref_c", 1, DB::UInt128(3), /*birth=*/false,
+              /*prev_epoch_seal=*/RefTxnId{1, 3});
+    publishAt(backend, layout, ns, RefTxnId{2, 2}, "ref_d", 2, DB::UInt128(4));
+}
+
+/// The number of rows in `cls` whose note mentions `needle`, over the whole report.
+size_t rowsMentioning(const FsckReport & rep, FsckClass cls, const String & needle)
+{
+    size_t n = 0;
+    for (const FsckObject & o : rep.objects)
+    {
+        if (o.cls != cls)
+            continue;
+        for (const String & note : o.reachable_from)
+            if (note.find(needle) != String::npos)
+                ++n;
+    }
+    return n;
+}
+
+}
+
+/// ---- REBUILD condemns nothing ----
+
+/// THE REGRESSION TEST FOR r5-finding-4. A blob pinned by a COMMITTED ref, whose ref-log record and
+/// whose manifest body the store both omit from every LIST while serving them perfectly by exact key.
+/// The catalog row plus `_ckpt` frontier make the ref-log record authoritative, so REBUILD must recover
+/// both owners despite the lying hint. The hidden manifest LIST still cannot justify condemnation: an
+/// omitted object costs retention and never data.
+TEST(CASRebuildCondemnNothing, HiddenLiveManifestBlobIsNotCondemned)
+{
+    auto backend = std::make_shared<HintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    /// Visible owner: ref_a pins blob 1.
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", /*build_sequence=*/1, DB::UInt128(1), /*birth=*/true);
+    /// HIDDEN owner: ref_b pins blob 2, and neither its record nor its manifest is ever listed.
+    const String hidden_manifest =
+        publishAtReturningManifestKey(*backend, layout, kNsA, RefTxnId{1, 2}, "ref_b", /*build_sequence=*/2, DB::UInt128(2));
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 2},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+    backend->hide(layout.refLogKey(fixture::fixtureLife(kNsA), RefTxnId{1, 2}));
+    backend->hide(hidden_manifest);
+
+    /// Precondition: both objects really are durable and really are hidden.
+    ASSERT_TRUE(backend->get(layout.refLogKey(fixture::fixtureLife(kNsA), RefTxnId{1, 2})).has_value());
+    ASSERT_TRUE(backend->get(hidden_manifest).has_value());
+
+    Gc gc(store, kGc);
+    const RebuildReport rep = gc.rebuildBaseline(/*force=*/true);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    ASSERT_GT(backend->holesServed(), 0u) << "the hidden keys were never actually omitted from a LIST";
+    EXPECT_EQ(rep.committed_refs, 2u)
+        << "the immutable checkpoint frontier, not the lying LIST, defines both committed owners";
+
+    EXPECT_FALSE(condemnedInSealedRuns(*backend, layout, DB::UInt128(2)))
+        << "the hidden owner's blob was condemned — that is acked data scheduled for deletion";
+    EXPECT_FALSE(loadMetaForTest(*backend, layout, DB::UInt128(2)).has_value())
+        << "a rebuild condemns nothing, so it publishes no condemn marker";
+
+    /// And it survives the pipeline: rounds run, nothing reclaims it.
+    for (int i = 0; i < 4; ++i)
+    {
+        gc.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(2))) << "acked data was deleted after a rebuild";
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(1)));
+}
+
+/// An ORPHAN blob — one no manifest anywhere names — is likewise left alone. This is the NAMED residual
+/// (register R4) stated as a test rather than as prose: until the build/upload registry can enumerate
+/// in-flight uploads, a manifest-less blob is unreclaimable, and the rebuild does NOT get to guess. The
+/// blob a live ref pins and the blob nothing pins are indistinguishable from a LIST, which is exactly
+/// why the old pass could not tell them apart either.
+TEST(CASRebuildCondemnNothing, OrphanBlobIsRetainedNotCondemned)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", /*build_sequence=*/1, DB::UInt128(1), /*birth=*/true);
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+    writeBlobBody(*backend, layout, DB::UInt128(2));   /// orphan: present, named by nothing
+
+    Gc gc(store, kGc);
+    const RebuildReport rep = gc.rebuildBaseline(/*force=*/true);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+
+    EXPECT_FALSE(condemnedInSealedRuns(*backend, layout, DB::UInt128(2)));
+    const GcState st = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const CasFoldSeal seal = decodeFoldSeal(backend->get(layout.foldSealKey(st.snap_generation, st.snap_attempt))->bytes);
+    ASSERT_TRUE(seal.condemned_summary.contains(0)) << "the summary stays TOTAL over gc_shards";
+    EXPECT_EQ(seal.condemned_summary.at(0).condemned_total, 0u) << "a rebuild condemns nothing";
+
+    for (int i = 0; i < 4; ++i)
+    {
+        gc.runRegularRound();
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(2)))
+        << "the residual is RETENTION: an orphan is kept, not quietly reclaimed by a substitute pass";
+}
+
+/// The rebuild is the `gc/state` disaster-recovery command, so it is the LAST thing that may refuse to
+/// run over a pool holding one bad key. A name-bearing segment under the opaque stream root is not a
+/// canonical physical life id. The key must be skipped -- no catalog entry can claim it -- and the
+/// rebuild must continue over unrelated cataloged lives.
+TEST(CASRebuildCondemnNothing, NonCanonicalLifeKeyDoesNotAbortTheRebuild)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", /*build_sequence=*/1, DB::UInt128(1), /*birth=*/true);
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    /// Hand-built: no helper can mint this shape any more.
+    const String noncanonical_life =
+        layout.casRefsPrefix() + kNsA.string() + "/_log/" + renderRefTxnId(RefTxnId{1, 1}) + ".zst";
+    ASSERT_EQ(backend->putIfAbsent(noncanonical_life, "garbage").outcome, PutOutcome::Done);
+
+    Gc gc(store, kGc);
+    RebuildReport rep;
+    ASSERT_NO_THROW(rep = gc.rebuildBaseline(/*force=*/true))
+        << "the recovery command must not be taken out by the damage it exists to recover from";
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+
+    /// The live namespace was still discovered and folded -- the bad key was skipped, not the pool.
+    EXPECT_EQ(rep.committed_refs, 1u) << "the malformed key must not hide the live namespace";
+}
+
+/// The SECOND way the same damage can reach the rebuild, and it is a different code path from the one
+/// above: the gen-0 health check LISTs each namespace's own life prefix and groups those keys to decide
+/// whether any table proves cleaned logs. `groupRefKeys` refuses a key that names no life, so a NESTED
+/// shape under the life prefix (`<life_id>/x/_log/<id>.zst`) reaches the refusal there instead of at
+/// `discoverUniverse`, which absorbs it. Same rule, same reason: the recovery command must not be taken
+/// out by the damage it exists to recover from.
+///
+/// A decodable `gc/state` at generation 0 is what makes that branch run at all -- with no state object
+/// the health check never reaches it. The state here comes from one real round (so the lease belongs to
+/// this identity) with `snap_generation` written back to 0.
+TEST(CASRebuildCondemnNothing, NestedLifelessKeyUnderTheLifePrefixDoesNotAbortTheRebuild)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", /*build_sequence=*/1, DB::UInt128(1), /*birth=*/true);
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    {
+        const auto got = backend->get(layout.gcStateKey());
+        ASSERT_TRUE(got.has_value());
+        GcState st = decodeGcState(got->bytes);
+        st.snap_generation = 0;
+        ASSERT_EQ(backend->putOverwrite(layout.gcStateKey(), encodeGcState(st), got->token).outcome,
+                  PutOutcome::Done);
+    }
+
+    /// Hand-built, and planted AFTER the round so the round itself is clean: one segment too deep under
+    /// the life prefix, so the segment where the incarnation belongs holds `x`. No helper mints this.
+    const String nested = layout.namespaceStreamPrefix(fixture::fixtureLife(kNsA))
+        + "x/_log/" + renderRefTxnId(RefTxnId{1, 1}) + ".zst";
+    ASSERT_EQ(backend->putIfAbsent(nested, "garbage").outcome, PutOutcome::Done);
+
+    RebuildReport rep;
+    ASSERT_NO_THROW(rep = gc.rebuildBaseline(/*force=*/false))
+        << "the recovery command must not be taken out by the damage it exists to recover from";
+    /// FORCE is deliberately NOT passed: a listing the check could not group proves nothing about the ref
+    /// baseline, so the pool cannot be declared healthy, and the un-forced rebuild must therefore RUN
+    /// rather than refuse.
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_EQ(rep.committed_refs, 1u) << "the live namespace must still be folded";
+}
+
+TEST(CASRebuildCondemnNothing, OneCatalogCutDrivesHealthCheckAndRebuild)
+{
+    auto backend = std::make_shared<CatalogChangesOnSecondReadBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", /*build_sequence=*/1,
+              DB::UInt128(1), /*birth=*/true);
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    /// A decoded generation-0 state exercises the health check's namespace walk before the rebuild
+    /// universe is consumed. The backend would erase the authority set on a second catalog GET.
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+    {
+        const auto got = backend->get(layout.gcStateKey());
+        ASSERT_TRUE(got);
+        GcState state = decodeGcState(got->bytes);
+        state.snap_generation = 0;
+        ASSERT_EQ(backend->putOverwrite(
+            layout.gcStateKey(), encodeGcState(state), got->token).outcome, PutOutcome::Done);
+    }
+
+    backend->armCatalogMutation(layout.refCatalogKey());
+    const RebuildReport rep = gc.rebuildBaseline(/*force=*/true);
+    EXPECT_EQ(backend->catalogReads(), 1u)
+        << "REBUILD must use its entry cut for both the generation-0 health check and the rebuild";
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+    EXPECT_EQ(rep.committed_refs, 1u);
+}
+
+/// Removing the condemnation must not disturb the other thing a rebuild owes: every hold in the prior
+/// seal rides through VERBATIM (Task 8). Asserted together with the condemn-nothing rule because the
+/// two used to be produced by the same pass, and a hold dropped here would hand back a baseline that
+/// claims a frontier proof it does not have.
+TEST(CASRebuildCondemnNothing, CarriesHoldsVerbatimWhileCondemningNothing)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", /*build_sequence=*/1, DB::UInt128(1), /*birth=*/true);
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+    writeBlobBody(*backend, layout, DB::UInt128(2));   /// an orphan alongside the held namespace
+
+    /// One real round first: it establishes the pool's `gc/state` and takes the lease under THIS
+    /// identity, so the rebuild below is the disaster-recovery path and not a lease conflict.
+    Gc gc(store, kGc);
+    ASSERT_TRUE(gc.runRegularRound().acquired_lease);
+
+    const RefHold planted{.reason = HoldReason::GapBelowWitness, .offending_position = RefTxnId{4, 9},
+                          .retry_count = 17, .next_retry_round = 23};
+    {
+        const GcState adopted = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+        seedFoldCursorForTest(*backend, layout, kNsA, RefTxnId{1, 1}, planted,
+                              adopted.snap_generation, adopted.snap_attempt);
+    }
+
+    const RebuildReport rep = gc.rebuildBaseline(/*force=*/true);
+    ASSERT_TRUE(rep.performed) << rep.refusal;
+
+    const GcState st = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const CasFoldSeal seal = decodeFoldSeal(backend->get(layout.foldSealKey(st.snap_generation, st.snap_attempt))->bytes);
+    const auto it = seal.ref_lives.find(catalogLifeIdForTest(*backend, layout, kNsA));
+    ASSERT_NE(it, seal.ref_lives.end());
+    EXPECT_EQ(it->second.coverage.classification, 4);
+    ASSERT_TRUE(it->second.coverage.hold.has_value());
+    EXPECT_EQ(*it->second.coverage.hold, planted)
+        << "a rebuild retried nothing, so it rewrites nothing about the hold";
+
+    EXPECT_FALSE(condemnedInSealedRuns(*backend, layout, DB::UInt128(2)));
+}
+
+/// ---- fsck: arithmetic streams ----
+
+/// A pool with nothing wrong reports nothing: no hole, no unproven namespace, a clean bill of health
+/// and a zero exit. `unchecked` is not a resting state — it is a verdict a healthy pool never reaches.
+TEST(CASRebuildCondemnNothingFsck, HealthyArithmeticPoolIsClean)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 2}, "ref_b", 2, DB::UInt128(2));
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 3}, "ref_c", 3, DB::UInt128(3));
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 3},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    const FsckReport rep = runFsck(*store, /*detail=*/true);
+    EXPECT_TRUE(rep.clean()) << formatFsckSummary(rep);
+    EXPECT_EQ(rep.chain_broken, 0u);
+    EXPECT_EQ(rep.unchecked, 0u) << "a healthy namespace is PROVEN, not merely uncomplained-about";
+    EXPECT_EQ(rep.ref_records_walked, 3u);
+    EXPECT_EQ(rep.dangling, 0u);
+}
+
+/// A 404 BELOW a durable same-epoch id. Ids are dense `1..T` within `(namespace, epoch)` (INV-1), so
+/// this cannot be the end of a stream: a durable record is missing and every transaction above it is
+/// unreachable. The verdict is FATAL — it appears in the machine-parseable summary line and it makes
+/// the report unclean, which is what turns into the command's nonzero exit.
+TEST(CASRebuildCondemnNothingFsck, MidChainHoleBelowAWitnessIsChainBroken)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 2}, "ref_b", 2, DB::UInt128(2));
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 3}, "ref_c", 3, DB::UInt128(3));
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 3},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    /// Punch the hole: {1,2} is gone while {1,3} stays durable and listed.
+    const String holed = layout.refLogKey(fixture::fixtureLife(kNsA), RefTxnId{1, 2});
+    const HeadResult h = backend->head(holed);
+    ASSERT_TRUE(h.exists);
+    backend->deleteExact(holed, h.token);
+
+    FsckReport rep;
+    ASSERT_NO_THROW(rep = runFsck(*store, /*detail=*/true))
+        << "a finding is RECORDED, never thrown — an fsck that dies reports nothing";
+    EXPECT_EQ(rep.chain_broken, 1u);
+    EXPECT_FALSE(rep.clean()) << "chain-broken is a hard finding";
+    EXPECT_NE(formatFsckSummary(rep).find("chain_broken=1"), String::npos) << formatFsckSummary(rep);
+
+    bool row = false;
+    for (const FsckObject & o : rep.objects)
+        if (o.cls == FsckClass::ChainBroken)
+            row = true;
+    EXPECT_TRUE(row) << "the fatal must name the position it was detected at";
+}
+
+/// The tail ABOVE `_ckpt.checkpoint` is WALKED, not assumed. Here the store lists neither of the two
+/// records above the checkpoint, so a listing-driven audit would see an empty tail and report a clean
+/// pool it never read. Arithmetic reads them by exact key: they are walked, counted, and the namespace
+/// comes back PROVEN — not `unchecked`, which is reserved for what cannot be proved at all.
+TEST(CASRebuildCondemnNothingFsck, TailAboveTheCheckpointIsWalkedNotUnchecked)
+{
+    auto backend = std::make_shared<HintHoleBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 2}, "ref_b", 2, DB::UInt128(2));
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 3}, "ref_c", 3, DB::UInt128(3));
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 4}, "ref_d", 4, DB::UInt128(4));
+
+    /// A published snapshot at {1,2}, named by the checkpoint. Its bytes are the codec's own view of
+    /// the state at {1,2}, so exact checkpoint-base validation accepts it.
+    writeRefSnapshotRaw(*backend, layout,
+                        snapshotOf(stateAfter(*backend, layout, kNsA, {RefTxnId{1, 1}, RefTxnId{1, 2}}), kNsA.string()));
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 4},
+                         .checkpoint_snapshot_id = RefTxnId{1, 2}, .last_epoch_seal = std::nullopt});
+
+    /// The store stops listing the tail. It stays perfectly readable by exact key.
+    backend->hide(layout.refLogKey(fixture::fixtureLife(kNsA), RefTxnId{1, 3}));
+    backend->hide(layout.refLogKey(fixture::fixtureLife(kNsA), RefTxnId{1, 4}));
+
+    const FsckReport rep = runFsck(*store, /*detail=*/true);
+    ASSERT_GT(backend->holesServed(), 0u) << "the tail was never actually hidden from a LIST";
+    EXPECT_EQ(rep.ref_records_walked, 2u) << "the two records above the checkpoint must be read by exact key";
+    EXPECT_EQ(rep.unchecked, 0u) << "a walked tail is PROVEN; `unchecked` is not a default";
+    EXPECT_EQ(rep.chain_broken, 0u);
+    EXPECT_TRUE(rep.clean()) << formatFsckSummary(rep);
+}
+
+/// A SEALED multi-epoch stream, walked. The epoch boundary is crossed the way the protocol proves it —
+/// through the next epoch's `prev_epoch_seal` back-chain — never by guessing `epoch + 1`. The
+/// checkpoint sits below the seal, so the tail the walk owes covers the boundary itself.
+TEST(CASRebuildCondemnNothingFsck, SealedStreamIsWalkedAcrossTheEpochBoundary)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    seedSealedTwoEpochStream(*backend, layout, kNsA);
+    writeRefSnapshotRaw(*backend, layout,
+                        snapshotOf(stateAfter(*backend, layout, kNsA, {RefTxnId{1, 1}, RefTxnId{1, 2}}), kNsA.string()));
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{2, 2},
+                         .checkpoint_snapshot_id = RefTxnId{1, 2}, .last_epoch_seal = RefTxnId{1, 3}});
+
+    const FsckReport rep = runFsck(*store, /*detail=*/true);
+    EXPECT_TRUE(rep.clean()) << formatFsckSummary(rep);
+    EXPECT_EQ(rep.chain_broken, 0u);
+    EXPECT_EQ(rep.unchecked, 0u) << "a PROVED crossing is not an unproven one";
+    EXPECT_EQ(rep.ref_records_walked, 3u) << "the seal plus both records of the epoch it opened";
+}
+
+/// An exact `_ckpt` frontier turns an impossible epoch crossing into a hard chain break. Here `ns_a`'s
+/// epoch 1 ends at the PRESENT ordinary record `{1,3}`, while both `_ckpt` and `{2,1}` falsely claim
+/// that position as the closing seal. The finite range is complete and proves the contradiction: this
+/// is NOT `unchecked`, and there is no earlier missing record that could make the test pass instead.
+/// The healthy `ns_b` in the same pool is unaffected — one broken namespace never spreads.
+TEST(CASRebuildCondemnNothingFsck, ExactFrontierMakesAnUnsealedEpochCrossingChainBroken)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 2}, "ref_b", 2, DB::UInt128(2));
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 3}, "ref_c", 3, DB::UInt128(3));
+    /// `{1,3}` exists but is an ordinary owner transaction, not an `EpochSeal`. Claiming it as the
+    /// predecessor must not authorize the transition to epoch 2.
+    publishAt(*backend, layout, kNsA, RefTxnId{2, 1}, "ref_d", 1, DB::UInt128(4), /*birth=*/false,
+              /*prev_epoch_seal=*/RefTxnId{1, 3});
+
+    publishAt(*backend, layout, kNsB, RefTxnId{1, 1}, "ref_z", 1, DB::UInt128(9), /*birth=*/true);
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{2, 1},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = RefTxnId{1, 3}});
+    writeCkptRaw(*backend, layout, kNsB,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    FsckReport rep;
+    ASSERT_NO_THROW(rep = runFsck(*store, /*detail=*/true));
+    EXPECT_EQ(rep.unchecked, 0u) << "the exact frontier proves this crossing inconsistent";
+    EXPECT_EQ(rep.chain_broken, 1u) << "exactly the malformed namespace must be reported";
+    const String missing_key = layout.refLogKey(fixture::fixtureLife(kNsA), RefTxnId{1, 4});
+    EXPECT_EQ(std::count_if(rep.objects.begin(), rep.objects.end(), [&](const FsckObject & object)
+    {
+        return object.cls == FsckClass::ChainBroken && object.key == missing_key;
+    }), 1u) << "the present ordinary 1-3 cannot close epoch 1, so arithmetic continuation requires 1-4";
+    EXPECT_GE(rowsMentioning(rep, FsckClass::ChainBroken, "checkpoint requires id 1-4"), 1u)
+        << "the verdict must expose that the claimed ordinary predecessor did not authorize a crossing";
+    EXPECT_GE(rowsMentioning(rep, FsckClass::ChainBroken, "inclusive frontier 2-1"), 1u)
+        << "the verdict must name the authority that made the absence a proven chain break";
+}
+
+/// W2 (Task-3 review): a holed namespace used to make the WHOLE scan throw — `applyOne` raises
+/// `CORRUPTED_DATA` on a non-contiguous replay and nothing caught it, so one bad table aborted the
+/// audit and every namespace after it went unexamined. For recovery, throwing is the correct
+/// fail-close; for a read-only diagnostic it violates "record and continue, never wedge".
+TEST(CASRebuildCondemnNothingFsck, OneBadNamespaceDoesNotAbortTheAudit)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds=*/0);
+    const Layout & layout = store->layout();
+
+    /// `ns_a` sorts FIRST, so a scan that dies on it never reaches `ns_b`.
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 1}, "ref_a", 1, DB::UInt128(1), /*birth=*/true);
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 2}, "ref_b", 2, DB::UInt128(2));
+    publishAt(*backend, layout, kNsA, RefTxnId{1, 3}, "ref_c", 3, DB::UInt128(3));
+    writeCkptRaw(*backend, layout, kNsA,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 3},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+    const String holed = layout.refLogKey(fixture::fixtureLife(kNsA), RefTxnId{1, 2});
+    const HeadResult h = backend->head(holed);
+    ASSERT_TRUE(h.exists);
+    backend->deleteExact(holed, h.token);
+
+    publishAt(*backend, layout, kNsB, RefTxnId{1, 1}, "ref_z", 1, DB::UInt128(9), /*birth=*/true);
+    writeCkptRaw(*backend, layout, kNsB,
+                 RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    FsckReport rep;
+    ASSERT_NO_THROW(rep = runFsck(*store, /*detail=*/true));
+    EXPECT_EQ(rep.chain_broken, 1u);
+    EXPECT_GE(rep.reachable, 1u) << "the namespace AFTER the broken one must still have been examined";
+    EXPECT_EQ(rep.dangling, 0u) << "`ns_b` is healthy; a wedged scan would have reported nothing about it";
+}
diff --git a/src/Disks/tests/gtest_cas_record_stream_format.cpp b/src/Disks/tests/gtest_cas_record_stream_format.cpp
new file mode 100644
index 000000000000..42e44585c357
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_record_stream_format.cpp
@@ -0,0 +1,255 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRecordStreamFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <IO/WriteBufferFromString.h>
+#include <fmt/format.h>
+#include <vector>
+
+using namespace DB;
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+    extern const int UNKNOWN_FORMAT_VERSION;
+}
+
+namespace
+{
+
+BlobRef chRef(uint64_t n)
+{
+    return BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(UInt128(n))};
+}
+
+SourceEdgeRecord edge(const BlobRef & ref, uint64_t source_id)
+{
+    return SourceEdgeRecord{.ref = ref, .source_id = UInt128(source_id), .marker = kEdgeActive};
+}
+
+SourceEdgeRecord zero(const BlobRef & ref)
+{
+    return SourceEdgeRecord{.ref = ref, .source_id = UInt128(0), .marker = kZeroMarker};
+}
+
+SourceEdgeRecord condemned(const BlobRef & ref, const Token & token, uint64_t size, uint64_t round, bool pend)
+{
+    return SourceEdgeRecord{.ref = ref, .source_id = UInt128(0), .marker = kCondemned,
+                            .delete_pending = pend, .token = token, .size = size, .condemn_round = round};
+}
+
+/// Encode a run from records already in (ref, source_id) order.
+String encodeRun(const std::vector<SourceEdgeRecord> & recs)
+{
+    DB::WriteBufferFromOwnString out;
+    SourceEdgeRunWriter writer(out);
+    for (const auto & r : recs)
+        writer.append(r);
+    writer.finish();
+    out.finalize();
+    return out.str();
+}
+
+/// Stream a run back to records; verifies the trailer count as a side effect.
+std::vector<SourceEdgeRecord> decodeRun(const String & bytes)
+{
+    ReadBufferFromMemory in(bytes.data(), bytes.size());
+    SourceEdgeRunReader reader(in);
+    std::vector<SourceEdgeRecord> out;
+    SourceEdgeRecord r;
+    while (reader.next(r))
+        out.push_back(r);
+    return out;
+}
+
+}
+
+TEST(CASRecordStream, EmptyRunRoundTripsAndChecksumMatches)
+{
+    const String bytes = encodeRun({});
+    EXPECT_EQ(bytes, fmt::format(
+        "{{\"type\":\"cas_run\",\"v\":{},\"kind\":\"source_edge\"}}\n{{\"n\":0}}\n", currentCompatibilityVersion()));
+
+    ReadBufferFromMemory in(bytes.data(), bytes.size());
+    SourceEdgeRunReader reader(in);
+    SourceEdgeRecord r;
+    EXPECT_FALSE(reader.next(r));
+    /// The read-side accumulated hash equals the write-side helper over the same bytes.
+    reader.verifyAgainst(sourceEdgeRunChecksum(bytes));
+}
+
+TEST(CASRecordStream, EdgeZeroCondemnedRoundTrip)
+{
+    const BlobRef a = chRef(1);
+    const BlobRef b = chRef(2);
+    const BlobRef c = chRef(3);
+    /// Sorted by (ref, source_id): b's condemned sentinel is at source_id 0 (sorts first for b); a has
+    /// an edge; c has a zero marker. Blobs ascend a < b < c, so the sequence is already non-decreasing.
+    std::vector<SourceEdgeRecord> recs = {
+        edge(a, 10),
+        condemned(b, Token{"e-1", TokenType::ETag}, 4242, 7, /*pend*/ true),
+        zero(c),
+    };
+    const String bytes = encodeRun(recs);
+    const std::vector<SourceEdgeRecord> back = decodeRun(bytes);
+    ASSERT_EQ(back.size(), 3u);
+
+    EXPECT_EQ(back[0].ref, a);
+    EXPECT_EQ(back[0].source_id, UInt128(10));
+    EXPECT_EQ(back[0].marker, kEdgeActive);
+
+    EXPECT_EQ(back[1].ref, b);
+    EXPECT_EQ(back[1].source_id, UInt128(0));
+    EXPECT_EQ(back[1].marker, kCondemned);
+    EXPECT_TRUE(back[1].delete_pending);
+    EXPECT_EQ(back[1].token, (Token{"e-1", TokenType::ETag}));
+    EXPECT_EQ(back[1].size, 4242u);
+    EXPECT_EQ(back[1].condemn_round, 7u);
+
+    EXPECT_EQ(back[2].ref, c);
+    EXPECT_EQ(back[2].marker, kZeroMarker);
+}
+
+TEST(CASRecordStream, WriterIsByteDeterministic)
+{
+    std::vector<SourceEdgeRecord> recs = {
+        edge(chRef(1), 5),
+        edge(chRef(1), 9),
+        condemned(chRef(2), Token{"t/with/slashes", TokenType::ETag}, 1, 2, false),
+    };
+    EXPECT_EQ(encodeRun(recs), encodeRun(recs));   /// pure function of the sorted record set
+}
+
+TEST(CASRecordStream, SortOrderAcrossAlgosFollowsAlgoByte)
+{
+    /// b = <algo byte 2-hex><digest hex>. The algo byte leads, so string-sorting b reproduces the
+    /// binary (algo, digest, source_id) order: ch128 (01) < xxh3 (02) < sha256 (03).
+    BlobDigest d16 = BlobDigest::fromU128(UInt128(7));
+    BlobDigest d32{};
+    d32.bytes[0] = 0x10;
+    const BlobRef ch{BlobHashAlgo::CityHash128, d16};
+    const BlobRef xx{BlobHashAlgo::XXH3_128, d16};
+    const BlobRef sha{BlobHashAlgo::Sha256, d32};
+
+    /// Accepted in algo-byte order without an out-of-order throw.
+    const String bytes = encodeRun({edge(ch, 1), edge(xx, 1), edge(sha, 1)});
+    const std::vector<SourceEdgeRecord> back = decodeRun(bytes);
+    ASSERT_EQ(back.size(), 3u);
+    EXPECT_EQ(back[0].ref.algo, BlobHashAlgo::CityHash128);
+    EXPECT_EQ(back[1].ref.algo, BlobHashAlgo::XXH3_128);
+    EXPECT_EQ(back[2].ref.algo, BlobHashAlgo::Sha256);
+}
+
+TEST(CASRecordStream, AppendOutOfOrderThrows)
+{
+    DB::WriteBufferFromOwnString out;
+    SourceEdgeRunWriter writer(out);
+    writer.append(edge(chRef(2), 1));
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            writer.append(edge(chRef(1), 1));
+        },
+        "records appended out of");   /// ref regression
+}
+
+TEST(CASRecordStream, SourceIdRendersAs32Hex)
+{
+    const String bytes = encodeRun({edge(chRef(1), 10)});
+    /// The source id 10 is a 32-char lowercase hex string ending in 'a'.
+    EXPECT_NE(bytes.find("\"s\":\"0000000000000000000000000000000a\""), String::npos);
+    /// The record key `b` for a ch128 ref is the algo byte 01 + a 32-hex digest (34 chars total).
+    EXPECT_NE(bytes.find("\"b\":\"01"), String::npos);
+}
+
+TEST(CASRecordStream, SealChecksumMismatchFailsClosed)
+{
+    const String bytes = encodeRun({edge(chRef(1), 10), edge(chRef(1), 20)});
+    const UInt128 good = sourceEdgeRunChecksum(bytes);
+
+    /// A correct verify passes.
+    {
+        ReadBufferFromMemory in(bytes.data(), bytes.size());
+        SourceEdgeRunReader reader(in);
+        SourceEdgeRecord r;
+        while (reader.next(r)) {}
+        reader.verifyAgainst(good);
+    }
+
+    /// Any byte flip either fails the parse or the whole-file checksum — never silently trusted.
+    String flipped = bytes;
+    flipped[flipped.size() / 2] ^= 0x20;
+    EXPECT_NE(sourceEdgeRunChecksum(flipped), good);
+    EXPECT_THROW({
+        ReadBufferFromMemory in(flipped.data(), flipped.size());
+        SourceEdgeRunReader reader(in);
+        SourceEdgeRecord r;
+        while (reader.next(r)) {}
+        reader.verifyAgainst(good);
+    }, DB::Exception);
+}
+
+TEST(CASRecordStream, TrailerCountMismatchIsCorruptData)
+{
+    String bytes = encodeRun({edge(chRef(1), 10)});
+    /// Rewrite the trailer count 1 -> 2.
+    const String from = "{\"n\":1}\n";
+    const String to = "{\"n\":2}\n";
+    const size_t at = bytes.rfind(from);
+    ASSERT_NE(at, String::npos);
+    bytes.replace(at, from.size(), to);
+    EXPECT_THROW(decodeRun(bytes), DB::Exception);
+}
+
+TEST(CASRecordStream, TruncationAtLineBoundaryFailsClosed)
+{
+    const String bytes = encodeRun({edge(chRef(1), 10), edge(chRef(1), 20)});
+    /// Drop the trailer line entirely (truncate after the last record's newline).
+    const size_t trailer = bytes.rfind("{\"n\":");
+    ASSERT_NE(trailer, String::npos);
+    EXPECT_THROW(decodeRun(bytes.substr(0, trailer)), DB::Exception);
+}
+
+TEST(CASRecordStream, HeaderGates)
+{
+    /// Wrong type.
+    {
+        const String s = "{\"type\":\"cas_pool_meta\",\"v\":3,\"kind\":\"source_edge\"}\n{\"n\":0}\n";
+        EXPECT_THROW(decodeRun(s), DB::Exception);
+    }
+    /// Wrong kind.
+    {
+        const String s = "{\"type\":\"cas_run\",\"v\":3,\"kind\":\"blob_delta\"}\n{\"n\":0}\n";
+        EXPECT_THROW(decodeRun(s), DB::Exception);
+    }
+    /// Future version -> UNKNOWN_FORMAT_VERSION.
+    {
+        const String s = fmt::format(
+            "{{\"type\":\"cas_run\",\"v\":{},\"kind\":\"source_edge\"}}\n{{\"n\":0}}\n", currentCompatibilityVersion() + 1);
+        ReadBufferFromMemory in(s.data(), s.size());
+        try
+        {
+            SourceEdgeRunReader reader(in);
+            FAIL() << "expected UNKNOWN_FORMAT_VERSION";
+        }
+        catch (const DB::Exception & e)
+        {
+            EXPECT_EQ(e.code(), DB::ErrorCodes::UNKNOWN_FORMAT_VERSION);
+        }
+    }
+    /// An out-of-range version must not narrow to a valid low u32 value.
+    {
+        const String s = "{\"type\":\"cas_run\",\"v\":4294967299,\"kind\":\"source_edge\"}\n{\"n\":0}\n";
+        try
+        {
+            decodeRun(s);
+            FAIL() << "expected CORRUPTED_DATA";
+        }
+        catch (const DB::Exception & e)
+        {
+            EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+        }
+    }
+}
diff --git a/src/Disks/tests/gtest_cas_recovery_grounding.cpp b/src/Disks/tests/gtest_cas_recovery_grounding.cpp
new file mode 100644
index 000000000000..5362813bdfa6
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_recovery_grounding.cpp
@@ -0,0 +1,701 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <algorithm>
+#include <functional>
+#include <limits>
+#include <map>
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int INVALID_STATE;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+using DB::Cas::tests::CountingBackend;
+using DB::Cas::tests::minimalLiveSnapshot;
+using DB::Cas::tests::namespaceBirthOp;
+using DB::Cas::tests::publishCommittedOps;
+using DB::Cas::tests::seedPoolMetaForRestart;
+using DB::Cas::tests::writeRefSnapshotRaw;
+
+enum class ListingMode : uint8_t
+{
+    Full,
+    Empty,
+    Partial,
+    Reordered,
+};
+
+class RecoveryListingBackend : public CountingBackend
+{
+public:
+    explicit RecoveryListingBackend(ListingMode mode_) : mode(mode_) { seedPoolMetaForRestart(*this); }
+
+    size_t list_calls = 0;
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ++list_calls;
+        ListPage page = CountingBackend::list(prefix, cursor, limit);
+        if (mode == ListingMode::Empty)
+            page.keys.clear();
+        else if (mode == ListingMode::Partial)
+        {
+            page.keys.erase(std::remove_if(page.keys.begin(), page.keys.end(), [](const ListedKey & key)
+            {
+                return key.key.find("/_log/") != String::npos;
+            }), page.keys.end());
+        }
+        else if (mode == ListingMode::Reordered)
+            std::reverse(page.keys.begin(), page.keys.end());
+        return page;
+    }
+
+private:
+    ListingMode mode;
+};
+
+RefLogTxn txn(const RootNamespace & ns, RefTxnId id, std::vector<RefOp> ops,
+              std::optional<RefTxnId> previous_seal = std::nullopt)
+{
+    return RefLogTxn{.ns = ns.string(), .txn_id = id, .ops = std::move(ops), .prev_epoch_seal = previous_seal};
+}
+
+std::map<String, ManifestRef> committedOf(const RefTableState & state)
+{
+    std::map<String, ManifestRef> result;
+    for (const auto [name, row] : state.getCommitted())
+        result.emplace(name, row.manifest_ref);
+    return result;
+}
+
+void seedAuthoritativeStream(Backend & backend, const Layout & layout, const RootNamespace & ns,
+                             RefTxnId committed_through, bool include_f_plus_one = false)
+{
+    const ManifestRef first{1, 1, 1};
+    std::vector<RefOp> birth{namespaceBirthOp()};
+    const auto first_publish = publishCommittedOps("a", first);
+    birth.insert(birth.end(), first_publish.begin(), first_publish.end());
+    const RefLogTxn first_txn = txn(ns, {1, 1}, std::move(birth));
+    DB::Cas::tests::fixture::writeRefLogRaw(backend, layout, first_txn);
+
+    if (committed_through > RefTxnId{1, 1})
+    {
+        RefOp seal_op;
+        seal_op.kind = RefOpKind::EpochSeal;
+        DB::Cas::tests::fixture::writeRefLogRaw(backend, layout, txn(ns, {1, 2}, {std::move(seal_op)}));
+        const ManifestRef second{2, 1, 1};
+        DB::Cas::tests::fixture::writeRefLogRaw(backend, layout,
+            txn(ns, {2, 1}, publishCommittedOps("b", second), RefTxnId{1, 2}));
+    }
+    if (include_f_plus_one)
+    {
+        const ManifestRef extra{1, 2, 1};
+        DB::Cas::tests::fixture::writeRefLogRaw(backend, layout, txn(ns, {1, 2}, publishCommittedOps("uncommitted", extra)));
+    }
+
+    RefTableState snapshot_state;
+    applyRefLogTxn(snapshot_state, first_txn);
+    writeRefSnapshotRaw(backend, layout, snapshotOf(snapshot_state, ns.string()));
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(backend, layout, ns);
+    const RefCkpt authority{
+        .life_epoch = 1,
+        .committed_through = committed_through,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = committed_through.writer_epoch > 1
+            ? std::optional<RefTxnId>{RefTxnId{1, 2}} : std::nullopt};
+    backend.putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(authority));
+}
+
+/// This is deliberately caller-side plumbing, not a convenience overload in `CasRefProtocol`: production
+/// callers obtain `entry` from their frozen `RefPlan::catalogCut` and sample `_ckpt` in the same plan.
+/// The API under test receives those exact values and performs no catalog or checkpoint resolution itself.
+RecoveredRefTable recoverFromCurrentCatalogCut(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const CasRefCatalog::Snapshot cut = CasRefCatalog::read(backend, layout);
+    std::optional<CatalogEntry> entry;
+    for (const CatalogEntry & candidate : cut.catalog.entries)
+    {
+        if (candidate.ns == ns)
+        {
+            entry = candidate;
+            break;
+        }
+    }
+    std::optional<RefCkpt> checkpoint;
+    if (entry)
+    {
+        const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry->ns, entry->incarnation);
+        if (const std::optional<CkptSample> sample = readCkpt(backend, layout, life))
+            checkpoint = sample->ckpt;
+    }
+    return recoverRefTableDetailedFromAuthority(backend, layout, entry, checkpoint);
+}
+
+CatalogEntry catalog(NsState state)
+{
+    return CatalogEntry{.ns = RootNamespace{"srv1/recovery_grounding"}, .state = state, .incarnation = 1};
+}
+
+RefCkpt ckpt(uint64_t life_epoch, std::optional<RefTxnId> committed_through,
+             std::optional<RefTxnId> checkpoint_snapshot_id = std::nullopt,
+             std::optional<RefTxnId> last_epoch_seal = std::nullopt)
+{
+    return RefCkpt{.life_epoch = life_epoch,
+                   .committed_through = committed_through,
+                   .checkpoint_snapshot_id = checkpoint_snapshot_id,
+                   .last_epoch_seal = last_epoch_seal};
+}
+
+void expectCode(const std::function<void()> & f, int code)
+{
+    try
+    {
+        f();
+        FAIL() << "expected DB::Exception";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), code);
+    }
+}
+
+TEST(CASRecoveryGrounding, CreatingAndAbsentCatalogEntriesAreNotRecovered)
+{
+    expectCode([&] { chooseRecoveryGrounding(catalog(NsState::Creating), ckpt(7, RefTxnId{7, 3})); },
+               DB::ErrorCodes::INVALID_STATE);
+    expectCode([&] { chooseRecoveryGrounding(std::nullopt, ckpt(7, RefTxnId{7, 3})); },
+               DB::ErrorCodes::INVALID_STATE);
+}
+
+TEST(CASRecoveryGrounding, LiveAndRemovingRequireCheckpointAndLifeEpoch)
+{
+    expectCode([&] { chooseRecoveryGrounding(catalog(NsState::Live), std::nullopt); },
+               DB::ErrorCodes::CORRUPTED_DATA);
+    expectCode([&] { chooseRecoveryGrounding(catalog(NsState::Removing), RefCkpt{}); },
+               DB::ErrorCodes::CORRUPTED_DATA);
+}
+
+TEST(CASRecoveryGrounding, MissingFrontierMeansNoCommittedTransaction)
+{
+    const RecoveryGrounding grounding = chooseRecoveryGrounding(catalog(NsState::Live), ckpt(7, std::nullopt));
+    EXPECT_FALSE(grounding.base);
+    EXPECT_FALSE(grounding.committed_through);
+}
+
+TEST(CASRecoveryGrounding, ChoosesCheckpointBaseAndArithmeticWalkStart)
+{
+    const RecoveryGrounding grounding = chooseRecoveryGrounding(
+        catalog(NsState::Live), ckpt(7, RefTxnId{7, 8}, RefTxnId{7, 4}));
+    EXPECT_EQ(grounding.base, (RefTxnId{7, 4}));
+    EXPECT_EQ(grounding.walk_from, (RefTxnId{7, 5}));
+    EXPECT_EQ(grounding.committed_through, (RefTxnId{7, 8}));
+}
+
+TEST(CASRecoveryGrounding, BaseAtFrontierStillStartsAtItsExactSuccessor)
+{
+    /// A writer recovery probes exactly this slot for its sole possible unfrontiered successor. The
+    /// grounding contract must supply the arithmetic start even when the committed replay tail is empty.
+    const RecoveryGrounding grounding = chooseRecoveryGrounding(
+        catalog(NsState::Live), ckpt(7, RefTxnId{7, 8}, RefTxnId{7, 8}));
+
+    EXPECT_EQ(grounding.base, (RefTxnId{7, 8}));
+    EXPECT_EQ(grounding.walk_from, (RefTxnId{7, 9}));
+    EXPECT_EQ(grounding.committed_through, (RefTxnId{7, 8}));
+}
+
+TEST(CASRecoveryGrounding, WalksFromLifeEpochWithoutCheckpointBase)
+{
+    const RecoveryGrounding grounding = chooseRecoveryGrounding(catalog(NsState::Removing), ckpt(9, RefTxnId{9, 3}));
+    EXPECT_EQ(grounding.walk_from, (RefTxnId{9, 1}));
+}
+
+TEST(CASRecoveryGrounding, RejectsBaseWithoutARepresentableSuccessor)
+{
+    expectCode([&]
+    {
+        chooseRecoveryGrounding(catalog(NsState::Live),
+            ckpt(7, RefTxnId{8, 1}, RefTxnId{7, std::numeric_limits<uint64_t>::max()}, RefTxnId{8, 1}));
+    }, DB::ErrorCodes::CORRUPTED_DATA);
+}
+
+TEST(CASRecoveryGrounding, RejectsCheckpointFieldsAboveCommittedFrontier)
+{
+    expectCode([&]
+    {
+        chooseRecoveryGrounding(catalog(NsState::Live), ckpt(7, RefTxnId{7, 3}, RefTxnId{7, 4}));
+    }, DB::ErrorCodes::CORRUPTED_DATA);
+    expectCode([&]
+    {
+        chooseRecoveryGrounding(catalog(NsState::Live), ckpt(7, RefTxnId{7, 3}, std::nullopt, RefTxnId{7, 4}));
+    }, DB::ErrorCodes::CORRUPTED_DATA);
+}
+
+TEST(CASRecoveryGrounding, RejectsIncoherentEpochBoundaryInCheckpointAuthority)
+{
+    expectCode([&]
+    {
+        chooseRecoveryGrounding(catalog(NsState::Live), ckpt(7, RefTxnId{10, 1}, std::nullopt, RefTxnId{7, 9}));
+    }, DB::ErrorCodes::CORRUPTED_DATA);
+    expectCode([&]
+    {
+        chooseRecoveryGrounding(catalog(NsState::Live), ckpt(7, RefTxnId{8, 5}, std::nullopt, RefTxnId{8, 1}));
+    }, DB::ErrorCodes::CORRUPTED_DATA);
+    expectCode([&]
+    {
+        chooseRecoveryGrounding(catalog(NsState::Live), ckpt(7, RefTxnId{8, 1}));
+    }, DB::ErrorCodes::CORRUPTED_DATA);
+}
+
+/// A life starts in its own writer epoch. Letting it start after the checkpoint's writer epoch makes
+/// `walk_from > committed_through`, so recovery silently returns an empty table instead of refusing the
+/// impossible authority. The codec and pure grounding entry point must reject the same sabotage.
+TEST(CASRecoveryGrounding, RejectsLifeEpochAboveCommittedFrontierOnDecodeAndGrounding)
+{
+    const RefCkpt invalid = ckpt(2, RefTxnId{1, 5});
+    String encoded = encodeRefCkpt(ckpt(1, RefTxnId{1, 5}));
+    const size_t life_epoch = encoded.find(R"("le":"1")");
+    ASSERT_NE(life_epoch, String::npos);
+    encoded.replace(life_epoch, String{R"("le":"1")"}.size(), R"("le":"2")");
+
+    expectCode([&] { (void)decodeRefCkpt(encoded); }, DB::ErrorCodes::CORRUPTED_DATA);
+    expectCode([&] { (void)chooseRecoveryGrounding(catalog(NsState::Live), invalid); },
+               DB::ErrorCodes::CORRUPTED_DATA);
+}
+
+TEST(CASRecoveryGrounding, RecoveryIsEquivalentUnderFullEmptyPartialAndReorderedList)
+{
+    struct Observation
+    {
+        std::map<String, ManifestRef> committed;
+        RefTxnId greatest_applied;
+        std::optional<RefTxnId> last_epoch_seal;
+        RefTxnId next_id;
+        uint64_t log_gets = 0;
+        uint64_t snapshot_gets = 0;
+        uint64_t list_calls = 0;
+    };
+
+    std::vector<Observation> observations;
+    for (const ListingMode mode : {ListingMode::Full, ListingMode::Empty, ListingMode::Partial, ListingMode::Reordered})
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(mode);
+        const Layout layout("p");
+        const RootNamespace ns{"srv1/list_equivalence"};
+        const RefTxnId frontier{2, 1};
+        seedAuthoritativeStream(*backend, layout, ns, frontier);
+        const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+        backend->resetCounts();
+        backend->list_calls = 0;
+
+        const RecoveredRefTable recovered = recoverFromCurrentCatalogCut(*backend, layout, ns);
+        const uint64_t log_gets = backend->getCount(layout.refLogKey(life, {1, 1}))
+            + backend->getCount(layout.refLogKey(life, {1, 2}))
+            + backend->getCount(layout.refLogKey(life, {2, 1}));
+        const uint64_t snapshot_gets = backend->getCount(layout.refSnapshotKey(life, {1, 1}));
+        observations.push_back(Observation{
+            .committed = committedOf(recovered.state),
+            .greatest_applied = recovered.state.getGreatestApplied(),
+            .last_epoch_seal = recovered.last_epoch_seal,
+            .next_id = recovered.state.nextTxnId(/*live_epoch=*/3),
+            .log_gets = log_gets,
+            .snapshot_gets = snapshot_gets,
+            .list_calls = backend->list_calls});
+    }
+
+    ASSERT_EQ(observations.size(), 4u);
+    for (size_t i = 1; i < observations.size(); ++i)
+    {
+        EXPECT_EQ(observations[i].committed, observations[0].committed);
+        EXPECT_EQ(observations[i].greatest_applied, observations[0].greatest_applied);
+        EXPECT_EQ(observations[i].last_epoch_seal, observations[0].last_epoch_seal);
+        EXPECT_EQ(observations[i].next_id, observations[0].next_id);
+    }
+    for (const Observation & observation : observations)
+        EXPECT_EQ(observation.log_gets, 3u)
+            << "recovery must fetch every exact log in the checkpoint-bounded frontier";
+    for (const Observation & observation : observations)
+        EXPECT_EQ(observation.snapshot_gets, 0u)
+            << "a snapshot not named by `_ckpt` is not a recovery base";
+    for (const Observation & observation : observations)
+        EXPECT_EQ(observation.list_calls, 0u)
+            << "recovery must not enumerate a stream whose exact checkpoint already supplies its base and frontier";
+}
+
+TEST(CASRecoveryGrounding, CatalogLifecycleAndCheckpointAreMandatoryForReadOnlyRecovery)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/mandatory_authority"};
+
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {1, 1}, {namespaceBirthOp()}));
+        expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::CORRUPTED_DATA);
+    }
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+        CasRefCatalog::casAdmitEntry(
+            *backend, layout, 1, CatalogEntry{.ns = ns, .state = NsState::Live, .incarnation = 8});
+        expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::CORRUPTED_DATA);
+    }
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+        const CatalogEntry live{.ns = ns, .state = NsState::Live, .incarnation = 9};
+        CasRefCatalog::casAdmitEntry(*backend, layout, 1, live);
+        const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(live.ns, live.incarnation);
+        ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), "not a sealed checkpoint").outcome,
+                  PutOutcome::Done);
+        expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::CORRUPTED_DATA);
+    }
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+        CatalogEntry creating{.ns = ns, .state = NsState::Creating, .incarnation = 7,
+            .creator = CreatorFence{"srv1", 1, 1}};
+        CasRefCatalog::casAdmitEntry(*backend, layout, 1, creating);
+        backend->putIfAbsent(layout.refCkptKey(NamespaceLifeId::fromCatalogEntry(creating.ns, creating.incarnation)),
+            encodeRefCkpt(RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                                  .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt}));
+        expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::INVALID_STATE);
+    }
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+        backend->putIfAbsent(layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns)),
+            encodeRefCkpt(RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 1},
+                                  .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt}));
+        expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::INVALID_STATE);
+    }
+}
+
+TEST(CASRecoveryGrounding, NonrecoverableAuthorityPerformsNoBackendRecoveryIo)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/nonrecoverable_authority"};
+    const RefCkpt valid_ckpt{
+        .life_epoch = 1,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt};
+
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+        backend->resetCounts();
+        const CatalogEntry creating{
+            .ns = ns, .state = NsState::Creating, .incarnation = 1, .creator = CreatorFence{"srv1", 1, 1}};
+        expectCode(
+            [&] { (void)recoverRefTableDetailedFromAuthority(*backend, layout, creating, valid_ckpt); },
+            DB::ErrorCodes::INVALID_STATE);
+        EXPECT_EQ(backend->list_calls, 0u);
+        EXPECT_EQ(backend->getTotal(), 0u);
+    }
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+        backend->resetCounts();
+        const CatalogEntry live{.ns = ns, .state = NsState::Live, .incarnation = 2};
+        expectCode(
+            [&] { (void)recoverRefTableDetailedFromAuthority(*backend, layout, live, std::nullopt); },
+            DB::ErrorCodes::CORRUPTED_DATA);
+        EXPECT_EQ(backend->list_calls, 0u);
+        EXPECT_EQ(backend->getTotal(), 0u);
+    }
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+        backend->resetCounts();
+        expectCode(
+            [&] { (void)recoverRefTableDetailedFromAuthority(*backend, layout, std::nullopt, valid_ckpt); },
+            DB::ErrorCodes::INVALID_STATE);
+        EXPECT_EQ(backend->list_calls, 0u);
+        EXPECT_EQ(backend->getTotal(), 0u);
+    }
+}
+
+TEST(CASRecoveryGrounding, ReadOnlyRecoveryNeverAdoptsFPlusOne)
+{
+    auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/read_only_excludes_f_plus_one"};
+    seedAuthoritativeStream(*backend, layout, ns, RefTxnId{1, 1}, /*include_f_plus_one=*/true);
+
+    const RecoveredRefTable recovered = recoverFromCurrentCatalogCut(*backend, layout, ns);
+    EXPECT_EQ(recovered.state.getGreatestApplied(), (RefTxnId{1, 1}));
+    EXPECT_TRUE(recovered.state.getCommitted().contains("a"));
+    EXPECT_FALSE(recovered.state.getCommitted().contains("uncommitted"));
+}
+
+/// A well-formed snapshot can describe a real but uncommitted transaction. If recovery merely treated
+/// `LIST` as a performance hint, it could still select this false base and skip the exact first log.
+/// The checkpoint names no snapshot, so every listing behaviour must leave the forged object unread.
+TEST(CASRecoveryGrounding, ForgedWellFormedListedSnapshotIsUnobservedAndRecoveryDoesNotList)
+{
+    for (const ListingMode mode : {ListingMode::Full, ListingMode::Empty, ListingMode::Partial, ListingMode::Reordered})
+    {
+        auto backend = std::make_shared<RecoveryListingBackend>(mode);
+        const Layout layout("p");
+        const RootNamespace ns{"srv1/forged_listed_snapshot"};
+        seedAuthoritativeStream(*backend, layout, ns, RefTxnId{1, 1}, /*include_f_plus_one=*/true);
+        const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+
+        RefTableState forged_state;
+        std::vector<RefOp> birth{namespaceBirthOp()};
+        const auto first_publish = publishCommittedOps("a", ManifestRef{1, 1, 1});
+        birth.insert(birth.end(), first_publish.begin(), first_publish.end());
+        applyRefLogTxn(forged_state, txn(ns, {1, 1}, std::move(birth)));
+        applyRefLogTxn(forged_state, txn(ns, {1, 2}, publishCommittedOps("uncommitted", ManifestRef{1, 2, 1})));
+        writeRefSnapshotRaw(*backend, layout, snapshotOf(forged_state, ns.string()));
+        const String forged_key = layout.refSnapshotKey(life, {1, 2});
+
+        backend->resetCounts();
+        backend->list_calls = 0;
+        const RecoveredRefTable recovered = recoverFromCurrentCatalogCut(*backend, layout, ns);
+
+        EXPECT_EQ(backend->list_calls, 0u);
+        EXPECT_EQ(backend->getCount(forged_key), 0u);
+        EXPECT_EQ(recovered.state.getGreatestApplied(), (RefTxnId{1, 1}));
+        EXPECT_TRUE(recovered.state.getCommitted().contains("a"));
+        EXPECT_FALSE(recovered.state.getCommitted().contains("uncommitted"));
+    }
+}
+
+/// A checkpoint-named snapshot is immutable lifecycle authority, not a list candidate. Its exact GET
+/// and semantic decode must therefore fail closed rather than falling back to replaying the same log.
+TEST(CASRecoveryGrounding, SemanticallyMalformedCheckpointSnapshotIsCorruptionAfterExactRead)
+{
+    auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Empty);
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/semantically_malformed_checkpoint"};
+    const ManifestRef manifest{1, 1, 1};
+    std::vector<RefOp> ops{namespaceBirthOp()};
+    const auto publish = publishCommittedOps("committed", manifest);
+    ops.insert(ops.end(), publish.begin(), publish.end());
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {1, 1}, std::move(ops)));
+
+    RefTableSnapshot malformed = minimalLiveSnapshot(
+        ns.string(), {1, 1}, {DB::Cas::tests::committedRow("committed", manifest)});
+    malformed.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "precommit", manifest});
+    writeRefSnapshotRaw(*backend, layout, malformed);
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    const String snapshot_key = layout.refSnapshotKey(life, {1, 1});
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = RefTxnId{1, 1},
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    backend->resetCounts();
+    try
+    {
+        (void)recoverFromCurrentCatalogCut(*backend, layout, ns);
+        FAIL() << "expected checkpoint-named malformed snapshot to fail closed";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+        EXPECT_NE(String(e.message()).find("stateFromSnapshot"), String::npos);
+    }
+    EXPECT_EQ(backend->getCount(snapshot_key), 1u)
+        << "the corruption must come from the checkpoint snapshot's exact decode";
+}
+
+TEST(CASRecoveryGrounding, CheckpointSnapshotEqualToLastEpochSealIsRejectedBeforeReadingItsLog)
+{
+    auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/checkpoint_base_seal"};
+    /// The checkpoint directly contradicts itself: its sole snapshot base names its terminal seal.
+    seedAuthoritativeStream(*backend, layout, ns, RefTxnId{1, 2});
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+
+    RefTableState through_seal;
+    std::vector<RefOp> birth{namespaceBirthOp()};
+    const auto first_publish = publishCommittedOps("a", ManifestRef{1, 1, 1});
+    birth.insert(birth.end(), first_publish.begin(), first_publish.end());
+    applyRefLogTxn(through_seal, txn(ns, {1, 1}, std::move(birth)));
+    RefOp seal;
+    seal.kind = RefOpKind::EpochSeal;
+    applyRefLogTxn(through_seal, txn(ns, {1, 2}, {std::move(seal)}));
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(through_seal, ns.string()));
+
+    const CkptSample before = *readCkpt(*backend, layout, life);
+    const RefCkpt with_sealed_base{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = RefTxnId{1, 2},
+        .last_epoch_seal = RefTxnId{1, 2}};
+    ASSERT_EQ(backend->casPut(layout.refCkptKey(life), encodeRefCkpt(with_sealed_base), before.token).outcome,
+              CasOutcome::Committed);
+
+    backend->resetCounts();
+    expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::CORRUPTED_DATA);
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, {1, 2})), 0u)
+        << "the contradictory checkpoint metadata is rejected before any matching-log read";
+    EXPECT_EQ(backend->getCount(layout.refSnapshotKey(life, {1, 2})), 0u)
+        << "the seal-kind witness must be checked before reading the forged same-id snapshot";
+}
+
+/// An `EpochSeal` terminates its numeric epoch.  A checkpoint frontier one sequence later in that
+/// same epoch is not an empty tail: no record can occupy that slot.  Recovery must diagnose the
+/// malformed authority instead of advancing to `{E+1,1}` and terminating because that id sorts above
+/// the bogus same-epoch frontier.
+TEST(CASRecoveryGrounding, SameEpochFrontierAfterDecodedEpochSealIsCorruption)
+{
+    auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/frontier_after_seal"};
+
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {1, 1}, {namespaceBirthOp()}));
+    RefOp seal;
+    seal.kind = RefOpKind::EpochSeal;
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {1, 2}, {std::move(seal)}));
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    String malformed_ckpt = encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2}});
+    const size_t frontier_sequence = malformed_ckpt.find(R"("cts":"2")");
+    ASSERT_NE(frontier_sequence, String::npos);
+    malformed_ckpt.replace(frontier_sequence, String{R"("cts":"2")"}.size(), R"("cts":"3")");
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), malformed_ckpt).outcome, PutOutcome::Done);
+
+    expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::CORRUPTED_DATA);
+}
+
+TEST(CASRecoveryGrounding, OlderCheckpointSnapshotAtSealIsCorruption)
+{
+    auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/older_checkpoint_base_seal"};
+    seedAuthoritativeStream(*backend, layout, ns, RefTxnId{2, 1});
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+
+    RefOp second_seal;
+    second_seal.kind = RefOpKind::EpochSeal;
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {2, 2}, {std::move(second_seal)}));
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout,
+        txn(ns, {3, 1}, publishCommittedOps("c", ManifestRef{3, 1, 1}), RefTxnId{2, 2}));
+
+    RefTableState through_first_seal;
+    std::vector<RefOp> birth{namespaceBirthOp()};
+    const auto first_publish = publishCommittedOps("a", ManifestRef{1, 1, 1});
+    birth.insert(birth.end(), first_publish.begin(), first_publish.end());
+    applyRefLogTxn(through_first_seal, txn(ns, {1, 1}, std::move(birth)));
+    RefOp first_seal;
+    first_seal.kind = RefOpKind::EpochSeal;
+    applyRefLogTxn(through_first_seal, txn(ns, {1, 2}, {std::move(first_seal)}));
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(through_first_seal, ns.string()));
+
+    const CkptSample before = *readCkpt(*backend, layout, life);
+    const RefCkpt with_old_sealed_base{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{3, 1},
+        .checkpoint_snapshot_id = RefTxnId{1, 2},
+        .last_epoch_seal = RefTxnId{2, 2}};
+    ASSERT_EQ(backend->casPut(layout.refCkptKey(life), encodeRefCkpt(with_old_sealed_base), before.token).outcome,
+              CasOutcome::Committed);
+
+    backend->resetCounts();
+    expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::CORRUPTED_DATA);
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, {1, 2})), 1u)
+        << "the old seal differs from `last_epoch_seal`, so only the matching-log proof can reject it";
+    EXPECT_EQ(backend->getCount(layout.refSnapshotKey(life, {1, 2})), 0u)
+        << "the old seal must be rejected before the forged same-id snapshot is read";
+}
+
+TEST(CASRecoveryGrounding, TerminalGapBelowFrontierIsCorruptionNotARebirth)
+{
+    auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/terminal_gap"};
+    const RefLogTxn birth = txn(ns, {1, 1}, {namespaceBirthOp()});
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, birth);
+    RefOp remove;
+    remove.kind = RefOpKind::RemoveNamespace;
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {1, 2}, {std::move(remove)}));
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {2, 1}, {namespaceBirthOp()}));
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    String malformed_ckpt = encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt});
+    const size_t frontier_epoch = malformed_ckpt.find(R"("cte":"1")");
+    ASSERT_NE(frontier_epoch, String::npos);
+    malformed_ckpt.replace(frontier_epoch, String{R"("cte":"1")"}.size(), R"("cte":"2")");
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), malformed_ckpt).outcome, PutOutcome::Done);
+
+    expectCode([&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); }, DB::ErrorCodes::CORRUPTED_DATA);
+}
+
+TEST(CASRecoveryGrounding, LaterEpochCheckpointBaseRequiresItsContextualBacklink)
+{
+    auto backend = std::make_shared<RecoveryListingBackend>(ListingMode::Full);
+    const Layout layout("p");
+    const RefTxnId seal_id{1, 2};
+    const RefTxnId base_id{2, 1};
+
+    const auto expect_rejected = [&](const RootNamespace & ns, std::optional<RefTxnId> backlink)
+    {
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {1, 1}, {namespaceBirthOp()}));
+        RefOp seal;
+        seal.kind = RefOpKind::EpochSeal;
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, seal_id, {std::move(seal)}));
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, base_id, {}, backlink));
+        writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), base_id));
+
+        const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+        ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+            .life_epoch = 1,
+            .committed_through = base_id,
+            .checkpoint_snapshot_id = base_id,
+            .last_epoch_seal = seal_id})).outcome, PutOutcome::Done);
+
+        expectCode(
+            [&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); },
+            DB::ErrorCodes::CORRUPTED_DATA);
+    };
+
+    expect_rejected(RootNamespace{"srv1/base_missing_backlink"}, std::nullopt);
+    expect_rejected(RootNamespace{"srv1/base_wrong_backlink"}, RefTxnId{1, 99});
+
+    const auto expect_predecessor_rejected = [&](const RootNamespace & ns, bool write_ordinary_predecessor)
+    {
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, {1, 1}, {namespaceBirthOp()}));
+        if (write_ordinary_predecessor)
+            DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, seal_id, {}));
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, txn(ns, base_id, {}, seal_id));
+        writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), base_id));
+
+        const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+        ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+            .life_epoch = 1,
+            .committed_through = base_id,
+            .checkpoint_snapshot_id = base_id,
+            .last_epoch_seal = seal_id})).outcome, PutOutcome::Done);
+
+        expectCode(
+            [&] { (void)recoverFromCurrentCatalogCut(*backend, layout, ns); },
+            DB::ErrorCodes::CORRUPTED_DATA);
+    };
+
+    expect_predecessor_rejected(RootNamespace{"srv1/base_predecessor_absent"}, false);
+    expect_predecessor_rejected(RootNamespace{"srv1/base_predecessor_not_seal"}, true);
+}
+
+}
diff --git a/src/Disks/tests/gtest_cas_recovery_streaming.cpp b/src/Disks/tests/gtest_cas_recovery_streaming.cpp
new file mode 100644
index 000000000000..ad2dbbcb9342
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_recovery_streaming.cpp
@@ -0,0 +1,647 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include "cas_test_helpers.h"
+
+#include <Common/Exception.h>
+#include <Common/scope_guard_safe.h>
+
+#include <atomic>
+#include <chrono>
+#include <cstdint>
+#include <future>
+#include <thread>
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int S3_ERROR;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+/// A deterministic accountant for the streaming-recovery memory probe: each recovery loop reports
+/// `+footprint` while one decoded transaction is resident and `-footprint` once it is discarded, so
+/// `peak` is the maximum summed decoded-transaction footprint ever resident at one instant. Streaming
+/// holds one transaction; the retired whole-tail materialiser -- and the test-local control that stands
+/// in for it -- held the entire tail. Deterministic (it accounts the footprints the probe is handed, a
+/// pure function of decoded content, not RSS), so it is stable under ASan quarantine noise.
+struct PeakTracker
+{
+    std::atomic<int64_t> alive_bytes{0};
+    std::atomic<int64_t> peak_bytes{0};
+
+    std::function<void(int64_t)> probe()
+    {
+        return [this](int64_t delta)
+        {
+            const int64_t now = alive_bytes.fetch_add(delta, std::memory_order_relaxed) + delta;
+            int64_t prev = peak_bytes.load(std::memory_order_relaxed);
+            while (now > prev && !peak_bytes.compare_exchange_weak(prev, now, std::memory_order_relaxed))
+            {
+            }
+        };
+    }
+
+    int64_t peak() const { return peak_bytes.load(std::memory_order_relaxed); }
+    int64_t alive() const { return alive_bytes.load(std::memory_order_relaxed); }
+};
+
+/// A distinct manifest per call: `build_sequence` carries the identity so every generated
+/// `(ref_name, manifest_ref)` add-precommit is a legal transition (no manifest is owned twice).
+ManifestRef mref(uint64_t seq)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = 1};
+}
+
+/// One maximum-shaped ref-log transaction: `num_ops` add-precommit ops over distinct
+/// `(ref_name, manifest_ref)` pairs (plus a leading `namespace_birth` for the first transaction of a
+/// never-born table). Each pair is unique across the whole tail (the running `manifest_seq`), so the
+/// tail replays cleanly and the candidate state simply grows -- the point is a large decoded body per
+/// transaction, which is what makes the whole-tail vector's resident footprint N times a single
+/// transaction's.
+RefLogTxn makeBigTxn(const String & ns, RefTxnId id, size_t num_ops, uint64_t & manifest_seq, bool birth)
+{
+    RefLogTxn txn;
+    txn.ns = ns;
+    txn.txn_id = id;
+    if (birth)
+        txn.ops.push_back(namespaceBirthOp());
+    for (size_t i = 0; i < num_ops; ++i)
+    {
+        const String ref_name = "rs_" + std::to_string(id.ref_sequence) + "_" + std::to_string(i);
+        txn.ops.push_back(ownerTransitionOp(
+            std::nullopt, RefOwnerBinding{RefOwnerKind::Precommit, ref_name, mref(manifest_seq)}));
+        ++manifest_seq;
+    }
+    return txn;
+}
+
+/// Seed `num_txns` maximum-shaped transactions at ids {1,1}..{1,num_txns} directly into `ns`'s `_log/`
+/// stream, and return the resident DECODED footprint (`decodedRefLogTxnFootprint`) of the largest single
+/// transaction plus the total across all. The largest single footprint is the streaming peak (one
+/// transaction resident at a time); the total is what a whole-tail materialiser holds resident at once.
+/// Footprint -- not the compressed stored size -- is the bound's currency: it is what actually sits in
+/// memory and what a materialising regression accumulates N-fold, and it is a deterministic function of
+/// the decoded content (identical whether computed on the built or the decoded transaction).
+struct SeededTail
+{
+    uint64_t max_single_footprint = 0;
+    uint64_t total_footprint = 0;
+};
+
+SeededTail seedBigTail(
+    InMemoryBackend & backend, const Layout & layout, const RootNamespace & ns,
+    size_t num_txns, size_t ops_per_txn, uint64_t & manifest_seq)
+{
+    SeededTail seeded;
+    for (size_t t = 0; t < num_txns; ++t)
+    {
+        const RefLogTxn txn = makeBigTxn(ns.string(), RefTxnId{1, t + 1}, ops_per_txn, manifest_seq, /*birth=*/t == 0);
+        const uint64_t footprint = decodedRefLogTxnFootprint(txn);
+        seeded.max_single_footprint = std::max(seeded.max_single_footprint, footprint);
+        seeded.total_footprint += footprint;
+        fixture::writeRefLogRaw(backend, layout, txn);
+    }
+    /// This helper always builds a recoverable `Live` life. Tests that need the distinct missing-
+    /// checkpoint corruption shape use the lower-level raw writers directly instead.
+    writeRecoverableCkptForRawFixture(
+        backend, layout, ns,
+        RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, static_cast<uint64_t>(num_txns)},
+                .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+    return seeded;
+}
+
+/// Bounded busy-wait on a test-observable predicate (the established `yield()`-poll idiom for recovery
+/// waiters, see `CasPool::refRecoveryWaitersForTest`). Bound is a generous wall-clock ceiling that only
+/// trips on a genuine hang, never in the normal fast path; returns false on timeout so the caller can
+/// release any blocked threads before asserting.
+template <typename Pred>
+bool pollUntil(Pred pred)
+{
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(30);
+    while (!pred())
+    {
+        if (std::chrono::steady_clock::now() > deadline)
+            return false;
+        std::this_thread::yield();
+    }
+    return true;
+}
+
+/// Backend that drops one selected `_log/` object on its FIRST GET (a concurrent-cleanup vanish),
+/// then serves it normally, and counts fresh (cursor-empty) LISTs of the ref prefix so a test can
+/// prove a stable checkpoint verdict did not spin on the advisory listing.
+class VanishMidTailOnceBackend : public InMemoryBackend
+{
+public:
+    using InMemoryBackend::get;   /// keep the one-arg convenience overload visible past our override
+
+    String target_log_key;
+    String refs_prefix;
+    std::atomic<bool> armed{false};
+    std::atomic<bool> vanished{false};
+    std::atomic<int> fresh_list_count{0};
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (armed.load() && key == target_log_key && !vanished.exchange(true))
+            return std::nullopt;   /// selected object gone between LIST and GET; recovery must re-LIST
+        return InMemoryBackend::get(key, range);
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (armed.load() && prefix == refs_prefix && cursor.empty())
+            fresh_list_count.fetch_add(1, std::memory_order_relaxed);
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+};
+
+/// Backend that replaces one selected `_log/` object's body with a valid-but-foreign ref-log object
+/// (a different namespace in the body): decoding it fails with CORRUPTED_DATA (body/key mismatch), the
+/// durable-corruption class recovery must fail fast on -- no re-LIST loop.
+class CorruptLogOnGetBackend : public InMemoryBackend
+{
+public:
+    using InMemoryBackend::get;   /// keep the one-arg convenience overload visible past our override
+
+    String target_log_key;
+    String corrupt_bytes;
+    String refs_prefix;
+    std::atomic<bool> armed{false};
+    std::atomic<int> refs_list_count{0};
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        auto got = InMemoryBackend::get(key, range);
+        if (armed.load() && got && key == target_log_key)
+            got->bytes = corrupt_bytes;
+        return got;
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (armed.load() && prefix == refs_prefix && cursor.empty())
+            refs_list_count.fetch_add(1, std::memory_order_relaxed);
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+};
+
+/// Backend that blocks the first exact log GET while recovery holds no state lock. A concurrent second
+/// caller can then reach `recovery_cv`, while the LIST counter proves neither caller enumerates the
+/// recovery stream.
+class BlockingFirstLogGetBackend : public InMemoryBackend
+{
+public:
+    using InMemoryBackend::get;
+
+    String refs_prefix;
+    String target_log_key;
+    std::atomic<bool> armed{false};
+    std::atomic<bool> blocked{false};
+    std::atomic<int> list_calls{0};
+    std::function<void()> on_first_target_get;
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (armed.load() && key == target_log_key && !blocked.exchange(true))
+            on_first_target_get();
+        return InMemoryBackend::get(key, range);
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (armed.load() && prefix == refs_prefix && cursor.empty())
+            list_calls.fetch_add(1, std::memory_order_relaxed);
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+};
+
+}
+
+/// Test 14 (load-bearing memory bound): a long tail of maximum-shaped transactions replays under a hard
+/// peak bound (twice the largest single transaction's decoded footprint) that a whole-tail materialiser
+/// -- which holds every decoded transaction resident at once -- provably exceeds. The bound is computed
+/// from the fixture's own footprints and the whole-tail total is asserted to exceed it, so the bound is
+/// a property of the fixture, not a lucky constant. Its materialising counterpart,
+/// `MaterializingControlExceedsMemoryBound`, trips this same bound.
+TEST(CASRecoveryStreaming, LongTailReplaysUnderMemoryBound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+    seedPoolMetaForRestart(*backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    constexpr size_t kTxns = 24;
+    constexpr size_t kOpsPerTxn = 250;
+    uint64_t manifest_seq = 1;
+    const SeededTail seeded = seedBigTail(*backend, layout, ns, kTxns, kOpsPerTxn, manifest_seq);
+
+    const uint64_t bound = 2 * seeded.max_single_footprint;
+    ASSERT_GT(seeded.total_footprint, bound)
+        << "fixture must make the whole tail (" << seeded.total_footprint
+        << " B) provably exceed the bound (" << bound << " B)";
+
+    PeakTracker tracker;
+    setRecoveryReplayMemoryProbeForTest(tracker.probe());
+    SCOPE_EXIT({ setRecoveryReplayMemoryProbeForTest({}); });
+
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(*backend, layout);
+    const RefTableState state = recoverRefTableDetailedAtCatalogCutForTest(*backend, layout, catalog_cut, ns).state;
+    EXPECT_EQ(state.getPrecommits().size(), kTxns * kOpsPerTxn) << "the whole tail must have replayed";
+    EXPECT_LE(tracker.peak(), static_cast<int64_t>(bound))
+        << "streaming recovery must hold at most ~one decoded transaction (peak " << tracker.peak()
+        << " B) not the whole " << kTxns << "-transaction tail (" << seeded.total_footprint << " B)";
+    /// Lower bound (the accountant's fail-close): the peak must reach at least one whole decoded
+    /// transaction's footprint. This couples the assertion to the production report calls -- delete them
+    /// and the peak collapses to zero, failing HERE instead of passing vacuously under the upper bound.
+    EXPECT_GE(tracker.peak(), static_cast<int64_t>(seeded.max_single_footprint))
+        << "the probe must observe at least one whole decoded transaction resident (peak " << tracker.peak()
+        << " B, one transaction " << seeded.max_single_footprint
+        << " B) -- a zero peak means the production report calls were removed and the bound guards nothing";
+    EXPECT_EQ(tracker.alive(), 0) << "every decoded transaction must be discarded after it is applied";
+}
+
+/// Test 14 (materialising RED control): the discriminating counterpart to the streaming bound above. A
+/// control that GETs+decodes the WHOLE tail into a vector BEFORE applying it -- the retired whole-tail
+/// shape -- holds every decoded transaction resident at once. Driven through the SAME memory probe as
+/// streaming recovery, its peak must EXCEED the same bound the streaming path stays under. This is the
+/// regression the memory guard exists to catch, and the guard discriminates precisely because the probe
+/// now accounts the caller's whole resident set (each decoded transaction for the span it is held), not
+/// one apply in isolation. Under the retired stored-byte-in-`applyOne` probe this control's peak stayed
+/// at one transaction (see the RED capture in the round-2 fix report); it now correctly trips.
+TEST(CASRecoveryStreaming, MaterializingControlExceedsMemoryBound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const Layout layout("p");
+    seedPoolMetaForRestart(*backend);
+    const RootNamespace ns{"00/aa@cas@"};
+
+    constexpr size_t kTxns = 24;
+    constexpr size_t kOpsPerTxn = 250;
+    uint64_t manifest_seq = 1;
+    const SeededTail seeded = seedBigTail(*backend, layout, ns, kTxns, kOpsPerTxn, manifest_seq);
+
+    const uint64_t bound = 2 * seeded.max_single_footprint;
+    ASSERT_GT(seeded.total_footprint, bound)
+        << "fixture must make the whole tail (" << seeded.total_footprint
+        << " B) provably exceed the bound (" << bound << " B)";
+
+    PeakTracker tracker;
+    setRecoveryReplayMemoryProbeForTest(tracker.probe());
+    SCOPE_EXIT({ setRecoveryReplayMemoryProbeForTest({}); });
+
+    /// Materialise the WHOLE tail first (retired shape): every decoded transaction stays resident in
+    /// `resident_txns`, and its footprint is reported to the probe up front, released only AFTER the whole
+    /// tail has been applied -- exactly the memory profile streaming recovery replaced.
+    std::vector<RefLogTxn> resident_txns;
+    int64_t held = 0;
+    for (size_t t = 1; t <= kTxns; ++t)
+    {
+        const auto got = backend->get(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, t}));
+        ASSERT_TRUE(got.has_value());
+        RefLogTxn txn = decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), RefTxnId{1, t});
+        const int64_t footprint = static_cast<int64_t>(decodedRefLogTxnFootprint(txn));
+        reportReplayMemoryDelta(footprint);
+        held += footprint;
+        resident_txns.push_back(std::move(txn));
+    }
+
+    RefReplayBuilder builder(std::nullopt);
+    for (RefLogTxn & txn : resident_txns)
+        builder.applyOne(std::move(txn), 0);
+    const RecoveryResult result = std::move(builder).finish();
+    EXPECT_EQ(result.state.getPrecommits().size(), kTxns * kOpsPerTxn) << "the whole tail must have applied";
+
+    EXPECT_GT(tracker.peak(), static_cast<int64_t>(bound))
+        << "the materialising control holds the whole tail resident; the probe must exceed the "
+           "single-transaction bound (peak " << tracker.peak() << " B, bound " << bound << " B)";
+
+    reportReplayMemoryDelta(-held);   /// release the whole tail
+    EXPECT_EQ(tracker.alive(), 0);
+}
+
+/// Test 14 (vanished-selected-object leg): once the exact `_ckpt` commits a finite frontier, a missing
+/// record inside it is not something a fresh LIST may reinterpret as a shorter stream. With the same
+/// checkpoint token still durable, recovery fails closed immediately instead of accepting incomplete
+/// state or spinning on an advisory enumeration.
+TEST(CASRecoveryStreaming, MidTailVanishedObjectFailsClosedAgainstStableAuthority)
+{
+    auto backend = std::make_shared<VanishMidTailOnceBackend>();
+    seedPoolMetaForRestart(*backend);
+    const Layout layout("p");
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const uint64_t seq1 = publishCommittedTransition(*backend, layout, ns, "a", std::nullopt, mref(1));
+    const uint64_t seq2 = publishCommittedTransition(*backend, layout, ns, "b", std::nullopt, mref(2));
+    const uint64_t seq3 = publishCommittedTransition(*backend, layout, ns, "c", std::nullopt, mref(3));
+    ASSERT_LT(seq1, seq2);
+    ASSERT_LT(seq2, seq3);
+    /// Semantic publication already durably advances the exact checkpoint frontier to `seq3`.
+
+    auto store = openPoolForTest(backend);
+    backend->refs_prefix = layout.namespaceStreamPrefix(fixture::fixtureLife(ns));
+    backend->target_log_key = layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, seq2});   /// vanish a mid-tail object
+    backend->armed = true;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)store->listRefs(ns); });
+    EXPECT_TRUE(backend->vanished.load()) << "the selected committed object must actually have vanished";
+    EXPECT_EQ(backend->fresh_list_count.load(), 0)
+        << "the exact checkpoint frontier makes recovery stream enumeration unnecessary";
+}
+
+/// Test 14 (durable-corruption leg): a `_log/` object whose body decodes to a foreign namespace is
+/// durable corruption, not a transient vanish -- recovery discards the candidate and fails fast with
+/// no re-LIST loop. Asserts the throw, zero restarts, and a single LIST.
+TEST(CASRecoveryStreaming, CorruptObjectFailsFast)
+{
+    auto backend = std::make_shared<CorruptLogOnGetBackend>();
+    seedPoolMetaForRestart(*backend);
+    const Layout layout("p");
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishCommittedTransition(*backend, layout, ns, "a", std::nullopt, mref(1));
+    const uint64_t seq2 = publishCommittedTransition(*backend, layout, ns, "b", std::nullopt, mref(2));
+    publishCommittedTransition(*backend, layout, ns, "c", std::nullopt, mref(3));
+    /// Semantic publication already durably advances the exact checkpoint frontier.
+
+    /// A structurally valid ref-log object for a DIFFERENT namespace: it decompresses and parses, but
+    /// its body namespace does not match the key, which `decodeRefLogTxn` rejects as CORRUPTED_DATA.
+    RefLogTxn foreign;
+    foreign.ns = "99/zz@cas@";
+    foreign.txn_id = RefTxnId{1, seq2};
+    foreign.ops = {namespaceBirthOp()};
+    backend->corrupt_bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(foreign));
+
+    auto store = openPoolForTest(backend);
+    backend->refs_prefix = layout.namespaceStreamPrefix(fixture::fixtureLife(ns));
+    backend->target_log_key = layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, seq2});
+    backend->armed = true;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->resolveRef(ns, "a"); });
+    /// The exact checkpoint frontier determines this GET. A corrupt committed object fails fast without
+    /// asking a stream enumeration to reinterpret the durable recovery boundary.
+    EXPECT_EQ(backend->refs_list_count.load(), 0) << "durable corruption must not trigger recovery LIST";
+}
+
+/// Test 14 (concurrent-waiter leg): while one caller is blocked in recovery's unlocked exact-log GET,
+/// a second caller for the same table parks on `recovery_cv` and is woken exactly once when recovery
+/// completes. Neither caller may race an independent stream LIST.
+TEST(CASRecoveryStreaming, ConcurrentWaiterUnblockedOnce)
+{
+    auto backend = std::make_shared<BlockingFirstLogGetBackend>();
+    seedPoolMetaForRestart(*backend);
+    const Layout layout("p");
+    const RootNamespace ns{"00/aa@cas@"};
+
+    publishCommittedTransition(*backend, layout, ns, "x", std::nullopt, mref(1));
+    publishCommittedTransition(*backend, layout, ns, "y", std::nullopt, mref(2));
+    /// Semantic publication already durably advances the exact checkpoint frontier.
+
+    auto store = openPoolForTest(backend);
+    backend->refs_prefix = layout.namespaceStreamPrefix(fixture::fixtureLife(ns));
+
+    /// Gate the first exact replay GET. The leader reaches it with `state_mutex` released, which is
+    /// the window in which a second caller must be able to park on `recovery_cv`.
+    std::atomic<bool> get_entered{false};
+    std::promise<void> entered_promise;
+    std::promise<void> release_promise;
+    std::shared_future<void> release_future = release_promise.get_future().share();
+    backend->target_log_key = layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 1});
+    backend->on_first_target_get = [&]
+    {
+        if (!get_entered.exchange(true))
+            entered_promise.set_value();
+        release_future.wait();
+    };
+    backend->armed = true;
+
+    std::thread t1([&] { store->listRefs(ns); });   /// exact GET blocks with recovery unlocked
+    entered_promise.get_future().wait();
+
+    std::thread t2([&] { store->listRefs(ns); });    /// second caller must park on recovery_cv
+    const bool parked = pollUntil([&] { return store->refRecoveryWaitersForTest(ns) >= 1; });
+
+    release_promise.set_value();
+    t1.join();
+    t2.join();
+
+    EXPECT_TRUE(parked) << "the second caller must reach recovery_cv while the first is in the retry window";
+    EXPECT_EQ(store->refRecoveryWaitersForTest(ns), 0u) << "no phantom waiter after recovery completes";
+    EXPECT_TRUE(store->resolveRef(ns, "x").has_value());
+    EXPECT_TRUE(store->resolveRef(ns, "y").has_value());
+    EXPECT_EQ(backend->list_calls.load(), 0)
+        << "the leader and parked waiter must both recover without stream enumeration";
+}
+
+/// Test 14 (other materializers leg): the orphan-sweep recovery (`recoverRefTableDetailedFromAuthority`)
+/// and fsck's exact-authority recovery stream through the SAME builder and hold under the SAME
+/// per-transaction bound as primary recovery.
+TEST(CASRecoveryStreaming, OrphanSweepAndFsckSameBound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    seedPoolMetaForRestart(*backend);
+    const Layout layout("p");
+    const RootNamespace ns_sweep{"00/sweep@cas@"};
+    const RootNamespace ns_fsck{"00/fsck@cas@"};
+
+    constexpr size_t kTxns = 16;
+    constexpr size_t kOpsPerTxn = 250;
+    uint64_t manifest_seq = 1;
+    const SeededTail sweep_tail = seedBigTail(*backend, layout, ns_sweep, kTxns, kOpsPerTxn, manifest_seq);
+    const SeededTail fsck_tail = seedBigTail(*backend, layout, ns_fsck, kTxns, kOpsPerTxn, manifest_seq);
+
+    const uint64_t max_single = std::max(sweep_tail.max_single_footprint, fsck_tail.max_single_footprint);
+    const uint64_t bound = 2 * max_single;
+    ASSERT_GT(sweep_tail.total_footprint, bound);
+    ASSERT_GT(fsck_tail.total_footprint, bound);
+
+    auto store = openPoolForTest(backend);
+
+    {
+        PeakTracker tracker;
+        setRecoveryReplayMemoryProbeForTest(tracker.probe());
+        SCOPE_EXIT({ setRecoveryReplayMemoryProbeForTest({}); });
+        const CasRefCatalog::Snapshot sweep_catalog_cut = CasRefCatalog::read(*backend, layout);
+        const RecoveredRefTable recovered =
+            recoverRefTableDetailedAtCatalogCutForTest(*backend, layout, sweep_catalog_cut, ns_sweep);
+        EXPECT_EQ(recovered.state.getPrecommits().size(), kTxns * kOpsPerTxn);
+        EXPECT_LE(tracker.peak(), static_cast<int64_t>(bound))
+            << "orphan-sweep recovery must stream: peak " << tracker.peak() << " B, bound " << bound << " B";
+        EXPECT_GE(tracker.peak(), static_cast<int64_t>(sweep_tail.max_single_footprint))
+            << "the probe must observe at least one decoded sweep transaction (peak " << tracker.peak()
+            << " B) -- a zero peak means the orphan-sweep report call was silently removed";
+        EXPECT_EQ(tracker.alive(), 0);
+    }
+
+    {
+        PeakTracker tracker;
+        setRecoveryReplayMemoryProbeForTest(tracker.probe());
+        SCOPE_EXIT({ setRecoveryReplayMemoryProbeForTest({}); });
+        const FsckReport report = runFsck(*store, /*detail=*/true);
+        EXPECT_TRUE(report.clean());
+        EXPECT_LE(tracker.peak(), static_cast<int64_t>(bound))
+            << "fsck exact-authority recovery must stream: peak " << tracker.peak() << " B, bound " << bound << " B";
+        EXPECT_GE(tracker.peak(), static_cast<int64_t>(fsck_tail.max_single_footprint))
+            << "the probe must observe at least one decoded fsck-recovery transaction (peak " << tracker.peak()
+            << " B) -- a zero peak means fsck stopped recovering catalog-authoritative namespaces";
+        EXPECT_EQ(tracker.alive(), 0);
+    }
+}
+
+/// Test 14 (writer-ledger leg -- the production recovery path): the writer ledger's OWN recovery loop
+/// (`CasRefLedger::ensureRefTableRecovered`, reached through any Pool touch) must stream the tail under
+/// the SAME per-transaction bound the free recovery does. This is the exact production path the original
+/// memory finding named; `LongTailReplaysUnderMemoryBound` above exercises the free authoritative recovery,
+/// NOT the ledger loop, so the ledger could regress to whole-tail materialisation while every other
+/// bound stayed green. Recovery is driven through the production non-minting namespace-file read path,
+/// which does NOT dispatch the stale-precommit sweep `listRefs` would (that sweep
+/// would append removals over the seeded epoch-1 precommit bindings and perturb both the count and the
+/// probe). The whole tail sits above a never-born base, so the retained tail count equals the whole tail.
+TEST(CASRecoveryStreaming, LedgerRecoveryReplaysUnderMemoryBound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    seedPoolMetaForRestart(*backend);
+    const Layout layout("p");
+    const RootNamespace ns{"00/ledger@cas@"};
+
+    constexpr size_t kTxns = 24;
+    constexpr size_t kOpsPerTxn = 250;
+    uint64_t manifest_seq = 1;
+    const SeededTail seeded = seedBigTail(*backend, layout, ns, kTxns, kOpsPerTxn, manifest_seq);
+
+    const uint64_t bound = 2 * seeded.max_single_footprint;
+    ASSERT_GT(seeded.total_footprint, bound)
+        << "fixture must make the whole tail (" << seeded.total_footprint
+        << " B) provably exceed the bound (" << bound << " B)";
+
+    auto store = openPoolForTest(backend);
+
+    PeakTracker tracker;
+    setRecoveryReplayMemoryProbeForTest(tracker.probe());
+    SCOPE_EXIT({ setRecoveryReplayMemoryProbeForTest({}); });
+
+    /// The production Task 4b reader drives `CasRefLedger::ensureRefTableRecovered` without the
+    /// stale-precommit sweep; the resident-only observer then reads the retained tail count.
+    ASSERT_TRUE(store->namespaceFilesLifeIfReadable(ns));
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), kTxns)
+        << "the whole tail must have replayed through the ledger's own recovery loop";
+    EXPECT_LE(tracker.peak(), static_cast<int64_t>(bound))
+        << "ledger recovery must hold at most ~one decoded transaction (peak " << tracker.peak()
+        << " B) not the whole " << kTxns << "-transaction tail (" << seeded.total_footprint << " B)";
+    /// Lower bound (the accountant's fail-close): a zero peak means the ledger loop's production report
+    /// call was removed and this bound would guard nothing -- the exact silent-decoupling this leg exists
+    /// to catch on the production path.
+    EXPECT_GE(tracker.peak(), static_cast<int64_t>(seeded.max_single_footprint))
+        << "the probe must observe at least one whole decoded transaction resident (peak " << tracker.peak()
+        << " B, one transaction " << seeded.max_single_footprint
+        << " B) -- a zero peak means the ledger's production report call was removed and the bound guards nothing";
+    EXPECT_EQ(tracker.alive(), 0) << "every decoded transaction must be discarded after it is applied";
+}
+
+/// Test 15 (publication inventory): after streaming recovery of a table with a non-trivial snapshot
+/// base, precommit bindings, and a tail of committed transactions, EVERY field the
+/// recovery publication seeds is asserted -- not just the two a prose inventory would keep. This is a
+/// regression guard: streaming recovery must install exactly what the whole-tail recovery installed.
+TEST(CASRecoveryStreaming, RecoveryResultInventoryComplete)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    seedPoolMetaForRestart(*backend);
+    const Layout layout("p");
+    const RootNamespace ns{"00/inv@cas@"};
+
+    /// A non-trivial base snapshot: two committed rows plus a stale predecessor precommit binding.
+    RefTableSnapshot base;
+    base.ns = ns.string();
+    base.snapshot_id = RefTxnId{1, 5};
+    base.committed = {committedRow("c_one", mref(11)), committedRow("c_two", mref(12))};
+    base.precommits = {RefOwnerBinding{RefOwnerKind::Precommit, "p_stale", mref(13)}};
+    RefLogTxn base_txn;
+    base_txn.ns = ns.string();
+    base_txn.txn_id = base.snapshot_id;
+    base_txn.ops = publishCommittedOps("c_two", mref(12));
+    fixture::writeRefLogRaw(*backend, layout, base_txn);
+    writeRefSnapshotRaw(*backend, layout, base);
+    const auto base_got = backend->get(layout.refSnapshotKey(fixture::fixtureLife(ns), base.snapshot_id));
+    ASSERT_TRUE(base_got.has_value());
+    const uint64_t base_stored_bytes = base_got->bytes.size();
+
+    /// Two committed transactions strictly above the base -- the tail.
+    RefLogTxn t6;
+    t6.ns = ns.string();
+    t6.txn_id = RefTxnId{1, 6};
+    t6.ops = publishCommittedOps("c_three", mref(21));
+    fixture::writeRefLogRaw(*backend, layout, t6);
+    RefLogTxn t7;
+    t7.ns = ns.string();
+    t7.txn_id = RefTxnId{1, 7};
+    t7.ops = publishCommittedOps("c_four", mref(22));
+    fixture::writeRefLogRaw(*backend, layout, t7);
+
+    writeRecoverableCkptForRawFixture(
+        *backend, layout, ns, RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, 7},
+                                       .checkpoint_snapshot_id = RefTxnId{1, 5},
+                                       .last_epoch_seal = std::nullopt});
+
+    const uint64_t tail6 = backend->get(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 6}))->bytes.size();
+    const uint64_t tail7 = backend->get(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, 7}))->bytes.size();
+
+    backend->resetCounts();
+    auto store = openPoolForTest(backend);
+
+    /// Drive recovery via the production namespace-file reader WITHOUT the stale-precommit sweep that
+    /// `resolveRef`/`listRefs` dispatch (that
+    /// sweep would clear `needs_stale_precommit_sweep` before it could be observed). Every inventory
+    /// field below is then read straight off the seeded runtime, and the read-path state assertion is
+    /// left for LAST (after `needs_stale_precommit_sweep` has been observed).
+
+    ASSERT_TRUE(store->namespaceFilesLifeIfReadable(ns));
+    const NamespaceLifeId life = fixture::fixtureLife(ns);
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, base.snapshot_id)), 1u)
+        << "recovery must validate the selected base's matching ordinary log";
+    EXPECT_EQ(backend->getCount(layout.refSnapshotKey(life, base.snapshot_id)), 1u)
+        << "the inventory must come from the selected snapshot, not a pre-snapshot failure";
+
+    /// newest snapshot identity: the recovered base id, no seal on this clean mount.
+    EXPECT_EQ(store->newestPublishedSnapshotIdForTest(ns), std::optional<RefTxnId>(base.snapshot_id));
+
+    /// last_epoch_seal: this mount's live epoch is the one the seeded stream was written in, so the
+    /// CAS-walk crossed no epoch transition and installed no chain link. Pins that the field IS part of
+    /// the published inventory (its non-empty counterpart lives in
+    /// `CASRefRecoveryCasWalk.DeadEpochIsClosedByOurOwnSealAtTPlusOne`).
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::nullopt);
+
+    /// stale-precommit sweep: recovery always arms it (asserted BEFORE any read-side sweep runs).
+    EXPECT_TRUE(store->needsStalePrecommitSweepForTest(ns));
+
+    /// tail count / bytes: exactly the two transactions above the base and their stored sizes.
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 2u);
+    EXPECT_EQ(store->refTailBytesSinceSnapshotForTest(ns), tail6 + tail7);
+
+    /// base snapshot bytes: the encoded body size of the recovered base snapshot.
+    EXPECT_EQ(store->refBaseSnapshotBytesForTest(ns), base_stored_bytes);
+
+    /// admission budgets: the raw hard limits minus this table's wire overhead and the safety margin.
+    const uint64_t overhead = 4 + ns.string().size() + 4096;
+    const uint64_t expected_budget = 64ULL * 1024 * 1024 - overhead;
+    EXPECT_EQ(store->refSnapshotBudgetForTest(ns), expected_budget);
+    EXPECT_EQ(store->refRemovalBudgetForTest(ns), expected_budget);
+
+    /// state: four committed rows (two from the base, two from the tail). This read dispatches the
+    /// read-side sweep, hence it comes last -- after the sweep flag has been observed above.
+    const auto refs = store->listRefs(ns);
+    EXPECT_EQ(refs.size(), 4u);
+    EXPECT_TRUE(store->resolveRef(ns, "c_one").has_value());
+    EXPECT_TRUE(store->resolveRef(ns, "c_two").has_value());
+    EXPECT_TRUE(store->resolveRef(ns, "c_three").has_value());
+    EXPECT_TRUE(store->resolveRef(ns, "c_four").has_value());
+}
diff --git a/src/Disks/tests/gtest_cas_ref_carve.cpp b/src/Disks/tests/gtest_cas_ref_carve.cpp
new file mode 100644
index 000000000000..e12706d64f69
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_carve.cpp
@@ -0,0 +1,366 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <exception>
+#include <future>
+#include <memory>
+#include <mutex>
+#include <new>
+#include <optional>
+#include <thread>
+#include <vector>
+
+/// Task 7 (stage-1 §2): the ref-flush two-phase carve and the validation loop's publish ordering.
+///
+/// Two exception-safety windows are pinned here, both driven through the `setCarveHookForTest` fault
+/// seam (which fires `std::bad_alloc` at named carve/validation phase points):
+///
+///  - The carve must PLAN (scan `pending` without popping; build the selection and every reservation)
+///    and only then PUBLISH (pop + append under the same continuous `ref_queue_mutex` hold, using only
+///    non-throwing moves/copies). A throw anywhere in the plan must leave the queue byte-for-byte intact
+///    so no already-selected item is stranded (removed from `pending` yet never completed) and no waiter
+///    hangs. The pre-fix carve interleaved pops with the allocating `seen_refs`/`batch` growth, so a
+///    throw after the first pop stranded popped items and hung their waiters forever — the behavioural
+///    signature this suite demonstrates.
+///  - The per-item validation loop must reserve `final_ops`/`survivors` growth BEFORE applying the item
+///    to `working`, and publish only past all throwing points. The pre-fix loop moved `working` before
+///    those allocations, so a failure there left a failed item's effects in `working` and — when the
+///    throw fell between the two accumulator writes — its ops already in the durably-committed
+///    transaction while its own caller was told the append failed.
+///
+/// The suite name is prefixed `RefWriter` so it is covered by the `RefWriter*` unit-test gate filter.
+
+using namespace DB::Cas;
+using Phase = CasRefLedger::CarvePhaseForTest;
+
+namespace
+{
+
+PoolPtr openPool(const BackendPtr & backend)
+{
+    /// A fresh pool with no residue: `seedPoolMetaForRestart` is idempotent and a no-op here (mirrors the
+    /// ref-writer suite's `openPool`), it just lets `beginPartWrite` bootstrap over a valid `_pool_meta`.
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// A legal blob-free part: stage an empty manifest, precommit, promote — enough to leave one committed
+/// ref in `ns` that a later `dropRef` can co-batch. Mirrors the ref-writer suite's `publishEmptyPart`.
+///
+/// Stage B (Task 4-C): pin `ns` to the sentinel before the first real touch, mirroring the same fix in
+/// `gtest_cas_ref_writer.cpp`'s `startBuildFor` and `gtest_cas_ref_chunked_flush.cpp`'s
+/// `publishEmptyPart` -- every test in this file births its namespace here before any fault
+/// injection/verification that separately computes a key via `DB::Cas::tests::fixture::fixtureLife(ns)`.
+void publishEmptyPart(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    DB::Cas::tests::casAdmitRecoverableEntry(s->backend(), s->layout(), ns, s->liveWriterEpoch());
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+}
+
+/// Shared, heap-backed synchronisation state for one case. Heap-backing (captured by `shared_ptr` into
+/// the hooks and caller threads) is what makes a leaked/detached hung thread safe on the RED path: the
+/// thread keeps its own references alive, so nothing it touches is destroyed underneath it.
+struct CaseSync
+{
+    std::mutex m;
+    std::condition_variable cv;
+    bool entered = false;                        /// guarded by m: the first flush reached the pre-carve hook
+    /// Per-`CarvePhaseForTest` invocation counter, indexed by `static_cast<int>(phase)`. Sized off the
+    /// enum's last enumerator rather than a literal: the array was already undersized once (it predates
+    /// `ChunkReseed`; `PostDurableInstall` and `PostInstallPreAck` followed), and it is out of bounds only
+    /// because every call site happens to filter to a lower-numbered phase first -- a trap for the next
+    /// phase added.
+    std::atomic<int> phase_hits[static_cast<size_t>(CasRefLedger::CarvePhaseForTest::PostInstallPreAck) + 1] = {};
+};
+
+/// The newest `_log/` transaction currently present for `ns`, decoded from the backend directly (no Pool
+/// cache). Used to inspect exactly what a flush durably committed.
+std::optional<RefLogTxn> newestLogTxn(DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const RootNamespace & ns)
+{
+    std::optional<RefTxnId> newest;
+    String cursor;
+    for (;;)
+    {
+        const ListPage page = backend.list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+        for (const ListedKey & lk : page.keys)
+        {
+            const auto parsed = layout.parseRefObjectKey(lk.key);
+            if (parsed && parsed->life_id == DB::Cas::tests::fixture::fixtureLife(ns).incarnation
+                && parsed->kind == RefObjectKind::Log
+                && (!newest || *newest < parsed->txn_id))
+                newest = parsed->txn_id;
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    if (!newest)
+        return std::nullopt;
+    const auto got = backend.get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), *newest));
+    if (!got)
+        return std::nullopt;
+    return decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), *newest);
+}
+
+/// Counts `OwnerTransition` removal ops (old binding present, no new binding) naming `ref_name` across
+/// EVERY committed `_log/` transaction of `ns`.
+size_t committedRemovalCountForRef(DB::Cas::Backend & backend, const DB::Cas::Layout & layout,
+                                   const RootNamespace & ns, const String & ref_name)
+{
+    size_t count = 0;
+    String cursor;
+    for (;;)
+    {
+        const ListPage page = backend.list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+        for (const ListedKey & lk : page.keys)
+        {
+            const auto parsed = layout.parseRefObjectKey(lk.key);
+            if (!parsed || parsed->life_id != DB::Cas::tests::fixture::fixtureLife(ns).incarnation
+                || parsed->kind != RefObjectKind::Log)
+                continue;
+            const auto got = backend.get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), parsed->txn_id));
+            if (!got)
+                continue;
+            const RefLogTxn txn = decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), parsed->txn_id);
+            for (const RefOp & op : txn.ops)
+                if (op.kind == RefOpKind::OwnerTransition && op.old_binding.has_value()
+                    && !op.new_binding.has_value() && op.old_binding->ref_name == ref_name)
+                    ++count;
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    return count;
+}
+
+/// One queued append driven on its own thread, with a future that becomes ready only when the caller's
+/// `dropRef` RETURNS (normally or by throwing). A caller whose item was stranded never returns, so its
+/// future stays not-ready — a bounded `wait_for` on it is the hung-waiter detector.
+struct Caller
+{
+    std::thread t;
+    std::future<std::exception_ptr> fut;
+    String ref;
+};
+
+Caller launchDrop(const PoolPtr & store, const RootNamespace & ns, const String & ref)
+{
+    auto prom = std::make_shared<std::promise<std::exception_ptr>>();
+    std::future<std::exception_ptr> fut = prom->get_future();
+    std::thread t([store, ns, ref, prom]
+    {
+        std::exception_ptr err;
+        try { store->dropRef(ns, ref); }
+        catch (...) { err = std::current_exception(); }
+        prom->set_value(err);
+    });
+    return Caller{std::move(t), std::move(fut), ref};
+}
+
+/// Stages three compatible drops (leader "a" plus followers "b","c") into one carve, injects
+/// `std::bad_alloc` at `target_phase` on its `target_ordinal`-th firing, then asserts that no caller
+/// hangs and the queue drains. Returns true iff a caller hung (the stranded-item signature).
+///
+/// On the fixed (two-phase) carve, a plan-phase throw pops nothing: the leader's own item is failed by
+/// the leadership-exit guard and the untouched followers commit on the next leader's retry. On the
+/// pre-fix interleaved carve, the same throw strands already-popped followers, whose waiters hang.
+bool runPlanPointCase(Phase target_phase, int target_ordinal, const char * label)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{String("srv1/carve_") + label};
+    publishEmptyPart(store, ns, "a");
+    publishEmptyPart(store, ns, "b");
+    publishEmptyPart(store, ns, "c");
+
+    auto sync = std::make_shared<CaseSync>();
+    /// Block the first flush's leader in the pre-carve window until all three items are queued, forcing a
+    /// deterministic three-item batch. Heap-backed captures (see `CaseSync`) keep this safe even if a
+    /// stranded follower later spins through it on the RED path.
+    store->setRefPreCarveHookForTest([sync, store, ns]
+    {
+        std::unique_lock lk(sync->m);
+        if (sync->entered)
+            return;   /// only the first carve blocks; retries proceed straight through
+        sync->entered = true;
+        sync->cv.notify_all();
+        /// Bounded (10s) so a staging bug bounds the wait instead of blocking the whole suite; the
+        /// predicate is normally satisfied well before the deadline.
+        sync->cv.wait_for(lk, std::chrono::seconds(10), [&] { return store->refQueuePendingForTest(ns) >= 3; });
+    });
+    store->setCarveHookForTest([sync, target_phase, target_ordinal](Phase ph)
+    {
+        if (ph != target_phase)
+            return;
+        if (sync->phase_hits[static_cast<int>(ph)].fetch_add(1) + 1 == target_ordinal)
+            throw std::bad_alloc{};
+    });
+
+    Caller ca = launchDrop(store, ns, "a");
+    {
+        std::unique_lock lk(sync->m);
+        sync->cv.wait_for(lk, std::chrono::seconds(10), [&] { return sync->entered; });
+    }
+    Caller cb = launchDrop(store, ns, "b");
+    Caller cc = launchDrop(store, ns, "c");
+    {
+        const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(10);
+        while (store->refQueuePendingForTest(ns) < 3 && std::chrono::steady_clock::now() < deadline)
+            std::this_thread::yield();
+    }
+    sync->cv.notify_all();   /// release the pre-carve hook now its (>=3 pending) predicate holds
+
+    bool hung = false;
+    std::vector<Caller *> callers = {&ca, &cb, &cc};
+    for (Caller * c : callers)
+    {
+        /// Bounded (5s): a stranded item never completes, so this is where the hang surfaces.
+        if (c->fut.wait_for(std::chrono::seconds(5)) != std::future_status::ready)
+        {
+            hung = true;
+            EXPECT_TRUE(false) << label << ": caller for ref '" << c->ref
+                               << "' never returned within 5s — its item was stranded (removed from "
+                                  "pending, never completed) and the waiter hung";
+            /// Cannot join a permanently-hung thread. Detach it; its heap-backed captures (including a
+            /// `store` copy) keep everything it touches alive until the process exits. Leave the hooks
+            /// installed — clearing them here would race the detached thread's `std::function` read.
+            c->t.detach();
+        }
+        else
+        {
+            c->t.join();
+        }
+    }
+    if (hung)
+        return true;
+
+    /// GREEN: everything joined, so clearing the hooks now cannot race any live flush.
+    store->setRefPreCarveHookForTest(nullptr);
+    store->setCarveHookForTest(nullptr);
+    EXPECT_EQ(store->refQueuePendingForTest(ns), 0u) << label << ": the append queue did not fully drain";
+    /// The leader ("a") is the item whose plan threw; the guard fails it, so its drop must NOT commit.
+    EXPECT_TRUE(store->resolveRef(ns, "a").has_value())
+        << label << ": the failed leader's drop must not have committed";
+    /// The untouched followers commit on retry.
+    EXPECT_FALSE(store->resolveRef(ns, "b").has_value()) << label << ": survivor 'b' must have been dropped";
+    EXPECT_FALSE(store->resolveRef(ns, "c").has_value()) << label << ": survivor 'c' must have been dropped";
+    return false;
+}
+
+}
+
+/// Test 7: a throw at any plan-phase point of the carve must leave the queue intact and hang no waiter.
+/// RED on the pre-fix interleaved carve (a plan-point throw after the first pop strands followers whose
+/// waiters then hang, tripping the 5s bounded wait); GREEN on the two-phase carve.
+TEST(CASRefWriterCarve, CarveThrowLeavesQueueIntact)
+{
+    /// `PlanSeenRefs`/`PlanBatchGrow` fire once per scanned item; injecting on the third firing strands a
+    /// popped follower under the old carve. `PlanReserveOwned` fires once, after the whole selection is
+    /// (under the old carve) already popped, stranding every follower. `ASSERT_FALSE` stops at the first
+    /// demonstrated hang so at most one case leaks a detached thread on the RED path.
+    ASSERT_FALSE(runPlanPointCase(Phase::PlanSeenRefs, 3, "PlanSeenRefs"));
+    ASSERT_FALSE(runPlanPointCase(Phase::PlanBatchGrow, 3, "PlanBatchGrow"));
+    ASSERT_FALSE(runPlanPointCase(Phase::PlanReserveOwned, 1, "PlanReserveOwned"));
+}
+
+/// Test 8: an allocation failure at the per-item accumulation point must leave the failed item's effects
+/// out of BOTH `working` (the in-memory committed state) and the durable transaction. RED on the pre-fix
+/// loop (which moved `working` and appended `final_ops` before the throwing point, so the failed drop
+/// committed while its caller was told it failed); GREEN on the reserve-before-publish loop.
+TEST(CASRefWriterCarve, ValidationAllocFailureLeavesWorkingClean)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const DB::Cas::Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/carve_validate"};
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+    ASSERT_TRUE(store->resolveRef(ns, "x").has_value());
+    ASSERT_TRUE(store->resolveRef(ns, "y").has_value());
+
+    auto sync = std::make_shared<CaseSync>();
+    store->setRefPreCarveHookForTest([sync, store, ns]
+    {
+        std::unique_lock lk(sync->m);
+        if (sync->entered)
+            return;
+        sync->entered = true;
+        sync->cv.notify_all();
+        sync->cv.wait_for(lk, std::chrono::seconds(10), [&] { return store->refQueuePendingForTest(ns) >= 2; });
+    });
+    /// Fail the FIRST admitted item (the leader's own drop of "x") at its accumulation point.
+    store->setCarveHookForTest([sync](Phase ph)
+    {
+        if (ph != Phase::ValidateFinalOps)
+            return;
+        if (sync->phase_hits[static_cast<int>(ph)].fetch_add(1) + 1 == 1)
+            throw std::bad_alloc{};
+    });
+
+    Caller cx = launchDrop(store, ns, "x");
+    {
+        std::unique_lock lk(sync->m);
+        sync->cv.wait_for(lk, std::chrono::seconds(10), [&] { return sync->entered; });
+    }
+    Caller cy = launchDrop(store, ns, "y");
+    {
+        const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(10);
+        while (store->refQueuePendingForTest(ns) < 2 && std::chrono::steady_clock::now() < deadline)
+            std::this_thread::yield();
+    }
+    sync->cv.notify_all();
+
+    ASSERT_EQ(cx.fut.wait_for(std::chrono::seconds(10)), std::future_status::ready) << "drop x must not hang";
+    ASSERT_EQ(cy.fut.wait_for(std::chrono::seconds(10)), std::future_status::ready) << "drop y must not hang";
+    const std::exception_ptr x_err = cx.fut.get();
+    const std::exception_ptr y_err = cy.fut.get();
+    cx.t.join();
+    cy.t.join();
+    store->setRefPreCarveHookForTest(nullptr);
+    store->setCarveHookForTest(nullptr);
+
+    /// The injected item's own caller was told the append failed.
+    ASSERT_TRUE(x_err != nullptr) << "drop x's caller must observe the injected allocation failure";
+    /// The co-batched survivor committed cleanly.
+    EXPECT_TRUE(y_err == nullptr) << "the co-batched survivor drop y must commit";
+
+    /// (1) `working`/committed state stays clean: x's drop, whose caller failed, must NOT have taken
+    /// effect — x remains resolvable.
+    EXPECT_TRUE(store->resolveRef(ns, "x").has_value())
+        << "the failed item's drop leaked into the committed state — `working` was not kept clean";
+    EXPECT_FALSE(store->resolveRef(ns, "y").has_value()) << "survivor drop y must be committed";
+
+    /// (2) Decode the committed object: no committed ref-log transaction may carry x's removal op, and
+    /// exactly the survivor's removal must be present.
+    EXPECT_EQ(committedRemovalCountForRef(*backend, layout, ns, "x"), 0u)
+        << "the failed item's removal op leaked into a durably-committed ref-log object";
+    EXPECT_EQ(committedRemovalCountForRef(*backend, layout, ns, "y"), 1u)
+        << "the survivor's removal op must be present in exactly one committed ref-log object";
+
+    const auto newest = newestLogTxn(*backend, layout, ns);
+    ASSERT_TRUE(newest.has_value());
+    for (const RefOp & op : newest->ops)
+        if (op.kind == RefOpKind::OwnerTransition && op.old_binding.has_value() && !op.new_binding.has_value())
+            EXPECT_NE(op.old_binding->ref_name, String("x"))
+                << "the newest committed transaction must not contain the failed item's removal";
+}
diff --git a/src/Disks/tests/gtest_cas_ref_catalog.cpp b/src/Disks/tests/gtest_cas_ref_catalog.cpp
new file mode 100644
index 000000000000..4a12eccb4a31
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_catalog.cpp
@@ -0,0 +1,1717 @@
+#include "cas_format_test_battery.h"
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Common/ProfileEvents.h>
+#include <Common/logger_useful.h>
+/// Explicit rather than relying on a transitive path: `DEBUG_OR_SANITIZER_BUILD` (used below to gate
+/// the `*DeathTest` split) must resolve in THIS translation unit.
+#include <base/defines.h>
+#include <Poco/AutoPtr.h>
+#include <Poco/StreamChannel.h>
+#include <fmt/format.h>
+#include <algorithm>
+#include <limits>
+#include <sstream>
+#include <type_traits>
+#include <utility>
+
+using namespace DB::Cas;
+
+namespace ProfileEvents
+{
+    extern const Event CASGCUnmatchedAdoptedParentLives;
+    extern const Event CASGCStuckRemovals;
+}
+
+namespace DB::Cas::tests
+{
+
+/// This friend-only compile pin is derived from the actual private production member pointers. It
+/// fails if a raw round carrier becomes separately pairable with `fold`.
+class GcRoundPlanSignatureAccess
+{
+public:
+    using FoldSignature = decltype(&Gc::fold);
+    using ExpectedFoldSignature = Gc::FoldResult (Gc::*)(
+        GcState &, Token &, RoundReport &, uint64_t, const RefPlan &, UniversePolicy, GcRoundWorkBudget &);
+    using BuilderSignature = decltype(&buildRefWalkPlan);
+    using ExpectedBuilderSignature = RefPlan (*)(RoundInput &&);
+
+    static_assert(std::is_same_v<FoldSignature, ExpectedFoldSignature>);
+    static_assert(std::is_same_v<BuilderSignature, ExpectedBuilderSignature>);
+};
+
+}
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int LOGICAL_ERROR;
+    extern const int LIMIT_EXCEEDED;
+    extern const int NETWORK_ERROR;
+    extern const int BAD_ARGUMENTS;
+}
+
+namespace
+{
+
+/// Hand-builds one raw "ent" line, bypassing `encodeRefCatalog` entirely -- used by the decode-side
+/// rejection tests, which must exercise bytes the encoder itself would refuse to produce.
+String rawEntLine(const String & ns, const String & state, const String & inc_hex,
+                   std::optional<std::tuple<String, uint64_t, uint64_t>> creator = std::nullopt)
+{
+    if (!creator)
+        return fmt::format(R"({{"k":"ent","ns":"{}","st":"{}","inc":"{}"}})", ns, state, inc_hex);
+    const auto & [srid, we, fg] = *creator;
+    return fmt::format(R"({{"k":"ent","ns":"{}","st":"{}","inc":"{}","csr":"{}","cwe":"{}","cfg":"{}"}})",
+                        ns, state, inc_hex, srid, we, fg);
+}
+
+/// Wraps `ent_lines` in the header/trailer a real `cas_ref_catalog` object carries. `v:1` always
+/// passes the header gate (any version <= the build's `G_BUILD` does), matching the convention
+/// `gtest_cas_fold_seal_format.cpp`'s `RejectsOutOfRangeNsCleanupState` uses for the same reason.
+String rawCatalog(const std::vector<String> & ent_lines)
+{
+    String out = R"({"type":"cas_ref_catalog","v":1})" "\n";
+    for (const String & l : ent_lines)
+        out += l + "\n";
+    out += fmt::format("{{\"n\":{}}}\n", ent_lines.size());
+    return out;
+}
+
+String withRemovalStartedRound(String line, uint64_t round)
+{
+    const size_t close = line.rfind('}');
+    EXPECT_NE(close, String::npos);
+    line.insert(close, fmt::format(R"(,"rsr":"{}")", round));
+    return line;
+}
+
+CatalogEntry liveEntry(const String & ns, uint64_t inc)
+{
+    return CatalogEntry{.ns = RootNamespace{ns}, .state = NsState::Live, .incarnation = UInt128(inc)};
+}
+
+CatalogEntry entryInState(const String & ns, NsState state, uint64_t inc)
+{
+    CatalogEntry entry{.ns = RootNamespace{ns}, .state = state, .incarnation = UInt128(inc)};
+    if (state == NsState::Creating)
+        entry.creator = CreatorFence{.server_root_id = "srv", .writer_epoch = 1, .fence_generation = 1};
+    if (state == NsState::Removing)
+        entry.removal_started_round = 1;
+    return entry;
+}
+
+class EraseWinnerBackend final : public DB::Cas::tests::CountingBackend
+{
+public:
+    using CountingBackend::casPut;
+    using CountingBackend::get;
+
+    void replaceOnNextCatalogCas(const String & key, std::optional<CatalogEntry> replacement_)
+    {
+        catalog_key = key;
+        replacement = std::move(replacement_);
+        armed = true;
+    }
+
+    bool fenceMoved() const { return fence_moved; }
+
+    CasResult casPut(
+        const String & key, const String & bytes, const std::optional<Token> & expected,
+        const ObjectMeta & meta) override
+    {
+        if (armed && key == catalog_key)
+        {
+            armed = false;
+            const auto current = CountingBackend::get(key);
+            if (!current)
+                throw std::runtime_error("test fixture lost mandatory catalog");
+            RefCatalog winner_catalog;
+            if (replacement)
+                winner_catalog.entries.push_back(*replacement);
+            const CasResult winner = CountingBackend::casPut(
+                key, encodeRefCatalog(winner_catalog), current->token, meta);
+            if (winner.outcome != CasOutcome::Committed)
+                throw std::runtime_error("test fixture winner failed to replace catalog");
+            fence_moved = true;
+        }
+        return CountingBackend::casPut(key, bytes, expected, meta);
+    }
+
+private:
+    String catalog_key;
+    std::optional<CatalogEntry> replacement;
+    bool armed = false;
+    bool fence_moved = false;
+};
+
+class CasPutThrowsOnceBackend final : public DB::Cas::tests::CountingBackend
+{
+public:
+    using CountingBackend::casPut;
+
+    void armCasPutThrow(const String & key)
+    {
+        throw_key = key;
+        armed = true;
+    }
+
+    CasResult casPut(
+        const String & key, const String & bytes, const std::optional<Token> & expected,
+        const ObjectMeta & meta) override
+    {
+        if (armed && key == throw_key)
+        {
+            armed = false;
+            throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR,
+                "injected casPut failure during completed-removal erase");
+        }
+        return CountingBackend::casPut(key, bytes, expected, meta);
+    }
+
+private:
+    String throw_key;
+    bool armed = false;
+};
+
+class ScopedCasGcLogCapture
+{
+public:
+    ScopedCasGcLogCapture()
+        : logger(getLogger("CasGc"))
+        , channel(new Poco::StreamChannel(stream))
+        , old_channel(logger->getChannel())
+        , old_level(logger->getLevel())
+    {
+        logger->setChannel(channel.get());
+        logger->setLevel("warning");
+    }
+
+    ~ScopedCasGcLogCapture()
+    {
+        logger->setChannel(old_channel);
+        logger->setLevel(old_level);
+    }
+
+    String captured() const { return stream.str(); }
+
+private:
+    LoggerPtr logger;
+    std::ostringstream stream; // STYLE_CHECK_ALLOW_STD_STRING_STREAM
+    Poco::AutoPtr<Poco::StreamChannel> channel;
+    Poco::Channel * old_channel;
+    int old_level;
+};
+
+}
+
+/// ---------- format-battery registration ----------
+
+TEST(CASFormatBattery, RefCatalog)
+{
+    RefCatalog c;
+    c.entries.push_back(CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Creating,
+        .incarnation = UInt128(1),
+        .creator = CreatorFence{.server_root_id = "srv1", .writer_epoch = 5, .fence_generation = 2}});
+    c.entries.push_back(liveEntry("b", 2));
+    runFormatBattery({FormatId::RefCatalog,
+        [&] { return sealObject(FormatId::RefCatalog, encodeRefCatalog(c)); },
+        [](std::string_view s) { decodeRefCatalog(std::string(openObject(FormatId::RefCatalog, s))); },
+        currentFormatHeader("cas_ref_catalog") +
+        "{\"k\":\"ent\",\"ns\":\"a\",\"st\":\"creating\",\"inc\":\"00000000000000000000000000000001\","
+        "\"csr\":\"srv1\",\"cwe\":\"5\",\"cfg\":\"2\"}\n"
+        "{\"k\":\"ent\",\"ns\":\"b\",\"st\":\"live\",\"inc\":\"00000000000000000000000000000002\"}\n"
+        "{\"n\":2}\n"});
+}
+
+/// ---------- codec round-trip ----------
+
+TEST(CASRefCatalogFormat, RoundTripsAllThreeStates)
+{
+    RefCatalog in;
+    in.entries.push_back(CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Creating,
+        .incarnation = UInt128(1),
+        .creator = CreatorFence{.server_root_id = "srv1", .writer_epoch = 5, .fence_generation = 2}});
+    in.entries.push_back(liveEntry("b", 2));
+    in.entries.push_back(CatalogEntry{
+        .ns = RootNamespace{"c"},
+        .state = NsState::Removing,
+        .incarnation = UInt128(3),
+        .removal_started_round = 11});
+
+    const RefCatalog out = decodeRefCatalog(encodeRefCatalog(in));
+    EXPECT_EQ(out, in);
+    EXPECT_EQ(out.entries[0].state, NsState::Creating);
+    EXPECT_EQ(out.entries[1].state, NsState::Live);
+    EXPECT_EQ(out.entries[2].state, NsState::Removing);
+}
+
+/// Mutation caught: making removal age caller-local or optional would let an adopted `Removing` row
+/// lose the immutable round from which stuck-removal diagnostics measure.
+TEST(CASRefCatalogFormat, RemovalStartedRoundIsRequiredExactlyForRemoving)
+{
+    CatalogEntry removing{
+        .ns = RootNamespace{"removing"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{7},
+        .removal_started_round = 19};
+    const RefCatalog catalog{.entries = {removing}};
+    const String encoded = encodeRefCatalog(catalog);
+    EXPECT_NE(encoded.find("\"rsr\":\"19\""), String::npos);
+    EXPECT_EQ(decodeRefCatalog(encoded), catalog);
+
+    const String inc = "00000000000000000000000000000009";
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeRefCatalog(rawCatalog({rawEntLine("missing", "removing", inc)})); });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        (void)decodeRefCatalog(rawCatalog({withRemovalStartedRound(rawEntLine("forbidden", "live", inc), 21)}));
+    });
+}
+
+TEST(CASRefCatalogFormat, EmptyCatalogRoundTrips)
+{
+    EXPECT_EQ(decodeRefCatalog(encodeRefCatalog(RefCatalog{})), RefCatalog{});
+}
+
+/// Mutation caught: replacing the reverse index with `emplace`-and-ignore would make the first row
+/// win. Every lifecycle state participates, both duplicate ids are unresolvable, and an unrelated
+/// unique row remains usable by point resolution.
+TEST(CASRefCatalogLifeIndex, DuplicatePhysicalIdsAreAmbiguousWithoutPoisoningUniquePointResolution)
+{
+    RefCatalog catalog;
+    catalog.entries = {
+        entryInState("a-creating", NsState::Creating, 7),
+        entryInState("b-live", NsState::Live, 7),
+        entryInState("c-removing", NsState::Removing, 8),
+        entryInState("d-live", NsState::Live, 8),
+        entryInState("e-unique", NsState::Live, 9),
+    };
+
+    const CatalogLifeIndex index(catalog);
+    EXPECT_TRUE(index.isAmbiguous(UInt128{7}));
+    EXPECT_TRUE(index.isAmbiguous(UInt128{8}));
+    EXPECT_THROW(index.resolve(UInt128{7}), DB::Exception);
+    EXPECT_THROW(index.resolve(UInt128{8}), DB::Exception);
+    const auto unique = index.resolve(UInt128{9});
+    ASSERT_TRUE(unique);
+    EXPECT_EQ(unique->ns.string(), "e-unique");
+}
+
+/// Catalog mutation is destructive authority: any ambiguous current id stops the mutation before a
+/// candidate can be written. An unrelated unique point lookup remains available from the same cut.
+TEST(CASRefCatalogLifeIndex, AmbiguityStopsCatalogMutationButNotUnrelatedPointLookup)
+{
+    InMemoryBackend backend;
+    const Layout layout("p");
+    RefCatalog catalog;
+    catalog.entries = {
+        entryInState("a", NsState::Live, 7),
+        entryInState("b", NsState::Removing, 7),
+        entryInState("c", NsState::Live, 9),
+    };
+    ASSERT_EQ(backend.putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(catalog)).outcome, PutOutcome::Done);
+    const auto before = backend.get(layout.refCatalogKey());
+    ASSERT_TRUE(before);
+
+    EXPECT_THROW(CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog & current) { return current; }), DB::Exception);
+    const auto after = backend.get(layout.refCatalogKey());
+    ASSERT_TRUE(after);
+    EXPECT_EQ(after->token, before->token);
+    EXPECT_EQ(after->bytes, before->bytes);
+
+    const auto unique = CasRefCatalog::lifeIfCataloged(backend, layout, RootNamespace{"c"});
+    ASSERT_TRUE(unique);
+    EXPECT_EQ(unique->incarnation, UInt128{9});
+}
+
+TEST(CASRefCatalogFormat, NamespaceAtExactByteBoundRoundTrips)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry(String(kMaxNamespaceBytes, 'a'), 1));
+    const RefCatalog out = decodeRefCatalog(encodeRefCatalog(c));
+    EXPECT_EQ(out, c);
+}
+
+/// ---------- strict rejections: encode side (LOGICAL_ERROR -- our own state, not yet durable) ----------
+
+/// Every `expectThrowsCode(LOGICAL_ERROR, ...)` in this block aborts the process in debug/sanitizer
+/// builds instead of behaving like a catchable exception (`Common/Exception.cpp`'s
+/// `handle_error_code`), so each test is split: the throw-and-catch form below runs only on a plain
+/// release build, and its `...DeathTest` counterpart (grouped after this block) proves the abort
+/// positively on debug/sanitizer builds instead.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+
+TEST(CASRefCatalogFormat, EncodeRejectsDuplicateNamespace)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry("a", 1));
+    c.entries.push_back(liveEntry("a", 2));
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&] { encodeRefCatalog(c); });
+}
+
+TEST(CASRefCatalogFormat, EncodeRejectsNonCanonicalOrder)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry("b", 1));
+    c.entries.push_back(liveEntry("a", 2));
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&] { encodeRefCatalog(c); });
+}
+
+TEST(CASRefCatalogFormat, EncodeRejectsCreatorPresentOnLive)
+{
+    RefCatalog c;
+    c.entries.push_back(CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Live, .incarnation = UInt128(1),
+        .creator = CreatorFence{.server_root_id = "srv", .writer_epoch = 1, .fence_generation = 1}});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&] { encodeRefCatalog(c); });
+}
+
+TEST(CASRefCatalogFormat, EncodeRejectsCreatorAbsentOnCreating)
+{
+    RefCatalog c;
+    c.entries.push_back(CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Creating, .incarnation = UInt128(1)});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&] { encodeRefCatalog(c); });
+}
+
+TEST(CASRefCatalogFormat, EncodeRejectsZeroIncarnation)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry("a", 0));
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&] { encodeRefCatalog(c); });
+}
+
+TEST(CASRefCatalogFormat, EncodeRejectsNameOverByteBound)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry(String(kMaxNamespaceBytes + 1, 'a'), 1));
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&] { encodeRefCatalog(c); });
+}
+
+TEST(CASRefCatalogFormat, EncodeRejectsEmptyNamespace)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry("", 1));
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&] { encodeRefCatalog(c); });
+}
+
+/// Mutation caught: making removal age caller-local or optional would let an adopted `Removing` row
+/// lose the immutable round from which stuck-removal diagnostics measure.
+TEST(CASRefCatalogFormat, EncodeRejectsLiveWithRemovalStartedRound)
+{
+    CatalogEntry live_with_round = liveEntry("live", 8);
+    live_with_round.removal_started_round = 20;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR,
+        [&] { (void)encodeRefCatalog(RefCatalog{.entries = {live_with_round}}); });
+}
+
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+
+TEST(CASRefCatalogFormatDeathTest, EncodeRejectsDuplicateNamespaceAborts)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry("a", 1));
+    c.entries.push_back(liveEntry("a", 2));
+    EXPECT_DEATH({ (void)encodeRefCatalog(c); }, "not canonically ordered");
+}
+
+TEST(CASRefCatalogFormatDeathTest, EncodeRejectsNonCanonicalOrderAborts)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry("b", 1));
+    c.entries.push_back(liveEntry("a", 2));
+    EXPECT_DEATH({ (void)encodeRefCatalog(c); }, "not canonically ordered");
+}
+
+TEST(CASRefCatalogFormatDeathTest, EncodeRejectsCreatorPresentOnLiveAborts)
+{
+    RefCatalog c;
+    c.entries.push_back(CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Live, .incarnation = UInt128(1),
+        .creator = CreatorFence{.server_root_id = "srv", .writer_epoch = 1, .fence_generation = 1}});
+    EXPECT_DEATH({ (void)encodeRefCatalog(c); }, "carries a creator fence");
+}
+
+TEST(CASRefCatalogFormatDeathTest, EncodeRejectsCreatorAbsentOnCreatingAborts)
+{
+    RefCatalog c;
+    c.entries.push_back(CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Creating, .incarnation = UInt128(1)});
+    EXPECT_DEATH({ (void)encodeRefCatalog(c); }, "lacks a creator fence");
+}
+
+TEST(CASRefCatalogFormatDeathTest, EncodeRejectsZeroIncarnationAborts)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry("a", 0));
+    EXPECT_DEATH({ (void)encodeRefCatalog(c); }, "zero incarnation");
+}
+
+TEST(CASRefCatalogFormatDeathTest, EncodeRejectsNameOverByteBoundAborts)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry(String(kMaxNamespaceBytes + 1, 'a'), 1));
+    EXPECT_DEATH({ (void)encodeRefCatalog(c); }, "admission bound");
+}
+
+TEST(CASRefCatalogFormatDeathTest, EncodeRejectsEmptyNamespaceAborts)
+{
+    RefCatalog c;
+    c.entries.push_back(liveEntry("", 1));
+    EXPECT_DEATH({ (void)encodeRefCatalog(c); }, "namespace must not be empty");
+}
+
+TEST(CASRefCatalogFormatDeathTest, EncodeRejectsLiveWithRemovalStartedRoundAborts)
+{
+    CatalogEntry live_with_round = liveEntry("live", 8);
+    live_with_round.removal_started_round = 20;
+    EXPECT_DEATH(
+        { (void)encodeRefCatalog(RefCatalog{.entries = {live_with_round}}); }, "removal_started_round");
+}
+
+#endif
+
+/// A namespace + creator server_root_id that both max out at their respective byte bounds (512 +
+/// 255), escaped worst-case, land one "ent" line over the 4 KiB line cap (~4.7 KiB) -- reachable
+/// because neither this codec nor `validateServerRootId` restricts the charset, only the length.
+/// The refusal must be `LIMIT_EXCEEDED` (a capacity refusal), not `LOGICAL_ERROR` (a bug report) --
+/// `encodeFoldSeal`'s own `checkLineBytes` raises `LIMIT_EXCEEDED` for the identical shape of gate.
+TEST(CASRefCatalogFormat, EncodeLineOverCapRaisesLimitExceeded)
+{
+    RefCatalog c;
+    c.entries.push_back(CatalogEntry{
+        .ns = RootNamespace{String(kMaxNamespaceBytes, '\x01')},
+        .state = NsState::Creating,
+        .incarnation = UInt128(1),
+        .creator = CreatorFence{.server_root_id = String(255, '\x01'), .writer_epoch = 1, .fence_generation = 1}});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LIMIT_EXCEEDED, [&] { encodeRefCatalog(c); });
+}
+
+/// ---------- strict rejections: decode side (CORRUPTED_DATA -- bytes may have come from anywhere) ----------
+
+TEST(CASRefCatalogFormat, DecodeRejectsDuplicateNamespace)
+{
+    const String bad = rawCatalog({rawEntLine("a", "live", u128ToHex(UInt128(1))),
+                                    rawEntLine("a", "live", u128ToHex(UInt128(2)))});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+TEST(CASRefCatalogFormat, DecodeRejectsNonCanonicalOrder)
+{
+    const String bad = rawCatalog({rawEntLine("b", "live", u128ToHex(UInt128(1))),
+                                    rawEntLine("a", "live", u128ToHex(UInt128(2)))});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+TEST(CASRefCatalogFormat, DecodeRejectsCreatorPresentOnLive)
+{
+    const String bad = rawCatalog({rawEntLine("a", "live", u128ToHex(UInt128(1)),
+                                               std::make_tuple(String("srv"), uint64_t(1), uint64_t(1)))});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+TEST(CASRefCatalogFormat, DecodeRejectsCreatorAbsentOnCreating)
+{
+    const String bad = rawCatalog({rawEntLine("a", "creating", u128ToHex(UInt128(1)))});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+TEST(CASRefCatalogFormat, DecodeRejectsZeroIncarnation)
+{
+    const String bad = rawCatalog({rawEntLine("a", "live", u128ToHex(UInt128(0)))});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+TEST(CASRefCatalogFormat, DecodeRejectsNameOverByteBound)
+{
+    const String too_long_ns(kMaxNamespaceBytes + 1, 'a');
+    const String bad = rawCatalog({rawEntLine(too_long_ns, "live", u128ToHex(UInt128(1)))});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+TEST(CASRefCatalogFormat, DecodeRejectsUnknownState)
+{
+    const String bad = rawCatalog({rawEntLine("a", "bogus", u128ToHex(UInt128(1)))});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+TEST(CASRefCatalogFormat, DecodeRejectsEmptyNamespace)
+{
+    const String bad = rawCatalog({rawEntLine("", "live", u128ToHex(UInt128(1)))});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+TEST(CASRefCatalogFormat, DecodeRejectsMissingNamespaceKey)
+{
+    /// No "ns" key at all -- must be refused exactly like an explicit empty one, not read as "".
+    const String bad = rawCatalog({R"({"k":"ent","st":"live","inc":")" + u128ToHex(UInt128(1)) + "\"}"});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCatalog(bad); });
+}
+
+/// `nsStateToWord`'s only reachable input is either a live `NsState` or one `nsStateFromWord` already
+/// validated on decode, so an unrecognized value is a bug in THIS process -- `LOGICAL_ERROR`, matching
+/// this file's own stated taxonomy for the encode-side helper it (indirectly, via `creatorPairingOk`'s
+/// error message) serves. Aborts under debug/sanitizer builds -- split like the block above;
+/// `CASRefCatalogFormatDeathTest.NsStateToWordRaisesLogicalErrorOnImpossibleValueAborts` covers it there.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASRefCatalogFormat, NsStateToWordRaisesLogicalErrorOnImpossibleValue)
+{
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR,
+        [&] { nsStateToWord(static_cast<NsState>(99)); });
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASRefCatalogFormatDeathTest, NsStateToWordRaisesLogicalErrorOnImpossibleValueAborts)
+{
+    EXPECT_DEATH({ (void)nsStateToWord(static_cast<NsState>(99)); }, "unknown ns state"); // NOLINT(clang-analyzer-optin.core.EnumCastOutOfRange): the whole point of this test is an impossible enum value
+}
+#endif
+
+/// ---------- registry row / raw-storage tripwire ----------
+
+/// The registry row is part of the contract, mirroring `gtest_cas_ref_ckpt.cpp`'s
+/// `RegistryRowIsControlStrictWithTightCaps`: Control/Strict decides how the decoder treats unknown
+/// keys, and the caps are the first thing that fires if a foreign object ever lands at the key.
+TEST(CASRefCatalogFormat, RegistryRowIsControlStrictWithRawStorage)
+{
+    const FormatTraits & traits = traitsFor(FormatId::RefCatalog);
+    EXPECT_EQ(traits.type, "cas_ref_catalog");
+    EXPECT_EQ(traits.family, TextFamily::Control);
+    EXPECT_EQ(traits.strictness, KeyStrictness::Strict);
+    EXPECT_EQ(traits.object_cap, 256u * 1024u * 1024u);
+    EXPECT_EQ(traits.line_cap, 4u * 1024u);
+    EXPECT_EQ(traitsForType("cas_ref_catalog"), &traits);
+    /// Raw, so the key has no suffix: `Pool/CasRefCatalog.cpp` hands bytes to/from the backend
+    /// directly, bypassing `sealObject`/`openObject` because both are the identity under
+    /// `CompressionPolicy::Never`. This line is the TRIPWIRE for that shortcut -- a policy flip to
+    /// `Always` would silently write uncompressed bodies under a `.zst` key, which this assertion
+    /// catches first (see `CasRefCatalogFormat.h`'s comment on `encodeRefCatalog`).
+    EXPECT_EQ(storedSuffix(FormatId::RefCatalog), "");
+    EXPECT_EQ(traits.compression, CompressionPolicy::Never);
+}
+
+/// ---------- capacity admission: per-predicate boundary tests [codex r2/r3 finding 9] ----------
+
+TEST(CASRefCatalogAdmission, Predicate1AcceptsEqualityRefusesCapPlusOne)
+{
+    const uint64_t cap = traitsFor(FormatId::RefCatalog).object_cap;
+    const RootNamespace ns{"admitted"};
+    EXPECT_NO_THROW(checkCatalogObjectBytes(cap, ns));
+    try
+    {
+        checkCatalogObjectBytes(cap + 1, ns);
+        FAIL() << "expected LIMIT_EXCEEDED";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::LIMIT_EXCEEDED);
+        EXPECT_NE(e.message().find("predicate 1"), String::npos) << e.message();
+        EXPECT_NE(e.message().find(ns.string()), String::npos) << e.message();
+    }
+}
+
+TEST(CASRefCatalogAdmission, Predicate2AcceptsEqualityRefusesOneEntryOver)
+{
+    const Layout layout("p");
+    constexpr uint64_t gc_shards = 1;
+    /// The exact boundary is expressed in ENTRIES (predicate (2) is a sum over admitted entries), so
+    /// the boundary count is derived from the real registry constants rather than assumed.
+    const uint64_t cap = foldSealCaps().object_cap;
+    const uint64_t fixed = foldSealFixedBytes();
+    const uint64_t reservation = worstCaseEntryFoldReservationBytes();
+    ASSERT_GT(reservation, 0u);
+    const uint64_t nonentry = widestBlobTargetRunReservationBytes(layout, gc_shards)
+        + widestCondemnedSummaryReservationBytes(gc_shards);
+    const uint64_t max_entries = (cap - fixed - nonentry) / reservation;
+
+    const RootNamespace ns{"admitted"};
+    EXPECT_NO_THROW(checkFoldSealReservation(max_entries, gc_shards, layout, ns));
+    try
+    {
+        checkFoldSealReservation(max_entries + 1, gc_shards, layout, ns);
+        FAIL() << "expected LIMIT_EXCEEDED";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::LIMIT_EXCEEDED);
+        EXPECT_NE(e.message().find("predicate 2"), String::npos) << e.message();
+        EXPECT_NE(e.message().find(ns.string()), String::npos) << e.message();
+    }
+}
+
+/// `entry_count * worstCaseEntryFoldReservationBytes()` must saturate, not wrap: choosing
+/// `entry_count` as the SMALLEST value whose true (unbounded) product with `reservation` crosses
+/// 2^64, an unsaturated `uint64_t` multiplication wraps to a remainder SMALLER than `reservation`
+/// itself (a few KiB) -- which reads as trivially "fits" a 256 MiB cap even though the real
+/// reservation this many entries demands is astronomically larger. A saturating multiply refuses it
+/// regardless of the wraparound arithmetic underneath.
+TEST(CASRefCatalogAdmission, Predicate2SaturatesEntryCountReservationInsteadOfWrapping)
+{
+    const Layout layout("p");
+    const uint64_t reservation = worstCaseEntryFoldReservationBytes();
+    const uint64_t entry_count = std::numeric_limits<uint64_t>::max() / reservation + 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LIMIT_EXCEEDED,
+        [&] { checkFoldSealReservation(entry_count, 1, layout, RootNamespace{"huge"}); });
+}
+
+TEST(CASRefCatalogAdmission, CombinedAdmissionPropagatesCandidateEntryCount)
+{
+    /// `checkCatalogAdmission` runs predicate (1) then predicate (2) against the SAME candidate; for
+    /// an ordinary small catalog both hold slack and it returns the exact bytes `encodeRefCatalog`
+    /// would produce.
+    RefCatalog candidate;
+    candidate.entries.push_back(liveEntry("a", 1));
+    candidate.entries.push_back(liveEntry("b", 2));
+    const Layout layout("p");
+    const String encoded = checkCatalogAdmission(candidate, 1, layout, RootNamespace{"b"});
+    EXPECT_EQ(encoded, encodeRefCatalog(candidate));
+}
+
+TEST(CASRefCatalogAdmission, ReservationCoversActualWidestLegalRowsAcrossDecimalTransitions)
+{
+    const Layout layout("p/quoted-\"prefix");
+    constexpr uint64_t gc_shards = 100;
+    constexpr uint64_t max = std::numeric_limits<uint64_t>::max();
+
+    for (const uint64_t entry_count : {9, 10, 99, 100})
+    {
+        CasFoldSeal seal;
+        seal.generation = max;
+        seal.parent_generation = max;
+        for (uint64_t i = 0; i < entry_count; ++i)
+        {
+            seal.ref_lives.emplace(std::numeric_limits<UInt128>::max() - i, RefLifeFoldState{
+                .coverage = RefCoverage{
+                    .classification = 4,
+                    .last_folded_ref_id = RefTxnId{max, max},
+                    .hold = RefHold{
+                        .reason = HoldReason::UnconsumedSealCrossing,
+                        .offending_position = RefTxnId{max, max},
+                        .retry_count = std::numeric_limits<uint32_t>::max(),
+                        .next_retry_round = max}},
+                .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{max, max}}});
+        }
+        for (uint64_t shard = 0; shard < gc_shards; ++shard)
+        {
+            /// Predicate 2 charges exactly `gc_shards` widest `btr` rows. This fixture is the maximum
+            /// legal cardinality, not an optimistic producer convention: authoritative fold-seal
+            /// grammar permits at most one run per shard and requires its canonical key to use seq 0.
+            seal.blob_target_runs.push_back(RunRef{
+                .key = layout.blobTargetRunKey(max, max, shard, 0),
+                .checksum = std::numeric_limits<UInt128>::max(),
+                .shard = shard,
+                .generation = max});
+            seal.condemned_summary.emplace(shard, CondemnedSummary{
+                .condemned_total = max,
+                .pending_total = max,
+                .oldest_nonpending_condemn_round = max});
+        }
+
+        ASSERT_EQ(seal.blob_target_runs.size(), gc_shards);
+        EXPECT_NO_THROW(validateFoldSealForWrite(seal, layout, gc_shards));
+
+        const uint64_t bound = foldSealFixedBytes()
+            + entry_count * worstCaseEntryFoldReservationBytes()
+            + gc_shards * widestBlobTargetRunReservationBytes(layout, gc_shards)
+            + gc_shards * widestCondemnedSummaryReservationBytes(gc_shards);
+        EXPECT_LE(encodeFoldSeal(seal).size(), bound) << "entry_count=" << entry_count;
+    }
+}
+
+/// ---------- Constraint 13: removal is never refused, even at the admission boundary ----------
+
+TEST(CASRefCatalogAdmission, RemovalNeverRefusedEvenAtCapacity)
+{
+    const Layout layout("p");
+    constexpr uint64_t gc_shards = 1;
+    const uint64_t cap = foldSealCaps().object_cap;
+    const uint64_t fixed = foldSealFixedBytes();
+    const uint64_t reservation = worstCaseEntryFoldReservationBytes();
+    const uint64_t nonentry = widestBlobTargetRunReservationBytes(layout, gc_shards)
+        + widestCondemnedSummaryReservationBytes(gc_shards);
+    const uint64_t max_entries = (cap - fixed - nonentry) / reservation;
+
+    /// Confirm the boundary is real: one entry beyond it is refused through admission.
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LIMIT_EXCEEDED,
+        [&] { checkFoldSealReservation(max_entries + 1, gc_shards, layout, RootNamespace{"z"}); });
+
+    /// Build a catalog carrying exactly `max_entries` Live entries -- as full as admission ever
+    /// permits -- directly (a fixture, not itself an admission call).
+    RefCatalog full;
+    full.entries.reserve(max_entries);
+    for (uint64_t i = 0; i < max_entries; ++i)
+        full.entries.push_back(liveEntry(fmt::format("ns{:012}", i), i + 1));
+
+    InMemoryBackend backend;
+    backend.putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(full));
+
+    /// The removal transition (Live -> Removing) on one entry goes through the PLAIN update path
+    /// (`casUpdate`, which runs no admission check at all) and succeeds even though the catalog is
+    /// already at the point where ANY growth would be refused.
+    const RefCatalog after = CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog & cur)
+    {
+        RefCatalog next = cur;
+        next.entries[0].state = NsState::Removing;
+        next.entries[0].removal_started_round = 1;
+        return next;
+    });
+    EXPECT_EQ(after.entries.size(), max_entries);
+    EXPECT_EQ(after.entries[0].state, NsState::Removing);
+}
+
+/// ---------- Pool/CasRefCatalog: token-CAS read / create / update / conflict-retry ----------
+
+TEST(CASRefCatalog, ReadAbsentFailsClosed)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    DB::Cas::tests::expectThrowsCode(
+        DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)CasRefCatalog::read(backend, layout); });
+}
+
+TEST(CASRefCatalog, CasUpdateRefusesWhenAbsent)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog & cur) { return cur; });
+    });
+    EXPECT_FALSE(backend.head(layout.refCatalogKey()).exists);
+}
+
+TEST(CASRefCatalog, CasUpdateAppliesOnTopOfExistingState)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+
+    const RefCatalog updated = CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog & cur)
+    {
+        RefCatalog next = cur;
+        next.entries[0].state = NsState::Removing;
+        next.entries[0].removal_started_round = 1;
+        return next;
+    });
+    ASSERT_EQ(updated.entries.size(), 1u);
+    EXPECT_EQ(updated.entries[0].ns.string(), "a");
+    EXPECT_EQ(updated.entries[0].state, NsState::Removing);
+}
+
+/// `CasRefCatalog::casUpdate`'s identity-preserving refusal throws `LOGICAL_ERROR`, which aborts the
+/// whole process in debug/sanitizer builds (`Common/Exception.cpp`'s `handle_error_code`) instead of
+/// behaving like a catchable exception -- so the throw-and-catch form below runs only on a plain
+/// release build, and `CASRefCatalogDeathTest.GenericCasUpdateCannotDeleteOrReplaceCatalogIdentityAborts`
+/// proves the abort positively on debug/sanitizer builds instead.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASRefCatalog, GenericCasUpdateCannotDeleteOrReplaceCatalogIdentity)
+{
+    const Layout layout("p");
+    {
+        InMemoryBackend backend;
+        CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+        CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+        DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&]
+        {
+            (void)CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog &) { return RefCatalog{}; });
+        });
+    }
+
+    {
+        InMemoryBackend backend;
+        CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+        CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+        DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&]
+        {
+            (void)CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog & current)
+            {
+                RefCatalog next = current;
+                next.entries[0] = liveEntry("b", 2);
+                return next;
+            });
+        });
+    }
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASRefCatalogDeathTest, GenericCasUpdateCannotDeleteOrReplaceCatalogIdentityAborts)
+{
+    const Layout layout("p");
+    {
+        InMemoryBackend backend;
+        CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+        CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+        EXPECT_DEATH(
+            { (void)CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog &) { return RefCatalog{}; }); },
+            "cannot add or delete catalog entries");
+    }
+
+    {
+        InMemoryBackend backend;
+        CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+        CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+        EXPECT_DEATH(
+            {
+                (void)CasRefCatalog::casUpdate(backend, layout, [](const RefCatalog & current)
+                {
+                    RefCatalog next = current;
+                    next.entries[0] = liveEntry("b", 2);
+                    return next;
+                });
+            },
+            "cannot replace catalog identity");
+    }
+}
+#endif
+
+TEST(CASRefCatalog, CasUpdateRetriesOnConflictAgainstFreshState)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+
+    backend.failNextCasPut(layout.refCatalogKey());   /// one-shot artificial Conflict on the next write
+
+    int mutate_calls = 0;
+    const RefCatalog result = CasRefCatalog::casUpdate(backend, layout, [&](const RefCatalog & cur)
+    {
+        ++mutate_calls;
+        RefCatalog next = cur;
+        next.entries[0].state = NsState::Removing;
+        next.entries[0].removal_started_round = 1;
+        return next;
+    });
+
+    EXPECT_EQ(mutate_calls, 2);   /// first attempt hit the injected conflict; the retry succeeded
+    ASSERT_EQ(result.entries.size(), 1u);
+    EXPECT_EQ(result.entries[0].state, NsState::Removing);
+
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    EXPECT_EQ(snap.catalog, result);
+}
+
+TEST(CASRefCatalog, BeginRemovingRechecksFenceAfterCatalogCasConflict)
+{
+    InMemoryBackend backend;
+    const Layout layout("p");
+    const CatalogEntry observed = liveEntry("a", 1);
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, observed);
+
+    uint64_t current_fence_generation = 7;
+    size_t fence_checks = 0;
+    const auto outcome = CasRefCatalog::beginRemoving(
+        backend, layout, observed, /*removal_started_round*/ 13, /*admitted_generation*/ 7,
+        [&](uint64_t admitted_generation)
+        {
+            ++fence_checks;
+            if (admitted_generation != current_fence_generation)
+                throw std::runtime_error("stale catalog mutation fence");
+            if (fence_checks == 1)
+            {
+                /// Move the caller fence after the first admission check and force that attempt's
+                /// catalog CAS to conflict. The next attempt must check the fence again before writing.
+                current_fence_generation = 8;
+                backend.failNextCasPut(layout.refCatalogKey());
+            }
+        });
+
+    EXPECT_EQ(outcome, CasRefCatalog::BeginRemovingOutcome::FencedOut);
+    EXPECT_EQ(fence_checks, 2u);
+    const CasRefCatalog::Snapshot after = CasRefCatalog::read(backend, layout);
+    EXPECT_EQ(after.catalog.entries, std::vector<CatalogEntry>{observed});
+}
+
+/// A re-read that finds the catalog genuinely ABSENT after it was previously observed present is a
+/// real concurrent delete, not a bootstrap -- `casUpdate` must refuse rather than silently create a
+/// fresh catalog containing only this one mutation's entry (which would drop every other namespace).
+/// Reproduced with a REAL delete (no fault injection needed): `mutate`'s first invocation deletes the
+/// seeded object using the token `casUpdate`'s own initial read observed, so the loop's own `casPut`
+/// against that now-stale token gets a genuine `Conflict`, and the follow-up re-read genuinely finds
+/// the key absent.
+/// Missing mandatory authority raises `CORRUPTED_DATA`; the split remains only because the debug
+/// variant historically lived in the death-test suite.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASRefCatalog, CasUpdateThrowsOnVanishMidRetryInsteadOfReplacingTheCatalog)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+    const CasRefCatalog::Snapshot seeded = CasRefCatalog::read(backend, layout);
+    ASSERT_TRUE(seeded.token.has_value());
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        CasRefCatalog::casUpdate(backend, layout, [&](const RefCatalog & cur)
+        {
+            backend.deleteExact(layout.refCatalogKey(), *seeded.token);
+            RefCatalog next = cur;
+            next.entries[0].state = NsState::Removing;
+            next.entries[0].removal_started_round = 1;
+            return next;
+        });
+    });
+
+    /// Nothing was written by the failed attempt: the object is exactly as the delete left it
+    /// (absent), never a fresh single-entry catalog.
+    EXPECT_FALSE(backend.head(layout.refCatalogKey()).exists);
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASRefCatalogDeathTest, CasUpdateThrowsOnVanishMidRetryInsteadOfReplacingTheCatalogAborts)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+    const CasRefCatalog::Snapshot seeded = CasRefCatalog::read(backend, layout);
+    ASSERT_TRUE(seeded.token.has_value());
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+            CasRefCatalog::casUpdate(backend, layout, [&](const RefCatalog & cur)
+            {
+                backend.deleteExact(layout.refCatalogKey(), *seeded.token);
+                RefCatalog next = cur;
+                next.entries[0].state = NsState::Removing;
+                next.entries[0].removal_started_round = 1;
+                return next;
+            });
+    });
+}
+#endif
+
+/// The retry loop is bounded (the same live-lock brake `publishCkpt`/`allocateWriterEpoch` use on
+/// their own contended token-CAS singletons) and ends in the typed retryable error, not an infinite
+/// spin. `mutate` re-arms the one-shot conflict injection on every call, so every attempt fails.
+TEST(CASRefCatalog, CasUpdateGivesUpAfterBoundedAttemptsWithRetryLaterError)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+
+    int mutate_calls = 0;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        CasRefCatalog::casUpdate(backend, layout, [&](const RefCatalog & cur)
+        {
+            ++mutate_calls;
+            backend.failNextCasPut(layout.refCatalogKey());
+            RefCatalog next = cur;
+            return next;
+        });
+    });
+    EXPECT_GT(mutate_calls, 1);   /// genuinely retried, not a single-shot failure
+}
+
+TEST(CASRefCatalog, CasAdmitEntryAcceptsAnOrdinaryCreation)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+
+    const RefCatalog created = CasRefCatalog::casAdmitEntry(backend, layout, 1,
+        CatalogEntry{.ns = RootNamespace{"a"}, .state = NsState::Creating, .incarnation = UInt128(1),
+            .creator = CreatorFence{.server_root_id = "srv", .writer_epoch = 1, .fence_generation = 1}});
+    ASSERT_EQ(created.entries.size(), 1u);
+    EXPECT_EQ(created.entries[0].state, NsState::Creating);
+}
+
+TEST(CASRefCatalog, CasAdmitEntryInsertsAtCanonicalPosition)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("b", 1));
+    const RefCatalog after = CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 2));
+    ASSERT_EQ(after.entries.size(), 2u);
+    EXPECT_EQ(after.entries[0].ns.string(), "a");   /// inserted BEFORE "b", not appended
+    EXPECT_EQ(after.entries[1].ns.string(), "b");
+}
+
+/// Caught by `encodeRefCatalog`'s own canonical-order/no-duplicate grammar check, inside
+/// `checkCatalogAdmission` -- no separate duplicate check needed here. That `LOGICAL_ERROR` aborts
+/// under debug/sanitizer builds -- split like the blocks above.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASRefCatalog, CasAdmitEntryRejectsADuplicateNamespace)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR,
+        [&] { CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 2)); });
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASRefCatalogDeathTest, CasAdmitEntryRejectsADuplicateNamespaceAborts)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 1));
+    EXPECT_DEATH({ CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("a", 2)); }, "not canonically ordered");
+}
+#endif
+
+TEST(CASRefCatalog, CasAdmitEntryRefusesOverCapacity)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+
+    const uint64_t cap = foldSealCaps().object_cap;
+    const uint64_t fixed = foldSealFixedBytes();
+    const uint64_t reservation = worstCaseEntryFoldReservationBytes();
+    const uint64_t nonentry = widestBlobTargetRunReservationBytes(layout, 1)
+        + widestCondemnedSummaryReservationBytes(1);
+    const uint64_t max_entries = (cap - fixed - nonentry) / reservation;
+
+    /// Seed the catalog directly at the admission boundary (a fixture -- not itself an admission call).
+    RefCatalog full;
+    full.entries.reserve(max_entries);
+    for (uint64_t i = 0; i < max_entries; ++i)
+        full.entries.push_back(liveEntry(fmt::format("ns{:012}", i), i + 1));
+    backend.putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(full));
+
+    /// Admitting ONE more namespace is refused -- the additive predicate is checked BEFORE the write,
+    /// so the backend object is untouched.
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LIMIT_EXCEEDED, [&]
+    {
+        CasRefCatalog::casAdmitEntry(backend, layout, 1, liveEntry("zzz", 999999999));
+    });
+
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    EXPECT_EQ(snap.catalog.entries.size(), max_entries);
+}
+
+TEST(CASRefCatalogRemoval, DeleteCompletedRemovingRequiresExactAdoptedProofAndLeaderFence)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("p");
+    const CatalogEntry removing{
+        .ns = RootNamespace{"a"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{7},
+        .removal_started_round = 13};
+    ASSERT_EQ(backend.putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(RefCatalog{.entries = {removing}})).outcome,
+        PutOutcome::Done);
+
+    CasFoldSeal held_parent;
+    held_parent.ref_lives.emplace(UInt128{7}, RefLifeFoldState{
+        .coverage = RefCoverage{
+            .classification = 4,
+            .last_folded_ref_id = RefTxnId{1, 2},
+            .hold = RefHold{.offending_position = RefTxnId{1, 3}}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 2}}});
+    EXPECT_EQ(CasRefCatalog::deleteCompletedRemoving(
+        backend, layout, removing, held_parent, 5,
+        [](uint64_t) { return CasRefCatalog::LeaderFenceStatus::Held; }),
+        CasRefCatalog::CompletedRemovingDeleteOutcome::ProofRefused);
+
+    CasFoldSeal mismatched_parent;
+    mismatched_parent.ref_lives.emplace(UInt128{8}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 2}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 2}}});
+    EXPECT_EQ(CasRefCatalog::deleteCompletedRemoving(
+        backend, layout, removing, mismatched_parent, 5,
+        [](uint64_t) { return CasRefCatalog::LeaderFenceStatus::Held; }),
+        CasRefCatalog::CompletedRemovingDeleteOutcome::ProofRefused);
+
+    CasFoldSeal ready_parent;
+    ready_parent.ref_lives.emplace(UInt128{7}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 2}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 2}}});
+
+    CatalogEntry live = removing;
+    live.state = NsState::Live;
+    live.removal_started_round.reset();
+    EXPECT_EQ(CasRefCatalog::deleteCompletedRemoving(
+        backend, layout, live, ready_parent, 5,
+        [](uint64_t) { return CasRefCatalog::LeaderFenceStatus::Held; }),
+        CasRefCatalog::CompletedRemovingDeleteOutcome::ProofRefused);
+    CatalogEntry creating = live;
+    creating.state = NsState::Creating;
+    creating.creator = CreatorFence{.server_root_id = "server", .writer_epoch = 3, .fence_generation = 4};
+    EXPECT_EQ(CasRefCatalog::deleteCompletedRemoving(
+        backend, layout, creating, ready_parent, 5,
+        [](uint64_t) { return CasRefCatalog::LeaderFenceStatus::Held; }),
+        CasRefCatalog::CompletedRemovingDeleteOutcome::ProofRefused);
+
+    EXPECT_EQ(CasRefCatalog::deleteCompletedRemoving(
+        backend, layout, removing, ready_parent, 5,
+        [](uint64_t) { return CasRefCatalog::LeaderFenceStatus::Moved; }),
+        CasRefCatalog::CompletedRemovingDeleteOutcome::FencedOut);
+    EXPECT_EQ(backend.casPutCount(layout.refCatalogKey()), 0);
+
+    EXPECT_EQ(CasRefCatalog::deleteCompletedRemoving(
+        backend, layout, removing, ready_parent, 5, [](uint64_t generation)
+        {
+            EXPECT_EQ(generation, 5);
+            return CasRefCatalog::LeaderFenceStatus::Held;
+        }),
+        CasRefCatalog::CompletedRemovingDeleteOutcome::Deleted);
+    EXPECT_TRUE(CasRefCatalog::read(backend, layout).catalog.entries.empty());
+    EXPECT_EQ(backend.casPutCount(layout.refCatalogKey()), 1);
+    EXPECT_EQ(backend.listTotal(), 0);
+    EXPECT_EQ(backend.deleteTotal(), 0);
+}
+
+TEST(CASRefCatalogRemoval, ExactDeletionRefusesChangedEntryAndAdmissionCannotCarryRemoval)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    const CatalogEntry removing{
+        .ns = RootNamespace{"a"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{7},
+        .removal_started_round = 13};
+#ifndef DEBUG_OR_SANITIZER_BUILD
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR,
+        [&] { (void)CasRefCatalog::casAdmitEntry(backend, layout, 1, removing); });
+#endif
+
+    const CatalogEntry current{
+        .ns = RootNamespace{"a"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{7},
+        .removal_started_round = 14};
+    ASSERT_EQ(backend.putIfAbsent("unrelated", "sentinel").outcome, PutOutcome::Done);
+    ASSERT_EQ(backend.casPut(layout.refCatalogKey(), encodeRefCatalog(RefCatalog{.entries = {current}}),
+        CasRefCatalog::read(backend, layout).token).outcome, CasOutcome::Committed);
+
+    CasFoldSeal ready_parent;
+    ready_parent.ref_lives.emplace(UInt128{7}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 2}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 2}}});
+    EXPECT_EQ(CasRefCatalog::deleteCompletedRemoving(
+        backend, layout, removing, ready_parent, 5,
+        [](uint64_t) { return CasRefCatalog::LeaderFenceStatus::Held; }),
+        CasRefCatalog::CompletedRemovingDeleteOutcome::EntryChanged);
+    EXPECT_EQ(CasRefCatalog::read(backend, layout).catalog.entries, std::vector<CatalogEntry>{current});
+}
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASRefCatalogRemovalDeathTest, AdmissionCannotCarryRemovalAborts)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("p");
+    CasRefCatalog::initializeEmptyForNewPool(backend, layout);
+    const CatalogEntry removing{
+        .ns = RootNamespace{"a"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{7},
+        .removal_started_round = 13};
+
+    EXPECT_DEATH(
+        { (void)CasRefCatalog::casAdmitEntry(backend, layout, 1, removing); },
+        "cannot admit namespace.*directly as Removing");
+}
+#endif
+
+/// Mutation caught: deriving the control outcome from the resolution snapshot would turn a stale
+/// leader's `FencedOut` into `Deleted` or `EntryChanged`. Resolution may prove the old life dead and
+/// carry its invalidation, but it cannot restore the caller's authority to continue the GC round.
+TEST(CASRefCatalogRemoval, FenceLossRemainsControlOutcomeWhenWinnerRemovesOrReplacesLife)
+{
+    for (const bool replace : {false, true})
+    {
+        EraseWinnerBackend backend;
+        const Layout layout(replace ? "replacement" : "absence");
+        const CatalogEntry removing{
+            .ns = RootNamespace{"a"},
+            .state = NsState::Removing,
+            .incarnation = UInt128{7},
+            .removal_started_round = 13};
+        ASSERT_EQ(backend.putIfAbsent(
+            layout.refCatalogKey(), encodeRefCatalog(RefCatalog{.entries = {removing}})).outcome,
+            PutOutcome::Done);
+
+        CasFoldSeal ready_parent;
+        ready_parent.ref_lives.emplace(UInt128{7}, RefLifeFoldState{
+            .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 2}},
+            .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 2}}});
+        std::optional<CatalogEntry> replacement;
+        if (replace)
+            replacement = CatalogEntry{
+                .ns = removing.ns,
+                .state = NsState::Live,
+                .incarnation = UInt128{8}};
+        backend.replaceOnNextCatalogCas(layout.refCatalogKey(), replacement);
+
+        const CasRefCatalog::CompletedRemovingDeleteResult result
+            = CasRefCatalog::deleteCompletedRemoving(
+                backend, layout, removing, ready_parent, 5, [&](uint64_t)
+                {
+                    if (backend.fenceMoved())
+                        return CasRefCatalog::LeaderFenceStatus::Moved;
+                    return CasRefCatalog::LeaderFenceStatus::Held;
+                });
+
+        EXPECT_EQ(result.outcome, CasRefCatalog::CompletedRemovingDeleteOutcome::FencedOut);
+        ASSERT_TRUE(result.invalidated_life);
+        EXPECT_EQ(*result.invalidated_life,
+            NamespaceLifeId::fromCatalogEntry(removing.ns, removing.incarnation));
+        const RefCatalog current = CasRefCatalog::read(backend, layout).catalog;
+        if (replace)
+            EXPECT_EQ(current.entries, std::vector<CatalogEntry>{*replacement});
+        else
+            EXPECT_TRUE(current.entries.empty());
+    }
+}
+
+/// Mutation caught: treating every authority-check exception as a moved fence hides corruption and
+/// backend/decode failures. Before any CAS, inability to evaluate authority must propagate unchanged.
+TEST(CASRefCatalogRemoval, NonFenceAuthorityExceptionPropagatesBeforeEraseCas)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("pre-cas-authority-error");
+    const CatalogEntry removing{
+        .ns = RootNamespace{"a"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{7},
+        .removal_started_round = 13};
+    ASSERT_EQ(backend.putIfAbsent(
+        layout.refCatalogKey(), encodeRefCatalog(RefCatalog{.entries = {removing}})).outcome,
+        PutOutcome::Done);
+    CasFoldSeal ready_parent;
+    ready_parent.ref_lives.emplace(UInt128{7}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 2}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 2}}});
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        (void)CasRefCatalog::deleteCompletedRemoving(
+            backend, layout, removing, ready_parent, 5, [](uint64_t) -> CasRefCatalog::LeaderFenceStatus
+            {
+                throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+                    "injected authority read failure before erase CAS");
+            });
+    });
+    EXPECT_EQ(backend.casPutCount(layout.refCatalogKey()), 0u);
+}
+
+/// The post-CAS authority check is distinct: the erase may already be durable and its mandatory
+/// resolution complete, but inability to evaluate authority is still the original error, not
+/// `FencedOut`.
+TEST(CASRefCatalogRemoval, NonFenceAuthorityExceptionPropagatesAfterEraseResolution)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("post-cas-authority-error");
+    const CatalogEntry removing{
+        .ns = RootNamespace{"a"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{7},
+        .removal_started_round = 13};
+    ASSERT_EQ(backend.putIfAbsent(
+        layout.refCatalogKey(), encodeRefCatalog(RefCatalog{.entries = {removing}})).outcome,
+        PutOutcome::Done);
+    CasFoldSeal ready_parent;
+    ready_parent.ref_lives.emplace(UInt128{7}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 2}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 2}}});
+
+    size_t authority_checks = 0;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        (void)CasRefCatalog::deleteCompletedRemoving(
+            backend, layout, removing, ready_parent, 5, [&](uint64_t)
+            {
+                if (++authority_checks == 2)
+                    throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+                        "injected authority read failure after erase resolution");
+                return CasRefCatalog::LeaderFenceStatus::Held;
+            });
+    });
+    EXPECT_EQ(authority_checks, 2u);
+    EXPECT_TRUE(CasRefCatalog::read(backend, layout).catalog.entries.empty());
+}
+
+/// Mutation caught: swallowing a synchronous `casPut` exception raised during the erase attempt
+/// itself (as opposed to the authority/fence check) and treating it as ordinary non-convergence
+/// would hide a real backend fault behind ProofRefused/EntryChanged, and would skip the mandatory
+/// resolution read that this branch's siblings above already prove runs before any conclusion.
+TEST(CASRefCatalogRemoval, CasPutExceptionPropagatesAfterMandatoryResolution)
+{
+    CasPutThrowsOnceBackend backend;
+    const Layout layout("cas-put-throw");
+    const CatalogEntry removing{
+        .ns = RootNamespace{"a"},
+        .state = NsState::Removing,
+        .incarnation = UInt128{7},
+        .removal_started_round = 13};
+    ASSERT_EQ(backend.putIfAbsent(
+        layout.refCatalogKey(), encodeRefCatalog(RefCatalog{.entries = {removing}})).outcome,
+        PutOutcome::Done);
+    CasFoldSeal ready_parent;
+    ready_parent.ref_lives.emplace(UInt128{7}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 2}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 2}}});
+
+    backend.armCasPutThrow(layout.refCatalogKey());
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        (void)CasRefCatalog::deleteCompletedRemoving(
+            backend, layout, removing, ready_parent, 5, [](uint64_t)
+            {
+                return CasRefCatalog::LeaderFenceStatus::Held;
+            });
+    });
+    /// The mandatory resolution read ran before the rethrow: the exact old row is still present,
+    /// unchanged by the failed attempt.
+    const RefCatalog current = CasRefCatalog::read(backend, layout).catalog;
+    EXPECT_EQ(current.entries, std::vector<CatalogEntry>{removing});
+}
+
+TEST(CASRefCatalogRemoval, CancelStalledCreatingRequiresExactRowAndTerminalCreatorFence)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("p");
+    const CatalogEntry creating{
+        .ns = RootNamespace{"a"},
+        .state = NsState::Creating,
+        .incarnation = UInt128{7},
+        .creator = CreatorFence{.server_root_id = "server", .writer_epoch = 3, .fence_generation = 4}};
+    ASSERT_EQ(backend.putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(RefCatalog{.entries = {creating}})).outcome,
+        PutOutcome::Done);
+
+    EXPECT_EQ(CasRefCatalog::cancelStalledCreating(
+        backend, layout, creating, [](const CreatorFence &) { return false; }, 5, [](uint64_t) {}),
+        CasRefCatalog::StalledCreatingCancelOutcome::CreatorFenceStillLive);
+    EXPECT_EQ(backend.casPutCount(layout.refCatalogKey()), 0);
+
+    CatalogEntry stale = creating;
+    stale.creator->writer_epoch = 2;
+    EXPECT_EQ(CasRefCatalog::cancelStalledCreating(
+        backend, layout, stale, [](const CreatorFence &) { return true; }, 5, [](uint64_t) {}),
+        CasRefCatalog::StalledCreatingCancelOutcome::EntryChanged);
+    EXPECT_EQ(backend.casPutCount(layout.refCatalogKey()), 0);
+
+    EXPECT_EQ(CasRefCatalog::cancelStalledCreating(
+        backend, layout, creating, [](const CreatorFence &) { return true; }, 5, [](uint64_t) {}),
+        CasRefCatalog::StalledCreatingCancelOutcome::Cancelled);
+    EXPECT_TRUE(CasRefCatalog::read(backend, layout).catalog.entries.empty());
+    EXPECT_EQ(backend.casPutCount(layout.refCatalogKey()), 1);
+    EXPECT_EQ(backend.listTotal(), 0);
+    EXPECT_EQ(backend.deleteTotal(), 0);
+}
+
+TEST(CASGCRefWalkPlan, CatalogIsSoleRowAdmissionAuthorityAcrossOrdinaryAndRebuildInputs)
+{
+    RefCatalog catalog;
+    catalog.entries = {
+        CatalogEntry{
+            .ns = RootNamespace{"creating"},
+            .state = NsState::Creating,
+            .incarnation = UInt128{1},
+            .creator = CreatorFence{.server_root_id = "server", .writer_epoch = 1, .fence_generation = 1}},
+        liveEntry("live", 2),
+        CatalogEntry{
+            .ns = RootNamespace{"removing"},
+            .state = NsState::Removing,
+            .incarnation = UInt128{3},
+            .removal_started_round = 8},
+    };
+    const CasRefCatalog::Snapshot cut{
+        .catalog = catalog, .token = std::nullopt, .life_index = CatalogLifeIndex(catalog)};
+
+    RefScanSummary ordinary_scan;
+    ordinary_scan.parent_ref_lives.emplace(UInt128{1}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 1}}});
+    ordinary_scan.parent_ref_lives.emplace(UInt128{3}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{3, 3}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{3, 3}}});
+    ordinary_scan.parent_ref_lives.emplace(UInt128{4}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{4, 4}}});
+    ordinary_scan.listed_lives = {UInt128{1}, UInt128{2}, UInt128{4}};
+    ordinary_scan.holds.emplace(UInt128{1}, RefHold{.offending_position = RefTxnId{1, 2}});
+    ordinary_scan.holds.emplace(UInt128{2}, RefHold{.offending_position = RefTxnId{2, 2}});
+    ordinary_scan.checkpoint_observations.emplace(UInt128{1}, RefTxnId{1, 9});
+    ordinary_scan.checkpoint_observations.emplace(UInt128{2}, RefTxnId{2, 9});
+    ordinary_scan.max_log_by_life.emplace(UInt128{1}, RefTxnId{1, 10});
+    ordinary_scan.max_log_by_life.emplace(UInt128{2}, RefTxnId{2, 10});
+
+    RefScanSummary rebuild_scan;
+    rebuild_scan.parent_ref_lives.emplace(UInt128{1}, ordinary_scan.parent_ref_lives.at(UInt128{1}));
+    rebuild_scan.parent_ref_lives.emplace(UInt128{5}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{5, 5}}});
+    rebuild_scan.listed_lives = {UInt128{1}, UInt128{3}, UInt128{5}};
+    rebuild_scan.holds.emplace(UInt128{1}, RefHold{.offending_position = RefTxnId{1, 3}});
+    rebuild_scan.holds.emplace(UInt128{3}, RefHold{.offending_position = RefTxnId{3, 4}});
+    rebuild_scan.checkpoint_observations.emplace(UInt128{1}, RefTxnId{1, 11});
+    rebuild_scan.checkpoint_observations.emplace(UInt128{3}, RefTxnId{3, 11});
+    rebuild_scan.max_log_by_life.emplace(UInt128{1}, RefTxnId{1, 12});
+    rebuild_scan.max_log_by_life.emplace(UInt128{3}, RefTxnId{3, 12});
+
+    const RefPlan ordinary = tests::buildRefWalkPlanForTest(ordinary_scan, cut);
+    const RefPlan rebuild = tests::buildRefWalkPlanForTest(rebuild_scan, cut);
+    const auto ordinary_parent_states = ordinary.parentFoldStates();
+    const auto rebuild_parent_states = rebuild.parentFoldStates();
+    const auto ordinary_successor_states = ordinary.successorFoldStates();
+    const auto rebuild_successor_states = rebuild.successorFoldStates();
+    EXPECT_EQ(ordinary_parent_states.size(), 1u);
+    EXPECT_TRUE(ordinary_parent_states.contains(UInt128{3}));
+    EXPECT_FALSE(ordinary_parent_states.contains(UInt128{2}));
+    EXPECT_TRUE(ordinary_successor_states.contains(UInt128{2}));
+    EXPECT_TRUE(ordinary_successor_states.contains(UInt128{3}));
+    EXPECT_TRUE(rebuild_parent_states.empty());
+    EXPECT_TRUE(rebuild_successor_states.contains(UInt128{2}));
+    EXPECT_TRUE(rebuild_successor_states.contains(UInt128{3}));
+    const std::set<UInt128> expected{UInt128{2}, UInt128{3}};
+    EXPECT_EQ(ordinary.lifeIds(), expected);
+    EXPECT_EQ(rebuild.lifeIds(), expected);
+
+    EXPECT_TRUE(ordinary.row(UInt128{2}).listed_hint);
+    ASSERT_TRUE(ordinary.row(UInt128{2}).fold_state.coverage.hold);
+    EXPECT_EQ(ordinary.row(UInt128{2}).checkpoint_observation, (RefTxnId{2, 9}));
+    EXPECT_EQ(ordinary.row(UInt128{2}).tail_observation, (RefTxnId{2, 10}));
+    const std::optional<RefCleanupEvidence> cleanup_evidence{
+        RefCleanupEvidence{.remove_txn_id = RefTxnId{3, 3}}};
+    EXPECT_EQ(ordinary.row(UInt128{3}).fold_state.cleanup_evidence, cleanup_evidence);
+    EXPECT_EQ(ordinary.row(UInt128{3}).removal_started_round, 8u);
+    EXPECT_FALSE(ordinary.contains(UInt128{1}));
+    EXPECT_FALSE(ordinary.contains(UInt128{4}));
+
+    EXPECT_TRUE(rebuild.row(UInt128{3}).listed_hint);
+    ASSERT_TRUE(rebuild.row(UInt128{3}).fold_state.coverage.hold);
+    EXPECT_EQ(rebuild.row(UInt128{3}).checkpoint_observation, (RefTxnId{3, 11}));
+    EXPECT_EQ(rebuild.row(UInt128{3}).tail_observation, (RefTxnId{3, 12}));
+    EXPECT_FALSE(rebuild.contains(UInt128{1}));
+    EXPECT_FALSE(rebuild.contains(UInt128{5}));
+}
+
+TEST(CASGCStuckRemoval, ThresholdAndRestartUseOnlyDurableRounds)
+{
+    const Layout layout("p");
+    RefWalkPlanRow row{
+        .life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"removing"}, UInt128{7}),
+        .fold_state = {},
+        .removal_started_round = 10,
+        .has_parent_fold_state = false,
+        .listed_hint = false,
+        .checkpoint_observation = std::nullopt,
+        .tail_observation = std::nullopt};
+
+    EXPECT_FALSE(stuckRemovalWarning(row, /*current_round=*/12, /*threshold_rounds=*/3, layout));
+    const auto at_threshold = stuckRemovalWarning(row, /*current_round=*/13, /*threshold_rounds=*/3, layout);
+    const auto next_round = stuckRemovalWarning(row, /*current_round=*/14, /*threshold_rounds=*/3, layout);
+    ASSERT_TRUE(at_threshold);
+    ASSERT_TRUE(next_round);
+    EXPECT_NE(at_threshold->find("age_rounds=3"), String::npos);
+    EXPECT_NE(next_round->find("age_rounds=4"), String::npos);
+
+    /// A fresh process given the same durable catalog row and adopted round produces the same signal.
+    EXPECT_EQ(stuckRemovalWarning(row, 13, 3, layout), at_threshold);
+
+    row.fold_state.cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 1}};
+    EXPECT_FALSE(stuckRemovalWarning(row, 100, 3, layout));
+}
+
+TEST(CASGCStuckRemoval, BoundaryAndAbsentVersusUnreadableMessagesAreExact)
+{
+    const Layout layout("p");
+    RefWalkPlanRow row{
+        .life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"removing"}, UInt128{7}),
+        .fold_state = {},
+        .removal_started_round = std::numeric_limits<uint64_t>::max(),
+        .has_parent_fold_state = false,
+        .listed_hint = false,
+        .checkpoint_observation = std::nullopt,
+        .tail_observation = std::nullopt};
+    EXPECT_FALSE(stuckRemovalWarning(row, 0, 1, layout));
+
+    row.removal_started_round = 1;
+    const auto absent = stuckRemovalWarning(row, 2, 1, layout);
+    ASSERT_TRUE(absent);
+    EXPECT_NE(absent->find("terminal has not folded"), String::npos);
+    EXPECT_EQ(absent->find("/_log/"), String::npos) << "an absent terminal has no exact id to name";
+
+    row.fold_state.coverage.classification = 4;
+    row.fold_state.coverage.hold = RefHold{
+        .reason = HoldReason::BodyUndecodable,
+        .offending_position = RefTxnId{5, 6}};
+    const auto unreadable = stuckRemovalWarning(row, 2, 1, layout);
+    ASSERT_TRUE(unreadable);
+    EXPECT_NE(unreadable->find(layout.refLogKey(row.life, RefTxnId{5, 6})), String::npos);
+    EXPECT_NE(unreadable->find("is unreadable"), String::npos);
+    EXPECT_NE(unreadable->find("restore the exact object"), String::npos);
+    EXPECT_NE(unreadable->find("recreate the pool"), String::npos);
+    EXPECT_EQ(unreadable->find("REBUILD"), String::npos)
+        << "the diagnostic must not promise a command that cannot recover this exact object";
+}
+
+TEST(CASGCStuckRemoval, DiagnosticDoesNotAppendOrMutateBackend)
+{
+    DB::Cas::tests::CountingBackend backend;
+    const Layout layout("p");
+    const RefWalkPlanRow row{
+        .life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"removing"}, UInt128{7}),
+        .fold_state = {},
+        .removal_started_round = 1,
+        .has_parent_fold_state = false,
+        .listed_hint = false,
+        .checkpoint_observation = std::nullopt,
+        .tail_observation = std::nullopt};
+    const uint64_t puts_before = backend.putTotal();
+    const uint64_t cas_before = backend.casPutTotal();
+    EXPECT_TRUE(stuckRemovalWarning(row, 11, 10, layout));
+    EXPECT_EQ(backend.putTotal(), puts_before);
+    EXPECT_EQ(backend.casPutTotal(), cas_before);
+    EXPECT_EQ(backend.deleteTotal(), 0u);
+}
+
+TEST(CASGCStuckRemoval, AdoptedRoundWarnsEveryRestartWithoutAppending)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p",
+        .server_root_id = "test",
+        .gc_stuck_removal_rounds = 10});
+    const Layout & layout = store->layout();
+    const UInt128 gc_id{99};
+    const UInt128 life_id{7};
+
+    const CatalogEntry removing{
+        .ns = RootNamespace{"removing"},
+        .state = NsState::Removing,
+        .incarnation = life_id,
+        .removal_started_round = 1};
+    const auto catalog = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog);
+    ASSERT_EQ(backend->casPut(
+        layout.refCatalogKey(), encodeRefCatalog(RefCatalog{.entries = {removing}}), catalog->token).outcome,
+        CasOutcome::Committed);
+
+    CasFoldSeal seal;
+    seal.generation = 1;
+    seal.ref_lives.emplace(life_id, RefLifeFoldState{
+        .coverage = RefCoverage{
+            .classification = 4,
+            .hold = RefHold{
+                .reason = HoldReason::BodyUndecodable,
+                .offending_position = RefTxnId{5, 6},
+                .retry_count = 0,
+                .next_retry_round = 12}}});
+    seal.condemned_summary[0] = CondemnedSummary{};
+    ASSERT_EQ(backend->putIfAbsent(layout.foldSealKey(1, 1), encodeFoldSeal(seal)).outcome, PutOutcome::Done);
+
+    GcState state;
+    state.lease = GcLease{.owner = gc_id, .seq = 1};
+    state.round = 11;
+    state.gc_shards = 1;
+    state.snap_generation = 1;
+    state.snap_attempt = 1;
+    ASSERT_EQ(backend->putIfAbsent(layout.gcStateKey(), encodeGcState(state)).outcome, PutOutcome::Done);
+
+    const uint64_t signals_before
+        = ProfileEvents::global_counters[ProfileEvents::CASGCStuckRemovals].load();
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(removing.ns, life_id);
+    const String unreadable_ref_log_key = layout.refLogKey(life, RefTxnId{5, 6});
+    const uint64_t append_puts_before = backend->putCount(unreadable_ref_log_key);
+    ScopedCasGcLogCapture log_capture;
+    Gc first_process(store, gc_id);
+    EXPECT_TRUE(first_process.runRegularRound().acquired_lease);
+    Gc restarted_process(store, gc_id);
+    EXPECT_TRUE(restarted_process.runRegularRound().acquired_lease);
+
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASGCStuckRemovals].load() - signals_before, 2u);
+    EXPECT_EQ(backend->putCount(unreadable_ref_log_key), append_puts_before)
+        << "the diagnostic cannot append the unreadable ref log";
+    const String captured = log_capture.captured();
+    EXPECT_EQ(std::count(captured.begin(), captured.end(), '\n'), 2u);
+    EXPECT_NE(captured.find(unreadable_ref_log_key), String::npos);
+    EXPECT_NE(captured.find("is unreadable"), String::npos);
+}
+
+TEST(CASGCStuckRemoval, ZeroThresholdIsRefusedAtGcConstruction)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p",
+        .server_root_id = "test",
+        .gc_stuck_removal_rounds = 0});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS,
+        [&] { Gc gc(store, UInt128{1}); });
+}
+
+TEST(CASGCRefWalkPlan, UnmatchedAdoptedParentLifeIsObservedWithoutEnteringThePlan)
+{
+    const NamespaceLifePhysicalId current_life{2};
+    const NamespaceLifePhysicalId unmatched_life =
+        hexToU128("fedcba98765432100123456789abcdef");
+    RefCatalog catalog{.entries = {liveEntry("live", 2)}};
+    const CasRefCatalog::Snapshot cut{
+        .catalog = catalog, .token = std::nullopt, .life_index = CatalogLifeIndex(catalog)};
+    RefScanSummary scan;
+    scan.parent_ref_lives.emplace(current_life, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{2, 3}}});
+    scan.parent_ref_lives.emplace(unmatched_life, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{9, 9}}});
+
+    const uint64_t events_before =
+        ProfileEvents::global_counters[ProfileEvents::CASGCUnmatchedAdoptedParentLives].load();
+    const RefPlan plan = tests::buildRefWalkPlanForTest(scan, cut);
+
+    EXPECT_EQ(
+        ProfileEvents::global_counters[ProfileEvents::CASGCUnmatchedAdoptedParentLives].load() - events_before,
+        1u);
+    EXPECT_EQ(plan.droppedParentRows(), 1u);
+    EXPECT_EQ(plan.size(), 1u);
+    EXPECT_TRUE(plan.contains(current_life));
+    EXPECT_FALSE(plan.contains(unmatched_life));
+    EXPECT_FALSE(plan.parentFoldStates().contains(unmatched_life));
+    EXPECT_FALSE(plan.successorFoldStates().contains(unmatched_life));
+}
+
+TEST(CASGCRefPlan, RoundInputOwnsObservationsAndSuccessorStateCannotChangePlan)
+{
+    /// This catches a plan that borrows the post-LIST observations or lets its successor state alias a
+    /// row. Replacing the owning `RoundInput`/`RefPlan` boundary with the former loose inputs, or
+    /// returning plan storage for the successor, must make this fail.
+    static_assert(!std::is_constructible_v<RoundInput, RefScanSummary, CasRefCatalog::Snapshot>);
+    static_assert(!std::is_default_constructible_v<RoundInput>);
+    static_assert(!std::is_default_constructible_v<RefPlan>);
+    static_assert(!std::is_assignable_v<RefPlan &, RefPlan>);
+    static_assert(!std::is_assignable_v<RoundInput &, RoundInput>);
+
+    RefCatalog catalog;
+    catalog.entries = {liveEntry("live", 2)};
+    CasRefCatalog::Snapshot cut{
+        .catalog = catalog, .token = std::nullopt, .life_index = CatalogLifeIndex(catalog)};
+
+    RefScanSummary observations;
+    observations.max_log_by_life.emplace(UInt128{2}, RefTxnId{2, 7});
+    observations.parent_ref_lives.emplace(UInt128{2}, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{2, 3}}});
+
+    const RefPlan plan = tests::buildRefWalkPlanForTest(observations, cut);
+
+    /// The caller may reuse and mutate the sources after its one post-LIST/catalog observation and
+    /// plan construction. Those mutations cannot retarget the plan DEFER, fold, and publication use.
+    observations.max_log_by_life.at(UInt128{2}) = RefTxnId{2, 99};
+    observations.parent_ref_lives.at(UInt128{2}).coverage.last_folded_ref_id = RefTxnId{2, 88};
+    cut.catalog.entries.clear();
+
+    ASSERT_TRUE(plan.contains(UInt128{2}));
+    EXPECT_EQ(plan.row(UInt128{2}).tail_observation, (RefTxnId{2, 7}));
+    EXPECT_EQ(plan.row(UInt128{2}).fold_state.coverage.last_folded_ref_id, (RefTxnId{2, 3}));
+
+    /// A fold/rebuild successor starts as a copy. It can earn a new cleanup state without changing the
+    /// immutable input that DEFER, the fold, and publication all consume.
+    auto successor_lives = plan.successorFoldStates();
+    successor_lives.at(UInt128{2}).coverage.last_folded_ref_id = RefTxnId{2, 9};
+    successor_lives.emplace(UInt128{9}, RefLifeFoldState{});
+    EXPECT_EQ(plan.row(UInt128{2}).fold_state.coverage.last_folded_ref_id, (RefTxnId{2, 3}));
+    EXPECT_FALSE(plan.contains(UInt128{9}));
+}
diff --git a/src/Disks/tests/gtest_cas_ref_catalog_birth_wiring.cpp b/src/Disks/tests/gtest_cas_ref_catalog_birth_wiring.cpp
new file mode 100644
index 000000000000..1ea4b8096e4d
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_catalog_birth_wiring.cpp
@@ -0,0 +1,512 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <limits>
+#include <memory>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int NETWORK_ERROR;
+}
+
+/// Stage B Task 4-C: production birth wiring. `CasRefLedger::resolveNamespaceLife`, called from
+/// `ensureRefTableRecovered`, resolves a namespace's real catalog life ONCE per table-open --
+/// create-if-absent, adopt an existing `Live`/`Removing` entry, or reconcile a stale `Creating` one via
+/// `CasRefCatalog::reconcileStaleCreator` + `isCreatorFenceTerminal` -- so every ref-layer object a
+/// mounted writer produces is keyed at a real, catalog-proven incarnation (spec INV-3), never the
+/// Stage-A sentinel.
+///
+/// OBLIGATION 3 (carried from Task 3's review, closed here): Task 3 could only enforce "`Creating`
+/// forbids publication" (`CasRefCatalog::checkPublicationAdmittedOrThrow`) AT THE CATALOG LEVEL, because
+/// nothing on the production ref-write path consulted the catalog at all. The refusal this suite pins
+/// below rests on CONSTRUCTION, not a check: there is no `if (state == Creating) throw` anywhere in
+/// `appendRefOps`'s path. `ensureRefTableRecovered` simply cannot make a table's runtime usable
+/// (`rt.recovered` never becomes `true`, `rt.life` never gets set) while the catalog entry is `Creating`
+/// under a fence that is not provably dead -- so no append can reach `commitRefChunk` for such a
+/// namespace, by construction, stronger than any per-write check could prove. Stated here so nobody
+/// later greps for a check and concludes the gap Task 3's review flagged is still open.
+///
+/// The suite name is prefixed `Cas` so it is covered by the `Cas*` unit-test gate filter.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+/// Fault the mandatory catalog's very first bootstrap write before it reaches durable storage. This
+/// models a definite write failure, distinct from the acknowledgement-loss shape below: a retry must
+/// still be allowed to prove a new pool, and the failed first attempt must not have published
+/// `_pool_meta` without the catalog it makes mandatory.
+class CatalogBootstrapPutFailsOnceBackend final : public CountingBackend
+{
+public:
+    using CountingBackend::putIfAbsent;
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (fail_once && key == Layout{"p"}.refCatalogKey())
+        {
+            fail_once = false;
+            throw Poco::TimeoutException("CatalogBootstrapPutFailsOnceBackend: catalog PUT did not land");
+        }
+        return CountingBackend::putIfAbsent(key, bytes, meta);
+    }
+
+private:
+    bool fail_once = true;
+};
+
+class CatalogCancellationRaceBackend final : public CountingBackend
+{
+public:
+    using CountingBackend::casPut;
+
+    CasResult casPut(
+        const String & key, const String & bytes, const std::optional<Token> & expected,
+        const ObjectMeta & meta) override
+    {
+        if (race_armed && key == Layout{"p"}.refCatalogKey())
+        {
+            race_armed = false;
+            on_catalog_cas();
+        }
+        return CountingBackend::casPut(key, bytes, expected, meta);
+    }
+
+    bool race_armed = false;
+    std::function<void()> on_catalog_cas;
+};
+
+PoolPtr openPoolForBirthTest(const BackendPtr & backend, const String & server_root_id = "test")
+{
+    seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = server_root_id});
+}
+
+const CatalogEntry * findEntry(const RefCatalog & catalog, const RootNamespace & ns)
+{
+    for (const CatalogEntry & e : catalog.entries)
+        if (e.ns.string() == ns.string())
+            return &e;
+    return nullptr;
+}
+
+/// The same one-transaction publish `gtest_cas_ref_ckpt.cpp`'s `publishRef` drives: a namespace's first
+/// append through the REAL append lane, which is also what triggers `resolveNamespaceLife`.
+RefTxnId publishBirth(const PoolPtr & store, const RootNamespace & ns, const String & ref)
+{
+    return store->appendRefOps(ns, MutationScope::ref(ref),
+        [&ref](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (const RefOp & op : publishCommittedOps(ref, ManifestRef{1, 1, 1}))
+                ops.push_back(op);
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Publish);
+}
+
+}
+
+/// The happy path: nothing to reconcile, no pre-existing entry. The first append mints a fresh `Live`
+/// catalog entry and keys the birth transaction at it -- not at the Stage-A sentinel.
+TEST(CASRefCatalogBirthWiring, FirstOpenMintsALiveCatalogEntryAndKeysTheBirthAtIt)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const RootNamespace ns{"srv1/birth_wiring"};
+
+    const RefTxnId id = publishBirth(store, ns, "a");
+    EXPECT_EQ(id, (RefTxnId{store->writerEpoch(), 1}));
+
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(*backend, store->layout());
+    const CatalogEntry * entry = findEntry(snap.catalog, ns);
+    ASSERT_NE(entry, nullptr) << "the first open must mint a catalog entry";
+    EXPECT_EQ(entry->state, NsState::Live);
+    EXPECT_NE(entry->incarnation, UInt128(0));
+    EXPECT_EQ(entry->creator, std::nullopt) << "creator is forbidden outside Creating (strict grammar)";
+
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry->ns, entry->incarnation);
+    EXPECT_TRUE(backend->head(store->layout().refLogKey(life, id)).exists)
+        << "the birth transaction must be keyed at the REAL minted incarnation, not the Stage-A sentinel";
+    EXPECT_FALSE(backend->head(store->layout().refLogKey(fixture::fixtureLife(ns), id)).exists)
+        << "and must NOT be keyed at the sentinel any more";
+}
+
+TEST(CASRefCatalogBirthWiring, CatalogLossAfterMountCannotRecreateAOneRowAuthority)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const Layout & layout = store->layout();
+
+    publishBirth(store, RootNamespace{"srv1/existing"}, "old");
+    const auto catalog = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog);
+    ASSERT_EQ(backend->deleteExact(layout.refCatalogKey(), catalog->token).kind,
+              DeleteOutcome::Kind::Deleted);
+    backend->resetCounts();
+
+    EXPECT_THROW(publishBirth(store, RootNamespace{"srv1/new"}, "new"), DB::Exception);
+    EXPECT_FALSE(backend->head(layout.refCatalogKey()).exists)
+        << "runtime loss must not be repaired with a one-row replacement authority";
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+    EXPECT_EQ(backend->putTotal(), 0u)
+        << "the failed birth must not publish a checkpoint or ref-log body";
+    EXPECT_EQ(backend->putOverwriteTotal(), 0u);
+}
+
+TEST(CASRefCatalogBirthWiring, FailedCatalogBootstrapDoesNotPublishPoolMetaAndRetryConverges)
+{
+    auto backend = std::make_shared<CatalogBootstrapPutFailsOnceBackend>();
+    const Layout layout{"p"};
+
+    EXPECT_ANY_THROW(Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}));
+    EXPECT_FALSE(backend->head(layout.poolMetaKey()).exists)
+        << "a failed mandatory catalog bootstrap must leave no authoritative pool meta behind";
+    EXPECT_FALSE(backend->head(layout.refCatalogKey()).exists);
+
+    PoolPtr retry;
+    ASSERT_NO_THROW(retry = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}));
+    EXPECT_TRUE(backend->head(layout.poolMetaKey()).exists);
+    EXPECT_TRUE(backend->head(layout.refCatalogKey()).exists);
+}
+
+TEST(CASRefCatalogBirthWiring, LostCatalogBootstrapAcknowledgementLeavesOnlyRetryableCatalogResidue)
+{
+    auto backend = std::make_shared<LandedButAckLostOnceBackend>();
+    const Layout layout{"p"};
+    backend->key_substr = layout.refCatalogKey();
+
+    EXPECT_ANY_THROW(Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}));
+    EXPECT_FALSE(backend->head(layout.poolMetaKey()).exists);
+    EXPECT_TRUE(backend->head(layout.refCatalogKey()).exists)
+        << "the injected write must land before its acknowledgement is lost";
+
+    PoolPtr retry;
+    ASSERT_NO_THROW(retry = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}));
+    EXPECT_TRUE(backend->head(layout.poolMetaKey()).exists);
+}
+
+TEST(CASRefCatalogBirthWiring, BootstrapConflictExactReadsTheCanonicalEmptyCatalog)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout{"p"};
+    const String canonical_empty = encodeRefCatalog(RefCatalog{});
+    ASSERT_EQ(backend->putIfAbsent(layout.refCatalogKey(), canonical_empty).outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::initializeEmptyForNewPool(*backend, layout);
+    EXPECT_TRUE(snap.catalog.entries.empty());
+    EXPECT_EQ(backend->putCount(layout.refCatalogKey()), 1u);
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), 1u)
+        << "a concurrent bootstrap winner must be exact-read before acceptance";
+}
+
+TEST(CASRefCatalogBirthWiring, BootstrapConflictRefusesANonemptyCatalog)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout{"p"};
+    const RefCatalog nonempty{.entries = {CatalogEntry{
+        .ns = RootNamespace{"test/nonempty"}, .state = NsState::Live, .incarnation = UInt128{1}, .creator = std::nullopt}}};
+    ASSERT_EQ(backend->putIfAbsent(layout.refCatalogKey(), encodeRefCatalog(nonempty)).outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { CasRefCatalog::initializeEmptyForNewPool(*backend, layout); });
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), 1u);
+}
+
+TEST(CASRefCatalogBirthWiring, ExistingPoolMetaWithMissingCatalogStillFailsClosed)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    PoolPtr first = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+    const auto catalog = backend->get(first->layout().refCatalogKey());
+    ASSERT_TRUE(catalog);
+    ASSERT_EQ(backend->deleteExact(first->layout().refCatalogKey(), catalog->token).kind,
+              DeleteOutcome::Kind::Deleted);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"}); });
+}
+
+TEST(CASRefCatalogBirthWiring, RestartFixturePreservesItsExistingNonemptyCatalog)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout{"p"};
+    seedPoolMetaForRestart(*backend);
+    const auto empty = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(empty);
+
+    const RefCatalog nonempty{.entries = {CatalogEntry{
+        .ns = RootNamespace{"test/preserved"}, .state = NsState::Live, .incarnation = UInt128{1}, .creator = std::nullopt}}};
+    const String bytes = encodeRefCatalog(nonempty);
+    ASSERT_EQ(backend->putOverwrite(layout.refCatalogKey(), bytes, empty->token).outcome, PutOutcome::Done);
+    const auto before = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(before);
+
+    seedPoolMetaForRestart(*backend);
+    const auto after = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(after);
+    EXPECT_EQ(after->bytes, before->bytes);
+    EXPECT_EQ(after->token, before->token);
+}
+
+/// A namespace whose catalog entry is ALREADY `Live` (e.g. admitted by an earlier mount that this
+/// runtime never cached) must be ADOPTED, never re-minted: `CasRefCatalog::createNamespace` refuses
+/// outright once any entry exists, so `resolveNamespaceLife` has no create branch left to take here --
+/// only the adopt branch can succeed.
+TEST(CASRefCatalogBirthWiring, AnExistingLiveEntryIsAdoptedRatherThanReminted)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/adopt_live"};
+
+    const CatalogEntry entry{.ns = ns, .state = NsState::Live, .incarnation = UInt128(0xcafe),
+                             .creator = std::nullopt};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, entry);
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = store->writerEpoch(),
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    const RefTxnId id = publishBirth(store, ns, "a");
+    EXPECT_EQ(id, (RefTxnId{store->writerEpoch(), 1}));
+
+    /// The read result must outlive the returned pointer -- findEntry points into its entries.
+    const auto after_cut = CasRefCatalog::read(*backend, layout);
+    const CatalogEntry * after = findEntry(after_cut.catalog, ns);
+    ASSERT_NE(after, nullptr);
+    EXPECT_EQ(after->incarnation, UInt128(0xcafe)) << "adopted, not re-minted";
+    EXPECT_EQ(after->state, NsState::Live);
+
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(ns, UInt128(0xcafe));
+    EXPECT_TRUE(backend->head(layout.refLogKey(life, id)).exists);
+}
+
+/// OBLIGATION 3, pinned through the PRODUCTION path: a `Creating` entry left by a DIFFERENT, still-live
+/// (or at least not provably dead) actor refuses every append -- no test-only seam, no direct call to
+/// `resolveNamespaceLife`/`reconcileStaleCreator`, just an ordinary `appendRefOps`.
+TEST(CASRefCatalogBirthWiring, ANamespaceStuckCreatingUnderALiveForeignFenceRefusesProductionPublicationByConstruction)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/stuck_creating"};
+
+    /// A DIFFERENT actor's `Creating` entry naming a server root that never mounted at all --
+    /// `isCreatorFenceTerminal`'s own doc: an ABSENT mount slot answers nothing about liveness, so it
+    /// is treated as NOT terminal (fail closed), never as proof of death.
+    const CreatorFence foreign_creator{.server_root_id = "ghost-server", .writer_epoch = 9, .fence_generation = 1};
+    const CatalogEntry entry{.ns = ns, .state = NsState::Creating, .incarnation = UInt128(0xdead),
+                             .creator = foreign_creator};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, entry);
+    backend->resetCounts();
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { publishBirth(store, ns, "a"); });
+
+    /// Nothing was written: the entry is exactly as observed, still Creating, still the foreign fence.
+    /// The read result must outlive the returned pointer -- findEntry points into its entries.
+    const auto still_cut = CasRefCatalog::read(*backend, layout);
+    const CatalogEntry * still = findEntry(still_cut.catalog, ns);
+    ASSERT_NE(still, nullptr);
+    EXPECT_EQ(*still, entry) << "a refused resolution must write nothing";
+    EXPECT_EQ(backend->putTotal(), 0u);
+    EXPECT_EQ(backend->putOverwriteTotal(), 0u);
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+}
+
+/// The mirror image, and Task 3's own deferred obligation ("wire `reconcileStaleCreator` and pin it
+/// with a test that drives reconciliation through the discovery path rather than by calling the
+/// primitive directly"): a dead predecessor's `Creating` entry is reconciled onto THIS mount and
+/// completed to `Live`, over the SAME incarnation -- resumption, not rebirth.
+TEST(CASRefCatalogBirthWiring, AStaleCreatingEntryFromATerminatedForeignFenceIsReconciledThroughTheProductionPath)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForBirthTest(backend, "this-server");
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/reconciled"};
+
+    /// A dead predecessor's `Creating` entry: its mount lease carries the clean-farewell sentinel
+    /// (`min_active == UINT64_MAX`), one of `isCreatorFenceTerminal`'s three certificates of death.
+    const CreatorFence dead_creator{.server_root_id = "dead-server", .writer_epoch = 3, .fence_generation = 1};
+    const CatalogEntry entry{.ns = ns, .state = NsState::Creating, .incarnation = UInt128(0xbeef),
+                             .creator = dead_creator};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, entry);
+    setWatermarkMinActive(*backend, layout, "dead-server", /*writer_epoch=*/3,
+                          /*min_active=*/std::numeric_limits<uint64_t>::max());
+
+    /// The production path resumes creation itself: reconciles the stale entry onto THIS mount's own
+    /// fence and completes it to `Live`, over the SAME incarnation the dead creator minted.
+    const RefTxnId id = publishBirth(store, ns, "a");
+    EXPECT_EQ(id, (RefTxnId{store->writerEpoch(), 1}));
+
+    /// The read result must outlive the returned pointer -- findEntry points into its entries.
+    const auto live_cut = CasRefCatalog::read(*backend, layout);
+    const CatalogEntry * live = findEntry(live_cut.catalog, ns);
+    ASSERT_NE(live, nullptr);
+    EXPECT_EQ(live->state, NsState::Live);
+    EXPECT_EQ(live->incarnation, UInt128(0xbeef)) << "the SAME incarnation throughout -- resumption, not rebirth";
+    EXPECT_EQ(live->creator, std::nullopt);
+
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(ns, UInt128(0xbeef));
+    EXPECT_TRUE(backend->head(layout.refLogKey(life, id)).exists);
+}
+
+TEST(CASRefCatalogBirthWiring, DropRefusesLiveCreatingFenceWithZeroCatalogMutation)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"drop_live_creator"};
+    const CatalogEntry creating{
+        .ns = ns,
+        .state = NsState::Creating,
+        .incarnation = UInt128{0xd001},
+        .creator = CreatorFence{.server_root_id = "unproven-live", .writer_epoch = 7, .fence_generation = 1}};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, creating);
+    backend->resetCounts();
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(ns); });
+    EXPECT_EQ(backend->putTotal(), 0u);
+    EXPECT_EQ(backend->putOverwriteTotal(), 0u);
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+    EXPECT_EQ(backend->deleteTotal(), 0u);
+    EXPECT_EQ(CasRefCatalog::read(*backend, layout).catalog.entries, std::vector<CatalogEntry>{creating});
+}
+
+TEST(CASRefCatalogBirthWiring, DropDeletesTerminalCreatingExactlyAndLeavesCkptForJanitor)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"drop_terminal_creator"};
+    const CatalogEntry creating{
+        .ns = ns,
+        .state = NsState::Creating,
+        .incarnation = UInt128{0xd002},
+        .creator = CreatorFence{.server_root_id = "dead-creator", .writer_epoch = 8, .fence_generation = 1}};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, creating);
+    setWatermarkMinActive(*backend, layout, "dead-creator", 8, std::numeric_limits<uint64_t>::max());
+    const NamespaceLifeId old_life = NamespaceLifeId::fromCatalogEntry(ns, creating.incarnation);
+    const String ckpt_key = layout.refCkptKey(old_life);
+    ASSERT_EQ(backend->putIfAbsent(ckpt_key, "stalled-ckpt").outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    store->dropNamespace(ns);
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), 1u);
+    EXPECT_EQ(backend->deleteTotal(), 0u);
+    EXPECT_EQ(backend->deleteCount(ckpt_key), 0u);
+    EXPECT_TRUE(backend->head(ckpt_key).exists);
+    EXPECT_TRUE(CasRefCatalog::read(*backend, layout).catalog.entries.empty());
+
+    const NamespaceLifeId reborn = store->namespaceLife(ns);
+    EXPECT_NE(reborn.incarnation, old_life.incarnation);
+}
+
+TEST(CASRefCatalogBirthWiring, DropLosesExactCreatingRaceToReconciliationWithoutDeletingCkpt)
+{
+    auto backend = std::make_shared<CatalogCancellationRaceBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"drop_reconcile_race"};
+    const CreatorFence old_creator{
+        .server_root_id = "dead-racing-creator", .writer_epoch = 9, .fence_generation = 1};
+    const CatalogEntry creating{
+        .ns = ns, .state = NsState::Creating, .incarnation = UInt128{0xd003}, .creator = old_creator};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, creating);
+    setWatermarkMinActive(
+        *backend, layout, old_creator.server_root_id, old_creator.writer_epoch,
+        std::numeric_limits<uint64_t>::max());
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(ns, creating.incarnation);
+    const String ckpt_key = layout.refCkptKey(life);
+    ASSERT_EQ(backend->putIfAbsent(ckpt_key, "stalled-ckpt").outcome, PutOutcome::Done);
+    backend->on_catalog_cas = [&]
+    {
+        EXPECT_EQ(CasRefCatalog::reconcileStaleCreator(
+            *backend, layout, creating,
+            CreatorFence{.server_root_id = "replacement", .writer_epoch = 10, .fence_generation = 1},
+            [](const CreatorFence &) { return true; }, store->fenceGeneration(),
+            [](uint64_t) {}), CasRefCatalog::ReconcileCreatorOutcome::Reconciled);
+    };
+    backend->race_armed = true;
+    backend->resetCounts();
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(ns); });
+    EXPECT_EQ(backend->deleteTotal(), 0u);
+    EXPECT_EQ(backend->deleteCount(ckpt_key), 0u);
+    EXPECT_TRUE(backend->head(ckpt_key).exists);
+    const CasRefCatalog::Snapshot after = CasRefCatalog::read(*backend, layout);
+    ASSERT_EQ(after.catalog.entries.size(), 1u);
+    ASSERT_TRUE(after.catalog.entries.front().creator);
+    EXPECT_EQ(after.catalog.entries.front().creator->server_root_id, "replacement");
+}
+
+TEST(CASRefCatalogBirthWiring, FencedDropCannotCancelTerminalCreating)
+{
+    auto backend = std::make_shared<CatalogCancellationRaceBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"fenced_drop_terminal_creator"};
+    const CatalogEntry creating{
+        .ns = ns,
+        .state = NsState::Creating,
+        .incarnation = UInt128{0xd004},
+        .creator = CreatorFence{.server_root_id = "dead-fenced-creator", .writer_epoch = 11, .fence_generation = 1}};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, creating);
+    setWatermarkMinActive(*backend, layout, "dead-fenced-creator", 11, std::numeric_limits<uint64_t>::max());
+    backend->resetCounts();
+
+    /// The first cancellation attempt passes its fence check, then loses its catalog CAS while the
+    /// local mount is re-armed at a new fence generation. The retry must re-check the caller fence and
+    /// refuse before another catalog mutation attempt.
+    backend->on_catalog_cas = [&]
+    {
+        rearmMountFenceAfterAnomalyForTest(store);
+        backend->failNextCasPut(layout.refCatalogKey());
+    };
+    backend->race_armed = true;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(ns); });
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), 1u);
+    EXPECT_EQ(backend->deleteTotal(), 0u);
+    EXPECT_EQ(CasRefCatalog::read(*backend, layout).catalog.entries, std::vector<CatalogEntry>{creating});
+}
+
+TEST(CASRefCatalogBirthWiring, ExactOldLifeCannotCancelReplacementTerminalCreating)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPoolForBirthTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"exact_old_life_terminal_creator"};
+    const NamespaceLifeId predecessor = NamespaceLifeId::fromCatalogEntry(ns, UInt128{0xd005});
+    const CatalogEntry successor{
+        .ns = ns,
+        .state = NsState::Creating,
+        .incarnation = UInt128{0xd006},
+        .creator = CreatorFence{.server_root_id = "dead-successor-creator", .writer_epoch = 12, .fence_generation = 1}};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, successor);
+    setWatermarkMinActive(*backend, layout, "dead-successor-creator", 12, std::numeric_limits<uint64_t>::max());
+    const String ckpt_key = layout.refCkptKey(NamespaceLifeId::fromCatalogEntry(ns, successor.incarnation));
+    ASSERT_EQ(backend->putIfAbsent(ckpt_key, "successor-ckpt").outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(predecessor); });
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+    EXPECT_EQ(backend->deleteTotal(), 0u);
+    EXPECT_EQ(backend->deleteCount(ckpt_key), 0u);
+    EXPECT_EQ(CasRefCatalog::read(*backend, layout).catalog.entries, std::vector<CatalogEntry>{successor});
+}
diff --git a/src/Disks/tests/gtest_cas_ref_chunk_preparation.cpp b/src/Disks/tests/gtest_cas_ref_chunk_preparation.cpp
new file mode 100644
index 000000000000..c9cc317ab813
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_chunk_preparation.cpp
@@ -0,0 +1,276 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+
+#include <Common/Exception.h>
+
+#include <optional>
+#include <string>
+#include <utility>
+#include <vector>
+
+/// `prepareRefChunk` is the pure half of `commitRefChunk` (Stage B directive
+/// `{#extract-prepare-ref-chunk}`): everything the append lane DECIDES before this chunk can have any
+/// durable effect. This TU is where that purity is exercised, and it is deliberately backend-free --
+/// nothing below names a backend, a pool, a ledger instance or a clock, and nothing constructs one. The
+/// mechanical guarantee is `static` on `prepareRefChunk` itself: with no `this` there is no member
+/// backend, runtime, clock or lock reachable from inside it, so a future edit cannot quietly reach for
+/// one and still compile here.
+///
+/// What that buys is exactly what shows up below: every case is a direct call, so INV-2's chain-link
+/// grammar is swept as a cross product -- including its negatives -- instead of being probed through
+/// I/O.
+///
+/// The value the extraction protects is pinned elsewhere on purpose: the equivalence fences in
+/// `gtest_cas_ref_ckpt.cpp` assert that the durable key, the sealed bytes and the per-key request
+/// counts a REAL append produces are unchanged. Those need a backend, so they live there and this TU
+/// stays pure.
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+const RootNamespace kNs{"srv1/prep@cas@"};
+const Layout kLayout{"p"};
+/// `prepareRefChunk` takes a resolved catalog life (Stage B, Task 4-C), not a bare namespace; this TU
+/// is deliberately backend-free (no catalog to resolve one from), so it threads the Stage-A sentinel
+/// through EXPLICITLY as its own test input -- the same value production minted internally before
+/// Task 4-C, so every golden byte/key assertion below is unchanged.
+const NamespaceLifeId kLife = DB::Cas::tests::fixture::fixtureLife(kNs);
+
+RefOp birthOp()
+{
+    RefOp op;
+    op.kind = RefOpKind::NamespaceBirth;
+    return op;
+}
+
+RefOp epochSealOp()
+{
+    RefOp op;
+    op.kind = RefOpKind::EpochSeal;
+    return op;
+}
+
+/// A minimal content op: the `AddPrecommit` shape (a pure add of a PRECOMMIT owner). A committed owner
+/// is only ever reached by promoting a precommit, so this is the smallest legal content transition.
+RefOp addPrecommitOp(const String & ref_name, const ManifestRef & manifest)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref_name, manifest};
+    return op;
+}
+
+ManifestRef mref(uint64_t seq)
+{
+    return ManifestRef{1, seq, 1};
+}
+
+/// One live namespace, born at `{1,1}`, as the state a later chunk prepares against.
+RefTableState bornState()
+{
+    RefTableState state;
+    applyRefLogTxn(state, RefLogTxn{kNs.string(), RefTxnId{1, 1}, {birthOp()}, std::nullopt});
+    return state;
+}
+
+/// Asserts that preparation REFUSES with `CORRUPTED_DATA` -- the code every ref-log grammar violation
+/// normalises to -- and that the message names the chain link, so a row cannot pass because some
+/// unrelated validator happened to throw first.
+template <class F>
+void expectGrammarRefusal(F && body, const char * what)
+{
+    try
+    {
+        std::forward<F>(body)();
+        FAIL() << "expected a grammar refusal: " << what;
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA) << what;
+        EXPECT_NE(e.message().find("prev_epoch_seal"), String::npos)
+            << what << " -- refused, but not by the chain-link rule; message: " << e.message();
+    }
+}
+
+/// `prepareRefChunk` CONSUMES its state, so this copies -- which also lets every caller below assert
+/// afterwards that its own state was left alone.
+CasRefLedger::PreparedRefChunk prepare(const RefTableState & state, const RefTxnId & id,
+                                       const std::optional<RefTxnId> & chain_link,
+                                       const std::vector<RefOp> & ops, uint64_t admitted_generation = 7)
+{
+    return CasRefLedger::prepareRefChunk(kLayout, kLife, state, id, chain_link, ops, admitted_generation);
+}
+
+}
+
+/// The two things that actually become durable -- the key and the sealed body -- are both derivable
+/// before any request, and both round-trip: the key parses back to the life and id it names, and the
+/// bytes decode back to the very transaction that was prepared.
+TEST(CASRefChunkPreparation, PreparedKeyAndSealedBytesAreCanonical)
+{
+    const RefTxnId id{1, 2};   /// the contiguous successor of the born state's `1-1`
+    const auto prepared = prepare(bornState(), id, std::nullopt, {addPrecommitOp("r1", mref(3))});
+
+    const auto parsed = kLayout.parseRefObjectKey(prepared.prepared_attempt.key);
+    ASSERT_TRUE(parsed.has_value()) << "the prepared key must be one of OUR ref-object keys";
+    EXPECT_EQ(parsed->life_id, kLife.incarnation);
+    EXPECT_EQ(parsed->kind, RefObjectKind::Log);
+    EXPECT_EQ(parsed->txn_id, id);
+    EXPECT_EQ(prepared.prepared_attempt.key, kLayout.refLogKey(kLife, id));
+
+    const RefLogTxn decoded = decodeRefLogTxn(
+        openObject(FormatId::RefLog, prepared.prepared_attempt.bytes), kNs.string(), id);
+    EXPECT_EQ(decoded, prepared.chunk_txn) << "the sealed bytes must decode back to the prepared transaction";
+    EXPECT_EQ(decoded.ns, kNs.string());
+    EXPECT_EQ(decoded.txn_id, id);
+    ASSERT_EQ(decoded.ops.size(), 1u);
+    EXPECT_EQ(decoded.ops.front().kind, RefOpKind::OwnerTransition);
+}
+
+/// The base id a later install re-presents is the greatest-applied of the state preparation STARTED
+/// from -- not of the candidate it produced. Getting this backwards would let an install adopt a
+/// candidate over a state that had moved on.
+TEST(CASRefChunkPreparation, CandidateBaseIdIsGreatestApplied)
+{
+    const RefTableState state = bornState();
+    const RefTxnId base = state.getGreatestApplied();
+    ASSERT_EQ(base, (RefTxnId{1, 1})) << "precondition: the born state's greatest-applied is its birth";
+
+    const auto prepared = prepare(state, RefTxnId{1, 2}, std::nullopt, {addPrecommitOp("r1", mref(3))});
+    EXPECT_EQ(prepared.candidate_base_id, base) << "the base id describes the state prepared FROM";
+    EXPECT_EQ(prepared.candidate.getGreatestApplied(), (RefTxnId{1, 2}))
+        << "the candidate itself has this chunk applied";
+    /// `prepare` handed over a COPY, so the caller's state cannot have been advanced -- the property the
+    /// real caller relies on when it re-presents `candidate_base_id` at install time.
+    EXPECT_EQ(state.getGreatestApplied(), base) << "preparation must not mutate the caller's state";
+}
+
+/// INV-2's chain-link grammar across the full cross product. Preparation runs the real validators, so
+/// this sweeps both directions: where the link is required or forbidden, an ill-formed combination must
+/// be REFUSED here -- before anything is durable -- rather than sealed into bytes and PUT. That
+/// two-sided sweep is what the extraction buys: it needs no backend, so there is no reason not to cover
+/// the negatives too.
+///
+/// The base state is built per row, because a transaction id is only meaningful as the contiguous
+/// successor of some stream (INV-1): a row cannot just assert a grammar rule on an id the stream would
+/// never reach.
+///
+/// Note which validator each row lands on, because the two halves of the rule are DISJOINT and live in
+/// different steps of preparation: the required-iff half is `validateEpochSealGrammarContextual`, run by
+/// the candidate apply; the forbidden-off-sequence-1 half is `validateEpochSealGrammarStructural`, run
+/// by `encodeRefLogTxn` during the seal. Both are inside preparation, which is the point -- a chunk that
+/// passes one and fails the other still fails before any durable effect.
+TEST(CASRefChunkPreparation, ChainLinkRequiredExactlyOnSequenceOneOfNonGenesisEpoch)
+{
+    const std::vector<RefOp> ops{addPrecommitOp("r1", mref(3))};
+    const RefTxnId epoch1_seal{1, 5};   /// the seal that closed epoch 1
+
+    /// From a namespace born at `1-1` (so `life_epoch == 1`).
+    /// Sequence > 1 of the genesis epoch: the link is FORBIDDEN.
+    EXPECT_NO_THROW(prepare(bornState(), RefTxnId{1, 2}, std::nullopt, ops))
+        << "seq >1 with no link is the ordinary case";
+    expectGrammarRefusal([&] { prepare(bornState(), RefTxnId{1, 2}, epoch1_seal, ops); },
+                         "a link at sequence >1 is forbidden and must be refused before any durable effect");
+
+    /// Sequence 1 of an epoch ABOVE genesis: the link is REQUIRED.
+    EXPECT_NO_THROW(prepare(bornState(), RefTxnId{2, 1}, epoch1_seal, ops))
+        << "seq 1 of a higher epoch names the seal that closed the previous one";
+    expectGrammarRefusal([&] { prepare(bornState(), RefTxnId{2, 1}, std::nullopt, ops); },
+                         "seq 1 of a higher epoch without a link must be refused -- 'no seal' is a fact "
+                         "about the stream, not a defaulted field");
+
+    /// Genesis itself: sequence 1 of the birth epoch has nothing to name, so a link is FORBIDDEN.
+    EXPECT_NO_THROW(prepare(RefTableState{}, RefTxnId{3, 1}, std::nullopt, {birthOp()}))
+        << "a genesis birth at sequence 1 finds nothing to name";
+    expectGrammarRefusal([&] { prepare(RefTableState{}, RefTxnId{3, 1}, epoch1_seal, {birthOp()}); },
+                         "a link on the birth transaction itself must be refused");
+
+    /// Whatever the grammar admitted, the sealed bytes carry exactly that link and nothing else.
+    const auto linked = prepare(bornState(), RefTxnId{2, 1}, epoch1_seal, ops);
+    ASSERT_TRUE(linked.chunk_txn.prev_epoch_seal.has_value());
+    EXPECT_EQ(*linked.chunk_txn.prev_epoch_seal, epoch1_seal);
+    const RefLogTxn decoded = decodeRefLogTxn(
+        openObject(FormatId::RefLog, linked.prepared_attempt.bytes), kNs.string(), RefTxnId{2, 1});
+    EXPECT_EQ(decoded.prev_epoch_seal, linked.chunk_txn.prev_epoch_seal)
+        << "the link must survive into the bytes that would become durable";
+}
+
+/// The birth `_ckpt` contribution is PREPARED here and published by `commitRefChunk`, because
+/// publishing it is a birth chunk's first durable effect. Preparation therefore owes two things: the
+/// value only for a birth, and the one fact no later writer can recover -- `life_epoch`.
+TEST(CASRefChunkPreparation, BirthContributionSetOnlyForNamespaceBirth)
+{
+    /// A birth chunk at epoch 3: the contribution exists and names THIS transaction's writer epoch.
+    const RefTxnId birth_id{3, 1};
+    const auto born = prepare(RefTableState{}, birth_id, std::nullopt, {birthOp()});
+    ASSERT_TRUE(born.birth_contribution.has_value());
+    ASSERT_TRUE(born.birth_contribution->life_epoch.has_value());
+    EXPECT_EQ(*born.birth_contribution->life_epoch, birth_id.writer_epoch);
+    EXPECT_FALSE(born.birth_contribution->checkpoint_snapshot_id.has_value())
+        << "the birth contributes life_epoch and nothing else -- the publisher owns the checkpoint field";
+    EXPECT_FALSE(born.birth_contribution->last_epoch_seal.has_value());
+
+    /// An ordinary content chunk contributes nothing: a second `_ckpt` write here would be a request the
+    /// append lane does not owe.
+    const auto ordinary = prepare(bornState(), RefTxnId{1, 2}, std::nullopt, {addPrecommitOp("r1", mref(3))});
+    EXPECT_FALSE(ordinary.birth_contribution.has_value());
+
+    /// A birth op mixed into a larger chunk still counts -- the check is over the whole chunk.
+    const auto mixed = prepare(RefTableState{}, RefTxnId{5, 1}, std::nullopt,
+                               {birthOp(), addPrecommitOp("r1", mref(3))});
+    ASSERT_TRUE(mixed.birth_contribution.has_value());
+    EXPECT_EQ(*mixed.birth_contribution->life_epoch, 5u);
+}
+
+TEST(CASRefChunkPreparation, CommitContributionCarriesFrontierAndOnlyMatchingSeal)
+{
+    const RefTxnId ordinary_id{1, 2};
+    const auto ordinary = prepare(bornState(), ordinary_id, std::nullopt,
+                                  {addPrecommitOp("r1", mref(3))});
+    EXPECT_EQ(ordinary.commit_contribution.committed_through, ordinary_id);
+    EXPECT_FALSE(ordinary.commit_contribution.life_epoch.has_value());
+    EXPECT_FALSE(ordinary.commit_contribution.checkpoint_snapshot_id.has_value());
+    EXPECT_FALSE(ordinary.commit_contribution.last_epoch_seal.has_value());
+
+    const RefTxnId seal_id{1, 2};
+    const auto seal = prepare(bornState(), seal_id, std::nullopt, {epochSealOp()});
+    EXPECT_EQ(seal.commit_contribution.committed_through, seal_id);
+    EXPECT_EQ(seal.commit_contribution.last_epoch_seal, seal_id)
+        << "an epoch seal and its committed frontier must be one checkpoint contribution";
+    EXPECT_FALSE(seal.commit_contribution.life_epoch.has_value());
+    EXPECT_FALSE(seal.commit_contribution.checkpoint_snapshot_id.has_value());
+}
+
+/// The attempt exists so that an `Unresolved` PUT -- an object that may be durable -- can be recorded by
+/// a MOVE and nothing else. That only holds if every field is already populated before the request goes
+/// out, so this asserts the whole struct is complete at the end of preparation.
+TEST(CASRefChunkPreparation, PreparedAttemptIsCompleteBeforeAnyDurableEffect)
+{
+    const RefTxnId id{1, 2};
+    const auto prepared = prepare(bornState(), id, std::nullopt, {addPrecommitOp("r1", mref(3))}, /*admitted_generation=*/42);
+
+    EXPECT_EQ(prepared.prepared_attempt.txn_id, id);
+    EXPECT_FALSE(prepared.prepared_attempt.key.empty());
+    EXPECT_FALSE(prepared.prepared_attempt.bytes.empty());
+    EXPECT_EQ(prepared.prepared_attempt.admitted_fence_generation, 42u)
+        << "the attempt carries the generation it was ADMITTED under, not a current reading";
+
+    /// Nothing left to build: the key and body the request will read are already the canonical ones, so
+    /// the arming block's only remaining work really is the move it declares itself to be.
+    EXPECT_EQ(prepared.prepared_attempt.key, kLayout.refLogKey(kLife, id));
+    EXPECT_EQ(prepared.prepared_attempt.bytes,
+              sealObject(FormatId::RefLog, encodeRefLogTxn(prepared.chunk_txn)));
+}
diff --git a/src/Disks/tests/gtest_cas_ref_chunked_flush.cpp b/src/Disks/tests/gtest_cas_ref_chunked_flush.cpp
new file mode 100644
index 000000000000..2cdc32ff1851
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_chunked_flush.cpp
@@ -0,0 +1,919 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+
+#include <IO/S3Common.h>
+
+#include <Poco/Exception.h>
+
+#include <algorithm>
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <exception>
+#include <future>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <string>
+#include <thread>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+extern const int LIMIT_EXCEEDED;
+extern const int CORRUPTED_DATA;
+extern const int NETWORK_ERROR;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASRefBatchFlushes;
+extern const Event CASRefBatchedMutations;
+extern const Event CASRefSnapshotPublishDispatched;
+}
+
+/// Task 8 (stage-1 §3 "Budget: counts only, chunked flush"): the counts-only admission caps --
+/// `ref_txn_max_ops` (5000), the carve item cap `kMaxRefBatch` (1000), and the per-op size cap
+/// `ref_op_max_bytes` (4096 bytes on normal-class ops) -- plus their failure-isolation contract: a
+/// single item whose own op count, or whose one op's encoded size, exceeds its cap fails ALONE; a
+/// neighbor co-batched into the same flush still commits. `ref_txn_max_ops` is checked exactly (the
+/// `build_ops` result's size), and the per-op cap is checked by encoding exactly one op at a time --
+/// no accumulation, matching the admission machinery this replaces. T9 (removal-class detection by
+/// op inspection) and T10 (chunked flush across a whole-batch op-count overflow) extend this file;
+/// this task adds only the per-item / per-op isolation tests and the canonical round-trip leg of
+/// test 12 (the maximum legally-admissible normal-class transaction).
+///
+/// The suite name is prefixed `RefWriter` so it is covered by the `RefWriter*` unit-test gate filter.
+
+using namespace DB::Cas;
+using DB::Cas::tests::committedRow;
+using DB::Cas::tests::minimalLiveSnapshot;
+using DB::Cas::tests::writeRefSnapshotRaw;
+
+namespace
+{
+
+PoolPtr openPool(const BackendPtr & backend)
+{
+    /// A fresh pool with no residue, mirroring the T7 carve suite's `openPool`.
+    ///
+    /// The FROZEN clock is load-bearing, not hygiene. `CasMountRuntime::refAppendFenceOk` gates every
+    /// controlled attempt against `boot_ms_fn`, and with the compiled defaults (mount_lease_ttl_ms
+    /// 30000, safety margin 7000) a pool opened on the REAL clock fences itself ~23s later — no
+    /// background renewal advances that deadline in a unit-test pool. Every test in this suite is
+    /// about chunking and op-caps, none about wall-clock lease behaviour, so any of them that runs
+    /// long enough simply dies of an unrelated fence trip: `DropNamespaceOverOpCapSucceeds` (5200
+    /// refs) takes 43-65s under a sanitizer and failed deterministically on all three sanitizer CI
+    /// builds with `txn is UNCERTAIN (retry budget exhausted)` — the pre-attempt fence reject, not a
+    /// real retry exhaustion. Same artifact, same fix as
+    /// `CASPartWriteTxn.ManifestCapEncodedBytesJustUnderStagesSuccessfully` (2026-07-18): decouple the
+    /// fence from execution speed. The waits in this file are `steady_clock` timeouts on futures and
+    /// condvars, which are unaffected by this injection.
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test", .boot_ms_fn = [] { return uint64_t{0}; }});
+}
+
+/// A legal blob-free part: stage an empty manifest, precommit, promote -- enough to leave one
+/// committed ref (and a `Live` table) that a later co-batched item can join.
+///
+/// Stage B (Task 4-C): pin `ns` to the sentinel before the first real touch -- the ONE choke point
+/// every test in this file uses to birth its namespace, before any `launchAppendOps`/`launchAppend`/
+/// `launchDrop` call. Several tests separately compute an expected key via
+/// `DB::Cas::tests::fixture::fixtureLife(ns)` for verification/fault injection; without this the real
+/// production birth mints a random incarnation and those computed keys land nowhere real.
+void publishEmptyPart(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    DB::Cas::tests::casAdmitRecoverableEntry(s->backend(), s->layout(), ns, s->liveWriterEpoch());
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+}
+
+/// One queued append (or drop) driven on its own thread; the future becomes ready only when the call
+/// RETURNS (normally or by throwing). Mirrors `gtest_cas_ref_carve.cpp`'s `Caller`/`launchDrop`.
+struct Caller
+{
+    std::thread t;
+    std::future<std::exception_ptr> fut;
+};
+
+Caller launchAppend(const PoolPtr & store, const RootNamespace & ns, MutationScope scope,
+                     std::function<std::vector<RefOp>(const RefTableState &)> build_ops)
+{
+    auto prom = std::make_shared<std::promise<std::exception_ptr>>();
+    std::future<std::exception_ptr> fut = prom->get_future();
+    std::thread t([store, ns, scope, build_ops, prom]
+    {
+        std::exception_ptr err;
+        try { store->appendRefOps(ns, scope, build_ops, RootMutationOrigin::Writer, RootMutationKind::Publish); }
+        catch (...) { err = std::current_exception(); }
+        prom->set_value(err);
+    });
+    return Caller{std::move(t), std::move(fut)};
+}
+
+Caller launchDrop(const PoolPtr & store, const RootNamespace & ns, const String & ref)
+{
+    auto prom = std::make_shared<std::promise<std::exception_ptr>>();
+    std::future<std::exception_ptr> fut = prom->get_future();
+    std::thread t([store, ns, ref, prom]
+    {
+        std::exception_ptr err;
+        try { store->dropRef(ns, ref); }
+        catch (...) { err = std::current_exception(); }
+        prom->set_value(err);
+    });
+    return Caller{std::move(t), std::move(fut)};
+}
+
+/// `n` filler ops for a `build_ops` result whose only purpose is to overflow the per-item op-count
+/// cap. They are NOT inert-when-applied: a default-constructed op is a `NamespaceBirth`, which throws
+/// `CORRUPTED_DATA` ("namespace_birth while already Live") if it were ever applied to the pre-published
+/// namespace. The load-bearing safety property is that the count check fires BEFORE any of these ops is
+/// applied or otherwise inspected.
+std::vector<RefOp> fillerOps(size_t n)
+{
+    return std::vector<RefOp>(n, RefOp{});
+}
+
+/// A zero-padded ref name for index `i`, so `kTotalRefs` names sort in the same order as their index
+/// (the snapshot fixture's committed rows must already be sorted by `ref_name`).
+String paddedRefName(size_t i)
+{
+    String s = std::to_string(i);
+    return "ref_" + String(6 - s.size(), '0') + s;
+}
+
+/// A single `SetPublishedAt` op whose `ref_name` is padded so its OWN encoded size (`encodedOpSize`)
+/// is exactly `target_bytes`. Every added 'a' is one un-escaped byte in the JSON ref-name string, so
+/// the size grows one-for-one; `checkCanonicalRefName` imposes no length limit, so this stays a
+/// valid, merely over-long, canonical ref name.
+RefOp paddedSetPublishedAtOp(size_t target_bytes)
+{
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r";
+    op.expected_manifest_ref = ManifestRef{1, 1, 1};
+    op.published_at_ms = 0;
+    const size_t base = encodedOpSize(op);
+    op.ref_name = "r" + String(target_bytes - base, 'a');
+    return op;
+}
+
+/// Blocks the FIRST flush's leader in the pre-carve window until `expected_pending` items are queued,
+/// forcing a deterministic multi-item batch (mirrors `gtest_cas_ref_carve.cpp`'s `CaseSync`/pre-carve
+/// hook pattern). Only the first carve blocks; retries proceed straight through.
+struct CaseSync
+{
+    std::mutex m;
+    std::condition_variable cv;
+    bool entered = false;
+};
+
+void armPreCarveBlock(const PoolPtr & store, const RootNamespace & ns, const std::shared_ptr<CaseSync> & sync, size_t expected_pending)
+{
+    store->setRefPreCarveHookForTest([sync, store, ns, expected_pending]
+    {
+        std::unique_lock lk(sync->m);
+        if (sync->entered)
+            return;
+        sync->entered = true;
+        sync->cv.notify_all();
+        /// Bounded (10s) so a staging bug bounds the wait instead of blocking the whole suite.
+        sync->cv.wait_for(lk, std::chrono::seconds(10), [&] { return store->refQueuePendingForTest(ns) >= expected_pending; });
+    });
+}
+
+void waitEntered(const std::shared_ptr<CaseSync> & sync)
+{
+    std::unique_lock lk(sync->m);
+    sync->cv.wait_for(lk, std::chrono::seconds(10), [&] { return sync->entered; });
+}
+
+void waitPendingAtLeast(const PoolPtr & store, const RootNamespace & ns, size_t n)
+{
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(10);
+    while (store->refQueuePendingForTest(ns) < n && std::chrono::steady_clock::now() < deadline)
+        std::this_thread::yield();
+}
+
+/// Asserts `err` is non-null and carries EXACTLY `expected_code` -- distinguishes the new counts-only
+/// admission checks (`LIMIT_EXCEEDED`) from any other per-item validation failure.
+void expectFailedWithCode(const std::exception_ptr & err, int expected_code, const char * what)
+{
+    ASSERT_TRUE(err != nullptr) << what << ": the caller must observe the admission-cap error";
+    try
+    {
+        std::rethrow_exception(err);
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), expected_code) << what;
+    }
+}
+
+}
+
+/// Test 10 (spec §3 "Oversized item / oversized op fail alone"): an item whose OWN op count exceeds
+/// `ref_txn_max_ops` fails alone -- its ops never enter the batch's transaction -- and a co-batched
+/// neighbor still commits.
+TEST(CASRefWriterChunkedFlush, OversizedItemFailsAlone)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/chunked_oversized_item"};
+    publishEmptyPart(store, ns, "neighbor");
+    ASSERT_TRUE(store->resolveRef(ns, "neighbor").has_value());
+
+    auto sync = std::make_shared<CaseSync>();
+    armPreCarveBlock(store, ns, sync, 2);
+
+    Caller oversized = launchAppend(store, ns, MutationScope::ref("oversized"),
+        [](const RefTableState &) -> std::vector<RefOp> { return fillerOps(ref_txn_max_ops + 1); });
+    waitEntered(sync);
+    Caller neighbor = launchDrop(store, ns, "neighbor");
+    waitPendingAtLeast(store, ns, 2);
+    sync->cv.notify_all();   /// release the pre-carve hook now its (>=2 pending) predicate holds
+
+    ASSERT_EQ(oversized.fut.wait_for(std::chrono::seconds(10)), std::future_status::ready) << "oversized item must not hang";
+    ASSERT_EQ(neighbor.fut.wait_for(std::chrono::seconds(10)), std::future_status::ready) << "neighbor must not hang";
+    const std::exception_ptr oversized_err = oversized.fut.get();
+    const std::exception_ptr neighbor_err = neighbor.fut.get();
+    oversized.t.join();
+    neighbor.t.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    expectFailedWithCode(oversized_err, DB::ErrorCodes::LIMIT_EXCEEDED, "oversized item (op count)");
+    EXPECT_TRUE(neighbor_err == nullptr) << "the co-batched neighbor must commit despite the oversized item";
+    EXPECT_FALSE(store->resolveRef(ns, "neighbor").has_value()) << "neighbor's drop must have committed";
+}
+
+/// Test 10, second leg: one op whose OWN encoded size exceeds `ref_op_max_bytes` (a maximum-length
+/// ref name -- `checkCanonicalRefName` imposes no length limit) fails only its item; a co-batched
+/// neighbor still commits.
+TEST(CASRefWriterChunkedFlush, OversizedOpFailsItsItemAlone)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/chunked_oversized_op"};
+    publishEmptyPart(store, ns, "neighbor");
+    ASSERT_TRUE(store->resolveRef(ns, "neighbor").has_value());
+
+    const RefOp oversized_op = paddedSetPublishedAtOp(ref_op_max_bytes + 1);
+    ASSERT_GT(encodedOpSize(oversized_op), ref_op_max_bytes);
+
+    auto sync = std::make_shared<CaseSync>();
+    armPreCarveBlock(store, ns, sync, 2);
+
+    Caller oversized = launchAppend(store, ns, MutationScope::ref("oversized_op"),
+        [oversized_op](const RefTableState &) -> std::vector<RefOp> { return {oversized_op}; });
+    waitEntered(sync);
+    Caller neighbor = launchDrop(store, ns, "neighbor");
+    waitPendingAtLeast(store, ns, 2);
+    sync->cv.notify_all();
+
+    ASSERT_EQ(oversized.fut.wait_for(std::chrono::seconds(10)), std::future_status::ready) << "oversized op item must not hang";
+    ASSERT_EQ(neighbor.fut.wait_for(std::chrono::seconds(10)), std::future_status::ready) << "neighbor must not hang";
+    const std::exception_ptr oversized_err = oversized.fut.get();
+    const std::exception_ptr neighbor_err = neighbor.fut.get();
+    oversized.t.join();
+    neighbor.t.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    expectFailedWithCode(oversized_err, DB::ErrorCodes::LIMIT_EXCEEDED, "oversized op");
+    EXPECT_TRUE(neighbor_err == nullptr) << "the co-batched neighbor must commit despite the oversized op";
+    EXPECT_FALSE(store->resolveRef(ns, "neighbor").has_value()) << "neighbor's drop must have committed";
+}
+
+/// Test 12, canonical round-trip leg: the maximum legally-admissible normal-class transaction under
+/// the new counts-only caps -- `ref_txn_max_ops` ops, each padded to exactly `ref_op_max_bytes` --
+/// round-trips comfortably under the whole-transaction `ref_txn_max_bytes` decode cap (5000 * 4096 =
+/// 20,480,000 bytes, with framing headroom to spare). Pure codec-level: proves the two counts-only
+/// caps compose without ever approaching the byte cap the encode-side estimation machinery used to
+/// police.
+TEST(CASRefWriterChunkedFlush, CanonicalMaxTransactionRoundTrips)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    txn.ops.reserve(ref_txn_max_ops);
+    for (size_t i = 0; i < ref_txn_max_ops; ++i)
+    {
+        RefOp op = paddedSetPublishedAtOp(ref_op_max_bytes);
+        ASSERT_EQ(encodedOpSize(op), ref_op_max_bytes);
+        txn.ops.push_back(std::move(op));
+    }
+
+    const String bytes = encodeRefLogTxn(txn);
+    /// Every op contributes exactly `ref_op_max_bytes`; header/meta/trailer framing adds strictly
+    /// more on top, and the whole thing still stays well under the 20 MiB decode cap.
+    EXPECT_GT(bytes.size(), ref_txn_max_ops * ref_op_max_bytes);
+    EXPECT_LT(bytes.size(), ref_txn_max_bytes);
+
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded.ops.size(), ref_txn_max_ops);
+    EXPECT_EQ(decoded, txn);
+}
+
+/// Test 11 (spec §3 "Removal-class detection, falsifiably"): `dropNamespace` over a table with
+/// > `ref_txn_max_ops` committed refs builds ONE transaction whose ops (one `owner_transition`
+/// removal per ref, plus a terminal `remove_namespace`) exceed the normal-class op-count cap --
+/// and must still succeed, because removal-class is byte-budgeted (`ref_removal_max_bytes`, 64 MiB)
+/// and has no op-count cap. Seeded via a raw snapshot (not `kTotalRefs` individual writer round-trips
+/// through `publishEmptyPart`) so the fixture stays fast; the writer never touches these rows until
+/// `dropNamespace` itself builds the one removal transaction.
+TEST(CASRefWriterChunkedFlush, DropNamespaceOverOpCapSucceeds)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/dropns_over_cap"};
+    constexpr size_t kTotalRefs = static_cast<size_t>(ref_txn_max_ops) + 200;
+
+    /// Open the store FIRST (still untouched for `ns`) so the seeded snapshot can use THIS mount's own
+    /// writer_epoch: namespace recovery is per-namespace and lazy (first touch), so writing the raw
+    /// fixture directly to `backend` after open, but before `ns` is ever touched, is observed identically
+    /// to writing it before open.
+    auto store = openPool(backend);
+    const uint64_t epoch = store->writerEpoch();
+    /// Stage B (Task 4-C): pin `ns` to the sentinel now, before the raw snapshot below -- `listRefs`/
+    /// `dropNamespace` further down are real production reads that trigger `resolveNamespaceLife`,
+    /// which for an UNADMITTED namespace mints a fresh RANDOM incarnation rather than adopting the
+    /// sentinel the raw fixture wrote at. Pinning first makes them adopt it instead.
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+
+    /// Ids are PER-NAMESPACE and derived from the table's own `greatest_applied` (INV-1), so seeding
+    /// `ns` at `{epoch, 1}` is all this fixture has to do: the `dropNamespace` below derives `{epoch, 2}`
+    /// from the seeded snapshot, and no other namespace's traffic can move it.
+    std::vector<RefCommittedRow> committed;
+    committed.reserve(kTotalRefs);
+    for (size_t i = 0; i < kTotalRefs; ++i)
+        committed.push_back(committedRow(paddedRefName(i), ManifestRef{epoch, i + 1, 1}));
+    ASSERT_GT(committed.size(), ref_txn_max_ops);
+
+    /// Recovery's checkpoint anchor includes the same-id ordinary log. The synthetic snapshot stands
+    /// for a long prior history, while this genesis record supplies the retained non-seal witness the
+    /// real publisher would necessarily leave at the selected id.
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(),
+        .txn_id = RefTxnId{epoch, 1},
+        .ops = {DB::Cas::tests::namespaceBirthOp()},
+        .prev_epoch_seal = std::nullopt});
+    writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), RefTxnId{epoch, 1}, committed));
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = epoch,
+        .committed_through = RefTxnId{epoch, 1},
+        .checkpoint_snapshot_id = RefTxnId{epoch, 1},
+        .last_epoch_seal = std::nullopt,
+    });
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, layout, ns).value();
+    backend->resetCounts();
+    ASSERT_EQ(store->listRefs(ns).size(), kTotalRefs);
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{epoch, 1})), 1u);
+    EXPECT_EQ(backend->getCount(layout.refSnapshotKey(life, RefTxnId{epoch, 1})), 1u);
+
+    DropNamespaceStats stats;
+    EXPECT_NO_THROW(stats = store->dropNamespace(ns));
+    EXPECT_EQ(stats.committed_refs, kTotalRefs);
+    EXPECT_EQ(CasRefCatalog::read(*backend, layout).catalog.entries.front().state, NsState::Removing);
+}
+
+/// Test 11, second leg: `WholeShard` scope ALONE is not the removal-class discriminator -- the
+/// stale-precommit reclaim sweep is also `WholeShard`-scoped but is not removal-class
+/// (`CasRefLedger.cpp` ~:1979). Only a SYNTHETIC item can pin this: the production stale-precommit
+/// sweep self-limits its own chunk size to the op cap, so running it proves nothing (spec's own
+/// warning). This item drives `MutationScope::wholeShard()` directly with ops that contain NO
+/// `RemoveNamespace` op -- if classification were keyed on scope instead of op inspection, this would
+/// be wrongly treated as removal-class and admitted; op-inspection correctly rejects it under the
+/// ordinary normal-class op-count cap, exactly like `OversizedItemFailsAlone` above.
+TEST(CASRefWriterChunkedFlush, SyntheticWholeShardNonRemovalRejected)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/synthetic_wholeshard_nonremoval"};
+
+    Caller synthetic = launchAppend(store, ns, MutationScope::wholeShard(),
+        [](const RefTableState &) -> std::vector<RefOp> { return fillerOps(ref_txn_max_ops + 1); });
+    ASSERT_EQ(synthetic.fut.wait_for(std::chrono::seconds(10)), std::future_status::ready)
+        << "synthetic WholeShard item must not hang";
+    const std::exception_ptr err = synthetic.fut.get();
+    synthetic.t.join();
+
+    expectFailedWithCode(err, DB::ErrorCodes::LIMIT_EXCEEDED,
+        "synthetic WholeShard-scoped item with non-removal ops over the op cap");
+}
+
+/// ===================================================================================
+/// Task 10 (spec §3 "Chunked flush, where each chunk is a complete commit boundary"): when admitting
+/// the next item's ops would exceed `ref_txn_max_ops`, the leader commits the accumulated chunk as a
+/// COMPLETE ref-log transaction (real id, PUT, apply, tail, metrics, survivor completion + waiter
+/// wakeups, snapshot scheduling), reseeds `working`/the trial-id high-water mark from the now-live
+/// state, and continues into a fresh chunk -- so one tenure can emit several transactions, each a valid
+/// persisted prefix. The failure-isolation and tenure-containment contracts are pinned below.
+/// ===================================================================================
+
+namespace
+{
+
+/// The `_log/`-PUT fault seam these tests are built on now lives in `cas_test_helpers.h`, next to
+/// `CountingBackend` it derives from: `gtest_cas_ref_install_safety.cpp` needs the SAME seam (spec §A1
+/// sites 2 and 3 both turn on what happens when a `_log/` PUT's response is lost), and two copies of a
+/// fault backend would drift apart.
+using DB::Cas::tests::ChunkFaultBackend;
+
+PoolPtr openPoolWith(const BackendPtr & backend, PoolConfig cfg)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    cfg.pool_prefix = "p";
+    cfg.server_root_id = "test";
+    /// Same frozen clock as `openPool` above, and for the same reason — see its comment. Defaulted
+    /// rather than forced, so a future test that IS about lease timing can still supply its own.
+    if (!cfg.boot_ms_fn)
+        cfg.boot_ms_fn = [] { return uint64_t{0}; };
+    return Pool::open(backend, cfg);
+}
+
+/// `num_pairs` add-then-remove precommit op pairs (2 * `num_pairs` ops total) for distinct refs
+/// (`prefix` + zero-padded index) each naming a distinct valid manifest. Every pair adds a precommit
+/// binding and immediately removes it, so the LIVE state (the `precommits` set, the committed COW map,
+/// the owned-manifest index) stays ~empty throughout the whole transaction -- keeping the per-op
+/// `admits` preview and the sanitizer-only body-counter assert O(1), so validating a maximal chunk of
+/// thousands of ops stays O(ops), not O(ops^2). It is the OP COUNT (not the resident state) that drives
+/// the chunk split under test; each op is tiny (well under `ref_op_max_bytes`), so the whole run is
+/// admissible on a `Live` namespace. The durable transaction still carries every op verbatim, so a
+/// chunk's ops can be compared against the exact expected vector.
+std::vector<RefOp> addRemovePrecommitPairs(const String & prefix, size_t num_pairs, uint64_t manifest_epoch)
+{
+    std::vector<RefOp> ops;
+    ops.reserve(num_pairs * 2);
+    for (size_t i = 0; i < num_pairs; ++i)
+    {
+        const String ref = prefix + paddedRefName(i);
+        const ManifestRef manifest{manifest_epoch, i + 1, 1};
+        RefOp add;
+        add.kind = RefOpKind::OwnerTransition;
+        add.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref, manifest};
+        ops.push_back(std::move(add));
+        RefOp remove;
+        remove.kind = RefOpKind::OwnerTransition;
+        remove.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref, manifest};
+        ops.push_back(std::move(remove));
+    }
+    return ops;
+}
+
+/// Every durable `_log/` transaction for `ns`, decoded, sorted ascending by transaction id. Undecodable
+/// objects (e.g. the foreign bytes a `ForeignConflict` fault lands) are skipped so a corrupt object never
+/// breaks the inventory. Reads the backend directly (no Pool cache).
+std::vector<RefLogTxn> listLogTxns(DB::Cas::Backend & backend, const DB::Cas::Layout & layout, const RootNamespace & ns)
+{
+    std::vector<RefTxnId> ids;
+    String cursor;
+    for (;;)
+    {
+        const ListPage page = backend.list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+        for (const ListedKey & lk : page.keys)
+        {
+            const auto parsed = layout.parseRefObjectKey(lk.key);
+            if (parsed && parsed->life_id == DB::Cas::tests::fixture::fixtureLife(ns).incarnation
+                && parsed->kind == RefObjectKind::Log)
+                ids.push_back(parsed->txn_id);
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    std::sort(ids.begin(), ids.end(), [](const RefTxnId & a, const RefTxnId & b) { return a < b; });
+    std::vector<RefLogTxn> txns;
+    for (const RefTxnId & id : ids)
+    {
+        const auto got = backend.get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), id));
+        if (!got)
+            continue;
+        try
+        {
+            txns.push_back(decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), id));
+        }
+        catch (...) // NOLINT(bugprone-empty-catch): best-effort helper -- an undecodable txn is simply skipped, not asserted on
+        {
+        }
+    }
+    return txns;
+}
+
+/// One queued append driven on its own thread, capturing BOTH the committed transaction id (on success)
+/// and the exception (on failure); `build_calls` (when non-null) counts `build_ops` invocations to pin
+/// the at-most-once contract across chunk boundaries. The ops are precomputed and returned verbatim, so a
+/// second invocation (a bug) is caught by the counter, not masked by a state-dependent rebuild.
+struct AppendResult
+{
+    std::exception_ptr err;
+    RefTxnId id{};
+};
+
+struct AppendCaller
+{
+    std::thread t;
+    std::future<AppendResult> fut;
+};
+
+AppendCaller launchAppendOps(const PoolPtr & store, const RootNamespace & ns, MutationScope scope,
+                             std::vector<RefOp> ops, std::shared_ptr<std::atomic<int>> build_calls)
+{
+    auto prom = std::make_shared<std::promise<AppendResult>>();
+    std::future<AppendResult> fut = prom->get_future();
+    auto build_ops = [captured_ops = std::move(ops), build_calls](const RefTableState &) -> std::vector<RefOp>
+    {
+        if (build_calls)
+            build_calls->fetch_add(1);
+        return captured_ops;
+    };
+    std::thread t([store, ns, scope, build_ops, prom]
+    {
+        AppendResult r;
+        try { r.id = store->appendRefOps(ns, scope, build_ops, RootMutationOrigin::Writer, RootMutationKind::Publish); }
+        catch (...) { r.err = std::current_exception(); }
+        prom->set_value(r);
+    });
+    return AppendCaller{std::move(t), std::move(fut)};
+}
+
+}
+
+/// Test 9 (happy path): a carve whose total ops exceed `ref_txn_max_ops` emits >= 2 ref-log transactions
+/// in ONE leader tenure. Three items (2000 ops each = 6000 > 5000) split into chunk 1 = {item_a,item_b}
+/// (4000 ops, one id) and chunk 2 = {item_c} (2000 ops, the next id). Per-chunk assertions: committed
+/// ids (co-chunk survivors share one real id; the next chunk allocates the next), tail counters (one per
+/// chunk), per-chunk metrics (`CASRefBatchFlushes` once per chunk, `CASRefBatchedMutations` counting
+/// survivors per chunk), follower wakeups (both followers return their correct real id -> completed +
+/// woken at their chunk's commit), `build_ops` at-most-once (invocation counters == 1), and folded state
+/// == the sequential result (the two durable transactions carry exactly item_a++item_b, then item_c).
+TEST(CASRefWriterChunkedFlush, ChunkedFlushCommitsPerChunk)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// Default thresholds: this handful of transactions never crosses the snapshot-publish threshold, so
+    /// no background publish interleaves and the tail/metric deltas below are exact.
+    auto store = openPool(backend);
+    const DB::Cas::Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/chunked_commits_per_chunk"};
+    publishEmptyPart(store, ns, "seed");
+    ASSERT_TRUE(store->resolveRef(ns, "seed").has_value());
+
+    /// 2000 ops per item (1000 add/remove pairs) -> 6000 > ref_txn_max_ops (5000): chunk 1 =
+    /// {item_a,item_b} (4000), chunk 2 = {item_c} (2000).
+    const std::vector<RefOp> ops1 = addRemovePrecommitPairs("aaa_", 1000, 900000001);
+    const std::vector<RefOp> ops2 = addRemovePrecommitPairs("bbb_", 1000, 900000002);
+    const std::vector<RefOp> ops3 = addRemovePrecommitPairs("ccc_", 1000, 900000003);
+    auto c1 = std::make_shared<std::atomic<int>>(0);
+    auto c2 = std::make_shared<std::atomic<int>>(0);
+    auto c3 = std::make_shared<std::atomic<int>>(0);
+
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+    const uint64_t flushes_before = ProfileEvents::global_counters[ProfileEvents::CASRefBatchFlushes].load();
+    const uint64_t mutations_before = ProfileEvents::global_counters[ProfileEvents::CASRefBatchedMutations].load();
+
+    auto sync = std::make_shared<CaseSync>();
+    armPreCarveBlock(store, ns, sync, 3);
+    /// Serialise the enqueue order so the batch is exactly [item_a(leader), item_b, item_c].
+    AppendCaller a = launchAppendOps(store, ns, MutationScope::ref("item_a"), ops1, c1);
+    waitEntered(sync);
+    AppendCaller b = launchAppendOps(store, ns, MutationScope::ref("item_b"), ops2, c2);
+    waitPendingAtLeast(store, ns, 2);
+    AppendCaller c = launchAppendOps(store, ns, MutationScope::ref("item_c"), ops3, c3);
+    waitPendingAtLeast(store, ns, 3);
+    sync->cv.notify_all();
+
+    ASSERT_EQ(a.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "item_a must not hang";
+    ASSERT_EQ(b.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "item_b must not hang";
+    ASSERT_EQ(c.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "item_c must not hang";
+    const AppendResult ra = a.fut.get();
+    const AppendResult rb = b.fut.get();
+    const AppendResult rc = c.fut.get();
+    a.t.join();
+    b.t.join();
+    c.t.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    ASSERT_TRUE(ra.err == nullptr) << "item_a must commit";
+    ASSERT_TRUE(rb.err == nullptr) << "item_b must commit";
+    ASSERT_TRUE(rc.err == nullptr) << "item_c must commit";
+
+    /// `build_ops` ran exactly once per item -- including item_c, the overflowing item validated once in
+    /// the fresh chunk it lands in.
+    EXPECT_EQ(c1->load(), 1);
+    EXPECT_EQ(c2->load(), 1);
+    EXPECT_EQ(c3->load(), 1);
+
+    /// Committed ids per chunk: item_a and item_b share chunk 1's real id (co-chunk survivors, both
+    /// woken with it); item_c gets chunk 2's id, exactly one sequence step above chunk 1.
+    EXPECT_EQ(ra.id, rb.id) << "co-chunk survivors must complete with the SAME real transaction id";
+    EXPECT_EQ(rc.id.writer_epoch, ra.id.writer_epoch);
+    EXPECT_EQ(rc.id.ref_sequence, ra.id.ref_sequence + 1) << "chunk 2 must allocate the id after chunk 1";
+
+    /// >= 2 durable transactions in the tenure, and the split is exactly the sequential result: chunk 1
+    /// carries item_a's then item_b's ops (survivor order), chunk 2 carries item_c's.
+    const std::vector<RefLogTxn> logs = listLogTxns(*backend, layout, ns);
+    std::optional<RefLogTxn> chunk1_txn;
+    std::optional<RefLogTxn> chunk2_txn;
+    for (const RefLogTxn & txn : logs)
+    {
+        if (txn.txn_id == ra.id)
+            chunk1_txn = txn;
+        if (txn.txn_id == rc.id)
+            chunk2_txn = txn;
+    }
+    ASSERT_TRUE(chunk1_txn.has_value()) << "chunk 1 must be durable";
+    ASSERT_TRUE(chunk2_txn.has_value()) << "chunk 2 must be durable (a second transaction in one tenure)";
+    std::vector<RefOp> expect_chunk1 = ops1;
+    expect_chunk1.insert(expect_chunk1.end(), ops2.begin(), ops2.end());
+    EXPECT_EQ(chunk1_txn->ops, expect_chunk1);
+    EXPECT_EQ(chunk2_txn->ops, ops3);
+
+    /// Tail counters advanced once per committed chunk.
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before + 2);
+
+    /// Per-chunk metrics: one batch-flush per chunk (2), survivors counted per chunk (2 + 1 = 3). The
+    /// snapshot-scheduling trigger is the final step of the SAME committed arm that increments
+    /// `CASRefBatchFlushes`, so == 2 also proves the scheduler was invoked per chunk;
+    /// `SnapshotPublisherLatchedAcrossChunks` proves that trigger actually re-fires across chunks.
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefBatchFlushes].load() - flushes_before, 2u);
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefBatchedMutations].load() - mutations_before, 3u);
+}
+
+namespace
+{
+
+/// Shared body for the three chunk-failure variants: two items (3000 ops each) -> chunk 1 = {item_a}
+/// (the leader's own item), chunk 2 = {item_b}. `mode` faults ONLY chunk 2's `_log/` PUT (skip chunk 1).
+/// In every variant chunk 1 commits and the leader's own call returns chunk 1's real id, while chunk 2's
+/// caller fails. Returns the two callers' results plus chunk 1's id for the per-variant assertions.
+struct ChunkFailureOutcome
+{
+    AppendResult leader;     /// item_a, chunk 1
+    AppendResult follower;   /// item_b, chunk 2
+    RefTxnId chunk1_id{};
+    std::shared_ptr<ChunkFaultBackend> backend;
+    PoolPtr store;
+};
+
+ChunkFailureOutcome runChunkFailureCase(const String & ns_suffix, ChunkFaultBackend::Mode mode)
+{
+    auto backend = std::make_shared<ChunkFaultBackend>();
+    PoolConfig cfg;
+    /// Single-attempt budget: one ambiguous PUT is a conclusive Unresolved (wedge) / DefiniteFailure,
+    /// with no inter-attempt sleep to serve.
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;   /// strictly above attempt_timeout_ms: equality is a wall-clock race (validateCasRequestBudget)
+    budget.lease_safety_margin_ms = 100;
+    cfg.cas_request_budget = budget;
+    auto store = openPoolWith(backend, cfg);
+    const DB::Cas::Layout & layout = store->layout();
+    const RootNamespace ns{String("srv1/") + ns_suffix};
+    publishEmptyPart(store, ns, "seed");
+
+    /// Fault ONLY chunk 2's `_log/` PUT: skip chunk 1's (the first match), fault the second. Armed AFTER
+    /// the seed so only the flush's two log PUTs are counted.
+    backend->fault_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = mode;
+    backend->fault_skip = 1;
+    backend->fault_count = 1;
+
+    auto sync = std::make_shared<CaseSync>();
+    armPreCarveBlock(store, ns, sync, 2);
+    /// 3000 ops per item (1500 add/remove pairs) -> 6000 > ref_txn_max_ops: chunk 1 = {item_a},
+    /// chunk 2 = {item_b}.
+    AppendCaller a = launchAppendOps(store, ns, MutationScope::ref("item_a"), addRemovePrecommitPairs("aaa_", 1500, 900000001), nullptr);
+    waitEntered(sync);
+    AppendCaller b = launchAppendOps(store, ns, MutationScope::ref("item_b"), addRemovePrecommitPairs("bbb_", 1500, 900000002), nullptr);
+    waitPendingAtLeast(store, ns, 2);
+    sync->cv.notify_all();
+
+    EXPECT_EQ(a.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "leader must not hang";
+    EXPECT_EQ(b.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "follower must not hang";
+    ChunkFailureOutcome out;
+    out.leader = a.fut.get();
+    out.follower = b.fut.get();
+    a.t.join();
+    b.t.join();
+    store->setRefPreCarveHookForTest(nullptr);
+    out.chunk1_id = out.leader.id;
+    out.backend = backend;
+    out.store = store;
+    return out;
+}
+
+}
+
+/// Test 9 (chunk-failure variant a -- definite failure): chunk 2's PUT is conclusively rejected
+/// (`CasWriteOutcome::DefiniteFailure`). Chunk 1's caller (the leader's own item) observes SUCCESS with
+/// chunk 1's real id; chunk 2's caller fails; the lane does NOT wedge (a definite rejection is a safe
+/// gap, not an uncertain PUT).
+TEST(CASRefWriterChunkedFlush, ChunkFailureDefinite)
+{
+#if !USE_AWS_S3
+    GTEST_SKIP() << "DefiniteFailure classification requires S3 error types (USE_AWS_S3 off)";
+#endif
+    ChunkFailureOutcome out = runChunkFailureCase("chunk_fail_definite", ChunkFaultBackend::Mode::Definite);
+    ASSERT_TRUE(out.leader.err == nullptr) << "chunk-1 caller must observe success even though chunk 2 failed";
+    ASSERT_TRUE(out.follower.err != nullptr) << "chunk-2 caller must observe the definite failure";
+    EXPECT_FALSE(out.store->refLaneWedgedForTest(RootNamespace{"srv1/chunk_fail_definite"}))
+        << "a definite failure is proven non-durable and must NOT wedge the lane";
+
+    const auto logs = listLogTxns(*out.backend, out.store->layout(), RootNamespace{"srv1/chunk_fail_definite"});
+    bool saw_chunk1 = false;
+    for (const RefLogTxn & txn : logs)
+        if (txn.txn_id == out.chunk1_id)
+            saw_chunk1 = true;
+    EXPECT_TRUE(saw_chunk1) << "chunk 1 must be durably committed";
+}
+
+/// Test 9 (chunk-failure variant b -- unresolved wedge): chunk 2's PUT is ambiguous and exhausts the
+/// budget, wedging the lane. Chunk 1's caller observes SUCCESS; chunk 2's caller fails; the wedge holds
+/// ONLY chunk 2's key (chunk 1 + 1), and chunk 1's object is durable while chunk 2's was never written.
+TEST(CASRefWriterChunkedFlush, ChunkFailureWedge)
+{
+    const RootNamespace ns{"srv1/chunk_fail_wedge"};
+    ChunkFailureOutcome out = runChunkFailureCase("chunk_fail_wedge", ChunkFaultBackend::Mode::Unresolved);
+    ASSERT_TRUE(out.leader.err == nullptr) << "chunk-1 caller must observe success even though chunk 2 wedged";
+    ASSERT_TRUE(out.follower.err != nullptr) << "chunk-2 caller must observe the append failure";
+
+    EXPECT_TRUE(out.store->refLaneWedgedForTest(ns)) << "chunk 2's unresolved PUT must wedge the lane";
+    RefTxnId chunk2_id = out.chunk1_id;
+    ++chunk2_id.ref_sequence;
+    EXPECT_EQ(out.store->wedgedKeyForTest(ns), out.store->layout().refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), chunk2_id))
+        << "the wedge must contain ONLY chunk 2's key";
+
+    const auto logs = listLogTxns(*out.backend, out.store->layout(), ns);
+    bool saw_chunk1 = false;
+    bool saw_chunk2 = false;
+    for (const RefLogTxn & txn : logs)
+    {
+        if (txn.txn_id == out.chunk1_id)
+            saw_chunk1 = true;
+        if (txn.txn_id == chunk2_id)
+            saw_chunk2 = true;
+    }
+    EXPECT_TRUE(saw_chunk1) << "chunk 1 must be durably committed";
+    EXPECT_FALSE(saw_chunk2) << "chunk 2's wedged object was never durably written";
+}
+
+/// Test 9 (chunk-failure variant c -- a throw): chunk 2's PUT surfaces a proven conflict (CORRUPTED_DATA
+/// thrown by the controller). Chunk 1's caller observes SUCCESS; chunk 2's caller fails with
+/// CORRUPTED_DATA; the lane does NOT wedge (a conclusive rejection).
+TEST(CASRefWriterChunkedFlush, ChunkFailureThrow)
+{
+    const RootNamespace ns{"srv1/chunk_fail_throw"};
+    ChunkFailureOutcome out = runChunkFailureCase("chunk_fail_throw", ChunkFaultBackend::Mode::ForeignConflict);
+    ASSERT_TRUE(out.leader.err == nullptr) << "chunk-1 caller must observe success even though chunk 2 threw";
+    ASSERT_TRUE(out.follower.err != nullptr) << "chunk-2 caller must observe the thrown failure";
+    expectFailedWithCode(out.follower.err, DB::ErrorCodes::CORRUPTED_DATA, "chunk-2 proven-conflict throw");
+    EXPECT_FALSE(out.store->refLaneWedgedForTest(ns)) << "a proven conflict is conclusive and must NOT wedge";
+
+    const auto logs = listLogTxns(*out.backend, out.store->layout(), ns);
+    bool saw_chunk1 = false;
+    for (const RefLogTxn & txn : logs)
+        if (txn.txn_id == out.chunk1_id)
+            saw_chunk1 = true;
+    EXPECT_TRUE(saw_chunk1) << "chunk 1 must be durably committed";
+}
+
+/// Test 9 (containment variant 1): the leader's OWN item lands in chunk 1; a throw is injected at the
+/// chunk boundary (simulating a reseed allocation failure) AFTER chunk 1 committed. Tenure containment
+/// (spec §3): the leader's own `appendRefOps` returns chunk 1's real id -- NOT the later exception --
+/// while the unattempted remainder (item_b) fails. This exercises the reworked outer catch, which no
+/// longer rethrows unconditionally over a durable own item.
+TEST(CASRefWriterChunkedFlush, LeaderOwnItemCommittedBeforeThrow)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const DB::Cas::Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/chunk_leader_own_committed"};
+    publishEmptyPart(store, ns, "seed");
+
+    auto c1 = std::make_shared<std::atomic<int>>(0);
+    auto c2 = std::make_shared<std::atomic<int>>(0);
+
+    /// Throw once at the first chunk boundary -- after chunk 1 (the leader's own item) is durable and
+    /// before the reseed completes.
+    auto boundary_hits = std::make_shared<std::atomic<int>>(0);
+    store->setCarveHookForTest([boundary_hits](CasRefLedger::CarvePhaseForTest ph)
+    {
+        if (ph == CasRefLedger::CarvePhaseForTest::ChunkReseed && boundary_hits->fetch_add(1) == 0)
+            throw std::bad_alloc{};
+    });
+
+    auto sync = std::make_shared<CaseSync>();
+    armPreCarveBlock(store, ns, sync, 2);
+    /// 3000 ops per item (1500 add/remove pairs) -> chunk 1 = {item_a}, boundary throw before chunk 2.
+    AppendCaller a = launchAppendOps(store, ns, MutationScope::ref("item_a"), addRemovePrecommitPairs("aaa_", 1500, 900000001), c1);
+    waitEntered(sync);
+    AppendCaller b = launchAppendOps(store, ns, MutationScope::ref("item_b"), addRemovePrecommitPairs("bbb_", 1500, 900000002), c2);
+    waitPendingAtLeast(store, ns, 2);
+    sync->cv.notify_all();
+
+    ASSERT_EQ(a.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "leader must not hang";
+    ASSERT_EQ(b.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "follower must not hang";
+    const AppendResult ra = a.fut.get();
+    const AppendResult rb = b.fut.get();
+    a.t.join();
+    b.t.join();
+    store->setCarveHookForTest(nullptr);
+    store->setRefPreCarveHookForTest(nullptr);
+
+    ASSERT_TRUE(ra.err == nullptr)
+        << "the leader's own committed-chunk item must return success, not the later boundary throw";
+    ASSERT_TRUE(rb.err != nullptr) << "the unattempted remainder must fail";
+
+    const std::vector<RefLogTxn> logs = listLogTxns(*backend, layout, ns);
+    std::optional<RefLogTxn> chunk1_txn;
+    for (const RefLogTxn & txn : logs)
+        if (txn.txn_id == ra.id)
+            chunk1_txn = txn;
+    ASSERT_TRUE(chunk1_txn.has_value()) << "chunk 1 must be durable";
+    EXPECT_EQ(chunk1_txn->ops, addRemovePrecommitPairs("aaa_", 1500, 900000001));
+    /// item_a's build_ops ran once (chunk 1); item_b's ran once (before the boundary throw preempted its
+    /// validation) and is NOT re-invoked -- the at-most-once contract holds through the failed tenure.
+    EXPECT_EQ(c1->load(), 1);
+    EXPECT_EQ(c2->load(), 1);
+}
+
+/// Test 9 (containment variant 2 -- snapshot coalescing): a snapshot publisher dispatched by chunk 1 is
+/// latched at its PUT AFTER capturing chunk 1's prefix; chunk 2 then commits and its publish trigger is
+/// discarded by the single-in-flight gate. When the latched publisher settles, settlement must re-fire
+/// the dropped trigger so a FOLLOW-UP publication covers chunk 2 -- otherwise chunk 2 would stay
+/// unsnapshotted until an unrelated later mutation. The chunk boundary is gated until the publisher has
+/// parked, so its captured candidate is provably chunk 1's prefix only.
+TEST(CASRefWriterChunkedFlush, SnapshotPublisherLatchedAcrossChunks)
+{
+    auto backend = std::make_shared<ChunkFaultBackend>();
+    PoolConfig cfg;
+    cfg.snapshot_log_count_threshold = 0;   /// every committed chunk crosses the tail-count threshold
+    auto store = openPoolWith(backend, cfg);
+    const DB::Cas::Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/chunk_snapshot_coalesce"};
+    publishEmptyPart(store, ns, "seed");
+    store->waitForSnapshotPublishSettleForTest(ns);   /// drain the seed's publish chain -> tail == 0
+
+    /// Latch the FIRST `_snap/` PUT (chunk 1's publisher) at its conditional PUT -- i.e. AFTER it has
+    /// captured chunk 1's prefix under state_mutex.
+    backend->armBlock(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_snap/");
+    /// Gate the leader at the chunk boundary until that publisher has parked on its PUT, so its captured
+    /// candidate is EXACTLY chunk 1's prefix (not chunk 1 + chunk 2).
+    store->setCarveHookForTest([backend](CasRefLedger::CarvePhaseForTest ph)
+    {
+        if (ph == CasRefLedger::CarvePhaseForTest::ChunkReseed)
+            backend->awaitBlockEntered();
+    });
+
+    auto sync = std::make_shared<CaseSync>();
+    armPreCarveBlock(store, ns, sync, 2);
+    /// 3000 ops per item (1500 add/remove pairs) -> chunk 1 = {item_a}, chunk 2 = {item_b}.
+    AppendCaller a = launchAppendOps(store, ns, MutationScope::ref("item_a"), addRemovePrecommitPairs("aaa_", 1500, 900000001), nullptr);
+    waitEntered(sync);
+    AppendCaller b = launchAppendOps(store, ns, MutationScope::ref("item_b"), addRemovePrecommitPairs("bbb_", 1500, 900000002), nullptr);
+    waitPendingAtLeast(store, ns, 2);
+    sync->cv.notify_all();
+
+    ASSERT_EQ(a.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "leader must not hang";
+    ASSERT_EQ(b.fut.wait_for(std::chrono::seconds(20)), std::future_status::ready) << "follower must not hang";
+    const AppendResult ra = a.fut.get();
+    const AppendResult rb = b.fut.get();
+    a.t.join();
+    b.t.join();
+    ASSERT_TRUE(ra.err == nullptr) << "chunk 1 must commit";
+    ASSERT_TRUE(rb.err == nullptr) << "chunk 2 must commit";
+    RefTxnId chunk2_id = ra.id;
+    ++chunk2_id.ref_sequence;
+    EXPECT_EQ(rb.id, chunk2_id);
+
+    /// Release the latched chunk-1 publisher. Its settlement must re-fire the chunk-2 trigger the
+    /// single-flight gate dropped -> a follow-up publication covers chunk 2.
+    backend->releaseBlock();
+    store->waitForSnapshotPublishSettleForTest(ns);
+    store->setCarveHookForTest(nullptr);
+    store->setRefPreCarveHookForTest(nullptr);
+
+    const std::optional<RefTxnId> newest = store->newestPublishedSnapshotIdForTest(ns);
+    ASSERT_TRUE(newest.has_value()) << "at least one snapshot must have been published";
+    EXPECT_FALSE(*newest < chunk2_id)
+        << "settlement must re-fire the dropped chunk-2 trigger so a snapshot covers chunk 2 (no lost trigger)";
+}
diff --git a/src/Disks/tests/gtest_cas_ref_ckpt.cpp b/src/Disks/tests/gtest_cas_ref_ckpt.cpp
new file mode 100644
index 000000000000..34bd84aee056
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_ckpt.cpp
@@ -0,0 +1,1270 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/MemoryTracker.h>
+#include <Common/SipHash.h>
+#include <base/hex.h>
+
+#include <Poco/Exception.h>
+
+#include <atomic>
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <optional>
+#include <string>
+#include <vector>
+
+/// Stage A task 5 (INV-4): the `_ckpt` object.
+///
+/// `_ckpt` exists because prefix cleaning made the ref stream unreadable from a LIST alone, so it is
+/// simultaneously the thing recovery point-reads to find its base AND the gate on what cleanup may
+/// delete. Both roles are only safe while three properties hold, and this suite pins exactly those:
+///
+///   1. the codec is STRICT in both directions -- a body that only partly decoded would be a cleanup
+///      decision taken from a partly-read object;
+///   2. there is ONE merge, by semantic maximum per field, used by BOTH writers -- a writer that
+///      wrote back the value it sampled earlier regresses the other writer's progress, which is TLC
+///      counterexample `_sab_sealclobbersbase` and costs an acked transaction;
+///   3. every CAS attempt re-checks the admitted fence generation AFTER its read and BEFORE its write,
+///      so a writer whose mount incarnation moved advances nothing.
+///
+/// The suite name is prefixed `Cas` so it is covered by the `Cas*` unit-test gate filter.
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int MEMORY_LIMIT_EXCEEDED;
+extern const int NETWORK_ERROR;
+extern const int UNKNOWN_FORMAT_VERSION;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::CountingBackend;
+using DB::Cas::tests::expectThrowsCode;
+using DB::Cas::tests::namespaceBirthOp;
+using DB::Cas::tests::publishCommittedOps;
+
+namespace
+{
+
+const RefTxnId ID_1_1{1, 1};
+const RefTxnId ID_1_2{1, 2};
+const RefTxnId ID_2_1{2, 1};
+
+PoolPtr openPool(const BackendPtr & backend)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// The same one-transaction publish the other ref suites drive, so a namespace reaches `Live` through
+/// the REAL append lane (which is also what creates its `_ckpt`).
+RefTxnId publishRef(const PoolPtr & store, const RootNamespace & ns, const String & ref, uint64_t ordinal)
+{
+    return store->appendRefOps(ns, MutationScope::ref(ref),
+        [&ref, ordinal](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (const RefOp & op : publishCommittedOps(ref, ManifestRef{1, ordinal, 1}))
+                ops.push_back(op);
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Publish);
+}
+
+/// A fence that never refuses, for the tests whose subject is not the fence.
+const std::function<void(uint64_t)> ALWAYS_ADMITTED = [](uint64_t) {};
+
+/// A deadline far enough out that only the test's own contention decides the outcome. The clock is
+/// frozen (a constant `now`), which is what makes every non-exhaustion test independent of wall time.
+CkptDeadline generousDeadline()
+{
+    return CkptDeadline{[] { return uint64_t{1000}; }, 60000};
+}
+
+/// Reads `life`'s `_ckpt` and returns its body, or a default-constructed one after failing the
+/// current test when the object is absent. Every assertion below goes through this rather than
+/// dereferencing the optional directly: a bare `->` on a disengaged optional ABORTS the whole test
+/// binary, so one regression would take every later suite's result with it instead of failing a test.
+RefCkpt readCkptOrFail(Backend & backend, const Layout & layout, const NamespaceLifeId & life)
+{
+    const std::optional<CkptSample> sample = readCkpt(backend, layout, life);
+    if (!sample)
+    {
+        ADD_FAILURE() << "expected a _ckpt for namespace '" << life.ns.string() << "', found none";
+        return RefCkpt{};
+    }
+    return sample->ckpt;
+}
+
+/// Stage B (Task 4-C): the incarnation `store`'s production birth wiring minted for `ns`, learned back
+/// from the catalog exactly as a real reader would (`NamespaceLifeId::fromCatalogEntry`) -- once a real
+/// `Pool`/`CasRefLedger` has opened the table, its ref-layer objects are no longer keyed at the
+/// Stage-A sentinel, so every test below that drives the REAL append lane must ask the catalog what
+/// incarnation it minted rather than assume the sentinel. Fails the current test (rather than
+/// dereferencing a disengaged optional) if the catalog carries no entry for `ns` -- e.g. called before
+/// the namespace's first append.
+NamespaceLifeId liveLifeOrFail(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    for (const CatalogEntry & entry : snap.catalog.entries)
+        if (entry.ns.string() == ns.string())
+            return NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation);
+    ADD_FAILURE() << "expected a catalog entry for namespace '" << ns.string() << "', found none";
+    return DB::Cas::tests::fixture::fixtureLife(ns);
+}
+
+/// Replaces the whole body of one key, minting a new incarnation -- how a test installs a deliberately
+/// malformed or concurrently-advanced object.
+void overwriteObject(Backend & backend, const String & key, const String & bytes)
+{
+    const HeadResult h = backend.head(key);
+    ASSERT_TRUE(h.exists) << "overwriteObject expects " << key << " to exist";
+    ASSERT_EQ(backend.putOverwrite(key, bytes, h.token).outcome, PutOutcome::Done);
+}
+
+/// Runs `on_get` right after every `get` of `watched_key` -- the deterministic way to act inside
+/// another component's read-then-write window without a sleep or a second thread. The hook is a public
+/// member rather than a constructor argument so it can be installed AFTER the backend exists (every
+/// interesting hook writes through that same backend) and only once the test's setup writes are done.
+class GetHookBackend : public CountingBackend
+{
+public:
+    using CountingBackend::get;
+
+    explicit GetHookBackend(String watched_key_) : watched_key(std::move(watched_key_)) {}
+
+    /// Stage B (Task 4-C): a test that must watch a namespace's `_ckpt` key can no longer compute it
+    /// before the pool exists -- the real incarnation is minted only once the namespace's first open
+    /// resolves it, which requires the pool (and so this backend) to already be constructed. Lets a
+    /// test retarget the watch once it has learned the real key, strictly before arming `on_get`.
+    void setWatchedKey(String watched_key_) { watched_key = std::move(watched_key_); }
+
+    std::function<void()> on_get;
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        auto result = CountingBackend::get(key, range);
+        if (key == watched_key && on_get)
+            on_get();
+        return result;
+    }
+
+private:
+    String watched_key;
+};
+
+/// Records the exact `_ckpt` recovery protocol and injects an ambiguous CAS response. The fault is
+/// armed only after fixture setup, so the journal contains solely the operation under test.
+class AmbiguousCkptBackend : public CountingBackend
+{
+public:
+    enum class Fault : uint8_t
+    {
+        None,
+        CommitThenThrow,
+        ThrowWithoutCommit,
+        AlwaysThrowWithoutCommit,
+    };
+
+    using CountingBackend::casPut;
+    using CountingBackend::get;
+
+    String watched_key;
+    Fault fault = Fault::None;
+    String dominating_bytes;
+    bool fail_resolution_get = false;
+    std::function<void()> before_resolution_get;
+    std::function<void()> after_resolution_get;
+    std::vector<String> journal;
+
+    void arm(const String & key, Fault fault_)
+    {
+        watched_key = key;
+        fault = fault_;
+        watched_get_count = 0;
+        journal.clear();
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (key != watched_key)
+            return CountingBackend::get(key, range);
+
+        journal.push_back("GET");
+        ++watched_get_count;
+        if (watched_get_count >= 2 && before_resolution_get)
+            before_resolution_get();
+        if (watched_get_count == 2 && fail_resolution_get)
+            throw Poco::TimeoutException("AmbiguousCkptBackend: exact-read response lost");
+        auto result = CountingBackend::get(key, range);
+        if (watched_get_count >= 2 && after_resolution_get)
+            after_resolution_get();
+        return result;
+    }
+
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        if (key != watched_key)
+            return CountingBackend::casPut(key, bytes, expected, meta);
+
+        journal.push_back("CAS");
+        if (fault == Fault::None)
+            return CountingBackend::casPut(key, bytes, expected, meta);
+        const Fault this_fault = fault;
+        if (fault != Fault::AlwaysThrowWithoutCommit)
+            fault = Fault::None;
+        if (this_fault == Fault::CommitThenThrow)
+        {
+            const CasResult result = CountingBackend::casPut(key, bytes, expected, meta);
+            if (result.outcome == CasOutcome::Committed && !dominating_bytes.empty())
+            {
+                const HeadResult head_result = CountingBackend::head(key);
+                EXPECT_EQ(CountingBackend::putOverwrite(key, dominating_bytes, head_result.token).outcome,
+                          PutOutcome::Done);
+            }
+        }
+        throw Poco::TimeoutException("AmbiguousCkptBackend: CAS response lost");
+    }
+
+private:
+    size_t watched_get_count = 0;
+};
+
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The codec
+/// ---------------------------------------------------------------------------------------------
+
+/// Every combination of the frontier and existing optionals survives a round trip. Both-absent is the shape a namespace
+/// carries from creation until its first snapshot, so it is a real state and not a degenerate one.
+TEST(CASRefCheckpoint, RoundTripsEveryFieldCombination)
+{
+    const std::vector<RefCkpt> cases = {
+        RefCkpt{.life_epoch = std::optional<uint64_t>{7}, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt},
+        RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_1_2, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = std::nullopt},
+        RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_2_1, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = ID_2_1},
+        RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_2_1, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = ID_2_1},
+        RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_2_1, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = ID_2_1},
+    };
+    for (const RefCkpt & ckpt : cases)
+        EXPECT_EQ(decodeRefCkpt(encodeRefCkpt(ckpt)), ckpt);
+}
+
+TEST(CASRefCheckpoint, CommittedThroughHasCanonicalExactWireEncoding)
+{
+    const RefCkpt ckpt{.life_epoch = std::optional<uint64_t>{7},
+                       .committed_through = RefTxnId{9, 11},
+                       .checkpoint_snapshot_id = RefTxnId{9, 10},
+                       .last_epoch_seal = RefTxnId{8, 12}};
+    const String expected = R"({"type":"cas_ref_ckpt","v":9}
+{"le":"7","cte":"9","cts":"11","cse":"9","css":"10","lse":"8","lss":"12"}
+)";
+
+    EXPECT_EQ(encodeRefCkpt(ckpt), expected);
+    EXPECT_EQ(decodeRefCkpt(expected), ckpt);
+}
+
+/// `last_epoch_seal` is chain evidence, not an arbitrary lower bound. It either names the frontier
+/// itself when that frontier is the terminal seal, or closes the immediately preceding numeric epoch.
+/// Accepting a gap or a later same-epoch frontier would manufacture a boundary that INV-2 never proved.
+TEST(CASRefCheckpoint, CodecRejectsIncoherentCommittedFrontierAndSealEpochs)
+{
+    const RefCkpt valid{.life_epoch = 7, .committed_through = RefTxnId{8, 5},
+                        .checkpoint_snapshot_id = RefTxnId{7, 4}, .last_epoch_seal = RefTxnId{7, 9}};
+    EXPECT_NO_THROW(encodeRefCkpt(valid));
+
+    const RefCkpt skipped_epoch{.life_epoch = 7, .committed_through = RefTxnId{10, 1},
+                                 .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = RefTxnId{7, 9}};
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefCkpt(skipped_epoch); });
+
+    const RefCkpt frontier_after_same_epoch_seal{.life_epoch = 7, .committed_through = RefTxnId{8, 5},
+                                                 .checkpoint_snapshot_id = std::nullopt,
+                                                 .last_epoch_seal = RefTxnId{8, 1}};
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+                     [&] { encodeRefCkpt(frontier_after_same_epoch_seal); });
+
+    const RefCkpt unsealed_non_genesis{.life_epoch = 7, .committed_through = RefTxnId{8, 1},
+                                        .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefCkpt(unsealed_non_genesis); });
+
+    String malformed = encodeRefCkpt(valid);
+    const size_t cte = malformed.find(R"("cte":"8")");
+    ASSERT_NE(cte, String::npos);
+    malformed.replace(cte, String{R"("cte":"8")"}.size(), R"("cte":"10")");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(malformed); });
+}
+
+/// STRICT means an unknown key is corruption, not something to skip. A `_ckpt` decides deletions, so a
+/// reader that ignored a field it did not understand would be authorizing them from a body it only
+/// partly read.
+TEST(CASRefCheckpoint, RejectsAnUnknownKey)
+{
+    const String good = encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_1_1, .checkpoint_snapshot_id = ID_1_1,
+                                              .last_epoch_seal = std::nullopt});
+    String with_unknown = good;
+    with_unknown.replace(with_unknown.rfind('}'), 1, R"(,"zz":"1"})");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(with_unknown); });
+
+    /// A `!`-prefixed key is a REQUIRED extension and reports the version, not corruption -- the
+    /// distinction is what lets an operator tell "this build is too old" from "this object is broken".
+    String with_critical = good;
+    with_critical.replace(with_critical.rfind('}'), 1, R"(,"!zz":"1"})");
+    expectThrowsCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION, [&] { decodeRefCkpt(with_critical); });
+}
+
+/// A duplicate key has no single meaning, so it can never be resolved by a reader's preference.
+TEST(CASRefCheckpoint, RejectsADuplicateKey)
+{
+    const String good = encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{7}, .checkpoint_snapshot_id = std::nullopt,
+                                              .last_epoch_seal = std::nullopt});
+    String duplicated = good;
+    duplicated.replace(duplicated.rfind('}'), 1, R"(,"le":"9"})");
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(duplicated); });
+}
+
+/// Truncation in each of its shapes. Half an optional pair is the dangerous one: silently dropping it
+/// would turn a truncated body into a well-formed `_ckpt` with NO checkpoint, which reads as
+/// "recovery has no base" and would be trusted.
+TEST(CASRefCheckpoint, RejectsTruncation)
+{
+    const String good = encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_1_2, .checkpoint_snapshot_id = ID_1_2,
+                                              .last_epoch_seal = std::nullopt});
+
+    const String header_only = good.substr(0, good.find('\n') + 1);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(header_only); });
+
+    /// The body line without its terminator: a read that stopped mid-object.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(good.substr(0, good.size() - 1)); });
+
+    /// An EMPTY body is not truncation, it is the legitimate "nobody knows anything yet" object -- the
+    /// shape a namespace carries between its creation and its first checkpoint. Asserted here, next to
+    /// the truncation cases, because the two are one character apart on the wire.
+    const String empty_body = good.substr(0, good.find('\n') + 1) + "{}\n";
+    EXPECT_EQ(decodeRefCkpt(empty_body), RefCkpt{});
+
+    const String half_pair = good.substr(0, good.find('\n') + 1) + R"({"le":"7","cse":"1"})" + "\n";
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(half_pair); });
+
+    const String other_half = good.substr(0, good.find('\n') + 1) + R"({"le":"7","lss":"2"})" + "\n";
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(other_half); });
+
+    const String frontier_half = good.substr(0, good.find('\n') + 1) + R"({"le":"7","cte":"1"})" + "\n";
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(frontier_half); });
+}
+
+TEST(CASRefCheckpoint, RejectsTrailingBytes)
+{
+    const String good = encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{7}, .checkpoint_snapshot_id = std::nullopt,
+                                              .last_epoch_seal = std::nullopt});
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(good + "junk\n"); });
+}
+
+/// The field-validity rule runs in BOTH directions: a struct this build refuses to read can never be
+/// written by it either, so a bug on the write side surfaces at the writer and not as an unreadable
+/// object discovered by a future recovery.
+TEST(CASRefCheckpoint, RejectsInvalidFieldsOnEncodeAndOnDecode)
+{
+    /// PRESENT means REAL: an absent field is legal, a present-but-impossible one is not. A zero
+    /// `life_epoch` would give the field two meanings ("unknown" and "epoch zero") on an object that
+    /// gates deletions.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [] { encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{0}, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt}); });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [] { encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{7}, .checkpoint_snapshot_id = RefTxnId{1, 0}, .last_epoch_seal = std::nullopt}); });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [] { encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{7}, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = RefTxnId{0, 1}}); });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [] { encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{7}, .checkpoint_snapshot_id = ID_1_1, .last_epoch_seal = std::nullopt}); });
+
+    const String header = encodeRefCkpt(RefCkpt{.life_epoch = std::optional<uint64_t>{7}, .checkpoint_snapshot_id = std::nullopt,
+                                                .last_epoch_seal = std::nullopt});
+    const String prefix = header.substr(0, header.find('\n') + 1);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefCkpt(prefix + R"({"le":"0"})" + "\n"); });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefCkpt(prefix + R"({"le":"7","cse":"1","css":"0"})" + "\n"); });
+}
+
+/// The registry row is part of the contract: Control/Strict decides how the decoder treats unknown
+/// keys, and the caps are the first thing that fires if a foreign object ever lands at the key.
+TEST(CASRefCheckpoint, RegistryRowIsControlStrictWithTightCaps)
+{
+    const FormatTraits & traits = traitsFor(FormatId::RefCkpt);
+    EXPECT_EQ(traits.type, "cas_ref_ckpt");
+    EXPECT_EQ(traits.family, TextFamily::Control);
+    EXPECT_EQ(traits.strictness, KeyStrictness::Strict);
+    EXPECT_EQ(traits.object_cap, 64u * 1024u);
+    EXPECT_EQ(traits.line_cap, 4u * 1024u);
+    EXPECT_EQ(traitsForType("cas_ref_ckpt"), &traits);
+    /// Raw, so the key has no suffix -- the Stage A shape is exactly `<ns>/_ckpt`. This line is also
+    /// the TRIPWIRE for the codec's shortcut: `encodeRefCkpt`/`decodeRefCkpt` hand bytes to and from
+    /// the backend directly, bypassing `sealObject`/`openObject` because both are the identity under
+    /// `CompressionPolicy::Never`. Flip the policy to `Always` and that bypass would silently write
+    /// uncompressed bodies under a `.zst` key -- which this assertion catches first.
+    EXPECT_EQ(storedSuffix(FormatId::RefCkpt), "");
+    EXPECT_EQ(traits.compression, CompressionPolicy::Never);
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The key
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASRefCheckpoint, KeyIsTheLifeLeafAndParsesBack)
+{
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_key"};
+    const NamespaceLifeId ns_id = DB::Cas::tests::fixture::fixtureLife(ns);
+    EXPECT_EQ(layout.refCkptKey(ns_id),
+        "p/cas/ns/state/" + renderIncarnation(ns_id.incarnation) + "/_ckpt");
+    EXPECT_EQ(layout.parseRefCkptKey(layout.refCkptKey(ns_id)), ns_id.incarnation);
+
+    /// `_ckpt` has no kind directory, so the id-bearing parser must NOT claim it -- and the `_ckpt`
+    /// parser must not claim the id-bearing keys either. Each key has exactly one classifier.
+    EXPECT_FALSE(layout.parseRefObjectKey(layout.refCkptKey(ns_id)).has_value());
+    EXPECT_FALSE(layout.parseRefCkptKey(layout.refLogKey(ns_id, ID_1_1)).has_value());
+    EXPECT_FALSE(layout.parseRefCkptKey(layout.refSnapshotKey(ns_id, ID_1_1)).has_value());
+    EXPECT_FALSE(layout.parseRefCkptKey(layout.refCkptKey(ns_id) + ".zst").has_value());
+    EXPECT_FALSE(layout.parseRefCkptKey("p/cas/ns/state/_ckpt").has_value());
+    EXPECT_FALSE(layout.parseRefCkptKey("q" + layout.refCkptKey(ns_id).substr(1)).has_value());
+}
+
+/// The hot stream grouping accepts logs and snapshots while ignoring a checkpoint from the separate
+/// state tree. An unrecognized key inside the stream tree still aborts the round.
+TEST(CASRefCheckpoint, GroupRefKeysScopesHotIntakeToTheStreamTree)
+{
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_group"};
+    const std::vector<String> keys = {
+        layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), ID_1_1),
+        layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), ID_1_1),
+        layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns)),
+    };
+
+    const auto grouped = groupRefKeys(layout, keys);
+    ASSERT_EQ(grouped.size(), 1u);
+    const RefTableListing & listing = grouped.at(DB::Cas::tests::fixture::fixtureLife(ns).incarnation);
+    EXPECT_EQ(listing.logs, std::vector<RefTxnId>{ID_1_1});
+    EXPECT_EQ(listing.snapshots, std::vector<RefTxnId>{ID_1_1});
+
+    /// A genuinely unrecognizable key inside this life stream is still corruption.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { groupRefKeys(layout, {layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_bogus"}); });
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The merge -- per field, both directions
+/// ---------------------------------------------------------------------------------------------
+
+/// The per-field table the ledger obligation from the TLA phase asks for: each field independently
+/// newer on either side, plus both-absent and equal bodies. A merge that is not per-field would pass
+/// some rows and fail others, which is the point of enumerating them.
+TEST(CASRefCheckpoint, MergeTakesThePerFieldSemanticMaximum)
+{
+    const RefCkpt low{.life_epoch = std::optional<uint64_t>{3}, .checkpoint_snapshot_id = ID_1_1, .last_epoch_seal = ID_1_1};
+    const RefCkpt high_ckpt{.life_epoch = std::optional<uint64_t>{3}, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = ID_1_1};
+    const RefCkpt high_seal{.life_epoch = std::optional<uint64_t>{3}, .checkpoint_snapshot_id = ID_1_1, .last_epoch_seal = ID_2_1};
+    const RefCkpt high_life{.life_epoch = std::optional<uint64_t>{9}, .checkpoint_snapshot_id = ID_1_1, .last_epoch_seal = ID_1_1};
+
+    /// Each field newer on the RIGHT, then the same case mirrored to the LEFT: the merge is symmetric,
+    /// which is exactly why the two writers need no ordering between them.
+    ///
+    /// The `life_epoch` rows stay mirrored, and that is a deliberate statement rather than an oversight:
+    /// a `life_epoch` that FALLS is refused, but the refusal lives in `publishCkpt`, which knows which
+    /// side is durable, and NOT here. This function stays commutative, so both directions must keep
+    /// yielding the maximum. See `CASRefCheckpointJoin` (`gtest_cas_ref_ckpt_join.cpp`) for the refusal itself
+    /// and for why it cannot be expressed at this level.
+    EXPECT_EQ(mergeCkpt(low, high_ckpt), high_ckpt);
+    EXPECT_EQ(mergeCkpt(high_ckpt, low), high_ckpt);
+    EXPECT_EQ(mergeCkpt(low, high_seal), high_seal);
+    EXPECT_EQ(mergeCkpt(high_seal, low), high_seal);
+    EXPECT_EQ(mergeCkpt(low, high_life), high_life);
+    EXPECT_EQ(mergeCkpt(high_life, low), high_life);
+
+    /// Fields advance INDEPENDENTLY: a merge of two bodies each newer in a different field keeps both.
+    const RefCkpt both = mergeCkpt(high_ckpt, high_seal);
+    EXPECT_EQ(both.checkpoint_snapshot_id, ID_1_2);
+    EXPECT_EQ(both.last_epoch_seal, ID_2_1);
+
+    /// An absent optional loses to a present one, whichever side it is on, and two absents stay absent.
+    const RefCkpt none{.life_epoch = std::optional<uint64_t>{3}, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    EXPECT_EQ(mergeCkpt(none, low), low);
+    EXPECT_EQ(mergeCkpt(low, none), low);
+    EXPECT_EQ(mergeCkpt(none, none), none);
+
+    /// Identical bodies merge to themselves -- the property `publishCkpt` turns into "no write".
+    EXPECT_EQ(mergeCkpt(low, low), low);
+
+    /// A contribution that knows NOTHING about `life_epoch` (the snapshot publisher's shape) must not
+    /// erase it. This is the case a plain assignment would get wrong.
+    const RefCkpt publisher_only{.life_epoch = std::nullopt, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = std::nullopt};
+    const RefCkpt advanced = mergeCkpt(low, publisher_only);
+    EXPECT_EQ(advanced.life_epoch, 3u);
+    EXPECT_EQ(advanced.checkpoint_snapshot_id, ID_1_2);
+    EXPECT_EQ(advanced.last_epoch_seal, ID_1_1) << "the publisher knows nothing about the seal and must "
+                                                   "not drag it backwards";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// publishCkpt
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASRefCheckpoint, CreatesTheObjectWhenItIsAbsent)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_create"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(ns);
+    const RefCkpt birth{.life_epoch = std::optional<uint64_t>{5}, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+
+    EXPECT_EQ(publishCkpt(*backend, layout, life, birth, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    const auto sample = readCkpt(*backend, layout, life);
+    ASSERT_TRUE(sample.has_value());
+    EXPECT_EQ(sample->ckpt, birth);
+}
+
+/// Any writer may CREATE the object, and none of them may complete it. A publisher knows only the
+/// checkpoint, so it creates an object that knows only the checkpoint; the birth transaction's
+/// `life_epoch` merges in afterwards. Order does not matter -- the merge is a per-field maximum, and
+/// no writer ever supplies a field it does not know (a guess here would be permanent, since the merge
+/// can never lower it).
+TEST(CASRefCheckpoint, EachWriterCreatesWithOnlyWhatItKnowsAndTheOtherFieldsMergeInLater)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_partial_create"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(ns);
+    const RefCkpt publisher{.life_epoch = std::nullopt, .committed_through = ID_1_1, .checkpoint_snapshot_id = ID_1_1, .last_epoch_seal = std::nullopt};
+
+    ASSERT_EQ(publishCkpt(*backend, layout, life, publisher, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    const auto created = readCkpt(*backend, layout, life);
+    ASSERT_TRUE(created.has_value());
+    EXPECT_EQ(created->ckpt.checkpoint_snapshot_id, ID_1_1);
+    EXPECT_FALSE(created->ckpt.life_epoch.has_value()) << "the publisher must not invent a genesis epoch";
+
+    const RefCkpt birth{.life_epoch = std::optional<uint64_t>{1}, .checkpoint_snapshot_id = std::nullopt,
+                        .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(publishCkpt(*backend, layout, life, birth, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    const auto completed = readCkpt(*backend, layout, life);
+    ASSERT_TRUE(completed.has_value());
+    EXPECT_EQ(completed->ckpt.life_epoch, 1u);
+    EXPECT_EQ(completed->ckpt.checkpoint_snapshot_id, ID_1_1) << "and must not lose the checkpoint on the way in";
+}
+
+/// The conflict path is the whole reason the algorithm re-READS instead of retrying its bytes: the
+/// winner's field must survive the loser's retry. Here a concurrent writer advances the seal between
+/// our read and our CAS; our retry must merge onto the new body, not overwrite it.
+TEST(CASRefCheckpoint, TokenConflictRereadsAndMergesOntoTheWinner)
+{
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_conflict"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(ns);
+    const String key = layout.refCkptKey(life);
+    const RefCkpt base{.life_epoch = std::optional<uint64_t>{1}, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+
+    auto backend = std::make_shared<GetHookBackend>(key);
+    ASSERT_EQ(backend->casPut(key, encodeRefCkpt(base), std::nullopt).outcome, CasOutcome::Committed);
+
+    /// The concurrent sealer lands exactly ONCE, immediately after our first read -- so our first CAS
+    /// carries a token that is no longer current, and our retry has to merge onto its body.
+    bool interfered = false;
+    backend->on_get = [&]
+    {
+        if (interfered)
+            return;
+        interfered = true;
+        const RefCkpt sealer{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_2_1, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = ID_2_1};
+        const HeadResult h = backend->head(key);
+        ASSERT_EQ(backend->putOverwrite(key, encodeRefCkpt(mergeCkpt(base, sealer)), h.token).outcome,
+                  PutOutcome::Done);
+    };
+
+    const RefCkpt publisher{.life_epoch = std::nullopt, .committed_through = ID_1_2, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = std::nullopt};
+    EXPECT_EQ(publishCkpt(*backend, layout, life, publisher, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+
+    const auto sample = readCkpt(*backend, layout, life);
+    ASSERT_TRUE(sample.has_value());
+    EXPECT_EQ(sample->ckpt.checkpoint_snapshot_id, ID_1_2) << "our own contribution must land";
+    EXPECT_EQ(sample->ckpt.last_epoch_seal, ID_2_1)
+        << "the concurrent writer's seal must survive our retry -- a retry that reused the body read "
+           "before the conflict would silently drop it (TLC `_sab_sealclobbersbase`)";
+    EXPECT_EQ(sample->ckpt.life_epoch, 1u);
+    EXPECT_GE(backend->casPutCount(key), 2u) << "the first CAS must have been rejected, not skipped";
+}
+
+/// A contribution that adds nothing issues NO write. This is a correctness property, not a saving:
+/// both writers publish on every snapshot and every seal, and a no-op write would mint a fresh token
+/// each time, turning every other writer's in-flight CAS into a conflict for identical bytes.
+TEST(CASRefCheckpoint, AnIdenticalMergedBodyIssuesNoWrite)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_noop"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(ns);
+    const String key = layout.refCkptKey(life);
+    const RefCkpt full{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_2_1, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = ID_2_1};
+
+    ASSERT_EQ(publishCkpt(*backend, layout, life, full, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    const uint64_t writes_after_create = backend->casPutCount(key);
+    const Token token_after_create = backend->head(key).token;
+
+    /// The same contribution again, and a strictly OLDER one: neither adds anything.
+    EXPECT_EQ(publishCkpt(*backend, layout, life, full, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::IdenticalSkip);
+    const RefCkpt older{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_1_1, .checkpoint_snapshot_id = ID_1_1, .last_epoch_seal = std::nullopt};
+    EXPECT_EQ(publishCkpt(*backend, layout, life, older, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::IdenticalSkip);
+
+    EXPECT_EQ(backend->casPutCount(key), writes_after_create) << "a skip must issue no CAS at all";
+    EXPECT_EQ(backend->head(key).token, token_after_create) << "and must not mint a new incarnation";
+}
+
+/// The fence is re-checked AFTER the read and BEFORE the write, on every attempt. A generation that
+/// moved means this writer's lease incarnation is gone, so its merged body is stale even if the fence
+/// is live again under a fresh incarnation.
+TEST(CASRefCheckpoint, AFenceBumpBetweenTheReadAndTheCasWritesNothing)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_fenced"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(ns);
+    const String key = layout.refCkptKey(life);
+    const RefCkpt base{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_1_1, .checkpoint_snapshot_id = ID_1_1, .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(publishCkpt(*backend, layout, life, base, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    const Token token_before = backend->head(key).token;
+    const uint64_t writes_before = backend->casPutCount(key);
+
+    /// The callback the pool wires from `CasMountRuntime::checkFenceOrThrow`: it throws when the
+    /// generation moved since admission. Mirrors the real site's class (the transient, upstream-retryable
+    /// one) so the stub cannot drift into testing a shape production never produces.
+    const auto moved_fence = [](uint64_t admitted)
+    {
+        throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR,
+            "fence generation moved since admission ({})", admitted);
+    };
+
+    const RefCkpt advance{.life_epoch = std::nullopt, .committed_through = ID_1_2, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = std::nullopt};
+    EXPECT_EQ(publishCkpt(*backend, layout, life, advance, 1, moved_fence, generousDeadline()),
+              CkptPublishOutcome::FencedOut);
+    EXPECT_EQ(backend->casPutCount(key), writes_before) << "the check precedes the CAS, so nothing is sent";
+    EXPECT_EQ(backend->head(key).token, token_before);
+    EXPECT_EQ(readCkptOrFail(*backend, layout, life), base);
+}
+
+/// Persistent contention fails CLOSED and says so. There is no partial state to clean up -- every
+/// attempt either committed the complete merged body or changed nothing -- but the caller must be told
+/// its contribution is unpublished rather than left to assume it landed.
+TEST(CASRefCheckpoint, AnExhaustedDeadlineUnderPersistentConflictThrowsRetryLater)
+{
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_exhausted"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(ns);
+    const String key = layout.refCkptKey(life);
+    const RefCkpt base{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_1_1, .checkpoint_snapshot_id = ID_1_1, .last_epoch_seal = std::nullopt};
+
+    auto backend = std::make_shared<GetHookBackend>(key);
+    ASSERT_EQ(backend->casPut(key, encodeRefCkpt(base), std::nullopt).outcome, CasOutcome::Committed);
+
+    /// Every read is followed by a rewrite of the SAME body under a fresh incarnation, so the token this
+    /// call holds is always stale and every CAS it issues conflicts. The clock advances one step per
+    /// read, so the DEADLINE is what ends the loop -- deterministically, with no sleeping and well
+    /// before the live-lock brake.
+    uint64_t now = 0;
+    backend->on_get = [&]
+    {
+        ++now;
+        const HeadResult h = backend->head(key);
+        if (h.exists)
+            backend->putOverwrite(key, encodeRefCkpt(base), h.token);
+    };
+
+    const RefCkpt advance{.life_epoch = std::nullopt, .committed_through = ID_1_2, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = std::nullopt};
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR,
+        [&] { publishCkpt(*backend, layout, life, advance, 1, ALWAYS_ADMITTED, CkptDeadline{[&] { return now; }, 5}); });
+    EXPECT_EQ(readCkptOrFail(*backend, layout, life), base) << "no partial state: every attempt either "
+                                                             "committed the complete merged body or wrote nothing";
+}
+
+TEST(CASRefCheckpoint, AmbiguousCommittedCasIsResolvedByOneExactReadWithoutBlindRetry)
+{
+    auto backend = std::make_shared<AmbiguousCkptBackend>();
+    const Layout layout{"p"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(RootNamespace{"srv1/ckpt_ambiguous_committed"});
+    const String key = layout.refCkptKey(life);
+    const RefCkpt base{.life_epoch = 1, .committed_through = ID_1_1,
+                       .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    const RefCkpt contribution{.life_epoch = std::nullopt, .committed_through = ID_1_2,
+                               .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(backend->casPut(key, encodeRefCkpt(base), std::nullopt).outcome, CasOutcome::Committed);
+    backend->arm(key, AmbiguousCkptBackend::Fault::CommitThenThrow);
+
+    EXPECT_EQ(publishCkpt(*backend, layout, life, contribution, 7, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    EXPECT_EQ(backend->journal, (std::vector<String>{"GET", "CAS", "GET"}));
+    EXPECT_EQ(readCkptOrFail(*backend, layout, life).committed_through, ID_1_2);
+}
+
+TEST(CASRefCheckpoint, AmbiguousUncommittedCasRetriesAgainstTheExactReadToken)
+{
+    auto backend = std::make_shared<AmbiguousCkptBackend>();
+    const Layout layout{"p"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(RootNamespace{"srv1/ckpt_ambiguous_retry"});
+    const String key = layout.refCkptKey(life);
+    const RefCkpt base{.life_epoch = 1, .committed_through = ID_1_1,
+                       .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    const RefCkpt contribution{.life_epoch = std::nullopt, .committed_through = ID_1_2,
+                               .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(backend->casPut(key, encodeRefCkpt(base), std::nullopt).outcome, CasOutcome::Committed);
+    backend->arm(key, AmbiguousCkptBackend::Fault::ThrowWithoutCommit);
+
+    EXPECT_EQ(publishCkpt(*backend, layout, life, contribution, 7, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    EXPECT_EQ(backend->journal, (std::vector<String>{"GET", "CAS", "GET", "CAS"}));
+    EXPECT_EQ(readCkptOrFail(*backend, layout, life).committed_through, ID_1_2);
+}
+
+TEST(CASRefCheckpoint, AmbiguousCasAcceptsAValidDominatingDurableFrontier)
+{
+    auto backend = std::make_shared<AmbiguousCkptBackend>();
+    const Layout layout{"p"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(RootNamespace{"srv1/ckpt_ambiguous_dominating"});
+    const String key = layout.refCkptKey(life);
+    const RefCkpt base{.life_epoch = 1, .committed_through = ID_1_1,
+                       .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    const RefCkpt contribution{.life_epoch = std::nullopt, .committed_through = ID_1_2,
+                               .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    const RefCkpt dominating{.life_epoch = 1, .committed_through = ID_2_1,
+                             .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = ID_2_1};
+    ASSERT_EQ(backend->casPut(key, encodeRefCkpt(base), std::nullopt).outcome, CasOutcome::Committed);
+    backend->dominating_bytes = encodeRefCkpt(dominating);
+    backend->arm(key, AmbiguousCkptBackend::Fault::CommitThenThrow);
+
+    EXPECT_EQ(publishCkpt(*backend, layout, life, contribution, 7, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    EXPECT_EQ(backend->journal, (std::vector<String>{"GET", "CAS", "GET"}));
+    EXPECT_EQ(readCkptOrFail(*backend, layout, life), dominating);
+}
+
+TEST(CASRefCheckpoint, FailedExactReadAfterAmbiguousCasFailsClosedWithoutAnotherCas)
+{
+    auto backend = std::make_shared<AmbiguousCkptBackend>();
+    const Layout layout{"p"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(RootNamespace{"srv1/ckpt_ambiguous_read_failed"});
+    const String key = layout.refCkptKey(life);
+    const RefCkpt base{.life_epoch = 1, .committed_through = ID_1_1,
+                       .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(backend->casPut(key, encodeRefCkpt(base), std::nullopt).outcome, CasOutcome::Committed);
+    backend->fail_resolution_get = true;
+    backend->arm(key, AmbiguousCkptBackend::Fault::ThrowWithoutCommit);
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        publishCkpt(*backend, layout, life, RefCkpt{.life_epoch = std::nullopt, .committed_through = ID_1_2,
+                    .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt}, 7,
+                    ALWAYS_ADMITTED, generousDeadline());
+    });
+    EXPECT_EQ(backend->journal, (std::vector<String>{"GET", "CAS", "GET"}));
+    EXPECT_EQ(readCkptOrFail(*backend, layout, life), base);
+}
+
+TEST(CASRefCheckpoint, FenceMovementAroundAmbiguityResolutionMakesTheExactReadInert)
+{
+    for (const bool move_before_read : {true, false})
+    {
+        auto backend = std::make_shared<AmbiguousCkptBackend>();
+        const Layout layout{"p"};
+        const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(
+            RootNamespace{move_before_read ? "srv1/ckpt_fence_before_resolution" : "srv1/ckpt_fence_after_resolution"});
+        const String key = layout.refCkptKey(life);
+        const RefCkpt base{.life_epoch = 1, .committed_through = ID_1_1,
+                           .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+        ASSERT_EQ(backend->casPut(key, encodeRefCkpt(base), std::nullopt).outcome, CasOutcome::Committed);
+        bool admitted = true;
+        const auto move_fence = [&] { admitted = false; };
+        if (move_before_read)
+            backend->before_resolution_get = move_fence;
+        else
+            backend->after_resolution_get = move_fence;
+        backend->arm(key, AmbiguousCkptBackend::Fault::CommitThenThrow);
+
+        const auto check_admission = [&](uint64_t)
+        {
+            if (!admitted)
+                throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR, "fence moved");
+        };
+        EXPECT_EQ(publishCkpt(*backend, layout, life,
+                              RefCkpt{.life_epoch = std::nullopt, .committed_through = ID_1_2,
+                                      .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt}, 7,
+                              check_admission, generousDeadline()), CkptPublishOutcome::FencedOut);
+        EXPECT_EQ(backend->journal, (std::vector<String>{"GET", "CAS", "GET"}));
+    }
+}
+
+TEST(CASRefCheckpoint, ContinuedAmbiguityStopsAtTheDeadlineAndNeverIssuesConsecutiveCasAttempts)
+{
+    auto backend = std::make_shared<AmbiguousCkptBackend>();
+    const Layout layout{"p"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(RootNamespace{"srv1/ckpt_ambiguity_deadline"});
+    const String key = layout.refCkptKey(life);
+    const RefCkpt base{.life_epoch = 1, .committed_through = ID_1_1,
+                       .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(backend->casPut(key, encodeRefCkpt(base), std::nullopt).outcome, CasOutcome::Committed);
+    uint64_t now = 0;
+    backend->after_resolution_get = [&] { ++now; };
+    backend->arm(key, AmbiguousCkptBackend::Fault::AlwaysThrowWithoutCommit);
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        publishCkpt(*backend, layout, life,
+                    RefCkpt{.life_epoch = std::nullopt, .committed_through = ID_1_2,
+                            .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt},
+                    7, ALWAYS_ADMITTED, CkptDeadline{[&] { return now; }, 3});
+    });
+    EXPECT_EQ(backend->journal,
+              (std::vector<String>{"GET", "CAS", "GET", "CAS", "GET", "CAS", "GET"}));
+    EXPECT_EQ(readCkptOrFail(*backend, layout, life), base);
+}
+
+/// A `_ckpt` that does not decode is NEVER overwritten. It is the only record of recovery's base and
+/// of what cleanup may delete, so replacing it with a body derived from the contribution alone would
+/// erase the base and leave a well-formed object a reader would trust.
+TEST(CASRefCheckpoint, ACorruptCheckpointIsNeverOverwritten)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout{"p"};
+    const RootNamespace ns{"srv1/ckpt_corrupt"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(ns);
+    const String key = layout.refCkptKey(life);
+    ASSERT_EQ(publishCkpt(*backend, layout, life,
+                          RefCkpt{.life_epoch = std::optional<uint64_t>{1}, .committed_through = ID_1_2, .checkpoint_snapshot_id = ID_1_2, .last_epoch_seal = std::nullopt},
+                          1, ALWAYS_ADMITTED, generousDeadline()), CkptPublishOutcome::Published);
+
+    const String garbage = "not a cas object\n";
+    overwriteObject(*backend, key, garbage);
+
+    const RefCkpt birth{.life_epoch = std::optional<uint64_t>{5}, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { publishCkpt(*backend, layout, life, birth, 1, ALWAYS_ADMITTED, generousDeadline()); });
+    EXPECT_EQ(backend->get(key)->bytes, garbage) << "corruption must be surfaced, never laundered into a "
+                                                    "well-formed object";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The reader-side rules Task 6 and the cleanup call sites consume
+/// ---------------------------------------------------------------------------------------------
+
+/// INV-4's three-way revalidation of a base that turned out to be missing.
+TEST(CASRefCheckpoint, AMissingSampledBaseRestartsOnAnAdvancedTokenAndIsCorruptionOnAnUnchangedOne)
+{
+    const Token sampled{"t1", TokenType::Emulated};
+    const Token advanced{"t2", TokenType::Emulated};
+
+    EXPECT_EQ(classifyMissingSampledBase(sampled, advanced), MissingBaseVerdict::RestartRecovery)
+        << "cleanup legitimately moved the checkpoint while we read; restart from the newer base";
+    EXPECT_EQ(classifyMissingSampledBase(sampled, sampled), MissingBaseVerdict::Corrupted)
+        << "the checkpoint still names an object that is not there, which the strictly-below deletion "
+           "gate makes unreachable in an honest run";
+    EXPECT_EQ(classifyMissingSampledBase(sampled, std::nullopt), MissingBaseVerdict::Corrupted)
+        << "a namespace with a sampled base and no checkpoint at all is worse, not better";
+}
+
+/// The deletion gate is STRICTLY below, because the checkpoint names the snapshot a recovery is
+/// entitled to fetch by exact key. At-or-below is TLC counterexample `_sab_staleckptcorruption`.
+TEST(CASRefCheckpoint, SnapshotsAreDeletableStrictlyBelowTheCheckpoint)
+{
+    EXPECT_TRUE(snapshotDeletableUnderCkpt(ID_1_1, ID_1_2));
+    EXPECT_FALSE(snapshotDeletableUnderCkpt(ID_1_2, ID_1_2)) << "the checkpoint's own base is off limits";
+    EXPECT_FALSE(snapshotDeletableUnderCkpt(ID_2_1, ID_1_2));
+    /// Fail closed: a namespace with no checkpoint has established no covering base, so nothing is
+    /// deletable -- a stale or absent pointer may only ever under-clean.
+    EXPECT_FALSE(snapshotDeletableUnderCkpt(ID_1_1, std::nullopt));
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The REAL call sites, through the ledger
+/// ---------------------------------------------------------------------------------------------
+
+/// The namespace-birth transaction creates the checkpoint, and it is the only writer that can: the
+/// `life_epoch` is this transaction's own writer epoch.
+TEST(CASRefCheckpoint, NamespaceBirthCreatesTheCheckpointCarryingItsLifeEpoch)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/ckpt_birth"};
+
+    /// Stage B (Task 4-C): the catalog carries no entry for `ns` before its first open, and the
+    /// namespace's real incarnation does not exist to name a key with yet -- the pre-birth analog of
+    /// "nothing exists" is "nothing is even NAMED", checked at the catalog rather than at a key this
+    /// test cannot yet compute.
+    EXPECT_TRUE(CasRefCatalog::read(*backend, store->layout()).catalog.entries.empty())
+        << "nothing exists before the birth";
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{store->writerEpoch(), 1}));
+
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    const auto sample = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(sample.has_value()) << "spec §3 creates the _ckpt before the namespace becomes Live";
+    EXPECT_EQ(sample->ckpt.life_epoch, store->writerEpoch());
+    EXPECT_FALSE(sample->ckpt.checkpoint_snapshot_id.has_value()) << "a newborn namespace has no base yet";
+    EXPECT_FALSE(sample->ckpt.last_epoch_seal.has_value());
+}
+
+/// The snapshot publisher is INV-4's second writer: the body PUT commits, then the checkpoint names it.
+TEST(CASRefCheckpoint, ACommittedSnapshotPublishAdvancesTheCheckpoint)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const uint64_t epoch = store->writerEpoch();
+    const RootNamespace ns{"srv1/ckpt_publish"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{epoch, 1}));
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    ASSERT_FALSE(readCkptOrFail(*backend, store->layout(), life).checkpoint_snapshot_id.has_value());
+
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    const auto published = store->newestPublishedSnapshotIdForTest(ns);
+    ASSERT_TRUE(published.has_value());
+
+    const auto sample = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(sample.has_value());
+    EXPECT_EQ(sample->ckpt.checkpoint_snapshot_id, published);
+    EXPECT_EQ(sample->ckpt.life_epoch, epoch) << "the publisher contributes nothing about life_epoch, so "
+                                                 "the merge must preserve what the birth wrote";
+    /// And the snapshot body it names really is there -- the checkpoint may never point at a key that
+    /// does not exist, which is the premise the missing-base rule reasons from.
+    EXPECT_TRUE(backend->head(store->layout().refSnapshotKey(life, *published)).exists);
+}
+
+/// The body-PUT/cleanup/`_ckpt` race, decided by the ORDER of the two writes: cleanup planned in the
+/// window between the snapshot body PUT and the checkpoint CAS still reads the OLD checkpoint, and the
+/// gate is strictly below it -- so it cannot delete the snapshot just published.
+TEST(CASRefCheckpoint, CleanupPlannedBetweenTheBodyPutAndTheCkptCasCannotDeleteTheNewSnapshot)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const uint64_t epoch = store->writerEpoch();
+    const RootNamespace ns{"srv1/ckpt_race"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{epoch, 1}));
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    const RefTxnId first_snapshot = *store->newestPublishedSnapshotIdForTest(ns);
+
+    ASSERT_EQ(publishRef(store, ns, "ref_2", 2), (RefTxnId{epoch, 2}));
+    /// The checkpoint a cleanup pass sampled BEFORE the second publication -- the stale reading the
+    /// race hands it.
+    const std::optional<RefTxnId> stale_checkpoint = readCkptOrFail(*backend, store->layout(), life).checkpoint_snapshot_id;
+    ASSERT_EQ(stale_checkpoint, first_snapshot);
+
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    const RefTxnId second_snapshot = *store->newestPublishedSnapshotIdForTest(ns);
+    ASSERT_LT(first_snapshot, second_snapshot);
+
+    /// Planning against the STALE checkpoint: the just-published snapshot is not deletable, and neither
+    /// is the one the stale checkpoint itself names. A stale pointer can only under-clean.
+    EXPECT_FALSE(snapshotDeletableUnderCkpt(second_snapshot, stale_checkpoint));
+    EXPECT_FALSE(snapshotDeletableUnderCkpt(first_snapshot, stale_checkpoint));
+    /// Once the checkpoint is re-read, the older snapshot becomes reclaimable and the base does not.
+    const std::optional<RefTxnId> fresh_checkpoint = readCkptOrFail(*backend, store->layout(), life).checkpoint_snapshot_id;
+    EXPECT_TRUE(snapshotDeletableUnderCkpt(first_snapshot, fresh_checkpoint));
+    EXPECT_FALSE(snapshotDeletableUnderCkpt(second_snapshot, fresh_checkpoint));
+}
+
+/// One `_ckpt` write per publication and not one more: the checkpoint is written where the snapshot is
+/// published, and a publisher with nothing above its newest snapshot touches it at all.
+TEST(CASRefCheckpoint, TheCheckpointIsWrittenOncePerPublicationAndNotOnIdleAttempts)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const uint64_t epoch = store->writerEpoch();
+    const RootNamespace ns{"srv1/ckpt_republish"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{epoch, 1}));
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    const String key = store->layout().refCkptKey(life);
+    const uint64_t writes_after_birth = backend->casPutCount(key);
+    EXPECT_EQ(writes_after_birth, 2u)
+        << "birth publishes `life_epoch` before its log, then the durable log's committed frontier";
+
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    EXPECT_EQ(backend->casPutCount(key), writes_after_birth + 1) << "one publication, one checkpoint CAS";
+    const uint64_t writes_after_publish = backend->casPutCount(key);
+    const auto after_publish = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(after_publish.has_value());
+
+    /// Nothing was appended since, so there is nothing above the newest snapshot: the publisher declines
+    /// before it reaches the checkpoint at all, and repeating the attempt changes nothing.
+    EXPECT_FALSE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    EXPECT_FALSE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    EXPECT_EQ(backend->casPutCount(key), writes_after_publish);
+    EXPECT_EQ(readCkptOrFail(*backend, store->layout(), life), after_publish->ckpt);
+}
+
+TEST(CASRefCheckpoint, SnapshotPublisherRefusesEpochSealCandidateWithoutAnyWrite)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const RootNamespace ns{"srv1/no_snapshot_at_seal"};
+    uint64_t predecessor_epoch = 0;
+    {
+        auto predecessor = openPool(backend);
+        predecessor_epoch = predecessor->writerEpoch();
+        ASSERT_EQ(publishRef(predecessor, ns, "ref_1", 1), (RefTxnId{predecessor_epoch, 1}));
+    }
+
+    auto store = openPool(backend);
+    ASSERT_GT(store->writerEpoch(), predecessor_epoch);
+    ASSERT_EQ(store->listRefs(ns).size(), 1u) << "recovery must close the predecessor epoch before publishing";
+
+    const RefTxnId seal_id{predecessor_epoch, 2};
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    const String snapshot_key = store->layout().refSnapshotKey(life, seal_id);
+    const String ckpt_key = store->layout().refCkptKey(life);
+    ASSERT_EQ(store->lastEpochSealForTest(ns), std::make_optional(seal_id));
+    ASSERT_EQ(readCkptOrFail(*backend, store->layout(), life).committed_through, std::make_optional(seal_id));
+    const uint64_t snapshot_puts_before = backend->putCount(snapshot_key);
+    const uint64_t ckpt_cas_before = backend->casPutCount(ckpt_key);
+
+    /// Recovery installed the epoch seal as the runtime's greatest applied transaction. The publisher
+    /// must decline it without reaching either durable write.
+    EXPECT_FALSE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    EXPECT_EQ(backend->putCount(snapshot_key), snapshot_puts_before);
+    EXPECT_EQ(backend->casPutCount(ckpt_key), ckpt_cas_before);
+
+    /// Once an ordinary transaction advances the candidate beyond the seal, normal publication resumes.
+    ASSERT_EQ(publishRef(store, ns, "ref_2", 2), (RefTxnId{store->writerEpoch(), 1}));
+    EXPECT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+}
+
+/// Publication replays a `NeedsRecovery` lane before it captures a snapshot and advances `_ckpt`.
+TEST(CASRefCheckpoint, NeedsRecoveryReplaysBeforeCheckpointAdvance)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/ckpt_poisoned"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{store->writerEpoch(), 1}));
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    const String key = store->layout().refCkptKey(life);
+    const auto before = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(before.has_value());
+    ASSERT_FALSE(before->ckpt.checkpoint_snapshot_id.has_value());
+    const uint64_t writes_before = backend->casPutCount(key);
+
+    /// Enter `NeedsRecovery`: an install throws after its transaction is
+    /// durable, leaving this cached table missing a transaction the log contains.
+    auto planned = std::make_exception_ptr(DB::Exception(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+        "simulated allocation failure inside the post-durable install region"));
+    auto fired = std::make_shared<std::atomic<bool>>(false);
+    store->setInstallRegionProbeForTest([planned, fired]
+    {
+        if (fired->exchange(true))
+            return;
+        ALLOW_ALLOCATIONS_IN_SCOPE;
+        std::rethrow_exception(planned);
+    });
+    expectThrowsCode(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED, [&] { publishRef(store, ns, "ref_2", 2); });
+    store->setInstallRegionProbeForTest(nullptr);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    /// The publish entry point recovers first, so the snapshot covers the stranded transaction.
+    EXPECT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    EXPECT_TRUE(store->resolveRef(ns, "ref_2", /*allow_stale=*/false).has_value())
+        << "the stranded transaction is durable; the re-derivation must have applied it";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+    EXPECT_GT(backend->casPutCount(key), writes_before)
+        << "and the checkpoint advances -- truthfully, over a snapshot that is not missing anything";
+    EXPECT_TRUE(readCkptOrFail(*backend, store->layout(), life).checkpoint_snapshot_id.has_value());
+
+}
+
+/// A publish admitted under an incarnation that is replaced mid-attempt advances NOTHING, and does not
+/// adopt the snapshot either -- adopting would suppress every later publication for it while the
+/// checkpoint still pointed below it, leaving recovery on an older base with nothing to fix it.
+TEST(CASRefCheckpoint, APublishFencedOutMidAttemptDoesNotAdvanceTheCheckpoint)
+{
+    const RootNamespace ns{"srv1/ckpt_stale_gen"};
+
+    /// The watched key cannot be computed yet -- the real incarnation is minted only once the pool
+    /// exists and this namespace's first open resolves it (`setWatchedKey` below, once it has).
+    auto backend = std::make_shared<GetHookBackend>("");
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    PoolPtr store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{store->writerEpoch(), 1}));
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    const String ckpt_key = store->layout().refCkptKey(life);
+    backend->setWatchedKey(ckpt_key);
+
+    const auto before = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(before.has_value());
+    ASSERT_FALSE(before->ckpt.checkpoint_snapshot_id.has_value());
+    const uint64_t writes_before = backend->casPutCount(ckpt_key);
+
+    /// Arm only after the precondition read above. The next watched `_ckpt` read is therefore the one
+    /// inside this publish's read-then-CAS window, after the attempt captured its immutable runtime
+    /// generation. Arming before `readCkpt` would stale the runtime before the operation began and test
+    /// entry admission instead of the intended mid-attempt recheck.
+    bool hook_fired = false;
+    backend->on_get = [&]
+    {
+        if (hook_fired)
+            return;
+        hook_fired = true;
+        DB::Cas::tests::rearmMountFenceAfterAnomalyForTest(store);
+    };
+
+    EXPECT_FALSE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns))
+        << "a publish whose checkpoint could not be advanced must not report success";
+    EXPECT_TRUE(hook_fired) << "the checkpoint read-then-CAS seam was never exercised";
+    backend->on_get = nullptr;
+    EXPECT_EQ(backend->casPutCount(ckpt_key), writes_before) << "nothing may be sent after the fence moved";
+    EXPECT_FALSE(readCkptOrFail(*backend, store->layout(), life).checkpoint_snapshot_id.has_value());
+    EXPECT_FALSE(store->newestPublishedSnapshotIdForTest(ns).has_value())
+        << "the snapshot must not be adopted as the newest while its checkpoint is unpublished";
+}
+
+/// ===================================================================================
+/// Equivalence fences for the `prepareRefChunk` extraction (Stage B `{#extract-prepare-ref-chunk}`)
+/// ===================================================================================
+///
+/// An extraction is only safe to review if something pins what crosses its boundary. These three
+/// fences are deliberately NOT red-first: they pass on the PRE-extraction tree and must keep passing
+/// after it, which is the whole point -- the literals below were captured from a real append on the
+/// pre-extraction tree and pasted in, so re-deriving them afterwards cannot silently measure the
+/// change against itself.
+///
+/// They live in this TU rather than beside the pure preparation tests because all three need a real
+/// backend and the real append lane, which this suite already drives through `publishRef` (including
+/// the namespace birth, the one chunk shape whose first durable effect is the `_ckpt` and not the
+/// ref-log `PUT`).
+TEST(CASRefCheckpoint, CommitRefChunkDurableBytesUnchangedByExtraction)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"test/golden@cas@"};
+
+    const RefTxnId id = publishRef(store, ns, "gold_ref", 7);
+    ASSERT_EQ(id.writer_epoch, 1u);
+    ASSERT_EQ(id.ref_sequence, 1u);
+
+    /// The KEY carries the namespace incarnation, so its life segment is rendered rather than pasted
+    /// (Task 1c re-keys it); every other segment is literal. Stage B (Task 4-C): the incarnation is now
+    /// a REAL, randomly minted catalog value rather than the Stage-A sentinel, so it is learned back
+    /// from the catalog (`liveLifeOrFail`) rather than pasted as a literal -- the shape assertion below
+    /// is unaffected, since it names every OTHER segment literally and renders this one dynamically.
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    const String key = store->layout().refLogKey(life, id);
+    EXPECT_EQ(key, "p/cas/ns/stream/" + renderIncarnation(life.incarnation)
+                   + "/_log/0000000000000001-0000000000000001.zst")
+        << "the canonical ref-log key the append lane derives";
+
+    /// The BODY is checked as exact length plus a 128-bit SipHash of it -- not literally byte for byte,
+    /// but any change that survives both is a 128-bit collision at a fixed length, which is the trade for
+    /// keeping the assertion readable. It is a function of `{format generation, ns, id, ops,
+    /// chain_link}` only -- no incarnation reaches it. The generation-9 committed-frontier cut updates
+    /// the hash while leaving the payload shape and exact length unchanged.
+    const auto got = backend->get(key);
+    ASSERT_TRUE(got.has_value()) << "the birth chunk must be durable at its canonical key";
+    EXPECT_EQ(got->bytes.size(), 177u) << "the sealed ref-log body changed size";
+    SipHash body_hash;
+    body_hash.update(got->bytes.data(), got->bytes.size());
+    EXPECT_EQ(getHexUIntLowercase(body_hash.get128()), "784e7fd1ae0010f9cffdac1796730070")
+        << "the sealed ref-log body changed content -- preparation must seal the same bytes it sealed "
+           "before the extraction";
+}
+
+/// The directive's "preserve backend request counts", asserted rather than assumed: preparation is pure,
+/// so lifting it out must not add or remove a single request. One birth chunk = exactly one write-once
+/// `PUT` at the ref-log key, no read-back, plus the two ordered `_ckpt` CASes required by the protocol:
+/// creation publishes `life_epoch` before the log and the append lane publishes `committed_through`
+/// after the log is durable.
+///
+/// COUNTS per key. Request ORDER is not checked here and cannot be with these counters; the ordering
+/// that matters for a birth -- `_ckpt` before the ref-log `PUT` -- is argued at the call site and would
+/// need a sequence-recording backend to pin.
+TEST(CASRefCheckpoint, AppendRequestCountUnchangedByExtraction)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"test/req@cas@"};
+
+    const RefTxnId id = publishRef(store, ns, "req_ref", 1);
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    const String log_key = store->layout().refLogKey(life, id);
+    const String ckpt_key = store->layout().refCkptKey(life);
+
+    EXPECT_EQ(backend->putCount(log_key), 1u) << "exactly one write-once PUT per committed chunk";
+    /// ONE GET, not zero, since Stage B (Task 4-C): `resolveNamespaceLife`'s `completeCreation` call
+    /// publishes this life's `_ckpt.life_epoch` BEFORE the birth chunk is prepared, so this table's
+    /// OWN recovery walk (also inside this `appendRefOps`, ahead of the commit) grounds itself at the
+    /// genesis position `_ckpt` now names and confirms it absent by exact key -- which is `log_key`
+    /// itself, the position the birth chunk is about to occupy. That GET precedes the Committed PUT;
+    /// the PUT itself still owes no read-back.
+    EXPECT_EQ(backend->getCount(log_key), 1u) << "one grounding probe from recovery, before the birth PUT";
+    EXPECT_EQ(backend->casPutCount(ckpt_key), 2u)
+        << "the birth contributes `life_epoch` before its log and `committed_through` after the durable "
+           "log; these are two different ordering obligations, not a duplicate publication";
+}
+
+/// The post-durable install region is the reason preparation has to happen where it does: once "this
+/// object may be durable", recording it must not fail. The extraction moves work EARLIER, never into that
+/// region.
+///
+/// The guarded region is NOT the whole window: between the `Committed` outcome and the swap,
+/// `carve_hook_for_test(PostDurableInstall)`, the `state_mutex` acquisition and the `state_unchanged`
+/// evaluation all run OUTSIDE `DENY_ALLOCATIONS_IN_SCOPE`. This fence does not prove allocation-freedom
+/// for them or for anything else.
+///
+/// WHAT THIS TEST PROVES, and what it does NOT -- stated precisely, because a fence trusted for more
+/// than it checks is worse than no fence.
+///
+/// `DENY_ALLOCATIONS_IN_SCOPE` is `static_assert(true)` unless `!defined(NDEBUG)` (`MemoryTracker.h`),
+/// so it is inert in every build that leaves `NDEBUG` defined -- which includes this gate and CI's
+/// sanitizer lanes, since those configure `CMAKE_BUILD_TYPE=None` and `CMakeLists.txt` maps that to
+/// `RelWithDebInfo`. Only a `Debug` build, or a tidy lane (which adds `-UNDEBUG`), has the
+/// no-allocation half live. Nothing here proves the region does not allocate.
+///
+/// What is left is weaker than "the install is still guarded": `install_region_probe_for_test` fires
+/// as the FIRST statement inside the guarded scope, BEFORE `rt->state.swap(*candidate)`, and the SAME
+/// probe is shared by BOTH probe-instrumented post-durable install regions (`CasRefLedger.cpp`: the
+/// wedge-resolution adoption and `commitRefChunk`'s `Committed` install). So `probe_hits > 0` proves
+/// only that SOME probe-instrumented region was entered on this path -- which on this path can only be
+/// the commit install, since nothing here wedges. It goes red if the region stops being entered at all
+/// (a lost commit path, a skipped install arm); a refactor that lifted the swap out of the scope while
+/// leaving the guard shell and the probe behind would keep it GREEN.
+TEST(CASRefCheckpoint, PostDurableInstallRegionStillEnteredAfterExtraction)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"test/region@cas@"};
+
+    unsigned probe_hits = 0;
+    store->setInstallRegionProbeForTest([&probe_hits] { ++probe_hits; });
+    const RefTxnId id = publishRef(store, ns, "region_ref", 1);
+    store->setInstallRegionProbeForTest(nullptr);
+
+    EXPECT_GT(probe_hits, 0u)
+        << "no probe-instrumented post-durable install region was entered on a committing append -- "
+           "the `Committed` install arm was not reached at all";
+    const NamespaceLifeId life = liveLifeOrFail(*backend, store->layout(), ns);
+    EXPECT_TRUE(backend->get(store->layout().refLogKey(life, id)).has_value());
+}
diff --git a/src/Disks/tests/gtest_cas_ref_ckpt_join.cpp b/src/Disks/tests/gtest_cas_ref_ckpt_join.cpp
new file mode 100644
index 000000000000..99f7f0042f99
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_ckpt_join.cpp
@@ -0,0 +1,548 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCatalogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <cstdint>
+#include <limits>
+#include <optional>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+/// The `_ckpt` JOIN law and its `O(1)` SIZE invariant.
+///
+/// `mergeCkpt` already has a suite (`CasRefCkpt` in `gtest_cas_ref_ckpt.cpp`) covering it as one step of
+/// the publish algorithm. This suite's subject is narrower and different: the JOIN LAW itself, per
+/// field, stated so that a later change to any one field's rule fails here rather than being absorbed
+/// into a publish-path assertion; plus the size invariant, which no existing test constrains at all.
+///
+/// The size half is a REGRESSION FENCE, not a fix -- nothing about today's `_ckpt` is non-`O(1)`. It
+/// exists to fail the day someone adds a map, a collection, or any per-ref/per-file term to an object
+/// that has no repair path and gates destructive cleanup.
+///
+/// Constraint 15 names four dimensions (refs, files, transactions, writer epochs) and they do NOT
+/// behave the same way, so they get two different assertions rather than one claim covering both:
+///
+///   - REFS and FILES never enter the body in any form, so the encoded size is BYTE-EQUAL between a
+///     namespace holding one and a namespace holding ten thousand. That is `EncodedCkptSizeIs...`
+///     below, and it drives the REAL append lane on purpose: a hand-built pair of `RefCkpt` structs
+///     would leave a newly-added collection field EMPTY in both and the equality would still hold,
+///     so the fence would not fire on the very change it exists to catch. Only a real producer
+///     populates a real field.
+///   - TRANSACTIONS and WRITER EPOCHS enter as the DECIMAL WIDTH of the two id pairs. That is not
+///     equality: `{cse=1,css=1}` and `{cse=1,css=10000}` differ by four bytes. It is `O(1)` because
+///     the fields are `uint64_t` and so the width is ceilinged at twenty digits, which is a bound a
+///     test asserts on a constructed worst case -- `EncodedCkptSizeHasAConstantCeiling...` below.
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int NETWORK_ERROR;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::CountingBackend;
+using DB::Cas::tests::namespaceBirthOp;
+using DB::Cas::tests::publishCommittedOps;
+
+/// Constraint 15's COMPILE-TIME half: `_ckpt` is a fixed-size product of scalar monotone facts. Any
+/// field that owns heap storage -- a map, a vector, a `String` -- makes `RefCkpt` non-trivially-copyable
+/// and fails the build here, which is the earliest and cheapest place the constraint can be enforced.
+/// The two runtime size tests below are the rest of the fence: this one cannot see a fixed-capacity
+/// array, and they cannot see a field that is never populated by the producers they drive.
+static_assert(std::is_trivially_copyable_v<RefCkpt>,
+              "Constraint 15: _ckpt is a fixed-size product of scalar monotone facts, so its encoded size is "
+              "O(1) in refs, files, transactions and writer epochs. A field with heap storage (a map, a "
+              "vector, a String) breaks that and belongs in a separate immutable object or ledger.");
+
+namespace
+{
+
+constexpr uint64_t U64_MAX = std::numeric_limits<uint64_t>::max();
+
+/// Constraint 15's bound, as a number: the encoded size of the WIDEST `_ckpt` this build can produce
+/// (all three fields present, every integer component at `UINT64_MAX`). Pinned as a literal so that
+/// adding a field, or widening one, fails a test rather than quietly moving the bound.
+constexpr size_t CKPT_WORST_CASE_ENCODED_BYTES = 234;
+
+/// The high-cardinality side of the size fence, in ONE transaction. Bounded above by the append lane's
+/// 5000-operation cap on a normal-class item (`publishCommittedOps` emits two ops per ref), and kept at
+/// one transaction on purpose: spreading a larger namespace over several of them costs tens of seconds
+/// in a debug build, and a size fence that cannot finish inside the harness budget fences nothing. Any
+/// per-ref term in `_ckpt` is as visible at this count as at any larger one.
+constexpr size_t MANY_REFS = 2000;
+
+/// Fixed-width, so the refs themselves cannot be what differs between the two namespaces: the claim
+/// under test is that ref cardinality does not reach `_ckpt`, and a name that grew with `i` would
+/// confound a size comparison if it ever did.
+String refName(size_t i)
+{
+    return fmt::format("r{:08}", i);
+}
+
+/// A fence that never refuses, and a deadline far enough out that only the test's own contention
+/// decides the outcome -- each `_ckpt`/catalog test file defines its own copy, matching the precedent
+/// `gtest_cas_ns_creation_lifecycle.cpp` states explicitly.
+const std::function<void(uint64_t)> ALWAYS_ADMITTED = [](uint64_t) {};
+
+CkptDeadline generousDeadline()
+{
+    return CkptDeadline{[] { return uint64_t{1000}; }, 60000};
+}
+
+/// Admits the FIRST call (spent by `completeCreation`'s step-2 `publishCkpt`) and refuses every call
+/// after (step 3's own `mutate`): "fenced out between the `_ckpt` create and the `Creating -> Live`
+/// CAS", deterministically and without a second thread. That is the durable shape a stalled creator
+/// leaves behind, and the starting state the resumption test needs.
+std::function<void(uint64_t)> admittedOnceThenFenced()
+{
+    auto calls = std::make_shared<int>(0);
+    return [calls](uint64_t admitted)
+    {
+        if (++*calls > 1)
+            throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR, "fence generation moved since admission ({})", admitted);
+    };
+}
+
+CreatorFence creatorFence(const String & srid, uint64_t writer_epoch, uint64_t fence_generation = 1)
+{
+    return CreatorFence{.server_root_id = srid, .writer_epoch = writer_epoch, .fence_generation = fence_generation};
+}
+
+/// A `is_creator_fence_terminal` stub answering one fixed verdict: terminality itself is not this
+/// suite's subject (its tests live next to the real predicate in `gtest_cas_mount.cpp`).
+std::function<bool(const CreatorFence &)> fixedTerminality(bool terminal)
+{
+    return [terminal](const CreatorFence &) { return terminal; };
+}
+
+const CatalogEntry * findEntryForTest(const RefCatalog & catalog, const RootNamespace & ns)
+{
+    for (const CatalogEntry & e : catalog.entries)
+        if (e.ns.string() == ns.string())
+            return &e;
+    return nullptr;
+}
+
+/// `life`'s durable `life_epoch`, failing the current test rather than dereferencing a disengaged
+/// optional -- a bare `->` on one aborts the whole binary and takes every later suite's result with it.
+uint64_t lifeEpochOrFail(Backend & backend, const Layout & layout, const NamespaceLifeId & life)
+{
+    const std::optional<CkptSample> sample = readCkpt(backend, layout, life);
+    if (!sample || !sample->ckpt.life_epoch)
+    {
+        ADD_FAILURE() << "expected a _ckpt carrying a life_epoch for namespace '" << life.ns.string() << "'";
+        return 0;
+    }
+    return *sample->ckpt.life_epoch;
+}
+
+/// `boot_ms_fn` defaults to the real clock. A caller whose test body does enough CPU-bound work
+/// against ONE open pool to risk outrunning `mount_lease_ttl_ms` on a slow sanitizer build should
+/// pass a frozen one instead of widening the TTL: the mount fence and the ref-log request controller
+/// both read time through this same seam (see `CasRefLedger`'s `controller_boot_ms_fn`), so freezing
+/// it removes the wall-clock race rather than merely giving it more room.
+PoolPtr openPool(const BackendPtr & backend, std::function<uint64_t()> boot_ms_fn = {})
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    PoolConfig config{.pool_prefix = "p", .server_root_id = "test"};
+    config.boot_ms_fn = std::move(boot_ms_fn);
+    return Pool::open(backend, std::move(config));
+}
+
+/// The incarnation the production birth wiring minted for `ns`, learned back from the catalog the way a
+/// real reader does. Fails the current test rather than dereferencing a disengaged optional, so one
+/// regression cannot abort the binary and take every later suite's result with it.
+NamespaceLifeId liveLifeOrFail(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const CasRefCatalog::Snapshot snap = CasRefCatalog::read(backend, layout);
+    for (const CatalogEntry & entry : snap.catalog.entries)
+        if (entry.ns.string() == ns.string())
+            return NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation);
+    ADD_FAILURE() << "expected a catalog entry for namespace '" << ns.string() << "', found none";
+    return DB::Cas::tests::fixture::fixtureLife(ns);
+}
+
+/// Births `ns` and publishes `ref_count` committed refs through the REAL append lane, in ONE
+/// transaction, and returns that namespace's durable `_ckpt` as encoded bytes.
+///
+/// One transaction also holds every OTHER dimension fixed while `ref_count` varies: two namespaces
+/// built this way end at the same transaction id, so a difference in their `_ckpt` bodies can only be
+/// the refs. `ref_count` must therefore stay within the append lane's per-item operation cap.
+String encodedCkptOfNamespaceWithRefs(const PoolPtr & store, Backend & backend, const Layout & layout,
+                                      const RootNamespace & ns, size_t ref_count)
+{
+    store->appendRefOps(ns, MutationScope::wholeShard(),
+        [ref_count](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (size_t i = 0; i < ref_count; ++i)
+                for (const RefOp & op : publishCommittedOps(refName(i), ManifestRef{1, i + 1, 1}))
+                    ops.push_back(op);
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Publish);
+
+    const NamespaceLifeId life = liveLifeOrFail(backend, layout, ns);
+    const std::optional<CkptSample> sample = readCkpt(backend, layout, life);
+    if (!sample)
+    {
+        ADD_FAILURE() << "expected a _ckpt for namespace '" << ns.string() << "' after its birth transaction";
+        return {};
+    }
+    return encodeRefCkpt(sample->ckpt);
+}
+
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The join law, per field
+/// ---------------------------------------------------------------------------------------------
+
+/// An absence is "this writer knew nothing", never "this writer says none". Exactly one writer ever
+/// knows a namespace's genesis epoch, so every other contribution is `nullopt` and must leave what is
+/// on record alone -- in BOTH argument orders, because the two `_ckpt` writers have no ordering
+/// between them and the merge is what makes that safe.
+TEST(CASRefCheckpointJoin, JoinUnknownLifeEpochWithPresentYieldsPresent)
+{
+    const RefCkpt unknown{.life_epoch = std::nullopt, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    const RefCkpt present{.life_epoch = 7, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+
+    EXPECT_EQ(mergeCkpt(unknown, present).life_epoch, std::optional<uint64_t>{7});
+    EXPECT_EQ(mergeCkpt(present, unknown).life_epoch, std::optional<uint64_t>{7})
+        << "the merge is commutative -- a writer that knows nothing must not be able to erase the "
+           "genesis epoch, whichever side it is on";
+
+    /// The other half of "absent loses": two absences stay absent. `life_epoch` has no floor to fall
+    /// back to, and a fabricated one is permanent -- the semantic-max merge can never lower it again.
+    EXPECT_EQ(mergeCkpt(unknown, unknown).life_epoch, std::nullopt);
+}
+
+/// The ordinary steady state: both writers agree. Asserted for its own sake because it is what
+/// `publishCkpt`'s may-not-decrease rule must keep admitting -- an equal republish is not a decrease --
+/// and it is also what `publishCkpt` turns into "no write at all".
+TEST(CASRefCheckpointJoin, JoinEqualLifeEpochsYieldsSame)
+{
+    const RefCkpt a{.life_epoch = 9, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    const RefCkpt b{.life_epoch = 9, .checkpoint_snapshot_id = RefTxnId{9, 4}, .last_epoch_seal = std::nullopt};
+
+    EXPECT_EQ(mergeCkpt(a, b).life_epoch, std::optional<uint64_t>{9});
+    EXPECT_EQ(mergeCkpt(b, a).life_epoch, std::optional<uint64_t>{9});
+    const std::optional<RefTxnId> b_checkpoint = RefTxnId{9, 4};
+    EXPECT_EQ(mergeCkpt(a, b).checkpoint_snapshot_id, b_checkpoint)
+        << "an equal life_epoch must not disturb the other fields' own join";
+}
+
+TEST(CASRefCheckpointJoin, CrossEpochFrontierRequiresAnImmediatelyAdjacentSeal)
+{
+    const RefCkpt older{.life_epoch = std::nullopt, .committed_through = RefTxnId{7, 9},
+                         .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    const RefCkpt transitioned{.life_epoch = std::nullopt, .committed_through = RefTxnId{8, 1},
+                                .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = RefTxnId{8, 1}};
+    EXPECT_EQ(mergeCkpt(older, transitioned).committed_through, transitioned.committed_through);
+    EXPECT_EQ(mergeCkpt(transitioned, older).committed_through, transitioned.committed_through);
+
+    /// Every committed epoch is materialized. A later frontier may advance only to the immediately
+    /// following numeric writer epoch, otherwise a missing epoch would be mistaken for a proved
+    /// boundary. The log grammar rejects this same skip at the record boundary; `_ckpt` must not
+    /// reintroduce it through its semantic merge.
+    const RefCkpt skipped_epoch{.life_epoch = std::nullopt, .committed_through = RefTxnId{10, 1},
+                                    .checkpoint_snapshot_id = std::nullopt,
+                                    .last_epoch_seal = RefTxnId{7, 9}};
+    EXPECT_THROW(mergeCkpt(older, skipped_epoch), DB::Exception);
+
+    const RefCkpt advanced{.life_epoch = std::nullopt, .committed_through = RefTxnId{8, 5},
+                            .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = RefTxnId{7, 9}};
+    EXPECT_EQ(mergeCkpt(advanced, older).committed_through, advanced.committed_through);
+
+    const RefCkpt unsealed{.life_epoch = std::nullopt, .committed_through = RefTxnId{8, 1},
+                           .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    EXPECT_THROW(mergeCkpt(older, unsealed), DB::Exception);
+    const RefCkpt stale_prior_seal{.life_epoch = std::nullopt, .committed_through = RefTxnId{10, 1},
+                                   .checkpoint_snapshot_id = std::nullopt,
+                                   .last_epoch_seal = RefTxnId{7, 8}};
+    EXPECT_THROW(mergeCkpt(older, stale_prior_seal), DB::Exception)
+        << "a seal below the lower durable frontier does not connect the two histories";
+    const RefCkpt seal_above_frontier{.life_epoch = std::nullopt, .committed_through = RefTxnId{8, 5},
+                                      .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = RefTxnId{8, 6}};
+    EXPECT_THROW(mergeCkpt(older, seal_above_frontier), DB::Exception);
+}
+
+/// THE FIRST OF THE TWO SEQUENCES THAT RAISE `life_epoch` HONESTLY, end to end through the production
+/// primitives rather than at the merge: a creator publishes `_ckpt` at E1 (`completeCreation` step 2)
+/// and dies before its `Creating -> Live` CAS (step 3), and a later actor reconciles the stalled entry
+/// and resumes over the SAME incarnation, contributing E2. Two different present values in one
+/// incarnation, and NOT a conflict -- the stored value must simply become E2, which is also what
+/// `CASNsCreationLifecycle.ReconcileSucceedsTokenExactlyAfterTheOriginalCreatorFenceIsTerminalThenResumesToLive`
+/// already pins from the catalog side ("the RESUMING actor's writer_epoch is the genesis epoch that
+/// actually landed"). Had the directive's literal rule landed, this sequence would raise
+/// `CORRUPTED_DATA` and, since `_ckpt` has no repair path, wedge the namespace forever.
+///
+/// Both fences share ONE `server_root_id` on purpose: every live namespace is rooted at its own pool
+/// member's `server_root_id`, so a creator and its reconciler are always actors of the same server root
+/// and draw from the same durable-monotone epoch counter. That is the whole basis for "contributions
+/// only ever rise", so the fixture must not quietly model two roots.
+TEST(CASRefCheckpointJoin, ResumedCreationRaisesLifeEpochWithoutRefusal)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    DB::Cas::tests::seedPoolMetaForRestart(backend);
+    const RootNamespace ns{"a"};
+
+    ASSERT_EQ(CasRefCatalog::createNamespace(backend, layout, 1, ns, creatorFence("srv1", 5),
+                                             /*admitted_generation=*/1, admittedOnceThenFenced(), generousDeadline()),
+              CasRefCatalog::NamespaceCreationOutcome::FencedOut);
+
+    /// Bound to a name, never chained through a temporary: a `const CatalogEntry *` taken from an
+    /// unbound `Snapshot` dangles the instant the full expression ends.
+    const CasRefCatalog::Snapshot stalled = CasRefCatalog::read(backend, layout);
+    const CatalogEntry * entry = findEntryForTest(stalled.catalog, ns);
+    ASSERT_NE(entry, nullptr);
+    ASSERT_EQ(entry->state, NsState::Creating);
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry->ns, entry->incarnation);
+    EXPECT_EQ(lifeEpochOrFail(backend, layout, life), 5u) << "step 2 landed before the creator stalled";
+
+    const CreatorFence resumer = creatorFence("srv1", 9);
+    ASSERT_EQ(CasRefCatalog::reconcileStaleCreator(backend, layout, *entry, resumer, fixedTerminality(true),
+                                                   /*admitted_generation=*/1, ALWAYS_ADMITTED),
+              CasRefCatalog::ReconcileCreatorOutcome::Reconciled);
+
+    CatalogEntry resumed = *entry;
+    resumed.creator = resumer;
+    EXPECT_EQ(CasRefCatalog::completeCreation(backend, layout, resumed, /*admitted_generation=*/1,
+                                              ALWAYS_ADMITTED, generousDeadline()),
+              CasRefCatalog::NamespaceCreationOutcome::Live)
+        << "the resumption must not be refused by the join";
+    EXPECT_EQ(lifeEpochOrFail(backend, layout, life), 9u)
+        << "the genesis epoch that actually landed is the resuming actor's, and the join must let it rise";
+}
+
+/// THE SECOND SEQUENCE, at the seam where the two `life_epoch`-knowing writers actually meet -- both of
+/// them reach this object only through `publishCkpt`, so driving that twice over one key IS the
+/// production interleaving, not a stand-in for it. `completeCreation` contributes the catalog creator's
+/// epoch; the mount's writer epoch then advances (a restart, a remount); the first precommit's birth
+/// chunk contributes the `NamespaceBirth` record's epoch. CREATE TABLE, restart, INSERT.
+TEST(CASRefCheckpointJoin, RestartBetweenCreationAndFirstWriteRaisesLifeEpochWithoutRefusal)
+{
+    InMemoryBackend backend;
+    Layout layout("p");
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"a"}, UInt128(42));
+
+    const RefCkpt from_creation{.life_epoch = 4, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(publishCkpt(backend, layout, life, from_creation, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+
+    const RefCkpt from_birth_chunk{.life_epoch = 7, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    EXPECT_EQ(publishCkpt(backend, layout, life, from_birth_chunk, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published)
+        << "the birth chunk's later epoch must be publishable, not refused as a conflict";
+    EXPECT_EQ(lifeEpochOrFail(backend, layout, life), 7u);
+}
+
+/// THE REFUSAL, and the state it constructs IS UNREACHABLE ON ANY HONEST PATH -- that is the point of
+/// the test, not a caveat on it. `writer_epoch` is durable-monotone per server root
+/// (`allocateWriterEpoch` CAS-bumps `<prefix>/gc/server-roots/<srid>/epoch`) and a namespace belongs to
+/// exactly one server root, so no live writer can contribute an epoch below one already durable. The
+/// only way to reach this is for the fence discipline itself to have failed and a SUPERSEDED writer's
+/// contribution to have landed anyway.
+///
+/// So this test does not model an operating condition; it asserts what happens if the guarantee above
+/// is ever violated -- `publishCkpt` refuses and names both values, rather than absorbing the violation
+/// into a maximum and leaving no trace. The state is built by publishing the two contributions in the
+/// order the fence discipline is supposed to prevent, which needs no seam that manufactures impossible
+/// states: `publishCkpt` is a public entry point and the order of two calls is the test's to choose.
+///
+/// It is driven through `publishCkpt` for a second reason, not just convenience: that IS where the rule
+/// lives and the only place it CAN live. `mergeCkpt` is commutative -- the stated reason the two writers
+/// need no ordering between them -- so it cannot tell a decrease from an increase, having no idea which
+/// of its arguments is durable. There is deliberately no merge-level counterpart to this test.
+TEST(CASRefCheckpointJoin, JoinDecreasingLifeEpochIsCorruptionAndPublishesNothing)
+{
+    CountingBackend backend;
+    Layout layout("p");
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"a"}, UInt128(42));
+    const String key = layout.refCkptKey(life);
+
+    const RefCkpt durable{.life_epoch = 9, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(publishCkpt(backend, layout, life, durable, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    const uint64_t cas_puts_before = backend.casPutCount(key);
+
+    const RefCkpt superseded{.life_epoch = 3, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    String message;
+    try
+    {
+        publishCkpt(backend, layout, life, superseded, 1, ALWAYS_ADMITTED, generousDeadline());
+        ADD_FAILURE() << "a contribution below the durable life_epoch must not be published";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+        message = e.message();
+    }
+
+    /// BOTH values, not just the offending one: an operator reading this has to be able to tell which
+    /// writer is the superseded one without going to the object. Matched as RENDERED substrings rather
+    /// than as bare digits -- a lone "9" would also be satisfied by a key or a byte count that happened
+    /// to contain it, so a bare-digit match would keep passing after the message stopped saying this.
+    EXPECT_NE(message.find("9 is durable"), String::npos) << "the durable value must be named: " << message;
+    EXPECT_NE(message.find("contributed 3"), String::npos) << "the contributed value must be named: " << message;
+    EXPECT_NE(message.find(key), String::npos) << "the key must be named: " << message;
+    /// And that the object cannot be repaired in place, which is the part an operator cannot derive
+    /// from the two numbers.
+    EXPECT_NE(message.find("NO in-place repair"), String::npos)
+        << "the message must say the object has no in-place repair: " << message;
+
+    /// And nothing was written. The refusal is decided before the body is built, so the durable object
+    /// is untouched and no write was even attempted.
+    EXPECT_EQ(backend.casPutCount(key), cas_puts_before) << "the publisher must not CAS on a refused publish";
+    EXPECT_EQ(lifeEpochOrFail(backend, layout, life), 9u) << "the durable value is unchanged";
+}
+
+/// The other half of the refusal, and the reason it consults the fence before classifying: the SAME
+/// decrease from a writer the fence is about to refuse is not corruption. That writer landed nothing
+/// anywhere, so what it gets is the transient control signal every other refusal in `publishCkpt`
+/// returns rather than throws. Reporting corruption for it would turn "your incarnation moved, retry"
+/// into a permanent verdict on the namespace, which is the opposite of what the detector means: the
+/// violation is a STILL-ADMITTED writer contributing a superseded epoch.
+TEST(CASRefCheckpointJoin, ADecreasingLifeEpochFromAFencedOutWriterIsReportedFencedOutNotCorruption)
+{
+    CountingBackend backend;
+    Layout layout("p");
+    const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(RootNamespace{"a"}, UInt128(42));
+    const String key = layout.refCkptKey(life);
+
+    const RefCkpt durable{.life_epoch = 9, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    ASSERT_EQ(publishCkpt(backend, layout, life, durable, 1, ALWAYS_ADMITTED, generousDeadline()),
+              CkptPublishOutcome::Published);
+    const uint64_t cas_puts_before = backend.casPutCount(key);
+
+    const std::function<void(uint64_t)> always_fenced = [](uint64_t admitted)
+    {
+        throw DB::Exception(DB::ErrorCodes::NETWORK_ERROR, "fence generation moved since admission ({})", admitted);
+    };
+    const RefCkpt superseded{.life_epoch = 3, .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt};
+    EXPECT_EQ(publishCkpt(backend, layout, life, superseded, 1, always_fenced, generousDeadline()),
+              CkptPublishOutcome::FencedOut);
+
+    EXPECT_EQ(backend.casPutCount(key), cas_puts_before);
+    EXPECT_EQ(lifeEpochOrFail(backend, layout, life), 9u);
+}
+
+/// `checkpoint_snapshot_id` and `last_epoch_seal` continue to merge by SEMANTIC MAXIMUM. Unlike
+/// `life_epoch` these two genuinely advance over a namespace's life, and the max is what stops a writer
+/// that sampled an older body from regressing the other writer's progress (TLC counterexample
+/// `_sab_sealclobbersbase`, which costs an acked transaction). Both directions and present-beats-absent,
+/// since the two writers have no ordering between them.
+TEST(CASRefCheckpointJoin, CheckpointAndSealStillMergeBySemanticMaximum)
+{
+    const RefCkpt lower{.life_epoch = std::nullopt, .checkpoint_snapshot_id = RefTxnId{3, 5}, .last_epoch_seal = RefTxnId{3, 4}};
+    const RefCkpt higher{.life_epoch = std::nullopt, .checkpoint_snapshot_id = RefTxnId{4, 1}, .last_epoch_seal = RefTxnId{4, 2}};
+
+    const std::optional<RefTxnId> higher_checkpoint = higher.checkpoint_snapshot_id;
+    const std::optional<RefTxnId> higher_seal = higher.last_epoch_seal;
+    const std::optional<RefTxnId> lower_checkpoint = lower.checkpoint_snapshot_id;
+    const std::optional<RefTxnId> lower_seal = lower.last_epoch_seal;
+
+    /// Ordered by writer_epoch FIRST: `{4,1}` beats `{3,5}` even though its sequence is smaller, which
+    /// is the intended timeline across an epoch restart that resets the sequence.
+    EXPECT_EQ(mergeCkpt(lower, higher).checkpoint_snapshot_id, higher_checkpoint);
+    EXPECT_EQ(mergeCkpt(higher, lower).checkpoint_snapshot_id, higher_checkpoint);
+    EXPECT_EQ(mergeCkpt(lower, higher).last_epoch_seal, higher_seal);
+    EXPECT_EQ(mergeCkpt(higher, lower).last_epoch_seal, higher_seal);
+
+    /// Present beats absent, both directions and both fields.
+    const RefCkpt nothing;
+    EXPECT_EQ(mergeCkpt(nothing, lower).checkpoint_snapshot_id, lower_checkpoint);
+    EXPECT_EQ(mergeCkpt(lower, nothing).checkpoint_snapshot_id, lower_checkpoint);
+    EXPECT_EQ(mergeCkpt(nothing, lower).last_epoch_seal, lower_seal);
+    EXPECT_EQ(mergeCkpt(lower, nothing).last_epoch_seal, lower_seal);
+    EXPECT_EQ(mergeCkpt(nothing, nothing).checkpoint_snapshot_id, std::nullopt);
+    EXPECT_EQ(mergeCkpt(nothing, nothing).last_epoch_seal, std::nullopt);
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Constraint 15: the `O(1)` size invariant
+/// ---------------------------------------------------------------------------------------------
+
+/// REFS and FILES: byte-equal, because they never enter the body. Driven through the REAL append lane
+/// (see `encodedCkptOfNamespaceWithRefs` on why a hand-built struct pair would not fence anything).
+TEST(CASRefCheckpointJoin, EncodedCkptSizeIsIndependentOfCardinality)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// `MANY_REFS` committed through ONE `appendRefOps` call is CPU-bound encoding, not I/O -- on a
+    /// slow sanitizer build (msan in particular) it can outrun the real-clock `mount_lease_ttl_ms`
+    /// this pool was opened under and trip the mount fence mid-publish. Freeze the pool's clock
+    /// instead of racing it (see `openPool`'s doc comment).
+    auto store = openPool(backend, [] { return uint64_t{0}; });
+    Layout layout("p");
+
+    const String one = encodedCkptOfNamespaceWithRefs(store, *backend, layout, RootNamespace{"srv1/one"}, 1);
+    const String many = encodedCkptOfNamespaceWithRefs(store, *backend, layout, RootNamespace{"srv1/many"}, MANY_REFS);
+
+    ASSERT_FALSE(one.empty());
+    ASSERT_FALSE(many.empty());
+    EXPECT_EQ(one, many)
+        << "not merely equal in SIZE: refs and files reach `_ckpt` in no form at all, so the two bodies "
+           "are byte-identical.";
+    /// The same claim stated so that it does not depend on the chosen cardinality at all: no ref
+    /// PUBLISHED into the namespace appears anywhere in its `_ckpt`. A count-based comparison can only
+    /// catch a term that grows; this catches one that is merely there.
+    EXPECT_EQ(many.find(refName(0)), String::npos)
+        << "a published ref's NAME appears in `_ckpt`: " << many;
+    EXPECT_EQ(many.find(refName(MANY_REFS - 1)), String::npos)
+        << "a published ref's NAME appears in `_ckpt`: " << many;
+    EXPECT_EQ(one.size(), many.size())
+        << "Constraint 15: `_ckpt`'s encoded size must not grow with the number of refs or files in the "
+           "namespace. A collection or per-ref term was added to an object that has NO repair path and "
+           "gates destructive cleanup; it belongs in a separate immutable object or ledger instead.\n"
+           "  1 ref:            " << one
+        << "  " << MANY_REFS << " refs: " << many;
+}
+
+/// TRANSACTIONS and WRITER EPOCHS: not equality -- they enter as the decimal width of the id pairs --
+/// but ceilinged, because the fields are `uint64_t`. The worst case is constructible exactly (every
+/// field present at `UINT64_MAX`), so the bound is asserted on it rather than believed about it.
+TEST(CASRefCheckpointJoin, EncodedCkptSizeHasAConstantCeilingAcrossTransactionsAndEpochs)
+{
+    /// The true worst case over every namespace history: all three fields present, every component at
+    /// the widest value its type can hold. No real `_ckpt` can encode larger, because there is no field
+    /// that is not one of these five integers.
+    const RefCkpt worst{.life_epoch = U64_MAX,
+                        .committed_through = RefTxnId{U64_MAX, U64_MAX},
+                        .checkpoint_snapshot_id = RefTxnId{U64_MAX, U64_MAX},
+                        .last_epoch_seal = RefTxnId{U64_MAX, U64_MAX}};
+    const size_t worst_bytes = encodeRefCkpt(worst).size();
+
+    /// Pinned as a literal, not merely compared against itself: this is the number Constraint 15's
+    /// `O(1)` claim reduces to, and a change to it means a field was added, removed or rewidened.
+    EXPECT_EQ(worst_bytes, CKPT_WORST_CASE_ENCODED_BYTES)
+        << "the widest `_ckpt` this build can encode changed size -- a field was added, removed, or "
+           "given a wider type. Constraint 15's O(1) bound is exactly this constant.";
+
+    /// The growth term is the decimal width, and it is bounded by that ceiling rather than proportional
+    /// to the number of transactions: four orders of magnitude of `ref_sequence` cost four bytes.
+    const RefCkpt at_sequence_1{.life_epoch = 1, .committed_through = RefTxnId{1, 1}, .checkpoint_snapshot_id = RefTxnId{1, 1}, .last_epoch_seal = RefTxnId{1, 1}};
+    const RefCkpt at_sequence_10k{.life_epoch = 1, .committed_through = RefTxnId{1, 10000}, .checkpoint_snapshot_id = RefTxnId{1, 10000}, .last_epoch_seal = RefTxnId{1, 10000}};
+    EXPECT_EQ(encodeRefCkpt(at_sequence_10k).size(), encodeRefCkpt(at_sequence_1).size() + 12);
+    EXPECT_LE(encodeRefCkpt(at_sequence_10k).size(), worst_bytes);
+    EXPECT_LE(encodeRefCkpt(at_sequence_1).size(), worst_bytes);
+
+    /// And the ceiling is far below the format registry's own object cap, so the cap is what it is
+    /// documented to be -- a corruption brake this object cannot approach -- and never the thing that
+    /// makes the size bounded.
+    EXPECT_LT(worst_bytes, traitsFor(FormatId::RefCkpt).object_cap);
+}
diff --git a/src/Disks/tests/gtest_cas_ref_contiguous_alloc.cpp b/src/Disks/tests/gtest_cas_ref_contiguous_alloc.cpp
new file mode 100644
index 000000000000..92f65c9f4b55
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_contiguous_alloc.cpp
@@ -0,0 +1,640 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasPoolMetaFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/MemoryTracker.h>
+#include <Common/ProfileEvents.h>
+
+#include <atomic>
+#include <chrono>
+#include <cstdint>
+#include <exception>
+#include <functional>
+#include <memory>
+#include <string>
+#include <thread>
+#include <vector>
+
+namespace ProfileEvents
+{
+extern const Event CASMountReleaseSkippedForeignOccupant;
+extern const Event CASMountExclusivityViolation;
+}
+
+/// Stage A task 3 (INV-1): ref-log transaction ids are PER-NAMESPACE and CONTIGUOUS.
+///
+/// The id an append persists is not drawn from a counter at all -- it is DERIVED from the table's own
+/// durable state: `{live_epoch, greatest_applied.ref_sequence + 1}` within one epoch, `{live_epoch, 1}`
+/// at an epoch change. Two consequences this suite pins, both of which the pool-wide counter this
+/// replaced made impossible:
+///
+///   1. namespaces are independent -- a busy table cannot push another table's ids up, so `(namespace,
+///      epoch)` ids are dense `1..T` and a reader can tell "this stream is complete" from the ids alone;
+///   2. an attempt that provably sent nothing consumes nothing -- the next caller re-derives the SAME
+///      id, so a refusal leaves no hole behind it.
+///
+/// The read side enforces exactly what the allocator produces: `RefTableState::applyTxnInPlace` rejects
+/// a non-successor id as `CORRUPTED_DATA`, so a hole can never become durable even if some future
+/// writer path forgot the rule.
+///
+/// The suite name is prefixed `Cas` so it is covered by the `Cas*` unit-test gate filter.
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int MEMORY_LIMIT_EXCEEDED;
+extern const int UNKNOWN_FORMAT_VERSION;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::namespaceBirthOp;
+using DB::Cas::tests::publishCommittedOps;
+
+namespace
+{
+
+PoolPtr openPool(const BackendPtr & backend)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// The fence-controlled pool of `gtest_cas_ref_install_safety.cpp`, for the pre-attempt refusal: the
+/// boot clock is frozen so `setMountDeadline` alone decides both fence predicates, renewal is parked an
+/// hour out so nothing re-arms the deadline underneath the test, and the single-attempt budget makes
+/// `attempt_timeout_ms + lease_safety_margin_ms` (200 ms) the window between "the flush is admitted"
+/// and "an attempt may start".
+PoolPtr openPoolFenceControlled(const BackendPtr & backend)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    PoolConfig cfg{.pool_prefix = "p", .server_root_id = "test"};
+    cfg.boot_ms_fn = [] { return uint64_t{0}; };
+    cfg.mount_renew_period = std::chrono::milliseconds{3600000};
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;   /// strictly above attempt_timeout_ms: equality is a wall-clock race (validateCasRequestBudget)
+    budget.lease_safety_margin_ms = 100;
+    cfg.cas_request_budget = budget;
+    return Pool::open(backend, cfg);
+}
+
+constexpr uint64_t FENCE_DEADLINE_HEALTHY_MS = 30000;
+constexpr uint64_t FENCE_DEADLINE_REFUSES_ATTEMPT_MS = 100;
+
+/// A bare `Pool::open` with no `_pool_meta` seeded: the path an operator's pool RECREATION takes, and
+/// the only one that runs the bootstrap residual + quiesce gates (`seedPoolMetaForRestart` mints the
+/// metadata directly and would bypass them).
+PoolPtr openPoolWithoutSeeding(const BackendPtr & backend, const String & srid)
+{
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = srid});
+}
+
+/// Deletes every object whose key contains `substr` ("" = the whole prefix), as an operator clearing
+/// the prefix would. Returns how many were removed.
+size_t eraseKeysContaining(Backend & backend, const String & substr)
+{
+    size_t removed = 0;
+    String cursor;
+    std::vector<String> keys;
+    while (true)
+    {
+        const ListPage page = backend.list("", cursor, 1000);
+        for (const ListedKey & listed : page.keys)
+            if (substr.empty() || listed.key.find(substr) != String::npos)
+                keys.push_back(listed.key);
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    for (const String & key : keys)
+    {
+        const HeadResult h = backend.head(key);
+        if (h.exists && backend.deleteExact(key, h.token).kind == DeleteOutcome::Kind::Deleted)
+            ++removed;
+    }
+    return removed;
+}
+
+String messageOfThrow(const std::function<void()> & fn)
+{
+    try
+    {
+        fn();
+    }
+    catch (const DB::Exception & e)
+    {
+        return e.message();
+    }
+    return {};
+}
+
+/// One ordinary publish transaction, driven straight through the append lane so the committed id is
+/// observable: `namespace_birth` while the table is not yet `Live`, then the precommit+promote pair for
+/// `ref`. Returns the id the append persisted under.
+RefTxnId publishRef(const PoolPtr & store, const RootNamespace & ns, const String & ref, uint64_t ordinal)
+{
+    return store->appendRefOps(ns, MutationScope::ref(ref),
+        [&ref, ordinal](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (const RefOp & op : publishCommittedOps(ref, ManifestRef{1, ordinal, 1}))
+                ops.push_back(op);
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Publish);
+}
+
+}
+
+/// INV-1, first half: each namespace has its OWN stream. Two tables are written strictly alternately,
+/// so a pool-wide counter would hand them 1,3,5 and 2,4 -- every id unique across the pool and dense
+/// nowhere. Per-namespace derivation gives each table 1,2,3.. of its own.
+TEST(CASRefContiguousAlloc, TwoNamespacesAllocateIndependently)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const uint64_t epoch = store->writerEpoch();
+    const RootNamespace ns_a{"srv1/contig_ns_a"};
+    const RootNamespace ns_b{"srv1/contig_ns_b"};
+
+    const RefTxnId a1 = publishRef(store, ns_a, "ref_1", 1);
+    const RefTxnId b1 = publishRef(store, ns_b, "ref_1", 1);
+    const RefTxnId a2 = publishRef(store, ns_a, "ref_2", 2);
+    const RefTxnId b2 = publishRef(store, ns_b, "ref_2", 2);
+    const RefTxnId a3 = publishRef(store, ns_a, "ref_3", 3);
+
+    EXPECT_EQ(a1, (RefTxnId{epoch, 1}));
+    EXPECT_EQ(a2, (RefTxnId{epoch, 2}));
+    EXPECT_EQ(a3, (RefTxnId{epoch, 3}))
+        << "ns_a's third transaction must be its own third id -- the two ns_b transactions interleaved "
+           "between them belong to a different stream and must not push it up";
+    EXPECT_EQ(b1, (RefTxnId{epoch, 1}));
+    EXPECT_EQ(b2, (RefTxnId{epoch, 2}));
+}
+
+/// INV-1, second half (the free half of the every-attempt rule): a refusal that PROVES nothing was sent
+/// consumes no id. The pre-attempt gate refuses while the flush is still admitted -- no fault injection,
+/// nothing reaches the backend -- and the very next append on that table commits under the SAME id the
+/// refused one would have used. Under the pool-wide counter that id was burned as a "safe gap", which is
+/// precisely what makes a durable stream unreadable as a contiguous chain.
+TEST(CASRefContiguousAlloc, PreAttemptRefusalConsumesNoId)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolFenceControlled(backend);
+    const uint64_t epoch = store->writerEpoch();
+    const RootNamespace ns{"srv1/contig_no_gap"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{epoch, 1}));
+
+    store->setMountDeadline(FENCE_DEADLINE_REFUSES_ATTEMPT_MS);
+    ASSERT_TRUE(store->mayMutate()) << "the flush must still be ADMITTED, or this exercises the "
+                                       "top-of-flush gate instead of the pre-attempt one";
+    const String refusal = messageOfThrow([&] { publishRef(store, ns, "ref_2", 2); });
+    ASSERT_NE(refusal, String()) << "the pre-attempt gate must refuse this append";
+    /// Pin WHICH refusal this is. The id-reuse below is only meaningful for a refusal that proves
+    /// nothing was sent; a different failure (an ambiguous PUT, say) would be free to have landed, and
+    /// re-deriving its id would then be a collision rather than the no-gap property under test.
+    EXPECT_NE(refusal.find("was refused BEFORE any request was sent"), String::npos)
+        << "this test is about the provably-sent-nothing refusal specifically: " << refusal;
+    EXPECT_NE(refusal.find("the txn id is not consumed"), String::npos) << refusal;
+    ASSERT_FALSE(store->refLaneWedgedForTest(ns)) << "a refusal that sent nothing must not wedge";
+
+    store->setMountDeadline(FENCE_DEADLINE_HEALTHY_MS);
+    EXPECT_EQ(publishRef(store, ns, "ref_2", 2), (RefTxnId{epoch, 2}))
+        << "the refused attempt sent nothing, so the next caller must re-derive the SAME id -- a refusal "
+           "must never leave a hole in the durable stream";
+}
+
+/// The epoch component is the second half of the id, and the sequence is dense WITHIN an epoch: a new
+/// mount incarnation restarts its table's sequence at 1 rather than continuing the dead incarnation's
+/// numbering. `{E1, 2}` -> `{E2, 1}` is therefore not a gap, and the apply-side check must admit it.
+TEST(CASRefContiguousAlloc, EpochChangeRestartsTheSequenceAtOne)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const RootNamespace ns{"srv1/contig_epoch_reset"};
+
+    uint64_t e1 = 0;
+    {
+        auto predecessor = openPool(backend);
+        e1 = predecessor->writerEpoch();
+        ASSERT_EQ(publishRef(predecessor, ns, "ref_1", 1), (RefTxnId{e1, 1}));
+        ASSERT_EQ(publishRef(predecessor, ns, "ref_2", 2), (RefTxnId{e1, 2}));
+    }   /// predecessor destroyed: its mount lease is released
+
+    auto successor = openPool(backend);
+    const uint64_t e2 = successor->writerEpoch();
+    ASSERT_GT(e2, e1);
+    EXPECT_EQ(publishRef(successor, ns, "ref_3", 3), (RefTxnId{e2, 1}))
+        << "a fresh incarnation starts this table's sequence over at 1";
+    EXPECT_EQ(publishRef(successor, ns, "ref_4", 4), (RefTxnId{e2, 2}));
+}
+
+/// The read side is what makes INV-1 an invariant rather than a convention: a transaction whose id is
+/// not the successor of `greatest_applied` is CORRUPTED_DATA, naming both ids. Before this task the
+/// state machine checked strict increase only, so a stream with a hole applied cleanly and no reader
+/// could tell a complete chain from a truncated one.
+TEST(CASRefContiguousAlloc, NonSuccessorIdIsRejectedOnApply)
+{
+    const String ns = "srv1/contig_density";
+    constexpr uint64_t kEpoch = 7;
+
+    RefTableState state = replay(DB::Cas::tests::minimalLiveSnapshot(ns, RefTxnId{kEpoch, 1}), {});
+    ASSERT_EQ(state.getGreatestApplied(), (RefTxnId{kEpoch, 1}));
+
+    /// Strictly greater, but skips {7,2}: admitted before this task, rejected now.
+    try
+    {
+        applyRefLogTxn(state, RefLogTxn{ns, RefTxnId{kEpoch, 3}, publishCommittedOps("r", ManifestRef{1, 1, 1}), std::nullopt});
+        FAIL() << "a non-successor id must be rejected";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+        EXPECT_NE(e.message().find("7-3"), String::npos) << "the offending id must be named: " << e.message();
+        EXPECT_NE(e.message().find("7-1"), String::npos) << "the greatest applied id must be named: " << e.message();
+    }
+    EXPECT_EQ(state.getGreatestApplied(), (RefTxnId{kEpoch, 1})) << "the rejected apply must change nothing";
+
+    /// A new epoch must ALSO start at 1: continuing the previous epoch's numbering is a hole in the new
+    /// epoch's stream, which reads exactly like a lost first transaction.
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        applyRefLogTxn(state, RefLogTxn{ns, RefTxnId{kEpoch + 1, 2}, publishCommittedOps("r", ManifestRef{1, 1, 1}), std::nullopt});
+    });
+
+    /// The two shapes the allocator can produce are the two the checker admits -- and `nextRefTxnId` is
+    /// the single rule both sides use, so they cannot drift apart.
+    EXPECT_EQ(nextRefTxnId(state.getGreatestApplied(), kEpoch), (RefTxnId{kEpoch, 2}));
+    EXPECT_NO_THROW(applyRefLogTxn(state, RefLogTxn{ns, nextRefTxnId(state.getGreatestApplied(), kEpoch),
+        publishCommittedOps("r", ManifestRef{1, 1, 1}), std::nullopt}));
+    ASSERT_EQ(state.getGreatestApplied(), (RefTxnId{kEpoch, 2}));
+
+    EXPECT_EQ(nextRefTxnId(state.getGreatestApplied(), kEpoch + 1), (RefTxnId{kEpoch + 1, 1}));
+    /// The id is admissible, but a Live table crossing into a new epoch also owes INV-2's chain link --
+    /// the seal that closed the epoch below, at the slot one past its last durable id.
+    EXPECT_NO_THROW(applyRefLogTxn(state, RefLogTxn{ns, nextRefTxnId(state.getGreatestApplied(), kEpoch + 1),
+        publishCommittedOps("r2", ManifestRef{1, 2, 1}), RefTxnId{kEpoch, 3}}));
+    EXPECT_EQ(state.getGreatestApplied(), (RefTxnId{kEpoch + 1, 1}));
+}
+
+/// The format floor. A pool written before contiguous ref streams holds ref logs whose ids this build
+/// would read as a corrupt (holed) chain, so opening it must fail closed at the pool metadata, naming
+/// recreation as the migration -- CAS is pre-release and has no in-place migration path.
+TEST(CASRefContiguousAlloc, OldPoolFormatIsRefusedNamingRecreation)
+{
+    PoolMeta pm;
+    pm.pool_id = UInt128{1, 2};
+    pm.blob_header_len = 256;
+    pm.min_reader_generation = G_BUILD;
+    pm.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128)};
+
+    const String current = encodePoolMeta(pm);
+    EXPECT_NO_THROW(decodePoolMeta(current));
+
+    /// Rewrite the header-line generation to the last pre-contiguous one, exactly as an older build
+    /// would have stamped it.
+    const String from = "\"v\":" + std::to_string(G_BUILD);
+    const String to = "\"v\":" + std::to_string(kContiguousRefStreamsGeneration - 1);
+    const size_t at = current.find(from);
+    ASSERT_NE(at, String::npos);
+    String old_format = current;
+    old_format.replace(at, from.size(), to);
+
+    try
+    {
+        decodePoolMeta(old_format);
+        FAIL() << "a pre-contiguous pool must not open";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::UNKNOWN_FORMAT_VERSION);
+        EXPECT_NE(e.message().find(fmt::format("CAS pool format {} predates generation-9 exact _ckpt committed_through recovery frontier",
+                                               kContiguousRefStreamsGeneration - 1)), String::npos)
+            << "the message must name the migration: " << e.message();
+    }
+}
+
+/// Generation 6 is a recreate-only physical-layout cut. A generation-5 pool has contiguous,
+/// incarnation-qualified streams but still repeats the logical namespace in every key; accepting it
+/// would silently run the generation-6 parsers over a different grammar.
+TEST(CASRefContiguousAlloc, GenerationFiveNamespaceBearingPoolIsRefusedNamingRecreation)
+{
+    PoolMeta pm;
+    pm.pool_id = UInt128{1, 2};
+    pm.blob_header_len = 256;
+    pm.min_reader_generation = G_BUILD;
+    pm.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128)};
+
+    const String current = encodePoolMeta(pm);
+    EXPECT_NO_THROW(decodePoolMeta(current));
+
+    /// Rewrite the header to the immediately preceding generation, which used
+    /// `cas/refs/<namespace>/<incarnation>/...`.
+    const String from = "\"v\":" + std::to_string(G_BUILD);
+    const String to = "\"v\":" + std::to_string(kNamespaceLifeKeyedGeneration);
+    const size_t at = current.find(from);
+    ASSERT_NE(at, String::npos);
+    String old_format = current;
+    old_format.replace(at, from.size(), to);
+    ASSERT_EQ(kNamespaceLifeKeyedGeneration + 1, kOpaqueNamespaceLifeLayoutGeneration)
+        << "this test pins the immediately preceding namespace-bearing generation";
+
+    try
+    {
+        decodePoolMeta(old_format);
+        FAIL() << "a generation-5 namespace-bearing pool must not open";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::UNKNOWN_FORMAT_VERSION);
+        EXPECT_NE(e.message().find(fmt::format("CAS pool format {} predates generation-9 exact _ckpt committed_through recovery frontier",
+                                               kNamespaceLifeKeyedGeneration)), String::npos)
+            << "the message must name the migration: " << e.message();
+    }
+}
+
+/// Mutation caught: leaving the pool floor at generation 6 would admit a seal whose independent
+/// name-keyed coverage and cleanup collections this build no longer has. Generation 7 is a
+/// recreate-only grammar cut, so the immediately preceding generation must fail at pool open.
+TEST(CASRefContiguousAlloc, GenerationSixSplitFoldSealPoolIsRefusedNamingRecreation)
+{
+    PoolMeta pm;
+    pm.pool_id = UInt128{1, 2};
+    pm.blob_header_len = 256;
+    pm.min_reader_generation = G_BUILD;
+    pm.algos_used = {static_cast<uint8_t>(BlobHashAlgo::CityHash128)};
+
+    const String current = encodePoolMeta(pm);
+    const String from = "\"v\":" + std::to_string(G_BUILD);
+    const String to = "\"v\":6";
+    const size_t at = current.find(from);
+    ASSERT_NE(at, String::npos);
+    String old_format = current;
+    old_format.replace(at, from.size(), to);
+
+    try
+    {
+        decodePoolMeta(old_format);
+        FAIL() << "a generation-6 split ref-life fold seal pool must not open";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::UNKNOWN_FORMAT_VERSION);
+        EXPECT_NE(e.message().find("CAS pool format 6 predates generation-9 exact _ckpt committed_through recovery frontier"), String::npos)
+            << "the message must name the recreate-only grammar cut: " << e.message();
+    }
+}
+
+TEST(CASPoolMeta, GcShardsIsPersistedAndOverridesMismatchedReopenConfig)
+{
+    InMemoryBackend backend;
+    const Layout layout("p");
+    const PoolMeta created = PoolMeta::createOrValidate(
+        backend, layout, /*blob_header_len=*/256, /*gc_shards=*/4,
+        BlobHashAlgo::CityHash128, /*allow_new=*/false, /*allow_mint=*/true);
+    EXPECT_EQ(created.gc_shards, 4u);
+
+    const PoolMeta reopened = PoolMeta::createOrValidate(
+        backend, layout, /*blob_header_len=*/256, /*gc_shards=*/1,
+        BlobHashAlgo::CityHash128, /*allow_new=*/false, /*allow_mint=*/false);
+    EXPECT_EQ(reopened.gc_shards, 4u);
+    EXPECT_EQ(decodePoolMeta(backend.get(layout.poolMetaKey())->bytes).gc_shards, 4u);
+}
+
+/// The one path where "an attempt that provably sent nothing consumes nothing" does not hold, and the
+/// A known-durable install failure must replay before the next id is derived. Replay installs the
+/// stranded transaction, so the next append derives its real contiguous successor.
+TEST(CASRefContiguousAlloc, NeedsRecoveryReplaysBeforeAllocatingTheNextId)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const uint64_t epoch = store->writerEpoch();
+    const RootNamespace ns{"srv1/contig_durable_floor"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{epoch, 1}));
+
+    /// One-shot throw inside the post-durable install region: txn {epoch, 2} commits durably and is
+    /// never installed. The exception is built OUTSIDE the region (building it inside would trip
+    /// `DENY_ALLOCATIONS_IN_SCOPE` and test the guard instead of the recovery transition).
+    auto planned = std::make_exception_ptr(DB::Exception(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+        "simulated allocation failure inside the post-durable install region"));
+    auto fired = std::make_shared<std::atomic<bool>>(false);
+    store->setInstallRegionProbeForTest([planned, fired]
+    {
+        if (fired->exchange(true))
+            return;
+        ALLOW_ALLOCATIONS_IN_SCOPE;
+        std::rethrow_exception(planned);
+    });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+        [&] { publishRef(store, ns, "ref_2", 2); });
+    store->setInstallRegionProbeForTest(nullptr);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    /// The next append first recovers `{epoch, 2}`, then lands at `{epoch, 3}`.
+    EXPECT_EQ(publishRef(store, ns, "ref_3", 3), (RefTxnId{epoch, 3}))
+        << "the stranded transaction is durable, so the next id must be its successor, not itself";
+    EXPECT_TRUE(store->resolveRef(ns, "ref_3", /*allow_stale=*/false).has_value())
+        << "the append may proceed only after recovery has repaired the cached state";
+
+    EXPECT_TRUE(store->resolveRef(ns, "ref_2", /*allow_stale=*/false).has_value())
+        << "the stranded transaction is back in this cache, which is what repairs the divergence";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+
+    /// The durable stream itself is dense: `1`, `2`, `3` all exist as objects. `ns` was born through
+    /// the REAL append lane (Stage B Task 4-C), so its objects sit at a real catalog-minted incarnation,
+    /// not the Stage-A sentinel -- resolve it the same way production discovery does.
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns).value();
+    for (uint64_t seq = 1; seq <= 3; ++seq)
+        EXPECT_TRUE(backend->head(store->layout().refLogKey(life, RefTxnId{epoch, seq})).exists)
+            << "log object " << epoch << "-" << seq << " must exist: the durable stream has no hole";
+}
+
+/// Snapshot publication also recovers a `NeedsRecovery` lane before it captures state.
+TEST(CASRefContiguousAlloc, NeedsRecoveryReplaysBeforeSnapshotPublication)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPool(backend);
+    const uint64_t epoch = store->writerEpoch();
+    const RootNamespace ns{"srv1/contig_poison_publish"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{epoch, 1}));
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns)) << "a healthy table must publish, or the refusal "
+                                                     "asserted below would prove nothing";
+    const auto published_before = store->newestPublishedSnapshotIdForTest(ns);
+    ASSERT_TRUE(published_before.has_value());
+
+    auto planned = std::make_exception_ptr(DB::Exception(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+        "simulated allocation failure inside the post-durable install region"));
+    auto fired = std::make_shared<std::atomic<bool>>(false);
+    store->setInstallRegionProbeForTest([planned, fired]
+    {
+        if (fired->exchange(true))
+            return;
+        ALLOW_ALLOCATIONS_IN_SCOPE;
+        std::rethrow_exception(planned);
+    });
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+        [&] { publishRef(store, ns, "ref_2", 2); });
+    store->setInstallRegionProbeForTest(nullptr);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    /// The append entry point replays before admitting this transaction.
+    EXPECT_EQ(publishRef(store, ns, "ref_3", 3), (RefTxnId{epoch, 3}));
+
+    /// Publication is safe because recovery installed the stranded transaction first.
+    EXPECT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    EXPECT_TRUE(store->resolveRef(ns, "ref_2", /*allow_stale=*/false).has_value())
+        << "the stranded transaction is durable and was re-derived -- publishing is safe precisely "
+           "because there is nothing left to omit";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+    EXPECT_NE(store->newestPublishedSnapshotIdForTest(ns), published_before);
+}
+
+/// Recreation quiesce, refusal leg. Refusing to OPEN an old-format pool fences nothing: the server that
+/// mounted it before the operator acted is still running, still holds its mount lease, and still has
+/// queued writes. If "recreate the pool" is followed literally -- clear the prefix, start fresh -- that
+/// writer's next flush lands its old-format transactions inside the NEW pool. So a recreation over a
+/// prefix whose mount slots are not terminal must fail closed, and must say why, BEFORE the operator
+/// clears anything.
+TEST(CASRefContiguousAlloc, RecreationRefusedWhileAMountSlotIsStillHeld)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto holder = openPool(backend);
+    const RootNamespace ns{"srv1/contig_quiesce"};
+    ASSERT_EQ(publishRef(holder, ns, "ref_1", 1), (RefTxnId{holder->writerEpoch(), 1}));
+
+    /// The operator removes the pool identity, intending to recreate -- but the holder is still up.
+    ASSERT_EQ(eraseKeysContaining(*backend, "_pool_meta"), 1u);
+
+    const String message = messageOfThrow([&] { openPoolWithoutSeeding(backend, "test2"); });
+    EXPECT_NE(message.find("mount lease(s) under this prefix are still held"), String::npos)
+        << "the refusal must name the held lease, not merely the residual data: " << message;
+    EXPECT_NE(message.find("do NOT clear the prefix first"), String::npos)
+        << "the remedy ordering is the whole point of this gate: " << message;
+    EXPECT_NE(message.find("server root 'test'"), String::npos)
+        << "the holder must be identified so the operator knows what to stop: " << message;
+
+    /// And the holder is untouched by the refused recreation: its own stream continues contiguously.
+    EXPECT_EQ(publishRef(holder, ns, "ref_2", 2), (RefTxnId{holder->writerEpoch(), 2}));
+}
+
+/// Recreation quiesce, acceptance leg. Once the holder is gone its slot carries the graceful-farewell
+/// marker -- one of the two clock-free certificates of death the mount protocol already recognises --
+/// so the quiesce gate stops firing and the ordinary bootstrap rules take over: clear the prefix, and
+/// the recreation mints a fresh pool.
+TEST(CASRefContiguousAlloc, RecreationProceedsOnceTheHolderIsTerminal)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    const RootNamespace ns{"srv1/contig_quiesce_ok"};
+    {
+        auto holder = openPool(backend);
+        publishRef(holder, ns, "ref_1", 1);
+    }   /// destroyed: the keeper stamps the farewell, making the slot terminal
+    ASSERT_EQ(eraseKeysContaining(*backend, "_pool_meta"), 1u);
+
+    /// The prefix still holds this pool's data, so the bootstrap still refuses -- but on the ORDINARY
+    /// residual rule, not the quiesce gate. That difference is the whole assertion: nothing is being
+    /// held any more.
+    const String residual = messageOfThrow([&] { openPoolWithoutSeeding(backend, "test2"); });
+    EXPECT_EQ(residual.find("still held"), String::npos)
+        << "a terminal slot must not block recreation: " << residual;
+    EXPECT_NE(residual.find("refusing to bootstrap over residual data"), String::npos) << residual;
+
+    /// The operator now clears the prefix -- in the order the refusal prescribed -- and the recreation
+    /// mints a fresh pool that starts its own ref stream at 1.
+    ASSERT_GT(eraseKeysContaining(*backend, ""), 0u);
+    auto recreated = openPoolWithoutSeeding(backend, "test");
+    EXPECT_EQ(publishRef(recreated, ns, "ref_1", 1), (RefTxnId{recreated->writerEpoch(), 1}));
+}
+
+/// The other half of the rule: if the prefix IS cleared while a writer survives (the mistake the
+/// refusal above exists to prevent, or a writer that was already mid-flight), the recreated pool's
+/// ordinary mount claim is what stops it. The survivor's next lease renewal finds a slot it can no
+/// longer hold, its local fence latches shut, and every later write is refused -- so a straggler can
+/// never append into the new pool.
+///
+/// The recreating mount here is a DIFFERENT server (its own `server_id`), which is what makes the
+/// survivor's renewal conclusive. Clearing the prefix also resets the durable writer-epoch counter, so
+/// a recreation by the SAME server uuid can be handed the very same `(uuid, epoch)` the survivor still
+/// holds -- and the two are then indistinguishable to the lease protocol, which reads the survivor's
+/// renewal as its own keeper adopting a refreshed body. That is precisely why the refusal above is the
+/// primary defence and this fence is only the backstop: quiescing the holder BEFORE the prefix is
+/// cleared is what keeps the ambiguous case from arising at all.
+TEST(CASRefContiguousAlloc, SurvivingWriterIsFencedByTheRecreatedPoolsMount)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// The survivor renews on its own thread, as a real mount does: the renewal loop is what latches the
+    /// write fence when a renewal fails, so a hand-driven `renewWatermarkOnce` would reproduce only the
+    /// failure and not the fencing it causes.
+    PoolConfig survivor_cfg{.pool_prefix = "p", .server_root_id = "test"};
+    survivor_cfg.background_watermark = true;
+    survivor_cfg.mount_renew_period = std::chrono::milliseconds{50};
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    auto survivor = Pool::open(backend, survivor_cfg);
+    const RootNamespace ns{"srv1/contig_survivor"};
+    ASSERT_EQ(publishRef(survivor, ns, "ref_1", 1), (RefTxnId{survivor->writerEpoch(), 1}));
+
+    /// The prefix is cleared and the pool recreated underneath the still-running survivor.
+    ASSERT_GT(eraseKeysContaining(*backend, ""), 0u);
+    PoolConfig recreated_cfg{.pool_prefix = "p", .server_root_id = "test"};
+    recreated_cfg.server_id = UInt128{7, 7};
+    auto recreated = Pool::open(backend, recreated_cfg);
+    ASSERT_TRUE(recreated->mayMutate());
+
+    /// The survivor's next renewal finds a slot held by a foreign server and fails closed, and the loop
+    /// latches the local write fence. Bounded wait: a real hang fails the test instead of stalling it.
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(10);
+    while (survivor->mayMutate() && std::chrono::steady_clock::now() < deadline)
+        std::this_thread::sleep_for(std::chrono::milliseconds(20));
+    EXPECT_FALSE(survivor->mayMutate())
+        << "a survivor whose slot was reclaimed must be fenced closed by its own failing renewal, not "
+           "left writing into the new pool";
+    EXPECT_NE(messageOfThrow([&] { publishRef(survivor, ns, "ref_2", 2); }), String())
+        << "the survivor's queued write must be refused";
+
+    /// The recreated pool is unaffected and owns the stream from 1.
+    EXPECT_EQ(publishRef(recreated, ns, "ref_1", 1), (RefTxnId{recreated->writerEpoch(), 1}));
+
+    /// The survivor's TEARDOWN is the other half, and it is asserted here rather than left to the
+    /// destructor at scope exit, because which arm of the release path it takes is exactly what a
+    /// regressed discriminator would get wrong — silently. A deposed writer meeting its successor in the
+    /// slot is the EXPECTED end of a failover (arm A: skip the farewell, leave the successor's slot
+    /// untouched); a writer that still believed it owned the mount meeting a stranger is
+    /// single-writer exclusivity BROKEN (arm B, must-always-be-zero). If `deposition_observed` ever stops
+    /// being set, every ordinary failover in production starts reporting itself as a broken guarantee —
+    /// and without the +0 assertion below, not one test would notice.
+    const String survivor_mount_key = recreated->layout().mountKey("test");
+    const auto successor_slot_before = backend->get(survivor_mount_key);
+    ASSERT_TRUE(successor_slot_before.has_value());
+    const uint64_t skipped_before
+        = ProfileEvents::global_counters[ProfileEvents::CASMountReleaseSkippedForeignOccupant].load();
+    const uint64_t violations_before
+        = ProfileEvents::global_counters[ProfileEvents::CASMountExclusivityViolation].load();
+
+    survivor.reset();
+
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMountReleaseSkippedForeignOccupant].load(),
+              skipped_before + 1)
+        << "a deposed writer's release must take the skip-the-farewell arm";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASMountExclusivityViolation].load(),
+              violations_before)
+        << "and must NOT report an exclusivity violation: this is a failover, not a broken guarantee";
+    const auto successor_slot_after = backend->get(survivor_mount_key);
+    ASSERT_TRUE(successor_slot_after.has_value());
+    EXPECT_EQ(successor_slot_after->bytes, successor_slot_before->bytes)
+        << "the deposed writer must not stamp its farewell over the successor's lease";
+    EXPECT_TRUE(recreated->mayMutate()) << "and must not disturb the live successor";
+}
diff --git a/src/Disks/tests/gtest_cas_ref_cow_manifest_set.cpp b/src/Disks/tests/gtest_cas_ref_cow_manifest_set.cpp
new file mode 100644
index 000000000000..5e1ddea64568
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_cow_manifest_set.cpp
@@ -0,0 +1,392 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowManifestSet.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <random>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::expectThrowsCode;
+
+namespace
+{
+
+ManifestRef mref(uint64_t epoch, uint64_t seq, uint32_t ordinal)
+{
+    return ManifestRef{epoch, seq, ordinal};
+}
+
+}
+
+/// ===================================================================================
+/// Keyed ops: contains/insert/erase across base+overlay (the "E2 owned-manifest index" work).
+/// ===================================================================================
+
+TEST(CASRefCowManifestSet, EmptySetHasNoMembers)
+{
+    RefCowManifestSet s;
+    EXPECT_TRUE(s.empty());
+    EXPECT_EQ(s.size(), 0u);
+    EXPECT_FALSE(s.contains(mref(1, 1, 1)));
+}
+
+TEST(CASRefCowManifestSet, InsertThenContains)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+    EXPECT_EQ(s.size(), 1u);
+    EXPECT_FALSE(s.contains(mref(2, 2, 2)));
+}
+
+TEST(CASRefCowManifestSet, InsertMultipleThenContainsEachIndependently)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.insert(mref(1, 1, 2));
+    s.insert(mref(2, 1, 1));
+    EXPECT_EQ(s.size(), 3u);
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+    EXPECT_TRUE(s.contains(mref(1, 1, 2)));
+    EXPECT_TRUE(s.contains(mref(2, 1, 1)));
+    EXPECT_FALSE(s.contains(mref(3, 3, 3)));
+}
+
+TEST(CASRefCowManifestSet, EraseRemovesAnOverlayOnlyMember)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.erase(mref(1, 1, 1));
+    EXPECT_FALSE(s.contains(mref(1, 1, 1)));
+    EXPECT_EQ(s.size(), 0u);
+    EXPECT_TRUE(s.empty());
+    EXPECT_EQ(s.overlayEntriesForTest(), 0u);   /// pure-overlay member: erase removes it outright
+}
+
+TEST(CASRefCowManifestSet, TombstoneThenReinsertWhilePurelyInOverlay)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.erase(mref(1, 1, 1));
+    s.insert(mref(1, 1, 1));   /// re-insert -- must not be treated as "still present"
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+    EXPECT_EQ(s.size(), 1u);
+}
+
+/// ===================================================================================
+/// materialize()
+/// ===================================================================================
+
+TEST(CASRefCowManifestSet, MaterializeFoldsOverlayIntoBaseAndEmptiesOverlay)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.insert(mref(1, 1, 2));
+    EXPECT_GT(s.overlayEntriesForTest(), 0u);
+
+    s.materialize();
+    EXPECT_EQ(s.overlayEntriesForTest(), 0u);
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+    EXPECT_TRUE(s.contains(mref(1, 1, 2)));
+    EXPECT_EQ(s.size(), 2u);
+}
+
+TEST(CASRefCowManifestSet, MaterializeOnAnEmptyOverlayIsANoOp)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.materialize();
+    const int64_t use_count_before = s.baseUseCountForTest();
+    s.materialize();   /// overlay is already empty
+    EXPECT_EQ(s.baseUseCountForTest(), use_count_before);
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+}
+
+TEST(CASRefCowManifestSet, EraseAfterMaterializeTombstonesABaseMember)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.insert(mref(1, 1, 2));
+    s.materialize();   /// both now live in `base`
+
+    s.erase(mref(1, 1, 1));
+    EXPECT_FALSE(s.contains(mref(1, 1, 1)));
+    EXPECT_TRUE(s.contains(mref(1, 1, 2)));
+    EXPECT_EQ(s.size(), 1u);
+
+    s.materialize();   /// tombstone folds away; base member actually removed
+    EXPECT_FALSE(s.contains(mref(1, 1, 1)));
+    EXPECT_EQ(s.size(), 1u);
+}
+
+TEST(CASRefCowManifestSet, TombstoneThenReinsertAcrossMaterializedBase)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.materialize();   /// mref(1,1,1) now lives in `base`
+
+    s.erase(mref(1, 1, 1));            /// tombstone shadowing the base member
+    s.insert(mref(1, 1, 1));           /// revive the tombstone -- must read as present again
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+    EXPECT_EQ(s.size(), 1u);
+
+    s.materialize();
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+    EXPECT_EQ(s.size(), 1u);
+}
+
+/// ===================================================================================
+/// materialize() fast path: fold into a uniquely-owned base IN PLACE, no O(N) copy (E5).
+/// ===================================================================================
+
+TEST(CASRefCowManifestSet, MaterializeReusesBaseWhenUniquelyOwned)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.materialize();                    /// mref(1,1,1) now in base; base is uniquely owned
+    const void * base_before = s.baseIdentityForTest();
+    ASSERT_EQ(s.baseUseCountForTest(), 1);
+
+    s.insert(mref(2, 2, 2));            /// pure-overlay addition
+    s.erase(mref(1, 1, 1));            /// tombstone a base member
+    s.materialize();
+
+    EXPECT_EQ(s.baseIdentityForTest(), base_before);   /// folded in place: same base allocation
+    EXPECT_EQ(s.overlayEntriesForTest(), 0u);
+    EXPECT_FALSE(s.contains(mref(1, 1, 1)));            /// tombstone erased from base
+    EXPECT_TRUE(s.contains(mref(2, 2, 2)));
+    EXPECT_EQ(s.size(), 1u);                            /// net_delta reset, size still exact
+}
+
+TEST(CASRefCowManifestSet, MaterializeBuildsFreshBaseWhenBaseIsShared)
+{
+    RefCowManifestSet original;
+    original.insert(mref(1, 1, 1));
+    original.materialize();
+    const void * shared_base = original.baseIdentityForTest();
+
+    RefCowManifestSet writer = original;   /// shares the base (use_count 2)
+    ASSERT_EQ(writer.baseUseCountForTest(), 2);
+    writer.insert(mref(9, 9, 9));
+    writer.erase(mref(1, 1, 1));
+    writer.materialize();                  /// base is shared -> must build a fresh one, mutate nothing shared
+
+    /// Load-bearing correctness pin: the OTHER holder's view is byte-unchanged.
+    EXPECT_EQ(original.baseIdentityForTest(), shared_base);
+    EXPECT_TRUE(original.contains(mref(1, 1, 1)));
+    EXPECT_FALSE(original.contains(mref(9, 9, 9)));
+    EXPECT_EQ(original.size(), 1u);
+
+    /// The writer folded its overlay into a fresh base of its own.
+    EXPECT_NE(writer.baseIdentityForTest(), shared_base);
+    EXPECT_FALSE(writer.contains(mref(1, 1, 1)));
+    EXPECT_TRUE(writer.contains(mref(9, 9, 9)));
+    EXPECT_EQ(writer.size(), 1u);
+}
+
+TEST(CASRefCowManifestSet, MaterializeEmptyOverlayIsANoOpEvenWhenUniquelyOwned)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.materialize();
+    ASSERT_EQ(s.baseUseCountForTest(), 1);
+    const void * base_before = s.baseIdentityForTest();
+    s.materialize();   /// overlay already empty: no fold, no reallocation
+    EXPECT_EQ(s.baseIdentityForTest(), base_before);
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+}
+
+/// ===================================================================================
+/// Copy-on-write isolation + O(1)-copy assertion.
+/// ===================================================================================
+
+TEST(CASRefCowManifestSet, CopyIsIsolatedFromOriginal)
+{
+    RefCowManifestSet original;
+    original.insert(mref(1, 1, 1));
+    original.materialize();
+
+    RefCowManifestSet copy = original;
+    copy.insert(mref(9, 9, 9));
+    copy.erase(mref(1, 1, 1));
+
+    EXPECT_TRUE(original.contains(mref(1, 1, 1)));
+    EXPECT_FALSE(original.contains(mref(9, 9, 9)));
+
+    EXPECT_FALSE(copy.contains(mref(1, 1, 1)));
+    EXPECT_TRUE(copy.contains(mref(9, 9, 9)));
+}
+
+TEST(CASRefCowManifestSet, CopySharesBaseUntilEitherSideMaterializesANewOne)
+{
+    RefCowManifestSet original;
+    original.insert(mref(1, 1, 1));
+    original.materialize();
+
+    RefCowManifestSet copy = original;
+    /// A copy shares the SAME base object (refcount bump, no per-element allocation) until a write
+    /// forces a new base into existence via `materialize()`.
+    EXPECT_EQ(original.baseUseCountForTest(), 2);
+    EXPECT_EQ(copy.baseUseCountForTest(), 2);
+
+    copy.insert(mref(2, 2, 2));   /// writes go to `copy`'s overlay; `base` is untouched
+    EXPECT_EQ(original.baseUseCountForTest(), 2);
+    EXPECT_EQ(copy.baseUseCountForTest(), 2);
+    EXPECT_FALSE(original.contains(mref(2, 2, 2)));
+
+    copy.materialize();   /// NOW `copy` points at a fresh base of its own
+    EXPECT_EQ(original.baseUseCountForTest(), 1);
+    EXPECT_EQ(copy.baseUseCountForTest(), 1);
+}
+
+/// ===================================================================================
+/// size()/net_delta correctness across a longer op sequence, mixing base and overlay changes.
+/// ===================================================================================
+
+TEST(CASRefCowManifestSet, SizeTracksNetDeltaAcrossMixedOps)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.insert(mref(1, 1, 2));
+    s.insert(mref(1, 1, 3));
+    EXPECT_EQ(s.size(), 3u);
+    s.materialize();
+    EXPECT_EQ(s.size(), 3u);
+
+    s.erase(mref(1, 1, 2));         /// base member removed via overlay tombstone
+    EXPECT_EQ(s.size(), 2u);
+    s.insert(mref(1, 1, 4));        /// pure-overlay addition
+    EXPECT_EQ(s.size(), 3u);
+    s.erase(mref(1, 1, 4));         /// pure-overlay addition removed outright
+    EXPECT_EQ(s.size(), 2u);
+    s.insert(mref(1, 1, 2));        /// revive the earlier tombstone
+    EXPECT_EQ(s.size(), 3u);
+
+    s.materialize();
+    EXPECT_EQ(s.size(), 3u);
+    EXPECT_TRUE(s.contains(mref(1, 1, 1)));
+    EXPECT_TRUE(s.contains(mref(1, 1, 2)));
+    EXPECT_TRUE(s.contains(mref(1, 1, 3)));
+    EXPECT_FALSE(s.contains(mref(1, 1, 4)));
+}
+
+/// ===================================================================================
+/// Drift-detection misuse (throws `CORRUPTED_DATA` in EVERY build, post-consult -- previously a
+/// debug-only `chassert`): `insert` requires absence, `erase` requires presence. The ref table's own
+/// uniqueness invariant guarantees both before either is ever called, so a violation here means the
+/// index has drifted, not that a legitimate caller can trigger it. Failing closed (rather than a silent
+/// release-build `net_delta` drift) is what keeps a corrupted history from later hiding a still-live
+/// owner.
+/// ===================================================================================
+
+TEST(CASRefCowManifestSet, InsertThrowsWhenAlreadyPresentInOverlay)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { s.insert(mref(1, 1, 1)); });
+}
+
+TEST(CASRefCowManifestSet, InsertThrowsWhenAlreadyPresentInBase)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.materialize();
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { s.insert(mref(1, 1, 1)); });
+}
+
+TEST(CASRefCowManifestSet, EraseThrowsWhenAbsent)
+{
+    RefCowManifestSet s;
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { s.erase(mref(1, 1, 1)); });
+}
+
+TEST(CASRefCowManifestSet, EraseThrowsWhenAlreadyTombstoned)
+{
+    RefCowManifestSet s;
+    s.insert(mref(1, 1, 1));
+    s.materialize();
+    s.erase(mref(1, 1, 1));
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { s.erase(mref(1, 1, 1)); });
+}
+
+/// ===================================================================================
+/// Fast-vs-forced-slow materialize parity (E5 xhigh review): the in-place fold (uniquely-owned base)
+/// and the build-fresh-and-swap fold (a copy still shares the base) must agree on membership and size
+/// across randomized op sequences. No iteration surface here, so membership is probed over a fixed
+/// keyspace. insert/erase preconditions are respected (guarded by the shared membership) so the two
+/// sets never drift and never trip the fail-closed CORRUPTED_DATA guards.
+/// ===================================================================================
+
+TEST(CASRefCowManifestSet, FastAndForcedSlowMaterializeAgreeOverRandomOps)
+{
+    std::mt19937 rng(20260722); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed for reproducible coverage.
+
+    std::vector<ManifestRef> keyspace;
+    for (uint64_t k = 0; k < 10; ++k)
+        keyspace.push_back(mref(1, k, 1));
+
+    for (int trial = 0; trial < 60; ++trial)
+    {
+        RefCowManifestSet fast;   /// never copied -> in-place (uniquely-owned) materialize
+        RefCowManifestSet slow;   /// a live copy is held across each materialize -> forced fresh-base path
+
+        for (int step = 0; step < 150; ++step)
+        {
+            const ManifestRef m = keyspace[rng() % keyspace.size()];
+            const bool present = fast.contains(m);   /// identical in both sets by construction
+            switch (rng() % 5)
+            {
+                case 0:
+                    if (!present)   /// respect the insert precondition (absent)
+                    {
+                        fast.insert(m);
+                        slow.insert(m);
+                    }
+                    break;
+                case 1:
+                    if (present)    /// respect the erase precondition (present)
+                    {
+                        fast.erase(m);
+                        slow.erase(m);
+                    }
+                    break;
+                case 2:   /// materialize both, each via its intended path
+                {
+                    ASSERT_EQ(fast.baseUseCountForTest(), 1) << "fast set must be uniquely owned";
+                    fast.materialize();   /// in-place fast path
+                    {
+                        RefCowManifestSet pin = slow;   /// shares slow's base
+                        ASSERT_EQ(slow.baseUseCountForTest(), 2) << "slow set must be forced onto the copy path";
+                        slow.materialize();             /// build-fresh-and-swap slow path
+                    }
+                    EXPECT_EQ(fast.overlayEntriesForTest(), 0u) << "trial " << trial << " step " << step;
+                    EXPECT_EQ(slow.overlayEntriesForTest(), 0u) << "trial " << trial << " step " << step;
+                    break;
+                }
+                default:
+                    break;   /// accumulate overlay without materializing
+            }
+
+            ASSERT_EQ(fast.size(), slow.size()) << "trial " << trial << " step " << step;
+            for (const auto & probe : keyspace)
+                ASSERT_EQ(fast.contains(probe), slow.contains(probe)) << "trial " << trial << " step " << step;
+        }
+
+        fast.materialize();
+        {
+            RefCowManifestSet pin = slow;
+            slow.materialize();
+        }
+        EXPECT_EQ(fast.overlayEntriesForTest(), 0u) << "trial " << trial;
+        EXPECT_EQ(slow.overlayEntriesForTest(), 0u) << "trial " << trial;
+        EXPECT_EQ(fast.size(), slow.size()) << "trial " << trial;
+        for (const auto & probe : keyspace)
+            EXPECT_EQ(fast.contains(probe), slow.contains(probe)) << "trial " << trial;
+    }
+}
diff --git a/src/Disks/tests/gtest_cas_ref_cow_map.cpp b/src/Disks/tests/gtest_cas_ref_cow_map.cpp
new file mode 100644
index 000000000000..40e4cb6b9cb5
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_cow_map.cpp
@@ -0,0 +1,516 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCowMap.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+
+#include <map>
+#include <random>
+#include <stdexcept>
+#include <string>
+#include <utility>
+#include <vector>
+
+using namespace DB::Cas;
+
+namespace
+{
+
+RefCommittedRow row(uint64_t epoch, uint64_t seq, uint32_t ordinal)
+{
+    RefCommittedRow r;
+    r.manifest_ref = ManifestRef{epoch, seq, ordinal};
+    return r;
+}
+
+}
+
+/// ===================================================================================
+/// Keyed ops
+/// ===================================================================================
+
+TEST(CASRefCowMap, EmptyMapHasNoEntries)
+{
+    RefCowMap m;
+    EXPECT_TRUE(m.empty());
+    EXPECT_EQ(m.size(), 0u);
+    EXPECT_FALSE(m.contains("a"));
+    EXPECT_FALSE(m.contains("a"));
+}
+
+TEST(CASRefCowMap, EmplaceThenFind)
+{
+    RefCowMap m;
+    const auto [it, inserted] = m.emplace("a", row(1, 1, 1));
+    EXPECT_TRUE(inserted);
+    EXPECT_EQ(m.size(), 1u);
+    ASSERT_TRUE(m.contains("a"));
+    EXPECT_EQ(it->second.manifest_ref, (ManifestRef{1, 1, 1}));
+    EXPECT_EQ(m.at("a").manifest_ref, (ManifestRef{1, 1, 1}));
+}
+
+TEST(CASRefCowMap, EmplaceDoesNotOverwriteExisting)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    const auto [it, inserted] = m.emplace("a", row(2, 2, 2));
+    EXPECT_FALSE(inserted);
+    EXPECT_EQ(it->second.manifest_ref, (ManifestRef{1, 1, 1}));
+    EXPECT_EQ(m.at("a").manifest_ref, (ManifestRef{1, 1, 1}));   /// unchanged
+}
+
+TEST(CASRefCowMap, InsertOrAssignOverwritesExisting)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    const auto [it, inserted] = m.insert_or_assign("a", row(2, 2, 2));
+    EXPECT_FALSE(inserted);
+    EXPECT_EQ(it->second.manifest_ref, (ManifestRef{2, 2, 2}));
+    EXPECT_EQ(m.at("a").manifest_ref, (ManifestRef{2, 2, 2}));
+}
+
+TEST(CASRefCowMap, InsertOrAssignInsertsWhenAbsent)
+{
+    RefCowMap m;
+    const auto [it, inserted] = m.insert_or_assign("a", row(1, 1, 1));
+    EXPECT_TRUE(inserted);
+    EXPECT_EQ(m.size(), 1u);
+    EXPECT_EQ(it->second.manifest_ref, (ManifestRef{1, 1, 1}));
+}
+
+TEST(CASRefCowMap, EraseByKey)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    EXPECT_EQ(m.erase("a"), 1u);
+    EXPECT_FALSE(m.contains("a"));
+    EXPECT_EQ(m.size(), 0u);
+    EXPECT_EQ(m.erase("a"), 0u);              /// already gone: no-op
+    EXPECT_EQ(m.erase("nonexistent"), 0u);
+}
+
+TEST(CASRefCowMap, AtThrowsOnMissingKey)
+{
+    RefCowMap m;
+    EXPECT_THROW(m.at("missing"), std::out_of_range);
+}
+
+TEST(CASRefCowMap, CountMatchesContains)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    EXPECT_EQ(m.count("a"), 1u);
+    EXPECT_EQ(m.count("b"), 0u);
+}
+
+/// ===================================================================================
+/// Ordered iteration -- overlay overrides/tombstones a materialized base (spec: "Ordered
+/// iteration: merge-iterate base and overlay ... a standard two-sorted-range merge").
+/// ===================================================================================
+
+TEST(CASRefCowMap, OrderedIterationOverAllBaseRowsIsSorted)
+{
+    RefCowMap m;
+    m.emplace("c", row(1, 3, 1));
+    m.emplace("a", row(1, 1, 1));
+    m.emplace("b", row(1, 2, 1));
+
+    std::vector<String> names;
+    for (const auto [name, r] : m)
+        names.push_back(name);
+    EXPECT_EQ(names, (std::vector<String>{"a", "b", "c"}));
+}
+
+TEST(CASRefCowMap, MergedIterationAppliesTombstonesAndOverrides)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    m.emplace("b", row(1, 2, 1));
+    m.emplace("c", row(1, 3, 1));
+    m.materialize();   /// a, b, c now live in `base`
+
+    m.insert_or_assign("b", row(9, 9, 9));   /// override b via the overlay
+    m.erase("c");                             /// tombstone c via the overlay
+    m.emplace("d", row(9, 9, 2));             /// pure-overlay addition (not in base)
+
+    std::vector<std::pair<String, ManifestRef>> seen;
+    for (const auto [name, r] : m)
+        seen.emplace_back(name, r.manifest_ref);
+
+    const std::vector<std::pair<String, ManifestRef>> expected = {
+        {"a", ManifestRef{1, 1, 1}},
+        {"b", ManifestRef{9, 9, 9}},
+        {"d", ManifestRef{9, 9, 2}},
+    };
+    EXPECT_EQ(seen, expected);
+    EXPECT_EQ(m.size(), 3u);
+}
+
+TEST(CASRefCowMap, FindOverlayOnlyKeyIteratesIntoBase)
+{
+    RefCowMap m;
+    m.emplace("A", row(1, 1, 1));
+    m.emplace("D", row(1, 4, 1));
+    m.materialize();   /// A, D now live in `base`
+
+    m.insert_or_assign("B", row(2, 2, 1));   /// overlay-only key between base keys "A" and "D"
+
+    auto it = m.find("B");
+    ASSERT_NE(it, m.end());
+    EXPECT_EQ(it->first, "B");
+    ++it;
+    ASSERT_NE(it, m.end());   /// must land on "D", not collapse straight to end()
+    EXPECT_EQ(it->first, "D");
+}
+
+TEST(CASRefCowMap, EraseByIteratorReturnsNextAndRemovesTheRow)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    m.emplace("b", row(1, 2, 1));
+    m.emplace("c", row(1, 3, 1));
+
+    auto it = m.find("b");
+    ASSERT_TRUE(it != m.end());
+    auto next = m.erase(it);
+    ASSERT_TRUE(next != m.end());
+    EXPECT_EQ(next->first, "c");
+    EXPECT_FALSE(m.contains("b"));
+    EXPECT_EQ(m.size(), 2u);
+}
+
+TEST(CASRefCowMap, EraseByIteratorOfLastElementReturnsEnd)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    auto it = m.find("a");
+    auto next = m.erase(it);
+    EXPECT_TRUE(next == m.end());
+    EXPECT_TRUE(m.empty());
+}
+
+/// ===================================================================================
+/// materialize() (spec §Materialization)
+/// ===================================================================================
+
+TEST(CASRefCowMap, MaterializeFoldsOverlayIntoFreshBaseAndKeepsValuesUnchanged)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    m.emplace("b", row(1, 2, 1));
+    m.erase("a");
+    EXPECT_GT(m.overlayEntriesForTest(), 0u);
+
+    m.materialize();
+    EXPECT_EQ(m.overlayEntriesForTest(), 0u);
+    EXPECT_FALSE(m.contains("a"));
+    ASSERT_TRUE(m.contains("b"));
+    EXPECT_EQ(m.at("b").manifest_ref, (ManifestRef{1, 2, 1}));
+    EXPECT_EQ(m.size(), 1u);
+}
+
+TEST(CASRefCowMap, MaterializeOnAnEmptyOverlayIsANoOp)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    m.materialize();
+    const int64_t use_count_before = m.baseUseCountForTest();
+    m.materialize();   /// overlay is already empty
+    EXPECT_EQ(m.baseUseCountForTest(), use_count_before);
+    EXPECT_TRUE(m.contains("a"));
+}
+
+TEST(CASRefCowMap, MaterializeDoesNotAffectACopyTakenBeforeIt)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    RefCowMap snapshot_before = m;   /// copy shares m's pre-materialize base, owns its own overlay
+    m.insert_or_assign("a", row(2, 2, 2));
+    m.materialize();
+
+    EXPECT_EQ(m.at("a").manifest_ref, (ManifestRef{2, 2, 2}));
+    EXPECT_EQ(snapshot_before.at("a").manifest_ref, (ManifestRef{1, 1, 1}));
+}
+
+/// ===================================================================================
+/// materialize() fast path: fold into a uniquely-owned base IN PLACE, no O(N) copy (E5).
+/// ===================================================================================
+
+TEST(CASRefCowMap, MaterializeReusesBaseWhenUniquelyOwned)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    m.materialize();                          /// "a" now in base; base is uniquely owned
+    const void * base_before = m.baseIdentityForTest();
+    ASSERT_EQ(m.baseUseCountForTest(), 1);
+
+    m.insert_or_assign("b", row(2, 2, 2));    /// pure-overlay addition
+    m.erase("a");                             /// tombstone a base member
+    m.materialize();
+
+    EXPECT_EQ(m.baseIdentityForTest(), base_before);   /// folded in place: same base allocation
+    EXPECT_EQ(m.overlayEntriesForTest(), 0u);
+    EXPECT_FALSE(m.contains("a"));                      /// tombstone erased from base
+    ASSERT_TRUE(m.contains("b"));
+    EXPECT_EQ(m.at("b").manifest_ref, (ManifestRef{2, 2, 2}));
+    EXPECT_EQ(m.size(), 1u);                            /// net_delta reset, size still exact
+}
+
+TEST(CASRefCowMap, MaterializeBuildsFreshBaseWhenBaseIsShared)
+{
+    RefCowMap original;
+    original.emplace("a", row(1, 1, 1));
+    original.materialize();
+    const void * shared_base = original.baseIdentityForTest();
+
+    RefCowMap writer = original;              /// shares the base (use_count 2)
+    ASSERT_EQ(writer.baseUseCountForTest(), 2);
+    writer.insert_or_assign("a", row(9, 9, 9));
+    writer.emplace("b", row(9, 9, 2));
+    writer.materialize();                     /// base is shared -> must build a fresh one, mutate nothing shared
+
+    /// Load-bearing correctness pin: the OTHER holder's view is byte-unchanged.
+    EXPECT_EQ(original.baseIdentityForTest(), shared_base);
+    EXPECT_EQ(original.at("a").manifest_ref, (ManifestRef{1, 1, 1}));
+    EXPECT_FALSE(original.contains("b"));
+    EXPECT_EQ(original.size(), 1u);
+
+    /// The writer folded its overlay into a fresh base of its own.
+    EXPECT_NE(writer.baseIdentityForTest(), shared_base);
+    EXPECT_EQ(writer.at("a").manifest_ref, (ManifestRef{9, 9, 9}));
+    EXPECT_TRUE(writer.contains("b"));
+    EXPECT_EQ(writer.size(), 2u);
+}
+
+TEST(CASRefCowMap, MaterializeEmptyOverlayIsANoOpEvenWhenUniquelyOwned)
+{
+    RefCowMap m;
+    m.emplace("a", row(1, 1, 1));
+    m.materialize();
+    ASSERT_EQ(m.baseUseCountForTest(), 1);
+    const void * base_before = m.baseIdentityForTest();
+    m.materialize();   /// overlay already empty: no fold, no reallocation
+    EXPECT_EQ(m.baseIdentityForTest(), base_before);
+    EXPECT_TRUE(m.contains("a"));
+}
+
+TEST(CASRefCowMap, EqualityComparesEffectiveContentsNotInternalLayout)
+{
+    RefCowMap a;
+    a.emplace("x", row(1, 1, 1));
+    a.materialize();   /// "x" lives in `base`
+
+    RefCowMap b;
+    b.emplace("x", row(1, 1, 1));   /// same logical content, but lives entirely in `overlay`
+
+    EXPECT_EQ(a.overlayEntriesForTest(), 0u);
+    EXPECT_GT(b.overlayEntriesForTest(), 0u);
+    EXPECT_TRUE(a == b);
+}
+
+/// ===================================================================================
+/// Copy-on-write isolation + O(1)-copy assertion (spec §Correctness & testing)
+/// ===================================================================================
+
+TEST(CASRefCowMap, CopyIsIsolatedFromOriginal)
+{
+    RefCowMap original;
+    original.emplace("a", row(1, 1, 1));
+    original.materialize();
+
+    RefCowMap copy = original;
+    copy.insert_or_assign("a", row(9, 9, 9));
+    copy.emplace("b", row(9, 9, 9));
+
+    EXPECT_EQ(original.at("a").manifest_ref, (ManifestRef{1, 1, 1}));
+    EXPECT_FALSE(original.contains("b"));
+
+    EXPECT_EQ(copy.at("a").manifest_ref, (ManifestRef{9, 9, 9}));
+    EXPECT_TRUE(copy.contains("b"));
+}
+
+TEST(CASRefCowMap, CopySharesBaseUntilEitherSideMaterializesANewOne)
+{
+    RefCowMap original;
+    original.emplace("a", row(1, 1, 1));
+    original.materialize();
+
+    RefCowMap copy = original;
+    /// A copy shares the SAME base object (refcount bump, no per-row allocation) until a write
+    /// forces a new base into existence via `materialize()` (spec §Mechanism: "Copy = O(1)").
+    EXPECT_EQ(original.baseUseCountForTest(), 2);
+    EXPECT_EQ(copy.baseUseCountForTest(), 2);
+
+    copy.insert_or_assign("a", row(2, 2, 2));   /// writes go to `copy`'s overlay; `base` is untouched
+    EXPECT_EQ(original.baseUseCountForTest(), 2);
+    EXPECT_EQ(copy.baseUseCountForTest(), 2);
+
+    copy.materialize();   /// NOW `copy` points at a fresh base of its own
+    EXPECT_EQ(original.baseUseCountForTest(), 1);
+    EXPECT_EQ(copy.baseUseCountForTest(), 1);
+}
+
+/// ===================================================================================
+/// Randomized exactness property test: RefCowMap must behave IDENTICALLY to
+/// std::map<String, RefCommittedRow> across randomized op sequences (spec §Correctness &
+/// testing: "random op sequences ... including copy-then-mutate isolation ... and
+/// tombstone/override correctness on the merged iterator").
+/// ===================================================================================
+
+TEST(CASRefCowMap, PropertyMatchesStdMapOverRandomOps)
+{
+    std::mt19937 rng(20260717); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+
+    for (int trial = 0; trial < 50; ++trial)
+    {
+        RefCowMap actual;
+        std::map<String, RefCommittedRow> oracle;
+
+        for (int step = 0; step < 200; ++step)
+        {
+            const String key = "ref" + std::to_string(rng() % 12);
+            const uint32_t action = rng() % 6;
+            switch (action)
+            {
+                case 0:   /// emplace
+                {
+                    RefCommittedRow r = row(1, static_cast<uint64_t>(step) + 1, 1);
+                    const bool oracle_inserted = oracle.emplace(key, r).second;
+                    const bool actual_inserted = actual.emplace(key, r).second;
+                    EXPECT_EQ(oracle_inserted, actual_inserted) << "trial " << trial << " step " << step;
+                    break;
+                }
+                case 1:   /// insert_or_assign
+                {
+                    RefCommittedRow r = row(2, static_cast<uint64_t>(step) + 1, 2);
+                    oracle[key] = r;
+                    actual.insert_or_assign(key, r);
+                    break;
+                }
+                case 2:   /// erase by key
+                {
+                    const size_t oracle_erased = oracle.erase(key);
+                    const size_t actual_erased = actual.erase(key);
+                    EXPECT_EQ(oracle_erased, actual_erased) << "trial " << trial << " step " << step;
+                    break;
+                }
+                case 3:   /// find/contains/at (read-only)
+                {
+                    EXPECT_EQ(oracle.contains(key), actual.contains(key)) << "trial " << trial << " step " << step;
+                    if (oracle.contains(key))
+                        EXPECT_EQ(oracle.at(key), actual.at(key)) << "trial " << trial << " step " << step;
+                    break;
+                }
+                case 4:   /// erase via a found iterator
+                {
+                    if (auto it = actual.find(key); it != actual.end())
+                    {
+                        oracle.erase(key);
+                        actual.erase(it);
+                    }
+                    break;
+                }
+                case 5:   /// materialize -- must not change observable content
+                {
+                    actual.materialize();
+                    break;
+                }
+                default:
+                    UNREACHABLE();
+            }
+
+            ASSERT_EQ(oracle.size(), actual.size()) << "trial " << trial << " step " << step;
+
+            auto oit = oracle.begin();
+            auto ait = actual.begin();
+            for (; oit != oracle.end() && ait != actual.end(); ++oit, ++ait)
+            {
+                ASSERT_EQ(oit->first, ait->first) << "trial " << trial << " step " << step;
+                ASSERT_EQ(oit->second, ait->second) << "trial " << trial << " step " << step;
+            }
+            ASSERT_TRUE(oit == oracle.end()) << "trial " << trial << " step " << step;
+            ASSERT_TRUE(ait == actual.end()) << "trial " << trial << " step " << step;
+        }
+    }
+}
+
+/// ===================================================================================
+/// Fast-vs-forced-slow materialize parity (E5 xhigh review): the in-place fold (uniquely-owned base)
+/// and the build-fresh-and-swap fold (a copy still shares the base) must produce IDENTICAL merged
+/// content, size, and empty overlay across randomized op sequences. This pins that the two code paths
+/// -- which handle `net_delta`, tombstones, and overrides differently -- never diverge.
+/// ===================================================================================
+
+TEST(CASRefCowMap, FastAndForcedSlowMaterializeAgreeOverRandomOps)
+{
+    std::mt19937 rng(20260722); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed for reproducible coverage.
+
+    for (int trial = 0; trial < 60; ++trial)
+    {
+        RefCowMap fast;   /// never copied -> `materialize` always takes the in-place (uniquely-owned) path
+        RefCowMap slow;   /// a live copy is held across each `materialize` -> forced fresh-base path
+
+        for (int step = 0; step < 150; ++step)
+        {
+            const String key = "ref" + std::to_string(rng() % 10);
+            switch (rng() % 5)
+            {
+                case 0:
+                {
+                    RefCommittedRow r = row(1, static_cast<uint64_t>(step) + 1, 1);
+                    fast.emplace(key, r);
+                    slow.emplace(key, r);
+                    break;
+                }
+                case 1:
+                {
+                    RefCommittedRow r = row(2, static_cast<uint64_t>(step) + 1, 2);
+                    fast.insert_or_assign(key, r);
+                    slow.insert_or_assign(key, r);
+                    break;
+                }
+                case 2:
+                {
+                    fast.erase(key);
+                    slow.erase(key);
+                    break;
+                }
+                case 3:   /// materialize both, each via its intended path
+                {
+                    ASSERT_EQ(fast.baseUseCountForTest(), 1) << "fast map must be uniquely owned";
+                    fast.materialize();   /// in-place fast path
+                    {
+                        RefCowMap pin = slow;   /// shares slow's base
+                        ASSERT_EQ(slow.baseUseCountForTest(), 2) << "slow map must be forced onto the copy path";
+                        slow.materialize();     /// build-fresh-and-swap slow path
+                    }
+                    EXPECT_EQ(fast.overlayEntriesForTest(), 0u) << "trial " << trial << " step " << step;
+                    EXPECT_EQ(slow.overlayEntriesForTest(), 0u) << "trial " << trial << " step " << step;
+                    break;
+                }
+                default:
+                    break;   /// accumulate overlay without materializing
+            }
+
+            /// Content + size parity holds at EVERY step, materialized or not.
+            ASSERT_EQ(fast.size(), slow.size()) << "trial " << trial << " step " << step;
+            auto fi = fast.begin();
+            auto si = slow.begin();
+            for (; fi != fast.end() && si != slow.end(); ++fi, ++si)
+            {
+                ASSERT_EQ(fi->first, si->first) << "trial " << trial << " step " << step;
+                ASSERT_EQ(fi->second, si->second) << "trial " << trial << " step " << step;
+            }
+            ASSERT_TRUE(fi == fast.end() && si == slow.end()) << "trial " << trial << " step " << step;
+        }
+
+        /// A final materialize of both via their two paths must leave identical, fully-folded state.
+        fast.materialize();
+        {
+            RefCowMap pin = slow;
+            slow.materialize();
+        }
+        EXPECT_EQ(fast.overlayEntriesForTest(), 0u) << "trial " << trial;
+        EXPECT_EQ(slow.overlayEntriesForTest(), 0u) << "trial " << trial;
+        EXPECT_TRUE(fast == slow) << "trial " << trial;
+        EXPECT_EQ(fast.size(), slow.size()) << "trial " << trial;
+    }
+}
diff --git a/src/Disks/tests/gtest_cas_ref_decode_bounds.cpp b/src/Disks/tests/gtest_cas_ref_decode_bounds.cpp
new file mode 100644
index 000000000000..a2301f88bb08
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_decode_bounds.cpp
@@ -0,0 +1,136 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <cstddef>
+#include <string>
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::expectThrowsCode;
+
+/// Stage-1 T11 (spec §3 "Byte limits: encode-side estimation machinery is what dies; the decode-side
+/// cap stays"). Two closures:
+///
+///  1. `openObject`'s raw (uncompressed) arm skipped `object_cap` entirely -- only the zstd arm checked
+///     the declared decompressed content size against it. A tolerated-unknown-field-padded or raw-body
+///     object up to `object_cap` would decode as though it were within budget just because it skipped
+///     compression. Fixed by gating the raw arm on the SAME cap.
+///  2. The writer's post-encode budget check (`checkBudget`, called from `encodeRefLogTxn`) must be a
+///     real `if`+`throw` (CORRUPTED_DATA), never a debug-only `chassert` -- verified here, not
+///     re-implemented (it was already a runtime throw as of stage-1 T8).
+
+namespace
+{
+
+/// A single `SetPublishedAt` op whose `ref_name` is padded so its own encoded size (`encodedOpSize`)
+/// is exactly `target_bytes` -- same construction as `gtest_cas_ref_chunked_flush.cpp`'s helper of
+/// the same shape (not shared: each test file owns its small fixture helpers).
+RefOp paddedSetPublishedAtOp(size_t target_bytes)
+{
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r";
+    op.expected_manifest_ref = ManifestRef{1, 1, 1};
+    op.published_at_ms = 0;
+    const size_t base = encodedOpSize(op);
+    op.ref_name = "r" + String(target_bytes - base, 'a');
+    return op;
+}
+
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// `openObject`: object_cap must gate a raw (uncompressed) body exactly as it gates a zstd frame's
+/// declared content size -- skipping compression must never also skip the size cap.
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASRefDecodeBounds, RawOverCapObjectRejected)
+{
+    const FormatTraits & t = traitsFor(FormatId::RefLog);
+    ASSERT_NE(t.object_cap, 0u);
+
+    /// A raw body strictly larger than the format's object cap. It carries no valid header at all --
+    /// the raw arm returns bytes verbatim (or, once fixed, rejects them by size) before any JSON
+    /// parsing happens, so the content need not be well-formed.
+    const String oversized(t.object_cap + 1, 'x');
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { openObject(FormatId::RefLog, oversized); });
+}
+
+TEST(CASRefDecodeBounds, RawAtCapObjectAccepted)
+{
+    /// The boundary itself must stay legal: exactly `object_cap` bytes, raw, still opens unchanged.
+    const FormatTraits & t = traitsFor(FormatId::RefLog);
+    const String at_cap(t.object_cap, 'x');
+    EXPECT_EQ(openObject(FormatId::RefLog, at_cap), at_cap);
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// `checkBudget` (decode side): the whole-object byte cap is measured over the ACTUAL decoded bytes,
+/// not accumulated per-op, so padding smuggled through a tolerant unknown field is caught exactly like
+/// padding smuggled through an oversized raw body.
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASRefDecodeBounds, PaddedNormalTxnOver20MiBRejected)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r";
+    op.expected_manifest_ref = ManifestRef{1, 1, 1};
+    op.published_at_ms = 1;
+    txn.ops.push_back(op);
+
+    const String text = encodeRefLogTxn(txn);
+    ASSERT_GE(text.size(), 2u);
+    ASSERT_EQ(text[text.size() - 1], '\n');
+    ASSERT_EQ(text[text.size() - 2], '}');
+
+    /// Pad the trailer line with an unknown tolerant field ("zz") -- legal per the wire's evolution
+    /// policy (`skipUnknown`) -- inflating the decoded object well past `ref_txn_max_bytes` without
+    /// touching a single op line or the op count. Padding an op line would only trip the per-op cap
+    /// and prove nothing about this (much larger) whole-transaction bound.
+    constexpr size_t pad_bytes = ref_txn_max_bytes + (1 << 20);
+    String padded = text.substr(0, text.size() - 2);
+    padded += ",\"zz\":\"" + String(pad_bytes, 'A') + "\"}\n"; // NOLINT(modernize-raw-string-literal): mixes '\"' quoting with '\n' line endings across this concatenated literal; a raw string can't hold the newline as-is.
+    ASSERT_GT(padded.size(), ref_txn_max_bytes);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(padded, txn.ns, txn.txn_id); });
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Writer side: the post-encode budget check is a real `if`+`throw`, never a debug-only `chassert` --
+/// a release build must reject an over-cap encode, not silently persist it.
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASRefDecodeBounds, WriterPostEncodeThrowIsRuntime)
+{
+    /// Constructed directly at the codec level -- bypassing the ledger's op-count admission gate
+    /// (`ref_txn_max_ops`) -- so the transaction's total encoded size alone drives the outcome: the
+    /// canonical writer can never reach this state through admission (at most `ref_txn_max_ops` ops at
+    /// `ref_op_max_bytes` each stays under `ref_txn_max_bytes`), but `encodeRefLogTxn`'s own post-encode
+    /// `checkBudget` call must still catch a direct over-cap construction as a real exception, not an
+    /// assert that a release build would silently skip.
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+
+    constexpr size_t op_count = ref_txn_max_bytes / ref_op_max_bytes + 16;
+    txn.ops.reserve(op_count);
+    for (size_t i = 0; i < op_count; ++i)
+        txn.ops.push_back(paddedSetPublishedAtOp(ref_op_max_bytes));
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
diff --git a/src/Disks/tests/gtest_cas_ref_epoch_seal_format.cpp b/src/Disks/tests/gtest_cas_ref_epoch_seal_format.cpp
new file mode 100644
index 000000000000..33d13ca10afa
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_epoch_seal_format.cpp
@@ -0,0 +1,491 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <vector>
+
+/// v3 text codec tests for the `EpochSeal` record kind + strict seal grammar added to `cas_ref_log`
+/// (stage A task 1, spec INV-2). Split into its own file per the plan's "prefer NEW test files"
+/// constraint, rather than extending `gtest_cas_ref_log_format.cpp`. Covers: the new op kind's round
+/// trip (including the meta-line `prev_epoch_seal` field), the context-free structural grammar
+/// (`validateEpochSealGrammarStructural`, run by both `encodeRefLogTxn` and `decodeRefLogTxn`), and
+/// the contextual required-iff rule (`validateEpochSealGrammarContextual`, exercised directly against
+/// explicit `life_epoch` values -- its writer-runtime call sites land in later tasks).
+
+using namespace DB::Cas;
+using DB::Cas::tests::expectThrowsCode;
+
+namespace
+{
+
+RefOp epochSealOp()
+{
+    RefOp op;
+    op.kind = RefOpKind::EpochSeal;
+    return op;
+}
+
+RefOp namespaceBirthOp()
+{
+    RefOp op;
+    op.kind = RefOpKind::NamespaceBirth;
+    return op;
+}
+
+}
+
+/// ===================================================================================
+/// refLogTxnIsEpochSeal / refLogTxnIsRemovalClass classification
+/// ===================================================================================
+
+TEST(CASRefEpochSealFormat, IsEpochSealTrueForSoleSealOp)
+{
+    RefLogTxn txn;
+    txn.ops.push_back(epochSealOp());
+    EXPECT_TRUE(refLogTxnIsEpochSeal(txn));
+}
+
+TEST(CASRefEpochSealFormat, IsEpochSealFalseForSealPlusOtherOp)
+{
+    RefLogTxn txn;
+    txn.ops.push_back(epochSealOp());
+    txn.ops.push_back(namespaceBirthOp());
+    EXPECT_FALSE(refLogTxnIsEpochSeal(txn));
+}
+
+TEST(CASRefEpochSealFormat, IsEpochSealFalseForNonSealOp)
+{
+    RefLogTxn txn;
+    txn.ops.push_back(namespaceBirthOp());
+    EXPECT_FALSE(refLogTxnIsEpochSeal(txn));
+}
+
+TEST(CASRefEpochSealFormat, IsEpochSealFalseForEmptyOps)
+{
+    RefLogTxn txn;
+    EXPECT_FALSE(refLogTxnIsEpochSeal(txn));
+}
+
+/// Step 3's explicit regression note: an `EpochSeal`-only op vector is not removal-class.
+TEST(CASRefEpochSealFormat, RemovalClassIsFalseForEpochSeal)
+{
+    std::vector<RefOp> ops{epochSealOp()};
+    EXPECT_FALSE(refLogTxnIsRemovalClass(ops));
+}
+
+/// ===================================================================================
+/// Round trip
+/// ===================================================================================
+
+TEST(CASRefEpochSealFormat, RoundTripSealAtSequenceOneWithPrevEpochSeal)
+{
+    /// An empty dead epoch (3) closes with a sequence-1 seal, which is therefore itself required to
+    /// carry `prev_epoch_seal` chaining to the seal that closed epoch 2 (spec INV-2's grammar: required
+    /// on exactly sequence 1 of every epoch above genesis).
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{3, 1};
+    txn.prev_epoch_seal = RefTxnId{2, 9};
+    txn.ops.push_back(epochSealOp());
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+    ASSERT_TRUE(decoded.prev_epoch_seal.has_value());
+    EXPECT_EQ(*decoded.prev_epoch_seal, (RefTxnId{2, 9}));
+    ASSERT_EQ(decoded.ops.size(), 1u);
+    EXPECT_EQ(decoded.ops[0].kind, RefOpKind::EpochSeal);
+}
+
+TEST(CASRefEpochSealFormat, RoundTripSealWithoutPrevEpochSeal)
+{
+    /// The common case: epoch 2 had real records (greatest applied sequence 5), so its closing seal
+    /// lands at sequence 6 -- not sequence 1 -- and therefore must NOT carry `prev_epoch_seal`.
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{2, 6};
+    txn.ops.push_back(epochSealOp());
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+    EXPECT_FALSE(decoded.prev_epoch_seal.has_value());
+}
+
+/// A re-encode of a decoded seal transaction is byte-identical (the encoder is a pure function of the
+/// txn), matching the pin `gtest_cas_ref_log_format.cpp` keeps for the other op kinds.
+TEST(CASRefEpochSealFormat, ByteIdenticalReencodeWithPrevEpochSeal)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{3, 1};
+    txn.prev_epoch_seal = RefTxnId{2, 9};
+    txn.ops.push_back(epochSealOp());
+
+    const String bytes1 = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes1, txn.ns, txn.txn_id);
+    const String bytes2 = encodeRefLogTxn(decoded);
+    EXPECT_EQ(bytes1, bytes2);
+}
+
+/// ===================================================================================
+/// Structural grammar (validateEpochSealGrammarStructural, via encode/decode -- context-free)
+/// ===================================================================================
+
+TEST(CASRefEpochSealFormat, EncodeRejectsSealTxnWithTwoSealOps)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    txn.ops.push_back(epochSealOp());
+    txn.ops.push_back(epochSealOp());
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefEpochSealFormat, EncodeRejectsSealTxnWithSecondNonSealOp)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    txn.ops.push_back(epochSealOp());
+    txn.ops.push_back(namespaceBirthOp());
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+/// Decode-side pin for the same op-count rule (review finding I1): `encodeRefLogTxn` can never
+/// produce a 2-op seal body, so only a decode-only splice proves `decodeRefLogTxn` independently
+/// re-derives the rule rather than trusting whatever the encoder produced -- deleting the structural
+/// validator's call site inside `decodeRefLogTxn` would leave this the only failing test.
+TEST(CASRefEpochSealFormat, DecodeRejectsSealTxnWithTwoOpsSpliced)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{2, 6};
+    txn.ops.push_back(epochSealOp());
+    const String bytes = encodeRefLogTxn(txn);
+
+    const String op_line = "{\"op\":\"epoch_seal\"}\n";
+    const auto op_pos = bytes.find(op_line);
+    ASSERT_NE(op_pos, String::npos);
+    String tampered = bytes;
+    tampered.insert(op_pos, op_line);   /// two consecutive "epoch_seal" op lines now
+
+    const String old_trailer = "{\"n\":1}\n";
+    const auto trailer_pos = tampered.find(old_trailer);
+    ASSERT_NE(trailer_pos, String::npos);
+    tampered.replace(trailer_pos, old_trailer.size(), "{\"n\":2}\n");   /// keep the trailer honest
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+/// Decode-side pin for the same op-count rule, with a DIFFERENT second op kind -- proves the rule
+/// rejects any companion op, not just a second `epoch_seal`.
+TEST(CASRefEpochSealFormat, DecodeRejectsSealTxnWithSecondNonSealOpSpliced)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{2, 6};
+    txn.ops.push_back(epochSealOp());
+    const String bytes = encodeRefLogTxn(txn);
+
+    const String op_line = "{\"op\":\"epoch_seal\"}\n";
+    const auto op_pos = bytes.find(op_line);
+    ASSERT_NE(op_pos, String::npos);
+    String tampered = bytes;
+    tampered.insert(op_pos + op_line.size(), "{\"op\":\"namespace_birth\"}\n");
+
+    const String old_trailer = "{\"n\":1}\n";
+    const auto trailer_pos = tampered.find(old_trailer);
+    ASSERT_NE(trailer_pos, String::npos);
+    tampered.replace(trailer_pos, old_trailer.size(), "{\"n\":2}\n");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+TEST(CASRefEpochSealFormat, EncodeRejectsPrevEpochSealAtNonUnitSequence)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 2};
+    txn.prev_epoch_seal = RefTxnId{1, 1};
+    txn.ops.push_back(namespaceBirthOp());
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+/// Decode-side pin for the sequence-1-only rule (review finding I1). `prev_epoch_seal`'s
+/// writer_epoch (1) is strictly below the transaction's own (5), satisfying the I3 chain-direction
+/// rule, so this isolates the sequence-1 rule specifically rather than incidentally also tripping I3.
+TEST(CASRefEpochSealFormat, DecodeRejectsPrevEpochSealAtNonUnitSequenceSpliced)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{5, 2};
+    txn.ops.push_back(namespaceBirthOp());
+    const String bytes = encodeRefLogTxn(txn);
+
+    const String needle = R"("rs":"2")";
+    const auto pos = bytes.find(needle);
+    ASSERT_NE(pos, String::npos);
+    String tampered = bytes;
+    tampered.insert(pos + needle.size(), R"(,"!pse":"1","!pss":"1")");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+/// Well-formedness (review finding M2): a zero component inside `prev_epoch_seal` is rejected the
+/// same way a zero component in the primary `txn_id` is (`checkRefTxnIdNonzero`, shared code path).
+TEST(CASRefEpochSealFormat, EncodeRejectsPrevEpochSealWithZeroWriterEpoch)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{3, 1};
+    txn.prev_epoch_seal = RefTxnId{0, 9};
+    txn.ops.push_back(epochSealOp());
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefEpochSealFormat, EncodeRejectsPrevEpochSealWithZeroRefSequence)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{3, 1};
+    txn.prev_epoch_seal = RefTxnId{2, 0};
+    txn.ops.push_back(epochSealOp());
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+/// Decode-side splice: `prev_epoch_seal` present as only one of its two wire fields ("!pse" without
+/// "!pss") -- a shape only reachable via corrupted bytes, since the encoder always writes both
+/// together. Boundary-plus-one for the additive-field decode contract (Constraint 7).
+TEST(CASRefEpochSealFormat, DecodeRejectsPrevEpochSealMissingPssComponent)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{3, 1};
+    txn.prev_epoch_seal = RefTxnId{2, 9};
+    txn.ops.push_back(epochSealOp());
+    const String bytes = encodeRefLogTxn(txn);
+
+    const String needle = R"(,"!pss":"9")";
+    const auto pos = bytes.find(needle);
+    ASSERT_NE(pos, String::npos);
+    String tampered = bytes;
+    tampered.erase(pos, needle.size());
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+/// Chain direction (review finding I3): a seal closing epoch E always has id `{E, T+1}`, and the
+/// sequence-1 transaction in the next numeric epoch must name it. This remains context-free (a
+/// property of one transaction), so it belongs in the structural half; Tasks 2/6 walk this pointer
+/// backwards over untrusted decoded bodies and must not have to re-derive the rule themselves.
+TEST(CASRefEpochSealFormat, EncodeRejectsPrevEpochSealPointingAtSameEpoch)
+{
+    /// Self-pointer: prev_epoch_seal names the SAME epoch this transaction is in.
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{5, 1};
+    txn.prev_epoch_seal = RefTxnId{5, 3};
+    txn.ops.push_back(epochSealOp());
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefEpochSealFormat, EncodeRejectsPrevEpochSealPointingAtFutureEpoch)
+{
+    /// Forward-pointer: prev_epoch_seal names an epoch AFTER this transaction's own.
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{5, 1};
+    txn.prev_epoch_seal = RefTxnId{9, 3};
+    txn.ops.push_back(epochSealOp());
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+/// INV-2 materializes every global writer epoch for an existing life.  A sequence-1 transaction in
+/// epoch E therefore chains to the seal of exactly E-1: accepting an older link would make an omitted
+/// epoch look like a proved boundary and let a fold bypass its missing seal.
+TEST(CASRefEpochSealFormat, EncodeRejectsPrevEpochSealSkippingImmediateEpoch)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{5, 1};
+    txn.prev_epoch_seal = RefTxnId{3, 7};
+    txn.ops.push_back(epochSealOp());
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+/// A damaged object bypasses the encoder, so the decoder must independently reject the same skipped
+/// link before any GC or recovery walker can treat it as boundary evidence.
+TEST(CASRefEpochSealFormat, DecodeRejectsPrevEpochSealSkippingImmediateEpochSpliced)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{5, 1};
+    txn.ops.push_back(namespaceBirthOp());
+    const String bytes = encodeRefLogTxn(txn);
+
+    const String needle = R"("rs":"1")";
+    const auto pos = bytes.find(needle);
+    ASSERT_NE(pos, String::npos);
+    String tampered = bytes;
+    tampered.insert(pos + needle.size(), R"(,"!pse":"3","!pss":"1")");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+/// Decode-side pin for the chain-direction rule (review finding I3): the encoder's own check would
+/// refuse to produce this shape (the two Encode* tests above pin that direction), so a splice into an
+/// otherwise-valid sequence-1 body proves decode re-derives the rule independently.
+TEST(CASRefEpochSealFormat, DecodeRejectsPrevEpochSealPointingAtSameOrFutureEpochSpliced)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{5, 1};
+    txn.ops.push_back(namespaceBirthOp());
+    const String bytes = encodeRefLogTxn(txn);   /// valid: sequence 1, no prev_epoch_seal
+
+    const String needle = R"("rs":"1")";
+    const auto pos = bytes.find(needle);
+    ASSERT_NE(pos, String::npos);
+    String tampered = bytes;
+    tampered.insert(pos + needle.size(), R"(,"!pse":"5","!pss":"1")");   /// self-pointer
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+/// ===================================================================================
+/// Contextual grammar (validateEpochSealGrammarContextual, called directly against explicit
+/// life_epoch values -- the writer-runtime call sites are wired by later tasks)
+/// ===================================================================================
+
+TEST(CASRefEpochSealFormat, ContextualRejectsMissingPrevEpochSealWhenRequired)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{3, 1};
+    txn.ops.push_back(namespaceBirthOp());
+    /// life_epoch 1 < writer_epoch 3: a sequence-1 txn above genesis MUST carry prev_epoch_seal.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { validateEpochSealGrammarContextual(txn, /*life_epoch=*/1); });
+}
+
+TEST(CASRefEpochSealFormat, ContextualRejectsPrevEpochSealWhenForbidden)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{5, 1};
+    txn.prev_epoch_seal = RefTxnId{4, 3};
+    txn.ops.push_back(namespaceBirthOp());
+    /// life_epoch == writer_epoch == 5: this IS the namespace's genesis sequence-1 txn, so
+    /// prev_epoch_seal is forbidden -- there is no preceding epoch to chain to.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { validateEpochSealGrammarContextual(txn, /*life_epoch=*/5); });
+}
+
+/// codex r2 finding 2: "genesis" is per-namespace. A namespace first born at global epoch 5 (not
+/// epoch 1) appends {5, 1} with NO prev_epoch_seal -- that IS its genesis, not a transition.
+TEST(CASRefEpochSealFormat, ContextualAllowsGenesisBirthAboveEpochOneWithoutPrevEpochSeal)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{5, 1};
+    txn.ops.push_back(namespaceBirthOp());
+    EXPECT_NO_THROW(validateEpochSealGrammarContextual(txn, /*life_epoch=*/5));
+}
+
+/// Review finding I2: the `ref_sequence != 1` early return is load-bearing for Task 4's encode call
+/// site, which calls this on every txn it mints, including ordinary sequence->=2 transactions in a
+/// post-transition epoch that legitimately carry no `prev_epoch_seal`. Pinned on both sides of the
+/// life_epoch relation to prove the early return fires regardless of it.
+TEST(CASRefEpochSealFormat, ContextualPassesThroughNonSequenceOneAboveLifeEpoch)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{3, 5};
+    txn.ops.push_back(namespaceBirthOp());
+    /// writer_epoch(3) > life_epoch(1): would be REQUIRED if this were sequence 1.
+    EXPECT_NO_THROW(validateEpochSealGrammarContextual(txn, /*life_epoch=*/1));
+}
+
+TEST(CASRefEpochSealFormat, ContextualPassesThroughNonSequenceOneAtOrBelowLifeEpoch)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 5};
+    txn.ops.push_back(namespaceBirthOp());
+    /// writer_epoch(1) == life_epoch(1): would be FORBIDDEN-if-present if this were sequence 1.
+    EXPECT_NO_THROW(validateEpochSealGrammarContextual(txn, /*life_epoch=*/1));
+}
+
+/// ===================================================================================
+/// Criticality of the prev_epoch_seal wire fields (review finding M4)
+/// ===================================================================================
+
+/// `!pse`/`!pss` are `!`-prefixed CRITICAL keys: `prev_epoch_seal` is INV-2 chain evidence, and a
+/// build that silently dropped it would still pass the structural grammar (absent field => no check)
+/// while losing the chain link. Proven here by splicing in a DIFFERENT, genuinely-unrecognized
+/// `!`-key (simulating a future critical field this build predates) rather than `!pse`/`!pss`
+/// themselves, which this build DOES recognize: `JsonObjectReader::skipUnknown` rejects any
+/// unrecognized `!`-prefixed key with `UNKNOWN_FORMAT_VERSION` (never a silent skip), so this pins
+/// the general mechanism the meta-line reader relies on to keep `!pse`/`!pss` safe against a decoder
+/// that doesn't (yet, or anymore) understand them.
+TEST(CASRefEpochSealFormat, DecodeRejectsUnknownCriticalKeyInMetaLine)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    txn.ops.push_back(namespaceBirthOp());
+    const String bytes = encodeRefLogTxn(txn);
+
+    const String needle = R"("rs":"1")";
+    const auto pos = bytes.find(needle);
+    ASSERT_NE(pos, String::npos);
+    String tampered = bytes;
+    tampered.insert(pos + needle.size(), R"(,"!future_critical_field":"1")");
+
+    expectThrowsCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+/// ===================================================================================
+/// Regression guard: existing unknown-op-word behavior stays intact after adding "epoch_seal"
+/// ===================================================================================
+
+TEST(CASRefEpochSealFormat, DecodeRejectsUnknownOpWordRegressionGuard)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    txn.ops.push_back(epochSealOp());
+    const String bytes = encodeRefLogTxn(txn);
+
+    const String needle = "\"epoch_seal\"";
+    const auto pos = bytes.find(needle);
+    ASSERT_NE(pos, String::npos);
+    String tampered = bytes;
+    tampered.replace(pos, needle.size(), "\"totally_bogus_op\"");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+/// ===================================================================================
+/// Shape-level failure-mode battery (truncation / v+1 gate / wrong type / leading garbage)
+/// ===================================================================================
+
+TEST(CASRefEpochSealFormat, FormatBatteryEpochSeal)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{3, 1};
+    txn.prev_epoch_seal = RefTxnId{2, 9};
+    txn.ops.push_back(epochSealOp());
+
+    const String ns = txn.ns;
+    const RefTxnId id = txn.txn_id;
+    runFormatBattery({FormatId::RefLog,
+        [txn] { return sealObject(FormatId::RefLog, encodeRefLogTxn(txn)); },
+        [ns, id](std::string_view s) { decodeRefLogTxn(openObject(FormatId::RefLog, s), ns, id); },
+        "{\"type\":\"cas_ref_log\",\"v\":9}\n"
+        "{\"ns\":\"ns\",\"we\":\"3\",\"rs\":\"1\",\"!pse\":\"2\",\"!pss\":\"9\"}\n"
+        "{\"op\":\"epoch_seal\"}\n"
+        "{\"n\":1}\n"});
+}
diff --git a/src/Disks/tests/gtest_cas_ref_gc.cpp b/src/Disks/tests/gtest_cas_ref_gc.cpp
new file mode 100644
index 000000000000..97d637982af6
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_gc.cpp
@@ -0,0 +1,987 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include "cas_test_helpers.h"
+
+#include <Common/ProfileEvents.h>
+
+#include <set>
+
+/// Task 12 required GC tests over the snapshot+log ref model (spec 2026-07-11-cas-ref-table-snapshot-log-design).
+/// Every fixture produces REAL wire-format ref logs (via the writer or `writeRefLogTxnRaw`, never hand-rolled
+/// bytes), and every test proves the fold actually consumed them (cursor advanced / nonzero in-degree), so a
+/// silent no-op fold cannot pass vacuously.
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASRefGlobalListPages;
+extern const Event CASRefLogBodyGets;
+extern const Event CASRefManifestBodyFoldGets;
+extern const Event CASRefEmittedEdges;
+extern const Event CASRefCleanupObjectsDeleted;
+}
+
+namespace
+{
+const UInt128 kGc = hexToU128("00000000000000000000000000000001");
+const UInt128 kGc2 = hexToU128("00000000000000000000000000000002");
+
+ManifestRef mref(uint64_t seq, uint32_t ord = 1)
+{
+    return ManifestRef{.writer_epoch = 1, .build_sequence = seq, .manifest_ordinal = ord};
+}
+
+/// Append a committed-ref log at an EXPLICIT sequence (no per-call LIST) -- fast bulk seeding of a
+/// >1000-key stream. The ops are replay-valid (birth on the first, then add-precommit + promote).
+void seedCommittedAt(
+    Backend & backend, const Layout & layout, const RootNamespace & ns, uint64_t seq,
+    const String & ref_name, const ManifestRef & mr, bool birth)
+{
+    std::vector<RefOp> ops;
+    if (birth)
+        ops.push_back(namespaceBirthOp());
+    const std::vector<RefOp> commit_ops = publishCommittedOps(ref_name, mr);
+    ops.insert(ops.end(), commit_ops.begin(), commit_ops.end());
+    RefLogTxn txn;
+    txn.ns = ns.string();
+    txn.txn_id = RefTxnId{1, seq};
+    txn.ops = std::move(ops);
+    fixture::writeRefLogRaw(backend, layout, txn);
+}
+
+/// Drive regular rounds, renewing the mount ack after each, until quiescent or `max_rounds`.
+size_t runToFixpoint(const PoolPtr & s, Gc & gc, size_t max_rounds = 64)
+{
+    size_t rounds = 0;
+    for (; rounds < max_rounds; ++rounds)
+    {
+        const RoundReport rep = runRegularRoundReclaiming(gc);
+        if (!rep.acquired_lease)
+            continue;
+        s->renewWatermarkOnce();
+        const bool no_work = rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0;
+        if (no_work && !anyCondemnedInSeal(s->backend(), s->layout()))
+            break;
+    }
+    return rounds;
+}
+
+bool blobPresent(Backend & b, const Layout & layout, const UInt128 & hash)
+{
+    return b.head(layout.blobKey(BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hash)})).exists;
+}
+
+/// Denies ONCE the single round-commit `gc/state` CAS that advances `snap_generation` (the losing
+/// leader deposed mid-round). The denied round leaves only never-adopted attempt-scoped debris.
+class DeposeRoundCommitBackend : public InMemoryBackend
+{
+public:
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        if (arm && key == "p/gc/state")
+        {
+            const auto stored = get(key);
+            const uint64_t stored_gen = stored ? decodeGcState(stored->bytes).snap_generation : 0;
+            if (decodeGcState(bytes).snap_generation > stored_gen)
+            {
+                arm = false;
+                throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+                    "test-injected: round-commit gc/state CAS denied (losing leader deposed mid-round)");
+            }
+        }
+        return InMemoryBackend::casPut(key, bytes, expected, meta);
+    }
+    bool arm = false;
+};
+
+/// Moves one of the two authorities `cleanupRefObjects` must revalidate at a precise ref-log delete
+/// boundary. The target object's own token is untouched, so only an authority check can refuse it.
+class RefCleanupAuthorityRaceBackend : public CountingBackend
+{
+public:
+    enum class Authority : uint8_t
+    {
+        Catalog,
+        GcFence,
+    };
+
+    enum class Timing : uint8_t
+    {
+        BeforeFirstDelete,
+        AfterFirstDelete,
+    };
+
+    void arm(
+        Authority authority_, Timing timing_, const Layout & layout,
+        const String & first_cleanup_key_)
+    {
+        authority = authority_;
+        timing = timing_;
+        catalog_key = layout.refCatalogKey();
+        gc_state_key = layout.gcStateKey();
+        first_cleanup_key = first_cleanup_key_;
+        armed = true;
+    }
+
+    HeadResult head(const String & key) override
+    {
+        HeadResult result = CountingBackend::head(key);
+        if (armed && timing == Timing::BeforeFirstDelete && key == first_cleanup_key)
+            moveAuthority();
+        return result;
+    }
+
+    DeleteOutcome deleteExact(const String & key, const Token & token) override
+    {
+        DeleteOutcome result = CountingBackend::deleteExact(key, token);
+        if (armed && timing == Timing::AfterFirstDelete && key == first_cleanup_key)
+            moveAuthority();
+        return result;
+    }
+
+private:
+    void moveAuthority()
+    {
+        armed = false;
+        const String & key = authority == Authority::Catalog ? catalog_key : gc_state_key;
+        const auto got = CountingBackend::get(key);
+        if (!got)
+            throw std::runtime_error("test-injected cleanup authority object is absent");
+
+        String bytes = got->bytes;
+        if (authority == Authority::GcFence)
+        {
+            GcState moved = decodeGcState(bytes);
+            ++moved.lease.seq;
+            bytes = encodeGcState(moved);
+        }
+        if (CountingBackend::casPut(key, bytes, got->token).outcome != CasOutcome::Committed)
+            throw std::runtime_error("test-injected cleanup authority move lost its CAS");
+    }
+
+    Authority authority = Authority::Catalog;
+    Timing timing = Timing::BeforeFirstDelete;
+    String catalog_key;
+    String gc_state_key;
+    String first_cleanup_key;
+    bool armed = false;
+};
+
+struct RefCleanupFixture
+{
+    String first_log_key;
+    String second_log_key;
+};
+
+RefCleanupFixture seedTwoCoveredLogs(
+    RefCleanupAuthorityRaceBackend & backend, const Layout & layout,
+    const RootNamespace & ns)
+{
+    fixture::admitLive(backend, layout, ns);
+    const ManifestRef r1 = mref(1);
+    const ManifestRef r2 = mref(2);
+    const ManifestRef r3 = mref(3);
+    writeManifestRaw(backend, layout, ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(backend, layout, ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    writeManifestRaw(backend, layout, ns, r3, {blobEntryFor("c", DB::UInt128(3))});
+    const uint64_t v1 = publishCommittedTransition(backend, layout, ns, "t1", std::nullopt, r1);
+    const uint64_t v2 = publishCommittedTransition(backend, layout, ns, "t2", std::nullopt, r2);
+    const uint64_t v3 = publishCommittedTransition(backend, layout, ns, "t3", std::nullopt, r3);
+    writeRefSnapshotRaw(backend, layout,
+        minimalLiveSnapshot(ns.string(), RefTxnId{1, v3},
+            {committedRow("t1", r1), committedRow("t2", r2), committedRow("t3", r3)}));
+    replaceRecoverableCkptForRawFixture(backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, v3},
+        .checkpoint_snapshot_id = RefTxnId{1, v3},
+        .last_epoch_seal = std::nullopt,
+    });
+    const NamespaceLifeId life = fixture::fixtureLife(ns);
+    return {
+        .first_log_key = layout.refLogKey(life, RefTxnId{1, v1}),
+        .second_log_key = layout.refLogKey(life, RefTxnId{1, v2})};
+}
+}
+
+/// (1) A >1000-key ref scan folds every pre-existing log exactly once: the cursor advances to the greatest
+/// id and every referenced blob has in-degree exactly 1 (folded once, not skipped, not doubled).
+TEST(CASRefGc, LargeRefScanFoldsEveryLogExactlyOnce)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    constexpr uint64_t N = 1200;   /// > 1000: forces multi-page LIST paging in the fold's global scan
+    for (uint64_t i = 1; i <= N; ++i)
+    {
+        const ManifestRef mr = mref(i);
+        writeManifestRaw(*backend, layout, ns, mr, {blobEntryFor("data", DB::UInt128(i))});
+        seedCommittedAt(*backend, layout, ns, /*seq*/ i, "t" + std::to_string(i), mr, /*birth*/ i == 1);
+    }
+    writeRecoverableCkptForRawFixture(
+        *backend, layout, ns,
+        RefCkpt{.life_epoch = 1, .committed_through = RefTxnId{1, N},
+                .checkpoint_snapshot_id = std::nullopt, .last_epoch_seal = std::nullopt});
+
+    Gc gc(store, kGc);
+    ASSERT_NO_THROW(gc.runRegularRound());
+
+    /// The durable cursor advanced to the greatest log id.
+    EXPECT_EQ(foldCursorOf(*backend, layout, ns, 0), N)
+        << "the fold must advance the per-table cursor to the greatest pre-existing log id";
+
+    /// Every referenced blob folded EXACTLY once (in-degree 1). Spot-check a spread across the >1000 set.
+    for (uint64_t i : {uint64_t{1}, uint64_t{2}, uint64_t{999}, uint64_t{1000}, uint64_t{1001}, N})
+        EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(i)), 1)
+            << "blob " << i << " must be folded exactly once (not skipped, not doubled)";
+}
+
+/// (2) A concurrent log appended AFTER the round's scan has passed its table is NOT skipped: the sealed
+/// cursor stays below it, and the next round folds it.
+TEST(CASRefGc, ConcurrentLogAfterScanIsFoldedNextRound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r1 = mref(1);
+    writeManifestRaw(*backend, layout, ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    const uint64_t v1 = publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r1);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();   /// round 1 folds v1
+    ASSERT_EQ(foldCursorOf(*backend, layout, ns, 0), v1);
+    ASSERT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 1);
+
+    /// A NEW log lands after the round sealed its cursor at v1 (a concurrent writer).
+    const ManifestRef r2 = mref(2);
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    const uint64_t v2 = publishCommittedTransition(*backend, layout, ns, "tbl2", std::nullopt, r2);
+    ASSERT_GT(v2, v1);
+
+    /// The sealed cursor is still v1 (< v2) -- the new log was never skipped past.
+    EXPECT_EQ(foldCursorOf(*backend, layout, ns, 0), v1)
+        << "a log that landed after the scan must remain below the durable cursor, never skipped";
+
+    gc.runRegularRound();   /// round 2 folds v2
+    EXPECT_EQ(foldCursorOf(*backend, layout, ns, 0), v2);
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(2)), 1)
+        << "the next round must fold the concurrently-appended log";
+}
+
+/// (3) Fold barrier: a live precommit whose manifest body is absent clamps the table cursor below its
+/// log (an anomaly is recorded), then folds once the body appears.
+TEST(CASRefGc, FoldBarrierClampsBelowMissingBodyThenFoldsOnAppear)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef pre = mref(7);
+    /// No writeManifestRaw for `pre`: its body is intentionally absent (the live precommit's barrier).
+    const uint64_t v = addPrecommitTransition(*backend, layout, ns, DB::UInt128(9), "part", std::nullopt, pre);
+
+    Gc gc(store, kGc);
+    RoundReport report;
+    ASSERT_NO_THROW(report = gc.runRegularRound());
+    EXPECT_TRUE(report.hasAnomaly(ns, /*shard*/0)) << "a missing live-precommit body must record an anomaly";
+    EXPECT_LT(foldCursorOf(*backend, layout, ns, 0), v)
+        << "the barrier must clamp the durable cursor BELOW the bodiless-precommit log";
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 0);
+
+    /// The body appears (the build finished staging): the next fold passes the barrier.
+    writeManifestRaw(*backend, layout, ns, pre, {blobEntryFor("p", DB::UInt128(1))});
+    gc.runRegularRound();
+    EXPECT_GE(foldCursorOf(*backend, layout, ns, 0), v) << "the barrier lifts once the body lands";
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 1);
+}
+
+/// (4) Edge cancellation: a manifest added then removed across a batch nets to zero in-degree and the
+/// exclusively-owned blob is reclaimed.
+TEST(CASRefGc, EdgeCancellationAddThenRemoveReclaimsBlob)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r = mref(1);
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeManifestRaw(*backend, layout, ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r);   /// +1 for r's blob
+    dropRefTransition(*backend, layout, ns, "tbl", r);                          /// -1: the add is cancelled
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runToFixpoint(store, gc) < 64u) << "the add+remove batch must converge to a fixpoint";
+
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 0)
+        << "an added-then-removed manifest nets to zero in-degree";
+    EXPECT_FALSE(blobPresent(*backend, layout, DB::UInt128(1)))
+        << "the net-zero blob is reclaimed";
+}
+
+/// (5) A losing generation commit adopts nothing and deletes nothing: a round whose single round-commit
+/// `gc/state` CAS is denied (deposed mid-round) must NOT advance the adopted (snap_generation, snap_attempt)
+/// and must NOT delete the condemned-but-unadopted blob. Its fold seal is durable only under its OWN
+/// never-adopted attempt (harmless debris).
+TEST(CASRefGc, LosingGenerationCommitAdoptsNothingDeletesNothing)
+{
+    auto backend = std::make_shared<DeposeRoundCommitBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r = mref(1);
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeManifestRaw(*backend, layout, ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    gc.runRegularRound();   /// round 1: folds the +1 and adopts it cleanly
+    store->renewWatermarkOnce();
+    const auto adopted = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    ASSERT_GT(adopted.snap_generation, 0u);
+
+    /// Drop the ref, then run the round whose commit is DENIED (losing leader).
+    dropRefTransition(*backend, layout, ns, "tbl", r);
+    backend->arm = true;
+    EXPECT_ANY_THROW(gc.runRegularRound());
+    backend->arm = false;
+
+    /// The deposed round adopted NOTHING: the durable pointers are unchanged...
+    const auto after = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    EXPECT_EQ(after.snap_generation, adopted.snap_generation)
+        << "a denied round-commit CAS must not advance the adopted generation";
+    EXPECT_EQ(after.snap_attempt, adopted.snap_attempt);
+    /// ...and it deleted NOTHING: the blob its unadopted fold condemned is still present.
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(1)))
+        << "a losing generation commit must never delete a blob against an unadopted fold";
+}
+
+/// (6) Ref-object cleanup trusts only a checkpoint-named recovery triple: an older `_log` and `_snap`
+/// are deleted after the durable cursor reaches them, while that triple remains intact.
+TEST(CASRefGc, RefObjectCleanupRetainsCheckpointNamedTriple)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    /// Two committed publishes -> logs {1,1} and {1,2}.
+    const ManifestRef r1 = mref(1);
+    const ManifestRef r2 = mref(2);
+    writeManifestRaw(*backend, layout, ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    const uint64_t v1 = publishCommittedTransition(*backend, layout, ns, "t1", std::nullopt, r1);
+    const uint64_t v2 = publishCommittedTransition(*backend, layout, ns, "t2", std::nullopt, r2);
+
+    /// Two observed snapshots: an OLD one covering only v1, and the NEWEST covering v2. Both are real
+    /// wire-format snapshot objects (the recovery codec reads them).
+    RefTableSnapshot old_snap = minimalLiveSnapshot(ns.string(), RefTxnId{1, v1},
+        {committedRow("t1", r1)});
+    RefTableSnapshot new_snap = minimalLiveSnapshot(ns.string(), RefTxnId{1, v2},
+        {committedRow("t1", r1), committedRow("t2", r2)});
+    writeRefSnapshotRaw(*backend, layout, old_snap);
+    writeRefSnapshotRaw(*backend, layout, new_snap);
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, v2},
+        .checkpoint_snapshot_id = RefTxnId{1, v2},
+        .last_epoch_seal = std::nullopt,
+    });
+
+    const String log_v1_key = layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, v1});
+    const String log_v2_key = layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, v2});
+    const String old_snap_key = layout.refSnapshotKey(fixture::fixtureLife(ns), RefTxnId{1, v1});
+    const String new_snap_key = layout.refSnapshotKey(fixture::fixtureLife(ns), RefTxnId{1, v2});
+    ASSERT_TRUE(backend->head(log_v1_key).exists);
+    ASSERT_TRUE(backend->head(log_v2_key).exists);
+    ASSERT_TRUE(backend->head(old_snap_key).exists);
+
+    Gc gc(store, kGc);
+    runToFixpoint(store, gc);   /// folds v1,v2 (cursor -> v2) then cleans covered ref objects post-CAS
+
+    /// The old log lies below both the durable cursor and the validated checkpoint base => DELETED.
+    EXPECT_FALSE(backend->head(log_v1_key).exists)
+        << "a log below the checkpoint-named snapshot base and durable cursor must be deleted";
+    /// The same-id ordinary log is part of recovery's triple and must survive.
+    EXPECT_TRUE(backend->head(log_v2_key).exists)
+        << "the checkpoint-named non-seal log must survive with its snapshot";
+    /// The older snapshot is deleted; the checkpoint-named snapshot is retained.
+    EXPECT_FALSE(backend->head(old_snap_key).exists) << "an older snapshot must be deleted";
+    EXPECT_TRUE(backend->head(new_snap_key).exists) << "the checkpoint-named snapshot must be retained";
+}
+
+/// `cleanupRefObjects`'s per-round cap. Five deletable logs share one
+/// namespace with a tiny `gc_round_ref_cleanup_budget`; the per-key fail-close validation
+/// (`deleteRefObject`'s catalog/lease revalidation before every exact delete) is untouched -- it is
+/// NOT amortized, only the cohort size per round is capped. `planRefCleanup` recomputes the same
+/// remaining candidates from durable state every round, so the excess needs no cursor of its own.
+TEST(CASRefGc, RefObjectCleanupRespectsRoundBudgetAndConvergesAcrossRounds)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .gc_round_ref_cleanup_budget = 1,
+        .gc_fold_max_defer_rounds = 0});
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+
+    /// Six sequential replacements of the SAME ref -> six committed logs {1,1}..{1,6}.
+    constexpr int kLogs = 6;
+    std::optional<ManifestRef> prev;
+    ManifestRef latest{};
+    uint64_t last_seq = 0;
+    for (int i = 1; i <= kLogs; ++i)
+    {
+        const ManifestRef r = mref(i);
+        writeManifestRaw(*backend, layout, ns, r, {blobEntryFor("a" + std::to_string(i), DB::UInt128(static_cast<uint64_t>(i)))});
+        last_seq = publishCommittedTransition(*backend, layout, ns, "t", prev, r);
+        prev = r;
+        latest = r;
+    }
+
+    /// A snapshot + checkpoint naming the LATEST row: every earlier log is below the checkpoint base.
+    RefTableSnapshot snap = minimalLiveSnapshot(ns.string(), RefTxnId{1, last_seq}, {committedRow("t", latest)});
+    writeRefSnapshotRaw(*backend, layout, snap);
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, last_seq},
+        .checkpoint_snapshot_id = RefTxnId{1, last_seq},
+        .last_epoch_seal = std::nullopt,
+    });
+
+    std::vector<String> deletable_log_keys;
+    for (int i = 1; i < kLogs; ++i)   /// {1,1}..{1,5}: strictly below the checkpoint base, hence deletable
+        deletable_log_keys.push_back(layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, static_cast<uint64_t>(i)}));
+
+    Gc gc(store, kGc);
+    auto countSurviving = [&]
+    {
+        size_t n = 0;
+        for (const String & k : deletable_log_keys)
+            if (backend->head(k).exists)
+                ++n;
+        return n;
+    };
+    ASSERT_EQ(countSurviving(), deletable_log_keys.size())
+        << "nothing cleaned before the first round even runs";
+
+    /// The SAME round that folds the whole tail also runs post-CAS cleanup, and with
+    /// `gc_round_ref_cleanup_budget = 1` deletes exactly one of the five deletable candidates.
+    runRegularRoundReclaiming(gc);
+    EXPECT_EQ(countSurviving(), deletable_log_keys.size() - 1)
+        << "a round with gc_round_ref_cleanup_budget=1 must delete exactly one ref object";
+
+    /// Repeated budgeted rounds converge: the whole deletable tail eventually drains, none stranded.
+    for (int i = 0; i < 10 && countSurviving() > 0; ++i)
+        runRegularRoundReclaiming(gc);
+    EXPECT_EQ(countSurviving(), 0u)
+        << "the whole deletable tail must eventually drain under repeated budgeted rounds";
+}
+
+TEST(CASRefGc, RefObjectCleanupRetainsCheckpointPredecessorSealProof)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/cross-epoch-cleanup@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+    const NamespaceLifeId life = fixture::fixtureLife(ns);
+    const RefTxnId birth_id{1, 1};
+    const RefTxnId seal_id{1, 2};
+    const RefTxnId base_id{2, 1};
+
+    const RefLogTxn birth{
+        .ns = ns.string(),
+        .txn_id = birth_id,
+        .ops = {namespaceBirthOp()},
+        .prev_epoch_seal = std::nullopt};
+    RefOp seal_op;
+    seal_op.kind = RefOpKind::EpochSeal;
+    const RefLogTxn seal{
+        .ns = ns.string(),
+        .txn_id = seal_id,
+        .ops = {std::move(seal_op)},
+        .prev_epoch_seal = std::nullopt};
+    const RefLogTxn base{
+        .ns = ns.string(),
+        .txn_id = base_id,
+        .ops = {},
+        .prev_epoch_seal = seal_id};
+    fixture::writeRefLogRaw(*backend, layout, birth);
+    fixture::writeRefLogRaw(*backend, layout, seal);
+    fixture::writeRefLogRaw(*backend, layout, base);
+
+    RefTableState state;
+    applyRefLogTxn(state, birth);
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(state, ns.string()));
+    applyRefLogTxn(state, seal);
+    applyRefLogTxn(state, base);
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(state, ns.string()));
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = base_id,
+        .checkpoint_snapshot_id = base_id,
+        .last_epoch_seal = seal_id});
+
+    Gc gc(store, kGc);
+    runToFixpoint(store, gc);
+
+    EXPECT_TRUE(backend->head(layout.refLogKey(life, seal_id)).exists)
+        << "cleanup must retain the predecessor seal that proves the checkpoint base's epoch transition";
+    const CasRefCatalog::Snapshot cut = CasRefCatalog::read(*backend, layout);
+    const auto entry = std::find_if(cut.catalog.entries.begin(), cut.catalog.entries.end(),
+        [&](const CatalogEntry & candidate) { return candidate.ns == ns; });
+    ASSERT_NE(entry, cut.catalog.entries.end());
+    const std::optional<CkptSample> checkpoint = readCkpt(*backend, layout, life);
+    ASSERT_TRUE(checkpoint);
+    EXPECT_NO_THROW((void)recoverRefTableDetailedFromAuthority(*backend, layout, *entry, checkpoint->ckpt));
+}
+
+TEST(CASRefGcCleanupAuthority, CatalogTokenMoveBeforeFirstDeleteRefusesEveryRefObjectDelete)
+{
+    auto backend = std::make_shared<RefCleanupAuthorityRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RefCleanupFixture keys = seedTwoCoveredLogs(*backend, layout, RootNamespace{"00/aa@cas@"});
+    backend->arm(
+        RefCleanupAuthorityRaceBackend::Authority::Catalog,
+        RefCleanupAuthorityRaceBackend::Timing::BeforeFirstDelete, layout, keys.first_log_key);
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    EXPECT_TRUE(backend->head(keys.first_log_key).exists);
+    EXPECT_TRUE(backend->head(keys.second_log_key).exists);
+    EXPECT_EQ(backend->deleteCount(keys.first_log_key), 0u);
+    EXPECT_EQ(backend->deleteCount(keys.second_log_key), 0u);
+}
+
+TEST(CASRefGcCleanupAuthority, CatalogTokenMoveBetweenKeysAllowsFirstAndRefusesSecondDelete)
+{
+    auto backend = std::make_shared<RefCleanupAuthorityRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RefCleanupFixture keys = seedTwoCoveredLogs(*backend, layout, RootNamespace{"00/aa@cas@"});
+    backend->arm(
+        RefCleanupAuthorityRaceBackend::Authority::Catalog,
+        RefCleanupAuthorityRaceBackend::Timing::AfterFirstDelete, layout, keys.first_log_key);
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    EXPECT_FALSE(backend->head(keys.first_log_key).exists);
+    EXPECT_TRUE(backend->head(keys.second_log_key).exists);
+    EXPECT_EQ(backend->deleteCount(keys.first_log_key), 1u);
+    EXPECT_EQ(backend->deleteCount(keys.second_log_key), 0u);
+}
+
+TEST(CASRefGcCleanupAuthority, GcFenceMoveBeforeFirstDeleteRefusesEveryRefObjectDelete)
+{
+    auto backend = std::make_shared<RefCleanupAuthorityRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RefCleanupFixture keys = seedTwoCoveredLogs(*backend, layout, RootNamespace{"00/aa@cas@"});
+    backend->arm(
+        RefCleanupAuthorityRaceBackend::Authority::GcFence,
+        RefCleanupAuthorityRaceBackend::Timing::BeforeFirstDelete, layout, keys.first_log_key);
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    EXPECT_TRUE(backend->head(keys.first_log_key).exists);
+    EXPECT_TRUE(backend->head(keys.second_log_key).exists);
+    EXPECT_EQ(backend->deleteCount(keys.first_log_key), 0u);
+    EXPECT_EQ(backend->deleteCount(keys.second_log_key), 0u);
+}
+
+TEST(CASRefGcCleanupAuthority, GcFenceMoveBetweenKeysAllowsFirstAndRefusesSecondDelete)
+{
+    auto backend = std::make_shared<RefCleanupAuthorityRaceBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RefCleanupFixture keys = seedTwoCoveredLogs(*backend, layout, RootNamespace{"00/aa@cas@"});
+    backend->arm(
+        RefCleanupAuthorityRaceBackend::Authority::GcFence,
+        RefCleanupAuthorityRaceBackend::Timing::AfterFirstDelete, layout, keys.first_log_key);
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+
+    EXPECT_FALSE(backend->head(keys.first_log_key).exists);
+    EXPECT_TRUE(backend->head(keys.second_log_key).exists);
+    EXPECT_EQ(backend->deleteCount(keys.first_log_key), 1u);
+    EXPECT_EQ(backend->deleteCount(keys.second_log_key), 0u);
+}
+
+/// Task 13 (spec §implementation-impact / §GC Budget): one fold+clean round increments every ref-intake
+/// observability counter -- global LIST pages (Q), log-body GETs (K), manifest-body fold GETs (H), emitted
+/// manifest edges, and cleaned old ref objects (D). Before/after deltas prove each site actually fires.
+TEST(CASRefGc, RefIntakeIncrementsObservabilityCounters)
+{
+    using ProfileEvents::global_counters;
+    const auto list_pages_before = global_counters[ProfileEvents::CASRefGlobalListPages].load();
+    const auto log_gets_before   = global_counters[ProfileEvents::CASRefLogBodyGets].load();
+    const auto mf_gets_before    = global_counters[ProfileEvents::CASRefManifestBodyFoldGets].load();
+    const auto edges_before      = global_counters[ProfileEvents::CASRefEmittedEdges].load();
+    const auto cleaned_before    = global_counters[ProfileEvents::CASRefCleanupObjectsDeleted].load();
+
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r1 = mref(1);
+    const ManifestRef r2 = mref(2);
+    writeManifestRaw(*backend, layout, ns, r1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    const uint64_t v1 = publishCommittedTransition(*backend, layout, ns, "t1", std::nullopt, r1);
+    const uint64_t v2 = publishCommittedTransition(*backend, layout, ns, "t2", std::nullopt, r2);
+    /// A checkpoint-named snapshot base makes older listed objects eligible for cleanup once folded.
+    writeRefSnapshotRaw(*backend, layout,
+        minimalLiveSnapshot(ns.string(), RefTxnId{1, v2}, {committedRow("t1", r1), committedRow("t2", r2)}));
+    replaceRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, v2},
+        .checkpoint_snapshot_id = RefTxnId{1, v2},
+        .last_epoch_seal = std::nullopt,
+    });
+    (void)v1;
+
+    Gc gc(store, kGc);
+    runToFixpoint(store, gc);
+
+    EXPECT_GT(global_counters[ProfileEvents::CASRefGlobalListPages].load(), list_pages_before);
+    EXPECT_GT(global_counters[ProfileEvents::CASRefLogBodyGets].load(), log_gets_before);
+    EXPECT_GT(global_counters[ProfileEvents::CASRefManifestBodyFoldGets].load(), mf_gets_before);
+    EXPECT_GT(global_counters[ProfileEvents::CASRefEmittedEdges].load(), edges_before);
+    EXPECT_GT(global_counters[ProfileEvents::CASRefCleanupObjectsDeleted].load(), cleaned_before);
+}
+
+/// Task 13 e2e (in-process regression twin of the rustfs integration test): the whole snapshot+log
+/// lifecycle over real wire-format objects and real GC rounds -- publish committed refs across two
+/// tables, replace one (dropping a blob), publish a covering snapshot, drive GC to a fixpoint, and
+/// assert the fold + ref-object cleanup + snapshot lifecycle plus the two read-only consumers:
+/// `runFsck(*store).clean()` (the fsck CLI's verdict, oracle included) and `gc.previewDeletes().empty()`
+/// (what `cas-gc-dryrun` reports). This is the deterministic permanent twin the unit sweep keeps running.
+TEST(CASRefGc, RefSnaplogLifecycleE2E)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns_a{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns_a);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    const RootNamespace ns_b{"00/bb@cas@"};
+
+    /// Two tables with committed refs naming present manifests + blobs (insert-like). ns_a's ref is then
+    /// re-published to a second manifest, dropping the first manifest's blob (a replace: -1 old, +1 new).
+    const ManifestRef a1 = mref(1);
+    const ManifestRef a2 = mref(2);
+    const ManifestRef b1 = mref(3);
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeBlobBody(*backend, layout, DB::UInt128(2));
+    writeBlobBody(*backend, layout, DB::UInt128(3));
+    writeManifestRaw(*backend, layout, ns_a, a1, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, layout, ns_a, a2, {blobEntryFor("a", DB::UInt128(2))});
+    writeManifestRaw(*backend, layout, ns_b, b1, {blobEntryFor("b", DB::UInt128(3))});
+    const uint64_t va1 = publishCommittedTransition(*backend, layout, ns_a, "t", std::nullopt, a1);
+    const uint64_t va2 = publishCommittedTransition(*backend, layout, ns_a, "t", a1, a2);   /// replace a1 -> a2
+    publishCommittedTransition(*backend, layout, ns_b, "t", std::nullopt, b1);
+    /// The semantic transition helper has already published the exact CTE for each life.
+
+    /// The writer's compaction: a snapshot of ns_a covering its greatest log (va2), the same
+    /// deterministic bytes the oracle recomputes.
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(*backend, layout);
+    const RefTableState sa = recoverRefTableDetailedAtCatalogCutForTest(*backend, layout, catalog_cut, ns_a).state;
+    writeRefSnapshotRaw(*backend, layout, snapshotOf(sa, ns_a.string()));
+    const NamespaceLifeId life_a = store->namespaceLife(ns_a);
+    const CkptSample before_snapshot_publish = *readCkpt(*backend, layout, life_a);
+    RefCkpt after_snapshot_publish = before_snapshot_publish.ckpt;
+    after_snapshot_publish.checkpoint_snapshot_id = RefTxnId{1, va2};
+    ASSERT_EQ(backend->casPut(
+        layout.refCkptKey(life_a), encodeRefCkpt(after_snapshot_publish), before_snapshot_publish.token).outcome,
+        CasOutcome::Committed);
+
+    Gc gc(store, kGc);
+    runToFixpoint(store, gc);
+
+    /// Snapshot lifecycle: the covering snapshot is retained; the covered logs (folded + snapshot-covered)
+    /// are cleaned; the replaced manifest's blob is reclaimed while the live blobs survive.
+    EXPECT_TRUE(backend->head(layout.refSnapshotKey(fixture::fixtureLife(ns_a), RefTxnId{1, va2})).exists)
+        << "covering snapshot retained";
+    EXPECT_FALSE(backend->head(layout.refLogKey(fixture::fixtureLife(ns_a), RefTxnId{1, va1})).exists) << "covered log cleaned";
+    EXPECT_FALSE(blobPresent(*backend, layout, DB::UInt128(1))) << "replaced blob reclaimed";
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(2))) << "live blob survives";
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(3))) << "other table's blob survives";
+
+    /// Read-only consumers agree: fsck recovers through the exact checkpoint base and reports no dangle,
+    /// while cas-gc-dryrun has no pending content deletes. Covered LIST debris is not diagnostic authority.
+    const FsckReport rep = runFsck(*store, /*detail*/true);
+    EXPECT_TRUE(rep.clean());
+    EXPECT_EQ(rep.dangling, 0u);
+    EXPECT_TRUE(gc.previewDeletes().empty()) << "cas-gc-dryrun equivalent: no pending content deletes";
+}
+
+/// (8) A malformed/adversarial ref key aborts ref folding for the round: no partial delta, no cursor
+/// advance. The malformed key is a real object under `cas/ns/stream/` whose `RefTxnId` render is invalid.
+TEST(CASRefGc, MalformedRefKeyAbortsRefFoldingNoPartialDelta)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r = mref(1);
+    writeManifestRaw(*backend, layout, ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r);
+    /// The semantic transition helper has already published the exact CTE.
+
+    /// Plant a malformed ref key under the ref prefix (a `_log` with a non-canonical id render).
+    const NamespaceLifeId life = store->namespaceLife(ns);
+    backend->putIfAbsent(layout.namespaceStreamPrefix(life) + "_log/not-a-valid-txn-id", "garbage");
+
+    Gc gc(store, kGc);
+    /// The fold's `groupRefKeys` rejects the unrecognized key and ABORTS ref folding for the round (spec
+    /// §Step 2: a malformed key cannot produce a partial ref delta or authorize destructive work). The
+    /// round CATCHES this internally and survives -- it must not propagate, and must not fold anything.
+    ASSERT_NO_THROW(gc.runRegularRound());
+
+    /// No partial delta, no cursor advance: the valid log's blob was NOT folded.
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 0)
+        << "a malformed ref key must abort the round before any partial ref delta lands";
+    EXPECT_EQ(foldCursorOf(*backend, layout, ns, 0), 0u)
+        << "the durable cursor must not advance on an aborted round";
+}
+
+/// (8b) A non-canonical physical life segment is the OTHER way a ref key can be malformed, and it must
+/// land on exactly the path (8) pins -- abort ref folding, record the anomaly, COMPLETE the round.
+///
+/// It gets its own test because the failure mode is worse than a lost round. The parser REFUSES this
+/// shape by name rather than returning `std::nullopt`, so it is the one malformed key that can throw
+/// from the round's global `cas/ns/stream/` enumeration, which runs in `defer_decision` -- before the fold,
+/// and outside the fold's catch. Escaping there does not merely fail one round: GC is the only thing
+/// that could ever delete the key, so a round that dies on it dies on it again every time, forever.
+/// The enumeration must therefore absorb the refusal per key and leave the key unindexed in
+/// `scan.keys`, exactly as it already does for every other malformed shape, and let `groupRefKeys`
+/// raise it once where the round is ready to catch it.
+TEST(CASRefGc, NonCanonicalLifeKeyAbortsRefFoldingWithoutWedgingTheRound)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+
+    const ManifestRef r = mref(1);
+    writeManifestRaw(*backend, layout, ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r);
+
+    /// A ref log whose supposed life segment contains logical namespace text rather than one canonical
+    /// opaque id. Only a foreign or corrupt writer can put this key here, and the pool must survive it.
+    const String noncanonical_life =
+        layout.casRefsPrefix() + ns.string() + "/_log/" + renderRefTxnId(RefTxnId{1, 1}) + ".zst";
+    ASSERT_EQ(backend->putIfAbsent(noncanonical_life, "garbage").outcome, PutOutcome::Done);
+
+    Gc gc(store, kGc);
+    RoundReport rep;
+    ASSERT_NO_THROW(rep = gc.runRegularRound())
+        << "the round must COMPLETE: a key GC alone could remove must never abort the round that would";
+    EXPECT_TRUE(rep.hasAnomaly(RootNamespace{}, /*shard*/ 0))
+        << "the refusal must surface as the fold's abort anomaly, not vanish";
+    EXPECT_EQ(rep.deleted, 0u);
+    EXPECT_EQ(rep.redeleted, 0u);
+
+    /// Same fail-close as (8): no partial delta, no cursor advance.
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 0)
+        << "an aborted ref fold must land no partial ref delta";
+    EXPECT_EQ(foldCursorOf(*backend, layout, ns, 0), 0u)
+        << "the durable cursor must not advance on an aborted round";
+
+    /// The wedge is only visible over time: the key is still there (nothing deletes it), so a second
+    /// round meets it again. It must survive that one too.
+    ASSERT_TRUE(backend->head(noncanonical_life).exists) << "precondition: nothing removed the key";
+    ASSERT_NO_THROW(gc.runRegularRound()) << "a round that dies on this key would die on it forever";
+}
+
+/// Coverage gap (Task 13a): a ref log at a CANONICAL key but with an undecodable BODY -- distinct from a
+/// malformed *key* (which aborts earlier at the group step, above). This exercises the
+/// GET-then-decode-throw path.
+///
+/// Its blast radius is the NAMESPACE, not the round (spec §5: the whole-round abort survives only for a
+/// key that cannot be attributed to any namespace). The body sits at the position the arithmetic walk
+/// reads next, so the walk stops there: everything below it stays folded (a transaction applies
+/// atomically -- there is no partial delta either way), the cursor never moves past it, and the recorded
+/// anomaly suppresses every destructive step of the round, so nothing the unfolded tail might still
+/// reference can be reclaimed.
+TEST(CASRefGc, InvalidRefLogBodyHoldsNamespaceNoPartialDelta)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    const ManifestRef r = mref(1);
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeManifestRaw(*backend, layout, ns, r, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns, "tbl", std::nullopt, r);
+
+    Gc gc(store, kGc);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).acquired_lease);
+    ASSERT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 1) << "published and folded";
+
+    /// Now DROP the ref, so the blob is genuinely unreferenced once that record folds, and only then
+    /// plant the invalid body at the walk's very next position. This ordering is what makes the
+    /// suppression assertion below mean something: asserting that a LIVE blob survives a held round
+    /// proves nothing, since a live blob is never reclaimable in the first place.
+    const uint64_t dropped = dropRefTransition(*backend, layout, ns, "tbl", r);
+
+    /// A canonical `_log` key (groupRefKeys accepts it) whose body cannot be decoded: the fold GETs it
+    /// and `decodeRefLogTxn` throws.
+    const String garbage_key = layout.refLogKey(fixture::fixtureLife(ns), RefTxnId{1, dropped + 1});
+    backend->putIfAbsent(garbage_key, "garbage-not-a-valid-reflog-body");
+    /// The corruption claims the next committed position. Advance only the durable frontier, not the
+    /// log body, so recovery must exact-GET and hold this malformed object instead of ignoring F+1.
+    advanceRecoverableCkptForRawFixture(*backend, layout, ns, RefTxnId{1, dropped + 1});
+
+    /// Eight rounds under the hold. Each one catches the hold internally and survives.
+    for (int i = 0; i < 8; ++i)
+    {
+        ASSERT_NO_THROW(runRegularRoundReclaiming(gc));
+        store->renewWatermarkOnce();
+    }
+
+    EXPECT_EQ(foldCursorOf(*backend, layout, ns, 0), dropped)
+        << "the durable cursor must stop BELOW the invalid record, and never advance past it";
+    EXPECT_EQ(inDegreeOf(*backend, layout, DB::UInt128(1)), 0)
+        << "the complete transaction below the invalid body folded -- the drop applied, so the blob is "
+           "unreferenced and would be reclaimed by any unsuppressed round";
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(1)))
+        << "the held namespace's anomaly suppresses graduation and pending deletes: an unreferenced "
+           "blob is NOT reclaimed while any namespace is held, because the unfolded tail behind the "
+           "hold may still name it";
+
+    /// DELETING THE EVIDENCE DOES NOT RELEASE THE HOLD. The hold is durable and clears by exactly one
+    /// event -- the fold resolving its offending position -- so an object that stops answering does not
+    /// turn the gap into a frontier. It is the same observation a lying store produces, and it is
+    /// precisely what made the hold necessary; if an absent could clear it, the whole mechanism would
+    /// be defeated by the corruption it exists to survive. (Before durable holds this delete DID
+    /// release the namespace, which is the hole Task 8 closed.)
+    const HeadResult h = backend->head(garbage_key);
+    ASSERT_TRUE(h.exists);
+    ASSERT_EQ(backend->deleteExact(garbage_key, h.token).kind, DeleteOutcome::Kind::Deleted);
+
+    for (int i = 0; i < 4; ++i)
+    {
+        ASSERT_NO_THROW(runRegularRoundReclaiming(gc));
+        store->renewWatermarkOnce();
+    }
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(1)))
+        << "the hold still stands: nothing resolved the offending position, an absent proved nothing";
+
+    /// REPAIR is the release: a DECODABLE record at the offending position. The fold reads it, folds
+    /// through it, seals a cursor above it -- and only then does the namespace stop being held and
+    /// destruction resumes. The CTE already claims this position, so this must replace the repaired
+    /// body at its exact id rather than use the semantic wrapper, which would attempt a non-monotone
+    /// checkpoint advance.
+    const ManifestRef r2 = mref(2);
+    writeBlobBody(*backend, layout, DB::UInt128(2));
+    writeManifestRaw(*backend, layout, ns, r2, {blobEntryFor("b", DB::UInt128(2))});
+    writeTxnAt(*backend, layout, ns, RefTxnId{1, dropped + 1}, publishCommittedOps("tbl2", r2));
+
+    ASSERT_TRUE(runToFixpoint(store, gc) < 64u) << "the released namespace must converge";
+    EXPECT_EQ(foldCursorOf(*backend, layout, ns, 0), dropped + 1) << "the walk folded through the hold";
+    EXPECT_FALSE(blobPresent(*backend, layout, DB::UInt128(1)))
+        << "once the hold clears, the unreferenced blob is reclaimed -- so the survival above was the "
+           "suppression doing its job, not the blob being unreclaimable";
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(2))) << "the repair's own blob is referenced";
+}
+
+/// Coverage gap (Task 13a): the per-table baseline guard (spec §Offline Recovery) has no positive-trip
+/// test at HEAD -- the adapted successor of the retired CASGCBaselineGuard.FreshStateOverTrimmedJournals
+/// contract. A table whose logs at/below its newest snapshot are gone and that has no sealed fold cursor
+/// is the "a prior fold advanced+cleaned covered logs, then gc/state was lost" signature: folding it from
+/// {0,0} would emit no edges and mass-condemn its still-referenced blob. GC must refuse the round before
+/// any delete. The existing CASGCBaselineGuard tests cover only the genuinely-fresh pass case and the
+/// adopted-seal-missing guard, not this branch.
+TEST(CASRefGc, BaselineGuardRefusesWhenSnapshotSurvivesWithoutLogsOrCursor)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+
+    /// Table A is healthy (a committed ref with its manifest+blob, no snapshot), giving GC a normal table
+    /// to fold in the same round.
+    const RootNamespace ns_a{"00/aa@cas@"};
+    const ManifestRef ra = mref(1);
+    writeBlobBody(*backend, layout, DB::UInt128(1));
+    writeManifestRaw(*backend, layout, ns_a, ra, {blobEntryFor("a", DB::UInt128(1))});
+    publishCommittedTransition(*backend, layout, ns_a, "ta", std::nullopt, ra);
+
+    /// Table B is poisoned: a durable snapshot survives, but its logs at/below it are GONE and B has no
+    /// sealed cursor (first round -> no adopted parent cursors). This is the exact baseline-guard input.
+    const RootNamespace ns_b{"00/bb@cas@"};
+    /// Stage B (Task 4-C): `writeRefSnapshotRaw` deliberately does NOT self-admit (several fixtures
+    /// build a table with no catalog entry on purpose), so without this `ns_b` would never enter the
+    /// catalog at all and would be invisible to the round -- the baseline guard below could then never
+    /// fire, since it never runs on a namespace outside the universe.
+    fixture::admitLive(*backend, layout, ns_b);
+    const ManifestRef rb = mref(2);
+    writeBlobBody(*backend, layout, DB::UInt128(2));
+    writeManifestRaw(*backend, layout, ns_b, rb, {blobEntryFor("b", DB::UInt128(2))});
+    writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns_b.string(), RefTxnId{1, 5},
+        {committedRow("tb", rb)}));
+
+    /// The baseline guard must fail closed BEFORE any destructive step (first round: no prior fold seal,
+    /// so the failure can only come from the baseline guard, not the seal-divergence guard).
+    Gc gc(store, kGc);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { gc.runRegularRound(); });
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(1))) << "table A's blob survives the refusal";
+    EXPECT_TRUE(blobPresent(*backend, layout, DB::UInt128(2)))
+        << "table B's blob must NOT be condemned -- the guard fires before any delete";
+}
+
+/// A catalog-admitted life without a parent cursor is a valid fresh fold target when it has no
+/// snapshot or logs. The fold must seed its successor seal from every plan row, not only the
+/// parent-cursor subset used by the baseline guard.
+TEST(CASRefGc, CatalogAdmittedFreshLifeWithoutParentSeedsSuccessorSeal)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend, /*gc_fold_max_defer_rounds*/ 0);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    fixture::admitLive(*backend, layout, ns);
+
+    const CasRefCatalog::Snapshot catalog_cut = CasRefCatalog::read(*backend, layout);
+    ASSERT_EQ(catalog_cut.catalog.entries.size(), 1u);
+    const UInt128 life_id = catalog_cut.catalog.entries.front().incarnation;
+
+    Gc gc(store, kGc);
+    ASSERT_NO_THROW(gc.runRegularRound());
+
+    const GcState state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const CasFoldSeal seal = decodeFoldSeal(
+        backend->get(layout.foldSealKey(state.snap_generation, state.snap_attempt))->bytes);
+    EXPECT_TRUE(seal.ref_lives.contains(life_id));
+}
diff --git a/src/Disks/tests/gtest_cas_ref_install_safety.cpp b/src/Disks/tests/gtest_cas_ref_install_safety.cpp
new file mode 100644
index 000000000000..d05ae091fd39
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_install_safety.cpp
@@ -0,0 +1,959 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefLedger.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/MemoryTracker.h>
+#include <Common/ProfileEvents.h>
+
+#include <atomic>
+#include <chrono>
+#include <cstddef>
+#include <exception>
+#include <functional>
+#include <memory>
+#include <string>
+
+/// Task 3 (spec §A1, site 1): the region of `CasRefLedger::commitRefChunk` between "this chunk's
+/// ref-log object is durable" and "the runtime records it".
+///
+/// Before the fix that region ran `applyRefLogTxn(rt->state, chunk_txn)`, which allocates (the COW
+/// containers build an overlay) and can therefore throw `MEMORY_LIMIT_EXCEEDED`. A throw there left the
+/// transaction durable but invisible to the writer -- and because a later transaction only needs
+/// `greatest_applied < its own id` (contiguity is never checked), a snapshot published afterwards is
+/// labelled with that LATER id, so recovery skips the stranded transaction permanently while GC, which
+/// folds the ref logs themselves, still applies it. That divergence loses data (a stranded removal
+/// leaves the writer holding a ref whose blobs GC deleted), which is why the install is now a
+/// prepared-candidate swap: allocation-free, hence non-throwing, and enforced as such by
+/// `DENY_ALLOCATIONS_IN_SCOPE`.
+///
+/// The suite name is prefixed `Cas` so it is covered by the `Cas*` unit-test gate filter.
+
+namespace DB::ErrorCodes
+{
+extern const int NETWORK_ERROR;
+extern const int CORRUPTED_DATA;
+extern const int LOGICAL_ERROR;
+extern const int MEMORY_LIMIT_EXCEEDED;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASRefNeedsRecovery;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+PoolPtr openPool(const BackendPtr & backend)
+{
+    /// A fresh pool with no residue, mirroring `gtest_cas_ref_chunked_flush.cpp`'s `openPool`.
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// As `openPool`, but with a SINGLE-attempt request budget, which is what makes one ambiguous `PUT`
+/// conclusive: with retries allowed the controller's resolve-before-reissue would either re-`PUT` (the
+/// object never landed) or prove the object durable (it did) and report `Committed`, and neither of the
+/// wedge arms under test would ever be reached. Same budget shape as
+/// `gtest_cas_ref_chunked_flush.cpp`'s `runChunkFailureCase`, including the short timeouts so there is
+/// no inter-attempt sleep to serve.
+PoolPtr openPoolSingleAttempt(const BackendPtr & backend)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    PoolConfig cfg{.pool_prefix = "p", .server_root_id = "test"};
+    CasRequestBudget budget;
+    /// ONE attempt is the whole mechanism these tests need: it is what turns an injected lost
+    /// acknowledgement into `Unresolved` instead of a transparent retry, and it does so independently of
+    /// how fast the machine is.
+    ///
+    /// The operation deadline must therefore NOT sit at `attempt_timeout_ms`, which is where it used to.
+    /// The controller's pre-send gate (`putIfAbsentControlled`: `now + attempt_timeout > deadline`
+    /// returns `Unresolved` WITHOUT sending) is then a zero-width race that passes only if no
+    /// millisecond tick elapses between the deadline capture and the gate. Under parallel-build load it
+    /// loses: the gate fires first, nothing is sent, the injected fault is never reached, and the flush
+    /// fails CLEAN -- so the product correctly does NOT wedge the lane and the wedge expectations flip.
+    /// `UncertainPrecommitKeepsItsCleanupOwnerAndItsBody` was observed failing exactly that way (Task 9,
+    /// `refLaneWedgedForTest` false at the wedge assertion), and every test on this fixture carries the
+    /// same razor. Same root cause and same fix as `8f9e63c7a19` for the sweep-interruption test.
+    ///
+    /// A WIDE deadline keeps the request always actually sent, so the injected fault decides the outcome
+    /// rather than the scheduler. Tests that want the pre-send REFUSAL instead use
+    /// `openPoolFenceControlled`, where a frozen clock makes that refusal deterministic rather than raced.
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;
+    budget.lease_safety_margin_ms = 100;
+    cfg.cas_request_budget = budget;
+    return Pool::open(backend, cfg);
+}
+
+/// The mount-fence deadlines the pre-attempt tests drive, in the FROZEN boot clock of
+/// `openPoolFenceControlled` (which is pinned at 0, so these are also the remaining lease budgets).
+///
+/// `CasMountRuntime` has TWO fence predicates and they are deliberately not the same:
+///   `mayMutate`         -- `now < deadline`; the top-of-flush gate in `flushRefBatch`.
+///   `refAppendFenceOk`  -- additionally `attempt_timeout_ms + lease_safety_margin_ms < deadline - now`,
+///                          i.e. "there is room for one whole controlled attempt"; the `fence_ok`
+///                          `commitRefChunk` hands to `putIfAbsentControlled`.
+/// With `openPoolFenceControlled`'s budget below that margin is 100 + 100 = 200 ms, so a 100 ms
+/// remaining lease sits BETWEEN them: the flush is admitted and then its very first pre-attempt gate
+/// refuses. That is
+/// exactly the production shape this task is about (a lease too short to start a write, not a lost
+/// one), and it needs no fault injection at all -- which is the point: nothing is sent.
+constexpr uint64_t FENCE_DEADLINE_HEALTHY_MS = 30000;
+constexpr uint64_t FENCE_DEADLINE_REFUSES_ATTEMPT_MS = 100;
+
+/// A legal blob-free part: stage an empty manifest, precommit, promote -- enough to drive real
+/// ref-log transactions through the append lane.
+void publishEmptyPart(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+}
+
+/// As `openPoolSingleAttempt`, but with the mount fence under the TEST's control instead of the wall
+/// clock's:
+///   - the boot clock is FROZEN at 0, so `setMountDeadline` alone decides both fence predicates and no
+///     elapsed real time can flip one of them mid-test (the same load-bearing injection, for the same
+///     reason, as `gtest_cas_ref_chunked_flush.cpp`'s `openPool`);
+///   - lease renewal is parked an hour out, so the keeper's background renew cannot re-arm the deadline
+///     underneath a test that just shortened it. Ten seconds (the default) would be enough in practice
+///     and flaky in principle; this removes the race rather than betting on it.
+PoolPtr openPoolFenceControlled(const BackendPtr & backend)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    PoolConfig cfg{.pool_prefix = "p", .server_root_id = "test"};
+    cfg.boot_ms_fn = [] { return uint64_t{0}; };
+    cfg.mount_renew_period = std::chrono::milliseconds{3600000};
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;   /// strictly above attempt_timeout_ms: equality is a wall-clock race (validateCasRequestBudget)
+    budget.lease_safety_margin_ms = 100;
+    cfg.cas_request_budget = budget;
+    return Pool::open(backend, cfg);
+}
+
+/// Runs `f`, requires it to throw the ref lane's retry-later condition, and returns the message so a
+/// caller can assert WHICH condition it was. The message is the only place the `CasUnresolvedReason`
+/// surfaces -- there is no accessor for it, by design (it is a diagnostic, not state) -- so this is how
+/// a test proves the reason actually reached the decision site instead of defaulting.
+String retryLaterMessageOf(const std::function<void()> & f)
+{
+    try
+    {
+        f();
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR) << e.message();
+        return e.message();
+    }
+    ADD_FAILURE() << "expected the CAS retry-later condition, but nothing was thrown";
+    return {};
+}
+
+/// Installs a ONE-SHOT throwing probe into the post-durable install regions (spec §A2): the next region
+/// entered throws, every later one runs normally -- which is what lets a terminality test drive a
+/// successful flush after the recovery transition.
+///
+/// The exception is built HERE, outside the region, and the probe only rethrows it: constructing a
+/// `DB::Exception` inside the region would allocate and trip `DENY_ALLOCATIONS_IN_SCOPE`, so the test
+/// would be exercising the guard instead of the recovery transition. `MEMORY_LIMIT_EXCEEDED` (what a
+/// real tracked allocation failure raises) is used deliberately instead of `LOGICAL_ERROR`, which
+/// aborts at construction in debug/sanitizer builds.
+///
+/// With §A1 landed this seam is the only way to reach `NeedsRecovery` from an install region:
+/// install regions are allocation-free and therefore cannot throw on their own.
+void armOneShotInstallFailure(const PoolPtr & store)
+{
+    auto planned = std::make_exception_ptr(DB::Exception(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+        "simulated allocation failure inside the post-durable install region"));
+    auto fired = std::make_shared<std::atomic<bool>>(false);
+    store->setInstallRegionProbeForTest([planned, fired]
+    {
+        if (fired->exchange(true))
+            return;
+        /// The throw itself allocates its exception object through `malloc`, which the memory tracker
+        /// does not see, so it would not trip the guard anyway -- re-allowing allocations for the
+        /// duration of the throw makes that a stated property of the test rather than a bet on a libc++
+        /// implementation detail.
+        ALLOW_ALLOCATIONS_IN_SCOPE;
+        std::rethrow_exception(planned);
+    });
+}
+
+}
+
+/// The post-durable install seam exists, is reached by an ordinary commit, and every transaction that
+/// reaches it is RECORDED: the tail counter advances exactly once per install, and the ref resolves.
+/// The equality is the point -- it is the invariant the old code could break, since there the install
+/// was an allocating apply that could throw between the durable `PUT` and the counter bump.
+TEST(CASRefInstallSafety, PostDurableInstallIsAllocationFree)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/install_safety_seam"};
+
+    /// Fired on the calling thread by the flush leader, which is this thread; `atomic` regardless, so
+    /// the assertions below cannot be read as depending on that.
+    std::atomic<size_t> installs{0};
+    store->setCarveHookForTest([&installs](CasRefLedger::CarvePhaseForTest phase)
+    {
+        if (phase == CasRefLedger::CarvePhaseForTest::PostDurableInstall)
+            installs.fetch_add(1);
+    });
+
+    publishEmptyPart(store, ns, "part_a");
+    store->setCarveHookForTest(nullptr);
+
+    const size_t seen = installs.load();
+    EXPECT_GT(seen, 0u) << "the post-durable install seam must be reached by an ordinary commit";
+    /// No snapshot publish can interfere: the thresholds are 256 logs / 1 MiB and this part is a
+    /// handful of tiny transactions, so the tail counter still holds every one of them.
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), seen)
+        << "every durable transaction that entered the install region must be recorded in the tail";
+    EXPECT_TRUE(store->resolveRef(ns, "part_a", /*allow_stale=*/false).has_value());
+}
+
+/// Task 4 (spec §A1, site 3). An `Unresolved` PUT must ALWAYS leave the lane wedged with the exact
+/// {id, key, bytes} of the in-doubt object -- the wedge is the only record that can ever resolve it, and
+/// on this path the object may already be durable. Building the wedge AFTER the PUT copies two `String`s
+/// and could therefore fail on allocation, recording NEITHER the transaction nor the wedge: strictly
+/// worse than a wedge, because the next append then mints a fresh id and proceeds against a state that
+/// is missing a landed transaction. It is now preconstructed before the PUT and installed by a
+/// non-throwing move.
+///
+/// `Mode::Unresolved` deliberately lands NOTHING, so this test also pins the other half of the wedge
+/// contract: an ambiguous outcome wedges even when the object turns out never to have existed. The tail
+/// counter must NOT advance -- an unproven transaction is not a recorded one.
+TEST(CASRefInstallSafety, UnresolvedAlwaysRecordsTheWedge)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/unresolved_wedge"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    /// Scoped to THIS namespace's ref log, so nothing else the part publish writes (the manifest, the
+    /// pool's own metadata) can consume the single fault.
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::Unresolved;
+    backend->fault_count = 1;
+
+    const String message = retryLaterMessageOf([&] { publishEmptyPart(store, ns, "part_a"); });
+
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns)) << "an Unresolved PUT must always leave a wedge";
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    EXPECT_FALSE(wedged_key.empty()) << "the wedge must retain the in-doubt object's key";
+    EXPECT_TRUE(wedged_key.starts_with(store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/"))
+        << "the wedged key must be this namespace's ref-log object, not some other key: " << wedged_key;
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 0u)
+        << "an UNPROVEN transaction must not be recorded as applied";
+
+    /// Task 18. The contrast half of `PreAttemptRefusalDoesNotWedgeTheLane` below, asserted on the ONE
+    /// artifact that carries the distinction: an attempt WAS sent here (the fault is thrown by the
+    /// backend's `putIfAbsent`, so the request reached it), the single-attempt budget is then spent, and
+    /// the lane wedges. The message must say so -- and must NOT say "no attempt was sent", which is the
+    /// only shape allowed to skip the wedge.
+    EXPECT_NE(message.find("attempt budget was exhausted"), String::npos)
+        << "the reason must reach the wedge message rather than defaulting: " << message;
+    EXPECT_EQ(message.find("no attempt was sent"), String::npos)
+        << "an ambiguous PUT is not a pre-attempt refusal: " << message;
+}
+
+/// Task 18 (finding #37 defect 3, behavioural half). A `NoAttemptSent` `Unresolved` must NOT wedge.
+///
+/// The wedge exists because an ambiguous PUT MAY HAVE LANDED, so the durable log may or may not contain
+/// the transaction and only an exact-key GET can settle it. That reasoning needs an attempt to have been
+/// SENT. Here both pre-attempt gates reject on the FIRST iteration -- the remaining lease has no room
+/// for one controlled attempt -- so nothing reaches the backend, the key is provably unwritten, and a
+/// wedge would protect against nothing while costing the table every ref append (inserts included)
+/// until a remount: an exact-key GET of a key that was never written reports `Unresolved` forever, so
+/// such a wedge can never clear itself.
+///
+/// No fault injection anywhere in this test, deliberately: the ZERO ref-log I/O assertion below is the
+/// direct proof that nothing was sent, and it would be meaningless if a fault backend were swallowing
+/// the request.
+TEST(CASRefInstallSafety, PreAttemptRefusalDoesNotWedgeTheLane)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = openPoolFenceControlled(backend);
+    const RootNamespace ns{"srv1/pre_attempt_refusal"};
+
+    publishEmptyPart(store, ns, "part_a");
+    const size_t tail_after_seed = store->tailSinceSnapshotCountForTest(ns);
+    const String log_prefix = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    const uint64_t log_io_after_seed = backend->ioCountForKeysContaining(log_prefix);
+
+    /// Shorten the lease to the window where the flush is admitted but no attempt may start.
+    store->setMountDeadline(FENCE_DEADLINE_REFUSES_ATTEMPT_MS);
+    /// Half of "the PRE-ATTEMPT gate is what refuses" is asserted here (the flush is admitted, so this
+    /// is not the top-of-flush `mayMutate` gate); the other half is asserted below, by the message
+    /// naming `NoAttemptSent` and by the ref-log I/O count not moving. `refAppendFenceOk` itself is
+    /// private to `Pool`, and is not worth widening for a test that can prove the same thing from the
+    /// outside.
+    ASSERT_TRUE(store->mayMutate()) << "the flush must still be ADMITTED, or this exercises the "
+                                       "top-of-flush gate instead of the pre-attempt one";
+
+    const String message = retryLaterMessageOf([&] { store->dropRef(ns, "part_a"); });
+
+    EXPECT_NE(message.find("no attempt was sent"), String::npos)
+        << "the caller must be told WHY, and this is the reason the no-wedge decision rests on: " << message;
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns))
+        << "nothing was sent, so nothing can be durable: there is no ambiguity for a wedge to resolve";
+    EXPECT_TRUE(store->wedgedKeyForTest(ns).empty());
+    EXPECT_EQ(backend->ioCountForKeysContaining(log_prefix), log_io_after_seed)
+        << "the refusal must be PRE-attempt: not one ref-log object may have been touched";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready)
+        << "no apply is owed for a transaction that was never sent -- leaving the marker pending would "
+           "claim this table may be missing a durable transaction for the rest of its life";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_after_seed)
+        << "nothing was committed, so nothing may be recorded";
+    EXPECT_TRUE(store->resolveRef(ns, "part_a", /*allow_stale=*/false).has_value())
+        << "the refused drop must not have taken effect";
+
+    /// The availability half of the claim: the lane is usable the moment the lease is healthy again --
+    /// no remount, no wedge resolution, nothing to clear. Before this task the same sequence left a
+    /// wedge over a key that was never written, and this append would have failed forever.
+    store->setMountDeadline(FENCE_DEADLINE_HEALTHY_MS);
+    store->dropRef(ns, "part_a");
+    EXPECT_FALSE(store->resolveRef(ns, "part_a", /*allow_stale=*/false).has_value())
+        << "the retry on the same lane must commit";
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+}
+
+/// Task 18, the same pair on the WEDGE-RESOLUTION path -- the negative half.
+///
+/// One flush does both things: it resolves an outstanding wedge over a genuinely durable object (the
+/// resolving GET proves it, so the transaction is installed and the lane unwedged), and then commits
+/// its own new chunk, which the pre-attempt gate refuses. The lane must come out CLEAN.
+///
+/// This is the worst pre-fix shape and the reason the case is worth its own test: the flush had just
+/// converted a resolvable wedge into a recorded transaction, and the old code immediately re-wedged the
+/// lane over an id whose object was never written -- turning a wedge that WOULD have cleared into one
+/// that never can.
+TEST(CASRefInstallSafety, PreAttemptRefusalAfterAWedgeResolutionLeavesTheLaneClean)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolFenceControlled(backend);
+    const RootNamespace ns{"srv1/pre_attempt_after_unwedge"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+    const size_t tail_after_seed = store->tailSinceSnapshotCountForTest(ns);
+
+    /// Wedge over an object that IS durable: the write lands, its acknowledgement is lost, and the
+    /// controller's own verifying read is lost too (the only mode that reaches the resolution install).
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::LandedThenLost;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    ASSERT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_after_seed);
+
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::None;
+    /// The wedge resolution is itself a conditional CREATE under the every-attempt rule, so it is
+    /// fence-gated like any other write: shortening the lease BEFORE the flush would refuse the
+    /// resolution too, and there would be no "after a wedge resolution" left to test. Shorten it
+    /// BETWEEN the two instead -- the pre-carve hook fires exactly there, after the wedge block and
+    /// before the batch is carved.
+    store->setRefPreCarveHookForTest([&] { store->setMountDeadline(FENCE_DEADLINE_REFUSES_ATTEMPT_MS); });
+    const String message = retryLaterMessageOf([&] { store->dropRef(ns, "y"); });
+    store->setRefPreCarveHookForTest(nullptr);
+
+    EXPECT_NE(message.find("no attempt was sent"), String::npos) << message;
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns))
+        << "the wedge that existed was RESOLVED, and the chunk that followed it was never sent -- the "
+           "lane must be left clean, not re-wedged over an id that can never resolve";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_after_seed + 1)
+        << "the resolved wedge must still have been installed exactly once";
+    EXPECT_FALSE(store->resolveRef(ns, "x", /*allow_stale=*/false).has_value())
+        << "the wedged drop was proven durable, so its removal must be visible";
+    EXPECT_TRUE(store->resolveRef(ns, "y", /*allow_stale=*/false).has_value())
+        << "the refused chunk must not have taken effect";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+
+    store->setMountDeadline(FENCE_DEADLINE_HEALTHY_MS);
+    store->dropRef(ns, "y");
+    EXPECT_FALSE(store->resolveRef(ns, "y", /*allow_stale=*/false).has_value());
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+}
+
+/// Task 18, the same pair on the wedge-resolution path -- the POSITIVE half, so the test above cannot
+/// pass by the fix having weakened the wedge generally. Same flush shape (resolve a durable wedge, then
+/// commit a new chunk), except the new chunk's PUT is genuinely ambiguous: an attempt WAS sent, so the
+/// lane must wedge again, now over the NEW transaction.
+TEST(CASRefInstallSafety, AmbiguousChunkAfterAWedgeResolutionRewedgesTheLane)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolFenceControlled(backend);
+    const RootNamespace ns{"srv1/ambiguous_after_unwedge"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+    const size_t tail_after_seed = store->tailSinceSnapshotCountForTest(ns);
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::LandedThenLost;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String first_wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_FALSE(first_wedged_key.empty());
+
+    /// The resolution is a conditional CREATE at the wedged key now, and that key already holds our
+    /// own landed object, so it conflicts and the follow-up read adopts it (`LandedThenLost`'s one-shot
+    /// lost read was consumed inside the previous attempt, so this read succeeds). `fault_skip` lets
+    /// that create through and puts the fault on this flush's OWN chunk PUT, which is the subject.
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::Unresolved;
+    backend->fault_skip = 1;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "y"); });
+
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns))
+        << "an attempt was sent for the new chunk, so its object may be durable: the lane must wedge";
+    EXPECT_NE(store->wedgedKeyForTest(ns), first_wedged_key)
+        << "the new wedge must describe the NEW transaction, not the resolved one";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_after_seed + 1)
+        << "only the resolved wedge is recorded; the in-doubt chunk is not";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Wedged)
+        << "a wedged lane may hold a durable transaction the runtime has not recorded";
+}
+
+/// Task 18's regression guard, asserted on the mapping itself rather than through six pieces of fault
+/// choreography. `unresolvedProvesNothingWasSent` is the whole decision: the ledger wedges unless it
+/// answers true, so this table IS the protocol.
+///
+/// What protects a future contributor who adds a `CasUnresolvedReason` member and forgets this file:
+/// the predicate is a switch with NO `default`, so the addition is a `-Wswitch` build error (a forced
+/// decision, not a silent one), and its trailing `return false` makes the runtime answer "wedge" even
+/// if that diagnostic is ever suppressed. Both directions fail closed; neither can widen the allow-list
+/// by omission. The `static_assert`s make the mapping a compile-time fact, and the `EXPECT`s repeat it
+/// so a break names the offending value in the test report.
+TEST(CASRefInstallSafety, OnlyNoAttemptSentMaySkipTheWedge)
+{
+    static_assert(unresolvedProvesNothingWasSent(CasUnresolvedReason::NoAttemptSent));
+    static_assert(!unresolvedProvesNothingWasSent(CasUnresolvedReason::NotUnresolved));
+    static_assert(!unresolvedProvesNothingWasSent(CasUnresolvedReason::FenceLostMidWay));
+    static_assert(!unresolvedProvesNothingWasSent(CasUnresolvedReason::DeadlineMidWay));
+    static_assert(!unresolvedProvesNothingWasSent(CasUnresolvedReason::FenceLostPostWrite));
+    static_assert(!unresolvedProvesNothingWasSent(CasUnresolvedReason::AttemptsExhausted));
+
+    EXPECT_TRUE(unresolvedProvesNothingWasSent(CasUnresolvedReason::NoAttemptSent))
+        << "the pre-attempt gates rejected before the first request: the key is provably unwritten";
+    /// `NotUnresolved` is reachable at the decision site if any path ever returns `Unresolved` without
+    /// recording a reason, so it is listed here as a real case, not as enum hygiene.
+    EXPECT_FALSE(unresolvedProvesNothingWasSent(CasUnresolvedReason::NotUnresolved))
+        << "an unrecorded reason proves nothing and must keep wedging";
+    EXPECT_FALSE(unresolvedProvesNothingWasSent(CasUnresolvedReason::FenceLostMidWay))
+        << "an attempt was already sent: its object may be durable";
+    EXPECT_FALSE(unresolvedProvesNothingWasSent(CasUnresolvedReason::DeadlineMidWay))
+        << "an attempt was already sent: its object may be durable";
+    EXPECT_FALSE(unresolvedProvesNothingWasSent(CasUnresolvedReason::FenceLostPostWrite))
+        << "the attempt COMMITTED and only the fence was lost afterwards -- the most durable case of all";
+    EXPECT_FALSE(unresolvedProvesNothingWasSent(CasUnresolvedReason::AttemptsExhausted))
+        << "every attempt is a candidate for having landed";
+}
+
+/// Task 5 (spec §A1, site 2). Resolving a wedge is a post-durable install too: the resolving GET PROVES
+/// the object landed, so the transaction MUST be recorded -- and recording it must be inseparable from
+/// clearing the wedge. It was not: the apply and the `materializeCommitted` fold sat between them
+/// WITHOUT the ordinary commit arm's swallow, so a fold failure left the transaction applied and the
+/// wedge still set, and the next resolution re-applied the same transaction and DOUBLE-bumped the tail
+/// counters. The candidate is now built before the GET and installed by a `noexcept` swap that clears
+/// the wedge in the same allocation-free region, with the fold outside it and swallowing.
+///
+/// Drives the real thing end to end (no seam beyond the `LandedThenLost` backend mode): a drop whose
+/// object landed but whose acknowledgement -- and whose immediate verification read -- were both lost,
+/// then a second append whose flush resolves it. The tail counter is the "exactly once" witness: it is
+/// bumped once per install, so a re-applied transaction shows up as one extra.
+TEST(CASRefInstallSafety, WedgeResolutionInstallsExactlyOnce)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/wedge_resolution"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+    /// No snapshot publish can interfere and reset these: the thresholds are 256 logs / 1 MiB and this
+    /// whole test is a handful of tiny transactions, so every delta below is exact.
+    const size_t tail_after_seed = store->tailSinceSnapshotCountForTest(ns);
+
+    /// Drop "x" through a PUT that LANDS and then loses its response, plus the one-shot lost read that
+    /// keeps the controller's own resolve-before-reissue from settling it inside the same attempt. One
+    /// attempt, so the lane wedges over an object that is genuinely durable -- the only way in.
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::LandedThenLost;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns)) << "the lost-response drop must wedge the lane";
+    ASSERT_FALSE(store->wedgedKeyForTest(ns).empty());
+    ASSERT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_after_seed)
+        << "the wedged transaction is durable but not yet PROVEN, so it must not be recorded yet";
+
+    /// A second append into the same table: its flush resolves the wedge first (+1 install) and then
+    /// commits its own transaction (+1). Nothing else can add a transaction in between.
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::None;
+    store->dropRef(ns, "y");
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "a wedge proven durable must be cleared";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_after_seed + 2)
+        << "the resolved transaction must be recorded EXACTLY once: +1 for it and +1 for the append that "
+           "resolved it (a double-apply would show as +3)";
+    /// Both drops took effect -- the wedged one via the resolution install, which is what proves that
+    /// install happened at all rather than the wedge merely being discarded.
+    EXPECT_FALSE(store->resolveRef(ns, "x", /*allow_stale=*/false).has_value())
+        << "the wedged drop was proven durable, so its removal must be visible in the cached state";
+    EXPECT_FALSE(store->resolveRef(ns, "y", /*allow_stale=*/false).has_value());
+}
+
+/// Negative control, part 1 of 2: does `DENY_ALLOCATIONS_IN_SCOPE` actually fire on an allocation in
+/// THIS binary? Gated on `MEMORY_TRACKER_DEBUG_CHECKS`, because that is the macro the guard itself is
+/// gated on (`MemoryTracker.h`: defined only under `!NDEBUG`, i.e. plain debug builds; everywhere else
+/// `DENY_ALLOCATIONS_IN_SCOPE` compiles to `static_assert(true)` and there is nothing to observe).
+/// An earlier version dispatched on `DEBUG_OR_SANITIZER_BUILD` instead — but sanitizer builds define
+/// NDEBUG, so the guard is a no-op there and the death test "failed to die" on all three sanitizer CI
+/// lanes. Note the implication chain: `MEMORY_TRACKER_DEBUG_CHECKS` ⇒ `!NDEBUG` ⇒
+/// `DEBUG_OR_SANITIZER_BUILD`, so whenever the guard exists its `LOGICAL_ERROR` aborts at Exception
+/// construction (`Exception.cpp`) — death is the only observable outcome, and a throw-only variant is
+/// dead code.
+#if defined(MEMORY_TRACKER_DEBUG_CHECKS)
+TEST(CASRefInstallSafetyDeathTest, DenyGuardStopsAnAllocation)
+{
+    EXPECT_DEATH(
+        {
+            DENY_ALLOCATIONS_IN_SCOPE;
+            volatile auto * p = new char[64];
+            (void)p;
+        },
+        "");
+}
+#endif
+
+/// Negative control, part 2 of 2: the region the guard protects is actually ENTERED, and the guard is
+/// armed at that exact point. A probe that only reads flags proves both without allocating, so unlike
+/// part 1 this assertion is immune to how a build type renders a `LOGICAL_ERROR`. Together the two
+/// parts give what a single death test was meant to give, and a failure now names WHICH half broke:
+/// "the guard does not fire" versus "the install region is never reached / not armed".
+TEST(CASRefInstallSafety, InstallRegionProbeIsInvokedAndTheGuardIsArmed)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/install_probe_diag"};
+
+    bool probe_ran = false;
+    [[maybe_unused]] bool guard_armed_when_probe_ran = false;
+    store->setInstallRegionProbeForTest([&]
+    {
+        probe_ran = true;
+#if defined(MEMORY_TRACKER_DEBUG_CHECKS)
+        guard_armed_when_probe_ran = memory_tracker_always_throw_logical_error_on_allocation;
+#endif
+    });
+
+    publishEmptyPart(store, ns, "part_a");
+    store->setInstallRegionProbeForTest(nullptr);
+
+    EXPECT_TRUE(probe_ran) << "the install-region probe was never invoked";
+#if defined(MEMORY_TRACKER_DEBUG_CHECKS)
+    EXPECT_TRUE(guard_armed_when_probe_ran) << "the probe ran but DENY_ALLOCATIONS_IN_SCOPE was not armed";
+#endif
+}
+
+/// ===================================================================================
+/// The append lane state machine.
+/// ===================================================================================
+
+/// The exact attempt is visible as `Writing` after durability and before installation.
+TEST(CASRefInstallSafety, WritingOwnsTheAttemptUntilInstall)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/apply_state_commit"};
+
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Closed)
+        << "a resident-only observer must not materialize a runtime for an untouched name";
+
+    /// Fired on the calling thread by the flush leader, which is this thread; `atomic` regardless, as in
+    /// `PostDurableInstallIsAllocationFree` above, so no assertion here reads as depending on that.
+    std::atomic<size_t> observations{0};
+    std::atomic<size_t> pending_observations{0};
+    store->setCarveHookForTest([&](CasRefLedger::CarvePhaseForTest phase)
+    {
+        if (phase != CasRefLedger::CarvePhaseForTest::PostDurableInstall)
+            return;
+        observations.fetch_add(1);
+        if (store->laneStateForTest(ns) == RefLaneState::Writing)
+            pending_observations.fetch_add(1);
+    });
+
+    publishEmptyPart(store, ns, "part_a");
+    store->setCarveHookForTest(nullptr);
+
+    EXPECT_GT(observations.load(), 0u) << "the post-durable seam must be reached by an ordinary commit";
+    EXPECT_EQ(pending_observations.load(), observations.load())
+        << "every durable-but-not-yet-installed transaction remains Writing";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready)
+        << "a completed install owes no apply: the marker must be back to Clean";
+}
+
+/// Ambiguity transfers the same exact attempt from `Writing` to `Wedged`.
+TEST(CASRefInstallSafety, UnresolvedTransfersWritingToWedged)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/apply_state_wedge"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::Unresolved;
+    backend->fault_count = 1;
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { publishEmptyPart(store, ns, "part_a"); });
+
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Wedged)
+        << "a wedged lane may hold a durable transaction the runtime has not recorded";
+}
+
+/// Durable resolution installs the attempt and returns the lane to `Ready`.
+TEST(CASRefInstallSafety, WedgeResolutionReturnsReady)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/apply_state_unwedge"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    /// The one mode that wedges over a GENUINELY durable object (see `ChunkFaultBackend`): the write
+    /// lands, its acknowledgement is lost, and the controller's own verifying read is lost too.
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::LandedThenLost;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::Wedged);
+
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::None;
+    store->dropRef(ns, "y");
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready)
+        << "the wedged transaction was proven durable AND installed, so nothing is owed any more";
+}
+
+/// A foreign occupant is a terminal `Faulted` verdict.
+TEST(CASRefInstallSafety, ConclusiveForeignConflictFaultsTheLane)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/apply_state_conflict"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::ForeignConflict;
+    backend->fault_count = 1;
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { publishEmptyPart(store, ns, "part_a"); });
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "a proven conflict is conclusive and must not wedge";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Faulted)
+        << "a foreign occupant is a terminal protocol verdict, not a retryable attempt";
+}
+
+/// `DefiniteFailure` proves nothing became durable and returns the lane to `Ready`. Needs S3 error
+/// classification: that is the only exception family
+/// `classifyConditionalWriteResult` will ever call definite (everything else is fail-safe Unresolved).
+TEST(CASRefInstallSafety, DefiniteFailureReturnsReady)
+{
+#if !USE_AWS_S3
+    GTEST_SKIP() << "DefiniteFailure classification requires S3 error types (USE_AWS_S3 off)";
+#else
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/apply_state_definite"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::Definite;
+    backend->fault_count = 1;
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { publishEmptyPart(store, ns, "part_a"); });
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "a definite failure is proven non-durable and must not wedge";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready)
+        << "a definitively rejected PUT is proven non-durable, so no apply is owed";
+#endif
+}
+
+/// A foreign occupant is the conclusive negative wedge resolution:
+/// the wedged (write-once) key prove our body never landed there. `resolveByExactGet` never reports a
+/// plain "absent" verdict -- absent or unreadable is `Unresolved`, since another attempt may still be
+/// legal -- so this arm is the whole of "a resolution that proves the key is not ours".
+///
+/// Runs in every build: the arm reports `CORRUPTED_DATA` (storage-controlled input must never be able
+/// to abort the server), where it used to raise the process-aborting `LOGICAL_ERROR` and this test had
+/// to be release-only with a death-test twin standing in. The marker is cleared BEFORE the anomaly
+/// reaction, which is what this test pins; the fence/audit half is
+/// `CASAnomalyPolicy.ForeignBytesAtWedgeKeyTripFenceAndRemount`'s.
+TEST(CASRefInstallSafety, WedgeResolutionProvenForeignFaultsTheLane)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/apply_state_foreign_wedge"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::Unresolved;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::Wedged);
+
+    /// Out of band, a foreign writer lands DIFFERENT bytes at the exact wedged key. The fault mode is
+    /// off first so this write is not itself intercepted.
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::None;
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_FALSE(wedged_key.empty());
+    ASSERT_EQ(backend->putIfAbsent(wedged_key, "a-different-object").outcome, PutOutcome::Done);
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "y"); });
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Faulted)
+        << "foreign interference is a terminal verdict, not an unresolved attempt";
+}
+
+/// `Writing -> NeedsRecovery` at the ordinary candidate install. Unreachable
+/// in production with §A1 landed -- the region allocates nothing -- so the probe seam simulates the
+/// post-durable failure. The transaction is durable at that point, but the runtime has not installed it:
+/// exactly the condition that `NeedsRecovery` names.
+TEST(CASRefInstallSafety, PostDurableInstallFailureRequiresRecovery)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/apply_state_poison"};
+
+    publishEmptyPart(store, ns, "x");
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+
+    const uint64_t needs_recovery_before = ProfileEvents::global_counters[ProfileEvents::CASRefNeedsRecovery].load();
+    armOneShotInstallFailure(store);
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED, [&] { store->dropRef(ns, "x"); });
+    store->setInstallRegionProbeForTest(nullptr);
+
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery)
+        << "an install that failed AFTER its object was durable must be visible, not silent";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefNeedsRecovery].load() - needs_recovery_before, 1u)
+        << "the transition to NeedsRecovery must be exported exactly once";
+}
+
+/// `Wedged -> NeedsRecovery` at the wedge-resolution install. Same class of failure
+/// one region over: the resolving GET already PROVED the object durable, so an install that does not
+/// complete there leaves the same missing transaction -- and the wedge survives, because the swap that
+/// would have cleared it is in the same region that threw.
+TEST(CASRefInstallSafety, WedgeResolutionInstallFailureRequiresRecovery)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/apply_state_poison_unwedge"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::LandedThenLost;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::None;
+    armOneShotInstallFailure(store);
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED, [&] { store->dropRef(ns, "y"); });
+    store->setInstallRegionProbeForTest(nullptr);
+
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns))
+        << "known durability transfers ownership from the attempt to recovery";
+}
+
+/// A later append may proceed only after the top-of-flush recovery has replayed the known-durable
+/// transaction and returned the lane to `Ready`.
+TEST(CASRefInstallSafety, NeedsRecoveryReplaysBeforeALaterFlush)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/apply_state_poison_terminal"};
+
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    const uint64_t needs_recovery_before = ProfileEvents::global_counters[ProfileEvents::CASRefNeedsRecovery].load();
+    armOneShotInstallFailure(store);
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED, [&] { store->dropRef(ns, "x"); });
+    store->setInstallRegionProbeForTest(nullptr);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    /// A perfectly ordinary, fully successful append afterwards.
+    store->dropRef(ns, "y");
+    EXPECT_FALSE(store->resolveRef(ns, "y", /*allow_stale=*/false).has_value())
+        << "the later flush must really have committed AND installed -- otherwise the assertion below "
+           "would pass for the wrong reason";
+
+    /// A flush's own success is not evidence that the stranded transaction was installed. What returns
+    /// the lane to `Ready` is the re-derivation that `ensureRefTableRecovered` performs at the top of
+    /// that same flush: the walk reads the stranded transaction from the durable log, then installs the
+    /// recovered state.
+    ///
+    /// Both halves are asserted, because only together do they mean the repair happened rather than the
+    /// state being relabeled: `x` really is gone (the stranded drop is applied at last), and the lane is
+    /// `Ready`.
+    EXPECT_FALSE(store->resolveRef(ns, "x", /*allow_stale=*/false).has_value())
+        << "the stranded drop of 'x' is durable, so the re-derivation must install it before returning "
+           "the lane to Ready";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefNeedsRecovery].load() - needs_recovery_before, 1u)
+        << "the event counts transitions, so the successful flush must not have added another";
+}
+
+/// Part B review, BLOCKER 2: an UNCERTAIN `precommitAdd` must keep its cleanup owner and its body.
+///
+/// `PartWriteTxn::precommitAdd` used to record `precommit_*` and set its `precommitted` flag only AFTER
+/// `appendRefOps` returned. But an `Unresolved` append MAY HAVE LANDED -- the `Unresolved` arm of
+/// `commitRefChunk` says exactly that, and wedges the lane for precisely that reason -- so on that path
+/// `abandon` ran against an object that believed it had never precommitted. It therefore queued NO
+/// removal, and `cleanupStagedManifestDebrisBestEffort`, deciding from the same unset state, DELETED the
+/// manifest body. When the wedge later resolved as committed, the table gained a live precommit with no
+/// cleanup owner and no body -- which clamps GC's fold barrier (a live precommit whose body is missing)
+/// forever.
+///
+/// The fix is the same discipline the wedge itself uses: record the intent BEFORE the ambiguous
+/// operation. Both assertions below fail against the old code -- the body is gone, and the precommit is
+/// still live once the wedge resolves.
+TEST(CASRefInstallSafety, UncertainPrecommitKeepsItsCleanupOwnerAndItsBody)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openPoolSingleAttempt(backend);
+    const RootNamespace ns{"srv1/uncertain_precommit"};
+    /// Stage B (Task 4-C): pin `ns` to the Stage-A sentinel BEFORE its first real touch, so
+    /// the fault injected below (computed from that same sentinel) lands on the key production
+    /// actually writes to.
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/part_a";
+    auto build = store->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+    const String manifest_key = store->layout().manifestKey(id);
+    ASSERT_TRUE(backend->head(manifest_key).exists) << "the staged body must exist before the precommit";
+
+    /// Scoped to THIS namespace's ref log so the manifest body's own PUT cannot consume the fault. The
+    /// object LANDS and only its acknowledgement is lost, which with the single-attempt budget wedges
+    /// the lane over a genuinely durable precommit -- the exact shape the old code mishandled.
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::LandedThenLost;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { build->precommitAdd(ns, "part_a", id); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns)) << "the lost-response precommit must wedge the lane";
+    EXPECT_EQ(build->precommitState(), PartWriteTxn::PrecommitState::Uncertain)
+        << "an append that may have landed is neither 'never precommitted' nor 'durably precommitted'";
+
+    /// The cleanup owner survives the uncertainty: this `abandon` resolves the wedge (proving the
+    /// precommit durable) and appends the exact removal in the same flush.
+    build->abandon();
+
+    EXPECT_TRUE(backend->head(manifest_key).exists)
+        << "abandon writer-deleted the body of a precommit that may be live -- GC's fold barrier would "
+           "clamp on it forever";
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty())
+        << "the uncertain precommit landed, so abandon owed its exact removal";
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "the abandon's own flush must have resolved the wedge";
+}
+
+/// The other side of the same state, and the reason it is a STATE and not just an extra bool: an
+/// `Uncertain` precommit that in fact never landed must not make `abandon` fail forever.
+///
+/// `RefTableState::applyOwnerTransition` rejects a removal whose `old_binding` names an absent
+/// precommit, so the removal is NOT unconditionally idempotent (the review's "it is idempotent" is only
+/// true with the presence check this test pins). Here the append is refused BEFORE any request is sent,
+/// which is provably-nothing-durable, and yet the transaction has already recorded the intent -- so the
+/// removal it owes must resolve to a no-op rather than to `CORRUPTED_DATA`.
+TEST(CASRefInstallSafety, UncertainPrecommitThatNeverLandedStillAbandonsCleanly)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = openPoolFenceControlled(backend);
+    const RootNamespace ns{"srv1/uncertain_precommit_absent"};
+
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/part_a";
+    auto build = store->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+
+    /// A lease with room for the flush but not for one whole controlled attempt: the pre-attempt gate
+    /// refuses, nothing is sent, and no wedge forms (`PreAttemptRefusalDoesNotWedgeTheLane`).
+    store->setMountDeadline(FENCE_DEADLINE_REFUSES_ATTEMPT_MS);
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { build->precommitAdd(ns, "part_a", id); });
+    ASSERT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(build->precommitState(), PartWriteTxn::PrecommitState::Uncertain);
+
+    store->setMountDeadline(FENCE_DEADLINE_HEALTHY_MS);
+    build->abandon();   /// must not throw: there is no binding to remove, and that is not an anomaly here
+    EXPECT_EQ(build->precommitState(), PartWriteTxn::PrecommitState::Settled);
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+}
diff --git a/src/Disks/tests/gtest_cas_ref_intake.cpp b/src/Disks/tests/gtest_cas_ref_intake.cpp
new file mode 100644
index 000000000000..49ae6bed4137
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_intake.cpp
@@ -0,0 +1,260 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Common/Exception.h>
+
+using namespace DB::Cas;
+
+namespace
+{
+
+ManifestRef mr(uint64_t epoch, uint64_t seq, uint32_t ordinal = 1)
+{
+    return ManifestRef{epoch, seq, ordinal};
+}
+
+RefTxnId rid(uint64_t epoch, uint64_t seq)
+{
+    return RefTxnId{epoch, seq};
+}
+
+RefOp addOwner(RefOwnerKind kind, const String & ref, const ManifestRef & manifest)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{kind, ref, manifest};
+    return op;
+}
+
+RefOp removeOwner(RefOwnerKind kind, const String & ref, const ManifestRef & manifest)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{kind, ref, manifest};
+    return op;
+}
+
+RefOp promote(const String & ref, const ManifestRef & manifest)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, ref, manifest};
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Committed, ref, manifest};
+    return op;
+}
+
+/// A raw `owner_transition` op from explicit optional bindings, bypassing every shape-builder above --
+/// used by the rejection tests to construct shapes `classifyOwnerTransitionShape` does not recognize.
+RefOp rawOwnerTransition(std::optional<RefOwnerBinding> old_binding, std::optional<RefOwnerBinding> new_binding)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = std::move(old_binding);
+    op.new_binding = std::move(new_binding);
+    return op;
+}
+
+RefLogTxn txn(const String & ns, RefTxnId id, std::vector<RefOp> ops)
+{
+    RefLogTxn t;
+    t.ns = ns;
+    t.txn_id = id;
+    t.ops = std::move(ops);
+    return t;
+}
+
+}
+
+/// spec §gc-step-produce-manifest-edge-delta: each explicit operation states its own edge change.
+TEST(CASRefIntake, ManifestEdgesPerOperationShape)
+{
+    /// Add precommit => one +1.
+    {
+        const auto edges = manifestEdgesOfTxn(txn("db/t", rid(1, 1), {addOwner(RefOwnerKind::Precommit, "p", mr(1, 5))}));
+        ASSERT_EQ(edges.size(), 1u);
+        EXPECT_EQ(edges[0].change, 1);
+        EXPECT_EQ(edges[0].manifest_id, (ManifestId{RootNamespace{"db/t"}, mr(1, 5)}));
+        EXPECT_EQ(edges[0].op_ordinal, 0u);
+        EXPECT_EQ(edges[0].edge_ordinal, 1u);
+    }
+    /// Remove committed => one -1.
+    {
+        const auto edges = manifestEdgesOfTxn(txn("db/t", rid(1, 2), {removeOwner(RefOwnerKind::Committed, "p", mr(1, 5))}));
+        ASSERT_EQ(edges.size(), 1u);
+        EXPECT_EQ(edges[0].change, -1);
+        EXPECT_EQ(edges[0].manifest_id, (ManifestId{RootNamespace{"db/t"}, mr(1, 5)}));
+    }
+    /// Remove precommit => one -1 (the fourth classified shape, distinct from remove committed only by
+    /// `old_binding.kind`).
+    {
+        const auto edges = manifestEdgesOfTxn(txn("db/t", rid(1, 25), {removeOwner(RefOwnerKind::Precommit, "p", mr(1, 5))}));
+        ASSERT_EQ(edges.size(), 1u);
+        EXPECT_EQ(edges[0].change, -1);
+        EXPECT_EQ(edges[0].owner_kind, RefOwnerKind::Precommit);
+        EXPECT_EQ(edges[0].manifest_id, (ManifestId{RootNamespace{"db/t"}, mr(1, 5)}));
+    }
+    /// Promote same manifest => no net edge (spec §Promote).
+    {
+        const auto edges = manifestEdgesOfTxn(txn("db/t", rid(1, 3), {promote("p", mr(1, 5))}));
+        EXPECT_TRUE(edges.empty());
+    }
+    /// set_published_at / namespace_birth / remove_namespace => no edge.
+    {
+        RefOp set_published_at;
+        set_published_at.kind = RefOpKind::SetPublishedAt;
+        set_published_at.ref_name = "p";
+        set_published_at.expected_manifest_ref = mr(1, 5);
+        EXPECT_TRUE(manifestEdgesOfTxn(txn("db/t", rid(1, 4), {set_published_at})).empty());
+
+        RefOp birth;
+        birth.kind = RefOpKind::NamespaceBirth;
+        EXPECT_TRUE(manifestEdgesOfTxn(txn("db/t", rid(1, 5), {birth})).empty());
+    }
+    /// Replace one manifest by a different one (two explicit ops) => -1 old, +1 new.
+    {
+        const auto edges = manifestEdgesOfTxn(txn("db/t", rid(1, 6),
+            {removeOwner(RefOwnerKind::Committed, "p", mr(1, 5)), addOwner(RefOwnerKind::Precommit, "p", mr(1, 6))}));
+        ASSERT_EQ(edges.size(), 2u);
+        EXPECT_EQ(edges[0].change, -1);
+        EXPECT_EQ(edges[0].manifest_id.ref, mr(1, 5));
+        EXPECT_EQ(edges[1].change, 1);
+        EXPECT_EQ(edges[1].manifest_id.ref, mr(1, 6));
+    }
+}
+
+/// `manifestEdgesOfTxn` rejects every `owner_transition` shape outside the four `classifyOwnerTransitionShape`
+/// recognizes (Pool/CasRefProtocol.cpp) -- it must never silently assign edge meaning to a shape the
+/// writer/replay state machine would refuse to apply. Each case throws `CORRUPTED_DATA`.
+TEST(CASRefIntake, ManifestEdgesRejectsUnrecognizedShapes)
+{
+    /// Neither binding: a degenerate owner_transition that names no owner change at all.
+    EXPECT_THROW(manifestEdgesOfTxn(txn("db/t", rid(1, 1), {rawOwnerTransition(std::nullopt, std::nullopt)})),
+                 DB::Exception);
+
+    /// old+new naming DIFFERENT manifests in ONE op (the never-legal "replace" shape; an atomic
+    /// manifest replace is always two ops -- an explicit removal then a same-manifest promote).
+    EXPECT_THROW(manifestEdgesOfTxn(txn("db/t", rid(1, 2),
+        {rawOwnerTransition(RefOwnerBinding{RefOwnerKind::Committed, "p", mr(1, 5)},
+                             RefOwnerBinding{RefOwnerKind::Precommit, "p", mr(1, 6)})})),
+                 DB::Exception);
+
+    /// Promote-shaped kinds (old=Precommit, new=Committed) but with MISMATCHED ref_names.
+    EXPECT_THROW(manifestEdgesOfTxn(txn("db/t", rid(1, 3),
+        {rawOwnerTransition(RefOwnerBinding{RefOwnerKind::Precommit, "p", mr(1, 5)},
+                             RefOwnerBinding{RefOwnerKind::Committed, "q", mr(1, 5)})})),
+                 DB::Exception);
+
+    /// Add with new.kind == Committed (only Precommit is a legal add target).
+    EXPECT_THROW(manifestEdgesOfTxn(txn("db/t", rid(1, 4),
+        {rawOwnerTransition(std::nullopt, RefOwnerBinding{RefOwnerKind::Committed, "p", mr(1, 5)})})),
+                 DB::Exception);
+
+    /// old+new both Committed, same manifest: not a promote (promote requires old.kind == Precommit).
+    EXPECT_THROW(manifestEdgesOfTxn(txn("db/t", rid(1, 5),
+        {rawOwnerTransition(RefOwnerBinding{RefOwnerKind::Committed, "p", mr(1, 5)},
+                             RefOwnerBinding{RefOwnerKind::Committed, "p", mr(1, 5)})})),
+                 DB::Exception);
+}
+
+/// Namespaces are edge-distinct even with identical ManifestRef tuples (spec §gc-inputs-and-output).
+TEST(CASRefIntake, EdgesAreNamespaceQualified)
+{
+    const auto a = manifestEdgesOfTxn(txn("db/a", rid(1, 1), {addOwner(RefOwnerKind::Precommit, "p", mr(1, 5))}));
+    const auto b = manifestEdgesOfTxn(txn("db/b", rid(1, 1), {addOwner(RefOwnerKind::Precommit, "p", mr(1, 5))}));
+    ASSERT_EQ(a.size(), 1u);
+    ASSERT_EQ(b.size(), 1u);
+    EXPECT_NE(a[0].manifest_id, b[0].manifest_id);
+}
+
+TEST(CASRefIntake, RemovalTxnIdDetection)
+{
+    RefOp remove_ns;
+    remove_ns.kind = RefOpKind::RemoveNamespace;
+    const auto with_removal = txn("db/t", rid(3, 8), {removeOwner(RefOwnerKind::Committed, "p", mr(1, 5)), remove_ns});
+    ASSERT_TRUE(removalTxnId(with_removal).has_value());
+    EXPECT_EQ(*removalTxnId(with_removal), rid(3, 8));
+
+    const auto ordinary = txn("db/t", rid(3, 9), {addOwner(RefOwnerKind::Precommit, "p", mr(1, 5))});
+    EXPECT_FALSE(removalTxnId(ordinary).has_value());
+}
+
+/// spec §Step 1: one global LIST groups by table, split by kind, sorted; the reconstructed namespace is
+/// re-validated (VERIFY-AT-T12) and a malformed ref key aborts ref folding (throws).
+TEST(CASRefIntake, GroupRefKeys)
+{
+    const Layout layout{"p"};
+    const RootNamespace ns{"db/t"};
+    const NamespaceLifeId life = DB::Cas::tests::fixture::fixtureLife(ns);
+
+    std::vector<String> keys{
+        layout.refSnapshotKey(life, rid(1, 4)),
+        layout.refLogKey(life, rid(1, 5)),
+        layout.refLogKey(life, rid(1, 3)),
+        layout.refCkptKey(life),        /// state-family keys are outside the hot stream LIST
+        "p/cas/manifests/db/t/foo",   /// outside the ref prefix -> ignored
+    };
+    const auto grouped = groupRefKeys(layout, keys);
+    ASSERT_EQ(grouped.size(), 1u);
+    const RefTableListing & t = grouped.at(life.incarnation);
+    EXPECT_EQ(t.logs, (std::vector<RefTxnId>{rid(1, 3), rid(1, 5)}));
+    EXPECT_EQ(t.snapshots, (std::vector<RefTxnId>{rid(1, 4)}));
+    /// Checkpoints live under `cas/ns/state/` and are deliberately absent from the hot stream listing.
+
+    /// A key under the ref prefix that is not a valid ref object aborts (a leftover old-format shard key).
+    EXPECT_THROW(groupRefKeys(layout, {"p/cas/ns/stream/0"}), DB::Exception);
+    /// A malformed physical id under a valid stream prefix aborts.
+    EXPECT_THROW(groupRefKeys(layout, {"p/cas/ns/stream/not-an-id/_log/" + renderRefTxnId(rid(1, 1))}), DB::Exception);
+}
+
+/// A LIST can observe a snapshot after its PUT but before the `_ckpt` CAS makes it a recovery base.
+/// That physical object proves nothing by itself: without a checkpoint-named triple, cleanup leaks
+/// rather than deleting either the genesis log or the unacknowledged snapshot.
+TEST(CASRefIntake, PlanRefCleanupRequiresCheckpointNamedBase)
+{
+    RefTableListing listing;
+    listing.logs = {rid(1, 1), rid(1, 2), rid(1, 3)};
+    listing.snapshots = {rid(1, 2)};   /// newest observed snapshot X = (1,2)
+
+    /// Even a complete-looking listing and cursor do not license cleanup without the checkpoint's
+    /// exact base. This is the snapshot-PUT-before-checkpoint-CAS sabotage.
+    {
+        const auto plan = planRefCleanup(listing, rid(1, 3), {});
+        EXPECT_TRUE(plan.deletable_logs.empty());
+        EXPECT_TRUE(plan.deletable_snapshots.empty());
+    }
+    /// A smaller cursor cannot turn that incomplete authority into a cleanup range.
+    {
+        const auto plan = planRefCleanup(listing, rid(1, 1), {});
+        EXPECT_TRUE(plan.deletable_logs.empty());
+    }
+    /// Nor may a newer listed snapshot reclaim an older listed snapshot before `_ckpt` names a base.
+    {
+        RefTableListing two_snaps = listing;
+        two_snaps.snapshots = {rid(1, 1), rid(1, 2)};
+        const auto plan = planRefCleanup(two_snaps, rid(1, 3), {});
+        EXPECT_TRUE(plan.deletable_snapshots.empty());
+    }
+    /// No snapshot => no coverage boundary => empty plan (condition 2).
+    {
+        RefTableListing no_snap;
+        no_snap.logs = {rid(1, 1)};
+        const auto plan = planRefCleanup(no_snap, rid(1, 5), {});
+        EXPECT_TRUE(plan.deletable_logs.empty());
+        EXPECT_TRUE(plan.deletable_snapshots.empty());
+    }
+}
+
+/// The checkpoint recovery anchor is a triple: `_ckpt`, its same-id `_snap`, and the same-id ordinary
+/// `_log` that proves the id is not an `EpochSeal`. Cleanup may reclaim older covered logs, but must
+/// retain that one witness for recovery and fsck.
+TEST(CASRefIntake, PlanRefCleanupRetainsCheckpointBaseLog)
+{
+    RefTableListing listing;
+    listing.logs = {rid(1, 1), rid(1, 2), rid(1, 3)};
+    listing.snapshots = {rid(1, 2)};
+
+    const RefCleanupPlan plan = planRefCleanup(listing, rid(1, 3), rid(1, 2));
+    EXPECT_EQ(plan.deletable_logs, (std::vector<RefTxnId>{rid(1, 1)}));
+    EXPECT_TRUE(plan.deletable_snapshots.empty());
+}
diff --git a/src/Disks/tests/gtest_cas_ref_lane_exception_safety.cpp b/src/Disks/tests/gtest_cas_ref_lane_exception_safety.cpp
new file mode 100644
index 000000000000..a88838dd7049
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_lane_exception_safety.cpp
@@ -0,0 +1,217 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Common/Exception.h>
+
+#include <atomic>
+#include <condition_variable>
+#include <mutex>
+#include <thread>
+#include <vector>
+
+/// Task 1: ref-lane exception-safety. A queue leader that throws BEFORE carving its compatible batch
+/// must not leave its own enqueued item stranded in `rt->pending`. If it does, a later leader (a woken
+/// follower) carves the stranded item and runs its `build_ops` closure long after the original caller's
+/// stack -- which the production `[&]` closures capture by reference -- has unwound: a use-after-free.
+///
+/// These tests drive the fault through the SAME pre-carve injection point production leaders pass
+/// (`setRefPreCarveHookForTest`, invoked inside `flushRefBatch` immediately before the batch is carved).
+/// The suite name is prefixed `RefWriter` so it is covered by the `RefWriter*` unit-test gate filter.
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+PoolPtr openPoolForRefLane(const BackendPtr & backend)
+{
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+}
+
+/// A SOLO faulted caller must not leave its own item behind in the pending queue. Before the fix, the
+/// leader's `appendRefOps` catch reset `leader_active` and rethrew but never completed / de-pended the
+/// leader's own item, so it was stranded in `rt->pending` with `done == false` forever (nothing left to
+/// carve it) -- the deterministic, sanitizer-independent shape of the stranded-item defect.
+TEST(CASRefWriterLaneExceptionSafety, SoloLeaderThrowBeforeCarveDrainsOwnItem)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForRefLane(backend);
+    const RootNamespace ns{"srv1/reflane_solo"};
+
+    std::atomic<int> fault_armed{1};
+    store->setRefPreCarveHookForTest([&]
+    {
+        if (fault_armed.exchange(0) == 1)
+            throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA, "injected pre-carve fault");
+    });
+
+    bool threw = false;
+    try
+    {
+        store->appendRefOps(ns, MutationScope::ref("ref_solo"),
+            [](const RefTableState &) -> std::vector<RefOp> { return {}; },
+            RootMutationOrigin::Writer, RootMutationKind::Publish);
+    }
+    catch (const DB::Exception &)
+    {
+        threw = true;
+    }
+    store->setRefPreCarveHookForTest(nullptr);
+
+    EXPECT_TRUE(threw) << "the faulted solo caller must observe the injected error";
+    EXPECT_EQ(store->refQueuePendingForTest(ns), 0u)
+        << "the leader's own item was left stranded in rt->pending after it threw before carving";
+}
+
+/// Two concurrent callers on one namespace. The first flush's leader throws before carving; a woken
+/// follower then leads. Before the fix, the follower carved the faulted leader's STILL-pending item and
+/// ran its `build_ops` closure -- the use-after-free window. This asserts, sanitizer-independently, that
+/// the follower never invokes the faulted caller's closure, that the queue drains, and that the
+/// non-faulted caller still completes.
+TEST(CASRefWriterLaneExceptionSafety, FollowerNeverRunsStrandedLeaderClosure)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForRefLane(backend);
+    const RootNamespace ns{"srv1/reflane_follower"};
+
+    std::atomic<int> fault_armed{1};
+    /// The leader parks HERE, at the pre-carve point, until the main thread has queued the follower
+    /// behind it -- and only then throws. Parking (rather than letting the leader race ahead while the
+    /// main thread polls the queue depth) is what makes the interleaving this test is about --
+    /// "the leader throws WHILE a follower is waiting for the baton" -- deterministic. The previous
+    /// formulation polled `refQueuePendingForTest(ns) >= 1` from the main thread AFTER starting t1,
+    /// which loses a race the scheduler decides: t1 could enqueue, take the baton, throw, and have its
+    /// item erased by `completeOwnedItemsAndReleaseLeadership` before the main thread was ever
+    /// scheduled to sample -- after which `pending` is 0 forever and the poll spins until the harness
+    /// is killed. Invisible on an idle 32-core box (10/10 green) and a guaranteed hang under
+    /// contention (8/8 when pinned to one CPU with `taskset -c 3`).
+    std::mutex hook_mutex;
+    std::condition_variable hook_cv;
+    bool leader_parked_at_precarve = false;   /// guarded by hook_mutex
+    bool release_leader = false;              /// guarded by hook_mutex
+    store->setRefPreCarveHookForTest([&]
+    {
+        /// Park+throw only on the FIRST leader flush, so the follower (or a re-drive) can proceed.
+        if (fault_armed.exchange(0) != 1)
+            return;
+        {
+            std::lock_guard announce(hook_mutex);
+            leader_parked_at_precarve = true;
+        }
+        hook_cv.notify_all();
+        {
+            std::unique_lock wait_for_follower(hook_mutex);
+            hook_cv.wait(wait_for_follower, [&] { return release_leader; });
+        }
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA, "injected pre-carve fault");
+    });
+
+    /// Set by the faulted caller's own closure iff a DIFFERENT thread (a follower leader) ever runs it --
+    /// i.e. the stranded item was carved by someone other than its owner. This is the direct, portable
+    /// signature of the use-after-free the fix prevents.
+    std::atomic<std::thread::id> faulted_owner{};
+    std::atomic<bool> faulted_closure_ran_on_follower{false};
+
+    std::atomic<int> ok{0};
+    auto caller = [&](int seq, bool is_faulted)
+    {
+        try
+        {
+            store->appendRefOps(ns, MutationScope::ref("ref_" + std::to_string(seq)),
+                [&, is_faulted](const RefTableState &) -> std::vector<RefOp>
+                {
+                    if (is_faulted && std::this_thread::get_id() != faulted_owner.load())
+                        faulted_closure_ran_on_follower.store(true);
+                    return {};
+                },
+                RootMutationOrigin::Writer, RootMutationKind::Publish);
+            ok.fetch_add(1);
+        }
+        catch (const DB::Exception &) // NOLINT(bugprone-empty-catch)
+        {
+            /// The faulted caller may see the injected error; that is expected.
+        }
+    };
+
+    /// Serialize the two callers so the fault deterministically lands on the FIRST one to lead: t1
+    /// enqueues, takes the baton, and PARKS at the pre-carve hook; t2 then queues behind it as a
+    /// follower; only then is t1 released to throw.
+    std::thread t1([&]
+    {
+        faulted_owner.store(std::this_thread::get_id());
+        caller(1, /*is_faulted=*/true);
+    });
+    {
+        std::unique_lock wait_for_leader(hook_mutex);
+        hook_cv.wait(wait_for_leader, [&] { return leader_parked_at_precarve; });
+    }
+    std::thread t2([&] { caller(2, /*is_faulted=*/false); });
+    /// The parked leader cannot drain anything, so this poll cannot miss its window: t1's own item is
+    /// already in `pending`, and the count reaches 2 as soon as t2 has enqueued.
+    while (store->refQueuePendingForTest(ns) < 2)
+        std::this_thread::yield();
+    {
+        std::lock_guard release(hook_mutex);
+        release_leader = true;
+    }
+    hook_cv.notify_all();
+
+    t1.join();
+    t2.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    EXPECT_FALSE(faulted_closure_ran_on_follower.load())
+        << "a follower leader carved and ran the stranded faulted caller's build_ops closure (use-after-free)";
+    EXPECT_EQ(store->refQueuePendingForTest(ns), 0u) << "an item was stranded in rt->pending";
+    EXPECT_GE(ok.load(), 1) << "the non-faulted caller must complete cleanly";
+}
+
+/// codex stage-1 review (Important): an allocation exception at the PRE-TENURE point -- the first
+/// allocation that builds the leader's responsibility set, BEFORE `leader_active` is published -- must
+/// not permanently strand the append-lane baton. Before the fix the throwing allocation fired AFTER
+/// `leader_active = true` (and after the queue mutex was released), leaving the baton held with no live
+/// leader and the caller's item stuck in `pending`: every later writer on the namespace would wait
+/// forever at the leader-election cv, and shutdown draining could only time out. This drives the fault
+/// through the dedicated pre-tenure seam and asserts, deterministically (no hang), that the lane is left
+/// idle: the item is un-enqueued and the baton is un-taken.
+TEST(CASRefWriterLaneExceptionSafety, PreTenureAllocFailureReleasesBaton)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForRefLane(backend);
+    const RootNamespace ns{"srv1/reflane_pretenure"};
+
+    std::atomic<int> fault_armed{1};
+    store->setRefPreTenureHookForTest([&]
+    {
+        if (fault_armed.exchange(0) == 1)
+            throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA, "injected pre-tenure fault");
+    });
+
+    bool threw = false;
+    try
+    {
+        store->appendRefOps(ns, MutationScope::ref("ref_pretenure"),
+            [](const RefTableState &) -> std::vector<RefOp> { return {}; },
+            RootMutationOrigin::Writer, RootMutationKind::Publish);
+    }
+    catch (const DB::Exception &)
+    {
+        threw = true;
+    }
+    store->setRefPreTenureHookForTest(nullptr);
+
+    EXPECT_TRUE(threw) << "the faulted caller must observe the injected error";
+    EXPECT_EQ(store->refQueuePendingForTest(ns), 0u)
+        << "a pre-tenure allocation failure left the caller's item stranded in rt->pending";
+    EXPECT_FALSE(store->refLeaderActiveForTest(ns))
+        << "a pre-tenure allocation failure left the append-lane baton held with no live leader";
+}
diff --git a/src/Disks/tests/gtest_cas_ref_log_format.cpp b/src/Disks/tests/gtest_cas_ref_log_format.cpp
new file mode 100644
index 000000000000..d5186e77aca1
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_log_format.cpp
@@ -0,0 +1,790 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <random>
+#include <vector>
+
+/// v3 text codec tests for `cas_ref_log` (codecs-v3 phase 3). Split out of the retired
+/// `gtest_cas_ref_codecs.cpp` and re-pointed at the TEXT codec: the encoder-side validation tests are
+/// format-agnostic (they only assert `encodeRefLogTxn` throws) and carry over verbatim; the old
+/// binary-offset byte-patch decode tests (`bytes[k] = 99`) are gone — the shape-level corruption
+/// classes (truncation, `v`+1 forward-gate, wrong type, leading garbage) are now covered by the
+/// `CASFormatBattery.RefLog` row below. `RefTxnId` render/parse coverage lives here too (it rode in
+/// the same suite and is independent of either ref codec).
+
+using namespace DB::Cas;
+using DB::Cas::tests::expectThrowsCode;
+
+namespace
+{
+
+ManifestRef manifestRef(uint64_t epoch, uint64_t seq, uint32_t ordinal)
+{
+    return ManifestRef{epoch, seq, ordinal};
+}
+
+}
+
+/// ===================================================================================
+/// RefTxnId: render / parse
+/// ===================================================================================
+
+TEST(CASRefCodec, RenderCanonicalForm)
+{
+    EXPECT_EQ(renderRefTxnId(RefTxnId{7, 0x8e}), "0000000000000007-000000000000008e");
+    EXPECT_EQ(renderRefTxnId(RefTxnId{1, 1}), "0000000000000001-0000000000000001");
+    EXPECT_EQ(renderRefTxnId(RefTxnId{0xffffffffffffffffULL, 0xffffffffffffffffULL}),
+        "ffffffffffffffff-ffffffffffffffff");
+}
+
+TEST(CASRefCodec, RenderRejectsZeroComponent)
+{
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            renderRefTxnId(RefTxnId{0, 1});
+        },
+        "RefTxnId: writer_epoch and ref_sequence must both be nonzero");
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            renderRefTxnId(RefTxnId{1, 0});
+        },
+        "RefTxnId: writer_epoch and ref_sequence must both be nonzero");
+    EXPECT_DEATH(
+        {
+            DB::abort_on_logical_error.store(true, std::memory_order_relaxed);
+            renderRefTxnId(RefTxnId{0, 0});
+        },
+        "RefTxnId: writer_epoch and ref_sequence must both be nonzero");
+}
+
+TEST(CASRefCodec, ParseRoundTrip)
+{
+    for (const RefTxnId id : {RefTxnId{7, 0x8e}, RefTxnId{1, 1}, RefTxnId{255, 2}, RefTxnId{0x100000000ULL, 3},
+                               RefTxnId{0x8000000000000000ULL, 0x8000000000000000ULL},
+                               RefTxnId{0xffffffffffffffffULL, 0xffffffffffffffffULL}})
+    {
+        const String rendered = renderRefTxnId(id);
+        const auto parsed = parseRefTxnId(rendered);
+        ASSERT_TRUE(parsed.has_value());
+        EXPECT_EQ(*parsed, id);
+    }
+}
+
+TEST(CASRefCodec, ParseRejectsShort)
+{
+    EXPECT_FALSE(parseRefTxnId("000000000000007-000000000000008e").has_value());   /// 32 chars, one short
+    EXPECT_FALSE(parseRefTxnId("7-8e").has_value());
+    EXPECT_FALSE(parseRefTxnId("").has_value());
+}
+
+TEST(CASRefCodec, ParseRejectsLong)
+{
+    EXPECT_FALSE(parseRefTxnId("00000000000000007-000000000000008e").has_value());  /// 34 chars, one long
+    EXPECT_FALSE(parseRefTxnId("0000000000000007-000000000000008e0").has_value());
+}
+
+TEST(CASRefCodec, ParseRejectsUppercase)
+{
+    EXPECT_FALSE(parseRefTxnId("0000000000000007-00000000000000AE").has_value());
+    EXPECT_FALSE(parseRefTxnId("0000000000000007-000000000000008E").has_value());
+    EXPECT_FALSE(parseRefTxnId("0000000000000007-00000000000000Ae").has_value());  /// mixed case
+}
+
+TEST(CASRefCodec, ParseRejectsZeroComponent)
+{
+    EXPECT_FALSE(parseRefTxnId("0000000000000000-000000000000008e").has_value());
+    EXPECT_FALSE(parseRefTxnId("0000000000000007-0000000000000000").has_value());
+    EXPECT_FALSE(parseRefTxnId("0000000000000000-0000000000000000").has_value());
+}
+
+TEST(CASRefCodec, ParseRejectsNonHexGarbage)
+{
+    EXPECT_FALSE(parseRefTxnId("000000000000000g-000000000000008e").has_value());
+    EXPECT_FALSE(parseRefTxnId("!!!!!!!!!!!!!!!!-000000000000008e").has_value());
+    EXPECT_FALSE(parseRefTxnId("0000000000000007_000000000000008e").has_value());  /// wrong separator
+}
+
+TEST(CASRefCodec, ParseRejectsMisplacedSeparator)
+{
+    /// 17 hex digits then '-' then 15: same total length (33), dash at the wrong index -- the kind of
+    /// shape that, read naively without a fixed dash position, could be mistaken for an in-range but
+    /// overflowing first component.
+    EXPECT_FALSE(parseRefTxnId("00000000000000078-00000000000000e").has_value());
+}
+
+TEST(CASRefCodec, OrderMatchesLexicalOrderOfRender)
+{
+    const std::vector<uint64_t> values{1, 2, 255, 1ULL << 32, 1ULL << 63};
+    std::vector<RefTxnId> ids;
+    for (uint64_t epoch : values)
+        for (uint64_t seq : values)
+            ids.push_back(RefTxnId{epoch, seq});
+
+    std::mt19937 rng(42); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    for (int iter = 0; iter < 200; ++iter)
+    {
+        const RefTxnId & a = ids[rng() % ids.size()];
+        const RefTxnId & b = ids[rng() % ids.size()];
+        const String ra = renderRefTxnId(a);
+        const String rb = renderRefTxnId(b);
+        EXPECT_EQ(a < b, ra < rb) << ra << " vs " << rb;
+        EXPECT_EQ(a == b, ra == rb);
+    }
+}
+
+/// ===================================================================================
+/// RefLogTxn: round trip
+/// ===================================================================================
+
+TEST(CASRefCodec, RoundTripNamespaceBirth)
+{
+    RefLogTxn txn;
+    txn.ns = "srv1/db/table@cas@";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::NamespaceBirth;
+    txn.ops.push_back(op);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+}
+
+TEST(CASRefCodec, RoundTripRemoveNamespace)
+{
+    RefLogTxn txn;
+    txn.ns = "srv1/db/table@cas@";
+    txn.txn_id = RefTxnId{1, 2};
+    RefOp op;
+    op.kind = RefOpKind::RemoveNamespace;
+    txn.ops.push_back(op);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+}
+
+TEST(CASRefCodec, RoundTripSetPublishedAt)
+{
+    RefLogTxn txn;
+    txn.ns = "srv1/db/table@cas@";
+    txn.txn_id = RefTxnId{3, 5};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "all_1_1_0";
+    op.expected_manifest_ref = manifestRef(3, 4, 1);
+    op.published_at_ms = 1717000000000ULL;
+    txn.ops.push_back(op);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+}
+
+/// No-tolerance decode pin (codex round-2, finding 3): the `"pl"` (payload) field was removed from the
+/// ref-op wire in stage-1 T12. Although the retired `set_payload` op WORD is already rejected by
+/// `opKindFromWord`, the generic op-record reader reads all field keys before switching on kind, so a
+/// `"pl"` field paired with a still-recognized op word would otherwise be `skipUnknown`'d. It is a
+/// removed field, not a genuinely-unknown one: decoding an op record that still carries `"pl"` must FAIL
+/// with `CORRUPTED_DATA` naming the removed field.
+TEST(CASRefCodec, DecodeRejectsRemovedPayloadFieldInOpRecord)
+{
+    RefLogTxn txn;
+    txn.ns = "srv1/db/table@cas@";
+    txn.txn_id = RefTxnId{3, 5};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "all_1_1_0";
+    op.expected_manifest_ref = manifestRef(3, 4, 1);
+    op.published_at_ms = 1717000000000ULL;
+    txn.ops.push_back(op);
+
+    const String bytes = encodeRefLogTxn(txn);
+    /// Splice the retired `"pl"` field back into the op record, just before its `"ts"` field.
+    const String needle = ",\"ts\":";
+    const auto pos = bytes.find(needle);
+    ASSERT_NE(pos, String::npos);
+    const String tampered = bytes.substr(0, pos) + R"(,"pl":"deadbeef")" + bytes.substr(pos);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { decodeRefLogTxn(tampered, txn.ns, txn.txn_id); });
+}
+
+TEST(CASRefCodec, RoundTripSetPublishedAtZeroTimestamp)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    op.published_at_ms = 0;
+    txn.ops.push_back(op);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+}
+
+TEST(CASRefCodec, RoundTripOwnerTransitionAdd)
+{
+    /// new-only = add: no old_binding, a fresh new_binding.
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "all_1_1_0", manifestRef(1, 1, 1)};
+    txn.ops.push_back(op);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+    ASSERT_TRUE(decoded.ops[0].new_binding.has_value());
+    EXPECT_FALSE(decoded.ops[0].old_binding.has_value());
+}
+
+TEST(CASRefCodec, RoundTripOwnerTransitionRemoval)
+{
+    /// old-only = removal: an old_binding, no new_binding.
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "all_1_1_0", manifestRef(1, 1, 1)};
+    txn.ops.push_back(op);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+    EXPECT_FALSE(decoded.ops[0].new_binding.has_value());
+    ASSERT_TRUE(decoded.ops[0].old_binding.has_value());
+}
+
+TEST(CASRefCodec, RoundTripOwnerTransitionReplace)
+{
+    /// both present = replace.
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "all_1_1_0", manifestRef(1, 1, 1)};
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "all_1_1_0", manifestRef(1, 1, 1)};
+    txn.ops.push_back(op);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+    ASSERT_TRUE(decoded.ops[0].old_binding.has_value());
+    ASSERT_TRUE(decoded.ops[0].new_binding.has_value());
+}
+
+TEST(CASRefCodec, RoundTripMultipleOpsInOneTransaction)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{9, 100};
+
+    RefOp birth;
+    birth.kind = RefOpKind::NamespaceBirth;
+    txn.ops.push_back(birth);
+
+    RefOp add;
+    add.kind = RefOpKind::OwnerTransition;
+    add.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "a/b/c", manifestRef(9, 1, 1)};
+    txn.ops.push_back(add);
+
+    RefOp set_published_at;
+    set_published_at.kind = RefOpKind::SetPublishedAt;
+    set_published_at.ref_name = "a/b/c";
+    set_published_at.expected_manifest_ref = manifestRef(9, 1, 1);
+    set_published_at.published_at_ms = 42;
+    txn.ops.push_back(set_published_at);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+    EXPECT_EQ(decoded.ops.size(), 3u);
+}
+
+/// A re-encode of a decoded transaction is byte-identical (the encoder is a pure function of the txn).
+TEST(CASRefCodec, ByteIdenticalReencode)
+{
+    RefLogTxn txn;
+    txn.ns = "srv1/db/table@cas@";
+    txn.txn_id = RefTxnId{9, 100};
+
+    RefOp birth;
+    birth.kind = RefOpKind::NamespaceBirth;
+    txn.ops.push_back(birth);
+
+    RefOp add;
+    add.kind = RefOpKind::OwnerTransition;
+    add.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "a/b/c", manifestRef(9, 1, 1)};
+    add.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "a/b/c", manifestRef(9, 1, 1)};
+    txn.ops.push_back(add);
+
+    RefOp set_published_at;
+    set_published_at.kind = RefOpKind::SetPublishedAt;
+    set_published_at.ref_name = "a/b/c";
+    set_published_at.expected_manifest_ref = manifestRef(9, 1, 1);
+    set_published_at.published_at_ms = 1717000000000ULL;
+    txn.ops.push_back(set_published_at);
+
+    const String bytes1 = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes1, txn.ns, txn.txn_id);
+    const String bytes2 = encodeRefLogTxn(decoded);
+    EXPECT_EQ(bytes1, bytes2);
+}
+
+/// ===================================================================================
+/// RefLogTxn: validation rejections (encoder-side + key/body binding + truncation)
+/// ===================================================================================
+
+TEST(CASRefCodec, EncodeRejectsZeroTxnId)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{0, 1};
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, DecodeRejectsTruncatedBuffer)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    const String bytes = encodeRefLogTxn(txn);
+
+    /// Dropping the trailing bytes leaves the final line without its '\n' terminator -> fail closed.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefLogTxn(bytes.substr(0, bytes.size() - 3), txn.ns, txn.txn_id); });
+}
+
+TEST(CASRefCodec, DecodeRejectsBodyNamespaceMismatch)
+{
+    RefLogTxn txn;
+    txn.ns = "ns-a";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::NamespaceBirth;
+    txn.ops.push_back(op);
+    const String bytes = encodeRefLogTxn(txn);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefLogTxn(bytes, "ns-b", txn.txn_id); });
+}
+
+TEST(CASRefCodec, DecodeRejectsBodyTxnIdMismatch)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::NamespaceBirth;
+    txn.ops.push_back(op);
+    const String bytes = encodeRefLogTxn(txn);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefLogTxn(bytes, txn.ns, RefTxnId{1, 2}); });
+}
+
+TEST(CASRefCodec, EncodeRejectsEmptyRefName)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsDotRefName)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = ".";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsDotDotSegment)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "a/../b";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsRepeatedSeparator)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "a//b";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsLeadingSlash)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "/a";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsTrailingSlash)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "a/";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsBackslash)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "a\\b";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsNonCanonicalOwnerBindingRefName)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "..", manifestRef(1, 1, 1)};
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsEmbeddedNulRefName)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = String("a\0b", 3);   /// embedded NUL byte -- never legitimate in a ref name
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsTooManyOps)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    for (size_t i = 0; i < ref_txn_max_ops + 1; ++i)
+    {
+        RefOp op;
+        op.kind = RefOpKind::NamespaceBirth;
+        txn.ops.push_back(op);
+    }
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeAllowsExactlyMaxOps)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    for (size_t i = 0; i < ref_txn_max_ops; ++i)
+    {
+        RefOp op;
+        op.kind = RefOpKind::NamespaceBirth;
+        txn.ops.push_back(op);
+    }
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded.ops.size(), ref_txn_max_ops);
+}
+
+TEST(CASRefCodec, EncodeRejectsOversizedNormalTransaction)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r" + String(ref_txn_max_bytes + 1, 'x');
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, RemovalClassTransactionLiftsByteBudgetAboveNormalLimit)
+{
+    /// A RemoveNamespace transaction carrying a ref_name bigger than the NORMAL limit but within the
+    /// REMOVAL limit must succeed -- proving the removal-class flag actually lifts the byte budget
+    /// rather than merely being ignored. The single set_published_at op here is also vastly bigger
+    /// than `ref_op_max_bytes`, so this doubles as proof that removal-class ops are exempt from the
+    /// per-op cap too.
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+
+    RefOp remove;
+    remove.kind = RefOpKind::RemoveNamespace;
+    txn.ops.push_back(remove);
+
+    RefOp set_published_at;
+    set_published_at.kind = RefOpKind::SetPublishedAt;
+    set_published_at.ref_name = "r" + String(ref_txn_max_bytes + 1024, 'x');
+    set_published_at.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(set_published_at);
+
+    const String bytes = encodeRefLogTxn(txn);
+    EXPECT_GT(bytes.size(), ref_txn_max_bytes);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+}
+
+TEST(CASRefCodec, RemovalClassTransactionStillRejectsBeyondRemovalLimit)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+
+    RefOp remove;
+    remove.kind = RefOpKind::RemoveNamespace;
+    txn.ops.push_back(remove);
+
+    RefOp set_published_at;
+    set_published_at.kind = RefOpKind::SetPublishedAt;
+    set_published_at.ref_name = "r" + String(ref_removal_max_bytes + 1, 'x');
+    set_published_at.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(set_published_at);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, RemovalClassTransactionNotCappedOnOpCount)
+{
+    /// A removal-class transaction may exceed `ref_txn_max_ops` -- only the (much larger) byte budget
+    /// bounds it, per spec ("its operation count is bounded by that byte limit").
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+
+    RefOp remove;
+    remove.kind = RefOpKind::RemoveNamespace;
+    txn.ops.push_back(remove);
+    for (size_t i = 0; i < ref_txn_max_ops + 10; ++i)
+    {
+        RefOp op;
+        op.kind = RefOpKind::NamespaceBirth;
+        txn.ops.push_back(op);
+    }
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded.ops.size(), txn.ops.size());
+}
+
+/// Stage-1 T8 (spec §3 "Budget: counts only, chunked flush") retires the scenario this test used to
+/// pin: ONE op carrying almost the whole `ref_txn_max_bytes` budget in its payload. The new per-op
+/// cap (`ref_op_max_bytes`, `EncodeAllowsExactlyMaxPerOpBytes` below) makes that construction illegal
+/// for a normal-class transaction — no single op may exceed `ref_op_max_bytes` regardless of the
+/// whole-transaction budget — so the exact-boundary pin moves to the per-op cap, the boundary a
+/// legally-admitted normal-class transaction can actually reach (`ref_txn_max_ops * ref_op_max_bytes`
+/// stays comfortably under `ref_txn_max_bytes`, pinned by `CanonicalMaxTransactionRoundTrips` in
+/// `gtest_cas_ref_chunked_flush.cpp`).
+
+TEST(CASRefCodec, EncodeAllowsExactlyMaxPerOpBytes)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+
+    const size_t base_size = encodedOpSize(op);
+    ASSERT_LE(base_size, ref_op_max_bytes);
+    /// Every added 'a' is one un-escaped byte inside the JSON ref-name string, so the encoded op size
+    /// grows one-for-one to exactly the per-op cap.
+    txn.ops[0].ref_name = "r" + String(ref_op_max_bytes - base_size, 'a');
+    ASSERT_EQ(encodedOpSize(txn.ops[0]), ref_op_max_bytes);
+
+    const String bytes = encodeRefLogTxn(txn);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+}
+
+TEST(CASRefCodec, EncodeRejectsOversizedOpOnNormalTransaction)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(op);
+
+    const size_t base_size = encodedOpSize(op);
+    ASSERT_LE(base_size, ref_op_max_bytes);
+    /// One byte past the per-op cap, well within the whole-transaction byte cap -- isolates the
+    /// per-op check from the (much larger) whole-transaction one.
+    txn.ops[0].ref_name = "r" + String(ref_op_max_bytes - base_size + 1, 'a');
+    ASSERT_GT(encodedOpSize(txn.ops[0]), ref_op_max_bytes);
+    ASSERT_LT(encodedOpSize(txn.ops[0]), ref_txn_max_bytes);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeAllowsExactlyMaxRemovalBytes)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp remove;
+    remove.kind = RefOpKind::RemoveNamespace;
+    txn.ops.push_back(remove);
+    RefOp ts_op;
+    ts_op.kind = RefOpKind::SetPublishedAt;
+    ts_op.ref_name = "r";
+    ts_op.expected_manifest_ref = manifestRef(1, 1, 1);
+    txn.ops.push_back(ts_op);
+
+    const size_t base_size = encodeRefLogTxn(txn).size();
+    ASSERT_LE(base_size, ref_removal_max_bytes);
+    /// Every added 'x' is one un-escaped byte inside the JSON ref-name string, so the encoded size
+    /// grows one-for-one to exactly the cap; the base "r" contributes 1 byte already counted in
+    /// base_size, so appending (rather than replacing) reaches the target exactly.
+    txn.ops[1].ref_name = "r" + String(ref_removal_max_bytes - base_size, 'x');
+
+    const String bytes = encodeRefLogTxn(txn);
+    EXPECT_EQ(bytes.size(), ref_removal_max_bytes);
+    const RefLogTxn decoded = decodeRefLogTxn(bytes, txn.ns, txn.txn_id);
+    EXPECT_EQ(decoded, txn);
+}
+
+/// ManifestRef field validation, enforced by the log codec (spec's "invalid identifiers are rejected"
+/// binds both codecs). Encoder-side only -- the decode path re-runs the identical checks and is
+/// covered by the round-trips + the battery.
+
+TEST(CASRefCodec, EncodeRejectsZeroManifestRefWriterEpochInOwnerBinding)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "r", manifestRef(0, 1, 1)};
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsZeroManifestRefBuildSequenceInOwnerBinding)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "r", manifestRef(1, 0, 1)};
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsOutOfRangeManifestOrdinalInOwnerBinding)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "r", manifestRef(1, 1, 0)};
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+TEST(CASRefCodec, EncodeRejectsZeroManifestRefInSetPublishedAt)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "r";
+    op.expected_manifest_ref = manifestRef(1, 1, 0);
+    txn.ops.push_back(op);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefLogTxn(txn); });
+}
+
+/// ===================================================================================
+/// Shape-level failure-mode battery (truncation / v+1 gate / wrong type / leading garbage)
+/// ===================================================================================
+
+TEST(CASFormatBattery, RefLog)
+{
+    RefLogTxn txn;
+    txn.ns = "ns";
+    txn.txn_id = RefTxnId{1, 1};
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = "all_1_1_0";
+    op.expected_manifest_ref = manifestRef(1, 1, 1);
+    op.published_at_ms = 42;
+    txn.ops.push_back(op);
+
+    const String ns = txn.ns;
+    const RefTxnId id = txn.txn_id;
+    runFormatBattery({FormatId::RefLog,
+        [txn] { return sealObject(FormatId::RefLog, encodeRefLogTxn(txn)); },
+        [ns, id](std::string_view s) { decodeRefLogTxn(openObject(FormatId::RefLog, s), ns, id); },
+        currentFormatHeader("cas_ref_log") +
+        "{\"ns\":\"ns\",\"we\":\"1\",\"rs\":\"1\"}\n"
+        "{\"op\":\"set_published_at\",\"rn\":\"all_1_1_0\",\"me\":\"1\",\"mb\":\"1\",\"mo\":1,\"ts\":42}\n"
+        "{\"n\":1}\n"});
+}
diff --git a/src/Disks/tests/gtest_cas_ref_read_contract.cpp b/src/Disks/tests/gtest_cas_ref_read_contract.cpp
new file mode 100644
index 000000000000..a86d2530d4e8
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_read_contract.cpp
@@ -0,0 +1,256 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+
+#include <algorithm>
+#include <memory>
+#include <optional>
+
+/// The ref-side read contract (the ten `CasRefCatalog::read` sites in `CasRefLedger.cpp` were
+/// classified elsewhere: every site is a mutation/admission authority or a per-key destructive
+/// revalidation, and every live table reader reaches `acquireReadableRefTableRuntime`, whose warm path
+/// returns the resident runtime before any catalog read). These are COVERAGE PINS for a contract the
+/// classification predicted already holds, not a fix: a held reader runtime answers stale-or-absent
+/// across a same-name rebirth, a warm read costs no catalog request, and the one held ref-writer seam
+/// that exists (`dropNamespace(const NamespaceLifeId &)`) refuses across the same rebirth rather than
+/// touching the successor.
+
+namespace DB::ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+    extern const int NETWORK_ERROR;
+}
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+namespace
+{
+
+/// One committed ref, born and published through the REAL production write path (`beginPartWrite` /
+/// `stageManifest` / `precommitAdd` / `promote`) -- this is what mints `ns`'s catalog life for real,
+/// exactly as an ordinary insert would, rather than a fixture sentinel.
+ManifestId publishRefThroughPool(const PoolPtr & store, const RootNamespace & ns, const String & ref_name)
+{
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref_name;
+    auto build = store->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref_name, id);
+    build->promote(ns, ref_name, build->buildId(), id);
+    return id;
+}
+
+/// Delete the current catalog life through the production exact-removal authority (`casUpdate` to
+/// `Removing`, then `deleteCompletedRemoving` under a held fence), retaining every old physical byte
+/// and any already-resident runtime. Mirrors `gtest_cas_ns_file_read_contract.cpp`'s
+/// `deleteCatalogLife` -- lifecycle-real, not a raw sentinel overwrite.
+void deleteCatalogLife(Backend & backend, const Layout & layout, const NamespaceLifeId & life)
+{
+    CasRefCatalog::casUpdate(backend, layout, [&](const RefCatalog & current)
+    {
+        RefCatalog next = current;
+        const auto it = std::find_if(next.entries.begin(), next.entries.end(), [&](const CatalogEntry & entry)
+        {
+            return entry.ns == life.ns && entry.incarnation == life.incarnation;
+        });
+        if (it == next.entries.end())
+            throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "Missing fixture catalog life '{}'", life.ns.string());
+        it->state = NsState::Removing;
+        it->removal_started_round = 1;
+        return next;
+    });
+
+    const CasRefCatalog::Snapshot snapshot = CasRefCatalog::read(backend, layout);
+    const auto it = std::find_if(snapshot.catalog.entries.begin(), snapshot.catalog.entries.end(), [&](const CatalogEntry & entry)
+    {
+        return entry.ns == life.ns && entry.incarnation == life.incarnation;
+    });
+    if (it == snapshot.catalog.entries.end())
+        throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "Missing Removing fixture catalog life '{}'", life.ns.string());
+
+    CasFoldSeal parent;
+    parent.ref_lives.emplace(life.incarnation, RefLifeFoldState{
+        .coverage = RefCoverage{.classification = 2, .last_folded_ref_id = RefTxnId{1, 1}},
+        .cleanup_evidence = RefCleanupEvidence{.remove_txn_id = RefTxnId{1, 1}}});
+    if (CasRefCatalog::deleteCompletedRemoving(
+            backend, layout, *it, parent, 1,
+            [](uint64_t) { return CasRefCatalog::LeaderFenceStatus::Held; })
+        != CasRefCatalog::CompletedRemovingDeleteOutcome::Deleted)
+        throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "Failed to delete fixture catalog life '{}'", life.ns.string());
+}
+
+/// Admit a fresh `Live` catalog row at the SAME logical name as `predecessor` -- the same-name
+/// rebirth every test below drives. Mirrors `gtest_cas_ns_file_read_contract.cpp`'s
+/// `admitReplacementLife`.
+NamespaceLifeId admitReplacementLife(
+    Backend & backend, const Layout & layout, uint64_t gc_shards,
+    const NamespaceLifeId & predecessor, UInt128 successor_incarnation)
+{
+    if (predecessor.incarnation == successor_incarnation)
+        throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "Fixture life ids unexpectedly collide");
+    const NamespaceLifeId successor = NamespaceLifeId::fromCatalogEntry(predecessor.ns, successor_incarnation);
+    CasRefCatalog::casAdmitEntry(backend, layout, gc_shards, CatalogEntry{
+        .ns = successor.ns, .state = NsState::Live, .incarnation = successor.incarnation});
+    return successor;
+}
+
+}
+
+/// This reader's runtime already holds life 1. Reusing it after a same-name rebirth is a retained
+/// life-handle operation, not a fresh logical-name admission: it may still answer life 1's committed
+/// value (or absent), but it must never surface life 2's -- the opaque physical life id makes the
+/// successor's bytes structurally unreachable through an unrefreshed handle.
+TEST(CASRefReadContract, HeldRuntimeAfterSameNameRebirthReadsStaleOrNotFoundNeverSuccessorRefs)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    /// A 1-byte whole-table cache budget is the production knob (`CASRefTableCacheEviction`) that lets a
+    /// single store instance both HOLD a table's runtime and, later, genuinely forget it by touching a
+    /// different table -- so the "fresh resolution" positive control below is a real re-recovery, not
+    /// a second mount.
+    auto store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .ref_table_cache_bytes = 1});
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/ref_read_contract_rebirth@cas@"};
+    const RootNamespace throwaway_ns{"00/ref_read_contract_rebirth_evictor@cas@"};
+    const String ref_name = "part_1";
+
+    const ManifestId life1_manifest = publishRefThroughPool(store, ns, ref_name);
+
+    /// Hold the reader runtime resident: one read.
+    const auto held_before = store->resolveRef(ns, ref_name);
+    ASSERT_TRUE(held_before.has_value());
+    EXPECT_EQ(held_before->manifest_id, life1_manifest);
+    ASSERT_TRUE(store->refTableLifeForTest(ns).has_value());
+    const NamespaceLifeId life1 = *store->refTableLifeForTest(ns);
+
+    /// Drop and re-admit under the SAME logical name, bypassing this store's own ledger entirely --
+    /// exactly as an independent actor's drop/rebirth would look from this reader's point of view.
+    deleteCatalogLife(*backend, layout, life1);
+    const NamespaceLifeId life2 = admitReplacementLife(*backend, layout, store->poolConfig().gc_shards, life1, UInt128{0xabc123});
+    ASSERT_NE(life1.incarnation, life2.incarnation);
+
+    const ManifestRef life2_ref{/*writer_epoch*/ 1, /*build_sequence*/ 777, /*manifest_ordinal*/ 1};
+    publishCommittedTransition(*backend, layout, ns, ref_name, std::nullopt, life2_ref);
+    const ManifestId life2_manifest{ns, life2_ref};
+    ASSERT_NE(life2_manifest, life1_manifest);
+
+    /// The held runtime never re-validates the catalog: it answers from its resident cache -- stale or
+    /// not-found -- but never the successor's value.
+    const auto held_after = store->resolveRef(ns, ref_name);
+    EXPECT_NE(
+        held_after.has_value() ? std::optional<ManifestId>(held_after->manifest_id) : std::nullopt,
+        std::optional<ManifestId>(life2_manifest));
+    if (held_after.has_value())
+        EXPECT_EQ(held_after->manifest_id, life1_manifest);
+
+    /// Force the cached runtime out: touch a different namespace under the 1-byte cache budget (the
+    /// production whole-table eviction path), so the NEXT access to `ns` re-recovers from scratch.
+    (void)publishRefThroughPool(store, throwaway_ns, "evict");
+    ASSERT_FALSE(store->refTableCachedForTest(ns));
+
+    /// Positive control: a fresh resolution -- through the SAME Pool, now cold -- sees life 2. Not
+    /// vacuous: the value really did move, and an unrefreshed handle really would have missed it.
+    const auto fresh = store->resolveRef(ns, ref_name);
+    ASSERT_TRUE(fresh.has_value());
+    EXPECT_EQ(fresh->manifest_id, life2_manifest);
+}
+
+/// The disjoint half of the read-side contract: once a table's runtime is resident, an ordinary read
+/// costs no catalog request at all -- the recovered-and-cached `RefTableState` is this process's
+/// sole authority for a table it has already opened.
+TEST(CASRefReadContract, HotRefReadsThroughHeldRuntimeIssueZeroCatalogRequests)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    PoolPtr store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/ref_read_contract_hot@cas@"};
+    const String ref_name = "part_1";
+
+    const ManifestId published = publishRefThroughPool(store, ns, ref_name);
+    const auto warm = store->resolveRef(ns, ref_name);
+    ASSERT_TRUE(warm.has_value());
+    EXPECT_EQ(warm->manifest_id, published);
+    ASSERT_TRUE(store->refTableLifeForTest(ns).has_value());
+    const NamespaceLifeId life = *store->refTableLifeForTest(ns);
+
+    /// Positive control, captured BEFORE the reset below: the cold admission above really did reach
+    /// the catalog and this namespace's own ref stream, so the upcoming zero is an absence and not a
+    /// recorder that never saw anything.
+    EXPECT_GT(
+        backend->headCount(layout.refCatalogKey()) + backend->getCount(layout.refCatalogKey())
+            + backend->casPutCount(layout.refCatalogKey()),
+        0u) << "the cold admission above must have reached the catalog at least once";
+    EXPECT_GT(backend->getCount(layout.refCkptKey(life)), 0u)
+        << "the cold recovery above must have read this namespace's own checkpoint at least once";
+
+    backend->resetCounts();
+
+    (void)store->resolveRef(ns, ref_name);
+    (void)store->listRefs(ns);
+    (void)store->hasAnyRefWithPrefix(ns, "");
+
+    EXPECT_EQ(backend->headCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->putCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->putOverwriteCount(layout.refCatalogKey()), 0u);
+    /// Stronger than the catalog-only clauses above: a warm ref read is a pure map lookup over the
+    /// recovered state (`ensureRefTableRecovered`'s early return once `rt.recovered`), so it issues no
+    /// backend request whatsoever, not merely none against the catalog.
+    EXPECT_TRUE(backend->touchedKeys().empty())
+        << "a warm ref read must issue no backend requests at all";
+}
+
+/// The one held ref-WRITER seam the classification found: `dropNamespace(const NamespaceLifeId &)`'s
+/// exact-incarnation guard. A stale holder can only be refused, never allowed to act on the successor
+/// -- there is no path by which it could target life 2's row or its ref data.
+TEST(CASRefReadContract, StaleLifeDropRefusesAfterRebirthAndNeverTouchesSuccessor)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/ref_read_contract_stale_drop@cas@"};
+    const String ref_name = "part_1";
+
+    (void)publishRefThroughPool(store, ns, ref_name);
+    ASSERT_TRUE(store->refTableLifeForTest(ns).has_value());
+    const NamespaceLifeId life1 = *store->refTableLifeForTest(ns);
+
+    deleteCatalogLife(*backend, layout, life1);
+    const NamespaceLifeId life2 = admitReplacementLife(*backend, layout, store->poolConfig().gc_shards, life1, UInt128{0xabc456});
+
+    const ManifestRef life2_ref{/*writer_epoch*/ 1, /*build_sequence*/ 999, /*manifest_ordinal*/ 1};
+    publishCommittedTransition(*backend, layout, ns, ref_name, std::nullopt, life2_ref);
+    const ManifestId life2_manifest{ns, life2_ref};
+
+    const HeadResult catalog_head_before = backend->head(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_head_before.exists);
+    const auto catalog_get_before = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_get_before.has_value());
+
+    /// The held life-1 handle names an incarnation the catalog no longer carries: refused, not
+    /// resolved against the current (life-2) row.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(life1); });
+
+    const HeadResult catalog_head_after = backend->head(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_head_after.exists);
+    EXPECT_EQ(catalog_head_after.token, catalog_head_before.token)
+        << "a refused stale-life drop must not touch the catalog object at all";
+    const auto catalog_get_after = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_get_after.has_value());
+    EXPECT_EQ(catalog_get_after->bytes, catalog_get_before->bytes);
+
+    /// Life 2's ref data is untouched: a fresh resolution (a separate mount over the same backend,
+    /// exactly like `CASRefWriterRuntimeIdentity.ColdReadRejectsReplacementByExternalPoolActor`'s
+    /// `external_store`) still sees exactly the value published above.
+    auto verify_store = Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "verify"});
+    const auto resolved = verify_store->resolveRef(ns, ref_name);
+    ASSERT_TRUE(resolved.has_value());
+    EXPECT_EQ(resolved->manifest_id, life2_manifest);
+}
diff --git a/src/Disks/tests/gtest_cas_ref_recovery_cas_walk.cpp b/src/Disks/tests/gtest_cas_ref_recovery_cas_walk.cpp
new file mode 100644
index 000000000000..2b75ae13ada0
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_recovery_cas_walk.cpp
@@ -0,0 +1,2010 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/MemoryTracker.h>
+#include <Common/ProfileEvents.h>
+
+#include <Poco/Exception.h>
+
+#include <algorithm>
+#include <atomic>
+#include <condition_variable>
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <set>
+#include <string>
+#include <thread>
+#include <vector>
+
+/// Stage A task 6: recovery is `_ckpt` + an ARITHMETIC tail + a seal CAS-walk, and it installs nothing
+/// without presenting the fence generation it was admitted under.
+///
+/// The one sentence this suite exists to defend: **recovery performs no stream `LIST`.** Everything the
+/// old recovery knew about a table's durable stream came from one `LIST`, so a listing that silently
+/// omitted a key produced a table missing an ACKED transaction and looked perfectly healthy. The
+/// checkpoint now supplies the only base and finite frontier; arithmetic exact GETs decide recovery.
+/// These list-liar fixtures are retained as sentinels: hiding or fabricating a listed key cannot affect
+/// recovery because recovery sends zero stream LIST requests.
+///
+/// The other half is INV-2: a dead epoch is closed IN-BAND, by a seal transaction the store's own
+/// conditional create places at exactly `{E, T+1}` -- the key a dying predecessor's in-flight PUT would
+/// have taken. That is why the walk WRITES, and why every write it performs is gated on the ONE fence
+/// generation captured when the recovery was admitted (slot-occupy, the `_ckpt` CAS, and the install
+/// recheck -- one capture, three checks).
+///
+/// The suite name is prefixed `Cas` so it is covered by the `Cas*` unit-test gate filter.
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+extern const int NETWORK_ERROR;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASRefRecoveryRestarts;
+extern const Event CASRefRecoveryEpochSealed;
+extern const Event CASRefRecoveryEpochSealAdopted;
+extern const Event CASRefRecoveryStragglerAdopted;
+extern const Event CASRefRecoveryCancelled;
+extern const Event CASRefCheckpointPublished;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::committedRow;
+using DB::Cas::tests::CountingBackend;
+using DB::Cas::tests::expectThrowsCode;
+using DB::Cas::tests::minimalLiveSnapshot;
+using DB::Cas::tests::namespaceBirthOp;
+using DB::Cas::tests::publishCommittedOps;
+using DB::Cas::tests::rearmMountFenceAfterAnomalyForTest;
+using DB::Cas::tests::writeRefSnapshotRaw;
+
+namespace
+{
+
+ManifestRef manifestRef(uint64_t epoch, uint64_t build_sequence, uint32_t ordinal)
+{
+    return ManifestRef{epoch, build_sequence, ordinal};
+}
+
+/// Make the durable mount immediately reclaimable so a test that deliberately moved the local fence
+/// generation can drive the production remount boundary without paying a live-lease expiry wait.
+void fenceOutMountForRemount(Backend & backend, const String & mount_key)
+{
+    const auto got = backend.get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    MountLease mount = decodeMountLease(got->bytes);
+    mount.gc_fenced = true;
+    mount.seq += 1;
+    ASSERT_EQ(backend.putOverwrite(mount_key, encodeMountLease(mount), got->token).outcome,
+        PutOutcome::Done);
+}
+
+/// A backend whose `LIST` can lie by omission. `hidden_keys` remain readable by exact key, so these
+/// fixtures prove the stronger modern rule: recovery sends no stream `LIST` at all and therefore cannot
+/// be affected by an enumeration inconsistency.
+///
+/// Deliberately NOT a "delete the object" fixture: an object that is genuinely gone is a different
+/// (and already covered) case. The blocker is an object that EXISTS and is invisible to enumeration.
+class HidingListBackend : public CountingBackend
+{
+public:
+    explicit HidingListBackend(bool seed_pool_meta = true)
+    {
+        if (seed_pool_meta)
+            DB::Cas::tests::seedPoolMetaForRestart(*this);
+    }
+
+    using CountingBackend::get;
+    using CountingBackend::list;
+    using CountingBackend::putIfAbsent;
+    using CountingBackend::casPut;
+
+    std::set<String> hidden_keys;
+    std::set<String> phantom_list_keys;
+
+    /// Every `putIfAbsent` of a key containing this substring throws a PLAIN (non-`DB::Exception`)
+    /// error, which `classifyConditionalWriteResult` can only ever classify `Unresolved` -- never
+    /// `DefiniteFailure`. Persistent rather than one-shot on purpose: the subject is what recovery does
+    /// when the store KEEPS refusing to say whether the write landed.
+    String ambiguous_put_substr;
+
+    /// Persistent thrown response for a matching mutable checkpoint CAS. The ref-log PUT has already
+    /// completed when tests arm this, producing the exact one-successor recovery window.
+    String ambiguous_cas_substr;
+    int ambiguous_cas_count = 0;
+
+    /// Runs after a checkpoint publisher read its expected token but before that publisher presents
+    /// its CAS. This is the exact window in which another admitted writer can advance the frontier.
+    std::function<void(const String &, const String &, const std::optional<Token> &)> before_cas_put;
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = CountingBackend::list(prefix, cursor, limit);
+        std::vector<ListedKey> kept;
+        kept.reserve(page.keys.size());
+        for (ListedKey & lk : page.keys)
+            if (!hidden_keys.contains(lk.key))
+                kept.push_back(std::move(lk));
+        if (cursor.empty())
+        {
+            for (const String & key : phantom_list_keys)
+            {
+                if (key.starts_with(prefix))
+                    kept.push_back(ListedKey{.key = key, .size = 0, .token = std::nullopt});
+            }
+        }
+        page.keys = std::move(kept);
+        return page;
+    }
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (!ambiguous_put_substr.empty() && key.find(ambiguous_put_substr) != String::npos)
+            throw std::runtime_error("injected ambiguous putIfAbsent");
+        return CountingBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        if (before_cas_put)
+            before_cas_put(key, bytes, expected);
+        if (ambiguous_cas_count > 0 && !ambiguous_cas_substr.empty()
+            && key.find(ambiguous_cas_substr) != String::npos)
+        {
+            --ambiguous_cas_count;
+            throw Poco::TimeoutException("HidingListBackend: simulated ambiguous checkpoint CAS");
+        }
+        return CountingBackend::casPut(key, bytes, expected, meta);
+    }
+};
+
+/// Fires `on_key` immediately AFTER a `putIfAbsent` whose key contains `watched_substr` -- the
+/// deterministic way to act inside recovery's own write window (bump a fence, land a straggler) with no
+/// sleep and no second thread. `skip` lets a test target the Nth such write.
+class PutHookBackend : public HidingListBackend
+{
+public:
+    using HidingListBackend::putIfAbsent;
+
+    using HidingListBackend::casPut;
+
+    String watched_substr;
+    uint64_t skip = 0;
+    std::function<void()> on_key;
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        PutResult result = HidingListBackend::putIfAbsent(key, bytes, meta);
+        fireIfWatched(key);
+        return result;
+    }
+
+    /// The `_ckpt` advance is a token-CAS, not a create, whenever the object already exists -- which is
+    /// the normal case, since the namespace birth creates it. Hooking only `putIfAbsent` would silently
+    /// never fire for it.
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        CasResult result = HidingListBackend::casPut(key, bytes, expected, meta);
+        fireIfWatched(key);
+        return result;
+    }
+
+private:
+    void fireIfWatched(const String & key)
+    {
+        if (!on_key || watched_substr.empty() || key.find(watched_substr) == String::npos)
+            return;
+        if (skip > 0)
+        {
+            --skip;
+            return;
+        }
+        auto hook = on_key;
+        on_key = nullptr;   /// one-shot: a hook that re-enters its own trigger would recurse
+        hook();
+    }
+};
+
+/// Materializes `late_bytes` at `late_key` at the instant the walk READS that key and finds it absent --
+/// i.e. strictly between the read and the conditional create that follows it.
+///
+/// This is the only faithful way to construct the race the `Occupied` arms exist for. Seeding the object
+/// up front does NOT work, and finding that out is the point: the walk fetches every id by EXACT KEY, so
+/// an object hidden from the listing is simply FOUND by the read and applied there. To meet it as an
+/// OCCUPANT of the slot, it has to arrive after the read said absent -- which is exactly what a
+/// straggler, or a concurrent recoverer's seal, does.
+class LateMaterializeBackend : public HidingListBackend
+{
+public:
+    using HidingListBackend::get;
+
+    String late_key;
+    String late_bytes;
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        std::optional<GetResult> result = HidingListBackend::get(key, range);
+        if (!result && !late_key.empty() && key == late_key)
+        {
+            CountingBackend::putIfAbsent(late_key, late_bytes);
+            late_key.clear();   /// one-shot: the walk must see it present from here on
+        }
+        return result;
+    }
+};
+
+/// Fires `on_key` immediately BEFORE a `get` whose key contains `watched_substr`, and can additionally
+/// FAULT that read with a transient object-store error -- the I/O seam the remount-barrier test pauses
+/// recovery at.
+class GetSeamBackend : public HidingListBackend
+{
+public:
+    using HidingListBackend::get;
+
+    String watched_substr;
+
+    /// Assigned from the test thread and read from whatever thread the recovery runs on, so the
+    /// read-and-move below is guarded. Today's tests all assign before starting the recovery thread and
+    /// clear after joining it, so there is no race to fix -- but this is the same seam that already
+    /// produced one use-after-free, and "the current tests happen not to race" is not a property a
+    /// future test author can see. The mutex makes the constraint enforced rather than remembered.
+    std::mutex hook_mutex;
+    std::function<void(const String &)> on_key;
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        std::unique_lock hook_lock(hook_mutex);
+        if (on_key && !watched_substr.empty() && key.find(watched_substr) != String::npos)
+        {
+            /// ONE-SHOT by moving the callback OUT before invoking it, and that is a correctness
+            /// requirement rather than a convenience. A hook that cleared `on_key` from inside its own
+            /// body would destroy the `std::function` whose closure it is still executing, and every
+            /// by-reference capture it touched afterwards would read freed heap. That is not
+            /// theoretical: it is what the first version of these tests did, and the ASan gate caught
+            /// it as a `heap-use-after-free` while a hook was parked on a condition variable.
+            auto hook = std::move(on_key);
+            on_key = nullptr;
+            /// Released before the hook runs: it parks on a condition variable, and holding the seam's
+            /// own mutex across that would deadlock the very thread meant to release it.
+            hook_lock.unlock();
+            hook(key);
+        }
+        return HidingListBackend::get(key, range);
+    }
+};
+
+/// Fires once after an exact GET has already fixed its result. This is the recovery authority seam:
+/// another actor advances the log+checkpoint after the walk observed its old end, but before the walk
+/// performs its final catalog/checkpoint validation.
+class AfterGetHookBackend : public HidingListBackend
+{
+public:
+    using HidingListBackend::get;
+
+    String watched_key;
+    std::function<void()> after_get;
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        std::optional<GetResult> result = HidingListBackend::get(key, range);
+        if (after_get && key == watched_key)
+        {
+            auto hook = std::move(after_get);
+            after_get = nullptr;
+            hook();
+        }
+        return result;
+    }
+};
+
+CasRequestBudget tinyBudget()
+{
+    return CasRequestBudget{
+        .attempt_timeout_ms = 50, .operation_deadline_ms = 500, .max_attempts = 1, .lease_safety_margin_ms = 50};
+}
+
+PoolConfig walkTestConfig()
+{
+    PoolConfig config;
+    config.pool_prefix = "p";
+    config.server_root_id = "test";
+    config.server_id = DB::UInt128(1);
+    config.cas_request_budget = tinyBudget();
+    config.wait_sleep_fn = [](uint64_t) {};
+    /// No background publication: every test here drives its own, so a threshold-triggered snapshot can
+    /// never move the base under an assertion about which base recovery chose.
+    config.snapshot_log_count_threshold = 1ULL << 40;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    return config;
+}
+
+PoolPtr openWalkPool(const BackendPtr & backend, PoolConfig config = walkTestConfig())
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend, config.pool_prefix);
+    return Pool::open(backend, std::move(config));
+}
+
+/// Burns durable writer epochs so a subsequent `Pool::open` allocates `target_live_epoch`. Epochs are
+/// minted, never reclaimed (`CasPool.cpp`'s allocator), so this is exactly what a pool that has been
+/// mounted `n` times looks like -- including the burned epochs in which nothing was ever written, which
+/// the seal chain must cross.
+void burnEpochsUpTo(Backend & backend, const Layout & layout, uint64_t target_live_epoch)
+{
+    for (uint64_t e = 1; e < target_live_epoch; ++e)
+        allocateWriterEpoch(backend, layout, "test", EpochMintPolicy::NormalMount, 0, [] { return RefCatalog{}; });
+}
+
+/// One ordinary transaction at `id`, publishing `ref` (prepending the birth op when `birth`).
+RefLogTxn makeOrdinaryTxn(const RootNamespace & ns, RefTxnId id, const String & ref, bool birth,
+                          std::optional<RefTxnId> prev_epoch_seal = std::nullopt)
+{
+    RefLogTxn txn;
+    txn.ns = ns.string();
+    txn.txn_id = id;
+    if (birth)
+        txn.ops.push_back(namespaceBirthOp());
+    for (const RefOp & op : publishCommittedOps(ref, manifestRef(id.writer_epoch, id.ref_sequence, 1u)))
+        txn.ops.push_back(op);
+    txn.prev_epoch_seal = prev_epoch_seal;
+    return txn;
+}
+
+/// The terminal `remove_namespace` op (this project's warning set requires every field named, so it is
+/// built field-by-field rather than by designated init).
+RefOp removeNamespaceOp()
+{
+    RefOp op;
+    op.kind = RefOpKind::RemoveNamespace;
+    return op;
+}
+
+/// One EPOCH SEAL transaction at `id` -- what a concurrent recoverer leaves behind.
+RefLogTxn makeSealTxn(const RootNamespace & ns, RefTxnId id,
+                      std::optional<RefTxnId> prev_epoch_seal = std::nullopt)
+{
+    RefLogTxn seal;
+    seal.ns = ns.string();
+    seal.txn_id = id;
+    RefOp op;
+    op.kind = RefOpKind::EpochSeal;
+    seal.ops.push_back(op);
+    seal.prev_epoch_seal = prev_epoch_seal;
+    return seal;
+}
+
+void seedTxn(Backend & backend, const Layout & layout, const RootNamespace & ns, RefTxnId id,
+             const String & ref, bool birth)
+{
+    DB::Cas::tests::fixture::writeRefLogRaw(backend, layout, makeOrdinaryTxn(ns, id, ref, birth));
+}
+
+/// Seeds the `_ckpt` a real namespace birth would have created, so recovery can ground its walk at the
+/// namespace's `life_epoch` without consulting the (untrusted) listing. Raw, because these fixtures
+/// never run a birth through the append lane.
+void seedCkpt(Backend & backend, const Layout & layout, const RootNamespace & ns, const RefCkpt & ckpt)
+{
+    backend.putIfAbsent(layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns)), encodeRefCkpt(ckpt));
+}
+
+RefCkpt lifeEpochCkpt(uint64_t life_epoch, std::optional<RefTxnId> committed_through = std::nullopt)
+{
+    return RefCkpt{.life_epoch = std::optional<uint64_t>{life_epoch},
+                   .committed_through = committed_through,
+                   .checkpoint_snapshot_id = std::nullopt,
+                   .last_epoch_seal = std::nullopt};
+}
+
+/// The decoded transaction at `id`, or `nullopt` when the object is absent. Never dereferences a
+/// disengaged optional: an aborted binary would take every later suite's result with it.
+std::optional<RefLogTxn> readLogTxn(Backend & backend, const Layout & layout, const RootNamespace & ns, RefTxnId id)
+{
+    const auto got = backend.get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), id));
+    if (!got)
+        return std::nullopt;
+    return decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), id);
+}
+
+uint64_t counterOf(ProfileEvents::Event event)
+{
+    return ProfileEvents::global_counters[event].load();
+}
+
+NamespaceLifeId catalogLife(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const CasRefCatalog::Snapshot catalog = CasRefCatalog::read(backend, layout);
+    for (const CatalogEntry & entry : catalog.catalog.entries)
+        if (entry.ns == ns)
+            return NamespaceLifeId::fromCatalogEntry(entry.ns, entry.incarnation);
+    throw std::runtime_error("test namespace has no catalog life");
+}
+
+NamespaceLifeId strandOneUnfrontieredSuccessor(
+    HidingListBackend & backend, const PoolPtr & store, const Layout & layout, const RootNamespace & ns)
+{
+    store->appendRefOps(ns, MutationScope::ref("a"),
+        [](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (const RefOp & op : publishCommittedOps("a", manifestRef(1, 1, 1)))
+                ops.push_back(op);
+            return ops;
+        }, RootMutationOrigin::Writer, RootMutationKind::Publish);
+
+    const NamespaceLifeId life = catalogLife(backend, layout, ns);
+    backend.ambiguous_cas_substr = layout.refCkptKey(life);
+    backend.ambiguous_cas_count = 200;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        store->appendRefOps(ns, MutationScope::ref("b"),
+            [](const RefTableState &) { return publishCommittedOps("b", manifestRef(1, 2, 1)); },
+            RootMutationOrigin::Writer, RootMutationKind::Publish);
+    });
+    backend.ambiguous_cas_count = 0;
+    return life;
+}
+
+CatalogEntry replaceCatalogLifeForTest(
+    Backend & backend, const Layout & layout, const CatalogEntry & predecessor, UInt128 successor_incarnation)
+{
+    const CasRefCatalog::Snapshot before_delete = CasRefCatalog::read(backend, layout);
+    RefCatalog without_predecessor = before_delete.catalog;
+    std::erase_if(without_predecessor.entries, [&](const CatalogEntry & entry)
+    {
+        return entry.ns == predecessor.ns && entry.incarnation == predecessor.incarnation;
+    });
+    if (backend.casPut(layout.refCatalogKey(), encodeRefCatalog(without_predecessor), before_delete.token).outcome
+        != CasOutcome::Committed)
+        throw std::runtime_error("test failed to retire exact predecessor catalog life");
+
+    CatalogEntry successor{
+        .ns = predecessor.ns,
+        .state = NsState::Live,
+        .incarnation = successor_incarnation,
+        .creator = std::nullopt};
+    const CasRefCatalog::Snapshot after_delete = CasRefCatalog::read(backend, layout);
+    RefCatalog reborn = after_delete.catalog;
+    reborn.entries.push_back(successor);
+    if (backend.casPut(layout.refCatalogKey(), encodeRefCatalog(reborn), after_delete.token).outcome
+        != CasOutcome::Committed)
+        throw std::runtime_error("test failed to publish successor catalog life");
+    return successor;
+}
+
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The checkpoint-bounded arithmetic tail: no recovery LIST
+/// ---------------------------------------------------------------------------------------------
+
+/// The durable stream is `{1,1} {1,2} {1,3}` while the backend hides the middle key from LIST.
+/// Recovery must make zero stream LIST requests and recover the same exact checkpoint range.
+TEST(CASRefRecoveryCasWalk, HiddenMiddleLogDoesNotAffectCheckpointRecovery)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/hint_middle"};
+
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 3}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+    seedTxn(*backend, layout, ns, RefTxnId{1, 2}, "b", /*birth=*/false);
+    seedTxn(*backend, layout, ns, RefTxnId{1, 3}, "c", /*birth=*/false);
+    backend->hidden_keys.insert(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{1, 2}));
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    backend->resetCounts();
+
+    const auto refs = store->listRefs(ns);
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns))), 0u);
+    EXPECT_EQ(refs.size(), 3u) << "the arithmetic walk must fetch {1,2} by exact key";
+    EXPECT_TRUE(refs.contains("a"));
+    EXPECT_TRUE(refs.contains("b")) << "'b' is the ref the omitted transaction published";
+    EXPECT_TRUE(refs.contains("c"));
+}
+
+/// The same sentinel at the tail. A hidden tail key is still found by the bounded exact walk, not by a
+/// stream enumeration.
+TEST(CASRefRecoveryCasWalk, HiddenTailLogDoesNotAffectCheckpointRecovery)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/hint_tail"};
+
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 2}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+    seedTxn(*backend, layout, ns, RefTxnId{1, 2}, "b", /*birth=*/false);
+    backend->hidden_keys.insert(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{1, 2}));
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    backend->resetCounts();
+
+    const auto refs = store->listRefs(ns);
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns))), 0u);
+    EXPECT_EQ(refs.size(), 2u) << "an omitted TAIL id is indistinguishable from the end of the stream to a "
+                                  "listing; recovery never enumerates it and exact-reads the checkpoint range";
+    EXPECT_TRUE(refs.contains("b"));
+}
+
+/// Hiding the checkpoint base snapshot from LIST cannot matter: the checkpoint names it, recovery
+/// exact-reads its matching non-seal log first, then exact-reads the snapshot.
+TEST(CASRefRecoveryCasWalk, CkptNamedBaseIsRecoveredWithoutStreamList)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/hint_snap"};
+
+    const RefTxnId base{1, 1};
+    seedTxn(*backend, layout, ns, base, "a", /*birth=*/true);
+    writeRefSnapshotRaw(*backend, layout,
+        minimalLiveSnapshot(ns.string(), base, {committedRow("a", manifestRef(1, 1, 1))}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 2}, "c", /*birth=*/false);
+    seedCkpt(*backend, layout, ns, RefCkpt{.life_epoch = std::optional<uint64_t>{1},
+                                           .committed_through = base,
+                                           .checkpoint_snapshot_id = base,
+                                           .last_epoch_seal = std::nullopt});
+    backend->hidden_keys.insert(layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), base));
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    backend->resetCounts();
+
+    const auto refs = store->listRefs(ns);
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns))), 0u);
+    EXPECT_EQ(refs.size(), 2u) << "the checkpoint names the base; the listing's omission is irrelevant";
+    EXPECT_TRUE(refs.contains("a")) << "'a' exists inside the checkpoint-named snapshot";
+    EXPECT_TRUE(refs.contains("c"));
+}
+
+TEST(CASRefRecoveryCasWalk, MissingExactIdAtOrBelowCommittedFrontierIsCorruption)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/missing_below_frontier"};
+    const RefTxnId frontier{1, 2};
+
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = std::optional<uint64_t>{1},
+        .committed_through = frontier,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    const auto ckpt_before = readCkpt(*backend, layout, life);
+    ASSERT_TRUE(ckpt_before);
+
+    auto store = openWalkPool(backend);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)store->listRefs(ns); });
+
+    const auto ckpt_after = readCkpt(*backend, layout, life);
+    ASSERT_TRUE(ckpt_after);
+    EXPECT_EQ(ckpt_after->token, ckpt_before->token)
+        << "an unchanged checkpoint makes the missing committed id corruption, not a shorter stream";
+}
+
+TEST(CASRefRecoveryCasWalk, UncommittedSnapshotIsUnobservedWithoutStreamList)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/hint_above_frontier"};
+    const RefTxnId frontier{1, 1};
+    const RefTxnId uncommitted_snapshot_id{1, 2};
+
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    seedTxn(*backend, layout, ns, frontier, "committed", /*birth=*/true);
+    writeRefSnapshotRaw(*backend, layout,
+        minimalLiveSnapshot(ns.string(), uncommitted_snapshot_id,
+            {committedRow("laundered", manifestRef(1, 2, 1))}));
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = std::optional<uint64_t>{1},
+        .committed_through = frontier,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    auto store = openWalkPool(backend);
+    backend->resetCounts();
+    const auto refs = store->listRefs(ns);
+
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamPrefix(life)), 0u);
+    EXPECT_TRUE(refs.contains("committed"));
+    EXPECT_FALSE(refs.contains("laundered"))
+        << "a physical snapshot not named by `_ckpt` cannot raise the recovered cut";
+}
+
+TEST(CASRefRecoveryCasWalk, ListingShapeDoesNotAffectCheckpointRecovery)
+{
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/list_equivalence"};
+    const RefTxnId base{1, 1};
+    const RefTxnId frontier{1, 2};
+    auto seed = std::make_shared<HidingListBackend>();
+
+    DB::Cas::tests::fixture::admitLive(*seed, layout, ns);
+    seedTxn(*seed, layout, ns, base, "a", /*birth=*/true);
+    writeRefSnapshotRaw(*seed, layout,
+        minimalLiveSnapshot(ns.string(), base, {committedRow("a", manifestRef(1, 1, 1))}));
+    seedTxn(*seed, layout, ns, frontier, "b", /*birth=*/false);
+    seedCkpt(*seed, layout, ns, RefCkpt{
+        .life_epoch = std::optional<uint64_t>{1},
+        .committed_through = frontier,
+        .checkpoint_snapshot_id = base,
+        .last_epoch_seal = std::nullopt});
+    const NamespaceLifeId life = catalogLife(*seed, layout, ns);
+
+    const auto clone_seed = [&]() -> std::shared_ptr<HidingListBackend>
+    {
+        /// A clone starts empty: constructing the normal fixture would pre-seed independent pool-meta
+        /// bytes before this loop could copy the source's identical durable image.
+        auto backend = std::make_shared<HidingListBackend>(/*seed_pool_meta=*/false);
+        String cursor;
+        do
+        {
+            const ListPage page = seed->list("", cursor, 1000);
+            for (const ListedKey & listed : page.keys)
+            {
+                const auto object = seed->get(listed.key);
+                if (!object)
+                    throw std::runtime_error("seed LIST returned a key that exact GET could not read");
+                const auto existing = backend->get(listed.key);
+                if (existing)
+                {
+                    if (existing->bytes != object->bytes || existing->attributes != object->attributes)
+                        throw std::runtime_error("clone backend constructor disagreed with seeded object");
+                }
+                else if (backend->putIfAbsent(listed.key, object->bytes, object->attributes).outcome != PutOutcome::Done)
+                    throw std::runtime_error("clone backend failed to copy seeded object");
+            }
+            cursor = page.next_cursor;
+        } while (!cursor.empty());
+        return backend;
+    };
+    const auto recover = [&](const std::shared_ptr<HidingListBackend> & backend)
+    {
+        auto store = openWalkPool(backend);
+        backend->resetCounts();
+        const auto refs = store->listRefs(ns);
+        EXPECT_EQ(backend->listCount(layout.namespaceStreamPrefix(life)), 0u);
+        return refs;
+    };
+
+    const auto full = recover(clone_seed());
+    auto empty_backend = clone_seed();
+    empty_backend->hidden_keys.insert(layout.refSnapshotKey(life, base));
+    empty_backend->hidden_keys.insert(layout.refLogKey(life, frontier));
+    const auto empty = recover(empty_backend);
+    ASSERT_EQ(full.size(), empty.size());
+    for (const auto & [name, resolved] : full)
+    {
+        ASSERT_TRUE(empty.contains(name));
+        EXPECT_EQ(resolved.manifest_id, empty.at(name).manifest_id);
+        EXPECT_EQ(resolved.manifest_size, empty.at(name).manifest_size);
+        EXPECT_EQ(resolved.published_at_ms, empty.at(name).published_at_ms);
+    }
+    EXPECT_TRUE(empty.contains("a"));
+    EXPECT_TRUE(empty.contains("b"));
+}
+
+TEST(CASRefRecoveryCasWalk, PhantomListedSnapshotIsUnobserved)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/stale_snapshot_hint"};
+    const RefTxnId checkpoint_base{1, 1};
+    const RefTxnId frontier{1, 2};
+
+    seedTxn(*backend, layout, ns, checkpoint_base, "a", /*birth=*/true);
+    writeRefSnapshotRaw(*backend, layout,
+        minimalLiveSnapshot(ns.string(), checkpoint_base, {committedRow("a", manifestRef(1, 1, 1))}));
+    seedTxn(*backend, layout, ns, frontier, "b", /*birth=*/false);
+    seedCkpt(*backend, layout, ns, RefCkpt{
+        .life_epoch = std::optional<uint64_t>{1},
+        .committed_through = frontier,
+        .checkpoint_snapshot_id = checkpoint_base,
+        .last_epoch_seal = std::nullopt});
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    backend->phantom_list_keys.insert(layout.refSnapshotKey(life, frontier));
+
+    auto store = openWalkPool(backend);
+    backend->resetCounts();
+    const auto refs = store->listRefs(ns);
+
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamPrefix(life)), 0u);
+    EXPECT_TRUE(refs.contains("a"));
+    EXPECT_TRUE(refs.contains("b"));
+}
+
+TEST(CASRefRecoveryCasWalk, ListedFPlusTwoWithoutFPlusOneIsInertUncommittedDebris)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/listed_uncommitted_debris"};
+    const RefTxnId frontier{1, 1};
+
+    seedTxn(*backend, layout, ns, frontier, "a", /*birth=*/true);
+    seedTxn(*backend, layout, ns, RefTxnId{1, 3}, "debris", /*birth=*/false);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, frontier));
+
+    auto store = openWalkPool(backend);
+    backend->resetCounts();
+    const auto refs = store->listRefs(ns);
+
+    EXPECT_EQ(backend->listCount(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns))), 0u);
+    EXPECT_TRUE(refs.contains("a"));
+    EXPECT_FALSE(refs.contains("debris"));
+}
+
+TEST(CASRefRecoveryCasWalk, DuplicateCatalogLifeIsCorruptionBeforeColdRuntimeAdmission)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/ambiguous_life_a"};
+    auto store = openWalkPool(backend);
+    const NamespaceLifeId life = strandOneUnfrontieredSuccessor(*backend, store, layout, ns);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    const CasRefCatalog::Snapshot sampled = CasRefCatalog::read(*backend, layout);
+    ASSERT_EQ(sampled.catalog.entries.size(), 1u);
+    RefCatalog ambiguous = sampled.catalog;
+    ambiguous.entries.push_back(CatalogEntry{
+        .ns = RootNamespace{"srv1/ambiguous_life_b"},
+        .state = NsState::Live,
+        .incarnation = life.incarnation});
+    std::sort(ambiguous.entries.begin(), ambiguous.entries.end(),
+        [](const CatalogEntry & lhs, const CatalogEntry & rhs) { return lhs.ns.string() < rhs.ns.string(); });
+    ASSERT_EQ(backend->casPut(layout.refCatalogKey(), encodeRefCatalog(ambiguous), sampled.token).outcome,
+              CasOutcome::Committed);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        auto cold_store = openWalkPool(backend);
+        (void)cold_store->listRefs(ns);
+    });
+}
+
+TEST(CASRefRecoveryCasWalk, CheckpointAdvanceAfterLastLogProbeRestartsBeforeInstall)
+{
+    auto backend = std::make_shared<AfterGetHookBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/final_authority_validation"};
+    const RefTxnId initial_frontier{1, 1};
+    const RefTxnId concurrent_frontier{1, 2};
+
+    seedTxn(*backend, layout, ns, initial_frontier, "a", /*birth=*/true);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, initial_frontier));
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    backend->watched_key = layout.refLogKey(life, concurrent_frontier);
+    backend->after_get = [&]
+    {
+        seedTxn(*backend, layout, ns, concurrent_frontier, "b", /*birth=*/false);
+        const auto sampled = readCkpt(*backend, layout, life);
+        ASSERT_TRUE(sampled);
+        RefCkpt advanced = sampled->ckpt;
+        advanced.committed_through = concurrent_frontier;
+        ASSERT_EQ(backend->casPut(layout.refCkptKey(life), encodeRefCkpt(advanced), sampled->token).outcome,
+                  CasOutcome::Committed);
+    };
+
+    auto store = openWalkPool(backend);
+    const uint64_t restarts_before = store->refRecoveryRestartsForTest(ns);
+    const auto refs = store->listRefs(ns);
+
+    EXPECT_TRUE(refs.contains("a"));
+    EXPECT_TRUE(refs.contains("b"))
+        << "the old private cut must be discarded when final exact authority moved after its last probe";
+    EXPECT_GT(store->refRecoveryRestartsForTest(ns), restarts_before)
+        << "the final authority observation is recovery's linearization point";
+}
+
+TEST(CASRefRecoveryCasWalk, LiveCatalogLifeWithoutReadableCheckpointIsCorruption)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/live_without_ckpt"};
+
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    seedTxn(*backend, layout, ns, RefTxnId{7, 1}, "hint-must-not-be-genesis", /*birth=*/true);
+    ASSERT_FALSE(readCkpt(*backend, layout, life));
+
+    auto store = openWalkPool(backend);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)store->listRefs(ns); });
+}
+
+/// A 404 BELOW the exact committed frontier is not the end of the stream -- it is a HOLE, and a hole
+/// in a dense stream is corruption. Recovery exact-reads the checkpoint token once to distinguish a
+/// concurrently moved cut from durable-data loss, then FAILS CLOSED while that token is unchanged. It
+/// must never fold what it has: that is precisely how an acknowledged transaction disappears.
+TEST(CASRefRecoveryCasWalk, AbsentIdBelowADurableHigherIdFailsClosed)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/hole"};
+
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 3}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+    /// {1,2} is MISSING while {1,3} is durable and listed: the listing itself witnesses the hole.
+    seedTxn(*backend, layout, ns, RefTxnId{1, 3}, "c", /*birth=*/false);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    store->setCasRetrySleepForTest([](uint64_t) {});
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->listRefs(ns); });
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The CAS-walk: closing dead epochs in-band
+/// ---------------------------------------------------------------------------------------------
+
+/// The ordinary case: one dead epoch, closed by OUR seal at `{E, T+1}` -- the exact key a dying
+/// predecessor's in-flight PUT would have taken, which is what makes the store's conditional create the
+/// fence (INV-2) rather than a detector after the fact.
+TEST(CASRefRecoveryCasWalk, DeadEpochIsClosedByOurOwnSealAtTPlusOne)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/seal_created"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    ASSERT_EQ(store->liveWriterEpoch(), 2u);
+
+    const uint64_t sealed_before = counterOf(ProfileEvents::CASRefRecoveryEpochSealed);
+    ASSERT_EQ(store->listRefs(ns).size(), 1u);
+    EXPECT_EQ(counterOf(ProfileEvents::CASRefRecoveryEpochSealed), sealed_before + 1);
+
+    const RefTxnId seal_id{1, 2};
+    const auto seal = readLogTxn(*backend, layout, ns, seal_id);
+    ASSERT_TRUE(seal.has_value()) << "epoch 1 is dead and must be closed at {1,2}";
+    EXPECT_TRUE(refLogTxnIsEpochSeal(*seal));
+    EXPECT_EQ(seal->prev_epoch_seal, std::nullopt) << "sequence 2 never carries a chain link";
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::optional<RefTxnId>(seal_id))
+        << "the chain link the next epoch's sequence-1 transaction must name";
+}
+
+/// A concurrent recoverer got there first. Its seal is already at `{E, T+1}`, so our conditional create
+/// loses -- and the right reaction is to ADOPT it, not to treat a peer's correct write as interference.
+/// The adopted seal is the same chain link ours would have been.
+TEST(CASRefRecoveryCasWalk, ConcurrentRecoverersSealIsAdoptedNotContested)
+{
+    auto backend = std::make_shared<LateMaterializeBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/seal_adopt"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+    /// The peer's seal lands between our read of {1,2} and our create of it, so we meet it as an
+    /// OCCUPANT rather than as a tail entry.
+    backend->late_key = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{1, 2});
+    backend->late_bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(makeSealTxn(ns, RefTxnId{1, 2})));
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    const uint64_t adopted_before = counterOf(ProfileEvents::CASRefRecoveryEpochSealAdopted);
+    ASSERT_EQ(store->listRefs(ns).size(), 1u);
+    EXPECT_GT(counterOf(ProfileEvents::CASRefRecoveryEpochSealAdopted), adopted_before);
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::optional<RefTxnId>(RefTxnId{1, 2}))
+        << "an adopted seal is this namespace's chain link exactly as a minted one is";
+}
+
+/// A STRAGGLER: an ordinary transaction of the dead epoch landed at `{E, T+1}` after our read of the
+/// tail and before our seal. The rule is state-derived ids (INV-2): adopt the transaction, advance `T`
+/// by ONE, and re-seal at the NEW `T+1`. Never mint `T+2` around it -- that writes a hole into the
+/// durable stream that no later reader can tell from a lost object.
+TEST(CASRefRecoveryCasWalk, StragglerAtTPlusOneIsAdoptedAndResealedAtTheNewTPlusOne)
+{
+    auto backend = std::make_shared<LateMaterializeBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/straggler"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+    /// The dying epoch's last append materializes between our read of {1,2} and our create of it -- the
+    /// straggler, arriving exactly where the every-attempt rule says it can.
+    backend->late_key = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{1, 2});
+    backend->late_bytes = sealObject(FormatId::RefLog,
+        encodeRefLogTxn(makeOrdinaryTxn(ns, RefTxnId{1, 2}, "late", /*birth=*/false)));
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    const uint64_t straggler_before = counterOf(ProfileEvents::CASRefRecoveryStragglerAdopted);
+    const auto refs = store->listRefs(ns);
+    EXPECT_EQ(refs.size(), 2u) << "the straggler's transaction is durable and must be applied, not skipped";
+    EXPECT_TRUE(refs.contains("late"));
+    EXPECT_GT(counterOf(ProfileEvents::CASRefRecoveryStragglerAdopted), straggler_before);
+
+    const auto seal = readLogTxn(*backend, layout, ns, RefTxnId{1, 3});
+    ASSERT_TRUE(seal.has_value()) << "the epoch must be re-sealed at the NEW T+1 = {1,3}, never at a blindly minted T+2";
+    EXPECT_TRUE(refLogTxnIsEpochSeal(*seal));
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::optional<RefTxnId>(RefTxnId{1, 3}));
+}
+
+TEST(CASRefRecoveryCasWalk, RecoveryPublishesEveryOccupiedObjectBeforeAdvancingPastIt)
+{
+    struct Case
+    {
+        String suffix;
+        uint64_t live_epoch;
+        RefTxnId occupant;
+        RefTxnId forbidden_successor;
+        bool occupant_is_seal;
+    };
+    const std::vector<Case> cases{
+        {"seal", 3, {1, 2}, {2, 1}, true},
+        {"straggler", 2, {1, 2}, {1, 3}, false},
+    };
+
+    for (const Case & test_case : cases)
+    {
+        SCOPED_TRACE(test_case.suffix);
+        auto backend = std::make_shared<LateMaterializeBackend>();
+        const Layout layout("p");
+        const RootNamespace ns{"srv1/occupied_frontier_" + test_case.suffix};
+        const RefTxnId initial_frontier{1, 1};
+
+        burnEpochsUpTo(*backend, layout, test_case.live_epoch);
+        seedTxn(*backend, layout, ns, initial_frontier, "a", /*birth=*/true);
+        seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, initial_frontier));
+        const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+        backend->late_key = layout.refLogKey(life, test_case.occupant);
+        const RefLogTxn occupant = test_case.occupant_is_seal
+            ? makeSealTxn(ns, test_case.occupant)
+            : makeOrdinaryTxn(ns, test_case.occupant, "late", /*birth=*/false);
+        backend->late_bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(occupant));
+
+        uint64_t fake_now = 1'000'000;
+        PoolConfig config = walkTestConfig();
+        config.boot_ms_fn = [&fake_now] { return fake_now; };
+        config.cas_request_budget.recovery_retry_budget_ms = 1;
+        config.cas_request_budget.recovery_retry_initial_backoff_ms = 1;
+        config.cas_request_budget.recovery_retry_max_backoff_ms = 1;
+        auto store = openWalkPool(backend, config);
+        store->setCasRetrySleepForTest([&fake_now](uint64_t ms) { fake_now += ms; });
+
+        backend->ambiguous_cas_substr = layout.refCkptKey(life);
+        backend->ambiguous_cas_count = 100'000;
+        expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->listRefs(ns); });
+
+        EXPECT_TRUE(backend->get(layout.refLogKey(life, test_case.occupant)));
+        EXPECT_FALSE(backend->get(layout.refLogKey(life, test_case.forbidden_successor)))
+            << "recovery advanced before exact _ckpt certified the occupied object";
+        EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, initial_frontier);
+        EXPECT_FALSE(store->refTableRecoveredForTest(ns));
+    }
+}
+
+/// Two BURNED epochs -- mounted, never written to, and abandoned. `CasPool`'s epoch allocator mints and
+/// never reclaims, so this is the normal shape of a pool that has restarted a few times, not an
+/// anomaly. Each empty epoch is closed by its own sequence-1 seal, and each carries the previous seal as
+/// its `prev_epoch_seal`: the chain is what makes a MISSING epoch detectable, which arithmetic within an
+/// epoch cannot do.
+TEST(CASRefRecoveryCasWalk, TwoBurnedEmptyEpochsProduceTwoChainedSequenceOneSeals)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/burned"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/4);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    ASSERT_EQ(store->liveWriterEpoch(), 4u);
+
+    ASSERT_EQ(store->listRefs(ns).size(), 1u);
+
+    const auto seal1 = readLogTxn(*backend, layout, ns, RefTxnId{1, 2});
+    ASSERT_TRUE(seal1.has_value()) << "epoch 1 closes at {1,2}";
+    EXPECT_EQ(seal1->prev_epoch_seal, std::nullopt);
+
+    const auto seal2 = readLogTxn(*backend, layout, ns, RefTxnId{2, 1});
+    ASSERT_TRUE(seal2.has_value()) << "empty epoch 2 still closes -- at its sequence 1";
+    EXPECT_TRUE(refLogTxnIsEpochSeal(*seal2));
+    EXPECT_EQ(seal2->prev_epoch_seal, std::optional<RefTxnId>(RefTxnId{1, 2}))
+        << "a sequence-1 seal MUST name the seal that closed the previous epoch";
+
+    const auto seal3 = readLogTxn(*backend, layout, ns, RefTxnId{3, 1});
+    ASSERT_TRUE(seal3.has_value()) << "empty epoch 3 closes too";
+    EXPECT_EQ(seal3->prev_epoch_seal, std::optional<RefTxnId>(RefTxnId{2, 1}));
+
+    EXPECT_FALSE(readLogTxn(*backend, layout, ns, RefTxnId{4, 1}).has_value())
+        << "epoch 4 is LIVE -- sealing it would close the epoch this mount writes in";
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::optional<RefTxnId>(RefTxnId{3, 1}));
+}
+
+TEST(CASRefRecoveryCasWalk, RecoveryPublishesEachCreatedSealBeforeCreatingTheNextEpochSeal)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/seal_frontier_before_next"};
+    const RefTxnId initial_frontier{1, 1};
+    const RefTxnId first_seal{1, 2};
+    const RefTxnId second_seal{2, 1};
+    const RefTxnId cold_remount_frontier{3, 1};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/3);
+    seedTxn(*backend, layout, ns, initial_frontier, "a", /*birth=*/true);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, initial_frontier));
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+
+    uint64_t fake_now = 1'000'000;
+    PoolConfig config = walkTestConfig();
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.cas_request_budget.recovery_retry_budget_ms = 1;
+    config.cas_request_budget.recovery_retry_initial_backoff_ms = 1;
+    config.cas_request_budget.recovery_retry_max_backoff_ms = 1;
+    auto store = openWalkPool(backend, config);
+    ASSERT_EQ(store->liveWriterEpoch(), 3u);
+    store->setCasRetrySleepForTest([&fake_now](uint64_t ms) { fake_now += ms; });
+
+    backend->ambiguous_cas_substr = layout.refCkptKey(life);
+    backend->ambiguous_cas_count = 100'000;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->listRefs(ns); });
+
+    EXPECT_TRUE(backend->get(layout.refLogKey(life, first_seal)))
+        << "the first recovery seal became durable before its frontier attempt";
+    EXPECT_FALSE(backend->get(layout.refLogKey(life, second_seal)))
+        << "recovery may not create a second object while the first is still above exact _ckpt";
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, initial_frontier);
+    EXPECT_FALSE(store->refTableRecoveredForTest(ns));
+
+    /// Restart cold, without the failed mount's `NeedsRecovery` attempt. The first seal is durable but
+    /// still outside `_ckpt`; the remount must recover and certify it before it may create `{2,1}`.
+    backend->ambiguous_cas_count = 0;
+    store.reset();
+    auto cold_store = openWalkPool(backend);
+    ASSERT_EQ(cold_store->liveWriterEpoch(), 4u);
+    ASSERT_EQ(cold_store->listRefs(ns).size(), 1u);
+    EXPECT_TRUE(backend->get(layout.refLogKey(life, second_seal)));
+    EXPECT_TRUE(backend->get(layout.refLogKey(life, cold_remount_frontier)));
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, cold_remount_frontier);
+}
+
+/// A straggler is not an exception to the recovered-successor rule. When it materializes in the seal
+/// slot, recovery adopts it as the one object above its accepted checkpoint and must certify that exact
+/// frontier before it can create the following seal at the new `T+1`.
+TEST(CASRefRecoveryCasWalk, RecoveryPublishesAnAdoptedStragglerBeforeCreatingItsFollowingSeal)
+{
+    auto backend = std::make_shared<LateMaterializeBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/straggler_frontier_before_seal"};
+    const RefTxnId initial_frontier{1, 1};
+    const RefTxnId straggler{1, 2};
+    const RefTxnId following_seal{1, 3};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedTxn(*backend, layout, ns, initial_frontier, "a", /*birth=*/true);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, initial_frontier));
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    backend->late_key = layout.refLogKey(life, straggler);
+    backend->late_bytes = sealObject(FormatId::RefLog,
+        encodeRefLogTxn(makeOrdinaryTxn(ns, straggler, "late", /*birth=*/false)));
+
+    uint64_t fake_now = 1'000'000;
+    PoolConfig config = walkTestConfig();
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.cas_request_budget.recovery_retry_budget_ms = 1;
+    config.cas_request_budget.recovery_retry_initial_backoff_ms = 1;
+    config.cas_request_budget.recovery_retry_max_backoff_ms = 1;
+    auto store = openWalkPool(backend, config);
+    store->setCasRetrySleepForTest([&fake_now](uint64_t ms) { fake_now += ms; });
+
+    backend->ambiguous_cas_substr = layout.refCkptKey(life);
+    backend->ambiguous_cas_count = 100'000;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->listRefs(ns); });
+
+    EXPECT_TRUE(backend->get(layout.refLogKey(life, straggler)))
+        << "the straggler occupied the recovery seal slot";
+    EXPECT_FALSE(backend->get(layout.refLogKey(life, following_seal)))
+        << "recovery may not create a seal after an adopted straggler above exact _ckpt";
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, initial_frontier);
+    EXPECT_FALSE(store->refTableRecoveredForTest(ns));
+
+    backend->ambiguous_cas_count = 0;
+    ASSERT_EQ(store->listRefs(ns).size(), 2u);
+    EXPECT_TRUE(backend->get(layout.refLogKey(life, following_seal)));
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, following_seal);
+}
+
+/// GENESIS. A namespace born at epoch 5 has no epochs 1-4 of its own: they are not "empty epochs it
+/// failed to close", they are epochs before it existed. The walk starts at the namespace's `life_epoch`
+/// and writes no phantom seals below it, and with no transition ever having happened it installs NO
+/// chain link -- `nullopt` means genesis and must mean it exactly, or the table's first transaction
+/// would be required to name a seal that never existed.
+TEST(CASRefRecoveryCasWalk, GenesisAtEpochFiveWritesNoPhantomSealsBelowLifeEpoch)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/genesis5"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/5);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(5, RefTxnId{5, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{5, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    ASSERT_EQ(store->liveWriterEpoch(), 5u);
+
+    ASSERT_EQ(store->listRefs(ns).size(), 1u);
+
+    for (uint64_t e = 1; e <= 4; ++e)
+        EXPECT_FALSE(readLogTxn(*backend, layout, ns, RefTxnId{e, 1}).has_value())
+            << "no seal may be written for epoch " << e << ", which predates this namespace";
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::nullopt)
+        << "no transition ever happened for this namespace: nullopt means GENESIS and must mean it exactly";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The trio: ONE captured generation, three checks
+/// ---------------------------------------------------------------------------------------------
+
+/// The GENERIC mid-walk bump: the fence moves while recovery is doing I/O, so the incarnation that
+/// admitted this work is gone. Nothing may be installed -- the recovered view belongs to a mount that no
+/// longer owns the namespace. The table stays unrecovered, and a retry under the CURRENT generation
+/// succeeds, which is what makes this a refusal rather than a wedge.
+TEST(CASRefRecoveryCasWalk, FenceBumpedMidWalkRefusesTheInstallAndTheRetrySucceeds)
+{
+    auto backend = std::make_shared<GetSeamBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/bump_midwalk"};
+
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    std::atomic<bool> bumped{false};
+    backend->watched_substr = "_log/";
+    backend->on_key = [&](const String &)
+    {
+        if (!bumped.exchange(true))
+            rearmMountFenceAfterAnomalyForTest(store);
+    };
+
+    EXPECT_ANY_THROW(store->listRefs(ns)) << "a recovery whose I/O window straddled a fence bump must install nothing";
+
+    backend->on_key = nullptr;
+    fenceOutMountForRemount(*backend, layout.mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce())
+        << "a generation bump cannot rebind the captured runtime; the production remount must publish "
+           "a distinct runtime at the accepted generation";
+    EXPECT_EQ(store->listRefs(ns).size(), 1u) << "the retry through the remounted runtime succeeds";
+}
+
+TEST(CASRefRecoveryCasWalk, RetiredLifePausedInRealRecoveryIoWritesAndInstallsNothing)
+{
+    auto backend = std::make_shared<GetSeamBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/recovery-retired-mid-io"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "predecessor", /*birth=*/true);
+    const CatalogEntry predecessor = CasRefCatalog::read(*backend, layout).catalog.entries.front();
+    const NamespaceLifeId predecessor_life
+        = NamespaceLifeId::fromCatalogEntry(predecessor.ns, predecessor.incarnation);
+    const auto predecessor_ckpt_before = backend->get(layout.refCkptKey(predecessor_life));
+    ASSERT_TRUE(predecessor_ckpt_before);
+
+    auto store = openWalkPool(backend);
+    ASSERT_EQ(store->liveWriterEpoch(), 2u);
+    const uint64_t recovery_installs_before = store->recoveryInstallCountForTest();
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool paused = false;
+    bool resume = false;
+    backend->watched_substr = "_log/";
+    backend->on_key = [&](const String &)
+    {
+        std::unique_lock lock(mutex);
+        paused = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return resume; });
+    };
+
+    std::exception_ptr recovery_error;
+    std::thread recovery([&]
+    {
+        try
+        {
+            (void)store->listRefs(ns);
+        }
+        catch (...)
+        {
+            recovery_error = std::current_exception();
+        }
+    });
+    {
+        std::unique_lock lock(mutex);
+        cv.wait(lock, [&] { return paused; });
+    }
+
+    const CatalogEntry successor = replaceCatalogLifeForTest(*backend, layout, predecessor, UInt128{0x5152});
+    const NamespaceLifeId successor_life
+        = NamespaceLifeId::fromCatalogEntry(successor.ns, successor.incarnation);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(successor_life), encodeRefCkpt(lifeEpochCkpt(2))).outcome,
+        PutOutcome::Done);
+    const auto successor_ckpt_before = backend->get(layout.refCkptKey(successor_life));
+    ASSERT_TRUE(successor_ckpt_before);
+    store->invalidateRemovedCatalogLife(predecessor_life);
+    backend->resetCounts();
+
+    {
+        std::lock_guard lock(mutex);
+        resume = true;
+    }
+    cv.notify_all();
+    recovery.join();
+
+    EXPECT_TRUE(recovery_error) << "the predecessor recovery must be refused, not exposed";
+    EXPECT_EQ(backend->putCount(layout.refLogKey(predecessor_life, RefTxnId{1, 2})), 0u)
+        << "no predecessor seal retry may be sent after exact retirement";
+    EXPECT_EQ(backend->casPutCount(layout.refCkptKey(predecessor_life)), 0u)
+        << "no predecessor checkpoint CAS may be sent after exact retirement";
+    const auto predecessor_ckpt_after = backend->get(layout.refCkptKey(predecessor_life));
+    ASSERT_TRUE(predecessor_ckpt_after);
+    EXPECT_EQ(predecessor_ckpt_after->token, predecessor_ckpt_before->token);
+    EXPECT_FALSE(store->refTableRecoveredForTest(ns)) << "the detached predecessor result was installed";
+    EXPECT_EQ(store->recoveryInstallCountForTest(), recovery_installs_before)
+        << "the detached predecessor reached the recovery publication point";
+    const String successor_prefix = layout.namespaceStreamPrefix(successor_life);
+    for (const String & key : backend->touchedKeys())
+        EXPECT_EQ(key.find(successor_prefix), String::npos)
+            << "predecessor recovery retargeted storage I/O into successor key " << key;
+    const auto successor_ckpt_after = backend->get(layout.refCkptKey(successor_life));
+    ASSERT_TRUE(successor_ckpt_after);
+    EXPECT_EQ(successor_ckpt_after->token, successor_ckpt_before->token);
+    EXPECT_EQ(successor_ckpt_after->bytes, successor_ckpt_before->bytes);
+}
+
+/// Bump point 1 of the trio's two interior seams: AFTER the slot-occupy landed, BEFORE the `_ckpt` CAS.
+/// The seal is durable (it was written under a generation that was still valid), but the checkpoint must
+/// NOT advance and nothing may be installed. This is the seam a single "check the fence at entry" would
+/// miss entirely.
+TEST(CASRefRecoveryCasWalk, FenceBumpedAfterSlotOccupyBeforeCkptCasAdvancesNoCheckpoint)
+{
+    auto backend = std::make_shared<PutHookBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/bump_after_seal"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    const auto ckpt_before = readCkpt(*backend, layout, DB::Cas::tests::fixture::fixtureLife(ns));
+    ASSERT_TRUE(ckpt_before.has_value());
+
+    backend->watched_substr = "_log/";
+    backend->on_key = [&] { rearmMountFenceAfterAnomalyForTest(store); };
+
+    EXPECT_ANY_THROW(store->listRefs(ns));
+
+    const auto ckpt_after = readCkpt(*backend, layout, DB::Cas::tests::fixture::fixtureLife(ns));
+    ASSERT_TRUE(ckpt_after.has_value());
+    EXPECT_EQ(ckpt_after->ckpt.last_epoch_seal, std::nullopt)
+        << "the seal is durable but the checkpoint must not record it under a generation that moved";
+    EXPECT_EQ(ckpt_after->token, ckpt_before->token) << "no CAS was sent at all";
+}
+
+/// Bump point 2: AFTER the `_ckpt` CAS, BEFORE the install. The checkpoint advance is harmless (the
+/// merge is a semantic maximum, so the retry re-derives the same or a greater value), but the STATE must
+/// not be published: this runtime's view belongs to a dead incarnation. Today there is no such recheck
+/// at all -- that gap is the whole reason this test exists.
+TEST(CASRefRecoveryCasWalk, FenceBumpedAfterCkptCasBeforeInstallPublishesNoState)
+{
+    auto backend = std::make_shared<PutHookBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/bump_after_ckpt"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    /// The `_ckpt` CAS is the LAST write recovery performs, so hooking it fires strictly between the
+    /// checkpoint advance and the install recheck.
+    backend->watched_substr = "/_ckpt";
+    backend->on_key = [&] { rearmMountFenceAfterAnomalyForTest(store); };
+
+    EXPECT_ANY_THROW(store->listRefs(ns)) << "the install recheck must refuse a result from a moved generation";
+
+    const auto ckpt_after = readCkpt(*backend, layout, DB::Cas::tests::fixture::fixtureLife(ns));
+    ASSERT_TRUE(ckpt_after.has_value());
+    EXPECT_EQ(ckpt_after->ckpt.last_epoch_seal, std::optional<RefTxnId>(RefTxnId{1, 2}))
+        << "the checkpoint advance already landed and is harmless -- the merge is a semantic maximum";
+
+    backend->on_key = nullptr;
+    fenceOutMountForRemount(*backend, layout.mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce())
+        << "a generation bump cannot rebind the captured runtime; the production remount must publish "
+           "a distinct runtime at the accepted generation";
+    EXPECT_EQ(store->listRefs(ns).size(), 1u) << "the retry through the remounted runtime installs normally";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// The self-remount barrier
+/// ---------------------------------------------------------------------------------------------
+
+/// Spec §3: "self-remount cancels or waits out recovery before rearming." The install recheck alone is
+/// not that rule -- it protects the install, not the WINDOW. A recovery paused in its I/O while the
+/// fence is re-armed would still be holding an admitted generation that is about to be superseded, and
+/// the barrier is what guarantees no `_ckpt` CAS and no install can follow the re-arm.
+///
+/// Driven at a real I/O seam: recovery blocks inside a `get`, the remount barrier is invoked from
+/// another thread and must BLOCK, the recovery is released, acknowledges the cancellation, and only then
+/// does the barrier return.
+TEST(CASRefRecoveryCasWalk, RemountBarrierBlocksUntilAPausedRecoveryAcknowledgesCancellation)
+{
+    auto backend = std::make_shared<GetSeamBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/remount_barrier"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    std::mutex m;
+    std::condition_variable cv;
+    bool recovery_parked = false;
+    bool release_recovery = false;
+
+    backend->watched_substr = "_log/";
+    /// `GetSeamBackend` moves the hook out before calling it, so this parks exactly once without the
+    /// hook having to clear itself -- see its `get` for why self-clearing is a use-after-free.
+    backend->on_key = [&](const String &)
+    {
+        std::unique_lock lock(m);
+        recovery_parked = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release_recovery; });
+    };
+
+    const uint64_t ckpt_before = counterOf(ProfileEvents::CASRefCheckpointPublished);
+    const uint64_t cancelled_before = counterOf(ProfileEvents::CASRefRecoveryCancelled);
+
+    std::thread recovery([&] { try { store->listRefs(ns); } catch (...) {} }); // NOLINT(bugprone-empty-catch): the outcome is asserted below via the ProfileEvents counters, not this thread's exception
+
+    {
+        std::unique_lock lock(m);
+        cv.wait(lock, [&] { return recovery_parked; });
+    }
+
+    std::atomic<bool> barrier_returned{false};
+    std::thread barrier([&]
+    {
+        store->cancelRefRecoveriesAndAwaitQuiescence();
+        barrier_returned.store(true);
+    });
+
+    /// Wait for the barrier's REQUEST to be visible before touching anything else. Releasing the parked
+    /// recovery any earlier would race it past a flag set a moment too late, and the test would observe
+    /// an ordinary completion and call it a missing cancellation.
+    while (!store->refRecoveryCancelRequestedForTest(ns))
+        std::this_thread::yield();
+
+    /// The request is published and the recovery is still parked, so the barrier is now provably inside
+    /// its wait. It must not have returned: fence re-arm may not proceed while a recovery is in flight.
+    EXPECT_FALSE(barrier_returned.load());
+
+    {
+        std::lock_guard lock(m);
+        release_recovery = true;
+    }
+    cv.notify_all();
+
+    barrier.join();
+    recovery.join();
+    EXPECT_TRUE(barrier_returned.load());
+
+    EXPECT_GT(counterOf(ProfileEvents::CASRefRecoveryCancelled), cancelled_before)
+        << "the released recovery must observe the cancellation rather than run to completion";
+    EXPECT_EQ(counterOf(ProfileEvents::CASRefCheckpointPublished), ckpt_before)
+        << "a cancelled recovery performs ZERO _ckpt CASes";
+    EXPECT_FALSE(store->refTableRecoveredForTest(ns)) << "and ZERO installs";
+}
+
+/// A `NeedsRecovery` lane replays the known-durable transaction before returning to `Ready`.
+TEST(CASRefRecoveryCasWalk, NeedsRecoveryReplaysTheStrandedTxn)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/poisoned"};
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    /// Publish one ref through the real lane, then fail the next commit's install region: the
+    /// transaction is durable and the install that would have recorded it throws.
+    ASSERT_NO_THROW(store->appendRefOps(ns, MutationScope::ref("a"),
+        [](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (const RefOp & op : publishCommittedOps("a", manifestRef(1, 1, 1)))
+                ops.push_back(op);
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Publish));
+
+    /// Built outside the region. One-shot, and re-allowing allocations for
+    /// the duration of the throw: `std::rethrow_exception` allocates through libc++'s
+    /// `__cxa_rethrow_primary_exception`, which the debug build's `DENY_ALLOCATIONS_IN_SCOPE` aborts on.
+    /// (Found by the debug gate -- the first cut of this probe took the whole binary down there.) Same
+    /// shape as `gtest_cas_ref_install_safety.cpp`'s `armOneShotInstallFailure`.
+    auto planned_failure = std::make_exception_ptr(DB::Exception(DB::ErrorCodes::CORRUPTED_DATA, "install probe"));
+    auto fired = std::make_shared<std::atomic<bool>>(false);
+    store->setInstallRegionProbeForTest([planned_failure, fired]
+    {
+        if (fired->exchange(true))
+            return;
+        ALLOW_ALLOCATIONS_IN_SCOPE;
+        std::rethrow_exception(planned_failure);
+    });
+    EXPECT_ANY_THROW(store->appendRefOps(ns, MutationScope::ref("b"),
+        [](const RefTableState &) { return publishCommittedOps("b", manifestRef(1, 2, 1)); },
+        RootMutationOrigin::Writer, RootMutationKind::Publish));
+    store->setInstallRegionProbeForTest(nullptr);
+
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    /// "Durable but not applied here", stated as the two facts it is made of: the object IS in the
+    /// store, and this runtime's floor is what keeps the allocator off its id. `ns` was born through
+    /// the REAL production lane (`appendRefOps`), not the raw `seedTxn`/`casAdmitEntry` fixtures this
+    /// file's OTHER tests use -- so its ref-layer objects sit at a REAL, catalog-minted incarnation,
+    /// not the Stage-A sentinel `readLogTxn` assumes. Resolved here rather than through `readLogTxn`.
+    {
+        const CasRefCatalog::Snapshot snap = CasRefCatalog::read(*backend, layout);
+        const CatalogEntry * entry = nullptr;
+        for (const CatalogEntry & e : snap.catalog.entries)
+            if (e.ns.string() == ns.string())
+                entry = &e;
+        ASSERT_NE(entry, nullptr) << "the birth above must have minted a catalog entry for " << ns.string();
+        const NamespaceLifeId life = NamespaceLifeId::fromCatalogEntry(entry->ns, entry->incarnation);
+        ASSERT_TRUE(backend->get(layout.refLogKey(life, RefTxnId{1, 2})).has_value())
+            << "the stranded transaction must be durable -- otherwise recovery is not owed";
+    }
+
+    /// The next touch drives recovery again -- this is the structural closure Task 3 deferred here.
+    const auto refs = store->listRefs(ns);
+    EXPECT_EQ(refs.size(), 2u);
+    EXPECT_TRUE(refs.contains("b")) << "the walk re-derived the stranded transaction from the durable log";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready)
+        << "only a completed recovery install returns the lane to Ready";
+}
+
+TEST(CASRefRecoveryCasWalk, WriterRecoveryAdoptsOneExactUnfrontieredSuccessorAndPublishesItsFrontier)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/recovery_one_successor"};
+    auto store = openWalkPool(backend);
+
+    ASSERT_NO_THROW(store->appendRefOps(ns, MutationScope::ref("a"),
+        [](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (const RefOp & op : publishCommittedOps("a", manifestRef(1, 1, 1)))
+                ops.push_back(op);
+            return ops;
+        }, RootMutationOrigin::Writer, RootMutationKind::Publish));
+
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    const String ckpt_key = layout.refCkptKey(life);
+    ASSERT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, (RefTxnId{1, 1}));
+    backend->ambiguous_cas_substr = ckpt_key;
+    backend->ambiguous_cas_count = 200;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        store->appendRefOps(ns, MutationScope::ref("b"),
+            [](const RefTableState &) { return publishCommittedOps("b", manifestRef(1, 2, 1)); },
+            RootMutationOrigin::Writer, RootMutationKind::Publish);
+    });
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    ASSERT_TRUE(backend->get(layout.refLogKey(life, RefTxnId{1, 2})))
+        << "the sole deterministic successor must be durable before recovery";
+    ASSERT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, (RefTxnId{1, 1}));
+
+    backend->ambiguous_cas_count = 0;
+    const auto refs = store->listRefs(ns);
+
+    EXPECT_TRUE(refs.contains("b"));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, (RefTxnId{1, 2}))
+        << "the successor is not installable until the current admitted fence publishes its frontier";
+}
+
+TEST(CASRefRecoveryCasWalk, ColdWriterRecoveryPublishesOneExactUnfrontieredSuccessorBeforeSealing)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/recovery_cold_successor"};
+    auto store = openWalkPool(backend);
+    const NamespaceLifeId life = strandOneUnfrontieredSuccessor(*backend, store, layout, ns);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    store.reset();
+    std::vector<RefCkpt> checkpoint_cas_bodies;
+    backend->before_cas_put = [&](const String & key, const String & bytes, const std::optional<Token> &)
+    {
+        if (key == layout.refCkptKey(life))
+            checkpoint_cas_bodies.push_back(decodeRefCkpt(bytes));
+    };
+
+    /// A remount/process restart has no in-memory `RefAppendAttempt`; the writer recovery entry point
+    /// still owns its one exact F+1 adoption duty from the durable checkpoint and log alone.
+    auto cold_store = openWalkPool(backend);
+    const auto refs = cold_store->listRefs(ns);
+
+    EXPECT_TRUE(refs.contains("b"));
+    EXPECT_EQ(cold_store->laneStateForTest(ns), RefLaneState::Ready);
+    EXPECT_TRUE(std::any_of(checkpoint_cas_bodies.begin(), checkpoint_cas_bodies.end(),
+        [](const RefCkpt & ckpt) { return ckpt.committed_through == std::make_optional(RefTxnId{1, 2}); }))
+        << "the exact F+1 frontier must publish before the remount seals its dead epoch";
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, (RefTxnId{1, 3}));
+}
+
+TEST(CASRefRecoveryCasWalk, WriterRecoveryAdoptsFirstCommittedTxnAboveLifeEpochOnlyCheckpoint)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/recovery_first_unfrontiered"};
+    auto store = openWalkPool(backend);
+
+    /// Create the catalog life explicitly, then retain exactly the checkpoint fragment published by
+    /// production birth before its first log. This makes `{1,1}` the first durable transaction above a
+    /// readable checkpoint whose `committed_through` is absent.
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(lifeEpochCkpt(1))).outcome,
+              PutOutcome::Done);
+    ASSERT_TRUE(readCkpt(*backend, layout, life)->ckpt.life_epoch);
+    ASSERT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, std::nullopt);
+
+    backend->ambiguous_cas_substr = layout.refCkptKey(life);
+    backend->ambiguous_cas_count = 200;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        store->appendRefOps(ns, MutationScope::ref("a"),
+            [](const RefTableState & state)
+            {
+                std::vector<RefOp> ops;
+                if (state.getLifecycle() != RefLifecycle::Live)
+                    ops.push_back(namespaceBirthOp());
+                for (const RefOp & op : publishCommittedOps("a", manifestRef(1, 1, 1)))
+                    ops.push_back(op);
+                return ops;
+            }, RootMutationOrigin::Writer, RootMutationKind::Publish);
+    });
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    ASSERT_TRUE(backend->get(layout.refLogKey(life, RefTxnId{1, 1})));
+    ASSERT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, std::nullopt);
+    ASSERT_FALSE(backend->get(layout.refSnapshotKey(life, RefTxnId{1, 1})))
+        << "the grounding test must exercise the exact log successor, not a hinted snapshot";
+
+    backend->ambiguous_cas_count = 0;
+    const auto refs = store->listRefs(ns);
+
+    EXPECT_TRUE(refs.contains("a"));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, (RefTxnId{1, 1}));
+}
+
+TEST(CASRefRecoveryCasWalk, WriterRecoveryRestartsWhenCheckpointAdvancesPastPrivateCandidate)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/recovery_checkpoint_moves"};
+    auto store = openWalkPool(backend);
+    const NamespaceLifeId life = strandOneUnfrontieredSuccessor(*backend, store, layout, ns);
+    const String ckpt_key = layout.refCkptKey(life);
+    const RefLogTxn later = makeOrdinaryTxn(ns, RefTxnId{1, 3}, "c", /*birth=*/false);
+    bool injected = false;
+
+    /// Recovery has fetched `{1,2}` and proved `{1,3}` absent. Another admitted writer can then append
+    /// `{1,3}` and publish its frontier before recovery's own checkpoint CAS. The stale private
+    /// candidate contains only `b`; it must restart and replay `c`, not accept an `IdenticalSkip` and
+    /// install below the exact checkpoint it just observed.
+    backend->before_cas_put = [&](const String & key, const String &, const std::optional<Token> & expected)
+    {
+        if (injected || key != ckpt_key)
+            return;
+        injected = true;
+        ASSERT_TRUE(expected);
+        ASSERT_EQ(backend->putIfAbsent(layout.refLogKey(life, later.txn_id),
+                                      sealObject(FormatId::RefLog, encodeRefLogTxn(later))).outcome,
+                  PutOutcome::Done);
+        const auto current = backend->get(key);
+        ASSERT_TRUE(current);
+        ASSERT_EQ(current->token, *expected);
+        const RefCkpt advanced = mergeCkpt(
+            decodeRefCkpt(current->bytes),
+            RefCkpt{.life_epoch = std::nullopt,
+                    .committed_through = later.txn_id,
+                    .checkpoint_snapshot_id = std::nullopt,
+                    .last_epoch_seal = std::nullopt});
+        ASSERT_EQ(backend->putOverwrite(key, encodeRefCkpt(advanced), current->token).outcome, PutOutcome::Done);
+    };
+
+    const auto refs = store->listRefs(ns);
+
+    EXPECT_TRUE(injected);
+    EXPECT_TRUE(refs.contains("b"));
+    EXPECT_TRUE(refs.contains("c")) << "recovery must restart from the newer exact frontier";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, later.txn_id);
+}
+
+TEST(CASRefRecoveryCasWalk, WriterRecoveryRejectsTwoUnfrontieredSuccessorsAfterExactCheckpointReread)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/recovery_two_successors"};
+    auto store = openWalkPool(backend);
+
+    ASSERT_NO_THROW(store->appendRefOps(ns, MutationScope::ref("a"),
+        [](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (const RefOp & op : publishCommittedOps("a", manifestRef(1, 1, 1)))
+                ops.push_back(op);
+            return ops;
+        }, RootMutationOrigin::Writer, RootMutationKind::Publish));
+
+    const NamespaceLifeId life = catalogLife(*backend, layout, ns);
+    const String ckpt_key = layout.refCkptKey(life);
+    backend->ambiguous_cas_substr = ckpt_key;
+    backend->ambiguous_cas_count = 200;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        store->appendRefOps(ns, MutationScope::ref("b"),
+            [](const RefTableState &) { return publishCommittedOps("b", manifestRef(1, 2, 1)); },
+            RootMutationOrigin::Writer, RootMutationKind::Publish);
+    });
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    const RefLogTxn second_successor = makeOrdinaryTxn(ns, RefTxnId{1, 3}, "c", /*birth=*/false);
+    ASSERT_EQ(backend->putIfAbsent(layout.refLogKey(life, second_successor.txn_id),
+                                  sealObject(FormatId::RefLog, encodeRefLogTxn(second_successor))).outcome,
+              PutOutcome::Done);
+    backend->ambiguous_cas_count = 0;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)store->listRefs(ns); });
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, (RefTxnId{1, 1}))
+        << "corruption must not launder either successor into the frontier";
+}
+
+TEST(CASRefRecoveryCasWalk, WriterRecoveryRejectsDifferentOrdinaryBytesAtTheRetainedSuccessorSlot)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/recovery_different_successor"};
+    auto store = openWalkPool(backend);
+    const NamespaceLifeId life = strandOneUnfrontieredSuccessor(*backend, store, layout, ns);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    const String successor_key = layout.refLogKey(life, RefTxnId{1, 2});
+    const auto original = backend->get(successor_key);
+    ASSERT_TRUE(original);
+    const RefLogTxn different = makeOrdinaryTxn(ns, RefTxnId{1, 2}, "different", /*birth=*/false);
+    ASSERT_EQ(backend->putOverwrite(successor_key,
+                                   sealObject(FormatId::RefLog, encodeRefLogTxn(different)),
+                                   original->token).outcome,
+              PutOutcome::Done);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)store->listRefs(ns); });
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, (RefTxnId{1, 1}));
+}
+
+TEST(CASRefRecoveryCasWalk, RetainedOldWriterAttemptLosesConclusiveToASuccessorSeal)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/recovery_successor_seal"};
+    auto store = openWalkPool(backend);
+    const NamespaceLifeId life = strandOneUnfrontieredSuccessor(*backend, store, layout, ns);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    const RefTxnId successor_id{1, 2};
+    const String successor_key = layout.refLogKey(life, successor_id);
+    const auto original = backend->get(successor_key);
+    ASSERT_TRUE(original);
+    const RefLogTxn successor_seal = makeSealTxn(ns, successor_id);
+    ASSERT_EQ(backend->putOverwrite(successor_key,
+                                   sealObject(FormatId::RefLog, encodeRefLogTxn(successor_seal)),
+                                   original->token).outcome,
+              PutOutcome::Done);
+
+    const auto refs = store->listRefs(ns);
+
+    EXPECT_FALSE(refs.contains("b")) << "the old writer's retained ordinary bytes lost at the sealed slot";
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::make_optional(successor_id));
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.committed_through, successor_id);
+    EXPECT_EQ(readCkpt(*backend, layout, life)->ckpt.last_epoch_seal, successor_id);
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Fail-closed on an unresolved slot
+/// ---------------------------------------------------------------------------------------------
+
+/// `Unresolved` from the slot-occupy means the store will not say whether our seal landed. That is not a
+/// state to guess about: recovery takes the transient-retry path and, once its budget is spent, fails
+/// closed with the table left unrecovered. Exposing a table whose dead epoch may or may not be closed is
+/// the one outcome that must be impossible.
+TEST(CASRefRecoveryCasWalk, UnresolvedSealSlotFailsClosedWithoutInstalling)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/unresolved"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    /// The backoff sleep ADVANCES the same fake clock the budget is measured against, so the retry
+    /// envelope is spent in a handful of iterations instead of spinning against a frozen clock. Not
+    /// cosmetic: with a frozen clock this test burns ~700k retries and the same number of log lines,
+    /// which is how a real regression in this arm would become invisible in the noise.
+    uint64_t fake_now = 1'000'000;
+    PoolConfig config = walkTestConfig();
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    auto store = openWalkPool(backend, config);
+    ASSERT_TRUE(store);
+
+    store->setCasRetrySleepForTest([&fake_now](uint64_t ms) { fake_now += ms; });
+    backend->ambiguous_put_substr = "/_log/";
+
+    EXPECT_ANY_THROW(store->listRefs(ns));
+    EXPECT_FALSE(store->refTableRecoveredForTest(ns))
+        << "a table whose dead epoch may or may not be closed must never be exposed as recovered";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// Carried forward from the retired `RefWriterRecoverySeal` suite
+/// ---------------------------------------------------------------------------------------------
+
+/// THE property the whole in-band design exists for, and the one the retired suite could only
+/// approximate with a detector: the Late Predecessor PUT is REFUSED, by the store, at the key it wanted.
+///
+/// A dying writer of epoch 1 has an append in flight for `{1,2}`. Recovery closes epoch 1 by occupying
+/// exactly that slot. When the ghost's conditional create finally reaches the store there is nothing for
+/// it to do -- the key is write-once and taken. The old sentinel seal was a SNAPSHOT at a synthetic id,
+/// which left `{1,2}` free: the ghost landed, and all anyone could do was notice afterwards.
+TEST(CASRefRecoveryCasWalk, ALatePredecessorPutAtTheSealedSlotIsRefusedByTheStore)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/ghost"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    ASSERT_EQ(store->listRefs(ns).size(), 1u);
+
+    /// The ghost: the exact append the dead epoch's writer had in flight, arriving late.
+    const RefTxnId ghost_id{1, 2};
+    const String ghost_bytes = sealObject(FormatId::RefLog,
+        encodeRefLogTxn(makeOrdinaryTxn(ns, ghost_id, "ghost", /*birth=*/false)));
+    const PutResult put = backend->putIfAbsent(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), ghost_id), ghost_bytes);
+    EXPECT_EQ(put.outcome, PutOutcome::PreconditionFailed)
+        << "the seal occupies the ghost's own key, so the store itself is the fence";
+
+    /// And the object at that key is still the seal, byte for byte -- nothing adopted the ghost.
+    const auto occupant = readLogTxn(*backend, layout, ns, ghost_id);
+    ASSERT_TRUE(occupant.has_value());
+    EXPECT_TRUE(refLogTxnIsEpochSeal(*occupant));
+}
+
+/// An occupant at the seal slot that this build cannot decode is NOT a straggler to adopt and NOT a
+/// peer's seal to defer to: it is an object at a key this namespace exclusively owns whose meaning is
+/// unknown. Recovery fails closed on it -- and, just as importantly, stays RESTARTABLE: the throw must
+/// leave `recovery_in_progress` cleared, or the table would be unrecoverable for the mount's life and
+/// every later toucher would park forever on a condition variable nobody will signal.
+TEST(CASRefRecoveryCasWalk, UndecodableOccupantAtTheSealSlotFailsClosedAndLeavesRecoveryRestartable)
+{
+    auto backend = std::make_shared<LateMaterializeBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/foreign_slot"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+    backend->late_key = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{1, 2});
+    backend->late_bytes = "not a ref-log object at all";
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->listRefs(ns); });
+    EXPECT_FALSE(store->refTableRecoveredForTest(ns));
+
+    /// Restartable: a second touch runs a WHOLE new attempt (it fails the same way, which is the point --
+    /// it reaches the failure again rather than hanging on a stuck `recovery_in_progress`).
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->listRefs(ns); });
+}
+
+/// A second caller that arrives while a recovery is mid-walk WAITS for it rather than racing an
+/// independent walk of its own. Two concurrent walks would both try to occupy the same seal slot, and
+/// while the loser adopts correctly, they would also both replay the whole tail and one would install a
+/// state the other's install immediately replaces -- work and I/O for nothing, on the path that is
+/// already the most expensive one in the system.
+TEST(CASRefRecoveryCasWalk, ASecondCallerWaitsForTheWalkInsteadOfRacingIt)
+{
+    auto backend = std::make_shared<GetSeamBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/serialized"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/2);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(1, RefTxnId{1, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+
+    std::mutex m;
+    std::condition_variable cv;
+    bool parked = false;
+    bool release = false;
+
+    backend->watched_substr = "_log/";
+    backend->on_key = [&](const String &)
+    {
+        std::unique_lock lock(m);
+        parked = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release; });
+    };
+
+    const uint64_t adopted_before = counterOf(ProfileEvents::CASRefRecoveryEpochSealAdopted);
+    std::thread first([&] { store->listRefs(ns); });
+    {
+        std::unique_lock lock(m);
+        cv.wait(lock, [&] { return parked; });
+    }
+
+    /// The second caller blocks on `recovery_in_progress`. Its own recovery would have to LIST, and the
+    /// walk holds no lock while parked, so nothing but the serialization flag can be keeping it out.
+    std::atomic<bool> second_done{false};
+    std::thread second([&] { store->listRefs(ns); second_done.store(true); });
+    for (int i = 0; i < 50 && !second_done.load(); ++i)
+        std::this_thread::yield();
+    EXPECT_FALSE(second_done.load()) << "a second caller must wait out the in-flight walk, not race it";
+
+    {
+        std::lock_guard lock(m);
+        release = true;
+    }
+    cv.notify_all();
+    first.join();
+    second.join();
+
+    EXPECT_TRUE(store->refTableRecoveredForTest(ns));
+    /// Exactly ONE walk minted the seal, and no second walk ever met it as an occupant. Adopting is the
+    /// CORRECT outcome for a concurrent recoverer -- it is just work this serialization exists to avoid
+    /// paying inside one process, so observing zero adoptions is what proves the second caller waited.
+    EXPECT_EQ(counterOf(ProfileEvents::CASRefRecoveryEpochSealAdopted), adopted_before);
+    const auto seal = readLogTxn(*backend, layout, ns, RefTxnId{1, 2});
+    ASSERT_TRUE(seal.has_value());
+    EXPECT_TRUE(refLogTxnIsEpochSeal(*seal));
+}
+
+/// Checkpoint-grounded recovery starts at the recreated life's own genesis and does not replay or
+/// extend the predecessor life's stream.
+///
+/// The old same-stream fixture claimed that recovery walked through the epoch-1 removal into epoch 2.
+/// With authoritative `_ckpt.life_epoch=2`, epoch 2 is instead the current life's genesis and the walk
+/// begins at `{2,1}`. Epoch-1 objects are inert predecessor-life debris: they neither supply state nor
+/// receive a recovery seal.
+TEST(CASRefRecoveryCasWalk, RecoveryStartsAtRecreatedLifeGenesisAndLeavesPredecessorStreamUntouched)
+{
+    auto backend = std::make_shared<HidingListBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/removed_then_reborn"};
+
+    burnEpochsUpTo(*backend, layout, /*target_live_epoch=*/3);
+    seedCkpt(*backend, layout, ns, lifeEpochCkpt(2, RefTxnId{2, 1}));
+    seedTxn(*backend, layout, ns, RefTxnId{1, 1}, "a", /*birth=*/true);
+
+    /// Epoch 1 ends with the terminal record: the ref is removed, then the namespace.
+    RefLogTxn removal;
+    removal.ns = ns.string();
+    removal.txn_id = RefTxnId{1, 2};
+    removal.ops = {DB::Cas::tests::ownerTransitionOp(
+                       RefOwnerBinding{RefOwnerKind::Committed, "a", manifestRef(1, 1, 1u)}, std::nullopt),
+                   removeNamespaceOp()};
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, removal);
+
+    /// The current life starts in epoch 2. Its birth is sequence 1 of its own genesis epoch, so it
+    /// carries no chain link to the predecessor life.
+    seedTxn(*backend, layout, ns, RefTxnId{2, 1}, "reborn", /*birth=*/true);
+
+    auto store = openWalkPool(backend);
+    ASSERT_TRUE(store);
+    ASSERT_EQ(store->liveWriterEpoch(), 3u);
+
+    const auto refs = store->listRefs(ns);
+    EXPECT_EQ(refs.size(), 1u);
+    EXPECT_TRUE(refs.contains("reborn")) << "recovery must begin at the recreated life's genesis";
+
+    EXPECT_FALSE(readLogTxn(*backend, layout, ns, RefTxnId{1, 3}).has_value())
+        << "recovery of life epoch 2 must not extend the predecessor-life stream";
+    const auto seal2 = readLogTxn(*backend, layout, ns, RefTxnId{2, 2});
+    ASSERT_TRUE(seal2.has_value()) << "epoch 2 IS live again by the time it dies, so it closes normally";
+    EXPECT_TRUE(refLogTxnIsEpochSeal(*seal2));
+    EXPECT_EQ(seal2->prev_epoch_seal, std::nullopt) << "sequence 2 carries no chain link";
+}
+
+/// `PutHookBackend::casPut` must route through its immediate parent `HidingListBackend::casPut`, not
+/// past it to `CountingBackend`, so that a test arming BOTH layers on one `PutHookBackend` instance
+/// gets both behaviors composed rather than one silently disabled by the other.
+TEST(CASRefRecoveryCasWalk, PutHookBackendComposesHidingListBackendCasPutFaultInjection)
+{
+    auto backend = std::make_shared<PutHookBackend>();
+
+    bool before_cas_put_fired = false;
+    backend->before_cas_put = [&](const String &, const String &, const std::optional<Token> &)
+    {
+        before_cas_put_fired = true;
+    };
+
+    backend->watched_substr = "probe";
+    bool on_key_fired = false;
+    backend->on_key = [&] { on_key_fired = true; };
+
+    ASSERT_EQ(backend->casPut("p/probe", "x", std::nullopt).outcome, CasOutcome::Committed);
+
+    EXPECT_TRUE(before_cas_put_fired)
+        << "HidingListBackend's before_cas_put hook must still fire for a PutHookBackend instance";
+    EXPECT_TRUE(on_key_fired) << "PutHookBackend's own on_key hook must still fire on top of it";
+}
diff --git a/src/Disks/tests/gtest_cas_ref_snapshot_format.cpp b/src/Disks/tests/gtest_cas_ref_snapshot_format.cpp
new file mode 100644
index 000000000000..2293c0bc166c
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_snapshot_format.cpp
@@ -0,0 +1,436 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <limits>
+
+/// v3 text codec tests for `cas_ref_snap` (codecs-v3 phase 3). Split out of the retired
+/// `gtest_cas_ref_codecs.cpp` and re-pointed at the TEXT codec. The encoder-side validation tests are
+/// format-agnostic and carry over verbatim; the old binary-offset byte-patch decode tests
+/// (`bytes[k] = 99`) are gone -- the shape-level corruption classes (truncation, `v`+1 forward-gate,
+/// wrong type, leading garbage) are covered by the `CASFormatBattery.RefSnapshot` row below, which also
+/// subsumes the old `DecodeRejectsFutureFormatVersion`/`DecodeRejectsFormatVersionOne` pair (there is
+/// no `format_version` byte any more -- the header `v` gate is the single forward-compat mechanism).
+
+using namespace DB::Cas;
+using DB::Cas::tests::expectThrowsCode;
+
+namespace
+{
+
+ManifestRef manifestRef(uint64_t epoch, uint64_t seq, uint32_t ordinal)
+{
+    return ManifestRef{epoch, seq, ordinal};
+}
+
+RefTableSnapshot makeLiveSnapshot()
+{
+    RefTableSnapshot s;
+    s.ns = "srv1/db/table@cas@";
+    s.snapshot_id = RefTxnId{5, 200};
+
+    RefCommittedRow c1;
+    c1.ref_name = "all_1_1_0";
+    c1.manifest_ref = manifestRef(5, 10, 1);
+    c1.published_at_ms = 1717000000000ULL;
+    s.committed.push_back(c1);
+
+    RefCommittedRow c2;
+    c2.ref_name = "all_2_2_0";
+    c2.manifest_ref = manifestRef(5, 11, 1);
+    c2.published_at_ms = 1717000000001ULL;
+    s.committed.push_back(c2);
+
+    RefOwnerBinding p1{RefOwnerKind::Precommit, "all_3_3_0", manifestRef(5, 12, 1)};
+    s.precommits.push_back(p1);
+
+    return s;
+}
+
+}
+
+/// ===================================================================================
+/// RefTableSnapshot: round trip
+/// ===================================================================================
+
+TEST(CASRefSnapshotCodec, RoundTripLive)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    const String bytes = encodeRefTableSnapshot(s);
+    const RefTableSnapshot decoded = decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id);
+    EXPECT_EQ(decoded, s);
+}
+
+TEST(CASRefSnapshotCodec, DecodeRequiresLifecycleField)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    String bytes = encodeRefTableSnapshot(s);
+    const String field = R"(,"lc":"live")";
+    const size_t at = bytes.find(field);
+    ASSERT_NE(at, String::npos);
+    bytes.erase(at, field.size());
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id); });
+}
+
+TEST(CASRefSnapshotCodec, DecodeRejectsTerminalLifecycleWord)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    String bytes = encodeRefTableSnapshot(s);
+    const String live = R"("lc":"live")";
+    const size_t at = bytes.find(live);
+    ASSERT_NE(at, String::npos);
+    bytes.replace(at, live.size(), R"("lc":"removed")");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id); });
+}
+
+TEST(CASRefSnapshotCodec, DecodeRejectsRetiredRemoveTxnEpochField)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    String bytes = encodeRefTableSnapshot(s);
+    const String live = R"("lc":"live")";
+    const size_t at = bytes.find(live);
+    ASSERT_NE(at, String::npos);
+    bytes.replace(at, live.size(), live + R"(,"rte":"7")");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id); });
+}
+
+TEST(CASRefSnapshotCodec, DecodeRejectsRetiredRemoveTxnSequenceField)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    String bytes = encodeRefTableSnapshot(s);
+    const String live = R"("lc":"live")";
+    const size_t at = bytes.find(live);
+    ASSERT_NE(at, String::npos);
+    bytes.replace(at, live.size(), live + R"(,"rts":"9")");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id); });
+}
+
+TEST(CASRefSnapshotCodec, DecodeRejectsRetiredRemoveTxnFieldPair)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    String bytes = encodeRefTableSnapshot(s);
+    const String live = R"("lc":"live")";
+    const size_t at = bytes.find(live);
+    ASSERT_NE(at, String::npos);
+    bytes.replace(at, live.size(), live + R"(,"rte":"7","rts":"9")");
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { (void)decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id); });
+}
+
+/// No-tolerance decode pin (codex round-2, finding 3): the `"pl"` (payload) field was removed from the
+/// committed-row wire in stage-1 T12. It is NOT a genuinely-unknown future field the tolerant reader may
+/// skip -- silently discarding a persisted payload would lose data -- so decoding a committed row that
+/// still carries `"pl"` must FAIL with `CORRUPTED_DATA` naming the removed field, not `skipUnknown` it.
+TEST(CASRefSnapshotCodec, DecodeRejectsRemovedPayloadFieldInCommittedRow)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow c;
+    c.ref_name = "all_1_1_0";
+    c.manifest_ref = manifestRef(5, 10, 1);
+    c.published_at_ms = 1717000000000ULL;
+    s.committed.push_back(c);
+
+    const String bytes = encodeRefTableSnapshot(s);
+    /// Splice the retired `"pl"` field back into the committed record, just before its `"ts"` field.
+    const String needle = ",\"ts\":";
+    const auto pos = bytes.find(needle);
+    ASSERT_NE(pos, String::npos);
+    const String tampered = bytes.substr(0, pos) + R"(,"pl":"deadbeef")" + bytes.substr(pos);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefTableSnapshot(tampered, s.ns, s.snapshot_id); });
+}
+
+TEST(CASRefSnapshotCodec, RoundTripLiveEmpty)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+
+    const String bytes = encodeRefTableSnapshot(s);
+    const RefTableSnapshot decoded = decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id);
+    EXPECT_EQ(decoded, s);
+    EXPECT_TRUE(decoded.committed.empty());
+    EXPECT_TRUE(decoded.precommits.empty());
+}
+
+TEST(CASRefSnapshotCodec, ByteIdenticalReencode)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    const String bytes1 = encodeRefTableSnapshot(s);
+    const RefTableSnapshot decoded = decodeRefTableSnapshot(bytes1, s.ns, s.snapshot_id);
+    const String bytes2 = encodeRefTableSnapshot(decoded);
+    EXPECT_EQ(bytes1, bytes2);
+}
+
+TEST(CASRefSnapshotCodec, RoundTripPrecommitsSameNameDifferentManifest)
+{
+    /// Two builds racing for the same final ref name: same ref_name, different manifest_ref, sorted
+    /// by manifest_ref as the tiebreak.
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "same", manifestRef(1, 1, 1)});
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "same", manifestRef(1, 2, 1)});
+
+    const String bytes = encodeRefTableSnapshot(s);
+    const RefTableSnapshot decoded = decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id);
+    EXPECT_EQ(decoded, s);
+    EXPECT_EQ(decoded.precommits.size(), 2u);
+}
+
+/// ===================================================================================
+/// Large ids
+/// ===================================================================================
+
+/// `ref_sequence` is a 64-bit counter and the codec writes it as a decimal STRING, so the top of the
+/// range survives a round trip without JSON's number semantics getting involved. This used to be pinned
+/// through the retired sentinel seal, whose synthetic `{E-1, UINT64_MAX}` id was the only place such a
+/// value arose; the representation guarantee is what actually mattered and it is pinned directly here.
+TEST(CASRefSnapshotFormat, MaximalRefSequenceRoundTripsAsADecimalString)
+{
+    RefTableSnapshot m;
+    m.ns = "ns";
+    m.snapshot_id = RefTxnId{5, std::numeric_limits<uint64_t>::max()};
+
+    const String text = encodeRefTableSnapshot(m);
+    const RefTableSnapshot back = decodeRefTableSnapshot(text, m.ns, m.snapshot_id);
+    EXPECT_EQ(back.snapshot_id.ref_sequence, std::numeric_limits<uint64_t>::max());
+    EXPECT_NE(text.find("\"rs\":\"18446744073709551615\""), String::npos);
+}
+
+/// ===================================================================================
+/// RefTableSnapshot: validation rejections (encoder-side + key/body binding + truncation)
+/// ===================================================================================
+
+TEST(CASRefSnapshotCodec, EncodeRejectsZeroSnapshotId)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{0, 1};
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsUnsortedCommitted)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow a;
+    a.ref_name = "b";
+    a.manifest_ref = manifestRef(1, 1, 1);
+    RefCommittedRow b;
+    b.ref_name = "a";
+    b.manifest_ref = manifestRef(1, 2, 1);
+    s.committed.push_back(a);
+    s.committed.push_back(b);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsDuplicateCommittedRefName)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow a;
+    a.ref_name = "same";
+    a.manifest_ref = manifestRef(1, 1, 1);
+    RefCommittedRow b;
+    b.ref_name = "same";
+    b.manifest_ref = manifestRef(1, 2, 1);
+    s.committed.push_back(a);
+    s.committed.push_back(b);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsUnsortedPrecommits)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "b", manifestRef(1, 1, 1)});
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "a", manifestRef(1, 2, 1)});
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsPrecommitsSameNameWrongManifestOrder)
+{
+    /// Same ref_name but the manifest_ref tiebreak is descending -- must be rejected even though the
+    /// names alone look sorted.
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "same", manifestRef(1, 2, 1)});
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "same", manifestRef(1, 1, 1)});
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsDuplicatePrecommitBinding)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "same", manifestRef(1, 1, 1)});
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "same", manifestRef(1, 1, 1)});
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsNonCanonicalCommittedRefName)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow row;
+    row.ref_name = "a/../b";
+    row.manifest_ref = manifestRef(1, 1, 1);
+    s.committed.push_back(row);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsNonCanonicalPrecommitRefName)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "", manifestRef(1, 1, 1)});
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsPrecommitWrongKind)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    s.precommits.push_back(RefOwnerBinding{RefOwnerKind::Committed, "r", manifestRef(1, 1, 1)});
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsZeroManifestRefFields)
+{
+    {
+        RefTableSnapshot s;
+        s.ns = "ns";
+        s.snapshot_id = RefTxnId{1, 1};
+        RefCommittedRow row;
+        row.ref_name = "r";
+        row.manifest_ref = manifestRef(0, 1, 1);
+        s.committed.push_back(row);
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+    }
+    {
+        RefTableSnapshot s;
+        s.ns = "ns";
+        s.snapshot_id = RefTxnId{1, 1};
+        RefCommittedRow row;
+        row.ref_name = "r";
+        row.manifest_ref = manifestRef(1, 1, 0);   /// ordinal 0 is out of range
+        s.committed.push_back(row);
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+    }
+}
+
+TEST(CASRefSnapshotCodec, EncodeRejectsOversizedSnapshot)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow row;
+    /// `ref_name` has no length limit (`checkCanonicalRefName`), so it is the padding field now that
+    /// `payload` is gone: a run of un-escaped 'x' bytes inflates the encoded row one-for-one.
+    row.ref_name = String(ref_snapshot_max_bytes + 1, 'x');
+    row.manifest_ref = manifestRef(1, 1, 1);
+    s.committed.push_back(row);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { encodeRefTableSnapshot(s); });
+}
+
+TEST(CASRefSnapshotCodec, DecodeRejectsTruncatedBuffer)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    const String bytes = encodeRefTableSnapshot(s);
+    /// Dropping the trailing bytes leaves the final line without its '\n' terminator -> fail closed.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefTableSnapshot(bytes.substr(0, bytes.size() - 3), s.ns, s.snapshot_id); });
+}
+
+TEST(CASRefSnapshotCodec, DecodeRejectsNamespaceMismatch)
+{
+    RefTableSnapshot s;
+    s.ns = "ns-a";
+    s.snapshot_id = RefTxnId{1, 1};
+    const String bytes = encodeRefTableSnapshot(s);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefTableSnapshot(bytes, "ns-b", s.snapshot_id); });
+}
+
+TEST(CASRefSnapshotCodec, DecodeRejectsSnapshotIdMismatch)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    const String bytes = encodeRefTableSnapshot(s);
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefTableSnapshot(bytes, s.ns, RefTxnId{1, 2}); });
+}
+
+TEST(CASRefSnapshotCodec, EncodeAllowsExactlySnapshotMaxBytes)
+{
+    RefTableSnapshot s;
+    s.ns = "ns";
+    s.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow row;
+    row.ref_name = "r";
+    row.manifest_ref = manifestRef(1, 1, 1);
+    s.committed.push_back(row);
+
+    const size_t base_size = encodeRefTableSnapshot(s).size();
+    ASSERT_LE(base_size, ref_snapshot_max_bytes);
+    /// Every added 'x' is one un-escaped byte inside the JSON ref_name string, so the encoded size
+    /// grows one-for-one to exactly the cap; +1 accounts for the base row's own 1-byte ref_name "r"
+    /// already counted in base_size.
+    s.committed[0].ref_name = String(ref_snapshot_max_bytes - base_size + 1, 'x');
+
+    const String bytes = encodeRefTableSnapshot(s);
+    EXPECT_EQ(bytes.size(), ref_snapshot_max_bytes);
+    const RefTableSnapshot decoded = decodeRefTableSnapshot(bytes, s.ns, s.snapshot_id);
+    EXPECT_EQ(decoded, s);
+}
+
+TEST(CASRefSnapshotCodec, DecodeRejectsOversizedBufferDirectly)
+{
+    /// A body with no line terminator inside the first `line_cap` bytes fails closed before any field
+    /// parsing (the text `readLine` line-cap guard, the text-codec analogue of the old early size guard).
+    const String oversized(ref_snapshot_max_bytes + 1, 'x');
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { decodeRefTableSnapshot(oversized, "ns", RefTxnId{1, 1}); });
+}
+
+/// ===================================================================================
+/// Shape-level failure-mode battery (truncation / v+1 gate / wrong type / leading garbage)
+/// ===================================================================================
+
+TEST(CASFormatBattery, RefSnapshot)
+{
+    const RefTableSnapshot s = makeLiveSnapshot();
+    const String ns = s.ns;
+    const RefTxnId id = s.snapshot_id;
+    runFormatBattery({FormatId::RefSnapshot,
+        [s] { return sealObject(FormatId::RefSnapshot, encodeRefTableSnapshot(s)); },
+        [ns, id](std::string_view d) { decodeRefTableSnapshot(openObject(FormatId::RefSnapshot, d), ns, id); },
+        currentFormatHeader("cas_ref_snap") +
+        "{\"ns\":\"srv1/db/table@cas@\",\"we\":\"5\",\"rs\":\"200\",\"lc\":\"live\"}\n"
+        "{\"k\":\"c\",\"rn\":\"all_1_1_0\",\"me\":\"5\",\"mb\":\"10\",\"mo\":1,\"ts\":1717000000000}\n"
+        "{\"k\":\"c\",\"rn\":\"all_2_2_0\",\"me\":\"5\",\"mb\":\"11\",\"mo\":1,\"ts\":1717000000001}\n"
+        "{\"k\":\"p\",\"rn\":\"all_3_3_0\",\"me\":\"5\",\"mb\":\"12\",\"mo\":1}\n"
+        "{\"n\":3}\n"});
+}
diff --git a/src/Disks/tests/gtest_cas_ref_snapshot_publish_ordering.cpp b/src/Disks/tests/gtest_cas_ref_snapshot_publish_ordering.cpp
new file mode 100644
index 000000000000..22be74028768
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_snapshot_publish_ordering.cpp
@@ -0,0 +1,501 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCkpt.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+
+#include <Poco/Exception.h>
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <string>
+#include <vector>
+
+namespace ProfileEvents
+{
+extern const Event CASRefSnapshotPublishDispatched;
+}
+
+namespace DB::ErrorCodes
+{
+extern const int NETWORK_ERROR;
+}
+
+/// Task 6b remainder (Stage B, `{#t2}`): the publication-ordering coverage that Task 6b's rename left
+/// undone. This suite PINS existing behavior of `CasRefLedger::tryPublishSnapshotAndAdvanceCheckpointOnce`
+/// (the one retry unit), `admitSnapshotPublishUnderStateLock`, `advancePublishBackoff`/
+/// `resetPublishBackoff`, and `dispatchSnapshotPublisher`/`settleSnapshotPublish`. It changes NO
+/// production code.
+///
+/// Normative ordering: (1) the immutable snapshot body becomes durable; (2) `_ckpt` advances; (3) the new
+/// snapshot is adopted in this cache's memory. `NeedsRecovery` (this campaign's `Poisoned`) blocks
+/// publication -- a durable transaction may be missing from the cached view -- and forces
+/// `ensureRefTableRecovered` to re-walk the durable stream on the very next touch.
+///
+/// The suite name is prefixed `Cas` so it is covered by the `Cas*` unit-test gate filter.
+
+using namespace DB::Cas;
+using DB::Cas::tests::CountingBackend;
+using DB::Cas::tests::expectThrowsCode;
+using DB::Cas::tests::namespaceBirthOp;
+using DB::Cas::tests::publishCommittedOps;
+
+namespace
+{
+
+/// Records the ORDER of body-PUT / `_ckpt`-CAS operations (so a test can compare indices) and lets a
+/// test inject a persistent `Conflict` on one chosen `_ckpt` key -- the same technique
+/// `gtest_cas_ref_writer.cpp`'s `RefWriterTestBackend::ckpt_conflict_key`/`ckpt_conflict_count` uses to
+/// drive the ledger into `NeedsRecovery`, reproduced here so this suite has no dependency on that file's
+/// internal (non-exported) test type. Delegates every operation to `CountingBackend` unchanged, so the
+/// per-key counters (`putCount`/`casPutCount`) remain available as the positive control.
+class OrderedFaultBackend : public CountingBackend
+{
+public:
+    using CountingBackend::casPut;
+    using CountingBackend::get;
+    using CountingBackend::putIfAbsent;
+
+    enum class Op : uint8_t { Put, Cas };
+    struct Entry
+    {
+        Op op;
+        String key;
+    };
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        record(Op::Put, key);
+        if (fail_put_count > 0 && !fail_put_substr.empty() && key.find(fail_put_substr) != String::npos)
+        {
+            --fail_put_count;
+            throw Poco::TimeoutException("OrderedFaultBackend: simulated PUT response lost, nothing landed");
+        }
+        return CountingBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                      const ObjectMeta & meta) override
+    {
+        record(Op::Cas, key);
+        if (key == fail_cas_key && fail_cas_count > 0)
+        {
+            --fail_cas_count;
+            /// A `Conflict` (not a thrown/ambiguous response): the caller's own re-read-and-merge loop
+            /// (`publishCkpt`) treats this exactly like a concurrent writer that landed first, and
+            /// exhausts `MAX_CKPT_CAS_ATTEMPTS` (100) without ever committing -- deterministically, with
+            /// no wall-clock wait, since the loop is attempt-bounded rather than only deadline-bounded.
+            return {CasOutcome::Conflict, {}};
+        }
+        return CountingBackend::casPut(key, bytes, expected, meta);
+    }
+
+    /// Arms a persistent CAS conflict at `key` for the next `count` attempts.
+    void armCasConflict(const String & key, size_t count)
+    {
+        fail_cas_key = key;
+        fail_cas_count = count;
+    }
+
+    /// Arms a persistent, never-committed PUT failure for the next `count` `putIfAbsent` calls whose key
+    /// contains `substr`: the object is never actually written (unlike a real ambiguous response, which
+    /// may or may not have landed), so the resolve-by-exact-GET a controlled `CasRequestBudget` with
+    /// `max_attempts = 1` performs always finds the key absent and classifies the attempt a definite,
+    /// non-`Committed` failure -- deterministically, with no internal retry and no wall-clock wait.
+    void armPutFailure(const String & substr, int count)
+    {
+        fail_put_substr = substr;
+        fail_put_count = count;
+    }
+
+    /// The current length of the journal -- a caller's baseline for `indicesFrom` below, so a query can
+    /// be scoped to "since I last looked" rather than "since the pool opened" (whose earlier entries
+    /// belong to unrelated setup writes, e.g. the birth transaction's own checkpoint CAS).
+    size_t journalSize() const
+    {
+        std::lock_guard lock(mutex);
+        return journal.size();
+    }
+
+    /// Every index at or after `from` where `op`/`key` matches, in order.
+    std::vector<size_t> indicesFrom(Op op, const String & key, size_t from) const
+    {
+        std::lock_guard lock(mutex);
+        std::vector<size_t> result;
+        for (size_t i = from; i < journal.size(); ++i)
+            if (journal[i].op == op && journal[i].key == key)
+                result.push_back(i);
+        return result;
+    }
+
+    /// The first index at or after `from` where `op`/`key` matches, if any.
+    std::optional<size_t> firstIndexFrom(Op op, const String & key, size_t from) const
+    {
+        const auto indices = indicesFrom(op, key, from);
+        return indices.empty() ? std::nullopt : std::make_optional(indices.front());
+    }
+
+private:
+    void record(Op op, const String & key)
+    {
+        std::lock_guard lock(mutex);
+        journal.push_back({op, key});
+    }
+
+    mutable std::mutex mutex;
+    std::vector<Entry> journal;
+    String fail_cas_key;
+    size_t fail_cas_count = 0;
+    String fail_put_substr;
+    int fail_put_count = 0;
+};
+
+PoolPtr openPool(const std::shared_ptr<OrderedFaultBackend> & backend, PoolConfig config = {})
+{
+    config.pool_prefix = "p";
+    config.server_root_id = "test";
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, std::move(config));
+}
+
+/// The same one-transaction publish every other ref suite drives, so a namespace reaches `Live` through
+/// the REAL append lane (which is also what creates its `_ckpt`).
+RefTxnId publishRef(const PoolPtr & store, const RootNamespace & ns, const String & ref, uint64_t ordinal)
+{
+    return store->appendRefOps(ns, MutationScope::ref(ref),
+        [&ref, ordinal](const RefTableState & state)
+        {
+            std::vector<RefOp> ops;
+            if (state.getLifecycle() != RefLifecycle::Live)
+                ops.push_back(namespaceBirthOp());
+            for (const RefOp & op : publishCommittedOps(ref, ManifestRef{1, ordinal, 1}))
+                ops.push_back(op);
+            return ops;
+        },
+        RootMutationOrigin::Writer, RootMutationKind::Publish);
+}
+
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// 1. Snapshot body durable strictly before `_ckpt` advances
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASRefSnapshotPublishOrdering, SnapshotBodyIsDurableBeforeCheckpointAdvances)
+{
+    auto backend = std::make_shared<OrderedFaultBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/order_body_before_ckpt"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{store->writerEpoch(), 1}));
+    const NamespaceLifeId life = *store->refTableLifeForTest(ns);
+    const String snapshot_key = store->layout().refSnapshotKey(life, RefTxnId{store->writerEpoch(), 1});
+    const String ckpt_key = store->layout().refCkptKey(life);
+
+    /// The birth transaction above already CAS'd `_ckpt` itself (once for its own `life_epoch`, once for
+    /// its committed frontier) -- ordinary append-commit traffic that has nothing to do with the snapshot
+    /// publisher. The comparison below must therefore look only at what happens FROM this offset, or it
+    /// would find the birth's ckpt writes (which precede the snapshot body by construction) and conclude
+    /// nothing about the publisher's own ordering.
+    const size_t offset = backend->journalSize();
+    const uint64_t put_before = backend->putCount(snapshot_key);
+    const uint64_t cas_before = backend->casPutCount(ckpt_key);
+
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns))
+        << "a healthy Ready-lane table with an uncovered tail must publish";
+
+    /// Positive control: this attempt touched each key exactly once (no retry, no redundant write) --
+    /// which is what makes the index comparison below meaningful rather than an artifact of a busy log.
+    EXPECT_EQ(backend->putCount(snapshot_key) - put_before, 1u);
+    EXPECT_EQ(backend->casPutCount(ckpt_key) - cas_before, 1u);
+
+    const auto body_index = backend->firstIndexFrom(OrderedFaultBackend::Op::Put, snapshot_key, offset);
+    const auto ckpt_index = backend->firstIndexFrom(OrderedFaultBackend::Op::Cas, ckpt_key, offset);
+    ASSERT_TRUE(body_index.has_value()) << "the snapshot body must have been PUT";
+    ASSERT_TRUE(ckpt_index.has_value()) << "the checkpoint must have been CAS-advanced";
+    EXPECT_LT(*body_index, *ckpt_index)
+        << "INV-4's second `_ckpt` writer runs strictly after the immutable body is durable";
+}
+
+
+/// ---------------------------------------------------------------------------------------------
+/// 2. Adoption happens last, and only once both durable effects landed
+/// ---------------------------------------------------------------------------------------------
+
+TEST(CASRefSnapshotPublishOrdering, AdoptionHappensLastAndOnlyAfterBothDurableEffects)
+{
+    auto backend = std::make_shared<OrderedFaultBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/order_adoption_after_both"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{store->writerEpoch(), 1}));
+    const NamespaceLifeId life = *store->refTableLifeForTest(ns);
+    const String snapshot_key = store->layout().refSnapshotKey(life, RefTxnId{store->writerEpoch(), 1});
+    const String ckpt_key = store->layout().refCkptKey(life);
+
+    /// Fail every one of the (attempt-bounded) 100 `_ckpt` CAS attempts `publishCkpt` will make: the
+    /// body PUT still commits (dedup: an identical, already-durable body resolves as `Committed` without
+    /// re-sending), but the checkpoint never advances within this call.
+    backend->armCasConflict(ckpt_key, 100);
+    EXPECT_FALSE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns))
+        << "a persistently conflicting checkpoint CAS must not be reported as a successful publish";
+
+    EXPECT_EQ(backend->putCount(snapshot_key), 1u) << "the body is durable regardless of the ckpt outcome";
+    EXPECT_FALSE(store->newestPublishedSnapshotIdForTest(ns).has_value())
+        << "in-memory adoption must NOT happen while the checkpoint has not advanced";
+
+    /// Disarm the fault and retry (the one retry unit): the retry issues its OWN `putIfAbsent` attempt at
+    /// the same content-addressed key with the same bytes (so `putCount`, a call counter, becomes 2 --
+    /// not a "no write happened" 1), but the backend resolves it as `Committed` against the already-durable
+    /// object rather than sending a distinct object, and the checkpoint CAS now succeeds.
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns))
+        << "the retry, with the fault cleared, must publish";
+    EXPECT_EQ(backend->putCount(snapshot_key), 2u)
+        << "the retry's body PUT is its own attempt, resolved via dedup against identical, "
+           "already-durable bytes rather than writing a second object";
+    EXPECT_EQ(store->newestPublishedSnapshotIdForTest(ns), std::make_optional(RefTxnId{store->writerEpoch(), 1}))
+        << "adoption happens exactly once, after both effects are durable";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// 3. `NeedsRecovery` ("Poisoned") lane: recovery precedes any snapshot publication
+/// ---------------------------------------------------------------------------------------------
+
+/// `Poisoned` is this task's plan's name for what the code spells `RefLaneState::NeedsRecovery` -- the
+/// state the header documents as "a transaction is known durable but cannot be installed in this cache
+/// ... a hard write and certification fence until replay completes". Recorded here as the vocabulary
+/// correction for later tasks: there is no state literally named `Poisoned` anywhere in `CasRefLedger`.
+/// This test is the plan's `PoisonedRefusesPublicationAndTriggersReRecovery`, renamed to state the actual
+/// pinned behavior precisely (recovery precedes publication, rather than an outright refusal).
+///
+/// It is reached here the same way `gtest_cas_ref_writer.cpp`'s
+/// `CASRefWriterAppendLane.CheckpointConflictAfterLogCommitRequiresRecoveryWithoutInstall` reaches it: a
+/// mutation's ref-log body commits durably while its OWN checkpoint-frontier CAS (`commitRefChunk`'s
+/// `commit_contribution`, not the snapshot publisher's) conflicts persistently.
+///
+/// The INVARIANT this pins (not a raw write count): a snapshot must never be published FROM AN
+/// UNRECOVERED CACHE -- a durable transaction may be missing from the cached view, and advancing `_ckpt`
+/// onto a snapshot built from that stale view is the data-loss shape `NeedsRecovery` exists to prevent.
+/// `tryPublishSnapshotAndAdvanceCheckpointOnce` calls `ensureRefTableRecovered` unconditionally, and that
+/// function re-walks the durable stream whenever the lane is `NeedsRecovery`, regardless of `recovered`.
+/// Recovery's own `_ckpt` catch-up write is NOT a violation of this invariant -- it is the remedy: it is
+/// how the cache stops being stale before anything is allowed to read it for a snapshot. So a request
+/// against a poisoned lane recovers first and MAY legitimately go on to publish (this table had never
+/// published a snapshot, so once recovered it has a real, uncovered candidate) -- "inert refusal with
+/// zero writes" is NOT what production implements, and recover-then-proceed is the correct behavior, not
+/// a deviation from it. What this test pins is: (a) no snapshot-publish effect (body PUT, publisher's own
+/// checkpoint-advance CAS) can ever appear in the journal before recovery's reconciliation CAS; (b)
+/// re-recovery is an observable state transition, never a silent skip; (c) if a snapshot IS published, it
+/// reflects the RECOVERED frontier -- the durable transaction the stale cache was missing is actually
+/// covered by it, not merely "some snapshot, from whichever view".
+TEST(CASRefSnapshotPublishOrdering, NeedsRecoveryLaneRecoversBeforeAnySnapshotPublication)
+{
+    auto backend = std::make_shared<OrderedFaultBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/order_poisoned_refuses"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{store->writerEpoch(), 1}));
+    const NamespaceLifeId life = *store->refTableLifeForTest(ns);
+    const String ckpt_key = store->layout().refCkptKey(life);
+    /// The durable transaction the stale cache will be missing: `dropRef`'s removal, sequence 2.
+    const RefTxnId missing_durable_txn{store->writerEpoch(), 2};
+    const String next_snapshot_key = store->layout().refSnapshotKey(life, missing_durable_txn);
+
+    /// Drive the very next mutation's OWN checkpoint-frontier CAS into persistent conflict: the log PUT
+    /// for `missing_durable_txn` commits durably, but its checkpoint never advances within this call, and
+    /// the lane is left `NeedsRecovery` rather than installing an uncertain result -- so the cached view
+    /// still reflects `ref_1` present, while the durable log already reflects it removed.
+    backend->armCasConflict(ckpt_key, 100);
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "ref_1"); });
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+
+    const uint64_t recovery_installs_before = store->recoveryInstallCountForTest();
+    backend->armCasConflict(ckpt_key, 0);   /// clear the fault so re-recovery's OWN catch-up CAN succeed
+    const size_t offset = backend->journalSize();
+
+    EXPECT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns))
+        << "recovery reconciles the durable gap and this table has never published a snapshot, so the "
+           "same call legitimately goes on to publish one -- see the invariant note above the test";
+
+    /// Re-recovery WAS triggered as an observable state transition (not a silent skip): the lane left
+    /// `NeedsRecovery`, and `recoveryInstallCountForTest` -- a counter of exact recovery-result
+    /// publications -- advanced.
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready)
+        << "ensureRefTableRecovered must have re-walked the durable stream and cleared the fence";
+    EXPECT_GT(store->recoveryInstallCountForTest(), recovery_installs_before)
+        << "a re-recovery install must be observable, not indistinguishable from never having run";
+
+    /// ORDER, not a global zero: recovery's OWN checkpoint catch-up CAS is the boundary marker. NO
+    /// snapshot-publish effect (the new snapshot's body PUT, nor the publisher's own checkpoint-advance
+    /// CAS) may appear at or before it.
+    const auto ckpt_cas_indices = backend->indicesFrom(OrderedFaultBackend::Op::Cas, ckpt_key, offset);
+    const auto snap_put_indices = backend->indicesFrom(OrderedFaultBackend::Op::Put, next_snapshot_key, offset);
+    ASSERT_GE(ckpt_cas_indices.size(), 2u)
+        << "expected one checkpoint CAS from recovery's catch-up and one from the snapshot publisher";
+    const size_t recovery_catchup_index = ckpt_cas_indices.front();
+    const size_t publisher_ckpt_index = ckpt_cas_indices.back();
+    ASSERT_FALSE(snap_put_indices.empty()) << "the recovered, uncovered candidate must have been published";
+    for (const size_t snap_put_index : snap_put_indices)
+        EXPECT_GT(snap_put_index, recovery_catchup_index)
+            << "no snapshot-publish body PUT may precede recovery's own checkpoint reconciliation";
+    EXPECT_LT(snap_put_indices.front(), publisher_ckpt_index)
+        << "the snapshot publisher's own checkpoint CAS still runs after ITS OWN body PUT (INV-4), even "
+           "immediately following recovery";
+
+    /// STRONGEST form: the published snapshot is not merely "some snapshot from whichever view" -- it
+    /// covers EXACTLY the recovered, previously-missing-from-cache frontier. Its id names the durable
+    /// removal transaction, and the recovered cache (which the snapshot was built from) no longer
+    /// resolves the removed ref.
+    EXPECT_EQ(store->newestPublishedSnapshotIdForTest(ns), std::make_optional(missing_durable_txn))
+        << "the published snapshot's frontier IS the durable transaction the stale cache was missing";
+    EXPECT_FALSE(store->resolveRef(ns, "ref_1").has_value())
+        << "the recovered (and now snapshotted) cache reflects the durable removal the stale view lacked";
+}
+
+/// ---------------------------------------------------------------------------------------------
+/// 4. Publish backoff: characterized against a controlled clock (`PoolConfig::boot_ms_fn`)
+/// ---------------------------------------------------------------------------------------------
+
+/// `admitSnapshotPublishUnderStateLock`, `advancePublishBackoff` and `resetPublishBackoff` are private
+/// to `CasRefLedger`, so they can only be characterized through the public dispatch surface
+/// (`appendRefOps`/`resolveRef` triggering `maybeScheduleSnapshotPublish`, and
+/// `waitForSnapshotPublishSettleForTest`/`ProfileEvents::CASRefSnapshotPublishDispatched` as the
+/// observables). `CASRequestControllerBackoff` is a DIFFERENT mechanism (the request controller's
+/// per-attempt retry backoff); this characterizes ONLY the per-table snapshot-publish dispatch backoff.
+///
+/// A controlled clock (`PoolConfig::boot_ms_fn`) DOES exist for this seam (`gtest_cas_ref_writer.cpp`'s
+/// `C4BackoffDefersThenRetriesAndPublishes` already relies on it) -- so unlike the plan's anticipated
+/// fallback, this pins literal accept/refuse decisions against exact clock offsets rather than only
+/// attempt counts.
+TEST(CASRefSnapshotPublishOrdering, PublishBackoffDecisionsAreCharacterized)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<OrderedFaultBackend>();
+
+    /// A single-attempt request budget, exactly as `gtest_cas_ref_writer.cpp`'s
+    /// `C4BackoffDefersThenRetriesAndPublishes` uses: with `max_attempts = 1` a faulted PUT resolves to a
+    /// definite, non-`Committed` outcome on its own attempt, with no internal retry loop and so no
+    /// wall-clock wait.
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;
+    budget.lease_safety_margin_ms = 100;
+
+    uint64_t fake_now = 1'000'000;
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 0;              /// any nonempty tail is over-threshold
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    config.snapshot_publish_backoff_initial_ms = 1000;
+    config.snapshot_publish_backoff_max_ms = 4000;
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(10'000'000);
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.cas_request_budget = budget;
+    auto store = openPool(backend, config);
+    const RootNamespace ns{"srv1/order_backoff"};
+
+    ASSERT_EQ(publishRef(store, ns, "ref_1", 1), (RefTxnId{store->writerEpoch(), 1}));
+    store->waitForSnapshotPublishSettleForTest(ns);   /// drain the birth's own auto-dispatched publish
+    /// The birth's own auto-dispatch already published a snapshot at this point (threshold 0); the
+    /// baseline every "no new publish yet" check below compares against.
+    const auto snapshot_after_birth = store->newestPublishedSnapshotIdForTest(ns);
+    ASSERT_TRUE(snapshot_after_birth.has_value());
+
+    /// Fault the snapshot BODY put (never the `_ckpt` CAS -- an append-commit's OWN checkpoint write
+    /// shares that key, and faulting it would drive the append lane into `NeedsRecovery` instead of
+    /// exercising the snapshot-publish backoff this test targets). Exactly 3 failures: the next 3
+    /// automatic dispatch attempts fail (arming, then doubling, then re-doubling the backoff); the 4th
+    /// finds the fault disarmed and succeeds.
+    backend->armPutFailure("_snap/", 3);
+
+    const auto dispatchCount = [&] { return global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(); };
+
+    /// Attempt 1: admitted immediately (no backoff armed yet). Fails -> backoff armed at the initial 1000ms.
+    ASSERT_EQ(publishRef(store, ns, "ref_2", 2), (RefTxnId{store->writerEpoch(), 2}));
+    store->waitForSnapshotPublishSettleForTest(ns);
+    const uint64_t d1 = dispatchCount();
+    EXPECT_EQ(store->newestPublishedSnapshotIdForTest(ns), snapshot_after_birth)
+        << "the failed attempt must not have advanced the published snapshot";
+
+    /// Still within the 1000ms window: a further trigger must NOT re-dispatch.
+    store->resolveRef(ns, "ref_1");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d1) << "a read within the initial backoff window must not re-dispatch";
+
+    /// Cross the 1000ms deadline: exactly one retry dispatches (and fails again, doubling to 2000ms).
+    fake_now += 1000;
+    store->resolveRef(ns, "ref_1");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d1 + 1) << "past the first deadline, exactly one retry dispatches";
+
+    /// Short of the DOUBLED (2000ms) deadline: still refused.
+    fake_now += 1000;
+    store->resolveRef(ns, "ref_1");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d1 + 1)
+        << "advancePublishBackoff doubled the interval to 2000ms; 1000ms elapsed is not enough";
+
+    /// Cross the doubled deadline: one more retry dispatches (and fails again -- the third and last armed
+    /// failure -- doubling to the 4000ms cap).
+    fake_now += 1000;
+    store->resolveRef(ns, "ref_1");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d1 + 2) << "past the doubled deadline, exactly one more retry dispatches";
+
+    /// Pin the 4000ms cap FROM BELOW: without this probe, a regression that stopped doubling at
+    /// 2000ms, or that read `initial` where it means `max`, would still pass -- the only check so far
+    /// is AT the +4000 crossing below. 2000ms past the doubled deadline is still short of the capped
+    /// 4000ms backoff, so no third retry may dispatch yet.
+    fake_now += 2000;
+    store->resolveRef(ns, "ref_1");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d1 + 2)
+        << "2000ms past the doubled deadline is still short of the capped 4000ms backoff";
+
+    /// Cross the (capped) 4000ms deadline: the retry's fault budget is exhausted, so this attempt
+    /// succeeds, and `resetPublishBackoff` clears the cooldown -- proved by the NEXT trigger dispatching
+    /// with no wait at all.
+    fake_now += 2000;
+    store->resolveRef(ns, "ref_1");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d1 + 3) << "past the second (capped) deadline, the retry dispatches and succeeds";
+    EXPECT_NE(store->newestPublishedSnapshotIdForTest(ns), snapshot_after_birth)
+        << "the fault budget is exhausted, so this attempt actually advances the published snapshot";
+
+    ASSERT_EQ(publishRef(store, ns, "ref_3", 3), (RefTxnId{store->writerEpoch(), 3}));
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d1 + 4)
+        << "resetPublishBackoff must have cleared the cooldown: the very next over-threshold trigger, at "
+           "the SAME clock reading as the successful publish, dispatches immediately with no wait";
+
+    /// The assertion just above cannot tell a real reset from a no-op: the successful publish and this
+    /// next trigger share one `fake_now`, so `now >= until` would still hold even with the stale
+    /// (pre-reset) deadline in place. Arm one more failure and check that the schedule restarts from
+    /// the INITIAL 1000ms interval rather than continuing from the 4000ms cap -- refused short of
+    /// 1000ms, admitted at 1000ms -- which a no-op reset cannot produce (it would refuse both probes,
+    /// since the stale deadline is still far in the future).
+    backend->armPutFailure("_snap/", 1);
+    ASSERT_EQ(publishRef(store, ns, "ref_4", 4), (RefTxnId{store->writerEpoch(), 4}));
+    store->waitForSnapshotPublishSettleForTest(ns);
+    const uint64_t d2 = dispatchCount();
+    fake_now += 500;
+    store->resolveRef(ns, "ref_1");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d2) << "short of 1000ms since the reset, no retry may dispatch yet";
+    fake_now += 500;
+    store->resolveRef(ns, "ref_1");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(dispatchCount(), d2 + 1)
+        << "resetPublishBackoff must have restarted the schedule at the INITIAL 1000ms interval, not "
+           "left it continuing from the 4000ms cap";
+}
diff --git a/src/Disks/tests/gtest_cas_ref_statemachine.cpp b/src/Disks/tests/gtest_cas_ref_statemachine.cpp
new file mode 100644
index 000000000000..af79232cde2e
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_statemachine.cpp
@@ -0,0 +1,1445 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <algorithm>
+#include <cstdint>
+#include <optional>
+#include <random>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+extern const int CORRUPTED_DATA;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::expectThrowsCode;
+
+namespace
+{
+
+/// ===================================================================================
+/// Small builders (mirrors gtest_cas_ref_codecs.cpp's local helpers)
+/// ===================================================================================
+
+ManifestRef manifestRef(uint64_t epoch, uint64_t seq, uint32_t ordinal)
+{
+    return ManifestRef{epoch, seq, ordinal};
+}
+
+RefLogTxn makeTxn(const String & ns, RefTxnId id, std::vector<RefOp> ops)
+{
+    RefLogTxn txn;
+    txn.ns = ns;
+    txn.txn_id = id;
+    txn.ops = std::move(ops);
+    return txn;
+}
+
+RefOp birthOp()
+{
+    RefOp op;
+    op.kind = RefOpKind::NamespaceBirth;
+    return op;
+}
+
+RefOp addPrecommitOp(const String & name, const ManifestRef & mref)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, name, mref};
+    return op;
+}
+
+RefOp removePrecommitOp(const String & name, const ManifestRef & mref)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, name, mref};
+    return op;
+}
+
+RefOp promoteOp(const String & name, const ManifestRef & mref)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, name, mref};
+    op.new_binding = RefOwnerBinding{RefOwnerKind::Committed, name, mref};
+    return op;
+}
+
+RefOp removeCommittedOp(const String & name, const ManifestRef & mref)
+{
+    RefOp op;
+    op.kind = RefOpKind::OwnerTransition;
+    op.old_binding = RefOwnerBinding{RefOwnerKind::Committed, name, mref};
+    return op;
+}
+
+RefOp setPublishedAtOp(const String & name, const ManifestRef & mref, uint64_t ts = 0)
+{
+    RefOp op;
+    op.kind = RefOpKind::SetPublishedAt;
+    op.ref_name = name;
+    op.expected_manifest_ref = mref;
+    op.published_at_ms = ts;
+    return op;
+}
+
+RefOp removeNamespaceOp()
+{
+    RefOp op;
+    op.kind = RefOpKind::RemoveNamespace;
+    return op;
+}
+
+/// Field-by-field comparison (via getters) rather than a `RefTableState::operator==` addition: the
+/// class is the plan's verbatim-normative interface and gains no member beyond what it specifies.
+void expectStatesEqual(const RefTableState & a, const RefTableState & b)
+{
+    EXPECT_EQ(a.getLifecycle(), b.getLifecycle());
+    EXPECT_EQ(a.getRemoveTxnId(), b.getRemoveTxnId());
+    EXPECT_EQ(a.getGreatestApplied(), b.getGreatestApplied());
+    EXPECT_EQ(a.getCommitted(), b.getCommitted());
+    EXPECT_EQ(a.getPrecommits(), b.getPrecommits());
+    /// Also compare the incremental budget counters: in release builds (no `debugAssertBodyCounters`)
+    /// this is the only cross-check that catches counter drift between two equal-looking states.
+    EXPECT_EQ(a.getSnapshotBodyBytes(), b.getSnapshotBodyBytes());
+    EXPECT_EQ(a.getRemovalBodyBytes(), b.getRemovalBodyBytes());
+}
+
+/// The spec's own construction for a hypothetical `remove_namespace` transaction (§Remove Namespace):
+/// an exact owner-removal op for every committed ref and precommit, then `remove_namespace`. Built
+/// independently of `CasRefStateMachine.cpp`'s internal helper of the same shape, purely from the
+/// public `RefTableState` fields, so the admission-budget property tests below measure against a
+/// ground truth this test file derives on its own.
+RefLogTxn buildRemovalTxnForTest(const RefTableState & state, const String & ns, RefTxnId id)
+{
+    std::vector<RefOp> ops;
+    for (const auto [name, row] : state.getCommitted())
+        ops.push_back(removeCommittedOp(name, row.manifest_ref));
+    for (const auto & [name, mref] : state.getPrecommits())
+        ops.push_back(removePrecommitOp(name, mref));
+    ops.push_back(removeNamespaceOp());
+    return makeTxn(ns, id, std::move(ops));
+}
+
+constexpr const char * kNs = "srv1/db/table@cas@";
+
+/// A validated state with "a" committed to manifest (1,1,1) -- the base the fail-closed replay/append
+/// tests below reuse to build a tail whose add-precommit op would collide cross-owner (name the SAME
+/// manifest under a DIFFERENT ref_name).
+RefTableSnapshot buildCollidingBaseSnapshotForTest()
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    return snapshotOf(state, kNs);
+}
+
+}
+
+/// ===================================================================================
+/// NamespaceBirth
+/// ===================================================================================
+
+TEST(CASRefStateMachine, BirthFromNeverBornAccepts)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    EXPECT_EQ(state.getLifecycle(), RefLifecycle::Live);
+    EXPECT_FALSE(state.getRemoveTxnId().has_value());
+    EXPECT_EQ(state.getGreatestApplied(), (RefTxnId{1, 1}));
+}
+
+TEST(CASRefStateMachine, BirthWhileLiveRejectedAndStateUnchanged)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {birthOp()})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, BirthAfterRemovalAccepts)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removeNamespaceOp()}));
+    ASSERT_EQ(state.getLifecycle(), RefLifecycle::Removed);
+    ASSERT_TRUE(state.getRemoveTxnId().has_value());
+
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {birthOp()}));
+    EXPECT_EQ(state.getLifecycle(), RefLifecycle::Live);
+    EXPECT_FALSE(state.getRemoveTxnId().has_value());
+}
+
+/// ===================================================================================
+/// Ops rejected outside Live (never-born and Removed) except birth
+/// ===================================================================================
+
+TEST(CASRefStateMachine, OwnerTransitionWhileNeverBornRejected)
+{
+    RefTableState state;
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {addPrecommitOp("a", manifestRef(1, 1, 1))})); });
+}
+
+TEST(CASRefStateMachine, SetPublishedAtWhileNeverBornRejected)
+{
+    RefTableState state;
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {setPublishedAtOp("a", manifestRef(1, 1, 1))})); });
+}
+
+TEST(CASRefStateMachine, RemoveNamespaceWhileNeverBornRejected)
+{
+    RefTableState state;
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {removeNamespaceOp()})); });
+}
+
+TEST(CASRefStateMachine, OpsWhileRemovedRejectedExceptBirth)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removeNamespaceOp()}));
+    const RefTableState after_removal = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {addPrecommitOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(after_removal, state);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 4}, {setPublishedAtOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(after_removal, state);
+
+    /// Repeated removal is corruption at THIS layer (spec §Remove Namespace: idempotent-success is
+    /// the API layer's job, not the state machine's).
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 5}, {removeNamespaceOp()})); });
+    expectStatesEqual(after_removal, state);
+}
+
+/// ===================================================================================
+/// Add precommit (spec §Add Precommit)
+/// ===================================================================================
+
+TEST(CASRefStateMachine, AddPrecommitAccepts)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    EXPECT_TRUE(state.getPrecommits().contains({"a", manifestRef(1, 1, 1)}));
+}
+
+TEST(CASRefStateMachine, AddPrecommitRejectsExactDuplicate)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, AddPrecommitRejectsConflictingManifestUnderDifferentName)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    const RefTableState before = state;
+
+    /// Same manifest_ref, a DIFFERENT ref_name: "no conflicting owner may name the same manifest".
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("b", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, AddPrecommitRejectsManifestAlreadyCommittedElsewhere)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    ASSERT_TRUE(state.getCommitted().contains("a"));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("b", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, AddPrecommitAllowsDifferentManifestsRacingForSameName)
+{
+    /// Two builds racing for the same final ref name (same shape gtest_cas_ref_codecs.cpp's
+    /// RoundTripPrecommitsSameNameDifferentManifest round-trips): distinct manifest_ref, no conflict.
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("same", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("same", manifestRef(1, 2, 1))}));
+    EXPECT_TRUE(state.getPrecommits().contains({"same", manifestRef(1, 1, 1)}));
+    EXPECT_TRUE(state.getPrecommits().contains({"same", manifestRef(1, 2, 1)}));
+}
+
+/// ===================================================================================
+/// Remove precommit / remove committed (spec §Remove Precommit, §Remove Committed Ref)
+/// ===================================================================================
+
+TEST(CASRefStateMachine, RemovePrecommitAccepts)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removePrecommitOp("a", manifestRef(1, 1, 1))}));
+    EXPECT_TRUE(state.getPrecommits().empty());
+}
+
+TEST(CASRefStateMachine, RemovePrecommitRejectsAbsentBinding)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removePrecommitOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, RemovePrecommitRejectsWrongManifest)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removePrecommitOp("a", manifestRef(1, 2, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, RemoveCommittedAccepts)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removeCommittedOp("a", manifestRef(1, 1, 1))}));
+    EXPECT_TRUE(state.getCommitted().empty());
+}
+
+TEST(CASRefStateMachine, RemoveCommittedRejectsAbsentRef)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removeCommittedOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, RemoveCommittedRejectsWrongManifest)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removeCommittedOp("a", manifestRef(9, 9, 9))})); });
+    expectStatesEqual(before, state);
+}
+
+/// ===================================================================================
+/// Promote (spec §Promote): exact precommit required, atomicity, invalid shapes
+/// ===================================================================================
+
+TEST(CASRefStateMachine, PromoteRejectsAbsentPrecommit)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {promoteOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, PromoteAtomicityNoOwnerlessIntermediate)
+{
+    /// A bare promote (no set_published_at in the same transaction) is itself a complete, valid, and
+    /// OBSERVABLE transaction -- there is no partial-op state exposed here, only the choice of
+    /// whether the timestamp arrives in this txn or a later one (spec §Promote).
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {promoteOp("a", manifestRef(1, 1, 1))}));
+
+    EXPECT_FALSE(state.getPrecommits().contains({"a", manifestRef(1, 1, 1)}));
+    ASSERT_TRUE(state.getCommitted().contains("a"));
+    EXPECT_EQ(state.getCommitted().at("a").manifest_ref, manifestRef(1, 1, 1));
+    EXPECT_EQ(state.getCommitted().at("a").published_at_ms, 0u);
+}
+
+TEST(CASRefStateMachine, PromoteWithSetPublishedAtInSameTxnInstallsTimestamp)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2},
+        {promoteOp("a", manifestRef(1, 1, 1)), setPublishedAtOp("a", manifestRef(1, 1, 1), 42)}));
+
+    ASSERT_TRUE(state.getCommitted().contains("a"));
+    EXPECT_EQ(state.getCommitted().at("a").published_at_ms, 42u);
+}
+
+TEST(CASRefStateMachine, PromoteRejectsDisplacingAnotherCommittedManifest)
+{
+    /// A challenger precommit under the SAME ref_name as an already-committed (different) manifest is
+    /// legal to stage (spec §Add Precommit only restricts manifest identity, not ref_name), but a bare
+    /// promote of it must not silently displace the stale committed row.
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("a", manifestRef(1, 2, 1))}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {promoteOp("a", manifestRef(1, 2, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, PromoteAcceptsAfterExplicitRemovalOfStaleCommitted)
+{
+    /// The correct atomic-replace sequence: an explicit removal of the old committed row, followed by
+    /// the promote, in the SAME transaction -- both ops are recorded, so GC sees the old manifest's
+    /// "-1" edge explicitly rather than losing it to a silent displacement.
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("a", manifestRef(1, 2, 1))}));
+
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3},
+        {removeCommittedOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 2, 1))}));
+
+    ASSERT_TRUE(state.getCommitted().contains("a"));
+    EXPECT_EQ(state.getCommitted().at("a").manifest_ref, manifestRef(1, 2, 1));
+    EXPECT_FALSE(state.getPrecommits().contains({"a", manifestRef(1, 2, 1)}));
+}
+
+TEST(CASRefStateMachine, OwnerTransitionRejectsInvalidCombinations)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+
+    /// old=None, new=Committed: not a recognized shape (committed rows are only reached via promote).
+    {
+        RefOp op;
+        op.kind = RefOpKind::OwnerTransition;
+        op.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "a", manifestRef(1, 1, 1)};
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+            [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {op})); });
+    }
+
+    /// A promote-shaped op (Precommit -> Committed) with mismatched ref_name is not a legal promote.
+    /// The rejected transaction above left `greatest_applied` untouched, so THIS one is still {1, 2}.
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    {
+        RefOp op;
+        op.kind = RefOpKind::OwnerTransition;
+        op.old_binding = RefOwnerBinding{RefOwnerKind::Precommit, "a", manifestRef(1, 1, 1)};
+        op.new_binding = RefOwnerBinding{RefOwnerKind::Committed, "b", manifestRef(1, 1, 1)};
+        const RefTableState before = state;
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+            [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {op})); });
+        expectStatesEqual(before, state);
+    }
+
+    /// old=Committed, new=Precommit: moving a committed ref "backwards" is not a recognized shape.
+    {
+        RefOp op;
+        op.kind = RefOpKind::OwnerTransition;
+        op.old_binding = RefOwnerBinding{RefOwnerKind::Committed, "a", manifestRef(1, 1, 1)};
+        op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "a", manifestRef(1, 1, 1)};
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+            [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {op})); });
+    }
+}
+
+/// ===================================================================================
+/// SetPublishedAt (spec §Update Payload)
+/// ===================================================================================
+
+TEST(CASRefStateMachine, SetPublishedAtRejectsWhenRefAbsent)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {setPublishedAtOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, SetPublishedAtRejectsManifestMismatch)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {setPublishedAtOp("a", manifestRef(9, 9, 9))})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, SetPublishedAtAcceptsAndReplacesTimestamp)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {setPublishedAtOp("a", manifestRef(1, 1, 1), 10)}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {setPublishedAtOp("a", manifestRef(1, 1, 1), 20)}));
+
+    EXPECT_EQ(state.getCommitted().at("a").published_at_ms, 20u);
+    EXPECT_EQ(state.getCommitted().at("a").manifest_ref, manifestRef(1, 1, 1));   /// unchanged: no edge move
+}
+
+/// ===================================================================================
+/// RemoveNamespace ordering lens (spec §Remove Namespace; codec deliberately doesn't check this)
+/// ===================================================================================
+
+TEST(CASRefStateMachine, RemoveNamespaceAloneOnEmptyTableAccepted)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removeNamespaceOp()}));
+    EXPECT_EQ(state.getLifecycle(), RefLifecycle::Removed);
+    ASSERT_TRUE(state.getRemoveTxnId().has_value());
+    EXPECT_EQ(*state.getRemoveTxnId(), (RefTxnId{1, 2}));
+}
+
+TEST(CASRefStateMachine, CatalogedNeverBornLifeAcceptsAtomicEmptyBirthAndRemoval)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), removeNamespaceOp()}));
+    EXPECT_EQ(state.getLifecycle(), RefLifecycle::Removed);
+    ASSERT_TRUE(state.getRemoveTxnId().has_value());
+    EXPECT_EQ(*state.getRemoveTxnId(), (RefTxnId{1, 1}));
+    EXPECT_TRUE(state.getCommitted().empty());
+    EXPECT_TRUE(state.getPrecommits().empty());
+}
+
+TEST(CASRefStateMachine, RemoveNamespaceDrainingOwnersInSameTxnAccepted)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), addPrecommitOp("b", manifestRef(1, 2, 1)),
+         promoteOp("b", manifestRef(1, 2, 1))}));
+    ASSERT_TRUE(state.getPrecommits().contains({"a", manifestRef(1, 1, 1)}));
+    ASSERT_TRUE(state.getCommitted().contains("b"));
+
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2},
+        {removePrecommitOp("a", manifestRef(1, 1, 1)), removeCommittedOp("b", manifestRef(1, 2, 1)),
+         removeNamespaceOp()}));
+    EXPECT_EQ(state.getLifecycle(), RefLifecycle::Removed);
+    EXPECT_TRUE(state.getCommitted().empty());
+    EXPECT_TRUE(state.getPrecommits().empty());
+}
+
+TEST(CASRefStateMachine, RemoveNamespaceRejectsWhenOwnersRemain)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), addPrecommitOp("b", manifestRef(1, 2, 1))}));
+    const RefTableState before = state;
+
+    /// Only "a" is drained; "b" remains -- remove_namespace's own precondition (empty owner sets)
+    /// must fail, and the WHOLE transaction (including the "a" removal) must not apply.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2},
+            {removePrecommitOp("a", manifestRef(1, 1, 1)), removeNamespaceOp()})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, RemoveNamespaceMustBeFinalOp)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removeNamespaceOp(), birthOp()})); });
+    expectStatesEqual(before, state);
+}
+
+TEST(CASRefStateMachine, RemoveNamespaceRejectsNonRemovalEarlierOp)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+    const RefTableState before = state;
+
+    /// set_published_at before remove_namespace: not an owner-removal transition.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2},
+            {setPublishedAtOp("a", manifestRef(1, 1, 1)), removeCommittedOp("a", manifestRef(1, 1, 1)),
+             removeNamespaceOp()})); });
+    expectStatesEqual(before, state);
+
+    /// An ADD (not a removal) owner_transition before remove_namespace: also rejected.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3},
+            {addPrecommitOp("c", manifestRef(1, 3, 1)), removeCommittedOp("a", manifestRef(1, 1, 1)),
+             removeNamespaceOp()})); });
+    expectStatesEqual(before, state);
+}
+
+/// ===================================================================================
+/// Whole-transaction atomicity: a failing LAST op leaves the whole txn (and earlier ops) unapplied
+/// ===================================================================================
+
+TEST(CASRefStateMachine, WholeTxnAtomicityLastOpFailureLeavesStateUntouched)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableState before = state;
+
+    /// ops[0] (add "a") would succeed in isolation; ops[1] (remove absent "b") fails -- the whole
+    /// transaction, including "a", must be rejected.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2},
+            {addPrecommitOp("a", manifestRef(1, 1, 1)), removePrecommitOp("b", manifestRef(9, 9, 9))})); });
+
+    expectStatesEqual(before, state);
+    EXPECT_FALSE(state.getPrecommits().contains({"a", manifestRef(1, 1, 1)}));
+}
+
+/// ===================================================================================
+/// Contiguous txn ids (INV-1)
+/// ===================================================================================
+
+/// A table's durable ids are DENSE within `(namespace, epoch)`: the only admissible id is the one
+/// `nextRefTxnId` derives from `greatest_applied`, which is also the only id the writer ever mints.
+/// Equal, lower, and skipped ids are all corruption -- the last of those is what makes "I can see ids
+/// 1..T" mean "nothing is missing", the property the whole invariant exists to provide.
+TEST(CASRefStateMachine, ContiguousTxnIdsRejectEqualLowerAndSkipped)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableState before = state;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {addPrecommitOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{0, 999}, {addPrecommitOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+
+    /// Strictly greater but SKIPPED: admitted before INV-1, corruption now.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {addPrecommitOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+
+    /// A new epoch restarts the sequence, so it must start at 1 -- carrying the previous epoch's
+    /// numbering forward would read exactly like a lost first transaction of the new stream.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{2, 2}, {addPrecommitOp("a", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+
+    /// The successor applies; then the next epoch's first id does.
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    EXPECT_EQ(state.getGreatestApplied(), (RefTxnId{1, 2}));
+
+    /// Crossing into a new epoch needs INV-2's chain link as well as INV-1's id: without it the reader
+    /// cannot tell an EMPTY epoch from a lost one, so a sequence-1 transaction that names no seal is
+    /// refused on a Live table. Both halves are pinned, since either alone would be silently weaker.
+    RefLogTxn crossing = makeTxn(kNs, RefTxnId{2, 1}, {addPrecommitOp("b", manifestRef(2, 1, 1))});
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { applyRefLogTxn(state, crossing); });
+    crossing.prev_epoch_seal = RefTxnId{1, 3};   /// the seal that closed epoch 1, one past its last id
+    applyRefLogTxn(state, crossing);
+    EXPECT_EQ(state.getGreatestApplied(), (RefTxnId{2, 1}));
+}
+
+/// ===================================================================================
+/// snapshotOf: canonical sort + terminal-state refusal
+/// ===================================================================================
+
+TEST(CASRefStateMachine, SnapshotOfSortsCommittedAndPrecommits)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("zzz", manifestRef(1, 3, 1)), addPrecommitOp("aaa", manifestRef(1, 1, 1)),
+         promoteOp("aaa", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("mmm", manifestRef(1, 2, 1))}));
+
+    const RefTableSnapshot snap = snapshotOf(state, kNs);
+    ASSERT_EQ(snap.committed.size(), 1u);
+    EXPECT_EQ(snap.committed[0].ref_name, "aaa");
+    ASSERT_EQ(snap.precommits.size(), 2u);
+    EXPECT_EQ(snap.precommits[0].ref_name, "mmm");
+    EXPECT_EQ(snap.precommits[1].ref_name, "zzz");
+    EXPECT_EQ(snap.snapshot_id, (RefTxnId{1, 2}));
+
+    /// The result must actually be encodable (canonical shape) -- a real round trip through the codec.
+    const String bytes = encodeRefTableSnapshot(snap);
+    const RefTableSnapshot decoded = decodeRefTableSnapshot(bytes, kNs, snap.snapshot_id);
+    EXPECT_EQ(decoded, snap);
+}
+
+TEST(CASRefStateMachine, SnapshotOfRefusesTerminalState)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {removeNamespaceOp()}));
+
+    EXPECT_EQ(state.getLifecycle(), RefLifecycle::Removed);
+    ASSERT_TRUE(state.getRemoveTxnId().has_value());
+    EXPECT_EQ(*state.getRemoveTxnId(), (RefTxnId{1, 2}));
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { snapshotOf(state, kNs); });
+}
+
+/// ===================================================================================
+/// replay: TableState = Replay(S_X.state, tail(X))
+/// ===================================================================================
+
+TEST(CASRefStateMachine, ReplayFromNoSnapshot)
+{
+    std::vector<RefLogTxn> tail{
+        makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}),
+        makeTxn(kNs, RefTxnId{1, 2}, {promoteOp("a", manifestRef(1, 1, 1))}),
+    };
+    const RefTableState state = replay(std::nullopt, tail);
+    EXPECT_EQ(state.getLifecycle(), RefLifecycle::Live);
+    EXPECT_TRUE(state.getCommitted().contains("a"));
+    EXPECT_EQ(state.getGreatestApplied(), (RefTxnId{1, 2}));
+}
+
+TEST(CASRefStateMachine, ReplayFromSnapshotPlusTail)
+{
+    RefTableState built;
+    applyRefLogTxn(built, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    const RefTableSnapshot snap = snapshotOf(built, kNs);
+
+    std::vector<RefLogTxn> tail{makeTxn(kNs, RefTxnId{1, 2}, {promoteOp("a", manifestRef(1, 1, 1))})};
+    const RefTableState state = replay(snap, tail);
+    EXPECT_TRUE(state.getCommitted().contains("a"));
+    EXPECT_EQ(state.getGreatestApplied(), (RefTxnId{1, 2}));
+}
+
+TEST(CASRefStateMachine, StateFromSnapshotConstructsLiveState)
+{
+    RefTableSnapshot snap;
+    snap.ns = kNs;
+    snap.snapshot_id = RefTxnId{1, 1};
+
+    const RefTableState state = stateFromSnapshot(snap);
+    EXPECT_EQ(state.getLifecycle(), RefLifecycle::Live);
+    EXPECT_FALSE(state.getRemoveTxnId().has_value());
+}
+
+TEST(CASRefStateMachine, ReplayRejectsTailNsMismatchAgainstSnapshot)
+{
+    RefTableState built;
+    applyRefLogTxn(built, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    const RefTableSnapshot snap = snapshotOf(built, kNs);
+
+    std::vector<RefLogTxn> tail{makeTxn("other-ns", RefTxnId{1, 2}, {addPrecommitOp("a", manifestRef(1, 1, 1))})};
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(snap, tail); });
+}
+
+TEST(CASRefStateMachine, ReplayRejectsTailNsMismatchAcrossEntries)
+{
+    std::vector<RefLogTxn> tail{
+        makeTxn("ns-a", RefTxnId{1, 1}, {birthOp()}),
+        makeTxn("ns-b", RefTxnId{1, 2}, {addPrecommitOp("a", manifestRef(1, 1, 1))}),
+    };
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(std::nullopt, tail); });
+}
+
+TEST(CASRefStateMachine, ReplayRejectsHandBuiltSnapshotWithDuplicateCommittedName)
+{
+    /// A hand-built RefTableSnapshot (never passed through decodeRefTableSnapshot -- exactly what
+    /// fsck hands to replay) with two committed rows sharing one ref_name must be rejected, not
+    /// silently collapsed to one row via std::map::emplace (the phantom-alive class of bug fixed in
+    /// stateFromSnapshot).
+    RefTableSnapshot snap;
+    snap.ns = kNs;
+    snap.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow row1;
+    row1.ref_name = "a";
+    row1.manifest_ref = manifestRef(1, 1, 1);
+    RefCommittedRow row2;
+    row2.ref_name = "a";
+    row2.manifest_ref = manifestRef(1, 2, 1);
+    snap.committed.push_back(row1);
+    snap.committed.push_back(row2);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(snap, {}); });
+}
+
+TEST(CASRefStateMachine, ReplayRejectsHandBuiltSnapshotWithUnsortedPrecommits)
+{
+    RefTableSnapshot snap;
+    snap.ns = kNs;
+    snap.snapshot_id = RefTxnId{1, 1};
+    snap.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "b", manifestRef(1, 1, 1)});
+    snap.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "a", manifestRef(1, 2, 1)});
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(snap, {}); });
+}
+
+/// Randomized replay equation: replay(snapshotOf(mid-state), tail) == full replay (spec §Table State).
+TEST(CASRefStateMachine, ReplayEquationPropertyTest)
+{
+    std::mt19937 rng(4242); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+    const std::vector<String> names{"a", "b", "c"};
+
+    for (int trial = 0; trial < 30; ++trial)
+    {
+        std::vector<RefLogTxn> history;
+        uint64_t seq = 1;
+        history.push_back(makeTxn(kNs, RefTxnId{1, seq++}, {birthOp()}));
+
+        /// Track our own model of legal next actions so every generated op is guaranteed valid --
+        /// this test exercises the replay equation, not the rejection paths (covered above).
+        std::vector<std::pair<String, ManifestRef>> open_precommits;
+        std::vector<std::pair<String, ManifestRef>> open_committed;
+        uint64_t next_build_seq = 1;
+
+        const int steps = 15;
+        for (int step = 0; step < steps; ++step)
+        {
+            const uint32_t choice = rng() % 4;
+            if (choice == 0 || (open_precommits.empty() && open_committed.empty()))
+            {
+                /// Add precommit under a fresh manifest_ref (never collides, so always legal).
+                const String & name = names[rng() % names.size()];
+                const ManifestRef mref = manifestRef(1, next_build_seq++, 1);
+                history.push_back(makeTxn(kNs, RefTxnId{1, seq++}, {addPrecommitOp(name, mref)}));
+                open_precommits.emplace_back(name, mref);
+            }
+            else if (choice == 1 && !open_precommits.empty())
+            {
+                /// Only a name NOT already committed is eligible for a BARE promote: promoting into an
+                /// already-committed name requires an explicit prior removal in the same transaction
+                /// (spec §Promote; see PromoteRejectsDisplacingAnotherCommittedManifest) -- a distinct
+                /// scenario from the one this equation test exercises.
+                std::vector<size_t> eligible;
+                for (size_t i = 0; i < open_precommits.size(); ++i)
+                {
+                    const bool already_committed = std::any_of(open_committed.begin(), open_committed.end(),
+                        [&](const auto & c) { return c.first == open_precommits[i].first; });
+                    if (!already_committed)
+                        eligible.push_back(i);
+                }
+                if (!eligible.empty())
+                {
+                    const size_t idx = eligible[rng() % eligible.size()];
+                    const auto [name, mref] = open_precommits[idx];
+                    open_precommits.erase(open_precommits.begin() + static_cast<int64_t>(idx));
+                    history.push_back(makeTxn(kNs, RefTxnId{1, seq++}, {promoteOp(name, mref)}));
+                    open_committed.emplace_back(name, mref);
+                }
+            }
+            else if (choice == 2 && !open_committed.empty())
+            {
+                const size_t idx = rng() % open_committed.size();
+                const auto & [name, mref] = open_committed[idx];
+                const uint64_t this_id = seq++;
+                history.push_back(makeTxn(kNs, RefTxnId{1, this_id},
+                    {setPublishedAtOp(name, mref, this_id)}));
+            }
+            else if (!open_precommits.empty())
+            {
+                const size_t idx = rng() % open_precommits.size();
+                const auto [name, mref] = open_precommits[idx];
+                open_precommits.erase(open_precommits.begin() + static_cast<int64_t>(idx));
+                history.push_back(makeTxn(kNs, RefTxnId{1, seq++}, {removePrecommitOp(name, mref)}));
+            }
+            else if (!open_committed.empty())
+            {
+                const size_t idx = rng() % open_committed.size();
+                const auto [name, mref] = open_committed[idx];
+                open_committed.erase(open_committed.begin() + static_cast<int64_t>(idx));
+                history.push_back(makeTxn(kNs, RefTxnId{1, seq++}, {removeCommittedOp(name, mref)}));
+            }
+        }
+
+        const RefTableState full = replay(std::nullopt, history);
+
+        const size_t cut = rng() % (history.size() + 1);
+        const std::vector<RefLogTxn> head(history.begin(), history.begin() + static_cast<int64_t>(cut));
+        const std::vector<RefLogTxn> tail(history.begin() + static_cast<int64_t>(cut), history.end());
+        const RefTableState mid = replay(std::nullopt, head);
+        const std::optional<RefTableSnapshot> mid_snapshot =
+            cut == 0 ? std::nullopt : std::make_optional(snapshotOf(mid, kNs));
+        const RefTableState resumed = replay(mid_snapshot, tail);
+
+        expectStatesEqual(full, resumed);
+    }
+}
+
+/// ===================================================================================
+/// Fail-closed replay + snapshot validation: a corrupted history or snapshot naming one manifest under
+/// two owners must be REJECTED in EVERY build (post-consult). The cross-owner uniqueness check is O(1)
+/// via `owned_manifests`, so it runs unconditionally -- on the writer's append path AND on replay --
+/// rather than being elided into a debug-only assertion. `stateFromSnapshot` enforces the same
+/// invariant across snapshot rows (the codec never did).
+/// ===================================================================================
+
+/// (Add path, committed collision) The writer's append-time contract rejects a fresh precommit that
+/// names a manifest already committed under a DIFFERENT ref_name, and leaves the state unchanged.
+TEST(CASRefStateMachine, LiveAppendRejectsAddPrecommitCollidingWithCommitted)
+{
+    RefTableState state = stateFromSnapshot(buildCollidingBaseSnapshotForTest());
+    const RefTableState before = state;
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("b", manifestRef(1, 1, 1))})); });
+    expectStatesEqual(before, state);
+}
+
+/// (Replay path, committed collision) A tail whose add-precommit collides cross-owner with an existing
+/// committed owner makes `replay` THROW -- it must NOT be silently accepted. This is the exact behavior
+/// the deleted `TrustedReplaySkipsCrossOwnerScanInRelease` test pinned as *desired*; post-consult it is
+/// the opposite: fail closed.
+TEST(CASRefStateMachine, ReplayRejectsTailAddPrecommitCollidingWithCommitted)
+{
+    const RefTableSnapshot snap = buildCollidingBaseSnapshotForTest();
+    const std::vector<RefLogTxn> tail{makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("b", manifestRef(1, 1, 1))})};
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(snap, tail); });
+}
+
+/// (Replay path, precommit collision) The same, but the base already holds a PRECOMMIT for the manifest
+/// and the tail adds a second precommit for it under another ref_name (precommit/precommit collision).
+TEST(CASRefStateMachine, ReplayRejectsTailAddPrecommitCollidingWithPrecommit)
+{
+    const std::vector<RefLogTxn> tail{
+        makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}),
+        makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("b", manifestRef(1, 1, 1))}),   // collides cross-owner
+    };
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(std::nullopt, tail); });
+}
+
+/// (Snapshot validation, committed/committed) A hand-built snapshot with two committed rows naming ONE
+/// manifest passes the codec (it checks only sortedness + no-duplicate ref_name) but must be rejected by
+/// `stateFromSnapshot`/`replay` as semantically corrupt.
+TEST(CASRefStateMachine, ReplayRejectsSnapshotWithTwoCommittedRowsNamingOneManifest)
+{
+    RefTableSnapshot snap;
+    snap.ns = kNs;
+    snap.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow row1;
+    row1.ref_name = "a";
+    row1.manifest_ref = manifestRef(1, 1, 1);
+    RefCommittedRow row2;
+    row2.ref_name = "b";                       // distinct ref_name (codec-legal)...
+    row2.manifest_ref = manifestRef(1, 1, 1);  // ...but the SAME manifest (corrupt)
+    snap.committed.push_back(row1);
+    snap.committed.push_back(row2);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(snap, {}); });
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)stateFromSnapshot(snap); });
+}
+
+/// (Snapshot validation, committed/precommit) A committed row and a precommit binding sharing one
+/// manifest -- also codec-legal (different owner kinds, sorted independently) but corrupt.
+TEST(CASRefStateMachine, ReplayRejectsSnapshotWithCommittedAndPrecommitSharingManifest)
+{
+    RefTableSnapshot snap;
+    snap.ns = kNs;
+    snap.snapshot_id = RefTxnId{1, 1};
+    RefCommittedRow row;
+    row.ref_name = "a";
+    row.manifest_ref = manifestRef(1, 1, 1);
+    snap.committed.push_back(row);
+    snap.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "b", manifestRef(1, 1, 1)});
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(snap, {}); });
+}
+
+/// (Snapshot validation, precommit/precommit) Two precommit bindings under different ref_names naming
+/// one manifest -- sorted by (ref_name, manifest_ref), so codec-legal, but corrupt.
+TEST(CASRefStateMachine, ReplayRejectsSnapshotWithTwoPrecommitsSharingManifest)
+{
+    RefTableSnapshot snap;
+    snap.ns = kNs;
+    snap.snapshot_id = RefTxnId{1, 1};
+    snap.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "a", manifestRef(1, 1, 1)});
+    snap.precommits.push_back(RefOwnerBinding{RefOwnerKind::Precommit, "b", manifestRef(1, 1, 1)});
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(snap, {}); });
+}
+
+/// Positive equivalence: a VALID tail replayed via `replay` (the in-place trusted path) produces a state
+/// byte-identical (getters + encoded snapshot) to the same tail applied via the public strong-guarantee
+/// `applyRefLogTxn` -- the apply strategy changes nothing a legal transaction produces.
+TEST(CASRefStateMachine, TrustedReplayEquivalentToLiveAppendOnValidTail)
+{
+    const std::vector<RefLogTxn> tail{
+        makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}),
+        makeTxn(kNs, RefTxnId{1, 2},
+            {promoteOp("a", manifestRef(1, 1, 1)), addPrecommitOp("b", manifestRef(1, 2, 1))}),
+        makeTxn(kNs, RefTxnId{1, 3}, {setPublishedAtOp("a", manifestRef(1, 1, 1), 7)}),
+        makeTxn(kNs, RefTxnId{1, 4}, {promoteOp("b", manifestRef(1, 2, 1))}),
+    };
+
+    RefTableState full_state;
+    for (const RefLogTxn & txn : tail)
+        applyRefLogTxn(full_state, txn);   // LiveAppend (default)
+
+    const RefTableState trusted_state = replay(std::nullopt, tail);   // replay uses the in-place trusted path internally
+
+    expectStatesEqual(full_state, trusted_state);
+    EXPECT_EQ(encodeRefTableSnapshot(snapshotOf(full_state, kNs)),
+              encodeRefTableSnapshot(snapshotOf(trusted_state, kNs)));
+}
+
+/// ===================================================================================
+/// E3: apply strategy per validation mode
+///   - LiveAppend: two-phase scratch copy, "throw => state byte-for-byte unchanged"
+///   - TrustedReplay (replay): in-place, poison-on-throw, discarded by the sole caller
+/// ===================================================================================
+
+namespace
+{
+/// A populated, MATERIALIZED Live state -- committed "a"->(1,1,1) plus a pending precommit
+/// ("p",(1,2,1)) -- built through the public LiveAppend path, then materialized so its COW overlays are
+/// empty (exactly the shape the writer's live state has at each flush boundary). The E3 LiveAppend-path
+/// tests mutate a COPY of this and assert the original-equivalent captured bytes/getters are intact
+/// after a rejected transaction.
+RefTableState buildPopulatedLiveState()
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1)),
+         addPrecommitOp("p", manifestRef(1, 2, 1))}));
+    state.materializeCommitted();
+    return state;
+}
+}
+
+/// LiveAppend-path atomicity, LATER-op throw ("populated" abort path): the first two ops touch committed,
+/// precommits, the owned-manifest index and the body counters; the third is illegal. The whole
+/// transaction is rejected and `state` is byte-for-byte unchanged -- getters AND encoded-snapshot
+/// bytes. This is the writer's live-state contract, preserved verbatim by E3's `LiveAppend` arm.
+TEST(CASRefStateMachine, E3LiveAppendLaterOpThrowLeavesPopulatedStateByteIdentical)
+{
+    RefTableState state = buildPopulatedLiveState();
+    const RefTableState before = state;
+    const String before_bytes = encodeRefTableSnapshot(snapshotOf(state, kNs));
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {
+            addPrecommitOp("q", manifestRef(1, 3, 1)),       // touches precommits + index + counters
+            removeCommittedOp("a", manifestRef(1, 1, 1)),     // touches committed + index + counters
+            removePrecommitOp("absent", manifestRef(9, 9, 9)) // ILLEGAL: exact binding absent -> throws
+        })); });
+
+    expectStatesEqual(before, state);
+    EXPECT_EQ(before_bytes, encodeRefTableSnapshot(snapshotOf(state, kNs)));
+    /// Neither surviving-looking earlier op leaked into the live state.
+    EXPECT_FALSE(state.getPrecommits().contains({"q", manifestRef(1, 3, 1)}));
+    EXPECT_TRUE(state.getCommitted().contains("a"));
+}
+
+/// LiveAppend-path atomicity, FIRST-op throw ("empty" abort path -- nothing applied before the throw): the
+/// symmetric guarantee still holds. Distinct from the case above because no op ever mutated the
+/// scratch, exercising the throw-before-any-effect branch.
+TEST(CASRefStateMachine, E3LiveAppendFirstOpThrowLeavesPopulatedStateByteIdentical)
+{
+    RefTableState state = buildPopulatedLiveState();
+    const RefTableState before = state;
+    const String before_bytes = encodeRefTableSnapshot(snapshotOf(state, kNs));
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA,
+        [&] { applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {
+            removeCommittedOp("absent", manifestRef(9, 9, 9)), // ILLEGAL first op
+            addPrecommitOp("q", manifestRef(1, 3, 1))
+        })); });
+
+    expectStatesEqual(before, state);
+    EXPECT_EQ(before_bytes, encodeRefTableSnapshot(snapshotOf(state, kNs)));
+}
+
+/// `admits` previews an op against `state` and must leave it byte-for-byte unchanged whether the op
+/// fits (true) or overflows (false) -- it is a pure query. Verified against both getters and encoded
+/// bytes, for both the accept and the reject verdicts.
+TEST(CASRefStateMachine, E3AdmitsPreviewLeavesStateByteIdentical)
+{
+    RefTableState state = buildPopulatedLiveState();
+    /// Must be an independent snapshot -- `state` is queried and potentially mutated below, and
+    /// comparing against a reference would make the check vacuous.
+    // NOLINTNEXTLINE(performance-unnecessary-copy-initialization)
+    const RefTableState before = state;
+    const String before_bytes = encodeRefTableSnapshot(snapshotOf(state, kNs));
+
+    const RefOp grow = addPrecommitOp("q", manifestRef(1, 3, 1));
+
+    /// Accept verdict (ample budget): state untouched.
+    EXPECT_TRUE(admits(state, grow, 1'000'000, 1'000'000));
+    expectStatesEqual(before, state);
+    EXPECT_EQ(before_bytes, encodeRefTableSnapshot(snapshotOf(state, kNs)));
+
+    /// Reject verdict (snapshot budget one byte short of the grown size): state STILL untouched.
+    RefTableState grown = state;
+    applyRefLogTxn(grown, makeTxn(kNs, RefTxnId{1, 2}, {grow}));
+    const size_t grown_size = encodeRefTableSnapshot(snapshotOf(grown, "")).size();
+    EXPECT_FALSE(admits(state, grow, grown_size - 1, 1'000'000));
+    expectStatesEqual(before, state);
+    EXPECT_EQ(before_bytes, encodeRefTableSnapshot(snapshotOf(state, kNs)));
+}
+
+/// TrustedReplay in-place apply, SUCCESS path across every `applyOp` arm: a tail that births, adds,
+/// promotes, replaces a committed manifest, removes a committed and a precommit, restamps a timestamp,
+/// and finally removes the namespace, replayed via `replay` (TrustedReplay, in place) must produce a
+/// state byte-identical to the SAME tail applied op-by-op through `LiveAppend` (scratch copy). This is the
+/// test only E3's in-place machinery can fail: a mis-maintained counter, a dropped owned-manifest
+/// index entry, or a lost `greatest_applied` update on the no-copy path would diverge here.
+TEST(CASRefStateMachine, E3TrustedReplayInPlaceMatchesLiveAppendAcrossAllArms)
+{
+    const std::vector<RefLogTxn> tail{
+        makeTxn(kNs, RefTxnId{1, 1}, {birthOp(),
+            addPrecommitOp("a", manifestRef(1, 1, 1)), addPrecommitOp("b", manifestRef(1, 2, 1))}),
+        makeTxn(kNs, RefTxnId{1, 2}, {
+            promoteOp("a", manifestRef(1, 1, 1)),                     // precommit -> committed
+            setPublishedAtOp("a", manifestRef(1, 1, 1), 42)}),        // restamp published_at_ms
+        makeTxn(kNs, RefTxnId{1, 3}, {removePrecommitOp("b", manifestRef(1, 2, 1))}),   // drop precommit
+        makeTxn(kNs, RefTxnId{1, 4}, {
+            removeCommittedOp("a", manifestRef(1, 1, 1)),             // evict stale committed...
+            addPrecommitOp("a", manifestRef(1, 9, 1)),                // ...then re-add under same name
+            promoteOp("a", manifestRef(1, 9, 1))}),                   // and promote the replacement
+        makeTxn(kNs, RefTxnId{1, 5}, {
+            removeCommittedOp("a", manifestRef(1, 9, 1)),             // drain the last owner...
+            removeNamespaceOp()}),                                     // ...then remove the namespace
+    };
+
+    RefTableState full_state;
+    for (const RefLogTxn & txn : tail)
+        applyRefLogTxn(full_state, txn);   // LiveAppend (default): two-phase scratch copy
+
+    const RefTableState replayed = replay(std::nullopt, tail);   // TrustedReplay in-place
+
+    expectStatesEqual(full_state, replayed);
+    EXPECT_THROW(encodeRefTableSnapshot(snapshotOf(full_state, kNs)), DB::Exception);
+    EXPECT_THROW(encodeRefTableSnapshot(snapshotOf(replayed, kNs)), DB::Exception);
+    EXPECT_EQ(replayed.getLifecycle(), RefLifecycle::Removed);
+    EXPECT_EQ(replayed.getRemoveTxnId(), std::make_optional(RefTxnId{1, 5}));
+}
+
+/// TrustedReplay in-place apply, THROW path: a tail whose LAST transaction is illegal makes `replay`
+/// throw `CORRUPTED_DATA`. The in-place apply poisons a state that is entirely internal to the failed
+/// `replay` call (it is never assigned to a caller on a throw), so an INDEPENDENT replay of just the
+/// valid prefix is completely unaffected -- pinning that the poison never escapes.
+TEST(CASRefStateMachine, E3TrustedReplayPoisonOnBadTailIsInternal)
+{
+    const std::vector<RefLogTxn> good_prefix{
+        makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}),
+        makeTxn(kNs, RefTxnId{1, 2}, {promoteOp("a", manifestRef(1, 1, 1))}),
+    };
+    std::vector<RefLogTxn> bad_tail = good_prefix;
+    /// A third txn whose op removes an absent precommit -- legal txn_id ordering, illegal effect, so it
+    /// throws mid-apply AFTER the good prefix has already been applied in place to the internal state.
+    bad_tail.push_back(makeTxn(kNs, RefTxnId{1, 3}, {removePrecommitOp("absent", manifestRef(9, 9, 9))}));
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { replay(std::nullopt, bad_tail); });
+
+    /// The failed replay's poisoned internal state never leaked: a fresh replay of the valid prefix is
+    /// byte-identical to one built entirely via LiveAppend, and reflects exactly the prefix.
+    const RefTableState from_prefix = replay(std::nullopt, good_prefix);
+    RefTableState full_prefix;
+    for (const RefLogTxn & txn : good_prefix)
+        applyRefLogTxn(full_prefix, txn);
+    expectStatesEqual(full_prefix, from_prefix);
+    EXPECT_TRUE(from_prefix.getCommitted().contains("a"));
+    EXPECT_EQ(from_prefix.getGreatestApplied(), (RefTxnId{1, 2}));
+}
+
+/// ===================================================================================
+/// admits(): dual-bound admission budget (spec §Snapshot Format)
+/// ===================================================================================
+
+TEST(CASRefStateMachine, AdmitsAcceptsWellUnderBudget)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    EXPECT_TRUE(admits(state, addPrecommitOp("a", manifestRef(1, 1, 1)), 1'000'000, 1'000'000));
+}
+
+TEST(CASRefStateMachine, AdmitsRejectsGrowthPastSnapshotBudgetOwnerTransitionAdd)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+
+    const RefOp op = addPrecommitOp("a", manifestRef(1, 1, 1));
+    RefTableState scratch = state;
+    applyRefLogTxn(scratch, makeTxn(kNs, RefTxnId{1, 2}, {op}));
+    const size_t true_size = encodeRefTableSnapshot(snapshotOf(scratch, "")).size();
+
+    EXPECT_TRUE(admits(state, op, true_size, 1'000'000));
+    EXPECT_FALSE(admits(state, op, true_size - 1, 1'000'000));
+}
+
+TEST(CASRefStateMachine, AdmitsRejectsGrowthPastSnapshotBudgetSetPublishedAt)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+
+    const RefOp op = setPublishedAtOp("a", manifestRef(1, 1, 1), 1700000000000ull);
+    RefTableState scratch = state;
+    applyRefLogTxn(scratch, makeTxn(kNs, RefTxnId{1, 2}, {op}));
+    const size_t true_size = encodeRefTableSnapshot(snapshotOf(scratch, "")).size();
+
+    EXPECT_TRUE(admits(state, op, true_size, 1'000'000));
+    EXPECT_FALSE(admits(state, op, true_size - 1, 1'000'000));
+}
+
+TEST(CASRefStateMachine, AdmitsRejectsGrowthPastSnapshotBudgetPromoteWithSetPublishedAt)
+{
+    /// The "promote-with-set_published_at" growth class: the owner_transition half of a promote is
+    /// admitted cheaply (published_at_ms starts unset), but the immediately-following set_published_at
+    /// that installs the REAL initial timestamp is where the growth actually happens.
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {promoteOp("a", manifestRef(1, 1, 1))}));
+    ASSERT_TRUE(state.getCommitted().contains("a"));
+    ASSERT_EQ(state.getCommitted().at("a").published_at_ms, 0u);
+
+    const RefOp op = setPublishedAtOp("a", manifestRef(1, 1, 1), 1700000000099ull);
+    RefTableState scratch = state;
+    applyRefLogTxn(scratch, makeTxn(kNs, RefTxnId{1, 3}, {op}));
+    const size_t true_size = encodeRefTableSnapshot(snapshotOf(scratch, "")).size();
+
+    EXPECT_TRUE(admits(state, op, true_size, 1'000'000));
+    EXPECT_FALSE(admits(state, op, true_size - 1, 1'000'000));
+}
+
+TEST(CASRefStateMachine, AdmitsRejectsGrowthPastRemovalBudget)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(), addPrecommitOp("a", manifestRef(1, 1, 1)), promoteOp("a", manifestRef(1, 1, 1))}));
+
+    const RefOp op = setPublishedAtOp("a", manifestRef(1, 1, 1), 1700000000300ull);
+    RefTableState scratch = state;
+    applyRefLogTxn(scratch, makeTxn(kNs, RefTxnId{1, 2}, {op}));
+    const String removal_bytes = encodeRefLogTxn(buildRemovalTxnForTest(scratch, "", RefTxnId{1, 1}));
+    const size_t true_removal_size = removal_bytes.size();
+
+    /// A generous snapshot budget isolates the removal-budget bound specifically.
+    EXPECT_TRUE(admits(state, op, 1'000'000, true_removal_size));
+    EXPECT_FALSE(admits(state, op, 1'000'000, true_removal_size - 1));
+}
+
+/// Randomized exactness property test: admits()'s internal size computation must exactly match the
+/// real encoders' output, for both bounds, across randomized states and candidate growing ops.
+TEST(CASRefStateMachine, AdmitsExactnessPropertyTest)
+{
+    std::mt19937 rng(777); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed is required for reproducible property coverage.
+
+    for (int trial = 0; trial < 20; ++trial)
+    {
+        RefTableState state;
+        applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+        uint64_t seq = 2;
+        uint64_t next_build_seq = 1;
+        std::vector<std::pair<String, ManifestRef>> open_precommits;
+        std::vector<std::pair<String, ManifestRef>> open_committed;
+
+        /// Build up a random but valid mid-state (a handful of precommits/committed rows/timestamps).
+        const int setup_steps = 1 + static_cast<int>(rng() % 5);
+        for (int i = 0; i < setup_steps; ++i)
+        {
+            const String name = "ref" + std::to_string(rng() % 4);
+            const ManifestRef mref = manifestRef(1, next_build_seq++, 1);
+            applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, seq++}, {addPrecommitOp(name, mref)}));
+            open_precommits.emplace_back(name, mref);
+
+            /// A bare promote may not target a name already committed under a different manifest
+            /// (spec §Promote; see PromoteRejectsDisplacingAnotherCommittedManifest) -- skip promoting
+            /// this iteration's precommit when an earlier iteration already committed the same name.
+            const bool name_already_committed = std::any_of(open_committed.begin(), open_committed.end(),
+                [&](const auto & c) { return c.first == name; });
+            if (!name_already_committed && rng() % 2 == 0)
+            {
+                const auto [pname, pmref] = open_precommits.back();
+                open_precommits.pop_back();
+                applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, seq++}, {promoteOp(pname, pmref)}));
+                open_committed.emplace_back(pname, pmref);
+            }
+        }
+
+        /// Pick a random candidate growing op against this state.
+        RefOp candidate;
+        const uint32_t kind = rng() % 3;
+        if (kind == 0 || open_committed.empty())
+        {
+            candidate = addPrecommitOp("fresh-" + std::to_string(trial), manifestRef(1, next_build_seq++, 1));
+        }
+        else if (kind == 1)
+        {
+            const auto & [name, mref] = open_committed[rng() % open_committed.size()];
+            candidate = setPublishedAtOp(name, mref, rng());
+        }
+        else
+        {
+            /// A genuinely distinct third shape: a racing precommit under an ALREADY-committed name
+            /// (legal -- spec §Add Precommit only restricts manifest identity, never ref_name).
+            const String & name = open_committed[rng() % open_committed.size()].first;
+            candidate = addPrecommitOp(name, manifestRef(1, next_build_seq++, 1));
+        }
+
+        RefTableState scratch = state;
+        applyRefLogTxn(scratch, makeTxn(kNs, RefTxnId{1, seq}, {candidate}));
+        const size_t true_snapshot_size = encodeRefTableSnapshot(snapshotOf(scratch, "")).size();
+        const size_t true_removal_size =
+            encodeRefLogTxn(buildRemovalTxnForTest(scratch, "", RefTxnId{1, 1})).size();
+
+        EXPECT_TRUE(admits(state, candidate, true_snapshot_size, true_removal_size));
+        EXPECT_FALSE(admits(state, candidate, true_snapshot_size - 1, true_removal_size));
+        EXPECT_FALSE(admits(state, candidate, true_snapshot_size, true_removal_size - 1));
+    }
+}
+
+/// ===================================================================================
+/// Snapshot size helpers: framing + Σ per-row must equal a full encode, byte for byte.
+/// ===================================================================================
+TEST(CASRefSnapshotSizeHelpers, FramingPlusRowsEqualsFullEncode)
+{
+    /// Build a non-trivial Live table: two committed rows (one with a stamped published_at_ms) and one
+    /// precommit.
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(),
+         addPrecommitOp("alpha", manifestRef(1, 1, 1)), promoteOp("alpha", manifestRef(1, 1, 1)),
+         addPrecommitOp("beta", manifestRef(1, 2, 1)), promoteOp("beta", manifestRef(1, 2, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2},
+        {setPublishedAtOp("alpha", manifestRef(1, 1, 1), 42)}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {addPrecommitOp("gamma", manifestRef(1, 3, 1))}));
+
+    const RefTableSnapshot snap = snapshotOf(state, "");
+    const size_t full = encodeRefTableSnapshot(snap).size();
+
+    size_t rebuilt = snapshotFramingSize("", snap.snapshot_id, snap.committed.size() + snap.precommits.size());
+    for (const RefCommittedRow & row : snap.committed)
+        rebuilt += committedRowEncodedSize(row);
+    for (const RefOwnerBinding & pc : snap.precommits)
+        rebuilt += precommitRowEncodedSize(pc);
+
+    EXPECT_EQ(rebuilt, full);
+}
+
+/// ===================================================================================
+/// Removal-txn size helpers: framing + Σ per-owner-op must equal a full removal-txn encode.
+/// ===================================================================================
+TEST(CASRefLogSizeHelpers, FramingPlusOpsEqualsFullRemovalEncode)
+{
+    RefTableState state;
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1},
+        {birthOp(),
+         addPrecommitOp("alpha", manifestRef(1, 1, 1)), promoteOp("alpha", manifestRef(1, 1, 1)),
+         addPrecommitOp("beta", manifestRef(1, 2, 1))}));
+
+    /// Ground truth: the whole-namespace removal txn this test file already builds independently.
+    const RefLogTxn removal = buildRemovalTxnForTest(state, "", RefTxnId{1, 1});
+    const size_t full = encodeRefLogTxn(removal).size();
+
+    size_t rebuilt = removalFramingSize("", RefTxnId{1, 1},
+                                        state.getCommitted().size() + state.getPrecommits().size() + 1);
+    for (const auto [name, row] : state.getCommitted())
+        rebuilt += removalOpEncodedSize(RefOwnerKind::Committed, name, row.manifest_ref);
+    for (const auto & [name, mref] : state.getPrecommits())
+        rebuilt += removalOpEncodedSize(RefOwnerKind::Precommit, name, mref);
+
+    EXPECT_EQ(rebuilt, full);
+}
+
+/// ===================================================================================
+/// Body-byte counters: snapshot_body_bytes / removal_body_bytes are a pure function of the rows.
+/// ===================================================================================
+namespace
+{
+uint64_t recomputeSnapshotBody(const RefTableState & s)
+{
+    uint64_t total = 0;
+    for (const auto [name, row] : s.getCommitted())
+        total += committedRowEncodedSize(row);
+    for (const auto & [name, mref] : s.getPrecommits())
+        total += precommitRowEncodedSize(RefOwnerBinding{RefOwnerKind::Precommit, name, mref});
+    return total;
+}
+uint64_t recomputeRemovalBody(const RefTableState & s)
+{
+    uint64_t total = 0;
+    for (const auto [name, row] : s.getCommitted())
+        total += removalOpEncodedSize(RefOwnerKind::Committed, name, row.manifest_ref);
+    for (const auto & [name, mref] : s.getPrecommits())
+        total += removalOpEncodedSize(RefOwnerKind::Precommit, name, mref);
+    return total;
+}
+}
+
+TEST(CASRefStateCounters, CountersTrackRowsThroughEveryOpKind)
+{
+    RefTableState state;
+    EXPECT_EQ(state.getSnapshotBodyBytes(), 0u);
+    EXPECT_EQ(state.getRemovalBodyBytes(), 0u);
+
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 2}, {addPrecommitOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 3}, {promoteOp("a", manifestRef(1, 1, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 4},
+        {setPublishedAtOp("a", manifestRef(1, 1, 1), 5)}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 5}, {addPrecommitOp("b", manifestRef(1, 2, 1))}));
+    EXPECT_EQ(state.getSnapshotBodyBytes(), recomputeSnapshotBody(state));
+    EXPECT_EQ(state.getRemovalBodyBytes(), recomputeRemovalBody(state));
+
+    /// Shrink back down: remove the precommit, then the committed row.
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 6}, {removePrecommitOp("b", manifestRef(1, 2, 1))}));
+    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 7}, {removeCommittedOp("a", manifestRef(1, 1, 1))}));
+    EXPECT_EQ(state.getSnapshotBodyBytes(), recomputeSnapshotBody(state));
+    EXPECT_EQ(state.getRemovalBodyBytes(), recomputeRemovalBody(state));
+    EXPECT_EQ(state.getSnapshotBodyBytes(), 0u);
+    EXPECT_EQ(state.getRemovalBodyBytes(), 0u);
+}
+
+/// ===================================================================================
+/// Budget-size accessors equal the real encoders across randomized states.
+/// ===================================================================================
+TEST(CASRefBudgetSize, AccessorsEqualFullEncodeRandomized)
+{
+    std::mt19937 rng(1234); // NOLINT(cert-msc32-c,cert-msc51-cpp): deterministic seed for reproducibility.
+    for (int trial = 0; trial < 30; ++trial)
+    {
+        RefTableState state;
+        applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, 1}, {birthOp()}));
+        uint64_t seq = 2;
+        uint64_t build = 1;
+        std::vector<std::pair<String, ManifestRef>> committed_names;
+
+        const int steps = 1 + static_cast<int>(rng() % 6);
+        for (int i = 0; i < steps; ++i)
+        {
+            const String name = "r" + std::to_string(rng() % 5);
+            const ManifestRef mref = manifestRef(1, build++, 1);
+            applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, seq++}, {addPrecommitOp(name, mref)}));
+            const bool already = std::any_of(committed_names.begin(), committed_names.end(),
+                [&](const auto & c) { return c.first == name; });
+            if (!already && rng() % 2 == 0)
+            {
+                applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, seq++}, {promoteOp(name, mref)}));
+                committed_names.emplace_back(name, mref);
+                if (rng() % 2 == 0)
+                    applyRefLogTxn(state, makeTxn(kNs, RefTxnId{1, seq++},
+                        {setPublishedAtOp(name, mref, rng())}));
+            }
+        }
+
+        const size_t true_snapshot = encodeRefTableSnapshot(snapshotOf(state, "")).size();
+        const size_t true_removal = encodeRefLogTxn(buildRemovalTxnForTest(state, "", RefTxnId{1, 1})).size();
+        EXPECT_EQ(encodedSnapshotBudgetSize(state), true_snapshot);
+        EXPECT_EQ(encodedRemovalBudgetSize(state), true_removal);
+    }
+}
diff --git a/src/Disks/tests/gtest_cas_ref_wedge_every_attempt.cpp b/src/Disks/tests/gtest_cas_ref_wedge_every_attempt.cpp
new file mode 100644
index 000000000000..ace02d24836e
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_wedge_every_attempt.cpp
@@ -0,0 +1,1476 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefCatalog.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/MemoryTracker.h>
+#include <Common/ProfileEvents.h>
+
+#include <Poco/Exception.h>
+
+#include <algorithm>
+#include <atomic>
+#include <condition_variable>
+#include <functional>
+#include <mutex>
+#include <optional>
+#include <stdexcept>
+#include <thread>
+
+/// ================================================================================================
+/// Task 4 (2026-07-28 CAS ref-chain Stage A streams, spec INV-1's every-attempt rule + INV-2's seal):
+/// the writer wedge.
+///
+/// An id is freed only when NOTHING WAS SENT, or when every sent attempt has its own CONCLUSIVE
+/// rejection. That is what these tests are about, and the second half is the part that changed: an
+/// ambiguous attempt used to be resolved by a bare exact GET, which can only ever report "absent" --
+/// and absent is not a rejection, because the ambiguous attempt may still land afterwards. The lane
+/// therefore stayed wedged FOREVER over a key nothing had written. The rule now runs one bounded
+/// `slotOccupy` per later caller's flush: the ref-log key is write-once, so a conditional CREATE of
+/// the SAME bytes either makes the transaction durable (adopt it) or conflicts with whatever is
+/// there, which the follow-up read then names -- our own earlier write (adopt), a successor's
+/// `EpochSeal` (the operation is conclusively rejected and never was acked), or a foreign object
+/// (impossible under mount-lease exclusivity: fail loud).
+///
+/// Two cross-cutting rules are exercised throughout rather than in one place:
+///   - the ADMISSION FENCE: a wedge carries the mount-fence generation it was admitted under, every
+///     retry is gated on THAT generation (never the current one), and every result is re-checked
+///     under `state_mutex` before anything acts on it. A result that returns after a fence
+///     bump/re-arm, or after the wedge it belonged to was replaced, must be INERT.
+///   - `prev_epoch_seal`: a seal observed at the wedged key IS this namespace's epoch-closing record,
+///     so it becomes the `prev_epoch_seal` the next sequence-1 append carries. `nullopt` means
+///     genesis, and means it exactly.
+/// ================================================================================================
+
+namespace ProfileEvents
+{
+extern const Event CASRefAppendSealRejected;
+extern const Event CASRefAppendOccupantUnreadable;
+extern const Event CASRefAppendWedged;
+extern const Event CASRefAppendDefiniteFailure;
+}
+
+namespace DB::ErrorCodes
+{
+extern const int BAD_ARGUMENTS;
+extern const int CORRUPTED_DATA;
+extern const int INVALID_STATE;
+extern const int MEMORY_LIMIT_EXCEEDED;
+extern const int NETWORK_ERROR;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::CountingBackend;
+using DB::Cas::tests::LandedButAckLostOnceBackend;
+using DB::Cas::tests::expectThrowsCode;
+
+namespace
+{
+
+PoolPtr openPool(const BackendPtr & backend, CasRequestBudget budget = {})
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .cas_request_budget = budget});
+}
+
+/// The budget every wedge test uses: ONE attempt, so a single injected ambiguity is the whole
+/// operation and the lane wedges deterministically instead of retrying its way out.
+CasRequestBudget singleAttemptBudget()
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;   /// strictly above attempt_timeout_ms: equality is a wall-clock race (validateCasRequestBudget)
+    budget.lease_safety_margin_ms = 100;
+    return budget;
+}
+
+/// TWO attempts of one logical operation, with the inter-attempt backoff disabled. Everything about the
+/// call-level verdict rule lives BETWEEN two attempts of a single call, so it cannot be reached with the
+/// one-attempt budget the tests above use; the backoff is switched off because the schedule is
+/// `gtest_cas_request_control.cpp`'s subject and a real sleep here would only slow the suite.
+///
+/// `[[maybe_unused]]`: both of its callers need a real S3-classified rejection to script their second
+/// attempt, so they compile away entirely in a build without S3.
+[[maybe_unused]] CasRequestBudget twoAttemptBudget()
+{
+    CasRequestBudget budget = singleAttemptBudget();
+    budget.max_attempts = 2;
+    budget.retry_initial_backoff_ms = 0;
+    budget.retry_max_backoff_ms = 0;
+    return budget;
+}
+
+PartWriteTxnPtr startBuildFor(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref;
+    return s->beginPartWrite(info);
+}
+
+void publishEmptyPart(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    auto build = startBuildFor(s, ns, ref);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+}
+
+/// A `CountingBackend` with the exact seams these tests need, all keyed by substring so a whole Pool's
+/// bootstrap traffic never consumes a fault meant for a `_log/` PUT.
+class WedgeTestBackend : public CountingBackend
+{
+public:
+    using CountingBackend::putIfAbsent;
+    using CountingBackend::get;
+
+    /// One-shot ambiguity that writes NOTHING: the response is lost and the key stays absent, which is
+    /// the input that makes a later `slotOccupy` report `Created`.
+    String ambiguous_substr;
+    int ambiguous_count = 0;
+
+    /// One-shot DETERMINISTIC LOCAL failure (`BAD_ARGUMENTS`, in `isDeterministicLocalFailure`'s set),
+    /// which `slotOccupy` rethrows unchanged -- a definite refusal of THIS attempt. Portable stand-in
+    /// for the S3 `DefiniteFailure` shape, which needs `USE_AWS_S3`; both are "proven never applied".
+    String definite_substr;
+    int definite_count = 0;
+
+    /// One-shot WHITELISTED SYNCHRONOUS REJECTION: the ONLY shape `classifyConditionalWriteResult`
+    /// answers `DefiniteFailure` for, and therefore the only way to drive the append lane's definite
+    /// arm. Distinct from `definite_substr` above on purpose -- that one is a deterministic LOCAL
+    /// failure, which `slotOccupy` rethrows but `putIfAbsentControlled` (no such special case) merely
+    /// classifies Unresolved, so it cannot script this arm at all.
+    String s3_definite_substr;
+    int s3_definite_count = 0;
+
+    /// A SUCCESSOR lands `conflict_bytes` at the key and only then is our response lost, so the
+    /// controller's resolve-before-reissue reads a different object and proves the conflict. This is how
+    /// the ordinary append site meets an occupant at the id it derived.
+    String conflict_substr;
+    int conflict_count = 0;
+    String conflict_bytes;
+
+    /// Fail GETs of matching keys after skipping the first `fail_get_skip` of them -- the resolve read
+    /// that PROVES the conflict must succeed, so only the adjudication read that follows it is faulted.
+    String fail_get_substr;
+    int fail_get_skip = 0;
+    int fail_get_count = 0;
+
+    String fail_cas_substr;
+    int fail_cas_count = 0;
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (fail_get_count > 0 && !fail_get_substr.empty() && key.find(fail_get_substr) != String::npos)
+        {
+            if (fail_get_skip > 0)
+                --fail_get_skip;
+            else
+            {
+                --fail_get_count;
+                throw Poco::TimeoutException("WedgeTestBackend: simulated lost GET (read response never arrived)");
+            }
+        }
+        return CountingBackend::get(key, range);
+    }
+
+    CasResult casPut(const String & key, const String & bytes, const std::optional<Token> & expected,
+                     const ObjectMeta & meta) override
+    {
+        if (fail_cas_count > 0 && !fail_cas_substr.empty() && key.find(fail_cas_substr) != String::npos)
+        {
+            --fail_cas_count;
+            throw Poco::TimeoutException("WedgeTestBackend: simulated ambiguous checkpoint CAS");
+        }
+        return CountingBackend::casPut(key, bytes, expected, meta);
+    }
+
+    /// Park a matching PUT until `releaseBlock()`, notifying `awaitBlockEntered()` on arrival, so a
+    /// test can drive a fence bump or a successor's write into the exact I/O window.
+    void armBlock(const String & substr)
+    {
+        std::lock_guard g(block_mutex);
+        block_substr = substr;
+        block_armed = true;
+        block_entered = false;
+    }
+    void awaitBlockEntered()
+    {
+        std::unique_lock lk(block_mutex);
+        block_cv.wait(lk, [&] { return block_entered; });
+    }
+    void releaseBlock()
+    {
+        {
+            std::lock_guard g(block_mutex);
+            block_armed = false;
+        }
+        block_cv.notify_all();
+    }
+
+    /// Write straight through, bypassing every fault and block seam above -- how a test models what a
+    /// SUCCESSOR (another process entirely) put at a key. Using the faulting entry point instead would
+    /// park the test's own write on the very gate it is trying to drive a scenario through. The
+    /// qualification must name the THREE-argument overload: `Backend`'s two-argument convenience
+    /// forwards to the VIRTUAL one, so `CountingBackend::putIfAbsent(key, bytes)` would dispatch right
+    /// back into the override above and deadlock the test against its own block.
+    PutResult putAsSuccessor(const String & key, const String & bytes)
+    {
+        return CountingBackend::putIfAbsent(key, bytes, ObjectMeta{});
+    }
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (ambiguous_count > 0 && !ambiguous_substr.empty() && key.find(ambiguous_substr) != String::npos)
+        {
+            --ambiguous_count;
+            throw Poco::TimeoutException("WedgeTestBackend: simulated ambiguous PUT (response lost, nothing landed)");
+        }
+        if (definite_count > 0 && !definite_substr.empty() && key.find(definite_substr) != String::npos)
+        {
+            --definite_count;
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS, "WedgeTestBackend: scripted deterministic local failure");
+        }
+        /// AFTER the ambiguity seam, so arming both scripts one call's attempts in order: the first
+        /// attempt goes ambiguous, the reissue is definitively refused.
+        if (s3_definite_count > 0 && !s3_definite_substr.empty() && key.find(s3_definite_substr) != String::npos)
+        {
+            --s3_definite_count;
+#if USE_AWS_S3
+            throw DB::S3Exception("WedgeTestBackend: simulated malformed request",
+                                  Aws::S3::S3Errors::UNKNOWN, "MalformedXML");
+#else
+            throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA,
+                "WedgeTestBackend: DefiniteFailure requires S3 error classification (USE_AWS_S3 off)");
+#endif
+        }
+        if (conflict_count > 0 && !conflict_substr.empty() && key.find(conflict_substr) != String::npos)
+        {
+            --conflict_count;
+            CountingBackend::putIfAbsent(key, conflict_bytes, meta);
+            throw Poco::TimeoutException("WedgeTestBackend: a successor's object landed; our response was lost");
+        }
+        {
+            std::unique_lock lk(block_mutex);
+            if (block_armed && !block_substr.empty() && key.find(block_substr) != String::npos)
+            {
+                block_entered = true;
+                block_cv.notify_all();
+                /// Bounded so a wiring bug bounds the wait instead of hanging the suite.
+                block_cv.wait_for(lk, std::chrono::seconds(20), [&] { return !block_armed; });
+            }
+        }
+        return CountingBackend::putIfAbsent(key, bytes, meta);
+    }
+
+private:
+    std::mutex block_mutex;
+    std::condition_variable block_cv;
+    String block_substr;
+    bool block_armed = false;
+    bool block_entered = false;
+};
+
+/// The `_log/` key prefix of one namespace -- what every fault seam here matches on.
+String logPrefix(const PoolPtr & store, const RootNamespace & ns)
+{
+    return store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+}
+
+CatalogEntry catalogEntryOrThrow(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const RefCatalog catalog = CasRefCatalog::read(backend, layout).catalog;
+    const auto it = std::find_if(catalog.entries.begin(), catalog.entries.end(), [&](const CatalogEntry & entry)
+    {
+        return entry.ns == ns;
+    });
+    if (it == catalog.entries.end())
+        throw std::runtime_error("catalog entry missing from wedge fixture");
+    return *it;
+}
+
+/// Wedge tests address raw ref-log keys at Stage A's deterministic sentinel identity, but their
+/// catalog fixture must still use production's `Creating -> _ckpt -> Live` birth order. A fixed
+/// creator identity makes the durable genesis checkpoint deterministic too.
+void admitProperlyBornEntry(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const CatalogEntry creating{
+        .ns = ns,
+        .state = NsState::Creating,
+        .incarnation = DB::Cas::tests::fixture::fixtureLife(ns).incarnation,
+        .creator = CreatorFence{.server_root_id = "test", .writer_epoch = 1, .fence_generation = 1},
+    };
+    CasRefCatalog::casAdmitEntry(backend, layout, /*gc_shards=*/1, creating);
+
+    const CkptDeadline deadline{.now_ms = [] { return uint64_t{1000}; }, .deadline_ms = 60000};
+    ASSERT_EQ(
+        CasRefCatalog::completeCreation(
+            backend, layout, creating, /*admitted_generation=*/1, [](uint64_t) {}, deadline),
+        CasRefCatalog::NamespaceCreationOutcome::Live);
+}
+
+CatalogEntry replaceCatalogLifeForWedgeRace(
+    Backend & backend, const Layout & layout, const CatalogEntry & predecessor, UInt128 successor_incarnation)
+{
+    const CasRefCatalog::Snapshot before_delete = CasRefCatalog::read(backend, layout);
+    RefCatalog without_predecessor = before_delete.catalog;
+    std::erase_if(without_predecessor.entries, [&](const CatalogEntry & entry)
+    {
+        return entry.ns == predecessor.ns && entry.incarnation == predecessor.incarnation;
+    });
+    if (backend.casPut(layout.refCatalogKey(), encodeRefCatalog(without_predecessor), before_delete.token).outcome
+        != CasOutcome::Committed)
+        throw std::runtime_error("test failed to retire exact predecessor catalog life");
+
+    CatalogEntry successor{
+        .ns = predecessor.ns,
+        .state = NsState::Live,
+        .incarnation = successor_incarnation,
+        .creator = std::nullopt};
+    const CasRefCatalog::Snapshot after_delete = CasRefCatalog::read(backend, layout);
+    RefCatalog reborn = after_delete.catalog;
+    reborn.entries.push_back(successor);
+    if (backend.casPut(layout.refCatalogKey(), encodeRefCatalog(reborn), after_delete.token).outcome
+        != CasOutcome::Committed)
+        throw std::runtime_error("test failed to publish successor catalog life");
+    return successor;
+}
+
+/// Decode the ref-log object at `id`, through the SAME codec the writer's recovery uses (never a
+/// hand-rolled parse), so an assertion about `prev_epoch_seal` is an assertion about the WIRE.
+RefLogTxn readRefLogTxn(Backend & backend, const Layout & layout, const RootNamespace & ns, const RefTxnId & id)
+{
+    const auto got = backend.get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), id));
+    if (!got)
+        throw DB::Exception(DB::ErrorCodes::CORRUPTED_DATA, "no ref-log object at {}-{}", id.writer_epoch, id.ref_sequence);
+    return decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), id);
+}
+
+/// The bytes of a real `EpochSeal` transaction closing `id.writer_epoch` at `id` -- what a SUCCESSOR
+/// writes into the dead epoch's next slot (spec INV-2). Grammar: exactly one `EpochSeal` op, and
+/// `prev_epoch_seal` on sequence 1 only, so callers pass it exactly when `id.ref_sequence == 1`.
+String epochSealBytes(const RootNamespace & ns, const RefTxnId & id, std::optional<RefTxnId> prev_epoch_seal = std::nullopt)
+{
+    RefOp op;
+    op.kind = RefOpKind::EpochSeal;
+    const RefLogTxn txn{ns.string(), id, {op}, prev_epoch_seal};
+    return sealObject(FormatId::RefLog, encodeRefLogTxn(txn));
+}
+
+/// Re-arm the mount fence WITHOUT a self-remount: the generation moves (twice -- trip, then re-arm)
+/// while the cached runtime survives, which is what isolates the generation check as the sole
+/// detector. A real self-remount also quiesces the runtimes; that path is Task 6's.
+void bumpFenceGeneration(const PoolPtr & store, uint64_t writer_epoch)
+{
+    store->tripMountLost();
+    store->armMountFence(DB::UInt128{0, 1}, writer_epoch, store->bootMsNow() + 600000);
+    /// The fence re-arm alone moves the GENERATION; the live incarnation's writer epoch is a separate
+    /// publication (`tryRemountOnce` does both), and the append lane derives its ids from that one.
+    store->setLiveWriterEpochForTest(writer_epoch);
+}
+
+/// Arm a one-shot throw inside the post-durable install regions. The exception is built OUTSIDE the
+/// region (building it inside would trip `DENY_ALLOCATIONS_IN_SCOPE` and test the guard instead), and
+/// `MEMORY_LIMIT_EXCEEDED` is what a real tracked allocation failure raises. Same shape as
+/// `gtest_cas_ref_install_safety.cpp`'s helper.
+void armOneShotInstallFailure(const PoolPtr & store)
+{
+    auto planned = std::make_exception_ptr(DB::Exception(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED,
+        "simulated allocation failure inside the post-durable install region"));
+    auto fired = std::make_shared<std::atomic<bool>>(false);
+    store->setInstallRegionProbeForTest([planned, fired]
+    {
+        if (fired->exchange(true))
+            return;
+        ALLOW_ALLOCATIONS_IN_SCOPE;
+        std::rethrow_exception(planned);
+    });
+}
+
+}
+
+/// ===================================================================================
+/// The every-attempt rule: an ambiguous attempt is resolved by a bounded CREATE, not a read
+/// ===================================================================================
+
+/// The headline change. Nothing landed, so the old bare-GET resolution reported "absent" forever and
+/// the lane never recovered without a remount. One conditional create of the SAME bytes settles it:
+/// the object becomes durable and the wedged transaction is adopted -- applied EXACTLY once, before
+/// the flush that resolved it allocates any new id.
+TEST(CASRefWedgeEveryAttempt, AmbiguousPutWedgesTheLaneAndTheNextFlushsCreateAdoptsItExactlyOnce)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_created"};
+    /// Stage B (Task 4-C): `logPrefix` below computes its fault-injection match at the sentinel;
+    /// pinning `ns` there BEFORE the first real touch keeps the real production birth landing on the
+    /// same key the fault targets.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    ASSERT_TRUE(store->resolveRef(ns, "x").has_value()) << "a wedged transaction is not applied";
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_FALSE(backend->get(wedged_key).has_value()) << "the ambiguous attempt wrote nothing";
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+
+    /// The next caller's flush resolves the wedge with ONE create, adopts it, and only then carves and
+    /// commits its own transaction.
+    EXPECT_NO_THROW(store->dropRef(ns, "y"));
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_FALSE(store->resolveRef(ns, "x").has_value()) << "the adopted wedge applied its drop";
+    EXPECT_FALSE(store->resolveRef(ns, "y").has_value()) << "the resolving flush committed its own drop";
+    EXPECT_TRUE(backend->get(wedged_key).has_value()) << "the wedged transaction is durable at its own key";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before + 2)
+        << "the adopted wedge and the ordinary commit must each join the tail exactly once";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+}
+
+TEST(CASRefWedgeEveryAttempt, DurableCreatedWedgeNeedsRecoveryWhenItsFrontierCannotBePublished)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_created_frontier_failed"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_FALSE(backend->get(wedged_key));
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+
+    const NamespaceLifeId life = *store->refTableLifeForTest(ns);
+    const String ckpt_key = store->layout().refCkptKey(life);
+    const RefCkpt ckpt_before = decodeRefCkpt(backend->get(ckpt_key)->bytes);
+    backend->fail_cas_substr = ckpt_key;
+    backend->fail_cas_count = 200;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "y"); });
+
+    EXPECT_TRUE(backend->get(wedged_key)) << "the exact wedged log was proven durable";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery)
+        << "a durable log without a confirmed frontier must not return to Ready";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before)
+        << "the unfrontiered wedge must not be installed into the resident table";
+    EXPECT_EQ(decodeRefCkpt(backend->get(ckpt_key)->bytes), ckpt_before);
+}
+
+TEST(CASRefWedgeEveryAttempt, RetiredLifeRefusesWedgeRetryBeforeAnyRequestOrAdoption)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge-retired-before-retry"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+    const CatalogEntry predecessor = catalogEntryOrThrow(*backend, store->layout(), ns);
+    const NamespaceLifeId predecessor_life
+        = NamespaceLifeId::fromCatalogEntry(predecessor.ns, predecessor.incarnation);
+
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_FALSE(backend->get(wedged_key));
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool paused = false;
+    bool resume = false;
+    store->setWedgeBeforeSlotOccupyHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        paused = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return resume; });
+    });
+
+    std::exception_ptr retry_error;
+    std::thread retry([&]
+    {
+        try
+        {
+            store->dropRef(ns, "y");
+        }
+        catch (...)
+        {
+            retry_error = std::current_exception();
+        }
+    });
+    {
+        std::unique_lock lock(mutex);
+        cv.wait(lock, [&] { return paused; });
+    }
+
+    const CatalogEntry successor
+        = replaceCatalogLifeForWedgeRace(*backend, store->layout(), predecessor, UInt128{0x71f2});
+    const NamespaceLifeId successor_life
+        = NamespaceLifeId::fromCatalogEntry(successor.ns, successor.incarnation);
+    ASSERT_EQ(backend->putIfAbsent(store->layout().refCkptKey(successor_life), encodeRefCkpt(RefCkpt{
+        .life_epoch = store->liveWriterEpoch(),
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    store->invalidateRemovedCatalogLife(predecessor_life);
+    backend->resetCounts();
+
+    {
+        std::lock_guard lock(mutex);
+        resume = true;
+    }
+    cv.notify_all();
+    retry.join();
+    store->setWedgeBeforeSlotOccupyHookForTest(nullptr);
+
+    EXPECT_TRUE(retry_error);
+    EXPECT_EQ(backend->putCount(wedged_key), 0u) << "retirement must refuse before the retry send";
+    EXPECT_EQ(backend->getCount(wedged_key), 0u) << "a refused retry needs no occupant resolution read";
+    EXPECT_FALSE(backend->get(wedged_key)) << "the predecessor wedge was adopted or made durable";
+    EXPECT_NO_THROW((void)store->listRefs(ns));
+    ASSERT_TRUE(store->refTableLifeForTest(ns));
+    EXPECT_EQ(*store->refTableLifeForTest(ns), successor_life);
+}
+
+/// The other adoption input, and the one that proves the identity rule is about BYTES: our own
+/// earlier attempt DID land (only its ack, and the controller's own resolve read, were lost). The
+/// retry's create conflicts with our own object, the follow-up read returns bytes equal to the
+/// wedge's, and the transaction is adopted -- ONCE, not once per attempt.
+TEST(CASRefWedgeEveryAttempt, OwnLandedAttemptIsAdoptedFromOccupiedWithoutDoubleApply)
+{
+    auto backend = std::make_shared<LandedButAckLostOnceBackend>();
+    /// Disarmed while the fixture is built: the one-shot fault matches ANY key until a substring is
+    /// set, and the pool's own bootstrap PUT would otherwise consume it.
+    backend->fired = true;
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_occupied_mine"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `logPrefix` below matches
+    /// its fault at that key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->key_substr = logPrefix(store, ns);
+    backend->lose_resolve_read = true;
+    backend->fired = false;   /// armed: the next `_log/` PUT lands and loses its ack
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_TRUE(backend->get(wedged_key).has_value()) << "this fault LANDS the write; only the ack was lost";
+    ASSERT_TRUE(store->resolveRef(ns, "x").has_value()) << "durable, but not applied while wedged";
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+    const uint64_t puts_before = backend->putCount(wedged_key);
+
+    EXPECT_NO_THROW(store->dropRef(ns, "y"));
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_FALSE(store->resolveRef(ns, "x").has_value()) << "the landed transaction is adopted on resolution";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before + 2)
+        << "adopted exactly once: a double-apply would bump the tail twice for one transaction";
+    EXPECT_EQ(backend->putCount(wedged_key), puts_before + 1)
+        << "the resolution costs exactly ONE conditional create at the wedged key";
+}
+
+/// `ambiguous-then-definite`, the control the phase-0 model singles out: a definite refusal of a LATER
+/// attempt says nothing about the EARLIER ambiguous one, which may still be in flight. The lane must
+/// stay wedged -- unwedging here is how an acked-then-lost transaction gets written around.
+TEST(CASRefWedgeEveryAttempt, DefiniteRefusalOfARetryAttemptKeepsTheLaneWedged)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_ambiguous_then_definite"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    const RefTxnId wedged_id = store->layout().parseRefObjectKey(wedged_key)->txn_id;
+
+    /// The retry's own create is definitively refused.
+    backend->definite_substr = logPrefix(store, ns);
+    backend->definite_count = 1;
+    EXPECT_ANY_THROW(store->dropRef(ns, "y"));
+
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns)) << "a definite refusal AFTER an ambiguous attempt must not unwedge";
+    EXPECT_EQ(store->wedgedKeyForTest(ns), wedged_key) << "the SAME wedge, not a fresh one";
+    EXPECT_TRUE(store->resolveRef(ns, "x").has_value()) << "nothing was adopted";
+    EXPECT_FALSE(backend->get(wedged_key).has_value()) << "and nothing became durable";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Wedged)
+        << "a wedged lane's steady state is 'may be durable, not applied'";
+
+    /// Still the same id afterwards: the definite refusal consumed nothing.
+    backend->definite_count = 0;
+    EXPECT_NO_THROW(store->dropRef(ns, "y"));
+    EXPECT_EQ(store->layout().parseRefObjectKey(
+        store->layout().refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), wedged_id))->txn_id, wedged_id);
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "the create-based resolution still settles it afterwards";
+}
+
+/// The SAME rule one level down, and the level where it was actually broken. The test above splits the
+/// two attempts across two CALLS, which the wedge already handles. Inside ONE call the controller used
+/// to report the LAST attempt's outcome: an ambiguous attempt followed by a definitively refused reissue
+/// came back `DefiniteFailure` -- the verdict that means "the key is provably unwritten". It is not. The
+/// refusal proves only that the SECOND request never applied; the first may still be in flight and may
+/// still land, and `unresolvedProvesNothingWasSent` is false for exactly that reason. So the CALL is
+/// unresolved, and a definite verdict is only ever the whole call's.
+///
+/// The new reason lands on the fail-close side of the predicate the ledger acts on. Asserted at compile
+/// time, beside the behaviour, because a member added to the enum without classifying it is precisely
+/// how the wedge would silently stop happening.
+static_assert(!unresolvedProvesNothingWasSent(CasUnresolvedReason::DefiniteFailureAfterAmbiguity));
+
+TEST(CASRefWedgeEveryAttempt, ADefiniteRefusalCannotSpeakForAnEarlierAmbiguousAttemptOfTheSameCall)
+{
+#if !USE_AWS_S3
+    GTEST_SKIP() << "DefiniteFailure classification requires S3 error types (USE_AWS_S3 off)";
+#else
+    auto backend = std::make_shared<WedgeTestBackend>();
+    CasRequestController controller(backend, twoAttemptBudget());
+    const std::function<bool()> fence_ok = [] { return true; };
+
+    /// One call, two attempts: ambiguous, then definitively refused.
+    backend->ambiguous_substr = "key/";
+    backend->ambiguous_count = 1;
+    backend->s3_definite_substr = "key/";
+    backend->s3_definite_count = 1;
+
+    CasUnresolvedReason reason = CasUnresolvedReason::NotUnresolved;
+    const CasWriteOutcome outcome =
+        controller.putIfAbsentControlled("key/haunted", "bytes", fence_ok, /*out_token=*/nullptr, &reason);
+
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved)
+        << "a definite refusal of the SECOND attempt cannot retire the first attempt's ambiguity";
+    EXPECT_EQ(reason, CasUnresolvedReason::DefiniteFailureAfterAmbiguity);
+    EXPECT_FALSE(unresolvedProvesNothingWasSent(reason))
+        << "the caller must keep protecting itself: an earlier attempt was sent and may yet land";
+    EXPECT_FALSE(backend->get("key/haunted").has_value())
+        << "and the key is still empty -- which is exactly why an absent read settles nothing";
+
+    /// THE CONTROL. Aggregation must not soften a definite refusal that speaks for the whole call: with
+    /// no ambiguous predecessor, the first attempt's whitelisted rejection is still `DefiniteFailure`,
+    /// and the ledger may still free the id on it.
+    backend->s3_definite_count = 1;
+    CasUnresolvedReason clean_reason = CasUnresolvedReason::NotUnresolved;
+    EXPECT_EQ(controller.putIfAbsentControlled("key/clean", "bytes", fence_ok, /*out_token=*/nullptr, &clean_reason),
+              CasWriteOutcome::DefiniteFailure);
+    EXPECT_EQ(clean_reason, CasUnresolvedReason::NotUnresolved);
+#endif
+}
+
+/// The ledger-side twin of the same call: what the append lane does with that verdict. On
+/// `DefiniteFailure` it returns the lane to `Ready` and tells callers the txn id was never used,
+/// so the next append re-derives that id -- which, with an earlier attempt still possibly in flight, is
+/// how an acked-then-lost transaction gets written around. The lane must wedge instead and stay pending
+/// until the key itself resolves.
+TEST(CASRefWedgeEveryAttempt, ADefiniteRefusalAfterAnAmbiguousAttemptOfTheSameCallStillWedgesTheLane)
+{
+#if !USE_AWS_S3
+    GTEST_SKIP() << "DefiniteFailure classification requires S3 error types (USE_AWS_S3 off)";
+#else
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, twoAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_one_call_ambiguous_then_definite"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `logPrefix` below matches
+    /// its fault at that key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    backend->s3_definite_substr = logPrefix(store, ns);
+    backend->s3_definite_count = 1;
+
+    const uint64_t wedged_before = ProfileEvents::global_counters[ProfileEvents::CASRefAppendWedged].load();
+    const uint64_t definite_before = ProfileEvents::global_counters[ProfileEvents::CASRefAppendDefiniteFailure].load();
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns))
+        << "one call whose first attempt is unresolved leaves an object that may become durable";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Wedged)
+        << "the id must NOT be declared never-used: the marker stands until the key itself resolves";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefAppendDefiniteFailure].load(), definite_before)
+        << "this append was never definitively rejected -- only one of its attempts was";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefAppendWedged].load(), wedged_before + 1);
+    EXPECT_TRUE(store->resolveRef(ns, "x").has_value()) << "nothing is applied while the lane is wedged";
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    EXPECT_FALSE(backend->get(wedged_key).has_value()) << "and nothing became durable";
+
+    /// And it still recovers by the ordinary route: the next flush's bounded create lands the wedged
+    /// transaction and adopts it, so wedging costs availability only until the next caller arrives.
+    EXPECT_NO_THROW(store->dropRef(ns, "y"));
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_FALSE(store->resolveRef(ns, "x").has_value()) << "the adopted wedge applied its drop";
+    EXPECT_FALSE(store->resolveRef(ns, "y").has_value());
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready);
+#endif
+}
+
+/// ===================================================================================
+/// A successor's `EpochSeal` is the conclusive rejection (spec INV-2)
+/// ===================================================================================
+
+/// The seal is the ONLY thing that can prove our transaction will never be durable: the key is
+/// write-once and a successor put its epoch-closing record there. The operation was never acked, so
+/// its callers get a permanent error; the wedge is cleared; and the seal becomes this namespace's
+/// `prev_epoch_seal`, which the first append of the NEXT epoch carries on the wire.
+TEST(CASRefWedgeEveryAttempt, SuccessorSealAtTheWedgedKeyRejectsConclusivelyAndSourcesPrevEpochSeal)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/wedge_sealed"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    const uint64_t epoch = store->liveWriterEpoch();
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    const RefTxnId seal_id = layout.parseRefObjectKey(wedged_key)->txn_id;
+    ASSERT_EQ(seal_id.writer_epoch, epoch);
+    ASSERT_GT(seal_id.ref_sequence, 1u) << "this namespace already has records, so its seal is not at sequence 1";
+    ASSERT_EQ(store->lastEpochSealForTest(ns), std::nullopt) << "nothing has closed an epoch for this namespace yet";
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+
+    /// A successor closes our epoch at exactly the slot our attempt was aiming at.
+    ASSERT_EQ(backend->putAsSuccessor(wedged_key, epochSealBytes(ns, seal_id)).outcome, PutOutcome::Done);
+
+    /// The next caller's resolution meets the seal. Its own items fail -- permanently, not "retry
+    /// later": nothing about this lane's epoch will ever accept a write again.
+    expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->dropRef(ns, "y"); });
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "a conclusive rejection clears the wedge";
+    EXPECT_TRUE(store->resolveRef(ns, "x").has_value()) << "the rejected transaction was never applied";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before) << "and never joined the tail";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Closed)
+        << "the successor seal closes this epoch's lane";
+    ASSERT_EQ(store->lastEpochSealForTest(ns), std::make_optional(seal_id))
+        << "the observed seal is this namespace's epoch-closing record";
+
+    /// INV-2's fence, stated as behaviour: a dying lane that observed the seal keeps deriving the SAME
+    /// `T+1` and keeps colliding with it -- it never mints `T+2` and writes its stream past the record
+    /// that closed its epoch. Ids are state-derived, so this falls out rather than being enforced.
+    ///
+    /// And the collision is adjudicated as the CONCLUSIVE REJECTION it is, not as foreign interference:
+    /// this is the designed path, so it must not fence the mount or raise an anomaly. The append site
+    /// reads the occupant and tells a seal of this namespace from a genuine breach, exactly as the
+    /// wedge-resolve site does.
+    const uint64_t remounts_before = store->scheduleRemountCallCountForTest();
+    expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->dropRef(ns, "y"); });
+    EXPECT_EQ(backend->get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{epoch, seal_id.ref_sequence + 1})), std::nullopt)
+        << "nothing of ours may exist above the seal in the closed epoch";
+    EXPECT_TRUE(store->mayMutate()) << "meeting a successor's seal is the protocol working, not an anomaly";
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), remounts_before)
+        << "and must not schedule a remount";
+
+    /// Merely changing the epoch counters does not reopen a cached runtime. Its immutable admitted
+    /// generation is stale, so the outer retry-safe fence refusal wins before the still-Closed lane is
+    /// consulted. Production reaches a new epoch through remount, which replaces the runtime and
+    /// recovers its chain link.
+    bumpFenceGeneration(store, epoch + 1);
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "y"); });
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Closed);
+}
+
+/// The wire round trip of the same rule, driven from the OTHER producer of `last_epoch_seal`:
+/// recovery's CAS-walk (Task 6), stood in for here by its test seam. The point is the encode call
+/// site, which is this task's.
+TEST(CASRefWedgeEveryAttempt, OrdinaryFirstAppendAfterASealedTransitionCarriesTheExactPrevEpochSeal)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/prev_epoch_seal_roundtrip"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `readRefLogTxn` above
+    /// reads that exact key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+
+    const uint64_t epoch = store->liveWriterEpoch();
+    publishEmptyPart(store, ns, "x");
+    const RefTxnId seal_id{epoch, 42};
+
+    /// A recovery that walked the dead epoch installs the seal it wrote; model its later epoch without
+    /// moving the mount-fence generation. This test is about the wire link, not runtime supersession.
+    store->setLastEpochSealForTest(ns, seal_id);
+    store->setLiveWriterEpochForTest(epoch + 1);
+
+    EXPECT_NO_THROW(store->dropRef(ns, "x"));
+
+    const RefLogTxn written = readRefLogTxn(*backend, layout, ns, RefTxnId{epoch + 1, 1});
+    EXPECT_EQ(written.prev_epoch_seal, std::make_optional(seal_id));
+
+    /// And it is carried on sequence 1 ONLY: the next transaction of the same epoch must not repeat it.
+    publishEmptyPart(store, ns, "z");
+    const RefLogTxn second = readRefLogTxn(*backend, layout, ns, RefTxnId{epoch + 1, 2});
+    EXPECT_EQ(second.prev_epoch_seal, std::nullopt)
+        << "prev_epoch_seal is required on sequence 1 of a non-genesis epoch and forbidden everywhere else";
+}
+
+/// GENESIS: `last_epoch_seal` is `nullopt` exactly for a namespace whose stream starts here, and a
+/// genesis birth carries NO `prev_epoch_seal` even though its epoch is far above 1. Nothing about the
+/// global epoch number makes a namespace non-genesis -- only a transition of its OWN stream does.
+TEST(CASRefWedgeEveryAttempt, GenesisBirthAtAHighEpochCarriesNoPrevEpochSeal)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/genesis_at_five"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `readRefLogTxn` above
+    /// reads that exact key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+
+    bumpFenceGeneration(store, 5);
+    ASSERT_EQ(store->liveWriterEpoch(), 5u);
+
+    publishEmptyPart(store, ns, "x");
+
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::nullopt)
+        << "a namespace with no recovered seal, whose greatest applied id is at its own life epoch, is genesis";
+    const RefLogTxn birth = readRefLogTxn(*backend, layout, ns, RefTxnId{5, 1});
+    EXPECT_EQ(birth.prev_epoch_seal, std::nullopt) << "a genesis stream opens; it does not continue one";
+}
+
+/// ===================================================================================
+/// A foreign occupant is impossible, so it is loud -- and the mount self-heals
+/// ===================================================================================
+
+/// Under mount-lease exclusivity the wedged key is exclusively ours, so a foreign non-seal object at
+/// it is corruption or a protocol breach. Fail closed with `CORRUPTED_DATA`, KEEP the wedge for
+/// inspection, and route the anomaly so the mount remounts itself rather than staying stuck until
+/// someone notices.
+TEST(CASRefWedgeEveryAttempt, ForeignNonSealOccupantIsCorruptedDataAndSchedulesARemount)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_foreign"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `logPrefix` below matches
+    /// its fault at that key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    const uint64_t remounts_before = store->scheduleRemountCallCountForTest();
+
+    /// Something that is neither our bytes nor a seal occupies the slot.
+    ASSERT_EQ(backend->putAsSuccessor(wedged_key, "not a ref-log object at all").outcome, PutOutcome::Done);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "y"); });
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Faulted);
+    EXPECT_GT(store->scheduleRemountCallCountForTest(), remounts_before)
+        << "the impossible-interference route must schedule a remount";
+}
+
+/// I5, the OTHER site with the same shape: the ordinary append's own conditional create can prove a
+/// different object sits at the id it derived. Task 3 made that fail closed -- correctly -- but it
+/// left the mount stuck there until a manual remount, unlike the wedge-resolution site. Both are the
+/// same impossibility and both must self-heal by remount.
+TEST(CASRefWedgeEveryAttempt, AppendSiteProvenDifferentObjectAlsoSchedulesARemount)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/append_site_foreign"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    publishEmptyPart(store, ns, "x");
+
+    /// Occupy the id the next append will derive with a foreign object, so its create conflicts and
+    /// the controller's resolve-before-reissue proves the occupant is not ours.
+    const RefTxnId next{store->liveWriterEpoch(), 3};
+    ASSERT_EQ(backend->putIfAbsent(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), next),
+                                   "a different object entirely").outcome, PutOutcome::Done);
+    const uint64_t remounts_before = store->scheduleRemountCallCountForTest();
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "x"); });
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "a proven different object is conclusive, never a wedge";
+    EXPECT_GT(store->scheduleRemountCallCountForTest(), remounts_before)
+        << "the append site must route through the same impossible-interference reaction as the wedge site";
+}
+
+/// ===================================================================================
+/// The admission fence
+/// ===================================================================================
+
+/// The old-generation-retry-inert rule. A wedge admitted under one mount incarnation may not send an
+/// attempt under another: the retry is refused BEFORE anything reaches the store, so the key is
+/// provably untouched and the wedge is intact for whoever recovers the lane properly.
+TEST(CASRefWedgeEveryAttempt, RetryUnderAnOlderAdmissionGenerationSendsNothing)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_old_generation"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `logPrefix` below matches
+    /// its fault at that key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    const uint64_t epoch = store->liveWriterEpoch();
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_EQ(store->wedgedAdmittedGenerationForTest(ns), store->fenceGeneration())
+        << "the wedge records the generation it was admitted under";
+    const uint64_t puts_before = backend->putCount(wedged_key);
+
+    /// The lease incarnation moves under the wedge; the mount is writable again, but not the same one.
+    bumpFenceGeneration(store, epoch);
+    ASSERT_NE(store->wedgedAdmittedGenerationForTest(ns), store->fenceGeneration());
+
+    EXPECT_ANY_THROW(store->dropRef(ns, "y"));
+
+    EXPECT_EQ(backend->putCount(wedged_key), puts_before)
+        << "the retry must be refused pre-attempt: nothing may reach the store under a foreign generation";
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns)) << "and the wedge is untouched";
+    EXPECT_FALSE(backend->get(wedged_key).has_value());
+}
+
+/// The post-I/O recheck, deterministically. The retry's create is parked mid-flight; while it is
+/// parked the fence is lost and re-armed AND a successor seals the slot. The released result is a
+/// perfectly real `Occupied`(seal) -- but it belongs to an incarnation that no longer exists, so this
+/// runtime must act on NOTHING: no acknowledgement, no unwedge, no install, and no adoption of the
+/// seal it just read.
+TEST(CASRefWedgeEveryAttempt, ResultReleasedAfterAFenceBumpAndSuccessorSealIsInert)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/wedge_blocked_io"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `logPrefix` below matches
+    /// its fault at that key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    const uint64_t epoch = store->liveWriterEpoch();
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    const RefTxnId seal_id = layout.parseRefObjectKey(wedged_key)->txn_id;
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+
+    backend->armBlock(logPrefix(store, ns));
+    std::exception_ptr caller_error;
+    std::thread resolver([&]
+    {
+        try { store->dropRef(ns, "y"); }
+        catch (...) { caller_error = std::current_exception(); }
+    });
+    backend->awaitBlockEntered();
+
+    /// Everything that makes this runtime superseded happens INSIDE the I/O window.
+    ASSERT_EQ(backend->putAsSuccessor(wedged_key, epochSealBytes(ns, seal_id)).outcome, PutOutcome::Done);
+    bumpFenceGeneration(store, epoch + 1);
+    backend->releaseBlock();
+    resolver.join();
+
+    ASSERT_TRUE(caller_error != nullptr) << "no acknowledgement: the caller must not be told this succeeded";
+    /// And it must be the RETRY-SAFE class. A moved incarnation is usually a routine lease blip, and the
+    /// storage layer classifies retry-safety on exactly `ABORTED || NETWORK_ERROR` — surfacing the fence
+    /// check's own `INVALID_STATE` here would turn every blip into a hard failure for the caller.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { std::rethrow_exception(caller_error); });
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns)) << "no unwedge";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before) << "no install";
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::nullopt)
+        << "and no adoption of the seal a superseded runtime happened to read";
+}
+
+/// The same recheck, on the identity leg rather than the generation leg. The fence never moves; only
+/// the installed wedge's BYTES change while the create is parked. Generation-equality alone would let
+/// the released result install a candidate built from the OTHER attempt's transaction -- the aliasing
+/// bug the phase-0 model found, which is why identity is (generation, id, bytes) and not any one of
+/// them. Production cannot reach this (one leader per table mutates a lane), so this is a white-box
+/// guard on the rule, driven through the force-wedge seam.
+TEST(CASRefWedgeEveryAttempt, ResultReleasedAfterTheWedgeIdentityChangedIsInert)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_identity_changed"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `logPrefix` below matches
+    /// its fault at that key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    const RefTxnId wedged_id = store->layout().parseRefObjectKey(wedged_key)->txn_id;
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+
+    backend->armBlock(logPrefix(store, ns));
+    std::exception_ptr caller_error;
+    std::thread resolver([&]
+    {
+        try { store->dropRef(ns, "y"); }
+        catch (...) { caller_error = std::current_exception(); }
+    });
+    backend->awaitBlockEntered();
+
+    /// Same id, same generation, DIFFERENT bytes.
+    store->forceWedgeForTest(ns, wedged_id.writer_epoch, wedged_id.ref_sequence, wedged_key, "different attempt bytes");
+    backend->releaseBlock();
+    resolver.join();
+
+    ASSERT_TRUE(caller_error != nullptr);
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns)) << "no unwedge";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before)
+        << "no install: the released result described a wedge that is no longer installed";
+    EXPECT_TRUE(store->resolveRef(ns, "x").has_value());
+}
+
+/// A resolution that proves the exact attempt durable but cannot install it has one successor:
+/// `NeedsRecovery`. It drops the attempt and forbids another write until replay catches the cache up.
+TEST(CASRefWedgeEveryAttempt, KnownDurableInstallFailureMovesDirectlyToRecovery)
+{
+    auto backend = std::make_shared<LandedButAckLostOnceBackend>();
+    backend->fired = true;   /// disarmed while the fixture is built (see the adoption test above)
+    auto store = openPool(backend, singleAttemptBudget());
+    const RootNamespace ns{"srv1/wedge_floor"};
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `logPrefix` below matches
+    /// its fault at that key.
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->key_substr = logPrefix(store, ns);
+    backend->lose_resolve_read = true;
+    backend->fired = false;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_TRUE(backend->get(wedged_key).has_value()) << "the wedged transaction is durable";
+    /// The adoption reaches its install region and the install throws.
+    armOneShotInstallFailure(store);
+    expectThrowsCode(DB::ErrorCodes::MEMORY_LIMIT_EXCEEDED, [&] { store->dropRef(ns, "y"); });
+    store->setInstallRegionProbeForTest(nullptr);
+    ASSERT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    ASSERT_FALSE(store->refLaneWedgedForTest(ns))
+        << "known durability transfers ownership to recovery; no uncertain attempt remains";
+    /// Do not call the tail-count seam here: it intentionally forces recovery, which is the transition
+    /// this assertion is proving has not happened yet.
+
+    /// The next flush first replays the durable drop of `x`, then admits the drop of `y`.
+    EXPECT_NO_THROW(store->dropRef(ns, "y"));
+    EXPECT_FALSE(store->resolveRef(ns, "x").has_value())
+        << "replay must install the already-durable drop of `x` before the next write";
+    EXPECT_FALSE(store->resolveRef(ns, "y").has_value());
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Ready)
+        << "only completed replay returns the lane to Ready";
+}
+
+/// ===================================================================================
+/// The append site owes the SAME three-way adjudication as the wedge site
+/// ===================================================================================
+
+/// No wedge is involved here at all: an ordinary append derives its next id and finds a successor's
+/// epoch seal sitting on it. That is not interference — it is INV-2's designed outcome for a lane that
+/// has been deposed without being told, and the lane will keep re-deriving that same id forever. So it
+/// must be adjudicated as the conclusive rejection it is: a permanent error for the callers, the seal
+/// recorded as this namespace's epoch-closing record, and NO fence and NO remount.
+TEST(CASRefWedgeEveryAttempt, AppendSiteMeetingASuccessorSealIsAConclusiveRejectionNotInterference)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/append_site_seal"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    publishEmptyPart(store, ns, "x");
+
+    const uint64_t epoch = store->liveWriterEpoch();
+    const RefTxnId next{epoch, 3};
+    ASSERT_EQ(store->lastEpochSealForTest(ns), std::nullopt);
+    const uint64_t remounts_before = store->scheduleRemountCallCountForTest();
+    const uint64_t sealed_before = ProfileEvents::global_counters[ProfileEvents::CASRefAppendSealRejected].load();
+
+    /// The successor's seal lands at exactly the id this table's next append derives.
+    backend->conflict_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), next);
+    backend->conflict_bytes = epochSealBytes(ns, next);
+    backend->conflict_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->dropRef(ns, "x"); });
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "a conclusive rejection is not an uncertain outcome";
+    EXPECT_TRUE(store->resolveRef(ns, "x").has_value()) << "the rejected transaction never applied";
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::make_optional(next))
+        << "the observed seal IS this namespace's epoch-closing record, whichever site observed it";
+    EXPECT_TRUE(store->mayMutate()) << "the designed path must not fence the mount";
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), remounts_before) << "nor schedule a remount";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefAppendSealRejected].load(), sealed_before + 1)
+        << "a deposed writer must still be COUNTED: this is the protocol working, and also the signal "
+           "that this mount has lost its lease and does not know it";
+}
+
+/// [CKPT-FAILED-BIRTH-DEBRIS] REVERSED (increment review Critical B; BACKLOG `{#ckpt-failed-birth-debris}`
+/// reopened, `{#ckpt-neverborn-gc-backstop}` filed). This test used to pin the OPPOSITE of what it now
+/// asserts: Task 3's `cleanupOrphanedBirthCkptBestEffort` deleted `_ckpt` here by a FRESH `head()` read
+/// at cleanup time, not a token captured from this attempt's own publish, and every branch that called it
+/// -- this one included -- had just PROVEN a different object occupies the derived key, directly
+/// contradicting the "reachable only while the ref-log has never durably held anything" argument that
+/// made the delete look safe. A successor that legitimately owns the same live incarnation (an ordinary
+/// INV-2 epoch-seal handoff, e.g. after a remount) may already have read this SAME `_ckpt` for its own
+/// recovery before this cleanup could run, and the delete could destroy the one genesis record
+/// (`life_epoch`) that successor's own future recovery still needs, with no way to tell that case apart
+/// from ordinary debris at cleanup time. The cleanup was removed entirely rather than patched (see
+/// `CasRefLedger.cpp`'s comment at the removed call sites for why a captured token does not close the
+/// gap either). The trade, named rather than hidden: a creation `_ckpt` whose first ref-log
+/// `NamespaceBirth` is conclusively rejected now SURVIVES -- a drained server root carrying it will refuse
+/// decommission (`claimOwnerOrThrow` -> `CORRUPTED_DATA`) until `{#ckpt-neverborn-gc-backstop}` lands --
+/// which is the right side of the trade against an unrecoverable delete of a live successor's only
+/// genesis record.
+TEST(CASRefWedgeEveryAttempt, CreationCkptSurvivesAConclusiveFirstRefLogRejection)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/birth_ckpt_debris"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+
+    const RefTxnId genesis{store->liveWriterEpoch(), 1};
+    const String ckpt_key = layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns));
+    const auto ckpt_before = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_before.has_value()) << "the fixture's creation checkpoint must exist before the first ref-log attempt";
+
+    /// A successor's epoch seal lands at exactly the id this first `NamespaceBirth` transaction derives.
+    backend->conflict_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), genesis);
+    backend->conflict_bytes = epochSealBytes(ns, genesis);
+    backend->conflict_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { publishEmptyPart(store, ns, "x"); });
+
+    const auto ckpt_after = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_after.has_value());
+    EXPECT_EQ(ckpt_after->bytes, ckpt_before->bytes)
+        << "the creation checkpoint must survive a conclusively rejected first ref-log PUT unchanged";
+}
+
+/// A creation `_ckpt` belonging to a `Live` namespace must survive no matter how a LATER transaction
+/// on that same namespace fails -- true unconditionally now that increment review Critical B removed
+/// the only code that ever deleted `_ckpt` on this path at all, but kept as its own pin: the fixture
+/// has already made `ns` `Live`; then two initial ref-log chunks (precommit-add, then promote) and a
+/// THIRD chunk meet the
+/// identical successor-seal conflict the test above exercises -- same conclusive rejection -- and the
+/// creation `_ckpt` must survive it byte-for-byte. `_ckpt` has no repair
+/// path (BACKLOG `{#ckpt-damage-no-repair-path}`), so this is the row that would catch a future
+/// reintroduction of the removed cleanup landing back on an already-Live namespace's `_ckpt`.
+TEST(CASRefWedgeEveryAttempt, CreationCkptSurvivesALaterConclusiveRejection)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/birth_ckpt_survives_live"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    /// ONE `publishEmptyPart` reaches sequence 2 (the precommit-add chunk at seq 1 carries the first
+    /// `NamespaceBirth`, the promote chunk lands at seq 2), so `next`
+    /// below is the SAME `{epoch, 3}` the sibling `AppendSiteMeetingASuccessorSealIsAConclusiveRejectionNotInterference`
+    /// test derives from the identical one-call setup -- copying THAT test's two-call variant here
+    /// (from a different test in this file) would derive a different id and never trigger the conflict.
+    publishEmptyPart(store, ns, "x");
+
+    const String ckpt_key = layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns));
+    const auto ckpt_before = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_before.has_value()) << "the fixture's creation step must have published a real _ckpt";
+
+    const RefTxnId next{store->liveWriterEpoch(), 3};
+    backend->conflict_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), next);
+    backend->conflict_bytes = epochSealBytes(ns, next);
+    backend->conflict_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->dropRef(ns, "x"); });
+
+    const auto ckpt_after = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_after.has_value()) << "a Live namespace's _ckpt must never be deleted by this path";
+    EXPECT_EQ(ckpt_after->bytes, ckpt_before->bytes)
+        << "not merely present but UNCHANGED -- no code anywhere on this path deletes _ckpt any more "
+           "(increment review Critical B), so it must not have been touched at all";
+}
+
+/// THE OTHER negative row (review C2): the AMBIGUOUS branch -- `Writing` -> `Wedged` -- is deliberately
+/// EXCLUDED from the cleanup call (see the lambda's own comment), and that exclusion is the
+/// load-bearing half of the whole safety story: it is the one branch where the ref-log bytes MIGHT
+/// still have landed. Nothing pinned that exclusion before this row; an edit that added the call here
+/// would be caught by no test. A one-shot ambiguous PUT on the first ref-log `NamespaceBirth`
+/// (`prepared->birth_contribution` set) writes NOTHING (the response is lost, the key
+/// stays absent) and wedges the lane -- `AmbiguousPutWedgesTheLaneAndTheNextFlushsCreateAdoptsItExactlyOnce`
+/// is the precedent this mirrors, adapted to a namespace's FIRST-ever transaction instead of its third.
+TEST(CASRefWedgeEveryAttempt, CreationCkptSurvivesWhenTheFirstNamespaceBirthIsAmbiguous)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend, singleAttemptBudget());
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/birth_ckpt_ambiguous"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    const String ckpt_key = layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns));
+    const auto ckpt_before = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_before.has_value()) << "the fixture's creation checkpoint must exist before the first ref-log attempt";
+
+    backend->ambiguous_substr = logPrefix(store, ns);
+    backend->ambiguous_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { publishEmptyPart(store, ns, "x"); });
+
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns)) << "an ambiguous outcome must WEDGE the lane, not "
+                                                     "resolve into one of the conclusive branches";
+    const auto ckpt_after = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_after.has_value());
+    EXPECT_EQ(ckpt_after->bytes, ckpt_before->bytes)
+        << "the creation checkpoint must survive an ambiguous first ref-log outcome unchanged";
+}
+
+/// Final review F5: the two other removed call sites, given their own first-`NamespaceBirth` survival rows.
+/// The reversed test above pins the `SuccessorSeal` branch; the sibling below it pins the ambiguous
+/// branch (never called it in the first place). The remaining two -- occupant-unreadable
+/// (`CORRUPTED_DATA` from a failed adjudication read) and genuine foreign interference -- had no
+/// first-`NamespaceBirth` row at all: `AppendSiteFaultsWhenTheOccupantCannotBeRead`,
+/// `ForeignNonSealOccupantIsCorruptedDataAndSchedulesARemount`, and `WellFormedNonSealOccupantIsStillForeign`
+/// all `publishEmptyPart` FIRST, so none of them ever carries a `birth_contribution` -- a reinstated
+/// GUARDED cleanup at either of these two sites would pass the whole suite with no first-transaction case to
+/// catch it. Mirrors `WellFormedNonSealOccupantIsStillForeign`'s occupant shape (a decodable, well-formed
+/// NON-seal transaction at the derived key), moved to sequence 1 of a namespace with no prior ref-log
+/// transaction, so this attempt's own PUT is its first.
+TEST(CASRefWedgeEveryAttempt, CreationCkptSurvivesWhenTheFirstNamespaceBirthOccupantCannotBeRead)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/birth_ckpt_occupant_unreadable"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+
+    const RefTxnId genesis{store->liveWriterEpoch(), 1};
+    const String ckpt_key = layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns));
+    const auto ckpt_before = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_before.has_value()) << "the fixture's creation checkpoint must exist before the first ref-log attempt";
+
+    backend->conflict_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), genesis);
+    backend->conflict_bytes = epochSealBytes(ns, genesis);
+    backend->conflict_count = 1;
+    /// Proper birth makes recovery first probe this absent log key. Skip that probe and the resolve
+    /// read that PROVES the conflict; fail only the adjudication read after it, so the occupant's
+    /// identity (seal vs. breach) cannot be determined.
+    backend->fail_get_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), genesis);
+    backend->fail_get_skip = 2;
+    backend->fail_get_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { publishEmptyPart(store, ns, "x"); });
+
+    const auto ckpt_after = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_after.has_value());
+    EXPECT_EQ(ckpt_after->bytes, ckpt_before->bytes)
+        << "the creation checkpoint must survive an occupant-unreadable first ref-log outcome unchanged";
+}
+
+/// The other former call site: a genuine breach of write-exclusivity at the first `NamespaceBirth`
+/// id, mirroring `WellFormedNonSealOccupantIsStillForeign`'s occupant shape but with no prior ref-log publish.
+TEST(CASRefWedgeEveryAttempt, CreationCkptSurvivesFirstNamespaceBirthForeignInterference)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/birth_ckpt_foreign_interference"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);
+
+    const RefTxnId genesis{store->liveWriterEpoch(), 1};
+    const String ckpt_key = layout.refCkptKey(DB::Cas::tests::fixture::fixtureLife(ns));
+    const auto ckpt_before = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_before.has_value()) << "the fixture's creation checkpoint must exist before the first ref-log attempt";
+
+    /// A perfectly decodable transaction for this exact namespace and id -- just not an epoch seal, and
+    /// not this attempt's own birth.
+    RefOp birth;
+    birth.kind = RefOpKind::NamespaceBirth;
+    const RefLogTxn foreign_txn{ns.string(), genesis, {birth}, std::nullopt};
+    backend->conflict_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), genesis);
+    backend->conflict_bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(foreign_txn));
+    backend->conflict_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { publishEmptyPart(store, ns, "x"); });
+
+    const auto ckpt_after = backend->get(ckpt_key);
+    ASSERT_TRUE(ckpt_after.has_value());
+    EXPECT_EQ(ckpt_after->bytes, ckpt_before->bytes)
+        << "the creation checkpoint must survive a foreign-interference first ref-log outcome unchanged";
+}
+
+/// The same conflict, but the read that would tell a seal from a breach fails. We must then decide
+/// NEITHER: fencing the mount would be a guess, and reporting a conclusive rejection would acknowledge
+/// a deposition nobody observed. The id is not consumed, so the next attempt re-derives it and
+/// classifies again — deferring costs one round trip and decides nothing wrongly.
+TEST(CASRefWedgeEveryAttempt, AppendSiteFaultsWhenTheOccupantCannotBeRead)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/append_site_unreadable"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    publishEmptyPart(store, ns, "x");
+
+    const RefTxnId next{store->liveWriterEpoch(), 3};
+    const uint64_t remounts_before = store->scheduleRemountCallCountForTest();
+
+    backend->conflict_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), next);
+    backend->conflict_bytes = epochSealBytes(ns, next);
+    backend->conflict_count = 1;
+    /// Skip the resolve read that PROVES the conflict; fail only the adjudication read after it.
+    backend->fail_get_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), next);
+    backend->fail_get_skip = 1;
+    backend->fail_get_count = 1;
+
+    const uint64_t deferred_before = ProfileEvents::global_counters[ProfileEvents::CASRefAppendOccupantUnreadable].load();
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "x"); });
+
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefAppendOccupantUnreadable].load(), deferred_before + 1)
+        << "the deferral is the one quiet arm here -- it must be counted or a starved loud path is invisible";
+    EXPECT_TRUE(store->mayMutate()) << "the table faults without guessing that the whole mount is corrupt";
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), remounts_before) << "nor schedule a remount";
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::nullopt)
+        << "nor record a deposition that was never actually observed";
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "nothing of ours became durable, so nothing is wedged";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Faulted);
+    expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->dropRef(ns, "x"); });
+}
+
+/// A WELL-FORMED ref-log transaction of this namespace at this id, which simply is not a seal, must be
+/// adjudicated `Foreign` on CONTENT — not because it failed to decode. The sibling test above reaches
+/// the same verdict through an undecodable body, so without this one the classifier could be deciding
+/// "foreign" purely from decode failures and nothing would notice.
+TEST(CASRefWedgeEveryAttempt, WellFormedNonSealOccupantIsStillForeign)
+{
+    auto backend = std::make_shared<WedgeTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/append_site_wellformed_foreign"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    publishEmptyPart(store, ns, "x");
+
+    const RefTxnId next{store->liveWriterEpoch(), 3};
+    const uint64_t remounts_before = store->scheduleRemountCallCountForTest();
+
+    /// A perfectly decodable transaction for this exact namespace and id — just not an epoch seal.
+    RefOp birth;
+    birth.kind = RefOpKind::NamespaceBirth;
+    const RefLogTxn foreign_txn{ns.string(), next, {birth}, std::nullopt};
+    backend->conflict_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), next);
+    backend->conflict_bytes = sealObject(FormatId::RefLog, encodeRefLogTxn(foreign_txn));
+    backend->conflict_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "x"); });
+
+    EXPECT_GT(store->scheduleRemountCallCountForTest(), remounts_before)
+        << "a well-formed non-seal occupant is still a breach of write-exclusivity";
+    EXPECT_EQ(store->lastEpochSealForTest(ns), std::nullopt) << "and is emphatically not an epoch seal";
+}
+
+/// The deposed-lane self-pointer. A successor that seals an EMPTY epoch writes its record at sequence 1
+/// of that epoch, and a lane still live there re-derives exactly that id. Stamping the seal as its own
+/// `prev_epoch_seal` would be a self-pointer, which the structural grammar (strictly-less by
+/// construction) refuses at ENCODE — so the lane would fail with a self-inflicted `CORRUPTED_DATA` on
+/// every attempt and never reach the seal collision that is supposed to fence it. The stamp is
+/// therefore conditioned on the seal's epoch being strictly BELOW the id's.
+TEST(CASRefWedgeEveryAttempt, ALiveEpochSealIsNeverStampedAsItsOwnPrevEpochSeal)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/live_epoch_seal"};
+    admitProperlyBornEntry(*backend, store->layout(), ns);   /// Stage B (Task 4-C): pin to the sentinel before the first real touch
+    publishEmptyPart(store, ns, "x");
+
+    const uint64_t epoch = store->liveWriterEpoch();
+    /// Keep this a local wire/encoder test: moving the mount-fence generation would correctly make the
+    /// immutable runtime stale before the self-pointer guard was reached.
+    store->setLiveWriterEpochForTest(epoch + 1);
+    /// A seal of the LIVE epoch — the deposed-lane shape the wedge rejection arm can record.
+    store->setLastEpochSealForTest(ns, RefTxnId{epoch + 1, 7});
+
+    /// The lane now holds NOTHING it can legally write. Its next id is sequence 1 of the new epoch, which
+    /// owes a link to the seal that closed the epoch BELOW -- and the only seal it has is of the epoch it
+    /// is trying to open. Stamping that one would be a self-pointer the ENCODER refuses; stamping nothing
+    /// leaves a crossing the READER refuses. So the append fails closed, locally, before anything is sent.
+    ///
+    /// That is a strictly better outcome than the one this test originally pinned (stamp nothing, send,
+    /// and let the successor's seal reject the attempt at the key): the deposed lane spends no request to
+    /// learn what it can already prove about itself. The property the test exists for is unchanged and is
+    /// asserted below in its strongest form -- NO object is written at that id at all, so no self-pointer
+    /// can have been stamped anywhere.
+    /// The refusal must reach the SAME TERMINAL OUTCOME the collision produced, not merely "an error".
+    /// Skipping the request must not skip the conclusion, so all four halves are pinned:
+    ///
+    ///   1. the class is INVALID_STATE -- the conclusive-rejection class the successor-seal arm uses,
+    ///      NOT the retry-later class. This is the one that matters most: a retryable error here would
+    ///      have every caller re-derive the same impossible transaction forever, and the deposition
+    ///      would never surface anywhere;
+    ///   2. the message says the lane resumes only under a later epoch -- that IS the deposition,
+    ///      reported to the caller and the operator in the same words the collision reported it;
+    ///   3. NOTHING is written, so no self-pointer can have been stamped and no request was spent;
+    ///   4. a SECOND flush behaves identically instead of looping or degrading.
+    const uint64_t remounts_before = store->scheduleRemountCallCountForTest();  /// NOLINT(clang-analyzer-deadcode.DeadStores)
+    const size_t puts_before = backend->putCount(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{epoch + 1, 1}));  /// NOLINT(clang-analyzer-deadcode.DeadStores)
+    try
+    {
+        store->dropRef(ns, "x");
+        FAIL() << "the deposed lane must reject conclusively, not succeed";
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), DB::ErrorCodes::INVALID_STATE) << "got: " << e.message();
+        EXPECT_NE(e.message().find("resumes only under a later epoch"), String::npos)
+            << "the deposition must be surfaced, not just the failure: " << e.message();
+    }
+    EXPECT_FALSE(backend->get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{epoch + 1, 1})).has_value())
+        << "nothing may be written: the lane could not construct a legal transaction, so it sent none";
+    EXPECT_EQ(backend->putCount(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{epoch + 1, 1})), puts_before)
+        << "and no request was spent learning what the lane could already prove about itself";
+
+    /// The second flush: same conclusive answer, still no traffic. A lane that re-derived and re-sent
+    /// here would be exactly the spin this arm exists to prevent.
+    expectThrowsCode(DB::ErrorCodes::INVALID_STATE, [&] { store->dropRef(ns, "x"); });
+    EXPECT_EQ(backend->putCount(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), RefTxnId{epoch + 1, 1})), puts_before);
+
+    /// NO remount is scheduled, matching the collision arm exactly. A successor closing our epoch is a
+    /// legitimate handover, not an anomaly to react to: the mount lease is what resolves it, and
+    /// scheduling a remount from here would turn every ordinary deposition into a self-inflicted
+    /// re-claim storm.
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), remounts_before);
+}
diff --git a/src/Disks/tests/gtest_cas_ref_writer.cpp b/src/Disks/tests/gtest_cas_ref_writer.cpp
new file mode 100644
index 000000000000..20bf4444cf97
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_ref_writer.cpp
@@ -0,0 +1,5299 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasEvent.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasRefProtocol.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefLogFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefCkptFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasRefSnapshotFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <base/scope_guard.h>
+
+#include <Poco/Exception.h>
+
+#include <algorithm>
+#include <atomic>
+#include <condition_variable>
+#include <future>
+#include <mutex>
+#include <thread>
+#include <utility>
+
+/// Task 10: the writer's ref persistence on the snapshot+log protocol. Covers the plan's Task 10
+/// failing-test list: empty+birth recovery; snapshot+tail recovery; recovery restart on a vanished
+/// object (converging on a newer snapshot); the append lane's wedge semantics (blocks the same table,
+/// leaves other tables free, applies a later-observed-durable append before unwedging); invalid batch
+/// entries failing in isolation; and the S3 request-cost contract (one create for a warm isolated
+/// mutation, one create shared by a compatible batch).
+
+namespace DB::ErrorCodes
+{
+extern const int ABORTED;
+extern const int FILE_DOESNT_EXIST;
+extern const int CORRUPTED_DATA;
+extern const int INVALID_STATE;
+extern const int LOGICAL_ERROR;
+extern const int NETWORK_ERROR;
+extern const int S3_ERROR;
+}
+
+namespace ProfileEvents
+{
+extern const Event CASRefSweepDeferred;
+extern const Event CASRefSweepRearmed;
+extern const Event CASRefStalePrecommitsReclaimed;
+extern const Event CASRefSnapshotPutBytes;
+extern const Event CASRefSnapshotTailLogs;
+extern const Event CASRefSnapshotPublishDispatched;
+extern const Event CASRefSnapshotPublishBackoff;
+extern const Event CASConditionalWriteFenceLostPostWrite;
+extern const Event CASRefRecoveryEpochSealed;
+extern const Event CASRefRecoveryRetries;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::CountingBackend;
+using DB::Cas::tests::committedRow;
+using DB::Cas::tests::expectThrowsCode;
+using DB::Cas::tests::minimalLiveSnapshot;
+using DB::Cas::tests::namespaceBirthOp;
+using DB::Cas::tests::publishCommittedOps;
+using DB::Cas::tests::runRegularRoundReclaiming;
+using DB::Cas::tests::writeRefSnapshotRaw;
+using DB::Cas::tests::writeSealAt;
+
+namespace
+{
+
+/// The operation deadline every SINGLE-ATTEMPT fixture in this file uses, and the reason it is
+/// deliberately NOT `attempt_timeout_ms`.
+///
+/// Those fixtures exist to make one injected ambiguous response conclusive, and `max_attempts = 1`
+/// alone achieves that: with retries allowed the controller would resolve-before-reissue and report a
+/// definite outcome instead. The deadline contributes nothing to that -- but setting it EQUAL to the
+/// attempt timeout collapses the controller's pre-send gate into a race. The deadline is captured as
+/// `now + operation_deadline_ms` and the gate asks `now + attempt_timeout_ms > deadline`
+/// (`CasRequestControl.cpp`), so equal values reduce it to `now_2 > now_1`: ONE elapsed millisecond
+/// between the two clock reads refuses the operation with NOTHING SENT, the injected fault is never
+/// reached, and the product correctly does not wedge -- flipping every wedge expectation downstream.
+///
+/// That is not hypothetical. It took down
+/// `CASRefWriterStalePrecommitSweep.BoundedBatchesAndInterruptionResumeAcrossMounts` on 5 of 6 sanitizer
+/// CI runs (fixed in `8f9e63c7a19`), `CASRefInstallSafety.UncertainPrecommitKeepsItsCleanupOwnerAndItsBody`
+/// under parallel-build load, and `CASRefWriterAppendLane.WedgedLaneBlocksSameTableWhileOtherTableProceeds`
+/// in a full-binary ASan run -- the last one with the mechanism named verbatim in the thrown message
+/// ("refused BEFORE any request was sent ... the operation deadline rejected before the first request").
+///
+/// A wide deadline keeps the request always actually sent, so what the test observes is the fault it
+/// injected rather than the machine it ran on. A fixture that genuinely wants the pre-send REFUSAL
+/// must drive it deterministically with a frozen clock (see `gtest_cas_ref_install_safety.cpp`'s
+/// `openPoolFenceControlled`), never by racing the wall clock.
+constexpr uint64_t kSingleAttemptDeadlineMs = 5000;
+
+/// A `CasEvent` sink safe to hand to `Pool::setEventSink`: the emit runs on whatever thread the pool's
+/// background syncer happens to be on, and the test reads the accumulated events afterward from the
+/// main test thread with no other ordering between the two -- a bare `std::vector` there is a real data
+/// race (the class this file's four `setEventSink` call sites all had, hidden because a debug/ASan build
+/// doesn't reliably catch an unsynchronized push_back/iterator-read pair on a small vector). `add` takes
+/// the lock only around the push; `snapshot` copies out under the lock and returns, so a caller iterating
+/// the result never holds the mutex across anything that could call back into the pool (which an
+/// event-sink callback legitimately can, on other seams in this file).
+class SynchronizedEventLog
+{
+public:
+    void add(const CasEvent & e)
+    {
+        std::lock_guard lock(mutex);
+        events.push_back(e);
+    }
+    std::vector<CasEvent> snapshot() const
+    {
+        std::lock_guard lock(mutex);
+        return events;
+    }
+private:
+    mutable std::mutex mutex;
+    std::vector<CasEvent> events;
+};
+
+PoolPtr openPool(const BackendPtr & backend, CasRequestBudget budget = {})
+{
+    /// Recovery tests seed ref-log/snapshot residue before opening; a pool with such residue always has a
+    /// `_pool_meta` in production, so establish it first (Task 7's zero-write bootstrap check refuses to
+    /// mint a fresh identity over residual data — see `seedPoolMetaForRestart`). Idempotent, and a no-op
+    /// for the fresh-open tests that seed nothing (the subsequent open validates the just-created meta).
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test", .cas_request_budget = budget});
+}
+
+/// Task 11: like `openPool`, but the caller supplies (and owns) the rest of the config -- snapshot
+/// thresholds, grace age, a fake `boot_ms_fn`, etc. `pool_prefix`/`server_root_id` are pinned so every
+/// test in this file addresses the same pool shape.
+PoolPtr openPoolWithConfig(const BackendPtr & backend, PoolConfig config)
+{
+    config.pool_prefix = "p";
+    config.server_root_id = "test";
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);   /// see `openPool` above
+    return Pool::open(backend, std::move(config));
+}
+
+/// Mirrors gtest_cas_part_write.cpp's startBuildFor/publishOneBlobPart, minus the blob (an empty-entry
+/// manifest is a legal, blob-free part -- the ref-writer tests only care about ref/manifest identity).
+///
+/// Stage B (Task 4-C): pin `ns` to the sentinel before the first real touch -- ONE choke point for
+/// every test in this file, since every real-path setup here funnels through `startBuildFor` (directly,
+/// or via `publishEmptyPart` below). Many of this file's tests separately compute an expected key via
+/// `DB::Cas::tests::fixture::fixtureLife(ns)` for fault injection/verification; without this the real
+/// production birth mints a random incarnation and those computed keys land nowhere real.
+PartWriteTxnPtr startBuildFor(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    DB::Cas::tests::casAdmitRecoverableEntry(s->backend(), s->layout(), ns, s->liveWriterEpoch());
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref;
+    return s->beginPartWrite(info);
+}
+
+ManifestId publishEmptyPart(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    auto build = startBuildFor(s, ns, ref);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+void publishWithProductionBirth(const PoolPtr & store, const RootNamespace & ns, const String & ref)
+{
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = store->beginPartWrite(info);
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+}
+
+CatalogEntry catalogEntryOrThrow(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    const RefCatalog catalog = CasRefCatalog::read(backend, layout).catalog;
+    const auto it = std::find_if(catalog.entries.begin(), catalog.entries.end(), [&](const CatalogEntry & entry)
+    {
+        return entry.ns == ns;
+    });
+    if (it == catalog.entries.end())
+        throw std::runtime_error("catalog entry missing from test fixture");
+    return *it;
+}
+
+CatalogEntry replaceCatalogLifeForRuntimeRace(
+    Backend & backend, const Layout & layout, const CatalogEntry & predecessor, UInt128 successor_incarnation)
+{
+    const CasRefCatalog::Snapshot before_delete = CasRefCatalog::read(backend, layout);
+    RefCatalog without_predecessor = before_delete.catalog;
+    std::erase_if(without_predecessor.entries, [&](const CatalogEntry & entry)
+    {
+        return entry.ns == predecessor.ns && entry.incarnation == predecessor.incarnation;
+    });
+    if (backend.casPut(layout.refCatalogKey(), encodeRefCatalog(without_predecessor), before_delete.token).outcome
+        != CasOutcome::Committed)
+        throw std::runtime_error("test failed to retire exact predecessor catalog life");
+
+    CatalogEntry successor{
+        .ns = predecessor.ns,
+        .state = NsState::Live,
+        .incarnation = successor_incarnation,
+        .creator = std::nullopt};
+    const CasRefCatalog::Snapshot after_delete = CasRefCatalog::read(backend, layout);
+    RefCatalog reborn = after_delete.catalog;
+    reborn.entries.push_back(successor);
+    if (backend.casPut(layout.refCatalogKey(), encodeRefCatalog(reborn), after_delete.token).outcome
+        != CasOutcome::Committed)
+        throw std::runtime_error("test failed to publish successor catalog life");
+    return successor;
+}
+
+std::optional<RefTxnId> listGreatestLogIdForTest(
+    Backend & backend, const Layout & layout, const RootNamespace & ns);
+
+std::optional<RefTxnId> listGreatestLogIdForLifeForTest(
+    Backend & backend, const Layout & layout, const NamespaceLifeId & life)
+{
+    std::optional<RefTxnId> greatest;
+    String cursor;
+    for (;;)
+    {
+        const ListPage page = backend.list(layout.namespaceStreamPrefix(life), cursor, 1000);
+        for (const ListedKey & listed : page.keys)
+        {
+            const auto parsed = layout.parseRefObjectKey(listed.key);
+            if (parsed && parsed->life_id == life.incarnation && parsed->kind == RefObjectKind::Log
+                && (!greatest || *greatest < parsed->txn_id))
+                greatest = parsed->txn_id;
+        }
+        if (page.next_cursor.empty())
+            return greatest;
+        cursor = page.next_cursor;
+    }
+}
+
+struct CompletedRemovingFixture
+{
+    CatalogEntry predecessor;
+    uint64_t writer_epoch = 0;
+    uint64_t runtime_identity = 0;
+};
+
+CompletedRemovingFixture prepareResidentRemovalForDrain(
+    const PoolPtr & store, Backend & backend, const RootNamespace & ns, Gc & gc)
+{
+    publishWithProductionBirth(store, ns, "predecessor");
+    const CatalogEntry predecessor = catalogEntryOrThrow(backend, store->layout(), ns);
+    const uint64_t writer_epoch = store->liveWriterEpoch();
+    const uint64_t runtime_identity = store->refTableRuntimeIdentityForTest(ns);
+
+    if (runRegularRoundReclaiming(gc).deferred)
+        throw std::runtime_error("fixture publish unexpectedly deferred");
+    store->dropNamespace(ns);
+    const CatalogEntry removing = catalogEntryOrThrow(backend, store->layout(), ns);
+    if (removing.state != NsState::Removing || removing.incarnation != predecessor.incarnation)
+        throw std::runtime_error("fixture removal did not publish the expected exact Removing row");
+    if (runRegularRoundReclaiming(gc).deferred)
+        throw std::runtime_error("fixture terminal fold unexpectedly deferred");
+
+    const GcState state = decodeGcState(backend.get(store->layout().gcStateKey())->bytes);
+    const CasFoldSeal seal = decodeFoldSeal(
+        backend.get(store->layout().foldSealKey(state.snap_generation, state.snap_attempt))->bytes);
+    const auto row = seal.ref_lives.find(predecessor.incarnation);
+    if (row == seal.ref_lives.end() || !row->second.cleanup_evidence)
+        throw std::runtime_error("fixture terminal fold produced no cleanup evidence");
+    return {predecessor, writer_epoch, runtime_identity};
+}
+
+ManifestRef manifestRef(uint64_t epoch, uint64_t seq, uint32_t ordinal)
+{
+    return ManifestRef{epoch, seq, ordinal};
+}
+
+/// Task 11: an INDEPENDENT ground truth for "cache-replay equivalence" tests -- lists every `_log/`
+/// key under `ns` directly off the backend (ignoring any snapshot), decodes and replays them in id
+/// order via the SAME shared state machine the writer uses, and returns the resulting state. A
+/// published snapshot's bytes must equal `encodeRefTableSnapshot(snapshotOf(replay-through-X, ns))`
+/// for this oracle's replay truncated at `X`.
+RefTableState independentFullReplayForTest(Backend & backend, const Layout & layout, const RootNamespace & ns,
+                                            std::optional<RefTxnId> up_to = std::nullopt)
+{
+    std::vector<RefTxnId> ids;
+    String cursor;
+    for (;;)
+    {
+        const ListPage page = backend.list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+        for (const ListedKey & lk : page.keys)
+        {
+            const auto parsed = layout.parseRefObjectKey(lk.key);
+            if (parsed && parsed->life_id == DB::Cas::tests::fixture::fixtureLife(ns).incarnation && parsed->kind == RefObjectKind::Log
+                && (!up_to || !(*up_to < parsed->txn_id)))
+                ids.push_back(parsed->txn_id);
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    std::sort(ids.begin(), ids.end());
+
+    RefTableState state;
+    for (const RefTxnId & id : ids)
+    {
+        const auto got = backend.get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), id));
+        applyRefLogTxn(state, decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), id));
+    }
+    return state;
+}
+
+/// The greatest `_snap/<id>.proto` key currently present for `ns`, found via a fresh LIST (independent
+/// of the Pool's own cached bookkeeping).
+std::optional<RefTxnId> listGreatestSnapshotIdForTest(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    std::optional<RefTxnId> greatest;
+    String cursor;
+    for (;;)
+    {
+        const ListPage page = backend.list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+        for (const ListedKey & lk : page.keys)
+        {
+            const auto parsed = layout.parseRefObjectKey(lk.key);
+            if (parsed && parsed->life_id == DB::Cas::tests::fixture::fixtureLife(ns).incarnation && parsed->kind == RefObjectKind::Snap
+                && (!greatest || *greatest < parsed->txn_id))
+                greatest = parsed->txn_id;
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    return greatest;
+}
+
+/// A backend that can (a) force one `get()` on a chosen exact key to return absent exactly once
+/// (simulating an object vanishing after recovery sampled its exact checkpoint, with an optional side effect
+/// fired at that exact moment -- e.g. publishing a covering newer snapshot, mirroring a concurrent GC
+/// cleanup+republish race), and (b) force `putIfAbsent` on keys matching a chosen substring to throw an
+/// ambiguous (Unresolved-classified) exception a bounded number of times, optionally still capturing
+/// the (key, bytes) so a test can later "deliver" it -- simulating a request whose RESPONSE was lost
+/// even though the write eventually landed server-side.
+class RefWriterTestBackend : public CountingBackend
+{
+public:
+    RefWriterTestBackend()
+    {
+        DB::Cas::tests::seedPoolMetaForRestart(*this);
+    }
+
+    using CountingBackend::get;
+    using CountingBackend::getStream;
+    using CountingBackend::putIfAbsent;
+    using CountingBackend::putIfAbsentStream;
+    using CountingBackend::putOverwrite;
+    using CountingBackend::casPut;
+
+    void clearRequestJournal()
+    {
+        std::lock_guard lock(request_journal_mutex);
+        request_journal.clear();
+    }
+
+    void recordRequestJournalEvent(String event)
+    {
+        std::lock_guard lock(request_journal_mutex);
+        request_journal.push_back(std::move(event));
+    }
+
+    std::vector<String> requestJournal() const
+    {
+        std::lock_guard lock(request_journal_mutex);
+        return request_journal;
+    }
+
+    std::set<String> vanish_once_keys;
+    std::function<void()> on_vanish_fire;
+
+    enum class CatalogCasFault : uint8_t
+    {
+        None,
+        CommitThenThrow,
+        OtherWriterReplacement,
+    };
+    CatalogCasFault catalog_cas_fault = CatalogCasFault::None;
+    String catalog_fault_key;
+    String catalog_replacement_bytes;
+    int catalog_resolution_get_fault_count = 0;
+    bool catalog_cas_fault_fired = false;
+    /// Fail one selected catalog GET after allowing an exact number of earlier catalog GETs through.
+    /// This reaches the removal lane's post-close observation without faulting its initial discovery.
+    int catalog_gets_before_fault = -1;
+    int catalog_get_fault_count = 0;
+
+    String fault_key_substr;
+    int fault_count = 0;
+    /// Let the first `fault_skip` matching PUTs through untouched before `fault_count` starts faulting.
+    /// Needed now that recovery's in-band epoch seal (INV-2) shares the `_log/` prefix with every other
+    /// write under a namespace: a test that wants to fault something LATER in the same prefix (e.g. the
+    /// stale-precommit sweep's removal chunk) must skip past recovery's own seal writes first. Same
+    /// seam as `ChunkFaultBackend::fault_skip` in `cas_test_helpers.h`.
+    int fault_skip = 0;
+    std::optional<std::pair<String, String>> pending_delayed_write;
+
+    /// (I1) On a matching `putIfAbsent`, a FOREIGN writer lands a DIFFERENT object at the exact key and
+    /// then this attempt's response is lost -- so the controller's resolve-before-reissue GET observes
+    /// different bytes and must raise CORRUPTED_DATA (a proven conflict, never a retry signal).
+    /// By default the foreign object is the attempt's own bytes plus a trailing marker -- UNDECODABLE
+    /// for zstd-framed objects (the frame size no longer matches), which is exactly right for tests
+    /// that pin fail-closed handling of a corrupt object. Tests that instead need a VALID foreign
+    /// object (e.g. a real cross-process seal to be adopted on retry) set `corrupt_foreign_bytes`.
+    String corrupt_key_substr;
+    int corrupt_count = 0;
+    String corrupt_foreign_bytes;
+
+    String ckpt_conflict_key;
+    size_t ckpt_conflict_count = 0;
+    String ckpt_get_hook_key;
+    std::function<void()> ckpt_get_hook;
+
+    /// Force a stream `LIST` to throw a transient object-store error (S3_ERROR) a bounded number of
+    /// times. Recovery must not consume this injection; callers that intentionally enumerate still do.
+    int list_fault_count = 0;
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (list_fault_count > 0)
+        {
+            --list_fault_count;
+            throw DB::Exception(DB::ErrorCodes::S3_ERROR, "RefWriterTestBackend: simulated transient LIST failure");
+        }
+        return CountingBackend::list(prefix, cursor, limit);
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        recordRequestJournalEvent("GET " + key);
+        if (key == ckpt_get_hook_key && ckpt_get_hook)
+        {
+            auto hook = std::exchange(ckpt_get_hook, nullptr);
+            hook();
+        }
+        if (key == catalog_fault_key && catalog_get_fault_count > 0 && catalog_gets_before_fault >= 0)
+        {
+            if (catalog_gets_before_fault == 0)
+            {
+                --catalog_get_fault_count;
+                throw std::runtime_error("RefWriterTestBackend: simulated catalog admission read failure");
+            }
+            --catalog_gets_before_fault;
+        }
+        if (catalog_cas_fault_fired && key == catalog_fault_key && catalog_resolution_get_fault_count > 0)
+        {
+            --catalog_resolution_get_fault_count;
+            throw std::runtime_error("RefWriterTestBackend: simulated catalog resolution read failure");
+        }
+        const auto it = vanish_once_keys.find(key);
+        if (it != vanish_once_keys.end())
+        {
+            vanish_once_keys.erase(it);
+            if (on_vanish_fire)
+            {
+                auto fire = std::move(on_vanish_fire);
+                on_vanish_fire = nullptr;
+                fire();
+            }
+            return std::nullopt;
+        }
+        return CountingBackend::get(key, range);
+    }
+
+    CasResult casPut(
+        const String & key, const String & bytes, const std::optional<Token> & expected,
+        const ObjectMeta & meta) override
+    {
+        recordRequestJournalEvent("CAS " + key);
+        if (key == ckpt_conflict_key && ckpt_conflict_count > 0)
+        {
+            --ckpt_conflict_count;
+            return {CasOutcome::Conflict, {}};
+        }
+        if (key == catalog_fault_key && catalog_cas_fault != CatalogCasFault::None)
+        {
+            const CatalogCasFault fault = std::exchange(catalog_cas_fault, CatalogCasFault::None);
+            catalog_cas_fault_fired = true;
+            if (fault == CatalogCasFault::CommitThenThrow)
+            {
+                const CasResult result = CountingBackend::casPut(key, bytes, expected, meta);
+                if (result.outcome != CasOutcome::Committed)
+                    return result;
+                throw Poco::TimeoutException(
+                    "RefWriterTestBackend: catalog CAS committed but its response was lost");
+            }
+
+            CasResult replacement = CountingBackend::casPut(
+                key, catalog_replacement_bytes, expected, meta);
+            if (replacement.outcome != CasOutcome::Committed)
+                return replacement;
+            return {CasOutcome::Conflict, {}};
+        }
+        return CountingBackend::casPut(key, bytes, expected, meta);
+    }
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        recordRequestJournalEvent("PUT " + key);
+        if (corrupt_count > 0 && !corrupt_key_substr.empty() && key.find(corrupt_key_substr) != String::npos)
+        {
+            --corrupt_count;
+            /// A foreign writer lands a DIFFERENT object at this exact key; then our own response is lost.
+            CountingBackend::putIfAbsent(
+                key, corrupt_foreign_bytes.empty() ? bytes + String("\x01_FOREIGN_DIFFERENT") : corrupt_foreign_bytes);
+            throw Poco::TimeoutException("RefWriterTestBackend: a foreign different object landed; response lost");
+        }
+        if (!fault_key_substr.empty() && key.find(fault_key_substr) != String::npos)
+        {
+            if (fault_skip > 0)
+            {
+                --fault_skip;
+            }
+            else if (fault_count > 0)
+            {
+                --fault_count;
+                pending_delayed_write = {key, bytes};
+                throw Poco::TimeoutException("RefWriterTestBackend: simulated ambiguous result (response lost)");
+            }
+        }
+        {
+            std::unique_lock lk(block_mutex);
+            bool block_this = false;
+            if (block_armed && key.find(block_substr) != String::npos)
+            {
+                if (!block_first_match_only)
+                    block_this = true;                 /// block EVERY matching put (the original mode)
+                else if (blocked_key.empty())
+                {
+                    blocked_key = key;                 /// first match: capture and block exactly this key
+                    block_this = true;
+                }
+                else if (key == blocked_key)
+                    block_this = true;                 /// the SAME captured key retried: keep blocking it
+                /// a DIFFERENT matching key under first-match-only mode falls through unblocked
+            }
+            /// (I1) Independent per-key blocking: every matching key parks on its OWN release, unlike
+            /// `block_armed` above (one shared gate released all-at-once). Lets a test park two DISTINCT
+            /// `_snap/<id>` PUTs concurrently and release them in a chosen order.
+            if (independent_block_armed && key.contains(independent_block_substr))
+            {
+                independent_blocked_keys.insert(key);
+                block_cv.notify_all();
+                block_cv.wait(lk, [&] { return independent_released_keys.contains(key); });
+            }
+            if (block_this)
+            {
+                block_entered = true;
+                block_cv.notify_all();
+                block_cv.wait(lk, [&] { return !block_armed; });
+                /// fix-round F3-1a (CRITICAL, unlock-throw race harness): on release, behave like
+                /// `corrupt_key_substr` above instead of proceeding normally -- a foreign writer landed
+                /// DIFFERENT bytes at this exact key while we were parked, so our own attempt is a
+                /// PROVEN conflict once `putIfAbsentControlled`'s resolve-before-reissue GETs it. Lets a
+                /// test make the recovery seal's PUT throw CORRUPTED_DATA from INSIDE the unlocked
+                /// window, deterministically, instead of merely returning a non-Committed outcome.
+                if (block_throw_corrupted_on_release)
+                {
+                    lk.unlock();
+                    CountingBackend::putIfAbsent(key, bytes + String("\x01_FOREIGN_DIFFERENT"));
+                    {
+                        std::lock_guard g(block_mutex);
+                        block_call_completed = true;
+                    }
+                    block_cv.notify_all();
+                    throw Poco::TimeoutException(
+                        "RefWriterTestBackend: a foreign different object landed on release; response lost");
+                }
+            }
+        }
+        const PutResult r = CountingBackend::putIfAbsent(key, bytes, meta);
+        {
+            std::lock_guard g(block_mutex);
+            block_call_completed = true;
+        }
+        block_cv.notify_all();
+        return r;
+    }
+    /// See `putIfAbsent`'s `block_this` branch. Set before spawning any thread that could race
+    /// `putIfAbsent`, like `corrupt_key_substr`/`fault_key_substr` above -- not itself lock-protected.
+    bool block_throw_corrupted_on_release = false;
+
+    /// "Deliver" the earlier ambiguous write: the request DID eventually land server-side, the caller
+    /// just never saw the ack. No-op if no fault has fired since the last delivery.
+    void materializePendingDelayedWrite()
+    {
+        if (pending_delayed_write)
+        {
+            CountingBackend::putIfAbsent(pending_delayed_write->first, pending_delayed_write->second);
+            pending_delayed_write.reset();
+        }
+    }
+
+    /// Task 11: blocks EVERY `putIfAbsent()` whose key contains `armed_block_substr` until
+    /// `releaseBlock()` is called, notifying `awaitBlockEntered()` the first time one is reached. Used
+    /// to prove snapshot publication never holds up an unrelated concurrent append.
+    void armPutBlock(const String & substr)
+    {
+        std::lock_guard g(block_mutex);
+        block_substr = substr;
+        block_armed = true;
+        block_entered = false;
+        block_call_completed = false;
+        block_first_match_only = false;
+        blocked_key.clear();
+    }
+
+    /// Task 11 (monotonic-adoption harness): block ONLY the FIRST `putIfAbsent` whose key contains
+    /// `substr`, capturing that exact key; every LATER put -- including a DIFFERENT `_snap/<id>` key --
+    /// proceeds unblocked. Lets a test pin one in-flight publish's PUT mid-flight while a second,
+    /// higher-id publish runs to completion, deterministically forcing the out-of-order overlap.
+    void armPutBlockFirstMatchOnly(const String & substr)
+    {
+        std::lock_guard g(block_mutex);
+        block_substr = substr;
+        block_armed = true;
+        block_entered = false;
+        block_call_completed = false;
+        block_first_match_only = true;
+        blocked_key.clear();
+    }
+    void awaitBlockEntered()
+    {
+        std::unique_lock lk(block_mutex);
+        block_cv.wait(lk, [&] { return block_entered; });
+    }
+    void releaseBlock()
+    {
+        {
+            std::lock_guard g(block_mutex);
+            block_armed = false;
+        }
+        block_cv.notify_all();
+    }
+    /// Blocks until the PREVIOUSLY-blocked `putIfAbsent` call has actually RETURNED (not merely been
+    /// unblocked) -- i.e. its underlying `CountingBackend::putIfAbsent` has completed. Deterministic,
+    /// sleep-free way to observe a detached background caller's own work finishing when the TEST no
+    /// longer holds anything (e.g. a Pool handle) that call would otherwise let it wait on.
+    void awaitBlockedCallCompleted()
+    {
+        std::unique_lock lk(block_mutex);
+        block_cv.wait(lk, [&] { return block_call_completed; });
+    }
+
+    /// (I1 regression harness) Arms independent per-key blocking for every `putIfAbsent` matching
+    /// `substr`: unlike `armPutBlock`/`armPutBlockFirstMatchOnly` (one shared release gate), each
+    /// blocked key parks on ITS OWN release (`releaseKey`), so two distinct `_snap/<id>` PUTs can be
+    /// parked concurrently -- both past their capture point, neither yet adopted -- and released in a
+    /// chosen order. Needed to construct the small-candidate-adopts-before-a-larger-one-already-in-flight
+    /// ordering that exercises `clampedCounterSub`'s actual clamp branch.
+    void armPutBlockIndependently(const String & substr)
+    {
+        std::lock_guard g(block_mutex);
+        independent_block_substr = substr;
+        independent_block_armed = true;
+        independent_blocked_keys.clear();
+        independent_released_keys.clear();
+    }
+    /// Blocks until at least `n` distinct matching keys are currently parked.
+    void awaitAtLeastNKeysBlocked(size_t n)
+    {
+        std::unique_lock lk(block_mutex);
+        block_cv.wait(lk, [&] { return independent_blocked_keys.size() >= n; });
+    }
+    /// A snapshot of the keys currently parked under independent blocking.
+    std::set<String> blockedKeysSnapshot()
+    {
+        std::lock_guard g(block_mutex);
+        return independent_blocked_keys;
+    }
+    /// Releases exactly the given key; every OTHER independently-blocked key stays parked.
+    void releaseKey(const String & key)
+    {
+        {
+            std::lock_guard g(block_mutex);
+            independent_released_keys.insert(key);
+        }
+        block_cv.notify_all();
+    }
+
+private:
+    mutable std::mutex request_journal_mutex;
+    std::vector<String> request_journal;
+    std::mutex block_mutex;
+    std::condition_variable block_cv;
+    String block_substr;
+    bool block_armed = false;
+    bool block_entered = false;
+    bool block_call_completed = false;
+    bool block_first_match_only = false;
+    String blocked_key;
+    String independent_block_substr;
+    bool independent_block_armed = false;
+    std::set<String> independent_blocked_keys;
+    std::set<String> independent_released_keys;
+};
+
+}
+
+/// ===================================================================================
+/// Recovery (spec §Recovery / exact checkpoint grounding)
+/// ===================================================================================
+
+TEST(CASRefWriterRecovery, EmptyNamespaceRecoversToEmptyState)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/never_touched"};
+
+    EXPECT_TRUE(store->listRefs(ns).empty());
+    EXPECT_FALSE(store->resolveRef(ns, "anything").has_value());
+    EXPECT_EQ(store->refRecoveryRestartsForTest(ns), 0u);
+}
+
+TEST(CASRefWriterNonMinting, ListRefsOnAbsentNamespaceDoesNotMutateCatalog)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/list_absent_non_minting"};
+    const auto catalog_before = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_before);
+    backend->resetCounts();
+
+    EXPECT_TRUE(store->listRefs(ns).empty());
+
+    EXPECT_EQ(backend->putCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->putOverwriteCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->deleteCount(layout.refCatalogKey()), 0u);
+    const auto catalog_after = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_after);
+    EXPECT_EQ(catalog_after->bytes, catalog_before->bytes);
+    EXPECT_EQ(catalog_after->token, catalog_before->token);
+}
+
+TEST(CASRefWriterRuntimeIdentity, ColdReadRejectsCatalogLifeReplacedWithoutLocalInvalidation)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/cold-read-catalog-aba"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, layout, ns, store->liveWriterEpoch());
+    const CatalogEntry predecessor = catalogEntryOrThrow(*backend, layout, ns);
+    ASSERT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool paused = false;
+    bool resume = false;
+    store->setReadableCatalogAfterObservationHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        paused = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return resume; });
+    });
+
+    std::exception_ptr stale_error;
+    std::thread stale_reader([&]
+    {
+        try
+        {
+            (void)store->listRefs(ns);
+        }
+        catch (...)
+        {
+            stale_error = std::current_exception();
+        }
+    });
+    {
+        std::unique_lock lock(mutex);
+        cv.wait(lock, [&] { return paused; });
+    }
+
+    const CatalogEntry successor
+        = replaceCatalogLifeForRuntimeRace(*backend, layout, predecessor, UInt128{0xabc002});
+    const NamespaceLifeId successor_life
+        = NamespaceLifeId::fromCatalogEntry(successor.ns, successor.incarnation);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(successor_life), encodeRefCkpt(RefCkpt{
+        .life_epoch = store->liveWriterEpoch(),
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    {
+        std::lock_guard lock(mutex);
+        resume = true;
+    }
+    cv.notify_all();
+    stale_reader.join();
+    store->setReadableCatalogAfterObservationHookForTest(nullptr);
+
+    EXPECT_TRUE(stale_error) << "the stale catalog life was published instead of refused";
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+    EXPECT_NO_THROW((void)store->listRefs(ns));
+    ASSERT_TRUE(store->refTableLifeForTest(ns));
+    EXPECT_EQ(*store->refTableLifeForTest(ns), successor_life);
+}
+
+TEST(CASRefWriterRuntimeIdentity, ColdReadRejectsReplacementByExternalPoolActor)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    PoolConfig external_config{.pool_prefix = "p", .server_root_id = "external-runtime-race"};
+    auto external_store = Pool::open(backend, std::move(external_config));
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/external-catalog-runtime-publication"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, layout, ns, store->liveWriterEpoch());
+    const CatalogEntry predecessor = catalogEntryOrThrow(*backend, layout, ns);
+    ASSERT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+
+    CatalogEntry successor;
+    store->setReadableCatalogAfterObservationHookForTest([&]
+    {
+        successor = replaceCatalogLifeForRuntimeRace(
+            external_store->backend(), external_store->layout(), predecessor, UInt128{0xabc003});
+        const NamespaceLifeId successor_life
+            = NamespaceLifeId::fromCatalogEntry(successor.ns, successor.incarnation);
+        if (external_store->backend().putIfAbsent(
+                external_store->layout().refCkptKey(successor_life),
+                encodeRefCkpt(RefCkpt{
+                    .life_epoch = external_store->liveWriterEpoch(),
+                    .checkpoint_snapshot_id = std::nullopt,
+                    .last_epoch_seal = std::nullopt})).outcome != PutOutcome::Done)
+            throw std::runtime_error("test failed to publish external successor checkpoint");
+    });
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->listRefs(ns); });
+    store->setReadableCatalogAfterObservationHookForTest(nullptr);
+
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+    EXPECT_NO_THROW((void)store->listRefs(ns));
+    ASSERT_TRUE(store->refTableLifeForTest(ns));
+    EXPECT_EQ(*store->refTableLifeForTest(ns),
+        NamespaceLifeId::fromCatalogEntry(successor.ns, successor.incarnation));
+}
+
+TEST(CASRefWriterRuntimeIdentity, ColdReadRejectsUnrelatedCatalogMutationBetweenObservations)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/unrelated-catalog-runtime-publication"};
+    const RootNamespace unrelated{"srv1/unrelated-catalog-row"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, layout, ns, store->liveWriterEpoch());
+    ASSERT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+
+    store->setReadableCatalogAfterObservationHookForTest([&]
+    {
+        DB::Cas::tests::fixture::admitLive(*backend, layout, unrelated);
+    });
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->listRefs(ns); });
+    store->setReadableCatalogAfterObservationHookForTest(nullptr);
+
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+    EXPECT_NO_THROW((void)store->listRefs(ns));
+}
+
+TEST(CASRefWriterRuntimeIdentity, WarmReadableRuntimeDoesNotReadCatalog)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/warm-runtime-zero-catalog-get"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, layout, ns, store->liveWriterEpoch());
+
+    EXPECT_NO_THROW((void)store->listRefs(ns));
+    ASSERT_NE(store->refTableRuntimeIdentityForTest(ns), 0u);
+    backend->resetCounts();
+
+    EXPECT_NO_THROW((void)store->listRefs(ns));
+    EXPECT_EQ(backend->getCount(layout.refCatalogKey()), 0u);
+}
+
+/// `DROP DETACHED PART` reaches this point lookup for a part that may already be absent. Its probe
+/// must not turn a missing table namespace into a new catalog life.
+TEST(CASRefWriterNonMinting, ResolveRefOnAbsentNamespaceDoesNotMutateCatalog)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/resolve_absent_non_minting"};
+    const auto catalog_before = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_before);
+    backend->resetCounts();
+
+    EXPECT_FALSE(store->resolveRef(ns, "detached_part").has_value());
+
+    EXPECT_EQ(backend->putCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->putOverwriteCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->deleteCount(layout.refCatalogKey()), 0u);
+    const auto catalog_after = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_after);
+    EXPECT_EQ(catalog_after->bytes, catalog_before->bytes);
+    EXPECT_EQ(catalog_after->token, catalog_before->token);
+}
+
+TEST(CASRefWriterNonMinting, DropNamespaceOnAbsentNamespaceDoesNotMutateCatalog)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/drop_absent_non_minting"};
+    const auto catalog_before = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_before);
+    backend->resetCounts();
+
+    store->dropNamespace(ns);
+
+    EXPECT_EQ(backend->putCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->putOverwriteCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->casPutCount(layout.refCatalogKey()), 0u);
+    EXPECT_EQ(backend->deleteCount(layout.refCatalogKey()), 0u);
+    const auto catalog_after = backend->get(layout.refCatalogKey());
+    ASSERT_TRUE(catalog_after);
+    EXPECT_EQ(catalog_after->bytes, catalog_before->bytes);
+    EXPECT_EQ(catalog_after->token, catalog_before->token);
+}
+
+/// A table born by a log tail alone (no snapshot yet): `namespace_birth` with nothing else is a legal
+/// Live-but-empty table.
+TEST(CASRefWriterRecovery, BirthOnlyLogNoSnapshotRecoversToEmptyLiveTable)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/birth_only"};
+
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{1, 1}, {namespaceBirthOp()}, std::nullopt});
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    auto store = openPool(backend);
+    EXPECT_TRUE(store->listRefs(ns).empty());
+}
+
+/// Empty base + birth log recovery (spec unit test list): birth and the first precommit->promote span
+/// TWO separate log transactions with no snapshot at all.
+TEST(CASRefWriterRecovery, BirthPlusPrecommitPromoteAcrossTwoLogsNoSnapshot)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/birth_then_promote"};
+    const ManifestRef m1 = manifestRef(1, 1, 1);
+
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{1, 1},
+        {namespaceBirthOp(), publishCommittedOps("part_1", m1)[0]}, std::nullopt});
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{1, 2},
+        {publishCommittedOps("part_1", m1)[1]}, std::nullopt});
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    auto store = openPool(backend);
+    const auto resolved = store->resolveRef(ns, "part_1");
+    ASSERT_TRUE(resolved.has_value());
+    EXPECT_EQ(resolved->manifest_id.ref, m1);
+    EXPECT_EQ(resolved->manifest_id.root_namespace, ns);
+
+    const auto refs = store->listRefs(ns);
+    ASSERT_EQ(refs.size(), 1u);
+    EXPECT_TRUE(refs.contains("part_1"));
+}
+
+TEST(CASRefWriterRecovery, TerminalGapBelowCheckpointFrontierIsCorruptionNotSameLifeRebirth)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/writer_terminal_gap"};
+
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        ns.string(), RefTxnId{1, 1}, {namespaceBirthOp()}, std::nullopt});
+    RefOp remove;
+    remove.kind = RefOpKind::RemoveNamespace;
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        ns.string(), RefTxnId{1, 2}, {std::move(remove)}, std::nullopt});
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        ns.string(), RefTxnId{2, 1}, {namespaceBirthOp()}, std::nullopt});
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{2, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = RefTxnId{1, 2},
+    });
+
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    const String next_log_key = layout.refLogKey(life, RefTxnId{2, 2});
+    auto store = openPool(backend);
+    const uint64_t installs_before = store->recoveryInstallCountForTest();
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { (void)store->listRefs(ns); });
+    EXPECT_FALSE(store->refTableRecoveredForTest(ns));
+    EXPECT_EQ(store->recoveryInstallCountForTest(), installs_before);
+
+    EXPECT_ANY_THROW((void)publishEmptyPart(store, ns, "must_not_allocate"));
+    EXPECT_EQ(backend->putCount(next_log_key), 0u)
+        << "an unrecovered malformed life must not allocate the next writer position";
+}
+
+/// Latest snapshot plus tail recovery (spec unit test list): a snapshot covering ref "a", a tail that
+/// drops "a" and publishes "b", and a STALE log at/below the snapshot id that must be ignored (its
+/// content, if replayed, would corrupt the result -- proving the "ignore log keys at or below the
+/// selected snapshot" rule).
+TEST(CASRefWriterRecovery, SnapshotPlusTailRecovery)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/snap_tail"};
+    const ManifestRef ma = manifestRef(1, 1, 1);
+    const ManifestRef mb = manifestRef(1, 2, 1);
+
+    /// A stale log BELOW the snapshot id would, if wrongly replayed, try to add "a" a second time
+    /// (the snapshot already contains it) and throw -- proving it must be ignored, not merely benign.
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{1, 3},
+        {namespaceBirthOp(), publishCommittedOps("a", ma)[0], publishCommittedOps("a", ma)[1]}, std::nullopt});
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(),
+        .txn_id = RefTxnId{1, 5},
+        .ops = publishCommittedOps("a", ma),
+        .prev_epoch_seal = std::nullopt});
+    writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), RefTxnId{1, 5}, {committedRow("a", ma)}));
+
+    std::vector<RefOp> tail_ops;
+    tail_ops.push_back([&] { RefOp op; op.kind = RefOpKind::OwnerTransition;
+        op.old_binding = RefOwnerBinding{RefOwnerKind::Committed, "a", ma}; return op; }());
+    tail_ops.push_back(publishCommittedOps("b", mb)[0]);
+    tail_ops.push_back(publishCommittedOps("b", mb)[1]);
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{1, 6}, tail_ops, std::nullopt});
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 6},
+        .checkpoint_snapshot_id = RefTxnId{1, 5},
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    backend->resetCounts();
+    auto store = openPool(backend);
+    EXPECT_FALSE(store->resolveRef(ns, "a").has_value());
+    const auto b = store->resolveRef(ns, "b");
+    ASSERT_TRUE(b.has_value());
+    EXPECT_EQ(b->manifest_id.ref, mb);
+    EXPECT_EQ(store->listRefs(ns).size(), 1u);
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{1, 5})), 1u)
+        << "recovery must validate the selected base's retained ordinary log";
+    EXPECT_EQ(backend->getCount(layout.refSnapshotKey(life, RefTxnId{1, 5})), 1u)
+        << "the fixture must reach and decode the selected base snapshot";
+}
+
+/// Restart-on-vanish (spec §Recovery): the checkpoint-named snapshot vanishes during its exact GET
+/// while concurrent cleanup publishes a newer checkpoint base. Recovery must restart from the newer
+/// exact checkpoint, not treat the vanish as corruption.
+TEST(CASRefWriterRecovery, RestartOnVanishConvergesOnNewerSnapshot)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/vanish_race"};
+    const ManifestRef ma = manifestRef(1, 1, 1);
+    const ManifestRef mb = manifestRef(1, 2, 1);
+
+    /// Stage B (Task 4-C): pin `ns` to the sentinel before the raw snapshot below -- `store->resolveRef`
+    /// further down is a real production read that triggers `resolveNamespaceLife`, which for an
+    /// UNADMITTED namespace mints a fresh RANDOM incarnation rather than adopting the sentinel the raw
+    /// fixture writes at.
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    const RefTxnId snap_x{1, 10};
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(),
+        .txn_id = snap_x,
+        .ops = publishCommittedOps("a", ma),
+        .prev_epoch_seal = std::nullopt});
+    writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), snap_x, {committedRow("a", ma)}));
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 10},
+        .checkpoint_snapshot_id = snap_x,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    backend->vanish_once_keys.insert(layout.refSnapshotKey(life, snap_x));
+    bool vanish_fired = false;
+    backend->on_vanish_fire = [&]
+    {
+        vanish_fired = true;
+        const RefTxnId snap_y{1, 20};
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+            .ns = ns.string(),
+            .txn_id = snap_y,
+            .ops = publishCommittedOps("b", mb),
+            .prev_epoch_seal = std::nullopt});
+        writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), snap_y, {committedRow("b", mb)}));
+        const auto before = backend->get(layout.refCkptKey(life));
+        ASSERT_TRUE(before);
+        ASSERT_EQ(backend->casPut(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+            .life_epoch = 1,
+            .committed_through = RefTxnId{1, 20},
+            .checkpoint_snapshot_id = snap_y,
+            .last_epoch_seal = std::nullopt}), before->token).outcome, CasOutcome::Committed);
+    };
+
+    backend->resetCounts();
+    auto store = openPool(backend);
+    const auto b = store->resolveRef(ns, "b");
+    ASSERT_TRUE(b.has_value());
+    EXPECT_EQ(b->manifest_id.ref, mb);
+    EXPECT_FALSE(store->resolveRef(ns, "a").has_value()) << "must converge on snapshot Y, not a mix of X and Y";
+    EXPECT_EQ(store->refRecoveryRestartsForTest(ns), 1u);
+    EXPECT_TRUE(vanish_fired) << "the fixture must reach the old snapshot GET and fire the replacement hook";
+    EXPECT_FALSE(backend->vanish_once_keys.contains(layout.refSnapshotKey(life, snap_x)));
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, snap_x)), 1u);
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, RefTxnId{1, 20})), 1u);
+}
+
+/// A DIFFERENT valid object at the exact snapshot key (not merely absent) is corruption, never a
+/// restart signal -- pins the boundary between "vanished" (restart) and "corrupt" (fail closed).
+TEST(CASRefWriterRecovery, DifferentBytesAtSelectedSnapshotIsCorruptionNotRestart)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/corrupt_snap"};
+    const RefTxnId snap_x{1, 10};
+
+    /// A structurally-valid snapshot BODY, but for a DIFFERENT namespace, placed under `ns`'s own key
+    /// (a copy-under-the-wrong-prefix scenario) -- decodeRefTableSnapshot's key/body cross-check must
+    /// reject it, never treat it as a restart signal.
+    /// Stage B (Task 4-C): pin `ns` to the sentinel before the raw write below -- `store->resolveRef`
+    /// further down is a real production read that would otherwise mint a fresh RANDOM incarnation
+    /// for this unadmitted namespace instead of adopting the sentinel the raw fixture writes at.
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, layout, ns);
+    const RootNamespace other_ns{"srv1/other"};
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{
+        .ns = ns.string(),
+        .txn_id = snap_x,
+        .ops = publishCommittedOps("anchor", manifestRef(1, 10, 1)),
+        .prev_epoch_seal = std::nullopt});
+    DB::Cas::RefTableSnapshot foreign;
+    foreign.ns = other_ns.string();
+    foreign.snapshot_id = snap_x;
+    const String snapshot_key = layout.refSnapshotKey(life, snap_x);
+    ASSERT_EQ(backend->putIfAbsent(snapshot_key,
+        DB::Cas::sealObject(DB::Cas::FormatId::RefSnapshot, DB::Cas::encodeRefTableSnapshot(foreign))).outcome,
+        PutOutcome::Done);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(life), encodeRefCkpt(RefCkpt{
+        .life_epoch = 1,
+        .committed_through = snap_x,
+        .checkpoint_snapshot_id = snap_x,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+
+    auto store = openPool(backend);
+    backend->resetCounts();
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->resolveRef(ns, "anything"); });
+    EXPECT_EQ(backend->getCount(layout.refLogKey(life, snap_x)), 1u)
+        << "the matching ordinary log must be validated before the selected snapshot";
+    EXPECT_EQ(backend->getCount(snapshot_key), 1u)
+        << "the corruption must come from decoding the required checkpoint snapshot";
+}
+
+/// ===================================================================================
+/// Append lane: request cost + batching (spec §Common Mutation Path / §Local Batching Queue)
+/// ===================================================================================
+
+TEST(CASRefWriterAppendLane, CommittedChunkPublishesFrontierBeforeInstallAndAck)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/warm"};
+    publishEmptyPart(store, ns, "part_1");
+    publishEmptyPart(store, ns, "part_2");
+    ASSERT_TRUE(store->resolveRef(ns, "part_1").has_value());
+    ASSERT_TRUE(store->resolveRef(ns, "part_2").has_value());
+
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns).value();
+    const String log_prefix = store->layout().namespaceStreamPrefix(life) + "_log/";
+    const String ckpt_key = store->layout().refCkptKey(life);
+    const auto ckpt_before = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(ckpt_before);
+    ASSERT_TRUE(ckpt_before->ckpt.committed_through);
+    const RefTxnId expected_frontier{
+        ckpt_before->ckpt.committed_through->writer_epoch,
+        ckpt_before->ckpt.committed_through->ref_sequence + 1};
+    backend->clearRequestJournal();
+    const uint64_t list_before = backend->listTotal();
+    const uint64_t put_before = backend->putTotal();
+    const uint64_t ckpt_get_before = backend->getCount(ckpt_key);
+    const uint64_t ckpt_cas_before = backend->casPutCount(ckpt_key);
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool pre_carve_entered = false;
+    bool post_install_entered = false;
+    bool release_post_install = false;
+    bool follower_returned = false;
+    store->setRefPreCarveHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        if (pre_carve_entered)
+            return;
+        pre_carve_entered = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return store->refQueuePendingForTest(ns) >= 2; });
+    });
+    store->setCarveHookForTest([&](CasRefLedger::CarvePhaseForTest phase)
+    {
+        if (phase != CasRefLedger::CarvePhaseForTest::PostInstallPreAck)
+            return;
+        backend->recordRequestJournalEvent("INSTALL");
+        std::unique_lock lock(mutex);
+        post_install_entered = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release_post_install; });
+    });
+
+    std::exception_ptr leader_error;
+    std::exception_ptr follower_error;
+    std::thread leader([&]
+    {
+        try
+        {
+            store->dropRef(ns, "part_1");
+        }
+        catch (...)
+        {
+            leader_error = std::current_exception();
+        }
+    });
+    {
+        std::unique_lock lock(mutex);
+        cv.wait(lock, [&] { return pre_carve_entered; });
+    }
+    std::thread follower([&]
+    {
+        try
+        {
+            store->dropRef(ns, "part_2");
+            backend->recordRequestJournalEvent("FOLLOWER ACK");
+            {
+                std::lock_guard lock(mutex);
+                follower_returned = true;
+            }
+            cv.notify_all();
+        }
+        catch (...)
+        {
+            follower_error = std::current_exception();
+        }
+    });
+    while (store->refQueuePendingForTest(ns) < 2)
+        std::this_thread::yield();
+    cv.notify_all();
+    {
+        std::unique_lock lock(mutex);
+        cv.wait(lock, [&] { return post_install_entered; });
+    }
+
+    bool follower_returned_before_release = false;
+    {
+        std::lock_guard lock(mutex);
+        follower_returned_before_release = follower_returned;
+    }
+    std::exception_ptr observation_error;
+    bool part_1_visible = true;
+    bool part_2_visible = true;
+    try
+    {
+        part_1_visible = store->resolveRef(ns, "part_1").has_value();
+        part_2_visible = store->resolveRef(ns, "part_2").has_value();
+    }
+    catch (...)
+    {
+        observation_error = std::current_exception();
+    }
+    {
+        std::lock_guard lock(mutex);
+        release_post_install = true;
+    }
+    cv.notify_all();
+    leader.join();
+    follower.join();
+    store->setRefPreCarveHookForTest(nullptr);
+    store->setCarveHookForTest(nullptr);
+
+    EXPECT_FALSE(follower_returned_before_release)
+        << "a co-batched waiter returned before the installed transaction was acknowledged";
+    EXPECT_FALSE(observation_error);
+    EXPECT_FALSE(part_1_visible);
+    EXPECT_FALSE(part_2_visible)
+        << "both co-batched mutations must be visible before either waiter can return success";
+    EXPECT_FALSE(leader_error);
+    EXPECT_FALSE(follower_error);
+    EXPECT_EQ(backend->listTotal(), list_before) << "a warm mutation performs no LIST";
+    EXPECT_EQ(backend->putTotal(), put_before + 1) << "exactly one body PUT with create-if-absent";
+    EXPECT_EQ(backend->getCount(ckpt_key), ckpt_get_before + 1)
+        << "one committed chunk pays exactly one checkpoint GET";
+    EXPECT_EQ(backend->casPutCount(ckpt_key), ckpt_cas_before + 1)
+        << "one committed chunk pays exactly one checkpoint CAS";
+
+    const std::vector<String> journal = backend->requestJournal();
+    ASSERT_EQ(journal.size(), 5u);
+    EXPECT_EQ(journal[0].find("PUT " + log_prefix), 0u) << journal[0];
+    EXPECT_EQ(journal[1], "GET " + ckpt_key);
+    EXPECT_EQ(journal[2], "CAS " + ckpt_key);
+    EXPECT_EQ(journal[3], "INSTALL");
+    EXPECT_EQ(journal[4], "FOLLOWER ACK");
+
+    const auto durable_ckpt = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(durable_ckpt);
+    EXPECT_EQ(durable_ckpt->ckpt.committed_through, expected_frontier);
+}
+
+TEST(CASRefWriterAppendLane, CheckpointConflictAfterLogCommitRequiresRecoveryWithoutInstall)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/frontier-conflict"};
+    publishEmptyPart(store, ns, "x");
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns).value();
+    const String ckpt_key = store->layout().refCkptKey(life);
+    const auto before = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(before);
+    ASSERT_TRUE(before->ckpt.committed_through);
+    const RefTxnId candidate{before->ckpt.committed_through->writer_epoch,
+                             before->ckpt.committed_through->ref_sequence + 1};
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+
+    backend->ckpt_conflict_key = ckpt_key;
+    backend->ckpt_conflict_count = 100;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before)
+        << "the durable log was not installed or acknowledged";
+    const auto after = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(after);
+    EXPECT_EQ(after->ckpt.committed_through, before->ckpt.committed_through);
+    EXPECT_TRUE(backend->get(store->layout().refLogKey(life, candidate)))
+        << "the log PUT committed before checkpoint publication failed";
+    EXPECT_FALSE(backend->get(store->layout().refLogKey(
+        life, RefTxnId{candidate.writer_epoch, candidate.ref_sequence + 1})))
+        << "no later id may be allocated above an unfrontiered durable transaction";
+}
+
+TEST(CASRefWriterAppendLane, FenceMovementAtCheckpointPublicationRequiresRecoveryWithoutInstall)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/frontier-fenced"};
+    publishEmptyPart(store, ns, "x");
+    const NamespaceLifeId life = CasRefCatalog::lifeIfCataloged(*backend, store->layout(), ns).value();
+    const String ckpt_key = store->layout().refCkptKey(life);
+    const auto before = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(before);
+    ASSERT_TRUE(before->ckpt.committed_through);
+    const RefTxnId candidate{before->ckpt.committed_through->writer_epoch,
+                             before->ckpt.committed_through->ref_sequence + 1};
+    const size_t tail_before = store->tailSinceSnapshotCountForTest(ns);
+
+    backend->ckpt_get_hook_key = ckpt_key;
+    backend->ckpt_get_hook = [&] { store->tripMountLost(); };
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::NeedsRecovery);
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_before)
+        << "the fenced frontier attempt must not install or acknowledge the durable log";
+    const auto after = readCkpt(*backend, store->layout(), life);
+    ASSERT_TRUE(after);
+    EXPECT_EQ(after->ckpt.committed_through, before->ckpt.committed_through);
+    EXPECT_TRUE(backend->get(store->layout().refLogKey(life, candidate)));
+    EXPECT_FALSE(backend->get(store->layout().refLogKey(
+        life, RefTxnId{candidate.writer_epoch, candidate.ref_sequence + 1})));
+}
+
+/// Phase 3 (reftable-cow-map materialization): each of these N
+/// publishes is its own isolated (unbatched) flush touching exactly one NEW ref -- if
+/// `flushRefBatch` did not materialize `rt->state.committed` after installing each flush's
+/// transaction, the overlay would grow by ~1 entry per flush and this would read back ~N,
+/// defeating the whole point of the COW map for a long-running table.
+TEST(CASRefWriterAppendLane, MaterializeKeepsOverlaySmallAcrossManyIsolatedFlushes)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/cowmap"};
+
+    constexpr int kRefs = 20;
+    for (int i = 0; i < kRefs; ++i)
+        publishEmptyPart(store, ns, "ref" + std::to_string(i));
+
+    EXPECT_LE(store->committedOverlayEntriesForTest(ns), 1u);
+    EXPECT_EQ(store->listRefs(ns).size(), static_cast<size_t>(kRefs));   /// sanity: all N really committed
+}
+
+/// `B` compatible queued mutations share one create (spec §Writer Budget).
+TEST(CASRefWriterAppendLane, CompatibleMutationsShareOneCreate)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/cobatch"};
+    publishEmptyPart(store, ns, "a");
+    publishEmptyPart(store, ns, "b");
+    ASSERT_TRUE(store->resolveRef(ns, "a").has_value());
+    ASSERT_TRUE(store->resolveRef(ns, "b").has_value());
+
+    std::mutex m;
+    std::condition_variable cv;
+    bool entered = false;
+    store->setRefPreCarveHookForTest([&]
+    {
+        std::unique_lock lk(m);
+        if (entered)
+            return;   /// only the leader's own first carve blocks; a second flush (if any) proceeds
+        entered = true;
+        cv.notify_all();
+        cv.wait(lk, [&] { return store->refQueuePendingForTest(ns) >= 2; });
+    });
+
+    const uint64_t put_before = backend->putTotal();
+    std::thread t_a([&] { store->dropRef(ns, "a"); });
+    {
+        std::unique_lock lk(m);
+        cv.wait(lk, [&] { return entered; });
+    }
+    std::thread t_b([&] { store->dropRef(ns, "b"); });
+    while (store->refQueuePendingForTest(ns) < 2)
+        std::this_thread::yield();
+    cv.notify_all();   /// wakes the pre-carve hook's own wait once its predicate (>=2 pending) holds
+    t_a.join();
+    t_b.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    EXPECT_EQ(backend->putTotal(), put_before + 1) << "both drops must land in ONE created log object";
+    EXPECT_FALSE(store->resolveRef(ns, "a").has_value());
+    EXPECT_FALSE(store->resolveRef(ns, "b").has_value());
+}
+
+/// An invalid queued request returns its own exception without entering the transaction; the
+/// co-batched neighbor still lands, in the SAME one create.
+TEST(CASRefWriterAppendLane, InvalidBatchEntryGetsOwnExceptionBatchSurvives)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/invalid_entry"};
+    publishEmptyPart(store, ns, "good");
+
+    std::mutex m;
+    std::condition_variable cv;
+    bool entered = false;
+    store->setRefPreCarveHookForTest([&]
+    {
+        std::unique_lock lk(m);
+        if (entered)
+            return;
+        entered = true;
+        cv.notify_all();
+        cv.wait(lk, [&] { return store->refQueuePendingForTest(ns) >= 2; });
+    });
+
+    const uint64_t put_before = backend->putTotal();
+    std::exception_ptr bad_error;
+    std::thread t_bad([&]
+    {
+        try { store->dropRef(ns, "does_not_exist"); }
+        catch (...) { bad_error = std::current_exception(); }
+    });
+    {
+        std::unique_lock lk(m);
+        cv.wait(lk, [&] { return entered; });
+    }
+    std::thread t_good([&] { store->dropRef(ns, "good"); });
+    while (store->refQueuePendingForTest(ns) < 2)
+        std::this_thread::yield();
+    cv.notify_all();
+    t_bad.join();
+    t_good.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    ASSERT_TRUE(bad_error != nullptr) << "the invalid item's OWN caller must receive its exception";
+    expectThrowsCode(DB::ErrorCodes::FILE_DOESNT_EXIST, [&] { std::rethrow_exception(bad_error); });
+    EXPECT_EQ(backend->putTotal(), put_before + 1) << "the survivor's own transaction still costs one create";
+    EXPECT_FALSE(store->resolveRef(ns, "good").has_value()) << "the innocent co-batched drop must land";
+}
+
+/// ===================================================================================
+/// Append lane: wedge semantics (spec §Writer-Side Linearization)
+/// ===================================================================================
+
+TEST(CASRefWriterAppendLane, WedgedLaneBlocksSameTableWhileOtherTableProceeds)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns_a{"srv1/wedge_a"};
+    const RootNamespace ns_b{"srv1/wedge_b"};
+    publishEmptyPart(store, ns_a, "x");
+    publishEmptyPart(store, ns_a, "x_second");
+    publishEmptyPart(store, ns_b, "y");
+
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns_a)) + "_log/";
+    backend->fault_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns_a, "x"); });
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns_a));
+
+    /// A different table proceeds normally while ns_a stays wedged.
+    EXPECT_NO_THROW(store->dropRef(ns_b, "y"));
+    EXPECT_FALSE(store->resolveRef(ns_b, "y").has_value());
+
+    /// Retrying ns_a does not allocate a later id -- it re-attempts the SAME one. The wedge's key was
+    /// never actually written (the fault never wrote through), and under the every-attempt rule the
+    /// retry is a conditional CREATE of the same bytes rather than a bare read: it lands, which makes
+    /// the wedged transaction durable and adopts it. That is the point of the rule -- a bare read could
+    /// only ever report "absent", which is not a rejection, and the lane would stay wedged forever over
+    /// a key nothing had written. See `gtest_cas_ref_wedge_every_attempt.cpp` for the full rule.
+    EXPECT_NO_THROW(store->dropRef(ns_a, "x_second"));
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns_a)) << "the retry's own create resolves the lane";
+    EXPECT_FALSE(store->resolveRef(ns_a, "x").has_value()) << "the wedged drop was adopted on resolution";
+}
+
+TEST(CASRefWriterAppendLane, WedgedAppendObservedDurableAppliesBeforeNextId)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/wedge_unwedge"};
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    ASSERT_TRUE(store->resolveRef(ns, "x").has_value()) << "not yet applied while wedged";
+
+    /// The earlier request eventually lands server-side; the caller just never saw the ack.
+    backend->materializePendingDelayedWrite();
+
+    /// A later mutation on the SAME table first resolves the wedge (applying "drop x" to cache) BEFORE
+    /// allocating its own next id (which drops "y").
+    EXPECT_NO_THROW(store->dropRef(ns, "y"));
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_FALSE(store->resolveRef(ns, "x").has_value()) << "the wedged drop was applied on resolution";
+    EXPECT_FALSE(store->resolveRef(ns, "y").has_value()) << "the next mutation committed normally afterward";
+}
+
+/// Wedge tail-counter accounting across the three states (xhigh review, item F): an UNRESOLVED wedge
+/// applied nothing, so it must NOT bump the applied-above-snapshot tail counters; a RESOLVED wedge is a
+/// commit like any other and MUST bump them (exactly once) alongside the ordinary commit that resolves
+/// it; and the resolution must fold its applied overlay in place (no residual committed overlay). Under
+/// the default 256-log / 1 MiB snapshot thresholds this handful of txns never triggers a publish, so the
+/// tail counter is a stable running count.
+TEST(CASRefWriterAppendLane, WedgeResolutionJoinsTailCountersAndFoldsOverlay)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/wedge_tail"};
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    const size_t tail_after_setup = store->tailSinceSnapshotCountForTest(ns);
+
+    /// Wedge the lane: the single-attempt budget turns the ambiguous log PUT into an Unresolved outcome.
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    ASSERT_TRUE(store->resolveRef(ns, "x").has_value()) << "not applied while merely wedged";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_after_setup)
+        << "an UNRESOLVED wedge applied nothing and must not join the tail counters";
+
+    /// The wedged PUT actually landed server-side; a later mutation resolves the wedge (applying drop x)
+    /// before committing its own drop y.
+    backend->materializePendingDelayedWrite();
+    EXPECT_NO_THROW(store->dropRef(ns, "y"));
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_FALSE(store->resolveRef(ns, "x").has_value()) << "the wedged drop was applied on resolution";
+    EXPECT_FALSE(store->resolveRef(ns, "y").has_value());
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), tail_after_setup + 2)
+        << "the RESOLVED wedge (drop x) and the ordinary commit (drop y) must each bump the tail once";
+    EXPECT_EQ(store->committedOverlayEntriesForTest(ns), 0u)
+        << "both the wedge resolution and the ordinary commit fold their overlay in place at install";
+}
+
+/// B3: `Pool::wedgedRefLaneCount()` (the accessor `CasGcScheduler::gcHealth()` reads for
+/// `system.cas_mounts.wedged_namespace_count`) must count EXACTLY the tables with a live
+/// wedge -- neither a cached-but-healthy table nor an unrelated table's own successful mutation may move
+/// it, and it must track the wedge's full lifecycle (0 -> 1 -> 0), not just a one-shot snapshot.
+TEST(CASRefWriterAppendLane, WedgedRefLaneCountTracksExactlyTheWedgedTableThroughItsLifecycle)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns_a{"srv1/wedge_count_a"};
+    const RootNamespace ns_b{"srv1/wedge_count_b"};
+    publishEmptyPart(store, ns_a, "x");
+    publishEmptyPart(store, ns_a, "y");
+    publishEmptyPart(store, ns_b, "p");
+    ASSERT_EQ(store->wedgedRefLaneCount(), 0u) << "both tables cached and healthy before the fault";
+
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns_a)) + "_log/";
+    backend->fault_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns_a, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns_a));
+    EXPECT_EQ(store->wedgedRefLaneCount(), 1u);
+
+    /// ns_b's own mutation succeeds and must not be swept into the count.
+    EXPECT_NO_THROW(store->dropRef(ns_b, "p"));
+    EXPECT_EQ(store->wedgedRefLaneCount(), 1u) << "an unrelated table's successful mutation must not move the count";
+
+    /// The earlier request eventually lands server-side; resolving ns_a's wedge on its next mutation
+    /// drops the count back to zero.
+    backend->materializePendingDelayedWrite();
+    EXPECT_NO_THROW(store->dropRef(ns_a, "y"));
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns_a));
+    EXPECT_EQ(store->wedgedRefLaneCount(), 0u);
+}
+
+/// ===================================================================================
+/// I1: a CORRUPTED_DATA from the retry controller (resolve-before-reissue observed a DIFFERENT object at
+/// the exact key) must be surfaced LOUDLY to the caller and never hang the table's append queue. The
+/// unfixed code let the throw propagate through the leader loop with `leader_active` still true, so every
+/// queued and future caller for that table blocked forever in `cv.wait`.
+/// ===================================================================================
+
+/// Append-site CORRUPTED_DATA: the offending caller gets the error, the lane is NOT wedged (a proven
+/// different-object conflict is conclusive, not uncertain), and no caller HANGS -- the queue's leader
+/// bookkeeping is restored, proven by a bounded wait on both a same-table and an independent-table
+/// append.
+///
+/// The reaction is now the mount's, not the table's [review I5]: a foreign object at a key that
+/// mount-lease exclusivity says is exclusively ours contradicts the exclusivity itself, so the append
+/// site routes through `reportImpossibleInterference` exactly as the wedge-resolve site does -- fence
+/// closed, remount scheduled. Before this task it failed closed and stayed closed, blocking the table
+/// until somebody remounted by hand. So there are two separate scopes to keep straight, and this test
+/// pins both:
+///   the FENCE is mount-wide -- while it is closed EVERY lane is refused, including untouched ones;
+///   the DAMAGE is per-namespace -- a real remount replaces both immutable runtimes, then recovery of
+///   the damaged stream still refuses while the unrelated table commits normally.
+TEST(CASRefWriterAppendLane, I1AppendCorruptionSurfacesAndFencesTheMountForRemount)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/i1_append"};
+    const RootNamespace other{"srv1/i1_other"};
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, other, "z");
+
+    /// The next `_log` PUT for `ns` has a foreign different object land at its key; resolve-before-reissue
+    /// then observes the mismatch and raises CORRUPTED_DATA.
+    backend->corrupt_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->corrupt_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "x"); });
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns)) << "a proven different-object conflict must not wedge the lane";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Faulted);
+    EXPECT_FALSE(store->mayMutate()) << "the impossible-interference reaction must fence this mount closed";
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), 1u)
+        << "the append site must schedule the remount that re-derives this table from the durable log";
+
+    /// Mount-wide while fenced -- and, crucially, PROMPT: a real cv hang would time out this wait, which
+    /// is the regression this test was written for.
+    auto fenced = std::async(std::launch::async, [&] { store->dropRef(other, "z"); });
+    ASSERT_EQ(fenced.wait_for(std::chrono::seconds(10)), std::future_status::ready)
+        << "the independent-table append hung -- the queue's leader bookkeeping was not restored";
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { fenced.get(); });
+
+    /// Drive the scheduled production recovery boundary. A direct fence re-arm is intentionally NOT a
+    /// substitute anymore: immutable runtimes retain the generation that admitted them and cannot be
+    /// rebound to the new one.
+    const String mount_key = layout.mountKey("test");
+    const auto mount = backend->get(mount_key);
+    ASSERT_TRUE(mount);
+    MountLease fenced_mount = decodeMountLease(mount->bytes);
+    fenced_mount.gc_fenced = true;
+    fenced_mount.seq += 1;
+    ASSERT_EQ(backend->putOverwrite(mount_key, encodeMountLease(fenced_mount), mount->token).outcome,
+        PutOutcome::Done);
+    ASSERT_TRUE(store->tryRemountOnce());
+
+    auto same = std::async(std::launch::async, [&] { store->dropRef(ns, "x"); });
+    ASSERT_EQ(same.wait_for(std::chrono::seconds(10)), std::future_status::ready)
+        << "the same-table append hung -- the queue's leader bookkeeping was not restored";
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { same.get(); });
+
+    auto indep = std::async(std::launch::async, [&] { store->dropRef(other, "z"); });
+    ASSERT_EQ(indep.wait_for(std::chrono::seconds(10)), std::future_status::ready);
+    indep.get();
+    EXPECT_FALSE(store->resolveRef(other, "z").has_value())
+        << "an unrelated table's stream is independent and must be entirely unaffected by the damage";
+}
+
+/// Wedge-resolve-site foreign interference: a wedged lane whose key a foreign writer overwrote must
+/// surface the anomaly to the triggering caller and fault the lane, without hanging.
+/// rev.6 Task 11 (spec §anomaly-policy): under the mount-lease exclusivity model this is no longer a
+/// possible protocol outcome (the wedged key is exclusively ours) -- it routes through
+/// `reportImpossibleInterference`, which fences the mount and schedules a remount.
+///
+/// It surfaces as `CORRUPTED_DATA`. It was `LOGICAL_ERROR` between rev.6 and the every-attempt rule,
+/// and that had a cost this test used to carry: `LOGICAL_ERROR` ABORTS the process in debug/sanitizer
+/// builds, so the whole test had to be release-only with a death-test twin standing in elsewhere.
+/// Storage-controlled input must never be able to abort the server, so the arm now reports the
+/// occupant for what it is -- corruption -- and one test covers every build.
+TEST(CASRefWriterAppendLane, I1WedgeResolveCorruptionSurfacesAndFaultsLane)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/i1_wedge"};
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    /// Wedge the lane with an ambiguous PUT that never landed.
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+
+    /// A foreign writer lands a DIFFERENT object at the exact wedged key; the next append's wedge resolve
+    /// observes the mismatch and must raise `CORRUPTED_DATA` to that caller while faulting the lane.
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_FALSE(wedged_key.empty());
+    ASSERT_EQ(backend->putIfAbsent(wedged_key, "a-different-object").outcome, PutOutcome::Done);
+
+    auto fut = std::async(std::launch::async, [&]
+    {
+        expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "y"); });
+    });
+    ASSERT_EQ(fut.wait_for(std::chrono::seconds(10)), std::future_status::ready)
+        << "the wedge-resolve anomaly hung the queue instead of surfacing to the caller";
+    fut.get();
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Faulted)
+        << "foreign interference is a terminal lane verdict";
+
+    /// The queue's leader bookkeeping was restored, so a SUBSEQUENT same-table caller does not hang: it
+    /// observes the terminal state and returns promptly (a real cv hang would time out this bounded
+    /// wait). This is the leg the unfixed code left blocked forever.
+    auto fut2 = std::async(std::launch::async, [&]
+    {
+        try
+        {
+            store->dropRef(ns, "y");
+        }
+        catch (...) // NOLINT(bugprone-empty-catch)
+        {
+            /// The anomaly is expected here; this future only verifies that the caller does not hang.
+        }
+    });
+    ASSERT_EQ(fut2.wait_for(std::chrono::seconds(10)), std::future_status::ready)
+        << "a later same-table append hung -- the leader bookkeeping was not restored after the anomaly";
+    fut2.get();
+}
+
+/// ===================================================================================
+/// rev.6 Task 11: wedge hard contract + anomaly policy (spec §anomaly-policy)
+/// ===================================================================================
+
+/// Foreign bytes at a wedge key (see `I1WedgeResolveCorruptionSurfacesAndFaultsLane` above for the
+/// hang-freedom coverage) must ALSO trip the local write fence closed and audit a `ForeignInterference`
+/// event -- the full anomaly-policy reaction, not just the throw. It runs in every build now that the
+/// arm reports `CORRUPTED_DATA` instead of the process-aborting `LOGICAL_ERROR`; the death twin that
+/// used to stand in for debug/sanitizer builds went with it.
+TEST(CASAnomalyPolicy, ForeignBytesAtWedgeKeyTripFenceAndRemount)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    SynchronizedEventLog seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/anomaly_wedge"};
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    store->setEventSink([&](const CasEvent & e) { seen.add(e); });
+
+    /// Wedge the lane with an ambiguous PUT that never landed.
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    ASSERT_TRUE(store->mayMutate()) << "the fence must not be tripped yet -- only an ordinary Unresolved wedge so far";
+    ASSERT_EQ(store->scheduleRemountCallCountForTest(), 0u) << "no remount must have been scheduled yet by the ordinary wedge alone";
+
+    /// Out-of-band, a foreign writer lands DIFFERENT bytes at the exact wedged key.
+    const String wedged_key = store->wedgedKeyForTest(ns);
+    ASSERT_FALSE(wedged_key.empty());
+    ASSERT_EQ(backend->putIfAbsent(wedged_key, "a-different-object").outcome, PutOutcome::Done);
+
+    /// The next append's wedge resolve observes the mismatch: CORRUPTED_DATA, the fence trips closed,
+    /// and a ForeignInterference event is audited.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "y"); });
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Faulted)
+        << "foreign interference must fault the lane";
+    EXPECT_FALSE(store->mayMutate()) << "the local write fence must trip closed on the anomaly";
+    /// Positively pins that `reportImpossibleInterference` called `scheduleRemount` (not just
+    /// `tripMountLost`, which alone already accounts for `mayMutate() == false` above). Counted at
+    /// `scheduleRemount`'s own entry regardless of `background_watermark` -- see that accessor's
+    /// comment for why this test deliberately does NOT enable `background_watermark` to observe a real
+    /// spawned thread: doing so was tried and makes the store's self-remount attempt race its own
+    /// still-live keeper for 30+ seconds per call (confirmed while building this test), which is not
+    /// something a fast unit test should be driving.
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), 1u)
+        << "reportImpossibleInterference must have called scheduleRemount exactly once";
+
+    const std::vector<CasEvent> observed = seen.snapshot();
+    const auto has_event = std::any_of(observed.begin(), observed.end(),
+        [](const CasEvent & e) { return e.type == CasEventType::ForeignInterference; });
+    EXPECT_TRUE(has_event) << "a ForeignInterference CasEvent must be audited";
+}
+
+/// An impossible non-`Ready` state at new-id allocation must refuse before minting an id, fault the
+/// lane, and trigger the anomaly policy. The synthetic wedge is injected after the top-of-flush
+/// resolver gate, so it represents an internal lifecycle contradiction rather than a normal wedge.
+TEST(CASAnomalyPolicy, NonReadyAtNewIdAllocationFaultsAndFailsClosed)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    SynchronizedEventLog seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/wedge_contract"};
+    publishEmptyPart(store, ns, "x");
+
+    store->setEventSink([&](const CasEvent & e) { seen.add(e); });
+
+    store->setRefPreCarveHookForTest([&]
+    {
+        store->forceWedgeForTest(ns, /*writer_epoch*/ 1, /*ref_sequence*/ 1, "bogus/_log/key", "bogus-bytes");
+    });
+
+    /// Ground truth: no NEW `_log` object may appear -- the guard must refuse BEFORE any id is minted
+    /// or PUT attempted.
+    auto countLogObjects = [&]
+    {
+        size_t n = 0;
+        String cursor;
+        for (;;)
+        {
+            const ListPage page = backend->list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+            for (const ListedKey & lk : page.keys)
+            {
+                const auto parsed = layout.parseRefObjectKey(lk.key);
+                if (parsed && parsed->life_id == DB::Cas::tests::fixture::fixtureLife(ns).incarnation && parsed->kind == RefObjectKind::Log)
+                    ++n;
+            }
+            if (page.next_cursor.empty())
+                break;
+            cursor = page.next_cursor;
+        }
+        return n;
+    };
+    const size_t log_objects_before = countLogObjects();
+
+    ASSERT_TRUE(store->mayMutate()) << "the fence must be armed BEFORE the wedge-contract violation, or the guard would trivially pass for the wrong reason";
+    ASSERT_EQ(store->scheduleRemountCallCountForTest(), 0u) << "no remount must have been scheduled yet";
+
+    /// BACKLOG `{#lane-terminal-reported-as-retryable}`: `Faulted` is a TERMINAL lane state, the same
+    /// one every OTHER `Faulted` arm in `commitRefChunk` reports as `CORRUPTED_DATA` -- reporting it as
+    /// `NETWORK_ERROR`/retry-later would tell the caller a state the lane can never leave on its own is
+    /// transient.
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->dropRef(ns, "x"); });
+
+    EXPECT_EQ(countLogObjects(), log_objects_before) << "the release guard must refuse before allocating/PUTting a new _log object";
+    EXPECT_EQ(store->laneStateForTest(ns), RefLaneState::Faulted)
+        << "the invariant violation must have one explicit terminal state";
+    EXPECT_FALSE(store->mayMutate()) << "the local write fence must trip closed on the wedge-contract violation";
+    /// See the sibling test's comment on why this checks the call-count seam (never `background_watermark`
+    /// + a real thread -- that combination makes the store's self-remount race its own still-live keeper).
+    EXPECT_EQ(store->scheduleRemountCallCountForTest(), 1u)
+        << "reportImpossibleInterference must have called scheduleRemount exactly once";
+
+    const std::vector<CasEvent> observed = seen.snapshot();
+    const auto has_event = std::any_of(observed.begin(), observed.end(),
+        [](const CasEvent & e) { return e.type == CasEventType::ForeignInterference; });
+    EXPECT_TRUE(has_event) << "a ForeignInterference CasEvent must be audited";
+}
+
+/// I3: a conditional write whose attempt classified Committed but whose FINAL post-write fence check
+/// failed (the mount fence was lost after the write may have landed) is counted separately, not folded
+/// into the generic Unresolved classifier (spec §Late Predecessor PUT best-effort diagnostic).
+TEST(CASRequestControllerFenceLoss, I3PostWriteFenceLossIsCounted)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<CountingBackend>();
+    CasRequestBudget budget;
+    budget.max_attempts = 3;
+    CasRequestController ctrl(backend, budget, [] { return static_cast<uint64_t>(0); });   // fixed clock
+
+    /// `fence_ok` holds for the pre-attempt check, then is lost by the post-write check.
+    int calls = 0;
+    auto fence_ok = [&calls] { return ++calls <= 1; };
+
+    const auto before = global_counters[ProfileEvents::CASConditionalWriteFenceLostPostWrite].load();
+    const CasWriteOutcome outcome = ctrl.putIfAbsentControlled("k", "v", fence_ok);
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved) << "a post-write fence loss must never be reported as Committed";
+    EXPECT_EQ(global_counters[ProfileEvents::CASConditionalWriteFenceLostPostWrite].load(), before + 1);
+}
+
+/// Task B (stageManifest rides the controller): a Committed return surfaces the committed
+/// incarnation's token — from the attempt's own PutResult, and equally from a resolve that proves an
+/// earlier ambiguous attempt landed — so audit emitters (`PartWriteTxn::stageManifest`'s `ManifestPut`
+/// event) keep their token without a follow-up HEAD.
+TEST(CASRequestController, CommittedSurfacesTokenFromPutAndFromResolve)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    CasRequestController ctrl(backend, CasRequestBudget{}, [] { return static_cast<uint64_t>(0); });
+    const auto fence_ok = [] { return true; };
+
+    Token direct_token;
+    ASSERT_EQ(ctrl.putIfAbsentControlled("k1", "v1", fence_ok, &direct_token), CasWriteOutcome::Committed);
+    EXPECT_EQ(direct_token, backend->head("k1").token) << "the direct-commit token is the PutResult's";
+
+    /// k2 already holds the IDENTICAL bytes (an earlier ambiguous attempt that landed): the attempt's
+    /// PreconditionFailed collapses to Unresolved and the resolve GET proves Committed — the token must
+    /// be the observed incarnation's, and no second incarnation is ever created.
+    const Token pre_existing = backend->putIfAbsent("k2", "v2").token;
+    Token resolved_token;
+    ASSERT_EQ(ctrl.putIfAbsentControlled("k2", "v2", fence_ok, &resolved_token), CasWriteOutcome::Committed);
+    EXPECT_EQ(resolved_token, pre_existing) << "the resolve-commit token is the observed incarnation's";
+}
+
+/// ===================================================================================
+/// Task 13: whole-table ref-cache eviction (spec §Byte, Memory, And CPU Budget)
+/// ===================================================================================
+
+/// A tiny cache budget forces WHOLE-TABLE eviction: publishing to several tables in turn keeps only the
+/// most-recently-touched one resident, and an evicted table re-recovers its exact committed state on the
+/// next touch (spec §Startup And Recovery: "Evicting the table drops the entire object; the next access
+/// repeats recovery").
+TEST(CASRefTableCacheEviction, WholeTableEvictionUnderBudgetReRecovers)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPoolWithConfig(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .ref_table_cache_bytes = 1});
+    const RootNamespace ns_a{"srv1/evict_a"};
+    const RootNamespace ns_b{"srv1/evict_b"};
+    const RootNamespace ns_c{"srv1/evict_c"};
+
+    publishEmptyPart(store, ns_a, "x");
+    publishEmptyPart(store, ns_b, "y");
+    publishEmptyPart(store, ns_c, "z");
+
+    /// A 1-byte budget is below one table's weight, so each new table evicts the prior idle ones: only
+    /// the last-touched table stays resident (the just-recovered table is never evicted).
+    EXPECT_EQ(store->refTablesCachedCountForTest(), 1u);
+    EXPECT_TRUE(store->refTableCachedForTest(ns_c));
+    EXPECT_FALSE(store->refTableCachedForTest(ns_a));
+    EXPECT_FALSE(store->refTableCachedForTest(ns_b));
+
+    /// The evicted table re-recovers its exact committed state on next touch.
+    const auto resolved = store->resolveRef(ns_a, "x");
+    ASSERT_TRUE(resolved.has_value()) << "an evicted table must re-recover its committed ref";
+    /// That touch, in turn, evicted the previously-resident table under the same budget.
+    EXPECT_TRUE(store->refTableCachedForTest(ns_a));
+    EXPECT_FALSE(store->refTableCachedForTest(ns_c));
+}
+
+/// A zero budget disables eviction entirely: every touched table stays resident.
+TEST(CASRefTableCacheEviction, ZeroBudgetDisablesEviction)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPoolWithConfig(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test", .ref_table_cache_bytes = 0});
+    for (const String & n : {String("srv1/keep_a"), String("srv1/keep_b"), String("srv1/keep_c")})
+        publishEmptyPart(store, RootNamespace{n}, "x");
+    EXPECT_EQ(store->refTablesCachedCountForTest(), 3u);
+}
+
+/// A table with a WEDGED append lane is never evicted, even when idle and over budget: its uncertain
+/// in-flight PUT is not reconstructable from the durable objects (spec §Writer-Side Linearization), so
+/// re-recovery must not be allowed to drop and re-materialize it (which could re-allocate an id).
+TEST(CASRefTableCacheEviction, WedgedTableIsNeverEvicted)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPoolWithConfig(backend,
+        PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                   .cas_request_budget = budget, .ref_table_cache_bytes = 1});
+    const Layout & layout = store->layout();
+    const RootNamespace ns_w{"srv1/wedged"};
+    publishEmptyPart(store, ns_w, "x");
+
+    /// Wedge ns_w's append lane with one ambiguous (Unresolved) PUT that exhausts the single-attempt budget.
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns_w)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns_w, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns_w));
+
+    /// Pressure the cache with other tables. ns_w is idle and over the 1-byte budget, but its wedged lane
+    /// makes it non-evictable, so its wedge state survives (a fresh runtime would report no wedge).
+    publishEmptyPart(store, RootNamespace{"srv1/other_a"}, "y");
+    publishEmptyPart(store, RootNamespace{"srv1/other_b"}, "z");
+
+    EXPECT_TRUE(store->refTableCachedForTest(ns_w)) << "a wedged table must never be evicted";
+    EXPECT_TRUE(store->refLaneWedgedForTest(ns_w)) << "and its wedge state survives";
+}
+
+/// ===================================================================================
+/// Task 11: snapshot publication (spec §writer-snapshot-publication)
+/// ===================================================================================
+
+/// The count threshold fires a background publish covering the whole retained tail; its bytes must
+/// equal an INDEPENDENT oracle's replay of the same logs through the published id (cache-replay
+/// equivalence), and the retained tail must be fully pruned afterward (spec: "Publication is
+/// background and never blocks an append").
+TEST(CASRefWriterSnapshotPublish, ThresholdTriggerPublishesCacheReplayEquivalentBytes)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 3;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+    const RootNamespace ns{"srv1/threshold_publish"};
+
+    publishEmptyPart(store, ns, "a");   /// tail: 2 (birth+add, promote)
+    publishEmptyPart(store, ns, "b");   /// tail: 3, then 4 (4 > 3 -> dispatches ONE background publish)
+
+    store->waitForSnapshotPublishSettleForTest(ns);
+
+    const auto snap_id = listGreatestSnapshotIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(snap_id.has_value()) << "the threshold trigger must have published a snapshot";
+    EXPECT_TRUE(store->newestPublishedSnapshotIdForTest(ns) == snap_id);
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 0u) << "a snapshot covering everything prunes the whole tail";
+
+    const auto got = backend->get(layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), *snap_id));
+    ASSERT_TRUE(got.has_value());
+
+    /// The independent oracle: replay every `_log/` object directly, ignoring the snapshot entirely.
+    const RefTableState oracle = independentFullReplayForTest(*backend, layout, ns, snap_id);
+    const String expected_bytes = encodeRefTableSnapshot(snapshotOf(oracle, ns.string()));
+    EXPECT_EQ(openObject(FormatId::RefSnapshot, got->bytes), expected_bytes)
+        << "published snapshot bytes must equal replay(logs through X)";
+}
+
+/// A publisher owns the runtime it captured, not the logical name. If that exact life is deleted and
+/// the name is reborn while snapshot bytes are still only local, the old attempt must become inert: in
+/// particular it must not recreate the predecessor's `_snap` or `_ckpt` after the GC retired them.
+TEST(CASRefWriterSnapshotPublish, CapturedPredecessorCannotPublishAfterSameNameRebirth)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    config.snapshot_log_count_threshold = 1ULL << 40;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/publisher-predecessor-rebirth"};
+
+    publishWithProductionBirth(store, ns, "predecessor");
+    const CatalogEntry predecessor = catalogEntryOrThrow(*backend, layout, ns);
+    const NamespaceLifeId predecessor_life
+        = NamespaceLifeId::fromCatalogEntry(ns, predecessor.incarnation);
+    const auto predecessor_snapshot
+        = listGreatestLogIdForLifeForTest(*backend, layout, predecessor_life);
+    ASSERT_TRUE(predecessor_snapshot);
+
+    Gc gc(store, UInt128{105});
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool captured = false;
+    bool release = false;
+    store->setSnapshotAfterCaptureHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        captured = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release; });
+    });
+
+    auto publisher = std::async(std::launch::async, [&]() -> bool
+    {
+        return store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns);
+    });
+    SCOPE_EXIT({
+        {
+            std::lock_guard lock(mutex);
+            release = true;
+        }
+        cv.notify_all();
+    });
+    {
+        std::unique_lock lock(mutex);
+        ASSERT_TRUE(cv.wait_for(lock, std::chrono::seconds(10), [&] { return captured; }));
+    }
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    EXPECT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    EXPECT_TRUE(runRegularRoundReclaiming(gc).deferred);
+    const RefCatalog after_removal = CasRefCatalog::read(*backend, layout).catalog;
+    EXPECT_TRUE(std::none_of(after_removal.entries.begin(), after_removal.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; }));
+
+    publishWithProductionBirth(store, ns, "successor");
+    const uint64_t successor_runtime = store->refTableRuntimeIdentityForTest(ns);
+    const CatalogEntry successor = catalogEntryOrThrow(*backend, layout, ns);
+    EXPECT_NE(successor.incarnation, predecessor.incarnation);
+    const auto predecessor_ckpt_before_resume = backend->get(layout.refCkptKey(predecessor_life));
+    ASSERT_TRUE(predecessor_ckpt_before_resume)
+        << "the removal protocol leaves this checkpoint as janitor-owned predecessor debris";
+
+    {
+        std::lock_guard lock(mutex);
+        release = true;
+    }
+    cv.notify_all();
+    ASSERT_EQ(publisher.wait_for(std::chrono::seconds(10)), std::future_status::ready);
+    EXPECT_FALSE(publisher.get());
+    store->setSnapshotAfterCaptureHookForTest(nullptr);
+
+    EXPECT_FALSE(backend->get(layout.refSnapshotKey(predecessor_life, *predecessor_snapshot)))
+        << "a stale publisher recreated the retired predecessor snapshot";
+    const auto predecessor_ckpt_after_resume = backend->get(layout.refCkptKey(predecessor_life));
+    ASSERT_TRUE(predecessor_ckpt_after_resume);
+    EXPECT_EQ(predecessor_ckpt_after_resume->token, predecessor_ckpt_before_resume->token)
+        << "a stale publisher replaced the retired predecessor checkpoint";
+    EXPECT_EQ(predecessor_ckpt_after_resume->bytes, predecessor_ckpt_before_resume->bytes)
+        << "a stale publisher changed the retired predecessor checkpoint";
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), successor_runtime);
+    EXPECT_TRUE(store->resolveRef(ns, "successor"));
+}
+
+/// The runtime admission check belongs inside every retrying `_ckpt` CAS attempt, not merely before
+/// calling the checkpoint helper. Retirement in the body-PUT/checkpoint gap leaves the already-written
+/// snapshot as harmless debris but must not advance or recreate the predecessor checkpoint.
+TEST(CASRefWriterSnapshotPublish, RetiredPredecessorCannotAdvanceCkptAfterSnapshotPut)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    config.snapshot_log_count_threshold = 1ULL << 40;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/publisher-predecessor-ckpt-race"};
+
+    publishWithProductionBirth(store, ns, "predecessor");
+    const CatalogEntry predecessor = catalogEntryOrThrow(*backend, layout, ns);
+    const NamespaceLifeId predecessor_life
+        = NamespaceLifeId::fromCatalogEntry(ns, predecessor.incarnation);
+    const auto candidate_id = listGreatestLogIdForLifeForTest(*backend, layout, predecessor_life);
+    ASSERT_TRUE(candidate_id);
+
+    Gc gc(store, UInt128{106});
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool before_ckpt_cas = false;
+    bool release = false;
+    store->setSnapshotBeforeCkptCasHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        before_ckpt_cas = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release; });
+    });
+
+    auto publisher = std::async(std::launch::async, [&]() -> bool
+    {
+        return store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns);
+    });
+    SCOPE_EXIT({
+        {
+            std::lock_guard lock(mutex);
+            release = true;
+        }
+        cv.notify_all();
+    });
+    {
+        std::unique_lock lock(mutex);
+        ASSERT_TRUE(cv.wait_for(lock, std::chrono::seconds(10), [&] { return before_ckpt_cas; }));
+    }
+    EXPECT_TRUE(backend->get(layout.refSnapshotKey(predecessor_life, *candidate_id)))
+        << "the hook must run after the snapshot body PUT and immediately before `_ckpt` admission";
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    EXPECT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    EXPECT_TRUE(runRegularRoundReclaiming(gc).deferred);
+    const RefCatalog after_removal = CasRefCatalog::read(*backend, layout).catalog;
+    EXPECT_TRUE(std::none_of(after_removal.entries.begin(), after_removal.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; }));
+
+    publishWithProductionBirth(store, ns, "successor");
+    const uint64_t successor_runtime = store->refTableRuntimeIdentityForTest(ns);
+    const CatalogEntry successor = catalogEntryOrThrow(*backend, layout, ns);
+    EXPECT_NE(successor.incarnation, predecessor.incarnation);
+    const auto predecessor_ckpt_before_resume = backend->get(layout.refCkptKey(predecessor_life));
+    ASSERT_TRUE(predecessor_ckpt_before_resume);
+
+    {
+        std::lock_guard lock(mutex);
+        release = true;
+    }
+    cv.notify_all();
+    ASSERT_EQ(publisher.wait_for(std::chrono::seconds(10)), std::future_status::ready);
+    EXPECT_FALSE(publisher.get());
+    store->setSnapshotBeforeCkptCasHookForTest(nullptr);
+
+    const auto predecessor_ckpt_after_resume = backend->get(layout.refCkptKey(predecessor_life));
+    ASSERT_TRUE(predecessor_ckpt_after_resume);
+    EXPECT_EQ(predecessor_ckpt_after_resume->token, predecessor_ckpt_before_resume->token);
+    EXPECT_EQ(predecessor_ckpt_after_resume->bytes, predecessor_ckpt_before_resume->bytes);
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), successor_runtime);
+    EXPECT_TRUE(store->resolveRef(ns, "successor"));
+}
+
+/// A read that already owns the predecessor runtime does not consult the name slot again after a
+/// same-name successor is published. Because removal applies the terminal state before retirement, a
+/// reader paused immediately before its state lock resumes with `NotFound`, never successor data.
+TEST(CASRefWriterRuntimeIdentity, CapturedReaderCannotRetargetSameNameSuccessor)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    config.snapshot_log_count_threshold = 1ULL << 40;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/captured-reader-rebirth"};
+
+    publishWithProductionBirth(store, ns, "shared");
+    ASSERT_TRUE(store->resolveRef(ns, "shared"));
+    const CatalogEntry predecessor = catalogEntryOrThrow(*backend, layout, ns);
+    Gc gc(store, UInt128{107});
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool captured = false;
+    bool release = false;
+    store->setReadBeforeStateLockHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        if (captured)
+            return;
+        captured = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release; });
+    });
+    auto reader = std::async(std::launch::async, [&]
+    {
+        return store->resolveRef(ns, "shared");
+    });
+    SCOPE_EXIT({
+        {
+            std::lock_guard lock(mutex);
+            release = true;
+        }
+        cv.notify_all();
+    });
+    {
+        std::unique_lock lock(mutex);
+        ASSERT_TRUE(cv.wait_for(lock, std::chrono::seconds(10), [&] { return captured; }));
+    }
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    EXPECT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    EXPECT_TRUE(runRegularRoundReclaiming(gc).deferred);
+    publishWithProductionBirth(store, ns, "shared");
+    const CatalogEntry successor = catalogEntryOrThrow(*backend, layout, ns);
+    EXPECT_NE(successor.incarnation, predecessor.incarnation);
+    const uint64_t successor_runtime = store->refTableRuntimeIdentityForTest(ns);
+    const auto successor_ref = store->resolveRef(ns, "shared");
+    ASSERT_TRUE(successor_ref);
+
+    {
+        std::lock_guard lock(mutex);
+        release = true;
+    }
+    cv.notify_all();
+    ASSERT_EQ(reader.wait_for(std::chrono::seconds(10)), std::future_status::ready);
+    EXPECT_FALSE(reader.get()) << "the captured predecessor reader retargeted through the name slot";
+    store->setReadBeforeStateLockHookForTest(nullptr);
+
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), successor_runtime);
+    const auto successor_after = store->resolveRef(ns, "shared");
+    ASSERT_TRUE(successor_after);
+    EXPECT_EQ(successor_after->manifest_id.ref, successor_ref->manifest_id.ref);
+}
+
+/// Ordinary append admission also owns the runtime it captured. If removal and rebirth complete before
+/// enqueue, the predecessor's closed lane returns retry-later; it cannot enqueue into or mutate the
+/// successor even when the successor deliberately reuses the same logical ref name.
+TEST(CASRefWriterRuntimeIdentity, CapturedAppendCannotEnqueueIntoSameNameSuccessor)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    config.snapshot_log_count_threshold = 1ULL << 40;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/captured-append-rebirth"};
+
+    publishWithProductionBirth(store, ns, "shared");
+    const CatalogEntry predecessor = catalogEntryOrThrow(*backend, layout, ns);
+    Gc gc(store, UInt128{108});
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool captured = false;
+    bool release = false;
+    store->setAppendAfterRuntimeCaptureHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        if (captured)
+            return;
+        captured = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release; });
+    });
+    auto append = std::async(std::launch::async, [&]
+    {
+        store->dropRef(ns, "shared");
+    });
+    SCOPE_EXIT({
+        {
+            std::lock_guard lock(mutex);
+            release = true;
+        }
+        cv.notify_all();
+    });
+    {
+        std::unique_lock lock(mutex);
+        ASSERT_TRUE(cv.wait_for(lock, std::chrono::seconds(10), [&] { return captured; }));
+    }
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    EXPECT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    EXPECT_TRUE(runRegularRoundReclaiming(gc).deferred);
+    publishWithProductionBirth(store, ns, "shared");
+    const CatalogEntry successor = catalogEntryOrThrow(*backend, layout, ns);
+    EXPECT_NE(successor.incarnation, predecessor.incarnation);
+    const uint64_t successor_runtime = store->refTableRuntimeIdentityForTest(ns);
+    const auto successor_ref = store->resolveRef(ns, "shared");
+    ASSERT_TRUE(successor_ref);
+
+    {
+        std::lock_guard lock(mutex);
+        release = true;
+    }
+    cv.notify_all();
+    ASSERT_EQ(append.wait_for(std::chrono::seconds(10)), std::future_status::ready);
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { append.get(); });
+    store->setAppendAfterRuntimeCaptureHookForTest(nullptr);
+
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), successor_runtime);
+    const auto successor_after = store->resolveRef(ns, "shared");
+    ASSERT_TRUE(successor_after);
+    EXPECT_EQ(successor_after->manifest_id.ref, successor_ref->manifest_id.ref);
+}
+
+/// Exact retirement is pointer/key scoped. A delayed notification for the predecessor may arrive after
+/// its same-name successor is already attached; it must not erase or poison that successor slot.
+TEST(CASRefWriterRuntimeIdentity, LatePredecessorInvalidationLeavesSuccessorAttached)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/late-predecessor-invalidation"};
+
+    publishWithProductionBirth(store, ns, "predecessor");
+    const CatalogEntry predecessor = catalogEntryOrThrow(*backend, layout, ns);
+    const NamespaceLifeId predecessor_life
+        = NamespaceLifeId::fromCatalogEntry(ns, predecessor.incarnation);
+    Gc gc(store, UInt128{109});
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    store->dropNamespace(ns);
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).deferred);
+
+    publishWithProductionBirth(store, ns, "successor");
+    const CatalogEntry successor = catalogEntryOrThrow(*backend, layout, ns);
+    ASSERT_NE(successor.incarnation, predecessor.incarnation);
+    const NamespaceLifeId successor_life
+        = NamespaceLifeId::fromCatalogEntry(ns, successor.incarnation);
+    const uint64_t successor_runtime = store->refTableRuntimeIdentityForTest(ns);
+    const auto successor_ref = store->resolveRef(ns, "successor");
+    ASSERT_TRUE(successor_ref);
+
+    store->invalidateRemovedCatalogLife(predecessor_life);
+
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), successor_runtime);
+    EXPECT_EQ(store->refTableLifeForTest(ns), successor_life);
+    const auto successor_after = store->resolveRef(ns, "successor");
+    ASSERT_TRUE(successor_after);
+    EXPECT_EQ(successor_after->manifest_id.ref, successor_ref->manifest_id.ref);
+}
+
+/// Task 13 (spec §implementation-impact): a threshold snapshot publish increments the writer-side
+/// observability counters -- snapshot PUT bytes and the tail-logs-compacted count
+/// (logs-per-table-after-snapshot). Before/after deltas prove both sites fire.
+TEST(CASRefWriterSnapshotPublish, PublishIncrementsSnapshotCounters)
+{
+    using ProfileEvents::global_counters;
+    const auto bytes_before = global_counters[ProfileEvents::CASRefSnapshotPutBytes].load();
+    const auto logs_before  = global_counters[ProfileEvents::CASRefSnapshotTailLogs].load();
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 3;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+    const RootNamespace ns{"srv1/counter_publish"};
+
+    publishEmptyPart(store, ns, "a");
+    publishEmptyPart(store, ns, "b");
+    store->waitForSnapshotPublishSettleForTest(ns);
+
+    ASSERT_TRUE(listGreatestSnapshotIdForTest(*backend, layout, ns).has_value())
+        << "the threshold trigger must have published a snapshot";
+    EXPECT_GT(global_counters[ProfileEvents::CASRefSnapshotPutBytes].load(), bytes_before);
+    EXPECT_GT(global_counters[ProfileEvents::CASRefSnapshotTailLogs].load(), logs_before);
+}
+
+/// A fresh mount that recovers a large PRE-EXISTING tail (left by a predecessor whose own thresholds
+/// never fired) retains that tail as trigger debt. Recovery ends at a terminal epoch seal, which is not
+/// snapshot-serializable; one ordinary successor makes the inherited over-threshold tail publishable.
+/// The single successor alone is below the threshold, so the dispatch still proves the mount-time tail
+/// was retained rather than forgotten during recovery.
+TEST(CASRefWriterSnapshotPublish, MountTimeRecoveredLargeTailPublishesAfterOrdinarySuccessor)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/mount_time_publish"};
+
+    {
+        /// Predecessor: default (high) thresholds, so nothing publishes yet. 3 parts -> 6 tail entries.
+        auto predecessor = openPool(backend);
+        publishEmptyPart(predecessor, ns, "a");
+        publishEmptyPart(predecessor, ns, "b");
+        publishEmptyPart(predecessor, ns, "c");
+    }   /// mount released; the tail is durable but nothing has published it
+
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 3;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto successor = openPoolWithConfig(backend, config);
+
+    /// A mere read triggers recovery. The recovered tail is already above threshold, but its greatest
+    /// applied record is the terminal seal, so there is deliberately no snapshot candidate yet.
+    EXPECT_EQ(successor->listRefs(ns).size(), 3u);
+    successor->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_GT(successor->tailSinceSnapshotCountForTest(ns), config.snapshot_log_count_threshold)
+        << "the mount must retain the predecessor's large uncovered tail";
+    EXPECT_FALSE(listGreatestSnapshotIdForTest(*backend, layout, ns).has_value())
+        << "a terminal recovery seal is not snapshot-serializable";
+
+    /// One ordinary transaction above the seal reopens the candidate. It cannot cross the threshold
+    /// by itself; publication therefore depends on the recovered mount-time tail asserted above.
+    successor->dropRef(ns, "c");
+    successor->waitForSnapshotPublishSettleForTest(ns);
+
+    const auto snap_id = listGreatestSnapshotIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(snap_id.has_value())
+        << "the ordinary successor must make the inherited mount-time tail publishable";
+    EXPECT_EQ(successor->tailSinceSnapshotCountForTest(ns), 0u);
+
+    const auto got = backend->get(layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), *snap_id));
+    ASSERT_TRUE(got.has_value());
+    const RefTableState oracle = independentFullReplayForTest(*backend, layout, ns, snap_id);
+    EXPECT_EQ(openObject(FormatId::RefSnapshot, got->bytes), encodeRefTableSnapshot(snapshotOf(oracle, ns.string())));
+}
+
+/// ===================================================================================
+/// rev.6 Task 10 (spec §publish-from-live): the grace-window machinery
+/// (`snapshot_min_log_age_ms`, the tail-replay-from-`snapshot_base_state` copy-once path,
+/// `CasRefLatePredecessorObserved`) is DELETED. The Task 8 recovery-seal plus the Task 6
+/// recovery seal already makes a late-arriving predecessor write born-covered for every
+/// observer by the time this writer could ever see it, so a young committed txn has nothing left to
+/// wait out -- it is immediately publish-eligible, with no time manipulation anywhere below.
+/// ===================================================================================
+
+/// A just-committed txn is covered by a publish forced immediately afterward -- no fake clock, no
+/// aging, no waiting: the OLD grace-window code would have published nothing here at all.
+TEST(CASRefWriterPublishFromLive, YoungTxnIsCoveredImmediately)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/publish_from_live_young"};
+    auto store = openPool(backend);
+
+    /// Setup: birth the namespace and add a precommit (not the txn under test).
+    auto build = startBuildFor(store, ns, "a");
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, "a", id);
+
+    /// The ONE committed txn under test.
+    build->promote(ns, "a", build->buildId(), id);
+
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns))
+        << "publish-from-live: a just-committed txn is immediately coverable, with no grace window";
+    const auto snap_id = listGreatestSnapshotIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(snap_id.has_value());
+    const auto got = backend->get(layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), *snap_id));
+    ASSERT_TRUE(got.has_value());
+    const RefTableSnapshot snap = decodeRefTableSnapshot(openObject(FormatId::RefSnapshot, got->bytes), ns.string(), *snap_id);
+    ASSERT_EQ(snap.committed.size(), 1u);
+    EXPECT_EQ(snap.committed.front().ref_name, "a")
+        << "the published snapshot body contains the just-promoted row";
+}
+
+/// The count trigger fires purely off the tail counters -- no aging involved -- even under a boot
+/// clock that never advances (the old code REQUIRED aging past `snapshot_min_log_age_ms` to fire).
+TEST(CASRefWriterSnapshotPublish, TriggerFiresOnCountAboveThresholdWithoutAging)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/publish_from_live_trigger"};
+    uint64_t fake_now = 1'000'000;   /// frozen: never advances
+
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 3;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(10'000'000);
+    auto store = openPoolWithConfig(backend, config);
+
+    const auto before = global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load();
+    publishEmptyPart(store, ns, "a");   /// tail: 2
+    publishEmptyPart(store, ns, "b");   /// tail: 4 > 3 -> dispatches, clock frozen throughout
+    store->waitForSnapshotPublishSettleForTest(ns);
+
+    EXPECT_GT(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), before)
+        << "the count trigger must fire without any aging, even under a frozen clock";
+}
+
+/// Adoption subtracts EXACTLY the counters captured at copy time, not whatever the counters read at
+/// adoption time: while a publish's PUT is in flight (captured count/bytes fixed), more commits land
+/// on the live counters. After adoption, the counters must equal precisely the amount appended AFTER
+/// the copy -- not zero (would drop the new txns from the next publish trigger) and not negative/
+/// wrapped (an unsigned underflow).
+TEST(CASRefWriterSnapshotPublish, AdoptionSubtractsCapturedCountersUnderConcurrentAppends)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/publish_from_live_adoption"};
+    auto store = openPool(backend);
+
+    publishEmptyPart(store, ns, "a");
+    ASSERT_EQ(store->tailSinceSnapshotCountForTest(ns), 2u);
+
+    backend->armPutBlock("_snap/");
+    std::thread publisher([&] { store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns); });
+    backend->awaitBlockEntered();   /// the candidate (count=2) is captured; the PUT is now in flight, no lock held
+
+    publishEmptyPart(store, ns, "b");   /// +2 more commits land WHILE the publish's PUT is in flight
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 4u);
+
+    backend->releaseBlock();
+    publisher.join();
+
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 2u)
+        << "adoption must subtract only the CAPTURED count (2), leaving exactly the 2 txns appended "
+           "after the copy";
+}
+
+/// Publication must never block a concurrent append on the SAME table (spec: "Publication is
+/// background and never blocks an append"): while a dispatched background publish is stuck mid-PUT, an
+/// ordinary mutation on the table must still complete promptly (a real deadlock would hang this test).
+TEST(CASRefWriterSnapshotPublish, PublicationNeverBlocksConcurrentAppend)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/publish_no_block"};
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 3;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+
+    backend->armPutBlock("_snap/");
+
+    publishEmptyPart(store, ns, "a");
+    publishEmptyPart(store, ns, "b");   /// tail reaches 4 (> 3) -> dispatches a background publish
+
+    backend->awaitBlockEntered();   /// the dispatched attempt is now stuck mid-PUT on the snapshot key
+
+    /// An unrelated mutation on the SAME table must complete without waiting for the stuck publish.
+    EXPECT_NO_THROW(store->dropRef(ns, "a"));
+    EXPECT_FALSE(store->resolveRef(ns, "a").has_value());
+
+    backend->releaseBlock();
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_TRUE(listGreatestSnapshotIdForTest(*backend, layout, ns).has_value());
+}
+
+/// Review caution (T10 review): a dispatched background publish must never outlive the Pool object
+/// it operates on -- `maybeScheduleSnapshotPublish` captures `shared_from_this()` BY VALUE into the
+/// dispatch lambda specifically to guarantee this (the classic "background thread references a
+/// dangling owner" shutdown segfault, avoided here since a shared_ptr copy keeps the object alive for
+/// as long as the thread holds it, regardless of what every OTHER holder does). Proves it directly:
+/// blocks a dispatched publish mid-PUT, drops the TEST's own (only) Pool handle while still blocked,
+/// and confirms via a `weak_ptr` that the Pool demonstrably survives on the blocked thread's own
+/// reference alone. Then unblocks it with no live Pool handle anywhere in this test any more -- a
+/// dangling-pointer crash here would abort the whole test binary, the strongest possible signal for
+/// this specific hazard.
+TEST(CASRefWriterSnapshotPublish, PublishThreadOutlivesDroppedPoolHandleWithoutCrashing)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/publish_outlives_store"};
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 3;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+
+    backend->armPutBlock("_snap/");
+    publishEmptyPart(store, ns, "a");
+    publishEmptyPart(store, ns, "b");   /// tail reaches 4 (> 3) -> dispatches a background publish
+    backend->awaitBlockEntered();       /// stuck mid-PUT, holding its OWN shared_ptr<Pool> copy
+
+    std::weak_ptr<Pool> weak_store = store;
+    store.reset();   /// drop the ONLY Pool handle this test holds
+    EXPECT_FALSE(weak_store.expired())
+        << "the blocked background thread's own shared_ptr copy must keep the Pool alive";
+
+    backend->releaseBlock();
+    /// Deterministic, sleep-free: waits for the blocked call to actually RETURN (not merely unblock),
+    /// entirely through the backend -- this test holds no Pool handle to wait on any more.
+    backend->awaitBlockedCallCompleted();
+}
+
+/// Review (T11) — CRITICAL: publishes are NOT serialized, so two overlapping attempts can finish out of
+/// order. An OLDER-candidate publish that lands its `_snap` PUT AFTER a newer one already adopted must
+/// NOT regress `newest_snapshot_id` back to its older id, and its (monotonically-skipped) adoption must
+/// NOT touch the tail counters a newer attempt already reset -- either would drop the txns committed in
+/// between, so the NEXT published snapshot would silently omit committed transactions and recovery
+/// would lose refs. Deterministic, sleep-free: the fake backend blocks publish #1's PUT (capturing
+/// exactly its key) while a higher-id publish #2 runs to completion, then unblocks #1.
+TEST(CASRefWriterSnapshotPublish, ConcurrentOutOfOrderPublishDoesNotRegressBaseNorDropCommittedTxns)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/concurrent_publish_monotonic"};
+    PoolConfig config;
+    /// High thresholds: NO automatic background dispatch -- we drive
+    /// `tryPublishSnapshotAndAdvanceCheckpointOnce` directly for full determinism.
+    config.snapshot_log_count_threshold = 1ULL << 40;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+
+    publishEmptyPart(store, ns, "a");   /// tail: 2 (birth+add, promote)
+    publishEmptyPart(store, ns, "b");   /// tail: 4 -- greatest_applied is publish #1's candidate
+
+    /// Block ONLY publish #1's own `_snap` PUT (its exact key is captured on first match); a later,
+    /// different `_snap/<id>` key proceeds unblocked.
+    backend->armPutBlockFirstMatchOnly("_snap/");
+
+    std::thread publisher1([&] { store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns); });
+    backend->awaitBlockEntered();   /// #1 is parked mid-PUT on `_snap/<older>`, holding no lock
+
+    /// While #1 is parked, commit more txns and run publish #2 to COMPLETION: it PUTs a strictly higher
+    /// `_snap/<newer>` (unblocked) and adopts it, resetting the tail counters through its own candidate.
+    publishEmptyPart(store, ns, "c");
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    const auto newest_after_2 = store->newestPublishedSnapshotIdForTest(ns);
+    ASSERT_TRUE(newest_after_2.has_value());
+    ASSERT_EQ(store->tailSinceSnapshotCountForTest(ns), 0u) << "publish #2 covers everything committed so far";
+
+    /// Release #1: on the BUGGY code it now adopts its OLDER candidate, regressing newest below #2
+    /// and/or double-subtracting from the counters #2 already reset. The monotonic guard must skip
+    /// that adoption entirely -- both the `newest_snapshot_id` write and the counter subtraction.
+    backend->releaseBlock();
+    publisher1.join();
+
+    const auto newest_after_1 = store->newestPublishedSnapshotIdForTest(ns);
+    ASSERT_TRUE(newest_after_1.has_value());
+    EXPECT_FALSE(*newest_after_1 < *newest_after_2)
+        << "a late-finishing OLDER publish must not regress newest_snapshot_id below the adopted newer one";
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 0u)
+        << "publish #1's skipped (monotonically-superseded) adoption must not subtract from counters "
+           "publish #2 already reset -- an unguarded subtraction here would corrupt or underflow them";
+
+    /// Independent proof no committed txn was lost: the NEXT publish's bytes must equal a full log replay.
+    /// A regressed base would omit the txns committed while publish #1 was parked.
+    publishEmptyPart(store, ns, "d");
+    ASSERT_TRUE(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+    const auto snap_id = listGreatestSnapshotIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(snap_id.has_value());
+    const auto got = backend->get(layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), *snap_id));
+    ASSERT_TRUE(got.has_value());
+    const RefTableState oracle = independentFullReplayForTest(*backend, layout, ns, snap_id);
+    EXPECT_EQ(openObject(FormatId::RefSnapshot, got->bytes), encodeRefTableSnapshot(snapshotOf(oracle, ns.string())))
+        << "published snapshot bytes must equal replay(all logs through X) -- a regressed base drops txns";
+}
+
+/// (I1, review of commit 9093482176a) `clampedCounterSub`'s actual clamp-to-zero branch -- the exact
+/// hazard it exists for -- was previously unpinned: `AdoptionSubtractsCapturedCountersUnderConcurrentAppends`
+/// subtracts from a counter that never goes below the captured amount (no clamp needed), and in
+/// `ConcurrentOutOfOrderPublish...` above the SMALLER candidate is the one parked, so its adoption is
+/// skipped entirely by the T11 monotonic guard BEFORE it would ever reach the subtraction -- the clamp
+/// is never exercised either way. This test forces the one ordering the guard does NOT catch: the
+/// SMALLER candidate adopts (and subtracts) FIRST, then a LARGER candidate -- captured earlier, while
+/// the counter still held the region the smaller one just subtracted -- adopts second. Its captured
+/// count therefore double-counts that already-subtracted region, and `clampedCounterSub` must clamp
+/// to zero rather than wrap a `uint64_t` to ~`UINT64_MAX` (which would permanently re-latch the C4
+/// storm trigger in a release build -- no `chassert` to catch it). Deterministic, sleep-free: two
+/// `_snap` PUTs are parked independently (both past their own capture, neither yet adopted) via
+/// `armPutBlockIndependently`, then released in the specific order that reproduces the hazard.
+TEST(CASRefWriterSnapshotPublish, ClampedCounterSubClampsInsteadOfUnderflowingOnOutOfOrderAdoption)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/clamp_out_of_order"};
+    PoolConfig config;
+    /// High thresholds: NO automatic background dispatch -- we drive
+    /// `tryPublishSnapshotAndAdvanceCheckpointOnce` directly for full determinism.
+    config.snapshot_log_count_threshold = 1ULL << 40;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+
+    publishEmptyPart(store, ns, "a");   /// tail: 2 -- publisher A's (smaller) candidate
+
+    backend->armPutBlockIndependently("_snap/");
+
+    std::thread publisher_a([&] { store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns); });
+    backend->awaitAtLeastNKeysBlocked(1);   /// A has captured (candidate=2 txns, count=2) and parked mid-PUT
+    const String key_a = *backend->blockedKeysSnapshot().begin();
+
+    publishEmptyPart(store, ns, "b");   /// tail: 4 -- publisher B's (larger) candidate, captured BELOW
+
+    std::thread publisher_b([&] { store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns); });
+    backend->awaitAtLeastNKeysBlocked(2);   /// B has ALSO captured (candidate=4 txns, count=4) and parked
+    const auto blocked = backend->blockedKeysSnapshot();
+    ASSERT_EQ(blocked.size(), 2u) << "both publishers must be parked past their own capture before either adopts";
+    String key_b;
+    for (const auto & k : blocked)
+        if (k != key_a)
+            key_b = k;
+    ASSERT_FALSE(key_b.empty());
+
+    /// Release the SMALLER candidate first: its monotonic guard passes (newest is still unset), so it
+    /// adopts -- newest becomes A's candidate, and the count drops from the live 4 to 2 (4 - captured_A=2).
+    backend->releaseKey(key_a);
+    publisher_a.join();
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 2u)
+        << "publisher A (smaller candidate) adopts first and subtracts its own captured count safely";
+
+    /// Release the LARGER candidate: its monotonic guard ALSO passes (newest=A's candidate < B's
+    /// candidate), so it reaches the subtraction with `captured_count_B == 4` -- but the live counter
+    /// is now only 2 (A's adoption already removed the overlapping region). A plain `fetch_sub` here
+    /// would wrap to ~UINT64_MAX; `clampedCounterSub` must clamp to 0 instead.
+    backend->releaseKey(key_b);
+    publisher_b.join();
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 0u)
+        << "clampedCounterSub must clamp to 0, not underflow/wrap, when B's captured count (4) already "
+           "includes the region A's earlier adoption already subtracted";
+
+    /// A wrapped counter would read as ~UINT64_MAX, permanently latching `over_threshold` (the C4
+    /// storm regression). With the huge threshold configured above, a dispatch firing here can ONLY
+    /// mean the counter is corrupted -- a clamped counter of 0 never crosses it.
+    const auto dispatched_before = global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load();
+    for (int i = 0; i < 5; ++i)
+        store->resolveRef(ns, "a");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), dispatched_before)
+        << "a correctly-clamped counter must never latch the threshold trigger";
+}
+
+/// ===================================================================================
+/// C4: bound the read-triggered snapshot-publish dispatch (spec §writer-snapshot-publication). A
+/// fold-heavy reader must not turn every ref read into a re-dispatched full-snapshot encode+PUT: an
+/// in-flight gate admits at most one publish per table, and a non-Committed outcome arms a bounded
+/// per-table backoff instead of re-triggering on the next read. The unfixed code dispatched a new
+/// publish on every trigger and never backed off, producing the soak's 46 GB/hr `_snap` PUT storm.
+/// ===================================================================================
+
+/// Under a saturated backend (every `_snap` PUT is Unresolved), the read path must NOT re-dispatch a
+/// publish on each read: the failure arms the backoff, and while it holds no read re-dispatches.
+TEST(CASRefWriterSnapshotPublish, C4LatchBoundedUnderSustainedNonCommittedPublish)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/c4_latch"};
+
+    CasRequestBudget budget;   /// one attempt per publish so a failure is a single PUT
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    uint64_t fake_now = 1'000'000;
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 1;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    config.snapshot_publish_backoff_initial_ms = 5000;   /// the frozen clock keeps the backoff armed
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(10'000'000);
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.cas_request_budget = budget;
+    auto store = openPoolWithConfig(backend, config);
+
+    /// Every `_snap` PUT throws Unresolved (backend saturated), from the very first publish attempt.
+    backend->fault_key_substr = "_snap/";
+    backend->fault_count = 100000;
+
+    publishEmptyPart(store, ns, "a");   /// crosses the threshold -> one dispatch -> fails -> backoff armed
+    store->waitForSnapshotPublishSettleForTest(ns);
+
+    const auto dispatched_before = global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load();
+    for (int i = 0; i < 30; ++i)
+    {
+        store->resolveRef(ns, "a");
+        store->waitForSnapshotPublishSettleForTest(ns);
+    }
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), dispatched_before)
+        << "reads within the backoff window must not re-dispatch a publish (the storm latch is broken)";
+}
+
+/// Recovery can leave the runtime at an epoch seal with a tail already above the threshold. The seal
+/// is not snapshot-serializable, so admission itself must reject it: letting execution reject it would
+/// make settlement immediately dispatch another identical background attempt. A later ordinary record
+/// must re-enable the same scheduler.
+TEST(CASRefWriterSnapshotPublish, RecoveredSealAboveThresholdDoesNotRedispatchUntilOrdinarySuccessor)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/recovered_seal_no_storm"};
+
+    {
+        PoolConfig predecessor_config;
+        predecessor_config.snapshot_log_count_threshold = 1ULL << 40;
+        predecessor_config.snapshot_log_bytes_threshold = 1ULL << 40;
+        auto predecessor = openPoolWithConfig(backend, predecessor_config);
+        DB::Cas::tests::fixture::admitLive(*backend, predecessor->layout(), ns);
+        const NamespaceLifeId life = *CasRefCatalog::lifeIfCataloged(*backend, predecessor->layout(), ns);
+        ASSERT_EQ(backend->putIfAbsent(predecessor->layout().refCkptKey(life), encodeRefCkpt(RefCkpt{
+            .life_epoch = predecessor->liveWriterEpoch(),
+            .committed_through = std::nullopt,
+            .checkpoint_snapshot_id = std::nullopt,
+            .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+        publishEmptyPart(predecessor, ns, "before_seal");
+        const auto before = backend->get(predecessor->layout().refCkptKey(life));
+        ASSERT_TRUE(before);
+        const RefCkpt before_seal = decodeRefCkpt(before->bytes);
+        ASSERT_TRUE(before_seal.committed_through);
+        const RefTxnId seal_id{before_seal.committed_through->writer_epoch,
+                               before_seal.committed_through->ref_sequence + 1};
+        writeSealAt(*backend, predecessor->layout(), ns, seal_id);
+
+        RefCkpt recovered_seal = before_seal;
+        recovered_seal.committed_through = seal_id;
+        recovered_seal.last_epoch_seal = seal_id;
+        ASSERT_EQ(backend->casPut(predecessor->layout().refCkptKey(life), encodeRefCkpt(recovered_seal), before->token).outcome,
+                  CasOutcome::Committed);
+    }
+
+    PoolConfig successor_config;
+    successor_config.snapshot_log_count_threshold = 0;
+    successor_config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto successor = openPoolWithConfig(backend, successor_config);
+
+    EXPECT_TRUE(successor->resolveRef(ns, "before_seal").has_value());
+    successor->waitForSnapshotPublishSettleForTest(ns);
+    const auto dispatched_at_seal = global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load();
+    for (int i = 0; i < 5; ++i)
+        EXPECT_TRUE(successor->resolveRef(ns, "before_seal").has_value());
+    successor->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), dispatched_at_seal)
+        << "a recovered seal must not dispatch or re-dispatch an unpublishable snapshot candidate";
+
+    /// One ordinary append transaction above the recovered seal must reopen the scheduler. `dropRef`
+    /// is exactly one ordinary ref-log append, unlike `publishEmptyPart`'s two-phase part publication.
+    successor->dropRef(ns, "before_seal");
+    successor->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), dispatched_at_seal + 1)
+        << "an ordinary successor above the seal must make the threshold candidate publishable again";
+}
+
+/// While one background publish is in flight (blocked mid-PUT), further reads must NOT dispatch a
+/// second: the single-in-flight gate holds `pending_snapshot_publishes` at one per table.
+TEST(CASRefWriterSnapshotPublish, C4InFlightGateAdmitsAtMostOne)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/c4_gate"};
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 0;   /// any nonempty tail triggers
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+
+    publishEmptyPart(store, ns, "a");
+    store->waitForSnapshotPublishSettleForTest(ns);   /// drain the setup publishes; tail is compacted
+
+    /// Block the first `_snap` PUT so one publisher parks in flight.
+    backend->armPutBlockFirstMatchOnly("_snap/");
+    std::thread mutator([&] { store->dropRef(ns, "a"); });   /// its detached publisher blocks mid-PUT
+    backend->awaitBlockEntered();
+
+    /// Many more reads while it is blocked must not admit a second publisher.
+    for (int i = 0; i < 20; ++i)
+        store->resolveRef(ns, "a");
+    EXPECT_EQ(store->pendingSnapshotPublishesForTest(ns), 1)
+        << "the in-flight gate must hold background publishes to at most one per table";
+
+    backend->releaseBlock();
+    mutator.join();
+    store->waitForSnapshotPublishSettleForTest(ns);
+}
+
+/// A non-Committed publish defers the next dispatch by the backoff, then a read past the backoff
+/// deadline dispatches exactly one retry that publishes a durable snapshot (freshness preserved).
+TEST(CASRefWriterSnapshotPublish, C4BackoffDefersThenRetriesAndPublishes)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/c4_backoff"};
+
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    uint64_t fake_now = 1'000'000;
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 1;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    config.snapshot_publish_backoff_initial_ms = 1000;
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(10'000'000);
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.cas_request_budget = budget;
+    auto store = openPoolWithConfig(backend, config);
+
+    /// Fail ONLY the first `_snap` PUT (arms the backoff); later PUTs succeed.
+    backend->fault_key_substr = "_snap/";
+    backend->fault_count = 1;
+
+    publishEmptyPart(store, ns, "a");   /// dispatch -> publish fails -> backoff armed
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_FALSE(listGreatestSnapshotIdForTest(*backend, layout, ns).has_value());
+
+    /// A read within the backoff window (frozen clock) must not re-dispatch.
+    const auto d1 = global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load();
+    store->resolveRef(ns, "a");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), d1)
+        << "a read within the backoff window must not re-dispatch";
+    EXPECT_FALSE(listGreatestSnapshotIdForTest(*backend, layout, ns).has_value());
+
+    /// Advance past the backoff: exactly one retry is dispatched and it publishes.
+    fake_now += 2000;
+    store->resolveRef(ns, "a");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), d1 + 1)
+        << "after the backoff elapses exactly one retry is dispatched";
+    EXPECT_TRUE(listGreatestSnapshotIdForTest(*backend, layout, ns).has_value())
+        << "the retry publishes a durable snapshot (freshness preserved)";
+}
+
+/// ===================================================================================
+/// rev.6 Task 10 (spec §publish-from-live): the tail counters count ONLY applied txns strictly above
+/// `newest_snapshot_id` -- incremented per commit, subtracted (clamped) exactly by adoption. This
+/// pins that a successful publish's adoption RESETS the counters rather than merely reducing them: a
+/// buggy "subtract a fixed prune count" scheme could let the table's already-covered history keep
+/// contributing to the trigger forever.
+/// ===================================================================================
+
+/// After a successful publish adopts `newest_snapshot_id`, the trigger arithmetic must restart from
+/// zero above it, not keep counting the table's already-covered history: 4 covered + 2 fresh entries
+/// must read as 2 (below a 3 threshold), never as 6.
+TEST(CASRefWriterSnapshotPublish, TriggerIgnoresEntriesCoveredByNewestSnapshot)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/trigger_covered"};
+    PoolConfig config;
+    config.snapshot_log_count_threshold = 3;
+    config.snapshot_log_bytes_threshold = 1ULL << 40;
+    auto store = openPoolWithConfig(backend, config);
+
+    const auto d0 = global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load();
+
+    /// Drive ONE successful publish: 4 entries (4 > 3).
+    publishEmptyPart(store, ns, "a");
+    publishEmptyPart(store, ns, "b");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    ASSERT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), d0 + 1);
+    const auto first_snap = listGreatestSnapshotIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(first_snap.has_value());
+    EXPECT_TRUE(store->newestPublishedSnapshotIdForTest(ns) == first_snap);
+    ASSERT_EQ(store->tailSinceSnapshotCountForTest(ns), 0u);
+
+    /// 2 fresh entries: 2 <= 3, while the covered history (4 entries at/below the snapshot) would push
+    /// a covered-counting trigger to 6 > 3. Must not dispatch.
+    publishEmptyPart(store, ns, "c");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), d0 + 1)
+        << "entries covered by the newest snapshot must not count toward the trigger";
+    EXPECT_TRUE(listGreatestSnapshotIdForTest(*backend, layout, ns) == first_snap);
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 2u);
+
+    /// Crossing the threshold with the fresh tail alone (4 > 3) dispatches exactly one more publish,
+    /// and it covers the whole uncovered tail.
+    publishEmptyPart(store, ns, "d");
+    store->waitForSnapshotPublishSettleForTest(ns);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSnapshotPublishDispatched].load(), d0 + 2);
+    const auto second_snap = listGreatestSnapshotIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(second_snap.has_value());
+    EXPECT_TRUE(*first_snap < *second_snap);
+    EXPECT_EQ(store->tailSinceSnapshotCountForTest(ns), 0u);
+}
+
+/// ===================================================================================
+/// Task 11: successor stale-precommit cleanup (spec §Clean Up Old Precommits)
+/// ===================================================================================
+
+/// A predecessor's dangling (never-promoted) precommits are swept by the successor mount's first touch
+/// of the table; a precommit the SUCCESSOR itself adds under its OWN (current) epoch must survive.
+TEST(CASRefWriterStalePrecommitSweep, SweepsOnlyStaleEpochPrecommitsKeepsCurrentEpoch)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/precommit_sweep_basic"};
+
+    {
+        /// A predecessor writer leaves THREE precommits dangling (a crash before promote).
+        auto predecessor = openPool(backend);
+        for (const String & name : {"stale_a", "stale_b", "stale_c"})
+        {
+            auto build = startBuildFor(predecessor, ns, name);
+            const ManifestId id = build->stageManifest({});
+            build->precommitAdd(ns, name, id);
+            /// no promote -- left dangling, as a crashed build would leave it
+        }
+    }   /// predecessor destroyed: its mount lease is released
+
+    /// The successor allocates a strictly higher durable writer_epoch; its own FRESH precommit must
+    /// survive the sweep its very first touch of the table triggers.
+    auto successor = openPool(backend);
+    auto build = startBuildFor(successor, ns, "fresh_x");
+    const ManifestId fresh_id = build->stageManifest({});
+    build->precommitAdd(ns, "fresh_x", fresh_id);   /// this call's own appendRefOps hoists the sweep first
+
+    const RefTableState replayed = independentFullReplayForTest(*backend, successor->layout(), ns);
+    EXPECT_EQ(replayed.getLifecycle(), RefLifecycle::Live);
+    EXPECT_TRUE(replayed.getCommitted().empty());
+    ASSERT_EQ(replayed.getPrecommits().size(), 1u);
+    EXPECT_EQ(replayed.getPrecommits().begin()->first, "fresh_x");
+    EXPECT_EQ(replayed.getPrecommits().begin()->second, fresh_id.ref);
+}
+
+/// The sweep chunks its removal to `ref_txn_max_ops` stale precommits per transaction (spec
+/// §Clean Up Old Precommits), and an interruption (an uncertain PUT, wedging the lane) leaves the
+/// remainder harmlessly for a LATER mount's own fresh recovery to finish -- "each chunk re-reads the
+/// LIVE state, so a partial sweep just leaves fewer stale bindings for the next chunk (a later retry
+/// on this mount, or the next mount's recovery) to find." (Same-mount retry is pinned separately by
+/// `FailedSweepRearmsAndRetriesUntilClean`.)
+TEST(CASRefWriterStalePrecommitSweep, BoundedBatchesAndInterruptionResumeAcrossMounts)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/precommit_sweep_bounded"};
+    /// Derived from `ref_txn_max_ops` (not a literal) so a future cap change cannot silently drop this
+    /// back to a single removal chunk: still > the cap, forcing at least two removal chunks.
+    constexpr int kTotalStale = static_cast<int>(ref_txn_max_ops) + 200;
+
+    uint64_t e1 = 0;
+    {
+        auto predecessor = openPool(backend);
+        e1 = predecessor->writerEpoch();
+    }   /// predecessor released; only its epoch is needed -- the stale precommits are seeded raw below
+
+    /// Seed kTotalStale precommits directly (bypassing any Pool) under the predecessor's epoch,
+    /// spread over two raw log objects (each within the per-transaction op ENCODE cap) so recovery
+    /// costs only two GETs, not kTotalStale of them.
+    {
+        std::vector<RefOp> ops1;
+        ops1.push_back(namespaceBirthOp());
+        for (int i = 0; i < 700; ++i)
+        {
+            RefOp op;
+            op.kind = RefOpKind::OwnerTransition;
+            op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "stale_" + std::to_string(i), manifestRef(e1, static_cast<uint64_t>(i) + 1, 1)};
+            ops1.push_back(op);
+        }
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{e1, 1}, ops1, std::nullopt});
+
+        std::vector<RefOp> ops2;
+        for (int i = 700; i < kTotalStale; ++i)
+        {
+            RefOp op;
+            op.kind = RefOpKind::OwnerTransition;
+            op.new_binding = RefOwnerBinding{RefOwnerKind::Precommit, "stale_" + std::to_string(i), manifestRef(e1, static_cast<uint64_t>(i) + 1, 1)};
+            ops2.push_back(op);
+        }
+        DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), RefTxnId{e1, 2}, ops2, std::nullopt});
+    }
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = e1,
+        .committed_through = RefTxnId{e1, 2},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    /// The successor: a tight retry budget so ONE simulated ambiguous response wedges rather than
+    /// transparently retries away. `8f9e63c7a19` widened `kSingleAttemptDeadlineMs` off a zero-width
+    /// race (equal attempt/operation deadlines), but it still measures the capture-to-gate window --
+    /// encoding the removal chunk (up to `ref_txn_max_ops` ops) -- against the REAL wall clock, so it
+    /// recurred (3 of 3 sanitizer lanes) once that encode step got slow enough on its own, independent
+    /// of scheduler contention: msan in particular. `ref_request_controller` reads its clock through
+    /// the same injectable seam as the mount fence (`CasRefLedger`'s `controller_boot_ms_fn` is the
+    /// pool's `boot_ms_fn`), so freeze it here instead of racing it -- the fault-injecting PUT below
+    /// still reaches the backend synchronously; only the deadline arithmetic stops moving.
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+    PoolConfig config;
+    config.cas_request_budget = budget;
+    config.boot_ms_fn = [] { return uint64_t{0}; };
+    auto successor = openPoolWithConfig(backend, config);
+
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    /// The successor's own recovery runs first and mints one in-band seal for the dead predecessor
+    /// epoch `e1` (its durable ids are `{e1,1}` and `{e1,2}`, so the seal lands at `{e1,3}`) -- that PUT
+    /// shares this same `_log/` prefix, so it would eat the fault before the sweep ever gets a chance.
+    /// Skip it and land the fault on the sweep's FIRST removal chunk's PUT, as intended.
+    backend->fault_skip = 1;
+    backend->fault_count = 1;   /// hits exactly the sweep's FIRST removal chunk's PUT
+
+    /// The sweep is piggybacked on this mount's very first touch; its (uncertain) failure is INSULATED
+    /// from the read (resolveRef/listRefs call `sweepStalePrecommitsForRead`, not
+    /// `maybeSweepStalePrecommits` directly): the read itself still succeeds, the failure is counted.
+    const uint64_t deferred_before = ProfileEvents::global_counters[ProfileEvents::CASRefSweepDeferred].load();
+    EXPECT_NO_THROW(successor->listRefs(ns));
+    const uint64_t deferred_after = ProfileEvents::global_counters[ProfileEvents::CASRefSweepDeferred].load();
+    EXPECT_EQ(deferred_after, deferred_before + 1)
+        << "the read-only caller must observe (and count) the deferred sweep failure, not throw";
+    EXPECT_TRUE(successor->refLaneWedgedForTest(ns));
+
+    /// The first chunk's request actually landed server-side; the caller just never saw the ack.
+    backend->materializePendingDelayedWrite();
+    successor.reset();   /// abandoned mid-sweep WITHOUT ever resolving its own wedge in-memory
+
+    /// A THIRD mount (successor-of-the-successor): fresh recovery replays the two raw seed logs PLUS the
+    /// first chunk's now-durable removal, sees `needs_stale_precommit_sweep` armed again, and finishes
+    /// the remaining stale precommits in exactly one further chunk (<= 1000 remain). `successor` was
+    /// abandoned mid-wedge above -- Task 5's drain fails closed on an unresolved PUT, so no clean
+    /// farewell was written -> this reclaim is `MountPriorState::UncleanObserved` (rev.6 Task 4), which
+    /// pays a real ~36.5s token-stability observation wait here. Inject a fake `boot_ms_fn` +
+    /// `wait_sleep_fn` (mirroring `CASMountOpenWaits.UncleanOpenPaysOnlyTheObservationWindow`) so it
+    /// resolves instantly.
+    uint64_t resumer_fake_boot = 0;
+    PoolConfig resumer_config;
+    resumer_config.boot_ms_fn = [&resumer_fake_boot] { return resumer_fake_boot; };
+    resumer_config.wait_sleep_fn = [&resumer_fake_boot](uint64_t ms) { resumer_fake_boot += ms; };
+    auto resumer = openPoolWithConfig(backend, resumer_config);
+    EXPECT_NO_THROW(resumer->listRefs(ns));
+
+    const RefTableState final_state = independentFullReplayForTest(*backend, layout, ns);
+    EXPECT_EQ(final_state.getLifecycle(), RefLifecycle::Live);
+    EXPECT_TRUE(final_state.getPrecommits().empty()) << "every stale precommit must eventually be swept";
+
+    /// Bounded batches: exactly THREE NEW `_log/` objects (epoch > e1) were needed -- never kTotalStale
+    /// individual removals, and one more than before INV-2 went in-band. In order: `{e1+1,1}` the
+    /// successor's own (delayed-delivered) FIRST removal chunk; `{e1+1,2}` the epoch seal that closes
+    /// the successor's own epoch once IT becomes dead in turn -- minted by `resumer`'s recovery, since
+    /// `successor` was abandoned mid-sweep without a clean farewell and never sealed itself; and
+    /// `{e1+2,1}` the resumer's own SECOND removal chunk, finishing the remaining stale precommits.
+    size_t new_log_objects = 0;
+    {
+        String cursor;
+        for (;;)
+        {
+            const ListPage page = backend->list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+            for (const ListedKey & lk : page.keys)
+            {
+                const auto parsed = layout.parseRefObjectKey(lk.key);
+                if (parsed && parsed->life_id == DB::Cas::tests::fixture::fixtureLife(ns).incarnation
+                    && parsed->kind == RefObjectKind::Log && parsed->txn_id.writer_epoch != e1)
+                    ++new_log_objects;
+            }
+            if (page.next_cursor.empty())
+                break;
+            cursor = page.next_cursor;
+        }
+    }
+    EXPECT_EQ(new_log_objects, 3u);
+}
+
+/// S13 regression fix (triage `.superpowers/sdd/s13-triage-report.md`, run 20260713T172032_S13_seed42):
+/// a FAILED sweep attempt must NOT consume the once-per-mount shot. The failure re-arms
+/// `needs_stale_precommit_sweep` (with a bounded backoff, so a saturated backend is not stormed), the
+/// read that piggybacked the sweep still succeeds (existing `CASRefSweepDeferred` contract), and a later
+/// trigger -- here a mutation -- retries until a pass completes verified clean, clearing the flag
+/// permanently. Each reclaimed binding is audited: one `precommit_reclaim` CA-log event + one
+/// `CASRefStalePrecommitsReclaimed` increment, exactly per binding.
+TEST(CASRefWriterStalePrecommitSweep, FailedSweepRearmsAndRetriesUntilClean)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/precommit_sweep_retry"};
+
+    /// One shared injected clock for both incarnations. The successor's wait hook below advances this
+    /// same clock, so both mount observation and the later sweep-backoff deadline are deterministic.
+    uint64_t fake_now = 1'000'000;
+    size_t mount_wait_calls = 0;
+    const auto fake_clock = [&fake_now] { return fake_now; };
+
+    {
+        /// A predecessor writer leaves THREE precommits dangling (a crash before promote).
+        PoolConfig pred_config;
+        pred_config.mount_lease_ttl_ms = std::chrono::milliseconds(10'000'000);
+        pred_config.boot_ms_fn = fake_clock;
+        auto predecessor = openPoolWithConfig(backend, pred_config);
+        std::vector<PartWriteTxnPtr> predecessor_builds;
+        for (const String & name : {"stale_a", "stale_b", "stale_c"})
+        {
+            auto build = startBuildFor(predecessor, ns, name);
+            const ManifestId id = build->stageManifest({});
+            build->precommitAdd(ns, name, id);
+            /// no promote -- left dangling, as a crashed build would leave it
+            predecessor_builds.push_back(std::move(build));
+        }
+    }   /// all three cleanup duties remain pending; predecessor publishes no clean farewell
+
+    /// The successor: a tight retry budget so ONE simulated ambiguous response wedges rather than
+    /// transparently retries away (mirrors the wedge-semantics tests in this file exactly).
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+    PoolConfig config;
+    config.cas_request_budget = budget;
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(10'000'000);
+    config.boot_ms_fn = fake_clock;
+    config.wait_sleep_fn = [&fake_now, &mount_wait_calls](uint64_t ms)
+    {
+        ++mount_wait_calls;
+        fake_now += ms;
+    };
+    SynchronizedEventLog seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto successor = openPoolWithConfig(backend, config);
+    EXPECT_GT(mount_wait_calls, 0u)
+        << "the unclean predecessor must exercise the injected mount-observation wait";
+
+    successor->setEventSink([&](const CasEvent & e) { seen.add(e); });
+
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    /// The successor's own recovery runs first and mints one in-band seal for the predecessor's now-dead
+    /// epoch (its three precommits are its only durable ids, so the seal takes the very next slot) --
+    /// that PUT shares this same `_log/` prefix, so it would eat the fault before the sweep gets a turn.
+    /// Skip it and land the fault on the sweep's FIRST removal chunk's PUT, as intended.
+    backend->fault_skip = 1;
+    backend->fault_count = 1;   /// hits exactly the sweep's FIRST removal chunk's PUT
+
+    /// FIRST trigger (read path): the sweep's removal PUT is uncertain -> the lane wedges; the read
+    /// itself still succeeds and counts the deferral (existing contract) -- but the shot must NOT be
+    /// consumed: the flag is re-armed for a later trigger.
+    const uint64_t deferred_before = global_counters[ProfileEvents::CASRefSweepDeferred].load();
+    const uint64_t rearmed_before = global_counters[ProfileEvents::CASRefSweepRearmed].load();
+    const uint64_t reclaimed_before = global_counters[ProfileEvents::CASRefStalePrecommitsReclaimed].load();
+    EXPECT_NO_THROW(successor->listRefs(ns));
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSweepDeferred].load(), deferred_before + 1);
+    EXPECT_TRUE(successor->refLaneWedgedForTest(ns));
+    EXPECT_TRUE(successor->needsStalePrecommitSweepForTest(ns))
+        << "a failed sweep must re-arm needs_stale_precommit_sweep, not consume the once-per-mount shot";
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSweepRearmed].load(), rearmed_before + 1);
+
+    /// Within the backoff window (the injected clock has not advanced) a read must NOT re-attempt --
+    /// the bounded-backoff storm latch: no new deferral, flag still armed.
+    EXPECT_NO_THROW(successor->listRefs(ns));
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefSweepDeferred].load(), deferred_before + 1)
+        << "within the backoff window the sweep must not re-attempt (PUT-storm latch)";
+    EXPECT_TRUE(successor->needsStalePrecommitSweepForTest(ns));
+
+    /// The lost response later lands server-side; past the backoff deadline the NEXT trigger (a
+    /// mutation this time) retries: the lane resolves its wedge (the first chunk's removals become
+    /// durable and applied), the re-pass verifies clean, and the flag clears permanently.
+    backend->materializePendingDelayedWrite();
+    fake_now += 60'000;   /// beyond any armed backoff (initial 200 ms, max 30 s)
+    EXPECT_NO_THROW(publishEmptyPart(successor, ns, "fresh"));
+    EXPECT_FALSE(successor->refLaneWedgedForTest(ns));
+    EXPECT_FALSE(successor->needsStalePrecommitSweepForTest(ns))
+        << "a verified-clean sweep clears the flag permanently";
+
+    /// Ground truth: every stale binding reclaimed; the successor's own committed work intact.
+    const RefTableState final_state = independentFullReplayForTest(*backend, layout, ns);
+    EXPECT_EQ(final_state.getLifecycle(), RefLifecycle::Live);
+    EXPECT_TRUE(final_state.getPrecommits().empty());
+    EXPECT_TRUE(final_state.getCommitted().contains("fresh"));
+
+    /// Audit (INTROSPECTION-1): exactly ONE `precommit_reclaim` event per reclaimed stale binding --
+    /// this is what makes the S13 card's "abandoned precommits reclaimed" counter falsifiable.
+    std::vector<String> reclaimed_refs;
+    for (const CasEvent & e : seen.snapshot())
+        if (e.type == CasEventType::PrecommitReclaim)
+            reclaimed_refs.push_back(e.ref_name);
+    std::sort(reclaimed_refs.begin(), reclaimed_refs.end());
+    EXPECT_EQ(reclaimed_refs, (std::vector<String>{"stale_a", "stale_b", "stale_c"}));
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefStalePrecommitsReclaimed].load(), reclaimed_before + 3);
+}
+
+/// Verified-clean semantics: a sweep that finds NOTHING stale clears the flag on its very first pass
+/// and emits no reclaim event (so "no abandons" and "reclaim broken" stay distinguishable in the
+/// audit log).
+TEST(CASRefWriterStalePrecommitSweep, VerifiedCleanSweepClearsFlagWithoutEvents)
+{
+    using ProfileEvents::global_counters;
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/precommit_sweep_clean"};
+
+    {
+        auto predecessor = openPool(backend);
+        publishEmptyPart(predecessor, ns, "committed_x");   /// committed work only; nothing dangles
+    }
+
+    SynchronizedEventLog seen;   /// declared BEFORE the Pool so it outlives the background syncer's emits (ASan 2026-07-09)
+    auto successor = openPool(backend);
+    successor->setEventSink([&](const CasEvent & e) { seen.add(e); });
+
+    const uint64_t deferred_before = ProfileEvents::global_counters[ProfileEvents::CASRefSweepDeferred].load();
+    const uint64_t reclaimed_before = global_counters[ProfileEvents::CASRefStalePrecommitsReclaimed].load();
+    EXPECT_NO_THROW(successor->listRefs(ns));
+    EXPECT_FALSE(successor->needsStalePrecommitSweepForTest(ns))
+        << "a clean first pass IS the verified-clean sweep: the flag clears without any removal";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefSweepDeferred].load(), deferred_before);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefStalePrecommitsReclaimed].load(), reclaimed_before);
+    const std::vector<CasEvent> observed = seen.snapshot();
+    EXPECT_EQ(std::count_if(observed.begin(), observed.end(),
+        [](const CasEvent & e) { return e.type == CasEventType::PrecommitReclaim; }), 0);
+}
+
+/// ===================================================================================
+/// C1: self-remount establishes a fresh ref-protocol incarnation (spec §Startup And Recovery /
+/// §write-fence). A self-remount bumps the durable writer_epoch, so every ref transaction it stamps
+/// afterward sorts strictly above any log a dead-incarnation or same-uuid twin left durable under an
+/// older epoch, and it drops its stale in-memory cache so the next touch re-recovers under the new
+/// epoch. The unfixed code kept the open-time `process_epoch` and the cached tables across the fence-out.
+/// ===================================================================================
+
+namespace
+{
+
+/// Fence out the mount lease so `tryRemountOnce` reclaims a fresh incarnation (mirrors
+/// gtest_cas_pool.cpp's fenceOutMount, without its ASSERT_ macros so it can run outside a fixture).
+void fenceOutRefMount(Backend & backend, const String & mount_key)
+{
+    const auto got = backend.get(mount_key);
+    MountLease m = decodeMountLease(got->bytes);
+    m.gc_fenced = true;
+    m.seq += 1;
+    backend.putOverwrite(mount_key, encodeMountLease(m), got->token);
+}
+
+/// The greatest `_log/<id>` transaction id currently present for `ns` (independent of any Pool cache).
+std::optional<RefTxnId> listGreatestLogIdForTest(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    std::optional<RefTxnId> greatest;
+    String cursor;
+    for (;;)
+    {
+        const ListPage page = backend.list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+        for (const ListedKey & lk : page.keys)
+        {
+            const auto parsed = layout.parseRefObjectKey(lk.key);
+            if (parsed && parsed->life_id == DB::Cas::tests::fixture::fixtureLife(ns).incarnation && parsed->kind == RefObjectKind::Log
+                && (!greatest || *greatest < parsed->txn_id))
+                greatest = parsed->txn_id;
+        }
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    return greatest;
+}
+
+/// Seed a same-uuid TWIN incarnation that bumped the durable writer_epoch and durably DROPPED `ref_name`
+/// (its committed binding `old_ref`) at `{twin_epoch, 1}` -- an id that sorts strictly above every log a
+/// Pool wrote under its own (lower) open-time epoch. Returns the twin's epoch.
+/// `prev_epoch_seal` is NOT optional decoration here. The twin's drop is sequence 1 of a new epoch, so
+/// INV-2's grammar requires it to name the seal that closed the epoch below -- and the reader enforces
+/// that, so a twin seeded without it describes a stream with an uncertified epoch boundary, which is
+/// exactly what recovery must refuse. The link names the id the recovering pool's own CAS-walk will mint
+/// for the dead epoch: one past that epoch's greatest durable id, which is what `seal_of_previous_epoch`
+/// derives by listing rather than hard-coding, so the fixture cannot drift from the walk's arithmetic.
+uint64_t seedTwinDrop(Backend & backend, const Layout & layout, const RootNamespace & ns,
+                      const String & ref_name, const ManifestRef & old_ref)
+{
+    uint64_t greatest_in_previous_epoch = 0;
+    uint64_t previous_epoch = 0;
+    forEachListedKey(backend, layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), [&](const ListedKey & lk)
+    {
+        const auto parsed = layout.parseRefObjectKey(lk.key);
+        if (!parsed || parsed->kind != RefObjectKind::Log)
+            return;
+        if (parsed->txn_id.writer_epoch > previous_epoch
+            || (parsed->txn_id.writer_epoch == previous_epoch && parsed->txn_id.ref_sequence > greatest_in_previous_epoch))
+        {
+            previous_epoch = parsed->txn_id.writer_epoch;
+            greatest_in_previous_epoch = parsed->txn_id.ref_sequence;
+        }
+    }, 1000);
+
+    const uint64_t twin_epoch = allocateWriterEpoch(backend, layout, "test", EpochMintPolicy::NormalMount, 0, [] { return RefCatalog{}; });
+    RefLogTxn twin;
+    twin.ns = ns.string();
+    twin.txn_id = RefTxnId{twin_epoch, 1};
+    twin.prev_epoch_seal = RefTxnId{previous_epoch, greatest_in_previous_epoch + 1};
+    RefOp drop;
+    drop.kind = RefOpKind::OwnerTransition;
+    drop.old_binding = RefOwnerBinding{RefOwnerKind::Committed, ref_name, old_ref};
+    twin.ops = {drop};
+    DB::Cas::tests::fixture::writeRefLogRaw(backend, layout, twin);
+    return twin_epoch;
+}
+
+}
+
+/// A fence-loss generation is a rejection marker, not a runtime admission token. If remount then loses
+/// to a foreign owner, neither a warm name nor a never-seen name may select/materialize a runtime under
+/// that intermediate generation; the predecessor remains only as a detached diagnostic object.
+TEST(CASRefWriterRemount, FailedRemountPublishesNoRuntimeUnderFenceLossGeneration)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace existing{"srv1/failed-remount-existing"};
+    const RootNamespace never_seen{"srv1/failed-remount-never-seen"};
+
+    publishWithProductionBirth(store, existing, "a");
+    const uint64_t predecessor_runtime = store->refTableRuntimeIdentityForTest(existing);
+    const uint64_t predecessor_generation
+        = store->refTableRuntimeAdmittedFenceGenerationForTest(existing);
+    const size_t cached_before = store->refTablesCachedCountForTest();
+    ASSERT_NE(predecessor_runtime, 0u);
+    ASSERT_EQ(predecessor_generation, store->fenceGeneration());
+
+    const String mount_key = layout.mountKey("test");
+    const auto got = backend->get(mount_key);
+    ASSERT_TRUE(got);
+    MountLease foreign = decodeMountLease(got->bytes);
+    foreign.server_uuid = foreign.server_uuid + UInt128{1};
+    foreign.seq += 1;
+    ASSERT_EQ(backend->putOverwrite(mount_key, encodeMountLease(foreign), got->token).outcome,
+        PutOutcome::Done);
+
+    store->tripMountLost();
+    const uint64_t rejected_generation = store->fenceGeneration();
+    ASSERT_NE(rejected_generation, predecessor_generation);
+    EXPECT_FALSE(store->tryRemountOnce());
+    EXPECT_FALSE(store->mayMutate());
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->resolveRef(existing, "a"); });
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->resolveRef(never_seen, "a"); });
+    EXPECT_EQ(store->refTablesCachedCountForTest(), cached_before);
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(existing), predecessor_runtime);
+    EXPECT_EQ(store->refTableRuntimeAdmittedFenceGenerationForTest(existing), predecessor_generation);
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(never_seen), 0u);
+    EXPECT_NE(store->refTableRuntimeAdmittedFenceGenerationForTest(existing), rejected_generation);
+
+    /// Make the foreign occupant terminal before teardown; it remains foreign and is never taken over.
+    fenceOutRefMount(*backend, mount_key);
+}
+
+/// C1/N1 (stale cache): a warm table whose committed ref a twin durably dropped must re-recover to the
+/// twin's view after a self-remount. The unfixed code kept the stale cache and still resolved the ref.
+TEST(CASRefWriterRemount, ReRecoversStaleCacheToTwinDrop)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/remount_twin_view"};
+
+    const ManifestId a_id = publishEmptyPart(store, ns, "a");
+    ASSERT_TRUE(store->resolveRef(ns, "a").has_value());
+    const uint64_t e1 = store->liveWriterEpoch();
+    const uint64_t predecessor_runtime = store->refTableRuntimeIdentityForTest(ns);
+    const NamespaceLifeId predecessor_life = *store->refTableLifeForTest(ns);
+    const uint64_t predecessor_generation = store->refTableRuntimeAdmittedFenceGenerationForTest(ns);
+
+    /// A same-uuid twin bumped the durable epoch and durably dropped "a"; this Pool's warm cache never
+    /// observed it.
+    const uint64_t twin_epoch = seedTwinDrop(*backend, layout, ns, "a", a_id.ref);
+    ASSERT_GT(twin_epoch, e1);
+    ASSERT_TRUE(store->resolveRef(ns, "a").has_value()) << "precondition: the warm cache is stale";
+
+    fenceOutRefMount(*backend, layout.mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce());
+    EXPECT_GT(store->liveWriterEpoch(), twin_epoch);
+
+    /// The remount dropped the stale runtime: the next read re-recovers from the durable objects and
+    /// adopts the twin's drop -- "a" is gone.
+    EXPECT_FALSE(store->resolveRef(ns, "a").has_value())
+        << "a self-remount must re-recover the table under the new epoch, adopting the twin's drop";
+    EXPECT_NE(store->refTableRuntimeIdentityForTest(ns), predecessor_runtime);
+    EXPECT_EQ(store->refTableLifeForTest(ns), predecessor_life);
+    EXPECT_NE(store->refTableRuntimeAdmittedFenceGenerationForTest(ns), predecessor_generation);
+    EXPECT_EQ(store->refTableRuntimeAdmittedFenceGenerationForTest(ns), store->fenceGeneration());
+}
+
+/// C1/N2 (epoch routing + ordering): a post-remount append must stamp its log with the fresh
+/// incarnation's live epoch, landing strictly above a twin's durable log (the pagination premise
+/// "a new log is never inserted at or below an already durable table log id"). The unfixed code stamped
+/// the stale open-time epoch, which sorts BELOW a higher-epoch twin log.
+TEST(CASRefWriterRemount, PostRemountAppendCarriesLiveEpochSortingAboveTwinLogs)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/remount_epoch_order"};
+
+    const ManifestId a_id = publishEmptyPart(store, ns, "a");
+    const uint64_t e1 = store->liveWriterEpoch();
+    const uint64_t twin_epoch = seedTwinDrop(*backend, layout, ns, "a", a_id.ref);
+    ASSERT_GT(twin_epoch, e1);
+
+    fenceOutRefMount(*backend, layout.mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce());
+    const uint64_t e2 = store->liveWriterEpoch();
+    ASSERT_GT(e2, twin_epoch);
+
+    publishEmptyPart(store, ns, "b");
+    const auto greatest = listGreatestLogIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(greatest.has_value());
+    EXPECT_EQ(greatest->writer_epoch, e2)
+        << "the newest ref log must carry the fresh incarnation's epoch and be the greatest id";
+    EXPECT_GT(*greatest, (RefTxnId{twin_epoch, 1}))
+        << "the post-remount append must sort strictly above the twin's log";
+}
+
+/// C1 (wedge disposition): a wedged append lane's runtime (and its wedge) is dropped on a self-remount,
+/// the next touch re-recovers a clean lane, and appends resume without hanging. The unfixed code kept the
+/// wedged runtime cached across the remount. The drop is a plain cache detach and needs to certify
+/// nothing: the undecided PUT the wedge describes is settled by the seal the next recovery writes into
+/// its slot -- see `quiesceRefTablesForRemount`'s doc comment (`CasPool.h`).
+TEST(CASRefWriterRemount, DiscardsWedgeAndLaneRemainsUsable)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    /// The self-remount below blocks on nothing (see
+    /// `CASRemountWaits.UnresolvedWedgeRemountPaysNoWaitEither`, `gtest_cas_pool.cpp`); the injected
+    /// `boot_ms_fn`/`wait_sleep_fn` keep this test off the real clock anyway.
+    uint64_t fake_boot = 0;
+    PoolConfig config;
+    config.cas_request_budget = budget;
+    config.boot_ms_fn = [&fake_boot] { return fake_boot; };
+    config.wait_sleep_fn = [&fake_boot](uint64_t ms) { fake_boot += ms; };
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/remount_wedge"};
+    publishEmptyPart(store, ns, "x");
+    publishEmptyPart(store, ns, "y");
+
+    /// Wedge the lane with an ambiguous PUT that never landed server-side.
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+
+    fenceOutRefMount(*backend, layout.mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce());
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns))
+        << "a self-remount discards the in-memory wedge with the detached runtime";
+
+    /// The lane is usable, not hung: a fresh append completes and carries the live epoch.
+    EXPECT_NO_THROW(store->dropRef(ns, "y"));
+    EXPECT_FALSE(store->resolveRef(ns, "y").has_value());
+    const auto greatest = listGreatestLogIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(greatest.has_value());
+    EXPECT_EQ(greatest->writer_epoch, store->liveWriterEpoch());
+}
+
+/// C1 residual: a flush leader that passed the top-of-flush gate BEFORE a self-remount and stalled
+/// mid-flush (here parked at the pre-carve hook, post-top-gate / pre-allocate) across the whole
+/// fence-loss + remount window must NOT, on resume, allocate an id and PUT a transaction validated
+/// against its now-stale detached cache. The pre-allocate `superseded_by_remount` re-check fails it
+/// closed: the caller gets the failure and no backend `_log` object is created.
+TEST(CASRefWriterRemount, SupersededLeaderMidFlushFailsClosedCreatesNoObject)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    /// This test parks a flush leader (`leader_active` stays true) across the ENTIRE `tryRemountOnce`
+    /// call below by construction (`release` is only set AFTER `tryRemountOnce` returns). Keep the
+    /// request budget at the file's usual tiny-wedge-test values so every bounded wait on that path
+    /// stays well under a second.
+    CasRequestBudget budget;
+    budget.attempt_timeout_ms = 100;
+    budget.lease_safety_margin_ms = 100;
+    uint64_t fake_boot = 0;
+    PoolConfig config;
+    config.cas_request_budget = budget;
+    config.boot_ms_fn = [&fake_boot] { return fake_boot; };
+    config.wait_sleep_fn = [&fake_boot](uint64_t ms) { fake_boot += ms; };
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/remount_midflush"};
+    publishEmptyPart(store, ns, "x");
+    const auto greatest_before = listGreatestLogIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(greatest_before.has_value());
+
+    /// Park the next flush leader at the pre-carve hook (post-top-gate, pre-allocate). Fires once.
+    std::mutex m;
+    std::condition_variable cv;
+    bool entered = false;
+    bool release = false;
+    std::atomic<bool> hook_fired{false};
+    store->setRefPreCarveHookForTest([&]
+    {
+        if (hook_fired.exchange(true))
+            return;
+        std::unique_lock<std::mutex> lk(m);
+        entered = true;
+        cv.notify_all();
+        cv.wait(lk, [&] { return release; });
+    });
+
+    auto fut = std::async(std::launch::async, [&]() -> std::string
+    {
+        try { store->dropRef(ns, "x"); return "committed"; }
+        catch (const DB::Exception & e) { return e.message(); }
+    });
+    { std::unique_lock<std::mutex> lk(m); cv.wait(lk, [&] { return entered; }); }   /// leader parked mid-flush
+
+    /// The remount completes while the leader is parked (the quiesce does not wait for leaders); it marks
+    /// the table superseded and re-arms the fence.
+    fenceOutRefMount(*backend, layout.mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce());
+
+    /// Unpark: the leader resumes, re-checks the flag before allocating, and fails closed.
+    { std::lock_guard<std::mutex> lk(m); release = true; }
+    cv.notify_all();
+
+    ASSERT_EQ(fut.wait_for(std::chrono::seconds(10)), std::future_status::ready)
+        << "the superseded leader hung instead of failing closed";
+    const std::string result = fut.get();
+    EXPECT_NE(result.find("superseded by a self-remount"), std::string::npos)
+        << "expected a superseded fail-closed, got: " << result;
+
+    /// No new ref-log object was created: the greatest durable log id is unchanged.
+    const auto greatest_after = listGreatestLogIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(greatest_after.has_value());
+    EXPECT_EQ(*greatest_after, *greatest_before)
+        << "a superseded leader must allocate no id and PUT no object";
+}
+
+/// ===================================================================================
+/// Task 11: namespace removal (spec §Namespace Removal)
+/// ===================================================================================
+
+/// A cached writer paused after its ordinary gates must re-check the exact catalog life immediately
+/// before id allocation. A concurrent `Live -> Removing` transition therefore admits no late owner.
+TEST(CASRefWriterNamespaceRemoval, CachedPositiveWriterCannotAppendAfterRemovingIsPublished)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/removing_blocks_cached_writer"};
+    publishEmptyPart(store, ns, "existing");
+
+    const CasRefCatalog::Snapshot before = CasRefCatalog::read(*backend, layout);
+    const auto observed = std::find_if(before.catalog.entries.begin(), before.catalog.entries.end(),
+        [&](const CatalogEntry & entry) { return entry.ns == ns; });
+    ASSERT_NE(observed, before.catalog.entries.end());
+    ASSERT_EQ(observed->state, NsState::Live);
+    const CatalogEntry & exact_live = *observed;
+    const auto greatest_before = listGreatestLogIdForTest(*backend, layout, ns);
+    ASSERT_TRUE(greatest_before);
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool entered = false;
+    bool release = false;
+    std::atomic<bool> hook_fired{false};
+    store->setRefPreCarveHookForTest([&]
+    {
+        if (hook_fired.exchange(true))
+            return;
+        std::unique_lock lock(mutex);
+        entered = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return release; });
+    });
+
+    auto writer = std::async(std::launch::async, [&]() -> String
+    {
+        try
+        {
+            publishEmptyPart(store, ns, "late");
+            return "committed";
+        }
+        catch (const DB::Exception & e)
+        {
+            return e.message();
+        }
+    });
+
+    bool writer_parked = false;
+    {
+        std::unique_lock lock(mutex);
+        writer_parked = cv.wait_for(lock, std::chrono::seconds(10), [&] { return entered; });
+    }
+    if (writer_parked)
+    {
+        CasRefCatalog::casUpdate(*backend, layout, [&](const RefCatalog & current)
+        {
+            RefCatalog next = current;
+            const auto it = std::find(next.entries.begin(), next.entries.end(), exact_live);
+            if (it == next.entries.end())
+                throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "exact Live row changed during test transition");
+            it->state = NsState::Removing;
+            it->removal_started_round = 0;
+            return next;
+        });
+    }
+    {
+        std::lock_guard lock(mutex);
+        release = true;
+    }
+    cv.notify_all();
+
+    EXPECT_TRUE(writer_parked) << "cached writer did not reach the deterministic pre-carve seam";
+    ASSERT_EQ(writer.wait_for(std::chrono::seconds(10)), std::future_status::ready);
+    const String result = writer.get();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    EXPECT_NE(result, "committed") << "cached positive ownership appended after `Removing` became visible";
+    EXPECT_FALSE(store->resolveRef(ns, "late"));
+    EXPECT_EQ(listGreatestLogIdForTest(*backend, layout, ns), greatest_before);
+}
+
+/// dropNamespace's ONE body transaction names an exact removal for every committed ref AND every
+/// dangling precommit, with `remove_namespace` as the FINAL op -- never any other shape.
+TEST(CASRefWriterNamespaceRemoval, TxnNamesEveryOwnerThenRemoveNamespace)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/remove_shape"};
+
+    publishEmptyPart(store, ns, "committed_1");
+    publishEmptyPart(store, ns, "committed_2");
+    /// One precommit left dangling (never promoted) so the removal txn must ALSO name it.
+    auto build = startBuildFor(store, ns, "dangling");
+    const ManifestId dangling_id = build->stageManifest({});
+    build->precommitAdd(ns, "dangling", dangling_id);
+
+    store->dropNamespace(ns);
+
+    /// The newest `_log/` object for `ns` is the removal transaction.
+    std::optional<RefTxnId> newest_log;
+    {
+        String cursor;
+        for (;;)
+        {
+            const ListPage page = backend->list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+            for (const ListedKey & lk : page.keys)
+            {
+                const auto parsed = layout.parseRefObjectKey(lk.key);
+                if (parsed && parsed->life_id == DB::Cas::tests::fixture::fixtureLife(ns).incarnation && parsed->kind == RefObjectKind::Log
+                    && (!newest_log || *newest_log < parsed->txn_id))
+                    newest_log = parsed->txn_id;
+            }
+            if (page.next_cursor.empty())
+                break;
+            cursor = page.next_cursor;
+        }
+    }
+    ASSERT_TRUE(newest_log.has_value());
+    const auto got = backend->get(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), *newest_log));
+    ASSERT_TRUE(got.has_value());
+    const RefLogTxn removal_txn = decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), *newest_log);
+
+    ASSERT_FALSE(removal_txn.ops.empty());
+    EXPECT_EQ(removal_txn.ops.back().kind, RefOpKind::RemoveNamespace);
+    size_t owner_removals = 0;
+    for (size_t i = 0; i + 1 < removal_txn.ops.size(); ++i)
+    {
+        const RefOp & op = removal_txn.ops[i];
+        EXPECT_EQ(op.kind, RefOpKind::OwnerTransition);
+        EXPECT_TRUE(op.old_binding.has_value());
+        EXPECT_FALSE(op.new_binding.has_value());
+        ++owner_removals;
+    }
+    EXPECT_EQ(owner_removals, 3u) << "2 committed + 1 dangling precommit";
+}
+
+/// The terminal transaction is the only durable removal record. Generation 7 never publishes a
+/// terminal `Removed` snapshot; the ordinary cleanup/janitor paths own old immutable stream debris.
+TEST(CASRefWriterNamespaceRemoval, RemovalPublishesTerminalLogWithoutTerminalSnapshot)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/remove_snapshot"};
+
+    publishEmptyPart(store, ns, "a");
+    const auto snapshot_before = store->newestPublishedSnapshotIdForTest(ns);
+    store->dropNamespace(ns);
+
+    EXPECT_EQ(store->newestPublishedSnapshotIdForTest(ns), snapshot_before)
+        << "removal must not publish a terminal snapshot";
+    EXPECT_GT(store->tailSinceSnapshotCountForTest(ns), 0u)
+        << "the terminal transaction remains ordinary immutable stream work until GC folds it";
+
+    size_t terminal_logs = 0;
+    for (const ListedKey & listed : backend->list(layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), "", 1000).keys)
+    {
+        const auto parsed = layout.parseRefObjectKey(listed.key);
+        if (!parsed || parsed->kind != RefObjectKind::Log)
+            continue;
+        const auto got = backend->get(listed.key);
+        ASSERT_TRUE(got.has_value());
+        const RefLogTxn txn = decodeRefLogTxn(openObject(FormatId::RefLog, got->bytes), ns.string(), parsed->txn_id);
+        if (!txn.ops.empty() && txn.ops.back().kind == RefOpKind::RemoveNamespace)
+            ++terminal_logs;
+    }
+    EXPECT_EQ(terminal_logs, 1u);
+}
+
+/// Review fix (prerequisite to this task's dropNamespace rewiring): `flushRefBatch`'s per-item
+/// validation previously previewed each op as its OWN single-op trial transaction, so a
+/// whole-transaction-shape rule ("remove_namespace must be the FINAL op") trivially passed on every
+/// singleton slice regardless of an item's REAL combined shape -- a malformed item would only have
+/// been caught by the post-persist apply, AFTER its transaction object was already durable (bricking
+/// the table on every future recovery and permanently wedging this table's lane). Drives
+/// `appendRefOps` directly with a deliberately malformed multi-op item (remove_namespace not last) to
+/// prove the whole-item shape check now rejects it BEFORE any backend object is created.
+TEST(CASRefWriterNamespaceRemoval, MalformedShapeWithRemoveNamespaceNotFinalRejectedBeforeAnyCreate)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/malformed_shape"};
+    publishEmptyPart(store, ns, "a");   /// births the table so the malformed item isn't ALSO rejected
+                                        /// for the unrelated reason "namespace_birth was needed first"
+
+    const uint64_t put_before = backend->putTotal();
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        store->appendRefOps(ns, MutationScope::wholeShard(),
+            [](const RefTableState &) -> std::vector<RefOp>
+            {
+                RefOp remove_ns_1;
+                remove_ns_1.kind = RefOpKind::RemoveNamespace;
+                RefOp remove_ns_2;
+                remove_ns_2.kind = RefOpKind::RemoveNamespace;
+                return {remove_ns_1, remove_ns_2};   /// remove_namespace NOT the final op -- malformed
+            },
+            /// Deliberately mislabel the malformed terminal as an ordinary mutation: this bypasses
+            /// the public removal-capability preflight and proves the txn-wide shape check itself
+            /// rejects the object before the later capability check or any backend mutation.
+            RootMutationOrigin::Writer, RootMutationKind::Publish);
+    });
+
+    EXPECT_EQ(backend->putTotal(), put_before) << "the malformed shape must be rejected before any object is created";
+    ASSERT_TRUE(store->resolveRef(ns, "a").has_value()) << "the malformed attempt left no trace on the table";
+}
+
+/// A caller cannot turn the generic append surface into a second namespace-removal capability, even
+/// when it disguises terminal operations as an ordinary mutation kind. Only `dropNamespace` may carry
+/// the exact runtime ownership established by the durable `Live -> Removing` transition.
+TEST(CASRefWriterNamespaceRemoval, GenericAppendCannotWriteTerminalWhileCatalogIsLive)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/unauthorized_terminal"};
+    publishEmptyPart(store, ns, "owned");
+    const CatalogEntry live = catalogEntryOrThrow(*backend, layout, ns);
+    ASSERT_EQ(live.state, NsState::Live);
+    const auto greatest_before = listGreatestLogIdForLifeForTest(
+        *backend, layout, NamespaceLifeId::fromCatalogEntry(ns, live.incarnation));
+    ASSERT_TRUE(greatest_before);
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        store->appendRefOps(ns, MutationScope::wholeShard(),
+            [](const RefTableState & state) -> std::vector<RefOp>
+            {
+                std::vector<RefOp> ops;
+                for (const auto [ref_name, row] : state.getCommitted())
+                {
+                    RefOp remove_owner;
+                    remove_owner.kind = RefOpKind::OwnerTransition;
+                    remove_owner.old_binding = RefOwnerBinding{
+                        RefOwnerKind::Committed, ref_name, row.manifest_ref};
+                    ops.push_back(std::move(remove_owner));
+                }
+                RefOp terminal;
+                terminal.kind = RefOpKind::RemoveNamespace;
+                ops.push_back(terminal);
+                return ops;
+            },
+            RootMutationOrigin::Writer, RootMutationKind::Publish,
+            /*skip_stale_precommit_sweep=*/true);
+    });
+
+    EXPECT_EQ(catalogEntryOrThrow(*backend, layout, ns), live);
+    EXPECT_EQ(listGreatestLogIdForLifeForTest(
+        *backend, layout, NamespaceLifeId::fromCatalogEntry(ns, live.incarnation)), greatest_before)
+        << "an unauthorized terminal must allocate no id and create no ref-log object";
+    EXPECT_TRUE(store->resolveRef(ns, "owned"));
+}
+
+/// The public generic surface must reject the terminal-capable operation kind before resolving or
+/// creating a life. Otherwise an absent name can acquire a catalog row and checkpoint before the
+/// internal terminal capability check rejects the actual operations.
+TEST(CASRefWriterNamespaceRemoval, GenericTerminalOnAbsentNamePerformsZeroDurableMutation)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/absent_unauthorized_terminal"};
+    const CasRefCatalog::Snapshot catalog_before = CasRefCatalog::read(*backend, store->layout());
+
+    backend->resetCounts();
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        store->appendRefOps(ns, MutationScope::wholeShard(),
+            [](const RefTableState &) -> std::vector<RefOp>
+            {
+                RefOp terminal;
+                terminal.kind = RefOpKind::RemoveNamespace;
+                return {terminal};
+            },
+            RootMutationOrigin::Writer, RootMutationKind::DropNamespace,
+            /*skip_stale_precommit_sweep=*/true);
+    });
+
+    EXPECT_EQ(backend->putTotal(), 0u);
+    EXPECT_EQ(backend->putOverwriteTotal(), 0u);
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+    EXPECT_EQ(backend->deleteTotal(), 0u);
+    const CasRefCatalog::Snapshot catalog_after = CasRefCatalog::read(*backend, store->layout());
+    EXPECT_EQ(catalog_after.token, catalog_before.token);
+    EXPECT_EQ(catalog_after.catalog, catalog_before.catalog);
+    EXPECT_FALSE(store->refTableLifeForTest(ns));
+}
+
+/// A namespace file births a catalog life and checkpoint without necessarily creating a ref stream.
+/// Removing that table must still publish terminal evidence and let GC retire the catalog row.
+TEST(CASRefWriterNamespaceRemoval, CatalogedNamespaceFilesOnlyLifeCompletesRemoval)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.gc_fold_max_defer_rounds = 0;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/files_only"};
+    const NamespaceLifeId life = store->namespaceLife(ns);
+    store->putNamespaceFile(life, "format_version.txt", "1\n");
+    ASSERT_TRUE(backend->list(layout.namespaceStreamPrefix(life), "", 100).keys.empty());
+    ASSERT_EQ(catalogEntryOrThrow(*backend, layout, ns).state, NsState::Live);
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    ASSERT_EQ(catalogEntryOrThrow(*backend, layout, ns).state, NsState::Removing);
+
+    const ListPage terminal_page = backend->list(layout.namespaceStreamPrefix(life), "", 100);
+    ASSERT_EQ(terminal_page.keys.size(), 1u);
+    const auto parsed = layout.parseRefObjectKey(terminal_page.keys.front().key);
+    ASSERT_TRUE(parsed);
+    const auto terminal_body = backend->get(terminal_page.keys.front().key);
+    ASSERT_TRUE(terminal_body);
+    const RefLogTxn terminal = decodeRefLogTxn(
+        openObject(FormatId::RefLog, terminal_body->bytes), ns.string(), parsed->txn_id);
+    ASSERT_EQ(terminal.ops.size(), 2u);
+    EXPECT_EQ(terminal.ops[0].kind, RefOpKind::NamespaceBirth);
+    EXPECT_EQ(terminal.ops[1].kind, RefOpKind::RemoveNamespace);
+
+    Gc gc(store, UInt128{181});
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    (void)runRegularRoundReclaiming(gc);
+    const RefCatalog after = CasRefCatalog::read(*backend, layout).catalog;
+    EXPECT_TRUE(std::none_of(after.entries.begin(), after.entries.end(), [&](const CatalogEntry & entry)
+    {
+        return entry.ns == ns;
+    }));
+}
+
+/// If the first catalog read after closing the positive lane fails, the catch-side authoritative read
+/// is still allowed to prove the exact original `Live` row and reopen admission.
+TEST(CASRefWriterNamespaceRemoval, PredurableCatalogReadFailureReopensExactLiveLane)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/predurable_read_failure"};
+    publishEmptyPart(store, ns, "owned");
+    const CatalogEntry live = catalogEntryOrThrow(*backend, layout, ns);
+
+    backend->catalog_fault_key = layout.refCatalogKey();
+    backend->catalog_gets_before_fault = 1;   /// initial discovery succeeds; post-close observation fails
+    backend->catalog_get_fault_count = 1;
+    EXPECT_THROW(store->dropNamespace(ns), std::runtime_error);
+    EXPECT_EQ(catalogEntryOrThrow(*backend, layout, ns), live);
+
+    EXPECT_NO_THROW(store->updateRefPublishedAt(ns, "owned", [](RefPublishedAtUpdate & update)
+    {
+        update.published_at_ms = 17;
+    })) << "a fresh exact Live observation must reopen the lane after a pre-durable failure";
+    EXPECT_EQ(catalogEntryOrThrow(*backend, layout, ns).state, NsState::Live);
+}
+
+/// spec §Namespace Removal (writer, line 666): "After the transaction is durable, it applies the same
+/// operations to memory, cancels local builds, and rejects further ordinary mutations." An in-flight
+/// build for the removed namespace must be cancelled once (and only once) the removal is durable: its
+/// next operation throws (ABORTED, from requireAlive) rather than promoting a fresh committed ref into
+/// the just-removed namespace.
+TEST(CASRefWriterNamespaceRemoval, DropNamespaceCancelsInFlightBuildAndNextOpThrows)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/remove_cancels_build"};
+
+    publishEmptyPart(store, ns, "committed");   /// births the table + one committed ref
+
+    /// An in-flight build for ns: staged + precommit-added, never promoted.
+    auto build = startBuildFor(store, ns, "inflight");
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, "inflight", id);
+
+    store->dropNamespace(ns);
+
+    /// The build is cancelled: EVERY subsequent operation fails fast at `requireAlive` with
+    /// NETWORK_ERROR (fix #37 phase 2's CAS write-retry-later reroute). `stageManifest` is the
+    /// discriminator -- it has NO namespace-lifecycle gate, so an UN-cancelled build would happily
+    /// execute it (staging more debris into a dead namespace); only cancellation stops it.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { build->stageManifest({}); });
+    /// And it certainly cannot promote a fresh committed ref into the removed namespace (the important
+    /// invariant -- though the old WPromote "precommit removed" guard also blocked this, less directly).
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { build->promote(ns, "inflight", build->buildId(), id); });
+
+    /// The cancelled build did not resurrect anything into the removed namespace.
+    EXPECT_FALSE(store->resolveRef(ns, "inflight").has_value());
+    EXPECT_FALSE(store->resolveRef(ns, "committed").has_value()) << "the whole namespace was removed";
+}
+
+/// The catalog transition precedes the terminal append. If that append is unresolved, the namespace
+/// remains `Removing`, positive ownership is refused, and a retry of the same removal resolves the wedge.
+TEST(CASRefWriterNamespaceRemoval, RemovalAppendFailureLeavesRemovingAndRetryCompletes)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/remove_fault_keeps_build"};
+
+    publishEmptyPart(store, ns, "committed");
+
+    auto build = startBuildFor(store, ns, "inflight");
+    const ManifestId id = build->stageManifest({});
+    build->precommitAdd(ns, "inflight", id);
+
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(ns); });
+
+    EXPECT_EQ(catalogEntryOrThrow(*backend, layout, ns).state, NsState::Removing);
+    EXPECT_FALSE(store->resolveRef(ns, "committed"))
+        << "a fresh name lookup must not expose a catalog-Removing life";
+    /// The build was NOT cancelled: a non-append operation (`stageManifest` -- it never touches the now
+    /// wedged ref-append lane) still succeeds; it would throw ABORTED had the build been cancelled.
+    EXPECT_NO_THROW(build->stageManifest({}));
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&]
+    {
+        build->promote(ns, "inflight", build->buildId(), id);
+    });
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { build->stageManifest({}); });
+}
+
+/// `namespaceStillLogicallyPresent` must stay `true` for the entire window between the catalog's
+/// durable `Live -> Removing` transition and the terminal `remove_namespace` append actually landing --
+/// the crash-shaped case the fix exists for. Reuses the injected stream-write fault shape from
+/// `RemovalAppendFailureLeavesRemovingAndRetryCompletes`.
+TEST(CASRefWriterNamespaceRemoval, PresenceProbeStaysTrueThroughRemovingUntilTerminalRetrySucceeds)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/presence_removing_no_terminal"};
+
+    publishEmptyPart(store, ns, "committed");
+    EXPECT_TRUE(store->namespaceStillLogicallyPresent(ns));
+
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(ns); });
+
+    ASSERT_EQ(catalogEntryOrThrow(*backend, layout, ns).state, NsState::Removing);
+    EXPECT_TRUE(store->namespaceStillLogicallyPresent(ns))
+        << "the catalog transitioned but the terminal append never landed -- cleanup is unproven";
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    EXPECT_FALSE(store->namespaceStillLogicallyPresent(ns))
+        << "the retried removal's terminal is now durable";
+}
+
+/// A `Creating` row is conservative in both directions -- present, and removal refuses to cancel it
+/// while its creator fence cannot be proven dead, then succeeds once a terminal certificate (here, a
+/// GC-fenced lease for the same server root) makes the fence provably terminal.
+TEST(CASRefWriterNamespaceRemoval, PresenceProbeCreatingIsPresentAndRemovalWaitsForCreatorFenceTerminality)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/presence_still_creating"};
+
+    CatalogEntry entry;
+    entry.ns = ns;
+    entry.state = NsState::Creating;
+    entry.incarnation = UInt128(99);
+    entry.creator = CreatorFence{.server_root_id = "srv1", .writer_epoch = store->liveWriterEpoch(), .fence_generation = 1};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, entry);
+
+    EXPECT_TRUE(store->namespaceStillLogicallyPresent(ns));
+
+    /// The creator fence names an unmounted server root: `isCreatorFenceTerminal` cannot certify it
+    /// dead (absence proves nothing), so removal fails closed rather than cancelling a `Creating` row a
+    /// live writer might still publish into.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(ns); });
+    EXPECT_EQ(catalogEntryOrThrow(*backend, layout, ns).state, NsState::Creating);
+    EXPECT_TRUE(store->namespaceStillLogicallyPresent(ns));
+
+    /// Publish a GC-fenced lease for the SAME server root -- one of `isCreatorFenceTerminal`'s accepted
+    /// certificates -- and removal now cancels the row outright (no `Removing` transition for a
+    /// namespace that never reached `Live`).
+    MountLease dead;
+    dead.writer_epoch = store->liveWriterEpoch();
+    dead.gc_fenced = true;
+    dead.seq = 1;
+    backend->putIfAbsent(layout.mountKey("srv1"), encodeMountLease(dead));
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    EXPECT_FALSE(store->namespaceStillLogicallyPresent(ns));
+}
+
+/// A "no catalog row" observation must never be turned into `false` by a race. Pausing the probe
+/// right after its first catalog read and admitting a fresh `Creating` row before it resumes must
+/// surface as a typed retry, not a stale absent answer; an unchanged catalog across both reads
+/// legitimately settles on absent.
+TEST(CASRefWriterNamespaceRemoval, PresenceProbeNoRowObservationRevalidatesRatherThanRacingToAbsent)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace racing{"srv1/presence_no_row_races_birth"};
+    const RootNamespace stable{"srv1/presence_no_row_stays_absent"};
+
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool paused = false;
+    bool resume = false;
+    store->setNamespacePresenceProbeAfterFirstReadHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        paused = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return resume; });
+    });
+
+    std::exception_ptr raced_error;
+    std::thread racer([&]
+    {
+        try
+        {
+            (void)store->namespaceStillLogicallyPresent(racing);
+        }
+        catch (...)
+        {
+            raced_error = std::current_exception();
+        }
+    });
+    {
+        std::unique_lock lock(mutex);
+        cv.wait(lock, [&] { return paused; });
+    }
+
+    CatalogEntry born;
+    born.ns = racing;
+    born.state = NsState::Creating;
+    born.incarnation = UInt128(1234);
+    born.creator = CreatorFence{.server_root_id = "srv1", .writer_epoch = store->liveWriterEpoch(), .fence_generation = 1};
+    CasRefCatalog::casAdmitEntry(*backend, layout, 1, born);
+
+    {
+        std::lock_guard lock(mutex);
+        resume = true;
+    }
+    cv.notify_all();
+    racer.join();
+    store->setNamespacePresenceProbeAfterFirstReadHookForTest(nullptr);
+
+    ASSERT_TRUE(raced_error) << "a namespace born after the first read must never resolve to a stale absent";
+    EXPECT_THROW(std::rethrow_exception(raced_error), DB::Exception);
+
+    /// Negative control: an unraced, genuinely absent namespace settles on `false`.
+    EXPECT_FALSE(store->namespaceStillLogicallyPresent(stable));
+}
+
+/// Every unreadable or ambiguous observation must throw, never answer `false`. Covers a catalog `GET`
+/// failure on the probe's very first read, and a lost mount fence discovered mid-probe. Not covered
+/// here: a missing checkpoint for a `Removing` row, and an ambiguous incarnation -- both would need a
+/// raw-catalog-write test helper this suite does not currently expose.
+TEST(CASRefWriterNamespaceRemoval, PresenceProbeCatalogReadFailurePropagatesRatherThanAnsweringAbsent)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/presence_catalog_read_fault"};
+
+    backend->catalog_fault_key = store->layout().refCatalogKey();
+    backend->catalog_gets_before_fault = 0;
+    backend->catalog_get_fault_count = 1;
+    EXPECT_THROW((void)store->namespaceStillLogicallyPresent(ns), std::runtime_error);
+}
+
+TEST(CASRefWriterNamespaceRemoval, PresenceProbeFenceLossPropagatesRatherThanAnsweringAbsent)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/presence_fence_loss"};
+    publishEmptyPart(store, ns, "x");
+
+    const String mount_key = store->layout().mountKey("test");
+    const auto got = backend->get(mount_key);
+    ASSERT_TRUE(got);
+    MountLease foreign = decodeMountLease(got->bytes);
+    foreign.server_uuid = foreign.server_uuid + UInt128{1};
+    foreign.seq += 1;
+    ASSERT_EQ(backend->putOverwrite(mount_key, encodeMountLease(foreign), got->token).outcome, PutOutcome::Done);
+    store->tripMountLost();
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->namespaceStillLogicallyPresent(ns); });
+}
+
+/// Pin all three facade states against each other on one namespace -- `Live` (present, content
+/// readable), incomplete `Removing` (present, content deliberately unreadable), and terminal `Removing`
+/// (absent, immediately, no GC).
+TEST(CASRefWriterNamespaceRemoval, PresenceProbeFacadeConsistencyAcrossRemovalLifecycle)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = kSingleAttemptDeadlineMs;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend, budget);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/presence_facade_consistency"};
+
+    publishEmptyPart(store, ns, "committed");
+    EXPECT_TRUE(store->namespaceStillLogicallyPresent(ns));
+    EXPECT_FALSE(store->listRefs(ns).empty());
+
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropNamespace(ns); });
+
+    EXPECT_TRUE(store->namespaceStillLogicallyPresent(ns))
+        << "present for cleanup, even though content below is about to prove unreadable";
+    EXPECT_FALSE(store->namespaceFilesLifeIfReadable(ns).has_value());
+    EXPECT_FALSE(store->resolveRef(ns, "committed").has_value());
+
+    EXPECT_NO_THROW(store->dropNamespace(ns));
+    EXPECT_FALSE(store->namespaceStillLogicallyPresent(ns));
+    EXPECT_FALSE(store->namespaceFilesLifeIfReadable(ns).has_value());
+}
+
+/// Fix-verify review finding: `namespaceStillLogicallyPresent`'s `Removing` branch proved the OBSERVED
+/// incarnation's terminal and returned `false` for the name without re-checking whether the catalog had
+/// moved on since. Proving one incarnation terminal is not proof the CURRENT logical namespace is
+/// absent: GC can delete the now-terminal row and a successor can be born under the same name while the
+/// probe's own recovery call (real I/O, no upper bound) is still in flight. Drives that exact
+/// interleaving deterministically via the terminal-proven hook: pause right after the predecessor's
+/// terminal is proven, drain GC to actually delete its row, birth a successor under the same name, then
+/// resume and require `true` (present) -- never the stale `false` the unfixed probe would answer.
+TEST(CASRefWriterNamespaceRemoval, PresenceProbeRevalidatesAfterTerminalProvenRatherThanRacingToStaleAbsent)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.gc_fold_max_defer_rounds = 8;
+    config.ref_table_cache_bytes = 0;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/presence_terminal_revalidate"};
+    const UInt128 gc_id = hexToU128("00000000000000000000000000000001");
+
+    const auto catalog_entry = [&]() -> std::optional<CatalogEntry>
+    {
+        const RefCatalog catalog = CasRefCatalog::read(*backend, layout).catalog;
+        const auto it = std::find_if(catalog.entries.begin(), catalog.entries.end(), [&](const CatalogEntry & entry)
+        {
+            return entry.ns == ns;
+        });
+        return it == catalog.entries.end() ? std::nullopt : std::optional<CatalogEntry>{*it};
+    };
+
+    /// `publishEmptyPart` pins its catalog entry to `fixture::fixtureLife(ns)`, a life derived from
+    /// `ns` alone -- the SAME value every time for the same name, which is exactly wrong for a test
+    /// whose whole point is that the successor's incarnation must differ from the predecessor's.
+    /// `publishWithProductionBirth` goes through the real birth path (`resolveNamespaceLife`'s random
+    /// mint), so both incarnations below are independently random.
+    publishWithProductionBirth(store, ns, "predecessor");
+    const std::optional<CatalogEntry> predecessor = catalog_entry();
+    ASSERT_TRUE(predecessor.has_value());
+
+    Gc gc(store, gc_id);
+    store->dropNamespace(ns);   /// terminal durable; catalog row still Removing until GC deletes it
+
+    /// Pause the probe right after it proves the predecessor's terminal, before its revalidation read.
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool paused = false;
+    bool resume = false;
+    store->setNamespacePresenceProbeAfterTerminalProvenHookForTest([&]
+    {
+        std::unique_lock lock(mutex);
+        paused = true;
+        cv.notify_all();
+        cv.wait(lock, [&] { return resume; });
+    });
+
+    std::optional<bool> probe_result;
+    std::exception_ptr probe_error;
+    std::thread prober([&]
+    {
+        try
+        {
+            probe_result = store->namespaceStillLogicallyPresent(ns);
+        }
+        catch (...)
+        {
+            probe_error = std::current_exception();
+        }
+    });
+    /// A fatal assertion below (predecessor/successor state, GC round shape) must not skip joining
+    /// `prober` -- it is still blocked on `cv` at that point, and destructing a joinable `std::thread`
+    /// calls `std::terminate`, aborting the whole binary and hiding every test queued after this one.
+    bool prober_joined = false;
+    SCOPE_EXIT({
+        if (!prober_joined)
+        {
+            {
+                std::lock_guard lock(mutex);
+                resume = true;
+            }
+            cv.notify_all();
+            prober.join();
+            store->setNamespacePresenceProbeAfterTerminalProvenHookForTest(nullptr);
+        }
+    });
+    {
+        std::unique_lock lock(mutex);
+        cv.wait(lock, [&] { return paused; });
+    }
+
+    /// While the probe is paused: drain GC to actually delete the predecessor's catalog row (fold the
+    /// terminal, then a drain-only round to adopt the evidence and delete the row -- same two-round
+    /// shape `SameNameSameWriterEpochRebirth...` uses), then birth a successor under the SAME name. The
+    /// row must be gone before a fresh creation is admitted at all, so this also proves the row really
+    /// was deleted, not merely that the test raced ahead of production's own invariants.
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred) << "the terminal delta must fold";
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).deferred) << "the drain-only round must adopt the evidence seal";
+    ASSERT_FALSE(catalog_entry().has_value()) << "control: the predecessor's row is really gone before rebirth";
+    publishWithProductionBirth(store, ns, "successor");
+    const std::optional<CatalogEntry> successor = catalog_entry();
+    ASSERT_TRUE(successor.has_value());
+    ASSERT_NE(successor->incarnation, predecessor->incarnation);
+
+    {
+        std::lock_guard lock(mutex);
+        resume = true;
+    }
+    cv.notify_all();
+    prober.join();
+    store->setNamespacePresenceProbeAfterTerminalProvenHookForTest(nullptr);
+    prober_joined = true;
+
+    ASSERT_FALSE(probe_error) << "a successor born under the same name must never surface as an error";
+    ASSERT_TRUE(probe_result.has_value());
+    EXPECT_TRUE(*probe_result)
+        << "the predecessor's proven terminal must not answer false once a successor occupies its name";
+}
+
+/// `StorageJoin`/`StorageSet::truncate` call `disk->removeRecursive(path)` then `disk->createDirectories
+/// (path)`. `createDirectories` is a CAS no-op (`ContentAddressedTransaction::createDirectory` only
+/// checks write admission, it never touches the catalog), so the actual re-mint happens lazily on the
+/// FIRST subsequent write, which resolves through `namespaceLife` exactly like this test does directly.
+/// Right after `TRUNCATE` the catalog row is still `Removing` -- GC has not yet folded and deleted it --
+/// so that first write throws a typed retry-later error rather than silently wedging or corrupting
+/// anything: the same self-healing window the presence-probe revalidation above depends on. Before the
+/// `existsDirectory` fix, the directory never reported as present in the first place, so `TRUNCATE`
+/// silently skipped `removeRecursive` entirely and the table kept its OLD contents -- a different,
+/// quieter wrong answer than this one, not a newly introduced break.
+TEST(CASRefWriterNamespaceRemoval, FilesOnlyNamespaceTruncateThrowsRetryLaterUntilGcReclaimsThenRebirths)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.gc_fold_max_defer_rounds = 8;
+    config.ref_table_cache_bytes = 0;
+    auto store = openPoolWithConfig(backend, config);
+    const RootNamespace ns{"srv1/truncate_retry_then_rebirth"};
+    const UInt128 gc_id = hexToU128("00000000000000000000000000000002");
+
+    /// Birth a files-only namespace: `StorageJoin`/`StorageSet` never publish a MergeTree part, their
+    /// table root only ever carries plain table files (`putNamespaceFile`'s shape, not a manifest ref).
+    const NamespaceLifeId predecessor_life = store->namespaceLife(ns);
+    store->putNamespaceFile(predecessor_life, "data.bin", "predecessor-contents");
+
+    Gc gc(store, gc_id);
+    store->dropNamespace(ns);   /// the TRUNCATE-shaped removeRecursive: terminal durable, row still Removing
+
+    /// The very next write CAS would attempt (the `createDirectories` no-op already ran; this is the
+    /// first real write) must not be told the namespace is gone, and must not silently mint into a
+    /// row still occupied by the predecessor -- it throws retry-later.
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->namespaceLife(ns); });
+
+    /// Drain GC (fold the terminal, then a drain-only round to delete the now-evidenced row) -- same
+    /// two-round shape the presence-probe revalidation test above uses.
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred) << "the terminal delta must fold";
+    ASSERT_TRUE(runRegularRoundReclaiming(gc).deferred) << "the drain-only round must adopt the evidence seal";
+
+    /// Self-healed: the same logical name now mints a fresh incarnation and accepts writes again,
+    /// exactly what a retried `INSERT` (or a retried `TRUNCATE`) after the CAS write's retry-later gets.
+    const NamespaceLifeId successor_life = store->namespaceLife(ns);
+    EXPECT_NE(successor_life.incarnation, predecessor_life.incarnation);
+    store->putNamespaceFile(successor_life, "data.bin", "successor-contents");
+    const auto successor_contents = store->getNamespaceFile(successor_life, "data.bin");
+    ASSERT_TRUE(successor_contents.has_value());
+    EXPECT_EQ(*successor_contents, "successor-contents");
+}
+
+/// Cancellation is namespace-scoped: dropping namespace N must not cancel an in-flight build targeting a
+/// DIFFERENT namespace M -- that build promotes normally.
+TEST(CASRefWriterNamespaceRemoval, DropNamespaceDoesNotCancelBuildsInOtherNamespaces)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns_dropped{"srv1/remove_me"};
+    const RootNamespace ns_other{"srv1/keep_me"};
+
+    publishEmptyPart(store, ns_dropped, "x");
+
+    /// An in-flight build in a DIFFERENT namespace.
+    auto other_build = startBuildFor(store, ns_other, "y");
+    const ManifestId id = other_build->stageManifest({});
+    other_build->precommitAdd(ns_other, "y", id);
+
+    store->dropNamespace(ns_dropped);
+
+    /// The other namespace's build is untouched: it promotes successfully and its ref resolves.
+    EXPECT_NO_THROW(other_build->promote(ns_other, "y", other_build->buildId(), id));
+    EXPECT_TRUE(store->resolveRef(ns_other, "y").has_value());
+}
+
+/// A writer-side create/resolution cannot reuse the predecessor while its catalog row is `Removing`.
+/// Both the resident-runtime and fresh-runtime paths return typed retry-later without a durable write.
+TEST(CASRefWriterNamespaceRemoval, CreateAgainstRemovingRetriesWithoutMutation)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const RootNamespace ns{"srv1/create-while-removing"};
+
+    {
+        auto store = openPool(backend);
+        publishEmptyPart(store, ns, "predecessor");
+        store->dropNamespace(ns);
+
+        backend->resetCounts();
+        expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)store->namespaceLife(ns); });
+        EXPECT_EQ(backend->putTotal(), 0u);
+        EXPECT_EQ(backend->putOverwriteTotal(), 0u);
+        EXPECT_EQ(backend->casPutTotal(), 0u);
+    }
+
+    auto fresh_store = openPool(backend);
+    backend->resetCounts();
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { (void)fresh_store->namespaceLife(ns); });
+    EXPECT_EQ(backend->putTotal(), 0u);
+    EXPECT_EQ(backend->putOverwriteTotal(), 0u);
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+}
+
+/// Same-name rebirth must not inherit the predecessor's physical life or folded cursor even when the
+/// writer mount and its per-name runtime stay resident throughout the complete real removal sequence.
+TEST(CASRefWriterNamespaceRemoval, SameNameSameWriterEpochRebirthInvalidatesResidentLifeAndStartsAtZeroCoverage)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.gc_fold_max_defer_rounds = 8;
+    config.ref_table_cache_bytes = 0;   /// unbounded: no eviction can explain a fresh resolution
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/same-name-rebirth"};
+    const UInt128 gc_id = hexToU128("00000000000000000000000000000001");
+
+    const auto publish_without_fixture_admission = [&](const String & ref)
+    {
+        PartWriteInfo info;
+        info.intended_namespace = ns;
+        info.intended_ref = ns.string() + "/" + ref;
+        auto build = store->beginPartWrite(info);
+        const ManifestId id = build->stageManifest({});
+        build->precommitAdd(ns, ref, id);
+        build->promote(ns, ref, build->buildId(), id);
+    };
+    const auto catalog_entry = [&]() -> std::optional<CatalogEntry>
+    {
+        const RefCatalog catalog = CasRefCatalog::read(*backend, layout).catalog;
+        const auto it = std::find_if(catalog.entries.begin(), catalog.entries.end(), [&](const CatalogEntry & entry)
+        {
+            return entry.ns == ns;
+        });
+        return it == catalog.entries.end() ? std::nullopt : std::optional<CatalogEntry>{*it};
+    };
+
+    publish_without_fixture_admission("predecessor");
+    const CatalogEntry predecessor = *catalog_entry();
+    const uint64_t writer_epoch = store->liveWriterEpoch();
+    ASSERT_EQ(store->refTableLifeForTest(ns)->incarnation, predecessor.incarnation);
+    const uint64_t runtime_identity = store->refTableRuntimeIdentityForTest(ns);
+    ASSERT_NE(runtime_identity, 0u);
+
+    Gc gc(store, gc_id);
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    GcState state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    CasFoldSeal seal = decodeFoldSeal(backend->get(layout.foldSealKey(state.snap_generation, state.snap_attempt))->bytes);
+    const auto predecessor_row = seal.ref_lives.find(predecessor.incarnation);
+    ASSERT_NE(predecessor_row, seal.ref_lives.end());
+    ASSERT_NE(predecessor_row->second.coverage.last_folded_ref_id, RefTxnId{});
+
+    const uint64_t puts_before_drop = backend->putTotal();
+    store->dropNamespace(ns);
+    ASSERT_GT(backend->putTotal(), puts_before_drop)
+        << "control: the real removal call returned after durably writing its terminal artifacts";
+    std::optional<RefTxnId> terminal_id;
+    for (const ListedKey & listed : backend->list(
+        layout.namespaceStreamPrefix(NamespaceLifeId::fromCatalogEntry(ns, predecessor.incarnation)),
+        "", 1000).keys)
+    {
+        const auto parsed = layout.parseRefObjectKey(listed.key);
+        if (parsed && parsed->kind == RefObjectKind::Log
+            && (!terminal_id || *terminal_id < parsed->txn_id))
+            terminal_id = parsed->txn_id;
+    }
+    ASSERT_TRUE(terminal_id.has_value());
+    const auto terminal_body = backend->get(layout.refLogKey(
+        NamespaceLifeId::fromCatalogEntry(ns, predecessor.incarnation), *terminal_id));
+    ASSERT_TRUE(terminal_body.has_value());
+    const RefLogTxn terminal = decodeRefLogTxn(
+        openObject(FormatId::RefLog, terminal_body->bytes), ns.string(), *terminal_id);
+    ASSERT_FALSE(terminal.ops.empty());
+    ASSERT_EQ(terminal.ops.back().kind, RefOpKind::RemoveNamespace)
+        << "control: the newest old-life log is the production terminal record";
+    const std::optional<CatalogEntry> removing = catalog_entry();
+    ASSERT_TRUE(removing.has_value());
+    ASSERT_EQ(removing->state, NsState::Removing)
+        << "the real terminal returned durable, but its catalog row stayed Live";
+    ASSERT_EQ(removing->incarnation, predecessor.incarnation);
+    ASSERT_EQ(store->refTableRuntimeIdentityForTest(ns), runtime_identity)
+        << "the removal path must invalidate the resident runtime's life, not pass through eviction";
+
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred) << "the terminal delta must fold";
+    state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    seal = decodeFoldSeal(backend->get(layout.foldSealKey(state.snap_generation, state.snap_attempt))->bytes);
+    ASSERT_TRUE(seal.ref_lives.at(predecessor.incarnation).cleanup_evidence.has_value());
+
+    const RoundReport drain = runRegularRoundReclaiming(gc);
+    ASSERT_TRUE(drain.deferred) << "the drain-only idle invocation must leave the evidence seal adopted";
+    ASSERT_FALSE(catalog_entry().has_value());
+    ASSERT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u)
+        << "catalog deletion must detach the predecessor from the name slot";
+
+    publish_without_fixture_admission("successor");
+    const CatalogEntry successor = *catalog_entry();
+    ASSERT_EQ(successor.ns, predecessor.ns) << "the exact same logical namespace must be reused";
+    ASSERT_NE(successor.incarnation, predecessor.incarnation);
+    ASSERT_EQ(store->liveWriterEpoch(), writer_epoch) << "rebirth must use the same mounted writer epoch";
+    ASSERT_NE(store->refTableRuntimeIdentityForTest(ns), runtime_identity)
+        << "rebirth must publish a distinct successor runtime, not reset the predecessor";
+    ASSERT_EQ(store->refTableLifeForTest(ns)->incarnation, successor.incarnation);
+
+    const NamespaceLifeId successor_life = NamespaceLifeId::fromCatalogEntry(ns, successor.incarnation);
+    const ListPage successor_stream = backend->list(layout.namespaceStreamPrefix(successor_life), "", 1000);
+    ASSERT_FALSE(successor_stream.keys.empty()) << "the real successor writer produced foldable stream work";
+    std::vector<GcPhaseRecord> successor_phases;
+    gc.setPhaseSink([&](const GcPhaseRecord & phase) { successor_phases.push_back(phase); });
+    const RoundReport successor_round = runRegularRoundReclaiming(gc);
+    const auto decision = std::find_if(successor_phases.begin(), successor_phases.end(), [](const GcPhaseRecord & phase)
+    {
+        return phase.phase == "defer_decision";
+    });
+    ASSERT_NE(decision, successor_phases.end());
+    ASSERT_FALSE(successor_round.deferred)
+        << "successor stream keys=" << successor_stream.keys.size()
+        << ", changed_shards=" << decision->metrics.at("changed_shards")
+        << ", dead_life_debris=" << decision->metrics.at("dead_life_debris");
+    state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    seal = decodeFoldSeal(backend->get(layout.foldSealKey(state.snap_generation, state.snap_attempt))->bytes);
+    EXPECT_FALSE(seal.ref_lives.contains(predecessor.incarnation));
+    const auto successor_row = seal.ref_lives.find(successor.incarnation);
+    ASSERT_NE(successor_row, seal.ref_lives.end());
+    EXPECT_EQ(successor_row->second.coverage.last_folded_ref_id.writer_epoch, writer_epoch);
+    EXPECT_EQ(successor_row->second.coverage.last_folded_ref_id.ref_sequence, 2u)
+        << "the successor starts at its own birth+publish stream, not the predecessor's cursor";
+    for (const String & key : backend->touchedKeys())
+        EXPECT_EQ(key.find("/_cleanup/"), String::npos) << key;
+}
+
+/// Losing the response to an erase that committed must not strand the same resident writer runtime
+/// behind its old removal-admission gate. A complete resolution read proves the exact old row absent,
+/// so the same name can be born immediately under a fresh incarnation without inheriting coverage.
+TEST(CASRefWriterNamespaceRemoval, CommitThenThrowEraseResolvesAndRebindsResidentRuntimeImmediately)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/removal-erase-lost-response"};
+    Gc gc(store, UInt128{101});
+    const CompletedRemovingFixture ready = prepareResidentRemovalForDrain(store, *backend, ns, gc);
+
+    backend->catalog_fault_key = layout.refCatalogKey();
+    backend->catalog_cas_fault = RefWriterTestBackend::CatalogCasFault::CommitThenThrow;
+    EXPECT_NO_THROW((void)runRegularRoundReclaiming(gc));
+
+    const RefCatalog after_erase = CasRefCatalog::read(*backend, layout).catalog;
+    EXPECT_TRUE(std::none_of(after_erase.entries.begin(), after_erase.entries.end(), [&](const CatalogEntry & entry)
+    {
+        return entry.ns == ns && entry.incarnation == ready.predecessor.incarnation;
+    }));
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+
+    EXPECT_NO_THROW(publishWithProductionBirth(store, ns, "successor"));
+    const CatalogEntry successor = catalogEntryOrThrow(*backend, layout, ns);
+    EXPECT_NE(successor.incarnation, ready.predecessor.incarnation);
+    EXPECT_EQ(store->liveWriterEpoch(), ready.writer_epoch);
+    EXPECT_NE(store->refTableRuntimeIdentityForTest(ns), ready.runtime_identity);
+
+    ASSERT_FALSE(runRegularRoundReclaiming(gc).deferred);
+    const GcState state = decodeGcState(backend->get(layout.gcStateKey())->bytes);
+    const CasFoldSeal seal = decodeFoldSeal(
+        backend->get(layout.foldSealKey(state.snap_generation, state.snap_attempt))->bytes);
+    EXPECT_FALSE(seal.ref_lives.contains(ready.predecessor.incarnation));
+    ASSERT_TRUE(seal.ref_lives.contains(successor.incarnation));
+    EXPECT_EQ(seal.ref_lives.at(successor.incarnation).coverage.last_folded_ref_id,
+        (RefTxnId{ready.writer_epoch, 2}));
+}
+
+/// If another actor wins the erase race by replacing the exact old row, `EntryChanged` still proves
+/// the predecessor life dead and must invalidate its resident runtime.
+TEST(CASRefWriterNamespaceRemoval, OtherWinnerReplacementInvalidatesExactPredecessorLife)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/removal-other-winner-replacement"};
+    Gc gc(store, UInt128{102});
+    const CompletedRemovingFixture ready = prepareResidentRemovalForDrain(store, *backend, ns, gc);
+
+    const CatalogEntry replacement{
+        .ns = ns,
+        .state = NsState::Live,
+        .incarnation = UInt128{0xfeed},
+        .creator = std::nullopt};
+    ASSERT_NE(replacement.incarnation, ready.predecessor.incarnation);
+    const NamespaceLifeId replacement_life = NamespaceLifeId::fromCatalogEntry(ns, replacement.incarnation);
+    ASSERT_EQ(backend->putIfAbsent(layout.refCkptKey(replacement_life), encodeRefCkpt(RefCkpt{
+        .life_epoch = ready.writer_epoch,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt})).outcome, PutOutcome::Done);
+    backend->catalog_fault_key = layout.refCatalogKey();
+    backend->catalog_replacement_bytes = encodeRefCatalog(RefCatalog{.entries = {replacement}});
+    backend->catalog_cas_fault = RefWriterTestBackend::CatalogCasFault::OtherWriterReplacement;
+
+    EXPECT_NO_THROW((void)runRegularRoundReclaiming(gc));
+    EXPECT_EQ(store->namespaceLife(ns), replacement_life);
+    EXPECT_NE(store->refTableRuntimeIdentityForTest(ns), ready.runtime_identity);
+}
+
+/// Failure to read the catalog while resolving a lost erase response is not success. A later fresh
+/// name lookup nevertheless observes the old exact row absent and reconciles the resident runtime.
+TEST(CASRefWriterNamespaceRemoval, LaterNameLookupReconcilesAfterEraseResolutionReadFailure)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/removal-resolution-read-failure-lookup"};
+    Gc gc(store, UInt128{103});
+    const CompletedRemovingFixture ready = prepareResidentRemovalForDrain(store, *backend, ns, gc);
+
+    backend->catalog_fault_key = layout.refCatalogKey();
+    backend->catalog_cas_fault = RefWriterTestBackend::CatalogCasFault::CommitThenThrow;
+    backend->catalog_resolution_get_fault_count = 1;
+    EXPECT_THROW((void)runRegularRoundReclaiming(gc), std::runtime_error);
+    EXPECT_TRUE(CasRefCatalog::read(*backend, layout).catalog.entries.empty());
+
+    std::optional<NamespaceLifeId> successor;
+    EXPECT_NO_THROW(successor = store->namespaceLife(ns));
+    ASSERT_TRUE(successor.has_value());
+    EXPECT_NE(successor->incarnation, ready.predecessor.incarnation);
+    EXPECT_NE(store->refTableRuntimeIdentityForTest(ns), ready.runtime_identity);
+}
+
+/// The normal post-LIST catalog cut is also a reconciliation point. It repairs a missed local
+/// invalidation before any later writer touches the name.
+TEST(CASRefWriterNamespaceRemoval, PostListCatalogCutReconcilesMissedEraseInvalidation)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    PoolConfig config;
+    config.gc_fold_threshold = 1;
+    config.ref_table_cache_bytes = 0;
+    auto store = openPoolWithConfig(backend, config);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv1/removal-resolution-read-failure-post-list"};
+    Gc gc(store, UInt128{104});
+    const CompletedRemovingFixture ready = prepareResidentRemovalForDrain(store, *backend, ns, gc);
+
+    backend->catalog_fault_key = layout.refCatalogKey();
+    backend->catalog_cas_fault = RefWriterTestBackend::CatalogCasFault::CommitThenThrow;
+    backend->catalog_resolution_get_fault_count = 1;
+    EXPECT_THROW((void)runRegularRoundReclaiming(gc), std::runtime_error);
+    EXPECT_TRUE(CasRefCatalog::read(*backend, layout).catalog.entries.empty());
+
+    EXPECT_NO_THROW((void)runRegularRoundReclaiming(gc));
+    EXPECT_NO_THROW(publishWithProductionBirth(store, ns, "successor"));
+    EXPECT_NE(catalogEntryOrThrow(*backend, layout, ns).incarnation, ready.predecessor.incarnation);
+    EXPECT_NE(store->refTableRuntimeIdentityForTest(ns), ready.runtime_identity);
+}
+
+/// ===================================================================================
+/// Task 11: namespace birth / the recreation gate (spec §Namespace Birth)
+/// ===================================================================================
+
+/// The writer assignment site may pin an already-`Live` catalog life, but recovering that empty life
+/// performs no catalog or stream mutation. It must install the exact incarnation from the observed row.
+TEST(CASRefWriterNamespaceBirth, ExistingLiveCatalogRowPinsExactLifeWithoutMutation)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/existing-live-assignment"};
+    CasRefCatalog::casAdmitEntry(*backend, store->layout(), 1, CatalogEntry{
+        .ns = ns, .state = NsState::Live, .incarnation = UInt128{41}});
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*backend, store->layout(), ns, RefCkpt{
+        .life_epoch = store->liveWriterEpoch(),
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    backend->resetCounts();
+    const NamespaceLifeId life = store->namespaceLife(ns);
+    EXPECT_EQ(life.incarnation, UInt128{41});
+    ASSERT_TRUE(store->refTableLifeForTest(ns));
+    EXPECT_EQ(store->refTableLifeForTest(ns)->incarnation, UInt128{41});
+    EXPECT_EQ(backend->putTotal(), 0u);
+    EXPECT_EQ(backend->putOverwriteTotal(), 0u);
+    EXPECT_EQ(backend->casPutTotal(), 0u);
+}
+
+/// A read of a never-born name may observe the catalog, but it must not allocate the local name slot
+/// or a life runtime. Otherwise arbitrary read traffic can fill the cache with identity-less runtimes,
+/// and a later birth has to mutate one of those objects into a different identity.
+TEST(CASRefWriterNamespaceBirth, NeverBornReadAllocatesNoRuntime)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/never-born-read-no-runtime"};
+
+    ASSERT_EQ(store->refTablesCachedCountForTest(), 0u);
+    EXPECT_FALSE(store->resolveRef(ns, "missing").has_value());
+    EXPECT_EQ(store->refTablesCachedCountForTest(), 0u)
+        << "catalog absence must be decided before a runtime is constructed";
+    EXPECT_EQ(store->refTableRuntimeIdentityForTest(ns), 0u);
+}
+
+/// A never-born namespace follows the ordinary catalog-first birth path.
+TEST(CASRefWriterNamespaceBirth, BirthFromNeverBornUsesOrdinaryPath)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/virgin"};
+
+    EXPECT_NO_THROW(publishEmptyPart(store, ns, "first"));
+    EXPECT_TRUE(store->resolveRef(ns, "first").has_value());
+}
+
+/// Coverage gap (Task 13a): the "one op per ref name per batch" cut in `flushRefBatch` (the `seen_refs`
+/// guard, `CASRefBatchScopeCuts`) had no test after the shard-lane `CasShardQueue.SameRefMutations
+/// SplitAcrossFlushes` was retired. Two payload mutations of the SAME committed ref, made co-pending by
+/// the pre-carve hook (mirrors `CompatibleMutationsShareOneCreate`), must NOT co-batch: per-request undo
+/// validates each op against the pre-batch state, so the batch carries at most one op per ref name and
+/// the two flush as two separate `_log` objects.
+TEST(CASRefWriterAppendLane, SameRefMutationsSplitAcrossFlushes)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/samerefsplit"};
+    publishEmptyPart(store, ns, "a");
+    ASSERT_TRUE(store->resolveRef(ns, "a").has_value());
+
+    std::mutex m;
+    std::condition_variable cv;
+    bool entered = false;
+    store->setRefPreCarveHookForTest([&]
+    {
+        std::unique_lock lk(m);
+        if (entered)
+            return;   /// only the leader's own first carve blocks; the second flush proceeds
+        entered = true;
+        cv.notify_all();
+        cv.wait(lk, [&] { return store->refQueuePendingForTest(ns) >= 2; });
+    });
+
+    const uint64_t put_before = backend->putTotal();
+    std::thread t_a([&] { store->updateRefPublishedAt(ns, "a", [](RefPublishedAtUpdate & r) { r.published_at_ms = 1; }); });
+    {
+        std::unique_lock lk(m);
+        cv.wait(lk, [&] { return entered; });
+    }
+    std::thread t_b([&] { store->updateRefPublishedAt(ns, "a", [](RefPublishedAtUpdate & r) { r.published_at_ms = 2; }); });
+    while (store->refQueuePendingForTest(ns) < 2)
+        std::this_thread::yield();
+    cv.notify_all();
+    t_a.join();
+    t_b.join();
+    store->setRefPreCarveHookForTest(nullptr);
+
+    EXPECT_EQ(backend->putTotal(), put_before + 2) << "same-ref mutations must flush as two separate logs";
+    /// Neither mutation was lost or corrupted -- the ref still resolves with one of the two writes.
+    const auto resolved = store->resolveRef(ns, "a");
+    ASSERT_TRUE(resolved.has_value());
+    EXPECT_TRUE(resolved->published_at_ms == 1 || resolved->published_at_ms == 2);
+}
+
+/// ===================================================================================
+/// rev.6 Task 8: the recovery seal (spec §recovery-seal / §seal-id / §seal-soundness). At an UNCLEAN
+/// mount, `ensureRefTableRecovered` must close every dead epoch it discovers with an immediate
+/// snapshot -- published at the UPPER BOUND of the dead-epoch region, `{liveWriterEpoch() - 1,
+/// UINT64_MAX}` -- BEFORE the table is exposed as recovered, so no late predecessor PUT from any dead
+/// epoch can ever surface to a cold fold or a fresh recovery.
+/// ===================================================================================
+
+namespace
+{
+
+/// Seeds crash-style predecessor debris for the seal tests: two DEAD epochs (1 and 2) of durable logs
+/// under `ns` -- epoch 1 births ref "a", epoch 2 adds ref "b" -- with no snapshot, and burns the
+/// durable epoch counter to exactly 2 so a subsequent `Pool::open` allocates epoch 3 (both dead
+/// epochs land strictly below the fresh writer's own, as `dead_region_nonempty` requires).
+void seedSealFixtureDeadEpochs(Backend & backend, const Layout & layout, const RootNamespace & ns)
+{
+    allocateWriterEpoch(backend, layout, "test", EpochMintPolicy::NormalMount, 0, [] { return RefCatalog{}; });   /// burns epoch 1
+    allocateWriterEpoch(backend, layout, "test", EpochMintPolicy::NormalMount, 0, [] { return RefCatalog{}; });   /// burns epoch 2
+
+    RefLogTxn birth;
+    birth.ns = ns.string();
+    birth.txn_id = RefTxnId{1, 1};
+    birth.ops = {namespaceBirthOp(), publishCommittedOps("a", manifestRef(1, 1, 1))[0],
+                 publishCommittedOps("a", manifestRef(1, 1, 1))[1]};
+    DB::Cas::tests::fixture::writeRefLogRaw(backend, layout, birth);
+
+    RefLogTxn mut;
+    mut.ns = ns.string();
+    mut.txn_id = RefTxnId{2, 1};
+    /// Sequence 1 of a new epoch names the seal that closed the one below -- `{1,2}`, the slot right
+    /// after epoch 1's only durable id, which is where the recovering mount's CAS-walk puts it. The seal
+    /// OBJECT is deliberately not seeded: this fixture's subject is a recovery that has to mint it.
+    mut.prev_epoch_seal = RefTxnId{1, 2};
+    mut.ops = {publishCommittedOps("b", manifestRef(2, 1, 1))[0],
+               publishCommittedOps("b", manifestRef(2, 1, 1))[1]};
+    DB::Cas::tests::fixture::writeRefLogRaw(backend, layout, mut);
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(backend, layout, ns, RefCkpt{
+        /// The namespace was born in epoch 1 and only `{1,1}` is fronted initially. Recovery must mint
+        /// the missing required seal `{1,2}` before it may adopt the already durable `{2,1}` successor.
+        .life_epoch = 1,
+        .committed_through = RefTxnId{1, 1},
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+}
+
+/// Plants a same-uuid, UNCLEAN (crash-style, no farewell) predecessor mount lease at `epoch`: a bare
+/// `claimMount` followed by a GC fence-out -- mirrors `CASMountOpenWaits.FencedPriorReclaimsWithoutAnyWait`. A fenced
+/// prior is an immediate certificate of death (`claimMountAwaitingExpiry` reclaims it on its FIRST
+/// attempt, no observation polling), so a fake-clocked successor `Pool::open` above it becomes
+/// unclean deterministically, without any real sleep.
+void seedUncleanPredecessorMount(Backend & backend, const Layout & layout, uint64_t epoch)
+{
+    claimMount(backend, layout, "test", UInt128(1), epoch, /*now_ms=*/1000, /*ttl_ms=*/500);
+    fenceOutRefMount(backend, layout.mountKey("test"));
+}
+
+/// The budget every seal test's successor `Pool::open` uses: a 500ms lease TTL needs a scaled-down
+/// budget (RFC cas-s3-timeout-retry-control §required-timeout-model: attempt_timeout + safety_margin <
+/// lease TTL) -- mirrors `CASMountOpenWaits.FencedPriorReclaimsWithoutAnyWait` exactly.
+CasRequestBudget sealTestTinyBudget()
+{
+    return CasRequestBudget{
+        .attempt_timeout_ms = 50, .operation_deadline_ms = kSingleAttemptDeadlineMs, .max_attempts = 1,
+        .lease_safety_margin_ms = 50};
+}
+
+}
+
+/// The `RefWriterRecoverySeal` suite is RETIRED with the sentinel seal it pinned, and the replacement is
+/// `gtest_cas_ref_recovery_cas_walk.cpp` (`CASRefRecoveryCasWalk`), which covers the same duties against
+/// the in-band mechanism: a dead epoch closed at `{E, T+1}`, a concurrent recoverer's seal adopted, a
+/// straggler adopted and re-sealed at the new `T+1`, chained seals across burned epochs, and genesis.
+///
+/// Three of its properties changed MEANING rather than mechanism, and a reader looking for them here
+/// should know where they went:
+///
+///   - "a clean boundary does not seal" is GONE as a rule. Sealing is now decided by `epoch < live_epoch`
+///     alone, never by how the predecessor died: the seal is the chain link that makes a MISSING epoch
+///     detectable, and a chain with holes in it wherever a mount shut down cleanly is not a chain.
+///   - "a late log below the seal is invisible to recovery" is replaced by something stronger, and the
+///     replacement is what makes the detector unnecessary: the seal occupies the ghost's own log key, so
+///     a late PUT is REFUSED by the store instead of landing somewhere a reader must learn to ignore.
+///   - the `sealed_from` inventory assertions are gone with the field; the chain link recovery installs
+///     is `last_epoch_seal`, asserted in the new suite and in `CASRecoveryStreaming`'s inventory test.
+///
+/// The fixtures above (`seedSealFixtureDeadEpochs`, `seedUncleanPredecessorMount`, `sealTestTinyBudget`)
+/// are KEPT: the recovery-retry suite below drives the same dead-epoch shape.
+
+/// ===================================================================================
+/// Layer 1 of the stuck-table-load fix: `ensureRefTableRecovered` retries a whole recovery attempt
+/// after a TRANSIENT object-store NETWORK_ERROR (bounded by `recovery_retry_budget_ms`), instead of
+/// failing the table's async load permanently. Non-transient errors and the terminal vanish-race
+/// brake still fail fast.
+/// ===================================================================================
+
+TEST(CASRefWriterRecoveryRetry, TransientSealFailureIsRetriedThenSucceeds)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/retry_ok"};
+
+    seedSealFixtureDeadEpochs(*backend, layout, ns);
+    seedUncleanPredecessorMount(*backend, layout, /*epoch=*/2);
+
+    uint64_t fake_now = 1'000'000;
+
+    PoolConfig config;
+    config.server_id = UInt128(1);
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(500);
+    config.cas_request_budget = sealTestTinyBudget();
+    config.cas_request_budget.recovery_retry_budget_ms = 120000;
+    config.cas_request_budget.recovery_retry_initial_backoff_ms = 1000;
+    config.cas_request_budget.recovery_retry_max_backoff_ms = 30000;
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.wait_sleep_fn = [](uint64_t) {};
+    auto store = openPoolWithConfig(backend, config);
+    ASSERT_TRUE(store);
+    ASSERT_EQ(store->liveWriterEpoch(), 3u);
+
+    /// No-op backoff and a frozen clock: retries run until the transient faults are exhausted, and the
+    /// frozen clock keeps the mount fence alive across them (advancing it past the tiny lease TTL would
+    /// drop the fence and abort recovery -- exercising the fence path, which is the budget test's job).
+    store->setCasRetrySleepForTest([](uint64_t) {});
+
+    /// Fail the epoch seal's conditional create twice with a transient (timeout) error; the third
+    /// attempt lands. The seal is a LOG transaction at `{2,2}` -- the slot after the dead epoch's last
+    /// durable id -- because INV-2 closes an epoch in-band, at the key a straggler would have taken.
+    const RefTxnId seal_id{2, 2};
+    backend->fault_key_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), seal_id);
+    backend->fault_count = 2;
+
+    using ProfileEvents::global_counters;
+    const auto retries_before = global_counters[ProfileEvents::CASRefRecoveryRetries].load();
+    const auto sealed_before = global_counters[ProfileEvents::CASRefRecoveryEpochSealed].load();
+
+    EXPECT_EQ(store->listRefs(ns).size(), 2u) << "recovery must succeed after retrying past the faults";
+
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefRecoveryRetries].load(), retries_before + 2);
+    /// TWO dead epochs (1 and 2) are closed by this walk, and a whole attempt is re-driven per transient
+    /// failure -- so the seals of the epochs a failed attempt already closed are ADOPTED on the retry
+    /// rather than minted again. Exactly two are minted in total.
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefRecoveryEpochSealed].load(), sealed_before + 2);
+}
+
+TEST(CASRefWriterRecoveryRetry, RecoveryDoesNotEnumerateItsStream)
+{
+    /// A recovery stream LIST used to be a transient failure leg. The checkpoint now names both the
+    /// base and frontier, so the same injected failures must remain untouched while recovery seals.
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/retry_list"};
+
+    seedSealFixtureDeadEpochs(*backend, layout, ns);
+    seedUncleanPredecessorMount(*backend, layout, /*epoch=*/2);
+
+    uint64_t fake_now = 1'000'000;
+
+    PoolConfig config;
+    config.server_id = UInt128(1);
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(500);
+    config.cas_request_budget = sealTestTinyBudget();
+    config.cas_request_budget.recovery_retry_budget_ms = 120000;
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.wait_sleep_fn = [](uint64_t) {};
+    auto store = openPoolWithConfig(backend, config);
+    ASSERT_TRUE(store);
+    ASSERT_EQ(store->liveWriterEpoch(), 3u);
+
+    store->setCasRetrySleepForTest([](uint64_t) {});
+
+    /// If recovery ever reintroduces a stream LIST, this injection turns the attempt into a retry and
+    /// consumes the counter. `namespaceFilesLifeIfReadable` reaches writer recovery without performing
+    /// the unrelated user-facing `listRefs` enumeration.
+    backend->list_fault_count = 2;
+
+    using ProfileEvents::global_counters;
+    const auto retries_before = global_counters[ProfileEvents::CASRefRecoveryRetries].load();
+    const auto sealed_before = global_counters[ProfileEvents::CASRefRecoveryEpochSealed].load();
+
+    ASSERT_TRUE(store->namespaceFilesLifeIfReadable(ns));
+    EXPECT_EQ(backend->list_fault_count, 2);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefRecoveryRetries].load(), retries_before);
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefRecoveryEpochSealed].load(), sealed_before + 2)
+        << "two dead epochs (1 and 2) are closed without enumerating their stream";
+}
+
+TEST(CASRefWriterRecoveryRetry, TransientFailureLongerThanBudgetPropagates)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/retry_budget"};
+
+    seedSealFixtureDeadEpochs(*backend, layout, ns);
+    seedUncleanPredecessorMount(*backend, layout, /*epoch=*/2);
+
+    uint64_t fake_now = 1'000'000;
+
+    PoolConfig config;
+    config.server_id = UInt128(1);
+    /// Lease TTL >> the recovery budget so the CLOCK-advancing backoff below trips the budget check,
+    /// not the mount fence -- this test specifically exercises the budget-exhaustion path.
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(600000);
+    config.cas_request_budget = sealTestTinyBudget();
+    config.cas_request_budget.recovery_retry_budget_ms = 5000;   /// small, deterministic
+    config.cas_request_budget.recovery_retry_initial_backoff_ms = 1000;
+    config.cas_request_budget.recovery_retry_max_backoff_ms = 30000;
+    config.boot_ms_fn = [&fake_now] { return fake_now; };
+    config.wait_sleep_fn = [](uint64_t) {};
+    auto store = openPoolWithConfig(backend, config);
+    ASSERT_TRUE(store);
+    store->setCasRetrySleepForTest([&fake_now](uint64_t ms) { fake_now += ms; });
+
+    /// The seal is an in-band LOG transaction at the slot after the dead epoch's last durable id, not a
+    /// snapshot at a synthetic id: epoch 1 closes at `{1,2}`, which is the FIRST write the walk attempts.
+    const RefTxnId seal_id{1, 2};
+    backend->fault_key_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), seal_id);
+    backend->fault_count = 1000;   /// never stops failing within the budget
+
+    expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->listRefs(ns); });
+}
+
+TEST(CASRefWriterRecoveryRetry, NonNetworkErrorIsNotRetried)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/retry_fatal"};
+
+    seedSealFixtureDeadEpochs(*backend, layout, ns);
+    seedUncleanPredecessorMount(*backend, layout, /*epoch=*/2);
+
+    PoolConfig config;
+    config.server_id = UInt128(1);
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(500);
+    config.cas_request_budget = sealTestTinyBudget();
+    config.wait_sleep_fn = [](uint64_t) {};
+    auto store = openPoolWithConfig(backend, config);
+    ASSERT_TRUE(store);
+
+    size_t sleep_calls = 0;
+    store->setCasRetrySleepForTest([&sleep_calls](uint64_t) { ++sleep_calls; });
+
+    /// A foreign writer lands DIFFERENT valid bytes at the seal key; resolve-before-reissue then throws
+    /// CORRUPTED_DATA (a real cross-process seal conflict), which must NOT be retried.
+    const RefTxnId seal_id{1, 2};
+    backend->corrupt_key_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), seal_id);
+    backend->corrupt_count = 1;
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->listRefs(ns); });
+    EXPECT_EQ(backend->corrupt_count, 0)
+        << "the test must reach the injected foreign seal conflict, not fail on fixture validation";
+    EXPECT_EQ(sleep_calls, 0u) << "a non-transient error must fail fast with zero backoff sleeps";
+}
+
+TEST(CASRefWriterRecoveryRetry, VanishBrakeStaysTerminalNotRetried)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/retry_vanish"};
+    const ManifestRef ma = manifestRef(1, 1, 1);
+
+    /// Stage B (Task 4-C): pin `ns` to the sentinel before the raw snapshot below -- `store->listRefs`
+    /// further down is a real production read that would otherwise mint a fresh RANDOM incarnation for
+    /// this unadmitted namespace instead of adopting the sentinel the raw fixture writes at.
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    const RefTxnId snap_x{1, 10};
+    std::vector<RefOp> base_ops{namespaceBirthOp()};
+    const auto publish_a = publishCommittedOps("a", ma);
+    base_ops.insert(base_ops.end(), publish_a.begin(), publish_a.end());
+    DB::Cas::tests::fixture::writeRefLogRaw(*backend, layout, RefLogTxn{ns.string(), snap_x, std::move(base_ops), std::nullopt});
+    writeRefSnapshotRaw(*backend, layout, minimalLiveSnapshot(ns.string(), snap_x, {committedRow("a", ma)}));
+    writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = snap_x,
+        .checkpoint_snapshot_id = snap_x,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    auto store = openPool(backend);
+
+    size_t sleep_calls = 0;
+    store->setCasRetrySleepForTest([&sleep_calls](uint64_t) { ++sleep_calls; });
+
+    /// A checkpoint-named snapshot belongs to the caller's immutable authority cut. If that exact
+    /// object is absent, recovery must report corruption immediately; it must neither reinterpret a
+    /// transient disappearance as a new authority cut nor enter the outer transient-retry loop.
+    const String vkey = layout.refSnapshotKey(DB::Cas::tests::fixture::fixtureLife(ns), snap_x);
+    backend->vanish_once_keys.insert(vkey);
+
+    using ProfileEvents::global_counters;
+    const auto retries_before = global_counters[ProfileEvents::CASRefRecoveryRetries].load();
+
+    expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { store->listRefs(ns); });
+
+    EXPECT_FALSE(backend->vanish_once_keys.contains(vkey))
+        << "the test must reach the checkpoint-named snapshot GET, not fail on earlier fixture validation";
+    EXPECT_EQ(global_counters[ProfileEvents::CASRefRecoveryRetries].load(), retries_before)
+        << "missing immutable checkpoint authority is terminal; the outer transient-retry loop must NOT re-drive it";
+    EXPECT_EQ(sleep_calls, 0u) << "no backoff sleep for missing immutable checkpoint authority";
+}
+
+TEST(CASRefWriterRecoveryRetry, ThrowingBackoffSleepDoesNotWedgeRecovery)
+{
+    /// If the backoff sleep itself throws (e.g. a clock syscall failure), the retry loop must re-acquire
+    /// state_mutex before unwinding so the SCOPE_EXIT that clears `recovery_in_progress` runs LOCKED --
+    /// otherwise a later touch would hang forever on the never-cleared flag. This drives that path and
+    /// then proves a second touch can still recover (the lane is not wedged).
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    const Layout layout("p");
+    const RootNamespace ns{"srv1/retry_sleep_throw"};
+
+    seedSealFixtureDeadEpochs(*backend, layout, ns);
+    seedUncleanPredecessorMount(*backend, layout, /*epoch=*/2);
+
+    PoolConfig config;
+    config.server_id = UInt128(1);
+    config.mount_lease_ttl_ms = std::chrono::milliseconds(500);
+    config.cas_request_budget = sealTestTinyBudget();
+    config.cas_request_budget.recovery_retry_budget_ms = 120000;
+    config.wait_sleep_fn = [](uint64_t) {};
+    auto store = openPoolWithConfig(backend, config);
+    ASSERT_TRUE(store);
+
+    /// First touch: the seal PUT fails transiently -> the loop enters backoff -> the sleep THROWS.
+    bool sleep_should_throw = true;
+    store->setCasRetrySleepForTest([&sleep_should_throw](uint64_t)
+    {
+        if (sleep_should_throw)
+            throw std::runtime_error("injected backoff-sleep failure");
+    });
+    const RefTxnId seal_id{1, 2};
+    backend->fault_key_substr = layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), seal_id);
+    backend->fault_count = 1;
+
+    EXPECT_ANY_THROW(store->listRefs(ns));   /// the sleep failure propagates
+
+    /// The lane must NOT be wedged: with the fault now spent and the sleep no longer throwing, a second
+    /// touch recovers cleanly. If recovery_in_progress had leaked (SCOPE_EXIT run unlocked / not run), a
+    /// concurrent-safe second recovery would deadlock or mis-behave.
+    sleep_should_throw = false;
+    EXPECT_EQ(store->listRefs(ns).size(), 2u) << "a second touch must recover; the retry lane is not wedged";
+}
+
+/// ===================================================================================
+/// Task 16: `hasAnyRefWithPrefix` -- pure existence probe, same recovery preamble as `listRefs` but
+/// without materializing the full ref map (an early-exit scan).
+/// ===================================================================================
+
+TEST(CASRefWriterListRefs, HasAnyRefWithPrefixMatchesListRefsEmptiness)
+{
+    auto backend = std::make_shared<RefWriterTestBackend>();
+    auto store = openPool(backend);
+    const RootNamespace ns{"srv1/prefix_probe"};
+    const RootNamespace empty_ns{"srv1/prefix_probe_empty"};
+
+    EXPECT_FALSE(store->hasAnyRefWithPrefix(empty_ns, "")) << "a never-touched namespace has no refs";
+
+    publishEmptyPart(store, ns, "all_1_1_0");
+    publishEmptyPart(store, ns, "detached-x");
+
+    EXPECT_TRUE(store->hasAnyRefWithPrefix(ns, "")) << "empty prefix means \"any ref at all\"";
+    EXPECT_TRUE(store->hasAnyRefWithPrefix(ns, "detached-"));
+    EXPECT_FALSE(store->hasAnyRefWithPrefix(ns, "moving-")) << "no ref carries this prefix";
+
+    store->dropNamespace(ns);
+    EXPECT_FALSE(store->hasAnyRefWithPrefix(ns, "")) << "a tombstoned namespace has no committed refs";
+}
diff --git a/src/Disks/tests/gtest_cas_repoint.cpp b/src/Disks/tests/gtest_cas_repoint.cpp
new file mode 100644
index 000000000000..9044a8eda895
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_repoint.cpp
@@ -0,0 +1,107 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Parts/PartFolderAccess.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/ProfileEvents.h>
+
+/// Task 3 (all-tree-part-files plan, 2026-07-15): `CachedPartFolderAccess::repointRef` -- the audited
+/// primitive a standalone write/remove on an already-COMMITTED part must go through once the mutable
+/// per-part file set is empty. It republishes
+/// the whole manifest with the new entry set, riding `PartWriteTxn::promote`'s `allow_repoint` mode (Task 2).
+
+namespace ProfileEvents
+{
+extern const Event CASRefRepoint;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+ManifestEntry inlineEntry(const String & path, const String & bytes)
+{
+    ManifestEntry e;
+    e.path = path;
+    e.placement = EntryPlacement::Inline;
+    e.ref = BlobRef{BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(DB::Cas::tests::u128Of(bytes))};
+
+    e.blob_size = bytes.size();
+    e.inline_bytes = bytes;
+    return e;
+}
+
+/// Publish `entries` as committed ref `ns/ref` through the real writer protocol.
+ManifestId publishPart(const PoolPtr & store, const RootNamespace & ns, const String & ref,
+                       std::vector<ManifestEntry> entries)
+{
+    auto build = store->beginPartWrite(PartWriteInfo{.intended_ref = ns.string() + "/" + ref,
+                                             .intended_namespace = ns, .op = ProvenanceOp::Insert});
+    const ManifestId id = build->stageManifest(entries);
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+}
+
+/// Byte-equal candidate: the exact same entries republished onto an already-committed ref must be a
+/// ZERO-mutation no-op -- no fresh manifest staged, no ref-log record appended, no `RefRepoint` event.
+/// `stageManifest` mints a non-content-derived `ManifestRef` AND durably PUTs the body on every call
+/// (CasPartWriteTxn.cpp), so this can only hold if the no-op check compares candidate `entries` directly
+/// against the currently-committed manifest's DECODED entries -- never by staging first (the same
+/// structural comparison `republishRef`'s BUG 1c fix uses).
+TEST(CASRepoint, ByteEqualIsNoOp)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    const RootNamespace ns{"srv/t1"};
+    DB::Cas::CachedPartFolderAccess access(store);
+    const auto id = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+    const DB::Cas::PartRefKey key{ns, "part_1"};
+
+    backend->resetCounts();
+    const uint64_t repoints_before = ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load();
+    const DB::Cas::CommitOutcome oc = access.repointRef(key, {inlineEntry("checksums.txt", "cs")}, ProvenanceOp::Other);
+    EXPECT_FALSE(oc.created);
+    EXPECT_EQ(oc.manifest_ref, id.ref) << "the byte-equal outcome must name the manifest ALREADY committed, unchanged";
+
+    EXPECT_EQ(backend->putTotal(), 0u) << "byte-equal repoint must perform ZERO pool mutations";
+    EXPECT_EQ(store->resolveRef(ns, "part_1")->manifest_id, id)
+        << "the committed manifest identity must be untouched";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load(), repoints_before);
+}
+
+/// A genuinely different entry set on an already-committed ref republishes the manifest: the returned
+/// `CommitOutcome` names a FRESH manifest (`created` still false -- the ref was already committed),
+/// the new content resolves, and the repoint is loud (ProfileEvent + the ref's cached view erased so a
+/// subsequent read serves the new manifest, not a stale retained one).
+TEST(CASRepoint, AddFileRepoints)
+{
+    auto backend = std::make_shared<DB::Cas::tests::CountingBackend>();
+    auto store = DB::Cas::tests::openPoolForTest(backend);
+    const RootNamespace ns{"srv/t1"};
+    DB::Cas::CachedPartFolderAccess access(
+        store, {.cache_bytes = 64ULL << 20, .max_entries = 10000, .max_entry_bytes = 16ULL << 20,
+                .explain_enabled = false, .validate = {}});
+    const auto id_before = publishPart(store, ns, "part_1", {inlineEntry("checksums.txt", "cs")});
+    const DB::Cas::PartRefKey key{ns, "part_1"};
+    /// Warm the retained view so the erase-on-success cache discipline is actually exercised.
+    ASSERT_NE(access.getView(key, DB::Cas::Freshness::CachedForLoad), nullptr);
+
+    const uint64_t repoints_before = ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load();
+    const std::vector<ManifestEntry> new_entries{inlineEntry("checksums.txt", "cs"), inlineEntry("metadata_version.txt", "7")};
+    const DB::Cas::CommitOutcome oc = access.repointRef(key, new_entries, ProvenanceOp::Other);
+    EXPECT_FALSE(oc.created);
+    EXPECT_NE(oc.manifest_ref, id_before.ref);
+
+    const auto resolved = store->resolveRef(ns, "part_1");
+    ASSERT_TRUE(resolved.has_value());
+    EXPECT_NE(resolved->manifest_id, id_before) << "a genuine content change must mint a fresh manifest";
+    EXPECT_EQ(ProfileEvents::global_counters[ProfileEvents::CASRefRepoint].load(), repoints_before + 1);
+
+    /// The view a caller reads next must reflect the new file, not a stale retained one.
+    auto view = access.getView(key, DB::Cas::Freshness::CachedForLoad);
+    ASSERT_NE(view, nullptr);
+    EXPECT_TRUE(view->hasFile("metadata_version.txt"));
+}
diff --git a/src/Disks/tests/gtest_cas_request_control.cpp b/src/Disks/tests/gtest_cas_request_control.cpp
new file mode 100644
index 000000000000..d3acdf287599
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_request_control.cpp
@@ -0,0 +1,833 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Common/ProfileEvents.h>
+
+#include <limits>
+
+#if USE_AWS_S3
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <IO/S3Common.h>
+#include <Poco/Net/NetException.h>
+#endif
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+    extern const int NETWORK_ERROR;
+    extern const int ABORTED;
+}
+
+namespace ProfileEvents
+{
+    extern const Event CASConditionalWriteAttempts;
+    extern const Event CASConditionalWriteCommitted;
+    extern const Event CASConditionalWriteDefiniteFailure;
+    extern const Event CASConditionalWriteUnresolved;
+}
+
+#if USE_AWS_S3
+namespace DB::ErrorCodes
+{
+    extern const int LOGICAL_ERROR;
+    extern const int CORRUPTED_DATA;
+    extern const int BAD_ARGUMENTS;
+    extern const int NOT_IMPLEMENTED;
+    extern const int UNKNOWN_EXCEPTION;
+}
+#endif
+
+/// The success path (buf.finalize() returned without throwing) is always Committed. No exception
+/// object is needed — the caller distinguishes success from failure before calling either overload.
+TEST(CASRequestControl, SuccessIsAlwaysCommitted)
+{
+    EXPECT_EQ(classifyConditionalWriteResult(), CasWriteOutcome::Committed);
+}
+
+/// Fix #37 phase 2: the retry-later throw must be NETWORK_ERROR, never ABORTED -- ABORTED is silently
+/// swallowed by ReplicatedMergeMutateTaskBase (no backoff, no last_exception), which is exactly the
+/// defect this fix closes.
+TEST(CASWriteRetryLater, ThrowsNetworkErrorNotAborted)
+{
+    bool threw = false;
+    try
+    {
+        throwCasWriteRetryLater("test cause");
+        FAIL() << "throwCasWriteRetryLater must always throw";
+    }
+    catch (const DB::Exception & e)
+    {
+        threw = true;
+        EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR);
+        EXPECT_NE(e.code(), DB::ErrorCodes::ABORTED);
+        EXPECT_NE(e.message().find("test cause"), String::npos) << e.message();
+        EXPECT_NE(e.message().find("retrying later"), String::npos) << e.message();
+    }
+    EXPECT_TRUE(threw);
+}
+
+/// The exception_ptr twin (for call sites that fail a pending future/promise rather than throw
+/// directly, e.g. CasRefLedger's queued-append completion paths) must carry the SAME classification.
+TEST(CASWriteRetryLater, ExceptionPtrVariantCarriesSameClassification)
+{
+    const std::exception_ptr eptr = makeCasWriteRetryLaterExceptionPtr("another cause");
+    bool threw = false;
+    try
+    {
+        std::rethrow_exception(eptr);
+        FAIL() << "expected a thrown exception";
+    }
+    catch (const DB::Exception & e)
+    {
+        threw = true;
+        EXPECT_EQ(e.code(), DB::ErrorCodes::NETWORK_ERROR);
+        EXPECT_NE(e.message().find("another cause"), String::npos) << e.message();
+    }
+    EXPECT_TRUE(threw);
+}
+
+#if USE_AWS_S3
+
+/// One row per RFC cas-s3-timeout-retry-control §operation-classes classification. PreconditionFailed
+/// is NEVER DefiniteFailure — it means the key exists, not that the request was rejected — and every
+/// unrecognized/ambiguous error also falls to Unresolved, never to a false DefiniteFailure.
+TEST(CASRequestControl, ClassifiesPreconditionFailedAsUnresolved)
+{
+    DB::S3Exception e("412 from backend", Aws::S3::S3Errors::UNKNOWN, "PreconditionFailed");
+    EXPECT_EQ(classifyConditionalWriteResult(e), CasWriteOutcome::Unresolved);
+}
+
+TEST(CASRequestControl, ClassifiesTimeoutAsUnresolved)
+{
+    Poco::TimeoutException e("simulated client-side receive timeout");
+    EXPECT_EQ(classifyConditionalWriteResult(e), CasWriteOutcome::Unresolved);
+}
+
+TEST(CASRequestControl, ClassifiesConnectionResetAsUnresolved)
+{
+    Poco::Net::ConnectionResetException e("simulated connection reset");
+    EXPECT_EQ(classifyConditionalWriteResult(e), CasWriteOutcome::Unresolved);
+}
+
+TEST(CASRequestControl, Classifies5xxAsUnresolved)
+{
+    DB::S3Exception e("simulated internal error", Aws::S3::S3Errors::INTERNAL_FAILURE, "InternalError");
+    EXPECT_EQ(classifyConditionalWriteResult(e), CasWriteOutcome::Unresolved);
+    /// SlowDown / ServiceUnavailable are also 5xx-class and equally Unresolved.
+    DB::S3Exception slow_down("simulated throttle", Aws::S3::S3Errors::SLOW_DOWN, "SlowDown");
+    EXPECT_EQ(classifyConditionalWriteResult(slow_down), CasWriteOutcome::Unresolved);
+}
+
+TEST(CASRequestControl, ClassifiesMalformedRequestAsDefiniteFailure)
+{
+    DB::S3Exception e("bad xml", Aws::S3::S3Errors::UNKNOWN, "MalformedXML");
+    EXPECT_EQ(classifyConditionalWriteResult(e), CasWriteOutcome::DefiniteFailure);
+    /// The modeled-enum path (no canonical name attached) must classify identically.
+    DB::S3Exception by_code("bad argument", Aws::S3::S3Errors::INVALID_REQUEST);
+    EXPECT_EQ(classifyConditionalWriteResult(by_code), CasWriteOutcome::DefiniteFailure);
+}
+
+TEST(CASRequestControl, ClassifiesEntityTooLargeAsDefiniteFailure)
+{
+    DB::S3Exception e("body exceeds the maximum object size", Aws::S3::S3Errors::UNKNOWN, "EntityTooLarge");
+    EXPECT_EQ(classifyConditionalWriteResult(e), CasWriteOutcome::DefiniteFailure);
+}
+
+TEST(CASRequestControl, ClassifiesAccessDeniedAsDefiniteFailure)
+{
+    DB::S3Exception e("simulated 403", Aws::S3::S3Errors::ACCESS_DENIED, "AccessDenied");
+    EXPECT_EQ(classifyConditionalWriteResult(e), CasWriteOutcome::DefiniteFailure);
+    /// The modeled-enum path (no canonical name attached) must classify identically.
+    DB::S3Exception by_code("simulated 403, no name", Aws::S3::S3Errors::ACCESS_DENIED);
+    EXPECT_EQ(classifyConditionalWriteResult(by_code), CasWriteOutcome::DefiniteFailure);
+}
+
+/// Anything the classifier does not recognize (an unmodeled/unnamed S3 error, or an entirely
+/// unrelated exception type) must fail toward Unresolved — never toward a false DefiniteFailure or a
+/// false Committed (RFC §resolve-before-reissuing: ambiguity always resolves toward "resolve before
+/// reissuing").
+TEST(CASRequestControl, UnrecognizedErrorsFailSafeToUnresolved)
+{
+    DB::S3Exception unknown_named("weird service error", Aws::S3::S3Errors::UNKNOWN, "SomeFutureErrorCode");
+    EXPECT_EQ(classifyConditionalWriteResult(unknown_named), CasWriteOutcome::Unresolved);
+
+    /// UNKNOWN_EXCEPTION (not LOGICAL_ERROR): any arbitrary non-S3 exception type works here -- the
+    /// point is that the classifier doesn't recognize it, not which specific code it carries.
+    /// LOGICAL_ERROR would abort the whole process under debug/sanitizer builds merely by being
+    /// constructed (Exception's constructor calls handle_error_code unconditionally).
+    DB::Exception unrelated(DB::ErrorCodes::UNKNOWN_EXCEPTION, "not an S3 error at all");
+    EXPECT_EQ(classifyConditionalWriteResult(unrelated), CasWriteOutcome::Unresolved);
+}
+
+/// recordConditionalWriteAttemptStarted / recordConditionalWriteOutcome bump the per-class counters
+/// (RFC §observability): attempts, and exactly one of Committed/DefiniteFailure/Unresolved per call.
+TEST(CASRequestControl, CountersHookupIncrementsPerClass)
+{
+    using ProfileEvents::global_counters;
+    const auto attempts_before = global_counters[ProfileEvents::CASConditionalWriteAttempts].load();
+    const auto committed_before = global_counters[ProfileEvents::CASConditionalWriteCommitted].load();
+    const auto definite_before = global_counters[ProfileEvents::CASConditionalWriteDefiniteFailure].load();
+    const auto unresolved_before = global_counters[ProfileEvents::CASConditionalWriteUnresolved].load();
+
+    recordConditionalWriteAttemptStarted();
+    recordConditionalWriteOutcome(CasWriteOutcome::Committed);
+    recordConditionalWriteAttemptStarted();
+    recordConditionalWriteOutcome(CasWriteOutcome::DefiniteFailure);
+    recordConditionalWriteAttemptStarted();
+    recordConditionalWriteOutcome(CasWriteOutcome::Unresolved);
+
+#if !WITH_COVERAGE
+    EXPECT_EQ(global_counters[ProfileEvents::CASConditionalWriteAttempts].load() - attempts_before, 3u);
+    EXPECT_EQ(global_counters[ProfileEvents::CASConditionalWriteCommitted].load() - committed_before, 1u);
+    EXPECT_EQ(global_counters[ProfileEvents::CASConditionalWriteDefiniteFailure].load() - definite_before, 1u);
+    EXPECT_EQ(global_counters[ProfileEvents::CASConditionalWriteUnresolved].load() - unresolved_before, 1u);
+#else
+    (void)attempts_before; (void)committed_before; (void)definite_before; (void)unresolved_before;
+#endif
+}
+
+/// Wiring smoke test: a real conditional write through ObjectStorageBackend (Native mode) counts one
+/// attempt and one Committed outcome via the SAME instrumented call site nativeConditionalPut uses —
+/// see finalizeConditionalWriteInstrumented in CasObjectStorageBackend.cpp.
+TEST(CASRequestControl, NativeConditionalPutCountsOneAttemptAndCommitted)
+{
+    using ProfileEvents::global_counters;
+    const auto attempts_before = global_counters[ProfileEvents::CASConditionalWriteAttempts].load();
+    const auto committed_before = global_counters[ProfileEvents::CASConditionalWriteCommitted].load();
+
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+    EXPECT_EQ(b->putIfAbsent("p/rc/one", "v1").outcome, PutOutcome::Done);
+
+#if !WITH_COVERAGE
+    EXPECT_EQ(global_counters[ProfileEvents::CASConditionalWriteAttempts].load() - attempts_before, 1u);
+    EXPECT_EQ(global_counters[ProfileEvents::CASConditionalWriteCommitted].load() - committed_before, 1u);
+#else
+    (void)attempts_before; (void)committed_before;
+#endif
+}
+
+/// Mechanism property (RFC §disable-transparent-conditional-write-retries), tested at the layer
+/// actually reachable from a unit-test binary: NO live/fake S3 endpoint is available here (the Native
+/// conditional-write path is exercised end-to-end only at M-W against RustFS — see the HONEST NOTE in
+/// CasObjectStorageBackend.cpp), so driving a real socket-level retry against a real client is not
+/// reachable from this binary. What IS reachable and asserted here: every Native conditional write
+/// selects the SingleAttempt object-storage retry profile, and a non-S3 backend such as
+/// LocalObjectStorage reports it as UNSUPPORTED via IObjectStorage::supportsRetryProfile — the property
+/// checkConditionalWriteSingleAttemptSupport's fail-closed mount-time gate relies on.
+TEST(CASRequestControl, SingleAttemptProfileRequestedAndLocalBackendRejected)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+    const auto ws = b->conditionalWriteSettingsForTest();
+    EXPECT_EQ(ws.object_storage_retry_profile, DB::ObjectStorageRetryProfile::SingleAttempt);
+    /// LocalObjectStorage does not implement the profile — the capability check must say no.
+    EXPECT_FALSE(DB::Cas::tests::makeLocalObjectStorageForTest()->supportsRetryProfile(DB::ObjectStorageRetryProfile::SingleAttempt));
+}
+
+/// The SECOND retry-affecting layer above the S3 client (review finding): WriteBufferFromS3's OWN
+/// makeSinglepartUpload/completeMultipartUpload retry loop reissues the identical conditional request
+/// on a NO_SUCH_KEY response, driven by S3RequestSetting::max_unexpected_write_error_retries (default
+/// 4) — a client-level override alone does not bound it (see WriteSettings::
+/// s3_max_unexpected_write_error_retries_override). Asserted at the reachable seam: no live/fake S3
+/// endpoint exists in this binary to drive the retry loop itself, so this proves the settings
+/// plumbing conditionalWriteSettings() -> WriteSettings produces the override value that
+/// S3ObjectStorage::writeObject then applies to request_settings — NOT a real single-attempt
+/// assertion against a live wire attempt.
+TEST(CASRequestControl, ConditionalWriteSettingsForceSingleUnexpectedWriteErrorRetry)
+{
+    auto b = std::make_shared<ObjectStorageBackend>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), ObjectStorageBackend::Mode::Native);
+    const auto ws = b->conditionalWriteSettingsForTest();
+    EXPECT_EQ(ws.s3_max_unexpected_write_error_retries_override, 1u);
+}
+
+/// ================================================================================================
+/// Task 5: CasRequestController — retry controller (deadlines, fence gating, exact-key resolution)
+/// ================================================================================================
+
+namespace
+{
+
+/// A per-call scripted Backend for CasRequestController tests: `putIfAbsent` optionally throws a
+/// caller-supplied exception (models one classified HTTP-attempt outcome) or returns a forced
+/// `PutOutcome` directly (models a `PreconditionFailed` observed WITHOUT an exception); with neither
+/// set it delegates to the real in-memory conditional-write semantics. `get` optionally returns a
+/// forced result, independent of what `putIfAbsent` actually did, so a test can drive exact-key
+/// resolution (identical / different / absent) without the scripted put and the resolve GET needing to
+/// agree on a shared, real backing store.
+class ScriptedControllerBackend : public InMemoryBackend
+{
+public:
+    std::function<void()> put_thrower;
+    std::optional<PutOutcome> put_forced_outcome;
+    std::atomic<uint64_t> put_attempts{0};
+
+    std::function<void()> put_overwrite_thrower;
+    std::optional<PutOutcome> put_overwrite_forced_outcome;
+    std::atomic<uint64_t> put_overwrite_attempts{0};
+
+    bool get_overridden = false;
+    std::optional<GetResult> get_override_value;   /// meaningful only when get_overridden
+
+    void setGetOverride(std::optional<GetResult> value)
+    {
+        get_overridden = true;
+        get_override_value = std::move(value);
+    }
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        ++put_attempts;
+        if (put_thrower)
+            put_thrower();
+        if (put_forced_outcome)
+            return {*put_forced_outcome, {}};
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+
+    PutResult putOverwrite(const String & key, const String & bytes, const Token & expected, const ObjectMeta & meta) override
+    {
+        ++put_overwrite_attempts;
+        if (put_overwrite_thrower)
+            put_overwrite_thrower();
+        if (put_overwrite_forced_outcome)
+            return {*put_overwrite_forced_outcome, {}};
+        return InMemoryBackend::putOverwrite(key, bytes, expected, meta);
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (get_overridden)
+            return get_override_value;
+        return InMemoryBackend::get(key, range);
+    }
+};
+
+GetResult resultWithBytes(const String & bytes)
+{
+    return GetResult{.bytes = bytes, .token = Token{"t", TokenType::Emulated}, .attributes = {}};
+}
+
+}
+
+TEST(CASRequestController, UncertainResolvesIdenticalAsCommitted)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_thrower = [] { throw Poco::TimeoutException("scripted: ambiguous"); };
+    backend->setGetOverride(resultWithBytes("payload"));
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", [] { return true; });
+    EXPECT_EQ(outcome, CasWriteOutcome::Committed);
+    EXPECT_EQ(backend->put_attempts.load(), 1u);
+}
+
+TEST(CASRequestController, UncertainResolvesDifferentThrowsCorruption)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_thrower = [] { throw Poco::TimeoutException("scripted: ambiguous"); };
+    backend->setGetOverride(resultWithBytes("someone-elses-bytes"));
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::CORRUPTED_DATA, [&]
+    {
+        controller.putIfAbsentControlled("k", "payload", [] { return true; });
+    });
+}
+
+/// GET-absent NEVER yields DefiniteFailure (spec §writer-side-linearization): the SAME (key, bytes) is
+/// retried up to `max_attempts`, and only THEN does the call give up with Unresolved.
+TEST(CASRequestController, UncertainResolvesAbsentRetriesSameKeyWithinBudget)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_thrower = [] { throw Poco::TimeoutException("scripted: ambiguous"); };
+    backend->setGetOverride(std::nullopt);   /// absent on every resolve
+
+    CasRequestBudget budget;
+    budget.max_attempts = 3;
+    budget.retry_initial_backoff_ms = 0;   /// backoff behavior is pinned by its own tests below
+    CasRequestController controller(backend, budget);
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", [] { return true; });
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_attempts.load(), 3u);   /// every attempt targeted the SAME key/bytes
+}
+
+/// The operation deadline — not just the attempt-count budget — cuts a retry loop short: a fake clock
+/// advances by a fixed step per now_ms() call (no sleeps), and max_attempts is generous enough that only
+/// the deadline check can be what stops the loop.
+TEST(CASRequestController, OperationDeadlineExhaustionReturnsUnresolvedBeforeMaxAttempts)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_thrower = [] { throw Poco::TimeoutException("scripted: ambiguous"); };
+    backend->setGetOverride(std::nullopt);   /// absent on every resolve
+
+    uint64_t clock = 0;
+    auto now_ms = [&clock]() -> uint64_t { const uint64_t t = clock; clock += 200; return t; };
+
+    CasRequestBudget budget;
+    budget.max_attempts = 10;
+    budget.attempt_timeout_ms = 50;
+    budget.operation_deadline_ms = 450;
+    budget.retry_initial_backoff_ms = 0;   /// isolate the deadline check from the backoff's own deadline guard
+    CasRequestController controller(backend, budget, now_ms);
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", [] { return true; });
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_attempts.load(), 2u);   /// cut off well before the 10-attempt budget
+}
+
+/// WHY `attempt_timeout_ms == operation_deadline_ms` IS REJECTED AT STARTUP, demonstrated on the
+/// mechanism itself before the rejection is asserted below.
+///
+/// The deadline is captured as `now + operation_deadline_ms` and the pre-send gate asks
+/// `now + attempt_timeout_ms > deadline`. Equal values collapse that to `now_2 > now_1`: ONE elapsed
+/// millisecond between the capture and the gate refuses the whole operation with NOTHING SENT. That is
+/// not a bounded operation, it is a coin flip on the scheduler -- "mostly works, occasionally refuses
+/// having sent nothing", which is the flakiness class validation exists to prevent. Single-attempt
+/// semantics is what `max_attempts = 1` is for; the equality contributes only the race.
+///
+/// The controller is constructed DIRECTLY here, bypassing `validateCasRequestBudget`, because the
+/// point is to show the behaviour the validator now forbids. Three tests were flaky on exactly this
+/// before it was forbidden: `8f9e63c7a19`'s sweep-interruption test,
+/// `CASRefInstallSafety.UncertainPrecommitKeepsItsCleanupOwnerAndItsBody`, and
+/// `CASRefWriterAppendLane.WedgedLaneBlocksSameTableWhileOtherTableProceeds`.
+TEST(CASRequestController, EqualAttemptTimeoutAndDeadlineWouldRefuseAfterASingleTick)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+
+    /// The smallest possible passage of time: one millisecond per clock read.
+    uint64_t clock = 0;
+    auto now_ms = [&clock]() -> uint64_t { const uint64_t t = clock; clock += 1; return t; };
+
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 100;
+    CasRequestController razor(backend, budget, now_ms);
+    EXPECT_EQ(razor.putIfAbsentControlled("k", "payload", [] { return true; }), CasWriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_attempts.load(), 0u)
+        << "the refusal came from the clock, not from the backend: nothing was sent at all";
+
+    /// STRICTLY LESS -- the shape the validator now requires -- sends the request over the SAME one-tick
+    /// clock. So what the inequality buys is the request actually happening, not merely a bigger number.
+    clock = 0;
+    budget.operation_deadline_ms = 5000;
+    CasRequestController wide(backend, budget, now_ms);
+    EXPECT_EQ(wide.putIfAbsentControlled("k", "payload", [] { return true; }), CasWriteOutcome::Committed);
+    EXPECT_EQ(backend->put_attempts.load(), 1u);
+}
+
+/// And the same equality is refused at startup, so no budget can reach the controller in that shape.
+/// The boundary is asserted from BOTH sides: equality throws, one millisecond more is accepted.
+TEST(CASRequestController, ValidateBudgetRejectsAttemptTimeoutEqualToOperationDeadline)
+{
+    CasRequestBudget budget;
+    budget.attempt_timeout_ms = 5000;
+    budget.operation_deadline_ms = 5000;
+    budget.lease_safety_margin_ms = 1000;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        validateCasRequestBudget(budget, /*mount_lease_ttl_ms=*/30000, /*mount_renew_period_ms=*/10000);
+    });
+
+    budget.operation_deadline_ms = 5001;
+    EXPECT_NO_THROW(validateCasRequestBudget(budget, /*mount_lease_ttl_ms=*/30000, /*mount_renew_period_ms=*/10000))
+        << "one millisecond of headroom is the whole requirement -- the rule is strictness, not size";
+}
+
+TEST(CASRequestController, OverwriteAmbiguousResolvesIntendedBytesAsCommitted)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    bool first_attempt = true;
+    backend->put_overwrite_thrower = [&first_attempt]
+    {
+        if (first_attempt)
+        {
+            first_attempt = false;
+            throw Poco::TimeoutException("scripted: ambiguous");
+        }
+    };
+    backend->setGetOverride(resultWithBytes("new-payload"));
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto result = controller.putOverwriteControlled(
+        "k", "new-payload", Token{"old", TokenType::Emulated}, [] { return true; });
+    EXPECT_EQ(result.outcome, CasOverwriteOutcome::Committed);
+    EXPECT_EQ(backend->put_overwrite_attempts.load(), 1u);
+    EXPECT_EQ(result.token, (Token{"t", TokenType::Emulated}));
+}
+
+TEST(CASRequestController, OverwriteAmbiguousResolvesExpectedTokenAndRetriesWithinBudget)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_overwrite_thrower = [] { throw Poco::TimeoutException("scripted: ambiguous"); };
+    const Token expected{"old", TokenType::Emulated};
+    backend->setGetOverride(GetResult{.bytes = "old-payload", .token = expected, .attributes = {}});
+
+    CasRequestBudget budget;
+    budget.max_attempts = 3;
+    budget.retry_initial_backoff_ms = 0;
+    CasRequestController controller(backend, budget);
+    const auto result = controller.putOverwriteControlled("k", "new-payload", expected, [] { return true; });
+    EXPECT_EQ(result.outcome, CasOverwriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_overwrite_attempts.load(), 3u);
+}
+
+TEST(CASRequestController, OverwriteAmbiguousResolvesDifferentTokenAndBytesAsConflict)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    bool first_attempt = true;
+    backend->put_overwrite_thrower = [&first_attempt]
+    {
+        if (first_attempt)
+        {
+            first_attempt = false;
+            throw Poco::TimeoutException("scripted: ambiguous");
+        }
+    };
+    backend->setGetOverride(GetResult{
+        .bytes = "someone-elses-payload", .token = Token{"other", TokenType::Emulated}, .attributes = {}});
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto result = controller.putOverwriteControlled(
+        "k", "new-payload", Token{"old", TokenType::Emulated}, [] { return true; });
+    EXPECT_EQ(result.outcome, CasOverwriteOutcome::Conflict);
+    EXPECT_EQ(backend->put_overwrite_attempts.load(), 1u);
+}
+
+TEST(CASRequestController, OverwriteOperationDeadlineExhaustionReturnsUnresolvedBeforeMaxAttempts)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_overwrite_thrower = [] { throw Poco::TimeoutException("scripted: ambiguous"); };
+    const Token expected{"old", TokenType::Emulated};
+    backend->setGetOverride(GetResult{.bytes = "old-payload", .token = expected, .attributes = {}});
+
+    uint64_t clock = 0;
+    auto now_ms = [&clock]() -> uint64_t { const uint64_t t = clock; clock += 200; return t; };
+
+    CasRequestBudget budget;
+    budget.max_attempts = 10;
+    budget.attempt_timeout_ms = 50;
+    budget.operation_deadline_ms = 450;
+    budget.retry_initial_backoff_ms = 0;
+    CasRequestController controller(backend, budget, now_ms);
+    const auto result = controller.putOverwriteControlled("k", "new-payload", expected, [] { return true; });
+    EXPECT_EQ(result.outcome, CasOverwriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_overwrite_attempts.load(), 2u);
+}
+
+TEST(CASRequestController, FenceLostBeforeAttemptSendsNoAttempt)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", [] { return false; });
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_attempts.load(), 0u);
+}
+
+/// The write itself may have landed, but a fence lost between the write and this call's own final
+/// check must never surface as Committed (RFC §ack-and-cache-rules: no ACK, no cache update on that
+/// path) — the caller sees Unresolved and must not treat the operation as acknowledged.
+TEST(CASRequestController, FenceLostAfterWriteNeverReturnsCommitted)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();   /// real in-memory commit path
+    int fence_calls = 0;
+    auto fence_ok = [&fence_calls] { return fence_calls++ == 0; };   /// true once, then false
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", fence_ok);
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_attempts.load(), 1u);   /// the write itself DID happen
+    EXPECT_TRUE(backend->head("k").exists);        /// ...it is durable; never claimed as Committed here
+}
+
+TEST(CASRequestController, DefiniteFailurePropagatesImmediatelyWithoutResolve)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_thrower = [] { throw DB::S3Exception("scripted: malformed", Aws::S3::S3Errors::UNKNOWN, "MalformedXML"); };
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", [] { return true; });
+    EXPECT_EQ(outcome, CasWriteOutcome::DefiniteFailure);
+    EXPECT_EQ(backend->put_attempts.load(), 1u);   /// no retry, no resolve GET issued
+}
+
+/// ================================================================================================
+/// Inter-attempt backoff (chaos-tolerance-report §Task B follow-up / stagefix-review M3): the
+/// controller paces reissues with a capped-exponential, fence-gated, deadline-aware sleep instead of
+/// hammering a recovering store with immediate retries.
+/// ================================================================================================
+
+/// The full event-ordered schedule: fence checked before EVERY attempt AND before EVERY sleep, sleeps
+/// strictly between attempts, capped exponential (initial 100ms, cap 200ms), no sleep after the final
+/// attempt. The exact interleaving is the contract — a sleep served before its fence check would keep
+/// a fenced writer dozing past its lease.
+TEST(CASRequestControllerBackoff, CappedExponentialSleepsAreFenceCheckedAndOrdered)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    std::vector<String> events;
+    backend->put_thrower = [&] { events.emplace_back("put"); throw Poco::TimeoutException("scripted: ambiguous"); };
+    backend->setGetOverride(std::nullopt);   /// absent on every resolve
+
+    CasRequestBudget budget;
+    budget.max_attempts = 5;
+    budget.attempt_timeout_ms = 1;
+    budget.operation_deadline_ms = 1000000;   /// never the binding constraint here
+    budget.retry_initial_backoff_ms = 100;
+    budget.retry_max_backoff_ms = 200;
+    CasRequestController controller(
+        backend, budget,
+        /*now_ms=*/[] { return static_cast<uint64_t>(0); },
+        /*sleep_ms=*/[&](uint64_t ms) { events.push_back("sleep:" + std::to_string(ms)); });
+
+    const auto fence_ok = [&] { events.emplace_back("fence"); return true; };
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", fence_ok);
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_attempts.load(), 5u);
+
+    const std::vector<String> expected{
+        "fence", "put", "fence", "sleep:100",
+        "fence", "put", "fence", "sleep:200",
+        "fence", "put", "fence", "sleep:200",
+        "fence", "put", "fence", "sleep:200",
+        "fence", "put"};   /// budget spent: no fence-for-sleep, no sleep after the last attempt
+    EXPECT_EQ(events, expected);
+}
+
+/// A fence lost between an ambiguous attempt's resolve and its backoff sleep aborts INSTANTLY: no
+/// sleep is served, no further attempt is sent, and the outcome is Unresolved (never a false
+/// Committed, never a retry under a lost lease).
+TEST(CASRequestControllerBackoff, FenceLostBeforeSleepAbortsWithoutSleeping)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_thrower = [] { throw Poco::TimeoutException("scripted: ambiguous"); };
+    backend->setGetOverride(std::nullopt);
+
+    CasRequestBudget budget;
+    budget.max_attempts = 5;
+    budget.retry_initial_backoff_ms = 100;
+    budget.retry_max_backoff_ms = 200;
+    uint64_t sleeps = 0;
+    int fence_calls = 0;
+    CasRequestController controller(
+        backend, budget, /*now_ms=*/[] { return static_cast<uint64_t>(0); },
+        /*sleep_ms=*/[&](uint64_t) { ++sleeps; });
+
+    /// True for the pre-attempt check (call 1), lost by the pre-sleep check (call 2).
+    const auto fence_ok = [&fence_calls] { return ++fence_calls <= 1; };
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", fence_ok);
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_attempts.load(), 1u) << "no attempt may be sent after the fence is lost";
+    EXPECT_EQ(sleeps, 0u) << "a fence lost mid-backoff must abort BEFORE the sleep, not after it";
+    EXPECT_EQ(fence_calls, 2);
+}
+
+/// A backoff sleep the operation deadline cannot afford is never served: when sleep + one more
+/// attempt would cross the deadline, the loop gives up immediately (Unresolved) instead of sleeping
+/// into a guaranteed exhaustion.
+TEST(CASRequestControllerBackoff, SleepThatWouldCrossOperationDeadlineIsSkipped)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    backend->put_thrower = [] { throw Poco::TimeoutException("scripted: ambiguous"); };
+    backend->setGetOverride(std::nullopt);
+
+    uint64_t clock = 0;
+    CasRequestBudget budget;
+    budget.max_attempts = 10;
+    budget.attempt_timeout_ms = 10;
+    budget.operation_deadline_ms = 100;
+    budget.retry_initial_backoff_ms = 1000;   /// any sleep would blow the 100ms deadline
+    budget.retry_max_backoff_ms = 1000;
+    uint64_t sleeps = 0;
+    CasRequestController controller(
+        backend, budget, /*now_ms=*/[&clock] { return clock; },
+        /*sleep_ms=*/[&](uint64_t) { ++sleeps; });
+
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", [] { return true; });
+    EXPECT_EQ(outcome, CasWriteOutcome::Unresolved);
+    EXPECT_EQ(backend->put_attempts.load(), 1u);
+    EXPECT_EQ(sleeps, 0u) << "the deadline guard must refuse the sleep, not serve it and then fail";
+}
+
+/// THE ENVELOPE CONTRACT (chaos-tolerance-report §Task B follow-up): the DEFAULT budget rides a
+/// simulated 60-second S3 outage — every conditional-write attempt fails (≈3s adaptive first-attempt
+/// timeout each, the observed incident shape) until the store recovers at t=60s, then the next
+/// attempt commits, all inside the default 90s operation deadline and 16-attempt budget. The fake
+/// clock advances 3s per failed attempt and by each backoff sleep, so this test pins the arithmetic
+/// documented on CasRequestBudget without any wall-clock waiting.
+TEST(CASRequestControllerBackoff, DefaultBudgetRidesSixtySecondOutage)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    uint64_t clock = 0;
+    backend->put_thrower = [&clock]
+    {
+        if (clock < 60000)
+        {
+            clock += 3000;   /// the failed attempt's own ~3s adaptive receive timeout
+            throw Poco::TimeoutException("scripted: store paused");
+        }
+        /// store recovered: fall through to the real in-memory conditional write (Done)
+    };
+    backend->setGetOverride(std::nullopt);   /// nothing ever landed while the store was paused
+
+    CasRequestController controller(
+        backend, CasRequestBudget{}, /*now_ms=*/[&clock] { return clock; },
+        /*sleep_ms=*/[&clock](uint64_t ms) { clock += ms; });
+
+    const auto outcome = controller.putIfAbsentControlled("k", "payload", [] { return true; });
+    EXPECT_EQ(outcome, CasWriteOutcome::Committed) << "the default budget must absorb a 60s outage";
+    /// Schedule: attempts fail at 3s each with sleeps 0.2,0.4,0.8,1.6,3.2 then 5s (cap); the first
+    /// attempt scheduled at clock >= 60000 (attempt 11, t=61.2s) commits — well inside 16 attempts
+    /// and the 90s deadline.
+    EXPECT_EQ(backend->put_attempts.load(), 11u);
+    EXPECT_LT(clock, CasRequestBudget{}.operation_deadline_ms);
+}
+
+/// Availfix review M1: deterministic CALLER/local bugs — `LOGICAL_ERROR` (a broken source),
+/// `NOT_IMPLEMENTED` (a mode/capability guard, e.g. promoteStaged on a backend without a native
+/// conditional copy), `BAD_ARGUMENTS` (a deterministic encode rejection escaping buildHeader's second
+/// encode), `CORRUPTED_DATA` (integrity) — propagate INSTANTLY from the create retry loop: exactly one
+/// attempt, no occupancy resolve, no backoff sleep. Retrying a deterministic failure only replays it
+/// (~12 minutes at the default budget through putBlob's outer loop) and buries the root cause behind a
+/// retryable ABORTED — the exact class the `PutBlobWrongSizeFailsClosed` sweep regression exposed.
+TEST(CASRequestControllerCreate, DeterministicLocalFailuresPropagateInstantly)
+{
+    /// LOGICAL_ERROR aborts the whole process in debug/sanitizer builds instead of behaving like a
+    /// catchable exception, so it's excluded from this loop there --
+    /// CASRequestControllerCreateDeathTest below proves the same instant-propagate contract for it
+    /// positively via EXPECT_DEATH instead.
+#ifdef DEBUG_OR_SANITIZER_BUILD
+    const std::vector<int> codes = {DB::ErrorCodes::NOT_IMPLEMENTED, DB::ErrorCodes::BAD_ARGUMENTS, DB::ErrorCodes::CORRUPTED_DATA};
+#else
+    const std::vector<int> codes = {DB::ErrorCodes::LOGICAL_ERROR, DB::ErrorCodes::NOT_IMPLEMENTED,
+                                     DB::ErrorCodes::BAD_ARGUMENTS, DB::ErrorCodes::CORRUPTED_DATA};
+#endif
+    for (const int code : codes)
+    {
+        SCOPED_TRACE("error code " + std::to_string(code));
+        auto backend = std::make_shared<ScriptedControllerBackend>();
+        uint64_t sleeps = 0;
+        int attempts = 0;
+        CasRequestController controller(
+            backend, CasRequestBudget{}, /*now_ms=*/[] { return static_cast<uint64_t>(0); },
+            /*sleep_ms=*/[&](uint64_t) { ++sleeps; });
+
+        bool threw = false;
+        try
+        {
+            controller.conditionalCreateControlled("k",
+                [&]() -> PutResult
+                {
+                    ++attempts;
+                    throw DB::Exception(code, "scripted deterministic local failure");
+                },
+                [] { return true; });
+        }
+        catch (const DB::Exception & e)
+        {
+            threw = true;
+            EXPECT_EQ(e.code(), code) << "the ORIGINAL exception must propagate, not a mapped outcome";
+        }
+        EXPECT_TRUE(threw) << "a deterministic local failure must propagate, never return an outcome";
+        EXPECT_EQ(attempts, 1) << "no reissue: retrying a deterministic failure only replays it";
+        EXPECT_EQ(sleeps, 0u) << "no backoff sleep may be served for a deterministic failure";
+    }
+}
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+/// Debug/sanitizer-build counterpart to DeterministicLocalFailuresPropagateInstantly's LOGICAL_ERROR
+/// case, excluded from that loop above: LOGICAL_ERROR aborts the process here instead of throwing a
+/// catchable exception, so the check must be a death test (same pattern as CASBlobDigestDeathTest in
+/// gtest_cas_blob_digest.cpp).
+TEST(CASRequestControllerCreateDeathTest, LogicalErrorPropagatesInstantlyAborts)
+{
+    auto backend = std::make_shared<ScriptedControllerBackend>();
+    CasRequestController controller(
+        backend, CasRequestBudget{}, /*now_ms=*/[] { return static_cast<uint64_t>(0); },
+        /*sleep_ms=*/[](uint64_t) {});
+    EXPECT_DEATH(
+        {
+            controller.conditionalCreateControlled("k",
+                [&]() -> PutResult { throw DB::Exception(DB::ErrorCodes::LOGICAL_ERROR, "scripted deterministic local failure"); },
+                [] { return true; });
+        }, "");
+}
+#endif
+
+/// Startup validation (RFC §required-timeout-model): a consistent default budget is accepted silently;
+/// either inequality violated on its own is rejected with BAD_ARGUMENTS.
+TEST(CASRequestController, ValidateBudgetAcceptsConsistentDefaults)
+{
+    EXPECT_NO_THROW(validateCasRequestBudget(CasRequestBudget{}, /*mount_lease_ttl_ms=*/30000, /*mount_renew_period_ms=*/10000));
+}
+
+TEST(CASRequestController, ValidateBudgetRejectsAttemptTimeoutPlusMarginAtOrAboveLeaseTtl)
+{
+    CasRequestBudget budget;
+    budget.attempt_timeout_ms = 25000;
+    budget.lease_safety_margin_ms = 5000;   /// sums to EXACTLY the lease TTL below — not strictly less
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        validateCasRequestBudget(budget, /*mount_lease_ttl_ms=*/30000, /*mount_renew_period_ms=*/10000);
+    });
+}
+
+TEST(CASRequestController, ValidateBudgetRejectsAttemptTimeoutAboveOperationDeadline)
+{
+    CasRequestBudget budget;
+    budget.attempt_timeout_ms = 6000;
+    budget.operation_deadline_ms = 5000;
+    budget.lease_safety_margin_ms = 1000;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        validateCasRequestBudget(budget, /*mount_lease_ttl_ms=*/30000, /*mount_renew_period_ms=*/10000);
+    });
+}
+
+/// max_attempts == 0 would let putIfAbsentControlled return Unresolved without ever sending an
+/// attempt — reject at startup rather than silently accepting a no-op budget.
+TEST(CASRequestController, ValidateBudgetRejectsZeroMaxAttempts)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 0;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        validateCasRequestBudget(budget, /*mount_lease_ttl_ms=*/30000, /*mount_renew_period_ms=*/10000);
+    });
+}
+
+/// A capped-exponential backoff whose cap sits below its own starting value is inconsistent — reject
+/// at startup (0/0 disables backoff and stays accepted, covered by the defaults test above since the
+/// defaults are nonzero and consistent).
+TEST(CASRequestController, ValidateBudgetRejectsInitialBackoffAboveMaxBackoff)
+{
+    CasRequestBudget budget;
+    budget.retry_initial_backoff_ms = 500;
+    budget.retry_max_backoff_ms = 100;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        validateCasRequestBudget(budget, /*mount_lease_ttl_ms=*/30000, /*mount_renew_period_ms=*/10000);
+    });
+}
+
+/// attempt_timeout_ms + lease_safety_margin_ms must not be computed by a wrapping uint64 sum: absurd
+/// config values near UINT64_MAX must fail validation (correctly, as inconsistent), never wrap around
+/// to a spuriously small sum that would pass the "< lease TTL" check.
+TEST(CASRequestController, ValidateBudgetRejectsOverflowingSumRatherThanWrapping)
+{
+    CasRequestBudget budget;
+    budget.attempt_timeout_ms = std::numeric_limits<uint64_t>::max() - 10;
+    budget.lease_safety_margin_ms = 20;   /// sum would wrap past UINT64_MAX to a tiny value
+    budget.operation_deadline_ms = std::numeric_limits<uint64_t>::max();
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::BAD_ARGUMENTS, [&]
+    {
+        validateCasRequestBudget(budget, /*mount_lease_ttl_ms=*/30000, /*mount_renew_period_ms=*/10000);
+    });
+}
+
+#endif
diff --git a/src/Disks/tests/gtest_cas_retirement_sweep.cpp b/src/Disks/tests/gtest_cas_retirement_sweep.cpp
new file mode 100644
index 000000000000..c058fa87bbbc
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_retirement_sweep.cpp
@@ -0,0 +1,420 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Common/ProfileEvents.h>
+#include <Poco/AutoPtr.h>
+#include <Poco/Util/XMLConfiguration.h>
+
+#include <algorithm>
+#include <mutex>
+#include <set>
+#include <sstream>
+#include <string>
+#include <vector>
+
+/// THE RETIREMENT SWEEP's executable half.
+///
+/// Two mechanisms of the pre-v9 ref protocol lost their premise when the ref stream became a
+/// contiguous, arithmetically-walkable chain, and this file is where each retirement is proved rather
+/// than asserted in a comment:
+///
+///   1. PROBE A's ABORT. The detector compared the round's two enumerations of `cas/ns/stream/` and, on any
+///      disagreement, aborted ref folding for the whole round -- because the fold ITERATED the listing,
+///      so a hole in it meant a record was about to be skipped forever. The intake reads by exact key
+///      now, so a hole folds through; a detector that still aborted would be halting a round that is
+///      provably doing the right thing. It was demoted to a sampled store-quality detector and then
+///      deleted outright: the round enumerates `cas/ns/stream/` exactly once, on every round.
+///   2. THE MATERIALIZATION GRACE (`T_mat`). A post-reclaim sleep, long enough for a straggler
+///      conditional `PUT` from a dying epoch to land or exhaust its retries BEFORE the successor
+///      trusted its recovery LISTINGS. Recovery does not trust listings; it closes every dead epoch
+///      with an in-band `EpochSeal` written as a conditional create, and the straggler's own create
+///      loses to it. The wait is deleted outright, setting and all -- the feature never shipped, so
+///      there is no config to protect and no parsed-but-inert period to serve.
+///
+/// The retirement rationale (premise / verdict / replacement / evidence, one row per retired item)
+/// is captured in `docs/en/antalya/cas/architecture/design-history.md`.
+
+namespace DB::ErrorCodes
+{
+    extern const int NETWORK_ERROR;
+}
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace
+{
+
+/// A backend that drops ONE chosen key from ONE chosen `list` call while exact `get`/`head` of that key
+/// keep working: the minimal realisation of "the store returned an incomplete answer". WHICH call is
+/// load-bearing here, because the round enumerates the ref prefix exactly once -- so `nth = 0` is
+/// always the round's own enumeration, the one the fold regroups.
+///
+/// `nth` counts only those `list` calls that WOULD have returned the key, so unrelated prefix
+/// enumerations cannot shift it; arm it AFTER every seeding write, since the writer's own namespace
+/// listings would otherwise consume a qualifying call.
+class HoleyListBackend : public InMemoryBackend
+{
+public:
+    void omitFromNthListCall(const String & key, size_t nth)
+    {
+        std::lock_guard lock(m);
+        omitted = key;
+        target_call = nth;
+        seen_calls = 0;
+        served = false;
+    }
+
+    /// Whether the hole was actually served. Asserted by every test that plants one, so a mistyped key
+    /// or a miscounted `nth` cannot let a test pass vacuously.
+    bool holeServed() const
+    {
+        std::lock_guard lock(m);
+        return served;
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        ListPage page = InMemoryBackend::list(prefix, cursor, limit);
+        std::lock_guard lock(m);
+        if (omitted.empty())
+            return page;
+        auto it = std::find_if(page.keys.begin(), page.keys.end(),
+                               [&](const ListedKey & k) { return k.key == omitted; });
+        if (it == page.keys.end())
+            return page;              /// not a qualifying call -- do not count it
+        if (seen_calls++ != target_call)
+            return page;
+        page.keys.erase(it);
+        served = true;
+        omitted.clear();              /// one hole only
+        return page;
+    }
+
+private:
+    mutable std::mutex m;
+    String omitted;
+    size_t target_call = 0;
+    size_t seen_calls = 0;
+    bool served = false;
+};
+
+/// Counts full enumerations of the ref prefix -- one increment per `list` call whose prefix is EXACTLY
+/// `cas/ns/stream/`, which is how "the round lists this prefix once" becomes an assertion instead of a
+/// claim. `janitor_prefix_lists` counts the bounded `cas/ns/` janitor page separately, by the same exact
+/// match: the two prefixes are distinct strings (`cas/ns/stream/` vs `cas/ns/`), so a hot scan and the
+/// janitor's own bounded page can never be conflated by this counter.
+class RefPrefixListCountingBackend : public InMemoryBackend
+{
+public:
+    String refs_prefix;
+    String janitor_prefix;
+    std::atomic<size_t> ref_prefix_lists{0};
+    std::atomic<size_t> janitor_prefix_lists{0};
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (!refs_prefix.empty() && prefix == refs_prefix)
+            ++ref_prefix_lists;
+        if (!janitor_prefix.empty() && prefix == janitor_prefix)
+            ++janitor_prefix_lists;
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+};
+
+/// Forces the FIRST `putIfAbsent` whose key contains `fault_key_substr` to throw an ambiguous
+/// (Unresolved-classified) exception, `fault_count` times -- the minimal fault injection needed to drive
+/// a ref-log append into the `Unresolved`/wedge outcome, with `max_attempts = 1` in the budget so the
+/// single failed attempt exhausts the retry budget immediately. (Same shape as `gtest_cas_pool.cpp`'s
+/// file-local backend of the same name; both are three lines of `throw` over `InMemoryBackend`, and
+/// hoisting a shared one would couple two suites' fault models for no gain.)
+class UnresolvedPutBackend final : public InMemoryBackend
+{
+public:
+    using Backend::putIfAbsent;
+
+    String fault_key_substr;
+    int fault_count = 0;
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (fault_count > 0 && !fault_key_substr.empty() && key.find(fault_key_substr) != String::npos)
+        {
+            --fault_count;
+            throw Poco::TimeoutException("UnresolvedPutBackend: simulated ambiguous result (response lost)");
+        }
+        return InMemoryBackend::putIfAbsent(key, bytes, meta);
+    }
+};
+
+/// GC's fence-out applied directly to the mount lease: preserve the body, set `gc_fenced`, bump `seq`
+/// (token-guarded). A subsequent `tryRemountOnce` then reclaims a fresh incarnation.
+void fenceOutMount(Backend & backend, const String & mount_key)
+{
+    const auto got = backend.get(mount_key);
+    ASSERT_TRUE(got.has_value());
+    MountLease m = decodeMountLease(got->bytes);
+    m.gc_fenced = true;
+    m.seq += 1;
+    ASSERT_EQ(backend.putOverwrite(mount_key, encodeMountLease(m), got->token).outcome, PutOutcome::Done);
+}
+
+/// Publish one part `ref` with a single content blob whose payload is `payload`.
+ManifestId publishOneBlobPart(const PoolPtr & s, const RootNamespace & ns, const String & ref,
+                              const String & payload)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    auto build = s->beginPartWrite(info);
+    build->putBlob(idOf(payload), BlobSource::fromString(payload));
+
+    ManifestEntry e;
+    e.path = "data.bin";
+    e.placement = EntryPlacement::Blob;
+    e.ref = BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(u128Of(payload))};
+    e.blob_size = payload.size();
+
+    const ManifestId id = build->stageManifest({e});
+    build->precommitAdd(ns, ref, id);
+    build->promote(ns, ref, build->buildId(), id);
+    return id;
+}
+
+/// Every ref-log key of `ns` currently listed, in key order.
+std::set<String> listRefLogKeys(Backend & b, const Layout & l, const RootNamespace & ns)
+{
+    std::set<String> out;
+    String cursor;
+    while (true)
+    {
+        const ListPage page = b.list(l.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)), cursor, 1000);
+        for (const ListedKey & k : page.keys)
+            if (const auto parsed = l.parseRefObjectKey(k.key); parsed && parsed->kind == RefObjectKind::Log)
+                out.insert(k.key);
+        if (page.next_cursor.empty())
+            break;
+        cursor = page.next_cursor;
+    }
+    return out;
+}
+
+/// The greatest ref-log id of `ns` in the current listing.
+RefTxnId greatestLoggedId(Backend & b, const Layout & l, const RootNamespace & ns)
+{
+    RefTxnId best{};
+    for (const String & key : listRefLogKeys(b, l, ns))
+        if (const auto parsed = l.parseRefObjectKey(key); parsed && best < parsed->txn_id)
+            best = parsed->txn_id;
+    return best;
+}
+
+Poco::AutoPtr<Poco::Util::XMLConfiguration> makeDiskConfig(const std::string & inner)
+{
+    std::istringstream iss("<clickhouse><disk>" + inner + "</disk></clickhouse>");
+    return new Poco::Util::XMLConfiguration(iss);
+}
+
+}
+
+/// The blob the hidden removal releases must actually be reclaimed. This is the retention half of the
+/// skipped-transaction class, and it is the outcome the abort used to buy at the price of a lost round:
+/// under arithmetic intake the very round that was served the hole folds the removal, so the blob dies
+/// on the normal schedule rather than waiting for a listing to become honest again.
+TEST(CASRetirementSweep, AHiddenRemovalStillReclaimsItsBlob)
+{
+    auto backend = std::make_shared<HoleyListBackend>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .gc_fold_max_defer_rounds = 0,
+    });
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv/tbl"};
+    const String payload = "reclaimed-payload";
+    /// Stage B (Task 4-C): pin to the sentinel before the first real touch -- `listRefLogKeys` below
+    /// lists at that exact prefix. The raw `Live` row also needs the same empty checkpoint authority
+    /// as a completed production birth before `publishOneBlobPart` invokes recovery.
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    publishOneBlobPart(store, ns, "part_a", payload);
+    Gc gc(store, hexToU128("00000000000000000000000000000012"));
+    /// Reclaiming rounds: Stage A's destructive gate refuses a universe it cannot enumerate, and this
+    /// test's subject IS the reclamation (see `runRegularRoundReclaiming`).
+    ASSERT_TRUE(DB::Cas::tests::runRegularRoundReclaiming(gc).acquired_lease);
+    store->renewWatermarkOnce();
+    const String blob_key = layout.blobKey(BlobRef{BlobHashAlgo::CityHash128,
+                                                   BlobDigest::fromU128(u128Of(payload))});
+    ASSERT_TRUE(backend->head(blob_key).exists);
+
+    const std::set<String> before_drop = listRefLogKeys(*backend, layout, ns);
+    store->dropRef(ns, "part_a");
+    String removal_key;
+    for (const String & k : listRefLogKeys(*backend, layout, ns))
+        if (!before_drop.contains(k))
+            removal_key = k;
+    ASSERT_FALSE(removal_key.empty());
+    store->renewWatermarkOnce();
+
+    backend->omitFromNthListCall(removal_key, /*nth=*/0);
+
+    /// condemn -> graduate -> exact-token delete needs several rounds; the first of them is the one
+    /// served the hole.
+    for (int i = 0; i < 12; ++i)
+    {
+        ASSERT_TRUE(DB::Cas::tests::runRegularRoundReclaiming(gc).acquired_lease);
+        store->renewWatermarkOnce();
+    }
+    ASSERT_TRUE(backend->holeServed()) << "the sabotage never fired";
+
+    EXPECT_FALSE(backend->head(blob_key).exists)
+        << "the removal was hidden from one enumeration and never folded -- the retention half of the "
+           "skipped-transaction class, which arithmetic intake is supposed to close";
+}
+
+/// THE RETIREMENT's whole point, made observable: a folding round enumerates `cas/ns/stream/` exactly
+/// ONCE, on EVERY round of a multi-round run -- not just the first, since a regression that quietly
+/// reintroduced a second enumeration only on a LATER round would pass a one-round check for the wrong
+/// reason. The bounded `cas/ns/` janitor page is a separate exact-string prefix and runs every round
+/// too; it must never be counted as, or mistaken for, a hot scan of the ref prefix. 32 rounds exercises
+/// the deleted detector's own cadence (every 16th folding round) twice over, so a regression that only
+/// reintroduces the second enumeration on that cadence cannot hide inside a shorter run.
+TEST(CASRetirementSweep, TheRoundEnumeratesTheRefPrefixExactlyOnce)
+{
+    auto backend = std::make_shared<RefPrefixListCountingBackend>();
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .gc_fold_max_defer_rounds = 0,
+    });
+    backend->refs_prefix = store->layout().casRefsPrefix();
+    backend->janitor_prefix = store->layout().namespaceRootPrefix();
+    const RootNamespace ns{"srv/tbl"};
+    publishOneBlobPart(store, ns, "part_a", "counted-payload");
+    store->renewWatermarkOnce();
+
+    Gc gc(store, hexToU128("00000000000000000000000000000013"));
+    for (int round = 0; round < 32; ++round)
+    {
+        backend->ref_prefix_lists.store(0);
+        backend->janitor_prefix_lists.store(0);
+        const RoundReport report = gc.runRegularRound();
+        ASSERT_TRUE(report.acquired_lease);
+        ASSERT_FALSE(report.deferred);
+        EXPECT_EQ(backend->ref_prefix_lists.load(), 1u)
+            << "round " << round << " enumerated cas/ns/stream/ a number of times other than once";
+        EXPECT_GT(backend->janitor_prefix_lists.load(), 0u)
+            << "round " << round << " never took the bounded cas/ns/ janitor page";
+        store->renewWatermarkOnce();
+    }
+}
+
+
+/// ==================== item 2: the materialization grace, retired ====================
+
+/// THE MECHANISM THAT REPLACED THE WAIT, tested directly. A ref lane is left holding an UNDECIDED
+/// conditional `PUT` when the fence trips -- the exact state `T_mat` was introduced to wait out. The
+/// remount proceeds with no wait at all, the next recovery closes the dead epoch with an in-band
+/// `EpochSeal` at the slot the straggler would have taken, and the straggler's own conditional create
+/// then LOSES to it.
+///
+/// The assertion is the conflict itself, not the absence of damage: "nothing bad happened" would also
+/// be true of a run where the straggler simply never arrived.
+TEST(CASRetirementSweep, AStragglerFromTheDyingEpochLosesItsCreateToTheRecoverySeal)
+{
+    CasRequestBudget budget;
+    budget.max_attempts = 1;
+    budget.attempt_timeout_ms = 100;
+    budget.operation_deadline_ms = 5000;
+    budget.lease_safety_margin_ms = 100;
+
+    auto backend = std::make_shared<UnresolvedPutBackend>();
+    uint64_t fake_boot = 1'000'000;
+    std::vector<uint64_t> waits;
+    auto store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p", .server_root_id = "test",
+        .mount_lease_ttl_ms = std::chrono::milliseconds(30000),
+        .cas_request_budget = budget,
+        .boot_ms_fn = [&] { return fake_boot; },
+        .wait_sleep_fn = [&](uint64_t ms) { fake_boot += ms; waits.push_back(ms); },
+    });
+    ASSERT_TRUE(store);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"srv/straggler"};
+    /// Pin to the transition life before the first real touch, and give that raw `Live` row the exact
+    /// empty checkpoint authority that production birth would have published before recovery.
+    DB::Cas::tests::fixture::admitLive(*backend, layout, ns);
+    DB::Cas::tests::writeRecoverableCkptForRawFixture(*backend, layout, ns, RefCkpt{
+        .life_epoch = 1,
+        .committed_through = std::nullopt,
+        .checkpoint_snapshot_id = std::nullopt,
+        .last_epoch_seal = std::nullopt,
+    });
+
+    publishOneBlobPart(store, ns, "x", "straggler-payload");
+    ASSERT_EQ(store->liveWriterEpoch(), 1u);
+
+    /// Drive the next ref-log append into the Unresolved/wedge outcome: the single attempt the budget
+    /// allows fails ambiguously, so this process can never learn whether its conditional PUT landed.
+    /// That undecidability is the whole reason the resolution is a conditional CREATE and not a GET.
+    backend->fault_key_substr = layout.namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NETWORK_ERROR, [&] { store->dropRef(ns, "x"); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+
+    /// The id the straggler would occupy: one past the greatest record that is actually durable in the
+    /// dying epoch. That is also, by construction, where the recovery seal goes.
+    const RefTxnId greatest = greatestLoggedId(*backend, layout, ns);
+    ASSERT_EQ(greatest.writer_epoch, 1u);
+    const RefTxnId straggler_slot{greatest.writer_epoch, greatest.ref_sequence + 1};
+    ASSERT_FALSE(backend->head(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), straggler_slot)).exists)
+        << "the slot must be empty before recovery -- otherwise this test proves nothing about who won";
+
+    /// Fence and remount. No wait: this is the case that used to cost 30 seconds.
+    fake_boot += 30001;
+    fenceOutMount(*backend, layout.mountKey("test"));
+    ASSERT_TRUE(store->tryRemountOnce());
+    ASSERT_EQ(store->liveWriterEpoch(), 2u);
+    EXPECT_TRUE(waits.empty())
+        << "the remount blocked on an operator-configured wait; the grace is supposed to be gone";
+
+    /// Touch the namespace so it re-recovers under the new epoch: the walk closes epoch 1 in band. The
+    /// ref itself is still THERE -- the removal's PUT was the undecided one and (in this fixture) never
+    /// landed, which is precisely the state that leaves a straggler outstanding.
+    backend->fault_key_substr.clear();
+    EXPECT_EQ(store->listRefs(ns).size(), 1u);
+    ASSERT_TRUE(backend->head(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), straggler_slot)).exists)
+        << "recovery did not seal the dead epoch at the slot a straggler would take -- without that "
+           "seal there is nothing for the straggler's create to lose to";
+
+    /// THE STRAGGLER ARRIVES. Its conditional create is refused, whenever it happens to land.
+    const PutResult put = backend->putIfAbsent(layout.refLogKey(DB::Cas::tests::fixture::fixtureLife(ns), straggler_slot), "ghost-body");
+    EXPECT_EQ(put.outcome, PutOutcome::PreconditionFailed)
+        << "the dying epoch's straggler overwrote (or joined) a slot the successor had already sealed";
+}
+
+/// THE FAIL-CLOSE THAT REPLACES THE DELETED SETTING. `materialization_grace_ms` is gone from the
+/// settings table outright -- no parsed-but-inert period, no deprecation log -- so a config that still
+/// asks for the wait is refused at disk open by the generic unknown-key path, loudly, instead of being
+/// silently ignored by a server that no longer honours it. The feature never shipped, so there is no
+/// deployed config this can break.
+TEST(CASRetirementSweep, AConfigStillAskingForTheMaterializationGraceIsRejected)
+{
+    auto cfg = makeDiskConfig(
+        "<server_root_id>srv1</server_root_id><materialization_grace_ms>30000</materialization_grace_ms>");
+    DB::ContentAddressedSettings s;
+    EXPECT_THROW(
+        s.loadFromConfig(*cfg, "disk", "/scratch", "/scratch", [](const std::string & v) { return v; }),
+        DB::Exception)
+        << "a retired setting must fail the disk open, not be quietly accepted and ignored";
+}
diff --git a/src/Disks/tests/gtest_cas_s3_staging.cpp b/src/Disks/tests/gtest_cas_s3_staging.cpp
new file mode 100644
index 000000000000..120bd8c5f5b4
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_s3_staging.cpp
@@ -0,0 +1,773 @@
+#include <gtest/gtest.h>
+#include "cas_test_helpers.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedMetadataStorage.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasProbe.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasServerRoot.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.h>
+#include <Disks/DiskCommitTransactionOptions.h>
+#include <IO/ReadHelpers.h>
+
+#include <Poco/AutoPtr.h>
+#include <Poco/Util/XMLConfiguration.h>
+
+#include <atomic>
+#include <chrono>
+#include <filesystem>
+#include <sstream>
+#include <string>
+
+/// Task 0 of the S3-native staging plan: pure config plumbing, ZERO behavior change.
+/// `staging_backend` (default `local`) is parsed
+/// from the CAS disk config; the parsed `StagingBackend` is exposed via
+/// `ContentAddressedMetadataStorage::stagingBackend()`. `::conditionalCopySupported()` is a stored
+/// bool, defaulting to `false` until a later task wires the mount-time capability probe.
+///
+/// The global constraint (OFF BY DEFAULT) is the DEFAULT arm below: absent config keys must parse to
+/// `StagingBackend::Local` with `conditionalCopySupported()==false`.
+
+namespace DB::ContentAddressedSetting
+{
+    extern const ContentAddressedSettingsString staging_backend;
+}
+
+namespace
+{
+
+/// Build a `Poco::Util::XMLConfiguration` with `inner_xml` nested under a `<disk>` element (mirrors
+/// the shape a real CAS disk config has under `storage_configuration.disks.<name>`, so
+/// `config_prefix = "disk"` reads exactly like the disk factory's `config_prefix`).
+Poco::AutoPtr<Poco::Util::XMLConfiguration> configWithDiskSection(const std::string & inner_xml)
+{
+    std::istringstream xml_stream( // STYLE_CHECK_ALLOW_STD_STRING_STREAM
+        "<clickhouse><disk>" + inner_xml + "</disk></clickhouse>");
+    return new Poco::Util::XMLConfiguration(xml_stream);
+}
+
+/// A test-only `LocalObjectStorage` subclass whose `copyObjectConditional` is configurable, so the
+/// Task 3 selection logic (`DB::Cas::probeConditionalCopy`) can be exercised without a live S3/RustFS
+/// backend (live enforcement is Task 7). `LocalObjectStorage` already implements every OTHER pure
+/// virtual (`writeObject`, `removeObjectIfExists`, `exists`, `copyObject`, ...) against real files
+/// under a fresh temp root, so overriding just `copyObjectConditional` is enough to fake either an
+/// ENFORCING or a NON-ENFORCING backend; a THROWING (default `NOT_IMPLEMENTED`) backend needs no
+/// fake at all — a plain `LocalObjectStorage` already exercises that path (see
+/// `DefaultCopyObjectConditionalThrowsNotImplemented` above).
+class FakeConditionalCopyObjectStorage : public DB::LocalObjectStorage
+{
+public:
+    enum class Mode
+    {
+        /// Real write-once semantics: creates the destination iff it was absent; a destination that
+        /// already exists is REJECTED (created=false), no bytes touched.
+        Enforcing,
+        /// A backend that silently ignores `If-None-Match`: every call overwrites the destination
+        /// and reports created=true, even when the destination already existed.
+        NonEnforcing,
+    };
+
+    FakeConditionalCopyObjectStorage(DB::LocalObjectStorageSettings settings_, Mode mode_)
+        : DB::LocalObjectStorage(std::move(settings_)), mode(mode_)
+    {
+    }
+
+    DB::ConditionalCopyResult copyObjectConditional(
+        const DB::StoredObject & object_from,
+        const DB::StoredObject & object_to,
+        const DB::ReadSettings & read_settings,
+        const DB::WriteSettings & write_settings,
+        std::optional<DB::ObjectAttributes> object_to_attributes) override
+    {
+        ++call_count;
+        if (mode == Mode::Enforcing && exists(object_to))
+            return {.created = false, .dest_etag = {}};
+
+        copyObject(object_from, object_to, read_settings, write_settings, object_to_attributes);
+        return {.created = true, .dest_etag = "fake-etag"};
+    }
+
+    int callCount() const { return call_count; }
+
+private:
+    Mode mode;
+    int call_count = 0;
+};
+
+/// Build a `FakeConditionalCopyObjectStorage` rooted at a fresh, unique temp directory (mirrors
+/// `DB::Cas::tests::makeLocalObjectStorageForTest`).
+std::shared_ptr<FakeConditionalCopyObjectStorage> makeFakeConditionalCopyStorage(FakeConditionalCopyObjectStorage::Mode mode)
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_s3_staging_probe_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    return std::make_shared<FakeConditionalCopyObjectStorage>(std::move(settings), mode);
+}
+
+/// A fake object-store sink for Task 4 of the S3-native staging plan (`DB::Cas::CaContentWriteBuffer`'s
+/// S3-staging constructor): an in-memory `WriteBufferFromFileBase` that records every byte written to
+/// it, plus whether `cancelImpl`/`finalizeImpl` ran. This is enough to prove the S3-staging mode
+/// streams to the SINK (not to a local temp file) while hashing, without needing a real object storage
+/// — the end-to-end wiring (`writeFile` choosing this mode, the promote path) lands in later tasks.
+class FakeStagingSink : public DB::WriteBufferFromFileBase
+{
+public:
+    explicit FakeStagingSink(std::string key_)
+        : DB::WriteBufferFromFileBase(/*buf_size=*/8192, nullptr, 0), key(std::move(key_))
+    {
+    }
+
+    void sync() override {}
+    std::string getFileName() const override { return key; }
+
+    const std::string & writtenBytes() const { return written; }
+    bool wasCancelled() const { return cancelled; }
+    bool wasFinalizedForTest() const { return did_finalize; }
+
+protected:
+    void nextImpl() override
+    {
+        if (!offset())
+            return;
+        written.append(working_buffer.begin(), offset());
+    }
+
+    void finalizeImpl() override
+    {
+        next();
+        did_finalize = true;
+    }
+
+    void cancelImpl() noexcept override
+    {
+        cancelled = true;
+    }
+
+private:
+    std::string key;
+    std::string written;
+    bool cancelled = false;
+    bool did_finalize = false;
+};
+
+/// Task 5 of the S3-native staging plan: the promote path (`PartWriteTxn::putBlob` with
+/// `BlobSource::server_side_copy_from` set) drives a WRITE-ONCE conditional server-side copy through
+/// the SAME condemn/resurrect gate the streaming path uses. This backend is a `DB::Cas::InMemoryBackend`
+/// (which models conditional create, so it honors both the write-once `promoteStaged` and the
+/// unconditional `resurrect` contracts) that RECORDS every server-side-copy call so a test can
+/// assert the copy source/destination and the conditional-vs-unconditional distinction — in particular
+/// that a condemned-blob RESURRECT copies FROM the staging key, NEVER from the condemned blob key
+/// (`feedback_ca_resurrect_invariant`), and that a live blob is NEVER unconditionally overwritten.
+class RecordingStagingBackend : public DB::Cas::InMemoryBackend
+{
+public:
+    struct CopyCall
+    {
+        std::string from;
+        std::string to;
+        bool conditional;   /// true = promoteStaged (write-once); false = resurrect (unconditional)
+    };
+
+    std::vector<CopyCall> copy_calls;
+
+    DB::Cas::PutResult promoteStaged(const String & staging_key, const String & blob_key) override
+    {
+        copy_calls.push_back({staging_key, blob_key, /*conditional=*/true});
+        return DB::Cas::InMemoryBackend::promoteStaged(staging_key, blob_key);
+    }
+
+    DB::Cas::Token resurrect(DB::ReadBuffer & payload, uint64_t payload_size, const String & blob_key,
+                             const String & fresh_header) override
+    {
+        /// The source is no longer an argument -- the caller opens the reader -- so `from` is recorded
+        /// as empty here and the "never the condemned key" invariant is asserted through `reads_of`
+        /// below, which counts what was actually READ. That is the stronger check: it observes the I/O
+        /// rather than a parameter the backend was told about.
+        copy_calls.push_back({String{}, blob_key, /*conditional=*/false});
+        return DB::Cas::InMemoryBackend::resurrect(payload, payload_size, blob_key, fresh_header);
+    }
+
+    /// Every key READ AS A STREAM, with a count. The resurrect opens its source with `getStream`, so
+    /// this counts exactly the reads that path performs -- and deliberately not the materializing
+    /// `get`, which the assertions themselves use to inspect bodies.
+    std::map<String, size_t> reads_of;
+
+    using DB::Cas::InMemoryBackend::getStream;
+    std::optional<DB::Cas::GetStreamResult> getStream(const String & key, DB::Cas::Range range) override
+    {
+        ++reads_of[key];
+        return DB::Cas::InMemoryBackend::getStream(key, range);
+    }
+
+
+    /// Every unconditional (resurrect) copy this backend saw — empty iff no live/condemned body was ever
+    /// overwritten. `assertNeverOverwritesLiveBlob` reads this to enforce invariant (d).
+    size_t unconditionalCopyCount() const
+    {
+        size_t n = 0;
+        for (const CopyCall & c : copy_calls)
+            n += c.conditional ? 0 : 1;
+        return n;
+    }
+};
+
+DB::Cas::PoolPtr openStagingPool(const std::shared_ptr<RecordingStagingBackend> & b)
+{
+    return DB::Cas::Pool::open(b, DB::Cas::PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// A build whose owning manifest namespace / final ref name are `ns`/`ref` (mirrors gtest_cas_build's
+/// `startBuildFor`: promote/stageManifest derive the namespace by splitting `intended_ref` on the LAST '/').
+DB::Cas::PartWriteTxnPtr startStagingBuild(const DB::Cas::PoolPtr & s, const DB::Cas::RootNamespace & ns, const String & ref)
+{
+    DB::Cas::PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    return s->beginPartWrite(info);
+}
+
+/// Stage a one-blob manifest and precommit it (so the EDGE-BEFORE-OBSERVE fail-closed check in
+/// `observeAndAdmit` holds), returning the build ready for a `putBlob` promote of `hash`.
+DB::Cas::PartWriteTxnPtr precommittedBuildFor(
+    const DB::Cas::PoolPtr & s, const DB::Cas::RootNamespace & ns, const String & ref,
+    const DB::UInt128 & hash, uint64_t blob_size)
+{
+    DB::Cas::PartWriteTxnPtr build = startStagingBuild(s, ns, ref);
+    const DB::Cas::ManifestId id = build->stageManifest({DB::Cas::tests::blobEntryFor("col.bin", hash, blob_size)});
+    build->precommitAdd(ns, ref, id);
+    return build;
+}
+
+/// A `BlobSource` that promotes via the S3 server-side-copy path (no local `open`).
+DB::Cas::BlobSource serverSideCopySource(const std::string & staging_key, uint64_t size)
+{
+    DB::Cas::BlobSource source;
+    source.size = size;
+    source.server_side_copy_from = staging_key;
+    return source;
+}
+
+}
+
+TEST(CASS3Staging, ParsesS3BackendFromConfig)
+{
+    auto config = configWithDiskSection("<staging_backend>s3</staging_backend>");
+
+    EXPECT_EQ(DB::ContentAddressedMetadataStorage::parseStagingBackend(*config, "disk"), DB::Cas::StagingBackend::S3);
+}
+
+TEST(CASS3Staging, DefaultConfigParsesToLocalBackend)
+{
+    /// No `staging_backend` key at all — the OFF BY DEFAULT arm.
+    auto config = configWithDiskSection("<scratch_path>/tmp/whatever</scratch_path>");
+
+    EXPECT_EQ(DB::ContentAddressedMetadataStorage::parseStagingBackend(*config, "disk"), DB::Cas::StagingBackend::Local);
+}
+
+TEST(CASS3Staging, UnknownBackendValueThrows)
+{
+    auto config = configWithDiskSection("<staging_backend>nfs</staging_backend>");
+    EXPECT_THROW(DB::ContentAddressedMetadataStorage::parseStagingBackend(*config, "disk"), DB::Exception);
+}
+
+TEST(CASS3Staging, DefaultConstructedStorageReportsLocalAndNoConditionalCopy)
+{
+    /// Constructed with no staging-related args at all (mirrors the existing gtest call sites, e.g.
+    /// gtest_ca_wiring.cpp, which stop at `context_`): the accessors must reflect the same
+    /// byte-for-byte-current-behavior defaults the config parser produces above.
+    auto settings = DB::Cas::tests::makeSettingsForTest(
+        "test", std::filesystem::temp_directory_path() / "cas_s3_staging_default_scratch");
+    auto storage = std::make_shared<DB::ContentAddressedMetadataStorage>(
+        DB::Cas::tests::makeLocalObjectStorageForTest(), "pool", "srv1", "", nullptr, settings);
+
+    EXPECT_EQ(storage->stagingBackend(), DB::Cas::StagingBackend::Local);
+    EXPECT_FALSE(storage->conditionalCopySupported());
+}
+
+/// Task 2 of the S3-native staging plan: `IObjectStorage::copyObjectConditional` (write-once
+/// conditional server-side copy) — the interface-level contract. Backends without an enforced,
+/// native conditional copy MUST NOT override the default: it fail-closes with `NOT_IMPLEMENTED`,
+/// exactly like the existing `IObjectStorage::removeObjectIfTokenMatches` default (never silently
+/// falls back to an unconditional overwrite). `LocalObjectStorage` (used by
+/// `makeLocalObjectStorageForTest`) does not override `copyObjectConditional`, so it exercises the
+/// base-class default directly. Live 412-vs-created S3 semantics are covered by the Task 7
+/// integration test (with_rustfs); this is deliberately just the fail-closed contract test.
+TEST(CASS3Staging, DefaultCopyObjectConditionalThrowsNotImplemented)
+{
+    auto storage = DB::Cas::tests::makeLocalObjectStorageForTest();
+
+    const DB::StoredObject from{"cas_s3_staging_conditional_copy_from"};
+    const DB::StoredObject to{"cas_s3_staging_conditional_copy_to"};
+
+    DB::Cas::tests::expectThrowsCode(DB::ErrorCodes::NOT_IMPLEMENTED, [&]
+    {
+        storage->copyObjectConditional(from, to, DB::ReadSettings{}, DB::WriteSettings{});
+    });
+}
+
+/// Task 3 of the S3-native staging plan: the mount-time capability probe (`DB::Cas::probeConditionalCopy`)
+/// for the OPTIONAL conditional-copy capability (distinct from the mandatory `runCapabilityProbe`
+/// battery). These three tests cover the fail-close SELECTION logic with fakes — live 412-vs-created
+/// enforcement against a real backend is Task 7 (with_rustfs integration test).
+
+TEST(CASS3Staging, ProbeConditionalCopyReturnsTrueForEnforcingBackend)
+{
+    auto storage = makeFakeConditionalCopyStorage(FakeConditionalCopyObjectStorage::Mode::Enforcing);
+
+    EXPECT_TRUE(DB::Cas::probeConditionalCopy(*storage, "probe_prefix"));
+    /// Both the "fresh destination" and the "already-existing destination" conditional copies ran.
+    EXPECT_EQ(storage->callCount(), 2);
+}
+
+TEST(CASS3Staging, ProbeConditionalCopyReturnsFalseForNonEnforcingBackend)
+{
+    auto storage = makeFakeConditionalCopyStorage(FakeConditionalCopyObjectStorage::Mode::NonEnforcing);
+
+    /// The backend silently overwrites the destination on the second call (created=true again) —
+    /// it does not enforce If-None-Match, so the probe must fail closed.
+    EXPECT_FALSE(DB::Cas::probeConditionalCopy(*storage, "probe_prefix"));
+}
+
+TEST(CASS3Staging, ProbeConditionalCopyReturnsFalseWhenCopyObjectConditionalThrows)
+{
+    /// A plain `LocalObjectStorage` does not override `copyObjectConditional` at all — it falls
+    /// through to the base-class default, which throws NOT_IMPLEMENTED (exactly what a real backend
+    /// without conditional-copy support does). The probe must never propagate this: it fails closed.
+    auto storage = DB::Cas::tests::makeLocalObjectStorageForTest();
+
+    EXPECT_FALSE(DB::Cas::probeConditionalCopy(*storage, "probe_prefix"));
+}
+
+/// Task 4 of the S3-native staging plan: `CaContentWriteBuffer`'s S3-staging constructor streams
+/// directly to an already-opened object-store sink while hashing, instead of spilling to a local temp
+/// file (see the constructor's doc comment in ContentAddressedWriteBuffers.h). These two tests
+/// exercise the buffer directly over a `FakeStagingSink` — no real object storage, disk, or
+/// `ContentAddressedTransaction` needed; `writeFile` choosing this mode is exercised together with the
+/// promote path in later tasks (S3 mode is off by default and not enabled by any existing test).
+
+TEST(CASS3Staging, ContentWriteBufferS3ModeStreamsToSinkAndFinalizes)
+{
+    const std::string staging_key = "staging/mount1/abc123.tmp";
+    auto * sink_ptr = new FakeStagingSink(staging_key);
+    std::unique_ptr<DB::WriteBufferFromFileBase> sink(sink_ptr);
+
+    std::string got_hash_hex;
+    size_t got_size = 0;
+    std::string got_key;
+    int on_finalized_calls = 0;
+
+    /// S3-native staging fix 2026-07-11: the S3 constructor takes a fixed-length envelope header that is
+    /// written to the sink FIRST, UNHASHED and excluded from the reported size. A distinctive 256-byte
+    /// filler stands in for the real CABL header here (this test exercises the buffer mechanics, not the
+    /// envelope encoder).
+    const std::string envelope_header(256, 'H');
+
+    auto buf = std::make_unique<DB::Cas::CaContentWriteBuffer>(
+        std::move(sink),
+        staging_key,
+        envelope_header,
+        DB::Cas::BlobHashAlgo::CityHash128,
+        /*buf_size=*/8192,
+        /*use_adaptive_buffer_size=*/false,
+        /*adaptive_buffer_initial_size=*/0,
+        [&](const std::string & hash_hex, size_t size, const std::string & key)
+        {
+            ++on_finalized_calls;
+            got_hash_hex = hash_hex;
+            got_size = size;
+            got_key = key;
+        });
+
+    /// Write in two chunks (exercises more than one nextImpl flush) and finalize.
+    const std::string payload_part1(4000, 'x');
+    const std::string payload_part2(1234, 'y');
+    buf->write(payload_part1.data(), payload_part1.size());
+    buf->write(payload_part2.data(), payload_part2.size());
+    buf->finalize();
+
+    const std::string payload = payload_part1 + payload_part2;
+
+    /// (a) the sink received the ENVELOPE HEADER FIRST, then EXACTLY the payload bytes — the staging
+    /// object holds `[header][payload]` so the promote can stay a verbatim server-side copy.
+    EXPECT_EQ(sink_ptr->writtenBytes(), envelope_header + payload);
+    EXPECT_TRUE(sink_ptr->wasFinalizedForTest());
+    EXPECT_FALSE(sink_ptr->wasCancelled());
+
+    /// (b) on_finalized fired exactly once with the correct cityHash128 hex, size, and staging key.
+    /// The pool-wide content hash is the STREAMING `HashingWriteBuffer` convention (chunked
+    /// cityHash128, block = 2048 B), which diverges from a one-shot `CityHash_v1_0_2::CityHash128`
+    /// call for a payload spanning more than one block (see `gtest_cas_part_write.cpp`'s
+    /// `CopyForwardMultiBlockPayloadVerifies`, which documents and exercises the same divergence).
+    /// This payload (5234 bytes) spans multiple 2048-byte blocks, so the expected hash must be
+    /// recomputed with the SAME streaming convention via `HashingReadBuffer`, not a one-shot call.
+    DB::ReadBufferFromMemory expected_in(payload.data(), payload.size());
+    DB::HashingReadBuffer expected_hashing(expected_in);
+    expected_hashing.ignoreAll();
+    const std::string expected_hash_hex = getHexUIntLowercase(expected_hashing.getHash());
+    EXPECT_EQ(on_finalized_calls, 1);
+    EXPECT_EQ(got_hash_hex, expected_hash_hex);
+    EXPECT_EQ(got_size, payload.size());
+    EXPECT_EQ(got_key, staging_key);
+    EXPECT_EQ(buf->getFileName(), staging_key);
+}
+
+TEST(CASS3Staging, ContentWriteBufferS3ModeCancelCancelsSinkAndSkipsFinalize)
+{
+    const std::string staging_key = "staging/mount1/cancelled.tmp";
+    auto * sink_ptr = new FakeStagingSink(staging_key);
+    std::unique_ptr<DB::WriteBufferFromFileBase> sink(sink_ptr);
+
+    bool on_finalized_called = false;
+
+    auto buf = std::make_unique<DB::Cas::CaContentWriteBuffer>(
+        std::move(sink),
+        staging_key,
+        /*envelope_header=*/std::string(256, 'H'),
+        DB::Cas::BlobHashAlgo::CityHash128,
+        /*buf_size=*/8192,
+        /*use_adaptive_buffer_size=*/false,
+        /*adaptive_buffer_initial_size=*/0,
+        [&](const std::string &, size_t, const std::string &)
+        {
+            on_finalized_called = true;
+        });
+
+    const std::string payload = "some bytes that must never be promoted";
+    buf->write(payload.data(), payload.size());
+    buf->cancel();
+
+    /// (c) cancel() before finalize cancels the sink and on_finalized is NEVER called — no partial
+    /// finalize (no promote-worthy hash/size is ever handed to the transaction for cancelled bytes).
+    EXPECT_TRUE(sink_ptr->wasCancelled());
+    EXPECT_FALSE(sink_ptr->wasFinalizedForTest());
+    EXPECT_FALSE(on_finalized_called);
+
+    /// The buffer's destructor calls cancel() again (defensive backstop) — already-cancelled, so this
+    /// must stay a no-op: still no on_finalized call, and no attempt to fs::remove a remote key.
+    buf.reset();
+    EXPECT_FALSE(on_finalized_called);
+}
+
+/// Task 5 of the S3-native staging plan: the promote path. `PartWriteTxn::putBlob` with
+/// `BlobSource::server_side_copy_from` set drives a WRITE-ONCE conditional SERVER-SIDE COPY through the
+/// SAME condemn/resurrect gate as the streaming path (spec 2026-07-11-cas-s3-native-staging §5/§9). The
+/// four cases below use `RecordingStagingBackend` (an emulated backend that models conditional create
+/// and records every server-side-copy call). Live 412-vs-created enforcement against a real backend is
+/// Task 7 (with_rustfs integration test).
+
+/// (a) Fresh blob key ⇒ the write-once conditional copy CREATES it; the tokened Blob dep is recorded at
+/// the copy's destination token (the new incarnation token). No unconditional copy is ever issued.
+TEST(CASS3Staging, PromoteViaServerSideCopyCreatesFreshBlobTokenedDep)
+{
+    auto backend = std::make_shared<RecordingStagingBackend>();
+    auto store = openStagingPool(backend);
+    const DB::Cas::RootNamespace ns{"srv1/nsA"};
+    const std::string ref = "part_a";
+
+    const DB::UInt128 hash = DB::Cas::tests::u128Of("payload-A");
+    const DB::Cas::BlobRef blob_id{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)};
+    const std::string blob_key = store->layout().blobKey(blob_id);
+    const std::string staging_key = "p/staging/mount1/aaa.tmp";
+    const std::string payload(300, 'a');
+    backend->putIfAbsent(staging_key, payload);
+
+    auto build = precommittedBuildFor(store, ns, ref, hash, payload.size());
+    const DB::Cas::PutBlobResult bref = build->putBlob(blob_id, serverSideCopySource(staging_key, payload.size()));
+
+    /// EXACTLY one CONDITIONAL server-side copy staging->blobKey; zero unconditional copies.
+    ASSERT_EQ(backend->copy_calls.size(), 1u);
+    EXPECT_TRUE(backend->copy_calls[0].conditional);
+    EXPECT_EQ(backend->copy_calls[0].from, staging_key);
+    EXPECT_EQ(backend->copy_calls[0].to, blob_key);
+    EXPECT_EQ(backend->unconditionalCopyCount(), 0u);
+
+    /// A TOKENED Blob dep was recorded (created path); the created blob carries the copy's dest token.
+    EXPECT_TRUE(build->depIsTokened(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)}));
+    const DB::Cas::HeadResult hr = backend->head(blob_key);
+    ASSERT_TRUE(hr.exists);
+    EXPECT_FALSE(hr.token.empty());
+    EXPECT_EQ(bref.size, payload.size());
+
+    /// The promoted blob body IS the staging bytes (server-side copy moved them verbatim).
+    const auto got = backend->get(blob_key);
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, payload);
+}
+
+/// (b) Blob key already exists and is CLEAN ⇒ the conditional copy 412s and the writer ADOPTS the
+/// existing incarnation. No copy of any kind lands over the live blob. Also covers invariant (d): NO
+/// unconditional copy is ever issued over a live (non-condemned) blob.
+TEST(CASS3Staging, PromoteOverExistingCleanBlobAdoptsAndNeverOverwrites)
+{
+    auto backend = std::make_shared<RecordingStagingBackend>();
+    auto store = openStagingPool(backend);
+    const DB::Cas::RootNamespace ns{"srv1/nsB"};
+    const std::string ref = "part_b";
+
+    const DB::UInt128 hash = DB::Cas::tests::u128Of("payload-B");
+    const DB::Cas::BlobRef blob_id{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)};
+    const std::string blob_key = store->layout().blobKey(blob_id);
+    const std::string staging_key = "p/staging/mount1/bbb.tmp";
+    backend->putIfAbsent(staging_key, std::string(300, 'b'));
+
+    /// A pre-existing, well-formed, CLEAN blob (envelope + payload) already at the content key.
+    DB::Cas::tests::writeBlobBody(*backend, store->layout(), hash);
+    DB::Cas::tests::writeMetaClean(*backend, store->layout(), hash, /*size=*/1);
+    const DB::Cas::HeadResult before = backend->head(blob_key);
+    ASSERT_TRUE(before.exists);
+
+    auto build = precommittedBuildFor(store, ns, ref, hash, 300);
+    build->putBlob(blob_id, serverSideCopySource(staging_key, 300));
+
+    /// Exactly one CONDITIONAL promote (which 412s) — then ADOPT. Invariant (d): zero unconditional copies.
+    ASSERT_EQ(backend->copy_calls.size(), 1u);
+    EXPECT_TRUE(backend->copy_calls[0].conditional);
+    EXPECT_EQ(backend->unconditionalCopyCount(), 0u);
+
+    /// The existing incarnation is untouched: same token, same bytes.
+    const DB::Cas::HeadResult after = backend->head(blob_key);
+    EXPECT_EQ(after.token, before.token);
+
+    /// The adopt recorded a TOKENED dep at the observed (existing) incarnation's token.
+    EXPECT_TRUE(build->depIsTokened(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)}));
+}
+
+/// (c) Blob key exists but is CONDEMNED ⇒ the writer RESURRECTS by re-uploading its OWN staging PAYLOAD
+/// under a FRESH-tagged envelope header — NEVER a read/copy of the condemned blob key
+/// (`feedback_ca_resurrect_invariant`) — and the resurrected body DIFFERS from the condemned incarnation
+/// (INV-NO-RETURN: a verbatim copy would reproduce identical bytes ⇒ identical ETag ⇒ the queued
+/// exact-token delete of the condemned incarnation would kill the live resurrection = data loss).
+TEST(CASS3Staging, PromoteOverCondemnedBlobResurrectsWithFreshTagNotVerbatim)
+{
+    auto backend = std::make_shared<RecordingStagingBackend>();
+    auto store = openStagingPool(backend);
+    const DB::Cas::RootNamespace ns{"srv1/nsC"};
+    const std::string ref = "part_c";
+
+    const DB::UInt128 hash = DB::Cas::tests::u128Of("payload-C");
+    const DB::Cas::BlobRef blob_id{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)};
+    const std::string blob_key = store->layout().blobKey(blob_id);
+    const std::string staging_key = "p/staging/mount1/ccc.tmp";
+    const std::string payload(300, 'c');
+
+    /// The staging object holds `[header][payload]` (as `writeFile` now emits it). The staging header is
+    /// a fixed 256-byte CABL envelope with its OWN incarnation_tag.
+    DB::Cas::EnvelopeHeader staging_h;
+    staging_h.kind = DB::Cas::ObjectKind::Blob;
+    staging_h.incarnation_tag = DB::UInt128(0xC0FFEE);   /// the create-time tag
+    const std::string staging_header = DB::Cas::encodeEnvelopeHeader(
+        staging_h, static_cast<uint32_t>(store->poolMeta().blob_header_len));
+    ASSERT_EQ(staging_header.size(), store->poolMeta().blob_header_len);
+    const std::string staging_bytes = staging_header + payload;
+    backend->putIfAbsent(staging_key, staging_bytes);
+
+    /// Seed the condemned blob body = EXACTLY what a verbatim promote of this staging object would have
+    /// produced (the writer's OWN create, later observed condemned). This is the adversarial shape: a
+    /// verbatim resurrect WOULD reproduce these identical bytes ⇒ identical ETag ⇒ collision.
+    backend->putIfAbsent(blob_key, staging_bytes);
+    DB::Cas::tests::writeMetaClean(*backend, store->layout(), hash, /*size=*/payload.size());
+    DB::Cas::tests::condemnMeta(*backend, store->layout(), hash, /*condemn_round=*/5);
+    const DB::Cas::HeadResult before = backend->head(blob_key);
+    ASSERT_TRUE(before.exists);
+
+    auto build = precommittedBuildFor(store, ns, ref, hash, payload.size());
+    build->putBlob(blob_id, serverSideCopySource(staging_key, payload.size()));
+
+    /// A CONDITIONAL promote (412 — blob present) FOLLOWED BY exactly one UNCONDITIONAL resurrect
+    /// whose SOURCE is the staging key, NEVER the condemned blob key.
+    ASSERT_EQ(backend->copy_calls.size(), 2u);
+    EXPECT_TRUE(backend->copy_calls[0].conditional);
+    EXPECT_EQ(backend->copy_calls[0].to, blob_key);
+    EXPECT_FALSE(backend->copy_calls[1].conditional);
+    EXPECT_EQ(backend->copy_calls[1].to, blob_key);
+    /// INV: the resurrect reads the STAGING object and never the condemned blob key. Asserted on the
+    /// reads themselves rather than on a source argument, because the caller now opens the reader.
+    EXPECT_GT(backend->reads_of[staging_key], 0u) << "the resurrect must read the writer's own staging object";
+    EXPECT_EQ(backend->reads_of[blob_key], 0u) << "the condemned blob key must never be read";
+    EXPECT_EQ(backend->unconditionalCopyCount(), 1u);
+
+    /// The incarnation token is REFRESHED (a fresh incarnation displaced the condemned one).
+    const DB::Cas::HeadResult after = backend->head(blob_key);
+    EXPECT_NE(after.token, before.token);
+    ASSERT_TRUE(after.exists);
+
+    const auto got = backend->get(blob_key);
+    ASSERT_TRUE(got.has_value());
+    const uint64_t header_len = store->poolMeta().blob_header_len;
+
+    /// INV-NO-RETURN — THE fresh-tag property: the resurrected body is NOT byte-identical to the
+    /// condemned incarnation (a verbatim copy would have been). The PAYLOAD is preserved exactly (the
+    /// resurrect read it from OUR staging object, skipping the staging header), but the envelope HEADER
+    /// differs — the writer minted a FRESH incarnation_tag — so on a real content-addressed store the
+    /// resurrected ETag differs and the queued exact-token delete of the condemned incarnation cannot
+    /// match the live resurrection.
+    EXPECT_NE(got->bytes, staging_bytes);
+    ASSERT_GE(got->bytes.size(), header_len);
+    EXPECT_EQ(got->bytes.substr(header_len), payload);              /// payload preserved
+    EXPECT_NE(got->bytes.substr(0, header_len), staging_header);    /// header freshly re-tagged
+
+    /// The resurrect recorded a tokened dep and flipped the meta back to Clean.
+    EXPECT_TRUE(build->depIsTokened(DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(hash)}));
+    const auto lm = DB::Cas::tests::loadMetaForTest(*backend, store->layout(), hash);
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(lm->meta.state, DB::Cas::MetaState::Clean);
+}
+
+/// ===========================================================================================
+/// Task 6 of the S3-native staging plan: staging cleanup after commit, read-your-writes over an S3
+/// pending blob, and the mount-lease-scoped sweeper (`CASStagingSweeper.h`).
+///
+/// The wiring-level tests below (cleanup-after-commit, read-your-writes) drive the REAL
+/// `ContentAddressedMetadataStorage` + `ContentAddressedTransaction` over `FakeConditionalCopyObjectStorage`
+/// in `Enforcing` mode. The fake's REAL `getType()` stays `Local`, so `Cas::ObjectStorageBackend` selects
+/// `EmulatedSingleProcess` for the CAS core protocol — the same fully-supported combination every other
+/// `gtest_ca_wiring.cpp` test uses — while the mount-time conditional-copy PROBE
+/// (`Cas::probeConditionalCopy`) is decoupled from that (it exercises `copyObjectConditional` directly
+/// against the object storage), so it reports S3 staging as usable independent of the backend mode. This
+/// lets `writeFile` take the S3-staging code path (stream to a staging object while hashing) WITHOUT
+/// needing a live/native conditional-copy backend for the promote: `PartWriteTxn::putBlob`'s
+/// `promoteStaged`/`resurrect` seams (Native-mode only — see `CasObjectStorageBackend.cpp`) are
+/// already covered directly against `Cas::PartWriteTxn`/`RecordingStagingBackend` above (Task 5) and against a
+/// live backend in Task 7's `with_rustfs` integration test; these two tests only ever exercise an S3
+/// pending blob that is either NEVER referenced (the B189 orphan shape — publishStaging skips its
+/// `putBlob`) or read BEFORE commit, so `promoteStaged` is never reached here.
+
+namespace
+{
+
+/// Construct a `ContentAddressedMetadataStorage` with `staging_backend=s3` over `object_storage`,
+/// mirroring `DefaultConstructedStorageReportsLocalAndNoConditionalCopy`'s settings defaults for every
+/// field this test suite does not care about — only `server_root_id` (the mount identity that names
+/// the staging prefix) and `staging_backend` differ.
+std::shared_ptr<DB::ContentAddressedMetadataStorage> makeS3StagingMetadataStorageForTest(
+    const DB::ObjectStoragePtr & object_storage, const std::string & server_root_id)
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto scratch = std::filesystem::temp_directory_path()
+        / ("cas_s3_staging_wiring_" + server_root_id + "_" + unique);
+    auto settings = DB::Cas::tests::makeSettingsForTest(server_root_id, scratch);
+    settings[DB::ContentAddressedSetting::staging_backend] = "s3";
+    settings.validate();
+    return std::make_shared<DB::ContentAddressedMetadataStorage>(
+        object_storage, "pool", "srv1", /*disk_name_=*/"", /*context_=*/nullptr, settings);
+}
+
+/// Mirrors gtest_ca_wiring.cpp's helper of the same shape.
+void writeThroughS3Transaction(DB::ContentAddressedTransaction & tx, const std::string & path, const std::string & bytes)
+{
+    auto buf = tx.writeFile(path, 65536, DB::WriteMode::Rewrite, {});
+    buf->write(bytes.data(), bytes.size());
+    buf->finalize();
+}
+
+}
+
+/// (a) A successful commit removes the S3 staging object of a pending blob it staged. Uses the B189
+/// orphan shape (the pending blob's entry is unlinked before commit) so `publishStaging` never calls
+/// `putBlob` for it — only `cleanupPendingTempFiles`'s Task 6 branch ever touches this staging object,
+/// which is exactly the seam this test targets.
+TEST(CASS3Staging, SuccessfulCommitRemovesOrphanedS3StagingObject)
+{
+    auto object_storage = makeFakeConditionalCopyStorage(FakeConditionalCopyObjectStorage::Mode::Enforcing);
+    auto metadata_storage = makeS3StagingMetadataStorageForTest(object_storage, "mountA");
+    metadata_storage->startup();
+    ASSERT_TRUE(metadata_storage->conditionalCopySupported());
+
+    auto tx = metadata_storage->createTransaction();
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+
+    /// orphan.bin forces the S3-staging blob path (a ".bin" suffix always stays a blob, per
+    /// `partFileMustStayBlob`); it is unlinked below before commit.
+    writeThroughS3Transaction(ca_tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/orphan.bin", std::string(300, 'x'));
+    /// checksums.txt is small and NOT blob-forcing: an INLINE entry that gives the part's PartWriteTxn a real
+    /// (non-orphaned) manifest entry, so `publishStaging` takes its normal path (not the early-return
+    /// mutable-only/no-PartWriteTxn branch).
+    writeThroughS3Transaction(ca_tx, "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/checksums.txt", "sums");
+
+    DB::RelativePathsWithMetadata staged_before;
+    object_storage->listObjects(metadata_storage->stagingKeyPrefix(), staged_before, /*max_keys=*/0);
+    ASSERT_EQ(staged_before.size(), 1u) << "exactly orphan.bin's S3 staging object should exist pre-commit";
+
+    tx->unlinkFile("a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/orphan.bin", false, false);
+
+    tx->commit(DB::NoCommitOptions{});
+
+    DB::RelativePathsWithMetadata staged_after;
+    object_storage->listObjects(metadata_storage->stagingKeyPrefix(), staged_after, /*max_keys=*/0);
+    EXPECT_TRUE(staged_after.empty())
+        << "cleanupPendingTempFiles must remove the orphaned S3 staging object after a successful commit";
+}
+
+/// (b) Read-your-writes over an S3 pending blob (before commit) returns the staged bytes from the S3
+/// staging object, not a local temp file.
+TEST(CASS3Staging, ReadYourWritesReturnsStagedBytesFromS3StagingObject)
+{
+    auto object_storage = makeFakeConditionalCopyStorage(FakeConditionalCopyObjectStorage::Mode::Enforcing);
+    auto metadata_storage = makeS3StagingMetadataStorageForTest(object_storage, "mountB");
+    metadata_storage->startup();
+    ASSERT_TRUE(metadata_storage->conditionalCopySupported());
+
+    auto tx = metadata_storage->createTransaction();
+    auto & ca_tx = dynamic_cast<DB::ContentAddressedTransaction &>(*tx);
+
+    const std::string path = "a11/a11a11a1-1111-4111-8111-111111111111/all_1_1_0/data.bin";
+    const std::string payload(5000, 'z');
+    writeThroughS3Transaction(ca_tx, path, payload);
+
+    auto read_buf = tx->tryReadFileInFlight(path, DB::ReadSettings{}, {});
+    ASSERT_NE(read_buf, nullptr);
+    std::string got;
+    DB::readStringUntilEOF(got, *read_buf);
+    EXPECT_EQ(got, payload);
+}
+
+/// (c) `sweepOwnMountStaging` removes only objects under the given mount prefix and leaves a DIFFERENT
+/// mount's staging objects untouched (the lease-fence — `CASStagingSweeper.h`).
+TEST(CASStagingSweeper, RemovesOnlyObjectsUnderGivenMountPrefix)
+{
+    auto storage = DB::Cas::tests::makeLocalObjectStorageForTest();
+    const std::string root = storage->getCommonKeyPrefix();
+
+    auto put = [&](const std::string & key, const std::string & bytes)
+    {
+        auto buf = storage->writeObject(DB::StoredObject(key), DB::WriteMode::Rewrite);
+        buf->write(bytes.data(), bytes.size());
+        buf->finalize();
+    };
+
+    put(root + "/p/staging/mountA/one.tmp", "a1");
+    put(root + "/p/staging/mountA/two.tmp", "a2");
+    put(root + "/p/staging/mountB/three.tmp", "b1");   /// a DIFFERENT mount's staging — must survive
+
+    DB::Cas::sweepOwnMountStaging(*storage, root + "/p/staging/mountA/");
+
+    EXPECT_FALSE(storage->exists(DB::StoredObject(root + "/p/staging/mountA/one.tmp")));
+    EXPECT_FALSE(storage->exists(DB::StoredObject(root + "/p/staging/mountA/two.tmp")));
+    EXPECT_TRUE(storage->exists(DB::StoredObject(root + "/p/staging/mountB/three.tmp")));
+}
+
+/// (d) GC's blob-discovery LISTs ONLY `Layout::blobsPrefix()` (`<pool>/blobs/`) — a top-level prefix
+/// strictly disjoint from the S3-staging area (`<pool>/staging/<mount_id>/`), so a staging object can
+/// never be listed, HEAD'd, or condemned as an orphan blob by GC's fold (`CasGc.cpp`, `CasFsck.cpp`).
+/// This is a prefix-separation assertion (the GC fold itself is not unit-testable in isolation from a
+/// full round — see `gtest_cas_gc_fold.cpp` for that machinery); it pins the invariant a refactor that
+/// nested `staging/` under `blobs/` (or vice versa) would violate.
+TEST(CASS3Staging, GcBlobDiscoveryPrefixExcludesStagingObjects)
+{
+    const DB::Cas::Layout layout("p");
+    const std::string blobs_prefix = layout.blobsPrefix();
+    const std::string staging_prefix = "p/staging/mountA/";
+    const std::string staging_key = staging_prefix + "aaa.tmp";
+
+    EXPECT_EQ(blobs_prefix, "p/blobs/");
+    EXPECT_FALSE(staging_prefix.starts_with(blobs_prefix));
+    EXPECT_FALSE(blobs_prefix.starts_with(staging_prefix));
+    EXPECT_FALSE(staging_key.starts_with(blobs_prefix));
+}
diff --git a/src/Disks/tests/gtest_cas_sentinel_probe.cpp b/src/Disks/tests/gtest_cas_sentinel_probe.cpp
new file mode 100644
index 000000000000..e4a50db58958
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_sentinel_probe.cpp
@@ -0,0 +1,264 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInstrumentedBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasObjectStorageBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasSentinelProbe.h>
+#include <Disks/DiskObjectStorage/ObjectStorages/Local/LocalObjectStorage.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <atomic>
+#include <filesystem>
+#include <memory>
+#include <unistd.h>
+
+#if USE_AWS_S3
+#include <IO/S3Common.h>
+#endif
+
+using namespace DB::Cas;
+
+/// Task 3 (spec §2): the typed sentinel probe below `Backend` must never conflate a transport error
+/// with absence. These tests exercise the free-function entry point `probeSentinel` against the generic
+/// `Backend::probeSentinelRaw` default (via `InMemoryBackend`, the "Emulated"-style in-memory backend used
+/// by CAS tests) and against `ObjectStorageBackend`'s `EmulatedSingleProcess` override, which is the REAL
+/// production mode for a content-addressed disk over `object_storage_type=local`.
+
+namespace
+{
+
+using DB::Cas::tests::nativeKeyUnder;
+
+/// A Backend decorator whose head/get/list all throw an untyped runtime error when armed — modelling
+/// a backend with no sharper evidence than "something went wrong" (a network timeout, a 5xx, an
+/// unclassifiable failure). Mirrors the existing MetaWriteFaultBackend fault-injection pattern
+/// (cas_test_helpers.h): every other operation delegates to InMemoryBackend unchanged.
+class TransportFaultBackend final : public InMemoryBackend
+{
+public:
+    /// Unhide the base convenience overloads, matching every other Backend subclass in this suite.
+    using Backend::get;
+    using Backend::getStream;
+    using Backend::putIfAbsent;
+    using Backend::putIfAbsentStream;
+    using Backend::putOverwrite;
+    using Backend::casPut;
+
+    HeadResult head(const String & key) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::head(key);
+    }
+
+    std::optional<GetResult> get(const String & key, Range range) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::get(key, range);
+    }
+
+    ListPage list(const String & prefix, const String & cursor, size_t limit) override
+    {
+        if (fail.load())
+            throw std::runtime_error("injected fault: transport error");
+        return InMemoryBackend::list(prefix, cursor, limit);
+    }
+
+    std::atomic<bool> fail{true};
+};
+
+}
+
+/// (a) A present key probes Present and carries the materialized body.
+TEST(CASSentinelProbe, PresentKeyReturnsPresentWithBody)
+{
+    InMemoryBackend backend;
+    ASSERT_EQ(backend.putIfAbsent("k", "hello").outcome, PutOutcome::Done);
+
+    const auto result = probeSentinel(backend, "k");
+    EXPECT_EQ(result.outcome, ProbeOutcome::Present);
+    ASSERT_TRUE(result.body.has_value());
+    EXPECT_EQ(*result.body, "hello");
+}
+
+/// (b) A deleted (never-written) key probes KeyAbsent while the container/backend is otherwise alive.
+TEST(CASSentinelProbe, AbsentKeyWithContainerAliveReturnsKeyAbsent)
+{
+    InMemoryBackend backend;
+    ASSERT_EQ(backend.putIfAbsent("other", "x").outcome, PutOutcome::Done);   // proves the backend is alive
+
+    const auto result = probeSentinel(backend, "missing");
+    EXPECT_EQ(result.outcome, ProbeOutcome::KeyAbsent);
+    EXPECT_FALSE(result.body.has_value());
+}
+
+/// (c) `ObjectStorageBackend::EmulatedSingleProcess` is the REAL production backend for a
+/// content-addressed disk over `object_storage_type=local` (ContentAddressedMetadataStorage.cpp
+/// selects it whenever the underlying storage is Local). Removing the WHOLE configured container
+/// directory (the disk root) must probe `ContainerAbsent`, distinct from an ordinary absent key —
+/// `LocalObjectStorage::listObjects` silently reports zero children for BOTH a missing directory and
+/// an empty one, so the distinction only exists because `probeSentinelRaw` stats the container first.
+TEST(CASSentinelProbe, ContainerDirectoryRemovedReturnsContainerAbsent)
+{
+    auto storage = tests::makeLocalObjectStorageForTest();
+    ObjectStorageBackend backend(storage, ObjectStorageBackend::Mode::EmulatedSingleProcess);
+
+    ASSERT_EQ(backend.putIfAbsent("k", "hello").outcome, PutOutcome::Done);
+
+    /// Sanity, container alive: Present vs. KeyAbsent are genuinely distinct before we remove anything.
+    EXPECT_EQ(probeSentinel(backend, "k").outcome, ProbeOutcome::Present);
+    EXPECT_EQ(probeSentinel(backend, "missing").outcome, ProbeOutcome::KeyAbsent);
+
+    std::filesystem::remove_all(storage->getCommonKeyPrefix());
+
+    const auto result = probeSentinel(backend, "k");
+    EXPECT_EQ(result.outcome, ProbeOutcome::ContainerAbsent);
+    EXPECT_FALSE(result.body.has_value());
+}
+
+/// `ObjectStorageBackend::Mode::Native` over a plain `LocalObjectStorage` (the same construction
+/// `gtest_cas_backend.cpp`'s Native-mode tests use to exercise the Native code path without a live S3
+/// endpoint): a present key must probe `Present` and carry the materialized body via the raw-HEAD ->
+/// `get` path, not just the EmulatedSingleProcess path already covered above.
+TEST(CASSentinelProbe, NativePresentKeyReturnsPresentWithBody)
+{
+    auto storage = tests::makeLocalObjectStorageForTest();
+    ObjectStorageBackend backend(storage, ObjectStorageBackend::Mode::Native);
+    const String key = nativeKeyUnder(storage, "some/key");
+
+    ASSERT_EQ(backend.putIfAbsent(key, "native body").outcome, PutOutcome::Done);
+
+    const auto result = probeSentinel(backend, key);
+    EXPECT_EQ(result.outcome, ProbeOutcome::Present);
+    ASSERT_TRUE(result.body.has_value());
+    EXPECT_EQ(*result.body, "native body");
+}
+
+/// (d) A backend forced to throw a transport error must probe Indeterminate — NEVER KeyAbsent, even
+/// though the failure looks superficially like "nothing there" from the caller's point of view.
+TEST(CASSentinelProbe, TransportErrorNeverClassifiesAsAbsent)
+{
+    TransportFaultBackend backend;
+    const auto result = probeSentinel(backend, "k");
+    EXPECT_EQ(result.outcome, ProbeOutcome::Indeterminate);
+    EXPECT_FALSE(result.body.has_value());
+}
+
+#if USE_AWS_S3
+
+namespace
+{
+
+/// A `LocalObjectStorage` whose `getObjectMetadata` can be armed to throw a configurable synthetic
+/// `S3Exception` — the same technique `gtest_cas_backend.cpp`'s `NativeReadThrowsNoSuchKeyObjectStorage`
+/// uses to exercise S3 error codes without a live S3 endpoint. Constructing `ObjectStorageBackend` in
+/// `Mode::Native` over this fake is the established pattern for testing the Native/S3 raw-error classifier
+/// in isolation (see also `gtest_cas_backend.cpp`'s `NativeRejectsWrongDialectTokenBeforeTouchingTheWire`).
+class ThrowingS3MetadataObjectStorage final : public DB::LocalObjectStorage
+{
+public:
+    using DB::LocalObjectStorage::LocalObjectStorage;
+
+    void throwOnGetObjectMetadata(Aws::S3::S3Errors code) { metadata_error = code; }
+
+    DB::ObjectMetadata getObjectMetadata(const std::string & path, bool with_tags) const override
+    {
+        if (metadata_error)
+            throw DB::S3Exception("injected fault: " + path, *metadata_error);
+        return DB::LocalObjectStorage::getObjectMetadata(path, with_tags);
+    }
+
+private:
+    std::optional<Aws::S3::S3Errors> metadata_error;
+};
+
+DB::ObjectStoragePtr makeThrowingS3MetadataStorageForTest()
+{
+    static std::atomic<uint64_t> counter{0};
+    const auto unique = std::to_string(::getpid()) + "_" + std::to_string(counter.fetch_add(1));
+    const auto root = (std::filesystem::temp_directory_path() / ("cas_sentinel_probe_unit_" + unique)).string();
+
+    std::error_code ec;
+    std::filesystem::remove_all(root, ec);
+    std::filesystem::create_directories(root, ec);
+
+    DB::LocalObjectStorageSettings settings("test", root, /*read_only_=*/false);
+    return std::make_shared<ThrowingS3MetadataObjectStorage>(std::move(settings));
+}
+
+}
+
+/// The full S3 IAM permutation table (spec §2): a raw NO_SUCH_KEY/NO_SUCH_BUCKET/ACCESS_DENIED HEAD
+/// error must classify EXACTLY, and anything unmodeled must fail closed to Indeterminate.
+TEST(CASSentinelProbe, NativeClassifiesNoSuchKeyAsKeyAbsent)
+{
+    auto storage = std::static_pointer_cast<ThrowingS3MetadataObjectStorage>(makeThrowingS3MetadataStorageForTest());
+    storage->throwOnGetObjectMetadata(Aws::S3::S3Errors::NO_SUCH_KEY);
+    ObjectStorageBackend backend(storage, ObjectStorageBackend::Mode::Native);
+
+    EXPECT_EQ(probeSentinel(backend, nativeKeyUnder(storage, "some/key")).outcome, ProbeOutcome::KeyAbsent);
+}
+
+/// A real S3 HEAD's 404 has no response body, so the SDK cannot parse a `NoSuchKey` `<Code>` and
+/// instead derives `RESOURCE_NOT_FOUND` straight from the HTTP status (see `isNotFoundError`,
+/// `src/IO/S3/getObjectInfo.cpp`) — THIS is the code a genuinely absent key throws on real S3, not
+/// `NO_SUCH_KEY`. Without classifying it, every real-S3 absence would be `Indeterminate` forever.
+TEST(CASSentinelProbe, NativeClassifiesResourceNotFoundAsKeyAbsent)
+{
+    auto storage = std::static_pointer_cast<ThrowingS3MetadataObjectStorage>(makeThrowingS3MetadataStorageForTest());
+    storage->throwOnGetObjectMetadata(Aws::S3::S3Errors::RESOURCE_NOT_FOUND);
+    ObjectStorageBackend backend(storage, ObjectStorageBackend::Mode::Native);
+
+    EXPECT_EQ(probeSentinel(backend, nativeKeyUnder(storage, "some/key")).outcome, ProbeOutcome::KeyAbsent);
+}
+
+TEST(CASSentinelProbe, NativeClassifiesNoSuchBucketAsContainerAbsent)
+{
+    auto storage = std::static_pointer_cast<ThrowingS3MetadataObjectStorage>(makeThrowingS3MetadataStorageForTest());
+    storage->throwOnGetObjectMetadata(Aws::S3::S3Errors::NO_SUCH_BUCKET);
+    ObjectStorageBackend backend(storage, ObjectStorageBackend::Mode::Native);
+
+    EXPECT_EQ(probeSentinel(backend, nativeKeyUnder(storage, "some/key")).outcome, ProbeOutcome::ContainerAbsent);
+}
+
+TEST(CASSentinelProbe, NativeClassifiesAccessDeniedAsAccessDenied)
+{
+    auto storage = std::static_pointer_cast<ThrowingS3MetadataObjectStorage>(makeThrowingS3MetadataStorageForTest());
+    storage->throwOnGetObjectMetadata(Aws::S3::S3Errors::ACCESS_DENIED);
+    ObjectStorageBackend backend(storage, ObjectStorageBackend::Mode::Native);
+
+    EXPECT_EQ(probeSentinel(backend, nativeKeyUnder(storage, "some/key")).outcome, ProbeOutcome::AccessDenied);
+}
+
+TEST(CASSentinelProbe, NativeClassifiesUnmodeledErrorAsIndeterminate)
+{
+    auto storage = std::static_pointer_cast<ThrowingS3MetadataObjectStorage>(makeThrowingS3MetadataStorageForTest());
+    storage->throwOnGetObjectMetadata(Aws::S3::S3Errors::SERVICE_UNAVAILABLE);
+    ObjectStorageBackend backend(storage, ObjectStorageBackend::Mode::Native);
+
+    EXPECT_EQ(probeSentinel(backend, nativeKeyUnder(storage, "some/key")).outcome, ProbeOutcome::Indeterminate);
+}
+
+/// Production wiring (`Pool::open`) ALWAYS wraps the real backend in `InstrumentedBackend` before
+/// anything calls it. `InstrumentedBackend` must forward `probeSentinelRaw` to `inner`, not fall
+/// through to `Backend::probeSentinelRaw`'s generic head/get-based default — the default would derive
+/// its answer from THIS object's own (correctly delegating, but non-typed) `head`/`get` overrides,
+/// silently discarding `ObjectStorageBackend`'s real S3-error classification. NO_SUCH_BUCKET is chosen
+/// deliberately: the generic default cannot produce `ContainerAbsent` at all (it only ever returns
+/// Present/KeyAbsent/Indeterminate), so this test can ONLY pass if the typed override is actually
+/// reached through the wrapper.
+TEST(CASSentinelProbe, InstrumentedBackendForwardsToInnerClassification)
+{
+    auto storage = std::static_pointer_cast<ThrowingS3MetadataObjectStorage>(makeThrowingS3MetadataStorageForTest());
+    storage->throwOnGetObjectMetadata(Aws::S3::S3Errors::NO_SUCH_BUCKET);
+    auto inner = std::make_shared<ObjectStorageBackend>(storage, ObjectStorageBackend::Mode::Native);
+    InstrumentedBackend instrumented(inner);
+
+    EXPECT_EQ(probeSentinel(instrumented, nativeKeyUnder(storage, "some/key")).outcome, ProbeOutcome::ContainerAbsent);
+}
+
+#endif
diff --git a/src/Disks/tests/gtest_cas_server_root_format.cpp b/src/Disks/tests/gtest_cas_server_root_format.cpp
new file mode 100644
index 000000000000..95bd33ad2d45
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_server_root_format.cpp
@@ -0,0 +1,94 @@
+#include "cas_format_test_battery.h"
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/Exception.h>
+#include <limits>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+}
+
+TEST(CASFormatBattery, Owner)
+{
+    OwnerObject o;
+    o.server_uuid = hexToU128("0123456789abcdeffedcba9876543210");
+    const String golden = currentFormatHeader("cas_owner") +
+        "{\"su\":\"0123456789abcdeffedcba9876543210\"}\n";
+    EXPECT_EQ(encodeOwner(o), golden);
+    EXPECT_FALSE(decodeOwner(golden).retired_at_ms.has_value());
+    runFormatBattery({FormatId::Owner,
+        [&] { return sealObject(FormatId::Owner, encodeOwner(o)); },
+        [](std::string_view s) { decodeOwner(std::string(openObject(FormatId::Owner, s))); },
+        golden});
+}
+
+TEST(CASOwnerFormat, RetiredAtRoundTrip)
+{
+    OwnerObject o;
+    o.server_uuid = hexToU128("0123456789abcdeffedcba9876543210");
+    o.retired_at_ms = 1752537600000ULL;
+
+    const OwnerObject back = decodeOwner(encodeOwner(o));
+    EXPECT_EQ(back.server_uuid, o.server_uuid);
+    EXPECT_EQ(back.retired_at_ms, o.retired_at_ms);
+}
+
+TEST(CASFormatBattery, ServerEpoch)
+{
+    ServerEpoch e;
+    e.next_writer_epoch = 7;
+    runFormatBattery({FormatId::ServerEpoch,
+        [&] { return sealObject(FormatId::ServerEpoch, encodeServerEpoch(e)); },
+        [](std::string_view s) { decodeServerEpoch(std::string(openObject(FormatId::ServerEpoch, s))); },
+        currentFormatHeader("cas_epoch") + "{\"nwe\":\"7\"}\n"});
+}
+
+TEST(CASFormatBattery, MountLease)
+{
+    MountLease m{hexToU128("0123456789abcdeffedcba9876543210"), 7, "host-1", 4242,
+                 1752537600000ULL, 5, 1752537630000ULL, 9, false};
+    runFormatBattery({FormatId::MountLease,
+        [&] { return sealObject(FormatId::MountLease, encodeMountLease(m)); },
+        [](std::string_view s) { decodeMountLease(std::string(openObject(FormatId::MountLease, s))); },
+        currentFormatHeader("cas_mount_lease") +
+        "{\"su\":\"0123456789abcdeffedcba9876543210\",\"we\":\"7\",\"hn\":\"host-1\",\"pid\":4242,"
+        "\"sat\":1752537600000,\"seq\":\"5\",\"eat\":1752537630000,\"ma\":\"9\",\"fen\":false}\n"});
+}
+
+TEST(CASMountLeaseFormat, FarewellSentinelAndFencedSurvive)
+{
+    MountLease m{hexToU128("0123456789abcdeffedcba9876543210"), 7, "h", 1,
+                 1, 5, 2, std::numeric_limits<uint64_t>::max(), true};
+    const MountLease back = decodeMountLease(encodeMountLease(m));
+    EXPECT_EQ(back.min_active, std::numeric_limits<uint64_t>::max());
+    EXPECT_TRUE(back.gc_fenced);
+    EXPECT_EQ(back.hostname, "h");
+    EXPECT_EQ(back.writer_epoch, 7u);
+    EXPECT_EQ(back.seq, 5u);
+}
+
+TEST(CASMountLeaseFormat, RejectsMissingIdentityFields)
+{
+    const String header = "{\"type\":\"cas_mount_lease\",\"v\":3}\n";
+    const String fields = "\"hn\":\"host-1\",\"pid\":4242,\"sat\":1752537600000,"
+                          "\"seq\":\"5\",\"eat\":1752537630000,\"ma\":\"9\",\"fen\":false}";
+
+    const auto expectCorrupted = [](const String & data)
+    {
+        try
+        {
+            decodeMountLease(data);
+            FAIL() << "expected CORRUPTED_DATA";
+        }
+        catch (const DB::Exception & e)
+        {
+            EXPECT_EQ(e.code(), DB::ErrorCodes::CORRUPTED_DATA);
+        }
+    };
+
+    expectCorrupted(header + R"({"we":"7",)" + fields + "\n");
+    expectCorrupted(header + R"({"su":"0123456789abcdeffedcba9876543210",)" + fields + "\n");
+}
diff --git a/src/Disks/tests/gtest_cas_settings.cpp b/src/Disks/tests/gtest_cas_settings.cpp
new file mode 100644
index 000000000000..124982d44cc6
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_settings.cpp
@@ -0,0 +1,146 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedSettings.h>
+#include <Common/Exception.h>
+#include <Poco/Util/XMLConfiguration.h>
+#include <Poco/AutoPtr.h>
+#include <sstream>
+
+using namespace DB;
+
+namespace DB::ErrorCodes
+{
+    extern const int NO_ELEMENTS_IN_CONFIG;
+    extern const int BAD_ARGUMENTS;
+}
+
+/// Per-TU extern declarations for the `ContentAddressedSetting` entries this file uses -- the
+/// established pattern for `BaseSettings`-derived classes in this codebase (see e.g.
+/// `RegisterDiskCache.cpp`'s `namespace FileCacheSetting` block): the entries are DEFINED once in
+/// `ContentAddressedSettings.cpp`, and each consumer TU declares only the ones it references.
+namespace DB::ContentAddressedSetting
+{
+    extern const ContentAddressedSettingsUInt64 gc_shards;
+    extern const ContentAddressedSettingsUInt64 gc_interval_sec;
+    extern const ContentAddressedSettingsUInt64 deduplication_cache_bytes;
+    extern const ContentAddressedSettingsString scratch_path;
+}
+
+namespace
+{
+Poco::AutoPtr<Poco::Util::XMLConfiguration> makeConfig(const std::string & inner)
+{
+    std::istringstream iss("<clickhouse><disk>" + inner + "</disk></clickhouse>");
+    return new Poco::Util::XMLConfiguration(iss);
+}
+const auto identity_macros = [](const std::string & s) { return s; };
+}
+
+TEST(CASContentAddressedSettings, DefaultsAndOverridesLand)
+{
+    auto cfg = makeConfig("<server_root_id>srv1</server_root_id><gc_shards>4</gc_shards>");
+    ContentAddressedSettings s;
+    s.loadFromConfig(*cfg, "disk", "/data", "/data/default_scratch", identity_macros);
+    EXPECT_EQ(s[ContentAddressedSetting::gc_shards].value, 4u);
+    EXPECT_EQ(s[ContentAddressedSetting::gc_interval_sec].value, 60u);          /// table default
+    EXPECT_EQ(s[ContentAddressedSetting::deduplication_cache_bytes].value, 64ULL << 20); /// table default
+    /// Absent key -> the verbatim default (never touches the anchor).
+    EXPECT_EQ(s[ContentAddressedSetting::scratch_path].value, "/data/default_scratch");
+}
+
+TEST(CASContentAddressedSettings, UnknownKeyRejected)
+{
+    auto cfg = makeConfig("<server_root_id>srv1</server_root_id><gc_shardz>4</gc_shardz>");
+    ContentAddressedSettings s;
+    EXPECT_THROW(s.loadFromConfig(*cfg, "disk", "/scratch", "/scratch", identity_macros), Exception);
+}
+
+TEST(CASContentAddressedSettings, ObjectStorageKeysSkipped)
+{
+    auto cfg = makeConfig(
+        "<metadata_type>cas</metadata_type><type>object_storage</type>"
+        "<object_storage_type>s3</object_storage_type><endpoint>http://x/y</endpoint>"
+        "<access_key_id>k</access_key_id><secret_access_key>s</secret_access_key>"
+        "<server_root_id>srv1</server_root_id>"
+        /// Regression pin (stateless-lane startup fix): the local-object-storage CAS disk config
+        /// (`tests/config/config.d/cas_storage_policy_for_merge_tree_by_default.xml`)
+        /// sets `path` -- the generic local-object-storage pool root, consumed by
+        /// `ObjectStorageFactory`/`IDisk`, same class as `endpoint`/`access_key_id` above -- and it
+        /// was missing from `non_cas_keys`, which threw `UNKNOWN_SETTING` at server startup.
+        "<path>cas_pool/</path>"
+        /// Regression pin: `name`, read generically by `DiskFromAST` for the inline SQL `disk(...)`
+        /// form used by the `05002`-`05015` CAS stateless tests, was likewise missing and threw
+        /// `UNKNOWN_SETTING` for every one of those tests (only the XML-config `path` gap was fixed
+        /// first; `name` surfaced once the stateless lane actually ran end to end).
+        "<name>cas_test_disk</name>"
+        /// Regression pin: `use_fake_transaction`, validated generically in
+        /// `RegisterDiskObjectStorage.cpp` for every metadata type that needs a real transaction (not
+        /// a CAS-specific check), must reach that check rather than being rejected here as unknown --
+        /// `05015_cas_reject_fake_transaction` depends on it doing so.
+        "<use_fake_transaction>1</use_fake_transaction>");
+    ContentAddressedSettings s;
+    EXPECT_NO_THROW(s.loadFromConfig(*cfg, "disk", "/scratch", "/scratch", identity_macros));
+}
+
+TEST(CASContentAddressedSettings, ValidateFailsClosed)
+{
+    {   /// missing server_root_id: ABSENT key -> typed NO_ELEMENTS_IN_CONFIG (distinct from a
+        /// present-but-invalid value, checked below), mirroring the pre-F4b factory behavior.
+        auto cfg = makeConfig("<gc_shards>1</gc_shards>");
+        ContentAddressedSettings s;
+        try
+        {
+            s.loadFromConfig(*cfg, "disk", "/scratch", "/scratch", identity_macros);
+            FAIL() << "expected an exception";
+        }
+        catch (const Exception & e)
+        {
+            EXPECT_EQ(e.code(), ErrorCodes::NO_ELEMENTS_IN_CONFIG);
+        }
+    }
+    {   /// present but invalid (empty) server_root_id -> BAD_ARGUMENTS from
+        /// `Cas::validateServerRootId`, not NO_ELEMENTS_IN_CONFIG.
+        auto cfg = makeConfig("<server_root_id></server_root_id>");
+        ContentAddressedSettings s;
+        try
+        {
+            s.loadFromConfig(*cfg, "disk", "/scratch", "/scratch", identity_macros);
+            FAIL() << "expected an exception";
+        }
+        catch (const Exception & e)
+        {
+            EXPECT_EQ(e.code(), ErrorCodes::BAD_ARGUMENTS);
+        }
+    }
+    {   /// zero gc_shards
+        auto cfg = makeConfig("<server_root_id>srv1</server_root_id><gc_shards>0</gc_shards>");
+        ContentAddressedSettings s;
+        EXPECT_THROW(s.loadFromConfig(*cfg, "disk", "/scratch", "/scratch", identity_macros), Exception);
+    }
+    {   /// unknown blob_hash spelling
+        auto cfg = makeConfig("<server_root_id>srv1</server_root_id><blob_hash>md5</blob_hash>");
+        ContentAddressedSettings s;
+        EXPECT_THROW(s.loadFromConfig(*cfg, "disk", "/scratch", "/scratch", identity_macros), Exception);
+    }
+}
+
+TEST(CASContentAddressedSettings, RelativeScratchPathAnchored)
+{
+    /// Reproduces the pre-F4b factory's anchor behavior (review finding, Critical): a relative
+    /// `scratch_path` override is anchored to the SERVER DATA PATH (`scratch_path_anchor_if_relative`)
+    /// -- NOT to `default_scratch_path`, which is itself a per-disk subdirectory
+    /// (`.../disks/<name>/cas_scratch`) that must never leak into an override's resolved path.
+    auto cfg = makeConfig("<server_root_id>srv1</server_root_id><scratch_path>rel/dir</scratch_path>");
+    ContentAddressedSettings s;
+    s.loadFromConfig(*cfg, "disk", "/data", "/data/disks/x/cas_scratch", identity_macros);
+    EXPECT_EQ(s[ContentAddressedSetting::scratch_path].value, "/data/rel/dir");
+}
+
+TEST(CASContentAddressedSettings, AbsentScratchPathUsesDefaultVerbatim)
+{
+    /// Absent key -> `default_scratch_path` verbatim, unaffected by the anchor (the per-disk default
+    /// already lives under the server data path; only an explicit relative OVERRIDE needs anchoring).
+    auto cfg = makeConfig("<server_root_id>srv1</server_root_id>");
+    ContentAddressedSettings s;
+    s.loadFromConfig(*cfg, "disk", "/data", "/data/disks/x/cas_scratch", identity_macros);
+    EXPECT_EQ(s[ContentAddressedSetting::scratch_path].value, "/data/disks/x/cas_scratch");
+}
diff --git a/src/Disks/tests/gtest_cas_slot_occupy.cpp b/src/Disks/tests/gtest_cas_slot_occupy.cpp
new file mode 100644
index 000000000000..6e65bfe333bf
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_slot_occupy.cpp
@@ -0,0 +1,334 @@
+#include <gtest/gtest.h>
+
+#include "config.h"
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasRequestControl.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <Poco/Exception.h>
+
+using namespace DB::Cas;
+using DB::Cas::tests::CountingBackend;
+using DB::Cas::tests::ChunkFaultBackend;
+using DB::Cas::tests::LandedButAckLostOnceBackend;
+
+namespace DB::ErrorCodes
+{
+    extern const int BAD_ARGUMENTS;
+}
+
+/// ================================================================================================
+/// Task 2 (2026-07-28 CAS ref-chain Stage A streams, spec INV-2): CasRequestController::slotOccupy --
+/// the dedicated RAW slot-occupy primitive every seal writer and wedge retry uses. ONE conditional
+/// create; on conflict, ONE raw exact GET of the occupant -- NEVER retries internally, NEVER lists,
+/// and NEVER composes putIfAbsentControlled (which retries the same (key, bytes) internally) or
+/// resolveByExactGet (which compares against an expected body and throws CORRUPTED_DATA on a
+/// mismatch) [codex finding 3]. Adjudicating whether an Occupied occupant is "mine" is entirely the
+/// CALLER's job (Task 4/6, the CaCasMountCore `mine` contract) -- these tests only pin the
+/// primitive's own three-way outcome and its op-count contract (Created=1, Occupied=2,
+/// Unresolved<=2 backend ops).
+/// ================================================================================================
+
+namespace
+{
+
+/// Deletes the key the INSTANT its own conditional create conflicts, modelling "the occupant that
+/// caused the conflict vanished before slotOccupy's single resolve GET" -- a race a real backend can
+/// produce (e.g. GC reclaiming an already-condemned object) that the primitive must survive by
+/// reporting Unresolved, NEVER a fabricated Created.
+class VanishOnConflictBackend : public CountingBackend
+{
+public:
+    using CountingBackend::putIfAbsent;
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        PutResult result = CountingBackend::putIfAbsent(key, bytes, meta);
+        if (result.outcome == PutOutcome::PreconditionFailed)
+        {
+            const HeadResult h = head(key);
+            if (h.exists)
+                deleteExact(key, h.token);
+        }
+        return result;
+    }
+};
+
+/// Throws a deterministic LOCAL failure (BAD_ARGUMENTS, in isDeterministicLocalFailure's set) on the
+/// first putIfAbsent -- models a backend-level programming bug, distinct from ChunkFaultBackend's
+/// Mode::Definite below, which is a whitelisted SYNCHRONOUS REJECTION
+/// (classifyConditionalWriteResult's DefiniteFailure). slotOccupy must rethrow both, unchanged, never
+/// folding either into Unresolved (SlotOccupyResult::Kind has no DefiniteFailure member to carry it).
+class LocalFailureOnceBackend : public CountingBackend
+{
+public:
+    using CountingBackend::putIfAbsent;
+    bool fail_once = true;
+
+    PutResult putIfAbsent(const String & key, const String & bytes, const ObjectMeta & meta) override
+    {
+        if (fail_once)
+        {
+            fail_once = false;
+            throw DB::Exception(DB::ErrorCodes::BAD_ARGUMENTS, "scripted deterministic local failure");
+        }
+        return CountingBackend::putIfAbsent(key, bytes, meta);
+    }
+};
+
+/// (`LandedButAckLostOnceBackend` -- "the write LANDS, then the ack is lost" -- was lifted into
+/// `cas_test_helpers.h` for Task 4, whose wedge-adoption tests need the identical seam through a whole
+/// Pool. Its `key_substr` defaults to empty, which is exactly this file's original behaviour: fault the
+/// first `putIfAbsent` of any key.)
+/// Delegates the FIRST putIfAbsent for a key to CountingBackend -- so the write actually LANDS -- and
+/// only THEN throws an ambiguous exception, modelling "our own PUT committed but its response was lost"
+/// (the Task-4 adoption input: plan's "Occupied + bytes == wedge.bytes -> an earlier attempt landed ->
+/// adopt"). Distinct from InMemoryBackend::injectAmbiguousPutIfAbsent, which never touches the store at
+/// all -- that hook models an attempt that did NOT land; this one models an attempt that DID.
+/// One-shot per backend instance: review finding I2 asked specifically for a ~10-line local backend rather than
+/// reusing ChunkFaultBackend::Mode::LandedThenLost, which also arms a one-shot lost-GET fault that would
+/// obscure whether slotOccupy's OWN immediate resolve (not just a later caller's retry) is correct too.
+
+}
+
+/// ---- Step 1 required scenarios ----
+
+TEST(CASSlotOccupy, AbsentKeyCreatesWithOneOp)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    CasRequestController controller(backend, CasRequestBudget{});
+
+    const auto result = controller.slotOccupy("k", "payload", [] { return true; });
+    EXPECT_EQ(result.kind, SlotOccupyResult::Kind::Created);
+    EXPECT_TRUE(result.occupant_bytes.empty());
+    EXPECT_TRUE(result.occupant_token.empty()) << "occupant_token is Occupied-only; must stay default on Created";
+    EXPECT_EQ(result.unresolved_reason, CasUnresolvedReason::NotUnresolved);
+
+    EXPECT_EQ(backend->putCount("k"), 1u);
+    EXPECT_EQ(backend->getCount("k"), 0u);
+    EXPECT_EQ(backend->headCount("k"), 0u);
+
+    const auto landed = backend->get("k");
+    ASSERT_TRUE(landed.has_value());
+    EXPECT_EQ(landed->bytes, "payload");
+}
+
+TEST(CASSlotOccupy, PreExistingKeyOccupiedWithExactBytesAndTokenTwoOps)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    const PutResult seeded = backend->putIfAbsent("k", "occupant-bytes");
+    ASSERT_EQ(seeded.outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto result = controller.slotOccupy("k", "my-attempt-bytes", [] { return true; });
+    EXPECT_EQ(result.kind, SlotOccupyResult::Kind::Occupied);
+    EXPECT_EQ(result.occupant_bytes, "occupant-bytes");
+    EXPECT_EQ(result.occupant_token, seeded.token);
+
+    EXPECT_EQ(backend->putCount("k"), 1u);
+    EXPECT_EQ(backend->getCount("k"), 1u);
+    EXPECT_EQ(backend->headCount("k"), 0u) << "exactly PUT+GET -- a HEAD-then-GET implementation must fail this";
+
+    /// A conflict never overwrites or appends -- the pre-existing object is untouched.
+    const auto current = backend->get("k");
+    ASSERT_TRUE(current.has_value());
+    EXPECT_EQ(current->bytes, "occupant-bytes");
+}
+
+TEST(CASSlotOccupy, InjectedAmbiguousPutResolvesUnresolvedWhenGetFindsNothing)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    backend->injectAmbiguousPutIfAbsent("k");
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto result = controller.slotOccupy("k", "payload", [] { return true; });
+    EXPECT_EQ(result.kind, SlotOccupyResult::Kind::Unresolved);
+    /// An attempt WAS sent (the ambiguous PUT itself) -- this is never the pre-attempt NoAttemptSent
+    /// case. Of the existing CasUnresolvedReason values, AttemptsExhausted is the one documented as
+    /// "the genuine case the 'retry budget exhausted' wording describes" -- exactly this call's single
+    /// (and only) attempt having nothing left to give once its resolve GET came up empty.
+    EXPECT_EQ(result.unresolved_reason, CasUnresolvedReason::AttemptsExhausted);
+    EXPECT_FALSE(unresolvedProvesNothingWasSent(result.unresolved_reason));
+
+    EXPECT_EQ(backend->putCount("k"), 1u);
+    EXPECT_EQ(backend->getCount("k"), 1u);
+    EXPECT_FALSE(backend->head("k").exists) << "the injected fault must not actually create anything";
+}
+
+TEST(CASSlotOccupy, ConflictThenVanishResolvesUnresolved)
+{
+    auto backend = std::make_shared<VanishOnConflictBackend>();
+    const auto seeded = backend->putIfAbsent("k", "occupant-bytes");
+    ASSERT_EQ(seeded.outcome, PutOutcome::Done);
+    backend->resetCounts();
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    const auto result = controller.slotOccupy("k", "my-attempt-bytes", [] { return true; });
+    EXPECT_EQ(result.kind, SlotOccupyResult::Kind::Unresolved);
+    EXPECT_EQ(result.unresolved_reason, CasUnresolvedReason::AttemptsExhausted);
+
+    EXPECT_EQ(backend->putCount("k"), 1u);
+    EXPECT_EQ(backend->getCount("k"), 1u);
+    /// No headCount assertion here (unlike the sibling Occupied test above): VanishOnConflictBackend's
+    /// OWN fixture issues a HEAD internally (to fetch the token before deleteExact) -- that HEAD belongs
+    /// to the test's vanish mechanism, not to slotOccupy, so asserting headCount==0 would be wrong, not
+    /// stronger. slotOccupy itself never calls head(); only put+get are its own ops.
+    EXPECT_FALSE(backend->head("k").exists) << "the occupant vanished between the conflict and the resolve GET";
+}
+
+TEST(CASSlotOccupy, FenceFlipMidCallRefusesPreAttemptNeverLiesCreated)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    CasRequestController controller(backend, CasRequestBudget{});
+
+    const auto result = controller.slotOccupy("k", "payload", [] { return false; });
+    EXPECT_EQ(result.kind, SlotOccupyResult::Kind::Unresolved);
+    /// The pre-attempt reason: fence_ok refused before anything was sent to the backend.
+    EXPECT_EQ(result.unresolved_reason, CasUnresolvedReason::NoAttemptSent);
+    EXPECT_TRUE(unresolvedProvesNothingWasSent(result.unresolved_reason));
+
+    EXPECT_EQ(backend->putTotal(), 0u);
+    EXPECT_EQ(backend->getTotal(), 0u);
+    EXPECT_FALSE(backend->head("k").exists) << "never a lie of Created -- the key must be untouched";
+}
+
+/// ---- Bonus coverage: the deadline pre-gate (the OTHER half of "fence/deadline-gated"), and the two
+/// rethrow paths this primitive shares with its sibling controlled ops.  ----
+
+/// The deadline gate is the SAME pre-attempt refusal as the fence gate above -- a fake clock proves it
+/// fires from elapsed time alone, with a fence that always says yes.
+TEST(CASSlotOccupy, OperationDeadlineExhaustedRefusesPreAttempt)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    uint64_t clock = 0;
+    auto now_ms = [&clock]() -> uint64_t { const uint64_t t = clock; clock += 1000; return t; };
+
+    CasRequestBudget budget;
+    budget.attempt_timeout_ms = 50;
+    budget.operation_deadline_ms = 500;   /// entry now_ms()==0 -> deadline_ms=500; the gate's OWN
+                                           /// now_ms() call then returns 1000 -> 1000+50 > 500 -> refuse
+    CasRequestController controller(backend, budget, now_ms);
+
+    const auto result = controller.slotOccupy("k", "payload", [] { return true; });
+    EXPECT_EQ(result.kind, SlotOccupyResult::Kind::Unresolved);
+    EXPECT_EQ(result.unresolved_reason, CasUnresolvedReason::NoAttemptSent);
+    EXPECT_EQ(backend->putTotal(), 0u);
+    EXPECT_EQ(backend->getTotal(), 0u) << "zero ops total -- the deadline gate must refuse before any I/O, same as the fence gate";
+}
+
+/// A whitelisted synchronous rejection (classifyConditionalWriteResult's DefiniteFailure) PROVES the
+/// request was never applied -- slotOccupy must surface it unchanged rather than resolving or folding
+/// it into Unresolved. Guarded to USE_AWS_S3 builds ONLY [review M6]: DefiniteFailure classification is
+/// structurally unreachable without it (classifyConditionalWriteResult's whitelist is entirely inside
+/// its own `#if USE_AWS_S3`), so on a no-S3 build ChunkFaultBackend::Mode::Definite instead throws a
+/// plain CORRUPTED_DATA DB::Exception -- which is in isDeterministicLocalFailure's set, meaning this
+/// test would silently exercise the SAME slotOccupy branch as DeterministicLocalFailurePropagatesWithoutResolve
+/// below rather than the DefiniteFailure branch it claims to cover. Better a visibly-absent test on that
+/// config than a passing one that isn't testing what its name says.
+#if USE_AWS_S3
+TEST(CASSlotOccupy, DefiniteFailurePropagatesWithoutResolve)
+{
+    auto backend = std::make_shared<ChunkFaultBackend>();
+    backend->fault_substr = "k";
+    backend->mode = ChunkFaultBackend::Mode::Definite;
+    backend->fault_count = 1;
+
+    CasRequestController controller(backend, CasRequestBudget{});
+    EXPECT_THROW(controller.slotOccupy("k", "payload", [] { return true; }), DB::Exception);
+    /// ChunkFaultBackend's fault check throws BEFORE delegating to CountingBackend::putIfAbsent, so
+    /// putCount stays 0 on this path -- fault_count reaching 0 is this backend's own proof the (one)
+    /// attempt was made and consumed the fault.
+    EXPECT_EQ(backend->fault_count, 0);
+    EXPECT_EQ(backend->getCount("k"), 0u) << "a whitelisted definite rejection must never trigger a resolve GET";
+}
+#endif
+
+/// A deterministic LOCAL failure (isDeterministicLocalFailure's set) is the OTHER rethrow path --
+/// distinct from DefiniteFailure above, and checked first in the implementation, so it needs its own
+/// backend-level fault to prove both branches are wired, not just one masking the other.
+TEST(CASSlotOccupy, DeterministicLocalFailurePropagatesWithoutResolve)
+{
+    auto backend = std::make_shared<LocalFailureOnceBackend>();
+    CasRequestController controller(backend, CasRequestBudget{});
+
+    bool threw = false;
+    try
+    {
+        controller.slotOccupy("k", "payload", [] { return true; });
+    }
+    catch (const DB::Exception & e)
+    {
+        threw = true;
+        EXPECT_EQ(e.code(), DB::ErrorCodes::BAD_ARGUMENTS) << "the ORIGINAL exception must propagate unchanged";
+    }
+    EXPECT_TRUE(threw) << "a deterministic local failure must propagate, never return an outcome";
+    /// LocalFailureOnceBackend throws BEFORE delegating to CountingBackend::putIfAbsent (same shape as
+    /// ChunkFaultBackend above), so putCount stays 0 here too -- fail_once flipping is this backend's
+    /// own proof the attempt was made.
+    EXPECT_FALSE(backend->fail_once);
+    EXPECT_EQ(backend->getCount("k"), 0u);
+}
+
+/// ---- Fix round 1 (review findings I1, I2): the two gaps the reviewer required landed before Task 4
+/// consumes this primitive. Both guard the design decisions the review approved -- see
+/// task-2-review.md concern (a) and finding I2's Task-4-adoption note. ----
+
+/// I1: pins the single-fence_ok()-call design (concern (a)) so a future contributor cannot silently
+/// "fix the inconsistency" by re-adding the sibling ops' post-write fence recheck -- that change would
+/// break Task 4's old-generation-retry semantics (resolveWedgeOnce deliberately calls slotOccupy under
+/// the wedge's ORIGINAL admitted_fence_generation, and relies on ITS OWN post-I/O checkFenceOrThrow,
+/// not a second internal check here, to decide whether the result is still relevant). A counting
+/// fence_ok that only answers true on its FIRST call: if slotOccupy ever called it again after the
+/// write landed, this test would see Unresolved instead of Created, OR (if the outcome happened to
+/// still read Created some other way) the call-count assertion below would catch the extra invocation
+/// either way.
+TEST(CASSlotOccupy, CreatedNeverRechecksFenceAfterTheWrite)
+{
+    auto backend = std::make_shared<CountingBackend>();
+    CasRequestController controller(backend, CasRequestBudget{});
+
+    int fence_calls = 0;
+    const auto fence_ok = [&fence_calls]
+    {
+        ++fence_calls;
+        return fence_calls == 1;
+    };
+
+    const auto result = controller.slotOccupy("k", "payload", fence_ok);
+    EXPECT_EQ(result.kind, SlotOccupyResult::Kind::Created);
+    EXPECT_EQ(fence_calls, 1) << "slotOccupy must call fence_ok() exactly ONCE (pre-attempt only) -- "
+                                 "a post-write recheck would falsely report Unresolved here (fence_calls's "
+                                 "SECOND answer is false) and would break Task 4's old-generation-retry design";
+}
+
+/// I2: proves Occupied is reachable for an occupant that is OUR OWN earlier ambiguous write, not only
+/// for a foreign pre-seeded one (PreExistingKeyOccupiedWithExactBytesAndTokenTwoOps above always seeds
+/// via a plain, unambiguous putIfAbsent). This is the exact input shape Task 4's resolveWedgeOnce
+/// adjudicates: "Occupied + bytes == wedge.bytes -> an earlier attempt landed -> adopt" (plan :329).
+TEST(CASSlotOccupy, OwnLandedAmbiguousWriteObservedAsOccupiedOnRetry)
+{
+    auto backend = std::make_shared<LandedButAckLostOnceBackend>();
+    CasRequestController controller(backend, CasRequestBudget{});
+
+    /// Call 1 -- the original attempt: the PUT's own response is lost, but the write DID commit, and
+    /// THIS call's own resolve GET (unfaulted) observes it immediately -- Occupied with OUR bytes,
+    /// proving the same-call resolve path works for a landed ambiguous write, not only a foreign one.
+    const auto first = controller.slotOccupy("k", "my-bytes", [] { return true; });
+    EXPECT_EQ(first.kind, SlotOccupyResult::Kind::Occupied);
+    EXPECT_EQ(first.occupant_bytes, "my-bytes");
+    EXPECT_EQ(backend->putCount("k"), 1u);
+    EXPECT_EQ(backend->getCount("k"), 1u);
+
+    /// Call 2 -- Task 4's resolveWedgeOnce pattern: a LATER caller's flush resolving the SAME logical
+    /// attempt via a FRESH slotOccupy call. The fault is already consumed (one-shot), so this PUT
+    /// conflicts cleanly (PreconditionFailed) and the resolve GET observes OUR OWN earlier bytes again --
+    /// the exact adoption input Task 4 is built on, and the SAME incarnation both calls saw.
+    const auto second = controller.slotOccupy("k", "my-bytes", [] { return true; });
+    EXPECT_EQ(second.kind, SlotOccupyResult::Kind::Occupied);
+    EXPECT_EQ(second.occupant_bytes, "my-bytes");
+    EXPECT_EQ(second.occupant_token, first.occupant_token) << "both calls must observe the SAME landed incarnation";
+    EXPECT_EQ(backend->putCount("k"), 2u);
+    EXPECT_EQ(backend->getCount("k"), 2u);
+}
diff --git a/src/Disks/tests/gtest_cas_sweep_deletion_premise.cpp b/src/Disks/tests/gtest_cas_sweep_deletion_premise.cpp
new file mode 100644
index 000000000000..a5af5889e495
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_sweep_deletion_premise.cpp
@@ -0,0 +1,433 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFoldSealFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasOrphanManifestSweep.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include "cas_sweep_test_support.h"
+#include "cas_test_helpers.h"
+
+#include <vector>
+
+using namespace DB::Cas;
+using namespace DB::Cas::tests;
+
+/// Spec §6, the sweep deletion premise. A manifest of an epoch-`E` build is deletable only when the
+/// namespace cursor has consumed epoch `E`'s seal AND no unconsumed tail record above the cursor names
+/// it as a removal target; on ANY uncertainty the sweep RETAINS and says why.
+///
+/// WHY THE CURSOR AND NOT A LISTING. The sweep's pre-existing protection view is assembled from an
+/// enumeration of the namespace's ref objects, and arithmetic ref intake demoted exactly that
+/// enumeration to a hint: a store may omit a durable key from a `LIST`. A hidden `+1` above the cursor
+/// therefore makes an owned manifest look unowned, and deleting it is data loss; a hidden `-1` makes a
+/// removal target look unprotected, and deleting it clamps the fold forever on the missing body. The
+/// premise closes the first by arithmetic (grants do not cross epochs, and an epoch is left only over
+/// its consumed seal) and the second by refusing whenever the tail is not decidable.
+namespace
+{
+
+/// The build's epoch. The namespace's seeded ref log lives at writer epoch 1 (`appendRefLogSeed`), so
+/// naming the build's epoch 1 as well keeps the fixture coherent: a cursor at `{2, _}` is then a cursor
+/// that genuinely crossed epoch 1's closing seal, not an invented number above an unrelated stream.
+constexpr uint64_t kBuildEpoch = 1;
+const String kServerRoot = "00";
+
+ManifestRef ref(uint64_t seq, uint64_t ordinal)
+{
+    return ManifestRef{.writer_epoch = kBuildEpoch, .build_sequence = seq,
+                       .manifest_ordinal = static_cast<uint32_t>(ordinal)};
+}
+
+BuildPrefix buildPrefix(uint64_t seq)
+{
+    return BuildPrefix{.writer_epoch = kBuildEpoch, .build_sequence = seq};
+}
+
+/// A pool with ONE eligible-but-unowned manifest body under build sequence 5: the shape the sweep is
+/// meant to reclaim, so that every test below differs only in the durable fold state.
+struct OrphanFixture
+{
+    std::shared_ptr<InMemoryBackend> backend = std::make_shared<InMemoryBackend>();
+    PoolPtr store;
+    RootNamespace ns{"00/aa@cas@"};
+    ManifestRef orphan = ref(5, 0xAB);
+
+    OrphanFixture()
+    {
+        store = openPoolForTest(backend);
+        /// This fixture has no ref transaction, but it is a normal empty catalog life rather than the
+        /// deliberate missing-checkpoint corruption shape. State that empty recovery frontier before
+        /// exercising the independent sweep-deletion premise.
+        casAdmitRecoverableEntry(*backend, store->layout(), ns);
+        writeManifestRaw(*backend, store->layout(), ns, orphan, {blobEntryFor("a", DB::UInt128(1))});
+        /// min_active 6 > build_sequence 5: the durable watermark fact makes the prefix ELIGIBLE, which
+        /// is the half the premise sits on top of.
+        setWatermarkMinActive(*backend, store->layout(), kServerRoot, kBuildEpoch, /*min_active*/6);
+    }
+
+    String orphanKey() const { return store->layout().manifestKey(ManifestId{ns, orphan}); }
+    bool orphanExists() const { return backend->head(orphanKey()).exists; }
+};
+
+}
+
+/// Rule (1), the load-bearing case. The cursor is still INSIDE the build's own epoch, so epoch 1's
+/// closing seal is not proven consumed and an unfolded `+1` naming this build may still exist above the
+/// cursor. The body survives, and the sweep says so through its `warnings` out-param.
+TEST(CASSweepDeletionPremise, AnUnconsumedEpochSealRetainsTheBuildsManifests)
+{
+    OrphanFixture f;
+    seedFoldCursorForTest(*f.backend, f.store->layout(), f.ns, RefTxnId{kBuildEpoch, 3});
+
+    std::vector<String> warnings;
+    const uint64_t deleted = sweepNamespace(*f.store, f.ns, buildPrefix(5), &warnings);
+
+    EXPECT_EQ(deleted, 0u);
+    EXPECT_TRUE(f.orphanExists())
+        << "the cursor sits at {1,3}, inside the build's own epoch -- epoch 1's closing seal is not "
+           "consumed, so a grant naming this build may still be unfolded above the cursor";
+    ASSERT_EQ(warnings.size(), 1u) << "a retained manifest is a visible decision, not a silent one";
+    EXPECT_NE(warnings[0].find(f.orphanKey()), String::npos);
+    EXPECT_NE(warnings[0].find("seal"), String::npos);
+}
+
+/// Rule (1) satisfied and the tail clean: the ordinary reclaim still happens, by exact token.
+TEST(CASSweepDeletionPremise, AConsumedEpochSealWithACleanTailDeletes)
+{
+    OrphanFixture f;
+    /// A cursor at `{2, 1}` is in an epoch strictly above the build's. An epoch is left ONLY over its
+    /// consumed `EpochSeal`, so this cursor is durable proof that every epoch-1 record is folded.
+    seedFoldCursorForTest(*f.backend, f.store->layout(), f.ns, RefTxnId{kBuildEpoch + 1, 1});
+
+    std::vector<String> warnings;
+    const uint64_t deleted = sweepNamespace(*f.store, f.ns, buildPrefix(5), &warnings);
+
+    EXPECT_EQ(deleted, 1u);
+    EXPECT_FALSE(f.orphanExists());
+    EXPECT_TRUE(warnings.empty()) << "nothing was retained, so nothing is warned about";
+}
+
+/// Uncertainty rule, hold arm. The cursor HAS consumed epoch 1's seal, so rule (1) alone would let the
+/// body go -- but the namespace is held, which means the fold could not account for everything at or
+/// above the held position. A held namespace retains everything under it.
+TEST(CASSweepDeletionPremise, AHeldNamespaceRetainsEvenAboveAConsumedSeal)
+{
+    OrphanFixture f;
+    const RefHold hold{.reason = HoldReason::GapBelowWitness,
+                       .offending_position = RefTxnId{kBuildEpoch + 1, 4},
+                       .retry_count = 2, .next_retry_round = 9};
+    seedFoldCursorForTest(*f.backend, f.store->layout(), f.ns, RefTxnId{kBuildEpoch + 1, 3}, hold);
+
+    std::vector<String> warnings;
+    const uint64_t deleted = sweepNamespace(*f.store, f.ns, buildPrefix(5), &warnings);
+
+    EXPECT_EQ(deleted, 0u);
+    EXPECT_TRUE(f.orphanExists());
+    ASSERT_EQ(warnings.size(), 1u);
+    EXPECT_NE(warnings[0].find("held"), String::npos);
+    EXPECT_NE(warnings[0].find(String{holdReasonToWord(HoldReason::GapBelowWitness)}), String::npos)
+        << "the retain reason names WHAT stopped the namespace, not just that something did";
+}
+
+/// Uncertainty rule, unreached-frontier arm in its most complete form: the adopted seal carries no row
+/// for this namespace at all, so no round has ever sealed a cursor for it and nothing about its ref
+/// stream is proven. This is also the state of a pool whose GC has never run.
+TEST(CASSweepDeletionPremise, ANamespaceWithNoSealedCursorRetains)
+{
+    OrphanFixture f;
+    /// A seal exists and is adopted, but it covers a DIFFERENT namespace.
+    seedFoldCursorForTest(*f.backend, f.store->layout(), RootNamespace{"00/zz@cas@"},
+                          RefTxnId{kBuildEpoch + 1, 1});
+
+    std::vector<String> warnings;
+    const uint64_t deleted = sweepNamespace(*f.store, f.ns, buildPrefix(5), &warnings);
+
+    EXPECT_EQ(deleted, 0u);
+    EXPECT_TRUE(f.orphanExists());
+    ASSERT_EQ(warnings.size(), 1u);
+    EXPECT_NE(warnings[0].find("coverage"), String::npos);
+}
+
+/// Rule (2). Removals cross epochs, so a record in a LATER epoch can name an earlier epoch's build as a
+/// removal target; deleting the body before that `-1` folds clamps the fold forever on the missing body.
+/// The predicate is exercised directly here because the sweep's own protection view already spares a
+/// listed tail removal before the premise is ever consulted -- the point of the rule is that the SAME
+/// answer is reached by the predicate both paths share, so neither path can lose it.
+TEST(CASSweepDeletionPremise, AnUnconsumedTailRemovalRetainsItsTarget)
+{
+    OrphanFixture f;
+    const String key = f.orphanKey();
+
+    NamespaceFoldView view;
+    RefCoverage cov;
+    cov.classification = 2;
+    cov.last_folded_ref_id = RefTxnId{kBuildEpoch + 1, 1};   /// rule (1) satisfied
+    view.coverage = cov;
+    view.tail_removal_targets.insert(key);
+
+    String reason;
+    EXPECT_FALSE(manifestDeletionPremise(view, ManifestKey{key, buildPrefix(5)}, &reason));
+    EXPECT_NE(reason.find("removal"), String::npos);
+
+    /// The same view without the removal target admits the deletion, so the retention above is the
+    /// removal target's doing and nothing else's.
+    view.tail_removal_targets.clear();
+    reason.clear();
+    EXPECT_TRUE(manifestDeletionPremise(view, ManifestKey{key, buildPrefix(5)}, &reason));
+    EXPECT_TRUE(reason.empty());
+}
+
+/// Both sweep paths call the ONE predicate: the cursor-paced page must refuse the same body the
+/// per-namespace sweep refuses, for the same reason.
+TEST(CASSweepDeletionPremise, TheCursorPagePathHonoursTheSamePremise)
+{
+    OrphanFixture f;
+    seedFoldCursorForTest(*f.backend, f.store->layout(), f.ns, RefTxnId{kBuildEpoch, 3});
+
+    const ManifestSweepResult held = sweepManifestCursorPageForTest(*f.store, "", /*list_budget*/100, /*delete_budget*/10);
+    EXPECT_EQ(held.deleted, 0u);
+    EXPECT_GE(held.skipped, 1u);
+    EXPECT_TRUE(f.orphanExists());
+
+    /// Consume the seal and the very same page deletes it.
+    seedFoldCursorForTest(*f.backend, f.store->layout(), f.ns, RefTxnId{kBuildEpoch + 1, 1});
+    const ManifestSweepResult freed = sweepManifestCursorPageForTest(*f.store, "", /*list_budget*/100, /*delete_budget*/10);
+    EXPECT_EQ(freed.deleted, 1u);
+    EXPECT_FALSE(f.orphanExists());
+}
+
+/// WHAT THE PREMISE COSTS, pinned so it is a stated behaviour rather than something a later reader
+/// discovers. The pure pre-precommit orphan -- a manifest body staged by a writer that crashed before
+/// appending any ref record for it -- lives under a namespace whose ref stream may not exist at all.
+/// Such a namespace never enters the fold's universe, so no round ever seals a cursor for it, so no
+/// epoch's closing seal is ever consumed for it, so the premise retains its debris INDEFINITELY. That
+/// is the safe direction and it is deliberate, but it is not "delay": reclaiming this class needs the
+/// sweep's own rework (registers R2/R3, Stage B) -- the writer duty queue that knows what it staged, and
+/// the nomination path. The premise ships as the safety floor, not as the reclaim policy.
+TEST(CASSweepDeletionPremise, DebrisUnderANamespaceTheFoldNeverWalksIsRetainedIndefinitely)
+{
+    OrphanFixture f;
+    /// No ref stream, no coverage row -- repeated passes change nothing.
+    std::vector<String> warnings;
+    for (int pass = 0; pass < 3; ++pass)
+        EXPECT_EQ(sweepNamespace(*f.store, f.ns, buildPrefix(5), &warnings), 0u) << "pass " << pass;
+
+    EXPECT_TRUE(f.orphanExists());
+    EXPECT_EQ(warnings.size(), 3u) << "every pass reports the retention rather than going quiet";
+}
+
+/// RETENTION IS VISIBLE ON THE PATH THAT ACTUALLY SWEEPS. `planManifestCursorPage` has no `warnings`
+/// out-param -- the background sweep answers to nobody but its phase row -- so the premise's refusals
+/// have to leave the process as COUNTERS or not at all. In Stage A that is nearly the whole story of
+/// the sweep, because rule (1) is satisfiable only for a closed-and-folded epoch.
+///
+/// Non-vacuous by construction: two namespaces on ONE page are retained for DIFFERENT reasons, so a
+/// counter wired to the wrong class, or one bucket catching everything, changes the answer. A
+/// single-reason page would pass against a single mislabelled counter.
+TEST(CASSweepDeletionPremise, DistinctRetainReasonsLandInDistinctCounters)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+
+    /// Namespace A: a cursor still INSIDE the build's epoch -> rule (1), `unconsumed_seal`.
+    const RootNamespace ns_a{"00/aa@cas@"};
+    const ManifestRef ref_a = ref(5, 0xA1);
+    casAdmitRecoverableEntry(*backend, layout, ns_a);
+    writeManifestRaw(*backend, layout, ns_a, ref_a, {blobEntryFor("a", DB::UInt128(1))});
+    seedFoldCursorForTest(*backend, layout, ns_a, RefTxnId{kBuildEpoch, 3});
+
+    /// Namespace B: a HELD row whose cursor is well above the build's epoch, so rule (1) is satisfied
+    /// and the hold is demonstrably what retained it.
+    const RootNamespace ns_b{"00/bb@cas@"};
+    const ManifestRef ref_b = ref(5, 0xB1);
+    casAdmitRecoverableEntry(*backend, layout, ns_b);
+    writeManifestRaw(*backend, layout, ns_b, ref_b, {blobEntryFor("b", DB::UInt128(2))});
+    const RefHold hold{.reason = HoldReason::BodyUndecodable,
+                       .offending_position = RefTxnId{kBuildEpoch + 1, 9},
+                       .retry_count = 1, .next_retry_round = 4};
+    seedFoldCursorForTest(*backend, layout, ns_b, RefTxnId{kBuildEpoch + 1, 8}, hold);
+
+    setWatermarkMinActive(*backend, layout, kServerRoot, kBuildEpoch, /*min_active*/6);
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget*/100, /*delete_budget*/10);
+
+    EXPECT_EQ(result.deleted, 0u);
+    EXPECT_EQ(result.retained_unconsumed_seal, 1u) << "namespace A's cursor is inside its build's epoch";
+    EXPECT_EQ(result.retained_hold, 1u) << "namespace B is held";
+    EXPECT_EQ(result.retained_no_coverage, 0u);
+    EXPECT_EQ(result.retained_tail_removal, 0u);
+    EXPECT_GE(result.skipped, 2u) << "both retentions are also ordinary skips";
+
+    /// The rollup an operator reads. The two classes tie at one each and the tie resolves by enum
+    /// order, which is what keeps an unchanged pool reporting an unchanged verdict pass after pass.
+    const auto top = result.topRetainReason();
+    EXPECT_EQ(top.second, 1u);
+    EXPECT_EQ(top.first, SweepRetainClass::Hold);
+    EXPECT_EQ(String{sweepRetainClassName(SweepRetainClass::UnconsumedSeal)}, "unconsumed_seal");
+
+    /// A page with no candidates reports nothing: the counters carry the premise's own refusals, not
+    /// ordinary skips.
+    auto empty_backend = std::make_shared<InMemoryBackend>();
+    auto empty_store = openPoolForTest(empty_backend);
+    const ManifestSweepResult nothing =
+        sweepManifestCursorPageForTest(*empty_store, "", /*list_budget*/100, /*delete_budget*/10);
+    EXPECT_EQ(nothing.topRetainReason().first, SweepRetainClass::None);
+    EXPECT_EQ(nothing.topRetainReason().second, 0u);
+}
+
+/// STAGE B SEAM (registers R2/R3). The premise is the per-manifest SAFETY floor and nothing else: it
+/// says when a body may go, never who nominates it or when. The sweep's own rework -- the writer duty
+/// queue that reclaims its own live epoch's debris, and the nomination path -- attaches here, and must
+/// satisfy this predicate rather than replace it.
+
+/// Uncertainty rule, budget arm. A candidate the page never DECIDED on -- the delete budget ran out
+/// before it -- is retained, and the cursor must not step over it: the sweep's cursor is a
+/// cleanup-progress hint whose skipped range is not revisited until a full wrap, so advancing past an
+/// undecided candidate converts "retained this round" into "unexamined for a whole cycle".
+TEST(CASSweepDeletionPremise, AnExhaustedDeleteBudgetRetainsAndDoesNotStepOverTheRest)
+{
+    OrphanFixture f;
+    const ManifestRef second = ref(5, 0xAC);
+    const ManifestRef third = ref(5, 0xAD);
+    writeManifestRaw(*f.backend, f.store->layout(), f.ns, second, {blobEntryFor("b", DB::UInt128(2))});
+    writeManifestRaw(*f.backend, f.store->layout(), f.ns, third, {blobEntryFor("c", DB::UInt128(3))});
+    seedFoldCursorForTest(*f.backend, f.store->layout(), f.ns, RefTxnId{kBuildEpoch + 1, 1});
+
+    const ManifestSweepResult first = sweepManifestCursorPageForTest(*f.store, "", /*list_budget*/100, /*delete_budget*/1);
+    EXPECT_EQ(first.deleted, 1u);
+    EXPECT_FALSE(first.wrapped)
+        << "the page stopped on an exhausted budget with candidates left, so it did not reach the end";
+    ASSERT_FALSE(first.next_cursor.empty());
+
+    /// Resume: the two survivors are still ahead of the cursor, so a budgeted continuation reaches them.
+    const ManifestSweepResult second_page =
+        sweepManifestCursorPageForTest(*f.store, first.next_cursor, /*list_budget*/100, /*delete_budget*/10);
+    EXPECT_EQ(second_page.deleted, 2u)
+        << "the cursor must not have stepped over the candidates the exhausted budget left undecided";
+
+    size_t surviving = 0;
+    for (const ManifestRef & r : {f.orphan, second, third})
+        if (f.backend->head(f.store->layout().manifestKey(ManifestId{f.ns, r})).exists)
+            ++surviving;
+    EXPECT_EQ(surviving, 0u);
+}
+
+/// MANDATORY liveness proof: a namespace whose committed-tail recovery walk
+/// can never finish within one round's `sweep_recovery_op_budget` must not wedge the cursor page for
+/// every subsequent round. Six eligible candidates share ONE namespace whose tail is ~200 unrelated
+/// committed transactions above the fold cursor -- far more than the tiny per-round recovery-op budget
+/// can traverse -- so `activeManifestKeys` reports `recovery_incomplete` on every attempt, every one of
+/// this namespace's candidates is retained (never nominated, never deleted), yet the page still DECIDES
+/// them (a retained candidate is a decision) and the cursor advances across pages until the whole
+/// keyspace is covered.
+TEST(CASSweepDeletionPremise, RecoveryWorkBudgetRetainsAndConvergesWithoutWedgingTheCursor)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+    const RootNamespace ns{"00/aa@cas@"};
+    /// `casAdmitEntry` (bare, no `_ckpt`) rather than `casAdmitRecoverableEntry` (which pre-seeds an
+    /// EMPTY `_ckpt`, `committed_through = nullopt`): the tail below is built entirely from real
+    /// `publishCommittedTransition` calls, whose first call needs `readCkpt` to see NOTHING yet so it
+    /// takes the fresh-`_ckpt` `putIfAbsent` path instead of `advanceRecoverableCkptForRawFixture`'s
+    /// monotonic-advance-from-existing-value path (which throws on a null `committed_through`).
+    casAdmitEntry(*backend, layout, ns);
+    setWatermarkMinActive(*backend, layout, kServerRoot, kBuildEpoch, /*min_active*/1000);
+
+    /// Six orphan candidates, all eligible (build_sequence << min_active), none owned by any ref.
+    constexpr int kCandidates = 6;
+    for (int i = 1; i <= kCandidates; ++i)
+        writeManifestRaw(*backend, layout, ns, ref(i, 1),
+                         {blobEntryFor("c" + std::to_string(i), DB::UInt128(static_cast<uint64_t>(i)))});
+
+    /// A committed tail of ~200 UNRELATED transactions above the fold cursor. None of these need a
+    /// manifest body of their own -- the recovery walk only GETs and decodes the ref-log transactions,
+    /// never the bodies they name.
+    constexpr int kTailSize = 200;
+    for (int i = 0; i < kTailSize; ++i)
+        publishCommittedTransition(*backend, layout, ns, "tail" + std::to_string(i), std::nullopt, ref(2000 + i, 1));
+    seedFoldCursorForTest(*backend, layout, ns, RefTxnId{kBuildEpoch, 1});
+
+    uint64_t total_retained_work_budget = 0;
+    uint64_t total_skipped = 0;
+    uint64_t total_deleted = 0;
+    String cursor;
+    bool wrapped = false;
+    int pages = 0;
+    for (; pages < 10 && !wrapped; ++pages)
+    {
+        /// A FRESH budget every page, exactly like production's one-instance-per-round contract --
+        /// the same namespace's recovery walk re-attempts and re-exhausts every time, by design (a
+        /// pathological namespace does not get to starve every OTHER page of budget forever).
+        GcRoundWorkBudget budget;
+        budget.max_sweep_recovery_ops = 5;   /// far below the ~200-record tail
+        const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, cursor, /*list_budget*/3, /*delete_budget*/10, &budget);
+
+        /// LIVENESS: every page decides at least one candidate (a retained one counts) or wraps.
+        EXPECT_TRUE(result.skipped > 0 || result.deleted > 0 || result.wrapped)
+            << "page " << pages << " decided nothing and did not wrap -- a wedge";
+        /// Every namespace this page touches hits the recovery-op-exhausted cause AT LEAST once --
+        /// only the FIRST candidate of an errored namespace on a page carries the specific retain-class
+        /// counter (the SAME pre-existing convention `retained_no_coverage`/`retained_hold` already
+        /// use); every other candidate of that namespace still lands in the generic `skipped` tally.
+        EXPECT_GE(result.retained_work_budget, 1u)
+            << "page " << pages << " never attributed a candidate to the recovery-budget cause";
+
+        total_retained_work_budget += result.retained_work_budget;
+        total_skipped += result.skipped;
+        total_deleted += result.deleted;
+        wrapped = result.wrapped;
+        ASSERT_NE(cursor, result.next_cursor) << "page " << pages << " made no cursor progress";
+        cursor = result.next_cursor;
+    }
+
+    EXPECT_TRUE(wrapped) << "the whole small keyspace must be fully covered well within 10 pages";
+    EXPECT_EQ(total_deleted, 0u) << "the pathological namespace's candidates are never safe to nominate";
+    EXPECT_EQ(total_skipped, static_cast<uint64_t>(kCandidates))
+        << "every one of the six candidates was decided (skipped), none silently dropped from the page";
+    EXPECT_GE(total_retained_work_budget, 1u);
+    for (int i = 1; i <= kCandidates; ++i)
+        EXPECT_TRUE(backend->head(layout.manifestKey(ManifestId{ns, ref(i, 1)})).exists)
+            << "candidate " << i << " must survive: it was never proven safe to delete";
+}
+
+/// The per-page NAMESPACE cap. Two otherwise-independently-deletable
+/// namespaces share one page; with `max_sweep_namespaces = 1`, only the first namespace this page
+/// touches gets a protection view built at all -- the second is retained under the work-budget cause,
+/// never given a partial or best-effort view.
+TEST(CASSweepDeletionPremise, NamespaceWorkBudgetCapsDistinctViewsPerPage)
+{
+    auto backend = std::make_shared<InMemoryBackend>();
+    auto store = openPoolForTest(backend);
+    const Layout & layout = store->layout();
+
+    const RootNamespace ns_a{"00/aa@cas@"};
+    const RootNamespace ns_b{"00/bb@cas@"};
+    const ManifestRef ref_a = ref(5, 0xA1);
+    const ManifestRef ref_b = ref(5, 0xB1);
+    casAdmitRecoverableEntry(*backend, layout, ns_a);
+    casAdmitRecoverableEntry(*backend, layout, ns_b);
+    writeManifestRaw(*backend, layout, ns_a, ref_a, {blobEntryFor("a", DB::UInt128(1))});
+    writeManifestRaw(*backend, layout, ns_b, ref_b, {blobEntryFor("b", DB::UInt128(2))});
+    /// Both namespaces satisfy rule (1) (cursor past the build's own epoch) and have no committed tail
+    /// at all (`_ckpt.committed_through` unset), so absent the namespace cap BOTH would delete.
+    seedFoldCursorForTest(*backend, layout, ns_a, RefTxnId{kBuildEpoch + 1, 1});
+    seedFoldCursorForTest(*backend, layout, ns_b, RefTxnId{kBuildEpoch + 1, 1});
+    setWatermarkMinActive(*backend, layout, kServerRoot, kBuildEpoch, /*min_active*/6);
+
+    GcRoundWorkBudget budget;
+    budget.max_sweep_namespaces = 1;
+
+    const ManifestSweepResult result = sweepManifestCursorPageForTest(*store, "", /*list_budget*/100, /*delete_budget*/10, &budget);
+
+    EXPECT_EQ(result.deleted, 1u) << "exactly one namespace's view could be built this page";
+    EXPECT_EQ(result.retained_work_budget, 1u)
+        << "the other namespace's candidate is retained, never decided from a missing view";
+    EXPECT_EQ(budget.sweep_namespaces_used, 1u);
+
+    size_t surviving = 0;
+    for (const auto & p : std::vector<std::pair<RootNamespace, ManifestRef>>{{ns_a, ref_a}, {ns_b, ref_b}})
+        if (backend->head(layout.manifestKey(ManifestId{p.first, p.second})).exists)
+            ++surviving;
+    EXPECT_EQ(surviving, 1u) << "exactly one candidate remains -- the one whose namespace had no budget left";
+}
diff --git a/src/Disks/tests/gtest_cas_text_format.cpp b/src/Disks/tests/gtest_cas_text_format.cpp
new file mode 100644
index 000000000000..4371afb3f9e8
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_text_format.cpp
@@ -0,0 +1,292 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Common/Exception.h>
+#include <IO/ReadBufferFromMemory.h>
+#include <fmt/format.h>
+#include <algorithm>
+#include <set>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes
+{
+    extern const int CORRUPTED_DATA;
+    extern const int UNKNOWN_FORMAT_VERSION;
+}
+
+namespace
+{
+/// Run `f` and require a DB::Exception with exactly `code`.
+template <typename F>
+void expectCode(int code, F && f)
+{
+    try
+    {
+        f();
+        FAIL() << "expected exception code " << code;
+    }
+    catch (const DB::Exception & e)
+    {
+        EXPECT_EQ(e.code(), code);
+    }
+}
+}
+
+/// ---- Task 2: FormatId entries for refsnaplog / blob meta / heartbeat ----
+
+TEST(CASFormatIds, NewIdsExistWithFrozenValues)
+{
+    EXPECT_EQ(static_cast<uint16_t>(FormatId::RefLog), 19);
+    EXPECT_EQ(static_cast<uint16_t>(FormatId::RefSnapshot), 20);
+    EXPECT_EQ(static_cast<uint16_t>(FormatId::BlobMeta), 21);
+    EXPECT_EQ(static_cast<uint16_t>(FormatId::GcHeartbeat), 22);
+    /// Every id, old and new, has a change-point ladder (BASELINE until a real bump).
+    for (auto id : {FormatId::RefLog, FormatId::RefSnapshot, FormatId::BlobMeta, FormatId::GcHeartbeat})
+        EXPECT_FALSE(changePoints(id).empty());
+}
+
+/// ---- Task 3: per-format traits registry ----
+
+TEST(CASFormatTraits, CompleteUniqueAndGated)
+{
+    /// Completeness: every FormatId except the reserved Roster has traits.
+    const FormatId all[] = {FormatId::Blob, FormatId::GcState, FormatId::PoolMeta,
+                            FormatId::GcOutcomes, FormatId::PartManifest, FormatId::RunFile,
+                            FormatId::FoldSeal, FormatId::Owner, FormatId::ServerEpoch, FormatId::MountLease,
+                            FormatId::RefLog, FormatId::RefSnapshot, FormatId::BlobMeta, FormatId::GcHeartbeat,
+                            FormatId::RefCkpt, FormatId::RefCatalog, FormatId::GcMaintenanceState};
+    std::set<std::string_view> types;
+    for (FormatId id : all)
+    {
+        const FormatTraits & t = traitsFor(id);
+        EXPECT_EQ(t.id, id);
+        EXPECT_TRUE(t.type.starts_with("cas_")) << t.type;
+        EXPECT_TRUE(types.insert(t.type).second) << "duplicate type " << t.type;
+        EXPECT_EQ(traitsForType(t.type), &t);
+    }
+    EXPECT_EQ(traitsForType("cas_nope"), nullptr);
+#ifndef DEBUG_OR_SANITIZER_BUILD
+    /// traitsFor(Roster) throws LOGICAL_ERROR (a reserved/unreachable FormatId), which aborts the
+    /// whole process in debug/sanitizer builds instead of behaving like a catchable exception --
+    /// CASFormatTraitsDeathTest below proves the abort positively in those builds instead.
+    EXPECT_THROW(traitsFor(FormatId::Roster), DB::Exception);
+#endif
+    /// Deterministic formats are pinned raw + strict; spot-check the two.
+    EXPECT_EQ(traitsFor(FormatId::RunFile).compression, CompressionPolicy::PinnedRaw);
+    EXPECT_EQ(traitsFor(FormatId::RunFile).strictness, KeyStrictness::Strict);
+    EXPECT_EQ(traitsFor(FormatId::FoldSeal).compression, CompressionPolicy::PinnedRaw);
+    EXPECT_EQ(traitsFor(FormatId::FoldSeal).strictness, KeyStrictness::Strict);
+    /// .zst key suffix is exactly the Always set (can-grow-large types).
+    EXPECT_EQ(storedSuffix(FormatId::RefSnapshot), ".zst");
+    EXPECT_EQ(storedSuffix(FormatId::RefLog), ".zst");
+    EXPECT_EQ(storedSuffix(FormatId::PartManifest), ".zst");
+    EXPECT_EQ(storedSuffix(FormatId::GcOutcomes), ".zst");
+    EXPECT_EQ(storedSuffix(FormatId::PoolMeta), "");
+    EXPECT_EQ(storedSuffix(FormatId::FoldSeal), "");
+    EXPECT_EQ(storedSuffix(FormatId::RunFile), "");
+}
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+/// Debug/sanitizer-build counterpart to CompleteUniqueAndGated's Roster check: LOGICAL_ERROR aborts
+/// the process here instead of throwing a catchable exception, so the check must be a death test
+/// (same pattern as CASBlobDigestDeathTest in gtest_cas_blob_digest.cpp).
+TEST(CASFormatTraitsDeathTest, TraitsForRosterAborts)
+{
+    EXPECT_DEATH({ (void)traitsFor(FormatId::Roster); }, "");
+}
+#endif
+
+/// ---- Task 4: JSON micro-vocabulary + JsonObjectReader ----
+
+TEST(CASJsonVocab, WriteAndReadBack)
+{
+    CasJsonWriter out;
+    bool first = true;
+    writeKey(out, "tag", first);
+    writeHex128Value(out, hexToU128("000102030405060708090a0b0c0d0e0f"));
+    writeKey(out, "seq", first);
+    writeU64StringValue(out, 18446744073709551615ULL);
+    writeKey(out, "n", first);
+    writeIntText(7, out);
+    writeKey(out, "ref", first);
+    writeStringValue(out, "t-1/all_1_2_0\n\"quoted\"");
+    closeObject(out, first);
+    const String rendered = std::move(out).take();
+    EXPECT_EQ(rendered.substr(0, 45), R"({"tag":"000102030405060708090a0b0c0d0e0f","se)");
+
+    DB::ReadBufferFromMemory in(rendered.data(), rendered.size());
+    JsonObjectReader r(in, KeyStrictness::Strict, "test");
+    String key;
+    ASSERT_TRUE(r.nextKey(key)); EXPECT_EQ(key, "tag");
+    EXPECT_EQ(r.readHex128(), hexToU128("000102030405060708090a0b0c0d0e0f"));
+    ASSERT_TRUE(r.nextKey(key)); EXPECT_EQ(key, "seq");
+    EXPECT_EQ(r.readU64String(), 18446744073709551615ULL);
+    ASSERT_TRUE(r.nextKey(key)); EXPECT_EQ(key, "n");
+    EXPECT_EQ(r.readU64Number(), 7u);
+    ASSERT_TRUE(r.nextKey(key)); EXPECT_EQ(key, "ref");
+    EXPECT_EQ(r.readString(), "t-1/all_1_2_0\n\"quoted\"");
+    EXPECT_FALSE(r.nextKey(key));
+}
+
+TEST(CASJsonVocab, FailClosedRules)
+{
+    auto reader = [](std::string_view text, KeyStrictness s, auto && consume)
+    {
+        DB::ReadBufferFromMemory in(text.data(), text.size());
+        JsonObjectReader r(in, s, "test");
+        consume(r);
+    };
+    /// duplicate key
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { reader(R"({"a":1,"a":2})", KeyStrictness::Tolerant, [](auto & r)
+    {
+        String k;
+        while (r.nextKey(k)) r.readU64Number();
+    }); });
+    /// unknown key: Tolerant skips (nested value), Strict rejects
+    reader(R"({"zz":{"deep":[1,2]},"n":5})", KeyStrictness::Tolerant, [](auto & r)
+    {
+        String k;
+        ASSERT_TRUE(r.nextKey(k)); r.skipUnknown(k);
+        ASSERT_TRUE(r.nextKey(k)); EXPECT_EQ(r.readU64Number(), 5u);
+        EXPECT_FALSE(r.nextKey(k));
+    });
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { reader(R"({"zz":1})", KeyStrictness::Strict, [](auto & r)
+    {
+        String k;
+        ASSERT_TRUE(r.nextKey(k)); r.skipUnknown(k);
+    }); });
+    /// critical key fails closed regardless of strictness
+    expectCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION, [&] { reader(R"({"!x":1})", KeyStrictness::Tolerant, [](auto & r)
+    {
+        String k;
+        ASSERT_TRUE(r.nextKey(k)); r.skipUnknown(k);
+    }); });
+    /// whitespace is not canonical
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { reader(R"({ "a":1})", KeyStrictness::Tolerant, [](auto & r)
+    {
+        String k;
+        r.nextKey(k);
+    }); });
+    /// bad hex width / junk in u64 string
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { reader(R"({"h":"0102"})", KeyStrictness::Tolerant, [](auto & r)
+    {
+        String k;
+        r.nextKey(k); r.readHex128();
+    }); });
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { reader(R"({"s":"12x"})", KeyStrictness::Tolerant, [](auto & r)
+    {
+        String k;
+        r.nextKey(k); r.readU64String();
+    }); });
+}
+
+/// ---- Task 5: header line, trailer line, readLine ----
+
+TEST(CASTextHeader, WriteExpectSniffGate)
+{
+    CasJsonWriter out;
+    writeHeaderLine(out, FormatId::PoolMeta);
+    const String rendered = std::move(out).take();
+    EXPECT_EQ(rendered, fmt::format("{{\"type\":\"cas_pool_meta\",\"v\":{}}}\n", currentCompatibilityVersion()));
+
+    DB::ReadBufferFromMemory in(rendered.data(), rendered.size());
+    const TextHeader h = expectHeaderLine(in, FormatId::PoolMeta);
+    EXPECT_EQ(h.type, "cas_pool_meta");
+    EXPECT_EQ(h.v, currentCompatibilityVersion());
+    EXPECT_TRUE(in.eof());
+
+    const auto sniffed = sniffHeaderLine(rendered);
+    ASSERT_TRUE(sniffed.has_value());
+    EXPECT_EQ(sniffed->type, "cas_pool_meta");
+    EXPECT_FALSE(sniffHeaderLine("PAR1 not a cas object").has_value());
+
+    /// wrong type -> CORRUPTED_DATA; future v -> UNKNOWN_FORMAT_VERSION
+    /// `v:3` is deliberate and must NOT follow a future `G_BUILD` bump: any version <= G_BUILD passes
+    /// the header gate, which is the point — the BODY is what has to fail here.
+    const String wrong = "{\"type\":\"cas_owner\",\"v\":3}\n";
+    DB::ReadBufferFromMemory in2(wrong.data(), wrong.size());
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { expectHeaderLine(in2, FormatId::PoolMeta); });
+    const String future = fmt::format("{{\"type\":\"cas_pool_meta\",\"v\":{}}}\n", currentCompatibilityVersion() + 1);
+    DB::ReadBufferFromMemory in3(future.data(), future.size());
+    expectCode(DB::ErrorCodes::UNKNOWN_FORMAT_VERSION, [&] { expectHeaderLine(in3, FormatId::PoolMeta); });
+
+    const String out_of_range = "{\"type\":\"cas_pool_meta\",\"v\":4294967299}\n";
+    DB::ReadBufferFromMemory in4(out_of_range.data(), out_of_range.size());
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { expectHeaderLine(in4, FormatId::PoolMeta); });
+}
+
+TEST(CASTextLines, ReadLineAndTrailer)
+{
+    CasJsonWriter out;
+    writeTrailerLine(out, 42);
+    EXPECT_EQ(std::move(out).take(), "{\"n\":42}\n");
+
+    const String two = "abc\ndef\n";
+    DB::ReadBufferFromMemory in(two.data(), two.size());
+    EXPECT_EQ(readLine(in, 16, "test"), "abc");
+    EXPECT_EQ(readLine(in, 16, "test"), "def");
+    /// missing terminator and over-cap both fail closed
+    const String noterm = "abc";
+    DB::ReadBufferFromMemory in2(noterm.data(), noterm.size());
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { readLine(in2, 16, "test"); });
+    DB::ReadBufferFromMemory in3(two.data(), two.size());
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { readLine(in3, 2, "test"); });
+}
+
+/// ---- Task 6: the zstd arm ----
+
+TEST(CASZstdArm, SealOpenPolicyAndCaps)
+{
+    /// Always types compress regardless of size (no threshold — the .zst key must be
+    /// constructible without knowing the body); a raw body is still readable (repair path).
+    /// `v:3` here is NOT the "any version <= G_BUILD passes" case the other negative bodies rely on:
+    /// `cas_ref_snap`'s own `changePoints` floor is generation 4, so a generation-3 ref snapshot is not
+    /// readable by this build in principle. It passes the header gate only because nothing consults
+    /// `changePoints` at decode time yet -- the gate is `v > G_BUILD` alone. Once a per-class floor is
+    /// wired in, this literal must move to `G_BUILD`; the test's subject is the truncated BODY, not the
+    /// version.
+    const String small = "{\"type\":\"cas_ref_snap\",\"v\":3}\n{}\n";
+    const String sealed_small = sealObject(FormatId::RefSnapshot, small);
+    ASSERT_TRUE(looksZstd(sealed_small));
+    EXPECT_EQ(openObject(FormatId::RefSnapshot, sealed_small), small);
+    EXPECT_EQ(openObject(FormatId::RefSnapshot, small), small);
+
+    String big = "{\"type\":\"cas_ref_snap\",\"v\":3}\n{\"pad\":\"";
+    big += String(8192, 'a');
+    big += "\"}\n";
+    const String sealed = sealObject(FormatId::RefSnapshot, big);
+    ASSERT_TRUE(looksZstd(sealed));
+    EXPECT_LT(sealed.size(), big.size());
+    EXPECT_EQ(openObject(FormatId::RefSnapshot, sealed), big);
+
+    /// Never and PinnedRaw formats never compress on write and reject compressed input on read.
+    EXPECT_EQ(sealObject(FormatId::FoldSeal, big), big);
+    EXPECT_EQ(sealObject(FormatId::PoolMeta, big), big);
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { openObject(FormatId::FoldSeal, sealed); });
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { openObject(FormatId::PoolMeta, sealed); });
+
+    /// Declared content size over the cap fails BEFORE the output allocation: 65 MiB of text
+    /// against RefSnapshot's 64 MiB cap (compresses to ~nothing, so the test is cheap on disk
+    /// bytes; the 65 MiB source string is the only big allocation).
+    const String over(65 * 1024 * 1024, 'b');
+    const String sealed_over = sealObject(FormatId::RefSnapshot, over);
+    ASSERT_TRUE(looksZstd(sealed_over));
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { openObject(FormatId::RefSnapshot, sealed_over); });
+
+    /// A flipped byte inside the frame is caught by zstd (frame checksum is on).
+    String corrupted = sealed;
+    corrupted[corrupted.size() / 2] ^= 0x01;
+    expectCode(DB::ErrorCodes::CORRUPTED_DATA, [&] { openObject(FormatId::RefSnapshot, corrupted); });
+}
+
+TEST(CASTextValueEscaping, ForwardSlashPinnedUnescaped)
+{
+    /// Goes RED if the global escape_forward_slashes default ever leaks back into CAS string values.
+    /// CAS values are dense with '/' (ref-paths, fold-seal keys); their bytes must be CAS-owned so
+    /// cas_fold_seal byte-determinism and every golden text file are independent of the global default.
+    CasJsonWriter out;
+    writeStringValue(out, "ns/shard/all_1_2_0");
+    EXPECT_EQ(std::move(out).take(), "\"ns/shard/all_1_2_0\"");
+}
diff --git a/src/Disks/tests/gtest_cas_truncate_reclaim.cpp b/src/Disks/tests/gtest_cas_truncate_reclaim.cpp
new file mode 100644
index 000000000000..0a76c338cdb5
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_truncate_reclaim.cpp
@@ -0,0 +1,282 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Tools/CasFsck.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <string>
+#include <vector>
+
+/// B140 regression guard. The soak's Phase-1 sync run did a `TRUNCATE TABLE` at op 450 and then
+/// observed fsck `unreachable` STUCK above zero (1751) while the incremental GC reported
+/// `candidates=0` — i.e. the GC believed it was done while orphaned blobs remained. This file
+/// reproduces the soak shape at the CORE level (no server, no docker): publish many parts that
+/// SHARE blobs (dedup), interleave regular GC rounds with the publishes (so trees get expanded
+/// into the durable snap exactly as they would during a steady-state insert workload), then
+/// perform the SAME removal a Replicated TRUNCATE issues — a per-ref `dropRef` for every part —
+/// and drive the GC to a fixpoint. The invariant under test: after the drops are folded and the
+/// cascade runs, `runFsck().unreachable` reaches 0 (every shared blob is reclaimed).
+///
+/// A `dropNamespace` variant is included as well (the path `removeRecursive` takes for a whole
+/// table dir, e.g. DROP TABLE): it journals one Remove per former ref, so the cascade should fold
+/// it identically.
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+
+namespace
+{
+
+PoolPtr openTestPool(std::shared_ptr<InMemoryBackend> & out_backend)
+{
+    out_backend = std::make_shared<InMemoryBackend>();
+    return Pool::open(out_backend, PoolConfig{.pool_prefix = "p", .server_root_id = "test"});
+}
+
+/// Publish one part `ref` with TWO content files whose payloads are passed in. Identical payloads
+/// across parts dedup to the SAME blob object (the soak's dedup_ratio ~3.8 comes from exactly this
+/// sharing). Returns the manifest id.
+ManifestId publishPart2(
+    const PoolPtr & s, const String & ns, const String & ref,
+    const String & payload_a, const String & payload_b)
+{
+    const RootNamespace nsr{ns};
+    PartWriteInfo info;
+    info.intended_ref = ns + "/" + ref;
+    auto build = s->beginPartWrite(info);
+
+    ManifestEntry ea;
+    ea.path = "data.bin";
+    ea.placement = EntryPlacement::Blob;
+    ea.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload_a))};
+
+    ea.blob_size = payload_a.size();
+
+    ManifestEntry eb;
+    eb.path = "data.cmrk3";
+    eb.placement = EntryPlacement::Blob;
+    eb.ref = DB::Cas::BlobRef{DB::Cas::BlobHashAlgo::CityHash128, DB::Cas::BlobDigest::fromU128(u128Of(payload_b))};
+
+    eb.blob_size = payload_b.size();
+
+    /// Wiring order (EDGE-BEFORE-OBSERVE): stageManifest -> precommitAdd -> putBlob -> promote.
+    const ManifestId id = build->stageManifest({ea, eb});
+    build->precommitAdd(nsr, ref, id);
+    build->putBlob(idOf(payload_a), BlobSource::fromString(payload_a));
+    build->putBlob(idOf(payload_b), BlobSource::fromString(payload_b));
+    build->promote(nsr, ref, build->buildId(), id);
+    return id;
+}
+
+/// Whether the CURRENT retired list (any gc-shard) still holds an entry — the ack-floor deletion pipeline
+/// (condemn -> graduate -> delete) is in flight while this is true.
+bool anyRetiredPending(const PoolPtr & s)
+{
+    /// Retired-in-snapshot (T4): condemned state rides the adopted fold seal's kCondemned rows, not a
+    /// separate retired list — reconstruct the in-flight set from the seal.
+    return DB::Cas::tests::anyCondemnedInSeal(s->backend(), s->layout());
+}
+
+/// Run regular GC rounds until a fixpoint over the ACK-FLOOR round. A condemned blob is deleted only a
+/// few rounds after its removal folds (condemn -> graduate once the ack floor passes it -> delete), so the
+/// loop advances the store's own mount ack after each round (`renewWatermarkOnce` runs the beat) and stays
+/// alive while ANY work counter is nonzero OR the current retired list still holds an in-flight entry.
+size_t runGcToFixpoint(const PoolPtr & s, Gc & gc, size_t max_rounds = 64)
+{
+    size_t rounds = 0;
+    for (; rounds < max_rounds; ++rounds)
+    {
+        const RoundReport rep = DB::Cas::tests::runRegularRoundReclaiming(gc);
+        if (!rep.acquired_lease)
+            continue;
+        s->renewWatermarkOnce();
+        const bool no_work = rep.candidates == 0 && rep.deleted == 0 && rep.absent == 0
+            && rep.replaced == 0 && rep.spared == 0;
+        if (no_work && !anyRetiredPending(s))
+            break;
+    }
+    return rounds;
+}
+
+}
+
+/// The faithful soak repro: many parts sharing blobs, GC interleaved with the publishes, then a
+/// per-ref drop of EVERY ref (Replicated TRUNCATE), then GC to a fixpoint. fsck.unreachable must
+/// reach 0 — no orphaned blob may survive.
+TEST(CASTruncateReclaim, PerRefDropOfSharedBlobsReclaimsToZero)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    constexpr int N = 32;
+
+    /// Publish N parts. Payloads are chosen so blobs are SHARED across parts: data.bin cycles
+    /// through 8 distinct contents, data.cmrk3 through 4 — heavy dedup, like the soak.
+    std::vector<String> refs;
+    for (int i = 0; i < N; ++i)
+    {
+        const String ref = "all_" + std::to_string(i) + "_" + std::to_string(i) + "_0";
+        refs.push_back(ref);
+        const String pa = "data-" + std::to_string(i % 8);
+        const String pb = "mark-" + std::to_string(i % 4);
+        publishPart2(s, ns.string(), ref, pa, pb);
+
+        /// Interleave a GC round every few publishes, so the live trees get EXPANDED into the
+        /// durable snap during the insert phase (steady-state GC, as in the soak).
+        if (i % 5 == 4)
+        {
+            Gc gc(s, hexToU128("00000000000000000000000000000001"));
+            DB::Cas::tests::runRegularRoundReclaiming(gc);
+        }
+    }
+
+    /// Steady-state GC has nothing to reclaim while the refs are live.
+    {
+        Gc gc(s, hexToU128("00000000000000000000000000000001"));
+        runGcToFixpoint(s, gc);
+        const FsckReport before = runFsck(*s, /*detail=*/false);
+        EXPECT_EQ(before.unreachable, 0u) << "live pool must have no unreachable debris";
+        EXPECT_EQ(before.dangling, 0u);
+        EXPECT_GT(before.reachable, 0u);
+    }
+
+    /// TRUNCATE: a Replicated TRUNCATE removes each part dir, which routes to dropRef per ref.
+    for (const String & ref : refs)
+        s->dropRef(ns, ref);
+
+    /// Every publishing build finished; advance the durable watermark floor past their seqs so the
+    /// Task 10 build-watermark guard no longer spares the now-dropped objects (production does this
+    /// via the background renewer ~2s; here the renewer is off, so drive it explicitly).
+    s->renewWatermarkOnce();
+
+    /// Drive GC to a fixpoint and require full reclamation — this is the B140 assertion.
+    {
+        Gc gc(s, hexToU128("00000000000000000000000000000001"));
+        const size_t rounds = runGcToFixpoint(s, gc);
+        const FsckReport after = runFsck(*s, /*detail=*/false);
+        EXPECT_EQ(after.dangling, 0u) << "TRUNCATE must never lose a reachable object";
+        EXPECT_EQ(after.unreachable, 0u)
+            << "B140: orphaned blobs survived TRUNCATE after " << rounds
+            << " GC rounds (reachable=" << after.reachable
+            << ", unreachable=" << after.unreachable << ")";
+        EXPECT_EQ(after.reachable, 0u) << "no refs remain, so nothing should be reachable";
+    }
+}
+
+/// Mirrors the soak exactly: TRUNCATE at "op 450" (drop every live ref), then CONTINUE inserting
+/// (the soak's ops 451..599 had min_op=451) while the GC keeps running, then a final drive to a
+/// fixpoint. The post-truncate inserts must not stall reclamation of the pre-truncate orphans.
+/// Also asserts a TIGHT bound on the number of rounds reclamation needs (the soak's 180s budget at
+/// gc_interval=30s only buys ~6 rounds, so the core must reach a fixpoint well inside that).
+TEST(CASTruncateReclaim, TruncateThenKeepInsertingStillReclaims)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    /// Pre-truncate generation (the soak's ops < 451).
+    std::vector<String> pre_refs;
+    for (int i = 0; i < 24; ++i)
+    {
+        const String ref = "pre_" + std::to_string(i);
+        pre_refs.push_back(ref);
+        publishPart2(s, ns.string(), ref, "p-data-" + std::to_string(i % 6), "p-mark-" + std::to_string(i % 3));
+        if (i % 5 == 4)
+        {
+            Gc gc(s, hexToU128("00000000000000000000000000000001"));
+            DB::Cas::tests::runRegularRoundReclaiming(gc);
+        }
+    }
+
+    /// TRUNCATE: drop every pre-truncate ref (per-ref dropRef).
+    for (const String & ref : pre_refs)
+        s->dropRef(ns, ref);
+
+    /// Continue inserting AFTER the truncate (the soak's ops 451..599), interleaving GC rounds.
+    for (int i = 0; i < 24; ++i)
+    {
+        publishPart2(s, ns.string(), "post_" + std::to_string(i),
+                     "q-data-" + std::to_string(i % 6), "q-mark-" + std::to_string(i % 3));
+        if (i % 5 == 4)
+        {
+            Gc gc(s, hexToU128("00000000000000000000000000000001"));
+            DB::Cas::tests::runRegularRoundReclaiming(gc);
+        }
+    }
+
+    /// All publishing builds finished; advance the durable watermark floor past their seqs so the
+    /// Task 10 build-watermark guard no longer spares the dropped objects (the background renewer is
+    /// off in this test, so drive it explicitly — production renews ~2s off the write path).
+    s->renewWatermarkOnce();
+
+    /// Drive to a fixpoint. unreachable must reach 0 (the pre-truncate orphans are gone) while the
+    /// post-truncate refs stay reachable.
+    Gc gc(s, hexToU128("00000000000000000000000000000001"));
+    const size_t rounds = runGcToFixpoint(s, gc);
+    const FsckReport after = runFsck(*s, /*detail=*/false);
+    EXPECT_EQ(after.dangling, 0u);
+    EXPECT_EQ(after.unreachable, 0u)
+        << "B140: pre-truncate orphans survived after " << rounds << " GC rounds";
+    EXPECT_GT(after.reachable, 0u) << "post-truncate refs must stay reachable";
+    /// Round bound: the ack-floor pipeline adds a bounded, constant number of rounds over the old
+    /// fold+delete (condemn -> graduate once the ack floor passes -> delete, with the ack kept current
+    /// each round). The dead subgraph still drains in a small, constant number of rounds — not O(orphans).
+    EXPECT_LE(rounds, 8u) << "reclamation took too many rounds (ack-floor pipeline is a small constant)";
+}
+
+/// The DROP TABLE path: removeRecursive of a table dir calls dropNamespace, which journals one
+/// Remove per former ref. Same reclamation invariant.
+TEST(CASTruncateReclaim, DropNamespaceLeavesSharedBlobDebrisForPerpetualSweep)
+{
+    std::shared_ptr<InMemoryBackend> b;
+    auto s = openTestPool(b);
+    const RootNamespace ns{"srv1/tbl"};
+
+    constexpr int N = 32;
+    for (int i = 0; i < N; ++i)
+    {
+        const String ref = "all_" + std::to_string(i) + "_" + std::to_string(i) + "_0";
+        const String pa = "data-" + std::to_string(i % 8);
+        const String pb = "mark-" + std::to_string(i % 4);
+        publishPart2(s, ns.string(), ref, pa, pb);
+        if (i % 5 == 4)
+        {
+            Gc gc(s, hexToU128("00000000000000000000000000000001"));
+            DB::Cas::tests::runRegularRoundReclaiming(gc);
+        }
+    }
+
+    {
+        Gc gc(s, hexToU128("00000000000000000000000000000001"));
+        runGcToFixpoint(s, gc);
+    }
+
+    /// DROP TABLE: the whole namespace is tombstoned at once (one Remove per ref in the journal).
+    s->dropNamespace(ns);
+
+    /// Every publishing build finished; advance the durable watermark floor past their seqs so the
+    /// Task 10 build-watermark guard no longer spares the dropped objects (renewer off here).
+    s->renewWatermarkOnce();
+
+    {
+        Gc gc(s, hexToU128("00000000000000000000000000000001"));
+        const size_t rounds = runGcToFixpoint(s, gc);
+        const FsckReport after = runFsck(*s, /*detail=*/false);
+        EXPECT_EQ(after.dangling, 0u);
+        /// Removal still performs no lifecycle-specific physical cleanup -- the perpetual sweep and the
+        /// janitor own the orphaned bytes. What changed is that they can now FINISH: dropping the last
+        /// namespace leaves an authoritative catalog that decodes to zero entries, which is a positive
+        /// proof of no live edge rather than the vacuous 0 == 0, so the round's frontier completes and
+        /// the sweep is no longer suppressed on an emptied pool.
+        EXPECT_EQ(after.unreachable, 0u)
+            << "an emptied pool must drain instead of standing still; the sweep owned these blobs and "
+               "reclaimed them within " << rounds << " GC rounds";
+        EXPECT_EQ(after.reachable, 0u);
+        EXPECT_FALSE(CasRefCatalog::lifeIfCataloged(s->backend(), s->layout(), ns))
+            << "physical debris must not keep the logical namespace life cataloged";
+    }
+}
diff --git a/src/Disks/tests/gtest_cas_txn_apply_ledger.cpp b/src/Disks/tests/gtest_cas_txn_apply_ledger.cpp
new file mode 100644
index 000000000000..80d262854555
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_txn_apply_ledger.cpp
@@ -0,0 +1,159 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasBlobInDegree.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGcShardPlan.h>
+
+using namespace DB::Cas;
+
+namespace
+{
+BlobRef bh(uint64_t n) { return BlobRef{BlobHashAlgo::CityHash128, BlobDigest::fromU128(DB::UInt128(n))}; }
+DB::UInt128 s(uint64_t n) { return DB::UInt128(n); }
+}
+
+/// The ledger is pure round-local bookkeeping; test it directly rather than trying to fabricate a
+/// lost bucket inside a real fold. The fold-side wiring is covered by the gate: every existing GC
+/// test now runs with the ledger armed and would throw if a delta went missing.
+TEST(CASTxnApplyLedger, HealthyRoundReportsNothingUnapplied)
+{
+    TxnApplyLedger ledger;
+    const uint32_t a = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 1});
+    const uint32_t b = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 2});
+    ledger.markProduced(a);
+    ledger.markCommitted(a);
+    ledger.markApplied(a);
+    ledger.markCommitted(b);          /// committed but produced no blob deltas — legitimate
+    EXPECT_TRUE(ledger.unapplied().empty());
+}
+
+TEST(CASTxnApplyLedger, CommittedAndProducedButNeverAppliedIsReported)
+{
+    TxnApplyLedger ledger;
+    const uint32_t a = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 1});
+    ledger.markProduced(a);
+    ledger.markCommitted(a);
+    ASSERT_EQ(ledger.unapplied().size(), 1u);
+    EXPECT_EQ(ledger.unapplied().front(), a);
+}
+
+TEST(CASTxnApplyLedger, ClampedTransactionIsNotReported)
+{
+    /// A clamped log emits deltas into the per-log staging buffer that is then DISCARDED; it is never
+    /// committed, so it must not be reported unapplied.
+    TxnApplyLedger ledger;
+    const uint32_t a = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 1});
+    ledger.markProduced(a);
+    EXPECT_TRUE(ledger.unapplied().empty());
+}
+
+/// The reducers mark `applied` by indexing the raw vector with `BlobDelta::txn_ordinal`, so the
+/// ledger's own vectors must stay index-parallel with the ordinals it hands out. Pin that: the
+/// ordinal is the position, and every parallel vector grows with it.
+TEST(CASTxnApplyLedger, OrdinalsIndexTheParallelVectors)
+{
+    TxnApplyLedger ledger;
+    EXPECT_EQ(ledger.open(RootNamespace{"a"}, RefTxnId{1, 7}), 0u);
+    EXPECT_EQ(ledger.open(RootNamespace{"b"}, RefTxnId{2, 3}), 1u);
+    ASSERT_EQ(ledger.applied.size(), 2u);
+    ASSERT_EQ(ledger.produced.size(), 2u);
+    ASSERT_EQ(ledger.committed.size(), 2u);
+    ASSERT_EQ(ledger.namespaces.size(), 2u);
+    EXPECT_EQ(ledger.namespaces[1], "b");
+    EXPECT_EQ(ledger.txns[1], (RefTxnId{2, 3}));
+}
+
+/// PROBE B2's reach, pinned as a property rather than left to prose: a delta consumed by a reducer
+/// clears its transaction, and only the transaction whose ordinal was never written stays reported.
+/// This is the exact shape a delta lost in gc-shard routing produces.
+TEST(CASTxnApplyLedger, OnlyTheTransactionWhoseDeltasVanishedIsReported)
+{
+    TxnApplyLedger ledger;
+    const uint32_t routed = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 1});
+    const uint32_t lost = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 2});
+    for (const uint32_t o : {routed, lost})
+    {
+        ledger.markProduced(o);
+        ledger.markCommitted(o);
+    }
+    /// The reducer's own write: a raw byte at the delta's ordinal, exactly as
+    /// `foldDeltasIntoGeneration` performs it.
+    ledger.applied[routed] = 1;
+
+    ASSERT_EQ(ledger.unapplied().size(), 1u);
+    EXPECT_EQ(ledger.unapplied().front(), lost);
+}
+
+/// The reducer-side half of probe B2, proven POSITIVELY rather than by the absence of a throw. The
+/// three tests above exercise the ledger's own arithmetic; this one exercises the write that
+/// `foldDeltasIntoGeneration` performs inside its delta-consumption loop — the only new code on the
+/// fold's hot path — and pins that a routed delta marks its ordinal while an ordinal no delta carries
+/// stays unmarked. Without this the fold-side wiring would only ever be covered negatively (the gate
+/// does not throw), which cannot distinguish "the probe is correct" from "the probe is inert".
+TEST(CASTxnApplyLedger, ReducerMarksTheOrdinalOfEveryDeltaItConsumes)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    TxnApplyLedger ledger;
+    const uint32_t routed = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 1});
+    const uint32_t absent = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 2});
+
+    /// Only `routed`'s transaction emitted deltas. `absent`'s ordinal is live in the ledger but no
+    /// delta carries it — exactly the shape a delta lost before the reducer produces.
+    std::vector<BlobDelta> deltas{
+        {bh(1), s(1), /*remove*/false, routed},
+        {bh(2), s(1), /*remove*/false, routed},
+    };
+    std::vector<RunRef> runs;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, /*new_generation*/1, /*attempt*/0,
+                             /*shard*/0, deltas, runs,
+                             /*current_round*/0, /*condemn_round*/0, /*head_blob*/{}, /*peek_head*/{},
+                             /*confirm_condemned_marker*/{}, /*out_retired*/nullptr,
+                             /*suppress_destructive*/false, &ledger.applied);
+
+    EXPECT_EQ(ledger.applied[routed], 1) << "the reducer consumed this transaction's deltas but did "
+                                            "not mark its ordinal — probe B2 is inert";
+    EXPECT_EQ(ledger.applied[absent], 0) << "an ordinal no delta carries must never be marked";
+
+    /// And the verdict follows from those bits: a committed+produced transaction whose deltas never
+    /// arrived is the one reported.
+    for (const uint32_t o : {routed, absent})
+    {
+        ledger.markProduced(o);
+        ledger.markCommitted(o);
+    }
+    ASSERT_EQ(ledger.unapplied().size(), 1u);
+    EXPECT_EQ(ledger.unapplied().front(), absent);
+}
+
+/// The reducer must mark a REMOVAL delta too. Removals are the direction that can legitimately
+/// collapse to nothing inside the set merge (an unmatched `-1` changes no state and emits no row), so
+/// a mark placed at run flush instead of at consumption would silently skip exactly this case and
+/// report a healthy round as lossy.
+TEST(CASTxnApplyLedger, ReducerMarksAnUnmatchedRemovalDelta)
+{
+    InMemoryBackend backend;
+    Layout layout{"pool"};
+
+    TxnApplyLedger ledger;
+    const uint32_t removal = ledger.open(RootNamespace{"ns"}, RefTxnId{1, 1});
+    ledger.markProduced(removal);
+    ledger.markCommitted(removal);
+
+    /// A `-1` for an edge no prior run ever activated: a per-key no-op by design.
+    std::vector<BlobDelta> deltas{{bh(1), s(1), /*remove*/true, removal}};
+    std::vector<RunRef> runs;
+    RetiredMergeResult merged;
+    foldDeltasIntoGeneration(backend, layout, /*prior_runs*/{}, /*new_generation*/1, /*attempt*/0,
+                             /*shard*/0, deltas, runs,
+                             /*current_round*/0, /*condemn_round*/0, /*head_blob*/{}, /*peek_head*/{},
+                             /*confirm_condemned_marker*/{}, &merged,
+                             /*suppress_destructive*/false, &ledger.applied);
+
+    EXPECT_EQ(merged.unmatched_removes, 1u) << "the fixture must actually stage an unmatched removal";
+    EXPECT_EQ(ledger.applied[removal], 1);
+    EXPECT_TRUE(ledger.unapplied().empty())
+        << "a legitimate no-op removal must not read as a lost transaction";
+}
diff --git a/src/Disks/tests/gtest_cas_upload_detached.cpp b/src/Disks/tests/gtest_cas_upload_detached.cpp
new file mode 100644
index 000000000000..d28ead312dbb
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_upload_detached.cpp
@@ -0,0 +1,640 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/Exception.h>
+#include <new>
+#include <vector>
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+using DB::Cas::tests::loadMetaForTest;
+using DB::Cas::tests::writeMetaClean;
+using DB::Cas::tests::condemnMeta;
+using DB::Cas::tests::blobEntryFor;
+using DB::Cas::tests::expectThrowsCode;  // NOLINT(misc-unused-using-decls): only used inside `#ifndef DEBUG_OR_SANITIZER_BUILD` -- unused in a sanitizer build's TU, used in a release build's
+
+namespace DB::ErrorCodes
+{
+extern const int LOGICAL_ERROR;
+}
+
+namespace
+{
+
+/// Open a Pool over `b`. `head_first_min_bytes` steers the HEAD-before-PUT size trigger: the default
+/// (1 MiB) keeps the trigger off for the small test payloads (so a branch is reached only via the dedup
+/// cache), while a value of 1 forces the HEAD-first path for any non-empty blob (the `HeadFirstHit` leg).
+PoolPtr openUploadPool(const std::shared_ptr<InMemoryBackend> & b, uint64_t head_first_min_bytes = (1ULL << 20))
+{
+    return Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                                    .deduplication_head_first_min_bytes = head_first_min_bytes});
+}
+
+/// Stage a one-blob manifest for `payload` and precommit it, so `observeAndAdmit`'s EDGE-BEFORE-OBSERVE
+/// fail-closed check holds on the adopt branches. Returns the build ready for an upload of `idOf(payload)`.
+PartWriteTxnPtr precommitBuildFor(
+    const PoolPtr & s, const RootNamespace & ns, const String & ref, const String & payload)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    PartWriteTxnPtr build = s->beginPartWrite(std::move(info));
+    const ManifestId id = build->stageManifest({blobEntryFor("col.bin", u128Of(payload), payload.size())});
+    build->precommitAdd(ns, ref, id);
+    return build;
+}
+
+/// Seed a present, well-formed blob body whose LOGICAL bytes are exactly `payload` (a fixed envelope
+/// header followed by the payload), so a later HEAD returns a token and a logical size of `payload.size()`.
+void seedPresentBody(
+    InMemoryBackend & b, const Layout & layout, const PoolMeta & pm, const BlobRef & ref, const String & payload)
+{
+    EnvelopeHeader h;
+    h.kind = ObjectKind::Blob;
+    h.incarnation_tag = DB::UInt128(0xABCD);
+    h.build_id = DB::UInt128(0x1111);
+    const String head = encodeEnvelopeHeader(h, static_cast<uint32_t>(pm.blob_header_len));
+    b.putIfAbsent(layout.blobKey(ref), head + payload);
+}
+
+/// The logical payload stored at `key` (object body minus the fixed blob header), or empty when absent.
+String logicalPayloadAt(InMemoryBackend & b, const String & key, uint64_t header_len)
+{
+    const auto got = b.get(key);
+    if (!got || got->bytes.size() < header_len)
+        return {};
+    return got->bytes.substr(header_len);
+}
+
+/// The blob's meta state, or nullopt when the meta object is absent.
+std::optional<MetaState> metaStateAt(InMemoryBackend & b, const Layout & layout, const String & payload)
+{
+    const auto lm = loadMetaForTest(b, layout, u128Of(payload));
+    return lm ? std::optional<MetaState>(lm->meta.state) : std::nullopt;
+}
+
+}
+
+/// dedup-cache hit: the ref is known-present in the dedup cache ⇒ HEAD-first ⇒ present ⇒ adopt.
+/// The returned result is a complete tokened adopt dep with the `DeduplicationCacheHit` outcome; the build's
+/// dep set stays untouched; the backend body/meta match the serial `putBlob` for the same input.
+TEST(CASUploadDetached, DeduplicationCacheHitAdoptsBuildUntouched)
+{
+    const RootNamespace ns{"srv1/nsDedup"};
+    const String ref_name = "part";
+    const String payload = "dedup-cache-hit-payload";
+    const BlobRef blob = idOf(payload);
+
+    auto arrange = [&](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openUploadPool(b);
+        seedPresentBody(*b, s->layout(), s->poolMeta(), blob, payload);
+        writeMetaClean(*b, s->layout(), u128Of(payload), payload.size());
+        s->dedupCacheAdd(blob);
+        build = precommitBuildFor(s, ns, ref_name, payload);
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    arrange(b1, s1, build1);
+    const String key = s1->layout().blobKey(blob);
+
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    const BlobUploadResult r = build1->uploadBlobDetached(
+        BlobUploadRequest{blob, BlobSource::fromString(payload), payload.size()});
+
+    EXPECT_EQ(r.outcome, BlobUploadOutcome::DeduplicationCacheHit);
+    EXPECT_EQ(r.ref, blob);
+    EXPECT_EQ(r.dep.kind, ObjectKind::Blob);
+    ASSERT_TRUE(r.dep.token.has_value());
+    EXPECT_FALSE(r.dep.token->value.empty());
+    EXPECT_FALSE(r.dep.adopted);
+    EXPECT_EQ(r.dep.size, payload.size());
+
+    /// Build UNTOUCHED: the detached primitive folded no dep.
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    /// Serial reference on an identically-arranged world: putBlob folds the dep; end-state matches.
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    arrange(b2, s2, build2);
+    const PutBlobResult pr = build2->putBlob(blob, BlobSource::fromString(payload));
+    EXPECT_TRUE(build2->depIsTokened(blob));
+    EXPECT_EQ(pr.size, r.dep.size);
+
+    EXPECT_EQ(logicalPayloadAt(*b1, key, s1->poolMeta().blob_header_len),
+              logicalPayloadAt(*b2, key, s2->poolMeta().blob_header_len));
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), metaStateAt(*b2, s2->layout(), payload));
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), std::optional<MetaState>(MetaState::Clean));
+}
+
+/// HEAD-first hit (size-triggered, not cached): a present body under the size trigger is adopted with
+/// the `HeadHit` outcome. Distinct from the dedup-cache leg by NOT being in the dedup cache.
+TEST(CASUploadDetached, HeadFirstHitAdopts)
+{
+    const RootNamespace ns{"srv1/nsHead"};
+    const String ref_name = "part";
+    const String payload = "head-first-hit-payload";
+    const BlobRef blob = idOf(payload);
+
+    auto arrange = [&](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openUploadPool(b, /*head_first_min_bytes=*/1);   /// force HEAD-first without the dedup cache
+        seedPresentBody(*b, s->layout(), s->poolMeta(), blob, payload);
+        writeMetaClean(*b, s->layout(), u128Of(payload), payload.size());
+        build = precommitBuildFor(s, ns, ref_name, payload);
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    arrange(b1, s1, build1);
+    const String key = s1->layout().blobKey(blob);
+
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    const BlobUploadResult r = build1->uploadBlobDetached(
+        BlobUploadRequest{blob, BlobSource::fromString(payload), payload.size()});
+
+    EXPECT_EQ(r.outcome, BlobUploadOutcome::HeadHit);
+    ASSERT_TRUE(r.dep.token.has_value());
+    EXPECT_FALSE(r.dep.adopted);
+    EXPECT_EQ(r.dep.size, payload.size());
+
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    arrange(b2, s2, build2);
+    build2->putBlob(blob, BlobSource::fromString(payload));
+    EXPECT_TRUE(build2->depIsTokened(blob));
+
+    EXPECT_EQ(logicalPayloadAt(*b1, key, s1->poolMeta().blob_header_len),
+              logicalPayloadAt(*b2, key, s2->poolMeta().blob_header_len));
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), std::optional<MetaState>(MetaState::Clean));
+}
+
+/// HEAD-first miss, then a live adopt via the conditional-create 412 path, with the meta point-read
+/// finding no meta and backfilling `Clean`. Outcome `HeadMissAdopted`.
+TEST(CASUploadDetached, HeadMissLiveAdoptBackfills)
+{
+    const RootNamespace ns{"srv1/nsAdopt"};
+    const String ref_name = "part";
+    const String payload = "head-miss-adopt-payload";
+    const BlobRef blob = idOf(payload);
+
+    auto arrange = [&](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openUploadPool(b);                        /// default trigger off ⇒ no HEAD-first
+        seedPresentBody(*b, s->layout(), s->poolMeta(), blob, payload);   /// body present, NO meta ⇒ backfill
+        build = precommitBuildFor(s, ns, ref_name, payload);
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    arrange(b1, s1, build1);
+    const String key = s1->layout().blobKey(blob);
+
+    ASSERT_FALSE(metaStateAt(*b1, s1->layout(), payload).has_value());   /// precondition: meta absent
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    const BlobUploadResult r = build1->uploadBlobDetached(
+        BlobUploadRequest{blob, BlobSource::fromString(payload), payload.size()});
+
+    EXPECT_EQ(r.outcome, BlobUploadOutcome::HeadMissAdopted);
+    ASSERT_TRUE(r.dep.token.has_value());
+    EXPECT_FALSE(r.dep.adopted);
+    EXPECT_EQ(r.dep.size, payload.size());
+
+    EXPECT_FALSE(build1->depIsTokened(blob));
+    /// The point-read backfilled a Clean meta.
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), std::optional<MetaState>(MetaState::Clean));
+
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    arrange(b2, s2, build2);
+    build2->putBlob(blob, BlobSource::fromString(payload));
+    EXPECT_TRUE(build2->depIsTokened(blob));
+
+    EXPECT_EQ(logicalPayloadAt(*b1, key, s1->poolMeta().blob_header_len),
+              logicalPayloadAt(*b2, key, s2->poolMeta().blob_header_len));
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), metaStateAt(*b2, s2->layout(), payload));
+}
+
+/// Fresh local streaming: nothing present ⇒ the write-once conditional create streams the body and
+/// creates the Clean meta. Outcome `FreshUpload`, a tokened dep sized to the source.
+TEST(CASUploadDetached, FreshLocalStreaming)
+{
+    const RootNamespace ns{"srv1/nsFresh"};
+    const String ref_name = "part";
+    const String payload = "fresh-local-streaming-payload";
+    const BlobRef blob = idOf(payload);
+
+    auto arrange = [&](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openUploadPool(b);
+        build = precommitBuildFor(s, ns, ref_name, payload);
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    arrange(b1, s1, build1);
+    const String key = s1->layout().blobKey(blob);
+
+    ASSERT_FALSE(b1->head(key).exists);   /// precondition: absent
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    const BlobUploadResult r = build1->uploadBlobDetached(
+        BlobUploadRequest{blob, BlobSource::fromString(payload), payload.size()});
+
+    EXPECT_EQ(r.outcome, BlobUploadOutcome::FreshUpload);
+    EXPECT_EQ(r.ref, blob);
+    ASSERT_TRUE(r.dep.token.has_value());
+    EXPECT_FALSE(r.dep.token->value.empty());
+    EXPECT_FALSE(r.dep.adopted);
+    EXPECT_EQ(r.dep.size, payload.size());
+
+    EXPECT_FALSE(build1->depIsTokened(blob));
+    EXPECT_TRUE(b1->head(key).exists);
+    EXPECT_EQ(logicalPayloadAt(*b1, key, s1->poolMeta().blob_header_len), payload);
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), std::optional<MetaState>(MetaState::Clean));
+
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    arrange(b2, s2, build2);
+    const PutBlobResult pr = build2->putBlob(blob, BlobSource::fromString(payload));
+    EXPECT_TRUE(build2->depIsTokened(blob));
+    EXPECT_EQ(pr.size, r.dep.size);
+
+    /// The envelope's fresh incarnation tag differs per upload, but the LOGICAL payload and meta match.
+    EXPECT_EQ(logicalPayloadAt(*b1, key, s1->poolMeta().blob_header_len),
+              logicalPayloadAt(*b2, key, s2->poolMeta().blob_header_len));
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), metaStateAt(*b2, s2->layout(), payload));
+}
+
+/// S3-native staging promotion: the source carries a server-side-copy descriptor and the blob key is
+/// absent ⇒ a write-once conditional server-side copy creates the blob. Outcome `StagingPromoted`.
+TEST(CASUploadDetached, S3StagingPromotion)
+{
+    const RootNamespace ns{"srv1/nsStaging"};
+    const String ref_name = "part";
+    const String payload = "s3-staging-promotion-payload";
+    const BlobRef blob = idOf(payload);
+    const String staging_key = "p/staging/mount1/promote.tmp";
+
+    auto arrange = [&](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build, String & staging_bytes)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openUploadPool(b);
+        /// The staging object holds [header][payload], exactly as the S3-staging writer emits it.
+        EnvelopeHeader h;
+        h.kind = ObjectKind::Blob;
+        h.incarnation_tag = DB::UInt128(0xC0FFEE);
+        staging_bytes = encodeEnvelopeHeader(h, static_cast<uint32_t>(s->poolMeta().blob_header_len)) + payload;
+        b->putIfAbsent(staging_key, staging_bytes);
+        build = precommitBuildFor(s, ns, ref_name, payload);
+    };
+
+    auto stagingSource = [&]() -> BlobSource
+    {
+        BlobSource src;
+        src.size = payload.size();
+        src.server_side_copy_from = staging_key;
+        return src;
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    String staging_bytes1;
+    arrange(b1, s1, build1, staging_bytes1);
+    const String key = s1->layout().blobKey(blob);
+
+    ASSERT_FALSE(b1->head(key).exists);
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    const BlobUploadResult r = build1->uploadBlobDetached(
+        BlobUploadRequest{blob, stagingSource(), payload.size()});
+
+    EXPECT_EQ(r.outcome, BlobUploadOutcome::StagingPromoted);
+    ASSERT_TRUE(r.dep.token.has_value());
+    EXPECT_FALSE(r.dep.token->value.empty());
+    EXPECT_FALSE(r.dep.adopted);
+    EXPECT_EQ(r.dep.size, payload.size());
+
+    EXPECT_FALSE(build1->depIsTokened(blob));
+    ASSERT_TRUE(b1->head(key).exists);
+    /// The server-side copy moved the staging bytes verbatim to the blob key.
+    const auto got = b1->get(key);
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes, staging_bytes1);
+
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    String staging_bytes2;
+    arrange(b2, s2, build2, staging_bytes2);
+    build2->putBlob(blob, stagingSource());
+    EXPECT_TRUE(build2->depIsTokened(blob));
+
+    const auto got2 = b2->get(key);
+    ASSERT_TRUE(got2.has_value());
+    EXPECT_EQ(got->bytes, got2->bytes);
+}
+
+/// Condemned-local resurrection: a present body observed condemned via the meta point-read is displaced
+/// by a fresh incarnation streamed from the writer's OWN source (`putOverwrite`), never a read of the
+/// dying object. Outcome `ResurrectedLocal`; the token is refreshed and the meta returns to Clean.
+TEST(CASUploadDetached, CondemnedLocalResurrection)
+{
+    const RootNamespace ns{"srv1/nsResLocal"};
+    const String ref_name = "part";
+    const String payload = "condemned-local-resurrect-payload";
+    const BlobRef blob = idOf(payload);
+
+    auto arrange = [&](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openUploadPool(b);
+        seedPresentBody(*b, s->layout(), s->poolMeta(), blob, payload);
+        writeMetaClean(*b, s->layout(), u128Of(payload), payload.size());
+        condemnMeta(*b, s->layout(), u128Of(payload), /*condemn_round=*/7);
+        build = precommitBuildFor(s, ns, ref_name, payload);
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    arrange(b1, s1, build1);
+    const String key = s1->layout().blobKey(blob);
+    const Token condemned_token = b1->head(key).token;
+
+    ASSERT_EQ(metaStateAt(*b1, s1->layout(), payload), std::optional<MetaState>(MetaState::Condemned));
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    const BlobUploadResult r = build1->uploadBlobDetached(
+        BlobUploadRequest{blob, BlobSource::fromString(payload), payload.size()});
+
+    EXPECT_EQ(r.outcome, BlobUploadOutcome::ResurrectedLocal);
+    ASSERT_TRUE(r.dep.token.has_value());
+    EXPECT_FALSE(r.dep.adopted);
+    EXPECT_EQ(r.dep.size, payload.size());
+
+    EXPECT_FALSE(build1->depIsTokened(blob));
+    /// The condemned incarnation was displaced by a fresh one (token changed) and the meta is Clean again.
+    const Token after_token = b1->head(key).token;
+    EXPECT_NE(after_token.value, condemned_token.value);
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), std::optional<MetaState>(MetaState::Clean));
+    EXPECT_EQ(logicalPayloadAt(*b1, key, s1->poolMeta().blob_header_len), payload);
+
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    arrange(b2, s2, build2);
+    build2->putBlob(blob, BlobSource::fromString(payload));
+    EXPECT_TRUE(build2->depIsTokened(blob));
+
+    EXPECT_EQ(logicalPayloadAt(*b1, key, s1->poolMeta().blob_header_len),
+              logicalPayloadAt(*b2, key, s2->poolMeta().blob_header_len));
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), metaStateAt(*b2, s2->layout(), payload));
+}
+
+/// Condemned-S3 resurrection: a present body observed condemned with an S3 staging source is displaced
+/// by an unconditional server-side copy from the SAME staging object under a fresh-tagged header, never
+/// a read/copy of the condemned blob key. Outcome `ResurrectedS3`.
+TEST(CASUploadDetached, CondemnedS3Resurrection)
+{
+    const RootNamespace ns{"srv1/nsResS3"};
+    const String ref_name = "part";
+    const String payload = "condemned-s3-resurrect-payload";
+    const BlobRef blob = idOf(payload);
+    const String staging_key = "p/staging/mount1/resurrect.tmp";
+
+    auto arrange = [&](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openUploadPool(b);
+        EnvelopeHeader h;
+        h.kind = ObjectKind::Blob;
+        h.incarnation_tag = DB::UInt128(0xC0FFEE);
+        const String staging_bytes = encodeEnvelopeHeader(h, static_cast<uint32_t>(s->poolMeta().blob_header_len)) + payload;
+        b->putIfAbsent(staging_key, staging_bytes);
+        /// Seed the condemned blob body = exactly a verbatim promote of the staging object would produce.
+        b->putIfAbsent(s->layout().blobKey(blob), staging_bytes);
+        writeMetaClean(*b, s->layout(), u128Of(payload), payload.size());
+        condemnMeta(*b, s->layout(), u128Of(payload), /*condemn_round=*/9);
+        build = precommitBuildFor(s, ns, ref_name, payload);
+    };
+
+    auto stagingSource = [&]() -> BlobSource
+    {
+        BlobSource src;
+        src.size = payload.size();
+        src.server_side_copy_from = staging_key;
+        return src;
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    arrange(b1, s1, build1);
+    const String key = s1->layout().blobKey(blob);
+    const Token condemned_token = b1->head(key).token;
+
+    ASSERT_EQ(metaStateAt(*b1, s1->layout(), payload), std::optional<MetaState>(MetaState::Condemned));
+    EXPECT_FALSE(build1->depIsTokened(blob));
+
+    const BlobUploadResult r = build1->uploadBlobDetached(
+        BlobUploadRequest{blob, stagingSource(), payload.size()});
+
+    EXPECT_EQ(r.outcome, BlobUploadOutcome::ResurrectedS3);
+    ASSERT_TRUE(r.dep.token.has_value());
+    EXPECT_FALSE(r.dep.adopted);
+    EXPECT_EQ(r.dep.size, payload.size());
+
+    EXPECT_FALSE(build1->depIsTokened(blob));
+    /// A fresh incarnation displaced the condemned one (INV-NO-RETURN: fresh tag ⇒ different token).
+    const Token after_token = b1->head(key).token;
+    EXPECT_NE(after_token.value, condemned_token.value);
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), std::optional<MetaState>(MetaState::Clean));
+
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    arrange(b2, s2, build2);
+    build2->putBlob(blob, stagingSource());
+    EXPECT_TRUE(build2->depIsTokened(blob));
+
+    EXPECT_EQ(metaStateAt(*b1, s1->layout(), payload), metaStateAt(*b2, s2->layout(), payload));
+}
+
+/// `mergeBlobUploadResults` folds N detached results in ONE call to EXACTLY the same deps a serial
+/// putBlob fold would produce. Both worlds run the identical sequence of backend calls (same
+/// precommit, same blobs in the same order), so their independently-minted tokens line up too -- the
+/// merge path adds no backend calls of its own, only in-memory bookkeeping, so a DEEP dep-map
+/// comparison (tokens included) is exact, not just per-ref.
+TEST(CASUploadDetached, MergeAppliesAllDeps)
+{
+    const RootNamespace ns{"srv1/nsMergeAll"};
+    const String ref_name = "part";
+    const std::vector<String> payloads = {"merge-fresh-a", "merge-fresh-b", "merge-fresh-c"};
+
+    auto arrange = [&](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openUploadPool(b);
+        build = precommitBuildFor(s, ns, ref_name, "manifest-seed");
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    arrange(b1, s1, build1);
+
+    std::vector<BlobUploadResult> results;
+    for (const auto & payload : payloads)
+    {
+        const BlobRef blob = idOf(payload);
+        EXPECT_FALSE(build1->depIsTokened(blob));
+        results.push_back(build1->uploadBlobDetached(
+            BlobUploadRequest{blob, BlobSource::fromString(payload), payload.size()}));
+    }
+    /// Still untouched before the merge -- uploadBlobDetached folds nothing.
+    for (const auto & payload : payloads)
+        EXPECT_FALSE(build1->depIsTokened(idOf(payload)));
+
+    build1->mergeBlobUploadResults(results);
+
+    for (const auto & payload : payloads)
+        EXPECT_TRUE(build1->depIsTokened(idOf(payload)));
+
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    arrange(b2, s2, build2);
+    for (const auto & payload : payloads)
+        build2->putBlob(idOf(payload), BlobSource::fromString(payload));
+
+    EXPECT_EQ(build1->depsSnapshotForTest(), build2->depsSnapshotForTest());
+}
+
+/// Merge exception safety (spec Test 16): a hook injected between per-result applications throws
+/// after the FIRST result would have applied; the SECOND result must never reach `deps`, and neither
+/// may a PRE-EXISTING unrelated dep be disturbed -- a DEEP snapshot (the whole map, not one ref probed
+/// at a time) proves the build is byte-for-byte at its pre-merge state, all-or-nothing observed.
+TEST(CASUploadDetached, MergeFailureLeavesBuildUntouched)
+{
+    const RootNamespace ns{"srv1/nsMergeFail"};
+    const String ref_name = "part";
+    const String payload_existing = "merge-fail-existing";
+    const String payload_a = "merge-fail-a";
+    const String payload_b = "merge-fail-b";
+
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openUploadPool(b);
+    auto build = precommitBuildFor(s, ns, ref_name, "manifest-seed");
+
+    /// A pre-existing folded dep the merge must leave completely alone.
+    build->putBlob(idOf(payload_existing), BlobSource::fromString(payload_existing));
+    ASSERT_TRUE(build->depIsTokened(idOf(payload_existing)));
+
+    std::vector<BlobUploadResult> results;
+    results.push_back(build->uploadBlobDetached(
+        BlobUploadRequest{idOf(payload_a), BlobSource::fromString(payload_a), payload_a.size()}));
+    results.push_back(build->uploadBlobDetached(
+        BlobUploadRequest{idOf(payload_b), BlobSource::fromString(payload_b), payload_b.size()}));
+
+    const auto pre_merge_snapshot = build->depsSnapshotForTest();
+    ASSERT_EQ(pre_merge_snapshot.size(), 1u);   /// only the pre-existing dep; the detached uploads folded nothing
+
+    build->setMergeHookForTest([](size_t applied_so_far)
+    {
+        if (applied_so_far == 1)
+            throw std::bad_alloc();
+    });
+
+    EXPECT_THROW(build->mergeBlobUploadResults(results), std::bad_alloc);
+
+    EXPECT_EQ(build->depsSnapshotForTest(), pre_merge_snapshot);
+    EXPECT_FALSE(build->depIsTokened(idOf(payload_a)));
+    EXPECT_FALSE(build->depIsTokened(idOf(payload_b)));
+}
+
+/// Duplicate-grouping consistency: two results for the SAME ref with conflicting sizes are rejected
+/// as a staging bug (LOGICAL_ERROR) BEFORE any result applies -- the fan-out's one-task-per-unique-ref
+/// invariant means this should never happen upstream, so merge itself is the backstop. LOGICAL_ERROR
+/// aborts the whole process in debug/sanitizer builds instead of behaving like a catchable exception
+/// (`Common/Exception.cpp`'s `handle_error_code`) -- `CASUploadDetachedDeathTest` below proves the
+/// abort positively in those builds instead (it cannot also verify the build-untouched postcondition,
+/// since there is no continuation after a real abort).
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASUploadDetached, MergeValidatesSizes)
+{
+    const RootNamespace ns{"srv1/nsMergeSizes"};
+    const String ref_name = "part";
+    const String payload = "merge-size-conflict";
+    const BlobRef blob = idOf(payload);
+
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openUploadPool(b);
+    auto build = precommitBuildFor(s, ns, ref_name, "manifest-seed");
+
+    const BlobUploadResult r = build->uploadBlobDetached(
+        BlobUploadRequest{blob, BlobSource::fromString(payload), payload.size()});
+    ASSERT_FALSE(build->depIsTokened(blob));
+
+    BlobUploadResult conflicting = r;
+    conflicting.dep.size = r.dep.size + 1;   /// same ref, conflicting declared size
+
+    const auto pre_merge_snapshot = build->depsSnapshotForTest();
+
+    expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&]
+    {
+        build->mergeBlobUploadResults(std::vector<BlobUploadResult>{r, conflicting});
+    });
+
+    EXPECT_EQ(build->depsSnapshotForTest(), pre_merge_snapshot);
+    EXPECT_FALSE(build->depIsTokened(blob));
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASUploadDetachedDeathTest, MergeValidatesSizesAborts)
+{
+    const RootNamespace ns{"srv1/nsMergeSizes"};
+    const String ref_name = "part";
+    const String payload = "merge-size-conflict";
+    const BlobRef blob = idOf(payload);
+
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openUploadPool(b);
+    auto build = precommitBuildFor(s, ns, ref_name, "manifest-seed");
+
+    const BlobUploadResult r = build->uploadBlobDetached(
+        BlobUploadRequest{blob, BlobSource::fromString(payload), payload.size()});
+
+    BlobUploadResult conflicting = r;
+    conflicting.dep.size = r.dep.size + 1;   /// same ref, conflicting declared size
+
+    EXPECT_DEATH(
+        { build->mergeBlobUploadResults(std::vector<BlobUploadResult>{r, conflicting}); }, "");
+}
+#endif
diff --git a/src/Disks/tests/gtest_cas_upload_fanout.cpp b/src/Disks/tests/gtest_cas_upload_fanout.cpp
new file mode 100644
index 000000000000..941c4f66c035
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_upload_fanout.cpp
@@ -0,0 +1,940 @@
+#include <gtest/gtest.h>
+#include <IO/ReadBufferFromString.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/ContentAddressedTransaction.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobUploadPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasBlobEnvelopeFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasBlobMeta.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasLayout.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/tests/cas_test_helpers.h>
+#include <Common/CurrentMetrics.h>
+#include <Common/Exception.h>
+#include <Common/ThreadPool.h>
+#include <base/scope_guard.h>
+
+#include <algorithm>
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <latch>
+#include <map>
+#include <mutex>
+#include <optional>
+#include <span>
+#include <thread>
+#include <tuple>
+#include <vector>
+
+using namespace DB::Cas;
+using DB::Cas::tests::idOf;
+using DB::Cas::tests::u128Of;
+using DB::Cas::tests::blobEntryFor;
+using DB::Cas::tests::writeMetaClean;
+using DB::Cas::tests::condemnMeta;
+using DB::Cas::tests::loadMetaForTest;
+using DB::Cas::tests::expectThrowsCode;
+using DB::Cas::tests::runRoundsUntilAbsent;
+using DB::Cas::tests::blobAbsent;
+using DB::Cas::tests::CountingBackend;
+
+namespace DB::ErrorCodes
+{
+extern const int LOGICAL_ERROR;
+extern const int INCORRECT_DATA;
+}
+
+namespace CurrentMetrics
+{
+    extern const Metric LocalThread;
+    extern const Metric LocalThreadActive;
+    extern const Metric LocalThreadScheduled;
+}
+
+namespace
+{
+
+/// A local upload pool of a chosen size. Task 5 takes the pool as a parameter (rather than reaching
+/// for the server-wide `Cas::blobUploadPool()`) precisely so a test can run the SAME fan-out through a
+/// size-1 pool (the serial reference) and a size-N pool (the fanned-out world) in ONE process -- the
+/// server-wide pool is once-only per binary and cannot be re-sized. The calling thread only submits
+/// and joins (it never occupies a pool slot), so size 1 is a valid fully-serial configuration.
+std::unique_ptr<ThreadPool> makePool(size_t size)
+{
+    return std::make_unique<ThreadPool>(
+        CurrentMetrics::LocalThread, CurrentMetrics::LocalThreadActive, CurrentMetrics::LocalThreadScheduled, size);
+}
+
+/// Open a Pool over any InMemoryBackend-derived backend (the plain one, or the CountingBackend that
+/// records per-key GET counts). `head_first_min_bytes` steers the HEAD-before-PUT size trigger, exactly
+/// as in `gtest_cas_upload_detached.cpp`.
+PoolPtr openPool(const std::shared_ptr<InMemoryBackend> & b, uint64_t head_first_min_bytes = (1ULL << 20))
+{
+    return Pool::open(b, PoolConfig{.pool_prefix = "p", .server_root_id = "test",
+                                    .deduplication_head_first_min_bytes = head_first_min_bytes});
+}
+
+/// Stage a one-blob seed manifest and precommit it, so every adopt branch of `uploadBlobDetached`
+/// passes its EDGE-BEFORE-OBSERVE fail-closed gate (which only checks the `precommitted` flag). One
+/// precommit covers an arbitrary number of subsequently-uploaded blobs, mirroring
+/// `precommitBuildFor`/`MergeAppliesAllDeps` in the detached suite.
+PartWriteTxnPtr precommitBuildFor(const PoolPtr & s, const RootNamespace & ns, const String & ref)
+{
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/" + ref;
+    PartWriteTxnPtr build = s->beginPartWrite(std::move(info));
+    const String seed = "seed-manifest-" + ns.string() + "/" + ref;
+    const ManifestId id = build->stageManifest({blobEntryFor("col.bin", u128Of(seed), seed.size())});
+    build->precommitAdd(ns, ref, id);
+    return build;
+}
+
+/// Seed a present, well-formed blob body whose LOGICAL bytes are exactly `payload`.
+void seedPresentBody(InMemoryBackend & b, const Layout & layout, const PoolMeta & pm, const String & payload)
+{
+    EnvelopeHeader h;
+    h.kind = ObjectKind::Blob;
+    h.incarnation_tag = DB::UInt128(0xABCD);
+    h.build_id = DB::UInt128(0x1111);
+    const String head = encodeEnvelopeHeader(h, static_cast<uint32_t>(pm.blob_header_len));
+    b.putIfAbsent(layout.blobKey(idOf(payload)), head + payload);
+}
+
+/// The logical payload stored at a blob key (object body minus the fixed blob header), or empty when absent.
+String logicalPayloadAt(InMemoryBackend & b, const String & key, uint64_t header_len)
+{
+    const auto got = b.get(key);
+    if (!got || got->bytes.size() < header_len)
+        return {};
+    return got->bytes.substr(header_len);
+}
+
+/// The blob's meta state, or nullopt when the meta object is absent.
+std::optional<MetaState> metaStateAt(InMemoryBackend & b, const Layout & layout, const String & payload)
+{
+    const auto lm = loadMetaForTest(b, layout, u128Of(payload));
+    return lm ? std::optional<MetaState>(lm->meta.state) : std::nullopt;
+}
+
+/// A local streaming source for `payload`, exactly as `ContentAddressedTransaction::uploadPendingBlobs`
+/// builds for a Local-staging pending blob.
+BlobUploadRequest localRequest(const String & payload)
+{
+    return BlobUploadRequest{idOf(payload), BlobSource::fromString(payload), payload.size()};
+}
+
+/// An S3-staging source: the bytes already live at `staging_key` and the upload is a server-side copy.
+BlobUploadRequest s3Request(const String & payload, const String & staging_key)
+{
+    BlobSource src;
+    src.size = payload.size();
+    src.server_side_copy_from = staging_key;
+    return BlobUploadRequest{idOf(payload), std::move(src), payload.size()};
+}
+
+/// The stable, deterministic part of a build's dep set: (kind, size, adopted, has-token) per ref.
+/// The token VALUE itself is intentionally excluded -- the InMemoryBackend mints tokens from ONE
+/// monotonic counter, so a fanned-out world's fresh/resurrect uploads land their tokens in a
+/// non-deterministic order relative to the serial world. Adopt branches reuse the seeded token, but
+/// keying the comparison off (kind,size,adopted,has-token) plus the backend end state (below) captures
+/// the behavioral equivalence without depending on token-mint ordering.
+using StableDep = std::tuple<ObjectKind, uint64_t, bool, bool>;
+std::map<BlobRef, StableDep> stableDeps(const PartWriteTxn & build)
+{
+    std::map<BlobRef, StableDep> out;
+    for (const auto & [ref, dep] : build.depsSnapshotForTest())
+        out.emplace(ref, StableDep{dep.kind, dep.size, dep.adopted, dep.token.has_value()});
+    return out;
+}
+
+/// The backend end state for a set of blob refs: (logical payload, meta state) per ref. Deterministic
+/// (content is the payload; meta settles to Clean), so it is compared byte-for-byte across worlds.
+using BackendState = std::map<BlobRef, std::pair<String, std::optional<MetaState>>>;
+BackendState backendState(InMemoryBackend & b, const PoolPtr & s, const std::vector<String> & payloads)
+{
+    BackendState out;
+    for (const auto & p : payloads)
+        out.emplace(idOf(p),
+            std::make_pair(logicalPayloadAt(b, s->layout().blobKey(idOf(p)), s->poolMeta().blob_header_len),
+                           metaStateAt(b, s->layout(), p)));
+    return out;
+}
+
+/// A one-shot event with a BOUNDED wait. Not a sleep-sequencer: the wait blocks only until the event
+/// fires; the bound exists solely so a design regression surfaces as a fast test failure instead of an
+/// infinite hang.
+struct BoundedEvent
+{
+    std::mutex m;
+    std::condition_variable cv;
+    bool fired = false;
+    void fire()
+    {
+        {
+            std::lock_guard l(m);
+            fired = true;
+        }
+        cv.notify_all();
+    }
+    bool wait(std::chrono::milliseconds bound)
+    {
+        std::unique_lock l(m);
+        return cv.wait_for(l, bound, [&] { return fired; });
+    }
+};
+
+/// Records the peak number of tasks simultaneously "inside" the rendezvous. A task calls `enter(want)`
+/// from the fan-out's in-task seam; it blocks (BOUNDED) until `want` tasks are inside together, OR every
+/// dispatched task has entered (so a final straggler is never stranded when the pool cannot form another
+/// pair), OR the bound elapses. A pool that CANNOT muster `want` concurrent tasks (size 1, where THIS
+/// task occupies the single worker) times out on the first waiter, marks the run serial, and every later
+/// task skips the wait -- so a too-small pool fails FAST and the whole run stays bounded, never
+/// deadlocked. `total` (the dispatched task count) is set before dispatch.
+struct ConcurrencyProbe
+{
+    std::mutex m;
+    std::condition_variable cv;
+    int current = 0;
+    int peak = 0;
+    int entered = 0;
+    int total = 0;
+    bool timed_out = false;
+    void enter(int want, std::chrono::milliseconds bound)
+    {
+        std::unique_lock l(m);
+        ++current;
+        ++entered;
+        peak = std::max(peak, current);
+        cv.notify_all();
+        const bool ok = cv.wait_for(l, bound,
+            [&] { return current >= want || entered == total || timed_out; });
+        if (!ok)
+            timed_out = true;   /// the pool cannot reach `want`; later tasks skip the wait
+        cv.notify_all();
+        --current;
+    }
+};
+
+}
+
+/// Test 1 (spec §1 "serial-vs-parallel equivalence for successful runs"): a multi-blob part that
+/// exercises every branch of `uploadBlobDetached` produces IDENTICAL recorded deps and IDENTICAL backend
+/// end state whether the fan-out runs serially (pool size 1) or in parallel (pool size 4). The seven
+/// branches split across two HEAD-first configurations (a single pool config cannot reach both the
+/// size-triggered `HeadHit` and the 412-path `HeadMissAdopted`), so the equivalence is proven under
+/// each config: pass A (HEAD-first off) covers dedup-cache-hit / head-miss-adopt / fresh / staging /
+/// condemned-local / condemned-S3; pass B (HEAD-first forced) covers the size-triggered head-hit.
+namespace
+{
+
+/// Arrange pass A's six-branch world (HEAD-first off) and return the payloads it uploads. Every branch
+/// is seeded on a DISTINCT ref so the one-task-per-unique-ref fan-out runs six independent tasks.
+struct WorldA
+{
+    std::shared_ptr<InMemoryBackend> b;
+    PoolPtr s;
+    PartWriteTxnPtr build;
+    std::vector<BlobUploadRequest> requests;
+    std::vector<String> payloads;
+};
+
+const char * const kDedup = "fanoutA-dedup-cache-hit";
+const char * const kAdopt = "fanoutA-head-miss-adopt";
+const char * const kFresh = "fanoutA-fresh-local";
+const char * const kStaging = "fanoutA-s3-staging";
+const char * const kResLocal = "fanoutA-condemned-local";
+const char * const kResS3 = "fanoutA-condemned-s3";
+
+WorldA arrangeWorldA()
+{
+    WorldA w;
+    w.b = std::make_shared<InMemoryBackend>();
+    w.s = openPool(w.b);   /// default trigger: HEAD-first only on a dedup-cache hit
+    const RootNamespace ns{"srv1/nsFanoutA"};
+    w.build = precommitBuildFor(w.s, ns, "part");
+
+    /// dedup-cache hit: present body + Clean meta + cache membership.
+    seedPresentBody(*w.b, w.s->layout(), w.s->poolMeta(), kDedup);
+    writeMetaClean(*w.b, w.s->layout(), u128Of(kDedup), std::string(kDedup).size());
+    w.s->dedupCacheAdd(idOf(kDedup));
+
+    /// HEAD-miss then 412-path live adopt with meta backfill: present body, NO meta, not cached.
+    seedPresentBody(*w.b, w.s->layout(), w.s->poolMeta(), kAdopt);
+
+    /// fresh local streaming: nothing present.
+
+    /// S3-native staging promotion: bytes live in a staging object, blob key absent.
+    {
+        EnvelopeHeader h;
+        h.kind = ObjectKind::Blob;
+        h.incarnation_tag = DB::UInt128(0xC0FFEE);
+        const String staging = encodeEnvelopeHeader(h, static_cast<uint32_t>(w.s->poolMeta().blob_header_len)) + kStaging;
+        w.b->putIfAbsent("p/staging/mount1/A-staging.tmp", staging);
+    }
+
+    /// condemned-local resurrection: present body + condemned meta, local source.
+    seedPresentBody(*w.b, w.s->layout(), w.s->poolMeta(), kResLocal);
+    writeMetaClean(*w.b, w.s->layout(), u128Of(kResLocal), std::string(kResLocal).size());
+    condemnMeta(*w.b, w.s->layout(), u128Of(kResLocal), /*condemn_round=*/7);
+
+    /// condemned-S3 resurrection: present body (= a verbatim promote of the staging object) + condemned
+    /// meta, S3 staging source.
+    {
+        EnvelopeHeader h;
+        h.kind = ObjectKind::Blob;
+        h.incarnation_tag = DB::UInt128(0xC0FFEE);
+        const String staging = encodeEnvelopeHeader(h, static_cast<uint32_t>(w.s->poolMeta().blob_header_len)) + kResS3;
+        w.b->putIfAbsent("p/staging/mount1/A-resurrect.tmp", staging);
+        w.b->putIfAbsent(w.s->layout().blobKey(idOf(kResS3)), staging);
+        writeMetaClean(*w.b, w.s->layout(), u128Of(kResS3), std::string(kResS3).size());
+        condemnMeta(*w.b, w.s->layout(), u128Of(kResS3), /*condemn_round=*/9);
+    }
+
+    w.requests = {
+        localRequest(kDedup),
+        localRequest(kAdopt),
+        localRequest(kFresh),
+        s3Request(kStaging, "p/staging/mount1/A-staging.tmp"),
+        localRequest(kResLocal),
+        s3Request(kResS3, "p/staging/mount1/A-resurrect.tmp"),
+    };
+    w.payloads = {kDedup, kAdopt, kFresh, kStaging, kResLocal, kResS3};
+    return w;
+}
+
+}
+
+TEST(CASUploadFanout, DepsEquivalenceAcrossBranches)
+{
+    /// Serial reference: pool size 1.
+    WorldA serial = arrangeWorldA();
+    auto serial_pool = makePool(1);
+    fanOutBlobUploads(*serial.build, serial.requests, *serial_pool);
+    const auto serial_deps = stableDeps(*serial.build);
+    const auto serial_backend = backendState(*serial.b, serial.s, serial.payloads);
+
+    /// Fanned-out: pool size 4, same inputs, freshly arranged world.
+    WorldA fanned = arrangeWorldA();
+    auto fanned_pool = makePool(4);
+    fanOutBlobUploads(*fanned.build, fanned.requests, *fanned_pool);
+    const auto fanned_deps = stableDeps(*fanned.build);
+    const auto fanned_backend = backendState(*fanned.b, fanned.s, fanned.payloads);
+
+    EXPECT_EQ(serial_deps.size(), 6u) << "one dep per unique ref";
+    EXPECT_EQ(serial_deps, fanned_deps) << "recorded deps must match across serial and fanned runs";
+    EXPECT_EQ(serial_backend, fanned_backend) << "backend end state must match across serial and fanned runs";
+
+    /// Every dep is a complete tokened blob dep (no branch left its effect behind as a side effect).
+    for (const auto & [ref, dep] : serial_deps)
+    {
+        EXPECT_EQ(std::get<0>(dep), ObjectKind::Blob);
+        EXPECT_TRUE(std::get<3>(dep)) << "every merged fan-out dep carries a token";
+    }
+    for (const auto & p : serial.payloads)
+        EXPECT_EQ(metaStateAt(*fanned.b, fanned.s->layout(), p), std::optional<MetaState>(MetaState::Clean));
+
+    /// Pass B: the size-triggered HeadHit branch, proven equivalent under its own (HEAD-first forced) config.
+    auto arrangeB = [](std::shared_ptr<InMemoryBackend> & b, PoolPtr & s, PartWriteTxnPtr & build)
+    {
+        b = std::make_shared<InMemoryBackend>();
+        s = openPool(b, /*head_first_min_bytes=*/1);
+        const RootNamespace ns{"srv1/nsFanoutB"};
+        build = precommitBuildFor(s, ns, "part");
+        seedPresentBody(*b, s->layout(), s->poolMeta(), "fanoutB-head-hit");
+        writeMetaClean(*b, s->layout(), u128Of("fanoutB-head-hit"), std::string("fanoutB-head-hit").size());
+    };
+
+    std::shared_ptr<InMemoryBackend> b1;
+    PoolPtr s1;
+    PartWriteTxnPtr build1;
+    arrangeB(b1, s1, build1);
+    std::vector<BlobUploadRequest> reqB{localRequest("fanoutB-head-hit")};
+    auto b_serial_pool = makePool(1);
+    fanOutBlobUploads(*build1, reqB, *b_serial_pool);
+
+    std::shared_ptr<InMemoryBackend> b2;
+    PoolPtr s2;
+    PartWriteTxnPtr build2;
+    arrangeB(b2, s2, build2);
+    auto b_fanned_pool = makePool(4);
+    fanOutBlobUploads(*build2, reqB, *b_fanned_pool);
+
+    EXPECT_EQ(stableDeps(*build1), stableDeps(*build2)) << "HeadHit branch: deps match serial vs fanned";
+    EXPECT_EQ(backendState(*b1, s1, {"fanoutB-head-hit"}), backendState(*b2, s2, {"fanoutB-head-hit"}))
+        << "HeadHit branch: backend end state matches serial vs fanned";
+}
+
+/// Test 1, GET-observability (routed from T3 review (a)): the resurrect invariant is that a condemned
+/// object is NEVER GET (revival is a fresh re-upload from the writer's own source). With a
+/// CountingBackend, assert ZERO get/getStream against the condemned blob keys through the whole fan-out.
+TEST(CASUploadFanout, CondemnedBranchesNeverGet)
+{
+    auto counting = std::make_shared<CountingBackend>();
+    auto s = openPool(counting);
+    const RootNamespace ns{"srv1/nsNoGet"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String local_payload = "noget-condemned-local";
+    const String s3_payload = "noget-condemned-s3";
+    const String s3_staging = "p/staging/mount1/noget-resurrect.tmp";
+
+    seedPresentBody(*counting, s->layout(), s->poolMeta(), local_payload);
+    writeMetaClean(*counting, s->layout(), u128Of(local_payload), local_payload.size());
+    condemnMeta(*counting, s->layout(), u128Of(local_payload), /*condemn_round=*/3);
+
+    {
+        EnvelopeHeader h;
+        h.kind = ObjectKind::Blob;
+        h.incarnation_tag = DB::UInt128(0xC0FFEE);
+        const String staging = encodeEnvelopeHeader(h, static_cast<uint32_t>(s->poolMeta().blob_header_len)) + s3_payload;
+        counting->putIfAbsent(s3_staging, staging);
+        counting->putIfAbsent(s->layout().blobKey(idOf(s3_payload)), staging);
+        writeMetaClean(*counting, s->layout(), u128Of(s3_payload), s3_payload.size());
+        condemnMeta(*counting, s->layout(), u128Of(s3_payload), /*condemn_round=*/5);
+    }
+
+    counting->resetCounts();   /// count only the fan-out's own backend traffic
+
+    std::vector<BlobUploadRequest> reqs{localRequest(local_payload), s3Request(s3_payload, s3_staging)};
+    auto pool = makePool(2);
+    fanOutBlobUploads(*build, reqs, *pool);
+
+    /// INV-1: revival is a fresh re-upload from the writer's own source; the condemned BODY object is
+    /// never read. (The fan-out DOES read the two condemned-META objects -- the meta point-read is how it
+    /// LEARNS an incarnation is condemned -- so the invariant is per-body-key, not a global GET count.)
+    const String local_key = s->layout().blobKey(idOf(local_payload));
+    const String s3_key = s->layout().blobKey(idOf(s3_payload));
+    EXPECT_EQ(counting->getCount(local_key), 0u) << "INV-1: the condemned local body is never read";
+    EXPECT_EQ(counting->getStreamCount(local_key), 0u) << "INV-1: the condemned local body is never streamed";
+    EXPECT_EQ(counting->getCount(s3_key), 0u) << "INV-1: the condemned S3 body is never read";
+    EXPECT_EQ(counting->getStreamCount(s3_key), 0u) << "INV-1: the condemned S3 body is never streamed";
+
+    /// Both resurrections still completed to Clean.
+    EXPECT_EQ(metaStateAt(*counting, s->layout(), local_payload), std::optional<MetaState>(MetaState::Clean));
+    EXPECT_EQ(metaStateAt(*counting, s->layout(), s3_payload), std::optional<MetaState>(MetaState::Clean));
+}
+
+/// Test 2: duplicate refs (staged-hardlink copies push a duplicate PendingBlob record) collapse to ONE
+/// task, and the merged build records exactly one dep for the ref.
+TEST(CASUploadFanout, DuplicateRefsLaunchOneTask)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsDup"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String payload = "dup-fresh-payload";
+    std::vector<BlobUploadRequest> reqs{localRequest(payload), localRequest(payload)};   /// same ref twice
+
+    std::atomic<int> dispatched{0};
+    std::map<BlobRef, int> per_ref;
+    std::mutex per_ref_m;
+    BlobUploadFanoutHooksForTest hooks;
+    hooks.on_dispatch = [&](const BlobRef & ref)
+    {
+        ++dispatched;
+        std::lock_guard l(per_ref_m);
+        ++per_ref[ref];
+    };
+
+    auto pool = makePool(4);
+    fanOutBlobUploads(*build, reqs, *pool, &hooks);
+
+    EXPECT_EQ(dispatched.load(), 1) << "two pending-blob records for one ref launch exactly one task";
+    EXPECT_EQ(per_ref[idOf(payload)], 1);
+    EXPECT_TRUE(build->depIsTokened(idOf(payload))) << "the one task's dep was merged";
+    EXPECT_EQ(build->depsSnapshotForTest().size(), 1u) << "exactly one dep for the unique ref";
+}
+
+/// Test 2, conflicting-size backstop: two records for the SAME ref with different declared sizes are a
+/// staging bug -- rejected with LOGICAL_ERROR before any task runs. LOGICAL_ERROR aborts under
+/// debug/sanitizer builds, so the abort is proven positively there (DeathTest) and the exception +
+/// build-untouched postcondition in a release build.
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASUploadFanout, ConflictingDuplicateSizesRejected)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsDupConflict"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String payload = "dup-conflict-payload";
+    BlobUploadRequest a = localRequest(payload);
+    BlobUploadRequest c = localRequest(payload);
+    c.declared_size = a.declared_size + 1;   /// same ref, conflicting declared size
+    c.source.size = c.declared_size;         /// keep declared == source so only the group conflict trips
+
+    const auto before = build->depsSnapshotForTest();
+    auto pool = makePool(4);
+    expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&]
+    {
+        fanOutBlobUploads(*build, std::vector<BlobUploadRequest>{a, c}, *pool);
+    });
+    EXPECT_EQ(build->depsSnapshotForTest(), before) << "a rejected fan-out merges nothing";
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASUploadFanoutDeathTest, ConflictingDuplicateSizesAbort)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsDupConflict"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String payload = "dup-conflict-payload";
+    BlobUploadRequest a = localRequest(payload);
+    BlobUploadRequest c = localRequest(payload);
+    c.declared_size = a.declared_size + 1;
+    c.source.size = c.declared_size;
+
+    auto pool = makePool(4);
+    EXPECT_DEATH({ fanOutBlobUploads(*build, std::vector<BlobUploadRequest>{a, c}, *pool); }, "");
+}
+#endif
+
+/// Test 2, declared_size == source.size fail-close (routed from T3 review (b)): a request whose grouping
+/// key (declared_size) disagrees with its streaming authority (source.size) is a wiring bug -- rejected
+/// with LOGICAL_ERROR before dispatch (DeathTest split for debug/sanitizer builds).
+#ifndef DEBUG_OR_SANITIZER_BUILD
+TEST(CASUploadFanout, DeclaredSizeMustMatchSourceSize)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsDeclared"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    BlobUploadRequest r = localRequest("declared-mismatch-payload");
+    r.declared_size = r.source.size + 7;   /// diverge the grouping key from the streaming authority
+
+    auto pool = makePool(2);
+    expectThrowsCode(DB::ErrorCodes::LOGICAL_ERROR, [&]
+    {
+        fanOutBlobUploads(*build, std::vector<BlobUploadRequest>{r}, *pool);
+    });
+}
+#endif
+
+#if defined(DEBUG_OR_SANITIZER_BUILD)
+TEST(CASUploadFanoutDeathTest, DeclaredSizeMismatchAborts)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsDeclared"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    BlobUploadRequest r = localRequest("declared-mismatch-payload");
+    r.declared_size = r.source.size + 7;
+
+    auto pool = makePool(2);
+    EXPECT_DEATH({ fanOutBlobUploads(*build, std::vector<BlobUploadRequest>{r}, *pool); }, "");
+}
+#endif
+
+/// The condemned-LOCAL displacement, end to end on the new unconditional streaming shape: the
+/// resurrected body is [fresh_header][payload], its token differs from the condemned one, and the
+/// meta flips back to Clean -- which is exactly what a later attempt reads to adopt instead of
+/// re-writing.
+TEST(CASUploadFanout, CondemnedLocalResurrectStreamsAndFlipsMetaClean)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsResLocalStream"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String payload = "condemned-local-streamed-payload";
+    seedPresentBody(*b, s->layout(), s->poolMeta(), payload);
+    writeMetaClean(*b, s->layout(), u128Of(payload), payload.size());
+    condemnMeta(*b, s->layout(), u128Of(payload), /*condemn_round=*/13);
+
+    const String blob_key = s->layout().blobKey(idOf(payload));
+    const Token condemned_token = b->head(blob_key).token;
+
+    std::vector<BlobUploadRequest> reqs{localRequest(payload)};
+    auto pool = makePool(2);
+    fanOutBlobUploads(*build, reqs, *pool, nullptr);
+
+    /// A fresh incarnation displaced the condemned one; INV-NO-RETURN: the queued exact-token delete
+    /// of the condemned incarnation must miss the resurrection.
+    const HeadResult after = b->head(blob_key);
+    ASSERT_TRUE(after.exists);
+    EXPECT_NE(after.token, condemned_token);
+    EXPECT_EQ(b->deleteExact(blob_key, condemned_token).kind, DeleteOutcome::Kind::TokenMismatch);
+    EXPECT_TRUE(b->head(blob_key).exists);
+
+    /// The payload survived verbatim under the fresh header.
+    const auto got = b->get(blob_key);
+    ASSERT_TRUE(got.has_value());
+    EXPECT_EQ(got->bytes.substr(s->poolMeta().blob_header_len), payload);
+
+    /// The meta flipped back to Clean -- the signal a later attempt adopts on.
+    const auto lm = loadMetaForTest(*b, s->layout(), u128Of(payload));
+    ASSERT_TRUE(lm.has_value());
+    EXPECT_EQ(lm->meta.state, MetaState::Clean);
+}
+
+/// `open` is the per-attempt unit of re-readability, and this pins the attempt count for the
+/// present-condemned shape: open #1 is the ordinary conditional-create attempt (streamed, refused at
+/// finalize because the body exists), open #2 is the resurrect itself. Anything ABOVE two would mean
+/// a hidden materialization pass or a mid-write re-open crept in; anything below would mean the
+/// create attempt stopped streaming (a protocol change, not an optimization to make silently).
+TEST(CASUploadFanout, CondemnedLocalResurrectOpensTheSourcePerAttempt)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsResLocalOpens"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String payload = "condemned-local-open-count-payload";
+    seedPresentBody(*b, s->layout(), s->poolMeta(), payload);
+    writeMetaClean(*b, s->layout(), u128Of(payload), payload.size());
+    condemnMeta(*b, s->layout(), u128Of(payload), /*condemn_round=*/17);
+
+    int opens = 0;
+    BlobSource source;
+    source.size = payload.size();
+    source.open = [&opens, payload]() -> std::unique_ptr<DB::ReadBuffer>
+    {
+        ++opens;
+        return std::make_unique<DB::ReadBufferFromOwnString>(payload);
+    };
+
+    std::vector<BlobUploadRequest> reqs{BlobUploadRequest{idOf(payload), std::move(source), payload.size()}};
+    auto pool = makePool(2);
+    fanOutBlobUploads(*build, reqs, *pool, nullptr);
+
+    EXPECT_EQ(opens, 2) << "conditional-create attempt + resurrect: exactly one open each, nothing extra";
+    EXPECT_TRUE(build->depIsTokened(idOf(payload)));
+}
+
+/// Test 2, condemned-S3 duplicate pair resurrects content-correctly: two duplicate S3-staging records
+/// for one condemned ref collapse to ONE resurrect task; the fresh incarnation displaces the condemned
+/// one (token changes, meta returns to Clean) and the content is the staging object's payload.
+TEST(CASUploadFanout, DuplicateCondemnedS3ResurrectsCorrectly)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsDupResS3"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String payload = "dup-condemned-s3-payload";
+    const String staging_key = "p/staging/mount1/dup-resurrect.tmp";
+    EnvelopeHeader h;
+    h.kind = ObjectKind::Blob;
+    h.incarnation_tag = DB::UInt128(0xC0FFEE);
+    const String staging_bytes = encodeEnvelopeHeader(h, static_cast<uint32_t>(s->poolMeta().blob_header_len)) + payload;
+    b->putIfAbsent(staging_key, staging_bytes);
+    b->putIfAbsent(s->layout().blobKey(idOf(payload)), staging_bytes);
+    writeMetaClean(*b, s->layout(), u128Of(payload), payload.size());
+    condemnMeta(*b, s->layout(), u128Of(payload), /*condemn_round=*/11);
+
+    const Token condemned_token = b->head(s->layout().blobKey(idOf(payload))).token;
+
+    std::atomic<int> dispatched{0};
+    BlobUploadFanoutHooksForTest hooks;
+    hooks.on_dispatch = [&](const BlobRef &) { ++dispatched; };
+
+    std::vector<BlobUploadRequest> reqs{s3Request(payload, staging_key), s3Request(payload, staging_key)};
+    auto pool = makePool(4);
+    fanOutBlobUploads(*build, reqs, *pool, &hooks);
+
+    EXPECT_EQ(dispatched.load(), 1) << "duplicate condemned records collapse to one resurrect task";
+    EXPECT_TRUE(build->depIsTokened(idOf(payload)));
+    const Token after_token = b->head(s->layout().blobKey(idOf(payload))).token;
+    EXPECT_NE(after_token.value, condemned_token.value) << "a fresh incarnation displaced the condemned one";
+    EXPECT_EQ(metaStateAt(*b, s->layout(), payload), std::optional<MetaState>(MetaState::Clean));
+    EXPECT_EQ(logicalPayloadAt(*b, s->layout().blobKey(idOf(payload)), s->poolMeta().blob_header_len), payload);
+}
+
+/// Test 3: one task fails (a poisoned source), one sibling succeeds. Merge-nothing means the build stays
+/// at its pre-fan-out state; the abandoned precommit turns the successful sibling's uploaded body into
+/// ORDINARY GC-reclaimable debris (NOT a new orphan class) -- a GC round reclaims it.
+TEST(CASUploadFanout, MergeNothingOnFailure)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsMergeNothing"};
+
+    const String good = "merge-nothing-good-sibling";
+    const String poisoned = "merge-nothing-poisoned";
+
+    PartWriteInfo info;
+    info.intended_ref = ns.string() + "/part";
+    auto build = s->beginPartWrite(info);
+    /// The precommit names BOTH blobs (the durable manifest edge the real writer establishes before any
+    /// upload), so the successful sibling's body is edge-protected until the precommit is abandoned.
+    const ManifestId id = build->stageManifest({blobEntryFor("data.bin", u128Of(good), good.size()),
+                                                blobEntryFor("data.cmrk3", u128Of(poisoned), poisoned.size())});
+    build->precommitAdd(ns, "part", id);
+
+    /// Poison the failing sibling via the in-task seam: throw a plain (non-LOGICAL, non-ABORTED)
+    /// exception so it is neither retried nor an abort under sanitizer builds.
+    BlobUploadFanoutHooksForTest hooks;
+    hooks.in_task = [&](const BlobRef & ref)
+    {
+        if (ref == idOf(poisoned))
+            throw DB::Exception(DB::ErrorCodes::INCORRECT_DATA, "poisoned upload source (test)");
+    };
+
+    std::vector<BlobUploadRequest> reqs{localRequest(good), localRequest(poisoned)};
+    auto pool = makePool(4);
+    expectThrowsCode(DB::ErrorCodes::INCORRECT_DATA, [&]
+    {
+        fanOutBlobUploads(*build, reqs, *pool, &hooks);
+    });
+
+    /// Merge-nothing: the build recorded NO dep, even though the good sibling's body was uploaded.
+    EXPECT_FALSE(build->depIsTokened(idOf(good)));
+    EXPECT_FALSE(build->depIsTokened(idOf(poisoned)));
+    EXPECT_EQ(build->depsSnapshotForTest().size(), 0u);
+
+    /// Abandon the precommit (the existing failure path), then GC reclaims the orphaned sibling body.
+    build->abandon();
+    s->renewWatermarkOnce();
+    Gc gc(s, DB::Cas::hexToU128("00000000000000000000000000000001"));
+    EXPECT_TRUE(runRoundsUntilAbsent(s, gc, *b, s->layout(), u128Of(good)))
+        << "the successful sibling's body is ordinary GC-reclaimable debris after abandon";
+    EXPECT_TRUE(blobAbsent(*b, s->layout(), u128Of(poisoned))) << "the poisoned sibling never uploaded a body";
+}
+
+/// Test 4: two DISTINCT-ref fresh uploads, latch-crossed so both are inside `uploadBlobDetached` (hence
+/// both touching the ONE shared dedup cache) at once. The cache's internal locking makes the concurrent
+/// insertion correct; on the TSan lane this pins that there is no data race on the shared cache.
+TEST(CASUploadFanout, ConcurrentDeduplicationCacheInsertion)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsCacheRace"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String pa = "cache-race-a";
+    const String pb = "cache-race-b";
+
+    /// A latch of 2 crossed with a pool of 2 cannot hang: both tasks are guaranteed to run concurrently
+    /// (the calling thread never occupies a pool slot), so both reach the latch and release together.
+    std::latch both_in{2};
+    BlobUploadFanoutHooksForTest hooks;
+    hooks.in_task = [&](const BlobRef &) { both_in.arrive_and_wait(); };
+
+    std::vector<BlobUploadRequest> reqs{localRequest(pa), localRequest(pb)};
+    auto pool = makePool(2);
+    fanOutBlobUploads(*build, reqs, *pool, &hooks);
+
+    EXPECT_TRUE(build->depIsTokened(idOf(pa)));
+    EXPECT_TRUE(build->depIsTokened(idOf(pb)));
+    EXPECT_TRUE(s->dedupCacheContains(idOf(pa))) << "concurrent insertion left both keys present";
+    EXPECT_TRUE(s->dedupCacheContains(idOf(pb)));
+}
+
+/// Test 5: pool saturation is bounded. Eight blobs run through a pool of 2 (peak concurrency 2 is
+/// observed) and a pool of 1 (peak concurrency 1 -- the single worker cannot self-wait for a second
+/// concurrent task, so it FAILS FAST via the bounded wait). Both configurations complete every upload:
+/// pool size 1 correctly degenerates to serial without deadlock.
+TEST(CASUploadFanout, PoolSaturationBounded)
+{
+    constexpr int kBlobs = 8;
+    /// Bounds are DECOUPLED by pool size. Pool 2 will reach the 2-task rendezvous in microseconds under
+    /// any realistic load, so its bound is generous (10s) purely as a hang guard -- it is essentially
+    /// never waited out (and `entered == total` releases any final straggler). Pool 1 CANNOT form a pair
+    /// (its single worker is occupied by the waiting task while the caller thread only joins), so its
+    /// first waiter must time out; 500ms is far above the microseconds a real pair needs, yet keeps the
+    /// serial run fast.
+    auto runEight = [](size_t pool_size, ConcurrencyProbe & probe, std::chrono::milliseconds bound)
+    {
+        auto b = std::make_shared<InMemoryBackend>();
+        auto s = openPool(b);
+        const RootNamespace ns{"srv1/nsSaturate"};
+        auto build = precommitBuildFor(s, ns, "part");
+
+        std::vector<BlobUploadRequest> reqs;
+        std::vector<String> payloads;
+        for (int i = 0; i < kBlobs; ++i)
+        {
+            payloads.push_back("saturate-payload-" + std::to_string(i));
+            reqs.push_back(localRequest(payloads.back()));
+        }
+        probe.total = kBlobs;
+
+        BlobUploadFanoutHooksForTest hooks;
+        hooks.in_task = [&, bound](const BlobRef &) { probe.enter(2, bound); };
+
+        auto pool = makePool(pool_size);
+        fanOutBlobUploads(*build, reqs, *pool, &hooks);
+
+        for (const auto & p : payloads)
+            EXPECT_TRUE(build->depIsTokened(idOf(p))) << "every blob uploaded (pool_size=" << pool_size << ")";
+    };
+
+    ConcurrencyProbe probe2;
+    runEight(2, probe2, std::chrono::seconds(10));
+    EXPECT_EQ(probe2.peak, 2) << "pool of 2 runs two blob uploads concurrently";
+    EXPECT_FALSE(probe2.timed_out) << "pool of 2 forms a pair without hitting the bound";
+
+    ConcurrencyProbe probe1;
+    runEight(1, probe1, std::chrono::milliseconds(500));
+    EXPECT_EQ(probe1.peak, 1) << "pool of 1 degenerates to serial (never occupies the caller thread's slot)";
+    EXPECT_TRUE(probe1.timed_out) << "the single worker fails fast on the bounded wait instead of hanging";
+}
+
+/// Test 6a: even when one task fails immediately, the join drains EVERY task before the failure surfaces.
+/// A failing task counts down an event and throws; a sibling waits for that event, then uploads. The
+/// fan-out rethrows only after the join, so the sibling's body is present in the backend by the time the
+/// caller observes the failure.
+TEST(CASUploadFanout, DrainPrecedesUnwind)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsDrain"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String failing = "drain-failing";
+    const String slow = "drain-slow-sibling";
+
+    BoundedEvent failing_threw;
+    BlobUploadFanoutHooksForTest hooks;
+    hooks.in_task = [&](const BlobRef & ref)
+    {
+        if (ref == idOf(failing))
+        {
+            failing_threw.fire();
+            throw DB::Exception(DB::ErrorCodes::INCORRECT_DATA, "drain-test failing task");
+        }
+        else
+        {
+            /// 5-second bound: the failing task fires the event in microseconds; the bound only guards
+            /// against a hang if the failing task never runs (a design regression).
+            (void)failing_threw.wait(std::chrono::seconds(5));
+        }
+    };
+
+    std::vector<BlobUploadRequest> reqs{localRequest(failing), localRequest(slow)};
+    auto pool = makePool(2);
+    expectThrowsCode(DB::ErrorCodes::INCORRECT_DATA, [&]
+    {
+        fanOutBlobUploads(*build, reqs, *pool, &hooks);
+    });
+
+    EXPECT_TRUE(b->head(s->layout().blobKey(idOf(slow))).exists)
+        << "the sibling's upload was drained by the join before the failure surfaced";
+    EXPECT_FALSE(build->depIsTokened(idOf(slow))) << "merge-nothing: the drained sibling's dep is not merged";
+}
+
+/// Test 6b: a throw injected DURING the dispatch loop (before all tasks are enqueued) still drains the
+/// tasks already scheduled -- the fan-out drains every already-scheduled task on the unwinding path
+/// before the captured storage is destroyed (the B90 lesson). The first task's body is present after the
+/// dispatch throw is caught.
+///
+/// The throw is GATED on the first task actually entering its body: the runner marks tasks that are
+/// still SCHEDULED as CANCELLED during unwind and skips waiting for a cancelled task, so a throw fired
+/// before the first task's body ran could cancel it and leave its body ABSENT -- a real flakiness the
+/// gate removes. Once the first task's `in_task` hook has fired, that task is past SCHEDULED (RUNNING),
+/// so it can no longer be cancelled and the drain deterministically waits for its upload.
+TEST(CASUploadFanout, DispatchThrowStillDrains)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsDispatchThrow"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String first = "dispatch-throw-first";
+    const String second = "dispatch-throw-second";
+    /// Dispatch runs in ascending-ref order, so the SMALLER-ref payload is the one enqueued before the
+    /// second dispatch throws.
+    const String enqueued = (idOf(first) < idOf(second)) ? first : second;
+
+    BoundedEvent first_task_running;
+    std::atomic<int> dispatch_calls{0};
+    BlobUploadFanoutHooksForTest hooks;
+    hooks.in_task = [&](const BlobRef & ref)
+    {
+        if (ref == idOf(enqueued))
+            first_task_running.fire();
+    };
+    hooks.on_dispatch = [&](const BlobRef &)
+    {
+        if (++dispatch_calls == 2)
+        {
+            /// Wait until the first task's body has entered before throwing, so it is RUNNING (not
+            /// SCHEDULED) and the unwind cannot cancel it. Pool size 2 guarantees the first task gets a
+            /// worker while this (dispatch) thread waits; 10s is a pure hang guard, never a sequencer.
+            EXPECT_TRUE(first_task_running.wait(std::chrono::seconds(10)))
+                << "the first dispatched task must reach its body before the dispatch throw";
+            throw DB::Exception(DB::ErrorCodes::INCORRECT_DATA, "dispatch-loop throw (test)");
+        }
+    };
+
+    std::vector<BlobUploadRequest> reqs{localRequest(first), localRequest(second)};
+    auto pool = makePool(2);
+    expectThrowsCode(DB::ErrorCodes::INCORRECT_DATA, [&]
+    {
+        fanOutBlobUploads(*build, reqs, *pool, &hooks);
+    });
+
+    EXPECT_EQ(dispatch_calls.load(), 2) << "the throw fired on the second dispatch";
+    /// The already-RUNNING first task was drained before the stack unwound, so its body is present
+    /// although nothing was merged.
+    EXPECT_TRUE(b->head(s->layout().blobKey(idOf(enqueued))).exists)
+        << "the already-dispatched task was drained before the stack unwound";
+    EXPECT_EQ(build->depsSnapshotForTest().size(), 0u) << "merge-nothing on a dispatch throw";
+}
+
+
+
+
+/// Test 6c (codex stage-1 review, Critical): a throw at the TRACKING-PUBLICATION seam still drains every
+/// already-scheduled task before the captured `results` storage is destroyed. In the broken form a task
+/// could be scheduled-but-untracked at the throw and run later against freed `results` (a
+/// heap-use-after-free); the fix schedules-and-tracks in ONE no-throw step (pre-reserved handle vector)
+/// and joins via a scope-exit drain guard, so a seam throw finds every scheduled task already tracked and
+/// drains it. The throw is gated on the first task RUNNING (same reason as `DispatchThrowStillDrains`) so
+/// its drain is deterministic; under ASan this run is UAF-clean -- the regression signature of a
+/// scheduled-but-untracked task is a heap-use-after-free on `results` here.
+TEST(CASUploadFanout, TrackingSeamThrowStillDrains)
+{
+    auto b = std::make_shared<InMemoryBackend>();
+    auto s = openPool(b);
+    const RootNamespace ns{"srv1/nsTrackSeam"};
+    auto build = precommitBuildFor(s, ns, "part");
+
+    const String first = "track-seam-first";
+    const String second = "track-seam-second";
+    /// Dispatch is ascending-ref, so the smaller ref is enqueued first.
+    const String smaller = (idOf(first) < idOf(second)) ? first : second;
+
+    BoundedEvent first_task_running;
+    std::atomic<int> enqueue_calls{0};
+    BlobUploadFanoutHooksForTest hooks;
+    hooks.in_task = [&](const BlobRef & ref)
+    {
+        if (ref == idOf(smaller))
+            first_task_running.fire();
+    };
+    hooks.after_enqueue = [&](const BlobRef &)
+    {
+        /// Throw at the tracking seam of the SECOND enqueue -- by then both tasks are scheduled and (in
+        /// the fixed code) tracked, so the drain guard must join both. Gate on the first task RUNNING so
+        /// the drain cannot race a still-SCHEDULED cancellation; 10s is a pure hang guard.
+        if (++enqueue_calls == 2)
+        {
+            EXPECT_TRUE(first_task_running.wait(std::chrono::seconds(10)))
+                << "the first task must be RUNNING before the tracking-seam throw";
+            throw DB::Exception(DB::ErrorCodes::INCORRECT_DATA, "tracking-seam throw (test)");
+        }
+    };
+
+    std::vector<BlobUploadRequest> reqs{localRequest(first), localRequest(second)};
+    auto pool = makePool(2);
+    expectThrowsCode(DB::ErrorCodes::INCORRECT_DATA, [&]
+    {
+        fanOutBlobUploads(*build, reqs, *pool, &hooks);
+    });
+
+    /// The already-scheduled first task was drained before `results` was destroyed, so its body is
+    /// present; nothing was merged (merge-nothing on any fan-out throw).
+    EXPECT_TRUE(b->head(s->layout().blobKey(idOf(smaller))).exists)
+        << "an already-scheduled task was not drained before the stack unwound";
+    EXPECT_EQ(build->depsSnapshotForTest().size(), 0u) << "merge-nothing on a tracking-seam throw";
+}
diff --git a/src/Disks/tests/gtest_cas_wire_vocab.cpp b/src/Disks/tests/gtest_cas_wire_vocab.cpp
new file mode 100644
index 000000000000..efbe3da7a0ae
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_wire_vocab.cpp
@@ -0,0 +1,53 @@
+#include <gtest/gtest.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasWireVocab.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasTextFormat.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Primitives/CasTypes.h>
+#include <IO/ReadBufferFromMemory.h>
+
+using namespace DB::Cas;
+
+namespace DB::ErrorCodes { extern const int CORRUPTED_DATA; }
+
+TEST(CASWireVocab, EnumWordsRoundTrip)
+{
+    for (TokenType t : {TokenType::ETag, TokenType::Generation, TokenType::Emulated})
+        EXPECT_EQ(tokenTypeFromWord(tokenTypeToWord(t), "t"), t);
+    for (BlobHashAlgo a : {BlobHashAlgo::CityHash128, BlobHashAlgo::XXH3_128, BlobHashAlgo::Sha256})
+        EXPECT_EQ(blobHashAlgoFromWord(blobHashAlgoName(a), "a"), a);
+    EXPECT_EQ(objectKindFromWord(objectKindToWord(ObjectKind::Blob), "k"), ObjectKind::Blob);
+    EXPECT_THROW(tokenTypeFromWord("nope", "t"), DB::Exception);
+    EXPECT_THROW(blobHashAlgoFromWord("nope", "a"), DB::Exception);
+}
+
+TEST(CASWireVocab, SiblingFieldsWriteAndReadBack)
+{
+    CasJsonWriter out;
+    bool first = true;
+    writeTokenFields(out, first, Token{"etag-abc\"x", TokenType::ETag});
+    const BlobRef ref{BlobHashAlgo::CityHash128, BlobDigest::fromU128(hexToU128("00112233445566778899aabbccddeeff"))};
+    writeBlobRefFields(out, first, ref);
+    closeObject(out, first);
+    const String rendered = std::move(out).take();
+    EXPECT_EQ(rendered,
+        R"({"tt":"etag","tv":"etag-abc\"x","ha":"ch128","h":"00112233445566778899aabbccddeeff"})");
+
+    DB::ReadBufferFromMemory in(rendered.data(), rendered.size());
+    JsonObjectReader r(in, KeyStrictness::Tolerant, "t");
+    String key;
+    String tv;
+    String ha;
+    String h;
+    TokenType tt{};
+    while (r.nextKey(key))
+    {
+        if (key == "tt") tt = tokenTypeFromWord(r.readString(), "t");
+        else if (key == "tv") tv = r.readString();
+        else if (key == "ha") ha = r.readString();
+        else if (key == "h") h = r.readString();
+        else r.skipUnknown(key);
+    }
+    EXPECT_EQ(tt, TokenType::ETag);
+    EXPECT_EQ(tv, "etag-abc\"x");
+    const BlobRef back{blobHashAlgoFromWord(ha, "a"), codecFor(blobHashAlgoFromWord(ha, "a")).fromHex(h)};
+    EXPECT_EQ(back, ref);
+}
diff --git a/src/Disks/tests/gtest_cas_writer_duties.cpp b/src/Disks/tests/gtest_cas_writer_duties.cpp
new file mode 100644
index 000000000000..662c0daa165e
--- /dev/null
+++ b/src/Disks/tests/gtest_cas_writer_duties.cpp
@@ -0,0 +1,545 @@
+#include <gtest/gtest.h>
+
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPartWriteTxn.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Pool/CasPool.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Backend/CasInMemoryBackend.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Formats/CasServerRootFormats.h>
+#include <Disks/DiskObjectStorage/MetadataStorages/ContentAddressed/Gc/CasGc.h>
+#include <Disks/tests/cas_test_helpers.h>
+
+#include <chrono>
+#include <limits>
+#include <memory>
+#include <vector>
+
+namespace DB::ErrorCodes
+{
+extern const int NETWORK_ERROR;
+}
+
+using namespace DB::Cas;
+
+namespace
+{
+
+PoolConfig singleAttemptConfig()
+{
+    PoolConfig config{
+        .pool_prefix = "p",
+        .server_root_id = "test",
+        .background_watermark = false,
+    };
+    config.cas_request_budget.max_attempts = 1;
+    config.cas_request_budget.attempt_timeout_ms = 100;
+    config.cas_request_budget.operation_deadline_ms = 5000;
+    config.cas_request_budget.lease_safety_margin_ms = 100;
+    return config;
+}
+
+PoolPtr openSingleAttemptPool(const BackendPtr & backend)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    return Pool::open(backend, singleAttemptConfig());
+}
+
+PoolPtr openFrozenSingleAttemptPool(const BackendPtr & backend)
+{
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    PoolConfig config = singleAttemptConfig();
+    config.boot_ms_fn = [] { return uint64_t{0}; };
+    config.mount_renew_period = std::chrono::hours{1};
+    return Pool::open(backend, config);
+}
+
+PartWriteTxnPtr stageEmptyManifest(
+    const PoolPtr & store, const RootNamespace & ns, const String & ref_name, ManifestId & id)
+{
+    PartWriteInfo info;
+    info.intended_namespace = ns;
+    info.intended_ref = ns.string() + "/" + ref_name;
+    auto build = store->beginPartWrite(std::move(info));
+    id = build->stageManifest({});
+    return build;
+}
+
+void publishEmptyRef(const PoolPtr & store, const RootNamespace & ns, const String & ref_name)
+{
+    ManifestId id;
+    auto build = stageEmptyManifest(store, ns, ref_name, id);
+    build->precommitAdd(ns, ref_name, id);
+    build->promote(ns, ref_name, build->buildId(), id);
+}
+
+uint64_t leaveRejectedCleanupDuty(const PoolPtr & store, const RootNamespace & ns)
+{
+    ManifestId rejected_id;
+    auto rejected = stageEmptyManifest(store, ns, "rejected", rejected_id);
+    const uint64_t rejected_seq = rejected->buildSeq();
+
+    store->setMountDeadline(100);
+    DB::Cas::tests::expectThrowsCode(
+        DB::ErrorCodes::NETWORK_ERROR,
+        [&] { rejected->precommitAdd(ns, "rejected", rejected_id); });
+    EXPECT_EQ(rejected->precommitState(), PartWriteTxn::PrecommitState::Uncertain);
+
+    rejected.reset();
+    EXPECT_EQ(store->minActive(), rejected_seq);
+    store->setMountDeadline(30000);
+    return rejected_seq;
+}
+
+}
+
+/// Removing the deferred-cleanup transfer from `~PartWriteTxn` makes this test fail at the first
+/// `minActive` assertion: the old unconditional destructor retirement advances the build floor while
+/// the owner-grant outcome is still unknown. The later assertions pin the other half of the duty: the
+/// next mutation resolves the durable wedge, removes the exact old precommit, and only then retires it.
+TEST(CASWriterDuties, UncertainAdoptedGrantStaysActiveUntilTheNextMutationRemovesIt)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openSingleAttemptPool(backend);
+    const RootNamespace ns{"srv1/writer_duty_adopt"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    ManifestId abandoned_id;
+    auto abandoned = stageEmptyManifest(store, ns, "abandoned", abandoned_id);
+    const uint64_t abandoned_seq = abandoned->buildSeq();
+    const String abandoned_manifest_key = store->layout().manifestKey(abandoned_id);
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::LandedThenLost;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(
+        DB::ErrorCodes::NETWORK_ERROR,
+        [&] { abandoned->precommitAdd(ns, "abandoned", abandoned_id); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    ASSERT_EQ(abandoned->precommitState(), PartWriteTxn::PrecommitState::Uncertain);
+
+    abandoned.reset();
+    EXPECT_EQ(store->minActive(), abandoned_seq)
+        << "an unresolved owner grant must keep its build active after the transaction object is gone";
+
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::None;
+    ManifestId successor_id;
+    auto successor = stageEmptyManifest(store, ns, "successor", successor_id);
+    const uint64_t successor_seq = successor->buildSeq();
+    successor->precommitAdd(ns, "successor", successor_id);
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->minActive(), successor_seq)
+        << "the abandoned build retires only after its exact cleanup duty settles";
+    EXPECT_EQ(
+        store->livePrecommitsForTest(ns),
+        (std::set<std::pair<String, ManifestRef>>{{"successor", successor_id.ref}}));
+    EXPECT_TRUE(backend->head(abandoned_manifest_key).exists)
+        << "the removed precommit body remains GC-owned until its decrement is sealed";
+
+    successor->abandon();
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+}
+
+/// Removing the absent-owner arm makes the deferred duty either stay forever or try to remove an owner
+/// that was never transmitted. A controller pre-attempt refusal proves the grant absent; the next
+/// healthy mutation must drain that duty as a no-op and retire the old build before publishing itself.
+TEST(CASWriterDuties, ProvenAbsentGrantDrainsAsNoOpBeforeTheNextMutation)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    PoolConfig config = singleAttemptConfig();
+    config.boot_ms_fn = [] { return uint64_t{0}; };
+    config.mount_renew_period = std::chrono::hours{1};
+    auto store = Pool::open(backend, config);
+    const RootNamespace ns{"srv1/writer_duty_reject"};
+
+    ManifestId rejected_id;
+    auto rejected = stageEmptyManifest(store, ns, "rejected", rejected_id);
+    const uint64_t rejected_seq = rejected->buildSeq();
+
+    store->setMountDeadline(100);
+    DB::Cas::tests::expectThrowsCode(
+        DB::ErrorCodes::NETWORK_ERROR,
+        [&] { rejected->precommitAdd(ns, "rejected", rejected_id); });
+    ASSERT_FALSE(store->refLaneWedgedForTest(ns));
+    ASSERT_EQ(rejected->precommitState(), PartWriteTxn::PrecommitState::Uncertain);
+
+    rejected.reset();
+    EXPECT_EQ(store->minActive(), rejected_seq)
+        << "the destructor cannot retire even an uncertain grant whose rejection has not been consumed";
+
+    store->setMountDeadline(30000);
+    ManifestId successor_id;
+    auto successor = stageEmptyManifest(store, ns, "successor", successor_id);
+    const uint64_t successor_seq = successor->buildSeq();
+    successor->precommitAdd(ns, "successor", successor_id);
+
+    EXPECT_EQ(store->minActive(), successor_seq);
+    EXPECT_EQ(
+        store->livePrecommitsForTest(ns),
+        (std::set<std::pair<String, ManifestRef>>{{"successor", successor_id.ref}}));
+
+    successor->abandon();
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+}
+
+/// The model gate recorded both wedge-resolution witnesses (adopt and reject); the C++ suite drove
+/// only the adopt arm above. This drives an uncertain grant into an ACTUAL wedged lane -- unlike
+/// `ProvenAbsentGrantDrainsAsNoOpBeforeTheNextMutation`'s controller pre-attempt refusal, which never
+/// wedges at all -- and resolves it as REJECT: `Mode::Unresolved` lands nothing, so the next attempt's
+/// resolve-before-reissue GET proves the key absent. The duty must then drain as a no-op: no
+/// `OwnerTransition` removal is owed for an absent precommit, the wedge clears, and `minActive` advances
+/// past the rejected build exactly as the no-wedge reject arm does.
+TEST(CASWriterDuties, WedgeResolvedAsRejectDrainsTheDutyAsNoOp)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openSingleAttemptPool(backend);
+    const RootNamespace ns{"srv1/writer_duty_wedge_reject"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+
+    ManifestId rejected_id;
+    auto rejected = stageEmptyManifest(store, ns, "rejected", rejected_id);
+    const uint64_t rejected_seq = rejected->buildSeq();
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::Unresolved;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(
+        DB::ErrorCodes::NETWORK_ERROR,
+        [&] { rejected->precommitAdd(ns, "rejected", rejected_id); });
+    ASSERT_TRUE(store->refLaneWedgedForTest(ns));
+    ASSERT_EQ(rejected->precommitState(), PartWriteTxn::PrecommitState::Uncertain);
+
+    rejected.reset();
+    EXPECT_EQ(store->minActive(), rejected_seq)
+        << "an unresolved owner grant must keep its build active after the transaction object is gone";
+
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::None;
+    ManifestId successor_id;
+    auto successor = stageEmptyManifest(store, ns, "successor", successor_id);
+    const uint64_t successor_seq = successor->buildSeq();
+    successor->precommitAdd(ns, "successor", successor_id);
+
+    EXPECT_FALSE(store->refLaneWedgedForTest(ns));
+    EXPECT_EQ(store->minActive(), successor_seq)
+        << "the rejected build retires only after its exact cleanup duty settles as a no-op";
+    EXPECT_EQ(
+        store->livePrecommitsForTest(ns),
+        (std::set<std::pair<String, ManifestRef>>{{"successor", successor_id.ref}}));
+
+    successor->abandon();
+    EXPECT_TRUE(store->livePrecommitsForTest(ns).empty());
+}
+
+/// Removing `mutateRefsAfterWriterCleanup` from the `dropRef` delegate leaves the rejected build at
+/// `minActive` even though the ref removal succeeds. The observable floor proves the direct API
+/// serviced the inherited cleanup duty before performing its own mutation.
+TEST(CASWriterDuties, DropRefServicesPendingDutyBeforeRemovingTheRef)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = openFrozenSingleAttemptPool(backend);
+    const RootNamespace ns{"srv1/writer_duty_drop_ref"};
+    publishEmptyRef(store, ns, "target");
+    leaveRejectedCleanupDuty(store, ns);
+
+    store->dropRef(ns, "target");
+
+    EXPECT_FALSE(store->resolveRef(ns, "target").has_value());
+    EXPECT_EQ(store->minActive(), store->peekNextBuildSeq());
+}
+
+/// Removing the shared drain seam from `updateRefPublishedAt` lets the timestamp mutation overtake a
+/// pending writer duty. The update remains observable, while the independent watermark assertion
+/// catches that bypass.
+TEST(CASWriterDuties, UpdateRefPublishedAtServicesPendingDutyBeforeUpdatingTheRef)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = openFrozenSingleAttemptPool(backend);
+    const RootNamespace ns{"srv1/writer_duty_update_ref"};
+    publishEmptyRef(store, ns, "target");
+    leaveRejectedCleanupDuty(store, ns);
+
+    store->updateRefPublishedAt(ns, "target", [](RefPublishedAtUpdate & update) { update.published_at_ms = 17; });
+
+    const auto resolved = store->resolveRef(ns, "target");
+    ASSERT_TRUE(resolved.has_value());
+    EXPECT_EQ(resolved->published_at_ms, 17);
+    EXPECT_EQ(store->minActive(), store->peekNextBuildSeq());
+}
+
+/// Each public namespace-removal overload has its own Pool delegate. Omitting the shared seam from
+/// either one still removes the namespace but strands the rejected build at the active floor, so the
+/// two independent cases protect both forwarding paths.
+TEST(CASWriterDuties, DropNamespaceOverloadsServicePendingDutyBeforeRemoval)
+{
+    {
+        auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+        auto store = openFrozenSingleAttemptPool(backend);
+        const RootNamespace ns{"srv1/writer_duty_drop_namespace"};
+        publishEmptyRef(store, ns, "target");
+        leaveRejectedCleanupDuty(store, ns);
+
+        store->dropNamespace(ns);
+
+        EXPECT_TRUE(store->listRefs(ns).empty());
+        EXPECT_EQ(store->minActive(), store->peekNextBuildSeq());
+    }
+
+    {
+        auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+        auto store = openFrozenSingleAttemptPool(backend);
+        const RootNamespace ns{"srv1/writer_duty_drop_namespace_life"};
+        publishEmptyRef(store, ns, "target");
+        const NamespaceLifeId life = store->namespaceLife(ns);
+        leaveRejectedCleanupDuty(store, ns);
+
+        store->dropNamespace(life);
+
+        EXPECT_TRUE(store->listRefs(ns).empty());
+        EXPECT_EQ(store->minActive(), store->peekNextBuildSeq());
+    }
+}
+
+/// The explicit snapshot/checkpoint attempt is the audited sibling mutation: without the common seam
+/// it may publish ledger state while leaving the older writer duty pinned. Its return value is allowed
+/// to be false; advancing the active floor is the cleanup contract under test.
+TEST(CASWriterDuties, SnapshotAttemptServicesPendingDutyBeforePublishingLedgerState)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    auto store = openFrozenSingleAttemptPool(backend);
+    const RootNamespace ns{"srv1/writer_duty_snapshot"};
+    publishEmptyRef(store, ns, "target");
+    leaveRejectedCleanupDuty(store, ns);
+
+    static_cast<void>(store->tryPublishSnapshotAndAdvanceCheckpointOnce(ns));
+
+    EXPECT_EQ(store->minActive(), store->peekNextBuildSeq());
+}
+
+/// Removing the pending-duty term from `Pool` teardown makes this test fail at the farewell
+/// assertion: a clean marker would falsely certify that the durable precommit below has no remaining
+/// writer work. The unclean handoff forces a fresh writer epoch; its arithmetic recovery seal then
+/// makes the ordinary stale-precommit sweep the crash-remnant cleanup path.
+TEST(CASWriterDuties, PendingDutySkipsCleanFarewellAndSuccessorSweepsTheCrashRemnant)
+{
+    auto backend = std::make_shared<DB::Cas::InMemoryBackend>();
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    const CasRequestBudget budget{
+        .attempt_timeout_ms = 50,
+        .operation_deadline_ms = 500,
+        .max_attempts = 1,
+        .lease_safety_margin_ms = 50,
+    };
+    const RootNamespace ns{"srv1/writer_duty_crash"};
+
+    auto predecessor = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p",
+        .server_id = UInt128(1),
+        .server_root_id = "test",
+        .background_watermark = false,
+        .mount_lease_ttl_ms = std::chrono::milliseconds(500),
+        .mount_renew_period = std::chrono::milliseconds(100),
+        .cas_request_budget = budget,
+    });
+    DB::Cas::tests::casAdmitRecoverableEntry(
+        *backend, predecessor->layout(), ns, predecessor->liveWriterEpoch());
+
+    ManifestId abandoned_id;
+    auto abandoned = stageEmptyManifest(predecessor, ns, "abandoned", abandoned_id);
+    abandoned->precommitAdd(ns, "abandoned", abandoned_id);
+    const uint64_t predecessor_epoch = predecessor->writerEpoch();
+    const Layout layout = predecessor->layout();
+    const String mount_key = layout.mountKey("test");
+
+    abandoned.reset();
+    predecessor.reset();
+
+    const auto mount = backend->get(mount_key);
+    ASSERT_TRUE(mount.has_value());
+    EXPECT_NE(decodeMountLease(mount->bytes).min_active, std::numeric_limits<uint64_t>::max())
+        << "a live writer-cleanup duty forbids the clean-release certificate";
+
+    uint64_t fake_boot = 0;
+    std::vector<uint64_t> waits;
+    auto successor_store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p",
+        .server_id = UInt128(1),
+        .server_root_id = "test",
+        .background_watermark = false,
+        .mount_lease_ttl_ms = std::chrono::milliseconds(500),
+        .mount_renew_period = std::chrono::milliseconds(100),
+        .cas_request_budget = budget,
+        .boot_ms_fn = [&] { return fake_boot; },
+        .wait_sleep_fn = [&](uint64_t ms) { fake_boot += ms; waits.push_back(ms); },
+    });
+    ASSERT_GT(successor_store->writerEpoch(), predecessor_epoch);
+    ASSERT_FALSE(waits.empty()) << "the predecessor supplied no clean-death certificate";
+
+    ManifestId successor_id;
+    auto successor = stageEmptyManifest(successor_store, ns, "successor", successor_id);
+    successor->precommitAdd(ns, "successor", successor_id);
+
+    EXPECT_EQ(
+        successor_store->livePrecommitsForTest(ns),
+        (std::set<std::pair<String, ManifestRef>>{{"successor", successor_id.ref}}));
+    const auto seal = successor_store->lastEpochSealForTest(ns);
+    ASSERT_TRUE(seal.has_value());
+    EXPECT_EQ(seal->writer_epoch, predecessor_epoch);
+
+    successor->abandon();
+}
+
+/// The duty queue above only ever resolves the ref
+/// table's precommit BINDING -- a rejected grant's manifest BODY is orphan from birth (no owner ever
+/// named it, so the edge-before-observe `+1` a durable precommit would have folded never landed
+/// either) and its reclaim is entirely the orphan sweep's job, gated on the one thing the duty queue
+/// cannot give it: the build's own epoch durably closed. This drives that closure (the same crash
+/// pattern as `PendingDutySkipsCleanFarewellAndSuccessorSweepsTheCrashRemnant`, but the predecessor's
+/// build is REJECTED rather than adopted) and then runs real GC rounds until the body is gone.
+TEST(CASWriterDuties, RejectedAttemptBodyIsEventuallyNominatedAndSwept)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    DB::Cas::tests::seedPoolMetaForRestart(*backend);
+    const CasRequestBudget budget{
+        .attempt_timeout_ms = 50,
+        .operation_deadline_ms = 500,
+        .max_attempts = 1,
+        .lease_safety_margin_ms = 50,
+    };
+    /// Rooted under the POOL's OWN `server_root_id` ("test", unlike this file's other fixtures, which
+    /// stay under "srv1" precisely because they never drive the orphan sweep): `prefixEligible`'s
+    /// watermark floor is looked up by walking the NAMESPACE's own prefix segments for a live mount
+    /// lease, so a namespace rooted under any other server-root would find no floor and retain forever
+    /// regardless of epoch/coverage.
+    const RootNamespace ns{"test/writer_duty_rejected_sweep"};
+
+    auto predecessor = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p",
+        .server_id = UInt128(1),
+        .server_root_id = "test",
+        .manifest_sweep_list_budget_keys = 100,
+        .manifest_sweep_delete_budget_keys = 100,
+        .gc_fold_max_defer_rounds = 0,
+        .background_watermark = false,
+        .mount_lease_ttl_ms = std::chrono::milliseconds(500),
+        .mount_renew_period = std::chrono::milliseconds(100),
+        .cas_request_budget = budget,
+    });
+
+    /// A real, fully-promoted ref through the ordinary production write path (no seeded catalog/ckpt)
+    /// gives the namespace genuine epoch-1 content, so the successor's recovery below has something
+    /// real to close -- unlike a pre-attempt-refused grant, which never touches the backend at all and
+    /// so leaves the namespace's fold coverage exactly where it started.
+    publishEmptyRef(predecessor, ns, "anchor");
+
+    ManifestId rejected_id;
+    auto rejected = stageEmptyManifest(predecessor, ns, "rejected", rejected_id);
+    const String rejected_manifest_key = predecessor->layout().manifestKey(rejected_id);
+    ASSERT_TRUE(backend->head(rejected_manifest_key).exists)
+        << "stageManifest's body write is unconditional; only the owner grant is refused below";
+
+    /// `Unresolved` lands nothing, so the wedge it leaves resolves as a conclusive REJECT once the
+    /// successor's own recovery walks past it -- unlike the ADOPT-arm crash-remnant test, this
+    /// manifest never becomes a live owner in any epoch. `anchor`'s real birth just above minted a
+    /// genuine (random) incarnation, so the fault key is computed from the namespace's ACTUAL life,
+    /// not the deterministic `fixtureLife` fallback a raw, never-touched fixture would use.
+    backend->fault_substr = predecessor->layout().namespaceStreamPrefix(predecessor->namespaceLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::Unresolved;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(
+        DB::ErrorCodes::NETWORK_ERROR,
+        [&] { rejected->precommitAdd(ns, "rejected", rejected_id); });
+    ASSERT_TRUE(predecessor->refLaneWedgedForTest(ns));
+    ASSERT_EQ(rejected->precommitState(), PartWriteTxn::PrecommitState::Uncertain);
+    const uint64_t predecessor_epoch = predecessor->writerEpoch();
+
+    rejected.reset();
+    predecessor.reset();
+
+    uint64_t fake_boot = 0;
+    std::vector<uint64_t> waits;
+    auto successor_store = Pool::open(backend, PoolConfig{
+        .pool_prefix = "p",
+        .server_id = UInt128(1),
+        .server_root_id = "test",
+        .manifest_sweep_list_budget_keys = 100,
+        .manifest_sweep_delete_budget_keys = 100,
+        .gc_fold_max_defer_rounds = 0,
+        .background_watermark = false,
+        .mount_lease_ttl_ms = std::chrono::milliseconds(500),
+        .mount_renew_period = std::chrono::milliseconds(100),
+        .cas_request_budget = budget,
+        .boot_ms_fn = [&] { return fake_boot; },
+        .wait_sleep_fn = [&](uint64_t ms) { fake_boot += ms; waits.push_back(ms); },
+    });
+    ASSERT_GT(successor_store->writerEpoch(), predecessor_epoch);
+    ASSERT_FALSE(waits.empty()) << "the predecessor supplied no clean-death certificate";
+
+    /// An ordinary successor mutation both drains the inherited duty as a no-op (the rejected grant
+    /// was never durable) and forces the predecessor's dead epoch to close with an arithmetic seal --
+    /// the fact rule (1) of the sweep's deletion premise reads.
+    ManifestId successor_id;
+    auto successor = stageEmptyManifest(successor_store, ns, "successor", successor_id);
+    successor->precommitAdd(ns, "successor", successor_id);
+
+    EXPECT_EQ(
+        successor_store->livePrecommitsForTest(ns),
+        (std::set<std::pair<String, ManifestRef>>{{"successor", successor_id.ref}}));
+    const auto seal = successor_store->lastEpochSealForTest(ns);
+    ASSERT_TRUE(seal.has_value());
+    EXPECT_EQ(seal->writer_epoch, predecessor_epoch);
+
+    Gc gc(successor_store, hexToU128("000000000000000000000000000000e1"));
+    for (int round = 0; round < 16 && backend->head(rejected_manifest_key).exists; ++round)
+        DB::Cas::tests::runRegularRoundReclaiming(gc);
+
+    EXPECT_FALSE(backend->head(rejected_manifest_key).exists)
+        << "the rejected attempt's orphan manifest must eventually be nominated and swept once its "
+           "build epoch is durably closed";
+
+    successor->abandon();
+}
+
+/// The settlement's own ordering is load-bearing: append the exact `OwnerTransition` removal (or
+/// observe conclusive absence), only then retire the build seq, only then drop the duty -- a throw
+/// between those steps must leave the duty owned by nobody but the queue. Faulting the SETTLEMENT's
+/// append (not the original grant, which is a plain pre-attempt refusal here) proves the retry path
+/// directly: the duty survives the throw and the mutation it was blocking aborts with it, then the
+/// very next drain -- once the fault clears -- settles the duty and lets that mutation proceed.
+TEST(CASWriterDuties, DutySurvivesSettlementFailureForRetry)
+{
+    auto backend = std::make_shared<DB::Cas::tests::ChunkFaultBackend>();
+    auto store = openFrozenSingleAttemptPool(backend);
+    const RootNamespace ns{"srv1/writer_duty_settlement_retry"};
+    DB::Cas::tests::casAdmitRecoverableEntry(*backend, store->layout(), ns, store->liveWriterEpoch());
+    publishEmptyRef(store, ns, "target");
+
+    /// A plain, unfaulted precommit that is simply destroyed without promote/abandon: Durable, never
+    /// settled, and (unlike a proven-absent grant) its duty's own settlement owes a REAL
+    /// `OwnerTransition` removal -- exactly the append this test needs to fault.
+    ManifestId durable_id;
+    auto durable = stageEmptyManifest(store, ns, "durable", durable_id);
+    const uint64_t durable_seq = durable->buildSeq();
+    durable->precommitAdd(ns, "durable", durable_id);
+    durable.reset();
+    ASSERT_TRUE(store->writerCleanupDutiesPendingForTest());
+
+    backend->fault_substr = store->layout().namespaceStreamPrefix(DB::Cas::tests::fixture::fixtureLife(ns)) + "_log/";
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::Unresolved;
+    backend->fault_count = 1;
+    DB::Cas::tests::expectThrowsCode(
+        DB::ErrorCodes::NETWORK_ERROR,
+        [&] { store->dropRef(ns, "target"); });
+
+    EXPECT_TRUE(store->writerCleanupDutiesPendingForTest())
+        << "a settlement that throws must retain the duty for retry, never lose it";
+    EXPECT_TRUE(store->resolveRef(ns, "target").has_value())
+        << "the settlement's failure must abort the mutation it was blocking too, not just its own append";
+    EXPECT_EQ(store->minActive(), durable_seq);
+
+    backend->mode = DB::Cas::tests::ChunkFaultBackend::Mode::None;
+    store->dropRef(ns, "target");
+
+    EXPECT_FALSE(store->writerCleanupDutiesPendingForTest());
+    EXPECT_FALSE(store->resolveRef(ns, "target").has_value());
+    EXPECT_EQ(store->minActive(), store->peekNextBuildSeq())
+        << "the retried drain settles the retained duty and lets the mutation proceed";
+}

From 2d5e0952376abe3ddc8f926860b752d04109cd57 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:40 +0200
Subject: [PATCH 26/30] CAS tests: stateless

New stateless tests for content-addressed storage.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../0_stateless/04278_cas_disk.reference      |   9 +
 tests/queries/0_stateless/04278_cas_disk.sql  |  49 +++++
 .../0_stateless/04279_cas_gc.reference        |   6 +
 tests/queries/0_stateless/04279_cas_gc.sql    |  66 ++++++
 .../04280_cas_clone_partition_works.reference |   8 +
 .../04280_cas_clone_partition_works.sql       |  54 +++++
 .../04282_cas_mutable_state.reference         |   7 +
 .../0_stateless/04282_cas_mutable_state.sql   |  57 +++++
 .../04283_cas_replicated_rejected.reference   |   5 +
 .../04283_cas_replicated_rejected.sql         |  47 +++++
 ...04284_cas_backup_pointer_holding.reference |   5 +
 .../04284_cas_backup_pointer_holding.sh       |  52 +++++
 ...deduplication_window_inline_disk.reference |   5 +
 ...5_cas_deduplication_window_inline_disk.sql |  45 ++++
 .../04286_cas_remote_data_paths.reference     |   2 +
 .../04286_cas_remote_data_paths.sql           |  41 ++++
 ...287_cas_detach_partition_listing.reference |   4 +
 .../04287_cas_detach_partition_listing.sql    |  38 ++++
 ..._detached_part_modification_time.reference |   4 +
 ...88_cas_detached_part_modification_time.sql |  40 ++++
 .../04289_cas_multi_detach_drop.reference     |   7 +
 .../04289_cas_multi_detach_drop.sql           |  43 ++++
 .../04290_cas_no_leftovers.reference          |   5 +
 .../0_stateless/04290_cas_no_leftovers.sh     | 132 ++++++++++++
 .../0_stateless/04292_cas_mutations.reference |   7 +
 .../0_stateless/04292_cas_mutations.sh        | 114 ++++++++++
 .../04293_cas_lightweight_delete.reference    |   6 +
 .../04293_cas_lightweight_delete.sh           | 101 +++++++++
 .../04294_cas_patch_parts.reference           |   6 +
 .../0_stateless/04294_cas_patch_parts.sh      |  88 ++++++++
 .../04295_cas_mutation_no_leftovers.reference |   5 +
 .../04295_cas_mutation_no_leftovers.sh        | 131 ++++++++++++
 ...04299_cas_projection_inline_disk.reference |  60 ++++++
 .../04299_cas_projection_inline_disk.sql      | 125 +++++++++++
 .../04300_cas_projection_multiblock.reference |  14 ++
 .../04300_cas_projection_multiblock.sql       |  48 +++++
 ...000_cas_projection_carry_forward.reference |  10 +
 .../05000_cas_projection_carry_forward.sql    |  45 ++++
 ..._cas_attach_partition_projection.reference |  11 +
 .../05001_cas_attach_partition_projection.sql |  46 ++++
 .../05002_cas_fetch_partition.reference       |   8 +
 .../0_stateless/05002_cas_fetch_partition.sql |  58 ++++++
 .../0_stateless/05003_cas_freeze.reference    |   7 +
 tests/queries/0_stateless/05003_cas_freeze.sh |  62 ++++++
 .../05004_cas_transactions.reference          |   7 +
 .../0_stateless/05004_cas_transactions.sh     |  62 ++++++
 .../05005_cas_backup_restore.reference        |   6 +
 .../0_stateless/05005_cas_backup_restore.sh   |  50 +++++
 ...06_cas_deduplication_blob_insert.reference |   1 +
 .../05006_cas_deduplication_blob_insert.sql   |  18 ++
 .../05007_cas_gc_introspection.reference      |   6 +
 .../0_stateless/05007_cas_gc_introspection.sh | 109 ++++++++++
 .../05008_cas_gc_snapshot_prune.reference     |   1 +
 .../05008_cas_gc_snapshot_prune.sh            |  77 +++++++
 .../0_stateless/05009_cas_event_log.reference |   4 +
 .../0_stateless/05009_cas_event_log.sql       |  44 ++++
 .../05010_cas_mounts_gc_health.reference      |   3 +
 .../0_stateless/05010_cas_mounts_gc_health.sh |  52 +++++
 .../05011_cas_gc_rebuild_access.reference     |   0
 .../05011_cas_gc_rebuild_access.sh            |  66 ++++++
 .../05012_cas_mounts_typed_columns.reference  |   3 +
 .../05012_cas_mounts_typed_columns.sql        |   8 +
 ...5013_system_cas_drop_pool_member.reference |   0
 .../05013_system_cas_drop_pool_member.sql     |   2 +
 ...sert_dedup_disk_commit_failpoint.reference |   1 +
 ...014_insert_dedup_disk_commit_failpoint.sql |  28 +++
 ...5015_cas_reject_fake_transaction.reference |   2 +
 .../05015_cas_reject_fake_transaction.sh      |  28 +++
 ...5016_cas_drop_pool_member_access.reference |   0
 .../05016_cas_drop_pool_member_access.sh      |  37 ++++
 ...17_lazy_load_tables_sync_replica.reference |   2 +
 .../05017_lazy_load_tables_sync_replica.sh    |  40 ++++
 .../05019_cas_fsck_access.reference           |   0
 .../0_stateless/05019_cas_fsck_access.sh      |  44 ++++
 .../0_stateless/05020_cas_fsck.reference      |   7 +
 tests/queries/0_stateless/05020_cas_fsck.sh   |  85 ++++++++
 ...05021_lazy_load_tables_mutations.reference |   2 +
 .../05021_lazy_load_tables_mutations.sh       |  41 ++++
 .../05022_cas_verb_access.reference           |   0
 .../0_stateless/05022_cas_verb_access.sh      |  69 ++++++
 ...5023_cas_dropns_leaked_namespace.reference |  14 ++
 .../05023_cas_dropns_leaked_namespace.sh      | 196 ++++++++++++++++++
 82 files changed, 2798 insertions(+)
 create mode 100644 tests/queries/0_stateless/04278_cas_disk.reference
 create mode 100644 tests/queries/0_stateless/04278_cas_disk.sql
 create mode 100644 tests/queries/0_stateless/04279_cas_gc.reference
 create mode 100644 tests/queries/0_stateless/04279_cas_gc.sql
 create mode 100644 tests/queries/0_stateless/04280_cas_clone_partition_works.reference
 create mode 100644 tests/queries/0_stateless/04280_cas_clone_partition_works.sql
 create mode 100644 tests/queries/0_stateless/04282_cas_mutable_state.reference
 create mode 100644 tests/queries/0_stateless/04282_cas_mutable_state.sql
 create mode 100644 tests/queries/0_stateless/04283_cas_replicated_rejected.reference
 create mode 100644 tests/queries/0_stateless/04283_cas_replicated_rejected.sql
 create mode 100644 tests/queries/0_stateless/04284_cas_backup_pointer_holding.reference
 create mode 100755 tests/queries/0_stateless/04284_cas_backup_pointer_holding.sh
 create mode 100644 tests/queries/0_stateless/04285_cas_deduplication_window_inline_disk.reference
 create mode 100644 tests/queries/0_stateless/04285_cas_deduplication_window_inline_disk.sql
 create mode 100644 tests/queries/0_stateless/04286_cas_remote_data_paths.reference
 create mode 100644 tests/queries/0_stateless/04286_cas_remote_data_paths.sql
 create mode 100644 tests/queries/0_stateless/04287_cas_detach_partition_listing.reference
 create mode 100644 tests/queries/0_stateless/04287_cas_detach_partition_listing.sql
 create mode 100644 tests/queries/0_stateless/04288_cas_detached_part_modification_time.reference
 create mode 100644 tests/queries/0_stateless/04288_cas_detached_part_modification_time.sql
 create mode 100644 tests/queries/0_stateless/04289_cas_multi_detach_drop.reference
 create mode 100644 tests/queries/0_stateless/04289_cas_multi_detach_drop.sql
 create mode 100644 tests/queries/0_stateless/04290_cas_no_leftovers.reference
 create mode 100755 tests/queries/0_stateless/04290_cas_no_leftovers.sh
 create mode 100644 tests/queries/0_stateless/04292_cas_mutations.reference
 create mode 100755 tests/queries/0_stateless/04292_cas_mutations.sh
 create mode 100644 tests/queries/0_stateless/04293_cas_lightweight_delete.reference
 create mode 100755 tests/queries/0_stateless/04293_cas_lightweight_delete.sh
 create mode 100644 tests/queries/0_stateless/04294_cas_patch_parts.reference
 create mode 100755 tests/queries/0_stateless/04294_cas_patch_parts.sh
 create mode 100644 tests/queries/0_stateless/04295_cas_mutation_no_leftovers.reference
 create mode 100755 tests/queries/0_stateless/04295_cas_mutation_no_leftovers.sh
 create mode 100644 tests/queries/0_stateless/04299_cas_projection_inline_disk.reference
 create mode 100644 tests/queries/0_stateless/04299_cas_projection_inline_disk.sql
 create mode 100644 tests/queries/0_stateless/04300_cas_projection_multiblock.reference
 create mode 100644 tests/queries/0_stateless/04300_cas_projection_multiblock.sql
 create mode 100644 tests/queries/0_stateless/05000_cas_projection_carry_forward.reference
 create mode 100644 tests/queries/0_stateless/05000_cas_projection_carry_forward.sql
 create mode 100644 tests/queries/0_stateless/05001_cas_attach_partition_projection.reference
 create mode 100644 tests/queries/0_stateless/05001_cas_attach_partition_projection.sql
 create mode 100644 tests/queries/0_stateless/05002_cas_fetch_partition.reference
 create mode 100644 tests/queries/0_stateless/05002_cas_fetch_partition.sql
 create mode 100644 tests/queries/0_stateless/05003_cas_freeze.reference
 create mode 100755 tests/queries/0_stateless/05003_cas_freeze.sh
 create mode 100644 tests/queries/0_stateless/05004_cas_transactions.reference
 create mode 100755 tests/queries/0_stateless/05004_cas_transactions.sh
 create mode 100644 tests/queries/0_stateless/05005_cas_backup_restore.reference
 create mode 100755 tests/queries/0_stateless/05005_cas_backup_restore.sh
 create mode 100644 tests/queries/0_stateless/05006_cas_deduplication_blob_insert.reference
 create mode 100644 tests/queries/0_stateless/05006_cas_deduplication_blob_insert.sql
 create mode 100644 tests/queries/0_stateless/05007_cas_gc_introspection.reference
 create mode 100755 tests/queries/0_stateless/05007_cas_gc_introspection.sh
 create mode 100644 tests/queries/0_stateless/05008_cas_gc_snapshot_prune.reference
 create mode 100755 tests/queries/0_stateless/05008_cas_gc_snapshot_prune.sh
 create mode 100644 tests/queries/0_stateless/05009_cas_event_log.reference
 create mode 100644 tests/queries/0_stateless/05009_cas_event_log.sql
 create mode 100644 tests/queries/0_stateless/05010_cas_mounts_gc_health.reference
 create mode 100755 tests/queries/0_stateless/05010_cas_mounts_gc_health.sh
 create mode 100644 tests/queries/0_stateless/05011_cas_gc_rebuild_access.reference
 create mode 100755 tests/queries/0_stateless/05011_cas_gc_rebuild_access.sh
 create mode 100644 tests/queries/0_stateless/05012_cas_mounts_typed_columns.reference
 create mode 100644 tests/queries/0_stateless/05012_cas_mounts_typed_columns.sql
 create mode 100644 tests/queries/0_stateless/05013_system_cas_drop_pool_member.reference
 create mode 100644 tests/queries/0_stateless/05013_system_cas_drop_pool_member.sql
 create mode 100644 tests/queries/0_stateless/05014_insert_dedup_disk_commit_failpoint.reference
 create mode 100644 tests/queries/0_stateless/05014_insert_dedup_disk_commit_failpoint.sql
 create mode 100644 tests/queries/0_stateless/05015_cas_reject_fake_transaction.reference
 create mode 100755 tests/queries/0_stateless/05015_cas_reject_fake_transaction.sh
 create mode 100644 tests/queries/0_stateless/05016_cas_drop_pool_member_access.reference
 create mode 100755 tests/queries/0_stateless/05016_cas_drop_pool_member_access.sh
 create mode 100644 tests/queries/0_stateless/05017_lazy_load_tables_sync_replica.reference
 create mode 100755 tests/queries/0_stateless/05017_lazy_load_tables_sync_replica.sh
 create mode 100644 tests/queries/0_stateless/05019_cas_fsck_access.reference
 create mode 100755 tests/queries/0_stateless/05019_cas_fsck_access.sh
 create mode 100644 tests/queries/0_stateless/05020_cas_fsck.reference
 create mode 100755 tests/queries/0_stateless/05020_cas_fsck.sh
 create mode 100644 tests/queries/0_stateless/05021_lazy_load_tables_mutations.reference
 create mode 100755 tests/queries/0_stateless/05021_lazy_load_tables_mutations.sh
 create mode 100644 tests/queries/0_stateless/05022_cas_verb_access.reference
 create mode 100755 tests/queries/0_stateless/05022_cas_verb_access.sh
 create mode 100644 tests/queries/0_stateless/05023_cas_dropns_leaked_namespace.reference
 create mode 100755 tests/queries/0_stateless/05023_cas_dropns_leaked_namespace.sh

diff --git a/tests/queries/0_stateless/04278_cas_disk.reference b/tests/queries/0_stateless/04278_cas_disk.reference
new file mode 100644
index 000000000000..b054d30e9cc7
--- /dev/null
+++ b/tests/queries/0_stateless/04278_cas_disk.reference
@@ -0,0 +1,9 @@
+basic	1000	499500	7	2020-01-01	2022-09-26
+oracle_full_match	1
+after_second_insert	2000
+after_merge	2000	999000
+active_parts	1
+slice	0	0	2
+slice	500	3	2
+slice	999	5	2
+dropped_ok
diff --git a/tests/queries/0_stateless/04278_cas_disk.sql b/tests/queries/0_stateless/04278_cas_disk.sql
new file mode 100644
index 000000000000..1bb3508876c2
--- /dev/null
+++ b/tests/queries/0_stateless/04278_cas_disk.sql
@@ -0,0 +1,49 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- Natural black-box oracle: a table on a `cas` disk must behave
+-- identically to a normal MergeTree table for the same data. We compare the two
+-- directly so the test is deterministic regardless of environment, and we also
+-- exercise INSERT (content-addressed write), SELECT (ref->part_id->footer->blob
+-- resolution), blob-level dedup of identical inserts, a merge, and DROP (removal).
+
+DROP TABLE IF EXISTS t_cas;
+DROP TABLE IF EXISTS t_ref;
+
+CREATE TABLE t_cas (a UInt64, s String, d Date)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04278',
+    name = '04278_cas',
+    path = '04278_cas_pool/');
+
+CREATE TABLE t_ref (a UInt64, s String, d Date)
+ENGINE = MergeTree ORDER BY a;
+
+INSERT INTO t_cas SELECT number, toString(number % 7), toDate('2020-01-01') + number FROM numbers(1000);
+INSERT INTO t_ref SELECT number, toString(number % 7), toDate('2020-01-01') + number FROM numbers(1000);
+
+SELECT 'basic', count(), sum(a), uniqExact(s), min(d), max(d) FROM t_cas;
+
+SELECT 'oracle_full_match',
+       (SELECT groupArray((a, s, d)) FROM (SELECT * FROM t_cas ORDER BY a))
+     = (SELECT groupArray((a, s, d)) FROM (SELECT * FROM t_ref ORDER BY a));
+
+-- Second identical insert: rows double (plain MergeTree does not dedup rows);
+-- the content blobs are deduplicated internally by content-addressing.
+INSERT INTO t_cas SELECT number, toString(number % 7), toDate('2020-01-01') + number FROM numbers(1000);
+SELECT 'after_second_insert', count() FROM t_cas;
+
+OPTIMIZE TABLE t_cas FINAL;
+SELECT 'after_merge', count(), sum(a) FROM t_cas;
+SELECT 'active_parts', count() FROM system.parts
+WHERE database = currentDatabase() AND table = 't_cas' AND active;
+
+SELECT 'slice', a, s, count() FROM t_cas WHERE a IN (0, 500, 999) GROUP BY a, s ORDER BY a;
+
+DROP TABLE t_cas;
+DROP TABLE t_ref;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04279_cas_gc.reference b/tests/queries/0_stateless/04279_cas_gc.reference
new file mode 100644
index 000000000000..d9fadb0b0b4d
--- /dev/null
+++ b/tests/queries/0_stateless/04279_cas_gc.reference
@@ -0,0 +1,6 @@
+count_match	1
+sum_match	1
+content_match	1
+point_match	1
+range_match	1
+dropped_ok
diff --git a/tests/queries/0_stateless/04279_cas_gc.sql b/tests/queries/0_stateless/04279_cas_gc.sql
new file mode 100644
index 000000000000..02dee1a95181
--- /dev/null
+++ b/tests/queries/0_stateless/04279_cas_gc.sql
@@ -0,0 +1,66 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- Correctness-under-active-GC oracle. The `cas` disk below opts in to the background
+-- reachability GC (`gc_enabled=1`, default OFF) and runs it aggressively
+-- and `old_parts_lifetime=1` drops the merged-away source parts quickly so their footers/blobs become
+-- unreferenced and turn into genuine GC fodder *during* the test. We assert the CA table stays
+-- byte-for-byte identical to a normal MergeTree table on the same data: if a concurrent sweep ever
+-- dropped a live blob, the oracle below would diverge. The test is fully deterministic — it never
+-- waits on GC and never asserts that GC has run by a deadline; correctness must hold regardless of
+-- whether (and how often) the background sweep fired.
+
+DROP TABLE IF EXISTS t_cas_gc;
+DROP TABLE IF EXISTS t_ref_gc;
+
+CREATE TABLE t_cas_gc (a UInt64, s String, d Date)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04279',
+    name = '04279_cas_gc',
+    path = '04279_cas_gc_pool/',
+    gc_enabled = 1,
+    gc_interval_sec = 1),
+    old_parts_lifetime = 1;
+
+CREATE TABLE t_ref_gc (a UInt64, s String, d Date)
+ENGINE = MergeTree ORDER BY a
+SETTINGS old_parts_lifetime = 1;
+
+-- First insert: rows 0..999.
+INSERT INTO t_cas_gc SELECT number, toString(number % 7), toDate('2020-01-01') + number FROM numbers(1000);
+INSERT INTO t_ref_gc SELECT number, toString(number % 7), toDate('2020-01-01') + number FROM numbers(1000);
+
+-- Second distinct insert: rows 1000..1999 (different data => different blobs, not deduped away).
+INSERT INTO t_cas_gc SELECT number, toString(number % 7), toDate('2020-01-01') + number FROM numbers(1000, 1000);
+INSERT INTO t_ref_gc SELECT number, toString(number % 7), toDate('2020-01-01') + number FROM numbers(1000, 1000);
+
+-- Merge both tables: on the CA table this leaves the source parts outdated; with old_parts_lifetime=1
+-- they are removed shortly, so their footers/blobs become unreferenced and the active GC may sweep them.
+OPTIMIZE TABLE t_cas_gc FINAL;
+OPTIMIZE TABLE t_ref_gc FINAL;
+
+-- Oracle: aggregates must match the normal table exactly.
+SELECT 'count_match',  (SELECT count() FROM t_cas_gc) = (SELECT count() FROM t_ref_gc);
+SELECT 'sum_match',    (SELECT sum(a)  FROM t_cas_gc) = (SELECT sum(a)  FROM t_ref_gc);
+
+-- Oracle: full ordered content must match exactly — proves no live blob was dropped by a concurrent sweep.
+SELECT 'content_match',
+       (SELECT groupArray((a, s, d)) FROM (SELECT * FROM t_cas_gc ORDER BY a, s, d))
+     = (SELECT groupArray((a, s, d)) FROM (SELECT * FROM t_ref_gc ORDER BY a, s, d));
+
+-- A few point/range reads (each resolves ref -> part_id -> footer -> blob) must also match the oracle.
+SELECT 'point_match',
+       (SELECT groupArray((a, s)) FROM (SELECT a, s FROM t_cas_gc WHERE a IN (0, 999, 1000, 1999) ORDER BY a, s))
+     = (SELECT groupArray((a, s)) FROM (SELECT a, s FROM t_ref_gc WHERE a IN (0, 999, 1000, 1999) ORDER BY a, s));
+
+SELECT 'range_match',
+       (SELECT groupArray((a, s)) FROM (SELECT a, s FROM t_cas_gc WHERE a BETWEEN 500 AND 1500 ORDER BY a, s))
+     = (SELECT groupArray((a, s)) FROM (SELECT a, s FROM t_ref_gc WHERE a BETWEEN 500 AND 1500 ORDER BY a, s));
+
+DROP TABLE t_cas_gc;
+DROP TABLE t_ref_gc;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04280_cas_clone_partition_works.reference b/tests/queries/0_stateless/04280_cas_clone_partition_works.reference
new file mode 100644
index 000000000000..13197e78dee8
--- /dev/null
+++ b/tests/queries/0_stateless/04280_cas_clone_partition_works.reference
@@ -0,0 +1,8 @@
+after_replace_dst_p1	100	4950
+after_attach_from_dst_p2	50	1225
+after_move_dst_p3	30	435
+after_move_src_p3	0
+after_detach_src	50
+after_reattach_src	100	4950
+after_drop_src_p2	0
+dropped_ok
diff --git a/tests/queries/0_stateless/04280_cas_clone_partition_works.sql b/tests/queries/0_stateless/04280_cas_clone_partition_works.sql
new file mode 100644
index 000000000000..82e18998cb59
--- /dev/null
+++ b/tests/queries/0_stateless/04280_cas_clone_partition_works.sql
@@ -0,0 +1,54 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- Positive test for the part-cloning partition commands on a cas disk.
+--
+-- History: these commands (MOVE PARTITION ... TO TABLE, REPLACE PARTITION, ATTACH PARTITION ... FROM,
+-- plain ATTACH PARTITION of a table's own detached parts) USED to be rejected with SUPPORT_IS_DISABLED
+-- on CA, because the file-by-file `createHardLink` clone path had no enclosing transaction and would
+-- corrupt the clone. CAS M9 W2 made that path transactional: `DataPartStorageOnDiskBase::freeze` runs
+-- the whole clone through ONE CA transaction and `moveDirectory` re-keys the detached-staging → active
+-- rename into a complete active ref. So the commands now SUCCEED and read back identical data. This test
+-- locks that they work and produce the correct rows (the gate at `checkAlterPartitionIsPossible` for the
+-- ContentAddressed metadata type now lists them as supported).
+--
+-- The tables use the DEFAULT MergeTree storage so this exercises whatever default disk the job installs:
+-- on the local-CA job that is a local cas disk, on the cas-over-S3 job it is
+-- a CA disk backed by minio. Both are the supported same-disk clone path.
+
+DROP TABLE IF EXISTS t_cas_clone_src;
+DROP TABLE IF EXISTS t_cas_clone_dst;
+
+CREATE TABLE t_cas_clone_src (a UInt64, p UInt8) ENGINE = MergeTree PARTITION BY p ORDER BY a;
+CREATE TABLE t_cas_clone_dst (a UInt64, p UInt8) ENGINE = MergeTree PARTITION BY p ORDER BY a;
+
+INSERT INTO t_cas_clone_src SELECT number, 1 FROM numbers(100);
+INSERT INTO t_cas_clone_src SELECT number, 2 FROM numbers(50);
+INSERT INTO t_cas_clone_src SELECT number, 3 FROM numbers(30);
+
+-- REPLACE PARTITION clones parts from another table into the destination.
+ALTER TABLE t_cas_clone_dst REPLACE PARTITION 1 FROM t_cas_clone_src;
+SELECT 'after_replace_dst_p1', count(), sum(a) FROM t_cas_clone_dst WHERE p = 1;
+
+-- ATTACH PARTITION ... FROM clones parts from another table (parses to REPLACE_PARTITION, replace=false).
+ALTER TABLE t_cas_clone_dst ATTACH PARTITION 2 FROM t_cas_clone_src;
+SELECT 'after_attach_from_dst_p2', count(), sum(a) FROM t_cas_clone_dst WHERE p = 2;
+
+-- MOVE PARTITION ... TO TABLE clones a partition to the destination and drops it from the source.
+ALTER TABLE t_cas_clone_src MOVE PARTITION 3 TO TABLE t_cas_clone_dst;
+SELECT 'after_move_dst_p3', count(), sum(a) FROM t_cas_clone_dst WHERE p = 3;
+SELECT 'after_move_src_p3', count() FROM t_cas_clone_src WHERE p = 3;
+
+-- Plain ATTACH PARTITION of the table's own detached part re-clones it back.
+ALTER TABLE t_cas_clone_src DETACH PARTITION 1;
+SELECT 'after_detach_src', count() FROM t_cas_clone_src;
+ALTER TABLE t_cas_clone_src ATTACH PARTITION 1;
+SELECT 'after_reattach_src', count(), sum(a) FROM t_cas_clone_src WHERE p = 1;
+
+-- The pointer-unlink command DROP PARTITION still works.
+ALTER TABLE t_cas_clone_src DROP PARTITION 2;
+SELECT 'after_drop_src_p2', count() FROM t_cas_clone_src WHERE p = 2;
+
+DROP TABLE t_cas_clone_src;
+DROP TABLE t_cas_clone_dst;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04282_cas_mutable_state.reference b/tests/queries/0_stateless/04282_cas_mutable_state.reference
new file mode 100644
index 000000000000..6291270c33ba
--- /dev/null
+++ b/tests/queries/0_stateless/04282_cas_mutable_state.reference
@@ -0,0 +1,7 @@
+oracle_full_match	1
+counts	1000	249500
+cas_active_parts	2
+cas_distinct_uuids	2
+cas_no_zero_uuid	0
+ref_distinct_uuids	2
+dropped_ok
diff --git a/tests/queries/0_stateless/04282_cas_mutable_state.sql b/tests/queries/0_stateless/04282_cas_mutable_state.sql
new file mode 100644
index 000000000000..aa9a780d0af2
--- /dev/null
+++ b/tests/queries/0_stateless/04282_cas_mutable_state.sql
@@ -0,0 +1,57 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- B23 mutable-per-part-state oracle: with assign_part_uuids=1, two INSERTs of IDENTICAL data produce
+-- two parts whose column content is byte-identical but whose per-part uuid.txt differs. On a
+-- cas disk the two parts dedup to ONE shared manifest, while their mutable per-part
+-- files (uuid.txt / txn_version.txt / metadata_version.txt) live in a per-ref sidecar and are
+-- overlaid on read. Before B23 the second part read the FIRST part's uuid (the shared manifest
+-- embedded one part's mutable files), so the two uuids collided. This is a natural black-box oracle:
+-- the cas table must behave exactly like a normal MergeTree table, and the two parts
+-- must carry two DISTINCT uuids.
+
+DROP TABLE IF EXISTS t_cas_mut;
+DROP TABLE IF EXISTS t_ref_mut;
+
+CREATE TABLE t_cas_mut (a UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS assign_part_uuids = 1, disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04282',
+    name = '04282_cas',
+    path = '04282_cas_pool/');
+
+CREATE TABLE t_ref_mut (a UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS assign_part_uuids = 1;
+
+-- Two IDENTICAL inserts → two parts with identical content but distinct per-part uuids.
+INSERT INTO t_cas_mut SELECT number, toString(number % 5) FROM numbers(500);
+INSERT INTO t_cas_mut SELECT number, toString(number % 5) FROM numbers(500);
+INSERT INTO t_ref_mut SELECT number, toString(number % 5) FROM numbers(500);
+INSERT INTO t_ref_mut SELECT number, toString(number % 5) FROM numbers(500);
+
+-- Data oracle: the cas table matches the normal table exactly.
+SELECT 'oracle_full_match',
+       (SELECT groupArray((a, s)) FROM (SELECT * FROM t_cas_mut ORDER BY a, s))
+     = (SELECT groupArray((a, s)) FROM (SELECT * FROM t_ref_mut ORDER BY a, s));
+
+SELECT 'counts', count(), sum(a) FROM t_cas_mut;
+
+-- Two active parts, each with its OWN non-empty, DISTINCT uuid (the B23 regression: no collision).
+SELECT 'cas_active_parts', count() FROM system.parts
+WHERE database = currentDatabase() AND table = 't_cas_mut' AND active;
+SELECT 'cas_distinct_uuids', uniqExact(uuid) FROM system.parts
+WHERE database = currentDatabase() AND table = 't_cas_mut' AND active;
+SELECT 'cas_no_zero_uuid', countIf(uuid = toUUID('00000000-0000-0000-0000-000000000000')) FROM system.parts
+WHERE database = currentDatabase() AND table = 't_cas_mut' AND active;
+
+-- The same property holds on the normal table (oracle for the uuid behaviour).
+SELECT 'ref_distinct_uuids', uniqExact(uuid) FROM system.parts
+WHERE database = currentDatabase() AND table = 't_ref_mut' AND active;
+
+DROP TABLE t_cas_mut;
+DROP TABLE t_ref_mut;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04283_cas_replicated_rejected.reference b/tests/queries/0_stateless/04283_cas_replicated_rejected.reference
new file mode 100644
index 000000000000..241bd6aee164
--- /dev/null
+++ b/tests/queries/0_stateless/04283_cas_replicated_rejected.reference
@@ -0,0 +1,5 @@
+repl_count	2
+repl_sum	30
+plain_count	100
+plain_sum	9900
+dropped_ok
diff --git a/tests/queries/0_stateless/04283_cas_replicated_rejected.sql b/tests/queries/0_stateless/04283_cas_replicated_rejected.sql
new file mode 100644
index 000000000000..5786b1644457
--- /dev/null
+++ b/tests/queries/0_stateless/04283_cas_replicated_rejected.sql
@@ -0,0 +1,47 @@
+-- Tags: no-fasttest, no-shared-merge-tree
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+--   no-shared-merge-tree: this exercises the open-source ReplicatedMergeTree path.
+
+-- B33 (lifted): ReplicatedMergeTree on a cas disk is now SUPPORTED. The earlier
+-- SUPPORT_IS_DISABLED gate in StorageReplicatedMergeTree was removed once replication-internal clones
+-- stopped corrupting content-addressed parts, so creating a ReplicatedMergeTree table on a
+-- cas disk now succeeds and works end-to-end. A plain (non-replicated) MergeTree on the
+-- same kind of disk must also still work.
+
+DROP TABLE IF EXISTS t_cas_repl;
+DROP TABLE IF EXISTS t_cas_plain;
+
+-- (1) A ReplicatedMergeTree table on a cas disk is now supported end-to-end.
+CREATE TABLE t_cas_repl (a UInt64, b UInt64)
+ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/t_cas_repl', 'r1')
+ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04283',
+    name = '04283_cas_repl',
+    path = '04283_cas_repl_pool/');
+
+INSERT INTO t_cas_repl VALUES (1, 10), (2, 20);
+SELECT 'repl_count', count() FROM t_cas_repl;
+SELECT 'repl_sum', sum(b) FROM t_cas_repl;
+DROP TABLE t_cas_repl;
+
+-- (2) A plain (non-replicated) MergeTree on a cas disk still works end-to-end.
+CREATE TABLE t_cas_plain (a UInt64, b UInt64)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04283',
+    name = '04283_cas_plain',
+    path = '04283_cas_plain_pool/');
+
+INSERT INTO t_cas_plain SELECT number, number * 2 FROM numbers(100);
+SELECT 'plain_count', count() FROM t_cas_plain;
+SELECT 'plain_sum', sum(b) FROM t_cas_plain;
+
+DROP TABLE t_cas_plain;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04284_cas_backup_pointer_holding.reference b/tests/queries/0_stateless/04284_cas_backup_pointer_holding.reference
new file mode 100644
index 000000000000..d1ea139d5213
--- /dev/null
+++ b/tests/queries/0_stateless/04284_cas_backup_pointer_holding.reference
@@ -0,0 +1,5 @@
+before	1000	499500	7
+BACKUP_CREATED
+RESTORED
+after	1000	499500	7
+dropped_ok
diff --git a/tests/queries/0_stateless/04284_cas_backup_pointer_holding.sh b/tests/queries/0_stateless/04284_cas_backup_pointer_holding.sh
new file mode 100755
index 000000000000..1b278d3f654a
--- /dev/null
+++ b/tests/queries/0_stateless/04284_cas_backup_pointer_holding.sh
@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# B34: BACKUP / RESTORE of a cas table in an Atomic (UUID) database — the default in
+# the stateless suite. Two behaviors:
+#   1. BACKUP uses the pointer-holding path (make_temporary_hard_links=false): it resolves the
+#      part's objects via getStorageObjects and never calls disk->createHardLink, so it succeeds on
+#      a cas disk. (The temporary-hard-link BACKUP path, used only by the deprecated
+#      Ordinary database engine, is fail-closed with a clear SUPPORT_IS_DISABLED message in
+#      DataPartStorageOnDiskBase::backup — see B34/B16.)
+#   2. RESTORE onto a cas disk now succeeds end-to-end: the part files are written back
+#      through the disk's write path and the restored table reads back identical to the original.
+#      (This used to fail closed with NOT_IMPLEMENTED until the whole-part write contract, B30,
+#      landed; restore-onto-CA is no longer an M1 gap.)
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+backup_name="Disk('backups', '${CLICKHOUSE_TEST_UNIQUE_NAME}.zip')"
+
+${CLICKHOUSE_CLIENT} --multiquery <<EOF
+DROP TABLE IF EXISTS t_cas_backup;
+DROP TABLE IF EXISTS t_cas_restored;
+
+CREATE TABLE t_cas_backup (a UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04284',
+    name = '04284_cas_backup',
+    path = '04284_cas_backup_pool/');
+
+INSERT INTO t_cas_backup SELECT number, toString(number % 7) FROM numbers(1000);
+SELECT 'before', count(), sum(a), uniqExact(s) FROM t_cas_backup;
+EOF
+
+# (1) Pointer-holding BACKUP of the Atomic-DB cas table succeeds.
+${CLICKHOUSE_CLIENT} -q "BACKUP TABLE t_cas_backup TO ${backup_name}" | cut -f2
+
+# (2) RESTORE onto a cas disk succeeds and the restored data is identical.
+${CLICKHOUSE_CLIENT} -q "RESTORE TABLE t_cas_backup AS t_cas_restored FROM ${backup_name}" | cut -f2
+${CLICKHOUSE_CLIENT} -q "SELECT 'after', count(), sum(a), uniqExact(s) FROM t_cas_restored"
+
+${CLICKHOUSE_CLIENT} --multiquery <<EOF
+DROP TABLE IF EXISTS t_cas_restored;
+DROP TABLE t_cas_backup;
+SELECT 'dropped_ok';
+EOF
diff --git a/tests/queries/0_stateless/04285_cas_deduplication_window_inline_disk.reference b/tests/queries/0_stateless/04285_cas_deduplication_window_inline_disk.reference
new file mode 100644
index 000000000000..5f758425afcc
--- /dev/null
+++ b/tests/queries/0_stateless/04285_cas_deduplication_window_inline_disk.reference
@@ -0,0 +1,5 @@
+after-two-identical	1
+after-new-block	2
+after-reload-same-block	2
+after-reload-new-block	3
+dropped_ok
diff --git a/tests/queries/0_stateless/04285_cas_deduplication_window_inline_disk.sql b/tests/queries/0_stateless/04285_cas_deduplication_window_inline_disk.sql
new file mode 100644
index 000000000000..007b238eefe2
--- /dev/null
+++ b/tests/queries/0_stateless/04285_cas_deduplication_window_inline_disk.sql
@@ -0,0 +1,45 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- A non_replicated_deduplication_window > 0 on a plain MergeTree keeps an on-disk deduplication log
+-- (deduplication_logs/deduplication_log_N.txt) at the table root. On a cas disk that log
+-- works the same way it does on a plain s3 disk: the disk cannot host append writes, so the log
+-- rewrites a fresh rotated log object per record, stored verbatim in the table's files/ namespace. This
+-- test uses an INLINE cas disk, so it exercises the CA path on any test config.
+
+DROP TABLE IF EXISTS t_cas_deduplication;
+
+CREATE TABLE t_cas_deduplication (a UInt64)
+ENGINE = MergeTree ORDER BY a
+SETTINGS non_replicated_deduplication_window = 100, disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04285',
+    name = '04285_cas_deduplication',
+    path = '04285_cas_deduplication_pool/');
+
+-- Identical inserts are deduplicated; each insert also writes a record to the on-disk log (the write
+-- that used to fail closed with a null writer on a cas disk).
+INSERT INTO t_cas_deduplication VALUES (1);
+INSERT INTO t_cas_deduplication VALUES (1);
+SELECT 'after-two-identical', count() FROM t_cas_deduplication;
+
+-- A distinct block is accepted.
+INSERT INTO t_cas_deduplication VALUES (2);
+SELECT 'after-new-block', count() FROM t_cas_deduplication;
+
+-- Reload the table: the deduplication log is re-read from the cas disk.
+DETACH TABLE t_cas_deduplication;
+ATTACH TABLE t_cas_deduplication;
+
+-- The first block is still deduplicated — its record was reloaded from the on-disk log, not memory.
+INSERT INTO t_cas_deduplication VALUES (1);
+SELECT 'after-reload-same-block', count() FROM t_cas_deduplication;
+
+-- A new distinct block is still accepted after the reload.
+INSERT INTO t_cas_deduplication VALUES (3);
+SELECT 'after-reload-new-block', count() FROM t_cas_deduplication;
+
+DROP TABLE t_cas_deduplication;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04286_cas_remote_data_paths.reference b/tests/queries/0_stateless/04286_cas_remote_data_paths.reference
new file mode 100644
index 000000000000..c3fdc13518a0
--- /dev/null
+++ b/tests/queries/0_stateless/04286_cas_remote_data_paths.reference
@@ -0,0 +1,2 @@
+1
+dropped_ok
diff --git a/tests/queries/0_stateless/04286_cas_remote_data_paths.sql b/tests/queries/0_stateless/04286_cas_remote_data_paths.sql
new file mode 100644
index 000000000000..2affab5a09df
--- /dev/null
+++ b/tests/queries/0_stateless/04286_cas_remote_data_paths.sql
@@ -0,0 +1,41 @@
+-- Tags: no-fasttest, no-cas-storage
+-- no-fasttest: cas is an object-storage metadata type; keep it off the minimal
+-- fasttest image.
+-- no-cas-storage: the coverage lives in the INLINE content-addressed disk created
+-- below, so the test is meaningful on every ordinary lane. On lanes where the DEFAULT MergeTree
+-- storage is itself content-addressed, `system.remote_data_paths` (whose `disk_name` filter is
+-- applied only after the traversal) also walks the huge shared default pool holding the whole
+-- run's data, and on the S3 (RustFS) variant that walk does not fit the 600s test timeout.
+
+-- B38: querying system.remote_data_paths traverses the disk and probes existsFile on pool sub-dirs
+-- (e.g. the "store" directory). Such a path resolves to a directory object key; existsFile must treat
+-- a directory as not-a-file and not let the raw filesystem "Is a directory" error escape. So a
+-- system.remote_data_paths query over a cas table must succeed (return the parts'
+-- remote paths) instead of throwing.
+
+DROP TABLE IF EXISTS t_cas_rdp;
+
+CREATE TABLE t_cas_rdp (a UInt64, b UInt64)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04286',
+    name = '04286_cas_rdp',
+    path = '04286_cas_rdp_pool/');
+
+INSERT INTO t_cas_rdp SELECT number, number * 2 FROM numbers(100);
+
+-- The traversal (with shadow paths) must be QUERYABLE: it used to throw `Is a directory` (Code 1001)
+-- when it probed the CA pool sub-dir (e.g. "store") via existsFile. After the B38 fix it returns a
+-- result without raising. We assert the query succeeds (count() is a non-negative number) rather than
+-- a specific row count: the CA disk's object-storage directory model determines how many rows the
+-- traversal yields, which is orthogonal to the not-throwing contract this test pins.
+SELECT count() >= 0
+FROM system.remote_data_paths
+WHERE disk_name = '04286_cas_rdp'
+SETTINGS traverse_shadow_remote_data_paths = 1;
+
+DROP TABLE t_cas_rdp;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04287_cas_detach_partition_listing.reference b/tests/queries/0_stateless/04287_cas_detach_partition_listing.reference
new file mode 100644
index 000000000000..a4f7c658a48c
--- /dev/null
+++ b/tests/queries/0_stateless/04287_cas_detach_partition_listing.reference
@@ -0,0 +1,4 @@
+count_before	100
+count_after	0
+detached	all_1_2_1
+dropped_ok
diff --git a/tests/queries/0_stateless/04287_cas_detach_partition_listing.sql b/tests/queries/0_stateless/04287_cas_detach_partition_listing.sql
new file mode 100644
index 000000000000..60e7825dc920
--- /dev/null
+++ b/tests/queries/0_stateless/04287_cas_detach_partition_listing.sql
@@ -0,0 +1,38 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- B36: after DETACH PARTITION on a cas disk, system.detached_parts must list the
+-- detached part DIRECTORY name (e.g. all_1_2_1), not a sidecar / mutable file (metadata_version.txt).
+-- The detached namespace is a container of detached part directories; the CA disk listing of the
+-- "detached" path must yield the part directory names, not the files inside them.
+
+DROP TABLE IF EXISTS t_cas_detach;
+
+CREATE TABLE t_cas_detach (a UInt64, b UInt64)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04287',
+    name = '04287_cas_detach',
+    path = '04287_cas_detach_pool/');
+
+INSERT INTO t_cas_detach SELECT number, number * 2 FROM numbers(50);
+INSERT INTO t_cas_detach SELECT number, number * 2 FROM numbers(50, 50);
+OPTIMIZE TABLE t_cas_detach FINAL;
+
+SELECT 'count_before', count() FROM t_cas_detach;
+
+ALTER TABLE t_cas_detach DETACH PARTITION tuple();
+
+SELECT 'count_after', count() FROM t_cas_detach;
+
+-- The detached parts listing must show the part directory name, not metadata_version.txt.
+SELECT 'detached', name
+FROM system.detached_parts
+WHERE database = currentDatabase() AND table = 't_cas_detach'
+ORDER BY name;
+
+DROP TABLE t_cas_detach;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04288_cas_detached_part_modification_time.reference b/tests/queries/0_stateless/04288_cas_detached_part_modification_time.reference
new file mode 100644
index 000000000000..180d4e299167
--- /dev/null
+++ b/tests/queries/0_stateless/04288_cas_detached_part_modification_time.reference
@@ -0,0 +1,4 @@
+count_before	50
+count_after	0
+detached_mtime_readable	all_1_1_0	1
+dropped_ok
diff --git a/tests/queries/0_stateless/04288_cas_detached_part_modification_time.sql b/tests/queries/0_stateless/04288_cas_detached_part_modification_time.sql
new file mode 100644
index 000000000000..a06818fdc071
--- /dev/null
+++ b/tests/queries/0_stateless/04288_cas_detached_part_modification_time.sql
@@ -0,0 +1,40 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- B44: reading system.detached_parts.modification_time of a part detached on a cas
+-- disk must NOT throw. The detached part is stored under a ref named "detached" whose manifest keys
+-- are shaped <detached_part>/<file>; system.detached_parts reads the modification time by calling
+-- IDisk::getLastModified on the detached part DIRECTORY (<table>/detached/<detached_part>). Before the
+-- fix, parsePartFilePath reported part_name="detached" + a non-empty file equal to the detached part
+-- directory name, so getLastModified fell through to the part-file manifest lookup and threw
+-- "ContentAddressed: file <detached_part> not in manifest". getLastModified now recognises the detached
+-- part directory and reports the "detached" ref manifest object's mtime.
+
+DROP TABLE IF EXISTS t_cas_detach_mtime;
+
+CREATE TABLE t_cas_detach_mtime (a UInt64, b UInt64)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04288',
+    name = '04288_cas_detach_mtime',
+    path = '04288_cas_detach_mtime_pool/');
+
+INSERT INTO t_cas_detach_mtime SELECT number, number * 2 FROM numbers(50);
+
+SELECT 'count_before', count() FROM t_cas_detach_mtime;
+
+ALTER TABLE t_cas_detach_mtime DETACH PARTITION tuple();
+
+SELECT 'count_after', count() FROM t_cas_detach_mtime;
+
+-- The modification_time read must succeed (be non-NULL) instead of throwing FILE_DOESNT_EXIST.
+SELECT 'detached_mtime_readable', name, modification_time IS NOT NULL
+FROM system.detached_parts
+WHERE database = currentDatabase() AND table = 't_cas_detach_mtime'
+ORDER BY name;
+
+DROP TABLE t_cas_detach_mtime;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04289_cas_multi_detach_drop.reference b/tests/queries/0_stateless/04289_cas_multi_detach_drop.reference
new file mode 100644
index 000000000000..f87bae700d97
--- /dev/null
+++ b/tests/queries/0_stateless/04289_cas_multi_detach_drop.reference
@@ -0,0 +1,7 @@
+active_parts	3
+detached_after	3
+detached_names	1_1_1_0
+detached_names	2_2_2_0
+detached_names	3_3_3_0
+detached_after_drop	0
+dropped_ok
diff --git a/tests/queries/0_stateless/04289_cas_multi_detach_drop.sql b/tests/queries/0_stateless/04289_cas_multi_detach_drop.sql
new file mode 100644
index 000000000000..3de4dd0a1906
--- /dev/null
+++ b/tests/queries/0_stateless/04289_cas_multi_detach_drop.sql
@@ -0,0 +1,43 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- B46/B47: multiple partitions detached on a cas disk must COEXIST under the one
+-- shared "detached" ref, and DROP DETACHED PARTITION ALL must remove them.
+--   B46: each DETACH PARTITION clones one part into detached/<detached_part>/ via a fresh CA commit;
+--        the commit used to REWRITE the shared "detached" ref, so each detach overwrote the previous
+--        one and only the last detached part was listed. commit now MERGES into the existing detached
+--        ref's manifest + sidecar, so all detached parts coexist.
+--   B47: DROP DETACHED PARTITION first renames the detached part to "deleting_<part>"
+--        (PartsTemporaryRename) then removes it; CA moveDirectory ignored a detached->detached rename
+--        (the rename was a no-op, so removeRecursive on the renamed dir found nothing). moveDirectory
+--        now re-keys the detached part dir within the shared detached ref, and removeRecursive handles a
+--        detached part directory by removing only that part's keys from the shared ref.
+
+DROP TABLE IF EXISTS t_cas_multi_detach;
+
+CREATE TABLE t_cas_multi_detach (p UInt64, v UInt64)
+ENGINE = MergeTree PARTITION BY p ORDER BY v
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04289',
+    name = '04289_cas_multi_detach',
+    path = '04289_cas_multi_detach_pool/');
+
+INSERT INTO t_cas_multi_detach VALUES (1, 1), (2, 2), (3, 3);
+SELECT 'active_parts', count() FROM system.parts WHERE database = currentDatabase() AND table = 't_cas_multi_detach' AND active;
+
+ALTER TABLE t_cas_multi_detach DETACH PARTITION ALL;
+
+-- All three partitions must be listed as detached parts (not just the last one detached).
+SELECT 'detached_after', count() FROM system.detached_parts WHERE database = currentDatabase() AND table = 't_cas_multi_detach';
+SELECT 'detached_names', name FROM system.detached_parts WHERE database = currentDatabase() AND table = 't_cas_multi_detach' ORDER BY name;
+
+ALTER TABLE t_cas_multi_detach DROP DETACHED PARTITION ALL SETTINGS allow_drop_detached = 1;
+
+-- DROP DETACHED PARTITION ALL must remove every detached part.
+SELECT 'detached_after_drop', count() FROM system.detached_parts WHERE database = currentDatabase() AND table = 't_cas_multi_detach';
+
+DROP TABLE t_cas_multi_detach;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04290_cas_no_leftovers.reference b/tests/queries/0_stateless/04290_cas_no_leftovers.reference
new file mode 100644
index 000000000000..ea1449c10eba
--- /dev/null
+++ b/tests/queries/0_stateless/04290_cas_no_leftovers.reference
@@ -0,0 +1,5 @@
+rows	600000
+grew_above_baseline 1
+fsck_unreachable 0
+fsck_dangling 0
+pool_meta_present 1
diff --git a/tests/queries/0_stateless/04290_cas_no_leftovers.sh b/tests/queries/0_stateless/04290_cas_no_leftovers.sh
new file mode 100755
index 000000000000..eda5ff7d4fbf
--- /dev/null
+++ b/tests/queries/0_stateless/04290_cas_no_leftovers.sh
@@ -0,0 +1,132 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest, no-parallel
+# ^ cas is an object-storage metadata type (keep it off the minimal fasttest image);
+#   no-parallel because we inspect a known on-disk pool directory from the shell and must not race
+#   another test sharing the same path.
+
+# North-star "no S3 leftovers" oracle for the content-addressed pool, exercised over a `local`
+# object_storage backend so the pool is a plain directory the test shell can inspect directly.
+#
+# We put the pool under CLICKHOUSE_USER_FILES_UNIQUE (an absolute path both the server and this
+# shell can see on a local run) and enable the background reachability GC aggressively
+# (gc_enabled=1, grace=2s, interval=1s). We then:
+#   (1) record the baseline blobs+parts object count (~0),
+#   (2) CREATE a MergeTree on the CA disk and INSERT several distinct batches to make many blobs,
+#   (3) assert the count rose above baseline,
+#   (4) DROP TABLE ... SYNC so the refs are unlinked and the blobs/footers become GC fodder,
+#   (5) drain the retire pipeline deterministically via `SYSTEM CAS GC RUN` (bounded
+#       loop on the `pending_*` gauges, NOT a fixed sleep), then run `FSCK` directly on the running
+#       disk (T13): a clean reachability audit reading back zero `unreachable`/`dangling` is a
+#       strictly stronger no-leftovers oracle than polling the pool directory ever was.
+# `_pool_meta` (durable single-owner marker) and the `store/` metadata tree are expected to remain.
+# Teardown is fail-closed (spec rev.8 §5/§9): `SYSTEM CAS FORGET` the disk (force-Vanish,
+# node-local), verify it reads `vanished(forgotten)` in system.cas_mounts, and only then
+# `rm -rf` — FORGET stopped and joined every CAS background thread for this disk.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+POOL_DIR="${CLICKHOUSE_USER_FILES_UNIQUE}_04290_${RANDOM}"
+
+# Fresh pool dir for this run.
+rm -rf "${POOL_DIR:?}"
+mkdir -p "${POOL_DIR}"
+
+# Count regular files (objects) currently living under blobs/ and parts/ in the pool.
+count_pool_objects() {
+    local n_blobs n_parts
+    n_blobs=$(find "${POOL_DIR}/ca/blobs" "${POOL_DIR}/ca/packs" -type f 2>/dev/null | wc -l)
+    n_parts=$(find "${POOL_DIR}/ca/trees" -type f 2>/dev/null | wc -l)
+    echo $(( n_blobs + n_parts ))
+}
+
+DISK_NAME="ca_04290_${CLICKHOUSE_TEST_UNIQUE_NAME}_${RANDOM}"
+DISK_DEF="disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04290',
+    name = '${DISK_NAME}',
+    path = '${POOL_DIR}/',
+    gc_enabled = 1,
+    gc_interval_sec = 1)"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_cas_leftovers SYNC"
+
+# (1) Baseline.
+BASELINE=$(count_pool_objects)
+
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_cas_leftovers (a UInt64, s String, d Date)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = ${DISK_DEF}"
+
+# (2) Several distinct inserts -> several distinct parts/blobs (distinct data => no dedup-away).
+for i in 0 1 2 3 4 5; do
+    $CLICKHOUSE_CLIENT --query "
+        INSERT INTO t_cas_leftovers
+        SELECT number + ${i} * 100000, toString(number + ${i} * 100000), toDate('2020-01-01') + (number % 1000)
+        FROM numbers(100000)"
+done
+
+$CLICKHOUSE_CLIENT --query "SELECT 'rows', count() FROM t_cas_leftovers"
+
+# (3) Pool must have grown above baseline.
+AFTER_INSERT=$(count_pool_objects)
+if [ "$AFTER_INSERT" -gt "$BASELINE" ]; then
+    echo "grew_above_baseline 1"
+else
+    echo "grew_above_baseline 0 (baseline=${BASELINE} after_insert=${AFTER_INSERT})"
+fi
+
+# (4) Drop: refs unlinked synchronously, blobs/footers become unreferenced GC fodder.
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_cas_leftovers SYNC"
+
+# (5) Drain GC deterministically: loop `SYSTEM CAS GC RUN` rounds until the retire
+#     pipeline's `pending_*` gauges (Task 7) read back to empty. Bounded (~60 rounds, half-second
+#     spacing), not a fixed sleep; column values are looked up BY HEADER NAME (not position) so the
+#     loop keeps working if the result set gains columns.
+PENDING=1
+for _ in $(seq 1 60); do
+    PENDING=$($CLICKHOUSE_CLIENT --query "SYSTEM CAS GC RUN '${DISK_NAME}'" --format TSVWithNames \
+        | awk -F'\t' 'NR==1 { for (i = 1; i <= NF; i++) col[$i] = i; next }
+                      { print $col["pending_condemned"] }   # already candidates+retired per its doc in Gc/CasGc.h; summing all three double-counts')
+    [ "${PENDING}" = "0" ] && break
+    sleep 0.5
+done
+
+if [ "${PENDING}" != "0" ]; then
+    echo "FAIL: GC did not drain the retire pipeline within the bounded loop (pending=${PENDING})" >&2
+    exit 1
+fi
+
+# (6) FSCK runs directly on the running disk (T13): a reachability audit that must read back zero
+#     unreachable/dangling objects. This is a strictly stronger no-leftovers oracle than the old
+#     dir-poll.
+$CLICKHOUSE_CLIENT --query "SYSTEM CAS FSCK '${DISK_NAME}'" --format TSVWithNames \
+    | awk -F'\t' 'NR==1 { for (i = 1; i <= NF; i++) col[$i] = i; next }
+                  { print "fsck_unreachable", $col["unreachable"]; print "fsck_dangling", $col["dangling"] }'
+
+# _pool_meta must still be present (durable single-owner marker is never GC'd).
+if [ -f "${POOL_DIR}/ca/_pool_meta" ]; then
+    echo "pool_meta_present 1"
+else
+    echo "pool_meta_present 0"
+fi
+
+# (7) Fail-closed teardown (spec rev.8 §5/§9): FORGET the disk (force-Vanish, node-local; the table is
+#     already dropped above), verify it reads exactly `vanished(forgotten)` in the mounts table, and
+#     only then rm. A failed FORGET or an unexpected lifecycle aborts with the pool dir left in place.
+#     FORGET logs an operator WARNING; the harness runs the client at --send_logs_level=warning, so that
+#     expected warning would stream to stderr and be flagged as a failure -- suppress it for this call.
+$CLICKHOUSE_CLIENT --allow_repeated_settings --send_logs_level=fatal \
+    --query "SYSTEM CAS FORGET '${DISK_NAME}'" || {
+    echo "FORGET failed — leaving pool dir in place (fail-closed)"; exit 1; }
+LIFECYCLE=$($CLICKHOUSE_CLIENT --query "
+    SELECT lifecycle || '(' || lifecycle_reason || ')' FROM system.cas_mounts
+    WHERE disk = '${DISK_NAME}'")
+[ "${LIFECYCLE}" = "vanished(forgotten)" ] || {
+    echo "unexpected lifecycle after FORGET: ${LIFECYCLE}"; exit 1; }
+
+rm -rf "${POOL_DIR:?}"   # safe: FORGET stopped and joined every CAS thread for this disk
diff --git a/tests/queries/0_stateless/04292_cas_mutations.reference b/tests/queries/0_stateless/04292_cas_mutations.reference
new file mode 100644
index 000000000000..98e15e398c73
--- /dev/null
+++ b/tests/queries/0_stateless/04292_cas_mutations.reference
@@ -0,0 +1,7 @@
+after_update_v: match
+after_delete: match
+after_update_s: match
+after_update_v_doubled: match
+after_multi_update: match
+final_rows_match	1
+final_data_match	1
diff --git a/tests/queries/0_stateless/04292_cas_mutations.sh b/tests/queries/0_stateless/04292_cas_mutations.sh
new file mode 100755
index 000000000000..02cb60321a53
--- /dev/null
+++ b/tests/queries/0_stateless/04292_cas_mutations.sh
@@ -0,0 +1,114 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# Correctness oracle for mutations on content-addressed disks (CAS M7).
+# After supportsHardLinks() was flipped to true, mutations are enabled on
+# content-addressed disks.  A mutation builds the new part through a
+# whole-part transaction: unchanged columns are carried forward by reference
+# (same blob) and changed columns are written fresh.
+#
+# Strategy: both tables receive identical data and identical mutations;
+# after each mutation we assert the full ordered contents are equal
+# (CA vs plain MergeTree).  Every assertion is a self-checking CA-vs-plain
+# equality so the reference file is trivially correct (no hand-computed
+# arithmetic needed).
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+DISK_CA="disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04292',
+    name = '04292_cas_mut',
+    path = '04292_cas_mut_pool/')"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_ca    SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_plain SYNC"
+
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_ca (id UInt64, v UInt64, s String)
+ENGINE = MergeTree ORDER BY id
+SETTINGS disk = ${DISK_CA}"
+
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_plain (id UInt64, v UInt64, s String)
+ENGINE = MergeTree ORDER BY id"
+
+# Seed both tables with identical deterministic data.
+$CLICKHOUSE_CLIENT --query "
+INSERT INTO t_ca    SELECT number, number * 10, toString(number) FROM numbers(100)"
+$CLICKHOUSE_CLIENT --query "
+INSERT INTO t_plain SELECT number, number * 10, toString(number) FROM numbers(100)"
+
+# Helper: compare full ordered contents.
+CMP_QUERY="SELECT if(
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_ca    ORDER BY id)) =
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_plain ORDER BY id)),
+  'match', 'DIFF')"
+
+# --- Mutation 1: UPDATE one column (id/s carry forward by reference on CA) ---
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_ca    UPDATE v = v + 1 WHERE id % 3 = 0 SETTINGS mutations_sync = 2"
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_plain UPDATE v = v + 1 WHERE id % 3 = 0 SETTINGS mutations_sync = 2"
+
+echo -n 'after_update_v: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Mutation 2: DELETE ---
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_ca    DELETE WHERE id % 7 = 0 SETTINGS mutations_sync = 2"
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_plain DELETE WHERE id % 7 = 0 SETTINGS mutations_sync = 2"
+
+echo -n 'after_delete: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Mutation 3: UPDATE string column for a range of rows ---
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_ca    UPDATE s = concat(s, '_x') WHERE id > 50 SETTINGS mutations_sync = 2"
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_plain UPDATE s = concat(s, '_x') WHERE id > 50 SETTINGS mutations_sync = 2"
+
+echo -n 'after_update_s: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# NOTE: a column-type change (`MODIFY COLUMN v Int64`) is deliberately NOT exercised here. It is a
+# data-`ALTER` that runs `checkAlterIsPossible`, which on a table created with an inline
+# `disk = disk(...)` setting trips a PRE-EXISTING, engine-agnostic bug: the `disk` value is stored as a
+# `CustomType` in `settings_changes` and several ALTER sub-checks read it as a `String` (`BAD_GET`).
+# That is orthogonal to content-addressing (it reproduces on any inline-disk table). `MODIFY COLUMN` on
+# a content-addressed disk is covered through the storage-policy path by the CA-default suite run.
+
+# --- Mutation 4: UPDATE the numeric column again (compounding the carry-forward) ---
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_ca    UPDATE v = v * 2 WHERE id % 2 = 0 SETTINGS mutations_sync = 2"
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_plain UPDATE v = v * 2 WHERE id % 2 = 0 SETTINGS mutations_sync = 2"
+
+echo -n 'after_update_v_doubled: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Mutation 5: multi-column UPDATE in one mutation (both data columns rewritten together) ---
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_ca    UPDATE v = v + id, s = concat('p_', s) WHERE id < 40 SETTINGS mutations_sync = 2"
+$CLICKHOUSE_CLIENT --query "
+ALTER TABLE t_plain UPDATE v = v + id, s = concat('p_', s) WHERE id < 40 SETTINGS mutations_sync = 2"
+
+echo -n 'after_multi_update: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Final sanity: row count and data equality ---
+$CLICKHOUSE_CLIENT --query "
+SELECT 'final_rows_match', count() = (SELECT count() FROM t_plain) FROM t_ca"
+$CLICKHOUSE_CLIENT --query "
+SELECT 'final_data_match',
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_ca    ORDER BY id)) =
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_plain ORDER BY id))"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_ca    SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_plain SYNC"
diff --git a/tests/queries/0_stateless/04293_cas_lightweight_delete.reference b/tests/queries/0_stateless/04293_cas_lightweight_delete.reference
new file mode 100644
index 000000000000..18c1efab4a4a
--- /dev/null
+++ b/tests/queries/0_stateless/04293_cas_lightweight_delete.reference
@@ -0,0 +1,6 @@
+after_delete_mod5: match
+after_delete_like: match
+after_delete_range: match
+after_optimize: match
+final_rows_match	1
+final_data_match	1
diff --git a/tests/queries/0_stateless/04293_cas_lightweight_delete.sh b/tests/queries/0_stateless/04293_cas_lightweight_delete.sh
new file mode 100755
index 000000000000..a034f9cc4f70
--- /dev/null
+++ b/tests/queries/0_stateless/04293_cas_lightweight_delete.sh
@@ -0,0 +1,101 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# Correctness oracle for lightweight DELETE on content-addressed disks (CAS M7).
+# After the supportsHardLinks() gate was lifted, lightweight DELETE is enabled on
+# content-addressed disks.  Unlike heavy mutations, lightweight DELETE uses row-
+# existence bitmaps stored alongside each part and is applied physically during
+# the next OPTIMIZE/merge.
+#
+# Strategy: both tables receive identical data and identical lightweight DELETEs;
+# after each DELETE (and after OPTIMIZE FINAL) we assert the full ordered contents
+# are equal (CA vs plain MergeTree).  Every assertion is a self-checking CA-vs-plain
+# equality so the reference file is trivially correct (no hand-computed arithmetic
+# needed).
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+DISK_CA="disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04293',
+    name = '04293_cas_lwd',
+    path = '04293_cas_lwd_pool/')"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_ca    SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_plain SYNC"
+
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_ca (id UInt64, v UInt64, s String)
+ENGINE = MergeTree ORDER BY id
+SETTINGS disk = ${DISK_CA}"
+
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_plain (id UInt64, v UInt64, s String)
+ENGINE = MergeTree ORDER BY id"
+
+# Seed both tables with identical deterministic data spread across two parts
+# (lightweight DELETEs across multiple parts are more meaningful than single-part).
+$CLICKHOUSE_CLIENT --query "
+INSERT INTO t_ca    SELECT number, number * 10, toString(number) FROM numbers(100)"
+$CLICKHOUSE_CLIENT --query "
+INSERT INTO t_ca    SELECT number, number * 10, toString(number) FROM numbers(100, 100)"
+$CLICKHOUSE_CLIENT --query "
+INSERT INTO t_plain SELECT number, number * 10, toString(number) FROM numbers(100)"
+$CLICKHOUSE_CLIENT --query "
+INSERT INTO t_plain SELECT number, number * 10, toString(number) FROM numbers(100, 100)"
+
+# Helper: compare full ordered contents of both tables.
+CMP_QUERY="SELECT if(
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_ca    ORDER BY id)) =
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_plain ORDER BY id)),
+  'match', 'DIFF')"
+
+# --- Lightweight DELETE 1: every 5th row ---
+$CLICKHOUSE_CLIENT --query "
+DELETE FROM t_ca    WHERE id % 5 = 0 SETTINGS lightweight_deletes_sync = 2"
+$CLICKHOUSE_CLIENT --query "
+DELETE FROM t_plain WHERE id % 5 = 0 SETTINGS lightweight_deletes_sync = 2"
+
+echo -n 'after_delete_mod5: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Lightweight DELETE 2: rows whose string starts with '1' (overlaps first delete) ---
+$CLICKHOUSE_CLIENT --query "
+DELETE FROM t_ca    WHERE s LIKE '1%' SETTINGS lightweight_deletes_sync = 2"
+$CLICKHOUSE_CLIENT --query "
+DELETE FROM t_plain WHERE s LIKE '1%' SETTINGS lightweight_deletes_sync = 2"
+
+echo -n 'after_delete_like: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Lightweight DELETE 3: rows with v > 1500 ---
+$CLICKHOUSE_CLIENT --query "
+DELETE FROM t_ca    WHERE v > 1500 SETTINGS lightweight_deletes_sync = 2"
+$CLICKHOUSE_CLIENT --query "
+DELETE FROM t_plain WHERE v > 1500 SETTINGS lightweight_deletes_sync = 2"
+
+echo -n 'after_delete_range: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- OPTIMIZE FINAL: force merge so lightweight deletes are physically applied ---
+$CLICKHOUSE_CLIENT --query "OPTIMIZE TABLE t_ca    FINAL"
+$CLICKHOUSE_CLIENT --query "OPTIMIZE TABLE t_plain FINAL"
+
+echo -n 'after_optimize: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Final sanity: row count and data equality ---
+$CLICKHOUSE_CLIENT --query "
+SELECT 'final_rows_match', count() = (SELECT count() FROM t_plain) FROM t_ca"
+$CLICKHOUSE_CLIENT --query "
+SELECT 'final_data_match',
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_ca    ORDER BY id)) =
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_plain ORDER BY id))"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_ca    SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_plain SYNC"
diff --git a/tests/queries/0_stateless/04294_cas_patch_parts.reference b/tests/queries/0_stateless/04294_cas_patch_parts.reference
new file mode 100644
index 000000000000..0f54f244519c
--- /dev/null
+++ b/tests/queries/0_stateless/04294_cas_patch_parts.reference
@@ -0,0 +1,6 @@
+after_patch_delete_1: match
+after_patch_delete_2: match
+ca_has_patch_part: 1
+after_optimize: match
+final_rows_match	1
+final_data_match	1
diff --git a/tests/queries/0_stateless/04294_cas_patch_parts.sh b/tests/queries/0_stateless/04294_cas_patch_parts.sh
new file mode 100755
index 000000000000..323ae0e46c9a
--- /dev/null
+++ b/tests/queries/0_stateless/04294_cas_patch_parts.sh
@@ -0,0 +1,88 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# Correctness oracle for PATCH PARTS (the native lightweight-update model, B5) on content-addressed
+# disks (CAS M7). The default lightweight DELETE mode is `alter_update` (a heavy mutation); this test
+# forces the lightweight-update path with `lightweight_delete_mode = 'lightweight_update_force'`, which
+# produces a PATCH PART (an `UPDATE _row_exists = 0`). `_force` THROWS if the table cannot do a
+# lightweight update, so a successful run is itself proof the patch-part path was exercised — on a
+# content-addressed disk the patch part is written through the same whole-part transaction as any part.
+#
+# Lightweight updates require materialized `_block_number` / `_block_offset` columns
+# (enable_block_number_column / enable_block_offset_column) and a non-UNIQUE-KEY custom-partitioned
+# table. Both tables get identical settings, data, and operations; every assertion is a self-checking
+# CA-vs-plain equality so the reference file is trivially correct.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+DISK_CA="disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04294',
+    name = '04294_cas_patch',
+    path = '04294_cas_patch_pool/')"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_ca    SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_plain SYNC"
+
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_ca (id UInt64, v UInt64, s String)
+ENGINE = MergeTree ORDER BY id
+SETTINGS disk = ${DISK_CA}, enable_block_number_column = 1, enable_block_offset_column = 1"
+
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_plain (id UInt64, v UInt64, s String)
+ENGINE = MergeTree ORDER BY id
+SETTINGS enable_block_number_column = 1, enable_block_offset_column = 1"
+
+# Seed both identically across two parts.
+$CLICKHOUSE_CLIENT --query "INSERT INTO t_ca    SELECT number, number * 10, toString(number) FROM numbers(100)"
+$CLICKHOUSE_CLIENT --query "INSERT INTO t_ca    SELECT number, number * 10, toString(number) FROM numbers(100, 100)"
+$CLICKHOUSE_CLIENT --query "INSERT INTO t_plain SELECT number, number * 10, toString(number) FROM numbers(100)"
+$CLICKHOUSE_CLIENT --query "INSERT INTO t_plain SELECT number, number * 10, toString(number) FROM numbers(100, 100)"
+
+CMP_QUERY="SELECT if(
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_ca    ORDER BY id)) =
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_plain ORDER BY id)),
+  'match', 'DIFF')"
+
+# Force the patch-part (lightweight-update) path. `_force` throws if unsupported, so success == patch path.
+LWU_SETTINGS="SETTINGS enable_lightweight_update = 1, lightweight_delete_mode = 'lightweight_update_force', lightweight_deletes_sync = 2"
+
+# --- Patch-part DELETE 1 ---
+$CLICKHOUSE_CLIENT --query "DELETE FROM t_ca    WHERE id % 5 = 0 ${LWU_SETTINGS}"
+$CLICKHOUSE_CLIENT --query "DELETE FROM t_plain WHERE id % 5 = 0 ${LWU_SETTINGS}"
+echo -n 'after_patch_delete_1: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Patch-part DELETE 2 (overlaps the first) ---
+$CLICKHOUSE_CLIENT --query "DELETE FROM t_ca    WHERE v > 1500 ${LWU_SETTINGS}"
+$CLICKHOUSE_CLIENT --query "DELETE FROM t_plain WHERE v > 1500 ${LWU_SETTINGS}"
+echo -n 'after_patch_delete_2: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# Prove a patch part really exists on the content-addressed table before it is merged away.
+echo -n 'ca_has_patch_part: '
+$CLICKHOUSE_CLIENT --query "
+SELECT count() > 0 FROM system.parts
+WHERE database = currentDatabase() AND table = 't_ca' AND active AND startsWith(name, 'patch')"
+
+# --- OPTIMIZE FINAL applies the patch parts during merge ---
+$CLICKHOUSE_CLIENT --query "OPTIMIZE TABLE t_ca    FINAL"
+$CLICKHOUSE_CLIENT --query "OPTIMIZE TABLE t_plain FINAL"
+echo -n 'after_optimize: '
+$CLICKHOUSE_CLIENT --query "$CMP_QUERY"
+
+# --- Final equality ---
+$CLICKHOUSE_CLIENT --query "SELECT 'final_rows_match', count() = (SELECT count() FROM t_plain) FROM t_ca"
+$CLICKHOUSE_CLIENT --query "
+SELECT 'final_data_match',
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_ca    ORDER BY id)) =
+  (SELECT groupArray((id, v, s)) FROM (SELECT id, v, s FROM t_plain ORDER BY id))"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_ca    SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_plain SYNC"
diff --git a/tests/queries/0_stateless/04295_cas_mutation_no_leftovers.reference b/tests/queries/0_stateless/04295_cas_mutation_no_leftovers.reference
new file mode 100644
index 000000000000..52b50e1048c9
--- /dev/null
+++ b/tests/queries/0_stateless/04295_cas_mutation_no_leftovers.reference
@@ -0,0 +1,5 @@
+grew_above_baseline 1
+rows_after_mutations_correct	1
+fsck_unreachable 0
+fsck_dangling 0
+pool_meta_present 1
diff --git a/tests/queries/0_stateless/04295_cas_mutation_no_leftovers.sh b/tests/queries/0_stateless/04295_cas_mutation_no_leftovers.sh
new file mode 100755
index 000000000000..e33c824c1023
--- /dev/null
+++ b/tests/queries/0_stateless/04295_cas_mutation_no_leftovers.sh
@@ -0,0 +1,131 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest, no-parallel
+# ^ cas is an object-storage metadata type (keep it off the minimal fasttest image);
+#   no-parallel because we inspect a known on-disk pool directory from the shell and must not race
+#   another test sharing the same path.
+
+# No-leftovers oracle for MUTATIONS + lightweight DELETE (patch parts) on the content-addressed pool
+# (CAS M7), exercised over a `local` object_storage backend so the pool is a plain directory the test
+# shell can inspect directly. Mirrors 04290 but adds heavy mutations and a patch-part lightweight
+# DELETE before the drop: a mutation supersedes the source part (its uniquely-owned blobs become
+# unreachable) and writes a new part; carried-forward columns stay referenced. We assert that after
+# DROP, draining the retire pipeline via `SYSTEM CAS GC RUN` then running `FSCK` on the
+# running disk (T13) reads back zero `unreachable`/`dangling` objects (no mutated-away or patch-part
+# blobs left behind), and that `_pool_meta` survives. Teardown is fail-closed (spec rev.8 §5/§9): FORGET
+# the disk, verify `vanished(forgotten)`, then rm.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+POOL_DIR="${CLICKHOUSE_USER_FILES_UNIQUE}_04295_${RANDOM}"
+
+rm -rf "${POOL_DIR:?}"
+mkdir -p "${POOL_DIR}"
+
+count_pool_objects() {
+    local n_blobs n_parts
+    n_blobs=$(find "${POOL_DIR}/ca/blobs" "${POOL_DIR}/ca/packs" -type f 2>/dev/null | wc -l)
+    n_parts=$(find "${POOL_DIR}/ca/trees" -type f 2>/dev/null | wc -l)
+    echo $(( n_blobs + n_parts ))
+}
+
+DISK_NAME="ca_04295_${CLICKHOUSE_TEST_UNIQUE_NAME}_${RANDOM}"
+DISK_DEF="disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04295',
+    name = '${DISK_NAME}',
+    path = '${POOL_DIR}/',
+    gc_enabled = 1,
+    gc_interval_sec = 1)"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_cas_mut_leftovers SYNC"
+
+BASELINE=$(count_pool_objects)
+
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_cas_mut_leftovers (a UInt64, v UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = ${DISK_DEF}, enable_block_number_column = 1, enable_block_offset_column = 1"
+
+# Several distinct inserts -> several distinct parts/blobs.
+for i in 0 1 2 3; do
+    $CLICKHOUSE_CLIENT --query "
+        INSERT INTO t_cas_mut_leftovers
+        SELECT number + ${i} * 100000, (number + ${i} * 100000) * 10, toString(number + ${i} * 100000)
+        FROM numbers(100000)"
+done
+
+AFTER_INSERT=$(count_pool_objects)
+if [ "$AFTER_INSERT" -gt "$BASELINE" ]; then
+    echo "grew_above_baseline 1"
+else
+    echo "grew_above_baseline 0 (baseline=${BASELINE} after_insert=${AFTER_INSERT})"
+fi
+
+# Heavy mutation (rewrites the v column; a/s carry forward by reference -> shared blobs).
+$CLICKHOUSE_CLIENT --query "ALTER TABLE t_cas_mut_leftovers UPDATE v = v + 1 WHERE a % 2 = 0 SETTINGS mutations_sync = 2"
+# Heavy mutation: delete part of the data.
+$CLICKHOUSE_CLIENT --query "ALTER TABLE t_cas_mut_leftovers DELETE WHERE a % 5 = 0 SETTINGS mutations_sync = 2"
+# Patch part: forced lightweight-update DELETE (throws if unsupported, so success == patch path).
+$CLICKHOUSE_CLIENT --query "
+    DELETE FROM t_cas_mut_leftovers WHERE a % 7 = 0
+    SETTINGS enable_lightweight_update = 1, lightweight_delete_mode = 'lightweight_update_force', lightweight_deletes_sync = 2"
+
+# Self-checking row count: a ranges over [0, 400000); the two deletes drop a%5=0 and a%7=0
+# (the UPDATE does not change the row count), so the survivors are exactly a%5!=0 AND a%7!=0.
+$CLICKHOUSE_CLIENT --query "
+SELECT 'rows_after_mutations_correct',
+       count() = (SELECT count() FROM numbers(400000) WHERE number % 5 != 0 AND number % 7 != 0)
+FROM t_cas_mut_leftovers"
+
+# Drop: every ref (original, mutated, and patch parts) is unlinked; all blobs/footers become GC fodder.
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_cas_mut_leftovers SYNC"
+
+# Drain GC deterministically: loop `SYSTEM CAS GC RUN` rounds until the retire
+# pipeline's `pending_*` gauges (Task 7) read back to empty. Bounded (~60 rounds, half-second
+# spacing), not a fixed sleep; column values are looked up BY HEADER NAME (not position) so the
+# loop keeps working if the result set gains columns.
+PENDING=1
+for _ in $(seq 1 60); do
+    PENDING=$($CLICKHOUSE_CLIENT --query "SYSTEM CAS GC RUN '${DISK_NAME}'" --format TSVWithNames \
+        | awk -F'\t' 'NR==1 { for (i = 1; i <= NF; i++) col[$i] = i; next }
+                      { print $col["pending_condemned"] }   # already candidates+retired per its doc in Gc/CasGc.h; summing all three double-counts')
+    [ "${PENDING}" = "0" ] && break
+    sleep 0.5
+done
+
+if [ "${PENDING}" != "0" ]; then
+    echo "FAIL: GC did not drain the retire pipeline within the bounded loop (pending=${PENDING})" >&2
+    exit 1
+fi
+
+# FSCK runs directly on the running disk (T13): a reachability audit that must read back zero
+# unreachable/dangling objects. This is a strictly stronger no-leftovers oracle than the old dir-poll.
+$CLICKHOUSE_CLIENT --query "SYSTEM CAS FSCK '${DISK_NAME}'" --format TSVWithNames \
+    | awk -F'\t' 'NR==1 { for (i = 1; i <= NF; i++) col[$i] = i; next }
+                  { print "fsck_unreachable", $col["unreachable"]; print "fsck_dangling", $col["dangling"] }'
+
+if [ -f "${POOL_DIR}/ca/_pool_meta" ]; then
+    echo "pool_meta_present 1"
+else
+    echo "pool_meta_present 0"
+fi
+
+# Fail-closed teardown (spec rev.8 §5/§9): FORGET the disk (force-Vanish, node-local; the table is
+# already dropped above), verify it reads exactly `vanished(forgotten)` in the mounts table, and only
+# then rm. A failed FORGET or an unexpected lifecycle aborts with the pool dir left in place. FORGET logs
+# an operator WARNING; the harness runs the client at --send_logs_level=warning, so that expected warning
+# would stream to stderr and be flagged as a failure -- suppress it for this call.
+$CLICKHOUSE_CLIENT --allow_repeated_settings --send_logs_level=fatal \
+    --query "SYSTEM CAS FORGET '${DISK_NAME}'" || {
+    echo "FORGET failed — leaving pool dir in place (fail-closed)"; exit 1; }
+LIFECYCLE=$($CLICKHOUSE_CLIENT --query "
+    SELECT lifecycle || '(' || lifecycle_reason || ')' FROM system.cas_mounts
+    WHERE disk = '${DISK_NAME}'")
+[ "${LIFECYCLE}" = "vanished(forgotten)" ] || {
+    echo "unexpected lifecycle after FORGET: ${LIFECYCLE}"; exit 1; }
+
+rm -rf "${POOL_DIR:?}"   # safe: FORGET stopped and joined every CAS thread for this disk
diff --git a/tests/queries/0_stateless/04299_cas_projection_inline_disk.reference b/tests/queries/0_stateless/04299_cas_projection_inline_disk.reference
new file mode 100644
index 000000000000..f45aa2ce0497
--- /dev/null
+++ b/tests/queries/0_stateless/04299_cas_projection_inline_disk.reference
@@ -0,0 +1,60 @@
+count	2000
+sum_b	9000
+by_b	0	200
+by_b	1	200
+by_b	2	200
+by_b	3	200
+by_b	4	200
+by_b	5	200
+by_b	6	200
+by_b	7	200
+by_b	8	200
+by_b	9	200
+after_merge_count	2000
+after_merge_by_b	0	200
+after_merge_by_b	1	200
+after_merge_by_b	2	200
+after_merge_by_b	3	200
+after_merge_by_b	4	200
+after_merge_by_b	5	200
+after_merge_by_b	6	200
+after_merge_by_b	7	200
+after_merge_by_b	8	200
+after_merge_by_b	9	200
+has_projection	1
+uses_projection	1
+after_merge_reload_projection	1
+after_merge_reload_uses_projection	1
+after_add_projection_count	2000
+projections_after_add	p_by_b	1
+projections_after_add	p_sum	1
+uses_p_sum	1
+projections_after_materialize_reload	p_by_b	1
+projections_after_materialize_reload	p_sum	1
+after_materialize_reload_uses_p_sum	1
+projections_after_update_reload	p_by_b	1
+projections_after_update_reload	p_sum	1
+after_update_reload_uses_p_sum	1
+after_drop_projection_count	2000
+projections_after_drop	p_sum	1
+after_reload_by_b	0	200
+after_reload_by_b	1	200
+after_reload_by_b	2	200
+after_reload_by_b	3	200
+after_reload_by_b	4	200
+after_reload_by_b	5	200
+after_reload_by_b	6	200
+after_reload_by_b	7	200
+after_reload_by_b	8	200
+after_reload_by_b	9	200
+after_reload_sum_b	0	199000
+after_reload_sum_b	1	199200
+after_reload_sum_b	2	199400
+after_reload_sum_b	3	199600
+after_reload_sum_b	4	199800
+after_reload_sum_b	5	200000
+after_reload_sum_b	6	200200
+after_reload_sum_b	7	200400
+after_reload_sum_b	8	200600
+after_reload_sum_b	9	200800
+dropped_ok
diff --git a/tests/queries/0_stateless/04299_cas_projection_inline_disk.sql b/tests/queries/0_stateless/04299_cas_projection_inline_disk.sql
new file mode 100644
index 000000000000..ed32d61ff357
--- /dev/null
+++ b/tests/queries/0_stateless/04299_cas_projection_inline_disk.sql
@@ -0,0 +1,125 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- Projections on a cas disk: the projection's files are stored as nested keys
+-- (<proj>.proj/<file>) in the parent part's manifest. Verify INSERT writes a projection, a
+-- projection-optimized SELECT returns correct results, and a merge (OPTIMIZE FINAL) rebuilds it.
+
+DROP TABLE IF EXISTS t_proj_cas;
+
+CREATE TABLE t_proj_cas (a UInt64, b UInt64, PROJECTION p_by_b (SELECT a, b ORDER BY b))
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '04299',
+    name = '04299_cas_projection',
+    path = '04299_cas_projection_pool/');
+
+INSERT INTO t_proj_cas SELECT number, number % 10 FROM numbers(1000);
+INSERT INTO t_proj_cas SELECT number, number % 10 FROM numbers(1000, 1000);
+
+SELECT 'count', count() FROM t_proj_cas;
+SELECT 'sum_b', sum(b) FROM t_proj_cas;
+SELECT 'by_b', b, count() FROM t_proj_cas GROUP BY b ORDER BY b;
+
+OPTIMIZE TABLE t_proj_cas FINAL;
+SELECT 'after_merge_count', count() FROM t_proj_cas;
+SELECT 'after_merge_by_b', b, count() FROM t_proj_cas GROUP BY b ORDER BY b;
+
+SELECT 'has_projection', countDistinct(name) FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_proj_cas' AND active;
+
+-- Prove the projection is actually selected by the optimizer (not a silent base-table fallback).
+SET optimize_use_projections = 1, force_optimize_projection = 1;
+SELECT 'uses_projection', countIf(explain LIKE '%p_by_b%') > 0
+FROM (EXPLAIN actions = 1 SELECT b, count() FROM t_proj_cas GROUP BY b);
+
+DROP TABLE t_proj_cas;
+
+-- ALTER ADD/DROP/MATERIALIZE PROJECTION + DETACH/ATTACH durability on the cas disk. We use
+-- the server's default cas storage policy here rather than an inline `disk = disk(...)`
+-- definition: an ALTER runs `checkColumnFilenamesForCollision`, which re-applies the table's raw
+-- `settings_changes` AST through the generic settings path, and the inline `disk(...)` function value
+-- is a CustomType that cannot be assigned to the String `disk` setting there (BAD_GET). That is a
+-- pre-existing, metadata-type-independent inline-disk-vs-ALTER issue, unrelated to content addressing;
+-- the projection ALTER mechanics on the CA disk are identical with the default-disk table. On the
+-- cas-default test job this plain table lands on a CA disk; on the normal job it lands on
+-- the local disk. The expected values below are the same on both (the oracle) — that equivalence is the
+-- whole point of B58: a merge/mutate-rebuilt projection must survive a reload on CA exactly as on a
+-- normal disk.
+DROP TABLE IF EXISTS t_proj_cas_alter;
+
+CREATE TABLE t_proj_cas_alter (a UInt64, b UInt64, PROJECTION p_by_b (SELECT a, b ORDER BY b))
+ENGINE = MergeTree ORDER BY a;
+
+INSERT INTO t_proj_cas_alter SELECT number, number % 10 FROM numbers(1000);
+INSERT INTO t_proj_cas_alter SELECT number, number % 10 FROM numbers(1000, 1000);
+OPTIMIZE TABLE t_proj_cas_alter FINAL;
+
+-- B58 DURABILITY (merge): the merge-rebuilt projection must survive a DETACH/ATTACH — it must live in the
+-- committed manifest, not only in memory. Reload and assert the projection is still active and usable.
+DETACH TABLE t_proj_cas_alter;
+ATTACH TABLE t_proj_cas_alter;
+SELECT 'after_merge_reload_projection', countDistinct(name) FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_proj_cas_alter' AND active;
+SET optimize_use_projections = 1, force_optimize_projection = 1;
+SELECT 'after_merge_reload_uses_projection', countIf(explain LIKE '%p_by_b%') > 0
+FROM (EXPLAIN actions = 1 SELECT b, count() FROM t_proj_cas_alter GROUP BY b);
+SET force_optimize_projection = 0;
+
+-- ALTER ADD PROJECTION on an existing table, then MATERIALIZE it on existing parts (rebuild path).
+-- This exercises the temp-projection (<proj>.tmp_proj -> <proj>.proj) flow inside the mutated part on
+-- the CA disk.
+ALTER TABLE t_proj_cas_alter ADD PROJECTION p_sum (SELECT b, sum(a) GROUP BY b);
+ALTER TABLE t_proj_cas_alter MATERIALIZE PROJECTION p_sum SETTINGS mutations_sync = 2;
+SELECT 'after_add_projection_count', count() FROM t_proj_cas_alter;
+-- After MATERIALIZE both the pre-existing p_by_b and the freshly built p_sum must be active. B58: the
+-- mutation must carry p_by_b forward and persist p_sum into the manifest of the rebuilt part.
+SELECT 'projections_after_add', name, count() FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_proj_cas_alter' AND active GROUP BY name ORDER BY name;
+
+-- The newly materialized projection must actually be selected by the optimizer.
+SELECT 'uses_p_sum', countIf(explain LIKE '%p_sum%') > 0
+FROM (EXPLAIN actions = 1 SELECT b, sum(a) FROM t_proj_cas_alter GROUP BY b);
+
+-- B58 DURABILITY (materialize): both projections must survive a DETACH/ATTACH after MATERIALIZE.
+DETACH TABLE t_proj_cas_alter;
+ATTACH TABLE t_proj_cas_alter;
+SELECT 'projections_after_materialize_reload', name, count() FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_proj_cas_alter' AND active GROUP BY name ORDER BY name;
+SET optimize_use_projections = 1, force_optimize_projection = 1;
+SELECT 'after_materialize_reload_uses_p_sum', countIf(explain LIKE '%p_sum%') > 0
+FROM (EXPLAIN actions = 1 SELECT b, sum(a) FROM t_proj_cas_alter GROUP BY b);
+SET force_optimize_projection = 0;
+
+-- B58 DURABILITY (data mutation): a mutation rebuilds the part; the surviving projections must be carried
+-- into the mutated part's manifest and stay usable after a reload. We use a DELETE that matches no rows so
+-- the row data (and therefore every expected value below) is unchanged across CA and non-CA — the part is
+-- still fully rewritten, exercising the mutation projection path.
+ALTER TABLE t_proj_cas_alter DELETE WHERE b = 999 SETTINGS mutations_sync = 2;
+DETACH TABLE t_proj_cas_alter;
+ATTACH TABLE t_proj_cas_alter;
+SELECT 'projections_after_update_reload', name, count() FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_proj_cas_alter' AND active GROUP BY name ORDER BY name;
+SET optimize_use_projections = 1, force_optimize_projection = 1;
+SELECT 'after_update_reload_uses_p_sum', countIf(explain LIKE '%p_sum%') > 0
+FROM (EXPLAIN actions = 1 SELECT b, sum(a) FROM t_proj_cas_alter GROUP BY b);
+SET force_optimize_projection = 0;
+
+-- DROP a projection: results unchanged, the projection's nested keys leave the new part version.
+ALTER TABLE t_proj_cas_alter DROP PROJECTION p_by_b SETTINGS mutations_sync = 2;
+SELECT 'after_drop_projection_count', count() FROM t_proj_cas_alter;
+SELECT 'projections_after_drop', name, count() FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_proj_cas_alter' AND active GROUP BY name ORDER BY name;
+
+-- Persistence: reload from the disk and re-read. `p_by_b` is gone, so the count() query falls back to the
+-- base table; the surviving `p_sum` still serves the sum(a) aggregation after the reload.
+DETACH TABLE t_proj_cas_alter;
+ATTACH TABLE t_proj_cas_alter;
+SELECT 'after_reload_by_b', b, count() FROM t_proj_cas_alter GROUP BY b ORDER BY b;
+SELECT 'after_reload_sum_b', b, sum(a) FROM t_proj_cas_alter GROUP BY b ORDER BY b;
+
+DROP TABLE t_proj_cas_alter;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/04300_cas_projection_multiblock.reference b/tests/queries/0_stateless/04300_cas_projection_multiblock.reference
new file mode 100644
index 000000000000..c16767ba74c9
--- /dev/null
+++ b/tests/queries/0_stateless/04300_cas_projection_multiblock.reference
@@ -0,0 +1,14 @@
+count	2600000
+by_b_top	1299999	3899998
+by_b_top	1299998	3899996
+by_b_top	1299997	3899994
+after_merge_by_b_top	1299999	3899998
+after_merge_by_b_top	1299998	3899996
+after_merge_by_b_top	1299997	3899994
+after_materialize_count	2600000
+projection_active	1
+uses_projection	1
+after_reload_by_b_top	1299999	3899998
+after_reload_by_b_top	1299998	3899996
+after_reload_by_b_top	1299997	3899994
+dropped_ok
diff --git a/tests/queries/0_stateless/04300_cas_projection_multiblock.sql b/tests/queries/0_stateless/04300_cas_projection_multiblock.sql
new file mode 100644
index 000000000000..e8dcb65642f0
--- /dev/null
+++ b/tests/queries/0_stateless/04300_cas_projection_multiblock.sql
@@ -0,0 +1,48 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- A projection built across MULTIPLE temp projection blocks (spill-and-merge) must read its own staged
+-- temp blocks back on a content-addressed disk (B59). MergeProjectionPartsTask only EXERCISES the
+-- read-back path when it has >1 temp projection part to merge (selected_parts.size() > 1); with a single
+-- temp part it just renames it. The temp-part flush threshold is min_insert_block_size_rows, and the
+-- background merge/mutation runs in the server's background context (NOT the client query settings), so
+-- the threshold is the server default (DEFAULT_INSERT_BLOCK_SIZE = 1048449). We therefore make the
+-- projection emit MORE rows than that: a high-cardinality GROUP BY key (1.3M distinct groups) forces >=2
+-- temp projection parts for BOTH an OPTIMIZE merge and an ALTER ... MATERIALIZE PROJECTION rebuild.
+
+DROP TABLE IF EXISTS t_pmb;
+CREATE TABLE t_pmb (a UInt64, b UInt64, PROJECTION p_by_b (SELECT b, sum(a) GROUP BY b))
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(type = object_storage, object_storage_type = local, metadata_type = cas,
+    name = '04300_pmb', server_root_id = '04300', path = '04300_pmb_pool/');
+
+-- 1.3M distinct b values, each appearing twice: a = number and a = number + 1300000, so for group b the
+-- two rows are b and b + 1300000 -> sum(a) = 2*b + 1300000. The projection emits 1.3M rows > 1048449 ->
+-- >= 2 temp projection parts on rebuild.
+INSERT INTO t_pmb SELECT number, number FROM numbers(1300000);
+INSERT INTO t_pmb SELECT number + 1300000, number FROM numbers(1300000);
+
+SELECT 'count', count() FROM t_pmb;
+SELECT 'by_b_top', b, sum(a) AS s FROM t_pmb GROUP BY b ORDER BY s DESC, b LIMIT 3;
+
+-- MERGE the parts: the projection rebuild merges >1 temp projection part (multi-block read-back).
+OPTIMIZE TABLE t_pmb FINAL;
+SELECT 'after_merge_by_b_top', b, sum(a) AS s FROM t_pmb GROUP BY b ORDER BY s DESC, b LIMIT 3;
+
+-- MUTATION that rebuilds the projection across >1 temp projection block:
+ALTER TABLE t_pmb MATERIALIZE PROJECTION p_by_b SETTINGS mutations_sync = 2;
+SELECT 'after_materialize_count', count() FROM t_pmb;
+SELECT 'projection_active', countDistinct(name) FROM system.projection_parts WHERE database = currentDatabase() AND table = 't_pmb' AND active;
+
+-- Prove the projection is actually selected by the optimizer (not a silent base-table fallback).
+SET optimize_use_projections = 1, force_optimize_projection = 1;
+SELECT 'uses_projection', countIf(explain LIKE '%p_by_b%') > 0
+FROM (EXPLAIN actions = 1 SELECT b, sum(a) FROM t_pmb GROUP BY b);
+SET force_optimize_projection = 0;
+
+-- survives reload:
+DETACH TABLE t_pmb; ATTACH TABLE t_pmb;
+SELECT 'after_reload_by_b_top', b, sum(a) AS s FROM t_pmb GROUP BY b ORDER BY s DESC, b LIMIT 3;
+
+DROP TABLE t_pmb;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/05000_cas_projection_carry_forward.reference b/tests/queries/0_stateless/05000_cas_projection_carry_forward.reference
new file mode 100644
index 000000000000..42430b9414c0
--- /dev/null
+++ b/tests/queries/0_stateless/05000_cas_projection_carry_forward.reference
@@ -0,0 +1,10 @@
+count	100000
+no_projection	1	0	0	1249950000
+no_projection	1	0	2	1250000000
+no_projection	1	1	1	1249975000
+no_projection	1	1	3	1250025000
+with_projection	1	0	0	1249950000
+with_projection	1	0	2	1250000000
+with_projection	1	1	1	1249975000
+with_projection	1	1	3	1250025000
+projection_parts	2	4
diff --git a/tests/queries/0_stateless/05000_cas_projection_carry_forward.sql b/tests/queries/0_stateless/05000_cas_projection_carry_forward.sql
new file mode 100644
index 000000000000..b36959270616
--- /dev/null
+++ b/tests/queries/0_stateless/05000_cas_projection_carry_forward.sql
@@ -0,0 +1,45 @@
+-- Tags: no-random-settings, no-random-merge-tree-settings
+
+-- B63: MATERIALIZE PROJECTION over a table with HETEROGENEOUS projection coverage. The first part
+-- predates ADD PROJECTION (it must BUILD the projection); a later part already has it (the mutation
+-- CARRIES IT FORWARD). On a content-addressed disk the carried-forward projection part was registered
+-- in-memory without its rows_count / index granularity (the hardlinked files are not yet committed, so
+-- it cannot reload them from disk), so a projection-served SELECT read back NOTHING from that part and
+-- silently dropped its rows from the aggregate. The fix copies the source projection part's already-loaded
+-- read-time state. This oracle compares the projection-served aggregate against the non-projection one in
+-- the same run, so it is correct on both a plain and a content-addressed default disk.
+
+DROP TABLE IF EXISTS t_proj_cf;
+
+CREATE TABLE t_proj_cf (k1 UInt32, k2 UInt32, k3 UInt32, value UInt32)
+ENGINE = MergeTree ORDER BY tuple();
+
+-- First part: NO projection yet.
+INSERT INTO t_proj_cf SELECT 1, number % 2, number % 4, number FROM numbers(50000);
+
+SYSTEM STOP MERGES t_proj_cf;
+
+ALTER TABLE t_proj_cf ADD PROJECTION aaaa (SELECT k1, k2, k3, sum(value) GROUP BY k1, k2, k3);
+
+-- Second part: built WITH the projection (INSERT after ADD PROJECTION).
+INSERT INTO t_proj_cf SELECT 1, number % 2, number % 4, number FROM numbers(100000) LIMIT 50000, 100000;
+
+SYSTEM START MERGES t_proj_cf;
+
+ALTER TABLE t_proj_cf MATERIALIZE PROJECTION aaaa SETTINGS mutations_sync = 2;
+
+SELECT 'count', count() FROM t_proj_cf;
+
+SELECT 'no_projection', k1, k2, k3, sum(value) v
+FROM t_proj_cf GROUP BY k1, k2, k3 ORDER BY k1, k2, k3
+SETTINGS optimize_use_projections = 0;
+
+SELECT 'with_projection', k1, k2, k3, sum(value) v
+FROM t_proj_cf GROUP BY k1, k2, k3 ORDER BY k1, k2, k3;
+
+-- Every active part must carry a non-empty projection part after MATERIALIZE.
+SELECT 'projection_parts', countDistinct(parent_name), min(rows)
+FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_proj_cf' AND active;
+
+DROP TABLE t_proj_cf;
diff --git a/tests/queries/0_stateless/05001_cas_attach_partition_projection.reference b/tests/queries/0_stateless/05001_cas_attach_partition_projection.reference
new file mode 100644
index 000000000000..1d143c7cd0d1
--- /dev/null
+++ b/tests/queries/0_stateless/05001_cas_attach_partition_projection.reference
@@ -0,0 +1,11 @@
+before_attach_rows	7
+after_attach_rows	7
+data	7	21	21
+projection_served	0	0
+projection_served	1	1
+projection_served	2	2
+projection_served	3	3
+projection_served	4	4
+projection_served	5	5
+projection_served	6	6
+1
diff --git a/tests/queries/0_stateless/05001_cas_attach_partition_projection.sql b/tests/queries/0_stateless/05001_cas_attach_partition_projection.sql
new file mode 100644
index 000000000000..5808b8d05efb
--- /dev/null
+++ b/tests/queries/0_stateless/05001_cas_attach_partition_projection.sql
@@ -0,0 +1,46 @@
+-- Tags: no-random-settings, no-random-merge-tree-settings
+
+-- B64: DETACH PARTITION + ATTACH PARTITION of a part that has a projection. On a content-addressed
+-- disk the part is re-attached from its detached STAGING directory (detached/attaching_<part>/), so
+-- the projection sub-directory is read as the NESTED path detached/attaching_<part>/<proj>.proj. The
+-- CA metadata storage recognized a projection directory only as a DIRECT child of a part
+-- (<part>/<proj>.proj), so the nested staging shape was missed: existsDirectory("<proj>.proj") returned
+-- false during the attach-time load, and IMergeTreeDataPart::loadProjections registered the surviving
+-- projection part with EMPTY columns and rows_count == 0 — making it unusable (PROJECTION_NOT_USED) and
+-- causing CHECK TABLE to throw BROKEN_PROJECTION (in-memory columns empty vs on-disk columns), even
+-- though the on-disk projection data was intact. Same projection-on-CA family as B58/B63, on the
+-- ATTACH-clone path. This oracle exercises DETACH+ATTACH PARTITION (no projection drop) and asserts the
+-- surviving projection re-attaches with the correct rows, is usable, and CHECK TABLE passes. It is
+-- correct on both a plain and a content-addressed default disk.
+
+DROP TABLE IF EXISTS t_attach_proj;
+
+CREATE TABLE t_attach_proj (x Int32, y Int32, PROJECTION p (SELECT x, y ORDER BY x))
+ENGINE = MergeTree() PARTITION BY intDiv(y, 100) ORDER BY y;
+
+INSERT INTO t_attach_proj SELECT number, number FROM numbers(7);
+
+SELECT 'before_attach_rows', min(rows)
+FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_attach_proj' AND active;
+
+ALTER TABLE t_attach_proj DETACH PARTITION 0;
+ALTER TABLE t_attach_proj ATTACH PARTITION 0;
+
+-- The surviving projection must re-attach with the correct row count (rows > 0), not empty.
+SELECT 'after_attach_rows', min(rows)
+FROM system.projection_parts
+WHERE database = currentDatabase() AND table = 't_attach_proj' AND active;
+
+-- Base data must be intact.
+SELECT 'data', count(), sum(x), sum(y) FROM t_attach_proj;
+
+-- The projection must be usable: force_optimize_projection requires a projection to serve the query,
+-- so this throws if the projection is broken/empty.
+SELECT 'projection_served', x, y FROM t_attach_proj ORDER BY x
+SETTINGS optimize_use_projections = 1, force_optimize_projection = 1;
+
+-- CHECK TABLE must pass (the projection's in-memory columns must match the on-disk columns).
+CHECK TABLE t_attach_proj SETTINGS check_query_single_value_result = 1;
+
+DROP TABLE t_attach_proj;
diff --git a/tests/queries/0_stateless/05002_cas_fetch_partition.reference b/tests/queries/0_stateless/05002_cas_fetch_partition.reference
new file mode 100644
index 000000000000..b883895433c7
--- /dev/null
+++ b/tests/queries/0_stateless/05002_cas_fetch_partition.reference
@@ -0,0 +1,8 @@
+src_parts	1
+detached_parts	1
+detached_after_attach	0
+attached_rows	3
+data_readback	0	a
+data_readback	2	b
+data_readback	4	c
+dropped_ok
diff --git a/tests/queries/0_stateless/05002_cas_fetch_partition.sql b/tests/queries/0_stateless/05002_cas_fetch_partition.sql
new file mode 100644
index 000000000000..cf81e488ca7a
--- /dev/null
+++ b/tests/queries/0_stateless/05002_cas_fetch_partition.sql
@@ -0,0 +1,58 @@
+-- Tags: no-fasttest, no-shared-merge-tree, no-replicated-database
+-- ^ no-fasttest: cas is an object-storage metadata type; keep it off the minimal image.
+--   no-shared-merge-tree: this exercises open-source ReplicatedMergeTree on a cas disk.
+--   no-replicated-database: the source replica_path is hard-coded per the database, not per the replica.
+
+-- ALTER TABLE ... FETCH PARTITION ... FROM '<zk_path>' on a cas disk: the gate is lifted
+-- and a to_detached fetch takes the byte-fetch path (the downloaded files content-address into the
+-- detached/ namespace; relink-into-detached is deferred). The fetched part must land usably in the CA
+-- detached/ namespace: system.detached_parts lists it, ATTACH publishes an active part out of it, and a
+-- SELECT reads back the exact source data. Both tables share one inline CA pool (a single server fetches
+-- from its own zk path, as 03350 does), so this also exercises the cross-table detached landing.
+
+DROP TABLE IF EXISTS t_cas_fetch_src;
+DROP TABLE IF EXISTS t_cas_fetch_dst;
+
+CREATE TABLE t_cas_fetch_src (key Int, s String)
+ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/t_cas_fetch_src', 'r1')
+PARTITION BY (key % 2) ORDER BY key
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05002',
+    name = '05002_cas_fetch',
+    path = '05002_cas_fetch_pool/');
+
+CREATE TABLE t_cas_fetch_dst (key Int, s String)
+ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/t_cas_fetch_dst', 'r1')
+PARTITION BY (key % 2) ORDER BY key
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05002',
+    name = '05002_cas_fetch',
+    path = '05002_cas_fetch_pool/');
+
+INSERT INTO t_cas_fetch_src VALUES (0, 'a'), (2, 'b'), (4, 'c');
+SELECT 'src_parts', count() FROM system.parts WHERE database = currentDatabase() AND table = 't_cas_fetch_src' AND active AND partition = '0';
+
+-- Fetch the single part of partition 0 into the destination's detached/ namespace.
+ALTER TABLE t_cas_fetch_dst FETCH PARTITION 0 FROM '/clickhouse/tables/{database}/t_cas_fetch_src'
+    SETTINGS insert_keeper_fault_injection_probability = 0;
+
+-- The fetched part must be present as a detached part.
+SELECT 'detached_parts', count() FROM system.detached_parts WHERE database = currentDatabase() AND table = 't_cas_fetch_dst';
+
+-- ATTACH publishes an active part out of the detached landing; SELECT must read back the exact data.
+ALTER TABLE t_cas_fetch_dst ATTACH PARTITION 0
+    SETTINGS insert_keeper_fault_injection_probability = 0;
+
+SELECT 'detached_after_attach', count() FROM system.detached_parts WHERE database = currentDatabase() AND table = 't_cas_fetch_dst';
+SELECT 'attached_rows', count() FROM t_cas_fetch_dst;
+SELECT 'data_readback', key, s FROM t_cas_fetch_dst ORDER BY key;
+
+DROP TABLE t_cas_fetch_src;
+DROP TABLE t_cas_fetch_dst;
+SELECT 'dropped_ok';
diff --git a/tests/queries/0_stateless/05003_cas_freeze.reference b/tests/queries/0_stateless/05003_cas_freeze.reference
new file mode 100644
index 000000000000..1740ea879482
--- /dev/null
+++ b/tests/queries/0_stateless/05003_cas_freeze.reference
@@ -0,0 +1,7 @@
+live_before_freeze	1	3
+live_before_freeze	2	2
+is_frozen	1
+live_after_drop	2	2
+command_type	partition_id	part_name	backup_name
+SYSTEM UNFREEZE	1	1_1_1_0	backup_05003
+dropped_ok
diff --git a/tests/queries/0_stateless/05003_cas_freeze.sh b/tests/queries/0_stateless/05003_cas_freeze.sh
new file mode 100755
index 000000000000..6468a1339c7b
--- /dev/null
+++ b/tests/queries/0_stateless/05003_cas_freeze.sh
@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# CA durability oracle: a FREEZE PARTITION snapshot is an independent GC root — it survives
+# ALTER TABLE ... DROP PARTITION on the same partition and remains independently recoverable.
+# This property is CA-specific: on a plain disk FREEZE makes a hard-link snapshot in shadow/ which
+# is independent by construction; on a CA disk the frozen part must be written as a separate shadow
+# ref (not merely an alias of the live part ref) so DROP PARTITION cannot destroy it.
+
+CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CURDIR"/../shell_config.sh
+
+UNFREEZE_STRUCTURE='command_type String, partition_id String, part_name String, backup_name String, backup_path String, part_backup_path String'
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS t_cas_freeze;"
+
+${CLICKHOUSE_CLIENT} --query "
+CREATE TABLE t_cas_freeze (k UInt32, v String)
+ENGINE = MergeTree ORDER BY k PARTITION BY k
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05003',
+    name = '05003_cas_freeze',
+    path = '05003_cas_freeze_pool/');"
+
+# Two partitions: k=1 (will be frozen then dropped) and k=2 (must survive untouched).
+${CLICKHOUSE_CLIENT} --query "SYSTEM STOP MERGES t_cas_freeze;"
+${CLICKHOUSE_CLIENT} --query "INSERT INTO t_cas_freeze VALUES (1, 'a'), (1, 'b'), (1, 'c');"
+${CLICKHOUSE_CLIENT} --query "INSERT INTO t_cas_freeze VALUES (2, 'x'), (2, 'y');"
+${CLICKHOUSE_CLIENT} --query "SYSTEM START MERGES t_cas_freeze;"
+
+${CLICKHOUSE_CLIENT} --query "SELECT 'live_before_freeze', k, count() FROM t_cas_freeze GROUP BY k ORDER BY k;"
+
+# Freeze only partition 1. The shadow ref becomes an independent GC root on the CA disk.
+${CLICKHOUSE_CLIENT} --query "ALTER TABLE t_cas_freeze FREEZE PARTITION 1 WITH NAME 'backup_05003';"
+
+${CLICKHOUSE_CLIENT} --query "
+SELECT 'is_frozen', count() FROM system.parts
+WHERE database = currentDatabase() AND table = 't_cas_freeze'
+  AND partition_id = '1' AND is_frozen AND active;"
+
+# Drop the live partition 1. On a CA disk this must NOT remove the shadow ref.
+${CLICKHOUSE_CLIENT} --query "ALTER TABLE t_cas_freeze DROP PARTITION 1;"
+
+# Live k=1 is gone; k=2 is untouched.
+${CLICKHOUSE_CLIENT} --query "SELECT 'live_after_drop', k, count() FROM t_cas_freeze GROUP BY k ORDER BY k;"
+
+# THE KEY ASSERTION: SYSTEM UNFREEZE finds and removes the frozen snapshot of partition 1,
+# proving it survived the DROP PARTITION as an independent shadow ref.
+# SYSTEM UNFREEZE does not accept a FORMAT clause; default output is TSV, piped through
+# clickhouse-local to filter to deterministic columns (backup_path/part_backup_path are
+# absolute paths; command_type/partition_id/part_name/backup_name are stable).
+${CLICKHOUSE_CLIENT} --query "SYSTEM UNFREEZE WITH NAME 'backup_05003';" \
+  | ${CLICKHOUSE_LOCAL} --structure "$UNFREEZE_STRUCTURE" \
+      --query "SELECT command_type, partition_id, part_name, backup_name FROM table ORDER BY partition_id FORMAT TSVWithNames"
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE t_cas_freeze;"
+${CLICKHOUSE_CLIENT} --query "SELECT 'dropped_ok';"
diff --git a/tests/queries/0_stateless/05004_cas_transactions.reference b/tests/queries/0_stateless/05004_cas_transactions.reference
new file mode 100644
index 000000000000..5c0d3efe9fd2
--- /dev/null
+++ b/tests/queries/0_stateless/05004_cas_transactions.reference
@@ -0,0 +1,7 @@
+base	1
+in_txn	2
+after_commit	2
+in_txn2	3
+after_rollback	2
+rolled_back_absent	0
+done
diff --git a/tests/queries/0_stateless/05004_cas_transactions.sh b/tests/queries/0_stateless/05004_cas_transactions.sh
new file mode 100755
index 000000000000..5c0e9e27493b
--- /dev/null
+++ b/tests/queries/0_stateless/05004_cas_transactions.sh
@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest, no-ordinary-database
+# no-fasttest: cas is an object-storage metadata type; not available on the minimal
+#              fasttest image.
+# no-ordinary-database: transactions require DatabaseAtomic (or similar); they are not supported
+#                       on DatabaseOrdinary.
+
+# CA transactions oracle: proves that transactional INSERT/COMMIT/ROLLBACK works correctly on a
+# content-addressed (CA) disk.  Three scenarios are verified:
+#   1. A committed transaction's rows become visible after COMMIT.
+#   2. A rolled-back transaction's rows are absent after ROLLBACK; prior data is intact.
+#   3. Counts are deterministic: base=1, after commit=2, after rollback=2, rolled-back row absent.
+#
+# MERGES ARE STOPPED immediately after CREATE to prevent any background merge from firing on
+# transactional parts during the test (transactional multi-part merges are not yet implemented on
+# CA disks — B53 in the backlog).
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS t_cas_txn;"
+
+${CLICKHOUSE_CLIENT} --query "
+CREATE TABLE t_cas_txn (k UInt32, v String)
+ENGINE = MergeTree ORDER BY k
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05004',
+    name = '05004_cas_transactions',
+    path = '05004_cas_transactions_pool/');"
+
+${CLICKHOUSE_CLIENT} --query "SYSTEM STOP MERGES t_cas_txn;"
+
+# ── Step 1: base row (outside any transaction) ──────────────────────────────
+${CLICKHOUSE_CLIENT} --query "INSERT INTO t_cas_txn VALUES (1, 'a');"
+${CLICKHOUSE_CLIENT} --query "SELECT 'base', count() FROM t_cas_txn;"
+
+# ── Step 2: committed transaction ───────────────────────────────────────────
+# BEGIN … COMMIT must share one client connection (one --query / multiquery block).
+${CLICKHOUSE_CLIENT} --query "
+BEGIN TRANSACTION;
+INSERT INTO t_cas_txn VALUES (2, 'b');
+SELECT 'in_txn', count() FROM t_cas_txn;
+COMMIT;"
+
+${CLICKHOUSE_CLIENT} --query "SELECT 'after_commit', count() FROM t_cas_txn;"
+
+# ── Step 3: rolled-back transaction ─────────────────────────────────────────
+${CLICKHOUSE_CLIENT} --query "
+BEGIN TRANSACTION;
+INSERT INTO t_cas_txn VALUES (3, 'c');
+SELECT 'in_txn2', count() FROM t_cas_txn;
+ROLLBACK;"
+
+${CLICKHOUSE_CLIENT} --query "SELECT 'after_rollback', count() FROM t_cas_txn;"
+${CLICKHOUSE_CLIENT} --query "SELECT 'rolled_back_absent', count() FROM t_cas_txn WHERE k = 3;"
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE t_cas_txn;"
+${CLICKHOUSE_CLIENT} --query "SELECT 'done';"
diff --git a/tests/queries/0_stateless/05005_cas_backup_restore.reference b/tests/queries/0_stateless/05005_cas_backup_restore.reference
new file mode 100644
index 000000000000..36da2c7b5808
--- /dev/null
+++ b/tests/queries/0_stateless/05005_cas_backup_restore.reference
@@ -0,0 +1,6 @@
+source	5	9	['a','b','c','d','e']
+restored	5	9	['a','b','c','d','e']
+projection	1	2
+projection	2	2
+projection	3	1
+done
diff --git a/tests/queries/0_stateless/05005_cas_backup_restore.sh b/tests/queries/0_stateless/05005_cas_backup_restore.sh
new file mode 100755
index 000000000000..5d3d51c2c88f
--- /dev/null
+++ b/tests/queries/0_stateless/05005_cas_backup_restore.sh
@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# no-fasttest: cas is an object-storage metadata type; not available on the minimal
+#              fasttest image.
+
+# CA BACKUP/RESTORE round-trip oracle: proves a table on a content-addressed (CA) disk survives a
+# full BACKUP -> DROP -> RESTORE cycle with byte-for-byte data equality, including a PROJECTION.
+# RESTORE materializes each part through one whole-part ContentAddressedTransaction
+# (restorePartFromBackup, commit d384298602b); BACKUP-read already worked. This is the inline-CA
+# oracle for B16/B34.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+backup_name="Disk('backups', '${CLICKHOUSE_TEST_UNIQUE_NAME}')"
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS t_cas_br;"
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS t_cas_br_restored;"
+
+${CLICKHOUSE_CLIENT} --query "
+CREATE TABLE t_cas_br (k UInt32, v String, PROJECTION p (SELECT k, count() GROUP BY k))
+ENGINE = MergeTree ORDER BY k
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05005',
+    name = '05005_cas_backup_restore',
+    path = '05005_cas_backup_restore_pool/');"
+
+# Two inserts -> two parts; deterministic rows.
+${CLICKHOUSE_CLIENT} --query "INSERT INTO t_cas_br VALUES (1, 'a'), (2, 'b'), (1, 'c');"
+${CLICKHOUSE_CLIENT} --query "INSERT INTO t_cas_br VALUES (3, 'd'), (2, 'e');"
+
+${CLICKHOUSE_CLIENT} --query "SELECT 'source', count(), sum(k), arraySort(groupArray(v)) FROM t_cas_br;"
+
+${CLICKHOUSE_CLIENT} --query "BACKUP TABLE t_cas_br TO ${backup_name} FORMAT Null;"
+
+${CLICKHOUSE_CLIENT} --query "RESTORE TABLE t_cas_br AS t_cas_br_restored FROM ${backup_name} FORMAT Null;"
+
+# Round-trip data equality on the restored table.
+${CLICKHOUSE_CLIENT} --query "SELECT 'restored', count(), sum(k), arraySort(groupArray(v)) FROM t_cas_br_restored;"
+
+# Projection-served query on the restored table (proves the projection round-tripped).
+${CLICKHOUSE_CLIENT} --query "SELECT 'projection', k, count() FROM t_cas_br_restored GROUP BY k ORDER BY k SETTINGS force_optimize_projection = 1;"
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE t_cas_br;"
+${CLICKHOUSE_CLIENT} --query "DROP TABLE t_cas_br_restored;"
+${CLICKHOUSE_CLIENT} --query "SELECT 'done';"
diff --git a/tests/queries/0_stateless/05006_cas_deduplication_blob_insert.reference b/tests/queries/0_stateless/05006_cas_deduplication_blob_insert.reference
new file mode 100644
index 000000000000..78bf4c7a8ea9
--- /dev/null
+++ b/tests/queries/0_stateless/05006_cas_deduplication_blob_insert.reference
@@ -0,0 +1 @@
+1000000	499999500000	499999500000	1000000
diff --git a/tests/queries/0_stateless/05006_cas_deduplication_blob_insert.sql b/tests/queries/0_stateless/05006_cas_deduplication_blob_insert.sql
new file mode 100644
index 000000000000..e7e82ea3bf40
--- /dev/null
+++ b/tests/queries/0_stateless/05006_cas_deduplication_blob_insert.sql
@@ -0,0 +1,18 @@
+-- Tags: long
+-- On a content-addressed S3 disk (the cas_s3 test lane), byte-identical column
+-- blobs deduplicate to a single object, so the second column's conditional PUT (If-None-Match: *) loses
+-- its precondition. Before the `Expect: 100-continue` fix the rejected large body triggered a
+-- 500/broken-pipe retry storm in the S3 client and this INSERT hung for tens of minutes (B118).
+-- Regression: the INSERT must complete and the data must round-trip. On non-CA storage this is a
+-- trivial fast insert.
+
+DROP TABLE IF EXISTS t_cas_deduplicated_blob;
+
+CREATE TABLE t_cas_deduplicated_blob (x UInt64, y UInt64) ENGINE = MergeTree ORDER BY x;
+
+-- x and y are byte-identical -> same content hash -> the second blob's conditional PUT 412s.
+INSERT INTO t_cas_deduplicated_blob SELECT number, number FROM numbers(1000000);
+
+SELECT count(), sum(x), sum(y), sum(x = y) FROM t_cas_deduplicated_blob;
+
+DROP TABLE t_cas_deduplicated_blob;
diff --git a/tests/queries/0_stateless/05007_cas_gc_introspection.reference b/tests/queries/0_stateless/05007_cas_gc_introspection.reference
new file mode 100644
index 000000000000..486914465921
--- /dev/null
+++ b/tests/queries/0_stateless/05007_cas_gc_introspection.reference
@@ -0,0 +1,6 @@
+1	1	1
+1
+1	1	1
+1	1	1
+1
+ok
diff --git a/tests/queries/0_stateless/05007_cas_gc_introspection.sh b/tests/queries/0_stateless/05007_cas_gc_introspection.sh
new file mode 100755
index 000000000000..0832a5b83a84
--- /dev/null
+++ b/tests/queries/0_stateless/05007_cas_gc_introspection.sh
@@ -0,0 +1,109 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# Introspection coverage for the content-addressed (CA) garbage collector: the
+# `SYSTEM CAS GC RUN <disk>` command runs one GC round synchronously and
+# the round is recorded in `system.cas_gc_log` (a Start + Finish row
+# per round, like `part_log`). We build a CA disk inline (named, so the SYSTEM command can target it),
+# create garbage by inserting then truncating, run the round a few times, flush the log, and assert
+# the rows are there with the right shape — including a non-empty per-round `ProfileEvents` delta
+# (the Manual round runs on the query thread, which always has an attached ThreadStatus that captures
+# ProfileEvents).
+#
+# This is a .sh test (not .sql) because `SYSTEM CAS GC RUN` now returns a
+# one-row-per-disk result set (UX pass); the three synchronous rounds below only care about their
+# side effects on the log, so their own output is redirected to /dev/null.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP TABLE IF EXISTS t_cas_gc_introspection;
+
+-- A named inline CA disk: the \`name\` is what \`SYSTEM CAS GC RUN <name>\`
+-- targets and what lands in the log's \`disk_name\` column.
+CREATE TABLE t_cas_gc_introspection (a UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05007',
+    name = '05007_cas_gc_introspection',
+    path = '05007_cas_gc_introspection_pool/',
+    gc_enabled = 1,
+    gc_interval_sec = 1),
+    old_parts_lifetime = 1;
+
+-- Two distinct inserts => distinct blobs (not deduped away), then TRUNCATE drops every ref so the
+-- blobs/trees become unreferenced GC fodder.
+INSERT INTO t_cas_gc_introspection SELECT number, toString(number) FROM numbers(1000);
+INSERT INTO t_cas_gc_introspection SELECT number, toString(number) FROM numbers(1000, 1000);
+TRUNCATE TABLE t_cas_gc_introspection;
+"""
+
+# Run several synchronous rounds: the first rounds mark the retired candidates, later rounds delete
+# them once the durable watermark floor advances past the builds (the background renewer does this).
+${CLICKHOUSE_CLIENT} -q "SYSTEM CAS GC RUN '05007_cas_gc_introspection'" > /dev/null
+${CLICKHOUSE_CLIENT} -q "SYSTEM CAS GC RUN '05007_cas_gc_introspection'" > /dev/null
+${CLICKHOUSE_CLIENT} -q "SYSTEM CAS GC RUN '05007_cas_gc_introspection'" > /dev/null
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+SYSTEM FLUSH LOGS cas_gc_log;
+
+-- A Start, a Finish, and a Manual-triggered row were all recorded for this disk.
+SELECT
+    countIf(event_type = 'Start') > 0,
+    countIf(event_type = 'Finish') > 0,
+    countIf(trigger = 'Manual') > 0
+FROM system.cas_gc_log
+WHERE disk_name LIKE '%05007_cas_gc_introspection%';
+
+-- A synchronous Manual Finish captured a non-empty per-round ProfileEvents delta (the round touches
+-- the object storage, so Cas*/Disk*/S3* counters are non-zero). The query thread is always attached,
+-- so capture is active for the Manual path.
+SELECT any(length(ProfileEvents)) > 0
+FROM system.cas_gc_log
+WHERE disk_name LIKE '%05007_cas_gc_introspection%'
+  AND event_type = 'Finish'
+  AND trigger = 'Manual';
+
+-- Per-phase rows: a folding round emits one Phase row per phase it reached (19 of them), so a run
+-- that folded at least once must show most of the phase vocabulary, including the fold's own
+-- ref-prefix enumeration and the round-commit CAS.
+SELECT countDistinct(phase) >= 10,
+       countIf(phase = 'fold_ref_group') > 0,
+       countIf(phase = 'round_commit') > 0
+FROM system.cas_gc_log
+WHERE disk_name LIKE '%05007_cas_gc_introspection%'
+  AND event_type = 'Phase';
+
+-- The correlator: every row of the most recent round of this disk -- its Start, each Phase, and its
+-- Finish -- shares one \`round_id\`, and that round emitted at least one Phase row between them.
+SELECT countIf(event_type = 'Start') = 1,
+       countIf(event_type = 'Finish') = 1,
+       countIf(event_type = 'Phase') > 0
+FROM system.cas_gc_log
+WHERE round_id = (
+    SELECT round_id FROM system.cas_gc_log
+    WHERE disk_name LIKE '%05007_cas_gc_introspection%' AND event_type = 'Finish'
+    ORDER BY event_time_microseconds DESC LIMIT 1);
+
+-- A Phase row's \`ProfileEvents\` is that phase's own delta, not the whole round's: the phase that
+-- enumerates the ref prefix must show fewer events than the round summary it is a part of. That
+-- enumeration lives in \`defer_decision\`, which owns the \`cas/refs/\` LIST. It is deliberately NOT
+-- \`fold_ref_group\`: that phase is I/O-free by construction -- the keys are already in hand -- so its
+-- own delta is empty and this assertion would answer 0 there no matter how healthy the round was.
+SELECT max(length(ProfileEvents)) > 0
+FROM system.cas_gc_log
+WHERE disk_name LIKE '%05007_cas_gc_introspection%'
+  AND event_type = 'Phase' AND phase = 'defer_decision';
+
+-- The error path: a non-CA disk (the always-present local \`default\`) is rejected.
+SYSTEM CAS GC RUN 'default'; -- { serverError BAD_ARGUMENTS }
+
+DROP TABLE t_cas_gc_introspection;
+SELECT 'ok';
+"""
diff --git a/tests/queries/0_stateless/05008_cas_gc_snapshot_prune.reference b/tests/queries/0_stateless/05008_cas_gc_snapshot_prune.reference
new file mode 100644
index 000000000000..9972842f9827
--- /dev/null
+++ b/tests/queries/0_stateless/05008_cas_gc_snapshot_prune.reference
@@ -0,0 +1 @@
+1	1
diff --git a/tests/queries/0_stateless/05008_cas_gc_snapshot_prune.sh b/tests/queries/0_stateless/05008_cas_gc_snapshot_prune.sh
new file mode 100755
index 000000000000..9478b4bc72c5
--- /dev/null
+++ b/tests/queries/0_stateless/05008_cas_gc_snapshot_prune.sh
@@ -0,0 +1,77 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# End-to-end: a content-addressed (CA) GC round physically deletes unreferenced objects ONLY through
+# the ack-floor retired-cursor pipeline — a blob is condemned (stage 1), floor-passed / republished as
+# `delete_pending` (stage 2), then exact-token deleted (stage 3), surfaced as `entries_redeleted` /
+# `objects_deleted` in `system.cas_gc_log`. We build a named inline CA
+# disk, create garbage (INSERT then TRUNCATE), then run synchronous GC rounds in a retry loop until a
+# round reports a physical delete — graduation (and so the physical delete) is round-paced, gated on
+# `condemn_round < current_round` in `settleEntry` (`renewWatermarkOnce` exists but does not gate it),
+# so we poll for a round boundary to pass rather than assume a fixed round count. Once a delete is
+# observed we assert every physical delete went through stage 3 (`entries_redeleted >= objects_deleted`):
+# the redelete loop is the SOLE content-delete site and counts one redelete per attempt
+# (Deleted/Absent/Replaced), so this inequality is a structural identity of the pipeline and an ad-hoc
+# delete that bypassed the graduate->redelete path would break it. This proves deletion fires
+# end-to-end through the real SystemLog path.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+DISK="05008_cas_gc_snapshot_prune"
+
+# CA-over-LOCAL object storage emits a one-time <Warning> about emulated conditional operations on
+# mount; the .sh harness fails on ANY client stderr, so send only error+ logs to the client (real
+# errors still surface and fail the test; the expected mount warning does not).
+CLIENT="$CLICKHOUSE_CLIENT --send_logs_level=error"
+
+$CLIENT -q "DROP TABLE IF EXISTS t_ca_p9"
+
+$CLIENT -q "
+CREATE TABLE t_ca_p9 (a UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '${DISK}',
+    name = '${DISK}',
+    path = '${DISK}_pool/',
+    gc_enabled = 1,
+    gc_interval_sec = 1),
+    old_parts_lifetime = 1"
+
+# Two distinct inserts => distinct blobs (not deduplicated away); TRUNCATE drops every ref so the
+# blobs/trees become unreferenced GC fodder.
+$CLIENT -q "INSERT INTO t_ca_p9 SELECT number, toString(number) FROM numbers(1000)"
+$CLIENT -q "INSERT INTO t_ca_p9 SELECT number, toString(number) FROM numbers(1000, 1000)"
+$CLIENT -q "TRUNCATE TABLE t_ca_p9"
+
+# Run synchronous rounds until a physical delete is observed (bounded retries; graduation is
+# round-paced, so we poll for a round boundary to pass rather than assume a fixed round count).
+deleted=0
+for _ in $(seq 1 40); do
+    $CLIENT -q "SYSTEM CAS GC RUN '${DISK}'" > /dev/null
+    $CLIENT -q "SYSTEM FLUSH LOGS cas_gc_log"
+    deleted=$($CLIENT -q "
+        SELECT sum(objects_deleted)
+        FROM system.cas_gc_log
+        WHERE disk_name LIKE '%${DISK}%' AND event_type = 'Finish'")
+    if [ "${deleted:-0}" -gt 0 ]; then break; fi
+    sleep 0.5
+done
+
+# A physical delete happened, and every physical delete went through stage 3 of the retired-cursor
+# pipeline (`entries_redeleted >= objects_deleted`, a structural identity: the redelete loop is the sole
+# content-delete site and increments `redeleted` once per attempt). Expect: "deleted>0 redeleted>=deleted"
+# => 1 1.
+$CLIENT -q "
+SELECT
+    sum(objects_deleted) > 0,
+    sum(entries_redeleted) >= sum(objects_deleted)
+FROM system.cas_gc_log
+WHERE disk_name LIKE '%${DISK}%' AND event_type = 'Finish'"
+
+$CLIENT -q "DROP TABLE t_ca_p9"
diff --git a/tests/queries/0_stateless/05009_cas_event_log.reference b/tests/queries/0_stateless/05009_cas_event_log.reference
new file mode 100644
index 000000000000..1ee92adcf718
--- /dev/null
+++ b/tests/queries/0_stateless/05009_cas_event_log.reference
@@ -0,0 +1,4 @@
+rows	2000
+1
+has_blob_put	1
+ok
diff --git a/tests/queries/0_stateless/05009_cas_event_log.sql b/tests/queries/0_stateless/05009_cas_event_log.sql
new file mode 100644
index 000000000000..e70ad7c95bc4
--- /dev/null
+++ b/tests/queries/0_stateless/05009_cas_event_log.sql
@@ -0,0 +1,44 @@
+-- Tags: no-fasttest
+-- ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+-- Default-ON contract for `system.cas_log`: the per-event content-addressed audit log is
+-- enabled by default. `programs/server/config.xml` ships a `<cas_log>` section because the
+-- CAS disk feature is experimental and this audit log is its primary forensic instrument (it costs
+-- nothing when no CAS disk is configured — events are emitted only by content-addressed disks). After we
+-- exercise a content-addressed disk end-to-end (INSERT, OPTIMIZE), the table exists and carries this
+-- disk's write-path events.
+
+DROP TABLE IF EXISTS t_cas_event_log;
+
+CREATE TABLE t_cas_event_log (a UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05009',
+    name = '05009_cas_event_log',
+    path = '05009_cas_event_log_pool/');
+
+-- Exercise the content-addressed write/merge path: this is exactly the work that emits put/ref events.
+INSERT INTO t_cas_event_log SELECT number, toString(number % 7) FROM numbers(1000);
+INSERT INTO t_cas_event_log SELECT number, toString(number % 7) FROM numbers(1000, 1000);
+OPTIMIZE TABLE t_cas_event_log FINAL;
+
+SELECT 'rows', count() FROM t_cas_event_log;
+
+-- Make the buffered events durable before we read them back.
+SYSTEM FLUSH LOGS cas_log;
+
+-- Default-on assertion #1: the table exists (the config ships the section).
+EXISTS TABLE system.cas_log;
+
+-- Default-on assertion #2: our disk's write path emitted at least one `blob_put` event. Filter by
+-- disk_name so parallel tests sharing this system table (e.g. the lane's own cas_s3 disk)
+-- cannot perturb the result.
+SELECT 'has_blob_put', count() > 0
+FROM system.cas_log
+WHERE disk_name = '05009_cas_event_log' AND event_type = 'blob_put';
+
+DROP TABLE t_cas_event_log;
+SELECT 'ok';
diff --git a/tests/queries/0_stateless/05010_cas_mounts_gc_health.reference b/tests/queries/0_stateless/05010_cas_mounts_gc_health.reference
new file mode 100644
index 000000000000..92f6b78e6c78
--- /dev/null
+++ b/tests/queries/0_stateless/05010_cas_mounts_gc_health.reference
@@ -0,0 +1,3 @@
+1	0
+1	1
+ok
diff --git a/tests/queries/0_stateless/05010_cas_mounts_gc_health.sh b/tests/queries/0_stateless/05010_cas_mounts_gc_health.sh
new file mode 100755
index 000000000000..f5a0a3a8be41
--- /dev/null
+++ b/tests/queries/0_stateless/05010_cas_mounts_gc_health.sh
@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# B3: system.cas_mounts exposes per-disk GC health (is_leader / pending_reclaim /
+# last_success_age_seconds / wedged_namespace_count), replacing the retired process-global
+# CasGcIsLeader / CasGcPendingReclaimEntries CurrentMetrics gauges (clobbered with >= 2 CAS disks).
+# Build one named inline CA disk, run a synchronous GC round so this process has led at least once,
+# then assert the column shapes on the healthy single-disk fixture.
+#
+# This is a .sh test (not .sql) because `SYSTEM CAS GC RUN` now returns a
+# one-row-per-disk result set (UX pass); the round below only cares about its side effect (leading
+# once), so its own output is redirected to /dev/null.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP TABLE IF EXISTS t_cas_mounts_gc_health;
+
+CREATE TABLE t_cas_mounts_gc_health (a UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05010',
+    name = '05010_cas_mounts_gc_health',
+    path = '05010_cas_mounts_gc_health_pool/',
+    gc_enabled = 1,
+    gc_interval_sec = 1),
+    old_parts_lifetime = 1;
+
+INSERT INTO t_cas_mounts_gc_health SELECT number, toString(number) FROM numbers(100);
+TRUNCATE TABLE t_cas_mounts_gc_health;
+"""
+
+${CLICKHOUSE_CLIENT} -q "SYSTEM CAS GC RUN '05010_cas_mounts_gc_health'" > /dev/null
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+SELECT is_leader, wedged_namespace_count
+FROM system.cas_mounts
+WHERE disk LIKE '%05010_cas_mounts_gc_health%';
+
+SELECT pending_reclaim >= 0, last_success_age_seconds < 60
+FROM system.cas_mounts
+WHERE disk LIKE '%05010_cas_mounts_gc_health%';
+
+DROP TABLE t_cas_mounts_gc_health;
+SELECT 'ok';
+"""
diff --git a/tests/queries/0_stateless/05011_cas_gc_rebuild_access.reference b/tests/queries/0_stateless/05011_cas_gc_rebuild_access.reference
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/queries/0_stateless/05011_cas_gc_rebuild_access.sh b/tests/queries/0_stateless/05011_cas_gc_rebuild_access.sh
new file mode 100755
index 000000000000..1c530430bed3
--- /dev/null
+++ b/tests/queries/0_stateless/05011_cas_gc_rebuild_access.sh
@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# Tags: no-parallel
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+# Intent (E1):
+#  1) A role granted only "SYSTEM CAS GC RUN" is REFUSED (ACCESS_DENIED)
+#     when it runs "SYSTEM CAS GC REBUILD <disk>", but ALLOWED to run the per-round
+#     "SYSTEM CAS GC RUN". Granting the new
+#     "SYSTEM CAS GC REBUILD" right then permits REBUILD.
+#  2) "SYSTEM CAS GC REBUILD" with NO disk is a SYNTAX_ERROR (required disk);
+#     naming a non-content-addressed disk yields BAD_ARGUMENTS (not a silent all-disks fan-out).
+#  3) A user with ZERO grants gets ACCESS_DENIED on the plain
+#     "SYSTEM CAS GC RUN 'no_such_disk'" -- the privilege check runs
+#     before disk resolution, so denial fires even though the named disk does not exist (it would
+#     otherwise be UNKNOWN_DISK).
+# (No CA disk needs to exist: the privilege check and the grammar/required-disk check both fire
+#  before any disk I/O; assert on the specific error codes. The `default` disk always exists and is
+#  never content-addressed, so it deterministically yields BAD_ARGUMENTS once a check is passed.)
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS user_test_05011;
+CREATE USER user_test_05011 IDENTIFIED WITH plaintext_password BY 'user_test_05011';
+REVOKE ALL ON *.* FROM user_test_05011;
+GRANT SYSTEM CAS GC RUN ON *.* TO user_test_05011;
+"""
+
+# GC-only role: REBUILD is refused; the per-round GC is allowed (fails later, on the disk-type check).
+${CLICKHOUSE_CLIENT} --multiline --user user_test_05011 --password user_test_05011 -q """
+SYSTEM CAS GC REBUILD default; -- { serverError ACCESS_DENIED }
+SYSTEM CAS GC RUN default; -- { serverError BAD_ARGUMENTS }
+"""
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+GRANT SYSTEM CAS GC REBUILD ON *.* TO user_test_05011;
+"""
+
+# Granting the new right permits REBUILD (fails later, on the disk-type check).
+${CLICKHOUSE_CLIENT} --multiline --user user_test_05011 --password user_test_05011 -q """
+SYSTEM CAS GC REBUILD default; -- { serverError BAD_ARGUMENTS }
+"""
+
+# REBUILD requires an explicit disk (syntax error), and never silently fans out across all disks.
+${CLICKHOUSE_CLIENT} --multiline -q """
+SYSTEM CAS GC REBUILD; -- { clientError SYNTAX_ERROR }
+SYSTEM CAS GC REBUILD default; -- { serverError BAD_ARGUMENTS }
+"""
+
+# A zero-grant user is denied before the disk is even resolved: naming a disk that does not exist
+# still yields ACCESS_DENIED, not UNKNOWN_DISK.
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS user_test_05011_zero_grants;
+CREATE USER user_test_05011_zero_grants IDENTIFIED WITH plaintext_password BY 'user_test_05011_zero_grants';
+REVOKE ALL ON *.* FROM user_test_05011_zero_grants;
+"""
+
+${CLICKHOUSE_CLIENT} --multiline --user user_test_05011_zero_grants --password user_test_05011_zero_grants -q """
+SYSTEM CAS GC RUN 'no_such_disk'; -- { serverError ACCESS_DENIED }
+"""
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS user_test_05011;
+DROP USER IF EXISTS user_test_05011_zero_grants;
+"""
diff --git a/tests/queries/0_stateless/05012_cas_mounts_typed_columns.reference b/tests/queries/0_stateless/05012_cas_mounts_typed_columns.reference
new file mode 100644
index 000000000000..87a5f014d9e5
--- /dev/null
+++ b/tests/queries/0_stateless/05012_cas_mounts_typed_columns.reference
@@ -0,0 +1,3 @@
+DateTime64(3)
+UUID
+DateTime64(3)
diff --git a/tests/queries/0_stateless/05012_cas_mounts_typed_columns.sql b/tests/queries/0_stateless/05012_cas_mounts_typed_columns.sql
new file mode 100644
index 000000000000..5b050154c846
--- /dev/null
+++ b/tests/queries/0_stateless/05012_cas_mounts_typed_columns.sql
@@ -0,0 +1,8 @@
+-- E3: system.cas_mounts exposes typed columns for the lease identity/timing fields
+-- (server_uuid as UUID, started_at/expires_at as DateTime64(3)) instead of raw String/UInt64.
+-- No CA disk needs to be mounted -- the table's ColumnsDescription is static.
+
+SELECT type FROM system.columns
+WHERE database = 'system' AND table = 'cas_mounts'
+  AND name IN ('server_uuid', 'started_at', 'expires_at')
+ORDER BY name;
diff --git a/tests/queries/0_stateless/05013_system_cas_drop_pool_member.reference b/tests/queries/0_stateless/05013_system_cas_drop_pool_member.reference
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/queries/0_stateless/05013_system_cas_drop_pool_member.sql b/tests/queries/0_stateless/05013_system_cas_drop_pool_member.sql
new file mode 100644
index 000000000000..e4732c06a050
--- /dev/null
+++ b/tests/queries/0_stateless/05013_system_cas_drop_pool_member.sql
@@ -0,0 +1,2 @@
+-- Grammar + dispatch only: execution needs a CA disk (covered by the integration test).
+SYSTEM CAS DROP POOL MEMBER 'srv1' FROM DISK 'no_such_disk'; -- { serverError UNKNOWN_DISK }
diff --git a/tests/queries/0_stateless/05014_insert_dedup_disk_commit_failpoint.reference b/tests/queries/0_stateless/05014_insert_dedup_disk_commit_failpoint.reference
new file mode 100644
index 000000000000..d00491fd7e5b
--- /dev/null
+++ b/tests/queries/0_stateless/05014_insert_dedup_disk_commit_failpoint.reference
@@ -0,0 +1 @@
+1
diff --git a/tests/queries/0_stateless/05014_insert_dedup_disk_commit_failpoint.sql b/tests/queries/0_stateless/05014_insert_dedup_disk_commit_failpoint.sql
new file mode 100644
index 000000000000..8188ac59c42a
--- /dev/null
+++ b/tests/queries/0_stateless/05014_insert_dedup_disk_commit_failpoint.sql
@@ -0,0 +1,28 @@
+-- Tags: zookeeper, no-fasttest, no-parallel
+-- no-fasttest: needs an object-storage disk (storage_policy 's3_cache').
+-- no-parallel: enables a server-global failpoint on the part disk-transaction commit.
+
+DROP TABLE IF EXISTS t_dedup_disk_commit SYNC;
+
+CREATE TABLE t_dedup_disk_commit (k UInt64, v String)
+ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/t_dedup_disk_commit', 'r1')
+ORDER BY k
+SETTINGS storage_policy = 's3_cache';
+
+SYSTEM ENABLE FAILPOINT part_storage_fail_commit_transaction;
+
+-- The disk-storage commit of the inserted part fails. The part must NOT be registered in Keeper:
+-- before the fix the disk commit ran only in MergeTreeData::Transaction::commit, AFTER the Keeper
+-- multi had durably created the block_id dedup znode, so this failure left a phantom dedup token.
+INSERT INTO t_dedup_disk_commit SETTINGS insert_deduplicate = 1, insert_keeper_fault_injection_probability = 0 VALUES (1, 'x'); -- { serverError FAULT_INJECTED }
+
+SYSTEM DISABLE FAILPOINT part_storage_fail_commit_transaction;
+
+-- Byte-identical retry of the failed INSERT: it must really insert. Before the fix it silently
+-- deduplicated against the phantom block_id ("already exists ... ignoring it") and was acked with
+-- zero rows written — the acked-then-lost data loss.
+INSERT INTO t_dedup_disk_commit SETTINGS insert_deduplicate = 1, insert_keeper_fault_injection_probability = 0 VALUES (1, 'x');
+
+SELECT count() FROM t_dedup_disk_commit;
+
+DROP TABLE t_dedup_disk_commit SYNC;
diff --git a/tests/queries/0_stateless/05015_cas_reject_fake_transaction.reference b/tests/queries/0_stateless/05015_cas_reject_fake_transaction.reference
new file mode 100644
index 000000000000..b261da18d51a
--- /dev/null
+++ b/tests/queries/0_stateless/05015_cas_reject_fake_transaction.reference
@@ -0,0 +1,2 @@
+1
+0
diff --git a/tests/queries/0_stateless/05015_cas_reject_fake_transaction.sh b/tests/queries/0_stateless/05015_cas_reject_fake_transaction.sh
new file mode 100755
index 000000000000..6f8af853ef2a
--- /dev/null
+++ b/tests/queries/0_stateless/05015_cas_reject_fake_transaction.sh
@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# An explicit `use_fake_transaction=1` on a `cas` disk would silently break the
+# atomic manifest/ref publish (per-file autocommit, no commit point for the transaction). The disk
+# factory must reject it at CREATE TABLE time with BAD_ARGUMENTS instead of silently corrupting
+# writes later -- mirrors the existing missing-`server_root_id` fail-close handling.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+${CLICKHOUSE_CLIENT} -q "
+DROP TABLE IF EXISTS t_cas_reject_fake_transaction;
+CREATE TABLE t_cas_reject_fake_transaction (a UInt64, s String)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05015',
+    name = '05015_cas_reject_fake_transaction',
+    path = '05015_cas_reject_fake_transaction_pool/',
+    use_fake_transaction = 1);
+" 2>&1 | grep -cm1 "use_fake_transaction. cannot be enabled for metadata type"
+
+${CLICKHOUSE_CLIENT} -q "SELECT count() FROM system.tables WHERE name = 't_cas_reject_fake_transaction'"
diff --git a/tests/queries/0_stateless/05016_cas_drop_pool_member_access.reference b/tests/queries/0_stateless/05016_cas_drop_pool_member_access.reference
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/queries/0_stateless/05016_cas_drop_pool_member_access.sh b/tests/queries/0_stateless/05016_cas_drop_pool_member_access.sh
new file mode 100755
index 000000000000..d5855c7dba48
--- /dev/null
+++ b/tests/queries/0_stateless/05016_cas_drop_pool_member_access.sh
@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+# Tags: no-parallel
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+# Intent: `SYSTEM CAS DROP POOL MEMBER` checks access BEFORE resolving the disk (same
+# pattern as the GC/GC REBUILD verbs covered by 05011_cas_gc_rebuild_access.sh), so this needs no
+# CA disk at all:
+#  1) A user with ZERO grants is refused with ACCESS_DENIED, even though the named disk does not
+#     exist (it would otherwise be UNKNOWN_DISK once past the access check).
+#  2) After granting "SYSTEM CAS DROP POOL MEMBER", the same query passes the access
+#     check and fails later with UNKNOWN_DISK -- proving that grant, and only that grant, is what
+#     unlocks the verb.
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS user_test_05016;
+CREATE USER user_test_05016 IDENTIFIED WITH plaintext_password BY 'user_test_05016';
+REVOKE ALL ON *.* FROM user_test_05016;
+"""
+
+${CLICKHOUSE_CLIENT} --multiline --user user_test_05016 --password user_test_05016 -q """
+SYSTEM CAS DROP POOL MEMBER 'x' FROM DISK 'y'; -- { serverError ACCESS_DENIED }
+"""
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+GRANT SYSTEM CAS DROP POOL MEMBER ON *.* TO user_test_05016;
+"""
+
+${CLICKHOUSE_CLIENT} --multiline --user user_test_05016 --password user_test_05016 -q """
+SYSTEM CAS DROP POOL MEMBER 'x' FROM DISK 'y'; -- { serverError UNKNOWN_DISK }
+"""
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS user_test_05016;
+"""
diff --git a/tests/queries/0_stateless/05017_lazy_load_tables_sync_replica.reference b/tests/queries/0_stateless/05017_lazy_load_tables_sync_replica.reference
new file mode 100644
index 000000000000..29b63ba2b7be
--- /dev/null
+++ b/tests/queries/0_stateless/05017_lazy_load_tables_sync_replica.reference
@@ -0,0 +1,2 @@
+TableProxy
+1
diff --git a/tests/queries/0_stateless/05017_lazy_load_tables_sync_replica.sh b/tests/queries/0_stateless/05017_lazy_load_tables_sync_replica.sh
new file mode 100755
index 000000000000..e60e86d28f41
--- /dev/null
+++ b/tests/queries/0_stateless/05017_lazy_load_tables_sync_replica.sh
@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Tags: zookeeper, no-replicated-database
+# no-replicated-database: the test creates its own Atomic database with `lazy_load_tables = 1`
+#   and an explicit ReplicatedMergeTree ZooKeeper path, which would conflict with the DDL
+#   replication mechanism of DatabaseReplicated.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+# Regression test: a table in a database created with `lazy_load_tables = 1` stays wrapped in a
+# `StorageTableProxy` until first access. Before this fix, `SYSTEM SYNC REPLICA` on such a table
+# failed with `BAD_ARGUMENTS: Table ... is not replicated`, because the interpreter cast the proxy
+# directly to `StorageReplicatedMergeTree` instead of materializing it first.
+
+LAZY_DB="${CLICKHOUSE_DATABASE}_lazy"
+
+${CLICKHOUSE_CLIENT} -q "DROP DATABASE IF EXISTS ${LAZY_DB}"
+${CLICKHOUSE_CLIENT} -q "CREATE DATABASE ${LAZY_DB} ENGINE = Atomic SETTINGS lazy_load_tables = 1"
+
+${CLICKHOUSE_CLIENT} -q "
+    CREATE TABLE ${LAZY_DB}.t (a UInt64)
+    ENGINE = ReplicatedMergeTree('/clickhouse/tables/${CLICKHOUSE_TEST_ZOOKEEPER_PREFIX}/lazy_sync_replica', 'r1')
+    ORDER BY a
+"
+${CLICKHOUSE_CLIENT} -q "INSERT INTO ${LAZY_DB}.t VALUES (1)"
+
+${CLICKHOUSE_CLIENT} -q "DETACH DATABASE ${LAZY_DB}"
+${CLICKHOUSE_CLIENT} -q "ATTACH DATABASE ${LAZY_DB}"
+
+# Confirm the table is still an unmaterialized proxy at this point.
+${CLICKHOUSE_CLIENT} -q "SELECT engine FROM system.tables WHERE database = '${LAZY_DB}' AND name = 't'"
+
+# This must succeed without first touching the table, i.e. without materializing the proxy
+# through any other path.
+${CLICKHOUSE_CLIENT} -q "SYSTEM SYNC REPLICA ${LAZY_DB}.t"
+
+${CLICKHOUSE_CLIENT} -q "SELECT count() FROM ${LAZY_DB}.t"
+
+${CLICKHOUSE_CLIENT} -q "DROP DATABASE ${LAZY_DB}"
diff --git a/tests/queries/0_stateless/05019_cas_fsck_access.reference b/tests/queries/0_stateless/05019_cas_fsck_access.reference
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/queries/0_stateless/05019_cas_fsck_access.sh b/tests/queries/0_stateless/05019_cas_fsck_access.sh
new file mode 100755
index 000000000000..1f3a08a69dcb
--- /dev/null
+++ b/tests/queries/0_stateless/05019_cas_fsck_access.sh
@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+# Tags: no-parallel
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+# Access control for `SYSTEM CAS FSCK` (mirrors 05011_cas_gc_rebuild_access.sh):
+#  1) A zero-grant user is denied before the disk is even resolved -- naming a disk that does not
+#     exist still yields ACCESS_DENIED, not UNKNOWN_DISK.
+#  2) Granting "SYSTEM CAS FSCK" permits the verb; it then fails later, on the
+#     disk-type check (the `default` disk always exists and is never content-addressed, so the
+#     query deterministically fails with BAD_ARGUMENTS instead).
+# (The UNMOUNT/MOUNT siblings this file once covered were removed with the Dormant lifecycle,
+#  spec rev.8 §9; FORGET / GC STOP / GC START access coverage is tracked for the acceptance task.)
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS user_test_05019;
+CREATE USER user_test_05019 IDENTIFIED WITH plaintext_password BY 'user_test_05019';
+REVOKE ALL ON *.* FROM user_test_05019;
+"""
+
+# Zero grants: denied before the disk is resolved.
+${CLICKHOUSE_CLIENT} --multiline --user user_test_05019 --password user_test_05019 -q """
+SYSTEM CAS FSCK 'no_such_disk'; -- { serverError ACCESS_DENIED }
+"""
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+GRANT SYSTEM CAS FSCK ON *.* TO user_test_05019;
+"""
+
+# Granting the FSCK right permits it (fails later, on the disk-type check).
+${CLICKHOUSE_CLIENT} --multiline --user user_test_05019 --password user_test_05019 -q """
+SYSTEM CAS FSCK default; -- { serverError BAD_ARGUMENTS }
+"""
+
+# The verb requires an explicit disk (syntax error).
+${CLICKHOUSE_CLIENT} --multiline -q """
+SYSTEM CAS FSCK; -- { clientError SYNTAX_ERROR }
+"""
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS user_test_05019;
+"""
diff --git a/tests/queries/0_stateless/05020_cas_fsck.reference b/tests/queries/0_stateless/05020_cas_fsck.reference
new file mode 100644
index 000000000000..2f34f4455804
--- /dev/null
+++ b/tests/queries/0_stateless/05020_cas_fsck.reference
@@ -0,0 +1,7 @@
+3
+0 0 0
+disk	reachable	dangling	unreachable	pending_gc	awaiting_gc	unaccounted	stale_edge	corrupted_runs	chain_broken	unchecked	lifeless_keys	namespace_janitor_pending	namespace_janitor_pending_bytes	namespace_janitor_pending_lives	ref_records_walked	physical_bytes	referenced_logical_bytes	distinct_blobs	total_blob_refs
+<disk>	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
+fsck_non_ca_disk_rejected: 1
+fsck_requires_disk: 1
+second_forget_idempotent: vanished(forgotten)
diff --git a/tests/queries/0_stateless/05020_cas_fsck.sh b/tests/queries/0_stateless/05020_cas_fsck.sh
new file mode 100755
index 000000000000..ea799db64469
--- /dev/null
+++ b/tests/queries/0_stateless/05020_cas_fsck.sh
@@ -0,0 +1,85 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type; keep it off the minimal fasttest image.
+
+# `SYSTEM CAS FSCK <disk>` (runs on a RUNNING disk, T13) + GC RUN's `pending_*` drain
+# columns + the fail-closed FORGET teardown (spec rev.8 §5/§9). FSCK is a read-only reachability audit
+# that now runs directly on the mounted, live disk and prints a clean one-row summary. The GC RUN result
+# set carries the retire pipeline's REMAINING (not this-round-delta) `pending_*` columns; on a disk with
+# nothing outstanding to reclaim they read 0. Teardown is fail-closed: DROP the table, `SYSTEM CONTENT
+# ADDRESSED FORGET` the disk (force-Vanish, node-local), verify via system.cas_mounts that
+# it reads exactly `vanished(forgotten)`, and only THEN `rm -rf` the pool dir (FORGET stopped and joined
+# every CAS background thread for this disk). A failed FORGET or an unexpected lifecycle aborts the test
+# with the pool dir left in place (the scripts have no `set -e`, so the checks are explicit).
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+DISK_NAME="ca_fsck_${CLICKHOUSE_TEST_UNIQUE_NAME}_${RANDOM}"
+POOL_DIR="${CLICKHOUSE_USER_FILES_UNIQUE}_fsck_${RANDOM}"
+rm -rf "${POOL_DIR:?}"
+mkdir -p "${POOL_DIR}"
+DISK_CA="disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '05020',
+    name = '${DISK_NAME}',
+    path = '${POOL_DIR}/')"
+
+${CLICKHOUSE_CLIENT} --query "DROP TABLE IF EXISTS t_fsck SYNC"
+
+${CLICKHOUSE_CLIENT} --query "
+CREATE TABLE t_fsck (id UInt64) ENGINE = MergeTree ORDER BY id
+SETTINGS disk = ${DISK_CA}"
+
+# --- GC RUN's result set carries the new pending_* columns while the disk is mounted, and they read 0
+#     on this fresh pool (nothing was ever written, so nothing was ever condemned) ---
+${CLICKHOUSE_CLIENT} --format TSVWithNames --query "SYSTEM CAS GC RUN '${DISK_NAME}'" \
+    | tr '\t' '\n' | grep -c "pending_candidates\|pending_condemned\|pending_retired"
+${CLICKHOUSE_CLIENT} --format TSV --query "SYSTEM CAS GC RUN '${DISK_NAME}'" \
+    | awk -F'\t' '{print $(NF-2), $(NF-1), $NF}'
+
+# --- FSCK on the RUNNING, healthy pool (T13: FSCK runs on a mounted disk): a clean one-row summary,
+#     no dangling/unreachable ---
+${CLICKHOUSE_CLIENT} --format TSVWithNames --query "SYSTEM CAS FSCK '${DISK_NAME}'" \
+    | sed "s/${DISK_NAME}/<disk>/"
+
+# --- A non-CA disk is rejected (the always-present local \`default\`) ---
+echo -n 'fsck_non_ca_disk_rejected: '
+${CLICKHOUSE_CLIENT} --query "SYSTEM CAS FSCK default" 2>&1 \
+    | grep -cm1 "is not a content-addressed disk"
+
+# --- FSCK requires an explicit disk (syntax error) ---
+echo -n 'fsck_requires_disk: '
+${CLICKHOUSE_CLIENT} --query "SYSTEM CAS FSCK" 2>&1 \
+    | grep -cm1 "Syntax error"
+
+# --- Fail-closed teardown (spec rev.8 §5/§9): DROP the table, FORGET the disk (force-Vanish, node-local),
+#     verify it reads exactly `vanished(forgotten)`, and only then rm. A failed FORGET or an unexpected
+#     lifecycle aborts here, leaving the pool dir in place. ---
+${CLICKHOUSE_CLIENT} --query "DROP TABLE t_fsck SYNC"
+# FORGET logs an operator WARNING (the decommission is deliberately prominent in the server log); the
+# clickhouse-test harness runs the client at --send_logs_level=warning, which would stream that expected
+# warning to stderr and be flagged as a failure. Suppress it on the client for the FORGET call only.
+${CLICKHOUSE_CLIENT} --allow_repeated_settings --send_logs_level=fatal \
+    --query "SYSTEM CAS FORGET '${DISK_NAME}'" || {
+    echo "FORGET failed — leaving pool dir in place (fail-closed)"; exit 1; }
+LIFECYCLE=$(${CLICKHOUSE_CLIENT} --query "
+    SELECT lifecycle || '(' || lifecycle_reason || ')' FROM system.cas_mounts
+    WHERE disk = '${DISK_NAME}'")
+[ "${LIFECYCLE}" = "vanished(forgotten)" ] || {
+    echo "unexpected lifecycle after FORGET: ${LIFECYCLE}"; exit 1; }
+
+# --- A second FORGET is idempotent: it succeeds and the disk stays `vanished(forgotten)` (an already
+#     terminal Vanished pool is the terminal truth — nothing to force, nothing to double-retire). ---
+${CLICKHOUSE_CLIENT} --allow_repeated_settings --send_logs_level=fatal \
+    --query "SYSTEM CAS FORGET '${DISK_NAME}'" || {
+    echo "second FORGET failed — leaving pool dir in place (fail-closed)"; exit 1; }
+LIFECYCLE_AGAIN=$(${CLICKHOUSE_CLIENT} --query "
+    SELECT lifecycle || '(' || lifecycle_reason || ')' FROM system.cas_mounts
+    WHERE disk = '${DISK_NAME}'")
+echo "second_forget_idempotent: ${LIFECYCLE_AGAIN}"
+
+rm -rf "${POOL_DIR:?}"   # safe: FORGET stopped and joined every CAS thread for this disk
diff --git a/tests/queries/0_stateless/05021_lazy_load_tables_mutations.reference b/tests/queries/0_stateless/05021_lazy_load_tables_mutations.reference
new file mode 100644
index 000000000000..8482a9714e8f
--- /dev/null
+++ b/tests/queries/0_stateless/05021_lazy_load_tables_mutations.reference
@@ -0,0 +1,2 @@
+1	11
+2	20
diff --git a/tests/queries/0_stateless/05021_lazy_load_tables_mutations.sh b/tests/queries/0_stateless/05021_lazy_load_tables_mutations.sh
new file mode 100755
index 000000000000..c43684319c66
--- /dev/null
+++ b/tests/queries/0_stateless/05021_lazy_load_tables_mutations.sh
@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Tags: zookeeper, no-replicated-database
+# no-replicated-database: the test creates its own Atomic database with `lazy_load_tables = 1`
+#   and an explicit ReplicatedMergeTree ZooKeeper path, which would conflict with the DDL
+#   replication mechanism of DatabaseReplicated.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+# Regression test: a table in a database created with `lazy_load_tables = 1` stays wrapped in a
+# `StorageTableProxy` until first access. Before this fix, any mutation (`ALTER ... UPDATE`,
+# `MATERIALIZE TTL`, ...) on such a table failed with NOT_IMPLEMENTED "Table engine
+# ReplicatedMergeTree doesn't support mutations": `StorageProxy` forwarded `mutate` but not
+# `checkMutationIsPossible`, so `IStorage`'s throwing default fired with the nested engine's name.
+
+LAZY_DB="${CLICKHOUSE_DATABASE}_lazy"
+
+${CLICKHOUSE_CLIENT} -q "DROP DATABASE IF EXISTS ${LAZY_DB}"
+${CLICKHOUSE_CLIENT} -q "CREATE DATABASE ${LAZY_DB} ENGINE = Atomic SETTINGS lazy_load_tables = 1"
+
+${CLICKHOUSE_CLIENT} -q "
+    CREATE TABLE ${LAZY_DB}.t (a UInt64, b UInt64)
+    ENGINE = ReplicatedMergeTree('/clickhouse/tables/${CLICKHOUSE_TEST_ZOOKEEPER_PREFIX}/lazy_mutations', 'r1')
+    ORDER BY a
+"
+${CLICKHOUSE_CLIENT} -q "INSERT INTO ${LAZY_DB}.t (a, b) VALUES (1, 10), (2, 20)"
+
+# DETACH + ATTACH the database so the table goes back to an unmaterialized proxy: the INSERT above
+# has already materialized it once, and the point is to mutate through the fresh proxy.
+${CLICKHOUSE_CLIENT} -q "DETACH DATABASE ${LAZY_DB}"
+${CLICKHOUSE_CLIENT} -q "ATTACH DATABASE ${LAZY_DB}"
+
+${CLICKHOUSE_CLIENT} -q "ALTER TABLE ${LAZY_DB}.t UPDATE b = b + 1 WHERE a = 1 SETTINGS mutations_sync = 2"
+# NOTE: MATERIALIZE TTL through a lazy proxy is still broken differently (the proxy's cached
+# in-memory metadata carries columns only, no TTL -- see the StorageProxy forwarding audit report);
+# this test deliberately pins only what the checkMutationIsPossible forward fixes.
+
+${CLICKHOUSE_CLIENT} -q "SELECT a, b FROM ${LAZY_DB}.t ORDER BY a"
+
+${CLICKHOUSE_CLIENT} -q "DROP DATABASE ${LAZY_DB}"
diff --git a/tests/queries/0_stateless/05022_cas_verb_access.reference b/tests/queries/0_stateless/05022_cas_verb_access.reference
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/queries/0_stateless/05022_cas_verb_access.sh b/tests/queries/0_stateless/05022_cas_verb_access.sh
new file mode 100755
index 000000000000..a1a750c724fe
--- /dev/null
+++ b/tests/queries/0_stateless/05022_cas_verb_access.sh
@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+# Access control for the three lifecycle verbs added in the rev.8 disk-lifecycle round -- SYSTEM CONTENT
+# ADDRESSED FORGET / GC STOP / GC START -- mirroring 05019_cas_fsck_access.sh. For each verb:
+#  1) A zero-grant user is denied BEFORE the disk is resolved -- naming a disk that does not exist still
+#     yields ACCESS_DENIED (the access check runs ahead of getDisk), not UNKNOWN_DISK.
+#  2) Granting the matching right permits the verb; it then fails later on the disk-type check (the always
+#     -present `default` disk exists and is never content-addressed, so it deterministically fails with
+#     BAD_ARGUMENTS without any lifecycle side effect).
+#  3) The verb requires an explicit disk (all three route through the target-required parser like FSCK, so
+#     omitting the disk is a client-side SYNTAX_ERROR, not a silent fan-out).
+# A unique user name keeps this parallel-safe (no global fixed-name object), and every verb run targets only
+# `no_such_disk`/`default`, so nothing is ever actually decommissioned or reconfigured.
+
+USER="user_test_${CLICKHOUSE_TEST_UNIQUE_NAME}"
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS ${USER};
+CREATE USER ${USER} IDENTIFIED WITH plaintext_password BY 'pw';
+REVOKE ALL ON *.* FROM ${USER};
+"""
+
+# (1) Zero grants: each verb is denied before the disk is resolved.
+${CLICKHOUSE_CLIENT} --multiline --user "${USER}" --password pw -q """
+SYSTEM CAS FORGET 'no_such_disk';   -- { serverError ACCESS_DENIED }
+"""
+${CLICKHOUSE_CLIENT} --multiline --user "${USER}" --password pw -q """
+SYSTEM CAS GC STOP 'no_such_disk';  -- { serverError ACCESS_DENIED }
+"""
+${CLICKHOUSE_CLIENT} --multiline --user "${USER}" --password pw -q """
+SYSTEM CAS GC START 'no_such_disk'; -- { serverError ACCESS_DENIED }
+"""
+
+# Grant each verb its matching right.
+${CLICKHOUSE_CLIENT} --multiline -q """
+GRANT SYSTEM CAS FORGET   ON *.* TO ${USER};
+GRANT SYSTEM CAS GC STOP  ON *.* TO ${USER};
+GRANT SYSTEM CAS GC START ON *.* TO ${USER};
+"""
+
+# (2) Granted: the verb is permitted, then fails later on the disk-type check against `default`.
+${CLICKHOUSE_CLIENT} --multiline --user "${USER}" --password pw -q """
+SYSTEM CAS FORGET default;   -- { serverError BAD_ARGUMENTS }
+"""
+${CLICKHOUSE_CLIENT} --multiline --user "${USER}" --password pw -q """
+SYSTEM CAS GC STOP default;  -- { serverError BAD_ARGUMENTS }
+"""
+${CLICKHOUSE_CLIENT} --multiline --user "${USER}" --password pw -q """
+SYSTEM CAS GC START default; -- { serverError BAD_ARGUMENTS }
+"""
+
+# (3) Each verb requires an explicit disk (syntax error).
+${CLICKHOUSE_CLIENT} --multiline -q """
+SYSTEM CAS FORGET;   -- { clientError SYNTAX_ERROR }
+"""
+${CLICKHOUSE_CLIENT} --multiline -q """
+SYSTEM CAS GC STOP;  -- { clientError SYNTAX_ERROR }
+"""
+${CLICKHOUSE_CLIENT} --multiline -q """
+SYSTEM CAS GC START; -- { clientError SYNTAX_ERROR }
+"""
+
+${CLICKHOUSE_CLIENT} --multiline -q """
+DROP USER IF EXISTS ${USER};
+"""
diff --git a/tests/queries/0_stateless/05023_cas_dropns_leaked_namespace.reference b/tests/queries/0_stateless/05023_cas_dropns_leaked_namespace.reference
new file mode 100644
index 000000000000..acfbe0e078ed
--- /dev/null
+++ b/tests/queries/0_stateless/05023_cas_dropns_leaked_namespace.reference
@@ -0,0 +1,14 @@
+empty_table_state_before_drop live
+empty_table_has_format_version 1
+empty_table_has_no_ref_stream_before_drop 1
+empty_table_state_after_sync_drop removing
+empty_table_terminal_stream_record_exists 1
+one_part_table_state_before_drop live
+one_part_table_state_after_sync_drop removing
+negative_control_state_after_truncate live
+negative_control_state_after_reinsert live
+negative_control_rows 1
+cycle_leaks_after_sync_drop 0
+captured_rows_absent_after_gc_fixpoint 1
+fsck_unreachable 0
+fsck_dangling 0
diff --git a/tests/queries/0_stateless/05023_cas_dropns_leaked_namespace.sh b/tests/queries/0_stateless/05023_cas_dropns_leaked_namespace.sh
new file mode 100755
index 000000000000..dcedd337a239
--- /dev/null
+++ b/tests/queries/0_stateless/05023_cas_dropns_leaked_namespace.sh
@@ -0,0 +1,196 @@
+#!/usr/bin/env bash
+# Tags: no-fasttest
+# ^ cas is an object-storage metadata type (keep it off the minimal fasttest image); this test uses its
+#   own unique local-object-storage pool and a per-run CAS disk name, so unlike 04290_cas_no_leftovers
+#   it does not need no-parallel.
+
+# FINDING #2 regression test: `DROP TABLE ... SYNC` on a content-addressed MergeTree used to leave the
+# table's CAS ref-catalog row `live` forever whenever `DirShape::TableDir`'s `existsDirectory` observed
+# zero committed refs -- an empty table, or one whose last part was just removed. `dropAllData`'s own
+# `existsDirectory` precheck skipped `removeRecursive`/`dropNamespace` entirely in that shape, so the
+# SQL-level drop completed normally while the CAS catalog row leaked, one per create/drop cycle.
+#
+# The primary oracle is the pool's OWN plain-text `cas/ref_catalog` object, read directly off disk: the
+# exact `st` (lifecycle) field recorded for the table's logical namespace. `SYSTEM CAS FSCK`'s
+# unreachable/dangling counts are a secondary check only -- fsck correctly regards a `live` leak as
+# CONSISTENT (nothing is unreachable; the row simply never dies), so it cannot detect this defect on its
+# own; `04290_cas_no_leftovers.sh`'s fsck-only oracle is exactly why FINDING #2 shipped unnoticed.
+
+CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CUR_DIR"/../shell_config.sh
+
+POOL_DIR="${CLICKHOUSE_USER_FILES_UNIQUE}_05023_${RANDOM}"
+DISK_NAME="ca_05023_${CLICKHOUSE_TEST_UNIQUE_NAME}_${RANDOM}"
+SERVER_ROOT_ID="dropns05023"
+
+rm -rf "${POOL_DIR:?}"
+mkdir -p "${POOL_DIR}"
+
+CATALOG_FILE="${POOL_DIR}/ca/cas/ref_catalog"
+
+# The pool's own plain-text catalog line for namespace $1, or empty if the namespace has no row at all.
+catalog_line() {
+    grep -F "\"ns\":\"$1\"" "${CATALOG_FILE}" 2>/dev/null || true
+}
+
+# The `st` (lifecycle) word recorded for namespace $1: "live"/"creating"/"removing", or "absent" if the
+# namespace has no catalog row (matches `04290`'s field-by-name discipline: never assume a position).
+catalog_state() {
+    local line
+    line=$(catalog_line "$1")
+    if [ -z "${line}" ]; then
+        echo "absent"
+        return
+    fi
+    echo "${line}" | grep -o '"st":"[a-z]*"' | head -1 | sed -E 's/"st":"([a-z]*)"/\1/'
+}
+
+# ClickHouse's own store/<u3>/<uuid> fanout with the CAS archive boundary marker, exactly as
+# `Cas::mirroredArchiveNamespace` builds it -- see PartPathParser.cpp. `$1` is the table's UUID.
+namespace_of() {
+    local uuid="$1"
+    echo "${SERVER_ROOT_ID}/store/${uuid:0:3}/${uuid}@cas@"
+}
+
+DISK_DEF="disk(
+    type = object_storage,
+    object_storage_type = local,
+    metadata_type = cas,
+    server_root_id = '${SERVER_ROOT_ID}',
+    name = '${DISK_NAME}',
+    path = '${POOL_DIR}/',
+    gc_enabled = 1,
+    gc_interval_sec = 100000)"
+# ^ gc_enabled=1 so `SYSTEM CAS GC RUN` is available; the interval is long enough that no background
+#   round can fire during the test's own window, so the post-drop catalog state read directly below is
+#   stable -- only the manual `GC RUN` loop at the end may advance it.
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_dropns_empty SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_dropns_one_part SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_dropns_negative_control SYNC"
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS t_dropns_cycle SYNC"
+
+# ---- (1) an EMPTY table: zero parts, zero namespace files beyond format_version.txt ----
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_dropns_empty (a UInt64)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = ${DISK_DEF}"
+
+EMPTY_UUID=$($CLICKHOUSE_CLIENT --query "SELECT uuid FROM system.tables WHERE database = currentDatabase() AND name = 't_dropns_empty'")
+EMPTY_NS=$(namespace_of "${EMPTY_UUID}")
+
+echo "empty_table_state_before_drop $(catalog_state "${EMPTY_NS}")"
+
+# Its only payload is the namespace-level format_version.txt: no ref stream object anywhere yet (a
+# files-only life never touched by a ref op has no `_log`/`_snap` at all).
+FORMAT_VERSION_HITS=$(find "${POOL_DIR}/ca/cas/ns/state" -path '*_files/format_version.txt' 2>/dev/null | wc -l)
+STREAM_HITS_BEFORE=$(find "${POOL_DIR}/ca/cas/ns/stream" -type f 2>/dev/null | wc -l)
+echo "empty_table_has_format_version $([ "${FORMAT_VERSION_HITS}" -ge 1 ] && echo 1 || echo 0)"
+echo "empty_table_has_no_ref_stream_before_drop $([ "${STREAM_HITS_BEFORE}" -eq 0 ] && echo 1 || echo 0)"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_dropns_empty SYNC"
+
+# The current branch fails here by leaving st:"live"; the fix must show "removing" (a terminal stream
+# record now exists but the catalog row itself is not deleted until GC folds and reclaims it).
+echo "empty_table_state_after_sync_drop $(catalog_state "${EMPTY_NS}")"
+STREAM_HITS_AFTER=$(find "${POOL_DIR}/ca/cas/ns/stream" -type f 2>/dev/null | wc -l)
+echo "empty_table_terminal_stream_record_exists $([ "${STREAM_HITS_AFTER}" -ge 1 ] && echo 1 || echo 0)"
+
+# ---- (2) same shape, but with one committed part: "parts removed first, files remain" is a separate
+#      path from the zero-part path above; pin it too. ----
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_dropns_one_part (a UInt64)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = ${DISK_DEF}"
+$CLICKHOUSE_CLIENT --query "INSERT INTO t_dropns_one_part VALUES (1)"
+
+ONE_PART_UUID=$($CLICKHOUSE_CLIENT --query "SELECT uuid FROM system.tables WHERE database = currentDatabase() AND name = 't_dropns_one_part'")
+ONE_PART_NS=$(namespace_of "${ONE_PART_UUID}")
+echo "one_part_table_state_before_drop $(catalog_state "${ONE_PART_NS}")"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_dropns_one_part SYNC"
+echo "one_part_table_state_after_sync_drop $(catalog_state "${ONE_PART_NS}")"
+
+# ---- (3) negative control: removing the ONLY part while keeping the table must never admit removal. ----
+$CLICKHOUSE_CLIENT --query "
+CREATE TABLE t_dropns_negative_control (a UInt64)
+ENGINE = MergeTree ORDER BY a
+SETTINGS disk = ${DISK_DEF}"
+$CLICKHOUSE_CLIENT --query "INSERT INTO t_dropns_negative_control VALUES (1)"
+
+NEGATIVE_UUID=$($CLICKHOUSE_CLIENT --query "SELECT uuid FROM system.tables WHERE database = currentDatabase() AND name = 't_dropns_negative_control'")
+NEGATIVE_NS=$(namespace_of "${NEGATIVE_UUID}")
+
+$CLICKHOUSE_CLIENT --query "TRUNCATE TABLE t_dropns_negative_control"
+echo "negative_control_state_after_truncate $(catalog_state "${NEGATIVE_NS}")"
+$CLICKHOUSE_CLIENT --query "INSERT INTO t_dropns_negative_control VALUES (2)"
+echo "negative_control_state_after_reinsert $(catalog_state "${NEGATIVE_NS}")"
+echo "negative_control_rows $($CLICKHOUSE_CLIENT --query "SELECT count() FROM t_dropns_negative_control")"
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE t_dropns_negative_control SYNC"
+
+# ---- (4) several same-SQL-name create/drop cycles: every fresh Atomic UUID gets its OWN namespace, so
+#      a stale predecessor cannot mask a fresh leak, and a same-name CREATE must never wait on the old
+#      UUID's GC. ----
+CYCLE_NS_LIST=()
+CYCLE_LEAK_COUNT=0
+for i in 1 2 3; do
+    $CLICKHOUSE_CLIENT --query "
+    CREATE TABLE t_dropns_cycle (a UInt64)
+    ENGINE = MergeTree ORDER BY a
+    SETTINGS disk = ${DISK_DEF}"
+    CYCLE_UUID=$($CLICKHOUSE_CLIENT --query "SELECT uuid FROM system.tables WHERE database = currentDatabase() AND name = 't_dropns_cycle'")
+    CYCLE_NS=$(namespace_of "${CYCLE_UUID}")
+    CYCLE_NS_LIST+=("${CYCLE_NS}")
+
+    $CLICKHOUSE_CLIENT --query "DROP TABLE t_dropns_cycle SYNC"
+    if [ "$(catalog_state "${CYCLE_NS}")" = "live" ]; then
+        CYCLE_LEAK_COUNT=$((CYCLE_LEAK_COUNT + 1))
+    fi
+done
+echo "cycle_leaks_after_sync_drop ${CYCLE_LEAK_COUNT}"
+
+# ---- (5) drive manual GC to a bounded fixpoint (Task 7 `pending_*` gauges, not a fixed sleep), then
+#      assert every captured namespace's catalog row is gone. ----
+ALL_NS=("${EMPTY_NS}" "${ONE_PART_NS}" "${CYCLE_NS_LIST[@]}")
+
+PENDING=1
+for _ in $(seq 1 60); do
+    PENDING=$($CLICKHOUSE_CLIENT --query "SYSTEM CAS GC RUN '${DISK_NAME}'" --format TSVWithNames \
+        | awk -F'\t' 'NR==1 { for (i = 1; i <= NF; i++) col[$i] = i; next }
+                      { print $col["pending_condemned"] }')
+    [ "${PENDING}" = "0" ] && break
+    sleep 0.5
+done
+if [ "${PENDING}" != "0" ]; then
+    echo "FAIL: GC did not drain within the bounded loop (pending=${PENDING})" >&2
+    exit 1
+fi
+
+ROWS_STILL_PRESENT=0
+for ns in "${ALL_NS[@]}"; do
+    if [ "$(catalog_state "${ns}")" != "absent" ]; then
+        ROWS_STILL_PRESENT=$((ROWS_STILL_PRESENT + 1))
+    fi
+done
+echo "captured_rows_absent_after_gc_fixpoint $([ "${ROWS_STILL_PRESENT}" -eq 0 ] && echo 1 || echo 0)"
+
+# ---- (6) SYSTEM CAS FSCK: secondary check only -- a `live` leak alone would read as CONSISTENT here,
+#      which is exactly why the fsck-only oracle in 04290_cas_no_leftovers.sh did not catch this defect. ----
+$CLICKHOUSE_CLIENT --query "SYSTEM CAS FSCK '${DISK_NAME}'" --format TSVWithNames \
+    | awk -F'\t' 'NR==1 { for (i = 1; i <= NF; i++) col[$i] = i; next }
+                  { print "fsck_unreachable", $col["unreachable"]; print "fsck_dangling", $col["dangling"] }'
+
+# ---- (7) fail-closed teardown (spec rev.8 §5/§9): FORGET the disk (all tables already dropped above),
+#      verify it, only then rm. ----
+$CLICKHOUSE_CLIENT --allow_repeated_settings --send_logs_level=fatal \
+    --query "SYSTEM CAS FORGET '${DISK_NAME}'" || {
+    echo "FORGET failed — leaving pool dir in place (fail-closed)"; exit 1; }
+LIFECYCLE=$($CLICKHOUSE_CLIENT --query "
+    SELECT lifecycle || '(' || lifecycle_reason || ')' FROM system.cas_mounts
+    WHERE disk = '${DISK_NAME}'")
+[ "${LIFECYCLE}" = "vanished(forgotten)" ] || {
+    echo "unexpected lifecycle after FORGET: ${LIFECYCLE}"; exit 1; }
+
+rm -rf "${POOL_DIR:?}"   # safe: FORGET stopped and joined every CAS thread for this disk

From 96cf457f1b7f99b548e7a7d2512a2a6b8b38aa0a Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:40 +0200
Subject: [PATCH 27/30] CAS tests: integration

Integration suites: shared pool, replicated relink, GC (sharded and S3),
insert fault recovery, lazy-load recovery, file cache, drop pool member,
ref snaplog, disks-app; plus shared helpers and compose files.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .../compose/docker_compose_rustfs.yml         |  17 +
 tests/integration/helpers/cluster.py          |  85 ++
 .../test_cas_drop_pool_member/__init__.py     |   0
 .../configs/server_root_id_node1.xml          |  16 +
 .../configs/server_root_id_node2.xml          |  16 +
 .../configs/storage_conf.xml                  |  46 +
 .../test_cas_drop_pool_member/test.py         | 323 +++++++
 .../test_cas_file_cache/__init__.py           |   0
 .../configs/storage_conf.xml                  |  34 +
 tests/integration/test_cas_file_cache/test.py | 117 +++
 tests/integration/test_cas_gc_s3/__init__.py  |   0
 .../test_cas_gc_s3/configs/storage_conf.xml   |  33 +
 tests/integration/test_cas_gc_s3/test.py      | 189 ++++
 .../test_cas_gc_sharded/__init__.py           |   0
 .../configs/server_root_id_node1.xml          |  12 +
 .../configs/server_root_id_node2.xml          |  12 +
 .../configs/storage_conf.xml                  |  38 +
 tests/integration/test_cas_gc_sharded/test.py | 362 +++++++
 .../__init__.py                               |   0
 .../configs/server_root_id_node1.xml          |  10 +
 .../configs/server_root_id_node2.xml          |  10 +
 .../configs/storage_conf.xml                  |  27 +
 .../test_cas_insert_fault_recovery/test.py    | 163 ++++
 .../test_cas_lazy_load_recovery/__init__.py   |   0
 .../configs/server_root_id_node1.xml          |  10 +
 .../configs/storage_conf.xml                  |  27 +
 .../test_cas_lazy_load_recovery/test.py       |  88 ++
 .../test_cas_ref_snaplog/__init__.py          |   0
 .../configs/storage_conf.xml                  |  43 +
 .../integration/test_cas_ref_snaplog/test.py  | 170 ++++
 .../test_cas_replicated_relink/__init__.py    |   0
 .../configs/server_root_id_node1.xml          |  12 +
 .../configs/server_root_id_node2.xml          |  12 +
 .../configs/storage_conf.xml                  |  36 +
 .../configs/storage_conf_other_pool.xml       |  31 +
 .../test_cas_replicated_relink/test.py        | 905 ++++++++++++++++++
 tests/integration/test_cas_s3/__init__.py     |   0
 .../test_cas_s3/configs/storage_conf.xml      |  28 +
 tests/integration/test_cas_s3/test.py         | 145 +++
 .../test_cas_shared_pool/__init__.py          |   0
 .../configs/server_root_id_node1.xml          |  12 +
 .../configs/server_root_id_node2.xml          |  12 +
 .../configs/storage_conf.xml                  |  36 +
 .../integration/test_cas_shared_pool/test.py  | 347 +++++++
 44 files changed, 3424 insertions(+)
 create mode 100644 tests/integration/compose/docker_compose_rustfs.yml
 create mode 100644 tests/integration/test_cas_drop_pool_member/__init__.py
 create mode 100644 tests/integration/test_cas_drop_pool_member/configs/server_root_id_node1.xml
 create mode 100644 tests/integration/test_cas_drop_pool_member/configs/server_root_id_node2.xml
 create mode 100644 tests/integration/test_cas_drop_pool_member/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_drop_pool_member/test.py
 create mode 100644 tests/integration/test_cas_file_cache/__init__.py
 create mode 100644 tests/integration/test_cas_file_cache/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_file_cache/test.py
 create mode 100644 tests/integration/test_cas_gc_s3/__init__.py
 create mode 100644 tests/integration/test_cas_gc_s3/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_gc_s3/test.py
 create mode 100644 tests/integration/test_cas_gc_sharded/__init__.py
 create mode 100644 tests/integration/test_cas_gc_sharded/configs/server_root_id_node1.xml
 create mode 100644 tests/integration/test_cas_gc_sharded/configs/server_root_id_node2.xml
 create mode 100644 tests/integration/test_cas_gc_sharded/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_gc_sharded/test.py
 create mode 100644 tests/integration/test_cas_insert_fault_recovery/__init__.py
 create mode 100644 tests/integration/test_cas_insert_fault_recovery/configs/server_root_id_node1.xml
 create mode 100644 tests/integration/test_cas_insert_fault_recovery/configs/server_root_id_node2.xml
 create mode 100644 tests/integration/test_cas_insert_fault_recovery/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_insert_fault_recovery/test.py
 create mode 100644 tests/integration/test_cas_lazy_load_recovery/__init__.py
 create mode 100644 tests/integration/test_cas_lazy_load_recovery/configs/server_root_id_node1.xml
 create mode 100644 tests/integration/test_cas_lazy_load_recovery/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_lazy_load_recovery/test.py
 create mode 100644 tests/integration/test_cas_ref_snaplog/__init__.py
 create mode 100644 tests/integration/test_cas_ref_snaplog/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_ref_snaplog/test.py
 create mode 100644 tests/integration/test_cas_replicated_relink/__init__.py
 create mode 100644 tests/integration/test_cas_replicated_relink/configs/server_root_id_node1.xml
 create mode 100644 tests/integration/test_cas_replicated_relink/configs/server_root_id_node2.xml
 create mode 100644 tests/integration/test_cas_replicated_relink/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_replicated_relink/configs/storage_conf_other_pool.xml
 create mode 100644 tests/integration/test_cas_replicated_relink/test.py
 create mode 100644 tests/integration/test_cas_s3/__init__.py
 create mode 100644 tests/integration/test_cas_s3/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_s3/test.py
 create mode 100644 tests/integration/test_cas_shared_pool/__init__.py
 create mode 100644 tests/integration/test_cas_shared_pool/configs/server_root_id_node1.xml
 create mode 100644 tests/integration/test_cas_shared_pool/configs/server_root_id_node2.xml
 create mode 100644 tests/integration/test_cas_shared_pool/configs/storage_conf.xml
 create mode 100644 tests/integration/test_cas_shared_pool/test.py

diff --git a/tests/integration/compose/docker_compose_rustfs.yml b/tests/integration/compose/docker_compose_rustfs.yml
new file mode 100644
index 000000000000..20651d5fbd2b
--- /dev/null
+++ b/tests/integration/compose/docker_compose_rustfs.yml
@@ -0,0 +1,17 @@
+services:
+  rustfs1:
+    # RustFS is an S3-compatible store that (unlike MinIO) correctly enforces the conditional-PUT
+    # (If-None-Match / exact-token) semantics the content-addressed (CAS) disk relies on. Content-
+    # addressed integration tests must run against RustFS, not MinIO.
+    image: rustfs/rustfs:1.0.0-beta.12
+    expose:
+      - "11121"
+    environment:
+      RUSTFS_ACCESS_KEY: clickhouse
+      RUSTFS_SECRET_KEY: clickhouse
+      # Single-disk ephemeral test pool: scanner + heal off (matches the ca-soak harness).
+      RUSTFS_SCANNER_ENABLED: "false"
+      RUSTFS_HEAL_ENABLED: "false"
+    command: server --address 0.0.0.0:11121
+    stop_grace_period: 5s
+    cpus: 3
diff --git a/tests/integration/helpers/cluster.py b/tests/integration/helpers/cluster.py
index 946ed4bf9ddf..6c07946477dd 100644
--- a/tests/integration/helpers/cluster.py
+++ b/tests/integration/helpers/cluster.py
@@ -648,6 +648,7 @@ def __init__(
 
         self.base_zookeeper_cmd = None
         self.base_minio_cmd = []
+        self.base_rustfs_cmd = []
         self.base_mysql57_cmd = []
         self.base_mysql8_cmd = []
         self.base_kafka_cmd = []
@@ -723,6 +724,18 @@ def __init__(
         self.minio_access_key = minio_access_key
         self.minio_secret_key = minio_secret_key
 
+        # available when with_rustfs == True. RustFS is an S3-compatible store that correctly
+        # enforces conditional-PUT semantics required by the content-addressed disk (MinIO does not).
+        self.with_rustfs = False
+        self.rustfs_dir = os.path.join(self.instances_dir, "rustfs")
+        self.rustfs_host = "rustfs1"
+        self.rustfs_ip = None
+        self.rustfs_bucket = "test"
+        self.rustfs_port = 11121
+        self.rustfs_access_key = "clickhouse"
+        self.rustfs_secret_key = "clickhouse"
+        self.rustfs_client = None  # type: Minio
+
         self.spark_session = None
         self.with_iceberg_catalog = False
         self._iceberg_rest_catalog_port = None
@@ -1827,6 +1840,19 @@ def setup_minio_cmd(self, instance, env_variables, docker_compose_yml_dir):
         )
         return self.base_minio_cmd
 
+    def setup_rustfs_cmd(self, instance, env_variables, docker_compose_yml_dir):
+        self.with_rustfs = True
+        self.base_cmd.extend(
+            ["--file", p.join(docker_compose_yml_dir, "docker_compose_rustfs.yml")]
+        )
+        self.base_rustfs_cmd = self.compose_cmd(
+            "--env-file",
+            instance.env_file,
+            "--file",
+            p.join(docker_compose_yml_dir, "docker_compose_rustfs.yml"),
+        )
+        return self.base_rustfs_cmd
+
     def setup_glue_catalog_cmd(self, instance, env_variables, docker_compose_yml_dir):
         self.with_glue_catalog = True
         env_variables["GLUE_CATALOG_PORT"] = str(self.glue_catalog_port)
@@ -2087,6 +2113,7 @@ def add_instance(
         with_nginx=False,
         with_redis=False,
         with_minio=False,
+        with_rustfs=False,
         # The config is defined in tests/integration/helpers/remote_database_disk.xml
         # However, some tests cannot use with_remote_database_disk by their configs: e.g using secure keeper
         # So, we set the default value of with_remote_database_disk to None and try to enable it if possible in ASAN build (i.e. if not explicitly set to false)
@@ -2229,6 +2256,7 @@ def add_instance(
             with_mongo=with_mongo,
             with_redis=with_redis,
             with_minio=with_minio,
+            with_rustfs=with_rustfs,
             with_remote_database_disk=with_remote_database_disk,
             with_azurite=with_azurite,
             with_jdbc_bridge=with_jdbc_bridge,
@@ -2457,6 +2485,11 @@ def add_instance(
                 self.setup_minio_cmd(instance, env_variables, docker_compose_yml_dir)
             )
 
+        if with_rustfs and not self.with_rustfs:
+            cmds.append(
+                self.setup_rustfs_cmd(instance, env_variables, docker_compose_yml_dir)
+            )
+
         if with_iceberg_catalog and not self.with_iceberg_catalog:
             cmds.append(
                 self.setup_iceberg_catalog_cmd(
@@ -3348,6 +3381,41 @@ def wait_minio_to_start(self, timeout=180, secure=False):
 
         raise Exception("Can't wait Minio to start")
 
+    def wait_rustfs_to_start(self, timeout=180):
+        self.rustfs_ip = self.get_instance_ip(self.rustfs_host)
+        rustfs_client = Minio(
+            f"{self.rustfs_ip}:{self.rustfs_port}",
+            access_key=self.rustfs_access_key,
+            secret_key=self.rustfs_secret_key,
+            secure=False,
+            http_client=urllib3.PoolManager(cert_reqs="CERT_NONE"),
+        )
+        start = time.time()
+        while time.time() - start < timeout:
+            try:
+                rustfs_client.list_buckets()
+                logging.debug("Connected to RustFS.")
+
+                if not rustfs_client.bucket_exists(self.rustfs_bucket):
+                    rustfs_client.make_bucket(self.rustfs_bucket)
+                logging.debug("RustFS bucket '%s' ready", self.rustfs_bucket)
+
+                self.rustfs_client = rustfs_client
+                return
+            except Exception as ex:
+                logging.debug("Can't connect to RustFS: %s", str(ex))
+                time.sleep(1)
+
+        try:
+            with open(os.path.join(self.rustfs_dir, "docker.log"), "w+") as f:
+                subprocess.check_call(  # STYLE_CHECK_ALLOW_SUBPROCESS_CHECK_CALL
+                    self.base_rustfs_cmd + ["logs"], stdout=f
+                )
+        except Exception:
+            logging.debug("Unable to get logs from docker.")
+
+        raise Exception("Can't wait RustFS to start")
+
     def create_minio_buckets(self, buckets, set_public_policy=False):
         """Create additional buckets on the standard MinIO (minio1).
 
@@ -4019,6 +4087,18 @@ def get_feature_flag_value(feature_flag):
                 logging.info("Trying to connect to Minio...")
                 self.wait_minio_to_start(secure=self.minio_certs_dir is not None)
 
+            if self.with_rustfs and self.base_rustfs_cmd:
+                os.makedirs(self.rustfs_dir, exist_ok=True)
+                rustfs_start_cmd = self.base_rustfs_cmd + common_opts
+                logging.info(
+                    "Trying to create RustFS instance by command %s",
+                    " ".join(map(str, rustfs_start_cmd)),
+                )
+                run_and_check(rustfs_start_cmd)
+                self.up_called = True
+                logging.info("Trying to connect to RustFS...")
+                self.wait_rustfs_to_start()
+
             # Catalogs use the standard MinIO (minio1). Create their
             # buckets on it before starting the catalog services.
             if self.with_glue_catalog or self.with_hms_catalog or self.with_iceberg_catalog:
@@ -4780,6 +4860,7 @@ def __init__(
         with_mongo,
         with_redis,
         with_minio,
+        with_rustfs,
         with_remote_database_disk,
         with_azurite,
         with_jdbc_bridge,
@@ -4908,6 +4989,7 @@ def __init__(
         self.with_arrowflight = with_arrowflight
         self.with_redis = with_redis
         self.with_minio = with_minio
+        self.with_rustfs = with_rustfs
         self.with_remote_database_disk = with_remote_database_disk
         self.with_azurite = with_azurite
         self.with_cassandra = with_cassandra
@@ -6337,6 +6419,9 @@ def write_embedded_config(name, dest_dir, fix_log_level=False):
         if self.with_minio:
             depends_on.append("minio1")
 
+        if self.with_rustfs:
+            depends_on.append("rustfs1")
+
         if self.with_azurite:
             depends_on.append("azurite1")
 
diff --git a/tests/integration/test_cas_drop_pool_member/__init__.py b/tests/integration/test_cas_drop_pool_member/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_drop_pool_member/configs/server_root_id_node1.xml b/tests/integration/test_cas_drop_pool_member/configs/server_root_id_node1.xml
new file mode 100644
index 000000000000..8b2f63dc5dd1
--- /dev/null
+++ b/tests/integration/test_cas_drop_pool_member/configs/server_root_id_node1.xml
@@ -0,0 +1,16 @@
+<clickhouse>
+    <!-- Phase 0 (mount safety): each server owning a DISTINCT layout subtree inside the shared pool.
+         Two servers must never write the same server_root_id tree (spec §mount-safety), so this is a
+         per-instance override merged onto the shared storage_conf.xml disk definitions. The read-only
+         fsck window shares node1's own srid: it never claims a mount, so no conflict. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_dpm>
+                <server_root_id>node1</server_root_id>
+            </disk_cas_dpm>
+            <disk_ca_ro>
+                <server_root_id>node1</server_root_id>
+            </disk_ca_ro>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_drop_pool_member/configs/server_root_id_node2.xml b/tests/integration/test_cas_drop_pool_member/configs/server_root_id_node2.xml
new file mode 100644
index 000000000000..65473eac537a
--- /dev/null
+++ b/tests/integration/test_cas_drop_pool_member/configs/server_root_id_node2.xml
@@ -0,0 +1,16 @@
+<clickhouse>
+    <!-- Phase 0 (mount safety): each server owning a DISTINCT layout subtree inside the shared pool.
+         Two servers must never write the same server_root_id tree (spec §mount-safety), so this is a
+         per-instance override merged onto the shared storage_conf.xml disk definitions. The read-only
+         fsck window shares node2's own srid: it never claims a mount, so no conflict. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_dpm>
+                <server_root_id>node2</server_root_id>
+            </disk_cas_dpm>
+            <disk_ca_ro>
+                <server_root_id>node2</server_root_id>
+            </disk_ca_ro>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_drop_pool_member/configs/storage_conf.xml b/tests/integration/test_cas_drop_pool_member/configs/storage_conf.xml
new file mode 100644
index 000000000000..4916ab840d81
--- /dev/null
+++ b/tests/integration/test_cas_drop_pool_member/configs/storage_conf.xml
@@ -0,0 +1,46 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <!-- Both nodes load this SAME base config, so they mount the SAME pool (the bucket is the
+                 single source of truth; no Keeper): exactly the shared-pool model test's topology.
+                 RustFS (not MinIO) enforces the conditional-PUT (If-None-Match) semantics the
+                 snapshot+log writer linearization relies on. -->
+            <disk_cas_dpm>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <endpoint>http://rustfs1:11121/test/cas_dpm_data/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <!-- Two servers mounting one pool: this acknowledgement is mandatory; without it the
+                     second mounter refuses to attach (see PoolMeta ownership check). -->
+                <!-- Background reachability GC enabled, short cadence so the decommissioned member's
+                     unreferenced content drains within the test. -->
+                <gc_enabled>1</gc_enabled>
+                <gc_interval_sec>1</gc_interval_sec>
+            </disk_cas_dpm>
+            <!-- Read-only window over the SAME pool for the read-only consumer (fsck). fsck refuses a
+                 writable pool; a read-only disk takes no mount lease, so it can inspect the pool the
+                 server writes without contention. -->
+            <disk_ca_ro>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <endpoint>http://rustfs1:11121/test/cas_dpm_data/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <readonly>true</readonly>
+                <gc_enabled>0</gc_enabled>
+            </disk_ca_ro>
+        </disks>
+        <policies>
+            <cas_dpm>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_dpm</disk>
+                    </main>
+                </volumes>
+            </cas_dpm>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_drop_pool_member/test.py b/tests/integration/test_cas_drop_pool_member/test.py
new file mode 100644
index 000000000000..6ac60271348d
--- /dev/null
+++ b/tests/integration/test_cas_drop_pool_member/test.py
@@ -0,0 +1,323 @@
+import re
+import shlex
+import time
+
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+from helpers.test_tools import assert_eq_with_retry
+
+cluster = ClickHouseCluster(__file__)
+
+# Both servers mount the SAME content-addressed pool over RustFS (not MinIO -- MinIO cannot serve CA
+# pools, see memory), distinct server_root_id (node1/node2) -- exactly the shared-pool model test's
+# two-node topology (test_cas_shared_pool), just on rustfs instead of minio (the model
+# test predates rustfs support; test_cas_ref_snaplog is the rustfs precedent copied here).
+STORAGE_POLICY = "cas_dpm"
+RO_DISK = "disk_ca_ro"
+CA_DISK = "disk_cas_dpm"
+
+SRID1 = "node1"
+SRID2 = "node2"
+
+POOL = "cas_dpm_data"
+BLOBS_PREFIX = POOL + "/blobs/"
+
+NUM_ROWS = 20000
+
+# Background GC: grace=2s, interval=1s (storage_conf.xml). After the drop-pool-member command removes
+# node2's namespaces the content that was only reachable through them becomes unreferenced GC fodder;
+# poll until it drains. Bounded wait on a known background process, not a race workaround.
+RECLAIM_RETRIES = 120
+RECLAIM_SLEEP = 1.0  # total bound ~= 120s
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node1",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node1.xml"],
+        with_rustfs=True,
+        stay_alive=True,
+    )
+    cluster.add_instance(
+        "node2",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node2.xml"],
+        with_rustfs=True,
+        stay_alive=True,
+    )
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def _count(prefix):
+    return len(
+        list(cluster.rustfs_client.list_objects(cluster.rustfs_bucket, prefix, recursive=True))
+    )
+
+
+def _disks(node, query):
+    # Run a clickhouse-disks command against the read-only CA window over the same pool — cas-fsck refuses
+    # a writable pool, so it must go through disk_ca_ro (the ref-snaplog integration test's idiom).
+    return node.exec_in_container(
+        [
+            "bash",
+            "-c",
+            "/usr/bin/clickhouse disks -C /etc/clickhouse-server/config.xml "
+            "--disk {} --save-logs --query {}".format(RO_DISK, shlex.quote(query)),
+        ]
+    )
+
+
+def test_drop_dead_pool_member_heals_the_pool():
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+
+    node1.query("DROP TABLE IF EXISTS t1 SYNC")
+    node2.query("DROP TABLE IF EXISTS t2 SYNC")
+
+    blobs_baseline = _count(BLOBS_PREFIX)
+
+    create_tpl = (
+        "CREATE TABLE {tbl} (id Int64, s String) ENGINE = MergeTree() ORDER BY id "
+        "SETTINGS storage_policy = '{policy}'"
+    )
+
+    # (2) node2 gets its own table with several parts -- this data must NOT survive (node2 is about to
+    #     be killed and decommissioned).
+    node2.query(create_tpl.format(tbl="t2", policy=STORAGE_POLICY))
+    for i in range(4):
+        node2.query(
+            "INSERT INTO t2 SELECT number + {off}, toString(number + {off}) "
+            "FROM numbers({rows})".format(off=i * NUM_ROWS, rows=NUM_ROWS)
+        )
+    assert int(node2.query("SELECT count() FROM t2")) == 4 * NUM_ROWS
+
+    # (3) node1 gets its own table -- this data MUST survive the whole flow untouched.
+    node1.query(create_tpl.format(tbl="t1", policy=STORAGE_POLICY))
+    node1.query(
+        "INSERT INTO t1 SELECT number, toString(number) FROM numbers({})".format(NUM_ROWS)
+    )
+    n1_count = int(node1.query("SELECT count() FROM t1"))
+    n1_sum = int(node1.query("SELECT sum(id) FROM t1"))
+    assert n1_count == NUM_ROWS
+
+    assert _count(BLOBS_PREFIX) > blobs_baseline, "expected content blobs after both nodes' inserts"
+
+    # (3b) T9: system.cas_mounts scopes the GC-health columns (is_leader et al.) to the
+    #      row for THIS server's own server_root_id; peer rows read NULL. Background GC (1s interval)
+    #      should have led at least one round on each node by now, but that is a background race, not
+    #      something this test synchronizes on directly -- poll rather than assume. For every disk
+    #      that reports any non-NULL is_leader row there must be exactly one such row, and it must
+    #      belong to the querying node's own server_root_id -- never a peer's.
+    for node, own_srid in ((node1, SRID1), (node2, SRID2)):
+        rows = []
+        for _ in range(30):
+            rows = (
+                node.query(
+                    "SELECT disk, server_root_id FROM system.cas_mounts "
+                    "WHERE is_leader IS NOT NULL ORDER BY disk"
+                )
+                .strip()
+                .splitlines()
+            )
+            if rows:
+                break
+            time.sleep(1.0)
+        assert rows, "expected at least one GC-health row with is_leader populated on {}".format(
+            node.name
+        )
+        seen_disks = set()
+        for row in rows:
+            disk, srid = row.split("\t")
+            assert srid == own_srid, "peer server_root_id '{}' leaked GC health on disk '{}': {}".format(
+                srid, disk, rows
+            )
+            assert disk not in seen_disks, "duplicate non-NULL is_leader row for disk '{}': {}".format(
+                disk, rows
+            )
+            seen_disks.add(disk)
+
+    # (4) Hard-kill node2: SIGKILL, no graceful farewell -- node2's mount lease is left to expire
+    #     naturally, exactly the scenario decommission exists for.
+    node2.stop_clickhouse(kill=True)
+
+    # (5) Wait until node1 observes node2's mount as no longer live (expired once its lease's TTL
+    #     elapses with no renewal, since there was no graceful farewell to mark it terminated instead).
+    #     min() because node1 sees the pool through TWO disks (the writable disk + the disk_ca_ro
+    #     fsck window), so the mounts table carries one row per disk view for the same server_root_id --
+    #     aggregate to a single row for the equality assert.
+    assert_eq_with_retry(
+        node1,
+        "SELECT min(state != 'live') FROM system.cas_mounts WHERE server_root_id = '{}'".format(
+            SRID2
+        ),
+        "1",
+        retry_count=90,
+        sleep_time=1.0,
+    )
+
+    # (6) Decommission the dead member from node1 -- PHASE 1 of a two-phase heal. SYSTEM queries do
+    #     not accept a FORMAT clause (ParserSystemQuery is not part of ParserQueryWithOutput), so parse
+    #     the default TSV row. Column order matches the interpreter's ColumnsDescription:
+    #     server_root_id, namespaces_removed, namespaces_already_removed, committed_refs_removed,
+    #     precommits_removed, manifest_debris_removed, staging_objects_removed,
+    #     mountpoint_objects_removed, slot_removed, warnings.
+    #
+    #     t2 is still `Live` at this point, so THIS call's normal drop path is what appends its
+    #     removal terminal and moves its catalog row to `Removing` -- catalog deletion stays GC's job
+    #     (`224aacd8eb9`), never the decommission command's, so a row this same call just legitimately
+    #     transitioned still counts as "owned" and the retirement fence correctly refuses the slot.
+    #     This is a success with GC completion pending, not a failure.
+    #     The mounts-table poll above and the decommission command judge liveness by DIFFERENT
+    #     predicates on purpose: the table renders TTL arithmetic over the last observed mount row,
+    #     while the command re-reads the mountpoint object and refuses while the lease could still
+    #     be live under its conservative safety margin. The destructive side being stricter is the
+    #     fail-close direction, so the table saying "not live" does not guarantee the command is
+    #     ready yet -- under sanitizer slowdowns the gap is wide enough to hit. Retry the command
+    #     itself through the documented "wait for its lease to lapse" refusal, bounded.
+    report_tsv = None
+    for _ in range(90):
+        try:
+            report_tsv = node1.query(
+                "SYSTEM CAS DROP POOL MEMBER '{}' FROM DISK '{}'".format(SRID2, CA_DISK)
+            ).rstrip("\n")
+            break
+        except Exception as e:
+            if "alive or contended" not in str(e):
+                raise
+            time.sleep(1.0)
+    assert report_tsv is not None, "decommission kept refusing: lease never lapsed within the bound"
+    fields = report_tsv.split("\t")
+    assert len(fields) == 10, report_tsv
+    assert fields[0] == SRID2, report_tsv
+    assert int(fields[1]) >= 1, report_tsv  # namespaces_removed: the drop half did its work
+    assert int(fields[8]) == 0, report_tsv  # slot_removed: not yet -- GC owns the row now
+    assert "pool member decommission underway" in fields[9], report_tsv
+
+    # (6b) PHASE 2: drive GC and re-run the decommission until the slot retires. Folding a fresh
+    #      terminal and pruning its catalog row are two separate GC rounds by design (a fold-then-prune
+    #      handoff -- see `CasDecommissionCatalogDuties.FoldedTerminalRemainsGcOwnedAndOnlyRequestsAnotherRound`),
+    #      so poll rather than assume one round suffices. The explicit `GC RUN` is the same idiom
+    #      `test_cas_replicated_relink` uses; it runs a synchronous round regardless of the background
+    #      cadence. Catalog-row pruning is Task 5's catalog-only pre-fold drain and does not consult
+    #      Stage A's destructive-reclaim suppression, so this heals under the current Stage-A posture.
+    for _ in range(30):
+        node1.query("SYSTEM CAS GC RUN '{}'".format(CA_DISK))
+        report_tsv = node1.query(
+            "SYSTEM CAS DROP POOL MEMBER '{}' FROM DISK '{}'".format(SRID2, CA_DISK)
+        ).rstrip("\n")
+        fields = report_tsv.split("\t")
+        assert len(fields) == 10, report_tsv
+        if int(fields[8]) == 1:
+            break
+        assert "pool member decommission underway" in fields[9], report_tsv
+    else:
+        pytest.fail("pool never healed after driving GC: {}".format(report_tsv))
+
+    assert fields[9] == "", report_tsv  # warnings: the pool healed cleanly
+
+    # (7) node1's own data survives the whole flow untouched.
+    assert int(node1.query("SELECT count() FROM t1")) == n1_count
+    assert int(node1.query("SELECT sum(id) FROM t1")) == n1_sum
+
+    # (8) node2's server_root_id is gone from the mounts table (only true once the slot above actually
+    #     retired -- checked after PHASE 2, not right after the first, still-pending call).
+    assert (
+        node1.query(
+            "SELECT count() FROM system.cas_mounts WHERE server_root_id = '{}'".format(SRID2)
+        ).strip()
+        == "0"
+    )
+
+    # (9) Drive GC (node1's background GC is already running against the shared pool) to reclaim
+    #     node2's now-unreferenced content, then poll for the blob count to drain back to baseline --
+    #     the authoritative "no content leftovers" proof, mirroring the ref-snaplog integration test's
+    #     idiom. node1's own t1 is still alive at this point and its blobs legitimately stay in the
+    #     pool, so a drain-to-baseline check is only meaningful after t1 is dropped too -- its survival
+    #     was already proven byte-for-byte in step 7, so drop it now and demand the pool drain to
+    #     EMPTY: node2's content via the decommission, t1's via the ordinary drop, no leftovers from
+    #     either. Then a read-only fsck over the drained pool must report clean (no dangling, no
+    #     unaccounted objects).
+    node1.query("DROP TABLE t1 SYNC")
+    at_drop = _count(BLOBS_PREFIX)
+
+    # THE RECLAMATION. Both contributions must go: node2's content via the decommission, t1's via the
+    # ordinary drop. Polled with an early exit, then cross-checked against GC's own bookkeeping so that
+    # a pool which shrank for some other reason cannot pass for a round that reclaimed it.
+    final = _count(BLOBS_PREFIX)
+    for _ in range(RECLAIM_RETRIES):
+        if final <= blobs_baseline:
+            break
+        time.sleep(RECLAIM_SLEEP)
+        final = _count(BLOBS_PREFIX)
+
+    assert final <= blobs_baseline, (
+        "the drained pool did not return to its baseline: baseline={}, at_drop={}, final={}".format(
+            blobs_baseline, at_drop, final
+        )
+    )
+
+    node1.query("SYSTEM FLUSH LOGS")
+    rounds = int(
+        node1.query(
+            "SELECT count() FROM system.cas_gc_log "
+            "WHERE event_type = 'Finish' AND outcome = 'Success'"
+        ).strip()
+        or 0
+    )
+    assert rounds > 0, "no successful GC round ran at all"
+    deleted = int(
+        node1.query(
+            "SELECT sum(objects_deleted + manifests_deleted) "
+            "FROM system.cas_gc_log WHERE event_type = 'Finish'"
+        ).strip()
+        or 0
+    )
+    assert deleted > 0, "the pool drained but GC's own bookkeeping reports no deletion"
+
+    # node2's decommissioned-and-healed namespace (t2) left canonical dead-life residue behind: its
+    # catalog row is gone (that is what let the slot retire above), but its `_ckpt`/`_files`/`_log`
+    # objects are the perpetual namespace janitor's job, not decommission's or GC's own destructive
+    # round. t1's row is pruned the same way once dropped. That residue must DRAIN to zero -- the
+    # janitor deletes one bounded page per round, so this is polled rather than read once -- and it
+    # must never be hard corruption on the way there, which is why `lifeless_keys` is checked on every
+    # poll and not only at the end.
+    for _ in range(RECLAIM_RETRIES):
+        fsck = _disks(node1, "cas-fsck")
+        assert "lifeless_keys=0" in fsck, fsck
+        janitor_pending_match = re.search(r"\bjanitor_pending=(\d+)", fsck)
+        assert janitor_pending_match, "cas-fsck summary is missing the janitor_pending field: {}".format(fsck)
+        if int(janitor_pending_match.group(1)) == 0:
+            break
+        time.sleep(RECLAIM_SLEEP)
+
+    assert "dangling=0" in fsck, fsck
+    assert "unaccounted=0" in fsck, fsck
+    assert int(janitor_pending_match.group(1)) == 0, (
+        "the dead-life residue from the healed decommission and the t1 drop never drained: {}".format(fsck)
+    )
+
+    # (10) Re-run the same command: decommission tombstones the owner anchor in place rather than
+    # deleting it, so the slot is not "unknown" -- the tombstone is found and the re-run is refused
+    # with CORRUPTED_DATA and a message telling the operator this server-root was explicitly
+    # decommissioned and will not silently resume.
+    err = node1.query_and_get_error(
+        "SYSTEM CAS DROP POOL MEMBER '{}' FROM DISK '{}'".format(SRID2, CA_DISK)
+    )
+    assert "explicitly decommissioned" in err, err
+
+
+def test_drop_pool_member_rejected_on_readonly_disk():
+    # disk_ca_ro is the fail-close guard's target: an observe-only window over the SAME pool (used
+    # elsewhere in this test only for fsck). Decommission is a mutating operation, so it must be
+    # rejected on this disk exactly like `createTransaction`/GC round/GC rebuild are -- READONLY,
+    # not a silent no-op or a crash further down the call chain.
+    node1 = cluster.instances["node1"]
+    err = node1.query_and_get_error(
+        "SYSTEM CAS DROP POOL MEMBER 'whatever' FROM DISK '{}'".format(RO_DISK)
+    )
+    assert "read-only" in err, err
diff --git a/tests/integration/test_cas_file_cache/__init__.py b/tests/integration/test_cas_file_cache/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_file_cache/configs/storage_conf.xml b/tests/integration/test_cas_file_cache/configs/storage_conf.xml
new file mode 100644
index 000000000000..fcf833676ffb
--- /dev/null
+++ b/tests/integration/test_cas_file_cache/configs/storage_conf.xml
@@ -0,0 +1,34 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <!-- Content-addressed disk over RustFS (RustFS, not MinIO, enforces the conditional-PUT
+                 semantics CAS relies on). -->
+            <disk_ca_s3>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <server_root_id>itest-cas-file-cache</server_root_id>
+                <endpoint>http://rustfs1:11121/test/cas_cache_data/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+            </disk_ca_s3>
+            <!-- File cache wrapping the CA disk. Before the fix this fails at startup with
+                 NOT_IMPLEMENTED (checkAccess → generic write path). -->
+            <disk_ca_s3_cache>
+                <type>cache</type>
+                <disk>disk_ca_s3</disk>
+                <path>/tmp/cas_file_cache/</path>
+                <max_size>1000000000</max_size>
+            </disk_ca_s3_cache>
+        </disks>
+        <policies>
+            <cas_cache>
+                <volumes>
+                    <main>
+                        <disk>disk_ca_s3_cache</disk>
+                    </main>
+                </volumes>
+            </cas_cache>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_file_cache/test.py b/tests/integration/test_cas_file_cache/test.py
new file mode 100644
index 000000000000..a08f173b2874
--- /dev/null
+++ b/tests/integration/test_cas_file_cache/test.py
@@ -0,0 +1,117 @@
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+STORAGE_POLICY = "cas_cache"
+NUM_ROWS = 100000
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node",
+        main_configs=["configs/storage_conf.xml"],
+        with_rustfs=True,
+        stay_alive=True,
+    )
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def test_cache_over_ca_startup_and_roundtrip():
+    # Before the fix the server fails to register the cache-over-CA disk (NOT_IMPLEMENTED at
+    # checkAccess), so this whole module fails at cluster.start(). After the fix, startup + a
+    # write/read round-trip succeed.
+    node = cluster.instances["node"]
+
+    node.query("DROP TABLE IF EXISTS cas_cache_test SYNC")
+    node.query(
+        """
+        CREATE TABLE cas_cache_test (id Int64, data String)
+        ENGINE = MergeTree() ORDER BY id
+        SETTINGS storage_policy = '{}'
+        """.format(
+            STORAGE_POLICY
+        )
+    )
+    node.query(
+        "INSERT INTO cas_cache_test SELECT number, toString(number) FROM numbers({})".format(
+            NUM_ROWS
+        )
+    )
+    expected_sum = (NUM_ROWS - 1) * NUM_ROWS // 2
+    assert int(node.query("SELECT count() FROM cas_cache_test")) == NUM_ROWS
+    assert int(node.query("SELECT sum(id) FROM cas_cache_test")) == expected_sum
+
+    node.query("DROP TABLE cas_cache_test SYNC")
+
+
+def _profile_event(node, query_id, event):
+    node.query("SYSTEM FLUSH LOGS")
+    v = node.query(
+        "SELECT sum(ProfileEvents['{}']) FROM system.query_log "
+        "WHERE query_id = '{}' AND type = 'QueryFinish'".format(event, query_id)
+    ).strip()
+    return int(v) if v else 0
+
+
+def test_cache_hits_on_repeated_reads():
+    # The point of the feature: a second full scan of the same data is served from the local file
+    # cache instead of re-fetching immutable content blobs from object storage.
+    node = cluster.instances["node"]
+
+    node.query("DROP TABLE IF EXISTS cas_cache_metrics SYNC")
+    node.query(
+        """
+        CREATE TABLE cas_cache_metrics (id Int64, data String)
+        ENGINE = MergeTree() ORDER BY id
+        SETTINGS storage_policy = '{}'
+        """.format(
+            STORAGE_POLICY
+        )
+    )
+    node.query(
+        "INSERT INTO cas_cache_metrics SELECT number, toString(number % 1000) FROM numbers(1000000)"
+    )
+    node.query("OPTIMIZE TABLE cas_cache_metrics FINAL")
+
+    # Start from a cold cache.
+    node.query("SYSTEM DROP FILESYSTEM CACHE")
+
+    q1 = "cas_cache_cold_scan"
+    node.query(
+        "SELECT sum(cityHash64(id, data)) FROM cas_cache_metrics",
+        query_id=q1,
+        settings={"enable_filesystem_cache": 1},
+    )
+    # The cold read must POPULATE the cache (read-through), not just read from source: pin the write
+    # side so a config where the cache never fills cannot pass on the warm-scan check alone.
+    assert int(node.query("SELECT count() FROM system.filesystem_cache")) > 0
+
+    q2 = "cas_cache_warm_scan"
+    node.query(
+        "SELECT sum(cityHash64(id, data)) FROM cas_cache_metrics",
+        query_id=q2,
+        settings={"enable_filesystem_cache": 1},
+    )
+
+    cold_source = _profile_event(node, q1, "CachedReadBufferReadFromSourceBytes")
+    warm_source = _profile_event(node, q2, "CachedReadBufferReadFromSourceBytes")
+    warm_cache = _profile_event(node, q2, "CachedReadBufferReadFromCacheBytes")
+
+    assert cold_source > 0, "cold scan should read from source"
+    assert warm_source * 10 < cold_source, (
+        "warm scan should read far fewer source bytes (cold={}, warm={})".format(
+            cold_source, warm_source
+        )
+    )
+    assert warm_cache > 0, "warm scan should read from the filesystem cache"
+
+    assert int(node.query("SELECT count() FROM system.filesystem_cache")) > 0
+
+    node.query("DROP TABLE cas_cache_metrics SYNC")
diff --git a/tests/integration/test_cas_gc_s3/__init__.py b/tests/integration/test_cas_gc_s3/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_gc_s3/configs/storage_conf.xml b/tests/integration/test_cas_gc_s3/configs/storage_conf.xml
new file mode 100644
index 000000000000..ac933c421ff5
--- /dev/null
+++ b/tests/integration/test_cas_gc_s3/configs/storage_conf.xml
@@ -0,0 +1,33 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <disk_cas_gc_s3>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- Phase 0 (mount safety): required, validated layout-subtree identity. Single server. -->
+                <server_root_id>itest-content-addressed-gc-s3</server_root_id>
+                <!-- RustFS (not MinIO) enforces the conditional-PUT/DELETE semantics the CA mount
+                     capability probe requires (CasProbe.cpp step 6): MinIO OSS silently honors a
+                     mismatched-token DELETE instead of rejecting it, so the fail-closed probe
+                     aborted server startup on MinIO (see PR#2073 CI triage). -->
+                <endpoint>http://rustfs1:11121/test/cas_gc_data/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <!-- Background GC is opt-in (default OFF); enable it and run aggressively so the
+                     background sweep actually reclaims orphaned blobs during the test. -->
+                <gc_enabled>1</gc_enabled>
+                <gc_interval_sec>1</gc_interval_sec>
+            </disk_cas_gc_s3>
+        </disks>
+        <policies>
+            <cas_gc_s3>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_gc_s3</disk>
+                    </main>
+                </volumes>
+            </cas_gc_s3>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_gc_s3/test.py b/tests/integration/test_cas_gc_s3/test.py
new file mode 100644
index 000000000000..7827ab80dbf9
--- /dev/null
+++ b/tests/integration/test_cas_gc_s3/test.py
@@ -0,0 +1,189 @@
+import time
+
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+STORAGE_POLICY = "cas_gc_s3"
+
+# Endpoint is http://rustfs1:11121/test/cas_gc_data/, so the pool's blobs and part footers live
+# under these key prefixes inside the `test` RustFS bucket. The authoritative "no S3 leftovers"
+# proof checks BOTH: a dropped table must leave neither content blobs nor part footers behind.
+BLOBS_PREFIX = "cas_gc_data/blobs/"
+PARTS_PREFIX = "cas_gc_data/parts/"
+
+# Enough rows / inserts to materialise several distinct blobs in the pool.
+NUM_ROWS = 100000
+NUM_INSERTS = 8
+
+# The background GC runs with grace=1s, interval=1s. After DROP TABLE ... SYNC the dropped table's
+# footers/blobs become unreferenced. How long we give the background GC to do its rounds; this is
+# waiting on a known background process, not papering over a race.
+RECLAIM_RETRIES = 60
+RECLAIM_SLEEP = 1.0  # seconds; total bound ~= RECLAIM_RETRIES * RECLAIM_SLEEP = 60s
+
+# The destructive phases of a GC round. Every one of them stamps `suppressed` into its phase_metrics,
+# which is how a suppressed round says so in a queryable way rather than only in the text log.
+DESTRUCTIVE_PHASES = (
+    "handoff_reclaim",
+    "manifest_deletes",
+    "namespace_cleanup",
+    "ref_object_cleanup",
+    "orphan_sweep",
+)
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node",
+        main_configs=["configs/storage_conf.xml"],
+        with_rustfs=True,
+        stay_alive=True,
+    )
+
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def count_prefix(prefix):
+    objects = cluster.rustfs_client.list_objects(
+        cluster.rustfs_bucket, prefix, recursive=True
+    )
+    return len(list(objects))
+
+
+def count_pool_objects():
+    # Both content blobs and part footers count: reclamation means BOTH drain.
+    return count_prefix(BLOBS_PREFIX) + count_prefix(PARTS_PREFIX)
+
+
+def gc_log_scalar(node, query):
+    node.query("SYSTEM FLUSH LOGS")
+    return int(node.query(query).strip())
+
+
+def test_gc_reclaims_dropped_blobs():
+    """
+    The background GC reclaims a dropped table's blobs and part footers.
+
+    A GC round may destroy only while holding a frontier proof for EVERY namespace that can hold a live
+    edge — reachability is a property of the whole pool, so deleting one blob asserts something about
+    every namespace at once, including the ones the round never looked at. The catalog supplies that set,
+    and each namespace's proof is one exact-key read at its cursor's successor.
+
+    So the reclamation below is asserted TOGETHER WITH the gate's own reason for permitting it: rounds
+    ran, every namespace in the universe reached a proven frontier, and no destructive phase reported
+    itself suppressed. Without that, a pool that shrank for some unrelated reason would read as a pass.
+    """
+    node = cluster.instances["node"]
+
+    node.query("DROP TABLE IF EXISTS cas_gc_test SYNC")
+
+    # (1) Baseline: how many objects (blobs + part footers) exist in the pool before our table.
+    baseline = count_pool_objects()
+
+    node.query(
+        """
+        CREATE TABLE cas_gc_test (
+            id Int64,
+            data String
+        ) ENGINE = MergeTree()
+        ORDER BY id
+        SETTINGS storage_policy = '{}'
+        """.format(
+            STORAGE_POLICY
+        )
+    )
+
+    # (2) Insert enough distinct rows across several inserts to produce several blobs.
+    for i in range(NUM_INSERTS):
+        node.query(
+            "INSERT INTO cas_gc_test "
+            "SELECT number + {offset}, toString(number + {offset}) "
+            "FROM numbers({rows})".format(offset=i * NUM_ROWS, rows=NUM_ROWS)
+        )
+
+    assert int(node.query("SELECT count() FROM cas_gc_test")) == NUM_INSERTS * NUM_ROWS
+
+    after_insert = count_pool_objects()
+    assert (
+        after_insert > baseline
+    ), "expected pool object count (blobs+parts) to rise above baseline {} after inserts, got {}".format(
+        baseline, after_insert
+    )
+
+    # (3) Drop the table: refs are unlinked synchronously; the blobs and part footers become
+    #     unreferenced GC fodder.
+    node.query("DROP TABLE cas_gc_test SYNC")
+
+    # (4) Poll for the reclamation, exiting as soon as it has happened (grace=1s, interval=1s, so
+    #     plenty of rounds run within the window).
+    final = count_pool_objects()
+    for _ in range(RECLAIM_RETRIES):
+        if final <= baseline:
+            break
+        time.sleep(RECLAIM_SLEEP)
+        final = count_pool_objects()
+
+    # (5) The dropped table's objects are GONE, back to the pre-table baseline.
+    assert final <= baseline, (
+        "the dropped table's objects were not reclaimed: baseline={}, after_insert={}, final={} "
+        "(blobs={}, parts={})".format(
+            baseline,
+            after_insert,
+            final,
+            count_prefix(BLOBS_PREFIX),
+            count_prefix(PARTS_PREFIX),
+        )
+    )
+
+    # (6) …AND FOR THE RIGHT REASON, which is what separates "the gate opened on a proven frontier"
+    #     from "the pool shrank for some other reason".
+    #
+    #     (a) Rounds actually ran and completed as the leader.
+    rounds = gc_log_scalar(
+        node,
+        "SELECT count() FROM system.cas_gc_log "
+        "WHERE event_type = 'Finish' AND outcome = 'Success'",
+    )
+    assert rounds > 0, "no successful GC round ran at all — this is not suppression, it is a wedge"
+
+    #     (b) The rounds report the deletion on their OWN bookkeeping, not only on the S3 object count.
+    deleted = gc_log_scalar(
+        node,
+        "SELECT sum(objects_deleted + manifests_deleted) "
+        "FROM system.cas_gc_log WHERE event_type = 'Finish'",
+    )
+    assert deleted > 0, "the pool shrank but no round reported deleting anything"
+
+    #     (c) At least one round proved EVERY namespace in its universe (frontier_proven ==
+    #         frontier_namespaces, both nonzero). A round held up by a clamp, a hold or an exhausted
+    #         probe budget would have frontier_proven < frontier_namespaces and could not have opened
+    #         the gate.
+    fully_proven_rounds = gc_log_scalar(
+        node,
+        "SELECT count() FROM system.cas_gc_log "
+        "WHERE phase = 'fold_ref_intake' "
+        "  AND phase_metrics['frontier_namespaces'] > 0 "
+        "  AND phase_metrics['frontier_proven'] = phase_metrics['frontier_namespaces']",
+    )
+    assert fully_proven_rounds > 0, (
+        "no round reached a fully proven frontier, so whatever removed those objects was not a round "
+        "acting on a complete frontier"
+    )
+
+    #     (d) And no destructive phase reported itself suppressed — the gate really did open.
+    suppressed_phases = gc_log_scalar(
+        node,
+        "SELECT uniqExact(phase) FROM system.cas_gc_log "
+        "WHERE phase IN {} AND phase_metrics['suppressed'] = 1".format(DESTRUCTIVE_PHASES),
+    )
+    assert suppressed_phases == 0, (
+        "a destructive phase reported itself suppressed on a pool whose frontier was complete"
+    )
diff --git a/tests/integration/test_cas_gc_sharded/__init__.py b/tests/integration/test_cas_gc_sharded/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_gc_sharded/configs/server_root_id_node1.xml b/tests/integration/test_cas_gc_sharded/configs/server_root_id_node1.xml
new file mode 100644
index 000000000000..92aea0f9cc5d
--- /dev/null
+++ b/tests/integration/test_cas_gc_sharded/configs/server_root_id_node1.xml
@@ -0,0 +1,12 @@
+<clickhouse>
+    <!-- Phase 0 (mount safety): each server owning a DISTINCT layout subtree inside the shared pool.
+         Two servers must never write the same server_root_id tree (spec §mount-safety), so this is a
+         per-instance override merged onto the shared storage_conf.xml disk definition. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_gc_sharded>
+                <server_root_id>node1</server_root_id>
+            </disk_cas_gc_sharded>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_gc_sharded/configs/server_root_id_node2.xml b/tests/integration/test_cas_gc_sharded/configs/server_root_id_node2.xml
new file mode 100644
index 000000000000..da16e054454a
--- /dev/null
+++ b/tests/integration/test_cas_gc_sharded/configs/server_root_id_node2.xml
@@ -0,0 +1,12 @@
+<clickhouse>
+    <!-- Phase 0 (mount safety): each server owning a DISTINCT layout subtree inside the shared pool.
+         Two servers must never write the same server_root_id tree (spec §mount-safety), so this is a
+         per-instance override merged onto the shared storage_conf.xml disk definition. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_gc_sharded>
+                <server_root_id>node2</server_root_id>
+            </disk_cas_gc_sharded>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_gc_sharded/configs/storage_conf.xml b/tests/integration/test_cas_gc_sharded/configs/storage_conf.xml
new file mode 100644
index 000000000000..e4236453a6ee
--- /dev/null
+++ b/tests/integration/test_cas_gc_sharded/configs/storage_conf.xml
@@ -0,0 +1,38 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <disk_cas_gc_sharded>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- A single shared bucket path; BOTH replicas mount the SAME content-addressed pool
+                     (one source of truth — shared-pool mode, spec §6). This is what makes a two-replica
+                     disjoint-shard GC scenario possible: both nodes see the same gc/state key and
+                     compete for the coordinator lease. -->
+                <!-- RustFS (not MinIO): the CA mount capability probe requires enforced conditional-DELETE
+                     semantics (CasProbe step 6); MinIO OSS silently honors a mismatched-token DELETE and the
+                     fail-closed probe aborts server startup (PR#2073 CI triage). -->
+                <endpoint>http://rustfs1:11121/test/cas_gc_sharded/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <!-- Phase 4: two blob-hash-prefix GC shards (gc_shards=2). Creation-time only; the
+                     pool is authoritative on reopen. With gc_shards=2 the coordinator splits the
+                     BlobDelta stream into two disjoint buckets (even/odd high-64 hash mod 2) and
+                     writes two blob_target shard keys per generation. -->
+                <gc_shards>2</gc_shards>
+                <!-- Fast GC for the integration soak: grace 3s, interval 1s. -->
+                <gc_enabled>1</gc_enabled>
+                <gc_interval_sec>1</gc_interval_sec>
+            </disk_cas_gc_sharded>
+        </disks>
+        <policies>
+            <cas_gc_sharded>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_gc_sharded</disk>
+                    </main>
+                </volumes>
+            </cas_gc_sharded>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_gc_sharded/test.py b/tests/integration/test_cas_gc_sharded/test.py
new file mode 100644
index 000000000000..a7c56edf2705
--- /dev/null
+++ b/tests/integration/test_cas_gc_sharded/test.py
@@ -0,0 +1,362 @@
+"""Phase 4 integration soak: two-replica disjoint-shard GC with gc_shards=2.
+
+Two ClickHouse nodes mount the SAME CA pool (shared-pool mode). The pool is configured with
+`gc_shards=2` — at first GC-state creation the coordinator writes two `blob_target/<shard>`
+runs (one per shard) per GC generation. Blob hashes route to shard 0 or shard 1 by
+`blobShard(blob_hash, 2) = high64(hash) % 2` (CasGcShardPlan::blobShard). Each generation
+therefore produces keys under both `blob_target/0/` and `blob_target/1/` (assuming the workload
+generates enough distinct blobs to cover both shard buckets — see the 2000-row inserts below).
+
+The pool runs on RustFS, not MinIO: the CA mount capability probe (`CasProbe::runCapabilityProbe`)
+requires an S3-compatible backend that enforces `DeleteObject If-Match` (conditional delete);
+MinIO OSS silently honors a mismatched-token DELETE instead of rejecting it, which the fail-closed
+probe treats as a fatal capability gap.
+
+The soak drives a blob-churn workload (INSERT x3 + OPTIMIZE FINAL + DROP x3 x2 rounds) on
+`node1`, then restarts `node2` (light chaos), quiesces (waits for GC to drain), and asserts:
+
+  A) No dangle / no loss — after quiesce both replicas return the same row counts for the live
+     table; no CA-layer exception or fatal error appears in either server log.
+
+  B) Single completion signal per generation — no partial-shard product was adopted before all
+     shards were committed. The pool's `gc/state` object names exactly one adopted
+     (generation, attempt) pair; that pair's fold-seal object must exist. A retry-created attempt
+     that never got adopted (and so is not named by `gc/state`) may have written its own seal too —
+     this is expected and must NOT be treated as a second completion signal; that is why this test
+     resolves the adopted pair from `gc/state` first, rather than counting every seal object it can
+     list under a generation. The soak waits for `gc/state` to adopt a nonzero generation before
+     asserting.
+
+  C) Disjoint-shard reduce progress — over the whole soak, `blob_target` keys were physically
+     written under BOTH shard 0 and shard 1 (proving the sharded path executed, not just the
+     gc_shards==1 fast-path). This is checked across every generation/attempt seen under
+     `gc/gen/`, not only the currently-adopted one: a seal's `blob_target_runs` carry a PARENT's
+     runs forward as references, but the physical objects stay under the (generation, attempt)
+     prefix where they were originally written -- a late-soak adopted attempt whose own round
+     produced no new deltas can have an empty `blob_target/` prefix of its own even though earlier
+     attempts wrote plenty. Scanning the whole `gc/gen/` subtree is the only check that matches
+     where the objects actually physically live.
+"""
+
+import re
+import time
+
+import pytest
+from minio.error import S3Error
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+STORAGE_POLICY = "cas_gc_sharded"
+
+# Pool bucket key prefixes (the endpoint is http://rustfs1:11121/test/cas_gc_sharded/).
+POOL_PREFIX = "cas_gc_sharded"
+GC_STATE_KEY = POOL_PREFIX + "/gc/state"
+
+# Workload parameters — enough rows to produce blobs routing to BOTH hash-mod-2 buckets.
+NUM_ROWS_PER_INSERT = 2000
+NUM_INSERTS = 4
+
+# GC quiesce: grace=3s, interval=1s. We poll up to 90 s for at least one completed generation.
+GC_POLL_RETRIES = 90
+GC_POLL_SLEEP = 1.0
+
+# Error patterns in server logs that must NOT appear in a healthy soak.
+CA_FATAL_LOG_KEYWORDS = [
+    "DANGLE",
+    "dangle",
+    "CorruptDangle",
+]
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node1",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node1.xml"],
+        macros={"replica": "node1"},
+        with_rustfs=True,
+        with_zookeeper=True,
+        stay_alive=True,
+    )
+    cluster.add_instance(
+        "node2",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node2.xml"],
+        macros={"replica": "node2"},
+        with_rustfs=True,
+        with_zookeeper=True,
+        stay_alive=True,
+    )
+
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+# ---------------------------------------------------------------------------
+# RustFS helpers
+# ---------------------------------------------------------------------------
+
+def list_rustfs_prefix(prefix, recursive=True):
+    """Return a list of object keys under `prefix` in the shared RustFS bucket."""
+    objects = cluster.rustfs_client.list_objects(
+        cluster.rustfs_bucket, prefix, recursive=recursive
+    )
+    return [o.object_name for o in objects]
+
+
+def get_rustfs_object(key):
+    """Return the raw bytes of `key`, or None if it does not exist."""
+    try:
+        resp = cluster.rustfs_client.get_object(cluster.rustfs_bucket, key)
+        try:
+            return resp.read()
+        finally:
+            resp.close()
+            resp.release_conn()
+    except S3Error:
+        return None
+
+
+# `gc/state`'s wire format is a plain JSON-like text object (CasGcStateFormat.cpp), not a binary
+# blob: the two fields this test needs are literally spelled `"sg":"<digits>"` (snap_generation)
+# and `"sa":"<digits>"` (snap_attempt) in the object bytes, so a direct regex read is exact without
+# needing the C++ decoder. This mirrors the production reader that resolves "the adopted seal"
+# (Gc/CasOrphanManifestSweep.cpp): read gc/state, take (snap_generation, snap_attempt), then look up
+# that exact fold seal -- the only two-hop lookup that names one authoritative adopted pair.
+_SNAP_GENERATION_RE = re.compile(r'"sg":"(\d+)"')
+_SNAP_ATTEMPT_RE = re.compile(r'"sa":"(\d+)"')
+
+
+def read_adopted_generation_and_attempt():
+    """
+    Return the (generation, attempt) pair `gc/state` currently names as adopted, or None if
+    `gc/state` does not exist yet or its `snap_generation` is still the "nothing adopted yet"
+    sentinel (0, GcState's documented default).
+    """
+    data = get_rustfs_object(GC_STATE_KEY)
+    if data is None:
+        return None
+    text = data.decode("utf-8", errors="replace")
+    sg_match = _SNAP_GENERATION_RE.search(text)
+    sa_match = _SNAP_ATTEMPT_RE.search(text)
+    if not sg_match or not sa_match:
+        return None
+    generation = int(sg_match.group(1))
+    if generation == 0:
+        return None
+    return generation, int(sa_match.group(1))
+
+
+def adopted_fold_seal_key(generation, attempt):
+    """The one fold-seal key `gc/state` names as adopted for (generation, attempt)."""
+    return "{}/gc/gen/{}/attempt/{}/fold_seal".format(POOL_PREFIX, generation, attempt)
+
+
+# Matches ".../gc/gen/<generation>/attempt/<attempt>/blob_target/<shard>/<seq>" -- the exact key
+# shape `Layout::blobTargetRunKey` writes -- capturing only the shard id. Deliberately NOT scoped
+# to one (generation, attempt): the objects a run key names stay physically where they were
+# WRITTEN, and a seal only carries a REFERENCE to an earlier attempt's runs forward, so "did the
+# sharded path ever write both shards over the whole soak" has to scan the whole gc/gen/ subtree.
+_BLOB_TARGET_SHARD_RE = re.compile(r"/gc/gen/\d+/attempt/\d+/blob_target/(\d+)/")
+
+
+def blob_target_shards_present():
+    """Return (all blob_target keys found under gc/gen/, set of shard ids covered by them)."""
+    keys = list_rustfs_prefix(POOL_PREFIX + "/gc/gen/", recursive=True)
+    blob_target_keys = []
+    shards = set()
+    for k in keys:
+        m = _BLOB_TARGET_SHARD_RE.search(k)
+        if m:
+            blob_target_keys.append(k)
+            shards.add(int(m.group(1)))
+    return blob_target_keys, shards
+
+
+# ---------------------------------------------------------------------------
+# Workload
+# ---------------------------------------------------------------------------
+
+def run_blob_churn_workload(node, table_name, rounds=2):
+    """
+    Insert rows + merge + drop in `rounds` cycles. Each cycle creates a fresh
+    `ReplicatedMergeTree`, inserts `NUM_INSERTS` batches of `NUM_ROWS_PER_INSERT` rows,
+    forces a merge, then drops the table. This generates blob churn: blobs are
+    referenced during the cycle, then become orphaned after the drop.
+    """
+    for i in range(rounds):
+        full_name = "{}_{}".format(table_name, i)
+        node.query("DROP TABLE IF EXISTS {} SYNC".format(full_name))
+        node.query(
+            "CREATE TABLE {name} (id Int64, v UInt64, s String) "
+            "ENGINE = ReplicatedMergeTree('/clickhouse/tables/{name}', '{{replica}}') "
+            "ORDER BY id "
+            "SETTINGS storage_policy = '{policy}'".format(
+                name=full_name, policy=STORAGE_POLICY
+            )
+        )
+        for j in range(NUM_INSERTS):
+            offset = (i * NUM_INSERTS + j) * NUM_ROWS_PER_INSERT
+            node.query(
+                "INSERT INTO {name} "
+                "SELECT number + {off}, number + {off}, toString(number + {off}) "
+                "FROM numbers({rows})".format(
+                    name=full_name, off=offset, rows=NUM_ROWS_PER_INSERT
+                )
+            )
+        node.query("OPTIMIZE TABLE {} FINAL".format(full_name))
+        node.query("DROP TABLE {} SYNC".format(full_name))
+
+
+# ---------------------------------------------------------------------------
+# Main soak test
+# ---------------------------------------------------------------------------
+
+def test_sharded_gc_soak():
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+
+    # Create a long-lived table to confirm row-count parity after the soak.
+    node1.query("DROP TABLE IF EXISTS live_table SYNC")
+    node2.query("DROP TABLE IF EXISTS live_table SYNC")
+    node1.query(
+        "CREATE TABLE live_table (id Int64, s String) "
+        "ENGINE = ReplicatedMergeTree('/clickhouse/tables/live_table', '{{replica}}') "
+        "ORDER BY id "
+        "SETTINGS storage_policy = '{}'".format(STORAGE_POLICY)
+    )
+    node2.query(
+        "CREATE TABLE live_table (id Int64, s String) "
+        "ENGINE = ReplicatedMergeTree('/clickhouse/tables/live_table', '{{replica}}') "
+        "ORDER BY id "
+        "SETTINGS storage_policy = '{}'".format(STORAGE_POLICY)
+    )
+
+    # Insert into the live table so it has real content during the soak.
+    node1.query(
+        "INSERT INTO live_table "
+        "SELECT number, toString(number) FROM numbers({rows})".format(
+            rows=NUM_ROWS_PER_INSERT * NUM_INSERTS
+        )
+    )
+    node2.query("SYSTEM SYNC REPLICA live_table", timeout=60)
+
+    live_count_before = int(node1.query("SELECT count() FROM live_table"))
+    assert live_count_before == NUM_ROWS_PER_INSERT * NUM_INSERTS
+
+    # --- WORKLOAD: blob churn on node1 ---
+    run_blob_churn_workload(node1, "churn", rounds=2)
+
+    # --- CHAOS: restart node2 ---
+    node2.restart_clickhouse(kill=True)
+    node2.query("SYSTEM SYNC REPLICA live_table", timeout=120)
+
+    # --- QUIESCE: wait for gc/state to adopt a generation ---
+    adopted = None
+    for _ in range(GC_POLL_RETRIES):
+        adopted = read_adopted_generation_and_attempt()
+        if adopted:
+            break
+        time.sleep(GC_POLL_SLEEP)
+
+    assert adopted, (
+        "gc/state never adopted a generation (snap_generation stayed at the zero sentinel) within "
+        "{} s; gc_shards=2 soak cannot proceed (check server logs for GC errors)".format(
+            GC_POLL_RETRIES * GC_POLL_SLEEP
+        )
+    )
+
+    # Allow one more GC interval for any in-progress round to finish, then re-resolve the adopted
+    # pointer (it may have advanced again).
+    time.sleep(5)
+    generation, attempt = read_adopted_generation_and_attempt() or adopted
+
+    # -----------------------------------------------------------------------
+    # ASSERTION A: no dangle / no loss
+    # -----------------------------------------------------------------------
+
+    # Both replicas must agree on the live row count.
+    count1 = int(node1.query("SELECT count() FROM live_table"))
+    count2 = int(node2.query("SELECT count() FROM live_table"))
+    assert count1 == live_count_before, (
+        "node1 live_table count changed: before {} after {}".format(
+            live_count_before, count1
+        )
+    )
+    assert count1 == count2, (
+        "replica row-count divergence: node1={} node2={}".format(count1, count2)
+    )
+
+    # No CA-layer dangle errors in either server log. `system.text_log`'s underlying table can
+    # take a moment after node2's restart above to become queryable ("Unknown table" briefly),
+    # independent of whether any row has actually been flushed to it yet. Wait that out with a
+    # SEPARATE, keyword-free readiness probe first, THEN run the real keyword checks with a plain
+    # query (no retry): retrying the keyword query itself would be self-defeating -- each failed
+    # attempt logs its own `<Error>` line that echoes the failing query's text (which contains the
+    # search keyword as a LIKE pattern), and that line then lands in system.text_log itself, so a
+    # later successful attempt of the SAME keyword query would count its own failed predecessors
+    # as matches.
+    for inst_name, inst in [("node1", node1), ("node2", node2)]:
+        inst.query_with_retry("SELECT count() FROM system.text_log")
+        for kw in CA_FATAL_LOG_KEYWORDS:
+            log_count = inst.query(
+                "SELECT count() FROM system.text_log "
+                "WHERE level IN ('Error', 'Fatal') "
+                "  AND message LIKE '%{}%'".format(kw)
+            )
+            assert int(log_count) == 0, (
+                "{} has CA fatal/error entries matching '{}' in system.text_log".format(
+                    inst_name, kw
+                )
+            )
+
+    # -----------------------------------------------------------------------
+    # ASSERTION B: the adopted (generation, attempt) has a durable fold seal
+    # -----------------------------------------------------------------------
+
+    # gc/state names exactly one adopted (generation, attempt) pair; this is the ONLY seal this
+    # test looks at -- a non-adopted retry attempt's own seal (if any) is never named by gc/state
+    # and so cannot be mistaken for a second completion signal.
+    seal_key = adopted_fold_seal_key(generation, attempt)
+    assert get_rustfs_object(seal_key) is not None, (
+        "gc/state adopted (generation={}, attempt={}) but its fold seal ('{}') does not exist -- "
+        "gc/state points at an attempt whose seal was never durably written".format(
+            generation, attempt, seal_key
+        )
+    )
+
+    # -----------------------------------------------------------------------
+    # ASSERTION C: disjoint-shard reduce progress
+    # -----------------------------------------------------------------------
+
+    # Over the WHOLE gc/gen/ subtree (every generation and attempt seen, not only the currently
+    # adopted one -- see the module docstring's point C for why), blob_target keys must cover
+    # BOTH shard 0 and shard 1.
+    blob_target_keys, shards_covered = blob_target_shards_present()
+    # Log the observed key set once: an assertion that passes on an empty listing (e.g. a further
+    # key-shape mismatch) would be a silent false pass, not evidence the sharded path ran.
+    print(
+        "test_sharded_gc_soak: blob_target keys under {}/gc/gen/: {}".format(
+            POOL_PREFIX, blob_target_keys
+        )
+    )
+    assert blob_target_keys, (
+        "no blob_target keys found anywhere under '{}/gc/gen/'; before checking shard coverage "
+        "the listing itself must be non-empty".format(POOL_PREFIX)
+    )
+    assert 0 in shards_covered and 1 in shards_covered, (
+        "blob_target keys under '{}/gc/gen/' cover shards {}, expected BOTH 0 and 1; the sharded "
+        "gc_shards=2 fold path may not have executed. Keys observed: {}".format(
+            POOL_PREFIX, sorted(shards_covered), blob_target_keys
+        )
+    )
+
+    # -----------------------------------------------------------------------
+    # Cleanup
+    # -----------------------------------------------------------------------
+    node1.query("DROP TABLE IF EXISTS live_table SYNC")
+    node2.query("DROP TABLE IF EXISTS live_table SYNC")
diff --git a/tests/integration/test_cas_insert_fault_recovery/__init__.py b/tests/integration/test_cas_insert_fault_recovery/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_insert_fault_recovery/configs/server_root_id_node1.xml b/tests/integration/test_cas_insert_fault_recovery/configs/server_root_id_node1.xml
new file mode 100644
index 000000000000..dd63460deccc
--- /dev/null
+++ b/tests/integration/test_cas_insert_fault_recovery/configs/server_root_id_node1.xml
@@ -0,0 +1,10 @@
+<clickhouse>
+    <!-- Each server owns a DISTINCT layout subtree inside the shared pool (mount safety). -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <server_root_id>node1</server_root_id>
+            </disk_cas_shared>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_insert_fault_recovery/configs/server_root_id_node2.xml b/tests/integration/test_cas_insert_fault_recovery/configs/server_root_id_node2.xml
new file mode 100644
index 000000000000..e1be581a8c9f
--- /dev/null
+++ b/tests/integration/test_cas_insert_fault_recovery/configs/server_root_id_node2.xml
@@ -0,0 +1,10 @@
+<clickhouse>
+    <!-- Each server owns a DISTINCT layout subtree inside the shared pool (mount safety). -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <server_root_id>node2</server_root_id>
+            </disk_cas_shared>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_insert_fault_recovery/configs/storage_conf.xml b/tests/integration/test_cas_insert_fault_recovery/configs/storage_conf.xml
new file mode 100644
index 000000000000..0149d398aa1c
--- /dev/null
+++ b/tests/integration/test_cas_insert_fault_recovery/configs/storage_conf.xml
@@ -0,0 +1,27 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- Both replicas load this same config, so they mount the SAME content-addressed pool.
+                     RustFS (not MinIO) enforces the conditional-PUT/DELETE semantics a CA pool requires
+                     to mount at all (B31). -->
+                <endpoint>http://rustfs1:11121/test/shared_pool/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <!-- Two replicas mounting one pool: the second mounter would otherwise refuse to attach. -->
+            </disk_cas_shared>
+        </disks>
+        <policies>
+            <cas_shared>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_shared</disk>
+                    </main>
+                </volumes>
+            </cas_shared>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_insert_fault_recovery/test.py b/tests/integration/test_cas_insert_fault_recovery/test.py
new file mode 100644
index 000000000000..e1f3ff4ee23a
--- /dev/null
+++ b/tests/integration/test_cas_insert_fault_recovery/test.py
@@ -0,0 +1,163 @@
+import time
+
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+# Two replicas of one ReplicatedMergeTree on a SHARED content-addressed pool.
+STORAGE_POLICY = "cas_shared"
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node1",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node1.xml"],
+        macros={"replica": "node1"},
+        with_rustfs=True,
+        with_zookeeper=True,
+        stay_alive=True,
+    )
+    cluster.add_instance(
+        "node2",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node2.xml"],
+        macros={"replica": "node2"},
+        with_rustfs=True,
+        with_zookeeper=True,
+        stay_alive=True,
+    )
+
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def _wait_until(predicate, timeout=180, interval=2, desc=""):
+    # Condition-based wait (systematic-debugging): the ordinary lost-part recovery is asynchronous
+    # (part-check retry/backoff), so gating on a fixed-timeout `SYSTEM SYNC REPLICA` is inherently flaky —
+    # that call blocks on the very recovery we are waiting for. Poll the actual OUTCOME instead, with a
+    # generous cap. Transient errors while node1 is mid-restart are swallowed and retried.
+    deadline = time.time() + timeout
+    last = None
+    while time.time() < deadline:
+        try:
+            last = predicate()
+        except Exception as e:  # node briefly unavailable during restart, etc.
+            last = e
+        if last is True:
+            return
+        time.sleep(interval)
+    raise AssertionError("timed out after {}s waiting for: {} (last={!r})".format(timeout, desc, last))
+
+
+def test_post_multi_termination_uses_ordinary_lost_part_recovery(start_cluster):
+    # HISTORY: this test was authored (2026-07-16) against the OLD commit ordering, where the disk
+    # commit ran AFTER the Keeper multi — the failpoint then left a phantom ZK part entry and the
+    # assertion was "ordinary lost-part recovery runs (ReplicatedDataLoss bumps, empty cover)".
+    # One day later the R3 acked-data-loss fix (`77484196b0d`) deliberately REVERSED that order:
+    # `renameParts` closes the part's disk-storage transaction BEFORE the Keeper multi, so a part
+    # must be durable before its block_id/part znode is registered. Under the new ordering the
+    # failpoint (`disk_object_storage_fail_commit_metadata_transaction`, fired from inside
+    # `renameParts`) aborts the INSERT BEFORE anything reaches ZK — there is no phantom part, no
+    # lost part, and NOTHING to recover. The old predicate waited forever (600s timeouts on all
+    # three sanitizer CI lanes of PR#2073 and on a local release build).
+    #
+    # The test now asserts the NEW invariant, which is strictly stronger for the user:
+    #   1. the failed INSERT leaves NO trace: no ZK part entry, no replication-queue debris,
+    #      count() stays 0 on both replicas after a node1 restart, and `ReplicatedDataLoss` does
+    #      NOT bump (nothing was ever lost);
+    #   2. THE R3 GUARD: retrying the SAME insert (same bytes => same block_id) actually lands —
+    #      a phantom block_id surviving the failed attempt would silently dedup the retry away
+    #      (the acked-data-loss class the reordering exists to prevent);
+    #   3. no CA-specific wedge: no LOGICAL_ERROR in either server's log, queues drained.
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+
+    node1.query("DROP TABLE IF EXISTS t SYNC")
+    node2.query("DROP TABLE IF EXISTS t SYNC")
+
+    create = (
+        "CREATE TABLE t (a UInt64) ENGINE = ReplicatedMergeTree('/clickhouse/tables/t', '{{replica}}') "
+        "ORDER BY a SETTINGS storage_policy = '{policy}'"
+    ).format(policy=STORAGE_POLICY)
+    node1.query(create)
+    node2.query(create)
+
+    def loss_count():
+        return int(
+            node1.query(
+                "SELECT sum(value) FROM system.events WHERE event = 'ReplicatedDataLoss'"
+            )
+            or 0
+        )
+
+    loss_before = loss_count()
+
+    # Force the disk commit to throw. Under the R3 ordering this fires inside `renameParts`,
+    # BEFORE the Keeper multi — the INSERT fails with nothing registered anywhere (ONCE failpoint).
+    node1.query("SYSTEM ENABLE FAILPOINT disk_object_storage_fail_commit_metadata_transaction")
+    node1.query_and_get_error("INSERT INTO t VALUES (1)")
+
+    # No phantom state may exist even across a restart: ZK has no part entry, so startup's
+    # `checkPartsImpl` has nothing to reconcile and no recovery runs.
+    node1.restart_clickhouse()
+
+    def node1_clean():
+        # The failed INSERT left no trace: nothing to recover (ReplicatedDataLoss unchanged),
+        # no rows, no replication-queue debris.
+        cnt = node1.query("SELECT count() FROM t").strip()
+        queue = node1.query(
+            "SELECT count() FROM system.replication_queue WHERE table = 't'"
+        ).strip()
+        return loss_count() == loss_before and cnt == "0" and queue == "0"
+
+    _wait_until(
+        node1_clean,
+        timeout=120,
+        desc="node1 restarts clean: no phantom part, no recovery triggered, queue empty",
+    )
+
+    # THE R3 GUARD (acked-data-loss class): retrying the SAME insert (same bytes => same block_id)
+    # must genuinely land. If the failed attempt had leaked its block_id into ZK, dedup would
+    # silently swallow this retry and count() would stay 0 — exactly the silent loss the
+    # renameParts-before-Keeper ordering exists to prevent.
+    node1.query("INSERT INTO t VALUES (1)")
+
+    def retry_landed_everywhere():
+        return (
+            node1.query("SELECT count() FROM t").strip() == "1"
+            and node2.query("SELECT count() FROM t").strip() == "1"
+        )
+
+    _wait_until(
+        retry_landed_everywhere,
+        timeout=120,
+        desc="the retried identical INSERT lands and replicates (no phantom-block_id dedup)",
+    )
+
+    # The regression guard: no CA-specific exception / LOGICAL_ERROR left either server wedged. The
+    # expected `FILE_DOESNT_EXIST` interserver miss is tolerated (it is not a LOGICAL_ERROR).
+    for node in (node1, node2):
+        assert not node.contains_in_log(
+            "LOGICAL_ERROR"
+        ), "unexpected LOGICAL_ERROR in {}'s log — a CA-specific failure, not ordinary lost-part recovery".format(
+            node.name
+        )
+
+    # Server is healthy (no wedge): a fresh, different INSERT also succeeds end to end.
+    node1.query("INSERT INTO t VALUES (2)")
+
+    def replicated_two_rows():
+        return (
+            node1.query("SELECT count() FROM t").strip() == "2"
+            and node2.query("SELECT count() FROM t").strip() == "2"
+        )
+
+    _wait_until(replicated_two_rows, timeout=120, desc="fresh INSERT replicates to both replicas")
+
+    node1.query("DROP TABLE IF EXISTS t SYNC")
+    node2.query("DROP TABLE IF EXISTS t SYNC")
diff --git a/tests/integration/test_cas_lazy_load_recovery/__init__.py b/tests/integration/test_cas_lazy_load_recovery/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_lazy_load_recovery/configs/server_root_id_node1.xml b/tests/integration/test_cas_lazy_load_recovery/configs/server_root_id_node1.xml
new file mode 100644
index 000000000000..dd63460deccc
--- /dev/null
+++ b/tests/integration/test_cas_lazy_load_recovery/configs/server_root_id_node1.xml
@@ -0,0 +1,10 @@
+<clickhouse>
+    <!-- Each server owns a DISTINCT layout subtree inside the shared pool (mount safety). -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <server_root_id>node1</server_root_id>
+            </disk_cas_shared>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_lazy_load_recovery/configs/storage_conf.xml b/tests/integration/test_cas_lazy_load_recovery/configs/storage_conf.xml
new file mode 100644
index 000000000000..0149d398aa1c
--- /dev/null
+++ b/tests/integration/test_cas_lazy_load_recovery/configs/storage_conf.xml
@@ -0,0 +1,27 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- Both replicas load this same config, so they mount the SAME content-addressed pool.
+                     RustFS (not MinIO) enforces the conditional-PUT/DELETE semantics a CA pool requires
+                     to mount at all (B31). -->
+                <endpoint>http://rustfs1:11121/test/shared_pool/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <!-- Two replicas mounting one pool: the second mounter would otherwise refuse to attach. -->
+            </disk_cas_shared>
+        </disks>
+        <policies>
+            <cas_shared>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_shared</disk>
+                    </main>
+                </volumes>
+            </cas_shared>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_lazy_load_recovery/test.py b/tests/integration/test_cas_lazy_load_recovery/test.py
new file mode 100644
index 000000000000..3eeae3debc7d
--- /dev/null
+++ b/tests/integration/test_cas_lazy_load_recovery/test.py
@@ -0,0 +1,88 @@
+import time
+
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+STORAGE_POLICY = "cas_shared"
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node1",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node1.xml"],
+        macros={"replica": "node1"},
+        with_rustfs=True,
+        with_zookeeper=True,
+        stay_alive=True,
+    )
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def _create(node):
+    # lazy_load_tables=1: the CAS table attaches as a proxy and its real storage is built on first
+    # access. A transient object-store outage during that build is ridden out / retried on a later
+    # access instead of being cached as a permanently-FAILED AsyncLoader job (which, for a non-lazy
+    # database, would strand the table until a full server restart).
+    node.query("CREATE DATABASE IF NOT EXISTS lazy_db ENGINE = Atomic SETTINGS lazy_load_tables = 1")
+    node.query(
+        "CREATE TABLE IF NOT EXISTS lazy_db.t (k UInt64, v UInt64) "
+        "ENGINE = ReplicatedMergeTree('/clickhouse/tables/lazy_t', '{replica}') "
+        "ORDER BY k SETTINGS storage_policy = '%s', min_bytes_for_wide_part = 0" % STORAGE_POLICY
+    )
+
+
+def test_lazy_cas_table_self_heals_after_s3_recovery(start_cluster):
+    node = cluster.instances["node1"]
+    _create(node)
+    node.query("INSERT INTO lazy_db.t SELECT number, number FROM numbers(100)")
+    assert node.query("SELECT count() FROM lazy_db.t").strip() == "100"
+
+    # Restart so the table re-attaches as a lazy proxy (its real storage is not yet constructed; the
+    # disk mounts at startup while S3 is up, the storage is built only on first access below).
+    node.restart_clickhouse()
+
+    # Touch the table while S3 is unreachable: the lazy first-access build (its CAS ref-recovery LIST
+    # over the object store) cannot complete, so the client query fails within its bounded timeout.
+    # Note: the build does NOT fail fast server-side -- it blocks on the object store's own retry until
+    # S3 returns (see the BACKLOG "block-until-recovered" note); the client-side timeout is what makes
+    # this probe short. We assert the probe DID hit the outage (raised): the build needs several object-
+    # store round-trips, so the freezer (effective within milliseconds of `pause_container` returning)
+    # reliably catches it -- if this ever flakes, the pause raced a sub-millisecond full build, not a
+    # real self-heal regression.
+    with cluster.pause_container("rustfs1", wait_for_paused=False):
+        probe_raised = False
+        try:
+            node.query("SELECT count() FROM lazy_db.t", timeout=30)
+        except Exception:
+            probe_raised = True  # expected while the object store is unreachable
+        assert probe_raised, "the probe should have failed while S3 was unreachable (did the pause race the build?)"
+
+    # S3 is back (context exit unpaused rustfs). WITHOUT a server restart and WITHOUT any DETACH, a
+    # later access must make the table usable again. This proves the key Layer 2 property: a transient
+    # object-store outage during a lazy CAS table's first-access build leaves NO permanently-cached
+    # AsyncLoader FAILED state (a non-lazy table whose load failed would stay FAILED until a full server
+    # restart). What actually recovers here is the original in-flight build completing once S3 returns
+    # (the block-until-recovered path), which is sufficient for "usable again without restart"; this
+    # test does not (and, given block-until-recovered, cannot) assert a proxy retry of a THROWN build.
+    deadline = time.time() + 180
+    last = None
+    while time.time() < deadline:
+        try:
+            last = node.query("SELECT count() FROM lazy_db.t").strip()
+        except Exception as e:
+            last = "err: " + str(e)
+        if last == "100":
+            break
+        time.sleep(3)
+    assert last == "100", (
+        "lazy CAS table must become usable again on a later access after S3 returns, with no server "
+        "restart (last=%r)" % last
+    )
diff --git a/tests/integration/test_cas_ref_snaplog/__init__.py b/tests/integration/test_cas_ref_snaplog/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_ref_snaplog/configs/storage_conf.xml b/tests/integration/test_cas_ref_snaplog/configs/storage_conf.xml
new file mode 100644
index 000000000000..cf842fcd4f6c
--- /dev/null
+++ b/tests/integration/test_cas_ref_snaplog/configs/storage_conf.xml
@@ -0,0 +1,43 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <!-- Content-addressed disk over RustFS. RustFS (not MinIO) enforces the conditional-PUT
+                 (If-None-Match) semantics the snapshot+log writer linearization relies on. Background
+                 GC on a tight cadence so a DROP is folded, cleaned, and reclaimed within the test. -->
+            <disk_ca>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <server_root_id>itest-ref-snaplog</server_root_id>
+                <endpoint>http://rustfs1:11121/test/cas_snaplog_data/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <gc_enabled>1</gc_enabled>
+                <gc_interval_sec>1</gc_interval_sec>
+            </disk_ca>
+            <!-- Read-only window over the SAME pool for the read-only consumers (fsck, cas-gc-dryrun).
+                 Those commands refuse a writable pool; a read-only disk takes no mount lease, so it can
+                 inspect the pool the server writes without contention. -->
+            <disk_ca_ro>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <server_root_id>itest-ref-snaplog</server_root_id>
+                <endpoint>http://rustfs1:11121/test/cas_snaplog_data/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <readonly>true</readonly>
+                <gc_enabled>0</gc_enabled>
+            </disk_ca_ro>
+        </disks>
+        <policies>
+            <ref_snaplog>
+                <volumes>
+                    <main>
+                        <disk>disk_ca</disk>
+                    </main>
+                </volumes>
+            </ref_snaplog>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_ref_snaplog/test.py b/tests/integration/test_cas_ref_snaplog/test.py
new file mode 100644
index 000000000000..0a9628673a26
--- /dev/null
+++ b/tests/integration/test_cas_ref_snaplog/test.py
@@ -0,0 +1,170 @@
+import shlex
+import time
+
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+STORAGE_POLICY = "ref_snaplog"
+RO_DISK = "disk_ca_ro"
+
+# Endpoint is http://rustfs1:11121/test/cas_snaplog_data/, so the pool lives under bucket `test`,
+# prefix `cas_snaplog_data/`. The snapshot+log ref protocol keeps a table's immutable transaction logs
+# and snapshots under cas/ns/stream/, part manifests under cas/manifests/, and content blobs under blobs/.
+POOL = "cas_snaplog_data"
+BLOBS_PREFIX = POOL + "/blobs/"
+REFS_PREFIX = POOL + "/cas/ns/stream/"
+MANIFESTS_PREFIX = POOL + "/cas/manifests/"
+
+NUM_ROWS = 20000
+NUM_INSERTS = 8
+
+# Background GC runs every 1s with a 2s grace. After DROP TABLE ... SYNC the dropped namespace's content
+# (blobs) and part manifests become GC fodder; we poll until they drain. Bounded wait on a known
+# background process, not a race hack.
+RECLAIM_RETRIES = 120
+RECLAIM_SLEEP = 1.0  # total bound ~= 120s
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node",
+        main_configs=["configs/storage_conf.xml"],
+        with_rustfs=True,
+        stay_alive=True,
+    )
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def _count(prefix):
+    return len(
+        list(
+            cluster.rustfs_client.list_objects(
+                cluster.rustfs_bucket, prefix, recursive=True
+            )
+        )
+    )
+
+
+def _content_objects():
+    # Content blobs + part manifests: the objects the GC fold + condemn/delete pipeline and the
+    # namespace-cleanup item reclaim after a namespace is removed.
+    return _count(BLOBS_PREFIX) + _count(MANIFESTS_PREFIX)
+
+
+def _disks(node, query):
+    # Run a clickhouse-disks command against the read-only CA window over the same pool.
+    return node.exec_in_container(
+        [
+            "bash",
+            "-c",
+            "/usr/bin/clickhouse disks -C /etc/clickhouse-server/config.xml "
+            "--disk {} --save-logs --query {}".format(RO_DISK, shlex.quote(query)),
+        ]
+    )
+
+
+def test_ref_snaplog_lifecycle_reclaims_and_fsck_clean():
+    node = cluster.instances["node"]
+
+    for t in ("ref_t1", "ref_t1_renamed", "ref_t2"):
+        node.query("DROP TABLE IF EXISTS {} SYNC".format(t))
+
+    content_baseline = _content_objects()
+
+    # (1) Two tables on the CA/rustfs policy.
+    for t in ("ref_t1", "ref_t2"):
+        node.query(
+            "CREATE TABLE {} (id Int64, data String) ENGINE = MergeTree() ORDER BY id "
+            "SETTINGS storage_policy = '{}'".format(t, STORAGE_POLICY)
+        )
+
+    # (2) Several inserts each -> distinct content blobs + one immutable ref-log transaction per insert.
+    for t in ("ref_t1", "ref_t2"):
+        for i in range(NUM_INSERTS):
+            node.query(
+                "INSERT INTO {} SELECT number + {off}, toString(number + {off}) "
+                "FROM numbers({rows})".format(t, off=i * NUM_ROWS, rows=NUM_ROWS)
+            )
+
+    assert int(node.query("SELECT count() FROM ref_t1")) == NUM_INSERTS * NUM_ROWS
+    assert int(node.query("SELECT count() FROM ref_t2")) == NUM_INSERTS * NUM_ROWS
+
+    # The snapshot+log ref format is actually in use: immutable ref objects exist under cas/ns/stream/.
+    assert _count(REFS_PREFIX) > 0, "expected ref log/snapshot objects under cas/ns/stream/"
+    assert (
+        _content_objects() > content_baseline
+    ), "expected content objects to rise above baseline after inserts"
+
+    # Read-only fsck agrees while data is present: no authoritative ref names a missing object.
+    live_fsck = _disks(node, "cas-fsck")
+    assert "dangling=0" in live_fsck, live_fsck
+
+    # (3) Rename one table: data must survive (its ref namespace and its logs/snapshots are unaffected).
+    node.query("RENAME TABLE ref_t1 TO ref_t1_renamed")
+    assert (
+        int(node.query("SELECT count() FROM ref_t1_renamed")) == NUM_INSERTS * NUM_ROWS
+    )
+
+    # (4) Drop both: the writer appends remove_namespace; background GC folds the -1 edges, condemns and
+    #     deletes the now-unreferenced blobs, and runs the namespace-cleanup item that reclaims the
+    #     removed namespace's physical @cas@ prefixes (part manifests + verbatim files).
+    node.query("DROP TABLE ref_t1_renamed SYNC")
+    node.query("DROP TABLE ref_t2 SYNC")
+    # The content count at the moment the refs are unlinked, so the reclamation below is measured
+    # against what was actually there to reclaim.
+    after_drop_content = _content_objects()
+
+    # (5) THE RECLAMATION: the pool's CONTENT (blobs + part manifests) drains back to baseline. Polled
+    #     with an early exit, then cross-checked against GC's own bookkeeping — a pool that shrank for
+    #     some other reason must not pass for a round that reclaimed it.
+    final = _content_objects()
+    for _ in range(RECLAIM_RETRIES):
+        if final <= content_baseline:
+            break
+        time.sleep(RECLAIM_SLEEP)
+        final = _content_objects()
+
+    assert final <= content_baseline, (
+        "the dropped namespaces' content was not reclaimed: baseline={}, "
+        "at_drop={}, final={} (blobs={}, manifests={})".format(
+            content_baseline,
+            after_drop_content,
+            final,
+            _count(BLOBS_PREFIX),
+            _count(MANIFESTS_PREFIX),
+        )
+    )
+
+    node.query("SYSTEM FLUSH LOGS")
+    rounds = int(
+        node.query(
+            "SELECT count() FROM system.cas_gc_log "
+            "WHERE event_type = 'Finish' AND outcome = 'Success'"
+        ).strip()
+    )
+    assert rounds > 0, "no successful GC round ran at all"
+
+    deleted = int(
+        node.query(
+            "SELECT sum(objects_deleted + manifests_deleted) "
+            "FROM system.cas_gc_log WHERE event_type = 'Finish'"
+        ).strip()
+        or 0
+    )
+    assert deleted > 0, "the pool's content drained but GC's own bookkeeping reports no deletion"
+
+    # (6) Read-only consumers on the DRAINED pool.
+    final_fsck = _disks(node, "cas-fsck")
+    assert "dangling=0" in final_fsck, final_fsck
+
+    #     `cas-gc-dryrun` on a fully drained pool has nothing left to preview.
+    dryrun = _disks(node, "cas-gc-dryrun")
+    assert "preview_deletes=0" in dryrun, dryrun
diff --git a/tests/integration/test_cas_replicated_relink/__init__.py b/tests/integration/test_cas_replicated_relink/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_replicated_relink/configs/server_root_id_node1.xml b/tests/integration/test_cas_replicated_relink/configs/server_root_id_node1.xml
new file mode 100644
index 000000000000..be23e47852bb
--- /dev/null
+++ b/tests/integration/test_cas_replicated_relink/configs/server_root_id_node1.xml
@@ -0,0 +1,12 @@
+<clickhouse>
+    <!-- Phase 0 (mount safety): each server owning a DISTINCT layout subtree inside the shared pool.
+         Two servers must never write the same server_root_id tree (spec §mount-safety), so this is a
+         per-instance override merged onto the shared storage_conf.xml disk definition. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <server_root_id>node1</server_root_id>
+            </disk_cas_shared>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_replicated_relink/configs/server_root_id_node2.xml b/tests/integration/test_cas_replicated_relink/configs/server_root_id_node2.xml
new file mode 100644
index 000000000000..e3b468aaa4e4
--- /dev/null
+++ b/tests/integration/test_cas_replicated_relink/configs/server_root_id_node2.xml
@@ -0,0 +1,12 @@
+<clickhouse>
+    <!-- Phase 0 (mount safety): each server owning a DISTINCT layout subtree inside the shared pool.
+         Two servers must never write the same server_root_id tree (spec §mount-safety), so this is a
+         per-instance override merged onto the shared storage_conf.xml disk definition. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <server_root_id>node2</server_root_id>
+            </disk_cas_shared>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_replicated_relink/configs/storage_conf.xml b/tests/integration/test_cas_replicated_relink/configs/storage_conf.xml
new file mode 100644
index 000000000000..0ce475f4bc80
--- /dev/null
+++ b/tests/integration/test_cas_replicated_relink/configs/storage_conf.xml
@@ -0,0 +1,36 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- A single shared bucket path. BOTH replicas load this same config, so they mount
+                     the SAME content-addressed pool (the bucket is the single source of truth). The
+                     shared pool is what makes fetch-by-relink possible: a part written by one replica
+                     is already present (as blobs + manifest) for the other to publish a ref to.
+                     RustFS (not MinIO) enforces the conditional-PUT/DELETE semantics CasProbe's
+                     fail-closed startup capability check (B31) requires: MinIO does not enforce
+                     conditional deletes, so a CA pool over MinIO refuses to mount at all. -->
+                <endpoint>http://rustfs1:11121/test/shared_pool/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <!-- Two replicas mounting one pool: mandatory, else the second mounter refuses to
+                     attach (PoolMeta ownership check). This is the N-mounter enablement (spec §6). -->
+                <!-- Background reachability GC enabled on BOTH replicas. The relink's pin-before-publish
+                     (spec §4) protects the blobs against a concurrent sweep in the fetch window. -->
+                <gc_enabled>1</gc_enabled>
+                <gc_interval_sec>1</gc_interval_sec>
+            </disk_cas_shared>
+        </disks>
+        <policies>
+            <cas_shared>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_shared</disk>
+                    </main>
+                </volumes>
+            </cas_shared>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_replicated_relink/configs/storage_conf_other_pool.xml b/tests/integration/test_cas_replicated_relink/configs/storage_conf_other_pool.xml
new file mode 100644
index 000000000000..416cbfca86ae
--- /dev/null
+++ b/tests/integration/test_cas_replicated_relink/configs/storage_conf_other_pool.xml
@@ -0,0 +1,31 @@
+<clickhouse>
+    <!-- A SECOND, independent content-addressed pool, loaded by node2 only. It exists for exactly one
+         assertion: a relink offer is gated on the receiver and the sender naming the SAME pool, so a
+         fetch into this pool from a part living in `shared_pool` must degrade to a byte transfer.
+         Only one server mounts it, so it needs no per-instance `server_root_id` override.
+         GC is off: nothing here is ever meant to be reclaimed,
+         and a second GC scheduler would only add background noise to the other tests' timing. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_other>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <endpoint>http://rustfs1:11121/test/other_pool/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <server_root_id>node2_other</server_root_id>
+                <gc_enabled>0</gc_enabled>
+            </disk_cas_other>
+        </disks>
+        <policies>
+            <cas_other>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_other</disk>
+                    </main>
+                </volumes>
+            </cas_other>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_replicated_relink/test.py b/tests/integration/test_cas_replicated_relink/test.py
new file mode 100644
index 000000000000..83d8d239ba92
--- /dev/null
+++ b/tests/integration/test_cas_replicated_relink/test.py
@@ -0,0 +1,905 @@
+import re
+import shlex
+import time
+
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+# Both replicas mount the SAME content-addressed pool (endpoint .../root/shared_pool/). A
+# ReplicatedMergeTree part written on one replica is therefore ALREADY present (as content blobs +
+# manifest) in the pool when the other replica needs it — so the "fetch" is a fetch-by-relink: the
+# fetching replica publishes its own ref to the existing blobs instead of downloading any bytes (the CA
+# analogue of zero-copy replication, spec §4).
+STORAGE_POLICY = "cas_shared"
+CA_DISK = "disk_cas_shared"
+
+# A second, independent pool mounted by node2 only (configs/storage_conf_other_pool.xml). Used for the
+# cross-pool leg of B66b: relink is gated on both sides naming the same pool, so a fetch into this one
+# must degrade to bytes.
+OTHER_STORAGE_POLICY = "cas_other"
+OTHER_CA_DISK = "disk_cas_other"
+
+# The shared pool's blob prefix inside the `test` RustFS bucket. The relink proof is that the fetch does
+# NOT create new objects under here: relink publishes a ref (per-server, under store/), never a blob.
+BLOBS_PREFIX = "shared_pool/blobs/"
+
+NUM_ROWS = 10000
+
+# ----------------------------------------------------------------------------------------------------
+# WHY EVERY RELINK ASSERTION BELOW IS A POSITIVE ONE
+#
+# "The fetch created no new blobs" is NOT by itself evidence that a relink happened. On a
+# content-addressed disk a BYTE fetch writes the very same content, which deduplicates against the
+# blobs already in the pool, so its blob-count delta is zero too. A test that only counts blobs is
+# therefore green whether the protocol worked or silently fell back — the single easiest worthless test
+# on this path.
+#
+# So each relink test asserts a signal that is reachable ONLY through the intended path:
+#
+#   RELINK RAN   -> the receiver's `Relink of part <p> onto disk <d> finished (no bytes transferred).`
+#                   That line is the last statement of `Fetcher::relinkPartToDisk` and is reachable only
+#                   after the confirm answered `yes` AND `promote()` returned `Committed` (taxonomy
+#                   row 4). Every other row returns or throws before it.
+#   BYTES RAN    -> the receiver's `Download of part <p> onto disk <d> finished.` from
+#                   `downloadPartToDisk`, plus the specific line naming WHY relink was declined.
+#
+# The blob-count / `CASBlobPut == 0` checks are kept as corroboration, never as the proof.
+# ----------------------------------------------------------------------------------------------------
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node1",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node1.xml"],
+        macros={"replica": "node1"},
+        with_rustfs=True,
+        with_zookeeper=True,
+        stay_alive=True,
+    )
+    cluster.add_instance(
+        "node2",
+        main_configs=[
+            "configs/storage_conf.xml",
+            "configs/server_root_id_node2.xml",
+            "configs/storage_conf_other_pool.xml",
+        ],
+        macros={"replica": "node2"},
+        with_rustfs=True,
+        with_zookeeper=True,
+        stay_alive=True,
+    )
+
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def blob_keys():
+    """Every object key under the shared pool's blob prefix, as a set."""
+    objects = cluster.rustfs_client.list_objects(
+        cluster.rustfs_bucket, BLOBS_PREFIX, recursive=True
+    )
+    return {obj.object_name for obj in objects}
+
+
+def count_blobs():
+    return len(blob_keys())
+
+
+def log_lines(node, pattern):
+    """Server-log lines matching an extended regular expression.
+
+    Deliberately NOT `instance.grep_in_log`: that one globs `clickhouse-server.log*`, which includes
+    `clickhouse-server.err.log`, so any warning-or-above line is counted twice. Several assertions here
+    are exact counts, and a doubled count is indistinguishable from a real second attempt.
+    """
+    out = node.exec_in_container(
+        [
+            "bash",
+            "-c",
+            "grep -a -E {} /var/log/clickhouse-server/clickhouse-server.log || true".format(
+                shlex.quote(pattern)
+            ),
+        ]
+    )
+    return [line for line in out.splitlines() if line.strip()]
+
+
+def wait_for_log_lines(node, pattern, timeout=60):
+    """Poll until at least one line matches, then return the matches. Fails loudly on timeout."""
+    deadline = time.time() + timeout
+    while True:
+        found = log_lines(node, pattern)
+        if found:
+            return found
+        assert time.time() < deadline, "timed out waiting for log lines matching {!r} on {}".format(
+            pattern, node.name
+        )
+        time.sleep(0.5)
+
+
+def relink_finished_pattern(table, part, disk=CA_DISK):
+    """The receiver-side proof that the publish→confirm→promote path completed for this exact part."""
+    return r"default\.{} .*Relink of part {} onto disk {} finished \(no bytes transferred\)".format(
+        table, re.escape(part), disk
+    )
+
+
+def download_finished_pattern(table, part, disk=CA_DISK):
+    """The receiver-side proof that the BYTE path completed for this exact part."""
+    return r"default\.{} .*Download of part {} onto disk {} finished".format(
+        table, re.escape(part), disk
+    )
+
+
+def relink_offer_pattern(table, part):
+    """The SENDER-side line, one per relink offer actually made. The attempt counter."""
+    return r"default\.{} .*Sending part {} by relink".format(table, re.escape(part))
+
+
+def assert_relinked(node, table, part, disk=CA_DISK, timeout=60):
+    wait_for_log_lines(node, relink_finished_pattern(table, part, disk), timeout=timeout)
+    assert not log_lines(node, download_finished_pattern(table, part, disk)), (
+        "part {} of {} was relinked AND byte-downloaded on {} — the relink proof is not exclusive".format(
+            part, table, node.name
+        )
+    )
+
+
+def assert_byte_downloaded(node, table, part, disk=CA_DISK, timeout=60):
+    wait_for_log_lines(node, download_finished_pattern(table, part, disk), timeout=timeout)
+    assert not log_lines(node, relink_finished_pattern(table, part, disk)), (
+        "part {} of {} was expected to arrive as bytes but a relink completed on {}".format(
+            part, table, node.name
+        )
+    )
+
+
+def assert_no_new_blobs(before_keys):
+    """Corroboration for a relink: the fetch added no object under the pool's blob prefix.
+
+    Phrased as "no NEW key" rather than "the same count" on purpose — background GC may reclaim
+    unrelated debris at any moment on this fixture (`gc_interval_sec` is 1), and a count that went DOWN
+    says nothing about whether the fetch wrote anything.
+    """
+    new_keys = sorted(blob_keys() - before_keys)
+    assert not new_keys, "the fetch wrote {} new blob(s), e.g. {}".format(len(new_keys), new_keys[:5])
+
+
+def cas_blob_puts(node):
+    return int(node.query("SELECT sum(value) FROM system.events WHERE event = 'CASBlobPut'") or 0)
+
+
+def active_part_names(node, table):
+    return node.query(
+        "SELECT name FROM system.parts WHERE database = 'default' AND table = '{}' AND active "
+        "ORDER BY name".format(table)
+    ).split()
+
+
+def any_state_part_count(node, table, part):
+    return int(
+        node.query(
+            "SELECT count() FROM system.parts WHERE database = 'default' AND table = '{}' "
+            "AND name = '{}'".format(table, part)
+        )
+    )
+
+
+def wait_until(predicate, timeout, what):
+    deadline = time.time() + timeout
+    while True:
+        if predicate():
+            return
+        assert time.time() < deadline, "timed out waiting for {}".format(what)
+        time.sleep(0.5)
+
+
+def fsck(node, disk=CA_DISK):
+    """`SYSTEM CAS FSCK <disk>` as a dict of column -> value.
+
+    Driven through `clickhouse-client --format` rather than a trailing `FORMAT` clause: `ASTSystemQuery`
+    is not an `ASTQueryWithOutput`, so `SYSTEM ... FORMAT TSVWithNames` is a syntax error. Reading the
+    header is what keeps this from depending on the column ORDER of the summary.
+    """
+    out = node.exec_in_container(
+        [
+            "bash",
+            "-c",
+            "clickhouse client --format TSVWithNames --query {}".format(
+                shlex.quote("SYSTEM CAS FSCK '{}'".format(disk))
+            ),
+        ]
+    ).splitlines()
+    header, row = out[0].split("\t"), out[1].split("\t")
+    summary = dict(zip(header, row))
+    assert "dangling" in summary, "unexpected FSCK summary shape: {}".format(out)
+    return summary
+
+
+def gc_round(node, disk=CA_DISK):
+    node.query("SYSTEM CAS GC RUN '{}'".format(disk))
+
+
+def drop_everywhere(table):
+    for node in (cluster.instances["node1"], cluster.instances["node2"]):
+        node.query("DROP TABLE IF EXISTS {} SYNC".format(table))
+
+
+def create_replicated(node, table, policy=STORAGE_POLICY, zk_path=None, extra_settings=""):
+    node.query(
+        "CREATE TABLE {table} (id Int64, v UInt64, s String) "
+        "ENGINE = ReplicatedMergeTree('{zk}', '{{replica}}') ORDER BY id "
+        "SETTINGS storage_policy = '{policy}'{extra}".format(
+            table=table,
+            zk=zk_path or "/clickhouse/tables/" + table,
+            policy=policy,
+            extra=(", " + extra_settings) if extra_settings else "",
+        )
+    )
+
+
+def insert_rows(node, table, start, rows=NUM_ROWS):
+    node.query(
+        "INSERT INTO {table} SELECT number, number * 10, toString(number) "
+        "FROM numbers({start}, {rows})".format(table=table, start=start, rows=rows)
+    )
+
+
+def test_replicated_fetch_by_relink():
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+
+    node1.query("DROP TABLE IF EXISTS r SYNC")
+    node2.query("DROP TABLE IF EXISTS r SYNC")
+
+    # Two replicas of ONE ReplicatedMergeTree table on the shared CA pool. Lifting B33 is what makes this
+    # CREATE succeed at all; the shared-pool mount is what makes the second replica start.
+    create_tpl = (
+        "CREATE TABLE r (id Int64, v UInt64, s String) "
+        "ENGINE = ReplicatedMergeTree('/clickhouse/tables/r', '{{replica}}') "
+        "ORDER BY id SETTINGS storage_policy = '{policy}'"
+    )
+    node1.query(create_tpl.format(policy=STORAGE_POLICY))
+    node2.query(create_tpl.format(policy=STORAGE_POLICY))
+
+    # (1) INSERT on replica node1. node2 must replicate the part.
+    node1.query(
+        "INSERT INTO r SELECT number, number * 10, toString(number) FROM numbers({rows})".format(
+            rows=NUM_ROWS
+        )
+    )
+
+    # Blob count after the insert, BEFORE node2 fetches. This is the relink baseline.
+    blobs_after_insert = count_blobs()
+    assert blobs_after_insert > 0, "insert must have written content blobs to the shared pool"
+
+    # (2) node2 fetches the part. SYNC REPLICA blocks until the queue (the fetch) drains.
+    node2.query("SYSTEM SYNC REPLICA r", timeout=60)
+
+    # (3) node2 reads the SAME rows back.
+    expected_sum_id = (NUM_ROWS - 1) * NUM_ROWS // 2
+    assert int(node2.query("SELECT count() FROM r")) == NUM_ROWS
+    assert int(node2.query("SELECT sum(id) FROM r")) == expected_sum_id
+    assert int(node2.query("SELECT sum(v) FROM r")) == expected_sum_id * 10
+
+    # (4) THE RELINK PROOF: the fetch created NO new blob objects. node2 published a ref to the blobs
+    #     node1 already wrote — it did not download/re-write them. (Relink, not byte download.)
+    blobs_after_fetch = count_blobs()
+    assert blobs_after_fetch == blobs_after_insert, (
+        "fetch-by-relink must not create new blob objects: had {} after insert, {} after node2 fetched "
+        "(a byte download would have re-written blobs)".format(
+            blobs_after_insert, blobs_after_fetch
+        )
+    )
+
+    # (5) A merge on node1 fetched-by-relink by node2: insert a second part on node1, OPTIMIZE to merge,
+    #     and confirm node2 picks up the merged part with still no new blobs beyond the merge's own.
+    node1.query(
+        "INSERT INTO r SELECT number, number * 10, toString(number) FROM numbers({a}, {rows})".format(
+            a=NUM_ROWS, rows=NUM_ROWS
+        )
+    )
+    node2.query("SYSTEM SYNC REPLICA r", timeout=60)
+    blobs_before_merge = count_blobs()
+
+    node1.query("OPTIMIZE TABLE r FINAL")
+    node1.query("SYSTEM SYNC REPLICA r", timeout=60)
+    blobs_after_merge_on_node1 = count_blobs()
+
+    # node2 fetches the merged part. The merge itself may write new blobs on node1 (the merged content),
+    # but node2's FETCH of that merged part must add NOTHING further (relink).
+    node2.query("SYSTEM SYNC REPLICA r", timeout=60)
+    blobs_after_merge_fetch = count_blobs()
+    assert blobs_after_merge_fetch == blobs_after_merge_on_node1, (
+        "fetch-by-relink of the merged part must not create new blobs: {} after node1 merged, {} after "
+        "node2 fetched".format(blobs_after_merge_on_node1, blobs_after_merge_fetch)
+    )
+
+    assert int(node2.query("SELECT count() FROM r")) == 2 * NUM_ROWS
+    assert int(node1.query("SELECT count() FROM r")) == 2 * NUM_ROWS
+
+    node1.query("DROP TABLE IF EXISTS r SYNC")
+    node2.query("DROP TABLE IF EXISTS r SYNC")
+
+
+def test_relink_happy_path_proof():
+    """Task 16 step 2 — the happy path, proved POSITIVELY.
+
+    Taxonomy row 4 (confirm `yes` -> `promote` -> `Committed`). The proof is the receiver's
+    `... finished (no bytes transferred)` line, which no other row can reach; `CASBlobPut == 0` and the
+    flat blob count are corroboration only (see the note at the top of this file).
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    table = "relink_happy"
+    drop_everywhere(table)
+
+    create_replicated(node1, table)
+    create_replicated(node2, table)
+
+    node2.query("SYSTEM STOP FETCHES {}".format(table))
+    insert_rows(node1, table, 0)
+    part = active_part_names(node1, table)[0]
+
+    blobs_before = blob_keys()
+    puts_before = cas_blob_puts(node2)
+
+    node2.query("SYSTEM START FETCHES {}".format(table))
+    node2.query("SYSTEM SYNC REPLICA {}".format(table), timeout=60)
+
+    # THE PROOF: reachable only after a confirm `yes` and a committed promote.
+    assert_relinked(node2, table, part)
+
+    # Corroboration, in the plan's own terms: the receiver issued no blob PUT at all, and the pool's
+    # blob set is byte-identical to what the sender's insert left behind.
+    assert cas_blob_puts(node2) == puts_before
+    assert_no_new_blobs(blobs_before)
+
+    assert int(node2.query("SELECT count() FROM {}".format(table))) == NUM_ROWS
+    assert int(node2.query("SELECT sum(v) FROM {}".format(table))) == int(
+        node1.query("SELECT sum(v) FROM {}".format(table))
+    )
+
+    drop_everywhere(table)
+
+
+def test_fetch_part_into_detached_relinks():
+    """Task 16 step 5 — B66b, manual caller #1: `ALTER TABLE ... FETCH PART ... FROM`.
+
+    Taxonomy row 4 with `to_detached=true`: the staged ref is `detached/tmp-fetch_<part>` and the
+    finalization is `renameTo(detached/<part>)`. Before B66b the relink capability was gated on
+    `!to_detached`, so this fetch could only ever be bytes.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    src, dst = "b66b_part_src", "b66b_part_dst"
+    drop_everywhere(src)
+    drop_everywhere(dst)
+
+    create_replicated(node1, src)
+    create_replicated(node2, dst)
+    insert_rows(node1, src, 0)
+    part = active_part_names(node1, src)[0]
+
+    blobs_before = blob_keys()
+    puts_before = cas_blob_puts(node2)
+
+    node2.query(
+        "ALTER TABLE {dst} FETCH PART '{part}' FROM '/clickhouse/tables/{src}'".format(
+            dst=dst, part=part, src=src
+        )
+    )
+
+    assert_relinked(node2, dst, part)
+    assert cas_blob_puts(node2) == puts_before
+    assert_no_new_blobs(blobs_before)
+
+    # ... and the detached part is a real, readable part once attached.
+    node2.query("ALTER TABLE {} ATTACH PART '{}'".format(dst, part))
+    assert int(node2.query("SELECT count() FROM {}".format(dst))) == NUM_ROWS
+    assert int(node2.query("SELECT sum(v) FROM {}".format(dst))) == int(
+        node1.query("SELECT sum(v) FROM {}".format(src))
+    )
+
+    drop_everywhere(src)
+    drop_everywhere(dst)
+
+
+def test_fetch_partition_into_detached_relinks():
+    """Task 16 step 5 — B66b, manual caller #2: `ALTER TABLE ... FETCH PARTITION ... FROM`.
+
+    Same taxonomy row as the FETCH PART leg; a separate test because it is a separate call site (it
+    fetches a whole partition through its own thread pool) and Task 15 changed both.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    src, dst = "b66b_partition_src", "b66b_partition_dst"
+    drop_everywhere(src)
+    drop_everywhere(dst)
+
+    create_replicated(node1, src)
+    create_replicated(node2, dst)
+    insert_rows(node1, src, 0)
+    part = active_part_names(node1, src)[0]
+
+    blobs_before = blob_keys()
+    puts_before = cas_blob_puts(node2)
+
+    node2.query(
+        "ALTER TABLE {dst} FETCH PARTITION ID 'all' FROM '/clickhouse/tables/{src}'".format(
+            dst=dst, src=src
+        )
+    )
+
+    assert_relinked(node2, dst, part)
+    assert cas_blob_puts(node2) == puts_before
+    assert_no_new_blobs(blobs_before)
+
+    node2.query("ALTER TABLE {} ATTACH PARTITION ID 'all'".format(dst))
+    assert int(node2.query("SELECT count() FROM {}".format(dst))) == NUM_ROWS
+
+    drop_everywhere(src)
+    drop_everywhere(dst)
+
+
+def test_detached_fetch_cross_pool_falls_back_to_bytes():
+    """Task 16 step 5 — the cross-pool leg: relink is gated on ONE pool, so this must be bytes.
+
+    Not a taxonomy row at all: the sender's pre-filter (`receiver_pool_uuid == getPoolUUID()`) declines
+    to make an offer, so the receiver never enters `relinkPartToDisk`. The positive signal is therefore
+    the byte path's own completion line plus the ABSENCE of any relink offer for this part.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    src, dst = "xpool_src", "xpool_dst"
+    drop_everywhere(src)
+    drop_everywhere(dst)
+
+    create_replicated(node1, src)
+    create_replicated(node2, dst, policy=OTHER_STORAGE_POLICY)
+    insert_rows(node1, src, 0)
+    part = active_part_names(node1, src)[0]
+
+    node2.query(
+        "ALTER TABLE {dst} FETCH PART '{part}' FROM '/clickhouse/tables/{src}'".format(
+            dst=dst, part=part, src=src
+        )
+    )
+
+    # The bytes really moved: the receiver ran `downloadPartToDisk` onto the OTHER pool's disk.
+    assert_byte_downloaded(node2, dst, part, disk=OTHER_CA_DISK)
+    # ... and the sender never offered a relink for it, which is what makes the byte path the *intended*
+    # outcome here rather than an accident of some later failure.
+    assert not log_lines(node1, relink_offer_pattern(src, part))
+
+    node2.query("ALTER TABLE {} ATTACH PART '{}'".format(dst, part))
+    assert int(node2.query("SELECT count() FROM {}".format(dst))) == NUM_ROWS
+    assert int(node2.query("SELECT sum(v) FROM {}".format(dst))) == int(
+        node1.query("SELECT sum(v) FROM {}".format(src))
+    )
+
+    drop_everywhere(src)
+    drop_everywhere(dst)
+
+
+def test_attach_partition_from_relinks_on_queue_fetch():
+    """Task 16 step 6 (RPL-5) — `ATTACH PARTITION ... FROM` replicates as `REPLACE_RANGE`.
+
+    The source table exists only on node1, so node2 cannot clone locally and its queue entry falls
+    through to `executeReplaceRange`'s `fetchSelectedPart` — a THIRD fetch call site, with its own
+    `tmp_replace_from_fetch_` prefix. Taxonomy row 4; the proof is the same relink-finished line.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    src, dst = "rpl5_attach_src", "rpl5_attach_dst"
+    drop_everywhere(src)
+    drop_everywhere(dst)
+
+    node1.query(
+        "CREATE TABLE {src} (id Int64, v UInt64, s String) ENGINE = MergeTree ORDER BY id "
+        "SETTINGS storage_policy = '{policy}'".format(src=src, policy=STORAGE_POLICY)
+    )
+    create_replicated(node1, dst)
+    create_replicated(node2, dst)
+    insert_rows(node1, src, 0)
+
+    node1.query("ALTER TABLE {dst} ATTACH PARTITION tuple() FROM {src}".format(dst=dst, src=src))
+    part = active_part_names(node1, dst)[0]
+
+    blobs_before = blob_keys()
+    puts_before = cas_blob_puts(node2)
+
+    node2.query("SYSTEM SYNC REPLICA {}".format(dst), timeout=90)
+
+    assert_relinked(node2, dst, part)
+    assert cas_blob_puts(node2) == puts_before
+    assert_no_new_blobs(blobs_before)
+    assert int(node2.query("SELECT count() FROM {}".format(dst))) == NUM_ROWS
+
+    node1.query("DROP TABLE IF EXISTS {} SYNC".format(src))
+    drop_everywhere(dst)
+
+
+def test_replace_partition_relinks_on_queue_fetch():
+    """Task 16 step 6 (RPL-5) — `REPLACE PARTITION`, i.e. the same entry with a drop range attached.
+
+    Separate from the ATTACH leg because the destination is non-empty: node2 must drop its own covering
+    part AND fetch the replacement, so the relink runs against a partition that already had a ref.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    src, dst = "rpl5_replace_src", "rpl5_replace_dst"
+    drop_everywhere(src)
+    drop_everywhere(dst)
+
+    node1.query(
+        "CREATE TABLE {src} (id Int64, v UInt64, s String) ENGINE = MergeTree ORDER BY id "
+        "SETTINGS storage_policy = '{policy}'".format(src=src, policy=STORAGE_POLICY)
+    )
+    create_replicated(node1, dst)
+    create_replicated(node2, dst)
+
+    # Destination starts non-empty and replicated, so REPLACE really replaces something.
+    insert_rows(node1, dst, 0)
+    node2.query("SYSTEM SYNC REPLICA {}".format(dst), timeout=60)
+
+    insert_rows(node1, src, 5 * NUM_ROWS)
+    node1.query("ALTER TABLE {dst} REPLACE PARTITION tuple() FROM {src}".format(dst=dst, src=src))
+    part = active_part_names(node1, dst)[0]
+
+    node2.query("SYSTEM SYNC REPLICA {}".format(dst), timeout=90)
+
+    assert_relinked(node2, dst, part)
+    assert int(node2.query("SELECT count() FROM {}".format(dst))) == NUM_ROWS
+    assert int(node2.query("SELECT sum(v) FROM {}".format(dst))) == int(
+        node1.query("SELECT sum(v) FROM {}".format(dst))
+    )
+
+    node1.query("DROP TABLE IF EXISTS {} SYNC".format(src))
+    drop_everywhere(dst)
+
+
+def interserver_request(node, target_host, params):
+    """One raw interserver request, straight at the sender's `DataPartsExchange` endpoint.
+
+    The version-mix behaviour lives on the wire and nowhere else: which protocol version the peer
+    advertises is not configurable, so the only way to exercise a NON-confirm-capable peer against this
+    build's sender is to be that peer. Returns (headers, body_size).
+    """
+    query = "&".join("{}={}".format(k, v) for k, v in params)
+    out = node.exec_in_container(
+        [
+            "bash",
+            "-c",
+            "curl -sS -o /tmp/ca_ism_body -D /tmp/ca_ism_hdr -w '%{{http_code}} %{{size_download}}' "
+            "{url} >/tmp/ca_ism_stat; cat /tmp/ca_ism_hdr; echo '--STAT--'; cat /tmp/ca_ism_stat".format(
+                url=shlex.quote("http://{}:9009/?{}".format(target_host, query))
+            ),
+        ]
+    )
+    headers, stat = out.split("--STAT--")
+    http_code, size = stat.split()
+    assert http_code == "200", "interserver request failed: {}\n{}".format(stat, headers)
+    return headers, int(size)
+
+
+def test_version_mix_legacy_peer_gets_bytes():
+    """Task 16 step 7 — version mix: a peer that does not promise to confirm is served BYTES.
+
+    This is the sender-side half of the mixed-build gate, and it is the half that is reachable without a
+    second binary: the offer is gated on `client_protocol_version >= 11` (`..._WITH_CA_CONFIRM`), so a
+    peer advertising 10 — a build that would relink WITHOUT confirming — must get the byte stream.
+    Degrading to bytes, never to an unconfirmed relink, is the whole point of moving the gate to 11.
+
+    The control request (identical, but advertising 11) is what makes the negative meaningful: it proves
+    the request is otherwise perfectly relinkable, so the absence of an offer in the v10 case is the
+    version gate and not a malformed request.
+
+    The receiver-side row-1 branch — a genuinely OLD sender that offers a relink with NO source token —
+    is NOT covered here; see the report accompanying this task.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    table = "vermix"
+    drop_everywhere(table)
+
+    create_replicated(node1, table)
+    create_replicated(node2, table)
+    insert_rows(node1, table, 0)
+    node2.query("SYSTEM SYNC REPLICA {}".format(table), timeout=60)
+    part = active_part_names(node1, table)[0]
+
+    # The pool identity as the SERVER reports it: taken from the sender's own offer line rather than
+    # re-derived from the pool metadata, so the value fed back in is exactly what `getPoolUUID` returns.
+    offers = wait_for_log_lines(node1, relink_offer_pattern(table, part))
+    pool_uuid = re.search(r"shared pool ([0-9a-f]+)\)", offers[-1]).group(1)
+
+    endpoint = "DataPartsExchange:/clickhouse/tables/{}/replicas/node1".format(table)
+    base = [
+        ("endpoint", endpoint),
+        ("part", part),
+        ("compress", "false"),
+        ("cas_pool_uuid", pool_uuid),
+    ]
+
+    # CONTROL — a confirm-capable peer: an offer, with a token, and a tiny manifest-only body.
+    headers_v11, size_v11 = interserver_request(
+        node2, "node1", base + [("client_protocol_version", "11")]
+    )
+    assert "cas_relink=part_manifest_v2" in headers_v11, headers_v11
+    assert "cas_source_token=" in headers_v11, headers_v11
+
+    # THE CASE UNDER TEST — a peer advertising the pre-confirm version: no offer, and the part's bytes.
+    headers_v10, size_v10 = interserver_request(
+        node2, "node1", base + [("client_protocol_version", "10")]
+    )
+    assert "cas_relink" not in headers_v10, headers_v10
+    assert "cas_source_token" not in headers_v10, headers_v10
+    assert "server_protocol_version=10" in headers_v10, headers_v10
+
+    # Positive proof that bytes ACTUALLY moved rather than the request merely succeeding: the v10
+    # response carries the whole part, orders of magnitude more than the manifest-only relink payload.
+    assert size_v10 > 20 * size_v11, (
+        "the v10 peer should have received the part's bytes, got {} bytes against the relink offer's "
+        "{}".format(size_v10, size_v11)
+    )
+
+    drop_everywhere(table)
+
+
+def test_recursion_brake_bounds_relink_to_one_attempt():
+    """Task 16 step 4 — the `allow_ca_relink` recursion brake.
+
+    A mechanism failure that is a property of the sender/receiver PAIR reproduces on every attempt, so
+    without the brake the byte-fetch fallback re-advertises the pool, is re-offered a relink, fails
+    again, and recurses until the stack is gone. The failpoint injects exactly that class of failure
+    (taxonomy rows 2 and 5 share this ACTION), because no configuration can produce one.
+
+    The assertion is a COUNT, not termination: exactly ONE relink offer is made for this part, and then
+    the bytes arrive. Termination alone would also hold for a brake that merely bounded the recursion at
+    some larger depth, and it would hold vacuously if the relink path were never entered at all.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    table = "brake"
+    drop_everywhere(table)
+
+    create_replicated(node1, table)
+    create_replicated(node2, table)
+
+    node2.query("SYSTEM STOP FETCHES {}".format(table))
+    insert_rows(node1, table, 0)
+    part = active_part_names(node1, table)[0]
+
+    node2.query("SYSTEM ENABLE FAILPOINT cas_relink_receiver_force_mechanism_failure")
+    try:
+        node2.query("SYSTEM START FETCHES {}".format(table))
+        node2.query("SYSTEM SYNC REPLICA {}".format(table), timeout=90)
+
+        # The receiver hit the injected failure exactly once...
+        hits = log_lines(
+            node2,
+            r"Failpoint cas_relink_receiver_force_mechanism_failure: abandoning the relink of part {}".format(
+                re.escape(part)
+            ),
+        )
+        assert len(hits) == 1, "expected exactly one relink attempt, got {}:\n{}".format(
+            len(hits), "\n".join(hits)
+        )
+
+        # ... and the SENDER, independently, made exactly one offer. This is the sharper of the two: the
+        # re-request is what would re-open the capability, and the sender is the only party that can say
+        # whether it did.
+        offers = log_lines(node1, relink_offer_pattern(table, part))
+        assert len(offers) == 1, "expected exactly one relink offer, got {}:\n{}".format(
+            len(offers), "\n".join(offers)
+        )
+
+        # And the fetch still succeeded, over the byte path.
+        assert_byte_downloaded(node2, table, part)
+        assert int(node2.query("SELECT count() FROM {}".format(table))) == NUM_ROWS
+        assert int(node2.query("SELECT sum(v) FROM {}".format(table))) == int(
+            node1.query("SELECT sum(v) FROM {}".format(table))
+        )
+    finally:
+        node2.query("SYSTEM DISABLE FAILPOINT cas_relink_receiver_force_mechanism_failure")
+
+    drop_everywhere(table)
+
+
+# Settings that make node1 drop an outdated part — and with it the CA ref the confirm asks about —
+# within a few seconds instead of the default eight minutes.
+FAST_OLD_PART_REMOVAL = (
+    "old_parts_lifetime = 1, cleanup_delay_period = 1, cleanup_delay_period_random_add = 1, "
+    "max_cleanup_delay_period = 1"
+)
+
+
+def open_publish_confirm_window(node1, node2, table, base):
+    """Drive a relink up to the paused point BETWEEN the receiver's durable `+1` and the confirm.
+
+    Returns `(part, part_blobs)`: the name of the part whose relink is now stalled, and the blob keys
+    that its insert ADDED to the pool. The delta matters — debris from earlier tests in this module may
+    still be sitting in the pool and may legitimately be reclaimed while the window is open, so only the
+    keys this part created can be asserted about. The caller MUST resume the failpoint.
+
+    `base` shifts the generated rows so this table's column data is unlike any other table's in this
+    module. Without it the content-addressed store deduplicates the insert against an earlier test's
+    identical blobs and the delta is EMPTY — which would make every blob assertion below vacuous.
+    """
+    create_replicated(node1, table, extra_settings=FAST_OLD_PART_REMOVAL)
+    create_replicated(node2, table, extra_settings=FAST_OLD_PART_REMOVAL)
+
+    node2.query("SYSTEM STOP FETCHES {}".format(table))
+    before_insert = blob_keys()
+    insert_rows(node1, table, base)
+    part = active_part_names(node1, table)[0]
+    part_blobs = blob_keys() - before_insert
+    assert part_blobs, "the insert wrote no new blob into the shared pool"
+
+    node2.query("SYSTEM ENABLE FAILPOINT cas_relink_receiver_pause_before_confirm")
+    node2.query("SYSTEM START FETCHES {}".format(table))
+    # Blocks until the fetch thread is parked inside `relinkPartToDisk`, after `prepareAdoptFromManifest`
+    # made the receiver's `+1` durable and before the confirm request is built.
+    node2.query("SYSTEM WAIT FAILPOINT cas_relink_receiver_pause_before_confirm PAUSE", timeout=120)
+    return part, part_blobs
+
+
+def merge_the_source_part_away(node1, table, part, base):
+    """While the receiver is parked: make the sender stop holding the exact binding it offered."""
+    insert_rows(node1, table, base + NUM_ROWS)
+    node1.query("OPTIMIZE TABLE {} FINAL".format(table))
+    # The confirm is answered from the sender's live state, so the test is only meaningful once the old
+    # part — and the ref naming its manifest — is really gone, not merely Outdated.
+    wait_until(
+        lambda: any_state_part_count(node1, table, part) == 0,
+        timeout=120,
+        what="node1 to drop the outdated part {}".format(part),
+    )
+
+
+def test_confirm_refuses_when_source_dropped_in_window():
+    """Task 16 step 1 — the race the confirm exists to lose safely.
+
+    Taxonomy row 3: the source cannot prove it still holds the offered manifest, so the receiver aborts
+    its durable `+1` and throws a retry-later `NETWORK_ERROR` INSTEAD of falling back to bytes. The two
+    assertions that matter are (a) the queue recovers by re-selecting — here, onto the covering part —
+    and (b) NO byte re-request ever went to the source whose state was in doubt. (b) is the entire
+    reason row 3 throws where rows 2 and 5 return `nullptr`.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    table = "race_confirm"
+    drop_everywhere(table)
+
+    try:
+        part, _ = open_publish_confirm_window(node1, node2, table, base=1_000_000)
+        merge_the_source_part_away(node1, table, part, base=1_000_000)
+    finally:
+        node2.query("SYSTEM DISABLE FAILPOINT cas_relink_receiver_pause_before_confirm")
+
+    # POSITIVE SIGNAL for row 3: the locally generated refusal, naming the source and the part.
+    wait_for_log_lines(
+        node2,
+        r"Source .* did not prove it still holds the manifest it offered for part {}".format(
+            re.escape(part)
+        ),
+        timeout=120,
+    )
+
+    # (a) the queue re-selects rather than losing the data.
+    node2.query("SYSTEM SYNC REPLICA {}".format(table), timeout=180)
+    assert int(node2.query("SELECT count() FROM {}".format(table))) == 2 * NUM_ROWS
+    assert int(node2.query("SELECT sum(v) FROM {}".format(table))) == int(
+        node1.query("SELECT sum(v) FROM {}".format(table))
+    )
+    assert active_part_names(node2, table) == active_part_names(node1, table)
+
+    # (b) the abandoned part was never re-requested as bytes from the same source, and it was never
+    #     promoted either — both would be a violation of the row-3 contract.
+    assert not log_lines(node2, download_finished_pattern(table, part)), (
+        "row 3 must not fall back to a byte re-request against the source it could not confirm"
+    )
+    assert not log_lines(node2, relink_finished_pattern(table, part))
+    assert any_state_part_count(node2, table, part) == 0
+
+    drop_everywhere(table)
+
+
+def test_stalled_publish_protects_source_blobs_and_commits_nothing():
+    """Task 16 step 3 — the codex-6 regression, which is why publish-then-confirm exists at all.
+
+    The receiver's `+1` is durable while the fetch is stalled. Across the stall the sender merges the
+    part away and GC runs to a fixpoint several times over: the offered manifest's blobs MUST survive,
+    because the stalled receiver's own binding protects them — that is what makes the later confirm a
+    meaningful question rather than a race against a sweep. And when the confirm finally answers
+    `unproven`, the stalled attempt must leave NOTHING committed.
+
+    The soundness guard is the last assertion: once the attempt is abandoned and the sender no longer
+    holds the part, GC DOES reclaim those same blobs. Without it, "the blobs survived" would also be
+    satisfied by a GC that never deletes anything.
+    """
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+    table = "codex6_stall"
+    drop_everywhere(table)
+
+    try:
+        # `part_blobs` is exactly what this part's insert added to the pool — see the helper for why it
+        # has to be the delta and not everything under the prefix.
+        part, part_blobs = open_publish_confirm_window(node1, node2, table, base=2_000_000)
+
+        merge_the_source_part_away(node1, table, part, base=2_000_000)
+
+        # Four full GC rounds on both mounters, spread well past the pool's 3-second condemn grace, so
+        # a blob that was NOT protected would have been condemned, aged out and deleted in the window.
+        for _ in range(4):
+            gc_round(node1)
+            gc_round(node2)
+            time.sleep(1.5)
+
+        missing = sorted(part_blobs - blob_keys())
+        assert not missing, (
+            "the stalled receiver's durable +1 must protect the offered manifest's blobs across GC; "
+            "{} of {} were reclaimed, e.g. {}".format(len(missing), len(part_blobs), missing[:5])
+        )
+    finally:
+        node2.query("SYSTEM DISABLE FAILPOINT cas_relink_receiver_pause_before_confirm")
+
+    wait_for_log_lines(
+        node2,
+        r"Source .* did not prove it still holds the manifest it offered for part {}".format(
+            re.escape(part)
+        ),
+        timeout=120,
+    )
+
+    # Nothing was committed by the stalled attempt.
+    assert any_state_part_count(node2, table, part) == 0
+    assert not log_lines(node2, relink_finished_pattern(table, part))
+
+    node2.query("SYSTEM SYNC REPLICA {}".format(table), timeout=180)
+    assert int(node2.query("SELECT count() FROM {}".format(table))) == 2 * NUM_ROWS
+
+    # No dangling reference anywhere in the pool, from either mounter's point of view.
+    for node in (node1, node2):
+        summary = fsck(node)
+        assert summary["dangling"] == "0", "{} fsck: {}".format(node.name, summary)
+
+    # THE SOUNDNESS GUARD, and it is what makes the survival asserted earlier mean anything: with the
+    # part gone from both replicas and the stalled attempt abandoned, its unique blobs are unreachable,
+    # so GC reclaiming them proves their survival DURING the stall was the relink pin and not GC
+    # inactivity. Without this, "the blobs were still there" would also be what a GC that never ran
+    # produces.
+    reclaimed = set()
+    for _ in range(8):
+        gc_round(node1)
+        gc_round(node2)
+        reclaimed = part_blobs - blob_keys()
+        if reclaimed == part_blobs:
+            break
+    # Pool-wide: the GC lease is held by ONE server and it need not be node1.
+    rounds = 0
+    for n in (node1, node2):
+        n.query("SYSTEM FLUSH LOGS")
+        rounds += int(
+            n.query(
+                "SELECT count() FROM system.cas_gc_log "
+                "WHERE event_type = 'Finish' AND outcome = 'Success'"
+            ).strip()
+            or 0
+        )
+    assert rounds > 0, "no successful GC round ran at all"
+    assert reclaimed, (
+        "none of the abandoned attempt's {} blob(s) were reclaimed, so their survival during the "
+        "stall does not distinguish the relink pin from an inactive GC".format(len(part_blobs))
+    )
+
+    drop_everywhere(table)
diff --git a/tests/integration/test_cas_s3/__init__.py b/tests/integration/test_cas_s3/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_s3/configs/storage_conf.xml b/tests/integration/test_cas_s3/configs/storage_conf.xml
new file mode 100644
index 000000000000..fdfa19ff5409
--- /dev/null
+++ b/tests/integration/test_cas_s3/configs/storage_conf.xml
@@ -0,0 +1,28 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <disk_cas_s3>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- Phase 0 (mount safety): required, validated layout-subtree identity. Single server. -->
+                <server_root_id>itest-content-addressed-s3</server_root_id>
+                <!-- RustFS (not MinIO): the CA mount capability probe requires enforced conditional-DELETE
+                     semantics (CasProbe step 6); MinIO OSS silently honors a mismatched-token DELETE and the
+                     fail-closed probe aborts server startup (PR#2073 CI triage). -->
+                <endpoint>http://rustfs1:11121/test/cas_data/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+            </disk_cas_s3>
+        </disks>
+        <policies>
+            <cas_s3>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_s3</disk>
+                    </main>
+                </volumes>
+            </cas_s3>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_s3/test.py b/tests/integration/test_cas_s3/test.py
new file mode 100644
index 000000000000..c5144bafb8a0
--- /dev/null
+++ b/tests/integration/test_cas_s3/test.py
@@ -0,0 +1,145 @@
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+STORAGE_POLICY = "cas_s3"
+NUM_ROWS = 1000
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    cluster.add_instance(
+        "node",
+        main_configs=["configs/storage_conf.xml"],
+        with_rustfs=True,
+        stay_alive=True,
+    )
+
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def test_cas_s3():
+    node = cluster.instances["node"]
+
+    node.query("DROP TABLE IF EXISTS cas_test SYNC")
+    node.query(
+        """
+        CREATE TABLE cas_test (
+            id Int64,
+            data String
+        ) ENGINE = MergeTree()
+        ORDER BY id
+        SETTINGS storage_policy = '{}'
+        """.format(
+            STORAGE_POLICY
+        )
+    )
+
+    # First insert of NUM_ROWS deterministic rows.
+    node.query(
+        "INSERT INTO cas_test SELECT number, toString(number) FROM numbers({})".format(
+            NUM_ROWS
+        )
+    )
+
+    expected_sum = (NUM_ROWS - 1) * NUM_ROWS // 2
+    assert int(node.query("SELECT count() FROM cas_test")) == NUM_ROWS
+    assert int(node.query("SELECT sum(id) FROM cas_test")) == expected_sum
+
+    # A second identical insert: the row count doubles. Each part's content is identical, so the
+    # content-addressed disk deduplicates the blobs, but the logical row count must still double.
+    node.query(
+        "INSERT INTO cas_test SELECT number, toString(number) FROM numbers({})".format(
+            NUM_ROWS
+        )
+    )
+    assert int(node.query("SELECT count() FROM cas_test")) == 2 * NUM_ROWS
+    assert int(node.query("SELECT sum(id) FROM cas_test")) == 2 * expected_sum
+
+    # Merge the two parts together.
+    node.query("OPTIMIZE TABLE cas_test FINAL")
+    assert int(node.query("SELECT count() FROM cas_test")) == 2 * NUM_ROWS
+    assert int(node.query("SELECT sum(id) FROM cas_test")) == 2 * expected_sum
+
+    # Persistence: after a restart the refs/footers/blobs in S3 must still resolve the data.
+    node.restart_clickhouse()
+
+    assert int(node.query("SELECT count() FROM cas_test")) == 2 * NUM_ROWS
+    assert int(node.query("SELECT sum(id) FROM cas_test")) == 2 * expected_sum
+
+    # Drop must complete without error (ref unlink + deferred GC).
+    node.query("DROP TABLE cas_test SYNC")
+    assert (
+        node.query(
+            "SELECT count() FROM system.tables WHERE database = currentDatabase() AND name = 'cas_test'"
+        ).strip()
+        == "0"
+    )
+
+
+def test_mutations_and_patch_parts_survive_restart():
+    # A mutated part and a patch part are ordinary content-addressed parts published as refs. After a
+    # restart the active set must be rediscovered from the refs in S3, so the post-mutation /
+    # post-lightweight-delete state must survive (CAS M7).
+    node = cluster.instances["node"]
+
+    node.query("DROP TABLE IF EXISTS cas_mut SYNC")
+    node.query(
+        """
+        CREATE TABLE cas_mut (
+            id Int64,
+            v UInt64,
+            s String
+        ) ENGINE = MergeTree()
+        ORDER BY id
+        SETTINGS storage_policy = '{}', enable_block_number_column = 1, enable_block_offset_column = 1
+        """.format(
+            STORAGE_POLICY
+        )
+    )
+
+    node.query(
+        "INSERT INTO cas_mut SELECT number, number * 10, toString(number) FROM numbers({})".format(
+            NUM_ROWS
+        )
+    )
+
+    # Heavy mutation: UPDATE one column (id/s carry forward by reference on the content-addressed disk).
+    node.query(
+        "ALTER TABLE cas_mut UPDATE v = v + 1 WHERE id % 2 = 0 SETTINGS mutations_sync = 2"
+    )
+    # Heavy mutation: DELETE.
+    node.query("ALTER TABLE cas_mut DELETE WHERE id % 100 = 0 SETTINGS mutations_sync = 2")
+    # Data-ALTER (column type change). Via a storage policy there is no inline-disk CustomType in
+    # settings_changes, so this works on the content-addressed disk (see backlog B53).
+    node.query("ALTER TABLE cas_mut MODIFY COLUMN v Int64 SETTINGS mutations_sync = 2")
+    # Patch part: a forced lightweight-update DELETE (throws if unsupported, so success == patch path).
+    node.query(
+        "DELETE FROM cas_mut WHERE id % 7 = 0 "
+        "SETTINGS enable_lightweight_update = 1, lightweight_delete_mode = 'lightweight_update_force', lightweight_deletes_sync = 2"
+    )
+
+    count_before = int(node.query("SELECT count() FROM cas_mut"))
+    sum_before = int(node.query("SELECT sum(v) FROM cas_mut"))
+    digest_before = node.query("SELECT sum(cityHash64(id, v, s)) FROM cas_mut").strip()
+
+    # Persistence: rediscover the active set (incl. the mutated and patch parts) from S3 refs.
+    node.restart_clickhouse()
+
+    assert int(node.query("SELECT count() FROM cas_mut")) == count_before
+    assert int(node.query("SELECT sum(v) FROM cas_mut")) == sum_before
+    assert node.query("SELECT sum(cityHash64(id, v, s)) FROM cas_mut").strip() == digest_before
+
+    node.query("DROP TABLE cas_mut SYNC")
+    assert (
+        node.query(
+            "SELECT count() FROM system.tables WHERE database = currentDatabase() AND name = 'cas_mut'"
+        ).strip()
+        == "0"
+    )
diff --git a/tests/integration/test_cas_shared_pool/__init__.py b/tests/integration/test_cas_shared_pool/__init__.py
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/tests/integration/test_cas_shared_pool/configs/server_root_id_node1.xml b/tests/integration/test_cas_shared_pool/configs/server_root_id_node1.xml
new file mode 100644
index 000000000000..be23e47852bb
--- /dev/null
+++ b/tests/integration/test_cas_shared_pool/configs/server_root_id_node1.xml
@@ -0,0 +1,12 @@
+<clickhouse>
+    <!-- Phase 0 (mount safety): each server owning a DISTINCT layout subtree inside the shared pool.
+         Two servers must never write the same server_root_id tree (spec §mount-safety), so this is a
+         per-instance override merged onto the shared storage_conf.xml disk definition. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <server_root_id>node1</server_root_id>
+            </disk_cas_shared>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_shared_pool/configs/server_root_id_node2.xml b/tests/integration/test_cas_shared_pool/configs/server_root_id_node2.xml
new file mode 100644
index 000000000000..e3b468aaa4e4
--- /dev/null
+++ b/tests/integration/test_cas_shared_pool/configs/server_root_id_node2.xml
@@ -0,0 +1,12 @@
+<clickhouse>
+    <!-- Phase 0 (mount safety): each server owning a DISTINCT layout subtree inside the shared pool.
+         Two servers must never write the same server_root_id tree (spec §mount-safety), so this is a
+         per-instance override merged onto the shared storage_conf.xml disk definition. -->
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <server_root_id>node2</server_root_id>
+            </disk_cas_shared>
+        </disks>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_shared_pool/configs/storage_conf.xml b/tests/integration/test_cas_shared_pool/configs/storage_conf.xml
new file mode 100644
index 000000000000..1620ff6d0797
--- /dev/null
+++ b/tests/integration/test_cas_shared_pool/configs/storage_conf.xml
@@ -0,0 +1,36 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <disk_cas_shared>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- A single shared bucket path. BOTH servers load this same config, so they mount
+                     the SAME pool (the bucket is the single source of truth; no Keeper). -->
+                <!-- RustFS (not MinIO): the CA mount capability probe requires enforced conditional-DELETE
+                     semantics (CasProbe step 6); MinIO OSS silently honors a mismatched-token DELETE and the
+                     fail-closed probe aborts server startup (PR#2073 CI triage). -->
+                <endpoint>http://rustfs1:11121/test/shared_pool/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <!-- Two servers mounting one pool: this acknowledgement is mandatory; without it the
+                     second mounter refuses to attach (see PoolMeta ownership check). -->
+                <!-- Background reachability GC enabled on BOTH servers, running aggressively so a
+                     sweep happens during the test. The grace window must be short enough to reclaim
+                     within the test, but the sweep must NOT reclaim a blob the OTHER server's live
+                     part still references. -->
+                <gc_enabled>1</gc_enabled>
+                <gc_interval_sec>1</gc_interval_sec>
+            </disk_cas_shared>
+        </disks>
+        <policies>
+            <cas_shared>
+                <volumes>
+                    <main>
+                        <disk>disk_cas_shared</disk>
+                    </main>
+                </volumes>
+            </cas_shared>
+        </policies>
+    </storage_configuration>
+</clickhouse>
diff --git a/tests/integration/test_cas_shared_pool/test.py b/tests/integration/test_cas_shared_pool/test.py
new file mode 100644
index 000000000000..5becc8dc049a
--- /dev/null
+++ b/tests/integration/test_cas_shared_pool/test.py
@@ -0,0 +1,347 @@
+import time
+
+import pytest
+
+from helpers.cluster import ClickHouseCluster
+
+cluster = ClickHouseCluster(__file__)
+
+# Both servers mount the SAME content-addressed pool (endpoint .../root/shared_pool/). The blob pool
+# (blobs/ + parts/) is shared across servers; refs are per-server under store/<server_id>/..., so the
+# two servers dedup identical content while keeping independent ref roots.
+STORAGE_POLICY = "cas_shared"
+
+# blobs/ holds content blobs, parts/ holds part footers. These are the shared pool's object prefixes
+# inside the `root` MinIO bucket. "No leftovers" means BOTH drain back to baseline.
+BLOBS_PREFIX = "shared_pool/blobs/"
+PARTS_PREFIX = "shared_pool/parts/"
+
+# Deterministic data. Identical rows on both nodes => identical content blobs => cross-server dedup.
+NUM_ROWS = 100000
+
+# Background GC: grace=3s, interval=1s. After both DROP ... SYNC the pool's objects become
+# unreferenced and a sweep (run by either server) reclaims them after grace. Bounded poll: this waits
+# on a known background process, it is not papering over a race.
+RECLAIM_RETRIES = 60
+RECLAIM_SLEEP = 1.0  # seconds; total bound ~= 60s
+
+
+@pytest.fixture(scope="module", autouse=True)
+def start_cluster():
+    # RustFS (not MinIO) backs the pool: the CA mount capability probe requires enforced
+    # conditional-DELETE semantics, which MinIO OSS lacks — the fail-closed probe aborted server
+    # startup there (PR#2073 CI triage). Both instances reach the shared rustfs1 and load the
+    # identical storage_conf.xml, so both mount the SAME shared pool.
+    cluster.add_instance(
+        "node1",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node1.xml"],
+        with_rustfs=True,
+        stay_alive=True,
+    )
+    cluster.add_instance(
+        "node2",
+        main_configs=["configs/storage_conf.xml", "configs/server_root_id_node2.xml"],
+        with_rustfs=True,
+        stay_alive=True,
+    )
+
+    try:
+        cluster.start()
+        yield cluster
+    finally:
+        cluster.shutdown()
+
+
+def count_prefix(prefix):
+    objects = cluster.rustfs_client.list_objects(
+        cluster.rustfs_bucket, prefix, recursive=True
+    )
+    return len(list(objects))
+
+
+
+def _gc_bookkeeping(*nodes):
+    """Pool-wide (successful rounds, objects deleted) from the CA GC log. The GC lease is held by ONE
+    server per pool and which one is not fixed, so both must be asked."""
+    rounds = deleted = 0
+    for n in nodes:
+        n.query("SYSTEM FLUSH LOGS")
+        rounds += int(
+            n.query(
+                "SELECT count() FROM system.cas_gc_log "
+                "WHERE event_type = 'Finish' AND outcome = 'Success'"
+            ).strip()
+            or 0
+        )
+        deleted += int(
+            n.query(
+                "SELECT sum(objects_deleted + manifests_deleted + entries_redeleted) "
+                "FROM system.cas_gc_log WHERE event_type = 'Finish'"
+            ).strip()
+            or 0
+        )
+    return rounds, deleted
+
+def count_pool_objects():
+    # The shared pool is empty only when BOTH content blobs and part footers are gone.
+    return count_prefix(BLOBS_PREFIX) + count_prefix(PARTS_PREFIX)
+
+
+def test_two_servers_share_one_pool():
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+
+    node1.query("DROP TABLE IF EXISTS t1 SYNC")
+    node2.query("DROP TABLE IF EXISTS t2 SYNC")
+
+    # (0) Baseline pool object count before either table exists.
+    baseline = count_pool_objects()
+
+    # (1) Each server creates its OWN MergeTree table on the shared pool. Distinct names => distinct
+    #     table UUIDs => independent per-server refs, but the SAME shared blob pool.
+    create_tpl = (
+        "CREATE TABLE {tbl} (id Int64, v UInt64, s String) "
+        "ENGINE = MergeTree() ORDER BY id "
+        "SETTINGS storage_policy = '{policy}'"
+    )
+    node1.query(create_tpl.format(tbl="t1", policy=STORAGE_POLICY))
+    node2.query(create_tpl.format(tbl="t2", policy=STORAGE_POLICY))
+
+    # (2) INSERT IDENTICAL deterministic data into both. The content blobs are byte-identical, so the
+    #     shared pool dedups them across the two servers. Logical reads must still be correct on each.
+    insert_tpl = (
+        "INSERT INTO {tbl} "
+        "SELECT number, number * 10, toString(number) FROM numbers({rows})"
+    )
+    node1.query(insert_tpl.format(tbl="t1", rows=NUM_ROWS))
+    node2.query(insert_tpl.format(tbl="t2", rows=NUM_ROWS))
+
+    expected_sum_id = (NUM_ROWS - 1) * NUM_ROWS // 2
+    assert int(node1.query("SELECT count() FROM t1")) == NUM_ROWS
+    assert int(node2.query("SELECT count() FROM t2")) == NUM_ROWS
+    assert int(node1.query("SELECT sum(id) FROM t1")) == expected_sum_id
+    assert int(node2.query("SELECT sum(id) FROM t2")) == expected_sum_id
+
+    # Cross-server dedup sanity: the two identical single-part inserts must NOT have doubled the pool's
+    # blob count. With dedup the blob count after both inserts is well below twice the per-server count.
+    after_insert = count_pool_objects()
+    assert after_insert > baseline, (
+        "expected pool object count to rise above baseline {} after inserts, got {}".format(
+            baseline, after_insert
+        )
+    )
+
+    # (3) Heavy mutations / merges on EACH server, in parallel ownership of the shared pool.
+    #     UPDATE (id/s carry forward by reference), DELETE, then OPTIMIZE FINAL.
+    node1.query("ALTER TABLE t1 UPDATE v = v + 1 WHERE id % 2 = 0 SETTINGS mutations_sync = 2")
+    node2.query("ALTER TABLE t2 UPDATE v = v + 1 WHERE id % 2 = 0 SETTINGS mutations_sync = 2")
+
+    node1.query("ALTER TABLE t1 DELETE WHERE id % 100 = 0 SETTINGS mutations_sync = 2")
+    node2.query("ALTER TABLE t2 DELETE WHERE id % 100 = 0 SETTINGS mutations_sync = 2")
+
+    node1.query("OPTIMIZE TABLE t1 FINAL")
+    node2.query("OPTIMIZE TABLE t2 FINAL")
+
+    # Post-mutation expected aggregates (identical recipe on both, so both must match).
+    count_after_mut = int(node1.query("SELECT count() FROM t1"))
+    sum_after_mut = int(node1.query("SELECT sum(v) FROM t1"))
+    digest_after_mut = node1.query("SELECT sum(cityHash64(id, v, s)) FROM t1").strip()
+
+    assert int(node2.query("SELECT count() FROM t2")) == count_after_mut
+    assert int(node2.query("SELECT sum(v) FROM t2")) == sum_after_mut
+    assert node2.query("SELECT sum(cityHash64(id, v, s)) FROM t2").strip() == digest_after_mut
+
+    # (4) Let the background GC (enabled on BOTH servers, short grace) run several sweep cycles while
+    #     both tables are still live. The cross-server safety property: a sweep run by either server
+    #     must NOT reclaim a blob that the OTHER server's live part references (deduped/shared blob).
+    #     Sleeping here is waiting on the known background sweep cadence, not a race workaround.
+    time.sleep(3 * RECLAIM_SLEEP + 3)  # > grace(3s) + a few interval(1s) cycles
+
+    # Re-read on BOTH servers: no data lost to the other server's GC.
+    assert int(node1.query("SELECT count() FROM t1")) == count_after_mut
+    assert int(node1.query("SELECT sum(v) FROM t1")) == sum_after_mut
+    assert node1.query("SELECT sum(cityHash64(id, v, s)) FROM t1").strip() == digest_after_mut
+
+    assert int(node2.query("SELECT count() FROM t2")) == count_after_mut
+    assert int(node2.query("SELECT sum(v) FROM t2")) == sum_after_mut
+    assert node2.query("SELECT sum(cityHash64(id, v, s)) FROM t2").strip() == digest_after_mut
+
+    # (5) Both servers drop their tables. Refs are unlinked synchronously; the shared pool's blobs and
+    #     footers become unreferenced GC fodder. Then poll until the pool drains back to baseline.
+    node1.query("DROP TABLE t1 SYNC")
+    node2.query("DROP TABLE t2 SYNC")
+
+    at_drop = count_pool_objects()
+
+    # THE RECLAMATION: both servers' content goes. Polled with an early exit, then cross-checked
+    # against GC's own bookkeeping so a pool that shrank for some other reason cannot pass for a round
+    # that reclaimed it.
+    final = count_pool_objects()
+    for _ in range(RECLAIM_RETRIES):
+        if final <= baseline:
+            break
+        time.sleep(RECLAIM_SLEEP)
+        final = count_pool_objects()
+
+    assert final <= baseline, (
+        "the shared pool did not drain after both servers dropped: "
+        "baseline={}, after_insert={}, at_drop={}, final={} (blobs={}, parts={})".format(
+            baseline,
+            after_insert,
+            at_drop,
+            final,
+            count_prefix(BLOBS_PREFIX),
+            count_prefix(PARTS_PREFIX),
+        )
+    )
+
+    # Counted POOL-WIDE: exactly one server holds the GC lease for a shared pool, and it need not be
+    # node1 — asking only node1 yields 0 rounds whenever node2 is the leader, which is how this
+    # assertion first failed.
+    rounds, deleted = _gc_bookkeeping(node1, node2)
+    assert rounds > 0, "no successful GC round ran at all"
+    assert deleted > 0, "the shared pool drained but GC's own bookkeeping reports no deletion"
+
+
+# Crash-resilience uses a SMALLER, DISTINCT dataset per node. Distinct content => node1's blobs are
+# NOT deduped with node2's, so "node2's GC must not reclaim node1's blobs while node1 is down" is a
+# real, observable invariant on the pool object count (node1's blobs cannot hide behind node2's).
+CRASH_ROWS = 50000
+
+
+def test_pool_survives_node_crash():
+    # Proves the bucket is self-describing and the pool survives a hard node crash:
+    #   (a) the surviving node keeps running with background GC on and loses no data;
+    #   (b) the hard-killed node recovers its data on restart (refs are durable in the bucket);
+    #   (c) any orphaned write-session the crash left behind is eventually reclaimed (its lease
+    #       expires; the pool drains to baseline after DROP).
+    # Lock-fencing safety (paused GC leader fenced by a peer's higher fence token) is covered by the
+    # gtest SweepStopsWhenLeadershipLost; this test focuses on crash-resilience.
+    node1 = cluster.instances["node1"]
+    node2 = cluster.instances["node2"]
+
+    node1.query("DROP TABLE IF EXISTS crash1 SYNC")
+    node2.query("DROP TABLE IF EXISTS crash2 SYNC")
+
+    # (0) Baseline pool object count before either table exists.
+    baseline = count_pool_objects()
+
+    create_tpl = (
+        "CREATE TABLE {tbl} (id Int64, v UInt64, s String) "
+        "ENGINE = MergeTree() ORDER BY id "
+        "SETTINGS storage_policy = '{policy}'"
+    )
+    node1.query(create_tpl.format(tbl="crash1", policy=STORAGE_POLICY))
+    node2.query(create_tpl.format(tbl="crash2", policy=STORAGE_POLICY))
+
+    # (1) DISTINCT deterministic data per node (different `v` recipe => different content blobs, so
+    #     node1's blobs are NOT shared with node2's and cannot be hidden behind dedup).
+    node1.query(
+        "INSERT INTO crash1 SELECT number, number * 10, toString(number) "
+        "FROM numbers({rows})".format(rows=CRASH_ROWS)
+    )
+    node2.query(
+        "INSERT INTO crash2 SELECT number, number * 7, concat('n2_', toString(number)) "
+        "FROM numbers({rows})".format(rows=CRASH_ROWS)
+    )
+
+    # Capture node1's authoritative aggregates BEFORE the crash; recovery must reproduce them exactly.
+    n1_count = int(node1.query("SELECT count() FROM crash1"))
+    n1_sum_id = int(node1.query("SELECT sum(id) FROM crash1"))
+    n1_digest = node1.query("SELECT sum(cityHash64(id, v, s)) FROM crash1").strip()
+    assert n1_count == CRASH_ROWS
+
+    n2_count = int(node2.query("SELECT count() FROM crash2"))
+    n2_digest = node2.query("SELECT sum(cityHash64(id, v, s)) FROM crash2").strip()
+    assert n2_count == CRASH_ROWS
+
+    # The pool now holds BOTH nodes' (distinct) blobs. Remember this high-water mark: after node1 is
+    # killed, node2's GC must NOT shrink the pool below the level needed to hold node1's blobs.
+    after_both_inserts = count_pool_objects()
+    assert after_both_inserts > baseline
+
+    # (2) HARD-KILL node1 (SIGKILL via pkill -9 => simulated crash). node2 stays up. A crash mid-flight
+    #     can leave node1 holding an unreleased write-session lease (an orphaned pin) on the pool.
+    node1.stop_clickhouse(kill=True)
+
+    # (3) With node1 down, keep node2 working AND let node2's background GC run several sweep cycles.
+    #     Two invariants:
+    #       - node2 reads its OWN data correctly (no loss while it owns the pool alone);
+    #       - node2's GC does NOT reclaim node1's blobs: node1's refs are durable roots in the bucket
+    #         even though node1's process is gone. Sleeping here waits on the known sweep cadence.
+    node2.query(
+        "INSERT INTO crash2 SELECT number, number * 7, concat('n2b_', toString(number)) "
+        "FROM numbers({rows})".format(rows=CRASH_ROWS)
+    )
+    node2.query("OPTIMIZE TABLE crash2 FINAL")
+    n2_count_after = int(node2.query("SELECT count() FROM crash2"))
+    assert n2_count_after == 2 * CRASH_ROWS
+
+    time.sleep(3 * RECLAIM_SLEEP + 3)  # > grace(3s) + a few interval(1s) cycles of node2's GC
+
+    # node2 lost nothing.
+    assert int(node2.query("SELECT count() FROM crash2")) == n2_count_after
+    # node1's blobs were NOT swept by node2's GC: the pool still holds at least node1's portion. node1
+    # contributed (after_both_inserts - baseline) objects on top of the empty baseline, so even if
+    # node2 had reclaimed every one of its own blobs the pool could not have dropped below that.
+    node1_contribution = after_both_inserts - baseline
+    pool_with_node1_down = count_pool_objects()
+    assert pool_with_node1_down >= baseline + node1_contribution, (
+        "node2's GC appears to have reclaimed node1's durable refs while node1 was down: "
+        "baseline={}, after_both_inserts={}, node1_contribution={}, pool_now={}".format(
+            baseline, after_both_inserts, node1_contribution, pool_with_node1_down
+        )
+    )
+
+    # (4) RESTART node1. The bucket is self-describing: node1 rebuilds its active set from the durable
+    #     refs and must re-read its table with the EXACT pre-crash count/sum/digest. After a hard kill
+    #     the harness reconnects on start_clickhouse via wait_start; use the instance object fresh.
+    #     150s, not the 60s default: a post-SIGKILL restart legitimately pays the unclean-reclaim
+    #     cost before serving — the stale-token observation window over its own unexpired lease
+    #     (~TTL + 5% + renew_period/2 ≈ 36.5s with defaults) plus the materialization grace
+    #     (30s default) plus the lease re-write; ~71s observed end-to-end. A bounded wait on a
+    #     known, by-design recovery protocol — not a race hack.
+    node1.start_clickhouse(start_wait_sec=150)
+
+    assert int(node1.query("SELECT count() FROM crash1")) == n1_count
+    assert int(node1.query("SELECT sum(id) FROM crash1")) == n1_sum_id
+    assert node1.query("SELECT sum(cityHash64(id, v, s)) FROM crash1").strip() == n1_digest
+
+    # node2 still consistent after node1 rejoined.
+    assert int(node2.query("SELECT count() FROM crash2")) == n2_count_after
+
+    # (5) DROP both tables. Refs are unlinked synchronously; the orphaned write-session that node1's
+    #     crash left behind no longer pins anything once its lease expires, so GC (run by either
+    #     server) reclaims the lot. Bounded-poll the pool until it drains back to baseline.
+    node1.query("DROP TABLE crash1 SYNC")
+    node2.query("DROP TABLE crash2 SYNC")
+
+    at_drop = count_pool_objects()
+
+    # THE RECLAMATION, and the point of this test: the hard kill left nothing behind that survives the
+    # drop. Once both tables are gone and the orphaned write-session's lease expires, nothing pins the
+    # content and the pool returns to baseline.
+    final = count_pool_objects()
+    for _ in range(RECLAIM_RETRIES):
+        if final <= baseline:
+            break
+        time.sleep(RECLAIM_SLEEP)
+        final = count_pool_objects()
+
+    assert final <= baseline, (
+        "the shared pool did not drain after the crash + both DROPs: "
+        "baseline={}, after_both_inserts={}, at_drop={}, "
+        "final={} (blobs={}, parts={})".format(
+            baseline,
+            after_both_inserts,
+            at_drop,
+            final,
+            count_prefix(BLOBS_PREFIX),
+            count_prefix(PARTS_PREFIX),
+        )
+    )
+
+    # Counted POOL-WIDE, for the same leader-may-be-either-node reason as the first test in this file.
+    rounds, deleted = _gc_bookkeeping(node1, node2)
+    assert rounds > 0, "no successful GC round ran at all"
+    assert deleted > 0, "the shared pool drained but GC's own bookkeeping reports no deletion"

From 8ad9b861f491407ced44767ca835c99c3b79330a Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:41 +0200
Subject: [PATCH 28/30] CAS CI wiring and test tags

CA-default stateless/integration lanes in praktika and workflows, the
content-addressed default-disk test configs, clickhouse-test support, and
tag edits to pre-existing tests (no-content-addressed-storage and
friends).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 .github/workflows/master.yml                  | 522 +++++++++++++++++-
 .github/workflows/pull_request.yml            | 502 ++++++++++++++++-
 .github/workflows/pull_request_community.yml  | 480 ++++++++++++++++
 .github/workflows/release_builds.yml          | 158 +++++-
 ci/defs/altinity_jobs.py                      |  51 ++
 ci/jobs/functional_tests.py                   |  20 +-
 ci/jobs/scripts/check_style/various_checks.sh |  26 +
 ci/jobs/scripts/clickhouse_proc.py            | 104 +++-
 ci/workflows/backport_branches.py             |   8 +-
 ci/workflows/master.py                        |   7 +-
 ci/workflows/pull_request.py                  |  13 +-
 ci/workflows/pull_request_community.py        |  12 +-
 ci/workflows/release_branches.py              |   8 +-
 ci/workflows/release_builds.py                |   7 +-
 tests/clickhouse-test                         |  42 +-
 ...orage_policy_for_merge_tree_by_default.xml |  66 +++
 ...orage_policy_for_merge_tree_by_default.xml |  35 ++
 tests/config/install.sh                       |  16 +
 .../01271_show_privileges.reference           |   7 +
 .../0_stateless/02253_empty_part_checksums.sh |   3 +-
 .../02254_projection_broken_part.sh           |   3 +-
 .../02255_broken_parts_chain_on_start.sh      |   3 +-
 .../02369_lost_part_intersecting_merges.sh    |   3 +-
 .../02370_lost_part_intersecting_merges.sh    |   3 +-
 ...2444_async_broken_outdated_part_loading.sh |   3 +-
 .../02486_truncate_and_unexpected_parts.sql   |   1 -
 .../02980_s3_plain_DROP_TABLE_MergeTree.sh    |   5 +-
 ...s3_plain_DROP_TABLE_ReplicatedMergeTree.sh |   3 +-
 ...lter_table_fetch_partition_thread_pool.sql |   1 +
 .../03352_allow_suspicious_ttl.sql            |   2 +-
 .../0_stateless/03541_rename_column_start.sql |   2 +-
 ...03829_insert_deduplication_info_memory.sql |  10 +-
 ...eplicated_missing_covered_part_on_start.sh |   3 +-
 .../04316_reader_executor_basic.sql           |   7 +-
 .../04327_reader_executor_metrics.sql         |   6 +-
 ...04328_reader_executor_kpi_async_metric.sql |   6 +-
 36 files changed, 2109 insertions(+), 39 deletions(-)
 create mode 100644 tests/config/config.d/cas_s3_storage_policy_for_merge_tree_by_default.xml
 create mode 100644 tests/config/config.d/cas_storage_policy_for_merge_tree_by_default.xml

diff --git a/.github/workflows/master.yml b/.github/workflows/master.yml
index 94c61cb6e696..f618bc8663c5 100644
--- a/.github/workflows/master.yml
+++ b/.github/workflows/master.yml
@@ -2860,6 +2860,516 @@ jobs:
           . ./ci/tmp/praktika_setup_env.sh
           PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (arm_binary, sequential)' --workflow "MasterCI" --ci --timestamp
 
+  stateless_tests_amd_binary_cas_s3_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_binary, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYmluYXJ5LCBjYXMgczMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (amd_binary, cas s3 storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_binary, cas s3 storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_BINARY_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_BINARY_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_binary, cas s3 storage, parallel)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_1_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_asan_ubsan, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYXNhbl91YnNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzIp') }}
+    name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_ASAN_UBSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_ASAN_UBSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_2_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_asan_ubsan, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYXNhbl91YnNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzIp') }}
+    name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_ASAN_UBSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_ASAN_UBSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_amd_tsan_cas_s3_storage_parallel_1_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester, 16c]
+    needs: [build_amd_tsan, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfdHNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzIp') }}
+    name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_TSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_TSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_amd_tsan_cas_s3_storage_parallel_2_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester, 16c]
+    needs: [build_amd_tsan, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfdHNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzIp') }}
+    name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_TSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_TSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_1_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_msan, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_MSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_2_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_msan, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_MSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_3_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_msan, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAzLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_MSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_arm_binary_cas_s3_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester-aarch64, 16c]
+    needs: [build_arm_binary, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhcm1fYmluYXJ5LCBjYXMgczMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (arm_binary, cas s3 storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (arm_binary, cas s3 storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_ARM_BIN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_ARM_BIN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (arm_binary, cas s3 storage, parallel)' --workflow "MasterCI" --ci --timestamp
+
+  stateless_tests_amd_binary_cas_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_binary, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYmluYXJ5LCBjYXMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (amd_binary, cas storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_binary, cas storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_BINARY_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_BINARY_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_binary, cas storage, parallel)' --workflow "MasterCI" --ci --timestamp
+
   stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_1_8:
     runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
     needs: [build_amd_llvm_coverage_per_test, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
@@ -5995,7 +6505,7 @@ jobs:
 
   finish_workflow:
     runs-on: [self-hosted, altinity-on-demand, altinity-style-checker]
-    needs: [ast_fuzzer_amd_debug, ast_fuzzer_amd_msan, ast_fuzzer_amd_tsan, ast_fuzzer_arm_asan_ubsan, build_amd_asan_ubsan, build_amd_binary, build_amd_debug, build_amd_llvm_coverage_per_test, build_amd_msan, build_amd_release, build_amd_tsan, build_arm_asan_ubsan, build_arm_binary, build_arm_debug, build_arm_msan, build_arm_release, build_arm_tsan, build_arm_ubsan, buzzhouse_amd_debug, buzzhouse_amd_msan, buzzhouse_amd_tsan, buzzhouse_arm_asan_ubsan, clickbench_amd_release, clickbench_arm_release, compatibility_check_amd_release, compatibility_check_arm_release, config_workflow, docker_keeper_image, docker_server_image, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, install_packages_amd_release, install_packages_arm_release, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_1_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_2_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_3_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_4_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_5_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_6_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_7_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_8_8, integration_tests_amd_msan_10_10, integration_tests_amd_msan_1_10, integration_tests_amd_msan_2_10, integration_tests_amd_msan_3_10, integration_tests_amd_msan_4_10, integration_tests_amd_msan_5_10, integration_tests_amd_msan_6_10, integration_tests_amd_msan_7_10, integration_tests_amd_msan_8_10, integration_tests_amd_msan_9_10, integration_tests_amd_tsan_1_6, integration_tests_amd_tsan_2_6, integration_tests_amd_tsan_3_6, integration_tests_amd_tsan_4_6, integration_tests_amd_tsan_5_6, integration_tests_amd_tsan_6_6, integration_tests_arm_binary_distributed_plan_1_4, integration_tests_arm_binary_distributed_plan_2_4, integration_tests_arm_binary_distributed_plan_3_4, integration_tests_arm_binary_distributed_plan_4_4, sign_release_amd_release, sign_release_arm_release, source_upload, sqllogic_test, sqlstorm_test, sqltest, stateless_tests_amd_asan_ubsan_db_disk_distributed_plan_sequential_1_2, stateless_tests_amd_asan_ubsan_db_disk_distributed_plan_sequential_2_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_distributed_plan_s3_storage_parallel, stateless_tests_amd_debug_distributed_plan_s3_storage_sequential, stateless_tests_amd_debug_parallel, stateless_tests_amd_debug_sequential, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_1_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_2_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_3_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_4_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_5_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_6_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_7_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_8_8, stateless_tests_amd_msan_wasmedge_parallel_1_4, stateless_tests_amd_msan_wasmedge_parallel_2_4, stateless_tests_amd_msan_wasmedge_parallel_3_4, stateless_tests_amd_msan_wasmedge_parallel_4_4, stateless_tests_amd_msan_wasmedge_sequential_1_2, stateless_tests_amd_msan_wasmedge_sequential_2_2, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_amd_tsan_s3_storage_parallel_1_2, stateless_tests_amd_tsan_s3_storage_parallel_2_2, stateless_tests_amd_tsan_s3_storage_sequential_1_2, stateless_tests_amd_tsan_s3_storage_sequential_2_2, stateless_tests_amd_tsan_sequential_1_2, stateless_tests_amd_tsan_sequential_2_2, stateless_tests_arm_asan_ubsan_azure_parallel, stateless_tests_arm_asan_ubsan_azure_sequential_1_2, stateless_tests_arm_asan_ubsan_azure_sequential_2_2, stateless_tests_arm_binary_parallel, stateless_tests_arm_binary_sequential, stress_test_amd_asan_ubsan, stress_test_amd_debug, stress_test_amd_msan, stress_test_amd_tsan, stress_test_arm_asan_ubsan, stress_test_arm_asan_ubsan_s3, stress_test_arm_debug, stress_test_arm_msan, stress_test_arm_release, stress_test_arm_tsan, stress_test_arm_ubsan, stress_test_azure_amd_msan, stress_test_azure_amd_tsan, unit_tests_asan_ubsan, unit_tests_asan_ubsan_function_prop_fuzzer, unit_tests_msan, unit_tests_msan_function_prop_fuzzer, unit_tests_tsan, unit_tests_tsan_function_prop_fuzzer]
+    needs: [ast_fuzzer_amd_debug, ast_fuzzer_amd_msan, ast_fuzzer_amd_tsan, ast_fuzzer_arm_asan_ubsan, build_amd_asan_ubsan, build_amd_binary, build_amd_debug, build_amd_llvm_coverage_per_test, build_amd_msan, build_amd_release, build_amd_tsan, build_arm_asan_ubsan, build_arm_binary, build_arm_debug, build_arm_msan, build_arm_release, build_arm_tsan, build_arm_ubsan, buzzhouse_amd_debug, buzzhouse_amd_msan, buzzhouse_amd_tsan, buzzhouse_arm_asan_ubsan, clickbench_amd_release, clickbench_arm_release, compatibility_check_amd_release, compatibility_check_arm_release, config_workflow, docker_keeper_image, docker_server_image, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, install_packages_amd_release, install_packages_arm_release, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_1_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_2_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_3_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_4_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_5_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_6_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_7_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_8_8, integration_tests_amd_msan_10_10, integration_tests_amd_msan_1_10, integration_tests_amd_msan_2_10, integration_tests_amd_msan_3_10, integration_tests_amd_msan_4_10, integration_tests_amd_msan_5_10, integration_tests_amd_msan_6_10, integration_tests_amd_msan_7_10, integration_tests_amd_msan_8_10, integration_tests_amd_msan_9_10, integration_tests_amd_tsan_1_6, integration_tests_amd_tsan_2_6, integration_tests_amd_tsan_3_6, integration_tests_amd_tsan_4_6, integration_tests_amd_tsan_5_6, integration_tests_amd_tsan_6_6, integration_tests_arm_binary_distributed_plan_1_4, integration_tests_arm_binary_distributed_plan_2_4, integration_tests_arm_binary_distributed_plan_3_4, integration_tests_arm_binary_distributed_plan_4_4, sign_release_amd_release, sign_release_arm_release, source_upload, sqllogic_test, sqlstorm_test, sqltest, stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_1_2, stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_2_2, stateless_tests_amd_asan_ubsan_db_disk_distributed_plan_sequential_1_2, stateless_tests_amd_asan_ubsan_db_disk_distributed_plan_sequential_2_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_binary_cas_s3_storage_parallel, stateless_tests_amd_binary_cas_storage_parallel, stateless_tests_amd_debug_distributed_plan_s3_storage_parallel, stateless_tests_amd_debug_distributed_plan_s3_storage_sequential, stateless_tests_amd_debug_parallel, stateless_tests_amd_debug_sequential, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_1_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_2_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_3_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_4_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_5_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_6_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_7_8, stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_8_8, stateless_tests_amd_msan_cas_s3_storage_parallel_1_3, stateless_tests_amd_msan_cas_s3_storage_parallel_2_3, stateless_tests_amd_msan_cas_s3_storage_parallel_3_3, stateless_tests_amd_msan_wasmedge_parallel_1_4, stateless_tests_amd_msan_wasmedge_parallel_2_4, stateless_tests_amd_msan_wasmedge_parallel_3_4, stateless_tests_amd_msan_wasmedge_parallel_4_4, stateless_tests_amd_msan_wasmedge_sequential_1_2, stateless_tests_amd_msan_wasmedge_sequential_2_2, stateless_tests_amd_tsan_cas_s3_storage_parallel_1_2, stateless_tests_amd_tsan_cas_s3_storage_parallel_2_2, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_amd_tsan_s3_storage_parallel_1_2, stateless_tests_amd_tsan_s3_storage_parallel_2_2, stateless_tests_amd_tsan_s3_storage_sequential_1_2, stateless_tests_amd_tsan_s3_storage_sequential_2_2, stateless_tests_amd_tsan_sequential_1_2, stateless_tests_amd_tsan_sequential_2_2, stateless_tests_arm_asan_ubsan_azure_parallel, stateless_tests_arm_asan_ubsan_azure_sequential_1_2, stateless_tests_arm_asan_ubsan_azure_sequential_2_2, stateless_tests_arm_binary_cas_s3_storage_parallel, stateless_tests_arm_binary_parallel, stateless_tests_arm_binary_sequential, stress_test_amd_asan_ubsan, stress_test_amd_debug, stress_test_amd_msan, stress_test_amd_tsan, stress_test_arm_asan_ubsan, stress_test_arm_asan_ubsan_s3, stress_test_arm_debug, stress_test_arm_msan, stress_test_arm_release, stress_test_arm_tsan, stress_test_arm_ubsan, stress_test_azure_amd_msan, stress_test_azure_amd_tsan, unit_tests_asan_ubsan, unit_tests_asan_ubsan_function_prop_fuzzer, unit_tests_msan, unit_tests_msan_function_prop_fuzzer, unit_tests_tsan, unit_tests_tsan_function_prop_fuzzer]
     if: ${{ !cancelled() && needs.config_workflow.outputs.pipeline_status != '' }}
     name: "Finish Workflow"
     outputs:
@@ -6148,6 +6658,16 @@ jobs:
       - stateless_tests_amd_tsan_s3_storage_sequential_2_2
       - stateless_tests_arm_binary_parallel
       - stateless_tests_arm_binary_sequential
+      - stateless_tests_amd_binary_cas_s3_storage_parallel
+      - stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_1_2
+      - stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_2_2
+      - stateless_tests_amd_tsan_cas_s3_storage_parallel_1_2
+      - stateless_tests_amd_tsan_cas_s3_storage_parallel_2_2
+      - stateless_tests_amd_msan_cas_s3_storage_parallel_1_3
+      - stateless_tests_amd_msan_cas_s3_storage_parallel_2_3
+      - stateless_tests_amd_msan_cas_s3_storage_parallel_3_3
+      - stateless_tests_arm_binary_cas_s3_storage_parallel
+      - stateless_tests_amd_binary_cas_storage_parallel
       - stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_1_8
       - stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_2_8
       - stateless_tests_amd_llvm_coverage_per_test_per_test_coverage_3_8
diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index 2b77e85439e8..708552d3ef2f 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -2405,6 +2405,496 @@ jobs:
           . ./ci/tmp/praktika_setup_env.sh
           PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (arm_binary, sequential)' --workflow "PR" --ci --timestamp
 
+  stateless_tests_amd_binary_cas_s3_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_binary, build_amd_debug, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYmluYXJ5LCBjYXMgczMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (amd_binary, cas s3 storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_binary, cas s3 storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_BINARY_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_BINARY_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_binary, cas s3 storage, parallel)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_1_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYXNhbl91YnNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzIp') }}
+    name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_ASAN_UBSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_ASAN_UBSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_2_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYXNhbl91YnNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzIp') }}
+    name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_ASAN_UBSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_ASAN_UBSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_amd_tsan_cas_s3_storage_parallel_1_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfdHNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzIp') }}
+    name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_TSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_TSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_amd_tsan_cas_s3_storage_parallel_2_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfdHNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzIp') }}
+    name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_TSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_TSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_1_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_msan, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_MSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_2_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_msan, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_MSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_3_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_msan, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAzLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_MSAN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_arm_binary_cas_s3_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester-aarch64, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhcm1fYmluYXJ5LCBjYXMgczMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (arm_binary, cas s3 storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (arm_binary, cas s3 storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_ARM_BIN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_ARM_BIN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (arm_binary, cas s3 storage, parallel)' --workflow "PR" --ci --timestamp
+
+  stateless_tests_amd_binary_cas_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_binary, build_amd_debug, build_amd_tsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYmluYXJ5LCBjYXMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (amd_binary, cas storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_binary, cas storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_BINARY_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_BINARY_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_binary, cas storage, parallel)' --workflow "PR" --ci --timestamp
+
   stateless_tests_arm_asan_ubsan_azure_parallel:
     runs-on: [self-hosted, altinity-on-demand, altinity-func-tester-aarch64, 16c]
     needs: [build_amd_asan_ubsan, build_amd_debug, build_amd_tsan, build_arm_asan_ubsan, build_arm_binary, ci_tests, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_parallel, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_arm_binary_parallel]
@@ -5545,7 +6035,7 @@ jobs:
 
   finish_workflow:
     runs-on: [self-hosted, altinity-on-demand, altinity-style-checker]
-    needs: [ast_fuzzer_amd_debug, ast_fuzzer_amd_debug_targeted, ast_fuzzer_amd_debug_targeted_old_compatibility, ast_fuzzer_amd_msan, ast_fuzzer_amd_tsan, ast_fuzzer_arm_asan_ubsan, build_amd_asan_ubsan, build_amd_binary, build_amd_debug, build_amd_msan, build_amd_release, build_amd_tsan, build_arm_asan_ubsan, build_arm_binary, build_arm_debug, build_arm_msan, build_arm_release, build_arm_tsan, build_arm_ubsan, build_toolchain_pgo_bolt_aarch64, build_toolchain_pgo_bolt_amd64, buzzhouse_amd_debug, buzzhouse_amd_msan, buzzhouse_amd_tsan, buzzhouse_arm_asan_ubsan, ci_tests, compatibility_check_amd_release, compatibility_check_arm_release, config_workflow, docker_keeper_image, docker_server_image, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, install_packages_amd_release, install_packages_arm_release, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_1_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_2_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_3_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_4_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_5_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_6_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_7_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_8_8, integration_tests_amd_asan_ubsan_targeted, integration_tests_amd_msan_10_10, integration_tests_amd_msan_1_10, integration_tests_amd_msan_2_10, integration_tests_amd_msan_3_10, integration_tests_amd_msan_4_10, integration_tests_amd_msan_5_10, integration_tests_amd_msan_6_10, integration_tests_amd_msan_7_10, integration_tests_amd_msan_8_10, integration_tests_amd_msan_9_10, integration_tests_amd_tsan_1_6, integration_tests_amd_tsan_2_6, integration_tests_amd_tsan_3_6, integration_tests_amd_tsan_4_6, integration_tests_amd_tsan_5_6, integration_tests_amd_tsan_6_6, integration_tests_arm_binary_distributed_plan_1_4, integration_tests_arm_binary_distributed_plan_2_4, integration_tests_arm_binary_distributed_plan_3_4, integration_tests_arm_binary_distributed_plan_4_4, keeper_stress_tests_pr, quick_functional_tests, source_upload, sqllogic_test, sqlstorm_test, stateless_tests_amd_asan_ubsan_db_disk_distributed_plan_sequential_1_2, stateless_tests_amd_asan_ubsan_db_disk_distributed_plan_sequential_2_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_debug_distributed_plan_s3_storage_parallel, stateless_tests_amd_debug_distributed_plan_s3_storage_sequential, stateless_tests_amd_debug_parallel, stateless_tests_amd_debug_sequential, stateless_tests_amd_msan_wasmedge_parallel_1_4, stateless_tests_amd_msan_wasmedge_parallel_2_4, stateless_tests_amd_msan_wasmedge_parallel_3_4, stateless_tests_amd_msan_wasmedge_parallel_4_4, stateless_tests_amd_msan_wasmedge_sequential_1_2, stateless_tests_amd_msan_wasmedge_sequential_2_2, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_amd_tsan_s3_storage_parallel_1_2, stateless_tests_amd_tsan_s3_storage_parallel_2_2, stateless_tests_amd_tsan_s3_storage_sequential_1_2, stateless_tests_amd_tsan_s3_storage_sequential_2_2, stateless_tests_amd_tsan_sequential_1_2, stateless_tests_amd_tsan_sequential_2_2, stateless_tests_arm_asan_ubsan_azure_parallel, stateless_tests_arm_asan_ubsan_azure_sequential_1_2, stateless_tests_arm_asan_ubsan_azure_sequential_2_2, stateless_tests_arm_asan_ubsan_targeted, stateless_tests_arm_binary_parallel, stateless_tests_arm_binary_sequential, stress_test_amd_asan_ubsan, stress_test_amd_debug, stress_test_amd_msan, stress_test_amd_tsan, stress_test_arm_asan_ubsan, stress_test_arm_asan_ubsan_s3, stress_test_arm_debug, stress_test_arm_msan, stress_test_arm_release, stress_test_arm_tsan, stress_test_arm_ubsan, unit_tests_asan_ubsan, unit_tests_asan_ubsan_function_prop_fuzzer, unit_tests_msan, unit_tests_msan_function_prop_fuzzer, unit_tests_tsan, unit_tests_tsan_function_prop_fuzzer]
+    needs: [ast_fuzzer_amd_debug, ast_fuzzer_amd_debug_targeted, ast_fuzzer_amd_debug_targeted_old_compatibility, ast_fuzzer_amd_msan, ast_fuzzer_amd_tsan, ast_fuzzer_arm_asan_ubsan, build_amd_asan_ubsan, build_amd_binary, build_amd_debug, build_amd_msan, build_amd_release, build_amd_tsan, build_arm_asan_ubsan, build_arm_binary, build_arm_debug, build_arm_msan, build_arm_release, build_arm_tsan, build_arm_ubsan, build_toolchain_pgo_bolt_aarch64, build_toolchain_pgo_bolt_amd64, buzzhouse_amd_debug, buzzhouse_amd_msan, buzzhouse_amd_tsan, buzzhouse_arm_asan_ubsan, ci_tests, compatibility_check_amd_release, compatibility_check_arm_release, config_workflow, docker_keeper_image, docker_server_image, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, fast_test, install_packages_amd_release, install_packages_arm_release, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_1_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_2_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_3_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_4_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_5_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_6_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_7_8, integration_tests_amd_asan_ubsan_db_disk_old_analyzer_8_8, integration_tests_amd_asan_ubsan_targeted, integration_tests_amd_msan_10_10, integration_tests_amd_msan_1_10, integration_tests_amd_msan_2_10, integration_tests_amd_msan_3_10, integration_tests_amd_msan_4_10, integration_tests_amd_msan_5_10, integration_tests_amd_msan_6_10, integration_tests_amd_msan_7_10, integration_tests_amd_msan_8_10, integration_tests_amd_msan_9_10, integration_tests_amd_tsan_1_6, integration_tests_amd_tsan_2_6, integration_tests_amd_tsan_3_6, integration_tests_amd_tsan_4_6, integration_tests_amd_tsan_5_6, integration_tests_amd_tsan_6_6, integration_tests_arm_binary_distributed_plan_1_4, integration_tests_arm_binary_distributed_plan_2_4, integration_tests_arm_binary_distributed_plan_3_4, integration_tests_arm_binary_distributed_plan_4_4, keeper_stress_tests_pr, quick_functional_tests, source_upload, sqllogic_test, sqlstorm_test, stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_1_2, stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_2_2, stateless_tests_amd_asan_ubsan_db_disk_distributed_plan_sequential_1_2, stateless_tests_amd_asan_ubsan_db_disk_distributed_plan_sequential_2_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_1_2, stateless_tests_amd_asan_ubsan_distributed_plan_parallel_2_2, stateless_tests_amd_binary_cas_s3_storage_parallel, stateless_tests_amd_binary_cas_storage_parallel, stateless_tests_amd_debug_distributed_plan_s3_storage_parallel, stateless_tests_amd_debug_distributed_plan_s3_storage_sequential, stateless_tests_amd_debug_parallel, stateless_tests_amd_debug_sequential, stateless_tests_amd_msan_cas_s3_storage_parallel_1_3, stateless_tests_amd_msan_cas_s3_storage_parallel_2_3, stateless_tests_amd_msan_cas_s3_storage_parallel_3_3, stateless_tests_amd_msan_wasmedge_parallel_1_4, stateless_tests_amd_msan_wasmedge_parallel_2_4, stateless_tests_amd_msan_wasmedge_parallel_3_4, stateless_tests_amd_msan_wasmedge_parallel_4_4, stateless_tests_amd_msan_wasmedge_sequential_1_2, stateless_tests_amd_msan_wasmedge_sequential_2_2, stateless_tests_amd_tsan_cas_s3_storage_parallel_1_2, stateless_tests_amd_tsan_cas_s3_storage_parallel_2_2, stateless_tests_amd_tsan_parallel_1_2, stateless_tests_amd_tsan_parallel_2_2, stateless_tests_amd_tsan_s3_storage_parallel_1_2, stateless_tests_amd_tsan_s3_storage_parallel_2_2, stateless_tests_amd_tsan_s3_storage_sequential_1_2, stateless_tests_amd_tsan_s3_storage_sequential_2_2, stateless_tests_amd_tsan_sequential_1_2, stateless_tests_amd_tsan_sequential_2_2, stateless_tests_arm_asan_ubsan_azure_parallel, stateless_tests_arm_asan_ubsan_azure_sequential_1_2, stateless_tests_arm_asan_ubsan_azure_sequential_2_2, stateless_tests_arm_asan_ubsan_targeted, stateless_tests_arm_binary_cas_s3_storage_parallel, stateless_tests_arm_binary_parallel, stateless_tests_arm_binary_sequential, stress_test_amd_asan_ubsan, stress_test_amd_debug, stress_test_amd_msan, stress_test_amd_tsan, stress_test_arm_asan_ubsan, stress_test_arm_asan_ubsan_s3, stress_test_arm_debug, stress_test_arm_msan, stress_test_arm_release, stress_test_arm_tsan, stress_test_arm_ubsan, unit_tests_asan_ubsan, unit_tests_asan_ubsan_function_prop_fuzzer, unit_tests_msan, unit_tests_msan_function_prop_fuzzer, unit_tests_tsan, unit_tests_tsan_function_prop_fuzzer]
     if: ${{ !cancelled() && needs.config_workflow.outputs.pipeline_status != '' }}
     name: "Finish Workflow"
     outputs:
@@ -5687,6 +6177,16 @@ jobs:
       - stateless_tests_amd_tsan_s3_storage_sequential_2_2
       - stateless_tests_arm_binary_parallel
       - stateless_tests_arm_binary_sequential
+      - stateless_tests_amd_binary_cas_s3_storage_parallel
+      - stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_1_2
+      - stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_2_2
+      - stateless_tests_amd_tsan_cas_s3_storage_parallel_1_2
+      - stateless_tests_amd_tsan_cas_s3_storage_parallel_2_2
+      - stateless_tests_amd_msan_cas_s3_storage_parallel_1_3
+      - stateless_tests_amd_msan_cas_s3_storage_parallel_2_3
+      - stateless_tests_amd_msan_cas_s3_storage_parallel_3_3
+      - stateless_tests_arm_binary_cas_s3_storage_parallel
+      - stateless_tests_amd_binary_cas_storage_parallel
       - stateless_tests_arm_asan_ubsan_azure_parallel
       - stateless_tests_arm_asan_ubsan_azure_sequential_1_2
       - stateless_tests_arm_asan_ubsan_azure_sequential_2_2
diff --git a/.github/workflows/pull_request_community.yml b/.github/workflows/pull_request_community.yml
index c576bd5afaba..89ec4bca1be8 100644
--- a/.github/workflows/pull_request_community.yml
+++ b/.github/workflows/pull_request_community.yml
@@ -2073,6 +2073,486 @@ jobs:
           . ./ci/tmp/praktika_setup_env.sh
           PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (arm_binary, sequential)' --workflow "Community PR" --ci --timestamp
 
+  stateless_tests_amd_binary_cas_s3_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_binary, build_amd_debug, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYmluYXJ5LCBjYXMgczMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (amd_binary, cas s3 storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_binary, cas s3 storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_BINARY
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_BINARY
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_binary, cas s3 storage, parallel)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_1_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYXNhbl91YnNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzIp') }}
+    name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_ASAN_UBSAN
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_ASAN_UBSAN
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 1/2)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_amd_asan_ubsan_cas_s3_storage_parallel_2_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_asan_ubsan, build_amd_debug, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYXNhbl91YnNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzIp') }}
+    name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_ASAN_UBSAN
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_ASAN_UBSAN
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_asan_ubsan, cas s3 storage, parallel, 2/2)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_amd_tsan_cas_s3_storage_parallel_1_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester, 16c]
+    needs: [build_amd_debug, build_amd_tsan, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfdHNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzIp') }}
+    name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_TSAN
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_TSAN
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_tsan, cas s3 storage, parallel, 1/2)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_amd_tsan_cas_s3_storage_parallel_2_2:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester, 16c]
+    needs: [build_amd_debug, build_amd_tsan, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfdHNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzIp') }}
+    name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_TSAN
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_TSAN
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_tsan, cas s3 storage, parallel, 2/2)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_1_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_debug, build_amd_msan, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAxLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_MSAN
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 1/3)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_2_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_debug, build_amd_msan, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAyLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_MSAN
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 2/3)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_amd_msan_cas_s3_storage_parallel_3_3:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester]
+    needs: [build_amd_debug, build_amd_msan, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfbXNhbiwgY2FzIHMzIHN0b3JhZ2UsIHBhcmFsbGVsLCAzLzMp') }}
+    name: "Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_MSAN
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_MSAN
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_msan, cas s3 storage, parallel, 3/3)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_arm_binary_cas_s3_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester-aarch64, 16c]
+    needs: [build_amd_debug, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhcm1fYmluYXJ5LCBjYXMgczMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (arm_binary, cas s3 storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (arm_binary, cas s3 storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_ARM_BIN
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_ARM_BIN
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (arm_binary, cas s3 storage, parallel)' --workflow "Community PR" --ci --timestamp
+
+  stateless_tests_amd_binary_cas_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_binary, build_amd_debug, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYmluYXJ5LCBjYXMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (amd_binary, cas storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_binary, cas storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_BINARY
+        uses: actions/download-artifact@v8
+        with:
+          name: CH_AMD_BINARY
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_binary, cas storage, parallel)' --workflow "Community PR" --ci --timestamp
+
   integration_tests_amd_asan_ubsan_db_disk_old_analyzer_1_8:
     runs-on: [self-hosted, altinity-on-demand, altinity-func-tester, 16c]
     needs: [build_amd_asan_ubsan, build_amd_debug, build_arm_binary, config_workflow, fast_test, stateless_tests_amd_debug_parallel, stateless_tests_arm_binary_parallel]
diff --git a/.github/workflows/release_builds.yml b/.github/workflows/release_builds.yml
index 8f6f6f22525d..5e943c0cd19e 100644
--- a/.github/workflows/release_builds.yml
+++ b/.github/workflows/release_builds.yml
@@ -1334,9 +1334,162 @@ jobs:
           . ./ci/tmp/praktika_setup_env.sh
           PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (arm_binary, sequential)' --workflow "Release Builds" --ci --timestamp
 
+  stateless_tests_amd_binary_cas_s3_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_binary, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYmluYXJ5LCBjYXMgczMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (amd_binary, cas s3 storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_binary, cas s3 storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_BINARY_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_BINARY_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_binary, cas s3 storage, parallel)' --workflow "Release Builds" --ci --timestamp
+
+  stateless_tests_arm_binary_cas_s3_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-func-tester-aarch64, 16c]
+    needs: [build_arm_binary, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhcm1fYmluYXJ5LCBjYXMgczMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (arm_binary, cas s3 storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (arm_binary, cas s3 storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_ARM_BIN_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_ARM_BIN_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (arm_binary, cas s3 storage, parallel)' --workflow "Release Builds" --ci --timestamp
+
+  stateless_tests_amd_binary_cas_storage_parallel:
+    runs-on: [self-hosted, altinity-on-demand, altinity-builder, 16c]
+    needs: [build_amd_binary, config_workflow, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest]
+    if: ${{ !cancelled() && !contains(needs.*.outputs.pipeline_status, 'failure') && !contains(needs.*.outputs.pipeline_status, 'undefined') && !contains(fromJson(needs.config_workflow.outputs.data).workflow_config.cache_success_base64, 'U3RhdGVsZXNzIHRlc3RzIChhbWRfYmluYXJ5LCBjYXMgc3RvcmFnZSwgcGFyYWxsZWwp') }}
+    name: "Stateless tests (amd_binary, cas storage, parallel)"
+    outputs:
+      data: ${{ steps.run.outputs.DATA }}
+      pipeline_status: ${{ steps.run.outputs.pipeline_status || 'undefined' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+        with:
+          ref: ${{ env.CHECKOUT_REF }}
+
+      - name: Setup
+        uses: ./.github/actions/runner_setup
+      - name: Docker setup
+        uses: ./.github/actions/docker_setup
+        with:
+          test_name: "Stateless tests (amd_binary, cas storage, parallel)"
+
+      - name: Prepare env script
+        run: |
+          rm -rf ./ci/tmp
+          mkdir -p ./ci/tmp
+          cat > ./ci/tmp/praktika_setup_env.sh << 'ENV_SETUP_SCRIPT_EOF'
+          export PYTHONPATH=./ci:.:
+          cat > ./ci/tmp/workflow_inputs.json << 'EOF'
+          ${{ toJson(github.event.inputs) }}
+          EOF
+          cat > ./ci/tmp/workflow_job.json << 'EOF'
+          ${{ toJson(job) }}
+          EOF
+          cat > ./ci/tmp/workflow_status.json << 'EOF'
+          ${{ toJson(needs) }}
+          EOF
+          ENV_SETUP_SCRIPT_EOF
+
+      - name: Download artifact CH_AMD_BINARY_GH
+        uses: actions/download-artifact@v8
+        continue-on-error: true
+        with:
+          name: CH_AMD_BINARY_GH
+          path: ./ci/tmp
+
+      - name: Run
+        id: run
+        run: |
+          . ./ci/tmp/praktika_setup_env.sh
+          PYTHONUNBUFFERED=1 python3 -m praktika run 'Stateless tests (amd_binary, cas storage, parallel)' --workflow "Release Builds" --ci --timestamp
+
   finish_workflow:
     runs-on: [self-hosted, altinity-on-demand, altinity-style-checker]
-    needs: [build_amd_asan_ubsan, build_amd_binary, build_amd_debug, build_amd_msan, build_amd_release, build_amd_tsan, build_arm_asan_ubsan, build_arm_binary, build_arm_debug, build_arm_msan, build_arm_release, build_arm_tsan, build_arm_ubsan, config_workflow, docker_keeper_image, docker_server_image, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, install_packages_amd_release, install_packages_arm_release, sign_release_amd_release, sign_release_arm_release, source_upload, stateless_tests_arm_binary_parallel, stateless_tests_arm_binary_sequential]
+    needs: [build_amd_asan_ubsan, build_amd_binary, build_amd_debug, build_amd_msan, build_amd_release, build_amd_tsan, build_arm_asan_ubsan, build_arm_binary, build_arm_debug, build_arm_msan, build_arm_release, build_arm_tsan, build_arm_ubsan, config_workflow, docker_keeper_image, docker_server_image, dockers_build_amd, dockers_build_arm, dockers_build_multiplatform_manifest, install_packages_amd_release, install_packages_arm_release, sign_release_amd_release, sign_release_arm_release, source_upload, stateless_tests_amd_binary_cas_s3_storage_parallel, stateless_tests_amd_binary_cas_storage_parallel, stateless_tests_arm_binary_cas_s3_storage_parallel, stateless_tests_arm_binary_parallel, stateless_tests_arm_binary_sequential]
     if: ${{ !cancelled() && needs.config_workflow.outputs.pipeline_status != '' }}
     name: "Finish Workflow"
     outputs:
@@ -1433,6 +1586,9 @@ jobs:
       - source_upload
       - stateless_tests_arm_binary_parallel
       - stateless_tests_arm_binary_sequential
+      - stateless_tests_amd_binary_cas_s3_storage_parallel
+      - stateless_tests_arm_binary_cas_s3_storage_parallel
+      - stateless_tests_amd_binary_cas_storage_parallel
       - finish_workflow
       - GrypeScanServer
       - GrypeScanKeeper
diff --git a/ci/defs/altinity_jobs.py b/ci/defs/altinity_jobs.py
index 1d736abaa17e..ab7db9c881be 100644
--- a/ci/defs/altinity_jobs.py
+++ b/ci/defs/altinity_jobs.py
@@ -1,6 +1,7 @@
 from praktika import Artifact, Job
 
 from ci.defs.defs import TEMP_DIR, ArtifactNames, RunnerLabels
+from ci.defs.job_configs import common_ft_job_config
 
 
 class AltinityArtifactNames:
@@ -69,3 +70,53 @@ class AltinityJobConfigs:
         command="python3 ./ci/jobs/source_upload.py",
         timeout=3600,
     )
+    # Stateless tests with a content-addressed disk as the default MergeTree storage.
+    cas_functional_tests_jobs = common_ft_job_config.parametrize(
+        # CAS over S3: RustFS, not MinIO OSS, because the incarnation pool needs
+        # enforced conditional deletes.
+        Job.ParamSet(
+            parameter="amd_binary, cas s3 storage, parallel",
+            runs_on=RunnerLabels.AMD_MEDIUM_CPU,
+            requires=[ArtifactNames.CH_AMD_BINARY_GH],
+        ),
+        # The sanitizer lanes are sharded because an unsharded one exceeds the 6h
+        # GitHub job timeout and is killed before it uploads any results.
+        *[
+            Job.ParamSet(
+                parameter=f"amd_asan_ubsan, cas s3 storage, parallel, {batch}/{total_batches}",
+                runs_on=RunnerLabels.AMD_MEDIUM_CPU,
+                requires=[ArtifactNames.CH_AMD_ASAN_UBSAN_GH],
+            )
+            for total_batches in (2,)
+            for batch in range(1, total_batches + 1)
+        ],
+        *[
+            Job.ParamSet(
+                parameter=f"amd_tsan, cas s3 storage, parallel, {batch}/{total_batches}",
+                runs_on=RunnerLabels.AMD_MEDIUM,
+                requires=[ArtifactNames.CH_AMD_TSAN_GH],
+            )
+            for total_batches in (2,)
+            for batch in range(1, total_batches + 1)
+        ],
+        *[
+            Job.ParamSet(
+                parameter=f"amd_msan, cas s3 storage, parallel, {batch}/{total_batches}",
+                runs_on=RunnerLabels.FUNC_TESTER_AMD,
+                requires=[ArtifactNames.CH_AMD_MSAN_GH],
+            )
+            for total_batches in (3,)
+            for batch in range(1, total_batches + 1)
+        ],
+        Job.ParamSet(
+            parameter="arm_binary, cas s3 storage, parallel",
+            runs_on=RunnerLabels.ARM_MEDIUM_CPU,
+            requires=[ArtifactNames.CH_ARM_BINARY_GH],
+        ),
+        # CAS over local object storage.
+        Job.ParamSet(
+            parameter="amd_binary, cas storage, parallel",
+            runs_on=RunnerLabels.AMD_MEDIUM_CPU,
+            requires=[ArtifactNames.CH_AMD_BINARY_GH],
+        ),
+    )
diff --git a/ci/jobs/functional_tests.py b/ci/jobs/functional_tests.py
index fcde1575154b..d66150839527 100644
--- a/ci/jobs/functional_tests.py
+++ b/ci/jobs/functional_tests.py
@@ -142,6 +142,8 @@ def run_tests(
     "old analyzer": "--analyzer",
     "WasmEdge": "--wasm-engine wasmedge",
     "s3 storage": "--s3-storage",
+    "cas storage": "--cas-storage",
+    "cas s3 storage": "--cas-s3-storage",
     "DatabaseReplicated": "--db-replicated",
     "DatabaseOrdinary": "--db-ordinary",
     "wide parts enabled": "--wide-parts",
@@ -155,6 +157,8 @@ def run_tests(
 
 OPTIONS_TO_TEST_RUNNER_ARGUMENTS = {
     "s3 storage": "--s3-storage --no-stateful",
+    "cas storage": "--cas-storage",
+    "cas s3 storage": "--cas-s3-storage",
     "ParallelReplicas": "--no-zookeeper --no-shard --no-parallel-replicas",
     "AsyncInsert": " --no-async-insert",
     "DatabaseReplicated": " --no-stateful --replicated-database",
@@ -242,6 +246,7 @@ def main():
     is_targeted_check = False
     is_bugfix_validation = False
     is_s3_storage = False
+    is_cas_s3 = False
     is_azure_storage = False
     is_database_replicated = False
     is_shared_catalog = False
@@ -295,8 +300,13 @@ def main():
             is_excluded_from_llvm = True
         if "per_test_coverage" in to:
             is_per_test_coverage = True
-        if "s3 storage" in to:
+        if "s3 storage" in to and "cas" not in to:
+            # The CAS-over-s3 variant ("cas s3 storage") installs
+            # only its own default policy and must not pull in the s3 stateful-data / encrypted
+            # storage machinery, so it is deliberately excluded from is_s3_storage.
             is_s3_storage = True
+        if "cas s3 storage" in to:
+            is_cas_s3 = True
         if "azure" in to:
             is_azure_storage = True
         if "DatabaseReplicated" in to:
@@ -637,6 +647,14 @@ def main():
 
         def start():
             res = CH.start_minio(test_type="stateless") and CH.start_azurite()
+            if res and is_cas_s3:
+                # The CA-over-S3 pool lives on RustFS (M-W D-W8): the incarnation pool
+                # needs ENFORCED conditional deletes, which MinIO OSS lacks (the
+                # fail-closed capability probe rejects it). start_rustfs wipes its data
+                # dir per run, so no pool state bleeds between runs (the local-CA
+                # analogue is the per-run server-store wipe). MinIO keeps the non-CA
+                # s3 disks.
+                res = CH.start_rustfs()
             res = res and CH.start()
             res = res and CH.wait_ready()
             if res:
diff --git a/ci/jobs/scripts/check_style/various_checks.sh b/ci/jobs/scripts/check_style/various_checks.sh
index 3660b8acd193..1d208fff563d 100755
--- a/ci/jobs/scripts/check_style/various_checks.sh
+++ b/ci/jobs/scripts/check_style/various_checks.sh
@@ -230,6 +230,32 @@ done
 # CLICKHOUSE_URL already includes "?"
 git grep -P 'CLICKHOUSE_URL(|_HTTPS)(}|}/|/|)\?' $ROOT_PATH/tests/queries/0_stateless/*.sh && echo "CLICKHOUSE_URL already includes '?', use '&' to append query parameters"
 
+# A bare double quote inside a `-q """ ... """` block ends the string early.
+# Bash treats """ as an empty string followed by an open quote, so everything up to the next quote is
+# one argument. A quote anywhere inside -- including in a SQL comment -- closes it there, and the rest
+# of the block becomes shell words. The script stays syntactically valid, so `bash -n` and shellcheck
+# both pass; the only symptom is that the client receives a truncated query and reports a syntax error
+# pointing at whatever followed the quote. Escape it as \" or use single quotes.
+python3 - "$ROOT_PATH" <<'PYEOF'
+import glob, os, re, sys
+
+for path in sorted(glob.glob(os.path.join(sys.argv[1], "tests/queries/0_stateless/*.sh"))):
+    inside = False
+    with open(path, encoding="utf-8", errors="replace") as handle:
+        for number, line in enumerate(handle, 1):
+            if not inside:
+                if re.search(r'(-q|--query)\s+"""\s*$', line.strip()):
+                    inside = True
+                continue
+            # The block can close mid-line (`... LIMIT 10;"""`), so only what precedes the
+            # closing delimiter is still inside it.
+            body, closed, _ = line.partition('"""')
+            if '"' in re.sub(r'\\"', "", body):
+                print(f"{path}:{number}: bare double quote inside a -q \"\"\" block ends the SQL early")
+            if closed:
+                inside = False
+PYEOF
+
 # Large files checked into git.
 # Every byte committed is cloned by every contributor forever and cannot be removed without history rewriting.
 # Binary blobs (JARs, archives, .so, datasets) should be downloaded at test time or built from source.
diff --git a/ci/jobs/scripts/clickhouse_proc.py b/ci/jobs/scripts/clickhouse_proc.py
index 88a4e56c97ba..cbc8ad5984b4 100644
--- a/ci/jobs/scripts/clickhouse_proc.py
+++ b/ci/jobs/scripts/clickhouse_proc.py
@@ -9,6 +9,7 @@
 import threading
 import traceback
 import uuid
+import zipfile
 from collections import defaultdict
 from pathlib import Path
 from typing import List
@@ -43,6 +44,7 @@ class ClickHouseProc:
     MINIO_LOG = f"{temp_dir}/minio.log"
     AZURITE_LOG = f"{temp_dir}/azurite.log"
     KAFKA_LOG = f"{temp_dir}/kafka.log"
+    RUSTFS_LOG = f"{temp_dir}/rustfs.log"
     LOGS_SAVER_CLIENT_OPTIONS = "--max_memory_usage 10G --max_threads 1 --max_rows_to_read=0 --max_result_rows 0 --max_result_bytes 0 --max_bytes_to_read 0 --max_execution_time 0 --max_execution_time_leaf 0 --max_estimated_execution_time 0"
     DMESG_LOG = f"{temp_dir}/dmesg.log"
     # TODO: run servers in  dedicated wds to keep trash localised
@@ -163,6 +165,77 @@ def start_minio(self, test_type):
         print("Failed to start minio")
         return False
 
+    RUSTFS_VERSION = "1.0.0-beta.12"
+
+    def download_rustfs(self, rustfs_bin):
+        machine = platform.machine()
+        if machine not in ("x86_64", "aarch64", "arm64"):
+            print(f"unsupported architecture for rustfs [{machine}]")
+            return False
+        arch = "aarch64" if machine in ("aarch64", "arm64") else "x86_64"
+        url = (
+            f"https://github.com/rustfs/rustfs/releases/download/{self.RUSTFS_VERSION}"
+            f"/rustfs-linux-{arch}-musl-v{self.RUSTFS_VERSION}.zip"
+        )
+        zip_path = f"{temp_dir}/rustfs.zip"
+        if not Shell.check(
+            f"curl -sSfL --retry 3 --retry-delay 5 -o {zip_path} {url}", verbose=True
+        ):
+            print(f"failed to download rustfs from {url}")
+            return False
+        # The release zip contains the single `rustfs` binary at its root.
+        with zipfile.ZipFile(zip_path) as archive:
+            archive.extract("rustfs", temp_dir)
+        os.remove(zip_path)
+        os.chmod(rustfs_bin, 0o755)
+        return True
+
+    def start_rustfs(self):
+        # RustFS backs the CAS-over-S3 pool because the incarnation pool needs enforced
+        # conditional operations (a wrong-token DELETE must fail with 412) that MinIO OSS lacks;
+        # MinIO keeps serving the non-CAS s3 disks on its own port. Binary and data dir live
+        # under ci/tmp, which CI wipes per run, so no pool state bleeds between runs.
+        rustfs_bin = f"{temp_dir}/rustfs"
+        if not Path(rustfs_bin).is_file() and not self.download_rustfs(rustfs_bin):
+            print(f"rustfs binary not found at {rustfs_bin} and download failed")
+            return False
+        data_dir = f"{temp_dir}/rustfs_data"
+        Shell.check(f"rm -rf {data_dir} && mkdir -p {data_dir}", verbose=True)
+        # The background data-scanner and auto-heal manager do no useful work on a single-disk
+        # ephemeral pool, but their namespace locks produced multi-minute bursts of 503
+        # ServiceUnavailable that stalled client I/O. Client GET/PUT/LIST/DELETE do not depend on
+        # either. The RUSTFS_ENABLE_* spellings are deprecated since 1.0.0-beta.8.
+        # Raise the open-files limit for the same reason start_azurite does: under parallel load
+        # the server holds thousands of S3 connections, and at the default soft limit (1024)
+        # rustfs runs out of fds and refuses new TCP connections in bursts.
+        command = (
+            "(ulimit -n 1048576 2>/dev/null || ulimit -n $(ulimit -Hn)) && "
+            f"RUSTFS_SCANNER_ENABLED=false RUSTFS_HEAL_ENABLED=false "
+            f"{rustfs_bin} server --address 0.0.0.0:11121 "
+            f"--access-key clickhouse --secret-key clickhouse {data_dir}"
+        )
+        with open(self.RUSTFS_LOG, "w") as log_file:
+            self.rustfs_proc = subprocess.Popen(
+                command, stdout=log_file, stderr=subprocess.STDOUT, shell=True
+            )
+        print(f"Started rustfs asynchronously with PID {self.rustfs_proc.pid}")
+
+        if not Shell.check(
+            "curl -s -o /dev/null -w '%{http_code}' http://127.0.0.1:11121/ | grep -qE '403|200'",
+            verbose=False,
+            retries=6,
+        ):
+            print("Failed to start rustfs")
+            return False
+        # The `test` bucket the storage policy expects.
+        res = Shell.check(
+            "/mc alias set carustfs http://localhost:11121 clickhouse clickhouse && /mc mb --ignore-existing carustfs/test",
+            verbose=True,
+        )
+        if not res:
+            print("Failed to create rustfs test bucket")
+        return res
+
     def start_azurite(self):
         # Raise the open files limit before launching azurite-rs.
         # Each concurrent test query opens a TCP connection plus an in-memory
@@ -938,6 +1011,8 @@ def prepare_logs(self, info, all=False):
                     res.append(self.AZURITE_LOG)
                 if Path(self.KAFKA_LOG).exists():
                     res.append(self.KAFKA_LOG)
+                if Path(self.RUSTFS_LOG).exists():
+                    res.append(self.RUSTFS_LOG)
                 if Path(self.DMESG_LOG).exists():
                     res.append(self.DMESG_LOG)
                 if Path(self.CH_LOCAL_ERR_LOG).exists():
@@ -1211,6 +1286,29 @@ def dump_system_tables(self):
         Shell.check(
             f"sed -i 's|<errorlog>.*</errorlog>|<errorlog>{self.CH_LOCAL_ERR_LOG}</errorlog>|' /etc/clickhouse-server/config.xml"
         )
+        # Open any CAS disk read-only: a writable open claims server-root ownership and fails
+        # closed against the real server's persisted owner uuid, while a read-only open skips the
+        # claim and is all a dump needs. Keyed on the `<metadata_type>cas</metadata_type>` marker
+        # rather than on disk names, so it covers every CAS disk however this job names it.
+        # `grep -R` and `sed --follow-symlinks` are required: `tests/config/install.sh` symlinks
+        # these configs into `config.d`, and `-r`/plain `sed` would silently match nothing.
+        Shell.check(
+            "grep -Rl '<metadata_type>cas</metadata_type>' /etc/clickhouse-server/ 2>/dev/null "
+            "| xargs -r sed -i --follow-symlinks 's|<metadata_type>cas</metadata_type>|<metadata_type>cas</metadata_type><readonly>true</readonly>|g'"
+        )
+        # Report loudly if the substitution stops matching: a declared but not read-only CAS disk
+        # means this scrape is about to die on ownership. Reports; does not abort the dump.
+        if Shell.check(
+            "grep -Rlq '<metadata_type>cas</metadata_type>' /etc/clickhouse-server/",
+            verbose=False,
+        ) and not Shell.check(
+            "grep -Rlq '<metadata_type>cas</metadata_type><readonly>true</readonly>' /etc/clickhouse-server/",
+            verbose=False,
+        ):
+            print(
+                "WARNING: a CAS disk is declared but the read-only marker was not inserted "
+                "-- `clickhouse local` will claim server-root ownership and this scrape will fail"
+            )
         # FIXME: Hack for s3_with_keeper (note, that we don't need the disk,
         # the problem is that whenever we need disks all disks will be
         # initialized [1])
@@ -1226,8 +1324,12 @@ def dump_system_tables(self):
 
         self.restore_system_metadata_files_from_remote_database_disk()
 
+        # `**`, not `*`: dynamic cache disks created by tests nest their path, e.g.
+        # `filesystem_caches/disks/cache_03517/status` — a one-level glob missed exactly that file,
+        # and the scrape died on its flock (`StatusFile.cpp` "Another server instance ... is already
+        # running") when the server had not released it.
         cache_status_files = glob.glob(
-            f"{self.ch_var_lib_dir}/filesystem_caches/*/status"
+            f"{self.ch_var_lib_dir}/filesystem_caches/**/status", recursive=True
         )
         if cache_status_files:
             print(
diff --git a/ci/workflows/backport_branches.py b/ci/workflows/backport_branches.py
index 528a47cbe32e..5f891943d5c9 100644
--- a/ci/workflows/backport_branches.py
+++ b/ci/workflows/backport_branches.py
@@ -1,9 +1,15 @@
 from praktika import Workflow
 
 from ci.defs.defs import DOCKERS, SECRETS, ArtifactConfigs
+from ci.defs.altinity_jobs import AltinityJobConfigs
 from ci.defs.job_configs import JobConfigs
 from ci.jobs.scripts.workflow_hooks.filter_job import should_skip_job
 
+FUNCTIONAL_TESTS_JOBS = [
+    *JobConfigs.functional_tests_jobs,
+    *AltinityJobConfigs.cas_functional_tests_jobs,
+]
+
 workflow = Workflow.Config(
     name="BackportPR",
     event=Workflow.Event.PULL_REQUEST,
@@ -24,7 +30,7 @@
         JobConfigs.docker_keeper,
         *JobConfigs.install_check_jobs,
         *JobConfigs.compatibility_test_jobs,
-        *[job for job in JobConfigs.functional_tests_jobs if "amd_asan_ubsan" in job.name],
+        *[job for job in FUNCTIONAL_TESTS_JOBS if "amd_asan_ubsan" in job.name],
         *[
             job
             for job in JobConfigs.unittest_jobs
diff --git a/ci/workflows/master.py b/ci/workflows/master.py
index b3ffe686f331..b98ffb96fa66 100644
--- a/ci/workflows/master.py
+++ b/ci/workflows/master.py
@@ -13,6 +13,11 @@
 from ci.jobs.scripts.workflow_hooks.filter_job import should_skip_job
 from ci.workflows.pull_request import REGULAR_BUILD_NAMES
 
+FUNCTIONAL_TESTS_JOBS = [
+    *JobConfigs.functional_tests_jobs,
+    *AltinityJobConfigs.cas_functional_tests_jobs,
+]
+
 # Add long retention tags to subset of artifacts
 clickhouse_binaries_with_tags = []
 for artifact in ArtifactConfigs.clickhouse_binaries + ArtifactConfigs.clickhouse_stripped_binaries:
@@ -55,7 +60,7 @@
         *JobConfigs.compatibility_test_jobs,
         *[
             j
-            for j in JobConfigs.functional_tests_jobs
+            for j in FUNCTIONAL_TESTS_JOBS
             if "coverage" not in j.name
         ],
         # *JobConfigs.functional_test_llvm_coverage_jobs,
diff --git a/ci/workflows/pull_request.py b/ci/workflows/pull_request.py
index ae88a3b3a244..a3d063400716 100644
--- a/ci/workflows/pull_request.py
+++ b/ci/workflows/pull_request.py
@@ -14,11 +14,16 @@
 from ci.jobs.scripts.workflow_hooks.filter_job import should_skip_job
 from ci.jobs.scripts.workflow_hooks.trusted import can_be_tested
 
-ALL_FUNCTIONAL_TESTS = [job.name for job in JobConfigs.functional_tests_jobs]
+FUNCTIONAL_TESTS_JOBS = [
+    *JobConfigs.functional_tests_jobs,
+    *AltinityJobConfigs.cas_functional_tests_jobs,
+]
+
+ALL_FUNCTIONAL_TESTS = [job.name for job in FUNCTIONAL_TESTS_JOBS]
 
 FUNCTIONAL_TESTS_PARALLEL_BLOCKING_JOB_NAMES = [
     job.name
-    for job in JobConfigs.functional_tests_jobs
+    for job in FUNCTIONAL_TESTS_JOBS
     if any(
         substr in job.name
         for substr in (
@@ -40,7 +45,7 @@
 REGULAR_BUILD_NAMES = [job.name for job in JobConfigs.build_jobs]
 
 PLAIN_FUNCTIONAL_TEST_JOB = [
-    j for j in JobConfigs.functional_tests_jobs if "amd_debug, parallel" in j.name
+    j for j in FUNCTIONAL_TESTS_JOBS if "amd_debug, parallel" in j.name
 ][0]
 
 workflow = Workflow.Config(
@@ -91,7 +96,7 @@
                 if j.name not in FUNCTIONAL_TESTS_PARALLEL_BLOCKING_JOB_NAMES
                 else []
             )
-            for j in JobConfigs.functional_tests_jobs
+            for j in FUNCTIONAL_TESTS_JOBS
             if "coverage" not in j.name
         ],
         *[
diff --git a/ci/workflows/pull_request_community.py b/ci/workflows/pull_request_community.py
index af76e6ac9c4b..71fe9bbca8ed 100644
--- a/ci/workflows/pull_request_community.py
+++ b/ci/workflows/pull_request_community.py
@@ -2,12 +2,18 @@
 from praktika import Workflow, Artifact
 
 from ci.defs.defs import BASE_BRANCH, DOCKERS, ArtifactConfigs, JobNames
+from ci.defs.altinity_jobs import AltinityJobConfigs
 from ci.defs.job_configs import JobConfigs
 from ci.jobs.scripts.workflow_hooks.filter_job import should_skip_job
 
+FUNCTIONAL_TESTS_JOBS = [
+    *JobConfigs.functional_tests_jobs,
+    *AltinityJobConfigs.cas_functional_tests_jobs,
+]
+
 FUNCTIONAL_TESTS_PARALLEL_BLOCKING_JOB_NAMES = [
     job.name
-    for job in JobConfigs.functional_tests_jobs
+    for job in FUNCTIONAL_TESTS_JOBS
     if any(
         substr in job.name
         for substr in (
@@ -24,7 +30,7 @@
 ]
 
 PLAIN_FUNCTIONAL_TEST_JOB = [
-    j for j in JobConfigs.functional_tests_jobs if "amd_debug, parallel" in j.name
+    j for j in FUNCTIONAL_TESTS_JOBS if "amd_debug, parallel" in j.name
 ][0]
 
 def _normalize_gh_aliases(items):
@@ -74,7 +80,7 @@ def _normalize_gh_aliases(items):
                 if j.name not in FUNCTIONAL_TESTS_PARALLEL_BLOCKING_JOB_NAMES
                 else []
             )
-            for j in JobConfigs.functional_tests_jobs if 'coverage' not in j.name
+            for j in FUNCTIONAL_TESTS_JOBS if 'coverage' not in j.name
         ],
         *[
             job.set_run_after(FUNCTIONAL_TESTS_PARALLEL_BLOCKING_JOB_NAMES)
diff --git a/ci/workflows/release_branches.py b/ci/workflows/release_branches.py
index 2969c8f10b30..78f43c15d600 100644
--- a/ci/workflows/release_branches.py
+++ b/ci/workflows/release_branches.py
@@ -1,9 +1,15 @@
 from praktika import Workflow
 
 from ci.defs.defs import BINARIES_WITH_LONG_RETENTION, DOCKERS, SECRETS, ArtifactConfigs
+from ci.defs.altinity_jobs import AltinityJobConfigs
 from ci.defs.job_configs import JobConfigs
 from ci.jobs.scripts.workflow_hooks.filter_job import should_skip_job
 
+FUNCTIONAL_TESTS_JOBS = [
+    *JobConfigs.functional_tests_jobs,
+    *AltinityJobConfigs.cas_functional_tests_jobs,
+]
+
 builds_for_release_branch = [
     job
     for job in JobConfigs.build_jobs
@@ -31,7 +37,7 @@
         JobConfigs.docker_server,
         JobConfigs.docker_keeper,
         *JobConfigs.install_check_master_jobs,
-        *[job for job in JobConfigs.functional_tests_jobs if "asan" in job.name],
+        *[job for job in FUNCTIONAL_TESTS_JOBS if "asan" in job.name],
         *[job for job in JobConfigs.unittest_jobs if "fuzzer" not in job.name],
         *[
             job
diff --git a/ci/workflows/release_builds.py b/ci/workflows/release_builds.py
index 5e6165f9a744..d11b1324968e 100644
--- a/ci/workflows/release_builds.py
+++ b/ci/workflows/release_builds.py
@@ -5,6 +5,11 @@
 from ci.defs.job_configs import JobConfigs
 from ci.jobs.scripts.workflow_hooks.filter_job import should_skip_job
 
+FUNCTIONAL_TESTS_JOBS = [
+    *JobConfigs.functional_tests_jobs,
+    *AltinityJobConfigs.cas_functional_tests_jobs,
+]
+
 # Add long retention tags to subset of artifacts
 clickhouse_binaries_with_tags = []
 for artifact in ArtifactConfigs.clickhouse_binaries + ArtifactConfigs.clickhouse_stripped_binaries:
@@ -45,7 +50,7 @@
         AltinityJobConfigs.source_upload_job,
         *[
             job
-            for job in JobConfigs.functional_tests_jobs
+            for job in FUNCTIONAL_TESTS_JOBS
             if any(t in job.name for t in ("release", "binary"))
         ],
     ],
diff --git a/tests/clickhouse-test b/tests/clickhouse-test
index a8bca36f4d0e..9979a8619ff0 100755
--- a/tests/clickhouse-test
+++ b/tests/clickhouse-test
@@ -1352,6 +1352,7 @@ class FailureReason(enum.Enum):
     OBJECT_STORAGE = "object-storage"
     COVERAGE = "coverage"
     S3_STORAGE = "s3-storage"
+    CAS_STORAGE = "cas-storage"
     AZURE_BLOB_STORAGE = "azure-blob-storage"
     BUILD = "not running for current build"
     NO_PARALLEL_REPLICAS = "smth is not supported with parallel replicas"
@@ -2877,18 +2878,32 @@ class TestCase:
 
         if tags and ("no-s3-storage" in tags) and args.s3_storage:
             return FailureReason.S3_STORAGE
+        if (
+            tags
+            and ("no-cas-storage" in tags)
+            and args.cas_storage
+        ):
+            return FailureReason.CAS_STORAGE
         if tags and ("no-azure-blob-storage" in tags) and args.azure_blob_storage:
             return FailureReason.AZURE_BLOB_STORAGE
         if (
             tags
             and ("no-object-storage" in tags)
-            and (args.azure_blob_storage or args.s3_storage)
+            and (
+                args.azure_blob_storage
+                or args.s3_storage
+                or args.cas_storage
+            )
         ):
             return FailureReason.OBJECT_STORAGE
         if (
             tags
             and "no-object-storage-with-slow-build" in tags
-            and (args.s3_storage or args.azure_blob_storage)
+            and (
+                args.s3_storage
+                or args.azure_blob_storage
+                or args.cas_storage
+            )
             and BuildFlags.RELEASE not in args.build_flags
         ):
             return FailureReason.OBJECT_STORAGE
@@ -5624,6 +5639,16 @@ def main(args):
     if args.s3_storage and (BuildFlags.RELEASE not in args.build_flags):
         args.no_random_settings = True
 
+    if args.cas_s3_storage:
+        # CA-over-S3 is a CAS disk AND it is backed by S3 (minio).
+        # Mirror the local-CA gating (no-cas-storage + no-object-storage)
+        # and additionally honour no-s3-storage, since this disk IS s3-backed.
+        # Done AFTER the no_random_settings block above on purpose: like the local-CA
+        # job we keep random (merge-tree) settings; we only want the s3/object-storage
+        # *gating*, not the s3 stateful-data / no-random-settings machinery.
+        args.cas_storage = True
+        args.s3_storage = True
+
     if args.skip:
         args.skip = set(args.skip)
 
@@ -6338,6 +6363,19 @@ def parse_args():
         default=False,
         help="Run tests over s3 storage",
     )
+    parser.add_argument(
+        "--cas-storage",
+        action="store_true",
+        default=False,
+        help="Run tests with a CAS disk as the default MergeTree storage",
+    )
+    parser.add_argument(
+        "--cas-s3-storage",
+        action="store_true",
+        default=False,
+        help="Run tests with a CAS disk backed by S3 (minio) as the default MergeTree storage. "
+        "Implies --cas-storage (object-storage gating) and --s3-storage (s3-storage gating).",
+    )
     parser.add_argument(
         "--distributed-cache",
         action="store_true",
diff --git a/tests/config/config.d/cas_s3_storage_policy_for_merge_tree_by_default.xml b/tests/config/config.d/cas_s3_storage_policy_for_merge_tree_by_default.xml
new file mode 100644
index 000000000000..c73dd01b2338
--- /dev/null
+++ b/tests/config/config.d/cas_s3_storage_policy_for_merge_tree_by_default.xml
@@ -0,0 +1,66 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <cas_s3>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- Phase 0 (mount safety): required, validated identity of the layout subtree this
+                     server owns. Single-server stateless lane → a stable literal id. -->
+                <server_root_id>stateless-ca-s3</server_root_id>
+                <!-- RustFS (M-W D-W8): the CA pool needs ENFORCED conditional ops (exact-token
+                     DELETE must 412 on mismatch) - MinIO OSS silently ignores If-Match on DELETE
+                     and the fail-closed capability probe rejects it. RustFS runs on a dedicated
+                     port (11121); MinIO keeps serving the non-CA s3 disks. Bucket `test`, a
+                     dedicated key prefix for this disk. -->
+                <endpoint>http://localhost:11121/test/cas_s3/</endpoint>
+                <access_key_id>clickhouse</access_key_id>
+                <secret_access_key>clickhouse</secret_access_key>
+                <!-- Real server-local scratch dir for the write-buffer spill. -->
+                <scratch_path>cas_s3_scratch/</scratch_path>
+                <!-- Background GC pacing: the incarnation protocol's fence/recheck handshake
+                     replaces the old grace window (M-W D-W5); a short interval so reclamation
+                     happens during the run. -->
+                <gc_enabled>1</gc_enabled>
+                <gc_interval_sec>5</gc_interval_sec>
+            </cas_s3>
+            <!-- Optional local filesystem cache in front of the cacheless CA-S3 disk (mirrors how plain-s3
+                 backs its policy with `cached_s3`). Validated: once warm it serves reads from local disk
+                 (0 S3 GETs, ~local latency) vs raw CA re-fetching every read from RustFS. NOT the lane
+                 default on purpose — it adds a one-time populate cost to single-pass reads and routes
+                 large single-pass WRITES (e.g. stateful dataset prep) through the cache; the stateless
+                 suite is mostly single-pass, so raw CA is the better default here. Available as an opt-in
+                 policy (`cas_s3_cache`) for workloads that re-read; the merge_tree default
+                 below can be flipped to it in one line. -->
+            <cas_s3_cache>
+                <type>cache</type>
+                <disk>cas_s3</disk>
+                <path>cas_s3_cache/</path>
+                <max_size>5368709120</max_size>
+            </cas_s3_cache>
+        </disks>
+        <policies>
+            <cas_s3>
+                <volumes>
+                    <main>
+                        <disk>cas_s3</disk>
+                    </main>
+                </volumes>
+            </cas_s3>
+            <cas_s3_cache>
+                <volumes>
+                    <main>
+                        <disk>cas_s3_cache</disk>
+                    </main>
+                </volumes>
+            </cas_s3_cache>
+        </policies>
+    </storage_configuration>
+    <merge_tree>
+        <storage_policy>cas_s3</storage_policy>
+    </merge_tree>
+    <!-- System logs are intentionally left on the cas_s3 default policy (the earlier B86
+         workaround that pinned every system log to the local `default` policy has been removed): the CA
+         write path must handle the frequent small-flush system-log workload directly. If a flush stall
+         resurfaces, fix the CA flush path rather than re-routing the logs off it. -->
+</clickhouse>
diff --git a/tests/config/config.d/cas_storage_policy_for_merge_tree_by_default.xml b/tests/config/config.d/cas_storage_policy_for_merge_tree_by_default.xml
new file mode 100644
index 000000000000..18a8aa0c824f
--- /dev/null
+++ b/tests/config/config.d/cas_storage_policy_for_merge_tree_by_default.xml
@@ -0,0 +1,35 @@
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <cas>
+                <type>object_storage</type>
+                <object_storage_type>local</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <!-- Phase 0 (mount safety): required, validated identity of the layout subtree this
+                     server owns. Single-server stateless lane → a stable literal id. -->
+                <server_root_id>stateless-ca-local</server_root_id>
+                <!-- Local object-storage pool root, under the server store. -->
+                <path>cas_pool/</path>
+                <!-- Real server-local scratch dir for the write-buffer spill. -->
+                <scratch_path>cas_scratch/</scratch_path>
+                <!-- Background GC pacing: the incarnation protocol's fence/recheck handshake
+                     replaces the old grace window (M-W D-W5); a short interval so reclamation
+                     happens during the run. -->
+                <gc_enabled>1</gc_enabled>
+                <gc_interval_sec>5</gc_interval_sec>
+            </cas>
+        </disks>
+        <policies>
+            <cas>
+                <volumes>
+                    <main>
+                        <disk>cas</disk>
+                    </main>
+                </volumes>
+            </cas>
+        </policies>
+    </storage_configuration>
+    <merge_tree>
+        <storage_policy>cas</storage_policy>
+    </merge_tree>
+</clickhouse>
diff --git a/tests/config/install.sh b/tests/config/install.sh
index 359cd9cfd6f8..3c3311a96271 100755
--- a/tests/config/install.sh
+++ b/tests/config/install.sh
@@ -29,6 +29,8 @@ while [[ "$#" -gt 0 ]]; do
         --fast-test) FAST_TEST=1 && EXPORT_S3_STORAGE_POLICIES=0 ;;
         --analyzer) USE_OLD_ANALYZER=1 ;;
         --s3-storage) EXPORT_S3_STORAGE_POLICIES=1 && USE_S3_STORAGE_FOR_MERGE_TREE=1 && RANDOMIZE_OBJECT_KEY_TYPE=1 ;;
+        --cas-storage) USE_CAS_STORAGE_FOR_MERGE_TREE=1 ;;
+        --cas-s3-storage) USE_CAS_S3_STORAGE_FOR_MERGE_TREE=1 ;;
         --parallel-rep) USE_PARALLEL_REPLICAS=1 ;;
         --db-replicated) USE_DATABASE_REPLICATED=1 ;;
         --distributed-plan) USE_DISTRIBUTED_PLAN=1 ;;
@@ -373,6 +375,20 @@ if [[ "$USE_S3_STORAGE_FOR_MERGE_TREE" == "1" ]]; then
     else
         ln -sf $SRC_PATH/config.d/s3_storage_policy_for_merge_tree_by_default.xml $DEST_SERVER_PATH/config.d/
     fi
+elif [[ "$USE_CAS_STORAGE_FOR_MERGE_TREE" == "1" ]]; then
+    # Content-addressed disk (over local object storage) as the default MergeTree policy.
+    # Installs ONLY this config so the suite runs with CA-as-default without disturbing
+    # other storage variants (no s3/azure default policies are set up in this mode).
+    ln -sf $SRC_PATH/config.d/cas_storage_policy_for_merge_tree_by_default.xml $DEST_SERVER_PATH/config.d/
+elif [[ "$USE_CAS_S3_STORAGE_FOR_MERGE_TREE" == "1" ]]; then
+    # Content-addressed disk over S3 (minio) as the default MergeTree policy. This is the
+    # real-S3 (north star) counterpart of --cas-storage, which uses local
+    # object storage. minio is started unconditionally by the stateless praktika job (see
+    # ci/jobs/functional_tests.py start_minio), so the disk just needs minio reachable at
+    # localhost:11111 with the `test` bucket and the clickhouse/clickhouse creds, which
+    # setup_minio.sh provides. Installs ONLY this policy to keep the variant clean (no s3/azure
+    # default policies are set up in this mode).
+    ln -sf $SRC_PATH/config.d/cas_s3_storage_policy_for_merge_tree_by_default.xml $DEST_SERVER_PATH/config.d/
 elif [[ "$USE_AZURE_STORAGE_FOR_MERGE_TREE" == "1" ]]; then
     if [[ -n "$USE_ENCRYPTED_STORAGE" ]] && [[ "$USE_ENCRYPTED_STORAGE" -eq 1 ]]; then
         ln -sf $SRC_PATH/config.d/azure_encrypted_storage_policy_by_default.xml $DEST_SERVER_PATH/config.d/
diff --git a/tests/queries/0_stateless/01271_show_privileges.reference b/tests/queries/0_stateless/01271_show_privileges.reference
index cc318eca54d0..b42b919113aa 100644
--- a/tests/queries/0_stateless/01271_show_privileges.reference
+++ b/tests/queries/0_stateless/01271_show_privileges.reference
@@ -158,6 +158,13 @@ SYSTEM RELOAD ASYNCHRONOUS METRICS	['RELOAD ASYNCHRONOUS METRICS']	GLOBAL	SYSTEM
 SYSTEM RECONNECT ZOOKEEPER	['SYSTEM RECONNECT ZOOKEEPER','RECONNECT ZOOKEEPER']	GLOBAL	SYSTEM
 SYSTEM RELOAD	[]	\N	SYSTEM
 SYSTEM RESTART DISK	['SYSTEM RESTART DISK']	GLOBAL	SYSTEM
+SYSTEM CAS GC RUN	['SYSTEM CAS GC RUN']	GLOBAL	SYSTEM
+SYSTEM CAS GC REBUILD	['SYSTEM CAS GC REBUILD']	GLOBAL	SYSTEM
+SYSTEM CAS DROP POOL MEMBER	['SYSTEM CAS DROP POOL MEMBER']	GLOBAL	SYSTEM
+SYSTEM CAS FSCK	['SYSTEM CAS FSCK']	GLOBAL	SYSTEM
+SYSTEM CAS FORGET	['SYSTEM CAS FORGET']	GLOBAL	SYSTEM
+SYSTEM CAS GC STOP	['SYSTEM CAS GC STOP']	GLOBAL	SYSTEM
+SYSTEM CAS GC START	['SYSTEM CAS GC START']	GLOBAL	SYSTEM
 SYSTEM WAIT BLOBS CLEANUP	['SYSTEM WAIT BLOBS CLEANUP']	GLOBAL	SYSTEM
 SYSTEM MERGES	['SYSTEM STOP MERGES','SYSTEM START MERGES','STOP MERGES','START MERGES']	TABLE	SYSTEM
 SYSTEM TTL MERGES	['SYSTEM STOP TTL MERGES','SYSTEM START TTL MERGES','STOP TTL MERGES','START TTL MERGES']	TABLE	SYSTEM
diff --git a/tests/queries/0_stateless/02253_empty_part_checksums.sh b/tests/queries/0_stateless/02253_empty_part_checksums.sh
index ba45d7d0b177..af4e1d896658 100755
--- a/tests/queries/0_stateless/02253_empty_part_checksums.sh
+++ b/tests/queries/0_stateless/02253_empty_part_checksums.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-# Tags: zookeeper, no-replicated-database, no-shared-merge-tree
+# Tags: zookeeper, no-replicated-database, no-shared-merge-tree, no-cas-storage
+# no-cas-storage: test asserts system.parts.path is an absolute local FS path; on a cas disk the path is a relative object-storage key (orthogonal part-file path-shape)
 # no-replicated-database because it adds extra replicas
 # no-shared-merge-tree do something with parts on local fs
 # add_minmax_index_for_numeric_columns=0: Adds extra files, which changes the hashes
diff --git a/tests/queries/0_stateless/02254_projection_broken_part.sh b/tests/queries/0_stateless/02254_projection_broken_part.sh
index 04a0c4fb0a19..84f4ef8bfe53 100755
--- a/tests/queries/0_stateless/02254_projection_broken_part.sh
+++ b/tests/queries/0_stateless/02254_projection_broken_part.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-# Tags: long, zookeeper, no-shared-merge-tree
+# Tags: long, zookeeper, no-shared-merge-tree, no-cas-storage
+# no-cas-storage: test asserts system.parts.path is an absolute local FS path; on a cas disk the path is a relative object-storage key (orthogonal part-file path-shape)
 
 CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 # shellcheck source=../shell_config.sh
diff --git a/tests/queries/0_stateless/02255_broken_parts_chain_on_start.sh b/tests/queries/0_stateless/02255_broken_parts_chain_on_start.sh
index 888ac73e4ab1..de16ba1a0bff 100755
--- a/tests/queries/0_stateless/02255_broken_parts_chain_on_start.sh
+++ b/tests/queries/0_stateless/02255_broken_parts_chain_on_start.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-# Tags: long, zookeeper, no-shared-merge-tree
+# Tags: long, zookeeper, no-shared-merge-tree, no-cas-storage
+# no-cas-storage: test asserts system.parts.path is an absolute local FS path; on a cas disk the path is a relative object-storage key (orthogonal part-file path-shape)
 
 CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 # shellcheck source=../shell_config.sh
diff --git a/tests/queries/0_stateless/02369_lost_part_intersecting_merges.sh b/tests/queries/0_stateless/02369_lost_part_intersecting_merges.sh
index 68ff2222dfec..cc4a3b53b957 100755
--- a/tests/queries/0_stateless/02369_lost_part_intersecting_merges.sh
+++ b/tests/queries/0_stateless/02369_lost_part_intersecting_merges.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-# Tags: zookeeper, no-shared-merge-tree, long
+# Tags: zookeeper, no-shared-merge-tree, long, no-cas-storage
+# no-cas-storage: test asserts system.parts.path is an absolute local FS path; on a cas disk the path is a relative object-storage key (orthogonal part-file path-shape)
 # no-shared-merge-tree: depend on local fs
 
 CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
diff --git a/tests/queries/0_stateless/02370_lost_part_intersecting_merges.sh b/tests/queries/0_stateless/02370_lost_part_intersecting_merges.sh
index dff5b5f5eccf..72a749f0ec8e 100755
--- a/tests/queries/0_stateless/02370_lost_part_intersecting_merges.sh
+++ b/tests/queries/0_stateless/02370_lost_part_intersecting_merges.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-# Tags: long, zookeeper, no-shared-merge-tree, no-parallel
+# Tags: long, zookeeper, no-shared-merge-tree, no-parallel, no-cas-storage
+# no-cas-storage: test asserts system.parts.path is an absolute local FS path; on a cas disk the path is a relative object-storage key (orthogonal part-file path-shape)
 # no-shared-merge-tree: depend on local fs (remove parts)
 
 CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
diff --git a/tests/queries/0_stateless/02444_async_broken_outdated_part_loading.sh b/tests/queries/0_stateless/02444_async_broken_outdated_part_loading.sh
index ff756fb620e8..2f3f38cfba4c 100755
--- a/tests/queries/0_stateless/02444_async_broken_outdated_part_loading.sh
+++ b/tests/queries/0_stateless/02444_async_broken_outdated_part_loading.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-# Tags: long, zookeeper, no-shared-merge-tree
+# Tags: long, zookeeper, no-shared-merge-tree, no-cas-storage
+# no-cas-storage: test asserts system.parts.path is an absolute local FS path; on a cas disk the path is a relative object-storage key (orthogonal part-file path-shape)
 
 CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 # shellcheck source=../shell_config.sh
diff --git a/tests/queries/0_stateless/02486_truncate_and_unexpected_parts.sql b/tests/queries/0_stateless/02486_truncate_and_unexpected_parts.sql
index 29946e315544..c9cec021a6ec 100644
--- a/tests/queries/0_stateless/02486_truncate_and_unexpected_parts.sql
+++ b/tests/queries/0_stateless/02486_truncate_and_unexpected_parts.sql
@@ -1,4 +1,3 @@
-
 create table rmt (n int) engine=ReplicatedMergeTree('/test/02468/{database}', '1') order by tuple() partition by n % 2 settings replicated_max_ratio_of_wrong_parts=0, max_suspicious_broken_parts=0, max_suspicious_broken_parts_bytes=0;
 create table rmt1 (n int) engine=ReplicatedMergeTree('/test/02468/{database}', '2') order by tuple() partition by n % 2 settings replicated_max_ratio_of_wrong_parts=0, max_suspicious_broken_parts=0, max_suspicious_broken_parts_bytes=0;
 
diff --git a/tests/queries/0_stateless/02980_s3_plain_DROP_TABLE_MergeTree.sh b/tests/queries/0_stateless/02980_s3_plain_DROP_TABLE_MergeTree.sh
index 5648db32d189..fe3dbd37e4d2 100755
--- a/tests/queries/0_stateless/02980_s3_plain_DROP_TABLE_MergeTree.sh
+++ b/tests/queries/0_stateless/02980_s3_plain_DROP_TABLE_MergeTree.sh
@@ -1,5 +1,8 @@
 #!/usr/bin/env bash
-# Tags: no-fasttest, no-random-settings, no-random-merge-tree-settings, no-encrypted-storage
+# Tags: no-fasttest, no-random-settings, no-random-merge-tree-settings, no-encrypted-storage, no-cas-storage
+# Tag no-cas-storage: the test uses an Ordinary database, whose BACKUP path goes via
+# temporary hard links - not supported on a cas disk (Code 344 SUPPORT_IS_DISABLED;
+# BACKUP/RESTORE, B16/B34). Re-checked on the T13 CA-S3 lane (2026-06-12): still fails for this reason.
 # Tag no-fasttest: requires S3
 # Tag no-random-settings, no-random-merge-tree-settings: to avoid creating extra files like serialization.json, this test too exocit anyway
 
diff --git a/tests/queries/0_stateless/02980_s3_plain_DROP_TABLE_ReplicatedMergeTree.sh b/tests/queries/0_stateless/02980_s3_plain_DROP_TABLE_ReplicatedMergeTree.sh
index 865a43d91ef3..023e6fd8c529 100755
--- a/tests/queries/0_stateless/02980_s3_plain_DROP_TABLE_ReplicatedMergeTree.sh
+++ b/tests/queries/0_stateless/02980_s3_plain_DROP_TABLE_ReplicatedMergeTree.sh
@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-# Tags: no-fasttest, no-random-settings, no-random-merge-tree-settings, no-shared-merge-tree, no-encrypted-storage
+# Tags: no-fasttest, no-random-settings, no-random-merge-tree-settings, no-shared-merge-tree, no-encrypted-storage, no-cas-storage
+# no-cas-storage: BACKUP via temporary hard links is not supported on a cas disk (Code 344 SUPPORT_IS_DISABLED; BACKUP/RESTORE, B16/B34)
 # Tag no-fasttest: requires S3
 # Tag no-random-settings, no-random-merge-tree-settings: to avoid creating extra files like serialization.json, this test too exocit anyway
 # Tag no-shared-merge-tree: use database ordinary
diff --git a/tests/queries/0_stateless/03350_alter_table_fetch_partition_thread_pool.sql b/tests/queries/0_stateless/03350_alter_table_fetch_partition_thread_pool.sql
index f5cbd809eef2..dacad1a30788 100644
--- a/tests/queries/0_stateless/03350_alter_table_fetch_partition_thread_pool.sql
+++ b/tests/queries/0_stateless/03350_alter_table_fetch_partition_thread_pool.sql
@@ -1,4 +1,5 @@
 -- Tags: no-parallel, no-replicated-database, no-shared-merge-tree
+-- no-cas-storage: FETCH PARTITION is supported on a cas disk (the gate is lifted, byte-fetch lands into detached/, see 05002), but this test fetches a 100-part partition CONCURRENTLY via the FETCH thread pool. Those parallel fetches all read-modify-write the SHARED "detached" ref object; the read side of that hot pointer object is not serialized against the truncating in-place rewrite of the local object storage, so a concurrent reader can see a torn ref/manifest (CANNOT_READ_ALL_DATA / NO_FILE_IN_DATA_PART). The atomic pointer-object publish needed to make concurrent fan-out safe is a deferred backlog item (B66a); single-part FETCH works (01650 + 05002).
 -- Tag: no-parallel - to avoid polluting FETCH PARTITION thread pool with other fetches
 -- Tag: no-replicated-database - replica_path is different
 
diff --git a/tests/queries/0_stateless/03352_allow_suspicious_ttl.sql b/tests/queries/0_stateless/03352_allow_suspicious_ttl.sql
index 5fd2bb3bf3a4..2a23baffe13c 100644
--- a/tests/queries/0_stateless/03352_allow_suspicious_ttl.sql
+++ b/tests/queries/0_stateless/03352_allow_suspicious_ttl.sql
@@ -1,4 +1,4 @@
- -- Tags: long, zookeeper
+-- Tags: long, zookeeper
 
 -- Replicated
 
diff --git a/tests/queries/0_stateless/03541_rename_column_start.sql b/tests/queries/0_stateless/03541_rename_column_start.sql
index b5d4fa03f18a..0fa8af8b26b8 100644
--- a/tests/queries/0_stateless/03541_rename_column_start.sql
+++ b/tests/queries/0_stateless/03541_rename_column_start.sql
@@ -1,4 +1,4 @@
- -- Tags: zookeeper
+-- Tags: zookeeper
 
 CREATE TABLE rmt (a UInt64, b UInt64)
 ENGINE=ReplicatedMergeTree('/clickhouse/tables/{database}/rmt', '1')
diff --git a/tests/queries/0_stateless/03829_insert_deduplication_info_memory.sql b/tests/queries/0_stateless/03829_insert_deduplication_info_memory.sql
index 2dc33bfd11c1..f03853a88cbe 100644
--- a/tests/queries/0_stateless/03829_insert_deduplication_info_memory.sql
+++ b/tests/queries/0_stateless/03829_insert_deduplication_info_memory.sql
@@ -8,9 +8,13 @@ DROP TABLE IF EXISTS t_dedup_memory;
 CREATE TABLE t_dedup_memory (x UInt32, fat FixedString(10000)) ENGINE = MergeTree ORDER BY x;
 
 -- 10 000 rows * 10 000 bytes FixedString ≈ 100 MB of column data.
--- With the bug, original_block doubles this to ~200 MB, exceeding the limit.
--- Without the bug, only the data columns are held, fitting within the limit.
-SET max_memory_usage = '150M';
+-- With the bug, original_block doubles this to ~200 MB, which must exceed the limit.
+-- Without the bug, only the data columns are held (peak ≈ 143 MB), which must fit under it.
+-- The limit sits between those two peaks with enough headroom to absorb small, fixed write-path
+-- buffering overhead (e.g. a content-addressed disk's insert path adds ~0.5 MB on top of the ~143 MB
+-- baseline) so the test still runs unchanged on such a storage backend, while remaining well below the
+-- ~200 MB doubling-bug peak it is meant to catch.
+SET max_memory_usage = '170M';
 
 INSERT INTO t_dedup_memory SELECT number, toString(number) FROM numbers(10000)
     SETTINGS max_insert_threads = 1, min_insert_block_size_rows = 0, min_insert_block_size_bytes = 0;
diff --git a/tests/queries/0_stateless/04215_replicated_missing_covered_part_on_start.sh b/tests/queries/0_stateless/04215_replicated_missing_covered_part_on_start.sh
index f7fb4b10817b..7c1a2df1c8fb 100755
--- a/tests/queries/0_stateless/04215_replicated_missing_covered_part_on_start.sh
+++ b/tests/queries/0_stateless/04215_replicated_missing_covered_part_on_start.sh
@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
-# Tags: long, zookeeper, no-shared-merge-tree
+# Tags: long, zookeeper, no-shared-merge-tree, no-cas-storage
 # no-shared-merge-tree: depends on local fs
+# no-cas-storage: test asserts system.parts.path is an absolute local FS path; on a cas disk the path is a relative object-storage key (orthogonal part-file path-shape)
 
 CUR_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
 # shellcheck source=../shell_config.sh
diff --git a/tests/queries/0_stateless/04316_reader_executor_basic.sql b/tests/queries/0_stateless/04316_reader_executor_basic.sql
index 950db07f0003..f063513d9199 100644
--- a/tests/queries/0_stateless/04316_reader_executor_basic.sql
+++ b/tests/queries/0_stateless/04316_reader_executor_basic.sql
@@ -1,9 +1,12 @@
--- Tags: no-distributed-cache, no-encrypted-storage
+-- Tags: no-distributed-cache, no-encrypted-storage, no-cas-storage
 -- The executor does not implement the distributed cache or decryption, so it
 -- falls back on those storage configs and the activation check below would not
 -- hold. Those stages can't be turned off from the test (unlike async prefetch
 -- and the filesystem cache), so skip them; the test still runs on local disk and
--- plain object storage where the executor engages.
+-- plain object storage where the executor engages. Content-addressed storage
+-- always adds a `file_view` stage (the payload is a byte window inside a shared
+-- blob), which the executor falls back on the same way -- see
+-- `ReadPipeline::tryBuildReaderExecutor` -- so it never engages there either.
 --
 -- Smoke test for the experimental ReaderExecutor read path. Reads a MergeTree
 -- table with `use_reader_executor = 1`, checks the data comes back correct (full
diff --git a/tests/queries/0_stateless/04327_reader_executor_metrics.sql b/tests/queries/0_stateless/04327_reader_executor_metrics.sql
index 9ec8766021d8..fead6d70ab72 100644
--- a/tests/queries/0_stateless/04327_reader_executor_metrics.sql
+++ b/tests/queries/0_stateless/04327_reader_executor_metrics.sql
@@ -1,8 +1,10 @@
--- Tags: no-distributed-cache, no-encrypted-storage
+-- Tags: no-distributed-cache, no-encrypted-storage, no-cas-storage
 -- Like 04316, the executor falls back on the distributed cache and decryption
 -- (which can't be disabled from the test), so its metrics would not be emitted on
 -- those storage configs. Skip them; the test still runs on local disk and plain
--- object storage where the executor engages.
+-- object storage where the executor engages. Content-addressed storage always
+-- adds a `file_view` stage (byte window inside a shared blob), which the
+-- executor falls back on the same way -- see `ReadPipeline::tryBuildReaderExecutor`.
 --
 -- Checks that the experimental ReaderExecutor emits its observability metrics.
 -- Reads a MergeTree table with `use_reader_executor = 1` and verifies, via the
diff --git a/tests/queries/0_stateless/04328_reader_executor_kpi_async_metric.sql b/tests/queries/0_stateless/04328_reader_executor_kpi_async_metric.sql
index 32988b1ea311..71063e563c4d 100644
--- a/tests/queries/0_stateless/04328_reader_executor_kpi_async_metric.sql
+++ b/tests/queries/0_stateless/04328_reader_executor_kpi_async_metric.sql
@@ -1,7 +1,9 @@
--- Tags: no-distributed-cache, no-encrypted-storage
+-- Tags: no-distributed-cache, no-encrypted-storage, no-cas-storage
 -- The executor falls back on the distributed cache and decryption (which can't be
 -- disabled from the test), so its metrics would not be emitted there; skip those
--- configs (as in 04316 / 04327).
+-- configs (as in 04316 / 04327). Content-addressed storage always adds a
+-- `file_view` stage (byte window inside a shared blob), which the executor
+-- falls back on the same way -- see `ReadPipeline::tryBuildReaderExecutor`.
 --
 -- End-to-end check that the modeled-cost KPI asynchronous metric
 -- `ReaderExecutorModeledCostMsPerRequestedMiB` moves when the executor does work.

From 14ecc1f9922a7be743c38a2d7a813ccbc87fc45c Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:41 +0200
Subject: [PATCH 29/30] CAS documentation: the Antalya CAS book

The dedicated documentation set under docs/en/antalya/cas/: architecture
(storage layout, manifests and refs, blob protocol, part lifecycle, read
path, replication, mounts and leases, namespaces, garbage collection,
correctness, design history) and operations (configuration, bucket
requirements, monitoring, debugging, troubleshooting, migration).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 docs/en/antalya/cas/architecture/backend.md   | 100 +++++++
 .../antalya/cas/architecture/blob-protocol.md | 232 +++++++++++++++
 .../antalya/cas/architecture/correctness.md   |  91 ++++++
 .../cas/architecture/design-history.md        |  61 ++++
 .../cas/architecture/garbage-collection.md    | 263 ++++++++++++++++++
 docs/en/antalya/cas/architecture/index.md     | 114 ++++++++
 .../cas/architecture/manifests-and-refs.md    | 260 +++++++++++++++++
 .../cas/architecture/mounts-and-leases.md     | 223 +++++++++++++++
 .../en/antalya/cas/architecture/namespaces.md | 172 ++++++++++++
 .../cas/architecture/part-lifecycle.md        | 148 ++++++++++
 docs/en/antalya/cas/architecture/read-path.md |  84 ++++++
 .../antalya/cas/architecture/replication.md   | 118 ++++++++
 .../cas/architecture/storage-layout.md        | 160 +++++++++++
 docs/en/antalya/cas/bucket-requirements.md    |  44 +++
 docs/en/antalya/cas/configuration.md          | 132 +++++++++
 docs/en/antalya/cas/index.md                  |  89 ++++++
 docs/en/antalya/cas/operations/debugging.md   | 225 +++++++++++++++
 docs/en/antalya/cas/operations/migration.md   | 209 ++++++++++++++
 docs/en/antalya/cas/operations/monitoring.md  | 101 +++++++
 .../antalya/cas/operations/troubleshooting.md |  29 ++
 docs/en/antalya/cas/quick-start.md            | 145 ++++++++++
 docs/en/antalya/cas/roadmap.md                | 108 +++++++
 22 files changed, 3108 insertions(+)
 create mode 100644 docs/en/antalya/cas/architecture/backend.md
 create mode 100644 docs/en/antalya/cas/architecture/blob-protocol.md
 create mode 100644 docs/en/antalya/cas/architecture/correctness.md
 create mode 100644 docs/en/antalya/cas/architecture/design-history.md
 create mode 100644 docs/en/antalya/cas/architecture/garbage-collection.md
 create mode 100644 docs/en/antalya/cas/architecture/index.md
 create mode 100644 docs/en/antalya/cas/architecture/manifests-and-refs.md
 create mode 100644 docs/en/antalya/cas/architecture/mounts-and-leases.md
 create mode 100644 docs/en/antalya/cas/architecture/namespaces.md
 create mode 100644 docs/en/antalya/cas/architecture/part-lifecycle.md
 create mode 100644 docs/en/antalya/cas/architecture/read-path.md
 create mode 100644 docs/en/antalya/cas/architecture/replication.md
 create mode 100644 docs/en/antalya/cas/architecture/storage-layout.md
 create mode 100644 docs/en/antalya/cas/bucket-requirements.md
 create mode 100644 docs/en/antalya/cas/configuration.md
 create mode 100644 docs/en/antalya/cas/index.md
 create mode 100644 docs/en/antalya/cas/operations/debugging.md
 create mode 100644 docs/en/antalya/cas/operations/migration.md
 create mode 100644 docs/en/antalya/cas/operations/monitoring.md
 create mode 100644 docs/en/antalya/cas/operations/troubleshooting.md
 create mode 100644 docs/en/antalya/cas/quick-start.md
 create mode 100644 docs/en/antalya/cas/roadmap.md

diff --git a/docs/en/antalya/cas/architecture/backend.md b/docs/en/antalya/cas/architecture/backend.md
new file mode 100644
index 000000000000..fab4d23e323b
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/backend.md
@@ -0,0 +1,100 @@
+---
+description: 'The Cas::Backend storage seam, its token contract, the per-provider conditional-write dialects, and the mount-time capability probe.'
+sidebar_label: 'Backend abstraction'
+sidebar_position: 11
+slug: /antalya/cas/architecture/backend
+title: 'CAS Architecture — Backend Abstraction'
+doc_type: 'reference'
+---
+
+# Backend abstraction {#backend-abstraction}
+
+Every protocol described elsewhere in this set — blobs, manifests, refs, mounts, GC — is written
+against one interface, `Cas::Backend` (`Backend/CasBackend.h`). It is a token-aware storage seam:
+every present key has exactly one current incarnation identified by an opaque `Token`, and
+`putOverwrite`/`casPut` succeed only against the expected current token (or expected absence).
+
+## The interface {#interface}
+
+| Method | Contract |
+|---|---|
+| `get` / `getStream` | Read bytes (or a forward-only stream, for write-once objects) plus the token of the incarnation read |
+| `head` | Existence, size, token, and metadata without reading the body |
+| `putIfAbsent` / `putIfAbsentStream` | Create-if-absent (`If-None-Match: *`); `PreconditionFailed` is a returned outcome, never an exception |
+| `putOverwrite` | Replace the current object only when its token equals `expected`; a mismatch is a returned outcome |
+| `casPut` | `expected == nullopt` ⇒ create-if-absent CAS (used for the first write of a root object); a set `expected` conditionally replaces that exact incarnation |
+| `deleteExact` | Delete only the incarnation named by `token`; a token mismatch (`TokenMismatch`) leaves the object untouched and is distinguished from `NotFound` |
+| `list` | One page of keys under a prefix, resumed by the backend's own cursor |
+| `supportsListTokens` | Whether `list` can surface a per-key incarnation token, letting GC discovery skip an unchanged root shard without a `GET` |
+| `promoteStaged` / `resurrect` | `promoteStaged`: write-once server-side copy from S3 staging (optional, defaults to `NOT_IMPLEMENTED`). `resurrect`: unconditional re-upload displacing a condemned incarnation from a caller-supplied reader (streamed on remote object storage, materialized one-at-a-time on the local emulated mode); size-checked before publication, and fresh-tagged so pending deletes of the old incarnation cannot remove it |
+
+`deleteExact`, `putIfAbsent`/`putIfAbsentStream`, and `putOverwrite`/`casPut` are safety-critical:
+they are what makes exact-token deletes, write-once creation, and mutual exclusion hold. Every
+other method is protocol hygiene.
+
+**`TOKEN ⟹ CONTENT`** is the one contract item the capability probe cannot check: a token must
+uniquely identify the byte content of the incarnation it labels, so that a repeated token never
+means different bytes. The read-path decode cache skips a re-read on a token match, so a backend
+that recycled tokens across different content would serve stale manifests — a wrong-result bug, not
+merely inefficiency. `S3` `ETag`s are content-derived; the in-memory and emulated backends mint a
+strictly monotonic sequence that is never reused. This remains a standing requirement of every
+backend implementation, not a property the probe verifies.
+
+## Provider dialects {#dialects}
+
+`ObjectStorageBackend` (`Backend/CasObjectStorageBackend.cpp`) wraps one `IObjectStorage` and picks
+its token dialect from `IObjectStorage::conditionalOpsUseGenerationTokens()`:
+
+| Dialect | Token type | How a conditional write is expressed |
+|---|---|---|
+| `AWS` (default) | `ETag` | `If-None-Match: *` / `If-Match: <etag>` sent as-is |
+| `GCS` | `Generation` | The backend rewrites conditional headers before the request goes out: `If-None-Match: *` becomes `x-goog-if-generation-match: 0`, and `If-Match: <etag>` becomes `x-goog-if-generation-match: <generation>` (`applyGcsConditionalDialectToRequest`, `IO/S3/GCSConditionalDialect.cpp`) |
+
+The GCS dialect is opted into by client configuration (`http_client = gcs_hmac` or `gcp_oauth`), not
+auto-detected from the endpoint host. It also rejects one shape outright: a **conditional
+`CompleteMultipartUpload`** throws rather than silently dropping the precondition, because GCS
+ignores preconditions on that call — a measured, documented gap, not a hypothetical one. `CAS`'s
+conditional writes therefore always take the single-`PUT` path on a generation-dialect backend.
+
+This bounds conditional writes only: the write-once create is therefore single-part and limited by
+`gcs_max_conditional_put_bytes` on a generation dialect, while the unconditional resurrect takes the
+ordinary multipart path and has no size limit on any backend.
+
+Every request carrying a rewritten header also has its AWS auth headers stripped and every
+remaining `x-amz-*` header renamed to `x-goog-*`, since GCS rejects a mixed header set.
+
+Azure Blob Storage's REST API documents equivalent conditional headers (`If-None-Match`,
+`If-Match`), but no third dialect exists in this backend yet — `IObjectStorage`'s Azure
+implementation does not currently wire up a `CAS` conditional path, so Azure is untested by the
+capability probe below, not merely a slower-verified third case.
+
+## Exact-token delete, per provider {#exact-token-delete}
+
+`deleteExact(key, token)` is realized as a conditional `DELETE` naming the token as a precondition:
+an `If-Match`-style delete on `AWS` (`ETag`), a generation-match delete on `GCS`. A precondition
+failure — `S3::isPreconditionFailedError` — is reported as `DeleteOutcome::TokenMismatch`, never as
+an exception; the object is left untouched. `DeleteOutcome::created_delete_marker` reports whether
+the backend created a delete marker instead of actually removing the object, which the capability
+probe rejects: a bucket with versioning enabled would let `CAS` "delete" a blob without freeing any
+storage, and GC would silently stop reclaiming.
+
+## The capability probe {#capability-probe}
+
+`runCapabilityProbe` (`Backend/CasProbe.cpp`) runs a throwaway-key battery against every writable
+mount, described in full on the [bucket requirements](/antalya/cas/bucket-requirements) page. It is
+fail-closed: any check that does not pass throws `NOT_IMPLEMENTED` naming the specific failure, and
+the mount refuses to become writable. Two mount-time gates sit alongside it:
+
+- `checkPoolPreconditions` — on the `GCS`-dialect combination only, verifies bucket versioning is
+  off (a confirmed `Enabled` throws; an inconclusive check proceeds under an assumption, logged at
+  `WARNING`, that versioning is off).
+- `checkConditionalWriteSingleAttemptSupport` — refuses to mount writable unless the underlying
+  object storage supports a single-HTTP-attempt retry profile for conditional writes. A hidden SDK
+  retry can outlive the writer's mount lease and obscure whether a conditional operation actually
+  committed, so retries on the conditional path must be explicit CAS state-machine transitions, not
+  transparent client behavior.
+
+A third, optional probe (`probeConditionalCopy`) checks whether the backend enforces a write-once
+conditional server-side copy. It only matters for `staging_backend = s3`: when the probe reports
+`false`, S3-native staging silently falls back to local staging rather than refusing to mount, since
+enforcement here is an optimization, not a correctness requirement of the disk itself.
diff --git a/docs/en/antalya/cas/architecture/blob-protocol.md b/docs/en/antalya/cas/architecture/blob-protocol.md
new file mode 100644
index 000000000000..64a62511afba
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/blob-protocol.md
@@ -0,0 +1,232 @@
+---
+description: 'How CAS writes, deduplicates, and reclaims a blob: conditional-write sequencing, the writer-versus-GC race, and the deterministic-artifact adoption pin.'
+sidebar_label: 'Blob protocol'
+sidebar_position: 3
+slug: /antalya/cas/architecture/blob-protocol
+title: 'CAS Architecture — Blob Protocol'
+doc_type: 'reference'
+---
+
+# CAS architecture — blob protocol {#blob-protocol}
+
+A blob is the unit of content-addressed storage: one part file's bytes, keyed by a hash of its
+own content. This page covers how a blob gets written exactly once, how a duplicate write is
+turned into a no-op, and how a writer and a `GC` round racing over the same blob are kept safe
+without ever comparing multi-gigabyte bodies. Object layout and the four durable object kinds
+are covered on the [overview page](/antalya/cas/architecture/); `GC`'s fold and round structure
+is covered on the GC page.
+
+## Conditional-write sequence {#conditional-write-sequence}
+
+Every blob body lives at a key derived purely from its content hash
+(`blobs/<algo>/<hex[0:2]>/<hex>`, `CasLayout::blobKey`), with a sidecar `.meta` object at the
+same key plus `.meta`. Because the key already encodes the digest, the backend never needs a
+compare-and-swap on content — only on *presence* (`PUT` with `If-None-Match: *`) or on a specific
+prior incarnation (`PUT`/`DELETE` with `If-Match: <token>`).
+
+```mermaid
+sequenceDiagram
+    autonumber
+    participant Writer
+    participant S3 as Object store
+
+    Writer->>Writer: hash source, derive key from digest
+    alt dedup cache hit OR size >= deduplication_head_first_min_bytes
+        Writer->>S3: HEAD blobs/algo/hex
+        alt body present
+            Writer->>S3: GET .meta (point read, body never streamed)
+            Writer->>Writer: adopt current token if Clean or absent
+        else body absent
+            Writer->>S3: putIfAbsentStream (If-None-Match: star)
+        end
+    else small, no cache hit
+        Writer->>S3: putIfAbsentStream (If-None-Match: star) directly
+    end
+    S3-->>Writer: Done -- fresh upload, write Clean meta
+    S3-->>Writer: PreconditionFailed -- someone occupies the key
+    opt on PreconditionFailed
+        Writer->>S3: HEAD blobs/algo/hex
+        Writer->>S3: GET .meta -- adopt the occupant's token as a dependency
+    end
+```
+
+Ordered steps (`Pool/CasPartWriteTxn.cpp:160-245` and `:427-779`):
+
+1. `requireAlive()` — the build is not abandoned, the namespace not dropped, the writer epoch
+   still live.
+2. **Adaptive dedup gate.** `HEAD` first if the dedup cache reports the content present, or the
+   object is at least `deduplication_head_first_min_bytes` (default 1 MiB). Below that threshold
+   a speculative conditional `PUT` is cheaper than a `HEAD` plus a `PUT`.
+3. On a `HEAD` hit, `observeAndAdmit` point-reads the `.meta` sidecar and adopts the live
+   incarnation — the body is never streamed for a dedup hit.
+4. Otherwise a bounded retry loop (up to 8 attempts) around `uploadFromSource`, which mints a
+   **fresh `incarnation_tag` per attempt** and does either a conditional server-side `COPY` from
+   S3 staging or a streaming `putIfAbsentStream`. The byte count is verified against the declared
+   source size.
+5. A 412 means someone occupies the key. Because the key embeds the content digest, **any
+   occupant is by definition the intended content** — ambiguity is resolved by one `HEAD`
+   (occupancy), never by comparing bodies.
+6. `Unresolved` (timeout, 5xx, connection loss) never acks. It throws retry-later — nothing was
+   published, so a body that lands late is inert debris for the orphan sweep.
+
+**Two writers uploading identical content** both derive the same key and both send
+`If-None-Match: *`. The object store serializes them: one gets `Done`, the other gets 412,
+`HEAD`s, point-reads `.meta`, and adopts the winner's token as its own dependency. The loser never
+published anything — a failed or cancelled sink publishes nothing — and its adopt is protected by
+its own durable precommit edge (see [the writer-versus-GC race](#writer-gc-race)). Both writers
+are safe; the only cost is one wasted upload attempt.
+
+## Dedup and the identity primitive {#dedup-identity}
+
+Two blobs are the same object if and only if they hash to the same digest under the pool's
+configured algorithm. Nothing else — not size, not `LIST` order, not a cheap prefix compare —
+is allowed to stand in for that check. This follows the same rule everywhere in CAS: identity
+is *proven* by hash equality, never *inferred* by a cheap signal, and re-hashing on read is the
+identity primitive wherever the correctness of a decision depends on it.
+
+The blob content hash is pluggable per pool, fixed at pool creation: `blob_hash` selects
+`cityhash128` (default), `xxh3-128`, or `sha256` (`parseBlobHashAlgo`,
+`Primitives/CasBlobDigest.h`). A blob is identified by the pair `BlobRef = (BlobHashAlgo, digest)`,
+never by a bare digest — a bare digest is ambiguous once more than one algorithm can appear in a
+pool. `blob_hash_allow_new` gates admitting a second algorithm into an already-populated pool's
+`algos_used` set; it defaults to off.
+
+`cityhash128` is not cryptographically collision-resistant. A pool shared across mutually
+untrusted writers should run `sha256` — CAS enforces no policy choice here; the operator picks
+the threat model via `blob_hash`. This is why the dedup admission gate is a `HEAD` (occupancy)
+rather than a body compare: it tells the writer *something* already claims this key, and the
+digest is the only claim CAS trusts.
+
+## The writer-versus-GC race {#writer-gc-race}
+
+This is the interleaving that gets the most reviewer attention, because a writer and a `GC` round
+can legitimately disagree about whether a blob is still needed.
+
+```mermaid
+sequenceDiagram
+    autonumber
+    participant W as Writer
+    participant S3 as Object store
+    participant GC as GC leader
+
+    Note over GC: round n -- fold finds in-degree 0
+    GC->>S3: HEAD blob -- capture exact token t1
+    GC->>S3: write .meta = Condemned round n
+
+    rect rgba(120,160,255,0.12)
+    Note over W,S3: a writer arrives wanting this content
+    W->>S3: append ref-log PRECOMMIT (durable +1 edge)
+    W->>S3: HEAD blob (present, token t1)
+    W->>S3: GET .meta
+    alt meta is Clean
+        W->>W: adopt t1 as dependency
+        Note over GC: next fold sees in-degree >= 1 -- spared
+    else meta is Condemned
+        W->>S3: PUT blob unconditional re-upload of writer own source, fresh incarnation tag -- token t2 not t1
+        W->>S3: CAS .meta back to Clean
+    end
+    end
+
+    Note over GC: round n+1 -- graduation, only if still zero
+    GC->>S3: re-verify in-degree, requires confirmed durable Condemned evidence for hash+token t1
+    Note over GC: publishes delete_pending
+
+    Note over GC: round n+2 -- the single content-delete site
+    GC->>S3: deleteExact(blob, t1)
+    alt writer resurrected
+        S3-->>GC: TokenMismatch -- nothing deleted, blob is live at t2
+    else genuinely dead
+        S3-->>GC: Deleted -- then drop the .meta
+    end
+```
+
+The invariant that makes every interleaving safe: **revival is re-upload only — never `GET` a
+condemned object to revive it.** A writer that finds `Condemned` metadata does not resurrect the
+existing body; it re-uploads its own source bytes under a fresh `incarnation_tag`, producing a
+new token that no prior `deleteExact` call can name. `GC` never streams a body it might delete,
+and a writer never trusts a body it did not itself just write.
+
+Why this closes the race in both directions:
+
+- A writer that **adopts** a token must have read a non-`Condemned` marker, and its precommit
+  edge was durable *before* that read. The next fold therefore sees in-degree ≥ 1 and spares the
+  blob.
+- A writer that **resurrects** changes the token. A stale `deleteExact(t1)` then returns
+  `TokenMismatch` and reclaims nothing — the delete names an exact incarnation, never "the object
+  at this key".
+- The delete lags condemnation by at least two full rounds, and publishing the one edge that
+  authorizes an irreversible delete requires confirmed durable `Condemned` evidence for that
+  exact `(hash, token)` pair. Without it `GC` never throws — it carries the entry and retries the
+  marker write on the next round.
+
+Both directions degrade to a spurious re-upload or a no-op delete. Neither can lose data or leave
+a dangling manifest entry.
+
+**One asymmetry worth flagging:** on a local (emulated) disk the resurrect path materializes the
+full `[header][payload]` in memory; resurrections are serialized, so at most one body is held whole
+in RAM at a time. On remote object storage the resurrect streams and holds nothing.
+
+### The `.meta` sidecar {#meta-sidecar}
+
+`.meta` has exactly two states: `Clean` (body present, may be referenced) and `Condemned`
+(`GC` observed zero in-degree; the body is still present and a writer may resurrect it). An
+*absent* `.meta` reads exactly like `Clean` — there is no third "unaccounted" state in the
+stored format; `unaccounted` is an `ca-fsck` classification, not something `GC` ever writes.
+
+The record carries `state`, `condemn_round`, and `size`, and deliberately carries **no token**:
+it is a per-hash hint, not a per-incarnation fact. All safety comes from the body's in-envelope
+`incarnation_tag` plus exact-token deletes; a stale marker costs at worst one spurious re-upload,
+never a lost delete or a false revival.
+
+## Deterministic artifacts and the adoption pin {#deterministic-artifacts}
+
+Some CAS objects are a pure function of their inputs: the `GC` source-edge run files (`cas_run`)
+and fold seals (`cas_fold_seal`). For these, `putDeterministicArtifact`
+(`Gc/CasBlobInDegree.cpp:341-352`) is the write-once helper:
+
+```cpp
+if (backend.putIfAbsent(key, bytes).outcome == PutOutcome::PreconditionFailed)
+{
+    const auto existing = backend.get(key);
+    if (!existing || existing->bytes != bytes)
+        throw Exception(ErrorCodes::CORRUPTED_DATA, ...);
+    /// byte-equal => our own deterministic replay; adopt (no-op).
+}
+```
+
+The idempotency argument: identical inputs produce byte-identical output, so a replayed round —
+leader deposed mid-round, round `CAS` aborted, crash-restart — re-derives exactly the same bytes.
+A 412 therefore means "already occupied by our own replay", verified by comparing the fetched
+bytes, not inferred from occupancy alone as blob uploads do. Divergent bytes are impossible under
+correct operation and fail closed as `CORRUPTED_DATA`.
+
+This is the format-evolution **adoption pin**, documented in the persisted-format registry
+(`Formats/README.md`): on a `putDeterministicArtifact` conflict, the writer re-encodes at the `v`
+of the *existing* object rather than at its own current build's version, so two writers on
+different builds replaying the same deterministic round still land on byte-identical output.
+
+The helper is explicitly **not** for observation-bearing artifacts — `GC` outcome logs carry
+`HEAD`-observed tokens on which two observers may legitimately disagree, so those use
+first-durable-write-wins byte-adopt semantics instead. And a blob body can never use this path:
+the fresh-tag rule means two attempts at the same logical create are allowed to legitimately
+differ, which is exactly what `putDeterministicArtifact`'s divergence check would reject.
+
+## Settings {#settings}
+
+All names below are unprefixed keys inside the disk's `cas` config block
+(`ContentAddressedSettings.cpp`, `LIST_OF_CONTENT_ADDRESSED_SETTINGS`); none carry a `cas_`/`ca_`
+prefix.
+
+| Setting | Controls | Default |
+|---|---|---|
+| `blob_hash` | Pool blob content-hash function (`cityhash128` \| `xxh3-128` \| `sha256`); fixed at pool creation | `cityhash128` |
+| `blob_hash_allow_new` | Explicit opt-in to admit a new hash algorithm into an existing pool's `algos_used` | `false` |
+| `deduplication_cache_bytes` | Byte budget of the blob-presence cache that feeds the dedup `HEAD`-first decision (`0` disables) | 64 MiB |
+| `deduplication_head_first_min_bytes` | Minimum blob size to try a `HEAD` before uploading the body | 1 MiB |
+| `staging_backend` | Blob staging backend (`local` \| `s3`); `s3` is opt-in | `local` |
+| `scratch_path` | Server-local scratch directory for the local-staging write-buffer spill; a relative value is anchored to the server data path | `""` |
+| `gcs_max_conditional_put_bytes` | Largest conditional write on a generation-token store (GCS forces those single-part); does not bound the unconditional resurrect | 1 GiB |
+
+`GC`-round budgets that gate condemnation and reclaim of these same blobs (graduation, redelete,
+sweep budgets) live on the GC architecture page, not here — they govern the `GC` side of the race
+in [Writer-versus-GC race](#writer-gc-race), not the write path.
diff --git a/docs/en/antalya/cas/architecture/correctness.md b/docs/en/antalya/cas/architecture/correctness.md
new file mode 100644
index 000000000000..1beb28b0039f
--- /dev/null
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+---
+description: 'What the TLA+ model corpus proves about CAS safety, the counterexamples that shaped the design, and how model-checking and soak/chaos testing complement each other.'
+sidebar_label: 'Correctness'
+sidebar_position: 12
+slug: /antalya/cas/architecture/correctness
+title: 'CAS Architecture — Correctness'
+doc_type: 'reference'
+---
+
+# CAS architecture — correctness {#correctness}
+
+`CAS` treats formal modelling as a pre-implementation gate, not after-the-fact documentation. No
+task that changes safety-relevant behavior starts until the relevant `TLA+` model is green, and
+"green" means every safety and liveness stage holds **and** every deliberately sabotaged variant
+(`sab_*`) violates the specific rule it targets. A sabotage that fails to reproduce its named
+counterexample is treated as seriously as a real violation — it means the model was not actually
+covering the case it claimed to cover. This is why every safety rule below ships with the
+counterexample that appears when you remove it.
+
+The full model index (source `.tla` files and proof-run records) lives at
+`docs/superpowers/models/`; this page is the reader-facing summary.
+
+## Model → invariant → counterexample {#model-invariant-counterexample}
+
+| Model (`docs/superpowers/models/`) | Invariant it proves | Counterexample it caught |
+|---|---|---|
+| `CaIncarnationCore.tla` | `INV_NO_DANGLE`, `INV_NO_LOSS`, `INV_NO_RETURN` — the safety spine for the whole GC core | `sab_unconddelete`: replacing the exact-token delete with an unconditional one lets a stale delete kill the live incarnation a resurrect just wrote |
+| `CaBuildRootPrecommit.tla` | `INV_NO_DANGLE_COMMITTED` — a committed manifest never references an absent blob | Reproduces the dangling-manifest hazard exactly: `WriteBlob → AdoptBlob → BuildDie → GcDelete → Commit` with no presence re-check publishes a manifest over a deleted blob |
+| `CaGcLeaseCore.tla` | `NoFalseSteal` — no leader steals leadership from a live, mid-round incumbent | Without the advisory heartbeat, a frozen `seq` during a round looks identical to a dead leader, and a second leader steals from the alive one |
+| `CaCasMountCore.tla` | Reclaim exclusivity for an expired mount | `sab_wallclockreclaim`: trusting the foreign mount body's wall-clock timestamp (instead of observing a stable token on the reclaimer's own monotonic clock) breaks exclusivity |
+| `CaB140DangleMerge.tla` | `INV_NO_LOSS` across a GC lease handoff | Trim-before-durable: a fold cursor trimmed from in-memory state (not the durable snapshot) skips a live edge across a lease handoff, and the referenced blob is deleted while still live |
+| `CaGcRootLocalPartManifestCore.tla` | `INV_NO_DANGLE` over the root-local part-manifest fold | `sab_lazyfenceunsafe`: reusing a stale parent fence position instead of a fresh all-shard fence dangles a live object |
+| `CaGcShardIncarnationCore.tla` | `INV_NO_DANGLING` — safety of registry-free namespace discovery | `sab_pathkeyedcursor`: dropping the per-shard incarnation from the fold cursor reintroduces an ABA hazard on delete-then-recreate at the same path |
+| `CaGcAckFloorZombie.tla` | `INV_NO_DANGLE` under two fully-interleaved GC leaders | `sab_eagerdelete`: a leader deleting its own fresh (not-yet-pending) graduations — the pre-amendment single-phase behavior — dangles when a deposed leader's pass overlaps a live one |
+| `CaGcRoundDeferCore.tla` | `NoOverDelete` — a deferred round may skip a rebuild only when nothing destructive is pending | `sab_graduate_on_stale`: dropping the "an unfolded delta covers this blob" guard lets a deferred round delete a blob its own unread history still protects |
+| `CaEdgeBeforeObserve.tla` | The writer/`GC` publish order is safe to simplify | `sab_late_edge`: allowing adoption before the precommit closure is durable (the pre-fix order) dangles |
+| `CaGcCondemnMarkerGate.tla` | Graduation requires confirmed durable `Condemned` evidence | Swallowing a failed asynchronous condemn-marker write let a writer adopt a token a later graduation was about to delete |
+| `CaRefTableSnapshotLogCore.tla` | Dense, per-life ref ids with an in-band `_ckpt` recovery frontier | `sab_scanistruth`: trusting a listing as the source of truth for "acked" reproduces the real production incident where a `LIST` omitted two already-durable, already-acknowledged ref entries |
+
+## Soak and chaos: the empirical oracle {#soak-and-chaos}
+
+Model-checking and the soak/chaos harness (`utils/ca-soak/`) catch different classes of error, and
+the design leans on both rather than either alone. `TLA+` proves a protocol's constraints before a
+line of `C++` exists — the two-coordinate namespace-incarnation proof and the build-root necessity
+proof were both design decisions made this way. The soak, running two `ReplicatedMergeTree`
+replicas against one shared pool under a seeded workload and a seeded fault injector, finds what an
+idealized model necessarily abstracts away: the dangling-manifest hazard and the resurrect-reupload orphan were
+both first observed live in `system.cas_log` during soak runs, before either got a focused model. Each
+quiesced soak checkpoint cross-checks `SQL` results against a model oracle and runs
+`clickhouse-disks ca-fsck` plus `ca-gc-dryrun`, asserting `dangling=0`.
+
+The relationship runs in both directions: the resurrect-reupload orphan (`utils/ca-soak` scenario
+S30, root-caused via `system.cas_log`) got a focused `TLA+` reproduction that proved the fix and
+was then retired once a deterministic `gtest` (`CASGCLeak.ResurrectReplacedIncarnationReclaimed`)
+covered the same scenario for less ongoing cost — the model did its job as a pre-implementation
+gate and the regression coverage moved to the cheaper, faster tool. A model's proven-safe shape
+also becomes the thing a later soak scenario is written to stress. The `0x1430c`
+incident — a `LIST` that omitted two already-durable ref entries, caught live by an instrumented
+probe rather than reproduced by brute-force enumeration — is the clearest example: it is what made
+`sab_scanistruth` a permanent, named counterexample rather than a one-off incident report.
+
+## What this buys a reader {#what-this-buys}
+
+None of this proves the shipped `C++` is bug-free — a model proves its own abstraction, and several
+entries in the index are annotated `MIXED` or `DRIFTED` where the concrete mechanism has moved on
+from what a model checks, with the audit trail kept precisely so that gap is visible rather than
+implied. What it does buy: every safety rule in the GC core has an explicit counterexample on
+record for the world where that rule is missing, and the corpus is itself periodically re-audited
+for faithfulness to the code — a model whose guarantee the code no longer needs is deleted rather
+than kept as false comfort.
+
+## Test coverage {#test-coverage}
+
+The implementation was built test-first (TDD), and the coverage is correspondingly dense:
+
+| Layer | Volume |
+|---|---|
+| Unit tests (`gtest`, `CAS*` suites) | ~1,900 test cases across ~130 files, covering formats, the write and read paths, the ref machinery, `GC`, recovery, and the backend contract |
+| Integration tests | 10 dedicated `test_cas_*` suites (shared pools, `GC` on S3, sharded `GC`, relink replication, fault-injected `INSERT` recovery, member decommission, and more) |
+| Stateless tests | dozens of dedicated `CAS` tests (pool integrity, leftovers, fsck, GC), in addition to the whole standard suite running on a `CAS`-default server (below) |
+
+## The whole test suite, on CAS by default {#stateless-suite-on-cas}
+
+Beyond the model corpus and the soak harness, the standard ClickHouse **stateless test suite runs
+green with `CAS` as the default `MergeTree` storage**: dedicated CI lanes
+(the `cas storage` and `cas s3 storage` job families — the latter covering ASan/TSan/MSan/UBSan
+and ARM against a real S3-compatible store) run every stateless test against a server whose default disk is
+a `CAS` pool. A small set of tests carries the `no-cas-storage` tag and is skipped in those lanes —
+tests that exercise a mechanism a content-addressed disk deliberately does not have (for example,
+`s3_plain` layouts or deliberately corrupted on-disk part chains). Everything else — the thousands
+of tests that define what `MergeTree` is supposed to do — passes unchanged on top of `CAS`.
diff --git a/docs/en/antalya/cas/architecture/design-history.md b/docs/en/antalya/cas/architecture/design-history.md
new file mode 100644
index 000000000000..1587b2fdd58a
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/design-history.md
@@ -0,0 +1,61 @@
+---
+description: 'A condensed record of the paths CAS explored and rejected, and the major design pivots that produced the current architecture.'
+sidebar_label: 'Design history'
+sidebar_position: 13
+slug: /antalya/cas/architecture/design-history
+title: 'CAS Architecture — Design History'
+doc_type: 'reference'
+---
+
+# CAS architecture — design history {#design-history}
+
+This page is a condensed record of the roads not taken: what was tried, why it was abandoned, and
+the sequence of pivots that produced the architecture described elsewhere in this section.
+
+## Rejected paths {#rejected-paths}
+
+| What it was | Why it was abandoned | What replaced it |
+|---|---|---|
+| **Generation-in-the-key** (Epoch-Based Reclamation core; blob keys carried a generation, `blobs/<hash>/<gen>`) | Required `O(files)` persistent `Keeper` writes per commit and colliding intent keys across writers building identical content; a stuck writer stalled reclamation pool-wide | The incarnation-token design: identity moved into the object body and delete precision into the backend token, removing the generation from every key |
+| **Merkle tree layer** (a `Tree` object kind, `trees/<hash>` prefix, `child_gen` carried inside a tree's own identity) | Depended on the generation-in-the-key core: a reclaim at any child propagated a new generation up the entire tree chain, and the tree layer was itself an extra surface for the same class of bug | Removed entirely; trees became manifest-internal, and `Blob` is the sole durable object kind besides the manifest and the ref |
+| **Integer in-degree refcount** (a mutable counter, incremented per reference, decremented per release) | The decide-to-reference-then-not-yet-durable window let the fold observe in-degree 0 for a still-live blob; a mutable counter also costs a `CAS` round-trip proportional to write volume | A derived count: `GC` folds a multiset of `+`/`-` source-edge deltas, so losing or duplicating a record can only delay reclamation, never accelerate one |
+| **Extending zero-copy replication instead of a new mechanism** | Zero-copy's structural costs (a commit spanning local disk, S3, and `Keeper`; a mutable refcount) are inherent to its design, not a bug to patch | `CAS` is an alternative to zero-copy, not a replacement: both remain available, `metadata_type = cas` is opt-in per disk, and no existing deployment needs to migrate |
+| **Per-incarnation body keys** (`blobs/xx/<hash>.<incarnation>`, an alternative to the in-body incarnation tag) | A resurrect reusing the condemned incarnation instead of minting a fresh one reintroduced the shared-key race; structurally this was generation-in-the-key again | The in-body `incarnation_tag` plus exact-token body delete, which keeps the generation out of every object key |
+| **Meta as the lifecycle linearizer** (a per-hash `.meta` object whose presence/absence *was* the authority for a blob's lifetime) | The marker is a point-read hint only, never consulted by reads; treating it as the linearizer would assert a guarantee the design does not make | The meta stays advisory: the in-body incarnation tag and exact-token delete are the real authority, and an absent meta reads identically to `Clean` |
+| **Raw immutable bodies with a three-state tombstone meta** | A resurrect displacing the body forced a terminal-tombstone handshake — a writer↔`GC` liveness coupling that could re-enable data loss | The settled one-key-per-hash design with an in-body incarnation tag |
+| **A persistent, append-only namespace registry for `GC` discovery** | Never deregistered on drop, so it grew monotonically forever; its fence cost scaled with namespaces ever created, not namespaces live | Discovery from the ref data itself, made safe by two independent coordinates: a durable per-shard incarnation plus a pool-global round |
+| **A separate all-shard fence-and-recheck phase per `GC` round** | Both phases cost `O(pool size)` GET+CAS every round regardless of churn — roughly 2.4 million requests at 100k tables | A causal ack-floor: one streaming merge per round, with no separate fence or recheck phase, cutting the request count by roughly three orders of magnitude |
+| **A pool-wide sparse ref-id allocator with completeness certificates bolted on** | Successive additive fixes kept growing without closing the root cause: absence is undecidable in a sparse id space | The invariants were changed instead of patched: dense per-life ids derived from applied state, an in-band epoch seal, and a `_ckpt` head object carrying the exact acknowledged frontier |
+
+## Turns at a glance {#turns-at-a-glance}
+
+| Date | Turn |
+|---|---|
+| 2026-06-01 | Starting point: "content-addressed storage for `MergeTree`" thesis and a working proof of concept |
+| 2026-06-07 – 10 | The generation-in-the-key core is abandoned; the incarnation-token design replaces it |
+| 2026-06-11 | The incarnation model passes exhaustive model checking with zero violations |
+| 2026-06-18 | A dangling manifest reference — a committed manifest naming an already-deleted blob — leads to replacing per-blob protection hints with structural build-root reachability |
+| 2026-06-24 – 26 | Formats begin converging on a single self-describing envelope (completed in July as the all-text, JSON-based codec set) |
+| 2026-06-26 | Root-local full-tree manifests collapse a forest of small `GC` objects into one hot/cold split |
+| 2026-07-01 | The namespace registry is deleted; discovery moves to the two-coordinate incarnation-and-round scheme |
+| 2026-07-02 | Fence-and-recheck `GC` rounds are replaced by the one-pass, causal ack-floor round |
+| 2026-07-06 – 10 | Writer/`GC` simplification: promote-time revalidation of tokened dependencies is proved redundant |
+| 2026-07-13 | Mount-lease handover becomes boundary-exclusive, closing the cross-epoch grace window without a timeout |
+| 2026-07-15 | All part files become content-addressed: the mutable file set drops to empty, and disk-transaction dispatch collapses to one precommit contract |
+| 2026-07-17 | An acknowledged `INSERT` that could be lost is traced to a removed durability guard and fixed |
+| 2026-07-26 | A `LIST` omitting two already-durable ref entries is caught during a soak run — the incident that settles the trust model for listings |
+| 2026-07-27 – 29 | The sparse-id certificate stack is abandoned; the invariants change instead — dense ids, an in-band epoch seal, and a `_ckpt` recovery frontier |
+| 2026-08-01 – 03 | Recovery stops reading listings entirely and works from authoritative objects; the listing trust model is finalized |
+
+## The pattern underneath {#the-pattern}
+
+A few reflexes recur across these pivots and still apply to new design work:
+
+- **Re-derive the invariant, don't patch the mechanism.** Every durable fix came from asking what
+  property must hold, not from patching the specific failure observed.
+- **Delay is acceptable, authorization is not.** A stale-but-honest observation can only ever
+  postpone a decision; the design consistently rejects any mechanism that could *accelerate* a
+  destructive action past its safety gates — a delay is a latency cost, a wrongful authorization is
+  data loss.
+- **A model that no longer matches the code is worse than no model.** Superseded models are
+  removed rather than kept: an unfaithful proof is false comfort, not documentation.
diff --git a/docs/en/antalya/cas/architecture/garbage-collection.md b/docs/en/antalya/cas/architecture/garbage-collection.md
new file mode 100644
index 000000000000..48be9c4e6b24
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/garbage-collection.md
@@ -0,0 +1,263 @@
+---
+description: 'The CAS garbage collector: leadership as work de-duplication, the 18-phase round pipeline, condemnation and exact-token deletion, sharding, and observability.'
+sidebar_label: 'Garbage collection'
+sidebar_position: 8
+slug: /antalya/cas/architecture/garbage-collection
+title: 'CAS Architecture — Garbage Collection'
+doc_type: 'reference'
+---
+
+# CAS architecture — garbage collection {#garbage-collection}
+
+`GC` is the only place in `CAS` that ever deletes a blob body or a manifest body. It runs as a
+background, lease-paced loop per mount (`Gc::runRegularRound`, `Gc/CasGc.cpp`), folding ref-log
+history into blob in-degree, condemning what reaches zero, and deleting only after that
+condemnation has survived a full extra round. This page covers leadership, the round's 18 phases,
+condemnation and deletion, sharding, pruning, round cost, and observability. Manifest and ref
+mechanics that `GC` folds are covered on the
+[manifests-and-refs page](/antalya/cas/architecture/manifests-and-refs); the writer-versus-`GC`
+race over one blob is covered on the
+[blob-protocol page](/antalya/cas/architecture/blob-protocol#writer-gc-race).
+
+## Leadership {#leadership}
+
+There is **no separate `GC` lease object**. The lease lives inside `gc/state` itself as
+`{owner, seq}`.
+
+```mermaid
+stateDiagram-v2
+    [*] --> Reading: GET gc/state
+    Reading --> Creating: object absent, never observed before
+    Creating --> Leader: casPut create-if-absent, gc_shards fixed here, once
+    Reading --> Renewing: lease owner is me
+    Renewing --> Leader: casPut seq+1, guarded by the observed token
+    Reading --> Evaluating: foreign owner
+    Evaluating --> NotLeader: incumbent lease moved, or heartbeat moved, or steal not allowed
+    Evaluating --> Stealing: both frozen across a full observation window
+    Stealing --> Leader: casPut owner=me seq+1, on the observed token
+    Stealing --> NotLeader: lost the CAS, re-read and re-arm
+    Leader --> [*]: run the round
+```
+
+Two independent liveness signals are consulted before a steal: whether `(owner, seq)` moved since
+the last tick, and whether the separate `gc/hb` heartbeat moved. The heartbeat is compared only
+under the same remembered heartbeat owner, deliberately not against `lease.owner` — a deposed
+leader's heartbeat thread keeps pulsing, and that must not cause a live new leader's lease to be
+stolen. The paced background loop may steal; a manual `SYSTEM CAS GC RUN` may not, because the
+safety argument needs two observations separated by real wall time. Because every renew or steal
+bumps `seq`, `seq` doubles as the round's attempt id.
+
+**A deposed leader that keeps running cannot corrupt anything**, and the argument does not rely on
+exclusivity at all:
+
+1. `gc/state` is published by exactly **one** `CAS` per round; a deposed leader's `CAS` fails and
+   its entire round evaporates.
+2. Every fold artifact is written under that leader's own attempt number, invisible to every
+   reader, and reclaimed later by wholesale generation pruning.
+3. Destructive pre-`CAS` actions are justified only by previously published durable state, so they
+   are replay-idempotent.
+4. Deletes are exact-token, so a stale leader can never delete a newer incarnation.
+
+The lease is therefore **work de-duplication, not mutual exclusion**.
+
+## The round {#the-round}
+
+A round is one pass of 18 named phases ending in exactly one `gc/state` `CAS`
+(`Gc::runRegularRound`, `Gc/CasGc.cpp`).
+
+| # | Phase (`GcPhaseTimer` name) | What it does |
+|---|---|---|
+| 1 | `lease` | Acquire, renew or steal the lease inside `gc/state`. The only phase a not-a-leader round emits |
+| 2 | `pre_fold_ref_drain` | Resolve catalog `Removing` rows whose cleanup evidence the adopted parent already sealed; exact-CAS-delete the completed ones before anything else can act |
+| 3 | `heartbeat_floor` | One `LIST` of `gc/server-roots/`, one `GET` per mount slot, fence-out `PUT` for any mount whose write-token has held stable past the threshold |
+| 4 | `defer_decision` | One full `LIST` of `cas/ns/stream/`, build the catalog-keyed ref walk plan; decide `DEFER` (nothing changed, no graduation due) or continue to a full fold. A `DEFER` verdict still runs one namespace-janitor page — the same work phase 16 does on a folding round — with its deletes suppressed |
+| 5 | `parent_seal_read` | Capture the parent fold seal's run references before the fold mutates the in-memory generation/attempt, to detect a ref that moved off an already-pruned generation |
+| 6 | `fold_ref_group` | Regroup the one `LIST` from phase 4 into per-table listings — no I/O, the keys are already in hand |
+| 7 | `fold_seal_read` | `GET` and decode the adopted fold seal that anchors this fold's coverage |
+| 8 | `fold_ref_intake` | `GET` every new ref-log record and every referenced manifest, extracting blob source edges |
+| 9 | `fold_reduce` | The three-cursor merge over prior edges, new deltas and the parent's condemned rows: spare, condemn, graduate or redelete each candidate |
+| 10 | `fold_seal_write` | Write the new fold seal once, write-once deterministic, adopting a byte-identical replay instead of rewriting it |
+| 11 | `pending_deletes` | The single content-delete site: exact-token `deleteExact` of every entry the *previous* round marked `delete_pending`, plus the forensic outcome-log writes |
+| 12 | `meta_pool_wait` | Drain the bounded pool of async `.meta` condemn-marker writes queued during the fold |
+| 13 | `round_commit` | Retention-prune old generations, then publish the single `gc/state` `CAS` that adopts the whole round |
+| 14 | `handoff_reclaim` | Post-`CAS`: reclaim any generation that a ref moved off during this very round, before the ordinary wholesale prune would reach it |
+| 15 | `manifest_deletes` | Delete manifest bodies whose owner-removal minus-one edge the `CAS` in phase 13 just adopted |
+| 16 | `namespace_cleanup` | One bounded page of the perpetual namespace janitor, reclaiming dead-life debris |
+| 17 | `ref_object_cleanup` | Prune ref logs and snapshots once both fold coverage and a live snapshot make them safe to delete |
+| 18 | `orphan_sweep` | One cursor-paced page of the [orphan-manifest sweep](/antalya/cas/architecture/manifests-and-refs#orphan-sweep); wrapped so it can never fail the round |
+
+Phases 5 through 18 run only when phase 4 decides to fold. A `DEFER` verdict is not a bare no-op:
+it still runs one bounded namespace-janitor page with `suppress_destructive = true` — cursor
+progress and diagnostics only, no deletes — and then returns, publishing no fold artifact and no
+`gc/state` `CAS` at all:
+
+```mermaid
+flowchart LR
+    D4{"4 defer_decision"} -->|"nothing changed, no graduation due"| DEF["DEFER: one suppressed<br/>namespace-janitor page, then return"]
+    D4 -->|"changed shards, or graduation due"| FOLD["phases 5 through 18: full fold and round commit"]
+```
+
+Orderings that are load-bearing:
+
+- **2 before 4** — a row proved complete by the adopted parent is resolved before `DEFER` or any
+  successor plan can publish.
+- **15 after 13** — manifest bodies are deleted only after the `CAS` adopted their decrements.
+- **13's prune before the `CAS`** — a pre-`CAS` destructive action may rely only on already-
+  published state.
+
+**Clamp suppression.** `suppress_destructive = !anomalies.empty() || !carried_holds.empty() ||
+!frontier_complete` is computed once and threaded into the merge, current-life ref cleanup and the
+perpetual namespace janitor, so they cannot desynchronize. Under suppression there is no
+graduation, no redelete, and no ref or namespace deletion; condemnation and sparing continue,
+because both are non-destructive.
+
+**Fail-closed aborts.** A throw before the `CAS` means nothing is adopted: unapplied transactions,
+a cursor/apply mismatch, a missing adopted seal, a table with a snapshot but no surviving log and
+no cursor, a non-total condemned summary, and an observed delete marker (bucket versioning is on).
+
+## The one-pass commit {#gc-state}
+
+`gc/state` is the durable safety and round-adoption state: `round`, `gc_shards`,
+`snap_generation`, `snap_pruned_through`, `snap_attempt`, `manifest_sweep_cursor`, and the lease.
+Exactly one `CAS` per round publishes it; the fold itself performs no `CAS` of its own.
+
+**The fold seal *is* the coverage record**: generation, parent generation, one `ref_lives` row per
+catalog-admitted opaque life (coverage plus optional cleanup evidence), references to the
+source-edge run segments, and a per-shard condemned summary. It is encoded deterministically, so a
+replayed round produces byte-identical bytes and adopts its own output through the
+`putDeterministicArtifact` adoption pin (see the [blob-protocol page](/antalya/cas/architecture/blob-protocol#deterministic-artifacts)).
+There is **no separate retired-list object** — condemned entries ride the source-edge run as
+sentinel rows at `source_id = 0` — and **no run-file list outside the seal**; runs are resolved
+*through* the seal's references, never by key construction.
+
+## Finding orphans {#finding-orphans}
+
+In-degree is a set of source edges, not a refcount. A blob becomes a candidate when its edge set
+becomes empty and it was touched this pass: one `HEAD` captures the exact incarnation token and
+size that a future delete will name. A blob merely carried from the parent run pays no `HEAD`.
+
+**The grace period is measured in rounds, not acks:** an entry graduates once it has survived one
+full round (`condemn_round < current_round`). The heartbeat floor is liveness only and **never**
+gates graduation.
+
+**The 404 rule.** A body that is present but invalid is `CORRUPTED_DATA`, hard. A body that is
+missing is **never** a throw — the fold records and continues, and the caller decides by position:
+a precommit activation clamps as a barrier; a committed or removal fold clamps only that table.
+Prunes are likewise fail-open on 404.
+
+## Condemnation and deletion {#condemn-delete}
+
+```mermaid
+flowchart LR
+    A["round n: in-degree hits zero<br/>HEAD -- exact token t"] --> B["write .meta = Condemned round n<br/>async, bounded pool, drained pre-CAS"]
+    B --> C["retired with condemn_round = n+1"]
+    C --> D{"round n+1: re-verify"}
+    D -->|"in-degree recovered"| S["SPARED -- recovery wins, even past the floor"]
+    D -->|"still zero, confirmed durable Condemned evidence for hash and t"| G["GRADUATED -- delete_pending"]
+    D -->|"still zero, evidence unconfirmed"| C2["carried unchanged, retry the marker, never throw"]
+    D -->|"current token not equal to t"| SUP["SUPERSEDED -- a writer resurrected, re-condemn the CURRENT token"]
+    G --> E["round n+2, pre-CAS: deleteExact blob, t"]
+    E -->|"Deleted or Absent"| F["then drop the .meta"]
+    E -->|TokenMismatch| H["nothing deleted -- live at a newer token, leave the .meta alone"]
+```
+
+The `.meta` sidecar carries **no token** — it is a per-hash hint. The exact incarnation token lives
+in the condemned sentinel row inside the run, together with the condemn round and two flags,
+`delete_pending` and `marker_confirmed`. `GC`'s marker is add-only: `Clean → Condemned` yes, the
+reverse never, not even when sparing — only a writer that has already displaced the body may clear
+it. Minimum two full rounds separate condemnation from deletion, and `delete_pending` is terminal —
+an entry is never un-pended.
+
+## Sharding {#sharding}
+
+`gc_shards` is fixed at first lease acquire and immutable; decoders reject `0`. A blob routes by
+the **high** 64 bits of its digest, read big-endian.
+
+The role split is worth internalizing: the **coordinator** — the lease holder — owns discovery,
+round visibility, the single global fence, and the generation advance, because a publish into
+*one* namespace can protect a blob owned by *any* shard, so these span the whole universe and must
+not be sharded. **Reducers** own only their disjoint shard; their run-key namespaces never
+collide, so two servers could reduce different shards concurrently and reducer work needs no
+lease.
+
+A shard with an empty delta bucket and no condemned entries in the parent summary copies the
+parent's run references verbatim — zero run I/O, a "pure carry". A missing parent summary entry on
+a non-fresh pool is `CORRUPTED_DATA`, never silently treated as zero.
+
+## Pruning old objects {#pruning}
+
+- **Current-life ref logs and snapshots** (phase 17) — a log is deletable only when covered by
+  both durable fold coverage and a durable live snapshot; snapshots strictly older than the newest
+  observed one are deletable. There is no batch delete; it is `HEAD` plus `deleteExact` per key.
+- **Generations** (phase 13) — keep the last `gc_snapshot_generations_to_keep` (default 3; `0`
+  means keep everything, for forensics). Pruning is wholesale: `LIST` the generation prefix and
+  delete everything under it, including deposed-leader debris and attempt-scoped outcome sets. A
+  generation still referenced by the live seal is skipped, but the cursor still advances past it —
+  leak-freedom then rests on the post-`CAS` hand-off reclaim in phase 14.
+- **Manifests** — owner-removed bodies delete in phase 15; never-precommitted bodies go through the
+  [orphan-manifest sweep](/antalya/cas/architecture/manifests-and-refs#orphan-sweep) in phase 18.
+
+## What a round costs {#round-cost}
+
+Per **folding** round, with `N` live mounts, `S` ref tables and `S_changed` tables carrying new
+logs:
+
+| Operation | Count |
+|---|---|
+| `LIST cas/ns/stream/` | 1 full enumeration |
+| `LIST gc/server-roots/` | 1, plus 1 `GET` per mount |
+| `GET` the adopted fold seal | 5, explicitly instrumented |
+| `GET` ref logs | 1 per new log |
+| `GET` manifests | 1 per emitted edge — no manifest-body cache within a round |
+| `PUT` run segments | 1 per non-pure-carry shard, plus 1 fold seal |
+| `HEAD` blobs | 1 per newly condemned |
+| `DELETE` | 1 per graduate |
+| `CAS gc/state` | 1 |
+
+The measured `GET` formula is exact: total `GET`s equal ref-log body `GET`s plus manifest body
+`GET`s, i.e. `1 + edges_per_log`. An idle round is one `LIST` sweep, `N` heartbeat `GET`s, and one
+`CAS`. A deferred round is cheaper still: one `LIST`, three seal `GET`s, the lease `GET`/`PUT` and
+the heartbeat floor — no `gc/state` `CAS` at all.
+
+The round's work is internally self-regulated: anything a pass cannot finish is carried and retried
+by the next round's cursors, never dropped. The internal pacing knobs are deliberately not part of
+the user-facing configuration surface.
+
+| Setting | Default | Bounds |
+|---|---|---|
+| `gc_meta_pool_size` | 16 | bounded pool for condemn-marker writes |
+
+## Observability {#observability}
+
+`system.cas_gc_log` emits `Start`, `Finish` and per-`Phase` rows, correlated by `round_id` — not
+`round`, which is `0` on `Start` and does not exist at all on a not-a-leader round. Phase rows
+carry no verb columns by design: per-phase operation counts ride the row's own `ProfileEvents`
+delta, so grouping by phase over an S3 event attributes the LIST/GET/PUT/DELETE budget without
+inventing schema. `phase_metrics` carries the semantic counts no counter can supply (clamped
+tables, dead precommits skipped, pure-carry shards, generations visited). `Deferred` is kept
+distinct from `Success` precisely so "folded and found nothing" is distinguishable from "never
+folded". Every `GC`-related `ProfileEvent` carries the uppercase `CAS`/`CASGC` prefix — for example
+`CASGCRetiredCondemned`, `CASGCRetiredGraduated`, `CASGCRetiredRedeleted`,
+`CASGCClampSuppressedPasses`, `CASGCHeartbeatFenceOuts`.
+
+Alongside it, `system.cas_log` carries the audit trail: the condemn chain, fence-outs, anomalies
+(capped per round, each carrying the true total), and manifest deletes.
+
+`ca-fsck` distinguishes two classes that are easy to conflate: `dangling` — referenced but missing,
+data loss — versus `unreachable`/`awaiting-gc` — present, unreferenced, and
+simply waiting for graduation.
+
+## Operational surface {#operational-surface}
+
+| Command | Effect |
+|---|---|
+| `SYSTEM CAS GC RUN '<disk>'` | One synchronous round on the contacted node; only the lease holder makes progress |
+| `SYSTEM CAS GC STOP` / `SYSTEM CAS GC START` | Stop or resume future rounds on the same scheduler, preserving its identity |
+| `SYSTEM CAS GC REBUILD` (`clickhouse-disks ca-gc-rebuild`) | Fail-closed disaster-recovery path that every "GC refuses to run" error points at; deliberately over-protects — it prefers bounded leaks over risking an under-count. It cannot delete live data directly: deletions it produces still flow through the normal round's condemn, graduate, exact-token path |
+| `clickhouse-disks ca-gc-dryrun` | Opens the disk read-only, constructs a non-leader `GC`, and prints what would be deleted with a reason per entry. Write-free, resolves runs through the seal's references. Documented caveat: it does not fold new owner events, so away from quiescence it can **over-report** — the subset guarantee holds only at quiescence, and its output must never feed a real delete |
+
+`SYSTEM CAS DROP POOL MEMBER '<server_root_id>' FROM DISK '<disk>'` — permanent removal of a dead
+replica, distinct from ordinary `GC` — is covered on the
+[mounts-and-leases page](/antalya/cas/architecture/mounts-and-leases#mount-lifecycle). `SYSTEM CAS
+FSCK` and its `dangling`/`unreachable` vocabulary are a read-only diagnostic pass, not part of the
+`GC` protocol itself.
diff --git a/docs/en/antalya/cas/architecture/index.md b/docs/en/antalya/cas/architecture/index.md
new file mode 100644
index 000000000000..15a9516c9ae7
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/index.md
@@ -0,0 +1,114 @@
+---
+description: 'What CAS is, the Git-analogy mental model, the object model, and the safety invariants a reviewer should hold every CAS protocol against.'
+sidebar_label: 'Architecture overview'
+sidebar_position: 1
+slug: /antalya/cas/architecture/
+title: 'CAS Architecture — Overview'
+doc_type: 'reference'
+---
+
+# CAS architecture — overview {#overview}
+
+`CAS` ("content-addressed storage") is a `MetadataStorage` back-end for object-storage disks
+(`metadata_type = cas`) that stores every `MergeTree` part file once, addressed by the hash of
+its content. Many servers share one object-storage pool with no byte duplication, no zero-copy
+bookkeeping in `Keeper`, no per-replica local-disk reference state that grows with data volume,
+and no mutable per-blob refcount.
+
+It is still experimental — that is deliberate, not a caveat to apologize for. Pre-release means
+the format can still change cheaply, with zero compatibility scaffolding, and the design can
+still be iterated on invariants rather than migrations. The bet: all you need underneath is a
+good S3 bucket. No external coordinator, no metadata service, no Keeper state proportional to
+data — the pool is self-describing, and everything CAS needs to agree on (refs, leases, GC
+leadership, fencing tokens) is an object in the bucket.
+
+This page is the entry point of a 4-page set: it gives the mental model. Deeper detail on
+storage layout, the write/read protocols, and GC lives in the other three pages.
+
+## The Git analogy {#git-analogy}
+
+The fastest way to load the model is Git, which most readers already carry:
+
+| Git | CAS |
+|---|---|
+| blob (file content by hash) | **blob** — one part file's bytes, keyed by content hash |
+| tree (directory listing) | **part manifest** — the immutable file list of one part |
+| ref (`refs/heads/main`) | **ref** — `part name → manifest id`, the only mutable state |
+| `gc` / reachability | **GC round** — an in-degree fold over refs → manifests → blobs |
+
+Where the analogy breaks: Git's objects are locally addressed and GC runs against a single
+repository with no concurrent writers; CAS objects are addressed inside a shared, multi-writer
+object-storage pool, and its GC round has to reason about ambiguity (crashed writers,
+in-flight precommits, eventually-consistent `LIST`) that a local Git repository never faces.
+Git also has no equivalent of a CAS ref's precommit state — a CAS ref transition is durable
+before the blob it names is guaranteed reachable, never the other way round.
+
+## The object model {#object-model}
+
+Four durable object kinds exist in a pool: one mutable (the ref), three immutable
+(part manifest, blob, and a blob's condemnation-marker sidecar).
+
+```mermaid
+graph TD
+    R["Ref: part name maps to manifest id"]
+    M["Part manifest: file list of one part"]
+    B["Blob: one part file's bytes, keyed by content hash"]
+    BM["Blob meta: condemnation marker sidecar"]
+
+    R -->|names| M
+    M -->|entry references| B
+    B -.->|sidecar| BM
+```
+
+**The reachability rule, stated once:** a blob is live if and only if some live manifest names
+it, and a manifest is live if and only if some ref — committed or precommitted — names it. `GC`
+computes exactly this and nothing else.
+
+## Safety invariants {#safety-invariants}
+
+The full numbered list lives in the CAS agent guide; this is the reader-facing summary of the
+substance:
+
+| Invariant | What it means |
+|---|---|
+| No silent data loss | No path may delete an object a committed reference still names |
+| Revival is re-upload only | A condemned blob is never revived by copying it — only by re-uploading the original bytes under a fresh identity |
+| Exact-token deletes | Every delete names the exact object incarnation it removes, never "the object at this key" |
+| `TOKEN ⟹ CONTENT` | A repeated write token implies unchanged bytes — the backend must never let a token be reused over different content |
+| Fail closed on ambiguity | An operation that may have landed is never treated as one that did not |
+| One content-delete site | Exactly one place in the whole codebase ever deletes a blob body, gated on a previously published `GC` round |
+| `GC` never invents history | Cleaning up an abandoned write is the writer's job, not `GC`'s |
+| Over-count only | A lost or duplicated `GC` fold can only delay a reclaim, never bring one forward |
+| No dangle / no loss / no return | A live ref always resolves through present objects; a delete requires proven unreachability at an exact token; a retired object identity is never valid again (though the same logical key can return under a new token) |
+
+## Positioning: shared-nothing, not shared-state {#positioning}
+
+Each server owns the catalog rows under its own identity and writes only its own state objects
+— that part is shared-nothing, same as `ReplicatedMergeTree` today. What CAS adds is a single
+**shared** resource: the blob content space, addressed purely by content hash, which is
+write-once and conflict-free by construction — two servers writing the same content write the
+same key with the same bytes, so there is nothing to reconcile. The only mutual exclusion CAS
+needs anywhere is a conditional write (create-if-absent, or compare-and-swap on a token) against
+a single object.
+
+That is deliberately not a coordinator or a serializable metadata service: there is no external
+coordinator, and no `ZooKeeper`/`Keeper` usage inside the pool protocol itself. `Keeper` stays
+exactly where `ReplicatedMergeTree` already used it — replication log and part-set consensus —
+and its load does not grow with pool size, because the pool's own bookkeeping never touches it.
+
+## The subsystem pages {#subsystem-pages}
+
+| Page | Covers |
+|---|---|
+| [Storage layout](/antalya/cas/architecture/storage-layout) | Every S3 key shape, the object envelope, codecs, a worked example tree |
+| [Namespaces](/antalya/cas/architecture/namespaces) | Namespaces, `life_id`, the catalog, and their lifetime |
+| [Blob protocol](/antalya/cas/architecture/blob-protocol) | Conditional writes, deduplication, the writer-vs-GC race |
+| [Part lifecycle](/antalya/cas/architecture/part-lifecycle) | Build, precommit, upload, promote; crash points and their cleaners |
+| [Manifests and refs](/antalya/cas/architecture/manifests-and-refs) | Part manifests and the ref machinery: publish, fold, recovery |
+| [Mounts and leases](/antalya/cas/architecture/mounts-and-leases) | Server identity, the owner claim, the mount lease, fencing |
+| [Replication](/antalya/cas/architecture/replication) | Fetch-by-relink between replicas sharing one pool |
+| [Read path](/antalya/cas/architecture/read-path) | Ref resolution, manifest reads, ranged blob reads, the caches |
+| [Garbage collection](/antalya/cas/architecture/garbage-collection) | Leadership, the round, sharding, cost, observability |
+| [Backend abstraction](/antalya/cas/architecture/backend) | Provider dialects for conditional writes, the capability probe |
+| [Correctness](/antalya/cas/architecture/correctness) | TLA+ models, counterexamples, soak methodology, test coverage |
+| [Design history](/antalya/cas/architecture/design-history) | The rejected designs and the major pivots |
diff --git a/docs/en/antalya/cas/architecture/manifests-and-refs.md b/docs/en/antalya/cas/architecture/manifests-and-refs.md
new file mode 100644
index 000000000000..bd23b51e9a5f
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/manifests-and-refs.md
@@ -0,0 +1,260 @@
+---
+description: 'Part manifest structure and lifecycle, the ref table as the only mutable state in a CAS pool, the publish protocol, and the orphan-manifest sweep.'
+sidebar_label: 'Manifests and refs'
+sidebar_position: 5
+slug: /antalya/cas/architecture/manifests-and-refs
+title: 'CAS Architecture — Manifests and Refs'
+doc_type: 'reference'
+---
+
+# CAS architecture — manifests and refs {#manifests-and-refs}
+
+A part manifest is the immutable file list of one `MergeTree` part; a ref is the mutable pointer
+from a part name to the manifest that currently backs it. Together they are the two object kinds
+that make a CAS pool's state machine: manifests never change, refs are the only place anything
+moves. This page covers what a manifest contains, how a manifest becomes reachable or becomes an
+orphan, how a ref mutation is published durably, and how a mounted server recovers a ref table
+after a crash or a fresh mount. The write/promote sequence that drives these primitives is on the
+[part-lifecycle page](/antalya/cas/architecture/part-lifecycle); how `GC` folds ref history into
+blob liveness is on the [garbage-collection page](/antalya/cas/architecture/garbage-collection).
+
+## Part manifests {#part-manifests}
+
+A manifest (`cas_part_manifest`, `Formats/CasPartManifestFormat.h`) has four top-level fields:
+`ref` (its own id, repeated in the body for fail-closed validation), `root_namespace_id` (the
+owning namespace, likewise repeated), `payload_digest` (integrity/debug only — never a key, never
+a dedup input, never a `GC` edge), and `entries` — strictly ascending by path after decode. Each
+entry is `{path, placement, BlobRef, blob_size, inline_bytes}`; the hash algorithm travels **per
+entry**, so one manifest may legitimately mix algorithms if the pool has more than one enabled.
+
+A manifest deliberately holds **no** offsets, no packed-file support, no projections field, no
+codec info, no parent-manifest link, no source edges, and no incarnation token. One blob is one
+file's bytes; a read window is `{blobKey, blob_header_len, blob_size}`. A projection is an
+ordinary entry whose path has a `.proj` component. The incarnation token is the backend `ETag`
+observed by a `HEAD`, never stored in the manifest.
+
+**The manifest id is neither a content hash nor random.** It is
+`ManifestRef = {writer_epoch, build_sequence, manifest_ordinal}` — durable writer epoch times
+monotone per-incarnation build sequence times monotone per-build ordinal — which gives "no
+manifest id reuse" by construction with no randomness needed. The `GC`-level identity is the pair
+`ManifestId = (RootNamespace, ManifestRef)`; two namespaces may legally carry the same
+`ManifestRef`.
+
+Backpressure caps are enforced before the body is written (`Pool/CasPartWriteTxn.cpp`):
+
+| Cap | Limit |
+|---|---|
+| Entries per manifest | 1 048 576 |
+| Encoded manifest text | 256 MiB |
+| Total inline bytes | 16 MiB |
+| Largest single inline entry | 1 MiB |
+
+A manifest is written once with a conditional create (`putIfAbsentStream`) and **never rewritten**.
+A different object at that key would be an id collision and is `CORRUPTED_DATA`, fail-closed,
+before any owner transition names it. Rewriting a part therefore writes a **new** manifest over
+the **same** blobs and moves the ref in one ref-log record — a repoint, covered in full on the
+[part-lifecycle page](/antalya/cas/architecture/part-lifecycle#repoint).
+
+## Manifest lifecycle and the orphan sweep {#manifest-lifecycle}
+
+```mermaid
+stateDiagram-v2
+    [*] --> Staged: stageManifest, body PUT write-once
+    Staged --> PrecommitOwned: precommitAdd, ref-log OwnerTransition, plus-one edges on fold
+    PrecommitOwned --> Committed: promote, Precommit to Committed, no edge, net zero
+    Committed --> OwnerRemoved: drop or repoint or namespace removal, minus-one edges
+    OwnerRemoved --> [*]: GC deletes the body after the decrements are sealed
+
+    Staged --> OrphanA: writer died before precommitAdd
+    OrphanA --> [*]: writer best-effort delete, else the orphan sweep
+
+    PrecommitOwned --> DanglingPrecommit: writer died before promote
+    DanglingPrecommit --> OwnerRemoved: binding removed by abandon or a successor stale-precommit sweep
+```
+
+Two disjoint failure classes matter here:
+
+- **Pre-precommit orphan.** The body exists but no ref-log record ever named it. It contributes no
+  edges and nobody protects it — this is exactly what the orphan sweep below reclaims.
+- **Dangling precommit.** The transaction died between `precommitAdd` and `promote`. Nothing wakes
+  it up on its own: a `PartWriteTxn` is never persisted. The binding must be removed by a ref-log
+  transaction — either the live writer's own `abandon`, or a fenced successor's stale-precommit
+  sweep, which removes precommits whose `manifest_ref.writer_epoch < live_epoch`
+  (`Pool/CasRefLedger.cpp`). Only after that minus-one folds does `GC` delete the body, on the
+  ordinary owner-removal path.
+
+The writer's own best-effort cleanup deliberately **skips** the precommit target once a precommit
+was even attempted — including an uncertain outcome — because deleting a body that turns out to be
+a live precommit would clamp `GC`'s fold barrier forever.
+
+### The orphan-manifest sweep {#orphan-sweep}
+
+Runs as the last phase of the `GC` round, cursor-paced and budgeted, wrapped so it can never fail
+a round (`Gc/CasOrphanManifestSweep.cpp`). Eligibility comes **exclusively** from the durable
+watermark in the mount lease — there is no age threshold and no time-based grace period anywhere
+in this protocol. No mount lease for the `server_root_id` means no deletion authority means
+nothing is swept for that root.
+
+```mermaid
+flowchart TD
+    A["LIST one page of cas/manifests/<br/>budget: manifest_sweep_list_budget_keys"] --> B{"build-prefix eligible?<br/>durable watermark fact only"}
+    B -->|"epoch less than lease epoch"| ELIG["eligible, old-epoch debris"]
+    B -->|"same epoch, min_active clears build_seq"| ELIG
+    B -->|"no lease, or epoch ahead, or build may be live"| SKIP["skip"]
+    ELIG --> C["protection view: committed manifests<br/>plus live precommits<br/>plus manifests with an unfolded minus-one"]
+    C -->|"key protected"| SKIP2["skip"]
+    C -->|"not protected"| D["deleteExact key, token"]
+    D -->|Deleted| E["emit ManifestDelete audit event"]
+    D -->|"NotFound or TokenMismatch"| SKIP3["spared, a fresh owner reclaimed the key"]
+    E --> F["CAS gc/state with the advanced cursor"]
+```
+
+The protection view is built from the **same complete replay** that writer recovery uses, and a
+namespace whose view fails to build is added to an errored set with **all** of its deletions
+skipped — an empty owner set is never substituted for a failed one. The sweep deletes only
+manifest bodies and emits no blob deltas, correct precisely because a pre-precommit body never
+contributed a `+1`. Contrast with the owner-removal path, which is ordered the other way: fold the
+`-1` edges, adopt the decrements in the round `CAS`, *then* delete the body — a crash there leaks
+a body to this sweep, never a dangle.
+
+## Source edges: how a manifest makes blobs live {#source-edges}
+
+Blob liveness is a **set of source edges**, not a counter (`Gc/CasBlobInDegree.h`) — which is what
+makes `GC`'s fold idempotent. An edge id is `sourceEdgeId(ManifestId, path)`, a deterministic hash
+over the namespace, epoch, build sequence, ordinal and path — an edge *identity*, deliberately not
+a content hash and not reconstructable.
+
+Edges are never written at manifest-write time. They materialize only when `GC` folds a ref-log
+transaction that changes ownership: add-precommit means `+1` per blob entry; either removal means
+`-1`; **promote means no edge at all**, because the manifest never loses an owner, so it is net
+zero. Inline entries produce no edges — they have no separate object to reclaim.
+
+## The ref table {#ref-table}
+
+A ref is the only mutable state in the whole system, so this is where the concurrency design is
+concentrated.
+
+- **Name** — a canonical clean relative path, in practice the part directory name with an optional
+  `detached/` or `moving/` prefix.
+- **Value** — `{ref_name, ManifestRef, published_at_ms}`. There is **no** token/`ETag` in a ref
+  row; the cross-server "confirm token" is the text form `epoch:build:ordinal`.
+- **Scope** — one ref table per `RootNamespace`, i.e. per table per server root.
+- **Ownership slots** — a `ManifestRef` has at most one owner across the table, in one of two
+  slots: `Committed` or `Precommit`. Precommits are keyed by the pair `(ref_name, manifest_ref)`,
+  so several in-flight builds may legitimately contend for one ref name.
+
+In memory, `RefTableState` holds a copy-on-write map of committed rows, a set of precommits, an
+ownership index enforcing the one-owner rule, a lifecycle (`Live`/`Removed`), the greatest applied
+transaction id, and byte-size counters used for admission. Copying a state is a refcount bump, so
+a flush's trial and candidate copies cost proportional to touched rows, not the whole table.
+Network I/O is never performed while holding the state lock, so a reader sees either a whole
+transaction or none of it.
+
+Two immutable object kinds carry the durable form under `cas/ns/stream/<life_id>/` (see the
+[storage-layout key table](/antalya/cas/architecture/storage-layout#key-table)): a log object
+holds exactly one transaction, `{namespace, txn_id, ops[]}`; a snapshot object holds one live table image
+— sorted committed rows plus precommits. Mutable/path-addressed state lives separately under
+`cas/ns/state/<life_id>/`: the per-life `_ckpt` checkpoint and any namespace-owned `_files/`.
+
+`RefTxnId = {writer_epoch, ref_sequence}` renders as two fixed-width hex fields, so lexical key
+order equals tuple order. Ids are per-namespace and contiguous: within one `(namespace,
+writer_epoch)` they run `1, 2, 3, …` with no holes, and a new mount epoch restarts the sequence at
+`1`. A hole is therefore corruption, not an allocation artifact, and a non-successor id is rejected
+as `CORRUPTED_DATA`.
+
+The op vocabulary is deliberately tiny: `NamespaceBirth`, `OwnerTransition{old?, new?}`,
+`SetPublishedAt`, `RemoveNamespace`. There are exactly four legal `OwnerTransition` shapes — add
+precommit, remove precommit, remove committed, and promote — enumerated identically by the state
+machine and by `GC`'s edge extractor, so the two readers of the format cannot drift.
+
+Logs are pure conditional creates on write-once keys. There is no append-to-object and no
+`CAS`-swapped mutable pointer anywhere in the ref lane. The writer never deletes ref objects; only
+`GC` does, once coverage and a live snapshot both make a log safe to remove.
+
+Snapshots publish in the background, best-effort, one in flight per table, when the tail exceeds a
+log-count or log-byte threshold.
+
+## Publishing a ref mutation {#publish-protocol}
+
+All mutations funnel through one flat-combining lane, `CasRefLedger::appendRefOps`. A single flush
+carves a batch out of the queue and commits it as one or more transactions.
+
+```mermaid
+flowchart TD
+    Q["appendRefOps enqueues ops"] --> REC["ensure the table is recovered"]
+    REC --> FEN{"mount fence still live?"}
+    FEN -->|no| FAIL0["fail the whole carved queue, retry error"]
+    FEN -->|yes| W{"outstanding wedge?"}
+    W -->|yes| WR["resolve the wedge by its exact key first"]
+    WR -->|resolved durable| INST0["install candidate, clear wedge"]
+    WR -->|still unresolved| FAIL1["fail the queue, stay wedged, never allocate a new id"]
+    W -->|no| CARVE["two-phase carve: plan may throw, publish never throws"]
+    CARVE --> VAL["per-item validation: caps, shape, byte budget<br/>a failing item fails alone"]
+    VAL --> PREP["build candidate state and the complete wedge before the PUT"]
+    PREP --> PUT["putIfAbsent the ref-log key"]
+    PUT -->|Committed| OK["allocation-free install: swap state, bump counters, complete waiters"]
+    PUT -->|DefiniteFailure| GAP["fail survivors, id not consumed"]
+    PUT -->|"Unresolved, provably nothing sent"| NOSEND["do not wedge"]
+    PUT -->|"Unresolved, otherwise"| WEDGE["install the prepared wedge, survivors fail Uncertain"]
+    OK --> SNAP["maybe schedule a snapshot publish"]
+```
+
+The **wedge** is the mechanism that makes fail-closed ambiguity concrete: at most one per table,
+recording the single conditional `PUT` whose outcome is unknown, complete with the key and the
+sealed bytes. The next flush must resolve *that exact key* before it may allocate a new transaction
+id — an unresolved write can never silently become a gap, and the ledger never double-publishes.
+
+Crash points: between the `PUT` and the install, the object is durable and unapplied — the next
+mount's recovery replays it. Between a precommit and its promote, a dangling precommit is reclaimed
+by the successor's stale-precommit sweep, described above.
+
+## Recovery {#recovery}
+
+Recovery is lazy per table, on first touch (`Pool/CasRefLedger.cpp`), and reads only named,
+authoritative objects — there is no `LIST` anywhere in this path:
+
+1. **Exact `GET` of `_ckpt`.** The durable checkpoint is the sole source of the recovery grounding:
+   `chooseRecoveryGrounding` derives the base (a snapshot id, or genesis if there is none) and the
+   exact transaction to walk from purely from the checkpoint's own fields
+   (`committed_through`/`checkpoint_snapshot_id`/`life_epoch`) — recovery never enumerates its own
+   stream to find them.
+2. If the grounding names a snapshot, `GET` and decode it as the replay base.
+3. Walk forward by exact key from there, one transaction resident at a time: `GET`
+   `cas/ns/stream/<life_id>/<epoch>-<sequence>`, decode, apply, discard, advance to the next
+   arithmetic id. Every key this walk touches is a dense, deterministic successor of the last —
+   never a listed or guessed one.
+4. **Absence is a decision point, not an error.** Finding a slot empty is either the live epoch's
+   stream legitimately ending there, or — for a dead predecessor epoch — the exact slot where its
+   closing `EpochSeal` must be written before the table may be trusted; the two cases are
+   distinguished by whether the epoch being walked is still live, not by retrying a listing.
+5. Recovery may itself advance `_ckpt` as it replays, each time via a conditional write against the
+   checkpoint it last read; the write is re-verified with a fresh exact `GET` afterward, and a
+   concurrent winner's farther frontier is honored by restarting from that newer checkpoint rather
+   than trusting the write blindly.
+6. Transient network errors retry the whole attempt with capped backoff; corruption and logic
+   errors fail fast.
+
+For a mounted writer the recovered in-memory table is authoritative for reads of its own
+namespaces — there is no other writer of that namespace. S3 is authoritative for durability:
+in-memory state advances only after a durable `PUT`, and a caller's `appendRefOps` returns only
+after the durable install. In-flight precommits are visible only through the precommit set, never
+through an ordinary ref resolve.
+
+Two cross-process readers see a different, colder view, but only at the discovery boundary: `GC`
+and `ca-fsck` `LIST` once to discover which namespaces exist, staleness-bounded by whatever was
+durable at `LIST` time, so a namespace born after that `LIST` is invisible to this pass. Within
+each discovered namespace, the replay itself is not `LIST`-driven — it is the same exact-`GET`,
+`_ckpt`-grounded arithmetic walk described above, just called from a caller-supplied catalog entry
+instead of a live mount. The relink-confirm handshake (see the
+[replication page](/antalya/cas/architecture/replication#relink-gates)) does zero object-store I/O
+and answers `Yes` only against the resident, warm, fence-live in-memory table — `No` is not proof
+of the negative, only `Yes` is fence-gated.
+
+## Namespace removal {#namespace-removal}
+
+Namespace removal has no physical-empty handshake. The writer changes the catalog row from `Live`
+to `Removing`, appends the exact removals plus `RemoveNamespace`, and deletes nothing itself. The
+`GC` fold attaches cleanup evidence to that life row; a later invocation's pre-fold drain exact-CAS
+-deletes the matching `Removing` catalog row before any successor plan publishes. A perpetual
+namespace janitor and the orphan-manifest sweep reclaim physical debris independently — a same-name
+birth waits only for the catalog row to disappear, never for physical emptiness.
diff --git a/docs/en/antalya/cas/architecture/mounts-and-leases.md b/docs/en/antalya/cas/architecture/mounts-and-leases.md
new file mode 100644
index 000000000000..468a783297a6
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/mounts-and-leases.md
@@ -0,0 +1,223 @@
+---
+description: 'How a CAS server establishes identity, claims its mount slot, and holds a renewable lease that fences stale writers out of the pool.'
+sidebar_label: 'Mounts and leases'
+sidebar_position: 4
+slug: /antalya/cas/architecture/mounts-and-leases
+title: 'CAS Architecture — Mounts and Leases'
+doc_type: 'reference'
+---
+
+Page 4 of 4 in the CAS architecture set. Covers server identity, the mount lease that fences
+writers, and the server-scoped control-plane objects. No external coordinator is involved: there
+is no ZooKeeper/Keeper client anywhere in this protocol — `MountLeaseKeeper` is a local lease
+*renewer*, not a Keeper client.
+
+## `server_root_id` — the identity {#server-root-id}
+
+Every content-addressed disk must be configured with an explicit `server_root_id`. It is
+validated and immutable, and deliberately **not** derived from `ServerUUID` — two replicas can
+otherwise regenerate the same `ServerUUID` from a wiped local state directory, which must not
+silently steal an existing identity.
+
+Validation (`validateServerRootId`, `Pool/CasServerRoot.h`) is fail-closed `BAD_ARGUMENTS`, no
+sanitizing fallback: non-empty, at most 255 bytes, no empty/`.`/`..` path segment, no `_files` or
+`_manifests` segment.
+
+It roots four subtrees and owns catalog names at or below `<server_root_id>`:
+
+| Subtree | Contents |
+|---|---|
+| `gc/server-roots/<server_root_id>/` | owner, epoch, mount — the three control-plane objects below |
+| `roots/<server_root_id>/` | loose mountpoint objects, no namespace/catalog association |
+| `cas/manifests/<server_root_id>/` | part manifests |
+| `staging/<server_root_id>/` | S3-staging debris, outside every GC `LIST`, reclaimed only by this server's next mount |
+
+`blobs/` is **not** under the `server_root_id` — content is pool-global, which is what makes cross-server
+dedup work. Ref/namespace keys are also deliberately opaque and do not embed the `server_root_id`.
+
+Each replica sharing a backend endpoint must use a distinct `server_root_id`; omitting the setting
+is a startup error.
+
+## The owner claim {#owner-claim}
+
+`claimOwnerOrThrow` binds `server_root_id` ↔ `server_uuid` **permanently**. The owner object is never deleted
+and never reassigned — decommission only tombstones it in place.
+
+| Observed at `gc/server-roots/<server_root_id>/owner` | Action |
+|---|---|
+| present, same `server_uuid`, not tombstoned | proceed |
+| present, `retired_at_ms` set | `CORRUPTED_DATA` — explicitly decommissioned, refuses to resume |
+| present, different `server_uuid` | `CORRUPTED_DATA` — names the regenerated-uuid-file cause |
+| absent, subtree provably empty | `putIfAbsent` the owner (claim) |
+| absent, subtree non-empty | `CORRUPTED_DATA` — identity lost over existing data |
+| lost the `putIfAbsent` race | re-read; equal uuid proceeds, else `CORRUPTED_DATA` |
+
+"Provably empty" requires both an authoritative decoded catalog naming no life owned by `server_root_id` and
+a 1-key `LIST` probe finding nothing under `cas/manifests/<server_root_id>/` or `roots/<server_root_id>/`.
+
+Two failure modes this closes:
+
+- A **second server with a different `server_uuid`** is refused at this gate and can never take
+  over, regardless of lease expiry.
+- A **same-uuid live twin** (two processes sharing one uuid file and `server_root_id`) is caught separately, by
+  the mount claim's token-stability observation, and aborts with an operator-facing message rather
+  than corrupting the pool.
+
+## The mount lease {#mount-lease}
+
+One object, `gc/server-roots/<server_root_id>/mount`, carries **both** the liveness lease and the build
+watermark — there is no separate watermark object. `MountLease` fields: `server_uuid`,
+`writer_epoch`, `hostname`, `pid`, `started_at_ms`, renewal `seq`, `expires_at_ms`, `min_active`
+(the build-watermark floor), and `gc_fenced`.
+
+- **Cadence.** Renew every `mount_renew_period` (default 10 s), TTL `mount_lease_ttl_ms` (default
+  30 s, TTL/3 renewal ratio). Each beat is a token-guarded `putOverwrite` bumping `seq + 1` —
+  `MountLeaseKeeper` never re-mints the object.
+- **Local fence clock.** `CLOCK_BOOTTIME`, not `CLOCK_MONOTONIC`, so a VM resumed from suspend
+  correctly observes itself expired. The deadline anchors at attempt-*start*, never at response
+  time.
+- **Per-write recheck.** Every durable write or delete captures the fence generation at admission
+  and rechecks it immediately before the object-store call and on every conditional retry. Reads
+  are not gated.
+- **Request-budget admission.** `refAppendFenceOk` refuses to *start* a ref-log attempt unless
+  `attempt_timeout + safety_margin` fits inside the remaining lease, rejecting with
+  `BAD_ARGUMENTS` at request-admission time rather than mid-flight.
+
+**Losing the lease is neither read-only mode nor an abort.** It trips the local fence (latches
+`lost`, bumps the fence generation, moves the in-process runtime to `TransientNotLive`) and
+schedules a self-remount with exponential backoff from 1 s to 30 s. Only a *foreign* `server_uuid`
+observed on the mount body is `LOGICAL_ERROR` — the owner anchor makes a foreign claim
+protocol-unreachable, so seeing one is an invariant violation, not a recoverable race. A
+`putOverwrite` that threw *before* observing any outcome does not fence while the confirmed
+deadline is still comfortably ahead — only a **confirmed** mismatch is immediately terminal. A real
+fence still costs only an epoch: recovery re-claims with a fresh one, bounded at 3 attempts. This
+is the fail-closed posture from the general CAS invariant: doubt about the source aborts, doubt
+about the mechanism may retry.
+
+GC's own view of a dead server is symmetric and clock-skew-immune: a slot becomes fence-eligible
+only after the leader observes the *same* renewal token hold stable, on its own monotonic clock,
+for `TTL + TTL/20 + cadence` — the identical formula a re-mounting server uses to wait out a
+predecessor. The stamped `expires_at_ms` never participates in that decision; wall-clock `now` is
+audit-only.
+
+## The two monotone counters {#counters}
+
+| Counter | Storage | Scope | Protects against |
+|---|---|---|---|
+| `writer_epoch` | durable, `gc/server-roots/<server_root_id>/epoch` (`ServerEpoch::next_writer_epoch`, CAS-bumped by `allocateWriterEpoch`) | across crashes and restarts | a same-`(uuid, epoch)` twin: a present mount under a normal claim attempt is `CORRUPTED_DATA` |
+| `build_seq` | in-memory only, `CasMountRuntime::next_build_seq`, reset to 1 on every process start | one process incarnation | orders builds *within* an epoch; combined with `writer_epoch` it gives GC a total order |
+
+The absent-epoch branch of `allocateWriterEpoch` is deliberately paranoid: absent with a
+non-empty subtree is `CORRUPTED_DATA` (reset hazard); absent with an empty subtree decides by an
+authoritative probe, never by plain-`get` absence, because a transport fault must not be flattened
+into "not found".
+
+Global build ordering is the **pair** `(writer_epoch, build_seq)` compared lexicographically — the
+exact comparison GC uses for eligibility. The durable authority for both is the mount object
+itself: no mount means no deletion authority means nothing is swept. `min_active`, the oldest
+in-flight `build_seq`, rides in the same mount object as the watermark floor; `UINT64_MAX` in
+`min_active` is the farewell/retired sentinel, not a real build.
+
+## Mount claim outcomes {#claim-outcomes}
+
+The implementation does not expose a single named durable-slot enum; `claimMount` instead returns
+a `MountClaimResult::Kind` together with a `MountPriorState` describing which certificate of death
+(if any) justified a reclaim:
+
+| `Kind` | Meaning |
+|---|---|
+| `Claimed` | fresh claim (absent slot), same-`(uuid, epoch)` refresh, or a certified reclaim |
+| `LiveDoubleStart` | same `server_uuid`, different `writer_epoch`, and no certificate of death yet — a live twin, wait it out |
+| `ForeignOwner` | different `server_uuid` — refused unconditionally |
+| `FencedSelf` | same `(uuid, epoch)`, but `gc_fenced` — terminal for *this* epoch; the caller must mint a fresh one |
+
+| `MountPriorState` | Certificate that justified the reclaim |
+|---|---|
+| `None` | no reclaim needed (fresh claim or same-epoch refresh) |
+| `Clean` | the predecessor's own graceful farewell (`min_active == UINT64_MAX`) |
+| `Fenced` | GC's own threshold-gated fence-out (`gc_fenced`) |
+| `UncleanObserved` | this claimant's own token-stability observation held for the full `TTL + drift` window |
+
+## Behavioral mount-slot model {#mount-state-machines}
+
+Two coupled state pictures. Neither is a literal source enum — the durable slot is derived from
+the claim outcomes above and is shown here as behavior, not as a type in the code:
+
+```mermaid
+stateDiagram-v2
+    [*] --> Absent
+    Absent --> Live: claimMount putIfAbsent, seq=1
+    Live --> Live: keeper beat, putOverwrite seq+1
+    Live --> Fenced: GC observes a stable token past threshold, gc_fenced=1, body preserved
+    Live --> Terminated: certified drain, terminal farewell (expires_at=now, min_active=MAX)
+    Fenced --> Live: same-uuid claim with a fresh writer_epoch, instant reclaim
+    Terminated --> Live: same-uuid claim with a fresh writer_epoch, instant reclaim
+    Live --> Live: same-uuid claim, proven-dead token via UncleanObserved
+    Fenced --> Fenced: same uuid and epoch claim, FencedSelf, no write
+    Live --> Absent: decommission tail, mount then epoch then owner tombstone
+    Terminated --> [*]
+```
+
+The in-process `PoolLifecycle` runtime, by contrast, is a literal enum (`CasMountRuntime.h`):
+
+```mermaid
+stateDiagram-v2
+    [*] --> Live: Pool constructed, fence unarmed
+    Live --> Live: mountWritable arms the fence
+    Live --> TransientNotLive: renewal failure, tripMountLost, lost=true
+    TransientNotLive --> Live: self-remount succeeds with a fresh epoch
+    TransientNotLive --> TransientNotLive: probe inconclusive, retry with backoff
+    TransientNotLive --> IdentityLost: pool meta and owner both authoritatively absent
+    TransientNotLive --> VanishedReplaced: foreign pool_id observed
+    Live --> VanishedForgotten: SYSTEM CAS FORGET
+    IdentityLost --> [*]
+    VanishedReplaced --> [*]
+    VanishedForgotten --> [*]
+```
+
+`IdentityLost`, `VanishedReplaced` and `VanishedForgotten` are terminal and absorbing: the remount
+and GC threads self-exit, and there is deliberately no auto-revive — an identity disappearing
+under a live mount is an operator-level event.
+
+## Mount, unmount, crash {#mount-lifecycle}
+
+**Writable open** runs in a strict order: bootstrap-residual proof, capability probe under a
+random per-mount prefix, pool-meta create-or-validate, `validateServerRootId`, owner claim,
+`allocateWriterEpoch`, mount claim and keeper adopt, materialization grace if the predecessor was
+unclean (default 30 s), arm the fence, start background renewal. If the grace period consumed the
+TTL, one fresh renewal re-anchors the deadline before the fence is armed.
+
+**Clean unmount:** stop and join the remount thread, drain the ref lanes, and only if the drain
+*certified* quiescence write the terminal farewell (`expires_at_ms` already-expired,
+`min_active = UINT64_MAX`). That sentinel is what lets a successor reclaim instantly. If the drain
+did not certify, the keeper stops renewing and writes no farewell — an unearned farewell would let
+a successor start mutating while a stale conditional write from the predecessor is still in
+flight.
+
+**Crash:** no farewell; the renewal token freezes. Recovery is either the same server restarting
+and waiting out the token-stability observation, or the GC leader fencing the slot first, after
+which any reclaim is instant.
+
+**Permanent removal** of a dead replica (`Cas::decommissionPoolMember`, driven by
+`SYSTEM CAS DROP POOL MEMBER '<server_root_id>' FROM DISK '<disk>'`) claims the victim's mount slot
+as an administrative writer with a no-wait policy (refuses immediately if the member is alive),
+drops every ref-bearing namespace, sweeps manifest debris before the slot (deleting the mount
+removes the watermark authority), drains staging and roots, then — only with zero warnings —
+retires in order: mount, epoch, a final liveness re-check, owner tombstone.
+
+## `system.cas_mounts` {#mounts-table}
+
+A read-only view of the same heartbeat-floor computation GC uses: one `LIST` of
+`gc/server-roots/` plus one `GET` per slot, zero writes, per-row fail-open (an undecodable body
+becomes `state = 'corrupt'`, never an exception). Shows every `server_root_id` in the pool, including peers.
+
+| Column | Notes |
+|---|---|
+| `disk`, `server_root_id`, `server_uuid`, `hostname`, `process_id` | identity |
+| `writer_epoch`, `renewal_sequence`, `started_at`, `expires_at`, `min_active_build_sequence`, `gc_fenced` | lease state (`DateTime64(3)` columns; the millisecond-integer field names live only in the internal `MountLease` struct and the on-disk body) |
+| `state` | one of `live`, `expired`, `terminated`, `fenced`, `corrupt` |
+| `is_leader`, `pending_reclaim`, `last_success_age_seconds`, `wedged_namespace_count` | GC health, process-local; **`NULL` on every peer row** — a process-local fact must never be stamped onto another server's row |
+| `lifecycle`, `lifecycle_reason`, `lifecycle_detail`, `lifecycle_since` | the SQL surface for the in-process `PoolLifecycle` runtime above: `lifecycle` is one of `live`, `not_live`, `identity_lost`, `vanished`, `constructing`, `shutdown`; `lifecycle_reason` distinguishes `replaced` from `forgotten` for a `vanished` disk; `lifecycle_detail` carries the full diagnosis text; `lifecycle_since` is when the current non-live state began (`NULL` while live) |
+
+The lifecycle snapshot is I/O-free and ungated, so a not-live, never-started, or vanished disk
+still produces a row instead of silently disappearing from the table.
diff --git a/docs/en/antalya/cas/architecture/namespaces.md b/docs/en/antalya/cas/architecture/namespaces.md
new file mode 100644
index 000000000000..2f622ff0e329
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/namespaces.md
@@ -0,0 +1,172 @@
+---
+description: 'What a namespace is, the opaque life_id that qualifies every object it owns, the pool-wide namespace catalog, and a namespace lifetime end to end from first write to catalog-row deletion.'
+sidebar_label: 'Namespaces'
+sidebar_position: 10
+slug: /antalya/cas/architecture/namespaces
+title: 'CAS Architecture — Namespaces'
+doc_type: 'reference'
+---
+
+# CAS architecture — namespaces {#namespaces}
+
+A namespace (`Cas::RootNamespace`) is the opaque, per-table, per-server-root string under which one
+table's part manifests and one ref table live — in practice something the wiring layer composes,
+such as `srv1/<table_uuid>` for an ordinary table or `shadow/<backup>/<table_uuid>` for a `FREEZE`
+shadow. `CAS` never interprets its contents beyond a shape check (non-empty, no empty or reserved
+path segment, at most 512 bytes). The [manifests-and-refs page](/antalya/cas/architecture/manifests-and-refs#ref-table)
+covers the ref table one namespace owns; this page covers the namespace itself — its physical
+identity, the catalog that is the sole authority for whether it exists, and its full lifetime from
+first write to the catalog row's deletion.
+
+## `life_id`: the physical identity {#life-id}
+
+A namespace **name** can be reused — a table dropped and recreated keeps the same name. What must
+never be reused is the **physical identity** any durable object under that name is keyed by, so
+that a stale reader of the old incarnation can never be handed bytes belonging to the new one. That
+identity is `life_id`: an opaque, pool-wide, randomly minted 128-bit value (two `thread_local_rng`
+draws; retried on the astronomically unlikely zero draw, since `0` is reserved as "never a valid
+life"). Internally it is the catalog's `incarnation` field, aliased as `NamespaceLifePhysicalId`;
+paired with the namespace name it forms `NamespaceLifeId{ns, incarnation}`
+(`Primitives/CasNamespaceLifeId.h`).
+
+`NamespaceLifeId` deliberately has no default construction and no conversion from a bare namespace
+name: code holding only the name cannot address a ref object or a namespace file at all, so
+forgetting the life qualifier is a compile error, not a runtime aliasing bug. The only legitimate
+source of a `NamespaceLifeId` is `fromCatalogEntry` — reading it off one immutable catalog cut —
+which is what makes "this life belongs to this name" a catalog fact rather than something a caller
+could reconstruct incorrectly.
+
+`life_id` renders as 32 fixed-width lowercase hex digits and appears in exactly the two subtrees
+that are life-owned (see the [storage-layout key table](/antalya/cas/architecture/storage-layout#key-table)):
+
+| Subtree | Contents |
+|---|---|
+| `cas/ns/stream/<life_id>/` | The immutable `_log`/`_snap` ref-transaction history |
+| `cas/ns/state/<life_id>/` | The mutable `_ckpt` checkpoint and any namespace-owned `_files/` |
+
+Part manifests deliberately do **not** carry `life_id` — a manifest already has its own globally
+unique identity (`{writer_epoch, build_sequence, manifest_ordinal}` under the server root, see the
+[manifests-and-refs page](/antalya/cas/architecture/manifests-and-refs#part-manifests)) and needs no
+further qualification. Loose mountpoint objects under `roots/` are outside namespace ownership
+altogether and carry no `life_id` either.
+
+## The namespace catalog {#catalog}
+
+One pool-wide object, `cas/ref_catalog` (`Layout::refCatalogKey`), is the sole authority for which
+namespaces exist. It is read on every fold round and every ref-table recovery, and mutated by one
+token-`CAS` write per lifecycle transition. Its entries are canonically ordered by namespace bytes,
+strictly ascending, with no duplicate name — both the encoder and the decoder enforce this, so an
+out-of-order or duplicate-keyed catalog can never become durable.
+
+Each row (`CatalogEntry`) carries:
+
+| Field | Meaning |
+|---|---|
+| `ns` | The namespace name |
+| `state` | `Creating`, `Live`, or `Removing` — see below |
+| `incarnation` | The `life_id` for this row, nonzero, never reused |
+| `creator` | The mounted writer's fence identity (server root, writer epoch, admission fence generation) that is creating this row — **required** iff `state == Creating`, **forbidden** otherwise |
+| `removal_started_round` | The `GC` round observed when removal began — **required** iff `state == Removing`, absent otherwise |
+
+`NsState`'s three wire values (`Creating = 1`, `Live = 2`, `Removing = 3`) are append-only, exactly
+like every other persisted enum in `CAS`: a catalog object written by one build is read by another,
+so a value is never renumbered or repurposed.
+
+```mermaid
+stateDiagram-v2
+    [*] --> Creating: casAdmitEntry -- fresh random life_id, creator fence stamped
+    Creating --> Live: completeCreation -- publish genesis _ckpt, then flip, clear creator
+    Creating --> Creating: a live foreign creator fence -- retry later, no steal
+    Creating --> Live: reconcileStaleCreator finds the creator fence provably dead,<br/>a fresh opener steals and completes it
+    Live --> Removing: beginRemoving -- table drop, stamps removal_started_round
+    Removing --> [*]: GC drains the row once a fold sealed positive cleanup evidence
+    [*] --> Creating: a fresh createNamespace call, only once the old row is fully absent -- brand new life_id
+```
+
+A row's own state machine is linear per row (`Creating → Live → Removing → gone`); what makes the
+catalog non-linear as a whole is that a stalled `Creating` row can resolve two different ways
+depending on whether its creator fence is still alive, and that a name only becomes creatable again
+once its prior row is completely gone — both shown above.
+
+## Lifetime end to end {#lifetime}
+
+### Creation, on first write {#creation}
+
+There is no explicit "create namespace" statement; a namespace is born the first time anything
+resolves its ref table (`CasRefLedger::resolveNamespaceLife`, bounded at 32 loop attempts). If the
+catalog has no row for the name at all, the resolving mount admits a `Creating` entry stamped with
+its own creator fence and a freshly minted `life_id`
+(`CasRefCatalog::createNamespace` → `casAdmitEntry`). Two more steps make it usable:
+
+1. **Publish the genesis checkpoint.** The first `_ckpt` ever written for this `life_id` carries
+   `life_epoch = creator.writer_epoch` — the only writer that will ever know this namespace's
+   genesis epoch.
+2. **Flip to `Live`.** One token-`CAS` moves the row from `Creating` to `Live` and clears `creator`.
+
+Both steps re-check the resolving mount's own fence before writing, so a mount that lost its lease
+mid-creation reports `FencedOut` rather than silently completing. Several openers racing the same
+brand-new name all observe "no entry", but only one wins the admit; the rest see `Superseded` and
+simply re-read the catalog, landing on the winner's `Creating` row.
+
+A `Creating` row under a **different** mount's creator fence is not this opener's problem to force:
+if that fence is still provably alive, the opener retries later; only once the fence is provably
+dead (the same mount-lease terminality check `GC`'s heartbeat floor uses) does
+`reconcileStaleCreator` let a fresh opener steal the row onto its own fence and finish the two steps
+above itself.
+
+### Removal {#removal}
+
+Dropping a table (`DROP TABLE`, and every operation that reduces to it) calls
+`CasRefLedger::dropNamespace`. It closes the namespace's local positive-mutation lane first — new
+positive writers are refused while the in-flight ones drain — then transitions the catalog row from
+`Live` to `Removing` in one token-`CAS` (`beginRemoving`, stamping `removal_started_round` from the
+currently observed `GC` round), then appends **one** ref-log transaction that removes every current
+committed and precommit binding and ends with a terminal `RemoveNamespace` op. Removal is never
+refused by an admission check — Constraint 13 in the catalog's own spec — it always succeeds once
+the fence holds.
+
+Nothing is deleted by the writer at this point. No blob, no manifest, no ref-log object physically
+disappears here — only pointers move, exactly like an ordinary [`DROP TABLE`](/antalya/cas/architecture/part-lifecycle#operation-mapping)
+on any other ref.
+
+### What `GC` does with a `Removing` namespace {#gc-and-removal}
+
+The terminal `RemoveNamespace` transaction is folded like any other ref-log record, during the
+[round's fold phases](/antalya/cas/architecture/garbage-collection#the-round). Folding it stamps
+positive **cleanup evidence** directly onto that `life_id`'s row in the new fold seal — there is no
+physical listing and no `Pending`/`Completed` handshake; the evidence is a pure fact about which
+ref-log transaction folded.
+
+The **next** round's `pre_fold_ref_drain` phase is what actually removes the catalog row: it reads
+the just-adopted parent fold seal, and for every `Removing` row whose life carries durable cleanup
+evidence, it exact-`CAS`-deletes the catalog entry before that round does anything else. This
+two-round shape — evidence sealed in round *n*, catalog row deleted in round *n+1* — is why removal
+needs no separate physical-emptiness proof: by the time the row is deleted, a fold has already
+proven its ref history is fully drained.
+
+### What disappears, and when {#what-disappears}
+
+| Object class | Reclaimed by | When |
+|---|---|---|
+| Catalog row (`cas/ref_catalog` entry) | `GC` phase 2, `pre_fold_ref_drain` | The round after the fold that sealed cleanup evidence for this life |
+| Part manifest bodies | Ordinary owner-removal ([phase 15](/antalya/cas/architecture/garbage-collection#the-round)) for anything that had a committed or precommit binding, the [orphan-manifest sweep](/antalya/cas/architecture/manifests-and-refs#orphan-sweep) for anything that never got that far | As each owning ref is dropped by the removal transaction itself, independent of the catalog row |
+| Blob bodies | The ordinary condemn/graduate/delete pipeline | Whenever the manifests that named them stop being live, same as any other blob |
+| Ref stream/state objects (`_log`, `_snap`, `_ckpt`, `_files`) under the dead `life_id` | The perpetual namespace janitor ([phase 16](/antalya/cas/architecture/garbage-collection#the-round)) | Best-effort, one bounded `LIST` page at a time, whenever it next lists a key whose `life_id` a fresh catalog cut no longer names — independent of, and not gated on, catalog-row deletion |
+
+The janitor is leak-only: it never fails a round, never blocks progress on an unreadable key, and a
+crash mid-page simply leaves debris for its next page.
+
+### Recreate while removing {#recreate-while-removing}
+
+A fresh `createNamespace` call for a name whose catalog row is still `Live` or `Removing` is
+refused outright — internally this is a misuse `LOGICAL_ERROR`, because the higher-level open loop
+(`resolveNamespaceLife`) filters that case out first and reports a typed retry-later error instead:
+"creation waits for its terminal fold and catalog removal to complete". A caller that keeps
+resolving the same name simply keeps retrying until the row is gone.
+
+Once `pre_fold_ref_drain` has deleted the row, the name is free again, and the very next opener mints
+a **brand new**, independently random `life_id` — never the retired one. That is the whole answer to
+"what happens on recreate": the old physical identity is never revived, so every key ever written
+under it — its `_log`, its `_snap`, its `_ckpt`, its `_files` — stays permanently addressed by a
+value nothing will ever mint again, and a reader still holding the old `NamespaceLifeId` observes
+only stale-or-absent data, never a byte that belongs to the new incarnation.
diff --git a/docs/en/antalya/cas/architecture/part-lifecycle.md b/docs/en/antalya/cas/architecture/part-lifecycle.md
new file mode 100644
index 000000000000..5a61d0490dc1
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/part-lifecycle.md
@@ -0,0 +1,148 @@
+---
+description: 'The part-add protocol from local build through blob upload to promote, its nine crash points and their cleaners, and how each MergeTree operation maps onto it.'
+sidebar_label: 'Part lifecycle'
+sidebar_position: 6
+slug: /antalya/cas/architecture/part-lifecycle
+title: 'CAS Architecture — Part Lifecycle'
+doc_type: 'reference'
+---
+
+# CAS architecture — part lifecycle {#part-lifecycle}
+
+Publishing a `MergeTree` part on a `CAS` disk is one durable protocol,
+`stageManifest → precommitAdd → putBlob → promote`, driven by `Cas::PartWriteTxn`
+(`Pool/CasPartWriteTxn.cpp`). This page walks that protocol end to end: local build, the durable
+order and why each step is where it is, every crash window and who cleans it up, and how each
+`MergeTree`-level operation (insert, merge, mutation, detach, …) maps onto it. Manifest structure
+and the ref table it writes into are covered on the
+[manifests-and-refs page](/antalya/cas/architecture/manifests-and-refs); the fetch-side protocol
+for replicated parts is on the [replication page](/antalya/cas/architecture/replication).
+
+## The protocol {#protocol}
+
+```mermaid
+sequenceDiagram
+    autonumber
+    participant MT as MergeTree
+    participant TX as CA transaction overlay
+    participant PW as PartWriteTxn
+    participant S3 as Object store
+
+    rect rgba(140,190,140,0.12)
+    Note over MT,S3: Phase A -- local build, nothing durable, nothing visible
+    MT->>TX: writeFile data.bin
+    TX->>TX: classify: blob class spills and hashes to scratch or S3 staging
+    MT->>TX: writeFile count.txt, columns.txt, ...
+    TX->>TX: buffer small files in memory as inline candidates
+    MT->>TX: moveDirectory tmp_insert to final name
+    Note over TX: pure overlay re-key, not a publish
+    end
+
+    rect rgba(120,160,255,0.12)
+    Note over MT,S3: Phase B -- publish, per part, serially
+    MT->>TX: commit
+    TX->>PW: stageManifest entries
+    PW->>S3: PUT manifest, write-once, no preliminary HEAD
+    PW->>S3: append ref-log PRECOMMIT, plus NamespaceBirth if needed
+    Note over PW: precommit durable, the observe gate opens
+    TX->>PW: fan out blob uploads, one task per unique BlobRef
+    par blob 1
+        PW->>S3: HEAD / conditional PUT / adopt
+    and blob 2
+        PW->>S3: ...
+    end
+    PW->>PW: merge upload results on the owning thread, one no-throw swap
+    TX->>PW: promote
+    PW->>S3: GET and validate the precommit manifest body
+    PW->>S3: append ref-log txn: retire old committed, Precommit to Committed, SetPublishedAt
+    Note over PW: commit durable, then retire the build sequence
+    end
+```
+
+**Phase A — staging.** The transaction is an eager overlay, not a queue: `writeFile` immediately
+classifies the path and either spills bytes to a hashing buffer or holds them in memory as an
+inline candidate. Blob-class files stage to local scratch by default, or — when `staging_backend`
+is `s3` and the mount-time conditional-copy probe passed — to an S3 staging object written as
+`[header][payload]`, so that the later promote is a verbatim server-side copy. The `tmp_ → final`
+rename is a pure overlay re-key; the durable publish happens only in `commit`.
+
+**Step 1 — `stageManifest`.** Caps (see the
+[manifests-and-refs page](/antalya/cas/architecture/manifests-and-refs#part-manifests)) are
+checked before the write; the id is minted as `{epoch, build_seq, ordinal++}`; the body goes out
+with a conditional create and no preliminary `HEAD`. Both a definite failure and an unresolved
+outcome throw retry-later.
+
+**Step 2 — `precommitAdd`.** The intent — target namespace, final ref name, manifest — is recorded
+before the append, because an unresolved append may have landed anyway. One ref-log transaction
+adds the precommit binding. A same-name birth is refused with retry-later while the catalog still
+says `Removing`; once the predecessor row is absent, creation receives a new opaque life id and
+starts its own stream. On return the precommit is durable, and only now may the writer adopt
+existing blobs.
+
+**Step 3 — blob upload fan-out.** One task per unique `BlobRef`, deterministic dispatch order, one
+pre-sized result slot per ref (see the write-path sequence on the
+[blob-protocol page](/antalya/cas/architecture/blob-protocol#conditional-write-sequence)). The
+calling thread only submits and joins, never occupies a pool slot, so a pool of size one degenerates
+to a correct serial run and can never deadlock. The contract is merge-nothing: if any task threw,
+nothing is merged and the first error in dispatch order is rethrown. Results are folded into the
+dependency set on the owning thread, into a copy, committed by one no-throw swap. Pool size is the
+server setting `cas_blob_upload_pool_size` (default 16).
+
+**Step 4 — `promote`.** Reads and revalidates the precommit manifest body once; sets the commit
+state to Uncertain before the append — past that point, failure is no longer proof of the negative
+— then checks that the precommit is still the live owner and revalidates leaves. Tokened leaves
+are skipped because they are edge-protected; tokenless leaves must be evidence adopts, trusted
+through the durable manifest edge with no per-file `HEAD`; anything else is a `LOGICAL_ERROR`. The
+whole thing lands as one ref-log record: optional retirement of the old committed binding, the pure
+Precommit-to-Committed owner move, and `SetPublishedAt`. Promotion emits no blob deltas — the
+manifest never loses an owner, so it is net zero.
+
+## Crash points and their cleaners {#crash-points}
+
+This table is the single best summary of the design's crash-safety story: every row leaks
+something recoverable; no row loses data or leaves a dangling reference.
+
+| # | Crash window | Left behind | Who cleans it |
+|---|---|---|---|
+| C1 | During staging | Local temp files, or S3 staging objects | Local: unconditional cleanup plus buffer destructor. S3: the mount's own staging sweep at next mount — never deleted on abort |
+| C2 | After `stageManifest`, before `precommitAdd` | An unreferenced manifest body | Writer's best-effort exact-token delete; durable backstop is the orphan-manifest sweep |
+| C3 | `precommitAdd` returned Unresolved | A possibly-live precommit binding | Intent recorded pre-append; `abandon` appends the exact removal, tolerating absence. The body is never writer-deleted |
+| C4 | Between `precommitAdd` and `promote` | A live precommit plus uploaded blobs | No resume path exists. Removed by `abandon`, else by a fenced successor's stale-precommit sweep |
+| C5 | Mid blob fan-out | Already-uploaded blobs | Nothing merged; blobs become `GC`-reclaimable debris; the part is not published |
+| C6 | `promote` append Unresolved | The ref may or may not be committed | Commit state Uncertain — the relink layer maps this to "retry the whole fetch", never to a byte fetch |
+| C7 | A later part throws after earlier parts published | A partial multi-part commit | Precise rollback: drop only the refs this call created, matching the exact manifest — never clobbers a concurrent writer's repoint |
+| C8 | Transaction destroyed uncommitted | Open builds | Destructor abandons every build |
+| C9 | Namespace dropped mid-build | — | One atomic flag; every further op fails closed at the alive check |
+
+## The repoint {#repoint}
+
+Writing into an already-committed part — an `ALTER`-style metadata rewrite, or any standalone
+write against a committed source — never mutates the existing manifest. It writes a **new**
+manifest over the (possibly partly reused) blob set and moves the ref to it in one ref-log record.
+Unchanged columns are adopted by hash through a tokenless evidence dependency with no `HEAD` and no
+`GET`; changed columns are fresh uploads. A repoint therefore costs zero bytes moved for the
+carry-forward portion of the file set — only the changed content re-uploads.
+
+## How each MergeTree operation maps {#operation-mapping}
+
+| Operation | CAS mechanics |
+|---|---|
+| `INSERT` | The canonical path above. Projections ride the parent part's transaction |
+| Merge | Identical for the output part. `<proj>.tmp_proj → <proj>.proj` is an entry-prefix re-key inside the staged manifest, not a rename |
+| Mutation | `createHardLink` per unchanged file: a source staged in *this* transaction copies the entry and its pending-blob record; a **committed** source records a tokenless evidence dependency with no `HEAD` and no `GET`. A mutation is a manifest rewrite where zero bytes move for the carry-forward |
+| `ALTER` / metadata rewrites | Standalone writes into a committed part, i.e. a repoint |
+| `DROP PART` | `removeDirectory` drops the ref and clears any per-file removal marks — one ref-drop, zero repoints |
+| `DROP TABLE` / `DETACHED` / `UNFREEZE` | A namespace or prefixed-ref drop. Blobs are never deleted here — removal is pointer-unlink plus deferred `GC` |
+| `RENAME TABLE` | Republishes every ref and verbatim file into the new namespace, then drops the old one. Not atomic across namespaces, but idempotent and re-drivable — true atomicity would need a move journal and is out of scope |
+| `FREEZE` / `BACKUP` / `RESTORE` / cross-disk `MOVE` | Each wraps the whole clone in one disk transaction, because a CAS part is one atomic unit |
+
+`FREEZE` is the one operation that materializes real bytes into a genuinely separate shadow
+namespace rather than reusing a table's own ref names — that shadow namespace is a `GC`
+reachability root, and `UNFREEZE` releases its refs.
+
+## Reads while a part is in flight {#in-flight-reads}
+
+Read-your-writes for a part still inside an open transaction is served by an explicit overlay
+rather than by any durable object — `tryGetInFlightStorageObjects`, `tryReadFileInFlight`,
+`listInFlightDirectory`. One deliberate subtlety: the bare part directory reports as absent in the
+overlay, so cleanup of a deduplication-rejected temporary part does not mistake it for a real part.
diff --git a/docs/en/antalya/cas/architecture/read-path.md b/docs/en/antalya/cas/architecture/read-path.md
new file mode 100644
index 000000000000..c92634dd9e74
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/read-path.md
@@ -0,0 +1,84 @@
+---
+description: 'How a CAS read resolves a ref to a manifest and then to ranged blob reads, and the two caches — manifest decode and part-folder view — that sit on that path.'
+sidebar_label: 'Read path'
+sidebar_position: 9
+slug: /antalya/cas/architecture/read-path
+title: 'CAS Architecture — Read Path'
+doc_type: 'reference'
+---
+
+# CAS architecture — read path {#read-path}
+
+A `CAS` read never touches a classical local-metadata path: there is no local directory listing to
+consult, only a ref resolve followed by object-store reads. This page covers the three ways a file
+access is served, the full chain for the common case, the two caches that sit on that chain, and
+how a part still open inside a write transaction serves its own reads.
+
+## How a file access is served {#access-kinds}
+
+| Access kind | How it is served | S3 cost |
+|---|---|---|
+| Inline entry — small files such as `count.txt`, `columns.txt` | Decoded straight out of the manifest body | Zero additional operations |
+| Blob-backed file — `.bin`, marks, large `primary.idx` | Ranged `GET` bounded by `[header_len, header_len + blob_size)` | One `GET` per column file per part open |
+| Verbatim file — `roots/…` objects | Plain object read, no `CAS` indirection | One `GET` |
+
+The full chain for a blob-backed file is: resolve the ref, read the manifest, look up the path,
+build a blob view plan, ranged `GET`, then `ReadBufferFromFileView`. Because the payload always
+starts at a pool-constant offset (the manifest's `blob_header_len`), no header parse is needed to
+locate content — see the [envelope format](/antalya/cas/architecture/storage-layout#envelope-format)
+on the storage-layout page.
+
+Part manifests themselves are read whole after opening the object: there is no on-disk random
+access, `seek`, or streaming requirement for their entry records — a manifest is small enough that
+decoding the whole body is cheaper than any partial-read machinery would be.
+
+## The two caches {#caches}
+
+| Cache | Keyed by | Setting | Default | What still hits the network |
+|---|---|---|---|---|
+| Manifest decode cache | `(ManifestId, Token)` | `manifest_decode_cache_bytes` | 128 MiB | A mandatory `HEAD` on **every** access, cache hit or miss |
+| Part-folder view cache (`Cas::CachedPartFolderAccess`, `Parts/PartFolderAccess.h`) | Part ref key | `part_folder_cache_bytes`, `part_folder_cache_max_entries`, `part_folder_cache_max_entry_bytes` | 64 MiB / 10 000 entries / 16 MiB | Its `ForceFresh` policy re-proves the manifest body via that same mandatory `HEAD`, paced by `part_folder_validate` (`always` \| `never` \| `age <seconds>`) |
+
+**The `HEAD` is mandatory even on a cache hit** — the page's most counter-intuitive fact, because it
+means a cache hit still costs one object-store round trip:
+
+```mermaid
+flowchart TD
+    A["readManifestShared(ManifestId)"] --> B["HEAD the manifest key"]
+    B -->|"absent"| C["throw FILE_DOESNT_EXIST --<br/>a live ref must never name a missing object"]
+    B -->|"present, token t"| D{"cache lookup (ManifestId, t)"}
+    D -->|hit| E["return the cached decode -- no GET"]
+    D -->|miss| F["GET the body"]
+    F --> G{"body's own ref and namespace<br/>match the key?"}
+    G -->|no| H["throw CORRUPTED_DATA"]
+    G -->|yes| I["decode, insert into cache keyed by (ManifestId, t), return"]
+```
+
+The `HEAD` is what proves the live ref still names an existing object — the no-dangle invariant —
+and it supplies the token that keys the cache; only then is the decode cache consulted. On a miss,
+the `GET` is followed by the two identity checks in the diagram, each `CORRUPTED_DATA` on failure.
+Only a fully validated decode enters the cache. Setting either cache's byte budget to `0` disables
+retention while leaving the `HEAD`-and-validate sequence intact — a cache is purely an
+optimization, never a trust boundary.
+
+The part-folder view cache is invalidated on every promote and repoint, and is single-flight on a
+cold build: concurrent readers of the same not-yet-cached view coalesce into one build rather than
+racing independent `GET`s.
+
+## Reads while a part is still being written {#in-flight-reads}
+
+An in-flight part inside an open write transaction is not yet visible through the ordinary ref
+resolve — reading it goes through the same explicit overlay used for read-your-writes, covered on
+the [part-lifecycle page](/antalya/cas/architecture/part-lifecycle#in-flight-reads). The bare part
+directory itself reports as absent in that overlay, precisely so that cleanup of a rejected
+temporary part is never mistaken for a real, resolvable part.
+
+## Diagnostic and read-only access {#read-only-access}
+
+A read-only or diagnostic opener of a `CAS` disk (`ca-fsck`, `ca-gc-dryrun`, and similar tools)
+must not claim mount ownership, schedule `GC`, or mint writer state — read-only enforcement sits
+below the ordinary facade checks, at the backend layer itself. A mounted `Pool` caches its ref
+table and does not re-recover it on every read; a diagnostic tool that deliberately performs a
+fresh cold recovery on each pass can therefore observe a **less** stale ref table than a live
+mounted read, which is intentional for tools whose entire purpose is catching drift a live mount
+would not notice.
diff --git a/docs/en/antalya/cas/architecture/replication.md b/docs/en/antalya/cas/architecture/replication.md
new file mode 100644
index 000000000000..cc767f7fe115
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/replication.md
@@ -0,0 +1,118 @@
+---
+description: 'Fetch by relink between two replicas sharing a pool: the gates in order, what actually seals commit-before-release, and detach/attach/drop.'
+sidebar_label: 'Replication'
+sidebar_position: 7
+slug: /antalya/cas/architecture/replication
+title: 'CAS Architecture — Replication'
+doc_type: 'reference'
+---
+
+# CAS architecture — replication {#replication}
+
+When two `ReplicatedMergeTree` replicas share a `CAS` pool, a fetch should move **no bytes** — the
+receiver already has access to the same blobs the sender does. The mechanism is a three-phase
+handshake, fetch by relink, layered directly on the ordinary interserver part-fetch protocol. This
+page covers the handshake, the gates that decide whether it fires, what actually makes it safe
+against a concurrent `GC` round, and how detach/attach/drop reduce to the same primitives. The
+writer owns table semantics and part publication (see the
+[part-lifecycle page](/antalya/cas/architecture/part-lifecycle)); `GC` owns ref-log folding and
+physical cleanup (see the [garbage-collection page](/antalya/cas/architecture/garbage-collection))
+— ordinary replication traffic never reads `gc/state` or waits on a `GC` round.
+
+## The handshake {#handshake}
+
+Only two of the three phases are round trips to the sender — the offer and the confirm. The
+publish and the promote are the receiver's own writes to the pool.
+
+```mermaid
+sequenceDiagram
+    autonumber
+    participant R as Receiver
+    participant Snd as Sender
+    participant S3 as Shared pool
+
+    R->>Snd: GET part, cas_pool_uuid = R's pool uuid, client_protocol_version = 11
+    Note over R: advertising 11 is a promise to confirm before promoting
+    Snd->>Snd: same disk pool uuid? identity, never endpoint plus prefix
+    Snd->>S3: resolve the offer once -- manifest bytes and confirm token from the SAME view
+    Snd-->>R: cookie cas_relink = part_manifest_v2, cookie cas_source_token = ..., body = manifest bytes
+    Note over Snd: sender is fire-and-forget -- it releases the part here
+
+    rect rgba(120,160,255,0.12)
+    Note over R,S3: T1 -- publish, the plus-one lands first
+    R->>S3: adopt entries by evidence, no HEAD, no bytes, stageManifest fresh receiver-local id, precommitAdd
+    Note over R: the sender's ManifestRef, namespace and digest are ignored -- only entries are used
+    end
+
+    rect rgba(255,190,120,0.15)
+    Note over R,Snd: T2 -- confirm
+    R->>Snd: POST cas_confirm = token
+    Snd->>Snd: confirmExactRef, zero object-store I/O, never throws
+    Snd-->>R: cookie cas_confirm_answer = yes or unproven
+    end
+
+    alt answer is yes
+        R->>S3: T3 -- promote, ref published
+    else anything else -- unproven, missing cookie, timeout, transport error
+        R->>R: throw a locally generated NETWORK_ERROR, retry later
+        Note over R: never a byte re-request -- that would go back to the very source whose state is in doubt
+    end
+```
+
+## The gates, in order {#relink-gates}
+
+| # | Gate | What it enforces |
+|---|---|---|
+| 1 | Pool identity | The receiver advertises `cas_pool_uuid`; the sender offers relink only if its own disk's pool uuid is **equal**. Matching by endpoint and prefix was tried and rejected — a minted pool uuid is the identity |
+| 2 | Protocol version 11 | On the receiver side, advertising it is a promise to run the confirm round trip before promoting |
+| 3 | One resolution for two outputs | The manifest bytes and the confirm token come from the **same** view. Two separate calls would allow a repoint in between and hand the receiver a token naming a manifest whose entries it never adopted |
+| 4 | The receiver trusts nothing from the wire but the entry list | The sender's manifest id, namespace and payload digest are ignored; the target namespace and ref come from the receiver's own router, and manifest path hygiene is validated at decode |
+| 5 | The confirm is I/O-free and fail-closed | A cold, evicted, unfenced or terminal mount answers `Unknown`. `No` and `Unknown` both go on the wire as `unproven`, because the fence check is evaluated last, so a `No` cannot be distinguished from "cannot prove it right now" |
+| 6 | Only the literal `yes` authorizes promotion | Everything else — including a timeout — is one outcome: throw and retry later |
+| 7 | Promote outcomes are three-way | `Committed` proceeds; a **proven** not-committed state (body-absent precommit, precommit no longer live owner, ref conflict) falls back to a byte fetch; `Unresolved` **throws**, because returning "fall back" there would publish the part twice |
+
+The byte-fetch fallback is bounded: it re-invokes the fetch with relink disabled, which stops the
+receiver advertising its pool uuid, which stops the sender offering relink — so the relink path
+cannot be entered twice for one fetch. Byte-fetched files content-address and dedup on arrival
+anyway, so falling back never loses the dedup property, only the zero-byte-move property for that
+one fetch.
+
+## What actually seals "commit before release" {#relink-seal}
+
+The receiver's `+1` — its precommit binding — is durable **before** the sender is asked anything,
+and any removal of the sender's own binding is appended strictly after that `+1` is in the ref
+log. That ordering, steps T1 then T2 then T3, is the whole seal.
+
+This does **not** establish that every subsequent `GC` fold *sees* that `+1` under every listing
+behavior: a configuration with one incomplete listing page can, in principle, let a fold miss a
+freshly published edge. A confirmed relink therefore proves only "the source still holds exactly
+this manifest right now", not "no future fold can ever miss this edge" — `ca-fsck`'s
+reachable-but-absent scan is the backstop for that gap, not the relink protocol itself. Relink
+also races `GC` in the ordinary sense any writer does: between the sender encoding its offer and
+the receiver's promote, `GC` on the shared pool may condemn a blob that was live only through the
+sender's own ref. The [writer-versus-GC race](/antalya/cas/architecture/blob-protocol#writer-gc-race)
+on the blob-protocol page is what makes that interleaving safe — revival is re-upload only, and the
+receiver's evidence-adopt is protected by its own durable precommit edge exactly like any other
+writer's adopt.
+
+A fetch whose source part is still a live, held `DataPartPtr` on the sender's own replica — the
+common case for a local, same-process relink — keeps the source pinned through the destination's
+commit by ordinary part-lifetime rules, independent of the ref-log seal above.
+
+## Detach, attach, drop {#detach-attach-drop}
+
+A detached part is **not** a separate namespace — it is a ref in the table's own namespace with a
+`detached/` prefix (the same is true of `moving/`). Only `FREEZE` uses a genuinely separate shadow
+namespace, which the ownership check deliberately refuses to claim, so a frozen part can never be
+relink-confirmed.
+
+`DETACH`, `ATTACH`, `delete_tmp_` cleanup, and merge-result renames all reduce to the same two
+moves: re-key any *staged* source into the destination, then `republishRef(src → dst)` for any
+*committed* source. `republishRef` re-reads the source manifest freshly, publishes an
+equivalent-entry manifest under the destination ref — a **new** manifest id, with blobs untouched
+and adopted by evidence — then drops the source ref. A destination that already exists with
+identical entries just drops the source, an idempotent re-drive; one with different entries
+throws.
+
+Manifests are therefore per-ref and never moved: a detach creates a new manifest for
+`detached/<part>` and retires the old one, and the blobs' net in-degree is unchanged.
diff --git a/docs/en/antalya/cas/architecture/storage-layout.md b/docs/en/antalya/cas/architecture/storage-layout.md
new file mode 100644
index 000000000000..9b4336a6bf2f
--- /dev/null
+++ b/docs/en/antalya/cas/architecture/storage-layout.md
@@ -0,0 +1,160 @@
+---
+description: 'S3 key layout and on-disk text-object formats used by the content-addressed storage (CAS) MergeTree disk backend.'
+sidebar_label: 'Storage layout'
+sidebar_position: 2
+slug: /antalya/cas/architecture/storage-layout
+title: 'CAS Architecture — Storage Layout'
+doc_type: 'reference'
+---
+
+# CAS architecture — storage layout {#storage-layout}
+
+Every key in a pool is built by one class, `Cas::Layout` (`Formats/CasLayout.h`), which owns
+exactly the pool prefix. Every persisted object opens with a one-line JSON envelope header, and
+control-plane bodies are JSON Lines — one JSON object per line, sorted where the object is a log
+or a set of entries (`Formats/README.md`; see [Envelope format](#envelope-format) below for which
+parts are a single JSON object versus JSON Lines versus raw payload bytes). The format is
+deliberately this plain: any object can be fetched and read with ordinary line-oriented tools
+while debugging, and a new field is additive — a tolerant reader skips it — so the format evolves
+without a migration.
+
+## Key table {#key-table}
+
+All key patterns are shown under the pool prefix. A **namespace** is the opaque per-table string
+under which one `MergeTree` table's part manifests and ref history live: for a live table it is
+the table's canonical disk path (`store/<xx>/<uuid>`, `@cas@`-marked) prefixed by the owning
+server's `server_root_id`, and a backup gets its own `shadow/…` namespace instead; `Cas::Layout`
+only validates a namespace's shape and never interprets its contents.
+
+| Key pattern | Object | Codec | Writer |
+|---|---|---|---|
+| `_pool_meta` | pool identity + floors | `cas_pool_meta` | pool create/admit |
+| `blobs/<algo>/<hex[0:2]>/<hex>` | blob envelope + payload | `cas_blob` | uploads |
+| `blobs/<algo>/<hex[0:2]>/<hex>.meta` | blob freshness sidecar | `cas_blob_meta` | dedup/GC |
+| `cas/ns/stream/<life_id>/_log/<epoch-hex>-<seq-hex>.zst` | ref transaction log | `cas_ref_log` | writer commit path |
+| `cas/ns/stream/<life_id>/_snap/<epoch-hex>-<seq-hex>.zst` | complete ref table snapshot | `cas_ref_snap` | writer/GC fold |
+| `cas/ns/state/<life_id>/_ckpt` | mutable per-life checkpoint | `cas_ref_ckpt` | writer/GC fold |
+| `cas/ns/state/<life_id>/_files/<relative-name>` | namespace-owned verbatim file | — (raw passthrough) | upper layers |
+| `cas/manifests/<namespace>/<epoch-hex>-<seq-hex>/<ordinal>.zst` | part manifest | `cas_part_manifest` | part build |
+| `gc/state` | GC state (incl. GC lease) | `cas_gc_state` | GC |
+| `gc/hb` | GC leader heartbeat | `cas_gc_hb` | GC |
+| `gc/maintenance_state` | leak-only namespace-janitor cursor | `cas_gc_maintenance_state` | future janitor |
+| `gc/gen/<gen>/attempt/<att>/fold_seal` | fold seal (deterministic) | `cas_fold_seal` | GC |
+| `gc/gen/<gen>/attempt/<att>/blob_target/<shard>/<seq>` | GC source-edge run segment | `cas_run` | GC |
+| `gc/gen/<gen>/attempt/<att>/outcomes/<round>/<shard>.zst` | GC outcome log | `cas_gc_outcomes` | GC |
+| `gc/server-roots/<server_root_id>/owner` | server-root owner singleton | `cas_owner` | mount |
+| `gc/server-roots/<server_root_id>/epoch` | server-root epoch singleton | `cas_epoch` | mount |
+| `gc/server-roots/<server_root_id>/mount` | mount lease (incl. `min_active` watermark) | `cas_mount_lease` | mount |
+| `roots/<key>` | loose mountpoint object, verbatim | — (never interpreted) | upper layers |
+| `staging/<server_root_id>/…` | S3-native upload staging scratch | — | writer, own mount only |
+
+`<algo>` is `ch128`, `xxh3`, or `sha256` — the hash algorithm is a path segment because one pool may
+legally hold blobs under several algorithms at once. `<hex[0:2]>` is a flat two-character S3 key
+shard for request-fan-out, unrelated to the separate `gc_shards` GC-internal reduction fan-out
+(which appears only inside `gc/gen/…` keys and routes by the digest's high 64 bits, read
+big-endian). Discovery LISTs use fixed prefixes: `cas/ns/stream/`, `cas/ns/`, `cas/manifests/`,
+`blobs/` (deliberately without the algorithm segment, so one recursive LIST covers every
+algorithm), `roots/`, `gc/server-roots/`. `staging/` is a top-level sibling that no GC LIST ever
+touches — it is reclaimed only by its own server's next mount.
+
+## Envelope format {#envelope-format}
+
+Every persisted CAS metadata object is text: a header line, a body, and an optional trailer.
+
+```
+{"type":"cas_<object>","v":N}          <- header line, always present
+<body>                                  <- one JSON object, sorted NDJSON records,
+                                            or a descriptor + raw payload zone
+{"n":…}                                 <- optional trailer (record/entry count)
+```
+
+`v` is the only version field; a reader rejects `v` above what the build supports with
+`UNKNOWN_FORMAT_VERSION`, checked before the body. A `.zst` key suffix means, exactly, that the
+object kind's compression policy is `Always`: the object is stored as one zstd frame with the
+checksum flag on, and its declared content size is checked against a per-kind cap before
+allocation. Always-small and deterministic kinds (`cas_ref_ckpt`, `cas_blob_meta`, `cas_fold_seal`,
+`cas_run`, …) are stored raw, with no `.zst` suffix.
+
+The blob envelope is a special case of the header/body shape: a JSON descriptor padded with ASCII
+spaces to a pool-constant `blob_header_len` (256 bytes, a `cas_pool_meta` field), terminated by
+`\n`, so the raw payload always starts at that fixed offset with no header parse needed to locate
+it. The part manifest is the other `PayloadHybrid` kind: text header, descriptor, sorted NDJSON
+entry records, `{"n":…}` trailer, then a banner-framed raw payload zone for small inline file
+bytes.
+
+## Codec table {#codec-table}
+
+Condensed from the authoritative traits table in `CasFormat.cpp` (`TRAITS`, asserted complete by
+`gtest_cas_text_format.cpp`).
+
+| Type string | Family | Key strictness | Compression |
+|---|---|---|---|
+| `cas_blob` | `PayloadHybrid` | tolerant | never (raw, fixed offset) |
+| `cas_blob_meta` | `Control` | tolerant | never |
+| `cas_pool_meta` | `Control` | tolerant | never |
+| `cas_ref_log` | `Control` | tolerant | always (`.zst`) |
+| `cas_ref_snap` | `Control` | tolerant | always (`.zst`) |
+| `cas_ref_ckpt` | `Control` | strict | never |
+| `cas_ref_catalog` | `Control` | strict | never |
+| `cas_part_manifest` | `PayloadHybrid` | tolerant | always (`.zst`) |
+| `cas_run` | `RecordStream` | strict | pinned raw |
+| `cas_fold_seal` | `Control` | strict | pinned raw |
+| `cas_gc_state` | `Control` | tolerant | never |
+| `cas_gc_hb` | `Control` | tolerant | never |
+| `cas_gc_outcomes` | `Control` | tolerant | always (`.zst`) |
+| `cas_gc_maintenance_state` | `Control` | strict | never |
+| `cas_owner` | `Control` | tolerant | never |
+| `cas_epoch` | `Control` | tolerant | never |
+| `cas_mount_lease` | `Control` | tolerant | never |
+
+"Strict" means unknown keys are rejected rather than skipped, used for objects where every field
+decides a durability or cleanup decision (`cas_ref_ckpt`, `cas_ref_catalog`, `cas_fold_seal`,
+`cas_run`, `cas_gc_maintenance_state`); a `!`-prefixed key is always critical regardless of the
+kind's strictness. "Pinned raw" objects (`cas_run`, `cas_fold_seal`) need stable bytes across
+re-encodes for deterministic-artifact adoption, so their bytes are never recompressed once
+written. `cas_blob` and `cas_part_manifest` are the `PayloadHybrid` family: a text descriptor
+followed by a raw payload zone, rather than a single JSON body.
+
+## Worked example tree {#worked-example}
+
+Pool prefix `ca-pool`, server root `srv1`, one `Atomic` table, one part `all_1_1_0` with one blob
+column file, written at `writer_epoch = 1, sequence = 3`:
+
+```
+ca-pool/_pool_meta
+
+ca-pool/cas/ns/stream/0123456789abcdef0123456789abcdef/_log/0000000000000001-0000000000000003.zst
+ca-pool/cas/ns/stream/0123456789abcdef0123456789abcdef/_snap/0000000000000001-0000000000000003.zst
+ca-pool/cas/ns/state/0123456789abcdef0123456789abcdef/_ckpt
+
+ca-pool/cas/manifests/srv1/store/3f2/3f2a1b7c-…-abcdefabcdef@cas@/0000000000000001-0000000000000003/000001.zst
+
+ca-pool/blobs/xxh3/a1/a1b2c3d4e5f60708b1c2d3e4f5061728
+ca-pool/blobs/xxh3/a1/a1b2c3d4e5f60708b1c2d3e4f5061728.meta
+
+ca-pool/roots/srv1/clickhouse_access_check_8f3a1c2d
+
+ca-pool/gc/state
+ca-pool/gc/hb
+ca-pool/gc/server-roots/srv1/{owner,epoch,mount}
+ca-pool/gc/gen/7/attempt/1/fold_seal
+ca-pool/gc/gen/7/attempt/1/blob_target/0/1
+ca-pool/gc/gen/7/attempt/1/outcomes/1/0.zst
+
+ca-pool/staging/srv1/<upload scratch>
+```
+
+`0123456789abcdef0123456789abcdef` is the opaque physical `life_id` the catalog maps the table's
+namespace to; the ref log and snapshot keys reuse the same `RefTxnId` rendering
+(`0000000000000001-0000000000000003`) as the manifest's build-scoped directory, but they are
+different counters with different semantics, not the same identifier. The `data.bin` entry inside
+the part manifest names the blob by `{XXH3_128, a1b2…1728}`, which is what resolves to the
+`blobs/xxh3/a1/…` key above. A small file such as `count.txt` has no object of its own — it is
+inline inside the manifest's raw payload zone, not a separate key.
+
+## Notes {#notes}
+
+- `cas/ns/state/<life_id>/_ckpt` carries **no** `.zst` suffix: `cas_ref_ckpt`'s compression policy
+  is `never`, while its `_log`/`_snap` siblings in the same `cas/ns/` tree compress `always`.
+- The namespace-stream tree is `cas/ns/stream/` (immutable `_log`/`_snap` objects) and
+  `cas/ns/state/` (mutable `_ckpt`, verbatim `_files/`).
diff --git a/docs/en/antalya/cas/bucket-requirements.md b/docs/en/antalya/cas/bucket-requirements.md
new file mode 100644
index 000000000000..ca05ad903aaf
--- /dev/null
+++ b/docs/en/antalya/cas/bucket-requirements.md
@@ -0,0 +1,44 @@
+---
+description: 'The object-store contract a bucket must satisfy to host content-addressed storage, and which providers qualify.'
+sidebar_label: 'Bucket requirements'
+sidebar_position: 4
+slug: /antalya/cas/bucket-requirements
+title: 'CAS Bucket Requirements'
+doc_type: 'reference'
+---
+
+# Bucket requirements {#bucket-requirements}
+
+`CAS` is built on a small object-store contract (`Backend/CasBackend.h`), checked by a capability
+probe that runs at every writable mount and fails closed: an object store that does not enforce
+these conditions is refused rather than trusted.
+
+## The capability table {#capability-table}
+
+| Requirement | Interface method | Why it is needed |
+|---|---|---|
+| Read-after-write on a fresh key | `Backend::get` / `Backend::head` | Recovery listings and point reads must see what was just written |
+| Conditional create (`If-None-Match: *`) | `Backend::putIfAbsent`, `Backend::putIfAbsentStream` | Write-once creation of blobs, manifests, and ref-log entries |
+| Conditional overwrite (`If-Match: <token>`) | `Backend::putOverwrite`, `Backend::casPut` | The one mutual-exclusion primitive: mount leases, `gc/state` |
+| Exact-token delete | `Backend::deleteExact` | GC must delete only the incarnation it condemned, never a resurrected replacement |
+| Ranged `GET` | `Backend::get` / `Backend::getStream` with a `Range` | Opening one column file of a part costs one bounded read, not a whole-object fetch |
+| `LIST` with a resumable cursor | `Backend::list` | GC discovery and the orphan-manifest sweep page through the pool without a separate index |
+| No versioning / no delete markers | probed by `runCapabilityProbe`; `created_delete_marker` on `DeleteOutcome` | A delete marker over a live key would break exact-token semantics — GC would archive instead of reclaim |
+| `TOKEN ⟹ CONTENT` (a repeated token implies unchanged bytes) | standing requirement on every `Backend` implementation | Not probed — it cannot be tested cheaply. A backend that recycled tokens would serve stale manifests, i.e. wrong query results, not merely an inefficiency |
+
+Bucket **versioning is not required** — in fact it must be **disabled** on the generation-token
+dialect (see below), because a token-exact delete on a versioned bucket archives a noncurrent
+generation instead of reclaiming storage, silently stopping GC reclamation.
+
+## Platform support {#platform-support}
+
+| Platform | Status | Notes |
+|---|---|---|
+| AWS S3 | ✓ | Native `ETag`-based conditional dialect: `If-None-Match` / `If-Match` used directly |
+| Google Cloud Storage | ✓ | Generation-token dialect: conditional headers are rewritten to `x-goog-if-generation-match`, opted into via `http_client = gcs_hmac` or `gcp_oauth` |
+| Azure Blob Storage | probably | Azure's REST API documents the equivalent conditional headers, but ClickHouse's Azure object-storage backend does not yet wire up a `CAS` conditional dialect the way the S3 and GCS paths do — untested, not validated by the capability probe |
+| Other S3-compatible stores | only with enforced conditional operations | The capability probe is the actual gate: a store that silently ignores `If-None-Match`/`If-Match` (accepting and applying the write regardless) fails the probe and is refused. `RustFS` passes the full battery and is used as the project's test backend; `Garage` was evaluated and rejected because it silently ignores conditional operations |
+
+The full mechanics of the two dialects — how the backend detects which one a given endpoint speaks,
+what the capability probe actually checks, and how exact-token deletes map onto each provider's
+primitives — are in [the Backend architecture page](/antalya/cas/architecture/backend).
diff --git a/docs/en/antalya/cas/configuration.md b/docs/en/antalya/cas/configuration.md
new file mode 100644
index 000000000000..8e551ef292be
--- /dev/null
+++ b/docs/en/antalya/cas/configuration.md
@@ -0,0 +1,132 @@
+---
+description: 'Every disk-level and server-level setting content-addressed storage exposes, generated from ContentAddressedSettings and ServerSettings at HEAD.'
+sidebar_label: 'Configuration'
+sidebar_position: 3
+slug: /antalya/cas/configuration
+title: 'CAS Configuration Reference'
+doc_type: 'reference'
+---
+
+# Configuration reference {#configuration-reference}
+
+## The disk config block {#disk-config}
+
+A `CAS` disk is an `object_storage` disk with `metadata_type` set to `cas` and an explicit
+`server_root_id`. The recommended shape layers a `type=cache` disk in front of it — the local
+filesystem cache absorbs repeated reads of the same blob, while the `CAS` disk underneath stays the
+single source of truth the pool's other members and GC also read from. The storage policy references
+the **cached** disk, not the raw `CAS` disk directly:
+
+```xml
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <cas>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <server_root_id>{replica}</server_root_id>
+                <endpoint>https://bucket.s3.amazonaws.com/cas/</endpoint>
+                <access_key_id>...</access_key_id>
+                <secret_access_key>...</secret_access_key>
+            </cas>
+            <cas_cache>
+                <type>cache</type>
+                <disk>cas</disk>
+                <path>/var/lib/clickhouse/cas_cache/</path>
+                <max_size>10Gi</max_size>
+            </cas_cache>
+        </disks>
+        <policies>
+            <cas>
+                <volumes>
+                    <main>
+                        <disk>cas_cache</disk>
+                    </main>
+                </volumes>
+            </cas>
+        </policies>
+    </storage_configuration>
+</clickhouse>
+```
+
+`path` and `max_size` are ordinary `type=cache` disk settings (see
+[external disk cache](/operations/storing-data#using-local-cache)), not `CAS`-specific — size the
+cache to the working set of blobs a node reads repeatedly, not to the pool's total size. `type`,
+`object_storage_type`, `metadata_type`, `endpoint`, `access_key_id`, `secret_access_key`, and the
+other generic object-storage/disk keys (`path`, `name`, `region`, `use_environment_credentials`,
+`readonly`, `use_fake_transaction`, and a handful more) belong to the shared disk layer, not to
+`CAS` — they are accepted inside the `cas` disk's own block but are not `CAS` settings. Every key
+below this line, and every key not in that shared set, is rejected as unknown.
+
+The bare, uncached form — a storage policy pointing directly at the `CAS` disk, as used by
+[quick start](/antalya/cas/quick-start) — remains valid and is the minimal way to try `CAS` out:
+
+```xml
+<policies>
+    <cas>
+        <volumes>
+            <main>
+                <disk>cas</disk>
+            </main>
+        </volumes>
+    </cas>
+</policies>
+```
+
+## Disk-level settings {#disk-settings}
+
+None of these keys carry a `cas_`/`ca_` prefix — the disk block already scopes them.
+
+`CAS` is experimental: any setting below may change semantics, change its default, or disappear
+entirely before release. Treat this table as a snapshot of the current build, not a stable contract.
+
+| Setting | Default | Description |
+|---|---|---|
+| `server_root_id` | — (required) | Explicit layout subtree identity; macros expand as in the `s3` `endpoint`. Anchored in the pool by a write-once owner claim — a colliding identity is refused at mount |
+| `scratch_path` | server data path | Server-local scratch dir for the write-buffer spill; a relative value is anchored to the server data path |
+| `gc_enabled` | `true` | Run the background GC scheduler on this disk. `false` is a debugging aid, not an operating mode: garbage then accumulates indefinitely and silently — watch `system.cas_gc_log` for round activity if you ever toggle it |
+| `gc_interval_sec` | `60` | Seconds between background GC rounds (≥ 1) |
+| `blob_hash` | `cityhash128` | Pool blob content-hash function (`cityhash128` \| `xxh3-128` \| `sha256`). Recorded in the pool at creation; a mismatching config is refused at mount |
+| `blob_hash_allow_new` | `false` | Explicit opt-in to admit a new hash algorithm into an existing pool. One-way: once admitted, the pool carries both algorithms permanently |
+| `skip_access_check` | `false` | Skip the boot-time capability probe (start now, fix later). Safer than the name suggests: only the preflight probe is skipped — the conditional-write correctness check still runs unconditionally on every writable mount |
+| `deduplication_cache_bytes` | 64 MiB | Byte budget of the blob presence cache (`0` disables) |
+| `deduplication_head_first_min_bytes` | 1 MiB | Minimum blob size to try a `HEAD` before uploading the body |
+| `gc_snapshot_generations_to_keep` | `3` | GC snapshot generations retained |
+| `gc_shards` | `1` | Blob-hash-prefix reducer shards (≥ 1). Recorded in the pool at creation; a mismatching config is refused at mount |
+| `gcs_max_conditional_put_bytes` | 1 GiB | Largest conditional write on a generation-token store (GCS forces those single-part); does not bound the unconditional resurrect |
+| `part_folder_cache_bytes` | 64 MiB | Part-folder view cache byte budget (`0` disables retention) |
+| `part_folder_cache_max_entries` | `10000` | Part-folder view cache entry cap |
+| `part_folder_cache_max_entry_bytes` | 16 MiB | Oversized part-folder views bypass retention above this size |
+| `part_folder_validate` | `always` | Cache body re-proof policy (`always` \| `never` \| `age <seconds>`). **Leave at `always`**: the other modes trade the fail-closed body-existence check for an optimization — this is a trust decision about unverified data, not a performance knob |
+| `manifest_decode_cache_bytes` | 128 MiB | Manifest decode cache byte budget (`0` disables) |
+| `gc_meta_pool_size` | `16` | Bounded pool size for GC per-hash freshness-meta writes |
+| `staging_backend` | `local` | Blob staging backend (`local` \| `s3`); `s3` is opt-in |
+
+### Choosing `blob_hash` {#choosing-blob-hash}
+
+`blob_hash` is fixed at pool creation, so pick it deliberately. `blob_hash_allow_new` is the
+escape hatch — it admits a second algorithm into an existing pool's `algos_used` rather than
+requiring a fresh pool.
+
+| Algorithm | Pick it for | Trade-off |
+|---|---|---|
+| `sha256` | Maximum safety | No known collision classes; slightly slower than the other two |
+| `xxh3-128` | Maximum speed | Fastest, 128-bit, no known collision classes |
+| `cityhash128` (default) | ClickHouse-ecosystem compatibility, and a possible future hash-reuse mode that avoids recomputation | Fast, but has a known class of collisions that occurs far more often than an ideal hash function would predict |
+
+## Server-level settings {#server-settings}
+
+Source: `ServerSettings.cpp`. Unlike the disk-level list, these carry the `cas_` prefix because they
+are process-wide, not scoped to one disk block.
+
+| Setting | Default | Description |
+|---|---|---|
+| `cas_blob_upload_pool_size` | `16` | Size of the dedicated server-wide thread pool used to upload blobs in parallel when committing a `CAS` part. Zero is rejected: the pool must have at least one thread |
+
+## `SYSTEM CAS` commands {#system-commands}
+
+`SYSTEM CAS GC RUN`, `SYSTEM CAS GC STOP`, `SYSTEM CAS GC START`, `SYSTEM CAS GC REBUILD`,
+`SYSTEM CAS FSCK`, `SYSTEM CAS FORGET`, and `SYSTEM CAS DROP POOL MEMBER '<server_root_id>' FROM
+DISK '<disk>'` operate on a mounted `CAS` disk. Introspection lives in `system.cas_log`,
+`system.cas_gc_log`, and `system.cas_mounts`.
diff --git a/docs/en/antalya/cas/index.md b/docs/en/antalya/cas/index.md
new file mode 100644
index 000000000000..2bc71046494b
--- /dev/null
+++ b/docs/en/antalya/cas/index.md
@@ -0,0 +1,89 @@
+---
+description: 'What content-addressed storage is, the problem it solves, its current status, and where to go next.'
+sidebar_label: 'Overview'
+sidebar_position: 1
+slug: /antalya/cas
+title: 'Content-Addressed Storage'
+doc_type: 'guide'
+---
+
+# Content-addressed storage {#content-addressed-storage}
+
+`ReplicatedMergeTree` on object storage has two unattractive options today. Plain replication
+stores a byte-identical copy of every part on every replica, so storage cost multiplies with the
+replication factor. Zero-copy replication shares the bytes, but at a structural price: every
+replica keeps local metadata referencing each shared S3 object, and that state grows with the
+data; a commit spans three independent systems — local disk, S3, and `Keeper` — whose interleaving
+is easy to get subtly wrong, and a failure in any one of the three hurts availability; sharing is
+tracked by a numeric refcount, so a lost or duplicated retry can corrupt the count; and the
+special cases supporting all of this are scattered widely through the `MergeTree` code.
+
+Content-addressed storage (`CAS`) is a `MetadataStorage` back-end for object-storage disks
+(`metadata_type = cas`) that takes the same sharing goal and collapses it onto one system: every
+`MergeTree` part file is stored once, keyed by the hash of its content, in the object-storage pool
+itself. There is no `CAS` state in `Keeper` at all — a commit is one conditional write against a
+single object in the pool — and the reachability accounting is a derived in-degree edge set folded
+from append-only deltas, not a mutable refcount a lost message can corrupt.
+
+```mermaid
+graph LR
+    subgraph today["Today: zero-copy replication"]
+        R1["Replica 1<br/>local disk: object refs<br/>(grows with data)"] -->|"in-flight ops only"| K["Keeper"]
+        R2["Replica 2<br/>local disk: object refs<br/>(grows with data)"] -->|"in-flight ops only"| K
+        R1 -.->|"shares bytes"| S1["S3"]
+        R2 -.->|"shares bytes"| S1
+    end
+    subgraph cas["CAS: content-addressed pool"]
+        C1["Replica 1"] -->|"publish a ref"| P["S3 pool<br/>(refs, leases, GC — all in-bucket)"]
+        C2["Replica 2"] -->|"publish a ref"| P
+    end
+```
+
+Every CAS bookkeeping object — refs, mount leases, GC leadership, fencing tokens — lives in the
+bucket. There is no external coordinator, and no `Keeper` usage inside the pool protocol; `Keeper`
+stays exactly where `ReplicatedMergeTree` already used it, for replication log and part-set
+consensus, and its load does not grow with pool size.
+
+## Deployment guidance {#deployment-guidance}
+
+`GC` throughput is proportional to how much changes in the pool: a pool holding a very large
+number of parts from many servers, or data that churns very quickly, means longer `GC` rounds.
+Two consequences for planning:
+
+- **The preferred deployment is a second tier for cold data**: hot, fast-churning parts stay on
+  the local (or plain S3) tier, and `CAS` holds the large, slow-moving cold tail — where
+  deduplication pays the most and `GC` traffic is minimal.
+- **At large scale, shard the pool by key prefix.** With tens of servers, or thousands of tables
+  and millions of parts, split the deployment into several independent pools by giving each shard
+  its own prefix — the shards can share one bucket:
+
+  ```xml
+  <endpoint>https://bucket.s3.amazonaws.com/cas/{shard}</endpoint>
+  ```
+
+  Each prefix is a fully independent pool (its own refs, leases, and `GC`), so rounds stay short
+  regardless of the total fleet size.
+
+## Status {#status}
+
+`CAS` is **experimental**. It ships in Altinity Antalya builds. Experimental means the on-disk
+format and the SQL surface can still change between releases — that is deliberate, not a caveat to
+apologize for. Pre-release means the format can change cheaply, with zero compatibility
+scaffolding, and the design can keep being iterated on invariants rather than migrations. The bet
+underneath it: all you need is a good S3 bucket. See [bucket requirements](/antalya/cas/bucket-requirements)
+for exactly what "good" means.
+
+`CAS` coexists with zero-copy replication; it does not replace it. `metadata_type = cas` is opt-in
+per disk, so adopting it never requires migrating an existing deployment.
+
+## Where to go next {#nav}
+
+| Page | Covers |
+|---|---|
+| [Quick start](/antalya/cas/quick-start) | A minimal disk config and the first `CREATE TABLE` / `INSERT` / `SELECT` |
+| [Configuration](/antalya/cas/configuration) | Every disk-level and server-level setting |
+| [Bucket requirements](/antalya/cas/bucket-requirements) | What an object store must support, and which providers qualify |
+| [Architecture overview](/antalya/cas/architecture/) | The object model, the Git analogy, and the safety invariants |
+| [Correctness](/antalya/cas/architecture/correctness) | How the design was verified: TLA+ models, counterexamples, soak methodology |
+| [Design history](/antalya/cas/architecture/design-history) | What earlier designs were tried and rejected, and why |
+| [Roadmap](/antalya/cas/roadmap) | What is shipped, planned, and deliberately not pursued |
diff --git a/docs/en/antalya/cas/operations/debugging.md b/docs/en/antalya/cas/operations/debugging.md
new file mode 100644
index 000000000000..b1ed38791341
--- /dev/null
+++ b/docs/en/antalya/cas/operations/debugging.md
@@ -0,0 +1,225 @@
+---
+description: 'SQL-first CAS debugging: live investigation queries against cas_log/cas_gc_log/cas_mounts/blob_storage_log, SYSTEM CAS FSCK/GC RUN/GC STOP-START/FORGET, and the offline clickhouse-disks tools for when the server cannot answer.'
+sidebar_label: 'Debugging'
+sidebar_position: 4
+slug: /antalya/cas/operations/debugging
+title: 'CAS Operations — Debugging'
+doc_type: 'guide'
+---
+
+# Operations — debugging {#debugging}
+
+Debugging a content-addressed (`CAS`) incident starts on a **live server**, with SQL: the three
+system tables plus `SYSTEM CAS` commands cover reachability checks, forced GC rounds, and
+per-object/per-round forensics without ever touching the bucket directly. The offline
+`clickhouse-disks` tools at the [end of this page](#offline-tools) are the fallback for when SQL
+cannot reach the pool at all — the server is down, or the access is deliberately read-only forensic.
+
+## Investigating on a live server {#live-investigation}
+
+See [monitoring](/antalya/cas/operations/monitoring#system-tables) for the three system tables'
+grain and general health queries; this section is investigation queries for a specific incident,
+not a health dashboard.
+
+### What happened to this part or blob {#part-blob-history}
+
+`system.cas_log` carries one row per writer/GC decision, keyed by `ref_name` (a part name) or
+`object_hash` (a blob's content hash):
+
+```sql
+SELECT event_time_microseconds, event_type, outcome, reason, object_kind, object_hash, token, round, detail
+FROM system.cas_log
+WHERE disk_name = 'cas' AND ref_name = '<part_name>'
+ORDER BY event_time_microseconds;
+```
+
+```sql
+SELECT event_time_microseconds, event_type, outcome, reason, ref_name, round, detail
+FROM system.cas_log
+WHERE disk_name = 'cas' AND object_kind = 'blob' AND object_hash = '<hex_digest>'
+ORDER BY event_time_microseconds;
+```
+
+`outcome` (`ok`, `adopt`, `resurrect`, `deleted`, `replaced`, `spared`, `absent`, `zeroed`,
+`skipped`) and `reason` are the two columns to read first; `detail` is a
+`Map(LowCardinality(String), String)` of decision-specific facts (`condemn_round`,
+`superseded_token`, `code`, `site`) worth `arrayJoin(detail)` when the summary columns alone do not
+explain the decision. See [`system.cas_log`](/operations/system-tables/cas_log) for the full column
+reference.
+
+### Why GC is not reclaiming {#why-not-reclaiming}
+
+Two questions, in order: is this node's scheduler leading, and did its recent rounds actually fold?
+
+```sql
+SELECT server_root_id, is_leader, state, last_success_age_seconds, pending_reclaim
+FROM system.cas_mounts WHERE disk = 'cas';
+
+SELECT event_time, outcome, candidates_marked, entries_condemned, entries_graduated,
+       entries_redeleted, anomalies
+FROM system.cas_gc_log
+WHERE event_type = 'Finish' AND disk_name = 'cas'
+ORDER BY event_time DESC LIMIT 10;
+```
+
+A `0`/`false` `is_leader` means this node never reclaims for this disk — check the peer that holds
+leadership instead. A steady `entries_condemned` with `entries_graduated` stuck at `0` means objects
+are being found but never crossing the safety floor (recall the grace period is measured in full
+rounds, not acks — see [condemnation and deletion](/antalya/cas/architecture/garbage-collection#condemn-delete)).
+A specific blob's own story — was it ever condemned, spared, or is it not being seen at all — is the
+per-object query in the previous section, filtered to `object_kind = 'blob'`.
+
+### What one GC round did {#gc-round-detail}
+
+Every round writes a `Start` and a `Finish` row to
+[`system.cas_gc_log`](/operations/system-tables/cas_gc_log), correlated by `round_id` (not `round`,
+which is `0` on `Start` and absent on a round that never led). One `Phase` row per phase reached
+carries that phase's own `phase_duration_microseconds`, `ProfileEvents` delta, and `phase_metrics` —
+group by `round_id` to reconstruct one round in order:
+
+```sql
+SELECT event_type, outcome, phase, phase_duration_microseconds, duration_ms
+FROM system.cas_gc_log
+WHERE round_id = '<round_id>'
+ORDER BY event_time_microseconds;
+```
+
+### Who holds the mount {#who-holds-mount}
+
+```sql
+SELECT server_root_id, hostname, process_id, state, writer_epoch, renewal_sequence,
+       expires_at, is_leader
+FROM system.cas_mounts
+WHERE disk = 'cas'
+ORDER BY is_leader DESC;
+```
+
+Every `server_root_id` sharing the pool shows up here, not just this node's own — a `state` other
+than `live` (`expired`, `terminated`, `fenced`, `corrupt`) on a member that should be up is the first
+thing to check before assuming a lease problem is this node's own. `is_leader` and the other
+process-local columns are `NULL` on every peer's row; run the query on that peer to see its own view.
+
+## SQL commands for live diagnosis {#sql-commands}
+
+### SYSTEM CAS FSCK {#sql-fsck}
+
+The online consistency check — unlike the offline tools below, this runs against a disk the server
+already has **mounted and serving traffic**; the scan re-validates every finding against a fresh
+authoritative read, so it needs no quiesce:
+
+```sql
+SYSTEM CAS FSCK cas;
+```
+
+Returns one row: `disk`, `reachable`, `dangling`, `unreachable`, `pending_gc`, `awaiting_gc`,
+`unaccounted`, `stale_edge`, `corrupted_runs`, `chain_broken`, `unchecked`, `lifeless_keys`,
+`namespace_janitor_pending` (+`_bytes`/`_lives`), `ref_records_walked`, `physical_bytes`,
+`referenced_logical_bytes`, `distinct_blobs`, `total_blob_refs`. `dangling` is the one column that
+means data loss — `unreachable`, `pending_gc`, and `awaiting_gc` are objects still
+moving through the normal condemn/graduate/delete pipeline, not a problem on their own.
+`chain_broken` and `corrupted_runs` are the other two hard findings: a hole in a ref-log stream and a
+GC source-edge run that failed its checksum, respectively. This summary-only form has no
+per-object `--detail` equivalent yet — for that, the offline `cas-fsck --detail` below is still
+needed.
+
+### SYSTEM CAS GC RUN {#sql-gc-run}
+
+Runs one round synchronously and returns exactly the shape of a `cas_gc_log` `Finish` row — driving
+a round on demand while watching its outcome interactively is one of the most direct diagnostics
+available:
+
+```sql
+SYSTEM CAS GC RUN cas;
+```
+
+One row per disk it ran on: `disk`, `acquired_lease`, `deferred`, `round`, `candidates_marked`,
+`objects_deleted`, `objects_absent`, `objects_replaced`, `objects_spared`, `manifests_deleted`,
+`entries_condemned`, `entries_graduated`, `entries_redeleted`, `fence_outs`, `anomalies`,
+`pending_candidates`, `pending_condemned`, `pending_retired`. Omitting
+the disk name runs one round on every content-addressed disk on the node. A manual run executes
+regardless of `SYSTEM CAS GC STOP` — `STOP` pauses only the background scheduler.
+
+### SYSTEM CAS GC STOP / START {#sql-gc-stop-start}
+
+Pause the background scheduler on one disk while investigating a suspect object, so it cannot be
+condemned or deleted mid-investigation, then resume it:
+
+```sql
+SYSTEM CAS GC STOP cas;
+-- investigate, e.g. cas-inspect a specific blob's raw key
+SYSTEM CAS GC START cas;
+```
+
+`STOP` is idempotent and stops-in-place (the same scheduler instance resumes on `START`, keeping its
+`gc_id` and lease-observation history); it works even on a not-live disk. It does not stop a manual
+`SYSTEM CAS GC RUN`. See the [operational surface](/antalya/cas/architecture/garbage-collection#operational-surface)
+table for the full command list.
+
+### SYSTEM CAS FORGET {#sql-forget}
+
+Node-local operator assertion that a disk is permanently gone — the "fire marshal" verb for a stuck
+disk (a transient/`IdentityLost` pool, an operator-asserted decommission):
+
+```sql
+SYSTEM CAS FORGET cas;
+```
+
+It is an assertion, not a proof of erasure: the disk stays registered and answers further store-class
+access with a typed error, and a server restart re-registers the name. This is different from
+[`SYSTEM CAS DROP POOL MEMBER`](/antalya/cas/operations/migration#decommission), which permanently
+retires one pool *member*'s identity across the whole shared pool — `FORGET` only affects this node's
+own local view of one disk.
+
+## Offline tools {#offline-tools}
+
+When the server cannot answer — it is down, or the access needs to be read-only forensic against the
+bucket directly, disaster recovery of `gc/state`, or a raw object decode — `clickhouse-disks` runs
+these against the pool's backend without a live server. All five require the disk to be opened with
+`<readonly>true</readonly>` in the `clickhouse-disks` config; they must never claim a live server's
+mount.
+
+| Command | Use it for |
+|---|---|
+| `cas-fsck [--detail] [--timeout N] [--namespace PREFIX] [--partial]` | The same reachability scan as `SYSTEM CAS FSCK`, offline. `--detail` adds a per-object `<class>\t<key>\t<size>` listing (`reachable`, `dangling`, `unreachable`, `pending-gc`, `awaiting-gc`, `unaccounted`, `stale-edge`, `corrupted-run`, `chain-broken`, `unchecked`, `lifeless-key`, `janitor-pending`) — the only way to get per-object, not just per-pool, findings. `--timeout`/`--partial` bound a scan on a large pool |
+| `cas-gc-dryrun` | Previews the next round's deletes, read-only, no lease. Over-reports away from quiescence (does not fold new owner events) — a diagnostic only, never a delete source |
+| `cas-inspect '<raw-object-key>'` | Decodes one raw object-storage key (as printed by `cas-fsck`/`cas-gc-dryrun`) straight to JSON |
+| `cas-gc-rebuild [--force]` | Disaster recovery: rebuilds a `gc/state` baseline from raw owner state after the GC guard has refused every regular round. `--force` bypasses only the healthy-state refusal, never a competing leader or a failed `CAS`. See [`SYSTEM CAS GC REBUILD`](/sql-reference/statements/system#system-cas-gc-rebuild) for the destructive-tool caveats |
+
+```bash
+clickhouse-disks -C config.xml --disk cas cas-fsck --detail
+clickhouse-disks -C config.xml --disk cas cas-gc-dryrun
+clickhouse-disks -C config.xml --disk cas cas-inspect '<raw-object-key>'
+clickhouse-disks -C config.xml --disk cas cas-gc-rebuild --force
+```
+
+`cas-drop-member` — the offline twin of `SYSTEM CAS DROP POOL MEMBER` — is covered on the
+[migration page](/antalya/cas/operations/migration#decommission) alongside the SQL form, since
+decommissioning a pool member is a migration/scale-down operation, not an incident-time tool.
+
+## The CLICKHOUSE_USER_FILES gotcha when reproducing a test manually {#user-files-gotcha}
+
+Running a `CAS` stateless test directly with `tests/clickhouse-test` against a manually started
+`clickhouse-server` (outside a configured praktika lane) requires exporting `CLICKHOUSE_USER_FILES`
+to match the server's actual data path. The harness's default,
+`/var/lib/clickhouse/user_files`, will not match a custom data path, which makes the pool directory
+invisible to the server — the symptom is an `Unknown disk` error together with a diagnostic that
+reads like an empty pool (e.g. `baseline=0 after_insert=0`) even though the server is otherwise
+healthy.
+
+## What to collect before filing a bug {#filing-a-bug}
+
+- `SYSTEM CAS FSCK '<disk>'` output (or `clickhouse-disks cas-fsck --detail`, if the server cannot
+  answer or a per-object listing is needed) — the authoritative reachability snapshot at the time of
+  the incident.
+- The `system.cas_gc_log` rows for the relevant `round_id`(s): `Start`, every `Phase`, and `Finish`.
+- The `system.cas_log` rows for the specific ref name, blob hash, or object key involved, filtered by
+  `event_time` around the incident.
+- `system.cas_mounts` output from every node sharing the pool, to capture lease/epoch state at
+  incident time — it is a live view and will not reflect a state that has since changed.
+- For a suspected object-store issue, `system.blob_storage_log` rows for the affected `disk_name`
+  with a nonzero `error_code`, and the relevant `CAS*` `ProfileEvents` (`system.query_log`'s
+  `ProfileEvents` map for one query, or `system.metric_log`'s `ProfileEvent_*` columns for a window —
+  see [monitoring](/antalya/cas/operations/monitoring#key-metrics) for which counters matter and the
+  restart-resets-`system.events` caveat).
+- The server version and, if the incident is reproducible, the exact `CREATE TABLE` / `INSERT` /
+  `ALTER` sequence that triggers it.
diff --git a/docs/en/antalya/cas/operations/migration.md b/docs/en/antalya/cas/operations/migration.md
new file mode 100644
index 000000000000..0d4f954c2dae
--- /dev/null
+++ b/docs/en/antalya/cas/operations/migration.md
@@ -0,0 +1,209 @@
+---
+description: 'Adding a content-addressed disk to an existing deployment, moving a partition onto it with ALTER TABLE MOVE PARTITION, rolling back, and permanently decommissioning a pool member.'
+sidebar_label: 'Migration'
+sidebar_position: 1
+slug: /antalya/cas/operations/migration
+title: 'CAS Operations — Migration'
+doc_type: 'guide'
+---
+
+# Operations — migration {#migration}
+
+This page walks through moving `MergeTree` data onto a content-addressed (`CAS`) disk from an
+existing disk, and the reverse. `metadata_type = cas` is opt-in per disk (see the
+[overview](/antalya/cas)), so this is an additive change to a running deployment: the existing
+disk and its data are untouched until a partition is explicitly moved.
+
+## Add a CAS disk alongside an existing one {#add-disk}
+
+A storage policy can carry both an ordinary disk and a `CAS` disk as separate volumes. `ALTER TABLE
+... MOVE PARTITION ... TO DISK` then moves data between them without an `INSERT`/`DROP` cycle. As on
+the [configuration](/antalya/cas/configuration#disk-config) page, the recommended shape layers a
+`type=cache` disk over the `CAS` disk, and the policy's volume references the **cached** disk name:
+
+```xml
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <local_disk>
+                <type>local</type>
+                <path>/var/lib/clickhouse/local_disk/</path>
+            </local_disk>
+            <cas>
+                <type>object_storage</type>
+                <object_storage_type>s3</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <server_root_id>{replica}</server_root_id>
+                <endpoint>https://bucket.s3.amazonaws.com/cas/</endpoint>
+                <access_key_id>...</access_key_id>
+                <secret_access_key>...</secret_access_key>
+            </cas>
+            <cas_cache>
+                <type>cache</type>
+                <disk>cas</disk>
+                <path>/var/lib/clickhouse/cas_cache/</path>
+                <max_size>10Gi</max_size>
+            </cas_cache>
+        </disks>
+        <policies>
+            <tiered>
+                <volumes>
+                    <hot>
+                        <disk>local_disk</disk>
+                    </hot>
+                    <cas_volume>
+                        <disk>cas_cache</disk>
+                    </cas_volume>
+                </volumes>
+            </tiered>
+        </policies>
+    </storage_configuration>
+</clickhouse>
+```
+
+See [configuration](/antalya/cas/configuration) for the full disk-level settings surface and
+[bucket requirements](/antalya/cas/bucket-requirements) for what the target bucket needs to
+support. A table does not need to be created for the first time on `CAS` to use it — an existing
+table just needs its storage policy widened to include a volume backed by a `CAS` disk, which is a
+metadata-only change (`ALTER TABLE ... MODIFY SETTING storage_policy = ...`, subject to the usual
+constraint that the new policy must still contain every volume and disk of the old one — a storage
+policy can only grow, never lose a disk it once had).
+
+## Move a partition onto CAS {#move-partition}
+
+`ALTER TABLE ... MOVE PARTITION ... TO DISK` moves every part of one partition to the named disk in
+place — the ordinary `MergeTree` partition-move mechanism, unchanged by `CAS`:
+
+```sql
+CREATE TABLE events (event_date Date, event_id UInt64, payload String)
+ENGINE = MergeTree ORDER BY event_id PARTITION BY event_date
+SETTINGS storage_policy = 'tiered';
+
+INSERT INTO events VALUES ('2026-08-04', 1, 'hello'), ('2026-08-04', 2, 'world');
+
+SELECT name, partition, disk_name FROM system.parts WHERE table = 'events' AND active;
+```
+
+```text
+Row 1:
+──────
+name:      20260804_1_1_0
+partition: 2026-08-04
+disk_name: local_disk
+```
+
+The partition starts on `local_disk`, the first volume in the policy. Moving it onto `CAS` uploads
+each part's files as content-addressed blobs, writes a part manifest, and publishes a ref — the same
+write path an `INSERT` directly onto `CAS` takes (see
+[what just happened](/antalya/cas/quick-start#what-happened) in the quick start). `TO DISK` names the
+disk actually listed in the policy's volume — with a cache layered in front, that is the **cache**
+disk's name (`cas_cache`), not the raw `CAS` disk's name (`cas`) underneath it; naming the raw disk
+is refused, because it is not a member of the table's storage policy:
+
+```sql
+ALTER TABLE events MOVE PARTITION '2026-08-04' TO DISK 'cas_cache';
+
+SELECT name, partition, disk_name FROM system.parts WHERE table = 'events' AND active;
+```
+
+```text
+Row 1:
+──────
+name:      20260804_1_1_0
+partition: 2026-08-04
+disk_name: cas_cache
+```
+
+```sql
+SELECT * FROM events ORDER BY event_id;
+```
+
+```text
+2026-08-04	1	hello
+2026-08-04	2	world
+```
+
+`system.parts.disk_name` reports the cache disk's name, not the underlying `CAS` disk's — this is
+the ordinary `type=cache` disk behavior (the same happens layering a cache over any other disk type)
+and is not `CAS`-specific. `system.filesystem_cache` shows the part's files populated into the
+`cas_cache` cache on this read-through.
+
+## Roll back {#rollback}
+
+The move is symmetric: `MOVE PARTITION ... TO DISK` back onto the original disk name returns the
+partition to its previous location, with the data intact throughout:
+
+```sql
+ALTER TABLE events MOVE PARTITION '2026-08-04' TO DISK 'local_disk';
+
+SELECT name, partition, disk_name FROM system.parts WHERE table = 'events' AND active;
+```
+
+```text
+Row 1:
+──────
+name:      20260804_1_1_0
+partition: 2026-08-04
+disk_name: local_disk
+```
+
+Moving a partition off `CAS` does not itself delete the blobs it stops referencing — dropping the
+old ref makes them eligible for reclamation by the next
+[GC round](/antalya/cas/architecture/garbage-collection), the same as dropping a part.
+
+This exact three-disk, cache-over-`CAS` configuration and the forward/rollback `ALTER TABLE ... MOVE
+PARTITION` sequence above were run against a live server before publication, using the `local`
+object-storage backend for the `cas` disk: `CREATE TABLE`, `INSERT`, both `MOVE PARTITION`
+directions, the `system.parts` checks, the `system.filesystem_cache` check, and the `SELECT` all
+completed with zero errors and the shown output. A prior attempt to move onto `TO DISK 'cas'`
+directly (the raw disk, not the cache) was refused with `All parts of partition '20260804' are
+already on disk 'cas_cache'. (UNKNOWN_DISK)` — a real error message from the run, kept here because
+it is exactly what an operator sees after guessing the wrong disk name.
+
+## Permanently removing a pool member {#decommission}
+
+A `CAS` pool can be shared by several servers (see [`server_root_id`](/antalya/cas/architecture/mounts-and-leases#server-root-id)).
+Scaling down — permanently removing a server that will never rejoin the pool — is a distinct,
+irreversible operation from an ordinary restart or a temporary outage: it fences the member's
+`server_root_id` and reclaims the storage attributable only to it.
+
+`SYSTEM CAS DROP POOL MEMBER` claims the victim's mount slot as an administrative writer (refusing
+immediately if the member is still alive), drops every table namespace the member owned, sweeps
+manifest debris, drains its staging and mountpoint objects, and — only once every drain is
+confirmed — retires the mount slot itself. It emits ordinary ref-edge deltas rather than a GC
+transition: it does not synchronously reclaim shared blob content, it only makes the now-unreferenced
+blobs eligible for an ordinary GC round to reclaim later.
+
+```sql
+SYSTEM CAS DROP POOL MEMBER 'server_root_id' FROM DISK 'disk_name' [ON CLUSTER cluster_name]
+```
+
+Both `server_root_id` and `disk_name` are required string literals. The offline CLI twin,
+`clickhouse-disks cas-drop-member <server_root_id>`, does the same work against a disk opened
+read-only — the pool-admin claim happens internally, so the disk it runs against must not be the
+live server's own mount:
+
+```bash
+clickhouse-disks -C config.xml --disk cas cas-drop-member 'replica-2'
+```
+
+The command returns one row (or, offline, one line per field) with `namespaces_removed`,
+`namespaces_already_removed`, `committed_refs_removed`, `precommits_removed`,
+`manifest_debris_removed`, `staging_objects_removed`, `mountpoint_objects_removed`, and
+`slot_removed`. It is resumable: a rerun skips namespaces already marked removed and reports them
+under `namespaces_already_removed` rather than redoing the work. A per-object drain failure is
+recorded as a `warning` rather than raised as an exception, leaving the slot terminated but not
+fully drained so a later invocation can resume; a non-empty `warnings` means exactly that, and the
+mount slot stays in place as a resume anchor rather than being fully retired.
+
+**Preconditions.** Confirm the member is actually and permanently dead before running this: the
+operation fences that `server_root_id` out even if the server comes back online, and it deletes
+namespace and drain state that cannot be recovered. Check `system.cas_mounts` for the member's
+`state` and `last_success_age_seconds` first — a `live` row, or one with a recent lease renewal,
+means the member is not a decommission candidate yet.
+
+**Verification.** After the command reports `slot_removed = true` with no warnings, the member's
+`server_root_id` no longer appears as a row in `system.cas_mounts` on any peer, and a subsequent
+`SYSTEM CAS GC RUN` on the pool will no longer wait on or fence its heartbeat. See
+[mount, unmount, crash](/antalya/cas/architecture/mounts-and-leases#mount-lifecycle) for how the
+claim, drain, and retirement steps fit into the mount-slot lifecycle.
diff --git a/docs/en/antalya/cas/operations/monitoring.md b/docs/en/antalya/cas/operations/monitoring.md
new file mode 100644
index 000000000000..6ab8b3dd9bb0
--- /dev/null
+++ b/docs/en/antalya/cas/operations/monitoring.md
@@ -0,0 +1,101 @@
+---
+description: 'The three content-addressed system tables, a key-metrics table with healthy ranges, and queries for reading GC health from cas_gc_log.'
+sidebar_label: 'Monitoring'
+sidebar_position: 2
+slug: /antalya/cas/operations/monitoring
+title: 'CAS Operations — Monitoring'
+doc_type: 'guide'
+---
+
+# Operations — monitoring {#monitoring}
+
+Content-addressed (`CAS`) storage exposes three system tables and a family of `CAS`-prefixed
+`ProfileEvents`. This page is the entry point for day-to-day health checks; see
+[debugging](/antalya/cas/operations/debugging) for incident-time tooling and
+[troubleshooting](/antalya/cas/operations/troubleshooting) for symptom-driven diagnosis.
+
+## The three system tables {#system-tables}
+
+| Table | Grain | Use it for |
+|---|---|---|
+| [`system.cas_mounts`](/operations/system-tables/cas_mounts) | One row per mount slot in the pool, read live from the backend on every query | Who is in the pool right now, lease/epoch state, which node holds GC leadership |
+| [`system.cas_gc_log`](/operations/system-tables/cas_gc_log) | One `Start`/`Finish` row per GC round, plus one `Phase` row per phase reached | GC round outcomes, duration, and where a round's `LIST`/`GET`/`PUT`/`DELETE` budget went |
+| [`system.cas_log`](/operations/system-tables/cas_log) | One row per writer/GC decision (blob puts, dedup adoptions, retire decisions, dangling-access findings) | Fine-grained forensics for one part, one blob hash, or one round |
+
+`system.cas_mounts` is the only one of the three with no persisted backing log — it is a live view,
+so a transient backend error on one disk is skipped rather than blinding the whole query. The other
+two are ordinary `system.*_log` tables and follow the usual flush/retention settings.
+
+## Key metrics {#key-metrics}
+
+Every `CAS`-related `ProfileEvent` carries the uppercase `CAS`/`CASGC` prefix. This is a curated
+subset for a first health pass; the full list groups by object class (`CASBlob*`, `CASManifest*`,
+`CASRoot*`, `CASGC*`, `CASServer*`, `CASOther*`, `CASRef*`, `CASMeta*`) and is enumerated in
+`src/Common/ProfileEvents.cpp`.
+
+| Metric | Healthy range | A spike or nonzero means |
+|---|---|---|
+| `CASBlobCompareSwapConflict` | Near zero relative to `CASBlobCompareSwap` | Concurrent-update contention on blob metadata |
+| `CASBlobHeadFirst` vs `CASBlobBodyPutAvoided` | `CASBlobBodyPutAvoided` tracks `CASBlobHeadFirst` closely | A widening gap means the dedup `HEAD`-before-`PUT` gate is firing but not finding matches — expected for genuinely new content, worth checking if it dominates |
+| `CASRefAppendWedged` | Zero | A ref-log append lane exhausted its retries after an uncertain `PUT`; ref-log progress on that namespace may be stalled |
+| `CASRefNeedsRecovery` | Zero | A ref-append lane could not install a known-durable transaction and now refuses writes, snapshots, and confirmation until durable replay completes |
+| `CASRefAppendSealRejected` | Occasional (a deposed writer losing a race is the protocol working); sustained growth is not | A writer keeps retrying after losing its mount and does not yet know it |
+| `CASGCHeartbeatFenceOuts` | Zero on a healthy pool | GC fenced an expired mount; check `system.cas_mounts` for a member that should have cleanly unmounted |
+| `CASGCUnmatchedRemoveDeltas` | Occasional (benign per-key no-op by design) | A persistent nonzero rate means removal deltas are reaching the reducer without their matching activation — a correctness signal worth a look, not an automatic false deletion |
+| `CASGCCondemnMarkerUnconfirmedCarry` | Zero | A durable condemn marker could not be confirmed; deletion is safely postponed but investigate marker write/read failures |
+| `CASGCMetaWriteAnomaly` | Zero | The bounded GC metadata pool failed an operation; backend or pool pressure may delay metadata convergence |
+| `CASRefRollbackBestEffortDropFailed` | Zero | A rollback cleanup drop hit a backend failure; refs may remain live and GC may be delayed on that namespace |
+
+Two counter-reading caveats that apply to `system.events`-backed metrics generally, not only `CAS`
+ones: a counter that has never incremented can be **absent** from `system.events` rather than
+present at zero — query with `system_events_show_zero_values = 1` to tell "never happened" from "not
+shown". A server restart resets `system.events` to zero, so a cumulative `CAS` total across a
+restart has to be computed from summed per-second deltas in `system.metric_log`, not read directly
+off `system.events`.
+
+## Reading GC health from cas_gc_log {#gc-health}
+
+Round outcomes over the last day, per disk:
+
+```sql
+SELECT disk_name, outcome, count() AS rounds, avg(duration_ms) AS avg_ms
+FROM system.cas_gc_log
+WHERE event_type = 'Finish' AND event_time > now() - INTERVAL 1 DAY
+GROUP BY disk_name, outcome
+ORDER BY disk_name, rounds DESC;
+```
+
+A steady stream of `Success` and `Deferred` rows is healthy; `Deferred` means the round found no
+changed shard needing a fold and no graduation was due — a cheap round, not a stuck one (see
+[the round](/antalya/cas/architecture/garbage-collection#the-round)). Recurring `Error` rows, or
+`NotALeader` outcomes for the disk's own scheduler, warrant investigation. `anomalies` in the
+`Finish` row is worth a steady watch: it is fold clamps surfaced and survived, so a non-zero value
+that persists across rounds is more interesting than an isolated one.
+
+Which phase dominates round duration or the `LIST` budget — reproduced from the
+[per-phase rows](/operations/system-tables/cas_gc_log#per-phase-rows) reference:
+
+```sql
+SELECT phase,
+       count() AS rounds,
+       quantile(0.99)(phase_duration_microseconds) AS p99_microseconds,
+       sum(ProfileEvents['S3ListObjects']) AS lists
+FROM system.cas_gc_log
+WHERE event_type = 'Phase' AND disk_name = 'cas'
+GROUP BY phase
+ORDER BY p99_microseconds DESC;
+```
+
+Pending-reclaim backlog and time since a disk's GC last led, from the live mount view:
+
+```sql
+SELECT disk, server_root_id, is_leader, pending_reclaim, last_success_age_seconds, wedged_namespace_count
+FROM system.cas_mounts
+WHERE is_leader IS NOT NULL
+ORDER BY disk, server_root_id;
+```
+
+`is_leader`, `pending_reclaim`, `last_success_age_seconds`, and `wedged_namespace_count` are
+process-local — `NULL` on every row describing a peer's mount — so this query is only informative
+run against the node whose GC leadership you are checking; run it on each node to see the whole
+pool's view of itself.
diff --git a/docs/en/antalya/cas/operations/troubleshooting.md b/docs/en/antalya/cas/operations/troubleshooting.md
new file mode 100644
index 000000000000..94c29a8d077c
--- /dev/null
+++ b/docs/en/antalya/cas/operations/troubleshooting.md
@@ -0,0 +1,29 @@
+---
+description: 'Symptom-to-action table for common content-addressed storage incidents: mount lease loss, stalled GC, startup failures, fsck timeouts, and read-only pools.'
+sidebar_label: 'Troubleshooting'
+sidebar_position: 3
+slug: /antalya/cas/operations/troubleshooting
+title: 'CAS Operations — Troubleshooting'
+doc_type: 'guide'
+---
+
+# Operations — troubleshooting {#troubleshooting}
+
+Start from the symptom, not the mechanism. Each row below names a concrete diagnostic query or
+command and the action it points to; see [monitoring](/antalya/cas/operations/monitoring) for the
+system tables referenced and [debugging](/antalya/cas/operations/debugging) for the underlying
+tools.
+
+| Symptom | Diagnosis | Action |
+|---|---|---|
+| A server keeps losing its mount lease and self-remounting | Check `system.cas_mounts` for the server's own row's `state`/`expires_at`; losing the lease is neither read-only mode nor an abort, it trips a local fence and retries with backoff from 1s to 30s | Look for clock issues (the fence clock is `CLOCK_BOOTTIME`) or network latency exceeding `mount_lease_ttl_ms` (default 30s); see [the mount lease](/antalya/cas/architecture/mounts-and-leases#mount-lease) |
+| Writes slow down or stall under load, with no exception reaching the client | S3 `SlowDown`/`ServiceUnavailable`/`RequestTimeout`/`InternalError` (5xx) responses are not on `CasRequestController`'s definite-failure whitelist (only malformed-request, entity-too-large, and access-denied are), so they classify as `Unresolved` and are retried automatically. Confirm with `sum(ProfileEvents['CASConditionalWriteUnresolved'])` rising alongside `sum(ProfileEvents['CASConditionalWriteAttempts'])` over `system.query_log` for the affected window (or `ProfileEvent_CASConditionalWriteUnresolved` in `system.metric_log` for a cumulative view across queries), and check `system.blob_storage_log` for `disk_name = '<cas>'` rows with a nonzero `error_code` around the same window | Nothing to configure per-request: the controller retries the same `(key, bytes)` with capped-exponential backoff (200ms initial, capped at 5s) for up to 16 attempts inside a 90-second operation deadline, and the mount-lease renewer keeps extending the fence across the disruption — this is the "blips, throttling, partial outages" case the write path is built to survive. Confirm the mount lease itself is still renewing (`system.cas_mounts.expires_at` moving forward, `last_success_age_seconds` not climbing) — if it is, this is expected and self-resolving. If `SlowDown` responses are sustained rather than transient, check the bucket's request-rate limits against the pool's actual PUT/GET rate (see [bucket requirements](/antalya/cas/bucket-requirements)) and consider lowering `cas_blob_upload_pool_size` to reduce concurrent upload traffic; a write only surfaces a client-visible `NETWORK_ERROR` if the 90-second deadline is exhausted before the store recovers, and that error is retried by the ordinary merge/insert backoff, not silently dropped |
+| `GC` never seems to reclaim space after tables are dropped | `SELECT * FROM system.cas_gc_log WHERE event_type='Finish' ORDER BY event_time DESC LIMIT 5` — check `outcome`; also `SELECT is_leader FROM system.cas_mounts` on this node | If `outcome != 'Success'`/`'Deferred'`, see [reading GC health](/antalya/cas/operations/monitoring#gc-health); if this node is not the leader (`is_leader = 0`), it never reclaims for this disk — check the peer holding leadership. Reclamation also needs at least two full rounds past condemnation by design (the grace period is rounds, not acks) — a single manual `SYSTEM CAS GC RUN` will not finish it |
+| A dangling-access exception or `CORRUPTED_DATA` on read | Run `clickhouse-disks cas-fsck --detail` and check `dangling` specifically — it is the one class that means data loss, distinct from `unreachable`/`awaiting-gc`, which are just waiting for graduation | A nonzero `dangling` count is a real incident: collect the `--detail` output (see [what to collect before filing a bug](/antalya/cas/operations/debugging#filing-a-bug)) before taking any destructive action |
+| `SYSTEM CAS FSCK` or `clickhouse-disks cas-fsck` times out on a large pool | The scan is bounded by `--timeout` (default 600s / the `SYSTEM` form has no override); a large `roots/` prefix can make the scan slow | Retry with `--partial` to see the counts accumulated so far instead of aborting empty-handed, or `--namespace <prefix>` to scope the scan to a subset of namespaces |
+| `SYSTEM CAS DROP POOL MEMBER` returns a non-empty `warnings` column | A per-object drain step could not confirm emptiness; the mount slot is left terminated but not fully drained, as a resume anchor | Rerun the same command — it is resumable and skips namespaces already marked removed, reporting them under `namespaces_already_removed` |
+| Writes or `ALTER`s on a `CAS` disk fail with a `READONLY`-class error | The disk's metadata storage rejects every mutating entry point; this is deliberate for a disk opened with `<readonly>true</readonly>`, used by every offline `clickhouse-disks` tool | Confirm whether the disk was intentionally configured read-only (offline inspection, `cas-fsck`, `cas-gc-dryrun`, `cas-gc-rebuild`, `cas-drop-member` all require it); a production disk serving writes must not carry `<readonly>true</readonly>` |
+| A table stays unavailable after a transient network error during startup | `AsyncLoader` has no retry/requeue path for a failed table load job: a transient S3 `NETWORK_ERROR` during `CAS` ref-table startup recovery can leave the job permanently `FAILED` | Restart the server, or issue a fresh load for the table; this is a one-shot job design, not a `CAS`-specific bug |
+| A `CAS` server process aborts after its pool directory was removed or renamed while mounted | The lease-renewal thread observes a confirmed mismatch and raises `LOGICAL_ERROR`; in debug/sanitizer builds constructing that exception aborts the process | Never remove or rename a mounted pool's storage path while a server has it live; to retire a member permanently use [`SYSTEM CAS DROP POOL MEMBER`](/antalya/cas/operations/migration#decommission) instead of raw filesystem operations |
+| Stale-looking part metadata after an out-of-band change to the pool | The part-folder view cache may be serving a retained (not re-validated) view | Set the disk-level `part_folder_cache_bytes = 0` as a diagnostic kill switch to disable retention, and run `fsck`/integrity checks with `part_folder_validate = always` so every read re-proves the body |
+| A wide merge (many thousands of columns) fails with a port-exhaustion error from the network layer | Each column in a wide part can cost a separate object-store operation in one merge, and a very wide part can issue on the order of the column count in requests, exhausting local ephemeral TCP ports under load | Reduce concurrent merge parallelism on that table, or increase the host's ephemeral port range; this is a general high-fan-out-merge limit, not specific to content addressing |
diff --git a/docs/en/antalya/cas/quick-start.md b/docs/en/antalya/cas/quick-start.md
new file mode 100644
index 000000000000..b40e7d547121
--- /dev/null
+++ b/docs/en/antalya/cas/quick-start.md
@@ -0,0 +1,145 @@
+---
+description: 'A minimal content-addressed storage disk config and the first CREATE TABLE, INSERT, and SELECT against it, executed live before publication.'
+sidebar_label: 'Quick start'
+sidebar_position: 2
+slug: /antalya/cas/quick-start
+title: 'CAS Quick Start'
+doc_type: 'guide'
+---
+
+# Quick start {#quick-start}
+
+## The disk config {#disk-config}
+
+A `CAS` disk is an `object_storage` disk with `metadata_type` set to `cas` and an explicit,
+per-server `server_root_id`. This example uses the `local` object-storage backend so it needs
+nothing beyond a `ClickHouse` binary — no bucket, no credentials:
+
+```xml
+<clickhouse>
+    <storage_configuration>
+        <disks>
+            <cas>
+                <type>object_storage</type>
+                <object_storage_type>local</object_storage_type>
+                <metadata_type>cas</metadata_type>
+                <server_root_id>quickstart-demo</server_root_id>
+                <path>cas_pool/</path>
+            </cas>
+            <cas_cache>
+                <type>cache</type>
+                <disk>cas</disk>
+                <path>cas_cache/</path>
+                <max_size>10Gi</max_size>
+            </cas_cache>
+        </disks>
+        <policies>
+            <cas>
+                <volumes>
+                    <main>
+                        <disk>cas_cache</disk>
+                    </main>
+                </volumes>
+            </cas>
+        </policies>
+    </storage_configuration>
+</clickhouse>
+```
+
+The `cas_cache` disk layers a local filesystem cache over `cas`: it absorbs repeated reads of the
+same blob while `cas` stays the source of truth, and the policy's volume points at the cached disk
+— see [configuration](/antalya/cas/configuration#disk-config) for the sizing note.
+
+`server_root_id` must be unique per server sharing a pool. On a single, non-replicated server a
+literal string, as above, is enough; on a replicated cluster where every replica shares one config,
+`<server_root_id>{replica}</server_root_id>` expands through the same macro substitution an `s3`
+disk's `endpoint` already uses, giving each replica a distinct subtree from one template.
+
+**S3 endpoint variant.** Swap `object_storage_type` to `s3` and add the usual object-storage
+connection keys; nothing else in this config changes:
+
+```xml
+<cas>
+    <type>object_storage</type>
+    <object_storage_type>s3</object_storage_type>
+    <metadata_type>cas</metadata_type>
+    <server_root_id>quickstart-demo</server_root_id>
+    <endpoint>https://bucket.s3.amazonaws.com/cas/</endpoint>
+    <access_key_id>...</access_key_id>
+    <secret_access_key>...</secret_access_key>
+</cas>
+```
+
+`cas_cache` is unaffected by this swap — it wraps `disk cas` regardless of which object-storage
+backend `cas` itself uses. See [bucket requirements](/antalya/cas/bucket-requirements) for what the
+target bucket needs to support, and [configuration](/antalya/cas/configuration) for the full
+settings surface.
+
+## First table {#first-table}
+
+```sql
+CREATE TABLE events (event_date Date, event_id UInt64, payload String)
+ENGINE = MergeTree ORDER BY event_id
+SETTINGS storage_policy = 'cas';
+
+INSERT INTO events VALUES ('2026-08-04', 1, 'hello'), ('2026-08-04', 2, 'world');
+
+SELECT * FROM events ORDER BY event_id;
+```
+
+```text
+   ┌─event_date─┬─event_id─┬─payload─┐
+1. │ 2026-08-04 │        1 │ hello   │
+2. │ 2026-08-04 │        2 │ world   │
+   └────────────┴──────────┴─────────┘
+```
+
+An ordinary `MergeTree` table on a `CAS` disk. `INSERT`, `SELECT`, merges, and mutations all work
+exactly as on any other `MergeTree` — the content-addressing is invisible at the SQL surface.
+
+## Checking the mount {#checking-the-mount}
+
+```sql
+SELECT disk, server_root_id, state, is_leader FROM system.cas_mounts;
+```
+
+```text
+Row 1:
+──────
+disk:           cas
+server_root_id: quickstart-demo
+state:          live
+is_leader:      0
+
+Row 2:
+──────
+disk:           cas_cache
+server_root_id: quickstart-demo
+state:          live
+is_leader:      0
+```
+
+`system.cas_mounts` shows every server currently sharing this pool, not just the local one. With a
+cache layered in front, the same mount shows up **twice** — once under each configured disk name
+(`cas` and `cas_cache`), both reporting the one underlying `server_root_id` — because the table
+lists a row per configured disk, not per mount; this is the one visible change the cache layer adds
+to this page's output. `is_leader` is `0` on both rows because `GC` leader election is asynchronous
+and had not yet run at query time on this freshly mounted disk — see
+[mounts and leases](/antalya/cas/architecture/mounts-and-leases) for the full column reference and
+[garbage collection](/antalya/cas/architecture/garbage-collection) for leadership.
+
+## What just happened {#what-happened}
+
+The `INSERT` wrote two part files as content-addressed blobs, a part manifest listing them, and a
+ref pointing the part name at that manifest — the only mutable object the write touched. On a
+second replica sharing this same pool, inserting or fetching the identical content publishes a ref
+without re-uploading a single byte; see
+[garbage collection](/antalya/cas/architecture/garbage-collection) for how a dropped part's blobs
+get reclaimed once nothing references them anymore.
+
+This exact cache-layered configuration and SQL were run against a live server before publication:
+`CREATE TABLE`, `INSERT`, `SELECT`, and the `system.cas_mounts` query above all completed with zero
+errors, with the two-row `system.cas_mounts` output shown above captured from that run. The
+`INSERT`/`SELECT` output is unaffected by the cache — the one visible difference the cache layer
+adds anywhere on this page is that second `system.cas_mounts` row.
+
diff --git a/docs/en/antalya/cas/roadmap.md b/docs/en/antalya/cas/roadmap.md
new file mode 100644
index 000000000000..0c8b14110d89
--- /dev/null
+++ b/docs/en/antalya/cas/roadmap.md
@@ -0,0 +1,108 @@
+---
+description: 'What CAS ships today, what is still planned, known platform limitations, and design directions deliberately not taken.'
+sidebar_label: 'Roadmap'
+sidebar_position: 5
+slug: /antalya/cas/roadmap
+title: 'CAS Roadmap'
+doc_type: 'guide'
+---
+
+# CAS roadmap {#cas-roadmap}
+
+CAS is experimental (see [status](/antalya/cas/)): the format and SQL surface can still change.
+This page tracks what already works, what is still ahead, and — since a project this deep in
+adversarial verification collects real dead ends — what was tried and deliberately not shipped.
+
+## Shipped {#shipped}
+
+**Storage and object model.** Content-addressed blobs deduplicated across every replica sharing
+a pool; immutable part manifests; a pluggable blob-hash algorithm (`cityhash128` default,
+`xxh3-128`, or `sha256`) fixed per pool at creation; a JSON-text object format end to end (no
+binary framing, no protobuf) so any object can be read with ordinary line-oriented tools.
+
+**Write path.** Conditional writes (create-if-absent, compare-and-swap) as the only mutual
+exclusion primitive the pool needs; an adaptive HEAD-before-PUT dedup gate; a bounded thread pool
+fanning out multi-blob part uploads in parallel; carry-forward on mutation for `Wide` parts (an
+untouched column is re-referenced, not re-hashed).
+
+**Read path.** Ref resolution to manifest to ranged blob reads, with a manifest-decode cache and
+a part-folder view cache sitting on that path.
+
+**Replication.** Fetch by relink between replicas sharing a pool — a replicated fetch publishes a
+ref pointing at blobs the pool already has, at zero bytes on the wire — with a publish-then-confirm
+protocol that closes the sender-crash and stale-cache races a naive relink would be exposed to.
+
+**Garbage collection.** An 18-phase round built on a causal ack-floor (no separate fence-and-recheck
+phase); sharded folding (`gc_shards`); condemn/spare bookkeeping; generation pruning with a
+configurable retention window; a dry-run mode and a rebuild path for recovery.
+
+**Mounts and identity.** Explicit `server_root_id` per disk; a renewable mount lease with
+observation-based reclaim of an expired predecessor (never trusting a foreign body's wall-clock
+timestamp); clean decommission of a permanently departed pool member
+(`SYSTEM CAS DROP POOL MEMBER`).
+
+**Backends.** AWS S3 (`ETag`-based conditional dialect) and Google Cloud Storage (generation-token
+dialect) both live-validated; a capability probe that runs at every writable mount and refuses to
+proceed on a backend that does not enforce the conditions CAS depends on.
+
+**Operability.** `system.cas_log`, `system.cas_gc_log`, and `system.cas_mounts` for introspection;
+`clickhouse-disks` commands `ca-fsck`, `ca-inspect`, `ca-gc-dryrun`, and `ca-gc-rebuild`; the
+`SYSTEM CAS` SQL control surface (`GC RUN`/`STOP`/`START`/`REBUILD`, `FSCK`, `FORGET`, `DROP POOL
+MEMBER`).
+
+**Coexistence.** `metadata_type = cas` is opt-in per disk; zero-copy replication keeps working
+unmodified on disks that do not opt in — see [why CAS exists](/antalya/cas/) for the fuller
+positioning.
+
+## In progress / planned {#in-progress}
+
+- **Azure real-store validation.** AWS and GCS are live-validated; Azure is not — see
+  [known limitations](#known-limitations) below.
+- **WORM deployments.** A read-only disk mode exists today; a fuller write-once story — a pool
+  served immutably, with pinned snapshots for read-only replicas — has a draft design and is not
+  yet implemented.
+- **Backup and restore.** See [Backups](#backups) below — this is further along as a design than as
+  an implementation.
+- **First-class local-disk pools.** Today a pool over local paths runs a minimal best-effort
+  emulation of the conditional-write dialect (single-process, serialized resurrections). Making the
+  local mode efficient in its own right is under consideration: a local CAS tier is a natural target
+  for backups, pinned snapshots, and moving data between CAS tiers.
+
+## Known limitations {#known-limitations}
+
+- **Azure Blob Storage's REST API documents the equivalent conditional headers CAS needs, but no
+  CAS conditional-write dialect is wired up for it yet** — untested, not validated by the capability
+  probe. See [bucket requirements](/antalya/cas/bucket-requirements) and
+  [the backend page](/antalya/cas/architecture/backend) for the AWS/GCS dialects that are wired.
+- **Other S3-compatible object stores qualify only if they pass the capability probe** — a store
+  that silently ignores conditional writes is refused at mount time rather than trusted. Bucket
+  versioning must be off; it is not required to be on.
+- **The format and settings surface can still change.** CAS is pre-release: there is no persisted
+  production data to keep compatible, so a format change costs a version bump, not a migration.
+  Treat every detail on these pages as subject to change until the format is declared stable.
+
+## Backups {#backups}
+
+A `snapshot` / `mirror` / `fetch` / `restore` design is **approved but not implemented**. The
+model is deliberately git-shaped: `snapshot` is instant and free (like `git tag` — it references
+existing manifests, copies nothing); `mirror` is a continuous pull from a production pool into a
+backup pool (like `git push --mirror`); `fetch` is a selective pull from a backup pool into a
+fresh pool (a partial clone); `restore` is an in-pool relink (like `git checkout`, instant). One
+closure-walk-and-hash-verification primitive is meant to serve all three pool-to-pool movements.
+None of this is wired into the `BACKUP`/`RESTORE` SQL surface yet.
+
+## Deliberately rejected directions {#rejected}
+
+A short pointer list; the reasons and the counterexamples that drove each decision are in
+[design history](/antalya/cas/architecture/design-history).
+
+- A Merkle tree layer as a distinct object kind.
+- Epoch-based reclamation as the GC core.
+- An integer in-degree refcount instead of a folded edge set.
+- A persistent, append-only namespace registry for GC discovery.
+- Per-incarnation body keys as an alternative to an in-body incarnation tag.
+- Using a blob's freshness metadata as the authority for its lifecycle instead of an advisory hint.
+- A separate all-shard fence-and-recheck phase per GC round.
+- A sparse ref-id allocator with a certificate stack bolted on to prove completeness.
+- Extending zero-copy replication instead of building a new mechanism — CAS is an alternative to
+  zero-copy, not a replacement; both remain available.

From 056488b47a0b3f32ea2ebf83d508b697ff4c0c17 Mon Sep 17 00:00:00 2001
From: Mikhail Filimonov <mfilimonov@altinity.com>
Date: Wed, 5 Aug 2026 00:07:41 +0200
Subject: [PATCH 30/30] CAS documentation

User-facing documentation: content-addressed storage in storing-data, the
SYSTEM statements, and the three system-table pages.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Mikhail Filimonov <mfilimonov@altinity.com>
---
 docs/en/operations/storing-data.md            | 102 +++++++++++-
 .../en/operations/system-tables/cas_gc_log.md | 156 ++++++++++++++++++
 docs/en/operations/system-tables/cas_log.md   |  62 +++++++
 .../en/operations/system-tables/cas_mounts.md |  81 +++++++++
 docs/en/sql-reference/statements/system.md    | 145 ++++++++++++++++
 5 files changed, 545 insertions(+), 1 deletion(-)
 create mode 100644 docs/en/operations/system-tables/cas_gc_log.md
 create mode 100644 docs/en/operations/system-tables/cas_log.md
 create mode 100644 docs/en/operations/system-tables/cas_mounts.md

diff --git a/docs/en/operations/storing-data.md b/docs/en/operations/storing-data.md
index 8a2903cd1ebb..c26316f5cdff 100644
--- a/docs/en/operations/storing-data.md
+++ b/docs/en/operations/storing-data.md
@@ -46,7 +46,7 @@ It requires specifying:
 
 <br/>
 
-Optionally, `metadata_type` can be specified (it is equal to `local` by default), but it can also be set to `plain`, `web` and, starting from `24.4`, `plain_rewritable`.
+Optionally, `metadata_type` can be specified (it is equal to `local` by default), but it can also be set to `plain`, `web`, `plain_rewritable` (starting from `24.4`) and `cas`.
 Usage of `plain` metadata type is described in [plain storage section](/operations/storing-data#plain-storage), `web` metadata type can be used only with `web` object storage type, `local` metadata type stores metadata files locally (each metadata files contains mapping to files in object storage and some additional meta information about them).
 
 For example:
@@ -452,6 +452,106 @@ is equal to
 Starting from `24.5` it is possible to configure any object storage disk 
 (`s3`, `azure`, `local`) using the `plain_rewritable` metadata type.
 
+### Using Content-Addressed Storage {#content-addressed-storage}
+
+Setting `metadata_type` to `cas` turns a disk into a content-addressed (CAS) disk: every
+object is addressed by the hash of its content rather than by a randomly generated blob name, so
+identical content written by different parts (or different tables) is stored once and shared. A
+background garbage collector reclaims objects once no part references them anymore; see
+[`SYSTEM CAS GC RUN`](/sql-reference/statements/system#system-cas-gc-run),
+[`SYSTEM CAS GC REBUILD`](/sql-reference/statements/system#system-cas-gc-rebuild),
+[`SYSTEM CAS DROP POOL MEMBER`](/sql-reference/statements/system#system-cas-drop-pool-member),
+and the [`system.cas_gc_log`](/operations/system-tables/cas_gc_log),
+[`system.cas_mounts`](/operations/system-tables/cas_mounts), and
+[`system.cas_log`](/operations/system-tables/cas_log) system tables. See the
+[content-addressed storage documentation](/antalya/cas) for the architecture, operations
+runbooks, and a live-validated quick start.
+
+Configuration:
+
+```xml
+<s3_cas>
+    <type>object_storage</type>
+    <object_storage_type>s3</object_storage_type>
+    <metadata_type>cas</metadata_type>
+    <endpoint>https://s3.eu-west-1.amazonaws.com/clickhouse-eu-west-1.clickhouse.com/data/</endpoint>
+    <use_environment_credentials>1</use_environment_credentials>
+
+    <server_root_id>server-{replica}</server_root_id>
+    <scratch_path>disks/s3_cas/cas_scratch/</scratch_path>
+    <staging_backend>local</staging_backend>
+    <blob_hash>cityhash128</blob_hash>
+    <gc_enabled>true</gc_enabled>
+    <gc_interval_sec>60</gc_interval_sec>
+    <gc_shards>1</gc_shards>
+    <deduplication_cache_bytes>67108864</deduplication_cache_bytes>
+    <part_folder_cache_bytes>67108864</part_folder_cache_bytes>
+    <part_folder_validate>always</part_folder_validate>
+</s3_cas>
+```
+
+The CAS-specific settings are written directly inside the disk element, alongside
+`<type>object_storage</type>` / `<object_storage_type>` and the connection settings, which are the
+same as for any other `object_storage` disk. Since the disk element already scopes every key to this
+disk, none of the keys below carry a redundant `cas_`/`ca_` prefix.
+
+#### Required parameters {#required-parameters-content-addressed}
+
+- `server_root_id` — the subtree of the shared pool that this server owns. When several replicas
+  mount the same pool (same `endpoint`), each one must own a distinct subtree, so this is normally
+  written with a macro, e.g. `<server_root_id>server-{replica}</server_root_id>`. Missing this key is
+  a startup error.
+
+#### Optional parameters {#optional-parameters-content-addressed}
+
+These are the commonly used settings; see [Configuration](/antalya/cas/configuration) for the full
+disk-level and server-level settings surface.
+
+- `scratch_path` — a real, server-local filesystem directory used to spill the write buffer before it
+  is committed to the pool (never the object-storage key prefix). Defaults to
+  `<clickhouse-path>/disks/<disk_name>/cas_scratch/`. A relative override is anchored to the server
+  data path, not the process's current working directory.
+- `staging_backend` — `local` (default) or `s3`. Selects where in-flight part data is staged before
+  being committed into the pool; `local` is byte-for-byte the original write path, `s3` enables
+  S3-native staging.
+- `blob_hash` — `cityhash128` (default), `xxh3-128`, or `sha256`. Selects the pool's blob
+  content-hash function. The choice is fixed at pool creation; a reopen whose `blob_hash` disagrees
+  with the pool's recorded algorithm fails closed. See
+  [choosing `blob_hash`](/antalya/cas/configuration#choosing-blob-hash) for the trade-offs between
+  the three.
+- `blob_hash_allow_new` — `false` by default. Admits a new hash algorithm into an existing pool's set
+  of recorded algorithms; without it, a `blob_hash` that disagrees with what the pool already recorded
+  fails closed instead of silently turning the pool mixed-algorithm.
+- `gc_enabled` — `true` by default. Enables the background garbage collector for this disk.
+- `gc_interval_sec` — `60` by default; must be `>= 1`. Interval between background GC rounds.
+- `gc_shards` — `1` by default; must be `>= 1`. Number of blob-hash-prefix shards the GC reducer
+  splits work across. This is a creation-time-only setting: on reopen the pool's persisted GC state is
+  authoritative.
+- `deduplication_cache_bytes` — `64` MiB by default. Size of the in-memory deduplication lookup cache.
+- `deduplication_head_first_min_bytes` — `1` MiB by default. Minimum blob size at which a `HEAD` is sent
+  before the body, so that an upload of already-present content can be skipped. `0` disables it.
+- `gc_snapshot_generations_to_keep` — `3` by default. Number of past GC snapshot generations retained.
+- `gcs_max_conditional_put_bytes` — `1` GiB by default. On generation-token backends (Google Cloud
+  Storage), the body of a conditional write is RAM-buffered up to this size; a larger conditional
+  write throws `NOT_IMPLEMENTED`. Irrelevant on `ETag`-based backends such as AWS S3.
+- `part_folder_cache_bytes` — `64` MiB by default. Size of the part-folder view cache. `0` disables
+  retention; this is a supported permanent operational configuration, not only a debug aid.
+- `part_folder_cache_max_entries` — `10000` by default. Maximum number of entries in the part-folder
+  view cache.
+- `part_folder_cache_max_entry_bytes` — `16` MiB by default. Maximum size of a single cached
+  part-folder view entry.
+- `part_folder_validate` — `always` (default), `never`, or `age <seconds>`. Controls how often a
+  `ForceFresh` read re-proves a cached manifest body via a `HEAD` request: `always` re-proves every
+  time (the original, pre-optimization behavior), `never` trusts the cache without re-proving, and
+  `age <seconds>` re-proves only once the cached entry is older than the given number of seconds.
+- `manifest_decode_cache_bytes` — `128` MiB by default. Byte bound for the decoded-manifest cache.
+  `0` disables decode caching entirely (a diagnostic mode).
+- `gc_meta_pool_size` — `16` by default. Bounded thread-pool size for the GC's per-hash freshness-meta
+  writes (condemn/spare/delete), so a mass `DROP` condemning millions of blobs does not run fully
+  sequentially.
+- `skip_access_check` — `false` by default. Skips the disk's startup access check ("start now, fix
+  later"), unlike the generic disk-wide startup flag.
+
 ### Using Azure Blob Storage {#azure-blob-storage}
 
 `MergeTree` family table engines can store data to [Azure Blob Storage](https://azure.microsoft.com/en-us/services/storage/blobs/) 
diff --git a/docs/en/operations/system-tables/cas_gc_log.md b/docs/en/operations/system-tables/cas_gc_log.md
new file mode 100644
index 000000000000..5fd04b4fb11d
--- /dev/null
+++ b/docs/en/operations/system-tables/cas_gc_log.md
@@ -0,0 +1,156 @@
+---
+description: 'System table containing per-round records of the content-addressed (CAS) MergeTree garbage collector.'
+sidebar_label: 'cas_gc_log'
+sidebar_position: 30
+slug: /operations/system-tables/cas_gc_log
+title: 'system.cas_gc_log'
+doc_type: 'reference'
+---
+
+## Description {#description}
+
+The `system.cas_gc_log` table contains per-round records of the
+content-addressed (CAS) MergeTree garbage collector. For every garbage-collection round it stores a
+`Start` row and a `Finish` row (like `system.part_log` stores events per data part), with the counts
+of objects marked and deleted, the round duration, the outcome, and a per-round `ProfileEvents`
+delta.
+
+Between them it also stores one `Phase` row per GC phase the round reached, each carrying that
+phase's own duration, its `ProfileEvents` delta, and its phase-specific counts. All rows of one round
+share a `round_id`. See [Per-phase rows](#per-phase-rows).
+
+Rounds are emitted both by the background GC scheduler (`trigger = 'Scheduled'`) and by the
+synchronous [`SYSTEM CAS GC RUN`](/sql-reference/statements/system#system-cas-gc-run)
+command (`trigger = 'Manual'`).
+
+The table is created only if the `cas_gc_log` server setting is
+specified (it is enabled by default in the shipped `config.xml`).
+
+## Columns {#columns}
+
+- `hostname` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — Host name of the server executing the round.
+- `event_date` ([Date](/sql-reference/data-types/date)) — Event date.
+- `event_time` ([DateTime](/sql-reference/data-types/datetime)) — Event time.
+- `event_time_microseconds` ([DateTime64(6)](/sql-reference/data-types/datetime64)) — Event time with microseconds precision.
+- `event_type` ([Enum8](/sql-reference/data-types/enum)) — `Start` or `Finish` of a GC round, or one `Phase` of it.
+- `disk_name` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — The content-addressed disk the round ran on.
+- `server_root_id` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — Identifies the mount whose GC scheduler ran this round. Distinguishes concurrent mounters of the same shared pool; join on this column when correlating rounds against [`system.cas_mounts`](/operations/system-tables/cas_mounts).
+- `gc_id` ([String](/sql-reference/data-types/string)) — The GC scheduler instance id (which mounter ran the round).
+- `trigger` ([Enum8](/sql-reference/data-types/enum)) — `Scheduled` (background tick) or `Manual` (`SYSTEM` command).
+- `round` ([UInt64](/sql-reference/data-types/int-uint)) — The GC round number (`0` on a `Start` row).
+- `outcome` ([Enum8](/sql-reference/data-types/enum)) — `Unknown` (on a `Start` row), `Success` (led, folded, and completed), `NotALeader` (another replica holds the GC lease), `Deferred` (led but took the skip-unchanged fast path — no fold ran, because no changed shard reached the fold threshold and no graduation was due), or `Error` (the round threw).
+- `candidates_marked` ([UInt64](/sql-reference/data-types/int-uint)) — Objects retired (marked) this round.
+- `objects_deleted` ([UInt64](/sql-reference/data-types/int-uint)) — Objects physically deleted this round.
+- `objects_absent` ([UInt64](/sql-reference/data-types/int-uint)) — Retire candidates found already absent.
+- `objects_replaced` ([UInt64](/sql-reference/data-types/int-uint)) — `412`-saves (a resurrection won the race against the delete).
+- `objects_spared` ([UInt64](/sql-reference/data-types/int-uint)) — Candidates spared because their in-degree was greater than zero at recheck.
+- `manifests_deleted` ([UInt64](/sql-reference/data-types/int-uint)) — Owner-removed manifest bodies physically deleted this round, counted separately from blob deletes.
+- `entries_condemned` ([UInt64](/sql-reference/data-types/int-uint)) — Retired entries newly condemned this round (retired-cursor pipeline stage 1).
+- `entries_graduated` ([UInt64](/sql-reference/data-types/int-uint)) — Retired entries newly floor-passed and republished `delete_pending` this round (pipeline stage 2; deleted the next round).
+- `entries_redeleted` ([UInt64](/sql-reference/data-types/int-uint)) — Pending exact-token blob deletes executed this round (pipeline stage 3).
+- `fence_outs` ([UInt64](/sql-reference/data-types/int-uint)) — Expired mounts fenced out by this round's heartbeat floor.
+- `anomalies` ([UInt64](/sql-reference/data-types/int-uint)) — Fold clamps surfaced (and survived) this round. A steady non-zero value warrants a look at the round log details.
+- `duration_ms` ([UInt64](/sql-reference/data-types/int-uint)) — The round wall-clock duration (on a `Finish` row).
+- `error` ([String](/sql-reference/data-types/string)) — The exception text when `outcome = 'Error'`.
+- `ProfileEvents` ([Map(LowCardinality(String), UInt64)](/sql-reference/data-types/map)) — On a `Start`/`Finish` row, the per-round `ProfileEvents` delta (the `CAS*` counters and S3/disk events for this round). On a `Phase` row, **that phase's** delta, so `GROUP BY phase` over `ProfileEvents['S3ListObjects']` attributes the round's `LIST` budget to the phase that spent it.
+- `round_id` ([String](/sql-reference/data-types/string)) — The correlator for every row of one round attempt: its `Start`, each of its `Phase` rows, and its `Finish`. Minted per attempt, so unlike `round` it exists even for a round that never committed and for a round that never led. Group by this column to reconstruct one round.
+- `phase` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — The GC phase this row describes; empty on `Start`/`Finish`. See [Per-phase rows](#per-phase-rows) for the phase list.
+- `phase_duration_microseconds` ([UInt64](/sql-reference/data-types/int-uint)) — The wall-clock duration of this phase, in microseconds (`Phase` rows only). Microseconds rather than milliseconds because several phases are routinely sub-millisecond and the point of the row is to see when they are not.
+- `phase_metrics` ([Map(LowCardinality(String), UInt64)](/sql-reference/data-types/map)) — Phase-specific semantic counts (`Phase` rows only) that a phase computes for itself and no `ProfileEvents` counter can supply. The verb counts ride the `ProfileEvents` column of the same row.
+
+## Per-phase rows {#per-phase-rows}
+
+Besides the `Start` and `Finish` row of each round, the collector emits one `Phase` row per GC phase.
+Every row of one round attempt — `Start`, each `Phase`, and `Finish` — shares a `round_id`. A round
+that defers, or that never acquires the GC lease, emits only the phases it actually reached; a round
+that throws still emits the row of the phase it died in.
+
+The phases, in execution order:
+
+| `phase` | What it covers | Dominant I/O |
+|---|---|---|
+| `lease` | Acquire, renew, or observe the GC lease. The only phase a `NotALeader` round emits. | `gc/state` `GET` + compare-and-swap |
+| `pre_fold_ref_drain` | Resolve catalog rows whose terminal fold evidence is already adopted before this invocation publishes or defers. | catalog `GET` + exact compare-and-swap |
+| `heartbeat_floor` | Classify every mount slot and fence out the dead ones. | `LIST` of the mount prefix, one `GET` per mount, one `PUT` per fence |
+| `defer_decision` | The skip-unchanged decision: graduation check plus the round's one enumeration of the ref prefix. | one full ref-prefix `LIST`, two fold-seal `GET`s |
+| `parent_seal_read` | Capture the pre-fold seal's run refs for the hand-off reclaim. | one fold-seal `GET` |
+| `fold_ref_group` | Regroup the round's enumeration into per-table listings — what this round will fold. | none |
+| `fold_seal_read` | The adopted fold seal, read twice at the same generation and attempt. | two fold-seal `GET`s |
+| `fold_ref_intake` | Read and fold every new ref log and the manifest bodies its edges name. | one `GET` per new log, one `GET` per manifest edge |
+| `fold_reduce` | The per-shard in-degree merge: condemn, spare, graduate. | prior-run streaming `GET`s, one `HEAD` per zero-transition candidate, run `PUT`s |
+| `fold_seal_write` | Publish the new fold seal. | one `PUT` |
+| `pending_deletes` | The single content-delete site: exact-token deletes of previously published `delete_pending` entries, plus the outcome logs. | one `DELETE` per entry, one outcome-log `PUT` per shard |
+| `meta_pool_wait` | Drain the round's per-hash freshness-meta writes. | none on this thread — see the caveat below |
+| `round_commit` | The generation-retention prune and the round's single `gc/state` compare-and-swap. | prune `LIST`s and deletes, one compare-and-swap |
+| `handoff_reclaim` | Wholesale-reclaim generations a moved run ref stranded below the retention cursor. | prefix `LIST`s and deletes |
+| `manifest_deletes` | Exact-token deletes of owner-removed manifest bodies, after their decrements were adopted. | one `DELETE` per body |
+| `namespace_cleanup` | Run one bounded `cas/ns/` page across the stream and state subtrees for the perpetual dead-life janitor. This phase is physical reclamation, not a lifecycle gate. | one namespace-root page `LIST`, catalog cut, exact-token deletes |
+| `ref_object_cleanup` | Delete ref logs covered by both the durable fold cursor and a durable snapshot, plus superseded snapshots. | one `HEAD` + one `DELETE` per deletable object |
+| `orphan_sweep` | The budgeted, cursor-paced orphan part-manifest backstop. | budgeted `LIST` and deletes |
+
+Which phase dominates a round:
+
+```sql
+SELECT phase,
+       count() AS rounds,
+       quantile(0.5)(phase_duration_microseconds) AS p50_microseconds,
+       quantile(0.99)(phase_duration_microseconds) AS p99_microseconds,
+       sum(phase_duration_microseconds) AS total_microseconds
+FROM system.cas_gc_log
+WHERE event_type = 'Phase' AND disk_name = 'ca'
+GROUP BY phase
+ORDER BY total_microseconds DESC;
+```
+
+Which phase spends the `LIST` budget:
+
+```sql
+SELECT phase, sum(ProfileEvents['S3ListObjects']) AS lists
+FROM system.cas_gc_log
+WHERE event_type = 'Phase' AND disk_name = 'ca'
+GROUP BY phase
+ORDER BY lists DESC;
+```
+
+One round, in order — including a round that failed, which is why the correlator is `round_id` and
+not `round`:
+
+```sql
+SELECT phase, phase_duration_microseconds, phase_metrics, ProfileEvents['S3ListObjects'] AS lists
+FROM system.cas_gc_log
+WHERE round_id = '...' AND event_type = 'Phase'
+ORDER BY event_time_microseconds;
+```
+
+Two caveats when reading these rows:
+
+- Work scheduled onto the GC meta pool runs on other threads, so the `meta_pool_wait` row's
+  `ProfileEvents` delta is **empty by construction**. Read its `phase_metrics` `jobs_scheduled` /
+  `jobs_completed` next to its duration instead: they distinguish a deep queue from a slow endpoint.
+- Phase durations do not sum to the round's `duration_ms`. The round also performs untimed
+  bookkeeping between phases, and the `Finish` row's `duration_ms` remains the authority on total
+  round time.
+
+## Example {#example}
+
+```sql
+SELECT
+    event_type,
+    disk_name,
+    trigger,
+    outcome,
+    candidates_marked,
+    objects_deleted,
+    duration_ms
+FROM system.cas_gc_log
+ORDER BY event_time_microseconds DESC
+LIMIT 2
+FORMAT Vertical;
+```
+
+## See Also {#see-also}
+
+- [`SYSTEM CAS GC RUN`](/sql-reference/statements/system#system-cas-gc-run) — run one GC round synchronously.
+- [`system.cas_mounts`](/operations/system-tables/cas_mounts) — live per-`server_root_id` mount and GC-health state.
+- [`system.cas_log`](/operations/system-tables/cas_log) — per-decision event log for the CAS garbage collector and writer.
+- [`system.part_log`](/operations/system-tables/part_log) — the analogous per-part event log.
diff --git a/docs/en/operations/system-tables/cas_log.md b/docs/en/operations/system-tables/cas_log.md
new file mode 100644
index 000000000000..315842d3b9b4
--- /dev/null
+++ b/docs/en/operations/system-tables/cas_log.md
@@ -0,0 +1,62 @@
+---
+description: 'System table containing a per-decision event log for the content-addressed (CAS) MergeTree writer and garbage collector.'
+sidebar_label: 'cas_log'
+sidebar_position: 32
+slug: /operations/system-tables/cas_log
+title: 'system.cas_log'
+doc_type: 'reference'
+---
+
+## Description {#description}
+
+The `system.cas_log` table contains a per-decision event log for the content-addressed
+(CAS) MergeTree storage engine: blob puts and dedup adoptions, root/ref transitions, in-degree changes,
+garbage-collector retire decisions and recheck verdicts, blob deletes, and dangling-access/corruption
+findings. It is a much finer-grained, per-event complement to
+[`system.cas_gc_log`](/operations/system-tables/cas_gc_log),
+which only records one `Start`/`Finish` row per GC round.
+
+## Columns {#columns}
+
+- `hostname` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — Host name of the server that emitted the event.
+- `event_date` ([Date](/sql-reference/data-types/date)) — Event date.
+- `event_time` ([DateTime](/sql-reference/data-types/datetime)) — Event time.
+- `event_time_microseconds` ([DateTime64(6)](/sql-reference/data-types/datetime64)) — Event time with microseconds precision.
+- `event_type` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — The CAS decision/event, e.g. `blob_put`, `blob_reuse_adopt`, `root_remove`, `indegree_zero`, `gc_retire_decision`, `gc_recheck_verdict`, `blob_delete`, `dangling_access`, `corrupt_dangle`.
+- `disk_name` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — The content-addressed disk / pool the event belongs to.
+- `namespace` ([String](/sql-reference/data-types/string)) — `roots/<namespace>` (server/table); empty if not applicable.
+- `ref_name` ([String](/sql-reference/data-types/string)) — Part name / ref the event concerns; empty if not applicable.
+- `object_kind` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — One of `none`, `blob`, `manifest`, `root`, `snapshot`.
+- `object_hash` ([String](/sql-reference/data-types/string)) — Content hash (lowercase hex) of the object; empty if not applicable.
+- `token` ([String](/sql-reference/data-types/string)) — Incarnation token (`ETag`) involved; empty if not applicable.
+- `round` ([UInt64](/sql-reference/data-types/int-uint)) — GC round (`0` if not applicable).
+- `generation` ([UInt64](/sql-reference/data-types/int-uint)) — GC snapshot generation (`0` if not applicable).
+- `at_version` ([UInt64](/sql-reference/data-types/int-uint)) — Manifest `shard_version` of the driving journal record (`0` if not applicable).
+- `outcome` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — Decision outcome, e.g. `ok`, `adopt`, `resurrect`, `deleted`, `replaced`, `spared`, `absent`, `zeroed`, `skipped`.
+- `reason` ([LowCardinality(String)](/sql-reference/data-types/lowcardinality)) — Human-readable rationale for the decision. Templated across rows, so it is `LowCardinality`.
+- `thread_id` ([UInt64](/sql-reference/data-types/int-uint)) — OS thread that emitted the event.
+- `query_id` ([String](/sql-reference/data-types/string)) — Query id for correlation with [`system.query_log`](/operations/system-tables/query_log); empty if not applicable.
+- `detail` ([Map(LowCardinality(String), String)](/sql-reference/data-types/map)) — Structured event-specific facts, e.g. `condemn_round`, `superseded_token`, `code`, `site`.
+
+## Example {#example}
+
+```sql
+SELECT
+    event_time_microseconds,
+    event_type,
+    disk_name,
+    ref_name,
+    object_kind,
+    outcome,
+    reason
+FROM system.cas_log
+ORDER BY event_time_microseconds DESC
+LIMIT 10
+FORMAT Vertical;
+```
+
+## See Also {#see-also}
+
+- [`system.cas_gc_log`](/operations/system-tables/cas_gc_log) — per-round GC event log.
+- [`system.cas_mounts`](/operations/system-tables/cas_mounts) — live per-`server_root_id` mount and GC-health state.
+- [`system.query_log`](/operations/system-tables/query_log) — correlate via `query_id`.
diff --git a/docs/en/operations/system-tables/cas_mounts.md b/docs/en/operations/system-tables/cas_mounts.md
new file mode 100644
index 000000000000..5a9465db04af
--- /dev/null
+++ b/docs/en/operations/system-tables/cas_mounts.md
@@ -0,0 +1,81 @@
+---
+description: 'System table containing the live mount and GC-health state of every server mounted onto a content-addressed (CAS) disk pool.'
+sidebar_label: 'cas_mounts'
+sidebar_position: 31
+slug: /operations/system-tables/cas_mounts
+title: 'system.cas_mounts'
+doc_type: 'reference'
+---
+
+## Description {#description}
+
+The `system.cas_mounts` table contains one row per mount slot discovered on every
+content-addressed (CAS) disk configured on the node. A pool may be shared by several servers (or
+several `server_root_id` mounts on the same server), and this table lists every mount visible in
+the pool's backend at query time, not only the querying server's own mount — it exists for
+incident-time diagnosis of leases, epochs, and GC leadership across a shared pool.
+
+The table is read directly from the CAS disk's backend on every query (there is no persisted log
+behind it); a transient backend error on one disk is skipped and does not blind the rest of the
+rows.
+
+## Columns {#columns}
+
+- `disk` ([String](/sql-reference/data-types/string)) — Name of the content-addressed disk.
+- `server_root_id` ([String](/sql-reference/data-types/string)) — Server root id owning the mount slot.
+- `server_uuid` ([UUID](/sql-reference/data-types/uuid)) — UUID of the server incarnation holding the lease.
+- `hostname` ([String](/sql-reference/data-types/string)) — Hostname recorded in the lease body.
+- `process_id` ([UInt64](/sql-reference/data-types/int-uint)) — Process id recorded in the lease body.
+- `writer_epoch` ([UInt64](/sql-reference/data-types/int-uint)) — Fenced writer epoch of the incarnation.
+- `renewal_sequence` ([UInt64](/sql-reference/data-types/int-uint)) — Lease renewal sequence number.
+- `started_at` ([DateTime64(3)](/sql-reference/data-types/datetime64)) — Time when the lease started.
+- `expires_at` ([DateTime64(3)](/sql-reference/data-types/datetime64)) — Time when the lease expires.
+- `min_active_build_sequence` ([UInt64](/sql-reference/data-types/int-uint)) — Oldest in-flight build sequence (`UINT64_MAX` means the mount said farewell).
+- `gc_fenced` ([UInt8](/sql-reference/data-types/int-uint)) — `1` if GC fenced this slot out (terminal).
+- `state` ([String](/sql-reference/data-types/string)) — One of `live`, `expired`, `terminated`, `fenced`, `corrupt`.
+- `is_leader` ([Nullable(UInt8)](/sql-reference/data-types/nullable)) — `1` if this server's GC scheduler currently holds this disk's leadership lease.
+- `pending_reclaim` ([Nullable(Int64)](/sql-reference/data-types/nullable)) — Cumulative two-phase deletion backlog observed by this process's GC on this disk (condemned entries minus executed exact-token deletes).
+- `last_success_age_seconds` ([Nullable(UInt64)](/sql-reference/data-types/nullable)) — Seconds since this disk's GC last led a round (`0` if it has never led or GC is not running here).
+- `wedged_namespace_count` ([Nullable(UInt64)](/sql-reference/data-types/nullable)) — Ref-append lanes currently wedged on this disk (an uncertain `PUT` exhausted its retry budget).
+- `lifecycle` ([String](/sql-reference/data-types/string)) — This server's content-addressed pool lifecycle for the disk (a non-gated snapshot, always populated so a not-live disk stays visible): one of `live`, `not_live`, `identity_lost`, `vanished`, `constructing` (never started), or `shutdown` (torn down).
+- `lifecycle_reason` ([String](/sql-reference/data-types/string)) — The enum-clean sub-state word for a `vanished` disk: `replaced` or `forgotten`. Empty for every other lifecycle, so `lifecycle || '(' || lifecycle_reason || ')'` reads e.g. `vanished(forgotten)`.
+- `lifecycle_detail` ([String](/sql-reference/data-types/string)) — The full typed reason text naming the actual cause when not live: the vanish diagnosis (a data root replaced by a foreign pool, or decommissioned by `SYSTEM CAS FORGET` at a given time) or the identity-loss message. Empty when live.
+- `lifecycle_since` ([Nullable(DateTime)](/sql-reference/data-types/nullable)) — When this server entered the current non-live lifecycle state. `NULL` when live, or when the state has no backing pool to date from.
+
+`lifecycle`/`lifecycle_reason`/`lifecycle_detail`/`lifecycle_since` are the SQL surface for
+diagnosing an identity-lost or forgotten disk without reading server logs — see the
+`IdentityLost`/`VanishedReplaced`/`VanishedForgotten` states on the
+[mount-slot behavioral model](/antalya/cas/architecture/mounts-and-leases#mount-state-machines) for
+what each lifecycle value means, and [`SYSTEM CAS FORGET`](/antalya/cas/operations/debugging#sql-forget)
+for the command that produces `vanished(forgotten)`.
+
+:::note Local-only GC-health columns
+`is_leader`, `pending_reclaim`, `last_success_age_seconds`, and `wedged_namespace_count` are process-local
+facts about *this* server's own GC scheduler. They are populated **only** on the row whose `server_root_id` matches
+this server's own mount, and are `NULL` on every row describing another server's mount — stamping a local
+health fact onto a peer's row would misread as "the peer is the GC leader" during an incident. To see the
+peer's own view of these columns, query `system.cas_mounts` on that server.
+:::
+
+## Example {#example}
+
+```sql
+SELECT
+    disk,
+    server_root_id,
+    state,
+    writer_epoch,
+    is_leader,
+    pending_reclaim,
+    last_success_age_seconds
+FROM system.cas_mounts
+ORDER BY disk, server_root_id
+FORMAT Vertical;
+```
+
+## See Also {#see-also}
+
+- [`system.cas_gc_log`](/operations/system-tables/cas_gc_log) — per-round GC event log.
+- [`system.cas_log`](/operations/system-tables/cas_log) — per-decision event log for the CAS garbage collector and writer.
+- [`SYSTEM CAS GC RUN`](/sql-reference/statements/system#system-cas-gc-run) — run one GC round synchronously.
+- [`SYSTEM CAS DROP POOL MEMBER`](/sql-reference/statements/system#system-cas-drop-pool-member) — permanently decommission a dead pool member's `server_root_id`.
diff --git a/docs/en/sql-reference/statements/system.md b/docs/en/sql-reference/statements/system.md
index 69b2b59cb0ae..ed3a2b97cf87 100644
--- a/docs/en/sql-reference/statements/system.md
+++ b/docs/en/sql-reference/statements/system.md
@@ -457,6 +457,151 @@ Wait until all asynchronously loading data parts of a table (outdated data parts
 SYSTEM WAIT LOADING PARTS [ON CLUSTER cluster_name] [db.]merge_tree_family_table_name
 ```
 
+### SYSTEM CAS GC RUN {#system-cas-gc-run}
+
+Runs one garbage-collection round of the content-addressed (CAS) MergeTree garbage collector synchronously and node-local: it reclaims content-addressed objects that are no longer referenced by any part. This is the on-demand counterpart of the background GC scheduler; it is useful for tests and diagnostics.
+
+```sql
+SYSTEM CAS GC RUN [ON CLUSTER cluster_name] [disk_name]
+```
+
+When `disk_name` is given, the round runs on that content-addressed disk only; targeting a non-content-addressed disk raises an exception. When `disk_name` is omitted, one round runs on every content-addressed disk configured on the node; if none are configured, the command raises an exception.
+
+Each round is recorded in [`system.cas_gc_log`](/operations/system-tables/cas_gc_log) as a `Start` and a `Finish` row (with `trigger = 'Manual'`).
+
+The command returns one row per disk it ran on (multiple rows when `disk_name` is omitted), with columns `disk`, `acquired_lease`, `deferred`, `round`, `candidates_marked`, `objects_deleted`, `objects_absent`, `objects_replaced`, `objects_spared`, `manifests_deleted`, `entries_condemned`, `entries_graduated`, `entries_redeleted`, `fence_outs`, `anomalies`, `pending_candidates`, `pending_condemned`, and `pending_retired`, describing the outcome of that round. The `pending_*` columns are the retire pipeline's remaining backlog sizes read from the `gc/state` this round's own commit just published (not this round's own delta, unlike the columns before them) — `0` on a non-authoritative row (`acquired_lease = 0` or `deferred = 1`), same as every other counter.
+
+A manual run always executes, regardless of [`SYSTEM CAS GC STOP`](#system-cas-gc-stop-start): `STOP` pauses only the background scheduler on that disk.
+
+### SYSTEM CAS GC REBUILD {#system-cas-gc-rebuild}
+
+Disaster-recovery command for the content-addressed (CAS) MergeTree garbage collector. It rebuilds a
+CAS disk's `gc/state` baseline from scratch, by re-discovering the whole ref universe and re-folding
+manifest edges into a fresh generation. It writes only the GC plane (`gc/state` and the `gc/gen/*`
+artifacts) and never touches ref shards, manifests, or blobs, and it never deletes anything itself —
+but the rebuilt baseline drives every subsequent GC round's retire decisions, so this is a
+**destructive disaster-recovery tool**, not something to run routinely: a rebuild performed against
+a state that was not actually corrupted discards live bookkeeping, and an incorrect rebuild can make
+a later round delete objects that are still referenced.
+
+```sql
+SYSTEM CAS GC REBUILD [FORCE] [ON CLUSTER cluster_name] disk_name
+```
+
+Unlike `SYSTEM CAS GC RUN`, `disk_name` is **required**: the destructive
+baseline rebuild must never fan out across every content-addressed disk on the node from a bare
+command; targeting a non-content-addressed disk raises an exception.
+
+By default the command refuses to run when the disk's existing `gc/state` and every artifact it
+references decode successfully and are present — a rebuild would needlessly discard healthy live
+bookkeeping. Add `FORCE` to rebuild deliberately even though the existing state looks healthy. The
+command also refuses (regardless of `FORCE`) when another GC leader currently holds the disk's
+lease. In both refusal cases it raises an exception instead of returning a row.
+
+On success it returns one row with columns `disk`, `performed`, `round`, `generation`, `namespaces`,
+`shards`, `committed_refs`, `live_precommits`, `unowned_alive_manifests`, `edges`,
+`clamped_shards`, `virgin_by_enumeration`, and `adopted_seal_generation`, describing the freshly
+rebuilt baseline. `virgin_by_enumeration = 1` means the rebuild found no fold seal at all and
+carried no durable hold forward, concluding from enumeration alone that the pool never sealed a
+baseline — on a pool that has ever completed a GC round this means the object listing lied.
+`adopted_seal_generation` names which generation's fold seal the rebuild carried holds from; `0`
+when it carried none.
+
+### SYSTEM CAS GC STOP / SYSTEM CAS GC START {#system-cas-gc-stop-start}
+
+Pause or resume the background GC scheduler on one content-addressed disk, without affecting reads
+or writes on that disk. This is granular operator control of GC alone — for example to pause
+reclamation during an incident — not a lifecycle transition; the disk stays fully usable throughout.
+
+```sql
+SYSTEM CAS GC STOP [ON CLUSTER cluster_name] disk_name
+SYSTEM CAS GC START [ON CLUSTER cluster_name] disk_name
+```
+
+`disk_name` is **required** for both — unlike `SYSTEM CAS GC RUN`, there is no fan-out form, since
+each command targets exactly one disk's scheduler.
+
+`GC STOP` stops in place: the scheduler object is retained, so a later `GC START` resumes the *same*
+instance, preserving its `gc_id` and lease-observation history. It is idempotent, and works even on
+a disk that is not currently live (stopping GC on a sick disk is a legitimate operation). It does
+not stop a manual [`SYSTEM CAS GC RUN`](#system-cas-gc-run) on the same disk.
+
+`GC START` re-enters that same scheduler instance rather than creating a new one; leadership is
+**not** automatically restored — the scheduler re-acquires the durable `gc/state` lease through the
+next round's normal acquisition, the same as any other contender. It is idempotent (a no-op on an
+already-running scheduler), and refuses with a typed error on a decommissioned or uncertain pool,
+since restarting GC there would only spin failing rounds.
+
+Neither command returns a result set.
+
+### SYSTEM CAS FSCK {#system-cas-fsck}
+
+Independently verifies content-addressed pool reachability against a **running, mounted** disk — the
+scan re-validates every finding against a fresh authoritative read, so unlike the offline
+`clickhouse-disks cas-fsck` tool it needs no quiesce and no read-only mount.
+
+```sql
+SYSTEM CAS FSCK [ON CLUSTER cluster_name] disk_name
+```
+
+`disk_name` is **required**. The command returns one row with columns `disk`, `reachable`,
+`dangling`, `unreachable`, `pending_gc`, `awaiting_gc`, `unaccounted`, `stale_edge`,
+`corrupted_runs`, `chain_broken`, `unchecked`, `lifeless_keys`, `namespace_janitor_pending`,
+`namespace_janitor_pending_bytes`, `namespace_janitor_pending_lives`, `ref_records_walked`,
+`physical_bytes`, `referenced_logical_bytes`, `distinct_blobs`, and `total_blob_refs`. `dangling` is
+the one column that means data loss; `unreachable`, `pending_gc`, and `awaiting_gc` are objects
+still moving through the normal condemn/graduate/delete pipeline, not a problem on their own. This
+is a summary-only scan; per-object detail requires the offline `clickhouse-disks cas-fsck --detail`.
+
+### SYSTEM CAS FORGET {#system-cas-forget}
+
+Node-local operator assertion that a content-addressed disk is permanently gone — the "fire marshal"
+verb for a stuck disk (a transient or identity-lost pool, or an operator-asserted decommission).
+Unlike the other `SYSTEM CAS` commands, it deliberately works on a disk that is **not** live, since
+that is its whole purpose.
+
+```sql
+SYSTEM CAS FORGET [ON CLUSTER cluster_name] disk_name
+```
+
+`disk_name` is **required**. It is an assertion, not a proof of erasure: the disk stays registered
+and answers further store-class access with a typed error, and a server restart re-registers the
+name. Returns no result set. This is different from
+[`SYSTEM CAS DROP POOL MEMBER`](#system-cas-drop-pool-member), which permanently retires one pool
+*member's* identity across the whole shared pool — `FORGET` only affects this node's own local view
+of one disk.
+
+### SYSTEM CAS DROP POOL MEMBER {#system-cas-drop-pool-member}
+
+Permanently decommissions a dead member (`server_root_id`) of a content-addressed disk
+pool. It claims the member's mount slot as an administrative writer — fencing that `server_root_id` from ever
+writing again — then drops every table namespace the member owned, drains eligible manifest debris,
+staging objects, and mountpoint objects belonging to it, and retires the mount slot once all drains
+are confirmed. This is a **destructive, irreversible** operation: only run it once the `server_root_id` is
+confirmed permanently dead, since it fences the member out even if it later comes back online, and
+it deletes namespace and drain state that cannot be recovered.
+
+It is a writer operation, not GC: it emits ordinary ref-edge deltas rather than inventing GC
+transitions, and it does not synchronously reclaim shared blob content — removing the ref edges only
+makes the now-unreferenced blobs eligible for an ordinary GC round to reclaim later.
+
+```sql
+SYSTEM CAS DROP POOL MEMBER 'server_root_id' FROM DISK 'disk_name' [ON CLUSTER cluster_name]
+```
+
+Both `server_root_id` and `disk_name` are required string literals (a `server_root_id` is an opaque server-root path
+that may contain `/`, not a plain identifier, so it cannot be written unquoted).
+
+The operation is resumable: a rerun skips namespaces already marked `Removed` and reports them
+separately from namespaces newly removed by this invocation. Per-object drain failures are recorded
+as warnings and leave the slot in a terminated-but-not-fully-drained state that a later invocation
+can resume from, rather than raising an exception.
+
+The command returns one row with columns `server_root_id`, `namespaces_removed`, `namespaces_already_removed`,
+`committed_refs_removed`, `precommits_removed`, `manifest_debris_removed`, `staging_objects_removed`,
+`mountpoint_objects_removed`, `slot_removed`, and `warnings`. A non-empty `warnings` means some
+drain was not confirmed and the mount slot was left in place as a resume anchor.
+
 ## Managing ReplicatedMergeTree Tables {#managing-replicatedmergetree-tables}
 
 ClickHouse can manage background replication related processes in [ReplicatedMergeTree](/engines/table-engines/mergetree-family/replication) tables.