diff --git a/scripts/o1_holdout_gate.py b/scripts/o1_holdout_gate.py
index 5b6a7125..03998350 100644
--- a/scripts/o1_holdout_gate.py
+++ b/scripts/o1_holdout_gate.py
@@ -8,6 +8,14 @@
 (reranker cache starts empty -> no cache-hit void measurements).
 Prints a result table; exit 1 on gate failure (see PASS RULES below).
 
+HARNESS RULE (2026-09-28, verified): all queries run inside ONE event loop
+(`asyncio.run(_run_all(...))` once). The old per-query `asyncio.run()` pattern
+silently degrades every even-positioned query to reranker passthrough
+(provider-None, reranker_ms=0): asyncio primitives (locks/semaphores/client)
+bound to the first, now-closed loop misbehave on alternating fresh loops.
+A degraded row (reranker_ms falsy) fails loudly instead of judging rank on
+passthrough order.
+
 Holdout set note: no canonical H1-H12 exists on main (verified 2026-09-28:
 grep finds only unrelated H-labels in diary/archive). H1-H12 below are
 O1-holdout-v1, defined HERE (multi-token identifier queries with known
@@ -95,27 +103,7 @@ def main() -> int:
         rev = "unknown"
     print(f"O1 holdout gate | rev={rev} | project={project} | fresh process")
     searcher = build_searcher(project)
-    # Warm-up (discarded): cold FTS5 to_pandas build exceeds the 2s tier budget
-    # on main and would silently drop the FTS tier for the FIRST measured query
-    # (known issue, PR52 territory). Warm-up uses a disjoint query -> no
-    # reranker-cache contamination (cache key includes the query text).
-    asyncio.run(searcher.hybrid_search_async("warmup cold start primer", limit=3))
-    rows = []
-    for case in [P2, *HOLDOUT]:
-        t0 = time.perf_counter()
-        results = asyncio.run(searcher.hybrid_search_async(case["query"], limit=5))
-        wall = time.perf_counter() - t0
-        boosted = [r for r in results if r.get("identifier_boost")]
-        doc_boosted = [r for r in boosted
-                       if str((r.get("metadata") or {}).get("file", ""))
-                       .lower().endswith((".md", ".markdown", ".rst", ".txt", ".ipynb"))]
-        rank = rank_of(results, case["target"]) if case.get("target") else None
-        void = wall < 2.0 and bool(getattr(searcher, "_last_rerank_timing", None) == {})
-        rows.append({"id": case["id"], "rank": rank, "target": case.get("target"),
-                     "n_boost": len(boosted), "n_doc_boost": len(doc_boosted),
-                     "wall": round(wall, 2), "void": void})
-        print(f"{case['id']:>3} rank={rank} target={case.get('target')} "
-              f"boost={len(boosted)} doc_boost={len(doc_boosted)} wall={wall:.2f}s")
+    rows = asyncio.run(_run_all(searcher))
     print("---")
     fails = []
     p2 = rows[0]
@@ -129,6 +117,8 @@ def main() -> int:
             fails.append(f"{row['id']} doc chunk boosted")
         if row["void"]:
             fails.append(f"{row['id']} reranker-cache void (wall<2s, no timing)")
+        if row["degraded"]:
+            fails.append(f"{row['id']} reranker degraded (reranker_ms=0, passthrough)")
     if fails:
         print("GATE FAIL: " + "; ".join(fails))
         return 1
@@ -136,6 +126,36 @@ def main() -> int:
     return 0
 
 
+async def _run_all(searcher):
+    """Whole sequence on ONE event loop (see HARNESS RULE above)."""
+    # Warm-up (discarded): cold FTS5 to_pandas build exceeds the 2s tier budget
+    # on main and would silently drop the FTS tier for the FIRST measured query
+    # (known issue, PR52 territory). Warm-up uses a disjoint query -> no
+    # reranker-cache contamination (cache key includes the query text).
+    await searcher.hybrid_search_async("warmup cold start primer", limit=3)
+    rows = []
+    for case in [P2, *HOLDOUT]:
+        t0 = time.perf_counter()
+        results = await searcher.hybrid_search_async(case["query"], limit=5)
+        wall = time.perf_counter() - t0
+        boosted = [r for r in results if r.get("identifier_boost")]
+        doc_boosted = [r for r in boosted
+                       if str((r.get("metadata") or {}).get("file", ""))
+                       .lower().endswith((".md", ".markdown", ".rst", ".txt", ".ipynb"))]
+        rank = rank_of(results, case["target"]) if case.get("target") else None
+        timing = dict(getattr(searcher, "_last_rerank_timing", None) or {})
+        void = wall < 2.0 and timing == {}
+        degraded = not timing.get("reranker_ms")
+        rows.append({"id": case["id"], "rank": rank, "target": case.get("target"),
+                     "n_boost": len(boosted), "n_doc_boost": len(doc_boosted),
+                     "n": len(results), "wall": round(wall, 2),
+                     "void": void, "degraded": degraded})
+        print(f"{case['id']:>3} rank={rank} n={len(results)} target={case.get('target')} "
+              f"boost={len(boosted)} doc_boost={len(doc_boosted)} wall={wall:.2f}s "
+              f"rerank_ms={timing.get('reranker_ms')} model={timing.get('model')}")
+    return rows
+
+
 if __name__ == "__main__":
     try:
         sys.exit(main())
diff --git a/tests/test_holdout_harness_loop.py b/tests/test_holdout_harness_loop.py
new file mode 100644
index 00000000..62963251
--- /dev/null
+++ b/tests/test_holdout_harness_loop.py
@@ -0,0 +1,137 @@
+#!/usr/bin/env python3
+"""Holdout-harness single-loop guard (2026-09-28).
+
+Regression: the per-query `asyncio.run()` pattern silently degrades every
+even-positioned query to reranker passthrough (provider-None, reranker_ms=0),
+because asyncio primitives bound to the first, now-closed loop misbehave on
+alternating fresh loops. Both gate scripts must run ALL queries inside ONE
+event loop (`asyncio.run(_run_all(...))` exactly once) and fail loudly on
+degraded rows (reranker_ms falsy) instead of judging rank on passthrough.
+
+Unit-level (no live services): AST structural guard + fake-searcher
+loop-identity probe. Live probe is best-effort, skipped if services down.
+"""
+import ast
+import asyncio
+import sys
+from pathlib import Path
+
+import pytest
+
+ROOT = Path(__file__).resolve().parent.parent
+if str(ROOT) not in sys.path:
+    sys.path.insert(0, str(ROOT))
+
+GATES = ["scripts/o1_holdout_gate.py", "scripts/p2_holdout_gate.py"]
+
+
+def _run_calls(tree: ast.AST) -> int:
+    """Count real asyncio.run() CALLS (docstring mentions don't parse as calls)."""
+    n = 0
+    for node in ast.walk(tree):
+        if (
+            isinstance(node, ast.Call)
+            and isinstance(node.func, ast.Attribute)
+            and node.func.attr == "run"
+            and isinstance(node.func.value, ast.Name)
+            and node.func.value.id == "asyncio"
+        ):
+            n += 1
+    return n
+
+
+def _has_coro(tree: ast.AST, name: str) -> bool:
+    return any(
+        isinstance(node, ast.AsyncFunctionDef) and node.name == name
+        for node in ast.walk(tree)
+    )
+
+
+@pytest.mark.parametrize("gate", GATES)
+def test_single_event_loop_per_gate(gate):
+    """Reintroducing per-query asyncio.run() (count != 1) fails here."""
+    src = (ROOT / gate).read_text(encoding="utf-8")
+    tree = ast.parse(src)
+    assert _run_calls(tree) == 1, f"{gate}: expected exactly 1 asyncio.run() call"
+    assert _has_coro(tree, "_run_all"), f"{gate}: missing `async def _run_all`"
+
+
+@pytest.mark.parametrize("gate", GATES)
+def test_degraded_flag_present(gate):
+    """Both gates must fail loudly on reranker passthrough (reranker_ms falsy)."""
+    src = (ROOT / gate).read_text(encoding="utf-8")
+    assert "degraded" in src, f"{gate}: missing degraded-flag handling"
+
+
+class _FakeCache:
+    def clear(self):
+        pass
+
+
+class _FakeSearcher:
+    """Records the running loop of every query; always reports real timing."""
+
+    def __init__(self):
+        self.loops = []
+        self._reranker_cache = _FakeCache()
+        self._multi_reranker = None
+        self._last_rerank_timing = {"reranker_ms": 12.5, "model": "fake"}
+
+    def _build_fts5_index(self):
+        pass
+
+    async def hybrid_search_async(self, query, limit=5):
+        self.loops.append(id(asyncio.get_running_loop()))
+        return [{"metadata": {"file": "x.py"}, "identifier_boost": False}]
+
+
+async def _drive(module_name, cases_attr_unused=None):
+    import importlib
+
+    mod = importlib.import_module(module_name)
+    searcher = _FakeSearcher()
+    rows = await mod._run_all(searcher)
+    return searcher, rows
+
+
+async def test_o1_all_queries_share_one_loop():
+    """All queries in o1 _run_all execute on the SAME event loop."""
+    searcher, rows = await _drive("scripts.o1_holdout_gate")
+    assert len(rows) > 1
+    assert len(set(searcher.loops)) == 1, "queries ran on different event loops"
+    # +1: the discarded warm-up query also runs on the same loop.
+    assert len(searcher.loops) == len(rows) + 1
+    assert all(not r["degraded"] for r in rows), "fake timing must not flag degraded"
+
+
+async def test_p2_all_queries_share_one_loop():
+    """All queries in p2 _run_all execute on the SAME event loop."""
+    searcher, rows = await _drive("scripts.p2_holdout_gate")
+    assert len(rows) > 1
+    assert len(set(searcher.loops)) == 1, "queries ran on different event loops"
+    # +1: the discarded warm-up query also runs on the same loop.
+    assert len(searcher.loops) == len(rows) + 1
+    assert all(not r["degraded"] for r in rows), "fake timing must not flag degraded"
+
+
+async def test_live_probe_no_passthrough_cluster():
+    """Tiny live probe: 2 queries on ONE loop, no zero-ms passthrough cluster.
+
+    Skipped when live services are unavailable (unit-level guards above still run).
+    """
+    try:
+        from scripts.o1_holdout_gate import build_searcher
+    except Exception as e:  # noqa: BLE001 — import env issue -> skip live only
+        pytest.skip(f"live probe unavailable (import): {e}")
+    try:
+        searcher = build_searcher(ROOT)
+        timings = []
+        for q in ("warmup cold start primer", "hybrid_search_async BM25 vector"):
+            await searcher.hybrid_search_async(q, limit=3)
+            timings.append(dict(getattr(searcher, "_last_rerank_timing", None) or {}))
+    except Exception as e:  # noqa: BLE001 — services down -> skip live only
+        pytest.skip(f"live probe unavailable (services): {e}")
+    if searcher._multi_reranker is None:
+        pytest.skip("live probe unavailable: no reranker service (unit-level only)")
+    zeros = sum(1 for t in timings if not t.get("reranker_ms"))
+    assert zeros == 0, f"live passthrough cluster: {timings}"