diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index dd6fc47..048ddaa 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -65,6 +65,33 @@ jobs:
       - name: Tests
         run: uv run pytest
 
+  # uv.lock pins the newest releases, so the matrix never sees the oldest ones
+  # pyproject still admits. The floors were once set by hand and were wrong for
+  # three packages at once. This job installs the oldest direct dependency each
+  # floor allows and runs the suite on it, so a floor that no longer holds fails
+  # here, not on a user's machine.
+  lowest:
+    name: the oldest dependencies pyproject allows
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v7
+        with:
+          python-version: "3.12"
+
+      - name: Resolve the lowest direct dependencies
+        # uv sync alone keeps a lockfile that already satisfies pyproject, so
+        # the lowest resolution is locked first and then installed as written.
+        run: uv lock --resolution lowest-direct
+
+      - name: Sync dependencies
+        run: uv sync --frozen
+
+      - name: Tests
+        run: uv run --frozen pytest
+
   # The unit suite has twice now passed while the path a new user takes first
   # was broken: an adapter that raised a bare TypeError when a setting was
   # missing, and a local arm that failed every case with the reason visible
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0626c50..64cfd3d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -83,6 +83,19 @@ different event from one that moved because it was wrong.
 
 ### Fixed
 
+- The dependency floors in pyproject were never tested, and three were wrong:
+  scipy 1.14.0 has no wheel for Python 3.13, anthropic before 0.77 lacks the
+  structured output types the generative adapter uses, and typer before 0.16
+  breaks against click 8.2 (#47). The floors are now the oldest releases the
+  whole suite passes on: numpy 2.1, scipy 1.14.1, typer 0.16, typesafe-sdk
+  0.5.7, and anthropic 0.77. The two SDKs are capped below their next breaking
+  series, httpx is no longer declared (nothing imported it), and a new CI job
+  runs the suite on the oldest direct dependencies pyproject allows.
+- A broken or missing SDK took down every command, the mock included, because
+  the registry imported all four adapters at startup (#47). An adapter is now
+  imported the first time it is created, and one that cannot be says which
+  module is missing; the other adapters, and the adapters list, are unaffected.
+
 - A row whose response named no model was never priced, because pricing
   looked up only the model reported; the guard, which prices the requested
   model, and the report then disagreed about the same run (#36). Such a row is
diff --git a/pyproject.toml b/pyproject.toml
index b593acc..bb52ca6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,13 +13,15 @@ classifiers = [
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
 ]
+# Each floor is the oldest release the whole test suite passes on, checked by
+# the lowest-direct job in CI. The SDKs are capped below their next breaking
+# series, since an adapter depends on their request and response shapes.
 dependencies = [
     "numpy>=2.1",
-    "scipy>=1.14",
-    "typer>=0.15",
-    "httpx>=0.28",
-    "typesafe-sdk>=0.1",
-    "anthropic>=0.40",
+    "scipy>=1.14.1",
+    "typer>=0.16",
+    "typesafe-sdk>=0.5.7,<0.8",
+    "anthropic>=0.77,<2",
 ]
 
 [project.optional-dependencies]
@@ -43,6 +45,9 @@ dev = [
     "pytest-cov>=6.0",
     "ruff>=0.8",
     "mypy>=1.13",
+    # The CLI tests read stderr apart from stdout, which Click's test runner
+    # does from 8.2.
+    "click>=8.2",
 ]
 
 [build-system]
diff --git a/src/plumbline/adapters/__init__.py b/src/plumbline/adapters/__init__.py
index b0c8a6f..c8774c1 100644
--- a/src/plumbline/adapters/__init__.py
+++ b/src/plumbline/adapters/__init__.py
@@ -1,11 +1,30 @@
-"""Adapters, organized by transport rather than by vendor."""
+"""Adapters, organized by transport rather than by vendor.
+
+The adapter classes are imported on first access, not with the package, so a
+broken or missing SDK affects only the adapter that needs it.
+"""
+
+from __future__ import annotations
+
+import importlib
+from typing import Any
 
 from plumbline.adapters.base import Adapter, check_probability_semantics
-from plumbline.adapters.generative import GenerativeAdapter
-from plumbline.adapters.local_logits import LocalLogitsAdapter
-from plumbline.adapters.mock import MockAdapter
 from plumbline.adapters.registry import available, create, register
 
+_LAZY = {
+    "GenerativeAdapter": "plumbline.adapters.generative",
+    "LocalLogitsAdapter": "plumbline.adapters.local_logits",
+    "MockAdapter": "plumbline.adapters.mock",
+}
+
+
+def __getattr__(name: str) -> Any:
+    if name in _LAZY:
+        return getattr(importlib.import_module(_LAZY[name]), name)
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+
 __all__ = [
     "Adapter",
     "GenerativeAdapter",
diff --git a/src/plumbline/adapters/registry.py b/src/plumbline/adapters/registry.py
index c216223..25ddbff 100644
--- a/src/plumbline/adapters/registry.py
+++ b/src/plumbline/adapters/registry.py
@@ -8,33 +8,39 @@
 
 from __future__ import annotations
 
+import importlib
 import inspect
 from collections.abc import Callable
 from typing import Any
 
 from plumbline.adapters.base import Adapter
-from plumbline.types import UnknownAdapterError
+from plumbline.types import PlumblineError, UnknownAdapterError
 
 AdapterFactory = Callable[..., Adapter]
 
 _REGISTRY: dict[str, AdapterFactory] = {}
 
+#: Built-in adapters by module and class, imported the first time they are
+#: created. Importing all four at startup meant one broken or missing SDK took
+#: the whole command line down, the mock included.
+_BUILTINS: dict[str, tuple[str, str]] = {
+    "generative": ("plumbline.adapters.generative", "GenerativeAdapter"),
+    "local_logits": ("plumbline.adapters.local_logits", "LocalLogitsAdapter"),
+    "mock": ("plumbline.adapters.mock", "MockAdapter"),
+    "typesafe_wire": ("plumbline.adapters.typesafe_wire", "TypeSafeWireAdapter"),
+}
+
 
 def register(name: str, factory: AdapterFactory) -> None:
     """Bind ``name`` to a factory. Re-registering the same name is an error."""
-    if name in _REGISTRY:
+    if name in _REGISTRY or name in _BUILTINS:
         raise ValueError(f"adapter {name!r} is already registered")
     _REGISTRY[name] = factory
 
 
 def create(name: str, **config: Any) -> Adapter:
     """Build a configured adapter by registered name."""
-    try:
-        factory = _REGISTRY[name]
-    except KeyError:
-        raise UnknownAdapterError(
-            f"no adapter registered as {name!r}. Available: {available()!r}"
-        ) from None
+    factory = _factory(name)
 
     missing = _missing_arguments(factory, config)
     if missing:
@@ -63,6 +69,26 @@ def create(name: str, **config: Any) -> Adapter:
     return factory(**config)
 
 
+def _factory(name: str) -> AdapterFactory:
+    """The factory registered as ``name``, importing a built-in on first use."""
+    if name in _REGISTRY:
+        return _REGISTRY[name]
+    if name not in _BUILTINS:
+        raise UnknownAdapterError(f"no adapter registered as {name!r}. Available: {available()!r}")
+    module_name, class_name = _BUILTINS[name]
+    try:
+        module = importlib.import_module(module_name)
+    except ImportError as missing:
+        raise PlumblineError(
+            f"the {name} adapter could not be loaded, because a module it needs could not "
+            f"be imported: {missing}. The other adapters are unaffected. Reinstall "
+            "plumbline's dependencies (uv sync) to restore it."
+        ) from missing
+    factory: AdapterFactory = getattr(module, class_name)
+    _REGISTRY[name] = factory
+    return factory
+
+
 def _missing_arguments(factory: AdapterFactory, config: dict[str, Any]) -> list[str]:
     """Required parameters of ``factory`` that ``config`` does not supply.
 
@@ -85,20 +111,5 @@ def _missing_arguments(factory: AdapterFactory, config: dict[str, Any]) -> list[
 
 
 def available() -> tuple[str, ...]:
-    """Registered adapter names, sorted."""
-    return tuple(sorted(_REGISTRY))
-
-
-def _register_builtins() -> None:
-    from plumbline.adapters.generative import GenerativeAdapter
-    from plumbline.adapters.local_logits import LocalLogitsAdapter
-    from plumbline.adapters.mock import MockAdapter
-    from plumbline.adapters.typesafe_wire import TypeSafeWireAdapter
-
-    register("generative", GenerativeAdapter)
-    register("local_logits", LocalLogitsAdapter)
-    register("mock", MockAdapter)
-    register("typesafe_wire", TypeSafeWireAdapter)
-
-
-_register_builtins()
+    """Registered adapter names, sorted, without importing any of them."""
+    return tuple(sorted({*_REGISTRY, *_BUILTINS}))
diff --git a/tests/test_registry.py b/tests/test_registry.py
index 8154225..2c7d20f 100644
--- a/tests/test_registry.py
+++ b/tests/test_registry.py
@@ -33,3 +33,29 @@ def test_unknown_name_names_what_is_available() -> None:
 def test_duplicate_registration_is_an_error() -> None:
     with pytest.raises(ValueError, match="already registered"):
         registry.register("mock", MockAdapter)
+
+
+def test_a_broken_optional_sdk_breaks_only_the_adapter_that_needs_it() -> None:
+    """With anthropic unimportable, the mock must still run and generative must say why (#47).
+
+    Run in a fresh interpreter, so the SDK can be made to fail to import without
+    disturbing the modules this test session already loaded.
+    """
+    import subprocess
+    import sys
+
+    script = """
+import sys
+sys.modules["anthropic"] = None  # makes `import anthropic` raise ImportError
+from plumbline.adapters import registry
+assert "generative" in registry.available()
+registry.create("mock", gold_by_text={"t": "a"})
+try:
+    registry.create("generative")
+except Exception as error:
+    print(type(error).__name__, "|", error)
+"""
+    done = subprocess.run([sys.executable, "-c", script], capture_output=True, text=True)
+
+    assert done.returncode == 0, done.stderr
+    assert "PlumblineError" in done.stdout and "anthropic" in done.stdout
diff --git a/uv.lock b/uv.lock
index 83bb78d..a3380d8 100644
--- a/uv.lock
+++ b/uv.lock
@@ -1116,7 +1116,6 @@ version = "0.1.0"
 source = { editable = "." }
 dependencies = [
     { name = "anthropic" },
-    { name = "httpx" },
     { name = "numpy" },
     { name = "scipy" },
     { name = "typer" },
@@ -1131,6 +1130,7 @@ local = [
 
 [package.dev-dependencies]
 dev = [
+    { name = "click" },
     { name = "mypy" },
     { name = "pytest" },
     { name = "pytest-cov" },
@@ -1139,19 +1139,19 @@ dev = [
 
 [package.metadata]
 requires-dist = [
-    { name = "anthropic", specifier = ">=0.40" },
-    { name = "httpx", specifier = ">=0.28" },
+    { name = "anthropic", specifier = ">=0.77,<2" },
     { name = "numpy", specifier = ">=2.1" },
-    { name = "scipy", specifier = ">=1.14" },
+    { name = "scipy", specifier = ">=1.14.1" },
     { name = "torch", marker = "extra == 'local'", specifier = ">=2.5" },
     { name = "transformers", marker = "extra == 'local'", specifier = ">=4.46" },
-    { name = "typer", specifier = ">=0.15" },
-    { name = "typesafe-sdk", specifier = ">=0.1" },
+    { name = "typer", specifier = ">=0.16" },
+    { name = "typesafe-sdk", specifier = ">=0.5.7,<0.8" },
 ]
 provides-extras = ["local"]
 
 [package.metadata.requires-dev]
 dev = [
+    { name = "click", specifier = ">=8.2" },
     { name = "mypy", specifier = ">=1.13" },
     { name = "pytest", specifier = ">=8.3" },
     { name = "pytest-cov", specifier = ">=6.0" },