diff --git a/pyproject.toml b/pyproject.toml
index 4f89956..9ecae01 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,7 +13,11 @@ dependencies = ["boto3>=1.40.0", "botocore>=1.43.0", "pyyaml>=6.0"]
 # and coverage; pytest-randomly surfaces cross-file import-order coupling. Install
 # with `pip install -e '.[test]'` (mirrors the Makefile/CI invocation).
 [project.optional-dependencies]
-test = ["pytest>=8.0", "hypothesis>=6.0", "pytest-randomly>=3.15", "coverage>=7.0", "mcp>=1.0", "anyio[trio]>=4.0", "pytest-anyio>=0.0.0"]
+# `tomli` is the 3.10-only TOML reader (stdlib `tomllib` arrives in 3.11). Without it the
+# packaging tripwire — which reads pyproject.toml as data — silently SKIPPED its whole
+# file on 3.10, the project's own declared minimum, taking the INV-MCP-2 wheel guard with
+# it. Marker-gated so 3.11+ does not install a redundant dependency.
+test = ["pytest>=8.0", "hypothesis>=6.0", "pytest-randomly>=3.15", "coverage>=7.0", "mcp>=1.0", "anyio[trio]>=4.0", "pytest-anyio>=0.0.0", "tomli>=2.0; python_version<'3.11'"]
 mcp = ["mcp>=1.0"]
 
 [project.scripts]
diff --git a/tests/test_packaging.py b/tests/test_packaging.py
index 1ee0738..cc18c6c 100644
--- a/tests/test_packaging.py
+++ b/tests/test_packaging.py
@@ -17,10 +17,21 @@
 
 import pytest
 
+# TOML reader. `tomllib` is stdlib from 3.11; on 3.10 — which `requires-python = ">=3.10"`
+# declares as supported — the standard backport `tomli` provides the identical API.
+#
+# This used to be `tomllib = None` on 3.10, and the fixture skipped. The consequence was
+# that EVERY test reading pyproject.toml — 7 of this file's 12, including the INV-MCP-2
+# packaging guard added in round 20 — was inert on the project's own minimum version. CI
+# covers 3.11+ so the guard still had teeth there, but a guard that evaporates on the
+# floor version is a guard with a documented hole in it.
 if sys.version_info >= (3, 11):
     import tomllib
-else:  # pragma: no cover — py3.10 fallback
-    tomllib = None
+else:  # pragma: no cover — py3.10 path, exercised in the 3.10 CI leg
+    try:
+        import tomli as tomllib  # type: ignore[no-redef]
+    except ImportError:  # pragma: no cover
+        tomllib = None  # type: ignore[assignment]
 
 _REPO_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 PYPROJECT = os.path.join(_REPO_ROOT, "pyproject.toml")