diff --git a/.github/workflows/mutation.yml b/.github/workflows/mutation.yml
index d5e2d09e..b2e6c6aa 100644
--- a/.github/workflows/mutation.yml
+++ b/.github/workflows/mutation.yml
@@ -74,8 +74,8 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 30
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-python@v7
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
         with:
           python-version: "3.12"
       - run: pip install -r ci/poetry-pin.txt --require-hashes
@@ -94,7 +94,7 @@ jobs:
     outputs:
       matrix: ${{ steps.groups.outputs.matrix }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
       - id: groups
         # The `groups` dispatch input was DECLARED and read nowhere: `-f groups=fee` was
         # silently ignored and the operator got the whole matrix. Same class as a CLI flag
diff --git a/tests/test_workflow_actions_are_sha_pinned.py b/tests/test_workflow_actions_are_sha_pinned.py
new file mode 100644
index 00000000..222c75e1
--- /dev/null
+++ b/tests/test_workflow_actions_are_sha_pinned.py
@@ -0,0 +1,80 @@
+"""Every `uses:` in every GitHub workflow must be pinned to a full commit SHA.
+
+A floating tag (`actions/checkout@v7`) resolves at run time to whatever that tag points at THEN.
+Whoever controls the tag controls what executes in CI, with the repository checked out and the
+job's secrets in scope. A SHA cannot be moved.
+
+WHY THIS IS A TEST AND NOT A SCRIPT. The repo already learned that a checker wired to nothing is
+not a checker: two mutation groups were added to `scripts/mutation_groups.py` and not to the CI
+matrix, and a coverage-omission script sat as a taskipy task nothing invoked. `pytest tests/` runs
+in CI unconditionally, so a test has no separate wiring to forget — the weakest link in
+"detect" is the wiring, and this removes it.
+
+HOW IT WAS FOUND. Dependabot #550 proposed bumping `actions/checkout@v4` -> `@v7` in
+`mutation.yml`. Merging it would have been an improvement in version and a regression in posture:
+every other checkout in the repo is SHA-pinned, and that PR would have left two floating. Pinning
+them to the SHA the rest of the repo already uses achieves the bump AND the consistency. A third
+straggler — `setup-python@v7`, the last floating tag in the repository — turned up only because
+fixing the two prompted a sweep for the whole set rather than the two the PR named.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+import pytest
+
+_WORKFLOWS = sorted((Path(__file__).resolve().parent.parent / ".github" / "workflows").glob("*.yml"))
+
+#: `uses: owner/repo@ref` or `uses: ./local/path`. Docker refs (`docker://`) are out of scope.
+_USES = re.compile(r"^\s*-?\s*uses:\s*(?P<ref>\S+)")
+_SHA = re.compile(r"^[0-9a-f]{40}$")
+
+
+def test_there_are_workflows_to_check() -> None:
+    """Guards the guard. A glob that silently matches nothing passes every assertion below."""
+    assert len(_WORKFLOWS) >= 5, f"only found {len(_WORKFLOWS)} workflow files; the glob is wrong"
+
+
+@pytest.mark.parametrize("path", _WORKFLOWS, ids=lambda p: p.name)
+def test_every_action_reference_is_sha_pinned(path: Path) -> None:
+    floating: list[tuple[int, str]] = []
+    for lineno, line in enumerate(path.read_text(encoding="utf-8").splitlines(), start=1):
+        m = _USES.match(line)
+        if not m:
+            continue
+        ref = m.group("ref")
+        if ref.startswith("./") or ref.startswith("docker://"):
+            continue  # a local composite action is this repo's own code; a docker ref is digest-pinned separately
+        _, _, version = ref.partition("@")
+        if not _SHA.match(version):
+            floating.append((lineno, line.strip()))
+
+    assert not floating, (
+        f"{path.name} references actions by a MOVABLE ref, so whoever controls the tag controls "
+        f"what runs in CI with this repo checked out:\n"
+        + "\n".join(f"  {path.name}:{n}  {text}" for n, text in floating)
+        + "\n\nPin to the full commit SHA and keep the version in a trailing comment, e.g.\n"
+        "  - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1"
+    )
+
+
+def test_the_same_action_is_pinned_to_ONE_sha_everywhere() -> None:
+    """Two SHAs for one action means a bump landed in some workflows and not others — the drift
+    that leaves a stale, possibly-vulnerable version running in the lane nobody looked at."""
+    seen: dict[str, set[str]] = {}
+    for path in _WORKFLOWS:
+        for line in path.read_text(encoding="utf-8").splitlines():
+            m = _USES.match(line)
+            if not m:
+                continue
+            ref = m.group("ref")
+            if ref.startswith("./") or ref.startswith("docker://"):
+                continue
+            name, _, version = ref.partition("@")
+            if _SHA.match(version):
+                seen.setdefault(name, set()).add(version)
+
+    split = {name: shas for name, shas in seen.items() if len(shas) > 1}
+    assert not split, f"these actions are pinned to more than one SHA across workflows: {split}"