diff --git a/.github/workflows/cowork-auto-pr.yml b/.github/workflows/cowork-auto-pr.yml
index b27f04e..3a455a8 100644
--- a/.github/workflows/cowork-auto-pr.yml
+++ b/.github/workflows/cowork-auto-pr.yml
@@ -16,7 +16,7 @@ jobs:
       # without this step every run failed with "not a git repository" and no
       # PR was ever opened (fleet-wide defect: 11/11 seeded copies lacked it).
       - name: Check out the pushed branch
-        uses: actions/checkout@v4
+        uses: actions/checkout@d23441a48e516b6c34aea4fa41551a30e30af803 # v6
         with:
           ref: ${{ github.ref_name }}
           fetch-depth: 0
diff --git a/tests/test_ci_hygiene.py b/tests/test_ci_hygiene.py
new file mode 100644
index 0000000..707bd6b
--- /dev/null
+++ b/tests/test_ci_hygiene.py
@@ -0,0 +1,85 @@
+"""CI hygiene regression tests.
+
+Ensures workflow files follow security best practices:
+- All GitHub Actions are SHA-pinned (no mutable tags like @v4)
+- No silent-failure traps (|| true on validation steps)
+"""
+
+from __future__ import annotations
+
+import pytest
+import re
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+WORKFLOWS_DIR = REPO_ROOT / ".github" / "workflows"
+
+# Pattern: uses: OWNER/ACTION@REF
+# SHA-pinned refs are exactly 40 hex chars.
+# Mutable tags look like @v4, @v4.2.2, @main, @release/v1, etc.
+USES_PATTERN = re.compile(r"uses:\s*([^@\s]+)@(\S+)")
+SHA_PATTERN = re.compile(r"^[0-9a-f]{40}$")
+
+# Local/composite actions (e.g. ./.github/actions/foo) don't need SHA pins.
+LOCAL_ACTION_PREFIX = "./"
+
+
+class TestWorkflowHygiene:
+    """Regression guards for CI workflow security and correctness."""
+
+    @pytest.fixture
+    def workflow_files(self) -> list[Path]:
+        files = list(WORKFLOWS_DIR.glob("*.yml")) + list(WORKFLOWS_DIR.glob("*.yaml"))
+        if not files:
+            pytest.skip("No workflow files found")
+        return files
+
+    def test_all_actions_sha_pinned(self, workflow_files: list[Path]) -> None:
+        """Every remote action reference must use a 40-char SHA, not a mutable tag.
+
+        Mutable tags like @v4 can be silently moved to point at different commits,
+        creating a supply-chain attack vector. SHA pins lock the exact commit.
+        """
+        violations: list[str] = []
+        for wf in workflow_files:
+            for lineno, line in enumerate(wf.read_text(encoding="utf-8").splitlines(), 1):
+                match = USES_PATTERN.search(line)
+                if not match:
+                    continue
+                action, ref = match.group(1), match.group(2)
+                # Strip inline comments (e.g. "# v4.2.2")
+                ref = ref.split("#")[0].strip()
+                if action.startswith(LOCAL_ACTION_PREFIX):
+                    continue
+                if not SHA_PATTERN.match(ref):
+                    violations.append(f"{wf.name}:{lineno} {action}@{ref}")
+
+        assert not violations, (
+            f"Found {len(violations)} mutable action reference(s). "
+            "Pin to a 40-char SHA instead:\n" + "\n".join(violations)
+        )
+
+    def test_no_silent_failure_on_validation_steps(self, workflow_files: list[Path]) -> None:
+        """Validation/lint/test steps must not suppress failures with '|| true'.
+
+        A step whose purpose is to fail the build on defects (linters, type
+        checkers, security scanners) must not hide failures. This catches the
+        'validation theater' trap where a real check is neutered.
+        """
+        validation_keywords = ("lint", "check", "test", "audit", "scan", "format", "typecheck")
+        violations: list[str] = []
+        for wf in workflow_files:
+            lines = wf.read_text(encoding="utf-8").splitlines()
+            for lineno, line in enumerate(lines, 1):
+                stripped = line.strip()
+                if "|| true" not in stripped:
+                    continue
+                # Check if this line or the step name above contains a validation keyword
+                context = " ".join(lines[max(0, lineno - 5) : lineno]).lower()
+                if any(kw in context for kw in validation_keywords):
+                    violations.append(f"{wf.name}:{lineno} {stripped[:80]}")
+
+        assert not violations, (
+            f"Found {len(violations)} validation step(s) with '|| true' suppression. "
+            "Remove the suppression so failures are visible:\n" + "\n".join(violations)
+        )