diff --git a/scripts/check_readme_version.py b/scripts/check_readme_version.py
index 4f618529..9e17875b 100644
--- a/scripts/check_readme_version.py
+++ b/scripts/check_readme_version.py
@@ -69,9 +69,12 @@ def main(argv: list[str] | None = None) -> int:
     parser.add_argument(
         "--check",
         action="store_true",
-        help="Audit only and exit non-zero on drift (the CI-gate mode).",
+        help=(
+            "Exit non-zero on drift (the CI-gate mode). Without it, drift is "
+            "reported but the exit code stays 0 for informational, non-gating runs."
+        ),
     )
-    parser.parse_args(argv)
+    args = parser.parse_args(argv)
 
     root = _repo_root()
     drift = audit(root / "pyproject.toml", root / "README.md")
@@ -84,7 +87,7 @@ def main(argv: list[str] | None = None) -> int:
             "the README sample to match. See ADR 0013.",
             file=sys.stderr,
         )
-        return 1
+        return 1 if args.check else 0
     print("README sample-output version is in sync with the pyproject.toml anchor.")
     return 0
 
diff --git a/test/unit/test_check_readme_version.py b/test/unit/test_check_readme_version.py
new file mode 100644
index 00000000..c1d50ef5
--- /dev/null
+++ b/test/unit/test_check_readme_version.py
@@ -0,0 +1,59 @@
+"""Regression tests for ``scripts/check_readme_version.py``.
+
+AAASM-4942: the ``--check`` flag used to be a no-op — its parsed value was
+discarded and drift always forced a non-zero exit. These tests pin the two
+distinct modes so the flag can't silently regress back to a single mode:
+``--check`` gates (exit 1 on drift) while the default reports drift but exits 0.
+"""
+
+from __future__ import annotations
+
+import importlib.util
+from pathlib import Path
+from types import ModuleType
+
+import pytest
+
+_SCRIPT = Path(__file__).resolve().parents[2] / "scripts" / "check_readme_version.py"
+
+
+def _load_module() -> ModuleType:
+    spec = importlib.util.spec_from_file_location("check_readme_version", _SCRIPT)
+    assert spec is not None and spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+def _write_tree(root: Path, *, anchor: str, sample: str) -> None:
+    (root / "pyproject.toml").write_text(f'[project]\nversion = "{anchor}"\n', encoding="utf-8")
+    (root / "README.md").write_text(f"aasm --version   # e.g. aasm {sample}\n", encoding="utf-8")
+
+
+@pytest.fixture
+def script(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> tuple[ModuleType, Path]:
+    module = _load_module()
+    monkeypatch.setattr(module, "_repo_root", lambda: tmp_path)
+    return module, tmp_path
+
+
+def test_check_exits_nonzero_on_drift(script: tuple[ModuleType, Path]) -> None:
+    module, root = script
+    _write_tree(root, anchor="0.0.1rc6", sample="9.9.9-STALE")
+
+    assert module.main(["--check"]) == 1
+
+
+def test_default_reports_drift_but_exits_zero(script: tuple[ModuleType, Path]) -> None:
+    module, root = script
+    _write_tree(root, anchor="0.0.1rc6", sample="9.9.9-STALE")
+
+    assert module.main([]) == 0
+
+
+def test_both_modes_pass_when_in_sync(script: tuple[ModuleType, Path]) -> None:
+    module, root = script
+    _write_tree(root, anchor="0.0.1rc6", sample="0.0.1rc6")
+
+    assert module.main(["--check"]) == 0
+    assert module.main([]) == 0