diff --git a/scripts/generate_skill_index.py b/scripts/generate_skill_index.py
index e63501a4..69df952c 100755
--- a/scripts/generate_skill_index.py
+++ b/scripts/generate_skill_index.py
@@ -94,6 +94,34 @@ def build_catalog() -> dict:
     }
 
 
+def load_existing_catalog() -> dict | None:
+    """Return the currently committed catalog, or None if it doesn't exist/parse."""
+    if not INDEX_PATH.exists():
+        return None
+    try:
+        return json.loads(INDEX_PATH.read_text())
+    except json.JSONDecodeError:
+        return None
+
+
+def pin_unchanged_generated_at(catalog: dict, existing: dict | None) -> None:
+    """Reuse the committed `generated_at` when the actual catalog content
+    (schema_version + skills) hasn't changed.
+
+    `generated_at` is a "last regenerated" timestamp, not build-time metadata:
+    it should only advance when a skill doc actually changes. Without this,
+    every run stamps today's date, which makes `--check` fail on pure
+    calendar drift (e.g. a PR branch cut before today, or CI running a day
+    after the last regeneration) even though no skill content is stale.
+    """
+    if (
+        existing is not None
+        and existing.get("schema_version") == catalog["schema_version"]
+        and existing.get("skills") == catalog["skills"]
+    ):
+        catalog["generated_at"] = existing.get("generated_at", catalog["generated_at"])
+
+
 def validate_catalog(catalog: dict) -> None:
     schema = json.loads(SCHEMA_PATH.read_text())
     validator = Draft202012Validator(schema)
@@ -137,6 +165,7 @@ def main() -> int:
 
     try:
         catalog = build_catalog()
+        pin_unchanged_generated_at(catalog, load_existing_catalog())
         validate_catalog(catalog)
     except ValueError as e:
         print(f"error: {e}", file=sys.stderr)
diff --git a/tests/test_skill_docs.py b/tests/test_skill_docs.py
index ed3d4aad..b1cfcee2 100644
--- a/tests/test_skill_docs.py
+++ b/tests/test_skill_docs.py
@@ -251,6 +251,62 @@ def test_generate_skill_index_round_trip(tmp_path: Path, monkeypatch: pytest.Mon
     assert mod.main() == 0
 
 
+def test_generate_skill_index_check_tolerates_stale_generated_at(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """A `--check` run must not fail purely because `generated_at` no longer
+    matches today's date. Real-world PR CI runs on a different calendar day
+    than the last regeneration on main, and that alone is not staleness.
+    """
+    repo_root = make_skill_tree(tmp_path)
+    mod = load_generate_skill_index()
+    patch_skill_index_paths(mod, repo_root)
+
+    monkeypatch.setattr(sys, "argv", ["generate_skill_index.py", "--write"])
+    assert mod.main() == 0
+
+    index_path = repo_root / "docs" / "skills" / "index.json"
+    md_path = repo_root / "docs" / "skills" / "index.md"
+
+    # Simulate calendar drift: back-date the committed files without
+    # touching any skill content.
+    data = json.loads(index_path.read_text())
+    data["generated_at"] = "2020-01-01"
+    index_path.write_text(json.dumps(data, indent=2) + "\n")
+    md_text = md_path.read_text().replace(
+        f"Generated: {mod.date.today().isoformat()}", "Generated: 2020-01-01"
+    )
+    md_path.write_text(md_text)
+
+    monkeypatch.setattr(sys, "argv", ["generate_skill_index.py", "--check"])
+    assert mod.main() == 0
+
+
+def test_generate_skill_index_check_still_fails_on_real_drift(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """A genuine content change without regeneration must still fail
+    `--check` — the date-drift tolerance must not weaken this protection.
+    """
+    repo_root = make_skill_tree(tmp_path)
+    mod = load_generate_skill_index()
+    patch_skill_index_paths(mod, repo_root)
+
+    monkeypatch.setattr(sys, "argv", ["generate_skill_index.py", "--write"])
+    assert mod.main() == 0
+
+    # Modify a skill doc's front matter without regenerating the index.
+    zeta_path = repo_root / "docs" / "skills" / "zeta.md"
+    zeta_path.write_text(
+        zeta_path.read_text().replace("Zeta purpose", "Totally different purpose")
+    )
+
+    monkeypatch.setattr(sys, "argv", ["generate_skill_index.py", "--check"])
+    assert mod.main() == 1
+    captured = capsys.readouterr()
+    assert "index.json is stale" in captured.err
+
+
 def test_generate_skill_index_rejects_missing_entry_point(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
 ) -> None: