diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 9c9d083b..2cfb3a59 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -65,3 +65,8 @@
 **Vulnerability:** Path traversal in `media_shrinker.py` via unresolved `..` segments or symlink escapes before deriving conversion output paths.
 **Learning:** `Path.relative_to()` is only a lexical containment check unless both the source and root have first been resolved into canonical absolute paths. Relative paths and symlinks can otherwise bypass root-boundary assumptions.
 **Prevention:** Resolve both source and root once, reject sources outside the resolved root with a sanitized `MediaShrinkerError`, and derive `rel_source` from the resolved paths before planning outputs.
+
+## 2026-07-28 - [Sentinel: FastAPI hmac.compare_digest TypeError DoS]
+**Vulnerability:** Unhandled Exception causing Denial of Service (DoS) when comparing non-ASCII strings in `hmac.compare_digest`.
+**Learning:** Python's `hmac.compare_digest()` raises a `TypeError` if either argument is a string containing non-ASCII characters. Passing unsanitized user input (like HTTP headers) directly to this function can crash the request handling, leading to a potential DoS vulnerability.
+**Prevention:** Always encode user-provided strings and configured secrets to bytes (e.g., using `.encode('utf-8')`) before passing them to `hmac.compare_digest()`.
diff --git a/saas_web.py b/saas_web.py
index 63265e94..5b280f05 100644
--- a/saas_web.py
+++ b/saas_web.py
@@ -114,7 +114,7 @@ async def require_api_key(request: Request, call_next):
     if configured_keys and not (request.method == "GET" and request.url.path == "/"):
         provided_key = request.headers.get("x-api-key", "")
         if not any(
-            hmac.compare_digest(provided_key, key) for key in configured_keys
+            hmac.compare_digest(provided_key.encode('utf-8'), key.encode('utf-8')) for key in configured_keys
         ):
             return JSONResponse(
                 status_code=401,
diff --git a/tests/test_saas_web.py b/tests/test_saas_web.py
index 3b57e033..d08c2216 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -1222,6 +1222,25 @@ def test_video_content_type_accepted_by_validator(self):
             )
         )
 
+class TestSaaSHmacSecurity(unittest.TestCase):
+    def test_hmac_compare_digest_non_ascii(self):
+        from unittest import mock
+        from fastapi.testclient import TestClient
+        import saas_web
+
+        with mock.patch.dict("os.environ", {"CODEC_CARVER_API_KEYS": "test-key-1, test-key-2"}):
+            client = TestClient(saas_web.app)
+
+            # Test valid request (to ensure endpoint logic doesn't block it initially)
+            response = client.get("/jobs/123", headers={"X-API-Key": "test-key-1"})
+            self.assertNotEqual(response.status_code, 401)
+
+            # Test request with non-ASCII header
+            # Using raw bytes tuples to prevent client string decoding errors
+            headers = [(b"x-api-key", "한글키".encode("utf-8"))]
+            response = client.get("/jobs/123", headers=headers)
+            self.assertEqual(response.status_code, 401)
+            self.assertEqual(response.json(), {"error": "Invalid or missing API key"})
 
 if __name__ == "__main__":
     unittest.main()