diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 9c9d083b..796330f1 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.
+
+## 2026-10-08 - [Sentinel: Unhandled Exception DoS via hmac.compare_digest]
+**Vulnerability:** Denial of Service (CWE-400/CWE-754) due to unhandled `TypeError` when comparing non-ASCII API keys using `hmac.compare_digest()`.
+**Learning:** Passing strings containing non-ASCII characters directly to Python's `hmac.compare_digest()` raises a `TypeError`. In a FastAPI web application middleware, this results in an unhandled 500 Internal Server Error, allowing an attacker to DoS the application by sending malformed or international characters in the HTTP headers.
+**Prevention:** Always encode string arguments to bytes (e.g., using `.encode('latin1')` for Starlette request headers, or `.encode('utf-8')` for general strings) before comparing them with `hmac.compare_digest()`.
diff --git a/saas_web.py b/saas_web.py
index 63265e94..8067c609 100644
--- a/saas_web.py
+++ b/saas_web.py
@@ -113,8 +113,10 @@ async def require_api_key(request: Request, call_next):
     configured_keys = get_configured_api_keys()
     if configured_keys and not (request.method == "GET" and request.url.path == "/"):
         provided_key = request.headers.get("x-api-key", "")
+        # Starlette decodes headers using latin1 (ISO-8859-1) by default as per RFC.
+        provided_key_bytes = provided_key.encode("latin1", errors="replace")
         if not any(
-            hmac.compare_digest(provided_key, key) for key in configured_keys
+            hmac.compare_digest(provided_key_bytes, 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..cf7486c4 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -1222,6 +1222,22 @@ def test_video_content_type_accepted_by_validator(self):
             )
         )
 
+class TestUnicodeAPIKeys(unittest.TestCase):
+    @patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "한글키"})
+    def test_unicode_api_key_handles_safely(self):
+        from fastapi.testclient import TestClient
+        import saas_web
+        client = TestClient(saas_web.app)
+
+        # Test valid unicode key
+        response = client.get("/jobs/missing", headers=[(b"X-API-Key", "한글키".encode("utf-8"))])
+        self.assertNotEqual(response.status_code, 500)
+        self.assertNotEqual(response.status_code, 401)
+
+        # Test invalid unicode key
+        response = client.get("/jobs/missing", headers=[(b"X-API-Key", "틀린키".encode("utf-8"))])
+        self.assertEqual(response.status_code, 401)
+
 
 if __name__ == "__main__":
     unittest.main()