diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 9c9d083b..39fc1121 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.
+
+## 2026-07-20 - [Sentinel: Unhandled Exception DoS in HMAC Comparison]
+**Vulnerability:** Uncontrolled Resource Consumption (DoS) via Unhandled `TypeError` Exception (CWE-400 / CWE-754) when validating API keys.
+**Learning:** Python's `hmac.compare_digest` function throws a `TypeError: comparing strings with non-ASCII characters is not supported` if either string argument contains non-ASCII characters. Because the `X-API-Key` HTTP header is user-controlled, an attacker can send a request with a non-ASCII key (e.g., `X-API-Key: ö`), causing the FastAPI application to crash with a 500 Internal Server Error, bypassing the intended 401 Unauthorized response and potentially leading to a Denial of Service.
+**Prevention:** Always encode user-controlled strings to bytes (e.g., `.encode("utf-8")`) before passing them to `hmac.compare_digest` to ensure safe, constant-time comparison regardless of the input character set.
diff --git a/saas_web.py b/saas_web.py
index 63265e94..53b7e923 100644
--- a/saas_web.py
+++ b/saas_web.py
@@ -112,9 +112,9 @@ 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", "")
+        provided_key = request.headers.get("x-api-key", "").encode("utf-8")
         if not any(
-            hmac.compare_digest(provided_key, key) for key in configured_keys
+            hmac.compare_digest(provided_key, 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..78aa4af1 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -28,6 +28,34 @@
     client = TestClient(app)
 
 
+@unittest.skipUnless(
+    _HAS_FASTAPI, "fastapi not installed (optional integration dependency)"
+)
+class TestApiKeyAuthDoSMitigation(unittest.IsolatedAsyncioTestCase):
+    async def test_non_ascii_header_does_not_crash(self):
+        from fastapi import Request
+        import saas_web
+
+        # Setup a mock request with a non-ASCII API key header
+        scope = {
+            "type": "http",
+            "method": "POST",
+            "path": "/shrink",
+            "headers": [(b"x-api-key", "wrong-key-ö".encode("utf-8"))],
+        }
+        request = Request(scope)
+
+        async def mock_call_next(request):
+            return "SUCCESS"
+
+        with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret-key"}):
+            response = await saas_web.require_api_key(request, mock_call_next)
+
+        self.assertEqual(response.status_code, 401)
+        body = json.loads(response.body.decode('utf-8'))
+        self.assertEqual(body["error"], "Invalid or missing API key")
+
+
 @unittest.skipUnless(
     _HAS_FASTAPI, "fastapi not installed (optional integration dependency)"
 )