diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 858d9d42..b72e7612 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -1,3 +1,8 @@
+## 2026-08-09 - [Sentinel: Unhandled TypeError in hmac.compare_digest]
+**Vulnerability:** Unhandled Exception (CWE-754) / DoS via non-ASCII characters in `hmac.compare_digest`.
+**Learning:** `hmac.compare_digest` requires ASCII-only strings or bytes. Passing an HTTP header containing non-ASCII characters (parsed as a Python string by ASGI servers like Uvicorn) causes a `TypeError`, resulting in an unhandled 500 error instead of a secure 401 rejection.
+**Prevention:** When comparing potentially untrusted strings using `hmac.compare_digest`, encode both strings to bytes (e.g., using `.encode("utf-8")`) beforehand to safely handle any character input and prevent runtime exceptions.
+
 ## 2026-05-28 - [Sentinel Fixes: Temp Files & Injection]
 **Vulnerability:** Predictable Temp Files (CWE-377) and Insecure Default Permissions (CWE-276), plus Command Injection via FFmpeg Filtergraph (CWE-20).
 **Learning:** Python's `Path.with_name` plus a suffix string to make a temp file opens a race condition because it's predictable and the permissions default to system `umask` which might expose secret `0600` data. Additionally, interpolating variables directly into FFmpeg filtergraph strings allows arbitrary filter injection.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4cca1ced..1f1e16cb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,3 +11,4 @@
 
 ### Fixed
 - 단일·일괄 대상 크기 입력을 비웠을 때 이전 custom validity와 `aria-invalid` 상태를 즉시 초기화해 현재 필수 입력 상태를 정확히 전달합니다.
+- `x-api-key` 헤더에 ASCII가 아닌 문자가 포함될 때 `hmac.compare_digest`에서 발생하던 `TypeError`를 수정했습니다.
diff --git a/saas_web.py b/saas_web.py
index 0ef95a1e..192ec5fd 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/test_direct.py b/test_direct.py
new file mode 100644
index 00000000..e5f6b575
--- /dev/null
+++ b/test_direct.py
@@ -0,0 +1,11 @@
+from fastapi import Request
+from starlette.testclient import TestClient
+from saas_web import app
+import os
+from unittest.mock import patch
+
+with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret-key"}):
+    client = TestClient(app)
+    req = client.build_request("POST", "/shrink", headers=[(b"x-api-key", b"\xff")])
+    response = client.send(req)
+    print(response.status_code)
diff --git a/test_encode.py b/test_encode.py
new file mode 100644
index 00000000..c3561af4
--- /dev/null
+++ b/test_encode.py
@@ -0,0 +1,10 @@
+import hmac
+
+provided_key = "\xff"
+configured_keys = ["secret-key"]
+
+try:
+    any(hmac.compare_digest(provided_key.encode("utf-8"), key.encode("utf-8")) for key in configured_keys)
+    print("UTF-8 works")
+except Exception as e:
+    print(repr(e))
diff --git a/test_hmac_direct.py b/test_hmac_direct.py
new file mode 100644
index 00000000..7a27425c
--- /dev/null
+++ b/test_hmac_direct.py
@@ -0,0 +1,33 @@
+from fastapi import FastAPI, Request
+from fastapi.testclient import TestClient
+from fastapi.responses import JSONResponse
+import hmac
+import os
+
+app = FastAPI()
+
+@app.middleware("http")
+async def require_api_key(request: Request, call_next):
+    configured_keys = ["secret-key"]
+    provided_key = request.headers.get("x-api-key", "")
+    try:
+        if not any(
+            hmac.compare_digest(provided_key, key) for key in configured_keys
+        ):
+            return JSONResponse(status_code=401, content={"error": "Invalid"})
+    except Exception as e:
+        return JSONResponse(status_code=500, content={"error": repr(e)})
+    return await call_next(request)
+
+@app.get("/")
+def read_root():
+    return {"Hello": "World"}
+
+client = TestClient(app, raise_server_exceptions=False)
+import requests
+# Using requests to bypass httpx's strict ASCII header checks to see if the server handles it
+try:
+    response = requests.get("http://localhost:8000/", headers={"x-api-key": "안녕"})
+    print(f"Status Code: {response.status_code}")
+except Exception as e:
+    print(e)
diff --git a/test_hmac_direct2.py b/test_hmac_direct2.py
new file mode 100644
index 00000000..3dc9fe8d
--- /dev/null
+++ b/test_hmac_direct2.py
@@ -0,0 +1,44 @@
+from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse
+import hmac
+import os
+import uvicorn
+import threading
+import time
+import socket
+
+app = FastAPI()
+
+@app.middleware("http")
+async def require_api_key(request: Request, call_next):
+    configured_keys = ["secret-key"]
+    provided_key = request.headers.get("x-api-key", "")
+    try:
+        if not any(
+            hmac.compare_digest(provided_key, key) for key in configured_keys
+        ):
+            return JSONResponse(status_code=401, content={"error": "Invalid"})
+    except Exception as e:
+        print(f"Server caught exception: {repr(e)}")
+        return JSONResponse(status_code=500, content={"error": repr(e)})
+    return await call_next(request)
+
+@app.get("/")
+def read_root():
+    return {"Hello": "World"}
+
+def run_server():
+    uvicorn.run(app, host="127.0.0.1", port=8000, log_level="error")
+
+t = threading.Thread(target=run_server, daemon=True)
+t.start()
+time.sleep(1)
+
+# Manually send a raw HTTP request with non-ASCII header
+req = b"GET / HTTP/1.1\r\nHost: localhost:8000\r\nx-api-key: \xff\r\n\r\n"
+s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+s.connect(("127.0.0.1", 8000))
+s.sendall(req)
+resp = s.recv(4096)
+print(resp.decode("latin-1"))
+s.close()
diff --git a/test_hmac_encode.py b/test_hmac_encode.py
new file mode 100644
index 00000000..e8a11a88
--- /dev/null
+++ b/test_hmac_encode.py
@@ -0,0 +1,6 @@
+import hmac
+try:
+    hmac.compare_digest("hello".encode('utf-8'), "안녕".encode('utf-8'))
+    print("Bytes work")
+except Exception as e:
+    print(f"Exception: {repr(e)}")
diff --git a/test_hmac_exception.py b/test_hmac_exception.py
new file mode 100644
index 00000000..5782743e
--- /dev/null
+++ b/test_hmac_exception.py
@@ -0,0 +1,7 @@
+import hmac
+try:
+    hmac.compare_digest("hello", "world")
+    print("ASCII works")
+    hmac.compare_digest("hello", "안녕")
+except Exception as e:
+    print(f"Exception: {repr(e)}")
diff --git a/test_hmac_header.py b/test_hmac_header.py
new file mode 100644
index 00000000..56b32c07
--- /dev/null
+++ b/test_hmac_header.py
@@ -0,0 +1,27 @@
+from fastapi import FastAPI, Request
+from fastapi.testclient import TestClient
+from fastapi.responses import JSONResponse
+import hmac
+import os
+
+app = FastAPI()
+
+@app.middleware("http")
+async def require_api_key(request: Request, call_next):
+    configured_keys = ["secret-key"]
+    provided_key = request.headers.get("x-api-key", "")
+    if not any(
+        hmac.compare_digest(provided_key, key) for key in configured_keys
+    ):
+        return JSONResponse(status_code=401, content={"error": "Invalid"})
+    return await call_next(request)
+
+@app.get("/")
+def read_root():
+    return {"Hello": "World"}
+
+client = TestClient(app)
+response = client.get("/", headers={"x-api-key": "안녕"})
+print(f"Status Code: {response.status_code}")
+if response.status_code == 500:
+    print("Vulnerability confirmed!")
diff --git a/test_hmac_latin1.py b/test_hmac_latin1.py
new file mode 100644
index 00000000..5df5ae3f
--- /dev/null
+++ b/test_hmac_latin1.py
@@ -0,0 +1,27 @@
+from fastapi import FastAPI, Request
+from fastapi.testclient import TestClient
+from fastapi.responses import JSONResponse
+import hmac
+import os
+
+app = FastAPI()
+
+@app.middleware("http")
+async def require_api_key(request: Request, call_next):
+    configured_keys = ["secret-key"]
+    provided_key = request.headers.get("x-api-key", "")
+    if not any(
+        hmac.compare_digest(provided_key, key) for key in configured_keys
+    ):
+        return JSONResponse(status_code=401, content={"error": "Invalid"})
+    return await call_next(request)
+
+@app.get("/")
+def read_root():
+    return {"Hello": "World"}
+
+client = TestClient(app, raise_server_exceptions=False)
+response = client.get("/", headers={"x-api-key": b"\xff".decode("latin-1")})
+print(f"Status Code: {response.status_code}")
+if response.status_code == 500:
+    print("Vulnerability confirmed!")
diff --git a/tests/test_saas_web.py b/tests/test_saas_web.py
index cd45dbc3..d36466ff 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -639,6 +639,17 @@ def test_wrong_key_rejected(self):
         self.assertEqual(response.json(), {"error": "Invalid or missing API key"})
         self.assertNotIn("secret-key", response.text)
 
+    def test_non_ascii_key_rejected_safely(self):
+        with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret-key"}):
+            # httpx test client refuses to send non-ASCII strings as headers;
+            # we must bypass its validation by explicitly passing raw bytes for the request
+            # to simulate an attacker sending raw bytes over the wire.
+            req = client.build_request("POST", "/shrink", headers=[(b"x-api-key", b"\xff")])
+            response = client.send(req)
+
+        self.assertEqual(response.status_code, 401)
+        self.assertEqual(response.json(), {"error": "Invalid or missing API key"})
+
     def test_correct_key_reaches_handler(self):
         with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret-key"}):
             response = self._post_shrink(headers={"X-API-Key": "secret-key"})