diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 9c9d083b..ae0f636d 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -1,3 +1,8 @@
+## 2026-07-28 - [Sentinel: Unhandled Exception in hmac.compare_digest]
+**Vulnerability:** Unhandled Exception (CWE-754) leading to 500 Internal Server Error via non-ASCII header injection.
+**Learning:** `hmac.compare_digest` throws a `TypeError` when comparing strings containing non-ASCII characters. If client headers are decoded and contain such characters, comparing them against ASCII keys causes the application to crash instead of failing securely (401).
+**Prevention:** Always encode strings to `utf-8` bytes before passing them to `hmac.compare_digest` to ensure robust comparison and secure failure for malformed inputs.
+
 ## 2026-07-25 - [Cross-platform upload basename normalization]
 **Behavior:** Upload metadata now interprets both forward slashes and backslashes as path separators before extracting a basename.
 **Learning:** On POSIX systems, `pathlib.Path(filename).name` retains backslashes because they are ordinary characters there. That caused inconsistent manifest and converter filenames for Windows-style client paths. The upload itself is still written inside a trusted temporary workspace, and batch archive entry names are generated outputs; this change does not establish a filesystem traversal or archive-entry escape.
diff --git a/saas_web.py b/saas_web.py
index 63265e94..071b1419 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_hmac.py b/test_hmac.py
new file mode 100644
index 00000000..70d20985
--- /dev/null
+++ b/test_hmac.py
@@ -0,0 +1,53 @@
+from fastapi import FastAPI, Request
+from fastapi.testclient import TestClient
+from fastapi.responses import JSONResponse
+import hmac
+import traceback
+import uvicorn
+import httpx
+import asyncio
+
+app = FastAPI()
+
+configured_keys = ["validkey"]
+
+@app.middleware("http")
+async def require_api_key(request: Request, call_next):
+    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 or missing API key"})
+    except Exception as e:
+        print("Exception:", e)
+        traceback.print_exc()
+        return JSONResponse(status_code=500, content={"error": "Server error"})
+    return await call_next(request)
+
+@app.get("/test")
+def test():
+    return {"status": "ok"}
+
+async def run_test():
+    config = uvicorn.Config(app, port=8888, log_level="info")
+    server = uvicorn.Server(config)
+    task = asyncio.create_task(server.serve())
+    await asyncio.sleep(1) # wait for server to start
+
+    # Use raw socket to bypass httpx ascii check
+    import socket
+    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    s.connect(("127.0.0.1", 8888))
+
+    # send raw bytes
+    req = b"GET /test HTTP/1.1\r\nHost: 127.0.0.1:8888\r\nx-api-key: invalid\xc3\xb1\r\n\r\n"
+    s.sendall(req)
+
+    resp = s.recv(4096)
+    print("Response:\n", resp.decode('latin1'))
+    s.close()
+
+    server.should_exit = True
+    await task
+
+if __name__ == "__main__":
+    asyncio.run(run_test())
diff --git a/test_hmac_2.py b/test_hmac_2.py
new file mode 100644
index 00000000..bdf3244b
--- /dev/null
+++ b/test_hmac_2.py
@@ -0,0 +1,9 @@
+import hmac
+
+provided_key = "invalid\xc3\xb1" # simulated latin-1 decode from ASGI
+key = "validkey"
+
+try:
+    hmac.compare_digest(provided_key, key)
+except Exception as e:
+    print(e)
diff --git a/test_hmac_3.py b/test_hmac_3.py
new file mode 100644
index 00000000..baa1b23d
--- /dev/null
+++ b/test_hmac_3.py
@@ -0,0 +1,10 @@
+import hmac
+
+provided_key = "invalid\xc3\xb1" # simulated latin-1 decode from ASGI
+key = "validkey"
+
+try:
+    hmac.compare_digest(provided_key.encode('utf-8'), key.encode('utf-8'))
+    print("Success")
+except Exception as e:
+    print(e)