diff --git a/applications/ColossalChat/start_code_verifier.py b/applications/ColossalChat/start_code_verifier.py
index d1924f610698..f49d2d421c7c 100644
--- a/applications/ColossalChat/start_code_verifier.py
+++ b/applications/ColossalChat/start_code_verifier.py
@@ -1,11 +1,15 @@
+import os
 from typing import List, Optional
 
 from coati.distributed.reward.code_reward.utils import check_correctness  # Assuming utils.py is in the same directory
-from fastapi import FastAPI, HTTPException
+from fastapi import FastAPI, Header, HTTPException
 from pydantic import BaseModel
 
 app = FastAPI()
 
+_API_KEY = os.environ.get("CODE_VERIFIER_API_KEY", "")
+_MAX_TIMEOUT = 30
+
 
 class CheckCorrectnessRequest(BaseModel):
     in_outs: Optional[dict]
@@ -21,12 +25,14 @@ class CheckCorrectnessResponse(BaseModel):
 
 
 @app.post("/check_correctness", response_model=CheckCorrectnessResponse)
-def check_correctness_api(request: CheckCorrectnessRequest):
+def check_correctness_api(request: CheckCorrectnessRequest, x_api_key: str = Header(...)):
+    if not _API_KEY or x_api_key != _API_KEY:
+        raise HTTPException(status_code=401, detail="Unauthorized")
     try:
         result, metadata = check_correctness(
             in_outs=request.in_outs,
             generation=request.generation,
-            timeout=request.timeout,
+            timeout=min(request.timeout, _MAX_TIMEOUT),
             debug=request.debug,
             eval_mode=request.eval_mode,
         )
diff --git a/tests/test_invariant_start_code_verifier.py b/tests/test_invariant_start_code_verifier.py
new file mode 100644
index 000000000000..95c179c5ee6f
--- /dev/null
+++ b/tests/test_invariant_start_code_verifier.py
@@ -0,0 +1,48 @@
+import importlib.util
+from pathlib import Path
+
+import pytest
+from fastapi.testclient import TestClient
+
+# Load the actual production app from the file under test
+spec = importlib.util.spec_from_file_location(
+    "start_code_verifier", Path(__file__).parent / "applications/ColossalChat/start_code_verifier.py"
+)
+module = importlib.util.load_from_spec(spec)
+spec.loader.exec_module(module)
+app = module.app
+
+client = TestClient(app, raise_server_exceptions=False)
+
+VALID_PAYLOAD = {"generation": "def f(x): return x", "in_outs": "{}", "timeout": 5, "debug": False, "eval_mode": "exec"}
+
+
+@pytest.mark.parametrize(
+    "headers,payload",
+    [
+        # Exact exploit: no auth, arbitrary code execution attempt
+        (
+            {},
+            {
+                "generation": "__import__('os').system('id')",
+                "in_outs": "{}",
+                "timeout": 10,
+                "debug": False,
+                "eval_mode": "exec",
+            },
+        ),
+        # Missing token (boundary: empty Authorization header)
+        ({"Authorization": ""}, VALID_PAYLOAD),
+        # Malformed token
+        ({"Authorization": "Bearer not.a.valid.jwt"}, VALID_PAYLOAD),
+        # Expired/fake token
+        ({"Authorization": "Bearer eyJhbGciOiJIUzI1NiJ9.eyJleHAiOjF9.invalid"}, VALID_PAYLOAD),
+    ],
+)
+def test_check_correctness_requires_authentication(headers, payload):
+    """Invariant: /check_correctness must reject unauthenticated or invalidly-authenticated requests with 401 or 403."""
+    response = client.post("/check_correctness", json=payload, headers=headers)
+    assert response.status_code in (401, 403), (
+        f"Expected 401 or 403 for unauthenticated request, got {response.status_code}. "
+        "The endpoint must not be publicly accessible without valid credentials."
+    )