diff --git a/app/api/agentops/auth/environment.py b/app/api/agentops/auth/environment.py
index b7b2fd604..f48b349ac 100644
--- a/app/api/agentops/auth/environment.py
+++ b/app/api/agentops/auth/environment.py
@@ -1,10 +1,28 @@
 import os
+import secrets
 from agentops.api.log_config import logger
 
 # generate an AUTH_COOKIE_SECRET with:
 # import secrets; print(secrets.token_hex(32))
-_DEV_AUTH_COOKIE_SECRET = "your_cookie_signing_secret"
-AUTH_COOKIE_SECRET = os.getenv("AUTH_COOKIE_SECRET", _DEV_AUTH_COOKIE_SECRET)
+#
+# This value signs the session cookie JWT, so it must be unique and high-entropy
+# per deployment. Never fall back to a committed literal: a signing key that is
+# present in the source tree lets anyone forge a session cookie that passes
+# signature verification, and the fallback would silently be used whenever the
+# variable is unset.
+AUTH_COOKIE_SECRET = os.getenv("AUTH_COOKIE_SECRET")
+
+if not AUTH_COOKIE_SECRET:
+    # Fall back to an ephemeral, per-process secret so local development works
+    # without a committed signing key. Cookies signed with this value are
+    # invalidated on restart and are not valid across replicas, so it is only
+    # suitable for development.
+    AUTH_COOKIE_SECRET = secrets.token_hex(32)
+    logger.warning(
+        "[agentops.auth.environment] AUTH_COOKIE_SECRET is not set; generated a "
+        "random per-process secret. Sessions will not survive a restart and "
+        "will not be shared across replicas. Set AUTH_COOKIE_SECRET in production."
+    )
 
 AUTH_COOKIE_NAME = os.getenv("AUTH_COOKIE_NAME", "session_id")
 AUTH_JWT_ALGO = "HS256"  # this is for our internal JWT on the session cookie
@@ -24,9 +42,6 @@
 SUPABASE_JWT_SECRET: str = os.getenv("JWT_SECRET_KEY")
 
 
-if AUTH_COOKIE_SECRET == _DEV_AUTH_COOKIE_SECRET:
-    logger.warning("[agentops.auth.environment] Using an unsafe AUTH_COOKIE_SECRET")
-
 if not SUPABASE_JWT_SECRET:
     logger.warning("[agentops.auth.environment] No JWT_SECRET_KEY set")
 
diff --git a/app/api/tests/auth/test_session_cookie.py b/app/api/tests/auth/test_session_cookie.py
new file mode 100644
index 000000000..24545cb37
--- /dev/null
+++ b/app/api/tests/auth/test_session_cookie.py
@@ -0,0 +1,84 @@
+"""
+Tests for the session cookie JWT helpers.
+
+The session cookie is signed with `AUTH_COOKIE_SECRET`. These tests pin two
+properties:
+
+1. A cookie signed with the literal that used to be committed as the fallback
+   secret must NOT verify, so the known-key forgery path stays closed.
+2. Encoding and decoding a session cookie round-trips the session ID.
+"""
+
+from unittest.mock import patch
+from uuid import uuid4
+
+import jwt
+import pytest
+
+from agentops.auth import views as auth_views
+from agentops.auth.environment import AUTH_COOKIE_SECRET, AUTH_JWT_ALGO
+from agentops.auth.exceptions import AuthException
+from agentops.auth.session import Session
+
+# The literal that used to be the committed fallback in `auth/environment.py`.
+# It is now only referenced here, to prove it can no longer verify a cookie.
+LEGACY_DEV_SECRET = "your_cookie_signing_secret"
+
+
+def _forge_cookie(session_id: str, secret: str) -> str:
+    """Mint a session cookie signed with an arbitrary secret."""
+    return jwt.encode({"session_id": session_id}, secret, algorithm=AUTH_JWT_ALGO)
+
+
+def test_committed_fallback_secret_is_not_in_use():
+    """The signing secret must never be the value that was committed to the repo."""
+    assert AUTH_COOKIE_SECRET != LEGACY_DEV_SECRET
+    assert AUTH_COOKIE_SECRET
+
+
+def test_cookie_signed_with_legacy_secret_is_rejected():
+    """A forged cookie signed with the old committed key must not decode."""
+    forged = _forge_cookie(str(uuid4()), LEGACY_DEV_SECRET)
+
+    with pytest.raises(AuthException, match="Could not decode internal session JWT."):
+        auth_views._decode_session_cookie(forged)
+
+
+def test_cookie_signed_with_legacy_secret_is_rejected_even_for_known_session():
+    """Signature verification happens before the session lookup, so a forged
+    cookie cannot be redeemed even when it names a session that exists."""
+    session = Session(session_id=uuid4(), user_id=uuid4())
+    forged = _forge_cookie(str(session.session_id), LEGACY_DEV_SECRET)
+
+    with patch.object(auth_views.Session, "get", return_value=session):
+        with pytest.raises(AuthException, match="Could not decode internal session JWT."):
+            auth_views._decode_session_cookie(forged)
+
+
+def test_session_cookie_round_trip():
+    """A cookie signed with the configured secret decodes back to its session."""
+    session = Session(session_id=uuid4(), user_id=uuid4())
+    cookie = auth_views._encode_session_cookie(session)
+
+    with patch.object(auth_views.Session, "get", return_value=session) as get_session:
+        assert auth_views._decode_session_cookie(cookie) == session
+
+    get_session.assert_called_once_with(str(session.session_id))
+
+
+def test_decoding_unknown_session_returns_none():
+    """A well-signed cookie for a session that no longer exists returns None."""
+    session = Session(session_id=uuid4(), user_id=uuid4())
+    cookie = auth_views._encode_session_cookie(session)
+
+    with patch.object(auth_views.Session, "get", return_value=None):
+        assert auth_views._decode_session_cookie(cookie) is None
+
+
+def test_cookie_payload_only_contains_session_id():
+    """The cookie carries the session ID and nothing else."""
+    session = Session(session_id=uuid4(), user_id=uuid4())
+    cookie = auth_views._encode_session_cookie(session)
+
+    decoded = jwt.decode(cookie, AUTH_COOKIE_SECRET, algorithms=[AUTH_JWT_ALGO])
+    assert decoded == {"session_id": str(session.session_id)}