diff --git a/api/.env.example b/api/.env.example
index ae76209..568fc81 100644
--- a/api/.env.example
+++ b/api/.env.example
@@ -21,8 +21,15 @@ SLACK_WEBHOOK_URL=
 # SLACK_WEBHOOK_SECURITY=
 # SLACK_WEBHOOK_PAYMENTS=
 # SLACK_WEBHOOK_INTEREST=
-# Dev-only session token signing (see api/auth.py) — replaced by Clerk later. Use a random value, never reuse across environments.
+# Dev-only session token signing (see api/auth.py). Only used when DEV_MODE=true.
+# Use a random value, never reuse across environments.
 SESSION_SECRET=dev-secret-change-me
+# Clerk. REQUIRED in production: with DEV_MODE unset or false, the API refuses to
+# boot without CLERK_JWKS_URL, because auth would otherwise fall back to the
+# dev-only HMAC token above. Leave unset for local work and CI.
+# CLERK_JWKS_URL=https://<your-clerk-subdomain>/.well-known/jwks.json
+# CLERK_ISSUER=https://<your-clerk-subdomain>
+# CLERK_SECRET_KEY=
 # Encrypts nfc_tags.aes_key_encrypted at rest (see api/services/nfc_crypto.py).
 # Generate with: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
 NFC_KEY_ENCRYPTION_SECRET=
diff --git a/api/auth.py b/api/auth.py
index 3fbba9e..4a33445 100644
--- a/api/auth.py
+++ b/api/auth.py
@@ -1,8 +1,11 @@
-"""Platform authentication — venue/role identity for /dashboard and /admin routes.
+"""Platform authentication: venue/role identity for /dashboard and /admin routes.
 
-DEV-ONLY STUB: gamespec.md specifies Clerk for real auth (2FA, sessions,
-roles). Until a Clerk dev instance is wired in, sessions are simple
-HMAC-signed tokens issued by POST /api/auth/dev-login (DEV_MODE only).
+Two modes. Production verifies real Clerk RS256 JWTs against CLERK_JWKS_URL.
+DEV_MODE additionally accepts HMAC-signed tokens issued by POST
+/api/auth/dev-login, for local work, CI and the sim tools. _verify_token refuses
+the HMAC path outright outside DEV_MODE, so it is unreachable in production even
+when CLERK_JWKS_URL is missing; that case warns at import and fails every
+dashboard and admin token, while leaving the patron game routes untouched.
 
 The contract below (get_current_user / require_role) is the permanent
 pattern every dashboard/admin route should depend on. Swapping in real
@@ -13,6 +16,7 @@
 import hashlib
 import hmac
 import os
+import sys
 import time
 from typing import Optional
 
@@ -28,11 +32,21 @@
 TOKEN_TTL_SECONDS = 60 * 60 * 12  # 12 hours — dev convenience only
 
 # Clerk mode: set CLERK_JWKS_URL (+ CLERK_ISSUER) to verify real Clerk RS256 JWTs.
-# Unset -> falls back to the dev-login HMAC token, so dev/CI are unaffected and prod
-# "just works" once these env vars exist (the swap this module was designed for).
+# Unset -> falls back to the dev-login HMAC token, which is correct for dev/CI and
+# unacceptable in production, where _verify_token refuses it outright.
 CLERK_JWKS_URL = os.getenv("CLERK_JWKS_URL")
 CLERK_ISSUER = os.getenv("CLERK_ISSUER")
 CLERK_SECRET_KEY = os.getenv("CLERK_SECRET_KEY")
+
+# Outside DEV_MODE, Clerk is the only acceptable auth path. Warn loudly here and
+# refuse the HMAC path in _verify_token, rather than raising: this module is
+# imported by the whole app, so raising would take the patron game routes down
+# too, and they never touch Clerk. A missing JWKS URL must break owner and admin
+# login only, never a table full of people mid-session.
+if os.getenv("DEV_MODE") != "true" and not CLERK_JWKS_URL:
+    print("WARNING: CLERK_JWKS_URL is not set outside DEV_MODE -- dashboard and "
+          "admin auth will reject every token until it is configured",
+          file=sys.stderr)
 # Auto-provisioning allowlist: a first-login user whose email is here becomes an
 # 'admin'; everyone else becomes a 'venue_owner' (no venue yet -> setup wizard).
 ADMIN_EMAILS = {e.strip().lower() for e in os.getenv("ADMIN_EMAILS", "").split(",") if e.strip()}
@@ -82,6 +96,13 @@ def _verify_token(token: str) -> str:
             # tools + the dashboard dev-login). In production, Clerk is the only path.
             if os.getenv("DEV_MODE") != "true":
                 raise HTTPException(status_code=401, detail="Invalid token")
+
+    # Defence in depth. The boot guard makes an unconfigured production
+    # unreachable, but the HMAC path below must never run outside DEV_MODE even
+    # if that guard is later loosened.
+    if os.getenv("DEV_MODE") != "true":
+        raise HTTPException(status_code=401, detail="Invalid token")
+
     try:
         payload, expires_at, signature = base64.urlsafe_b64decode(token.encode()).decode().rsplit(":", 2)
     except (ValueError, UnicodeDecodeError):
diff --git a/api/tests/test_auth_production_guard.py b/api/tests/test_auth_production_guard.py
new file mode 100644
index 0000000..7f34aba
--- /dev/null
+++ b/api/tests/test_auth_production_guard.py
@@ -0,0 +1,74 @@
+"""Production must never fall back to the dev HMAC token.
+
+api/auth.py has two auth paths: real Clerk JWTs (CLERK_JWKS_URL) and dev-login
+HMAC tokens. Previously, an unconfigured production silently used the second
+one, making SESSION_SECRET the only thing protecting a venue dashboard.
+
+The fix is a refusal inside _verify_token, not a refusal to start: this module
+is imported by every router, so a hard failure here would also take down the
+patron game routes, which never use Clerk at all. A missing JWKS URL therefore
+warns loudly and breaks owner and admin login only.
+
+The import checks run in a subprocess so import-time behaviour is observed
+cleanly, without touching the suite's already-imported api.auth.
+"""
+import os
+import subprocess
+import sys
+
+import pytest
+from fastapi import HTTPException
+
+import api.auth
+
+
+REPO_ROOT = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+
+def _import_auth_with(env_overrides):
+    """Import api.auth in a clean subprocess. Returns (returncode, stderr)."""
+    env = {**os.environ, **env_overrides}
+    for key, value in env_overrides.items():
+        if value is None:
+            env.pop(key, None)
+    proc = subprocess.run(
+        [sys.executable, "-c", "import api.auth"],
+        cwd=REPO_ROOT, env=env, capture_output=True, text=True,
+    )
+    return proc.returncode, proc.stderr
+
+
+def test_production_without_clerk_jwks_warns_but_still_boots():
+    """A missing JWKS URL must be loud, but must not take the app down.
+
+    api.auth is imported by every router, so raising here would kill the patron
+    game routes, which never touch Clerk. The security guarantee is enforced in
+    _verify_token instead (see the HMAC test below), not by refusing to start.
+    """
+    code, stderr = _import_auth_with({"DEV_MODE": "false", "CLERK_JWKS_URL": None})
+    assert code == 0, f"api.auth refused to import, taking patron routes with it:\n{stderr}"
+    assert "CLERK_JWKS_URL is not set" in stderr
+
+
+def test_production_with_clerk_jwks_boots():
+    code, stderr = _import_auth_with({
+        "DEV_MODE": "false",
+        "CLERK_JWKS_URL": "https://example.test/.well-known/jwks.json",
+    })
+    assert code == 0, f"api.auth failed to import with CLERK_JWKS_URL set:\n{stderr}"
+
+
+def test_dev_mode_without_clerk_jwks_still_boots():
+    code, stderr = _import_auth_with({"DEV_MODE": "true", "CLERK_JWKS_URL": None})
+    assert code == 0, f"api.auth failed to import in DEV_MODE:\n{stderr}"
+
+
+def test_hmac_token_is_rejected_outside_dev_mode(monkeypatch):
+    """A validly-signed dev token must not authenticate when DEV_MODE is off."""
+    token = api.auth.issue_dev_token("user_whatever")
+    assert api.auth._verify_token(token) == "user_whatever"  # DEV_MODE on: accepted
+
+    monkeypatch.setenv("DEV_MODE", "false")
+    with pytest.raises(HTTPException) as exc:
+        api.auth._verify_token(token)
+    assert exc.value.status_code == 401