diff --git a/api/routes/auth.py b/api/routes/auth.py
index 6083b875..baf5bdde 100644
--- a/api/routes/auth.py
+++ b/api/routes/auth.py
@@ -5,7 +5,11 @@ from api.db import db_client
 from api.db.models import UserModel
 from api.enums import OrganizationConfigurationKey, PostHogEvent
 from api.schemas.auth import AuthResponse, LoginRequest, SignupRequest, UserResponse
-from api.services.auth.depends import create_user_configuration_with_mps_key, get_user
+from api.services.auth.depends import (
+    create_user_configuration_with_mps_key,
+    get_user,
+    require_local_auth,
+)
 from api.services.configuration.ai_model_configuration import (
     convert_legacy_ai_model_configuration_to_v2,
 )
@@ -15,6 +19,7 @@ from api.utils.auth import create_jwt_token, hash_password, verify_password
 router = APIRouter(
     prefix="/auth",
     tags=["auth"],
+    dependencies=[Depends(require_local_auth)],
 )
 
 
diff --git a/api/services/auth/depends.py b/api/services/auth/depends.py
index 4225b6f0..488d45a4 100644
--- a/api/services/auth/depends.py
+++ b/api/services/auth/depends.py
@@ -20,6 +20,19 @@ from api.services.posthog_client import (
 )
 from api.utils.auth import decode_jwt_token
 
+
+async def require_local_auth() -> None:
+    """Reject email/password auth requests outside OSS (local) deployments.
+
+    The auth router stays mounted in every mode so the OpenAPI spec — and the
+    clients generated from it — don't vary with AUTH_PROVIDER; the gate has to
+    happen at request time. Without it, the SaaS deployment accepts
+    unauthenticated signups that mint oss_* users bypassing Stack Auth.
+    """
+    if AUTH_PROVIDER != "local":
+        raise HTTPException(status_code=404, detail="Not found")
+
+
 POSTHOG_ORGANIZATION_GROUP_TYPE = "organization"
 POSTHOG_ORGANIZATION_USES_MPS_BILLING_V2_PROPERTY = "uses_mps_billing_v2"