fix: address Greptile round 3 feedback

- initialize_guardrail: validate mode='pre_mcp_call' at init time — misconfigured
  mode silently bypasses JWT injection, which is a zero-trust bypass
- _build_claims: remove duplicate inline 'import re' (module-level import already present)
- _types.py: add TODO comment explaining jwt_claims is forward-compat plumbing
  for a follow-up PR that will forward upstream IdP claims into outbound MCP JWTs

litellm/proxy/_types.py

@@ -2471,6 +2471,9 @@ class UserAPIKeyAuth(
         Any
     ] = None  # Expanded created_by user when expand=user is used
     end_user_object_permission: Optional[LiteLLM_ObjectPermissionTable] = None
+    # TODO: jwt_claims carries decoded upstream IdP claims (groups, roles, etc.) so
+    # guardrails can forward them into outbound tokens (e.g. MCPJWTSigner). Currently
+    # populated but not yet consumed — forward-compat hook for a follow-up PR.
     jwt_claims: Optional[Dict] = None
 
     model_config = ConfigDict(arbitrary_types_allowed=True)

litellm/proxy/guardrails/guardrail_hooks/mcp_jwt_signer/__init__.py

@@ -19,6 +19,13 @@ def initialize_guardrail(
     if not guardrail_name:
         raise ValueError("MCPJWTSigner guardrail requires a guardrail_name")
 
+    mode = litellm_params.mode
+    if mode != "pre_mcp_call":
+        raise ValueError(
+            f"MCPJWTSigner guardrail '{guardrail_name}' has mode='{mode}' but must use "
+            "mode='pre_mcp_call'. JWT injection only fires for MCP tool calls."
+        )
+
     optional_params = getattr(litellm_params, "optional_params", None)
 
     def _get(key):  # type: ignore[no-untyped-def]

litellm/proxy/guardrails/guardrail_hooks/mcp_jwt_signer/mcp_jwt_signer.py

@@ -277,8 +277,6 @@ class MCPJWTSigner(CustomGuardrail):
         # tool call JWTs should not carry enumeration permissions.
         # Tool names are sanitized (alphanumeric + _ and -) before embedding
         # so path-traversal or malformed scope values cannot be injected.
-        import re
-
         raw_tool_name: str = data.get("mcp_tool_name", "")
         tool_name = re.sub(r"[^a-zA-Z0-9_\-]", "_", raw_tool_name) if raw_tool_name else ""
         if tool_name: