fix(mcp): enforce required user_fields in MCPServerManager.call_tool

The streamable-HTTP entrypoint (server.execute_mcp_tool) gated tool
calls on a server's admin-declared user_fields, but the Responses API
path (LiteLLM_Proxy_MCP_Handler) bypassed that gate by calling
MCPServerManager.call_tool directly. With required fields unset, the
manager just resolved an empty value map and dispatched anyway, so the
upstream call went out without the configured headers/env.

Move the enforcement into MCPServerManager.call_tool itself so every
dispatch path raises the same friendly 401 (user_fields_missing +
config_url) when required values are absent.

litellm/proxy/_experimental/mcp_server/mcp_server_manager.py

@@ -1371,6 +1371,44 @@ class MCPServerManager:
         _, values = await _get_user_field_values_cached(mcp_server, user_api_key_auth)
         return values or {}
 
+    async def _enforce_required_user_fields(
+        self,
+        mcp_server: MCPServer,
+        user_api_key_auth: Optional["UserAPIKeyAuth"],
+    ) -> None:
+        """Raise the user_fields_missing 401 when required values are absent.
+
+        ``server.execute_mcp_tool`` runs the same gate for the streamable-HTTP
+        entrypoint, but ``call_tool`` is also called directly by the
+        Responses API path (``LiteLLM_Proxy_MCP_Handler``). Running the check
+        here covers every dispatch path through the manager so a caller
+        cannot skip enforcement by going around ``execute_mcp_tool``.
+        """
+        from litellm.proxy._experimental.mcp_server.user_fields import (
+            build_user_fields_missing_error,
+            compute_missing_user_fields,
+            server_has_user_fields,
+        )
+
+        if not server_has_user_fields(mcp_server):
+            return
+
+        stored_values = await self._resolve_user_field_values(
+            mcp_server, user_api_key_auth
+        )
+        missing = compute_missing_user_fields(mcp_server, stored_values or None)
+        if not missing:
+            return
+
+        from litellm.proxy._experimental.mcp_server.oauth_utils import (
+            _resolve_proxy_base_url_env,
+        )
+
+        detail = build_user_fields_missing_error(
+            mcp_server, missing, _resolve_proxy_base_url_env()
+        )
+        raise HTTPException(status_code=401, detail=detail)
+
     async def _create_mcp_client(
         self,
         server: MCPServer,
@@ -2956,6 +2994,8 @@ class MCPServerManager:
         if mcp_server is None:
             raise ValueError(f"Tool {name} not found")
 
+        await self._enforce_required_user_fields(mcp_server, user_api_key_auth)
+
         #########################################################
         # Pre MCP Tool Call Hook
         # Allow validation and modification of tool calls before execution

tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_user_fields.py

@@ -500,6 +500,74 @@ async def test_enforce_user_fields_no_user_id_raises():
 # ---------------------------------------------------------------------------
 
 
+@pytest.mark.asyncio
+async def test_manager_call_tool_enforces_required_user_fields():
+    """``MCPServerManager.call_tool`` is invoked directly by the Responses
+    API path, so it must raise the same friendly 401 as ``execute_mcp_tool``
+    when a required user-field has no stored value — otherwise a caller
+    can bypass enforcement and dispatch with the value silently dropped.
+    """
+    from fastapi import HTTPException
+
+    from litellm.proxy._experimental.mcp_server.mcp_server_manager import (
+        MCPServerManager,
+    )
+    from litellm.proxy._experimental.mcp_server.server import _user_fields_cache
+    from litellm.proxy._types import UserAPIKeyAuth
+
+    _user_fields_cache.clear()
+    srv = _gmail_server()
+    mgr = MCPServerManager()
+    mgr.registry["s1"] = srv
+    mgr.tool_name_to_mcp_server_name_mapping["gmail-prod-send"] = srv.name
+    mgr.tool_name_to_mcp_server_name_mapping["send"] = srv.name
+    user = UserAPIKeyAuth(api_key="hashed", user_id="responses-user")
+    _user_fields_cache[("responses-user", "s1")] = (None, 1e18)
+
+    with pytest.raises(HTTPException) as exc_info:
+        await mgr.call_tool(
+            server_name="gmail-prod",
+            name="send",
+            arguments={},
+            user_api_key_auth=user,
+        )
+    assert exc_info.value.status_code == 401
+    assert exc_info.value.detail["error"] == "user_fields_missing"
+    assert "GMAIL_TOKEN" in [
+        m["field_key"] for m in exc_info.value.detail["missing_fields"]
+    ]
+
+
+@pytest.mark.asyncio
+async def test_manager_call_tool_enforces_when_no_user_id():
+    """Anonymous callers cannot satisfy required user-fields, so the manager
+    must still surface ``user_fields_missing`` rather than silently dispatch
+    without the configured headers."""
+    from fastapi import HTTPException
+
+    from litellm.proxy._experimental.mcp_server.mcp_server_manager import (
+        MCPServerManager,
+    )
+    from litellm.proxy._types import UserAPIKeyAuth
+
+    srv = _gmail_server()
+    mgr = MCPServerManager()
+    mgr.registry["s1"] = srv
+    mgr.tool_name_to_mcp_server_name_mapping["gmail-prod-send"] = srv.name
+    mgr.tool_name_to_mcp_server_name_mapping["send"] = srv.name
+    user = UserAPIKeyAuth(api_key="hashed")  # no user_id
+
+    with pytest.raises(HTTPException) as exc_info:
+        await mgr.call_tool(
+            server_name="gmail-prod",
+            name="send",
+            arguments={},
+            user_api_key_auth=user,
+        )
+    assert exc_info.value.status_code == 401
+    assert exc_info.value.detail["error"] == "user_fields_missing"
+
+
 @pytest.mark.asyncio
 async def test_build_stdio_env_merges_user_field_env_over_static():
     """Stored user_field env vars must take precedence over static server.env."""