From 7ba28bb6dd4cc7926c6792b479086891b048af56 Mon Sep 17 00:00:00 2001
From: jibanez-staticduo <staticduo@gmail.com>
Date: Wed, 13 May 2026 17:40:04 +0200
Subject: [PATCH] fix(mcp): share invalid MCP client IP sentinel

Define a shared fail-closed MCP client IP sentinel and reuse it across Responses utilities, LazyMCP routing, and tests to avoid security-critical string drift.
---
 litellm/responses/mcp/litellm_proxy_mcp_handler.py         | 3 ++-
 litellm/responses/utils.py                                 | 4 +++-
 .../proxy/_experimental/mcp_server/test_mcp_server.py      | 7 +++++--
 .../responses/mcp/test_litellm_proxy_mcp_handler.py        | 5 +++--
 4 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/litellm/responses/mcp/litellm_proxy_mcp_handler.py b/litellm/responses/mcp/litellm_proxy_mcp_handler.py
index 7f3ad81434a..e6e0f47b008 100644
--- a/litellm/responses/mcp/litellm_proxy_mcp_handler.py
+++ b/litellm/responses/mcp/litellm_proxy_mcp_handler.py
@@ -20,6 +20,7 @@ from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLogging
 from litellm.proxy._experimental.mcp_server.utils import split_server_prefix_from_name
 from litellm.responses.main import aresponses
 from litellm.responses.streaming_iterator import BaseResponsesAPIStreamingIterator
+from litellm.responses.utils import INVALID_MCP_CLIENT_IP_SENTINEL
 from litellm.types.llms.openai import ResponsesAPIResponse
 from litellm.types.utils import (
     CallTypes,
@@ -463,7 +464,7 @@ class LiteLLM_Proxy_MCP_Handler:
             )
 
         standard_client_ip = (
-            None if client_ip == "__invalid_mcp_client_ip__" else client_ip
+            None if client_ip == INVALID_MCP_CLIENT_IP_SENTINEL else client_ip
         )
         return await LiteLLM_Proxy_MCP_Handler._get_standard_mcp_tools(
             user_api_key_auth=user_api_key_auth,
diff --git a/litellm/responses/utils.py b/litellm/responses/utils.py
index f2ccdc4cc7f..c19dabb0fec 100644
--- a/litellm/responses/utils.py
+++ b/litellm/responses/utils.py
@@ -32,6 +32,8 @@ from litellm.types.utils import (
     Usage,
 )
 
+INVALID_MCP_CLIENT_IP_SENTINEL = "__invalid_mcp_client_ip__"
+
 
 class ResponsesAPIRequestUtils:
     """Helper utils for constructing ResponseAPI requests"""
@@ -900,7 +902,7 @@ class ResponsesAPIRequestUtils:
             client_ip = secret_fields.get("mcp_client_ip")
             if isinstance(client_ip, str) and client_ip.strip():
                 return client_ip.strip()
-        return "__invalid_mcp_client_ip__"
+        return INVALID_MCP_CLIENT_IP_SENTINEL
 
 
 class ResponseAPILoggingUtils:
diff --git a/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py b/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
index 4e210e698eb..10a5d741a7f 100644
--- a/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
+++ b/tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_server.py
@@ -21,7 +21,10 @@ from litellm.proxy._types import (
     MCPTransport,
     UserAPIKeyAuth,
 )
-from litellm.responses.utils import ResponsesAPIRequestUtils
+from litellm.responses.utils import (
+    INVALID_MCP_CLIENT_IP_SENTINEL,
+    ResponsesAPIRequestUtils,
+)
 from litellm.types.mcp import MCPAuth
 from litellm.types.mcp_server.mcp_server_manager import MCPServer
 
@@ -967,7 +970,7 @@ def test_verified_mcp_client_ip_returns_value_or_fail_closed():
     )
     assert (
         ResponsesAPIRequestUtils.get_verified_mcp_client_ip({})
-        == "__invalid_mcp_client_ip__"
+        == INVALID_MCP_CLIENT_IP_SENTINEL
     )
 
 
diff --git a/tests/test_litellm/responses/mcp/test_litellm_proxy_mcp_handler.py b/tests/test_litellm/responses/mcp/test_litellm_proxy_mcp_handler.py
index 0155802415e..16080e01d08 100644
--- a/tests/test_litellm/responses/mcp/test_litellm_proxy_mcp_handler.py
+++ b/tests/test_litellm/responses/mcp/test_litellm_proxy_mcp_handler.py
@@ -10,6 +10,7 @@ import importlib
 from litellm.responses.mcp.litellm_proxy_mcp_handler import (
     LiteLLM_Proxy_MCP_Handler,
 )
+from litellm.responses.utils import INVALID_MCP_CLIENT_IP_SENTINEL
 from typing import Any, cast
 from litellm.types.llms.openai import ResponseAPIUsage, ResponsesAPIResponse
 from litellm.types.utils import ModelResponse
@@ -354,7 +355,7 @@ async def test_lazymcp_catalog_uses_fail_closed_client_ip(monkeypatch):
         client_ip=ResponsesAPIRequestUtils.get_verified_mcp_client_ip(None),
     )
 
-    assert captured["client_ip"] == "__invalid_mcp_client_ip__"
+    assert captured["client_ip"] == INVALID_MCP_CLIENT_IP_SENTINEL
 
 
 @pytest.mark.asyncio
@@ -950,7 +951,7 @@ async def test_standard_mcp_preserves_missing_client_ip_behavior(monkeypatch):
         mcp_tools_with_litellm_proxy=[
             {"type": "mcp", "server_url": "litellm_proxy/mcp/standard"}
         ],
-        client_ip="__invalid_mcp_client_ip__",
+        client_ip=INVALID_MCP_CLIENT_IP_SENTINEL,
     )
 
     assert captured["client_ip"] is None