fix(mcp): respect stateful routing and harden initialize detection

Ensure streamable MCP requests are dispatched via the computed target session manager, and guard initialize detection against non-object JSON bodies. Update stale-session test patches to target the stateful manager so routing assertions remain correct.

Made-with: Cursor

litellm/proxy/_experimental/mcp_server/server.py

@@ -2544,7 +2544,7 @@ if MCP_AVAILABLE:
             return False
         try:
             data = json.loads(body)
-            return data.get("method") == "initialize"
+            return isinstance(data, dict) and data.get("method") == "initialize"
         except (json.JSONDecodeError, TypeError):
             return False
 
@@ -2848,7 +2848,7 @@ if MCP_AVAILABLE:
                 mcp_servers,
                 _client_ip,
             ):
-                await session_manager.handle_request(scope, receive, send)
+                await target_manager.handle_request(scope, receive, send)
         except HTTPException:
             # Re-raise HTTP exceptions to preserve status codes and details
             raise

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

@@ -384,12 +384,12 @@ async def test_valid_mcp_session_id_is_preserved():
             True,
         ),
         patch.object(
-            session_manager,
+            session_manager_stateful,
             "handle_request",
             side_effect=mock_handle_request,
         ),
         patch.object(
-            session_manager,
+            session_manager_stateful,
             "_server_instances",
             mock_instances,
         ),