ProxyUser Serialization Error When Invoking MCP Tools on LangGraph Cloud

[Insomniacboy]

When invoking MCP tools in a LangGraph deployed to LangGraph Cloud, tool executions fail with a serialization error:

{"error": "Unable to serialize unknown type: <class 'langgraph_api.auth.custom.ProxyUser'>"}

Environment

• Platform: LangGraph Cloud
• Tools: MCP tools (via langchain-mcp-adapters.MultiServerMCPClient)
• Transport: SSE (Server-Sent Events)
• Python Version: 3.1.12

Root Cause
LangGraph Cloud injects authentication context (a ProxyUser object) into the RunnableConfig under config['configurable'] with private keys (e.g., __auth). When tools are invoked via tool.ainvoke(args, config=config), the full config object (including the non-serializable ProxyUser) is passed through to the tool execution layer. During tool execution, LangChain or the MCP adapter attempts to serialize the config for logging/tracing/checkpointing, which fails because ProxyUser is not JSON-serializable.

• The error only occurs on LangGraph Cloud; local/self-hosted LangGraph runs work fine (no ProxyUser injection)
• The error occurs regardless of tool transport type (SSE, stdio, streamable_http)

[Insomniacboy]

Also see the following error after I stripped ProxyUser:
{
"error": "Unable to serialize unknown type: <class 'langchain_core.callbacks.manager.AsyncCallbackManager'>"
}