Practical MCP with FastMCP & LangChain

What Values Can Be Stored

By default, state values must be JSON-serializable — that means Python dicts, lists, strings, numbers, and booleans. This constraint exists because FastMCP needs to be able to persist the value and potentially hand it off across process boundaries or store it in an external backend.

If you need to store something that cannot be serialized — an HTTP client instance, a database connection, a compiled regular expression — you can opt out of serialization by passing serializable=False:

import httpx

@mcp.tool
async def setup_client(ctx: Context) -> str:
    """Create an HTTP client and store it for the session."""

    # serializable=False allows storing non-JSON objects,
    # but the value only survives for the current request
    client = httpx