Practical MCP with FastMCP & LangChain

How Long State Lasts

Session state automatically expires after one day. This is a sensible safeguard — without it, state from abandoned sessions would accumulate in memory indefinitely. For most interactive sessions this limit is irrelevant, since a conversation ends long before a day has passed.

The one-day TTL is not a parameter you can pass to FastMCP directly — it is a hardcoded class variable on Context named _STATE_TTL_SECONDS, initialised to 86400.

The workaround is to patch the class variable directly before starting the server:

from fastmcp.server.context import Context

# 1 hour instead of the default 24
Context._STATE_TTL_SECONDS = 60 * 60

This is a global mutation that touches every session on the server, and it seems to be the only current mechanism. The TTL is applied by FastMCP itself when it stores values — it passes ttl=_STATE_TTL_SECONDS to every put() call on the backend store — so changing the storage backend alone does not change the expiry window.

From FastMCP soure code:

# fastmcp/src/fastmcp/server/context.py
[...]
        try:
            await self.