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MCP Python SDK 1.x now expires idle Streamable HTTP sessions after 30 minutes and caps concurrent sessions at 10,000 by default
PR #3426, merged 2026-09-02T15:06Z, backports session lifecycle ownership to the 1.x line: `session_idle_timeout` existed on `StreamableHTTPSessionManager` but defaulted to `None` and could not be set from `FastMCP`, so at stock settings a session whose client never sent `DELETE` stayed registered until the process exited. Both defaults are now active where there was previously no limit, and a manager beyond `max_sessions` answers 503. This is a behavior change for anyone running a stateful MCP server today, and pass `None` on either keyword to restore the old unbounded behaviour.
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