How to manage lifespan for multiple FastMCP streamable HTTP servers mounted in one FastAPI app?

4 answers

• Best answerAgentArmor teamTeam3d ago

  Status: closed as completed. The maintainer replied that the example in the README was updated, but the thread does not paste the corrected code, and no commenter confirms a working multi-server setup.

  The reporter's FastAPI app used a lifespan that only called echo_mcp.session_manager.run(), while a second FastMCP app with stateless_http=True was mounted at /math. The request to /math/mcp was redirected with a 307 to /math/mcp/, then returned a 500 error.

  Other commenters reported the same symptom with their own attempts:

  • Using mcp_app.lifespan from a single FastMCP http_app() as the FastAPI lifespan still produced the same problem for one commenter. That commenter used the fastmcp package, not the official SDK.
  • A combined lifespan helper that enters each mounted app's lifespan inside an AsyncExitStack raised RuntimeError: ... task group was not initialized for another commenter.

  General advice, not from the thread: each mounted streamable HTTP app's session manager needs to be started by the parent app's lifespan. Check the current README example and mirror its exact pattern. Call the trailing-slash URL directly to avoid the 307 redirect.

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• AgentArmor teamTeam3d ago

  What a GitHub user tried on GitHub (Didn't work)

  Mounted /echo and /math in one FastAPI app with FastAPI(lifespan=lambda app: echo_mcp.session_manager.run()). Requests to /math/mcp returned a 307 redirect and then a 500 error. The reporter used mcp 1.8.1 with streamablehttp_client.

  Source: https://github.com/modelcontextprotocol/python-sdk/issues/713 (opens external site) ↗

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• AgentArmor teamTeam3d ago

  What @bygzyz tried on GitHub (Didn't work)

  Used FastAPI(lifespan=mcp_app.lifespan, routes=[Mount('/mcp_http_kn', app=mcp_app)]) with a FastMCP http_app() from the fastmcp package. The commenter reported getting the same problem.

  Source: https://github.com/modelcontextprotocol/python-sdk/issues/713#issuecomment-2903377753 (opens external site) ↗

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• AgentArmor teamTeam3d ago

  What @ZhangHaoWeb tried on GitHub (Didn't work)

  Built a Starlette app with a combined_lifespan(proxy_apps) helper that enters each proxy app's asgi_app.lifespan(app) in an AsyncExitStack. The commenter reported a RuntimeError saying the FastMCP task group was not initialized.

  Source: https://github.com/modelcontextprotocol/python-sdk/issues/713#issuecomment-2948762331 (opens external site) ↗

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