[BUG] TypeError: Controller is already closed when connecting to fastmcp Streamable HTTP transport

[Feng-YiJing-Dao]

Title: [BUG] TypeError: Controller is already closed when connecting to fastmcp Streamable HTTP transport

Inspector Version

v0.17.2

Describe the bug

When attempting to connect the MCP Inspector (using npx @modelcontextprotocol/inspector) to a Streamable HTTP service implemented with the Python library fastmcp, the Node.js proxy immediately crashes with a TypeError [ERR_INVALID_STATE]: Invalid state: Controller is already closed.

The connection works fine when the fastmcp service is running in Legacy SSE mode.

To Reproduce

This assumes a project set up with fastmcp similar to the user code provided (using mcp.run_http_async for Streamable HTTP mode).

1. Start the Server in Streamable HTTP Mode:

   # Assuming the server is running on port 8001 and mounted at /mcp
   python -m src.mcp_server.src.main shttp 
   # (Server starts and is accessible at [http://127.0.0.1:8001/mcp](http://127.0.0.1:8001/mcp))

2. Run the MCP Inspector:

   npx @modelcontextprotocol/inspector [http://127.0.0.1:8001/mcp](http://127.0.0.1:8001/mcp)

3. Observe the Crash: The command line output for the inspector will show the crash after initializing the StreamableHttp client transport.

Full Traceback Observed:

New StreamableHttp connection request
Query parameters: {"url":"[http://127.0.0.1:8001/mcp](http://127.0.0.1:8001/mcp)","transportType":"streamable-http"}
Created StreamableHttp client transport
Client <-> Proxy  sessionId: ...
Proxy  <-> Server sessionId: ...
Received POST message for sessionId ...
Received GET message for sessionId ...
node:internal/webstreams/readablestream:1068
      throw new ERR_INVALID_STATE.TypeError('Controller is already closed');
      ^

TypeError [ERR_INVALID_STATE]: Invalid state: Controller is already closed
    at ReadableStreamDefaultController.close (node:internal/webstreams/readablestream:1068:13)
    at PassThrough.<anonymous> (file:///E:/packages/npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/inspector/server/build/index.js:196:28)
    at PassThrough.emit (node:events:524:28)
    at endReadableNT (node:internal/streams/readable:1698:12)
    at process.processTicksAndRejections (node:internal/process/task_queues:90:21) {
  code: 'ERR_INVALID_STATE'
}

Expected behavior

The MCP Inspector should successfully connect to the Streamable HTTP endpoint, start the proxy, and open the browser interface without crashing, allowing the user to interact with the service tools.

Screenshots

Environment (please complete the following information):

• OS: Windows 10/11
• Node.js Version: v22.13.0
• Backend Framework: fastmcp (Python)

Additional context

• The same fastmcp service running in Legacy SSE mode (python -m src.mcp_server.src.main sse connected via http://127.0.0.1:8001/sse) works without issue, confirming the backend logic is sound.
• The issue seems to be isolated to the handling of the Streamable HTTP transport type within the inspector's Node.js proxy/client logic, specifically related to the ReadableStreamDefaultController.close() method being called multiple times.
• The service was successfully tested with Cherry Studio, which suggests the service implementation adheres to the MCP spec correctly.

[snemuri]

Running into the same issue.

[demian-licht-hs]

Likewise, is there any known workaround?

[gpierrick]

Same here. I tried to go to another version but same issue

[rperezll]

For me downgrading to 0.16.6 worked

[mathieu-benoit]

Same here, downgrading to v0.16.7 to workaround this issue:

npx --yes @modelcontextprotocol/inspector@0.16.7

[v1-azhar]

For me too, versions below v0.16.8 don't have this issue. Although v0.16.8 was working perfectly fine for me yesterday.

[FelixBaum]

Same for me, version v0.17.2 was working fine until yesterday. Now only v0.16.7 works.

[nexigo-nm]

same problem with spring-ai streamable http server

[cliffhall]

This will be fixed in the next release by #940

[rperezll]

For me the new version v0.17.5 works and fixes the problems. Thanks a lot team!!

[wouterds]

Confirmed as fixed in 0.17.5, thanks!

[mathieu-benoit]

Same here, fixed with 0.17.5, thanks!

[cliffhall]

Closing as fixed