MCP python server doesn't work in claude desktop app

[DanieleMorotti]

Hi,

I didn't find any similar issues, but please let me know if there are any existing ones.

I implemented a Python server as follows:

from mcp.server.fastmcp import FastMCP

# Create server
fast_mcp = FastMCP("MN API")

@fast_mcp.tool()
async def fetch_api_info(...):
...

@fast_mcp.tool()
async def get_user_info(...):
...


if __name__ == "__main__":
    fast_mcp.run(transport="sse")

When I run this file directly with python, it works without any issues. However, when I load it in the Claude desktop app using the following configuration:

{
    "mcpServers": {
        "Custom Server": {
            "command": "/path/to/.local/bin/uv",
            "args": [
                "--directory",
                "/path/to/mcp_test",
                "run",
                "server.py"
            ]
        }
    }
}

I receive the following error log:

2025-04-01T16:09:59.795Z [MN API Server] [info] Initializing server...
2025-04-01T16:09:59.804Z [MN API Server] [info] Server started and connected successfully
2025-04-01T16:09:59.805Z [MN API Server] [info] Message from client: {"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"claude-ai","version":"0.1.0"}},"jsonrpc":"2.0","id":0}
2025-04-01T16:10:01.152Z [MN API Server] [info] Initializing server...
2025-04-01T16:10:01.161Z [MN API Server] [info] Server started and connected successfully
2025-04-01T16:10:01.164Z [MN API Server] [info] Message from client: {"method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"claude-ai","version":"0.1.0"}},"jsonrpc":"2.0","id":0}
INFO:     Started server process [43926]
INFO:     Waiting for application startup.
INFO:     Started server process [43940]
INFO:     Application startup complete.
INFO:     Waiting for application startup.
INFO:     Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
INFO:     Application startup complete.
ERROR:    [Errno 48] error while attempting to bind on address ('0.0.0.0', 8000): address already in use
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
2025-04-01T16:10:05.018Z [MN API Server] [info] Server transport closed
2025-04-01T16:10:05.018Z [MN API Server] [info] Client transport closed
2025-04-01T16:10:05.018Z [MN API Server] [info] Server transport closed unexpectedly, this is likely due to the process exiting early. If you are developing this MCP server you can add output to stderr (i.e. `console.error('...')` in JavaScript, `print('...', file=sys.stderr)` in python) and it will appear in this log.
2025-04-01T16:10:05.019Z [MN API Server] [error] Server disconnected. For troubleshooting guidance, please visit our [debugging documentation](https://modelcontextprotocol.io/docs/tools/debugging) {"context":"connection"}
2025-04-01T16:10:05.019Z [MN API Server] [info] Client transport closed
2025-04-01T16:10:59.807Z [MN API Server] [info] Message from client: {"jsonrpc":"2.0","method":"notifications/cancelled","params":{"requestId":0,"reason":"Error: MCP error -32001: Request timed out"}}
2025-04-01T16:10:59.808Z [MN API Server] [info] Client transport closed
2025-04-01T16:10:59.809Z [MN API Server] [info] Server transport closed
2025-04-01T16:10:59.809Z [MN API Server] [info] Client transport closed
2025-04-01T16:10:59.809Z [MN API Server] [info] Server transport closed unexpectedly, this is likely due to the process exiting early. If you are developing this MCP server you can add output to stderr (i.e. `console.error('...')` in JavaScript, `print('...', file=sys.stderr)` in python) and it will appear in this log.
2025-04-01T16:10:59.809Z [MN API Server] [error] Server disconnected. For troubleshooting guidance, please visit our [debugging documentation](https://modelcontextprotocol.io/docs/tools/debugging) {"context":"connection"}
INFO:     Shutting down
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
INFO:     Finished server process [43926]
2025-04-01T16:10:59.946Z [MN API Server] [info] Server transport closed
2025-04-01T16:10:59.946Z [MN API Server] [info] Client transport closed

It appears that the server is started correctly, but then a parallel instance is launched, causing an address already in use error . This error doesn't occur when running the server.py file directly with Python.

Am I doing something wrong, or should it work as expected?

Thank you

[ishswar]

You are starting the server with SSE but in Client config you are not using that - so either change sse from code to stdio and try or use https://www.npmjs.com/package/mcp-remote - and then use that to connect to SSE server in Claud

[DanieleMorotti]

Okay, it works with stdio, thank you

[NilayYadav]

I'm getting Error: MCP error -32001: Request timed out even though I'm using server.run("stdio")

and in my claude config its

{
  "mcpServers": {
    "mcp-server": {
      "command": "/Library/Frameworks/Python.framework/Versions/3.11/bin/uv",
      "args": [
        "--directory",
       "/path/to/sever//server.py",
        "run",
        "server.py"
      ]
    }
  }
}

[ishswar]

@NilayYadav Config does not look correct - /path/to/sever//server.py looks like place holder you need to replace that with the correct path - and it is just a dir name the actual python file also needs to be replaced that is server.py in the above example

[NilayYadav]

I just used that dummy path as an example. I'm on a Mac and was getting a UV-related error, so I replaced 'uv' with my actual UV path using which uv.

…

On Thu, Apr 3, 2025, 1:25 AM Pranay Shah ***@***.***> wrote: @NilayYadav <https://github.com/NilayYadav> Config does not look correct - /path/to/sever//server.py looks like place holder you need to replace that with the correct path - and it is just a dir name the actual python file also needs to be replaced that is server.py in the above example — Reply to this email directly, view it on GitHub <#406 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/A4L3JGPIQPGITPTB52YR2HL2XQ6DJAVCNFSM6AAAAAB2HL5VT6VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDONZTGU4DGNZSGA> . You are receiving this because you were mentioned.Message ID: ***@***.***> [image: ishswar]*ishswar* left a comment (modelcontextprotocol/python-sdk#406) <#406 (comment)> @NilayYadav <https://github.com/NilayYadav> Config does not look correct - /path/to/sever//server.py looks like place holder you need to replace that with the correct path - and it is just a dir name the actual python file also needs to be replaced that is server.py in the above example — Reply to this email directly, view it on GitHub <#406 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/A4L3JGPIQPGITPTB52YR2HL2XQ6DJAVCNFSM6AAAAAB2HL5VT6VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDONZTGU4DGNZSGA> . You are receiving this because you were mentioned.Message ID: ***@***.***>