Python FastMCP stdio server doesn't initialize

[unoti]

Describe the bug
Python MCP client cannot successfully initialize an MCP Stdio server.

To Reproduce

I have two reproduction steps. One for windows, and one for Linux. Additional notes and details are here in my reproduction notes but I will reproduce them here as well.

Windows Reproduction Steps

Create a new empty directory and cd to it, then:

1. Save this file as mcp_echo_server.py. This is intended to be the same as the sdk docs for echo server.
2. Save this file as mcp_client.py. This is intended to be as close as practical to the sdk docs for writing mcp clients.
   Install mcp and run:

uv venv --python=3.10 venv
.\venv\Scripts\activate.ps1
uv pip install mcp
python mcp_client.py

The output I get is this:

(venv) PS C:\Users\rilack\code\mcp-client-python-issue\src> python mcp_client.py
Ultra Basic MCP Client
sys.executable='C:\\Users\\rilack\\code\\mcp-client-python-issue\\venv\\Scripts\\python.exe'
Python version 3.10.16 (main, Mar 17 2025, 20:54:03) [MSC v.1943 64 bit (AMD64)]
Initializing session...

Note that we never see the message indicating that it finished initializing the session.

    print("Initializing session...") # This is the last thing we will see.
    await session.initialize()
    print("Session initialized successfully.") # If it's broken we won't see this.

Linux Container Reproduction Steps

There is a repo with these files but I am placing the info here as well.

Make an empty directy and go to it, then:

1. Save this as DOCKERFILE
2. Save this as src/mcp_client.py
3. Save this as src/mcp_echo_server.py
4. Build the dockerfile, from the root of the project use the following. I used podman but you can use docker:

   podman build --tag mcp-lab-08c --file DOCKERFILE .
   

5. Run the container. Replace your-src-path with the full pathname of the root above src:

   podman run -it --rm -v "your-src-path:/usr/code" mcp-lab-08c bash
   

   So for me, I used this to replace the path:

   podman run -it --rm -v "c:\Users\rilack\code\mcp-client-python-issue:/usr/code" mcp-lab-08c bash
   

6. Then from within the container:

   pip install mcp
   cd src
   python mcp_client.py

Observed output:

root@85b6bb0ac0e8:/usr/code/src# python mcp_client.py
Ultra Basic MCP Client
sys.executable='/usr/bin/python'
Python version 3.10.12 (main, Feb  4 2025, 14:57:36) [GCC 11.4.0]
Initializing session...

Note that we do not see further output indicating a successful initialization.

Expected behavior
We expect to see the message Session initialized successfully.

    print("Initializing session...") # This is the last thing we will see.
    await session.initialize()
    print("Session initialized successfully.") # If it's broken we won't see this.

Desktop (please complete the following information):

• OS: Windows 11, for container we used Ubuntu 22.04.

Additional context
I have tried this in many configurations, including Python 3.10 windows, Python 3.12 windows, Python 3.12 under WSL, and running Linux in a container, using the latest main branch and also the latest releases, and various forms of UV and global Python installations. I have also tried it using asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy()) in various Windows configurations. Under some configurations this leads to a new NotImplementedError, but under no circumstances was I able to make it work.

I haven't been able to make a Python client work yet. I can get node client and server working without difficulties, though.

It's possible that issue 382 is similar, although its
code for the repro is much more complex. I have endeavored to strip this down to the bare minimum of what is shown
in the sdk documentation. In fact the only difference in these repros versus the sdk documentation on clients is the example there shows something about handle_sampling_message and indicates it is optional. I was unable to figure out what that's for either by reading the docs or
studying the source for a while, so I removed that, but it doesn't help either way.

I am able to make node clients work without difficulty, but I really need a Python client, so I'm sticking with this.
I have not yet tried an HTTP/SSE client and will do so next, but I already have a stdio-only server that works
well for my use case in Claude desktop that I am trying to get working from a Python client.

[unoti]

This appears to be similar to #382 but the repro code here is smaller and as close as practical to the sdk docs. The repro steps here also fail both under Windows and Linux.

[unoti]

Update: Was able to make a Python client that connects to a node-based server.
Whatever is wrong here appears to be with *MCP Server rather than the client.
I was able to get mcp_client2.py to work on my machine under Windows, connecting against a node-based MCP server that I have installed which is working well with Claude Desktop.

Ultra Basic MCP Client
sys.executable='C:\\Users\\rilack\\code\\mcp-client-python-issue\\venv\\Scripts\\python.exe'
Python version 3.10.16 (main, Mar 17 2025, 20:54:03) [MSC v.1943 64 bit (AMD64)]
Initializing session...
Session initialized successfully.
Available tools:
- execute_command: Execute a terminal command with timeout. Command will continue running in background if it doesn't complete within timeout.
- read_output: Read new output from a running terminal session.
- force_terminate: Force terminate a running terminal session...

[danthekishan]

I am also having the similar issue in Mac (M2 Pro),

import asyncio
from mcp import ClientSession, StdioServerParameters, types
from mcp.client.stdio import stdio_client

# Create server parameters for stdio connection
server_params = StdioServerParameters(
    command="node",
    args=["/Users/xxx/src/server.js",],
    env=None,
)


async def run():
    async with stdio_client(server_params) as (read, write):
        async with ClientSession(read, write) as session:
            # Initialize the connection
            await session.initialize()
            tools = await session.list_tools()
            print(tools)



if __name__ == "__main__":
    asyncio.run(run())

I create a simple client like above, and used the MCP server that really worked other MCP clients. But for some reason, initialize() function is hanging, no responsive.

[danthekishan]

if I keep the script on running, I get this error

Error: Argument list too long (os error 7)

It seems like it is failing to because of very large args or env payload

[BenSmith]

For the server, the major thing I missed, embarrassingly, was that I needed to mcp install scripty.py it for claude, or run it with mcp dev script.py. Posting here so others can avoid this. It's really right up front in the readme

https://github.com/modelcontextprotocol/python-sdk?tab=readme-ov-file#quickstart

[iShaka]

You can try to add the following codes on the file mcp_echo_server.py

if __name__ == '__main__': mcp.run()

[Denrusn]

You can try to add the following codes on the file mcp_echo_server.py

if __name__ == '__main__': mcp.run()

我使用此方法运行成功

[Elijas]

It previously worked, now it doesn't anymore

Also all the other Python MCP Clients I tried (like the official MCP Python SDK and FastMCP MCP Client) don't work either.