MCP SDK Client Python Issue

Created Issue 395 on MCP Python Github

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.

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...

This repo is to demonstrate and diagnose an issue we're having with the Python MCP stdio client. What happens is it never finishes initializes the server. What I expect to happen is for it to initialize the server successfully so that we can start communicating with it.

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.

Repro steps, Windows and UV

From the root directory of the project (above src):

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

Output under windows:

(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...

And that is the last thing that we see.

Repro steps, Docker, Ubuntu

To eliminate the variables that this could be an issue with Windows or UV, I also tested this with Linux. I have tried this both with Python 3.12 and 3.10 with the same results.

Build container

I used podman to build and run my containers, but you can use docker instead.

From the root of the project:

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

Run container

Replace your-src-path to the directory of your local machine where this project is installed.

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

So for me, I used:

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

Then from the container:

pip install mcp
cd src
python mcp_client.py

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...