Describe the bug

When using a stdio_client to connect to a FastMCP server on Windows, the connection fails with an McpError: Invalid request parameters. This occurs even with a minimal, "hello world" server setup, which suggests a fundamental incompatibility between these components on Windows.

Affected version

Note: I am not using Docker. I installed the library using pip.

To find the version, please run this command in your terminal and paste the output below:

pip show mcp

(If your package is named differently, like github-mcp-server, please use that name instead of mcp)

Steps to reproduce the behavior

1. Create a file named test_server.py with the following code:

   Python

   # test_server.py
   from mcp.server.fastmcp import FastMCP
   mcp = FastMCP("simple-test-server")
   @mcp.tool()
   async def hello(name: str) -> str:
    """A simple tool that returns a greeting."""
    return f"Hello, {name}"
   if __name__ == "__main__":
    mcp.run()
   

2. Create another file in the same directory named test_client.py with the following code:

   Python

   # test_client.py
   import asyncio
   import sys
   from types import SimpleNamespace
   from mcp import ClientSession
   from mcp.client.stdio import stdio_client
   async def run_test_client():
    server_config = SimpleNamespace(
    command=sys.executable,
    args=["test_server.py"], 
    env=None, 
    cwd=None, 
    encoding='utf-8',
    encoding_error_handler='ignore'
    )
    print("--- Starting simple server ---")
    async with stdio_client(server_config) as (read_stream, write_stream):
    async with ClientSession(read_stream, write_stream) as session:
    tools = await session.list_tools()
    print("Available tools:", [tool.name for tool in tools])
    result = await session.call_tool("hello", {"name": "World"})
    print("\nServer says:", result.output_text)
   if __name__ == "__main__":
    asyncio.run(run_test_client())
   

3. From the terminal, run the client: python test_client.py

4. See the error traceback in the output.

Expected vs actual behavior

Expected behavior: The client should connect successfully, print Available tools: ['hello'], and then print Server says: Hello, World.

Actual behavior: The client fails immediately when trying to list the tools, raising an McpError: Invalid request parameters.

Logs

--- Starting simple server ---

• Exception Group Traceback (most recent call last):
  | File "D:\CODE\projects\Currency_Agent\answer\test_client.py", line 32, in
  | asyncio.run(run_test_client())
  | File "C:\Users[username]\AppData\Local\Programs\Python\Python311\Lib\asyncio\runners.py", line 190, in run
  | return runner.run(main)
  | ^^^^^^^^^^^^^^^^
  | File "C:\Users[username]\AppData\Local\Programs\Python\Python311\Lib\asyncio\runners.py", line 118, in run
  | return self.loop.run_until_complete(task)
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  | File "C:\Users[username]\AppData\Local\Programs\Python\Python311\Lib\asyncio\base_events.py", line 653, in run_until_complete
  | return future.result()
  | ^^^^^^^^^^^^^^^
  | File "D:\CODE\projects\Currency_Agent\answer\test_client.py", line 21, in run_test_client
  | async with stdio_client(server_config) as (read_stream, write_stream):
  | File "C:\Users[username]\AppData\Local\Programs\Python\Python311\Lib\contextlib.py", line 222, in aexit
  | await self.gen.athrow(typ, value, traceback)
  | File "D:\CODE\projects\Currency_Agent.venv\Lib\site-packages\mcp\client\stdio_init.py", line 180, in stdio_client
  | async with (
  | File "D:\CODE\projects\Currency_Agent.venv\Lib\site-packages\anyio_backends_asyncio.py", line 772, in aexit
  | raise BaseExceptionGroup(
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
  | Exception Group Traceback (most recent call last):
  | File "D:\CODE\projects\Currency_Agent.venv\Lib\site-packages\mcp\client\stdio_init_.py", line 187, in stdio_client
  | yield read_stream, write_stream
  | File "D:\CODE\projects\Currency_Agent\answer\test_client.py", line 22, in run_test_client
  | async with ClientSession(read_stream, write_stream) as session:
  | File "D:\CODE\projects\Currency_Agent.venv\Lib\site-packages\anyio_backends_asyncio.py", line 772, in aexit
  | raise BaseExceptionGroup(
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
  | Traceback (most recent call last):
  | File "D:\CODE\projects\Currency_Agent\answer\test_client.py", line 24, in run_test_client
  | tools = await session.list_tools()
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^
  | File "D:\CODE\projects\Currency_Agent.venv\Lib\site-packages\mcp\client\session.py", line 386, in list_tools
  | result = await self.send_request(
  | ^^^^^^^^^^^^^^^^^^^^^^^^
  | File "D:\CODE\projects\Currency_Agent.venv\Lib\site-packages\mcp\shared\session.py", line 286, in send_request
  | raise McpError(response_or_error.error)
  | mcp.shared.exceptions.McpError: Invalid request parameters
  +------------------------------------

test_client.py
test_server.py