On Windows 11 when you initialize an mcp client it hangs indefinitely

[BenMawnMahlauNBTC]

Describe the bug
On Windows 11 when you initialize an mcp client it hangs indefinitely.

To Reproduce
Code to reproduce:

# ruff: noqa
import asyncio

from mcp import ClientSession, StdioServerParameters
from mcp.client.sse import sse_client
from mcp.client.stdio import stdio_client


async def run():
 params = StdioServerParameters(
 command='bunx', args=['@playwright/mcp@latest']
 )
 async with stdio_client(params) as (read, write):
 print('inside client')
 async with ClientSession(read, write) as c:
 print('inside ClientSession')
 await c.initialize()
 print('exit ClientSession')
 print('exit stdio_client')


async def run_sse():
 async with sse_client('http://localhost:8931/sse') as (read, write):
 async with ClientSession(read, write) as c:
 await c.initialize()
 print('exit ClientSession')
 print('exit sse_client')


if __name__ == '__main__':
 asyncio.run(run_sse()) # works
 asyncio.run(run()) # does not work

Expected behavior
in both cases it should print both exit statements

Desktop (please complete the following information):

• OS: Windows 11
• Python Version: Tested on both 3.13.1 and 3.12.7

Additional context
counterintuitively commenting out the code meant to support windows fixes this issue:

import os
import sys
from contextlib import asynccontextmanager
from pathlib import Path
from typing import Literal, TextIO

import anyio
import anyio.lowlevel
from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
from anyio.streams.text import TextReceiveStream
from pydantic import BaseModel, Field

import mcp.types as types

# from .win32 import (
# create_windows_process,
# get_windows_executable_command,
# terminate_windows_process,
# )

# Environment variables to inherit by default
DEFAULT_INHERITED_ENV_VARS = (
 [
 "APPDATA",
 "HOMEDRIVE",
 "HOMEPATH",
 "LOCALAPPDATA",
 "PATH",
 "PROCESSOR_ARCHITECTURE",
 "SYSTEMDRIVE",
 "SYSTEMROOT",
 "TEMP",
 "USERNAME",
 "USERPROFILE",
 ]
 if sys.platform == "win32"
 else ["HOME", "LOGNAME", "PATH", "SHELL", "TERM", "USER"]
)
print( sys.platform)

def get_default_environment() -> dict[str, str]:
 """
 Returns a default environment object including only environment variables deemed
 safe to inherit.
 """
 env: dict[str, str] = {}

 for key in DEFAULT_INHERITED_ENV_VARS:
 value = os.environ.get(key)
 if value is None:
 continue

 if value.startswith("()"):
 # Skip functions, which are a security risk
 continue

 env[key] = value

 return env


class StdioServerParameters(BaseModel):
 command: str
 """The executable to run to start the server."""

 args: list[str] = Field(default_factory=list)
 """Command line arguments to pass to the executable."""

 env: dict[str, str] | None = None
 """
 The environment to use when spawning the process.

 If not specified, the result of get_default_environment() will be used.
 """

 cwd: str | Path | None = None
 """The working directory to use when spawning the process."""

 encoding: str = "utf-8"
 """
 The text encoding used when sending/receiving messages to the server

 defaults to utf-8
 """

 encoding_error_handler: Literal["strict", "ignore", "replace"] = "strict"
 """
 The text encoding error handler.

 See https://docs.python.org/3/library/codecs.html#codec-base-classes for
 explanations of possible values
 """


@asynccontextmanager
async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stderr):
 """
 Client transport for stdio: this will connect to a server by spawning a
 process and communicating with it over stdin/stdout.
 """
 read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception]
 read_stream_writer: MemoryObjectSendStream[types.JSONRPCMessage | Exception]

 write_stream: MemoryObjectSendStream[types.JSONRPCMessage]
 write_stream_reader: MemoryObjectReceiveStream[types.JSONRPCMessage]

 read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
 write_stream, write_stream_reader = anyio.create_memory_object_stream(0)

 command = _get_executable_command(server.command)

 # Open process with stderr piped for capture
 process = await _create_platform_compatible_process(
 command=command,
 args=server.args,
 env=(
 {**get_default_environment(), **server.env}
 if server.env is not None
 else get_default_environment()
 ),
 errlog=errlog,
 cwd=server.cwd,
 )

 async def stdout_reader():
 assert process.stdout, "Opened process is missing stdout"

 try:
 async with read_stream_writer:
 buffer = ""
 async for chunk in TextReceiveStream(
 process.stdout,
 encoding=server.encoding,
 errors=server.encoding_error_handler,
 ):
 lines = (buffer + chunk).split("\n")
 buffer = lines.pop()

 for line in lines:
 try:
 message = types.JSONRPCMessage.model_validate_json(line)
 except Exception as exc:
 await read_stream_writer.send(exc)
 continue

 await read_stream_writer.send(message)
 except anyio.ClosedResourceError:
 await anyio.lowlevel.checkpoint()

 async def stdin_writer():
 assert process.stdin, "Opened process is missing stdin"

 try:
 async with write_stream_reader:
 async for message in write_stream_reader:
 json = message.model_dump_json(by_alias=True, exclude_none=True)
 await process.stdin.send(
 (json + "\n").encode(
 encoding=server.encoding,
 errors=server.encoding_error_handler,
 )
 )
 except anyio.ClosedResourceError:
 await anyio.lowlevel.checkpoint()

 async with (
 anyio.create_task_group() as tg,
 process,
 ):
 tg.start_soon(stdout_reader)
 tg.start_soon(stdin_writer)
 try:
 yield read_stream, write_stream
 finally:
 # Clean up process to prevent any dangling orphaned processes
 # if sys.platform == "win32":
 # await terminate_windows_process(process)
 # else:
 process.terminate()


def _get_executable_command(command: str) -> str:
 """
 Get the correct executable command normalized for the current platform.

 Args:
 command: Base command (e.g., 'uvx', 'npx')

 Returns:
 str: Platform-appropriate command
 """
 # if sys.platform == "win32":
 # return get_windows_executable_command(command)
 # else:
 return command


async def _create_platform_compatible_process(
 command: str,
 args: list[str],
 env: dict[str, str] | None = None,
 errlog: TextIO = sys.stderr,
 cwd: Path | str | None = None,
):
 """
 Creates a subprocess in a platform-compatible way.
 Returns a process handle.
 """
 # if sys.platform == "win32":
 # print('attempting create windows process')
 # process = await create_windows_process(command, args, env, errlog, cwd)
 # print('created windows process')
 # else:
 process = await anyio.open_process(
 [command, *args], env=env, stderr=errlog, cwd=cwd
 )

 return process

[Dadiya-Harsh]

@BenMawnMahlauNBTC yes i am facing the same issue, if you overcome this issue please tell me, how to do it.

[BenMawnMahlauNBTC]

@BenMawnMahlauNBTC yes i am facing the same issue, if you overcome this issue please tell me, how to do it.

I included the fix that worked for me in the issue -> additional context. if you paste that code into .venv\Lib\site-packages\mcp\client\stdio\__init__.py it should stop hanging

[Dadiya-Harsh]

@BenMawnMahlauNBTC but what should i paste?

[BenMawnMahlauNBTC]

@BenMawnMahlauNBTC but what should i paste?

import os
import sys
from contextlib import asynccontextmanager
from pathlib import Path
from typing import Literal, TextIO

import anyio
import anyio.lowlevel
from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
from anyio.streams.text import TextReceiveStream
from pydantic import BaseModel, Field

import mcp.types as types

# from .win32 import (
# create_windows_process,
# get_windows_executable_command,
# terminate_windows_process,
# )

# Environment variables to inherit by default
DEFAULT_INHERITED_ENV_VARS = (
 [
 "APPDATA",
 "HOMEDRIVE",
 "HOMEPATH",
 "LOCALAPPDATA",
 "PATH",
 "PROCESSOR_ARCHITECTURE",
 "SYSTEMDRIVE",
 "SYSTEMROOT",
 "TEMP",
 "USERNAME",
 "USERPROFILE",
 ]
 if sys.platform == "win32"
 else ["HOME", "LOGNAME", "PATH", "SHELL", "TERM", "USER"]
)
print( sys.platform)

def get_default_environment() -> dict[str, str]:
 """
 Returns a default environment object including only environment variables deemed
 safe to inherit.
 """
 env: dict[str, str] = {}

 for key in DEFAULT_INHERITED_ENV_VARS:
 value = os.environ.get(key)
 if value is None:
 continue

 if value.startswith("()"):
 # Skip functions, which are a security risk
 continue

 env[key] = value

 return env


class StdioServerParameters(BaseModel):
 command: str
 """The executable to run to start the server."""

 args: list[str] = Field(default_factory=list)
 """Command line arguments to pass to the executable."""

 env: dict[str, str] | None = None
 """
 The environment to use when spawning the process.

 If not specified, the result of get_default_environment() will be used.
 """

 cwd: str | Path | None = None
 """The working directory to use when spawning the process."""

 encoding: str = "utf-8"
 """
 The text encoding used when sending/receiving messages to the server

 defaults to utf-8
 """

 encoding_error_handler: Literal["strict", "ignore", "replace"] = "strict"
 """
 The text encoding error handler.

 See https://docs.python.org/3/library/codecs.html#codec-base-classes for
 explanations of possible values
 """


@asynccontextmanager
async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stderr):
 """
 Client transport for stdio: this will connect to a server by spawning a
 process and communicating with it over stdin/stdout.
 """
 read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception]
 read_stream_writer: MemoryObjectSendStream[types.JSONRPCMessage | Exception]

 write_stream: MemoryObjectSendStream[types.JSONRPCMessage]
 write_stream_reader: MemoryObjectReceiveStream[types.JSONRPCMessage]

 read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
 write_stream, write_stream_reader = anyio.create_memory_object_stream(0)

 command = _get_executable_command(server.command)

 # Open process with stderr piped for capture
 process = await _create_platform_compatible_process(
 command=command,
 args=server.args,
 env=(
 {**get_default_environment(), **server.env}
 if server.env is not None
 else get_default_environment()
 ),
 errlog=errlog,
 cwd=server.cwd,
 )

 async def stdout_reader():
 assert process.stdout, "Opened process is missing stdout"

 try:
 async with read_stream_writer:
 buffer = ""
 async for chunk in TextReceiveStream(
 process.stdout,
 encoding=server.encoding,
 errors=server.encoding_error_handler,
 ):
 lines = (buffer + chunk).split("\n")
 buffer = lines.pop()

 for line in lines:
 try:
 message = types.JSONRPCMessage.model_validate_json(line)
 except Exception as exc:
 await read_stream_writer.send(exc)
 continue

 await read_stream_writer.send(message)
 except anyio.ClosedResourceError:
 await anyio.lowlevel.checkpoint()

 async def stdin_writer():
 assert process.stdin, "Opened process is missing stdin"

 try:
 async with write_stream_reader:
 async for message in write_stream_reader:
 json = message.model_dump_json(by_alias=True, exclude_none=True)
 await process.stdin.send(
 (json + "\n").encode(
 encoding=server.encoding,
 errors=server.encoding_error_handler,
 )
 )
 except anyio.ClosedResourceError:
 await anyio.lowlevel.checkpoint()

 async with (
 anyio.create_task_group() as tg,
 process,
 ):
 tg.start_soon(stdout_reader)
 tg.start_soon(stdin_writer)
 try:
 yield read_stream, write_stream
 finally:
 # Clean up process to prevent any dangling orphaned processes
 # if sys.platform == "win32":
 # await terminate_windows_process(process)
 # else:
 process.terminate()


def _get_executable_command(command: str) -> str:
 """
 Get the correct executable command normalized for the current platform.

 Args:
 command: Base command (e.g., 'uvx', 'npx')

 Returns:
 str: Platform-appropriate command
 """
 # if sys.platform == "win32":
 # return get_windows_executable_command(command)
 # else:
 return command


async def _create_platform_compatible_process(
 command: str,
 args: list[str],
 env: dict[str, str] | None = None,
 errlog: TextIO = sys.stderr,
 cwd: Path | str | None = None,
):
 """
 Creates a subprocess in a platform-compatible way.
 Returns a process handle.
 """
 # if sys.platform == "win32":
 # print('attempting create windows process')
 # process = await create_windows_process(command, args, env, errlog, cwd)
 # print('created windows process')
 # else:
 process = await anyio.open_process(
 [command, *args], env=env, stderr=errlog, cwd=cwd
 )

 return process

[Dadiya-Harsh]

@BenMawnMahlauNBTC, thanks but still not working and when i am using stdio transport mechanism then only i am facing this issue, when i am running with sse transport mechanism it is working normally as intended

[Dadiya-Harsh]

@BenMawnMahlauNBTC , thank you i appreciate your help but issue is still there

[Dadiya-Harsh]

@modelcontextprotocol , please help

[BenMawnMahlauNBTC]

is the mcp client hanging or is it giving you an error

[Dadiya-Harsh]

look it is not stuck , when i have implemented the following code server is running but not showing any response(logs like sse mechanisms).

server.py:

import json
import os
from mcp.server.fastmcp import FastMCP
import nest_asyncio
nest_asyncio.apply()
server = FastMCP(name = "LLM-Server", host = "0.0.0.0", port = 8050)


@server.tool()
def knowledge_base() -> str:
 """Retrieve the entire knowledge base as a formatted string.

 Returns:
 A formatted string containing all Q&A pairs from the knowledge base.
 """
 try:
 kb_path = os.path.join(os.path.dirname(__file__), "data", "kb.json")
 with open(kb_path, "r") as f:
 kb_data = json.load(f)

 # Format the knowledge base as a string
 kb_text = "Here is the retrieved knowledge base:\n\n"

 if isinstance(kb_data, list):
 for i, item in enumerate(kb_data, 1):
 if isinstance(item, dict):
 question = item.get("question", "Unknown question")
 answer = item.get("answer", "Unknown answer")
 else:
 question = f"Item {i}"
 answer = str(item)

 kb_text += f"Q{i}: {question}\n"
 kb_text += f"A{i}: {answer}\n\n"
 else:
 kb_text += f"Knowledge base content: {json.dumps(kb_data, indent=2)}\n\n"

 return kb_text
 except FileNotFoundError:
 return "Error: Knowledge base file not found"
 except json.JSONDecodeError:
 return "Error: Invalid JSON in knowledge base file"
 except Exception as e:
 return f"Error: {str(e)}"
 
if __name__ == "__main__":
 server.run(transport="stdio")

client.py

import asyncio
import json
from contextlib import AsyncExitStack
import os
from typing import Any, Dict, List, Optional

import nest_asyncio
from dotenv import load_dotenv
from mcp import ClientSession, StdioServerParameters
from mcp.client.stdio import stdio_client
# from openai import AsyncOpenAI
from groq import AsyncGroq

# Apply nest_asyncio to allow nested event loops (needed for Jupyter/IPython)
nest_asyncio.apply()

# Load environment variables
load_dotenv()


class MCPGroqClient:
 """Client for interacting with groq models using MCP tools."""

 def __init__(self, model: str = "llama-3.3-70b-versatile"):
 """Initialize the groq MCP client.

 Args:
 model: The groq model to use.
 """
 # Initialize session and client objects
 self.session: Optional[ClientSession] = None
 self.exit_stack = AsyncExitStack()
 self.groq_client = AsyncGroq(api_key=os.getenv("GROQ_API_KEY"))
 self.model = model
 self.stdio: Optional[Any] = None
 self.write: Optional[Any] = None

 async def connect_to_server(self, server_script_path: str = "server.py"):
 """Connect to an MCP server.

 Args:
 server_script_path: Path to the server script.
 """
 # Server configuration
 server_params = StdioServerParameters(
 command="python",
 args=[server_script_path],
 )

 # Connect to the server
 stdio_transport = await self.exit_stack.enter_async_context(
 stdio_client(server_params)
 )
 self.stdio, self.write = stdio_transport
 self.session = await self.exit_stack.enter_async_context(
 ClientSession(self.stdio, self.write)
 )

 # Initialize the connection
 await self.session.initialize()

 # List available tools
 tools_result = await self.session.list_tools()
 print("\nConnected to server with tools:")
 for tool in tools_result.tools:
 print(f" - {tool.name}: {tool.description}")

 async def get_mcp_tools(self) -> List[Dict[str, Any]]:
 """Get available tools from the MCP server in groq format.

 Returns:
 A list of tools in groq format.
 """
 tools_result = await self.session.list_tools()
 return [
 {
 "type": "function",
 "function": {
 "name": tool.name,
 "description": tool.description,
 "parameters": tool.inputSchema,
 },
 }
 for tool in tools_result.tools
 ]

 async def process_query(self, query: str) -> str:
 """Process a query using groq and available MCP tools.

 Args:
 query: The user query.

 Returns:
 The response from groq.
 """
 # Get available tools
 tools = await self.get_mcp_tools()

 # Initial groq API call
 response = await self.groq_client.chat.completions.create(
 model=self.model,
 messages=[{"role": "user", "content": query}],
 tools=tools,
 tool_choice="auto",
 )

 # Get assistant's response
 assistant_message = response.choices[0].message

 # Initialize conversation with user query and assistant response
 messages = [
 {"role": "user", "content": query},
 assistant_message,
 ]

 # Handle tool calls if present
 if assistant_message.tool_calls:
 # Process each tool call
 for tool_call in assistant_message.tool_calls:
 # Execute tool call
 result = await self.session.call_tool(
 tool_call.function.name,
 arguments=json.loads(tool_call.function.arguments),
 )

 # Add tool response to conversation
 messages.append(
 {
 "role": "tool",
 "tool_call_id": tool_call.id,
 "content": result.content[0].text,
 }
 )

 # Get final response from groq with tool results
 final_response = await self.groq_client.chat.completions.create(
 model=self.model,
 messages=messages,
 tools=tools,
 tool_choice="none", # Don't allow more tool calls
 )

 return final_response.choices[0].message.content

 # No tool calls, just return the direct response
 return assistant_message.content

 async def cleanup(self):
 """Clean up resources."""
 await self.exit_stack.aclose()


async def main():
 """Main entry point for the client."""
 client = MCPGroqClient()
 await client.connect_to_server()

 # Example: Ask about company vacation policy
 query = "What is our company's vacation policy?"
 print(f"\nQuery: {query}")

 response = await client.process_query(query)
 print(f"\nResponse: {response}")


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

i have run both server and client file while i can't see anything for server file in command prompt but i got this for client.py

PS E:\Projects\MCP\mcp-demo\learning\groq> uv run client.py

Connected to server with tools:
 - knowledge_base: Retrieve the entire knowledge base as a formatted string.

 Returns:
 A formatted string containing all Q&A pairs from the knowledge base.


Query: What is our company's vacation policy?

Response: Our company's vacation policy is as follows: Full-time employees are entitled to 20 paid vacation days per year. Vacation days can be taken after completing 6 months of employment. Unused vacation days can be carried over to the next year up to a maximum of 5 days. Vacation requests should be submitted at least 2 weeks in advance through the HR portal.
Exception ignored in: <async_generator object stdio_client at 0x000002025AE0EFC0>
Traceback (most recent call last):
 File "C:\Program Files\Python312\Lib\asyncio\tasks.py", line 314, in __step_run_and_handle_result
 result = coro.send(None)
 ^^^^^^^^^^^^^^^
RuntimeError: async generator ignored GeneratorExit
Exception ignored in: <coroutine object terminate_windows_process at 0x000002025AF5F3E0>
Traceback (most recent call last):
 File "E:\Projects\MCP\mcp-demo\.venv\Lib\site-packages\mcp\client\stdio\win32.py", line 105, in terminate_windows_process 
 with anyio.fail_after(2.0):
 File "C:\Program Files\Python312\Lib\contextlib.py", line 158, in __exit__
 self.gen.throw(value)
 File "E:\Projects\MCP\mcp-demo\.venv\Lib\site-packages\anyio\_core\_tasks.py", line 112, in fail_after
 with get_async_backend().create_cancel_scope(
 File "E:\Projects\MCP\mcp-demo\.venv\Lib\site-packages\anyio\_backends\_asyncio.py", line 456, in __exit__
 if current_task() is not self._host_task:
 ^^^^^^^^^^^^^^
RuntimeError: no running event loop

so in short server is running in stdio transport mechanism but there are no logs like sse transport mechanism

[BenMawnMahlauNBTC]

try building from this pr #555

[ashrobertsdragon]

This is related to Issue #391. I'm currently working on a PR to fix this.

[Dadiya-Harsh]

This is related to Issue #391. I'm currently working on a PR to fix this.

@ashrobertsdragon thanks, if your pr is merged please let me know thanks for trying to solve the issue

[Naitik4516]

Use bun x instead of bunx

 params = StdioServerParameters(
 command='bun', args=['x', '@playwright/mcp@latest']
 )

Unexpectedly, this resolved the issue for me. I'm not sure why, but it might be worth trying if you're encountering similar behavior.