Bug: RuntimeError during cleanup on Windows – no running event loop

[Dadiya-Harsh]

---

Describe the bug

When running the mcp client using uv run client.py on Windows, the client connects and runs correctly, but after the response is returned, a RuntimeError: no running event loop is thrown during shutdown. The error occurs inside the terminate_windows_process function in win32.py, which uses anyio.fail_after() after the event loop has already closed.

---

To Reproduce

Steps to reproduce the behavior:

1. Use the following Python code to create a minimal MCP client that connects to a server and runs a tool-enabled Groq chat query:

# save this as `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 groq import AsyncGroq

nest_asyncio.apply()
load_dotenv()

class MCPGroqClient:
 def __init__(self, model: str = "llama-3.3-70b-versatile"):
 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"):
 server_params = StdioServerParameters(command="python", args=[server_script_path])
 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))
 await self.session.initialize()
 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]]:
 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:
 tools = await self.get_mcp_tools()
 response = await self.groq_client.chat.completions.create(
 model=self.model,
 messages=[{"role": "user", "content": query}],
 tools=tools,
 tool_choice="auto",
 )
 assistant_message = response.choices[0].message
 messages = [{"role": "user", "content": query}, assistant_message]

 if assistant_message.tool_calls:
 for tool_call in assistant_message.tool_calls:
 result = await self.session.call_tool(
 tool_call.function.name,
 arguments=json.loads(tool_call.function.arguments),
 )
 messages.append({
 "role": "tool",
 "tool_call_id": tool_call.id,
 "content": result.content[0].text,
 })

 final_response = await self.groq_client.chat.completions.create(
 model=self.model,
 messages=messages,
 tools=tools,
 tool_choice="none",
 )
 return final_response.choices[0].message.content

 return assistant_message.content

 async def cleanup(self):
 await self.exit_stack.aclose()

async def main():
 client = MCPGroqClient()
 await client.connect_to_server()
 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())

2. Set your environment variable GROQ_API_KEY.
3. Start the server with a valid tool (like knowledge_base).
4. Run the client using:

   uv run client.py

5. Observe the traceback at shutdown related to RuntimeError: no running event loop.

---

Expected behavior

The client should shut down gracefully without throwing exceptions after processing a response.

---

Screenshots / Traceback

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 "...\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 "...\Lib\site-packages\anyio\_core\_tasks.py", line 112, in fail_after
 with get_async_backend().create_cancel_scope(
 File "...\Lib\site-packages\anyio\_backends\_asyncio.py", line 456, in __exit__
 if current_task() is not self._host_task:
 ^^^^^^^^^^^^^^
RuntimeError: no running event loop

---

Desktop (please complete the following information):

• OS: Windows 11
• Python Version: 3.12
• MCP Version: [insert version]
• AnyIO Version: [insert version]
• Terminal: Windows PowerShell
• Command Used: uv run client.py

---

Additional context

• This issue seems to be a Windows-specific async shutdown bug.
• Happens when anyio.fail_after() is used after the event loop is already closed.
• Suggest adding a check before using async context tools during shutdown, such as:

  if asyncio.get_event_loop().is_running():
   with anyio.fail_after(2.0):
   ...

• Or use try/except RuntimeError to suppress it gracefully.

---