RuntimeError: Attempted to exit cancel scope in a different task when cleaning up multiple MCPClient instances out-of-order

[HMJiangGatech]

Describe the bug
If two MCPClient objects are instantiated and cleaned up in non-FILO order (i.e., the first-created client is cleaned up before the second), teardown fails with a cascade of RuntimeError/CancelledError exceptions coming from anyio and mcp.client.stdio.

To Reproduce
Minimal repro:

import os, asyncio, json
from typing import Optional
from contextlib import AsyncExitStack
from mcp import ClientSession, StdioServerParameters
from mcp.types import TextContent
from mcp.client.stdio import stdio_client

class MCPClient:
    def __init__(self, command: str, args: list[str], env: Optional[dict] = None):
        self.session: Optional[ClientSession] = None
        self.command, self.args, self.env = command, args, env
        self._cleanup_lock = asyncio.Lock()
        self.exit_stack: Optional[AsyncExitStack] = None

    async def connect_to_server(self):
        await self.cleanup()
        self.exit_stack = AsyncExitStack()

        server_params = StdioServerParameters(
            command=self.command, args=self.args, env=self.env
        )
        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()

    async def cleanup(self):
        if self.exit_stack:
            async with self._cleanup_lock:
                await self.exit_stack.aclose()
                self.session = None
            self.exit_stack = None

async def main():
    cfg = {
        "command": "npx",
        "args": ["-y", "@adenot/mcp-google-search"],
        "env": {
            "GOOGLE_API_KEY": os.environ["GOOGLE_API_KEY"],
            "GOOGLE_SEARCH_ENGINE_ID": os.environ["GOOGLE_SEARCH_ENGINE_ID"],
        },
    }

    c1, c2 = MCPClient(**cfg), MCPClient(**cfg)
    await c1.connect_to_server()
    await c2.connect_to_server()

    # Works (FILO)
    # await c2.cleanup()
    # await c1.cleanup()

    # Fails (FIFO)
    await c1.cleanup()      # <-- boom
    await c2.cleanup()

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

Expected behavior

cleanup() should succeed regardless of the order in which multiple MCPClient instances are closed, as long as each instance’s own exit_stack is intact. A single client ought to manage its own lifetime without depending on external FILO discipline.

Actual Traceback

RuntimeError: Attempted to exit cancel scope in a different task than it was entered in
...
asyncio.exceptions.CancelledError: Cancelled by cancel scope ...
...
RuntimeError: Attempted to exit a cancel scope that isn't the current task's current cancel scope

Environment

Item	Version
mcp	1.6.0
Python	3.12.10
anyio	4.9.0
OS	macOS 14.4 (Apple Silicon)

[mashriram]

Facing the same issue when connectng trying to use FastAPI and async methods with mcp to work with frontend
The error simply like this and it starts at mcp libraries sse.py and flows down to below
``

~~~~~~~~~~~~~~~~~~~~~~~^^
File "/home/mukundan/projects/mcp-sse/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 773, in aexit
if self.cancel_scope.exit(type(exc), exc, exc.traceback):
~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/mukundan/projects/mcp-sse/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 456, in exit
raise RuntimeError(
...<2 lines>...
)
RuntimeError: Attempted to exit cancel scope in a different task than it was entered in```

[pitta-bread]

Faced the same issue. My custom context manager class was doing simple iteration of the clients list to attempt cleanup (FIFO). In line with the workaround detailed above, fixed with FILO:

e.g.

finally:
    # Clean up in the same task context
    # MUST BE FILO, FIFO fails
    # https://github.com/modelcontextprotocol/python-sdk/issues/577
    for client in reversed(self.clients):
        try:
            await client.cleanup()

[CuriousTank]

How to achieve free management of multiple MCPClient instances under such conditions, enabling instant exit and connection establishment

[jhaoming-oai]

Trying OpenAI Codex to fix the issue:
https://chatgpt.com/s/cd_682dec27cf788191b2892b8097d45248