ARM64 RuntimeError: Attempted to exit cancel scope in a different task than it was entered in

[davebirr]

RuntimeError: Attempted to exit cancel scope in a different task than it was entered in

Update - This appears to be an ARM64 issue. FastMCP works in the exact same environment on a AMD64 machine.

Summary:
When running the CIPP-MCP Proxy (Python 3.13.4, fastmcp, anyio), the server fails after initialization with repeated RuntimeError: Attempted to exit cancel scope in a different task than it was entered in exceptions. This occurs during normal JSON-RPC message exchange between the client and server.

---

Proxy Config (local_mcp.py):

import sys
print("PYTHON EXECUTABLE:", sys.executable, file=sys.stderr)
print("PYTHON VERSION:", sys.version, file=sys.stderr)
from fastmcp import FastMCP

# Create a proxy directly from a config dictionary
config = {
    "mcpServers": {
        "default": {  # For single server configs, 'default' is commonly used
            "url": "http://localhost:3001/mcp",
            "transport": "streamable-http"
        }
    }
}

# Create a proxy to the configured server
proxy = FastMCP.as_proxy(config, name="CIPP-MCP Proxy")

# Run the proxy with stdio transport for local access
if __name__ == "__main__":
    proxy.run()

---

Log Excerpt:

2025-06-08T23:24:00.697Z [CIPP-MCP Proxy] [info] Initializing server...
2025-06-08T23:24:00.718Z [CIPP-MCP Proxy] [info] Server started and connected successfully
2025-06-08T23:24:00.719Z [CIPP-MCP Proxy] [info] Message from client: {"method":"initialize", ...}
PYTHON EXECUTABLE: C:\Users\davidb\AppData\Local\Programs\Python\Python313-arm64\python.exe
PYTHON VERSION: 3.13.4 (tags/v3.13.4:8a526ec, Jun  3 2025, 18:23:08) [MSC v.1943 64 bit (ARM64)]
[06/08/25 19:24:01] INFO     Starting MCP server 'MCP Proxy' server.py:799
                             with transport 'stdio'
2025-06-08T23:24:01.965Z [CIPP-MCP Proxy] [info] Message from server: {"jsonrpc":"2.0","id":0,"result":{...}}
2025-06-08T23:24:01.966Z [CIPP-MCP Proxy] [info] Message from client: {"method":"notifications/initialized", ...}
2025-06-08T23:24:01.972Z [CIPP-MCP Proxy] [info] Message from client: {"method":"tools/list", ...}
Task exception was never retrieved
future: <Task finished name='Task-8' coro=<<async_generator_athrow without __name__>()> exception=RuntimeError('Attempted to exit cancel scope in a different task than it was entered in')
...
RuntimeError: Attempted to exit cancel scope in a different task than it was entered in

---

Details:

• The proxy bridges stdio to HTTP and relays JSON-RPC messages as expected.
• The error occurs deep in the async stack (anyio, fastmcp, mcp), not in the proxy or client/server message handling.
• This results in request timeouts and subsequent cancellation notifications from the client.

---

Question:
Is this a me issue or a known issue with FastMCP or its use of anyio cancellation scopes? Are there recommended workarounds or fixes for this context management problem?

---

Environment:

• Python 3.13.4 (ARM64, Windows)
• FastMCP version: 2.5.2
• MCP version: 1.9.3
• Platform: Windows-11-10.0.26100-SP0
• anyio (latest pip version)

Thanks for your help!

---

[davebirr]

Update: Installation Error on ARM64 (Python 3.13)

While trying to install fastmcp on Windows ARM64 using Python 3.13, I ran into a build failure related to the cffi dependency. The error ends with:

LINK : fatal error LNK1104: cannot open file 'python313t.lib'

This suggests the build process is looking for a debug version of the Python library (python313t.lib), which isn’t included in standard Python distributions. This issue appears specific to ARM64 and Python 3.13, as the same setup works fine on AMD64.

I was able to work around the install issue using:

pip install --only-binary=:all: fastmcp

This does bypass the build step and installs successfully, but runtime issue described above persists on ARM64, indicating deeper compatibility problems.

Sharing here in case others hit similar issues or have insight into how fastmcp handles native extensions like cryptography and cffi on ARM64.