beta_tool generates a type: array input_schema and an uncallable tool for positional-only parameters (and *args)

[azrabano23]

Summary

@beta_tool / @beta_async_tool on a function with a positional-only parameter (def f(a: int, /, b: str)) produces an input_schema of type: "array" and a tool whose .call() can never succeed. A function with *args gets the same type: "array" schema. Nothing raises at decoration time.

Reproduction

import json
from anthropic import beta_tool

@beta_tool
def lookup(user_id: int, /, field: str = "name") -> str:
    """Look up a field on a user record."""
    return f"{user_id}:{field}"

print(json.dumps(lookup.input_schema))
lookup.call({"user_id": 7, "field": "email"})

Output on anthropic==1.3.0 (same code on current main, src/anthropic/lib/tools/_beta_functions.py):

{"maxItems": 2, "minItems": 1, "prefixItems": [{"title": "User Id", "type": "integer"}, {"default": "name", "title": "Field", "type": "string"}], "type": "array"}
ValueError: Invalid arguments for function lookup
  (cause: 2 validation errors for lookup — missing_positional_only_argument / unexpected_keyword_argument)

def collect(*values: int) gives {"items": {"type": "integer"}, "type": "array"}.

Why this is wrong

• InputSchema in this SDK is typed with type: Required[Literal["object"]] (src/anthropic/types/tool_param.py), and the API only accepts object schemas for tools[].input_schema. The generated type: array schema violates the SDK's own type and is sent to the API as-is.
• Even with a hand-written input_schema=, BetaFunctionTool.call does self._func_with_validate(**input), so a positional-only parameter is always reported as unexpected_keyword_argument and the tool cannot be invoked by the tool runner.
• Positional-only plus keyword-only in the same signature already fails loudly (PydanticInvalidForJsonSchema), so only the all-positional-only / *args cases fail silently.

Root cause

_create_schema_from_function delegates to pydantic's GenerateJsonSchema.arguments_schema, which renders the positional form (an array schema) whenever a signature has positional-only parameters or *args, and no keyword-only parameters. The docstring hook in kw_arguments_schema is never reached for those signatures. call() then forwards the input object purely by keyword.

Proposed fix

Tool inputs are JSON objects passed by name, so:

• treat positional-only parameters as named properties when generating the schema, and have call() route those values back into positional slots (missing/extra values still go through validate_call);
• raise a TypeError at decoration time for *args, which a JSON object cannot represent.

I have a small patch with tests for this and will open a PR.