Add Annotated[T, Field(...)] syntax for tool arguments (@function_tool)

[georg-wolflein]

Summary

This PR adds support for Annotated[T, Field(...)] on tool parameters in addition to the existing = Field(...) syntax. The original = Field(...) support was introduced in #1124.

Why add this variant?

1. Pydantic supports it. Pydantic allows Field either as the default (x: int = Field(ge=1)) or inside Annotated (x: Annotated[int, Field(ge=1)]). Supporting both in function_schema keeps the SDK aligned with how users define constrained fields in Pydantic models.

2. Parameter ordering in Python. A parameter like x: int = Field(..., ge=1) still has a default value as far as Python is concerned (the default is the Field(...) object). So you cannot put another parameter without a default after it — that would be a SyntaxError:

   # SyntaxError: parameter without default follows parameter with default
   @function_tool
   def bad(
       positive_number: int = Field(..., ge=1),
       another_arg: str,
   ) -> str:
       ...

   With Annotated, the constrained parameter has no default in the signature, so you can have multiple required arguments (some with Field constraints) in any order:

   @function_tool
   def good(
       positive_number: Annotated[int, Field(..., ge=1)],
       another_arg: str,
   ) -> str:
       ...

Examples

Required argument with constraints:

from typing import Annotated
from pydantic import Field
from agents import function_tool

@function_tool
def score_task(score: Annotated[int, Field(..., ge=0, le=100)]) -> str:
    return f"Score recorded: {score}"

Optional argument with default and constraints:

@function_tool
def search(
    query: str,
    limit: Annotated[int, Field(default=10, ge=1, le=100)] = 10,
) -> str:
    return f"Searching for {query} with limit {limit}"

Multiple required arguments where one has constraints (only possible without SyntaxError using Annotated):

@function_tool
def process(
    positive_number: Annotated[int, Field(..., ge=1)],
    another_arg: str,
) -> str:
    return f"{positive_number}: {another_arg}"

Implementation: we preserve per-parameter Annotated metadata in function_schema, detect FieldInfo in that metadata, and use it when building the Pydantic model and JSON schema so that constraints, defaults, and descriptions are applied the same way as for = Field(...).

---

Test plan

Documentation: the Pydantic Field feature for tool arguments was not documented before. As requested in the PR #1124 review, a new subsection "Constraining and describing arguments with Pydantic Field" in docs/tools.md now documents both forms (default-based = Field(...) and Annotated[T, Field(...)]) with example code for each.

The new unit tests mirror the existing tests for the = Field(...) format: required constraints (test_function_with_field_required_constraints → test_function_with_annotated_field_required_constraints), optional with default and constraints, string constraints (min/max length, pattern), and multiple parameters with mixed constraints. Each new test uses the same assertions (JSON schema shape, valid input round-trip, invalid input raises ValidationError) as its counterpart, so both syntaxes are covered consistently.

---

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

these may be fine, but can you apply these unrelated changes separately?

[seratch]

Having the document change in this PR requires us to immediately release a new version when merging it. Can you create a separate PR for doc chnages?

[haasonsaas]

I can’t push updates to the fork branch from here, so I opened split PRs instead per the review request to keep docs separate:

• feat: support Annotated[T, Field(...)] in function schema #2435 (code-only): support Annotated[T, Field(...)] in function_schema
• docs: describe Pydantic Field annotations for tool args #2436 (docs-only): tools.md subsection

If you prefer updating this PR directly, the minimal change is to revert docs/tools.md back to main and keep only the src/ + tests/ changes here.