[Somay-kousis]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: FunctionTool passes a plain str to an Enum-typed parameter, so the tool crashes #7635

2. Or, if no issue exists, describe the change:

Problem:

A function tool with an Enum parameter got the raw value ("celsius") instead of the member on the default path (FUNCTION_TOOL_ARG_VALIDATION off). The declaration lists the enum's values, so the model sends one back, and tool code that uses the member (unit.value, is Unit.X) raised and ended the run.

Solution:

_schema_utils.preprocess_args now converts the argument to its Enum member when the parameter is annotated with an Enum subclass, Optional[...] of one, or ... | None. It sits next to the existing pydantic model and int handling on the default path.

• A valid value ("celsius") becomes the member (Unit.CELSIUS).
• An IntEnum value that came back as a float (2.0, from a proto Struct round trip) is matched too.
• None and values that already are the member are left alone.
• An unknown value passes through unchanged with a warning, the same as a failed pydantic conversion. Rejecting it stays the job of FUNCTION_TOOL_ARG_VALIDATION.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

New tests in tests/unittests/tools/test_function_tool.py:

• test_run_async_converts_enum_value_to_member
• test_run_async_converts_optional_enum_value_to_member (also checks that None logs no warning)
• test_run_async_converts_int_enum_value_from_float
• test_run_async_passes_through_unknown_enum_value
• test_run_async_keeps_enum_member_arg

main: 3 failed, 105 passed
fix:  108 passed

The two that pass on main are guards (unknown value passes through, an existing member is kept). Breaking each part of the change (the lookup, the try/except, the None guard) makes at least one new test fail. tests/unittests/tools and tests/unittests/utils pass (3883 tests). isort and pyink are clean.

Manual End-to-End (E2E) Tests:

The repro in the issue (Agent + InMemoryRunner, scripted model):

before: AttributeError: 'str' object has no attribute 'value'
after:  {'city': 'Paris', 'unit': 'celsius'}

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

Containers of enums (list[Unit]) are not converted here.