DecomposedType.is_instance fails for union types, so functions with an optional single input cannot be invoked with a value

Version

develop (013255a), also reproduces on 1.9.0

Which installation method(s) does this occur on?

Source, PyPI

Describe the bug.

DecomposedType.is_instance (packages/nvidia_nat_core/src/nat/utils/type_utils.py, around line 380) takes the union's root type and passes it to isinstance. For str | None the root is types.UnionType, so the check is always False; for typing.Optional[str] the root is typing.Union, and isinstance raises TypeError: typing.Union cannot be used with isinstance().

is_instance is what Function._convert_input, _needs_conversion and TypeConverter use to decide whether a value already matches the declared input type, so a function whose single input is annotated str | None or Optional[str] cannot be invoked with a plain value at all. Passing None works, passing "hi" fails. The same happens when requesting an optional output via to_type.

Minimum reproducible example

import asyncio
import typing

from nat.utils.type_utils import DecomposedType

print(DecomposedType(str | None).is_instance("hi"))  # False, expected True
print(DecomposedType(typing.Optional[str]).is_instance("hi"))  # TypeError, expected True

# End to end, through a registered function
from nat.builder.builder import Builder
from nat.builder.workflow_builder import WorkflowBuilder
from nat.cli.register_workflow import register_function
from nat.data_models.function import FunctionBaseConfig


class DummyConfig(FunctionBaseConfig, name="dummy_optional"):
    pass


@register_function(config_type=DummyConfig)
async def _register(config: DummyConfig, b: Builder):

    async def _inner(message: str | None) -> str:
        return f"got {message!r}"

    yield _inner


async def main():
    async with WorkflowBuilder() as builder:
        fn = await builder.add_function(name="f", config=DummyConfig())
        print(await fn.ainvoke(None))  # got None
        print(await fn.ainvoke("hi"))  # TypeError, expected "got 'hi'"


asyncio.run(main())

Relevant log output

TypeError: typing.Union cannot be used with isinstance()

(for the str | None spelling the failure surfaces as a conversion TypeError from Function.ainvoke because is_instance returned False and no converter exists)

Other/Misc.

Proposed fix: when the base type is a union, check the instance against each member (any(DecomposedType(arg).is_instance(instance) for arg in args)), which also handles NoneType and typing.Any members. I have this implemented with unit tests and will open a PR. Report prepared with AI assistance (Claude Code); reproduced locally on develop.

Code of Conduct

• I agree to follow this project's Code of Conduct

Start in packages/nvidia_nat_core/src/nat/utils/type_utils.py around DecomposedType.is_instance, then trace its use from Function._convert_input, _needs_conversion, and TypeConverter. Reproduce the str | None and typing.Optional[str] cases from the issue, and run or extend the unit tests so plain values and None are accepted for optional inputs and outputs.