Function tools don't support datetime arguments

[tibbe]

Description

I wrote a function tool that accepts a datetime as one of its arguments e.g.

def create_task(ctx: RunContext, name: str, due_date: datetime) -> None:
    ...

(I use this as Tool(create_task).)

Unfortunately the schema generator gets very sad:

.venv/lib/python3.13/site-packages/pydantic_ai/tools.py:246: in __init__
    self.function_schema = function_schema or _function_schema.function_schema(
.venv/lib/python3.13/site-packages/pydantic_ai/_function_schema.py:167: in function_schema
    fields[field_name] = td_schema = gen_schema._generate_td_field_schema(  # pyright: ignore[reportPrivateUsage]
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:1189: in _generate_td_field_schema
    common_field = self._common_field_schema(name, field_info, decorators)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:1372: in _common_field_schema
    schema = self._apply_annotations(
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:2297: in _apply_annotations
    schema = get_inner_schema(source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_schema_generation_shared.py:83: in __call__
    schema = self._handler(source_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:2279: in inner_handler
    schema = self._generate_schema_inner(obj)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:1009: in _generate_schema_inner
    return self.match_type(obj)
           ^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:1123: in match_type
    return self._match_generic_type(obj, origin)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:1146: in _match_generic_type
    return self._union_schema(obj)
           ^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:1434: in _union_schema
    choices.append(self.generate_schema(arg))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:711: in generate_schema
    schema = self._generate_schema_inner(obj)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:1009: in _generate_schema_inner
    return self.match_type(obj)
           ^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.13/site-packages/pydantic/_internal/_generate_schema.py:1127: in match_type
    return self._unknown_type_schema(obj)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <pydantic._internal._generate_schema.GenerateSchema object at 0x10ad9fd30>
obj = <class 'datetime.datetime'>

    def _unknown_type_schema(self, obj: Any) -> CoreSchema:
>       raise PydanticSchemaGenerationError(
            f'Unable to generate pydantic-core schema for {obj!r}. '
            'Set `arbitrary_types_allowed=True` in the model_config to ignore this error'
            ' or implement `__get_pydantic_core_schema__` on your type to fully support it.'
            '\n\nIf you got this error by calling handler(<some type>) within'
            ' `__get_pydantic_core_schema__` then you likely need to call'
            ' `handler.generate_schema(<some type>)` since we do not call'
            ' `__get_pydantic_core_schema__` on `<some type>` otherwise to avoid infinite recursion.'
        )
E       pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'datetime.datetime'>. Set `arbitrary_types_allowed=True` in the model_config to ignore this error or implement `__get_pydantic_core_schema__` on your type to fully support it.
E       
E       If you got this error by calling handler(<some type>) within `__get_pydantic_core_schema__` then you likely need to call `handler.generate_schema(<some type>)` since we do not call `__get_pydantic_core_schema__` on `<some type>` otherwise to avoid infinite recursion.
E       
E       For further information visit https://errors.pydantic.dev/2.11/u/schema-for-unknown-type

Is this supposed to work? If not, is the recommended practice to accept datetimes as strings and hope that the model passes them correctly as ISO 8601?

References

No response

[DouweM]

@tibbe Hmm, Pydantic supports datetime (https://docs.pydantic.dev/2.0/usage/types/datetime/), so I'm not sure why it wouldn't work here.

@Viicos Can you have a look here please?

[Viicos]

@tibbe please provide a MRE of your issue (standalone script, with imports).

[tibbe]

I can't reproduce this. I suspect that I had used the freezegun library to mock datetime and thus what pydantic saw was not a real datetime object.

[Viicos]

I can't reproduce this. I suspect that I had used the freezegun library to mock datetime and thus what pydantic saw was not a real datetime object.

This is most likely it (see pydantic/pydantic#9343). time-machine is a great alternative that iirc doesn't suffer from this monkeypatching issue.