If you're always inserting a string there, it might be better to use a computed_field. Something along this, maybe?

class MyModel(BaseModel):
    input: str

    @computed_field
    def x(self) -> int:
        return len(self.input)

I think it's very counterintuitive if you see the model with the int declaration while it would raise type errors if you put an integer inside a JSON at that place.