def test_plain_validator_plain_serializer_single_ser_call() -> None:
    """https://github.com/pydantic/pydantic/issues/10385"""

    ser_count = 0

    def ser(v):
        nonlocal ser_count
        ser_count += 1
        return v

    class Model(BaseModel):
        foo: Annotated[bool, PlainSerializer(ser), PlainValidator(lambda v: v)]

    model = Model(foo=True)
    data = model.model_dump()

    assert data == {'foo': True}
    assert ser_count == 1
https://github.com/pydantic/pydantic/issues/10385
tests/test_validators.py