the JSON sent to the client will only contain the data (fields) defined in the `response_model`. If you returned an object that contains an attribute `password` but the `response_model` does not include that field, the JSON sent to the client would not have that `password`. * Validation: whatever you return will be serialized with the `response_model`, converting any data as necessary to generate the corresponding JSON. But if the data in the object returned is not valid, that would mean a violation of the contract with the client, so it's an error from the API developer. So, FastAPI will raise an error and return a 500 error code (Internal Server Error). Read more about it in the [FastAPI docs for Response Model](https://fastapi.tiangolo.com/tutorial/response-model/). aM