Request and Response validation in controllers

[Aspire Higher]

I am currently working on a project using Flask to make REST apis, blablabla. But, working with request validation tools that we use, I am thinking of whether it would be necessary to validate the responses of the controllers.

I read a bit about Design by contracts and I saw how it was done in Eiffel programming language but, I was thinking of a way this could be applied to back-end development.

Is there any need to validate the request and the response in app code or this could be handled by the API specs?