Your tests are an assertion that 'no matter what this will never change'. If your interface can change then you are testing implementation details instead of the behavior users care about.
replythe above is really hard. A lot of tdd 'experts' don't understand is and teach fragile tests that are not worth having.
https://www.hyrumslaw.com/
replyyour implementation is your interface. its a bit naive or hating-your-users to assume your tests are what your users care about. theyre dealing with everything, regardless of what youve tested or not.
Refactoring is changing the design of the code without affecting the behaviour.
replyYou can change an interface and not change the behaviour.
I have rarely heard such a rigid interpretation such as this.