If you write your tests the Test-Driven Development way in that they first fail before production changes are introduced, you will be able to trust them a lot more. Especially if they are well-written tests that test behavior or contracts, not implementation details. I find that dependency injection helps a lot with this. I try to avoid mocking and complex dependencies as much as possible. This also allows me to easily refactor the code without having to worry about breaking anything if all the tests still pass.

When it comes to agentic coding. I created an open source tool that enforces those practices. The agent gets blocked by a hook if it tries to do anything that violates those principles. I think it helps a lot if I may say so myself.

https://github.com/nizos/tdd-guard

Edit: I realize now that I misunderstood your comment. I was quick to respond.