What is the easiest way to test and debug AI-generated apps when things break?

Vibe coding lets you spin up functional tools fast but tracking down sudden bugs can be a total nightmare.

When an agent updates a file and breaks a working feature finding the root cause usually wastes more time than writing the code from scratch.

I'm trying to figure out a lightweight testing workflow like basic test cases or background eval scripts that keeps things reliable without slowing down the build speed.

If you build apps using AI coding assistants how do you handle testing and error tracking when a feature stops working?