Ngl I’m doing this right now for a client. Part of my strategy is to write out e2e tests that get a certain baseline of functionality, and then use that as the check for any change that I make to the codebase to make sure it continues to work.

So workflow for a full web app is make e2e tests for all use cases. Then add a very strict duplication checker, and linter, and then just tell the ai to hit a certain duplication limit like 3%, check the linter, and add unit tests to ~95% or greater of the code.

With the right CI and other checks that are deterministic you can really do a lot with a codebase.

I have also experimented with mutation testing. But the side effect of this approach is that it keeps public interfaces intact, and most of cases you don't want that.