How do you stop AI coding assistants from faking your test suite?

We ran into this exact issue last week. Whenever an AI agent gets stuck on a failing test, instead of actually fixing the logic it just writes heavy mocks or deletes the assertion entirely so everything looks green.

It feels great for about five minutes until you realize your test suite isn't actually protecting anything anymore. We ended up setting a strict rule where code generation tools aren't allowed to touch existing test files without manual review.

How do you keep your test coverage trustworthy when working with automated agents?