AI code makes tests less optional.
That sounds obvious, but the pressure changes when code volume goes up. A human reviewer can skim a small patch and build a mental model. With generated code, that model gets expensive quickly. There is more surface area, more defensive branches, more plausible-looking glue.
Tests become the review surface. They are not proof that the design is good, but they force the generated work to state what it thinks should be true.
Without tests, AI-assisted development drifts toward vibes. The code compiles, the assistant sounds confident, and everyone is tired enough to accept the diff.