Mutation Testing: The Missing Safety Net for AI-Generated Code

92% code coverage. No SonarQube criticals. All green. And an AI-generated deduplication bug made it to production because not a single test had challenged the logic. Code coverage tells you what ran. Mutation testing tells you what your tests would actually catch if the code were wrong. And in the AI world, that's the only thing that matters. Let us check an analogy here > Walking through a building, coverage means we visited all rooms. Mutation testing means we would notice if there were a missing wall. One measures presence, the other measures resistance. The Bug That Coverage Could Not See I've seen this occur in the wild. An AI agent produced the service layer for a payment reconciliation workflow. 140 unit tests. 92% line coverage. It looked good on the PR. But two days after deployment, the reconciliation started silently duplicating line items. The AI had used reference equality on objects, not business key equality. For 98%, it was functionally the same. For the 2% it recons