The agent patch passed the gates I ran on it. Its unit tests were green, fixtures matched, nothing was flaky. Then I seeded four faults into the implementation, one at a time. Two survived. That gap is what this article is about. A green suite is a claim, not a measurement. Mutation testing turns it into a measurement: introduce a fault, run the suite, and see whether the suite notices. I now run this loop before merging any agent-written patch, and the whole thing costs a few rebuilds. ...