Test reality’s acceptance, not our intent
A green suite proves the code does what the tests expect. It does not prove the system does what the world requires. Six defects once shipped through a fully green suite; the remedy was to test trajectories against reality, not snapshots against intent.
The lesson arrived as a streak of bugs that every test had blessed. Each test asserted the behaviour we had intended, and the intent was wrong in ways only the running system could show. The practice that followed: test what reality accepts — the real wire, the real operator, the real failure — and test trajectories, not snapshots. Unwired tests are not coverage.
go-crap and gen-contract exist to hold that line mechanically: mutation testing asks whether the tests would notice a wrong change; contract validation asks whether the services still agree.