A/B Test Only Changed an API Parameter, Yet Error Rates Doubled: Which Downstream Assumptions Did You Break?
1. Introduction: “We Only Changed One Parameter” The A/B test looked harmless. You changed one API parameter.No new endpoints.No schema changes.No traffic increase. Yet after rollout: This feels irrational. How can a single parameter cause so much damage? The answer is simple but uncomfortable:you didn’t break the API — you broke downstream assumptions that were…