API Contract Testing: Why Safe Changes Still Break Clients

APIs often break clients in ways that don’t show up in tests. A change looks safe inside the service: remove an unused field tighten validation adjust an error response Everything still works locally. Tests pass. Deployment succeeds. Then a mobile app crashes. A partner integration fails. An older frontend silently breaks. Nothing is “wrong” in the service - but the contract changed . The Problem: Internal Changes, External Breakage Consider a response: { "id" : "ord_123" , "status" : "paid" , "