You configured your app to use gpt-4o . Your provider returned a response from gpt-4o-mini . Same HTTP 200. Same JSON structure. But 10x the error rate and half the quality. This isn't a hypothetical. It's happening every day in production AI systems. The Scale of the Problem When a provider changes the model serving your request without notice, it's called a silent model swap . And it's remarkably common: Provider-side upgrades : "We've upgraded you to a faster mode...