Prompt caching: what actually gets cached, and when it silently misses
Prompt caching can cut your input-token bill by 90% on the reused part of a request — or do nothing at all, with no error to tell you which. It’s an exact-prefix match with a short TTL, and small, common mistakes in how agents build requests break it silently. Here’s what actually gets cached, the five ways real agents lose the discount without noticing, and how to check whether yours is.