Prompt-Caching

The per-token rate on Anthropic’s pricing page is real, but it’s the least useful number for predicting your actual invoice. Three multipliers move the bill far more than model choice does — a cache that silently misses, a transcript that grows quadratically with run length, and a best-of-N pattern that prepays for latency you may not need. Here’s the arithmetic on all three, combined, with real numbers.

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.