The Prompt Caching Mistake That's Costing You 70% More Than You Need to Pay Here is a specific claim: most teams building on Claude or GPT-4o are paying three to ten times more per token than they need to, and the reason is not model selection or request volume — it is prompt construction. Prompt caching has been available on Anthropic's API since August 2024 and on OpenAI's since October 2024. The economics are not subtle: Anthropic cache reads cost $0.30 per million tokens versus $3.00 per million for fresh tokens, a 90 percent reduction. OpenAI's automatic caching runs at roughly 50 percent off. If your application sends any system prompt longer than a few hundred tokens — and most do — you are either capturing that discount or you are not, and the line between those two outcomes is exactly one architectural decision most teams have never made deliberately. Why most applications do not actually capture the discount Prefix caching works by hashing the leading bytes of your prompt against a stored version.…