Prompt Caching With the Claude API: A Practical Guide

I noticed a pattern looking at three months of Anthropic invoices. The same 8 KB system prompt was getting billed full price on every request. Same instructions, same tool definitions, same RAG context, charged again every turn. The fix takes about ten lines of code and cuts the input bill by roughly 90 percent on cached tokens. This guide is the version of that fix I wish I had bookmarked a year ago. TL;DR Question Short answer What does it do? Stores a prefix of your prompt...