The original headline is: "Prompt caching cut my Claude API bill by 85%. Here's the exact setup."

Short summary

Claude API prompt caching cuts cached token costs by 90% ($3.00 to $0.30 per million) with a one-line code addition: cache_control in your system message. This approach works best for static content like tool definitions, examples, and rules in production agents making repeated requests within the 5-minute cache window. Concrete result: a production agent's daily API costs dropped from $47 to $6.80.

• •Prompt caching reduces cached token cost by 90% ($3.00 → $0.30 per million tokens)
• •One-line implementation: add cache_control to system message blocks with static content
• •Strong ROI for production agents making 2+ requests within the 5-minute cache window

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