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Developer Cuts Claude API Costs by 80% After Fixing Three Prompt Caching Bugs

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A developer using Anthropic's Claude API was unknowingly paying full price for repeated input tokens because silent bugs prevented prompt caching from working. The core issue is that Claude's cache relies on exact byte-for-byte prefix matching, meaning any change — however small — invalidates the entire cache. Three common mistakes were identified: embedding a dynamic timestamp in the system prompt, serializing config objects without sorted keys, and building per-user tool lists that varied at the front of the prompt. Fixing these issues involved moving volatile data to the user message, sorting JSON keys deterministically, and using a single stable tool list for all users. Once the prefix was consistent and a cache_control marker was added, cache hit rates became reliable and input-side API costs dropped by approximately 80%.

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