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Claude prompt caching fails due to timestamp bug, costing users more

A developer discovered that Claude's prompt caching feature was not working as intended, resulting in a 0% hit rate and increased costs. The issue stemmed from a dynamic timestamp included in the system prompt, which made each request unique and thus ineligible for caching. After relocating the timestamp to a user turn and addressing a secondary bug related to Python's set ordering, the cache hit rate improved significantly, leading to substantial cost savings. AI

IMPACT This highlights potential pitfalls in using LLM caching features and the importance of careful monitoring of usage metrics for cost optimization.

RANK_REASON The item details a bug in a specific feature of an AI model's API and its impact on a user's application and costs.

Read on dev.to — LLM tag →

AI-generated summary · Google Gemini · from 1 sources. How we write summaries →

Claude prompt caching fails due to timestamp bug, costing users more

COVERAGE [1]

  1. dev.to — LLM tag TIER_1 English(EN) · jidonglab ·

    Claude Prompt Caching Hit Rate Was 0%. One Timestamp Did It

    <p>I turned on Claude prompt caching, shipped it, and moved on. Nine days later I looked at the usage logs and saw <code>cache_read_input_tokens: 0</code> on every one of 3,412 calls.</p> <p>Not a low hit rate. Zero. And it was worse than zero, because every one of those calls pa…