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Prompt Caching: Why cache_control Writes But Never Reads

This story is from 2026-09-12. It is preserved in the archive; the latest stories are on the live feed.

I turned on prompt caching for an agent loop that resends a 12K-token system prompt on every turn. Obvious win, right? Input tokens are the whole bill in a tool loop. The bill went up. Not a little. Roughly a quarter. And nothing in the logs looked wrong. Every request returned 200. Latency was the…

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  1. 2026-09-12 16:50 · DEV Community — AI
    Prompt Caching: Why cache_control Writes But Never Reads

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