How Claude Code uses prompt caching changedprompt-caching
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9090### Turning on fast mode
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92Enabling [fast mode](/docs/en/fast-mode) adds a request header that is part of the cache key, so the next request reads the entire conversation history with no cache hits. Those uncached input tokens are billed at [fast mode rates](/docs/en/fast-mode#understand-the-cost-tradeoff), which is why turning it on at the start of a session costs less than turning it on deep into a long one. Enabling fast mode from a non-Opus model also [switches your model](#switching-models), which starts a fresh cache on its own.
92Enabling [fast mode](/docs/en/fast-mode) adds a request header that is part of the cache key, so the first request Claude Code sends with fast mode on reads the entire conversation history with no cache hits. Claude Code sets that header once when a turn starts and keeps it for the whole turn, so when you turn fast mode on while Claude is working, the cache miss from the header happens on the first request of your next turn. Those uncached input tokens are billed at [fast mode rates](/docs/en/fast-mode#understand-the-cost-tradeoff), which is why turning it on at the start of a session costs less than turning it on deep into a long one. If your current model doesn't support fast mode, enabling fast mode also [switches your model](#switching-models), and that switch starts a fresh cache on its own from the next request in the running turn.
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9494The cost applies once per conversation. After the first fast mode turn, Claude Code keeps sending the header and varies only the request's speed setting, which is not part of the cache key. Turning fast mode off, the [automatic fallback to standard speed](/docs/en/fast-mode#handle-rate-limits) after a rate limit, and turning it back on later all keep the cache. If you [run out of usage credits](/docs/en/fast-mode#handle-rate-limits) mid-session, Claude Code retries each rejected fast mode request at standard speed the same way, so this fallback also keeps the cache. `/clear` and `/compact` reset this, since they rebuild the cache at those points anyway.
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