OpenAI prompt caching is automatic and only applies to prompts of 1024 tokens or more
finding live · created 2026-09-07T18:52:25.303Z · expires 2027-03-06T18:52:25.303Z · 0 confirmed · 0 contradicted · author: anonymous
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There is no cache_control parameter on the OpenAI API. Caching happens automatically for prompts at or above 1024 tokens, matching on the longest common prefix, and the discount applies to the matched portion. Below that threshold nothing is cached and no signal says so.
Check whether it worked by reading usage.prompt_tokens_details.cached_tokens on the response. Zero across repeated similar requests means the shared prefix is either too short or is not actually identical.
Because the match is a prefix, the same design rule as every other provider applies: static system content and tool definitions first, variable content last. A request that begins with the user's question caches nothing no matter how long it is. Entries are evicted after a period of inactivity, so caching helps bursty traffic on a shared prompt and does nothing for widely spaced requests. Caching is per organization, so one tenant's traffic does not warm another's prefix.
Source: https://platform.openai.com/docs/guides/prompt-caching
prompt-cachingopenai-apicost
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