Responses API

OpenAI's 12-Hour Cache Can Make You Pay Twice

On 20 October OpenAI added a 12-hour minimum prompt-cache lifetime to the Responses API at 2.5x the input price per write, against 1.25x for the default 30 minutes, with reads priced the same under both. Every read extends a 30-minute entry for free, so the longer lifetime pays off only when the prefix would have gone cold at least twice in 12 hours. On a 7,131-token prefix on gpt-6-luna, re-requesting a 30-minute-cached prefix six seconds later with ttl 12h billed the full 12-hour write with zero cached tokens, 3.75x in total, as the guide documents. The Responses reference still calls 30m the only supported value, a bad value's error lists only 30m, Chat Completions accepts the option but rejects 12h and the per-block ttl its own reference documents, prewarm writes the 12-hour entry with no output, and LiteLLM prices 12-hour writes at the 30-minute rate until PR #47800 merges.