Day 16 of Tibo Sottiaux's 28-day Codex improvement run, on October 20, was about caching. Logged-in users across Codex and ChatGPT Work now keep their prompt cache with no expiry, so returning to an old conversation hours later, or resuming after a break, is guaranteed to hit the cache with no penalty.

The same day, OpenAI announced a 12-hour prompt caching option in the API, aimed at persistent agents that reuse the same instructions and tools all day. Cache writes for the long option cost more than the default 30-minute cache, while reads stay at the discounted cached-input rate.

It is a practical, money-and-limits item rather than a model launch: cache hits mean repeated context is cheaper and, users hope, lighter on usage allowances. Day 17, October 21, had not landed at sweep time.