Sounds like that's the problem then, 275k is a tiny context window. I regularly have sessions that go to 450k or even up to 700k for an unattended overnight Claude Opus session.

Apparently OpenAI makes you manually setup their 1 Million context window, and it seems to be only documented on X:

https://x.com/thsottiaux/status/2089082893804896524

There's at least a forum thread about it here:

https://community.openai.com/t/why-does-codex-report-a-258-4...

But that 250k context worth way more than 1M in terms of how well it's utilized, so actually I do like codex trying to keep you at that sweet spot.