Isn’t codex open source and you can just go read what they do?

I have read the gemini source and it’s a pretty simple prompt to summarize everything when the context window is full

It should be noted that OpenAI now has a specific compaction API which returns opaque encrypted items. This is AFAICT different from deciding when to compact, and many open source tools should indeed be inspectable to that regard.

It's likely to either be an approach like this [0] or something even less involved.

0: https://github.com/apple/ml-clara