Agents can use git FWIW, and you can tell them to search old sessions by saying "Search through sessions in ~/.codex/sessions" and it'll find the most appropriate tools for doing so that is installed already. You can even add this to your system prompt or AGENTS.md and now you don't even have to prompt for it, it'll just look up the session history by itself.

Why this isn't built-in, I dunno, but been possible and easy for a very long time already, and works for any agent harness out there (as long as they persist sessions that is).

Personally I make the agent justify and explain things in the git commits, where is where that info went before agents anyways too, then have some sentences in my AGENTS.md about reading recent commits before doing changes, and using it whenever I prompt for history that isn't part of the current session. Seems to work perfectly fine.