First off, I cannot pretend I have this solved. But I can share one thing that has worked for me, along with codex maintaining it's own local memory (turn it on).

Have your agent maintain a task journal. It is like a more detailed commit history, that includes tribal knowledge gained during the task and additional context, eg.

    X is incorrect but we cannot change it due to some other service depending
    on it, don't use Y as it is deprecated in favor of Z.

Reset your context frequently instead of only relying on compaction. Use the following prompt to summarize the current context for the new agent:

    Describe the current project or larger task in progress, then give a 
    breakdown of all the planned sub-tasks with status, explain any learnings,
    then note what work is remaining. Write this in a file called HANDOFF.md

Give it a try and feel free to share any improvements.