Perhaps, there's a small change that might work better here - if we use a different agent (by creating custom agents) for plan and implementation, then the switch to the implementation agent wouldn't end up re-reading a lot as the new agent would have fresh empty context. I implemented this[1] from one of the posters on HN and I think it'd work well with this problem too. Would be great if you could point out if my understanding is wrong.

[1]: https://www.stavros.io/posts/how-i-write-software-with-llms/

The issue with this is just how much context it consumes. Generating the plan could easily get me to 30% of a 1M context window—the point at which I normally clear. Most of that context fill is not necessary for execution, and the model degrades as context fills. So starting from 30% (or passing a fresh agent all of that context) is not ideal. But maybe I'm misunderstanding what you're proposing?