OpenAI Codex released a new feature in the latest version v0.122.0
Plan Mode can start implementation in a fresh context, with context-usage shown before deciding whether to carry the planning thread forward
I consistently use the Opus model to create a highly comprehensive plan, and then switch to Auto mode for the build phase. This approach gives me the best balance across quality, time, and cost.
The only downside is that, to avoid context contamination, I have to manually save the plan and start a new session each time, which is quite cumbersome. It would be ideal if the planning mode could natively support something like Codex—executing the build in a fresh context.