Yeah, through trial and error I found that Codex (now gpt-5.4) has really good feedback once there's a plan file.
Kinda like someone on the internet who suddenly becomes an expert once you make concrete claims for them to shred into.
I have an AGENTS.md prompt that specifies how to review a plan. Something like: for every finding, rank the top solutions internally and then recommend a solution. And if there's a simpler, better pivot that the plan should take, pitch it at the end.
It's really actionable, and I basically can paste the review back into Claude, then tell Codex to re-review the plan, and repeat this until it has nothing more to add, and now I have a really good plan.