I have been using GPT 5.5 to review Opus implementation and vice versa.

This does not require any special tools, the skill creators in Claude Code or Codex can set this up for you in five minutes.

It is good for catching bugs, particularly edge cases, and it often suggests abstractions.

It does noting to make Opus deliver the more usable results Fable gives me for user facing features, where the UI typically looked and worked better out of the box with Fable. With Opus, I have to test it myself and give it my feedback first.