You can get fable-ish performance with gpt 5.5 watching over opus output. Although it fundementally cannot work as well because gpt 5.5 doesn't see the thinking process behind opus 4.8 unlike fable which presumeably self-steers and is natively trained for it.
See more: https://omp.sh - turn on advisor and set advisor role to gpt 5.5 xhigh thinking.
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.
Advisor is such a killer hidden feature
Also burns through token budgets