Fable feels like a slightly more advanced 4.5/4.6 (less verbose than 4.7 and 4.8) with more adversial work checking. And a lot more compute to be more thorough from the first prompt. I feel it would be possible to get pretty much the same results with 4.6 with enough back and forth iterations. It kind of makes sense to me that this is the 'magic' behind Mythos and its cyber capabilities too. Just a massive iterative loop and really going into a lot more detail on edge cases.