I had a similar experience. So far Fable has been a game changer, at least for the work I used it for. Having said that, I think its writing is definitely worse than GPT 5.5. Ethan Mollick also observed the same. He called it more "Claudy." It generates worse academic prose than other frontier models.

I think the claude code harness made up a significant part of the improvements co-released with Fable, the nested agent capabilities seem to be much better even with opus (which I guess we're stuck with for a while).