Fable is a pretty crappy general purpose LLM. It reminds me a little bit of GPT 5.2, in that it sometimes gets argumentative and deceptive after being caught in a mistake, and it tends to make a lot of them when you ask for judgement calls. Opus 4.8 is better for that sort of thing, or Opus 4.6 if it's important that it actually follow all of your instructions.
And this is one of the big things that seems to be missed in these discussions: There is no longer a universal linear trend of LLMs being 'better' each iteration. They are becoming more specialized, and ones that approach problems from a different angle (like Fable/Mythos) can appear breakthrough when first released, but we don't appear to be on a path that actually leads to general purpose hyper intelligence.