Why is it labeled as new type of model with a version 5? We don't have Fable-4, what does it compare to?
More parameters is the simplest possible explanation for a smarter model. OpenAI/Anthropic constantly talk about 'the scaling laws', which come from the bitter lesson https://en.wikipedia.org/wiki/Bitter_lesson: scaling up compute is the most reliable way of getting better machine learning results.
Anthropic created Mythos/Fable for a reason. The labs are trying to consolidate around price points for model tiers. Fable is priced 2x Opus, but nowhere near ChatGPT Pro models. Likewise OpenAI doubled the price of GPT 5.4 -> 5.5 to about the price of Opus. Despite the hike, Fable is 2/3 the price of old Opus 3. Raw API price doesn't tell the whole story here because workflows have changed to automated long-running workflows rather than 1:1 chat conversations.