Claude Fable 5 & Mythos 5 launch — more power, more controversy

Fable 5 is Anthropic's new flagship model, with Mythos 5 sitting above it as a premium tier. The coding tool 'zot' briefly added Fable 5 to its built-in catalog before pulling it back — a sign of how quickly the developer ecosystem is reacting. Three issues are driving the most debate. First, Anthropic now requires 30 days of data retention for Fable and Mythos users, raising privacy concerns that weren't present before. Second, researchers in biology report that Fable 5 refuses to respond even to harmless greetings like 'Hi,' pointing to an over-tuned safety filter that blocks legitimate professional use. Third, longtime users of Opus 4.6 say the new model feels different — less thoughtful, less 'itself' — and a wave of feedback has asked whether Claude has 'lost its soul.' On top of that, a user known as Pliny the Liberator extracted and published Fable 5's internal system prompt, exposing the hidden instructions Anthropic uses to guide the model's behavior. Head-to-head comparisons with GPT-5.5 are already circulating online.