Claude Fable 5

Claude Fable 5 (`claude-fable-5`) was Anthropic's most capable widely released model at its June 9, 2026 launch and the first public Mythos-class release. It ships with a 1M token context window, 128K max output, always-on adaptive thinking, and pricing of $10 per million input tokens and $50 per million output tokens - double Opus 4.8. The headline integration change is safety classifiers: requests in offensive cyber, most biology and chemistry, and model distillation return a `refusal` stop reason or route to Opus 4.8, with new server-side fallback and fallback-credit mechanics in the API. It is GA on the Claude API, Amazon Bedrock, Vertex AI, and Microsoft Foundry, and included on Pro, Max, Team, and seat-based Enterprise plans at no extra cost through June 22, 2026, after which it requires usage credits. It was the top of the Claude family for multi-day autonomous runs and the hardest reasoning work until Claude Fable 5.1 shipped on September 1, 2026.