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Claude Fable 5 and Mythos 5 Released

Summary

Anthropic released Claude Fable 5, a Mythos-class model with safety safeguards for general use, alongside Claude Mythos 5, an unrestricted variant for authorized cybersecurity and research professionals. Fable 5 shows state-of-the-art performance across coding, knowledge work, vision, and long-context benchmarks, at $10 per M input tokens and $50 per M output tokens (less than half the cost of Mythos Preview). Safety classifiers auto-route sensitive requests (cybersecurity, biology/chemistry, distillation attempts) to Opus 4.8, with over 95% of Fable sessions requiring no safeguard fallback.

Key Details

  • Positioning: Fable 5 = safety-gated Mythos-class model for public/enterprise use; Mythos 5 = unrestricted variant for Project Glasswing cyber defenders + limited biomedical research access
  • Performance highlights: 50-million-line Ruby migration in one day; top scores on finance benchmarks; scientific hypothesis generation across 138 animal species
  • Pricing: $10/M input, $50/M output — ~2× Opus 4.8 cost, ~3× Sonnet 4.6 cost
  • Safety architecture: 3 classifier categories (cybersecurity / biology-chemistry / distillation-prevention) route to Opus 4.8; 1000+ hours external red-teaming, no universal jailbreaks
  • Data retention: 30-day retention for all Mythos-class traffic; no training use except safety
  • Availability: Everywhere from 2026-06-09; free on Pro/Max/Team/Enterprise through 2026-06-22, then requires credits

Why Rolf Thinks This Matters

Further Reading