Anthropic shipped Claude Fable 5.1 on September 1–2, 2026 at $10/$50 per million tokens with cache reads cut 75% to $0.25/MTok. Artificial Analysis's Intelligence Index and Coding Agent Index currently favor Fable 5.1 over GPT-6 Astra on aggregate scores. Mythos 5.1 shares the same underlying model with select safeguards lifted for Project Glasswing participants. Enterprise Frontier Safeguards (EFS) adds customer-cloud misuse detection for enterprises.