Claude Fable 5 and Mythos 5 Overview

Model Segmentation and Access

Anthropic has introduced two distinct model tiers: Fable 5 and Mythos 5. Fable 5 is the public-facing iteration designed for complex, long-running agentic workflows. Mythos 5 is a restricted, high-capacity model with fewer built-in safeguards, reserved exclusively for participants in Project Glasswing. This project is a collaborative initiative involving organizations like Amazon Web Services, Anthropic, and Apple, focused on leveraging advanced AI to defend against cybersecurity threats and secure software infrastructure. Fable 5 includes automated fallback mechanisms that revert to Opus 4.8 when a user query triggers specific safety boundaries.

Agentic Capabilities and Performance

Fable 5 is optimized for "Ultra Code" and "Dynamic Workflows," which allow the model to spawn hundreds of sub-agents that perform self-verification and inter-agent communication to complete multi-step tasks. Benchmarks indicate that Fable 5 significantly outperforms Opus 4.8 in accuracy across complex domains, including multi-disciplinary reasoning, biology, spatial reasoning, and legal analysis. The pricing for both models is set at $10 per million input tokens and $50 per million output tokens. Practical applications demonstrated include codebase-wide migrations, 3D CAD design, and complex fluid simulations. Notably, Stripe utilized these agentic capabilities to complete a large-scale codebase migration in one day, a task estimated to require two months of manual engineering effort.