Claude Fable 5 and Claude Mythos 5

Summary

Claude Fable 5 and Claude Mythos 5 are presented as a joint launch of the same underlying model, with Fable 5 intended for general use and Mythos 5 initially reserved for selected cyberdefenders and infrastructure providers. Fable 5 uses conservative safeguards that route some sensitive queries to Claude Opus 4.8, while Mythos 5 lifts safeguards in selected areas through Project Glasswing and a planned trusted-access program. The post reports strong results in software engineering, knowledge work, vision, long-context tasks, cybersecurity, protein design, molecular biology, and genomics, including a reported 50-million-line Ruby migration completed in a day. Both models are priced at $10 per million input tokens and $50 per million output tokens, with Fable 5 available through the Claude API and staged subscription access.

Takeaways

1. 1

   Fable 5 reportedly completed a codebase-wide migration in a 50-million-line Ruby codebase in one day, compared with more than two months of manual work for a whole team.

2. 2

   Fable 5’s safeguards trigger in fewer than 5% of sessions on average, but the post notes that they can also block harmless requests.

3. 3

   Mythos 5’s protein-design work produced strong drug-design candidates for nine of 14 targets, while internal experts reported that the model accelerated aspects of the process by around 10 times.