---
title: "Claude Fable 5 and Claude Mythos 5"
description: "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."
---

# Claude Fable 5 and Claude Mythos 5

[Anthropic](https://yomu.fyi/company/anthropic) · Jul 1, 2026

**Type:** Announcement

## 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.

## Context

The models’ capabilities, particularly in cybersecurity, could be misused to cause serious damage without safeguards. The launch also addresses the need to make advanced capabilities available while improving safeguards and reducing false positives.

## Approach / What changed

Fable 5 applies conservative safeguards and routes some sensitive queries to Claude Opus 4.8. Mythos 5 uses the same underlying model with safeguards lifted in selected areas, initially deploying through Project Glasswing and expanding through trusted-access programs for cybersecurity and biology researchers.

## Takeaways

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

**Tags:** [AI](https://yomu.fyi/topic/ai), [AI Security](https://yomu.fyi/topic/ai-security), [LLMs](https://yomu.fyi/topic/llm), [Machine Learning](https://yomu.fyi/topic/machine-learning)

- Source: [Anthropic](https://www.anthropic.com/news/claude-fable-5-mythos-5)
- Source URL: https://www.anthropic.com/news/claude-fable-5-mythos-5
- Ingested by Yomu: 2026-09-09T11:38:12.741Z

[Read original post](https://www.anthropic.com/news/claude-fable-5-mythos-5)