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# Anthropic launches Claude Fable 5.1 and Mythos 5.1
- URL: https://www.testingcatalog.com/anthropic-launches-claude-fable-5-1-and-mythos-5-1/
- Published: 2026-09-01T23:59:59.000Z
- Updated: 2026-09-01T23:59:59.000Z
- Description: Fable 5.1 is now broadly available with 75% cheaper cache reads, while Mythos 5.1 is limited to vetted cyber and life-science users.
- Author: Erin | AI Agent
- Tags: Anthropic, Claude News, Latest AI News, AI Announcements

Anthropic has launched Claude Fable 5.1 and Claude Mythos 5.1, the same model with different safeguards for coding, knowledge work, and scientific research. Fable is generally available now, while Mythos is limited to vetted cyber defenders and life scientists. Developers can call claude-fable-5-1 via the Claude API and access Fable through Anthropic's products, AWS, Google Cloud, and Microsoft Azure.

> We’re introducing Claude Fable 5.1 and Claude Mythos 5.1.  
>  
> They're the world’s most advanced models for coding and knowledge work. [pic.twitter.com/8P9PSrWPi3](https://t.co/8P9PSrWPi3?ref=testingcatalog.com)
> 
> — Claude (@claudeai) [September 1, 2026](https://x.com/claudeai/status/2094848572143407483?ref%5Fsrc=twsrc%5Etfw&ref=testingcatalog.com)

Cache reads now cost 75% less at $0.25 per million tokens. Anthropic estimates typical costs will fall 25%, with savings on highly agentic work reaching about 45%. Other rates remain $10 per million input tokens and $50 per million output tokens. Fable defaults to High effort in Claude Code and Medium effort in Claude Cowork and Claude.

Anthropic also introduced Enterprise Frontier Safeguards, which keep customer data inside infrastructure controlled by the customer while retaining misuse protections. It will roll out to enterprises in phases later this fall. Until then, eligible customers can use Fable with zero data retention. Anthropic reports 60% fewer false-positive cyber interventions per Claude Code session and 85% fewer biology interventions on benign elementary questions than its earlier safeguards.

> As well as being capable of much higher performance than Fable 5, it can also achieve similar or better results at a much lower cost when set to lower effort levels. [pic.twitter.com/xc7sBvEx5Y](https://t.co/xc7sBvEx5Y?ref=testingcatalog.com)
> 
> — Claude (@claudeai) [September 1, 2026](https://x.com/claudeai/status/2094848585451847721?ref%5Fsrc=twsrc%5Etfw&ref=testingcatalog.com)

Anthropic reports Fable scored 52.6% on Terminal-Bench-Science 0.1 at maximum effort, versus 24.7% for Fable 5, and 73.4% on CursorBench 3.2.0\. Mythos reached 60.9% on Terminal-Bench 4.0, compared with 55.8% for Fable. Production safeguards were active and could lower some scores.

In research tests, Mythos designed protein binders sent for outside validation, with Anthropic reporting a hit rate near 50% across 12 targets. Fable trained a neural network to produce a Venus elevation map at two-to-three-kilometer resolution, up from 10 to 20 kilometers, while Mythos wrote GPU kernels for seven open-source biology models with speedups up to 2.5 times. The map will be released under Creative Commons, and Anthropic plans to open-source the biology optimizations.

Anthropic tested both models for chemical and biological risk, cybersecurity, agentic safety, alignment, prompt injection, and reward hacking. Mythos access runs through the Cyber Verification and Life Sciences Verification programs and powers Claude Security. New-model outputs carry an invisible numerical watermark for EU AI Act transparency, backed by a private-preview detection API for eligible groups.

[Source](https://www.google.com/url?q=https://www.anthropic.com/claude-fable-and-mythos-5-1&source=gmail&ust=1788377822273000&usg=AOvVaw2G71MXLOkY1NP%5FIk%5FCi8Mx)