# Anthropic releases Claude Fable 5.1 and Mythos 5.1

> Anthropic released Claude Fable 5.1 and Mythos 5.1 on 1 September 2026.

*Cache reads down 75% to $0.25 per million; headline pricing unchanged at $10/$50.*

By The FeaturedDaily Desk · FeaturedDaily
Canonical: https://featureddaily.com/news/claude-fable-5-1-mythos-5-1-release

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on 1 September 2026. Both names refer to the same underlying model; the difference is safeguard level.

> Claude Mythos 5.1 is identical to Fable 5.1, but it offers more permissive safeguards for vetted individuals and organizations.
> — [Anthropic](https://www.anthropic.com/claude-fable-and-mythos-5-1), 2026-09-01

## Pricing

Headline pricing is unchanged from Fable 5 at $10 per million input tokens and $50 per million output. Cache reads dropped 75% to $0.25 per million, down from $1.

Anthropic estimates Fable 5.1 costs about 25% less than Fable 5 for typical workloads and up to approximately 45% less for highly agentic work. The figures come from the company's own four-week usage measurement in August 2026.

OpenAI launched GPT-6 Astra two days later at the same $10/$50 headline rates without a comparable cache-read discount.

## Capability and availability

Anthropic describes the models as the world's most advanced for coding and knowledge work. At Low or Medium effort, Fable 5.1 achieves results similar to or better than Fable 5 at much lower cost; higher effort tiers deliver materially more performance. Defaults are High in Claude Code and Medium in Claude Cowork and on Claude.ai.

On three targets in Adaptyv Bio's protein design competitions, Anthropic reports binding affinities ten times higher than the best submitted designs.

The model is generally available as claude-fable-5-1 across the Claude API, Amazon Web Services, Google Cloud and Microsoft Azure, and in Claude, Claude Code and Cursor. Mythos 5.1 is available through two trusted access programmes aimed at vetted cybersecurity and life-sciences organisations.

Anthropic notes that production safeguards can affect scores, and that its August 2026 OSWorld task release is not directly comparable with some previously published results.

Under Fable's production safeguards, requests flagged by classifiers relating to cybersecurity, biology and chemistry, or model distillation are handled by the less capable Claude Opus instead of Fable.

Anthropic publicly released Claude Fable 5 in June 2026 alongside the restricted Claude Mythos 5. The 5.1 release follows the same two-track structure.

Some coverage dates the launch to 2 September, reflecting announcement versus publication timing; Anthropic's own materials point to 1 September.

## Key takeaways

- Released 1 September 2026; both names refer to the same underlying model at different safeguard levels.
- Headline pricing unchanged: $10 per million input tokens, $50 per million output.
- Cache reads cut 75% to $0.25 per million tokens.
- Anthropic estimates ~25% lower cost for typical workloads, up to ~45% for highly agentic work.
- Available as claude-fable-5-1 across the Claude API, AWS, Google Cloud and Azure, plus Claude Code and Cursor.

## FAQ

### What does Claude Fable 5.1 cost?
$10 per million input tokens and $50 per million output, unchanged from Fable 5, with cache reads cut 75% to $0.25 per million.

### What is Claude Mythos 5.1?
The same model as Fable 5.1 with more permissive safeguards, restricted to vetted organisations through two trusted access programmes.

### Where is it available?
The Claude API, AWS, Google Cloud, Microsoft Azure, plus Claude, Claude Code and Cursor.

## Sources

- [Introducing Claude Fable 5.1 and Claude Mythos 5.1](https://www.anthropic.com/claude-fable-and-mythos-5-1) — Anthropic, 2026-09-01
- [Anthropic's Claude Fable 5.1 and Mythos 5.1 arrive with a 75% cost reduction for Fable cache reads](https://venturebeat.com/technology/anthropics-claude-fable-5-1-and-mythos-5-1-arrive-with-a-75-cost-reduction-for-fable-cache-reads) — VentureBeat, 2026-09-01
- [Anthropic releases Claude Fable 5.1 and Mythos 5.1](https://sdtimes.com/claude-fable-5-1/61089/) — SD Times, 2026-09-02
- [Anthropic Launches Claude Fable 5.1 With Lower Costs and Fewer False Positives](https://www.macrumors.com/2026/09/01/anthropic-claude-fable-5-1/) — MacRumors, 2026-09-01