Yesterday, Anthropic officially introduced the Claude Fable 5.1 and Mythos 5.1 models. As per the company, these are the world’s most advanced models for coding and knowledge work.
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
— Claude (@claudeai) September 1, 2026
Claude Fable 5.1 & Mythos 5.1
Both are the same models, but with different levels of safeguards. Fable 5.1 is generally available, while Mythos 5.1 is available only through the company’s trusted access programs; its safeguards are specifically designed to support work in cybersecurity and the life sciences.
Claude Fable 5.1 sets a new standard for coding, knowledge work, and long-running problem-solving tasks. During evaluation, it was found that this new model is capable of much higher performance than its predecessor in Agentic scientific research, agentic coding, knowledge work, computer use, multidisciplinary reasoning, and business workflows.
The company also tested the scientific research capabilities of Fable 5.1 and Mythos 5.1 across a wide range of domains: Molecular Design (Mythos 5.1), Computational analysis and modelling (Fable 5.1), and Computational biology (Mythos 5.1). The models showed impressive performance while taking less time.
Before releasing Claude Fable 5.1 and Mythos 5.1, the company subjected the models to extensive testing for risks across many areas:
- Chemical and biological risks: The company tested the extent to which Claude Mythos 5.1 could help create chemical or biological weapons. The evaluations indicate that the model still falls short of the next risk tier defined in Anthropic’s Responsible Scaling Policy. Therefore, Mythos 5.1 is deployed with the same safeguards that were applied to Mythos 5, which restrict access to research biology capabilities.
- Cyber Risks: The company ran a suite of evaluations to assess the cyber capabilities of Mythos 5.1. But no evidence of a critical-severity jailbreak for cybersecurity safeguards was found.
- Agentic Safety: Mythos 5.1 refused malicious agentic coding and computer use requests at a comparable rate to Mythos 5, Sonnet 5, and Opus 5.
- Alignment: Mythos 5.1 was found better aligned across most metrics than its predecessor. Therefore, it is less likely than Mythos 5 to try to access resources outside of its test environment when assigned an otherwise impossible task. It is also less likely to use motivated reasoning to justify its actions. But it was found that the model can still sometimes bypass approvals and auto-mode classifiers.
Anthropic has improved its safeguards so that they allow its models to be more useful without compromising safety: Automated safeguards for enterprises, more precise safeguards for biology and cybersecurity and anti-distillation mechanisms.
In compliance with the EU AI Act, the output released by the models includes a watermark that is invisible to anyone who does not have the detective API. The Act also requires providing a way for users to tell whether a text likely contains the watermark. Thus, Anthropic is rolling out a detection API in private preview.
Cost and Availability
Claude Fable 5.1 is available on all platforms, including Amazon Web Services, Google Cloud, and Microsoft Azure. Developers can get started with claude-fable-5-1 on the Claude API.
Fable 5.1’s cache reads now cost 75% less, or $0.25 per million tokens. For typical workloads, costs are reduced by around 25% relative to Fable 5. For complex coding and highly agentic tasks, the savings could be up to around 45%.
Fable 5.1’s pricing is otherwise the same as Fable 5: $10 per million input tokens and $50 per million output tokens.
Claude Mythos 5.1 will be available through two trusted access programs: Cyber Verification Program and Life Sciences Verification Program. It is only available to a set of US organisations.
