Claude Fable 5.1 launches broadly; Mythos 5.1 remains trusted-access

Quick answer

Anthropic released Claude Fable 5.1 for general use and Claude Mythos 5.1 through trusted-access programs on September 1, 2026. The two names refer to the same underlying model with different safeguard configurations: Fable is the broadly available version, while Mythos gives vetted cyberdefenders and life-sciences professionals access to capabilities that Fable restricts. Fable 5.1 is available across Anthropic's platforms and cloud partners with the API identifier claude-fable-5-1. Anthropic kept standard input and output token prices unchanged, cut cache-read pricing, and reports lower typical workload cost. Neither exact 5.1 model is verified in Chat AI's current model directory.

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Fable 5.1 and Mythos 5.1 are one model with different safeguards

Anthropic says Claude Fable 5.1 and Claude Mythos 5.1 use the same underlying model. Fable 5.1 is the generally available version, with safeguards intended for broad coding and knowledge work. Mythos 5.1 applies more permissive cybersecurity and life-sciences boundaries for vetted people and organizations. The naming therefore describes access and safeguard levels, not two separately trained capability tiers.

Sources: Anthropic, Anthropic Help Center

Fable 5.1 is available across Claude products, the API, and cloud platforms

Anthropic lists Fable 5.1 across Claude on the web, mobile, and desktop, as well as Claude Cowork, Claude Code, Claude Design, Microsoft 365 integrations, and Claude Tag. It is available on all paid Claude plans, but how usage is billed depends on the plan and seat type. Developers can call the model as claude-fable-5-1 through the Claude API, and Anthropic says it is also available through Amazon Web Services, Google Cloud, and Microsoft Azure.

Sources: Anthropic, Anthropic Help Center

Cache reads cost less, while standard input and output prices stay the same

Anthropic prices Fable 5.1 at $10 per million input tokens and $50 per million output tokens, matching Fable 5. Cache reads cost $0.25 per million tokens after a 75% reduction. Based on four weeks of Anthropic's August 2026 usage data at default effort, the company estimates that typical token-billed workloads cost about 25% less than Fable 5 and that context-heavy, highly agentic work can save up to about 45%. Those percentages are first-party estimates, not guarantees for every request pattern.

Sources: Anthropic

Anthropic reports gains in coding, research, and long-running tasks

Anthropic's launch evaluations report higher Fable 5.1 scores than Fable 5 across its selected coding, scientific-research, knowledge-work, computer-use, and workflow benchmarks. Results vary by effort setting, tool access, and safeguard intervention, and the company notes statistical uncertainty for Terminal-Bench-Science. These provider-run measurements are useful for forming a test plan, but teams should compare Fable 5.1 on their own repositories, documents, tools, latency targets, and spending limits before changing production routing.

Sources: Anthropic

Fable now supports vulnerability discovery but still restricts exploit work

Anthropic says Fable 5.1 may be used to identify software vulnerabilities for defensive purposes. It still redirects or blocks several dual-use tasks, including penetration testing, exploit generation, and binary-based vulnerability scanning. The company estimates that its revised cyber safeguards cause about 60% fewer interventions per Claude Code session than the safeguards that launched with Fable 5. If automatic switching is enabled in Claude products, a flagged request may be rerun on an Opus model and that response will be labeled accordingly.

Sources: Anthropic, Anthropic Help Center

Mythos 5.1 access is limited to vetted cyber and life-sciences work

Anthropic says Mythos 5.1 access is organized through its Cyber Verification Program and Life Sciences Verification Program. The life-sciences program has enrolled its first participants, while Anthropic says the cyber program will add Mythos-class access in the near future. Current availability is limited to selected U.S. organizations, with broader domestic and international access planned. Claude Security now uses Mythos 5.1 for codebase vulnerability scanning and suggested patches that remain subject to human review. General availability, standard consumer-plan access, and an unrestricted public API route are not announced.

Sources: Anthropic

The exact 5.1 models are not yet verified in Chat AI

Claude Fable 5.1 and Claude Mythos 5.1 do not appear as exact entries in Chat AI's current verified model directory. This article therefore does not claim that either model is available in Chat AI. Model access can differ among Anthropic's own products, cloud platforms, trusted-access programs, and third-party multi-model apps, so the precise model name should be checked before assuming feature or pricing parity.

Sources: Chat AI, Anthropic

Frequently asked questions

What readers usually ask

What is the difference between Claude Fable 5.1 and Mythos 5.1?

Anthropic says they are the same underlying model with different safeguards. Fable 5.1 is broadly available, while Mythos 5.1 uses trusted-access programs for vetted cybersecurity and life-sciences work.

Is Claude Fable 5.1 available now?

Yes. Anthropic says it is available across its paid Claude products, the Claude API, and supported AWS, Google Cloud, and Microsoft Azure platforms. Plan inclusion and billing differ by product and seat type.

What is the Claude Fable 5.1 API model name?

Anthropic documents the API identifier as claude-fable-5-1.

How much does Fable 5.1 cost through the API?

Anthropic lists $10 per million input tokens, $50 per million output tokens, and $0.25 per million cache-read tokens. Actual workload cost depends on token use, cache reuse, tools, effort, and platform charges.

Can everyone use Claude Mythos 5.1?

No. Anthropic limits it to vetted cyberdefenders and life-sciences professionals through trusted-access programs, currently covering selected U.S. organizations.

Are Claude Fable 5.1 or Mythos 5.1 available in Chat AI?

Availability has not been verified. Neither exact 5.1 model appears in Chat AI's current verified model directory.

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