Claude Mythos 5.1
Claude Mythos 5.1 is Anthropic's trusted-access sibling of Claude Fable 5.1, available only through approved programs rather than general self-serve access.
Mythos 5.1 defines an access and safeguard boundary, not a normal public upgrade tier. Anthropic says it uses the same underlying model as Fable 5.1 but different safeguards for vetted cybersecurity and life-science work, so most teams should plan around Fable or Opus unless they qualify for a trusted-access program.
Anthropic's launch post, Fable 5.1 model documentation, Project Glasswing information, and system card define the same-underlying-model relationship, restricted access, specifications, and safeguard posture. Anthropic's scientific and security results are publisher-reported. Public community reports cannot establish Mythos access or general availability.
- Approved defensive cybersecurity research and evaluation.
- Vetted life-science research under a trusted-access program.
- Controlled scientific investigation requiring the Mythos safeguard profile.
- Governed frontier-model evaluation by eligible organizations.
Claude Mythos 5.1 shares Claude Fable 5.1's underlying model and published specification profile, including a 1M-token context window, 128K maximum output, adaptive thinking, and text and image input. The difference is the safeguard and access path: Mythos 5.1 is not generally available.
- Model ID: claude-mythos-5-1 for approved accounts.
- Pricing follows the Fable 5.1 $10 input, $0.25 cache-read, and $50 output rates per million tokens.
- Access is routed through Anthropic, AWS, or Google Cloud account teams rather than public self-service signup.
Anthropic positions Mythos 5.1 for vetted cyberdefenders and life-science researchers. Current program language describes trusted access and a staged enrollment path, so an organization should not build a production plan that assumes immediate approval.
- Cybersecurity use is intended for approved defensive work under Project Glasswing.
- Life-science access uses a separate program developed with the US government.
- Anthropic says enrollment for scientists is planned, which is not the same as open enrollment today.
The practical difference is safeguard coverage and eligibility, not a separate public architecture or pricing tier. Use Fable 5.1 for generally available workloads. Treat Mythos 5.1 as an approved research route only when the task, organization, and provider account satisfy the program boundary.
Claude Mythos 5.1 FAQ
Common questions about Claude Mythos 5.1.
Is Claude Mythos 5.1 publicly available?+
No. Claude Mythos 5.1 is available only through Anthropic trusted-access programs for approved organizations and researchers. Most users should evaluate Claude Fable 5.1 or Claude Opus 5 instead.
Is Mythos 5.1 a different model from Fable 5.1?+
Anthropic says Claude Mythos 5.1 and Claude Fable 5.1 use the same underlying model. They differ in safeguards and access: Fable 5.1 is generally available, while Mythos 5.1 is restricted to approved programs.
How can an organization get Mythos 5.1 access?+
Eligible organizations must work through Anthropic, AWS, or Google Cloud account teams and satisfy the relevant trusted-access program requirements. A published model ID does not guarantee account eligibility or immediate approval.