Sources & verification notes

Claude Mythos 5.1

• Exact identifier & access

  Identity and lifecycle apply to this exact identifier and serving surface.

  • Pricing ↗Anthropic · provider-authored · checked Sep 23, 2026

• First availability

  Effective Sep 1, 2026. Checked Sep 23, 2026.

  Available to Project Glasswing participants only.

  • Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Claude Platform release notes ↗Anthropic · provider-authored · checked Sep 23, 2026

• Availability status

  Effective date not established; this is an observation. Checked Sep 23, 2026.

  Active, invite only.

  • Model deprecations ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026

• Capabilities & limits

  Effective date not established; this is an observation. Checked Sep 23, 2026.

  Forced tool use (tool_choice any/tool) returns a 400 error; auto and none work.

  • Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Claude Platform release notes ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Structured outputs ↗Anthropic · provider-authored · checked Sep 23, 2026

• Input price

  Effective date not established; this is an observation. Checked Sep 23, 2026.

  Derived from the captured rate card's standard token row.

  • Pricing ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026

• Output price

  Effective date not established; this is an observation. Checked Sep 23, 2026.

  Derived from the captured rate card's standard token row.

  • Pricing ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026

• Model card

  Effective date not established; this is an observation. Checked Sep 23, 2026.

  • Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Anthropic Transparency Hub ↗Anthropic · provider-authored · checked Sep 23, 2026

• Official announcement

  Effective Sep 1, 2026. Checked Sep 23, 2026.

  • Introducing Claude Fable 5.1 and Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026

• Full rate card

  Effective date not established; this is an observation. Checked Sep 23, 2026.

  • Pricing ↗Anthropic · provider-authored · checked Sep 23, 2026
  • Claude Mythos 5.1 ↗Anthropic · provider-authored · checked Sep 23, 2026

• License

  Effective date not established; this is an observation. Checked Sep 23, 2026.

  Not verified. These sources establish the identifier, not this missing fact.

  • Pricing ↗Anthropic · provider-authored · checked Sep 23, 2026