Anthropic
Claude Mythos 5.1 is distinguished by its access boundary. It follows Claude Mythos 5, shares a model core with Claude Fable 5.1 according to Anthropic, and is available only through Project Glasswing’s trusted programs.
Anthropic describes safeguards for cybersecurity and life-science work on Mythos 5.1. Fable 5.1 is generally available, so Mythos 5.1 is not an openly purchasable alternative.
| Specification | Value and source |
|---|---|
| Weights | ProprietarySource |
| Access | APISource |
| Context window | 1,000,000 tokensSource |
| Knowledge cutoff | June 2026Source |
| Notes | Released September 1, 2026. Shares the model, specifications, and pricing of Fable 5.1, but uses fewer restrictive safeguards. Available by invitation only through Project Glasswing. Official API ID: claude-mythos-5-1.Source |
| API model ID | claude-mythos-5-1Source |
| Source status | active (invite only)Source |
| Release | 2026-09-01Source |
| Maximum output | 128,000 tokensSource |
| Input | Text, imagesSource |
| Output | TextSource |
| Reasoning mode | adaptive (always on); default highSource |
| Knowledge cutoff | 2026-06Source |
| Tool calling | YesSource |
| Structured output | YesSource |
| Availability | invite onlySource |
These measurements have no matching peer values under the same test conditions. Their original values and sources remain available here.
Each test shows the latest attempt, with no subsequent fixes to the generated code. Earlier runs remain available in the test details. These results do not contribute to an overall score.
A community classic for free-form SVG drawing.
Claude Mythos 5.1
No available test access
Not yet visually reviewed. A successful recording does not confirm correct physics or full compliance with the prompt.
Generate an SVG of a pelican riding a bicycle
Token limit including reasoning: 8,192 tokens
Task origin (Simon Willison)6.7 / 100
| Type | Evidence and data date |
|---|---|
| Price | Base API prices without caching or batch discounts. Higher context tiers and other rates are listed under Costs. |
| Input | Documented modalities. Retrieved September 7, 2026. |
| Output | Documented modalities. Retrieved September 7, 2026. |
| Research status | Research date September 8, 2026. 2 source URLs checked. This documents the inspected sources, not an exhaustive inventory of every publication. |
| Not found in the inspected sources | parameters-billions Original wording Anthropic does not publish parameter counts for this model. |
| Not found in the inspected sources | active-parameters-billions Original wording Anthropic does not publish active parameter counts for this model. |
| Not found in the inspected sources | profile-benchmarks-additional Original wording The shared-model table reports most rows under Fable 5.1; only the explicitly annotated Mythos 5.1 Terminal-Bench value is attributed here. |
| Additional source | Anthropic (retrieved September 8, 2026) · Anthropic, Fable 5.1 and Mythos 5.1 · Editorial description reviewed October 2, 2026 |
| Additional source | Migrating to Fable 5.1 and Mythos 5.1 · Editorial description reviewed October 2, 2026 |
| Additional source | OpenRouter: Popularity index (derived), CC BY 4.0 |
| Additional source | Vercel AI Gateway: Popularity index (derived), CC BY 4.0 |
| Additional source | Creative Commons Attribution 4.0 |