Model · Multimodal language model
Claude Mythos 5.1
An invite-only pinned Mythos-class Claude release with a 1M-token context, available through Anthropic's developer API under Project Glasswing and documented third-party cloud platforms, without downloadable model weights.
- Developer
- Anthropic
- Distribution
- Developer API · Third-party hosting
- Artifact licensing
- Proprietary access
- License scope
- Reviewed release artifacts and mandatory terms; not an assessment of training code or training data openness.
- Licenses
- Anthropic Commercial Terms of Service
- Boundary
- This record represents the Claude Mythos 5.1 release identified by the dateless pinned ID claude-mythos-5-1, which Anthropic documents as the same model as Claude Fable 5.1 offered by invitation only through Project Glasswing. Anthropic's API, Amazon Bedrock, Google Cloud, Microsoft Foundry, Claude Fable 5.1, Claude Mythos 5, and later Mythos releases remain separate service or product boundaries.
Risk statements
“CB-1” · Precautionary
We apply the same CB-1 designation and safeguards to Mythos 5.1 as we applied to Mythos 5. We emphasize that although we have not applied the CB-2 designation to either Mythos 5 or Mythos 5.1, we have conservatively expanded the safeguards applied to these models relative to those we applied to weaker models, including Opus 5.
- Risk areas
- Biological or chemical
- Covers
- The model itself
Read the developer's statement ↗ · 2026-10-07 · Research confidence: High
Anthropic names this release against “CB-1” in biological or chemical capability, as a precaution. The statement covers the model itself. This is the developer's own statement, not an Atlas risk rating.
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