Model · Multimodal language model
Claude Fable 5.1
A pinned Mythos-class Claude release in current status with a 1M-token context, available through Anthropic's developer API 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 Fable 5.1 release identified by the dateless pinned ID claude-fable-5-1, the current model of the Fable line with a documented breaking-change surface against Fable 5. Anthropic's API, Amazon Bedrock, Google Cloud, Microsoft Foundry, Claude Platform on AWS, Claude applications, Claude Fable 5, and later releases remain separate service or product boundaries.
Risk statements
“CB-1” · Precautionary
On chemical and biological risks, we judge that the model has CB-1 capabilities—meaning it could meaningfully help someone with a basic technical background synthesize a known weapon—but falls short of the CB-2 threshold for functionally replacing rare expert talent, which is the limiting factor in developing novel weapons. We hold this judgment with some uncertainty, and are deploying Claude Fable 5.1 with the same biological safeguards we deployed with Claude Fable 5.
- Risk areas
- Biological or chemical
- Covers
- Safeguards on a release channel
Read the developer's statement ↗ · 2026-10-07 · Research confidence: Low
Anthropic names this release against “CB-1” in biological or chemical capability, as a precaution. The statement covers safeguards on a release channel. This is the developer's own statement, not an Atlas risk rating.
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