Claude Fable 5.1 and Claude Mythos 5.1 launch
Fable 5.1 is generally available and up to 45% cheaper for agentic work; Mythos 5.1 stays limited to trusted cybersecurity and biology programs.
Claude adds two new models built for coding and knowledge work: Fable 5.1, generally available now, and Mythos 5.1, available only through Anthropic's trusted access programs for cybersecurity and life-sciences work. The two are the same underlying model, differing only in the safeguards applied.
Alongside the launch, Anthropic is addressing customer feedback on price, data retention, and safeguards for Fable 5.1:
- Price: Fable 5.1 costs an estimated 25% less than Fable 5 for typical token-billed workloads, due to cheaper cache reads. For highly agentic work, savings can reach roughly 45%.
- Data retention: a new system called Enterprise Frontier Safeguards will let enterprise customers store data on infrastructure they control, giving them the privacy of a zero data retention policy. It rolls out in phases starting this autumn; until then, eligible customers can use Fable 5.1 with zero data retention.
- Safeguards: cybersecurity safeguards now produce 60% fewer false positives, partly because Fable 5.1 can be used to find software vulnerabilities, though not to develop exploits for them. A biology access program built with the US government will give scientists access to Mythos 5.1's biology capabilities, with enrollment opening soon.
Fable 5.1 defaults to High effort in Claude Code and Medium effort in Claude Cowork and on Claude.ai. Full details, including benchmark comparisons against Fable 5, are in the original announcement.
