Anthropic launches Claude Fable 5.1 and restricted Mythos 5.1

Anthropic introduced Claude Fable 5.1 and Claude Mythos 5.1, describing them as the same underlying model with different safeguards. Fable 5.1 is generally available for coding and knowledge work, while Mythos 5.1 is restricted to trusted access programs for vetted cybersecurity and life sciences users.

The release emphasizes lower operating costs and enterprise controls. Anthropic says Fable 5.1 will cost an estimated 25% less than Fable 5 for typical token-billed workloads, with savings up to approximately 45% for highly agentic work. Cache reads now cost 75% less, or $0.25 per million tokens. Enterprise Frontier Safeguards will let customers store data in their own cloud infrastructure while maintaining misuse detection, with rollout beginning later this fall.

Anthropic reports gains across coding, reasoning, business workflow, and scientific research benchmarks, alongside early examples in molecular design, Venus elevation mapping, and computational biology. Mythos 5.1 designed binders across 12 targets and sped up seven open-source deep learning models by up to 2.5 times, according to the company’s testing.

Safety updates focus on more precise safeguards, reduced false positives, and restricted access to higher-risk capabilities. Cybersecurity safeguards now block 60% fewer false positives than before, while biology safeguards fire 85% less often for benign requests related to elementary biology and medical questions.