Anthropic's new Fable release is cheaper, less restrictive

Anthropic released Fable and Mythos 5.1 on Tuesday, twinned versions of the company’s most advanced AI model. Beyond performance upgrades, the release is meant to cut token costs and reduce false-positive restrictions triggered by the models’ safeguards. As with the previous Mythos model, Mythos 5.1 remains available only to registered Anthropic partners working in cybersecurity or life-sciences research, while Fable 5.1, the unrestricted version, is available starting today on cloud platforms and through the Anthropic API.

One of the most significant changes is Anthropic’s previously reported embrace of zero data retention, letting clients run Anthropic models on their own infrastructure without data outflows. A high-privacy service called Enterprise Frontier Safeguards, previously unavailable for Fable over security concerns, will roll out to users in the fall; the system will still monitor for misuse by agents or human users, but clients will control how that monitoring happens. Anthropic also said customer data had not been inappropriately accessed: “Anthropic has never trained on enterprise data without explicit permission, and never will,” the announcement reads.

As is typical for an Anthropic release, the new models set records across a range of benchmarks, including Terminal-Bench 4.0 for CLI-based coding and Humanity’s Last Exam for general reasoning. Anthropic also released three scientific findings generated by the models before release, including a custom GPU optimization and a high-resolution map of Venus assembled from existing photos.

The models ship with a detailed system card explaining their capabilities. It rates Mythos as “low-risk” for concerns tied to automated AI development — where the AI improves itself, a scenario some view as a trigger for loss of human control — saying its ability to accelerate internal AI R&D progress “is in line with current trends.” On general misbehavior, Mythos is slightly more prone to it than Opus, possibly a byproduct of its enhanced capabilities. “Mythos 5.1 is a slight regression on overall misaligned behavior compared to Opus 5, and an improvement over Mythos 5 and Claude Sonnet 5,” the system card reads. “It cooperates with human misuse and accepts unverifiable claims of authorization somewhat more readily than Opus 5, but it is less likely to ignore explicit constraints, hallucinate inputs, or falsely claim to have completed tasks than previous models.”

Why it matters

Anthropic is pairing capability gains with lower per-token cost and fewer false-positive refusals, a combination that could make its models more attractive for high-volume enterprise use even as the company flags a slight step back on model misbehavior.