Anthropic has launched Fable 5.1 and Mythos 5.1, two versions of its most advanced model released in tandem. Fable 5.1 is available immediately on cloud platforms and through the Anthropic API, with a lower token cost and fewer false-positive restrictions in its safety filters. Mythos 5.1 is limited to registered Anthropic partners working in cybersecurity or life sciences.
The release is the first time Anthropic has offered an enterprise-tier service for its flagship model on privacy grounds that previously blocked it. A high-privacy offering called Enterprise Frontier Safeguards is planned for the fall.
What Fable 5.1 changes for users
Fable 5.1 reduces the per-token cost compared with earlier versions, lowering the barrier for teams running high-volume workloads. The release also trims the rate of false-positive restrictions produced by the model’s safeguards, meaning fewer requests are incorrectly flagged or refused.
Developers can run the model on their own infrastructure under a zero data retention arrangement, so inputs and outputs stay within the client’s environment and do not flow back to Anthropic. Anthropic has also stated that it has never trained on enterprise data without explicit permission.
How Mythos 5.1 fits the release
Mythos 5.1 is the restricted-access twin of Fable 5.1. Access is gated to partners registered with Anthropic who operate in cybersecurity or life sciences, sectors where the model’s capabilities are most directly relevant and most carefully scoped.
The system card accompanying the release describes Mythos as low-risk for automated AI development and notes a slight regression on misaligned behaviour compared with Opus 5. Records on standard evaluations including Terminal-Bench 4.0 and Humanity’s Last Exam are included in the system card.
Enterprise Frontier Safeguards and privacy controls
Enterprise Frontier Safeguards is a new tier designed for organizations that need stronger privacy guarantees. It arrives in the fall and represents a category that was previously unavailable for Fable on security grounds.
Misuse monitoring continues under the new tier, but clients control how that monitoring is configured within their own deployments. Combined with the option to self-host the model, the design gives enterprises a way to use Fable 5.1 without sending sensitive data outside their infrastructure.
Scientific findings from pre-release testing
Three novel scientific findings emerged from the models during pre-release testing. One is a GPU optimization that improves compute efficiency. Another is a high-resolution map of Venus assembled from existing photographs, raising the quality of surface data available for the planet.
These results point to a model that is already producing usable research output in domains that range from hardware engineering to planetary science.
FAQ
What is the difference between Fable 5.1 and Mythos 5.1?
Fable 5.1 is the unrestricted version available on cloud platforms and via the Anthropic API. Mythos 5.1 is limited to registered Anthropic partners in cybersecurity or life sciences.
How does Fable 5.1 reduce costs?
Fable 5.1 lowers the per-token cost compared with prior releases, and the release also reduces false-positive restrictions in its safeguards so fewer requests are unnecessarily refused.
Can enterprises run Fable 5.1 on their own infrastructure?
Yes. Clients can run the models on their own infrastructure with zero data retention, and the upcoming Enterprise Frontier Safeguards tier adds further privacy controls with client-configured misuse monitoring.