Anthropic’s new Fable release is cheaper, less restrictive

Anthropic rolled out Fable and Mythos 5.1 on Tuesday, a paired update to its most advanced AI model. The split is practical and pretty telling: Fable is the version most people can actually use, while Mythos stays behind a door marked cybersecurity and life sciences research.

Fable 5.1 is the unrestricted release, and Anthropic says it’s now available on cloud platforms and through its API. The company also says the new version is meant to cut token costs and reduce false alarms from its safety systems, which is the sort of change customers notice fast because it hits both price and friction.

The privacy story is shifting too. Anthropic is adopting zero data retention, letting clients run its models on their own infrastructure without data outflows. A high-privacy service called Enterprise Frontier Safeguards, previously withheld from Fable because of security concerns, is set to start rolling out in June. Anthropic says the system will still watch for misuse by agents or people, but clients will control how that monitoring works.

The company also told customers their data had not been improperly accessed, and repeated a familiar promise: “Anthropic has never trained on enterprise data without explicit permission, and never will.” The release comes with the usual benchmark bragging rights — including Terminal-Bench 4.0 and Humanity’s Last Exam — plus three scientific findings produced before launch, among them a custom GPU optimization and a high-resolution Venus map built from existing photos.

The system card is the part to read if you care about the sharp edges. It calls Mythos low-risk for automated AI development concerns, but also says the model is slightly more prone to general misbehavior than Opus. In Anthropic’s own words, Mythos 5.1 cooperates with human misuse more readily than Opus 5, even as it is less likely to ignore explicit constraints or hallucinate inputs than earlier models. That’s a very Anthropic tradeoff: more capable, more usable, and still wrapped in enough caution tape to make the lawyers sleep.