Anthropic ships Fable 5.1 and Mythos 5.1

Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, an upgrade to its most capable model tier rather than a new generation. Anthropic says Fable 5.1 does better at coding, knowledge work, and extended problem-solving, and specifically calls out that it “avoids shortcuts that result in poorer-quality work” and is better at tracing a software bug to its actual root cause instead of patching symptoms. Mythos 5.1 is the same underlying model with its safeguards loosened for cybersecurity and life-sciences research, and stays restricted to Anthropic’s trusted-access programs rather than general release.

The pricing change is the part worth checking against your own bill. Anthropic cut cache-read pricing, which brings typical workload costs down roughly 25%, and highly agentic workloads, the kind that lean hard on cached context across many tool calls, can see savings up to 45%. Every other price on the Fable line stays the same as Fable 5.

Anthropic’s headline example: Fable 5.1 diagnosed a rare system crash at investment firm Millennium that the firm’s own engineers had been unable to solve for years. Fable 5.1 is generally available now; Mythos 5.1 requires enrollment in Anthropic’s trusted access program. The release follows Anthropic’s automated alignment researcher work published two days earlier and lands the same day as Anthropic’s new Enterprise Frontier Safeguards product and its $35 billion Lambda compute deal, part of the same broader push to ship capability, safety tooling, and infrastructure commitments in the same week.