Anthropic released Claude Fable 5.1 and Claude Mythos 5.1 on Sept. 1. They’re the same underlying model sold with different safeguards. Fable 5.1 is generally available through the API and cloud platforms. Mythos 5.1, which has fewer restrictions for cybersecurity and biology work, is limited to vetted organizations, TechCrunch reported.
“Claude Fable 5.1 sets a new standard on coding, knowledge work, and long-running problem-solving tasks,” Anthropic said, according to 9to5Mac. The company said that at low or medium effort, the model “achieves similar or better results than Fable 5 at a much lower cost.” In one example Anthropic gave, investment firm Millennium used Fable 5.1 to find the cause of a rare crash in its internal systems that its engineers and other models hadn’t been able to explain for several years.
Cheaper to run
List pricing hasn’t changed: Mythos 5.1 starts at $10 per million input tokens and $50 per million output tokens, according to Anthropic’s Mythos page. Anthropic cut the price of cache reads, though, and said Fable 5.1 will cost an estimated 25% less than Fable 5 for typical token-billed workloads. For highly agentic work it said savings could reach about 45%, since those sessions reread a lot of stored context.
More precise safeguards
Fable 5.1 can now identify software vulnerabilities in source code. Anthropic said its biology safeguards step in on benign requests 85% less often than the ones Fable 5 launched with. The safeguards still block penetration testing, exploit generation and binary-based vulnerability scanning, and they route dual-use biology and chemistry questions to Anthropic’s Opus models. Using Mythos 5.1 requires accepting a 30-day data retention policy for safety monitoring by default.
For enterprises, the bigger change may be Enterprise Frontier Safeguards. That’s a zero-data-retention option, rolling out this fall, that lets customers run the model on their own infrastructure while keeping control of how misuse monitoring works. “Anthropic has never trained on enterprise data without explicit permission, and never will,” the announcement said.
The system card calls Mythos 5.1 “a slight regression on overall misaligned behavior compared to Opus 5, and an improvement over Mythos 5 and Claude Sonnet 5.” It adds that the model “cooperates with human misuse and accepts unverifiable claims of authorization somewhat more readily than Opus 5.”
The release comes after a rocky summer for the Mythos line. Anthropic took Mythos 5 offline on June 12 and restored access for a set of U.S. organizations on July 1, after export controls on the model were lifted with U.S. government approval.
WebProNews is an iEntry Publication