Anthropic Launches Claude Fable 5.1 With Lower Costs and New Privacy Controls

Anthropic introduced Claude Fable 5.1 and Claude Mythos 5.1 in September 2026, calling them the world’s most advanced models for coding and knowledge work. [1] Claude Fable 5.1 and Claude Mythos 5.1 are the same model with different safeguard levels. [1] Fable 5.1 is generally available, while Mythos 5.1 is restricted to Anthropic’s trusted access programs, with safeguards designed for cybersecurity and life-sciences work. [1] Anthropic says Fable 5.1 delivers much higher performance than Fable 5 on coding, knowledge work, and long-running problem-solving tasks. [1]

Anthropic says Fable 5.1 will cost an estimated 25% less than Fable 5 for typical token-billed workloads because cache-read pricing is lower. [1] For highly agentic work, Anthropic estimates savings can reach approximately 45%. [1]

Enterprise Frontier Safeguards stores data in cloud infrastructure controlled entirely by the customer rather than Anthropic and is scheduled for phased enterprise availability beginning later this fall. [1] Until EFS becomes available, eligible customers can use Fable 5.1 with zero data retention. [1] Anthropic says its newest cybersecurity safeguards block 60% fewer false positives and permit vulnerability discovery but not exploit development. [1]

The launch separates broad availability from controlled access while combining Anthropic’s performance claims with changes to pricing, privacy, and safeguards. Compared with Fable 5, Fable 5.1 is positioned as both higher-performing and less expensive for typical token-billed workloads. The practical test will be whether customer-controlled EFS deployment delivers the promised privacy while preserving protection against adversarial use, and whether fewer false positives improve legitimate cybersecurity work without weakening exploit restrictions. What the sources don't address: how Anthropic will measure EFS performance or determine eligibility for its trusted access programs.