Anthropic Ships Claude Fable 5.1 and Mythos 5.1: Cheaper, More Private, Better at Code
Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, cutting costs by up to 45% for agentic work, adding customer-controlled data retention, and posting big gains on coding and science benchmarks.
What happened
Anthropic introduced Claude Fable 5.1 and Claude Mythos 5.1 on September 1. The two are the same model with different levels of safeguards: Fable 5.1 is generally available, while Mythos 5.1 ships only through trusted access programs for work in cybersecurity and the life sciences.
Lower price, stronger privacy
Fable 5.1 costs an estimated 25% less than Fable 5 for typical token-billed workloads, and up to roughly 45% less for highly agentic work, thanks to cheaper cache reads. The new Enterprise Frontier Safeguards framework promises zero-data-retention-level privacy while keeping data in cloud infrastructure the customer controls; it rolls out in phases starting later this fall. Until then, eligible customers can run Fable 5.1 with zero data retention.
Benchmarks and safeguards
Anthropic reports 52.6% on Terminal-Bench-Science 0.1 versus 24.7% for Fable 5, 55.8% on Terminal-Bench 4.0 and 31.4% on AutomationBench. Cybersecurity safeguards now produce 60% fewer false positives, and the model can find software vulnerabilities, though not build exploits. In a test at the investment firm Millennium, it traced a rare crash that had puzzled engineers for years.