Anthropic said in their announcement, “Claude Fable 5.1 and Claude Mythos 5.1 are the same model, but with different levels of safeguards. Fable 5.1 is generally available, while Mythos 5.1 is available only through our trusted access programs; its safeguards are specifically designed to support work in cybersecurity and the life sciences. Alongside its increased capabilities, Fable 5.1 takes important steps towards addressing the feedback we’ve received from customers on price, data retention, and safeguards.”

May be related to Anthropic’s previous announcement, “Fable 5.1 will cost an estimated 25% less than Fable 5 for typical workloads, wherever usage is billed by token. This is because we’re reducing our pricing on cache reads (where the model reads inputs that have already been processed and stored). For highly agentic work, the savings will often be much larger—up to approximately 45%.”

Key Upgrades & Technical Performance

• Next-Level Coding & Benchmarks: Fable 5.1 sets new performance records, scoring 52.6% on the Terminal-Bench-Science 0.1 benchmark (compared to 24.7% for Fable 5) and reaching 73.4% on CursorBench 3.2.0. The model excels at finding root causes in complex codebases—in one instance identifying a rare bug in an external vendor library that had eluded human engineers for years.
• Mythos 5.1 for Specialized Research: Available strictly through trusted access programs, Mythos 5.1 utilizes specialized, high-tier safeguards designed for advanced cybersecurity vulnerability discovery and scientific research in the life sciences.
• Significant Cost Reduction: Typical token-based workloads will see an estimated 25% cost reduction compared to Fable 5 due to lower pricing on cache reads. Highly agentic workflows can yield total savings of up to 45%.
• Enterprise Frontier Safeguards (EFS): Anthropic introduced EFS to deliver zero data retention capabilities by allowing enterprise users to store data directly within their own cloud infrastructure rather than on Anthropic servers.

Benchmark Comparison