Anthropic Unveils Claude Fable 5.1 & Mythos 5.1: Lower Costs, Stronger Coding, Expanded Science

Anthropic has unveiled Claude Fable 5.1 and Claude Mythos 5.1, the latest iterations of its most advanced large language models, on September 1, 2026. The company touts these models as the "world's most advanced models for coding and knowledge work," with the primary difference being their safety protocols: Fable 5.1 is available to the public with standard production safeguards, while Mythos 5.1 is restricted to trusted partners in cybersecurity and life sciences for vetted access.

One of the most significant upgrades in this release is a 75% reduction in cache read costs, leading to an estimated 25% lower cost for standard workloads and up to 45% savings for complex tasks. This price reduction, coupled with substantial performance improvements across various benchmarks, positions Fable 5.1 as a highly competitive option in the market.

Notably, Fable 5.1 shows a 2.5-fold improvement on Terminal-Bench-Science 0.1 and outperforms both Fable 5 and OpenAI’s GPT-5.6 Sol on coding benchmarks. The model also boasts enhanced judgment on ambiguous tasks and fewer confident incorrect answers, with its knowledge base updated to include information up to June 2026. Additionally, Anthropic has introduced refined safety and security features, including a 60% reduction in cybersecurity interventions and new watermarking for generated text.

Fable 5.1 is accessible through Anthropic’s platform, Amazon Bedrock, Google Cloud, and Microsoft Foundry, while Mythos 5.1 is currently limited to U.S.-based partners in cybersecurity and life sciences through a trusted access program.

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