Anthropic launches Fable 5.1 and Mythos 5.1 with lower costs and refined safeguards

Anthropic has released its latest pair of models, Fable 5.1 and Mythos 5.1, explicitly targeting feedback from enterprise customers regarding cost efficiency, data privacy, and the friction caused by overly broad safety measures. The company positions Fable 5.1 as a performance upgrade over its predecessor that is also significantly cheaper, claiming typical savings of around 25 percent and up to 45 percent for complex agentic workflows. This reduction is driven largely by lower pricing for cached data, a critical factor for developers running long-running agents that process large context windows repeatedly.

Performance and Cost Efficiency

Early reactions from industry leaders suggest the efficiency gains are tangible. Dan Shipper, CEO of Every, noted that the model is not only the strongest coding option his team has used but also more token-efficient and natural in its output. Aaron Levie of Box highlighted that his company’s agents using Fable 5.1 detected subtleties and ambiguities in data that the previous version missed in identical tests.

Benchmark data indicates that Mythos 5.1, when set to low reasoning modes, performs on par with the previous generation’s maximum reasoning settings, suggesting a more efficient inference path for developers who need high performance without the latency or cost of extended thinking chains.

Safeguards and Data Retention

Anthropic has also tweaked its safety architecture. Fable 5.1 features "more precise safeguards" designed to reduce false positives, such as blocking basic biology questions, whereas Mythos 5.1 retains the same biology restrictions as its predecessor. In a significant move for enterprise trust, Anthropic announced Enterprise Frontier Safeguards, which will store data on the customer’s own cloud servers rather than Anthropic’s infrastructure. This "complete privacy" feature is slated for a rollout later this fall.

Regarding cybersecurity applications, Anthropic now permits Fable 5.1 to be used for identifying software vulnerabilities. However, high-risk tasks such as penetration testing, exploit generation, and binary-based vulnerability scanning are still redirected to the Opus model line, maintaining a tiered approach to sensitive capabilities.

Availability

Claude Fable 5.1 is currently available on all platforms. Mythos 5.1 is restricted to participants in Project Glasswing, Anthropic’s early access program for advanced model testing.