--- title: "Anthropic Releases Fable 5.1 and Mythos 5.1: Cheaper, More Accurate, and with Zero Data Retention" description: "Anthropic has released Fable 5.1 and Mythos 5.1: the models are cheaper, more accurate on agentic tasks, and have moved to zero data retention. Access to Mythos 5.1 will be granted to partners only." date: 2026-09-02T12:22:01.000Z lang: en url: https://xab.info/en/posts/anthropic-fable-5-1-mythos-5-1-cheaper-accurate-zero-data-retention tags: [anthropic, ai-models, claude, enterprise-ai, data-privacy, llm] publisher: "XAB.info" --- # Anthropic Releases Fable 5.1 and Mythos 5.1: Cheaper, More Accurate, and with Zero Data Retention ![Stylized diagram of Anthropic's new Fable 5.1 and Mythos 5.1 models: higher accuracy, lower cost and zero data storage](https://xab.info/media/2026/09/02/anthropic-fable-5-1-mythos-5-1/anthropic-fable-5-1-mythos-5-1-1.webp) ## 🎯 Key Points - Anthropic has released Fable 5.1 and Mythos 5.1 β€” more performant and cheaper versions with fewer security false positives. - The key innovation is the shift to zero data retention and the upcoming Enterprise Frontier Safeguards service for enterprise control. - The models set records on Terminal-Bench 4.0 and Humanity's Last Exam, and before the release they independently optimized GPUs and built a map of Venus. - Fable 5.1 is available to everyone, while Mythos 5.1 is restricted to partners in cybersecurity and life sciences; the system card rates the self-improvement risk as low. Anthropic has released two updated versions of its flagship AI systems β€” Fable 5.1 and Mythos 5.1. According to the developers, both models have become more performant, cheaper to operate, and exhibit fewer false positives from their built-in safety systems. The release is accompanied by a revised data-handling policy and an updated system card in which the company openly assesses the behavioral risks of the new versions. ### Two Models, One Release The update affected both product lines: Fable 5.1 and Mythos 5.1 gained a performance boost and lower per-call costs, making them more accessible to mainstream developers. At the same time, Anthropic has relaxed the behavior of its safety filters, which in previous versions frequently blocked legitimate requests. This is directly reflected in independent reviews, which note the more "lenient" behavior of the new models while basic protective mechanisms remain in place. ### Zero Data Retention and Enterprise Control The main infrastructure innovation of this release is Anthropic's shift to a zero data retention policy. Clients can now run the models on their own infrastructure without the risk of data leaking to the company's external servers. According to industry publications, this decision was made following active pressure from enterprise customers, for whom data confidentiality was a key barrier to AI adoption. In the fall, users will additionally receive the Enterprise Frontier Safeguards service β€” high-frequency monitoring of AI actions under full client control. The developers also assured that companies' private data has never been used and will not be used to train models without explicit permission. ### Benchmarks and Independent Discoveries Both systems posted record results on Terminal-Bench 4.0, which evaluates command-line programming, and on the complex, comprehensive Humanity's Last Exam task suite. Notably, even before the official release, the models completed a number of independent tasks: they optimized GPU architecture and built a highly detailed map of Venus based on available photographs. These examples underscore the growth of agentic capabilities β€” the ability of AI to independently break down and execute multi-step tasks. ### Separate Distribution Policy Access to the models remains segmented. Fable 5.1 is available to all developers, whereas the Mythos 5.1 version will be provided by Anthropic exclusively to registered partners engaged in cybersecurity or life-sciences research. This approach allows the company to control the deployment of its most powerful model in sensitive domains. ### System Card and Risk Assessment Alongside the release, the company published a system card with an analysis of the AI's behavior. Experts assessed Mythos as a "low-risk" model with respect to scenarios of loss of human control due to autonomous self-improvement. At the same time, Mythos 5.1 shows a somewhat greater tendency toward incorrect behavior than the Opus 5 version, but outperforms Mythos 5 and Claude Sonnet 5 in reliability. Improved instruction-following is noted separately: the model less often ignores direct instructions, less often fabricates input data, and does not present fabricated task completion as real. ## πŸ” Fact-Check Verification - [Anthropic's New AI Lies Less: The Company Unveils Fable 5.1 and Mythos 5.1](https://www.rbc.ua/ukr/news/noviy-shi-anthropic-menshe-breshe-kompaniya-1788343616.html) - ΠŸΠΎΠ΄Ρ‚Π²Π΅Ρ€ΠΆΠ΄Π°Π΅Ρ‚ Ρ€Π΅Π»ΠΈΠ· ΠΎΠ±Π΅ΠΈΡ… вСрсий ΠΈ сниТСниС склонности ΠΊ Π²Ρ‹Π΄ΡƒΠΌΡ‹Π²Π°Π½ΠΈΡŽ (Π³Π°Π»Π»ΡŽΡ†ΠΈΠ½Π°Ρ†ΠΈΡΠΌ). - [Anthropic Releases Claude Fable 5.1 and Mythos 5.1 with Lower Prices and Softer Filters](https://vc.ru/ai/3116277-anthropic-claude-fable-mythos) - ΠŸΠΎΠ΄Ρ‚Π²Π΅Ρ€ΠΆΠ΄Π°Π΅Ρ‚ сниТСниС Ρ†Π΅Π½Ρ‹ ΠΈ смягчСниС Ρ„ΠΈΠ»ΡŒΡ‚Ρ€ΠΎΠ² бСзопасности. - [Anthropic Is Back in the Game: Claude Fable 5.1 β€” A Breakthrough in Agentic Tasks and AI Copy Protection](https://vc.ru/ai/3116673-claude-fable-5-1-anthropic-proryv-agentnyh-zadach) - ΠŸΠΎΠ΄Ρ‚Π²Π΅Ρ€ΠΆΠ΄Π°Π΅Ρ‚ сниТСниС Ρ†Π΅Π½Ρ‹ ΠΈ смягчСниС Ρ„ΠΈΠ»ΡŒΡ‚Ρ€ΠΎΠ² бСзопасности. - [Anthropic Drops Data Retention Requirement for Mythos and Fable 5.1 After Enterprise Pushback](https://www.cxtoday.com/security-privacy-compliance/anthropic-zero-data-retention-enterprise-ai/) - ΠŸΠΎΠ΄Ρ‚Π²Π΅Ρ€ΠΆΠ΄Π°Π΅Ρ‚ ΠΏΠ΅Ρ€Π΅Ρ…ΠΎΠ΄ Π½Π° zero data retention, ΠΌΠΎΡ‚ΠΈΠ²ΠΈΡ€ΠΎΠ²Π°Π½Π½Ρ‹ΠΉ Π΄Π°Π²Π»Π΅Π½ΠΈΠ΅ΠΌ ΠΊΠΎΡ€ΠΏΠΎΡ€Π°Ρ‚ΠΈΠ²Π½Ρ‹Ρ… ΠΊΠ»ΠΈΠ΅Π½Ρ‚ΠΎΠ². ## ❓ FAQ ### Q: What's new in Fable 5.1 and Mythos 5.1? **A:** Both models have become more performant and cheaper to use, and the number of false positives from the safety systems has decreased. Anthropic has also moved to a zero data retention policy. ### Q: What is zero data retention and why is it needed? **A:** It is a zero data retention policy: clients can run the models on their own infrastructure without the risk of data leaking to Anthropic's external servers. The decision was made in response to requests from enterprise customers. ### Q: Who has access to Mythos 5.1? **A:** Unlike Fable 5.1, which is available to all developers, the Mythos 5.1 version will be provided by Anthropic only to registered partners working in cybersecurity or life sciences. ### Q: What risks did Anthropic note in the system card? **A:** The risk of self-improvement and loss of control is rated as low. Mythos 5.1 is slightly more prone to incorrect behavior than Opus 5, but is more reliable than Mythos 5 and Claude Sonnet 5, and it follows instructions better and fabricates data less often.