# Claude Fable 5.1 and Claude Mythos 5.1 Announced A new era begins in artificial intelligence models - **Source:** [Hacker News (news.ycombinator.com)](https://brifing.si/source/13c923cd-6d99-442b-9b8b-1181f4d8cd91) - **Published:** 2026-09-01 - **Topic:** [Data & AI](https://brifing.si/c/data-ai) - **Original article:** - **On Brifing:** https://brifing.si/s/ec5c253e-e530-4d26-b8a4-8fcb840a0fc0 ## Next-Generation AI Models Anthropic has announced Claude Fable 5.1 and Claude Mythos 5.1 models. These are introduced as the world's most advanced AI solutions for coding and knowledge work. Based on the same core model, the two versions come with different security measures. While Fable 5.1 is generally available, Mythos 5.1 is accessible only through trusted access programs for cybersecurity and life sciences applications. ## Significant Price Reductions Claude Fable 5.1 will be approximately 25% more cost-effective than Fable 5 for typical workloads. The price reduction in cache reads enables these savings. For highly autonomous tasks, savings can reach up to 45%. This innovation offers substantial cost advantages, particularly for organizations running large-scale AI projects. ## Data Privacy Revolution The Enterprise Frontier Safeguards (EFS) system provides complete privacy with a zero data retention policy while effectively preventing malicious uses. Data will be stored entirely in customer-controlled cloud infrastructure. EFS will be rolled out to enterprise customers in phases starting this fall. Until implementation, eligible customers can use Fable 5.1 with zero data retention. ## Enhanced Security Measures New safeguards significantly reduce false positives. In cybersecurity, false positive blocking rates have been reduced by 60%. Fable 5.1 can now detect software vulnerabilities but cannot be used to develop exploit tools. In biology, a government-partnered access program will enable scientists to utilize Mythos 5.1's advanced capabilities. ## Performance Comparison Fable 5.1 shows marked performance improvements over Fable 5 in Terminal-Bench-Science tests. At low and medium effort levels, Fable 5.1 achieves similar or better results at lower costs. High effort is default in Claude Code, while medium effort is standard in Claude Cowork and Claude.ai. ## Real-World Success Stories Investment firm Millennium reported that Fable 5.1 identified the cause of a rare system crash that had eluded their engineers and other models for years. The model detected a bug in a third-party library, demonstrating capabilities beyond human analysts. Manual analysis would have required significantly more time and resources. ## Revolution in Coding and Prototyping Users report Fable 5.1 developed a complex prototype in approximately three days. The model researched service codes and documentation to create an innovative, extensible design. It operated unattended for hours, completing phases overnight and delivering visual walkthroughs of its progress each morning. ## Stability in Long Tasks While previous models became difficult to follow during extended operations, Fable 5.1 maintains readability throughout multi-step tasks. This capability provides major advantages for complex system designs and code reviews. Users also note qualitative improvements in output coherence. ## Enterprise Migration Begins Some companies are preparing to shift Opus 5 traffic to Fable 5.1. The model outperformed Fable 5 at lower costs in testing, offering an economical solution particularly for code review workloads. New cache read pricing makes Fable-class models viable for workloads previously maintained on Opus. ## New Tool for Scientific Progress Claude Mythos 5.1 is specifically designed for scientific research. Its advanced biology capabilities hold potential for groundbreaking discoveries in life sciences. A partnership with the US government will facilitate scientist access to the model, with enrollment expected to open soon. ## Industry Impact Fable 5.1's release may significantly alter software development and data analysis processes. Lower costs and enhanced capabilities could democratize AI access, especially for SMEs. The model's ability to detect rare bugs may improve software reliability and reduce crash rates in critical systems. ## New Stage in AI Evolution Claude Fable 5.1 and Mythos 5.1 demonstrate AI's potential to contribute to scientific advancement. These models set new industry standards not only through technical capabilities but also via security and privacy-focused approaches. Coming years will likely see increased application of these technologies in critical sectors like healthcare, finance, and security. ## Sources covering this story - https://www.anthropic.com/claude-fable-and-mythos-5-1 — --- _Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._