Claude Fable 5.1 and Claude Mythos 5.1 (platform.claude.com)

Claude Fable 5.1 has been launched, enhancing its predecessor by offering the same pricing for input and output while significantly reducing cache read costs to a quarter of previous rates. This upgrade focuses on long-running agentic coding, advanced multistep research, and varying document types like spreadsheets and slides. Users are encouraged to choose Claude Opus 5 for general tasks but should switch to Claude Fable 5.1 for more complex reasoning needs. Notably, the update includes breaking changes such as alterations in forced tool usage and incompatibility with earlier models' thinking blocks. Additive features like per-message effort options and displayable progress updates also appear in the beta stage. The significance of Claude Fable 5.1 lies in its ability to boost productivity and efficiency in demanding AI tasks while making operations cost-effective. Key technical advancements include preserved thinking across conversation sessions, enabling a more seamless user experience when switching between different Claude models. Moreover, the addition of a statistical text watermark across outputs reinforces content authenticity without compromising quality. The model is designed to cater to intense workloads while maintaining low operational costs, making it an indispensable tool for AI practitioners and researchers.