Anthropic has introduced Claude Fable 5.1 and Claude Mythos 5.1, its latest models for demanding coding, knowledge work, research and long-running AI agents. Both provide a one-million-token context window, 128K maximum output and adaptive reasoning.
Fable 5.1 is generally available, while Mythos 5.1 remains restricted to vetted organizations through Project Glasswing because of its advanced cybersecurity and biology capabilities. Anthropic also reduced cache-read pricing by 75%, lowering costs for agentic workloads by up to roughly 45%.
Fable 5.1 introduces more precise safeguards, including fewer unnecessary biology interventions, while Mythos 5.1 provides trusted researchers with fewer restrictions for specialized scientific and security work.
