Anthropic launches Claude Fable 5.1 and Mythos 5.1, its most capable AI models yet. Along with gains in agentic coding and text quality, the company cuts costs by up to 45 percent.
Like their predecessors, both 5.1 models share the same base model but differ in safety guardrails. Fable 5.1 is broadly available, while Mythos 5.1 is restricted to special access programs for cybersecurity and life sciences.
They're also the first Claude models to ship with built-in watermarks. Anthropic is launching a detection API in private preview that lets regulators, media outlets, fact-checkers, and research institutions verify whether a text contains the watermark. The company plans to expand access over time, and interested parties can sign up here.
Fable 5.1 costs about 25 percent less than Fable 5 for typical workloads, with savings climbing to roughly 45 percent for heavily agentic tasks involving long, autonomous runs with many tool calls. Anthropic made that possible by slashing cache reads from $1 to $0.25 per million tokens. All other API prices remain unchanged at $10 per million input tokens and $50 per million output tokens. For comparison, Opus 5 runs at half that price with $5 input and $25 output per million tokens.
