CONFIRMED: Anthropic launches Claude Fable 5.1

Anthropic officially launched Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026. They share the same underlying model but use different safeguard levels: Fable 5.1 is generally available, while Mythos 5.1 is restricted to trusted-access programs for cybersecurity and life sciences.

Built for long-running agentic work

Fable 5.1 targets demanding coding, research and multistep knowledge work. It offers a 1 million token context window, up to 128,000 output tokens and adaptive thinking that is always on. Anthropic says it sets a new performance frontier on long-running problem solving, with especially strong gains on agentic terminal and scientific research evaluations.

Cache reads are 75% cheaper

Base pricing remains $10 per million input tokens and $50 per million output tokens, but prompt-cache reads fall from $1 to $0.25 per million tokens. Anthropic estimates typical workloads will cost around 25% less than Fable 5 and highly agentic, context-heavy sessions may cost up to about 45% less.

Availability and model ID

Developers can access the model as claude-fable-5-1 through the Claude API, Amazon Web Services, Google Cloud and Microsoft Foundry. Fable 5.1 defaults to High effort in Claude Code and Medium effort in Claude Cowork and Claude.ai.

What this changes in practice

The launch arrives at exactly the point where companies are trying to move from chatbots to agents that can hold a goal across long sequences of tools and obstacles. The practical signal is not only the benchmark lead: lower cache-read pricing makes long sessions substantially more economical. Teams should still rerun their own evaluations before migrating production workflows, especially around tool use and refusal behavior. Official source: https://www.anthropic.com/claude-fable-and-mythos-5-1