Anthropic releases Claude Fable 5.1
Anthropic releases Claude Fable 5.1 publicly on September 1, 2026, for long-horizon agentic coding, deep reasoning and multistep scientific research.
What happened
Event details
Anthropic announced Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026. They share the same underlying model with different safeguards: Fable 5.1 is publicly available, while Mythos 5.1 is offered only through trusted access programs. Official documentation lists a one-million-token context window, a 128K-token maximum output and a default effort setting of high for Fable 5.1. It accepts text and image input, produces text and uses the API model ID claude-fable-5-1. The launch page reports results on Terminal-Bench-Science 0.1, Terminal-Bench 4.0, OSWorld 2.0, Humanity's Last Exam, AutomationBench and CursorBench, alongside research examples in protein design, Venus topographic mapping and computational biology optimization. Pricing is $10 per million input tokens, $50 per million output tokens and $0.25 per million cache-read tokens. Anthropic says typical workloads cost about 25% less than with Fable 5, with savings of up to about 45% on highly agentic workloads.
Assessment
Why it matters
Anthropic introduces public and trusted-access release paths for the shared Fable 5.1/Mythos 5.1 model, changing support for long-horizon agentic work and scientific research, cache pricing, and enterprise privacy and security boundaries across APIs, cloud platforms and Claude products.
Availability
Access notes
Fable 5.1 is publicly available. Claude Mythos 5.1 is restricted to vetted cybersecurity and life sciences organizations.