Claude Fable 5.1
Anthropic · 2026-09-01
Claude Fable 5.1 and Claude Mythos 5.1 are deployments of Anthropic's September 2026 checkpoint with identical model weights. The default Fable 5.1 deployment is generally available with production safeguards; Mythos 5.1 relaxes domain-specific safeguards through trusted access programs. Safeguard interventions, fallback behavior, and API or consumer system prompts can change observed results and depend on the evaluation configuration.