Anthropic · Model release
Claude Fable 5.1 and Mythos 5.1: the September refresh
Fable 5.1's 2026-09-01 launch announcement reported a leading position on the Artificial Analysis Intelligence Index v4.1.1 at $10/$50 pricing, with a restricted-access Mythos 5.1 sibling for cyber and bio research. Later index snapshots use different versions and model rows; this guide does not claim a current top-ranked model.
What changed
- At launch, Anthropic reported #1 on LiveBench (83.4, Max Effort) and AA Intelligence Index v4.1.1 (66). In the publisher-rounded v4.3.2 snapshot dated 2026-09-26, Fable 5.1 scores 53 at Max; this is a dated source-specific comparison, not a current ranking claim.
- Cache reads cut 75% to $0.25/MTok — roughly 25% savings on typical chat and ~45% on agentic workloads.
- Defaults to 'high' effort with adaptive thinking always on; knowledge cutoff June 2026. Mythos 5.1 (same underlying model, fewer cyber/bio safeguards) is invite-only for vetted US organisations.
Best fit
- Frontier agents
- Hardest reasoning work
- Fable 5 workloads at the same price
What to be careful about
- Anthropic now recommends Opus 5.5 by default and positions Fable 5.1 for demanding reasoning and long-horizon agentic work — at 2.5x Opus 5.5's token price.
- Cache-read savings only materialise when your harness actually reuses prompts — the default in Claude Code does.
Dataset snapshot: Claude Fable 5.1 is also represented in the checked comparison dataset. Prices and benchmark figures carry their own field dates.