Claude Fable 5.1

Claude Fable 5.1 launched September 1, 2026 alongside the access-gated Mythos 5.1 — the same underlying model with different safeguard levels. Fable is the broadly available version of Anthropic's new tier above Opus, engineered for long-horizon tasks: multi-day software development, infrastructure management, and heavy data analysis without losing context. It thinks on every request by default, with an effort parameter replacing the old extended-thinking toggle.

Reported results include 95.0% on SWE-bench Verified, a perfect 100% on ProofBench v1.1 formal math proofs, and Terminal-Bench-Science more than doubling from Fable 5 to 52.6%. Pricing is $10/$50 per million tokens with cache reads cut 75% to $0.25. The honest picture: it is Anthropic's most expensive current model, Pro usage is metered through credits from the first token, safeguards route some cybersecurity and biology work to other models, and the scores are Anthropic's own. It also briefly vanished for foreign nationals in June 2026 under US export controls before access was restored July 1 — a reminder that flagship access can be geopolitically fragile.