Anthropic launched Claude Fable 5.1 for coding and knowledge work and the reporting compared it with Fable 5 using a set of practical tasks. Terminal-Bench-Science shows 52.6% for 5.1 versus 24.7% for 5. Fable 5 and 5.1 matched on four real-work tests, so any upgrade advantage may depend on whether your use case resembles the benchmark’s long, messy research agents, with day-to-day results looking similar.
Artificial Analysis released Intelligence Index v4.2, reworking the benchmark to emphasize harder tasks and more private held-out test data. Private, held-out test sets now make up 40% of the Index weighting, up from the 20% used in v4.1. The leaderboard order stays the same with Anthropic’s Claude Fable 5.1 first and OpenAI’s GPT-6 Astra second, while grading and several benchmarks are updated (with GPQA Diamond removed).
Anthropic used Claude to turn Andrew Wiles’ Fermat’s Last Theorem proof into computer-verifiable Lean code. The formalization consists of 13 million lines of Lean code and was completed in 11 days. The result is a large Lean proof file that can be checked automatically by computers, reducing human-error risk and easing sharing.
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