GPT-6 Astra leads Epoch Capabilities Index with ECI of 166

OpenAI’s GPT-6 Astra has claimed the top spot on the Epoch Capabilities Index, posting a composite score of 166 and outranking all 249 models currently tracked by the research organization. The model, which began rolling out on September 3, 2026, represents the highest single-score performance Epoch AI has ever recorded, surpassing both GPT-5.5 Pro (162) and Anthropic’s Claude Fable 5.1 (164). But there’s a catch buried in the numbers. Astra’s software engineering score sits at just 47% on the MirrorCode benchmark, a category where Claude Fable 5.1 posts 73%. For a model billing itself as the most capable AI system on the planet, that gap is hard to ignore. What the numbers actually show The Epoch Capabilities Index isn’t a single test. It aggregates performance across 37 to 59 benchmarks into one composite score, normalizing results so that comparisons across model generations are meaningful. A difference of roughly 10 points on the ECI scale is considered statistically significant, which means Astra’s two-point lead over Claude Fable 5.1 is notable but not exactly a blowout. Where Astra truly flexes is in math, continual learning, and game-puzzle categories. The model achieved 98...