
GPT-6 Astra Tops Epoch AI's Capabilities Index, but Claude Fable 5.1 Leads on Software Engineering
OpenAI's GPT-6 Astra holds the highest general score on Epoch AI's Capabilities Index at 166.31, with a 90% confidence interval of 163.00 to 171.88, and also sets a new math record with a Math-ECI of 169.83. However, Anthropic's Claude Fable 5.1 leads on software engineering with a SWE-ECI of 167.44, reversing the overall ranking. The September 16, 2026 Data Insight also covers GPT-5.6 Sol and Moonshot's Kimi K3, and notes that Astra's ECI fell 2.9 points from its launch snapshot as more software engineering benchmarks were added.