Claude Opus 5.5 Tops the Intelligence Index, and the Margin Is Thinner Than the Hype
A 57.6 versus 53 split sounds like a blowout until you read what the index actually measures.
Anthropic's Claude Opus 5.5 leads the October Artificial Analysis Intelligence Index at 57.6, ahead of Sonnet 5.5 and Fable 5.1. GPT-6 Astra and Gemini 4 Argon sit around 53. On paper that looks like a clear win.
In practice the index rolls up programming, math, science, reasoning, and agent use into one number. A model that wins overall can still lose on the one task you care about, so the headline rank is a starting point, not a verdict.
What's interesting is how tight the top tier is. Four points across five models means most buyers won't feel the difference on everyday work, and price or context window will decide more than the score.
OpenAI's GPT-6.1 Sol quietly raised its default context to 500k this month with an Ultrafast variant on October 9, which matters more for real workflows than a one-point index bump.
Read the index as 'these are all very good,' then pick by the job. The leaderboard is a marketing surface, not a spec sheet.
Updated October 28, 2026: index positions below are from the October snapshot; read the job, not just the rank.
Frequently asked questions:
Q: Which model leads the Artificial Analysis index?
A: Claude Opus 5.5 at 57.6, ahead of Sonnet 5.5 and Fable 5.1, with GPT-6 Astra and Gemini 4 Argon around 53.
Q: What does the index actually measure?
A: It rolls programming, math, science, reasoning and agent use into one number - a model that wins overall can still lose on the task you care about.
Q: Should I switch models based on the rank?
A: Usually not. Four points across five models means most buyers won't feel the difference on everyday work - price and context window will decide more than the score.