OpenAI's GPT-5.6 Sol scored 59 on the Artificial Analysis Intelligence Index, one point behind Anthropic's Claude Fable 5 at 60, with Claude Opus 4.8 third at 56. GPT-5.6 Terra and GPT-5.5 tied at 55, and GPT-5.6 Luna scored 51. Sol at max effort costs $1.04 per Intelligence Index task against Fable 5’s $2.75, offering nearly identical intelligence for roughly a third of the spend. Terra costs $0.55 per task and Luna $0.21, undercutting Sol by about 50% and 80% respectively. On the Artificial Analysis Coding Agent Index, GPT-5.6 Sol running in Codex scored 80, the highest of any model tested, leading in all three underlying evaluations. Sol’s per-task cost in Codex runs about 40% below Fable 5 in Claude Code and 10% below Opus 4.8. Terra and Luna scored 77 and 75 on the same index, with cost reductions of roughly 60% and 80% against Sol.