OpenAI GPT-5.6 Sol rivals Claude Fable 5 intelligence at one-third the cost

OpenAI released its GPT-5.6 model family this week, comprising three tiers — Sol, Terra, and Luna — targeting different price-performance trade-offs. According to benchmarks by Artificial Analysis, the flagship GPT-5.6 Sol scores 59 on the Intelligence Index, just one point below Claude Fable 5, while costing roughly one-third as much per task. In coding agent evaluations, Sol leads all models tested on the new Artificial Analysis Coding Agent Index, scoring 80 and outperforming Claude Fable 5 at approximately 40% lower cost. The release also introduces cache-write pricing for OpenAI, charging 1.25 times the base input rate for cached tokens — a structure already used by Anthropic that reflects real infrastructure costs. The entry-level Luna tier, priced at $1 per million input tokens, is positioned as a cost-efficient option that outperforms several comparable models including GPT-5.5 and Gemini 3.5 Flash.