GPT-5.6 Sol is the top of OpenAI's new three-tier lineup, designed to succeed GPT-5.5 as the go-to model when quality matters more than cost. It shares the same 1M+ context window and 128K output limit as its siblings but allocates more compute per token for deeper reasoning chains. Sol is the right pick for autonomous agent loops, complex code generation, and research synthesis where nuance matters. At $5/$30 per million tokens it matches GPT-5.5 pricing while delivering a meaningful capability jump.