GPT-5.5 dropped in the OpenAI API this week — more token-efficient, better at agentic workflows, already live on Vercel's AI Gateway. But the first line of OpenAI's own migration guide is a warning: "Treat it as a new model family to tune for, not a drop-in replacement." That's the part worth paying attention to. What actually changed Two variants: gpt-5.5 for agentic coding and multi-step too