GPT-5.6 Luna is the entry tier of OpenAI's mid-2026 frontier line, sitting below Terra and the flagship Sol. It keeps the family's full 1.05-million-token context window and 128,000-token output limit, but is priced roughly twenty-five times cheaper than Sol on input, which makes it the volume model of the range rather than its showpiece.
Its significance is less technical than commercial. On 6 August 2026 OpenAI made Luna the default model for ChatGPT's Free and Go tiers and removed the cap on text conversations there, so the weakest member of the family became the one most people actually talk to. A Think button, which spends more reasoning effort on a single message, follows from the week of 10 August. OpenAI's own testing claims Luna makes 62 per cent fewer factual errors than the model it replaces on legal, medical and financial prompts - a vendor figure, not an independent benchmark.