GPT-5.6 Luna is the fastest and most cost-efficient model in OpenAI's GPT-5.6 family, designed for high-volume, latency-sensitive tasks. It is well-suited for chat, classification, and lightweight agentic workflows where speed and affordability are priorities. Luna is part of the GPT-5.6 series alongside Sol (flagship reasoning) and Terra (balanced performance), and advances the frontier on software engineering, professional knowledge work, and scientific research. During its initial preview, access is limited to a select group of trusted partners and organizations through the OpenAI API and Codex.