Quoting OpenAI

OpenAI announced a limited preview of the GPT-5.6 series, including flagship model Sol, balanced model Terra, and fast/affordable model Luna. Terra matches GPT-5.5 performance at half the cost, while Luna offers strong capability at the lowest price. The preview, coordinated with the U.S. government, involves a small group of trusted partners before broader release.

We're beginning a limited preview of the GPT‑5.6 series: Sol, our flagship model; Terra, a balanced model for everyday work; and Luna, a fast and affordable model. Terra has competitive performance to GPT‑5.5 while being 2x cheaper and Luna brings strong capability at our lowest cost. ... We believe in broad access, and we plan to make GPT‑5.6 Sol, Terra, and Luna generally available in the coming weeks. As part of our ongoing engagement with the U.S. government, we previewed our plans and the models’ capabilities ahead of today’s launch. At their request, we are starting with a limited preview for a small group of trusted partners whose participation has been shared with the government, before releasing more broadly. — OpenAI https://openai.com/index/previewing-gpt-5-6-sol/ , Previewing GPT‑5.6 Sol: a next-generation model Tags: gpt https://simonwillison.net/tags/gpt , generative-ai https://simonwillison.net/tags/generative-ai , ai-security-research https://simonwillison.net/tags/ai-security-research , openai https://simonwillison.net/tags/openai , llms https://simonwillison.net/tags/llms