OpenAI Previews GPT-5.6 Series: Sol, Terra, and Luna
OpenAI announced a limited preview of the GPT-5.6 series, including the flagship model Sol, a balanced model Terra, and a fast, affordable model Luna. Terra matches GPT-5.5 performance at half the cost, while Luna delivers strong capability at the lowest price. Pricing per 1M tokens: Sol $5 input / $30 output; Terra $2.50 / $15; Luna $1 / $6. The series also introduces improved prompt caching with explicit breakpoints and a 30-minute minimum cache life. Due to U.S. government engagement, the release begins with a limited preview for trusted partners before broader availability.
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Simon Willison’s Weblog
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26th June 2026
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. [...]
GPT‑5.6 is priced per 1M tokens across three model sizes: Sol is $5 input / $30 output; Terra is $2.50 input / $15 output; and Luna is $1 input / $6 output. GPT‑5.6 also introduces more predictable prompt caching, including support for explicit cache breakpoints and a 30-minute minimum cache life. For GPT‑5.6 and later models, cache writes are billed at 1.25x the model’s uncached input rate, while cache reads continue to receive the 90% cached-input discount.
— OpenAI, Previewing GPT‑5.6 Sol: a next-generation model
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This is a quotation collected by Simon Willison, posted on 26th June 2026.
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