Quoting OpenAI

OpenAI announced a limited preview of three new GPT-5.6 models: Sol (high-performance), Terra (balanced), and Luna (fast and affordable), with pricing ranging from $1-$30 per million tokens depending on the model and whether the input or output is being processed. The company is starting with a limited preview for trusted partners approved by the U.S. government before making the models more broadly available, and the new models include improved prompt caching (a feature that stores frequently used inputs to speed up responses) with explicit cache breakpoints and longer minimum cache duration.