OpenAI fully released the GPT-5.6 series after a limited preview that began June 27: three capability tiers in one generation. Sol is the flagship, Terra the everyday model, and Luna the most cost-efficient — at $1/$6 per million input/output tokens, roughly a sixth of Sol's price.

The launch also folded Codex into ChatGPT itself, which Sam Altman called 'a really big deal' on the launch livestream — merging the conversational assistant and the coding agent into one surface.

Chief scientist Jakub Pachocki had previewed GPT-5.6 as a 'meaningful improvement' over GPT-5.5, with gains aimed at multi-hour agentic workflows rather than single-turn chat quality.