Three tiers, one generation
GPT-5.6 is the model family OpenAI released in 2026. It is not one model but three tiers differing in capability and cost: Sol, Terra, and Luna.
Beyond writing and research, it targets agentic work that uses tools across long tasks, software development, and specialist analysis. It is available through ChatGPT, Codex, and the OpenAI API.
The tiers
- Sol: the top tier, for difficult and complex work
- Terra: the everyday tier, balancing quality, speed, and cost
- Luna: fast and inexpensive, for routine tasks and high-volume processing
The generation number 5.6 and the tier names Sol, Terra, and Luna are separate axes. OpenAI positions the tier names as continuing across generations.
Using it with Codex
GPT-5.6 is available in Codex for agentic development work — reading a repository, editing files, running tests and commands. Sol in particular suits multi-stage implementation and long-running tasks.
There is no need to use Sol for everything. Terra for well-defined everyday work and Luna for simple conversion or repetition keeps speed and usage in balance.
Things to check
Separately from the tier, a reasoning effort setting controls how much time and computation goes into thinking. The same Sol behaves quite differently depending on it, so check the setting as well as the model name.
Pricing, plan availability, and which tiers appear in each app can change. Check OpenAI’s current documentation for availability.