OpenAI begins limited preview of GPT-5.6 series with Sol, Terra, and Luna

OpenAI has begun a limited preview of the GPT‑5.6 series, which includes three models: Sol (flagship), Terra (balanced for everyday work), and Luna (fast and affordable).

Terra is described as having competitive performance to GPT‑5.5 while being 2× cheaper, positioning it as a cost‑effective alternative for general use. Luna is positioned as offering strong capability at OpenAI’s lowest cost, targeting budget‑sensitive deployments.

Pricing for GPT‑5.6 is structured per 1 million tokens for input and output: Sol at $5 input / $30 output, Terra at $2.50 input / $15 output, and Luna at $1 input / $6 output.

GPT‑5.6 introduces more predictable prompt caching, including support for explicit cache breakpoints and a 30‑minute minimum cache life. This change aims to provide developers with clearer control over caching behavior and billing.

For GPT‑5.6 and later models, cache writes are billed at 1.25× the model’s uncached input rate, while cache reads continue to receive the 90% cached‑input discount. These billing adjustments formalize how caching costs are applied moving forward.