OpenAI Previews GPT-5.6 With Sol, Terra, and Luna: Tiered Models, New Reasoning Modes, Limited Access

OpenAI has begun a limited preview of GPT-5.6 , its next-generation model series. The lineup splits into three named tiers: Sol, Terra, and Luna. Sol is the flagship. Terra targets everyday production work. Luna is the fast, low-cost option.

OpenAI is starting with a small group of trusted partners through the API and Codex. According to OpenAI post, they shared the models and plans with the U.S. government first. Broader access in ChatGPT, Codex, and the API is planned in the coming weeks.

The change is mostly structural. GPT-5.6 introduces tiered models, two new reasoning modes, and a heavier safety stack.

GPT-5.6 is a family, not a single model. OpenAI also changed how it names releases. The number now marks the generation. The names mark durable capability tiers.

Each tier can advance on its own schedule. That gives developers a clearer choice across intelligence, speed, and cost.

OpenAI calls Sol its strongest model yet. It cites gains in coding, biology, and cybersecurity. Terra matches GPT-5.5 performance while costing roughly half as much. Luna brings strong capability at OpenAI’s lowest price.

GPT-5.6 adds two reasoning controls. The first is a new max reasoning effort. It gives Sol the most time to reason deeply.

The second is ultra mode. Instead of one model working alone, ultra leverages subagents. These subagents split complex work to accelerate it.

Think of it this way. The max setting deepens a single chain of reasoning. The ultra mode coordinates several workers on one task. Both trade latency and cost for accuracy on long-horizon problems.

OpenAI shared a preview set of evaluations.

Sol sets a new state of the art on Terminal-Bench 2.1. The benchmark tests command-line workflows that need planning, iteration, and tool coordination.

On Agent’s Last Exam, Sol was the only model past the halfway mark. It reached 50.9% in ‘code mode,’. On GeneBench v1, Sol beat GPT-5.5 on long-horizon genomics analysis. It did so while using fewer tokens. On ExploitBench, OpenAI reports Sol was competitive with Mythos Preview using about one-third of the output tokens.

GPT-5.6 is priced per one million tokens. Caching behavior also changes.

Sol’s $5/$30 matches GPT-5.5’s pricing. Terra is about 2x cheaper than GPT-5.5. Prompt caching now supports explicit cache breakpoints and a 30-minute minimum cache life. Cache writes cost 1.25x the uncached input rate. Cache reads keep the 90% discount.