The new GPT-5.6 family: Luna, Terra, Sol

Simon Willison · Simon Willison · 9 July 2026, 19:46 ● Covered by 6 sources

OpenAI released three new GPT-5.6 models—Luna, Terra, and Sol—with input/output token prices of $1/$6, $2.50/$15, and $5/$30 respectively. On Agents' Last Exam benchmark measuring long-running professional workflows across 55 fields, GPT-5.6 Sol scored 53.6 compared to Claude Fable 5's 40.5, while Luna and Terra matched Fable 5's performance at one-sixteenth the cost. The models include new API features for programmatic tool composition, multi-agent spawning, and explicit prompt cache breakpoints, expanding capabilities for agentic workflows.

Why it matters

OpenAI's latest flagship model hit general availability this morning, and comes in three sizes: Luna, Terra, and Sol (from smallest to largest). The new models are priced per 1M input/output tokens as Luna $1/$6, Terra $2.50/$15, Sol $5/$30. For comparison, the Claude Opus series are $5/$25 and the Claude Fable 5 is $10/$50, but price-per-million tokens doesn't tell us much now that the number of reasoning tokens can differ so much between models for the same task. All three models have a February 16th 2026 knowledge cutoff, a million token context window, and 128,000 maximum output tokens. OpenAI's biggest benchmark claim concerns long-running agentic performance, with one benchmark showing all three models outperforming Claude Fable 5: We trained GPT-5.6 to get more useful work from every token. On Agents’ Last Exam, an evaluation of long-running professional workflows across 55 fields, GPT-5.6 Sol sets a new high of 53.6, eclipsing Claude Fable 5 (adaptive reasoning) by 13.1 points.