OpenAI Releases GPT-5.6 (Sol, Terra, Luna): A Three-Tier Model Family With Programmatic Tool Calling in the Responses API

OpenAI moved GPT-5.6 to general availability on July 9, 2026, shipping three tiers instead of one model. Sol is $5/$30 per 1M tokens, Terra is $2.50/$15, and Luna is $1/$6. Sol sets the Artificial Analysis Coding Agent Index at 80, 2.8 points above Claude Fable 5, and reaches 62.6% on OSWorld 2.0 using 85% fewer output tokens than Opus 4.8. The substantive developer change is Programmatic Tool Calling, which runs model-written JavaScript in an isolated V8 runtime to orchestrate tools without returning every intermediate result to the model. Clio reports a 38% cut in prompt tokens, PlayCo 63.5% fewer total tokens. The gaps are real too: Fable 5 still leads the Artificial Analysis Intelligence Index, GDPval-AA v2, and Toolathlon, and Claude Mythos 5 leads SWE-Bench Pro by roughly 15 points.

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