OpenAI Releases GPT-5.6 (Sol, Terra, Luna): A Three-Tier Model Family With Programmatic Tool Calling in the Responses API
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OpenAI just moved the GPT-5.6 family to general availability, following a limited preview. The release ships three models rather than one. Sol is the flagship, Terra is the balanced everyday tier, and Luna is the most cost-efficient.
To sum up
- GPT-5.6 ships three tiers — Sol, Terra, Luna — priced from $1/$6 to $5/$30 per 1M tokens.
- Sol leads the Artificial Analysis Coding Agent Index at 80, 2.8 points above Claude Fable 5.
- Programmatic Tool Calling runs model-written JavaScript in an isolated V8 runtime with no network access.
ultraruns four agents in parallel, lifting Terminal-Bench 2.1 from 88.8% to 91.9%.- SWE-Bench Pro remains a gap: Sol’s 64.6% trails Claude Mythos 5’s 80.3% by roughly 15 points.
What is GPT-5.6?
Three models, one generation, priced per 1M tokens. Sol is $5 input and $30 output. Terra is $2.50 and $15. Luna is $1 and $6.
Availability differs by surface:
- Chat: Plus, Pro, Business, and Enterprise users access Sol at medium and higher effort. Pro and Enterprise can also select GPT-5.6 Sol Pro.
- ChatGPT Work and Codex: Free and Go users access Terra. Paid users choose among all three and set effort per model.
maxis available to all users with GPT-5.6 access and is toggled in settings. - API: All three tiers are available. Programmatic Tool Calling and a multi-agent beta both live in the Responses API.
Prompt caching also changed. GPT-5.6 supports explicit cache breakpoints and a 30-minute minimum cache life. Cache writes are billed at 1.25x the model’s uncached input rate. Cache reads continue to receive the 90% cached-input discount.
Performance
Furthermore, Agents’ Last Exam evaluates long-running professional workflows across 55 fields. OpenAI reports a new high of 53.6 for Sol. It describes this as eclipsing Claude Fable 5 (adaptive reasoning) by 13.1 points.
OpenAI’s own eval table lists Sol at 52.7% and Fable 5 at 40.5%. The 13.1-point gap matches 53.6 minus 40.5, so the Fable 5 baseline is consistent across both. Only Sol’s figure differs. OpenAI does not label which reasoning configuration produced 53.6.
On the Artificial Analysis Coding Agent Index v1.1, Sol at max reasoning scores 80. That is 2.8 points above Fable 5. OpenAI reports it does so using less than half the output tokens and less than half the time.
Sol sets new state-of-the-art results on Terminal-Bench 2.1 and DeepSWE. It reaches 92.2% on BrowseComp and 62.6% on OSWorld 2.0. On OSWorld it surpasses Claude Opus 4.8 while using 85% fewer output tokens.
| Eval | GPT-5.6 Sol | GPT-5.6 Terra | GPT-5.6 Luna | GPT-5.5 | Claude Fable 5 | Claude Opus 4.8 | Gemini 3.1 Pro Preview |
|---|---|---|---|---|---|---|---|
| AA Coding Agent Index v1.1 | 80 | 77.4 | 74.6 | 76.4 | 77.2 | 72.5 | 42.7 |
| AA Intelligence Index v4.1 | 58.9 | 55 | 51.2 | 54.8 | 59.9 | 55.7 | 46.5 |
| Terminal-Bench 2.1 | 88.8% | 87.4% | 84.7% | 85.6% | 83.1% | 78.9% | 70.7% |
| DeepSWE v1.1 | 72.7% | 69.6% | 67.2% | 67% | 69.7% | 59% | 11.8% |
| SWE-Bench Pro | 64.6% | 63.4% | 62.7% | 59.4% | 80% | 69.2% | 54.2% |
| Agents’ Last Exam | 52.7% | 50.4% | 50.3% | 46.9% | 40.5% | 45.2% | 32.1% |
| GDPval-AA v2 (Elo) | 1,747.8 | 1,593 | 1,591.8 | 1,493.7 | 1,759.6 | 1,600.1 | 962.3 |
| BrowseComp | 90.4% | 87.5% | 83.3% | 84.4% | — | 84.3% | 85.9% |
| OSWorld 2.0 | 62.6% | 50.2% | 45.6% | 47.5% | — | 54.8% | — |
| Toolathlon | 58% | 53.1% | 53.4% | 55.6% | 61.7% | 59.9% | 48.8% |
Where GPT-5.6 Does Not Lead
However, four gaps are worth naming:
- SWE-Bench Pro: Sol scores 64.6%. Claude Mythos 5 scores 80.3% and Fable 5 scores 80%. That is a roughly 15-point deficit on a widely watched coding eval.
- Broad intelligence and knowledge work: Fable 5 leads the Artificial Analysis Intelligence Index v4.1, 59.9 to 58.9. Fable 5 also leads GDPval-AA v2 by about 12 Elo. On HealthBench Professional, Fable 5 scores 60.9% against Sol’s 60.5%.
- Tool use: On Toolathlon, Sol scores 58%. Fable 5 reaches 61.7% and Opus 4.8 reaches 59.9%. Luna also edges out Terra here, inverting the tier order.
- Long context: Luna drops to 41.3% on OpenAI MRCR v2 8-needle, at both 256K–512K and 512K–1M. Sol scores 73.8% at 512K–1M, slightly below GPT-5.5’s 74%.
Interactive Explainer
GPT‑5.5
Anthropic
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