GPT-5.6 Has Landed: Sol vs Luna vs Terra — and How They Stack Up Against Claude and Grok

TL;DR: OpenAI released the GPT-5.6 family — Sol (frontier), Terra (balanced) and Luna (cost-optimized). Per Artificial Analysis, Sol scores 59 on the Intelligence Index — a close second to Claude Fable 5's 60 at roughly a third of the cost — and takes the #1 spot on the Coding Agent Index. All three are live on TulexAI today: Sol at 12 PT, Terra at 6 PT and Luna at 2.5 PT per message, on every plan from $11/mo.

Three models, one family

Unlike previous releases, GPT-5.6 isn't one model — it's a pricing ladder. All three share the same 1,050,000-token context window, 128K max output, February 16, 2026 knowledge cutoff, and text + image input. What changes is the intelligence-per-dollar point:

Model	Positioning (OpenAI's words)	Input / 1M	Output / 1M	On TulexAI
GPT-5.6 Sol	"Frontier model for complex professional work"	$5.00	$30.00	12 PT / message
GPT-5.6 Terra	"Balances intelligence and cost"	$2.50	$15.00	6 PT / message
GPT-5.6 Luna	"Optimized for cost-sensitive workloads"	$1.00	$6.00	2.5 PT / message

All prices verified against OpenAI's developer docs (July 2026). Cached input is a flat 10% of the list rate on all three — the deepest cache discount OpenAI has shipped. One API fine-print item worth knowing: prompts above 272K input tokens reprice the entire request at 2× input / 1.5× output, so the million-token window has a toll booth in the middle.

What do the benchmarks actually say?

The numbers below come from Artificial Analysis' launch analysis ("GPT-5.6 has landed", July 2026), plus their earlier Grok 4.5 report for the non-OpenAI rows. Intelligence Index scores are at maximum reasoning settings.

Model	AA Intelligence Index	AA Coding Agent Index	Cost per Index task
Claude Fable 5	60 (leader)	—	~3× Sol
GPT-5.6 Sol	59	80 (#1)	$1.04
Claude Opus 4.8	56	—	—
GPT-5.6 Terra	55	77	$0.55
GPT-5.5	55	—	—
Grok 4.5	54	—	—
GPT-5.6 Luna	51	75	$0.21

Three things stand out. First, Sol comes within one point of Fable 5 — Artificial Analysis' framing is that it delivers near-Fable intelligence at about a third of Fable's cost. Second, the whole family leads on coding: Sol's 80 tops the Coding Agent Index outright (DeepSWE, Terminal-Bench v2 and SWE-Atlas-QnA, run in Codex), with Terra at 77 and even budget Luna at 75, at roughly 10% lower cost than Fable 5 and Opus 4.8 running in Claude Code. Third, AA says Sol is more intelligent and more token-efficient than Claude Opus 4.8 — about 15K output tokens per Index task versus GPT-5.5's 16K.

And a practical note on the leader: Claude Fable 5 has been suspended from public API access since June 2026, so for most teams it's a scoreboard entry, not an option. Sol is currently the highest-scoring model you can actually call.

The caveats nobody headlines

Artificial Analysis is unusually direct about the fine print, so we will be too:

• The jump over GPT-5.5 is modest. AA calls the AA-Omniscience improvement "minor" — and notes the small accuracy uplift is coupled with an increase in hallucination rate. If your work is fact-critical, pair Sol with web search or a second-model check (both one click on TulexAI).
• Fable 5 still wins long-horizon agent work. On AA-Briefcase, the rubric-scored agentic eval, Sol lands at 42% versus Fable 5's 56% — a real 14-point gap. GPT-5.6 doesn't win everything.
• Terra is the odd one out. On AA's cost-versus-intelligence chart, "Luna and Sol are always on the Pareto frontier ahead of Terra" — meaning on pure math you'd pick Luna for cheap work and Sol for hard work, and skip the middle. Terra's case is simpler: one notch below Sol's reasoning at exactly half the price, without dropping to Luna's tier.

Who should use which?

• Pick Sol when: you're doing serious coding (it's #1 on the Coding Agent Index), multi-step professional work, or anything where you'd previously have reached for Opus 4.8 — Sol scores higher and burns fewer tokens.
• Pick Luna when: it's everyday drafting, summarizing, Q&A or high-volume workloads. At 51 it matches or exceeds GLM-5.2 and Gemini 3.5 Flash at lower cost — and at 2.5 PT it's the cheapest way into the 5.6 family.
• Pick Terra when: you want one model for everything and Sol's price on every message stings. It's the compromise pick — a deliberate one.
• Stay on Claude Opus 4.8 when: careful long-form writing and instruction-following reliability matter more than leaderboard position — and for long-horizon agent tasks, where Anthropic's models still lead on AA-Briefcase.
• What about Grok 4.5? At 2 PT it's still the value pick for frontier-adjacent reasoning — but the honest scoreboard now reads Sol 59, Terra 55, Grok 54.

The part where one subscription beats four

GPT-5.6 launched this week. Grok 4.5 launched last week. The "best model" changes monthly — which is exactly why locking into one vendor's $20–30/mo subscription is the wrong architecture for 2026. On TulexAI, all three GPT-5.6 models sit next to Claude Opus 4.8, Gemini 3.1 Pro, Grok 4.5 and 50+ other models on one login, with a live token meter showing what every message costs before you send it. Run the same prompt through Sol, Opus and Grok, and pick the winner yourself.

Try GPT-5.6 free on TulexAI — no card, first prompts on us. Coming from ChatGPT? Bring your chat history with you.

Frequently Asked Questions

Is GPT-5.6 Sol better than Claude Fable 5?

Not quite — Fable 5 still leads the Artificial Analysis Intelligence Index 60 to 59, and wins long-horizon agent work (56% vs 42% on AA-Briefcase). But Sol costs roughly a third as much per task, leads the Coding Agent Index outright — and Fable 5 has been suspended from public API access since June 2026, so Sol is the strongest model you can actually use today.

How much does GPT-5.6 cost?

Via the OpenAI API: Sol is $5/$30 per million input/output tokens, Terra $2.50/$15, Luna $1/$6, with cached input at 10% of list on all three (verified July 2026). On TulexAI: Sol 12 PT, Terra 6 PT and Luna 2.5 PT per message, included in every plan from $11/mo — no separate ChatGPT subscription needed.

What is the difference between GPT-5.6 Sol, Luna and Terra?

Same family, same 1,050,000-token context window and 128K max output — different intelligence-per-dollar points. Sol is the frontier model (Intelligence Index 59), Terra the balanced middle (55), Luna the cost-optimized entry (51). Coding scores stay remarkably close: 80, 77 and 75 on the Coding Agent Index.

Is GPT-5.6 better than GPT-5.5?

Modestly. Artificial Analysis calls the knowledge-reliability improvement "minor," and flags that the accuracy uplift comes with a higher hallucination rate. The clearer wins are coding (Coding Agent Index leader) and token efficiency (~15K output tokens per Index task vs GPT-5.5's 16K). Sol also pushes past GPT-5.5 on AA's cost-intelligence Pareto frontier.

Which GPT-5.6 model should I pick?

Default to Luna for everyday work and Sol for hard problems — Artificial Analysis notes those two sit on the Pareto frontier, while Terra is the deliberate middle option (one notch below Sol at half the price). On TulexAI you can switch per message, so the honest answer is: try the same prompt on each at 2.5 / 6 / 12 PT and see where your work lands.

Can I use GPT-5.6 without a ChatGPT subscription?

Yes. TulexAI serves all three GPT-5.6 models through the OpenAI API on every plan — from $11/mo, next to Claude, Gemini, Grok and 50+ other models. Your first prompts are free, no card required.