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GPT-5.6: Sol, Terra and Luna Explained
GPT-5.6 was the first OpenAI release to come as a family of three tiers instead of one model. It has since been overtaken by GPT-6, but it is still in the API and in many tools. Here is what each model is, what it costs and when to use it.
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Facts checked against official announcements and documentation, October 2026 · Last updated October 2026
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01 · What it is
The GPT-5.6 family
One generation, three tiers.
What is GPT-5.6?
GPT-5.6 is a generation of OpenAI models released as three tiers: Sol, the flagship for the hardest work; Terra, a balanced model for everyday tasks at lower cost; and Luna, the fastest and most affordable. OpenAI previewed the family to a limited group first, then made all three generally available in ChatGPT, ChatGPT Work, Codex and the API on July 9, 2026.
Source: OpenAI: GPT-5.6, frontier intelligence that scales with your ambition; OpenAI: Previewing GPT-5.6 Sol
What do Sol, Terra and Luna mean?
They are capability tiers that OpenAI introduced with GPT-5.6. The number identifies the model's generation, while Sol, Terra and Luna are durable tiers that can each be upgraded on their own schedule. That is why GPT-6.1 Sol and GPT-6.1 Terra exist alongside GPT-6 Luna: each tier moves forward independently.
Source: OpenAI: Previewing GPT-5.6 Sol; OpenAI: Introducing GPT-6.1 Terra (Oct 26, 2026)
What is the difference between GPT-5.6 Sol, Terra and Luna?
Sol is the most capable, with the best results on hard coding, research and security tasks. Terra trades a little capability for a much lower price and performs competitively with GPT-5.5. Luna is the fastest and cheapest and suits high-volume chat, classification and extraction. On the Artificial Analysis Intelligence Index v4.1, OpenAI reported Sol at 58.9, Terra at 55 and Luna at 51.2. Our Sol vs Terra vs Luna comparison goes task by task.
Source: OpenAI: GPT-5.6, frontier intelligence that scales with your ambition
02 · Specs and prices
GPT-5.6 prices, context and benchmarks
Current figures from OpenAI's model pages.
How much does GPT-5.6 cost?
Per million tokens on OpenAI's model pages in November 2026: GPT-5.6 Sol costs $4 input, $0.40 cached input and $20 output, a promotional rate OpenAI says is available at least through November 21, 2026; GPT-5.6 Terra costs $2 input, $0.20 cached and $12 output; and GPT-5.6 Luna costs $0.20 input, $0.02 cached and $1.20 output. For comparison, the newer GPT-6.1 Terra costs $1 and $5.
Source: OpenAI API: GPT-5.6 Sol model; OpenAI API: GPT-5.6 Terra model; OpenAI API: GPT-5.6 Luna model; OpenAI: Introducing GPT-6.1 Terra (Oct 26, 2026)
What is GPT-5.6's context window?
All three GPT-5.6 models accept up to 1,050,000 tokens of context and can write up to 128,000 tokens in one response. GPT-5.6 Sol's knowledge cutoff is February 16, 2026.
Source: OpenAI API: GPT-5.6 Sol model; OpenAI API: GPT-5.6 Terra model; OpenAI API: GPT-5.6 Luna model
How good is GPT-5.6 on benchmarks?
OpenAI's launch figures for Sol include 88.8% on Terminal-Bench 2.1 (91.9% in Sol Ultra mode), 64.6% on SWE-Bench Pro, 94.6% on GPQA Diamond, 90.4% on BrowseComp and 62.6% on OSWorld 2.0 for computer use. Terra scored 87.4% and Luna 84.7% on Terminal-Bench 2.1, showing how close the cheaper tiers came on coding. These are OpenAI's own results.
Source: OpenAI: GPT-5.6, frontier intelligence that scales with your ambition
03 · Access
How to use GPT-5.6 and its effort levels
Which plans got which tier, and how to control how hard it thinks.
Which ChatGPT plans got GPT-5.6?
At launch, Plus, Pro, Business and Enterprise users used GPT-5.6 Sol in chat at medium and higher effort, and Pro and Enterprise users could select GPT-5.6 Sol Pro for the highest-quality answers on complex tasks. In ChatGPT Work and Codex, Free and Go users got GPT-5.6 Terra, while paid plans could choose Sol, Terra or Luna and set an effort level for each.
Source: OpenAI: GPT-5.6, frontier intelligence that scales with your ambition
What are max and ultra effort in GPT-5.6?
Effort controls how long the model reasons before answering: more effort means better results on hard problems, slower replies and more usage. GPT-5.6 added a max setting, available to everyone with GPT-5.6 in ChatGPT Work and Codex and switched on in settings, and an ultra setting, available to Pro and Enterprise users in ChatGPT Work and to Plus and higher plans in Codex. Sol Ultra reached 91.9% on Terminal-Bench 2.1 against 88.8% for standard Sol.
Source: OpenAI: GPT-5.6, frontier intelligence that scales with your ambition
What is Programmatic Tool Calling in GPT-5.6?
A capability introduced with GPT-5.6 Sol that lets the model write JavaScript which runs in a network-less V8 sandbox, so it can process data, chain tool results and do calculations in code rather than in its own reasoning. It makes multi-step tool use faster and more reliable for developers building agents.
Source: Gradually AI: ChatGPT models, all GPT versions up to GPT-6.1 Terra
04 · Today
Should you still use GPT-5.6?
GPT-6 has taken over most of its jobs.
Is GPT-5.6 still available in ChatGPT?
GPT-6 models are now the defaults: GPT-6 Astra on paid plans, GPT-6 Luna on Free and Go, and GPT-6 Sol replaced GPT-5.6 Sol in regular ChatGPT chat in October 2026. GPT-5.6 models remain available through the API, where OpenAI's model pages list no deprecation date as of November 2026.
Source: Gradually AI: ChatGPT models, all GPT versions up to GPT-6.1 Terra; OpenAI API: GPT-5.6 Sol model; OpenAI: GPT-6 Astra, a new generation of intelligence
GPT-5.6 vs GPT-6: which should I use?
GPT-6 for new work. GPT-6.1 Sol costs $2 input and $10 output against $4 and $20 for GPT-5.6 Sol even at its promotional rate, and GPT-6.1 Terra at $1 and $5 is half the price of GPT-5.6 Terra. Keep GPT-5.6 only if an existing app is tuned to it and you have not yet re-tested on GPT-6. Some platforms are already retiring it; GitHub scheduled GPT-5.6 Terra for deprecation in Copilot on November 16, 2026.
Source: OpenAI API: Pricing; OpenAI: Introducing GPT-6.1 Terra (Oct 26, 2026); OpenAI API: GPT-5.6 Sol model; Shattered: GitHub Copilot retires 6 models, shifts to GPT-6.1 Sol
What models did GPT-5.6 replace?
In the API, OpenAI listed GPT-5.6 Sol as the replacement for several retired models, including gpt-5-codex, gpt-5-chat-latest, gpt-5.1-codex and gpt-5.1-chat-latest (shut down July 23, 2026) and gpt-5.2-chat-latest and gpt-5.3-chat-latest (shut down August 10, 2026). If you are migrating from one of those, move straight to GPT-6.1 Sol instead.
Source: just4o: Model deprecation calendar (citing OpenAI API deprecations); OpenAI API: Deprecations
Sources
Read and checked October 2026. Policies change, so confirm anything time-sensitive on the source page.
- OpenAI: GPT-5.6, frontier intelligence that scales with your ambition
- OpenAI: Previewing GPT-5.6 Sol
- OpenAI: Introducing GPT-6.1 Terra (Oct 26, 2026)
- OpenAI API: GPT-5.6 Sol model
- OpenAI API: GPT-5.6 Terra model
- OpenAI API: GPT-5.6 Luna model
- Gradually AI: ChatGPT models, all GPT versions up to GPT-6.1 Terra
- OpenAI: GPT-6 Astra, a new generation of intelligence
- OpenAI API: Pricing
- Shattered: GitHub Copilot retires 6 models, shifts to GPT-6.1 Sol
- just4o: Model deprecation calendar (citing OpenAI API deprecations)
- OpenAI API: Deprecations