GPT-6 Sol vs GPT-6 Astra vs GPT-5.6 Sol: OpenAI's model names explained
Sol, Astra, Terra and Luna: how OpenAI's model names work, where GPT-6 Sol and Luna fit, and which models you get on which ChatGPT plan.
Start with the naming rule. When word got round that gpt-6-sol had appeared in OpenAI's API, a Hacker News commenter asked how GPT-5.6 Sol and GPT-6 Sol would be differentiated. Another commenter supplied the useful answer: GPT-6 extends the 5.6 naming scheme and adds Astra as a tier above Sol.
That answer held. On 22 September 2026 OpenAI released GPT-6 Sol and GPT-6 Luna, filling two more rungs of the ladder. Once you know the rule, the names are straightforward to read.
The rule: the number is the generation, the name is the tier
OpenAI explained the system when it previewed GPT-5.6 on 26 June 2026: "the number identifies a model's generation, while Sol, Terra, and Luna identify durable capability tiers that can advance on their own cadence."
So a name has two parts. GPT-5.6 Sol is the Sol tier of generation 5.6. GPT-6 Sol is the Sol tier of generation 6. Because each tier "can advance on their own cadence", a generation can arrive one tier at a time.
That is how generation 6 has come out. Astra arrived first, on 3 September. Sol and Luna followed on 22 September. Terra is the one tier with no GPT-6 model yet.
The tiers, top to bottom
Tier | Generation 6 | API price | Generation 5.6 | API price |
|---|---|---|---|---|
Astra | GPT-6 Astra, 3 September | $10 / $50 | None | None |
Sol | GPT-6 Sol, 22 September | $2 / $10 | GPT-5.6 Sol, 9 July | $4 / $20, promotional until at least 21 November |
Terra | None yet | None | GPT-5.6 Terra, 9 July | $2 / $12 |
Luna | GPT-6 Luna, 22 September | $0.10 / $0.50 | GPT-5.6 Luna, 9 July | $0.20 / $1.20 |
Prices are per million input and output tokens, at the short-context rates for standard processing on OpenAI's API pricing page, checked 22 September 2026.
One oddity stands out. The tiers describe capability, and generation 6 is priced lower than generation 5.6 at the same tier. GPT-6 Sol costs less than GPT-5.6 Terra, the tier below it, on output.
GPT-6 Sol vs GPT-5.6 Sol
Same tier, different generation.
GPT-6 Sol costs half as much as GPT-5.6 Sol's promotional price and has a newer knowledge cutoff, 20 April 2026. On the Artificial Analysis Intelligence Index, it scores 48 at max effort, one point above GPT-5.6 Sol's 47. OpenAI's own claim is about accuracy more than raw capability: on its internal factuality evaluation, GPT-6 Sol "makes about half as many mistakes as its predecessor".
GPT-5.6 Sol is still available in the API as gpt-5.6-sol, and OpenAI has not announced a retirement date for it.
GPT-6 Sol vs GPT-6 Astra
Same generation, different tiers.
Astra is the top of the ladder, and OpenAI's launch post says it "continues to be our best model across the board". Artificial Analysis scores GPT-6 Astra at 53 and GPT-6 Sol at 48, both at max effort. Sol is the faster of the two, at about 104 output tokens per second against about 58 for Astra, and it costs a fifth as much per token.
Astra also comes with a safety classification of its own. OpenAI's safety overview says "Astra is our first model to reach the Critical level of cybersecurity capability under our Preparedness Framework". We covered the pause in Astra work that preceded its launch in our report from August.
GPT-6 Luna
Luna is the smallest and cheapest GPT-6 model, at $0.10 per million input tokens and $0.50 per million output tokens. Artificial Analysis scores it at 37 and measured it at about 157 output tokens per second, the fastest of the GPT-6 models. OpenAI says that at higher effort levels it matches GPT-5.6 Sol on its factuality evaluation "at about a hundredth its cost".
Which model you get on which plan
OpenAI's launch post and its help centre, checked 22 September 2026, describe two different places in ChatGPT.
In ChatGPT Work and Codex, from 22 September:
Plus, Pro, Business, Enterprise and Edu get GPT-6 Sol and GPT-6 Luna, rolling out through launch day. Plus also includes GPT-6 Astra here.
Free and Go can use GPT-6 Luna in the desktop app.
In ordinary ChatGPT chats, OpenAI says the new models are "not yet available", so the GPT-5.6 line-up still applies:
Free and Go: GPT-5.6 Luna is the default model and also powers Think.
Plus: GPT-5.6 Sol powers Instant, Medium and High. Extra High and Pro are not included.
Pro, Business and Enterprise: GPT-5.6 Sol at every thinking level, plus GPT-6 Pro, "powered by GPT-6 Astra", within weekly or monthly allowances.
In the API, all six models are available under their own ids: gpt-6-astra, gpt-6-sol, gpt-6-luna, gpt-5.6-sol, gpt-5.6-terra and gpt-5.6-luna.
In MultiChats, we are adding GPT-6 Sol and GPT-6 Luna on our paid plans, next to GPT-5.6 Terra and models from Anthropic, Google, xAI and more. We do not carry GPT-6 Astra.
Frequently asked questions
Is GPT-6 Sol the same as GPT-5.6 Sol?
No. They share a tier name, one generation apart. GPT-5.6 Sol came out on 9 July 2026 and GPT-6 Sol on 22 September 2026, at half GPT-5.6 Sol's promotional price.
Is GPT-6 Astra better than GPT-6 Sol?
Yes. OpenAI calls Astra its best model across the board, and Artificial Analysis scores it 53 against Sol's 48 at max effort. Sol is faster and costs a fifth as much per token.
Is Sol still OpenAI's flagship?
No. Sol was the top tier when GPT-5.6 launched in July. Since 3 September, GPT-6 Astra sits above it.
What do Terra and Luna mean?
They are the lower tiers. OpenAI calls Terra "a balanced model for everyday work" and Luna "a fast and affordable model". Luna now exists in both generations, while Terra so far exists only as GPT-5.6 Terra.
Will there be a GPT-6 Terra?
OpenAI has not said. It launched Sol and Luna together on 22 September without a Terra, and the naming scheme leaves room for one. DevDay is on 29 September.
Update log
22 September 2026, 21:30 UTC. OpenAI released GPT-6 Sol and GPT-6 Luna. Added both to the tier table, updated plan availability from the launch post and the new help centre page, and added Artificial Analysis scores.
21 September 2026, 20:22 UTC. Page opened, with GPT-6 Sol still unreleased.