Guide · AI Assistants
ChatGPT models explained: GPT-6 Astra, Sol and Luna, plus GPT-5.6 — which to use
GPT-6 Astra, Sol and Luna and the GPT-5.6 models explained: which ChatGPT model each plan gets, thinking levels, Pro limits and the GPT-5.5 retirement date.
The short answer
ChatGPT chat runs on GPT-5.6 Sol and Luna, with GPT-6 Pro, powered by GPT-6 Astra, on Pro plans. In ChatGPT Work and Codex, OpenAI added GPT-6 Sol and GPT-6 Luna on September 22, 2026 and GPT-6.1 Sol on September 29, alongside GPT-6 Astra and the GPT-5.6 models. Free and Go use Luna, Plus adds Sol, and GPT-5.5 retires from ChatGPT on October 14, 2026.
ChatGPT no longer runs on one model. OpenAI now offers the GPT-6 family — Astra, Sol and Luna — alongside three models in the GPT-5.6 family — Sol, Terra and Luna — and the one you use depends on your plan, the tool and how much you ask ChatGPT to think. This guide explains what each model is for, which plans get which, how the thinking levels and Pro limits work, and what is being retired. It was updated on September 24, 2026 for the launch of GPT-6 Sol and Luna.
The current models
| Model | What OpenAI says it is for | Where you can use it |
|---|---|---|
| GPT-6 Astra | “Our most capable model for complex work across code, apps, and research, combining advanced reasoning, computer use, and stronger judgment” | GPT-6 Pro in chat on Pro, Business and Enterprise; ChatGPT Work and Codex on Plus and above |
| GPT-6.1 Sol | An upgrade to GPT-6 Sol that OpenAI says nearly matches GPT-6 Astra on agentic coding, computer use and professional work at one-fifth of Astra’s price | Work and Codex on Plus and above, starting with Pro; API. Not yet in chat |
| GPT-6 Sol | Built for complex coding and agentic workflows, with stronger factual reliability and clearer communication than GPT-5.6 Sol | Work and Codex on Plus and above; API. Not yet in chat |
| GPT-6 Luna | OpenAI’s most efficient model for focused, high-volume tasks such as summarisation, extraction and focused coding | Work and Codex on Plus and above, and the desktop app on Free and Go; API. Not yet in chat |
| GPT-5.6 Sol | The most capable GPT-5.6 model for complex coding, computer use, research and cybersecurity | ChatGPT on Plus and above; Work; Codex; API |
| GPT-5.6 Terra | A balanced model for everyday work, with performance competitive with GPT-5.5 at a lower cost | Work; Codex, including Free and Go; API. Not selectable in chat |
| GPT-5.6 Luna | A fast and affordable model with strong capability at the lowest cost in the family | Default for Free and Go in chat; Work; Codex; API |
OpenAI introduced Astra on September 3, 2026, saying at first that access was rolling out to a limited set of organisations. On September 22 it added GPT-6 Sol and GPT-6 Luna, cutting API prices for both by 50% against GPT-5.6; read our report on GPT-6 Sol and Luna. On September 29 it added GPT-6.1 Sol, covered in GPT-6.1 Sol, and an Ultrafast speed tier for Astra on the new Pro 500 plan. The GPT-5.6 models remain available during the rollout and are still the models in ordinary chat. It says Astra can create documents, spreadsheets and presentations that follow your templates and instructions, and it includes extra safety monitoring for agents that may not have interpreted instructions correctly.
Which model you get on each plan
| Plan | In chat | In Work and Codex | GPT-6 Astra |
|---|---|---|---|
| Free | GPT-5.6 Luna | GPT-6 Luna in the desktop app, subject to rollout | Not included |
| Go ($8) | GPT-5.6 Luna; the Think button also uses Luna | GPT-6 Luna in the desktop app, subject to rollout | Not included |
| Plus ($20) | GPT-5.6 Sol at Instant, Medium and High | GPT-6 Sol and Luna, plus GPT-6 Astra; GPT-5.6 models remain during the rollout | In ChatGPT Work and Codex |
| Pro ($100, $200 or $500) | Sol at Instant, Medium, High, Extra High and Pro; GPT-6 Pro | GPT-6 Sol, Luna and Astra; GPT-5.6 models remain during the rollout | In chat as GPT-6 Pro, and in Work and Codex |
| Business and Enterprise | The same options as Pro; admins may limit which models members can use | GPT-6 Sol, Luna and Astra, subject to workspace settings | GPT-6 Pro in chat; Enterprise access also depends on workspace permissions |
Prices are US monthly prices. For a deeper look at Go and Plus, see ChatGPT Go vs Plus.
What the thinking levels mean
On eligible paid plans, a thinking slider in the model picker decides how much ChatGPT reasons before it answers:
- Instant: fast responses for everyday questions, powered by GPT-5.6 Sol on paid plans and GPT-5.6 Luna on Free and Go.
- Medium: standard thinking with GPT-5.6 Sol.
- High: extended thinking with GPT-5.6 Sol.
- Extra High: the highest thinking level available with GPT-5.6 Sol.
- Pro: models for difficult tasks and longer-running workflows, including GPT-5.6 Sol Pro and, on eligible plans, GPT-6 Pro.
Plus doesn’t include Extra High or Pro. On Free and Go there is no slider; you get a Think button for harder questions, and it uses Luna, not Sol.
Since September 14, 2026, Instant no longer switches to a higher level on its own for Plus and Pro users. Asking ChatGPT to “think harder” won’t change the level either; choose an option in the model picker instead. ChatGPT can still switch automatically for safety.
GPT-6 Pro limits
A Pro subscription does not give unlimited use of GPT-6 Pro. In chat, the allowances are:
| Plan | GPT-6 Pro in chat | GPT-5.6 Sol Pro |
|---|---|---|
| Pro $200 | 200 messages a week | Separate 170 messages a day; both models together are limited to 200 messages a day |
| Pro $100 | 50 messages a week | Shares that one weekly allowance |
| Business Standard | 15 messages a month | Shares that one monthly allowance |
| Business Premium | 50 messages a week | Shares that one weekly allowance |
On Pro $200, when you reach the GPT-6 Pro weekly limit, ChatGPT switches automatically to GPT-5.6 Thinking at Medium. ChatGPT Work and Codex have their own allowances, which you can see in our guide to the best AI coding assistants. Our guide to ChatGPT Pro explained covers the $100 and $200 tiers and the pause on new Pro $200 sign-ups.
How much ChatGPT can read at once
OpenAI’s pricing page lists the total context window, the amount of text ChatGPT can hold in mind in one conversation:
| Plan | Instant (total context) | Reasoning (total context) |
|---|---|---|
| Free | 27K | Varies |
| Go | 54K | 256K |
| Plus | 54K | 256K |
| Pro | 128K | 400K |
A short timeline
| Date (2026) | What changed |
|---|---|
| July 9 | GPT-5.6 Sol begins rolling out in ChatGPT to eligible paid plans |
| August 6 | An updated Sol and a thinking slider arrive for Plus and Pro; Luna becomes the default for Free and Go |
| September 3 | OpenAI introduces GPT-6 Astra, at first to a limited set of organisations |
| September 9 | ChatGPT Voice can use GPT-5.6 or GPT-6 Astra for harder questions |
| September 14 | Automatic switching from Instant to Thinking ends for Plus and Pro |
| September 22 | GPT-6 Sol and GPT-6 Luna arrive in ChatGPT Work and Codex, and API prices for Sol and Luna fall 50% |
| September 29 | GPT-6.1 Sol arrives in Work and Codex; GPT-6 Astra Ultrafast launches on Pro 500 and Enterprise |
| October 14 | GPT-5.5 retires from ChatGPT, ChatGPT Work and Codex |
What is being retired
- GPT-5.5: OpenAI says it retires from ChatGPT, ChatGPT Work and Codex on all plans on October 14, 2026, including Business, Enterprise and Edu. It stays in the API. If you use it in workspace configurations, custom agents or scripts, OpenAI now advises choosing GPT-6 Sol (
gpt-6-sol) on Plus, Pro, Business, Enterprise and Edu, or GPT-6 Luna (gpt-6-luna) in the desktop app on Free and Go, when available. - Custom GPTs: OpenAI plans to retire them and provide a migration path to plugins, and its FAQ gives December 11, 2026 as the retirement date. Timing can vary by plan and workspace, and plugins combine reusable instructions with connected apps.
Which model should you use?
- Everyday questions, writing and planning: the default. On Free and Go that is Luna; on paid plans it is Sol at Instant.
- Hard reasoning, coding or research: on Plus, choose Sol at High; on Pro, try Extra High or Pro.
- Long agent jobs in Work or Codex: GPT-6 Astra for the hardest work, or GPT-6 Sol for everyday and complex coding at a lower usage cost. Watch the usage limits, and see What is ChatGPT Work? for how Work usage works.
- Cost-sensitive developer work: GPT-6 Luna for focused, repeatable tasks and the lowest price, or GPT-6 Sol when you need more judgment, both through Codex or the API.
If you mostly want to talk, our guide to ChatGPT Voice covers the GPT-Live models, and ChatGPT for Excel, Word and PowerPoint shows the models at work in Office. For the assistant overall, see the ChatGPT profile, or compare it with rivals in ChatGPT vs Claude.
Frequently asked questions
What is the difference between GPT-5.6 Sol and GPT-6 Astra?
GPT-6 Astra is OpenAI's most capable model, introduced on September 3, 2026, for complex work across code, apps and research. GPT-5.6 Sol is the most capable GPT-5.6 model and powers ChatGPT on paid plans. Astra appears in chat as GPT-6 Pro on Pro, Business and Enterprise plans, and in ChatGPT Work and Codex on Plus.
What are GPT-6 Sol and GPT-6 Luna?
They are models OpenAI introduced on September 22, 2026, trained with similar methods to GPT-6 Astra but faster and cheaper. GPT-6 Sol is for complex coding and agentic workflows and GPT-6 Luna is for focused, repeatable tasks. They are in ChatGPT Work and Codex for Plus and above, with Luna also on Free and Go in the desktop app, but not yet in Chat.
Which ChatGPT model do free users get?
Free and Go users get GPT-5.6 Luna, with unlimited everyday text chats subject to abuse-prevention safeguards. The Think button for harder questions also uses Luna rather than Sol. Separate limits apply to file uploads, image generation, voice and other tools.
Can I pick GPT-5.6 Terra or Luna in ChatGPT?
Not in standard ChatGPT conversations. Terra and Luna are available in ChatGPT Work and Codex depending on your plan, and to developers through the OpenAI API. Since September 22, Codex also offers GPT-6 Luna on Free and Go in the desktop app, and GPT-6 Sol and Luna on Plus and above, with the GPT-5.6 models remaining during the rollout.
Is GPT-5.5 going away?
Yes. OpenAI says GPT-5.5 retires from ChatGPT, ChatGPT Work and Codex on all plans on October 14, 2026. It remains available in the OpenAI API. In Codex configurations and scripts, OpenAI now advises choosing GPT-6 Sol (gpt-6-sol) on Plus, Pro, Business, Enterprise and Edu, or GPT-6 Luna (gpt-6-luna) in the desktop app on Free and Go, when available.
Does ChatGPT Plus include GPT-6 Astra?
In part. Plus includes GPT-6 Astra in ChatGPT Work and Codex, but not in ordinary chat. GPT-6 Pro, the chat option powered by Astra, is included with Pro ($100 and $200), Business and Enterprise plans.
Why doesn't ChatGPT switch to a thinking model on its own anymore?
On September 14, 2026 OpenAI retired automatic switching from Instant to Thinking for Plus and Pro users and removed the Higher intelligence setting from ChatGPT on the web for those plans. You now choose an option in the model picker when you want ChatGPT to think for longer.
Sources
Product facts are checked against the vendor's own announcements and documentation.
- GPT-5.6 and GPT-6 Pro in ChatGPT — OpenAI Help Centerhttps://help.openai.com/en/articles/20001354-gpt-56-in-chatgpt
- Introducing GPT-6 Sol and Luna — OpenAIhttps://openai.com/index/introducing-gpt-6-sol-and-luna/
- ChatGPT — Release Notes — OpenAI Help Centerhttps://help.openai.com/en/articles/6825453-chatgpt-release-notes
- Model Release Notes — OpenAI Help Centerhttps://help.openai.com/en/articles/9624314-model-release-notes
- Models — OpenAIhttps://learn.chatgpt.com/docs/models
- ChatGPT pricing — OpenAIhttps://chatgpt.com/pricing/
- Custom GPT retirement and migration FAQ — OpenAI Help Centerhttps://help.openai.com/en/articles/20001519-custom-gpt-retirement-and-migration-faq
