ChatGPT Models Explained: Which One Powers AI Search and What It Means for SEO
ChatGPT is built by OpenAI, and as of September 2026 its current lineup is GPT-6 Astra plus the GPT-5.6 family: Sol, Terra and Luna. The defaults are what matter for search visibility: GPT-5.6 Luna answers free-tier queries and GPT-5.6 Sol answers Plus and Pro queries, per OpenAI's own documentation. This page compares the lineup, tracks the timeline through the retirement of the numbered GPT-5.x line, and explains what it all means for SEO.
Test whether an AI browsing agent can use your website with the GPT-6 Astra readiness audit.
The Current ChatGPT Model Lineup
The lineup reset in mid-2026. The numbered GPT-5.x models and the entire o-series are retired or scheduled for shutdown on OpenAI's deprecations page, replaced by the named GPT-5.6 family launched July 9, 2026 and the GPT-6 Astra flagship launched September 3, 2026. All four current models share a 1,050,000 token context window, text and image input, and web search support, per OpenAI's model documentation.
| Model | Released | Context | Modalities | Where it is used | Best fit |
|---|---|---|---|---|---|
| GPT-6 Astra | Sep 3, 2026 | 1.05M | Text + image | Paid ChatGPT plans, API; limited rollout | Frontier reasoning, state-of-the-art browsing |
| GPT-5.6 Sol | Jul 9, 2026 | 1.05M | Text + image | Default on ChatGPT Plus and Pro | Complex professional work; the paid default |
| GPT-5.6 Terra | Jul 9, 2026 | 1.05M | Text + image | API and third-party products | Balanced everyday work at scale |
| GPT-5.6 Luna | Jul 9, 2026 | 1.05M | Text + image | Default on ChatGPT Free and Go | The volume tier; most ChatGPT answers |
Release Timeline
Most recent first. Dates are from OpenAI's announcements, release notes and deprecations page, linked in the sources below.
GPT-6 Astra released in limited rollout
OpenAI's new flagship for paid plans and the API, with state-of-the-art browsing and a 1.05M token context window. Not yet generally available; not on the free tier.
GPT-5.6 Sol and Luna became the ChatGPT defaults
A chat-tuned Sol became the Plus and Pro default with a thinking-effort slider; Luna became the free-tier default with unlimited text chats and a Think button.
The GPT-5.6 family launched
Sol, Terra and Luna went GA after a limited preview, followed by aggressive price cuts across the family in July and August.
The numbered GPT-5.x line wound down in ChatGPT
gpt-5-chat, 5.1, 5.2 and 5.3 chat models were shut down in ChatGPT in July and August 2026, with API snapshots scheduled through December 11, 2026, all pointing at GPT-5.6 replacements.
The o-series exits
Every o-series reasoning model carries a shutdown date on OpenAI's deprecations page: o1, o3-mini and o4-mini in October 2026, o3 in December 2026. No current o-series model remains in the lineup.
How Model Choice Affects Retrieval, Citation and Freshness
The short answer: ChatGPT's search pipeline is shared across models, so model choice changes who is asking and how hard the answer is verified, not where retrieval comes from. Search is available on every tier including signed-out users, triggers automatically or manually, and draws on third-party search providers including Microsoft, which keeps Bing indexation the quiet dependency under everything.
What changes by model: Luna answers the mass market fast and quotes the most liftable source; Sol adds a thinking-effort dial that reads and verifies more at higher settings; Astra brings state-of-the-art browsing that opens and evaluates your actual pages. Freshness is uniform in one sense, every current model has a February to April 2026 cutoff, so anything newer than that reaches users only through live search, which is exactly the surface you can optimize.
What This Means for SEO
One playbook covers the family. Allow GPTBot, OAI-SearchBot and ChatGPT-User, keep Bing healthy, write answer-first pages, and build the third-party mention layer ChatGPT verifies against; the complete method is my ChatGPT SEO guide, the commercial version is the ChatGPT SEO expert service, and multi-engine programs run under the AI SEO expert program.
Test both defaults. Baseline free-tier answers on Luna and paid-tier answers on Sol monthly, because your customers split across both. This is the discipline behind my chiropractic clinic ChatGPT visibility case study, where a local business went from invisible to cited, and the entity-led approach in my Arshay Cooper technical SEO, AEO and GEO case study.
Every ChatGPT Model, Explained
Each current model gets its own page: what it is, its key facts with sources, its retrieval behavior, and the checklist for staying visible when it handles the query.
Frequently Asked Questions
Which model powers ChatGPT right now?
Is GPT-5 still available in ChatGPT?
What happened to the o-series reasoning models?
Does free ChatGPT search the web?
Do I optimize differently for Astra, Sol, Terra and Luna?
Why does Bing matter for ChatGPT visibility?
Anshul Rana
I am an AI SEO, AEO and GEO specialist with 8+ years of experience helping businesses get found on Google and AI platforms like ChatGPT, Gemini, Claude and Perplexity. Top Rated Plus on Upwork with a 100% Job Success Score and 1,000+ websites optimized, working with clients across the US, the UK, Australia and India. I run The Digital Geek for agency-level engagements.
Sources
Every model fact on this page comes from OpenAI's own documentation and announcements, listed below and linked inline where each fact appears.
- OpenAI: current models list
- OpenAI: GPT-6 Astra announcement
- OpenAI: GPT-5.6 family announcement
- OpenAI: improving GPT-5.6 Sol in ChatGPT (default changes)
- OpenAI: ChatGPT release notes
- OpenAI: ChatGPT free tier FAQ
- OpenAI: Searching the web with ChatGPT
- OpenAI: model deprecations
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