GPT-6 vs GPT-5.6: OpenAI's Astra, Sol and Luna models explained

Updated November 5, 2026. Researched and drafted with AI help; every figure is checked against the linked source.

OpenAI released GPT-6 Astra on September 3, 2026, added the smaller GPT-6 Sol and Luna on September 22, and replaced GPT-6 Sol with GPT-6.1 Sol on September 29. GPT-5.6, released in July, is still on sale and is still what most ChatGPT users talk to. That makes OpenAI’s lineup confusing right now, so here is the whole picture.

The names

Both generations use the same naming pattern:

Sol is the main model.
Luna is the small, fast, cheap one.
Terra is the middle option: GPT-5.6 Terra, and since October 26, 2026, GPT-6.1 Terra. GPT-6 adds Astra at the top.

API prices

Model	Input / output per 1M tokens	Cached input	Context window
GPT-6 Astra	$10 / $50	$1.00	1.05M tokens
GPT-6.1 Sol	$2 / $10	$0.10	1.05M tokens
GPT-6 Sol (previous)	$2 / $10	$0.20	1.05M tokens
GPT-6.1 Terra (new)	$1 / $5	$0.05	1.05M tokens
GPT-6 Luna	$0.10 / $0.50	$0.01	1.05M tokens
GPT-5.6 Sol	$5 / $30 (list)	$0.50	1.05M tokens
GPT-5.6 Terra (previous)	$2 / $12	$0.20	1.05M tokens
GPT-5.6 Luna	$0.20 / $1.20	$0.02	1.05M tokens

Prompts over 272K tokens are billed at 2x input and 1.5x output on GPT-6.1 Sol, GPT-6.1 Terra, GPT-6 Sol, GPT-6 Astra and GPT-5.6 Sol.

On October 26, 2026 OpenAI added GPT-6.1 Terra, a middle option at $1 / $5 per million tokens with $0.05 cached input. It is half the price of GPT-6.1 Sol and succeeds GPT-5.6 Terra as the middle model; OpenAI’s pricing page no longer lists GPT-5.6 Terra.

GPT-6.1 Sol keeps GPT-6 Sol’s $2 / $10 price but halves the cached input price to $0.10 per million tokens. OpenAI’s GPT-6 Sol model page now points to GPT-6.1 Sol as the newer Sol model. OpenAI says GPT-6.1 Sol matches GPT-6 Astra on DeepSWE v1.1 and beats GPT-6 Sol by seven percentage points on OSWorld 2.0. We compare it with Anthropic’s new model at the same price in Claude Sonnet 5.5 vs GPT-6.1 Sol.

The surprise is that GPT-6 is cheaper than GPT-5.6. GPT-6.1 Sol costs half of GPT-5.6 Sol’s current promotional price ($4 / $20), or a third of its $30 list output price, and GPT-6 Luna costs about half of GPT-5.6 Luna. If you are on the API and GPT-6 handles your task, switching is an easy saving.

One caveat on GPT-5.6 Sol: OpenAI’s main pricing page used to list $5 / $30 but no longer shows per-token prices, while the developer pricing page and the model’s own page show a promotional $4 / $20, available until November 30, 2026. Check your invoice.

GPT-6 Astra

Astra is OpenAI’s new top model, which the company calls “the world’s most intelligent and aligned model.” At $10 / $50 per million tokens it is priced like the other frontier flagships, such as Anthropic’s Claude Fable 5.1. Since its September 3 launch, OpenAI has rolled it out to ChatGPT Plus, Pro, Business and Enterprise users. It also offers a Fast mode at twice the price.

Which model does ChatGPT use?

This is where most “it got dumber” confusion comes from, because the plan decides the model.

ChatGPT plan	Monthly price (US)	Main chat model
Free	$0	GPT-5.6 Luna
Go	$8	GPT-5.6 Luna, with Think mode
Plus	$20	GPT-5.6 Sol
Pro	$100, $200 or $500	GPT-5.6 Sol plus GPT-6 Pro, with weekly message caps

GPT-6 Sol and Luna are rolling out first in ChatGPT’s work features and in Codex rather than in regular chat. OpenAI says: “These models are not yet available in Chat.”

The August 6 change

On August 6, 2026 OpenAI updated GPT-5.6 Sol in ChatGPT for “more focused answers” and added a slider for how much the model thinks (Plus and Pro). At the same time, Free and Go users got GPT-5.6 Luna as their default with unlimited text chats. If ChatGPT felt different to you that week, that’s a documented reason.

Price cuts in July

GPT-5.6 launched on July 9 at $2.50 / $15 for Terra and $1 / $6 for Luna. On July 30 OpenAI cut those to $2 / $12 and $0.20 / $1.20. Sol’s price did not change.

Quick recommendations

New API project: try GPT-6.1 Sol first. It’s cheaper than GPT-5.6 Sol.
High volume, simple tasks: GPT-6 Luna at $0.10 / $0.50 is one of the cheapest capable models anywhere. Compare it on the price list.
ChatGPT user wondering what you’re getting: Plus gets you Sol; Free and Go get Luna.

See how people rate each OpenAI model today on the dumb meter, or compare plans on the plans page.