AI Tools
GPT-5.6 Sol vs Luna: Which Model Should You Use?
GPT-5.6 Sol vs Luna: OpenAI's frontier and nano-tier models, split across ChatGPT plans on August 6, 2026 when Luna became the Free and Go default and Plus and Pro users got an improved Sol with a reasoning slider. In the API, Sol costs $5.00 per 1M input tokens and $30.00 per 1M output, versus $0.20 and $1.20 for Luna.
What matters
- GPT-5.6 Luna became the default ChatGPT model for Free and Go users in the week of August 6, 2026; unlimited text chats and a Think button follow the next week.
- API pricing gap is 25x: GPT-5.6 Sol costs $5.00 in and $30.00 out per 1M tokens, GPT-5.6 Luna $0.20 and $1.20 (short context, standard pricing, verified August 8, 2026).
- All three GPT-5.6 models share a 1,050,000-token context window, 128,000 max output tokens, and a February 16, 2026 knowledge cutoff.
- Plus and Pro users get the updated Chat Sol with a new effort slider; the Sol powering Work and Codex is unchanged, per OpenAI.
- OpenAI's internal evaluation reports factually error-prone responses fell about 68% with Sol and 62% with Luna versus GPT-5.5 Instant.
What are GPT-5.6 Sol and GPT-5.6 Luna?
GPT-5.6 Sol is OpenAI's frontier model, described on its official model card as the default model for complex professional work. The gpt-5.6 API alias routes requests to GPT-5.6 Sol, so most developer documentation that says "GPT-5.6" means Sol. It accepts text and image input and returns text output.
GPT-5.6 Luna sits at the opposite end of the family. OpenAI's model card says it is designed for cost-sensitive, high-volume workloads and roughly corresponds to the nano model tier used in earlier GPT-5 families. Between them is GPT-5.6 Terra, which roughly corresponds to the mini tier and balances intelligence and cost. For the full family picture, see our GPT-5.6 explainer.
All three models share the same core specifications: a 1,050,000-token context window, 128,000 max output tokens, and a February 16, 2026 knowledge cutoff, per the official model cards.
What changed in the August 6, 2026 ChatGPT update?
OpenAI's August 6, 2026 announcement updates ChatGPT in two directions. For Plus and Pro users, GPT-5.6 Sol in Chat is tuned to give more focused answers, be more reliable with facts, and avoid extra formatting. A new slider on web, mobile, and desktop lets them choose how much thought ChatGPT puts into each response, and the same model now powers both Instant answers and deeper reasoning.
For Free users, the default model becomes GPT-5.6 Luna with unlimited text chats and a new Think button for harder questions. The rollout is staged: the updated Sol and slider went live for Plus and Pro on August 6, Luna becomes the default for Free and Go users during the week of August 6, and unlimited text chats plus the Think button arrive the following week, subject to abuse guardrails. Limits still apply to file uploads, images, and other tools.
OpenAI also published an internal evaluation: across financial, medical, and legal prompts, responses containing at least one factual error were about 68% less common with GPT-5.6 Sol and 62% less common with GPT-5.6 Luna than with GPT-5.5 Instant. These are the company's own numbers, not an independent benchmark.
GPT-5.6 Sol vs Luna: API pricing, verified
As of August 8, 2026, the standard API prices per 1 million tokens at short context are $5.00 input and $30.00 output for GPT-5.6 Sol, $2.00 and $12.00 for GPT-5.6 Terra, and $0.20 and $1.20 for GPT-5.6 Luna. Luna is 25 times cheaper than Sol on both input and output tokens.
Long-context pricing roughly doubles input and lifts output by 50%: Sol runs $10.00 in and $45.00 out, Terra $4.00 and $18.00, and Luna $0.40 and $1.80 per 1M tokens. The model cards carry the same headline prices for short context, so those figures are cross-verified across two official sources.
Which ChatGPT plan gets which model?
- Free and Go: GPT-5.6 Luna becomes the default model this week, with unlimited text chats and the Think button following next week. File uploads, images, and other tools remain limited.
- Plus and Pro: the updated GPT-5.6 Sol powers Chat with the new effort slider, covering both quick answers and deeper reasoning.
- Work and Codex: these keep their own build of GPT-5.6 Sol, which OpenAI says is not changing in this release.
The announcement does not mention any change to ChatGPT subscription pricing, and OpenAI does not publish per-plan model prices. What changed is access: the free tier's default moved from the GPT-5.5 line to GPT-5.6 Luna, and paid users get the improved Chat Sol. Our GPT-5.6 review covers the family pricing in more depth.
Should you pick Sol or Luna?
For API builders, this is a cost-versus-quality tradeoff. Luna is built for cost-sensitive, high-volume workloads: at $0.20 per 1M input tokens it suits classification, extraction, summarization, and user-facing chat at scale. Sol, at 25 times the price, is for complex professional work where answer quality matters more than the token bill. Terra is the middle option at $2.00 input.
For ChatGPT users, the plan decides it. Free users get Luna plus a Think button for harder questions, and Plus and Pro subscribers get Sol at every effort level through the new slider. If you only need everyday answers, the free tier now covers more than it did before this update. Claude from Anthropic remains the main paid alternative for reasoning-heavy chat, and we compare the two assistants in our Claude tool profile.
Sol is the wrong choice for high-volume, low-complexity calls, and Luna is the wrong choice where a single wrong answer is expensive, such as legal or financial analysis. OpenAI's own evaluation shows Sol still ahead of Luna on factual reliability.
The honest catch
Three caveats are easy to miss. First, Luna is explicitly the nano-tier model. The free default is the entry-level member of the GPT-5.6 family, not a frontier model repackaged for free. In the Hacker News discussion, commenters debated whether the switch is an upgrade or a quiet downgrade for free users compared with the older GPT-5.5 default; OpenAI positions Luna as the successor to the previous Instant tier.
Second, the improved Chat version of Sol is a separate build from the one powering Work and Codex. OpenAI says the Work and Codex version is not changing, which means two models share the GPT-5.6 Sol name with no public way to tell which build you are talking to.
Third, the headline free-tier perks are staged. Unlimited text chats and the Think button land the week after Luna becomes the default, abuse guardrails apply, and the error-rate statistics come from OpenAI's internal evaluation. More AI model coverage lives on the radar index.
At a glance
| GPT-5.6 Sol | GPT-5.6 Terra | GPT-5.6 Luna | |
|---|---|---|---|
| Tier positioning | Frontier, complex professional work | Mid-tier, balances intelligence and cost | Nano-tier, cost-sensitive high-volume |
| API input per 1M tokens (short context) | $5.00 | $2.00 | $0.20 |
| API output per 1M tokens (short context) | $30.00 | $12.00 | $1.20 |
| API input per 1M tokens (long context) | $10.00 | $4.00 | $0.40 |
| API output per 1M tokens (long context) | $45.00 | $18.00 | $1.80 |
| Context window | 1,050,000 tokens | 1,050,000 tokens | 1,050,000 tokens |
| Max output tokens | 128,000 | 128,000 | 128,000 |
| Knowledge cutoff | February 16, 2026 | February 16, 2026 | February 16, 2026 |
| ChatGPT access (Aug 6, 2026 update) | Plus and Pro, updated Chat build | Not covered by the Aug 6 update | Free and Go default |
FAQ
Is GPT-5.6 Luna free?
Yes. GPT-5.6 Luna became the default ChatGPT model for Free and Go users during the week of August 6, 2026, and unlimited text chats plus a Think button arrive the following week, subject to abuse guardrails. Limits still apply to file uploads, images, and other tools.
Is GPT-5.6 Luna better than GPT-5.6 Sol?
No. Luna roughly corresponds to the nano model tier of earlier GPT-5 families, while Sol is the frontier model for complex professional work. The API prices reflect the gap: Luna is 25 times cheaper than Sol per token, and OpenAI's internal evaluation shows Sol with fewer factual errors.
How much does the GPT-5.6 API cost?
Standard pricing per 1M tokens at short context is $5.00 input and $30.00 output for GPT-5.6 Sol, $2.00 and $12.00 for GPT-5.6 Terra, and $0.20 and $1.20 for GPT-5.6 Luna, per OpenAI's pricing page as of August 2026.
Related reading
What Is GPT-5.6? Family explainer, GPT-5.6 review: pricing and models, Claude tool profile, Radar index