# GPT-6 vs GPT-5.6: prices, specs and what changed

By SeedRouter · Published 2026-09-28 · Updated 2026-09-28

GPT-6 is cheaper and newer than GPT-5.6 without being bigger. OpenAI cut the list price of GPT-6 Sol and GPT-6 Luna to half of GPT-5.6 Sol and GPT-5.6 Luna's promotional prices, kept the same 1.05M-token context and 128K-token output, and moved the knowledge cutoff from February to April and May 2026. On OpenAI's own tests the GPT-6 models also make fewer mistakes and do more per task.

If you are on GPT-5.6 today, the move is mostly a change to `model`, plus a few request details covered below.

## How much cheaper is GPT-6 than GPT-5.6?

How OpenAI's list prices changed, for prompts up to 272K input tokens:

| From → to                             | Input | Cached input | Output              |
| ------------------------------------- | ----- | ------------ | ------------------- |
| GPT-5.6 Sol (promotional) → GPT-6 Sol | Half  | Half         | Half                |
| GPT-5.6 Luna → GPT-6 Luna             | Half  | Half         | 42% of the old rate |
| GPT-5.6 Terra → GPT-6 Sol             | Same  | Same         | 83% of the old rate |

For scale, GPT-6 Astra's rates are five times GPT-6 Sol's on every line. The live table at the end of this guide shows what each GPT-6 model costs on SeedRouter today.

The prices come from OpenAI's [pricing page](https://developers.openai.com/api/docs/pricing) and the [GPT-6 Sol and Luna announcement](https://openai.com/index/introducing-gpt-6-sol-and-luna/), which calls it "reducing API prices for Sol and Luna by 50% compared with their GPT‑5.6 promotional pricing". The GPT-5.6 Sol page notes that its promotional price runs at least through November 21, 2026.

There is no GPT-6 Terra. GPT-6 Sol now costs the same as GPT-5.6 Terra on input and less on output, so a Terra workload can move up to Sol without paying more per input token.

The long-context rule did not change: above 272K input tokens the whole request is billed at 2x input and 1.5x output, on both generations.

## What stayed the same and what changed?

|                         | GPT-5.6 Sol, Terra, Luna | GPT-6 Sol, Luna                     | GPT-6 Astra            |
| ----------------------- | ------------------------ | ----------------------------------- | ---------------------- |
| Context window          | 1,050,000 tokens         | 1,050,000 tokens                    | 1,050,000 tokens       |
| Max output              | 128,000 tokens           | 128,000 tokens                      | 128,000 tokens         |
| Knowledge cutoff        | February 16, 2026        | April 20 (Sol), May 18 (Luna), 2026 | April 30, 2026         |
| Input / output          | Text and images / text   | Text and images / text              | Text and images / text |
| Lowest reasoning effort | Model-dependent          | `none`                              | `low`                  |

The figures are from OpenAI's model pages for [GPT-5.6 Sol](https://developers.openai.com/api/docs/models/gpt-5.6-sol), [GPT-6 Sol](https://developers.openai.com/api/docs/models/gpt-6-sol), [GPT-6 Luna](https://developers.openai.com/api/docs/models/gpt-6-luna) and [GPT-6 Astra](https://developers.openai.com/api/docs/models/gpt-6-astra).

## Is GPT-6 better than GPT-5.6?

On the results OpenAI published, yes. These are OpenAI's own numbers:

* **Mistakes.** On its factuality evaluation, GPT-6 Sol makes "about half as many mistakes as its predecessor", and GPT-6 Luna at higher effort matches GPT-5.6 Sol at about a hundredth of its cost.
* **Business workflows.** On AutomationBench, GPT-6 Luna at high effort improves on GPT-5.6 Luna by 5.4 percentage points at 58% lower cost per task.
* **Coding.** On FrontierCode, GPT-6 Sol "improves substantially over GPT‑5.6 Sol". In the Astra launch table, GPT-6 Astra scores 57.9% on Terminal-Bench 4.0 against 37.3% for GPT-5.6 Sol.
* **Computer use.** GPT-6 Luna at max effort exceeds GPT-5.6 Sol at medium effort on OSWorld 2.0 "at one tenth of its cost". GPT-6 Astra scored 72.6% in about 40 minutes per task where GPT-5.6 Sol scored 65.7% in about 75.

OpenAI also says the GPT-6 models write "slightly shorter answers overall without losing substance", with less jargon in technical conversations.

## Is GPT-6 Luna cheaper than GPT-5.6 Luna?

Yes. On OpenAI's list, GPT-6 Luna's input and cached-input rates are half of GPT-5.6 Luna's, and its output rate is 42% of it, so a typical request costs less than half as much.

## What should I change when moving from GPT-5.6?

OpenAI's [GPT-6 migration notes](https://developers.openai.com/api/docs/guides/latest-model) list the checks:

1. **Model ID.** Use `gpt-6-sol`, `gpt-6-luna` or `gpt-6-astra`.
2. **Reasoning effort.** Keep your current effort where it exists. GPT-6 Astra has no `none`; use `low`. If you used `minimal`, start with `low`.
3. **Sampling.** Unless effort is `none`, remove `temperature`, `top_p` and `top_logprobs` (and `logprobs` on Chat Completions).
4. **Tools.** Use the Responses API for reasoning with tools. On Chat Completions, GPT-6 Sol and Luna call functions only at `none` effort, and GPT-6 Astra not at all.
5. **Effort mid-conversation.** To change effort between turns without losing the prompt cache, append a `configuration_update` input item instead of changing the request-level effort.

The request and response formats are otherwise the ones you already use.

## Frequently asked questions

### Is GPT-6 Sol better than GPT-5.6 Sol?

Yes, on OpenAI's published results: about half the factual mistakes, better coding scores, and half the token price of GPT-5.6 Sol's promotional rate.

### Is GPT-6 Astra better than GPT-5.6 Sol?

Yes. GPT-6 Astra is OpenAI's most capable model and leads GPT-5.6 Sol on every coding benchmark in OpenAI's launch table, from Terminal-Bench 4.0 to DeepSWE. It also costs more: its list rates are 2.5 times GPT-5.6 Sol's promotional rates on every line.

### Does GPT-6 have a bigger context window than GPT-5.6?

No. Both generations read up to 1,050,000 tokens, with 922,000 of them input, and write up to 128,000.

### Can I still use GPT-5.6 on SeedRouter?

SeedRouter sells the GPT-6 models: [GPT-6 Sol](https://seedrouter.ai/models/gpt-6-sol), [GPT-6 Luna](https://seedrouter.ai/models/gpt-6-luna) and [GPT-6 Astra](https://seedrouter.ai/models/gpt-6-astra). GPT-5.6 is not offered.

## What does GPT-6 cost on SeedRouter?

### GPT-6 Astra

Current rates are temporarily unavailable. No price estimate is provided; unavailable rates must not be interpreted as free usage.

### GPT-6 Sol

Current rates are temporarily unavailable. No price estimate is provided; unavailable rates must not be interpreted as free usage.

### GPT-6 Luna

Current rates are temporarily unavailable. No price estimate is provided; unavailable rates must not be interpreted as free usage.

## Try GPT-6 on your own prompts

Run a prompt you already send to GPT-5.6 through [GPT-6 Sol](https://seedrouter.ai/models/gpt-6-sol) or [GPT-6 Luna](https://seedrouter.ai/models/gpt-6-luna) in the browser and compare the answer and the token count. The [Astra vs Sol vs Luna guide](https://seedrouter.ai/blog/gpt-6-astra-vs-sol-vs-luna) helps you pick between them.