GPT-6 Sol vs 5.6 Sol: Price, Specs and Which Is Better

GPT-6 Sol vs 5.6 Sol comes down to one large change and several small ones. OpenAI lists GPT-6 Sol at $2 per million input tokens and $10 per million output tokens, half of GPT-5.6 Sol's current $4 and $20^[1]. The context window, output ceiling and reasoning-effort ladder did not change^[2][3].

The small changes are the ones that break requests: how function calling works on Chat Completions, which sampling parameters are accepted, and the fact that OpenAI already shipped GPT-6.1 Sol a week later^[4]. This article lays out the documented differences, the price history behind the "50% cheaper" claim, and what to change before you switch.

TL;DR

• Price: GPT-6 Sol is $2 input, $0.20 cached input and $10 output per million tokens. GPT-5.6 Sol is $4, $0.40 and $20 on its current promotional price^[1].
• Same limits: both have a 1,050,000-token context window, 922,000 max input tokens, 128,000 max output tokens and six effort levels from none to max^[2][3].
• Newer knowledge: the cutoff moved from February 16, 2026 to April 20, 2026^[2][3].
• Tool calling: on OpenAI's own Chat Completions endpoint, GPT-6 Sol supports function calling only with reasoning_effort set to none^[2].
• Quality: OpenAI reports about half as many factual mistakes and substantially better FrontierCode results than GPT-5.6 Sol^[5]. Early user reports are mixed.
• Newer Sol: GPT-6.1 Sol shipped on September 29, 2026, and OpenAI's GPT-6 Sol page points to it as "the newer Sol model"^[2][4].

What changed from GPT-5.6 Sol to GPT-6 Sol

OpenAI released GPT-5.6 Sol on July 9, 2026 as the flagship of the GPT-5.6 family, and GPT-6 Sol on September 22, 2026 together with GPT-6 Luna^[4]. GPT-5.6 Sol's page calls it a model "for complex professional work". GPT-6 Sol's page says it is "built to power complex coding and agentic workflows"^[2][3].

The token budgets, the effort names and the default stay the same. What a request costs, what the model knows and how tool calls work on Chat Completions are the parts that moved.

GPT-6 Sol vs 5.6 Sol price: where the 50% comes from

OpenAI's launch post states the cut directly: Sol and Luna API prices fell "by 50% compared with their GPT‑5.6 promotional pricing", with GPT-5.6 Sol at $4 → $2 for input and $20 → $10 for output^[5]. The word "promotional" matters. GPT-5.6 Sol has had two prices:

1. At launch it listed higher. On August 21, 2026 OpenAI cut it to $4 input and $20 output and called that "20% lower input pricing and 33% lower output pricing"^[4]. Working back from those percentages, the earlier price was $5 and $30.
2. That cut is promotional. OpenAI says GPT-5.6 Sol's promotional pricing "is available at least through November 21, 2026"^[1]. It has not said what the price becomes afterwards.

Against the launch price, GPT-6 Sol is 60% cheaper on input and about 67% cheaper on output. Against today's price it is exactly half. The same 50% ratio holds on every other line of OpenAI's rate card^[1]:

For an illustrative month of 50 million input tokens and 10 million output tokens with no caching and no long prompts, GPT-5.6 Sol costs $200 + $200 = $400 at OpenAI's list price and GPT-6 Sol costs $100 + $100 = $200. That is per-token math only. Reasoning tokens bill as output, so a model that thinks longer on your task can narrow the gap. OpenAI's own task-level comparisons (below) were run in its research environment, not on your workload.

For the full GPT-6 Sol rate card and more worked examples, see GPT-6 Sol pricing.

Is GPT-6 Sol better than 5.6 Sol?

OpenAI's claims are specific but come from its own evaluations. In its launch post OpenAI says GPT-6 Sol makes "about half as many mistakes as its predecessor" on an internal factuality evaluation built from conversations where users had flagged errors, and that it "improves substantially over GPT‑5.6 Sol" on FrontierCode, a benchmark that grades whether code changes are ready to merge^[5]. It also reports alignment improvements over the GPT-5.6 models and says Sol's answers are slightly shorter, with less jargon^[5].

Early user reports are mixed. In a Reddit thread from September 29, one user described a Codex task that GPT-6 Sol at extra-high effort had barely started after 33 minutes, while GPT-5.6 Sol finished the plan in about 38 seconds^[6]. Another user reported that GPT-6 Sol at xhigh introduced regressions while reviewing six projects and went back to 5.6 Sol^[7]. These are single accounts from Codex, not measurements, but they show what people are checking: speed per task and instruction following.

One known issue has been fixed. On September 25, OpenAI corrected an image-encoding bug that had degraded image understanding in GPT-6 Sol and GPT-6 Luna, and recommended rerunning evaluations that involve image inputs^[4]. If you compared the two models on screenshots before that date, run the comparison again.

Request changes when you switch from GPT-5.6 Sol

On OpenAI's API, three things need attention:

• Tools on Chat Completions. GPT-6 Sol supports function calling there only with reasoning_effort: "none". OpenAI recommends the Responses API for reasoning with tools^[2][8].
• Sampling parameters. OpenAI's GPT-6 migration notes say to remove temperature, top_p and top_logprobs when effort is not none, and logprobs as well on Chat Completions^[8].
• Effort. Both models accept the same six values, so you can keep your current setting and compare on the same tasks^[2][3].

On reAPI, both models are served through the same POST /api/v1/chat/completions endpoint, and the switch is a change of the model string from gpt-5.6-sol to gpt-6-sol. The parameter surface differs in a few places, as measured and documented in reAPI's API docs^[9][10]:

The practical rule: strip temperature, top_p and both penalties from requests before you point them at gpt-6-sol. The GPT-6 Sol docs list every field.

FAQ

Is GPT-6 Sol cheaper than 5.6 Sol?

Yes. On OpenAI's list, GPT-6 Sol costs $2 input and $10 output per million tokens against $4 and $20 for GPT-5.6 Sol, and the 50% gap holds for cached input, cache writes, long context, Batch and Fast mode^[1]. Total cost per task also depends on how many reasoning tokens each model spends.

Is GPT-6 Sol or 5.6 Sol better?

OpenAI reports GPT-6 Sol ahead on its factuality evaluation and on FrontierCode^[5]. Some early Codex users reported slower or less reliable results than with 5.6 Sol^[6][7]. Neither settles your workload, so run the same tasks on both before you switch production traffic.

Is GPT-6 Sol or 5.6 Sol discounted?

GPT-5.6 Sol is on promotional pricing of $4 and $20, available at least through November 21, 2026^[1]. We found no promotional label or end date for GPT-6 Sol's $2 and $10 on OpenAI's pricing page, model page or changelog^[1][2][4].

GPT-6 Sol vs GPT-6.1 Sol

GPT-6.1 Sol was released on September 29, 2026 at the same $2 input and $10 output, with cached input at $0.10 instead of $0.20 and an April 30, 2026 knowledge cutoff. It does not support the none effort, and on Chat Completions it does not support tool calling at all^[11][4]. OpenAI has published no shutdown date for GPT-6 Sol.

GPT-6 Sol vs 5.6 Terra

GPT-5.6 Terra lists at $2 input and $12 output per million tokens, so GPT-6 Sol matches it on input and is cheaper on output at $10^[1]. OpenAI introduced Terra for "a balance of intelligence and cost" and 5.6 Sol for "frontier capability"^[4].

GPT-6 Sol context window

GPT-6 Sol has a 1,050,000-token context window, with up to 922,000 input tokens and 128,000 output tokens, the same as GPT-5.6 Sol^[2][3]. Prompts over 272K input tokens are billed at the long-context rate for the whole request^[2].

Has GPT 6 been released?

Yes. GPT-6 Sol and GPT-6 Luna were released in the OpenAI API on September 22, 2026, and GPT-6.1 Sol followed on September 29, 2026^[4]. On reAPI, GPT-6 Sol is available on the GPT-6 Sol model page.

Should you migrate from GPT-5.6 Sol?

For new work, start with GPT-6 Sol: it costs half as much per token, has the same limits and a newer knowledge cutoff. For an existing GPT-5.6 Sol pipeline, the price case is strong but not urgent, because 5.6 Sol's promotional price runs at least to November 21, 2026. Use that window to rerun your own tasks on both models, remove the sampling parameters GPT-6 Sol rejects, and check tool calls and time per task. Then compare GPT-6.1 Sol too, since it is OpenAI's newer Sol at the same input and output price. The current per-token rates are on the GPT-6 Sol model page and the GPT-5.6 Sol model page. The GPT-6 Sol vs 5.6 Sol decision should rest on those results, not on the version number.

References

1. OpenAI. API pricing. Retrieved October 2026 from developers.openai.com/api/docs/pricing
2. OpenAI. GPT-6 Sol model page. Retrieved October 2026 from developers.openai.com/api/docs/models/gpt-6-sol
3. OpenAI. GPT-5.6 Sol model page. Retrieved October 2026 from developers.openai.com/api/docs/models/gpt-5.6-sol
4. OpenAI. API changelog. Retrieved October 2026 from developers.openai.com/api/docs/changelog
5. OpenAI. Introducing GPT-6 Sol and Luna. Retrieved October 2026 from openai.com/index/introducing-gpt-6-sol-and-luna
6. Reddit. GPT-6 Sol: 33 minutes to do what GPT-5.6 Sol did in 38 seconds — what is going on? Retrieved October 2026 from reddit.com/gallery/1wtbec9
7. Reddit r/codex. GPT-6 Sol is... not good. Retrieved October 2026 from reddit.com/r/codex/comments/1wp5x1a
8. OpenAI. Using GPT-6. Retrieved October 2026 from developers.openai.com/api/docs/guides/latest-model
9. reAPI. GPT-6 Sol API docs. Retrieved October 2026 from reapi.ai/docs/gpt-6-sol
10. reAPI. GPT-5.6 Sol API docs. Retrieved October 2026 from reapi.ai/docs/gpt-5-6-sol
11. OpenAI. GPT-6.1 Sol model page. Retrieved October 2026 from developers.openai.com/api/docs/models/gpt-6.1-sol