FastMetal

Model comparison

GPT-5.6 Luna vs GPT-6.1 Sol

Pricing, context window and real answers (September 2026)

GPT-5.6 Luna (OpenAI) and GPT-6.1 Sol (OpenAI), compared as you can actually call them on FastMetal. Both are served from the same OpenAI-compatible endpoint and API key; switching is a change to the model string.

Specs and pricing

openai logoGPT-5.6 Lunaopenai logoGPT-6.1 Sol
ProviderOpenAIOpenAI
Input (per 1M tokens)¥35.74¥357.4
Output (per 1M tokens)¥214.44¥1,787
Typical cost (1,000 in + 500 out tokens × 1,000 calls)¥143¥1,251
Context window1,050,000 tokens1,050,000 tokens
Release date7/9/20269/29/2026
Input modalitiesfile, image, textfile, image, text
Throughput (p50)310 tokens/s (n=17)—
Arena · overall#70 · ELO 1,454Unranked
Arena · Japanese#49 · ELO 1,433Unranked
Arena · coding#73 · ELO 1,501Unranked

Prices are FastMetal's yen rates before tax.

Rankings: LMArena leaderboard dataset (lmarena-ai/leaderboard-dataset), CC BY 4.0, snapshot 2026-09-25; category names mapped and ratings rounded by FastMetal.

Which should you pick?

  • On price, GPT-5.6 Luna: ¥214.44 per 1M output tokens, 88% less than GPT-6.1 Sol.
  • If in doubt, try both on the same key. Switching is a change to the model string, and each is billed at its own rate.

Real answers to the same prompts

Exactly what the FastMetal gateway returned, side by side. Not benchmark scores: actual output.

Count the number of 'r's in 'strawberry'

Count the number of 'r's in 'strawberry'. Explain your reasoning step by step.

GPT-5.6 Luna

The word **“strawberry”** is spelled: **s – t – r – a – w – b – e – r – r – y** The letter **“r”** appears at positions **3, 8, and 9**. **Answer: 3**
3.0s120 output tokens

GPT-6.1 Sol

“strawberry” is spelled **s-t-r-a-w-b-e-r-r-y**. The letter **r** appears at positions **3, 8, and 9**. So, there are **3 r’s** in “strawberry.”
2.8s88 output tokens

Debug This Error

I'm getting the following error in my Node.js application: TypeError: Cannot read properties of undefined (reading 'map') at UserList (/app/components/UserList.js:12:25) at renderWithHooks (/app/node_modules/rea…

GPT-5.6 Luna

`users` is `undefined` when `UserList` renders, so calling `.map()` on it fails. `.map()` only exists on arrays. Common causes include: - The parent does not pass a `users` prop. - The prop name differs, such as `userList` instead of `users`. - Users are loaded asynchronously and are initially undefined. - The API response has a different shape, such as `{ data: [...] }`. ### Provide a default value ```jsx function UserList({ users = [] }) { return ( <ul> {users.map(user => ( <li key={user.id}>{user.name}</li> ))} </ul> ); } ``` The parent should pass an array: ```jsx <UserList users={users} /> ``` If the list comes from state, initialize it as an empty…
4.8s507 output tokens

GPT-6.1 Sol

The error means **`users` is `undefined` when `UserList` renders**. Calling `.map()` on `undefined` throws this error. This commonly happens when the parent doesn’t pass the `users` prop, or when data hasn’t finished loading yet. ### Fix: provide a default empty array ```jsx function UserList({ users = [] }) { return ( <ul> {users.map(user => ( <li key={user.id}>{user.name}</li> ))} </ul> ); } ``` This renders an empty list if `users` is omitted or `undefined`. It does **not** handle `null`. ### Check the parent component too Make sure it passes an array: ```jsx <UserList users={users} /> ``` If you’re fetching users asynchronously, initialize the stat…
5.8s306 output tokens

Code Review

Please review the following Python function and suggest improvements for readability, performance, and best practices: def get_data(url, retries=3): import requests import time for i in range(retries):…

GPT-5.6 Luna

### Main issues - Imports should be at module level, not inside the function. - A bare `except:` catches everything, including programming errors and `KeyboardInterrupt`. - The function sleeps even after the final failed attempt. - It retries all HTTP statuses, including permanent errors such as `404`. - `status_code == 200` excludes other successful `2xx` responses. - `if item["active"] == True` should generally be `if item.get("active") is True`. - A `requests.Session` can reuse connections and improve performance. - Returning `[]` for every failure makes it impossible to distinguish “no active items” from “request failed.” - Retries should generally use backoff rather than a fixed delay.…
23.3s1978 output tokens

GPT-6.1 Sol

The function is short, but its main weakness is that it hides failures: a network error, invalid JSON, and a genuinely empty result all return `[]`. ### Suggested improvements - **Move imports to module scope.** This is conventional and makes dependencies visible. Repeated imports are cached, so the performance benefit is minor. - **Avoid bare `except:`.** It catches everything, including `KeyboardInterrupt` and `SystemExit`, and can hide programming errors. - **Handle HTTP errors explicitly.** `raise_for_status()` provides useful exceptions. Retry transient failures such as timeouts, connection errors, HTTP 429, and HTTP 5xx—not permanent errors such as HTTP 404. - **Don’t sleep after the…
19.0s1094 output tokens

Compare on more prompts →

Frequently asked questions

Which is cheaper, GPT-5.6 Luna or GPT-6.1 Sol?
Per 1M output tokens, GPT-5.6 Luna is ¥214.44 and GPT-6.1 Sol is ¥1,787 on FastMetal (yen, before tax), so GPT-5.6 Luna is cheaper.
How do the context windows of GPT-5.6 Luna and GPT-6.1 Sol compare?
GPT-5.6 Luna takes 1,050,000 tokens; GPT-6.1 Sol takes 1,050,000.
Can I use GPT-5.6 Luna and GPT-6.1 Sol with the same API key?
Yes. On FastMetal's OpenAI-compatible endpoint, switch by passing "gpt-5.6-luna" or "gpt-6.1-sol" as the model. Each is billed at its own rate from the same prepaid balance.

Try both on one API key

Create an account and add credit to call GPT-5.6 Luna and GPT-6.1 Sol from the browser chat and the API. No monthly fee.

Create a free accountTry GPT-5.6 Luna in chatTry GPT-6.1 Sol in chat

More comparisons with GPT-5.6 Luna

More comparisons with GPT-6.1 Sol