Claude Opus 4.8 vs Kimi K3
Pricing, context window and real answers (August 2026)
Claude Opus 4.8 (Anthropic) and Kimi K3 (Moonshot), 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
 Claude Opus 4.8 |  Kimi K3 | |
|---|---|---|
| Provider | Anthropic | Moonshot |
| Input (per 1M tokens) | ¥893.5 | ¥536.1 |
| Output (per 1M tokens) | ¥4,467.5 | ¥2,680.5 |
| Typical cost (1,000 in + 500 out tokens × 1,000 calls) | ¥3,127 | ¥1,876 |
| Context window | 1,000,000 tokens | 1,048,576 tokens |
| Release date | 5/27/2026 | 7/16/2026 |
| Input modalities | text, image, file | text, image |
| Arena · overall | #22 · ELO 1,474 | #11 · ELO 1,486 |
| Arena · Japanese | #26 · ELO 1,448 | Unranked |
| Arena · coding | #12 · ELO 1,528 | #8 · ELO 1,530 |
Prices are FastMetal's yen rates before tax. Arena ranks are as of the latest leaderboard scrape.
Which should you pick?
- On price, Kimi K3: ¥2,680.5 per 1M output tokens, 40% less than Claude Opus 4.8.
- On the Overall arena board, Kimi K3 ranks higher (#11; Claude Opus 4.8 is #22).
- On the Coding arena board, Kimi K3 ranks higher (#8; Claude Opus 4.8 is #12).
- For long documents, Kimi K3: a 1,048,576-token context window against 1,000,000 for Claude Opus 4.8.
- 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.
Claude Opus 4.8
# Counting 'r's in "strawberry"
## Step-by-step process:
Let me go through each letter one at a time:
| Position | Letter | Is it 'r'? |
|----------|--------|-----------|
| 1 | s | No |
| 2 | t | No |
| 3 | r | ✓ Yes (1) |
| 4 | a | No |
| 5 | w | No |
| 6 | b | No |
| 7 | e | No |
| 8 | r | ✓ Yes (2) |
| 9 | r | ✓ Yes (3) |
| 10 | y | No |
## Answer:
There are **3** 'r's in "strawberry".
The three 'r's appear at positions 3, 8, and 9 — one after the "st" and two together in the "rry" ending.
4.5s262 output tokens
Kimi K3
# Counting the 'r's in "strawberry"
**Step 1: Write out each letter with its position**
| Position | Letter |
|----------|--------|
| 1 | s |
| 2 | t |
| 3 | **r** ✓ |
| 4 | a |
| 5 | w |
| 6 | b |
| 7 | e |
| 8 | **r** ✓ |
| 9 | **r** ✓ |
| 10 | y |
**Step 2: Identify each 'r'**
- Position 3: the 'r' in "st**r**"
- Position 8: the first 'r' in "be**r**ry"
- Position 9: the second 'r' in "ber**r**y"
**Step 3: Verify by splitting the word**
"Strawberry" = "straw" + "berry"
- "straw" contains **1** r
- "berry" contains **2** r's
**Answer: There are 3 r's in "strawberry."** 🍓
12.2s594 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…
Claude Opus 4.8
# What's Causing the Error
The error tells you exactly what's happening:
```
TypeError: Cannot read properties of undefined (reading 'map')
at UserList (/app/components/UserList.js:12:25)
```
At the moment `UserList` renders, the `users` prop is `undefined`, and you can't call `.map()` on `undefined`. This is extremely common when data is fetched asynchronously—the component renders **before** the data arrives.
Common reasons `users` is undefined:
- The parent hasn't passed the `users` prop at all
- Data is still loading from an API (initial state is `undefined`)
- The API response shape differs from what you expect (e.g. `data.users` vs `data`)
---
# How to Fix It
## Option 1: De…
11.3s761 output tokens
Kimi K3
## What's causing it
The error means `users` is `undefined` when the component renders — `.map()` only exists on arrays, so calling it on `undefined` throws. Your component code is fine; the problem is **upstream**: whatever renders `<UserList />` isn't passing a valid array.
The most common culprit is **async data fetching with uninitialized state** in the parent:
```jsx
function App() {
const [users, setUsers] = useState(); // ← undefined!
useEffect(() => {
fetch('/api/users')
.then(res => res.json())
.then(setUsers);
}, []);
return <UserList users={users} />; // undefined on first render
}
```
The component renders *before* the fetch resolves, so `users` is `…
47.5s1756 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):…
Claude Opus 4.8
# Code Review
Here's a breakdown of issues and suggested improvements for your function.
## Key Issues
1. **Imports inside the function** — should be at module level (runs on every call, harder to see dependencies).
2. **Bare `except:`** — catches everything, including `KeyboardInterrupt` and `SystemExit`. Silently swallowing errors makes debugging hard.
3. **Magic numbers** — `10`, `2`, `200` scattered around; extract as parameters/constants.
4. **`== True` comparison** — non-Pythonic; use the value directly (or `is True` if strict).
5. **Blocking `time.sleep(2)`** — fixed delay; exponential backoff is a better practice.
6. **`r.status_code == 200`** — misses other success codes; use `ra…
16.9s1403 output tokens
Kimi K3
This works, but there are several issues worth addressing. Here's a breakdown, followed by an improved version.
## Issues
### Correctness & best practices
1. **Bare `except:`** — catches *everything*, including `KeyboardInterrupt` and programming bugs like `KeyError` on a malformed item (which then get pointlessly retried). Catch `requests.RequestException` specifically.
2. **Silent failures** — errors are swallowed with no logging, and returning `[]` makes "API is down" indistinguishable from "no active items." At minimum, log; ideally, raise after the final attempt.
3. **Retrying non-retryable errors** — a 400/401/404 will never succeed on retry, but the code sleeps and retries anyway. O…
57.6s8060 output tokens
Frequently asked questions
- Which is cheaper, Claude Opus 4.8 or Kimi K3?
- Per 1M output tokens, Claude Opus 4.8 is ¥4,467.5 and Kimi K3 is ¥2,680.5 on FastMetal (yen, before tax), so Kimi K3 is cheaper.
- How do the context windows of Claude Opus 4.8 and Kimi K3 compare?
- Claude Opus 4.8 takes 1,000,000 tokens; Kimi K3 takes 1,048,576.
- Which ranks higher, Claude Opus 4.8 or Kimi K3?
- Kimi K3 ranks higher on the public arena (Claude Opus 4.8 #26, Kimi K3 #11). Ranks move as the leaderboard updates.
- Can I use Claude Opus 4.8 and Kimi K3 with the same API key?
- Yes. On FastMetal's OpenAI-compatible endpoint, switch by passing "anthropic-claude-opus-4-8" or "kimi-k3" 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 Claude Opus 4.8 and Kimi K3 from the browser chat and the API. No monthly fee.