Claude Opus 5 vs Gemini 3.1 Pro Preview
Pricing, context window and real answers (September 2026)
Claude Opus 5 (Anthropic) and Gemini 3.1 Pro Preview (Google), 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 5 |  Gemini 3.1 Pro Preview | |
|---|---|---|
| Provider | Anthropic | |
| Input (per 1M tokens) | ¥893.5 | ¥357.4 |
| Output (per 1M tokens) | ¥4,467.5 | ¥2,144.4 |
| Typical cost (1,000 in + 500 out tokens × 1,000 calls) | ¥3,127 | ¥1,430 |
| Context window | 1,000,000 tokens | 1,048,576 tokens |
| Release date | 7/24/2026 | 2/19/2026 |
| Input modalities | text, image, file | audio, file, image, text, video |
| Throughput (p50) | — | — |
| Arena · overall | #10 · ELO 1,493 | #15 · ELO 1,487 |
| Arena · Japanese | #6 · ELO 1,503 | #9 · ELO 1,494 |
| Arena · coding | #12 · ELO 1,533 | #28 · ELO 1,520 |
Prices are FastMetal's yen rates before tax. Arena ranks are as of the latest leaderboard scrape.
Which should you pick?
- On price, Gemini 3.1 Pro Preview: ¥2,144.4 per 1M output tokens, 52% less than Claude Opus 5.
- On the Overall arena board, Claude Opus 5 ranks higher (#10; Gemini 3.1 Pro Preview is #15).
- On the Japanese arena board, Claude Opus 5 ranks higher (#6; Gemini 3.1 Pro Preview is #9).
- On the Coding arena board, Claude Opus 5 ranks higher (#12; Gemini 3.1 Pro Preview is #28).
- For long documents, Gemini 3.1 Pro Preview: a 1,048,576-token context window against 1,000,000 for Claude Opus 5.
- 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 5
## Counting the 'r's in "strawberry"
**Step 1: Break the word into individual letters**
| 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 occurrence of 'r'**
- Position 3: the 'r' in the "str-" beginning
- Position 8: the first 'r' in the "-rry" ending
- Position 9: the second 'r' in the "-rry" ending
**Step 3: Tally them up**
3 + 8 + 9 → that's **3 occurrences**.
## Answer: 3
The tricky part is that "strawberry" contains a *double* 'r' near the end ("berry"), which is easy to skim over as a single letter. Splitting the word into "straw" (1 'r') + "berry"…
7.1s466 output tokens
Gemini 3.1 Pro Preview
There are **3** 'r's in the word 'strawberry'.
Here is the step-by-step breakdown of the spelling:
1. **s** - not an 'r'
2. **t** - not an 'r'
3. **r** - **1st 'r'**
4. **a** - not an 'r'
5. **w** - not an 'r'
6. **b** - not an 'r'
7. **e** - not an 'r'
8. **r** - **2nd 'r'**
9. **r** - **3rd 'r'**
10. **y** - not an 'r'
Therefore, the total count is 3.
5.3s503 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 5
## What's happening
The error means `users` is `undefined` when `UserList` renders — so calling `.map()` on it throws. React isn't the problem; the prop simply isn't there yet (or ever).
The three most common causes:
1. **The parent isn't passing the prop** (or passes it under a different name / typo):
```jsx
<UserList /> // users is undefined
<UserList user={data} /> // wrong name
```
2. **The data is fetched asynchronously**, and the first render happens before it arrives:
```jsx
const [users, setUsers] = useState(); // undefined on first render!
useEffect(() => { fetch('/api/users').then(r => r.json()).then(setUsers); }, []);
return <UserList user…
15.1s1191 output tokens
Gemini 3.1 Pro Preview
### The Cause
This error occurs because the `users` prop being passed to your `UserList` component is currently `undefined`.
In JavaScript, you can only use the `.map()` method on arrays. Because `users` is `undefined` (often due to data not finishing an API fetch yet, or the parent component forgetting to pass the prop), Node/React throws an error when it tries to read `.map`.
### How to Fix It
You can fix this in a few different ways, depending on how you want your application to behave.
#### Fix 1: Assign a Default Value (Recommended)
The cleanest way to handle this is to assign a default empty array to `users` in your component's parameters. If `users` is undefined, it defaults to…
13.2s1511 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 5
# Code Review: `get_data`
Overall, the function works for the happy path, but there are several issues worth addressing — the most serious being the bare `except`, the silent failure mode, and the conflation of two responsibilities (fetching and transforming).
## Critical Issues
### 1. Bare `except` swallows everything
```python
except:
time.sleep(2)
```
This catches `KeyboardInterrupt`, `SystemExit`, `MemoryError`, and — importantly — bugs in your own code. If `item["active"]` raises a `KeyError` because the API changed its schema, you'll retry three times and return `[]`, giving you no indication anything went wrong. Catch `requests.RequestException` instead (the base class for all…
34.7s2823 output tokens
Gemini 3.1 Pro Preview
Here is a review of your function. While the original code works, there are several areas where it can be improved regarding **best practices**, **readability**, and **performance**.
### 1. Refactored Code
Here is the improved version of your function. I will explain the specific changes below.
```python
import time
import requests
import logging
from typing import Any
# Optional: Set up a logger
logger = logging.getLogger(__name__)
def get_data(url: str, retries: int = 3, delay: int = 2, timeout: int = 10) -> list[dict[str, Any]]:
"""
Fetches JSON data from a URL and extracts active items.
Args:
url: The API endpoint to fetch data from.
retries: Number o…
23.6s3094 output tokens
Frequently asked questions
- Which is cheaper, Claude Opus 5 or Gemini 3.1 Pro Preview?
- Per 1M output tokens, Claude Opus 5 is ¥4,467.5 and Gemini 3.1 Pro Preview is ¥2,144.4 on FastMetal (yen, before tax), so Gemini 3.1 Pro Preview is cheaper.
- How do the context windows of Claude Opus 5 and Gemini 3.1 Pro Preview compare?
- Claude Opus 5 takes 1,000,000 tokens; Gemini 3.1 Pro Preview takes 1,048,576.
- Which ranks higher, Claude Opus 5 or Gemini 3.1 Pro Preview?
- Claude Opus 5 ranks higher on the public arena (Claude Opus 5 #6, Gemini 3.1 Pro Preview #9). Ranks move as the leaderboard updates.
- Can I use Claude Opus 5 and Gemini 3.1 Pro Preview with the same API key?
- Yes. On FastMetal's OpenAI-compatible endpoint, switch by passing "anthropic-claude-opus-5" or "gemini-3.1-pro-preview" 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 5 and Gemini 3.1 Pro Preview from the browser chat and the API. No monthly fee.