Claude Opus 4.6 vs Mercury 2.5
Pricing, context window and real answers (September 2026)
Claude Opus 4.6 (Anthropic) and Mercury 2.5 (Inception), 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 4.6 | Mercury 2.5 | |
|---|---|---|
| Provider | Anthropic | Inception |
| Input (per 1M tokens) | ¥850 | ¥6.77 |
| Output (per 1M tokens) | ¥4,200 | ¥25.38 |
| Typical cost (1,000 in + 500 out tokens × 1,000 calls) | ¥2,950 | ¥19 |
| Context window | 1,000,000 tokens | 260,000 tokens |
| Release date | 2/4/2026 | 9/8/2026 |
| Input modalities | text, image | text |
| Arena · overall | #6 · ELO 1,498 | Unranked |
| Arena · Japanese | #16 · ELO 1,481 | Unranked |
| Arena · coding | #5 · ELO 1,546 | Unranked |
Prices are FastMetal's yen rates before tax. Arena ranks are as of the latest leaderboard scrape.
Which should you pick?
- On price, Mercury 2.5: ¥25.38 per 1M output tokens, 99% less than Claude Opus 4.6.
- For long documents, Claude Opus 4.6: a 1,000,000-token context window against 260,000 for Mercury 2.5.
- 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 4.6
I need to count the number of 'r's in the word 'strawberry'. Let me go through it letter by letter.
**s** - not an r
**t** - not an r
**r** - this is an r ✓ (count: 1)
**a** - not an r
**w** - not an r
**b** - not an r
**e** - not an r
**r** - this is an r ✓ (count: 2)
**r** - this is an r ✓ (count: 3)
**y** - not an r
There are **3** r's in 'strawberry'.
3.2s158 output tokens
Mercury 2.5
To determine the number of 'r's in the word "strawberry", we can examine each letter in sequence:
1. **s** - Not 'r'
2. **t** - Not 'r'
3. **r** - First 'r' found
4. **a** - Not 'r'
5. **w** - Not 'r'
6. **b** - Not 'r'
7. **e** - Not 'r'
8. **r** - Second 'r' found
9. **r** - Third 'r' found
10. **y** - Not 'r'
Counting these occurrences, there are **3** 'r's in "strawberry".
1.8s968 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 4.6
# Understanding the Error
The error `Cannot read properties of undefined (reading 'map')` means that **`users` is `undefined`** when the component renders. You're trying to call `.map()` on a value that doesn't exist yet.
This typically happens when:
- The parent component doesn't pass the `users` prop
- The data is being fetched asynchronously and hasn't loaded yet
- The prop name is misspelled
## Solutions
### 1. Add a Default Parameter Value (Simplest Fix)
```javascript
function UserList({ users = [] }) {
return (
<ul>
{users.map(user => <li key={user.id}>{user.name}</li>)}
</ul>
);
}
```
### 2. Add a Guard with Optional Chaining
```javascript
function UserList({…
10.5s599 output tokens
Mercury 2.5
This error occurs because the `users` prop is `undefined` when the component renders. Since the code calls `.map()` on `users`, JavaScript throws a TypeError when `users` is not an array.
Here is how to fix it:
1. **Set a Default Value**
Update the function signature to default `users` to an empty array if it is not provided:
```javascript
function UserList({ users = [] }) {
// ...
}
```
2. **Add a Conditional Check**
If `users` might be null or undefined during data loading, guard the map operation:
```javascript
{users && users.map(user => <li key={user.id}>{user.name}</li>)}
```
3. **Verify Parent Props**
Ensure the parent component pas…
2.4s1028 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 4.6
# Code Review: `get_data` Function
## Original Issues Identified
1. **Imports inside function** – reduces clarity and adds per-call overhead
2. **Bare `except`** – catches everything including `KeyboardInterrupt` and `SystemExit`
3. **`== True` comparison** – non-Pythonic
4. **Manual list building** – could use a list comprehension
5. **No logging** – failures happen silently
6. **No response validation** – assumes JSON is always a list of dicts with specific keys
7. **Magic numbers** – `timeout=10`, `sleep(2)` unexplained
8. **Vague function/variable names** – `r`, `data`, `get_data`
---
## Improved Version
```python
import logging
import time
from typing import Any
import requests
fr…
19.9s1223 output tokens
Mercury 2.5
Here is a review of the function with suggestions for improvement.
### Key Observations
1. **Imports:** Libraries should be imported at the module level, not inside functions.
2. **Exception Handling:** Bare `except:` clauses catch system exits and interrupts. Use specific exceptions like `requests.exceptions.RequestException`.
3. **Logic & Pythonicity:**
* Avoid `== True`; rely on truthiness or `item.get("active")`.
* List comprehensions are more concise and efficient than manual appending.
4. **Retry Strategy:** Implement exponential backoff instead of fixed sleeps, and avoid sleeping after the final failed attempt.
5. **Documentation:** Add type hints and a docstring fo…
4.6s1999 output tokens
Frequently asked questions
- Which is cheaper, Claude Opus 4.6 or Mercury 2.5?
- Per 1M output tokens, Claude Opus 4.6 is ¥4,200 and Mercury 2.5 is ¥25.38 on FastMetal (yen, before tax), so Mercury 2.5 is cheaper.
- How do the context windows of Claude Opus 4.6 and Mercury 2.5 compare?
- Claude Opus 4.6 takes 1,000,000 tokens; Mercury 2.5 takes 260,000.
- Can I use Claude Opus 4.6 and Mercury 2.5 with the same API key?
- Yes. On FastMetal's OpenAI-compatible endpoint, switch by passing "anthropic-claude-opus-4-6" or "mercury-2.5" 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 4.6 and Mercury 2.5 from the browser chat and the API. No monthly fee.