Claude Sonnet 5.5 vs DeepSeek V4 Flash
Pricing, context window and real answers (September 2026)
Claude Sonnet 5.5 (Anthropic) and DeepSeek V4 Flash (DeepSeek), 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 Sonnet 5.5 |  DeepSeek V4 Flash | |
|---|---|---|
| Provider | Anthropic | DeepSeek |
| Input (per 1M tokens) | ¥357.4 | ¥15.12 |
| Output (per 1M tokens) | ¥1,787 | ¥30.24 |
| Typical cost (1,000 in + 500 out tokens × 1,000 calls) | ¥1,251 | ¥30 |
| Context window | 1,000,000 tokens | 1,048,576 tokens |
| Release date | 9/28/2026 | 4/24/2026 |
| Input modalities | text, image, file | text |
| Throughput (p50) | — | 87 tokens/s (n=16) |
| Arena · overall | Unranked | #99 · ELO 1,436 |
| Arena · Japanese | Unranked | #81 · ELO 1,398 |
| Arena · coding | Unranked | #98 · ELO 1,484 |
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, DeepSeek V4 Flash: ¥30.24 per 1M output tokens, 98% less than Claude Sonnet 5.5.
- For long documents, DeepSeek V4 Flash: a 1,048,576-token context window against 1,000,000 for Claude Sonnet 5.5.
- 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 Sonnet 5.5
Let me go through the word letter by letter.
**strawberry**
| Position | Letter |
|---|---|
| 1 | s |
| 2 | t |
| 3 | **r** ← 1st |
| 4 | a |
| 5 | w |
| 6 | b |
| 7 | e |
| 8 | **r** ← 2nd |
| 9 | **r** ← 3rd |
| 10 | y |
**Step-by-step reasoning:**
1. I split the word into individual letters: s-t-r-a-w-b-e-r-r-y (10 letters total).
2. I checked each letter for 'r'.
3. The first 'r' is in position 3, in "str…".
4. The second and third 'r's are in positions 8 and 9, in the "…rry" ending.
**Answer: There are 3 r's in "strawberry".**
A common mistake is to see "berry" as having only one 'r' (or to treat the double-r as a single letter), which leads to the incorrect answer of 2.
3.1s332 output tokens
DeepSeek V4 Flash
The word "strawberry" is spelled as s-t-r-a-w-b-e-r-r-y. I go through each letter: s (no), t (no), r (yes, first), a (no), w (no), b (no), e (no), r (yes, second), r (yes, third), y (no). Thus, there are three 'r's.
3.9s251 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 Sonnet 5.5
## Cause
`users` is `undefined` when the component renders, so `users.map(...)` throws. This happens when:
1. **The parent doesn't pass the prop**, or passes it under a different name (e.g. `<UserList data={x} />` or a typo like `user=`).
2. **The data loads asynchronously.** On the first render, the state or fetch result hasn't arrived yet.
3. **The value is wrong.** For example, the API returns `{ users: [...] }` and you passed the whole object, or you're reading a property that doesn't exist (`response.data.users` vs `response.users`).
## Fixes
### 1. Default value (simplest)
```jsx
function UserList({ users = [] }) {
return (
<ul>
{users.map(user => <li key={user.id}>{use…
6.7s824 output tokens
DeepSeek V4 Flash
The error occurs because `users` is `undefined` when the component tries to call `.map()` on it.
## Why this happens
Your component expects `users` to be an array, but it's receiving `undefined` as a prop. This typically happens when:
1. The parent component doesn't pass the `users` prop
2. The data fetching hasn't completed yet (async data loading)
3. The API response is missing the expected data structure
## How to fix it
### Option 1: Default prop value (Recommended for initial state)
```jsx
function UserList({ users = [] }) {
return (
<ul>
{users.map(user => <li key={user.id}>{user.name}</li>)}
</ul>
);
}
```
### Option 2: Guard clause (More explicit)
```jsx
func…
6.3s422 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 Sonnet 5.5
# Code Review
## Main Issues
### 1. Bare `except:` hides bugs (most important)
It catches everything, including `KeyboardInterrupt` and `SystemExit`. It also wraps your parsing logic, so a missing `"active"` key (`KeyError`) or bad JSON is treated as a network failure, retried 3 times with sleeps, and then silently becomes `[]`. Catch only `requests.RequestException`, and keep parsing outside the `try`.
### 2. Silent failure
Returning `[]` after all retries fails is indistinguishable from "the server returned no active items." Callers can't tell whether something went wrong. Raise an exception (or at least log) once retries are exhausted.
### 3. Retry logic
- It sleeps after the final at…
20.9s3003 output tokens
DeepSeek V4 Flash
Here's a comprehensive review with suggested improvements:
## Issues Identified:
1. **Import at wrong location** - Imports inside functions are inefficient and non-standard
2. **Bare `except:` clause** - Catches all exceptions including `KeyboardInterrupt`, `SystemExit`
3. **Missing error handling granularity** - Should distinguish between retryable and non-retryable errors
4. **Duplicate sleep logic** - Same sleep in `else` and `except` blocks
5. **Unnecessary variable** - `data` is only used once
6. **Inefficient list building** - Manual loop instead of list comprehension
7. **Hardcoded error message** - No logging or error context
8. **Missing type hints** - No type annotations
9. **Fun…
12.6s987 output tokens
Frequently asked questions
- Which is cheaper, Claude Sonnet 5.5 or DeepSeek V4 Flash?
- Per 1M output tokens, Claude Sonnet 5.5 is ¥1,787 and DeepSeek V4 Flash is ¥30.24 on FastMetal (yen, before tax), so DeepSeek V4 Flash is cheaper.
- How do the context windows of Claude Sonnet 5.5 and DeepSeek V4 Flash compare?
- Claude Sonnet 5.5 takes 1,000,000 tokens; DeepSeek V4 Flash takes 1,048,576.
- Can I use Claude Sonnet 5.5 and DeepSeek V4 Flash with the same API key?
- Yes. On FastMetal's OpenAI-compatible endpoint, switch by passing "anthropic-claude-sonnet-5-5" or "deepseek-v4-flash" 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 Sonnet 5.5 and DeepSeek V4 Flash from the browser chat and the API. No monthly fee.