Claude Opus 5 vs gpt-oss-20b
Pricing, context window and real answers (September 2026)
Claude Opus 5 (Anthropic) and gpt-oss-20b (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
 Claude Opus 5 |  gpt-oss-20b | |
|---|---|---|
| Provider | Anthropic | OpenAI |
| Input (per 1M tokens) | ¥893.5 | ¥5.36 |
| Output (per 1M tokens) | ¥4,467.5 | ¥25.02 |
| Typical cost (1,000 in + 500 out tokens × 1,000 calls) | ¥3,127 | ¥18 |
| Context window | 1,000,000 tokens | 131,072 tokens |
| Release date | 7/24/2026 | 8/5/2025 |
| Input modalities | text, image, file | text |
| Throughput (p50) | — | — |
| Arena · overall | #10 · ELO 1,493 | #252 · ELO 1,317 |
| Arena · Japanese | #6 · ELO 1,503 | #179 · ELO 1,256 |
| Arena · coding | #12 · ELO 1,533 | #232 · ELO 1,369 |
Prices are FastMetal's yen rates before tax. Arena ranks are as of the latest leaderboard scrape.
Which should you pick?
- On price, gpt-oss-20b: ¥25.02 per 1M output tokens, 99% less than Claude Opus 5.
- On the Overall arena board, Claude Opus 5 ranks higher (#10; gpt-oss-20b is #252).
- On the Japanese arena board, Claude Opus 5 ranks higher (#6; gpt-oss-20b is #179).
- On the Coding arena board, Claude Opus 5 ranks higher (#12; gpt-oss-20b is #232).
- For long documents, Claude Opus 5: a 1,000,000-token context window against 131,072 for gpt-oss-20b.
- 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
gpt-oss-20b
**Count of ‘r’ in “strawberry”: 3**
---
### Step‑by‑step reasoning
1. **Write out the word in individual letters**
```
s t r a w b e r r y
```
2. **Identify each ‘r’**
- The first ‘r’: appears after the “t” → **r**
- The second ‘r’: appears in the part “berry” → **r**
- The third ‘r’: the next letter after that → **r**
3. **Verify there are no more letters left**
The remaining letters are “b”, “e”, “y”, none of which are “r”.
4. **Count the identified ‘r’s**
1 (first) + 1 (second) + 1 (third) = **3**
---
### Quick recap
- Word: **strawberry**
- Letters containing “r”: positions 3, 8, and 9.
- Total count: **3**.
11.6s371 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
gpt-oss-20b
### Why does the error happen?
`users.map(...)` can only run when **`users` is an array**.
The stack trace shows that the crash happens on line 12 of `UserList.js`, which is the
exact spot where the code tries to call `.map`.
The value that is being passed to `UserList` (or the initial value that the parent
creates) is **`undefined`** (or it could be `null`/`{}`). When JavaScript tries to
read `users.map`, it throws:
```
TypeError: Cannot read properties of undefined (reading 'map')
```
Typical reasons for this are:
1. The parent component **didn’t pass the `users` prop** at all.
2. The parent passed it but the value was **`undefined` / `null`** because data
hasn’t been loaded ye…
11.4s1303 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
gpt-oss-20b
Below is a quick “code‑review” style list that touches the three axes you asked for – **readability**, **performance**, and **best practices**.
After the bullet‑list I’ll paste a refactored version that incorporates the most common fixes.
---
## 1. Readability
| Issue | Why it hurts | Quick fix |
|-------|--------------|-----------|
| **Imports inside the function** | Every call recompiles the import tables and makes the module’s top‑level slower. | Move `import requests` and `import time` to the module’s global header. |
| **Bare `except:`** | Catches *everything* (KeyboardInterrupt, SystemExit, MemoryError, etc.) and swallows useful tracebacks. | Catch only the expected network-relate…
24.6s2267 output tokens
Frequently asked questions
- Which is cheaper, Claude Opus 5 or gpt-oss-20b?
- Per 1M output tokens, Claude Opus 5 is ¥4,467.5 and gpt-oss-20b is ¥25.02 on FastMetal (yen, before tax), so gpt-oss-20b is cheaper.
- How do the context windows of Claude Opus 5 and gpt-oss-20b compare?
- Claude Opus 5 takes 1,000,000 tokens; gpt-oss-20b takes 131,072.
- Which ranks higher, Claude Opus 5 or gpt-oss-20b?
- Claude Opus 5 ranks higher on the public arena (Claude Opus 5 #6, gpt-oss-20b #179). Ranks move as the leaderboard updates.
- Can I use Claude Opus 5 and gpt-oss-20b with the same API key?
- Yes. On FastMetal's OpenAI-compatible endpoint, switch by passing "anthropic-claude-opus-5" or "gpt-oss-20b" 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 gpt-oss-20b from the browser chat and the API. No monthly fee.