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Opus 5.5 vs Sonnet 5.5: which Claude model to use
Both came out in September 2026, a week apart. They share the same 1M context window, so the choice comes down to price, speed and how hard the task is.
Side by side
| Sonnet 5.5 | Opus 5.5 | |
|---|---|---|
| Released | 2026-09-28 | 2026-09-22 |
| Input / output per 1M tokens | $2.00 / $10.00 | $4.00 / $20.00 |
| Cache hit per 1M | $0.20 | $0.20 |
| Context window | 1M | 1M |
| Max output | 128K | 128K |
| Speed (Anthropic) | 30%+ faster than Sonnet 5 | 30%+ faster than Opus 5 |
| Free Claude plan | Yes | No (Pro and above) |
| API model ID | claude-sonnet-5-5 | claude-opus-5-5 |
Benchmarks
From Anthropic's Sonnet 5.5 launch post, which lists both models (Opus 5.5 at its highest effort).
| Benchmark | Sonnet 5.5 | Opus 5.5 | Sonnet 5 |
|---|---|---|---|
| Terminal-Bench 4.0 | 70.6% | 66.4% | 10.3% |
| FrontierCode 1.1 (xhigh) | 52.1% | 54.4% | 42.4% |
| CursorBench 4.0 | 55.5% | 57.8% | 34.1% |
| OSWorld 2.1 | 80.1% | 81.8% | 57.0% |
| Humanity's Last Exam | 64.5% | 67.7% | 54.9% |
| GDPval-AA v2.1 (Elo) | 1844 | 1846 | 1449 |
Source: Anthropic: Introducing Claude Sonnet 5.5
Cost per coding task
List prices with our calculator's assumptions, per agent task:
| Codebase size | Sonnet 5.5 | Opus 5.5 |
|---|---|---|
| Small (< 10k lines) | $0.115 | $0.223 |
| Medium (10k-100k lines) | $0.258 | $0.498 |
| Large / monorepo (> 100k lines) | $0.618 | $1.188 |
At 173 tasks a month on a medium codebase: about $45 on Sonnet 5.5 and $86 on Opus 5.5.
Price your own workload in the calculator
Which one to pick
Pick Sonnet 5.5 for
- Everyday agent coding in Claude Code
- Terminal-heavy tasks: tests, builds, scripts
- High-volume API work where cost matters
- Free-plan users (Opus is not included)
Pick Opus 5.5 for
- Hard, open-ended problems Anthropic says it is still clearly stronger at
- Large refactors across many files
- Bugs Sonnet 5.5 has already failed to fix
- Security work: Sonnet 5.5 sends some higher-risk cyber requests back to Sonnet 5
Using both in Claude Code
You can switch models mid-session with /model. A practical pattern is to run on Sonnet 5.5 by default and move to Opus 5.5 only when a task stalls.
FAQ
Is Opus 5.5 better than Sonnet 5.5?
Only slightly, and not everywhere. Opus 5.5 leads on most of Anthropic's benchmarks by 3.2 points or less, but Sonnet 5.5 leads on Terminal-Bench 4.0 (70.6% vs 66.4%) at half the price.
Is Opus 5.5 worth twice the price?
For routine coding, usually not. For the hardest long-running tasks, or when Sonnet gets stuck, the extra cost can pay for itself.
How does Sonnet 5 compare?
Sonnet 5 scored 10.3% on Terminal-Bench 4.0, far below both. Sonnet 5.5 has the same price as Sonnet 5, so there is no reason to stay on it.
Which is better for Claude Code?
Sonnet 5.5 as the default, Opus 5.5 for the hard cases. Both are available in Claude Code on paid plans.
Independent page. Not affiliated with Anthropic. Benchmarks are Anthropic's; we have not run our own tests.