Sonnet 5.5 vs Opus 5.5: Price, Benchmarks, Which to Use
Updated 2026-10-09
Short answer: Claude Sonnet 5.5 costs exactly half of Opus 5.5 per token ($2 / $10 vs $4 / $20 per million input / output tokens) and, on Anthropic's own benchmark table, trails it by only about 1–8 points – it even wins Terminal-Bench 4.0. But Sonnet tends to use more output tokens on long agentic tasks, so the real saving per task is usually smaller than 50%. Use Sonnet 5.5 for well-scoped, high-volume work; keep Opus 5.5 for open-ended engineering, high-risk reviews and questions that depend on the model's own knowledge.
Both models are new: Anthropic released Opus 5.5 on September 22, 2026 and Sonnet 5.5 six days later, on September 28, 2026, describing Sonnet as "a faster, lower-cost complement to Claude Opus 5.5." For the background on Opus, see our Claude Opus 5.5 guide; for the general rule of thumb across versions, see Opus vs Sonnet.
Sonnet 5.5 vs Opus 5.5 at a glance
| Claude Sonnet 5.5 | Claude Opus 5.5 | |
|---|---|---|
| Release date | September 28, 2026 | September 22, 2026 |
| API model ID | claude-sonnet-5-5 |
claude-opus-5-5 |
| Input / output price (per 1M tokens) | $2 / $10 | $4 / $20 |
| Cache reads (per 1M tokens) | $0.20 | $0.20 |
| Batch API | 50% off | 50% off |
| Context window | 1M tokens | 1M tokens |
| Max output | 128K tokens | 128K tokens |
| Default effort (API) | high | medium |
| Default effort (Claude Code) | medium | medium |
Figures as published by Anthropic and summarised in coverage dated September 30, 2026. Two things stand out: cache reads cost the same on both, so for coding agents that mostly re-read a large repository the price gap shrinks; and the API default effort differs, so an untuned side-by-side test is not a fair comparison – set the effort explicitly.
Benchmarks: how close is Sonnet 5.5?
On the table Anthropic published with the Sonnet 5.5 launch, Opus 5.5 leads on seven of eight rows, mostly by small margins:
- Terminal-Bench 4.0: Sonnet 70.6% vs Opus 66.4% – Sonnet ahead (Anthropic notes the Opus figure was run at a different effort setting).
- FrontierCode 1.1: Sonnet 46.2% (max effort) vs Opus 54.4% – the largest gap, and the benchmark closest to hard, open-ended engineering. Anthropic reports 52.1% for Sonnet at xhigh effort.
- CursorBench 4.0: 55.5% vs 57.8%.
- OSWorld 2.1: 80.1% vs 81.8%.
- Humanity's Last Exam (with tools): 64.5% vs 67.7%.
Independent testing by Artificial Analysis (as reported in early October 2026) broadly agrees, and adds one interesting split: Opus 5.5 answered more factual-knowledge questions correctly, while Sonnet 5.5 was more likely to say it didn't know and showed a lower hallucination rate on that test. For apps that answer from your own documents, Sonnet's profile is attractive; for questions answered from the model's own memory, Opus is stronger.
Does Sonnet 5.5 really cost half?
Per token, yes. Per task, not always. In Artificial Analysis's max-effort runs, Sonnet 5.5 used noticeably more output tokens per task than Opus 5.5, and longer runs also mean more input re-sent on every turn – at max effort its measured all-in cost per task came out higher than Opus.
At normal effort on well-scoped jobs, the saving is real. A simple way to think about it:
- Same token counts: Sonnet costs about 50% of Opus.
- Sonnet uses ~60% more tokens: Sonnet costs roughly 80–85% of Opus.
- Heavily cached workloads: the gap narrows further, because cache reads are $0.20 per million on both.
So budget Sonnet at roughly half to most of Opus's cost, not a flat 50%, and measure on your own tasks – log usage for both models with the same effort setting.
Which is better for coding?
For day-to-day coding – bug fixes, small features, terminal and DevOps work – Sonnet 5.5 is close enough, and it is faster. Anthropic itself describes Sonnet 5.5 as strongest at "well-scoped everyday tasks, fixing bugs, and creating polished documents, slides, and spreadsheets," while Opus 5.5 "remains clearly stronger at complex, open-ended work requiring sustained judgment."
A code-review test published by CodeRabbit points the same way: on its small 13-case benchmark, Opus 5.5 caught more of the known issues than Sonnet 5.5, and CodeRabbit kept Opus for high-risk changes while using Sonnet for routine pull requests. Thirteen cases is a small sample, so treat it as directional.
In Claude Code
Recent Claude Code versions use Opus 5.5 as the default Opus model and Sonnet 5.5 as the default Sonnet model on the Anthropic API, both at medium effort. You can switch with /model sonnet or /model opus, or use the opusplan alias: Opus plans, Sonnet writes the code. On other clouds the sonnet alias may still point to an older Sonnet, so pin the full model ID there.
Sonnet 5 vs Sonnet 5.5
If you are upgrading from Sonnet 5: Anthropic's "up to 30% less per task" and "30%+ faster" claims compare Sonnet 5.5 with Sonnet 5, not with Opus 5.5 – some coverage mixes the two up. Sonnet 5.5's effort levels were recalibrated, so re-test your old effort settings instead of copying them over.
Which one should you pick?
| Task | Pick |
|---|---|
| Well-scoped bug fixes, small features | Sonnet 5.5 (medium effort) |
| Terminal / DevOps agents | Sonnet 5.5 |
| Routine PR review at volume | Sonnet 5.5 |
| High-risk review (auth, payments, migrations) | Opus 5.5 |
| Architecture, large refactors, open-ended engineering | Opus 5.5 |
| Documents, slides, spreadsheets | Sonnet 5.5 |
| Knowledge-heavy Q&A from model memory | Opus 5.5 |
| Plan, then implement | Both (opusplan) |
When in doubt, Anthropic's docs still suggest starting with Opus 5.5 and moving well-defined, repetitive work to Sonnet 5.5 once you have measured it.
FAQ
Is Claude Sonnet 5.5 better than Opus 5.5? Not overall. Opus 5.5 leads on most published benchmarks, but Sonnet 5.5 is close, faster, half the per-token price and ahead on Terminal-Bench 4.0.
How much cheaper is Sonnet 5.5 than Opus 5.5? Half per token: $2 / $10 vs $4 / $20 per million input / output tokens. Cache reads cost $0.20 per million on both. Per task, the saving depends on how many more tokens Sonnet spends.
Do Sonnet 5.5 and Opus 5.5 have the same context window? Yes – 1M tokens each, with up to 128K output tokens.
Is Sonnet 5.5 faster than Opus 5.5? Yes. Anthropic lists Sonnet 5.5 as fast and Opus 5.5 as moderate in latency, and independent measurements show higher output speed for Sonnet.
Which model does Claude Code use by default?
That depends on your plan and settings; on the Anthropic API the opus and sonnet aliases now resolve to Opus 5.5 and Sonnet 5.5. Check /model in your session.
Prices and benchmark figures are as published by Anthropic and reported by independent testers in late September / early October 2026. Check Anthropic's pricing page before making cost decisions.