Claude Opus 4.5 vs Claude Sonnet 4.5 Reasoning: which model should you choose?
We compared Claude Opus 4.5 and Claude Sonnet 4.5 Reasoning on price, context window and 12 benchmarks. Claude Opus 4.5 and Claude Sonnet 4.5 Reasoning are effectively tied on the index average; Claude Sonnet 4.5 Reasoning is cheaper on a blended 3:1 basis.
Claude Opus 4.5
Claude Opus 4.5 is a frontier model from Anthropic. It offers a 400,000-token context window, costs $19.42 per million input tokens and $75.27 per million output tokens, and averages 87.7 across the index's benchmark suite.
- Input
- $19.42 / 1M
- Output
- $75.27 / 1M
- Context
- 400,000
- Avg score
- 87.7
- Open weights
- No
Claude Sonnet 4.5 Reasoning
Claude Sonnet 4.5 Reasoning is a frontier model from Anthropic. It offers a 400,000-token context window, costs $19.48 per million input tokens and $68.20 per million output tokens, and averages 87.7 across the index's benchmark suite.
- Input
- $19.48 / 1M
- Output
- $68.20 / 1M
- Context
- 400,000
- Avg score
- 87.7
- Open weights
- No
Specification comparison
| Spec | Claude Opus 4.5 | Claude Sonnet 4.5 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $19.42 | $19.48 | A |
| Output price / 1M | $75.27 | $68.20 | B |
| Context window | 400,000 | 400,000 | Tie |
| Max output | 4,096 | 8,192 | B |
| Average score | 87.7 | 87.7 | Tie |
| Arena Elo | 1491.2 | 1486.5 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Opus 4.5 wins 6, Claude Sonnet 4.5 Reasoning wins 6, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Opus 4.5 | Claude Sonnet 4.5 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 87.0 | 87.6 | 0.6 | Claude Sonnet 4.5 Reasoning |
| GPQA Diamond | 81.7 | 85.2 | 3.5 | Claude Sonnet 4.5 Reasoning |
| SWE-bench Verified | 77.1 | 79.6 | 2.5 | Claude Sonnet 4.5 Reasoning |
| HumanEval | 93.7 | 90.3 | 3.4 | Claude Opus 4.5 |
| MATH-500 | 93.7 | 89.7 | 4.0 | Claude Opus 4.5 |
| AIME 2025 | 89.8 | 87.3 | 2.5 | Claude Opus 4.5 |
| LiveCodeBench | 85.0 | 85.1 | 0.1 | Claude Sonnet 4.5 Reasoning |
| IFEval | 92.5 | 91.9 | 0.6 | Claude Opus 4.5 |
| TruthfulQA | 88.1 | 87.6 | 0.5 | Claude Opus 4.5 |
| HellaSwag | 92.4 | 95.2 | 2.8 | Claude Sonnet 4.5 Reasoning |
| MMMU | 84.2 | 85.2 | 1.0 | Claude Sonnet 4.5 Reasoning |
| Arena Elo | 1491.2 | 1486.5 | 4.7 | Claude Opus 4.5 |
Price analysis
On a blended 3:1 input-to-output basis, Claude Opus 4.5 costs about $33.38 per million tokens and Claude Sonnet 4.5 Reasoning costs about $31.66. Claude Sonnet 4.5 Reasoning is roughly 5% cheaper than Claude Opus 4.5. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
Both models list a 400,000-token context window. That is enough for long documents, large codebases and multi-step agent traces.
Verdict
Across 12 shared benchmarks, Claude Opus 4.5 wins 6, Claude Sonnet 4.5 Reasoning wins 6, and 0 are tied. The two models are effectively tied on the index average at 87.7.
The price picture favours Claude Sonnet 4.5 Reasoning. The context picture is effectively a tie. Those two dimensions often matter more than a few benchmark points once a model is good enough for the task.
If you need a single default, start with Claude Sonnet 4.5 Reasoning if cost is the constraint and Claude Opus 4.5 if context is the constraint. Test both on a sample of your own traffic before committing.
Choose Claude Opus 4.5 if
- You prefer Anthropic's tooling or already use it.
Choose Claude Sonnet 4.5 Reasoning if
- Unit cost matters and you want the cheaper blended price.
- You need the larger 8,192-token output limit.
FAQ
Is Claude Opus 4.5 better than Claude Sonnet 4.5 Reasoning?
The two models are effectively tied on the index average at 87.7. The better choice depends on price, context and your task. Claude Opus 4.5 wins 6 benchmarks, Claude Sonnet 4.5 Reasoning wins 6.
Which is cheaper, Claude Opus 4.5 or Claude Sonnet 4.5 Reasoning?
Claude Sonnet 4.5 Reasoning is cheaper on a blended 3:1 basis at about $31.66 per million tokens, versus $33.38 for Claude Opus 4.5.
Which has the larger context window?
Both models list a 400,000-token context window.
Are these models open weights?
Claude Opus 4.5 is proprietary; Claude Sonnet 4.5 Reasoning is proprietary.
Which should I choose for production?
If you need a single default, start with Claude Sonnet 4.5 Reasoning if cost is the constraint and Claude Opus 4.5 if context is the constraint. Test both on a sample of your own traffic before committing.