Claude Sonnet 4.5 vs o3 Reasoning: which model should you choose?
We compared Claude Sonnet 4.5 and o3 Reasoning on price, context window and 12 benchmarks. Claude Sonnet 4.5 has the stronger average benchmark score; Claude Sonnet 4.5 is cheaper on a blended 3:1 basis.
Claude Sonnet 4.5
Claude Sonnet 4.5 is a frontier model from Anthropic. It offers a 400,000-token context window, costs $17.84 per million input tokens and $70.50 per million output tokens, and averages 86.0 across the index's benchmark suite.
- Input
- $17.84 / 1M
- Output
- $70.50 / 1M
- Context
- 400,000
- Avg score
- 86.0
- Open weights
- No
o3 Reasoning
o3 Reasoning is a frontier model from OpenAI. It offers a 200,000-token context window, costs $18.07 per million input tokens and $70.65 per million output tokens, and averages 85.3 across the index's benchmark suite.
- Input
- $18.07 / 1M
- Output
- $70.65 / 1M
- Context
- 200,000
- Avg score
- 85.3
- Open weights
- No
Specification comparison
| Spec | Claude Sonnet 4.5 | o3 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $17.84 | $18.07 | A |
| Output price / 1M | $70.50 | $70.65 | A |
| Context window | 400,000 | 200,000 | A |
| Max output | 128,000 | 16,384 | A |
| Average score | 86.0 | 85.3 | A |
| Arena Elo | 1478.2 | 1456.9 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Sonnet 4.5 wins 9, o3 Reasoning wins 3, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Sonnet 4.5 | o3 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 88.0 | 87.7 | 0.3 | Claude Sonnet 4.5 |
| GPQA Diamond | 82.6 | 79.6 | 3.0 | Claude Sonnet 4.5 |
| SWE-bench Verified | 74.2 | 74.1 | 0.1 | Claude Sonnet 4.5 |
| HumanEval | 92.7 | 91.3 | 1.4 | Claude Sonnet 4.5 |
| MATH-500 | 87.0 | 92.0 | 5.0 | o3 Reasoning |
| AIME 2025 | 86.3 | 80.5 | 5.8 | Claude Sonnet 4.5 |
| LiveCodeBench | 82.5 | 82.7 | 0.2 | o3 Reasoning |
| IFEval | 91.1 | 90.6 | 0.5 | Claude Sonnet 4.5 |
| TruthfulQA | 87.2 | 86.5 | 0.7 | Claude Sonnet 4.5 |
| HellaSwag | 93.0 | 90.9 | 2.1 | Claude Sonnet 4.5 |
| MMMU | 80.9 | 82.3 | 1.4 | o3 Reasoning |
| Arena Elo | 1478.2 | 1456.9 | 21.3 | Claude Sonnet 4.5 |
Price analysis
On a blended 3:1 input-to-output basis, Claude Sonnet 4.5 costs about $31.00 per million tokens and o3 Reasoning costs about $31.22. Claude Sonnet 4.5 is roughly 1% cheaper than o3 Reasoning. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
Claude Sonnet 4.5 has the larger context window at 400,000 tokens, compared with 200,000 for o3 Reasoning. That matters for long documents, large codebases and multi-step agent traces.
Verdict
Across 12 shared benchmarks, Claude Sonnet 4.5 wins 9, o3 Reasoning wins 3, and 0 are tied. On the index average, Claude Sonnet 4.5 leads o3 Reasoning by 0.7 points (86.0 versus 85.3).
The price picture favours Claude Sonnet 4.5. The context picture favours Claude Sonnet 4.5. 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 for quality-sensitive work, and check whether its price holds at your expected volume before committing.
Choose Claude Sonnet 4.5 if
- You want the higher average benchmark score in this index.
- Unit cost matters and you want the cheaper blended price.
- You need the larger 400,000-token context window.
- You need the larger 128,000-token output limit.
Choose o3 Reasoning if
- You prefer OpenAI's tooling or already use it.
FAQ
Is Claude Sonnet 4.5 better than o3 Reasoning?
On the index average, Claude Sonnet 4.5 leads o3 Reasoning by 0.7 points (86.0 versus 85.3). The better choice depends on price, context and your task. Claude Sonnet 4.5 wins 9 benchmarks, o3 Reasoning wins 3.
Which is cheaper, Claude Sonnet 4.5 or o3 Reasoning?
Claude Sonnet 4.5 is cheaper on a blended 3:1 basis at about $31.00 per million tokens, versus $31.22 for o3 Reasoning.
Which has the larger context window?
Claude Sonnet 4.5 has the larger context window at 400,000 tokens. o3 Reasoning has 200,000 tokens.
Are these models open weights?
Claude Sonnet 4.5 is proprietary; o3 Reasoning is proprietary.
Which should I choose for production?
If you need a single default, start with Claude Sonnet 4.5 for quality-sensitive work, and check whether its price holds at your expected volume before committing.