Claude Sonnet 4.5 Pro vs o3 Reasoning: which model should you choose?
We compared Claude Sonnet 4.5 Pro and o3 Reasoning on price, context window and 12 benchmarks. Claude Sonnet 4.5 Pro has the stronger average benchmark score; o3 Reasoning is cheaper on a blended 3:1 basis.
Claude Sonnet 4.5 Pro
Claude Sonnet 4.5 Pro is a frontier model from Anthropic. It offers a 1,000,000-token context window, costs $18.73 per million input tokens and $82.29 per million output tokens, and averages 86.7 across the index's benchmark suite.
- Input
- $18.73 / 1M
- Output
- $82.29 / 1M
- Context
- 1,000,000
- Avg score
- 86.7
- 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 Pro | o3 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $18.73 | $18.07 | B |
| Output price / 1M | $82.29 | $70.65 | B |
| Context window | 1,000,000 | 200,000 | A |
| Max output | 16,384 | 16,384 | Tie |
| Average score | 86.7 | 85.3 | A |
| Arena Elo | 1469.0 | 1456.9 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Sonnet 4.5 Pro wins 7, o3 Reasoning wins 5, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Sonnet 4.5 Pro | o3 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 86.3 | 87.7 | 1.4 | o3 Reasoning |
| GPQA Diamond | 83.1 | 79.6 | 3.5 | Claude Sonnet 4.5 Pro |
| SWE-bench Verified | 73.9 | 74.1 | 0.2 | o3 Reasoning |
| HumanEval | 92.7 | 91.3 | 1.4 | Claude Sonnet 4.5 Pro |
| MATH-500 | 90.1 | 92.0 | 1.9 | o3 Reasoning |
| AIME 2025 | 88.4 | 80.5 | 7.9 | Claude Sonnet 4.5 Pro |
| LiveCodeBench | 81.9 | 82.7 | 0.8 | o3 Reasoning |
| IFEval | 94.1 | 90.6 | 3.5 | Claude Sonnet 4.5 Pro |
| TruthfulQA | 86.1 | 86.5 | 0.4 | o3 Reasoning |
| HellaSwag | 92.6 | 90.9 | 1.7 | Claude Sonnet 4.5 Pro |
| MMMU | 84.5 | 82.3 | 2.2 | Claude Sonnet 4.5 Pro |
| Arena Elo | 1469.0 | 1456.9 | 12.1 | Claude Sonnet 4.5 Pro |
Price analysis
On a blended 3:1 input-to-output basis, Claude Sonnet 4.5 Pro costs about $34.62 per million tokens and o3 Reasoning costs about $31.22. o3 Reasoning is roughly 10% cheaper than Claude Sonnet 4.5 Pro. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
Claude Sonnet 4.5 Pro has the larger context window at 1,000,000 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 Pro wins 7, o3 Reasoning wins 5, and 0 are tied. On the index average, Claude Sonnet 4.5 Pro leads o3 Reasoning by 1.4 points (86.7 versus 85.3).
The price picture favours o3 Reasoning. The context picture favours Claude Sonnet 4.5 Pro. 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 Pro for quality-sensitive work and o3 Reasoning for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.
Choose Claude Sonnet 4.5 Pro if
- You want the higher average benchmark score in this index.
- You need the larger 1,000,000-token context window.
Choose o3 Reasoning if
- Unit cost matters and you want the cheaper blended price.
FAQ
Is Claude Sonnet 4.5 Pro better than o3 Reasoning?
On the index average, Claude Sonnet 4.5 Pro leads o3 Reasoning by 1.4 points (86.7 versus 85.3). The better choice depends on price, context and your task. Claude Sonnet 4.5 Pro wins 7 benchmarks, o3 Reasoning wins 5.
Which is cheaper, Claude Sonnet 4.5 Pro or o3 Reasoning?
o3 Reasoning is cheaper on a blended 3:1 basis at about $31.22 per million tokens, versus $34.62 for Claude Sonnet 4.5 Pro.
Which has the larger context window?
Claude Sonnet 4.5 Pro has the larger context window at 1,000,000 tokens. o3 Reasoning has 200,000 tokens.
Are these models open weights?
Claude Sonnet 4.5 Pro is proprietary; o3 Reasoning is proprietary.
Which should I choose for production?
If you need a single default, start with Claude Sonnet 4.5 Pro for quality-sensitive work and o3 Reasoning for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.