Claude Sonnet 4.5 vs o3 Pro: which model should you choose?
We compared Claude Sonnet 4.5 and o3 Pro 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 Pro
o3 Pro is a frontier model from OpenAI. It offers a 400,000-token context window, costs $17.89 per million input tokens and $81.48 per million output tokens, and averages 85.8 across the index's benchmark suite.
- Input
- $17.89 / 1M
- Output
- $81.48 / 1M
- Context
- 400,000
- Avg score
- 85.8
- Open weights
- No
Specification comparison
| Spec | Claude Sonnet 4.5 | o3 Pro | Edge |
|---|---|---|---|
| Input price / 1M | $17.84 | $17.89 | A |
| Output price / 1M | $70.50 | $81.48 | A |
| Context window | 400,000 | 400,000 | Tie |
| Max output | 128,000 | 8,192 | A |
| Average score | 86.0 | 85.8 | A |
| Arena Elo | 1478.2 | 1471.9 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Sonnet 4.5 wins 8, o3 Pro wins 4, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Sonnet 4.5 | o3 Pro | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 88.0 | 84.3 | 3.7 | Claude Sonnet 4.5 |
| GPQA Diamond | 82.6 | 78.7 | 3.9 | Claude Sonnet 4.5 |
| SWE-bench Verified | 74.2 | 76.9 | 2.7 | o3 Pro |
| HumanEval | 92.7 | 90.6 | 2.1 | Claude Sonnet 4.5 |
| MATH-500 | 87.0 | 92.4 | 5.4 | o3 Pro |
| AIME 2025 | 86.3 | 83.5 | 2.8 | Claude Sonnet 4.5 |
| LiveCodeBench | 82.5 | 82.4 | 0.1 | Claude Sonnet 4.5 |
| IFEval | 91.1 | 90.9 | 0.2 | Claude Sonnet 4.5 |
| TruthfulQA | 87.2 | 88.4 | 1.2 | o3 Pro |
| HellaSwag | 93.0 | 92.5 | 0.5 | Claude Sonnet 4.5 |
| MMMU | 80.9 | 83.1 | 2.2 | o3 Pro |
| Arena Elo | 1478.2 | 1471.9 | 6.3 | 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 Pro costs about $33.79. Claude Sonnet 4.5 is roughly 8% cheaper than o3 Pro. 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 Sonnet 4.5 wins 8, o3 Pro wins 4, and 0 are tied. On the index average, Claude Sonnet 4.5 leads o3 Pro by 0.2 points (86.0 versus 85.8).
The price picture favours Claude Sonnet 4.5. 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 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 128,000-token output limit.
Choose o3 Pro if
- You prefer OpenAI's tooling or already use it.
FAQ
Is Claude Sonnet 4.5 better than o3 Pro?
On the index average, Claude Sonnet 4.5 leads o3 Pro by 0.2 points (86.0 versus 85.8). The better choice depends on price, context and your task. Claude Sonnet 4.5 wins 8 benchmarks, o3 Pro wins 4.
Which is cheaper, Claude Sonnet 4.5 or o3 Pro?
Claude Sonnet 4.5 is cheaper on a blended 3:1 basis at about $31.00 per million tokens, versus $33.79 for o3 Pro.
Which has the larger context window?
Both models list a 400,000-token context window.
Are these models open weights?
Claude Sonnet 4.5 is proprietary; o3 Pro 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.