Claude Sonnet 4.5 Pro vs GPT-5.1: which model should you choose?
We compared Claude Sonnet 4.5 Pro and GPT-5.1 on price, context window and 12 benchmarks. Claude Sonnet 4.5 Pro has the stronger average benchmark score; GPT-5.1 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
GPT-5.1
GPT-5.1 is a frontier model from OpenAI. It offers a 2,000,000-token context window, costs $19.01 per million input tokens and $69.84 per million output tokens, and averages 86.0 across the index's benchmark suite.
- Input
- $19.01 / 1M
- Output
- $69.84 / 1M
- Context
- 2,000,000
- Avg score
- 86.0
- Open weights
- No
Specification comparison
| Spec | Claude Sonnet 4.5 Pro | GPT-5.1 | Edge |
|---|---|---|---|
| Input price / 1M | $18.73 | $19.01 | A |
| Output price / 1M | $82.29 | $69.84 | B |
| Context window | 1,000,000 | 2,000,000 | B |
| Max output | 16,384 | 128,000 | B |
| Average score | 86.7 | 86.0 | A |
| Arena Elo | 1469.0 | 1462.5 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Sonnet 4.5 Pro wins 9, GPT-5.1 wins 3, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Sonnet 4.5 Pro | GPT-5.1 | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 86.3 | 86.1 | 0.2 | Claude Sonnet 4.5 Pro |
| GPQA Diamond | 83.1 | 82.3 | 0.8 | Claude Sonnet 4.5 Pro |
| SWE-bench Verified | 73.9 | 76.1 | 2.2 | GPT-5.1 |
| HumanEval | 92.7 | 90.6 | 2.1 | Claude Sonnet 4.5 Pro |
| MATH-500 | 90.1 | 90.5 | 0.4 | GPT-5.1 |
| AIME 2025 | 88.4 | 85.6 | 2.8 | Claude Sonnet 4.5 Pro |
| LiveCodeBench | 81.9 | 78.9 | 3.0 | Claude Sonnet 4.5 Pro |
| IFEval | 94.1 | 90.9 | 3.2 | Claude Sonnet 4.5 Pro |
| TruthfulQA | 86.1 | 89.2 | 3.1 | GPT-5.1 |
| HellaSwag | 92.6 | 92.2 | 0.4 | Claude Sonnet 4.5 Pro |
| MMMU | 84.5 | 83.3 | 1.2 | Claude Sonnet 4.5 Pro |
| Arena Elo | 1469.0 | 1462.5 | 6.5 | 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 GPT-5.1 costs about $31.72. GPT-5.1 is roughly 8% 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.
GPT-5.1 has the larger context window at 2,000,000 tokens, compared with 1,000,000 for Claude Sonnet 4.5 Pro. That matters for long documents, large codebases and multi-step agent traces.
Verdict
Across 12 shared benchmarks, Claude Sonnet 4.5 Pro wins 9, GPT-5.1 wins 3, and 0 are tied. On the index average, Claude Sonnet 4.5 Pro leads GPT-5.1 by 0.7 points (86.7 versus 86.0).
The price picture favours GPT-5.1. The context picture favours GPT-5.1. 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 GPT-5.1 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.
Choose GPT-5.1 if
- Unit cost matters and you want the cheaper blended price.
- You need the larger 2,000,000-token context window.
- You need the larger 128,000-token output limit.
FAQ
Is Claude Sonnet 4.5 Pro better than GPT-5.1?
On the index average, Claude Sonnet 4.5 Pro leads GPT-5.1 by 0.7 points (86.7 versus 86.0). The better choice depends on price, context and your task. Claude Sonnet 4.5 Pro wins 9 benchmarks, GPT-5.1 wins 3.
Which is cheaper, Claude Sonnet 4.5 Pro or GPT-5.1?
GPT-5.1 is cheaper on a blended 3:1 basis at about $31.72 per million tokens, versus $34.62 for Claude Sonnet 4.5 Pro.
Which has the larger context window?
GPT-5.1 has the larger context window at 2,000,000 tokens. Claude Sonnet 4.5 Pro has 1,000,000 tokens.
Are these models open weights?
Claude Sonnet 4.5 Pro is proprietary; GPT-5.1 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 GPT-5.1 for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.