Claude Sonnet 4.5 Reasoning vs GPT-5.1: which model should you choose?
We compared Claude Sonnet 4.5 Reasoning and GPT-5.1 on price, context window and 12 benchmarks. Claude Sonnet 4.5 Reasoning has the stronger average benchmark score; Claude Sonnet 4.5 Reasoning is cheaper on a blended 3:1 basis.
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
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 Reasoning | GPT-5.1 | Edge |
|---|---|---|---|
| Input price / 1M | $19.48 | $19.01 | B |
| Output price / 1M | $68.20 | $69.84 | A |
| Context window | 400,000 | 2,000,000 | B |
| Max output | 8,192 | 128,000 | B |
| Average score | 87.7 | 86.0 | A |
| Arena Elo | 1486.5 | 1462.5 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Sonnet 4.5 Reasoning 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 Reasoning | GPT-5.1 | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 87.6 | 86.1 | 1.5 | Claude Sonnet 4.5 Reasoning |
| GPQA Diamond | 85.2 | 82.3 | 2.9 | Claude Sonnet 4.5 Reasoning |
| SWE-bench Verified | 79.6 | 76.1 | 3.5 | Claude Sonnet 4.5 Reasoning |
| HumanEval | 90.3 | 90.6 | 0.3 | GPT-5.1 |
| MATH-500 | 89.7 | 90.5 | 0.8 | GPT-5.1 |
| AIME 2025 | 87.3 | 85.6 | 1.7 | Claude Sonnet 4.5 Reasoning |
| LiveCodeBench | 85.1 | 78.9 | 6.2 | Claude Sonnet 4.5 Reasoning |
| IFEval | 91.9 | 90.9 | 1.0 | Claude Sonnet 4.5 Reasoning |
| TruthfulQA | 87.6 | 89.2 | 1.6 | GPT-5.1 |
| HellaSwag | 95.2 | 92.2 | 3.0 | Claude Sonnet 4.5 Reasoning |
| MMMU | 85.2 | 83.3 | 1.9 | Claude Sonnet 4.5 Reasoning |
| Arena Elo | 1486.5 | 1462.5 | 24.0 | Claude Sonnet 4.5 Reasoning |
Price analysis
On a blended 3:1 input-to-output basis, Claude Sonnet 4.5 Reasoning costs about $31.66 per million tokens and GPT-5.1 costs about $31.72. Claude Sonnet 4.5 Reasoning is roughly 0% cheaper than GPT-5.1. 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 400,000 for Claude Sonnet 4.5 Reasoning. That matters for long documents, large codebases and multi-step agent traces.
Verdict
Across 12 shared benchmarks, Claude Sonnet 4.5 Reasoning wins 9, GPT-5.1 wins 3, and 0 are tied. On the index average, Claude Sonnet 4.5 Reasoning leads GPT-5.1 by 1.7 points (87.7 versus 86.0).
The price picture favours Claude Sonnet 4.5 Reasoning. 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 Reasoning for quality-sensitive work, and check whether its price holds at your expected volume before committing.
Choose Claude Sonnet 4.5 Reasoning if
- You want the higher average benchmark score in this index.
- Unit cost matters and you want the cheaper blended price.
Choose GPT-5.1 if
- 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 Reasoning better than GPT-5.1?
On the index average, Claude Sonnet 4.5 Reasoning leads GPT-5.1 by 1.7 points (87.7 versus 86.0). The better choice depends on price, context and your task. Claude Sonnet 4.5 Reasoning wins 9 benchmarks, GPT-5.1 wins 3.
Which is cheaper, Claude Sonnet 4.5 Reasoning or GPT-5.1?
Claude Sonnet 4.5 Reasoning is cheaper on a blended 3:1 basis at about $31.66 per million tokens, versus $31.72 for GPT-5.1.
Which has the larger context window?
GPT-5.1 has the larger context window at 2,000,000 tokens. Claude Sonnet 4.5 Reasoning has 400,000 tokens.
Are these models open weights?
Claude Sonnet 4.5 Reasoning 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 Reasoning for quality-sensitive work, and check whether its price holds at your expected volume before committing.