Claude Sonnet 4.5 Max vs GPT-5.1 Reasoning: which model should you choose?
We compared Claude Sonnet 4.5 Max and GPT-5.1 Reasoning on price, context window and 12 benchmarks. GPT-5.1 Reasoning has the stronger average benchmark score; GPT-5.1 Reasoning is cheaper on a blended 3:1 basis.
Claude Sonnet 4.5 Max
Claude Sonnet 4.5 Max is a frontier model from Anthropic. It offers a 1,000,000-token context window, costs $19.90 per million input tokens and $95.47 per million output tokens, and averages 87.2 across the index's benchmark suite.
- Input
- $19.90 / 1M
- Output
- $95.47 / 1M
- Context
- 1,000,000
- Avg score
- 87.2
- Open weights
- No
GPT-5.1 Reasoning
GPT-5.1 Reasoning is a frontier model from OpenAI. It offers a 400,000-token context window, costs $20.27 per million input tokens and $82.11 per million output tokens, and averages 88.3 across the index's benchmark suite.
- Input
- $20.27 / 1M
- Output
- $82.11 / 1M
- Context
- 400,000
- Avg score
- 88.3
- Open weights
- No
Specification comparison
| Spec | Claude Sonnet 4.5 Max | GPT-5.1 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $19.90 | $20.27 | A |
| Output price / 1M | $95.47 | $82.11 | B |
| Context window | 1,000,000 | 400,000 | A |
| Max output | 65,536 | 128,000 | B |
| Average score | 87.2 | 88.3 | B |
| Arena Elo | 1474.9 | 1485.9 | B |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Sonnet 4.5 Max wins 3, GPT-5.1 Reasoning wins 9, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Sonnet 4.5 Max | GPT-5.1 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 86.1 | 90.0 | 3.9 | GPT-5.1 Reasoning |
| GPQA Diamond | 84.4 | 85.6 | 1.2 | GPT-5.1 Reasoning |
| SWE-bench Verified | 80.2 | 76.0 | 4.2 | Claude Sonnet 4.5 Max |
| HumanEval | 92.4 | 92.9 | 0.5 | GPT-5.1 Reasoning |
| MATH-500 | 92.8 | 95.9 | 3.1 | GPT-5.1 Reasoning |
| AIME 2025 | 85.4 | 85.0 | 0.4 | Claude Sonnet 4.5 Max |
| LiveCodeBench | 82.8 | 80.9 | 1.9 | Claude Sonnet 4.5 Max |
| IFEval | 92.9 | 94.6 | 1.7 | GPT-5.1 Reasoning |
| TruthfulQA | 87.3 | 90.0 | 2.7 | GPT-5.1 Reasoning |
| HellaSwag | 93.1 | 94.4 | 1.3 | GPT-5.1 Reasoning |
| MMMU | 82.0 | 86.2 | 4.2 | GPT-5.1 Reasoning |
| Arena Elo | 1474.9 | 1485.9 | 11.0 | GPT-5.1 Reasoning |
Price analysis
On a blended 3:1 input-to-output basis, Claude Sonnet 4.5 Max costs about $38.79 per million tokens and GPT-5.1 Reasoning costs about $35.73. GPT-5.1 Reasoning is roughly 8% cheaper than Claude Sonnet 4.5 Max. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
Claude Sonnet 4.5 Max has the larger context window at 1,000,000 tokens, compared with 400,000 for GPT-5.1 Reasoning. That matters for long documents, large codebases and multi-step agent traces.
Verdict
Across 12 shared benchmarks, Claude Sonnet 4.5 Max wins 3, GPT-5.1 Reasoning wins 9, and 0 are tied. On the index average, GPT-5.1 Reasoning leads Claude Sonnet 4.5 Max by 1.1 points (88.3 versus 87.2).
The price picture favours GPT-5.1 Reasoning. The context picture favours Claude Sonnet 4.5 Max. 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 GPT-5.1 Reasoning for quality-sensitive work, and check whether its price holds at your expected volume before committing.
Choose Claude Sonnet 4.5 Max if
- You need the larger 1,000,000-token context window.
Choose GPT-5.1 Reasoning 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.
FAQ
Is Claude Sonnet 4.5 Max better than GPT-5.1 Reasoning?
On the index average, GPT-5.1 Reasoning leads Claude Sonnet 4.5 Max by 1.1 points (88.3 versus 87.2). The better choice depends on price, context and your task. Claude Sonnet 4.5 Max wins 3 benchmarks, GPT-5.1 Reasoning wins 9.
Which is cheaper, Claude Sonnet 4.5 Max or GPT-5.1 Reasoning?
GPT-5.1 Reasoning is cheaper on a blended 3:1 basis at about $35.73 per million tokens, versus $38.79 for Claude Sonnet 4.5 Max.
Which has the larger context window?
Claude Sonnet 4.5 Max has the larger context window at 1,000,000 tokens. GPT-5.1 Reasoning has 400,000 tokens.
Are these models open weights?
Claude Sonnet 4.5 Max is proprietary; GPT-5.1 Reasoning is proprietary.
Which should I choose for production?
If you need a single default, start with GPT-5.1 Reasoning for quality-sensitive work, and check whether its price holds at your expected volume before committing.