Claude Opus 4.5 Reasoning vs GPT-5.1 Reasoning: which model should you choose?
We compared Claude Opus 4.5 Reasoning 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 Opus 4.5 Reasoning
Claude Opus 4.5 Reasoning is a frontier model from Anthropic. It offers a 2,000,000-token context window, costs $20.91 per million input tokens and $108.13 per million output tokens, and averages 88.2 across the index's benchmark suite.
- Input
- $20.91 / 1M
- Output
- $108.13 / 1M
- Context
- 2,000,000
- Avg score
- 88.2
- 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 Opus 4.5 Reasoning | GPT-5.1 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $20.91 | $20.27 | B |
| Output price / 1M | $108.13 | $82.11 | B |
| Context window | 2,000,000 | 400,000 | A |
| Max output | 16,384 | 128,000 | B |
| Average score | 88.2 | 88.3 | B |
| Arena Elo | 1497.5 | 1485.9 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Opus 4.5 Reasoning wins 6, GPT-5.1 Reasoning wins 6, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Opus 4.5 Reasoning | GPT-5.1 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 90.7 | 90.0 | 0.7 | Claude Opus 4.5 Reasoning |
| GPQA Diamond | 82.6 | 85.6 | 3.0 | GPT-5.1 Reasoning |
| SWE-bench Verified | 79.5 | 76.0 | 3.5 | Claude Opus 4.5 Reasoning |
| HumanEval | 92.1 | 92.9 | 0.8 | GPT-5.1 Reasoning |
| MATH-500 | 91.6 | 95.9 | 4.3 | GPT-5.1 Reasoning |
| AIME 2025 | 88.9 | 85.0 | 3.9 | Claude Opus 4.5 Reasoning |
| LiveCodeBench | 81.8 | 80.9 | 0.9 | Claude Opus 4.5 Reasoning |
| IFEval | 95.9 | 94.6 | 1.3 | Claude Opus 4.5 Reasoning |
| TruthfulQA | 89.0 | 90.0 | 1.0 | GPT-5.1 Reasoning |
| HellaSwag | 93.8 | 94.4 | 0.6 | GPT-5.1 Reasoning |
| MMMU | 84.6 | 86.2 | 1.6 | GPT-5.1 Reasoning |
| Arena Elo | 1497.5 | 1485.9 | 11.6 | Claude Opus 4.5 Reasoning |
Price analysis
On a blended 3:1 input-to-output basis, Claude Opus 4.5 Reasoning costs about $42.72 per million tokens and GPT-5.1 Reasoning costs about $35.73. GPT-5.1 Reasoning is roughly 16% cheaper than Claude Opus 4.5 Reasoning. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
Claude Opus 4.5 Reasoning has the larger context window at 2,000,000 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 Opus 4.5 Reasoning wins 6, GPT-5.1 Reasoning wins 6, and 0 are tied. On the index average, GPT-5.1 Reasoning leads Claude Opus 4.5 Reasoning by 0.1 points (88.3 versus 88.2).
The price picture favours GPT-5.1 Reasoning. The context picture favours Claude Opus 4.5 Reasoning. 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 Opus 4.5 Reasoning if
- You need the larger 2,000,000-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 Opus 4.5 Reasoning better than GPT-5.1 Reasoning?
On the index average, GPT-5.1 Reasoning leads Claude Opus 4.5 Reasoning by 0.1 points (88.3 versus 88.2). The better choice depends on price, context and your task. Claude Opus 4.5 Reasoning wins 6 benchmarks, GPT-5.1 Reasoning wins 6.
Which is cheaper, Claude Opus 4.5 Reasoning 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 $42.72 for Claude Opus 4.5 Reasoning.
Which has the larger context window?
Claude Opus 4.5 Reasoning has the larger context window at 2,000,000 tokens. GPT-5.1 Reasoning has 400,000 tokens.
Are these models open weights?
Claude Opus 4.5 Reasoning 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.