Claude Opus 4.5 Reasoning vs GPT-5.1 Pro: which model should you choose?
We compared Claude Opus 4.5 Reasoning and GPT-5.1 Pro on price, context window and 12 benchmarks. Claude Opus 4.5 Reasoning has the stronger average benchmark score; GPT-5.1 Pro 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 Pro
GPT-5.1 Pro is a frontier model from OpenAI. It offers a 200,000-token context window, costs $19.77 per million input tokens and $100.32 per million output tokens, and averages 87.8 across the index's benchmark suite.
- Input
- $19.77 / 1M
- Output
- $100.32 / 1M
- Context
- 200,000
- Avg score
- 87.8
- Open weights
- No
Specification comparison
| Spec | Claude Opus 4.5 Reasoning | GPT-5.1 Pro | Edge |
|---|---|---|---|
| Input price / 1M | $20.91 | $19.77 | B |
| Output price / 1M | $108.13 | $100.32 | B |
| Context window | 2,000,000 | 200,000 | A |
| Max output | 16,384 | 32,768 | B |
| Average score | 88.2 | 87.8 | A |
| Arena Elo | 1497.5 | 1490.0 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Opus 4.5 Reasoning wins 6, GPT-5.1 Pro wins 5, and 1 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Opus 4.5 Reasoning | GPT-5.1 Pro | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 90.7 | 86.2 | 4.5 | Claude Opus 4.5 Reasoning |
| GPQA Diamond | 82.6 | 83.4 | 0.8 | GPT-5.1 Pro |
| SWE-bench Verified | 79.5 | 80.1 | 0.6 | GPT-5.1 Pro |
| HumanEval | 92.1 | 91.1 | 1.0 | Claude Opus 4.5 Reasoning |
| MATH-500 | 91.6 | 95.1 | 3.5 | GPT-5.1 Pro |
| AIME 2025 | 88.9 | 90.6 | 1.7 | GPT-5.1 Pro |
| LiveCodeBench | 81.8 | 82.2 | 0.4 | GPT-5.1 Pro |
| IFEval | 95.9 | 94.8 | 1.1 | Claude Opus 4.5 Reasoning |
| TruthfulQA | 89.0 | 86.9 | 2.1 | Claude Opus 4.5 Reasoning |
| HellaSwag | 93.8 | 93.8 | 0.0 | Tie |
| MMMU | 84.6 | 81.6 | 3.0 | Claude Opus 4.5 Reasoning |
| Arena Elo | 1497.5 | 1490.0 | 7.5 | 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 Pro costs about $39.91. GPT-5.1 Pro is roughly 7% 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 200,000 for GPT-5.1 Pro. 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 Pro wins 5, and 1 are tied. On the index average, Claude Opus 4.5 Reasoning leads GPT-5.1 Pro by 0.4 points (88.2 versus 87.8).
The price picture favours GPT-5.1 Pro. 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 Claude Opus 4.5 Reasoning for quality-sensitive work and GPT-5.1 Pro for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.
Choose Claude Opus 4.5 Reasoning if
- You want the higher average benchmark score in this index.
- You need the larger 2,000,000-token context window.
Choose GPT-5.1 Pro if
- Unit cost matters and you want the cheaper blended price.
- You need the larger 32,768-token output limit.
FAQ
Is Claude Opus 4.5 Reasoning better than GPT-5.1 Pro?
On the index average, Claude Opus 4.5 Reasoning leads GPT-5.1 Pro by 0.4 points (88.2 versus 87.8). The better choice depends on price, context and your task. Claude Opus 4.5 Reasoning wins 6 benchmarks, GPT-5.1 Pro wins 5.
Which is cheaper, Claude Opus 4.5 Reasoning or GPT-5.1 Pro?
GPT-5.1 Pro is cheaper on a blended 3:1 basis at about $39.91 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 Pro has 200,000 tokens.
Are these models open weights?
Claude Opus 4.5 Reasoning is proprietary; GPT-5.1 Pro is proprietary.
Which should I choose for production?
If you need a single default, start with Claude Opus 4.5 Reasoning for quality-sensitive work and GPT-5.1 Pro for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.