Claude Opus 4.5 Pro vs GPT-5.1 Reasoning: which model should you choose?
We compared Claude Opus 4.5 Pro and GPT-5.1 Reasoning on price, context window and 12 benchmarks. Claude Opus 4.5 Pro has the stronger average benchmark score; GPT-5.1 Reasoning is cheaper on a blended 3:1 basis.
Claude Opus 4.5 Pro
Claude Opus 4.5 Pro is a frontier model from Anthropic. It offers a 200,000-token context window, costs $20.98 per million input tokens and $85.59 per million output tokens, and averages 89.1 across the index's benchmark suite.
- Input
- $20.98 / 1M
- Output
- $85.59 / 1M
- Context
- 200,000
- Avg score
- 89.1
- 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 Pro | GPT-5.1 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $20.98 | $20.27 | B |
| Output price / 1M | $85.59 | $82.11 | B |
| Context window | 200,000 | 400,000 | B |
| Max output | 32,768 | 128,000 | B |
| Average score | 89.1 | 88.3 | A |
| Arena Elo | 1483.9 | 1485.9 | B |
| Open weights | No | No | Tie |
Benchmark comparison
Claude Opus 4.5 Pro wins 8, GPT-5.1 Reasoning wins 4, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Claude Opus 4.5 Pro | GPT-5.1 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 91.2 | 90.0 | 1.2 | Claude Opus 4.5 Pro |
| GPQA Diamond | 86.0 | 85.6 | 0.4 | Claude Opus 4.5 Pro |
| SWE-bench Verified | 81.1 | 76.0 | 5.1 | Claude Opus 4.5 Pro |
| HumanEval | 93.4 | 92.9 | 0.5 | Claude Opus 4.5 Pro |
| MATH-500 | 91.2 | 95.9 | 4.7 | GPT-5.1 Reasoning |
| AIME 2025 | 91.4 | 85.0 | 6.4 | Claude Opus 4.5 Pro |
| LiveCodeBench | 84.1 | 80.9 | 3.2 | Claude Opus 4.5 Pro |
| IFEval | 96.1 | 94.6 | 1.5 | Claude Opus 4.5 Pro |
| TruthfulQA | 86.7 | 90.0 | 3.3 | GPT-5.1 Reasoning |
| HellaSwag | 95.2 | 94.4 | 0.8 | Claude Opus 4.5 Pro |
| MMMU | 83.3 | 86.2 | 2.9 | GPT-5.1 Reasoning |
| Arena Elo | 1483.9 | 1485.9 | 2.0 | GPT-5.1 Reasoning |
Price analysis
On a blended 3:1 input-to-output basis, Claude Opus 4.5 Pro costs about $37.13 per million tokens and GPT-5.1 Reasoning costs about $35.73. GPT-5.1 Reasoning is roughly 4% cheaper than Claude Opus 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 Reasoning has the larger context window at 400,000 tokens, compared with 200,000 for Claude Opus 4.5 Pro. That matters for long documents, large codebases and multi-step agent traces.
Verdict
Across 12 shared benchmarks, Claude Opus 4.5 Pro wins 8, GPT-5.1 Reasoning wins 4, and 0 are tied. On the index average, Claude Opus 4.5 Pro leads GPT-5.1 Reasoning by 0.8 points (89.1 versus 88.3).
The price picture favours GPT-5.1 Reasoning. The context picture favours GPT-5.1 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 Pro for quality-sensitive work and GPT-5.1 Reasoning 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 Pro if
- You want the higher average benchmark score in this index.
Choose GPT-5.1 Reasoning if
- Unit cost matters and you want the cheaper blended price.
- You need the larger 400,000-token context window.
- You need the larger 128,000-token output limit.
FAQ
Is Claude Opus 4.5 Pro better than GPT-5.1 Reasoning?
On the index average, Claude Opus 4.5 Pro leads GPT-5.1 Reasoning by 0.8 points (89.1 versus 88.3). The better choice depends on price, context and your task. Claude Opus 4.5 Pro wins 8 benchmarks, GPT-5.1 Reasoning wins 4.
Which is cheaper, Claude Opus 4.5 Pro 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 $37.13 for Claude Opus 4.5 Pro.
Which has the larger context window?
GPT-5.1 Reasoning has the larger context window at 400,000 tokens. Claude Opus 4.5 Pro has 200,000 tokens.
Are these models open weights?
Claude Opus 4.5 Pro is proprietary; GPT-5.1 Reasoning is proprietary.
Which should I choose for production?
If you need a single default, start with Claude Opus 4.5 Pro for quality-sensitive work and GPT-5.1 Reasoning for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.