GPT-5.1 Pro vs Grok 4 Reasoning: which model should you choose?
We compared GPT-5.1 Pro and Grok 4 Reasoning on price, context window and 12 benchmarks. GPT-5.1 Pro and Grok 4 Reasoning are effectively tied on the index average; Grok 4 Reasoning is cheaper on a blended 3:1 basis.
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
Grok 4 Reasoning
Grok 4 Reasoning is a frontier model from xAI. It offers a 2,000,000-token context window, costs $19.67 per million input tokens and $93.29 per million output tokens, and averages 87.8 across the index's benchmark suite.
- Input
- $19.67 / 1M
- Output
- $93.29 / 1M
- Context
- 2,000,000
- Avg score
- 87.8
- Open weights
- No
Specification comparison
| Spec | GPT-5.1 Pro | Grok 4 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $19.77 | $19.67 | B |
| Output price / 1M | $100.32 | $93.29 | B |
| Context window | 200,000 | 2,000,000 | B |
| Max output | 32,768 | 65,536 | B |
| Average score | 87.8 | 87.8 | Tie |
| Arena Elo | 1490.0 | 1487.6 | A |
| Open weights | No | No | Tie |
Benchmark comparison
GPT-5.1 Pro wins 7, Grok 4 Reasoning wins 5, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | GPT-5.1 Pro | Grok 4 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 86.2 | 88.5 | 2.3 | Grok 4 Reasoning |
| GPQA Diamond | 83.4 | 81.9 | 1.5 | GPT-5.1 Pro |
| SWE-bench Verified | 80.1 | 78.0 | 2.1 | GPT-5.1 Pro |
| HumanEval | 91.1 | 93.8 | 2.7 | Grok 4 Reasoning |
| MATH-500 | 95.1 | 94.4 | 0.7 | GPT-5.1 Pro |
| AIME 2025 | 90.6 | 85.6 | 5.0 | GPT-5.1 Pro |
| LiveCodeBench | 82.2 | 85.1 | 2.9 | Grok 4 Reasoning |
| IFEval | 94.8 | 93.4 | 1.4 | GPT-5.1 Pro |
| TruthfulQA | 86.9 | 89.4 | 2.5 | Grok 4 Reasoning |
| HellaSwag | 93.8 | 95.2 | 1.4 | Grok 4 Reasoning |
| MMMU | 81.6 | 80.8 | 0.8 | GPT-5.1 Pro |
| Arena Elo | 1490.0 | 1487.6 | 2.4 | GPT-5.1 Pro |
Price analysis
On a blended 3:1 input-to-output basis, GPT-5.1 Pro costs about $39.91 per million tokens and Grok 4 Reasoning costs about $38.08. Grok 4 Reasoning is roughly 5% cheaper than GPT-5.1 Pro. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
Grok 4 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, GPT-5.1 Pro wins 7, Grok 4 Reasoning wins 5, and 0 are tied. The two models are effectively tied on the index average at 87.8.
The price picture favours Grok 4 Reasoning. The context picture favours Grok 4 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 Grok 4 Reasoning; it is cheaper on a blended basis and has the larger context window. Test it on your own traffic before committing.
Choose GPT-5.1 Pro if
- You prefer OpenAI's tooling or already use it.
Choose Grok 4 Reasoning if
- Unit cost matters and you want the cheaper blended price.
- You need the larger 2,000,000-token context window.
- You need the larger 65,536-token output limit.
FAQ
Is GPT-5.1 Pro better than Grok 4 Reasoning?
The two models are effectively tied on the index average at 87.8. The better choice depends on price, context and your task. GPT-5.1 Pro wins 7 benchmarks, Grok 4 Reasoning wins 5.
Which is cheaper, GPT-5.1 Pro or Grok 4 Reasoning?
Grok 4 Reasoning is cheaper on a blended 3:1 basis at about $38.08 per million tokens, versus $39.91 for GPT-5.1 Pro.
Which has the larger context window?
Grok 4 Reasoning has the larger context window at 2,000,000 tokens. GPT-5.1 Pro has 200,000 tokens.
Are these models open weights?
GPT-5.1 Pro is proprietary; Grok 4 Reasoning is proprietary.
Which should I choose for production?
If you need a single default, start with Grok 4 Reasoning; it is cheaper on a blended basis and has the larger context window. Test it on your own traffic before committing.