GPT-5.1 Reasoning vs o4 Reasoning: which model should you choose?
We compared GPT-5.1 Reasoning and o4 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.
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
o4 Reasoning
o4 Reasoning is a frontier model from OpenAI. It offers a 1,000,000-token context window, costs $19.69 per million input tokens and $103.53 per million output tokens, and averages 87.5 across the index's benchmark suite.
- Input
- $19.69 / 1M
- Output
- $103.53 / 1M
- Context
- 1,000,000
- Avg score
- 87.5
- Open weights
- No
Specification comparison
| Spec | GPT-5.1 Reasoning | o4 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $20.27 | $19.69 | B |
| Output price / 1M | $82.11 | $103.53 | A |
| Context window | 400,000 | 1,000,000 | B |
| Max output | 128,000 | 128,000 | Tie |
| Average score | 88.3 | 87.5 | A |
| Arena Elo | 1485.9 | 1472.2 | A |
| Open weights | No | No | Tie |
Benchmark comparison
GPT-5.1 Reasoning wins 9, o4 Reasoning wins 3, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | GPT-5.1 Reasoning | o4 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 90.0 | 86.1 | 3.9 | GPT-5.1 Reasoning |
| GPQA Diamond | 85.6 | 84.0 | 1.6 | GPT-5.1 Reasoning |
| SWE-bench Verified | 76.0 | 76.4 | 0.4 | o4 Reasoning |
| HumanEval | 92.9 | 90.2 | 2.7 | GPT-5.1 Reasoning |
| MATH-500 | 95.9 | 94.8 | 1.1 | GPT-5.1 Reasoning |
| AIME 2025 | 85.0 | 90.3 | 5.3 | o4 Reasoning |
| LiveCodeBench | 80.9 | 83.4 | 2.5 | o4 Reasoning |
| IFEval | 94.6 | 94.5 | 0.1 | GPT-5.1 Reasoning |
| TruthfulQA | 90.0 | 89.0 | 1.0 | GPT-5.1 Reasoning |
| HellaSwag | 94.4 | 92.4 | 2.0 | GPT-5.1 Reasoning |
| MMMU | 86.2 | 81.8 | 4.4 | GPT-5.1 Reasoning |
| Arena Elo | 1485.9 | 1472.2 | 13.7 | GPT-5.1 Reasoning |
Price analysis
On a blended 3:1 input-to-output basis, GPT-5.1 Reasoning costs about $35.73 per million tokens and o4 Reasoning costs about $40.65. GPT-5.1 Reasoning is roughly 12% cheaper than o4 Reasoning. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
o4 Reasoning 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, GPT-5.1 Reasoning wins 9, o4 Reasoning wins 3, and 0 are tied. On the index average, GPT-5.1 Reasoning leads o4 Reasoning by 0.8 points (88.3 versus 87.5).
The price picture favours GPT-5.1 Reasoning. The context picture favours o4 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 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.
Choose o4 Reasoning if
- You need the larger 1,000,000-token context window.
FAQ
Is GPT-5.1 Reasoning better than o4 Reasoning?
On the index average, GPT-5.1 Reasoning leads o4 Reasoning by 0.8 points (88.3 versus 87.5). The better choice depends on price, context and your task. GPT-5.1 Reasoning wins 9 benchmarks, o4 Reasoning wins 3.
Which is cheaper, GPT-5.1 Reasoning or o4 Reasoning?
GPT-5.1 Reasoning is cheaper on a blended 3:1 basis at about $35.73 per million tokens, versus $40.65 for o4 Reasoning.
Which has the larger context window?
o4 Reasoning has the larger context window at 1,000,000 tokens. GPT-5.1 Reasoning has 400,000 tokens.
Are these models open weights?
GPT-5.1 Reasoning is proprietary; o4 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.