GPT-5.1 Turbo vs o4 Reasoning: which model should you choose?
We compared GPT-5.1 Turbo and o4 Reasoning on price, context window and 12 benchmarks. o4 Reasoning has the stronger average benchmark score; GPT-5.1 Turbo is cheaper on a blended 3:1 basis.
GPT-5.1 Turbo
GPT-5.1 Turbo is a frontier model from OpenAI. It offers a 2,000,000-token context window, costs $19.19 per million input tokens and $89.83 per million output tokens, and averages 86.1 across the index's benchmark suite.
- Input
- $19.19 / 1M
- Output
- $89.83 / 1M
- Context
- 2,000,000
- Avg score
- 86.1
- 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 Turbo | o4 Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $19.19 | $19.69 | A |
| Output price / 1M | $89.83 | $103.53 | A |
| Context window | 2,000,000 | 1,000,000 | A |
| Max output | 128,000 | 128,000 | Tie |
| Average score | 86.1 | 87.5 | B |
| Arena Elo | 1482.0 | 1472.2 | A |
| Open weights | No | No | Tie |
Benchmark comparison
GPT-5.1 Turbo wins 4, o4 Reasoning wins 8, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | GPT-5.1 Turbo | o4 Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 85.2 | 86.1 | 0.9 | o4 Reasoning |
| GPQA Diamond | 81.3 | 84.0 | 2.7 | o4 Reasoning |
| SWE-bench Verified | 74.7 | 76.4 | 1.7 | o4 Reasoning |
| HumanEval | 91.5 | 90.2 | 1.3 | GPT-5.1 Turbo |
| MATH-500 | 91.4 | 94.8 | 3.4 | o4 Reasoning |
| AIME 2025 | 84.7 | 90.3 | 5.6 | o4 Reasoning |
| LiveCodeBench | 84.6 | 83.4 | 1.2 | GPT-5.1 Turbo |
| IFEval | 93.4 | 94.5 | 1.1 | o4 Reasoning |
| TruthfulQA | 89.2 | 89.0 | 0.2 | GPT-5.1 Turbo |
| HellaSwag | 91.3 | 92.4 | 1.1 | o4 Reasoning |
| MMMU | 79.7 | 81.8 | 2.1 | o4 Reasoning |
| Arena Elo | 1482.0 | 1472.2 | 9.8 | GPT-5.1 Turbo |
Price analysis
On a blended 3:1 input-to-output basis, GPT-5.1 Turbo costs about $36.85 per million tokens and o4 Reasoning costs about $40.65. GPT-5.1 Turbo is roughly 9% cheaper than o4 Reasoning. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
GPT-5.1 Turbo has the larger context window at 2,000,000 tokens, compared with 1,000,000 for o4 Reasoning. That matters for long documents, large codebases and multi-step agent traces.
Verdict
Across 12 shared benchmarks, GPT-5.1 Turbo wins 4, o4 Reasoning wins 8, and 0 are tied. On the index average, o4 Reasoning leads GPT-5.1 Turbo by 1.4 points (87.5 versus 86.1).
The price picture favours GPT-5.1 Turbo. The context picture favours GPT-5.1 Turbo. 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 o4 Reasoning for quality-sensitive work and GPT-5.1 Turbo for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.
Choose GPT-5.1 Turbo if
- Unit cost matters and you want the cheaper blended price.
- You need the larger 2,000,000-token context window.
Choose o4 Reasoning if
- You want the higher average benchmark score in this index.
FAQ
Is GPT-5.1 Turbo better than o4 Reasoning?
On the index average, o4 Reasoning leads GPT-5.1 Turbo by 1.4 points (87.5 versus 86.1). The better choice depends on price, context and your task. GPT-5.1 Turbo wins 4 benchmarks, o4 Reasoning wins 8.
Which is cheaper, GPT-5.1 Turbo or o4 Reasoning?
GPT-5.1 Turbo is cheaper on a blended 3:1 basis at about $36.85 per million tokens, versus $40.65 for o4 Reasoning.
Which has the larger context window?
GPT-5.1 Turbo has the larger context window at 2,000,000 tokens. o4 Reasoning has 1,000,000 tokens.
Are these models open weights?
GPT-5.1 Turbo is proprietary; o4 Reasoning is proprietary.
Which should I choose for production?
If you need a single default, start with o4 Reasoning for quality-sensitive work and GPT-5.1 Turbo for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.