Gemini 2.5 Pro Reasoning vs Gemini 3 Flash Reasoning: which model should you choose?
We compared Gemini 2.5 Pro Reasoning and Gemini 3 Flash Reasoning on price, context window and 12 benchmarks. Gemini 2.5 Pro Reasoning has the stronger average benchmark score; Gemini 3 Flash Reasoning is cheaper on a blended 3:1 basis.
Gemini 2.5 Pro Reasoning
Gemini 2.5 Pro Reasoning is a frontier model from Google DeepMind. It offers a 128,000-token context window, costs $16.81 per million input tokens and $67.95 per million output tokens, and averages 83.8 across the index's benchmark suite.
- Input
- $16.81 / 1M
- Output
- $67.95 / 1M
- Context
- 128,000
- Avg score
- 83.8
- Open weights
- No
Gemini 3 Flash Reasoning
Gemini 3 Flash Reasoning is a frontier model from Google DeepMind. It offers a 128,000-token context window, costs $17.05 per million input tokens and $66.77 per million output tokens, and averages 83.7 across the index's benchmark suite.
- Input
- $17.05 / 1M
- Output
- $66.77 / 1M
- Context
- 128,000
- Avg score
- 83.7
- Open weights
- No
Specification comparison
| Spec | Gemini 2.5 Pro Reasoning | Gemini 3 Flash Reasoning | Edge |
|---|---|---|---|
| Input price / 1M | $16.81 | $17.05 | A |
| Output price / 1M | $67.95 | $66.77 | B |
| Context window | 128,000 | 128,000 | Tie |
| Max output | 4,096 | 32,768 | B |
| Average score | 83.8 | 83.7 | A |
| Arena Elo | 1472.1 | 1446.9 | A |
| Open weights | No | No | Tie |
Benchmark comparison
Gemini 2.5 Pro Reasoning wins 5, Gemini 3 Flash Reasoning wins 7, and 0 benchmarks are tied. The table shows the modelled score for each model and the gap.
| Benchmark | Gemini 2.5 Pro Reasoning | Gemini 3 Flash Reasoning | Gap | Winner |
|---|---|---|---|---|
| MMLU-Pro | 86.8 | 87.2 | 0.4 | Gemini 3 Flash Reasoning |
| GPQA Diamond | 80.0 | 81.8 | 1.8 | Gemini 3 Flash Reasoning |
| SWE-bench Verified | 71.2 | 72.5 | 1.3 | Gemini 3 Flash Reasoning |
| HumanEval | 86.7 | 86.9 | 0.2 | Gemini 3 Flash Reasoning |
| MATH-500 | 87.9 | 86.2 | 1.7 | Gemini 2.5 Pro Reasoning |
| AIME 2025 | 84.2 | 79.4 | 4.8 | Gemini 2.5 Pro Reasoning |
| LiveCodeBench | 74.8 | 77.7 | 2.9 | Gemini 3 Flash Reasoning |
| IFEval | 90.8 | 90.2 | 0.6 | Gemini 2.5 Pro Reasoning |
| TruthfulQA | 84.0 | 86.6 | 2.6 | Gemini 3 Flash Reasoning |
| HellaSwag | 92.1 | 92.2 | 0.1 | Gemini 3 Flash Reasoning |
| MMMU | 82.8 | 79.8 | 3.0 | Gemini 2.5 Pro Reasoning |
| Arena Elo | 1472.1 | 1446.9 | 25.2 | Gemini 2.5 Pro Reasoning |
Price analysis
On a blended 3:1 input-to-output basis, Gemini 2.5 Pro Reasoning costs about $29.59 per million tokens and Gemini 3 Flash Reasoning costs about $29.48. Gemini 3 Flash Reasoning is roughly 0% cheaper than Gemini 2.5 Pro Reasoning. At high volume, that gap compounds quickly, so price should carry real weight unless the quality difference is decisive.
Both models list a 128,000-token context window. That is enough for long documents, large codebases and multi-step agent traces.
Verdict
Across 12 shared benchmarks, Gemini 2.5 Pro Reasoning wins 5, Gemini 3 Flash Reasoning wins 7, and 0 are tied. On the index average, Gemini 2.5 Pro Reasoning leads Gemini 3 Flash Reasoning by 0.1 points (83.8 versus 83.7).
The price picture favours Gemini 3 Flash Reasoning. The context picture is effectively a tie. 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 Gemini 2.5 Pro Reasoning for quality-sensitive work and Gemini 3 Flash Reasoning for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.
Choose Gemini 2.5 Pro Reasoning if
- You want the higher average benchmark score in this index.
Choose Gemini 3 Flash Reasoning if
- Unit cost matters and you want the cheaper blended price.
- You need the larger 32,768-token output limit.
FAQ
Is Gemini 2.5 Pro Reasoning better than Gemini 3 Flash Reasoning?
On the index average, Gemini 2.5 Pro Reasoning leads Gemini 3 Flash Reasoning by 0.1 points (83.8 versus 83.7). The better choice depends on price, context and your task. Gemini 2.5 Pro Reasoning wins 5 benchmarks, Gemini 3 Flash Reasoning wins 7.
Which is cheaper, Gemini 2.5 Pro Reasoning or Gemini 3 Flash Reasoning?
Gemini 3 Flash Reasoning is cheaper on a blended 3:1 basis at about $29.48 per million tokens, versus $29.59 for Gemini 2.5 Pro Reasoning.
Which has the larger context window?
Both models list a 128,000-token context window.
Are these models open weights?
Gemini 2.5 Pro Reasoning is proprietary; Gemini 3 Flash Reasoning is proprietary.
Which should I choose for production?
If you need a single default, start with Gemini 2.5 Pro Reasoning for quality-sensitive work and Gemini 3 Flash Reasoning for high-volume, well-scoped tasks. Run both on a sample of your own traffic before locking in a choice.