GPT-5.1 Codex vs o3 for coding
GPT-5.1 Codex is the stronger coder of the two on benchmarks, but o3 can be the better pick when cost, speed, or context window matter more. Below: a side-by-side spec table and exactly when to pick each.
At a glance
| Spec | GPT-5.1 Codex | o3 |
|---|---|---|
| Provider | OpenAI | OpenAI |
| Released | Nov 2025 | Apr 2025 |
| SWE-bench Verified | 75% | 71% |
| HumanEval | 95% | 96% |
| MMLU | 88% | 91% |
| Context window | 400K | 200K |
| Max output | 128K | 100K |
| Input price (per 1M) | $1.25 | $10 |
| Output price (per 1M) | $10 | $40 |
| Price tier | Mid | Premium |
| Speed | Medium | Slow/Reasoning |
| Hosting | Closed/API | Closed/API |
| Modality | Text + Vision | Multimodal (vision) |
| Knowledge cutoff | Oct 2025 | Jun 2024 |
Pick GPT-5.1 Codex if…
- It scores higher on SWE-bench Verified (75% vs 71%), the best proxy for real-world coding.
- It's cheaper (Mid tier vs Premium).
- It has a larger context window (400K vs 200K).
- It responds faster (Medium).
Pick o3 if…
- It's tuned for hardest coding problems, complex multi-step reasoning, advanced debugging.
GPT-5.1 Codex vs o3: which is better for coding?
GPT-5.1 Codex is the stronger coder of the two on benchmarks, but o3 can be the better pick when cost, speed, or context window matter more. See the full spec table for SWE-bench, HumanEval, MMLU, context window, and pricing on both. Benchmarks are a directional signal, not a guarantee for your codebase — the most reliable test is running both on a real task you care about.
Compare these head-to-head with live data, or build a full stack around your pick — Flowpicker shows compatibility and monthly cost.
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See the full model leaderboard ranked by SWE-bench, HumanEval, and MMLU.