GPT-5.1 Codex vs GPT-5.5 for coding
GPT-5.1 Codex and GPT-5.5 are closely matched on coding benchmarks; GPT-5.1 Codex wins on price, while GPT-5.5 may edge ahead on other specs below. Below: a side-by-side spec table and exactly when to pick each.
At a glance
| Spec | GPT-5.1 Codex | GPT-5.5 |
|---|---|---|
| Provider | OpenAI | OpenAI |
| Released | Nov 2025 | Apr 2026 |
| SWE-bench Verified | 75% | 75% |
| HumanEval | 95% | 95% |
| MMLU | 88% | 89% |
| Context window | 400K | 1M+ |
| Max output | 128K | 128K |
| Input price (per 1M) | $1.25 | $5 |
| Output price (per 1M) | $10 | $30 |
| Price tier | Mid | Premium |
| Speed | Medium | Standard |
| Hosting | Closed/API | Closed/API |
| Modality | Text + Vision | Multimodal (vision) |
| Knowledge cutoff | Oct 2025 | Jan 2026 |
Pick GPT-5.1 Codex if…
- It's cheaper (Mid tier vs Premium).
- It's tuned for codex CLI and long-horizon coding agents; engineered for terminal-driven workflows.
Pick GPT-5.5 if…
- It has a larger context window (1M+ vs 400K).
- It's tuned for frontier reasoning, agentic coding, long-context refactors, multimodal analysis, replaces GPT-5.4 as default flagship.
GPT-5.1 Codex vs GPT-5.5: which is better for coding?
GPT-5.1 Codex and GPT-5.5 are closely matched on coding benchmarks; GPT-5.1 Codex wins on price, while GPT-5.5 may edge ahead on other specs below. 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.