GPT-5.1 Codex vs GPT-5.5
Head-to-head specifications
| Metric | GPT-5.1 Codex | GPT-5.5 | Difference |
|---|---|---|---|
| Intelligence Index | 35.0 | 55.0 | -36.4% |
| Context window | 922K tokens | 1M tokens | — |
| Blended price ($/1M tokens) | $1.14 | $1.54 | -26.0% |
| Output speed (tokens/s) | 200 | 67 | +198.5% |
| Access | Proprietary API | Proprietary API | — |
- GPT-5.5 leads overall capability (Intelligence Index 55.0 vs 35.0).
- GPT-5.1 Codex is the cheaper model to run at $1.14/1M blended tokens — about 1.4× cheaper.
- GPT-5.5 offers the larger context window (1M tokens), useful for long documents and codebases.
Verdict: GPT-5.1 Codex or GPT-5.5?
GPT-5.1 Codex advantages
- Affordability (+26%)
- Output speed (+67%)
GPT-5.5 advantages
- General intelligence (+36%)
- Context window (+8%)
Which should you choose?
- Choose the GPT-5.1 Codex if you want the lowest cost per token at scale.
- Choose the GPT-5.5 if you need the strongest overall reasoning and accuracy.
- Choose the GPT-5.1 Codex if low latency and fast generation matter for your application.
Value for money
GPT-5.5 offers more intelligence per dollar (1.2× the Intelligence-Index-per-cost of the alternative), making it the stronger value for high-volume use.
GPT-5.1 Codex vs GPT-5.5: which should you choose?
GPT-5.1 Codex — OpenAI multimodal model with an Intelligence Index of 35, a 922K-token context window and a blended price of $1.14/1M tokens.
GPT-5.5 — OpenAI multimodal model with an Intelligence Index of 55, a 1M-token context window and a blended price of $1.54/1M tokens.
GPT-5.1 Codex vs GPT-5.5: GPT-5.5 scores higher on the Intelligence Index. GPT-5.5 leads overall capability (Intelligence Index 55.0 vs 35.0). GPT-5.1 Codex is the cheaper model to run at $1.14/1M blended tokens — about 1.4× cheaper.
Capability: intelligence, coding and agentic work
On the composite Intelligence Index the GPT-5.5 scores 55.0 versus 35.0. Composite indices summarize many evaluations, but always test on your own workload before committing.
Context window and speed
The GPT-5.5 accepts up to 1 million tokens per request, which sets how much documentation, transcript or code it can reason over at once. In measured throughput, GPT-5.1 Codex generates faster (200 vs 67 tokens/s), which matters for interactive apps and high-volume pipelines.
Pricing and access
At blended per-token rates, GPT-5.1 Codex is the cheaper model to run ($1.14 vs $1.54 per 1M tokens). GPT-5.1 Codex is proprietary api and GPT-5.5 is proprietary api. Open-weight models can be self-hosted, trading per-call cost for infrastructure you manage; for production also weigh rate limits, throughput and data-residency requirements.
The verdict
Both are credible choices in the ai model comparison space; the specification table above lays out every metric so you can weigh the trade-offs that matter to you. Pick the one whose strengths line up with how you will actually use it.
Frequently asked questions
Is the GPT-5.1 Codex better than the GPT-5.5?
These two are closely matched — the right pick comes down to which specific strengths you value and the price you actually pay. GPT-5.5 leads overall capability (Intelligence Index 55.0 vs 35.0).
What is the main difference between the GPT-5.1 Codex and the GPT-5.5?
GPT-5.5 leads overall capability (Intelligence Index 55.0 vs 35.0). GPT-5.1 Codex is the cheaper model to run at $1.14/1M blended tokens — about 1.4× cheaper.
Which is better value?
GPT-5.5 offers more intelligence per dollar (1.2× the Intelligence-Index-per-cost of the alternative), making it the stronger value for high-volume use.
Which should I choose?
Choose the GPT-5.1 Codex if you want the lowest cost per token at scale. Choose the GPT-5.5 if you need the strongest overall reasoning and accuracy.
Methodology
Large language models are compared on independent leaderboard metrics: an Intelligence Index (a composite of reasoning and knowledge evaluations), Coding and Agentic indices where measured, community arena Elo, maximum context window, a blended API price per million tokens (weighted across cache-hit, input and output rates), and measured output speed in tokens per second. Where a model ships multiple reasoning-effort variants, we report its strongest variant. Benchmarks capture only part of real-world quality, which also depends on tool use, latency, safety and task fit — and this space moves quickly, so figures reflect the leaderboard snapshot on the page date.