GPT-5.3 Codex vs Gemini 3.5 Flash vs Gemini 3.1 Pro Preview
Too close to call on our weighted score (Gemini 3.5 Flash 69, Gemini 3.1 Pro Preview 68, GPT-5.3 Codex 64). The right pick depends on what you value most.
OpenAIGPT-5.3 Codex
64/100- ECI156.8
- Price$1.75 / $14.00
- Context400K
GoogleGemini 3.5 Flash
69/100- ECI154.5
- Price$1.50 / $9.00
- Context1.05M
- Google
Gemini 3.1 Pro Preview
68/100- ECI154.8
- Price$2.00 / $12.00
- Context1.05M
Too close to call
It is close. Our weighted score puts them within 1 points (Gemini 3.5 Flash 69/100, Gemini 3.1 Pro Preview 68/100, GPT-5.3 Codex 64/100), so choose by what matters most for your work: GPT-5.3 Codex for raw capability and Gemini 3.5 Flash on price. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
- CapabilityGPT-5.3 CodexCapabilities Index (ECI): GPT-5.3 Codex 156.8 · Gemini 3.1 Pro Preview 154.8 · Gemini 3.5 Flash 154.5
- Lowest priceGemini 3.5 FlashGemini 3.5 Flash $3.38 · Gemini 3.1 Pro Preview $4.50 · GPT-5.3 Codex $4.81 per 1M tokens (3:1 blend)
- Longest contextGemini 3.5 Flash and Gemini 3.1 Pro PreviewGemini 3.5 Flash 1,048,576 · Gemini 3.1 Pro Preview 1,048,576 · GPT-5.3 Codex 400,000 tokens
- Widest inputsGemini 3.5 Flash and Gemini 3.1 Pro PreviewGPT-5.3 Codex: Text, Images, PDFs · Gemini 3.5 Flash: Text, Images, PDFs, Audio, Video · Gemini 3.1 Pro Preview: Text, Images, PDFs, Audio, Video
- Self-hostingNo open weightsAll three are available only through APIs
| Measure | Weight | GPT-5.3 Codex | Gemini 3.5 Flash | Gemini 3.1 Pro Preview |
|---|---|---|---|---|
| CapabilityCapabilities Index (ECI) | 50% | 87 | 84 | 84 |
| Price | 25% | 18 | 25 | 19 |
| Inputs & features | 15% | 80 | 100 | 100 |
| Context window | 10% | 44 | 61 | 61 |
| Overall | 100% | 64/100 | 69/100 | 68/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | GPT-5.3 CodexOpenAI | Gemini 3.5 FlashGoogle | Gemini 3.1 Pro PreviewGoogle |
|---|---|---|---|
| Capability | |||
| Capabilities Index (ECI) | 156.8 (best) | 154.5 | 154.8 |
| ECI rank | #18 of 148 (best) | #33 of 148 | #31 of 148 |
| GPQA DiamondGraduate-level science questions | — | 92.8% | 94.4% (best) |
| FrontierMath Tiers 1–3Research-level mathematics | — | 62.8% (best) | 59.7% |
| OTIS Mock AIME 2024–2025Competition mathematics | — | 95.6% | 95.6% (best) |
| SWE-bench VerifiedFixing real GitHub issues | 74.8% | 79.3% (best) | 75.6% |
| SimpleQA VerifiedShort factual questions | — | 66.2% | 73.5% (best) |
| Price per million tokens | |||
| Input | $1.75 | $1.50 (best) | $2.00 |
| Output | $14.00 | $9.00 (best) | $12.00 |
| Cached input | $0.175 | $0.15 (best) | $0.20 |
| Blended (3:1) | $4.81 | $3.38 (best) | $4.50 |
| Long-context rate | Same rate | Same rate | Over 200K: $4.00 / $18.00 |
| Price source | Official OpenAI API | Official Google API | Official Google API |
| Limits | |||
| Context window | 400,000 tokens | 1,048,576 tokens (best) | 1,048,576 tokens (best) |
| Max output | 128,000 tokens (best) | 65,536 tokens | 65,536 tokens |
| Inputs and features | |||
| Text | Yes | Yes | Yes |
| Images | Yes | Yes | Yes |
| PDFs | Yes | Yes | Yes |
| Audio | No | Yes | Yes |
| Video | No | Yes | Yes |
| Reasoning | Yeslow · medium · high · xhigh | Yesminimal · low · medium · high | Yeslow · medium · high |
| Tool calling | Yes | Yes | Yes |
| Structured output | Yes | Yes | Yes |
| Availability | |||
| Weights | Proprietary | Proprietary | Proprietary |
| API model ID | gpt-5.3-codex | gemini-3.5-flash | gemini-3.1-pro-preview |
| API providers | 19 | 32 (best) | 26 |
| Released | Feb 5, 2026 | May 19, 2026 | Feb 19, 2026 |
| Knowledge cutoff | Aug 31, 2025 | Jan 2025 | Jan 2025 |
What would a month cost?
Enter your expected volume in millions of tokens. List prices only; caching and batch discounts would lower these.
- GPT-5.3 Codex$45.50
- Gemini 3.5 Flash$33.00
- Gemini 3.1 Pro Preview$44.00
Which should you choose?
Which is better: GPT-5.3 Codex, Gemini 3.5 Flash or Gemini 3.1 Pro Preview?
It is close. Our weighted score puts them within 1 points (Gemini 3.5 Flash 69/100, Gemini 3.1 Pro Preview 68/100, GPT-5.3 Codex 64/100), so choose by what matters most for your work: GPT-5.3 Codex for raw capability and Gemini 3.5 Flash on price. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
Which is cheaper, GPT-5.3 Codex, Gemini 3.5 Flash or Gemini 3.1 Pro Preview?
Gemini 3.5 Flash is cheaper at $1.50 input / $9.00 output per million tokens (official Google API price). Gemini 3.1 Pro Preview costs $2.00 input / $12.00 output per million tokens (official Google API price); GPT-5.3 Codex costs $1.75 input / $14.00 output per million tokens (official OpenAI API price). At a typical mix of three input tokens to one output token, that is $3.38 per million tokens for Gemini 3.5 Flash versus $4.50 for Gemini 3.1 Pro Preview (1.3× as much) and $4.81 for GPT-5.3 Codex (1.4× as much).
Which scores higher on benchmarks?
GPT-5.3 Codex scores higher on the Capabilities Index (ECI): GPT-5.3 Codex 156.8 (#18 of 148), Gemini 3.1 Pro Preview 154.8 (#31 of 148) and Gemini 3.5 Flash 154.5 (#33 of 148). The confidence ranges of the top two overlap (153.5–160.8 vs 152.6–157.3), so treat the gap as small. On individual benchmarks: SWE-bench Verified — Gemini 3.5 Flash 79.3%, Gemini 3.1 Pro Preview 75.6%, GPT-5.3 Codex 74.8%.
Which is better for coding?
Gemini 3.5 Flash resolves more real GitHub issues on SWE-bench Verified: Gemini 3.5 Flash 79.3%, Gemini 3.1 Pro Preview 75.6% and GPT-5.3 Codex 74.8%. All three support tool calling for agent workflows.
Which has the bigger context window?
Gemini 3.5 Flash and Gemini 3.1 Pro Preview have the largest context windows (1,048,576 and 1,048,576 tokens), against 400,000 for GPT-5.3 Codex. Maximum output per response: GPT-5.3 Codex up to 128,000, Gemini 3.5 Flash up to 65,536, Gemini 3.1 Pro Preview up to 65,536 tokens.
Which can read images, PDFs, audio or video?
GPT-5.3 Codex accepts text, images and PDFs; Gemini 3.5 Flash accepts text, images, PDFs, audio and video; Gemini 3.1 Pro Preview accepts text, images, PDFs, audio and video. Gemini 3.5 Flash handles the widest range of inputs.
Are any of these open source?
No. GPT-5.3 Codex, Gemini 3.5 Flash and Gemini 3.1 Pro Preview are proprietary and only available through APIs and apps.
Which is newer?
Gemini 3.5 Flash is the newest, released May 19, 2026. Gemini 3.1 Pro Preview came out Feb 19, 2026; GPT-5.3 Codex came out Feb 5, 2026. Knowledge cutoff: GPT-5.3 Codex Aug 31, 2025, Gemini 3.5 Flash Jan 2025, Gemini 3.1 Pro Preview Jan 2025.
How do you decide the winner?
Each model gets a 0–100 score on capability (50%, independent benchmark results); price (25%, blended price per million tokens (3 input : 1 output), log scale); inputs & features (15%, image, PDF, audio and video input, tool calling, structured output and reasoning); context window (10%, maximum tokens per request, log scale). Dimensions missing for any model are dropped and the remaining weights rescaled, so every model is judged on the same evidence. Specs and prices come from public model listings and the labs’ own API pages; capability scores come from independent benchmark runs. Data updated Oct 4, 2026.
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