ZMIME
Comparison · 2 models · Updated Oct 4, 2026

GPT-5.3 Codex vs Gemini 3.5 Flash

Gemini 3.5 Flash comes out ahead, 69 to 64 on our weighted score, and it is the cheaper option too.

  1. OpenAI

    GPT-5.3 Codex

    Released Feb 5, 2026

    64/100
    • ECI156.8
    • Price$1.75 / $14.00
    • Context400K
    Full specs Remove GPT-5.3 Codex
  2. Our pick

    Google

    Gemini 3.5 Flash

    Released May 19, 2026

    69/100
    • ECI154.5
    • Price$1.50 / $9.00
    • Context1.05M
    Full specs Remove Gemini 3.5 Flash

  3. Add a model

    Make it a three-way comparison.

01 — Verdict

Gemini 3.5 Flash is our pick

Gemini 3.5 Flash is the better all-round choice, scoring 69/100 against GPT-5.3 Codex (64). It leads on price, inputs & features and context window. GPT-5.3 Codex wins on capability. 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 3.5 Flash 154.5
  • Lowest priceGemini 3.5 FlashGemini 3.5 Flash $3.38 · GPT-5.3 Codex $4.81 per 1M tokens (3:1 blend)
  • Longest contextGemini 3.5 FlashGemini 3.5 Flash 1,048,576 · GPT-5.3 Codex 400,000 tokens
  • Widest inputsGemini 3.5 FlashGPT-5.3 Codex: Text, Images, PDFs · Gemini 3.5 Flash: Text, Images, PDFs, Audio, Video
  • Self-hostingNo open weightsBoth are available only through APIs
How the score is built
MeasureWeightGPT-5.3 CodexGemini 3.5 Flash
CapabilityCapabilities Index (ECI)50%8784
Price25%1825
Inputs & features15%80100
Context window10%4461
Overall100%64/10069/100
02 — Side by side

Every spec in one table

Highlighted cells lead their row. Dashes mean the data is not published.

GPT-5.3 Codex vs Gemini 3.5 Flash specifications side by side
SpecificationGPT-5.3 CodexOpenAIGemini 3.5 FlashGoogle
Capability
Capabilities Index (ECI)156.8 (best)154.5
ECI rank#18 of 148 (best)#33 of 148
GPQA DiamondGraduate-level science questions—92.8%
FrontierMath Tiers 1–3Research-level mathematics—62.8%
OTIS Mock AIME 2024–2025Competition mathematics—95.6%
SWE-bench VerifiedFixing real GitHub issues74.8%79.3% (best)
SimpleQA VerifiedShort factual questions—66.2%
Price per million tokens
Input$1.75$1.50 (best)
Output$14.00$9.00 (best)
Cached input$0.175$0.15 (best)
Blended (3:1)$4.81$3.38 (best)
Long-context rateSame rateSame rate
Price sourceOfficial OpenAI APIOfficial Google API
Limits
Context window400,000 tokens1,048,576 tokens (best)
Max output128,000 tokens (best)65,536 tokens
Inputs and features
TextYesYes
ImagesYesYes
PDFsYesYes
AudioNoYes
VideoNoYes
ReasoningYeslow · medium · high · xhighYesminimal · low · medium · high
Tool callingYesYes
Structured outputYesYes
Availability
WeightsProprietaryProprietary
API model IDgpt-5.3-codexgemini-3.5-flash
API providers1932 (best)
ReleasedFeb 5, 2026May 19, 2026
Knowledge cutoffAug 31, 2025Jan 2025
03 — Cost

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
04 — Questions

Which should you choose?

Which is better: GPT-5.3 Codex or Gemini 3.5 Flash?

Gemini 3.5 Flash is the better all-round choice, scoring 69/100 against GPT-5.3 Codex (64). It leads on price, inputs & features and context window. GPT-5.3 Codex wins on capability. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.

Which is cheaper, GPT-5.3 Codex or Gemini 3.5 Flash?

Gemini 3.5 Flash is cheaper at $1.50 input / $9.00 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 $4.81 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) and Gemini 3.5 Flash 154.5 (#33 of 148). The confidence ranges of the top two overlap (153.5–160.8 vs 152.5–156.6), so treat the gap as small. On individual benchmarks: SWE-bench Verified — Gemini 3.5 Flash 79.3%, 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% and GPT-5.3 Codex 74.8%. Both support tool calling for agent workflows.

Which has the bigger context window?

Gemini 3.5 Flash has the largest context window at 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 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 3.5 Flash handles the widest range of inputs.

Are any of these open source?

No. GPT-5.3 Codex and Gemini 3.5 Flash are proprietary and only available through APIs and apps.

Which is newer?

Gemini 3.5 Flash is the newest, released May 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.

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.

05 — Keep comparing

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