Gemini 3.1 Flash Image parameters

Use this exact path ID:

gemini-3.1-flash-image

The stable ID does not include -preview.

Output controls

Field	Value through this API
`responseModalities`	Include `IMAGE`; `TEXT` is optional
`candidateCount`	Omit or use `1`
`imageSize`	Omit for 1K, or use uppercase `1K`, `2K`, `4K`
`aspectRatio`	Optional; choose one of fourteen values
Upstream 0.5K / 512px	Not exposed in this API’s current public contract
Top-level `n` or `resolution`	Not supported

Supported ratios:

1:1   1:4   1:8   2:3   3:2   3:4   4:1
4:3   4:5   5:4   8:1   9:16  16:9  21:9

The model-specific API contract exposes the ratio values above. Use only those values; support for one ratio does not imply an unlisted size-ratio value.

Generation body

{
  "contents": [{
    "role": "user",
    "parts": [{"text": "A wide editorial illustration with room for a headline"}]
  }],
  "generationConfig": {
    "responseModalities": ["TEXT", "IMAGE"],
    "imageConfig": {"aspectRatio": "16:9", "imageSize": "2K"},
    "candidateCount": 1
  }
}

Editing body

{
  "contents": [{
    "role": "user",
    "parts": [
      {"text": "Keep the product geometry and replace the background with pale stone"},
      {
        "inlineData": {
          "mimeType": "image/jpeg",
          "data": "BASE64_SOURCE_IMAGE_1"
        }
      },
      {"inlineData": {"mimeType": "image/png", "data": "BASE64_SOURCE_IMAGE_2"}}
    ]
  }],
  "generationConfig": {
    "responseModalities": ["TEXT", "IMAGE"],
    "imageConfig": {"aspectRatio": "4:3", "imageSize": "1K"},
    "candidateCount": 1
  }
}

Repeat inlineData for multiple references. Unified asynchronous editing accepts 1–16 images, with a 20 MiB decoded limit per file and 64 MiB total. Grounding, thinking, and other official fields remain governed by the current Gemini protocol and the selected model’s support. A fixed maximum source-image byte or pixel size is not currently published. Total request-body, proxy, and upstream limits may differ; reduce the source image after HTTP 413, and contact API support before relying on a guaranteed production envelope.

Size and transport

The requested tier determines billing; compressed file bytes do not. Higher tiers can create larger Base64 responses, so align client and proxy limits with the actual delivery.

Copy Flash examples

Run complete synchronous, asynchronous, editing, and parsing flows.

Check the current price

Use the dated YingTu-only table.

​Output controls

​Generation body

​Editing body

​Size and transport