Aspect ratios

The aspect_ratio field is required on every generation request. The supported set depends on the model.

Aspect ratio	gemini-2.5-flash-image	gemini-3.1-flash-image / gemini-3-pro-image
`1:1`	✅	✅
`2:3`	✅	✅
`3:2`	✅	✅
`3:4`	✅	✅
`4:3`	✅	✅
`4:5`	✅	✅
`5:4`	✅	✅
`9:16`	✅	✅
`16:9`	✅	✅
`21:9`	✅	✅
`1:4`	❌	✅
`1:8`	❌	✅
`4:1`	❌	✅
`8:1`	❌	✅
`auto`	✅	✅

Special values

auto string

Resolve the aspect ratio from the prompt text (for example “16:9”). If the prompt contains no ratio: 9:16 is used for text-to-image, and no ratio is sent upstream for image-to-image. Supported by all image models.