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The aspect_ratio field is required on every generation request. The supported set depends on the model.
Aspect ratioNanoBananaNanoBanana 2 / Pro
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❌✅
from_prompt✅✅

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Special values

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auto
string
Infer the ratio from the first input image. Image-to-image only, and not supported by NanoBanana.
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from_prompt
string
Let the model choose the ratio based on the prompt.
Passing a ratio that the model doesn’t support — for example auto or 4:1 to NanoBanana — results in a 422 validation error.