Nano banana 2 (gemini-3.1-flash-image-preview) cannot generate 0.5K images.

[Ricardo1980]

I'm using genkit js and the plugin @genkit-ai/google-genai

I tried generating a 0.5K image, but it is totally impossible. It always returns a 1K image.

The documentation is not very clear about the parameter we have to send to create a 512px image:

https://ai.google.dev/gemini-api/docs/image-generation

Gemini 3 image models generate 1K images by default but can also output 2K, 4K, and 512px (05.K) (Gemini 3.1 Flash Image only) images. To generate higher resolution assets, specify the image_size in the generation_config.

You must use an uppercase 'K' (e.g. 512px (05.K), 1K, 2K, 4K). Lowercase parameters (e.g., 1k) will be rejected.

I tried "512px", "0.5K", and "05.K", but nothing, it always returns a 1024 image.

I used curl and all those possible values. None of the worked. So, I assume the problem is in the model itself. (or google apis)

Do you see the same issue?

curl -s -X POST \
  "https://generativelanguage.googleapis.com/v1beta/models/gemini-3.1-flash-image-preview:generateContent" \
  -H "x-goog-api-key: XXXXXXXX" \
  -H "Content-Type: application/json" \
  -d '{
    "contents": [{
      "parts": [
        {"text": "Create a picture of a nano banana dish in a fancy restaurant with a Gemini theme"}
      ]
    }],
    "generationConfig": {
      "responseModalities": ["IMAGE"],
      "imageConfig": {
        "imageSize": "0.5K",
        "aspectRatio": "1:1"
      }
    }
  }' | python3 -c "
import sys, json, base64
data = json.load(sys.stdin)
for part in data['candidates'][0]['content']['parts']:
    if 'inlineData' in part:
        img = base64.b64decode(part['inlineData']['data'])
        with open('/tmp/test.png', 'wb') as f:
            f.write(img)
        print(f'Saved to /tmp/test.png ({len(img)} bytes)')
        break
"

EDIT: I just saw this:
https://discuss.ai.google.dev/t/imagesize-512px-not-applied-in-gemini-3-1-flash-image-preview-always-returns-1k-resolution/127332