Nano Banana Pro (Gemini 3 Pro Image)

Nano Banana Pro is Google's Gemini 3 Pro Image model, built for professional assets and complex instructions. It thinks before it draws, so responses report reasoning tokens. Send Google's generateContent request body with a model field, keep the returned task ID, and check that task for the finished image. Reference images go in contents as fileData URLs.

Model IDs

Both IDs accept the same parameters. See the model page for current prices.

Quick example

curl https://api.seedrouter.ai/v1/images/generations \
  -H "Authorization: Bearer $SEEDROUTER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gemini-3-pro-image",
    "contents": [{"parts": [{"text": "A ceramic teapot on a linen tablecloth, soft window light"}]}],
    "generationConfig": {
      "responseModalities": ["IMAGE"],
      "imageConfig": {"aspectRatio": "16:9", "imageSize": "2K"}
    }
  }'

Endpoint

POST https://api.seedrouter.ai/v1/images/generations

The body is Google's generateContent request with one addition: model, because this endpoint carries no model in its path. The response contains a task ID, not the finished image. Keep API keys in server-side code. Calling /v1beta/models/...:generateContent directly is not supported; use this endpoint.

Parameters

Safety categories: HARM_CATEGORY_HARASSMENT, HARM_CATEGORY_HATE_SPEECH, HARM_CATEGORY_SEXUALLY_EXPLICIT, HARM_CATEGORY_DANGEROUS_CONTENT. Thresholds: BLOCK_NONE, BLOCK_ONLY_HIGH, BLOCK_MEDIUM_AND_ABOVE, BLOCK_LOW_AND_ABOVE, OFF.

Unknown fields are rejected. Not yet available: Google Search grounding (tools) and cached content; thinkingLevel is not documented for this model. inlineData is not accepted; pass media as fileData URLs. responseFormat.image.delivery accepts only INLINE: finished images are always returned as hosted URLs.

Output size

Other aspect ratios keep the same token count; for example 16:9 at 1K is 1376×768.

Modes

There is no separate mode parameter or editing endpoint.

To continue a conversation, rebuild the model turn from the previous task's output.parts, in order: a text part becomes {"text": ..., "thoughtSignature": ...} and an image part becomes {"fileData": {"mimeType": "image/<output_format>", "fileUri": <data[image].url>}, "thoughtSignature": ...}. Keep each thoughtSignature exactly as returned: it is the URL of the signature we stored for you (a 4K image's signature is several megabytes), and we restore it before the request reaches the model. Only signatures from your own task results are accepted.

Edit with a reference image

curl https://api.seedrouter.ai/v1/images/generations \
  -H "Authorization: Bearer $SEEDROUTER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gemini-3-pro-image",
    "contents": [{
      "role": "user",
      "parts": [
        {"text": "Turn this photo into a watercolor painting. Keep the composition."},
        {"fileData": {"mimeType": "image/jpeg", "fileUri": "https://example.com/photo.jpg"}}
      ]
    }]
  }'

Replace the example URL with your own accessible image.

Media inputs

This API accepts URL references only. Base64 inlineData, data: URLs, and multipart uploads are not accepted. The Playground uploads selected files to storage before submitting their URLs.

Reference images must be public HTTP(S) URLs to PNG, JPEG, WebP, HEIC, or HEIF files, each smaller than 50 MB and 100 MB in total. mimeType must match the file. URLs are fetched during processing; an inaccessible image fails the task, and a failed task is not charged.

Pricing dimensions

Check the model pricing section for current rates. gemini-3-pro-image charges one flat price per delivered image, whatever the size or prompt. gemini-3-pro-image-official charges by usage: input tokens (text and reference images), text and thinking output tokens, and image output tokens, each at its own rate. The image size is the main factor; see the table above.

View final charges in your account usage history. Failed tasks are not charged.

Output schema

Submission returns a task reference:

{
  "id": "task_...",
  "model": "gemini-3-pro-image",
  "status": "processing",
  "created_at": 1790310979
}

Poll the task

curl https://api.seedrouter.ai/v1/tasks/YOUR_TASK_ID \
  -H "Authorization: Bearer $SEEDROUTER_API_KEY"

Poll every few seconds until status is completed or failed. A network timeout while polling does not mean generation failed: keep the task ID and resume checking it. Do not create another task to check progress.

Complete polling example

Run this after the Python submission example above.

import time

deadline = time.monotonic() + 600
while time.monotonic() < deadline:
    result = requests.get(
        f"https://api.seedrouter.ai/v1/tasks/{task_id}",
        headers={"Authorization": f"Bearer {os.environ['SEEDROUTER_API_KEY']}"},
        timeout=30,
    )
    result.raise_for_status()
    task = result.json()
    if task["status"] == "completed":
        for image in task["output"]["data"]:
            print(image["url"])
        break
    if task["status"] == "failed":
        raise RuntimeError(task["error"]["message"])
    time.sleep(3)
else:
    raise TimeoutError(f"Still waiting. Resume polling task {task_id}.")

Completed task

{
  "id": "task_...",
  "model": "gemini-3-pro-image",
  "status": "completed",
  "created_at": 1790310979,
  "finished_at": 1790311001,
  "output": {
    "created": 1790310999,
    "data": [{"url": "https://static.seedrouter.ai/media/tasks/task_example/0.jpg"}],
    "output_format": "jpeg",
    "usage": {
      "input_tokens": 27,
      "output_tokens": 1366,
      "total_tokens": 1393,
      "output_tokens_details": {"image_tokens": 1120, "text_tokens": 95, "reasoning_tokens": 151}
    }
  }
}

Streaming (streamGenerateContent) is not supported; results are delivered through the task.

Errors

Requests rejected before a task is created return an HTTP error with an error object. A task that fails after acceptance returns HTTP 200 when queried, with status: "failed" and an error object. An image withheld by the model's safety filters fails with content_policy_violation; a response with no image fails with no_output.

See the shared error catalog for codes, HTTP statuses, and retry guidance.

{
  "id": "task_...",
  "status": "failed",
  "error": {
    "code": 60001,
    "message": "The request was rejected by the content policy. Please revise the prompt or input images."
  }
}

If submission itself times out, check your task history before submitting again: the first request may have been accepted.

Tips

• Describe subject, setting, lighting, and style in full sentences.
• For an edit, say what should change and what must stay the same.
• 2K costs the same image tokens as 1K; use 4K for print-sized assets.
• Save returned images to your own storage when you need a durable copy.

Related

• Nano Banana Pro Playground and pricing
• Download OpenAPI
• Copyable Markdown