OpenRouter Gemini Image Generation

Problem

OpenRouter's Gemini image models (Nano Banana, Nano Banana Pro) don't work with the standard

/images/generations

endpoint. Using this endpoint returns 404 errors, and even when the endpoint seems correct, aspect ratio validation and response parsing can fail.

Context / Trigger Conditions

• Error:

  OpenRouter API error 404 Not Found

  when calling

  /api/v1/images/generations

• Error:

  400 Bad Request

  with message about invalid aspect_ratio values
• Error:

  Failed to parse response: error decoding response body

• Using models:

  google/gemini-3-pro-image-preview

  (Nano Banana Pro) or

  google/gemini-2.5-flash-image

  (Nano Banana)

Solution

1. Use Chat Completions Endpoint

Instead of

/api/v1/images/generations

, use

/api/v1/chat/completions

:

let response = client
    .post("https://openrouter.ai/api/v1/chat/completions")
    .header("Authorization", format!("Bearer {}", api_key))
    .header("Content-Type", "application/json")
    .json(&request)
    .send()
    .await?;

2. Include Modalities Parameter

The request must include

modalities: ["image", "text"]

:

{
  "model": "google/gemini-3-pro-image-preview",
  "messages": [
    {"role": "user", "content": "Create a professional blog header image..."}
  ],
  "modalities": ["image", "text"],
  "image_config": {
    "aspect_ratio": "16:9"
  }
}

3. Use Valid Aspect Ratios Only

Valid values:

1:1

,

2:3

,

3:2

,

3:4

,

4:3

,

4:5

,

5:4

,

9:16

,

16:9

,

21:9

Invalid values that will cause 400 errors:

• 1.91:1

  (use

  16:9

  instead for social media images)
• Custom ratios like

  1200x630

  format

4. Parse Nested Response Structure

Images are nested in the response:

{
  "choices": [{
    "message": {
      "role": "assistant",
      "content": "I've generated the image.",
      "images": [{
        "type": "image_url",
        "image_url": {
          "url": "data:image/png;base64,iVBORw0KGgo..."
        }
      }]
    }
  }]
}

Extract image bytes from:

choices[0].message.images[0].image_url.url

Verification

• API returns 200 status
• Response contains

  choices[0].message.images

  array with at least one image
• Image URL starts with

  data:image/png;base64,

  (or similar)
• After base64 decoding, bytes represent valid image data

Example

// Request structure
#[derive(Serialize)]
struct ChatCompletionsRequest {
    model: String,
    messages: Vec<ChatMessage>,
    modalities: Vec<String>,
    #[serde(skip_serializing_if = "Option::is_none")]
    image_config: Option<ImageConfig>,
}

#[derive(Serialize)]
struct ImageConfig {
    aspect_ratio: String,
}

// Response structure
#[derive(Deserialize)]
struct ChatCompletionsResponse {
    choices: Vec<ChatChoice>,
}

#[derive(Deserialize)]
struct ChatChoice {
    message: AssistantMessage,
}

#[derive(Deserialize)]
struct AssistantMessage {
    #[serde(default)]
    images: Vec<ImageObject>,
}

#[derive(Deserialize)]
struct ImageObject {
    image_url: ImageUrlData,
}

#[derive(Deserialize)]
struct ImageUrlData {
    url: String,  // "data:image/png;base64,..."
}

// Usage
let request = ChatCompletionsRequest {
    model: "google/gemini-3-pro-image-preview".to_string(),
    messages: vec![ChatMessage {
        role: "user".to_string(),
        content: prompt.to_string(),
    }],
    modalities: vec!["image".to_string(), "text".to_string()],
    image_config: Some(ImageConfig {
        aspect_ratio: "16:9".to_string(),
    }),
};

Notes

• Nano Banana Pro (

  google/gemini-3-pro-image-preview

  ) is paid ($2/M input, $12/M output tokens)
• Nano Banana (

  google/gemini-2.5-flash-image

  ) is cheaper ($0.30/M input, $2.50/M output tokens)
• Image generation can take 20-30+ seconds
• The

  content

  field in the response may contain descriptive text about the generated image

References

• OpenRouter Image Generation Docs
• Nano Banana Pro Model Page
• Nano Banana Model Page