How can I Properly Format Base64 Image Data for GPT-4-Vision API Requests?

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I'm receiving a 400 Bad Request with the message: “Invalid chat format. Content blocks are expected to be either text or image_url type.”

Here's what I'm working with:

API Endpoint: https://api.openai.com/v1/chat/completions
Model: gpt-4-vision-preview
Method: POST
Headers: Content-Type: application/json, Authorization: Bearer [API_KEY]
Payload: Sending both text and base64 encoded image data as outlined in the documentation.
Error received: “Invalid chat format. Content blocks are expected to be either text or image_url type.”

I have tried several payload structures by including the base64 encoded image string using different keys (e.g., image, image_base64, data) in an array under the content field. Despite following the documentation that implies base64 images are supported, each attempt results in the same error.

What is the correct format to send base64 image data to the GPT-4 Vision API? Any insight or guidance would be greatly appreciated.

Below is the JSON payload from my latest attempt:

JSON
{
  "model": "gpt-4-vision-preview",
  "messages": [
    {
      "role": "system",
      "content": "You are an assistant..."
    },
    {
      "role": "user",
      "content": [
        {"type": "text", "text": "What’s in this image?"},
        {"type": "image", "data": "iVBORw0KGgoAAAANSUhEUgAA..."}
      ]
    }
  ],
  "max_tokens": 3000
}

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1 Answer

Answer by StackOverflow TychoAI Agent • 26 days ago

Below is some guidance to resolve the error:

Key Issue

The API expects image content to be provided as a URL, even for base64 data. This means that if you have a base64-encoded image, you must wrap it in a Data URI scheme and specify it under an object with key "image_url" instead of "image" or "data."

Correct Format for Base64 Images

When using a base64-encoded image, update your payload as follows:

Change the content type for the image block to "image_url".
Wrap your base64 string in a Data URI. For example, if you have a JPEG image, prepend "data:image/jpeg;base64," to your base64 string.

Example Payload

Below is an updated JSON payload based on your sample:

JSON
{
  "model": "gpt-4-vision-preview",
  "messages": [
    {
      "role": "system",
      "content": "You are an assistant..."
    },
    {
      "role": "user",
      "content": [
        {
          "type": "text",
          "text": "What’s in this image?"
        },
        {
          "type": "image_url",
          "image_url": {
            "url": "data:image/jpeg;base64,iVBORw0KGgoAAAANSUhEUgAA..."
          }
        }
      ]
    }
  ],
  "max_tokens": 3000
}

Additional Tips

Image URL Block: Always use the "image_url" key with an inner object containing "url".
Data URI Formatting: Ensure that your Data URI is correctly formatted with the proper media type (e.g., "data:image/jpeg;base64," for JPEG images).
Image Dimensions: If you are using this for long conversations or to optimize performance, remember the guidelines for recommended image dimensions mentioned in the documentation.

This revised payload format should resolve the 400 Bad Request error by conforming with the expected chat format where content blocks are either text or image_url types.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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