Image alt-text generation APIs

If you want to automatically generate descriptive image alt text for accessibility and SEO, there are several strong API options. The main distinction is between vision-language APIs that write natural-language descriptions and computer vision APIs that identify objects, labels, and text.

For most websites, I recommend a vision-language API that can generate a concise, context-aware sentence, followed by a review step before publishing. Microsoft's documentation specifically describes automated image captioning as a way to generate alt text for accessibility and image SEO.learn.microsoft.com+1

Best APIs for automatic alt text

1. OpenAI API

(Best for natural descriptions)

Use a vision-capable model to analyze an image and generate a concise alt attribute tailored to its purpose. You can instruct it to describe the main subject, relevant action, setting, and important visual details without keyword stuffing.

• Best for: E-commerce, blogs, editorial images, and custom content pipelines.
• Strengths: Flexible descriptions, prompt control, and integration with other text-generation workflows.
• Watch out for: Descriptions may occasionally infer details that are not visible.
• Docs: Images and vision API guide.learn.microsoft.com+1

2. Azure AI Vision

(Best for built-in captioning)

Its Image Analysis API can generate human-readable image captions and return descriptive tags. This is a natural fit when your primary requirement is a straightforward image description.

• Best for: Content management systems, media libraries, and enterprise applications.
• Strengths: Dedicated captioning features and confidence scores.
• Watch out for: Captioning availability depends on supported regions and languages.
• Docs: Generate alt text.learn.microsoft.com+2

3. Google Cloud Vision API

Best when you need image labels, object recognition, and OCR to support an alt-text workflow. You can combine its analysis results with a language model to produce a natural sentence.

• Best for: Large image libraries, tagging pipelines, and images containing text.
• Strengths: Label detection, object localization, and text detection.
• Watch out for: Labels alone are not finished, reader-friendly alt text.
• Docs: Cloud Vision documentation.docs.cloud.google.com+2

4. Amazon Rekognition

A computer vision API that detects labels, objects, and text. It works well as an input to a separate alt-text generation step.

• Best for: AWS-hosted websites, image processing pipelines, and large-scale media catalogs.
• Strengths: Label confidence scores, object bounding boxes, and text detection.
• Watch out for: You will generally need another step to turn labels into polished descriptions.
• Docs: Rekognition API reference.docs.aws.amazon.com+2

Which API should you choose?

My default recommendation is OpenAI for generating the final text, or Azure AI Vision if you prefer a dedicated captioning feature. Google Cloud Vision and Rekognition are particularly useful when image labeling and OCR are also part of your requirements.

Example: Generate alt text automatically

With a vision-language API, you can pass an image and ask for a short description suitable for an HTML alt attribute.

Example prompt:

ALT-TEXT GENERATION PROMPT

Generate concise, accurate alt text for this image for a website.

• Describe the main subject and any meaningful action.

• Include relevant visual details that help convey the image's purpose.

• Use the surrounding page context when provided.

• Do not start with "Image of" or "Picture of."

• Do not invent details that cannot be verified visually.

• Avoid keyword stuffing and keep the result concise.

• If the image is purely decorative, recommend empty alt text.

  [Copy prompt]

For example, an image of a person using a laptop in a coffee shop might receive:

• Good alt text: Person working on a laptop at a café table beside a window.
• Poor alt text: Laptop computer café business work productivity best laptop deals SEO.

Recommended production workflow

1. Image uploaded

Receive the image and its page or product context.

2. AI generates alt text

Return a short description as structured output.

3. Validate the result

Check accuracy, context, length, and sensitive or uncertain claims.

4. Save to your CMS

Store the description and render it in the HTML alt attribute.

For example, the final HTML might look like this:

<img
  src="/images/workspace.jpg"
  alt="Person working on a laptop at a café table beside a window."
>

Important accessibility and SEO considerations

• Use context: A product image should describe the product and relevant distinguishing features; an editorial image should describe what matters in the story.
• Don't force alt text on every image: Decorative images generally need alt="" so screen readers can skip them.
• Review important images: AI can misidentify objects, text, and visual relationships. Human review is especially useful for product listings and information-rich graphics.
• Don't keyword-stuff: Alt text should communicate meaningful image content to people using assistive technology, not function as a list of SEO keywords.
• Measure the right outcomes: Better alt text supports accessibility and helps search engines understand images, but it does not guarantee higher rankings.

One question to help me narrow this down: What platform are you building on?

1. Website or technology

WordPress

Shopify / e-commerce

Custom website / API

Another CMS

2. Approximate image volume

Under 100 images

Hundreds of images

Thousands of images

Continuous new uploads

3. Desired automation

Generate and publish automatically

Generate, then review before publishing

[Find the best solution]