🤖 💬 AI Alt Text

Generate alt text for CraftCMS Asset Images using OpenAI's API.

Plugin Store | GitHub Repository

Video demo

Watch on GitHub

📋 Requirements

This plugin requires:

• Craft CMS 5.0.0 or later
• PHP 8.2 or later
• An OpenAI API key

📥 Installation

You can install this plugin from the Plugin Store or with Composer.

📦 With Composer

Open your terminal and run the following commands:

# tell Composer to load the plugin
composer require heavymetalavo/craft-aialttext
# or
ddev composer require heavymetalavo/craft-aialttext

Then:

# tell Craft to install the plugin
./craft plugin/install ai-alt-text
# or
ddev craft plugin/install ai-alt-text

🤖 Setup OpenAI API Key

1. Visit https://platform.openai.com/ and sign up in the top-right.
2. Revisit the API platform home page again
3. Click the ⚙️ icon in the top-right
4. Left menu > Organization > API keys > + Create new secret key (top-right)
5. Create a name and assign to a suitable project
6. Permissions > Restricted > set "Model capabilities" to "Write" and "Responses API" to "Write"
7. Save API key to an env var, you wont get to see it again!
8. Make sure you have a credit balance! Left menu > Organization > Billing, loading $5 is probably going to get you quite far, disabling auto recharge might be safer though that's up to you!

🚀 How to use

1. Check the plugin settings are suitable for your project (and your API key is added)
2. Ensure your volumes have the native alt field assigned to the field layout
3. Ensure your templates are updated to use the alt field, you could consider a fallback asset.alt ?: asset.title if that what was used before
4. Then generate some AI Alt text by performing one of the following actions:
   1. Triggering a bulk action in the bulk actions table
   2. For individual or a group of specific assets find them in the Assets manager section clicking the checkbox on a row, clicking the cog icon to reveal the Element actions menu and select Generate AI Alt Text
   3. When viewing a single asset's page, open the action menu and select Generate AI Alt Text
   4. Upload a new asset (if the upload setting is enabled)
5. The plugin will queue jobs to generate alt text for each selected asset

Example twig:

{% set asset = craft.assets.one() %}
<img src="{{ asset.url }}" alt="{{ asset.alt ?: asset.title }}">

⚙️ Plugin settings

After installation, configure the plugin at Settings → AI Alt Text:

📊 Settings overview

Setting	Description	Default
OpenAI API Key	Your OpenAI API key. You can get one from OpenAI's API Platform.	None (required)
Prompt	The text prompt sent to the AI to generate alt text. Supports {asset.property} and {site.property}	See below
Open AI Model	The OpenAI model to use for generating alt text.	gpt-4.1-nano
Open AI Image Input Detail Level	How detailed the image analysis should be.	low
Propagate	Whether the asset should be saved across all of its supported sites, if enabled it could save the same initial alt text value across all sites.	false
Generate for new image assets (on upload)	Whether to automatically generate alt text when new assets are created.	false
Save translated results for each site	Whether to save translated results to an Asset's translatable alt text field for each site.	false

🧠 Model Options

Some models that support vision capabilities:

• gpt-4.1-nano - Fast, affordable small model for focused tasks (default)
• gpt-4o - Fast, intelligent, flexible GPT model
• o1 - High-intelligence reasoning model

To find out which models are capable of vision, check the models page, click into a model's detail page (e.g., GPT-4.1-nano) and look for "Input: Text, image" in the features columns at the top.

💬 Default prompt

Generate a brief (roughly 150 characters maximum) alt text description focusing on the main subject and overall composition. Do not add a prefix of any kind (e.g. alt text: AI content) so the value is suitable for the alt text attribute value of the image. Output in {site.language}

🔍 Image detail options

• low - Less detailed, faster and cheaper (default to protect against unexpected costs)
• high - More detailed, slower and more expensive (higher resolution analysis)
• auto - Let OpenAI decide

For more information about these settings, refer to the OpenAI API documentation.

🏷️ Field requirements

This plugin requires a native CraftCMS field for alt text with the handle alt to be added to all asset volumes where you want to generate alt text. The plugin will use this field to store the generated alt text.

To add this field:

1. Go to Settings → Assets → Volume name → + Add → search for the alt field and click → save
2. Scroll to Field Layout section
3. Click the + Add button
4. Search for the alt field and click
5. Save changes to the volume
6. Update your templates to use the new alt field

Limitations

• The OpenAI API has image input requirements which have changed in the past month (2025-05), however these requirements don't appear to be enforced, e.g. sending a base64 image above required image dimensions will be accepted by the API.
• Where an unsupported file type is requested the plugin will attempt an image transform to a jpg to be sent instead
• The plugin checks a file's mimetype to see if it's valid, a filename which contains the wrong extension could return the wrong file type until Craft v5.8.0 is released
• If an asset's dimensions are larger than the dimensions required by the API an image transform is sent instead
• If an asset has no URL (private) and requires a transform (e.g. if the original asset is an unsupported mime type, or, the dimensions are too large) the plugin cannot retrieve the transform's file contents to send a base64 encoded version of the image to the OpenAI API.
• Where an alternative image transformer is used, e.g. when an application is hosted on Servd and assets are processed through their asset platform this may not support svg -> raster transforms

Supported file types

• PNG (.png)
• JPEG (.jpeg and .jpg)
• WEBP (.webp)
• Non-animated GIF (.gif)

Size limits

• Up to 20MB per image
• Low-resolution: 512px x 512px
• High-resolution: 768px (short side) x 2000px (long side)

Other requirements

• No watermarks or logos
• No text
• No NSFW content
• Clear enough for a human to understand

🛠️ Troubleshooting

• If the plugin returns errors about API authentication, verify your API key.
• For "bad request" errors, ensure your selected model supports vision capabilities.
• Alt text generation is processed through Craft's queue system for bulk operations, so check the queue if generation seems to be taking a long time.
• Any errors should be logged, check your queue.log files!

⚠️ Disclaimer

We've taken some steps to try prevent unexpected costs with default plugin settings (e.g. detail: low and model: gpt-4.1-nano) though we take no responsibility for excessive API token usage that may result from mistakes, bugs, or security vulnerabilities within this plugin so use at your own risk.

If you are concerned about unexpected charges we recommend:

• Set up rate limits and spending caps at the API account level in your OpenAI account settings
• Start with smaller batches when using bulk generation until you're comfortable with the costs
• Consider using the default low setting, which significantly reduces token usage
• Monitor your OpenAI API usage regularly

📈 Example usage statistics

When testing using the default settings (gpt-4o-mini model, low detail level):

Metric	Value
March budget	$0.03 / $120
Total tokens used	163,713
Total requests	29

🙋 Support

If you encounter any issues or have questions, please submit them on GitHub.

Credits

The eye icon used in this project is from SVG Repo and is available under the CC0 1.0 Universal (Public Domain) license.