AI Alt Text for OpenCart Product Images: Describe Every Photo in Every Language With Claude (Free Extension)

Open any product page in OpenCart 4’s default theme and look at the image tags. The main image, every additional image and every thumbnail in product lists has the same alt text: the product name. This is catalog/view/template/product/product.twig in OpenCart 4.1:

<img src="{{ thumb }}" title="{{ heading_title }}" alt="{{ heading_title }}" class="img-thumbnail mb-3"/>

So a product with five photos, showing the front, the back, the label, the size chart and someone wearing it, tells a screen reader user “Linen Shirt, Linen Shirt, Linen Shirt, Linen Shirt, Linen Shirt”. Google Images learns nothing about what each photo shows.

Writing alt text by hand for every image in every language is the job nobody gets to. This free extension has Claude look at each product image and suggest a description in each of your store’s languages. You read the suggestions, edit them, and save. The storefront then uses them in place of the product name.

What good alt text is

The W3C’s alt text decision tree gives simple rules. For an image that carries information:

Use a brief description of the image in a way that conveys that meaning in the alt attribute.

W3C Web Accessibility Initiative, An alt Decision Tree

And for purely decorative images, “Use an empty alt attribute.” Product photos are never decorative: they’re the main information on the page for many shoppers. Some examples:

Image	Bad alt	Good alt
Main photo of a shirt	Linen Shirt	Men's white linen shirt, front view, button-down collar
Back view	Linen Shirt	Back of the white linen shirt with a box pleat
Label close-up	Linen Shirt	Care label: 100% linen, machine wash 30°C
Size chart	Linen Shirt	Size chart: chest and length in cm for sizes S to XXL
Lifestyle photo	Image of linen shirt	Man wearing the linen shirt with rolled sleeves on a beach
Any photo	best linen shirt cheap linen shirt buy	(keyword stuffing helps nobody and can hurt)

Alt text is required for accessibility under WCAG success criterion 1.1.1 (Non-text Content), which is the basis of most accessibility laws for online stores. Our post on accessibility for ecommerce stores covers the legal side. It also helps image search: Google uses alt text to understand images.

How the extension works

1. In the admin, the Images tab lists your products with all their images (main image first) and one text field per language for each image.
2. Click Generate next to an image, or Generate missing on this page to do every image that has an empty field.
3. The extension scales the image down to at most 1,024 pixels, sends it to Claude with the product name and model, and asks for alt text in each selected language, returned as structured JSON (one string per language).
4. The suggestions appear in the fields, highlighted. Nothing is saved yet.
5. Read them, fix anything wrong, and click Save alt text.
6. With Show on store on, product pages and product lists use the saved text instead of the product name.

The storefront part uses two OpenCart events (product/product and product/thumb view output). It finds each product image’s resized URL, such as image/cache/catalog/demo/shirt-1-500x500.webp, maps it back to the original image (catalog/demo/shirt-1), and replaces the alt attribute with the saved text for the visitor’s language. If no text is saved, the product name stays, as before. The match ignores the file extension, so it keeps working with our WebP and AVIF extension.

Why there’s a review step

AI image descriptions are good, and sometimes wrong in ways that matter for a store: a color that looks different under studio light, a material that can’t be seen in a photo, a model number guessed from a similar product. The prompt tells Claude to use the product name for brand and model and not to guess details it can’t see, but you know your products. Anthropic’s own vision documentation says: “Always carefully review and verify Claude’s image interpretations, especially for high-stakes use cases.” Reviewing a suggestion takes seconds; writing from scratch takes minutes.

Installation

1. Create an API key in the Claude Console under API Keys (it starts with sk-ant-), and set a spend limit.
2. Download ai_alt_text.ocmod.zip with the button above.
3. Extensions > Installer, upload, and click the green Install button in the list.
4. Extensions > Extensions > Modules, find AI Image Alt Text, click +, then the pencil.
5. On the Settings tab, paste the API key, choose the model and languages, and save.
6. On the Images tab, try Generate on a few images first.
7. When you’re happy, switch Show on store on in Settings.

Installing creates one table (oc_image_alt) and two events. Uninstalling removes them and all saved texts, so export your database first if you might reinstall.

Settings

Setting	What it does
Anthropic API key	Used only from the admin, never on the storefront
Model	claude-opus-5-5 (default), claude-sonnet-5-5 or claude-haiku-5-5
Languages	Which languages to write. None ticked means all enabled languages.
Extra instructions	Store-specific guidance, for example “Mention the fabric for clothing” or “We sell refurbished phones; say Refurbished”
Show on store	Use the saved texts on the storefront

What it costs

You pay Anthropic per request with your own key; there’s one request per image, covering all languages. Images cost tokens by size: Anthropic’s vision documentation counts a 28 × 28-pixel patch as one token, so the 1,024-pixel square image the extension sends is about 1,400 tokens. With the prompt and the answer, a rough estimate per image at October 2026 list prices:

Model	Price per 1M input / output tokens	About per 1,000 images
Claude Opus 5.5	$4 / $20	$12 to $16
Claude Sonnet 5.5	$2 / $10	$6 to $8
Claude Haiku 5.5	$0.10 / $0.50	under $1

More languages mean more output tokens, and Claude’s thinking is billed as output too, so treat these as ballpark figures. Try 20 images with each model and compare the texts and the cost in the Claude Console before running your catalog.

Tips for better results

• Filter by “Only products whose main image has no alt text” to work through the catalog in batches of 10 products.
• Use extra instructions for your category: furniture stores might ask for material and dimensions when visible; fashion stores for fit and how it’s worn.
• Check additional images first: main images are often similar packshots, while additional images (labels, details, size charts) are where alt text adds the most.
• Product names still matter: the extension doesn’t change titles. For names, descriptions and meta tags, see our AI description generator extension.

Privacy

Product images are sent to Anthropic’s API to generate the text. According to Anthropic’s documentation, images sent through the API aren’t used to train models. Only send images you have the right to process this way, which for product photos from suppliers is usually covered by your reseller agreement; check if unsure.

Limitations

• Product images only: banners, category images, manufacturer logos and images inside descriptions aren’t included.
• Themes that build image URLs differently from OpenCart’s resize (image/cache/...-WIDTHxHEIGHT.ext) won’t be matched. Check one product page after switching it on.
• The admin list shows 10 products per page; very large catalogs take a while.
• Built for OpenCart 4.1.0.x.

The code is on GitHub with our other free OpenCart extensions. For more on AI and product data, see AI shopping product feeds and how AI crawlers read your website.

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