The tool
What the Claude alt text generator does
Type what is in your image and why it is on the page. The Claude alt text generator returns one short sentence that names what matters, leaves out what does not, and reads properly when a screen reader announces it.
The writing happens on our server, so the text you paste is sent to us. That buys 5 free runs every rolling 24 hours, no account, and an input of 1 to 12,000 characters. The output arrives as bare text with no preamble and no quotation marks around it, ready to paste into an alt attribute.
The difference
This alt text generator reads a description, not an image
Every hosted tool competing for this term takes a file. AltText.ai accepts PNG, JPG, GIF, WebP and BMP up to 16MB and will also scan 25 pages of a site. Ahrefs takes a JPG, PNG or GIF under 10MB. Both run a vision model over the pixels. This route does not: no upload, no drag and drop, no image URL field.
That sounds like a limitation until you look at what alt text is for. A vision model can tell you there is a bar chart with blue bars. It cannot tell you the chart is there because revenue grew, which is the sentence a listener needs. The published Claude alt text skill on karozieminski.substack.com states the same rule bluntly: context over appearance. You already know the context, and it is the part no image analysis can recover.
How it works
How to generate alt text in three steps
The quality of the sentence you get back is set almost entirely by step one.
- Step 1
- Describe the image in plain words, then say why it is on the page. A photograph illustrating a news story and the same photograph in a product listing need different alt text.
- Step 2
- Run it. If you want a particular angle, say so in your own words in the instruction field, because that instruction is added to the brief the server works from.
- Step 3
- Read the result against the actual picture before you paste it. You can see the image and the tool cannot, so you are the check.
Length
How long should alt text be?
The usual target is 125 characters, and the reason is mechanical: screen readers are widely reported to chunk announcements around that length, so a longer description can be cut mid-sentence for the person who most needs it. Neither AltText.ai nor the Ahrefs generator states a number anywhere on its page, which is why the question keeps getting asked.
It is a convention, not a rule. WCAG sets no character limit. Treat 125 as the target, go longer only when the image carries essential detail, and when it genuinely will not fit, stop stretching the attribute.
That is the case with a chart or a diagram. Name the chart type and the direction of the trend in the alt attribute, then write the actual numbers into the body copy beside it, where a screen reader user, a reader on a slow connection and a crawler can all reach them.
The craft
What makes alt text good
Four habits do most of the work, and you should be able to spot them in the output.
- Do not open with 'image of'
- A screen reader has already said this is an image. Starting with 'image of' or 'photo of' spends your first words repeating what the listener was just told.
- Be specific, not evocative
- 'Runners training' says almost nothing. Four women doing lunges beside a track says something. Words like stylish and stunning describe your opinion, not the picture.
- Say what it is doing there
- The same photograph earns different alt text in an article and a shop listing, which is why the tool asks you for the purpose as well as the contents.
- Do not repeat the caption
- If the caption beside the image already describes it, the alt text should not repeat it. A screen reader user hears both in a row.
The common mistake
Images that need no alt text at all
A divider, a background flourish, or an icon beside a label that already says the same word should carry an empty alt attribute instead of a description. Writing alt="" tells a screen reader to skip it. Writing a description forces someone to listen to a sentence about a decorative line.
None of the hosted generators we read mentions this, and the reason is structural: a tool metered by the image has no incentive to say that some of your images need none. Sort decorative from informative first, then generate for the informative ones.
The honest answer
Does alt text actually help SEO?
It gives a crawler words where there was only a picture, and that is worth doing. What we will not do is put a number on it. Ahrefs hedges its own version as 'potentially driving more organic traffic', which is about as far as anyone honestly can go.
Write the sentence for the listener and a search engine gets a usable description as a by-product. Write it for the search engine and you get keyword soup that fails the only audience the attribute exists for.
Limits, stated
What this Claude alt text generator will not do
No image upload, no folder, no CSV, no crawler. AltText.ai advertises bulk CSV import, Dropbox and Cloudinary sources and more than twenty integrations including WordPress, Shopify, a REST API and browser extensions. This route is one web page talking to one endpoint, one pasted run at a time.
It also cannot certify compliance, which is what AltText.ai sells on. Whether a description serves a purpose equivalent to the image depends on the page it sits on, and this tool never sees that page. What comes back is a well-written draft, and the comparison against the real picture is the step that makes it accessible.