Claude alt text generator

This route accepts a written description of an image and returns an alt attribute value. It is a generator in the strict sense: the request goes to our server with a brief, and the model returns finished content with no preamble, commentary or markdown fence around it. Input is bounded at 1 to 12,000 characters and the allowance is 5 runs per rolling 24 hours.

What it does not do is look at a file. There is no image pipeline on this route, so the picture is never analysed, never stored and never seen. That is a design consequence worth being explicit about, because every hosted competitor for this term is built the other way round.

The output is bounded by the description, so the first step is where the quality is decided.

Step 1

State the contents and the function separately. Contents is what a camera recorded; function is why an editor put it on this page. Both are needed and only one is visible in the file.

Step 2

Run it, adding an instruction if you need a particular register. The instruction is appended to the system brief, so it shapes the output rather than being answered as a question.

Step 3

Verify against the image. The tool cannot perform this check, which makes it the one step that cannot be automated away here.

AltText.ai takes PNG, JPG, GIF, WebP and BMP up to 16MB and will scan 25 pages of a site on its free tier. The Ahrefs generator takes JPG, PNG or GIF under 10MB with an optional 2,048-character description field and a tone selector. Both infer the description from the image itself, which is the obvious architecture and the reason the category looks the way it does.

The weakness is structural rather than a question of model quality. Pixels encode contents, not purpose. A published Claude alt text skill on karozieminski.substack.com names the same failure in its rules: a chart showing revenue growth matters more than a bar chart with blue bars. Only the page's author knows which of those two sentences is correct, and supplying that judgement costs a sentence of typing.

The working target is 125 characters, and the basis is screen reader behaviour: announcements are widely reported to be chunked around that length, so an attribute longer than it can be truncated for the reader who depends on it. That is a behavioural convention, not a conformance requirement. WCAG specifies no character limit anywhere.

It is worth noting who does not say this. Neither of the hosted generators read for this page states a target length at all: AltText.ai gives none, Ahrefs gives none and offers only 'informative and concise'. A generator that will not tell you how long the output should be has left the only measurable part of the brief undefined.

Four tests, each of which can be applied to a generated line in a couple of seconds.

No redundant opener

A screen reader announces the element type before the attribute, so 'image of' and 'photo of' restate what the listener was just told.

Specificity over evaluation

Counted, named subjects carry information. Adjectives about quality carry the author's opinion, which is not what the attribute is for.

Function stated, not implied

If the sentence would be equally correct on a different page, it has described the file rather than the image's role in this document.

No duplication of the caption

The caption and the attribute are announced consecutively. Overlap is dead air for the listener.

A decorative image takes alt="", which instructs a screen reader to pass over it. Describing it is not a harmless extra; it inserts noise into a linear reading of the page. The test is whether the image carries information the surrounding text does not, and an icon repeating an adjacent label fails that test.

None of the pages read for this route raises the point, and the commercial reason is not hard to see: metering is per image, and a page explaining that a share of your images need nothing described reduces the count. Stating it here costs us nothing, because the allowance is fixed at 5 runs whether you use them well or badly.

It converts an image into indexable text, which is a genuine mechanism. Quantifying the effect is where claims start outrunning evidence, which is why even Ahrefs hedges to 'potentially driving more organic traffic'. We state the mechanism and stop.

The related claim to be wary of is compliance. AltText.ai markets on WCAG and ADA conformance across thousands of images. Conformance is a property of a page, judged by whether the alternative serves an equivalent purpose in context. A generator that never sees the page cannot establish that, and neither can this one.

No image upload, no drag and drop, no image URL. No bulk queue, no CSV, no folder processing, no site crawl. No CMS integration, no browser extension, no API and no MCP server. AltText.ai advertises more than twenty integrations and 130 or more languages; this route makes no language claim, because nothing in the system establishes one.

One output per run, not a set of variants. No tone dropdown, though a free-text instruction does reach the brief. No stored history, no account and nothing to buy when the 5 runs are gone. The list is longer than most competitors would print, and printing it is the point: the things above are absent by design, not pending.