AI Mastodon Image Caption Generator

Alt text is the point here, not an extra

Mastodon has an unusually strong accessibility culture. Posting an image without alt text is noticed, commented on and occasionally called out, in a way that simply does not happen on commercial platforms. That makes an image reading tool useful here for a different reason than elsewhere: the alt text is not a bonus output, it is arguably the main one. The post text matters too, but the community norm is clear enough that many users will not boost an image post that has no description attached to it.

How to write good alt text

The point is conveying what a sighted viewer takes from the image.

• Say what the picture contains in plain words, the way you would explain it to someone who cannot see the screen.
• Transcribe any text in the image, since a screen reader cannot extract words from a picture.
• Describe the finding for charts rather than the chart type, because the data is the information.
• Skip "image of" and "photo of", which screen readers already announce.
• Include detail that carries meaning and leave out detail that does not, since exhaustive alt text is tiring to listen to.
• Write it every time. Mastodon has no automatic fallback worth relying on, and the community expects it.

Fediverse conventions around images

Several norms apply here that are stronger than on other platforms.

• Content warnings are used broadly, covering politics, food, eye contact in photos and long posts, not only distressing material.
• Photos where someone is looking directly at the camera are often given an eye contact warning, which surprises newcomers.
• CamelCase hashtags are expected so screen readers pronounce multi word tags correctly.
• Tagging is essential for discovery, since Mastodon has no algorithm surfacing your post to anyone.
• Boosting is how content spreads, and a well described image post is considerably more likely to be boosted.

Who uses this tool

People who want to meet the norm and are unsure how.

• New Mastodon users who have been asked for alt text and did not know it was expected.
• Photographers and artists posting work who need every image described.
• Academics and researchers sharing charts and figures where the data is the content.
• Open source projects posting screenshots of interfaces and releases.
• Anyone who wants to write alt text properly and finds starting from blank difficult.

Mastodon image post specs

The defaults and the conventions.

Instances set their own character limits and some have explicit rules about alt text and content warnings. Checked September 2026.

When this is not the right tool

Check the generated alt text rather than pasting it unread. An AI description that is slightly wrong is worse than a short accurate one written by you, because the person relying on it has no way to know. It also cannot judge whether your image needs a content warning, which depends on community norms and on your instance rules rather than on the picture alone. For text posts the Mastodon caption generator applies the same conventions, and the Mastodon hashtag generator handles CamelCase tagging.