Definition
Plain language
The hidden text description attached to an image so screen readers and software can tell what the picture shows.
As stated in the literature
Alternative text metadata associated with an image element; treated by agents as low-authority page content, which is why alt-text prompt injections landed far less often than in-body text in the study.
Also called: alt text, image alt-text, image alt text
Why it matters: It lets people who can't see images still understand what a picture shows, and its low-authority treatment by AI agents affects how easily hidden instructions inside it can slip through.
For example, a photo of a golden retriever catching a ball might carry the hidden alt-text "a dog catching a red ball in a park," which a blind user's screen reader reads aloud.
Heard on the show
“Image alt-text — the little description tucked behind an image — only landed sixteen percent of the time, because the agents treat alt-text as low-authority page metadata, not instructions.”Episode 227 — Poisoned Bug Reports Fooled Coding Agents Two Times Out of Three