Glossary
Accessibility tree
The browser's structured list of named controls. A browser agent reads it so a click can name a button instead of guessing a pixel.
The accessibility tree is the browser’s structured list of what a page exposes to assistive technology: roles, names, and states. A browser agent reads that list so the next tool can target a control by reference.
Anthropic’s browser-use tool, read November 13, 2026, says read_page returns the tree as text, with each element tagged by a reference such as [ref_2]. A later left_click or form_input can pass that reference. The same page says the computer-use tool does not work this way. It uses screenshots and coordinates only. That tool is computer use. The web-page tool is browser use.
Browser Use 1.0, published November 10, 2026, says its ultrafast mode works with accessibility-tree ids. The closed list of calls the model may make against those ids is the action space. How the loop uses the tree, where a screenshot is still required, and what to put in the tree if the page is yours, is How do AI browser agents complete tasks?.