Glossary
Action space
The closed list of actions a harness lets a model request on a page or a screen, such as click, type, scroll, or navigate.
An action space is the set of actions a harness will accept from the model. If the action is not in the set, the model cannot take it, no matter how it describes the goal.
Anthropic’s browser tool, read November 13, 2026, declares 31 member tools and leaves four off unless you enable them, including javascript_exec and file_upload. The default calls include navigate, read_page, left_click, type, and form_input. Google’s computer-use page, last updated 2026-11-12 UTC, lets the client remove verbs with excluded_predefined_functions. A click in that set still needs a target, either a reference from the accessibility tree or a coordinate.
Each call spends the tool-call budget. A wider list is a larger attack surface, because a page the agent is reading can try to steer the next call. The loop those calls sit in is How do AI browser agents complete tasks?. The general mechanism is tool use. The web-only case is browser use.