What is token budget?
A token budget caps the serialized page state regardless of the page’s real size. Elements are indexed in priority order and the list is trimmed to fit, so the planner’s prompt has a known upper bound rather than one set by whatever the site happens to render.
Why it matters
It is what makes cost predictable on pages you do not control. The trade is real and worth stating: on a very large page the trimmed tail is genuinely gone, which is why a long listing is better read with row expansion than with a whole-page plan.