The last three Runtime Snapshots posts built a stack. #15 said your agent is blind and needs eyes and hands. #16 named the three ways it can see: vision, accessibility tree, and runtime perception. #17 asked what happens when more than one agent shares the same live browser space. This post goes underneath all of them, to the question those posts assume an answer to: what does an agent actually perceive in the half-second before it acts?
The asymmetry nobody states
Most browser-agent failures get filed as model failures. The model clicked the wrong button, missed the menu, filled the wrong field, thought the page had loaded when it hadn't, couldn't recover when the UI shifted. Sometimes that diagnosis is right. Often the model was handed the wrong surface and asked to reason from it.
Here's the part that gets skipped: agents don't fail the way people do. You open a tab and quietly compensate for everything the page doesn't say out loud - a greyed-out button you don't click, a spinner still turning so you wait, a modal over the page so you know the thing underneath isn't live yet. You get that continuity for free, from being a human looking at a rendered page. An LLM gets none of it for free. It gets exactly the representation we hand it, and nothing else. So the representation is the whole game.
