Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so the safety profile is covered. The description adds the useful fact that the image is a crop of the visible rectangle from the saved screenshot, but it says nothing about resolution, viewport/DPR handling, or what happens when the element is clipped or off-screen.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.