Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover the safety profile (readOnlyHint, destructiveHint, openWorldHint), so the bar is lower. The description adds genuinely useful behavioral context beyond annotations: the data is synthetic/illustrative (not real client or live measurement), and visual series may be trimmed for the response-size budget. These are non-obvious traits an agent should know before relying on output.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.