Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover read-only, idempotent, and non-destructive traits, so the bar is lower and the description clears it: it discloses that output is labelled per Location, that unreachable Locations are reported as skipped, and that IDs are scoped to a single Location. Those are real behavioral details not present in structured fields, though return granularity/pagination is untouched.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.