Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already establish readOnlyHint=true and openWorldHint=false, so safety is covered. The description adds real behavioral context beyond that: the shape of a hold/carry measure_progression (target_unit_measure, target_duration_s, target_distance_m, suggested_weight in kg), which an agent needs to interpret the response.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.