Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover the safety profile (readOnlyHint=true, idempotentHint=true, destructiveHint=false), so the description's job is to add context beyond them. It does: it discloses the exact status values the agent should expect (queued/sent/delivered/undelivered/failed) and the presence of carrier error codes, which helps an agent interpret results. This is non-redundant, useful behavioral information consistent with the annotations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.