Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint and idempotentHint, so safety is covered. The description adds meaningful context by scoping behavior to syntax checking and common domain typos, which clarifies that it is not a deliverability or mailbox-existence check.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.