Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover readOnlyHint, idempotentHint, destructiveHint and openWorldHint, so the safety bar is lower. The description adds real value beyond them by disclosing empty-result semantics and the non-exhaustive nature of the registry, which prevents a common misinterpretation.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.