Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations provide hints (readOnlyHint=false, destructiveHint=false, idempotentHint=true, openWorldHint=true), but the description adds valuable context: it's a batch operation affecting all computers simultaneously, which implies potential downtime or coordination needs. It doesn't contradict annotations and supplements them with operational details.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.