Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations cover safety (readOnlyHint=true, destructiveHint=false, idempotentHint=true, openWorldHint=true), so the agent knows it's a safe, repeatable read operation. The description adds no behavioral context beyond the HTTP method, such as rate limits, authentication needs, or what metrics are returned. It doesn't contradict annotations, but adds minimal value beyond them.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.