Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations cover the safety profile (readOnlyHint=false, openWorldHint=true, idempotentHint=false, destructiveHint=false). The description adds behavior the annotations cannot express: the URL is one-time-use and requires a human to open it in a browser and authorize. It stops short of stating URL expiry or the post-authorization flow, but the human-in-the-loop disclosure is genuinely additive.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.