Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the annotations, which already declare read-only, idempotent, and non-destructive behavior, the description discloses a moderate token cost (2-5k depending on subject size) and the specific content of the response. This adds helpful context for the agent to manage expectations.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.