Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the readOnly/idempotent/destructive annotations, the description discloses valuable behavioral details: exactly which parts are returned (claims, elements, evidence, claim maps), what is excluded (no _computed block), and the consequence (smaller response payload). This gives the agent a clear expectation of the response content.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.