Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the description's 'read-only' adds little. However, it contributes valuable extras: 'free, keyless' reveals access requirements, and 'honest pay status (funded vs proposed)' discloses a nuance not present in annotations. This goes beyond structured data.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.