Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare the tool as read-only, idempotent, and non-destructive, so the description's 'List' verb matches. It adds slight value by clarifying that the posts can be blog posts and/or pages, which is not explicit in the schema's type parameter description.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.