Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover read-only and non-destructive behavior, but the description adds useful context: execution is synchronous, the output is internal state for orchestration, and it instructs the agent on how to present results (e.g., suggest next action). This goes beyond the structured fields and helps the agent behave correctly with the returned data.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.