Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations are neutral (readOnlyHint=false, destructiveHint=false, openWorldHint=false), so the description carries the burden of disclosing effects. It adds useful behavioral context by stating it saves locally and never sends a message, which is not present in annotations. However, it does not disclose whether the baseline overwrites previous data, persists across sessions, or triggers external side effects.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.