Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations only declare readOnlyHint and openWorldHint; the description adds the genuinely important behavioral traits beyond that — non-blocking, repeatable, and 'never consumes the result' (contrasting with elicit_result, which does). It also enumerates the possible status values the caller will observe.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.