Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already mark the tool read-only, idempotent, and non-destructive, and the description adds a specific side-effect guarantee not fully captured by those hints: it does not send a message. This is useful because the mention of an 'invitation' could otherwise suggest an outgoing message; the description removes that ambiguity.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.