Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly/idempotent/non-destructive, but the description adds a genuinely valuable caveat: headline metrics are the original backtest, not re-measured, and each row's headline_scope tells you the basis. That is data-provenance context the annotations cannot supply. 'Read-only' merely repeats the annotation, adding no new information.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.