Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the annotations, the description discloses that 'Entries are appended, so saving one preference never overwrites another,' which is important persistence behavior. It also provides real examples of acceptable and unacceptable entries, clarifying the expected effect of the operation.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.