Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover readOnly/idempotent/non-destructive, but the description adds materially new behavior: results are newest-first, an absent key yields an empty list rather than an error ('absence is a first-class answer'), and there is an access-tier gate ('sp500+, sample rejected'). That empty-result-not-error contract is exactly the kind of detail that prevents an agent from treating a miss as a failure.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.