Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already carry the safety profile (readOnlyHint=true, idempotentHint=true, destructiveHint=false), so the bar is lower. The description adds value beyond annotations by disclosing the mathematical transformation behavior, the matrix-mode expansion when target is omitted, and the critical non-obvious return trait: 'USD purchasing-power equivalents, not currency-converted amounts.' This prevents a common misuse.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.