Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond annotations, the description adds meaningful behavioral context: auth requirements (X-Vibes-Key or x402), approximate cost (~$0.02), advisory-only nature, no local writes, and the return mode (JSON assessing key presence/durability, or payment_required). This substantially expands on the readOnlyHint/idempotentHint annotations without contradicting them.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.