Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond annotations (readOnlyHint=false, etc.), the description discloses that code is never executed, lists specific checks (e.g., unbalanced do/end, missing return), and provides constraints (max 128 KB, cost $0.002). This adds significant context for agent behavior.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.