Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the annotations (readOnly/idempotent/non-destructive), the description discloses non-obvious behavior: tallies hidden until voted unless author opted in, poll closed, or own poll, with counts returning zero; percentages summing past 100 for multi-choice; auth optional; errors only for missing/non-poll posts. This is exactly the context annotations cannot carry.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.