Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full burden. It only says 'Get a user by user_id.' It does not disclose whether the operation is read-only, what the return format is, or any permissions needed. The term 'Get' implies idempotency, but no explicit behavioral context is added.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.