Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond annotations (readOnlyHint, destructiveHint), the description adds critical behavioral context: downloadUrl behavior depends on startup stage and user access, with locked=true when access is restricted. This explains conditional output, which is valuable for the agent.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.