Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true, destructiveHint=false, and openWorldHint=false. The description adds a billing guarantee ('does not charge') that annotations do not cover, which is genuinely valuable for an agent deciding whether it is safe to call this before committing to a generation.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.