Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description must disclose behavioral traits. It only states the tool returns a regex pattern but omits details like pattern format (anchors, flags), whether it's for validation (matching full string) or extraction, and the response structure.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.