Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full burden, but it only states the intended effect. It does not disclose whether the directory must already exist, whether it is created, whether the change is persistent, or what happens on invalid paths. This is a minimal behavioral disclosure for a setter.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.