Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full burden; 'List' implies a safe read, but it says nothing about whether inactive/current devices are included, whether results are scoped to the authenticated user, or what fields a device entry carries. For a zero-parameter read this is an acceptable but thin disclosure.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.