Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
readOnlyHint=true already tells the agent this is a safe read, so the bar is lower, and the description adds the return contract that the annotations do not: the 0-100 scale and the boolean mute state. With no output schema, disclosing the shape of the answer is exactly the added value needed. It does not mention permissions or any failure mode, keeping it short of a 5.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.