Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint and idempotentHint, so the description doesn't need to repeat those. It adds useful behavioral detail by specifying the exact pattern (BBBB0NNNNNN) and the invariant 5th character, making what the validator checks explicit.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.