Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the read-only annotations, the description adds meaningful behavioral context: no API key is needed, each leg has independent as_of/availability, and a chainless token yields no TVL rather than a fabricated value. These details materially change how an agent interprets results and are exactly the kind of non-obvious behavior worth stating.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.