Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
For a simple single-parameter read-only tool, the description covers the main functionality, the exact checks performed, the no-external-API behavior, and the per-call cost. It does not detail the return structure, but the description enumerates the output dimensions (country, format validity, 010 mobile) sufficiently for an agent to infer it.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.