Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
For a 4-required-param read tool with full annotation coverage, complete schema descriptions, and an output schema, the description supplies the remaining pieces: cost/auth, path acquisition, fail-closed behavior, and the disallowed alternative input. Nothing needed to call it correctly is missing.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.