Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is known; the description restates it but also adds genuinely new operational facts: hard limits (64 KB input, 1 s per call, 1 MB result) and determinism. It does not contradict the annotations, though the side-effects sentence largely duplicates them.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Well front-loaded: purpose, then when, then not-for, then side effects and limits. However the ES text is duplicated almost verbatim in English and again in inputSchema.description, which inflates length without adding information for the reader.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists, so return values need no explanation; the description still supplies algorithm, prefix interpretation, determinism, timeouts and size bounds. Nothing an agent needs in order to call this correctly is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% so the baseline is a 3, but the description adds meaning the schema lacks: the 11-digit requirement, the mod-11 check-digit weights, and the prefix-to-taxpayer-type mapping (10/15/17/20). It stops short of describing accepted formatting variants (dots, dashes, whitespace) given the 20-char maxLength.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb+resource ('Valida un RUC peruano'), the exact algorithm (SUNAT mod-11, weights 5432765432) and the secondary behavior (taxpayer type from prefix). It also names the sibling it is not (ec-id-validate), so an agent can distinguish it without opening the schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicit 'Cuándo' clause gives the triggering context (offline check of a user-supplied LatAm/Spanish ID before storing or using it) and an explicit 'No para' clause naming the alternative tool (ec-id-validate). Both when-to-use and when-not-to-use are covered.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.