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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint=false; the description adds genuinely new behavioral context by confirming the check is purely local/offline and that official sources plus a web calculator link are returned. It stops short of describing the response shape in detail or what happens on invalid input.

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?

Two sentences, front-loaded with the verb and the four document types, with the key limitation placed early in the second sentence. The first sentence is dense with parenthetical qualifiers but every clause carries discriminating information, so there is little waste.

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?

With no output schema, the description carries the return-value burden and does so partially: it says parts are separated and official sources plus a calculator link are returned. For a two-parameter, four-variant validator this is nearly sufficient, though it could say more about the success/failure shape.

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 description coverage is 100% and the enum values already map to their meanings, so baseline is 3. The description does add some value by attaching digit lengths (44/49) and the módulo 11 rule to specific types, though it does not restate the whitespace/punctuation tolerance for 'value'.

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 ('Valida la estructura y el dígito verificador') plus every resource it covers: CDC SIFEN 44 dígitos, clave SRI 49 dígitos, RUC Paraguay módulo 11, RUC/cédula Ecuador. The addition of 'y separa sus partes' makes clear it also decomposes the document, and the scope is unmistakable against calculator siblings like calcular_iva or calcular_retenciones.

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?

Explicitly sets the usage boundary: 'Es un cálculo local: no consulta a SIFEN ni al SRI si el documento existe' tells the agent when-not to rely on it (existence verification). It does not name an alternative tool, but no sibling performs validation, so there is nothing to route away from.

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