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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false and destructiveHint=false, so the safety profile is pre-covered. The description nonetheless adds real context beyond them: determinism, no network/stored state, and operational limits (64 KB input, 1 s per call, 1 MB result) that no annotation conveys.

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?

The Spanish block is tightly structured with explicit labels (Cuándo, No para, Efectos secundarios, Determinista, Límites), so it is front-loaded and scannable. The full English duplicate plus the identical text repeated in inputSchema.description is some redundancy, but it does not obscure the key information.

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-value details are unnecessary in the description. Between the algorithm, the negative case, the side-effect profile and the resource limits, an agent has everything needed to decide and call correctly.

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?

With one parameter at 100% schema coverage, the baseline is 3. The description still adds meaning beyond the schema's bare 'up to 20 chars': the 11-digit plus two-check-digit structure and the expected formatted output '529.982.247-25', which tells the agent what accepted input looks like.

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?

The description names a specific verb and resource ('Valida un CPF brasileño'), specifies the algorithm (11 digits, two mod-11 check digits), the rejection rule (repeated digits) and the formatting behavior. It also explicitly distinguishes itself from the sibling br-cnpj-validate, so an agent can tell the two apart without opening a 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?

It has an explicit 'Cuándo' clause (checking a user-supplied LatAm/Spanish ID offline before storing or using it) and an explicit 'No para' clause routing Brazilian CNPJ checks to br-cnpj-validate. When-to-use, when-not, and the alternative are all stated outright.

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