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

Annotations already declare readOnlyHint, openWorldHint=false, idempotentHint and destructiveHint=false, and the description reinforces this with concrete operational facts the annotations cannot express: no network, no stored state, deterministic output, and hard limits (64 KB input, 1 s per call, 1 MB result). That is genuine added context, though the 'solo lectura' restatement partially duplicates the annotations.

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 and partitioned into labeled clauses (purpose, when, not-for, side effects, determinism, limits), so scanning is easy. It loses a point for shipping the entire content twice (ES and EN), which roughly doubles length without adding 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?

The tool is simple, has an output schema (so return values need no explanation), full annotation coverage and full parameter coverage. Purpose, triggering condition, exclusion, side-effect profile and resource limits are all present — nothing an agent needs 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.

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

Only one parameter and schema coverage is 100%, so the baseline is 3. The description does add meaning the schema lacks — that the value is exactly 9 characters and includes a check digit — but it also conflicts mildly with the schema's maxLength of 12, which an agent may notice; that ambiguity keeps it from a 4.

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 and resource ('Valida un CUSIP de 9 caracteres con su dígito de control') and even names the algorithm (modulus-10 double-add-double), plus parenthetical scope '(US/Canada security identifier)'. It explicitly contrasts itself with iban-validate, so an agent can pick it out of a very large sibling list without opening schemas.

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?

Has explicit 'Cuándo' (verify format and check digit offline before the identifier is used) and 'No para' clauses naming the alternative tool (iban-validate). Both the positive trigger and the exclusion are stated, leaving nothing to inference.

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