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

Annotations already declare readOnly, idempotent, closed-world and non-destructive, so the description earns credit for the extra context: no network access, no stored state, deterministic output, and concrete limits (64 KB input, 1 s per call, 1 MB result). It repeats the read-only point but the determinism and limit disclosures are genuinely additive.

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?

Front-loaded with the purpose, then labeled sections for when/not-for/side-effects/limits, so an agent can scan quickly. The cost is that the entire text is duplicated in Spanish and English, roughly doubling length for no added 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 values need not be explained, and the description covers purpose, alternatives, side effects, determinism and operational limits. 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 baseline would be 3, but the description adds real meaning by stating the value must be a 12-character ISIN with an ISO 6166 structure — usefully resolving the schema's looser 'up to 20 chars' constraint. It does not discuss case handling or what happens on malformed input.

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 (validate a 12-character ISIN per ISO 6166) and enumerates the components checked: country prefix, NSIN and Luhn check digit. That level of specificity separates it cleanly from the many other identifier validators in the sibling list.

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 'When' clause (verify format and check digit offline before using an identifier) plus an explicit 'Not for' clause naming the sibling cusip-validate and the distinguishing property (9 characters vs 12). Routing between the two tools requires no inference.

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