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

Annotations only declare readOnlyHint and openWorldHint; the description adds substantive behavior: it is pure arithmetic against registry data, it discloses which registries are consulted (SWIFT, Banco de España), and it explicitly caps its own guarantees by saying it does not verify account existence or ownership. That last point is exactly the kind of limitation an agent needs and is not available anywhere in the structured fields.

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?

Three dense sentences, front-loaded with the core check and then narrowing to the Spanish-specific behavior and the limitation. Every clause carries information; nothing is restated from the name or title.

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?

For a single-parameter, read-only validation tool, the description covers inputs, checks performed, and limits. The only gap is that, with no output schema, it never indicates the shape of the result (e.g., valid/invalid plus reasons), which an agent may need to interpret the response.

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 is 3. The description goes beyond the schema by clarifying that a 20-digit Spanish account (CCC) is accepted and internally converted to its IBAN, and that spaces are tolerated, which disambiguates the single parameter's accepted inputs.

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 (validate/check) and resource (IBAN), then enumerates exactly which checks are performed: SWIFT country length, ISO 13616 check digits, and Spanish DC/entidad/BIC. This is clearly distinct from siblings like check_vat_number, so an agent can route correctly without opening any 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?

The description gives a clear context of use (well-formedness validation) and draws an explicit boundary: it does NOT confirm the account exists or who owns it. It also notes the CCC-to-IBAN conversion path. It stops short of naming alternatives or stating when-not-to-use conditions, so it is strong but not fully routing-aware.

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