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

Annotations already establish read-only, idempotent, closed-world behavior, yet the description adds substantial extra context: it performs no registry lookup, stores no personal data, costs $0.05 per successful call, and is paid over x402 (USDC on Base). These are meaningful operational facts not derivable from the annotations or schema.

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 tightly packed sentences with no filler: purpose first, coverage list second, scope/limitations and pricing last. Every sentence carries information an agent needs.

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 three-parameter validation tool with full schema coverage and clear annotations, the description supplies the remaining essentials: covered countries, identifier types, the structural-only limitation, and the payment model. No output schema exists, but a boolean-style validation result is self-evident from 'validate ... against checksum rule'.

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 already 100%, so the baseline is 3, but the description adds genuine value for the 'kind' parameter by enumerating which identifier types each country supports (BSN, NIF/NIE/CIF, codice fiscale, SIREN/SIRET, etc.). This helps the agent map country to a valid kind beyond the schema's generic hint.

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) and resource (national identity, tax or company registration number) and scopes it to a country's official checksum rule. The enumeration of 23 countries and example identifier types clearly distinguishes it from sibling validators like id_gs1 and id_security.

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 clarifies context with 'Structure and check digit only — no registry lookup,' which implicitly tells the agent when this tool is appropriate and when it is not. It does not name a specific alternative tool for registry lookups, so it stops short of explicit routing.

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