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

Annotations declare read-only and idempotent, and the description adds substantial non-annotation behavior: no registry lookup, no storage of the number, log retention limited to length and character types, and a $0.05 per-successful-call price via x402 (USDC on Base). Privacy, pricing, and side-effect profile are all disclosed.

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 purpose and scope in the first sentence, with coverage, behavioral caveats, and pricing following in a logical order. The country/identifier enumeration is dense but earns its place by telling the agent which inputs are in scope.

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?

With no output schema, the description carries the return-value burden; it conveys that validation is checksum-based and that country echoes back, which implies a validity response, though the exact response shape is not spelled out. Combined with the full behavioral disclosure, it is nearly complete for a stateless validator.

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 description coverage is 100%, so all three parameters are already documented, including separator stripping, the 64-character limit, case-insensitivity, and UK/GB handling. The description adds only a broad inventory of supported countries/kind examples, which is useful orientation but not new per-parameter semantics.

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+resource ('check a national identity, tax or company registration number against its country's official checksum rule') and enumerates the identifier families and countries covered. An agent can immediately distinguish it from siblings like iban_validate, gtin_barcode_validate, and crypto_address_validate.

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 phrase 'Structure and check digit only, no registry lookup' clearly delimits when this tool applies versus a registry/verification service, and the country/kind list signals the input domain. It does not, however, name a sibling alternative or state explicit exclusion conditions (e.g., what to use for IBANs instead).

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