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

Adds substantial context beyond the annotations: explicit per-call cost ($0.001 USDC on Base via x402) and the failure/retry behavior on an unpaid call. Annotations only cover the read-only, closed-world profile. It stops short of describing the response shape (valid/invalid, extracted check digit), which the absence of an output schema leaves undocumented.

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 compact sentences, front-loaded with the purpose before cost and payment mechanics. Every sentence carries distinct information: what it validates, what it costs and how to pay, and a callable example.

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 validator this covers purpose, pricing, and the payment retry path, which is the hard part. The main remaining gap is return-value semantics (what 'valid' looks like, and whether the check-digit derivation is exposed), which matters because there is no output schema.

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?

One parameter with 100% schema description coverage, so the schema already documents `number`. The description's example args duplicate the schema's example value (9429041535134) rather than adding constraints such as accepted length, formatting tolerance, or whether the check digit is required to be present.

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 (NZBN), plus the actual mechanism (check digit), which is exactly what an agent needs to distinguish it from nz_validate_bank, nz_validate_ird and nz_validate_phone. The parenthetical 'GTIN-13' is a slightly loose framing of the 13-digit format but does not obscure intent.

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?

No explicit when-to-use or when-not-to-use guidance relative to the sibling validators; selection is implied by the resource name. It does give clear operational guidance for the payment path (unpaid call returns PaymentRequired, retry with _meta["x402/payment"]), which is genuinely useful but is about invocation mechanics rather than tool choice.

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