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

Annotations already declare readOnlyHint=true and openWorldHint=false, and the description goes beyond them by disclosing the x402 payment requirement, the exact price, and the concrete recovery path (_meta["x402/payment"]) after a PaymentRequired response. That is meaningful behavioral context; what is missing is what the tool returns for a valid vs. invalid number.

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, followed by payment mechanics and a worked example in a compact single paragraph. The example duplicates the schema's own example, which is the only wasted element.

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?

There is no output schema, so the description should describe the return value (boolean? check-digit detail? error shape for malformed input) and it does not. Payment behavior is covered well, but the result semantics an agent needs to interpret the call are absent.

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% and the schema already carries the example '49091850', so the description's example args duplicate rather than extend the schema. With one fully documented parameter, the baseline 3 applies.

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') plus a precise resource ('NZ IRD/GST number check digit'), and the resource type cleanly distinguishes it from siblings like nz_validate_bank, nz_validate_nzbn, and nz_validate_phone. An agent can route to it 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?

Usage is implied by the name and the single 'number' argument, and the description does surface a genuinely useful operational fact (cost per call, payment failure behavior). However, it never states when to prefer this over a sibling validator or what happens for non-IRD inputs, so guidance remains implicit rather than explicit.

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