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

Annotations only cover readOnlyHint and openWorldHint; the description goes further by disclosing the paid model ($0.001 USDC on Base via x402), the PaymentRequired response on unpaid calls, and the _meta["x402/payment"] retry path. That is real behavioural detail an agent needs and cannot get from the annotations. It omits what happens on a malformed or non-NZ number, keeping it out of 5 territory.

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?

Two tight sentences with no filler, and the core purpose is front-loaded before the payment mechanics. The inline example is slightly redundant against the schema's own example, which is the only minor waste.

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 still conveys the return shape (E.164 plus type) and the payment/retry protocol, which is the highest-risk unknown for this tool. What's missing is failure behaviour for invalid input, but for a one-parameter validator this is close to complete.

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% and the single parameter already carries the same example format shown in the description ("021 123 4567"), so the example adds essentially no new meaning beyond the schema. Baseline 3 is appropriate when the schema does all the work.

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 pair (validate, normalise), the resource (NZ phone number), and the output format (E.164 with type). This cleanly separates it from the sibling validators for bank, IRD, and NZBN, so an agent can route without opening the 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 purpose implies the usage context, and the description explains payment/retry behaviour, but it never states when to pick this over the sibling validators or what an invalid number yields. Usage is inferable rather than explicit.

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