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

Annotations already cover readOnlyHint and openWorldHint, so the safety profile is settled; the description adds genuinely new behavioral context by disclosing per-call cost ($0.002 USDC, x402 payment) and the data source (Google libphonenumber). It stops short of saying what happens on an invalid or malformed 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?

Two sentences, front-loaded with purpose and return values, with cost tacked on last. Every clause carries information an agent needs; nothing is padding.

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 well by naming the exact fields returned, and it discloses the cost model. It is only slightly incomplete in not covering invalid-input behavior or the `country` fallback for numbers without a +code.

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 50%: `country` is documented in the schema while `number` is not. The description mentions no input parameters at all, so it adds nothing beyond the schema for `country` and nothing for the self-evident required `number`. Baseline 3 is appropriate given the schema handles the one non-obvious field.

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 and resource ("validates a number" / "Phone number check") and enumerates exactly what comes back: country, line type, E.164, international and national formats. Neither `postal` nor `zip` does this, so the agent can distinguish it from neighbors 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?

Usage is implied rather than stated: you call it when you need to validate or normalize a phone number, and the $0.002 USDC (x402) cost line gives a useful invocation-economics signal. But there is no explicit when-to-use/when-not, no mention of the alternative with `country` for numbers lacking a +code, and no error/invalid-number guidance.

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