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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds the traits that actually matter for this paid tool: exact cost (0.003 USDC via x402 or 2 credits), the x402/API-key auth paths, and the first-5-calls-per-day free tier. It does not cover error behavior or latency, keeping it below 5.

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 sentences, front-loaded with the use case, then the input rule, then the cost. Every sentence carries information an agent needs before invoking a paid tool; nothing is padded.

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?

Only two parameters with full schema coverage, no nested objects, and no output schema needed since the description itself enumerates the returned fields. Payment, auth, and free-tier context fill the remaining gap, so an agent can decide and invoke without further information.

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%, so baseline is 3, but the description adds genuine meaning: national-format input requires the country parameter, with a concrete example pairing 0612345678 with country=NL. That disambiguates a real input pitfall the schema alone leaves implicit.

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 ('check') and resource, then enumerates exactly what is returned: validity/possibility against allocated ranges, E.164 and national format, country, line type, and an SMS-capable hint. Distinguished from sibling verifiers like email_verify or iban_check by naming the phone-specific output set.

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?

'Use when you need to check a phone number from any country' gives a clear triggering context, and 'National numbers need a country' gives the prerequisite for the main ambiguity. No explicit when-not or named alternative (e.g., vs email_verify) is offered, so it falls short of a 5.

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