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

Annotations already establish read-only, idempotent, non-destructive safety. The description adds substantive non-schema behavior: pricing via x402 or API credits, a 5-free-call daily allowance, and the returned validation attributes. It does not cover error modes, but the cost and free-tier disclosure is strong.

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?

Four sentences, front-loaded with the use case and result details, then payment information. Every sentence carries scope, parameter guidance, or cost context without filler.

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 3-parameter paid read-only tool with full schema coverage and no output schema, the description supplies the missing return-field details, national-number handling, and pricing. Nothing critical for an agent to call it correctly is 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?

With 100% schema description coverage, the baseline is 3. The description's national-number example reinforces how to pass country, but it largely mirrors schema text and adds no extra syntax for phone or api_key.

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?

The description opens with "check a phone number from any country" and enumerates the checks performed: validity, possible allocation, E.164/national format, country, line type, and SMS-capable hint. That is a specific verb+resource with clear scope; no sibling tool performs phone validation, so implicit differentiation is sufficient.

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?

It gives a clear trigger ("Use when you need to check a phone number from any country") and a specific condition for national numbers with an example. It does not state when not to use the tool or name alternatives, so the guidance is clear but not exhaustive.

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