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

Annotations only declare readOnlyHint and openWorldHint, so the description carries the interesting load and does it well: it discloses that the carrier field is range-allocation data rather than a live network query, and it spells out the paid path — $0.05 USDC per successful call via x402 on Base or Solana when no API key is present. Cost model and payment rails are exactly the kind of operational context an agent needs before invoking.

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: the capability and its outputs come first, then the pricing fallback. The scope caveat about carrier data is embedded inline rather than padding a third sentence.

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 one-parameter, no-output-schema tool, the description must explain what comes back and it does — validity, type, region, location, allocated carrier — plus the billing conditions. There is nothing an agent needs in order to call this correctly that is missing.

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 single parameter already documents the 'digits only, country code first, no + or leading zeros' format with an example. The description adds only the notion of 'its country's numbering plan', which is modest added meaning, so 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?

The description states a specific verb and resource — validating a phone number against a country's numbering plan — and enumerates exactly what is returned (validity, type, region, location, carrier allocation). It does not name the closest sibling (osint-phone), so the agent must infer the split, which keeps it off a 5.

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 only implied by the phrase 'Validate a phone number'; there is no explicit when-to-use or when-not, and no pointer to osint-phone or whatsapp_business for adjacent phone tasks. The parenthetical '(not a live network query)' does sharpen expectations about the result, but it is a scope caveat, not guidance on tool selection.

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