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

Annotations already declare readOnlyHint and openWorldHint, but the description adds genuinely non-structured behavior: a per-call price ($0.002 USDC via x402 on Base/Solana/Arc), the billing rule that failed calls are free, and that the lookup itself makes no network call. That pricing/payment information is exactly the kind of auth/cost context annotations cannot convey. Minor deduction: 'No network call' sits awkwardly against openWorldHint=true and the x402 payment step, which does involve an HTTP exchange, leaving the agent slightly unsure what 'no network' scopes to.

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?

Front-loaded with the core verb+resource, then outputs, then the country rule, then cost — a sensible importance ordering with little waste. The output enumeration is somewhat redundant given an output schema exists, which keeps it from a 5.

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 two-parameter, read-only lookup with a full schema, an output schema, and annotations covering safety, the description supplies everything else an agent needs: what it returns, the required country fallback, the fact that no external lookup occurs, and the payment model. Nothing material to correct invocation 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 both parameters are documented, including the country's 'when the number has no +' semantics, so the baseline is 3. The description's 'Without a + sign, give the country' restates that same dependency rather than adding format, precedence, or error-handling detail, so it adds no value beyond the schema.

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 (checks and formats) plus the exact resource (a phone number) and names the underlying library (libphonenumber), then enumerates the outputs it produces (valid/possible flags, E.164, international/national, country, calling code, line type). No sibling in the very large list competes for this resource, so it is trivially distinguishable from email_check, iban, vat or the generic validate.

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?

There is one operational rule — 'Without a + sign, give the country' — which tells the agent how to pair the two arguments, but nothing states when to prefer this tool over the generic `validate` sibling or when it is inappropriate (e.g. partial numbers, extensions). Usage is implied by the purpose rather than spelled out.

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