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

With no annotations, the description carries the full burden and does so well: it discloses that prices are in credits, that values are approximate (typical mobile number, carrier variance), that data is refreshed at least daily via `checked_at` in UTC, and that billing is metered (~0.41 avg per request). It stops short of describing pagination or the full response shape, but the behavioral profile is unusually rich.

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?

Every sentence is front-loaded and earns its place: content first, then freshness caveat, then the sibling routing, then exclusions, then group and billing metadata. No filler or repetition of the name.

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?

There is no output schema and no annotations, so the description must supply everything, and it does: what is returned, in what unit, sorted how, how fresh it is, what is omitted, and the cost of calling it. An agent can call this correctly with no 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?

The tool takes zero parameters, so per the rubric the baseline is 4. The description instead adds semantic meaning to the returned fields (credits, checked_at, per-country typical-number basis), which goes beyond an empty schema without being a parameter explanation.

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 exactly what the tool returns — per-country credit pricing for verification SMS, sorted by country name — which is a specific resource and scope. It implicitly distinguishes itself from the cost-estimation sibling by noting that countries are priced for a 'typical mobile number' while 'Estimate Cost' gives an exact number.

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 names the alternative tool explicitly ('use Estimate Cost for an exact number') and gives the condition that selects it (carrier-level precision needed). It also states the exclusion rule — countries SMS cannot be sent to are omitted — so an agent knows what absence of a country means.

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