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

The annotations already declare readOnlyHint and destructiveHint, so the safety profile is covered. The description adds essentially no behavioral context beyond that — no hint about whether results are cached, scoped to an account balance, or region-limited. The output schema carries return shape, but the description still leaves the discovery semantics unexplained.

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?

It is short, and the operative sentence is front-loaded enough. But the opening noun fragment is dead weight and actively muddies the meaning, and the two sentences don't form a coherent structure.

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?

An output schema exists, so return values needn't be described, but for a discovery tool the description should at minimum say the service_id comes from the rental services list and that this is the step preceding sms_rental_buy_number. Neither is present, leaving an agent to infer the workflow.

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?

There is one required parameter (service_id) with 0% schema description coverage, so the description must compensate. It only weakly implies the parameter identifies a service ('for a service'), without saying where valid IDs come from or that it filters the country list.

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 second sentence does state a specific verb and resource ('list countries available to rent for a service'), but the leading fragment 'Rented SMS numbers.' is ambiguous and reads more like a category label than a purpose statement. Nothing in the text distinguishes it from the sibling sms_activation_list_countries, which has an almost identical name and role.

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?

'Available to rent for a service' implies this is a pre-purchase discovery step, which is useful implied usage. However, it never states that service_id must come from sms_rental_list_services, nor does it rule out using this for activation flows rather than rental.

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