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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the read-only nature is covered. The description adds valuable behavioral details beyond annotations: sorting by success rate, inclusion of VoIP/mobile distinction, and the bracket format for countryId to be used in order_sms_verification. No contradiction with annotations.

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 well-structured sentences front-load the core purpose, then add sorting and selection guidance, and finally mention the return format. Every sentence earns its place with zero redundancy or fluff.

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 simple list tool with one parameter and no output schema, the description fully conveys what the tool returns (country list with live price, success rate, VoIP status), how it's sorted, and how to use the output (bracket countryId for order_sms_verification). No critical information 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 coverage is 100% (single serviceId parameter with description). The tool description does not add extra meaning beyond the schema, but since the schema fully documents the parameter, a baseline of 3 is appropriate. The description implicitly confirms that 'service' refers to the serviceId parameter.

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 clearly states the tool lists countries for one SMS service with specific attributes (price, success rate, VoIP status) and explicitly mentions the countryId bracket format for use with order_sms_verification. This distinguishes it from sibling tools like search_sms_services and check_country_coverage by focusing on country availability for a single service.

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?

The description provides clear usage context: use this to get country options for a specific service before ordering. It even gives selection guidance ('prefer high success rates, prefer non-VoIP when the target service blocks virtual numbers'), but it does not explicitly name alternative tools or state when not to use this tool.

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