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

With no annotations, the description carries the full burden and does useful work: it discloses local parsing via libphonenumber metadata, names the data source, and explicitly limits the result to format/number-plan validity. It leaves minor gaps around authentication, rate limits, and whether the operation is strictly read-only, but the core behavioral scope is clear.

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?

Three tight sentences with no waste, and the core scope and limitation are front-loaded before the data-source note. Every sentence contributes to correct tool selection or interpretation.

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 two-parameter lookup with no output schema or annotations, the description adequately explains what is returned (format and number-plan validity) and what is not queried. The main missing piece is country-parameter semantics, which prevents a perfect score.

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 0%, so the description must explain the two parameters, but it only implies the 'number' parameter. The 'country' parameter is never explained, including whether it is a default region for parsing or required in some cases, leaving a meaningful gap for a low-coverage 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 and resource: parse a phone number locally against the numbering plan. It also distinguishes the scope from verification tools by limiting results to format and number-plan validity, so an agent can tell it apart from siblings like email-verify or domain-lookup.

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?

Clearly says the tool checks format and number-plan validity only, and explicitly excludes current network assignment and subscriber status. That gives a strong when-not signal, though it does not name any alternative tool for those excluded cases.

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