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

Annotations already establish read-only, idempotent, non-destructive behavior. The description adds valuable functional transparency by listing exact return categories, noting carrier data is provided 'where libphonenumber has data,' and describing the risk band purpose. It does not contradict annotations and provides context beyond the safety flags.

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?

The description is a single dense sentence that front-loads the core action and then efficiently lists the enrichment outputs. Every clause adds useful information, with no filler or repetition of the tool name or title.

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?

Given there is no output schema, the description does a good job enumerating return fields and including caveats such as carrier data availability. It could be more complete by specifying validity/error behavior for invalid numbers, but the listed fields give an agent sufficient understanding of the tool's scope and result shape.

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%, so the schema already documents both phone and default_region, including formats and the default country code. The description reinforces that the tool handles E.164/national/international inputs but does not add meaningful new parameter-level semantics 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?

The description opens with a specific verb and resource: 'Validate + enrich a phone number in 200+ countries.' It enumerates concrete enrichment outputs (formatting, country, number type, carrier, timezones, risk band), which clearly distinguishes it from sibling tools like email_validate or address_validate. The purpose is unambiguous.

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 clearly implies usage for phone number validation and sign-up fraud screening via the risk band mention. It does not explicitly name alternatives or state when not to use the tool, so it falls short of the 'explicit alternatives' bar, but the context is clear enough for an agent to select it appropriately.

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