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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered. The description adds useful cost information (1 credit; 10 with line type), which is real behavioral context beyond annotations, but it does not clarify auth requirements or output freshness despite the open-world hint.

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 short and front-loads return fields and cost, but it is written as two fragments rather than a coherent sentence. The phrase 'with line type 10' is not self-explanatory and would benefit from clearer phrasing.

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 the description need not enumerate return values, yet it does so instead of covering the missing essentials: acceptable phone number format, auth requirements for line_type, and any ambiguity around credit cost. For a 2-parameter lookup tool with only 50% schema description coverage, this leaves significant gaps.

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 only 50%, and the description does not compensate for the undocumented `number` parameter—there is no format guidance such as E.164 or country-code expectations. The `line_type` parameter is described in the schema, and the description's phrase 'with line type 10' ambiguously relates to cost rather than parameter semantics.

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 lists what the lookup returns (country, number type, blocklist status, verdict) but never states a clear verb or scope such as validating or enriching a phone number. The title and name make the resource obvious, but the description itself only implies the purpose.

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?

There is no guidance on when to use this tool versus siblings such as lookup_email or lookup_iban, nor any prerequisite or exclusion. The only usage-adjacent information is pricing, which does not tell an agent when to select this tool.

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