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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint=false, so the safety profile is covered by structured data. The description adds genuinely useful non-schema behavior: it is a paid call ($0.001 USDC per call via x402 on Base mainnet), which an agent must budget for, and it discloses the return shape. It does not say what happens on an unrecognized or invalid number, which is the main behavioral gap.

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?

Front-loaded with purpose and country scope, then output, then use case, then price — a logical ordering with no filler. The 17-country list is long but earns its place by defining coverage boundaries; the only mild redundancy is restating operator names already implied by the 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?

With no output schema, the description correctly states what is returned (operator + E.164 number), and it discloses pricing and coverage for a two-parameter lookup. Missing only failure behavior for invalid or ambiguous numbers, which is a minor gap for a simple read-only lookup.

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%, and the schema already explains the international-vs-local number formats and when the country code is required. The description only reinforces the E.164 output format and country coverage; it adds no syntax or edge-case meaning beyond the schema, so baseline 3 applies.

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+resource ('Mobile network operator lookup for phone numbers') plus explicit geographic scope of 17 named countries and the returned fields (operator and E.164 number). This clearly separates it from the South Africa-specific siblings such as za_normalize_phone or za_validate_company_number.

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?

Gives a concrete usage context: 'Useful before sending airtime, mobile money or SMS', which tells the agent the downstream decision this feeds. It stops short of naming an alternative tool or stating when not to use it (e.g. non-African numbers, or using za_normalize_phone for South African-only normalization).

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