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

Annotations already cover the safety profile (readOnlyHint, idempotentHint, openWorldHint=false), so the bar is lower; the description adds genuinely useful behavior: exact pricing ($0.05 per successful call, paid over x402 in USDC on Base) and case/accent-insensitive name matching. It does not describe failure behavior for unmatched codes or whether subdivision resolution returns a different shape.

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 the lookup capability, then return fields, then edge cases, then price. Dense but every sentence carries information; the pricing sentence is somewhat tangential but materially affects invocation decisions for a paid tool.

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 carries the return-value burden and does so thoroughly: all code systems, official/common names, UN M49 region, currencies, TLD, and the optional subdivision list with types. Combined with the paid-call and matching-behavior notes, an agent has everything needed to invoke it correctly.

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%, so the baseline is 3, but the description adds real meaning: it clarifies that 'include' controls the optional full ISO 3166-2 subdivision list with each subdivision's type, and that 'code' also accepts subdivision codes like NL-NH. It does not add syntax or default details beyond the schema's own descriptions.

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 precise verb+resource ('find a country or territory') and enumerates exactly what is returned: codes, names, UN M49 region, currencies, TLD, and optional ISO 3166-2 subdivisions. It also distinguishes itself from the ISO 3166-2 subdivision-resolution case and from siblings like nuts_region_lookup and currency_lookup by scoping to ISO 3166 data.

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 clear context for when the tool applies: lookups by alpha-2/alpha-3/numeric code, by name, or by an ISO 3166-2 subdivision code, and notes that names are matched case- and accent-insensitively. It stops short of explicitly naming an alternative (e.g., nuts_region_lookup) or stating when not to use it, so it lacks true when-not routing.

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