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

Adds substantial context beyond annotations: keyless and free of quota cost, a 30-distinct-pairs/day anonymous cap per network, repeat pairs are always free, English only, one pair per call, and that the verification date is only present when that pair was individually re-verified. It also discloses the primary rate-limit dimension (distinct vs. repeated), which no annotation conveys.

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?

Well front-loaded: what it answers, what it returns, then routing, then limits. The closing API-key solicitation with a tracking-style URL is promotional and the longest single clause, which slightly dilutes an otherwise tight definition.

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 supplies the return fields and the conditional verification date, plus rate limits, language scope, and a fallback route. An agent has everything needed to decide and call 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% and the schema already specifies ISO 3166-1 alpha-3 with examples, so baseline is 3. The description adds only the call-shape constraint 'one pair per call' and English-only, without new detail on the parameters themselves.

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?

Opens with the exact question it answers and enumerates what it returns: requirement, visa-free days, and verification date. It names the sibling it is not (check_visa_requirement) so an agent can separate the two without opening schemas.

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 an explicit selection rule — prefer this for a plain yes/no, switch to check_visa_requirement when the user needs documents, fees, processing times, transit rules, or another language. Both the use case and the exclusion criteria are stated.

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