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

Annotations cover the safety profile (readOnly, idempotent, openWorld), and the description adds behavior they cannot express: no personal data, paid access via x402 USDC on Base at $0.001 per successful call, and the failure mode (calling without payment yields a PaymentRequired object). Cost and auth semantics are exactly the kind of context an agent needs before invoking.

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?

Three short sentences, front-loaded with the usage directive, then what is returned, then the payment model — no filler. It is slightly dense and packs a lot of unrelated commercial detail into one sentence, but nothing is redundant.

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?

For a zero-parameter, no-output-schema enumeration tool, the description supplies everything needed: what it returns, approximately how many items, the field names, and the payment prerequisite. Nothing an agent must know to call it correctly is missing.

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?

There are zero parameters, so the baseline is 4 and there is nothing for the description to disambiguate. It does describe the shape of the returned records (countryCode plus English name), which is mildly useful but strictly about output, not parameters.

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 and resource ('discover supported country codes... returns every country Nager.Date covers (~100), each with countryCode and English name'), and the enumeration is concrete enough to separate it from its sibling 'holidays'. An agent can tell instantly that this is the lookup/enumeration tool rather than a query tool.

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?

Opens with an explicit precondition: 'Use before calling /v1/holidays or /v1/holidays/check', which names the exact endpoints (matching the holidays and holiday_check siblings) whose country codes this tool supplies. This is a clear when-to-use statement rather than inferred guidance.

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