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

Annotations cover only the safety profile (readOnly/openWorld/idempotent); the description goes well beyond them, disclosing the x402 payment handshake and retry flow, that 402 challenges are free, that >=400 answers are never charged, and that failures after payment settle nothing. It also discloses coverage variability and 404/502 semantics — exactly the kind of operational context annotations cannot carry.

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-loads the paid status and then the functionality before the coverage and error caveats, which is a sensible ordering. It is long, and the payment/error prose runs several clauses, but each sentence carries information an agent needs to invoke the tool correctly.

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?

Complete for a paid, read-only lookup: pricing, coverage boundaries, defaults, error-charging policy and the per-date return fields are all covered, and an output schema exists so the actual result shape need not be re-explained.

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 description coverage is 100%, so the schema already documents date, year, country and payment in full detail (including defaults and the PAYMENT-SIGNATURE header). The description mostly restates rather than adds syntax or format meaning, so the 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 resource (Nager.Date public holidays for a country/year, plus a per-date isHoliday check) with the exact scope of each return field. No sibling tool covers holidays, so it is trivially distinguished from the rest of the catalog, and the 'nationwide vs regional vs observance' distinction is spelled out.

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: defaults (date = today UTC, year derived from date), the covered country list URL, and explicit exclusions (Saudi Arabia, UAE, India return 404). It does not name an alternative tool, but this is a niche capability with no real sibling to route against, so the absence is minor.

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