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

No annotations exist, so the description carries the burden, and it does disclose the return value (365/366) and the leap-year rule. It omits date input format, error behavior for invalid dates, and timezone handling, and the trailing trial-call line is promotional rather than behavioral.

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?

The core behavior is front-loaded in one tight sentence with no wasted wording. The emoji-and-free-trial clause is marketing noise appended to an otherwise efficient 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?

For a trivial single-param tool with no output schema and no annotations, the description covers what is returned and the default-date behavior, which is most of what an agent needs. It stops short of documenting the date string format or failure modes.

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 0% for the single optional 'date' param, but the description compensates partially by clarifying that the date is optional and defaults to the current date. It never specifies the accepted date format (ISO, epoch, etc.), leaving a meaningful gap.

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?

The description states a specific verb and resource ('Returns 365 or 366 for the year of a date') and names the governing rule ('leap-year rules'), so the agent knows exactly what comes back. It does not name a sibling (e.g. x402-is-leap-year or x402-days-in-month) to disambiguate among the many adjacent date tools.

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?

'for the year of a date or the current date' implies the parameter is optional and defaults to today, which is real usage guidance. There is no statement of when to prefer this over x402-day-of-year, x402-days-in-month-year, or x402-is-leap-year, so routing is left to inference.

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