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

Annotations cover the safety profile (readOnly, idempotent, openWorld, non-destructive), and the description layers on operational facts annotations cannot express: pricing ($0.002 per call over x402 on Base), the prepaid x-credit-token header for unlimited calls, a 1 free call/day allowance, and that no API key is needed. That is exactly the auth/cost/limit 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?

Front-loaded with the resource and filters, then routing guidance, then the commercial terms — a logical order with little wasted text. It is dense with pricing detail in one block, which slightly dilutes the core purpose statement.

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 simple two-parameter read tool with an output schema, this covers everything an agent needs: what it returns, which country set is supported, the sibling relationship, and the payment/auth model. No material gap remains.

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 defines country as ISO-2 and year as a year string. The description restates the same inputs and adds only the coverage fact (~110 countries), which is marginal beyond the structured data. Baseline 3 is appropriate.

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?

Names the specific resource (public holidays), the two filters (country and year), the upstream source (Nager.Date, ~110 countries), and the extra semantic it returns (national vs regional marking). It also distinguishes itself from its sibling by noting it is the raw calendar that business_days is built on.

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?

"Same source business-days uses — for agents that need the raw calendar" clearly positions it against the sibling tool and tells the agent when the raw list is the right pick. It stops short of an explicit exclusion (e.g. when to prefer business_days instead), so it is strong context rather than a full when/when-not rule.

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