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

Beyond the readOnly/idempotent/openWorld annotations, the description discloses cost ($0.01 per call), the two auth paths (api_key from card checkout at /pricing, otherwise x402 payment instructions returned in the response), and the pricing of the downstream registration/delivery. That is exactly the behavioral context an agent needs before invoking a paid, open-world tool.

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 price and purpose, then contents, then auth, then follow-up actions — a logical ordering with no filler sentences. The parenthetical pricing clauses add density but each carries concrete information.

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 an output schema present, return shape need not be explained; the description covers the remaining gaps an agent faces — payment/auth mechanics, aggregate vs per-series economics, and what to bind afterward. Nothing material for correct invocation 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?

Schema coverage is 100%, so both params are already documented, but the description adds real meaning to api_key by explaining the fallback x402 flow and to country by implying the full bundle is fetched for that one jurisdiction. It stops short of adding format detail beyond ISO codes already in the enum.

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 ('Country brief: every reference series we hold for one country ... in one call') and enumerates the contents (policy rate, VAT/GST, CPI, taxes, minimum wage, holidays). This clearly separates it from per-series siblings like get_series, get_series_history and list_series, which it explicitly compares against ('Cheaper than buying the series one by one').

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 a clear selection criterion: use this aggregate call instead of purchasing series one by one, and routes to the correct follow-up tools after a successful call (get_changes for free polling, register_alert for HMAC push). It does not name a hard exclusion case (e.g. when a single-series tool is preferable), so it falls just short of a 5.

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