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

Annotations only say readOnlyHint/openWorldHint, but the description discloses a highly non-obvious behavioral trait: the tool is paid ($0.002 USDC via x402), and calling it returns the HTTP request and x402 payment requirements rather than the data directly, requiring an x402 client to actually retrieve results. It also states the return includes the rate date. This is exactly the kind of extra context the annotations cannot convey.

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?

Dense and front-loaded, leading with the paid/cost context and data source before params and return behavior. The 'Params: from, to, amount' line is mildly redundant with the schema but everything else earns its place.

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 payment-gated API with no output schema, the description covers cost, payment mechanism, data source, and a key return field (rate date). Only the precise shape of the successful response and accepted currency-code format are left unspecified.

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 0%, so the description must carry parameter meaning. It only lists the names ('from, to, amount') with no format guidance (e.g., ISO currency codes, how amount is expressed as a string, defaults). The '~30 currencies' hint is the only semantic value added.

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+resource (currency conversion) and the data source (official ECB euro reference rates, ~30 currencies, daily), which is far more informative than the bare name 'currency'. It is not confused with any sibling tool (country, holidays, base_gas), though it does not explicitly name an alternative.

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?

Usage is implied by the description (convert between currencies) but there is no explicit when-to-use/when-not guidance or named alternative. It does explain the payment precondition and how to complete a call, which is closer to behavioral than selection guidance.

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