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

With no annotations, the description carries the full behavioral burden. It does disclose the result size (20 pairs), the optional filtering, and a useful economic constraint (5 free trial calls per signed wallet). It does not say whether this is a safe read (implied by "returns"), whether results are capped at 20 with no pagination, or what auth a "signed wallet" requires in practice.

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?

A single front-loaded sentence with the resource stated first and the trial note last. No padding, though the emoji pricing tag is noise rather than signal.

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?

No output schema and no annotations means the description must cover everything, and it leaves the duplicate q/query parameters unexplained and provides no differentiation from the large currency sibling family. For a 2-param tool in a crowded namespace, this is under-specified.

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% and the schema exposes two very confusingly similar parameters, "q" and "query", with no descriptions. The description mentions "a query substring match" but never clarifies which parameter that is or why both exist — the exact ambiguity the description should resolve.

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 concrete verb and resource plus scope: "Returns a list of 20 common currency code and name pairs." An agent knows exactly what comes back. It does not, however, distinguish itself from the many currency siblings (x402-currency-code, x402-currency-by-country, x402-currency-format), so sibling differentiation is missing.

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?

"optionally filtered by a query substring match" describes the filter but gives no when-to-use guidance and no alternative routing against the currency siblings. There is no statement of when-not to use it or which sibling handles code lookups vs formatting vs conversion.

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