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

No annotations are supplied, so the description carries the full behavioral burden. It does disclose pricing ($0.002 USDC/call via x402 on eip155:8453), the data source, the peg fallback, the reported method (direct, cross via EUR, peg-derived), and that no estimation occurs — solid disclosure on those axes. But it omits freshness/staleness behavior (fixing publication cadence, weekend and holiday handling) and error behavior, which matter for a rate lookup with no annotations to fall back on.

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?

Two sentences, front-loaded with the payment terms and then the resource definition, with no filler. The bracketed payment prefix is denser than ideal for a purpose statement but is material information for an x402-paid tool.

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 no output schema, the description compensates by enumerating what is returned (the rate, rate date, derivation method, source attribution), which covers the return contract for a zero-parameter tool. The only meaningful gap, fixing cadence and non-publication days, is secondary for a reference-rate lookup.

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?

The tool takes zero parameters, which is the baseline-4 case; the description correctly does not invent or discuss inputs. Pair selection is baked into the tool name rather than a parameter, and the description is consistent with that design.

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 precise resource (official KRW/CNY reference rate), its source (latest ECB fixing, with peg fallback), and derived output fields. The currency pair in the name already distinguishes it from the hundreds of sibling fx_<base>_<quote> tools, but the description does not explicitly contrast itself with aggregators like fx_convert, fx_latest, or fx_timeseries.

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?

Context is implied rather than stated: the words 'reference rate', 'deterministic', and 'no estimate' tell an agent this is an official-fixing lookup rather than a live mid-market quote. However, there is no explicit when-to-use or when-not-to-use guidance, and no pointer to fx_convert or fx_timeseries for conversion or historical work.

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