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

With no annotations, the description carries the full burden and does well: it discloses the paid cost ($0.002 USDC), the x402 protocol and chain (eip155:8453), and the response contents (rate, rate date, derivation method, source attribution). What it omits is freshness cadence — an ECB fixing is published on a daily schedule, so an agent cannot tell how stale the 'latest' value might be.

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?

Roughly forty words across two sentences plus a bracketed payment tag, with the identifying fact ('Official USD/KRW reference rate') front-loaded and every clause adding information the agent cannot get from structured fields. No filler or restatement of the tool name.

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 zero-parameter, zero-annotation tool with no output schema, the description supplies cost, provenance, derivation logic and the shape of the return value — a strong package. The one real gap is update timing/publishing cadence, which matters for a 'latest fixing' tool an agent may call repeatedly.

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, so there is nothing for the description to disambiguate beyond what the schema already conveys. Baseline 4 applies; the description correctly spends its words on output semantics instead.

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 names a concrete verb+resource pair — the official USD/KRW reference rate — and pins the direction (USD base, KRW quote), which is enough to distinguish it from the inverse sibling fx_krw_usd and from the other ~400 pair tools. It stops short of positioning itself against non-pair siblings like fx_convert, fx_latest or fx_timeseries, which also surface rates.

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?

'Deterministic, sourced, no estimate' implies this is the reference-rate option rather than an indicative or computed one, so the when-to-use is inferable. However, there is no explicit when-not-to-use or pointer to fx_convert for actual conversion math or fx_timeseries for historical data, so the agent must infer the boundary itself.

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