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

With no annotations, the description carries the full burden and does a good job: it discloses the $0.002 USDC per-call price, the x402 payment rail and chain (eip155:8453), the upstream source (ECB), and the derivation method including peg-derived rates. It omits freshness semantics, such as what a weekend or TARGET holiday returns, which matters for a daily fixing series.

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?

One dense sentence plus a four-word qualifier, with the payment/cost constraint front-loaded in brackets and the substantive definition immediately after. Nothing is repeated from the schema and no sentence is filler.

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, read-only lookup with no output schema, the description covers the essentials: what is returned (rate, date, method, attribution), where it comes from, and what it costs to call. The remaining gap is temporal behavior (fixing cadence, behavior on non-publishing days) that an agent pricing a paid call would want to know.

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 no parameter meaning to convey and the baseline of 4 applies. The description correctly implies the pair is fixed by the tool name rather than supplied as an argument.

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: the official JPY/KRW reference rate, sourced to the ECB fixing, including what comes back (rate date, method, source attribution). The currency pair is unambiguous, so an agent can distinguish it from other fx_* pair tools by name alone. It does not, however, differentiate itself from aggregation siblings 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?

Usage is only implied: 'Deterministic, sourced, no estimate' signals to prefer this over estimated or derived rates, and the paid-per-call tag implies a cost tradeoff. There is no explicit when-to-use or when-not guidance and no alternative tool is named, so the agent must infer that fx_convert is the wrong choice for a raw reference rate.

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