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 access model ($0.002 USDC/call, x402, eip155:8453), the data source (ECB), the derivation method (direct, cross via EUR, or peg-derived), and guarantees determinism with no estimation. It does not explain failure behavior when a fixing is unavailable or how recent the returned fixing is relative to the call, which keeps it below 5.

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 dense sentence delivers purpose, source, method, and output fields, with the payment/transport tag front-loaded in brackets. The nested parenthetical about the peg adds real information rather than filler, though the sentence is packed enough that an agent must parse several clauses at once.

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 lookup with no output schema, the description covers what matters: what rate is returned, where it comes from, how it was derived, that it carries a rate date and attribution, and that calls are paid via x402. Only the freshness cadence and unavailable-rate behavior remain unstated.

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 the baseline is 4 and there is nothing the schema could document. The description usefully clarifies that the currency pair is fixed by the tool itself and that the response carries the rate date, method, and source attribution.

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 the exact resource (official NOK/KRW reference rate) and its provenance (latest ECB fixing, or the official peg when KRW is pegged), so an agent knows precisely which of the hundreds of fx_* pair tools returns this data. It stops short of a full 5 because it never contrasts itself with the related aggregate tools (fx_convert, fx_latest, fx_timeseries), leaving the agent to infer that this is a single-pair spot lookup rather than a conversion or time-series endpoint.

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: the phrase "latest ... fixing" signals a current reference-rate lookup, and "no estimate" hints at when not to expect an approximation. There is no explicit when-to-use statement, no mention that fx_convert should be used for amount conversion or fx_timeseries for historical rates, and no note on staleness/frequency of the fixing.

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