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 on economics and provenance: it discloses the price ($0.002 USDC per call over x402 on eip155:8453) and the upstream source (ECB fixing / official peg), plus the derived method. It does not cover freshness cadence, behaviour on non-publishing days or market holidays, caching, or whether a peg-derived value is flagged differently in the response, which for a paid data call are meaningful gaps.

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 bracketed price tag, with the resource named early and the return payload enumerated compactly. The parenthetical enumeration of return fields ('rate date, method (direct, cross via EUR or peg-derived), source attribution') is information-dense rather than padded, though the single sentence is long enough that the return-value portion is easy to skim past.

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, no-annotation tool with no output schema, the description supplies the essentials an agent needs: what value comes back, that it is deterministic and sourced rather than estimated, the payment requirement, and the fallback logic (direct vs cross via EUR vs peg-derived). Absent only are freshness/cadence and failure modes for days when the ECB does not fix, which are the remaining completeness gaps.

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 at the argument level and the baseline of 4 applies. The pair being fixed in the tool name means an agent never has to construct currency codes, which is a sound design choice given the sibling family.

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+resource: the official PLN/QAR reference rate, naming the exact source (latest ECB fixing, or the official peg where QAR is pegged) and the fields returned. It is unmistakably distinct from non-FX siblings, but it never states the base/quote direction convention (PLN per QAR vs QAR per PLN), which matters given a ~600-tool fx_* family with pair-named siblings and no inverse fx_qar_pln to disambiguate.

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 implied rather than stated: 'reference rate' plus 'no estimate' signals this is for authoritative, auditable single-pair rates, not conversions or history. It never names alternatives such as fx_convert (for arithmetic conversion), fx_latest (for a broad snapshot), or fx_timeseries (for historical series), nor does it say when this pair tool should be skipped in favour of a cross via a more liquid pair.

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