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

With no annotations, the description carries the full burden and does so well: it discloses the paid x402 pricing model and exact cost, the source (ECB), the derivation method (direct, cross via EUR, or peg-derived), that a rate date is attached, and that values are deterministic rather than estimated. It stops short of describing rate limits, caching/freshness cadence, or payment-failure behavior.

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 resource and followed by the qualifiers that matter (source, method, determinism). The bracketed payment preamble is boilerplate but carries genuinely decision-relevant cost information; 'Deterministic, sourced, no estimate' is slightly redundant with the preceding clause but is a useful trust signal.

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, no-output-schema lookup, the description covers the essential unknowns: what the number represents, where it comes from, how it was derived, when it is dated, and what it costs. Only the freshness/publication cadence and the concrete response field shape remain unspecified.

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 semantics to explain; the schema is trivially complete. Baseline 4 applies.

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 PLN/SAR reference rate') and its provenance (ECB fixing, or peg when SAR is pegged), which is specific enough for an agent to know precisely what value comes back. It does not, however, differentiate itself from related siblings such as fx_convert, fx_latest, or fx_timeseries, leaving that routing to the pair-encoded name alone.

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: an agent infers 'call this to get the PLN-to-SAR rate.' There is no explicit when-to-use, no statement of when a sibling like fx_convert (conversion) or fx_timeseries (history) is the better choice, and no note on whether a stale/non-published day yields an error.

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