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

With no annotations at all, the description carries the full behavioral burden and does a decent job: it discloses the pay-per-call cost and protocol ($0.002 USDC, x402, eip155:8453), the data provenance (ECB), the derivation path, and that results are deterministic with no estimation. It omits failure behavior (missing fixing, weekends/holidays), staleness or update cadence, and caching, which keeps it below a 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?

Two sentences, front-loaded with the paid/x402 tag (which an agent needs before calling) and then the substantive description. Density is high but nearly every clause adds a distinct fact; the only excess is stacking cost, protocol, and chain namespace into one bracketed preface.

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?

There is no output schema, and the description compensates by naming the returned elements (rate, rate date, method, source attribution). For a zero-parameter rate lookup this covers what an agent needs to call and interpret it; only the absence of update cadence and non-trading-day behavior leaves a real gap.

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 input schema has zero parameters, so there is nothing for the description to disambiguate. The baseline of 4 applies; the description correctly avoids inventing parameters and instead spends its words on provenance.

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 pair ("Official IDR/PLN reference rate") and pins the source (latest ECB fixing), the fallback case (official peg when PLN is pegged), and the derivation methods (direct, cross via EUR, peg-derived). An agent can distinguish this from fx_pln_idr, fx_eur_pln, or fx_convert from the description 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: "reference rate" plus "no estimate" signals this is for authoritative sourcing rather than indicative pricing, but the description never names an alternative (fx_convert for conversions, fx_timeseries for history, fx_latest for a snapshot) or a condition that selects one over the other. The agent must infer routing from the pair name.

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