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

With no annotations, the description carries the full burden and does most of it: it discloses the payment gate ($0.002 USDC/call, x402, eip155:8453) — a real precondition an agent must satisfy before calling — plus determinism, source attribution, the reported method (direct/cross/peg-derived), and that estimates are excluded. It stops short of covering failure modes, staleness windows, or what is returned when no fixing exists for the current date.

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?

Dense and front-loaded — the paid/x402 prefix comes first, then the substantive purpose sentence. The second half is a long appositive chain ('with the rate date, the method ... and the source attribution') that reads as a run-on but every clause carries distinct information.

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, so the description must describe the return payload — it does, listing rate date, derivation method, and source attribution. Payment requirement, determinism, and peg fallback are all covered. What is missing is the response's numeric shape/units and behavior when the fixing is unavailable.

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 beyond the pair encoded in the name. 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?

Names a specific resource (PLN/USD reference rate) with a precise provenance (latest ECB fixing, or the official peg when USD is pegged), which is far more than a restatement of the name. It does not reference the pair-family siblings (fx_latest, fx_convert, fx_timeseries), but the currency pair itself is unambiguous from the name, so an agent can still select it correctly.

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: 'Official ... reference rate' and 'no estimate' suggest this is the authoritative-source choice over any estimated or aggregated rate, and therefore the right pick for accounting/compliance work. However, no sibling is named and there is no explicit statement of when to prefer fx_latest, fx_convert, or fx_timeseries instead, nor any note about ECB publication cadence (weekends/holidays).

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