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

No annotations exist, so the description carries the full burden and does a decent job: it discloses the cost and payment rail ($0.002 USDC/call, x402, eip155:8453), the source (ECB), the fallback path (peg-derived rate), the derivation method (direct, cross via EUR), and that results are deterministic rather than estimated. It does not state update cadence, weekend/holiday staleness behavior, or error cases when no fixing exists.

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 with the paid-endpoint tag front-loaded, then the resource, source, fallback and output attributes in priority order. Nothing is redundant, though the clause list (rate date, method, source attribution) is packed enough that it reads as an enumeration rather than prose.

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 source, derivation method, determinism, and payment mechanics, which is most of what an agent needs. The notable omission is freshness semantics — how recent the fixing is and what happens on non-publication days — which matters for a rate an agent may use in a calculation.

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 is empty with zero required parameters, so there is nothing for the description to disambiguate; the baseline for a no-parameter tool applies. The description correctly implies the pair is fixed by the tool identity rather than passed as an argument.

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 the exact resource (Official XOF/NZD reference rate) and the specific provenance (latest ECB fixing, or the official peg when NZD is pegged), which is far more than the tautological name would give. It stops short of naming the sibling it competes with (fx_convert, fx_latest, fx_xof_eur for cross-derived values), so pair selection relies on the tool name rather than the prose.

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?

"Reference rate" and "no estimate" implicitly tell the agent this is for authoritative/accounting-grade lookups rather than indicative market quotes, but there is no explicit when-to-use or when-not-to-use statement. With fx_convert, fx_latest and fx_timeseries in the sibling set, the agent is left to infer that this is the single-pair snapshot endpoint.

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