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

With no annotations provided, the description carries the full burden and does substantial work: it discloses payment cost and protocol, the data source, the method resolution chain (direct, cross via EUR, peg-derived), and explicitly asserts determinism with 'no estimate'. Gaps remain around publication cadence (ECB fixing timing), weekend/holiday behavior, and failure modes.

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 tightly packed sentences: the payment/protocol tag is bracketed and front-loaded so cost is seen before commitment, then the core behavior follows. Every clause adds distinct information — source, peg fallback, return metadata, determinism — with no repetition of the tool name.

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 parameterless, schema-less, annotation-less tool the description is unusually complete: it covers cost, source, computation method, and the descriptive fields returned, which substitutes for the missing output schema. Only freshness/publication timing and error semantics are absent.

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 schema is an empty object with zero parameters, so there is nothing for the description to disambiguate; the baseline of 4 applies. Currency pair and date are fixed by the tool identity, and the description confirms the returned metadata fields (rate date, method, attribution) that a caller would otherwise expect as parameters.

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 a specific verb+resource pair (official PLN/NZD reference rate) and pins down the exact source (European Central Bank fixing) plus the fallback mechanism (official peg when NZD is pegged). That level of specificity separates it from generic spot/estimate rate tools and from the mirrored fx_nzd_pln sibling even without naming it.

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?

Pricing context ($0.002 USDC, x402 on eip155:8453) implies the call is metered and should not be spammed, and 'reference rate' implies official/accounting use rather than trading. However, it never states when to prefer this over fx_convert, fx_latest, fx_timeseries, or the inverse pair, so the agent must infer routing from the tool name alone.

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