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

With no annotations, the description carries the full burden and does well: it discloses the paid access model (x402, $0.002 USDC/call on eip155:8453), the upstream source (ECB), the fallback behavior when NZD is pegged, and that output is deterministic with no estimation. It does not cover freshness/latency (how stale the latest fixing may be) or failure behavior, which is why it falls short of 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?

A single tight sentence with no filler; the pairing and the nature of the value are stated immediately. The bracketed cost preamble is slightly front-loaded ahead of the purpose, but it is genuinely decision-relevant given the paid x402 model.

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 tool with no output schema, the description supplies what the agent would otherwise lack: the returned fields (rate, rate date, derivation method, source attribution) and the guarantee of determinism. Only the exact response shape and freshness window remain unspecified, a minor gap for this complexity level.

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; the schema is an empty object and the description correctly implies the pair is fixed by the tool identity. 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 (USD/NZD) and a specific kind of value (official ECB reference fixing, or the official peg), so the agent knows this is a spot reference-rate lookup rather than a conversion. It does not explicitly distinguish itself from siblings like fx_convert, fx_latest, fx_timeseries, or fx_nzd_usd, which is the only thing keeping it from a 5.

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 implied rather than stated: 'reference rate ... no estimate' hints this is the tool for an authoritative single-pair quote, but there is no explicit when-to-use, no mention of the inverse pair (fx_nzd_usd), the general converter (fx_convert), or the time-series tool as alternatives. An agent must infer routing from the name.

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