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

With no annotations at all, the description carries the full burden and does well: it discloses the price ($0.002 USDC/call), the payment rail (x402, eip155:8453), determinism ('no estimate'), the provenance of the figure, the derivation method (direct, cross via EUR, or peg-derived), and the fallback when NZD is pegged. It stops short of describing freshness/publishing cadence or error behaviour when a fixing is unavailable.

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?

One dense, front-loaded sentence with the payment terms up front and no redundant restatement of the name. The closing tagline 'Deterministic, sourced, no estimate' is compact and adds trust signals rather than padding, though the bracketed metadata prefix is heavy.

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 read tool with no output schema, the description adequately conveys what comes back (rate, rate date, method, source attribution). Missing only secondary details such as the publication cadence of ECB fixings and what happens when no rate is available.

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 the baseline is 4; there is nothing for the description to document. The pair is fixed by the tool name itself, which the description reinforces.

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+resource: 'Official AUD/NZD reference rate: latest European Central Bank fixing', including fallback behaviour (the official peg when NZD is pegged). Because the tool name encodes the exact currency pair, the agent can distinguish it from the hundreds of other fx_* pair tools without opening a schema.

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: an agent can infer it should call this to obtain the AUD/NZD reference rate. There is no explicit guidance about when to prefer it over the inverse pair (fx_nzd_aud), the generic fx_convert, fx_latest, or fx_timeseries for historical series, nor any note about which pairs exist as dedicated tools.

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