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 x402 cost (0.002 USDC on eip155:8453), the data source, the derivation method, the peg fallback, and that a rate date is returned. It stops short of stating behavior on non-publishing days (weekends/holidays) or what an error/empty response looks like, which matters for a rate that updates only on ECB fixing days.

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 bracketed pricing tag plus a single sentence that packs source, fallback, returned fields, and guarantees with no filler. Information is front-loaded and every clause (method, peg, source attribution) maps to something the caller receives.

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 and no annotations, so the prose effectively serves as the return contract by enumerating rate, rate date, method, and source attribution, which is sufficient for a zero-parameter lookup. The remaining gap is failure/timing behavior around ECB publication schedules.

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 and the baseline of 4 applies. The description correctly adds no parameter narrative, keeping the pair fixed by tool name.

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 the exact resource (NZD/AUD reference rate) and its provenance (official ECB fixing, peg fallback), so the agent knows precisely what datum comes back. It does not explicitly contrast itself with siblings such as fx_convert or fx_timeseries, leaving differentiation to the encoded currency pair in the name.

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?

'Deterministic, sourced, no estimate' implies this is the authoritative choice when a verified reference rate is needed rather than a computed conversion, which is useful implied guidance. However, there is no explicit statement of when to prefer this over fx_convert, fx_timeseries, or the inverse pair fx_aud_nzd, and no mention of prerequisites beyond the payment notice.

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