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

With no annotations supplied, the description carries the full burden and does well: it discloses the paid x402 model ($0.002 USDC/call, eip155:8453), the authoritative source, the fallback branch for a pegged NZD, and that results are deterministic with no estimation. It stops short of stating failure behavior when no fixing exists or how stale a rate can be before it is refused.

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 payment tag followed by one dense sentence that front-loads the resource and then qualifies source, fallback, and output fields. Every clause carries information, though the parenthetical about pegging and the method list make it slightly packed for a single sentence.

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 and no annotations, it covers the essentials: payment/auth mechanics, provenance, determinism, and the conceptual return fields (rate date, method, attribution). Only the practical edges, such as expected response shape and behavior when the fixing is unavailable, are unstated.

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 and there is nothing the description needs to compensate for. The pair and source are fully baked into the tool identity.

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 resource ("Official CHF/NZD reference rate") and names the exact data source and variant (latest ECB fixing, or peg when NZD is pegged), so the agent knows precisely what value it gets. It does not address the obvious siblings, fx_nzd_chf (inverse) or fx_convert, but the pair is unambiguous from the description plus 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?

Usage is only implied: the description clarifies which fixing you receive, which lets an agent self-select when it needs an official reference rate rather than a conversion. There is no explicit when-not guidance, no note that fx_chf_nzd is the inverse of fx_nzd_chf, and no pointer to fx_convert or fx_timeseries for derived or historical needs.

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