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 a per-call price ($0.002 USDC), the payment protocol (x402, eip155:8453), and that the value is deterministic, sourced and not an estimate. What it does not cover is failure behavior when payment is missing, rate limits, or refresh cadence of the ECB fixing.

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 sentence plus a bracketed payment prefix, front-loaded with price and protocol before the substantive description. Nearly every clause earns its place, though the parenthetical about pegs and cross-via-EUR methods is compressed almost to the point of being hard to parse.

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?

No input schema, no output schema, no annotations, so the description must supply everything. It covers cost, payment mechanism, data source, derivation methods and the fields returned, which is close to complete for a single-value lookup. Only failure and freshness behavior 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 tool takes zero parameters, so there is no parameter semantics to explain; baseline 4 applies. The description instead spends its words on what the returned figure contains (rate date, method, source attribution), which is the useful equivalent for a no-arg call.

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 resource and scope: the official AUD/CVE reference rate, ECB fixing or peg, with rate date, derivation method and source. That is well beyond a restatement of the name. It does not, however, contrast itself with the very similar siblings (fx_convert, fx_latest, fx_timeseries, fx_aud_eur) that an agent choosing among hundreds of fx_* tools would also consider.

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?

There is no when-to-use or when-not-to-use guidance and no named alternative. An agent gets no signal about when to prefer this single-pair endpoint over a generic conversion tool or a multi-pair lookup. Usage is only implied by the pair encoded in the name.

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