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 substantial work: it discloses the pricing model and payment protocol ($0.002 USDC/call, x402, eip155:8453), the derivation method (direct, cross via EUR, or peg-derived), and explicitly states the result is deterministic, sourced and not an estimate. It stops short of explaining freshness cadence or error behavior 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?

The cost/payment bracket is front-loaded so an agent sees the paid nature before anything else, and the substantive sentence is dense but single-pass readable. Minor density cost from cramming source, method, peg fallback and determinism claim into one sentence, but nothing is wasted or repeated.

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 annotations and no output schema, the description covers source, method, fallback behavior, cost and determinism. It even sketches the return payload (rate, rate date, method, source attribution), which is what an agent needs given the absence of an output schema; only freshness/latency expectations are left out.

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 and the baseline of 4 applies. The description correctly adds no parameter discussion.

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 the exact resource (CAD/AUD reference rate) and its provenance (latest ECB fixing, or the official peg if AUD is pegged), which lets an agent distinguish it from the hundreds of fx_<pair> siblings purely by the pair it serves. The verb/resource pairing is specific rather than tautological.

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?

The word 'latest' and the ECB-fixing framing imply this is for spot/reference lookups rather than historical series, which implicitly separates it from fx_timeseries and fx_convert. However, no alternative is named and there is no explicit when-not-to-use guidance, which matters in a catalog where fx_convert and fx_latest also exist.

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