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

With no annotations at all, the description carries the full load and does well: it discloses the paid, x402-metered access model ($0.002 USDC/call on eip155:8453), the authoritative ECB source, that the rate is deterministic rather than estimated, and how the value is derived (direct, cross via EUR, or peg-derived). It stops short of stating staleness behavior (weekends, holidays, last-fixing fallback) or what happens if no fixing exists, which is the main remaining gap for a financial data tool.

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?

Two tight sentences with no filler; the payment tag is front-loaded in brackets so callers see the cost before anything else. Density is high and slightly jargon-heavy (x402, eip155:8453) but each clause (source, method, date, attribution, no-estimate) maps to a real caller question.

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 description must convey the return payload — it does, naming the rate, rate date, derivation method, and source attribution. For a zero-parameter single-value lookup with declared return fields, that is nearly complete; only failure/staleness handling is unaddressed.

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?

Zero parameters, so there is nothing for the description to disambiguate and the baseline applies. The schema is empty and fully covered.

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 AUD/KRW reference rate) and an unambiguous source (latest ECB fixing, or the official peg when KRW is pegged), plus the exact fields returned: rate date, derivation method, and source attribution. Because the sibling set is a mechanical fx_<base>_<quote> matrix, the pair is already pinned by the name, and the description adds the detail that actually differentiates it from generic rate tools: ECB-sourced, deterministic, no estimate.

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 by the phrase 'reference rate' — the description never says when to prefer this over siblings such as fx_convert, fx_latest, or fx_timeseries, nor does it say when not to use it. There is no exclusion of alternatives, so an agent must infer the routing itself.

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