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 a per-call cost ($0.002 USDC via x402 on eip155:8453), the authoritative upstream source (ECB), the rate construction method, and the determinism guarantee. Gaps remain around what happens on payment failure or stale fixings, and whether the charge is incurred per call regardless of outcome.

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 single sentence, front-loaded with the payment/transport block in brackets followed by the substantive rate definition and its provenance attributes. Nothing is redundant, though the bracketed pricing prefix is dense and competes with the purpose statement for first-read attention.

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, no-output-schema lookup, the description covers the essentials an agent needs: what rate, from where, how derived, what accompanies the value (rate date, method, attribution), and the cost model. The only real omission is failure behavior, such as missing fixing or payment rejection.

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; the baseline of 4 applies. The description correctly signals a parameterless call and instead spends its words on what the caller cannot express via inputs (source, method, cost).

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 (AED/IDR reference rate) and a specific source and construction method (ECB fixing, direct/cross-via-EUR/peg-derived), which is far more informative than the bare pair name. It does not, however, explicitly distinguish this from neighboring tools like fx_convert, fx_latest, or fx_timeseries, leaving the agent to infer that this returns a single spot rate rather than a conversion or history.

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 words 'reference rate' and 'deterministic, sourced, no estimate' — the agent can infer this is for authoritative single-pair lookups rather than arbitrary amount conversion. There is no explicit when-to-use/when-not statement and no named alternative for conversion (fx_convert) or historical series (fx_timeseries), which matters given hundreds of near-identical fx siblings.

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