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

With no annotations, the description carries the full load and does well: it discloses the payment model and price ($0.002 USDC/call, x402, eip155:8453), the authoritative source, the derivation method (direct, cross via EUR, or peg-derived), and guarantees determinism with no estimation. It stops short of describing caching/staleness windows or how the x402 payment is settled before the call.

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 plus a bracketed cost/protocol tag, with no filler. The purpose statement is front-loaded after the payment tag, and every clause (rate date, method, attribution, no estimate) conveys distinct information an agent would otherwise lack.

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-annotation, no-output-schema tool, the description covers the essential contract: what value is returned, its provenance, and the fields bundled with it (rate date, method, source). Only minor operational details such as data update cadence and payment prerequisites 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 nothing for the description to disambiguate; the baseline for a parameterless tool applies. The description correctly frames the result as an always-latest lookup, which is consistent with the empty schema.

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 (PLN/SGD reference rate), a specific source (European Central Bank fixing, or the official peg for SGD), and the fact that it is the latest value. The pair is unambiguous and an agent can select it among the hundreds of fx_* siblings directly from the description.

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?

Implies usage by saying 'latest ECB fixing' and 'no estimate', so an agent can infer this is for spot reference-rate lookups rather than historical series or conversions. However, it never names alternatives such as fx_convert, fx_latest, or fx_timeseries, nor does it state when not to use it.

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