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

With no annotations supplied, the description carries the behavioral burden and does well: it discloses the paid cost ($0.002 USDC/call), payment protocol (x402, eip155:8453), ECB source, peg fallback, rate date, calculation method, attribution, and that the result is deterministic with no estimate. It does not explain how payment must be attached or what happens on payment failure, which keeps it from a 5.

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 sentences, front-loaded with the payment and protocol tag, then the substantive description of the rate source, fallback, output fields, and determinism. Every clause adds useful information and there is no filler.

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?

With no output schema, the description must convey what comes back, and it does: the rate, rate date, method (direct/cross/peg-derived), and source attribution. It is slightly incomplete for a paid tool because it does not explain the payment handshake or error/missing-fixing behavior, though the payment tag and sibling tools provide surrounding context.

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 meaning to add beyond the schema. Per the baseline for zero-parameter tools, a 4 is appropriate; the description does not need to compensate for any schema coverage gap.

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 verb+resource: the 'Official AUD/SGD reference rate' from the 'latest European Central Bank fixing.' It distinguishes the nature of the tool from conversion siblings by calling it a reference rate and noting the peg fallback and cross-rate method, which an agent can match to the unique fx_aud_sgd pair.

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 implied: an agent should call this when it needs a sourced AUD/SGD reference rate rather than a conversion. However, the description never explicitly says when to prefer it over fx_convert, fx_latest, or fx_timeseries, nor does it state exclusions or prerequisites beyond the payment tag.

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