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

With no annotations, the description carries the full behavioral burden and does disclose real substance: cost ($0.002 USDC/call via x402 on eip155:8453), data provenance (ECB fixing), peg handling, and what accompanies the rate (rate date, method, source attribution). It stops short of describing error behavior, staleness/caching, or what the payload looks like, which matters for a paid call with no output schema.

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 price/payment bracket so the cost is visible before the agent commits. The remaining clauses about method (direct, cross via EUR, peg-derived) and 'deterministic, sourced, no estimate' each carry information, though the method enumeration is slightly dense for one sentence.

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 FX lookup with no output schema, the description covers the essentials an agent needs: source, pair, return fields, payment model, and reliability claims. The missing piece is the quote direction convention, which is consequential given the mirror-image sibling fx_aud_sgd.

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; baseline 4 applies. The description correctly implies a parameterless fixed-pair lookup.

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 ("Official SGD/AUD reference rate: latest European Central Bank fixing") and pins the data source, which clearly separates it from the hundreds of other fx_* pair tools. The one gap is direction convention: with both fx_sgd_aud and fx_aud_sgd in the sibling list, "SGD/AUD" does not state whether the rate is SGD-per-AUD or AUD-per-SGD.

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: an agent can infer this is the tool for the official SGD/AUD reference rate, but the description never says when to pick it over fx_convert, fx_latest, or fx_timeseries, nor does it exclude anything. No prerequisites or conditions are stated beyond the implicit 'you want this specific pair'.

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