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

With no annotations, the description carries the full burden and does well: it discloses the paid x402 flow, the exact price and chain, the authoritative source (ECB), the peg fallback rule, and that output is deterministic and estimated-free. Remaining gaps are secondary behavior: precision/rounding, staleness or holiday behavior, and error semantics if no fixing exists — a mutation-free read tool, so these are less critical.

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 dense sentences, with the payment metadata front-loaded and the substantive purpose immediately following. Every clause (source, fallback, method, attribution, determinism) does work. The bracketed payment tag is slightly token-heavy but is justified for a paid endpoint.

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 compensates by enumerating the key return fields — rate date, derivation method (direct / cross via EUR / peg-derived), and source attribution — which is exactly what an agent needs to interpret the result. It stops short of stating the quote direction convention (units of SGD per DKK) and precision, the one substantive gap for a rate tool.

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 the schema-based baseline is 4 and there is nothing for the description to disambiguate. It correctly does not waste space restating currency arguments that don't exist, and the fixed pair is baked into the tool identity.

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 verb+resource: the official DKK/SGD reference rate, with the data source (ECB fixing) and the peg fallback. The pair is unambiguous from both the name and the sentence. It does not, however, name or distinguish itself from related siblings such as fx_convert, fx_latest, or fx_timeseries, so an agent must infer that this returns a single pair's spot reference rather than performing 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 implied rather than stated: request the official DKK/SGD reference rate when that pair is needed. There is no explicit when-to-use vs. when-not guidance, no pointer to fx_convert for arithmetic or fx_timeseries for history, and no note on what to do when a rate is unavailable. The upfront price disclosure does at least help an agent weigh cost against alternatives, which lifts it above a bare 2.

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