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 cost ($0.002 USDC/call on eip155:8453), the upstream authority (ECB fixing vs official peg), the returned metadata (rate date, derivation method, source attribution), and the determinism guarantee. The only unstated behavior is freshness/staleness — no mention that ECB fixings are not published on weekends or TARGET holidays.

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, cost/transport front-loaded in the bracket, then the data contract. Every clause adds information (source, peg fallback, derivation method, attribution, no estimation) with no restatement of the tool name.

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 characterize return values and it does: rate, rate date, method, and attribution are all named. The gap is operational freshness — an agent cannot tell from this whether calling on a Sunday returns Friday's fixing or an error.

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 baseline is 4 and the schema has nothing to explain. The description usefully resolves the pair orientation and explains that the rate may be direct, EUR-crossed, or peg-derived, which is the only semantic ambiguity such a parameterless call could carry.

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 (ISK/DKK reference rate), a specific source (European Central Bank fixing), and the direction of the pair, which is the key differentiator against the hundreds of sibling fx_* tools. An agent can tell fx_isk_dkk from fx_dkk_isk or fx_isk_eur without opening the schema.

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 by the pair name and the 'reference rate' framing, but there is no explicit routing to siblings such as fx_convert (for conversion math), fx_timeseries (for history) or fx_latest (for multi-pair snapshots). The peg-handling parenthetical is a data property, not a when-to-use rule.

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