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 model ($0.002 USDC/call via x402 on eip155:8453), the authoritative source, the publication basis (latest ECB fixing or official peg), and asserts determinism / no estimation. It stops short of stating behavior on non-fixing days, staleness windows, or failure/refund handling.

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?

A single dense sentence group, front-loaded with the billing notice then the substantive definition; each clause adds information (source, peg fallback, date, derivation method, attribution, determinism). The price prefix is somewhat noisy but is genuinely relevant to an agent that must decide whether to spend a call.

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?

No output schema exists, so the description responsibly enumerates the returned fields — rate, rate date, derivation method, and source attribution — which is what an agent needs to consume the result. It is nearly complete; only edge-case behavior (holidays, stale fixings) is left undefined.

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 there is nothing for the description to disambiguate. The pair and source are fixed by the name, which is consistent with an 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?

Names the exact resource (EUR/NOK) and the specific source (European Central Bank fixing), plus the fallback case for a pegged NOK, so the agent knows precisely which rate it gets. It does not differentiate itself from the numerous fx_* pair siblings or from fx_convert / fx_latest, so an agent must infer the distinction from the name alone.

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 and the 'reference rate' framing, but there is no explicit when-to-use or when-not-to-use guidance, and no pointer to fx_convert for conversions or fx_latest / fx_timeseries for other shapes of the same data. In a namespace of hundreds of near-identical pair tools, that omission leaves routing to inference.

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