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 model and price ($0.002 USDC/call on eip155:8453), the upstream source (ECB), the fallback behavior when IDR is pegged, the derivation method (direct, cross via EUR, or peg-derived), and that no estimation is involved. Missing are failure modes when no fixing exists and any freshness/staleness window.

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 with no filler, though the bracketed payment metadata is placed first rather than the purpose, which front-loads cost over function. Every clause (source, method, fallback, attribution, determinism) adds distinct information.

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 tool with no output schema, the description usefully enumerates what comes back: the rate, the rate date, the derivation method, and source attribution. Only the numeric format/precision and behavior when a fixing is unavailable remain unstated.

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?

This tool takes zero parameters, so the baseline is 4 and there is nothing for the description to disambiguate. The description does not need to explain inputs, and the pair is already encoded in the tool name.

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 the exact resource ('Official DKK/IDR reference rate') and pins the source to the 'latest European Central Bank fixing', which is far more specific than the bare pair name and separates it from the dozens of sibling fx_* pairs. It stops short of explicitly contrasting itself with siblings like fx_convert, fx_latest or fx_timeseries, which is the only thing keeping it off a 5.

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 a sourced DKK-to-IDR reference rate suitable for compliance/audit work ('deterministic, sourced, no estimate'). There is no explicit when-to-use versus when-not-to-use statement and no pointer to fx_convert or fx_timeseries for alternative needs.

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