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

With no annotations present, the description carries the full burden and does reasonably well: it discloses the cost ($0.002 USDC/call), the payment protocol and network (x402, eip155:8453), the upstream source (ECB), and that the value is deterministic rather than estimated. It does not state what happens when the ECB has not published for a given day (weekends/holidays) or how auth/payment failures surface, which are the main remaining behavioral gaps.

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 paragraph with the payment terms bracketed off and the core purpose front-loaded after them. Every clause adds something (source, method, peg handling, determinism), though the price/protocol metadata occupying the first clause slightly delays the actual purpose.

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 no-parameter, no-output-schema tool, the description covers what matters: source, methodology (direct/cross/peg-derived), and the fields returned (rate date, method, attribution). Only the fallback behavior on non-publishing days and the concrete response shape are 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?

The tool takes zero parameters, so there is nothing for the description to disambiguate; the baseline of 4 applies. The description correctly implies no inputs are needed by presenting the pair as fixed in 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?

The description names a specific verb and resource: the official DKK/KMF reference rate from the latest ECB fixing, including the peg path for KMF. It is clearly distinct from fx_convert and fx_latest in substance, though it never explicitly names those siblings or states that this is a single pre-computed pair rather than a conversion.

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-specific name and the 'reference rate' framing, and the bracketed x402 price tag signals that this is a paid call requiring payment setup. However, there is no explicit when-to-use or when-not-to-use guidance, and no routing to alternatives such as fx_convert (for arithmetic) or fx_timeseries (for history).

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