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

With no annotations supplied, the description carries the behavioral burden and does well: it discloses the paid x402 access model ($0.002 USDC/call on eip155:8453), the authoritative data source, the peg fallback path, and asserts determinism ('no estimate'). It does not state what happens on rate unavailability or how stale a fixing could be, so it stops short of full transparency.

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?

One bracketed cost/access tag followed by a single dense sentence; the paid-call caveat is front-loaded and every clause (source, date, method, attribution, determinism) adds information the caller cannot get elsewhere.

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?

There is no output schema, so the description must sketch the response, and it does name the key returned fields: rate date, computation method (direct, cross via EUR, or peg-derived) and source attribution. It omits numeric precision/units convention and error behavior for missing fixings, which leaves a small but real gap for a zero-arg paid call.

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 and the input schema is an empty object, so there is no parameter meaning for the description to add. Baseline 4 applies; the description correctly asks nothing of the caller.

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 (DKK/CNY reference rate) and a specific provenance (latest European Central Bank fixing, or the official peg when pegged), which distinguishes it from generic siblings like fx_convert, fx_latest and fx_timeseries even though the tool name alone would not. The verb/resource pairing and stated source make the tool's identity unambiguous.

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: an agent can infer it should call this when it needs an authoritative DKK/CNY rate rather than a computed conversion. There is no explicit 'do not use for X, use fx_convert instead' routing, which matters given hundreds of near-identical fx_* siblings and a fx_convert alternative.

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