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

With no annotations the description carries the full burden and does substantial work: it discloses the paid x402 model with an exact price ($0.002 USDC) and chain (eip155:8453), the upstream source, how the figure is derived (direct, cross via EUR, or peg-derived), that a rate date accompanies it, and that no estimation occurs. What it omits is edge behavior — what happens on non-fixing days, staleness bounds, or error conditions.

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?

Effectively one front-loaded sentence: the paid-endpoint tag is bracketed up front, then the resource, then the source and derivation, then the trust guarantee. The trailing 'Deterministic, sourced, no estimate' slightly restates earlier content, but it reinforces the key differentiator rather than padding.

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-param, no-output-schema lookup this is close to complete — it tells the agent what the payload describes (rate, date, method, attribution) and that the figure is authoritative. Without an output schema, the exact field names and numeric formatting of the response remain unstated, which is the only real gap.

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 to select a pair or date.

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 (DKK/INR reference rate), the producing authority (European Central Bank fixing, or the official peg when INR is pegged), and the returned facets (rate date, derivation method, source attribution). Because siblings are hundreds of currency-pair permutations of the same tool, the pair encoded in the name plus this ECB framing is enough for an agent to identify exactly which slice of the family this is.

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 'call this when you need the DKK/INR rate,' and the phrase 'no estimate' signals a hard requirement for sourced data. But it never routes against siblings such as fx_convert (for actual conversion math), fx_timeseries (for history), or fx_latest, and gives no when-not condition.

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