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

With no annotations at all, the description carries the full burden and does it well: it discloses the payment requirement ($0.002 USDC/call via x402 on eip155:8453), the upstream authority (ECB fixing), the peg-handling contingency, and that the value is a deterministic non-estimated figure. It stops short of stating staleness behavior on non-publication days (weekends/holidays) or error responses.

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 dense sentence plus a bracketed payment tag, front-loaded with cost/transport before the actual function. Every clause earns its place, though the pricing bracket sits ahead of the purpose statement, which slightly delays the core answer.

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?

With no output schema and no annotations, the description compensates by enumerating the returned content (rate, rate date, derivation method, source attribution). That is close to complete for a zero-parameter lookup; only freshness/error semantics are unaddressed.

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; the currency pair is baked into the endpoint and the description makes that explicit by naming AUD/DKK up front. Nothing further is needed on the input side.

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 (AUD/DKK reference rate) and pins the exact semantics: latest ECB fixing, with peg fallback when DKK is pegged. The pair is named explicitly, which disambiguates it from the inverse fx_dkk_aud and from the hundreds of other fx_* pair tools.

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?

There is implicit routing value in 'Deterministic, sourced, no estimate' — an agent can infer this is for audit-grade official rates rather than market quotes. But no sibling is ever named (fx_convert, fx_latest, fx_timeseries, or the inverse pair), so the agent must infer when to prefer this over them.

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