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

With no annotations, the description carries the full burden, and it discloses meaningful behavior: the value is deterministic and sourced, it is not an estimate, and the response distinguishes direct, EUR-cross, and peg-derived methods. It also surfaces the paid-per-call x402 model (USDC, eip155:8453), which is important operational context, though it omits failure modes such as a stale or missing fixing.

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?

It is a single dense sentence-complex with no filler, and the core purpose is stated first. The bracketed payment preamble leads with pricing rather than the tool's job, which slightly buries the most useful information for selection.

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 has to hint at the return shape, and it does list the rate date, the derivation method, and source attribution. For a zero-parameter lookup this is nearly sufficient; a note on data freshness expectations or behaviour when no fixing exists is the only 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 beyond what the empty schema already shows; baseline 4 applies. The description's only obligation is to make the implied fixed pair clear, which it does.

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 (DKK/XAF) and a specific verb-equivalent ('reference rate: latest European Central Bank fixing'), and clarifies the peg fallback case. Against hundreds of identically-shaped fx_* siblings, this leaves no ambiguity about which pair and which data source the tool returns.

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: the phrase 'latest ECB fixing' signals a spot-rate lookup, and 'no estimate' signals a preference for authoritative data. However, the description never tells the agent when to pick this over fx_convert, fx_latest, or fx_timeseries, nor any precondition such as needing a paid x402 balance.

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