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

With no annotations supplied, the description carries the full burden and does substantial work: it discloses that the call is paid ($0.002 USDC via x402 on eip155:8453), that the value comes from the official ECB fixing rather than a market mid, and that it is deterministic with no estimation. It still omits error/failure behavior and rate-limiting or caching semantics.

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 sentence with the payment metadata bracketed and front-loaded, followed by source, fallback, and return-content details. Every clause conveys information an agent would need, and nothing is repeated from structured fields.

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 parameterless lookup with no output schema, the description covers source, fallback path, and the conceptual return contents (rate, rate date, derivation method, attribution). It stops short of describing the actual response field names or what a pegged versus floating case looks like in practice.

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 per the baseline this scores 4. The description correctly implies no input is required, since the currency pair is fixed by the tool name.

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 resource (the XOF/IDR official reference rate) and the exact source and derivation (European Central Bank fixing, peg fallback). It is clearly not a generic converter, but it never explicitly contrasts itself with near-neighbours like fx_convert, fx_latest, fx_timeseries, or the inverse fx_idr_xof, so sibling differentiation rests mostly on the tool name.

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 no when-to-use or when-not-to-use guidance: nothing says to prefer this over fx_convert for arbitrary pairs, fx_latest for newest data, or fx_timeseries for history. The only implicit selection signal is the stated per-call price, which at least tells the agent a cost is incurred.

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