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

With no annotations, the description carries the behavioral burden well by disclosing the $0.002 USDC per-call charge, x402 payment mechanism, and eip155:8453 network. It also explains determinism, source attribution, possible calculation methods, and the absence of estimation, while listing the rate date, method, and source metadata returned.

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?

The payment prefix and two compact statements convey cost, scope, provenance, methodology, and quality guarantees without filler. The lead payment tag is operationally important, and the dense parentheticals contain relevant selection and invocation information.

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-argument fixed-pair lookup, this is nearly sufficient to select and invoke the tool, including its expected cost and principal result metadata. With no output schema, it still leaves the precise rate encoding and publication or freshness timing unspecified, and selection against the generic FX siblings requires inference.

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?

There are no parameters, so the zero-input baseline applies. The fixed pair and “latest” wording reinforce that callers should not supply currencies, an amount, or a historical 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?

The description identifies the exact resource, an official XOF/MYR reference rate, and narrows it to the latest ECB fixing or an official-peg derivation. It is highly specific about the pair, rate date, and calculation method, but does not explicitly differentiate itself from generic siblings such as fx_convert, fx_latest, or fx_timeseries.

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?

“Latest,” “official,” “reference rate,” and “no estimate” establish a clear use case: obtaining a current authoritative fixing rather than an estimated value. There is no explicit when-not guidance or routing to fx_convert for amount conversion or fx_timeseries for historical data.

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