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

With no annotations, the description carries the full burden and does well: it discloses the cost and payment mechanism ($0.002 USDC/call, x402, eip155:8453), the upstream source (ECB), the peg fallback, and the computation method vocabulary (direct, cross via EUR, peg-derived). It stops short of describing failure modes (no fixing available, stale rate) or freshness/caching behavior, which matters for a paid call.

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?

Two packed sentences, front-loaded with price/network then the substantive description of what is returned. 'Deterministic, sourced, no estimate' is largely redundant with the ECB-fixing claim already made, so a small amount of wording does not earn its place.

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-parameter, no-output-schema tool the description supplies everything needed to call and interpret it: the exact pair, the rate provenance, the fallback behavior, the method taxonomy, and the payment overhead. Only error/availability behavior is left implicit, which is minor at this complexity.

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 no parameter meaning to add and the baseline applies. The description instead spends its words on the return contract (rate date, method, source attribution), which is the useful analogue here.

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 verb+resource: the 'Official XOF/KRW reference rate' with its source (ECB fixing) and fallback (official peg). The pair identity is unambiguous, though the description never names related siblings (fx_convert for arbitrary pairs, fx_xof_eur for a cross) that an agent might otherwise prefer.

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: 'Official ... reference rate ... latest ECB fixing' tells the agent this is for authoritative reference rates, not conversions or historical series. It gives no explicit when-not guidance and no routing to alternatives such as fx_convert or fx_timeseries; the paid nature is disclosed but framed as a tag, not as a decision criterion.

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