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 most of it: it discloses the metered cost ($0.002 USDC per call via x402 on eip155:8453), the upstream authority, that a peg may override the fixing, and that the value is deterministic rather than estimated. Missing are the payment/auth mechanics in more detail and any freshness or rate-limit bounds.

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 compact sentences with no filler, and the important qualifiers (ECB authority, peg fallback, method, attribution) are front-loaded. The bracketed pricing/transport prefix is boilerplate but genuinely required for an x402 metered call, so it earns 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-argument, no-output-schema tool, the description covers source, fallback behavior, and the fields a caller will receive, which is close to sufficient. It stops short of describing the response envelope or error conditions, which an agent pricing a $0.002 call might want to know.

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, which is the correct baseline of 4; the pair is fully encoded in the tool name and the description confirms it. Nothing further about parameters could usefully be added.

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 (the XOF/KMF reference rate), its authority (European Central Bank fixing, or the official peg when one applies), and the companion fields returned (rate date, method, attribution). That is specific enough to identify the tool without opening the schema, but it never explicitly contrasts itself with the nearby fx_convert, fx_latest, or fx_timeseries tools, which is what a 5 would require in this dense fx_* family.

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?

'Deterministic, sourced, no estimate' implies the intended use case (a reference/compliance rate rather than a market quote), but the agent must infer that. There is no explicit statement of when to prefer this pair-specific tool over fx_convert for amount conversion or fx_latest for a snapshot, and no exclusions.

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