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

With no annotations present, the description carries the full burden and does well: it discloses the paid cost ($0.002 USDC/call via x402 on eip155:8453), the upstream source (ECB fixing or official peg), the resolution method (direct, cross via EUR, or peg-derived), and that the result is deterministic and non-estimated. It omits operational traits such as error behavior when the fixing is unavailable or how stale the 'latest' fixing can be.

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 sentences, front-loaded with the payment model and then the substantive description of what is returned. Every clause adds information; the bracketed payment prefix is arguably operational noise for tool selection but is legitimately needed before an agent commits to a paid call.

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 tool with no output schema, the description covers the essentials an agent needs: what rate, from which source, by what derivation method, and what metadata accompanies it. Only minor gaps remain, such as update cadence and failure modes, which keep it short of full completeness.

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 the baseline is 4; there is nothing in the schema for the description to clarify. The description appropriately spends its words on output semantics (rate date, method, source attribution) instead of parameters.

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 official XAF/KMF reference rate) and states its provenance (ECB fixing or the official KMF peg), which is far more specific than a generic 'get exchange rate'. It does not explicitly contrast itself with the many sibling pair tools or with fx_convert/fx_latest, but the currency pair in the name plus the 'reference rate' framing makes the target unambiguous.

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: 'Deterministic, sourced, no estimate' hints at using this when an authoritative reference is required instead of a market/estimated quote, and 'latest ECB fixing' implies a daily-fixing cadence. There is no explicit when-to-use vs. fx_convert or fx_latest, and no statement of when this tool is the wrong choice (e.g. historical or converted amounts).

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