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

With no annotations, the description carries the full behavioral burden and does real work: it discloses cost ($0.002 USDC/call), payment protocol (x402 on eip155:8453), data provenance (ECB fixing), the derivation method (direct, cross via EUR, or peg-derived), and that values are deterministic rather than estimated. The main gap is failure behavior — what happens on weekends/holidays or when no fixing exists for the pair.

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 dense sentence, front-loaded with the payment tag and then the substantive content, with no filler. It is slightly overloaded — cost metadata, provenance, method taxonomy, and a quality claim all in one sentence — but every clause carries information an agent needs.

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 reference-rate call with no output schema, the description usefully enumerates the returned components (rate, rate date, method, source attribution), which compensates for the missing return schema. Only the absence of failure/staleness behavior keeps it from being fully complete.

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 of 4 applies; the pair is fully encoded in the tool name and the description never implies configurable inputs. There is nothing extra the description could add on parameter semantics.

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 states a specific verb+resource: the official XOF/SGD reference rate, sourced from the latest ECB fixing, with the peg fallback explicitly noted. That is far more precise than a tautology like 'get FX rate'. It does not mention the sibling fx_convert or fx_latest, which an agent might reasonably choose instead for conversion or multi-pair needs, so it stops short of a 5.

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: 'latest ... fixing' signals a spot reference rate, and 'no estimate' signals when not to use it (i.e., don't use it for projected/forecast values). There is no explicit 'use this when X, use fx_convert when Y' routing and no statement of when the rate is stale or unavailable.

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