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 well: it discloses the paid access model ($0.002 USDC per call via x402 on eip155:8453), the authoritative source, the acceptable derivation methods (direct, cross via EUR, peg-derived), and that the value is deterministic rather than estimated. It does not cover publication cadence, weekend/holiday staleness, or caching, which are the remaining behavioral gaps for a rate tool.

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 sentence, front-loaded with the cost/access bracket and then the resource and its returned attributes. Every clause adds information — cost, source, method, attribution, determinism — with no filler or repetition of the name.

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?

There is no output schema and no input schema, so the description must convey what comes back, and it does: rate, rate date, derivation method, and source attribution. Missing are quoting convention (units per unit), numeric precision, and freshness behavior, which an agent aggregating rates would want, but the definition is otherwise complete for a zero-argument lookup.

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 nothing for the description to disambiguate and the schema is trivially complete. Baseline 4 applies; the description usefully implies the pair is fixed by the tool identity rather than passed as an argument.

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 resource (the official IDR/GBP reference rate), a specific source (European Central Bank fixing, or the official peg), and what the result carries (rate date, derivation method, source attribution). Because the pair is encoded in the tool name and each sibling covers a distinct pair, an agent can select this one without opening a schema.

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?

Implied usage is clear — fetch the current official reference rate for this pair — but there is no explicit when-to-use or when-not guidance against siblings such as fx_convert, fx_latest, or fx_timeseries. The paid-call context is stated, which helps the agent judge cost, but routing between the FX tools is left to inference.

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