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

With no annotations supplied, the description carries the full burden and does so well: it discloses the paid model ($0.002 USDC/call via x402 on eip155:8453), the data authority (ECB fixing), the methodology (direct, cross via EUR, or peg-derived), and that the output is deterministic and sourced rather than estimated. It stops short of stating update cadence, weekend/holiday or stale-rate behavior, and failure modes when no fixing exists.

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 with the payment/protocol banner front-loaded, then the resource, then the return composition. Every clause carries information (cost, source, fallback, method, attribution), with only mild compression cost from packing four ideas into one sentence.

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-annotation, no-output-schema tool, the description covers what an agent needs: cost and payment path before committing, the authoritative source, the peg fallback, and the shape of the returned fields (rate, date, method, attribution). The remaining gap is freshness semantics — how often the fixing updates and what a stale or missing fixing looks like.

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 schema has nothing to explain and the baseline is 4. The description correctly needs no parameter guidance and instead spends its words on provenance and output composition.

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 precise verb+resource: the official GBP/CNY reference rate sourced from the latest ECB fixing, plus the fallback case when CNY is pegged. The pair is unambiguous from the name, but the description never contrasts itself with nearby siblings such as fx_cny_gbp (inverse), fx_convert, fx_latest, or fx_timeseries, so sibling differentiation is left to inference.

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: calling out 'official reference rate' and 'no estimate' signals this is for authoritative reference/accounting figures rather than ad-hoc conversion, which is where fx_convert would apply. There is no explicit when-to-use, when-not, or named alternative for historical series or inverse rates, so an agent must infer the routing.

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