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

With no annotations, the description carries the full burden and does well: it discloses the paid model ($0.002 USDC/call, x402, eip155:8453), the upstream authority (ECB), the possible derivation methods (direct, cross via EUR, peg-derived), and asserts determinism with no estimation. It does not say what happens on days ECB does not publish a fixing, or whether results are cached, which is the remaining behavioural gap.

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 front-loaded sentence plus a bracketed payment tag; the cost, the pair, the source and the method variants all arrive early with no filler. Nothing could be removed without losing 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 parameterless, annotation-free, output-schema-free lookup, the description covers provenance, derivation method, rate date, attribution, determinism and price. What is missing is edge-case behaviour around non-publication days and stale-fixing handling, which matters for a reference-rate tool but is a minor gap overall.

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 for the description to disambiguate beyond fixing the pair, which the title and description jointly do.

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 and direction ("Official CNY/CVE reference rate") and enumerates what the returned figure is: latest ECB fixing, or the official peg when CVE is pegged. An agent can identify the exact pair and data source. It does not explicitly differentiate itself from near-neighbours such as fx_convert, fx_latest or fx_timeseries, 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 by the currency pair in the name and by the mention of a peg-derived method, but there is no explicit statement of when to prefer this tool over fx_convert (for arithmetic), fx_latest (for a bare current quote) or fx_timeseries (for history). The agent must infer the routing decision from the pair alone.

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