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

With no annotations, the description carries the full burden and does substantial work: it discloses the paid x402 model ($0.002 USDC/call, eip155:8453), the upstream source (ECB), the derivation method (direct, cross via EUR, or peg-derived), and that output is deterministic with no estimation. It stops short of stating refresh cadence, error behavior when no fixing exists, or payment failure handling.

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 compact sentence plus a bracketed price tag, with the resource front-loaded and the fallback/method/source details following in a logical order. Every clause adds information an agent would otherwise have to guess.

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-output-schema tool the description covers the essentials: what the number is, where it comes from, how it was derived, and what it costs. The remaining gap is operational detail (freshness/update cadence and behavior on missing fixings) rather than anything needed to select the tool.

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?

Zero parameters, so there is nothing to disambiguate and the baseline applies; the description correctly implies a call with no inputs. It adds the useful nuance that the pair and method are fixed by the tool itself rather than selectable.

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 (official CHF/CNY reference rate) and a specific provenance (latest European Central Bank fixing, or the official peg when CNY is pegged). An agent knows exactly which pair and which kind of number it is getting without opening anything else; the currency pair in the name does the sibling differentiation against the hundreds of other fx_* pairs.

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 only implied: an agent can infer this returns a single CHF/CNY rate rather than performing a conversion, but the description never names an alternative such as fx_convert, fx_latest, or fx_timeseries, nor does it say when to prefer those. There are no explicit when-not conditions either.

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