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 so well: it discloses the price ($0.002 USDC/call), the payment protocol and chain (x402, eip155:8453), the upstream data source (ECB), the fallback behavior for pegged currencies, and the derivation method. It does not cover error behavior or what happens on stale/no-fixing days, which keeps it below a 5.

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?

One dense sentence, front-loaded with the cost/payment tag, then the resource, then the source and method. Every clause adds information; the only mild cost is that the bracketed payment preamble sits ahead of the actual purpose statement.

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 lookup, the description covers source, fallback semantics, method disclosure, and the returned metadata fields, which is what an agent needs to trust and use the value. The one gap is that the x402 payment flow itself (how settlement is triggered) is named but not explained.

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 to disambiguate beyond the currency pair baked into the name. The description correctly spends no words on parameters and instead documents the returned fields (rate date, method, attribution).

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 (CNY/AUD reference rate) and a specific source (ECB fixing), and clarifies the fallback case (official peg when AUD is pegged). It is clearly distinguishable from other fx_* pair tools by the currency pair, but it never acknowledges the general-purpose siblings (fx_convert, fx_latest, fx_timeseries) that an agent might otherwise choose.

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: 'Deterministic, sourced, no estimate' suggests this is the tool for an authoritative reference rate rather than a converted estimate, but there is no explicit 'use this when / use fx_convert instead when' guidance. Given roughly 300 sibling fx pair tools plus fx_convert and fx_latest, an explicit routing rule would add real value.

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