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 x402 gate ($0.002 USDC per call on eip155:8453), the data provenance, the method used to derive the rate, and that values are deterministic with no estimation. It does not state what happens when no fresh fixing exists (staleness or error behavior), which is the main remaining 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?

Cost and payment protocol are bracketed up front, followed by one dense sentence covering source, fallback, and returned fields. Every clause earns its place and nothing is repeated.

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 single-value tool with no output schema, the description explains provenance, cost, and the qualitative contents of the result. Only the response shape and the no-data/freshness behavior are left implicit.

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 the baseline is 4. The description still adds meaning by enumerating what the response conveys: rate date, derivation method (direct, cross via EUR, or peg-derived), and source 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?

Names the exact resource (CNY/USD) and the exact product: the official ECB reference fixing, including the peg fallback case. That is far more specific than the pair name alone and tells an agent this is an official reference rate, not a market quote.

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?

The single-pair, zero-argument nature implies usage, but there is no explicit when-to-use or when-not guidance against siblings such as fx_convert (arbitrary pair conversion), fx_latest, or fx_timeseries (historical series). An agent must infer that this is the point lookup and the others are for other shapes of request.

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