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

With no annotations, the description carries the burden and does so well: it discloses the paid x402 payment requirement with price and chain (eip155:8453), the data provenance (ECB fixing, or official peg when CNY is pegged), and that results are deterministic with no estimation. It does not state failure behavior when no fixing exists or whether the response is cached/stale, which is why it isn't 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?

A single dense sentence plus a bracketed payment tag – no filler, and every clause (source, peg fallback, method, attribution) adds information. The payment bracket is front-loaded ahead of the purpose, which is slightly awkward for an agent scanning for what the tool does, costing one point.

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 no-parameter, no-output-schema tool, the description covers the essentials: what the rate is, where it comes from, its fallback path, and the cost of calling it. It stops short of describing response shape or error conditions for missing fixings, which would fully close the gap.

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 no parameter semantics to document; the baseline for a no-param tool is 4. The description does compensate usefully by describing what the returned rate represents (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?

States a specific verb+resource: 'Official JPY/CNY reference rate' sourced from the ECB fixing, with method and attribution. The currency pair is unambiguous against the hundreds of fx_* siblings because the direction is named in the title itself. It does not explicitly point to fx_convert, fx_latest or fx_timeseries as different-scope siblings, which keeps it at 4.

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 resource: an agent wanting a single official JPY/CNY rate knows this is it, and the clause 'Deterministic, sourced, no estimate' hints at when to prefer it over derived data. There is no explicit when-to-use/when-not statement, no mention of fx_convert for cross-currency math, and no note on how it differs from fx_timeseries or fx_latest.

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