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 access model ($0.002 USDC/call on eip155:8453), the authoritative source, the peg fallback path, the derived-method variants (direct, cross via EUR, peg-derived), and that results are deterministic with no estimation. It does not state rate direction (RON per CNY vs CNY per RON) or precision, which is the main residual 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 dense sentence preceded by a bracketed payment/transport tag, with the core resource front-loaded ahead of the qualifiers. Every clause (source, peg fallback, method, attribution, determinism) 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 cost, source, fallback logic, and the returned fields (rate, rate date, method, attribution). It falls short of fully complete because the numeric semantics of the returned rate — quote direction and rounding — are left unstated, and there is no output schema to compensate.

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 for the description to disambiguate; baseline 4 applies. The description correctly implies a parameterless call by describing a fixed pair and a fixed source.

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/RON reference rate) and pins down the provenance (latest ECB fixing, or the official peg when RON is pegged), so it is clearly distinct from generic siblings like fx_convert or fx_timeseries. An agent can tell exactly what value this returns without opening any schema.

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 rather than stated: fetch the official reference rate for this one pair. There is no explicit routing guidance against near-neighbours such as fx_convert (conversion), fx_latest, or fx_timeseries, and no note about when the peg fallback applies versus a normal fixing. The pegged-rate clause is the only conditional behavior explained.

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