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 payment requirement and price ($0.002 USDC/call via x402 on eip155:8453), the authoritative source (ECB fixing), the peg fallback rule, and the derivation method. It stops short of stating response field names, staleness/publication-lag policy, or failure behavior, which keeps it from 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?

Two compact sentences: a bracketed cost/payment prefix followed by the substantive definition. The key facts — rate, source, method, attribution, determinism — are front-loaded and every clause adds information; nothing is redundant.

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 tool with no output schema and no annotations, the description covers the essential decision inputs: what the number is, where it comes from, how it was derived, what metadata accompanies it, and what it costs. Only the concrete response shape and publication-lag semantics are left unstated.

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 beyond confirming the pair is fixed by the tool name. Baseline 4 applies for a parameterless tool.

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 and scope: the official CNY/SGD reference rate, specifically the latest ECB fixing with peg fallback. It also states what comes back (rate date, derivation method, source attribution), which lets an agent distinguish this from fx_latest, fx_convert, and fx_timeseries without opening a 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: an agent can infer this is the right tool when it needs a single, authoritative CNY/SGD reference rate. However, in a sibling set containing fx_convert, fx_latest, fx_timeseries, and hundreds of pair tools, no explicit when-to-use or when-not-to-use guidance is given, and the deterministic/no-estimate line is the only implicit routing signal.

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