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 a paid x402 call at $0.002 USDC on eip155:8453, the upstream source (ECB), the derivation method (direct, cross via EUR, or peg-derived), and that values are deterministic rather than estimated. It does not cover behavior on non-fixing days (weekends/holidays) or failure/payment-failure modes, which keeps it short of 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, front-loaded with the pricing/payment tag before the capability statement, which is the right order for a metered tool. Every clause (source, method, determinism) adds information; the bracketed payment prefix is slightly noisy but operationally necessary.

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?

There is no output schema and no parameters, so the description must convey the return shape, and it does: rate value plus rate date, method, and source attribution. The one real gap is that the quote direction (1 CNY = X INR versus the inverse) is never stated, which is material for an FX rate consumer.

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 usefully reinforces that there is nothing to specify by framing the result as always the latest fixing, so an agent will not look for a date argument. No further parameter meaning is needed.

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 resource (CNY/INR reference rate) and a specific provenance-backed operation (latest ECB fixing or official peg), including what metadata accompanies it (rate date, derivation method, source attribution). Because each sibling is a distinct pair or aggregate tool (fx_cny_eur, fx_convert, fx_timeseries), the pair encoded in the name plus this description makes the tool trivially distinguishable.

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: 'latest' fixing, no parameters, no date selection. The description never tells the agent when to prefer fx_convert or fx_timeseries instead, nor what to do if a historical rate is needed. An agent can infer this is a spot-reference lookup, but no exclusions or alternatives are stated.

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