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-usage model (PAID $0.002 USDC/call, x402, eip155:8453), which is critical for an agent that must arrange payment before calling. It also discloses the data methodology (direct, cross via EUR, or peg-derived) and that results are deterministic and sourced rather than estimated. Gaps remain around staleness on non-fixing days and error behavior, keeping 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?

Compact, front-loaded with the billing/transport tag an agent must see before deciding to call, then the substantive content in a single dense sentence. Nothing is redundant, though the reading is dense and directionality is never stated plainly.

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, the description compensates well by naming the returned elements (rate, rate date, method, source attribution) and the pricing context. The main omission is quote convention and behavior when no fixing exists (weekends/holidays).

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 the baseline is 4. The description correctly implies no input is needed and instead spends its words on output semantics and methodology.

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 and provenance: 'Official CNY/BRL reference rate: latest European Central Bank fixing', plus what accompanies the rate (date, derivation method, attribution). That is far more than a restatement of the name. It stops short of 5 because it never clarifies quote direction/convention (CNY per BRL vs BRL per CNY) against the reverse sibling fx_brl_cny, nor names any alternative like fx_convert.

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?

Implicitly routes the agent well: 'Deterministic, sourced, no estimate' tells it to use this when an authoritative fixing is required rather than a computed estimate, and the ECB attribution signals the authoritative source. However, there is no explicit when-not guidance or pointer to fx_convert, fx_latest, or fx_timeseries for derived or historical needs.

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