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

With no annotations provided, the description carries the full behavioral burden and discloses key traits: the official source, weekend/holiday fallback behavior, and the pricing model ($0.005 USDC per call via x402, first call daily free). It also gives an example response, though it omits error conditions or authentication details beyond the payment mechanism.

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?

The description is front-loaded with the core purpose, followed by pricing and a compact example response. Every sentence earns its place: the fallback behavior, cost, and example return shape are all necessary for correct invocation and interpretation.

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?

Given that there is no output schema and no annotations, the description is complete enough: it explains the data source, fallback semantics, cost, and provides a representative response example. No critical information needed to call or interpret the tool is missing.

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?

Schema coverage is 100%, so the baseline is 3, but the description adds meaningful semantic context beyond the schema: the date parameter falls back to the last bulletin before a weekend/holiday, and the currency parameter represents a foreign currency unit quoted in BRL. The example response also clarifies the distinction between requested_date and quote_date.

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 states a specific verb (retrieves) and resource (official PTAX exchange rate published by Banco Central do Brasil), with clear scope (BRL per unit of foreign currency, for a given date). It clearly distinguishes itself from sibling tools like selic_rate and ipca_inflation, which cover different economic indicators.

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?

The description implies usage by specifying that it returns a rate for a given date and falls back to the last bulletin before weekends/holidays. However, it does not explicitly state when to choose this tool over alternatives such as bank_lookup or selic_rate, leaving usage guidance to inference.

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