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

With no annotations, the description carries the burden and does disclose a genuinely useful behavioral trait: the $0.005 USDC-per-call cost via x402, which tells the agent this is a paid invocation. It also characterizes the values fetched, though it does not describe return shape or freshness guarantees.

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 sentences, front-loaded with the resource and source, then the pricing detail. No filler or redundancy.

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-param lookup with no output schema, the description identifies the specific quantities returned (Selic target, effective daily Selic, CDI, annualized basis), which is enough for correct invocation. It omits whether values are a single current snapshot versus a series, a minor gap.

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 has zero parameters, so the baseline of 4 applies; there is nothing for the description to disambiguate beyond the resource scope it already states.

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 (the Brazilian policy rate, including Copom's Selic target, effective daily Selic and CDI, annualized) and its authoritative source (Banco Central do Brasil). An agent can distinguish this cleanly from siblings like ipca_inflation or ptax_exchange_rate.

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?

No explicit when-to-use guidance or named alternatives, but the tool takes zero parameters and has a single unambiguous data-lookup purpose, so the omission is low-cost. Usage is implied by the resource named rather than stated.

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