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

With no annotations, the description carries the full burden, and it does add real context: the $0.005 USDC charge via x402, the free first call per tool per day, and the upstream source. However, it omits rate limits, freshness/update cadence, and failure behavior, so a mutation-free read tool is still only partly characterized.

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?

Front-loaded with what the tool returns, followed by cost and a concrete example. The example JSON is long but earns its place because no output schema exists; the sentences are tight with no filler.

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, no-output-schema read tool, the description supplies the units, annualization basis, source, cost, and an exact sample payload, which is nearly everything needed to call and consume it. Only cadence and error semantics are 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?

The schema has zero parameters, so per the rubric the baseline is 4. There is nothing for the description to disambiguate, and it correctly avoids inventing parameter guidance.

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?

Names a precise resource set — Selic target set by Copom, daily annualized Selic, and CDI — with the issuing authority (Banco Central do Brasil). An agent immediately knows this is the Brazilian policy-rate data source, but the description never references siblings like ipca_inflation or ptax_exchange_rate to route the agent explicitly.

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?

There is no when-to-use guidance, no exclusions, and no mention of alternative rate/inflation tools. The only usage-adjacent content is the pricing model, which tells the agent about cost but not when this tool is the right choice.

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