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

With no annotations available, the description carries the burden and does disclose the key credential requirement (token 'cred_…'). It does not go further to state read-only behavior, rate limits, or error conditions, though 'saldo e extrato' strongly implies a safe query.

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 a single, front-loaded sentence: the purpose comes first and the credential requirement is parenthesized. No filler or repetition.

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 read tool, the description covers purpose and authentication in one line. It could also describe the response format or confirm that the call does not consume credit, but those are minor for a balance/statement query.

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 input schema has zero parameters, so there is nothing to document beyond the schema. The description correctly adds the only relevant context — the token requirement — rather than inventing parameter semantics.

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 clearly identifies the resource (prepaid credit) and the operation (returning balance and statement), so an agent can tell what the tool does. It does not explicitly contrast with the sibling credito_recarregar, but the sibling's name makes the distinction obvious.

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?

It provides clear context: use this to inspect prepaid credit balance/extract, and it explicitly warns that a 'cred_…' token is required. It does not name alternatives or exclusion conditions, but no-argument usage makes the context sufficient.

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