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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds an auth requirement (the cred_… token) that is not in the structured data, which is genuine behavioral context. Return format is not described, but no output schema exists and the read-only nature makes that a minor gap.

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?

One sentence, front-loading the resource (balance/statement) before the credential caveat. Nothing is wasted and the key constraint is not buried.

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-only tool with no output schema, the description covers purpose and the one non-obvious prerequisite (the cred_ token). It is complete enough to invoke correctly; only richer return/format detail is absent, which the missing output schema does not require.

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 of 4 applies. Schema coverage is 100% (trivially, empty object) and there is nothing for the description to compensate for.

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 content ('Saldo e extrato do crédito pré-pago'), so an agent knows it retrieves the pre-paid credit balance and statement. It is reasonably distinguishable from credito_recarregar (which tops up credit), though it never names the sibling 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?

The parenthetical supplies a real precondition ('precisa do token cred_…'), which tells the agent this call needs a specific credential. However, there is no explicit when-to-use vs. when-not guidance or reference to alternatives such as billing or credito_recarregar.

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