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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), so the description's added value is in disclosing the backing service (Correios via ViaCEP) and the fallback resolution rule: if the street has no CEP of its own, the general district/locality CEP is returned. That fallback is behavior an agent cannot infer from annotations or schema. Minor gap: no mention of failure modes or what happens with partial/ambiguous address input.

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?

Four sentences with no filler, and the source and the two modes are front-loaded ahead of the fallback rule and the privacy warning. Slightly dense in the third sentence, but every clause carries information an agent needs.

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?

With no output schema, the description takes on the burden of explaining returns and does so ('o CEP do logradouro ou, se a rua não tiver CEP próprio, o CEP geral do distrito ou da localidade'). The address-mode return value is only implied ('obter o endereço'), and error/empty-result behavior is unaddressed, which keeps it short of a 5 for a six-parameter open-world lookup.

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 83%, so the baseline is 3 and the schema already documents cep, city, number, street, and neighborhood. The description nonetheless adds the crucial semantic the schema does not express: the parameters form two mutually exclusive modes rather than one combined query (notable because the schema marks all six as required, which would otherwise mislead an agent into always sending everything).

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 verb and resource ('Consulta a base de CEPs dos Correios') and immediately clarifies that the tool is bidirectional: CEP in → address out, or address fields in → CEP out. That bidirectional framing separates it cleanly from the only nearby sibling, cnpj_lookup, without the agent needing to open either schema.

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?

Explicitly names both invocation modes ('Informe cep para obter o endereço, ou uf, city, street, neighborhood e number para obter o CEP'), so an agent knows which parameter set to populate for each goal. It also adds a hard exclusion — 'Nunca envie nome, CPF ou outro dado da pessoa' — which is exactly the when-not guidance that prevents misuse.

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