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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so safety is covered by structured data. The description adds genuine context beyond that: the data source is the public Receita Federal base and the output is already qualified for use in a legal document. It does not say what happens with an invalid/not-found CNPJ, latency, or rate limits.

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 compact sentences that front-load the returned fields and then state the input requirement. Nothing is redundant, though 'com a qualificação pronta para a peça' is slightly vague about what the output actually contains.

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 simple read-only, single-parameter lookup with no output schema, the description does the important work: it lists the data returned, which substitutes for a missing output schema, and names the authoritative source. The remaining gap is CNPJ input formatting and not-found behavior.

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?

One parameter with 0% schema description coverage, so the description must compensate: 'Leva só o número do CNPJ' does establish that only the CNPJ number is needed, no extra filters. However, it never specifies the accepted format (14 raw digits vs. the 18-character masked form implied by minLength 14 / maxLength 18), which is the single most useful detail an agent needs here.

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+resource: fetches company registration data from Receita Federal's public CNPJ database, and enumerates the returned fields (razão social, situação, endereço, atividade, sócios). It is clearly distinguishable from siblings like cep_lookup or oab_processes_lookup, though it never explicitly contrasts itself with them.

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?

Usage is only implied — an agent infers it should call this when it needs company registration data for a legal filing, reinforced by 'pronta para a peça'. There is no explicit when-to-use, when-not-to-use, or named alternative (e.g., cep_lookup for address-only needs).

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