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

With no annotations, the description carries the full burden and does notable work: it discloses the x402 payment model ($0.01 USDC per call, first call per tool per day free), the data provenance, and a full example response showing the return shape. It omits error behavior for invalid CNPJs and any rate/latency notes, keeping it short of a 5.

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 the core purpose and field list, then pricing, then the example. The example response block is sizable but earns its place since no output schema exists. Slight density cost but no wasted sentences.

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 single-parameter read tool with no output schema, the inline example response supplies the return shape an agent would otherwise lack, and the pricing model covers cost expectations. Remaining gaps (error handling, invalid CNPJ behavior) are minor.

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 description coverage is 100%, so the single `cnpj` parameter is already documented as 14 chars with or without punctuation. The description's 'Supports alphanumeric CNPJ' merely restates what the schema already says, adding no meaning beyond structured data.

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 (registry lookup), a specific resource (Brazilian company/CNPJ), the data source (Receita Federal open data), and enumerates the returned fields. It is easily distinguished from siblings like cep_lookup or bank_lookup, which target postal codes and bank codes respectively.

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 scope (CNPJ input only, alphanumeric supported) implies when to use it, and the sibling names make the domain obvious, but there is no explicit when/when-not guidance or named alternative. Usage is inferred rather than stated.

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