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

With no annotations, the description carries the full burden and does disclose meaningful behavioral facts: the data provenance (Receita Federal open data), the normalized result fields, support for alphanumeric CNPJ, and the paid access model ($0.01 USDC per call via x402). What is missing is anything about rate limits, errors, or latency for a non-CNPJ 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?

A single dense sentence that front-loads purpose, then enumerates returned fields, then lists two operational caveats (alphanumeric support, price). Nothing is wasted, though the field enumeration in the middle is a long list without structure.

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?

There is no output schema, so the description usefully compensates by naming the returned data (legal/trade name, status, CNAE, address, Simples/MEI, QSA) and by disclosing the payment mechanism. For a single-parameter lookup this is close to complete, with only error/miss handling unaddressed.

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?

Only one parameter and schema description coverage is 100%, so the schema fully documents the cnpj string, its 14-char length, punctuation tolerance and alphanumeric support. The description's note about alphanumeric CNPJ merely repeats the schema, adding no new syntax or format detail, so the baseline 3 is appropriate.

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 (Brazilian company/CNPJ registry lookup) plus the authoritative data source (Receita Federal open data). The CNPJ domain is clearly distinct from siblings like bank_lookup and cep_lookup, so an agent can disambiguate immediately.

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 implied rather than stated: it is evidently the tool for looking up a company by CNPJ. However, it never states when to prefer it over siblings such as bank_lookup or cep_lookup, nor any prerequisites or failure conditions (e.g. invalid/unknown CNPJ).

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