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

Beyond the readOnlyHint and idempotentHint annotations, the description discloses the algorithm used, input format flexibility, the return value (mathematical validity and cleaned CNPJ), and a key limitation (no active status verification). This is rich behavioral context with no contradiction to the annotations.

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?

The description is three concise sentences, front-loaded with the main action, then usage context, then input/output specifics. Every sentence earns its place with no wasted words.

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 tool with detailed annotations and no output schema, the description is complete. It covers what the tool does, when to use it, input requirements, return value, and a key limitation, making it fully actionable for an agent.

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 input schema already provides 100% coverage with a description of the 'cnpj' parameter. The description adds the specific constraint 'Input must be a 14-digit string,' which is not explicitly stated in the schema. This adds meaningful clarification beyond the schema's example.

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?

The description clearly states the tool 'Validates a Brazilian CNPJ' using the official Receita Federal checksum algorithm, with a specific resource and scope. It distinguishes from sibling validation tools (e.g., validate_cpf, validate_cuit) by naming the CNPJ specifically and mentioning it returns validity and the cleaned CNPJ.

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 states when to use the tool: 'Use this tool when processing Brazilian company registrations, B2B invoices, supplier onboarding, e-commerce orders, or any document requiring a valid Brazilian company taxpayer number.' It also provides an exclusion: 'Does not verify if the CNPJ is active in the Receita Federal database,' guiding users away from using it for that purpose.

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