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

Beyond the annotations (readOnlyHint=true, idempotentHint=true), the description discloses the verification algorithm, the return behavior ('Returns whether the CPF is mathematically valid, along with the cleaned CPF'), and the key limitation that it only validates format/checksum, not database existence. This provides full transparency for a read-only validation tool.

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, front-loaded sentences. The first sentence states the primary purpose, the second gives usage context, and the third details input/output and limitations. Every sentence contributes unique information, with no redundancy or fluff.

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?

Given the tool's simplicity (1 parameter, no output schema, strong annotations), the description covers all essential aspects: purpose, usage scenarios, input format, return value, and limitations. It is fully self-contained and sufficient for an agent to select and invoke the tool correctly, especially compared to similar sibling validation tools.

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 schema already documents the 'cpf' parameter well with an example and notes formatting is automatically removed (100% coverage). The description adds the 11-digit length constraint and explicitly allows 'with or without formatting', which reinforces schema semantics without conflicting. Minor added value, hence 4 rather than 5.

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 verb 'Validates' with the specific resource 'Brazilian CPF' and the method 'official Receita Federal checksum algorithm', distinguishing it from sibling validation tools like validate_cnpj and validate_cuil. It also specifies the exact use cases (Brazilian user registrations, invoices, tax forms), leaving no ambiguity about the tool's purpose.

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 description explicitly states when to use the tool: 'processing Brazilian user registrations, invoices, tax forms, e-commerce orders, or any document requiring a valid Brazilian individual taxpayer number'. It also provides a critical exclusion: 'Does not verify if the CPF exists in the Receita Federal database', which tells users when not to use it and implies an alternative for existence checks.

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