Validate Pix Key

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

With no annotations, the description carries the behavioral disclosure burden. It explains accepted key formats, CPF/CNPJ check-digit validation, and the type-detection behavior. It does not state precisely what happens for invalid keys or how type detection is returned, but the core behavior is transparent.

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 compact and front-loaded with the main purpose. The background sentence on PIX is brief and relevant, and the closing usage sentence adds practical value. No major redundancy, though it could trim small redundancies around key types.

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 validator with an output schema, the description covers accepted inputs, validation strength, type detection, and intended use context. It is complete enough for an agent to confidently call the tool, though explicit error/return behavior would add more color.

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 only defines 'key' as a string with 0% coverage, so the description must compensate. It does by detailing that the key can be CPF, CNPJ, e-mail, phone, or EVP and giving digit lengths and format hints. It stops short of providing concrete examples or exact patterns.

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 states a clear verb ('validate') and specific resource ('Brazilian PIX key') and adds that it detects the key type. It also distinguishes itself from siblings like validate_tax_id and validate_phone_number by covering PIX-specific formats including EVP and phone keys.

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 phrase 'Lets a payments agent confirm a PIX key is well-formed before initiating a transfer' gives a clear use context. It does not explicitly list alternatives or say when not to use it, but the PIX-specific framing is enough to signal it over generic validators.

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