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

Annotations already declare readOnlyHint=true and idempotentHint=true, covering safety. The description adds the return behavior—'Returns whether the key is valid and the detected key type'—and enumerates supported formats, which enriches understanding beyond the structured 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 two sentences, front-loaded with the core purpose, followed by usage context and supported formats. It is concise, with no redundant information, and every sentence contributes to understanding the tool.

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 simple validation tool with one parameter, no output schema, and clear annotations, the description is complete. It covers purpose, usage context, supported input formats, and return value, making it fully sufficient for an agent to invoke correctly.

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 coverage is 100%, with the key parameter described in detail including examples. The description adds a list of accepted formats (CPF, CNPJ, email, phone, EVP) and explains the phone format (+55), complementing the schema but not carrying the full burden since the schema already documents the parameter well.

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 'Validates a Brazilian PIX key format' with a specific verb and resource. It distinguishes itself from sibling validators by focusing on PIX keys, even listing all supported key types, making the tool's purpose unambiguous.

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 'Use this tool when processing Brazilian payments, validating payment forms, or any fintech application handling Brazilian transfers,' providing clear usage context. However, it does not explicitly mention alternatives for standalone CPF/CNPJ validation, though the sibling list implies such tools exist.

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