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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint. The description goes well beyond these by explaining exactly what it returns and what it never returns (not the PIX key, full name, or tax number), and by detailing error semantics: e2e_invalido means malformed and correctable, while nao_encontrado does not prove fraud because another institution may have settled it. This is substantial behavioral context.

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 longer than average, but nearly every sentence earns its place with use cases, return fields, security properties, and error handling. Minor redundancy exists, such as repeating the free/open/no-API-key point already in the title, and the rhetorical 'that is the point of this tool' phrase. Overall, it is well-structured and front-loaded.

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?

With one parameter, a fully descriptive schema, an output schema, and rich annotations, the description still adds critical context: the counterparty trust scenario, the exact return fields, the warning about amount matching, and detailed error interpretation. Nothing material is missing for an agent to select and invoke this tool 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?

The input schema already fully defines the e2e parameter with pattern, length, and composition, so baseline is 3. The description adds meaning beyond the schema: the E2E is the only lookup key, a valid E2E for R$1 is not proof of a R$1000 payment, and users must verify amount and timestamp themselves. This is genuinely useful semantic guidance not present in the schema.

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 a specific verb ('verifies'/'confirms') and a specific resource (a PIX payment settled in Brazil, identified by the Central Bank E2E id). It explicitly differentiates itself from siblings by explaining it is not for following a sale started in Lunium — that is lunium_get_crypto_sale — and by saying it cannot search by amount, name, or date. This fully satisfies a specific verb+resource plus sibling differentiation.

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 gives explicit when-to-use scenarios: proof before releasing goods/credit/access, reconciling a receipt, or final post-settlement verification. It also provides explicit when-not-to-use guidance: not for searching by amount/name/date, and not for following a sale started locally, directing the agent to lunium_get_crypto_sale. Exclusions and alternatives are named directly.

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