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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and non-destructive. The description adds valuable behavioral context: it detects compra-format IDs and returns an explicit diagnostic, and it mentions a 15-minute cache. These are beyond what annotations provide, enhancing transparency about the tool's behavior in edge cases.

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 concise and well-structured. It front-loads the main purpose, then provides the endpoint, return content, a special behavior note, and cache info. Every sentence adds value without redundancy. It's appropriately sized for a single-parameter 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?

The description covers the essential aspects: what it does, what it returns (header fields), and a specific edge case. Given the tool has an output schema (not shown but referenced), the description doesn't need to detail return structure. It lacks explicit error handling beyond the format issue, but that's minor. Overall, it's complete enough for an agent to call it 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%, and the schema already provides a detailed description of the parameter, including format, example, and a warning about confusion with compra ID. The description adds a bit about the diagnostic behavior, but that is more behavioral than parameter semantics. Since the schema does the heavy lifting, a baseline of 3 is appropriate.

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's purpose: 'Consulta uma ARP específica pelo identificador PNCP.' It specifies the exact resource (a specific ARP) and the identifier format, and it mentions the endpoint and return content. This distinguishes it from sibling tools like compras_arp_listar (which lists many) and compras_arp_por_fim_vigencia (which filters by date).

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 implies when to use this tool: when you have a specific ARP identifier. It also provides a clear usage warning: if the identifier is in compra format (without the '-NNNNNN' suffix), the tool returns a diagnostic instead of a silent false. This is a practical guideline. However, it does not explicitly mention alternatives or when not to use it, though the specificity is clear.

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