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

Annotations already cover read-only, open-world, and idempotent behavior, so the description needn't repeat those. It adds meaningful context by specifying the validation method and that it returns a 'signed XDR-1 receipt,' which is useful behavioral information beyond the annotations. It stops short of describing receipt contents or any rate limits, but that's acceptable given the output schema.

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?

Three sentences: first defines the tool, second gives use cases, third states the return. It is front-loaded with the core function and contains no redundant information. Every sentence contributes value.

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 validation tool with annotations covering safety and an output schema defining the return, the description provides everything needed: what it does, why it matters, and what it returns. No critical gaps.

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 fully documents the single 'cnpj' parameter, including format and punctuation allowance (100% coverage). The description adds no further semantics about the parameter, so the baseline 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 states a specific verb ('Validate') and resource ('Brazilian corporate tax ID (CNPJ)'), and even specifies the algorithm ('official dual mod-11 check digits'). This clearly distinguishes it from sibling validators for other countries or formats.

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?

It lists concrete use cases ('Brazilian B2B payments, PIX verification, and vendor due diligence') that tell when to use the tool. However, it does not explicitly contrast with alternatives (e.g., 'use this for Brazil, not for other country IDs') or mention when not to use it, leaving a small gap in routing guidance.

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