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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds real value beyond that: it discloses the returned fields, notes that the count is delivered in the response rather than in the description, and states the tool is free with no key required — an auth fact the annotations do not carry.

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 purpose and scope are front-loaded in the first sentence, and the exclusion rule follows immediately. The middle sentence about the count being served in the response is slightly meta, but it is short and informs AI clients why no count is hardcoded, so it largely earns its place.

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 zero-parameter, read-only list tool whose annotations cover the full safety profile, the description is sufficient: it states the content, the exclusions, the alternative tools, and the auth status, and even anticipates the missing return schema by naming the fields returned.

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 tool takes zero parameters, so there is nothing to disambiguate; baseline for a parameterless tool is 4. The description correctly adds no parameter detail because none exists.

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?

Names a specific verb (Lista) and resource (companhias abertas brasileiras / AÇÕES) and states the returned fields (nome, CNPJ, ticker). It explicitly distinguishes itself from the FII siblings by declaring that real estate funds are excluded, so an agent can tell it apart without opening other schemas.

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?

Gives an explicit when-to-use with routing: use this for stocks, and for FII use `fiis_ranking` (the list) or `fii` (a single fund). It names the alternatives and the condition that selects them, leaving nothing to inference.

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