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

Annotations already indicate readOnly and idempotent behavior, and the description adds crucial context: it validates format only and does not verify against the SAT registry. This warns the agent about a potential false sense of completeness. Also specifies the exact return values (validity, type, cleaned RFC), going well beyond 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 three sentences, starts with the core action, provides practical use cases, and closes with an important limitation. Every sentence earns its place with no filler, making it efficient and well-organized.

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?

Given the simple single-parameter tool with no output schema, the description fully covers what the tool does, when to use it, what it returns, and its limitation. It is complete and self-contained for this straightforward validator.

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 schema description for 'rfc' already includes an example and notes that spaces are automatically removed, giving high coverage. The tool description further adds meaning by explaining the 13-character individual and 12-character company formats, which helps the agent understand validation expectations and form valid inputs.

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 validates Mexican RFC format for both individuals and companies, with specific character counts. It distinguishes from siblings by explicitly focusing on Mexican taxpayer identifiers and mentions return values, making the 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 provides explicit use cases such as processing Mexican invoices (CFDI), tax forms, and supplier registrations. While it does not explicitly list when-not-to-use or name alternatives, the sibling context and 'Mexican RFC' phrasing make the domain clear. Lacks explicit exclusions but still offers strong guidance.

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