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

Annotations already declare readOnly, idempotent, and openWorld, so the safety profile is covered. The description adds genuine value beyond them by disclosing the business requirement driving the call and that a signed XDR-1 receipt is produced; it stops short of saying what an invalid RFC yields or whether a receipt is returned on failure, but the output schema covers return shape.

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 tight sentences: what is validated, why it matters, what comes back. Scope and format variants are front-loaded with no filler or repetition of the title.

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 one-parameter, annotation-rich tool with an output schema, the description supplies purpose, format variants, and business context. The only minor gap is error behavior on a malformed or invalid RFC, which the output schema likely handles.

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% (baseline 3), and the description goes further by mapping the two accepted lengths to corporate (12-char) versus individual (13-char) RFC forms and calling out the homoclave component, which the schema does not explain. That is meaningful semantic enrichment of the single parameter.

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?

Uses a specific verb (Validate) against a precisely scoped resource: Mexican SAT RFC, 12-char corporate vs 13-char individual, including homoclave. That specificity alone separates it from the many other country-identifier siblings (abn_check, cnpj_check, gstin_check) without needing to name them.

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?

States the concrete trigger context: required for Mexican contractor invoicing and SPEI cross-border settlements, which tells the agent when this check is mandated. It does not name alternatives or exclusions (e.g., when another country check is the right sibling), but the country scoping makes the choice largely self-evident.

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