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

Annotations already declare readOnlyHint and idempotentHint, but the description adds valuable context: the modulo-11 algorithm, automatic formatting removal, return value (valid status and cleaned RUT), and the explicit limitation that it does not verify SII registry activity. These go beyond annotation hints and inform the agent of exact behavior.

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 front-loaded with the primary purpose, followed by usage guidance and behavioral notes. Each sentence serves a distinct function, and the overall length (about 70 words) is appropriate for a simple validation tool with one parameter.

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 tool is simple: one parameter, no output schema. The description covers the return value and a key limitation (SII registry), which is sufficient for an agent to select and invoke the tool correctly. No critical information is missing.

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 single parameter 'rut' is fully described in the schema (100% coverage), including formatting removal and an example. The description repeats the formatting acceptance but adds no new semantic information about the parameter itself. Baseline 3 is appropriate as the schema already handles the heavy lifting.

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 it 'Validates a Chilean RUT' using the 'official Chilean modulo-11 checksum algorithm.' The verb is specific, the resource is clear, and the algorithm details distinguish it from sibling validators for other countries.

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 gives explicit use cases: 'when processing Chilean invoices, tax forms, user registrations, e-commerce orders, or any document requiring a valid Chilean taxpayer identifier.' However, it does not explicitly name alternative tools or when NOT to use it, though it does mention a limitation about SII registry verification.

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