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 genuinely new operational context: a free demo with a daily quota and a production path via POST /cl/rut with x402. The one behavioral gap is what happens on a failed check digit (error thrown vs. a false validity flag), which the agent would need to know.

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 short sentences, front-loaded with the core check-then-normalize behavior, followed by usage and then limitations. The final sentence packs quota and x402 payment details together slightly tersely, but no sentence is redundant.

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 validator with no output schema, the description supplies purpose, a usage cue, an important scope limit, and access/quota context. The only missing piece is how invalid RUTs are surfaced, which matters because there is no output schema to reveal it.

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?

There is a single parameter with 100% schema description coverage, and the schema already supplies both accepted forms with examples. The description adds only the normalized output format, which is about the return value rather than the input, so it does not meaningfully extend parameter meaning beyond the schema.

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?

States a specific verb and resource: checks the modulo 11 check digit of a Chilean RUT and returns it normalized, with a concrete output example (12.345.678-5). The Chile-specific scope implicitly separates it from the generic validate_tax_id / vat_validate siblings, but it never names those alternatives explicitly.

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?

"Use it to catch typos before saving a record" gives a concrete triggering context, and "Does not confirm the person or company exists with the SII" sets a clear boundary on what the result does not prove. It stops short of routing the agent to a sibling for cross-country tax ID validation or for SII existence lookups.

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