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

Annotations only declare readOnlyHint, and the description adds meaningful behavior: validation happens fully offline without contacting SAT, and each call costs $0.002 USDC via x402, which the agent must know before invoking. It does not explain how the x402 payment flow works, which is a residual gap for a paid tool.

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?

Two tight sentences: capability and checks first, then the operational caveats (offline, price). No filler or repetition.

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?

There is no output schema, and the description does not say what a validation result looks like (boolean, per-check breakdown, error reasons), leaving the agent to guess the response shape. The pricing and offline claims do, however, cover the most consequential operational facts for a one-parameter tool.

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% and already documents the single 'rfc' parameter as 12 or 13 characters. The description adds only indirect meaning by naming person type (física/moral), which explains the length split, but supplies no format syntax or examples 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 (validate a Mexican RFC tax ID) and enumerates exactly what is checked: structure, person type física/moral, embedded date, and SAT check digit. This is clearly distinguishable from sibling validators like curp_validar or sat_cfdi_verificar.

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?

'Offline, no SAT call' gives clear context for choosing this over SAT-dependent siblings, implying it checks format validity rather than real-world existence. It stops short of explicitly naming an alternative or stating when not to use it.

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