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

Beyond the readOnlyHint/openWorldHint annotations, the description discloses concrete operational traits: a 20-call-per-client-per-UTC-day free quota shared across the Mexico tools, an error on quota exhaustion, and the exact paid endpoint with pricing model (x402/USDC on Base, no key). It also states the epistemic limit of the result. It does not describe what a successful response contains, which matters given there is no output schema.

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?

Front-loaded with the checks performed, then limitations, then commercial/quota mechanics — a sensible order with no filler sentences. The pricing sentence is dense but each clause (endpoint, price, protocol, no-key) carries decision-relevant information for an agent that hits the quota.

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 validator with no output schema, the description fully covers inputs and failure modes but never explains the shape or meaning of a success response (e.g., whether it returns per-check booleans, a matched 69-B status, or a single verdict). The deliberate boundary statement and quota disclosure are strong, but the return side is the remaining hole.

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% for the single rfc parameter, so the baseline is 3. The description does add value by mapping the 12-character form to persona moral and the 13-character form to persona física, which is semantic meaning the schema's "12 or 13 characters" does not convey — but it offers no normalization, casing, or whitespace guidance.

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 names a specific verb and resource ("Validate a Mexican RFC") and enumerates exactly what is checked: format with 12/13-character moral/física split, the YYMMDD date part, the homoclave check digit, generic RFCs, and a 69-B cross-check. It also scopes negatively ("Does not prove the RFC is registered with the SAT"). It stops short of explicitly separating itself from sibling mx_sat_69b_lookup, whose name overlaps with the cross-check this tool claims to perform.

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?

Clear context for use: offline validation of the RFC's internal structure plus a 69-B cross-check, with an explicit exclusions statement that this does not prove SAT registration. The quota and paid-fallback path tell an agent what happens when the tool is unavailable. It does not say when to prefer mx_sat_69b_lookup over this tool, so the boundary with that sibling is left to inference.

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