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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds genuinely useful context beyond that: the call hits an external SAT web service, it costs $0.03 USDC per call via x402, and it lists the specific status fields returned — cost and payment protocol are the kind of detail annotations cannot carry.

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 sentences, both dense: the first states the operation and its outputs, the second states the required inputs and the price. The verification outcome is front-loaded and nothing is wasted.

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?

With no output schema, the description compensates by enumerating what the response answers (existence, vigente/cancelado, cancellation status, EFOS flag) and adds the pricing model an agent needs before invoking a paid call. Only the concrete response shape or error behavior is left unspecified.

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 description coverage is 100%, so all five parameters are already documented in the schema. The description restates the same five inputs in prose without adding format or validation detail beyond what the schema provides, so the baseline 3 applies.

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 gives a specific verb and resource ('Verifies a CFDI 3.3/4.0 invoice against the SAT web service') and enumerates the exact questions answered: existence, vigente/cancelado, cancellation status and EFOS flag. It does not name or distinguish itself from any sibling such as sat_69b, so it lands just short of the top mark.

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?

Usage is implied by the enumeration of required inputs ('Needs UUID, issuer RFC, receiver RFC, total and the last 8 characters of the seal'), which tells an agent what must be on hand before calling. There is no explicit when-to-use, when-not-to-use, or routing to an alternative tool for related SAT checks.

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