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

Annotations already declare the safe/idempotent/closed-world profile, yet the description adds substantial non-obvious behavior: validation is performed offline, nothing is stored, cost is 0.01 USDC across three chains with no account required, and it precisely describes both the failure shape (isError plus structuredContent with x402Version 2 and `accepts`) and where settlement lands (`result._meta["x402/payment-response"]`). That is exactly 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?

Front-loads the purpose and return contract before the payment mechanics, and every sentence carries operative information (checks, storage, cost, retry protocol, settlement location). The payment segment is long but justified for an x402 tool; a slightly tighter phrasing of the retry steps would earn a 5.

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 still specifies the return content (normalized and formatted values, entity type, JSON text) and the metadata location for settlement. Combined with the fully specified payment/retry loop, an agent has everything needed to invoke and recover from the 402 path without guessing.

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 the schema itself documents the `cuit` field as 'CUIT or CUIL, with or without dashes' with maxLength 60. The description's 'Send {cuit}' restates rather than enriches this, so baseline 3 applies — no added syntax guidance 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 an Argentine tax ID (CUIT/CUIL)') and enumerates the actual checks performed (length, prefix, modulo-11 check digit) plus the return payload (normalized/formatted values, entity type). This makes it trivially distinguishable from siblings like ar_cbu or ar_phone, which cover different Argentine identifiers.

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?

Gives explicit invocation instruction ('Send {cuit}') and a complete conditional flow for the paid path: unpaid call returns a payment-required result, sign one of the `accepts`, retry with PaymentPayload in `_meta`. It does not name when to prefer a sibling (e.g. ar_cbu for bank codes), but the resource is narrow enough that misuse is unlikely.

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