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

Annotations already cover readOnly, idempotent, non-destructive and closed-world, yet the description adds substantial context: validation is offline, no data is stored, pricing (0.01 USDC per call across three chains, no account/key), the shape of a payment-required failure (isError, structuredContent with x402Version 2 and accepts), and where the settlement receipt lands (result._meta["x402/payment-response"]). This is exactly the kind of behavior an agent cannot infer from structured fields.

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?

Purpose and validation scope are front-loaded in the first sentence; the payment flow follows in the order an agent would encounter it (call, unpaid result, sign, retry, settlement). No filler sentences.

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 fully covers the return shape (type, entity/branch codes, bank name for major banks, formatted value, JSON text) as well as the failure/retry path. An agent has everything needed to call it and handle both success and payment-required outcomes.

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% and the single param already documents '22-digit CBU or CVU'. The description's 'Send {cbu}' and mention of 'Argentine bank account number (CBU) or virtual account (CVU)' add context but no format or syntax detail beyond the schema, 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?

States a specific verb (validate) and resource (Argentine CBU/CVU) and enumerates the outputs: check-digit verification, type, entity/branch codes, bank name, formatted value. This clearly distinguishes it from siblings like ar_cuit (tax ID) and ar_phone (phone numbers).

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 operational guidance: send {cbu}, expects offline validation, nothing stored, and lays out exactly what to do on an unpaid call (sign an accepts entry and retry with the PaymentPayload in params._meta["x402/payment"]). It does not explicitly say when to choose this over sibling ar_cuit, so it stops short of a 5.

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