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

Goes far beyond the annotations: it discloses subscription vs. one-off billing per payment rail, the 402-with-price failure mode, the `account_required` precondition, the `compra` marker against repeat credit debits, the single-settlement x402 nonce, and the advice to keep receipts after a delivery error. None of that is recoverable from readOnlyHint=false / idempotentHint=false alone.

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 purpose, then payment rails, then preconditions and caveats — each sentence carries distinct information with no filler. It is dense with domain jargon (`compra`, `X-Credito`, `X-PAYMENT`) but that density buys real detail rather than 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?

For a payment/mutation tool with no output schema, it covers the failure path (402 with price), the auth precondition, and idempotency hazards thoroughly. The one gap is the success response shape — an agent is told what happens when payment is missing but not what a completed purchase returns.

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 single enum value `carteira-100` is already explained in the schema (99 more monitored companies). The description adds no further meaning about what the plan tier entitles or costs, so the schema does all the work — baseline 3.

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?

Opens with a specific verb+resource: "Buys a plan for the signed-in account," which clearly separates it from list_plans, pricing, consulta_planos and cancel_plan. It never explicitly names a sibling or restates how it differs from them, so it stops just short of a 5.

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?

Provides real usage context: sign in first, choose a payment rail (prepaid credit, x402 v1/v2, card), and know that Pix/USDC give 30 days while card auto-renews. Missing an explicit pointer to alternatives (e.g. "use list_plans to see options, cancel_plan to stop renewal"), so it is clear context without exclusions.

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