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

Annotations already mark it as a non-readonly, non-destructive, non-idempotent, closed-world write, and the description adds real context on top: renewal semantics per rail, the 402-with-price error, accepted payment headers (X-Credito, PAYMENT-SIGNATURE v2 / X-PAYMENT v1), the `compra` mark guarding against repeat credit debits, single-settle x402 nonces, and receipt-retention after a delivery error. It also clarifies the consequence of idempotentHint=false by naming the exact repeat-debit guard.

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?

Every sentence carries operational content and the purchase act plus payment rails are front-loaded; there is no filler. It is dense and shifts topic rapidly (renewal, headers, guest ownership, unit costs), which is slightly harder to parse than an ideally partitioned description.

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?

Despite no output schema, the description covers auth paths (wallet agent auth, claim after sign-in), guest purchase ownership, error behavior, and even the shape of an active plan (usage.total / usage.used / usage.remaining). Nothing an agent needs to invoke this correctly appears to be missing.

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?

The single `plan` parameter is fully documented in the schema (100% coverage) including the allowance sizes for each enum value, so the baseline is 3. The description's unit-cost figures (1 page = 1 unit, price report 25, alert 5, tender watch 3) let an agent reason about tier fit but do not add per-parameter syntax or constraints.

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?

Specific verb plus resource plus scope: 'Buys a plan for the account or paying wallet.' This clearly separates it from read-only siblings like list_plans, consulta_planos, pricing, and from cancel_plan, so an agent knows this is the purchase action.

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 routing by payment rail (Pix and USDC = 30 days no renewal; card = monthly auto-renewing until cancelled) and states the fallback when unpaid: a 402 carrying the price. It also implies when not to buy ('Original PDFs are free'). It stops short of explicitly directing the agent to list_plans/consulta_planos first to pick a tier.

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