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

Annotations already establish a safe read, so the description spends its space on genuinely additive behavior: unauthenticated calls return `account: null`, Pix/USDC purchases are 30-day non-renewing while card subscriptions auto-renew until cancelled, and allowance is drawn down before prepaid credit. These are non-obvious operational facts an agent could not derive from the schema.

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?

The purpose is front-loaded in the first sentence and every subsequent sentence carries distinct information (auth nullability, renewal terms, unit costs, credit consumption order). It is dense but not padded, though the unit-cost list is close to reference material that could live elsewhere.

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 steps in to describe the shape of the active plan (usage.total/used/remaining) and the null-account case, plus the billing model. It stops short of naming the fields describing the plans for sale themselves, which leaves some return-shape uncertainty.

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 tool takes zero parameters, so there is nothing to disambiguate and the baseline of 4 applies. The description instead spends its budget on response and billing semantics, which is the right allocation here.

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 opening sentence names the resource precisely: the plans available for sale plus the caller's active plan, with the auth-dependent variant called out. It's clear enough that an agent knows this returns plan data rather than performing a purchase or cancellation. It does not, however, differentiate itself from near-neighbours like `pricing`, `consulta_planos`, `billing` or `my_account`.

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?

There is no statement of when to call this versus `pricing`, `consulta_planos`, `billing` or `my_account`, all of which plausibly overlap. The description is entirely about what comes back and how billing works, leaving the agent to infer the selection criteria from the sibling list alone.

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