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

Annotations already declare readOnlyHint=false and idempotentHint=false, and the description sensibly adds behavior the annotations cannot: payment happens once through x402 and the call produces a token. It omits failure/refund behavior and whether the token expires, but the added context is meaningful for a zero-annotation-style mutation.

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?

One front-loaded sentence that opens with the action and packs mechanism and outcome without filler. Slightly dense with parenthetical detail but nothing is wasted.

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 single-parameter purchase tool with no output schema, the description usefully discloses the return value (a token that discounts across the house APIs), covering the main gap an absent output schema creates. Remaining omissions like token lifetime or payment failure handling are minor.

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 usd parameter is already documented as '1, 5, 10 ou 25'. The description's package list merely restates that enum of allowed values, adding no syntax, currency, or formatting detail 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 (recarrega crédito) plus the payment mechanism (x402) and the result (a token), which is well beyond a tautology. It does not explicitly name the alternative credito_saldo, so an agent must infer the buy-vs-check distinction from the sibling list rather than the text.

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?

Usage is implied: this is the tool that purchases credit, as opposed to credito_saldo which reports balance. There is no explicit when-to-use/when-not statement, no pruning against api_access_buy or billing, and no stated prerequisites for the x402 payment step.

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