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

With no annotations, the description carries the full burden and does add real value by clarifying that a stored card grants no API credit and that usage must be funded via credits or an authorized postpay allowance. It still omits auth requirements, idempotency, and reversibility, so a mutation tool's behavior is only partially disclosed.

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?

Three sentences, front-loaded with the action, and the funding caveat earns its place by preventing a common misconception. The final sentence mostly duplicates the schema's parameter description, which is minor redundancy.

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 mutation tool with no annotations and no output schema, the billing-model context is useful but incomplete: auth/permission needs, failure modes, and the result of a successful setup are not covered. It is adequate but has clear gaps.

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%, so the schema already documents payment_method_id fully. The description largely restates that same pm_xxx/Stripe.js guidance rather than adding format or constraint detail beyond it, so the baseline 3 is appropriate.

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 ('Set up') and resource ('pay-per-use billing') plus the mechanism (Stripe payment method), so the agent knows this is the initial billing-enablement action. It does not explicitly name or distinguish itself from siblings like get_billing_status or purchase_credits, keeping it 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?

The description implies this is the setup step and hints that funding comes from credits or postpay allowance, which lightly signals the boundary with purchase_credits. However it never states when to use this tool versus get_billing_status or purchase_credits, leaving usage to inference.

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