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

Beyond the annotations (destructiveHint, idempotentHint, openWorldHint), the description discloses that write operations need operator confirmation, must be polled, that prepaid funds are a separate track, and that no live presented-credential rail exists. This is meaningful behavioral context a caller cannot get from the annotations 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?

Four short, front-loaded sentences with no wasted text; the primary read operations come first and the write caveats follow. The telegraphic style is dense but every clause carries information.

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 an eight-operation, nested-parameter, destructive tool with no output schema, the description is incomplete: it never explains status, plans, checkout_status, or funding_purchase_status, nor what polling returns. The safety and approval guidance is present, but the operation catalog is only partially covered.

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 parameters like confirm, idempotency_key, and instrument are already well documented in the schema. The description only lightly reinforces this ('payment_methods reads current cards') and adds no syntax or format detail beyond the schema, making the baseline 3 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?

The description names the resource (billing) and the primary operations: reading features/plans, reading payment methods, and checkout/add_funds. It is clearly distinct from the campaign/segment/contact siblings. It stops short of covering the status/checkout_status/funding_purchase_status operations, so an agent must open the enum to see the full surface.

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?

It offers some operational guidance: checkout and add_funds require operator approval and polling afterward, and prepaid funds are separate. However, it does not tell the agent when to choose each of the eight operations over the others; the 'Start with status or features' routing hint lives only in the schema, not the description.

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