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

The description discloses the full purchase lifecycle: checkout creation, no money moves until a human pays, units become available upon payment, are consumed after quota, and remain valid for 12 months. It also notes the +5% bonus and API key requirement, adding considerable context beyond the annotations.

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?

Despite being multi-sentence, every clause provides necessary details: pricing, trigger conditions, exclusions, return format, payment/availability behavior, and API key requirement. Nothing is redundant, and the key actions are front-loaded.

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 mutation tool with no output schema, the description is complete: it covers when to use, when not to use, parameter values, side effects, return JSON structure, and auth. An agent does not need to open schemas or inspect siblings.

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 schema only says size can be 's', 'm', or 'l'. The description expands each value with specific unit counts and prices, and adds the bonus-units behavior, so an agent knows exactly what each size means in practice.

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 opens with a specific verb and resource: 'Start the purchase of a prepaid unit pack' and immediately enumerates exact sizes and prices. It explicitly excludes changing the plan and reading balances, and routes the reading task to get_billing_status, making it unmistakable from siblings.

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 gives an explicit trigger: 'Use it when what_can_i_do or a refusal says the monthly budget is running out.' It also names clear exclusions: not for changing the plan via web console and not for reading balances, providing the alternative.

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