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

With no annotations, the description carries full disclosure responsibility. It reveals that a stored payment method will be used, which signals a financial transaction, and states it returns success/failure. However, it does not mention irreversibility or additional costs, though the use of 'stored payment method' is a strong warning.

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 description is a single sentence that front-loads the core action, and the API key requirement is appended as a practical note. No words are wasted, and all information is relevant and directly usable.

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?

The tool is simple, with no output schema, but the description covers the return type. It directly addresses the main risk (payment) and supporting auth requirement. It could mention checking domain availability first, but that is not essential for basic operation. Overall, it is sufficiently complete for an agent to act on.

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?

Both parameters are fully described in the schema (domain and years), achieving 100% coverage. The tool description adds no extra parameter detail beyond what the schema already provides, so the baseline of 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?

The description states 'Purchase a domain using the stored payment method' with a specific verb and resource, clearly differing from sibling tools like check_domain or buy_link. It also mentions the return value, fully defining the tool's action.

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 implicitly indicates when to use the tool (to purchase a domain) and provides a prerequisite (API key), but it does not explain when to choose this over alternatives or mention any exclusions. The context is clear but lacks explicit comparative guidance.

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