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

Annotations already declare readOnlyHint=true and destructiveHint=false, and the description reinforces this usefully with 'Nothing is bought by this tool' plus the per-call payment model for agents and the human request path. It goes beyond the annotations by clarifying the informational nature and the cost model, though it doesn't explain what the tool actually returns.

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?

Two short sentences, front-loaded with the usage trigger, and every clause carries some information. The 'AI agents pay per call; people ask us' construction is compact but slightly cryptic.

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?

With no output schema, the description should clarify what the agent receives back from an 'explanatory' tool, and it doesn't. It covers when to use it and that nothing is purchased, which is the safety-relevant core, but the return behavior remains unspecified for a tool whose entire purpose is to convey information.

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?

There is a single parameter with 100% schema description coverage ('The domain to register'), so the schema already carries the semantics. The description adds no format, TLD, or availability requirements beyond that, making the baseline 3 correct.

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 that the tool 'explains how to register' a domain rather than performing the registration, which is a specific verb+resource, but it sits awkwardly against the name 'register_domain' and the title 'How to register a domain'. It never explicitly distinguishes itself from the sibling check_domain, so an agent must infer whether this checks, explains, or performs registration.

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?

'Use when the user wants to buy an available domain' gives a clear triggering condition, and the trailing note separates agent and human paths. However, no exclusion or explicit hand-off to check_domain (for availability) or request_service is stated, so the routing is left partly implicit.

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