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

Annotations cover the read-only/open-world profile, and the description adds substantial extra context: the paid x402 model, exact cost (0.005 USDC on Base per call), the 404-equals-available heuristic, and the caveat that this is not a binding quote. These are precisely the operational facts an agent needs before invoking.

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?

Front-loaded with purpose and result interpretation, then cost, then payment mechanics. Dense and information-rich; the enumerated TLD and variation lists cost some length but are genuinely useful scope information rather than filler.

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?

No output schema exists, and the description compensates by explaining what a 404 implies, the limits of the result, and how the payment handshake works. For a two-parameter paid lookup tool, nothing essential is missing.

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 coverage is 100%, so the baseline is 3, but the description goes beyond the schema by explaining the payment flow for x_payment (call without it to obtain requirements) and by clarifying that 'name' accepts a bare name or idea. That adds real meaning the schema's short field descriptions do not.

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 ('Check') and resource ('domain name availability'), plus the mechanism (public registry RDAP) and the exact scope of TLDs and prefix/suffix variations covered. An agent can tell immediately what this does and that it is not the sibling read_url.

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?

Gives clear usage context: how to interpret a 404, that results are not a registrar quote and must be confirmed at checkout, and that calling without x_payment returns payment requirements. It does not explicitly contrast with the sibling read_url, so it stops short of a full when/when-not routing statement.

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