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

Annotations already cover the read-only, open-world profile, and the description adds genuinely non-obvious behavior: redacted placeholders are omitted rather than returned, a `privacy` flag signals a hidden owner, and unauthenticated calls cost $0.05 USDC per successful call via x402. What is missing is any mention of rate limits, error cases (e.g., unregistered domain), or latency.

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 the core question, then the field list, then the redaction caveat and pricing. Dense but every clause carries information; only minor cost is the slightly compressed pricing phrasing.

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 usefully serves as return-value documentation by listing the fields an agent will receive, and it discloses the redaction behavior and payment requirement. For a one-parameter lookup that is nearly complete; only edge-case behavior (invalid or unregistered domains, response shape) is left uncovered.

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 only one parameter and the schema documents it at 100% coverage, including that a bare domain, URL, or email on the domain is accepted. The description adds no extra semantics about the domain argument, so the baseline of 3 applies.

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 and resource ('Who owns a domain') and enumerates the exact record types returned (owner, registrar, registration/expiry dates, status, nameservers). This enumeration distinguishes it clearly from sibling lookups like domain_emails, domain_ips, and domain_urls without needing to name them.

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?

Usage is implied by the field list – an agent can infer it should be called when ownership/registrar data is needed – but there is no explicit when-to-use or when-not-to-use guidance, and no reference to the domain-scoped siblings that query adjacent data. The x402 pricing note describes cost, not selection criteria.

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