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

Annotations already cover read-only/idempotent/non-destructive, and the description adds material context beyond them: unsupported TLDs return 400 uncharged, the $0.01 USDC cost, the x402 PaymentRequired handshake and retry with params._meta['x402/payment'], and the absence of personal contact data. These are exactly the operational facts an agent needs before invoking a paid, open-world tool.

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-loads what is returned, then usage, then error and payment behavior. Dense but every clause carries information; the payment-handshake sentence is verbose but necessary for correct invocation.

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 enumerates the returned fields, and it additionally covers the error case (400 for unsupported TLDs), the cost, and the two-step payment flow. An agent has everything needed to call it correctly.

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 description coverage is 100% and the schema already explains normalization of URLs/hostnames/emails to a registrable domain plus the 'name' alias. The description adds nothing about the single domain parameter, 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 (looks up a domain in its registry's RDAP server) and enumerates the returned data fields (registration date, age, newlyRegistered, expiry, registrar, EPP status, nameservers, DNSSEC). It also distinguishes itself from siblings like dns_lookup by noting RDAP is the WHOIS successor and by returning registrar/registration metadata rather than DNS records.

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?

Explicitly states when to use it: judging whether a domain, sender or vendor is brand-new as a phishing/fraud signal. It does not name alternatives (e.g. dns_lookup or url_unshorten) or state when not to use it, but the triggering context is clear and actionable.

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