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

Annotations already declare readOnlyHint and openWorldHint, so the description's real contribution is the cost and payment behavior: $0.002 USDC on Base per call, unpaid calls return x402 PaymentRequired, and x402 MCP clients pay automatically. That is exactly the kind of operational trait annotations cannot express. It omits failure behavior for invalid/nonexistent domains, which keeps it from a 5.

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 capability and return fields, then the cost/payment caveat in a short second sentence. Dense but every clause carries information; no 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?

With no output schema, enumerating the returned fields is the right compensation, and the payment/error flow is disclosed. An agent knows what it gets back, what it costs, and what happens on non-payment, which is 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?

There is a single parameter with 100% schema description coverage ('Domain name, e.g. example.com'), so the schema already carries the semantics. The description adds nothing about accepted formats (bare domain vs URL vs subdomain), so baseline 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?

Names a specific verb+resource (WHOIS lookup via RDAP) and enumerates the exact data returned (registrar, dates, age, status codes, nameservers, DNSSEC, abuse contact), which clearly separates it from dns_lookup or domain_report. It stops short of explicitly naming the sibling it differs from, so it lands at clear-but-not-differentiated.

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?

'Spot newly registered (phishing) domains' implies a use case, and the payment note implies when x402 clients should call it. However, there is no explicit guidance on when to prefer this over domain_report, dns_lookup, or email_domain_check, so routing is left to inference.

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