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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds material context beyond them: the RDAP data source, the flagging behavior for domains under 30 days old and expiring ones, punycode/IDN handling, and crucially the per-call cost of $0.005 USDC on Base via x402, which an agent must know before invoking. It does not explain what happens for unregistered or malformed domains, which keeps it at a 4.

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 purpose and the data source, then the field enumeration, then the vetting use case, then the price. Every sentence carries information a caller needs. The field list is a dense run-on that could be tightened, but no sentence is 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, the description carries the burden of telling the agent what comes back, and it does so by enumerating the return fields and the risk flags. The payment requirement is also disclosed, which matters for a paid x402 tool. Missing only edge-case behavior for invalid or unregistered domains.

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 required parameter with 100% schema description coverage, so the schema already tells the agent it accepts "domain or host name, e.g. github.com or www.github.com." The description mentions punycode/IDN names but does not clarify whether subdomains or bare hostnames are normalized, so it adds little meaning beyond the structured field. Baseline 3 is appropriate.

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 ("Domain registration lookup") and pins the data source (RDAP, WHOIS successor), then enumerates the returned fields: registration date, expiry, registrar, abuse email, EPP status, nameservers, DNSSEC, and derived flags. This clearly separates it from sibling tools like domain_dns, domain_email_check, and ens_resolve, which handle different facets of a domain.

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 an explicit use case with rationale: "Use to vet links, senders and counterparties (phishing domains are usually days old)." That is a concrete trigger condition. It stops short of naming alternatives (e.g. domain_dns for records-only checks, web_link_check for URL-level vetting) or stating when not to bother, so it falls just shy of a 5.

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