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

Annotations only declare readOnly and openWorld; the description adds real context beyond them — the underlying data provenance (certificate logs, web crawls, archives, passive DNS) and the paid-call model ($0.05 USDC per successful call via x402 on Base or Solana). It does not cover rate limits, caching, or what a partial/empty result looks like.

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?

Two dense sentences, front-loaded with what is returned, followed by the pivot suggestion and the pricing caveat. Every clause carries information; nothing is redundant with the title or schema.

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 and minimal annotations, the description must carry the return-shape burden, and it does — enumerating subdomains, IPs, network owner/ASN/country, and DNS records. Payment terms close the remaining gap for a paid, open-world tool.

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 schema description coverage is 100% ('A domain name, or a URL or email address on it' is already in the schema). The description adds no further parameter-level meaning, so the high-coverage 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 ('Every subdomain found for a domain ... with the IPv4 addresses each resolves to') plus secondary outputs (NS, MX, TXT). The scope is precise enough to separate it from sibling tools like domain_whois (registration data) and domain_emails/domain_urls (harvesting different artifacts).

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 routes downstream work: 'Pivot any IP into ip_lookup for open ports and CVEs.' It also gives the cost context for callers without an API key. It stops short of stating when *not* to use this (e.g., vs domain_whois for registration, or ip_geolocation for a bare IP), so a 4 rather than 5.

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