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

With no annotations supplied, the description carries the full behavioral burden and does so: NXDOMAIN returns 200 with exists:false rather than an error, responses are under 10 s, cost is €0.002 per domain charged to the Bearer token, and WHOIS results are cached for 24h. Error semantics, latency, billing, and auth are all disclosed.

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?

A single dense paragraph where every clause carries distinct information — no filler. The 'Parameters —' tail is telegraphic and slightly cramped, but it is front-loaded with the capability list before the behavioral caveats.

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?

Return values are covered by an output schema, and the description still adds everything an agent needs to invoke correctly: billing, auth header, latency budget, cache behavior, and error semantics. Nothing material is missing for a three-parameter diagnostic 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?

Schema description coverage is 0%, so the description must compensate, and it does: domain (domain name), url (page url, an alternative to domain), and checks (comma list of dns, whois, ssl, mail with default all). The enum values and default, absent from the schema, are supplied here.

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?

The description names the concrete resource set precisely: DNS (A, AAAA, MX, TXT, NS, CAA), SPF/DMARC parsing, WHOIS, and TLS certificate validity. An agent knows exactly what signal comes back. It does not differentiate against adjacent siblings such as uptime, email_verify, or url_safety, which is the only thing keeping it from a 5.

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 content listing and the note that url is an alternative to domain, but there is no explicit 'use this when' or 'use X instead' framing against the overlapping sibling tools. The agent must infer suitability from the enumerated checks.

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