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

With no annotations, the description carries the full burden and does well: it discloses live DNS rather than cache, no API key or account required, per-call cost of $0.005 settled over x402 in USDC, and that calls are one-domain-at-a-time. Gaps remain around rate limits, behavior on an unregistered domain (error vs. structured result), and latency/timeout.

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 trigger condition, then the returned data set, then operational facts (auth, cost, batching) — a logical progression with no filler. The data enumeration is long but every item is decision-relevant for trust evaluation.

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 must convey what comes back, and it does by listing each DNS signal collected. It is nearly complete for a single-parameter tool; only the result shape (structured object vs. text) and unregistered-domain behavior are left unspecified.

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?

A single required parameter with 100% schema description coverage, so the schema already explains that URLs and email addresses are reduced to their host. The description's framing ("whether a domain is real and reachable") reinforces intent but adds no new input semantics. Baseline for this configuration is met.

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 uses a specific verb and resource ("check a domain") and enumerates exactly what the call returns: registration status, A/AAAA, nameservers, MX, SPF/DMARC, DNSSEC, and parked detection. That enumeration lets an agent distinguish it from red_dnsbl (blocklists) and red_whois (registration records) without opening a schema.

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?

"Use when an agent must decide whether a domain is real and reachable before trusting it" gives a clear use condition, and "built for going through a list of domains one at a time" sets the batching expectation. It stops short of naming when to prefer red_dnsbl, red_whois, or red_email_check instead, so no explicit exclusions are offered.

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