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

Annotations already cover the safety profile (readOnly, non-destructive, openWorld), so the description's job is to add operational context — which it does by disclosing the external data sources consulted (RDAP/WHOIS, DNS, TLS) and by signalling best-effort behavior with 'when it can be identified', implying some fields may be absent. It does not mention latency, rate limits, or handling of partially resolvable domains.

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 resource and its scope, followed by a compact usage clause — two sentences, no filler. The long enumeration of record types is dense but each item is meaningful and justified for a tool whose whole value is the breadth of its lookup.

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 usefully compensates by enumerating the categories of data returned, and annotations carry the safety profile. It is a single-parameter, read-only lookup, so remaining gaps (response shape, how missing data is represented, behavior on malformed input) are minor.

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 100% and the schema already defines the single 'domain' parameter precisely (public domain name, IDN accepted, no URL/port/IP, maxLength 253). The description adds no parameter-level detail beyond the schema, so the baseline of 3 applies — nothing is lost, but nothing is added either.

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 a specific resource and enumerates exactly what intelligence it returns (age/expiry, registrar, DNS record types, SPF/DMARC/MTA-STS, DNSSEC, TLS, hosting, company). It also implicitly separates itself from the sibling 'company' and 'network' tools by scoping itself to a single domain and treating the owning company as a conditional extra rather than the primary subject.

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?

It gives an explicit 'Use when:' clause covering the two core scenarios — judging legitimacy of a domain/link/email sender, and auditing mail configuration. That is clear triggering context, but it names no alternatives or when-not conditions (e.g. use 'company' for org-level profiles, or what to do with an IP address instead of a domain).

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