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

Annotations already declare readOnlyHint, openWorldHint and idempotentHint, so the safety profile is covered. The description adds useful context beyond that: it is free-tier preview and lists the specific DNS/email-auth signals returned, though it does not discuss rate limits, failure behavior, or output format.

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 with no wasted wording. The primary use case and scope are front-loaded, and the paid-endpoint alternative follows immediately.

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 one parameter, clear annotations and no output schema, the description provides enough context for correct invocation by enumerating the checks returned and clarifying the free-tier limitation plus where to go for paid enrichment.

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 one parameter with 100% schema description coverage; the schema already explains that it accepts a domain, URL or email address. The description adds no further syntax or format guidance, so the baseline of 3 is appropriate when the schema does the heavy lifting.

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 the exact checks performed (existence, mail routing, SPF/DMARC, nameservers, DNSSEC). It clearly distinguishes this from the paid x402 endpoint for deeper reputation data, but it does not explicitly differentiate from the sibling tool check_email.

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 states when to use the tool ('before your agent trusts a link, a sender or a counterparty') and gives a clear alternative for registration age, registrar, parking and risk scoring (the paid /v1/domain endpoint). It does not mention when to prefer the sibling check_email instead.

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