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

Annotations already declare readOnlyHint and openWorldHint, yet the description still adds real context: the free-tier quota, the x402/card payment path with the exact in-band parameter, the DoH provider and its fallback, and what the mail-auth check reports (MX hosts, SPF, DMARC policy, warnings). It does not describe failure behavior or rate limiting beyond the daily quota, which keeps it short of a 5.

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?

Dense but nearly waste-free, with the usage guidance and alternative clearly signposted. Minor deduction: the first sentence is pricing/payment boilerplate rather than the tool's purpose, so the front-loading is inverted relative to what an agent most needs first.

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 an output schema present, return-value explanation is unnecessary, and the description covers capabilities, scope, payment prerequisite, and the sibling alternative. Nothing an agent needs to call it correctly is missing.

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 coverage is 100%, so the baseline is 3, but the description adds two things the schema does not make obvious: only one domain is handled per call, and the full supported record-type vocabulary (SOA, SRV, HTTPS, DS) beyond the schema's default list. It does not explain the semantics of the string-typed mail enum beyond what the schema already says.

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: DNS lookup over DNS-over-HTTPS with an explicit record-type list, plus a mail-auth check (MX, SPF, DMARC) and DNSSEC-validated flag. It also names the sibling it is not (email-verify), so an agent can route without opening either 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?

Contains an explicit 'When to use' clause covering both capabilities and a contrasting alternative ('Use email-verify to check a single address'). The selection condition between the two tools is stated rather than inferred.

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