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

Annotations already establish read-only, idempotent, non-destructive behavior, so the safety profile is covered. The description adds genuinely new context beyond the schema: a per-call cost of $0.001 native USDC on Base, and an explicit disclaimer that no mailbox-existence claim is made. It omits any hint about result shape or latency.

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 tight sentences, zero filler, with the capability statement front-loaded and the scope caveat and cost trailing. Every clause carries information an agent needs.

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?

No output schema exists, so the description should ideally characterize what comes back (DNS records, authentication results, a posture verdict) but does not. Scope, cost, and safety are adequately conveyed; the return-value gap and the single undocumented parameter leave it only minimally complete.

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?

One required 'email' parameter with 0% schema description coverage; only minLength/maxLength constraints are machine-readable. The phrase 'practical email syntax' signals that input is validated rather than strictly RFC-parsed, which adds mild meaning, but nothing about accepted formats or normalization.

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?

Names specific verbs (validate syntax, inspect DNS, mail routing, authentication, domain-security posture) and the resource is unambiguous. It also delimits scope against the sibling concern 'domain_preflight' by being email-scoped, though it never names a sibling explicitly.

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?

The clause 'DNS-level only; no SMTP mailbox-existence claim' tells the agent what this tool will not answer, which is useful routing guidance away from an expected-but-unavailable capability. There is no explicit statement of when to pick this over domain_email_security or domain_preflight, so usage is only implied.

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