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

Annotations already establish readOnlyHint=true, destructiveHint=false and openWorldHint=true, so safety is covered. The description adds genuinely useful non-structured context: no credit is consumed and a Pro plan is required, plus an implicit sketch of what the response reports ('what's missing').

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 compact sentences, front-loaded with the core check and followed by scope qualifiers. The second sentence partly restates the first ('plain deliverability health check'), a minor redundancy, but nothing is padded out.

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 required parameter, full schema coverage, annotations covering the safety profile, and no output schema to explain, the description supplies what an agent needs: domain-level scope, what is evaluated, cost, and plan gating. Only the lack of routing to sibling tools leaves a small gap.

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 a single parameter with 100% schema description coverage, so the schema already documents the domain argument and its format. The description adds no syntax or normalization guidance beyond it, making the baseline of 3 appropriate.

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 verb (check) and resource (domain) plus the concrete facets examined: mail servers, SPF, DMARC, and gaps. It implicitly separates itself from mailbox-level siblings like verify_email, though it never names an alternative 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?

It frames the use case ('a plain deliverability health check for a sending domain') and adds plan and cost context ('No credit used', 'Qualisend Pro plan'), which helps an agent decide when it is callable. However it gives no when-not guidance and does not point to verify_email or check_blacklist for adjacent questions.

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