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

No annotations are provided, so the description carries the full burden. It discloses the paid two-step process (call without x_payment to get terms, sign, then call again), the DNS-over-HTTPS method, and the important limitation that it is not a guaranteed inbox verdict. This is substantive behavioral context beyond the schema.

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?

The description is densely packed but efficient: two sentences cover purpose, checks, limitations, and the paid workflow. It is front-loaded with the core deliverability information and has zero wasted words.

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?

Despite no output schema or annotations, the description covers what is checked, the evidence-based nature, the payment workflow, and a key limitation. It does not describe the exact output structure, but for a tool with this complexity, the provided context is sufficient for an agent to call and interpret results correctly.

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% for both parameters, so the baseline is 3. The description adds critical semantics for x_payment by explaining the exact payment flow and the meaning of omitting it, and clarifies email is for one address, which enriches the schema's simple descriptions.

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 clearly states what the tool does: deliverability signals for one address, listing specific checks (RFC5322 syntax, MX/SPF/DMARC, disposable/role-account flags). It differentiates from siblings by focusing on single-address email deliverability, not broader domain intelligence or screening.

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 description provides clear context: it is for checking a single address and returns signals with evidence, not a guaranteed verdict. It does not explicitly name alternatives or exclusion criteria, but the 'one address' scope and the specific signal list imply appropriate use cases.

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