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

With no annotations, the description carries the full behavioral disclosure burden. It reveals the paid nature, exact pricing, supported payment chains, live MX-record reads, and the fact that a second call completes the purchase. This is substantial transparency, though it omits edge-case behavior such as handling of invalid addresses or response format details.

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?

Three information-dense sentences with no fluff. Capabilities are front-loaded, the pricing and payment mechanics are stated compactly, and the operational steps are clear. Every sentence earns its place.

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?

For a paid tool with no output schema, the description covers the core behavior, the cost, the payment workflow, and the result concept ('verdict per address'). It is slightly light on the exact shape of the returned verdict and on failure behavior, but it is adequate for an agent to know how to invoke and complete the call.

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 real value beyond the schema: it explains the email limit, the x402 payment flow, the two-call sequence, and what the `payment` parameter represents in practice. This elevates it above the baseline.

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 action ('Check up to 100 email addresses'), a precise resource, and enumerates concrete checks: shape, live MX records, disposable domains, role accounts, and a per-address verdict. It also distinguishes itself from mailbox probing and from domain-level verification, which separates it from relevant siblings.

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 clearly explains the two-step paid usage flow: call once without `payment` to obtain terms, sign with the wallet, then call again with `payment`. It also states that it checks domain deliverability rather than doing a mailbox probe, which provides useful scoping, though it does not name sibling alternatives explicitly.

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