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

Adds substantial behavior beyond the readOnlyHint/openWorldHint annotations: it is paid ($0.005 USDC via x402), explains that the call returns the HTTP request and payment requirements rather than data directly, requires an x402 client to settle, and states addresses are not stored or logged. These are exactly the operational details an agent needs before calling.

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 front-loaded: cost and the no-SMTP constraint lead, followed by the checks, verdicts, and limitations. Nearly every sentence carries distinct information, though the enumeration-of-flags style is slightly list-heavy.

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 no output schema, the description shoulders the return value burden and does so: it names the possible verdicts (deliverable_domain | risky | invalid) and explains the unusual x402 payment-first response flow. An agent knows both what it gets and what it must do to get it.

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?

Only one parameter with 100% schema description coverage ('one email address'), so the schema already carries the semantics. The description adds no syntax or format detail for the email argument, making the baseline 3 correct.

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+resource ('Email address check') and immediately scopes it ('WITHOUT sending mail or SMTP probing'). It enumerates exactly what is checked (RFC syntax, MX/A DNS-over-HTTPS, disposable, role-address, free-mail) and the verdicts, which cleanly separates it from siblings like link_check, page_check and robots_check.

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?

Clear context for use and an explicit boundary ('Cannot confirm that the mailbox itself exists'), which tells the agent not to expect mailbox-level certainty. It does not name an alternative tool for cases this doesn't cover, so it stops short of full when/when-not routing.

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