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

Annotations only say readOnly/openWorld; the description adds the critical non-obvious behavior: a $0.002 USDC cost on Base via x402, that unpaid calls return PaymentRequired, and that a compliant x402 MCP client pays automatically. That is exactly the payment/auth context an agent needs before invoking and is not derivable from 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?

Two sentences, no filler; the check list is front-loaded and the payment caveat is placed after the core purpose. The capability enumeration is dense but each item maps to a real returned signal, so little could be cut without losing information.

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 does useful work by naming the returned signals (MX status, disposable flag, provider, SPF/DMARC, risk flags). It stops short of describing the shape of the result (e.g., how risk flags are represented or what an inconclusive domain-only check returns).

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% and the single parameter is already documented as 'Email address or bare domain'. The description's 'email address or domain' restates the schema rather than adding format, validation, or normalization guidance, so the baseline of 3 applies.

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?

Specific verb ('check') plus resource ('email address or domain') followed by an enumerated capability list: syntax, MX, mail acceptance, disposable/free provider, provider identity, SPF/DMARC, risk flags. This is clearly distinguishable from siblings like dns_lookup or domain_report, which do not cover mailbox-level deliverability signals.

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?

'before you send' gives an explicit trigger context: pre-send deliverability validation. It does not, however, name what to use instead for related tasks (url_check for URLs, domain_report for broader domain intelligence), so routing against siblings is left to inference.

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