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

Adds substantive context beyond annotations: no message is sent, the mail server is not contacted, results are DNS/syntax signals not proof of mailbox existence, and pricing ($0.01/call, first 25 free, failed calls free, repeats not free). The per-call cost caveat meaningfully informs an agent about repeat invocation, complementing the idempotentHint=false annotation without contradicting it.

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?

Front-loads purpose and what is checked, then covers limits and pricing. Dense but each sentence carries information; the pricing and billing sentences could be trimmed slightly but are relevant to call behavior.

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?

No output schema exists, and the description still names the return classifications (likely-deliverable, risky, undeliverable, invalid) with reasons, plus caveats about what the result does not prove. Nothing an agent needs to invoke or interpret it is missing.

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?

Single parameter at 100% schema description coverage, so the schema already does the work. The inline example (address=support@cloudflare.com) adds slight format confirmation but nothing beyond that; baseline 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?

States a specific verb and resource ('Check email syntax, DNS and MX records...') and enumerates the exact signal set (disposable, role, free provider, SPF/DMARC). This clearly separates it from generic siblings like dns_lookup or domain_report, and from pack_email_deliverability.

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?

Gives explicit trigger conditions: 'Use before sending mail or saving a signup.' It does not name an alternative sibling (e.g. pack_email_deliverability vs this tool) or state when-not-to-use, so it falls short of a 5.

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