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

Annotations already declare read-only, non-destructive, open-world, non-idempotent, but the description adds substantial behavior the schema does not: no mailbox is contacted, DNS/syntax signals only, a $0.01/call charge via x402 USDC or card key, first 25 free, failed calls free, and that repeats are not free because each call is billed. The billing model is material to an agent deciding whether to loop over addresses, and it is stated plainly rather than buried.

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-loaded with what is checked and what is returned, followed by the caveat, usage contexts, example, and pricing in a logical order with no filler. The semicolon-separated clause about pricing and read-only behavior is dense but each element carries information an agent needs.

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 single-parameter tool with no output schema, the description supplies the verdict taxonomy, the reasons field, the caveat that mailbox existence is unproven, usage contexts, and cost/billing semantics. Nothing an agent would need in order to invoke it correctly or interpret the result 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?

With one parameter at 100% schema coverage, the schema already defines 'address' as the email address to verify, so little is required. The description adds a concrete worked example (address=support@cloudflare.com) that clarifies format expectations beyond the field description.

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 verb and resource plus the exact signals checked (syntax, domain/MX, disposable/free providers, role addresses, SPF/DMARC) and the four verdict values returned. It is unmistakable what the tool does, though it never explicitly names the sibling it differs from (e.g. pack_email_deliverability, email, dns_lookup) to route the agent.

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 concrete usage contexts: 'Use before sending mail, saving a signup or scoring a lead.' It also sets an explicit boundary on interpretation ('this is not proof that a mailbox exists'), which functions as guidance on when the result should not be trusted for gating. What's missing is an explicit alternative tool for mailbox-level checks or bulk use.

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