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

Goes well past the readOnly/idempotent/openWorld annotations by disclosing the verification method (no mail sent, no SMTP probing), the source of the verdict (live MX lookup plus a ~100k disposable-domain list), billing mechanics ($0.001/call, 10 free/day, x402 payment-required result), and error handling (isError on invalid input or upstream failure, not charged).

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 the core question and the list of checks, then dense supplementary detail on pricing and errors. Everything is relevant, though the billing/error sentences are somewhat packed and could be tightened.

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?

An output schema exists, so return values need not be described. Between method, verdict source, cost model, and error semantics, the description gives an agent everything needed to decide whether and how to call 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?

Schema coverage is 100% with a single required parameter, and the 'no mail is sent' note is already echoed in the schema's own description. The description adds no format, syntax, or constraint detail beyond what structured data provides, so the 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?

Opens with a concrete question the tool answers ('Will this email address work?') and enumerates exactly what is evaluated: syntax, MX records, disposable providers, role addresses, free-mail providers. This is clearly distinguishable from siblings like dns_lookup, address_check, or http_probe.

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 implies when to use it (validating deliverability without sending mail) and rules out the SMTP-probing alternative that would burn server reputation. However, it never names a sibling tool or states an explicit when-not-to-use condition, leaving the agent to infer usage from context.

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