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

Annotations already cover the read-only/idempotent/open-world safety profile, and the description goes well beyond them: no SMTP probe, no mail sent, no storage or logging, exactly one live DNS lookup via Cloudflare DoH, verdict vocabulary, and per-call cost with payment placement in _meta and the no-charge-on-error rule.

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 action and checks, then limitations, then cost and error semantics. It is dense but every clause carries distinct, decision-relevant information with no filler or repetition.

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?

There is no output schema, yet the description supplies the return vocabulary (ok/risky/undeliverable with plain reasons), the data-retention and privacy stance, billing mechanics, and failure behavior — everything an agent needs to invoke and interpret it correctly.

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?

One parameter with 100% schema description coverage, so the schema already documents name, type and example. The description adds no format, normalization, or length constraints beyond what is already in the schema, 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?

Specific verb and resource ('check one email address') with an explicit enumeration of what is checked (syntax, MX/null MX/implicit A, disposable, role-account, free-provider). It clearly differentiates itself from the sibling validate_email by declaring it does far more than syntax validation.

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?

Explicitly routes the agent: 'Use validate_email first if you only need syntax,' and states the exclusion condition ('no SMTP probe is made... cannot confirm the mailbox exists'). It also covers the error/charging condition, which is a routing-relevant constraint.

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