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

Annotations already cover safety (readOnly, idempotent, non-destructive, openWorld), but the description adds behavior the annotations cannot: no mail is sent and no SMTP connection is opened, the possible verdict values, the explicit epistemic limit (mailbox existence is not provable), the $0.01 USDC cost, and the two-step x402 payment retry flow. Cost and payment mechanics are exactly the kind of hidden behavior agents need disclosed.

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?

Information-dense with no filler: the check enumeration, verdict set, limitation, usage trigger, price, and payment handshake each occupy one clause. The usage cue and payment caveat are front-loaded in positions an agent will read before invoking.

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 verification tool with no output schema, the description supplies everything needed: what is checked, what verdicts come back, the confidence ceiling, when to use it, and the paid-invocation protocol. No return-shape documentation is needed because verdicts are already named inline.

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?

Only one parameter and schema description coverage is 100% (the schema documents it with an example and length bounds), so the schema does the heavy lifting. The description adds no syntax, normalization, or format guidance beyond what the schema already states, which matches the baseline for high-coverage schemas.

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 (checks) plus a fully enumerated resource scope: syntax, domain, MX records, A/AAAA fallback, RFC 7505 null MX, disposable list, role account and free provider. This is clearly distinguishable from siblings like dns_lookup (generic DNS) and email_auth_check (SPF/DKIM/DMARC), which the description implicitly excludes by scoping itself to MX-only verification.

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 a concrete usage context ('use when an agent validates a signup, lead or contact email before using it') and a clear limitation boundary ('cannot prove the mailbox itself exists', 'no SMTP'). It does not name a sibling tool as an alternative nor say when to prefer email_auth_check, so it stops short of full when/when-not routing.

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