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

Goes well beyond the annotations (readOnly/idempotent/non-destructive/openWorld) by disclosing the SMTP-probe limitation, the pricing model ($0.02/call over x402 on Base), the prepaid x-credit-token path for unlimited calls, the 1-free-call/day allowance, and that no API key is required. These are material operational facts an agent needs before invoking.

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, then capabilities, then limitations, then pricing/auth in descending priority. Every sentence carries distinct information (what it checks, what it catches, what it does not do, how it is billed) with no filler.

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 no explanation. The description covers capability, detection scope, explicit limitations, cost, and auth requirements, which is everything needed to call this 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?

Only one parameter ('email') with 100% schema description coverage, so the schema already carries the semantics; the description merely restates 'Required input: email'. It adds no format or normalization guidance, so the baseline 3 is appropriate.

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 resource (email address) and enumerates exactly what is checked: syntax, live DNS MX/A, and disposable/role/consumer-mailbox detection, producing a GO/HOLD/STOP verdict. The sibling set contains no other email tool, and the description clearly separates it from phone_check/address_check by naming the email-specific signals.

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?

Opens with the decision context ('Is this address worth sending to?') and lists concrete triggering cases (throwaway inboxes, support@-style shared mailboxes, typos like gmial.com). It also sets the boundary that no SMTP probe is performed, so mailbox existence is explicitly not claimed. No named alternative tool, but there is no close sibling to route against.

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