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

With no annotations, the description carries the full burden and does substantial work: it discloses the SMTP limitation (mailbox itself is not verified), the refusal-before-payment behavior for malformed addresses, and the pricing/rails. It omits rate limits, latency, and what an unverified mailbox means for the verdict's reliability in edge cases.

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 usage condition, then input, output, limits, error behavior, example, and price in compact order. Every clause is load-bearing for an agent deciding whether and how to call it.

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?

No output schema exists, yet the description enumerates the verdict values and the fields returned (reasons, MX hosts, null MX, SPF/DMARC, disposable, role, free provider). For a single-parameter tool this is complete enough to call without further discovery.

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?

Single parameter with 100% schema description coverage, so the baseline is already high. The description adds an inline example ("?email=info@example.com") showing the exact query-string form and reinforces that only a well-formed address is accepted, which the schema does not state.

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?

Names the specific resource and the exact checks performed (syntax, MX, null MX, SPF/DMARC, disposable domain, role account, free provider) plus the final verdict type (deliverable/risky/undeliverable). Nothing about the operation is left ambiguous, and the scope boundary (no SMTP mailbox probe) is stated outright.

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?

"Use when an agent is about to email or sign up a contact" gives a clear triggering condition and is well matched to the tool's purpose. There is no explicit when-not guidance or named alternative, but among the sibling list no other tool overlaps on email validation, so the omission is minor.

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