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

Goes well beyond the annotations: discloses that no mail is sent, lists the exact check pipeline, names the four verdict outcomes (deliverable, risky, undeliverable, invalid), and — critically — states the payment requirement and rate ($0.005 USDC per call via x402 on eip155:8453). The x402 pricing/auth detail is exactly the kind of context annotations cannot convey.

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?

Three dense sentences, front-loaded with the check list and verdict categories with no filler. Minor waste: the first sentence is a near-verbatim repeat of the title field, and the check list is stated twice (once as a summary, once in the detailed clause).

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?

With no output schema, the description usefully names the verdict vocabulary and the checks performed, so an agent knows what comes back conceptually. It stops short of describing the response shape (per-check booleans, suggested correction field for typos, normalized domain value), which would fully close the gap given there is no output schema.

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 exists and the schema gives it zero description coverage (just type string, maxLength 320). The description never says what input form is accepted — bare addr-spec vs display-name form like 'Foo <a@b.com>' — nor does it mention normalization behavior applied to input, so it adds little beyond the schema.

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+resource (an e-mail address check) and enumerates exactly what is checked: RFC syntax, IDN normalization, MX/A records, null-MX, disposable lists, role accounts, free providers, typo suggestions. This clearly separates it from siblings like dns_mx (MX only) and email_security (domain policy records).

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?

"Validates an address without sending mail" gives clear context that this is a non-invasive, no-email-sent check with a bounded set of criteria. It does not, however, explicitly route the agent between this and siblings such as dns_mx or email_security, so no exclusions or named alternatives are provided.

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