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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds the meaningful behavioral limit: it performs only syntax and MX resolution and explicitly does NOT detect disposable/role addresses. It omits return shape and any rate/network-failure behavior, but that is a minor gap given the annotations.

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?

Two tight sentences, zero filler. The capability and its boundary are front-loaded before the alternative-tool pointer.

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 read-only validator with no output schema, the description covers what is checked and what is excluded, which is sufficient for correct invocation. It does not describe the return value, which is the one small gap since no output schema exists.

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 with 100% schema coverage, so the baseline is 4 and the schema already defines it fully. The description restates the check semantics ('correct syntax and a resolvable domain') but adds nothing beyond what the schema wording provides.

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 and resource (email check) and immediately narrows the scope to syntax plus MX-record resolution. It explicitly differentiates itself from the sibling email_verify, so an agent can choose between them without reading either schema.

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 gives the when-not case ('Does not detect disposable or role addresses') and names the alternative tool for the fuller check ('use email_verify for the combined check'). Routing guidance is unambiguous.

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