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

Goes well beyond the annotations: discloses the side-effect profile ('Sends no email'), the auth situation ('No login or API key required'), and a rate limit. It also defines the result vocabulary ('unknown means inconclusive or mailbox not checked; likely_deliverable is evidence, not a delivery guarantee'), which is exactly the interpretive context an agent needs.

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?

Six short sentences, each carrying distinct information: what is checked, no send, DNS limitation, result interpretation, auth, and rate limit. The most important scope statement is front-loaded and nothing is redundant.

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 structure needn't be restated, yet the description still supplies the semantic meaning of key result values. Combined with the side-effect, auth, and rate-limit disclosures, an agent has everything required to call and interpret this tool.

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?

Schema description coverage is 0%, so the schema only declares a bare string with min/max length. The description compensates slightly by stating it takes 'one email,' implying a single address rather than a list, but adds no format or constraint detail. Adequate but leaves the parameter largely to inference.

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 (Check) plus the exact resource (one email) and enumerates the concrete checks performed: syntax, mail routing, disposable/free-provider/role flags, and SMTP recipient acceptance. The scope is unmistakable and no sibling tool overlaps this domain.

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?

Clearly frames the operation as a non-sending verification and warns that 'DNS alone does not verify a mailbox,' which tells the agent what this result does and does not establish. There are no sibling alternatives to route against, so only the absence of explicit when-to-use phrasing keeps it from a 5.

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