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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so safety is covered structurally. The description still adds real value by confirming no email is sent and no DNS requests are made, i.e. no network side effects and no deliverability implication. It stops just short of describing the exact result shape, so it is not a full 5.

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 sentences, zero padding, front-loaded with the scope limitation before the usage guidance and the negative capability disclosure. Every sentence earns its place.

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 boolean-style validator with no output schema, the description covers scope, the negative case (no mailbox verification, no DNS), and the return nature ('syntax-only result'). An agent has everything needed to invoke and interpret it 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?

There is one parameter with 100% schema description coverage, including the maxLength and ASCII-only constraint, so the schema already carries the semantics. The description's 'ASCII format only' phrasing corroborates but does not extend the schema, which is the baseline-3 case.

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?

The description names a concrete verb-and-resource action (reject malformed email strings / return a syntax-only result) and explicitly bounds the scope as 'ASCII email format, without mailbox verification.' That scope statement cleanly separates it from unrelated validators in the sibling list (is-phone-e164, is-iban-safe, is-ip-safe) without needing their schemas.

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?

It states the use condition ('before a form or fixture is processed') and an explicit exclusion with a routing instruction ('For mailbox existence or deliverability, use a separate verification service'). An agent knows both when to call it and when not to, even though the alternative is external rather than a sibling tool.

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