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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint, and destructiveHint=false. The description goes further by disclosing the return of a normalized value, the DNS/MX side effect of check_dns, and explicit non-capabilities (no email sent, no mailbox verification) — useful context an agent cannot infer from 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?

Three tight sentences: capability first, optional mode second, limitations third. Every sentence earns its place with no repetition of the title or schema.

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 correctly supplies the return behavior (normalized version) and the boundaries of the validation. For a two-parameter read-only tool, nothing an agent needs to call it correctly is missing.

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 coverage is 50%: check_dns is documented in the schema as 'Also look up the domain's mail servers. Default false,' which the description enriches with MX wording, but the required email parameter has no description in either place (maxLength 512 and accepted format are never stated). The description partly compensates but leaves a gap for the required parameter.

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 (checks/validates) and resource (an email address) plus the return behavior ('returns a normalized version'). The scope boundaries ('Does not send email and cannot confirm that a specific mailbox exists') cleanly separate it from the sibling validators validate_iban, validate_phone, and validate_url.

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?

Gives a clear conditional for the optional mode: 'Set check_dns=true to also confirm the domain exists and can receive mail (MX records).' It also states a meaningful limitation (cannot confirm a specific mailbox exists), but it does not explicitly say when to prefer this over sibling validation tools or when validation is unnecessary.

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