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

Annotations already indicate read-only, idempotent, open-world, and non-destructive behavior, and the description adds substantial behavioral detail beyond that: no mail sent, no SMTP probe, live MX DNS lookup, provider naming, and the explicit caveat that domain reachability is confirmed but not mailbox existence. It also enumerates exact verdict values and output components, providing rich transparency for an agent.

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?

The description is dense yet well-structured, front-loading the primary use case ('Validate an email before spending a send, token, or signup') and then systematically listing checks, outputs, and limitations. Every clause contributes useful information, and there is no filler or repetition of schema fields.

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?

Despite having no output schema, the description fully enumerates the return values: verdict, 0-100 score, canonical mailbox, and verdict hash, including all possible verdict strings. It also clarifies the critical limitation that mailbox existence is not confirmed. For a single-parameter tool with read-only annotations, this is complete enough for an agent to select and invoke 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?

The input schema already fully documents the single `email` parameter with a description and maxLength, giving 100% schema coverage. The tool description adds helpful context about validation behavior, but does not add new meaning specific to the `email` parameter beyond what the schema states. This meets the baseline but does not exceed it.

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 opens with a clear verb and resource: 'Validate an email' before a send, token, or signup. It then enumerates exactly what checks are performed (RFC-5322 syntax, MX, disposable domains, role mailboxes, typos) and what the tool returns, making its purpose unmistakable. Although siblings are mostly unrelated, the description strongly differentiates the tool by stating it sends no mail and does not confirm mailbox existence.

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?

The description clearly states when to use the tool: before spending a send, token, or signup, and without sending mail or probing SMTP. It does not explicitly name alternatives or exclusions, but given the sibling list is largely unrelated, this is sufficient context. The phrase 'FREE' also signals that cost is not a concern for this validation step.

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