Validate one email address

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

Adds significant behavioral context beyond the readOnlyHint and openWorldHint annotations: explains the meaning of 'Unknown' status, lists reasons (catch-all, greylisting, blocked SMTP), and warns never to treat Unknown as invalid. This is valuable operational guidance.

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 four sentences, each serving a distinct purpose: action, statuses, clarification, and sibling alternative. No fluff or redundancy, and the most critical information is front-loaded.

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 tool with an output schema, the description covers the action, result interpretation, edge cases, and tool alternative. The 'Unknown' handling is especially important and fully addressed, leaving no significant gaps.

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 provides a description and example for the email parameter, with 100% coverage. The tool description adds minimal extra parameter info, so the baseline score of 3 is appropriate.

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 clearly states the tool checks a single email address for deliverability, enumerating specific checks (syntax, MX, disposable-domain, SMTP probe). It also explicitly distinguishes from sibling validate_emails by noting the single-address scope, making the purpose unambiguous.

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?

Provides explicit guidance to use validate_emails when more than one address is present, which serves as an exclusion criterion. It also implies this tool is for single-address checks, and offers interpretive guidance on handling Unknown results, meeting the dimension fully.

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