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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds context annotations cannot carry: cost (0.005 USDC per call), p95 latency (800 ms), and a hard capability boundary (no SMTP probe, so mailbox existence is not confirmed). That capability limit is the single most decision-relevant behavior for this tool.

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?

Front-loaded with the returned fields, then usage, then caveats, cost and latency — no filler sentences. The opening enumeration of return fields is partly redundant with the existing output schema, which is the only real slack.

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 an output schema, full parameter coverage, and annotations covering the safety profile, the description fills the remaining gaps an agent needs: scope limits, cost, and latency. Nothing required to select or invoke 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?

One parameter with 100% schema description coverage, including an example and length bounds, so the schema carries the burden. The description adds no syntax or format detail beyond what the schema already states; baseline 3 applies.

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 and resource ('Returns JSON with an email address check') and enumerates the exact facets checked (syntax, MX, disposable/free/role flags, SPF/DMARC, deliverability verdict). This is clearly distinguishable from every sibling, none of which validate email addresses.

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 explicit when-to-use ('before sending, signup or lead scoring') and when-not-to-use ('no SMTP probe is done', 'not to send email'). The exclusion of mailbox-existence checking is exactly the boundary an agent would otherwise get wrong.

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