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

Annotations only cover safety (readOnlyHint) and openness (openWorldHint), so the description correctly carries the rest: it enumerates the returned status, sub-status, free-mailbox flag, suggested correction, domain age, and mail servers, and discloses the cost model ('1 credit, charged only when a call succeeds'). It does not mention rate limits or authentication needs, but for a read-only lookup that is a minor gap.

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-loads what the tool determines, then the return payload, then the usage rule and alternative, then the cost caveat. Every sentence carries distinct information with no redundancy.

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?

There is no output schema, yet the description explicitly enumerates the response fields an agent needs in order to interpret results, and it covers the selection decision and cost. Nothing required to call or act on the tool 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?

There is a single required parameter with 100% schema description coverage, so the schema already documents it fully. The description adds no syntax, format, or normalization guidance for the email input, 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?

States a specific verb (check) and resource (an email address) plus the exact judgment it renders — deliverable, catch-all, or disposable. It is clearly distinguishable from the sibling 'emails' tool, which is named directly.

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 an explicit trigger ('Use before emailing an address you were given or found') and routes the agent away from this tool with a named alternative ('To find an address from a person's name use emails'). Both when-to-use and when-not-to-use are covered.

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