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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false; the description adds the crucial behavioral limit that mailbox existence is not verified and that no SMTP probe occurs, plus the return shape (verdict/score/reason codes). It does not cover latency, quotas, or error behavior, keeping it below 5.

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?

Two tight sentences, front-loaded with the capability list and followed by the return contract and the single most important limitation. No filler, no restatement of the tool name, and the caveat is placed where it is read before invocation.

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 compensates by naming the returned fields (verdict values, 0-100 score, reason codes), stating the per-call limit, and disclosing the key verification gap. An agent has enough to call the tool correctly and interpret results.

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?

Only one parameter exists and schema description coverage is 100%, with the schema itself documenting array form, 'Name <email>'/'mailto:' formats, minItems 1 and maxItems 10. The description's 'up to 10' restates the schema cap rather than adding new semantics, so the baseline of 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?

Starts with a specific verb ('Check') and resource ('email addresses'), then enumerates the exact checks performed (syntax, MX, provider, disposable, role account, typo, domain age, SPF/DMARC). Scope is bounded ('up to 10') and the negative claim ('Does not verify that the mailbox itself exists') pins down what the tool is not.

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 and title jointly define the boundary of the check ('no SMTP probing', 'does not verify that the mailbox itself exists'), which tells the agent when this tool is appropriate versus a deeper verification approach. No alternatives are named, but none exist among siblings, so the guidance is clear without exclusions.

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