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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds substantial context beyond them: no email is sent, the 100-address cap, the four possible verdicts with their meanings, and the important caveat that 'valid' is not proof the mailbox exists. That is rich behavioral disclosure an agent cannot get from the annotations or schema.

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?

Opens with the core action and its safety property ('BEFORE writing... without sending anything'), then enumerates verdicts, then closes with the usage trigger. Every sentence carries information; no filler or restatement of the title.

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?

No output schema exists, and the description supplies the return semantics (the four verdicts) that an agent needs to interpret results, plus scope limits and preconditions. Nothing material is missing for a one-parameter read-only verification tool.

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 with 100% schema coverage, and the schema already states '1 to 100' addresses, so the description's batch-size mention adds nothing new. Baseline 3 is correct when the schema fully documents the single parameter.

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 (verifies) and resource (email addresses), plus the exact checks performed: syntax, MX/mail-receiving domain, prior hard bounces, disposable/role mailboxes, DNS dead-mailbox signals. It clearly separates itself from siblings like add_subscriber or send_email by noting it writes nothing and sends nothing.

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 concrete trigger conditions: 'before prospecting, or before importing a list you did not collect yourself,' which implicitly excludes self-collected opted-in lists. It does not name an alternative tool explicitly, but the positioning relative to the write tools is unambiguous enough to route correctly.

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