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

Annotations only declare readOnly/openWorld/non-idempotent/non-destructive; the description goes well beyond by disclosing that nothing is sent, enumerating the checks performed, and specifying the credit model including which outcomes are free and which are refunded. It also defines the four possible verdict values and the accompanying `reason`, which the annotations cannot convey.

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 sentences, both front-loaded: the first states the decision it supports, the second gives mechanics, return values and cost. No sentence is redundant with the title or the schema.

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` with its four enumerated values, plus `reason`). It also covers the cost model, so an agent has everything needed to call it and interpret the result.

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?

Schema description coverage is 100% and the single `email` parameter is already documented with an example, so the description adds no syntax or format guidance beyond it. Baseline 3 applies for a fully-covered schema.

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: check whether an email address exists and is deliverable, and spells out the three checks (format, domain mail servers, mailbox). Distinguishes itself from siblings like find_work_email and reverse_email_lookup, which search for or reverse-lookup addresses rather than validating one.

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 clear timing context ("before it bounces") that tells the agent when this tool is the right pick, and the credit/refund note signals a cost tradeoff to weigh. However, it never names an alternative or an explicit when-not-to-use case among the twelve siblings.

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