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

Annotations already declare destructiveHint=true, idempotentHint=true, openWorldHint=true and readOnlyHint=false, and the description corroborates this by disclosing two real behaviors: it consumes one verification credit and it overwrites the previous verification status on any matching lead. That side-effect and cost disclosure is genuinely additive. It does not explicitly address idempotency or the auth/permission requirements.

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 core action, then the verdict set, then cost, then side effect — a logical order with little waste. It is slightly dense (enumerating six verdicts plus the accept_all caveat), but each sentence carries distinct information.

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 correctly supplies the full return vocabulary (valid, invalid, accept_all, webmail, disposable, unknown) and flags the ambiguous accept_all case. Combined with the credit cost and the lead-status overwrite, 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?

One parameter with 100% schema description coverage, so the schema already documents 'email'. The description adds no syntax, format, or validation detail beyond what the schema says, so the baseline of 3 is appropriate.

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+resource ('Check whether one email address can receive mail') and scopes it to a single address, which implicitly distinguishes it from the bulk sibling verify_emails. An agent can tell what it does and that it operates on one address without opening the schema.

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?

Usage is implied by the single-address scope and by the credit-cost note, but there is no explicit when-to-use/when-not guidance and no direct routing to the sibling verify_emails. The accept_all caveat ('cannot be confirmed — do not treat it as valid') is useful handling guidance rather than selection guidance.

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