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

Annotations only declare readOnlyHint=false and openWorldHint=true; the description adds genuinely new behavioral facts: the billing model (billable valid/invalid/catch_all/do_not_mail vs free `unknown`, cache does not change billing), the auth requirement, and the return shape. This is exactly the context an agent needs before invoking a credit-consuming tool.

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?

Three sentences, front-loaded with purpose and checks, then billing, then auth and return shape. No filler, and the parenthetical enumeration of checks is dense but readable.

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?

Low-complexity tool (one required param, no output schema) and the description covers purpose, checks performed, cost, auth, and return keys. What is missing is marginal: how errors surface and whether any rate limits apply.

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 parameter with 0% schema description coverage, so the description carries the burden — it clarifies that the input is the email address being verified. Beyond that it adds no format or validation guidance (syntax expectations, normalization), which is the minimum viable for a one-field tool.

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 ('verify a single email's deliverability') and then enumerates exactly what checks run: syntax, DNS/MX, SMTP mailbox, disposable/role/catch-all detection, and a confidence score. The word 'single' plus the enumeration makes it readily separable from the bulk and demo siblings without opening any 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?

Offers cost-based context (1 credit for billable results, free `unknown`) and the API-key prerequisite, which implicitly helps an agent weigh this against cheaper alternatives. However, it never names `verify_email_demo` or `submit_bulk_verification`, so the decision of when to use this versus those siblings is left to inference.

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