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

Annotations already declare readOnly/idempotent/openWorld/non-destructive, but the description adds substantial context beyond them: DNS-only with no mail-server contact, that mailbox existence is explicitly not verified, typical latency 0.1-1 s, cost USD 0.001, and a free tier of 5 per IP per UTC day. It also warns to treat returned page text and on-chain strings as untrusted.

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-loads the purpose, then layers scope, latency, pricing and safety guidance in distinct sentences with no filler. The one weak spot is the 'returned page text and on-chain strings' untrusted-data warning, which is generic boilerplate poorly matched to an email-verification tool.

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?

An output schema exists so return values need no explanation, the single parameter is fully documented, and annotations cover the safety profile. Adding latency, price, rate limit and scope caveats makes it complete for correct invocation.

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 description coverage, so the schema already carries the semantics. The description adds no format or syntax guidance for the email argument beyond what the schema states, making the baseline 3 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 ('verify an email address') and enumerates exactly what is checked (syntax, MX, null MX, A/AAAA fallback, disposable list, role parts, free providers) and what is not (no SMTP/RCPT, mailbox existence unverified). It explicitly names the sibling inspect_domain as the tool for SPF/DMARC/DKIM, so the agent can separate them without opening either 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?

Gives concrete when-to-use contexts ('before you send to it or accept it at sign-up') and routes domain-level email auth checks to inspect_domain. The scope boundary (DNS-only, no mailbox probing) tells the agent when this tool is insufficient.

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