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

With no annotations, the description carries the full burden, but it does disclose the checking dimensions and the return shape (verdict + agent_action + reasons). It does not mention that a DNS/mail-server lookup happens over the network, nor rate limits, failures, or auth needs — meaningful gaps for a tool that touches DNS.

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 tightly packed sentences, front-loaded with the action and checks, then the return values. No filler.

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, so the description usefully names the return fields (verdict, agent_action, reasons). It covers the validation surface well, though the check_dns parameter's effect and failure behavior remain unexplained.

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 coverage is 0%, so the description must compensate. 'Check one email address' clarifies the email parameter as a single address, and 'whether the domain has a working mail server' loosely maps to the check_dns toggle, but it never states that check_dns controls that lookup or its default. Partial compensation only.

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 one email address') and enumerates exactly what is checked: syntax, typo suggestion, disposable inbox, role account, and domain mail server. The 'one email address' phrasing implicitly distinguishes it from the bulk-oriented sibling validate_contacts.

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 only implied: 'one email address' suggests single-address validation versus validate_contacts, but there is no explicit when-to-use, when-not-to-use, or named alternative. No prerequisites or cost/latency guidance is given.

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