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

Annotations already cover read-only, idempotent, open-world, and non-destructive behavior, so the bar is lower. The description adds critical operational context beyond annotations: it does not probe the mailbox, and valid is null when MX lookup is incomplete, with retry guidance instead of treating the address as invalid.

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, then alternatives, then the key failure mode. Every sentence earns its place with no redundancy.

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?

For a single-parameter, read-only, open-world tool with no output schema, the description covers purpose, alternatives, scope limitation (no mailbox probe), and the crucial null-valid edge case. Nothing an agent needs to select or correctly interpret the call is missing.

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%, so the single email parameter is already fully documented in the schema. The description restates that email is run through the checks but adds no syntax or format meaning beyond what the schema provides, matching the baseline for high coverage.

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 combined action (syntax, MX, disposable, role/generic checks) and explicitly names the sibling tools it supersedes. An agent can distinguish it from email_validate and email_disposable 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?

Explicitly says to prefer this over email_validate and email_disposable unless a single signal is needed, and adds that it does not probe the mailbox. This gives clear when-to-use and when-not-to-use guidance.

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