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

Annotations already mark read-only, open-world, idempotent, and non-destructive behavior. The description adds important traits annotations cannot express: that it does not probe the mailbox, and that valid is null rather than false when MX lookup cannot complete, which prevents misinterpreting the result.

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?

Four tight sentences, front-loaded with the tool's main value and followed by routing, behavioral, and null-handling details. Every sentence adds actionable information and none is 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?

For a one-parameter, read-only lookup with rich annotations and no output schema, the description supplies the routing context, scope, and null-result interpretation an agent needs. Nothing material is missing for correct selection or 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?

There is one required parameter with 100% schema description coverage, so the schema already documents it adequately. The description restates the checks but adds no parameter syntax, format, or constraints beyond what the schema provides, 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 resource and enumerates the exact checks performed: syntax, MX record, disposable-domain detection, and role/generic account detection. It also names the sibling tools it supersedes, making the tool's distinct scope clear 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?

Gives explicit routing guidance: prefer this over email_validate and email_disposable unless only a single signal is needed. It also tells the agent how to treat a null valid result, including a retry-later instruction, so the usage boundary is fully specified.

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