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

With annotations already declaring non-readOnly, non-destructive, non-idempotent and closed-world, the description adds real context beyond them: it discloses the return payload (DNS records, valuable since there is no output schema), that verification is deferred to a separate action, and that an existing domain is reused. It does not mention auth/connection requirements or what happens to an existing domain's purpose configuration.

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 short sentences, front-loaded with the action and scope, followed by the return value and the verification caveat. No filler or repetition.

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 4-parameter mutation with no output schema, the description covers the essential gaps: what is created, that DNS records come back, and that verification is out of scope. It is only marginally short on failure/precondition behavior and how it differs from register/reuse siblings.

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% — name examples, region enum with default, purposes enum with per-value meanings and tracking default are all documented in the schema. The description only restates 'one domain' and 'the chosen purposes', adding no syntax or constraint detail beyond the schema.

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?

Clear specific verb+resource: adds one domain to the workspace's Resend account for chosen purposes. It does not explicitly distinguish itself from close siblings like register_mail_domain or reuse_resend_domain, which handle the same resource — the phrase 'or reuse it if it is there' actually blurs the boundary with reuse_resend_domain rather than clarifying it.

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?

It gives one useful routing cue — 'Domain verification is a separate action' — steering the agent toward verify_mail_domain afterward. But it offers no guidance on when to prefer this over register_mail_domain or reuse_resend_domain, and no prerequisites (e.g., Resend connection required) are stated.

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