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

Annotations only indicate that the tool is not read-only and not destructive. The description adds one useful behavioral fact: the tool returns DNS records for configuration, implying it does not perform the DNS setup itself. It does not address idempotency, authentication, or duplicate-domain behavior, but for a simple creation tool this is acceptable.

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?

One short sentence that front-loads the verb and resource and places the side effect in a parenthetical. Every word 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?

Given a single required parameter, an output schema, and annotations covering the basic safety profile, the description supplies the essential action and its key outcome. Nothing critical is missing for an agent to select and invoke this tool correctly.

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?

The only parameter is fully covered by the schema, which provides a type and example ('mail.acme.com'). The description adds no additional parameter semantics beyond labeling it a sending domain, so the baseline of 3 for high schema coverage applies.

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?

The description states a specific verb and resource: 'Add a sending domain' and tells the agent the key outcome, that DNS records are returned. The qualifier 'sending' differentiates it from the sibling add_domain, while the DNS-record detail separates it from create_email_sender or verify_email_domain.

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?

The phrase 'sending domain' and the DNS-record side effect imply the tool is for initiating email-domain setup, but the description gives no explicit when-to-use or when-not-to-use guidance. It does not mention alternatives like verify_email_domain or how to proceed after DNS records are configured.

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