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

Annotations already disclose that the operation is non-read-only, non-destructive, and idempotent. The description adds that the domain is 'registered' and that it is a precursor to DNS setup, implying it does not perform DNS configuration or verification itself. This is consistent with annotations but does not provide additional behavioral insight beyond what the annotations convey, such as whether an existing domain is overwritten or how idempotency manifests. Given annotations cover the core traits, a 3 is appropriate.

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?

The description is two concise sentences with no superfluous information. The primary purpose is front-loaded, and the usage guidance is neatly appended. Every word adds value, making it efficiently scannable for an agent.

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 the tool's low complexity (one parameter, output schema present), the description provides sufficient context: what it does, when to use it, and its place in the workflow. The presence of an output schema means return details are already structured, and the description covers the essential purpose and sequencing. Nothing crucial is missing for an agent to call it 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 schema provides 100% coverage for the single 'domain' parameter with a clear description ('Domain name to add, such as mail.example.com or example.com.'). The description does not add any supplementary semantics beyond the schema, though it aligns with the generic domain usage. Since the schema fully documents the parameter, the baseline of 3 is maintained; no extra meaning is contributed by the description.

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 uses the specific verb 'Register' with the resource 'custom sending domain', clearly indicating the action and object. It states the domain is registered 'in the workspace' and 'so it can be configured for email traffic', which distinguishes it from related siblings like domain_verify (verification) and domain_zone_file (DNS file) by emphasizing the creation step. The sequencing clue 'before DNS setup and verification' further clarifies its unique role.

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 description explicitly states when to use the tool ('Use this before DNS setup and verification'), giving clear timing for invocation. It implies that this is the initial registration step and that subsequent tools handle DNS and verification, but it does not name those alternatives explicitly. The guidance is clear enough for an agent to sequence it correctly, but it lacks an explicit 'when not to use' or mention of sibling tools.

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