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

Annotations already declare readOnlyHint=false and non-idempotent, so the agent knows this is a mutating write. The description adds genuinely useful behavioral context beyond that: the tool returns DNS records the user must act on externally, and the verification is a separate step. It doesn't address duplicate-registration behavior or auth, but it adds real value over the annotations.

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?

Two tight sentences, front-loaded with the purpose and then the output/workflow. No filler, every clause earns its place.

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?

There is no output schema, so the description carries the burden of explaining returns and it does so concisely (DNS records to add). It also covers the required next step, making it complete for a create-domain tool.

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 both parameters (apiKey, domain) are already documented in the schema, giving a baseline of 3. The description adds no syntax or format details about the domain FQDN beyond what the schema provides.

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 verb+resource ('Register a new sending domain') that clearly distinguishes it from siblings like list_domains and check_domain_health. An agent can tell this is a create operation for a domain 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?

Provides a clear follow-up workflow by naming check_domain_health and the condition for calling it (after adding DNS records). It doesn't state prerequisites (e.g., API key needs) or when-not-to-use, but the workflow context is explicit.

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