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

Annotations cover the safety profile (readOnlyHint=false, destructiveHint=false), and the description adds real context beyond them: this registers an ownership challenge, and it discloses that mailbox passwords, message content and the private service credential are never returned. It stops short of saying what happens to an existing domain or what the challenge response is.

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 short clauses, purpose front-loaded, then permissions, then data-handling caveats. No filler, though the permission sentence is dense and slightly out of place for a create call.

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?

With no output schema, the description should carry return-value expectations, and it only tells the agent what is *not* returned. It never describes the ownership challenge payload or the logical next step (verification), leaving a gap for a three-parameter mutation.

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 the baseline is 3. The description adds nothing about domain format, organization_id or the idempotency_key, so it does not improve on the schema's own parameter docs.

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 first sentence gives a specific verb and resource ('Register a domain and its ownership challenge'), so an agent can distinguish it from siblings like domain_get, domain_verify and domain_setup. It never explicitly names those alternatives, but the create-vs-read/verify split is inferable.

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 supplies an access model ('No guest access. Members can read. Owners, admins and organization keys can manage email') but frames it as permissions rather than when to call this tool instead of domain_verify or domain_setup. Usage is implied, not routed.

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