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

Annotations already declare destructiveHint, idempotentHint, and openWorldHint, so the safety profile is covered. The description adds real context beyond that: attaching does not publish, the returned DNS values are project-specific, and it decodes the two failure modes (409 = variant on another Vercel project, 403 = plan domain limit). It doesn't cover replacement semantics if a different domain is attached later, which keeps it from a 5.

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 tightly packed sentences, front-loaded with the action and its outcome, then the lifecycle note, then error decoding, then a detail pointer. Every clause earns its place and nothing is redundant.

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 mutation tool with no output schema, the description covers the return (DNS records to create), the non-effect (not published), the detach path, and ambiguous error codes. An agent has everything needed to call it and interpret common failures.

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%, including the redirectMode enum semantics and the apex/www registration behavior, so the schema already carries the parameter burden. The description reinforces the apex-or-www input form but adds no syntax or format detail beyond it. Baseline 3 is correct.

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 and resource ('Attach a custom domain'), names the accepted shapes (apex+www pair, single subdomain), and distinguishes itself from the clear_domain sibling that detaches. An agent can identify the operation and its scope without opening the 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?

Explicit about when to use it (attach), the alternative to use instead for the inverse operation (clear_domain detaches), a key non-obvious condition ('attaching doesn't publish' — publishing is a separate sibling call), and a pointer to get_style_reference for deeper detail. Nothing is left to inference.

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