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

Annotations already declare destructive/idempotent/openWorld, so the description correctly focuses on adding context: it registers both apex+www variants, returns DNS records rather than publishing, and decodes 409 (collision on another project) and 403 (plan limit). No contradiction with the annotation profile.

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?

Front-loads the action and return value, then stacks routing, error, and reference clauses. Dense and telegraphic ('409 = ...; 403 upgrade = ...') but every clause carries information; not padded.

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?

No output schema exists, and the description compensates by stating the tool returns DNS records to create. With annotations covering the safety profile and error codes covering failure modes, an agent has enough to call this correctly; only the redirect-mode sibling relationship is unaddressed.

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 coverage is 100%, so the baseline is 3. The description's mention of the apex+www pair and the one-subdomain case largely restates the schema's own note that 'both variants are registered,' and redirectMode is left entirely to the enum 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?

States a specific verb (Attach) and resource (custom domain) plus what comes back (the DNS records to create, specific to this project). It explicitly distinguishes itself from clear_domain (detach) and the check/get status siblings, so an agent can route 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?

Names the detach alternative (clear_domain) and rules out a mistaken expectation with 'Doesn't publish.' The 409/403 error semantics tell the agent when the call will fail and why. It stops short of positioning against check_domain, get_domain_status, or set_domain_redirect_mode, which is the only real gap.

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