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

Annotations already declare readOnlyHint=false, destructiveHint=false, openWorldHint=true, so the mutation/open-world nature is covered. The description adds a useful routing fact ('the website serves /api too') but omits what an agent would want for an open-world DNS operation: whether verification is required, what happens on duplicate hostnames, and whether the call blocks on propagation.

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 with the primary action front-loaded and the conditional parameter guidance second. No filler, nothing repeated from the title.

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 an open-world mutation with no output schema, the description should cover prerequisites and follow-ups (DNS records, verify_domain, primary-domain designation). It covers the service-vs-website distinction well but stops short of the surrounding workflow an agent needs to call this 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?

Schema coverage is 67%, so service and project_id are documented in the schema and the description reinforces service semantics. The required hostname has no schema description and the prose does not specify its expected format (bare hostname vs URL, subdomain rules), leaving the one required parameter under-explained.

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 ('Add a custom domain to the published app') and immediately clarifies the two modes of operation (website vs backend service). An agent can distinguish it from list_domains/remove_domain/verify_domain by the write framing, though no sibling is named explicitly.

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?

Gives conditional guidance on the service parameter ('omit for the website' vs 'pass a backend service name'), which is real usage direction. But it never says when to use this versus set_primary_domain, or whether verify_domain must follow, so the workflow context is only implied.

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