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

Annotations only state readOnlyHint=false and destructiveHint=false. The description does not say what happens to the previously primary domain, whether the target must first be verified or attached, or whether DNS/routing changes propagate — all material for a mutation tool.

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 words with no padding, but the brevity comes from under-specification rather than efficiency. It is front-loaded but contains almost no information.

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?

A mutation tool with no output schema, half its parameters undocumented, and only basic annotations — the description supplies none of the missing context an agent needs to invoke it safely or 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 50%: project_id is documented in the schema, but domain_id is an undescribed string. The description adds no clarification of either parameter, such as where domain_id comes from (e.g., list_domains).

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?

"Set primary custom domain" restates the tool name and title almost verbatim, adding only the word "custom". It conveys a mutation on a domain resource, but nothing tells an agent how this differs from add_domain, verify_domain, or list_domains.

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?

There is no guidance on when to call this versus the other domain tools in the sibling set. An agent must infer that this switches which existing domain is primary, rather than adding or verifying one.

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