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

Annotations indicate this is a write operation (readOnlyHint=false) but not destructive or idempotent. The description adds that it affects 'generated links and custom-domain redirects,' which is useful behavioral context. However, it doesn't disclose prerequisites (e.g., domain must already be associated/accepted) or side effects (e.g., immediate changes to existing links). No contradiction with annotations.

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?

The description is one concise sentence that front-loads the core action and purpose. No filler or redundant phrasing. It efficiently communicates the tool's function without unnecessary elaboration.

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?

The description covers the primary purpose but lacks important contextual details such as whether the domain must already be added, whether verification is required, and what happens after setting (e.g., immediate effect on links). Given the tool's simplicity (2 params, no output schema), the description is somewhat adequate but leaves gaps that an agent might need to guess at.

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%, with both parameters well-documented. The description adds no extra parameter-specific information beyond what the schema provides. Since the schema already explains app_id and domain, the baseline of 3 applies; the description adds no additional detail about parameter usage or constraints.

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 description clearly states the action ('set') and the resource ('primary domain for generated links and custom-domain redirects'). It distinguishes from siblings like add_custom_domain (which adds a domain) and configure_domain_redirects (which configures redirects), so an agent can infer the specific role of this tool.

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 does not provide any explicit guidance on when to use this tool versus alternatives. It doesn't mention that domains must be added first, whether this tool should be called after adding a custom domain, or when not to use it. It merely states the function without context, forcing the agent to infer from sibling names.

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