list_domains

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

Annotations already declare readOnlyHint=true and idempotentHint=true, so the agent knows this is a safe, read-only operation. The description adds valuable context about what status means ('verified' domains can host mailboxes) and the implication of an empty list (only shared platform domain available). This exceeds the annotation baseline.

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?

All three sentences are meaningful and non-redundant. The purpose is front-loaded, usage guidance follows, and the status constraint is stated clearly. No wasted words.

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?

With no parameters and basic output, the description covers all needed information: purpose, when to use, status semantics, and downstream usage. The only minor gap is not explaining what happens if the account has no custom domains (returns empty list), but the description does describe 'an empty list means only the shared platform domain is available'.

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?

The input schema has no parameters, so the description doesn't need to document any. However, it goes beyond by explaining what the returned data includes (id, name, verification status) and how to use it downstream, which is especially helpful with no output schema.

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 uses 'List the account's custom sending domains' which clearly identifies the verb and resource. It distinguishes itself from siblings like list_mailboxes by specifying it returns domains with id, name, and verification status.

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 explicitly states when to call this tool: 'when you want to create a mailbox on a custom domain' and 'to check whether a domain has finished verifying'. It provides alternatives by explaining the shared platform domain is available when the list is empty, and specifies that only verified domains can be used.

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