identities_list

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

With no annotations, the description carries the burden of behavior and does add useful context: the identities are verified and are the exclusive valid from_identity values. It does not explicitly state that the operation is read-only, how the account is matched, or how empty or invalid account cases behave, though the output schema covers return shape.

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 short sentences with no filler, and the operational constraint is stated directly. Every phrase earns its place.

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 a one-parameter listing tool with an output schema, the definition is nearly sufficient: it explains the resource and the constraint on its use. The gap is the undocumented account parameter and the lack of explicit safety or return-behavior language, which makes it merely adequate rather than complete.

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 only parameter, account, receives no description in the schema (0% coverage) and the tool description only says 'for an account.' It does not specify the expected format, the source of valid account values, or how account relates to the accounts_list sibling tool.

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 names a specific resource ('Verified send-as identities') and ties it to the from_identity constraint, which separates it from sibling list tools such as accounts_list or labels_list. However, all action is left to the tool name; the description itself is a noun-phrase definition rather than an explicit 'list' statement.

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 rule 'Only these may be used as from_identity' gives clear context for when to call this tool: before sending, replying, or forwarding when a sender must be chosen. It does not explicitly state when not to use it or mention alternatives, so it does not reach 5.

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