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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds valuable context that the operation is fast because it is cached and does not lock Mail, which are behavioral traits beyond the annotations. It does not detail return format or error handling, but the output schema exists to cover that.

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 three sentences, front-loaded with the core action and performance trait, then gives usage guidance and a clear alternative. Every sentence contributes distinct value with no redundancy or fluff.

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 simple zero-parameter read-only tool with an output schema, the description is fully complete. It states what is returned (account names), how it behaves (cached, no lock), when to use it (before list_emails), and when to use an alternative (list_accounts). No critical gaps remain.

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 tool has zero parameters, so the schema coverage is 100% and the baseline for a 0-param tool is 4. The description doesn't add parameter-level semantics, but none are needed; it does clarify the output is just account names, which indirectly helps the agent understand what to expect.

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 tool lists all Mail.app account names, with a specific verb ('Lists') and resource ('Mail.app account NAMES'). It also distinguishes itself from the sibling tool list_accounts by noting that this returns only names, not addresses or type.

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?

Explicitly says this is the preferred way to get account names and instructs to call it first, then use list_emails(account=name) with the discovered names. It also names an alternative tool (list_accounts) for when email addresses or account type are needed.

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