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 still earns credit by disclosing a non-obvious behavioral trait beyond the annotations: unverified mailboxes appear in the listing but cannot actually send. That is exactly the kind of surprise an agent needs warned about.

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?

Three short sentences, no filler, and the core identity of the tool comes first, followed by the consumer instruction and the caveat. Every sentence 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 zero-parameter, read-only listing with no output schema, the description supplies the essentials: what is returned (mailboxes and their ids) and the verified/unverified distinction. It could be marginally better by hinting at the shape of each entry, but nothing needed to call it correctly is missing.

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?

There are zero parameters, so the baseline is 4. The description goes slightly beyond baseline by explaining the meaning of the returned id and its linkage to the mailbox_id field in send_lead_email, which is semantics rather than schema repetition.

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 the resource precisely: 'The verified mailboxes this account can send email from.' It is not a literal verb-first phrasing, but the list semantics are unmistakable and it is clearly differentiated from send_lead_email, which it names as the consumer. The added distinction between verified and unverified mailboxes sharpens the purpose further.

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?

It tells the agent exactly what to do with the result: 'Use one of these ids as mailbox_id for send_lead_email.' That is a concrete downstream routing instruction, not just implied usage. It stops short of stating when NOT to call it, but for a zero-parameter listing tool feeding a single sibling, the context is effectively complete.

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