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

Annotations already establish it is a non-read-only, non-idempotent, closed-world write, and the description adds real value on top: the message is stored in Sent Items, it charges $0.001 per call and nothing on failure, and the reply variant keeps the thread. The cost statement is repeated twice, which dilutes an otherwise informative disclosure.

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?

Front-loaded with the core action and the default outcome (sent, saved to Sent Items), with the reply variant as a clarifying second sentence. The trailing '$0.001. Costs $0.001 per call' duplicates information already implied and wastes a line.

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 6-parameter mutation with no output schema, the definition covers destination, side effect, billing and the reply mode, which is respectable. It leaves out recipient format limits and what the caller gets back (e.g. a message id), so an agent still has gaps before calling confidently.

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 coverage is only 17%, so the description carries much of the burden, and it does enumerate the fields (to, cc, subject, body, reply_to_message_id) and specify 'text body'. It never says to/cc are arrays of up to 50 addresses and gives no format hint for reply_to_message_id, so it only partially compensates for the coverage gap.

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?

Concrete verb plus resource ('Send an email as one of the company's mailboxes') with the fields covered, and it distinguishes the send mode from the reply mode in the same thread. It does not name the adjacent mail-sending siblings (email_send, google_mail_send), so the differentiation is achieved by the 'company's mailboxes' scoping rather than explicit routing.

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 reply path is well framed: 'With reply_to_message_id it replies to that message's sender in the same thread instead' tells the agent which parameter switches modes. There is no guidance on when to choose this over the sibling mail-send tools, nor prerequisites such as the mailbox needing to be connected.

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