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

Adds substantial behavior beyond the annotations: the call is asynchronous and returns a queued operation that is explicitly NOT confirmation of provider completion, an uncertain SMTP attempt is never auto-resent, and operationId reuse with different content is rejected. Note a mild tension with idempotentHint=false, since the description says retries with the same operationId are deduplicated; this is best read as key-opt-in idempotency rather than an outright contradiction of the annotation.

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?

Purpose is front-loaded in the first sentence and every subsequent sentence concerns retry/async semantics. There is slight redundancy across the three operationId sentences, but each states a distinct rule (reuse, rejection on content change, never use a new key after an uncertain send).

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 output schema, the description correctly explains the return (a queued operation) and how to follow up via get_mail_action, which is the key gap a caller would otherwise hit. It is thin on the other eight parameters and on where the mailbox/accountId/senderIdentityId come from, but for an async send tool the critical context is present.

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 11%, so the description carries the burden, and it fully explains the most consequential parameter (operationId: high-entropy stable key, reuse only for the exact same send). It adds no meaning for the remaining eight parameters, including accountId, senderIdentityId, or the cc/bcc/htmlBody/textBody fields, so the compensation is partial.

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?

States a specific verb and resource ("Send email using the selected mailbox") and immediately clarifies the delivery model (queued operation, not provider confirmation). However, it never distinguishes itself from sibling tools that also produce outgoing mail such as create_draft, reply_to_email, or forward_email, leaving the agent to infer the boundary.

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?

Gives concrete operational guidance: reuse the operationId for retries of the exact message, poll get_mail_action with the returned operationId, and never retry an uncertain send with a new key. It does not, however, say when to prefer this tool over reply_to_email/forward_email/create_draft, which are the realistic selection alternatives.

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