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

Beyond the destructiveHint annotation, the description adds meaningful behavior: the email is sent immediately, the action cannot be undone, and a suppressed address fails the entire call with its reason. This gives the agent a strong safety and failure-mode picture.

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?

Four short sentences, each contributing distinct value: core purpose, prerequisite, failure behavior, and irreversibility. Nothing is redundant or padded.

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?

The description, combined with a fully self-documenting schema and informative annotations, gives the agent everything needed to invoke the tool correctly: verified domains, suppression atomicity, required sender, and destructive consequence. No missing critical context.

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 100%, and the schema already documents from, to, html, text, subject, and replyTo clearly. The description adds general context like verified-domain requirements and suppression behavior, but does not add parameter-level meaning beyond what the schema provides, so the baseline of 3 is appropriate.

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 opens with a specific verb and resource: 'Sends a single (transactional) email to one or more addresses.' This clearly distinguishes it from the sibling send_campaign by emphasizing single and transactional, making the tool's scope immediately unambiguous.

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 description gives clear operational context: it is for one-off transactional sends, requires a verified-domain sender, and tells the agent to call get_account to discover valid domains. It implies the boundary against send_campaign but does not explicitly name the alternative or state when not to use this tool.

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