Send an email

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

Beyond annotations, the description discloses irreversibility, no-retry semantics, Gmail threading prerequisites, multipart/alternative body handling, return fields, and daily sending limits. This is exactly the behavioral context needed for a destructive, non-idempotent tool.

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 dense but every sentence carries essential information, and the layout front-loads the core send semantics before threading, retry, and rate-limit guidance. The all-caps cues ('TO REPLY IN A THREAD', 'NEVER retried') make the critical instructions easy to scan.

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 complex send operation with no output schema, the description covers inputs, output fields, failure handling, verification path, and rate limits. An agent has enough to invoke correctly and know what to do when the outcome is unclear.

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?

Although the schema already documents each parameter, the description adds cross-parameter rules: at least one recipient across to/cc/bcc, bcc-only allowed, subject or body required, and how thread_id/in_reply_to/subject must align for threading. It also clarifies references defaults to in_reply_to.

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: it sends an email from the authenticated mailbox, and adds 'immediately and irreversibly' to define the behavior precisely. This clearly distinguishes send_message from sibling draft tools and read-only tools such as list_messages or get_message.

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 gives explicit when-to-use guidance for replies: call get_message first, then pass threadId, in_reply_to, and 'Re: ' subject. It also tells the agent not to retry after timeout or 5xx and to verify via list_messages, but it does not explicitly route users to create_draft/send_draft for non-immediate sends.

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