send_message

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

With no annotations provided, the description carries the full burden and does well: it discloses that the mail is 'threaded properly (In-Reply-To/References + Gmail threadId)' and that 'to'/'subject' default to the original sender and 'Re: <original subject>'. It does not cover failure modes or delivery semantics, but the key non-obvious threading and defaulting behaviors are transparently stated.

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 a compact block that front-loads the core action ('Send an email.') before moving to attachments and the reply mechanism. The reply sentence is dense but every clause earns its place; there is no fluff or repetition of schema data.

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 covers the core action, attachments, and the reply threading path well. However, with no output schema and no annotations, it omits what the tool returns (e.g., message id or thread id) and leaves recipient address formatting unspecified — meaningful gaps for an agent invoking a 7-parameter mutating tool.

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 description coverage is only 29%, so the description must compensate. It adds real meaning for reply_to_message_id (makes 'to'/'subject' optional, threads the mail) and attachments ('local files by absolute path'), but recipient fields (to, cc, bcc) and subject receive no format clarification — e.g., whether multiple addresses are comma-separated is left ambiguous.

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 'Send an email' — a specific verb and resource that clearly identifies the core function, and it goes beyond the generic name 'send_message' by specifying email. It is easily distinguished from read-oriented siblings like search_messages and get_message, though it does not explicitly contrast with the overlapping sibling create_draft.

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 explicitly explains the reply scenario: 'To reply inside an existing conversation, pass reply_to_message_id' and details the resulting defaults for 'to' and 'subject'. This gives clear context for the threading path, though it stops short of naming alternatives like create_draft for drafting-without-sending.

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