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

With no annotations, the description carries the burden of behavioral disclosure. It states that recipients and subject are auto-populated, that default reply is to sender only, and that replyAll is an opt-in flag. It also mandates user confirmation, which is a critical behavioral safeguard. While it doesn't mention side effects like email delivery, it covers the essential behaviors for safe invocation.

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 three concise sentences: the first states the core action, the second explains the replyAll behavior, and the third gives a safety instruction. No filler; every sentence adds necessary information.

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 tool with fully documented parameters and no output schema, the description covers the essential details: what it does, how recipients are determined, default behavior for replyAll, and a confirmation requirement. An agent has everything needed to invoke it correctly without seeking additional 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%, so the baseline is 3. The description adds value by clarifying that the user only supplies the body and that replyAll toggles inclusion of all original recipients. This explains the relationship between parameters (e.g., messageId identifies the original, body is the only content) beyond the schema's individual parameter descriptions.

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 clearly states the tool replies in an existing thread, with recipients and subject taken from the original message. This distinguishes it from send_gmail_message (which sends new messages), and the verb+resource combo is 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 implies this is for replying to existing messages versus composing new ones, and explicitly instructs the user to confirm before calling, providing a usage guardrail. It doesn't name an alternative tool, but the thread-reply context is clear enough to avoid confusion.

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