Reply to a message

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

Annotations declare destructiveHint=true, and the description adds significant context: immediate send, no undo, and the Mail.Send permission requirement. It also explains how the comment is placed above the quoted original and that Graph builds subject and recipients. This goes beyond the annotations and covers key behavioral traits, though it doesn't mention potential failures or idempotency nuances (already covered by idempotentHint=false).

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 sentences with no filler. The most critical caveat (immediate send, no undo, confirm first) is front-loaded, followed by formatting details and permission requirements. Every sentence earns its place and the structure is logical and efficient.

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 simple 3-parameter tool with no output schema, the description covers the essential aspects: behavior, permission, formatting, and recipient semantics. It lacks explicit mention of return values or error handling, but these are not critical for a basic reply action. The description is sufficiently complete for an agent to call the tool correctly.

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 baseline is 3. The description adds value by explaining that the comment is placed above the quoted original as plain text, and that replyAll includes every original recipient. This clarifies the semantics of comment and replyAll beyond their schema descriptions. The id parameter is self-explanatory and needs no extra context.

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 sends a reply to an existing message immediately, and explicitly contrasts with creating a draft. It names the specific resource (message) and the action (reply), making it distinct from siblings like mail_create_draft and mail_send. The verb 'sends' is specific and 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 provides clear usage guidance by stating it sends immediately, has no undo, and requires confirmation with the user first. It implies when to use this tool (when an immediate send is desired) and contrasts with draft creation. However, it does not explicitly name alternative sibling tools (e.g., mail_create_draft) as the recommended path for draft scenarios, leaving some inference to the agent.

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