reply-mail-message

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

Beyond the destructiveHint annotation, the description reveals that the tool saves the message in the Sent Items folder, which implies it sends the reply. It also details behavioral constraints like the mutual exclusivity of comment and body properties in JSON format, and the requirement for base64 encoding in MIME format. This adds useful context for the agent.

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 front-loaded with the main purpose and uses bullet points for the two formats, making it scannable. It avoids excessive verbosity, though some repetition could be trimmed (e.g., repeated mentions of 'save the message in Sent Items'). Overall, it is well-structured and concise.

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?

Given the complexity (two formats, nested schema, no output schema), the description covers the essential behaviors: sending, format options, constraints, and the alternative draft flow. The optional parameters includeHeaders and excludeResponse are documented in the schema but not mentioned in the description, which is acceptable as they are self-explanatory from their descriptions.

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?

The input schema has high coverage (75%), so the schema itself documents many properties. However, the description adds critical parameter semantics: it clarifies that in JSON format, either Comment or body.Message.body must be specified (not both), and explains how to use the MIME format with base64 encoding. This provides meaningful guidance beyond the schema.

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 'Reply to the sender of a message', which is a specific verb+resource. It distinguishes from sibling tools like reply-all-mail-message and create-reply-draft by specifying the scope (sender) and the action (send immediately rather than create draft). The tip about preserving HTML formatting further clarifies the tool's functionality.

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 context on when to use the tool: it covers both JSON and MIME formats, and mentions the alternative of creating a draft instead of sending directly. However, it does not explicitly exclude usage cases like replying to all recipients or forwarding, which would be handled by sibling tools. The tip about not reconstructing the email manually is a helpful guideline.

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