reply_to_email

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

Discloses two behavioral traits (setting Reply-To headers, preserving the thread) beyond what annotations provide (none). However, it omits that this tool sends the email (vs. drafting) and does not mention other potential side effects like authentication or irreversible changes. With no annotations, more disclosure would be expected.

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?

Two concise sentences: the first states the core purpose, the second adds key behavioral details. No filler, perfectly front-loaded and efficiently written.

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 reply tool, the description covers the main intent and two behaviors, but lacks explicit confirmation that the email is actually sent (critical given send_email and create_draft siblings). With no output schema or annotations, the agent may not know the full side effects or whether a draft is created instead. More context on sending behavior would improve completeness.

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 provides 100% coverage of parameter descriptions, so the baseline is 3. The tool description adds no extra parameter semantics; it only indirectly references messageId via 'thread' but does not explain parameter relationships or formatting beyond 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?

Clearly states the action ('Reply to an existing email thread') and distinguishes this from sending a new email or creating a draft. The mention of 'automatically sets Reply-To headers and preserves the thread' adds specificity beyond mere naming.

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 the primary use case (replying to an existing thread) but does not explicitly contrast with sibling tools like send_email or create_draft. It gives clear context but lacks explicit when-not-to-use guidance or alternative tool names.

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