reply_to_message

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

Without annotations, the description carries the behavioral transparency burden, and it does add meaningful detail: it sets threading headers so the recipient's client groups the response with the existing exchange. It does not cover side effects, permissions, or failure modes, but the core behavior is clearly disclosed.

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 tight sentences: the action, the behavioral mechanism, and the routing guidance. Every sentence earns its place, with the core purpose front-loaded.

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 fully covers selection and the key threading behavior, which is the tool's differentiator. However, with no output schema and no annotation safety profile, it omits practical invocation details like response format, error behavior, and how to handle content/recipient fields.

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 33%, yet the description provides almost no parameter-level guidance. It alludes to the message being replied to but does not clarify how to populate to, subject, html, or text, so the description does not compensate for the schema's gaps.

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 a specific action ('Reply to a message, keeping it on the same conversation') and explains the mechanism that distinguishes it from a new email. It also explicitly positions the tool relative to send_email, making its purpose 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?

It explicitly says to prefer this tool over send_email whenever answering something, providing a clear decision rule. This directly addresses when to use this tool versus the most likely sibling alternative.

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