reply

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

Since no annotations are provided, the description carries the burden of behavioral disclosure. It usefully reveals that the tool fills in the recipient, adds Re: subject, and sets threading headers. However, it does not state the actual side effect of sending a message, possible error conditions, or response behavior.

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 compact and front-loaded with the most important concept. The second sentence earns its place by explicitly contrasting with send_email, and there is no redundant schema repetition or filler.

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 tool has 6 parameters, no annotations, no output schema, and 0% schema description coverage. The description explains the conversation threading intent but does not clarify how an agent should fill required parameters, what reply output is returned, or what happens when the reply cannot be sent. This is significantly incomplete for safe invocation.

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 0% description coverage, and the description does not enumerate or explain the parameters. It gives a partial hint about auto-filled recipient/subject behavior, but leaves address, message_id, cc, bcc, body, and html semantically unexplored. This is insufficient for six parameters with no schema 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 uses a specific verb and resource ('Reply to a message') and immediately defines the key scoping property: the reply stays in the SAME conversation. It also distinguishes itself from send_email by explaining the behavioral difference, so an agent can tell them apart.

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 when answering, and explains why send_email is inappropriate because it starts a new conversation. This gives the agent a clear when-to-use rule and names the alternative.

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