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

With no annotations, the description carries the safety burden, and it does disclose the most important trait: 'Nothing is sent,' so the agent knows this is non-destructive. It also notes subject defaults to 'Re: last subject.' It says nothing about required permissions, what happens if thread_id is invalid, or error/rate-limit 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?

Roughly two sentences with zero filler. The non-destructive guarantee and the sequencing instruction are front-loaded, and every clause adds information.

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 doubles as a return-value summary (recipient, subject, text) which partially substitutes for the missing output schema. But for a 4-parameter tool at 25% coverage with no annotations, the undefined thread_id and body_html handling leaves gaps an agent may hit at call time.

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 only 25%, so the description must compensate, and it partially does: it documents that subject defaults to 'Re: last subject' and that the body is 'the exact text' rendered. However, thread_id and body_html are undocumented in both schema and description, leaving half the surface to inference.

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 states a dry-run preview of a reply showing recipient, subject, and exact text, which makes the outcome concrete. It also names the sibling reply_in_campaign_thread, so an agent can separate preview from send. It stops short of a clean verb+resource sentence, but the resource is unambiguous from name and title.

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 gives an explicit sequencing rule — 'Call before reply_in_campaign_thread' — and an explicit post-step: show the text to the user. The alternative tool is named and the condition for choosing this one versus it is stated outright.

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