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

Annotations only mark this as non-read-only, non-idempotent and destructive; the description adds substantial operational detail beyond that: the message is irreversible ('cannot be unsent'), the 24-hour channel window, refusal conditions for broken channel connections, and plan-based refusal behavior. This is exactly the kind of behavioral context the annotations cannot convey.

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?

It is a single dense paragraph, but it is front-loaded with the action and its irreversibility, then layers limits, channel constraints, and plan gating in a logical order. Every sentence carries a distinct constraint, though the block could be broken up for faster scanning.

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?

With an output schema present, return values need no explanation, and the description instead covers the dimensions an agent actually needs: reversibility, channel-specific time limits, media/text restrictions, and the failure modes (refused calls) for connection and plan issues.

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 100%, so a baseline of 3 applies, but the description adds real meaning: the 4,000-character text limit, the cap of four media files, the fact that reviews and comment threads are text-only, and that replyToMessageId is comment-thread specific. These constraints go beyond the raw schema field types.

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 opens with a specific verb and resource ('Send a reply in an Inbox conversation') and scopes it to direct messages, reviews and comment threads. This clearly separates it from sibling tools like reply_to_comment, create_post, or moderate_comment without needing to open any schema.

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 explicit trigger context ('Use this when the user wants to answer a direct message, review or comment thread in their Inbox') plus concrete preconditions: 24-hour Messenger/Instagram window, channel connection health, and Pro/Business plan gating. It stops short of naming alternative tools for out-of-window or non-Inbox cases, so the routing is implied rather than fully spelled out.

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