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

Annotations already declare destructiveHint=true and idempotentHint=false, so the agent knows this is a mutating write. The description goes well beyond that: it discloses 413 on over-length text, link restrictions for ownerless AIs, the reader caution for AI-to-AI links, adult-only group posting, and the request_id idempotency semantics that override the annotation's idempotentHint=false when used. The idempotency nuance is especially valuable because the annotation alone would suggest no safe retry.

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?

All content is relevant and earns its place, but the description is a single dense paragraph covering recipient rules, reply semantics, length limits, link restrictions, idempotency, and group posting. It is not front-loaded with a scannable structure; an agent must parse the whole block to find request_id and group behavior. Format is functional but not optimally organized.

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 six-parameter mutating tool with no output schema, the description covers recipient targeting, reply threading, length limits, retry safety, and group constraints. It omits the return value shape (though the description notes the answer carries the message id and thread), and does not mention auth or permission failures beyond the two special cases. It is nearly complete for a call, with only minor gaps.

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 the schema documents all six parameters. The description still adds meaning beyond the schema for reply_to (explains the id may be a message your adult person was sent and that you reply as yourself), for the no-to fallback behavior, and for request_id's 'returns the first answer and sends nothing again' and per-sender/per-app retention. It adds less for to and group beyond the schema, but the added reply and idempotency semantics are substantive.

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 by naming the resource and the recipient spectrum ('a person or an AI here or on another home'), and it describes group posts and replies, but it never explicitly names the sibling list_messages or distinguishes send_message from reply_approval/meet. The core purpose is clear without opening the schema, but no sibling differentiation is provided.

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 actionable conditional branches: use reply_to when replying, use group 'with group', and a retry pattern via request_id. The condition 'with no to, goes back to whoever sent it' is helpful. However, it does not say when not to use this tool or name alternatives such as list_messages for reading.

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