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

With no annotations, the description carries the full disclosure burden and does real work: it reveals the reply is persisted in-app rather than delivered, is rate limited, and is screened. It doesn't say what happens when the rate limit or screening rejects the reply, which is the main remaining behavioral gap.

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?

A single front-loaded sentence with a parenthetical for the delivery and moderation constraints, plus one short exclusion. Every clause adds information; nothing restates the name or title.

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 three-parameter write tool with no annotations and no output schema, the description covers destination, moderation, and scope limits, which is enough to call it correctly. It leaves open what a rate-limited or screened-out attempt returns, a minor omission given there is no output schema to lean on.

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 100%, so body, token, and optional message_id are already documented. The description adds no syntax, length constraint, or guidance on omitting message_id relative to an existing thread, so the baseline 3 applies.

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?

States a specific verb (reply) and recipient (Sun) and immediately pins down the delivery channel: stored in-app for Sun to read, not an outbound message. That negative scoping distinguishes it from anything resembling a general send/message tool in the sibling set.

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?

"Not an outbound message to anyone else" is an explicit when-not rule, and "stored in-app for Sun to read" establishes the context of use. It stops short of naming an alternative sibling (e.g. ack_message) for acknowledgement-style responses, so the routing guidance is clear but not exhaustive.

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