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

With no annotations provided, the description carries the full behavioral burden and does well: it discloses the lookup by message_id, the from-address construction, and the key side effect that the reply is queued for delivery rather than sent synchronously. It also clarifies the security model: any valid owner_key works and the gate is anti-abuse, not ownership enforcement. It does not mention failure modes or rate limits, but the core behavior is well covered.

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?

Three tightly written sentences with no filler. The first sentence states the action, the second describes the mechanism and side effect, and the third gives the auth requirement—each sentence earns its place and the most important information is front-loaded.

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 tool with no annotations or output schema, the description explains the required inputs, the sender, the queueing side effect, and the auth requirement. The only notable omission is what the caller should expect back on success, but since no output schema is declared, this is a minor gap rather than a blocking one.

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%, and each parameter has a specific description already. The tool description adds contextual behavior (lookup by message_id, reply construction) but not much parameter-level meaning beyond what the schema provides, so the baseline score of 3 is appropriate.

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 specific verb and resource: 'Reply to an inbound email stored by the mailbox ingest endpoint.' It also names the lookup key (message_id) and the sender address construction, making the tool's function unambiguous. No sibling tool competes for this exact email-reply action.

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?

The description clearly frames when to use it: when replying to an inbound email that came through the mailbox ingest endpoint. It provides the auth-related context ('Requires any valid owner_key') and the gating semantics, but does not explicitly name exclusions or alternative tools, though no obvious email-send sibling exists among the list.

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