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

Annotations already flag destructiveHint=true and openWorldHint=true, so the description's real contribution is spelling out what that means in practice: one real email leaves the system, the user must see recipient/subject/body and give an explicit yes, and it is only for addresses the user controls. It does not say anything about rate limits or what comes back on success, which keeps it below a 5.

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 sentences, front-loaded with what the tool does, then the safety gate, then the alternative. Every sentence earns its place and nothing is repeated from the schema.

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 single-fire side-effecting tool with no output schema, the description covers purpose, consent requirements and the safer alternative. The only soft gap is that it does not say what the agent gets back or how failures surface, though annotations and the simple shape make that low-risk.

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 every parameter including the campaignId and emailAccountId cross-references is already documented. The description only mentions recipient, subject and body in the confirmation flow and adds no format or validation details beyond the schema — baseline 3.

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 and resource ('Send one real email ... using a campaign mailbox') and scopes it to a testing purpose, which cleanly separates it from campaign_preview_message and from the prospect-messaging tools. The sibling it is not is named explicitly.

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?

Gives concrete when-to-use conditions (check rendering and deliverability before launch), an explicit when-not-to-use ('not a way to message a prospect, and there is no tool for that'), and names the alternative for the read-only case: campaign_preview_message.

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