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

Annotations already establish read-only, idempotent, non-destructive behavior, so the description earns credit for adding real behavioral context: the `download_url` is short-lived, has an `expires_at`, and the attachment bytes are not inlined. This is exactly the kind of operational detail an agent needs beyond the annotations.

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?

Four sentences, each earning its place: purpose, returned fields, critical expiration caveat, and source/alternative guidance. The most important constraint is front-loaded and there is no filler.

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?

Given that an output schema exists and the annotations cover safety, the description supplies everything missing for correct invocation: return field list, URL lifetime behavior, ID provenance, and sibling differentiation. Nothing critical is left implicit.

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 baseline is 3, but the description meaningfully supplements the schema by explaining that `attachment_id` values come from the draft's `attachments` array and that the draft is an unsent one. This adds provenance that the bare parameter descriptions lack.

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: 'Fetches metadata for one attachment on an unsent draft.' It clearly distinguishes this tool from the sent-message sibling by naming the twin, so an agent can identify the correct scope without opening the 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?

The description explicitly narrows use to unsent drafts and tells the agent where the required IDs come from: the draft's `attachments` array on `get_agentmail_inbox_draft`. It also names the alternative for sent messages, `get_agentmail_inbox_message_attachment`, providing clear routing guidance.

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