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

Beyond the readOnlyHint/idempotentHint annotations, the description discloses that the download_url is short-lived with expires_at, that bytes are not inlined, and that the URL must be fetched before expiration. This is valuable behavioral context that is not available from annotations alone.

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, no filler: action and return fields come first, followed by the expiration caveat, the source of IDs, and the sibling pointer. Every sentence earns its place and the 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?

With an output schema present, the return format is already documented; the description covers the tool's distinguishing scope, the short-lived download behavior, and the related sibling tool. Nothing essential is missing for correct invocation.

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% and each parameter is described, so the baseline is 3. The description adds meaning by explaining that IDs come from the thread's attachments array on get_agentmail_inbox_thread, clarifying the relationship between thread_id and attachment_id beyond the generic schema descriptions.

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–resource pair: 'Fetches metadata for one attachment anywhere in a thread,' and lists the exact returned fields. It distinguishes itself from the message-level sibling by naming it directly, so an agent can tell this tool apart from alternatives without guessing.

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 gives clear operational context: IDs come from the thread's attachments array, and the message-level twin is explicitly named. It does not state explicit when-to-use versus when-not-to-use conditions, but the source-of-IDs guidance and sibling pointer make the intended usage clear.

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