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

With no annotations, the description carries the full burden. It usefully discloses the return encoding (base64) and that exactly one attachment is retrieved, but says nothing about attachment size limits, failure modes, or permission/auth requirements for downloading attachments.

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?

One tight sentence with the action, the result format, and the selection options front-loaded. No filler or redundancy.

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?

There is no output schema, and the description correctly supplies the key return detail (base64), which is what an agent needs to handle the result. It stops short of covering error/size behavior, but is adequate for a simple single-attachment fetch tool.

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 adds meaning: drive_id and index are presented as alternative selectors on the required message_id, clarifying they are two addressing paths rather than required companions.

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 ('Download one email attachment') plus the return encoding ('as base64') and the two addressing modes. It is clearly distinguishable from read_message and list_messages, which surface message metadata rather than attachment content.

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?

Usage is implied rather than stated: the agent can infer this is the tool to fetch attachment bytes, and the description notes the two ways to select an attachment. There is no explicit when-to-use/when-not guidance nor any mention of the sibling tools.

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