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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), so the bar is lower. The description adds genuinely useful behavior beyond them: the payload is base64-encoded with a MIME type, which the agent must know to interpret the result, especially since there is no output schema. It omits size limits and failure modes, keeping it from 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?

Two short sentences, front-loaded with the action and followed by the critical return-format detail. No filler; every clause earns its place.

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?

Covering the base64/MIME return shape is the right call given no output schema exists. However, with three undocumented required parameters and no mention of size limits, missing-attachment behavior, or how the attachment is located, the definition is only minimally sufficient.

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 0% with three required parameters, and the description never mentions accountId, messageId, or attachmentId or their roles. Parameter meaning must be inferred purely from names, so the description does not compensate for the coverage gap.

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?

Specific verb+resource: 'Fetch one attachment' paired with the return format (base64 + MIME type) makes it unambiguous against get_email or get_thread, which deal with message content. It does not explicitly name a sibling to contrast with, so it falls short of a 5.

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?

'On demand' implies this is a lazy/secondary fetch you call only when you actually need attachment bytes, as opposed to listing metadata via get_email. No explicit when/when-not condition or named alternative is given, so usage is only implied.

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