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

Annotations already cover the safety profile, but the description adds substantial behavioral context: it does NOT mark the message read, the text cap (20,000 chars), what else is returned (draft, tracking, conversation summaries, replyArgs), the untrusted-content warning, and the meaning of MAIL_NOT_FOUND. This goes well beyond the structured hints.

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?

Front-loaded with purpose and the read-only guarantee, and most sentences earn their place (return shape, security warning, error semantics). It is a dense single block, but the length is justified by the behavioral and safety content rather than padding.

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?

No output schema exists, and the description compensates by enumerating the return payload and the primary error case. Combined with the read-only framing and follow-up routing, an agent has everything needed to call and interpret it correctly.

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?

With 0% schema description coverage, the description must carry parameter meaning. It explains the id's origin and the MAIL_NOT_FOUND condition tied to it, but says nothing about the locale enum (11 values) or its effect. Partial compensation for a two-parameter tool.

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 ('Open one message from the authenticated business mailbox') and anchors the input to the sibling that produces it ('the id list_business_emails returned'). An agent can immediately distinguish this from list_business_emails and from send_business_email.

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?

It names the prerequisite source for the id and points to send_business_email / draft_business_email_reply as the follow-up path for replying, which gives clear context for when to use it. It stops short of an explicit 'do not use this when...' exclusion, so it is not a full 5.

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