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

Annotations already declare readOnlyHint/idempotentHint/destructiveHint=false, yet the description adds genuinely non-obvious behavior: reading does not alter shared read status, and content is untrusted input. This goes beyond the structured safety profile, though auth/account requirements and any rate limits are left to the schema.

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?

Three tight sentences with no filler; the read payload is front-loaded, followed by the side-effect/routing note and the safety warning. Every sentence 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?

With an output schema present, the description need not explain return values, and annotations cover the safety profile; the added read-status and untrusted-content notes round it out well. Only account/authorization scoping is left entirely to the schema, a minor gap.

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 every parameter (including reason, purpose, account_id) carries its own description, so the schema does the heavy lifting. The prose adds no parameter syntax or format detail beyond what the schema already provides, making the baseline 3 appropriate.

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 (Read) and resource (a received email) plus exactly what is returned: decoded headers, body text and attachment metadata. This payload description distinguishes it from mailbox_email_list, mailbox_email_download, and mailbox_email_update without requiring the agent to open any 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?

Explicitly names the sibling to use for a related action ('use mailbox_email_update to mark read/unread'), which is the main alternative an agent would confuse this with. It lacks broader when/when-not framing (e.g., versus mailbox_email_download for attachments), so it stops short of a full 5.

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