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

Beyond the annotations (readOnlyHint=false, idempotentHint=false, destructiveHint=false), the description discloses that the copy is server-side ('bytes are never downloaded or reuploaded') and that archived sources are usable. It does not say what happens when the destination path already exists (overwrite vs. error), which is the main behavioral unknown for a non-idempotent copy.

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 tight sentences with the core action and scope front-loaded, followed by the key constraint. No filler or restatement of the title.

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, return values need no explanation, and the description covers action, source scope, and a usage constraint. The remaining gap is destination-collision behavior and the target_path vs. target_folder choice, minor but relevant for a mutating 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 every parameter is already documented, making 3 the baseline. The description adds only the 'storage reference' framing and the archived-source note; it does not clarify the relationship between target_path, target_folder, and target_mailbox_id, which is where an agent could mis-select inputs.

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 states a specific verb (copy) and resource (one existing file) plus the destination scope ('another path or mailbox') and source scope ('including archived sources'). 'Copy' inherently distinguishes it from the sibling mailbox_file_move, mailbox_file_write, and mailbox_file_rename without ambiguity.

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 gives a clear precondition for correct use ('The target mailbox must be writable'), which tells the agent when this call will succeed. It stops short of naming alternatives or exclusion cases (e.g. use mailbox_file_move to relocate instead of duplicate), so it is clear context without routing guidance.

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