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

Annotations already flag destructiveHint=true and readOnlyHint=false, and the description adds non-obvious consequences: where the item lands per provider (Gmail bin, Outlook Deleted Items, imap trash), retention behavior, label retention on Gmail/imap, and a new message id on non-Gmail imap. The id-mutation side effect is exactly the kind of trait annotations cannot convey.

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?

The primary action is front-loaded and the sentences carry dense, useful information with little filler. The second sentence is a convoluted run-on with nested clauses, which slightly hurts readability, but nothing is wasted.

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 needn't be explained, and the description correctly defers to it ('which the answer names'). It covers destination, retention, undo, and id behavior, though it leaves the relationship to threads_archive implicit.

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 100%, so the schema already documents id, scope, and mailbox semantics (including the scope enum). The description reinforces the thread-vs-message distinction but adds no format or syntax detail beyond the schema, so the baseline 3 is correct.

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+resource ('Move one thread, or one message of it, to the trash') and names the exact sibling/reverse operation (threads_untrash). An agent can distinguish it from threads_archive, threads_modify, and the messages_* tools without opening 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?

Names threads_untrash as the operation that reverses this one, giving clear recovery context, and the 'one thread, or one message of it' phrasing signals the message-scope case. There is no explicit statement of when to prefer this over threads_archive, so it stops short of full when/when-not guidance.

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