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

Annotations already declare readOnlyHint=false, destructiveHint=false, openWorldHint=false, so the safety profile is covered. The description adds meaningful behavioral context beyond that: the 1000-message batch ceiling and the combined add/remove semantics. It does not say whether the operation is idempotent or what happens if a label ID is invalid.

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 sentences, front-loaded with the action and scale, followed by the folder-move analogy. No filler or restatement of the tool name.

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?

For a 4-parameter mutation tool with 25% schema coverage and no output schema, the description covers the operational model and batch limit but leaves the mechanics of messageIds and label IDs unaddressed. Adequate as a minimum, not complete.

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 only 25% (mailbox only), so the description must carry more weight. It explains the intent of addLabelIds/removeLabelIds qualitatively and caps messageIds scope at 1000, but adds no format or source details — e.g., where label IDs come from (list_gmail_labels) or what messageIds accepts.

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 pair (add/remove), the resource (labels), and the scope (up to 1000 messages at once). The 'Gmail's equivalent of moving messages between folders' framing also separates it from sibling label tools like create_gmail_label or rename_gmail_label.

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?

Gives an explicit operating pattern — 'add the destination label and remove INBOX (or the source label) in the same call' — which is real when-to-use guidance. It stops short of naming a sibling alternative (e.g., move_messages or update_agentmail_labels) or stating when not to use this tool.

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