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addLabel applies one existing label to Gmail messages. Use it with user labels to categorize processed mail, route work, or record an automation stage. Gmail system labels can also change message state or placement.

Example

Apply the Processed label to each newly received message:
The label must already exist in the selected Gmail account.

Inputs

Every input field also accepts a compatible Signal; the example passes email.messageId directly. Label IDs must match exactly. Display names are matched after trimming surrounding whitespace and ignoring capitalization. If the label does not exist in the selected account, the action fails without modifying the messages.

Output

The action returns a Signal<string[]> containing the modified message IDs in input order. Pass that signal directly to another batch-friendly Gmail action.
This action updates messages, not entire threads, and does not create the label. Prefer dedicated actions such as archive, markAsSpam, and markAsRead when they express the system-label change you need.