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 theProcessed label to each newly received message:
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 aSignal<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.