Composing draft messages

In a Google Workspace Add-on you can create widgets that have linked actions. You can use an action to compose new email drafts, optionally filling them using information entered into the add-on UI or information from an open message. For example, you can have a button in your add-on's message UI that creates a reply to the currently opened message prepopulated with information from the add-on.

When an action that builds messages is triggered, Gmail executes a callback function to build and return the draft. Gmail then displays that draft in its UI in a standard email compose window, which the user can then edit and send as needed.

Configuring an action to build a draft message

To configure a widget to start a draft-building action when selected, you must do the following:

  1. Make sure your manifest includes the action.compose scope:

    You can use more a permissive scope instead, but should only do so if that scope is absolutely necessary.

  2. Create an Action object and associate it with a callback function you define.

  3. Call the widget's setComposeAction() widget handler function, providing it the Action object and specifying the ComposeEmailType.

  4. Implement the callback function that executes the draft-building action. This function is given an event object as an argument. The callback function must do the following:

    1. Create a GmailDraft object.
    2. Build a ComposeActionResponse object using the ComposeActionResponseBuilder class and the GmailDraft object.
    3. Return the built ComposeActionResponse.

You can prefill the GmailDraft you create in the callback function with recipients, a subject, a message body, and attachments. To fill in the draft, data can come from any source, but typically it derives from information provided to the add-on itself, information in the open message, or information gathered from a third-party service. The event object passed to the callback function contains the open message ID and other add-on information you can use to prefill the draft.

You can create the draft as a new standalone message or a reply to an existing message. This is controlled by the ComposeEmailType enum given to the setComposeAction(). You can create reply drafts as single replies or 'reply-all' messages.

Standalone drafts

A standalone draft starts a new thread and isn't a reply to any existing message. You can create a standalone draft with one of the following Gmail service functions:

Reply drafts

A reply draft is part of an existing message thread. Reply drafts are either single replies that only get sent to the sender of a message or "reply all" drafts that get sent to everyone who received that message. You can create a reply draft with one of these Gmail service functions:


The following code snippet shows how to assign an action that builds a reply draft to a button.

  var composeAction = CardService.newAction()
  var composeButton = CardService.newTextButton()
      .setText('Compose Reply')

  // ...

   *  Creates a draft email (with an attachment and inline image)
   *  as a reply to an existing message.
   *  @param {Object} e An event object passed by the action.
   *  @return {ComposeActionResponse}
  function createReplyDraft(e) {
    // Activate temporary Gmail scopes, in this case to allow
    // a reply to be drafted.
    var accessToken =;

    // Creates a draft reply.
    var messageId =;
    var message = GmailApp.getMessageById(messageId);
    var draft = message.createDraftReply('',
            htmlBody: "Kitten! <img src='cid:kitten'/>",
            attachments: [
            inlineImages: {
              "kitten": UrlFetchApp.fetch('')

    // Return a built draft response. This causes Gmail to present a
    // compose window to the user, pre-filled with the content specified
    // above.
    return CardService.newComposeActionResponseBuilder()