We are building a Gmail addon with Google Apps Script. We have run into a strange problem where Apps Script inserts HTML twice into the composed email. This is example code where “HELLO” is inserted twice into the email:

function insertHtml(e) {
  var html = "<span>HELLO</span>"

  var response = CardService.newUpdateDraftActionResponseBuilder()
    .setUpdateDraftBodyAction(CardService.newUpdateDraftBodyAction()
      .addUpdateContent(html, CardService.ContentType.IMMUTABLE_HTML)     
      .setUpdateType(CardService.UpdateDraftBodyType.IN_PLACE_INSERT))
    .build();
  return response;
}

This screenshot shows how “HELLO” is inserted twice:

The weird thing is that if you put a style on <span>, it works as intended:

function insertHtml(e) {
  var html = "<span style="color: red;">HELLO</span>"

  var response = CardService.newUpdateDraftActionResponseBuilder()
    .setUpdateDraftBodyAction(CardService.newUpdateDraftBodyAction()
      .addUpdateContent(html, CardService.ContentType.IMMUTABLE_HTML)     
      .setUpdateType(CardService.UpdateDraftBodyType.IN_PLACE_INSERT))
    .build();
  return response;
}

It also works if you use ContentType.MUTABLE_HTML.

This looks like a bug in addons. There is no way for us to make sure that the inserted HTML does not contain <span> without attributes.

So how can we use IMMUTABLE_HTML?