Gmail API thread replies missing when fetching thread messages after UI response

Other Questions Gmail
1

My Setup:

I’m working with two Gmail accounts (let’s call them User X and User Y) from different Google Workspace domains.

What I’m doing:

  1. I send an email from User X to User Y using Gmail API through Google Apps Script:
function createAndSendMessage(toAddress, emailSubject, messageBody) {
  
  // Build the email message
  var messageText = [
    'To: ' + toAddress,
    'Subject: ' + emailSubject,
    '',
    messageBody
  ].join('\r\n');

  var encodedMessage = Utilities.base64EncodeWebSafe(messageText);

  // Send using Gmail API
  var response = Gmail.Users.Messages.send({
    raw: encodedMessage
  }, 'me');

  return response.threadId
}
  1. User Y replies to this email through the regular Gmail web interface
  2. The reply shows up correctly in Gmail UI and appears threaded with the original message
  3. I try to fetch all messages in the thread using the threadId from step 1

The Issue:

When I call users.threads.get with the threadId, I only get back the original message I sent programmatically. The reply sent through Gmail UI doesn’t appear in the API response, even though it’s clearly part of the same thread in the Gmail interface.

My Question:

What do I need to include in my initial email to ensure that replies get properly threaded when retrieved via the API? Are there specific headers I should add when sending the original message?

2

This happens when you’re missing the Message-ID header in your original email. Gmail creates its own Message-ID through the API, but replies don’t link back to your thread properly. I manually set a Message-ID header - it fixes threading issues. Also check your authentication. If you send as User X but fetch the thread as User X, you won’t see User Y’s replies. You need to fetch from User Y’s mailbox or make sure both users can access the thread.

3

yea, i had that problem too. just add a little delay before you fetch the thread; sometimes the api just needs time to catch up. also, check the oauth scopes, make sure you’ve got gmail.readonly or gmail.modify. without the right scopes, it can get messy.

4

I’ve hit this exact issue building automated email workflows. The problem is you’re fetching the thread from the wrong user’s perspective. When User X sends an email and User Y replies, the thread exists in both mailboxes but with different messages. Your threadId from the send operation belongs to User X’s mailbox, so users.threads.get only shows what User X can see. To get the full thread with User Y’s reply, you need to authenticate as User Y and fetch from their mailbox. Threading works fine - it’s just about API permissions and whose data you’re querying.