How To Send A Reply With Gmail API

The answer of @Tholle (thanks!) was correct and put me on the right track, but after the recent changes:

https://gsuiteupdates.googleblog.com/2019/03/threading-changes-in-gmail-conversation-view.html

I had to conflate his two paths.

In my program I had to reply to a thread, but if I included only the threadId (as #2) the new message is put in thread by Gmail only in the mail of the replier, while in the mail of the original sender (also gmail) is appeared as a new thread.

I resolved by including both the threadId and the headers "References" and "In-Reply-To" with the id of the last message.

As the docs say, if you're trying to send a reply and want the email to thread, make sure that:

1. The Subject headers match
2. The References and In-Reply-To headers follow the RFC 2822 standard.

If you want to do this yourself, you could get the Subject, References and Message-ID-headers of the message you want to respond to:

Request:

userId = me
id = 14fd1c555a1352b7 // id of the message I want to respond to.
format = metadata
metadataHeaders = Subject,References,Message-ID

GET https://www.googleapis.com/gmail/v1/users/me/messages/14fd1c555a1352b7?format=metadata&metadataHeaders=Subject&metadataHeaders=References&metadataHeaders=Message-ID

Response:

{
 "id": "14fd1c555a1352b7",
 "threadId": "14fd1c52911f0f64",
 "labelIds": [
  "SENT",
  "INBOX",
  "IMPORTANT",
  "UNREAD"
 ],
 "snippet": "Next level dude 2015-09-15 18:10 GMT+02:00 Emil Tholin <[email protected]>: wow 2015-09-15 18:",
 "historyId": "575289",
 "internalDate": "1442333414000",
 "payload": {
  "mimeType": "multipart/alternative",
  "headers": [
   {
    "name": "In-Reply-To",
    "value": "<CADsZLRyzVPLRQuTthGSHKMCXL7Ora1jNW7h0jvoNgR+hU59BYg@mail.gmail.com>"
   },
   {
    "name": "References",
    "value": "<CADsZLRxZDUGn4Frx80qe2_bE5H5bQhgcqGk=GwFN9gs7Z_8oZw@mail.gmail.com> <CADsZLRyzVPLRQuTthGSHKMCXL7Ora1jNW7h0jvoNgR+hU59BYg@mail.gmail.com>"
   },
   {
    "name": "Message-ID", // This is the same for both users, as you were asking about.
    "value": "<CADsZLRwQWzLB-uq4_4G2E64NX9G6grn0cEeO0L=avY7ajzuAFg@mail.gmail.com>"
   },
   {
    "name": "Subject",
    "value": "Re: Cool"
   }
  ]
 },
 "sizeEstimate": 1890
}

To follow the RFC 2822 standard we have added the Message-ID of the message we want to respond to to the References-header, separated with a space. The In-Reply-To-header also has the value of message we want to respond to. We also add Re: to our Subject-header to indicate that it is a response.

// Base64-encode the mail and make it URL-safe 
// (replace "+" with "-", replace "/" with "_", remove trailing "=")
var encodedResponse = btoa(
  "Content-Type: text/plain; charset=\"UTF-8\"\n" +
  "MIME-Version: 1.0\n" +
  "Content-Transfer-Encoding: 7bit\n" +
  "References: <CADsZLRxZDUGn4Frx80qe2_bE5H5bQhgcqGk=GwFN9gs7Z_8oZw@mail.gmail.com> <CADsZLRyzVPLRQuTthGSHKMCXL7Ora1jNW7h0jvoNgR+hU59BYg@mail.gmail.com> <CADsZLRwQWzLB-uq4_4G2E64NX9G6grn0cEeO0L=avY7ajzuAFg@mail.gmail.com>\n" +
  "In-Reply-To: <CADsZLRwQWzLB-uq4_4G2E64NX9G6grn0cEeO0L=avY7ajzuAFg@mail.gmail.com>\n" +
  "Subject: Re:Cool\n" +
  "From: [email protected]\n" +
  "To: [email protected]\n\n" +

  "This is where the response text will go"
).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');

$.ajax({
  url: "https://www.googleapis.com/gmail/v1/users/me/messages/send?access_token=<USER_ACCESS_TOKEN>",
  method: "POST",
  contentType: "application/json",
  data: JSON.stringify({
    raw: encodedResponse
  })
});

As you can see, this is a pain in the backside to to manually. You could also just respond to the thread. This might not be enough for your use case however.

This way, you just have to supply the mail and the threadId, and make sure the Subject is the same, and Google will display it for you correctly.

// Base64-encode the mail and make it URL-safe 
// (replace "+" with "-", replace "/" with "_", remove trailing "=")
var encodedResponse = btoa(
  "Content-Type: text/plain; charset=\"UTF-8\"\n" +
  "MIME-Version: 1.0\n" +
  "Content-Transfer-Encoding: 7bit\n" +
  "Subject: Subject of the original mail\n" +
  "From: [email protected]\n" +
  "To: [email protected]\n\n" +

  "This is where the response text will go"
).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');

$.ajax({
  url: "https://www.googleapis.com/gmail/v1/users/me/messages/send?access_token=<USER_ACCESS_TOKEN>",
  method: "POST",
  contentType: "application/json",
  data: JSON.stringify({           
    raw: encodedResponse,
    threadId: "<THREAD_ID_OF_MESSAGE_TO_RESPOND_TO>"
  })
});