fix(Gmail Node): Do not break threads while creating drafts (#16272)

Co-authored-by: Shireen Missi <94372015+ShireenMissi@users.noreply.github.com>
Co-authored-by: cubic-dev-ai[bot] <191113872+cubic-dev-ai[bot]@users.noreply.github.com>

packages/nodes-base/nodes/Google/Gmail/v2/DraftDescription.ts

@@ -66,6 +66,13 @@ export const draftFields: INodeProperties[] = [
 		},
 		placeholder: 'Hello World!',
 	},
+	{
+		displayName: 'To reply to an existing thread, specify the exact subject title of that thread.',
+		name: 'threadNotice',
+		type: 'notice',
+		default: '',
+		displayOptions: { show: { resource: ['draft'], operation: ['create'] } },
+	},
 	{
 		displayName: 'Email Type',
 		name: 'emailType',

packages/nodes-base/nodes/Google/Gmail/v2/GmailV2.node.ts

@@ -13,6 +13,7 @@ import { labelFields, labelOperations } from './LabelDescription';
 import { getGmailAliases, getLabels, getThreadMessages } from './loadOptions';
 import { messageFields, messageOperations } from './MessageDescription';
 import { threadFields, threadOperations } from './ThreadDescription';
+import { addThreadHeadersToEmail } from './utils/draft';
 import { configureWaitTillDate } from '../../../../utils/sendAndWait/configureWaitTillDate.util';
 import { sendAndWaitWebhooksDescription } from '../../../../utils/sendAndWait/descriptions';
 import type { IEmail } from '../../../../utils/sendAndWait/interfaces';
@@ -560,6 +561,12 @@ export class GmailV2 implements INodeType {
 							attachments,
 						};
 
+						if (threadId && options.replyTo) {
+							// If a threadId is set, we need to add the Message-ID of the last message in the thread
+							// to the email so that Gmail can correctly associate the draft with the thread
+							await addThreadHeadersToEmail.call(this, email, threadId);
+						}
+
 						const body = {
 							message: {
 								raw: await encodeEmail(email),

packages/nodes-base/nodes/Google/Gmail/v2/utils/draft.ts

@@ -0,0 +1,30 @@
+import type { IExecuteFunctions } from 'n8n-workflow';
+
+import type { IEmail } from '@utils/sendAndWait/interfaces';
+
+import { googleApiRequest } from '../../GenericFunctions';
+
+/**
+ * Adds inReplyTo and reference headers to the email if threadId is provided.
+ */
+export async function addThreadHeadersToEmail(
+	this: IExecuteFunctions,
+	email: IEmail,
+	threadId: string,
+): Promise<void> {
+	const thread = await googleApiRequest.call(
+		this,
+		'GET',
+		`/gmail/v1/users/me/threads/${threadId}`,
+		{},
+		{ format: 'metadata', metadataHeaders: ['Message-ID'] },
+	);
+
+	if (thread?.messages) {
+		const lastMessage = thread.messages.length - 1;
+		const messageId: string = thread.messages[lastMessage].payload.headers[0].value;
+
+		email.inReplyTo = messageId;
+		email.reference = messageId;
+	}
+}