diff --git a/api/server/services/Endpoints/custom/initialize.js b/api/server/services/Endpoints/custom/initialize.js
index d2b7549699..de1417a78b 100644
--- a/api/server/services/Endpoints/custom/initialize.js
+++ b/api/server/services/Endpoints/custom/initialize.js
@@ -28,7 +28,10 @@ const initializeClient = async ({ req, res, endpointOption, optionsOnly, overrid
   const CUSTOM_API_KEY = extractEnvVariable(endpointConfig.apiKey);
   const CUSTOM_BASE_URL = extractEnvVariable(endpointConfig.baseURL);
 
-  let resolvedHeaders = resolveHeaders(endpointConfig.headers, req.user);
+  const customUserVars = {};
+  customUserVars.LIBRECHAT_CONVERSATION_ID = req.body.conversationId;
+
+  let resolvedHeaders = resolveHeaders(endpointConfig.headers, req.user, customUserVars);
 
   if (CUSTOM_API_KEY.match(envVarRegex)) {
     throw new Error(`Missing API Key for ${endpoint}.`);
diff --git a/api/server/services/Endpoints/custom/initialize.spec.js b/api/server/services/Endpoints/custom/initialize.spec.js
index 7e28995127..42125e6373 100644
--- a/api/server/services/Endpoints/custom/initialize.spec.js
+++ b/api/server/services/Endpoints/custom/initialize.spec.js
@@ -64,6 +64,20 @@ describe('custom/initializeClient', () => {
     jest.clearAllMocks();
   });
 
+  it('calls resolveHeaders with conversation ID when provided', async () => {
+    const { resolveHeaders } = require('@librechat/api');
+    const requestWithConversationId = {
+      ...mockRequest,
+      body: { ...mockRequest.body, conversationId: 'existing-conversation-123' },
+    };
+    await initializeClient({ req: requestWithConversationId, res: mockResponse, optionsOnly: true });
+    expect(resolveHeaders).toHaveBeenCalledWith(
+      { 'x-user': '{{LIBRECHAT_USER_ID}}', 'x-email': '{{LIBRECHAT_USER_EMAIL}}' },
+      { id: 'user-123', email: 'test@example.com' },
+      { LIBRECHAT_CONVERSATION_ID: 'existing-conversation-123' },
+    );
+  });
+
   it('calls resolveHeaders with headers and user', async () => {
     const { resolveHeaders } = require('@librechat/api');
     await initializeClient({ req: mockRequest, res: mockResponse, optionsOnly: true });