diff --git a/x-pack/solutions/security/plugins/elastic_assistant/server/routes/chat/chat_complete_route.test.ts b/x-pack/solutions/security/plugins/elastic_assistant/server/routes/chat/chat_complete_route.test.ts
index 8cd2f0fd801d0..d129ae58ca658 100644
--- a/x-pack/solutions/security/plugins/elastic_assistant/server/routes/chat/chat_complete_route.test.ts
+++ b/x-pack/solutions/security/plugins/elastic_assistant/server/routes/chat/chat_complete_route.test.ts
@@ -44,6 +44,7 @@ jest.mock('../helpers', () => {
   };
 });
 const mockAppendAssistantMessageToConversation = appendAssistantMessageToConversation as jest.Mock;
+const mockCreateConversationWithUserInput = createConversationWithUserInput as jest.Mock;
 
 const mockLangChainExecute = langChainExecute as jest.Mock;
 const mockStream = jest.fn().mockImplementation(() => new PassThrough());
@@ -150,7 +151,7 @@ describe('chatCompleteRoute', () => {
     jest.clearAllMocks();
     mockAppendAssistantMessageToConversation.mockResolvedValue(true);
     license.hasAtLeast.mockReturnValue(true);
-    (createConversationWithUserInput as jest.Mock).mockResolvedValue({ id: 'something' });
+    mockCreateConversationWithUserInput.mockResolvedValue({ id: 'something' });
     mockLangChainExecute.mockImplementation(
       async ({
         connectorId,
@@ -166,12 +167,14 @@ describe('chatCompleteRoute', () => {
         ) => Promise<void>;
       }) => {
         if (!isStream && connectorId === 'mock-connector-id') {
+          onLlmResponse('Non-streamed test reply.', {}, false).catch(() => {});
           return {
             connector_id: 'mock-connector-id',
             data: mockActionResponse,
             status: 'ok',
           };
         } else if (isStream && connectorId === 'mock-connector-id') {
+          onLlmResponse('Streamed test reply.', {}, false).catch(() => {});
           return mockStream;
         } else {
           onLlmResponse('simulated error', {}, true).catch(() => {});
@@ -399,4 +402,141 @@ describe('chatCompleteRoute', () => {
       mockGetElser
     );
   });
+
+  it('should add assistant reply to existing conversation when `persist=true`', async () => {
+    const mockRouter = {
+      versioned: {
+        post: jest.fn().mockImplementation(() => {
+          return {
+            addVersion: jest.fn().mockImplementation(async (_, handler) => {
+              await handler(
+                mockContext,
+                {
+                  ...mockRequest,
+                  body: {
+                    ...mockRequest.body,
+                    conversationId: existingConversation.id,
+                  },
+                },
+                mockResponse
+              );
+              expect(mockAppendAssistantMessageToConversation).toHaveBeenCalledWith(
+                expect.objectContaining({
+                  messageContent: 'Non-streamed test reply.',
+                  isError: false,
+                })
+              );
+              expect(mockCreateConversationWithUserInput).toHaveBeenCalledTimes(0);
+            }),
+          };
+        }),
+      },
+    };
+
+    chatCompleteRoute(
+      mockRouter as unknown as IRouter<ElasticAssistantRequestHandlerContext>,
+      mockGetElser
+    );
+  });
+
+  it('should not add assistant reply to existing conversation when `persist=false`', async () => {
+    const mockRouter = {
+      versioned: {
+        post: jest.fn().mockImplementation(() => {
+          return {
+            addVersion: jest.fn().mockImplementation(async (_, handler) => {
+              await handler(
+                mockContext,
+                {
+                  ...mockRequest,
+                  body: {
+                    ...mockRequest.body,
+                    conversationId: existingConversation.id,
+                    persist: false,
+                  },
+                },
+                mockResponse
+              );
+              expect(mockAppendAssistantMessageToConversation).toHaveBeenCalledTimes(0);
+              expect(mockCreateConversationWithUserInput).toHaveBeenCalledTimes(0);
+            }),
+          };
+        }),
+      },
+    };
+
+    chatCompleteRoute(
+      mockRouter as unknown as IRouter<ElasticAssistantRequestHandlerContext>,
+      mockGetElser
+    );
+  });
+
+  it('should add assistant reply to new conversation when `persist=true`', async () => {
+    const mockRouter = {
+      versioned: {
+        post: jest.fn().mockImplementation(() => {
+          return {
+            addVersion: jest.fn().mockImplementation(async (_, handler) => {
+              await handler(
+                mockContext,
+                {
+                  ...mockRequest,
+                  body: {
+                    ...mockRequest.body,
+                    conversationId: undefined,
+                    persist: true,
+                  },
+                },
+                mockResponse
+              );
+              expect(mockAppendAssistantMessageToConversation).toHaveBeenCalledWith(
+                expect.objectContaining({
+                  messageContent: 'Non-streamed test reply.',
+                  isError: false,
+                })
+              );
+              expect(mockCreateConversationWithUserInput).toHaveBeenCalledTimes(1);
+            }),
+          };
+        }),
+      },
+    };
+
+    chatCompleteRoute(
+      mockRouter as unknown as IRouter<ElasticAssistantRequestHandlerContext>,
+      mockGetElser
+    );
+  });
+
+  it('should not create a new conversation when `persist=false`', async () => {
+    const mockRouter = {
+      versioned: {
+        post: jest.fn().mockImplementation(() => {
+          return {
+            addVersion: jest.fn().mockImplementation(async (_, handler) => {
+              await handler(
+                mockContext,
+                {
+                  ...mockRequest,
+                  body: {
+                    ...mockRequest.body,
+                    conversationId: undefined,
+                    persist: false,
+                  },
+                },
+                mockResponse
+              );
+              expect(mockAppendAssistantMessageToConversation).toHaveBeenCalledTimes(0);
+              expect(mockCreateConversationWithUserInput).toHaveBeenCalledTimes(0);
+            }),
+          };
+        }),
+      },
+    };
+
+    chatCompleteRoute(
+      mockRouter as unknown as IRouter<ElasticAssistantRequestHandlerContext>,
+      mockGetElser
+    );
+  });
 });
diff --git a/x-pack/solutions/security/plugins/elastic_assistant/server/routes/chat/chat_complete_route.ts b/x-pack/solutions/security/plugins/elastic_assistant/server/routes/chat/chat_complete_route.ts
index 726ea224692ee..cc39ce04add17 100644
--- a/x-pack/solutions/security/plugins/elastic_assistant/server/routes/chat/chat_complete_route.ts
+++ b/x-pack/solutions/security/plugins/elastic_assistant/server/routes/chat/chat_complete_route.ts
@@ -93,7 +93,7 @@ export const chatCompleteRoute = (
             await ctx.elasticAssistant.getAIAssistantAnonymizationFieldsDataClient();
 
           let messages;
-          const conversationId = request.body.conversationId;
+          const existingConversationId = request.body.conversationId;
           const connectorId = request.body.connectorId;
 
           let latestReplacements: Replacements = {};
@@ -159,11 +159,10 @@ export const chatCompleteRoute = (
           });
 
           let newConversation: ConversationResponse | undefined | null;
-          if (conversationsDataClient && !conversationId && request.body.persist) {
+          if (conversationsDataClient && !existingConversationId && request.body.persist) {
             newConversation = await createConversationWithUserInput({
               actionTypeId,
               connectorId,
-              conversationId,
               conversationsDataClient,
               promptId: request.body.promptId,
               replacements: latestReplacements,
@@ -178,6 +177,11 @@ export const chatCompleteRoute = (
             }));
           }
 
+          // Do not persist conversation messages if `persist = false`
+          const conversationId = request.body.persist
+            ? existingConversationId ?? newConversation?.id
+            : undefined;
+
           const contentReferencesStore = newContentReferencesStore();
 
           const onLlmResponse = async (
@@ -185,11 +189,11 @@ export const chatCompleteRoute = (
             traceData: Message['traceData'] = {},
             isError = false
           ): Promise<void> => {
-            if (newConversation?.id && conversationsDataClient) {
+            if (conversationId && conversationsDataClient) {
               const contentReferences = pruneContentReferences(content, contentReferencesStore);
 
               await appendAssistantMessageToConversation({
-                conversationId: newConversation?.id,
+                conversationId,
                 conversationsDataClient,
                 messageContent: content,
                 replacements: latestReplacements,
@@ -207,7 +211,7 @@ export const chatCompleteRoute = (
             actionTypeId,
             connectorId,
             isOssModel,
-            conversationId: conversationId ?? newConversation?.id,
+            conversationId,
             context: ctx,
             getElser,
             logger,