fix(cli): prevent duplicate SessionStart systemMessage render (#25827)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/cli/src/ui/AppContainer.test.tsx

@@ -1267,6 +1267,42 @@ describe('AppContainer State Management', () => {
     });
   });
 
+  describe('SessionStart Hook Rendering', () => {
+    it('does not render systemMessage directly (avoids duplicate with HookSystemMessage event)', async () => {
+      const mockAddItem = vi.fn();
+      mockedUseHistory.mockReturnValue({
+        history: [],
+        addItem: mockAddItem,
+        updateItem: vi.fn(),
+        clearItems: vi.fn(),
+        loadHistory: vi.fn(),
+      });
+
+      const fireSessionStartEvent = vi.fn().mockResolvedValue({
+        systemMessage: 'Hello from SessionStart hook',
+        getAdditionalContext: vi.fn(() => undefined),
+      });
+      vi.spyOn(mockConfig, 'getHookSystem').mockReturnValue({
+        fireSessionEndEvent: vi.fn().mockResolvedValue(undefined),
+        fireSessionStartEvent,
+      } as unknown as ReturnType<Config['getHookSystem']>);
+
+      const { unmount } = await act(async () => renderAppContainer());
+      await waitFor(() => expect(fireSessionStartEvent).toHaveBeenCalled());
+
+      // The direct-render path (the bug) would call addItem with the
+      // systemMessage text and no `source` field. The HookSystemMessage
+      // event-listener path (the correct one) always sets `source`.
+      const directRenderCall = mockAddItem.mock.calls.find(
+        ([item]) =>
+          item?.text === 'Hello from SessionStart hook' && !item?.source,
+      );
+      expect(directRenderCall).toBeUndefined();
+
+      unmount();
+    });
+  });
+
   describe('Token Counting from Session Stats', () => {
     it('tracks token counts from session messages', async () => {
       // Session stats are provided through the SessionStatsProvider context

packages/cli/src/ui/AppContainer.tsx

@@ -497,16 +497,6 @@ export const AppContainer = (props: AppContainerProps) => {
         ?.fireSessionStartEvent(sessionStartSource);
 
       if (result) {
-        if (result.systemMessage) {
-          historyManager.addItem(
-            {
-              type: MessageType.INFO,
-              text: result.systemMessage,
-            },
-            Date.now(),
-          );
-        }
-
         const additionalContext = result.getAdditionalContext();
         const geminiClient = config.getGeminiClient();
         if (additionalContext && geminiClient) {
@@ -549,12 +539,6 @@ export const AppContainer = (props: AppContainerProps) => {
         debugLogger.error('Error during cleanup:', e),
       );
     };
-    // Disable the dependencies check here. historyManager gets flagged
-    // but we don't want to react to changes to it because each new history
-    // item, including the ones from the start session hook will cause a
-    // re-render and an error when we try to reload config.
-    //
-    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [config, resumedSessionData]);
 
   useEffect(