diff --git a/packages/cli/src/ui/AppContainer.test.tsx b/packages/cli/src/ui/AppContainer.test.tsx
index 101725d0cc..88779bb760 100644
--- a/packages/cli/src/ui/AppContainer.test.tsx
+++ b/packages/cli/src/ui/AppContainer.test.tsx
@@ -956,6 +956,109 @@ describe('AppContainer State Management', () => {
     });
   });
 
+  describe('Queue Error Message', () => {
+    beforeEach(() => {
+      vi.useFakeTimers();
+    });
+
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+
+    it('should set and clear the queue error message after a timeout', async () => {
+      const { rerender } = render(
+        <AppContainer
+          config={mockConfig}
+          settings={mockSettings}
+          version="1.0.0"
+          initializationResult={mockInitResult}
+        />,
+      );
+
+      expect(capturedUIState.queueErrorMessage).toBeNull();
+
+      capturedUIActions.setQueueErrorMessage('Test error');
+      rerender(
+        <AppContainer
+          config={mockConfig}
+          settings={mockSettings}
+          version="1.0.0"
+          initializationResult={mockInitResult}
+        />,
+      );
+      expect(capturedUIState.queueErrorMessage).toBe('Test error');
+
+      vi.advanceTimersByTime(3000);
+      rerender(
+        <AppContainer
+          config={mockConfig}
+          settings={mockSettings}
+          version="1.0.0"
+          initializationResult={mockInitResult}
+        />,
+      );
+      expect(capturedUIState.queueErrorMessage).toBeNull();
+    });
+
+    it('should reset the timer if a new error message is set', async () => {
+      const { rerender } = render(
+        <AppContainer
+          config={mockConfig}
+          settings={mockSettings}
+          version="1.0.0"
+          initializationResult={mockInitResult}
+        />,
+      );
+
+      capturedUIActions.setQueueErrorMessage('First error');
+      rerender(
+        <AppContainer
+          config={mockConfig}
+          settings={mockSettings}
+          version="1.0.0"
+          initializationResult={mockInitResult}
+        />,
+      );
+      expect(capturedUIState.queueErrorMessage).toBe('First error');
+
+      vi.advanceTimersByTime(1500);
+
+      capturedUIActions.setQueueErrorMessage('Second error');
+      rerender(
+        <AppContainer
+          config={mockConfig}
+          settings={mockSettings}
+          version="1.0.0"
+          initializationResult={mockInitResult}
+        />,
+      );
+      expect(capturedUIState.queueErrorMessage).toBe('Second error');
+
+      vi.advanceTimersByTime(2000);
+      rerender(
+        <AppContainer
+          config={mockConfig}
+          settings={mockSettings}
+          version="1.0.0"
+          initializationResult={mockInitResult}
+        />,
+      );
+      expect(capturedUIState.queueErrorMessage).toBe('Second error');
+
+      // 5. Advance time past the 3 second timeout from the second message
+      vi.advanceTimersByTime(1000);
+      rerender(
+        <AppContainer
+          config={mockConfig}
+          settings={mockSettings}
+          version="1.0.0"
+          initializationResult={mockInitResult}
+        />,
+      );
+      expect(capturedUIState.queueErrorMessage).toBeNull();
+    });
+  });
+
   describe('Terminal Height Calculation', () => {
     const mockedMeasureElement = measureElement as Mock;
     const mockedUseTerminalSize = useTerminalSize as Mock;
diff --git a/packages/cli/src/ui/AppContainer.tsx b/packages/cli/src/ui/AppContainer.tsx
index 4eab2524f9..14101c352f 100644
--- a/packages/cli/src/ui/AppContainer.tsx
+++ b/packages/cli/src/ui/AppContainer.tsx
@@ -93,6 +93,7 @@ import { useExtensionUpdates } from './hooks/useExtensionUpdates.js';
 import { ShellFocusContext } from './contexts/ShellFocusContext.js';
 
 const CTRL_EXIT_PROMPT_DURATION_MS = 1000;
+const QUEUE_ERROR_DISPLAY_DURATION_MS = 3000;
 
 function isToolExecuting(pendingHistoryItems: HistoryItemWithoutId[]) {
   return pendingHistoryItems.some((item) => {
@@ -154,6 +155,10 @@ export const AppContainer = (props: AppContainerProps) => {
     config.isTrustedFolder(),
   );
 
+  const [queueErrorMessage, setQueueErrorMessage] = useState<string | null>(
+    null,
+  );
+
   const extensions = config.getExtensions();
   const {
     extensionsUpdateState,
@@ -815,6 +820,17 @@ Logging in with Google... Please restart Gemini CLI to continue.
     }
   }, [ideNeedsRestart]);
 
+  useEffect(() => {
+    if (queueErrorMessage) {
+      const timer = setTimeout(() => {
+        setQueueErrorMessage(null);
+      }, QUEUE_ERROR_DISPLAY_DURATION_MS);
+
+      return () => clearTimeout(timer);
+    }
+    return undefined;
+  }, [queueErrorMessage, setQueueErrorMessage]);
+
   useEffect(() => {
     if (isInitialMount.current) {
       isInitialMount.current = false;
@@ -1131,6 +1147,7 @@ Logging in with Google... Please restart Gemini CLI to continue.
       currentLoadingPhrase,
       historyRemountKey,
       messageQueue,
+      queueErrorMessage,
       showAutoAcceptIndicator,
       showWorkspaceMigrationDialog,
       workspaceExtensions,
@@ -1212,6 +1229,7 @@ Logging in with Google... Please restart Gemini CLI to continue.
       currentLoadingPhrase,
       historyRemountKey,
       messageQueue,
+      queueErrorMessage,
       showAutoAcceptIndicator,
       showWorkspaceMigrationDialog,
       workspaceExtensions,
@@ -1276,6 +1294,7 @@ Logging in with Google... Please restart Gemini CLI to continue.
       onWorkspaceMigrationDialogOpen,
       onWorkspaceMigrationDialogClose,
       handleProQuotaChoice,
+      setQueueErrorMessage,
     }),
     [
       handleThemeSelect,
@@ -1301,6 +1320,7 @@ Logging in with Google... Please restart Gemini CLI to continue.
       onWorkspaceMigrationDialogOpen,
       onWorkspaceMigrationDialogClose,
       handleProQuotaChoice,
+      setQueueErrorMessage,
     ],
   );
 
diff --git a/packages/cli/src/ui/components/Composer.tsx b/packages/cli/src/ui/components/Composer.tsx
index d87737619a..0cd7f0ad98 100644
--- a/packages/cli/src/ui/components/Composer.tsx
+++ b/packages/cli/src/ui/components/Composer.tsx
@@ -89,6 +89,8 @@ export const Composer = () => {
             </Text>
           ) : uiState.showEscapePrompt ? (
             <Text color={theme.text.secondary}>Press Esc again to clear.</Text>
+          ) : uiState.queueErrorMessage ? (
+            <Text color={theme.status.error}>{uiState.queueErrorMessage}</Text>
           ) : (
             !settings.merged.ui?.hideContextSummary && (
               <ContextSummaryDisplay
@@ -149,6 +151,8 @@ export const Composer = () => {
               ? "  Press 'i' for INSERT mode and 'Esc' for NORMAL mode."
               : '  Type your message or @path/to/file'
           }
+          setQueueErrorMessage={uiActions.setQueueErrorMessage}
+          streamingState={uiState.streamingState}
         />
       )}
 
diff --git a/packages/cli/src/ui/components/InputPrompt.test.tsx b/packages/cli/src/ui/components/InputPrompt.test.tsx
index 4c4580b18a..c5021f68da 100644
--- a/packages/cli/src/ui/components/InputPrompt.test.tsx
+++ b/packages/cli/src/ui/components/InputPrompt.test.tsx
@@ -28,6 +28,7 @@ import { useKittyKeyboardProtocol } from '../hooks/useKittyKeyboardProtocol.js';
 import { createMockCommandContext } from '../../test-utils/mockCommandContext.js';
 import stripAnsi from 'strip-ansi';
 import chalk from 'chalk';
+import { StreamingState } from '../types.js';
 
 vi.mock('../hooks/useShellHistory.js');
 vi.mock('../hooks/useCommandCompletion.js');
@@ -2167,7 +2168,58 @@ describe('InputPrompt', () => {
     expect(mockBuffer.handleInput).toHaveBeenCalled();
     unmount();
   });
+  it('should prevent slash commands from being queued while streaming', async () => {
+    props.onSubmit = vi.fn();
+    props.buffer.text = '/help';
+    props.setQueueErrorMessage = vi.fn();
+    props.streamingState = StreamingState.Responding;
+    const { stdin, unmount } = renderWithProviders(<InputPrompt {...props} />);
+    await wait();
+    stdin.write('/help');
+    stdin.write('\r');
+    await wait();
+
+    expect(props.onSubmit).not.toHaveBeenCalled();
+    expect(props.setQueueErrorMessage).toHaveBeenCalledWith(
+      'Slash commands cannot be queued',
+    );
+    unmount();
+  });
+  it('should prevent shell commands from being queued while streaming', async () => {
+    props.onSubmit = vi.fn();
+    props.buffer.text = 'ls';
+    props.setQueueErrorMessage = vi.fn();
+    props.streamingState = StreamingState.Responding;
+    props.shellModeActive = true;
+    const { stdin, unmount } = renderWithProviders(<InputPrompt {...props} />);
+    await wait();
+    stdin.write('ls');
+    stdin.write('\r');
+    await wait();
+
+    expect(props.onSubmit).not.toHaveBeenCalled();
+    expect(props.setQueueErrorMessage).toHaveBeenCalledWith(
+      'Shell commands cannot be queued',
+    );
+    unmount();
+  });
+  it('should allow regular messages to be queued while streaming', async () => {
+    props.onSubmit = vi.fn();
+    props.buffer.text = 'regular message';
+    props.setQueueErrorMessage = vi.fn();
+    props.streamingState = StreamingState.Responding;
+    const { stdin, unmount } = renderWithProviders(<InputPrompt {...props} />);
+    await wait();
+    stdin.write('regular message');
+    stdin.write('\r');
+    await wait();
+
+    expect(props.onSubmit).toHaveBeenCalledWith('regular message');
+    expect(props.setQueueErrorMessage).not.toHaveBeenCalled();
+    unmount();
+  });
 });
+
 function clean(str: string | undefined): string {
   if (!str) return '';
   // Remove ANSI escape codes and trim whitespace
diff --git a/packages/cli/src/ui/components/InputPrompt.tsx b/packages/cli/src/ui/components/InputPrompt.tsx
index a4a5c35fe4..18d623fb58 100644
--- a/packages/cli/src/ui/components/InputPrompt.tsx
+++ b/packages/cli/src/ui/components/InputPrompt.tsx
@@ -38,6 +38,8 @@ import * as path from 'node:path';
 import { SCREEN_READER_USER_PREFIX } from '../textConstants.js';
 import { useShellFocusState } from '../contexts/ShellFocusContext.js';
 import { useUIState } from '../contexts/UIStateContext.js';
+import { StreamingState } from '../types.js';
+import { isSlashCommand } from '../utils/commandUtils.js';
 
 /**
  * Returns if the terminal can be trusted to handle paste events atomically
@@ -71,6 +73,8 @@ export interface InputPromptProps {
   onEscapePromptChange?: (showPrompt: boolean) => void;
   vimHandleInput?: (key: Key) => boolean;
   isEmbeddedShellFocused?: boolean;
+  setQueueErrorMessage: (message: string | null) => void;
+  streamingState: StreamingState;
 }
 
 // The input content, input container, and input suggestions list may have different widths
@@ -107,6 +111,8 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
   onEscapePromptChange,
   vimHandleInput,
   isEmbeddedShellFocused,
+  setQueueErrorMessage,
+  streamingState,
 }) => {
   const kittyProtocol = useKittyKeyboardProtocol();
   const isShellFocused = useShellFocusState();
@@ -221,6 +227,31 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
     ],
   );
 
+  const handleSubmit = useCallback(
+    (submittedValue: string) => {
+      const trimmedMessage = submittedValue.trim();
+      const isSlash = isSlashCommand(trimmedMessage);
+
+      const isShell = shellModeActive;
+      if (
+        (isSlash || isShell) &&
+        streamingState === StreamingState.Responding
+      ) {
+        setQueueErrorMessage(
+          `${isShell ? 'Shell' : 'Slash'} commands cannot be queued`,
+        );
+        return;
+      }
+      handleSubmitAndClear(trimmedMessage);
+    },
+    [
+      handleSubmitAndClear,
+      shellModeActive,
+      streamingState,
+      setQueueErrorMessage,
+    ],
+  );
+
   const customSetTextAndResetCompletionSignal = useCallback(
     (newText: string) => {
       buffer.setText(newText);
@@ -514,7 +545,7 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
 
       // If the command is a perfect match, pressing enter should execute it.
       if (completion.isPerfectMatch && keyMatchers[Command.RETURN](key)) {
-        handleSubmitAndClear(buffer.text);
+        handleSubmit(buffer.text);
         return;
       }
 
@@ -625,7 +656,7 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
             buffer.backspace();
             buffer.newline();
           } else {
-            handleSubmitAndClear(buffer.text);
+            handleSubmit(buffer.text);
           }
         }
         return;
@@ -706,6 +737,7 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
       onClearScreen,
       inputHistory,
       handleSubmitAndClear,
+      handleSubmit,
       shellHistory,
       reverseSearchCompletion,
       handleClipboardImage,
diff --git a/packages/cli/src/ui/contexts/UIActionsContext.tsx b/packages/cli/src/ui/contexts/UIActionsContext.tsx
index c9e7432fba..096128143f 100644
--- a/packages/cli/src/ui/contexts/UIActionsContext.tsx
+++ b/packages/cli/src/ui/contexts/UIActionsContext.tsx
@@ -45,6 +45,7 @@ export interface UIActions {
   onWorkspaceMigrationDialogOpen: () => void;
   onWorkspaceMigrationDialogClose: () => void;
   handleProQuotaChoice: (choice: 'auth' | 'continue') => void;
+  setQueueErrorMessage: (message: string | null) => void;
 }
 
 export const UIActionsContext = createContext<UIActions | null>(null);
diff --git a/packages/cli/src/ui/contexts/UIStateContext.tsx b/packages/cli/src/ui/contexts/UIStateContext.tsx
index fcc86f02e2..76c0de298f 100644
--- a/packages/cli/src/ui/contexts/UIStateContext.tsx
+++ b/packages/cli/src/ui/contexts/UIStateContext.tsx
@@ -89,6 +89,7 @@ export interface UIState {
   currentLoadingPhrase: string;
   historyRemountKey: number;
   messageQueue: string[];
+  queueErrorMessage: string | null;
   showAutoAcceptIndicator: ApprovalMode;
   showWorkspaceMigrationDialog: boolean;
   // eslint-disable-next-line @typescript-eslint/no-explicit-any