feat(cli): Introduce toggleable shell mode with enhanced UI

- Implements a toggleable shell mode, removing the need to prefix every command with `!`.
- Users can now enter and exit shell mode by typing `!` as the first character in an empty input prompt.
- The input prompt visually indicates active shell mode with a distinct color and `! ` prefix.
- Shell command history items (`user_shell`) are now visually differentiated from regular user messages.
- This provides a cleaner and more streamlined user experience for frequent shell interactions.

Fixes https://b.corp.google.com/issues/418509745

packages/cli/src/ui/App.tsx

@@ -61,6 +61,7 @@ export const App = ({
   const [themeError, setThemeError] = useState<string | null>(null);
   const [footerHeight, setFooterHeight] = useState<number>(0);
   const [corgiMode, setCorgiMode] = useState(false);
+  const [shellModeActive, setShellModeActive] = useState(false);
 
   const toggleCorgiMode = useCallback(() => {
     setCorgiMode((prev) => !prev);
@@ -152,10 +153,16 @@ export const App = ({
     (submittedValue: string) => {
       const trimmedValue = submittedValue.trim();
       if (trimmedValue.length > 0) {
-        submitQuery(submittedValue);
+        if (shellModeActive && !trimmedValue.startsWith('!')) {
+          // TODO: Don't prefix (hack) and properly submit pass throughs to a dedicated hook:
+          // https://b.corp.google.com/issues/418509745
+          submitQuery(`!${trimmedValue}`);
+        } else {
+          submitQuery(trimmedValue);
+        }
       }
     },
-    [submitQuery],
+    [submitQuery, shellModeActive],
   );
 
   const userMessages = useMemo(
@@ -364,6 +371,8 @@ export const App = ({
                   resetCompletion={completion.resetCompletionState}
                   setEditorState={setEditorState}
                   onClearScreen={handleClearScreen} // Added onClearScreen prop
+                  shellModeActive={shellModeActive}
+                  setShellModeActive={setShellModeActive}
                 />
                 {completion.showSuggestions && (
                   <Box>

packages/cli/src/ui/components/HistoryItemDisplay.tsx

@@ -7,6 +7,7 @@
 import React from 'react';
 import type { HistoryItem } from '../types.js';
 import { UserMessage } from './messages/UserMessage.js';
+import { UserShellMessage } from './messages/UserShellMessage.js';
 import { GeminiMessage } from './messages/GeminiMessage.js';
 import { InfoMessage } from './messages/InfoMessage.js';
 import { ErrorMessage } from './messages/ErrorMessage.js';
@@ -28,6 +29,7 @@ export const HistoryItemDisplay: React.FC<HistoryItemDisplayProps> = ({
   <Box flexDirection="column" key={item.id}>
     {/* Render standard message types */}
     {item.type === 'user' && <UserMessage text={item.text} />}
+    {item.type === 'user_shell' && <UserShellMessage text={item.text} />}
     {item.type === 'gemini' && (
       <GeminiMessage
         text={item.text}

packages/cli/src/ui/components/InputPrompt.tsx

@@ -26,6 +26,8 @@ interface InputPromptProps {
   navigateSuggestionDown: () => void;
   setEditorState: (updater: (prevState: EditorState) => EditorState) => void;
   onClearScreen: () => void;
+  shellModeActive: boolean;
+  setShellModeActive: (value: boolean) => void;
 }
 
 export interface EditorState {
@@ -48,6 +50,8 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
   resetCompletion,
   setEditorState,
   onClearScreen,
+  shellModeActive,
+  setShellModeActive,
 }) => {
   const handleSubmit = useCallback(
     (submittedValue: string) => {
@@ -116,6 +120,11 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
       _currentText?: string,
       _cursorOffset?: number,
     ) => {
+      if (input === '!' && query === '' && !showSuggestions) {
+        setShellModeActive(!shellModeActive);
+        onChangeAndMoveCursor(''); // Clear the '!' from input
+        return true;
+      }
       if (showSuggestions) {
         if (key.upArrow) {
           navigateSuggestionUp();
@@ -186,12 +195,21 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
       inputHistory,
       setEditorState,
       onClearScreen,
+      shellModeActive,
+      setShellModeActive,
+      onChangeAndMoveCursor,
     ],
   );
 
   return (
-    <Box borderStyle="round" borderColor={Colors.AccentBlue} paddingX={1}>
+    <Box
-      <Text color={Colors.AccentPurple}>&gt; </Text>
+      borderStyle="round"
+      borderColor={shellModeActive ? Colors.AccentYellow : Colors.AccentBlue}
+      paddingX={1}
+    >
+      <Text color={shellModeActive ? Colors.AccentYellow : Colors.AccentPurple}>
+        {shellModeActive ? '! ' : '> '}
+      </Text>
       <Box flexGrow={1}>
         <MultilineTextEditor
           key={editorState.key.toString()}

packages/cli/src/ui/components/messages/UserShellMessage.tsx

@@ -0,0 +1,25 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import React from 'react';
+import { Box, Text } from 'ink';
+import { Colors } from '../../colors.js';
+
+interface UserShellMessageProps {
+  text: string;
+}
+
+export const UserShellMessage: React.FC<UserShellMessageProps> = ({ text }) => {
+  // Remove leading '!' if present, as App.tsx adds it for the processor.
+  const commandToDisplay = text.startsWith('!') ? text.substring(1) : text;
+
+  return (
+    <Box>
+      <Text color={Colors.AccentCyan}>$ </Text>
+      <Text>{commandToDisplay}</Text>
+    </Box>
+  );
+};

packages/cli/src/ui/hooks/shellCommandProcessor.ts

@@ -38,7 +38,10 @@ export const useShellCommandProcessor = (
       const commandToExecute = rawQuery.trim().slice(1).trimStart();
 
       const userMessageTimestamp = Date.now();
-      addItemToHistory({ type: 'user', text: rawQuery }, userMessageTimestamp);
+      addItemToHistory(
+        { type: 'user_shell', text: rawQuery },
+        userMessageTimestamp,
+      );
 
       if (!commandToExecute) {
         addItemToHistory(

packages/cli/src/ui/types.ts

@@ -85,12 +85,18 @@ export type HistoryItemToolGroup = HistoryItemBase & {
   tools: IndividualToolCallDisplay[];
 };
 
+export type HistoryItemUserShell = HistoryItemBase & {
+  type: 'user_shell';
+  text: string;
+};
+
 // Using Omit<HistoryItem, 'id'> seems to have some issues with typescript's
 // type inference e.g. historyItem.type === 'tool_group' isn't auto-inferring that
 // 'tools' in historyItem.
 // Individually exported types extending HistoryItemBase
 export type HistoryItemWithoutId =
   | HistoryItemUser
+  | HistoryItemUserShell
   | HistoryItemGemini
   | HistoryItemGeminiContent
   | HistoryItemInfo