feat(cli): support @-mentioning files in AskUser custom input

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

@@ -22,7 +22,7 @@ import type { SelectionListItem } from '../hooks/useSelectionList.js';
 import { TabHeader, type Tab } from './shared/TabHeader.js';
 import { useKeypress, type Key } from '../hooks/useKeypress.js';
 import { Command } from '../key/keyMatchers.js';
-import { TextInput } from './shared/TextInput.js';
+import { AutocompleteTextInput } from './shared/AutocompleteTextInput.js';
 import { formatCommand } from '../key/keybindingUtils.js';
 import {
   useTextBuffer,
@@ -396,10 +396,12 @@ const TextQuestionView: React.FC<TextQuestionViewProps> = ({
 
       <Box flexDirection="row" marginBottom={1}>
         <Text color={theme.status.success}>{'> '}</Text>
-        <TextInput
+        <AutocompleteTextInput
           buffer={buffer}
           placeholder={placeholder}
           onSubmit={handleSubmit}
+          availableWidth={availableWidth}
+          suggestionsPosition="below"
         />
       </Box>
 
@@ -948,10 +950,12 @@ const ChoiceQuestionView: React.FC<ChoiceQuestionViewProps> = ({
                   />
                 )}
                 <Text color={theme.text.primary}> </Text>
-                <TextInput
+                <AutocompleteTextInput
                   buffer={customBuffer}
                   placeholder={placeholder}
                   focus={context.isSelected}
+                  availableWidth={availableWidth}
+                  suggestionsPosition="below"
                   onSubmit={(val) => {
                     if (question.multiSelect) {
                       const fullAnswer = buildAnswerString(

packages/cli/src/ui/components/shared/AutocompleteTextInput.tsx

@@ -0,0 +1,96 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type React from 'react';
+import { Box } from 'ink';
+import { TextInput, type TextInputProps } from './TextInput.js';
+import { useCommandCompletion } from '../../hooks/useCommandCompletion.js';
+import { SuggestionsDisplay } from '../SuggestionsDisplay.js';
+import { useConfig } from '../../contexts/ConfigContext.js';
+import { useKeypress, type Key } from '../../hooks/useKeypress.js';
+import { useKeyMatchers } from '../../hooks/useKeyMatchers.js';
+import { Command } from '../../key/keyMatchers.js';
+import type { CommandContext } from '../../commands/types.js';
+
+export interface AutocompleteTextInputProps extends TextInputProps {
+  suggestionsPosition?: 'above' | 'below';
+  availableWidth?: number;
+}
+
+/**
+ * A wrapper around TextInput that provides @-mention autocomplete for files.
+ */
+export function AutocompleteTextInput(
+  props: AutocompleteTextInputProps,
+): React.JSX.Element {
+  const {
+    suggestionsPosition = 'above',
+    availableWidth = 80,
+    ...textInputProps
+  } = props;
+  const config = useConfig();
+  const keyMatchers = useKeyMatchers();
+
+  const completion = useCommandCompletion({
+    buffer: props.buffer,
+    cwd: process.cwd(),
+    slashCommands: [],
+    // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+    commandContext: {} as unknown as CommandContext,
+    shellModeActive: false,
+    config,
+    active: props.focus ?? true,
+  });
+
+  const handleKeypress = (key: Key) => {
+    if (!completion.showSuggestions) return false;
+
+    if (key.name === 'tab') {
+      completion.handleAutocomplete(completion.activeSuggestionIndex);
+      return true;
+    }
+    if (keyMatchers[Command.MOVE_UP](key)) {
+      completion.navigateUp();
+      return true;
+    }
+    if (keyMatchers[Command.MOVE_DOWN](key)) {
+      completion.navigateDown();
+      return true;
+    }
+    if (keyMatchers[Command.SUBMIT](key) && !completion.isPerfectMatch) {
+      completion.handleAutocomplete(completion.activeSuggestionIndex);
+      return true;
+    }
+    return false;
+  };
+
+  useKeypress(handleKeypress, {
+    isActive: props.focus ?? true,
+    priority: true,
+  });
+
+  const suggestionsNode = completion.showSuggestions ? (
+    <Box paddingRight={2}>
+      <SuggestionsDisplay
+        suggestions={completion.suggestions}
+        activeIndex={completion.activeSuggestionIndex}
+        isLoading={completion.isLoadingSuggestions}
+        width={availableWidth}
+        scrollOffset={completion.visibleStartIndex}
+        userInput={props.buffer.text}
+        mode="reverse"
+      />
+    </Box>
+  ) : null;
+
+  return (
+    <Box flexDirection="column">
+      {suggestionsPosition === 'above' && suggestionsNode}
+      <TextInput {...textInputProps} />
+      {suggestionsPosition === 'below' && suggestionsNode}
+    </Box>
+  );
+}