fix(ui): prevent over-eager slash subcommand completion (#20136)

packages/cli/src/ui/hooks/useCommandCompletion.tsx

@@ -1,16 +1,17 @@
 /**
  * @license
- * Copyright 2025 Google LLC
+ * Copyright 2026 Google LLC
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import type React from 'react';
 import { useCallback, useMemo, useEffect, useState } from 'react';
 import type { Suggestion } from '../components/SuggestionsDisplay.js';
 import type { CommandContext, SlashCommand } from '../commands/types.js';
 import type { TextBuffer } from '../components/shared/text-buffer.js';
 import { logicalPosToOffset } from '../components/shared/text-buffer.js';
-import { isSlashCommand } from '../utils/commandUtils.js';
 import { toCodePoints } from '../utils/textUtils.js';
+import { isSlashCommand } from '../utils/commandUtils.js';
 import { useAtCompletion } from './useAtCompletion.js';
 import { useSlashCompletion } from './useSlashCompletion.js';
 import { useShellCompletion } from './useShellCompletion.js';
@@ -436,10 +437,23 @@ export function useCommandCompletion({
 
       const lineCodePoints = toCodePoints(buffer.lines[cursorRow] || '');
       const charAfterCompletion = lineCodePoints[end];
+
+      let shouldAddSpace = true;
+      if (completionMode === CompletionMode.SLASH) {
+        const command =
+          slashCompletionRange.getCommandFromSuggestion(suggestion);
+        // Don't add a space if the command has an action (can be executed)
+        // and doesn't have a completion function (doesn't REQUIRE more arguments)
+        const isExecutableCommand = !!(command && command.action);
+        const requiresArguments = !!(command && command.completion);
+        shouldAddSpace = !isExecutableCommand || requiresArguments;
+      }
+
       if (
         charAfterCompletion !== ' ' &&
         !suggestionText.endsWith('/') &&
-        !suggestionText.endsWith('\\')
+        !suggestionText.endsWith('\\') &&
+        shouldAddSpace
       ) {
         suggestionText += ' ';
       }