2 files changed, 105 insertions, 0 deletions
diff --git a/packages/cli/src/ui/commands/extensionsCommand.test.ts b/packages/cli/src/ui/commands/extensionsCommand.test.ts
new file mode 100644
index 00000000..a989d9b0
--- /dev/null
+++ b/packages/cli/src/ui/commands/extensionsCommand.test.ts
@@ -0,0 +1,66 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import { extensionsCommand } from './extensionsCommand.js';
+import { type CommandContext } from './types.js';
+import { createMockCommandContext } from '../../test-utils/mockCommandContext.js';
+import { MessageType } from '../types.js';
+
+describe('extensionsCommand', () => {
+  let mockContext: CommandContext;
+
+  it('should display "No active extensions." when none are found', async () => {
+    mockContext = createMockCommandContext({
+      services: {
+        config: {
+          getActiveExtensions: () => [],
+        },
+      },
+    });
+
+    if (!extensionsCommand.action) throw new Error('Action not defined');
+    await extensionsCommand.action(mockContext, '');
+
+    expect(mockContext.ui.addItem).toHaveBeenCalledWith(
+      {
+        type: MessageType.INFO,
+        text: 'No active extensions.',
+      },
+      expect.any(Number),
+    );
+  });
+
+  it('should list active extensions when they are found', async () => {
+    const mockExtensions = [
+      { name: 'ext-one', version: '1.0.0' },
+      { name: 'ext-two', version: '2.1.0' },
+    ];
+    mockContext = createMockCommandContext({
+      services: {
+        config: {
+          getActiveExtensions: () => mockExtensions,
+        },
+      },
+    });
+
+    if (!extensionsCommand.action) throw new Error('Action not defined');
+    await extensionsCommand.action(mockContext, '');
+
+    const expectedMessage =
+      'Active extensions:\n\n' +
+      `  - \u001b[36mext-one (v1.0.0)\u001b[0m\n` +
+      `  - \u001b[36mext-two (v2.1.0)\u001b[0m\n`;
+
+    expect(mockContext.ui.addItem).toHaveBeenCalledWith(
+      {
+        type: MessageType.INFO,
+        text: expectedMessage,
+      },
+      expect.any(Number),
+    );
+  });
+});
diff --git a/packages/cli/src/ui/commands/extensionsCommand.ts b/packages/cli/src/ui/commands/extensionsCommand.ts
new file mode 100644
index 00000000..87d23afb
--- /dev/null
+++ b/packages/cli/src/ui/commands/extensionsCommand.ts
@@ -0,0 +1,39 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { type CommandContext, type SlashCommand } from './types.js';
+import { MessageType } from '../types.js';
+
+export const extensionsCommand: SlashCommand = {
+  name: 'extensions',
+  description: 'list active extensions',
+  action: async (context: CommandContext): Promise<void> => {
+    const activeExtensions = context.services.config?.getActiveExtensions();
+    if (!activeExtensions || activeExtensions.length === 0) {
+      context.ui.addItem(
+        {
+          type: MessageType.INFO,
+          text: 'No active extensions.',
+        },
+        Date.now(),
+      );
+      return;
+    }
+
+    const extensionLines = activeExtensions.map(
+      (ext) => `  - \u001b[36m${ext.name} (v${ext.version})\u001b[0m`,
+    );
+    const message = `Active extensions:\n\n${extensionLines.join('\n')}\n`;
+
+    context.ui.addItem(
+      {
+        type: MessageType.INFO,
+        text: message,
+      },
+      Date.now(),
+    );
+  },
+};