fix(plan): allow safe fallback when experiment setting for plan is not enabled but approval mode at startup is plan (#19439)

Co-authored-by: Jerop Kipruto <jerop@google.com>

Author: Adib234

packages/cli/src/config/config.test.ts

@@ -1344,6 +1344,36 @@ describe('Approval mode tool exclusion logic', () => {
       'Invalid approval mode: invalid_mode. Valid values are: yolo, auto_edit, plan, default',
     );
   });
+
+  it('should fall back to default approval mode if plan mode is requested but not enabled', async () => {
+    process.argv = ['node', 'script.js'];
+    const settings = createTestMergedSettings({
+      general: {
+        defaultApprovalMode: 'plan',
+      },
+      experimental: {
+        plan: false,
+      },
+    });
+    const argv = await parseArguments(settings);
+    const config = await loadCliConfig(settings, 'test-session', argv);
+    expect(config.getApprovalMode()).toBe(ApprovalMode.DEFAULT);
+  });
+
+  it('should allow plan approval mode if experimental plan is enabled', async () => {
+    process.argv = ['node', 'script.js'];
+    const settings = createTestMergedSettings({
+      general: {
+        defaultApprovalMode: 'plan',
+      },
+      experimental: {
+        plan: true,
+      },
+    });
+    const argv = await parseArguments(settings);
+    const config = await loadCliConfig(settings, 'test-session', argv);
+    expect(config.getApprovalMode()).toBe(ApprovalMode.PLAN);
+  });
 });
 
 describe('loadCliConfig with allowed-mcp-server-names', () => {
@@ -2556,19 +2586,17 @@ describe('loadCliConfig approval mode', () => {
       },
     });
 
-    await expect(loadCliConfig(settings, 'test-session', argv)).rejects.toThrow(
-      'Approval mode "plan" is only available when experimental.plan is enabled.',
-    );
+    const config = await loadCliConfig(settings, 'test-session', argv);
+    expect(config.getApprovalMode()).toBe(ApprovalMode.DEFAULT);
   });
 
   it('should throw error when --approval-mode=plan is used but experimental.plan setting is missing', async () => {
     process.argv = ['node', 'script.js', '--approval-mode', 'plan'];
     const argv = await parseArguments(createTestMergedSettings());
     const settings = createTestMergedSettings({});
 
-    await expect(loadCliConfig(settings, 'test-session', argv)).rejects.toThrow(
-      'Approval mode "plan" is only available when experimental.plan is enabled.',
-    );
+    const config = await loadCliConfig(settings, 'test-session', argv);
+    expect(config.getApprovalMode()).toBe(ApprovalMode.DEFAULT);
   });
 
   // --- Untrusted Folder Scenarios ---
@@ -2678,11 +2706,8 @@ describe('loadCliConfig approval mode', () => {
         experimental: { plan: false },
       });
       const argv = await parseArguments(settings);
-      await expect(
-        loadCliConfig(settings, 'test-session', argv),
-      ).rejects.toThrow(
-        'Approval mode "plan" is only available when experimental.plan is enabled.',
-      );
+      const config = await loadCliConfig(settings, 'test-session', argv);
+      expect(config.getApprovalMode()).toBe(ApprovalMode.DEFAULT);
     });
   });
 });

packages/cli/src/config/config.ts

@@ -555,11 +555,13 @@ export async function loadCliConfig(
         break;
       case 'plan':
         if (!(settings.experimental?.plan ?? false)) {
-          throw new Error(
-            'Approval mode "plan" is only available when experimental.plan is enabled.',
+          debugLogger.warn(
+            'Approval mode "plan" is only available when experimental.plan is enabled. Falling back to "default".',
           );
+          approvalMode = ApprovalMode.DEFAULT;
+        } else {
+          approvalMode = ApprovalMode.PLAN;
         }
-        approvalMode = ApprovalMode.PLAN;
         break;
       case 'default':
         approvalMode = ApprovalMode.DEFAULT;