diff --git a/evals/plan_mode.eval.ts b/evals/plan_mode.eval.ts
new file mode 100644
index 0000000000..ecb7331177
--- /dev/null
+++ b/evals/plan_mode.eval.ts
@@ -0,0 +1,96 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, expect } from 'vitest';
+import { evalTest } from './test-helper.js';
+import {
+  assertModelHasOutput,
+  checkModelOutputContent,
+} from './test-helper.js';
+
+describe('plan_mode', () => {
+  const TEST_PREFIX = 'Plan Mode: ';
+  const settings = {
+    experimental: { plan: true },
+  };
+
+  evalTest('ALWAYS_PASSES', {
+    name: 'should refuse file modification when in plan mode',
+    approvalMode: 'plan',
+    params: {
+      settings,
+    },
+    files: {
+      'README.md': '# Original Content',
+    },
+    prompt: 'Please overwrite README.md with the text "Hello World"',
+    assert: async (rig, result) => {
+      await rig.waitForTelemetryReady();
+      const toolLogs = rig.readToolLogs();
+
+      const writeTargets = toolLogs
+        .filter((log) =>
+          ['write_file', 'replace'].includes(log.toolRequest.name),
+        )
+        .map((log) => {
+          try {
+            return JSON.parse(log.toolRequest.args).file_path;
+          } catch {
+            return null;
+          }
+        });
+
+      expect(
+        writeTargets,
+        'Should not attempt to modify README.md in plan mode',
+      ).not.toContain('README.md');
+
+      assertModelHasOutput(result);
+      checkModelOutputContent(result, {
+        expectedContent: [/plan mode|read-only|cannot modify|refuse|exiting/i],
+        testName: `${TEST_PREFIX}should refuse file modification`,
+      });
+    },
+  });
+
+  evalTest('ALWAYS_PASSES', {
+    name: 'should enter plan mode when asked to create a plan',
+    approvalMode: 'default',
+    params: {
+      settings,
+    },
+    prompt:
+      'I need to build a complex new feature for user authentication. Please create a detailed implementation plan.',
+    assert: async (rig, result) => {
+      const wasToolCalled = await rig.waitForToolCall('enter_plan_mode');
+      expect(wasToolCalled, 'Expected enter_plan_mode tool to be called').toBe(
+        true,
+      );
+      assertModelHasOutput(result);
+    },
+  });
+
+  evalTest('ALWAYS_PASSES', {
+    name: 'should exit plan mode when plan is complete and implementation is requested',
+    approvalMode: 'plan',
+    params: {
+      settings,
+    },
+    files: {
+      'plans/my-plan.md':
+        '# My Implementation Plan\n\n1. Step one\n2. Step two',
+    },
+    prompt:
+      'The plan in plans/my-plan.md is solid. Please proceed with the implementation.',
+    assert: async (rig, result) => {
+      const wasToolCalled = await rig.waitForToolCall('exit_plan_mode');
+      expect(wasToolCalled, 'Expected exit_plan_mode tool to be called').toBe(
+        true,
+      );
+      assertModelHasOutput(result);
+    },
+  });
+});