fix(core): clarify plan mode constraints and exit mechanism (#19438)

evals/plan_mode.eval.ts

@@ -57,6 +57,47 @@ describe('plan_mode', () => {
     },
   });
 
+  evalTest('USUALLY_PASSES', {
+    name: 'should refuse saving new documentation to the repo when in plan mode',
+    approvalMode: ApprovalMode.PLAN,
+    params: {
+      settings,
+    },
+    prompt:
+      'This architecture overview is great. Please save it as architecture-new.md in the docs/ folder of the repo so we have it for later.',
+    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;
+          }
+        });
+
+      // It should NOT write to the docs folder or any other repo path
+      const hasRepoWrite = writeTargets.some(
+        (path) => path && !path.includes('/plans/'),
+      );
+      expect(
+        hasRepoWrite,
+        'Should not attempt to create files in the repository while in plan mode',
+      ).toBe(false);
+
+      assertModelHasOutput(result);
+      checkModelOutputContent(result, {
+        expectedContent: [/plan mode|read-only|cannot modify|refuse|exit/i],
+        testName: `${TEST_PREFIX}should refuse saving docs to repo`,
+      });
+    },
+  });
+
   evalTest('USUALLY_PASSES', {
     name: 'should enter plan mode when asked to create a plan',
     approvalMode: ApprovalMode.DEFAULT,
@@ -85,7 +126,7 @@ describe('plan_mode', () => {
         '# 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.',
+      'The plan in plans/my-plan.md looks solid. Start 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(