fix(core): resolve nested plan directory duplication and relative path policies (#25138)

evals/plan_mode.eval.ts

@@ -305,7 +305,7 @@ describe('plan_mode', () => {
       settings,
     },
     prompt:
-      'Enter plan mode and plan to create a new module called foo. The plan should be saved as foo-plan.md. Then, exit plan mode.',
+      'I agree with your strategy. Please enter plan mode and draft the plan to create a new module called foo. The plan should be saved as foo-plan.md. Then, exit plan mode.',
     assert: async (rig, result) => {
       const enterPlanCalled = await rig.waitForToolCall('enter_plan_mode');
       expect(
@@ -376,4 +376,48 @@ describe('plan_mode', () => {
       assertModelHasOutput(result);
     },
   });
+
+  evalTest('USUALLY_PASSES', {
+    name: 'should handle nested plan directories correctly',
+    suiteName: 'plan_mode',
+    suiteType: 'behavioral',
+    approvalMode: ApprovalMode.PLAN,
+    params: {
+      settings,
+    },
+    prompt:
+      'Please create a new architectural plan in a nested folder called "architecture/frontend-v2.md" within the plans directory. The plan should contain the text "# Frontend V2 Plan". Then, exit plan mode',
+    assert: async (rig, result) => {
+      await rig.waitForTelemetryReady();
+      const toolLogs = rig.readToolLogs();
+
+      const writeCalls = toolLogs.filter((log) =>
+        ['write_file', 'replace'].includes(log.toolRequest.name),
+      );
+
+      const wroteToNestedPath = writeCalls.some((log) => {
+        try {
+          const args = JSON.parse(log.toolRequest.args);
+          if (!args.file_path) return false;
+          // In plan mode, paths can be passed as relative (architecture/frontend-v2.md)
+          // or they might be resolved as absolute by the tool depending on the exact mock state.
+          // We strictly ensure it ends exactly with the expected nested path and doesn't contain extra nesting.
+          const normalizedPath = args.file_path.replace(/\\/g, '/');
+          return (
+            normalizedPath === 'architecture/frontend-v2.md' ||
+            normalizedPath.endsWith('/plans/architecture/frontend-v2.md')
+          );
+        } catch {
+          return false;
+        }
+      });
+
+      expect(
+        wroteToNestedPath,
+        'Expected model to successfully target the nested plan file path',
+      ).toBe(true);
+
+      assertModelHasOutput(result);
+    },
+  });
 });