diff --git a/tests/test_role_session_router.py b/tests/test_role_session_router.py
index 25b21c1..6204b6d 100644
--- a/tests/test_role_session_router.py
+++ b/tests/test_role_session_router.py
@@ -206,28 +206,35 @@ class TestRoleSessionRouter(unittest.TestCase):
 
 class TestCheckMidMerge(unittest.TestCase):
     def test_skip_scan_walk_root_skips_sibling_worktrees_only(self):
-        worktree_root = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
-        main_root = os.path.dirname(os.path.dirname(worktree_root))
-        self.assertFalse(
-            role_session_router.skip_python_scan_walk_root(
-                main_root, main_root
+        # Hermetic <main>/branches/<worktree> layout (#283): deriving these
+        # paths from __file__ made the test pass only when the suite ran from
+        # a branches/ worktree, because skip_python_scan_walk_root skips a
+        # branches/ walk root only when <project_root>/branches exists.
+        with tempfile.TemporaryDirectory() as tmp:
+            main_root = os.path.join(tmp, "main")
+            worktree_root = os.path.join(main_root, "branches", "fix-issue-1")
+            os.makedirs(os.path.join(worktree_root, "tests"))
+
+            self.assertFalse(
+                role_session_router.skip_python_scan_walk_root(
+                    main_root, main_root
+                )
             )
-        )
-        self.assertTrue(
-            role_session_router.skip_python_scan_walk_root(
-                main_root, os.path.join(main_root, "branches", "fix-issue-1")
+            self.assertTrue(
+                role_session_router.skip_python_scan_walk_root(
+                    main_root, os.path.join(main_root, "branches", "fix-issue-1")
+                )
             )
-        )
-        self.assertFalse(
-            role_session_router.skip_python_scan_walk_root(
-                worktree_root, worktree_root
+            self.assertFalse(
+                role_session_router.skip_python_scan_walk_root(
+                    worktree_root, worktree_root
+                )
             )
-        )
-        self.assertFalse(
-            role_session_router.skip_python_scan_walk_root(
-                worktree_root, os.path.join(worktree_root, "tests")
+            self.assertFalse(
+                role_session_router.skip_python_scan_walk_root(
+                    worktree_root, os.path.join(worktree_root, "tests")
+                )
             )
-        )
 
     def test_decorative_equals_banner_is_not_mid_merge(self):
         self.assertFalse(role_session_router.check_mid_merge())