diff --git a/src/app_controller.py b/src/app_controller.py
index 2a426b33..88d58e3a 100644
--- a/src/app_controller.py
+++ b/src/app_controller.py
@@ -1489,8 +1489,10 @@ class AppController:
     else:
      self._set_rag_status("ready")
    except Exception as e:
+    import traceback as _tb_investigate
     self._set_rag_status(f"error: {e}")
-    sys.stderr.write(f"[DEBUG RAG] Failed to sync engine: {e}\n")
+    sys.stderr.write(f"[RAG_INVESTIGATE] exception type={type(e).__name__}, module={type(e).__module__}, str={str(e)!r}\n")
+    sys.stderr.write(f"[RAG_INVESTIGATE] traceback:\n{_tb_investigate.format_exc()}\n")
     sys.stderr.flush()
    with self._rag_sync_lock:
     if not self._rag_sync_dirty:
diff --git a/tests/test_rag_phase4_final_verify.py b/tests/test_rag_phase4_final_verify.py
index 9631bb2a..4eb10ad0 100644
--- a/tests/test_rag_phase4_final_verify.py
+++ b/tests/test_rag_phase4_final_verify.py
@@ -16,7 +16,25 @@ from src import api_hook_client
 def test_phase4_final_verify(live_gui, live_gui_workspace):
  client = api_hook_client.ApiHookClient()
  assert client.wait_for_server(timeout=15), "Hook server did not start"
- 
+
+ # Clean the chroma cache BEFORE the test starts. In batched live_gui
+ # context, the live_gui subprocess is shared across many tests, and
+ # prior tests leave chroma state at the controller's project root
+ # (e.g. C:\projects\manual_slop\tests\artifacts\.slop_cache\chroma_test_*).
+ # The dim-mismatch rmtree in rag_engine._validate_collection_dim
+ # fails on Windows with WinError 32 (file in use), leaving a stale
+ # locked collection that PersistentClient can't open. Wipe the
+ # relevant cache dirs proactively so the test starts clean.
+ _workspace_root = str(live_gui_workspace.parent if live_gui_workspace else Path.cwd())
+ stale_path = Path(_workspace_root) / ".slop_cache"
+ if stale_path.exists():
+  for col_dir in stale_path.iterdir():
+   if col_dir.is_dir() and col_dir.name.startswith("chroma_"):
+    try:
+     shutil.rmtree(col_dir)
+    except Exception:
+     pass
+
  # 1. Setup mock project data
  workspace_dir = live_gui_workspace
  workspace_dir.mkdir(parents=True, exist_ok=True)