diff --git a/src/app_controller.py b/src/app_controller.py
index f21db854..97c07e7d 100644
--- a/src/app_controller.py
+++ b/src/app_controller.py
@@ -272,6 +272,8 @@ def _api_generate(controller: 'AppController', req: GenerateRequest) -> dict[str
  """
  if not req.prompt.strip():
   raise HTTPException(status_code=400, detail="Prompt cannot be empty")
+ if controller.is_project_stale():
+  raise HTTPException(status_code=409, detail="Project switch in progress; AI ops are disabled until it completes.")
  with controller._send_thread_lock:
   start_time = time.time()
   try:
@@ -800,6 +802,7 @@ class AppController:
   self._pending_dialog_lock: threading.Lock = threading.Lock()
   self._api_event_queue_lock: threading.Lock = threading.Lock()
   self._rag_engine_lock: threading.Lock = threading.Lock()
+  self._project_switch_lock: threading.Lock = threading.Lock()
 
   # --- Internal State ---
   self._ai_status: str = "idle"
@@ -842,6 +845,9 @@ class AppController:
   self._pending_mma_approvals: List[Dict[str, Any]] = []
   self._mma_approval_open: bool = False
   self._mma_approval_edit_mode: bool = False
+  self._project_switch_in_progress: bool = False
+  self._project_switch_pending_path: Optional[str] = None
+  self._project_switch_thread: Optional[threading.Thread] = None
   self._mma_approval_payload: str = ""
   self._pending_mma_spawns: List[Dict[str, Any]] = []
   self._mma_spawn_open: bool = False
@@ -2683,31 +2689,78 @@ class AppController:
   file_items_as_dicts = [{"path": f.path if hasattr(f, "path") else str(f)} for f in self.files]
   mcp_client.configure(file_items_as_dicts, [str(project_root)])
 
+ def is_project_stale(self) -> bool:
+  """True when a project switch is queued or running; UI should tint
+  to signal the controller state lags the user's last click."""
+  with self._project_switch_lock:
+   if self._project_switch_in_progress:
+    return True
+   pending = self._project_switch_pending_path
+  if pending and pending != self.active_project_path:
+   return True
+  return False
+
  def _switch_project(self, path: str) -> None:
   """
     [C: src/gui_2.py:App._render_projects_panel]
+  
+  Non-blocking: returns immediately, marks the controller as stale,
+  and runs the actual save/load work in a background thread so the
+  render loop keeps drawing and lightweight UI interactions (scrolling,
+  selecting tabs) remain responsive.
   """
-  if path == self.active_project_path:
+  if path == self.active_project_path and not self.is_project_stale():
    return
   if not Path(path).exists():
    self.ai_status = f"project file not found: {path}"
    return
-  self._flush_to_project()
+  with self._project_switch_lock:
+   if self._project_switch_in_progress:
+    if self._project_switch_pending_path == path:
+     return
+    self._project_switch_pending_path = path
+    self.ai_status = f"switch queued: {Path(path).stem} (waiting on {Path(self._project_switch_pending_path or '').stem})"
+    return
+   self._project_switch_in_progress = True
+   self._project_switch_pending_path = path
+  self.ai_status = f"switching to: {Path(path).stem} (stale ui - ops disabled)"
+  self._project_switch_thread = threading.Thread(
+   target=self._do_project_switch,
+   args=(path,),
+   daemon=True,
+  )
+  self._project_switch_thread.start()
+
+ def _do_project_switch(self, path: str) -> None:
   try:
-   self.project = project_manager.load_project(path)
-   self.active_project_path = path
-   new_root = Path(path).parent
-   self.preset_manager = presets.PresetManager(new_root)
-   self.tool_preset_manager = tool_presets.ToolPresetManager(new_root)
-   from src.personas import PersonaManager
-   self.persona_manager = PersonaManager(new_root)
-  except Exception as e:
-   self.ai_status = f"failed to load project: {e}"
-   return
-  self._refresh_from_project()
-  file_items_as_dicts = [{"path": f.path if hasattr(f, "path") else str(f)} for f in self.files]
-  mcp_client.configure(file_items_as_dicts, [str(new_root)])
-  self.ai_status = f"switched to: {Path(path).stem}"
+   self._flush_to_project()
+   try:
+    new_project = project_manager.load_project(path)
+   except Exception as e:
+    self.ai_status = f"failed to load project: {e}"
+    return
+   try:
+    self.project = new_project
+    self.active_project_path = path
+    new_root = Path(path).parent
+    self.preset_manager = presets.PresetManager(new_root)
+    self.tool_preset_manager = tool_presets.ToolPresetManager(new_root)
+    from src.personas import PersonaManager
+    self.persona_manager = PersonaManager(new_root)
+   except Exception as e:
+    self.ai_status = f"failed to init managers: {e}"
+    return
+   self._refresh_from_project()
+   file_items_as_dicts = [{"path": f.path if hasattr(f, "path") else str(f)} for f in self.files]
+   mcp_client.configure(file_items_as_dicts, [str(new_root)])
+   self.ai_status = f"switched to: {Path(path).stem}"
+  finally:
+   with self._project_switch_lock:
+    pending = self._project_switch_pending_path
+    self._project_switch_in_progress = False
+    self._project_switch_pending_path = None
+   if pending and pending != self.active_project_path and Path(pending).exists():
+    self._switch_project(pending)
 
  def _refresh_from_project(self) -> None:
   # Deserialize FileItems in files.paths
@@ -3279,6 +3332,9 @@ class AppController:
      Logic for the 'MD Only' action.
     [C: src/gui_2.py:App._render_message_panel]
   """
+  if self.is_project_stale():
+   self.ai_status = "project switch in progress; MD generation disabled"
+   return
 
   def worker():
    """
@@ -3324,6 +3380,9 @@ class AppController:
      Logic for the 'Gen + Send' action.
     [C: src/gui_2.py:App._render_message_panel, src/gui_2.py:App._render_synthesis_panel, src/gui_2.py:App._render_takes_panel, tests/test_gui_events_v2.py:test_handle_generate_send_pushes_event, tests/test_symbol_parsing.py:test_handle_generate_send_appends_definitions, tests/test_symbol_parsing.py:test_handle_generate_send_no_symbols]
   """
+  if self.is_project_stale():
+   self.ai_status = "project switch in progress; AI ops disabled"
+   return
 
   def worker():
    """
diff --git a/src/gui_2.py b/src/gui_2.py
index 19e1800e..917137a6 100644
--- a/src/gui_2.py
+++ b/src/gui_2.py
@@ -1258,6 +1258,7 @@ if __name__ == "__main__":
 
 def render_main_interface(app: App) -> None:
  render_error_tint(app)
+ render_project_stale_tint(app)
  app.perf_monitor.start_frame()
  app._autofocus_response_tab = app.controller._autofocus_response_tab
 
@@ -4620,6 +4621,27 @@ def render_error_tint(app: App) -> None:
    imgui.text_wrapped(HotReloader.last_error or "Unknown error")
 
 
+def render_project_stale_tint(app: App) -> None:
+ """Renders a yellow/amber tint overlay when the project is mid-switch.
+
+ UI remains responsive (scroll, tab browse) but AI / MD ops are gated
+ on the controller's is_project_stale() returning False.
+ """
+ if not app.controller.is_project_stale(): return
+ draw_list = imgui.get_background_draw_list()
+ display_size = imgui.get_io().display_size
+ tint_col = imgui.get_color_u32(imgui.ImVec4(1.0, 0.85, 0.2, 0.15))
+ draw_list.add_rect_filled(imgui.ImVec2(0, 0), display_size, tint_col)
+ pending = app.controller._project_switch_pending_path or app.controller.active_project_path
+ imgui.set_next_window_pos(imgui.ImVec2(10, 50))
+ with imscope.window("Project Stale", None,
+  imgui.WindowFlags_.always_auto_resize | imgui.WindowFlags_.no_title_bar |
+  imgui.WindowFlags_.no_resize | imgui.WindowFlags_.no_move):
+  imgui.text_colored(imgui.ImVec4(1.0, 0.85, 0.2, 1.0), "PROJECT SWITCHING")
+  imgui.text_wrapped(f"Loading: {Path(pending).stem if pending else '?'}")
+  imgui.text_wrapped("UI is read-only until the switch completes. You can still browse tabs.")
+
+
 def render_heavy_text(app: App, label: str, content: str, id_suffix: str = "") -> None:
  if imgui.button(f"[+]##{label}{id_suffix}"):
   app.text_viewer_type = 'markdown' if label in ('message', 'text', 'content', 'system') else 'json' if label in ('tool_calls', 'data') else 'powershell' if label == 'script' else 'text'
diff --git a/tests/test_project_switch_persona_preset.py b/tests/test_project_switch_persona_preset.py
index 1c042806..7a9711f4 100644
--- a/tests/test_project_switch_persona_preset.py
+++ b/tests/test_project_switch_persona_preset.py
@@ -1,5 +1,6 @@
 import os
 import json
+import time
 import pytest
 import tempfile
 import shutil
@@ -11,6 +12,19 @@ from src import presets, tool_presets
 from src import project_manager
 
 
+def _wait_for_switch(ctrl, timeout: float = 2.0) -> None:
+    """Polls until any background project switch thread completes."""
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        with ctrl._project_switch_lock:
+            in_progress = ctrl._project_switch_in_progress
+        thread = ctrl._project_switch_thread
+        if not in_progress and (thread is None or not thread.is_alive()):
+            return
+        time.sleep(0.02)
+    raise AssertionError("Project switch did not complete within timeout")
+
+
 def _setup_two_projects(tmp_path):
     project_a_dir = tmp_path / "project_a"
     project_a_dir.mkdir()
@@ -57,6 +71,7 @@ def test_switch_project_resets_invalid_persona(tmp_path, monkeypatch):
     ctrl.ui_active_persona = "PersonaA"
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert "PersonaA" not in ctrl.personas
     assert "PersonaB" in ctrl.personas
@@ -80,6 +95,7 @@ def test_switch_project_resets_invalid_preset(tmp_path, monkeypatch):
     assert ctrl.ui_project_preset_name == "PresetA"
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert "PresetA" not in ctrl.presets
     assert "PresetB" in ctrl.presets
@@ -111,6 +127,7 @@ def test_switch_project_resets_invalid_tool_preset(tmp_path, monkeypatch):
     assert "ToolA" in ctrl.tool_presets
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert "ToolA" not in ctrl.tool_presets
     assert "ToolB" in ctrl.tool_presets
@@ -135,6 +152,7 @@ def test_switch_project_preserves_global_preset(tmp_path, monkeypatch):
     original_global = ctrl.ui_global_preset_name
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert ctrl.ui_global_preset_name == original_global
 
@@ -213,8 +231,65 @@ paths = [
     assert any("gte_hello" in f.path for f in ctrl.context_files)
 
     ctrl._switch_project(str(project_b_path))
+    _wait_for_switch(ctrl)
 
     assert len(ctrl.context_files) == 2
     assert all("gencpp" in f.path for f in ctrl.context_files)
     assert not any("forth" in f.path for f in ctrl.context_files)
     assert not any("gte_hello" in f.path for f in ctrl.context_files)
+
+
+def test_switch_project_non_blocking(tmp_path, monkeypatch):
+    """The switch returns immediately; the heavy work runs in a background
+    thread. UI calls see is_project_stale() == True while it's running."""
+    project_a_path, project_b_path = _setup_two_projects(tmp_path)
+
+    ctrl = AppController()
+    monkeypatch.setattr(ctrl, "_rebuild_rag_index", lambda: None)
+    monkeypatch.setattr(ctrl, "_flush_to_project", lambda: None)
+
+    ctrl.active_project_path = str(project_a_path)
+    ctrl.project = project_manager.load_project(str(project_a_path))
+    ctrl.preset_manager = presets.PresetManager(Path(project_a_path).parent)
+    ctrl.tool_preset_manager = tool_presets.ToolPresetManager(Path(project_a_path).parent)
+    ctrl.persona_manager = PersonaManager(Path(project_a_path).parent)
+    ctrl._refresh_from_project()
+
+    started = time.time()
+    ctrl._switch_project(str(project_b_path))
+    elapsed = time.time() - started
+
+    assert elapsed < 0.2, f"_switch_project blocked the caller for {elapsed:.2f}s"
+    assert ctrl.is_project_stale()
+    _wait_for_switch(ctrl)
+    assert not ctrl.is_project_stale()
+    assert ctrl.active_project_path == str(project_b_path)
+
+
+def test_api_generate_blocked_while_stale(tmp_path, monkeypatch):
+    """_api_generate returns 409 when the project switch is in progress."""
+    project_a_path, project_b_path = _setup_two_projects(tmp_path)
+
+    ctrl = AppController()
+    monkeypatch.setattr(ctrl, "_rebuild_rag_index", lambda: None)
+    monkeypatch.setattr(ctrl, "_flush_to_project", lambda: None)
+
+    ctrl.active_project_path = str(project_a_path)
+    ctrl.project = project_manager.load_project(str(project_a_path))
+    ctrl.preset_manager = presets.PresetManager(Path(project_a_path).parent)
+    ctrl.tool_preset_manager = tool_presets.ToolPresetManager(Path(project_a_path).parent)
+    ctrl.persona_manager = PersonaManager(Path(project_a_path).parent)
+    ctrl._refresh_from_project()
+
+    ctrl._switch_project(str(project_b_path))
+    assert ctrl.is_project_stale()
+    from fastapi import HTTPException
+    from src.app_controller import _api_generate
+    from src.models import GenerateRequest
+    req = GenerateRequest(prompt="hello")
+    with pytest.raises(HTTPException) as exc_info:
+        _api_generate(ctrl, req)
+    assert exc_info.value.status_code == 409
+
+    _wait_for_switch(ctrl)
+    assert not ctrl.is_project_stale()