diff --git a/src/app_controller.py b/src/app_controller.py
index 7d1cee32..dc1e1f82 100644
--- a/src/app_controller.py
+++ b/src/app_controller.py
@@ -837,6 +837,14 @@ class AppController:
   # sub-track 4 lands GUI-side error display); the instance list is
   # the durable data plane for sub-track 4 to drain.
   self._startup_timeline_errors:        List[Tuple[str, ErrorInfo]] = []  # (op_name, error)
+  # --- GUI state-setter error state (Phase 6: Group 6.3) ---
+  # Each carries the first ErrorInfo when the corresponding setter/operation
+  # fails. The legacy wrapper stores the error here for sub-track 4 GUI to
+  # display; stderr write IS the visible-but-incomplete drain (full drain =
+  # GUI modal in sub-track 4).
+  self._inject_preview_error:           Optional[ErrorInfo] = None
+  self._mcp_config_parse_error:         Optional[ErrorInfo] = None
+  self._save_project_error:             Optional[ErrorInfo] = None
   # --- Shared background pool + proactive warmup (startup_speedup_20260606) ---
   self._io_pool = make_io_pool()
   _install_sigint_exit_handler(self)
@@ -1518,15 +1526,27 @@ class AppController:
     Updates the preview content based on the selected file and injection mode.
     [C: src/gui_2.py:App._render_text_viewer_window, tests/test_skeleton_injection.py:test_update_inject_preview_full, tests/test_skeleton_injection.py:test_update_inject_preview_skeleton, tests/test_skeleton_injection.py:test_update_inject_preview_truncation]
   """
+  result = self._update_inject_preview_result()
+  if result.ok:
+   self._inject_preview = result.data
+   self._inject_preview_error = None
+  else:
+   err = result.errors[0]
+   self._inject_preview_error = err
+   self._inject_preview = f"Error reading file: {err.message}"
+
+ def _update_inject_preview_result(self) -> "Result[str]":
+  """Read the file for the inject preview. Returns Result[str] (Phase 6 Group 6.3).
+  On failure: OSError/IOError/UnicodeDecodeError -> ErrorInfo(original=e).
+  Caller (`_update_inject_preview`) stores the error on
+  `self._inject_preview_error` for sub-track 4 GUI."""
   if not self._inject_file_path:
-   self._inject_preview = ""
-   return
+   return Result(data="")
   target_path = self._inject_file_path
   if not os.path.isabs(target_path):
    target_path = os.path.join(self.active_project_root, target_path)
   if not os.path.exists(target_path):
-   self._inject_preview = ""
-   return
+   return Result(data="")
   try:
    with open(target_path, "r", encoding="utf-8") as f:
     content = f.read()
@@ -1538,10 +1558,14 @@ class AppController:
    lines = preview.splitlines()
    if len(lines) > 500:
     preview = "\n".join(lines[:500]) + "\n... (truncated)"
-   self._inject_preview = preview
+   return Result(data=preview)
   except (OSError, IOError, UnicodeDecodeError) as e:
-   logging.getLogger(__name__).debug("inject preview file read failed: %s", e, extra={"source": "app_controller._update_inject_preview"})
-   self._inject_preview = f"Error reading file: {e}"
+   return Result(data="", errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message=str(e),
+    source="app_controller._update_inject_preview_result",
+    original=e,
+   )])
 
  @property
  def ai_status(self) -> str:
@@ -1684,11 +1708,30 @@ class AppController:
   return json.dumps(self.mcp_config.to_dict()) if self.mcp_config else "{}"
  @mcp_config_json.setter
  def mcp_config_json(self, value: str) -> None:
+  result = self._set_mcp_config_json_result(value)
+  if not result.ok:
+   err = result.errors[0]
+   self._mcp_config_parse_error = err
+   sys.stderr.write(f"mcp_config parse failed: {err.ui_message()}\n")
+   sys.stderr.flush()
+
+ def _set_mcp_config_json_result(self, value: str) -> "Result[None]":
+  """Parse and assign mcp_config from a JSON string. Returns Result[None] (Phase 6 Group 6.3).
+  On failure: json.JSONDecodeError/ValueError/TypeError/KeyError/AttributeError -> ErrorInfo(original=e).
+  Property setters cannot return values; this sibling helper carries the
+  structured result. Caller (`mcp_config_json` setter) stores the error
+  on `self._mcp_config_parse_error` for sub-track 4 GUI."""
   try:
    data = json.loads(value)
    self.mcp_config = models.MCPConfiguration.from_dict(data)
+   return OK
   except (json.JSONDecodeError, ValueError, TypeError, KeyError, AttributeError) as e:
-   logging.getLogger(__name__).debug("mcp config parse failed: %s", e, extra={"source": "app_controller.mcp_config_json"})
+   return Result(data=None, errors=[ErrorInfo(
+    kind=ErrorKind.INVALID_INPUT,
+    message=str(e),
+    source="app_controller._set_mcp_config_json_result",
+    original=e,
+   )])
 
  @property
  def ui_file_paths(self) -> list[str]:
@@ -3125,13 +3168,32 @@ class AppController:
   """
     [C: src/gui_2.py:App.delete_context_preset, src/gui_2.py:App.save_context_preset]
   """
-  if self.active_project_path:
-   try:
-    cleaned = project_manager.clean_nones(self.project)
-    project_manager.save_project(cleaned, self.active_project_path)
-   except (OSError, IOError, ValueError, TypeError, KeyError, AttributeError) as e:
-    logging.getLogger(__name__).debug("save_project failed: %s", e, extra={"source": "app_controller._save_active_project"})
-    self.ai_status = f"save error: {e}"
+  result = self._save_active_project_result()
+  if not result.ok:
+   err = result.errors[0]
+   self._save_project_error = err
+   self.ai_status = f"save error: {err.message}"
+   sys.stderr.write(f"save_project failed: {err.ui_message()}\n")
+   sys.stderr.flush()
+
+ def _save_active_project_result(self) -> "Result[None]":
+  """Save the active project to its current path. Returns Result[None] (Phase 6 Group 6.3).
+  On failure: OSError/IOError/ValueError/TypeError/KeyError/AttributeError -> ErrorInfo(original=e).
+  Caller (`_save_active_project`) stores the error on
+  `self._save_project_error` for sub-track 4 GUI and updates self.ai_status."""
+  if not self.active_project_path:
+   return OK
+  try:
+   cleaned = project_manager.clean_nones(self.project)
+   project_manager.save_project(cleaned, self.active_project_path)
+   return OK
+  except (OSError, IOError, ValueError, TypeError, KeyError, AttributeError) as e:
+   return Result(data=None, errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message=str(e),
+    source="app_controller._save_active_project_result",
+    original=e,
+   )])
 
  #endregion: Project
 
diff --git a/tests/test_app_controller_result.py b/tests/test_app_controller_result.py
index 696f9187..efbfb84f 100644
--- a/tests/test_app_controller_result.py
+++ b/tests/test_app_controller_result.py
@@ -278,3 +278,130 @@ def test_warmup_complete_timeline_records_error_on_stderr_failure():
  assert len(warmup_errors) >= 1
  assert isinstance(warmup_errors[0][1], ErrorInfo)
  assert "stderr closed" in warmup_errors[0][1].message
+
+
+# --- Phase 6: Group 6.3 (GUI state setters / property setters) ---
+
+def test_update_inject_preview_result_returns_empty_when_no_path():
+ """
+ _update_inject_preview_result returns Result(data="") when no file path is set.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ ctrl._inject_file_path = None
+ result = ctrl._update_inject_preview_result()
+ assert isinstance(result, Result)
+ assert result.ok is True
+ assert result.data == ""
+
+
+def test_update_inject_preview_result_returns_error_on_read_failure():
+ """
+ _update_inject_preview_result converts OSError to ErrorInfo(original=e) and
+ returns Result[data=""]. The legacy wrapper stores the error on
+ self._inject_preview_error and sets self._inject_preview to a user-facing
+ message.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ ctrl._inject_file_path = "/nonexistent/path/that/does/not/exist.py"
+ result = ctrl._update_inject_preview_result()
+ assert isinstance(result, Result)
+ # When file doesn't exist, returns empty data (no error)
+ assert result.ok is True
+ assert result.data == ""
+
+
+def test_update_inject_preview_stores_error_on_read_failure():
+ """
+ When file read fails (e.g. permission), the legacy wrapper stores the
+ error on self._inject_preview_error and shows a user-facing message.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ ctrl._inject_file_path = "/tmp/test_inject.py"
+ # Force the file-existence check to pass and the open call to fail.
+ with patch("src.app_controller.os.path.exists", return_value=True):
+  with patch("builtins.open", side_effect=PermissionError("denied")):
+   ctrl._update_inject_preview()
+ assert ctrl._inject_preview_error is not None
+ assert isinstance(ctrl._inject_preview_error, ErrorInfo)
+ assert "denied" in ctrl._inject_preview_error.message
+ assert ctrl._inject_preview.startswith("Error reading file:")
+
+
+def test_set_mcp_config_json_result_returns_ok_on_valid_json():
+ """
+ _set_mcp_config_json_result returns Result[None] on valid JSON.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ result = ctrl._set_mcp_config_json_result('{"servers": {}}')
+ assert isinstance(result, Result)
+ assert result.ok is True
+ assert result.errors == []
+
+
+def test_set_mcp_config_json_result_returns_error_on_invalid_json():
+ """
+ _set_mcp_config_json_result converts JSONDecodeError to ErrorInfo(original=e).
+ The legacy setter stores the error on self._mcp_config_parse_error.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ result = ctrl._set_mcp_config_json_result("not valid json")
+ assert isinstance(result, Result)
+ assert result.ok is False
+ assert len(result.errors) == 1
+ assert isinstance(result.errors[0], ErrorInfo)
+
+
+def test_mcp_config_json_setter_stores_error_on_parse_failure():
+ """
+ The property setter stores the first error on self._mcp_config_parse_error
+ when parsing fails.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ ctrl.mcp_config_json = "not valid json"
+ assert ctrl._mcp_config_parse_error is not None
+ assert isinstance(ctrl._mcp_config_parse_error, ErrorInfo)
+
+
+def test_mcp_config_json_setter_no_error_on_valid_json():
+ """
+ On valid JSON, _mcp_config_parse_error stays as None.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ ctrl.mcp_config_json = '{"servers": {}}'
+ assert ctrl._mcp_config_parse_error is None
+
+
+def test_save_active_project_result_returns_ok_when_no_active_path():
+ """
+ _save_active_project_result returns OK when no active_project_path is set.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ ctrl.active_project_path = None
+ result = ctrl._save_active_project_result()
+ assert isinstance(result, Result)
+ assert result.ok is True
+ assert result.errors == []
+
+
+def test_save_active_project_stores_error_on_save_failure():
+ """
+ When save_project raises, the legacy wrapper stores the error on
+ self._save_project_error and updates self.ai_status.
+ """
+ from src.app_controller import AppController
+ ctrl = AppController()
+ ctrl.active_project_path = "/tmp/test_save.toml"
+ with patch("src.app_controller.project_manager.save_project", side_effect=PermissionError("denied")):
+  ctrl._save_active_project()
+ assert ctrl._save_project_error is not None
+ assert isinstance(ctrl._save_project_error, ErrorInfo)
+ assert "denied" in ctrl._save_project_error.message
+ assert "save error" in ctrl.ai_status