diff --git a/pkg/browser/service_test.go b/pkg/browser/service_test.go
index 9565c61..c025b5d 100644
--- a/pkg/browser/service_test.go
+++ b/pkg/browser/service_test.go
@@ -3,10 +3,9 @@ package browser
 
 import (
 	"context"
-	"errors"
+	core "dappco.re/go/core"
 	"testing"
 
-	core "dappco.re/go/core"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -58,7 +57,7 @@ func TestTaskOpenURL_Good(t *testing.T) {
 }
 
 func TestTaskOpenURL_Bad_PlatformError(t *testing.T) {
-	mp := &mockPlatform{urlErr: errors.New("browser not found")}
+	mp := &mockPlatform{urlErr: core.NewError("browser not found")}
 	_, c := newTestBrowserService(t, mp)
 
 	r := c.Action("browser.openURL").Run(context.Background(), core.NewOptions(
@@ -79,7 +78,7 @@ func TestTaskOpenFile_Good(t *testing.T) {
 }
 
 func TestTaskOpenFile_Bad_PlatformError(t *testing.T) {
-	mp := &mockPlatform{fileErr: errors.New("file not found")}
+	mp := &mockPlatform{fileErr: core.NewError("file not found")}
 	_, c := newTestBrowserService(t, mp)
 
 	r := c.Action("browser.openFile").Run(context.Background(), core.NewOptions(
diff --git a/pkg/display/display_test.go b/pkg/display/display_test.go
index fe99925..6afd85b 100644
--- a/pkg/display/display_test.go
+++ b/pkg/display/display_test.go
@@ -3,7 +3,6 @@ package display
 import (
 	"context"
 	"os"
-	"path/filepath"
 	"testing"
 
 	core "dappco.re/go/core"
@@ -437,8 +436,8 @@ func TestWSEventManager_Good(t *testing.T) {
 func TestLoadConfig_Good(t *testing.T) {
 	// Create temp config file
 	dir := t.TempDir()
-	cfgPath := filepath.Join(dir, ".core", "gui", "config.yaml")
-	require.NoError(t, os.MkdirAll(filepath.Dir(cfgPath), 0o755))
+	cfgPath := core.JoinPath(dir, ".core", "gui", "config.yaml")
+	require.NoError(t, os.MkdirAll(core.PathDir(cfgPath), 0o755))
 	require.NoError(t, os.WriteFile(cfgPath, []byte(`
 window:
   default_width: 1280
@@ -460,7 +459,7 @@ menu:
 
 func TestLoadConfig_Bad_MissingFile(t *testing.T) {
 	s, _ := New()
-	s.loadConfigFrom(filepath.Join(t.TempDir(), "nonexistent.yaml"))
+	s.loadConfigFrom(core.JoinPath(t.TempDir(), "nonexistent.yaml"))
 
 	// Should not panic, configData stays at empty defaults
 	assert.Empty(t, s.configData["window"])
@@ -470,7 +469,7 @@ func TestLoadConfig_Bad_MissingFile(t *testing.T) {
 
 func TestHandleConfigTask_Persists_Good(t *testing.T) {
 	dir := t.TempDir()
-	cfgPath := filepath.Join(dir, "config.yaml")
+	cfgPath := core.JoinPath(dir, "config.yaml")
 
 	s, _ := New()
 	s.loadConfigFrom(cfgPath) // Creates empty config (file doesn't exist yet)
diff --git a/pkg/notification/service_test.go b/pkg/notification/service_test.go
index 1ee6e23..88f4ee6 100644
--- a/pkg/notification/service_test.go
+++ b/pkg/notification/service_test.go
@@ -3,10 +3,9 @@ package notification
 
 import (
 	"context"
-	"errors"
+	core "dappco.re/go/core"
 	"testing"
 
-	core "dappco.re/go/core"
 	"forge.lthn.ai/core/gui/pkg/dialog"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -86,7 +85,7 @@ func TestTaskSend_Good(t *testing.T) {
 
 func TestTaskSend_Fallback_Good(t *testing.T) {
 	// Platform fails -> falls back to dialog via IPC
-	mockNotify := &mockPlatform{sendErr: errors.New("no permission")}
+	mockNotify := &mockPlatform{sendErr: core.NewError("no permission")}
 	mockDlg := &mockDialogPlatform{}
 	c := core.New(
 		core.WithService(dialog.Register(mockDlg)),
@@ -135,7 +134,7 @@ func TestTaskRevokePermission_Good(t *testing.T) {
 
 func TestTaskRevokePermission_Bad(t *testing.T) {
 	mock, c := newTestService(t)
-	mock.revokeErr = errors.New("cannot revoke")
+	mock.revokeErr = core.NewError("cannot revoke")
 	r := c.Action("notification.revokePermission").Run(context.Background(), core.NewOptions())
 	assert.False(t, r.OK)
 }
@@ -250,7 +249,7 @@ func TestQueryPermission_Bad(t *testing.T) {
 
 func TestQueryPermission_Ugly(t *testing.T) {
 	// Platform returns error — QUERY returns OK=false (framework does not propagate Value for failed queries)
-	mock := &mockPlatform{permErr: errors.New("platform error")}
+	mock := &mockPlatform{permErr: core.NewError("platform error")}
 	c := core.New(
 		core.WithService(Register(mock)),
 		core.WithServiceLock(),
diff --git a/pkg/window/persistence_test.go b/pkg/window/persistence_test.go
index eed02de..d438240 100644
--- a/pkg/window/persistence_test.go
+++ b/pkg/window/persistence_test.go
@@ -3,7 +3,7 @@ package window
 
 import (
 	"os"
-	"path/filepath"
+	core "dappco.re/go/core"
 	"testing"
 	"time"
 
@@ -181,7 +181,7 @@ func TestStateManager_Persistence_Good(t *testing.T) {
 
 func TestStateManager_SetPath_Good(t *testing.T) {
 	dir := t.TempDir()
-	path := filepath.Join(dir, "custom", "window-state.json")
+	path := core.JoinPath(dir, "custom", "window-state.json")
 
 	sm := NewStateManagerWithDir(dir)
 	sm.SetPath(path)