diff --git a/pkg/contextmenu/messages.go b/pkg/contextmenu/messages.go
index 32e7037..f7a78f3 100644
--- a/pkg/contextmenu/messages.go
+++ b/pkg/contextmenu/messages.go
@@ -1,8 +1,8 @@
 package contextmenu
 
-import "errors"
+import core "dappco.re/go/core"
 
-var ErrorMenuNotFound = errors.New("contextmenu: menu not found")
+var ErrorMenuNotFound = core.E("contextmenu", "menu not found", nil)
 
 // QueryGet returns a named context menu definition. Result: *ContextMenuDef (nil if not found)
 type QueryGet struct {
diff --git a/pkg/display/display.go b/pkg/display/display.go
index e6af355..1a71337 100644
--- a/pkg/display/display.go
+++ b/pkg/display/display.go
@@ -2,9 +2,6 @@ package display
 
 import (
 	"context"
-	"encoding/json"
-	"os"
-	"path/filepath"
 	"runtime"
 
 	"forge.lthn.ai/core/config"
@@ -331,9 +328,12 @@ func (s *Service) handleWSMessage(msg WSMessage) core.Result {
 		return c.QUERY(dock.QueryVisible{})
 	case "contextmenu:add":
 		name, _ := msg.Data["name"].(string)
-		menuJSON, _ := json.Marshal(msg.Data["menu"])
+		marshalResult := core.JSONMarshal(msg.Data["menu"])
 		var menuDef contextmenu.ContextMenuDef
-		_ = json.Unmarshal(menuJSON, &menuDef)
+		if marshalResult.OK {
+			menuJSON, _ := marshalResult.Value.([]byte)
+			core.JSONUnmarshal(menuJSON, &menuDef)
+		}
 		return c.Action("contextmenu.add").Run(ctx, core.NewOptions(
 			core.Option{Key: "task", Value: contextmenu.TaskAdd{Name: name, Menu: menuDef}},
 		))
@@ -580,11 +580,11 @@ func (s *Service) handleTrayAction(actionID string) {
 }
 
 func guiConfigPath() string {
-	home, err := os.UserHomeDir()
-	if err != nil {
-		return filepath.Join(".core", "gui", "config.yaml")
+	home := core.Env("HOME")
+	if home == "" {
+		return core.JoinPath(".core", "gui", "config.yaml")
 	}
-	return filepath.Join(home, ".core", "gui", "config.yaml")
+	return core.JoinPath(home, ".core", "gui", "config.yaml")
 }
 
 func (s *Service) loadConfig() {
diff --git a/pkg/display/events.go b/pkg/display/events.go
index 0dd2002..0498bb4 100644
--- a/pkg/display/events.go
+++ b/pkg/display/events.go
@@ -1,12 +1,12 @@
 package display
 
 import (
-	"encoding/json"
 	"net/http"
 	"strconv"
 	"sync"
 	"time"
 
+	core "dappco.re/go/core"
 	"forge.lthn.ai/core/gui/pkg/window"
 	"github.com/gorilla/websocket"
 )
@@ -134,10 +134,11 @@ func (em *WSEventManager) sendEvent(conn *websocket.Conn, event Event) {
 		return
 	}
 
-	data, err := json.Marshal(event)
-	if err != nil {
+	marshalResult := core.JSONMarshal(event)
+	if !marshalResult.OK {
 		return
 	}
+	data, _ := marshalResult.Value.([]byte)
 
 	conn.SetWriteDeadline(time.Now().Add(10 * time.Second))
 	if err := conn.WriteMessage(websocket.TextMessage, data); err != nil {
@@ -178,7 +179,7 @@ func (em *WSEventManager) handleMessages(conn *websocket.Conn) {
 			EventTypes []EventType `json:"eventTypes,omitempty"`
 		}
 
-		if err := json.Unmarshal(message, &msg); err != nil {
+		if unmarshalResult := core.JSONUnmarshal(message, &msg); !unmarshalResult.OK {
 			continue
 		}
 
@@ -224,8 +225,10 @@ func (em *WSEventManager) subscribe(conn *websocket.Conn, id string, eventTypes
 		"id":         id,
 		"eventTypes": eventTypes,
 	}
-	data, _ := json.Marshal(response)
-	conn.WriteMessage(websocket.TextMessage, data)
+	if marshalResult := core.JSONMarshal(response); marshalResult.OK {
+		responseData, _ := marshalResult.Value.([]byte)
+		conn.WriteMessage(websocket.TextMessage, responseData)
+	}
 }
 
 // unsubscribe removes a subscription for a client.
@@ -247,8 +250,10 @@ func (em *WSEventManager) unsubscribe(conn *websocket.Conn, id string) {
 		"type": "unsubscribed",
 		"id":   id,
 	}
-	data, _ := json.Marshal(response)
-	conn.WriteMessage(websocket.TextMessage, data)
+	if marshalResult := core.JSONMarshal(response); marshalResult.OK {
+		responseData, _ := marshalResult.Value.([]byte)
+		conn.WriteMessage(websocket.TextMessage, responseData)
+	}
 }
 
 // listSubscriptions sends a list of active subscriptions to a client.
@@ -272,8 +277,10 @@ func (em *WSEventManager) listSubscriptions(conn *websocket.Conn) {
 		"type":          "subscriptions",
 		"subscriptions": subs,
 	}
-	data, _ := json.Marshal(response)
-	conn.WriteMessage(websocket.TextMessage, data)
+	if marshalResult := core.JSONMarshal(response); marshalResult.OK {
+		responseData, _ := marshalResult.Value.([]byte)
+		conn.WriteMessage(websocket.TextMessage, responseData)
+	}
 }
 
 // removeClient removes a client and its subscriptions.
diff --git a/pkg/keybinding/messages.go b/pkg/keybinding/messages.go
index 4d82713..4590604 100644
--- a/pkg/keybinding/messages.go
+++ b/pkg/keybinding/messages.go
@@ -1,9 +1,9 @@
 package keybinding
 
-import "errors"
+import core "dappco.re/go/core"
 
-var ErrorAlreadyRegistered = errors.New("keybinding: accelerator already registered")
-var ErrorNotRegistered = errors.New("keybinding: accelerator not registered")
+var ErrorAlreadyRegistered = core.E("keybinding", "accelerator already registered", nil)
+var ErrorNotRegistered = core.E("keybinding", "accelerator not registered", nil)
 
 // BindingInfo describes a registered global key binding.
 type BindingInfo struct {
diff --git a/pkg/mcp/tools_contextmenu.go b/pkg/mcp/tools_contextmenu.go
index 9c8baf2..3413ede 100644
--- a/pkg/mcp/tools_contextmenu.go
+++ b/pkg/mcp/tools_contextmenu.go
@@ -3,7 +3,6 @@ package mcp
 
 import (
 	"context"
-	"encoding/json"
 
 	core "dappco.re/go/core"
 	coreerr "dappco.re/go/core/log"
@@ -26,13 +25,15 @@ type ContextMenuAddOutput struct {
 
 func (s *Subsystem) contextMenuAdd(_ context.Context, _ *mcp.CallToolRequest, input ContextMenuAddInput) (*mcp.CallToolResult, ContextMenuAddOutput, error) {
 	// Convert map[string]any to ContextMenuDef via JSON round-trip
-	menuJSON, err := json.Marshal(input.Menu)
-	if err != nil {
-		return nil, ContextMenuAddOutput{}, coreerr.E("mcp.contextMenuAdd", "failed to marshal menu definition", err)
+	marshalResult := core.JSONMarshal(input.Menu)
+	if !marshalResult.OK {
+		return nil, ContextMenuAddOutput{}, coreerr.E("mcp.contextMenuAdd", "failed to marshal menu definition", marshalResult.Value.(error))
 	}
+	menuJSON := marshalResult.Value.([]byte)
 	var menuDef contextmenu.ContextMenuDef
-	if err := json.Unmarshal(menuJSON, &menuDef); err != nil {
-		return nil, ContextMenuAddOutput{}, coreerr.E("mcp.contextMenuAdd", "failed to unmarshal menu definition", err)
+	unmarshalResult := core.JSONUnmarshal(menuJSON, &menuDef)
+	if !unmarshalResult.OK {
+		return nil, ContextMenuAddOutput{}, coreerr.E("mcp.contextMenuAdd", "failed to unmarshal menu definition", unmarshalResult.Value.(error))
 	}
 	r := s.core.Action("contextmenu.add").Run(context.Background(), core.NewOptions(
 		core.Option{Key: "task", Value: contextmenu.TaskAdd{Name: input.Name, Menu: menuDef}},
@@ -93,13 +94,15 @@ func (s *Subsystem) contextMenuGet(_ context.Context, _ *mcp.CallToolRequest, in
 		return nil, ContextMenuGetOutput{}, nil
 	}
 	// Convert to map[string]any via JSON round-trip to avoid cyclic type in schema
-	menuJSON, err := json.Marshal(menu)
-	if err != nil {
-		return nil, ContextMenuGetOutput{}, coreerr.E("mcp.contextMenuGet", "failed to marshal context menu", err)
+	marshalResult := core.JSONMarshal(menu)
+	if !marshalResult.OK {
+		return nil, ContextMenuGetOutput{}, coreerr.E("mcp.contextMenuGet", "failed to marshal context menu", marshalResult.Value.(error))
 	}
+	menuJSON := marshalResult.Value.([]byte)
 	var menuMap map[string]any
-	if err := json.Unmarshal(menuJSON, &menuMap); err != nil {
-		return nil, ContextMenuGetOutput{}, coreerr.E("mcp.contextMenuGet", "failed to unmarshal context menu", err)
+	unmarshalResult := core.JSONUnmarshal(menuJSON, &menuMap)
+	if !unmarshalResult.OK {
+		return nil, ContextMenuGetOutput{}, coreerr.E("mcp.contextMenuGet", "failed to unmarshal context menu", unmarshalResult.Value.(error))
 	}
 	return nil, ContextMenuGetOutput{Menu: menuMap}, nil
 }
@@ -124,13 +127,15 @@ func (s *Subsystem) contextMenuList(_ context.Context, _ *mcp.CallToolRequest, _
 		return nil, ContextMenuListOutput{}, coreerr.E("mcp.contextMenuList", "unexpected result type", nil)
 	}
 	// Convert to map[string]any via JSON round-trip to avoid cyclic type in schema
-	menusJSON, err := json.Marshal(menus)
-	if err != nil {
-		return nil, ContextMenuListOutput{}, coreerr.E("mcp.contextMenuList", "failed to marshal context menus", err)
+	marshalResult := core.JSONMarshal(menus)
+	if !marshalResult.OK {
+		return nil, ContextMenuListOutput{}, coreerr.E("mcp.contextMenuList", "failed to marshal context menus", marshalResult.Value.(error))
 	}
+	menusJSON := marshalResult.Value.([]byte)
 	var menusMap map[string]any
-	if err := json.Unmarshal(menusJSON, &menusMap); err != nil {
-		return nil, ContextMenuListOutput{}, coreerr.E("mcp.contextMenuList", "failed to unmarshal context menus", err)
+	unmarshalResult := core.JSONUnmarshal(menusJSON, &menusMap)
+	if !unmarshalResult.OK {
+		return nil, ContextMenuListOutput{}, coreerr.E("mcp.contextMenuList", "failed to unmarshal context menus", unmarshalResult.Value.(error))
 	}
 	return nil, ContextMenuListOutput{Menus: menusMap}, nil
 }
diff --git a/pkg/window/layout.go b/pkg/window/layout.go
index 9bdfc2b..373f4e8 100644
--- a/pkg/window/layout.go
+++ b/pkg/window/layout.go
@@ -2,12 +2,10 @@
 package window
 
 import (
-	"encoding/json"
-	"os"
-	"path/filepath"
 	"sync"
 	"time"
 
+	core "dappco.re/go/core"
 	coreio "dappco.re/go/core/io"
 	coreerr "dappco.re/go/core/log"
 )
@@ -40,9 +38,8 @@ func NewLayoutManager() *LayoutManager {
 	lm := &LayoutManager{
 		layouts: make(map[string]Layout),
 	}
-	configDir, err := os.UserConfigDir()
-	if err == nil {
-		lm.configDir = filepath.Join(configDir, "Core")
+	if configDir := core.Env("DIR_CONFIG"); configDir != "" {
+		lm.configDir = core.JoinPath(configDir, "Core")
 	}
 	lm.load()
 	return lm
@@ -60,7 +57,7 @@ func NewLayoutManagerWithDir(configDir string) *LayoutManager {
 }
 
 func (lm *LayoutManager) filePath() string {
-	return filepath.Join(lm.configDir, "layouts.json")
+	return core.JoinPath(lm.configDir, "layouts.json")
 }
 
 func (lm *LayoutManager) load() {
@@ -73,7 +70,7 @@ func (lm *LayoutManager) load() {
 	}
 	lm.mu.Lock()
 	defer lm.mu.Unlock()
-	_ = json.Unmarshal([]byte(content), &lm.layouts)
+	_ = core.JSONUnmarshalString(content, &lm.layouts)
 }
 
 func (lm *LayoutManager) save() {
@@ -81,11 +78,12 @@ func (lm *LayoutManager) save() {
 		return
 	}
 	lm.mu.RLock()
-	data, err := json.MarshalIndent(lm.layouts, "", "  ")
+	result := core.JSONMarshal(lm.layouts)
 	lm.mu.RUnlock()
-	if err != nil {
+	if !result.OK {
 		return
 	}
+	data := result.Value.([]byte)
 	_ = coreio.Local.EnsureDir(lm.configDir)
 	_ = coreio.Local.Write(lm.filePath(), string(data))
 }
diff --git a/pkg/window/state.go b/pkg/window/state.go
index 2ce339b..358ada5 100644
--- a/pkg/window/state.go
+++ b/pkg/window/state.go
@@ -2,12 +2,10 @@
 package window
 
 import (
-	"encoding/json"
-	"os"
-	"path/filepath"
 	"sync"
 	"time"
 
+	core "dappco.re/go/core"
 	coreio "dappco.re/go/core/io"
 )
 
@@ -38,9 +36,8 @@ func NewStateManager() *StateManager {
 	sm := &StateManager{
 		states: make(map[string]WindowState),
 	}
-	configDir, err := os.UserConfigDir()
-	if err == nil {
-		sm.configDir = filepath.Join(configDir, "Core")
+	if configDir := core.Env("DIR_CONFIG"); configDir != "" {
+		sm.configDir = core.JoinPath(configDir, "Core")
 	}
 	sm.load()
 	return sm
@@ -61,12 +58,12 @@ func (sm *StateManager) filePath() string {
 	if sm.statePath != "" {
 		return sm.statePath
 	}
-	return filepath.Join(sm.configDir, "window_state.json")
+	return core.JoinPath(sm.configDir, "window_state.json")
 }
 
 func (sm *StateManager) dataDir() string {
 	if sm.statePath != "" {
-		return filepath.Dir(sm.statePath)
+		return core.PathDir(sm.statePath)
 	}
 	return sm.configDir
 }
@@ -96,7 +93,7 @@ func (sm *StateManager) load() {
 	}
 	sm.mu.Lock()
 	defer sm.mu.Unlock()
-	_ = json.Unmarshal([]byte(content), &sm.states)
+	_ = core.JSONUnmarshalString(content, &sm.states)
 }
 
 func (sm *StateManager) save() {
@@ -104,11 +101,12 @@ func (sm *StateManager) save() {
 		return
 	}
 	sm.mu.RLock()
-	data, err := json.MarshalIndent(sm.states, "", "  ")
+	result := core.JSONMarshal(sm.states)
 	sm.mu.RUnlock()
-	if err != nil {
+	if !result.OK {
 		return
 	}
+	data := result.Value.([]byte)
 	if dir := sm.dataDir(); dir != "" {
 		_ = coreio.Local.EnsureDir(dir)
 	}