diff --git a/local-tool/frontend/src/hooks/use-flash-progress.ts b/local-tool/frontend/src/hooks/use-flash-progress.ts
index 2c24270..f6da4d7 100644
--- a/local-tool/frontend/src/hooks/use-flash-progress.ts
+++ b/local-tool/frontend/src/hooks/use-flash-progress.ts
@@ -44,7 +44,7 @@ export function useFlashProgress(deviceId: string) {
         const ws = createWebSocket(
           `/ws/devices/${deviceId}/flash-progress`,
           (data) => {
-            updateProgress(data as FlashProgress);
+            updateProgress(deviceId, data as FlashProgress);
           },
           () => {
             doResolve();
diff --git a/local-tool/frontend/src/stores/flash-store.ts b/local-tool/frontend/src/stores/flash-store.ts
index 227300d..96e71aa 100644
--- a/local-tool/frontend/src/stores/flash-store.ts
+++ b/local-tool/frontend/src/stores/flash-store.ts
@@ -4,26 +4,31 @@ import type { FlashProgress } from '@/types/device';
 import { showApiError } from '@/lib/toast';
 import { useActivityStore } from './activity-store';
 
+// M4 fix: flash state 以 deviceId 區分，避免多裝置 UI 互相覆蓋。
+// activeDeviceId 記錄當前正在 flash 的裝置，updateProgress 會比對確保不混。
+
 interface FlashState {
+  activeDeviceId: string | null;
   isFlashing: boolean;
   progress: FlashProgress | null;
   error: string | null;
   lastFlashParams: { deviceId: string; modelId: string } | null;
   startFlash: (deviceId: string, modelId: string) => Promise<void>;
-  updateProgress: (progress: FlashProgress) => void;
+  updateProgress: (deviceId: string, progress: FlashProgress) => void;
   setError: (error: string) => void;
   retryFlash: () => Promise<void>;
   reset: () => void;
 }
 
 export const useFlashStore = create<FlashState>((set, get) => ({
+  activeDeviceId: null,
   isFlashing: false,
   progress: null,
   error: null,
   lastFlashParams: null,
 
   startFlash: async (deviceId, modelId) => {
-    set({ isFlashing: true, progress: null, error: null, lastFlashParams: { deviceId, modelId } });
+    set({ activeDeviceId: deviceId, isFlashing: true, progress: null, error: null, lastFlashParams: { deviceId, modelId } });
     const res = await api.post(`/devices/${deviceId}/flash`, { modelId });
     if (!res.success) {
       const errorMsg = res.error?.message || 'Flash failed';
@@ -35,7 +40,12 @@ export const useFlashStore = create<FlashState>((set, get) => ({
     }
   },
 
-  updateProgress: (progress) => {
+  updateProgress: (deviceId, progress) => {
+    // M4 fix: 只更新對應裝置的 progress，忽略其他裝置的 WebSocket 訊息
+    if (get().activeDeviceId !== null && get().activeDeviceId !== deviceId) {
+      return;
+    }
+
     if (progress.error) {
       set({ isFlashing: false, error: progress.error });
       useActivityStore.getState().addActivity('flash_error', `Flash failed: ${progress.error}`);
@@ -59,6 +69,6 @@ export const useFlashStore = create<FlashState>((set, get) => ({
   },
 
   reset: () => {
-    set({ isFlashing: false, progress: null, error: null, lastFlashParams: null });
+    set({ activeDeviceId: null, isFlashing: false, progress: null, error: null, lastFlashParams: null });
   },
 }));
diff --git a/local-tool/server/internal/api/handlers/device_handler.go b/local-tool/server/internal/api/handlers/device_handler.go
index 24253cc..afd1a95 100644
--- a/local-tool/server/internal/api/handlers/device_handler.go
+++ b/local-tool/server/internal/api/handlers/device_handler.go
@@ -133,12 +133,13 @@ func (h *DeviceHandler) FlashDevice(c *gin.Context) {
 		return
 	}
 
-	// Forward progress to WebSocket
+	// Forward progress to WebSocket, then cleanup task (M2 fix)
 	go func() {
 		room := "flash:" + id
 		for progress := range progressCh {
 			h.wsHub.BroadcastToRoom(room, progress)
 		}
+		h.flashSvc.CleanupTask(taskID)
 	}()
 
 	c.JSON(200, gin.H{"success": true, "data": gin.H{"taskId": taskID}})
diff --git a/local-tool/server/internal/api/ws/flash_ws.go b/local-tool/server/internal/api/ws/flash_ws.go
index 712aafb..e4e7bad 100644
--- a/local-tool/server/internal/api/ws/flash_ws.go
+++ b/local-tool/server/internal/api/ws/flash_ws.go
@@ -20,9 +20,8 @@ func FlashProgressHandler(hub *Hub) gin.HandlerFunc {
 		hub.RegisterSync(sub)
 		defer hub.Unregister(sub)
 
-		// Read pump — drain incoming messages (ping/pong, close frames)
+		// Read pump — drain incoming messages; close handled by outer defer
 		go func() {
-			defer conn.Close()
 			for {
 				if _, _, err := conn.ReadMessage(); err != nil {
 					break
diff --git a/local-tool/server/internal/flash/progress.go b/local-tool/server/internal/flash/progress.go
index bd9e3c1..646ba80 100644
--- a/local-tool/server/internal/flash/progress.go
+++ b/local-tool/server/internal/flash/progress.go
@@ -1,8 +1,9 @@
 package flash
 
 import (
-	"visiona-local/server/internal/driver"
 	"sync"
+
+	"visiona-local/server/internal/driver"
 )
 
 type FlashTask struct {
@@ -15,7 +16,7 @@ type FlashTask struct {
 
 type ProgressTracker struct {
 	tasks map[string]*FlashTask
-	mu    sync.RWMutex
+	mu    sync.Mutex
 }
 
 func NewProgressTracker() *ProgressTracker {
@@ -24,26 +25,35 @@ func NewProgressTracker() *ProgressTracker {
 	}
 }
 
+// Create 建立新 flash task。如果同 taskID 已存在且未完成，回傳 nil 表示拒絕。
 func (pt *ProgressTracker) Create(taskID, deviceID, modelID string) *FlashTask {
 	pt.mu.Lock()
 	defer pt.mu.Unlock()
+
+	// M3 fix: 防止同裝置重複 flash — 舊 task 未完成就拒絕
+	if existing, ok := pt.tasks[taskID]; ok && !existing.Done {
+		return nil
+	}
+
 	task := &FlashTask{
 		ID:         taskID,
 		DeviceID:   deviceID,
 		ModelID:    modelID,
 		ProgressCh: make(chan driver.FlashProgress, 20),
+		Done:       false,
 	}
 	pt.tasks[taskID] = task
 	return task
 }
 
 func (pt *ProgressTracker) Get(taskID string) (*FlashTask, bool) {
-	pt.mu.RLock()
-	defer pt.mu.RUnlock()
+	pt.mu.Lock()
+	defer pt.mu.Unlock()
 	t, ok := pt.tasks[taskID]
 	return t, ok
 }
 
+// Remove 清除已完成的 task，釋放 map entry。
 func (pt *ProgressTracker) Remove(taskID string) {
 	pt.mu.Lock()
 	defer pt.mu.Unlock()
diff --git a/local-tool/server/internal/flash/service.go b/local-tool/server/internal/flash/service.go
index 4826bbe..80fa41b 100644
--- a/local-tool/server/internal/flash/service.go
+++ b/local-tool/server/internal/flash/service.go
@@ -12,9 +12,6 @@ import (
 	"visiona-local/server/internal/model"
 )
 
-// isCompatible checks if any of the model's supported hardware types match
-// the device type. The match is case-insensitive and also checks if the
-// device type string contains the hardware name (e.g. "kneron_kl720" contains "KL720").
 func isCompatible(modelHardware []string, deviceType string) bool {
 	dt := strings.ToUpper(deviceType)
 	for _, hw := range modelHardware {
@@ -25,11 +22,6 @@ func isCompatible(modelHardware []string, deviceType string) bool {
 	return false
 }
 
-// resolveModelPath checks if a chip-specific NEF file exists for the given
-// model. For cross-platform models whose filePath points to a KL520 NEF,
-// this tries to find the equivalent KL720 NEF (and vice versa).
-//
-// Resolution: data/nef/kl520/kl520_20001_... → data/nef/kl720/kl720_20001_...
 func resolveModelPath(filePath string, deviceType string) string {
 	if filePath == "" {
 		return filePath
@@ -45,12 +37,10 @@ func resolveModelPath(filePath string, deviceType string) string {
 		return filePath
 	}
 
-	// Already points to the target chip directory — use as-is.
 	if strings.Contains(filePath, "/"+targetChip+"/") {
 		return filePath
 	}
 
-	// Try to swap chip prefix in both directory and filename.
 	dir := filepath.Dir(filePath)
 	base := filepath.Base(filePath)
 
@@ -87,6 +77,11 @@ func NewService(deviceMgr *device.Manager, modelRepo *model.Repository) *Service
 	}
 }
 
+// CleanupTask 清除已完成的 flash task（由 handler goroutine 在讀取完 progressCh 後呼叫）。
+func (s *Service) CleanupTask(taskID string) {
+	s.tracker.Remove(taskID)
+}
+
 func (s *Service) StartFlash(deviceID, modelID string) (string, <-chan driver.FlashProgress, error) {
 	session, err := s.deviceMgr.GetDevice(deviceID)
 	if err != nil {
@@ -101,39 +96,47 @@ func (s *Service) StartFlash(deviceID, modelID string) (string, <-chan driver.Fl
 		return "", nil, fmt.Errorf("model not found: %w", err)
 	}
 
-	// Check hardware compatibility
 	deviceInfo := session.Driver.Info()
 	if !isCompatible(m.SupportedHardware, deviceInfo.Type) {
 		return "", nil, fmt.Errorf("model not compatible with device type %s", deviceInfo.Type)
 	}
 
-	// Use the model's .nef file path if available, otherwise fall back to modelID.
 	modelPath := m.FilePath
 	if modelPath == "" {
-		modelPath = modelID
+		return "", nil, fmt.Errorf("model %s has no .nef file path", modelID)
 	}
 
-	// Resolve chip-specific NEF (e.g. KL520 path → KL720 equivalent).
 	modelPath = resolveModelPath(modelPath, deviceInfo.Type)
 
 	taskID := fmt.Sprintf("flash-%s-%s", deviceID, modelID)
+
+	// M3 fix: 防止同裝置同模型重複 flash
 	task := s.tracker.Create(taskID, deviceID, modelID)
+	if task == nil {
+		return "", nil, fmt.Errorf("flash already in progress for device %s model %s", deviceID, modelID)
+	}
 
 	go func() {
-		defer func() {
-			task.Done = true
-			close(task.ProgressCh)
-		}()
-		// Brief pause to allow the WebSocket client to connect before
-		// progress messages start flowing.
+		// M1 fix: 先跑 driver.Flash，收集 error，最後才寫 error message + close channel。
+		// driver.Flash 內部會多次寫入 task.ProgressCh（進度更新），我們不能在它還在寫的時候 close。
+		// driver.Flash 返回時保證不會再寫入 progressCh。
+
 		time.Sleep(500 * time.Millisecond)
-		if err := session.Driver.Flash(modelPath, task.ProgressCh); err != nil {
+
+		flashErr := session.Driver.Flash(modelPath, task.ProgressCh)
+
+		// Flash 完成或失敗後，driver 不會再寫 progressCh，安全地寫 error 訊息然後 close。
+		if flashErr != nil {
 			task.ProgressCh <- driver.FlashProgress{
 				Percent: -1,
 				Stage:   "error",
-				Error:   err.Error(),
+				Error:   flashErr.Error(),
 			}
 		}
+
+		task.Done = true
+		close(task.ProgressCh)
+		// M2 note: 不在這裡 Remove — 讓 handler 讀完 progressCh 後呼叫 CleanupTask
 	}()
 
 	return taskID, task.ProgressCh, nil