diff --git a/pkg/service/handlers/handlers_sync_destructive_guard_test.go b/pkg/service/handlers/handlers_sync_destructive_guard_test.go
new file mode 100644
index 0000000..2794dde
--- /dev/null
+++ b/pkg/service/handlers/handlers_sync_destructive_guard_test.go
@@ -0,0 +1,153 @@
+package handlers
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+
+	"github.com/gesellix/bose-soundtouch/pkg/models"
+	"github.com/gesellix/bose-soundtouch/pkg/service/datastore"
+	"github.com/gesellix/bose-soundtouch/pkg/service/setup"
+	"github.com/go-chi/chi/v5"
+)
+
+// TestHandleInitialSync_DestructiveSyncReturns409ThenAppliesWhenConfirmed is
+// an HTTP-level regression test for #614's Sync-button data-loss bug (see
+// setup.TestSyncDeviceData_DestructiveSyncRequiresConfirmation for the
+// lower-level coverage of the same fix): a device already has more presets
+// stored than the mock speaker's live /presets now reports. The first,
+// unconfirmed sync request must come back 409 with the diff and must not
+// write anything; a retry with ?confirmed=true must apply it.
+func TestHandleInitialSync_DestructiveSyncReturns409ThenAppliesWhenConfirmed(t *testing.T) {
+	const (
+		accountID = "1234567"
+		deviceID  = "AABBCCDDEEFF"
+	)
+
+	// A real local server, not a black-hole IP: notifySpeakerSourcesUpdated
+	// (part of the confirmed-apply path) uses its own HTTP client rather
+	// than the injectable sm.HTTPGet, so it needs somewhere real to fail
+	// fast against (404) instead of timing out.
+	mockDevice := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/info":
+			w.Header().Set("Content-Type", "application/xml")
+			fmt.Fprintf(w, `<?xml version="1.0" encoding="UTF-8"?><info deviceID="%s"><name>Test Device</name><type>SoundTouch 20</type><margeAccountUUID>%s</margeAccountUUID></info>`, deviceID, accountID)
+		case "/presets":
+			w.Header().Set("Content-Type", "application/xml")
+			fmt.Fprint(w, `<?xml version="1.0" encoding="UTF-8"?><presets><preset id="1"><ContentItem source="LOCAL_INTERNET_RADIO" type="stationurl" location="/x" isPresetable="true"><itemName>Station 1</itemName></ContentItem></preset></presets>`)
+		case "/recents":
+			w.Header().Set("Content-Type", "application/xml")
+			fmt.Fprint(w, `<?xml version="1.0" encoding="UTF-8"?><recents></recents>`)
+		default:
+			w.WriteHeader(http.StatusNotFound)
+		}
+	}))
+	defer mockDevice.Close()
+
+	deviceIP := mockDevice.Listener.Addr().String()
+
+	tempDir, err := os.MkdirTemp("", "handlers-sync-destructive-guard-*")
+	if err != nil {
+		t.Fatalf("tempdir: %v", err)
+	}
+	defer func() { _ = os.RemoveAll(tempDir) }()
+
+	ds := datastore.NewDataStore(tempDir)
+
+	seeded := []models.ServicePreset{
+		{ID: "1", ButtonNumber: "1", ServiceContentItem: models.ServiceContentItem{Name: "Station 1"}},
+		{ID: "2", ButtonNumber: "2", ServiceContentItem: models.ServiceContentItem{Name: "Station 2"}},
+	}
+	if err := ds.SavePresets(accountID, deviceID, seeded); err != nil {
+		t.Fatalf("seed SavePresets: %v", err)
+	}
+
+	if err := ds.SaveDeviceInfo(accountID, deviceID, &models.ServiceDeviceInfo{
+		DeviceID:  deviceID,
+		AccountID: accountID,
+		IPAddress: deviceIP,
+	}); err != nil {
+		t.Fatalf("SaveDeviceInfo: %v", err)
+	}
+
+	sm := setup.NewManager("http://localhost:8000", ds, nil)
+
+	server := NewServer(ds, sm, "http://localhost:8000", false, false, false)
+
+	r := chi.NewRouter()
+	r.Post("/api/setup/sync/{deviceId}", server.HandleInitialSync)
+
+	ts := httptest.NewServer(r)
+	defer ts.Close()
+
+	// First, unconfirmed request: must be refused with 409.
+	resp, err := http.Post(ts.URL+"/api/setup/sync/"+deviceID, "application/json", nil)
+	if err != nil {
+		t.Fatalf("POST sync (unconfirmed): %v", err)
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusConflict {
+		body, _ := io.ReadAll(resp.Body)
+		t.Fatalf("expected 409 for a destructive unconfirmed sync, got %d: %s", resp.StatusCode, body)
+	}
+
+	var result setup.SyncResult
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		t.Fatalf("decode 409 body: %v", err)
+	}
+
+	if result.Applied {
+		t.Fatal("expected Applied=false in the 409 response")
+	}
+
+	if !result.Destructive {
+		t.Fatal("expected Destructive=true in the 409 response")
+	}
+
+	presetsAfterRefusal, err := ds.GetPresets(accountID, deviceID)
+	if err != nil {
+		t.Fatalf("GetPresets after refused sync: %v", err)
+	}
+
+	if len(presetsAfterRefusal) != 2 {
+		t.Fatalf("expected the original 2 presets to survive the refused sync, got %d", len(presetsAfterRefusal))
+	}
+
+	// Retry, confirmed: must apply.
+	resp2, err := http.Post(ts.URL+"/api/setup/sync/"+deviceID+"?confirmed=true", "application/json", nil)
+	if err != nil {
+		t.Fatalf("POST sync (confirmed): %v", err)
+	}
+
+	defer func() { _ = resp2.Body.Close() }()
+
+	if resp2.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(resp2.Body)
+		t.Fatalf("expected 200 for a confirmed sync, got %d: %s", resp2.StatusCode, body)
+	}
+
+	var confirmedResult setup.SyncResult
+	if err := json.NewDecoder(resp2.Body).Decode(&confirmedResult); err != nil {
+		t.Fatalf("decode 200 body: %v", err)
+	}
+
+	if !confirmedResult.Applied {
+		t.Fatal("expected Applied=true after confirming")
+	}
+
+	presetsAfterConfirm, err := ds.GetPresets(accountID, deviceID)
+	if err != nil {
+		t.Fatalf("GetPresets after confirmed sync: %v", err)
+	}
+
+	if len(presetsAfterConfirm) != 1 {
+		t.Fatalf("expected confirmed sync to shrink to 1 preset, got %d", len(presetsAfterConfirm))
+	}
+}
diff --git a/pkg/service/handlers/web/js/script.js b/pkg/service/handlers/web/js/script.js
index 78667ec..38f00c1 100644
--- a/pkg/service/handlers/web/js/script.js
+++ b/pkg/service/handlers/web/js/script.js
@@ -876,6 +876,37 @@ function getDeviceDisplayName(deviceId) {
     return deviceId;
 }
 
+// buildSyncConfirmMessage renders a human-readable summary of a destructive
+// SyncResult (see setup.SyncResult/SyncResourceDiff) for window.confirm() —
+// e.g. "Sync would remove 1 preset: Ici Roussillon. Continue?".
+function buildSyncConfirmMessage(result) {
+    const lines = ["This Data Sync would remove data that's currently stored:"];
+    for (const diff of result.diffs || []) {
+        if (!diff.destructive) {
+            continue;
+        }
+        const removedNote = diff.removed && diff.removed.length ? ": " + diff.removed.join(", ") : "";
+        lines.push("- " + diff.resource + ": " + diff.currentCount + " → " + diff.incomingCount + removedNote);
+    }
+    lines.push("This usually means the speaker's own live data was incomplete at this moment. Continue anyway?");
+    return lines.join("\n");
+}
+
+async function requestSync(deviceId, confirmed) {
+    let url = "/api/setup/sync/" + encodeURIComponent(deviceId);
+    if (confirmed) {
+        url += "?confirmed=true";
+    }
+    const response = await fetch(url, {method: "POST"});
+    let result = null;
+    try {
+        result = await response.clone().json();
+    } catch (e) {
+        // Non-JSON error body (e.g. a plain-text 500) — handled below via response.text().
+    }
+    return {response, result};
+}
+
 async function startSync() {
     const deviceId = document.getElementById("sync-device-list").value;
     if (!deviceId) {
@@ -895,14 +926,29 @@ async function startSync() {
     log.innerHTML = "";
 
     try {
-        const response = await fetch("/api/setup/sync/" + encodeURIComponent(deviceId), {method: "POST"},);
-        if (response.ok) {
+        let {response, result} = await requestSync(deviceId, false);
+
+        if (response.status === 409 && result) {
+            if (!confirm(buildSyncConfirmMessage(result))) {
+                status.style.backgroundColor = "#eef";
+                status.textContent = "Sync cancelled for " + display + " — nothing was changed.";
+                return;
+            }
+
+            ({response, result} = await requestSync(deviceId, true));
+        }
+
+        if (response.ok && result) {
             status.style.backgroundColor = "#dfd";
             status.textContent = "✅ Sync completed successfully for " + display + "!";
             results.style.display = "block";
-            log.textContent = "Data fetched and saved to local datastore for " + display + ".\nPresets: OK\nRecents: OK\nSources: OK";
+            const lines = (result.diffs || []).map(
+                (diff) => diff.resource + ": " + diff.currentCount + " → " + diff.incomingCount,
+            );
+            lines.push("sources: synced");
+            log.textContent = "Data fetched and saved to local datastore for " + display + ".\n" + lines.join("\n");
         } else {
-            const err = await response.text();
+            const err = result ? JSON.stringify(result) : await response.text();
             throw new Error(err);
         }
     } catch (error) {