Add support for fusing/unfusing JellyLMS zones into synchronized

multi-room groups, addressed by MAC (derived from the `lms-{mac}` device id).

src-tauri/src/commands/playback_mode.rs

@@ -51,6 +51,22 @@ pub async fn playback_mode_transfer_to_remote(
     manager.0.transfer_to_remote(session_id, position).await
 }
 
+/// Set the transferring flag on the playback mode manager.
+///
+/// Used by the frontend remote->local flow to mark the whole two-step sequence
+/// as a transfer, so `player_play_tracks` starts LOCAL playback instead of
+/// casting back to the remote session it's leaving. Always pair `true` with a
+/// later `false` (including on error) so the flag can't stick.
+#[tauri::command]
+#[specta::specta]
+pub async fn playback_mode_set_transferring(
+    manager: State<'_, PlaybackModeManagerWrapper>,
+    transferring: bool,
+) -> Result<(), String> {
+    manager.0.set_transferring(transferring);
+    Ok(())
+}
+
 /// Transfer playback from remote session back to local device
 ///
 /// Parameters:

src-tauri/src/commands/player/remote.rs

@@ -6,6 +6,7 @@
 use tauri::State;
 
 use super::PlayerStateWrapper;
+use crate::jellyfin::client::LmsSyncGroup;
 
 /// Play items on a remote Jellyfin session (casting)
 #[tauri::command]
@@ -129,3 +130,92 @@ pub async fn remote_session_toggle_mute(
         Err("Jellyfin client not configured".to_string())
     }
 }
+
+// --- JellyLMS multi-room sync groups (fuse / unfuse LMS zones) --------------
+//
+// The frontend addresses LMS players by MAC address, which it derives from a
+// session's device id (`lms-{mac}`). These commands forward to the JellyLMS
+// plugin REST API via the configured JellyfinClient.
+
+/// List current LMS sync groups.
+#[tauri::command]
+#[specta::specta]
+pub async fn lms_get_sync_groups(
+    player: State<'_, PlayerStateWrapper>,
+) -> Result<Vec<LmsSyncGroup>, String> {
+    let client_opt = {
+        let controller = player.0.lock().await;
+        controller.jellyfin_client().lock().map_err(|e| e.to_string())?.clone()
+    };
+
+    if let Some(client) = client_opt {
+        client.lms_get_sync_groups().await
+    } else {
+        Err("Jellyfin client not configured".to_string())
+    }
+}
+
+/// Fuse LMS zones into a sync group. `master_mac` keeps playing and the
+/// `slave_macs` zones join it in sync.
+#[tauri::command]
+#[specta::specta]
+pub async fn lms_create_sync_group(
+    player: State<'_, PlayerStateWrapper>,
+    master_mac: String,
+    slave_macs: Vec<String>,
+) -> Result<(), String> {
+    log::info!("[LmsSync] Fusing zones: master={}, slaves={:?}", master_mac, slave_macs);
+
+    let client_opt = {
+        let controller = player.0.lock().await;
+        controller.jellyfin_client().lock().map_err(|e| e.to_string())?.clone()
+    };
+
+    if let Some(client) = client_opt {
+        client.lms_create_sync_group(&master_mac, slave_macs).await
+    } else {
+        Err("Jellyfin client not configured".to_string())
+    }
+}
+
+/// Remove a single LMS zone from its sync group (decouple one player).
+#[tauri::command]
+#[specta::specta]
+pub async fn lms_unsync_player(
+    player: State<'_, PlayerStateWrapper>,
+    mac: String,
+) -> Result<(), String> {
+    log::info!("[LmsSync] Decoupling zone {}", mac);
+
+    let client_opt = {
+        let controller = player.0.lock().await;
+        controller.jellyfin_client().lock().map_err(|e| e.to_string())?.clone()
+    };
+
+    if let Some(client) = client_opt {
+        client.lms_unsync_player(&mac).await
+    } else {
+        Err("Jellyfin client not configured".to_string())
+    }
+}
+
+/// Dissolve an entire LMS sync group, identified by its master's MAC.
+#[tauri::command]
+#[specta::specta]
+pub async fn lms_dissolve_sync_group(
+    player: State<'_, PlayerStateWrapper>,
+    master_mac: String,
+) -> Result<(), String> {
+    log::info!("[LmsSync] Dissolving group with master {}", master_mac);
+
+    let client_opt = {
+        let controller = player.0.lock().await;
+        controller.jellyfin_client().lock().map_err(|e| e.to_string())?.clone()
+    };
+
+    if let Some(client) = client_opt {
+        client.lms_dissolve_sync_group(&master_mac).await
+    } else {
+        Err("Jellyfin client not configured".to_string())
+    }
+}

src-tauri/src/jellyfin/client.rs

@@ -384,6 +384,82 @@ impl JellyfinClient {
         let sessions = self.get_sessions().await?;
         Ok(sessions.into_iter().find(|s| s.id.as_deref() == Some(session_id)))
     }
+
+    // --- JellyLMS multi-room sync groups -----------------------------------
+    //
+    // The JellyLMS plugin exposes a REST API under `/JellyLms` for grouping LMS
+    // players ("zones") into synchronized multi-room sync groups. Players are
+    // addressed by MAC address; JellyTau maps a Jellyfin session to a MAC by
+    // stripping the `lms-` prefix off the session's device id (see
+    // LmsDeviceDiscoveryService in the jellyLMS repo, which registers each player
+    // with deviceId = "lms-{MacAddress}").
+
+    /// List current LMS sync groups.
+    pub async fn lms_get_sync_groups(&self) -> Result<Vec<LmsSyncGroup>, String> {
+        self.get("/JellyLms/SyncGroups").await
+    }
+
+    /// Fuse LMS zones: create a sync group with `master_mac` as the sync master
+    /// and `slave_macs` joining it. The master keeps playing; slaves follow.
+    pub async fn lms_create_sync_group(
+        &self,
+        master_mac: &str,
+        slave_macs: Vec<String>,
+    ) -> Result<(), String> {
+        let payload = serde_json::json!({
+            "MasterMac": master_mac,
+            "SlaveMacs": slave_macs,
+        });
+        self.post("/JellyLms/SyncGroups", &payload).await
+    }
+
+    /// Remove a single LMS player from whatever sync group it's in.
+    pub async fn lms_unsync_player(&self, mac: &str) -> Result<(), String> {
+        self.delete(&format!("/JellyLms/SyncGroups/Players/{}", mac)).await
+    }
+
+    /// Dissolve an entire LMS sync group, identified by its master's MAC.
+    pub async fn lms_dissolve_sync_group(&self, master_mac: &str) -> Result<(), String> {
+        self.delete(&format!("/JellyLms/SyncGroups/{}", master_mac)).await
+    }
+
+    /// Make a DELETE request to the Jellyfin API (used by the JellyLMS endpoints).
+    async fn delete(&self, endpoint: &str) -> Result<(), String> {
+        let url = format!("{}{}", self.config.server_url, endpoint);
+
+        log::debug!("[JellyfinClient] DELETE {}", endpoint);
+
+        let response = self.http_client
+            .delete(&url)
+            .header("X-Emby-Authorization", self.get_auth_header())
+            .send()
+            .await
+            .map_err(|e| format!("Network request failed: {}", e))?;
+
+        let status = response.status();
+        if !status.is_success() {
+            let error_text = response.text().await.unwrap_or_else(|_| "Unknown error".to_string());
+            return Err(format!("Jellyfin API error {}: {}", status.as_u16(), error_text));
+        }
+        Ok(())
+    }
+}
+
+/// An LMS multi-room sync group, as returned by JellyLMS `/JellyLms/SyncGroups`.
+///
+/// Mirrors the plugin's `SyncGroup` model. The master is the sync source; slaves
+/// follow it in lockstep.
+#[derive(specta::Type, Debug, Clone, Deserialize, serde::Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct LmsSyncGroup {
+    #[serde(alias = "MasterMac")]
+    pub master_mac: String,
+    #[serde(default, alias = "MasterName")]
+    pub master_name: String,
+    #[serde(default, alias = "SlaveMacs")]
+    pub slave_macs: Vec<String>,
+    #[serde(default, alias = "SlaveNames")]
+    pub slave_names: Vec<String>,
 }
 
 /// Default value for supports_remote_control when missing from API

src-tauri/src/lib.rs

@@ -52,11 +52,13 @@ use commands::{
     // Remote session control commands
     remote_play_on_session, remote_send_command, remote_session_seek, remote_session_set_volume,
     remote_session_toggle_mute,
+    // LMS multi-room sync group commands
+    lms_get_sync_groups, lms_create_sync_group, lms_unsync_player, lms_dissolve_sync_group,
     // Session polling commands
     sessions_set_polling_hint, sessions_poll_now, SessionPollerWrapper,
     // Playback mode commands
     playback_mode_get_current, playback_mode_set, playback_mode_is_transferring,
-    playback_mode_transfer_to_remote, playback_mode_transfer_to_local,
+    playback_mode_transfer_to_remote, playback_mode_transfer_to_local, playback_mode_set_transferring,
     playback_mode_get_remote_status,
     // Playback reporting commands
     playback_reporter_init, playback_reporter_destroy,
@@ -417,6 +419,11 @@ fn specta_builder() -> Builder<tauri::Wry> {
             remote_session_seek,
             remote_session_set_volume,
             remote_session_toggle_mute,
+            // LMS multi-room sync group commands
+            lms_get_sync_groups,
+            lms_create_sync_group,
+            lms_unsync_player,
+            lms_dissolve_sync_group,
             // Session polling commands
             sessions_set_polling_hint,
             sessions_poll_now,
@@ -427,6 +434,7 @@ fn specta_builder() -> Builder<tauri::Wry> {
             playback_mode_transfer_to_remote,
             playback_mode_get_remote_status,
             playback_mode_transfer_to_local,
+            playback_mode_set_transferring,
             // Playback reporting commands
             playback_reporter_init,
             playback_reporter_destroy,

src-tauri/src/playback_mode/mod.rs

@@ -81,6 +81,19 @@ impl PlaybackModeManager {
         self.is_transferring.load(Ordering::Relaxed)
     }
 
+    /// Set the transferring flag directly.
+    ///
+    /// The remote->local transfer is driven from the frontend in two steps
+    /// (`player_play_tracks` to start local playback, then
+    /// `playback_mode_transfer_to_local` to stop the remote). The first step's
+    /// routing depends on this flag: while it's set, `player_play_tracks` plays
+    /// locally instead of casting back to the remote session. The frontend must
+    /// raise the flag *before* that first call and lower it when the sequence is
+    /// done (or aborts), so it can't be left stuck on.
+    pub fn set_transferring(&self, transferring: bool) {
+        self.is_transferring.store(transferring, Ordering::Relaxed);
+    }
+
     /// Send volume command to remote session
     /// Commands: "SetVolume", "VolumeUp", "VolumeDown"
     #[allow(dead_code)] // Called from Android JNI callback

src/lib/api/bindings.ts

@@ -270,6 +270,31 @@ async remoteSessionSetVolume(sessionId: string, volume: number) : Promise<null>
 async remoteSessionToggleMute(sessionId: string) : Promise<null> {
     return await TAURI_INVOKE("remote_session_toggle_mute", { sessionId });
 },
+/**
+ * List current LMS sync groups.
+ */
+async lmsGetSyncGroups() : Promise<LmsSyncGroup[]> {
+    return await TAURI_INVOKE("lms_get_sync_groups");
+},
+/**
+ * Fuse LMS zones into a sync group. `master_mac` keeps playing and the
+ * `slave_macs` zones join it in sync.
+ */
+async lmsCreateSyncGroup(masterMac: string, slaveMacs: string[]) : Promise<null> {
+    return await TAURI_INVOKE("lms_create_sync_group", { masterMac, slaveMacs });
+},
+/**
+ * Remove a single LMS zone from its sync group (decouple one player).
+ */
+async lmsUnsyncPlayer(mac: string) : Promise<null> {
+    return await TAURI_INVOKE("lms_unsync_player", { mac });
+},
+/**
+ * Dissolve an entire LMS sync group, identified by its master's MAC.
+ */
+async lmsDissolveSyncGroup(masterMac: string) : Promise<null> {
+    return await TAURI_INVOKE("lms_dissolve_sync_group", { masterMac });
+},
 /**
  * Set polling frequency hint based on UI state
  */
@@ -322,6 +347,17 @@ async playbackModeGetRemoteStatus() : Promise<RemoteSessionStatus> {
 async playbackModeTransferToLocal(currentItemId: string, positionTicks: number) : Promise<null> {
     return await TAURI_INVOKE("playback_mode_transfer_to_local", { currentItemId, positionTicks });
 },
+/**
+ * Set the transferring flag on the playback mode manager.
+ * 
+ * Used by the frontend remote->local flow to mark the whole two-step sequence
+ * as a transfer, so `player_play_tracks` starts LOCAL playback instead of
+ * casting back to the remote session it's leaving. Always pair `true` with a
+ * later `false` (including on error) so the flag can't stick.
+ */
+async playbackModeSetTransferring(transferring: boolean) : Promise<null> {
+    return await TAURI_INVOKE("playback_mode_set_transferring", { transferring });
+},
 /**
  * Initialize playback reporter (called after login)
  */
@@ -1503,6 +1539,13 @@ export type ImageType = "Primary" | "Backdrop" | "Banner" | "Thumb" | "Logo"
  * Library (media collection)
  */
 export type Library = { id: string; name: string; collectionType: string; imageTag?: string | null }
+/**
+ * An LMS multi-room sync group, as returned by JellyLMS `/JellyLms/SyncGroups`.
+ * 
+ * Mirrors the plugin's `SyncGroup` model. The master is the sync source; slaves
+ * follow it in lockstep.
+ */
+export type LmsSyncGroup = { masterMac: string; masterName?: string; slaveMacs?: string[]; slaveNames?: string[] }
 /**
  * Media item
  */

src/lib/components/sessions/SessionPickerModal.svelte

@@ -4,6 +4,7 @@
   import { sessions, controllableSessions, selectedSession } from "$lib/stores";
   import { playbackMode, isTransferring, transferError } from "$lib/stores/playbackMode";
   import { playbackPosition } from "$lib/stores/player";
+  import { lmsSync, isLmsSession, macForSession } from "$lib/stores/lmsSync";
   import type { Session } from "$lib/api/types";
 
   interface Props {
@@ -14,6 +15,20 @@
 
   let { isOpen = false, onClose, onSelectSession }: Props = $props();
 
+  // MACs currently part of any sync group, for rendering toggle state.
+  const groupedMacs = $derived(new Set(($lmsSync.groups ?? []).flatMap(
+    (g) => [g.masterMac, ...(g.slaveMacs ?? [])]
+  )));
+
+  // The sync master is the LMS zone we're currently controlling. Zones can only
+  // be fused once we have a master to fuse them into.
+  const masterMac = $derived(isLmsSession($selectedSession) ? macForSession($selectedSession) : null);
+
+  function isZoneFused(session: Session): boolean {
+    const mac = macForSession(session);
+    return mac !== null && groupedMacs.has(mac);
+  }
+
   async function handleSessionSelect(session: Session) {
     try {
       // Transfer playback to remote session, resuming at our current position
@@ -31,6 +46,32 @@
     }
   }
 
+  // Toggle an LMS zone in/out of the current master's sync group. If there's no
+  // master yet (we're not controlling an LMS zone), the click falls back to a
+  // normal transfer so the user can start playing on a zone first.
+  async function handleLmsZoneToggle(session: Session) {
+    const zoneMac = macForSession(session);
+    if (!zoneMac) return;
+
+    if (!masterMac) {
+      // No master yet — start playback on this zone; it becomes the master.
+      await handleSessionSelect(session);
+      await lmsSync.refresh();
+      return;
+    }
+
+    try {
+      if (isZoneFused(session)) {
+        await lmsSync.decoupleZone(zoneMac);
+      } else {
+        await lmsSync.fuseZone(masterMac, zoneMac);
+      }
+    } catch (error) {
+      console.error("Failed to toggle LMS zone:", error);
+      // Error is surfaced via the lmsSync store
+    }
+  }
+
   async function handleTransferToLocal() {
     try {
       await playbackMode.transferToLocal();
@@ -80,6 +121,7 @@
   $effect(() => {
     if (isOpen) {
       sessions.refresh();
+      lmsSync.refresh();
     }
   });
 </script>
@@ -167,14 +209,24 @@
             <!-- Available sessions -->
             {#each $controllableSessions as session (session.id)}
               {#if session.id !== $selectedSession?.id}
+                {@const lmsZone = isLmsSession(session)}
+                {@const fused = lmsZone && isZoneFused(session)}
                 <button
-                  onclick={() => handleSessionSelect(session)}
+                  onclick={() => (lmsZone ? handleLmsZoneToggle(session) : handleSessionSelect(session))}
-                  class="w-full p-4 rounded-lg border border-gray-800 hover:border-[var(--color-jellyfin)]/50 hover:bg-[var(--color-jellyfin)]/5 transition-all text-left"
+                  disabled={$lmsSync.isBusy}
+                  aria-pressed={lmsZone ? fused : undefined}
+                  class="w-full p-4 rounded-lg border transition-all text-left disabled:opacity-60
+                    {fused
+                      ? 'border-[var(--color-jellyfin)] bg-[var(--color-jellyfin)]/10'
+                      : 'border-gray-800 hover:border-[var(--color-jellyfin)]/50 hover:bg-[var(--color-jellyfin)]/5'}"
                 >
                   <div class="flex items-center gap-3">
                     <div class="w-10 h-10 rounded-lg bg-gray-800 flex items-center justify-center flex-shrink-0">
                       <svg class="w-5 h-5 text-gray-400" fill="currentColor" viewBox="0 0 24 24">
-                        {#if getSessionIcon(session.client) === "tv"}
+                        {#if lmsZone}
+                          <!-- speaker icon for LMS zones -->
+                          <path d="M12 2a1 1 0 011 1v18a1 1 0 01-1 1H6a2 2 0 01-2-2V4a2 2 0 012-2h6zm0 2H6v16h6V4zm-3 9a2 2 0 110 4 2 2 0 010-4zm0-2a4 4 0 100 8 4 4 0 000-8zm0-4a1 1 0 110 2 1 1 0 010-2zm9 0h2v2h-2V7zm0 4h2v2h-2v-2z" />
+                        {:else if getSessionIcon(session.client) === "tv"}
                           <path d="M21 3H3c-1.1 0-2 .9-2 2v12c0 1.1.9 2 2 2h5v2h8v-2h5c1.1 0 1.99-.9 1.99-2L23 5c0-1.1-.9-2-2-2zm0 14H3V5h18v12z" />
                         {:else if getSessionIcon(session.client) === "web"}
                           <path d="M20 4H4c-1.1 0-1.99.9-1.99 2L2 18c0 1.1.9 2 2 2h16c1.1 0 2-.9 2-2V6c0-1.1-.9-2-2-2zm-5 14H4v-4h11v4zm0-5H4V9h11v4zm5 5h-4V9h4v9z" />
@@ -187,14 +239,33 @@
                     </div>
                     <div class="flex-1 min-w-0">
                       <h3 class="font-medium text-white truncate">{session.deviceName}</h3>
-                      <p class="text-sm text-gray-400 truncate">{session.client}</p>
+                      <p class="text-sm text-gray-400 truncate">
+                        {#if lmsZone}
+                          {fused ? "In sync group — tap to remove" : masterMac ? "Tap to add to group" : "Speaker zone"}
+                        {:else}
+                          {session.client}
+                        {/if}
+                      </p>
                       {#if session.nowPlayingItem}
                         <p class="text-xs text-gray-500 truncate mt-1">
                           Playing: {session.nowPlayingItem.name}
                         </p>
                       {/if}
                     </div>
-                    {#if session.playState}
+                    {#if lmsZone}
+                      <!-- Fuse toggle indicator -->
+                      <div class="flex-shrink-0">
+                        {#if fused}
+                          <svg class="w-5 h-5 text-[var(--color-jellyfin)]" fill="currentColor" viewBox="0 0 24 24" aria-hidden="true">
+                            <path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm-2 15l-5-5 1.41-1.41L10 14.17l7.59-7.59L19 8l-9 9z" />
+                          </svg>
+                        {:else}
+                          <svg class="w-5 h-5 text-gray-600" fill="none" stroke="currentColor" viewBox="0 0 24 24" aria-hidden="true">
+                            <circle cx="12" cy="12" r="9" stroke-width="2" />
+                          </svg>
+                        {/if}
+                      </div>
+                    {:else if session.playState}
                       <div class="flex-shrink-0">
                         {#if session.playState.isPaused}
                           <svg class="w-4 h-4 text-gray-500" fill="currentColor" viewBox="0 0 24 24">
@@ -295,6 +366,27 @@
         </div>
       {/if}
 
+      <!-- LMS sync error -->
+      {#if $lmsSync.error}
+        <div class="absolute bottom-0 left-0 right-0 bg-red-500/90 text-white px-6 py-3 flex items-center justify-between z-10 rounded-b-2xl">
+          <div class="flex items-center gap-2">
+            <svg class="w-5 h-5 flex-shrink-0" fill="currentColor" viewBox="0 0 24 24">
+              <path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm1 15h-2v-2h2v2zm0-4h-2V7h2v6z" />
+            </svg>
+            <span class="text-sm">{$lmsSync.error}</span>
+          </div>
+          <button
+            onclick={() => lmsSync.clearError()}
+            class="text-white hover:text-gray-200 transition-colors"
+            aria-label="Dismiss error"
+          >
+            <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+              <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12" />
+            </svg>
+          </button>
+        </div>
+      {/if}
+
       <!-- Error Display -->
       {#if $transferError}
         <div class="absolute bottom-0 left-0 right-0 bg-red-500/90 text-white px-6 py-3 flex items-center justify-between z-10 rounded-b-2xl">

src/lib/stores/lmsSync.test.ts

@@ -0,0 +1,91 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { get } from "svelte/store";
+import type { Session } from "$lib/api/types";
+
+const mockInvoke = vi.fn();
+vi.mock("$lib/api/bindings", () => ({
+  commands: {
+    lmsGetSyncGroups: () => mockInvoke("lms_get_sync_groups"),
+    lmsCreateSyncGroup: (masterMac: string, slaveMacs: string[]) =>
+      mockInvoke("lms_create_sync_group", { masterMac, slaveMacs }),
+    lmsUnsyncPlayer: (mac: string) => mockInvoke("lms_unsync_player", { mac }),
+    lmsDissolveSyncGroup: (masterMac: string) =>
+      mockInvoke("lms_dissolve_sync_group", { masterMac }),
+  },
+}));
+
+import { lmsSync, isLmsSession, macForSession } from "./lmsSync";
+
+function session(deviceId: string | null): Session {
+  return { id: "s", deviceId } as unknown as Session;
+}
+
+describe("LMS session detection", () => {
+  it("detects LMS zones by the lms- device-id prefix", () => {
+    expect(isLmsSession(session("lms-aa:bb:cc:dd:ee:ff"))).toBe(true);
+    expect(isLmsSession(session("chromecast-123"))).toBe(false);
+    expect(isLmsSession(session(null))).toBe(false);
+    expect(isLmsSession(null)).toBe(false);
+  });
+
+  it("recovers the MAC from an LMS session, null otherwise", () => {
+    expect(macForSession(session("lms-aa:bb:cc:dd:ee:ff"))).toBe("aa:bb:cc:dd:ee:ff");
+    expect(macForSession(session("web-xyz"))).toBeNull();
+    expect(macForSession(null)).toBeNull();
+  });
+});
+
+describe("lmsSync store", () => {
+  beforeEach(() => {
+    mockInvoke.mockReset();
+    lmsSync.reset();
+  });
+
+  it("fusing preserves existing slaves and adds the new zone", async () => {
+    // Initial group: master M with slave A.
+    mockInvoke.mockResolvedValueOnce([{ masterMac: "M", masterName: "M", slaveMacs: ["A"], slaveNames: [] }]);
+    await lmsSync.refresh();
+
+    // create returns void; refresh after returns the updated group.
+    mockInvoke.mockResolvedValueOnce(undefined);
+    mockInvoke.mockResolvedValueOnce([{ masterMac: "M", masterName: "M", slaveMacs: ["A", "B"], slaveNames: [] }]);
+
+    await lmsSync.fuseZone("M", "B");
+
+    expect(mockInvoke).toHaveBeenCalledWith("lms_create_sync_group", {
+      masterMac: "M",
+      slaveMacs: ["A", "B"],
+    });
+    expect(get(lmsSync).groups[0].slaveMacs).toEqual(["A", "B"]);
+  });
+
+  it("decoupling a slave unsyncs just that player", async () => {
+    mockInvoke.mockResolvedValueOnce([{ masterMac: "M", masterName: "M", slaveMacs: ["A"], slaveNames: [] }]);
+    await lmsSync.refresh();
+
+    mockInvoke.mockResolvedValueOnce(undefined); // unsync
+    mockInvoke.mockResolvedValueOnce([{ masterMac: "M", masterName: "M", slaveMacs: [], slaveNames: [] }]);
+
+    await lmsSync.decoupleZone("A");
+
+    expect(mockInvoke).toHaveBeenCalledWith("lms_unsync_player", { mac: "A" });
+  });
+
+  it("decoupling the master dissolves the whole group", async () => {
+    mockInvoke.mockResolvedValueOnce([{ masterMac: "M", masterName: "M", slaveMacs: ["A"], slaveNames: [] }]);
+    await lmsSync.refresh();
+
+    mockInvoke.mockResolvedValueOnce(undefined); // dissolve
+    mockInvoke.mockResolvedValueOnce([]);
+
+    await lmsSync.decoupleZone("M");
+
+    expect(mockInvoke).toHaveBeenCalledWith("lms_dissolve_sync_group", { masterMac: "M" });
+  });
+
+  it("treats a missing plugin (refresh error) as no groups, not fatal", async () => {
+    mockInvoke.mockRejectedValueOnce(new Error("404"));
+    await lmsSync.refresh();
+    expect(get(lmsSync).groups).toEqual([]);
+  });
+});

src/lib/stores/lmsSync.ts

@@ -0,0 +1,133 @@
+/**
+ * LMS multi-room sync ("fuse zones") store.
+ *
+ * The JellyLMS plugin registers each LMS player as a Jellyfin session whose
+ * device id is `lms-{MacAddress}`. We use that prefix both to detect which cast
+ * targets are fuseable LMS zones and to recover the MAC the SyncGroups API needs.
+ *
+ * Fusing model (per product decision): the currently-controlled zone is the sync
+ * master; other selected zones join it. Toggling an already-grouped zone off
+ * decouples just that zone; toggling the master off dissolves the group.
+ *
+ * TRACES: UR-010 | JA-021, JA-025 | DR-037
+ */
+
+import { writable, get } from "svelte/store";
+import { commands } from "$lib/api/bindings";
+import type { Session } from "$lib/api/types";
+import type { LmsSyncGroup } from "$lib/api/bindings";
+
+const LMS_DEVICE_PREFIX = "lms-";
+
+/** Is this cast target an LMS zone (and therefore fuseable)? */
+export function isLmsSession(session: Session | null | undefined): boolean {
+  return !!session?.deviceId?.startsWith(LMS_DEVICE_PREFIX);
+}
+
+/** Recover the LMS player MAC from a session, or null if it isn't an LMS zone. */
+export function macForSession(session: Session | null | undefined): string | null {
+  const deviceId = session?.deviceId;
+  if (!deviceId?.startsWith(LMS_DEVICE_PREFIX)) return null;
+  return deviceId.slice(LMS_DEVICE_PREFIX.length);
+}
+
+interface LmsSyncState {
+  groups: LmsSyncGroup[];
+  isBusy: boolean;
+  error: string | null;
+}
+
+function createLmsSyncStore() {
+  const { subscribe, update, set } = writable<LmsSyncState>({
+    groups: [],
+    isBusy: false,
+    error: null,
+  });
+
+  /** Refresh the list of current sync groups from the server. */
+  async function refresh(): Promise<void> {
+    try {
+      const groups = await commands.lmsGetSyncGroups();
+      update((s) => ({ ...s, groups, error: null }));
+    } catch (error) {
+      // The plugin may not be installed; treat as "no groups" rather than fatal.
+      console.warn("[LmsSync] Failed to load sync groups:", error);
+      update((s) => ({ ...s, groups: [] }));
+    }
+  }
+
+  /** All MACs that are part of any sync group (master or slave). */
+  function groupedMacs(): Set<string> {
+    const macs = new Set<string>();
+    for (const g of get({ subscribe }).groups) {
+      macs.add(g.masterMac);
+      for (const slave of g.slaveMacs ?? []) macs.add(slave);
+    }
+    return macs;
+  }
+
+  /**
+   * Add an LMS zone to the master's sync group (fuse it in).
+   * Preserves any zones already grouped with the master.
+   */
+  async function fuseZone(masterMac: string, zoneMac: string): Promise<void> {
+    if (masterMac === zoneMac) return;
+    update((s) => ({ ...s, isBusy: true, error: null }));
+    try {
+      const existing = get({ subscribe }).groups.find((g) => g.masterMac === masterMac);
+      const slaves = new Set(existing?.slaveMacs ?? []);
+      slaves.add(zoneMac);
+      await commands.lmsCreateSyncGroup(masterMac, [...slaves]);
+      await refresh();
+    } catch (error) {
+      const message = error instanceof Error ? error.message : "Failed to fuse zone";
+      update((s) => ({ ...s, error: message }));
+      throw error;
+    } finally {
+      update((s) => ({ ...s, isBusy: false }));
+    }
+  }
+
+  /**
+   * Remove a single zone from its group. If the zone is a group's master, the
+   * whole group is dissolved (a group can't outlive its master).
+   */
+  async function decoupleZone(zoneMac: string): Promise<void> {
+    update((s) => ({ ...s, isBusy: true, error: null }));
+    try {
+      const asMaster = get({ subscribe }).groups.find((g) => g.masterMac === zoneMac);
+      if (asMaster) {
+        await commands.lmsDissolveSyncGroup(zoneMac);
+      } else {
+        await commands.lmsUnsyncPlayer(zoneMac);
+      }
+      await refresh();
+    } catch (error) {
+      const message = error instanceof Error ? error.message : "Failed to decouple zone";
+      update((s) => ({ ...s, error: message }));
+      throw error;
+    } finally {
+      update((s) => ({ ...s, isBusy: false }));
+    }
+  }
+
+  function clearError(): void {
+    update((s) => ({ ...s, error: null }));
+  }
+
+  function reset(): void {
+    set({ groups: [], isBusy: false, error: null });
+  }
+
+  return {
+    subscribe,
+    refresh,
+    groupedMacs,
+    fuseZone,
+    decoupleZone,
+    clearError,
+    reset,
+  };
+}
+
+export const lmsSync = createLmsSyncStore();

src/lib/stores/playbackMode.ts

@@ -198,6 +198,12 @@ function createPlaybackModeStore() {
       // Get repository for handle (backend will fetch playback info via player_play_tracks)
       const repository = auth.getRepository();
 
+      // Mark the whole sequence as a transfer in the *Rust* manager too. Without
+      // this, player_play_tracks sees mode=Remote and casts the track back to the
+      // remote session instead of playing it locally (the frontend's own
+      // isTransferring flag is invisible to Rust). Cleared in `finally`.
+      await commands.playbackModeSetTransferring(true);
+
       // Start local playback (events allowed through because isTransferring=true)
       // Use player_play_tracks - backend fetches all metadata from single ID
       const repositoryHandle = repository.getHandle();
@@ -248,6 +254,14 @@ function createPlaybackModeStore() {
       console.error("Transfer to local failed:", error);
       throw error;
     } finally {
+      // Always lower the Rust transferring flag so it can't stick on if any step
+      // above threw (transfer_to_local lowers it on success, but not if we never
+      // reached it). Safe to call unconditionally.
+      try {
+        await commands.playbackModeSetTransferring(false);
+      } catch (e) {
+        console.warn("[PlaybackMode] Failed to clear transferring flag:", e);
+      }
       currentTransferAbort = null;
     }
   }