diff --git a/packages/web/src/stores/spaceStore.dmAlternatives.test.ts b/packages/web/src/stores/spaceStore.dmAlternatives.test.ts
new file mode 100644
index 00000000..d08b7e78
--- /dev/null
+++ b/packages/web/src/stores/spaceStore.dmAlternatives.test.ts
@@ -0,0 +1,116 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+
+// Stub AudioManager to avoid AudioWorkletNode reference error in jsdom
+// (spaceStore → authStore → voiceStore → AudioManager → AudioWorkletNode)
+vi.mock('../audio/AudioManager', () => ({
+  AudioManager: {
+    getInstance: vi.fn().mockReturnValue({
+      setOutputDevice: vi.fn(),
+      setVolume: vi.fn(),
+    }),
+  },
+}));
+
+// Stub instanceStore to avoid initialization ordering issues
+// (spaceStore → authStore → socialStore → instanceStore → setApiForOriginResolver)
+vi.mock('./instanceStore', () => ({
+  useInstanceStore: Object.assign(
+    (selector: (s: unknown) => unknown) => selector({ instances: [], _autoConnectDone: true }),
+    {
+      getState: () => ({ instances: [], _autoConnectDone: true }),
+      setState: vi.fn(),
+      subscribe: vi.fn(),
+    }
+  ),
+}));
+
+// Stub authStore to avoid localStorage access during module init
+vi.mock('./authStore', () => ({
+  useAuthStore: Object.assign(
+    (selector: (s: unknown) => unknown) => selector({ user: null, token: null }),
+    {
+      getState: () => ({ user: null, token: null }),
+      setState: vi.fn(),
+      subscribe: vi.fn(),
+    }
+  ),
+}));
+
+import { useSpaceStore } from './spaceStore';
+import type { DmChannel } from '@backspace/shared';
+
+function makeDm(id: string, federatedId: string | null, extras: Partial<DmChannel> = {}): DmChannel {
+  return {
+    id,
+    federatedId,
+    createdAt: 1000,
+    members: [],
+    ...extras,
+  };
+}
+
+beforeEach(() => {
+  useSpaceStore.getState().reset();
+});
+
+describe('spaceStore.dmAlternatives', () => {
+  it('records origin + local channel id for each DM with federatedId on populateFromReady', () => {
+    const dmsFromHome: DmChannel[] = [
+      makeDm('home-1', 'fed-aaa'),
+      makeDm('home-2', 'fed-bbb'),
+      makeDm('home-3', null), // no federatedId — not indexed
+    ];
+    useSpaceStore.getState().populateFromReady('', [], [], dmsFromHome, null, 0);
+
+    const alts = useSpaceStore.getState().dmAlternatives;
+    expect(alts.get('fed-aaa')?.get('')).toBe('home-1');
+    expect(alts.get('fed-bbb')?.get('')).toBe('home-2');
+    expect(alts.has(null as any)).toBe(false);
+  });
+
+  it('accumulates entries across multiple origins for the same federatedId', () => {
+    useSpaceStore.getState().populateFromReady(
+      '',
+      [],
+      [],
+      [makeDm('home-1', 'fed-aaa')],
+      null,
+      0,
+    );
+    useSpaceStore.getState().populateFromReady(
+      'https://remote.example',
+      [],
+      [],
+      [makeDm('remote-1', 'fed-aaa')],
+      null,
+      0,
+    );
+
+    const byOrigin = useSpaceStore.getState().dmAlternatives.get('fed-aaa');
+    expect(byOrigin?.get('')).toBe('home-1');
+    expect(byOrigin?.get('https://remote.example')).toBe('remote-1');
+  });
+
+  it('updates the local id if the same origin reports a different id for a federatedId', () => {
+    useSpaceStore.getState().populateFromReady(
+      'https://remote.example',
+      [],
+      [],
+      [makeDm('remote-old', 'fed-aaa')],
+      null,
+      0,
+    );
+    useSpaceStore.getState().populateFromReady(
+      'https://remote.example',
+      [],
+      [],
+      [makeDm('remote-new', 'fed-aaa')],
+      null,
+      0,
+    );
+
+    const byOrigin = useSpaceStore.getState().dmAlternatives.get('fed-aaa');
+    expect(byOrigin?.get('https://remote.example')).toBe('remote-new');
+    expect(byOrigin?.size).toBe(1);
+  });
+});
diff --git a/packages/web/src/stores/spaceStore.ts b/packages/web/src/stores/spaceStore.ts
index cafec7a1..e01d1d93 100644
--- a/packages/web/src/stores/spaceStore.ts
+++ b/packages/web/src/stores/spaceStore.ts
@@ -41,6 +41,8 @@ interface SpaceState {
   channelOriginMap: Map<string, string>; // channelId → instance origin ('' = home)
   voiceChannelIds: Set<string>; // channelIds that are voice channels (excluded from unread)
   categoryOriginMap: Map<string, string>; // categoryId → instance origin ('' = home)
+  /** federatedId → (origin → localChannelId). Every DM from every origin's ready payload is recorded here regardless of dedup outcome, so failover can re-point to an alternate origin's local channel ID. */
+  dmAlternatives: Map<string, Map<string, string>>;
   loadingSpaceId: string | null; // non-null while loadSpaceDetail is fetching
   _layoutUpdatedAt: number;
   setSpaces: (spaces: TaggedSpace[]) => void;
@@ -132,6 +134,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
   channelOriginMap: new Map(),
   voiceChannelIds: new Set(),
   categoryOriginMap: new Map(),
+  dmAlternatives: new Map(),
   loadingSpaceId: null,
   _layoutUpdatedAt: 0,
 
@@ -154,6 +157,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
       channelOriginMap: new Map(),
       voiceChannelIds: new Set(),
       categoryOriginMap: new Map(),
+      dmAlternatives: new Map(),
       loadingSpaceId: null,
       _layoutUpdatedAt: 0,
     });
@@ -584,6 +588,10 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
     const channelOriginMap = new Map(get().channelOriginMap);
     const voiceChannelIds = new Set(get().voiceChannelIds);
     const categoryOriginMap = new Map(get().categoryOriginMap);
+    const dmAlternatives = new Map<string, Map<string, string>>();
+    for (const [fid, byOrigin] of get().dmAlternatives) {
+      dmAlternatives.set(fid, new Map(byOrigin));
+    }
 
     // If home, clear home-origin entries first to avoid stale data
     if (isHome) {
@@ -688,6 +696,18 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
       }
     }
 
+    // Record every DM's (origin → localChannelId) for failover lookup,
+    // regardless of whether the dedup pass kept this copy in dmChannels.
+    for (const dm of incomingDms) {
+      if (!dm.federatedId) continue;
+      let byOrigin = dmAlternatives.get(dm.federatedId);
+      if (!byOrigin) {
+        byOrigin = new Map();
+        dmAlternatives.set(dm.federatedId, byOrigin);
+      }
+      byOrigin.set(origin, dm.id);
+    }
+
     // Merge: remove DMs belonging to this origin from existing state, then append incoming
     const existingDmsFromOtherOrigins = get().dmChannels.filter(dm => {
       const dmOrigin = get().channelOriginMap.get(dm.id);
@@ -711,6 +731,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
       channelOriginMap,
       voiceChannelIds,
       categoryOriginMap,
+      dmAlternatives,
     };
 
     // LWW layout merge: accept incoming layout only if its timestamp is >= ours