diff --git a/packages/web/src/stores/spaceStore.dmAlternatives.test.ts b/packages/web/src/stores/spaceStore.dmAlternatives.test.ts
index 073e96a4..e716da08 100644
--- a/packages/web/src/stores/spaceStore.dmAlternatives.test.ts
+++ b/packages/web/src/stores/spaceStore.dmAlternatives.test.ts
@@ -155,4 +155,61 @@ describe('spaceStore.dmAlternatives', () => {
 
     expect(useSpaceStore.getState().dmAlternatives.has('fed-solo')).toBe(false);
   });
+
+  it('resolveDmChannelId returns the id itself if it is already a primary dmChannels entry', async () => {
+    const { resolveDmChannelId } = await import('./spaceStore');
+    useSpaceStore.getState().populateFromReady(
+      '',
+      [],
+      [],
+      [makeDm('home-1', 'fed-aaa')],
+      null,
+      0,
+    );
+    expect(resolveDmChannelId('home-1')).toBe('home-1');
+  });
+
+  it('resolveDmChannelId resolves an alternative id to the primary via federatedId', async () => {
+    const { resolveDmChannelId } = await import('./spaceStore');
+    // Home loads first → home-1 becomes the primary in dmChannels.
+    useSpaceStore.getState().populateFromReady(
+      '',
+      [],
+      [],
+      [makeDm('home-1', 'fed-aaa')],
+      null,
+      0,
+    );
+    // Remote ready later → remote-1 recorded in dmAlternatives but deduped out of dmChannels.
+    useSpaceStore.getState().populateFromReady(
+      'https://remote.example',
+      [],
+      [],
+      [makeDm('remote-1', 'fed-aaa')],
+      null,
+      0,
+    );
+    expect(resolveDmChannelId('remote-1')).toBe('home-1');
+  });
+
+  it('resolveDmChannelId returns null if the id is unknown everywhere', async () => {
+    const { resolveDmChannelId } = await import('./spaceStore');
+    expect(resolveDmChannelId('nonexistent')).toBeNull();
+  });
+
+  it('resolveDmChannelId returns null if the alternative points to a federatedId no longer in dmChannels', async () => {
+    const { resolveDmChannelId } = await import('./spaceStore');
+    useSpaceStore.getState().populateFromReady(
+      'https://remote.example',
+      [],
+      [],
+      [makeDm('remote-1', 'fed-aaa')],
+      null,
+      0,
+    );
+    // Simulate the dmChannels entry getting removed without clearing dmAlternatives —
+    // resolveDmChannelId should gracefully return null rather than a stale pointer.
+    useSpaceStore.setState({ dmChannels: [] });
+    expect(resolveDmChannelId('remote-1')).toBeNull();
+  });
 });
diff --git a/packages/web/src/stores/spaceStore.ts b/packages/web/src/stores/spaceStore.ts
index 3987d0a6..c64023b3 100644
--- a/packages/web/src/stores/spaceStore.ts
+++ b/packages/web/src/stores/spaceStore.ts
@@ -924,6 +924,33 @@ export function getChannelOrigin(channelId: string): string {
   return useSpaceStore.getState().channelOriginMap.get(channelId) ?? '';
 }
 
+/**
+ * Resolves a raw DM channel ID to its primary `dmChannels` entry ID.
+ *
+ * - If `rawId` is already a primary entry: returns `rawId` unchanged.
+ * - If `rawId` is recorded in `dmAlternatives` as an alternate-origin local ID
+ *   for a DM whose primary is present in `dmChannels`: returns the primary's ID.
+ * - Otherwise: returns `null` (unknown ID — caller should no-op).
+ *
+ * Used by:
+ *  - `dm_message_created` WS handler to route messages arriving from alternate
+ *    origins to the primary entry (§3.11 of the failover spec).
+ *  - Future DM WS handlers that need to dedup alternate-origin deliveries.
+ */
+export function resolveDmChannelId(rawId: string): string | null {
+  const { dmChannels, dmAlternatives } = useSpaceStore.getState();
+  if (dmChannels.some(dm => dm.id === rawId)) return rawId;
+
+  for (const [federatedId, byOrigin] of dmAlternatives) {
+    for (const localId of byOrigin.values()) {
+      if (localId !== rawId) continue;
+      const primary = dmChannels.find(dm => dm.federatedId === federatedId);
+      return primary ? primary.id : null;
+    }
+  }
+  return null;
+}
+
 // ─── API client resolution ────────────────────────────────────────────────────
 // The actual resolver is registered by instanceStore on import, avoiding a
 // circular dependency (instanceStore → useWebSocket → chatStore → spaceStore).