diff --git a/apps/backend/src/db/schema.ts b/apps/backend/src/db/schema.ts
index 3305884..59488c4 100644
--- a/apps/backend/src/db/schema.ts
+++ b/apps/backend/src/db/schema.ts
@@ -110,6 +110,15 @@ export const messageEnvelopes = pgTable(
   ],
 );
 
+export const messageEnvelopes = pgTable('message_envelopes', {
+  id: uuid('id').primaryKey().defaultRandom(),
+  messageId: uuid('message_id')
+    .notNull()
+    .references(() => messages.id, { onDelete: 'cascade' }),
+  content: text('content').notNull(),
+  createdAt: timestamp('created_at').notNull().defaultNow(),
+});
+
 // ─── Devices & prekeys (issues #158, #159, #162) ─────────────────────────────
 //
 // Each user may register multiple devices. Each device has an Ed25519 identity
@@ -344,6 +353,11 @@ export const devicesRelations = relations(devices, ({ one, many }) => ({
   oneTimePreKeys: many(oneTimePreKeys),
 }));
 
+export const userDevicesRelations = relations(userDevices, ({ one, many }) => ({
+  user: one(users, { fields: [userDevices.userId], references: [users.id] }),
+  messages: many(messages),
+}));
+
 export const signedPreKeysRelations = relations(signedPreKeys, ({ one }) => ({
   device: one(devices, { fields: [signedPreKeys.deviceId], references: [devices.id] }),
 }));
diff --git a/apps/backend/src/lib/messages.ts b/apps/backend/src/lib/messages.ts
index 37601c2..1c27c58 100644
--- a/apps/backend/src/lib/messages.ts
+++ b/apps/backend/src/lib/messages.ts
@@ -11,7 +11,7 @@ type MessageLike = {
   [key: string]: any;
 };
 
-export function serializeMessage<T extends MessageLike>(
+export function serializeMessage<T extends MessageWithEnvelopes>(
   message: T,
 ): Omit<T, 'deletedAt' | 'envelopes' | 'ciphertext'> & {
   ciphertext: string | null;
diff --git a/apps/backend/src/routes/conversations.ts b/apps/backend/src/routes/conversations.ts
index 673385b..19c83f4 100644
--- a/apps/backend/src/routes/conversations.ts
+++ b/apps/backend/src/routes/conversations.ts
@@ -247,7 +247,9 @@ conversationsRouter.get('/:id/members', async (req: AuthRequest, res) => {
     with: {
       user: {
         columns: { id: true, username: true, avatarUrl: true },
-        with: { wallets: { columns: { address: true, isPrimary: true } } },
+        with: {
+          wallets: { columns: { address: true, isPrimary: true } },
+        },
       },
     },
   })) as ConversationMemberPayload[];
diff --git a/apps/backend/src/socket/messaging.ts b/apps/backend/src/socket/messaging.ts
index 844ce3f..4820b56 100644
--- a/apps/backend/src/socket/messaging.ts
+++ b/apps/backend/src/socket/messaging.ts
@@ -177,7 +177,11 @@ export function registerMessagingHandlers(io: Server, socket: AuthSocket): void
         : eq(messages.conversationId, conversationId),
       orderBy: desc(messages.createdAt),
       limit: PAGE_SIZE,
-      with: { sender: { columns: { id: true, username: true, avatarUrl: true } } },
+      with: {
+        envelopes: true,
+        senderDevice: true,
+        sender: { columns: { id: true, username: true, avatarUrl: true } },
+      },
     });
 
     socket.emit('message_history', {
@@ -446,6 +450,25 @@ export function registerMessagingHandlers(io: Server, socket: AuthSocket): void
         ON CONFLICT DO NOTHING
       `);
 
+      // Get or create an assistant device
+      let assistantDevice = await db.query.userDevices.findFirst({
+        where: eq(userDevices.userId, ASSISTANT_USER_ID),
+      });
+
+      if (!assistantDevice) {
+        const [newDevice] = await db
+          .insert(userDevices)
+          .values({
+            userId: ASSISTANT_USER_ID,
+            deviceId: 'assistant-device',
+            deviceName: 'Assistant',
+            platform: 'web',
+            identityPublicKey: 'assistant-public-key',
+          })
+          .returning();
+        assistantDevice = newDevice;
+      }
+
       // Post the reply
       const [replyMessage] = await db
         .insert(messages)