diff --git a/apps/backend/src/services/fanout.ts b/apps/backend/src/services/fanout.ts
new file mode 100644
index 0000000..3cd0ef0
--- /dev/null
+++ b/apps/backend/src/services/fanout.ts
@@ -0,0 +1,92 @@
+// #183 — fan-out service: resolve members -> devices -> envelopes
+//
+// Given an unpersisted message and a sender-provided map of
+// { recipientDeviceId -> ciphertext }, validates that the client encrypted
+// to exactly the conversation's current active recipient devices (including
+// the sender's *other* devices, for multi-device self-sync — but excluding
+// the device that is doing the sending). If the client's device set is
+// stale, returns device_set_mismatch with the authoritative device list
+// instead of guessing or dropping ciphertext. On success, the message and
+// its envelopes are persisted atomically.
+
+import { and, eq, inArray, isNull } from 'drizzle-orm';
+import { db } from '../db/index.js';
+import { conversationMembers, messages, messageEnvelopes, userDevices } from '../db/schema.js';
+import type { Message, NewMessage } from '../db/schema.js';
+
+export interface FanoutSuccess {
+  ok: true;
+  message: Message;
+}
+
+export interface FanoutDeviceSetMismatch {
+  ok: false;
+  error: 'device_set_mismatch';
+  expectedDeviceIds: string[];
+}
+
+export type FanoutResult = FanoutSuccess | FanoutDeviceSetMismatch;
+
+/**
+ * Persists `newMessage` and its per-device envelopes in a single transaction,
+ * after verifying `envelopeCiphertexts` covers exactly the conversation's
+ * current active recipient devices.
+ *
+ * @param newMessage - Message row to insert (id may be omitted; defaultRandom).
+ * @param senderDeviceId - The device sending this message; excluded from the
+ *   authoritative recipient set (it doesn't need its own envelope).
+ * @param envelopeCiphertexts - Sender-provided map of recipientDeviceId -> ciphertext.
+ */
+export async function fanoutMessage(
+  newMessage: NewMessage,
+  senderDeviceId: string | null,
+  envelopeCiphertexts: Record<string, string>,
+): Promise<FanoutResult> {
+  const members = await db.query.conversationMembers.findMany({
+    where: eq(conversationMembers.conversationId, newMessage.conversationId),
+    columns: { userId: true },
+  });
+  const memberIds = members.map((m) => m.userId);
+
+  const activeDevices = await db.query.userDevices.findMany({
+    where: and(inArray(userDevices.userId, memberIds), isNull(userDevices.revokedAt)),
+    columns: { id: true, userId: true },
+  });
+
+  const expectedDevices = activeDevices.filter((d) => d.id !== senderDeviceId);
+  const deviceToUser = new Map(expectedDevices.map((d) => [d.id, d.userId]));
+  const expectedDeviceIds = new Set(deviceToUser.keys());
+
+  const providedDeviceIds = Object.keys(envelopeCiphertexts);
+  const setsMatch =
+    providedDeviceIds.length === expectedDeviceIds.size &&
+    providedDeviceIds.every((id) => expectedDeviceIds.has(id));
+
+  if (!setsMatch) {
+    return {
+      ok: false,
+      error: 'device_set_mismatch',
+      expectedDeviceIds: [...expectedDeviceIds],
+    };
+  }
+
+  const message = await db.transaction(async (tx) => {
+    const [inserted] = await tx.insert(messages).values(newMessage).returning();
+    const persisted = inserted!;
+
+    const envelopeRows = providedDeviceIds.map((deviceId) => ({
+      messageId: persisted.id,
+      recipientDeviceId: deviceId,
+      recipientUserId: deviceToUser.get(deviceId)!,
+      ciphertext: envelopeCiphertexts[deviceId]!,
+    }));
+
+    if (envelopeRows.length > 0) {
+      await tx.insert(messageEnvelopes).values(envelopeRows);
+    }
+
+    return persisted;
+  });
+
+  return { ok: true, message };
+}