diff --git a/crates/quicproquo-server/src/domain/delivery.rs b/crates/quicproquo-server/src/domain/delivery.rs
index cefdc86..6883d89 100644
--- a/crates/quicproquo-server/src/domain/delivery.rs
+++ b/crates/quicproquo-server/src/domain/delivery.rs
@@ -1,6 +1,18 @@
 //! Delivery domain logic — enqueue, fetch, peek, ack.
 //!
 //! Pure business logic operating on `Store` trait and domain types.
+//!
+//! ## Multi-device delivery
+//!
+//! When a message is enqueued for a recipient identity, the service resolves
+//! all registered device IDs for that identity and enqueues a copy of the
+//! payload to each device-scoped queue. The queue key is a composite of
+//! `identity_key + device_id`, so each device maintains its own sequence
+//! counter and ack state.
+//!
+//! If the recipient has no registered devices, the message is delivered to
+//! the bare `identity_key` queue (backwards compatible with single-device
+//! clients).
 
 use std::sync::Arc;
 
@@ -11,6 +23,18 @@ use crate::storage::Store;
 
 use super::types::*;
 
+/// Build a device-scoped recipient key: `identity_key || device_id`.
+/// When `device_id` is empty, returns a clone of `identity_key` (single-device compat).
+fn device_recipient_key(identity_key: &[u8], device_id: &[u8]) -> Vec<u8> {
+    if device_id.is_empty() {
+        return identity_key.to_vec();
+    }
+    let mut key = Vec::with_capacity(identity_key.len() + device_id.len());
+    key.extend_from_slice(identity_key);
+    key.extend_from_slice(device_id);
+    key
+}
+
 /// Shared state needed by delivery operations.
 pub struct DeliveryService {
     pub store: Arc<dyn Store>,
@@ -18,7 +42,33 @@ pub struct DeliveryService {
 }
 
 impl DeliveryService {
-    /// Enqueue a payload for delivery.
+    /// Resolve the device-scoped recipient keys for an identity.
+    /// Returns a list of composite keys (identity_key + device_id) for each
+    /// registered device. If no devices are registered, returns a single-element
+    /// list with the bare identity_key for backwards compatibility.
+    fn resolve_device_keys(&self, identity_key: &[u8]) -> Vec<Vec<u8>> {
+        let devices = self.store.list_devices(identity_key).unwrap_or_default();
+        if devices.is_empty() {
+            vec![identity_key.to_vec()]
+        } else {
+            devices
+                .into_iter()
+                .map(|(device_id, _, _)| device_recipient_key(identity_key, &device_id))
+                .collect()
+        }
+    }
+
+    /// Wake any long-polling waiter for the given recipient key.
+    fn wake_waiter(&self, recipient_key: &[u8]) {
+        if let Some(notify) = self.waiters.get(recipient_key) {
+            notify.notify_one();
+        }
+    }
+
+    /// Enqueue a payload for delivery to all devices of the recipient.
+    ///
+    /// Returns the sequence number from the *first* device queue (for backwards
+    /// compatibility with single-device callers).
     pub fn enqueue(&self, req: EnqueueReq) -> Result<EnqueueResp, crate::storage::StorageError> {
         let ttl = if req.ttl_secs > 0 {
             Some(req.ttl_secs)
@@ -26,25 +76,35 @@ impl DeliveryService {
             None
         };
 
-        let seq = self.store.enqueue(
-            &req.recipient_key,
-            &req.channel_id,
-            req.payload,
-            ttl,
-        )?;
+        let device_keys = self.resolve_device_keys(&req.recipient_key);
+        let mut first_seq = 0;
 
-        // Wake any long-polling waiter for this recipient.
-        if let Some(notify) = self.waiters.get(&req.recipient_key) {
-            notify.notify_one();
+        for (i, dk) in device_keys.iter().enumerate() {
+            let seq = self.store.enqueue(
+                dk,
+                &req.channel_id,
+                req.payload.clone(),
+                ttl,
+            )?;
+            if i == 0 {
+                first_seq = seq;
+            }
+            self.wake_waiter(dk);
         }
 
+        // Also wake the bare identity_key waiter (legacy clients).
+        self.wake_waiter(&req.recipient_key);
+
         Ok(EnqueueResp {
-            seq,
+            seq: first_seq,
             delivery_proof: Vec::new(), // TODO: sign in Phase 2
         })
     }
 
-    /// Fetch and drain queued messages.
+    /// Fetch and drain queued messages for a specific device.
+    ///
+    /// The `recipient_key` should be the device-scoped composite key
+    /// (`identity_key + device_id`) or bare `identity_key` for single-device.
     pub fn fetch(&self, req: FetchReq) -> Result<FetchResp, crate::storage::StorageError> {
         let messages = if req.limit > 0 {
             self.store
@@ -84,7 +144,9 @@ impl DeliveryService {
         Ok(())
     }
 
-    /// Batch enqueue to multiple recipients.
+    /// Batch enqueue to multiple recipients (with multi-device fan-out for each).
+    ///
+    /// Returns one sequence number per recipient identity (from the first device queue).
     pub fn batch_enqueue(
         &self,
         req: BatchEnqueueReq,
@@ -97,14 +159,194 @@ impl DeliveryService {
 
         let mut seqs = Vec::with_capacity(req.recipient_keys.len());
         for rk in &req.recipient_keys {
-            let seq = self.store.enqueue(rk, &req.channel_id, req.payload.clone(), ttl)?;
-            seqs.push(seq);
+            let device_keys = self.resolve_device_keys(rk);
+            let mut first_seq = 0;
 
-            if let Some(notify) = self.waiters.get(rk) {
-                notify.notify_one();
+            for (i, dk) in device_keys.iter().enumerate() {
+                let seq = self.store.enqueue(dk, &req.channel_id, req.payload.clone(), ttl)?;
+                if i == 0 {
+                    first_seq = seq;
+                }
+                self.wake_waiter(dk);
             }
+
+            self.wake_waiter(rk);
+            seqs.push(first_seq);
         }
 
         Ok(BatchEnqueueResp { seqs })
     }
+
+    /// Build a device-scoped recipient key from identity_key and device_id.
+    /// Public helper for RPC handlers to build the correct fetch/ack key.
+    pub fn device_recipient_key(identity_key: &[u8], device_id: &[u8]) -> Vec<u8> {
+        device_recipient_key(identity_key, device_id)
+    }
+}
+
+#[cfg(test)]
+#[allow(clippy::unwrap_used)]
+mod tests {
+    use super::*;
+    use crate::storage::FileBackedStore;
+
+    fn test_service() -> (tempfile::TempDir, DeliveryService) {
+        let dir = tempfile::tempdir().unwrap();
+        let store = Arc::new(FileBackedStore::open(dir.path()).unwrap());
+        let svc = DeliveryService {
+            store,
+            waiters: Arc::new(DashMap::new()),
+        };
+        (dir, svc)
+    }
+
+    #[test]
+    fn enqueue_single_device_backwards_compat() {
+        let (_dir, svc) = test_service();
+        let ik = vec![1u8; 32];
+        let ch = vec![0u8; 16];
+
+        // No devices registered — should enqueue to bare identity_key.
+        let resp = svc
+            .enqueue(EnqueueReq {
+                recipient_key: ik.clone(),
+                payload: b"hello".to_vec(),
+                channel_id: ch.clone(),
+                ttl_secs: 0,
+            })
+            .unwrap();
+        assert_eq!(resp.seq, 0);
+
+        // Fetch from bare identity_key.
+        let fetched = svc
+            .fetch(FetchReq {
+                recipient_key: ik,
+                channel_id: ch,
+                limit: 10,
+            })
+            .unwrap();
+        assert_eq!(fetched.payloads.len(), 1);
+        assert_eq!(fetched.payloads[0].data, b"hello");
+    }
+
+    #[test]
+    fn enqueue_multi_device_fanout() {
+        let (_dir, svc) = test_service();
+        let ik = vec![2u8; 32];
+        let ch = vec![0u8; 16];
+        let dev_a = b"device-a".to_vec();
+        let dev_b = b"device-b".to_vec();
+
+        // Register two devices.
+        svc.store
+            .register_device(&ik, &dev_a, "Phone")
+            .unwrap();
+        svc.store
+            .register_device(&ik, &dev_b, "Laptop")
+            .unwrap();
+
+        // Enqueue a message.
+        svc.enqueue(EnqueueReq {
+            recipient_key: ik.clone(),
+            payload: b"fanout-msg".to_vec(),
+            channel_id: ch.clone(),
+            ttl_secs: 0,
+        })
+        .unwrap();
+
+        // Each device should receive the message on its own queue.
+        let key_a = device_recipient_key(&ik, &dev_a);
+        let key_b = device_recipient_key(&ik, &dev_b);
+
+        let msgs_a = svc
+            .fetch(FetchReq {
+                recipient_key: key_a,
+                channel_id: ch.clone(),
+                limit: 10,
+            })
+            .unwrap();
+        assert_eq!(msgs_a.payloads.len(), 1);
+        assert_eq!(msgs_a.payloads[0].data, b"fanout-msg");
+
+        let msgs_b = svc
+            .fetch(FetchReq {
+                recipient_key: key_b,
+                channel_id: ch.clone(),
+                limit: 10,
+            })
+            .unwrap();
+        assert_eq!(msgs_b.payloads.len(), 1);
+        assert_eq!(msgs_b.payloads[0].data, b"fanout-msg");
+
+        // Bare identity_key queue should be empty (not used when devices exist).
+        let msgs_bare = svc
+            .fetch(FetchReq {
+                recipient_key: ik,
+                channel_id: ch,
+                limit: 10,
+            })
+            .unwrap();
+        assert!(msgs_bare.payloads.is_empty());
+    }
+
+    #[test]
+    fn batch_enqueue_multi_device() {
+        let (_dir, svc) = test_service();
+        let ik1 = vec![3u8; 32];
+        let ik2 = vec![4u8; 32];
+        let ch = vec![0u8; 16];
+        let dev = b"dev1".to_vec();
+
+        // ik1 has a device, ik2 has none.
+        svc.store
+            .register_device(&ik1, &dev, "Phone")
+            .unwrap();
+
+        let resp = svc
+            .batch_enqueue(BatchEnqueueReq {
+                recipient_keys: vec![ik1.clone(), ik2.clone()],
+                payload: b"batch-msg".to_vec(),
+                channel_id: ch.clone(),
+                ttl_secs: 0,
+            })
+            .unwrap();
+        assert_eq!(resp.seqs.len(), 2);
+
+        // ik1 device should have the message.
+        let key_1 = device_recipient_key(&ik1, &dev);
+        let msgs_1 = svc
+            .fetch(FetchReq {
+                recipient_key: key_1,
+                channel_id: ch.clone(),
+                limit: 10,
+            })
+            .unwrap();
+        assert_eq!(msgs_1.payloads.len(), 1);
+
+        // ik2 (no devices) should have it on bare key.
+        let msgs_2 = svc
+            .fetch(FetchReq {
+                recipient_key: ik2,
+                channel_id: ch,
+                limit: 10,
+            })
+            .unwrap();
+        assert_eq!(msgs_2.payloads.len(), 1);
+    }
+
+    #[test]
+    fn device_recipient_key_construction() {
+        let ik = vec![1u8; 32];
+        let dev = b"my-device".to_vec();
+
+        // With device_id.
+        let key = device_recipient_key(&ik, &dev);
+        assert_eq!(key.len(), 32 + dev.len());
+        assert_eq!(&key[..32], &ik[..]);
+        assert_eq!(&key[32..], dev.as_slice());
+
+        // Empty device_id returns bare identity_key.
+        let bare = device_recipient_key(&ik, &[]);
+        assert_eq!(bare, ik);
+    }
 }
diff --git a/crates/quicproquo-server/src/storage.rs b/crates/quicproquo-server/src/storage.rs
index 303baaa..1b5b071 100644
--- a/crates/quicproquo-server/src/storage.rs
+++ b/crates/quicproquo-server/src/storage.rs
@@ -429,6 +429,18 @@ pub struct FileBackedStore {
 }
 
 impl FileBackedStore {
+    /// Create an ephemeral in-memory store backed by a tempdir.
+    /// Intended for unit tests only — the tempdir is leaked (cleaned up by OS).
+    #[cfg(test)]
+    #[allow(dead_code)]
+    pub fn ephemeral() -> Self {
+        let dir = tempfile::tempdir().expect("failed to create tempdir");
+        let store = Self::open(dir.path()).expect("failed to open ephemeral store");
+        // Leak the tempdir so it isn't deleted while the store is alive.
+        std::mem::forget(dir);
+        store
+    }
+
     pub fn open(dir: impl AsRef<Path>) -> Result<Self, StorageError> {
         let dir = dir.as_ref();
         if !dir.exists() {