diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/events/MessageDeliveredEvent.kt b/backend/src/main/java/net/buzzert/kordophone/backend/events/MessageDeliveredEvent.kt
index eff9dd2..75e063d 100644
--- a/backend/src/main/java/net/buzzert/kordophone/backend/events/MessageDeliveredEvent.kt
+++ b/backend/src/main/java/net/buzzert/kordophone/backend/events/MessageDeliveredEvent.kt
@@ -1,9 +1,11 @@
 package net.buzzert.kordophone.backend.events
 
 import net.buzzert.kordophone.backend.model.Conversation
+import net.buzzert.kordophone.backend.model.GUID
 import net.buzzert.kordophone.backend.model.Message
 
 data class MessageDeliveredEvent(
+    val guid: GUID,
     val message: Message,
     val conversation: Conversation,
 )
diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/model/Conversation.kt b/backend/src/main/java/net/buzzert/kordophone/backend/model/Conversation.kt
index 44bef55..947f222 100644
--- a/backend/src/main/java/net/buzzert/kordophone/backend/model/Conversation.kt
+++ b/backend/src/main/java/net/buzzert/kordophone/backend/model/Conversation.kt
@@ -3,6 +3,8 @@ package net.buzzert.kordophone.backend.model
 import com.google.gson.annotations.SerializedName
 import java.util.Date
 
+typealias GUID = String
+
 data class Conversation(
     @SerializedName("date")
     val date: Date,
@@ -20,5 +22,5 @@ data class Conversation(
     val lastMessagePreview: String,
 
     @SerializedName("guid")
-    val guid: String,
+    val guid: GUID,
 )
diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/model/Message.kt b/backend/src/main/java/net/buzzert/kordophone/backend/model/Message.kt
index 3aff5a4..283ad98 100644
--- a/backend/src/main/java/net/buzzert/kordophone/backend/model/Message.kt
+++ b/backend/src/main/java/net/buzzert/kordophone/backend/model/Message.kt
@@ -8,7 +8,7 @@ data class Message(
     val text: String,
 
     @SerializedName("guid")
-    val guid: String,
+    val guid: GUID,
 
     @SerializedName("sender")
     val sender: String?, // optional: nil means "from me"
diff --git a/backend/src/main/java/net/buzzert/kordophone/backend/server/ChatRepository.kt b/backend/src/main/java/net/buzzert/kordophone/backend/server/ChatRepository.kt
index ec549b1..adefe66 100644
--- a/backend/src/main/java/net/buzzert/kordophone/backend/server/ChatRepository.kt
+++ b/backend/src/main/java/net/buzzert/kordophone/backend/server/ChatRepository.kt
@@ -5,17 +5,25 @@ import kotlinx.coroutines.channels.Channel
 import kotlinx.coroutines.runBlocking
 import net.buzzert.kordophone.backend.events.MessageDeliveredEvent
 import net.buzzert.kordophone.backend.model.Conversation
+import net.buzzert.kordophone.backend.model.GUID
 import net.buzzert.kordophone.backend.model.Message
 import java.net.URL
 import java.util.Queue
+import java.util.UUID
 import java.util.concurrent.ArrayBlockingQueue
 
 const val REPO_LOG: String = "ChatRepository"
 
 class ChatRepository(private val apiInterface: APIInterface) {
-    public val messageDeliveredChannel = Channel<MessageDeliveredEvent>()
+    val messageDeliveredChannel = Channel<MessageDeliveredEvent>()
 
-    private val outgoingMessageQueue: ArrayBlockingQueue<Pair<Message, Conversation>> = ArrayBlockingQueue(16)
+    private data class OutgoingMessageInfo (
+        val message: Message,
+        val conversation: Conversation,
+        val guid: GUID,
+    )
+
+    private val outgoingMessageQueue: ArrayBlockingQueue<OutgoingMessageInfo> = ArrayBlockingQueue(16)
     private var outgoingMessageThread: Thread? = null
 
     suspend fun getVersion(): String {
@@ -30,14 +38,18 @@ class ChatRepository(private val apiInterface: APIInterface) {
         return apiInterface.getMessages(conversation.guid).body()!!
     }
 
-    suspend fun enqueueOutgoingMessage(message: Message, conversation: Conversation) {
-        Log.d(REPO_LOG, "Enqueuing outgoing message: $message")
-        outgoingMessageQueue.add(Pair(message, conversation))
+    suspend fun enqueueOutgoingMessage(message: Message, conversation: Conversation): GUID {
+        val guid = UUID.randomUUID().toString()
+
+        Log.d(REPO_LOG, "Enqueuing outgoing message: $message ($guid)")
+        outgoingMessageQueue.add(OutgoingMessageInfo(message, conversation, guid))
 
         if (outgoingMessageThread == null) {
             outgoingMessageThread = Thread { outgoingMessageQueueMain() }
             outgoingMessageThread?.start()
         }
+
+        return guid
     }
 
     // - private
@@ -47,7 +59,10 @@ class ChatRepository(private val apiInterface: APIInterface) {
         while (true) {
             val outgoingMessageRequest = outgoingMessageQueue.poll()?.let {
                 runBlocking {
-                    val (message, conversation) = it
+                    val guid = it.guid
+                    val message = it.message
+                    val conversation = it.conversation
+
                     Log.d(REPO_LOG, "Sending message to $conversation: $message")
 
                     val result = apiInterface.sendMessage(
@@ -60,7 +75,7 @@ class ChatRepository(private val apiInterface: APIInterface) {
 
                     if (result.isSuccessful) {
                         Log.d(REPO_LOG, "Successfully sent message.")
-                        messageDeliveredChannel.send(MessageDeliveredEvent(message, conversation))
+                        messageDeliveredChannel.send(MessageDeliveredEvent(guid, message, conversation))
                     } else {
                         Log.e(REPO_LOG, "Error sending message. Enqueuing for retry.")
                         outgoingMessageQueue.add(it)
diff --git a/backend/src/test/java/net/buzzert/kordophone/backend/BackendTests.kt b/backend/src/test/java/net/buzzert/kordophone/backend/BackendTests.kt
index 75992f3..1d77776 100644
--- a/backend/src/test/java/net/buzzert/kordophone/backend/BackendTests.kt
+++ b/backend/src/test/java/net/buzzert/kordophone/backend/BackendTests.kt
@@ -78,9 +78,10 @@ class BackendTests {
             sender = null,
         )
 
-        repository.enqueueOutgoingMessage(outgoingMessage, conversation)
+        val guid = repository.enqueueOutgoingMessage(outgoingMessage, conversation)
 
         val event = repository.messageDeliveredChannel.receive()
+        assertEquals(event.guid, guid)
         assertEquals(event.message.text, outgoingMessage.text)
     }
 }
\ No newline at end of file