diff --git a/api.py b/api.py
index 0a99e22..91111f5 100644
--- a/api.py
+++ b/api.py
@@ -65,16 +65,17 @@ def Pin(google_user, client, client_address, instance_id, args):
   sender_message_id = args['sender_message_id']
 
   try:
-    models.Subject.FindOrCreate(subject).Pin(
+    pin = models.Subject.FindOrCreate(subject).Pin(
         message,
         models.Client.profile.get_value_for_datastore(client),
         sender_message_id,
         client_address,
         instance)
-  except models.DuplicateMessage:
+  except models.DuplicateMessage as e:
     logging.warning('Duplicate pin: %s', sender_message_id)
     return {
       'result': 'duplicate_message',
+      'message': e.original,
     }
   except models.AccessDenied:
     logging.warning('Pin access denied')
@@ -84,6 +85,7 @@ def Pin(google_user, client, client_address, instance_id, args):
 
   return {
     'result': 'ok',
+    'pin': pin,
   }
 
 
@@ -93,15 +95,16 @@ def SendMessage(google_user, client, client_address, instance_id, args):
   sender_message_id = args['sender_message_id']
 
   try:
-    models.Subject.FindOrCreate(subject).SendMessage(
+    msg = models.Subject.FindOrCreate(subject).SendMessage(
         message,
         models.Client.profile.get_value_for_datastore(client),
         sender_message_id,
         client_address)
-  except models.DuplicateMessage:
+  except models.DuplicateMessage as e:
     logging.warning('Duplicate message: %s', sender_message_id)
     return {
       'result': 'duplicate_message',
+      'message': e.original,
     }
   except models.AccessDenied:
     logging.warning('SendMessage access denied')
@@ -111,6 +114,7 @@ def SendMessage(google_user, client, client_address, instance_id, args):
 
   return {
     'result': 'ok',
+    'message': msg,
   }
 
 
diff --git a/lib/models.py b/lib/models.py
index ae9bf2b..e395964 100644
--- a/lib/models.py
+++ b/lib/models.py
@@ -38,7 +38,9 @@ import utils
 
 
 class DuplicateMessage(Exception):
-  pass
+  def __init__(self, original):
+    self.original = original
+    super(DuplicateMessage, self).__init__(original)
 
 
 class AccessDenied(Exception):
@@ -205,7 +207,7 @@ class Subject(db.Model):
         .filter('sender_message_id =', sender_message_id)
         .fetch(1))
     if messages:
-      raise DuplicateMessage(sender_message_id)
+      raise DuplicateMessage(messages[0].ToEvent())
 
     message_id = subject.next_message_id
     subject.next_message_id += 1
@@ -243,6 +245,7 @@ class Subject(db.Model):
     event = obj.ToEvent()
     for subscription in subscriptions:
       subscription.SendMessage(event)
+    return event
 
   @db.transactional()
   def PutPin(self, message, sender, sender_message_id,
@@ -257,7 +260,7 @@ class Subject(db.Model):
         .filter('instance =', instance)
         .fetch(1))
     if pins:
-      raise DuplicateMessage(sender_message_id)
+      raise DuplicateMessage(pins[0].ToEvent())
 
     obj = Pin(
         parent=self,
@@ -277,6 +280,7 @@ class Subject(db.Model):
     event = obj.ToEvent()
     for subscription in subscriptions:
       subscription.SendMessage(event)
+    return event
 
   @db.transactional()
   def RemovePin(self, sender, sender_message_id, instance_key):
diff --git a/static/cosmopolite.js b/static/cosmopolite.js
index 457bf4d..6e3fac0 100644
--- a/static/cosmopolite.js
+++ b/static/cosmopolite.js
@@ -752,7 +752,8 @@ Cosmopolite.prototype.registerMessageHandlers_ = function() {
  * Callback for a sendMessage RPC ack by the server.
  *
  * @param {Cosmopolite.typeMessage} message Message details.
- * @param {?function()} resolve Promise resolution callback.
+ * @param {?function(Cosmopolite.typeMessage)} resolve Promise resolution
+ *     callback.
  * @param {?function(Error)} reject Promise rejection callback.
  * @param {Object} response Server RPC response.
  * @private
@@ -768,7 +769,9 @@ Cosmopolite.prototype.onMessageSent_ = function(
   var result = response['result'];
   if (result == 'ok' || result == 'duplicate_message') {
     if (resolve) {
-      resolve();
+      response['message']['message'] = JSON.parse(
+          response['message']['message']);
+      resolve(response['message']);
     }
   } else {
     if (reject) {
diff --git a/static/test.js b/static/test.js
index 00086c1..314875c 100644
--- a/static/test.js
+++ b/static/test.js
@@ -193,14 +193,16 @@ asyncTest('Complex object', function() {
 });
 
 asyncTest('sendMessage Promise', function() {
-  expect(1);
+  expect(3);
 
   var subject = randstring();
   var message = randstring();
 
   var cosmo = new Cosmopolite({}, null, randstring());
-  cosmo.sendMessage(subject, message).then(function() {
+  cosmo.sendMessage(subject, message).then(function(msg) {
     ok(true, 'sendMessage Promise fulfilled');
+    equal(msg['subject']['name'], subject);
+    equal(msg['message'], message);
     cosmo.shutdown();
     start();
   });