diff --git a/lib/AppForServer.js b/lib/AppForServer.js
index 7d5211c3..2910bd4a 100644
--- a/lib/AppForServer.js
+++ b/lib/AppForServer.js
@@ -340,15 +340,37 @@ AppForServer.prototype._updateScriptViews = function() {
 };
 
 AppForServer.prototype._autoRefresh = function(backend) {
-  var agents = this.agents = {};
+  this.agents = {};
   var app = this;
 
+  // Auto-refresh is implemented on top of ShareDB's messaging layer.
+  //
+  // However, ShareDB wasn't originally designed to support custom message types, so ShareDB's
+  // Agent class will log out "Invalid or unknown message" warnings if it encounters a message
+  // it doesn't recognize.
+  //
+  // A workaround is to register a "receive" middleware, which fires when a ShareDB server
+  // receives a message from a client. If the message is Derby-related, the middleware will
+  // "exit" the middleware chain early by not calling `next()`. That way, the custom message never
+  // gets to the ShareDB Agent and won't result in warnings.
+  //
+  // However, multiple Derby apps can run together on the same ShareDB backend, each adding a
+  // "receive" middleware, and they all need to be notified of incoming Derby messages. This
+  // solution combines the exit-early approach with a custom event to accomplish that.
   backend.use('receive', function(request, next) {
     var data = request.data;
     if (data.derby) {
-      app._handleMessage(request.agent, data.derby, data);
+      // Derby-related message, emit custom event and "exit" middleware chain early.
+      backend.emit('derby:_messageReceived', request.agent, data.derby, data);
+      return;
+    } else {
+      // Not a Derby-related message, pass to next middleware.
+      next();
     }
-    next();
+  });
+
+  backend.on('derby:_messageReceived', function(agent, action, message) {
+    app._handleMessage(agent, action, message);
   });
 };