diff --git a/app/controllers/Dev.scala b/app/controllers/Dev.scala
index b3486db0c1..a5f96352d7 100644
--- a/app/controllers/Dev.scala
+++ b/app/controllers/Dev.scala
@@ -19,7 +19,8 @@ object Dev extends LilaController {
     Env.report.scoreThresholdSetting,
     Env.api.cspEnabledSetting,
     Env.streamer.alwaysFeaturedSetting,
-    Env.rating.ratingFactorsSetting
+    Env.rating.ratingFactorsSetting,
+    Env.socket.socketDebugSetting
   )
 
   def settings = Secure(_.Settings) { implicit ctx => me =>
diff --git a/build.sbt b/build.sbt
index 4f1f334719..9b3efd3c74 100644
--- a/build.sbt
+++ b/build.sbt
@@ -391,7 +391,7 @@ lazy val tree = module("tree", Seq(common)).settings(
 )
 
 lazy val socket = module("socket", Seq(common, hub, memo, tree)).settings(
-  libraryDependencies ++= provided(play.api)
+  libraryDependencies ++= provided(play.api, reactivemongo.driver)
 )
 
 lazy val hub = module("hub", Seq(common)).settings(
diff --git a/modules/round/src/main/Env.scala b/modules/round/src/main/Env.scala
index 18bc51ea57..910a65c592 100644
--- a/modules/round/src/main/Env.scala
+++ b/modules/round/src/main/Env.scala
@@ -36,7 +36,8 @@ final class Env(
     evalCacheHandler: lila.evalCache.EvalCacheSocketHandler,
     isBotSync: lila.common.LightUser.IsBotSync,
     slackApi: lila.slack.SlackApi,
-    ratingFactors: () => lila.rating.RatingFactors
+    ratingFactors: () => lila.rating.RatingFactors,
+    val socketDebug: () => Boolean
 ) {
 
   private val settings = new {
@@ -278,6 +279,7 @@ object Env {
     evalCacheHandler = lila.evalCache.Env.current.socketHandler,
     isBotSync = lila.user.Env.current.lightUserApi.isBotSync,
     slackApi = lila.slack.Env.current.api,
-    ratingFactors = lila.rating.Env.current.ratingFactorsSetting.get
+    ratingFactors = lila.rating.Env.current.ratingFactorsSetting.get,
+    socketDebug = lila.socket.Env.current.socketDebugSetting.get
   )
 }
diff --git a/modules/round/src/main/History.scala b/modules/round/src/main/History.scala
index b3c4d283ec..4d9f43d01f 100644
--- a/modules/round/src/main/History.scala
+++ b/modules/round/src/main/History.scala
@@ -27,6 +27,12 @@ private final class History(
     events.headOption.fold(SocketVersion(0))(_.version)
   }
 
+  def versionDebugString: String = {
+    s"${events.lastOption.fold(-1)(_.version.value)}:${
+      events.headOption.fold(-1)(_.version.value)
+    }"
+  }
+
   def getEventsSince(v: SocketVersion, mon: Option[lila.mon.round.history.PlatformHistory]): EventResult = {
     val version = getVersion
     if (v > version) VersionTooHigh
diff --git a/modules/round/src/main/RoundSocket.scala b/modules/round/src/main/RoundSocket.scala
index 8d90a95a9b..a995cd3269 100644
--- a/modules/round/src/main/RoundSocket.scala
+++ b/modules/round/src/main/RoundSocket.scala
@@ -210,13 +210,13 @@ private[round] final class RoundSocket(
         case None =>
           lila.mon.round.history(mobile).versionCheck.getEventsTooFar()
           logger.info(s"Lost mobile:$mobile $version < ${history.getVersion} $gameId $member")
-          member push SocketTrouper.resyncMessage
+          member push SocketTrouper.resyncMsgWithDebug(s"sv(${history.versionDebugString}),cv($version)")
         case Some(Nil) => // all good, nothing to do
         case Some(evs) =>
           lila.mon.round.history(mobile).getEventsDelta(evs.size)
           lila.mon.round.history(mobile).versionCheck.lateClient()
           logger.info(s"Late mobile:$mobile $version < ${evs.lastOption.??(_.version)} $gameId $member")
-          batchMsgs(member, evs) foreach member.push
+          batchMsgsDebug(member, evs, s"sv(${history.versionDebugString}),cv($version)") foreach member.push
       }
 
     case eventList: EventList => notify(eventList.events)
@@ -297,6 +297,11 @@ private[round] final class RoundSocket(
     case many => makeMessage("b", many map (_ jsFor member)).some
   }
 
+  def batchMsgsDebug(member: Member, vevents: List[VersionedEvent], debug: => String) = {
+    if (Env.current.socketDebug()) makeMessageDebug("b", vevents map (_ jsFor member), debug).some
+    else batchMsgs(member, vevents)
+  }
+
   def notifyOwner[A: Writes](color: Color, t: String, data: A) =
     withOwnerOf(color) {
       _ push makeMessage(t, data)
diff --git a/modules/socket/src/main/Env.scala b/modules/socket/src/main/Env.scala
index 9f5f8d9de7..2880534881 100644
--- a/modules/socket/src/main/Env.scala
+++ b/modules/socket/src/main/Env.scala
@@ -6,7 +6,8 @@ import com.typesafe.config.Config
 import actorApi._
 
 final class Env(
-    system: ActorSystem
+    system: ActorSystem,
+    settingStore: lila.memo.SettingStore.Builder
 ) {
 
   import scala.concurrent.duration._
@@ -18,11 +19,18 @@ final class Env(
   private val userRegister = new UserRegister(system)
 
   system.scheduler.schedule(5 seconds, 1 seconds) { population ! PopulationTell }
+
+  val socketDebugSetting = settingStore[Boolean](
+    "socketDebug",
+    default = false,
+    text = "Send extra debugging to websockets.".some
+  )
 }
 
 object Env {
 
   lazy val current = "socket" boot new Env(
-    system = lila.common.PlayApp.system
+    system = lila.common.PlayApp.system,
+    settingStore = lila.memo.Env.current.settingStore
   )
 }
diff --git a/modules/socket/src/main/Socket.scala b/modules/socket/src/main/Socket.scala
index 2fc99b7b4b..509dd0b6e1 100644
--- a/modules/socket/src/main/Socket.scala
+++ b/modules/socket/src/main/Socket.scala
@@ -29,6 +29,11 @@ object Socket extends Socket {
 }
 
 private[socket] trait Socket {
+  def makeMessageDebug[A](t: String, data: A, debug: String)(implicit writes: Writes[A]): JsObject =
+    JsObject(new Map.Map3("t", JsString(t), "d", writes.writes(data), "debug", JsString(debug)))
+
+  def makeMessageDebug[A](t: String, debug: String): JsObject =
+    JsObject(new Map.Map2("t", JsString(t), "debug", JsString(debug)))
 
   def makeMessage[A](t: String, data: A)(implicit writes: Writes[A]): JsObject =
     JsObject(new Map.Map2("t", JsString(t), "d", writes.writes(data)))
diff --git a/modules/socket/src/main/SocketTrouper.scala b/modules/socket/src/main/SocketTrouper.scala
index 996b8e27d3..b2b6b7cbea 100644
--- a/modules/socket/src/main/SocketTrouper.scala
+++ b/modules/socket/src/main/SocketTrouper.scala
@@ -178,6 +178,10 @@ object SocketTrouper extends Socket {
   case class GetNbMembers(promise: Promise[Int])
 
   val resyncMessage = makeMessage("resync")
+
+  def resyncMsgWithDebug(debug: => String) =
+    if (Env.current.socketDebugSetting.get) makeMessageDebug("resync", debug)
+    else resyncMessage
 }
 
 // Not managed by a TrouperMap