Plugins can get plugin messages now.

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -103,10 +103,12 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
                 (ClientPlaySessionHandler) connection.getPlayer().getConnection().getSessionHandler();
         if (connection.getMinecraftConnection().getProtocolVersion() <= ProtocolConstants.MINECRAFT_1_12_2) {
             return message.getChannel().startsWith("MC|") ||
-                    playerHandler.getClientPluginMsgChannels().contains(message.getChannel());
+                    playerHandler.getClientPluginMsgChannels().contains(message.getChannel()) ||
+                    VelocityServer.getServer().getChannelRegistrar().registered(message.getChannel());
         } else {
             return message.getChannel().startsWith("minecraft:") ||
-                    playerHandler.getClientPluginMsgChannels().contains(message.getChannel());
+                    playerHandler.getClientPluginMsgChannels().contains(message.getChannel()) ||
+                    VelocityServer.getServer().getChannelRegistrar().registered(message.getChannel());
         }
     }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -42,6 +42,17 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
         this.player = player;
     }
 
+    @Override
+    public void activated() {
+        PluginMessage message;
+        if (player.getProtocolVersion() >= ProtocolConstants.MINECRAFT_1_13) {
+            message = PluginMessageUtil.constructChannelsPacket("minecraft:register", VelocityServer.getServer().getChannelRegistrar().getModernChannelIds());
+        } else {
+            message = PluginMessageUtil.constructChannelsPacket("REGISTER", VelocityServer.getServer().getChannelRegistrar().getLegacyChannelIds());
+        }
+        player.getConnection().write(message);
+    }
+
     @Override
     public void handle(MinecraftPacket packet) {
         if (packet instanceof KeepAlive) {
@@ -174,11 +185,17 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
         serverBossBars.clear();
 
         // Tell the server about this client's plugin messages. Velocity will forward them on to the client.
-        if (!clientPluginMsgChannels.isEmpty()) {
+        Collection<String> toRegister = new HashSet<>(clientPluginMsgChannels);
+        if (player.getProtocolVersion() >= ProtocolConstants.MINECRAFT_1_13) {
+            toRegister.addAll(VelocityServer.getServer().getChannelRegistrar().getModernChannelIds());
+        } else {
+            toRegister.addAll(VelocityServer.getServer().getChannelRegistrar().getLegacyChannelIds());
+        }
+        if (!toRegister.isEmpty()) {
             String channel = player.getConnection().getProtocolVersion() >= ProtocolConstants.MINECRAFT_1_13 ?
                     "minecraft:register" : "REGISTER";
-            player.getConnectedServer().getMinecraftConnection().delayedWrite(
-                    PluginMessageUtil.constructChannelsPacket(channel, clientPluginMsgChannels));
+            player.getConnectedServer().getMinecraftConnection().delayedWrite(PluginMessageUtil.constructChannelsPacket(
+                    channel, toRegister));
         }
 
         // Flush everything

proxy/src/main/java/com/velocitypowered/proxy/messages/VelocityChannelRegistrar.java

@@ -6,8 +6,10 @@ import com.velocitypowered.proxy.protocol.packet.PluginMessage;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+import java.util.Collection;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
 
 public class VelocityChannelRegistrar implements ChannelRegistrar {
     private static final Logger logger = LogManager.getLogger(VelocityChannelRegistrar.class);
@@ -55,4 +57,22 @@ public class VelocityChannelRegistrar implements ChannelRegistrar {
             identifierMap.remove(identifier.getId());
         }
     }
+
+    public Collection<String> getLegacyChannelIds() {
+        return identifierMap.values().stream()
+                .filter(i -> i instanceof LegacyChannelIdentifier)
+                .map(ChannelIdentifier::getId)
+                .collect(Collectors.toList());
+    }
+
+    public Collection<String> getModernChannelIds() {
+        return identifierMap.values().stream()
+                .filter(i -> i instanceof MinecraftChannelIdentifier)
+                .map(ChannelIdentifier::getId)
+                .collect(Collectors.toList());
+    }
+
+    public boolean registered(String id) {
+        return identifierMap.containsKey(id);
+    }
 }