Implement Player#spoofChatInput method

api/src/main/java/com/velocitypowered/api/proxy/Player.java

@@ -82,4 +82,11 @@ public interface Player extends CommandSource, InboundConnection, ChannelMessage
      * @param reason component with the reason
      */
     void disconnect(Component reason);
+
+    /**
+     * Sends chat input onto the players current server as if they typed it
+     * into the client chat box.
+     * @param input the chat input to send
+     */
+    void spoofChatInput(String input);
 }

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

@@ -247,7 +247,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
                 connection.closeWith(Disconnect.create(friendlyReason));
             }
         } else {
-            connection.write(Chat.create(friendlyReason));
+            connection.write(Chat.createClientbound(friendlyReason));
         }
     }
 
@@ -350,6 +350,12 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
         return true;
     }
 
+    @Override
+    public void spoofChatInput(String input) {
+        Preconditions.checkArgument(input.length() <= Chat.MAX_SERVERBOUND_MESSAGE_LENGTH, "input cannot be greater than " + Chat.MAX_SERVERBOUND_MESSAGE_LENGTH + " characters in length");
+        connectedServer.getMinecraftConnection().write(Chat.createServerbound(input));
+    }
+
     private class ConnectionRequestBuilderImpl implements ConnectionRequestBuilder {
         private final RegisteredServer server;
 

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/Chat.java

@@ -10,6 +10,8 @@ import net.kyori.text.serializer.ComponentSerializers;
 
 public class Chat implements MinecraftPacket {
     public static final byte CHAT = (byte) 0;
+    public static final int MAX_SERVERBOUND_MESSAGE_LENGTH = 256;
+
     private String message;
     private byte type;
 
@@ -61,12 +63,16 @@ public class Chat implements MinecraftPacket {
         }
     }
 
-    public static Chat create(Component component) {
-        return create(component, CHAT);
+    public static Chat createClientbound(Component component) {
+        return createClientbound(component, CHAT);
     }
 
-    public static Chat create(Component component, byte type) {
+    public static Chat createClientbound(Component component, byte type) {
         Preconditions.checkNotNull(component, "component");
         return new Chat(ComponentSerializers.JSON.serialize(component), type);
     }
+
+    public static Chat createServerbound(String message) {
+        return new Chat(message, CHAT);
+    }
 }