Handshaking should fix on the encoding.

See f8916a839c5a7bfe6d0de100a33cf23de33de6e4 for the corresponding change in the remoting.

Since the error messages only use ASCII

core/src/main/java/hudson/TcpSlaveAgentListener.java

@@ -26,8 +26,10 @@ package hudson;
 import hudson.slaves.OfflineCause;
 import jenkins.AgentProtocol;
 
+import java.io.BufferedWriter;
 import java.io.DataInputStream;
 import java.io.IOException;
+import java.io.OutputStreamWriter;
 import java.io.PrintWriter;
 import java.net.BindException;
 import java.net.ServerSocket;
@@ -139,7 +141,9 @@ public final class TcpSlaveAgentListener extends Thread {
                 LOGGER.info("Accepted connection #"+id+" from "+s.getRemoteSocketAddress());
 
                 DataInputStream in = new DataInputStream(s.getInputStream());
-                PrintWriter out = new PrintWriter(s.getOutputStream(),true); // DEPRECATED: newer protocol shouldn't use PrintWriter but should use DataOutputStream
+                PrintWriter out = new PrintWriter(
+                        new BufferedWriter(new OutputStreamWriter(s.getOutputStream(),"UTF-8")),
+                        true); // DEPRECATED: newer protocol shouldn't use PrintWriter but should use DataOutputStream
 
                 String s = in.readUTF();
 

core/src/main/java/hudson/cli/CliProtocol.java

@@ -9,7 +9,9 @@ import jenkins.model.Jenkins;
 
 import java.io.BufferedInputStream;
 import java.io.BufferedOutputStream;
+import java.io.BufferedWriter;
 import java.io.IOException;
+import java.io.OutputStreamWriter;
 import java.io.PrintWriter;
 import java.net.Socket;
 
@@ -39,7 +41,7 @@ public class CliProtocol extends AgentProtocol {
         }
 
         public void run() throws IOException, InterruptedException {
-            PrintWriter out = new PrintWriter(socket.getOutputStream(),true);
+            PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream(),"UTF-8")),true);
             out.println("Welcome");
             runCli(new Connection(socket));
         }

core/src/main/java/jenkins/slaves/JnlpSlaveAgentProtocol.java

@@ -12,10 +12,12 @@ import jenkins.security.HMACConfidentialKey;
 
 import java.io.BufferedInputStream;
 import java.io.BufferedOutputStream;
+import java.io.BufferedWriter;
 import java.io.DataInputStream;
 import java.io.DataOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
+import java.io.OutputStreamWriter;
 import java.io.PrintWriter;
 import java.net.Socket;
 import java.util.logging.Level;
@@ -81,7 +83,7 @@ public class JnlpSlaveAgentProtocol extends AgentProtocol {
         public Handler(Socket socket) throws IOException {
             this.socket = socket;
             in = new DataInputStream(socket.getInputStream());
-            out = new PrintWriter(socket.getOutputStream(),true);
+            out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream(),"UTF-8")),true);
         }
 
         protected void run() throws IOException, InterruptedException {