Javassist: How to add agent to classpath?

I have an agent that I'm dynamically loading into a running Java application which opens a simple Swing JFrame when it's attached. It also allows appending new lines into a TextArea inside that JFrame.

My goal is to change how some methods work inside the application the agent is loaded into.

public class MyAgent {
    public static void agentmain(String args, Instrumentation instrumentation) {
        UI.openWindow();
        UI.addMessage("Agent loaded: %s", args);
        instrumentation.addTransformer(new MyTransformer());
        instrumentation.redefineClasses(new ClassDefinition(Class.forName("app.TargetClass"), ...));
    }
}

The UI window is managed in another class accessible from the agent. It successfully opens a window and appends a text message when the agent is loaded.

public class UI {
    private static SwingWindow swingWindow;
    
    public static void addMessage(String format, Object... args) {
        System.out.println("UI: " + String.format(format, args));
        swingWindow.appendToTextArea(format, args);
    }
    
    public static void openWindow() {
        try {
            SwingUtilities.invokeAndWait(() -> swingWindow = new SwingWindow());
        }
        catch (Exception e) {}
    }
}

I'm using Javassist to generate bytecode inside my transformer.

public class MyTransformer implements ClassFileTransformer {
    @Override
    public byte[] transform(ClassLoader loader, String className, ..., byte[] classBuffer) {
        if (className.equals("app/TargetClass")) {
            UI.addMessage("Now transforming the class that I need!");

            try {
                ClassPool classPool = ClassPool.getDefault();
                CtClass targetClass = classPool.get("app.TargetClass");
                CtMethod targetMethod = targetClass.getDeclaredMethod("importantMethod");
                targetMethod.insertBefore("me.domain.agent.ui.UI.addMessage(\"Hello from Javassist!\")");
                byte[] byteCode = targetClass.toBytecode();
                targetClass.detach();
                return byteCode;
            }
            catch (Exception e) {
                UI.addMessage("Couldn't transform the class I needed.");
            }
        }

        return classBuffer;
    }
}

The target class gets found, but the bytecode does not compile:

UI: Failed transforming class app.TargetClass: [source error] no such class: me$domain.agent.ui.UI

However, the UI class is inside the agent:

agent.jar
├── META-INF
└── me.domain.agent
    ├── ui
    │   └── UI.class
    ├── MyTransformer.class
    └── Agent.class

I've tried adding the agent's ClassLoader to Javassist's ClassPool:

classPool.insertClassPath(new LoaderClassPath(Agent.class.getClassLoader()));

But it doesn't work. How can I add a call to my agent UI into the bytecode?

I've decided to use ASM to call my agent UI from bytecode. There are no issues with finding the class.

Here's how the ASM-based class transformer looks like:

public class MyTransformer implements ClassFileTransformer {
    public void transformClass(ClassNode classNode) {
        MethodNode methodNode = findMethodNodeOfClass(classNode, "importantMethod", "()V");
        if (methodNode == null) {
            throw new TransformerException("app.TargetClass#importantMethod not found");
        }

        AbstractInsnNode firstInsn = findFirstInstruction(methodNode);
        if (firstInsn == null) {
            throw new TransformerException("No instructions in app.TargetClass#importantMethod");
        }

        InsnList insnList = new InsnList();
        insnList.add(new LdcInsnNode("Hello from ASM!"));
        insnList.add(new MethodInsnNode(INVOKESTATIC, Type.getInternalName(UI.class), "addMessage", "(Ljava/lang/String;)V"));
        methodNode.instructions.insertBefore(firstInsn, insnList);
    }

    @Override
    public byte[] transform(ClassLoader classLoader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] byteCode) {
        if (className.equals("app/TargetClass")) {
            try {
                ClassNode classNode = new ClassNode();
                ClassReader classReader = new ClassReader(byteCode);
                classReader.accept(classNode, 0);
                this.transformClass(classNode);
                ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_MAXS | ClassWriter.COMPUTE_FRAMES);
                classNode.accept(classWriter);
                return classWriter.toByteArray();
            }
            catch (Exception e) {
                e.printStackTrace();
            }
        }

        return byteCode;
    }
}

This will insert the static call UI.addMessage("Hello from ASM!") before the first non-label instruction of the app.TargetClass#importantMethod.