When you forget to make the builder factory static:

Author: Gaming32

impl-classtransform/src/main/java/net/raphimc/javadowngrader/impl/classtransform/JavaDowngraderTransformer.java

@@ -93,7 +93,7 @@ public byte[] transform(String className, byte[] bytecode, boolean calculateStac
         }
     }
 
-    public Builder builder(TransformerManager transformerManager) {
+    public static Builder builder(TransformerManager transformerManager) {
         return new Builder(transformerManager);
     }