diff --git a/src/test/java/codechicken/multipart/asm/ASMMixinFactoryCharacterizationTest.java b/src/test/java/codechicken/multipart/asm/ASMMixinFactoryCharacterizationTest.java
new file mode 100644
index 0000000..1f29dda
--- /dev/null
+++ b/src/test/java/codechicken/multipart/asm/ASMMixinFactoryCharacterizationTest.java
@@ -0,0 +1,135 @@
+package codechicken.multipart.asm;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
+import java.util.Arrays;
+import java.util.BitSet;
+import java.util.NoSuchElementException;
+import java.util.Set;
+import java.util.TreeSet;
+
+import org.junit.jupiter.api.Test;
+import org.objectweb.asm.Type;
+import org.objectweb.asm.tree.ClassNode;
+
+import scala.Predef$;
+import scala.collection.Seq;
+
+/**
+ * Registration and construction need the mixin compiler, which cannot run outside Forge; the Forge suite covers those
+ * paths. What is checked here is the surface downstream mods link against and the state the factory owns.
+ */
+class ASMMixinFactoryCharacterizationTest {
+
+    @Test
+    void keepsExactPublicSurface() throws Exception {
+        assertEquals(Modifier.PUBLIC, ASMMixinFactory.class.getModifiers());
+        assertSame(Object.class, ASMMixinFactory.class.getSuperclass());
+        assertEquals(0, ASMMixinFactory.class.getInterfaces().length);
+        assertEquals(
+                signatures(
+                        "baseType()Ljava/lang/Class;",
+                        "construct(Ljava/util/BitSet;Lscala/collection/Seq;)Ljava/lang/Object;",
+                        "getId(Ljava/lang/String;)I",
+                        "registerTrait(Ljava/lang/Class;)I",
+                        "registerTrait(Ljava/lang/String;)I",
+                        "onCompiled(Ljava/lang/Class;Ljava/util/BitSet;)V",
+                        "autoCompleteJavaTrait(Lorg/objectweb/asm/tree/ClassNode;)V",
+                        "codechicken$multipart$asm$ASMMixinFactory$$concreteParent$1"
+                                + "(Lcodechicken/multipart/asm/ASMMixinCompiler$ClassInfo;)"
+                                + "Lcodechicken/multipart/asm/ASMMixinCompiler$ClassInfo;",
+                        "codechicken$multipart$asm$ASMMixinFactory$$checkParent$1"
+                                + "(Lcodechicken/multipart/asm/ASMMixinCompiler$ClassInfo;Ljava/lang/String;)Z"),
+                publicMethods(ASMMixinFactory.class));
+
+        assertEquals(1, ASMMixinFactory.class.getDeclaredConstructors().length);
+        assertEquals(
+                "(Ljava/lang/Class;Lscala/collection/Seq;)V",
+                Type.getConstructorDescriptor(ASMMixinFactory.class.getDeclaredConstructor(Class.class, Seq.class)));
+
+        // MicroblockGenerator and MultipartMixinFactory extend this, and both override the two callbacks.
+        for (String name : new String[] { "onCompiled", "autoCompleteJavaTrait" }) {
+            for (Method method : ASMMixinFactory.class.getDeclaredMethods()) {
+                if (method.getName().equals(name)) {
+                    assertTrue(Modifier.isPublic(method.getModifiers()));
+                    assertFalse(Modifier.isFinal(method.getModifiers()));
+                }
+            }
+        }
+    }
+
+    @Test
+    void keepsTheSixFieldsThatHoldTheTraitAndClassCaches() throws Exception {
+        assertField("baseType", Class.class, Modifier.PRIVATE | Modifier.FINAL);
+        assertField("paramTypes", Seq.class, Modifier.PRIVATE | Modifier.FINAL);
+        assertField("traitMap", scala.collection.mutable.Map.class, Modifier.PRIVATE | Modifier.FINAL);
+        assertField("traits", scala.collection.mutable.ArrayBuffer.class, Modifier.PRIVATE | Modifier.FINAL);
+        assertField("classMap", scala.collection.mutable.Map.class, Modifier.PRIVATE | Modifier.FINAL);
+        assertField("ugenid", int.class, Modifier.PRIVATE);
+        assertEquals(6, ASMMixinFactory.class.getDeclaredFields().length);
+    }
+
+    @Test
+    void generatesClassNamesFromTheBaseTypeInRegistrationOrder() throws Exception {
+        ASMMixinFactory<String> factory = factory(String.class);
+        assertSame(String.class, factory.baseType());
+
+        Method nextName = ASMMixinFactory.class.getDeclaredMethod("nextName");
+        nextName.setAccessible(true);
+        assertEquals("String_cmp$$0", nextName.invoke(factory));
+        assertEquals("String_cmp$$1", nextName.invoke(factory));
+        assertEquals("String_cmp$$2", nextName.invoke(factory));
+
+        assertEquals("Object_cmp$$0", nextName.invoke(factory(Object.class)));
+    }
+
+    @Test
+    void reportsAnUnregisteredTraitAsAMissingMapKey() {
+        ASMMixinFactory<String> factory = factory(String.class);
+        NoSuchElementException error = assertThrows(NoSuchElementException.class, () -> factory.getId("test/Missing"));
+        assertEquals("key not found: test/Missing", error.getMessage());
+    }
+
+    @Test
+    void leavesBothCallbacksAsNoOps() {
+        ASMMixinFactory<String> factory = factory(String.class);
+        ClassNode cnode = new ClassNode();
+        cnode.visit(50, 0, "test/JTrait", null, "java/lang/Object", null);
+
+        factory.autoCompleteJavaTrait(cnode);
+        factory.onCompiled(String.class, new BitSet());
+
+        assertEquals(0, cnode.methods.size());
+    }
+
+    private static <T> ASMMixinFactory<T> factory(Class<T> baseType) {
+        return new ASMMixinFactory<>(baseType, Predef$.MODULE$.wrapRefArray(new Class<?>[0]));
+    }
+
+    private static void assertField(String name, Class<?> type, int modifiers) throws Exception {
+        Field field = ASMMixinFactory.class.getDeclaredField(name);
+        assertSame(type, field.getType());
+        assertEquals(modifiers, field.getModifiers());
+    }
+
+    private static Set<String> publicMethods(Class<?> type) {
+        Set<String> result = new TreeSet<>();
+        for (Method method : type.getDeclaredMethods()) {
+            if (Modifier.isPublic(method.getModifiers())) {
+                result.add(method.getName() + Type.getMethodDescriptor(method));
+            }
+        }
+        return result;
+    }
+
+    private static Set<String> signatures(String... values) {
+        return new TreeSet<>(Arrays.asList(values));
+    }
+}