diff --git a/src/test/java/codechicken/multipart/asm/ScalaSigReaderCharacterizationTest.java b/src/test/java/codechicken/multipart/asm/ScalaSigReaderCharacterizationTest.java
new file mode 100644
index 0000000..f99447a
--- /dev/null
+++ b/src/test/java/codechicken/multipart/asm/ScalaSigReaderCharacterizationTest.java
@@ -0,0 +1,162 @@
+package codechicken.multipart.asm;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotSame;
+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.io.InputStream;
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Base64;
+import java.util.List;
+import java.util.Objects;
+import java.util.Scanner;
+import java.util.Set;
+import java.util.TreeSet;
+
+import org.junit.jupiter.api.Test;
+import org.objectweb.asm.ClassReader;
+import org.objectweb.asm.Type;
+import org.objectweb.asm.tree.AnnotationNode;
+import org.objectweb.asm.tree.ClassNode;
+
+import scala.Option;
+
+class ScalaSigReaderCharacterizationTest {
+
+    private static final String SIGNATURE_DESC = "Lscala/reflect/ScalaSignature;";
+
+    @Test
+    void keepsExactFacadeAndCompanionSurface() throws Exception {
+        Set<String> expected = signatures(
+                "decode(Ljava/lang/String;)[B",
+                "encode([B)Ljava/lang/String;",
+                "read(Lorg/objectweb/asm/tree/AnnotationNode;)Lcodechicken/multipart/asm/ScalaSignature;",
+                "write(Lcodechicken/multipart/asm/ScalaSignature;Lorg/objectweb/asm/tree/AnnotationNode;)Ljava/lang/Object;",
+                "ann(Lorg/objectweb/asm/tree/ClassNode;)Lscala/Option;");
+        for (Class<?> type : new Class<?>[] { ScalaSigReader.class, ScalaSigReader$.class }) {
+            assertEquals(Modifier.PUBLIC | Modifier.FINAL, type.getModifiers());
+            assertSame(Object.class, type.getSuperclass());
+            assertEquals(0, type.getInterfaces().length);
+            assertEquals(expected, publicMethods(type));
+        }
+        for (Method method : ScalaSigReader.class.getDeclaredMethods()) {
+            assertTrue(Modifier.isStatic(method.getModifiers()));
+        }
+
+        assertEquals(0, ScalaSigReader.class.getDeclaredFields().length);
+        Field module = ScalaSigReader$.class.getDeclaredField("MODULE$");
+        assertEquals(Modifier.PUBLIC | Modifier.STATIC | Modifier.FINAL, module.getModifiers());
+        assertSame(ScalaSigReader$.class, module.getType());
+        assertSame(ScalaSigReader$.MODULE$, module.get(null));
+        assertTrue(Modifier.isPrivate(ScalaSigReader$.class.getDeclaredConstructor().getModifiers()));
+    }
+
+    @Test
+    void encodingDropsTheTrailingByteAndSurvivesADecodeRoundTrip() {
+        byte[] payload = { 0, 1, 2, 3, 100, -1, -128, 127 };
+        String encoded = ScalaSigReader.encode(payload);
+
+        assertEquals(9, encoded.length());
+        for (int i = 0; i < encoded.length(); i++) {
+            assertTrue(encoded.charAt(i) <= 0x7f);
+        }
+        assertArrayEquals(payload, ScalaSigReader.decode(encoded));
+
+        assertEquals("", ScalaSigReader.encode(new byte[0]));
+        assertArrayEquals(new byte[0], ScalaSigReader.decode(""));
+
+        // The dropped trailing group is empty for a short payload, but carries the high bits of a long one.
+        byte[] single = { 42 };
+        assertArrayEquals(single, ScalaSigReader.decode(ScalaSigReader.encode(single)));
+        byte[] highBit = { 0, (byte) 0xfe };
+        assertFalse(Arrays.equals(highBit, ScalaSigReader.decode(ScalaSigReader.encode(highBit))));
+    }
+
+    @Test
+    void encodingLeavesItsInputAlone() {
+        byte[] payload = { 1, 2, 3 };
+        byte[] original = payload.clone();
+        assertNotSame(payload, ScalaSigReader.decode(ScalaSigReader.encode(payload)));
+        assertArrayEquals(original, payload);
+    }
+
+    @Test
+    void annReturnsTheFirstScalaSignatureAnnotationOrNone() {
+        ClassNode cnode = new ClassNode();
+        assertFalse(ScalaSigReader.ann(cnode).isDefined());
+
+        cnode.visibleAnnotations = new ArrayList<>();
+        cnode.visibleAnnotations.add(new AnnotationNode("Lscala/reflect/ScalaLongSignature;"));
+        assertFalse(ScalaSigReader.ann(cnode).isDefined());
+
+        AnnotationNode first = new AnnotationNode(SIGNATURE_DESC);
+        AnnotationNode second = new AnnotationNode(SIGNATURE_DESC);
+        cnode.visibleAnnotations.add(first);
+        cnode.visibleAnnotations.add(second);
+        Option<AnnotationNode> found = ScalaSigReader.ann(cnode);
+        assertTrue(found.isDefined());
+        assertSame(first, found.get());
+    }
+
+    @Test
+    void readsAndRewritesTheAnnotationValueOfAFrozenScalaClass() {
+        ClassNode cnode = new ClassNode();
+        new ClassReader(fixture()).accept(cnode, 0);
+        AnnotationNode ann = ScalaSigReader.ann(cnode).get();
+        String encoded = (String) ann.values.get(1);
+
+        ScalaSignature sig = ScalaSigReader.read(ann);
+        assertEquals(0, sig.bytes().pos());
+        assertEquals(sig.bytes().arr().length, sig.bytes().len());
+        assertArrayEquals(ScalaSigReader.decode(encoded), sig.bytes().arr());
+
+        // write returns the replaced list element and re-encodes to the same string.
+        Object replaced = ScalaSigReader.write(sig, ann);
+        assertEquals(encoded, replaced);
+        assertEquals(encoded, ann.values.get(1));
+    }
+
+    @Test
+    void readFailsOnAnAnnotationWithoutAValueList() {
+        assertThrows(Exception.class, () -> ScalaSigReader.read(new AnnotationNode(SIGNATURE_DESC)));
+
+        AnnotationNode wrongType = new AnnotationNode(SIGNATURE_DESC);
+        List<Object> values = new ArrayList<>();
+        values.add("bytes");
+        values.add(Integer.valueOf(1));
+        wrongType.values = values;
+        assertThrows(ClassCastException.class, () -> ScalaSigReader.read(wrongType));
+    }
+
+    private static byte[] fixture() {
+        InputStream input = Objects.requireNonNull(
+                ScalaSigReaderCharacterizationTest.class
+                        .getResourceAsStream("/compat/ReferenceScalaEdgePart.class.b64"));
+        try (Scanner scanner = new Scanner(input, StandardCharsets.US_ASCII.name()).useDelimiter("\\A")) {
+            return Base64.getMimeDecoder().decode(scanner.next());
+        }
+    }
+
+    private static Set<String> publicMethods(Class<?> type) {
+        Set<String> actual = new TreeSet<>();
+        for (Method method : type.getDeclaredMethods()) {
+            if (Modifier.isPublic(method.getModifiers())) {
+                actual.add(method.getName() + Type.getMethodDescriptor(method));
+            }
+        }
+        return actual;
+    }
+
+    private static Set<String> signatures(String... expected) {
+        return new TreeSet<>(Arrays.asList(expected));
+    }
+}