diff --git a/src/test/java/codechicken/multipart/asm/ByteCodeReaderCharacterizationTest.java b/src/test/java/codechicken/multipart/asm/ByteCodeReaderCharacterizationTest.java
new file mode 100644
index 0000000..7c82bc0
--- /dev/null
+++ b/src/test/java/codechicken/multipart/asm/ByteCodeReaderCharacterizationTest.java
@@ -0,0 +1,155 @@
+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.nio.charset.StandardCharsets;
+import java.util.Arrays;
+import java.util.Set;
+import java.util.TreeSet;
+
+import org.junit.jupiter.api.Test;
+import org.objectweb.asm.Type;
+
+import codechicken.multipart.asm.ScalaSignature.Bytes;
+
+class ByteCodeReaderCharacterizationTest {
+
+    @Test
+    void keepsExactReaderSurface() throws Exception {
+        assertEquals(Modifier.PUBLIC, ByteCodeReader.class.getModifiers());
+        assertSame(Object.class, ByteCodeReader.class.getSuperclass());
+        assertEquals(0, ByteCodeReader.class.getInterfaces().length);
+        assertEquals(
+                signatures(
+                        "advance(ILjava/lang/Object;)Ljava/lang/Object;",
+                        "bc()Lcodechicken/multipart/asm/ScalaSignature$Bytes;",
+                        "more()Z",
+                        "pos()I",
+                        "pos_$eq(I)V",
+                        "readByte()B",
+                        "readLong()J",
+                        "readNat()I",
+                        "readString(I)Ljava/lang/String;"),
+                publicMethods(ByteCodeReader.class));
+
+        Field bc = ByteCodeReader.class.getDeclaredField("bc");
+        assertEquals(Modifier.PRIVATE | Modifier.FINAL, bc.getModifiers());
+        assertSame(Bytes.class, bc.getType());
+        Field pos = ByteCodeReader.class.getDeclaredField("pos");
+        assertEquals(Modifier.PRIVATE, pos.getModifiers());
+        assertSame(int.class, pos.getType());
+
+        assertEquals(1, ByteCodeReader.class.getDeclaredConstructors().length);
+        assertEquals(
+                "(Lcodechicken/multipart/asm/ScalaSignature$Bytes;)V",
+                Type.getConstructorDescriptor(ByteCodeReader.class.getDeclaredConstructor(Bytes.class)));
+    }
+
+    @Test
+    void startsAtTheSectionOffsetAndTracksItsOwnPosition() {
+        Bytes bytes = new Bytes(bytes(1, 2, 3, 4), 1, 2);
+        ByteCodeReader reader = bytes.reader();
+
+        assertSame(bytes, reader.bc());
+        assertEquals(1, reader.pos());
+        assertTrue(reader.more());
+
+        assertEquals((byte) 2, reader.readByte());
+        assertEquals(2, reader.pos());
+        assertTrue(reader.more());
+
+        assertEquals((byte) 3, reader.readByte());
+        assertEquals(3, reader.pos());
+        assertFalse(reader.more());
+
+        reader.pos_$eq(1);
+        assertTrue(reader.more());
+        assertEquals((byte) 2, reader.readByte());
+    }
+
+    @Test
+    void readByteKeepsSignedValues() {
+        ByteCodeReader reader = new Bytes(bytes(0x80, 0xff, 0x7f), 0, 3).reader();
+        assertEquals((byte) -128, reader.readByte());
+        assertEquals((byte) -1, reader.readByte());
+        assertEquals((byte) 127, reader.readByte());
+    }
+
+    @Test
+    void readNatDecodesSevenBitGroupsAndOverflowsSilently() {
+        ByteCodeReader reader = new Bytes(bytes(0x7f, 0x81, 0x00, 0xff, 0xff, 0x7f), 0, 6).reader();
+        assertEquals(127, reader.readNat());
+        assertEquals(128, reader.readNat());
+        assertEquals(2097151, reader.readNat());
+
+        // Five continuation groups overflow an int rather than failing.
+        assertEquals(-1, new Bytes(bytes(0x8f, 0xff, 0xff, 0xff, 0x7f), 0, 5).reader().readNat());
+    }
+
+    @Test
+    void readLongConsumesTheRestOfTheSectionUnsigned() {
+        assertEquals(0x01ffL, new Bytes(bytes(1, 0xff), 0, 2).reader().readLong());
+        assertEquals(0L, new Bytes(bytes(1, 2), 0, 0).reader().readLong());
+
+        Bytes section = new Bytes(bytes(9, 1, 2, 9), 1, 2);
+        ByteCodeReader reader = section.reader();
+        assertEquals(0x0102L, reader.readLong());
+        assertEquals(3, reader.pos());
+    }
+
+    @Test
+    void readStringUsesTheDefaultCharsetAndClampsToTheArray() {
+        ByteCodeReader reader = new Bytes("abcd".getBytes(StandardCharsets.UTF_8), 1, 3).reader();
+        assertEquals("bc", reader.readString(2));
+        assertEquals(3, reader.pos());
+
+        // A section longer than its array yields the remaining bytes but still advances by the requested length.
+        ByteCodeReader wide = new Bytes("abc".getBytes(StandardCharsets.UTF_8), 0, 10).reader();
+        assertEquals("abc", wide.readString(5));
+        assertEquals(5, wide.pos());
+        assertEquals("", wide.readString(0));
+    }
+
+    @Test
+    void advanceRefusesToLeaveTheSectionAndReturnsItsArgument() {
+        ByteCodeReader reader = new Bytes(bytes(1, 2, 3, 4), 1, 2).reader();
+        Object marker = new Object();
+        assertSame(marker, reader.advance(2, marker));
+        assertEquals(3, reader.pos());
+
+        IllegalArgumentException error = assertThrows(IllegalArgumentException.class, () -> reader.advance(1, marker));
+        assertEquals("Ran off the end of bytecode", error.getMessage());
+        assertEquals(3, reader.pos());
+        assertThrows(IllegalArgumentException.class, reader::readByte);
+        assertThrows(IllegalArgumentException.class, () -> reader.readString(1));
+    }
+
+    private static byte[] bytes(int... values) {
+        byte[] array = new byte[values.length];
+        for (int i = 0; i < values.length; i++) {
+            array[i] = (byte) values[i];
+        }
+        return array;
+    }
+
+    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));
+    }
+}