diff --git a/jetty-util/src/main/java/org/eclipse/jetty/util/CharsetStringBuilder.java b/jetty-util/src/main/java/org/eclipse/jetty/util/CharsetStringBuilder.java
index 341f39fda072..b31c70f41a98 100644
--- a/jetty-util/src/main/java/org/eclipse/jetty/util/CharsetStringBuilder.java
+++ b/jetty-util/src/main/java/org/eclipse/jetty/util/CharsetStringBuilder.java
@@ -23,7 +23,7 @@
 import java.util.Objects;
 
 /**
- * <p>Build a string from a sequence of bytes.</p>
+ * <p>Build a string from a sequence of bytes and/or characters.</p>
  * <p>Implementations of this interface are optimized for processing a mix of calls to already decoded
  * character based appends (e.g. {@link #append(char)} and calls to undecoded byte methods (e.g. {@link #append(byte)}.
  * This is particularly useful for decoding % encoded strings that are mostly already decoded but may contain
@@ -36,15 +36,29 @@
  */
 public interface CharsetStringBuilder
 {
+    /**
+     * @param b An encoded byte to append
+     */
     void append(byte b);
 
+    /**
+     * @param c A decoded character to append
+     */
     void append(char c);
 
+    /**
+     * @param bytes Array of encoded bytes to append
+     */
     default void append(byte[] bytes)
     {
         append(bytes, 0, bytes.length);
     }
 
+    /**
+     * @param b Array of encoded bytes
+     * @param offset offset into the array
+     * @param length the number of bytes to append from the array.
+     */
     default void append(byte[] b, int offset, int length)
     {
         int end = offset + length;
@@ -52,6 +66,11 @@ default void append(byte[] b, int offset, int length)
             append(b[i]);
     }
 
+    /**
+     * @param chars sequence of decoded characters
+     * @param offset offset into the array
+     * @param length the number of character to append from the sequence.
+     */
     default void append(CharSequence chars, int offset, int length)
     {
         int end = offset + length;
@@ -59,6 +78,9 @@ default void append(CharSequence chars, int offset, int length)
             append(chars.charAt(i));
     }
 
+    /**
+     * @param buf Buffer of encoded bytes to append. The bytes are consumed from the buffer.
+     */
     default void append(ByteBuffer buf)
     {
         int end = buf.position() + buf.remaining();
@@ -75,6 +97,10 @@ default void append(ByteBuffer buf)
 
     void reset();
 
+    /**
+     * @param charset The charset
+     * @return A {@link CharsetStringBuilder} suitable for the charset.
+     */
     static CharsetStringBuilder forCharset(Charset charset)
     {
         Objects.requireNonNull(charset);
@@ -106,7 +132,7 @@ public void append(char c)
         @Override
         public void append(CharSequence chars, int offset, int length)
         {
-            _builder.append(chars, offset, length);
+            _builder.append(chars, offset, offset + length);
         }
 
         @Override
@@ -145,7 +171,7 @@ public void append(char c)
         @Override
         public void append(CharSequence chars, int offset, int length)
         {
-            _builder.append(chars, offset, length);
+            _builder.append(chars, offset, offset + length);
         }
 
         @Override
diff --git a/jetty-util/src/test/java/org/eclipse/jetty/util/CharsetStringBuilderTest.java b/jetty-util/src/test/java/org/eclipse/jetty/util/CharsetStringBuilderTest.java
new file mode 100644
index 000000000000..246a9a009399
--- /dev/null
+++ b/jetty-util/src/test/java/org/eclipse/jetty/util/CharsetStringBuilderTest.java
@@ -0,0 +1,104 @@
+//
+// ========================================================================
+// Copyright (c) 1995 Mort Bay Consulting Pty Ltd and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
+// which is available at https://www.apache.org/licenses/LICENSE-2.0.
+//
+// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
+// ========================================================================
+//
+
+package org.eclipse.jetty.util;
+
+import java.nio.ByteBuffer;
+import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.is;
+
+// @checkstyle-disable-check : AvoidEscapedUnicodeCharactersCheck
+public class CharsetStringBuilderTest
+{
+    public static Stream<Arguments> tests()
+    {
+        return Stream.of(
+            Arguments.of("Hello World \uC2B5@\uC39F\uC3A4\uC3BC\uC3A0\uC3A1-UTF-16 Æ\tÿ!!!", StandardCharsets.UTF_16),
+            Arguments.of("Hello World \uC2B5@\uC39F\uC3A4\uC3BC\uC3A0\uC3A1-UTF-8 Æ\tÿ!!!", StandardCharsets.UTF_8),
+            Arguments.of("Now is the time for all good men to test US_ASCII \r\n\t!", StandardCharsets.US_ASCII),
+            Arguments.of("How Now Brown Cow. Test iso 8859 Æ\tÿ!", StandardCharsets.ISO_8859_1)
+        );
+    }
+
+    @ParameterizedTest
+    @MethodSource("tests")
+    public void testBuilder(String test, Charset charset) throws Exception
+    {
+        byte[] bytes = test.getBytes(charset);
+
+        CharsetStringBuilder builder = CharsetStringBuilder.forCharset(charset);
+
+        builder.append(bytes);
+        assertThat(builder.build(), equalTo(test));
+
+        for (byte b : bytes)
+            builder.append(b);
+        assertThat(builder.build(), equalTo(test));
+
+        builder.append(bytes[0]);
+        builder.append(bytes, 1, bytes.length - 1);
+        assertThat(builder.build(), equalTo(test));
+    }
+
+    public static Stream<Charset> charsets()
+    {
+        return Stream.of(
+            StandardCharsets.UTF_8,
+            StandardCharsets.ISO_8859_1,
+            StandardCharsets.US_ASCII,
+            StandardCharsets.UTF_16
+        );
+    }
+
+    @ParameterizedTest
+    @MethodSource("charsets")
+    public void testBasicApi(Charset charset) throws Exception
+    {
+        CharsetStringBuilder builder = CharsetStringBuilder.forCharset(charset);
+        ByteBuffer encoded = charset.encode("1");
+        while (encoded.hasRemaining())
+            builder.append(encoded.get());
+
+        builder.append('2');
+
+        builder.append(charset.encode("34"));
+
+        encoded = charset.encode("abc");
+        int offset = encoded.remaining();
+        encoded = charset.encode("abc56");
+        int length = encoded.remaining() - offset;
+        encoded = charset.encode("abc56xyz");
+        byte[] bytes = new byte[1028];
+        encoded.get(bytes, 0, encoded.remaining());
+        builder.append(bytes, offset, length);
+
+        encoded = charset.encode("abc78xyz");
+        encoded.position(offset);
+        encoded.limit(offset + length);
+        builder.append(encoded);
+
+        builder.append("9A", 0, 2);
+        builder.append("xyzBCpqy", 3, 2);
+
+        assertThat(builder.build(), is("123456789ABC"));
+    }
+}