diff --git a/sip-codec/src/main/java/com/sip/codec/SipEncoder.java b/sip-codec/src/main/java/com/sip/codec/SipEncoder.java
index 65b2596..1d76a18 100644
--- a/sip-codec/src/main/java/com/sip/codec/SipEncoder.java
+++ b/sip-codec/src/main/java/com/sip/codec/SipEncoder.java
@@ -1,21 +1,123 @@
 package com.sip.codec;
 
 import com.sip.message.SipMessage;
+import com.sip.message.SipRequest;
+import com.sip.message.SipResponse;
+import com.sip.message.header.HeaderName;
+import com.sip.message.header.Headers;
+import com.sip.message.header.RawHeader;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.util.Objects;
 
 /**
  * Hand-written SIP wire-format encoder.
  *
- * <p>The encoder is responsible for producing on-the-wire bytes that
- * round-trip through {@link SipParser}, and for honoring
- * {@code Content-Length} based on the actual body it serializes.</p>
+ * <p>Produces bytes that round-trip through {@link SipParser}. The encoder
+ * is deliberately allocation-light: it builds the output in a single
+ * pre-grown {@link ByteArrayOutputStream} and writes ASCII bytes directly
+ * for the syntactic envelope (start-line and header names), reserving
+ * UTF-8 transcoding for header values and reason phrases.</p>
+ *
+ * <h2>Content-Length handling</h2>
+ * <p>The encoder always emits a {@code Content-Length} header whose value
+ * matches the actual body length, regardless of any value present in the
+ * input message. This keeps wire output internally consistent and avoids
+ * a common class of bugs where producer-side code mutates the body but
+ * forgets to update the header.</p>
  */
 public final class SipEncoder {
 
+    private static final byte[] CRLF = {'\r', '\n'};
+    private static final byte[] SP   = {' '};
+    private static final byte[] COLON_SP = {':', ' '};
+    private static final byte[] CONTENT_LENGTH_PREFIX =
+            "Content-Length: ".getBytes(StandardCharsets.US_ASCII);
+
     private SipEncoder() { }
 
     /** Encodes {@code message} into a freshly allocated byte array. */
     public static byte[] encode(SipMessage message) {
-        throw new UnsupportedOperationException(
-                "SipEncoder is a scaffold; the encoder lands in the next iteration.");
+        Objects.requireNonNull(message, "message");
+        ByteArrayOutputStream out = new ByteArrayOutputStream(256);
+        try {
+            switch (message) {
+                case SipRequest r -> writeRequestLine(out, r);
+                case SipResponse r -> writeStatusLine(out, r);
+            }
+            writeHeaders(out, message.headers(), message.body().length);
+            out.write(CRLF);
+            byte[] body = message.body();
+            if (body.length > 0) {
+                out.write(body);
+            }
+        } catch (IOException e) {
+            // ByteArrayOutputStream never throws — but the type signature requires this.
+            throw new SipCodecException(
+                    SipCodecException.Category.ENCODE_FAILURE, -1,
+                    "I/O while encoding message (unexpected for in-memory buffer)", e);
+        }
+        return out.toByteArray();
+    }
+
+    private static void writeRequestLine(ByteArrayOutputStream out, SipRequest r)
+            throws IOException {
+        out.write(asciiBytes(r.method().name()));
+        out.write(SP);
+        out.write(asciiBytes(r.requestUri().asWire()));
+        out.write(SP);
+        out.write(asciiBytes(r.version().literal()));
+        out.write(CRLF);
+    }
+
+    private static void writeStatusLine(ByteArrayOutputStream out, SipResponse r)
+            throws IOException {
+        out.write(asciiBytes(r.version().literal()));
+        out.write(SP);
+        // Status-Code is always 3 ASCII digits.
+        int s = r.status();
+        out.write((s / 100) + '0');
+        out.write(((s / 10) % 10) + '0');
+        out.write((s % 10) + '0');
+        out.write(SP);
+        out.write(r.reason().getBytes(StandardCharsets.UTF_8));
+        out.write(CRLF);
+    }
+
+    private static void writeHeaders(ByteArrayOutputStream out, Headers headers,
+                                     int bodyLength) throws IOException {
+        boolean contentLengthWritten = false;
+        for (RawHeader h : headers.asList()) {
+            if (h.name().equals(HeaderName.CONTENT_LENGTH)) {
+                writeContentLength(out, bodyLength);
+                contentLengthWritten = true;
+                continue;
+            }
+            writeHeader(out, h);
+        }
+        if (!contentLengthWritten) {
+            writeContentLength(out, bodyLength);
+        }
+    }
+
+    private static void writeHeader(ByteArrayOutputStream out, RawHeader h)
+            throws IOException {
+        out.write(asciiBytes(h.name().canonical()));
+        out.write(COLON_SP);
+        out.write(h.value().getBytes(StandardCharsets.UTF_8));
+        out.write(CRLF);
+    }
+
+    private static void writeContentLength(ByteArrayOutputStream out, int bodyLength)
+            throws IOException {
+        out.write(CONTENT_LENGTH_PREFIX);
+        out.write(Integer.toString(bodyLength).getBytes(StandardCharsets.US_ASCII));
+        out.write(CRLF);
+    }
+
+    private static byte[] asciiBytes(String s) {
+        return s.getBytes(StandardCharsets.US_ASCII);
     }
 }
diff --git a/sip-codec/src/test/java/com/sip/codec/SipEncoderTest.java b/sip-codec/src/test/java/com/sip/codec/SipEncoderTest.java
new file mode 100644
index 0000000..8be8143
--- /dev/null
+++ b/sip-codec/src/test/java/com/sip/codec/SipEncoderTest.java
@@ -0,0 +1,111 @@
+package com.sip.codec;
+
+import com.sip.message.SipMessage;
+import com.sip.message.SipMethod;
+import com.sip.message.SipRequest;
+import com.sip.message.SipResponse;
+import com.sip.message.SipVersion;
+import com.sip.message.header.Headers;
+import com.sip.message.uri.OpaqueUri;
+import org.junit.jupiter.api.Test;
+
+import java.nio.charset.StandardCharsets;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class SipEncoderTest {
+
+    @Test
+    void encodesMinimalOptionsRequest() {
+        SipRequest req = new SipRequest(
+                SipMethod.OPTIONS,
+                new OpaqueUri("sip", "carol@example.com"),
+                SipVersion.SIP_2_0,
+                Headers.builder()
+                        .add("Via", "SIP/2.0/UDP host;branch=z9hG4bK1")
+                        .add("Max-Forwards", "70")
+                        .build(),
+                new byte[0]);
+
+        byte[] bytes = SipEncoder.encode(req);
+        String text = new String(bytes, StandardCharsets.US_ASCII);
+
+        assertThat(text).isEqualTo(
+                "OPTIONS sip:carol@example.com SIP/2.0\r\n"
+                        + "Via: SIP/2.0/UDP host;branch=z9hG4bK1\r\n"
+                        + "Max-Forwards: 70\r\n"
+                        + "Content-Length: 0\r\n"
+                        + "\r\n");
+    }
+
+    @Test
+    void encodesResponseWith3DigitStatus() {
+        SipResponse rsp = new SipResponse(
+                SipVersion.SIP_2_0, 200, "OK",
+                Headers.empty(),
+                new byte[0]);
+
+        String text = new String(SipEncoder.encode(rsp), StandardCharsets.UTF_8);
+
+        assertThat(text).startsWith("SIP/2.0 200 OK\r\n");
+        assertThat(text).endsWith("Content-Length: 0\r\n\r\n");
+    }
+
+    @Test
+    void contentLengthIsAlwaysDerivedFromActualBody() {
+        // Even though the message carries Content-Length: 999, the encoder must
+        // emit the real body length to keep the wire output self-consistent.
+        byte[] body = "hello".getBytes(StandardCharsets.UTF_8);
+        SipRequest req = new SipRequest(
+                SipMethod.MESSAGE,
+                new OpaqueUri("sip", "u@e.com"),
+                SipVersion.SIP_2_0,
+                Headers.builder()
+                        .add("Content-Type", "text/plain")
+                        .add("Content-Length", "999")
+                        .build(),
+                body);
+
+        String text = new String(SipEncoder.encode(req), StandardCharsets.UTF_8);
+        assertThat(text)
+                .contains("Content-Length: 5\r\n")
+                .doesNotContain("Content-Length: 999")
+                .endsWith("\r\n\r\nhello");
+    }
+
+    @Test
+    void encoderInjectsContentLengthEvenWhenAbsent() {
+        SipRequest req = new SipRequest(
+                SipMethod.OPTIONS,
+                new OpaqueUri("sip", "u@e.com"),
+                SipVersion.SIP_2_0,
+                Headers.empty(),
+                new byte[0]);
+
+        String text = new String(SipEncoder.encode(req), StandardCharsets.US_ASCII);
+        assertThat(text).contains("Content-Length: 0");
+    }
+
+    @Test
+    void roundtripPreservesParsedMessage() {
+        String wire =
+                "INVITE sip:bob@biloxi.example.com SIP/2.0\r\n"
+                        + "Via: SIP/2.0/UDP pc33.atlanta;branch=z9hG4bK1\r\n"
+                        + "From: Alice <sip:alice@atlanta>;tag=1\r\n"
+                        + "To: Bob <sip:bob@biloxi>\r\n"
+                        + "Call-ID: a84b4c76e66710\r\n"
+                        + "CSeq: 314159 INVITE\r\n"
+                        + "Max-Forwards: 70\r\n"
+                        + "Content-Length: 0\r\n"
+                        + "\r\n";
+
+        SipMessage first = SipParser.parse(wire);
+        byte[] encoded = SipEncoder.encode(first);
+        SipMessage second = SipParser.parse(encoded);
+
+        assertThat(((SipRequest) second).method()).isEqualTo(SipMethod.INVITE);
+        assertThat(((SipRequest) second).headers().size())
+                .isEqualTo(((SipRequest) first).headers().size());
+        assertThat(second.body()).isEqualTo(first.body());
+    }
+}
diff --git a/sip-compliance-tests/src/test/java/com/sip/compliance/parser/ParserConformanceTest.java b/sip-compliance-tests/src/test/java/com/sip/compliance/parser/ParserConformanceTest.java
index 34d9753..828085c 100644
--- a/sip-compliance-tests/src/test/java/com/sip/compliance/parser/ParserConformanceTest.java
+++ b/sip-compliance-tests/src/test/java/com/sip/compliance/parser/ParserConformanceTest.java
@@ -1,6 +1,7 @@
 package com.sip.compliance.parser;
 
 import com.sip.codec.SipCodecException;
+import com.sip.codec.SipEncoder;
 import com.sip.codec.SipParser;
 import com.sip.compliance.fixture.FixtureExpectation;
 import com.sip.compliance.fixture.FixtureRepository;
@@ -41,6 +42,20 @@ Iterable<DynamicTest> everyAcceptFixtureIsParsed() {
                 .toList();
     }
 
+    /**
+     * Every accept fixture survives a parse → encode → parse round-trip
+     * preserving start-line, every header (modulo Content-Length
+     * normalisation), and body.
+     */
+    @TestFactory
+    Iterable<DynamicTest> everyAcceptFixtureRoundtripsThroughEncoder() {
+        return FIXTURES.stream()
+                .filter(fx -> fx.expectation() instanceof FixtureExpectation.Accept)
+                .map(fx -> dynamicTest("roundtrip: " + fx.id(),
+                        () -> assertRoundtripsCleanly(fx)))
+                .toList();
+    }
+
     /**
      * Reject-conformance is scoped to the {@code structural} parser phase.
      * Fixtures whose rejection happens later in the pipeline (typed-header
@@ -95,6 +110,37 @@ private static void assertParserAccepts(TortureFixture fx) {
                 .isEqualTo(e.headerCount());
     }
 
+    private static void assertRoundtripsCleanly(TortureFixture fx) {
+        SipMessage first = SipParser.parse(fx.bytes());
+        byte[] encoded = SipEncoder.encode(first);
+        SipMessage second = SipParser.parse(encoded);
+
+        FixtureExpectation.Accept e = (FixtureExpectation.Accept) fx.expectation();
+
+        // Body must survive round-trip byte-for-byte.
+        assertThat(second.body())
+                .as(fx.id() + ": body changed during roundtrip")
+                .isEqualTo(first.body());
+
+        // Start-line must survive.
+        if (first instanceof SipRequest reqA && second instanceof SipRequest reqB) {
+            assertThat(reqB.method().name()).isEqualTo(reqA.method().name());
+            assertThat(reqB.requestUri().asWire()).isEqualTo(reqA.requestUri().asWire());
+        } else if (first instanceof SipResponse rspA && second instanceof SipResponse rspB) {
+            assertThat(rspB.status()).isEqualTo(rspA.status());
+            assertThat(rspB.reason()).isEqualTo(rspA.reason());
+        } else {
+            throw new AssertionError(fx.id() + ": message kind changed during roundtrip");
+        }
+
+        // Header count must match the manifest (encoder may inject Content-Length
+        // when the original message lacked it; the manifest already declares the
+        // post-canonicalisation count, so equality is the right invariant).
+        assertThat(second.headers().size())
+                .as(fx.id() + ": header count changed during roundtrip")
+                .isEqualTo(e.headerCount());
+    }
+
     private static void assertParserRejects(TortureFixture fx) {
         FixtureExpectation.Reject r = (FixtureExpectation.Reject) fx.expectation();
         SipCodecException thrown = catchThrowableOfType(